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Over the last half century, durable synthetic polymers including nylon and polyester have derived from petroleum products and have been used for different industrial applications and for making clothes. However, despite their durability, Mochizuki (1) notes that, synthetic polymers have been blamed for various adverse environmental effects because they are a major problem in solid waste management. With increased awareness about the need to protect the environment, attention in polymer production has shifted to the development of biodegradable materials, which do not increase the amount of greenhouse gases in the environment upon incineration. The linear aliphatic thermoplastic polyester, otherwise referred to as Polylactic acid (PLA), is an excellent example of a biodegradable polymer derived from renewable sources such as corn, sugarcane, and tapioca products.
The fibers derived from PLA are not only compostable and biodegradable, but they are also highly functional, because they possess a wide range of properties including weathering resistance, bacteriostatic, bactericidal, and flame retardant as opposed to petrochemical fibers. Drumright et al. (2) indicates that PLA polymers were initially used in making biomedical products such as drug delivery systems and sutures because they were cost effective and readily available. However, the applications of PLA polymers have increased considerably over the past few years, and now, they can be used in making packaging materials and in other textile applications [see Schmack (3) for more details]. This paper reviews the synthesis and production of PLA fibers, their chemical and physical properties, and their applications. While recognizing that PLA is a relatively new polymer with unique properties and characteristics, this paper will highlight the advantages and disadvantages of PLA fibers in relation to other fibers already in the market.
Synthesis and Production of PLA Fibers
As opposed to other synthetic polymers, which are manufactured from oil and gas products with the use of non-renewable energy, PLA is manufactured from renewable crops with the aid of energy from the sun. According to Woodings (4), solar energy promotes photosynthesis in plant cells, whereby carbon dioxide and water are converted into starch, preferably cornstarch. The cornstarch is then extracted from plant cells and used in making fermentable sugar such as glucose (dextrose) and saccharose through enzymatic hydrolysis. Subsequently, through bacterial fermentation, the natural sugar is converted into lactic acid. Dugan (5) provides an elaborate discussion regarding the production of lactic acid from renewable natural resources. Lactic acid is the building block for PLA, which is prepared by two major processes; direct condensation of the acid or ring opening of the lactide dimer. Figure 1 below shows the two major polymerization processes involved in the production of PLA from lactic acid.
The direct condensation process involves the removal of water from lactic acid using a solvent under high vacuum and temperature conditions in order to produce PLA polymers. This process has many disadvantages, because it is difficult to remove trace amounts of water from the final product, and, hence, it is impossible to produce polymers with the required molecular weight. Another disadvantage of this approach is that it requires a large reactor besides the need for evaporation. Moreover, there is increased color and racemization when this approach is used to produce PLA. Therefore, Drumright et al. (2) posit that the most preferable method of PLA production is ring-opening polymerization of the lactide polymer. This method is based on advances in fermentation technologies, which promote production of the stereoisomers of lactic acid, and, thus, the production of high molecular weight PLA polymers. Generally, the process involves removal of water from L-lactic acid without the use of a solvent in order to produce a cyclic pre-polymer referred to as L-lactide. This is followed by purification of the monomer through vacuum distillation before the ring is opened under heat; a process that requires no solvent. Henton (6) provides an overview of the conventional ring-opening polymerization process. Here, polymers with different molecular weights can be produced from the intermediate dimer by controlling its purity. Furthermore, three products are derived from L-lactic acid through the intermediary route. L-lactic is converted into D-lactide, L-lactide, and meso-lactide. However, as opposed to meso-lactide, D- and L-lactide are optically active, and hence, they can be converted into polymers with different molecular weights through ring-opening polymerization. According to Drumright et al. (2) and Henton (6), this process entails varying the amount and the distribution of D-lactide in the PLA polymer. As a result, crystalline polymers can be produced by increasing the level of L-lactide in the polymer backbone. On the other hand, amorphous polymers can be produced by increasing the level of D-lactide in the polymer backbone. Overall, Henton (6) notes that the purity of the lactide plays a major role in determining the molecular weight of the final product. Most importantly, the properties of the final product such as melt behavior, ductility, barrier properties, and thermal properties can be manipulated in the same way described above.
