Process-Property Relationships for Melt-Spun Poly(lactic acid) Yarn.

Poly(lactic acid) (PLA) is an attractive biomaterial due to its biocompatibility, biodegradability, and fiber-forming ability. However, the polymer is highly susceptible to both hydrolytic and thermal degradation during processing. Melt processing conditions typically involve high temperature and shear, whereas to prevent premature degradation, PLA needs to be processed under the mildest conditions that still yield the desired yarn properties. Thus, there is a need to determine the optimum processing conditions to achieve the desired properties of extruded PLA yarn. This study focuses on the effect of melt-spinning process parameters on the mechanical and physicochemical properties of the resulting PLA yarn and to derive their process-property relationships. The study compares the effect of process parameters like melt temperature, throughput through the spinneret, take-up speed at the wind-up roller, draw ratio, and drawing temperature on the yarn properties such as the yarn size (linear mass density), tenacity, elongation at break, crystallinity, and molecular weight. Depending on the combination of process parameters, the resulting PLA yarn had a yarn size ranging from 6.2 to 101.6 tex, tenacity ranging from 2.5 to 34.1 gf/tex, elongation at break ranging from 4 to 480%, and degree of crystallinity ranging from 14.6 to 62.2%. Certain combinations of processing parameters resulted in higher process-induced degradation, as evident from the reduction in molecular weight, ranging from 7.6% reduction to 20.5% reduction. Findings from this study increase our understanding on how different process parameters can be utilized to achieve the desired properties of the as-spun and drawn PLA yarn while controlling process-induced premature degradation.

Introduction

Poly(lactic acid) (PLA) is an attractive biomaterial for regenerative medicine and tissue-engineering applications due to its inherent property of in vivo resorption over time.[1−4] PLA is also an eco-friendly alternative to conventional polymers like polyethylene or polypropylene since PLA is synthesized from nonpetroleum sources such as corn starch,[5,6] and also because PLA tends to decompose completely over time through hydrolytic degradation.[7,8] PLA is also a distinctive nonpetroleum-based polymer because of its ability to form continuous fibers, also known as yarn, with desirable mechanical strengths for a variety of applications such as medical sutures or textiles that can be used in a circular economy.[9,10]

Any polymer that can form a viscous melt or a solution like PLA can theoretically form fibers. For example, even a low molecular weight sugar solution can be spun into fibers to make cotton candy, but the resulting fibers do not have adequate stability or mechanical strength to be spun into yarn. Depending upon the end use, the yarn should possess a particular modulus, rigidity, or stiffness, and several factors pose limits on which polymers can form continuous fibers in practice. First, the degree of polymerization should be sufficiently high to enable fiber formation; a fiber-forming polymer typically possesses a high molecular weight and long linear molecular chain length. The minimum molecular weight required for fiber formation depends on the chemical nature of the polymer. In general, the lower the interchain cohesive forces, the higher the minimum molecular weight needed for fiber formation. Moreover, reactive side chains tend to result in a cross-linked, three-dimensional polymer network, which leads to insoluble polymers, infusible gels, or rubbers that cannot be spun into functional fibers.[11] To achieve desirable mechanical strength, a fiber-forming polymer also needs to have a chemical structure that restricts localized segmental mobility or overall chain mobility when spun into yarn, and enables an oriented crystalline structure to form upon stretching or drawing.[12] Usually, linear polymers without bulky side groups have this capability since bulky side groups in the polymer chain reduce the ease of crystallization. The rate and extent of crystallization including the shape, dimensions, and orientation of the crystals that are formed during cooling below the spinneret (process known as quenching) impact the thermal, mechanical, and physical properties of the yarn. The glass transition temperature (Tg) of a polymer also plays an important role in whether the yarn is stiff or flexible at the normal-use temperatures.

For thermoplastic polymers such as PLA that can form a viscous melt, melt spinning (illustrated in Figure ) is a common approach to forming yarn. As the name suggests, an extruder heats the polymer resin pellets above its melting-point temperature (Tm) and then a metering pump forces the polymer melt through a spin pack that contains a spinneret with one or more fine holes. The emerging molten thread lines solidify into yarn as cold air quenches them; the multifilament as-spun yarn is then wound on a take-up roller. For effective melt extrusion, the Tm of the polymer should be lower than its decomposition temperature (Td) and it should not be too high to affect the process capabilities. However, for thermal stability of the yarn, the Tm should be much higher than its normal-use temperature. Thus, the Tg, Tm, and Td are all critical factors in determining the processability of fiber-forming polymers and their resulting properties and potential end-use applications.

