Detailed analysis of the two main raw materials of wet-laid nonwovens, fiber raw materials and chemical additives

The two main raw materials of wet-laid nonwovens, fiber raw materials and chemical auxiliaries, are analyzed in detail. The process principle and production process of wet-laid fiber raw materials are roughly the same as that of papermaking, but there are obvious differences in fiber requirements and bonding methods (Table 10 -1). Since the fibers are networked in an aqueous solution, this requires that the fibers must have good dispersibility in water. The specific requirements for fibers are as follows. (1) The fiber length should not be too long, and the proper slenderness ratio should be maintained. The longer the fiber, the worse the web uniformity. And the narrower the dispersion of fiber length, the better the dispersion of fiber in water. Chemical fibers require good cutting quality, and double-length fibers and super-long fibers should be avoided as much as possible. The slenderness ratio of the fiber refers to the ratio of the length of the fiber to its radial maximum dimension. The larger the slenderness ratio, the more difficult it is for the fiber to disperse in water and the easier it is to form agglomerates. Usually fiber length and fineness have the following relationship: cut length (mm) u003d 5 × (fiber count) 2 (2) The fiber is relatively straight and has a small degree of crimp. The greater the degree of crimp of the fiber, the more complex the crimp, the easier it is to entangle the fibers, and the more difficult it is to disperse in water, especially for three-dimensional fibers. (3) The fiber should have a certain degree of moisture absorption. The better the moisture absorption of the fiber, the stronger the swelling ability of the fiber in water and the better the dispersion. (4) The surface friction factor of the fiber should be small. The greater the surface friction factor of the fiber, the more entangled the fiber, and the harder it is to disperse in water. The dispersion properties of the fibers can be improved by adding moisture absorbents, dispersants, etc. into the slurry. Chemical additives The main function of the dispersing agent is to promote the uniform dispersion of fibers in water, reduce fiber flocculation, improve the forming quality, and obtain uniform products. Dispersion is to distribute small solid particles as evenly as possible in another incompatible material. For the wet production process, fibers, fillers and some additives are all water-insoluble, and they aggregate by themselves in the aqueous solution. In addition, different materials are often kept away from each other due to incompatibility, which makes it difficult to obtain products with uniform performance and ideal strength. Adding a dispersant can form a bi-molecular structure on the surface of the fiber. The polar end of the outer dispersant has a strong affinity with water, which increases the degree of wetting of the solid particles by water. The solid particles are kept away from each other due to electrostatic repulsion to achieve a good dispersion effect. The fiber dispersant requires a certain degree of pyrolysis, that is, it gradually decomposes as the temperature increases, and the viscosity decreases accordingly, so that it will not stay on the dried paper and will not affect the fiber due to the use of fiber dispersant The original performance. 2. Strengthening agent (1) Dry strengthening agent. The function of the dry strengthening agent is to increase the strength of the wet-laid nonwoven fabric. At present, the commonly used dry enhancers include modified starch (anionic starch, cationic starch), acrylamide polymer and polyethanol. The mechanism of the cationic polyacrylamide polymer dry enhancer is: polyacrylamide has a strong flocculation effect, can bridge between particles, and has different binding mechanisms according to different ionic properties, and the same molecule can be adsorbed Several particles cause coagulation. The polar groups on polyacrylamide can also form strong hydrogen bonds and electrostatic bonds with fibers. (2) Wet enhancer. Adding a wet strengthening agent before forming the web, on the one hand, it can give some products the function of wet strength, such as paper towel paper bag paper, wet photosensitive paper, etc.; on the other hand, provide sufficient wet strength to the wet fiber web to ensure blending. The net has sufficient drafting stress. Currently commonly used wet enhancers are cationic porphyraldehyde resin (UF resin), melamine formaldehyde resin (MF resin), polyamine resin (PAE resin), polyethyleneimine (PEI resin), polyamide epichlorohydrin Resin (PE resin), etc. Free formaldehyde is precipitated when the resin containing formaldehyde is used, which is harmful to the human body. In recent years, its application has been decreasing. PPE resin is an efficient wet strengthening agent that can be cured under alkaline conditions, and the wet strength can be as high as 50% of the dry strength. 3. Defoamer In pulp production, foam handling is a thorny issue. From the operation of the pulping unit after steaming the ball to the coating process (washing, filtering, bleaching, dehydration, papermaking, sizing, coating, etc.), there are different degrees of foam, which seriously affects In accordance with normal production and product quality. It can be treated by chemical method, the principle is: in the production process, add a special kind of surface active agent one by one defoamer to control foam. This type of substance has strong surface activity, which can destroy the foaming agent in the production system, reduce the surface tension, and make it unable to form a strong foam film, which will cause the foam to burst. There are many organic compounds suitable for defoaming agents, including silicone oil, poly, alcohol, fatty acid, acid amine ester, phosphate and metal soap, etc. Correspondingly, there are mainly silicone oil type silicone defoamers and emulsion type silicone defoamers. Polyether and other modified silicone defoamers. 4. The bonding agent can be added before the production of the wet-laid nonwoven fabric, or after the production of the web. See section 4 Adhesive reinforcement for details.