Types of adhesives for dry compounding, blending, coating process

The adhesive for dry compounding is very versatile and should be selected according to the needs of different products. If no aluminum foil is compounded, a universal adhesive can be used. If aluminum foil is compounded and the end use is to package hot and sour foods containing liquids (such as pickled mustard, juice, soy sauce, peanut, etc.), aluminum foil adhesives must be used. Also consider adhesives with slightly larger molecular weights, relatively uniform molecular weights, good solvent release, and good coating properties. Although the adhesive having a small molecular weight has good coating performance, it has a strong molecular mobility, and it causes a decrease in the peel strength through the extremely thin aluminum layer, resulting in a decrease in the adhesive strength. Among the currently used polyurethane adhesives, there are two-component reflective and one-component components. Two-fluid reactive adhesives have the advantage of long shelf life. One-liquid adhesives have a short residence time, and they cure slowly after curing and have a long curing time. They are not suitable for high-temperature cooking and other products requiring high compounding. The two-component reactive adhesive solvent uses a thinner of ethyl acetate, and the one-component hot melt adhesive uses a thinner of toluene or ethyl acetate.

Preparation of glue

The correct operation of the glue is to add the appropriate amount of diluent to the main agent and mix it evenly, then add the curing agent and stir it evenly. Instead of mixing the main agent and the curing agent together, add a thinner. This is determined by the nature of the speed of the chemical reaction. The speed of the chemical reaction depends on the effective collisions between the molecules. At the same temperature and pressure (eg, normal temperature), the effective collision between molecules depends on their concentration. The higher the concentration, the greater the probability of an effective collision and the faster the reaction and speed. The main agent and curing agent in the absence of diluent, high concentrations, the faster the reaction rate. Rapidly generate larger molecules and condense into clumps, adding slower speed after dilution. In this way, the molecular weight of the glue coated on the film during the compounding process is extremely uneven, which may easily cause surface quality problems and also affect the fastness of the adhesive. The deployment of glue must be strictly in accordance with the instructions, otherwise it will cause quality accidents. In particular, it is necessary to remind that, according to abnormal operation, the glue molecules will crystallize rapidly, and products with high transparency will cause pitting.

Adhesive coating

Two-component polyurethane adhesive is applied using a concave eye coating roll. Its characteristic is that the coating amount is uniform, the coating amount is easy to control and accurate, and it can meet the requirements of compounding. The amount of sizing is determined by the concentration of the job and the depth of the screen roller. The depth of sizing on the stencil is high, and the amount of glue on the job is high, but the amount is small. Typical applications such as white film and smooth film standard coating weight is 1.5g/m2 -2.5g/m2. If the ink area is large, it should be increased to 2.5Z/m2 - 3.5g/m2 cryogenic distillation bag (100 temperature) is 3.5g/m2 -4.0g/m2, transparent cooking bag should be 3.5g/m2 -4.0g/m2, aluminum foil structure of high temperature cooking bag and aluminum foil container should be 4.0g/m2 -5.0g/m2. In general, Zahn Cup No. 3 is 15 to 30 seconds in viscosity, which is suitable for dry composite coating. At this time, the solid content is as large as 20% to 35%. Squeegee type and non-scraper type wire rolls are now available for coating and gluing. The angle of blade installation for wire roller coating is important. It should be between 30 and 60 degrees, preferably 45 degrees, between the squeegee blade and the tangent line contact point of the screen roller. Less than 30 degrees glue can not be scraped, gluing uneven quality problems. Above 60 degrees, both the doctor blade and the cable roller wear seriously, shortening the service life. In the transfer process, the cable roller uniformly applies the adhesive to the substrate. If there is material in the glue, it will leave a streaky state, which causes uneven wrinkles caused by uneven coating. If the screen roller is jammed, there will be uneven coating of the dry solid material in the hole, which will cause local glue, which will seriously affect the peel strength. Therefore, the adhesive should be clean and free of foreign materials when applying the glue. When the coating is finished, it should be cleaned with a thinner and washed with a copper wire brush. The general coating amount can be selected 110 lines / 60mm-80mm. If you encounter high temperature cooking bag, then choose 80 lines -100 lines /80mm-90mm on it. The blade used for coating is prone to wear over a long period of time, causing quality problems due to uneven sizing. The solution is to replace or grind the doctor blade.

Process aspects

In order to obtain the desired coating effect, the quality of the finished product does not occur, we must do a good job in every aspect of the work. The coating should be evenly baked, the control should be appropriate, the winding should be neat, and the tension should be well controlled. The drying temperature of the compounding machine is divided into three zones, generally controlled at: 60 degrees in zone 1, 65 degrees to 70 degrees in zone 2, and 70 degrees to 80 degrees in zone 3. The temperature increases in a gradient, and improper control of the temperature can easily lead to peel strength problems and surface quality problems. Such as bubbles, pits, low peel strength and other issues. It is strictly prohibited that the temperature in Zone 1 is too high, because the temperature is too high and the surface is dried, the inner layer is still wet, and (crater) problems often occur. Therefore, the temperature control is very critical. The tension refers to three aspects: the unwinding tension of the substrate, the unwinding tension of the composite substrate, and the finished product winding tension. The size is usually completed by the magnetic powder brake, tension adjustment should be appropriate, too large and small will cause quality accidents. The appearance of wrinkles, deformation, reduced peel strength and so on.

Aging problem

The purpose of maturation is to further improve the cross-linking ability of glue molecules and enhance peel strength. At the same time, the solvent can be further volatilized and cured without odor. Under normal circumstances, the aging temperature should be 45 degrees -50 degrees, aging time should be controlled at 48 hours or longer. Incomplete curing, incomplete solvent evaporation, low peel strength and odor. Therefore, it should be decided according to the actual situation. Sometimes the aluminum plating film is compounded and high-temperature short-term curing methods should be adopted. The purpose is to reduce some of the quality accidents caused by chemical reactions. In general, aging time and temperature control are directly proportional to peel strength.

Author / (Chen Quandong)