8. Conclusion

For static and dynamic measurements of samples, experimental devices were designed and implemented. Results showed that mechanical properties of PU foams are dependent on the strain rate, because the air in the initial stage cannot escape from the structure under dynamic loading, thus the air participates on the resulting foam properties. These results in increase of the main stress in the peripheral parts of the PU foam sample, as shown in the simulation of the sample loaded with harmonic signal. From the results of measurement of the relaxation, it was observed. Using the FEM simulation, the distribution of contact pressures was also compared. It was showed that the strain of the PU foam in directions perpendicular to the direction of compression leads to uneven stress distribution, which is reflected by the uneven distribution of contact pressures.

The described behavior influences properties of products containing PU foams. This behavior is significantly reflected in the construction of car seats. For example, headrests are used to capture the head in the rear impact (so-called whiplash). The energy absorption may be insufficient due to the setting of the headrest. If the backrest is fully inserted, a significant deflection of the spine in the area of the cervical vertebrae occurs, while if the headrest is correctly set, the spine is not deflected. Also, a dependence of material stiffness on strain rate has certain influence. The solution could be not only in the modification of the geometry and the anatomical design of the seat, but also in the change of the headrest stuff material which is not so dependent on the strain rate. The solution could be in a usage of foams with a highly porous structure and foams in the combination with fibrous structures that do not exhibit strong dependence of stiffness on the strain rate.

Furthermore, it has been shown that the PU foam is not capable to transmit tensile stresses. If the foam is under tensile stress, the stiffness will be very high with very low ability to deform. This means that low deformation and also the force cause a tearing of the cell structure. If the torn foam is further stressed, complete destruction occurs. This is especially important for mounting and assembling, such as inserting a seat foam, backrest, or mattress into the cover. This problem is further accentuated by relatively high surface friction of the foam to other materials. Therefore, it is suitable to use an interlayer that reduces the friction between the surfaces, e.g., interposed polyethylene film, which is removed after assembly or layer, which is inserted permanently. On the other hand, the foam that is already torn can transmit a pressure load without significant impact if the foam is placed in a cover or box that does not allow its distortion in a direction that is perpendicular to acting load.

Also, it has shown that molded polyurethane foam has a qualitatively different surface compared to the inner structure of the foam. The foam surface is closed, its porosity is very low, and its character approaches the integral foam. The surface also exhibits other mechanical properties, especially significantly higher stiffness in all kinds of loadings. This affects the load behavior, which is further influenced by the low permeability of the foam surface. As mentioned, the response to strain rate is a specific characteristic of foams and it is related to the cellular structure, especially its openness. If the surface is closed or only partially permeable to air, it influences the stiffness of the foam at a fast compression. The air is not able to immediately escape from the structure, and due to limited air compressibility, the rigidity of the foam increases. This can be seen especially in the first loading cycle, which is a crucial for impact during the crash when the human body is pushed into the foam. Therefore, it is appropriate to finish the surface in a suitable way, for example, by a grinding, perforating, grooving, etc. However, it should be noted that PU foam is invaluable material for static or quasi-static type of loadings. In particular, it is necessary to highlight behavior under compression loading. The strain-stress curve course is characterized by almost flat shape, so the force increases very slowly in large range of the deformation. This leads to the fact that the foam provides high comfort, because the foam very easily surrounds the body and reduces the contact pressure. This is not practically achievable for other materials and therefore PU foam is a material that is not easy to replace. Above all, PU foams exhibit balanced mechanical, physical, and thermal properties at low bulk densities and low weight. Also prduction costs are very low. That are the reasons for their use.