Figure 1

Schematic of the two stages of the melt-spinning process for producing yarn with desired properties. Also indicated in the figure are the processing parameters that are the focus of this work.

A second process called drawing (Stage 2 in Figure ) often follows melt extrusion to both generate yarn with a finer diameter and to improve mechanical properties such as the tensile strength and modulus. At this stage, as-spun yarn passes through a series of hot drum-rolls, or godets, rotating at different speeds to stretch the yarn and increase polymer alignments. Depending on the difference in the rotation speed between the two rollers, different draw ratios can be achieved, leading to partially drawn or fully drawn yarn. Other subsequent processing stages are also commonly done,[13] such as lubricating and/or twisting to reduce yarn friction and improve yarn handling and efficiency during subsequent textile processes such as texturizing, weaving, and knitting, but these processing conditions are not the subject of the current study.

Thus, PLA is a distinctive nonpetroleum-based polymer, which satisfies all of the conditions discussed above to be able to form continuous fibers. It can also undergo hydrolytic degradation over time, which is a desirable property for many medical device applications such as absorbable sutures. However, many process conditions such as elevated temperature and pressure,[14] atmospheric moisture,[15] shear stress at various stages of processing,[16] etc. can induce premature degradation (process-induced degradation) and affect the final properties and performance of PLA yarn.

Several researchers have performed studies evaluating the effect of different process parameters on structural changes, properties, and degradation of melt-spun PLA yarn. In one such study, the researchers evaluated the effect of melt temperature, residual moisture, and residence time in the extruder on the molecular weight of PLA.[17] They showed that the residence time had the most significant impact followed by the melt temperature and moisture. In another study, the researchers investigated the correlation between melt extrusion process parameters and the degradation of various grades of PLA with different melt flow indices.[18] They studied the effect of melt temperature in the extruder and rotational speed of the extruder screw and concluded that the PLA with a high melt flow index (i.e., low viscosity) was less susceptible to degradation during extrusion with a high throughput (i.e., shorter residence time). In another study, the researchers showed that the degree of crystallinity of the PLA yarn was affected by different spinning speeds at the drawing stage.[19] In all of these prior studies, researchers have focused only on the process variables either at the melt extrusion stage or at the drawing stage, and they evaluated the effect on a few key properties such as crystallinity or molecular weight of the PLA yarn. However, in this study, we have investigated the effect of process parameters throughout the entire melt-spinning process, both at the extrusion stage and the drawing stage. We have also evaluated the physical, mechanical, and physicochemical properties of both the as-spun PLA yarn and the drawn PLA yarn. This has resulted in an enhanced insight into the process–property relationship for melt-spun PLA yarn and a better understanding of the impact of melt-spinning process parameters on the premature degradation of the polymer at each stage.

These insights are particularly important for hydrolytically sensitive polymers like PLA where processing needs to be under the mildest conditions possible, which is contradictory to the optimal melt processing requirements, including high temperature to reduce melt viscosity and high pressure to push the polymer melt through the fine spinneret holes. Thus, there is a need to study process–property relationships for such polymers to achieve the desired properties of the extruded yarn while controlling the process-induced degradation. The process–property relationships derived from the experimental data for melt-spun PLA yarn could enable the use of these process parameters to adjust the final properties of the yarn with a better control on the process-induced degradation. In the long term, with additional study, the process parameters could also be used to fine-tune the degradation profile of PLA yarn to match the requirements of the desired application.

Materials and Methods

Materials Selection

Because of the chirality of the polylactide molecule, PLA has four stereoisomeric forms: poly(l-lactic acid) (PLLA), poly(d-lactic acid) (PDLA), meso-poly(lactic acid), and poly(d,l-lactic acid) (PDLLA), which is a racemic mixture of PLLA and PDLA. l-lactide is a naturally occurring isomer. Poly(l-lactic acid) (PLLA) was chosen for this study since it is a well-studied aliphatic polyester and is widely used for absorbable medical devices such as sutures,[20] tissue-engineering scaffolds,[21] and orthopedic fixation devices.[22] Moreover, it is easily available commercially. The PLA resin we chose is NatureWorks 6100HP, which contains greater than 98% of the l-isomer and a small amount of the d-isomer.

