Product Application & Research CentreMumbai Stretched Tapes (Raffia) and Monofilaments Introduction Stretched tapes are uniaxially oriented thermoplastic semi finished products with a high width to thickness ratio. These tapes can be converted into twines, ropes, woven and knitted fabrics. A range of applications for stretched tapes have expanded considerably from woven sacks to tarpaulins, primary carpet backing, industrial fabric, carpet yarn, ropes, geotextile fabrics, concrete reinforcement etc. Monofilaments are uniaxially oriented wirelike polymer strands having a circular cross section. They are manufactured by melt spinning process. The use of monofilaments has steadily increased as a substitute for natural fibres. The size of monofilaments ranges from 0.1mm to 2.5mm in diameter depending upon the end use application. Polyethylene, polypropylene, nylon and polyesters are commonly used raw materials for making monofilaments. Polypropylene has emerged as a leader in the monofilament industry because of its light weight, ease of extrusion and orientation, higher strength and low cost. Monofilaments and multifilaments can be distinguished by the filament size. The size of monofilaments varies from 0.1mm to 2mm whereas that of a single filament in multifilaments ranges from 5 microns to 50 microns. Theory of orientation Partially crystalline thermoplastics, namely eg. Polyethylene (HDPE) and Polypropylene (PP) are ideal materials for making oriented products. In a stretching process, the macro molecules are given an orientation in the direction of draw. The orienting ability of polymer is determined by its spherulite structure. When tension is applied, amorphous regions get oriented first between folded lamellae and the spherulite boundaries. During drawing or stretching, energy conversion takes place. The oriented structure is heated nearly to its melting temperature to reduce amorphous region tension and subsequent shrinkage of tapes is avoided. Manufacture of Tapes The principal stages involved in tape manufacture are : • Extrusion of film • Quenching of film • Slitting of film into tapes • Orientation of tapes • Annealing of tapes • Winding 2 0 . Quenching of the film The polymer melt is partly oriented during extrusion through a die. higher will be the output per revolution of screw. For good melt homogeniety and optimum output. To achieve optimum strength and elongation. the cooling is done by quenching the film in a water tank. feed. with higher strength. more and more processors are shifting to the cast film process owing to its better gauge control and higher outputs. the significant parameters which control the physicals of the tapes are die-water distance (air gap) and quench water temperature. apart from rendering better physicals. Screw diameters range from 65 upwards. air gap of 20-35 mm and quench water temperature of 30-40°C is recommended. In cast film. Die The type of die used is referred to as a coat hanger die / T-die which provides a good streamlined flow.5 should be used. 1.The equipment and processing requirements for conversion of PP into tapes are quite similar to those for HDPE. Equipment for Cast Film Extrusion Extruder PP can be processed on conventional extruders with three zone screws viz. 3 . The film from the die is directly taken into the tank filled with water. than a film which is quenched at slower rates. whilst compression ratio's between 3. An illustration of a typical stretched tape plant is given in Fig. Operation with a lower air gap will reduce the time for melt relaxation and result in films. During quenching.3. extruders with L/D ratios of 24:1 upto 30:1 are preferred. Film quality and performance of the resulting tapes mainly depend on the quenching conditions. To prevent melt relaxation the melt is quenched rapidly after exiting from the die. The output obtained from PP depends upon the L/D ratio (Length to Diameter Ratio). compression and metering. which will give higher clarity and strength. Although the blown film process has not phased out completely. Fast cooling promotes a finer crystalline structure of polymer in the film which in turn improves the performance of film during the stretching operation. A very fast rate of quench will result in a very fine crystal structure in the film. Higher the L/D ratio. uniform heating of the tapes takes place. Hence. Hot air circulation in the oven should be adequately controlled to avoid excessive turbulence. Hot air is blown in the oven. for all the tapes to traverse freely. Ratio