Most specifier complaints about acrylic come from reading the TDS in the wrong order. The first page is usually a product family, the last page is usually the test method, and the number the project actually depends on sits in the middle, frequently with a footnote.
Clause one: process, because everything else follows
Cast and extruded sheet do not share a property table. Cast PMMA has a slightly lower water absorption, better solvent-bonding behaviour and lower internal stress; extruded has tighter thickness control and a lower price. A TDS that lists one transmission figure and one heat deflection figure without stating the process is describing one of the two and selling it as both.
Clause two: transmission, with the thickness it was measured at
“92% light transmission” means nothing until you know it was measured on 3 mm. Transmission falls with thickness, and on a 20 mm cast panel the realistic figure is closer to 88%. If your job is an aquarium window or a lightbox diffuser, demand the transmission at your thickness, otherwise you are comparing a 3 mm sample against a 20 mm order.
Clause three: thermal limits, and which one you need
Vicat softening point around 105 °C and heat deflection around 95 °C at 1.8 MPa are typical for cast PMMA. Neither is a service temperature. The continuous service limit for a load-bearing outdoor panel is nearer 60 °C, because the number that matters is where creep begins under the actual load, not where the sheet deforms under a standard test bar.
Clause four to six: water, UV and tolerance
- Water absorption 0.3–0.4% by mass after 24 hours — relevant to aquarium bonding and to dimensional change on a wet site, not relevant to a dry signage panel
- UV stabilisation stated as a wavelength cut-off and an expected outdoor life; demand both, since “UV resistant” alone covers a 3-year film and a 10-year grade equally
- Thickness tolerance expressed as a percentage above 25 mm and as an absolute figure below it, because a blanket ±5% turns into ±1 mm at 20 mm and ±10 mm at 200 mm
Where a TDS and a quotation disagree, the quotation wins on what you will receive and the TDS wins on what the material can do. Keep both, and mark the grade on the drawing. For the thickness side of the same argument, thickness tolerance explained goes further, and cast versus extruded covers the process choice.
One clause that buyers should read twice is the one on residual monomer and internal stress. A cast sheet with high residual monomer crazes earlier under solvent cleaning, and the TDS figure is usually a maximum rather than a typical. For a panel that will be cleaned daily, ask for the typical value and the test method, not the limit.
The last practical point is that a TDS covers a grade family, and the grade you receive is the one on the carton. Print the grade designation on the drawing and on the purchase order, and ask for the retained sample. When a failure is investigated a year later, that designation is the only thing that ties the installed panel to the tested material.
Ask for a specimen TDS with your grade and thickness at oversales@rpplexiglass.com; we will mark which clauses apply to the sheet you are buying.
The three misreadings below account for most disputes over material performance.
Frequently Asked Questions
Q: What does a TDS tell me that a data sheet from a distributor does not?
A: A TDS states the grade, the process and the thickness at which each figure was measured. A distributor sheet usually repeats the headline numbers without those three conditions.
Q: Is 92% light transmission accurate for thick acrylic?
A: Only at 3 mm. At 20 mm cast sheet the realistic figure is about 88%, so always ask for transmission at the thickness you are ordering.
Q: Can I use the Vicat softening point as a service temperature?
A: No. Vicat is a test condition. Continuous service for a loaded outdoor panel is nearer 60 °C, where creep rather than softening becomes the limit.