Fiber Production

Prior to the melt-spinning process, the PLA resin pellets were dried under vacuum at 80 °C for 6 h to minimize the residual moisture content. The PLA yarn was then produced via melt spinning using the Hills Multifilament Research Line at The Nonwovens Institute, North Carolina State University. A spin pack with a 69-hole spinneret was used, resulting in a multifilament yarn. The process parameters that were varied during the melt extrusion and drawing stages are summarized in Figure .

Figure 2

Process parameters of interest during both the melt extrusion and drawing stages (see Figure ), with their corresponding low and high values.

During the melt extrusion stage, we investigated three processing parameters (also known as predictor variables) by incorporating a low and a high value for each: the effects of the maximum temperature in the extruder, the throughput through the metering pump (grams per hole per minute (GHM)), and the speed of the take-up rollers (m/min) after quenching. The Tm of PLA resin is around 180 °C, while its Td is above 260 °C, and in practice, the melt processing of PLA is done around 230 °C. Thus, the maximum extrusion temperatures were set to the low and high values of 220 and 250 °C, respectively. The metering pump attached to the extruder controls the amount of resin passing through the spinneret, where higher throughput means less exposure time for the polymer melt to the elevated temperature of the extruder. Low and high throughput values of 0.3 and 0.6 GHM, respectively, were chosen based on machine capabilities. The as-spun yarn formed after quenching was collected on rollers at two different take-up speeds of 500 and 1500 m/min, where higher take-up speeds provide improved orientation of the polymer chains while the polymer is still being quenched. Thus, eight as-spun yarn sample types were produced, each with a different combination of independent variables as described in Table .

Table 1

As-Spun and Drawn Yarn Samples with Corresponding Melt Extrusion and Drawing Process Parametersa

			drawing parameters
		as-spun	DL–RH	DL–RL	DH–RH	DH–RL
melt extrusion parameters	TL–PL–SL	220 °C, 0.3 GHM, 500 m/min	40 °C, 1.05	40 °C, 1*	75 °C, 3	75 °C, 2
	TL–PL–SH	220 °C, 0.3 GHM, 1500 m/min	40 °C, 2	40 °C, 1.5	75 °C, 2.1	75 °C, 1.5
	TL–PH–SL	220 °C, 0.6 GHM, 500 m/min	40 °C, 2.5	40 °C, 1.75	75 °C, 3.7	75 °C, 2.3
	TL–PH–SH	220 °C, 0.6 GHM, 1500 m/min	40 °C, 3.88	40 °C, 2.45	75 °C, 2.8	75 °C, 1.9
	TH–PL–SL	240 °C, 0.3 GHM, 500 m/min	40 °C, 1*	40 °C, 1*	75 °C, 2.8	75 °C, 1.9
	TH–PL–SH	240 °C, 0.3 GHM, 1500 m/min	40 °C, 1.38	40 °C, 1.05	75 °C, 1.7	75 °C, 1.3
	TH–PH–SL	240 °C, 0.6 GHM, 500 m/min	40 °C, 1.05	40 °C, 1*	75 °C, 4.4	75 °C, 2.7
	TH–PH–SH	240 °C, 0.6 GHM, 1500 m/min	40 °C, 1.25	40 °C, 1*	75 °C, 3.25	75 °C, 2.13

* indicates that the sample could not be successfully drawn under given processing conditions, and a draw ratio of 1 was assigned for analysis.

During the drawing stage, the temperature of the drawing rollers and the effects of the draw ratio were studied by incorporating a high and a low value for each of these parameters. The Tg of PLLA is around 60 °C, so the temperatures of the drawing rollers were chosen to be 40 °C (below Tg) and 70 °C (above Tg). The draw ratio quantifies the amount of drawing applied to the yarn, and the higher the draw ratio, the greater the orientation of the polymer chains and the lower the linear density or tex of the drawn yarn. For example, a draw ratio of two would imply that the yarn has been drawn to twice its original length. Since the maximum draw ratio for a given yarn is dependent on the melt extrusion parameters, the high value of the draw ratio for a given sample was chosen as the maximum possible draw ratio (a point beyond which the yarn would break and could not be drawn further), while the low value of draw ratio was chosen as half of that maximum value. Thus, during the drawing stage, there were two process parameters, each with two levels. Hence, 32 drawn yarn samples were obtained, each with a different combination of process variables from melt extrusion and drawing, as described in Table . It should be noted that 5 of the 32 samples could not be drawn under the given process conditions, and for statistical analysis, a draw ratio of one was assigned to those samples.

