1Natural Rubber and Recycled Materials William Klingensmith Akron Consulting Company, Akron, Ohio, U.S.A. Brendan Rodgers The Goodyear Tire & Rubber Company, Akron, Ohio, U.S.A. I. INTRODUCTION The nature of the tire and rubber industry has changed over the last 30 to 40 years in that, like all other industries, it has come to recognize the value of using renewable sources of raw materials, recycling materials whenever possible, and examining the potential of reclaiming used materials for fresh applications. Renewable raw materials range from natural rubber, more of which is used than any other elastomer, naturally occurring process aids such as pine tars and resins, and novel biological materials such as silica derived from the ash of burned rice husks. Naturally occurring materials include inorganic ﬁllers such as calcium carbonate, which is distinct from naturally occurring organic material, whose total supply may be restricted. Considerable work is underway today to develop markets and applications where rubber products can be recycled into existing new products and new applications developed for discarded rubber products such as tires. Given the desire to maximize the content of renewable, recycled, and reclaimed materials in rubber compounds, this review merges these topics under one title and treats each in turn. Copyright © 2004 by Taylor & Francis II. NATURAL RUBBER Of the range of elastomers available to technologists, natural rubber (NR) is among the most important, because it is the building block of most rubber compounds used in products today. In the previous edition of this text (1) Barlow presented a good introductory discussion of this strategic raw material. Roberts (2) edited a very thorough review of natural rubber covering topics ranging from basic chemistry and physics to production and applications. Natural rubber, which is a truly renewable resource, comes primarily from Indonesia, Malaysia, India, and the Philippines, though many more additional sources of good quality rubber are becoming available. It is a material that is capable of rapid deformation and recovery, and it is insoluble in a range of solvents, though it will swell when immersed in organic solvents at elevated temperatures. Its many attributes include abrasion resistance, good hysteretic properties, high tear strength, high tensile strength, and high green strength. However, it may also display poor fatigue resistance. It is diﬃcult to process in factories, and it can show poor tire performance in areas such as traction or wet skid compared to selected synthetic elastomers. Given the importance of this material, this section discusses 1. 2. 3. The biosynthesis and chemical composition of natural rubber Industry classiﬁcation, descriptions, and speciﬁcations Typical applications of natural rubber A. Chemistry of Natural Rubber Natural rubber is a polymer of isoprene (methylbuta-1,3-diene). It is a polyterpene synthesized in vivo via enzymatic polymerization of isopentenyl pyrophosphate. Isopentenyl pyrophosphate undergoes repeated condensation to yield cis-polyisoprene via the enzyme rubber transferase. Though bound to the rubber particle, this enzyme is also found in the latex serum. Structurally, cis-polyisoprene is a highly stereoregular polymer with an UOH group at the alpha terminal and three to four trans units at the omega end of the molecule (Fig. 1). Molecular weight distribution of Hevea brasiliensis rubber shows considerable variation from clone to clone, ranging from 100,000 to over 1,000,000. Natural rubber has a broad bimodal molecular weight distribution. The polydispersity or ratio of weight-average molecular weight to number-average molecular weight, Mw/Mn, can be as high 9.0 for some variety of natural rubber (3,4). This tends to be of considerable signiﬁcance in that the lower molecular weight fraction will facilitate ease of processing in end product manufacturing, while the higher molecular Copyright © 2004 by Taylor & Francis Figure 1 Cis and trans isomers of natural rubber. weight fraction contributes to high tensile strength, tear strength, and abrasion resistance. The biosynthesis or polymerization to yield polyisoprene, illustrated in Figure 2, occurs on the surface of the rubber particle(s) (5). The isopentyl pyrophosphate starting material is also used in the formation of farnesyl pyrophosphate. Subsequent condensation of transfarnesyl pyrophosphate yields trans-polyisoprene or gutta percha. Gutta percha is an isomeric polymer in which the double bonds have a trans conﬁguration. It is obtained from trees of the genus Dichopsis found in southeast Asia. This polymer is synthesized from isopentenyl pyrophosphate via a pathway similar to that for the biosynthesis of terpenes such as geraniol and farnasol. Gutta percha is more crystalline in its relaxed state, much harder, and less elastic. Natural rubber is obtained by ‘‘tapping’’ the tree Hevea brasiliensis. Tapping starts when the tree is 5–7 years old and continues until it reaches around 20–25 years of age. A knife is used to make a downward cut from left to right and at about a 20–30j angle to the horizontal plane, to a depth approximately 1.0 mm from the cambium. Latex then exudes from the cut and can ﬂow from the incision into a collecting cup. Rubber occurs in the trees in the form of particles suspended in a protein-containing serum, the whole Copyright © 2004 by Taylor & Francis Figure 2 Simpliﬁed schematic of the biosynthesis of natural rubber. constituting latex, which in turn is contained in speciﬁc latex vessels in the tree or other plant. Latex constitutes the protoplasm of the latex vessel. Tapping or cutting of the latex vessel creates a hydrostatic pressure gradient along the vessel, with consequent ﬂow of latex through the cut. In this way a portion of the contents of the interconnected latex vessel system can be drained from the tree. Eventually the ﬂow ceases, turgor is reestablished in the vessel, and the rubber content of the latex is restored to its initial level in about 48 hr. The tapped latex consists of 30–35% rubber, 60% aqueous serum, and 5–10% other constituents such as fatty acids, amino acids and proteins, starches, sterols, esters, and salts. Some of the nonrubber substances such as Copyright © 2004 by Taylor & Francis lipids, carotenoid pigments, sterols, triglycerides, glycolipids, and phospholipids can inﬂuence the ﬁnal properties of rubber such as its compounded vulcanization characteristics and classical mechanical properties. Hasma and Subramanian (6) conducted a comprehensive study characterizing these materials to which further reference should be made. Lipids can also aﬀect the mechanical stability of the latex while it is in storage, because lipids are a major component of the membrane formed around the rubber particle (7). Natural rubber latex is typically coagulated, washed, and then dried in either the open air or a ‘‘smokehouse.’’ The processed material consists of 93% rubber hydrocarbon; 0.5% moisture; 3% acetone-extractable materials such as sterols, esters, and fatty acids; 3% proteins; and 0.5% ash. Raw natural rubber gel can range from 5% to as high as 30%, which in turn can create processing problems in tire or industrial products factories. Nitrogen content is typically in the range of 0.3–0.6%. For clarity a number of deﬁnitions are given in Table 1. The rubber from a tapped tree is collected in three forms: latex, cuplump, and lace. It is collected as follows: 1. Latex collected in cups is coagulated with formic acid, crumbed, or sheeted. The sheeted coagulum can be immediately crumbed, aged and then crumbed, or smoke-dried at around 60jC to produce typically ribbed smoked sheet (RSS) rubber. Table 1 Deﬁnitions of Natural Rubber Terms Latex Fluid in the tree obtained by tapping or cutting the tree at a 20–30j angle to allow the latex to ﬂow into a collecting cup. Serum Aqueous component of latex that consists of lower molecular weight materials such as terpenes, fatty acids, proteins, and sterols. Whole ﬁeld latex Fresh latex collected from trees. Cup-lump Bacterially coagulated polymer in the collection cup. Lace Trim from the edge of collecting vessels and cut on tree. Earth scrap Collecting vessel overﬂow material collected from the tree base. Ribbed smoked sheets (RSS) Sheets produced from whole ﬁeld latex. LRP Large rubber particles. NSR Nigerian standard rubber. SIR Standard Indonesian rubber. SLR Standard Lanka rubber. SMR Standard Malaysian rubber. SRP Serum rubber particles. SSR Standard Singapore rubber. TSR Technically speciﬁed rubber. TTR Thai tested rubber. Copyright © 2004 by Taylor & Francis B.0 million tonnes (40%) was NR and the remaining was synthetic rubber (9). The traditional preservative is ammonia. Crumb rubber can be dried at temperatures up to 100jC.2. with all the major producing countries decreasing their output. natural rubber consumption is split—with tires consuming around 75%.98 (3. cleaned. and China having only limited impact on increased global consumption. due to bacterial action.0 and a density of 0. For the period 2002–2007 it is anticipated that Western European and Japanese consumption will increase due to economic recoveries in both areas. Ironfree water is necessary to minimize rubber oxidation. The major regional consumers of natural rubber are North America and eastern Asia. Natural rubber consumption will then increase slowly toward 8. Cup-lump is produced when the latex is left uncollected and allowed to coagulate.0 million tonnes per year. and Copyright © 2004 by Taylor & Francis .5 million metric tons (tonnes) of which 7. The lower grades of material are prepared from cup-lump. 3). The processing factories receive natural rubber in one of two forms: ﬁeld coagula or ﬁeld latex. and earth scrap after cleaning. nonrubber solids. cut. Japan. on the side of the collecting cup. automotive mechanical goods at 5%. Production of Natural Rubber Total global rubber consumption in 2001 was approximately 17. 3. which in concentrated solution is added in small quantities to the latex collected from the cup. Most latex concentrates are produced to meet the International Standard Organization’s ISO 2004 (8). dry rubber content. The formation of lace reseals the latex vessels and stops the ﬂow of rubber latex. with sustained economic activity in the United States. Globally. Field coagula consists of cup-lump and tree lace (Table 1).5–7. World production of NR was down by 3% from the same period in 2000. Lace is the coagulated residue left around the bark of the tree where the cut has been made for tapping. and crumbed.5 million tonnes. led predominantly by China and Japan. this being dependent on global economic conditions (Fig. rubber tree lace. nonautomotive mechanical goods at 10%. The net impact will be further growth in consumption toward 8. and alkalinity (as NH3). This standard deﬁnes the minimum content for total solids. Tetramethylthiuram disulﬁde (TMTD) and zinc oxide are also used as preservatives because of their greater eﬀectiveness as bactericides. partially dried small holders of rubber. Field coagulum or cup-lump is eventually collected. It is normally processed with cup-lump. Field coagula and latex are the base raw materials for the broad range of natural grades described in this review.4). creped. Fresh Hevea latex has a pH of 6. C. unsmoked sheet rubber. which will be discussed later. miscellaneous applications such as medical and health-related products consuming the remaining 10% (10). which is used in block rubber. Many times. There are around 25 million acres planted with rubber trees.. Smallholdings produce mainly cup-lump. Natural Rubber Products and Grades Natural rubber is available in six basic forms: 1.12). aged sheet rubber. Sheet rubber is generally regarded to be of higher quality. The three sources leading to crumb rubber (i. technically speciﬁed Copyright © 2004 by Taylor & Francis . and West Africa. India. and consistency and developing additional uses for contaminated material (11. one grade of technically speciﬁed rubber (TSR L) is produced from coagulated ﬁeld latex. and bale weights and dimensions. Malaysia. TSR 5 is produced from unsmoked sheets. Sheets Crepes Sheet rubber. the dominance of smallholdings has raised issues regarding quality and consistency. wrapping. with the objectives of improving rubber quality. A simpliﬁed schematic of the production process is presented in Figure 4. and lower grades such as TSR 10 and 20 are produced from ﬁeld coagulum. 