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Currently, ring-opening polymerization is a patented method for the production of low cost Polylactic acid polymers. Figure 2 below shows the commercial process developed by NatureWorks LLC for production of high molecular weight PLA in which aqueous lactic acid is continuously condensed into a low molecular weight pre-polymer. This is followed by intra-molecular cyclization whereby the pre-polymer is subjected to tin catalysis in order to ensure that the amount and sequence of lactide stereoisomers in the polymer backbone is maintained as required. Next, vacuum distillation is the method used to purify the end products from the lactide mixture. In fact, the final step entails ring-opening polymerization in which high molecular weight polymers are produced by tin-catalysis without the use of solvents. The remaining part of the mixture, which consists of monomers, is recycled and re-used in a fresh cycle.
Subsequently, Avinc (7) and Eustathios (8) indicate that PLA polymers are used to manufacture fibers through the process of melt-spinning. Here, PLA polymers are melt-spun into different categories of fibers such as monofilaments, multi-filaments, spunbound fabrics, staple fibers, bulked continuous filaments, and short-cut fibers with the aid of machines. This is followed by the annealing process in which the fibers are interwoven to produce the required mechanical properties in relation to tenacity, toughness, and dimensional stability. As stated earlier, the amount and sequence of D- and L-isomers in the polymer backbone is very important, because it determines the crystalline melting points for different types of PLA fibers. For instance, Huihui (9) reveals that fibers with high levels of D-lactide, approximately 10% can melt at 120-130%u02DAC. Furthermore, fibers with different thermal shrinkage properties can be used to produce conjugated fibers, which are known for their good resilience and high bulkiness, and thus, they are used as fiberfill.
Chemical and Physical Properties of PLA Fibers
Generally, PLA fibers share a number of similarities with different thermoplastic fibers. For instance, they both have controlled crimps, low moisture regain, and smooth surfaces. However, it is important to note that PLA fibers are unique in the sense that they are produced from renewable natural resources, and hence, their production does not impact negatively on the natural environment. Cicero (10) indicates that PLA fibers have a circular cross-section and a smooth surface. Moreover, the specific gravity of PLA fibers is 1.25 g cm-3, which is relatively lower compared to other fibers such as PET. Additionally, the refractive index for PLA fibers is 1.35-1.45 as opposed to 1.54 for PET fibers. As a result, PLA fibers can be used to make trilobal shapes, which impart strong anti-soiling properties to different materials. On the other hand, PLA fibers have many thermal properties in relation to different conditions. For instance, at room temperature, PLA fibers are always stiff. However, the glass transition temperature (Tg) for PLA fibers is approximately 55-65%u02DAC and their melting temperature (Tm) is 160-170%u02DAC, depending on the amount and sequence of D- and L-isomers in the polymer backbone. Compared to PET fibers with 254%u02DAC, the melting point of PLA fibers is relatively lower, and, hence, their applications can be restricted in this way. Nonetheless, by adjusting the level of D- and L-isomers in the polymer chain, the melting point for PLA fibers varies from 130%u02DAC to about 220%u02DAC [see Mochizuki (1) and Drumright et al. (2)].
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Furthermore, Kovacs and Tabi (11) reveal that PLA fibers can reach a good crimp degree and retention level after processing. Moreover, PLA can be used to make filament yarns and spun yarns in the same way PET has been used over the years. Most importantly, the tenacity at break for PLA fibres is 32-36 cN tex-1, which is relatively higher compared to other natural fibers. In addition, the tenacity of PLA fibers is not affected by humidity; however, they show increased elongation after manufacturing. Conversely, tenacity reduces with an increase in temperature, and the fibers tend to show increased fiber extension at high temperatures. Moisture regain properties in PLA fibers stands at 0.4-0.6%, which is extremely low compared to other natural fibers and higher than polyester. Further, PLA is a flammable polymer, but the fibers are known for their good self-extinguishing properties. For example, PLA fibers burn for a short time and they produce low smoke compared to other fibers. Moreover, the limiting oxygen index (LOI) for PLA fibers is relatively high compared to other fibers, and, thus, it is not easy to ignite them because they require higher oxygen levels. It is also worth noting that PLA fibers do not absorb UV rays, and, hence, their strength is less compromised when exposed to UV light. Moreover, PLA has excellent moisture transport properties, meaning that PLA fibers dry rapidly. On the other hand, PLA is not a source of instant microbial food, and therefore, it exhibits good biological resistance even before after-finish treatment. Another important point to note is that PLA has poor chemical resistance properties, meaning that dyeing and other finishing treatments must be carried out with extreme caution. However, PLA is not affected by different dry-cleaning solvents due to its limited solubility [see Kovacs and Tabi (11)].
