Textile size plays an important role in textile mills, and various sizes and additives appear in the market. Starch (including modified starch), Polyvinyl Alcohol (PVA) and polyacrylic ACID size are still the three main sizes at present.
The PVA used at present is white or yellowish particles, as well as flake, flocculent or powder. The degree of polymerization and alcoholysis determine the properties of PVA. For example, the solubility of PVA in water depends on the degree of polymerization and alcoholysis, especially the degree of alcoholysis.
PVA molecule is a water-soluble polymer compound because it contains a large number of hydroxyl groups, which are strong hydrophilic groups. Complete alcoholysis PVA only absorbs moisture and swelling at room temperature but does not dissolve, and dissolves at high temperature (above 80 ℃); Complete alcoholysis PVA has strong intermolecular hydrogen bond, has great affinity for hydrophilic natural fibers (such as cotton, hemp, etc.), and the PVA film formed has water resistance; There are about 10% residual acetate groups in the molecule of partially alcoholized PVA. The acetate group is a hydrophobic functional group with good affinity and adhesion to hydrophobic materials; Partial alcoholysis PVA can dissolve slowly at room temperature.
PVA is easy to form a film. The formed film is colorless and transparent, has good mechanical strength, and the surface is smooth without sticking. The higher the degree of polymerization and alcoholysis, the greater the mechanical strength of PVA film.
PVA size has good miscibility. When mixed with other sizes (such as synthetic size, etc.), it can be well and evenly mixed. The mixed liquid is relatively stable and is not prone to delamination. Because PVA has such excellent performance, since PVA was used for warp sizing, it was soon adopted on a large scale by the textile industry as the sizing material for nylon and viscose silk. PVA was once considered as an "ideal" slurry because of its excellent film-forming, adhesion and compatibility with other sizes. So far, no natural or synthetic size can match it in the comprehensive indexes such as warp reinforcement, wear resistance and stretch reduction.
The fatal weakness of PVA is non environmental protection, which is called "dirty slurry". Some European countries have explicitly banned the import of grey cloth containing PVA slurry. Another problem is that PVA desizing is not easy to completely desize, which also hinders its use. The main disadvantages of polyvinyl alcohol size are easy foaming, peeling and poor yarn separation of size film. In order to overcome these shortcomings, polyvinyl alcohol can be denatured. The more mature denaturation methods include PVA acrylamide copolymerization denaturation, PVA lactonization denaturation, PVA sulfonation denaturation and PVA grafting denaturation.
In China, the use of PVA as textile size is still small compared with other industries such as construction, so PVA manufacturers do not produce PVA specially used for textile size, and textile factories have always used PVA 1799 with high degree of polymerization. However, PVA 1799 size film has poor yarn separation performance. When sizing is dry and twisted, the yarn resistance is large, the size film is easy to tear and the hairiness is increased. Therefore, some adhesive materials with low size film strength (such as starch, modified starch, etc.) are often mixed into PVA size to improve the yarn splitting performance of size film.
With the improvement of people's demand and the development of weaving technology, there are more and more ultra-fine and high-density fabrics, but PVA 1799 can not meet the sizing needs of ultra-fine and high-density fabrics, mainly for the following reasons:
(1) The adhesion between sizing molecules is too large, which is easy to produce pulp spots, and it is easy to break the warp yarn during dry splitting, especially the ultra-fine yarn;
(2) It is difficult to completely dissolve, and there are still undissolved PVA fragments in the slurry, which poses a threat to the yarn in the weaving process;
(3) The sizing film has high strength. The sizing film exposed outside the yarn body is tough and looks like a blade, which poses a threat to the adjacent yarn. The greater the proportion of PVA, the greater the toughness of this sizing film and the more serious the threat to adjacent yarns. Therefore, PVA 1799 is no longer suitable for use as the main slurry and should be reduced to auxiliary slurry or not. As a substitute for PVA 1799, PVA with low degree of oligomerization is more suitable because of its suitable sizing strength, good permeability, good dry separation, twisting and splitting, and less regenerated hairiness. Table L shows the sizing properties of PVA of different specifications.
At present, there is a high voice of opposition to the use of PVA, focusing on its poor biodegradability. From the perspective of reducing environmental pollution, although the degradation performance of low polymerization degree PVA is worse than that of starch, it has made great progress compared with high polymerization degree PVA. At present, the situation of sizing is that textile mills have to continue to use PVA. To improve this situation, it still needs the hard work of sizing researchers and sizing workers in China.
