Extruded vs. Cell cast Acrylic How to test for difference?

Just guessing, but extruded may show orientation under crossed polars.

Also, cast may contain crosslinking agents and would be insoluble or just swell with common solvents.

Frank

When you burn it, you can hear the difference: cast makes a sizzling sound, while extruded and injected PMMA burn almost completely silently. On top of that experienced people can feel it: stick the lighter in the melt and try to pull a thread; extrusion feels different from cast.

And before people start bashing me: this is actually the way we do it here in Hong Kong, in the plastic wastes trade. It's the fastest way to identify a polymer: burn it, look at it, smell it.

Wouter.

What is the shape of the PMMA? If it is an injection molded part, it is likely to be a copolymer of about 90-97% MMA and 3-10% EA (ethyl acrylate) of relatively modest molecular weight.

If it is a flat sheet, how thick is the sheet? If it is less than 1/4" (6mm) it is likely to be extruded sheet with composition similar to the injection molded PMMA. If it is thicker than 1/2" (12mm) it is likely to be cast, which is likely to be close to 100% MMA.

It the part was thermoformed from sheet, the sheet probably was cast (cell or continuous belt cast).

If you have a good DSC, and have lots known reference samples, the Tg can give you a rough indication of the % acrylate comonomer. NMR, or other techniques (GC, mass spec., etc.) can be used to determine comonomer content. If you have samples with known comonomer content, you could try FTIR. It may take sometime to find the right wavelengths, and establish calibration curves, but FTIR probably would be the best least expensive method for testing lots and lots of samples. You will need about 10 mg for DSC, about 100 mg for the other tests.

Ernie

What I have is two 1/8" sheets. On is cast one extruded. I'm aware that burining can identify the type, but the trick here is no burning or no other destruction of the material. The polarizer trick did show some difference when shining light into the edge of the acrylic, however this is not always consistant.

I sure OSHA would frown upon sniff testing of burned plastics. John

Polarized light will show stresses but if material is oriented and placed between crossed polars, you should see orientation as material being lighter in one direction. This works for fibers and films which are stretched but may not be as evident in acrylic sheet. Frank

John,

If you have access to Ostwald viscometers (my guess is $~50/tube), a constant temperature bath, a stop watch, an analytical balance, toluene or other solvent for PMMA, and can spare 0.1 g of each sheet, you can determine which sheet has a higher MW. The higher MW sheet is the cast sheet. (If one of the samples does not dissolve, that sample is cast.) For method see

other references, search Google for Ostwald viscometer PMMA

Alternatively, you can try to measure melt viscosity with a melt indexer, but you need a melt indexer;

something similar, and this will require 10-20 g samples.

If the sheets have clean cut edges, look at the sheets sideways, so that you are looking through a significant distance ( 20+ inches) of the sheet. A cast sheet should have better light transmission than an extruded sheet, but this may require practice to make it useful.

Ernie