Silane Guide

Combining technology expertise withmarket knowledge to help you develop new materials with greater reliability and improved performance. from Dow Corning Guide to Silane Solutions 3 Dow Corning – The Silane Technology Pioneer .......................................... 4 Your Continuing Resource for Innovation and Application Success ............ 5 The Basics of Silane Chemistry .................................................................... 6 The Concept of Coupling with Organofunctional Silanes .......................... 8 Silane Coupling Agents ............................................................................... 8 Why Silane Coupling Agents Are Used ....................................................... 8 The Silane Bond to the Inorganic Substrate ................................................ 9 The Silane Bond to the Polymer ................................................................ 10 How to Choose a Silane Coupling Agent ................................................... 10 Typical Silane Applications ........................................................................ 13 Silanes from Dow Corning ........................................................................... 14 Fiberglass and Composites ....................................................................... 15 Mineral and Filler Treatment ...................................................................... 16 Paints, Inks and Coatings .......................................................................... 18 Primers .................................................................................................. 19 Zinc-Rich Primers ................................................................................. 20 Chromium Replacement ....................................................................... 20 Industrial Maintenance .......................................................................... 20 Automotive Clearcoats .......................................................................... 20 Architectural Coatings ........................................................................... 21 Typical Coating Benefits ....................................................................... 21 Pharmaceutical Manufacturing .................................................................. 22 Plastics and Rubber .................................................................................. 22 Rubber Compounding ........................................................................... 22 Polymer Manufacturing ......................................................................... 24 Plastics Compounding .......................................................................... 24 Adhesives and Sealants ............................................................................ 25 Adhesion Promoters ............................................................................. 25 Crosslinkers .......................................................................................... 26 Water Scavengers ................................................................................ 26 Coupling Agents .................................................................................... 26 Water Repellents and Surface Protection .................................................. 26 General Construction Applications ........................................................ 26 Other Surface Protection Applications .................................................. 27 Other Applications ..................................................................................... 27 The Surface and Interface Solutions Center – A Valuable Resource for Customer Success ............................................. 28 More than Materials – Competitive Advantage .......................................... 28 Dow Corning – The Right Partner for You .................................................. 29 Visit Our Website ....................................................................................... 29 Contents Dow Corning – The Silane Technology Pioneer ow Corning pioneered the development of organosilane technology more than 50 years ago to provide new classes of materials – silicones and silanes – with special physical and chemical properties. This research led to a new industry based on the synergy of organic and silicon chemistries. Silicones and silanes are now essential components in many major applications; without them, many of the materials we rely on today would not exist. The value of silane coupling agents was first discovered in the 1940s in conjunction with the development of fiberglass-reinforced polyester composites. When initially fabricated, these new composites were very strong, but their strength declined rapidly during aging. This weaken- ing was caused by a loss of bond strength between the glass and resin. In seeking a solution, research- ers found that organofunctional silanes – silicon chemicals that contain both organic and inorganic reactivity in the same molecule – functioned as coupling agents in the composites. A very small amount of an organofunctional alkoxysilane at the glass-resin interface not only significantly increased initial composite strength; it also resulted in a dramatic retention of that strength over time. Subsequently, other applications for silane coupling agents were discovered, including mineral and filler reinforcement; mineral dispersion; adhesion of paints, inks and coatings; reinforcement and crosslinking of plastics and rubber; reinforcement and adhesion of sealants and adhesives; water repellents and surface protection. Your Continuing Resource for Innovation and Application Success Dow Corning continues to pioneer the development of innovative technologies and applications for organosilane and silicon-containing materials through our global research team and Surface and Interface Solutions Center (SISC). From automotive to marine to aerospace, from electronics to building construction to sporting goods, Dow Corning silanes are an important component of today’s sophisticated technologies. They enable new materials to be developed with greater reliability and improved performance. With a full range of silane product and application solutions, Dow Corning offers you technology leadership, reliable supply, world- class manufacturing and global reach. In addition to materials, we offer supportive services and solutions you may never have imagined. Silane solutions. Distinctly Dow Corning. 5 The Basics of Silane Chemistry ilicon is in the same family of elements as carbon in the periodic table. In their most stable state, silicon and carbon will both conveniently bond to four other atoms; but silicon-based chemicals exhibit significant physical and chemical differences compared to analogous carbon-based chemicals. Silicon is more electropositive than carbon, does not form stable double bonds, and is capable of very special and useful chemical reactions. Silicon-based chemicals include several types of monomeric and polymeric materials. Figure 1. Carbon vs. silicon chemistry. Organic (Carbon-Based) Chemical H (alkane hydrogen) (methyl) CH 3 � OCH 3 (methyl ether) CH 2 CH 2 CH 2 -NH 2 (aminopropyl) Silane (Silicon-Based) Chemical H (hydride) (methyl) CH 3 �� OCH 3 (methoxy) CH 2 CH 2 CH 2 -NH 2 (aminopropyl) 7 Monomeric silicon chemicals are known as silanes. A silane structure and an analogous carbon-based structure are shown in Figure 1. The four substituents have been chosen to demonstrate differences and similarities in physical and chemical properties between silicon- and carbon-based chemicals. A silane that contains at least one carbon- silicon bond (CH 3 -Si-) structure is known as an organosilane. The carbon-silicon bond is very stable, very non-polar and gives rise to low surface energy, non-polar, hydrophobic effects. Similar effects can be obtained from carbon-based compounds, although these effects are often enhanced with silanes. The silicon hydride (–Si-H) structure is very reactive. It reacts with water to yield reactive silanol (-Si-OH) species and, additionally, will add across carbon-carbon double bonds to form new carbon-silicon-based materials. The methoxy group on the carbon compound gives a stable methyl ether, while its attachment to silicon gives a very reactive and hydrolyzable methoxysilyl structure. The organofunctional group, the aminopropyl substituent, will act chemically the same in the organosilicon compound as it does in the carbon-based compound. The distance of the amine, or other organofunctional group, from silicon will determine whether the silicon atom affects the chemistry of the organofunctional group. If the organic spacer group is a propylene linkage (e.g., -CH 2 CH 2 CH 2 -), then the organic reactivity in the organofunctional silane will be similar to organic analogs in carbon chemistry. Certain reactive silanes, particularly vinyl silanes (-Si-CH=CH 2 ) and silicon hydrides (-Si-H), are useful reactive groups in silicon chemistry, even though the reactive group is attached directly to the silicon atom. Attachment of chlorine, nitrogen, methoxy, ethoxy or acetoxy directly to silicon yields chlorosilanes, silyl- amines (silazanes), alkoxysilanes and acyloxysilanes, respectively, that are very reactive and exhibit unique inorganic reactivity. Such molecules will react readily with water, even moisture adsorbed on a surface, to form silanols. These silanols then can react with other silanols to form a siloxane bond (-Si-O-Si-), a very stable structure; or in