Characterization of Fiber Properties

The 40 yarn samples (8 as-spun and 32 drawn) were tested for various physical, mechanical, and physicochemical properties using the following test methods and protocols.

Tex is the unit of linear mass density, which is weight in grams of 1000 m of a yarn. It is used as a measurement for yarn size. The yarn tex was calculated by measuring the weight of a 2 m length of the multifilament yarn on a scientific balance to the nearest 0.1 mg. The average of five measurements was calculated and recorded for each sample.

The mechanical properties such as tenacity (gf/tex) and elongation at break (%) were measured for each sample on an Instron Model 5544 mechanical tester, following the Standard ASTM Method D3822, “tensile properties of textile fibers.” A 100 N capacity load cell was used together with an original gauge length of 25 mm and a crosshead speed of 50 mm/min. The average of five measurements was calculated and recorded for each sample.

Changes in the thermal properties (Tg and Tm) and the crystallinity of the polymer chains due to the processing conditions were determined using a differential scanning calorimeter (DSC). This test was performed on 5 mg specimens using a PerkinElmer Diamond DSC. An initial heating thermograph was recorded from room temperature (25 °C) to 185 °C at a linear heating rate of 20 °C/min. Once the melt endotherm peak had been recorded, the specimen was rapidly cooled to room temperature to permit a second heating cycle at the same rate of 20 °C/min that permitted the measurement of Tg. The percent crystallinity of the polymer was calculated using the heat of melting (ΔHm) and the heat of crystallization (ΔHc) according to the followingwhere ΔHm° is the theoretical heat of melting for 100% crystalline PLLA (93 J/g).[23] The areas under the endotherm peaks were calibrated by running a thermograph to melt a known mass of indium under the same conditions.

Gel permeation chromatography (GPC) was performed using an Alliance Model 2695 Waters high-performance liquid chromatography (HPLC) unit with a Waters Model 2414 refractive index (RI) detector to determine the weight-average molecular weight (Mw) of the polymer. The fiber samples were dissolved in tetrahydrofuran (THF) at a concentration of 1 mg/mL. The Mw for the original PLA resin was 120 kDa, and the Mw values for the yarn samples after melt extrusion and drawing were used to calculate the percent reduction in Mw, which was assumed to correspond to the degree of degradation that had occurred during processing.

Statistical Analysis for Process–Property Relationships

Statistical analysis was performed using JMP Pro 14 software (SAS Institute). All factor values were converted to −1 and +1, for the low and high levels, respectively. The low and high values for draw ratio (R) depended on the levels of the other factors, so the −1 and +1 values do not correspond to the same original values. The expected change in the response from −1 to +1, however, was consistent across samples. In some cases, the sample could not be drawn (since they were too brittle), meaning both the low and high values would be 1. In these instances, both the low and high R values were set to 0, the midpoint between −1 and +1, and all responses, except for max draw ratio, were set to those for the as-spun samples (prior to drawing). For statistical analysis, the max draw ratio for such samples was set to 1.

A two-stage analysis was performed on the drawn samples, which come from a split-plot experiment.[24] First, we averaged over the settings of the drawing parameters (D and R), the split-plot factors, and inspected the half-normal plot of the melt extrusion parameter effects, the whole-plot factors. Then, a split-plot analysis was performed including main effects and two-factor interaction effects between the melt extrusion parameters deemed significant from the half-normal plot and the two drawing parameters. The analysis also considered three-factor interaction effects between the melt extrusion and drawing parameters. For the split-plot effects, effects having p-values less than α = 0.05 were deemed important; for whole-plot effects we used α = 0.10 (since the tests were prone to low power due to the limited number of samples). For the max draw ratio analysis, α = 0.10 was used for the split-plot effects since there were only 16 samples, not 32. Effects found to be statistically insignificant were then removed from the split-plot model unless they violated effect heredity rules. For example, if the three-factor interaction PDR was significant, then we included all three main effects PDR and two-factor interactions PD, PR, and DR, even if they are statistically insignificant.

