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Orientation Effects on the Weldability of Polypropylene Strapping Tape
Orientation Effects on the Weldability of Polypropylene Strapping Tape
MJ Oliveira Dept Eng Polimeros, Universidade do Minho, 4800 Guimardes, Portugal DA Hemsley Polymer Microscopy Services, 52 Springfield Close, Loughborough LE 12 5AN, UK
INTRODUCTION Polypropylene tape with high tensile strength is commonly used for strapping many products ranging from light cardboard packs to heavy loads such as pallets of bricks or bundles of pipes. It is produced by extrusion followed by drawing at moderate temperatures to achieve ,--+"=""5;.... ,,_ high molecular orientation. For reducing fibrillation and to improve the weldability, the tape is embossed after the drawing stage by means of (m) textured hot rolls. The strapping cycle comprises feeding, tenFigureL Welding sequence. sioning and sealing of the tape around the pack. Thermal welding is the more common sealing process of polypropylene tape, replacing the traditional steel stapling. The welding process involves four steps (Figure 1): i the heater blade moves in to between the tapes 11 the sealing block moves upwards slightly pressing both surfaces against the blade 111 after a fixed heating time the heater blade retracts IV the sealing block moves to squeeze the melted surfaces together and cut the unused tape. After a cooling period the welded strap is released. The production of a strong weld is critical for a good performance of the strap. In this study polypropylene tapes, produced with different draw ratios or having different types of embossing patterns, were welded with a strapping machine. The effect of the welding tem-
46
Plastics Failure Analysis and Prevention
perature and the surface profile of the sealing block on the morphology and failure behavior will be presented.
EXPERIMENTAL This study was carried out on polypropylene strapping tape, of cross-dimensions of 12 x 0.6 mm, supplied by Gerrard Industries (U.K.). All the tapes were produced from polypropylene homopolymer of MFI about 4 g/600 s (230 QC, 21.6 N). The identification of the tapes is shown in Table 1. It was known that the tapes T 5 to T 9 were drawn in a oven at 95 QC at draw ratios of 5: 1 to 9: 1, as is indicated by the subscript. A sample of the extrudate from which the tapes were drawn was included in the testing program. Tape Ts, of unknown draw ratio, was included in this study due to the interest in studying a tape with a different type of embossing.
Table 1. Properties of the tapes
• •
• • •
Sample indentity
Density, kg m· 3
Birefringence xl03
Ts
902
T6
Shrinkage, % 1300 e
1500 e
28
7.4±O.5
32±1.5
902
30
11.0±l.O
34±2.5
T7
901
36
12.6±O.5
38±3
Tg
901
34
15.8±O.5
39±1
T9
889
36
16.3±O.5
41±1
Ts
905
28
9.4±O.5
28±1
The following tests and equipments were used to characterize the tapes: density measurement by the column gradient method; microscopical observation and birefringence measurement, in cross-sections cut along the drawing direction, with a Zeiss Universal polarizing microscope, equipped with a Ehringhaus quartz compensator; scanning electron microscopy to observe the embossed surface, using a Leica S 360; determination of the shrinkage on annealing at 130Q C and 150QC in an air circulating temperature controlled oven; determination of the tensile strength and elongation at break using a JJ type T 5002 tensile testing machine at a rate of 200 mm/min. The effective cross- sectional area of the
47
Orientation Effects on the Weldability
tape was evaluated taking into account the weight and the density of a precisely measured length of tape The tapes were welded using a semi-automatic Gerrard SA 600-IZ strapping machine at temperatures between 340Q C and 490 Q C at 30Q C intervals. The complete welding cycle was 1.4 s. These welds were made using the sealing block with a serrated profile provided with the machine. For investigating the effect of the block profile and welding pressure some welds were produced with tape Ts at 400 Q C with a flat block and a block of higher height, respectively. The effect of misalignment on the strength of the seal was analyzed on tapes welded at 400 Q C by imposing to the tape ends the maximum misalignment allowed by the machine (~2 mm). The microstructure and the mechanical behavior of the welds were studied by means of microscopy and by shear and peel testing.