2. uniformity. with the majority coming from smallholdings in Indonesia. 3.Figure 3 Global natural rubber productions (millions of tonnes). and ﬁeld cup-lump) typically provide diﬀerent grades of technically speciﬁed rubbers. Thailand.e. For example. typically displaying higher tensile and tear strength. and production employs nearly 3 million workers. In 1964 the International Standards Organization published a set of draft technical speciﬁcations that deﬁned contamination. Block rubber. 4. Field coagulum grade block rubbers have essentially replaced brown crepes except in India. Figure 4 Copyright © 2004 by Taylor & Francis . the ﬁrst four are in a dry form and represent nearly 90% of the total NR produced in the world. air-dried sheets. 6. these three types of dry NR are available in over 40 grades. accounting for less than 75. technically speciﬁed Preserved latex concentrates Specialty rubbers that have been mechanically or chemically modiﬁed Among these six types. 5. consisting of ribbed smoked sheets. Only Sri Lanka and India continue to produce latex crepes. and TSR in block form. which include latex-based and ﬁeld coagulum–derived estate brown crepes and remilled crepes. In the commercial market. crepes are now of minor signiﬁcance in the world market. Among the three major types.000 tonnes per year. crepes.Figure 4 Schematic of the natural rubber production process. presents a simpliﬁed schematic of the process followed in the production of natural rubber. smallholders’ rubber in most countries is processed and marketed as sheet rubber. To establish acceptable grades for commercial purposes. Crepe Rubber Crepe is a crinkled lace rubber obtained when coagulated latex is selected from clones that have a low carotene content. and the lower quality grade is typically RSS4. A number of prohibitions are also applicable to the RSS grades. Table 2 provides a summary of the criteria followed by inspectors in grading ribbed smoked sheet. bleached. undercured. Such rubber therefore lacks the anti-oxidation protection aﬀorded by drying the rubber in a smokehouse. Airdried sheets are prepared under conditions very similar to those for smoked sheets but are dried in a shed without smoke or additives. ribbed smoked sheet is the most popular. Only deliberately coagulated rubber latex processed into rubber sheets. and the details are given in the Green Book (13). Sheets are then suspended on poles for drying in a smokehouse for 2–4 days. Prior to grading RSS. The darker the rubber. can be used in making RSS. They are classiﬁed by visual evaluation. After straining. the sheets are separated and inspected and any blemishes are removed by manually cutting and removing defective material. two types of sheet rubbers are produced for the commercial market: ribbed smoked sheets (RSS) and air-dried sheets (ADS). properly dried and smoked. the water is removed. the International Rubber Quality and Packing Conference prepared a description for grading. Sheet Rubber Natural rubber in sheet form is the oldest and most popular type. the latex is passed Copyright © 2004 by Taylor & Francis . with the exception of sodium bisulfate. Sodium bisulﬁte is also added to maintain color and prevent darkening. 1. the lower the grade. Skim rubber made of skim latex cannot be used in whole or in part in patches as required under packing speciﬁcations deﬁned in the Green Book. 2. Being the simplest and easiest to produce on a small scale. This material can be substituted for RSS1 or RSS2 grades in various applications. Wet. The coagulated material is then milled. and original rubber and rubber that is not completely visually dry at the time of the buyer’s inspection is not acceptable (except slightly undercured rubber as speciﬁed for RSS-5). Whole ﬁeld latex used to produce ribbed smoked sheet is ﬁrst diluted to 15% solids and then coagulated for around 16 hours with dilute formic acid. and the material is sheeted with a rough surface to facilitate drying. Of the two. The premium grade is RSS1. Ribbed smoked sheet rubbers are made from intentionally coagulated whole ﬁeld latex. From the end user’s perspective. There are eight grades in this category. must be precleaned to separate the rubber from the bark. Trees or clones from which the grade is obtained typically have low yellow pigment levels (carotenes) and greater resistance to oxidation and discoloration. no blemishes No sand or foreign matter No sand or foreign matter No sand or foreign matter N/A several times through heavy rolls called creepers and the resultant material is air-dried at ambient temperature. Estate brown crepes. Tree bark scrap. Copyright © 2004 by Taylor & Francis . clean. slight Slight Slight Slight Slight Opaque spots No No No Slight Slight Slight Oversmoked spots No No No No Slight Slight Oxidized spots No No No No No N/A Burned sheets No No No No No No RSS 1X 1 2 3 4 5 Comments Dry. All six grades are made from cup-lump and other higher grade rubber scrap (ﬁeld coagulum) generated on the rubber estates. 3. All these grades must be produced from the fresh coagula of natural liquid latex under conditions where all processes are quality controlled. There are diﬀerent types of crepe rubber depending upon the type of starting materials from which they are produced.Table 2 Grade Classiﬁcation of Ribbed Smoked Sheet Rubber (RSS) Rubber mold No V. These grades are manufactured on powerwash mills 2. The rubber is milled to produce both thin and thick crepes. Powerwash mills are to be used in milling these grades into both thick and thin brown crepes (Table 4). Sri Lanka is the largest producer of pale crepes and the sole producer of thick pale crepe. Pale latex crepes. There are four grades in this class or category. The speciﬁcations for the diﬀerent types of crepe rubbers for which grade descriptions are given in the Green Book are as follows: 1. Thin brown crepes (remills). no blemishes Dry. slight Slight Slight Slight Slight Wrapping mold No V. Pale crepe is used for light-colored products and therefore commands a premium price. clean. There are six grades in this category. Pale crepes are used in pharmaceutical appliances such as stoppers and adhesives (Table 3). if used. streaks. bark No No No No No No Slight. < 10% of bales OK if <20% of bales OK if <20% of bales Odor No No No No No No No No No No Dust.Table 3 White and Pale Crepes Discoloration Class 1X 1X 1X 1 1 1 2 2 3 3 Grade Thin white crepe Thick pale crepe Thin pale crepe Thin white crepe Thick pale crepe Thin pale crepe Thick pale crepe Thin pale crepe Thick pale crepe Thin pale crepe Color White Light Light White Light Light Slightly darker Slightly darker Yellowish Yellowish Uniformity Uniform Uniform Uniform Slight shade Slight shade Slight shade Slight shade Slight shade Variation Variation Spots. sand No No No No No No No No No No Oil stains No No No No No No No No No No Oxidation No No No No No No No No No No Copyright © 2004 by Taylor & Francis . < 10% of bales Slight. Table 4 Estate Brown Crepes Discoloration Class 1X 1X 2X 2X 3X 3X Grade Thick brown crepe Thin brown crepe Thick brown crepe Thin brown crepe Thick brown crepe Thin brown crepe Color Light brown Light brown Medium brown Medium brown Dark brown Dark brown Uniformity Uniform Uniform Uniform Uniform Variation Variation Spots. sand. streaks No No No No No No Odor No No No No No No Dust. bark No No No No Bark Bark Oil stains No No No No No No Oxidation No No No No No No Copyright © 2004 by Taylor & Francis . blue for fast cure.4. dirty packing. clean. ﬁrm. The two grades of rubber in this category are produced on powerwash mills out of all types of scrap natural rubber in uncompounded form. if used. 6. and tough and also must retain an easily detectable smoked sheet odor. No other type of rubber can be used. and foreign matter are not permissible. For example. 5. for a more quantitative classiﬁcation scheme is required for visually inspected grades of natural rubber. must be precleaned to separate the rubber from the bark. lump. rubber meeting a speciﬁc visually determined grade or classiﬁcation might display poor mechanical properties when compounded with carbon black and vulcanizing agents owing to a broad or lower molecular weight distribution. Upon cure classiﬁcation. 4. Sludge. and red for slow cure) (Table 6) when the rubber is compounded using the American Society for Testing and Materials (ASTM) No.. the rubbers are further tested. This grade is made by milling on powerwash mills smoked rubber derived from ribbed smoked sheet (including block sheets) or ribbed smoked sheet cuttings. including earth scrap (Table 5). 3. and other high-grade scrap (Table 5). and at 0. Tree bark scrap. This classiﬁcation scheme has not received wide acceptance. from wet slab unsmoked sheet at the estates or smallholdings. Flat bark crepes. 1A formulation (15). Technical Classification of Visually Inspected Rubbers The Malaysian Rubber Producers Research Association (MRPRA) has published a technical information sheet describing sheet rubbers that have been further tested and classiﬁed with respect to cure characteristics (14). Technically Specified Natural Rubber (TSR) The International Standards Organization (ISO) ﬁrst published a technical speciﬁcation (ISO 2000) for natural rubber in 1964 (11). yellow for medium cure. Rubber of this type must be dry.e. The three grades in this category are also produced on powerwash mills from wet slab unsmoked sheets. heat spots. Based on these speciﬁcations. Malaysia introduced a national Standard Malaysian Rubber Copyright © 2004 by Taylor & Francis . which is clearly unfortunate. This may in turn create factory processing diﬃculties and product performance deﬁciencies.49 MPa the strain on the sample is measured after 1 min. Pure smoked blanket crepe. This color coding is limited to RSS1 and air-dried sheets. oil spots. Color variation from brown to very dark brown is permissible (Table 5). sand. The cure or vulcanization classes are distinguished by a color coding (i. Inclusion of earth scrap and smoked scrap is not permissible in these grades (Table 5). Thick blanket crepes (ambers). Thick Blanket. Flat Bark. bark No No No No No No Bark No No No Fine bark Fine bark No Oil stains No No No No No No No No No No No No No Oxidation No No No No No No No No No No No No No Thin brown crepes Thick blanket crepes (ambers) Flat bark crepes Pure smoked blanket crepe Copyright © 2004 by Taylor & Francis . Thin Brown.Table 5 Compo. Pure Smoked Blanket Crepe Discoloration Type Compo crepes Grade 1 2 3 1 2 3 4 2 3 4 Standard Hard Pure smoked Color Light brown Brown Dark brown Light brown Medium brown Medium brown Dark brown Light brown Medium brown Dark brown Very dark brown Black Not speciﬁed Spots. sand. streaks Yes Yes Yes Slight Yes Yes Yes Slight Slight Slight No No No Odor No No No No No No No No No No No No Smoked odor Dust. max. the designation given is Standard Indonesian Rubber Table 7 Technically Classiﬁed Rubbers Deﬁned in ISO 2000 Grade Property Dirt content.6 0.5 0.6 0.6 0. max. Technically speciﬁed rubbers are shipped in ‘‘blocks. All the block rubbers are also guaranteed to conform to certain technical speciﬁcations.5 0.5 0.or four-letter country code followed by a numeral indicating the maximum permissible dirt content for that grade expressed as hundredths of 1%.5 1. In Malaysia.8 TSR L 0.3 kg bales in the international market and 25. max. % strain Blue Production classiﬁcation Consumer acceptance 55–73 55–79 Yellow 73–85 61–91 Red 85–103 79–103 (SMR) scheme in 1965.0 kg in India.05 0.8 30 60 TSR 10 0.8 30 50 TSR 20 0.2 1 0. wt% Volatile matter. In Indonesia. as deﬁned by the national schemes or by ISO 2000 (Table 7).6 0.’’ which are generally 33.8 30 40 TSR 50 0.75 0. max. max.1 0.8 30 30 Copyright © 2004 by Taylor & Francis .6 0. lovibond units Mooney viscosity TSR CV 0.05 0. wt% Initial Wallace plasticity P0.05 0. The nomenclature describing technically speciﬁed rubbers consists of a three. min Plasticity retention index (min) Color. and since then all the natural rubber–producing countries have started production and marketing of technically speciﬁed rubbers based on the ISO 2000 scheme.6 0. wt% Ash content.Table 6 Technical Certiﬁcation of Sheet Rubber Class limits.6 0. wt% Nitrogen content. the TSR is designated as Standard Malaysian Rubber (SMR).8 30 60 60 6 60 F 5 TSR S 0. 2. In India. which facilitates handling.2 tonne pallet. 3.(SIR). It is shipped in a 1. handling. and storage space utilization. The detailed characteristics of the diﬀerent grades of TSR are discussed in the following subsections TSR CV. 3. polyethylene-wrapped bales of standard weight. and transportation. the TSRs are designated as Indian Standard Natural Rubber (ISNR). ISO has speciﬁed six grades of TSR. the rubber forms a coherent band almost instantaneously. thus potentially improving milling throughput. 2. Each pallet consists of 36 bales of 33. again to ensure better uniformity. Reduction of mixing times. Dirt is considered to be the residue remaining when the rubber is dissolved in a solvent. Grading is based on the dirt content measured as a weight percent. giving higher throughput Reduction of scraps and rejects due to better material uniformity Better resistance to chipping and chunking for oﬀ-the-road (OTR) tires Better green strength Copyright © 2004 by Taylor & Francis . Technically speciﬁed rubber (TSR) accounts for approximately 60% of the natural rubber produced worldwide. Coupled with its constant-viscosity feature. TSR CV rubber is generally softer than conventional technically speciﬁed grades. TSR CV. 4. washed through a 45 Am sieve. and each bale is wrapped in a polyethylene bag that is dispersible and compatible with rubber when mixed in an internal mixer at temperatures exceeding 110jC. 5. transportation. The advantages claimed for the technically speciﬁed rubbers over the conventional sheet and crepe grades of rubbers are the following: 1. of course. The storage hardening of this grade of rubber must be within 8 hardness units. They are shipped as compact. sometimes denoted as TTR). intended to ensure a uniform. Additional claimed beneﬁts of TSR CV include 1.3 kg net weight. and dried. typical in any rubber-mixing facility. it can provide a cost advantage by eliminating premastication. the TSRs are called Standard Thai Rubber (STR.’’ is produced from ﬁeld latex and is stabilized to a speciﬁed Mooney viscosity. They have a standard bale size to enable ease of transport through mechanized handling and containerization. When used in open mills. They can be prepared to prevent degradation of the rubber during storage. the CV designating ‘‘constant viscosity. 4. deﬁned quality. which are. In Thailand. Data on the content of foreign and volatile matter can be provided. They are available in a limited number of well-deﬁned grades. TSR L shows high tensile strength. TSR 5. This is a large-volume grade of technically speciﬁed natural rubber. It is packed and shipped to the same speciﬁcations as TSR CV and TSR L. ebonite battery plates. 3. raincoats. This material can be used for high-quality products such as mechanical mountings for engines and machinery. It is produced mostly from ﬁeld coagulum. large-truck tire treads. it has low ash and dirt content and is packed and presented in the same way as TSR CV. TSR L. bicycle tubes. TSR 10. adhesives. and ultimate elongation at break for both black and nonblack mix. inner tubes. rubber bands. ribbed smoked sheets. and unsmoked sheets. or air-dried sheets. TSR 5 is typically used for general-purpose friction and extruded products. and brake seals. TSR 10 has good technological properties similar to those of RSS2 and RSS3. sealing rings. cable sheaths. This material can be used for light-colored and transparent products such as surgical or pressure-sensitive tape. Technologically. surgical and pharmaceutical products. TSR 10 is produced from clean and fresh ﬁeld coagulum or from unsmoked sheets. large industrial rollers for the paper printing industry. masking tapes. TSR L. with 50 and 60 being the more common. microcellular sheets. gaskets. sportswear. TSR 5. injection-molded products including rubber–metal bonded components. conveyor belt covers. joint rings by injection molding. and certain components in tires. TSR 5 is produced from fresh coagulum. conveyor belts. vehicle suspension systems and general automotive components. cushion gum stocks. bridge bearings. TSR 20 has good processing characteristics Copyright © 2004 by Taylor & Francis . railway buﬀers. Lower viscosity Easier mixing characteristics (more rapid breakdown) Technical speciﬁcations and packaging in 33. cushion gum. bridge bearings.TSR CV rubber is available in diﬀerent viscosities. and TSR 10. It is packed and shipped to the same speciﬁcations as TSR CV. 2. separators. The advantage of TSR L is its light color together with its cleanliness and better heat-aging resistance. inner tubes. and TSR 5. chewing gum. It is packed and shipped in the same way as TSR CV. and footwear. and cement. industrial rolls. hot water bottles. cushion gum for retreading. lower grades of RSS. and adhesive solutions and tapes. TSR L is a light-colored rubber produced from high-quality latex. small components in passenger vehicles such as mountings.3 kg bales It can be used for tires. but has an advantage over RSS because of its 1. TSR L. upholstery and packing. TSR 20. textiles. modulus. and that of SMR 20 to 0.0%. Malaysia has produced grades of rubber outside the speciﬁc scope of ISO 2000. The most popular is Mooney viscosity (Vr). SMR 10. Viscosity and Viscosity Stabilization of Natural Rubber The properties of natural rubber that are most important regarding its use in the manufacture of tires or other products include viscosity. bicycle tires.016. It is viscosity-stabilized at 65 Mooney units using hydroxylamine neutral sulﬁte (HNS).08. 5%. and other general products. This procedure is deﬁned in ASTM D 1646. Of these three parameters. For example. that of SMR 10 from 0. and L from 0. 1. microcellular sheet for upholstery and packing. Stress Relaxation. It should be noted that these speciﬁcations will continue to be improved as production methods improve. This is the lowest grade of TSR and is produced from old. and other natural products. To illustrate the distribution and consumption of these various grades. It is similar to SMR10 in speciﬁcation. This property relates to the molecular weight. footwear. TSR 50.10 to 0. Natural rubber viscosity is a function of two major factors: viscosity of the rubber produced by the speciﬁc clone and the viscosity stabilization method. cushion gum stock. raincoats. 7%. 60%.025. molecular weight distribution. Its low viscosity and easier mixing characteristics (compared with the RSS grades) can reduce the mastication and mixing period considerably. dry ﬁeld coagulum or partly degraded rubber. SMR GP is a standard general-purpose (GP) rubber made from a 60:40 mixture of latex-grade sheet rubber and ﬁeld coagulum. SMR CV and SMR L.and physical properties. ‘‘Standard Test Methods for Mooney Viscosity. D. A range of methods are available to characterize the viscosity of natural rubber. fatty acid bloom. It is packed and shipped in the same way as other grades of TSR. CV50. The viscosity Copyright © 2004 by Taylor & Francis . 27%. In addition. shipments of SMR from Malaysia are typically SMR 20. and Prevulcanization Characteristics (Mooney viscometer)’’ (16). and amounts of other materials present in the polymer such as low molecular weight resins. It aﬀects the initial mixing of the rubber with other compounding ingredients and subsequent processing of the compounded materials to form the ﬁnal manufactured product. viscosity is probably the most important. SMR GP. fatty acids. It is used mostly for tires. which is obtained by measuring the torque that is required to rotate a disk embedded in rubber or a compounded sample. in 1991 the Rubber Research Institute of Malaysia revised the dirt levels of SMR CV60. conveyor belts. and compliance with the technical speciﬁcations. and SMR 5.05 to 0. Copyright © 2004 by Taylor & Francis .e. Mooney viscosity can be expressed as ML 1 + 4 (i. 6). Mooney large rotor. The rate of stress relaxation can correlate with molecular structural characteristics such as molec- Figure 5 Mooney scorch typically conducted at 121jC and 135jC. Stress relaxation.. The information obtained from a Mooney viscometer can include 1. A slow rate of relaxation indicates a higher elastic component in the overall response. which is the initial peak viscosity at the start of the test and a function of the green strength and can be a measure of compound factory shelf life. which can provide information on gel (T-95). It depends on molecular weight and molecular weight distribution. Viscosity (Vr). and nonrubber constituents. 3. The stress relaxation of rubber is a combination of both elastic and viscous response. 4. 2. molecular structure such as stereochemistry and polymer chain branching. is deﬁned as the response to a cessation of sudden deformation when the rotor of the Mooney viscometer stops. with 1 min pause and 4 min test duration). Caution is always required when attempting to establish relationships between Mooney viscosity and molecular weight. whereas a rapid rate of relaxation indicates a more highly viscous component. Prevulcanization properties or scorch resistance for the compounded polymer. provides a measure of the ease with which the material can be processed (Fig.will typically range from 45 to over 100. Mooney peak. a test that is conducted at temperatures ranging from 120jC to 135jC (Fig. 5). typically measured at 100jC. ular weight distribution.g. Bonﬁls et al. Much work has been done to establish a relationship between the Mooney viscosity (ML) and molecular weight of natural rubber as well as the molecular weight distribution. It is determined by measuring the time for a 95% (T-95) decay of the torque at the conclusion of the viscosity test. This provides another measure of the processing characteristics of the rubber. typically run at 100jC. (17) measured the molecular weight and molecular weight distribution of a number of samples of rubber from a variety of clones of Hevea brasiliensis and noted the following trend: Sample 1 2 3 4 P0 32 41 54 62 ML 1 + 4 57 78 92 104 Mw (kg/mol) 746 739 799 834 Copyright © 2004 by Taylor & Francis . Delta Mooney. and gel content.. It indicates the ease of processing compounds that are milled before being extruded or calendered (e. It can be used to give an indication of polydispersity or Mn/Mw. chain branching. 