In principle, a sofa, and, more precisely, the soft part can be divided into two areas: the intensive and non-intensive workload. (As opposed to Tony Parker). The part where you sit, are regularly subjected to maximum loads (the weight of your body). Accordingly, it should not only be soft, but durable. There are important technical characteristics of foam rubber. Mark foam define its two main characteristics: apparent density (hereinafter – the density) and stiffness.
Density – the value of showing how much foam is contained in unit volume. The density of the foam depends on longevity. Under the influence of foam load, over time, changes its form (sags) and crumble. The higher density, the longer it will last. Rigidity – the physical quantity, equal to the force of resistance to compression of the foam. Stiffness of foam is characterized by tension and compression depends on its chemical structure (stiffness of cells). The higher the number, the more rigid foam. The marking is always present both of these indicators.
For example, grade ST 2534 is interpreted as follows: ST – standard foam (brand), 25 – weight (25 kg. In one m3), 34 – compression stress (34 kPa). Hardness and density – the indicators are not related to each other. Foam rubber can be high density and low stiffness (elastomeric) and vice versa (hard). Important conclusion. If an airbag soft, it does not mean that the foam quickly "prosyadet." Seat foam must be on one side – soft (for comfort and entertainment), on the other hand – rigid (able to hold and distribute the load).