# Static Factors in Sitting Comfort: Seat Foam Properties, Temperature, and Contact Pressure
## 2 Influence Parameters  
### Influence of Foam Properties in Sitting Comfort Impressions  
As previously described, foam cushions represent the primary contact between the user and the seat. Thus, some authors studied the relationship between foam's physical characteristics and sitting comfort. This connection was assessed by means of subjective questionnaires and interface pressure measurements.  
[PERSON] et al. [22] related hysteresis loss with seating comfort once it correlates with resilience. Hysteresis Loss (HL) is defined as the percentage ratio between the area under the loading (0-75%) and unloading (75-0%) compression cycles Equation (1):  
\[HL(\%)=\frac{A_{0-75\%}}{A_{75-0\%}} \tag{1}\]  
Ideally, foams should present a low hysteresis loss percentage. This value increases as resilience decreases [4; 7]. Hysteresis loss is highly influenced by the cellular foam structure, specifically the cell/wall area ratio. During the compression cycle, the energy loss happens primarily due to the buckling affecting unrecovered cell walls and struts. Thus, to obtain low hysteresis, low cell/wall area ratios are required [23].  
The SAG factor is another measure of foam's suitability for sitting applications, obtained by the ratio of the compressive load at 65%, deformation (\(F_{65\%}\)), and the load necessary for 25% foam deformation (\(F_{25\%}\)), according to Equation (2):  
\[SAG=\frac{F_{65\%}}{F_{25\%}} \tag{2}\]  
Once the SAG factor provides an indication of the foam-supporting property, high values are associated with higher comfort performance. The supportive factor is influenced by the foam's cell structure and density. Regarding seating applications, an SAG value higher than 2.8 is recommended. Otherwise, users may feel the seat pad bottom [4; 7; 13; 23; 24].  
[PERSON] and [PERSON] [25] identified foam thickness and hardness as significantly affecting sitting comfort. Hardness is defined by the 25% Indentation Load Deflection (ILD). This relates to foam stiffness and defines the foam's resistance to deformation when applying a load. High values correspond to high firmness, whereas low values are associated with lower firmness [6]. Using foam samples with equal density and hardness but varying thicknesses (50, 75, and 100 mm), [PERSON] [26] concluded that foam thickness does not influence hysteresis loss but influences the 25% load as thicker foams reported higher forces.  
[PERSON] and [PERSON] [13] conducted a study to relate the influence of foam properties, such as density, SAG factor, and hysteresis loss, in sitting comfort impressions. The sitting impressions of 12 subjects were obtained to judge the comfort of four foams with equal hardness (\(\approx\)205 N) but varying density (45-65 kg/m\({}^{3}\)). The high-durability seat (55 kg/m\({}^{3}\)) was rated as the most comfortable, whereas the lowest-density foam (45 kg/m\({}^{3}\)) was the least. Moreover, the low-density foam presented the highest hysteresis loss (34%). In contrast, the high-durability foam demonstrated a lower hysteresis (29%). A high correlation was observed between the comfort scores and the SAG factor. A second experiment was performed to evaluate the influence of the foam's hardness. The same 12 subjects rated the comfort perception of five foams with 25% ILD hardness between 120-285 N. Results suggested that hardness influences static comfort, but the preferred foam varied between subjects. Additionally, it should be highlighted that harder foams present the lowest hysteresis loss. Similar conclusions were reported by [PERSON] et al. [4] when studying the production of PU foams with multiple layers. This foam is considered the best for seating applications, demonstrating a SAG factor of around 3, hardness of 150 N, and hysteresis loss of approximately 22.5%. Analysing the aforementioned studies, the ideal foam is characterised by a high SAG factor, low hysteresis loss, and intermedium to high density.