of speed of second set of rollers. counter current to the movement of tapes. Hot Air Oven Tapes from the first set of godet are taken through an oven on to the second godet rollers. The blades are equally placed on a bar using spacers and are set at an angle of 30 ° and 60 ° with the film. it needs to be oriented at a higher temperatures than that for HDPE to fully develop the mechanical properties in stretched products. A good temperature control system with an accuracy of atleast ± 5 °C is required. The slitting tools generally used are industrial or surgical blades with sharp edges. which ensures better tape properties compared to the Hot plate system. Stretching of tapes in presence of heat media results in molecular / chain orientation and thus greatly increases the mechanical strength of tapes. operating at higher speed. the orientation temperature and the stretch ratio are kept constant and checked randomly throughout the process. PP has a higher melting point (160-165 °C) than HDPE. Once the required tape properties are obtained. Orientation of tapes Orientation is accomplished by stretching the tapes while passing them through a hot air oven or over a hot plate. placed on either side of the hot air oven / hot plate and operating at different speeds. Blunt blades produce poor cuts. maintained at a temperature just below the melting temperature of PP. Stretching of tapes is done by passing them over two sets of rollers. to that of first set is termed as stretch ratio. PP tapes have to be oriented at a stretch ratio of 5-6 and temperature of 135-155°C in hot air oven or at 125-160 °C on a hot plate. Initial tape width is adjusted by selecting appropriate spacer. which lead to problems in drawing. and recirculated through a blower . Higher flow rate of circulating air is preferred as it enables faster and more uniform heating of the tapes and lower the risk of leaving some areas in tapes undrawn or underdrawn. In hot air oven system. winding and weaving of tapes. called godet rollers. The length of this unit is around 3 meters and its width is slightly more than that of the godet rolls.heater system at linear flow rate of 10-30 mtrs/sec. 4 .Slitting of film into tapes The flat film after quenching is slit into tapes of specific width according to the end use requirements. For good winding. The annealing ratio is a function of second and third goddet rollers. which are usually heated electrically or by circulating hot oil. In this system. This helps to minimize tape shrinkage which may occur as a result of residual stresses in the oriented tapes.Hot Plate Hot plate is heated electrically or by circulating hot oil. Although hot plate offers 25% saving in terms of energy requirement. Winding After the final goddet stand. Annealing of tapes Drawn tapes are "annealed" immediately after stretching operation. while for HDPE it is between 100-110 °C. tapes are in close contact with the hot metal surface while they are oriented. 5 . controlled tension is essential. thereby enabling the converter to exploit the superior mechanicals of PP. it is between 125-145 °C. the latter being maintained at a slightly lower speed (5% less) than the former. The upper and lower hot plate design is preferred as it provides enough contact for uniform distribution of the tapes. giving better heat transfer to all tapes and less variations in mechanical properties. tapes can be annealed by using hot goddet rolls. For PP. Annealing is done by heating the stretched tapes while they are passing over from second goddet rollers to third godet rollers. the hot air circulating ovens provide uniform heating to the tapes. The annealing temperature is slightly lower (5-10°C) than the orientation temperature. since half the tapes can be stretched over top surface while the other half are stretched over lower surface. Alternatively. Relaxation takes place over a short gap (the distance between the rolls). the tape proceeds to a winder stand and is taken up by the bobbins. 6 . 7 . . The L/D ratio of the die is usually 8 to 10mm.5 The homogenised melt from the extruder is fed to a circular die or spinneret which is at 90° to the axis of the extruder. Usually these extruders have screw dia of 65. A cone with a smooth surface is fixed onto the die to ensure smooth and uniform flow of melt to all the holes. which may otherwise lead to breakage of filaments. Circular dies are preferred for uniform pressure drop throughout the die. The die is threaded on to the head which is connected to the extruder through an adaptor or neck. These holes are precisely machined to get a smooth and uniform monofilament surface. screw of L/D ratio from 24:1 to 30:1 is preferred with compression ratio of 3 to 3. The filterpack and breaker plate are mounted on the head. For good melt homogeneity and optimum output. 