5. is the ﬁnal viscosity after 15 min.Figure 6 Mooney plasticity and stress relaxation. hot feed extrusion systems). which may inﬂuence the vulcanization rate.87. where P0 is the initial plasticity and P30 is the plasticity after aging for 30 min typically at 140jC.5 phr). Copyright © 2004 by Taylor & Francis . and 2-mercaptobenzothiazole (MBT. Natural rubber is susceptible to oxidation. stearic acid (0. zinc oxide (6. natural rubbers with low PRI values tend to break down more rapidly than those with high values. and Wallace plasticity P0.* This formulation is very sensitive to the presence of contaminants or other materials such as fatty acids. The Wallace plasticity test reports two measures: 1. and determined a correlation coeﬃcient of 0.19) explored this. and viscosity stabilization agent. pH of the coagulant. and subsequently cooled.where P0 is initial Wallace plasticity. The cure characteristics of natural rubber are highly variable due to such factors as maturation of the speciﬁc trees from which the material was extracted.0 phr). Various equations have been proposed that provide an empirical relationship between Mooney viscosity Vr. These * phr = parts per hundred parts of rubber. preservatives used. Though clearly not linear. method of coagulation. sulfur (3. A standardized formulation has been developed to enable a comparative assessment of diﬀerent natural rubbers. 2. Natural antioxidants will oﬀer protection from the degradation of natural rubber. This can aﬀect both the processing qualities of the rubber and the mechanical properties of the ﬁnal compounded rubber. it is known as the ACS1 (American Chemical Society No. which can be measured by the change in the material’s plasticity. and Mw is molecular weight. P0.5 phr). The plasticity retention index (PRI) measures recovery after a sample has been compressed. there is an empirical relationship between Mooney viscosity and molecular weight. established a relationship between intrinsic viscosity and Mooney viscosity. During processing in. 1). Mastication or milling also narrows the molecular weight distribution. a tire factory. Nair (18. Plasticity. is the initial Wallace plasticity and a measure of the compression of a sample after a load has been applied for a deﬁned time. dry rubber content. which is an important factor in this respect (20). ML 1 + 4 is Mooney viscosity after 4 min. for example. 0. This correlation can be improved by mastication of the test samples. amines. The formulation consists of natural rubber (100 phr). which improves the homogeneity. and amino acids. PRI% is deﬁned as ( P30/P0) Â 100. heated.5 phr). This hardening phenomenon is manifested as an increase in viscosity.50 for coeﬃcient X and between 4. denoted as HNS. allow lower mixing temperatures. An accelerated storagehardening test can measure the hardening of CV rubber that will occur during normal storage. will also reduce the viscosity of the compound with little loss in other mechanical properties. improve mixing uniformity. which can last for 5–7 days at around 60jC.30 wt % and 0. Synthetic polyisoprene when added at levels of up to 25% of the total polyisoprene content. Hydroxylamine neutral sulfate (NH2OHÁH2SO4). and this has been the basis for the development of CV rubbers.5 for C (21). and reduce mixing energy. Copyright © 2004 by Taylor & Francis . CV).08–0.equations depict a linear relationship between these two parameters and are therefore typically of the form Vr ¼ X P0 þ constant C ð 1Þ The numerical coeﬃcient X and constant C are functions of the clone and grade of rubber but normally fall between 1. It also allows for better control of component tack. The aldehyde group can readily react with the –NH2 groups in proteins to form a gel and thereby increase polymer viscosity.40 wt %.25 phr can signiﬁcantly improve productivity of the mixers. which is due to oxidation of the polymer chain and cleavage to form the functional groups. Other materials can be added to assist in improving the processability of natural rubber. However. This occurs primarily during the latex drying process.2V-dibenzamidodiphenyl disulﬁde. Materials may be added to natural rubber to suppress this increase in viscosity. respectively. to prevent gel formation. they will tend to display a darker color due to the HNS addition. When HNS is added before coagulation. These include peptizers such as 2.20– 0. or propionic hydrazide (PHZ) O k H2 NÀNHÀCÀEt Propionic hydrazide (PHZ) can be added to natural rubber latex at levels of 0. Natural rubber tends to harden during processing and storage at the plantation processing factory and also during shipping and prior to use in a rubber products manufacturing facility.15 and 1. which when added at levels of around 0. which is important in subsequent product assembly steps such as those in tire building. ketones UC(CH3) = O and aldehydes UCUCH = O. treated rubbers will show a P0 change of 8 units or less (constant viscosity.0 and 12. Mooney viscosity will range from 65 to 85. 5. The eﬀect is small. 4. Ammonia. the polymer gel content or other cross-linking phenomenon may increase.50% can result in a Mooney viscosity increase of up to 10 Mooney units.01% to 0. Coagulation methods can range from natural or bacterial coagulation to the addition of formic acid or heating. yield stimulants. Drying temperature. and strong van der Waals forces. the rubber can take a considerable time to cool. The age of the tapped rubber tree. Maturation. Storage of latex prior to drying and sheeting can cause an increase in Mooney viscosity due to an increase in gel content. An increase in the ammonia level added initially for preservation from 0. Copyright © 2004 by Taylor & Francis . the term ‘‘bound rubber’’ has frequently been used to describe this cross-linking condition in both natural rubber and polymers such as polybutadiene. 1. When hot. 2. Coagulation method. or the application of fast to ultrafast accelerators such as zinc diethyldithiocarbamate found in vulcanization systems with low cure temperatures. If baled hot. with 1:1 dilutions required to have any measurable eﬀect. with higher Mooney viscosity values being obtained through the use of natural coagulation techniques. Latex dilution. 3. The formation of bound rubber can result from the use of high-structure carbon blacks. Above 60jC there is a slight increase in Mooney viscosity. A number of production techniques can have an impact on the ﬁnal viscosity of the rubber. the use of silane coupling agents. Bound rubber can be found in all synthetic unsaturated elastomers and is due to a variety of factors such as covalent bonding. It can be readily measured by solvent extraction to remove polymer and leave a swollen insoluble gel. The ﬁeld methods are documented as follows. hydrogen bonding.Both HNS and PHZ block the reaction of the aldehyde groups with UNH2 by reacting with the UC(CH3) = O group to form RVUCðCH3 Þ ¼ N À NHUCOUR RVUCðCH3 ÞUCH ¼ NUCOUR and In compounded rubber. Another factor that can aﬀect viscosity is baling temperature. and seasonal eﬀects may also play some role. This rise in gel content can be due to an increase in pH due to partial hydrolysis of protein and amino acids and subsequent cross-linking or to an increase in bacterial action. Fatty acids.000 and 50. and abrasion resistance compared to elastomers that do not experience this phenomenon. Liquid natural rubber can be produced by a combination of mechanical milling. processing aid. A number of polymers emerged from this work: Liquid low molecular weight rubber. tensile strength. 40. particularly stearic acid. Reference may be made to the work of Claramma et al. can act as a nucleating agent in strain-induced crystallization (22. respectively. The rate of crystallization varies by grade. Depolymerized natural rubber ﬁnds application in ﬂexible molds for ceramics. The rapid crystallization of natural rubber is also due to nonrubber constituents present in the rubber. This rubber is liquid at room temperature but is also available on a silica carrier (24). heat. and base polymer. These are prepared by polymerizing 30. with pale crepe rubbers tending to show the greatest degree of crystallization. It is susceptible to oxidation and therefore requires appropriate compounding techniques for adequate aging resistance. and 49 parts of methyl methacrylate. liquid low molecular weight rubber can be used as a reactive plasticizer. in the latex before coagulation. They have found application primarily in adhesives due to the eﬀectiveness of the polar methacrylate group and non- Copyright © 2004 by Taylor & Francis .23). Methyl methacrylate grafting. (25) for a discussion on the eﬀect of liquid low molecular weight natural rubber on compounded classical mechanical properties. Molecular weights range between 40. Special-Purpose Natural Rubbers A considerable amount of work has been directed toward enhancing the properties of natural rubber through chemical modiﬁcation.000. 40. natural rubber crystallizes when strained or when stored at low temperatures.Because of the stereoregular structure of the polymer. and sealants. binders for grinding wheels. and the addition of chemical peptizer. Three grades of rubber with diﬀerent levels of grafted methyl methacrylate are available (Heveaplus MG 30. This can inﬂuence the end product performance. and 49). The rate of crystallization is temperature dependent and is most rapid at between À20jC and À30jC. E. This phenomenon is reversible and is very diﬀerent from storage hardening. Produced by depolymerization of natural rubber. for example in tires where strain-crystallized rubber can display reduced fatigue resistance but improved green strength. vulcanizates with high stiﬀness are attained but display Mooney viscosities ranging from 60 to 80 at typical factory compound processing temperatures.e. Two grades are available. These polymers oﬀer a number of advantages such as improved oil resistance (ENR 50 is comparable to polychloroprene). 2) Banbury mixing of the oil and rubber. seals. IRHD). Compared with natural rubber. for example. epoxidized NR shows better oil resistance and damping and low gas permeability. When blended with regular grades of natural rubber such as RSS2. However. OENR 75/25N for a 75% rubber.. and enables greater control of product uniformity and consistency (26). deproteinized rubber contains typically 0. When compounded.07%. Deproteinized natural rubber has found application in medical gloves to protect workers from allergic reactions and in automotive applications. Deproteinized natural rubber. The ratio of rubber to oil and oil type are denoted by a code that would read.polar isoprene bonding dissimilar surfaces. ﬁller loadings can be higher than those typically found in non-oil-extended rubber. Epoxy groups are randomly distributed along the polymer chain. This is produced by treating NR latex with an enzyme that breaks down naturally occurring protein and other nonrubber material into water-soluble residues. Such polymers tend to have very high hardness (International Rubber Hardness Degrees. The residues are then washed out of the rubber to leave a polymer with low sensitivity to water. and 3) soaking of the rubber in oilpans followed by milling to facilitate further incorporation and sheeting. The polymer displays low creep. Typically. 