Can sheet extruded from Eastman Spectar™ copolyester be painted?
Many commercially available paints can be used to decorate Spectar sheet. As with any plastic substrate, there are factors that can affect adhesion. For example, Spectar inherently exhibits good chemical resistance; however, chemical resistance can cause marginal adhesion for certain paint types. Use only paint thinner designed for plastic (NOT metal), and evaluate its effect on the specific plastic sheet being used.
Ductile paints tend to work best on plastics and often use urethane-based chemistries. Acrylic paints, on the other hand, tend to be brittle. Some solvent-based paints designed for use with polycarbonate or acrylic may not be sufficiently active to provide good adhesion to Spectar sheet. Generally speaking, paint that produces a flexible, tough, dry film has better impact resistance than one yielding a rigid, brittle film.
Before applying paint, the surface needs to be cleaned and static buildup removed. This is easily done using a 50:50 mixture of isopropyl alcohol and water. (There are many commercial solutions that can be used to clean and remove static from plastic sheet. We encourage the use of these mixtures only after testing in an inconspicuous area and inspecting for visual crazing and hazing of the sheet surface.)
We also recommend testing adhesion on a sample sheet—but only after the paint has been applied and fully cured in an oven for 48 hours at 50°C (122°F). The “tape test” (ASTM 3359-B) can test adhesion by etching a checkerboard pattern on the decorated sheet and attempting to remove the paint in the etched area with tape, applying brisk, steady pulls.
Finally, when choosing paint, consider how your decorated part will be used. For example, will it be subjected to consumer abuse or will it encounter little to no contact? Then consider which side of the sheet you’ll decorate.
In general, second-surface painting or painting on the face of the sheet oriented inward is the most severe orientation because impacts on the outward-facing side tend to stretch the inward paint layer. Weak paint layers break easiest when stretched. In contrast, first surface painting or painting on the face of the sheet that is oriented outward is least severe because impacts on the side with the paint layer tend to compress the outer layer of paint. Compressed paint layers tend not to break as easily.
Whether spray painting or using other methods, always follow the paint manufacturer’s recommendations for proper paint thinner to paint ratio and drying times between coats.
To test paint:
- Coat paint on a sample sheet (include a flood coat if necessary).
- Cure or dry the prepared sample.
- Check adhesion (ASTM D3359-B) before aging in an oven at 50°C (122°F) for 48 hours.
- Check the impact of the aged sample at room temperature using ASTM D3763.
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