Development of advanced instrumentation for evaluating drying and stickiness of textile materials

Abstract

Constant Power Drying Rate Tester (CPDRT) offers fast (< 30 minutes) and versatile measurement for fabrics. The operating temperature and water supply rate of CPDRT are adjustable to simulate required skin temperature and sweat rate, respectively. The feature that weight change of the fabric is continuously measured on a heated plate is new. It allows real-time observation of the entire drying process when fabric is placed on heated plate supplied with constant power. This provides comprehensive information which is useful for fabric development. A set of 28 fabrics was tested by CPDRT. The temperature of bare sample platform was kept at 37 °C while water delivery rate was set at 10 ml/hr. The key parameter, drying rate (DRCP), ranges from 0.32 to 1.69 ml/hr. Statistical test are conducted to verify the validity and repeatability of CPDRT. A prototype of Constant Temperature Drying Rate Tester (CTDRT) was built to evaluate the drying rate of fabrics. The sample platform controlled at constant temperature, was placed onto the balance. Its temperature is adjustable to simulate different skin temperature. The heater was placed underneath the sample platform but it did not have any contact with the sample platform. This novel non-contact heating system demonstrated real-time measurement of water evaporation in the sample throughout the whole experiment. In addition to drying rate, comprehensive information can be obtained from CTDRT, ranging from absorption, spreading and other fabric drying features. In addition, negative pressure gradient system of CTDRT helps to maintain constant atmospheric condition at negligible wind speed, so enhancing stability and sensitivity of measurement. The drying rate (DRCT), a key parameter of CTDRT, of 28 fabrics investigated are found to range from 0.36 to 2.56 ml/hr at skin temperature of 37 °C. This range offers a high resolution to differentiate drying properties of fabrics. Validity and repeatability of CTDRT are also confirmed with statistical proofs. The testing duration for CTDRT ranges from 15 to 40 minutes, which is similar to other drying methods where heat is applied, but is much faster than the conventional non-heating methods. In comparing the drying rate testers, CTDRT is able to eliminate the contribution of moisture regain of fabric during measurement. However, CPDRT captures evaporation of moisture regain in drying curve. Without the effect of moisture regain, CTDRT gives better observation on water absorption and spreading properties of fabric than CPDRT. Based on comparing coefficient of variation of DRCT and DRCP, DRCT has better repeatability than DRCP. A subjective wet sensation assessment was conducted. 20 subjects participated in the assessment. They were presented with fabrics at different drying time. The fabrics at different drying time simulated garments dry during recovery period. The recovery period refers to the garment being dried after conducted light activities. A 2-arm fabrics driver was built to simulate human's daily movements. The driver is synchronized to drive specimen and reference fabrics on both forearms of the subjects. The 2-arm configuration can help to enhance reliability of assessment result. Since a garment should "dries fast" and "offers dry sensation" to offer the best wet comfort to wearer, a wetness factor (WF) is developed to include these two factors. WF is the area under curve of wetness rating against drying time of fabric. The smaller WF indicates that a user suffers less from wet sensation. WF ranges from 6190 to 9510 among seven tested fabrics. Fabric Drag Force Measurement System (FDFMS) was built to study the interaction between fabrics and a wetted simulated skin. It is equipped with a novel free-end dragging type sample holder. The sample holder facilitated intimate contact between fabric and simulated skin, and external pressure is not required to apply on fabric during measurement. This simulates natural elongation of fabric on skin in actual wear condition. FDFMS acquires drag force curve to show a full profile of "drag force against water applied to fabric". Therefore, FDFMS is a convenient tool to measure drag force at various water levels in a single trial. Static drag force (FS) from the curve simulates the force required for a wetted fabric started to be dragged along human skin. A large FS implies strong adhesion between fabric and skin. The FS and adhesion deform skin and muscle to evoke stickiness sensation. The peak drag force (FP) from the drag force curve is another parameter to compare stickiness between fabrics. The larger FP implies the stronger stickiness feel can be evoked by a fabric. 28 fabrics were tested by FDFMS. The FS ranges from 0.29 N to 2.37 N for simulated skin wetness at 6 mg/cm². Four of the 28 fabrics have no FP. The FP of remaining 24 fabrics is found to be between 0.51 N and 1.98 N. Validity and repeatability of FDFMS are also confirmed. A subjective stickiness sensation assessment is conducted. Its result, stickiness rating, represents the real stickiness sense against wetted fabrics. FDFMS results are correlated with stickiness rating. A multiple linear regression is conducted to predict stickiness rating by FDFMS's results. The R² value of linear regression is 0.79, so that FDFMS is able to predict stickiness sensation. It is found that stickiness and water absorption properties of fabric are closely related. In considering the drying and stickiness properties by the developed instruments, result of CTDRT and FDFMS may be used. In applying DRCT of CTDRT and a stickiness rating predicted by FDFMS, the fabric with the best drying and stickiness performance can be identified. A polyester fabric with mesh structure is found to have the best drying and stickiness performance among knitted fabrics in this study.