94874356 raffia-tape-line-process

Product Application & Research Centre
Mumbai

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

The equipment and processing requirements for conversion of PP into tapes are quite
similar to those for HDPE. Although the blown film process has not phased out
completely, more and more processors are shifting to the cast film process owing to its
better 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 per
revolution of screw. For good melt homogeniety and optimum output, extruders with
L/D ratios of 24:1 upto 30:1 are preferred, whilst compression ratio's between 3.0 - 3.5
should be used. Screw diameters range from 65 upwards. An illustration of a typical
stretched 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 good
streamlined flow.

Quenching of the film

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

In cast film, the cooling is done by quenching the film in a water tank. The film from the
die is directly taken into the tank filled with water. Film quality and performance of the
resulting tapes mainly depend on the quenching conditions. During quenching, the
significant parameters which control the physicals of the tapes are die-water distance (air
gap) 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 structure
in the film, which will give higher clarity and strength, than a film which is quenched at
slower rates. To achieve optimum strength and elongation, air gap of 20-35 mm and
quench water temperature of 30-40°C is recommended.

3

Slitting of film into tapes

The flat film after quenching is slit into tapes of specific width according to the end use
requirements. The slitting tools generally used are industrial or surgical blades with
sharp 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 and
are set at an angle of 30 ° and 60 ° with the film. Initial tape width is adjusted by selecting
appropriate spacer.

Orientation of tapes

Orientation is accomplished by stretching the tapes while passing them through a hot air
oven or over a hot plate, maintained at a temperature just below the melting temperature
of PP. PP has a higher melting point (160-165 °C) than HDPE. Hence, it needs to be
oriented at a higher temperatures than that for HDPE to fully develop the mechanical
properties in stretched products. Stretching of tapes is done by passing them over two
sets of rollers, called godet rollers, placed on either side of the hot air oven / hot plate and
operating at different speeds. Ratio of speed of second set of rollers, operating at higher
speed, to that of first set is termed as stretch ratio. Stretching of tapes in presence of heat
media results in molecular / chain orientation and thus greatly increases the mechanical
strength of tapes. 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.

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 recirculated
through a blower - heater system at linear flow rate of 10-30 mtrs/sec. 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. Once the
required tape properties are obtained, the orientation temperature and the stretch ratio are
kept constant and checked randomly throughout the process.

In hot air oven system, uniform heating of the tapes takes place, which ensures better tape
properties compared to the Hot plate system. The length of this unit is around 3 meters
and its width is slightly more than that of the godet rolls, for all the tapes to traverse
freely. 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 excessive
turbulence.

4

Hot Plate

Hot plate is heated electrically or by circulating hot oil. In this system, tapes are in close
contact with the hot metal surface while they are oriented. The upper and lower hot plate
design 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 stretched
over lower surface, giving better heat transfer to all tapes and less variations in
mechanical properties.

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

Annealing of tapes

Drawn tapes are "annealed" immediately after stretching operation. This helps to
minimize tape shrinkage which may occur as a result of residual stresses in the oriented
tapes. Annealing is done by heating the stretched tapes while they are passing over from
second goddet rollers to third godet rollers; the latter being maintained at a slightly lower
speed (5% less) than the former. The annealing ratio is a function of second and third
goddet rollers. The annealing temperature is slightly lower (5-10°C) than the orientation
temperature. 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 heated
electrically or by circulating hot oil. Relaxation takes place over a short gap (the distance
between the rolls).

Winding

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

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94874356 raffia-tape-line-process

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 extruders
have 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 at
90° to the axis of the extruder. The die is threaded on to the head which is connected to
the extruder through an adaptor or neck. The filterpack and breaker plate are mounted on
the head. The filter ensures that a contaminant free melt is fed to the die, which may
otherwise lead to breakage of filaments.

Monofilament dies usually have 180 to 200 holes which are even distributed on 2 or 3
pitch 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 of
melt to all the holes. Circular dies are preferred for uniform pressure drop throughout the
die.

