Extruded polystyrene

Extruded polystyrene (or XPS) foam insulation is very similar to Explanded polystyrene (EPS) but is in someways a higher quality denser product, so more hard wearing and water-resistant with equally or better insulating qualities. Often used in applications where moisture resistance and some strength is important such as below ground or below slabs.

XPS is made by heating polystyrene resin beads or granules to a high temperature in an extruder, blowing agents are added, along with colouring agents to create the denser closed cellular structure. Common blowing agents used to produce XPS have been hydrofluorocarbons; HFC-134a and 142b.

However much of the industry has moved away from the use of hydrofluorocarbons (HFCs). HFC-134a by example has a GWP of 1,430, meaning it traps 1,430 times as much heat as carbon dioxide does over 100 years. Other options also exist such as blends which contain hydrofluoroolefins (HFOs).

In the 1980s Chlorofluorocarbons (CFCs) which had both high ozone depletion (OD) and Global Warming Potential (GWP), were replaced by Hydrochlorofluorocarbons (HCFCs) which also deplete the ozone layer and had high GWP but to a lesser degree, these were then replaced by hydrofluorocarbons (HFCs) which had minimal effect on the ozone layer but are potent greenhouse gases, in turn and in some cases these have been replaced by Hydrochlorofluoroolefin (HFOs), sometimes used in combination with Pentane. Whilst Hydrochlorofluoroolefin is generally accepted as have far reduce environmentally damaging potential. The main variations used are HFO-1234yf, HFO-1234ze, and HCFO-1233zd. and whilst there are some criticisms regarding some of the variants it is currently considered the better alternative.

Importantly although polystyrene can be classified as either a thermoset plastic or a thermoplastic, XPS is classified as thermoplastics, meaning they can in theory be melted down and recycled so long as any site contamination and collection issues are overcome.

Another variation on cellular, blown or foam polystyrene which is extensively used in construction is Graphite Polystyrene (GPS). GPS has a polymer matrix which contains graphite, the high-purity graphite particles are introduced in to the raw material polystyrene beads, which are heated at high temperature and then treated with steam to create the cellular dense structure, in a similar way to standard polystyrene. The inclusion of graphite particles helps to increase the foam's resistance to the flow of heat by reflecting heat energy to a greater extent than standard polystyrene, also containing small pockets of air as with other cellular foam boards.

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