Environmental Engineering Reference
In-Depth Information
PLA and PLA copolymers have also been designed to replace metal and other non-absorbable
polymers as therapeutic aids in surgery, including pins [24], plates [25], screws [25, 26],
suture anchors [27], and intravascular stents [9, 28]. The advantages of PLA and PLA
glycolide copolymers for these applications comes from the ability of the body both to
degrade the polymers and to metabolize the degradation products over time, leaving no
residual foreign material in the body [8]. The ability to tune PLA degradation times is also
relevant to the field of drug delivery, as drugs encapsulated in polymers can be released based
on the known degradation time of PLA [29] or PLA copolymers [30, 31], even allowing the
specific targeting of organs [32]. Other medical applications currently being pursued include
dressings for burn victims [33], substrates for skin grafts [24], and dental applications [9].
Medical usage in devices, sutures and drug delivery systems, while technologically important,
nevertheless represents only a small opportunity for displacing less environmentally benign
plastics.
Fortunately, PLA also has properties suitable for commercial fiber products. Fibers can
be produced from either pure liquid polymer, in a melt-spun process [34, 35], or from
polymer dissolved in an organic solvent, in a solution-spun process [36, 37]. Advantageously,
fibers produced from PLA have lower processing temperatures than PET fibers and therefore
require lower energy input during processing. In applications, non-woven fibers are able to
wick moisture without absorbing it and can therefore be used in products such as diapers.
Materials produced from woven fibers have additional desirable properties, including
favorable hand and touch, drape, wrinkle resistance, rapid wicking, and low-moisture
absorbance. Woven PLA fiber-based materials are highly resilient to wear, have excellent UV
resistance, and low inflammability. Applications for the PLA fibers or blends of PLA fibers
with other natural fibers, such as silk, cotton, or wool, therefore include clothing, carpets,
upholstery, and draperies [38].
The greatest opportunity for PLA to displace less benign materials is in the area of
packaging for both food and consumer products. This opportunity arises because the
properties of PLA allow improved function as well as diminished environmental impact. PLA
can be used in traditional polymer processing operations such as injection molding, blow
molding, extrusion, and extrusion coating. As a result, lids, trays, and clamshells used in food
handling can be thermoformed from extruded sheet PLA, even yielding products with higher
flex-crack resistance in living hinges (thin sections of plastic that bridge two parts) than those
made of polystyrene. In addition, thin sheets of many PLA variants possess high gloss,
excellent heat sealability, and clarity, allowing extruded thin sheets of PLA to replace
cellophane and PET in transparent packaging [39]. Bags for yard and/or food wastes form
another set of applications in which the physical properties as well as the biodegradable
nature of PLA can be used to advantage; bags are tough and puncture resistant but are
completely degraded within 4 to 6 weeks in a composting environment [39].
Extrusion coating of PLA for paper products is another set of applications with several
benefits. For example, paper coated with PLA does not require pretreatment for ink
adherence, whereas polyethylene (PE) paper coatings often do. PLA coatings also possess
higher gloss, greater clarity, lower coefficients of friction, and greater stiffness than PE,
allowing thinner paper to be used [18].
Some limitations do still exist, however, to the use of PLA in polystyrene-like
applications. These include primarily the low-melt strength (extensibility without breaking of
the molten state) and the relatively low temperature at which heat distortion begins to occur.