9

Spinning pumps
Monofilament lines usually include a spinning pump. 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.

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.
Some lines also incorporate on line screen changes prior to the spinning pump.

Forming
The monofilaments emerging from the die are taken into a quench bath. The quench
medium is water which is maintained at around 25-35°C. The purpose of quenching is to
have a finer crystallite size which will facilitate stretching to produce high tenacity
filament. The filaments are taken over a deflecting roll in the quench bath and are drawn
continuously by Godet I.
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. The hot distance influences the
tenacity 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 are
separated. Absorbant cotton pads or brushes are also used to reduce water carry over.
Separators avoids entanglement of filament which also leads to considerable breakages.

Drawing
The drawing zone of a monofilament line consists of the first godet, a hot air oven and a
fast godet (2nd godet). Stretching is effected by the speed differential between the 1st and
IInd godet. The filaments are heated to just below their softening point in the hot air
oven (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.

Setting
Monofilaments are subjected to heat setting to stabilise the orientation and residual
stresses which lead to shrinkage. 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.

Winding
In most of the extrusion lines, the filaments are wound individually on separate spools or
bobbins. 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.

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Resin Selection

The advantages of using synthetic materials instead of natural fibres are better physical
properties, chemical and thermal resistance, negligible water absorption resistance to
rotting and fungus attack.
PP belongs to the family of polyolefin polymers comprising of low and high density
polyethylenes, polybutylenes etc. Although both HDPE & PP compete in the
manufacturing of stretched tapes, a number of factors are in favour of PP. The density of
PP is the lowest among all the synthetic polymers. Moreover, PP has a higher softening
point which gives it the advantage when it comes to hot filling of certain products in
woven sacks. PP being stiffer than HDPE and fabric made from PP has a higher
coefficient of friction, giving it the advantage of higher stackability during storage. On
the other hand, PP tapes tend to fibrillate during processing but this can be avoided by
adding antifibrillating additive.
For polymer to work well in a tape processing environment, it has to meet certain basic
requirements.

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 package
ie. antioxidants and acid acceptor. This grade is recommended for raffia tapes used in
making woven fabric for sacking, carpet backing, industrial fabric and geotextile
application.
• Repol H030SU : This is a UV stabilized version of Repol H030SG and its
recommended 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 on
processing 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 weight
decreases, tenacity and % elongation decreases.

Polypropylene grades of higher melt flow tend to process easier than lower melt flow
grades 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 an
optimum balance of tape properties and processability.

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

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

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

Xylene solubles determines the percentage of lower molecular weight fraction in
polypropylene. Higher xylene solubles will lead to decreased tenacity, increased
shrinkage, stickiness and weaving problems. On the other hand, lower solubles will lead
to 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 % elongation
decreases. 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, non fibrillating tendency and curl free tapes. 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.

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

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

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

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As the orientation temperature increases, tenacity increases and % elongation remains
relatively 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 orientation
temperature 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 oriented
tape. 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) : 4.2
Elongation 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 for
packing 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 better
bursting strength, low weight and low cost, but paper bags exhibit better printability and
lower seepage than PP bags.

Previously HDPE woven sacks were predominantly used for packing cement, but most of
the 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) therefore
giving higher yield per unit weight
• Polypropylene exhibits higher service temperature than HDPE, hence when cement is
filled at a temp.. of 85 °C-90 °C at a pressure of 6 kg/cm2 , 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. 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.

