Color changes caused by plastic molding processing 1. During high temperature molding, the matrix resin is oxidized, degraded and discolored When the heating ring or heating plate of plastic molding processing equipment has been in a heating state due to loss of control, it is easy to lead to local temperature is too high, making the resin oxidation and decomposition at high temperature, for those heat-sensitive plastics, such as PVC, etc., this phenomenon is more likely to occur during molding processing, when serious, it will burn yellow, or even black, and accompanied by a large number of low molecular volatiles escape. This degradation includes depolymerization, random chain breaking, side group and low molecular matter removal and other reactions. (1) Depolymerization The depolymerization system breaks at the end of the macromolecule first, and then removes the monomer rapidly according to the linkage mechanism, especially when the polymerization temperature is above the upper limit. (2) Random chain break (degradation) For polymers such as PE at high temperature molding, its main chain can be broken at any position, the molecular weight rapidly decreases, but the monomer yield is very small, this reaction is called random chain break, sometimes also known as degradation, polyethylene chain break formed after the free radical activity is very high, there are more secondary hydrogen around, easy to occur chain transfer reaction, almost no monomer production. (3) Removal of substituents When polyvinyl chloride, polyvinyl acetate, polyacrylonitrile, polyvinyl fluoride, etc., are heated, substituents will be removed. Taking polyvinyl chloride (PVC) as an example, PVC is processed at temperatures below 180~200 ° C, but at lower temperatures (such as 100~120 ° C), that is, begins to dehydrogenate (HCl), about 200 ° C loses HCl quickly, and the polymer becomes dark and the strength becomes low, the total reaction is summarized as follows: ~ CH2CHCICH2CHCl ~ → ~ CH=CHCH=CH ~ +2HCl Free HCl has a catalytic effect on dehydrochlorination, and metal chlorides, such as ferric chloride formed by hydrogen chloride and processing equipment, promote catalysis: 3HCl+Fe→FeCl3+3HCl PVC in hot processing must add a few percent of the acid absorbent, such as barium stearate, organotin, lead compounds, etc., to improve its stability. If the polyolefin layer on the copper wire is not stable, green copper carboxylate will be formed on the polymer-copper interface when the telecommunication cable is colored. These reactions promote the diffusion of copper into the polymer and accelerate the catalytic oxidation of copper. Therefore, in order to reduce the oxidative degradation rate of polyolefin, phenols or arylamine antioxidants (AH) are often added to terminate the above reaction and form inactive free radicals A· : ROO·+AH-→ROOH+A· (4) Oxidative degradation Polymers are exposed to oxygen in the air during processing and use, which accelerates oxidative degradation when heated. The thermal oxidation of polyolefin belongs to the free radical chain reaction mechanism and has automatic catalytic behavior, which can be divided into three steps: initiation, growth and termination. The chain fracture caused by hydroperoxide group leads to the reduction of molecular weight, and the main products of homolytic cleavage are alcohols, aldehydes, ketones, and finally oxidized into carboxylic acids. Carboxylic acid plays an important role in catalytic oxidation of metals. 2. When plastic molding is processed, the colorant is decomposed and discolored because it is not resistant to high temperature The pigments or dyes used for plastic coloring have a temperature limit, and when this limit temperature is reached, the pigments or dyes will undergo chemical changes and generate various low molecular weight compounds, and the reaction formulas are more complex; Different pigments have different reactions and products, and the temperature resistance of different pigments can be measured by weight loss analysis.