90 or 110mm. Monofilament dies usually have 180 to 200 holes which are even distributed on 2 or 3 pitch circles (concentric to one another). 9 . The filter ensures that a contaminant free melt is fed to the die.Manufacture of monofilaments The process essentially consists of the following steps • • • • • Extrusion Forming (Quenching) Drawing Heat setting Winding Extrusion Conventional extruders are used for extrusion of monofilaments. Each winder is fitted with a separate torque motor. Winding tension is electronically set to get a good package. The winders also have a large bobbin winder at the end to wind all the filaments together during start up. Stretching is effected by the speed differential between the 1st and IInd godet. Drawing The drawing zone of a monofilament line consists of the first godet. a hot air oven and a fast godet (2nd godet). At the quench bath outlet. Stretch ratio of 8 to 10 are set for PP filament. The filaments are heated to just below their softening point in the hot air oven (145-160 °C) and drawn. Quench baths used are height adjustable to alter the distance between the die face and the water surface which is called hot distance or air gap. Forming The monofilaments emerging from the die are taken into a quench bath. The hot distance influences the tenacity and stretchability of the filaments. the filaments are taken over a roll where the filaments are separated. The quench medium is water which is maintained at around 25-35°C. This is effected with godet three which runs at a speed of 10% lower than that of the 2nd godet and by passing the filaments through a setting oven which is usually at 80-100°C. Absorbant cotton pads or brushes are also used to reduce water carry over. Separators avoids entanglement of filament which also leads to considerable breakages. Winding In most of the extrusion lines. The filaments are taken over a deflecting roll in the quench bath and are drawn continuously by Godet I. The purpose of quenching is to have a finer crystallite size which will facilitate stretching to produce high tenacity filament.Spinning pumps Monofilament lines usually include a spinning pump. Some lines also incorporate on line screen changes prior to the spinning pump. line speeds are between 130mpm to 160 mpm. Usually higher pump inlet pressures are set to ensure proper filling of the pump and also to increase extruder counter pressure which results in better homogenisation of melt. 10 . Setting Monofilaments are subjected to heat setting to stabilise the orientation and residual stresses which lead to shrinkage. Normally. the filaments are wound individually on separate spools or bobbins. It is a gear pump which maintains a constant supply of melt to the die and isolates it from surging effects which can lead to denier or size variation and even filament breakage. Normally it is maintained between 25-40mm. Resin Selection The advantages of using synthetic materials instead of natural fibres are better physical properties. 11 . PP has a higher softening point which gives it the advantage when it comes to hot filling of certain products in woven sacks. chemical and thermal resistance. giving it the advantage of higher stackability during storage. antioxidants and acid acceptor. The density of PP is the lowest among all the synthetic polymers. negligible water absorption resistance to rotting and fungus attack. For polymer to work well in a tape processing environment. polybutylenes etc. On the other hand. a number of factors are in favour of PP. These are : • Capability of being processed easily into film • Good processing stability and melt strength to eliminate melt flow breaks and thus deteriorating physical properties • Polymer cleanliness to eliminate filter pack blockage and tape flaws • Very little water carry over for processes using quench baths. carpet backing. industrial fabric and geotextile application. However this is most commonly related to the additive package • The capability to orient readily to eliminate tape breakage while drawing • Good end use stability. particularly for outdoor use Resin Characteristics • • • • • • MFI : 2 to 4 Moderately broad molecular weight distribution Free of gels and fish eyes Low water carry over in water bath quench