the presence of metal hydroxyl groups on the surface of glass, minerals or metals, silanols will form very stable –Si-O-metal bonds to the surface. This is the key chemistry that allows silanes to function as valuable surface-treating and coupling agents. Chloro-, alkoxy-, and acetoxy- silanes, and silazanes (-Si-NH-Si) will react readily with an active hydrogen on any organic chemical (e.g., alcohol, carboxylic acid, amine, phenol or thiol) via a process called silylation. R 3 Si-Cl + R'OH ‡ R 3 Si-OR' + HCl Silylation is very useful in organic synthesis to protect functional groups while other chemical manipulations are being performed. The silylated organofunctional group can be converted back to the original functional group once the chemical operation is completed. Silylation is very important in the manufacture of pharmaceutical products. �� Without Silane With Silane The Concept of Coupling with Organofunctional Silanes Silane Coupling Agents ilane coupling agents are silicon-based chemicals that contain two types of reactivity – inorganic and organic – in the same molecule. A typical general structure is (RO) 3 SiCH 2 CH 2 CH 2 -X, where RO is a hydrolyzable group, such as methoxy, ethoxy, or acetoxy, and X is an organofunctional group, such as amino, methacryloxy, epoxy, etc. A silane coupling agent will act at an interface between an inorganic substrate (such as glass, metal or mineral) and an organic material (such as an organic polymer, coating or adhesive) to bond, or couple, the two dissimilar materials. A simplified picture of the coupling mechanism is shown in Figure 2. For a more detailed discussion of this mechanism, read “A Silane Primer: Chemistry and Applications of Alkoxy Silanes” by Gerald L. Witucki, Journal of Coatings Technology, Volume 65, Number 822, July 1993, pages 57-60. A reprint of this article is posted in the Technical Library in the Fiberglass and Composites section of the Dow Corning Silanes Solutions website, www.dowcorning.com/ silanes. Why Silane Coupling Agents Are Used When organic polymers are re- iforced with glass fibers or minerals, the interface, or interphase region, between the polymer and the inorganic substrate is involved in a complex interplay of physical and chemical factors. These factors are related to adhesion, physical Figure 2. The silane coupling mechanism. Figure 3. SEM of silica-filled epoxy resin. 9 �� CH 3 OH HO-Si-O-Si-O-Si-OH RSi(OCH 3 ) 3 �� RSi(OH) 3 H 2 O H 2 O R R R O O O H H H strength, coefficient of expansion, concentration gradients and retention of product properties. A very destructive force affecting adhesion is migration of water to the hydrophilic surface of the inorganic reinforcement. Water attacks the interface, destroying the bond between the polymer and reinforcement, but a “true” coupling agent creates a water-resistant bond at the interface between the inorganic and organic materials. Silane coupling agents have the unique chemical and physical properties not only to enhance bond strength but also, more importantly, to prevent de-bonding at the interface during composite aging and use. The coupling agent provides a stable bond between two otherwise poorly bonding surfaces. Figure 3 shows (via an SEM of the fracture surface) the difference in adhesion between a silica-filled epoxy resin with silane vs. without silane. With silane, the epoxy coating on the silica particles is apparent; without silane, clean silica particles can be seen in the epoxy matrix. In composites, a substantial increase in flexural strength is possible through the use of the right silane coupling agent. Silane coupling agents also increase the bond strength of coatings and adhesives as well as their resistance to humidity and other adverse environmental conditions. Other benefits silane coupling agents can provide include: • Better wetting of inorganic substrates • Lower viscosities during compounding • Smoother surfaces of composites • Less catalyst inhibition of thermoset composites • Clearer reinforced plastics Figure 4. Hydrolysis of alkoxysilanes. Figure 5. Bonding to an inorganic surface. The Silane Bond to the Inorganic Substrate Silane coupling agents that contain three inorganic reactive groups on silicon (usually methoxy, ethoxy or acetoxy) will bond well to the metal hydroxyl groups on most inorganic substrates, especially if the substrate contains silicon, aluminum or a heavy metal in its structure. The alkoxy groups on silicon hydrolyze to silanols, either through the addition of water or from residual water on the inorganic surface. Then the silanols coordinate with metal hydroxyl groups on the inorganic surface to form an oxane bond and eliminate water. See Figures 4 and 5. � � � � � � � � � �� Silane molecules also react with each other to give a multimolecular structure of bound silane coupling agent on the surface. More than one layer, or monolayer equiva- lents, of silane is usually applied to the surface. This results in a tight siloxane network close to the inorganic surface that becomes more diffuse away from the surface. The Silane Bond to the Polymer The bond to the organic polymer is complex. The reactivity of a thermoset polymer should be matched to the reactivity of the silane. For example, an epoxysilane or aminosilane will bond to an epoxy resin; an aminosilane will bond to a phenolic resin; and a methacrylate silane will bond through styrene crosslinking to an unsaturated polyester resin. With thermoplastic polymers, bonding through a silane coupling agent can be explained by inter-diffusion and inter-penetrating network (IPN) formation in the interphase region. See Figure 6. To optimize IPN formation, it is important that the silane and the resin be compatible. One method is to match the chemical characteristics of the two materials. This will help improve the chances of forming a good composite with optimum properties. Even with thermoset polymers, where reactivity plays an important role, chemical structure matching will enhance the physical properties of the composite. How to Choose a Silane Coupling Agent All silane coupling agents with three OR groups on silicon should bond equally well with an inorganic substrate. A variety of organofunctional alkoxysilanes is available. See Figures 7 and 8. Figure 6. The inter-penetrating network (IPN) bonding mechanism. �� Silica �� 11 Matching the organofunctional group on silicon with the resin polymer type to be bonded will dictate which silane coupling agent should be used in a particular application. The organic group on the silane can be either a reactive organic group (i.e., an organofunctional group), or it can be a non-reactive organic group. The groups can be hydrophobic or hydrophilic, with varying thermal stability characteristics. The solubility parameters of the groups will vary, depending on the organic structure; this will influence, to some extent, the interpenetration the polymer network will have into the siloxane network of the surface treatment. Table 1 lists some of the characteristics for common organic substituents attached to silicon. The choice of silane should involve matching chemical reactivity, solubility characteristics, structural characteristics and, possibly, the thermal stability of the organosilane with the same parameters in the polymer structure. Figure 7. Silane coupling agent variations – basic structure. Figure 8. Silane coupling agent variations – alternative “Bis” structure. Table 1. Characteristics of Various Organic Substituents on Silanes Organosilanes R-Si(OMe) 3 R Characteristics of “R” Me Hydrophobic, Organophilic Ph Hydrophobic, Organophilic, Thermal Stability i-Bu Hydrophobic, Organophilic Octyl Hydrophobic, Organophilic -NH(CH 2 ) 3 NH 2 Hydrophilic, Organoreactive Epoxy Hydrophilic, Organoreactive Methacryl Hydrophobic, Organoreactive Alternative “Bis” Structure Si OR' OR' OR' Si R R'O R'O R'O Z-6920 Z-6670 Si CH 2 C H 2 C H 2 S S C H 2 C H 2 C H 2 Si OEt EtO EtO OEt OEt OEt Si C H 2 C H 2 MeO MeO OMe C H 2 C H 2 C H 2 C H 2 Si OMe OMe OMe N H Si R OR' OR' R'O Si CH CH 2 MeO OMe MeO Z-6300 Z-6040 Z-6911 Si O OMe MeO MeO O CH 3 C H 2 Z-6030 Si EtO EtO OEt Z-6341 Si NH 2 EtO EtO OEt Z-6011 Basic Structure R = alkyl, aryl, or organofunctional group OR' = methoxy, ethoxy, or acetoxy Si C H 2 C H 2 C H 2 SH OEt OEt EtO Si O OMe MeO MeO O Dow Corning ® brand Silane Organic Group Alkoxy Group Chemical Name Z-6697 - Ethoxy TetraEthoxysilane Z-6070 Methyl Methoxy Methyltrimethoxysilane Z-6366 Methyl Methoxy Methyltrimethoxysilane (HP) Z-6370 Methyl Ethoxy Methyltriethoxysilane Z-6383 Methyl Ethoxy Methyltriethoxysilane (HP) Z-6194 Methyl Methoxy Dimethyldimethoxysilane Z-6265 Propyl Methoxy Propyltrimethoxysilane Z-6535 Propyl Ethoxy Propyltriethoxysilane Z-2306 i-Butyl Methoxy Isobutyltrimethoxysilane Z-6403 i-Butyl Ethoxy Isobutyltriethoxysilane Z-6124 Phenyl Methoxy Phenyltrimethoxysilane Z-6341 n-Octyl Ethoxy n-Octyltriethoxysilane Table 3. Silane Coupling Agent Recommendations for Various Polymers – Matching Organoreactivity to Polymer Type Table 2. Non-Organoreactive Alkoxysilanes Dow Corning ® brand Silane Organic Reactivity Application (suitable polymers) Z-6011 Amino Acrylic, Nylon, Epoxy, Phenolics, PVC, Urethanes, Melamines, Nitrile Rubber Z-6020 Amino Acrylic, Nylon, Epoxy, Phenolics, PVC, Melamines, Urethanes, Nitrile Rubber Z-6028 Benzylamino Epoxies for PCBs, Polyolefins, All Polymer Types Z-6030 Methacrylate Unsaturated Polyesters, Acrylics, EVA, Polyolefin Z-6032 Vinyl-benzyl-amino Epoxies for PCBs, Polyolefins, All Polymer Types Z-6040 Epoxy Epoxy, PBT, Urethanes, Acrylics, Polysulfides