Product Application & Research CentreMumbai

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Stretched Tapes (Raffia) and Monofilaments

Introduction

Stretched tapes are uniaxially oriented thermoplastic semi finished products with a highwidth to thickness ratio. These tapes can be converted into twines, ropes, woven andknitted fabrics. A range of applications for stretched tapes have expanded considerablyfrom 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 crosssection. They are manufactured by melt spinning process. The use of monofilaments hassteadily increased as a substitute for natural fibres. The size of monofilaments rangesfrom 0.1mm to 2.5mm in diameter depending upon the end use application.

Polyethylene, polypropylene, nylon and polyesters are commonly used raw materials formaking monofilaments. Polypropylene has emerged as a leader in the monofilamentindustry because of its light weight, ease of extrusion and orientation, higher strength andlow cost.

Monofilaments and multifilaments can be distinguished by the filament size. The size ofmonofilaments varies from 0.1mm to 2mm whereas that of a single filament inmultifilaments 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 macromolecules are given an orientation in the direction of draw. The orienting ability ofpolymer is determined by its spherulite structure. When tension is applied, amorphousregions get oriented first between folded lamellae and the spherulite boundaries. Duringdrawing or stretching, energy conversion takes place. The oriented structure is heatednearly to its melting temperature to reduce amorphous region tension and subsequentshrinkage 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

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The equipment and processing requirements for conversion of PP into tapes are quitesimilar to those for HDPE. Although the blown film process has not phased outcompletely, more and more processors are shifting to the cast film process owing to itsbetter gauge control and higher outputs.

Equipment for Cast Film Extrusion

Extruder

PP can be processed on conventional extruders with three zone screws viz. feed,compression and metering. The output obtained from PP depends upon the L/D ratio(Length to Diameter Ratio). Higher the L/D ratio, higher will be the output perrevolution of screw. For good melt homogeniety and optimum output, extruders withL/D ratios of 24:1 upto 30:1 are preferred, whilst compression ratio's between 3.0 - 3.5should be used. Screw diameters range from 65 upwards. An illustration of a typicalstretched tape plant is given in Fig. 1.

Die

The type of die used is referred to as a coat hanger die / T-die which provides a goodstreamlined flow.

Quenching of the film

The polymer melt is partly oriented during extrusion through a die. To prevent meltrelaxation the melt is quenched rapidly after exiting from the die. Fast cooling promotesa finer crystalline structure of polymer in the film which in turn improves theperformance of film during the stretching operation, apart from rendering betterphysicals.

In cast film, the cooling is done by quenching the film in a water tank. The film from thedie is directly taken into the tank filled with water. Film quality and performance of theresulting tapes mainly depend on the quenching conditions. During quenching, thesignificant parameters which control the physicals of the tapes are die-water distance (airgap) and quench water temperature.

Operation with a lower air gap will reduce the time for melt relaxation and result in films,with higher strength. A very fast rate of quench will result in a very fine crystal structurein the film, which will give higher clarity and strength, than a film which is quenched atslower rates. To achieve optimum strength and elongation, air gap of 20-35 mm andquench water temperature of 30-40°C is recommended.

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Slitting of film into tapes

The flat film after quenching is slit into tapes of specific width according to the end userequirements. The slitting tools generally used are industrial or surgical blades withsharp edges. Blunt blades produce poor cuts, which lead to problems in drawing,winding and weaving of tapes. The blades are equally placed on a bar using spacers andare set at an angle of 30° and 60° with the film. Initial tape width is adjusted by selectingappropriate spacer.

Orientation of tapes

Orientation is accomplished by stretching the tapes while passing them through a hot airoven or over a hot plate, maintained at a temperature just below the melting temperatureof PP. PP has a higher melting point (160-165°C) than HDPE. Hence, it needs to beoriented at a higher temperatures than that for HDPE to fully develop the mechanicalproperties in stretched products. Stretching of tapes is done by passing them over twosets of rollers, called godet rollers, placed on either side of the hot air oven / hot plate andoperating at different speeds. Ratio of speed of second set of rollers, operating at higherspeed, to that of first set is termed as stretch ratio. Stretching of tapes in presence of heatmedia results in molecular / chain orientation and thus greatly increases the mechanicalstrength of tapes. PP tapes have to be oriented at a stretch ratio of 5-6 and temperature of135-155°C in hot air oven or at 125-160°C on a hot plate.

