Regain
The percentage of moisture in textile materials to dry weight is called moisture regain. Moisture regain is widely used in textile industry. If the dry weight of the sample is G0 and the wet weight of the sample is Ga, then the moisture regain W is:
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1. Moisture regain concept
Standard moisture regain
The moisture regain of textile materials when they reach equilibrium in standard atmosphere is called standard moisture regain. The actual moisture regain of fibers and their products varies with temperature and humidity. In order to compare the moisture absorption capacity of various textile materials, the moisture regain (standard moisture regain) measured after a specified time (balance) under uniform standard atmospheric conditions is compared.
The moisture regain of several common fibers at different relative humidity is shown in the table below.
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Corrected moisture regain
Weight is an important basis for trade valuation and cost accounting. Weight varies with different regain rates. In trade and cost calculation, textile materials are not necessarily in the standard state, and even in the standard state, the actual moisture regain of the same fiber material is not necessarily a constant.
For the convenience of weighing and nuclear price, it is necessary to establish a standard value for various fibre materials and products. This standard value is called the prescribed value of the prescribed moisture regain of textile materials. It should be noted that the value of the prescribed moisture regain is purely selected for the convenience of work and is close to the standard value. Average moisture regain in the state.
The stipulations on the nominal moisture regain of textile materials in various countries are usually formulated according to their own actual conditions, so they are not consistent, but there is little difference.
The nominal moisture regain of common textile materials in China is shown in table.
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& nbsp; comparison of standard and nominal moisture regain
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practice moisture regain
The moisture regain of textile materials is also called the measured moisture regain. The actual moisture regain represents the moisture content of the material at that time.
Definite moisture regain of blended yarn
The nominal moisture regain of several fibre blends, carding wool or blended yarns can be calculated by weighted average of dry weight blending ratio (other weight blending ratios, such as nominal weight blending ratio, can be calculated by referring to the following method), and is about one decimal point behind. The following is an example of blended yarn.
Let P1, P2, & hellip; & Pn be the first, the second, & hellip; the dry weight percentage of the nth fiber component (%), W1, W2, & hellip; & W n be the first, the second, & hellip; & n be the corresponding raw material pure yarn's nominal moisture regain (%), then the nominal moisture regain W of blended yarn is:
W mixing = (P1W1 + P2W2 + & hellip; +PnWn) & times; 100%
For example, the nominal moisture regain of 65/35 polyester-cotton blended yarn is calculated according to the above formula.
W mixing = (65% & times; 0.4% +35% & times; 8.5% & times; 100% = 3).24%
2. Factors affecting moisture regain
There are internal and external factors affecting the moisture regain of fibers.
internal cause
The effect of hydrophilic groups-nbsp; & nbsp; the amount of hydrophilic groups and the strength of hydrophilicity in the fiber molecule can affect its hygroscopicity. The more hydrophilic groups, the stronger the hydrophilicity, the better the hygroscopicity; the fibers with low degree of macromolecule polymerization, if the macromolecule end groups are hydrophilic groups, the stronger the hygroscopicity.
The crystallinity of fibers - nbsp; & nbsp; the lower the crystallinity, the stronger the hygroscopicity.
Specific surface area and voids of fibers-nbsp; & nbsp; the larger the specific surface area of fibers, the stronger the surface adsorption ability and the better the moisture absorption ability; the more the voids in the fibers, the stronger the moisture absorption ability.
Accompanying organisms and impurities-nbsp; & nbsp; different accompanying organisms and impurities have different effects. Cotton wax in cotton fibers and oil in wool fibers weaken the hygroscopicity, while pectin in flax fibers and sericin in silk fibers enhance the hygroscopicity.
external cause
Relative humidity-nbsp; & nbsp; under certain temperature conditions, the higher the relative humidity, the better the moisture absorption of the fiber.
Temperature effects - nbsp; & nbsp; In general, with the increase of air and fiber material temperature, the equilibrium moisture regain of fibers will decrease.
When the air velocity is fast, the equilibrium moisture regain of fibers will decrease.
3. Effect of moisture regain on textile materials
Effect on weight
The change of moisture regain rate will certainly cause the change of textile material weight. What we need to emphasize here is that the fair weight is the basis of payment in trade. Otherwise, either the buyer loses (the material is wet) or the seller loses (the material is dry) and loses the fairness of trade. This is also the main reason for trade disputes. First, in production, weight control per unit length (quantitative control), the weight of fiber material increases proportionally with the increase of moisture absorption.
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Effect on length and cross-sectional area
The volume expansion of the fibers will increase with the increase of moisture regain, which means that the transverse expansion is much larger than the longitudinal expansion.
Reason: Macromolecules are arranged along the axis, and the distance between molecules increases after moisture absorption, while the length of macromolecules does not increase.
Changes of Fabric Structure before and after Hygroscopicity
Hygroscopic expansion of fibers is not only the coarsening and hardening of fibers, but also one of the causes of shrinkage (shrinkage) of fabrics. When the fabric is filled with water, the moisture absorption and expansion of the fiber make the yarn diameter thicker, the bending degree of the yarn in the fabric increase, and the colleagues squeeze each other. It is the yarn that takes up longer in the warp or weft direction than the moisture absorption money. As a result, the fabric shrinks. However, the moisture absorption and expansion of fibers also have advantages, such as taps and raincoats, which can use them to absorb moisture and coarsen the fabric, making it more compact and difficult for water to pass through. Expansion is also an important factor in the wrinkling, drying and dyeing of fabrics.
Effect of
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on density
You can see from the left picture. The density of fibers increases first and then decreases with the increase of moisture regain.
Effect on mechanical properties
For most fibers, the strength decreases with the increase of moisture regain. A few fibers remain almost unchanged, while the strength of individual fibers (cotton and hemp) increases. The elongation at break of most fibers increases with the increase of moisture regain, while a few fibers remain almost unchanged. This is mainly due to the breakdown of crosslinks between macromolecules after water molecules enter, the reduction of intermolecular force and the easy slippage of macromolecules, so the strength decreases.
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With the increase of moisture regain, the fibers become soft and easy to deform, modulus decreases and entanglement, while the denser fabrics become rigid due to the popularity of the fibers; the friction coefficient of the fibers surface increases with the increase of moisture regain.
The change of moisture regain leads to the change of mechanical properties of fibers, and the change of mechanical properties affects the processing of textiles and the quality of products. If the moisture regain rate is too low, it will make the rigidity of the fibers bigger and brittle, easy to break in processing, and the electrostatic phenomenon is obvious; if the moisture regain rate is too high, it will make the fibers not easy to loosen, the impurities in the fibers are difficult to remove, they are easy to entanglement and twist each other, and the components on the machine are easy to twist, which will cause the fluctuation of carding, drafting, weaving and other processes. 。 The change of cohesion will also change the yarn structure and fabric quality, resulting in instability or change of yarn strength, hairiness, evenness, fabric size, fabric density, etc.
Effect on Thermal Properties
With the increase of moisture regain, the thermal insulation performance of textile materials decreases gradually, the cold feeling increases, the ignition temperature increases, the glass transition temperature decreases, the heat shrinkage rate increases, and the melting resistance of textile materials improves. The change of moisture regain has a great influence on the thermal properties of materials.
Effect on Electrical Properties
Textile material belongs to insulation material, but its insulation performance will decrease with the increase of moisture regain, dielectric loss will increase, and electrostatic phenomenon will be reduced.
The influence on optical properties
When the moisture regain increases, the optical refractive index of the fibers decreases. Light absorption will increase, color will deepen, photodegradation and aging will intensify.