Z-6076 Chloropropyl Urethanes, Epoxy, Nylon, Phenolics, Polyolefins Z-6094 Amino Acrylic, Nylon, Epoxy, Phenolics, PVC, Melamines, Urethanes, Nitrile Rubber Z-6106 Epoxy/Melamine Epoxy, Urethane, Phenolic, PEEK, Polyester Z-6128 Benzylamino Epoxies for PCBs, Polyolefins, All Polymer Types Z-6137 Amino Acrylic, Nylon, Epoxy, Phenolics, PVC, Melamines, Urethanes, Nitrile Rubber (especially suited for water-based systems) Z-6224 Vinyl-benzyl-amino Epoxies for PCBs, Polyolefins, All Polymer Types Z-6300 Vinyl Graft to Polyethylene for Moisture Crosslinking, EPDM Rubber, SBR, Polyolefin Z-6376 Chloropropyl Urethanes, Epoxy, Nylon, Phenolics, Polyolefins Z-6518 Vinyl Graft to Polyethylene for Moisture Crosslinking, EPDM Rubber, SBR, Polyolefin Z-6675 Ureido Asphaltic Binders, Nylon, Phenolics; Urethane Z-6910 Mercapto Organic Rubber Z-6920 Disulfido Organic Rubber Z-6940 Tetrasulfido Organic Rubber A list of alkyl and aryl, non-organoreactive alkoxysilanes is provided in Table 2. Those silanes give modified characteristics to inorganic surfaces, including hydrophobicity, organic compatibility and lower surface energy. Based on experience and histori- cal applications of silanes, a list of silane coupling agents and recommendations for evaluation with various polymer types is provided in Table 3. A correlation can be seen between the chemistry and structural characteristics of the silane coupling agent and the chemistry and structural characteristics of the polymer. M o r e H y d r o p h o b i c 13 Typical Silane Applications Coupling Agent: Organofunctional alkoxysilanes are used to couple organic polymers to inorganic materials. Typical of this application are reinforcements, such as fiberglass and mineral fillers, incorporated into plastics and rubbers. They are used with both thermoset and thermoplastic systems. Mineral fillers, such as silica, talc, mica, wollastonite, clay and others, are either pretreated with silane or treated in situ during the compounding process. By applying an organofunctional silane to the hydrophilic, non- organoreactive filler, the surfaces are converted to reactive and organophilic. Fiberglass applications include auto bodies, boats, shower stalls, printed circuit boards, satellite dishes, plastic pipes and vessels, and many others. Mineral- filled systems include reinforced polypropylene, silica-filled molding compounds, silicon-carbide grinding wheels, aggregate-filled polymer concrete, sand-filled foundry resins and clay-filled EPDM wire and cable. Also included are clay- and silica-filled rubber for automobile tires, shoe soles, mechanical goods and many other applications. Adhesion Promoter: Silane coupling agents are effective adhesion promoters when used as integral additives or primers for paints, inks, coatings, adhesives and sealants. As integral additives, they must migrate to the interface between the adhered product and the substrate to be effective. As a primer, the silane coupling agent is applied to the inorganic substrate before the product to be adhered is applied. In this case, the silane is in the optimum position (in the interphase region), where it can be most effective as an adhesion promoter. By using the right silane coupling agent, a poorly adhering paint, ink, coating, adhesive or sealant can be converted to a material that often will maintain adhesion even if subjected to severe environmental conditions. Hydrophobing and Dispersing Agent: Alkoxysilanes with hydrophobic organic groups attached to silicon will impart that same hydrophobic character to a hydrophilic inorganic surface. They are used as durable hydrophobing agents in construction, bridge and deck applications. They are also used to hydrophobe inorganic powders to make them free- flowing and dispersible in organic polymers and liquids. Crosslinking Agent: Organo- functional alkoxysilanes can react with organic polymers to attach the trialkoxysilyl group onto the polymer backbone. The silane is then available to react with moisture to crosslink the silane into a stable, three-dimensional siloxane structure. Such a mechanism can be used to crosslink plastics, especially polyethylene, and other organic resins, such as acrylics and urethanes, to impart durability, water resistance and heat resistance to paints, coatings and adhesives. Moisture Scavenger: The three alkoxy groups on silanes will hydrolyze in the presence of moisture to convert water molecules to alcohol molecules. Organotrialkoxysilanes are often used in sealants and other moisture-sensitive formulations as water scavengers. Polypropylene Catalyst “Donor”: Organoalkoxysilanes are added to Ziegler-Natta catalyzed polymerization of propylene to control the stereochemistry of the resultant polypropylene. The donors are usually mono- or di-organo silanes with corresponding tri- or di-alkoxy substitution on silicon. By using specific organosilanes, the tacticity (and hence the properties) of the polypropylene is controlled. Silicate Stabilizer: A siliconate derivative of a phosphonate- functional trialkoxysilane functions as a silicate stabilizer to prevent agglomeration and precipitation of silicates during use. The predomi- nant application is in engine coolant formulations to stabilize the silicate corrosion inhibitors. Silanes from Dow Corning ow Corning is the industry leader in supplying silane and intermediate product solutions; this is one of our company’s core businesses. Our silanes business unit encompasses the following product groups: • Chlorosilanes • Organofunctional silanes • Specialty silanes • Alkylsilanes Methylchlorosilanes are the basic building blocks of all of our silicon- based materials. They are used in basic synthesis of silanes and siloxanes, as protecting agents for intermediates in pharmaceutical synthesis, and as precursors in the manufacture of silicon-carbide coatings. Chlorosilanes are essential raw materials in the electronics and telecommunications industries and for the production of optical fibers, silicon wafers and chips, as well as the starting materials for fumed silica. Alkylsilanes, specialty silanes and organofunctional silanes have alkyl, aryl or organofunctional groups attached to silicon and have methoxy, ethoxy or acetoxy groups attached to silicon to allow them to function in the manner described in this brochure. Lists of silanes commercially available from Dow Corning can be found at www.dowcorning.com/silanes. Data sheets for these products can be viewed and downloaded from the website. We have many other silicon-based materials that may be of value to you as well. Information about these products can be obtained by contacting Dow Corning Customer Support either by e-mail or telephone. 15 0 100 200 300 400 500 600 700 Dry Strength Wet Strength, 72-hour water boil F l e x u r a l S t r e n g t h , M P a None Z-6040 (Epoxy) Z-6032 (Vinyl- benzylamino) Figure 9. Effect of silane coupling agents on the strength of glass-reinforced epoxy. Fiberglass and Composites Silane coupling agents are a critical component of fiberglass-reinforced polymers. The glass is very hydrophilic and attracts water to the interface. Without silane treatment on the surface, the bond between the glass fiber and the resin would weaken and eventually fail. Silane coupling agents are used on fiberglass for general-purpose reinforced plastic applications, such as automotive, marine, sporting goods and building construction, as well as for high-performance applications in printed circuit boards and aerospace composites. Dow Corning ® brand silanes figure prominently in the trend toward increasingly more-durable, higher- strength plastic composites. The chemical structure of the organic group in a silane coupling agent has a great effect on its performance in a composite, as measured by improvement of strength properties under wet and dry conditions. A wet-aging test, usually in boiling water, will show differences in the effectiveness of various silanes. The effect of the organic structure of the coupling agent on improving the flexural strength of a glass- reinforced, unsaturated polyester composite is shown in Figure 9. The vinylbenzyl-functional silane coupling agent (Dow Corning ® Z-6032 Silane, in this case) yields greater improvement in the flexural strength of a glass-reinforced epoxy system than does the epoxy- functional silane coupling agent (Dow Corning ® Z-6040 Silane). More significantly, the retention of strength after aging for 72 hours in boiling water is better with either silane than if no silane coupling agent is used; but Z-6032 Silane provides better retention of flexural strength. These are the types of effects generally expected from the use of silane coupling agents. Fiberglass for general-purpose applications is treated with a dilute aqueous sizing bath consisting of a combination of ingredients (organic film formers, lubricants, antistats and a silane coupling agent). The silane must be soluble in the aqueous bath at levels of 0.2 to 1 percent. Normally, if a water bath is acidified with acetic acid to a pH of 4, even hydrophobic silanes will dissolve in the bath at low concentrations and give the stability needed to treat the fiberglass. Certain silanes, such as aminosi- lanes, are more hydrophilic and will dissolve at high concentrations in water even without pH adjustment. The size is applied to the fiberglass at the glass fiber manufacturing plant immediately after the glass fibers are extruded and bundled into glass fiber rovings. Fiberglass for high-performance electronics, such as printed circuit boards, is processed differently. The