Evaluation Of Fabric Contact Thermal Sensation And How It Influenced By Clothing Pressure

Thermal comfort is a subjective thermal judgment, and is originated from thermal stimuli signals transmitted to the brain through cutaneous thermal receptors when human skin touches with fabric. Thermal judgment is decided by the transient thermal transfer at skin touching with fabric, and influenced easily by the temperature difference and the contact area between skin and fabric, fabric thermal properties and cutaneous thermal sensitivity of skin. It has generally been recognized that thermal judgment changes with fabric thermal properties and surface characteristics. However, this opinion comes from studies neglecting skin thermal sensation so that less helpful for an accurate evaluation on fabric thermal feeling. Skin temperature (Tsk) changes with skin blood flow (SkBF), which is controlled by human thermoregulation. But previous researches make a simple assumption that Tsk is stable. Thermoregulation is a typical function of human being as homoeothermic animal, which controls heat dissipation from skin by regulation of SkBF to keep relatively constant core temperature. To be more complicated, thermoregulation is obstructed easily when blood circulation is affected by clothing pressure, then Tsk will change after invalidation of thermoregulation, the distal skin touch thermal judgment could shift with the fluctuation of Tsk. In the field of textile engineering several critical problems include:mechanism of fabric-skin touch thermal sensation, fabric tactile thermal threshold, blood circulation and thermal sensation of distal skin under clothing pressure.Evaluation of fabric-skin touch thermal sensation needs two steps. By recording intensity of thermal stimuli when different fabric touches with skin, firstly, we focus on fabric thermal properties and the main related factors, which influence fabric-skin touch thermal judgment Secondly, it is necessary for accurate characterization of fabric touch thermal threshold to evaluate the fabric-skin touch thermal feeling by the individual temperatures of fabric and skin. Thermal threshold and Tsk are strongly coupled and Tsk easily influenced by clothing pressure, then thermal judgment is affected. This study investigated the distal skin circulation and Tsk under different pressure simulation, and discussed the fingertip thermal sensation after a50mmHg external pressure on upper arm. It has been proved that Tsk difference is the main reason for the distinction of skin touch thermal sensation in gender.The impacts of fabric on both Tsk and the SkBF are significant, and the impacts intensity depends primarily upon moisture level and surface characteristics. In order to identify human capability in distinguishing the difference between different fabrics, three different factors related to fabric thermal conductive are chosen:fiber types, moisture level and surface structure. Five different fabrics are chose by the three fabric thermal properties, and then contact with skin. Fluctuation of local SkBF and Tsk are recorded before and during fabric contacting skin. There is a marginaf difference between the thermal conductive of cotton, polyester and wool fiber, and this marginal difference can’t lead to an appreciable distinction in the thermal stimuli. But, the five thermal stimuli of chosen fabrics are discrepancy due to the moisture regain of each fiber type. The moisture level of the cotton fabric samples has a considerable influence on Tsk and SkBF, and the higher the moisture level, the longer the duration of the impact. Wool fabric surface characteristics play a very important role in heat transient exchange between skin touching fabric, and more hairy fabric can constrain heat loss by holding more stable air. Therewith, the congregated heat between fabric and skin makes skin feel warm.Except for above-mentioned three factors related to fabric thermal properties, thermal stimuli intensity is affected by fabric temperature. If the fabric is conditioned at two temperatures with big enough difference, the thermal judgments from skin contacting each of fabrics will be contrary. From subjective thermal judgments at the instant of contact with one woven cotton fabric at various temperatures, fabric thermal thresholds are obtained and analyzed. The results indicate that the thermal responses from the subjects are in high agreement with each other at extreme temperature levels. As the temperature moves to more moderate level, there is significant difference between the thermal judgments by subjects. Besides the fabric temperature, fluctuation of Tsk also contributes to the uncertainty of thermal judgments among all of subjects for the one fabric at same temperature. From the above-mentioned experiments, this dissertation carries out a deep analysis on the tactile thermal threshold under cold conditions. It is observed that the thermal thresholds by females appear is lower than that by males. Meanwhile, relative of fingertip temperature to ambient temperature, the temperature difference of females is lower than that of male, so that the Tsk of female fingertip is more stable. And also, the higher Tsk of male than female in cold ambient showed that more heat between skin and fabric in the instant of contact is collected, and this results make male feel warm easier than female when contact with one fabric at same temperature.In addition, Tsk is regulated by SkBF, and SkBF is easily influenced by the external pressure. To investigate the thermal response of skin to cold stimuli at an equivalent clothing pressure, SkBF and Tsk of distal skin are recorded via a tourniquet at the right upper arm. The external clothing pressure below30mmHg is applied at the upper arm, and the results show no visible effect on the Tsk and SkBF at the index fingertip. For clothing pressure above30mmHg, SkBF fluctuations at the index fingertip became noticeable, and lead to dynamic change of Tsk of the fingertip. And also, due to the deformation of arm skin at increasing external pressure, the SkBF signals fluctuation rather than monotonously decrease, and this reflects the dynamic self-adaptive rebound of the muscle and the blood vessel. On the other hand, the thermal response of right forearm to cold stimuli is delayed by the50mmHg external pressure applied at right upper arm. The delayed responses and the partial obstacle of blood circulation of distal skin indicate that higher clothing pressure affects the wearer’s thermoregulation.Base on the significant influence of higher clothing pressure on the distal skin circulation and the Tsk, it is hypothesized that fabric-skin touch thermal judgments will shift. To test this hypothesis, both The Tsk and the tactile thermal judgments at right index fingertip are recorded at normal and under the influence of applied50mmHg external pressure. The experiment results showed that the temperature difference between skin and ambient is lower, the smaller the thermal threshold. Moreover, the volunteers’ Tsk at the index fingertip reduced significantly with higher external pressure applied at the upper arm because the Tsk approaches ambient temperature, which is lower than Tsk. And also, the drop in Tsk suppresses the variations in subjective sensory responses, and it makes more subjects felt warmer. Additionally, tactile thermal thresholds of female without external pressure are consistent with those of male under external pressure, and it demonstrate that female is not always more sensitive than male. In other word, Tsk difference results in the difference of thermal feeling perception in gender.Together the above results, it can be concluded that the perceived thermal sensation, the Tsk and SkBF form such a cross-coupled triad that jointly affects our thermal feeling of fabric tactile, and it is not enough to just emphasize fabric thermal properties for an evaluate on fabric-skin touch thermal sensation. Although the fabric-skin touch thermal sensation is influenced by fabric properties, skin thermal threshold and clothing pressure are understand better than before, however, it needs more careful examination of each single factor for fabric thermal property and fabric-skin touch thermal threshold under clothing pressure.