The Effects of Annealing Conditions on the Structure and Properties of Polypropylene Fibers

Medical Plastics and Biomaterials Magazine
MPB Article Index

Originally published November 1997

TECHNICAL PAPER SERIES

Polypropylene (PP) fibers have been widely used in the medical industry in the production of sutures, surgical mesh, and personal hygiene products. These diverse applications require a large range of properties, which can be obtained by varying processing conditions. Made by extrusion, drawing, and annealing, PP fibers exhibit performance that is strongly dependent on their specific chemical structure and morphology.^1,2 Whereas the chemical structure of PP fibers is controlled by resin composition such as homopolymer (HPP) or copolymer (CPP), morphological structure is related to processing parameters, such as draw ratio and annealing conditions.^3—5

It has been observed, during annealing of oriented polymers, that the shrinkage and relaxation of molecular orientation occur much faster than does the increase in material density.⁶ Annealing of cold-drawn PP fibers at 140°C has been shown to reestablish their monoclinic structure, the formation of which had been disrupted during cold drawing.⁴ Most of the previous studies focused on the effect of annealing on crystal structure, crystallinity, and molecular orientation, using techniques such as x-ray diffraction and birefringence. However, little information is available with regard to the effect of annealing conditions on the dynamic mechanical properties of oriented PP fibers.

In this study, dynamic mechanical analysis (DMA) tests, including time and temperature sweeps, were used to identify annealing conditions for extruded fibers or filaments made from PP homopolymer and copolymer targeted for medical applications. The effect of various annealing parameters—such as time, temperature, and degree of constraint—on the physical and morphological properties of the HPP and CPP polypropylene fibers were investigated using DMA, differential scanning calorimetry (DSC), wide-angle x-ray diffraction (WAXD), and tensile tests. Annealing was done under both relaxed and constrained conditions.

EXPERIMENTAL

Two types of isotactic PP resins were used in this study: a homopolymer (melt index = 2.0) and a random copolymer (melt index = 2.0) that contained 97% PP and 3% polyethylene.

The PP fibers were melt-extruded at 230°C using a typical horizontal fiber-extrusion line with a 25-mm screw extruder. The as-spun fibers were then drawn uniaxially at 100°C with a draw ratio of 6:1 to achieve full orientation. Finally, annealing of the fully oriented fibers was conducted at both 110° and 130°C with the fibers wound on a rack with either full or partial constraint—corresponding to 0 or 10% relaxation, respectively.

Tensile modulus, strength, and elongation at break of unannealed and annealed fibers were measured using an Instron mechanical tester (Model 4201). The gauge length of the test samples was 50 mm, and the crosshead speed was 25.4 mm/min. The average and the standard deviation of the tensile properties were calculated using a minimum of eight samples.

A Seiko dynamic mechanical spectrometer (DMS 210) was used to obtain the dynamic mechanical properties—storage modulus (E') and loss tangent (tan *)—of the PP fibers. Time sweeps were conducted on unannealed HPP and CPP fibers at 100°, 120°, and 140°C for a period of 4 hours at frequencies of 0.2 and 1.0 Hz. The temperature sweeps were conducted on the unannealed and annealed PP fibers between —60° and 175°C, at a heating rate of 2°C/min and frequencies of 0.2 and 1.0 Hz. In both DMA time and temperature sweeps, the sample gauge length was 10 mm and the vibration amplitude was 20 µm.

DSC measurements were made using a Model 912 machine from TA Instruments. The scans of the unannealed and annealed PP fibers were obtained at 10°C/min.

WAXD measurements were carried out using a Siemens x-ray unit with GADDS software. The distance between detector and sample was 30.0 cm. The x-ray unit was operated at 40 kV, 40 mA, with an exposure time of 300 seconds.

RESULTS AND DISCUSSION

Figures 1 and 2 show the changes in E' and tan *, respectively, with time for both the HPP and the CPP fibers at 100° and 140°C. It can be observed that, at both temperatures, most of the change in E' occurs rapidly in the first 30 minutes. Both E' and tan * start to level off approximately 21/2 hours after the samples have reached the desired annealing temperatures. (Results of a time sweep at 120°C, though not shown in the figures, reveal the same trend.) Similar results derived from density measurements also have shown rapid change in crystallinity in the first 30 minutes.⁷ For the present study, therefore, it was decided that the annealing time would be limited to 30 minutes since the largest and most rapid changes in dynamic properties are seen within this time period.

Figure 1. Results of storage modulus versus time using DMA time sweep at 100° and 140°C.

Figure 2. Results of loss tangent versus time using DMA time sweep at 100° and 140°C.