glass fiber is treated with a starch size at the glass manufacturing plant, after which a “fiberglass weaver” weaves the fiber into glass cloth. The weaver then burns off the starch size at high temperature, producing “heat-cleaned” glass cloth. This clean cloth is then passed through a bath containing 0.2 to 0.5 percent silane coupling agent. Usually, no other significant sizing chemical is in the bath. The glass cloth is dried, inspected for flaws and supplied to a fabricator who makes epoxy, or other polymer, prepregs and laminates for printed circuit boards. This application requires excellent coupling agent technology to provide the flaw-free benefits required. Dow Corning Z-6032 Silane, and variations on this product, have been developed to provide the necessary quality and performance for printed circuit boards. Depositing the silane as a silse- quioxane (organosilicon with three oxygen atoms shared with other silicon atoms) on a surface and measuring the weight loss by thermal gravimetric analysis (TGA) Figure 10. Thermal stability of silanes at 300ºC (572ºF), TGA. Table 4. Thermal Stability of Mixed Silanes – Phenyl + Amino, S-Glass/Polyimide Laminates functional silanes, can provide benefits. The improvement in thermal stability of a fiberglass- polyimide composite is shown in Table 4. Some of the benefits imparted to fiberglass-reinforced plastics by Dow Corning silanes include: • Improved mechanical strength of the composites • Improved electrical properties • Improved resistance to moisture attack at the interface • Improved wet-out of the glass fiber • Improved fiber strand integrity, protection and handling • Improved resistance to hot solder during fabrication • Improved performance in cycling tests from hot to cold extremes Table 3 on page 12 suggests silanes for evaluation with various fiberglass-reinforced polymer systems. Product data sheets are available at www.dowcorning. com/silanes. Mineral and Filler Treatment Mineral fillers have become increasingly important additives and modifiers for organic polymers. The metal hydroxyl groups on the surface of minerals are usually hydrophilic and incompatible with organic polymers. Alkoxysilanes are a natural fit to treat the surface of the mineral to Coupling Agents on Glass Properties of Laminates, MPa 9:1 Blend, Silane A and C Aminosilane Alone, Silane B Flexural Strength, initial 544 476 1000 hr @ 260°C (500°F) 409 258 2000 hr @ 260°C (500°F) 306 134 Silane A: Z-6124 Ph-Si(OCH 3 ) 3 Silane B: Z-6011 H 2 N(CH 2 ) 3 Si(OCH 2 CH 3 ) Silane C: Z-6020 H 2 N(CH 2 ) 3 NH(CH 2 ) 2 Si(OCH 3 ) 3 can determine the thermal stability of the silane. Results of isothermal TGA at 300ºC (572ºF) for several silanes are shown in Figure 10. The diaminosilane (Dow Corning ® Z-6020 Silane) exhibited very poor thermal stability. As expected, the phenyl silane (Dow Corning ® Z-6124 Silane) showed excellent thermal stability. Surprisingly, the complex vinylbenzyl silane (Z-6032), based on Z-6020, showed very good thermal stability. These data suggest that for high- temperature applications, Z-6032, or blends of Z-6124 with other �� (CH2)3NHCH2CH2NHCH2 -CH=CH2 -CH2CH2CH2O- -CH2CH2CH2NH- -(CH2)3NHCH2CH2NH2 -NH3 -NH3 HCl Stability of RSiO 3/2 in Air Hours at 300°C (572°F) % R R e m a i n i n g R - On Silicon 17 make it more compatible and dispersible in the polymer, or even to make the filler a reinforcing additive. In addition to plastics applications, the use of silane-modified minerals in organic rubber, especially tires, has become increasingly important. Minerals with silicon and aluminum hydroxyl groups on their surfaces are generally very receptive to bonding with alkoxysilanes. The treatment of a mineral surface by an organosilane is depicted in Figure 11. Silica (both fumed and precipitated), glass beads, quartz, sand, talc, mica, clay and wollastonite have all effectively used silane coupling agents in filled polymer systems. Other metal hydroxyl groups, such as magnesium hydroxide, iron oxide, copper oxide, and tin oxide, may be reactive to a lesser extent, but often benefit from silane treatment. Traditionally, silane coupling agents give poor bonding to carbon black, graphite and calcium carbonate. Silane treatment can improve processing, performance and durability of mineral-modified products by: • Improving adhesion between the mineral and the polymer • Improving wet-out of the mineral by the polymer • Improving dispersion of the mineral in the polymer • Improving electrical properties • Increasing mechanical properties • Reducing the viscosity of the filler/polymer mix An example of the benefit of silane treatment of a silica filler used in an unsaturated polyester resin composite is shown in Figure 12. As is generally the case, the silane treatment results in higher initial strength and better retention of strength after humidity aging. The silane also can reduce the viscosity of the uncured resin/filler mixture, to allow easier processing, with different silanes giving different effects. In this case Dow Corning Z-6032 Silane (vinylbenzyl-amine) reduced viscosity by 65 percent while Dow Corning Figure 11. Filler surface treatment. Figure 12. Viscosity and coupling effect – polyester castings with 50% silica. Z-6030 Silane (methacrylate) reduced viscosity by only 10 percent. Similarly, the ability of silane coupling agents to impart improved electrical properties is shown in Table 5 on page 18. An epoxy resin was cured with and without quartz filler as the reinforcement. Without filler, the epoxy resin showed good electrical properties, dielectric constant and dissipation factor, even after aging for 72 hours in boiling water. However, once quartz filler was added, the hydrophilic F l e x u r a l S t r e n g t h , M P a No Silane – 24,500 Pa•s Z-6030 (Methacrylate) – 22,000 Pa•s Z-6032 (ViBz Amine) – 8,700 Pa•s �� Inorganic Surface Mineral, Metal, Glass �� + Surface Is Hydrophilic Converted to Organoreactive Surface + �� surface of the quartz led to severe loss of electrical properties during the water boil test. With either epoxy-silane (Dow Corning Z-6040 Silane) or aminosilane (Dow Corning Z-6011 Silane), the quartz-filled composite exhibited improved retention of electrical properties. Minerals are treated with either neat silane or a solution of silane in water and/or alcohol. With a neat silane, the adsorbed water on the filler surface is often sufficient to hydrolyze the alkoxysilane and simultaneously bond the silane to the filler surface. It is important that the filler be coated uniformly through the use of intensive mixing, such as with a Henschel mixer. Commercial processes are continu- ous, often in a heated chamber, followed by further heat treatment to remove byproducts of alcohol and water and to complete the bonding of the silane to the surface. The loading level of silane on the filler surface is a function of the surface area of the filler. While it was thought that one monolayer of silane should be sufficient, experi- mentation has shown that several layers of silane give optimal results. For example, typical fillers with average particle sizes of 1 to 5 microns often give best results when treated with about 1 percent silane. The optimal level of silane treatment should be determined experimentally. The choice of which silane to use in a particular application is determined by the nature of the benefit that is to be derived from the silane. All alkoxysilanes will bond to a receptive filler or mineral surface. If the silane treatment is designed to provide surface hydrophobicity, then a silane with a hydrophobic group, such as butyl, octyl, fluorocarbon or phenyl, should be chosen. If the silane treatment is designed to provide compatibility of the mineral in a polymer matrix, then the nature of the organic group on the silane should be similar to the chemical structure of the polymer (i.e., an octyl or longer- chain alkyl group will help provide compatibility and dispersibility of the mineral in a polyolefin matrix). If the silane treatment is to bond a filler to Dielectric Constant Dissipation Factor System 1 Initial Water Boil 2 Initial Water Boil 2 Unfilled Resin 3.44 3.43 0.007 0.005 Quartz, no Silane 3.39 14.60 0.017 0.305 Quartz, Z-6040 3.40 3.44 0.016 0.024 Quartz, Z-6011 3.46 3.47 0.013 0.023 1 Z-6040 = Epoxysilane; Z-6011 = Aminosilane 2 72-hour water boil Table 5. Ability of Silane Coupling Agents to Impart Electrical Properties a polymer matrix, then an organoreactive silane should be chosen that would bond chemically to reactive sites present in the polymer. A list of some mineral/filler applications is shown in Table 6. Table 3 on page 12 suggests silanes for evaluation with various filled polymer systems. Product data sheets are available at www.dowcorning.com/silanes. Paints, Inks and Coatings Tightening volatile organic compound (VOC) regulations in the coatings industry, along with demand for improved physical properties and extended performance life, have spurred interest in silane technology. The unique capability of silanes to create covalent bonds between inorganic and organic compounds, and the inherent stability of the siloxane (Si-O-Si) bond, make this technology a key component in high-performance paints and coatings. These properties lie at the heart of the ability of these materials to withstand physical, chemical, environmental and thermal degradation. Silane monomers, in the form of organofunctional alkoxysilanes, are utilized widely in coatings as adhesion promoters, pigment treatments and crosslinkers. Inorganic alkoxy functionality coupled with a wide range of