INFLUENCE OF PROCESSING PARAMETERS ON THE MOLECULAR-WEIGHT AND MECHANICAL-PROPERTIES OF POLY(3-HYDROXYBUTYRATE-CO-3-HYDROXYVALERATE)

The influence of film blowing and melt extrusion under different proce ssing conditions on the mechanical properties and molecular weight of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) have been characterized. Natural polymers are generally more sensitive to the high temperatures used in processing than synthetic ones. Size exclusion chromatography (SEC) showed that the weight average molecular weight (M) over bar(w) decreased with increasing processing temperature and decreasing screw speed. A 50% difference in (M) over bar(w) could be achieved. The mod ulus of elasticity (E) exhibited only minor changes with processing co nditions and was consequently, not affected by the (M) over bar(w). Th e elongation at yield (epsilon(y)) and break (epsilon(b)) decreased si gnificantly with increasing processing temperature with a maximum at a screw speed of approximately 25 rpm. The stress at yield (sigma(y)) d ecreased with increasing processing temperature. The stress at break ( sigma(b)) showed a larger variation with processing conditions with va lues between 21 and 31 MPa. The blown film exhibited values of (M) ove r bar(w), E and sigma(b), comparable with those of the extruded sample s The film showed no yield before break and was rather brittle. Both e psilon(y) and epsilon(b) of the extruded samples decreased approximate ly linearly with decreasing (M) over bar(w), and a breaking point in t he tensile strength was evident at a value of about 150 000 g/mol belo w which the mechanical properties decreased rapidly. In addition, sigm a(y) and sigma(b) decreased with decreasing (M) over bar(w). The mecha nical properties are optimized at a processing temperature of 160-165 degrees C and a screw speed at about 25 rpm, although it is suggested that these conditions are not optimal if the desire is to achieve rapi d embrittlement and disintegration of the polymer. (C) 1997 Elsevier S cience Limited.