2002 Annual Report
Research
Viscoelastic Properties of Extrusion Cast Sheets of Natural and Synthetic
Aliphatic Polyesters
Mrinal Bhattacharya, Professor
Lon Johnson, Assistant Scientist
Funding Source
USDA National Research Initiative
Objective
The objective of this study is to determine the viscoelastic properties
of natural (starch and protein) and synthetic biodegradable aliphatic
copolyester blends.
Project Description
Due to thermo-mechanical processing and viscoelastic effects, stresses
develop in the manufacture of sheets. The manufacturing process for polymer
sheets involves blending and extrusion operations through a “coat-hanger”
die at elevated temperature, calendering to reduce thickness, and winding
on rolls for storage and transport. In secondary manufacturing, the plastic
roll is uncoiled and cut into sheets. Evolution of viscoelastic properties
during the processing affects the final characteristics of the sheets.
Polymers are viscoelastic materials and exhibit a time-dependent relationship
between stress and strain. This is particularly true of blended materials,
where often each material creeps or relaxes at different rates.
Results
Blends of natural and synthetic biodegradable polybutylene succinate were cast into sheets using a coathanger die and then subjected to stress relaxation and creep tests at various temperatures. The natural content was varied at 10%, 30% and 50% by weight. In some formulations, a small (5% by weight) amount of compatibilizer was added. The materials were blended using a twin screw-extruder, pelletized, and sheeted using a coathanger die. The decay of stress upon the imposition of constant strain showed two regions: an exponential and power law; the stresses relaxed sharply at the initial stage and then decayed at a reduced rate for the duration of the experiment. The addition of compatibilizers increased the time required for the stresses to relax compared to uncompatibilized blends of the same compositions. Similarly, as the natural content increased, the time taken to relax to a specified stress level decreased. Increased temperature enhanced the relaxation process. The relaxation can be modeled using a Maxwell Model. The initial strain of the creep curves was affected by the natural content; the higher the natural content, the lower the initial strain for the samples upon imposition of a constant stress. Similarly, the presence of compatibilizer in the blend reduced the initial strain for samples containing the same natural content. As the natural content of the blend decreases, the time required to attend the plateau compliance is reduced. Increased temperature increased the equilibrium compliance. These behaviors are described in terms of blend morphology. The empirical Struik and power law models can be used to fit the compliance data well.