organofunctional 19 Fillers Comments Kaolin Clay Reinforced Nylon, Wire and Cable (EPDM) Talc Stiffness, Abrasion Resistance – Polypropylene (auto) Mica Stiffness – Polypropylene (auto) Silica Reinforced Rubber, Epoxy PCBs Wollastonite Reinforced Plastics, Coatings Glass Fiber/Beads Reinforced Plastics Aluminum Trihydrate Flame Retardance Magnesium Hydroxide Flame Retardance Crystobalite Abrasion Resistance – Plastics Titanium Dioxide Colorant, Filler – Plastics Table 6. Mineral/Filler Applications groups allows for covalent bonding between organic polymers and inorganic surfaces (e.g., pigments, fillers, and glass and metal substrates). The same coupling agent mechanisms described earlier allow for bonding between organic polymers and inorganic surfaces. All alkoxysilanes will bond essentially identically to inorganic surfaces, but the organofunctionality of the silane must be matched with the chemistry of the organic polymer in the paint, ink or coating to obtain optimum performance from the silane. The use of silanes in coatings can provide improvements in adhesion; resistance to moisture, chemicals, ultraviolet (UV) rays and abrasion; and improved dispersion of fillers. Alkoxysilane monomers (which are not silicones, per se) are completely miscible with many organic resins. In fact, silanes are reasonably strong polar solvents. Polymeriza- tion of the silanes into silicone resins and fluids impacts the compatibility and performance of the resulting polymer. Silanes are also used as intermediates to produce silicates and siliconates via reaction with metal hydroxide (e.g., sodium or potas- sium hydroxide). These materials are used in protective finishes, such as zinc-rich primers, masonry treatments for water repellency, or compounded directly into concrete coatings for improved physical properties and water repellency. Silicates are derived primarily from tetra-alkoxysilanes. In contrast, siliconates are produced via reactions of mono- or di-organo (e.g., methyl or other alkyl moieties) alkoxysilanes, which allow a broader range of performance properties, such as water repellency and substrate penetration. Primers Silanes provide crucial functionality in the primer segment of the coatings industry. Alkoxysilanes have broad utility in formulating primers for a variety of metal and siliceous substrates. Especially attractive to the formulator is the wide range of organo-reactive and non-reactive moieties attached to the silicon atom, which allows formulas to be tailored to specific application performance requirements. Widely known as adhesion promoters, alkoxysilane primers also offer controlled hydrophobicity, excellent UV and thermal stability, surface activity, chemical resistance and corrosion protection. The silane coupling agent must act at the interface between the sealant or adhesive and the substrate. It is chosen by matching its organic functionality to the organic moiety in the coating that is to be bonded. Table 3 on page 12 suggests silanes for evaluation based on the nature of the organic moiety in the coating. Often, mixtures of silanes are used as adhesion promoters to provide enhanced hydrophobicity, thermal stability or crosslinking at the bonding site. Using a silane as a primer ensures that the silane will be at the substrate- polymer interface where it can enhance adhesion. Silane primers are often dilute solutions of silanes, 0.5 to 5 percent, in an alcohol or water/alcohol solvent. They are wiped or sprayed on the substrate followed by solvent evaporation. Zinc-Rich Primers As early as 1962, partial hydrolyzates of alkoxysilanes (e.g., tetraethoxysilane), or alkali silicates, combined with zinc metal powder were found to provide galvanic protection of ferrous substrates beyond that imparted by organic resin-based zinc primers. 1 Initially, this technology was limited by its inherently short pot and shelf life. Later, the stability and overall performance of the primer was greatly improved by trans-esterifying the silicate with organic polyols (e.g., ethylene glycol or glycerol). 2 This innovation is one of the most widely cited silicon-based inventions (34 citations). These materials, based on partial hydrolyzates of tetra-ethoxy silane, are available as either one- or two-part systems and have been the dominant galvanic primer used in the paint industry. They are characterized by tolerance to high humidity and low-temperature application. Solvent-based primers are best suited for on-site application under difficult weather conditions. Chromium Replacement State-of-the-art metal surface preparations for adhesive bonding con- sist mainly of anodization or etching processes employing strong acids. Many of these surface preparations also contain hexavalent chromium. Surface treatment is followed by the application of a corrosion-inhibiting adhesive primer that typically contains high levels of volatile organic compounds (VOCs) and additional hexavalent chromium. Alternatives to chromium compounds are being sought due to new regulations, the increased cost of hazardous waste disposal and the increased aware- ness of the costs associated with employee health and safety. In 1983, a primer composed of an acrylic copolymer, an epoxy resin, a silica sol and a trialkoxysilane compound was developed. The primer provided superior paint- ability, degreaser resistance and corrosion resistance after painting. 3 Twelve years later, a wash primer, without the acrylic copolymer or the epoxy resin, was developed that provided similar benefits. 4 Metal was pretreated with an alkaline solution containing at least one of a dissolved inorganic silicate, a dissolved inorganic aluminate, an organofunctional silane, and a crosslinking agent containing trialkoxysilyl groups. The metal was then dried to completely cure the functional silane, resulting in an insoluble primer layer bonded tightly to the metal substrate. Industrial Maintenance Combining the cure profiles and barrier properties of organic resins with the thermal and UV stability of silanes, formulators have created high-performance coatings with excellent resistance to corrosion and chemical attack as well as thermal and UV degradation. A blend consisting of an epoxy resin, an epoxy resin curing agent, an organofunctional alkoxysilane and a catalyst for condensation polymerization of a silane compound can provide high heat resistance and excellent mechanical strength. 5 Similarly, epoxy resins can be reacted with hydrolyzed alkyl and phenyl alkoxysilanes to produce copolymers with improved water and moisture resistance. 6,7 Utilizing the functional groups available from silane monomers, resin formulators have created organofunctional (e.g., epoxy and amine) silicone resins for epoxy resin modification. 8,9 Automotive Clearcoats Color-plus-clear coating systems involving the application of a colored or pigmented base coat to a substrate followed by application of a clear topcoat have become the standard as OEM finishes for automobiles. Color-plus-clear systems have outstanding appearance properties (such as gloss and distinctness of image) due, in large part, to the clear coat. These clear coatings are, however, subject to damage from environmental elements, such as acid rain, UV degradation, high relative humidity and temperatures, stone chipping and abrasive scratching of the coating surface. 21 Typically, a harder, more highly crosslinked film may exhibit improved scratch resistance; however, high crosslink density embrittles the film, making it much more susceptible to chipping and/or thermal cracking. A softer, less-crosslinked film, while not prone to chipping or thermal cracking, is susceptible to scratching, water spotting and acid etch. Clear coats in color-plus- clear systems have demonstrated improved scratch resistance with the inclusion of surface-reactive, inorganic microparticles, such as silane coupling agent treated colloidal silica. 10 Architectural Coatings Changes in building practices, including concrete facades on multi- floor buildings and shifts in the eco- nomics of material and labor costs, have contributed to the trend toward silane-modified architectural paints. By using a reactive organic group on a trialkoxysilane to react into a latex polymer backbone, the latex polymer has the ability to crosslink via a moisture crosslinking mechanism once the coating is applied. A primary concern for water-based formulations is the stability of alkoxysilanes in an aqueous envi- ronment. Alkoxysilane adhesion promoters (also known as coupling agents) do react with water. For silanes to provide the intended benefits of adhesion or crosslinking, the hydrolysis reaction is a necessary and desired process step. Modifying the silane, via transesteri- fication, from methoxy functionality to longer alkoxy groups (e.g., isopropoxy) can slow, but not prevent, hydrolysis. Attaching an alkoxy chain length sufficient to eliminate hydrolysis would essentially deactivate the silane. By formulating to conpensate for the inevitable hydrolysis and subsequent condensation of alkoxysilanes, coating formulators can still utilize this technology to improve the performance of many water-based coatings. Many coatings fail because water is absorbed by or penetrates the film, ultimately reaching the coating- substrate interface. Alkoxysilanes are well known for improving the adhesion of coatings to metal or siliceous substrates by forming covalent bonds via dual organic- inorganic reactivity. This is one of several mechanisms by which alkoxysilanes provide benefit. In addition to chemical bonding, silanes improve the