Results and Discussion

Effect of Melt Extrusion Parameters and Drawing Temperature on the Drawing Performance

The draw ratio impacts the properties of the yarn due to both changes to the microstructure (polymer alignments and crystallinity) as well as other properties such as the diameter. Thus, it is considered a predictor variable in this study. However, as Figure illustrates, the drawing performance is heavily dependent on the melt extrusion parameters (TL/H, PL/H, SL/H) as well as drawing temperature (DL/H). Hence, we analyzed the factors affecting the drawing performance in terms of the maximum draw ratio. Regression analysis indicates that DL/H, PL/H, and TL/H are the most significant factors in determining RL/H. Higher drawing temperature, higher throughput, and lower melt temperature allow higher draw ratio for the PLA yarn. It should be noted that all of those samples that were either unable to be drawn or had a maximum draw ratio of 1 had low drawing temperature (DL) as a common parameter.

Figure 3

Maximum draw ratio of the PLA yarn as a function of the melt extrusion parameters and drawing temperature.

Physiochemical Properties of Fibers

Yarn Size

Figure presents the effect of the processing parameters on the linear density of the bundle of all of the filaments in a yarn, also called tex. The higher the tex value, the thicker is the yarn. The as-spun yarn varied from 16.2 to 101.6 tex (with standard deviations of 0.5–1.6), whereas the drawn yarn varied from 6.2 to 99.0 tex (with standard deviations of 0.1–1.6). Interestingly, at the melt-spinning stage, throughput (P) and take-up speed (S) had a significant impact on tex, while at drawing stage, both the draw temperature (D) and draw ratio (R) significantly impacted the tex. There was a significant 3-factor interaction between throughput (P), drawing temperature (D), and draw ratio (R), and take-up speed (S), drawing temperature (D), and draw ratio (R). A high drawing temperature is observed to decrease the tex from the as-spun values in all cases. As-spun and drawn samples with the highest values for tex are the ones with a high throughput and low take-up speed, and for the drawn samples, also a low drawing temperature. On the contrary, those with the lowest tex values are the ones with low throughput and high take-up speed; for the drawn samples, it appears that DH and RH decreased the tex even more than for DL and RL. Thus, these four predictor variables played a significant role in determining the yarn size; in other words, while the yarn thickness increased with higher throughput, it decreased with the higher take-up speed, higher drawing temperature, and higher draw ratio. The melt temperature (TL/H) is not observed to have a significant impact on the yarn size.

Figure 4

Yarn size of the PLA yarn as a function of the melt extrusion and drawing parameters. Error bars represent the standard deviation for each average measurement; note that the standard deviation for each sample is ≤1.6 tex, which is smaller than the size of the points.

Crystallinity

The melt extrusion parameters for a yarn, namely, the maximum temperature set in the extruder, the residence time in the extruder (which is controlled by the throughput), and the quench time (governed by the speed of the take-up rolls) determine the degree of crystallinity of polymer chains. Subsequent drawing at high temperature also affects the molecular chain orientation and thus the crystallinity of the polymer. The DSC thermographs for the as-spun PLA yarn as compared to the resin processed at low extrusion temperature (TL = 220 °C) in Figure a and high extrusion temperature (TH = 240 °C) in Figure b indicate that the Tg peak (around 60 °C) becomes more prominent for TH, which suggests an increased amorphous content. At the same time, the crystallization peak (around 80–95 °C) shifted toward a higher temperature, and the melting peak (around 175 °C) became broader, again indicating a higher amorphous content compared to samples processed at TL. In addition, samples processed at the higher take-up speed resulted in a less prominent Tg peak, a crystallization peak that shifted toward a lower temperature, and a narrower melting peak, indicating that these samples increased in chain alignments and crystalline content. In terms of throughput (PL/H), a lower throughput enables a longer residence time in the extruder for the polymer and facilitates better quenching. For samples processed at a lower throughput (PL), the crystallization peak shifts toward a lower temperature, and the melting peak became narrower, which indicates increased crystalline content. Figure c provides a typical DSC thermograph for PLA yarn after drawing. Prominent changes that can be observed are the reduction of the Tg peak and the appearance of a small peak above Tg (higher than 60 °C) as well as a narrowing of the melting peak (around 175 °C). This observation suggests that the drawing process increased the crystallinity of the polymer chains, which has also been reported in another study.[19] Note that the appearance of a small peak above the glass transition temperature may be due to the relaxation of stress caused by the thermal processing history. Moreover, the PLA resin did not show any peaks for Tg or crystallization, and had a broad melting peak, which indicate the lack of structure and symmetry in the polymer chains in the resin.

