2002 Annual Report
Research
Processing and Process Modeling of Extrusion Cast Films/Sheets from
Agricultural Materials
Mrinal Bhattacharya, Professor
Ramaswamy Mani, Research Associate
Lon Johnson, Assistant Scientist
Funding Source
USDA National Research Initiative
Objective
The objective of this research is to evaluate the physical, morphological,
and functional properties of these films as a function of various process
conditions and blend compositions. The polyester will be modified by incorporating
a functional group capable of reacting with functional groups on natural
polymers. Numerical simulation of the calendering process will be attempted
using two different classes of rheological constitutive equations. Development
and validation of process models would aid in efficient product development
and result in a cheaper product.
Project Description
Packaging films and sheets constitute a sizeable portion of our municipal
solid waste, and their inherent non-biodegradability is a major source
of pollution. In this process, the melt from the extruder is used for
making large-volume, high-quality sheets and films continuously. Since
it is difficult to cast films from purely agricultural material in the
melt-state, blends of biodegradable synthetic aliphatic polyesters and
agricultural polymers such as starch or gluten proteins will be extrusion-cast
into films. It is expected that approximately 50% of the synthetic polyester
could be replaced with natural polymers leading to a significant value
addition. These blends would serve as alternatives to pure petroleum-based
polymers.
Results
Natural and synthetic polymers of various compositions were blended in a twin-screw extruder. These blends were then thinly sheeted using a coat hanger die attached to a single-screw extruder. The natural content in the blend varied between 5 to 50 % by weight. The mechanical and morphological properties were evaluated. At 50% natural content, the tensile strength decreased to a third that of the synthetic polymer. The use of a compatibilizer doubled the tensile strength for the 50% natural content blend. The sheets displayed equal strengths in the machine and transverse direction. The tear strength decreased as the natural content increased, the decrease being greater in the anhydride compatibilized blends than the un-compatibilized blends. The blends displayed two distinct glass transitions, one for each component, indicating phase separation. The crystallinity of the blends decreased as the starch content increased. This was confirmed using differential scanning calorimetry (DSC), where the melting endotherm decreased as the starch content increased. Gel-permeation chromatography (GPC) indicated that the peak position was at the same location irrespective of blend composition, indicating minimal degradation of starch moeities. The water absorption is diffusion controlled with a sharp initial burst of water uptake. Scanning electron microscopy (SEM) showed melting of starch granules that formed a co-continuous phase with the synthetic polyester. Increasing the natural content also increased the surface roughness of the sheets.