25 mol% epoxidized and 50 mol% epoxidized. with values up to 96 and have thus had no application in pneumatic tires (7. OENR is produced by one of several methods: 1) cocoagulation of latex with an oil emulsion prior to coagulation or with the dried ﬁeld coagulum. Copyright © 2004 by Taylor & Francis . natural rubber contains around 0. and compatibility with PVC. Both aromatic (A) and naphthenic (N) oils are used at loadings typically around 65 phr. When compounded with silica. and bushings. low gas permeability equivalent to that of butyl rubber. Calcium stearate is required as a stabilizer. Epoxidized natural rubber. ENR 25 and ENR 50.4% nitrogen as protein. its tear strength is low. Oil-extended natural rubber. which has prevented its use in pneumatic tires. Oil-extended natural rubber (OENR) treads are very eﬀective in improving ice grip and snow traction of tires and have been reported to be used for service in northern Europe.8). 25% naphthenic oil material. i. exhibits strain relaxation. The principal use of ebonite materials is in battery boxes.epoxidized NR has reinforcement properties equivalent to those of carbon black but without the use of silane coupling agents (27). Thermoplastic NR materials consist of blends of natural rubber and polypropylene. These two grades contain 80% crosslinked rubber. Superior processing rubber. The term ‘‘pseudoebonite’’ has been used to describe rubber with a Shore A hardness. Guayule is a shrub that grows in the southern region of the United States and northern Mexico. True ebonite has a Young’s modulus of 500 MPa and Shore D hardness of typically 75. Ebonite is a rubber vulcanized with very high levels of sulfur. or IRHD (International Rubber Hardness degrees). Ebonite has a sulfur content of 25–50 phr. in height and have a dry weight of approximately 20% resinous rubber. Guayule. respectively). These two-phase polymer systems display high stiﬀness with good ﬂow and process qualities. Two grades are also available (PA 57 and PA 80) with a processing aid added to further facilitate factory handling. Though work of any signiﬁcance has not been conducted in this area for many years. SP 50 with 20%. though that remains one area that oﬀers considerable potential for the future (27). Thermoplastic natural rubber. This consists of a mixture of two types of natural rubber. it is an area that merits further exploration. It is prepared by blending vulcanized latex with diluted ﬁeld latex in levels according to the grade being prepared (SP 20. linings. and 50% cross-linked phase. Ebonite. and resins may also be used to obtain the required hardness or meet any compounding constraints of concern to the technologist. New clones could be developed that might have improved output and supplement current supplies of NR extracted from Hevea brasiliensis (28). given the advances in genetic engineering and related ﬁelds in biotechnology. one cross-linked and the other not. No application in tires or other major elastomer-based products has been developed. A typical 5–10-year old plant will grow to about 30 in. 40%. of 98 or Shore D hardness of 60. Rubber of reasonable quality can be obtained after extraction. piping valves and Copyright © 2004 by Taylor & Francis . SP 40. and TiCl4 (HAIN-i-C3H7)6. and coverings for rollers. packaging. mix consistency. supply. however. high-speed extrusions.4. When the required conversion is complete. Titanium-based catalysts will produce a polymer typically 96% cis-1. Such polymers have a much higher glass transition temperature (Tg) and therefore tend to ﬁnd application in tire tread compounds where traction is a required performance parameter. camphor. and its Mooney viscosity (ML 1 + 4) ranges from 55 to 95.’’ such as consistency or uniformity. SBR.4isoprene. Polyisoprene is more uniform than natural rubber and thus lends itself better to applications requiring good mixing eﬃciency. tear strength. a terminator (short stop) to deactivate the catalyst and a stabilizer are added. and PBD. These are n-BuLi. Such projected shortages could be met by the use of synthetic polymers such as polyisoprene. With appropriate levels of modiﬁers. Quality A number of factors can be considered under the broad category of ‘‘quality. and other natural products. the level of trans-1. It is used in applications requiring high tensile strength. TiCl4R3Al anisole and CS2.pumps. Three organometallic initiators have attained commercial signiﬁcance in cispolyisoprene production. Isoprene for chemical synthesis is typically recovered from the C5 streams obtained in the thermal cracking of naphtha. and minimum contamination. and 7% vinyl-3. where chemical and corrosion resistance is required (29). Demand. The following discussion will highlight some general qualities that Copyright © 2004 by Taylor & Francis . depending on the commercial grade. Global production of natural rubber is expected to stabilize at 8. 4-isoprene can be increased.000–400. The lithium catalyst will produce a polymer with a microstructure that is 92% cis-1. Synthetic polyisoprene. where HAIN is a poly (Nalkylimino alane).0 million tonnes/year in 2003–2004. The glass transition temperature is around À70jC.4-isoprene and 4% vinyl or isopropenyl. and abrasion resistance (30. The numberaverage molecular weight (Mn) of polyisoprene is 350. though there may be trace amounts of trans-1.000. with a ﬁnal total capacity of about 8. The isoprene unit not only exists in natural rubber but is also the building block for terpenes.5 million tonnes within a few years after that. and component uniformity as in tires.31).4-isoprene.500 million tonnes by the year 2020. is projected to grow to 10. resilience. F.4-isoprene and 1% trans-1. In an eﬀort to provide consistency and stability. The only physical measures that are used to quantify the processing characteristics of natural rubber are original Wallace rapid plasticity ( P0) and the plasticity retention index (PRI). It is also possible to stabilize other grades to a Mooney of 65 F 10 units using HNS if necessary. Shipping in metal containers avoids wood contamination and is recommended. P0 tested via the Wallace Plastimeter is used as a rapid means of measuring plasticity. Contamination Considerable work has been done to lower the dirt level in both technically speciﬁed and visually inspected rubbers. which in turn yields consistent processing characteristics.e. hydroxylamine neutral sulfate (HNS) is added to grades such as TSR 10 CV and TSR 20 CV that have been stabilized. natural rubber uniformity is required for ﬁnal compound consistency. Copyright © 2004 by Taylor & Francis . and little or no need to warm the rubber prior to mixing. The last revision to the Standard Malaysian Rubber (SMR) scheme (12) introduced the following revisions: 1. a P0 of 30 would suggest a Mooney viscosity of about 60. were deﬁned whose Mooney viscosities (ML 1 + 4) are 50 F 5 and 60 F 5.. Two constant viscosity grades. Packaging Bales must be wrapped properly to prevent moisture penetration and mold growth. to maintain the quality levels of the rubber at time of purchase. The level of P0 has been determined to represent approximately half the level of Mooney viscosity. i. increasing with time (storage between processing at source and use at tire plant delayed by ocean transit) and unfortunately at an inconsistent rate and level. 3. SMR 10 CV and SMR 20 CV. Consistency and Uniformity Within a grade. tack. and to avoid contamination. and product component properties. extrudate uniformity. Lack of consistency will result in variation in mixing speciﬁcations. respectively. Mooney viscosity and P0 alter with storage hardening (as a result of the cross-linking of random functional groups such as aldehyde groups in the polyisoprene chain). 2.rubber product manufacturers should expect from the raw natural rubber producers. In tire and industrial goods manufacturing. little spread in properties such as plasticity retention index. end users of natural rubber require uniformity. 1. Fatty Acids Excessive levels of fatty acids such as palmitic acid. Tack-inducing resins such as ExxonMobil Escorez 1102 may also be used to correct bloom. Synthetic polyisoprene can be used to control bloom. In the washing and cleaning of NR at the processor’s factory the sedimentation process separates heavy material from the ﬂoating light rubber crumbs. leaves. High levels of fatty acids can also aﬀect vulcanization kinetics. for example.8%. Dirt level speciﬁcations were reduced from 0. a TSR 20 with fatty acid levels of 0. This is in recognition that the ‘‘dirt’’ level has improved signiﬁcantly over the last few years for all TSR grades. and leaf stems. Addition of a polymer such as Natsyn 2200 (Goodyear Tire & Rubber Company) of up to 50% of the ﬁnal product can be used where there are concerns regarding excessive fatty acid levels. The NR industry has focused on reduction of dirt content by the use of sedimentation within the process. In consequence. Table 8 presents total fatty acid levels for a variety of natural rubbers and shows that they can vary from 0.16 for SMR 20 CV. SMR 60 CV). emphasis must be placed on reducing contamination. CV grades of SMR 5 were speciﬁed.0 wt%. and stearic acid can bloom to the surface of compounded rubber components prepared for tire building or other engineered products. Malaysian rubbers are produced to clearly deﬁned dirt levels and thus require little washing. again with viscosities of 50 and 60 (SMR 50 CV. but contamination with foreign matter is generally caused by material that ﬂoats and therefore is not controlled in the traditional process. In the future.03% are now typical. but it does not separate the light. twigs. Such bloom can later cause component separations. This is due to bloom. wood. and viscosity. 3. 4. Fatty acid levels are to a large degree a function of the amount of washing the raw materials undergo prior to shipping. oleic acid. Dirt levels of 0.9–1. Contaminants include foreign material originating from the ﬁeld in the form of bark. These have the potential to cause ﬁnal product failure.08 for SMR 10 CV and from 0.20 to 0.10 to 0. ﬂoating contaminants satisfactorily.2. materials from other regional sources such as Indonesia initially contain much higher dirt Copyright © 2004 by Taylor & Francis . Although the level of dirt may be measured by the residue weight and as such can be included in technical speciﬁcations. because large foreign particles do not disperse during compounding and can provide sites for crack initiation. Tire plants may have component tack diﬃculties when. contamination by foreign light matter such as wood chips and plastic material are not speciﬁable at this time. However. fatty acid levels can be relatively high. tack.3% to 0.25 wt% is changed to a TSR 20 grade with a fatty acid level of 0. which reduces dirt content. 08 0. which range in size from 12 in. require more washing. It is a function of tire operating temperature and durability. This is a measure of the number of tons a tire conﬁguration on a vehicle is capable of hauling at the average vehicle speed for the operation shift. truck tires use approximately 35%.31 Thailand (TTR-20) 0.05 0. Smither’s Scientiﬁc Services publishes an analysis of the materials used in various tire lines. Reduced rolling resistance.20 0.40 0. at the lower end up to 16 in.15 0. consumption of natural rubber in tires is divided as follows: automobile tire production uses around 45%. particularly for tires operating in oﬀ road conditions.20 0.05 0.20). ton miles per hour (TMPH). 