Semester

Fall

Date of Graduation

2024

Document Type

Thesis

Degree Type

MS

College

Statler College of Engineering and Mineral Resources

Department

Chemical and Biomedical Engineering

Committee Chair

Dr. Rakesh Gupta,

Committee Co-Chair

Dr. Ray Liang

Committee Member

Dr. Srinivas Palanki

Committee Member

Dr. Oishi Sanyal

Committee Member

Dr. Todd Stueckle

Abstract

Polylactic acid (PLA) is a biobased and compostable polymer that is increasingly being used as an alternative to petroleum-based, non-biodegradable polymers for single-use food packaging due to its environmental benefits. It is also commercially available in large quantities, and it can be processed like polyethylene. However, it has a higher water-vapor permeability (WVP) as compared to polyolefins and polyethylene terephthalate that are the incumbent materials. This results in a competitive disadvantage in employing PLA for packaging baked goods since the shelf life of the food is likely to be reduced. In the past, barrier properties of PLA were improved by incorporating nanoclay in PLA, resulting in the formation of a nanocomposite, but there is a concern regarding the potential adverse health effects from the use of nanomaterials. Additionally, PLA’s ductility is significantly lower than that of traditional polymers, and small deformations can cause rupture of the package; incorporation of nanomaterials further reduces the elongation-to-break.

Here, talc microparticles have been employed in place of clay nanoparticles as diffusion barriers in PLA. Indeed, talc is approved by the FDA for a variety of applications, making it safe for use with food. Since water molecules are expected to be transported through a nanocomposite or a microcomposite by the same solution-diffusion mechanism, the barrier properties in the two cases should be the same as long as the volume fraction of the two fillers is the same and the aspect ratio of the two platelets is the same. Validating this hypothesis was one of the objectives of the research. The other major objective of the current study was to enhance the ductility of the microcomposite, and this was done by blending amorphous polyhydroxyalkanoate (aPHA) with PLA for use as the matrix polymer. The aPHA was received from CJ Biomaterials as a masterbatch (P3HB4HB) composed of a mixture of 55% PLA and 45% PHA. This was an immiscible blend in which the glass transition temperature of the PHA was about 2 ºC, making it a rubbery polymer. Note that PHA is also a sustainable and compostable material.

To reduce WVP through PLA, talc microparticles were incorporated into PLA at weight percentages of 2.5%, 5%, 7.5%, 10%, 15%, and 20% using an internal mixer. Films were compression molded using a hydraulic press to have thickness values of 50-100 µm. WVP testing followed the ASTM E96 standard, known as the cup method; a desiccant is put in a cup, the test film is glued on top, and the cup is placed in a controlled humidity-and-temperature chamber. Moisture permeation takes place through the film, and the weight gain of the assembly over a period of 72 hours is measured. This allows for the calculation of the WVP through the PLA film. Results showed a progressive reduction in WVP with talc content, with a maximum reduction of 47% at a talc concentration of 10% by weight (4.99 vol%). Beyond this, WVP increased due to particle agglomeration. These results are comparable to those obtained by dispersing nanoclay in the same polymer and are the result of water vapor molecules traveling a longer distance due to the presence of moisture barriers. These results could also be explained in a quantitative manner by the theory proposed by Gupta and coworkers, employing aspect ratio values determined from SEM images.

PLA-aPHA blends were prepared with aPHA concentrations of 20%, 40%, 60%, 80% by weight and subjected to mechanical testing. It was found that the addition of aPHA improved PLA ductility, due possibly to the rubbery nature of aPHA. The best results were obtained at aPHA concentrations between 40% and 60%, and it was to this matrix that talc microparticles were added. Mechanical and barrier property measurements were done on this microcomposite. An elongation-to-break value that was 11.7% higher than neat PLA could be attained. Furthermore, at the same talc concentration, the WVP was 36% lower when compared to pure PLA and 53% lower than the PLA-aPHA blend. Comparisons could not be made with barrier property theory because SEM images revealed the presence of numerous cracks and microvoids that are present on the matrix of the composite. The results of this research, though, show that polymer microcomposites can be formulated by dispersing talc in a blend of PLA and aPHA. Thin films made from this material have desirable barrier and mechanical properties, and the films are both sustainable and compostable.

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