RESULTS PROPERTIES OF THE TAPES The Tables 1 and 2 include the results of the measurements performed on the tapes. The density and the birefringence of the undrawn extrudate was 905 kg m- 3 and 3x 10- 3 , respectively. Table 2 - Mechanical properties of the tapes Sample
Break load, N
Break strength, MPa
Elongation at break, %
Ts
1509±35
24l±6
45±2
T6
1974±38
315±6
40±1
T7
1761±48
368±10
34±1
Tg
2395±81
398±13
32±2
T9
2367±49
406±8
3l±1
Ts
1447±47
349±11
40±2
The effect of the draw ratio on the birefringence is shown in Figure 2. The birefringence of the extrudate is very low, compared to that of the tapes, evidencing the strong effect of the drawing operation on the molecular orientation. The increase in birefringence with the draw ratio is sharper for the lower draw ratios. Above the draw ratio of 8: 1 the bire-
48
Plastics Failure Analysis and Prevention
50
50·,.------------~
45
eR l%)
4Q G!;
(
so 25
200+----,--.--..,.....----,-
2
3
4
5
6
7
8
9
10
4
189 tlmW~
Figure 3. Effect of the draw ratio on the mechanical properties of the tapes. Figure 2. Effect of the draw ratio on the birefringence of the tapes.
00,.------------,
fringence tends to level off. Similar behavior was observed by Fransen et a1. 1 in polypropylene tapes and by Pezzuti et a1. 2 in polyethylene films. It can be observed in the Figure 3 that the tensile strength of the tapes increases steadily with the draw ratio up to the value of 8: I and then begins to level off similarly the birefringence behavior. The strain at break III decreases with the draw ratio displaying a pattern that is l) nearly a mirror image ofthe tensile strength vs. draw ratio 4 Ii & ., II 9 III curve. An identical behavior was observed by Ram et a1. 3 OfawfllljQ in polypropylene tapes. The accentuated decrease in denFigure 4. Variation of shrinkage with draw sity shown by tape T 9 was certainly caused by voiding and ratio. splitting within the structure, suggesting that the improvement in strength by increasing the draw ratio is reaching a limit. As was shown by Mahajan 4 for HDPE tapes drawn at 95 Q C and 120 QC those defects increase with draw ratio and with the decrease of the drawing temperature. As it is shown in Figure 4, the shrinkage increases with draw ratio. The values obtained at 150QC are particularly relevant, from the user's point of view, as they indicate the tendency of the tape to contract near the weld zone. The high shrinkage shown by the tapes at this temperature, between 32 and 41 %, suggests that the depth of the heated zone must be kept to a minimum and the tape ends tightly gripped to avoid spoiling the seal by retraction at the welding stage.
49
Orientation Effects on the Weldability
I Figure 5.Types of embossing: (a) - T s-T 9 , (b) - T s .
I mm
Figure 6. (a) -Example of a bad weld (T s -340"C). (b)Example of a good weld (Ts-370"C).
The Figure 5 shows the two types of embossing patterns of the tapes. The embossing pattern of tape Ts is deeper and has sharper comers than in the other tapes. The embossing induced splitting of the tapes, this being more severe in the tapes drawn at higher ratios. WELDING BEHAVIOR OF THE TAPES
The welding temperature has a marked effect on the morphology, strength and failure behavior of the welds. When the heating tool temperature is too low (below 400 Q C for most of the tapes) the welds show voids and splits at the interface with the unmelted material. As is illustrated in Figure 6-a, the low temperature and scarcity of the melt prevented the complete filling of the gaps between the matting surfaces resulting in a poor weld. The welds made on tape TS showed higher splitting than the others for identical welding temperatures. This behavior is certainly caused by the deeper embossing of this tape that demands a higher amount of melt to fill the gaps at the weld zone. The joints welded at too low temperatures showed low strength (Figure 8) and elongation at break. Depending on the type and amount of defects, the welds broke either at the
50
Plastics Failure Analysis and Prevention
1750 1500 1250
_1$ ~.~
fa
,,<> ""17
---·n
750
-<:I-f"
+--......---....---.---i
500 300
350
400
450
-'IS
500
WeldlnglllmPl/rllW,.. l"C)
Figure H. Effect of the welding temperature on the breaking load under shear testing.
