Figure 1. Polymerization of Poly(lactic acid).
A comparison of the properties of PLA with those of other common petroleum based thermoplastics is given in Figure 2 below. PLA can be processed on standard commercial extrusion, calendaring, thermoforming and injection molding equipment. However, there are some fundamental differences in processing characteristics and product performance arising from the inherent material properties of the resin. As the market for PLA grows, there is increasing interest in improving PLA properties to expand the available range of applications. Two areas of interest are impact modification to overcome the extremely brittle nature of the polymer, and increasing the melt strength of the molten polymer to access applications such as blown film and foaming which require a high degree of melt strength and melt elasticity during processing.
Figure 2. Comparison of PLA properties to several petroleum based resins (commodity resin pricing from Plastics News, March 30, 2009).
Thermoplastics are typically toughened through making blends with rubbery polymers such as low modulus polyesters, linear low Tg elastomers, or cross-linked core-shell impact modifiers. As is the case for many thermoplastics, core-shell impact modifiers have been observed to impart the highest degree of toughening in PLA4. These modifiers typically consist of a low Tg rubbery core encapsulated by a glassy shell that has good interfacial adhesion with the matrix polymer. When well dispersed, these modifiers act as nanoscale or microscale rubbery domains that dissipate mechanical energy to retard or arrest crack initiation and propagation through the polymer5,6.
Figure 3. A) AFM phase image of core-shell impact modifier dispersed in PLA. Dark regions (soft) correspond to rubbery domains. B) 40-mil-thick injection-molded PLA disk without impact modifier (right) and with 5% of a butadiene-rubber based core-shell impact modifier (left).
The addition of impact modifiers can dramatically increase the impact toughness of PLA by as much as several orders of magnitude. This mitigates cracking and chipping during processes such as thermoforming as well as improving the performance of the finished article. Impact toughening becomes increasingly critical for durable goods applications requiring higher impact strength and good low-temperature impact. The decrease in tensile and flexural modulus is proportional to the amount of modifier added (Figure 4) and can decrease the stiffness of PLA in applications such as blown film.
As can be seen in Figure 3B, this addition of impact modifier often results in a substantial decrease in clarity of the toughened blend. For many widely used PLA applications, such as thermoformed food packaging, high clarity in addition to improved toughening is required. Haze in a polymer arises from the scattering of light as it encounters regions of dissimilar refractive index materials such as dispersed modifiers or filler particles7. Particle size is also a factor as particles much smaller than the wavelength of light will not result in light scattering. Thus to minimize haze, smaller modifier particles with refractive indices similar to PLA are desired. To better match the refractive index of PLA, low Tg acrylics such as ethyl acrylate or butyl acrylate can be used to replace butadiene for the rubber core of the impact modifier. This optimization results in an impact modifier with very low haze (see Figure 5), although the toughening efficiency of the acrylic-based modifiers is generally not as high as that of the butadiene-based modifiers.
Figure 4. Mechanical properties of PLA with butadiene-based core-shell impact modifier.
Figure 5. A) 20-mil-thick extruded PLA sheet without impact modifier (left) and with 5% acrylic rubber-based core-shell impact modifier (right). B) Gardner Impact data of modified PLA on 15 mil extruded sheet
Improving Melt Processing
Another area of interest for PLA modification is in increasing the melt strength of the polymer. PLA has very low melt strength resulting in difficulties in processing the polymer with techniques such as blown film, deep draw thermoforming, or foaming, which rely on large draw down ratios or rapid controlled expansion of the melt. The melt strength of PLA can be improved by the addition of small amounts of linear high molecular weight acrylic copolymers. These copolymers are miscible with PLA resulting in a blend that is optically transparent. In this manner the melt strength of the blend (Figure 6A) can be increased by 50-100% over the neat PLA. Figure 6B qualitatively illustrates the effect of addition of acrylic melt strengthener to the PLA melt. The PLA melt containing additive is noticeably stiffer and holds its shape better than the neat PLA.
Figure 6. A) PLA strands analyzed on a Rheotens apparatus with and without acrylic copolymer. B) PLA without additive (left) and with 4% melt strengthener (right).
The melt strength of PLA decreases with decreasing molecular weight. As with other condensation polymers, PLA is subject to degradation through hydrolysis when melt processed in the presence of moisture. Thus drying of PLA pellets and PLA regrind is required prior to processing. When drying equipment is not available, the use of melt-strengthening additives has been shown to compensate for losses in melt strength due to hydrolysis (see Figure 7). Thus, the addition of 4% of an acrylic melt strengthener can improve the melt strength of PLA that has been processed without drying to levels above those of the virgin resin.
Figure 7. Rheotens data showing effect of acrylic melt strengthening additive on PLA processed without drying.
PLA is still a very new plastic. Its range of applications is rapidly growing and developing as processors look towards using this bioplastic in areas traditionally dominated by petroleum based resins. In addition to the additives presented here, much work is being devoted to addressing the issues of the low heat distortion temperature of PLA and increasing the rate of crystallization of PLA from the melt. There are also trends towards making blends of PLA with starch and other degradable bioplastics for completely biodegradable articles. On the other end of the spectrum, manufacturers are looking to blend PLA with thermoplastics such as polycarbonate or PMMA for making durable goods with an increased biobased content. With the current push towards sustainability coupled with steadily increasing global PLA production capacity the applications and innovations around PLA will undoubtedly grow in the coming years.
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