3.20 0. Component-to-component adhesion for durability and tire retreadability. and the remaining 20% is used for farm. From this information an average NR content may be estimated for each class of tire as illustrated in Figure 7. Use of Natural Rubber in Tires The amount of natural rubber used in speciﬁc tires varies according to the design and size of the tire. or oﬀ-the-road (OTR) vehicles and aircraft (10.10 0. Lower operating temperatures. and as a result have a greater amount of fatty acids removed before baling and shipping. Copyright © 2004 by Taylor & Francis .80 Malaysia (SMR-20) 0. The higher natural rubber levels found in commercial tires are required to meet the following performance needs: 1.75 Thailand (RSS-2) 0.Table 8 Examples of Fatty Acid Levels in TSR 20 Natural Rubber Weight percent fatty acid Acid Linoleic Palmitic Oleic Stearic Other Total Indonesia (SIR-20) 0. 4.15 0.30 0. 6. earthmover.08 0. For example.32). Improved OTR tire rating.10 0.20 0.10 0.15 0. Larger tires for commercial applications will have natural rubber contents within a range of 32–40 wt% (19. In terms of tonnage. Tear strength.10 0.90 levels. automobile and radial light truck tires. 5. 2.05 0.05 0.10 0. G. Tread wear. at the upper end contain from 10% to 15% by weight natural rubber. RMT. radial medium truck.) (From Ref. radial light truck. 36.) Copyright © 2004 by Taylor & Francis . (RLT.Figure 7 Natural rubber content by tire line (% of total tire weight) and relationship to tire performance triangles. oﬀ-the-road. OTR. as when it is used in commercial radial truck tires for good hysteresis and tear strength. The change in bound rubber content can be readily estimated from Mooney viscosity and Mooney peak data. strong interactions can occur such as hydrogen bonding. and van der Waals forces. sidewall. truck tire tread compounds containing 100 phr natural rubber. and thus save energy. Radialization has led to signiﬁcant increases in the use of natural rubber. The natural rubber content in tread compounds can range from 10 phr. Uncured compound that has been stored for long periods will show an increase in bound rubber content with consequent loss in ease of factory processing (33). when ﬁrst placed in a hothouse or broken down on warm-up mills. styrene butadiene rubber (SBR) for traction and stiﬀness. covalent bond formation. The Malaysian Natural Rubber Producers Association and Smithers Scientiﬁc Services have both reported on the use of natural rubber in the various components of a tire. The bulk of natural rubber is compounded with other elastomers to produce blends and thereby obtain the desired mechanical properties. permit shorter dwell time in internal mixers. and abrasion. However. When immersed in a solvent such as toluene. and 3–5 phr of process oil will have a shelf life of 3–5 days before extrusion due to the increase in bound rubber. Other polymers typically blended with natural rubber are polybutadiene (BR) for resistance to abrasion and fatigue. cut growth. and aircraft tires and the size of the bias truck tire market decreases. Typical levels of natural rubber in tread compound. to 100 phr. Tire sidewalls are typically 50:50 blends of natural rubber and polybutadiene for resistance to fatigue. or other processing step. This provides the factory rubber technologist with a simple tool to determine factory compound shelf life and times between compound mixing and subsequent extrusion.At temperatures below 100jC. and synthetic polyisoprene (IR) for processing qualities. In the absence of refrigeration. large earthmovers. bound rubber). leaving a swollen rubber-ﬁller gel (i. A comparative study was undertaken to obtain an overall assessment of natural rubber use by tire component for both RMT and automobile tires. 50 phr carbon black. calendering. Bound rubber content is not a constant property but will evolve until a ﬁxed value is attained. natural rubber– based compounds can be processed quite easily.e. free rubber can be extracted. Peptizers enable a lowering of compound mixing temperatures. When natural rubber is compounded with a highly reinforcing carbon black such as N121 or N134.. BIIR) for enhanced tire traction performance. butyl rubber (IIR) and halobutyl rubber (CIIR. when it has been added to improve processing qualities. and this will continue as the use of radial tires increases in farm equipment. Copyright © 2004 by Taylor & Francis . and wire coat compounds of three major classes of tires are presented in Table 9 (34–36). base compound. natural rubber can be diﬃcult to break down on mills or internal mixers and subsequently process. tear strength. and gum strips typically contain 100 phr natural rubber for component-to-component adhesion. OTHER NATURALLY OCCURRING MATERIALS Naturally occurring materials fall into two fundamental classes: organic or biotechnology products and inorganic materials. III. Table 10 provides a simple overview of the range of materials of interest that are either already available Table 10 Examples of Naturally Ocurring Materials for Use in Rubber Compounds Material Guayule Rice husks Starch Bamboo ﬁllers Pine tar Rosin Coumarone Indene Waxes Fatty acids Cotton Compounding ingredient type Replacement for natural rubber Filler Filler Filler Tackifying resin Tackifying resin Resin Processing aid Vulcanization system Filler. 34–36. reinforcement Potential replacement for synthetic material Synthetic elastomers Silica Silica Clay. Automobile tires 45 70 45 100 Radial medium truck tires 90 100 55 100 Bias truck tires 50 — 40 70 Internal components of tires such as wedges. nylon Copyright © 2004 by Taylor & Francis . polyester. silica Synthetic resins Synthetic resins Synthetic resins Synthetic oils N/A Carbon black. fabric skim compounds. wire skim or wire coat compounds.Table 9 Natural Rubber Levels (phr) in Selected Types of Tires and Tire Components Component Tread Base Sidewall Wire coat (breaker coat) Source: Refs. and hysteretic qualities. or are under investigation. The reinforcement properties of black rice husk ash are comparable to those of calcium carbonate and not as eﬀective as those of carbon black or silica. Clays such as kaolin and bentonite can also be used in combination with silica or carbon black. abrasion resistance. Starch has considerable potential when blended with carbon black or silica to improve the hysteretic properties of compounded rubber. Calcium carbonate can be used as a ﬁller even though its reinforcing properties are negligible. Rice husks that have been milled. it is appropriate to list them and note their potential application. 1. Though all require various degrees of processing prior to use in rubber compounds.5 to 5. and Na2O. starch. bioﬁllers hold the most promise for future increases in Copyright © 2004 by Taylor & Francis .5 to 10. White rice husk ash when added up to 20 phr in a natural rubber based compound did show good compound properties that were nearly equivalent to those found for silica-loaded compounds (37–39). MgO. This has implications for improvement in. Fe2O. hydrochloric acid. Of the range of materials. and water produce a hydrated silica that when compounded can produce a material with mechanical properties similar to those of silica and carbon black. they still represent renewable sources of raw materials available to the rubber technologist. and then treated with sodium hydroxide. High surface area chemically modiﬁed clays will improve the tensile strength. and tear strength of the rubber product. Though many of these will be discussed elsewhere in this text. ﬁltered. Particle sizes tend to range from 0. tire rolling resistance. 2. Biotechnology ﬁllers oﬀer considerable potential and have attracted attention in recent scientiﬁc literature. it is most eﬀective when blended with carbon black or silica. whereas black rice husk silica is approximately 55% silica and 45% carbon.0 Am. Inorganic mineral ﬁllers already ﬁnd extensive use in rubber compounds. and bamboo ﬁbers.0 Am. for example. CaO. Burning of rice husks leaves a waste consisting of SiO2 (95%). Residual carbon cannot be completely eliminated because it is trapped within the amorphous silica structure or is completely coated with silica so it is impossible to remove it by thermal processes. Talc is used in products such as carpet backing and can be eﬀective when blended with reinforcing ﬁllers such as silica or carbon black. Surface modiﬁcation by use of coupling agents can enhance the properties of compounds containing calcium carbonate. At this point they fall into three primary categories: silica ash derived from rice husk waste. Particle sizes can range from 0. K2O. 3. White rice husk silica contains around 95% SiO2. However. The automotive industry has set targets for recycle content of 25% of post-consumer and industrial scrap in their products with no increase in cost or loss in performance. there are both environmental and economic reasons to recycle and reclaim scrap rubber. Post-consumer scrap recycling is the reuse of products that have completed their service life. tear strength. With the use of a silane coupling agent. workers were reported to obtain good tensile strength. However. with no toxicity concerns (42). IV. rayon. This material was evaluated in natural rubber compounds containing a conventional cure system and was found to be an eﬀective substitute for more expensive antioxidants and processing aids and as a coactivator in place of stearic acid. and as an antioxidant for natural rubber.41). Industrial scrap is the waste material generated in the original manufacturing process. as a coactivator. It is anticipated that there will be a growing emphasis on the use of naturally occurring materials. In this instance the goal of recycling is to ensure that all this material is used in the production of high quality goods. to achieve the most from such systems. Raw rice bran oil contains fatty acids. and cotton. The purpose of this discussion is to provide the rubber technologist with introductory information on how to be compliant with these new environmental objectives and contain cost while satisfying the endproduct design and performance criteria.consumption. either a resorcinol/hexamethylenetetramine system or silane coupling agent is required. These products can be ground into a powder or returned to their original state via a devulcanization process. Also of considerable importance are waxes and fatty acids. Rice bran oil has been evaluated as a substitute for process oils. and hardness due to bonding between the polymer matrix and ﬁber. calcium carbonate. phosphatides. RECYCLING OF RUBBER Not only is there interest in the use of renewable raw materials such as natural rubber and ﬁllers such as calcium carbonate. and wax. Work of this nature merits further investigation. particularly in tires where vehicle manufacturers desire their products to have a deﬁned level of recycled or potentially renewable resource content. The discussion will describe the various forms and types of rubber recyclates available to the compounder Copyright © 2004 by Taylor & Francis . which are discussed elsewhere in this volume. Bamboo ﬁber–ﬁlled natural rubber has been investigated (40. Other naturally occurring materials that have found application in rubber-based products include silicates. Future government regulations may also require that automotive products and parts contain such materials. Filler loadings up to 50 phr are feasible. discusses all aspects of rubber recycling (44). i.g. The eﬀect of these rubber recyclates on the rubber compound will also be demonstrated in the form of physical and performance data. This was augmented by the development of wet process grinding of rubber in a water medium to produce very ﬁne particle sizes.0331 0. recycling should be a technologist’s objective.0059 in.0165 0. in.e.. of all facets of rubber recycling (43). chemical devulcanization. microwave. solution swelling in active solvents. The ﬁrst attempt at reusing rubber was through reclaiming. surface modiﬁcation. 