During our work experience we often answer to questions from our customers about the cover material suitable for pergolas and canopies. Every day we face questions as: -I have to put an canopy over the door to protect it against wind and rain, which material could be the best for the cover? -I need a pergola of size 400 cm x 600 cm: I can’t decide between plexiglas, polycarbonate or glass, what are the differences between these materials?
In this article we will try to illustrate the difference between these three materials.
Transparency. The transparency of the glass is very good with a transparency coefficient ranging from 0.80 to 0.90. Resistance. The glass normally used in pergolas and canopies is thermally treated. The treatment is used to increase the resistance, in this case it can reach a mechanical strength of 170 N/mm2, it resists to thermal shock, and in the event of breakage the fragments are little sharp. A 4 mm sheet of tempered glass has an impact resistance of a force equal to 10 joule. Regarding the load borne, a good glass canopy can support from 80 to 150 kg/mq depending on the models of cover used. Nevertheless, the glass has two major defects, first of all is a very heavy material, having a specific weight of 26,490 N/m3, so it is needed an important support for the plates that guarantees the seal once occurred the installation. Another “defect” of the glass is its easy to break, in fact, the tempered glass is likely to create the so-called “cobweb effect ” giving those unsightly signs of breakage even when subjected to a little stress, this effect occurs even after only a few years installation. In our opinion it isn’t the most suitable material for outdoor covers and canopies.
Polymethylmethacrylate (pmma) also called plexiglas from the name of manufacturer, is one of the most consumer plastic materials. Transparency. It is very transparent with a transparency coefficient about 0.93. Resistance. It has a good resistance to impact, weathering, moisture and an excellent resistance to chemicals. It is unbreakable and it can be extruded or cast. The extrudate has more uniform thickness but is less transparent contrary of the cast. A plate of 4 mm when subjected to impact has a resistance of 12 joules. The supported load in the event of a snow subjected to appropriate tests can be up to 150 kg/mq. The plexiglass is a good material for roofing, although it is industrially used in the furniture sector since if it is used outside over time it can harden and crack. It can be also easily scraped so it needs a good cleaning and maintenance. It also tends to yellow over time. For those who haven’t big needs can be a material adoperabile for outdoor covers.
Why the polycarbonate?
The polycarbonate, a polyester of carbonic acid, resulting from the organic compound called bisphenol, used in the synthesis of plastics. When we speak of polycarbonate we use two key words: strength and hardness. The polycarbonate material used in the construction of awnings and roofs is the compact polycarbonate (PCC). Its processing is simplified because you can bend it cold given a strong malleability. For our canopies and covers after several studies, we decided to use compact polycarbonate sheets of 4 mm thick which are UV resistant. This seems to be the best solution and our customers confirm that every day. Transparency. It has a transparency index of 0.86. Resistance. It has an excellent impact resistance, it is unbreakable and very weather resistant and does not require thermoforming. A sheet of 4 mm of compact polycarbonate resists to a force applied to 200 Joule. The load supported by an awning subjected to rigorous tests in the event of snow can be up to 170 kg/mq. The sheets have a sure guarantee of ten years. It is the plastic material more resistant to scratches and bumps, in addition to having an elasticity higher than the other plastic materials and has less tendency to yellow compared to the plexiglas. However, the possible yellowing over time is solved by UV treatment of the plates, which resist in this way to the effects due to sun exposure.
To the moon with the polycarbonate!
Yes, it is considered the best material even by the space plane industry. Indeed, the new copolymers of polycarbonate for extrusions are used in the aircraft, reducing their weight while ensuring an optimum safety. Thanks to its good light transmission and protection against the UV it is indicated in the production of rails, seat parts or lenses. Also according to the standard ASTM E662 polycarbonate has a very low level of toxicity satisfying the requirements of toxicity provided by airlines, contrary to the plexiglas. It is also used as a component of the suits of astronauts during spacewalks for the isolation from outside.