Figure 5

DSC thermographs for PLA resin (black line) and as-spun yarn as a function of a variety of processing parameters, (a) low and (b) high extrusion temperature, and (c) representative DSC thermograph for drawn PLA yarn that were produced from a low extrusion temperature.

Figure provides the crystalline content calculated from the DSC thermographs. DL/H, SL/H, RL/H, PL/H, and the two-factor and three-factor interactions among these variables had a significant impact on the degree of crystallinity. Standardized parameter estimates from the regression analysis show that the drawing temperature had the greatest influence on the degree of crystallinity. A higher drawing temperature facilitates better polymer chain orientation and hence a higher degree of crystallinity. The take-up speed was the second most influential factor, followed by the draw ratio. Both these factors enable better polymer chain alignment resulting in a higher degree of crystallinity. Thus, PLA yarn can be made either highly crystalline or amorphous by adjusting the drawing temperature, take-up speed, draw ratio, and the throughput. Low throughput, high take-up speed, high drawing temperature, and a high draw ratio promote better polymer chain orientation, resulting in the PLA yarn with a high crystalline content.

Figure 6

Crystalline content of the PLA yarn as a function of the melt extrusion and drawing parameters.

Molecular Weight

A reduction in the weight average molecular weight (Mw) of the PLA polymer is an indication of chain scission, a form of polymer degradation, caused by hydrolysis. In the case of PLA yarn, 7.6–20.5% reduction in Mw was observed after extrusion and drawing, which is attributed to process-induced degradation. Figure contains the percent reduction in Mw for PLA yarn after melt extrusion and drawing. For the as-spun samples processed at TH, a greater reduction in Mw is observed compared to those at TL. This indicates the importance of the temperature in the melt extruder to limit the process-induced degradation of PLA yarn, which has previously been reported by other studies.[17,18] The regression analysis further points out that the DL/H and SL/H also had a significant impact on Mw. A higher drawing temperature and a higher take-up speed exert more processing stress on polymer chains leading to a larger number of chain scission and increased amount of process-induced degradation.

Figure 7

Percent change (reduction) in Mw from the original PLA resin Mw (=120 kDa) for the PLA yarn as a function of the melt extrusion and drawing parameters.

Regression analysis suggests that to minimize the process-induced degradation, the PLA yarn should be processed using mild process conditions such as low melt temperature, low take-up speed, and low drawing temperature. However, a low R squared value (R2 = 0.44) in the regression analysis indicates that more variables contribute to process-induced degradation than those included in this study. For example, residual moisture in the polymer resin and the atmospheric ambient moisture can accelerate hydrolytic degradation during processing and alter the final properties of the fiber. Hence, the polymer resin should be dried carefully before processing and all moisture should be avoided. To prevent excessive degradation during processing, it is recommended that any residual moisture does not exceed 0.02%.[25] Premature degradation of the polymer during processing can also be triggered by higher residual monomer content as well as process-induced monomers. Processing factors that can induce monomer formation include high temperature, high shear forces, long exposure time to high temperature, screw design, and speed, as well as the catalyst content (usually Sn) of the polymer.[26,27] Furthermore, polymer resins with higher intrinsic viscosities require higher temperature and pressure and thus result in more monomer formation.[28] Higher monomer content has a plasticizing effect and can catalyze the degradation process. Hence, melt processing of polymers such as PLA that are susceptible to process-induced degradation requires a delicate balance of process parameters and an in-depth understanding of process–property relationships.