20–80 mesh) produced by ambient and cryogenic processes emerged (Table 11). and bacterial degradation. 2002. due to the growth in the radial tire market. However. in. published yearly by the Recycling Research Institute lists all grinders and processors of scrap tire and rubber products (45). and wet ground rubber the latter being similar to that produced by the ambient grind process. in. There have been numerous attempts to produce reusable rubber through devulcanization by using some of the following methods and techniques. 8). in. 60–200 mesh.0098 0.and show how they can be incorporated into a rubber compounder. in. Ultrasonic. the use of ﬁnely ground rubber (e. 9). More recently. published in 1997. Copyright © 2004 by Taylor & Francis . in the 1970s reclaim use declined. Table 11 Mesh Size Mesh size 10 20 30 40 60 80 100 Dimension 2.. Best Practices in Scrap Tires and Rubber Recycling by Klingensmith and Baranwal.00 mm 850 Am 600 Am 425 Am 250 Am 180 Am 150 Am 0. and many other methods have been evaluated or are in various stages of development and use.0787 0. Though achievable levels of recycled materials will most likely be lower than the initial target of 25%.0070 0.0234 0. The Scrap Tire Users Directory. in. Rubber recyclates include ambient ground rubber (Fig. Three publications worth noting for rubber compounders trying to utilize recycled rubber are Myhre and MacKillop’s review in Rubber Chemistry and Technology Annual Rubber Reviews. cryogenic ground rubber (Fig. Copyright © 2004 by Taylor & Francis .Figure 8 Simpliﬁed schematic of typical ambient grinding and reclaim system. Figure 9 Schematic of typical cryogenic grinding system. mechanical. and the material is the least expensive to produce. Steel wires are removed by using a magnetic separator. B. This has largely been facilitated through new manufacturing systems and new designs. The balance were recycled into other uses. A. cryogenic grinding. this is normally accomplished by shredding. Each of these will be considered in turn. It is common to produce 10–40 mesh material using this method. in size. C.00R20 up to 12. Retreading of aircraft and commercial truck tires is probably the most ideal use of worn products. popularly characterized as reduction. The ﬁner the desired particle. For commercial truck tires from size 9. The ambient process uses conventional high-powered mills with close nips that shear the rubber and grind it into small particles. the longer the rubber is kept on the mill.5. multiple grinds can be used to reduce the particle size. and pneumatic separators to remove metals and ﬁbers. In the case of aircraft tires. Recycle The major methods for recycling existing rubber are ambient grinding. up to four or ﬁve retreads are possible. The lower practical limit for the process is the production of 40 mesh material (Table 11). In ambient grinding.00R24 or 315/80R22. vulcanized scrap rubber is ﬁrst reduced to chips on the order of 1–2 in. two retreads are not unusual. Any ﬁber and extraneous material must be removed using an air separator. and reclaim. and gauge optimization of product components. and wet grinding. recycle. reuse. Excluding the tires that go to landﬁlls or to stockpiles or other storage facilities.The recycling of rubber products can also be considered to fall into four basic categories. A ﬂow chart for an ambient grind process including a side stream for Copyright © 2004 by Taylor & Francis . Reduction Materials reduction eﬀorts have focused on optimum use of materials. These pieces can be reduced in size by further ambient grinding on mills or by freezing them with liquid nitrogen and then grinding them into ﬁne particles. 61% were used for fuel. For some rubber products such as tires. Reuse Reuse of tires and other industrial rubber products has been directed toward their use as fuel. The shredded rubber is then passed over magnetic. the weight reduction of tires and other engineered products. The resulting products are useful for controlling compound cost and improve processing when added to newly compounded rubbers. Alternatively. which also must be taken into consideration when selecting a material for inclusion in a compound formula. Cryogenic grinding uses rubber particles of up to 2 in. The cryogenic process produces fractured surfaces. and freezes them with liquid nitrogen. Some typical properties of an EPDM-based compound with cryogenically ground crumb added are displayed in Table 14. The most signiﬁcant feature of the process is that almost all ﬁber or steel is liberated from the rubber. The eﬀect of variation in recycle content on an SBR-based compound is illustrated in Table 12. The size of particles typically produced by this method ranges from 60 mesh to 80 mesh.1 MPa to 3. the process generates a signiﬁcant amount of heat in the rubber during processing. The process produces a material with an irregular jagged particle shape.9 MPa. and cryogenically ground rubber can now compete on a large scale with ambient ground products. Table 13 presents a simpliﬁed comparison of the two fundamental types of ground rubber. an increase in Mooney viscosity from 40. First. The cost of liquid nitrogen has dropped signiﬁcantly. This eliminates scrap disposal. The important observation from these data is that a consistent level of recycle material is essential to ensure consistency in a product’s design speciﬁcations. The frozen pellets are passed into a mill for further grinding. The following paragraph on the eﬀects of concentration and mesh size on rubber properties (46. and eﬃcient cooling systems are essential. and is an environmentally sound business practice. and a drop in ODR rheometer torque from 59 to 34.47) is based on the Cryoﬁne EPDM Handbook (48). provides better control over cost. resulting in a high yield of usable product. A second problem is the need for a recycling organization capable of working with small quantities of a given lot of waste material and keeping it in suitably clean condition. Excess heat can degrade the rubber. Mesh size of the crumb ranged from 40 to 100. it may be diﬃcult to accumulate suﬃcient clean scrap of a given compound or classiﬁcation type. A ﬂow chart for a typical cryogenic process is shown in Figure 9. Clearly. Many manufacturing organizations wish to incorporate their scrap back into original rubber compound formula. A third is that it is necessary to understand the eﬀects of mesh size and concentration on the rubber properties. and it was added at 10% and 20% Copyright © 2004 by Taylor & Francis . In addition.0.reclaiming is shown in Figure 8. The advantages of this technique are that 1) little heat is generated so there is no thermal degradation of the material such as is found with ambient grinding and 2) ﬁner particles are obtained. cryogenically ground rubber is preferable to ambient ground because it has no ﬁber or wire. However. Increasing the amount of 20 mesh crumb rubber from 0% to 50% results in a drop in tensile strength from 10.0 to 111. This is a signiﬁcant drawback owing to the desire to attain consistent properties and performance of the ﬁnal compound formulation. several practical problems arise in doing this. 0 33. and 50 Crumb Compound 1 Compound Ingredient (phr) SBR 1502 N660 Aromatic oil TMQ (polymerized dihydrotrimethylquinoline) Stearic acid Zinc oxide Sulfur MBTS TMTD Crumb (%) Property Mooney viscosity Rheometer max torque Tensile strength (MPa) Ultimate elongation (%) 100. 48.1 330.0 61.0 91. The data in Table 15 were extracted from the Cryoﬁne Butyl Compounding Handbook (49).0 0.0 40.0 1.0 50.0 2.0 5. However.0 5.0 1.0 2.0 10.0 47.0 2 100.0 3.0 2. In both cases loss in fundamental mechanical properties such as tensile strength was negligible.0 1.0 300.0 2. increase in loading as noted in the data displayed in Table 12 did lead to loss in properties.9 330.0 2.0 59. Cryogenically Ground Whole Tire Recycled Rubber Physical property Speciﬁc gravity Particle shape Fiber content Steel content Cost Source: Ref.0 1.0 111.0 1.0 6.5 0.5 17.0 50. They show the eﬀect that an 80 mesh cryogenically ground butyl rubber has on the mechanical and physical properties of a Table 13 Characteristics of Ambient vs.0 0.0 50. Ambient ground Same Irregular 0.Table 12 Properties of Ambient Ground Rubber (20 Mesh) SBR 1502 Compounds with 0%.5 50.0 90.0 50.0 90.0 2.0 34.1% Comparable Cryogenic ground Same Fractured nil nil Comparable Copyright © 2004 by Taylor & Francis .9 270.0 loading.0 90.0 7.0 5. 33%. 17%.0 2.0 0.0 90.5 33.5% 0.0 1.0 5.0 1.0 1.0 4 100.0 3 100.0 0.0 2. 3 8. These products therefore ﬁnd ready use in tire compounds due to their uniformity and Copyright © 2004 by Taylor & Francis .0 70.72 410 8.0 1.0 130.0 1.0 1.89 330 8.3 8. The eﬀects of 5%.0 5.3 8. Besides reducing compound cost.0 70.0 8.73 400 8.0 1. Wet grinding uses a water suspension of rubber particles and a grinding mill.0 8.0 5.0 5.0 5.0 8.0 5.0 70.86 340 8.93 380 8.0 1.9 70 4 100.4 70 40 8.Table 14 Properties of EPDM Compounds Containing Cryogenically Rubber Compound 1 2 100.0 8.0 5.0 8.0 130. This eﬀect of ground rubber is noted in all elastomers.3 8. A 5% level of ﬁnely ground butyl scrap is commonly added to innerliners.0 1.5 70 60 9.13 73 typical halobutyl tire innerliner.0 8. especially the highly impermeable ones like butyl and ﬂuoroelastomers.0 130.0 8.5 320 8.0 130.4 71 100 9.4 390 8.0 1.0 40 8.0 70.0 1.0 1.5 70 80 10.0 70.0 5.72 410 8.0 1.0 8.0 70.0 5. the ground butyl provides a path for trapped air to escape from the compound.0 80 10.0 60 9.0 5.7 390 7. and 15% loadings are shown.0 1.0 8.9 68 Basic compound EPDM N650 N774 Paraﬃnic oil Low MW Polyethylene TMQ (polymerized dihydrotrimethylquinoline) Stearic acid Zinc oxide Sulfur TBBS TMTD TDEDC (tellurium diethyldithiocarbamate) MBT Properties at 10% loading Mesh Tensile strength Ultimate elongation 300% Modulus Hardness (Shore A) Properties at 20% loading Mesh Tensile strength Ultimate elongation 300% Modulus Hardness (Shore A) 100.0 70.0 70. The material is ﬁnely ground to a mesh size of 60–120.0 1.0 1.13 73 Control 9. 10%.4 72 3 100.0 1.0 130.0 8.8 69 5 100.0 100 9.0 1.0 70.0 1.0 5.0 Control 9. The number of tires rejected due to blisters is reduced signiﬁcantly.0 1.0 1.1 390 8.3 8.0 1.0 1.0 70. Latex is added to the crumb rubber in an aqueous dispersion.0 90.50 10 47 8. liquid polymers.0 90. In some instances.00 0.00 3.00 3.8 6.9 4.2 80.00 3.5 4.7 minimal contamination. resin additives.0 90. The surface of crumb rubber can be activated by addition of unsaturated low molecular weight elastomers. when the speciﬁc chemical composition of the surface treatment is compatible with the materials to be reincorporated.00 8. For example.Table 15 Properties of Innerline Compounds Loaded with Cryogenically Ground Butyl Rubber (80 Mesh) Compound 1 Base compound component (phr) Chlorobutyl rubber (1066) Natural rubber (TSR 5) N650 Mineral rubber Phenolic resin Naphthenic oil Stearic acid Zinc oxide Sulfur MBTS Ground butyl rubber loading (%) Rheometer t90 Tensile strength (MPa) 300% Modulus Air permeability (Qa) Source: Ref.0 20. 