Consistent processability Good colour and processing stability Repol Grades • • Repol H030SG : This is a 3MFI. Although both HDPE & PP compete in the manufacturing of stretched tapes. Moreover. PP belongs to the family of polyolefin polymers comprising of low and high density polyethylenes. homopolymer grade with a general additive package ie. This grade is recommended for raffia tapes used in making woven fabric for sacking. Repol H030SU : This is a UV stabilized version of Repol H030SG and its recommended for outdoor applications. it has to meet certain basic requirements. PP being stiffer than HDPE and fabric made from PP has a higher coefficient of friction. PP tapes tend to fibrillate during processing but this can be avoided by adding antifibrillating additive. Hence broad MWD PP's are easier to process than one with narrow MWD. increased shrinkage. Molten PP is shear sensitive ie. apparent viscosity decreases as applied pressure increases. Hence isotactic PP is preferred. However as molecular weight decreases. better the physical properties of the tape. Additives Repol H030SG incorporates a general additive package consisting of • • Antioxidants Acid acceptors 12 . As molecular weight increases (melt flow decreases) tenacity increase and % elongation decreases at a particular draw ratio. Repol H030SG has a melt flow index of 3 gm/10min by virtue of which it gives an optimum balance of tape properties and processability. extrusion pressures are lower for a given extrusion rate. tenacity and % elongation decreases. PP cannot crystallise. Polypropylene grades of higher melt flow tend to process easier than lower melt flow grades ie. MWD is found to have little effect on physical properties of PP. Higher the isotactic content. B) Molecular weight distribution (MWD) : Molecular weight distribution is a function of catalyst system and polymerisation process. The crystallinity is responsible for the strength. PP with broad MWD is more shear sensitive than the one with narrow MWD. stiffness and solvent resistance of PP. Unless these methyl groups are arranged in one position relative to the chain (isotactic arrangement). Xylene solubles determines the percentage of lower molecular weight fraction in polypropylene. Repol H030SG is a moderately broad molecular weight distribution polymer.Effect of polymer variables on properties A) Molecular weight (Melt flow) : Molecular weight of PP has a significant effect on processing and tape properties. Higher xylene solubles will lead to decreased tenacity. On the other hand. Hence it exhibits good processability. stickiness and weaving problems. Melt flow index is an indication of molecular weight. (C) Stereoregularity : Polypropylene has a methyl group attached to every other carbon atom. lower solubles will lead to tape splitting and drawbreaks. 3 13 . In the stretched tape process. Quench temperature is also another important variable as the rate of cooling has a significant effect on the strength and characteristics of the product. Also the tendency to fibrillate is less. non fibrillating tendency and curl free tapes. The effect of orientation temperature on stretched tape properties is shown in the following Fig.Effect of processing variables on physical properties :A) Draw / Stretch Ratio : As draw ratio increases. The draw ratio also determines initial cross-section of the slit strip/monofilament which is required for obtaining final width of the tape or size of monofilament. the temperature profile on the extruder affects the melt temperature and extruder output. Higher draw ratio increases the alignment of the polymer molecules. A draw ratio between 5:1 to 6:1 is optimum for obtaining a tape with good combination of mechanical properties. rapid cooling with low quench temperature produces a film of higher crystallinity which results in poor orientation. B) Temperature : Extrusion temperature ie. tenacity increases and % elongation decreases. the residual shrinkage decreases. tapes with tenacity of 5.2 15-25 In general. Effect of orientation temperature on shrinkage is shown in Figure 4. for mono-axially oriented tape. Eventually a temperature is reached where tenacity decreases rapidly. finished tapes should have the following characteristics :Finished tape width (mm) Linear density (denier) Tenacity (g/denier) Elongation at break (%) : : : : 5 mm (max) 600 (min) 4. with corresponding increase in % elongation. Desired Characteristics of PP Tapes Tapes are required to be produced as per IS 11197 1985 (Spec.0-6. 