Coextrusion Makes Transparent Sheet More Economical
Polymethylmethacrylate (PMMA) and styrene-acrylonitrile (SAN) are important materials for transparent applications. SAN has an unusually high stiffness and low density, but exhibits little wear resistance. PMMA, in contrast, has exceptional abrasion resistance, but is not particularly stiff and has a high density. The strengths of both materials can be combined by coextruding a layer of PMMA on either side of a SAN core layer. For test purposes, sheet with a core of SAN and two-sided skin of PMMA were extruded at nominal thicknesses of 50, 100 and 200 µm.
The abrasion resistance of the coextruded sheet evaluated on the basis of abrasion and scratch tests. The SAN/PMMA sheet was found to be just as abrasion-resistant as pure PMMA sheet and clearly superior to SAN sheet. The scratch resistance of the coextruded sheet depends on the thickness of the PMMA layer. Starting at a sheet thickness of about 150 µm, the scratch resistance of pure PMMA sheet is reached. High UV resistance results if the PMMA layers absorb the UV radiation completely before it can penetrate to the SAN core layer. This requires either a high percentage of UV additives in a thin PMMA layer or a low percentage of UV additives in a thick PMMA layer.
On average, the flexural modulus of SAN/PMMA sheet is 11% higher than that of PMMA. This increase in modulus means increased stiffness of 3 to 4% compared to PMMA. While the stiffness and elongation at break of PMMA sheet drop noticeably with increasing temperature, this was not noticed with the SAN/PMMA sheet. Coextruded sheet was successfully thermoformed into samples without difficulty, regardless of the sheet thickness of the thickness of the coextruded PMMA layer.
Would you like to subscribe to our Newsletters on plastics technology and profit from the latest information?