hydrolytic stability and integrity of the film. Including alkoxysilanes in coating formulations can create a more tightly crosslinked, hydrophobic film that is much less susceptible to moisture attack. Significant benefit can be achieved by adding 0.5 percent silane (based on system solids) to acrylic latex-based coatings. Treatment of mineral pigments and fillers (e.g., silica, titanium dioxide, etc.) with alkoxysilanes is well known in the coatings industry. While pigment or filler suppliers often treat fillers with silanes, similar benefits can be observed by incorporating the alkoxysilane directly into a water-based coating formulation. The presence of water at typically high pH levels results in hydrolysis of the silane and condensation around the solid particles. The net effect is better integration of the inorganic particle into the binder matrix, improved dispersion and physical properties. Successful incorporation of silanes into water-based formulations requires good dispersion of the silane prior to complete hydrolysis and condensation. Adequate mixing is essential. Along with good mixing, pre-diluting the silane into a coalescing solvent or plasticizer before adding it to the latex will minimize condensation of the silane monomers (and potential gel formation) and encourage interaction with the other components of the coating formulation. Typical Coating Benefits Silanes can impart several benefits to coatings, including: • Abrasion resistance • Adhesion • Better flow • Crosslinking to improve thermal stability and durability • Pigment and filler dispersion • UV resistance • Water and chemical resistance A list of Dow Corning ® silanes for use in paints, inks and coatings is available at www.dowcorning. com/silanes. Pharmaceutical Manufacturing The pharmaceutical industry relies heavily on silane chemistry in the synthesis of antibiotics, drugs and medicines. Through a process called silylation, the chemistry of silanes allows them to be used as protecting groups that permit chemical procedures to be performed, while retaining the desired organic functionalities necessary in the pharmaceutical molecular structure. Silylation is the displacement of an active hydrogen in an organic molecule by a silyl (R 3 Si) group. The active hydrogen is usually -OH (alcohol, carboxylic acid, phenol), -NH (amine, amide, urea) or -SH (thiol). The silylating agent is often a trimethylsilylhalide, dimethylsilyldi- halide or a trimethylsilyl nitrogen- functional compound. However, often larger, bulkier groups (e.g., tert-butyl) are on the silylating agent to control the chemistry of the reaction. Newer silylating agents will cleave esters and ethers. A mixture of silylating agents may be used, such as trimethylchlorosilane plus hexamethyldisilazane. This blend is more reactive than either reagent alone. The byproducts combine to form neutral ammonium chloride, e.g., in the following reaction where the -Si(CH 3 ) 3 group replaces the active hydrogen in the R-OH molecule. R-OH + (CH 3 ) 3 SiNHSi(CH 3 ) 3 + (CH 3 ) 3 SiCl ‡ 3 RO-Si(CH 3 ) 3 + NH 4 Cl The unique chemical properties of silanes allow them to replace one or more active hydrogens during chemical synthesis to protect these groups, while subsequently allowing other chemistries to be performed on the molecules without destroying or altering the protected organic functionalities. After the desired chemical procedures are carried out in other parts of the molecules, the silane protective group can be removed to regenerate the original organic functionality. Silanes have been used for many years in the production of antibiotics, such as penicillin and cephalosporin- type medications. Tertiary- butyldimethylchlorosilane is used in anti-cholesterol drug production as a “super-protector” during the manufacturing process. Other silanes, such as chloromethylsi- lydimethylchlorosilanes, have been used in direct chemical synthesis of herbicides where the silicon atom becomes a chemical part of the final product. As the global market for biologi- cal and pharmaceutical products increases, due to population growth and increasing demand for health- care, manufacturers will rely on silanes as they develop the next generation of medicinal therapies. A list of Dow Corning ® brand silylating agents for use in pharmaceutical manufacturing is available at www.dowcorning.com/silanes. Plastics and Rubber The unique properties of silanes are used to enhance performance and improve processes in the plastics and rubber industries. Silanes function as coupling and dispersing agents for fillers in rubber and plastics formulations, as polymerization modifiers in the synthesis of polypropylene, and as crosslinking agents for polyethylene homopoly- mers and copolymers. Rubber Compounding A major use for silanes has developed in the organic rubber industry as a result of the benefits that can be obtained from the use of inorganic filler in place of carbon black in the reinforcement of rubber. Silica and other inorganic filler reinforcements for rubber provide unique physical properties and performance properties versus carbon black reinforcement; however, silane coupling agents are necessary for the non-black reinforcing fillers to be effective. Silanes are the key to providing a method of effectively bonding the inorganic fillers to organic elastomers. Silane-coupled, mineral-filled rubber products are used for automotive and off-road tires, shoe soles, belts, hoses and mechanical goods. The mechanism is similar to that described earlier under “Mineral and Filler Treatment.” Methoxy- or ethoxy-silanes will bond tenaciously to the silica or clay surface; then the organic portion of an organofunctional silane will bond to the rubber polymer. See Figure 13. 23 The silane is usually added during the compounding process to treat the filler in situ. It must have the proper rate of reactivity to spread and react over the filler surface and still be able to react with the elastomer at a rate that allows processing of the rubber to be Figure 13. Bonding organic rubber to silica with sulfur silanes. Figure 14. Structure of sulfidosilanes used in rubber compounds. completed. This can be done with silane coupling agents that have triethoxysilyl groups at both ends of a polysulfido (tetrasulfide, disulfide or mixture thereof) organic group. See Figure 14. These coupling agents are supplied as neat liquids or as blends with a carrier such as carbon black. See Table 7. Even though silica can be used as the only filler, rubber tires incorporate small levels of carbon black to give consumers the uniform black color they expect. Without carbon black in the rubber compound, it is possible to make tires in a variety of colors. A specific example of this application is the silica/silane technology used in “green” tires to impart: • Increased abrasion resistance • Reduced rolling resistance and improved fuel economy of tires • Better grip on wet and snow/ ice surfaces Silica-reinforced tires are known as “green” tires because they provide improved fuel economy while �� Si EtO EtO OEt Sx Si EtO OEt OEt Si Si OEt O Si O O OEt Si O OH OEt Si O Si O Si O Si O Si O Si O O O OEt Sx Sx Silica Silica Rubber S Si Si OEt O Si O O OEt Si O OH OEt Si O Si O Si O Si O Si O Si O O O OEt S Sx s s s s s s s The silane can react in the sulfur vulcanization �� Table 7. Sulfidosilanes for Rubber Dow Corning ® brand Silane Features Average Value of X Z-6920 Liquid TESPD 2.20 Z-6925 Solid TESPD, 50% on Carbon Black 2.20 Z-6940 Liquid TESPT 3.75 Z-6945 Solid TESPT, 50% on Carbon Black 3.75 Si C H 2 OEt OEt OEt C H 2 C H 2 Si C H 2 EtO EtO OEt C H 2 C H 2 Sx Si C H 2 OEt OEt OEt C H 2 C H 2 Si C H 2 EtO EtO OEt C H 2 C H 2 Sx x ranges from 2 to 10 These are termed S 2 , S 3 , etc., monomers Bis-TriEthoxy Silyl Propyl Polysulfide - TESPX maintaining or improving other tire properties (as listed above). They also use a mineral-derived filler rather than one derived from a fossil fuel (natural gas or oil). This is currently the largest market for silane coupling agents. The use of vinyl silanes as a coupling agent in kaolin clay reinforced EPDM wire and cable coatings is another important rubber application. The vinyl silane improves the electrical properties of the reinforced rubber so a stringent power- factor electrical test can be passed, but only when optimum silane coupling agent technology is used. In addition to silanes, Dow Corning is a major supplier of silicone rubber. Silicone rubber is made from silicone polymers compounded with non-black fillers, usually fumed or precipitated silica. These compounds require silanes and functional silicone fluids. Silanol-functional silicone fluids and vinyl-functional silanes are available for silicone rubber compounding. A list of Dow Corning silanes for rubber compounding is available at www.dowcorning.com/silanes. Information about our silicone rubber materials is available at www.dowcorning.com/rubber. Polymer Manufacturing Selected silanes, known as “external donors,” or electron donors, are used in conjunction with Ziegler- Natta catalysts in the manufacture of polypropylene. Ziegler-Natta catalysts are organometallic compounds. Organoal-koxysilanes can chemically coordinate with the organometallic catalyst to modify the course of the polymerization. Specific variations in the tacticity of the propylene polymer are possible by optimizing the use of a silane donor in the process. Different silane donors with differing organo- alkoxy structures are used depending on the exact nature of the catalyst and the type of polypropylene being