Method for the production of biodegradable foamed products
A method for the production of a biodegradable foamed product 7 from a base material and a blowing agent 1 . The base materials are mixed with a blowing agent and any other required additives 1 . The mixture 3 , after extrusion 2 , is placed in a microwave transparent mold 6 and processed in a microwave via distinct steps. The first step preheats the extrudate to a temperature just below the flash point 6 . The second step 7 rapidly heats the extrudate beyond the flash point causing the extrudate to foam in the mold 7 . By utilizing this method it is possible to produce shaped articles with uniform properties and with packaging properties such as compressibility, resilience and shock absorption.
1. A method of producing a thick-walled biodegradable foamed product, the method comprising the steps of:
a) extruding a starch-based biodegradable polymeric base material containing water as a blowing agent, at a temperature below the flash point of water and with a level of shear which causes only medium levels of macromolecular degradation of the composition, to obtain a product with a water content of between 20% and 50%;
b) conditioning the extrudate of step (a) at a controlled temperature and humidity to obtain a conditioned extrudate with a moisture content between 5% and 20%, wherein the moisture content of the conditioned extrudate is less than that of extrudate of step a) and with viscoelastic properties which are high enough to prevent rupture of the extrudate during heating and low enough to prevent shrinkage post heating;
c) transferring the conditioned extrudate of step b to a mold; and
d) heating the conditioned extrudate of step b using microwaves from a microwave heating device producing microwaves at a frequency of 2450 MHz characterized by the steps of:
i) preheating by applying said microwaves in a first heating step to the conditioned extrudate in the device of step d to a temperature immediately below the flash point of the blowing agent to achieve uniform temperature distribution throughout the extrudate;
ii) and then in a second heating step different from said first heating step and after achieving said uniform temperature distribution throughout the extrudate, by controlled rapid heating of the preheated conditioned extrudate of step d(i) by applying said microwaves in the device of step d through the flash point of the blowing agent;
thereby forming an expanded foam product with a uniform closed cell structure, a medium density and resilient properties.
2. The method of claim 1 , wherein microwave heating in step (d) is effected by one or more microwave generators.
3. The method of claim 1 , wherein more than one microwave generator is used in step (d), and the microwave generators are tuned to a frequency selected from the group consisting of: all generators at the same frequency, all generators at different frequencies, and combinations thereof.
4. The method of claim 1 , wherein the starches are selected from the group consisting of: cereal, root, tuber, modified starches any combination thereof.
5. The method of claim 1 , wherein the base material is tapioca starch.
6. The method of claim 1 , wherein at least one additive is included with the base material selected from the group consisting of: plasticizers, nucleating agents, strength adjusting agents, viscosity modifiers, adhesion promoters, processing aids, fillers and any combination thereof.
7. The method of claim 6 , wherein the plasticizer is polyvinyl alcohol.
8. The method of claim 6 , wherein the nucleating agent is magnesium silicate.
9. The method of claim 1 , wherein at least one additive is included with the base material selected from the group consisting of: other non-water blowing agents, flame retardant agents, anti-fungal agents, mold inhibitors, and any combination thereof.
10. The method of claim 1 , wherein the extrudate in step (b) is conditioned at a controlled temperature range of 15 C to 40 C—and a relative humidity between 25% and 80% to obtain a conditioned extrudate with a moisture content between 12% and 16%.
11. The method of claim 1 , wherein the conditioned extrudate in step (b) has a maximum moisture content of 16%.