Fertilizer Packing

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

New Applications for PP Woven Sacks

• Sugar bags
• Postal bags
• Tea bags

15

Flexible Intermediate Bulk Containers (FIBC) / Jumbo bags

Polypropylene woven fabric is used to fabricate FIBC's or jumbo bags. These bags have
carrying capacities of 500 kgs to 4000 kgs. The design and fabric of these bags varies
depending upon the requisite container strength.
These bags are used for packing agro products, chemicals, detergent, plastic raw
materials, petrochemical products and fertilizers.
UV stabilised PP grades are used for this application. The tape denier ranges from
2100D 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 cotton
canvas tarpaulin is comparative to low gsm HDPE tarpaulins, but they soil easily and are
not 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. UV
stabilised 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 by
reducing 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

16

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 after
cracking and improved flexural strength. It also gives saving in construction and
transportation cost.

Monofilaments

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

Nets : PP and HDPE monofilaments are used for making nets. PP nets are used for high
strength 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 mainly
preferred for fishing. PP nets are stiff and can damage the catch.
PP monofilaments are used for making bristles.

17

Trouble - Shooting Guide - Tape Plant

Sr. Problem Suggested Remedy
No.
1. Film puncture Clean die lip
Optimise temperature profile

2. Tape breakage See under film puncture
Check hot plate temperature
Reduce stretch ratio
Reduce quench tank temperature
Set die gap

3. Low tenacity Increase stretch ratio
(Tensile strength) Increase oven/hot plate temperature

4. High elongation Reduce quench tank temperature
Increase oven/hot plate temperature
Increase stretch ratio
Reduce air gap

5. Denier variation 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

6. Fibrillation of tapes 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

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

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Trouble - Shooting Guide - Monofilaments

Sr. No. Problem Suggested Remedy
1. Filament breakage a) Surging in extruder. Run at higher pressure, or
with better-mixing screw, or with cooled
screw, or with gear pump, or more slowly. Try
to determine period of the surging, and relate
to drive, controllers etc.
b) One or more holes partially blocked. The same
filament(s) will break every time
c) Uneven temperature or distribution of material
in head and die. The same group of filaments
will 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 draw
down. Raise temperature
e) Melt too hot. Too fluid to hold together under
drawing tension. Reduce temperature
f) Drawdown too great (use smaller holes in die)
or drawdown too fast (slow down whole
system)
g) Moisture in material. Use a hopper-dryer
h) Contamination in material. Inspect feed, stop
using reground or reworked plastic
i) Oxidation and weakening of filament surface.
Run at lower temperature, or reduce the gap
between die and quench bath
j) Decomposition in extruder, head, or die. Bits
of decomposed material clog holes, or come
through them but weaken the filaments. Clean
die, run at lower temperature; examine die
design for possible stagnation spots, change die
or head if necessary, use better-stabilized
plastic
k) Too much orientation. Ratio of roll speeds
may be too high causing excessive tensile
stress in orientation stage
l) Too cold orientation. If plastics is not hot
enough, too much tensile stress will be
produced in the filaments
m) Too hot orientation (air oven). If plastic
becomes too hot, it loses ductility and will
break 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 operation
of all moving parts

2. All filaments varying in a) Surging in extruder, drawdown too much or
diameter too fast, or erratic drive. See (a), (f) and (o)
above

3. Some filaments different a) Temperature gradients in die. Unequal flow in
from others, in size die. Check for drafts, burned-out heaters, poor
and/or strength die design. See (c) above

4. Poor surface appearance a) Moisture in material or any pigment. Dry the
feed
b) Melt fracture. Reduce linear speed, or use
larger holes (more drawdown), or reduce
entrance angle into die holes, or use lower
viscosity compound, or run hotter
c) Plastic too cold. Run hotter
d) Compound inherently dull. Change
compound, or run hotter, or flame polish

5. Low tenacity a) Not enough orientation. Increase ratio of roll
speeds. Use proper orientation temperature.
Note that changes of orientation may also
change the filament size, requiring
compensating adjustments elsewhere
b) Degraded plastic. Use lower material
temperature, requiring compensating
adjustments formula
c) Nicks or cuts in filaments. Examine breaks
and examine unbroken filaments. Look for a
repetitive pattern and examine take-off for the
source

20

94874356 raffia-tape-line-process

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