seam or at the interface with the unmelted material. The increase in welding temperature up to 430 QC or 460 QC in the case of tape T 7 resulted in I ll11m a reduction of the flaws and increase in the shear Figure 7. Typical fracture paths of a good weld (T s strength and ductility of the welds. Flow lines - 370 C). (a) shear test; (b) peel test. and swirls were observed at the widest regions, while at the thinner zones the material oriented in the axial direction of the tape. Figure 6-b shows a typical cross section. Most the welds showed the maximum strength and ductility for a welding temperature of 430 QC (Figure 8). A typical failure of these welds is shown in Figure 7-a. The fracture path runs alternately on both sides of the boundaries of the weld zone, evidencing that the adhesion at the mating surfaces was stronger than at interface with the basic material. The excessive heating of the tapes, by using tool temperatures of 460QC or higher changed the morphology and failure behavior of the weld and reduced its strength (Figure 8). The microstructure, that was too fine to be resolved by optical microscopy in the previously referred welds became spherulitic for some tapes and occasionally showed cracks at the mating layer. These cracks probably resulted from a combined effect of degradation of the polymer by excessive heating and increasing contraction upon cooling of the spherulitic structure. The failure generally started at one of the weld ends and moved soon to the basic material. Q
Orientation Effects on the Weldability
51
The peel tests were less effective in assessing the quality of the welds than the shear tests. Above an optimum temperature the peel strength is almost unaffected by the welding temperature. These results correlate well with the failure behavior of the welds under this test. Except in the case of the samples welded at the lowest temperature that fractured through the joint interface, all the others failed through the original material (Figure 7-b). This may be explained by the reduced interfibrillar strength arising from the high molecular orientation of the tape, which made it more susceptible to crack propagation than the unoriented material of the weld. The modifications made on the surface pattern of the pressing block, namely the flattening of the surface and the increase in the height to increase the welding pressure caused some modifications on the weld morphology. However it only had a marginal effect on the shear strength. The use of the flat block in the tape with deeper embossing caused the welds to be more uniform and this seems to reduce the dispersion of the results. As expected the misalignment of the tape ends reduced the shear strength of the joint. In the case of the machine used, the maximum misalignment caused a reduction in strength of4%. The draw ratio of the tapes appears to influence its welding behavior. The tapes with higher orientation (T7 - T 9) produced thinner welds and with more splits and voids than the less oriented samples. The increase in orientation increases the melting temperature. S However for the range of draw ratios used here the difference in melting temperature that could be expected is not enough «l1----..............,-.......-........_-..,...............j to explain the reduction in thickness observed. 10 7 5 The increase in stiffness with the draw ratio cer" tainly favored the squeezing of the melt out of Figure 9. Effect of the draw ratio on the maximum effithe weld region, and is probably the main cause ciency of the tapes. of the reduction in thickness observed. The high shrinkage coupled with the high stiffness of the more oriented tapes is also probably the cause of the higher incidence of voids at the weld zone. The joint efficiency, defined as the ratio between the forces to break the welded and unwelded tape, decreases with the draw ratio up to the draw ratio of 8: 1 (Figure 9). Thus, the improvement in strength achieved by drawing the tapes at higher ratios is lost on weldmg.
52
Plastics Failure Analysis and Prevention
CONCLUSIONS The study carried out in polypropylene tapes with different draw ratios to investigate its welding behavior allowed to draw the following conclusions: 1. The welding temperature has a strong influence on properties of the welds. An optimum welding tool temperature around 430 QC could be defined for most of the tapes. 2. The microstructure of the welds is in general very fine and shows a much lower orientation than the tapes. Welding temperatures of 460 QC or higher produce coarser textures and favor the occurrence of voids and splits at the joints. 3. The welding temperature influences the fracture path of the welds. Below the optimum temperature it runs along one of the interfaces, at the optimum range it alternates between the two interfaces and above the optimum temperature generally moves away from the weld zone. 4. The orientation of the tapes influences the morphology of the welds. The increase in orientation reduces the thickness of the weld zone and favors the formation of voids. 5. The welding efficiency decreases with increasing the orientation of the tapes.
ACKNOWLEDGEMENTS The authors express their appreciation to Gerrard Industries for supplying the samples and lending the equipment.
REFERENCES 2 3 4 5
P. J. Franssen, J. M. A. Jansen and B. C. Roest, Polypropylene Fibres and Textiles, 2nd Int. Conf. Plast. and Rubber Inst., London, 26-2X, Sept., 1979. J. L. Pezzuti and R. S. Porter, J. App. Polym. Sci., 30, 4251-4259, 19X5. A. Ram, J. Saker and J. Adorian, Plasi. Rubb. Pmc. Appl., 1, 363-36X, 19X 1. S. J. Mahajan, B. L. Deopura and Y. Wang, J. Appl. Polym. Sci., 60,1539-1549,1996. A. O. Ibhadon, J. App. Polym. Sci., 43, 567-571, 1991.