Hot Air Oven

Tapes from the first set of godet are taken through an oven on to the second godet rollers.Hot air is blown in the oven, counter current to the movement of tapes, and recirculatedthrough a blower - heater system at linear flow rate of 10-30 mtrs/sec. Higher flow rateof circulating air is preferred as it enables faster and more uniform heating of the tapesand lower the risk of leaving some areas in tapes undrawn or underdrawn. Once therequired tape properties are obtained, the orientation temperature and the stretch ratio arekept constant and checked randomly throughout the process.

In hot air oven system, uniform heating of the tapes takes place, which ensures better tapeproperties compared to the Hot plate system. The length of this unit is around 3 metersand its width is slightly more than that of the godet rolls, for all the tapes to traversefreely. A good temperature control system with an accuracy of atleast ±5°C is required.Hot air circulation in the oven should be adequately controlled to avoid excessiveturbulence.

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Hot Plate

Hot plate is heated electrically or by circulating hot oil. In this system, tapes are in closecontact with the hot metal surface while they are oriented. The upper and lower hot platedesign is preferred as it provides enough contact for uniform distribution of the tapes,since half the tapes can be stretched over top surface while the other half are stretchedover lower surface, giving better heat transfer to all tapes and less variations inmechanical properties.

Although hot plate offers 25% saving in terms of energy requirement, the hot aircirculating ovens provide uniform heating to the tapes, thereby enabling the converter toexploit the superior mechanicals of PP.

Annealing of tapes

Drawn tapes are "annealed" immediately after stretching operation. This helps tominimize tape shrinkage which may occur as a result of residual stresses in the orientedtapes. Annealing is done by heating the stretched tapes while they are passing over fromsecond goddet rollers to third godet rollers; the latter being maintained at a slightly lowerspeed (5% less) than the former. The annealing ratio is a function of second and thirdgoddet rollers. The annealing temperature is slightly lower (5-10°C) than the orientationtemperature. For PP, it is between 125-145°C, while for HDPE it is between 100-110°C.Alternatively, tapes can be annealed by using hot goddet rolls, which are usually heatedelectrically or by circulating hot oil. Relaxation takes place over a short gap (the distancebetween the rolls).

Winding

After the final goddet stand, the tape proceeds to a winder stand and is taken up by thebobbins. For good winding, controlled tension is essential.

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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. Usually these extrudershave screw dia of 65, 90 or 110mm. For good melt homogeneity and optimum output,screw of L/D ratio from 24:1 to 30:1 is preferred with compression ratio of 3 to 3.5

The homogenised melt from the extruder is fed to a circular die or spinneret which is at90° to the axis of the extruder. The die is threaded on to the head which is connected tothe extruder through an adaptor or neck. The filterpack and breaker plate are mounted onthe head. The filter ensures that a contaminant free melt is fed to the die, which mayotherwise lead to breakage of filaments.

Monofilament dies usually have 180 to 200 holes which are even distributed on 2 or 3pitch circles (concentric to one another). The L/D ratio of the die is usually 8 to 10mm.These holes are precisely machined to get a smooth and uniform monofilament surface.A cone with a smooth surface is fixed onto the die to ensure smooth and uniform flow ofmelt to all the holes. Circular dies are preferred for uniform pressure drop throughout thedie.

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Spinning pumpsMonofilament lines usually include a spinning pump. It is a gear pump which maintainsa constant supply of melt to the die and isolates it from surging effects which can lead todenier or size variation and even filament breakage.

Usually higher pump inlet pressures are set to ensure proper filling of the pump and alsoto increase extruder counter pressure which results in better homogenisation of melt.Some lines also incorporate on line screen changes prior to the spinning pump.

FormingThe monofilaments emerging from the die are taken into a quench bath. The quenchmedium is water which is maintained at around 25-35°C. The purpose of quenching is tohave a finer crystallite size which will facilitate stretching to produce high tenacityfilament. The filaments are taken over a deflecting roll in the quench bath and are drawncontinuously by Godet I.Quench baths used are height adjustable to alter the distance between the die face and thewater surface which is called hot distance or air gap. The hot distance influences thetenacity and stretchability of the filaments. Normally it is maintained between 25-40mm.At the quench bath outlet, the filaments are taken over a roll where the filaments areseparated. Absorbant cotton pads or brushes are also used to reduce water carry over.Separators avoids entanglement of filament which also leads to considerable breakages.

