Лако-красочные материалы — производство

Three types of adhesives are used in the manufacture of paper bags and sacks: side-seam adhesive, bottom paste, and cross paste. During production of a single-layer bag, the paper is first formed into a long tube held together by side-seam adhesive. To be compa­tible with machinery used in this process, the adhesive needs a viscosity in the range 2-4Pas with a solids content of 20-22%. Generally, the side-seam adhesive is made using high-soluble white dextrin or acid-modified starch, so the viscosity of the suspension remains low. Bottom paste adhesive is used to close the bottoms of the paper bags; it is much more viscous and has greater tack than side-seam adhesive. The high tack is neces­sary to keep the bags from opening after they have been formed. Bottom paste adhesives are usually composed of white dextrins or starch; viscosity may be as high as 140 Pas at 25°C. Cross paste adhesive is used only for the manufacture of multiwall bags. It is similar to side-seam adhesive, with the added requirement that it must not bleed through the paper to other layers. This is achieved by the addition of mineral fillers or poly(vinyl acetate). Water resistance of bag adhesives can be improved by the addition of urea — formaldehyde or poly(vinyl alcohol). Typical formulations for these adhesives are listed in Table 3.

Laminates prepared from paper or paperboard may be bonded with carbohydrate adhe­sives. The properties of the adhesive used with each will differ greatly depending on the surface of the material and the equipment to be used for laminating. However, all lami­nating adhesives should exhibit high tack, low penetration into the substrate, and non­curling behavior. White dextrin is often used at about 20% of the total adhesive; a typical formulation is given in Table 4.