PLA-starch-nanocrystalline cellulose films find biodegradable packaging applications.
The poor degradability of conventional plastics makes them one of the worst pollution menaces of modern times. On the other hand, polylactic acid (PLA)-based packaging systems are biodegradable and have additional benefits due to their high strength.
Despite the low cost, abundant availability and fast biodegradability of starch, starchbased films have poor mechanical and barrier properties. Scientists at Kansas State University sought to overcome this problem by blending starch with PLA and nanofillers, such as nanocrystalline cellulose (NCC).
The researchers also used a compatibilizer, methylene diphenyl diisocyanate (MDI), to improve the poor interfacial adhesion between hydrophobic PLA and hydrophilic starch. The researchers found that a PLA-starch-NCC nanocomposite could find use as a biodegradable food packaging film.
In experiments, up to 40% starch was melt-blended into PLA along with NCC, at 1% levels, and MDI, at 4% levels, before being pressed into 0.05-mm films. The scientists characterized mechanical, morphological and thermal properties using commercial testing systems, x-ray diffraction, transmission electron microscopy and differential scanning calorimetry. The water vapor transmission rates (WVTRs) and oxygen transmission rates (OTRs) of the films were studied to evaluate barrier properties.
XRD and TEM studies showed that the nanofillers were dispersed uniformly in the polymer matrix. The enthalpy--the thermodynamic quantity equivalent to the total heat content of a system--of crystallization decreased from 23.15 joules per gram to 14 joules per gram with the addition of starch to PLA. This indicated that the starch granules might restrict the molecular motion of the PLA matrix, resulting in decreased crystallinity.
Adding NCC and MDI had no significant effect on the crystallinity of PLA-cellulose starch (CS) blends. The tensile analysis showed an increase in tensile strength with the addition of NCC and MDI to PLA-CS blends.
However, tensile strength decreased with the addition of starch to PLA. OTRs and WVTRs increased with the addition of NCC and MDI to PLA-CS blends, but decreased with the addition of starch to PLA.
Further information. Sajid Alavi, PhD, Department of Grain Science and Industry, Kansas State University, 201 Shellenberger Hall, Manhattan, KS 66506; phone: 785-532-2403; fax: 785-532-7010; email: email@example.com.