MJ Oliveira Dept Eng Polimeros, Universidade do Minho, 4800 Guimardes, Portugal DA Hemsley Polymer Microscopy Services, 52 Springfield Close, Loughborough LE 12 5AN, UK
INTRODUCTION Polypropylene tape with high tensile strength is commonly used for strapping many products ranging from light cardboard packs to heavy loads such as pallets of bricks or bundles of pipes. It is produced by extrusion followed by drawing at moderate temperatures to achieve ,--+"=""5;.... ,,_ high molecular orientation. For reducing fibrillation and to improve the weldability, the tape is embossed after the drawing stage by means of (m) textured hot rolls. The strapping cycle comprises feeding, tenFigureL Welding sequence. sioning and sealing of the tape around the pack. Thermal welding is the more common sealing process of polypropylene tape, replacing the traditional steel stapling. The welding process involves four steps (Figure 1): i the heater blade moves in to between the tapes 11 the sealing block moves upwards slightly pressing both surfaces against the blade 111 after a fixed heating time the heater blade retracts IV the sealing block moves to squeeze the melted surfaces together and cut the unused tape. After a cooling period the welded strap is released. The production of a strong weld is critical for a good performance of the strap. In this study polypropylene tapes, produced with different draw ratios or having different types of embossing patterns, were welded with a strapping machine. The effect of the welding tem-
46
Plastics Failure Analysis and Prevention
perature and the surface profile of the sealing block on the morphology and failure behavior will be presented.
EXPERIMENTAL This study was carried out on polypropylene strapping tape, of cross-dimensions of 12 x 0.6 mm, supplied by Gerrard Industries (U.K.). All the tapes were produced from polypropylene homopolymer of MFI about 4 g/600 s (230 QC, 21.6 N). The identification of the tapes is shown in Table 1. It was known that the tapes T 5 to T 9 were drawn in a oven at 95 QC at draw ratios of 5: 1 to 9: 1, as is indicated by the subscript. A sample of the extrudate from which the tapes were drawn was included in the testing program. Tape Ts, of unknown draw ratio, was included in this study due to the interest in studying a tape with a different type of embossing.
Table 1. Properties of the tapes
• •
• • •
Sample indentity
Density, kg m· 3
Birefringence xl03
Ts
902
T6
Shrinkage, % 1300 e
1500 e
28
7.4±O.5
32±1.5
902
30
11.0±l.O
34±2.5
T7
901
36
12.6±O.5
38±3
Tg
901
34
15.8±O.5
39±1
T9
889
36
16.3±O.5
41±1
Ts
905
28
9.4±O.5
28±1
The following tests and equipments were used to characterize the tapes: density measurement by the column gradient method; microscopical observation and birefringence measurement, in cross-sections cut along the drawing direction, with a Zeiss Universal polarizing microscope, equipped with a Ehringhaus quartz compensator; scanning electron microscopy to observe the embossed surface, using a Leica S 360; determination of the shrinkage on annealing at 130Q C and 150QC in an air circulating temperature controlled oven; determination of the tensile strength and elongation at break using a JJ type T 5002 tensile testing machine at a rate of 200 mm/min. The effective cross- sectional area of the
47
Orientation Effects on the Weldability
tape was evaluated taking into account the weight and the density of a precisely measured length of tape The tapes were welded using a semi-automatic Gerrard SA 600-IZ strapping machine at temperatures between 340Q C and 490 Q C at 30Q C intervals. The complete welding cycle was 1.4 s. These welds were made using the sealing block with a serrated profile provided with the machine. For investigating the effect of the block profile and welding pressure some welds were produced with tape Ts at 400 Q C with a flat block and a block of higher height, respectively. The effect of misalignment on the strength of the seal was analyzed on tapes welded at 400 Q C by imposing to the tape ends the maximum misalignment allowed by the machine (~2 mm). The microstructure and the mechanical behavior of the welds were studied by means of microscopy and by shear and peel testing.