2 80.50 1.0 50.0 4 4.5 9.50 15 46.00 0.50 1.5 8.7 4.00 2.9 6.00 0.00 2.0 4 4.50 5 46. There is considerable scope for further development in this area.00 8.2 4.00 2.3 9.00 8.3 7.50 0 47.00 8.0 4 4.00 0.0 65.00 2. oligomers.7 3 100.00 3. and rubber curatives.50 1. Surface treatment and additives can enhance the mechanical properties of compounds containing recycled materials.0 50.0 50. 49. retention of the original mechanical properties of the compound can be achieved. This technique. nitrile compounds should be treated with acrylonitrile butadiene copolymers or block copolymers. The water is removed.0 4 4.5 4 100. which have similar solubility parameters (43). know as the surface-activated crumb process. has been commercialized by Vredestein Rubber Recycling Company in Europe. Additives include materials such as polyurethane precursors.7 7. leaving a coating around the ground rubber. It is reasonable to state that success of the rubber recycling industry will be dependent on developing economically eﬀective means by which the surface Copyright © 2004 by Taylor & Francis .50 1. but pilot plant facilities have been in operation. Sulfur–sulfur bonds have lower bond energy than carbon–carbon bonds. and/or mechanical shear. The purpose of this section is not to discuss the manufacturing methods of reclaim but its beneﬁts and uses. and other rubber articles into a form of rubber that can be incorporated into virgin rubber compounds. tubes. Such systems have ranged from the simple addition of vulcanization system ingredients to crumb rubber. ultrasonic waves can have enough energy to selectively break the sulfur bonds. a range of techniques are available to produce such materials: Ultrasonic devulcanization. and polymer surface modiﬁcation to add functionality to the surface of the particles to treatment at higher temperatures with the intent of activating the surface.of ground recycled rubber is chemically activated so as to enable attainment of the original compound’s mechanical properties. reference should be made to the RT Vanderbilt Handbook (49). though pilot plant facilities are in operation. any future system will most likely involve catalytic degradation in a solvent at high temperature and pressure. Chemical devulcanization methods involve mixing rubber peelings in a high-swelling solvent with a catalyst. Though it has not been achieved commercially. They include the 1) heater or pan process. There are many processes available to accomplish this. Owing to the cost of operating such systems. it is possible to convert used tires. Chemical devulcanization. For details on manufacturing. Copyright © 2004 by Taylor & Francis . Thermal devulcanization. Reclaim Reclaim of rubber refers to the recovery of original elastomers in a form in which they can be used to replace fresh polymer (48). Through the use of reclaiming agents. No commercially successful systems have been developed. and 5) Banbury process. This involves the use of microwaves. bladders. Chemomechanical and thermomechanical techniques. Though other chemical techniques are being investigated. 2) dynamic dry digester process. thereby devulcanizing the compound. D. ultrasonic devulcanization continues to be a potential method to allow reclamation of the original polymer. 3) wet digester process. Again. steam digestion. 4) Reclaimotor process. Heating brings about a signiﬁcant reduction in cross-link density. there has been no successful commercial system. Given this. inducing an increase in temperature with preferential breaking of sulfur–sulfur bonds. Reclaim rubber had several distinct potential advantages.Reclaim rubber was used in signiﬁcant volume up to the early 1970s in the United States. However. Shore A Tear strength (kNÁm) Source: Ref. reclaim is still widely used in footwear.9 875 58 20. Though caution should be exercised with regard to the impact on cut growth and fatigue resistance. Static or low performance applications are therefore still preferred. Basic mechanical properties quoted may be acceptable for this application. environmental regulations. durability. automotive parts. fenders. Substituting 10 parts GF-80 3. and the possible reduction in the need for curing agents in the compound. Then the proliferation of radial tires.7 15.6 740 59 20.7 Copyright © 2004 by Taylor & Francis . and tear strength led to its removal from radial tire compounds. An estimated 200–300 reclaimers are still operating. (MPa) Elongation (%) Hardness. less shrinkage. and other goods. The estimate for the year 2002 was 60 million pounds. China.9 Substituting 10 parts whole tire reclaim 3. its lower green strength. and reclaimed silicone for automotive and electrical applications. 50. mats. reclaimed NR for mats and low-end static applications. some classical mechanical properties are retained when reclaim is added. bias tire compounds. and ﬂaps.1 775 58 20. These included lower cost than original rubber. The deﬁnition of low performance product applications largely excludes products such as high-performance tires. consisting mostly of reclaimed butyl for tire innerliners.7 13. In the 1960s there were estimates that as much as 600 million pounds per year of reclaim rubber was used in the United States. Table 16 illustrates the eﬀect of adding reclaim to a radial tire carcass compound (50). Table 17 illustrates typical properties achieved when whole tire reclaim is added to either a natural rubber or SBR automotive mat compound. improved rheological characteristics found in manufacture. and the large economy of scale of the new or upgraded SBR and PBD manufacturing process resulted in low original rubber prices and signiﬁcant contraction of the reclaim rubber industry.7 3 14. high-pressure hoses. Table 16 Physical Properties of Radial Tire Casing Compound Containing Wet Ground and Reclaim Rubber Control Modulus at 300% (MPa) Tensile strength. and some southeastern Asian countries. and conveyor belt covers but does include mats. In many regions of the world such as India. 4 480.Table 17 Reclaim in Automotive Vehicle Mats—Properties Compound 1 2 Component Natural rubber SBR1502 Whole tire reclaim Paraﬃnic oil N550 TMQ (polymerized dihydrotrimethylquinoline) Stearic acid Zinc oxide Sulfur MBT MBTS TMTM Property Tensile strength (Mpa) Ultimate elongation (%) Hardness (Shore A) Compression set (%) 79. In cement and concrete the addition of recycled rubber reduces vibration transmission.5 E.8 490. there are still a wide variety of successful applications. improves fracture resistance. For example.2 41.0 12.6 1.3 65.0 57.9 0. 2.4 0. Copyright © 2004 by Taylor & Francis .0 19.0 80. and other organic solvents. and provides a more comfortable ride. 1.0 1.0 60.4 3.0 2.0 22.0 45. reduces the level of noise generated by vehicles traveling on it.0 5. Though at low levels there can be deterioration in the mechanical properties of a compound.2 0.0 0.0 1. and reduces cracking. The total elastomer content can be kept at 100 RHC.9 1. has better resistance to thermal and reﬂective cracking. toluene. Asphalt containing crumb rubber is more durable. 3. crumb rubber is eﬀective at absorbing heavy metals and organic solvents such as benzene. or the recycle content can be added on top of the original elastomer as a ﬁller.0 65. In ﬁll for sludge treatment plants.3 13. Compounding Application of Recycled Materials Recycled material can be added to the original compound formulation in one of two ways.6 9.0 5. walkway tiles.0 25.0 50.E.E. and sports surfaces such as running or jogging tracks constitute a growing market for recycled rubber. 1. Tables 18 and 19 list a range of applications for recycled materials in tires and industrial products.0 5. Source: Ref. 44.Table 18 Use of Reclaim and Recycled Materials in Tires Component Treads Subtread Casing plies Bead ﬁllera Sidewalla Wedges Squeegee Liner a From Ref.0 10. Table 18 displays tire components that have the Table 19 Products Product Target Levels for Use of Reclaim and Recycled Materials in Industrial Potential application No Yes Yes Yes Yes Yes Yes Yes Potential loading (%) 0 3. Passenger tires Yes No No Yes Yes Yes Yes Yes Light truck tires Yes No No Yes Yes No Yes Yes Commercial tires No No No No No No Yes No Retreads Yes Yes No No No No Yes No 4.) Carpet backing Railroad crossingsa O.0 Belt casing/carcass Conveyor belt covers Transmission belts (non-O. the most important technical issue is removal of all steel from the material. However. 44.) Hose covers and inner tubes Low operating pressure tubing Weatherstripping (non-O.0 25. Such surfaces are very eﬀective.E. = original equipment. Copyright © 2004 by Taylor & Francis .0 5. a Rubber blocks laid between rails at highway-railroad crossings and junctions. Cryogenic grinding is typically used to produce materials for such applications. Flooring. Source: Ref. Research institutes throughout the world are working on these issues. To improve the mechanical properties of current materials and enable their use in novel compounds. 3. Of the range of naturally occurring materials used in advanced engineered products. 4. This discussion has therefore attempted to provide a foundation for the rubber technologist to Copyright © 2004 by Taylor & Francis . additional sources of materials will be required to meet the shortages anticipated by the year 2007. Given the growth of the global economies and the automotive industry speciﬁcally. To address this. natural rubber is among the most extensively used. many components in tires cannot contain recycled material owing to potential deterioration in performance. It is anticipated that this will change.potential to contain varying levels of recycled material. Speciﬁcations will be needed for visually inspected rubbers such as RSS grades to meet the end users’ need for consistency and uniformity in their factories. End product speciﬁcations and performance requirements will continue to be reﬁned. However. Quality. and purity. and tear strength. Availability. SUMMARY This chapter has reviewed the classiﬁcation and major uses of natural rubber. Chemical modiﬁcation. 1) more ﬁnely ground material with better deﬁned particle dimensions and 2) new compounding ingredients to improve factors such as dispersion. 2. thereby necessitating continuing improvement in consistency. Table 19 similarly shows potential levels for recycled material in industrial products such as belts and hoses. which in turn is restricting growth in recycling opportunities. absence of foreign materials or other contaminants. fatigue resistance. The use of recycled rubber will continue to increase throughout the ﬁrst decades of the 21st century. V. and the materials scientist should have the appropriate technologies in place to take advantage of future demand. overcapacity in global vehicle production is having a detrimental impact on pricing. which will ensure the use of natural rubber products long into the future. Four key factors will determine its use in the future: 1. This growth will be driven by regulatory and economic factors rather than technological factors. These provide target levels for the materials scientist developing compounds with recycle content. new synthetic derivatives of polymers will be required to compete with new functionalized synthetic elastomers. Conversely. Technical speciﬁcations. two requirements may need to be addressed. 16. 10. New York: Wiley. Rubber—Evaluation of NR (Natural Rubber). Encyclopedia of Polymer Science and Engineering. New York: Oxford Univ Press. Annual Book of ASTM Standards. Cordes EH. 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