Both materials are of cognate chemistry as both are polystyrene compounds. However, performance characteristics of these materials as heat-insulators are non-comparable. The reason lies in fundamentally different production technologies of these materials.  

Plastic foam production process consists of several vapour steam curing cycles of polystyrene placed into a block mold. During this process initial pellets multiply boost and sinter. Pellet boosting obviously goes alongside with the process of pellet surface extension and micropores enlargement.    

With time links between pellets weaken and material falls into pieces (separate pellets). A weak physicochemical interaction of pellets accounts for a low flex strength of foam plastic in comparison to extruded foam polystyrene.

Extruded foam polystyrene production is based on the method of extrusion.

Specific conditions inside the extruder activate the conversion process of polystyrene.

In the process of extrusion the polymer is conversed in a way, different from foam plastic and thus, acquires different qualities and different material structure. At first, pellets melt and turn into a viscous mass – raw material passes into a plastic state. Therefore, further changes concern not separate “bodies”- pellets, but a single liquid-phase substance with a continuous cross-linkage.

Extruded foam polystyrene has a fine close cell structure. This is a solid chemical substance with intermolecular cross links which are much stronger than in foam plastic.

Cells are impermeable because they do not have micropores, as opposed to foam plastic, and thus, gas and water cannot penetrate through cells. Cell walls consist of continuous substance. Substances from outside can permeate only open cells which are located at the edge of an XPS board. I.e. the whole XPS board does not absorb water, vapor etc. from outside.

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