Polylactic acid (PLA) is an environmentally friendly, plant-derived thermoplastic. The fiber forming substance is a lactic acid polymer in which at least 85 % by weight are lactic acid ester units derived from naturally occuring sugars (sugar beets and corn). Although compostable, polylactic acid is quite durable in most applications. Infact, PLA does not readily degrade unless it is exposed to high humidity and elevated temperatures (≥ 60 °C) which results in rapid decomposition of the fiber. Thus, for most applications, its durability is acceptable or good. The tensile strength of PLA fibers is comparable to those of polyester fibers. However, PLA has a low Tg of only 55 - 60 °C and thus, is less heat resistant than polyester (PET). It is also more flammable and less abrasion resistant. PLA is the only melt-processable natural based thermoplastic that can be melt spun into filaments which is often the most ecconomic and convenient method. However, the fiber can be also spun by a number of other methods. Dying can be accomplished with disperse or direct dyes using conventional processes when avoiding (strong) alkaline conditions.
The monomer is typically made from fermented plant starch such as from corn, cassava, sugarcane or sugar beet pulp. Several industrial routes afford usable (i.e. high molecular weight) PLA. Two main monomers are used: lactic acid, and the cyclic di-ester, lactide. The most common route to PLA is the ring-opening polymerization of lactide with various metal catalysts (typically tin octoate) in solution or as a suspension. The metal-catalyzed reaction tends to cause racemization of the PLA, reducing its stereoregularity compared to the starting material (usually corn starch).
PLA is used as a feedstock material in desktop fused filament fabrication 3D printers. PLA-printed solids can be encased in plaster-like moulding materials, then burned out in a furnace, so that the resulting void can be filled with molten metal. This is known as lost PLA casting, a type of investment casting. PLA can degrade into innocuous lactic acid, so it is used as medical implants in the form of anchors, screws, plates, pins, rods, and as a mesh. Depending on the exact type used, it breaks down inside the body within 6 months to 2 years. This gradual degradation is desirable for a support structure, because it gradually transfers the load to the body (e.g. the bone) as that area heals. The strength characteristics of PLA and PLLA implants are well documented. PLA can also be used as a decomposable packaging material, either cast, injection-molded, or spun. Cups and bags have been made from this material. In the form of a film, it shrinks upon heating, allowing it to be used in shrink tunnels. It is useful for producing loose-fill packaging, compost bags, food packaging, and disposable tableware. In the form of fibers and nonwoven fabrics, PLA also has many potential uses, for example as upholstery, disposable garments, awnings, feminine hygiene products, and diapers. Thanks to its bio-compatibility and biodegradability, PLA has also found ample interest as a polymeric scaffold for drug delivery purposes.
PLA is fully biodgradable and biocompatible which makes this fiber attractive for medical applications like wound dressing. The fiber is also useful as an eco- and people-friendly alternative to existing textile fibers for industrial and consumer apparel applications such as outdoor furniture, automotive interior fabrics, activewear, shoe linings, and disposable products like diapers and wipes, either at 100% or in blends with natural fibers such as cotton.
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