Abstract:

Objective The current testing standards and methods for fabric water vapor permeability are only applicable to sheet fabrics withuniform thickness and flatness, and cannot measure fabrics with curved surfaces and fabrics with uneven thickness, which leads to the lack of systematic water vapor permeability measurement methods for bra products.

Method This paper proposes a method for measuring the water vapor permeability of bra cups to solve the above problems. The main body of the equipment adopts the idea of "differential method", and consists of multiple slender water vapor permeable columns with a ground area of 1 cm×1 cm that continuously change in inclination angles to form a breast model with an arc-shaped evaporation surface. The innovations of the equipment are as follows: 1. the evaporation surface is designed to fit the cup radian more closely; 2. the sodium polyacrylate solution with a mass ratio of m_PNNa∶m_H2O=1∶500 is used instead of the pure water, which approximately eliminates the water vapor resistance of the air layer above the evaporating liquid surface. According to the relationship between the evaporation rate of pure water and the thickness of the air layer, the relationship between the evaporation rate of pure water and the type of medium, the relationship between different concentrations of sodium polyacrylate solution and the obtained the corresponding curve fitting equations were adopted to calculate the corresponding evaporation coefficients K_air, K_angle and K_PNNNa. Then algorithm and program design were carried out and the Visual Basic programming language was adopted to write the model method cup water vapor permeability calculation program. The program realizes the selection of water vapor permeability area, the input of independent variables such as mass and time, and the calculation of water vapor permeability. During the measurement, under the environmental conditions of temperature of 25 ℃, humidity of 30%, and wind speed of 0.4 m/s, the water vapor permeable column covered by the cup sample with approximately m_PNNNa∶m_H2O=1∶500 sodium polyacrylate aqueous solution (keep the air layer height above the liquid level at 2 mm), the cup sample was fixed on the model with gauze to evaporate 6 h, mass the initial mass G₀, evaporation end mass G₁, evaporation time and evaporation area distribution were reoorded, and then input them into the user interface, and the permeability was calculate of by the program. The water vapor permeability of 4 cup samples with different air-ventilation hole areas was and compared the waster vapor permeability of fabrics measured by the upright cup method.

Results The results show that a strong correlation (R>0.6) exists between the water vapor permeability values of different cups measured by the model method and the value of the hole quantity (R>0.6), indicating that the measurement results of the model method proposed can characterize the differences in the water vapor permeability of cups with different hole areas. On the one hand, the cups of each type were measured three times, and the CV values obtained were all less than 6%. It can be seen that the coefficient of variation of the data measured by the model method is small, and the stability of the equipment is high. And the water vapor permeability of cups made of five kinds of fabrics plain cotton, flax, silk, wool and canvas cotton obtained by this method has a high consistency with the water vapor permeability of the corresponding five fabrics measured by the upright cup method (R>0.6).

Conclusion The water vapor permeability measured by the model method can be adopted to characterize the water vapor permeability of bra cups and fabrics. This test method solves the problem that the water vapor permeability of bra products cannot be measured and characterized at present, and can provide method support for the evaluation of the water vapor permeability of bra products for underwear companies, facilitating companies research on the heat and humidity comfort of bras. Enterprises can find out the problems and deficiencies of research and development products through measurement, and reduce the flow of bras that do not meet the water vapor permeability requirements into the market. For consumers,by marking the water vapor permeability performance data of bra products on product details, consumers can be provided with more convincing and objective consumption guidance.

Key words: water vapor permeability, bra, water vapor permeability evaluation of bra, thermal and wet comfort, facilitating permeability