# Development and Characterization of Tailored Polyurethane Foams for Shock Absorption
## 3 Results
### Mechanical Characterization  
#### 3.2.1 Compression Tests  
The characteristic force-displacement curve of polyurethane foam (Figure 10) shows the three strain regimes typical of viscoelastic materials. The first part of the curve is linear and corresponds to the elastic regime. When the constraint was removed, the deformation of the sample was reversible. After the yield point, which is the end of the linear part, the stress-strain curve shows a long plateau at almost constant stress. This part corresponds to the platform. It is associated with the plastic deformation structure through a polyurethane foam brittle to the rupture of the cell walls. As the strain increased, the walls of cells came into contact with each other. When all the voids were filled, the resistance of the foam increased rapidly, proportional to the measured stress. This last section of the curve is called the densification scheme [48]. The curve representative of cellular compression shows a linear phase shadowed by a load plateau (force) and a densification step. The load increased considerably with displacement, behavior in three stages [49]. A slight dispersion was observed between the six specimens carried out under the same production conditions and tests [50]. The mechanical properties of polyurethane foam depend strongly on density, cell size and form, and the percentage of close and open cells. The foam may have a preferential orientation of the structure of the cells. Cells frequently appear elongated along the direction of expansion. It has been demonstrated that when the density of the foam increases, the elastic limit and plateau height rise [51], while the elongation of the plateau decreases [52]. This means mean that high-density foam is better at resisting the restrictions applied than \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Foams** & **Tg (\({}^{\circ}\)C)** & **Tm1 (\({}^{\circ}\)C)** & **Tm2 (\({}^{\circ}\)C)** & **Tdmax (\({}^{\circ}\)C)** & **Resolute \%** & **Td5\% (\({}^{\circ}\)C)** \\ \hline PUR\_1 & \(-\)36 & - & - & 382 & 9.5 & 282 \\ PUR\_2 & \(-\)57 & 120 & 188 & 367 & 17.8 & 254 \\ PUR\_3 & \(-\)43 & 137 & & 384 & 25.9 & 279 \\ PUR\_4 & \(-\)35 & - & - & 378 & 14.3 & 271 \\ PUR\_5 & \(-\)34 & - & - & 390 & 11.8 & 292 \\ PUR\_6 & \(-\)49 & - & - & 388 & 21.0 & 291 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Glass transition and melting temperatures of our PUR foams determined by DSC (10 K\(\cdot\)min\({}^{-1}\)). low-density foam. However, the structure of a high-density foam reaches the densification regime at low deformations. For rigid foams, the pressure increases due to condensation, and then the stress drops sharply at the end of the plateau. This medium corresponds to when the cell walls rupture and the foam structure collapses [53]. It is necessary to point out that these tests must be consolidated by studying the dynamic behavior of these elaborate foams. The insertion of bentonite and alumina particles into the foam cells increased the initial module in the stress-stream curve. On the other hand, if the particles introduced are more significant than the cells' dimensions, reinforcement is not effective [54]. Polyurethane foams have both stiff and flexible domains. Temperature or humidity increases degrade foam mechanical characteristics [55]. This loss of mechanical properties corresponds to a reduction in compressive strength. It is probably associated with the tear of the chains belonging to rigid domains (urea and urethane bonds) and the breaking of hydrogen bonds [56]. Determining the bulk density of the different polyurethane foams studied consists of weighing samples by an electronic balance to the accuracy of 0.01 g and calculating the volume of the test pieces. Average results obtained for untransformed (original) and loaded polyurethane foams are summarized in Table 5. Changes in mechanical properties of all studied samples before and after the tests were traced.