Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)

Department / School

Mechanical Engineering

First Advisor

Todd Letcher


Single screw extruders are versatile and simple devices which can be used in multiple processes such as injection molding, fused layer modeling (FLM) 3D printing, and polymer foaming. Extrusion uses polymers as material. A common biopolymer used in extrusion is PLA which degrades into environmentally friendly compounds but has weaker properties compared to petro-polymers. Additives can be added to PLA to increase its properties to make a more competitive material. Additives could be plasticizers like corn oil, soybean oil, or citric acid, or chain modifiers like BTCA. The properties of PLA investigated in this paper are the ultimate tensile strength, strain, modulus of elasticity, and viscosity. Baseline properties of the Neat PLA pellets were found to be an ultimate tensile strength of 60.5 MPa, a strain at yield of 1.50%, and a viscosity of 56 kPa*s. Pellets of modified PLA are created by mixing PLA and additives in an extruder and repelletizing the filament. Repelletized PLA resulted in decreased ultimate tensile strength of 52.5 MPa caused by thermal degradation during extrusion. All degradations tested were photodegradation, hydrolysis, and thermal degradation. It was found that the extrusion process was not adequate to mix additives enough with PLA to produce the expected effects. None of the additives listed above were able to retain or increase the strain of the material. Professional Epoxidized Soybean Oil was able to retain the tensile strength of material, with a strength of 52.4 MPa, which is comparable to the strength of Repelletized PLA. Additives which decreased the viscosity of the material, Pure Citric Acid 38.0 kPa*s and BTCA 22.4 kPa*s, could result in improved properties if the extruder was run at lower temperatures.

Library of Congress Subject Headings

Polymers -- Deterioration.
Plastics -- Extrusion.
Polylactic acid.
Polylactic acid -- Biodegradation.
Polylactic acid -- Additives.


South Dakota State University

Available for download on Friday, May 15, 2026



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