14 . tenacity increases and % elongation remains relatively constant. As orientation temperature is increased.As the orientation temperature increases. As per IS 11197.0 g/d and elongation of 20-25% are preferred. 90 g/cc) than HDPE (0. Since HDPE exhibits better outdoor stability than PP and also because PE lamination grades are easily available. hence when cement is filled at a temp. lower bag weight. polypropylene woven sacks offer advantages such as better bursting strength. Fertilizer Packing Laminated HDPE bags are predominantly used for packing fertilizers. but most of the end users have shifted to polypropylene due to the following advantages :• • • • Polypropylene has a lower density (0. New Applications for PP Woven Sacks • • • Sugar bags Postal bags Tea bags 15 . the performance of PP woven sacks is better in terms of bursting strength Polypropylene has higher tensile strength than HDPE Easy availability Unlaminated gussetted polypropylene bags with a valve for filling cement are normally used.Applications Raffia Tapes 1. Previously HDPE woven sacks were predominantly used for packing cement. In comparison to paper bags. Cement is usually packed in 50 kg bags. The fabric used has a mesh size of 10x10 and the bag weight is @70 gms. The bags are not laminated to facilitate breathing of air during filling. low weight and low cost. but paper bags exhibit better printability and lower seepage than PP bags.. Packaging applications Woven Sacks .Cement Packaging Polypropylene woven sacks have increasingly replaced jute bags and paper bags for packing cement. low cost. They offer distinct advantages such as high strength.952 g/cc) therefore giving higher yield per unit weight Polypropylene exhibits higher service temperature than HDPE. of 85 °C-90 °C at a pressure of 6 kg/cm2 . fungus attack and low seepage of cement as compared to jute bags. PP till recent times did not make in roads into this sector due to unavailability of a suitable PP lamination grade. Some applications of woven fabric tarpaulins are :• Water proof liners for trucks • Railway wagon covers • Floor linings for storage • Shed. but they soil easily and are not weather proof and rot proof. Geotextiles Geotextile is a woven or non woven fabric that is designed to stop water erosion. It works by reducing amount of water in contact with the structure and preventing soil erosion. Tarpaulins Tarpaulins are usually produced from HDPE woven fabric. These bags are used for packing agro products. cross laminated PE film. The design and fabric of these bags varies depending upon the requisite container strength. heavy duty LDPE/LLDPE films and cotton canvas. chemicals. soil embankments and other construction application.Flexible Intermediate Bulk Containers (FIBC) / Jumbo bags Polypropylene woven fabric is used to fabricate FIBC's or jumbo bags. UV stabilised PP grades are used for this application. Constructions of woven sack tarpaulin range from 90 gsm to 200 gsm. UV stabilised PP can be used for tarpaulins but the cost of it is higher than HDPE tarpaulins. detergent. Cost of cotton canvas tarpaulin is comparative to low gsm HDPE tarpaulins. 3. The tape denier ranges from 2100D to 2600D for Jumbo bags. HDPE/PP woven sacks are mainly used for :• Separation • Reinforcement • Drainage and filtration • Erosion control 16 . cracking in roads. warehouse covering • Agricultural farms • Construction sites . These bags have carrying capacities of 500 kgs to 4000 kgs.stock pile cover for cement • Poultry shadding • Automobile covers • Baling cloth HDPE is preferred for tarpaulins due to superior stability to outdoor exposure. 2. petrochemical products and fertilizers. plastic raw materials. nets for defence application. PP is also popular because of its low density. HDPE nets are mainly preferred for fishing. Monofilaments Ropes : Ropes are usually manufactured from PP or HDPE but both these materials cater to different markets. PP ropes are used for industrial application like cargo handling. Denier of monofilaments used for ropes and nets is usually between 500 to 800 and tenacity between 5. marine application (mooring ropes). furniture and light fishing activities. PP nets are stiff and can damage the catch. Advantages of PP reinforced concrete are impact strength.4. It also gives saving in construction and transportation cost. ropes for trawling. HDPE ropes are used for domestic application like decorative household uses. mountaineering etc. PP nets are used for high strength application due to its good tensile strength. PP exhibits