manufactured. Organic substituents, such as cyclohexyl, cyclopentyl, methyl, isobutyl and phenyl, are some of the organic groups attached to silicon. The alkoxy groups are either methoxy or ethoxy with one, two, or three alkoxy groups on the silane molecule. Three of the more common silane donors are Donor C, cyclohexylmethyldimethoxysilane (Dow Corning ® Z-6187 Silane); Donor D, dicypentyldimethoxysilane (Dow Corning ® Z-6228 Silane); and di-isobutyldimethoxysilane (Dow Corning ® Z-6275 Silane). Reactive silicone polymers have also been used to produce thermoplastic vulcanizates (TPVs). TPVs are prepared by chemically crosslinking a rubbery phase in a thermoplastic matrix. TPVs are produced by dynamic vulcanization, and silane chemistry allows new and unique crosslinking chemistries to be used in the manufacturing process. A list of Dow Corning silanes for polymer manufacturing is available at www.dowcorning.com/silanes. Plastics Compounding Vinyl silanes have been used commercially since the 1970s to crosslink polyethylene homopolymer and its copolymers. Vinyltrimeth- oxysilane and vinyltriethoxysilane are the most common silanes used in the process. In an extruder in the presence of peroxide and heat, the vinyl group will graft to the polyethylene backbone, yielding a silane- modified polyethylene that contains pendant trialkoxysilyl functionality. The grafted polyethylene can then be immediately crosslinked in the presence of a tin catalyst, moisture and heat to create a silane- crosslinked product. Diagrams of the grafting of vinyltrimethoxysilane (VTMOS) to polyethylene and the moisture crosslinking process are shown in Figures 15 and 16. The ease of processing and the simple equipment required make this the preferred method of producing crosslinked ethylene polymers and copolymers. The process also allows crosslinking to be delayed until after the grafted product is transformed into its final product configuration. Using the same silanes, it is also possible to copoly- merize the vinyl silane with ethylene monomer to make trialkoxysilyl- functionalized polyethylene. This then can be crosslinked in the same manner as the graft version. Silane-crosslinked polyethylene is used for electrical wire and cable insulation and jacketing where ease of processing, increased temperature resistance, abrasion resistance, stress-crack resistance, improved low-temperature properties and 25 retention of electrical properties are needed. Other applications for this technology include: • Cold- and hot-water pipe where resistance to long- term pressure at elevated temperatures is essential • Natural gas pipe with good resistance to stress cracking • Foam for insulation and packaging with greater resiliency and heat resistance • Other product and process types, such as film, blow- molded articles, sheeting and thermoforming A list of Dow Corning silanes for plastics compounding is available at www.dowcorning.com/silanes. Additional information is available at www.dowcorning.com/plastics. Adhesives and Sealants Silanes are widely used to improve the adhesion of a broad range of sealants and adhesives to inorganic substrates, such as metals, glass and stone. Sealants are based on filled, curable elastomers and have the dual purpose of preventing passage of water, air and chemicals through the zone where applied; in some cases they also serve as an adhesive. Their usefulness in the aircraft, automotive and construction industries depends upon their ability to form durable bonds to metal, glass, ceramic and other surfaces – bonds that will withstand exposure to heat, ultraviolet radia- tion, humidity and water. Adhesion Promoters A silane coupling agent will function at the interface between the sealant or adhesive and the substrate to act as an adhesion promoter. An organofunctional silane uses a mechanism similar to that described earlier for bonding an inorganic substrate and a sealant or adhesive polymer. The silane coupling agent is chosen by matching its organic functionality to the polymer to optimize bonding. Figure 15. Grafting of VTMOS to polyethylene – Sioplas ® process. Table 3 on page 12 sugests silanes to evaluate for various polymer systems. Often, mixtures of silanes are used as adhesion promoters to provide enhanced hydrophobicity, thermal stability or crosslinking at the bonding site. The silanes can be blended into an adhesive formulation or used as primers on substrates. When added to the adhesive formulation, Figure 16. Crosslinking of polyethylene in the presence of moisture – Sioplas ® process. Silane-Grafted Polyethylene ROOR Heat Si OMe OMe OMe + Polyethylene VTMS Si OMe OMe OMe Si OMe MeO MeO �� Si OMe MeO MeO Si OMe O MeO Si OMe MeO DBTDL = Dibutyltindilaurate the silane must be free enough to migrate to the interphase region between the adhesive/sealant and the surface of attachment. The structure and reactivity of the silane will affect the ability of the silane to migrate. Usually more than one silane is evaluated for an application to empirically choose the best performing silane. The most effective way to promote adhesion is to apply the silane as a primer to the surface, followed by application of the adhesive/sealant. In this way, the silane will be on the surface and therefore at the interface where it can enhance adhesion between the polymer and the substrate. Silane primers are usually dilute solutions of 0.5 to 5 percent silane in alcohol or water/alcohol solvent. They are wiped or sprayed on the substrate, after which the solvent is allowed to evaporate. When added to sealants or adhesives or used as primers on substrates, an improvement in adhesion is often realized with the bond showing greater resistance to moisture attack at the interface. This can result in: • Increased initial adhesion • An adhesive bond with longer life • Greater temperature resistance • Greater chemical resistance Crosslinkers Silanes can be used to crosslink polymers such as acrylates, poly- ethers, polyurethanes and polyesters. The organofunctional portion of the silane can react, and bond to, the polymer backbone in a sealant or adhesive. The alkoxysilyl group on the silane should not crosslink prematurely in order to be available to provide crosslinking once the sealant or adhesive is applied in its intended application. A silane-crosslinked sealant or adhesive can show enhanced properties, such as: • Tear resistance • Elongation at break • Abrasion resistance • Thermal stability • Moisture resistance Water Scavengers The ability of alkoxysilanes to react very rapidly with water makes them useful in sealant and adhesive formulations to capture excess moisture. A very common moisture scavenger is vinyltrimethoxysilane. The presence of the vinyl group attached to silicon increases the rate of reaction of the methoxysilane with water to give efficient elimina- tion of water. Methanol is formed as a byproduct, and the vinyl silane crosslinks into an inactive species in the formulation. Other silanes, such as methyltrimethoxysilane, are also used as water scavengers. Silane water scavengers in a formulation can: • Prevent premature cure during compounding • Enhance uniform curing • Improve in-package stability Coupling Agents Silane coupling agents are used to increase adhesion between fillers and the polymer matrix in sealants and adhesives. The mechanism and mode of action was described earlier under “Mineral and Filler Treatment.” The silane coupling agent treatment on the filler can provide: • Better bonding of the pigment or filler to the resin • Improved mixing • Increased matrix strength • Reduced viscosity of the uncured sealant or adhesive A list of Dow Corning ® silanes for use in formulating sealants and adhesives is available at www.dowcorning.com/silanes. Water Repellents and Surface Protection General Construction Applications Silanes can be chosen to impart hydrophobic (water repellent) and/ or oleophobic (oil and stain repellent) characteristics to surfaces. Silanes with alkyl groups (such 27 as butyl and octyl) and aromatic groups (such as phenyl) and even some organofunctional groups (such as chloropropyl and methacrylate) are hydrophobic. Similarly, silanes containing fluoroalkyl groups are oleophobic (oil repellent). Alkoxysilyl groups attached to these silanes allow them to actually penetrate, cure in and even bond to many inorganic substrates. These unique properties allow for versatile and durable formulating solutions for protection against harmful water- and oil-borne elements. Dow Corning brand water and stain repellent materials can be used in solvent- or water-based systems to provide the formulating flexibility needed to meet VOC and ease- of-use requirements. These silane- based water and stain repellents are available for use in formulations that penetrate a broad range of substrates, including: • Poured-in-place or pre-cast concrete • Concrete block • Sandstone/granite • Brick/tile/grout • Wood • Gypsum/perlite • Limestone/marble Silane-based water repellents from Dow Corning create an envelope of protection that extends the life of substrates for years in challeng- ing environments. Potntial benefits include: • Excellent water repellency • Long-term durability • UV stability • Depth of penetration • Water vapor permeability • High dilution capability and stability • Clear, uniform, neutral appearance Benefits of protection include: • Reduced efflorescence • Reduced freeze-thaw damage • Chloride ion resistance to deter corrosion of reinforcing steel in concrete structures • Preservation of aesthetics Other Surface Protection Applications Dow Corning