Fine Processing. Extrusion Die. Company Profile. Melt spinning is a typical spinning of synthetic fiber such as Polyester, Nylon, Polypropylene.
Melted polymer is discharged from the spinneret. The discharged fiber is wound after quenching process and drawing process. Melt spinning is used for various polymers and applications. 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.
Certain combinations of processing parameters resulted in higher process-induced degradation, as evident from the reduction in molecular weight, ranging from 7. 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.
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. Figure 2. Process parameters of interest during both the melt extrusion and drawing stages see Figure 1 , with their corresponding low and high values.
Figure 3. Maximum draw ratio of the PLA yarn as a function of the melt extrusion parameters and drawing temperature.
Figure 4. Yarn size of the PLA yarn as a function of the melt extrusion and drawing parameters. 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 6. Crystalline content of the PLA yarn as a function of the melt extrusion and drawing parameters. Figure 7. 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. Figure 9. Elongation at break of the PLA yarn as a function of the melt extrusion and drawing parameters. Raw data from the initial split-plot model which gave the p -values and the final regression model used for the analysis and discussion PDF.
Such files may be downloaded by article for research use if there is a public use license linked to the relevant article, that license may permit other uses. The manuscript was completed through contributions of all authors. All authors have given approval to the final version of the manuscript. More by Chirag R. More by Jon W. More by Melissa A.
More by Martin W. Cite this: ACS Omega , 6 , 24 , — Published by American Chemical Society. Article Views Altmetric -. Abstract High Resolution Image.
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. 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. 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 T g 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 1 is a common approach to forming yarn.
As the name suggests, an extruder heats the polymer resin pellets above its melting-point temperature T m 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 T m of the polymer should be lower than its decomposition temperature T d and it should not be too high to affect the process capabilities. However, for thermal stability of the yarn, the T m should be much higher than its normal-use temperature.
Thus, the T g , T m , and T d are all critical factors in determining the processability of fiber-forming polymers and their resulting properties and potential end-use applications. High Resolution Image. A second process called drawing Stage 2 in Figure 1 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. 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. 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.
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. 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. 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. 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.
A spin pack with a 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 2. 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. Thus, eight as-spun yarn sample types were produced, each with a different combination of independent variables as described in Table 1. Table 1. 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 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 1. 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.
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 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. The average of five measurements was calculated and recorded for each sample.
Changes in the thermal properties T g and T m and the crystallinity of the polymer chains due to the processing conditions were determined using a differential scanning calorimeter DSC. The M w for the original PLA resin was kDa, and the M w values for the yarn samples after melt extrusion and drawing were used to calculate the percent reduction in M w , which was assumed to correspond to the degree of degradation that had occurred during processing.
In some cases, the sample could not be drawn since they were too brittle , meaning both the low and high values would be 1. 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. 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. Effects found to be statistically insignificant were then removed from the split-plot model unless they violated effect heredity rules.
Results and Discussion. 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.
Hence, we analyzed the factors affecting the drawing performance in terms of the maximum draw ratio. 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 D L as a common parameter. Figure 4 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 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 D H and R H decreased the tex even more than for D L and R L.
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 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. In addition, samples processed at the higher take-up speed resulted in a less prominent T g 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.
For samples processed at a lower throughput P L , the crystallization peak shifts toward a lower temperature, and the melting peak became narrower, which indicates increased crystalline content. The melted polymer fibers then pass through a cooling region and the fibers are combined to form a yarn, and a spinfinish is applied.
The yarn is then drawn using several godet rolls with very good speed and temperature control to orient the molecules in the fibers and eliminate voids, making the yarn stronger. Melt spinning is the fastest fiber production system available in current technology. Therefore, if the polymer is thermoplastic, then melt spinning should be used for higher productivity.
Most of the manmade fibers such as polyester, polypropylene, nylon, and PGA are produced using melt spinning. Melt spinning technology is very advanced, and parameter control of the process is strong. It is possible to produce shaped fibers such as circular, triangular, and hollow fibers using melt spinning. Production of biocomponent and tricomponent fibers is also possible using this process. Secant Group has state-of-the-art, versatile melt spinning equipment with very sensitive speed and temperature controls, along with several drafting zones.
0コメント