The moisture regain of textile materials when they reach equilibrium under standard atmosphere is called standard moisture regain. The actual moisture regain of each fiber and its products changes with the conditions of temperature and humidity. In order to compare the moisture absorption capabilities of various textile materials, the moisture regain measured after being placed under uniform standard atmospheric conditions for a specified period of time (equilibration) ( standard moisture regain) for comparison.
01 Effect on weight
Changes in moisture regain will of course cause changes in the weight of textile materials. What needs to be emphasized here is that in trade, the public weight is used as the basis for payment. Otherwise, either the buyer will suffer (the material is too moist) or the seller will suffer (the material is too dry). Loss of trade fairness is also one of the main causes of trade disputes; in production, weight control per unit length (quantitative control) is carried out, and the weight of fiber materials increases in proportion to the amount of moisture absorbed.
02 Effect on length and cross-sectional area
The increase in moisture regain will cause the fiber volume to expand, and the transverse expansion is much greater than the longitudinal expansion.
reason
The macromolecules are arranged along the axial direction, and the distance between the molecules increases after moisture absorption, but the length of the macromolecules does not increase.
Changes in fabric structure before and after fabric absorbs moisture
The hygroscopic expansion of fibers not only causes the fibers to become thicker and harder, but is also one of the causes of shrinkage (shrinkage) of the fabric. After water enters the fabric, the fibers absorb moisture and expand, making the diameter of the yarn thicker. The bending degree of the yarns in the fabric increases, and at the same time they squeeze each other, causing the fabric to occupy longer yarns in the warp or weft direction than before absorbing moisture. The result is shrinkage of the fabric. However, the hygroscopic expansion of fibers also has a beneficial side. For example, hoses and raincoats can use their fibers to absorb moisture and thicken when they encounter water, making the fabric tighter and making it more difficult for water to pass through. Swelling is also an important factor in processes such as creping, drying and dyeing of fabrics.
03 Effect on density
It can be seen from the picture above. The density of the fiber first increases and then decreases as the moisture regain increases.
04 Effect on mechanical properties
For most fibers, their strength decreases with the increase in moisture regain, a few fibers remain almost unchanged, and the strength of individual fibers (cotton, linen) increases. The elongation at break of most fibers increases with the increase in moisture regain, while a few fibers remain almost unchanged. This is mainly due to the fact that after water molecules enter, the cross-links between macromolecules are broken down, the intermolecular force is reduced, and the macromolecules are easy to slip, so the strength is reduced.
As the moisture regain increases, the fibers become soft and easy to deform, the modulus decreases, and they are easy to tangle, while denser fabrics become stiff due to the support of the fibers; the friction factor on the fiber surface changes as the moisture regain increases. big.
Changes in moisture regain lead to changes in the mechanical properties of the fiber, and changes in mechanical properties affect textile processing and product quality. If the moisture regain rate is too low, the fibers will become more rigid and brittle, making them easy to break during processing, and static electricity will also be evident. If the moisture regain rate is too high, the fibers will be difficult to loosen, impurities in them will be difficult to remove, and they will easily become entangled with each other. Kinks tend to wrap around parts on machines, causing fluctuations in carding, drafting, weaving and other processes. Changes in cohesion will also change the yarn structure and fabric quality, causing instability or changes in yarn strength, hairiness, evenness, fabric size, fabric density, etc.
05 Effect on thermal properties
As the moisture regain of textile materials increases, their thermal insulation properties gradually decrease, the cold feeling increases, the ignition temperature increases, the glass transition temperature decreases, the thermal shrinkage rate increases, and the resistance to melt holes improves. Changes in moisture regain have a great impact on the thermal properties of materials.
06 Impact on electrical properties
Textile materials are insulating materials, but their insulation properties will decrease as the moisture regain rate increases, the dielectric loss increases, and the electrostatic phenomenon will decrease.
07 Effect on optical properties
As the moisture regain increases, the optical refractive index of the fiber decreases. The absorption of light will increase, the color will darken, and photodegradation and aging will increase.
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