DrawingThe drawing zone of a monofilament line consists of the first godet, a hot air oven and afast godet (2nd godet). Stretching is effected by the speed differential between the 1st andIInd godet. The filaments are heated to just below their softening point in the hot airoven (145-160°C) and drawn. Stretch ratio of 8 to 10 are set for PP filament. Normally,line speeds are between 130mpm to 160 mpm.

SettingMonofilaments are subjected to heat setting to stabilise the orientation and residualstresses which lead to shrinkage. This is effected with godet three which runs at a speedof 10% lower than that of the 2nd godet and by passing the filaments through a settingoven which is usually at 80-100°C.

WindingIn most of the extrusion lines, the filaments are wound individually on separate spools orbobbins. Each winder is fitted with a separate torque motor. Winding tension iselectronically set to get a good package.

The winders also have a large bobbin winder at the end to wind all the filaments togetherduring start up.

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Resin Selection

The advantages of using synthetic materials instead of natural fibres are better physicalproperties, chemical and thermal resistance, negligible water absorption resistance torotting and fungus attack.PP belongs to the family of polyolefin polymers comprising of low and high densitypolyethylenes, polybutylenes etc. Although both HDPE & PP compete in themanufacturing of stretched tapes, a number of factors are in favour of PP. The density ofPP is the lowest among all the synthetic polymers. Moreover, PP has a higher softeningpoint which gives it the advantage when it comes to hot filling of certain products inwoven sacks. PP being stiffer than HDPE and fabric made from PP has a highercoefficient of friction, giving it the advantage of higher stackability during storage. Onthe other hand, PP tapes tend to fibrillate during processing but this can be avoided byadding antifibrillating additive.For polymer to work well in a tape processing environment, it has to meet certain basicrequirements.

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. 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, homopolymer grade with a general additive packageie. antioxidants and acid acceptor. This grade is recommended for raffia tapes used inmaking woven fabric for sacking, carpet backing, industrial fabric and geotextileapplication.

• Repol H030SU : This is a UV stabilized version of Repol H030SG and itsrecommended for outdoor applications.

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Effect of polymer variables on properties

A) Molecular weight (Melt flow) : Molecular weight of PP has a significant effect onprocessing and tape properties. Melt flow index is an indication of molecular weight.As molecular weight increases (melt flow decreases) tenacity increase and %elongation decreases at a particular draw ratio. However as molecular weightdecreases, tenacity and % elongation decreases.

Polypropylene grades of higher melt flow tend to process easier than lower melt flowgrades ie. extrusion pressures are lower for a given extrusion rate.

Repol H030SG has a melt flow index of 3 gm/10min by virtue of which it gives anoptimum balance of tape properties and processability.

B) Molecular weight distribution (MWD) : Molecular weight distribution is a functionof catalyst system and polymerisation process. Molten PP is shear sensitive ie.apparent viscosity decreases as applied pressure increases. PP with broad MWD ismore shear sensitive than the one with narrow MWD. Hence broad MWD PP's areeasier to process than one with narrow MWD. MWD is found to have little effect onphysical properties of PP.

Repol H030SG is a moderately broad molecular weight distribution polymer. Hence itexhibits good processability.

(C) Stereoregularity : Polypropylene has a methyl group attached to every other carbonatom. Unless these methyl groups are arranged in one position relative to the chain(isotactic arrangement), PP cannot crystallise. The crystallinity is responsible for thestrength, stiffness and solvent resistance of PP. Higher the isotactic content, better thephysical properties of the tape. Hence isotactic PP is preferred.

Xylene solubles determines the percentage of lower molecular weight fraction inpolypropylene. Higher xylene solubles will lead to decreased tenacity, increasedshrinkage, stickiness and weaving problems. On the other hand, lower solubles will leadto tape splitting and drawbreaks.

Additives

Repol H030SG incorporates a general additive package consisting of

• Antioxidants• Acid acceptors

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Effect of processing variables on physical properties :-

A) Draw / Stretch Ratio : As draw ratio increases, tenacity increases and % elongationdecreases. 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 combinationof mechanical properties, non fibrillating tendency and curl free tapes. The draw ratioalso determines initial cross-section of the slit strip/monofilament which is required forobtaining final width of the tape or size of monofilament.

B) Temperature : Extrusion temperature ie. the temperature profile on the extruderaffects the melt temperature and extruder output.

Quench temperature is also another important variable as the rate of cooling has asignificant effect on the strength and characteristics of the product. In the stretched tapeprocess, rapid cooling with low quench temperature produces a film of highercrystallinity which results in poor orientation. Also the tendency to fibrillate is less.

