Products Comprising Polylactide Plasticized with Poly(ethylene glycol) and its Derivatives
Publication Date: 2004-Jan-27
The IP.com Prior Art Database
http://www.cargilldow.com/corporate/release.asp?id=94: URL [+15]
Polylactide (PLA) plasticized with poly(ethylene glycol) (PEG) is unstable. In time, some of the PEG exudes to the surface of the composite; some PEG forms separate domains and / or crystals within the PLA; and a fraction of the PEG remains dissolved within the PLA. This disclosure teaches various products that may benefit from the fugitive plasticization of PLA by PEG.
Richard Tanzer, Vasily Topolkaraev, Michael Lostocco
29 December 2003
PLA, polylactide, poly(lactic acid), PEG, poly(ethylene glycol), plasticize, durable, blend, blend instability, migration
Polylactide, or poly(lactic acid), (PLA) is a commercially available, sustainable, biodegradable, aliphatic polyester attracting considerable interest as a replacement for polyolefin and aromatic polyesters within consumer products.1 PLA has physical properties, such as high strength, wettability and thermoplasticity, which make it desirable for many applications. However, with a glass transition temperature ranging between 55oC and 65oC, PLA is stiff and brittle for room temperature applications. In contrast, with an obvious deficiency with regards to wettability, polyolefins possess far greater physical durability at room temperature with glass transition temperatures below 0oC.2 Clearly, identifying materials with the durability of polyolefins and the wettability of PLA would be desirable.
To reduce the brittleness of PLA, while maintaining wettability and biodegradability, some investigators have compounded PLA with poly(ethylene glycol) (PEG) and/or poly(ethylene oxide) (PEO).3, 4, 5, 6, 7 Nakafuku refers to these mixtures as "semi-miscible".4 Sheth reported that PEG and PLA are miscible at certain composition ranges, depending on the ambient temperature and the glass transition temperature (Tg) of the blend.5 Chen teaches us that "... poly(ethylene glycol) is miscible with polylactide in the range of 0-30 wt. %, ..." and that PEG enhances the impact resistance of PLA.6 Moreover, Jacobsen reports a decrease in elastic modulus and maximum tensile stress with improved elongation at break at higher concentrations of PEG.2 Martin has observed similar effects.8
In a complementary study, Hu has found that PLA plasticized with PEG is relatively unstable. The physical characteristics of a film prepared from 70% PLA resin and 30% PEG (8,000 molecular weight) changed considerably over time. Over the course of a month, "the films changed from low modulus elastomer-like materials to thermoplastic-like materials" with the modulus increasing by an order of magnitude and the fracture strain decreasing.9 At 10% PEG, this aging phenomenon was not observed.10 Hu also observed that as the PLA/PEG blend ages, PEG segregates from the amorphous phase of the blend with the rate and extent of this aging phenomenon showing dependence upon the temperature and relative humidity of the system.
General Description of Plasticized Polymers
In general, it is desirable for a plasticized polymer be stable during use. For most applications, the polymer is compounded with a plasticizer (or a plasticizer "package" which incorporates compatibilizers, fillers, surfactants, etc.) so that the plasticizer is uniformly distributed within the polymer matrix.