Advantages and Disadvantages of PLA Fibers
The chemical and physical properties of PLA fibers influence the performance features of the fibers and this forms the basis for their advantages and disadvantages. One of the most important features of PLA fibers entails sustainability. As noted earlier, PLA is produced from renewable natural resources. This implies that the advantages of manufacturing PLA fibers are many. According to Mochizuki (1) and Lunt (12), the use of renewable and non-polluting raw materials eliminates the challenges associated with oil as a raw material in the production of synthetic fibers. Biodegradability is another important feature of PLA fibres, which can be related to sustainability. Here, it is important to note that PLA is fully compostable. In fact, Mochizuki (13) reveal that the stability of PLA is reduced at temperatures greater than 60%u02DAC and high humidity conditions, preferably more than 80%. With these conditions, which are common to many composting environments, PLA is susceptible to bacterial attack. This entails a two-phase biodegradation process with water and carbon dioxide as the final products. In the first phase, PLA undergoes chemical hydrolysis at a temperature of more than 60%u02DAC and humidity at more than 80%. With a micro-organism population of approximately 10,000-20,000, the second phase begins with digestion of lactic acid and other low molecular weight oligomers to produce carbon dioxide and water. This implies that final products of the biodegradation process can be used to grow corn, rice, and sugarcane, which can be used to make PLA fibers. Basically, the production of PLA fibers has minimal environmental consequences compared to other recyclable polymers. Therefore, PLA offers excellent sustainability and a reduced environmental impact as opposed to other synthetic polymers [see Mochizuki (1) and Woodings (4)].
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Furthermore, considering that sustainability and environmental impact play a major role in determining the production and consumption of different fibres and fabrics, PLA is a better option for those manufacturers who want to produce consumer products for their environmentally-conscious consumers. Despite the relatively higher price of PLA fibers when compared to other synthetic fibers, its production on a large-scale will expand the number of applications available for PLA fibers, and, thus, lower the prices. In fact, Dugan (5) and Yuan et al. (14) predict that PLA fibers will be used to make diapers and other disposable materials in order to reduce landfill spaces and the environmental impact at large. Most importantly, as the number of environmentally friendly consumers grows, manufacturers of fibers and fabrics should begin to see the need to use PLA fibers to produce a wide range of products, while maintaining the product’s value and cost competitiveness. However, Yamaguchi (15) argues that issues of durability may arise if the overall objective of producing PLA fibers will be limited to sustainability, and, hence, the range of applications of PLA fibers may be restricted to disposable products. This implies that PLA fibers will not be suitable for use in apparel and other applications, which require durable materials. On the other hand, PLA is more durable compared to other biodegradable polymers. In fact, other biodegradable fibers can be consumed by microorganisms upon encounter. However, PLA cannot be digested by microbes before undergoing hydrolysis [see Mochizuki (1)].
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Another compelling advantage of PLA fibers entails their natural hydrophilicity properties. As opposed to other polymers such as PET and nylon, PLA allows oxygen and water molecules to pass through the linkages between its molecules. As a result, Drumright et al. (2) and Woodings (4) indicate that PLA fibers have good wettability and excellent moisture transmission properties. This implies that PLA fibers are a better option to nylon and PET in the manufacture of under garments, shirts, dresses, and shoes. Moreover, most applications with high moisture transport requirements are currently using the PET-based 4DG fiber, which is not reliable due to different issues related with the performance of the fiber finish in transporting water. However, PLA fibers offer a permanent moisture transport system, which does not compromise moisture transfer, and thus, it can perform better in the above-mentioned applications. On the other hand, PLA fibers are more advantageous than PET fibers due to their good dyeability properties. As opposed to PET, the refractive index of PLA is relatively lower, meaning that it can be dyed to form deeper and brighter shades [see Mochizuki (1)].
Apart from dyeability, PLA fibers are very useful in many applications because of their flammability attributes. As stated earlier, PLA burns with a white flame, which releases minimal amounts of smoke. Furthermore, PLA shrinks very fast when heated above its heat-set temperature. This implies that PLA fibers are suitable in making flame-protection models in which materials are expected to shrink away from the flame and guard against the flame spreading to adjacent materials. Therefore, Lunt (12) notes that the burning properties of PLA fibers make them suitable for different applications, particularly in upholstery, making furniture fabrics, and window dressings. Despite having many advantages against other synthetic fibers in the market, PLA fibers possess some compelling disadvantages. For example, PLA has poor abrasion resistance properties, meaning that it is not suitable for applications that require high-performance characteristics such as in making ropes. However, it is worth noting that the application of PLA in making carpets is still feasible. On the other hand, PLA has a low melting temperature, and thus, its applicability in environments with high-temperature is limited. Nevertheless, ironing and drying of apparel are not compromised to this effect [see Mochizuki (1)].