The effect of orientation temperature on stretched tape properties is shown in thefollowing Fig. 3

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As the orientation temperature increases, tenacity increases and % elongation remainsrelatively constant. Eventually a temperature is reached where tenacity decreases rapidly,with corresponding increase in % elongation.

Effect of orientation temperature on shrinkage is shown in Figure 4. As orientationtemperature is increased, the residual shrinkage decreases.

Desired Characteristics of PP Tapes

Tapes are required to be produced as per IS 11197 1985 (Spec. for mono-axially orientedtape. As per IS 11197, finished tapes should have the following characteristics :-

Finished tape width (mm) : 5 mm (max)Linear density (denier) : 600 (min)Tenacity (g/denier) : 4.2Elongation at break (%) : 15-25

In general, tapes with tenacity of 5.0-6.0 g/d and elongation of 20-25% are preferred.

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Applications

Raffia Tapes

1. Packaging applications

Woven Sacks - Cement Packaging

Polypropylene woven sacks have increasingly replaced jute bags and paper bags forpacking cement. They offer distinct advantages such as high strength, lower bag weight,low cost, fungus attack and low seepage of cement as compared to jute bags.

In comparison to paper bags, polypropylene woven sacks offer advantages such as betterbursting strength, low weight and low cost, but paper bags exhibit better printability andlower seepage than PP bags.

Previously HDPE woven sacks were predominantly used for packing cement, but most ofthe end users have shifted to polypropylene due to the following advantages :-

• Polypropylene has a lower density (0.90 g/cc) than HDPE (0.952 g/cc) thereforegiving higher yield per unit weight

• Polypropylene exhibits higher service temperature than HDPE, hence when cement isfilled at a temp.. of 85°C-90°C at a pressure of 6 kg/cm2, the performance of PPwoven 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 normallyused. Cement is usually packed in 50 kg bags. The fabric used has a mesh size of 10x10and the bag weight is @70 gms. The bags are not laminated to facilitate breathing of airduring filling.

Fertilizer Packing

Laminated HDPE bags are predominantly used for packing fertilizers. Since HDPEexhibits better outdoor stability than PP and also because PE lamination grades are easilyavailable, PP till recent times did not make in roads into this sector due to unavailabilityof a suitable PP lamination grade.

New Applications for PP Woven Sacks

• Sugar bags• Postal bags• Tea bags

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Flexible Intermediate Bulk Containers (FIBC) / Jumbo bags

Polypropylene woven fabric is used to fabricate FIBC's or jumbo bags. These bags havecarrying capacities of 500 kgs to 4000 kgs. The design and fabric of these bags variesdepending upon the requisite container strength.These bags are used for packing agro products, chemicals, detergent, plastic rawmaterials, petrochemical products and fertilizers.UV stabilised PP grades are used for this application. The tape denier ranges from2100D to 2600D for Jumbo bags.

2. Tarpaulins

Tarpaulins are usually produced from HDPE woven fabric, cross laminated PE film,heavy duty LDPE/LLDPE films and cotton canvas.Constructions of woven sack tarpaulin range from 90 gsm to 200 gsm. Cost of cottoncanvas tarpaulin is comparative to low gsm HDPE tarpaulins, but they soil easily and arenot weather proof and rot proof.

Some applications of woven fabric tarpaulins are :-• Water proof liners for trucks• Railway wagon covers• Floor linings for storage• Shed, warehouse covering• Agricultural farms• Construction sites - stock pile cover for cement• Poultry shadding• Automobile covers• Baling cloth

HDPE is preferred for tarpaulins due to superior stability to outdoor exposure. UVstabilised PP can be used for tarpaulins but the cost of it is higher than HDPE tarpaulins.

3. Geotextiles

Geotextile is a woven or non woven fabric that is designed to stop water erosion,cracking in roads, soil embankments and other construction application. It works byreducing amount of water in contact with the structure and preventing soil erosion.

HDPE/PP woven sacks are mainly used for :-• Separation• Reinforcement• Drainage and filtration• Erosion control

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4. Concrete reinforcement

Fibrillated PP tapes can be blended with concrete to reinforce non load bearing structures.Advantages of PP reinforced concrete are impact strength, some residual strength aftercracking and improved flexural strength. It also gives saving in construction andtransportation cost.