· Colorant reacts with resin to cause color change The reaction of colorant and resin is mainly manifested in some pigments or dyes and resins during processing and molding, and these chemical reactions will lead to changes in hue and degradation of polymers, thus changing the performance of products. 1. Reduction reaction Certain polymers, such as nylon and amino plastics, are strong acid reducing agents in the molten state, which can reduce and fade pigments or dyes that are stable at processing temperatures. 2. Alkali exchange The alkali earth metals in the polyvinyl chloride emulsion polymer or some stabilized polypropylene can undergo "alkali exchange" with the alkali earth metals in the colorant, thus changing the color from blue-red to orange. PVC emulsion polymer is VC in the emulsifier (such as sodium dodecyl sulfonate C12H25SO3Na) aqueous solution by stirring polymerization method, the reaction contains Na+; In order to improve the heat resistant oxygen performance of PP, antioxidants such as 1010 and DLTDP are often added. Antioxidant 1010 is a transesterification reaction of 3,5 di-tert-butyl-4-monohydroxypropionate methyl ester and sodium pentaerythritol catalyzed, while DLTDP is a reaction of Na2S aqueous solution with acrylonitrile to prepare dipropionic thiodipropionic acid, which is hydrolyzed to thiodipropionic acid. Finally, the product is esterified by lauryl alcohol, and the reaction also contains Na+. During the molding process of plastic products, the residual Na+ in the resin will react with the lake pigment containing metal ions such as C.I.PGment ·Red48:2(BBC or 2BP) :XCa2++2Na+ +Ca2+ 3. Reaction between pigment and hydrogen halide (HX) PVC removes HCI when the temperature rises to 170℃ or under the action of light to form conjugate double bonds. Halogenated flame retardant polyolefin or colored flame retardant plastic products are also dehalogenated HX in high temperature molding. (1) The reaction between ultramarine and HX Ultramarine pigment widely used for coloring or eliminating yellow light in plastics. It is a sulfur-containing compound. (2) Copper and gold powder pigments accelerate the oxidation and decomposition of PVC resin Copper pigment can be oxidized to Cu+ and Cu2+ at high temperature, which will accelerate the decomposition of PVC. (3) The destruction of metal ions on polymers Some pigments have destructive effect on polymer, such as manganese deposit pigment C.I.P. igmentRed48:4 is not suitable for PP plastic product molding, the reason is that the varivalent metal manganese ion catalyzes the decomposition of hydroperoxide through electron transfer in the thermal oxidation or photooxidation of PP, leading to the accelerated aging of PP. The ester bond in polycarbonate is easy to be hydrolyzed and decomposed by alkali when heated, and it is easier to promote decomposition once metal ions are present in the pigment. Metal ions also promote the thermal oxygen decomposition of resins such as PVC and cause color changes. To sum up, when producing plastic products, we should avoid the use of color pigments that react with resins is the most feasible and effective way.
· Reactions between colorants and auxiliaries 1, the reaction between sulfur pigments and auxiliaries Sulfur-containing pigments, such as cadmium yellow (the solid solution of CdS and CdSe), should not be used for PVC due to poor acid resistance, and should not be used with lead additives. 2. Lead compounds react with sulfur stabilizer The lead component in chrome yellow pigments or molybdenum chrome red reacts with antioxidants such as thiodistearate DSTDP. 3. Reaction between pigment and antioxidant Resin with antioxidants, such as PP, some pigments and antioxidants will also react, thus weakening the function of antioxidants, so that the thermal oxygen stability of the resin deteriorates. For example, phenolic antioxidants are easily absorbed by or react with carbon black and lose their activity; In white or light-colored plastic products, phenolic antioxidants form phenolic aromatic complexes with titanium ions to make the products yellow. We can prevent the color change of white pigment (TiO2) by selecting suitable antioxidants or adding auxiliary additives, such as zinc antacid salt (zinc stearate) or P2 phosphite ester. 4. Reaction between pigment and light stabilizer The action of pigments and light stabilizers, in addition to the reaction between sulfur-containing pigments and nickel-containing light stabilizers described above, will generally reduce the effectiveness of light stabilizers, especially blocked the effect of amine light stabilizers and azo yellow and red pigments, its light stability decline effect is more obvious, not as stable as the uncolored, this phenomenon has no exact explanation.
· Reactions between auxiliaries If many auxiliaries are used improperly, unexpected reactions may occur and make the product color change. For example, flame retardant Sb2O3 reacts with sulfur-containing resistance to produce Sb2S3: Sb2O3+ -- S -- →Sb2S3+ -- O -- Therefore, when considering the production formula, the additives must be carefully selected.
· Color change caused by automatic oxidation of additives The automatic oxidation of phenolic stabilizers is an important factor in promoting the color change of white or light-colored products, which is often called "Pinking" (red) in foreign countries. It is conjugated by oxidation products such as BHT antioxidants (2-6-di-tert-butyl-4-methylphenol) and is shaped like a 3,3 ',5,5 'monostilbenone reddish reaction product. This discoloration occurs only in the presence of oxygen and water and no light. Exposed to ultraviolet light, the reddish stilbenone rapidly decomposes into a yellow monocyclic product.
· Color pigments cause color change under the action of light and heat Some colored pigments under the action of light and heat, molecular configuration tautomerism, such as the use of C.I.Pig.R2(BBC) pigment from azo type to quinone type, change the original conjugation effect, resulting in the reduction of conjugation bonds, resulting in color from dark blue red to light orange red. At the same time, under the catalytic action of light, it decomposes with water, which changes the co-crystalline water and causes fading.
· Color change caused by atmospheric pollutants When plastic products are stored or used, some reactive groups, whether resins or additives, or coloring pigments, under the action of light and heat, will interact with atmospheric moisture or chemical pollutants such as acids and bases, causing a variety of complex chemical reactions, which will lead to fading or discoloration over time. This situation can be avoided or mitigated by adding appropriate thermal oxygen stabilizer, light stabilizer, or selecting high-quality weather resistant additives and pigments.