RESULTS PROPERTIES OF THE TAPES The Tables 1 and 2 include the results of the measurements performed on the tapes. The density and the birefringence of the undrawn extrudate was 905 kg m- 3 and 3x 10- 3 , respectively. Table 2 - Mechanical properties of the tapes Sample
Break load, N
Break strength, MPa
Elongation at break, %
Ts
1509±35
24l±6
45±2
T6
1974±38
315±6
40±1
T7
1761±48
368±10
34±1
Tg
2395±81
398±13
32±2
T9
2367±49
406±8
3l±1
Ts
1447±47
349±11
40±2
The effect of the draw ratio on the birefringence is shown in Figure 2. The birefringence of the extrudate is very low, compared to that of the tapes, evidencing the strong effect of the drawing operation on the molecular orientation. The increase in birefringence with the draw ratio is sharper for the lower draw ratios. Above the draw ratio of 8: 1 the bire-
48
Plastics Failure Analysis and Prevention
50
50·,.------------~
45
eR l%)
4Q G!;
(
so 25
200+----,--.--..,.....----,-
2
3
4
5
6
7
8
9
10
4
189 tlmW~
Figure 3. Effect of the draw ratio on the mechanical properties of the tapes. Figure 2. Effect of the draw ratio on the birefringence of the tapes.
00,.------------,
fringence tends to level off. Similar behavior was observed by Fransen et a1. 1 in polypropylene tapes and by Pezzuti et a1. 2 in polyethylene films. It can be observed in the Figure 3 that the tensile strength of the tapes increases steadily with the draw ratio up to the value of 8: I and then begins to level off similarly the birefringence behavior. The strain at break III decreases with the draw ratio displaying a pattern that is l) nearly a mirror image ofthe tensile strength vs. draw ratio 4 Ii & ., II 9 III curve. An identical behavior was observed by Ram et a1. 3 OfawfllljQ in polypropylene tapes. The accentuated decrease in denFigure 4. Variation of shrinkage with draw sity shown by tape T 9 was certainly caused by voiding and ratio. splitting within the structure, suggesting that the improvement in strength by increasing the draw ratio is reaching a limit. As was shown by Mahajan 4 for HDPE tapes drawn at 95 Q C and 120 QC those defects increase with draw ratio and with the decrease of the drawing temperature. As it is shown in Figure 4, the shrinkage increases with draw ratio. The values obtained at 150QC are particularly relevant, from the user's point of view, as they indicate the tendency of the tape to contract near the weld zone. The high shrinkage shown by the tapes at this temperature, between 32 and 41 %, suggests that the depth of the heated zone must be kept to a minimum and the tape ends tightly gripped to avoid spoiling the seal by retraction at the welding stage.
49
Orientation Effects on the Weldability
I Figure 5.Types of embossing: (a) - T s-T 9 , (b) - T s .
I mm
Figure 6. (a) -Example of a bad weld (T s -340"C). (b)Example of a good weld (Ts-370"C).
The Figure 5 shows the two types of embossing patterns of the tapes. The embossing pattern of tape Ts is deeper and has sharper comers than in the other tapes. The embossing induced splitting of the tapes, this being more severe in the tapes drawn at higher ratios. WELDING BEHAVIOR OF THE TAPES
The welding temperature has a marked effect on the morphology, strength and failure behavior of the welds. When the heating tool temperature is too low (below 400 Q C for most of the tapes) the welds show voids and splits at the interface with the unmelted material. As is illustrated in Figure 6-a, the low temperature and scarcity of the melt prevented the complete filling of the gaps between the matting surfaces resulting in a poor weld. The welds made on tape TS showed higher splitting than the others for identical welding temperatures. This behavior is certainly caused by the deeper embossing of this tape that demands a higher amount of melt to fill the gaps at the weld zone. The joints welded at too low temperatures showed low strength (Figure 8) and elongation at break. Depending on the type and amount of defects, the welds broke either at the
50
Plastics Failure Analysis and Prevention
1750 1500 1250
_1$ ~.~
fa
,,<> ""17
---·n
750
-<:I-f"
+--......---....---.---i
500 300
350
400
450
-'IS
500
WeldlnglllmPl/rllW,.. l"C)
Figure H. Effect of the welding temperature on the breaking load under shear testing.