Monofilaments

Ropes : Ropes are usually manufactured from PP or HDPE but both these materials caterto different markets.PP exhibits very good tensile strength and abrasion resistance due to which PP ropescater to high performance application. PP is also popular because of its low density. PPropes are used for industrial application like cargo handling, marine application (mooringropes), ropes for trawling, mountaineering etc. On the other hand, HDPE ropes are usedfor domestic application like decorative household uses, furniture and light fishingactivities.Denier of monofilaments used for ropes and nets is usually between 500 to 800 andtenacity between 5.5 to 6 gpd.

Nets : PP and HDPE monofilaments are used for making nets. PP nets are used for highstrength application due to its good tensile strength, wear resistance and knot strength.eg. cargo handling net, safety nets, nets for defence application. HDPE nets are mainlypreferred for fishing. PP nets are stiff and can damage the catch.PP monofilaments are used for making bristles.

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Trouble - Shooting Guide - Tape Plant

Sr.No.

Problem Suggested Remedy

1. Film puncture Clean die lipOptimise temperature profile

2. Tape breakage See under film punctureCheck hot plate temperatureReduce stretch ratioReduce quench tank temperatureSet die gap

3. Low tenacity(Tensile strength)

Increase stretch ratioIncrease oven/hot plate temperature

4. High elongation Reduce quench tank temperatureIncrease oven/hot plate temperatureIncrease stretch ratioReduce air gap

5. Denier variation Adjust die gap and clean die if required to getuniform film thicknessCheck uniformity of temperature on hot plateCheck spacer thickness with VernierCheck pressure roller and its pressure

6. Fibrillation of tapes Check sharpness of blades on spacerReduce stretch ratioCheck for wrinkles in the tapeCheck winder tensionCheck traverse guides of winders for damages

7. Shrinkage of tapes Increase annealing temp, check speed of godet 3Improve heat conduction during annealing

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Trouble - Shooting Guide - Monofilaments

Sr. No. Problem Suggested Remedy1. Filament breakage a) Surging in extruder. Run at higher pressure, or

with better-mixing screw, or with cooledscrew, or with gear pump, or more slowly. Tryto determine period of the surging, and relateto drive, controllers etc.

b) One or more holes partially blocked. The samefilament(s) will break every time

c) Uneven temperature or distribution of materialin head and die. The same group of filamentswill break every time. Look at blueprint of die,to find asymmetrical construction, if any,which could cause the uneven patterns

d) Melt too cold. Not enough ductility to drawdown. Raise temperature

e) Melt too hot. Too fluid to hold together underdrawing tension. Reduce temperature

f) Drawdown too great (use smaller holes in die)or drawdown too fast (slow down wholesystem)

g) Moisture in material. Use a hopper-dryerh) Contamination in material. Inspect feed, stop

using reground or reworked plastici) Oxidation and weakening of filament surface.

Run at lower temperature, or reduce the gapbetween die and quench bath

j) Decomposition in extruder, head, or die. Bitsof decomposed material clog holes, or comethrough them but weaken the filaments. Cleandie, run at lower temperature; examine diedesign for possible stagnation spots, change dieor head if necessary, use better-stabilizedplastic

k) Too much orientation. Ratio of roll speedsmay be too high causing excessive tensilestress in orientation stage

l) Too cold orientation. If plastics is not hotenough, too much tensile stress will beproduced in the filaments

m) Too hot orientation (air oven). If plasticbecomes too hot, it loses ductility and willbreak under the normal orientation stresses

n) Nicks or abrasive areas or any of the rolls.Grind them smooth, or wrap them with tape

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o) Erratic drive. Check smoothness of operationof all moving parts

2. All filaments varying indiameter

a) Surging in extruder, drawdown too much ortoo fast, or erratic drive. See (a), (f) and (o)above

3. Some filaments differentfrom others, in sizeand/or strength

a) Temperature gradients in die. Unequal flow indie. Check for drafts, burned-out heaters, poordie design. See (c) above

4. Poor surface appearance a) Moisture in material or any pigment. Dry thefeed

b) Melt fracture. Reduce linear speed, or uselarger holes (more drawdown), or reduceentrance angle into die holes, or use lowerviscosity compound, or run hotter

c) Plastic too cold. Run hotterd) Compound inherently dull. Change

compound, or run hotter, or flame polish

5. Low tenacity a) Not enough orientation. Increase ratio of rollspeeds. Use proper orientation temperature.Note that changes of orientation may alsochange the filament size, requiringcompensating adjustments elsewhere

b) Degraded plastic. Use lower materialtemperature, requiring compensatingadjustments formula

c) Nicks or cuts in filaments. Examine breaksand examine unbroken filaments. Look for arepetitive pattern and examine take-off for thesource