EFFECTS OF THE ENVIRONMENT ON PLASTICS
Environmental factors, such as ambient moisture, chemicals
(liquid or vapor), exposure to sunlight, high temperatures, hot water and/or
steam, bacterial/fungi (underground conditions), and irradiation all tend to
attack plastic materials. Materials may not only change appearance, but have a
significant decrease in properties, such as impact and tensile strength. Again
check the supplier's literature carefully.
Plastic materials do not rust or corrode and many
plastics perform significantly better than metals in corrosive environments.
Also understand that the MORE CHEMICALLY RESISTANT a plastic is, the MORE
DIFFICULT it is to bond to since bonding generally requires some chemical
attack.
Chemical resistance is also a critical factor if the part
is to be PAINTED. The solvents in the paint must be compatible with the
material to be painted. It is best to use paints recommended by the material
supplier.
Gaskets, "0" rings, or other dissimilar materials that
will be in intimate contact with a plastic over a long period of time MUST not
contain chemicals,solvents, or plasticizers that will leach out and attack the
base material. Flexible vinyl is an example of a material softened by a
chemical additive. This vinyl is also a good example of plasticizer migration
(outgassing) from pieces inside a car and it ends up fogging the windows.
The outgassing of volatiles is accelerated when the
material is exposed to high temperatures and/or vacuum. In critical
applications requiring no outgassing, a material must be selected that does
not contain any plasticizers or other additives that can outgas. Often,
pre-baking the material at a temperature slightly above the application
temperature will drive out most of the volatiles. Check with the material
suppliers. Materials such as polycarbonate, acetals, nylons, and acrylics have
been used in these applications.
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