seam or at the interface with the unmelted material. The increase in welding temperature up to 430 QC or 460 QC in the case of tape T 7 resulted in I ll11m a reduction of the flaws and increase in the shear Figure 7. Typical fracture paths of a good weld (T s strength and ductility of the welds. Flow lines - 370 C). (a) shear test; (b) peel test. and swirls were observed at the widest regions, while at the thinner zones the material oriented in the axial direction of the tape. Figure 6-b shows a typical cross section. Most the welds showed the maximum strength and ductility for a welding temperature of 430 QC (Figure 8). A typical failure of these welds is shown in Figure 7-a. The fracture path runs alternately on both sides of the boundaries of the weld zone, evidencing that the adhesion at the mating surfaces was stronger than at interface with the basic material. The excessive heating of the tapes, by using tool temperatures of 460QC or higher changed the morphology and failure behavior of the weld and reduced its strength (Figure 8). The microstructure, that was too fine to be resolved by optical microscopy in the previously referred welds became spherulitic for some tapes and occasionally showed cracks at the mating layer. These cracks probably resulted from a combined effect of degradation of the polymer by excessive heating and increasing contraction upon cooling of the spherulitic structure. The failure generally started at one of the weld ends and moved soon to the basic material. Q
Orientation Effects on the Weldability
51
The peel tests were less effective in assessing the quality of the welds than the shear tests. Above an optimum temperature the peel strength is almost unaffected by the welding temperature. These results correlate well with the failure behavior of the welds under this test. Except in the case of the samples welded at the lowest temperature that fractured through the joint interface, all the others failed through the original material (Figure 7-b). This may be explained by the reduced interfibrillar strength arising from the high molecular orientation of the tape, which made it more susceptible to crack propagation than the unoriented material of the weld. The modifications made on the surface pattern of the pressing block, namely the flattening of the surface and the increase in the height to increase the welding pressure caused some modifications on the weld morphology. However it only had a marginal effect on the shear strength. The use of the flat block in the tape with deeper embossing caused the welds to be more uniform and this seems to reduce the dispersion of the results. As expected the misalignment of the tape ends reduced the shear strength of the joint. In the case of the machine used, the maximum misalignment caused a reduction in strength of4%. The draw ratio of the tapes appears to influence its welding behavior. The tapes with higher orientation (T7 - T 9) produced thinner welds and with more splits and voids than the less oriented samples. The increase in orientation increases the melting temperature. S However for the range of draw ratios used here the difference in melting temperature that could be expected is not enough «l1----..............,-.......-........_-..,...............j to explain the reduction in thickness observed. 10 7 5 The increase in stiffness with the draw ratio cer" tainly favored the squeezing of the melt out of Figure 9. Effect of the draw ratio on the maximum effithe weld region, and is probably the main cause ciency of the tapes. of the reduction in thickness observed. The high shrinkage coupled with the high stiffness of the more oriented tapes is also probably the cause of the higher incidence of voids at the weld zone. The joint efficiency, defined as the ratio between the forces to break the welded and unwelded tape, decreases with the draw ratio up to the draw ratio of 8: 1 (Figure 9). Thus, the improvement in strength achieved by drawing the tapes at higher ratios is lost on weldmg.
52
Plastics Failure Analysis and Prevention
CONCLUSIONS The study carried out in polypropylene tapes with different draw ratios to investigate its welding behavior allowed to draw the following conclusions: 1. The welding temperature has a strong influence on properties of the welds. An optimum welding tool temperature around 430 QC could be defined for most of the tapes. 2. The microstructure of the welds is in general very fine and shows a much lower orientation than the tapes. Welding temperatures of 460 QC or higher produce coarser textures and favor the occurrence of voids and splits at the joints. 3. The welding temperature influences the fracture path of the welds. Below the optimum temperature it runs along one of the interfaces, at the optimum range it alternates between the two interfaces and above the optimum temperature generally moves away from the weld zone. 4. The orientation of the tapes influences the morphology of the welds. The increase in orientation reduces the thickness of the weld zone and favors the formation of voids. 5. The welding efficiency decreases with increasing the orientation of the tapes.
ACKNOWLEDGEMENTS The authors express their appreciation to Gerrard Industries for supplying the samples and lending the equipment.
REFERENCES 2 3 4 5
P. J. Franssen, J. M. A. Jansen and B. C. Roest, Polypropylene Fibres and Textiles, 2nd Int. Conf. Plast. and Rubber Inst., London, 26-2X, Sept., 1979. J. L. Pezzuti and R. S. Porter, J. App. Polym. Sci., 30, 4251-4259, 19X5. A. Ram, J. Saker and J. Adorian, Plasi. Rubb. Pmc. Appl., 1, 363-36X, 19X 1. S. J. Mahajan, B. L. Deopura and Y. Wang, J. Appl. Polym. Sci., 60,1539-1549,1996. A. O. Ibhadon, J. App. Polym. Sci., 43, 567-571, 1991.