very good tensile strength and abrasion resistance due to which PP ropes cater to high performance application. On the other hand. some residual strength after cracking and improved flexural strength. eg. wear resistance and knot strength. safety nets. 17 . PP monofilaments are used for making bristles. Nets : PP and HDPE monofilaments are used for making nets. cargo handling net.5 to 6 gpd. Concrete reinforcement Fibrillated PP tapes can be blended with concrete to reinforce non load bearing structures. Shrinkage of tapes 18 . check speed of godet 3 Improve heat conduction during annealing 2.Shooting Guide . Tape breakage 3. Fibrillation of tapes 7. Problem Film puncture Suggested Remedy Clean die lip Optimise temperature profile See under film puncture Check hot plate temperature Reduce stretch ratio Reduce quench tank temperature Set die gap Increase stretch ratio Increase oven/hot plate temperature Reduce quench tank temperature Increase oven/hot plate temperature Increase stretch ratio Reduce air gap Adjust die gap and clean die if required to get uniform film thickness Check uniformity of temperature on hot plate Check spacer thickness with Vernier Check pressure roller and its pressure Check sharpness of blades on spacer Reduce stretch ratio Check for wrinkles in the tape Check winder tension Check traverse guides of winders for damages Increase annealing temp. 1. Denier variation 6. No. Low tenacity (Tensile strength) High elongation 4. 5.Trouble .Tape Plant Sr. or with cooled screw. to find asymmetrical construction. Too fluid to hold together under drawing tension. If plastic becomes too hot. or with gear pump. Problem Filament breakage Suggested Remedy a) Surging in extruder. too much tensile stress will be produced in the filaments m) Too hot orientation (air oven). Use a hopper-dryer h) Contamination in material. b) One or more holes partially blocked. or reduce the gap between die and quench bath j) Decomposition in extruder. controllers etc. which could cause the uneven patterns d) Melt too cold. The same group of filaments will break every time. examine die design for possible stagnation spots. Run at lower temperature.Trouble . Not enough ductility to draw down. or with better-mixing screw. No. Bits of decomposed material clog holes. Ratio of roll speeds may be too high causing excessive tensile stress in orientation stage l) Too cold orientation. Grind them smooth. Try to determine period of the surging. Inspect feed. Clean die. Reduce temperature f) Drawdown too great (use smaller holes in die) or drawdown too fast (slow down whole system) g) Moisture in material. it loses ductility and will break under the normal orientation stresses n) Nicks or abrasive areas or any of the rolls. change die or head if necessary. Look at blueprint of die.Monofilaments Sr. or die. or come through them but weaken the filaments. If plastics is not hot enough. head. and relate to drive. 1. run at lower temperature. The same filament(s) will break every time c) Uneven temperature or distribution of material in head and die. if any. use better-stabilized plastic k) Too much orientation. Raise temperature e) Melt too hot. Run at higher pressure. or wrap them with tape 19 . stop using reground or reworked plastic i) Oxidation and weakening of filament surface.Shooting Guide . or more slowly. Note that changes of orientation may also change the filament size. Look for a repetitive pattern and examine take-off for the source 3.o) Erratic drive. requiring compensating adjustments formula c) Nicks or cuts in filaments. or flame polish a) Not enough orientation. Change compound. 5. Low tenacity 20 . or use lower viscosity compound. Examine breaks and examine unbroken filaments. Increase ratio of roll speeds. Run hotter d) Compound inherently dull. or reduce entrance angle into die holes. burned-out heaters. See (a). Reduce linear speed. Check smoothness of operation of all moving parts 2. or erratic drive. (f) and (o) above Some filaments different a) Temperature gradients in die. Check for drafts. in size die. Dry the feed b) Melt fracture. or run hotter. Use proper orientation temperature. All filaments varying in a) Surging in extruder. requiring compensating adjustments elsewhere b) Degraded plastic. 4. or use larger holes (more drawdown). See (c) above Poor surface appearance a) Moisture in material or any pigment. poor and/or strength die design. Unequal flow in from others. drawdown too much or diameter too fast. or run hotter c) Plastic too cold. Use lower material temperature.