Fluoropolymer foams, process to make them and applications thereof

Abstract

Described is a process for making closed cell fluoropolymer foam, and the foam so made. The process includes subjecting a fluoropolymer resin an inert gas at a pressure higher than atmospheric to drive gas into the resin, raising the temperature of the resin to or above its softening point, and reducing the pressure while maintaining the temperature at or above the softening point of the resin, in order to expand the resin to result in closed cell fluoropolymer foam. The resin is cross-linked prior to expansion. The resulting foams can be used in various applications, such as in flotation devices and for making thermal and/or acoustic insulation.

The invention claimed is: 
     
       1. A process for making closed cell fluoropolymer foam comprising the steps of:
 (a) subjecting a fluoropolymer resin to at least one inert gas at a pressure higher than atmospheric in order to drive gas into the resin, 
 (a1) raising the temperature of the resin to or above its softening point, wherein steps (a) and (a1) can take place in any order or simultaneously or with step (a1) taking place before step (a), and 
 (b) reducing the pressure whilst maintaining the temperature at or above the softening point of the resin, in order to expand the resin to result in closed cell fluoropolymer foam, 
 wherein the resin is cross-linked prior to expansion. 
 
     
     
       2. A process as claimed in  claim 1  in which before step (b) the following steps are carried out:
 (a2) the pressure is lowered to a pressure higher than atmospheric pressure but lower than the pressure of step (a) and the resin is cooled to below its softening point to result in a partially expanded fluoropolymer resin, 
 (a3) the partially expanded fluoropolymer resin is heated to a temperature above its softening point under a pressure of inert gas or air. 
 
     
     
       3. A process as claimed in  claim 2  in which step (a) takes place with the resin in a first vessel and wherein, before step (a3) is carried out, the partially expanded resin is transferred to a second vessel. 
     
     
       4. A process as claimed in  claim 2 , wherein, the temperature of the partially expanded resin resulting from step (a2) is reduced to slow the loss of the gas from the resin. 
     
     
       5. A process as claimed in  claim 1  wherein the resin is cross-linked by irradiating it with an electron beam or with gamma radiation. 
     
     
       6. A process as claimed in  claim 5  wherein the radiation dose is from 5 to 200 kGy. 
     
     
       7. A process as claimed in  claim 1  wherein the closed cell fluoropolymer foam is substantially homogeneous. 
     
     
       8. A process as claimed in  claim 1  which is carried out only on said fluoropolymer resin and not on any other material simultaneously with said resin. 
     
     
       9. A process as claimed in  claim 1  wherein, a second non-cross linked resin is present whilst the process is being carried out, and the resin that is cross linked prior to expansion expands more than the second non-cross linked resin. 
     
     
       10. A process as claimed in  claim 1  which is a batch process. 
     
     
       11. A process as claimed in  claim 1  wherein the fluoropolymer resin is substantially free of nucleants. 
     
     
       12. A process as claimed in  claim 1  in which the fluoropolymer resin is, prior to step (a), extruded into the form of a sheet. 
     
     
       13. A process as claimed in  claim 1  in which the fluoropolymer resin is a PVDF homopolymer. 
     
     
       14. A process as claimed in  claim 1  in which the fluoropolymer resin is a PVDF copolymer and the proportion of VDF is at least 60% by weight. 
     
     
       15. A process as claimed in  claim 14  in which the fluoropolymer resin is a PVDF copolymer and the proportion of VDF is at least 85% by weight. 
     
     
       16. A process as claimed in  claim 1  in which in step (a) the temperature is from 15 to 135° C. above the melting point. 
     
     
       17. A process as claimed in  claim 1  in which in step (a) the pressure is from 20 to 1000 bar. 
     
     
       18. A process as claimed in  claim 17  in which in step (a) the pressure is from 20 to 800 bar. 
     
     
       19. A process as claimed in  claim 2  in which in step (a3) the pressure is from 5 to 200 bar. 
     
     
       20. A process as claimed in  claim 19  in which said pressure is from 5 to 40 bar. 
     
     
       21. A process as claimed in  claim 1  in which in step (b) the temperature is from 15 to 135° C. above the melting point.