Mechanical Properties of Fibers

Tenacity

Figure provides the measured tenacity of the PLA yarn based on different processing parameters. The regression analysis indicates that all of the process parameters (TL/H, SL/H, PL/H, DL/H, and RL/H) have a significant impact on determining the tenacity of the fibers. Standardized parameter estimates from the regression analysis show that the drawing temperature had the greatest influence on the tenacity, and the draw ratio was the second most influential factor, followed by the take-up speed. All of these factors facilitate better polymer chain orientation, which results in increased chain locking and thus higher tenacity. Samples that were measured to have the lowest tenacity are the ones produced with PH, SL, and DL. These samples were very brittle and had a high modulus. On the other hand, samples with the highest tenacity were formed using DH coupled with RH. Interestingly, for the DL systems, a high tenacity was produced for samples with TL, PL, SH, and RH. Thus, low melt extruder temperature, low throughput, high take-up speed, high drawing temperature, and high draw ratio are likely to produce a high tenacity yarn because these factors contributed to a better orientation of the polymer chains.

Figure 8

Tenacity of the PLA yarn as a function of the melt extrusion and drawing parameters. Error bars represent the standard deviation for each average measurement.

Elongation at Break

Figure presents the maximum elongation at break as a function of the different processing parameters. Regression analysis indicates that SL/H, PL/H, DL/H, and RL/H have a significant effect on the elongation at break, whereas a three-way interaction between throughput (P), take-up speed (S), and draw temperature (D) had the strongest effect. For the as-spun samples, the effect of take-up speed on the elongation at break is strongly apparent. Low throughput, high draw temperature, high draw ratio, and high take-up speed resulted in a fully drawn yarn and thus a yarn with low elongation at break. On the other hand, a low draw ratio, low take-up speed, and high throughput resulted in a partially drawn yarn with a high residual elongation at break. It should be noted that a few samples were an exception. They had a low elongation at break despite having the lower take-up speed and lower draw ratio because these samples were very brittle and had a low tenacity and high modulus.

Figure 9

Elongation at break of the PLA yarn as a function of the melt extrusion and drawing parameters. Error bars represent the standard deviation for each average measurement.

Conclusions

It was confirmed that the process parameters at the melt extrusion stage determined the maximum draw ratio that could be achieved for each yarn sample. The higher throughput and higher drawing temperature resulted in the high draw ratios. The draw ratio affected the tenacity, crystallinity, and other properties, and thus, it is an important process parameter. A high throughput, low take-up speed, and low draw ratio resulted in a very brittle yarn with a high tex and low tenacity. The drawing process increased the crystallinity of the polymer chains with drawing temperature as the most influential factor. The drawing temperature also had the greatest influence on the tenacity, and the draw ratio was the second most influential factor, followed by the take-up speed. A high drawing temperature, a high draw ratio, and a high take-up speed increased the fiber tenacity. Various combinations of process parameters included in this study reduced the molecular weight of the PLA chains by 7.6–20.5%, which confirms that a certain set of process parameters induced a higher degree of premature degradation and that it is possible to reduce the process-induced degradation by adjusting the processing parameters.

The unique design of our experiment and statistical analysis enabled a detailed study of the changes in yarn properties after melt extrusion as well as after drawing. This helped in mapping the effect of the melt-spinning process parameters on the physical, mechanical, and physicochemical properties of PLA yarn. The desired values for PLA yarn properties depend on the end application of the yarn. For example, certain biomedical applications may require PLA to degrade rapidly after implantation, while others may require PLA yarn to degrade slowly over time. Some applications may require a higher tenacity, while others may require a higher elongation at break. Thus, the optimum values for the process parameters will vary with the desired application of the PLA yarn. However, if we consider a general scenario where one wants to maximize the yarn tenacity, while minimizing process-induced degradation, the process–property relationship from this study shows that the yarn should be processed at a low melt extruder temperature (220 °C), high throughput (0.6 GHM), low take-up speed (500 m/min), high drawing temperature (75 °C), and the maximum possible draw ratio. This set of process variables resulted in the yarn with one of the highest tenacities in the study and a limited amount of process-induced degradation. On the other hand, the yarn processed at a low melt extruder temperature (220 °C), high throughput (0.6 GHM), high take-up speed (1500 m/min), high drawing temperature (75 °C), and the maximum possible draw ratio resulted in the highest tenacity in the study. However, it also had one of the highest extents of process-induced degradation.

Thus, this study clarified which process parameters can be utilized to achieve the desired properties of PLA yarn while controlling the process-induced degradation.