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Impermeable epoxy
- Thread starter jinhh
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No epoxy is completely impermeable to water or common solvents; all epoxies (and all useful thermosets) exhibit transport phenomena. You will probably need to quantify the level of permeability needed and then acquire quite hard-to-find permeability data.
Many thermoplastics will give improved performance over epoxies in this respect, but for effectively zero permeability with reasonable thickness a design with, for instance, a metallic layer in it might work. An FML such as Glare won't let moisture through, but even then any flaws such as cracks will do so.
I don't know much about amines (not any sort of a chemist). Having Googled them they look like they might diffuse through plastic a little bit more slowly than water does. Not at all sure about common plastics' chemical resistance to them.
Many thermoplastics will give improved performance over epoxies in this respect, but for effectively zero permeability with reasonable thickness a design with, for instance, a metallic layer in it might work. An FML such as Glare won't let moisture through, but even then any flaws such as cracks will do so.
I don't know much about amines (not any sort of a chemist). Having Googled them they look like they might diffuse through plastic a little bit more slowly than water does. Not at all sure about common plastics' chemical resistance to them.
Hey R, this is a tag team! The reason that epoxies can not be impervious to water and some solvents is that they are polar molecules, and so are many solvents including water. The important issue is the effect of the water/solvent on the material. Water reduces the glass transition temperature and hence reduces elevated temperature properties. These effects can be addressed by moisture conditioning specimens before testing.
However, in the case of epoxy adhesives, the effect of water is exacerbated because it may lead to hydration of oxides on the surface of metals leading to interfacial disbonding. The key to success is to treat the surface to prevent hydration of the oxides on the metal. The effect of water is slow and hence short term strength tests are not sufficiently discriminating to avoid selection of processes which may fail in later service.
The effects of other solvents is less pronounced unless the surface chemical bonds between the epoxy and the substrate are susceptible to chemical degradation, and this is usually addressed in the test program for selection of the adhesive.
Blakmax
However, in the case of epoxy adhesives, the effect of water is exacerbated because it may lead to hydration of oxides on the surface of metals leading to interfacial disbonding. The key to success is to treat the surface to prevent hydration of the oxides on the metal. The effect of water is slow and hence short term strength tests are not sufficiently discriminating to avoid selection of processes which may fail in later service.
The effects of other solvents is less pronounced unless the surface chemical bonds between the epoxy and the substrate are susceptible to chemical degradation, and this is usually addressed in the test program for selection of the adhesive.
Blakmax
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