The foaming processes of mixtures of cassava starch–water and cassava starch–
natural rubber latex blends have been carried out by compression moulding.
The appropriate conditions under which to produce expanded foam are as
follows: a temperature of 150°C, 10·8 MPa pressure, and a 2 min moulding
time. For the foam from the cassava starch with water as a blowing agent, it
was found that water levels in the range of 150–200% by weight of the dry
starch gave good conditions for foaming. The resulting foamed material has
a uniform closed cell structure. Regarding blending of cassava starch with
natural rubber, the natural rubber could not be dispersed in the gelatinised
starch when blended at a temperature of 70°C. To stabilise and prevent the
coagulation of natural rubber in the blending process, Nonidet P40, a nonionic
surfactant, was used. A suitable amount of Nonidet P40 was 1·5% by
weight of natural rubber latex. The compressive stress and the storage modulus
of the foam obtained increased (42–233%) with increasing natural rubber
content owing to the high elasticity of the natural rubber and its promotion
of more elasticity to the foams. When 2–5% of benzoyl peroxide by weight of
natural rubber was added to the rubber latex, the compressive stress of the
foam was further increased (20–118%) owing to vulcanisation of the natural
rubber. Furthermore, an addition of 15–30% of calcium carbonate by weight
of the dry starch of the blends was found to increase the compressive stress
and storage modulus of the foams (69–148%) and the hardness and brittleness
of the foams.