5mm perspex for glazing

[aidy55]

I have built a pergola will be closing in the sides with some perspex. Most pieces will be around 0.9mx0.3m. Have two large windows to do at 2mx1m

 

Is 5mm perspex going to be ok. I could get thicker or i could get polycarbonate but i think perspex will be strong enough and stay clearer for longer 

[SteamyTea]

Ask for cast, rather than extruded, acrylic.

5mm is really quite thick and will be plenty strong enough, you could probably use 3mm.

 

https://plasticonline.co.uk/clear-cast-acrylic-sheet.html

[Beau]

Yep, go for high cast acrylic.

 

Perspex yellows quite quickly IME and polycarbonate is mighty expensive and doesn't have the same clarity. 

[JohnMo]

Write here comparing properties of each material

 

https://www.theplasticpeople.co.uk/blog/polycarbonate-vs-acrylic/

[SteamyTea]

43 minutes ago, Beau said:

Perspex yellows quite quickly

Perspex is a trade name (like Fibreglass and Hoover), but if it is a proper acrylic it should not yellow (acrylic is the only true optically clear plastic).

You can get acrylics that have rubber nanoparticles (smaller diameter than visible light wavelength) in them, this improves toughness, these can discolour over time.

The reason for picking cast over extruded is to do with the way that the polymers are aligned during manufacturing and processing.

Cast has the polymer chains randomly 'scattered' during curing, this makes for a homogeneous mix (thermal and optical properties the same in all directions).

Extruded, on the other hand, has the chains aligned in one direction (as it is a hot process that rearranges the already cured polymer). This means that there is a difference between directions, especially with thermal expansion, this can cause the sheet to ripple more in one plane than the other.

Cast usually costs more than extruded, but is easier to cut.

 

If hot processing acrylics i.e. vacuum forming, the sheets have to be baked in a drying oven for many hours to expel moisture. 

 

Polycarbonates, while immensely strong, do weaken with age as they are susceptible to chemical attack (mainly petrochemicals) and ultraviolet light degradation.

 

Another alternative that is extremely cheap is polystyrene, I have made my secondary glazing from it. 2 years on and it is fine. There are UV stabilised mixes for outside usage but I did not bother as it was an experiment (and one that paid for itself in 2 years).

 

(Even after 25 years out of the plastics/polymer industry, I still miss it in some ways)

 

[aidy55]

Maybe i will call up and check it out. Our roof is 3mm EZ glaze and its flimsy, although it is 4.4m long

 

3mm is going to be considerably cheaper so i dont want to get any thicker than i need to

 

Should i be getting exact measurements or subtracting 5-10mm 

[dpmiller]

I wouldn't expect McMordie's to be the cheapest source ever, but they *do* know their stuff...

[Alan Ambrose]

Acrylic is a bit brittle if it’s going to get any wind on it, polycarb is much tougher.

[aidy55]

Do you anyone else in Northern Ireland who can supply it, preferably precut. 

[SteamyTea]

4 minutes ago, Alan Ambrose said:

Acrylic is a bit brittle if it’s going to get any wind on it, polycarb is much tougher.

Not a very large window though.

[dpmiller]

B&Q used to keep polystyrene in stock, dunno about nowadays. Does it all need to be window-clear or is twinwall an option for any of it?

 

[Alan Ambrose]

Suggest buying a small bit of each material and, say, tapping it lightly with a hammer to simulate a stone hit.

 

Acrylic can be cut by your local laser shop, polycarb by a cnc router shop or both materials by hand with a jigsaw on slow speed (to stop it melting and gumming up rhe blade). Or an online place like cutmy.co.uk. Or a hand saw.

Edited September 15 by Alan Ambrose