1. Field of the Invention
This invention relates to improvements in the thermal barrier properties of silicone resin/glass fiber composites. More particularly, it pertains to the use of a thermal treatment of composites comprising a polysiloxane (silicone resin) matrix with a glass or quartz fiber reinforcement embedded in such matrix in order to effect a chemical change in the structure of the composite from silicone resin into porous silicone dioxide, the latter having improved thermal barrier properties.
2. Description of the Related Art
Missile nosecones and other missile components have recently been manufactured as composite structures that consist of an outer thermal protection layer of a silicone resin/glass fiber material surrounding an inner structural layer of bismaleimide resin/graphite fiber. As the missile approaches high speeds, the surface of the composite structure can reach temperatures up to 1,200° C. for brief (less than 2 minutes) periods of time. These temperatures are well above the temperatures under which bismaleimide resin would undergo substantial degradation and destruction, even after short periods of exposure. In some applications other than in the field of space technologies, there exist polymeric surfaces subject to brief exposure to very high temperatures as well.
The purpose of the silicone resin-based layer is to act as a thermal barrier such that the underlying structural layers hopefully experience temperatures only below that at which degradation occurs. This thermal barrier is achieved when the precursor silicone resin is chemically changed to form a relatively thick layer of a porous silicone dioxide.
There is a need to enhance further the thermal protection properties of polysiloxane based composites. As will be seen, the creation of a porous matrix structure in general and of a porous silicone dioxide in particular is helpful in this regard. There have been efforts, however, to improve the susceptibility of this type of composite to erosion, mechanical damage (like impact) and chemical attack through surface modifications.
In particular, U.S. Pat. No. 5,824,404 teaches that oxygen plasma or quartz lamp treatment of an uncoated silicone resin composite can cause the conversion of the polysiloxane groups into silica in the first few micrometers just beneath the surface. This so formed silica is believed to improve the mechanical properties of the composite surface. But the prior art does not teach a method for improving thermal protection of the composite surface via such polysiloxane-silica transformation. The silicone resin composite layer itself provides only a primary, and insufficient, barrier against high temperatures.
While thermal protection is certainly known in the prior art, there remains a need for even better thermal protection technology. The method proposed herein provides such improved thermal protection.