8.2.1 Properties of Hyperelastic Polyurethane Foams
Hyperelastic polyurethane foams are produced in the process of foaming polyurethanes, with the use of cross-linkers, foaming agents and catalysts. In the production of these types of foams, it is important that after seasoning the block, a mechanical opening of closed pores takes place. Properly opened pores provide the level of flexibility of the foam.
Foams are one of the basic materials used in the construction of subassemblages of upholstered furniture, both with springs and without springs. In particular in the latter, polyurethane foams form an essential part of the upholstery construction. However, due to the variety and availability of these raw materials, one should specify the rules for their selection, depending on the size and nature of the operational loads.
From the point of view of human physiology, maintaining sitting position for a couple of hours is not beneficial for the nervous and musculoskeletal system. In addition, the wrong distribution of weight on a seat can cause point loads on the cardiovascular system. As a consequence, long-term positioning of the body in the wrong position, on an improperly fitted base, often causes pains, changes in degenerative arthritis and blood clots, as well as superficial inflammation of the venous system of lower limbs (Smardzewski et al. 2007).
In production practice, highly flexible K-type foams, standard T-type foams and flame resistant foams are commonly used. Elastic properties of some of them are summarised in Table 8.1. Foam manufacturer’s signatures contain a letter and numeric part. The letter part means type of foams, the first two digits of the numeric part inform of the foam density in kg/m3 and the last two of the foam stiffness in kPa.
A characteristic feature of highly flexible polyurethane foams is their degressive-progressive, nonlinear stiffness. The division of the foam stiffness curve into three characteristic parts allows you to determine the stiffness coefficients and elasticity modules for each of the characteristic stages of deformation (Fig. 8.7). Stiffness coefficient k is expressed as the quotient of the load value P to the displacement A l:
while the linear elasticity modules E1, E2, E3 are determined for each part of the function P = /(Al) located between the points of inflection of the function and the beginning and end points of the curve, according to the scheme shown in Fig. 8.7.
Figure 8.8 shows the stiffness characteristics of highly flexible foams, and Fig. 8.9 shows the stiffness characteristics of standard-type foams.
From the stiffness analysis of polyurethane foams for the furniture industry, it appears that they are materials of nonlinear stiffness characteristics. This means that in the process of designing upholstered furniture, elastic materials should be chosen based on the selection of the specific foam stiffness and not the foam density, as it usually takes place in practice.