Applications of PLA fibers
The unique range of properties and performance features make PLA fibers the most suitable material in many applications. First and foremost, PLA fibers are now in use in the apparel fiber sector. In the Nagano Winter Olympics (1998), several garments manufactured from PLA fibers or blends of PLA and other natural fibers were exhibited by Kanebo, Inc. followed by the development of NatureWorks LLC’s PLA fiber-based IngeoTM brand in 2003 [see Henton (6) and Takasaki (16)]. These initial PLA fiber-based apparel brands have since set the pace for the production of several garments, which are currently in the market. The most outstanding features of PLA fibers, which make them suitable in the production of attires, include good moisture management properties, excellent crimp retention characteristics, good thermosetting properties, and high resiliency. In fact, garments made from PLA fibers or blends of PLA and other natural fibers such as wool and cotton offer a lot of comfort and hydrophilicity. Moreover, PLA fibers have good anti-flammability properties, particularly the self-extinguishing characteristics. Furthermore, the crimp retention properties of PLA fibers enable garments with excellent shape retention characteristics to be manufactured. Currently, fabrics made from PLA are being used to make sports wear and other outerwear products.
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Another important application of PLA fibers entails home ware, which includes a wide range of products such as blankets, pillows, mattresses, mattress pads, duvets, carpet tiles, and office panel fabrics. As a result, Yamane (17) and Zell et al. (18) indicate that the market for PLA fiber-based products is being driven by the increased awareness about the need to protect the natural environment, and thus, environmentally friendly consumers are increasing the demand for sustainable fibers. Most importantly, PLA’s resistance to UV light and better anti-flammability characteristics are the most compelling features, which make PLA fibers more suitable in manufacturing home ware products compared to petrochemical-based fibers. Apart from home ware products, Moon et al. (19) reveals that PLA fibers have found extensive applications in the production of medical apparatus. Over the years, textile scaffolds have been used to culture and grow human cells from different organs. The development of textile scaffolds for this application requires biodegradable fibers produced from degradable polymers. As a result, PLA fibers have been in use as the major biodegradable fibers in various implants. Furthermore, a combination of PLA and Polyglycolic acid (PGA) polymers offers superior properties required for different medical applications because they can undergo biodegradation, and, thus, allowing fibrous connective tissues to grow. This implies that there is no need for surgery in order to remove the implanted fabric materials.
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In addition, Yasuniwa et al. (20) note that PLA fibers are used to manufacture industrial fabrics, particularly those used in the automobile industry to make floor mats, ceiling fabrics, partition boards, seat cushions, and seat fabrics. On the other hand, PLA fibers are used in making filters for organic wastes, tea, and even coffee. In fact, Yasuniwa et al. (20) reveal that PLA fibers are safe even when they come into contact with food, and, hence, they are commonly approved for use in making tea bags. Additionally, PLA fibers can be used to make air filters in air conditioning systems because they have excellent antibacterial and antifungal properties. Finally, PLA fibers are used to make towels and wipes. These products are generally comfortable to use in drying surfaces. As a result, PLA fibers can be used to make a wide range of nonwoven materials, including body towels and wipes for different purposes.
Polylactic acid (PLA) is a synthetic polymer produced from renewable natural resources such as corn, rice, and sugarcane. Fibers made from PLA have a wide range of properties, including a relatively low density and refractive index, good thermal properties, good crimp retention characteristics, good tensile properties, good self-extinguishing attributes, low moisture regain, and UV resistance. Among other chemical and physical, these properties make PLA fibers suitable for different domestic and industrial applications, including apparel and medical applications, besides the production of industrial fabrics, filters, home furnishings, towels/wipes, and other personal belongings. However, PLA fibers are relatively new in the market compared to petrochemical-based fibers and even natural fibers. Therefore, there is the need for increased technological improvements in the production and processing of PLA fibers in order to gain full entry into the market considering that the current cost of production and processing is relatively high.