(1) The urea-formaldehyde resin used as the compression-molding powder of the resin requires a polycondensate having a relatively low degree of reaction, in which the viscosity of the resin is small, the impregnation of the filler is facilitated, and the proper flow is ensured after a long production cycle and drying. Sexually, industrially, a condensate of urea and formaldehyde at a low temperature (a mixture of mono- and dimethylol urea) is used. Usually, the ratio of urea to formaldehyde is 1:1.5 (molar ratio), and after all the dissolution at pH=8 (and temperature) 30-35 degrees, the amount of urea is added 0.3% - 0.54% of oxalic acid and 0.33% - 0.88% of oxalate, an exothermic reaction occurs immediately, the temperature rises, the temperature is maintained at 55-60 degrees, and the pH is controlled to 5.5-6.5, and the desired urea-formaldehyde resin is obtained after 60-75 minutes. Since the degree of polycondensation is low, it is only just after the addition reaction stage, and the main polycondensation reaction is carried out during the curing process.
(2) The curing agent used in the curing agent compression molding powder is required to have a certain latent property, and commonly used are oxalic acid, phthalic acid, benzoic acid, monochloroacetic acid and the like.
(3) The most commonly used filler for fillers is pulp, followed by wood flour or inorganic fillers (asbestos, fiberglass, mica, etc.). The pulp used is pure cellulose which is obtained by using wood as a raw material, which is treated with sulfite to dissolve non-cellulosic impurities in the wood and then bleached. The amount of filler is 25%-32% of the total material amount. If the dosage is too small, the flowability of the compression molding powder is large, and the strength of the product is low. Conversely, when the dosage is too much, the fluidity of the compression molding powder is reduced, and the surface of the product is not smooth and water resistant. reduce.
(4) Colorant The colorant can impart vivid color to the plastic. When selecting a coloring agent, it should be noted that the coloring agent used has strong coloring ability, can be uniformly dispersed in plastic, and does not change color at processing temperature and long-term sunlight. Precipitated from the product. The usual colorant is a pigment, and the dye is used less, in an amount of from 0.01% to 0.2% by weight of the material.
(5) Lubricant lubricant can improve the fluidity of the material when the finished product is pressed, and can be precipitated from the product, forming a separator between the product and the mold, so that the product is not easy to stick. Commonly used lubricants are metal salts of stearic acid (such as metal salts of zinc, calcium, aluminum, magnesium, etc.), esters of organic acids (such as cyclohexyl stearate, glyceryl stearate, etc.). The amount of addition is 0.1%-1.5% of the amount of the material. When too much, it will pollute the appearance of the product and reduce the gloss; if too small, the product is difficult to demould.
(6) Stabilizer Although the catalyst added to the compression molding powder is a latent catalyst, there is still a small amount of acid released during storage at room temperature, which affects the quality of the compression molding powder. Therefore, some alkaline is usually added. The substance is used to absorb the evolved acid, and the usual base is hexamethylenetetramine or ammonium carbonate.
(7) Plasticizers generally do not use plasticizers in compression molding powders and are used only in special occasions in order to improve the fluidity of the materials and reduce the shrinkage rate during curing. Useful plasticizers are urea and thiourea.
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