Two temperatures for annealing were selected from the range—100° to 140°C—within which stabilization of dynamic properties occurs at similar times. The lower temperature of 110°C was sufficiently above 80°C, the point around which secondary relaxation is known to occur within PP crystals. The upper temperature of 130°C was selected to be well below the PP melting point of 165°C in order to avoid rapid relaxation that can occur at temperatures close to T_m.⁸ To evaluate the effects of constraint, annealing was conducted with the fibers fully or partially constrained—that is, with 0 or 10% of relaxation, respectively.

The variation of E' and tan * with temperature for the HPP fibers annealed under different conditions was measured using DMA. Results are shown in Figures 3 and 4. At temperatures below the glass-transition temperature (T_g), the E' of the unannealed fibers is higher than that of the fully constrained fibers, but there is no significant difference at higher temperatures (see Figure 3). The E' of the partially constrained fibers is significantly lower than those of the unannealed and fully constrained fibers, a finding applicable to the entire temperature range scanned. The results indicate that the effect of constraint is more significant than the annealing temperature on the values of E', with partial constraint (10% relaxation) leading to lower E' values.

Figure 3. Results of storage modulus of HPP fibers versus temperature under different annealing conditions.

Figure 4. Results of loss tangent of HPP fibers versus temperature under different annealing conditions.

Figure 4 plots the tan * versus temperature curves of the HPP fibers with different annealing histories, while the glass-transition temperatures measured from peak in tan * via DMA measurements are reported in Table I. The shape of the damping peak in the glass-transition region changes from a point of inflection for the unannealed and constrained fibers to a sharp peak for the partially constrained fibers. The T_g shows no appreciable change between the unannealed and the fully constrained annealed fibers, either at 110° or 130°C. However, the T_g drops from 2° to —5°C or below for the partially constrained annealed fibers. These results suggest that the amorphous phase is more relaxed and thus capable of enhanced damping in the partially constrained fibers, at both temperatures. This relaxation phenomenon also translates to slightly lower T_g values. As with E', the degree of constraint during annealing has a larger effect on a fiber's T_g than does the annealing temperature.

Table I. Thermal properties of polypropylene fibers. Materials are designated as homopolymer (HPP) or copolymer (CPP) with 0 or 10% relaxation and 110° or 130°C annealing temperature.

For CPP fibers, it was observed that the degree of constraint and the annealing temperature have similar effects on E' and tan *. DMA results show that the CPP fibers have lower T_g and E' values compared with those of the HPP fibers. As seen in Figure 5, the E' for the partially constrained annealed CPP fibers is lower than those of the unannealed and the fully constrained annealed fibers for the entire temperature range scanned. At both annealing temperatures, partially constrained fibers show more pronounced damping compared with unannealed and fully constrained annealed fibers (see Figure 6). For a partially constrained fiber sample, the damping is higher and the T_g is lower at 130° than at 110°C.

Figure 5. Results of storage modulus of CPP fibers versus temperature under different annealing conditions.

Figure 6. Results of loss tangent of CPP fibers versus temperature under different annealing conditions.

Table I summarizes the heat of fusion values and melting peaks (T_m) for the HPP and CPP fibers. Representative DSC scans for partially and fully constrained annealed fibers are compared with those of unannealed fibers in Figures 7 and 8, respectively. The values for heat of fusion and melting peak are based on averages of two to four repeat runs. HPP fibers have higher heat of fusion values than do CPP fibers, although their melting points are quite similar.

Figure 7. Results of DSC scans of partially constrained (10% relaxation) HPP and CPP fibers under different annealing conditions.

Figure 8. Results of DSC scans of fully constrained (0% relaxation) HPP and CPP fibers under different annealing conditions.

The DSC scan was not able to detect any appreciable changes in the crystallinity or the melting point of either the HPP or CPP fibers annealed under different conditions. However, certain trends can be observed in Figures 7 and 8. The lower-temperature shoulder for the unannealed HPP fibers broadens and moves up slightly to higher temperatures compared with those for the fully constrained annealed fibers. For the partially constrained fibers, not only does the position of the shoulder change, but it also loses its sharpness. When compared with unannealed samples, the changes in the position and shape of the low-temperature shoulder for the constrained annealed CPP fibers are negligible, whereas a slight broadening can be detected for the unconstrained fibers. The different behavior in the two PP fibers can be ascribed to the homopolymer's potential to form more perfect crystals, and to the fact that it is more difficult for the copolymer to crystallize. As observed with the DMA results, the annealing temperature seems to have less effect than does the degree of constraint.