also manufactures a range of silicates and siliconates for use in formulating pore-blocking sealers and consolidators. These silicates and siliconates are alkali metal salts of hydrophobic silane oligomers and adhere tenaciously to inorganic substrates and surfaces in much the same way as simple alkoxysilanes do. Applications for these materials fall into two groups: sealers and consolidators. Sealers fall into two sub-groups: • Pore blockers provide little penetration and, instead, form a resin barrier on the concrete’s surface. Pore blockers are further distinguished by their ability to partially or fully fill the surface pores, a capability not shared by hydrophobing agents. • Hydrophobing agents, on the other hand, penetrate the material deeply. They allow the concrete to breathe and do not interfere with concrete cure. Consolidators can extend the life of stone and concrete because they penetrate and cure in and through these materials to help bind them together. They are used in a variety of restoration and flooring applications. A list of Dow Corning ® silanes for water repellents and surface protection is available at www.dowcorning. com/silanes. Additional information is available at www.dowcorning. com/construction. Other Applications The possible applications for silanes are certainly not limited to those provided in this brochure. Silanes bring performance- enhancing and problem-solving benefits to a wide array of specialty applications. Whether your application is typical or unique, Dow Corning can provide the silane solution and technical support you require, either through the proven resources of our Application Engineering Technical Service department or through the innovation expertise of our Surface and Interface Solutions Center. The Surface and Interface Solutions Center – A Valuable Resource for Customer Success ow Corning’s Surface and Interface Solutions Center (SISC) in Seneffe, Belgium, is pioneering the development of next-generation technologies and applications for organosilane and silicon-containing chemicals. The SISC designs innovative molecules, composites, processes, and surface interface and interphase technologies, including material science for filler reinforcement, crosslinking and adhesion. The center serves the needs of customers in multiple markets, including plastics, rubber, adhesives, sealants, coatings, textiles and electronics. Because it is located in Europe, the SISC complements our other silanes technology facilities in Midland, Michigan, USA, and Chiba, Japan, and expands our ability to provide you with advanced application and development support, worldwide. More than Materials – Competitive Advantage The scientists and engineers at the SISC are linked to Dow Corning’s global network of technology experts and to external sources of expertise. Because the center combines technology expertise with market knowledge, it enables us to identify previously unimagined opportunities to meet new and emerging customer needs. The SISC can provide you with novel materials that open the doors to new markets and applications. We can help you achieve a competitive advantage in other ways as well, by engineering solutions tailor-made to help you achieve your specific business goals and objectives. Whether you are looking for innovation support, performance improvement, increased productivity or business growth, the SISC can help. More information on the SISC is available at www.dowcorning. com/silanes/siscmain.asp. 29 Dow Corning – The Right Partner for You ore than 50 years ago, Dow Corning pioneered the development of organosilane technology. Today, we are recognized in the industry for our innovations, technical achievements and competence in silicon technology. Our exclusive focus on silicon-based chemistry guarantees state-of-the-art material, manufacturing and expertise. We have world-class facilities to study, handle and produce these materials. We have made significant investments to support the silanes market. These investments will enable us to further grow our silanes product line and identify new opportunities to provide you with performance-enhancing solutions. We invite your inquiries. We are anxious to discuss your opportunities, to assist you in optimizing your current applications, and to counsel you in the use of silane solutions in the development of emerging technologies. Our goal is to help you use the best silane technology to satisfy the needs of your customers, and thereby maximize your business potential. Visit Our Website Visit our website, www.dowcorning.com/silanes, and explore the silanes and other silicon-based technologies we have to offer. There you will find links to technical papers, data sheets, product and technology brochures, and other information that can assist you in finding solutions to your needs. Dow Corning is pleased to offer you “Silane Solutions.” Footnote References 1 S.L. Lapata and W.R. Keithler; Carboline Company; U.S. Patent 3,056,684, October 2, 1962. 2 G.D. McCleod; G.D. McCleod & Sons Inc.; U.S. Patent 3,917,648, November 4, 1975. 3 T. Hara; M. Ogawa; M. Yamashita; Y. Tajiri; Nippon Kokan Kabushiki Kaisha; U.S. Patent 4,407,899, October 4, 1983. 4 Wim J. van Ooij; Ashok Sabata; Armco, Inc.; U.S. Patent 5,433,976, July 18, 1995. 5 Y. Murata, et al.; Shell Oil Company; U.S. Patent 6,005,060 – “Epoxy Resin Composition and Cured Composite Product,” December 21, 1999. 6 R. Mikami; Toray Silicone Co. Ltd.; U.S. Patent 4,283,513 – “Siloxane-Modified Epoxy Resin Composition,” August 11, 1981. 7 R. Mikami; Toray Silicone Co. Ltd.; U.S. Patent 4,287,326 – “Siloxane-Modified Epoxy Resin Composition,” August 11, 1981. 8 G. Decker, et al.; Dow Corning Corp., Toray Industries; U.S. Patent 5,135,993 – “High Modulus Silicones as Toughening Agents for Epoxy Resins,” August 4, 1992. 9 G. Witucki, et al.; Dow Corning Corp.; U.S. Patent 5,280,098 – “Epoxy-functional Silicone Resin,” January 18, 1994. 10 Donald H. Campbell; Janice E. Echols; Walter H. Ohrbom; BASF Corporation; U.S. Patent 5,853,809, December 29, 1998. Additional References 1. E.P. Plueddemann; Silane Coupling Agents, 2 nd ed., Plenum Press, NY, 1991. 2. M.K. Chaudhury; T.M. Gentle; E.P. Plueddemann; J. Adhes. Sci. Technol., 1(1), 29-38, 1987. 3. Y.K. Lee and J.D. Craig; The Electrochem. Soc. 159 th Mtg., Paper 141, Minneapolis, 1981. 4. E.P. Plueddemann; H.A. Clark; L.E. Nelson; K.R. Hoffmann; Mod. Plast., 39, 136, 1962. 5. L.H. Lee; Adhesion Sci. & Technol., Vol. 9B, 647, Plenum, NY, 1975. 6. E.P. Plueddemann; Proc. Am. Soc. for Composites 1 st Tech. Conf., Technomic Publ. Co., 264-279, 1985. 7. P.G. Pape; J. Vinyl Additive Technol., 6(1), 49-52, 2000. 8. B. Thomas and M. Bowery; “Crosslinked Polyethylene Insulations Using the Sioplas Technology,” Wire J., May, 1977. 9. P.G. Pape and E.P. Plueddemann; “History of Silane Coupling Agents in Polymer Composites,” History of Polymer Composites, VNU Science Press, 105-139, 1987. 10. P.G. Pape and E.P. Plueddemann; “Methods of Improving the Performance of Silane Coupling Agents,” Silanes and Other Coupling Agents, K.L. Mittal, ed., VSP, Utrecht, 1992. 11. E.P. Plueddemann and P.G. Pape; “The Use of Mixed Silane Coupling Agents,” SPI Reinforced Plastics Technical Conference, Session 17-F, 1-4, 1985. 12. C.A. Roth; “Silylation Chemistry,” Ind. Eng. Chem. Prod. Res. Develop, 11, 134, 1972. 13. N.C. Angelotti and P.G. Pape; “Analytical Methods for Identification of Silanes and Silicones in Plastics,” Soc. Plastics Engineers RETEC, Atlantic City, NJ, 187-196, 1996. How to Contact Us Dow Corning has sales offices, manufacturing sites, and science and technology laboratories around the globe. Telephone numbers of locations near you are available on the World Wide Web at www.dowcorning.com, or by calling one of our primary locations listed below. Your Global Connection Asia Dow Corning Asia Ltd. – Japan Tel: +81 3 3287 8300 Dow Corning Asia – China Tel: +86 21 3774 7110 Australia & New Zealand Dow Corning Australia Pty. Ltd. Tel: +61 1300 360 732 Europe Dow Corning S.A. Tel: +32 64 88 80 00 North America Dow Corning Corporation World Headquarters Tel: +1 989 496 6000 South America Dow Corning do Brasil Tel: +55 11 3759 4300 LIMITED WARRANTY INFORMATION – PLEASE READ CAREFULLY The information contained herein is offered in good faith and is believed to be accurate. However, because conditions and methods of use of our products are beyond our control, this information should not be used in substitution for customer’s tests to ensure that Dow Corning’s products are safe, effective, and fully satisfactory for the intended end use. Suggestions of use shall not be taken as inducements to infringe any patent. Dow Corning’s sole warranty is that the product will meet the Dow Corning sales specifications in effect at the time of shipment. Your exclusive remedy for breach of such warranty is limited to refund of purchase price or replacement of any product shown to be other than as warranted. DOW CORNING SPECIFICALLY DISCLAIMS ANY OTHER EXPRESS OR IMPLIED WARRANTY OF FITNESS FOR A PARTICULAR PURPOSE OR MERCHANTABILITY. DOW CORNING DISCLAIMS LIABILITY FOR ANY INCIDENTAL OR CONSEQUENTIAL DAMAGES. Dow Corning is a registered trademark of Dow Corning Corporation. We help you invent the future is a trademark of Dow Corning Corporation. All other trademarks are the property of their respective owners. ©2005 Dow Corning Corporation. All rights reserved. Printed in USA AGP7436 Form No. 26-1328-01 Contact Dow Corning Photos: Front cover: AV06799 Page 4: AV04743

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