The WAXD results reinforce the DSC and DMA data, with the degree of constraint having a more significant effect than does annealing temperature. In Figure 9, the partially constrained annealed HPP fibers show relaxation of crystalline orientation when compared with the fully constrained annealed fibers at 130°C. However, there is not much noticeable difference in WAXD patterns between unannealed and fully constrained annealed fibers at 130°C. The CPP fibers manifest a trend similiar to that of the HPP fibers (see Figure 10).

Figure 9. WAXD patterns of (a) fully constrained and (b) partially constrained HPP fibers at 130°C.

Figure 10. WAXD patterns of (a) fully constrained and (b) partially constrained CPP fibers at 130°C.

Tensile properties of the HPP and CPP samples are presented in Table II. HPP fibers have higher modulus and strength than do CPP fibers. The tensile moduli of both the HPP and CPP samples drop from the unannealed to the fully constrained annealed samples, and decrease further for the partially constrained annealed samples. Elongation to break increases from the unannealed to the fully constrained annealed fibers, and increases further for the partially constrained annealed fibers. For both HPP and CPP fibers, the tensile strength does not change significantly when annealing is done under fully constrained conditions, but drops when the fibers are annealed under partial constraint. Annealing temperature does not have any noticeable effect on the tensile properties of either of the two polymers.

Table II. Tensile properties of polypropylene fibers. Standard deviations are given in parentheses. Materials are designated as homopolymer (HPP) or copolymer (CPP) with 0 or 10% relaxation and 110° or 130°C annealing temperature.

The difference in mechanical properties of the fibers can be explained by the results obtained from DMA, WAXD, and DSC. The homopolymer, which has higher crystallinity, also has higher tensile modulus and strength compared with the copolymer, which has lower crystallinity. The increase in elongation to break and decrease in modulus and strength are observed concurrently with lowering of the E' over a wide temperature range and lowering of T_g with a sharper damping peak when partial-constraint annealing takes place. These phenomena indicate that there is possible relaxation or reorientation in the amorphous phase, which leads to lower tensile modulus and strength and higher elongation to break. In addition, WAXD patterns indicate relaxation of the crystalline phase. Significant changes in tensile modulus and E' over a wide temperature range suggest that the crystalline phase is also being reorganized and perfected with no corresponding change in overall crystallinity, as can be seen in the DSC scan data plotted in Figures 7 and 8.

CONCLUSION

Dynamic mechanical analysis was used in determining annealing conditions of polypropylene homopolymer and copolymer fibers. For both types of PP fibers, most of the changes in dynamic properties occurred during the first 30 minutes of annealing.

The effect of degree of constraint was more significant than that of the annealing temperature on tensile and dynamic mechanical properties. For both the homopolymer and the copolymer, annealing of partially constrained annealed fibers led to lower tensile modulus, strength, and E' values compared with unannealed and fully constrained annealed fibers. The glass-transition temperature showed no appreciable change between the unannealed fibers and the fully constrained fibers for the homopolymer and the copolymer. However, for partially constrained fibers, the amorphous phase was more relaxed, resulting in sharper tan * peaks and a shift to lower temperatures.

The crystalline phase was reorganized and showed relaxation of crystalline orientation during annealing of partially constrained homopolymer fibers. These facts are supported by observations of the change in shape and position of the melting peak in DSC scans and the WAXD scans. However, the DSC scans showed no substantial change in crystallinity or melting point upon annealing for either polymer.

The relaxation and reorientation that occurs in the amorphous phase during partial-constraint annealing resulted in a significant change in the shape and position of the tan * peak. However, there was no appreciable change in the tan * peak between the unannealed and the fully constrained fiber annealed at either 110° or 130°C. Therefore, one can conclude that the degree of constraint during annealing has a larger effect on the amorphous phase in the oriented PP fibers. During partial-constraint annealing, the decrease of both tensile modulus and strength after annealing is caused mainly by relaxation and molecular reorientation in both the amorphous and the crystalline phase.

ACKNOWLEDGMENTS

The authors would like to thank Shaon X. Liu for supplying the WAXD data.

REFERENCES

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Arindam Datta, PhD, is a senior scientist at the Johnson & Johnson Corporate Biomaterials Center (Somerville, NJ), where he works on developing biocompatible materials used in a wide variety of implants. In his former position with Ethicon (owned by Johnson & Johnson, and also in Somerville), he was involved in the design of bioabsorbable devices for surgical applications. Jianguo J. Zhou, PhD, is an Ethicon staff engineer currently working on the research and development of surgical devices. A scientist at the Corporate Biomaterials Center, J. Jenny Yuan, PhD, has focused primarily on investigating the structure-property relationships of advanced composite materials and on developing innovative biomaterials. Also at the Corporate Biomaterials Center, Andrea Monisera specializes in the characterization of polymer systems for medical applications.

Copyright ©1997 Medical Plastics and Biomaterials