When coloring plastic products with dry powder or masterbatch, color change may occur, which may affect product quality.
Discoloration may be caused by the following reasons:
(1) Oxidative degradation of the matrix resin caused by high temperature molding;
(2) Due to chemical reactions between certain components of plastic products, such as matrix and additives, or between matrix and coloring pigments, or between additives and pigments;
(3) As coloring pigments or auxiliaries are not resistant to high temperatures, etc. We analyze the mechanism of the discoloration caused by these factors and provide reference for many plastic product manufacturers so that the right raw materials can be selected to produce qualified plastic products. .
Color change caused by plastic molding processing
1.Oxidation, degradation and discoloration of matrix resin during high temperature molding
When the heating ring or heating plate of the plastic molding processing equipment is in a heated state due to loss of control, the local temperature is too high and the resin is oxidized and decomposed at a high temperature. For those heat-sensitive plastics such as PVC, it is easier to perform the molding process. This phenomenon occurs. When it is serious, the char burns yellow or even black, and a large number of low molecular volatiles escape.
This degradation includes reactions such as depolymerization, random chain scission, side groups and low molecular removal.
(1) Disaggregation
The depolymerization reaction first breaks at the end of macromolecules, and then quickly removes the monomers according to the chaining mechanism, which is particularly easy to perform above the polymerization ceiling temperature.
(2) Anomalous chain breaking (degradation)
When a polymer such as PE is molded at a high temperature, its main chain may be broken at any position, and the molecular weight rapidly decreases, but the yield of the monomer is small. This type of reaction is called random chain breaking and is sometimes referred to as degradation. Polyethylene The radical activity formed after the chain scission is very high, there are more secondary hydrogens around, and the chain transfer reaction occurs easily, and almost no monomer is generated.
(3) Removal of Substituents
Polyvinyl chloride, polyvinyl acetate, polyacrylonitrile, polyvinyl fluoride, etc. When heated, the substituents will be removed. Take polyvinyl chloride (PVC) as an example, PVC is processed at a temperature of 180~200°C, but it is relatively At low temperatures (eg, 100-120° C.), dehydrogenation (HCl) begins, and HCl is lost sooner or later at about 200° C., causing the polymer to become darker and lower in strength. The overall reaction is shown as follows: _CH2CHCIH2CHCl~ ~ →~ ~ CH=CHCH=CH~ ~ +2HCl
Free HCl catalyzes dehydrochlorination. Metal chlorides, such as ferric chloride formed by the interaction of hydrogen chloride and processing equipment, promote catalysis. 3HCl+Fe→FeCl3+3HCl
When heat processing, PVC must add a few percent of acid absorbent, such as barium stearate, organic tin, lead compounds, etc., in order to improve its stability.
When using a communication cable to color a local communication cable, if the polyolefin layer on the copper wire is not stabilized well, a green copper carboxylate will form on the polymer-copper interface. These reactions promote the diffusion of copper into the polymer. Accelerated catalytic oxidation of copper.
Therefore, in order to reduce the oxidative degradation rate of polyolefins, phenolic or aromatic amine antioxidants (AH) are often added to terminate the above reaction to form inactive radicals A·:ROO·+AH-→ROOH+A·
(4) Oxidative degradation
The polymer is exposed to oxygen in the air during processing and use, and accelerates oxidative degradation when heated.
The thermal oxidation of polyolefins belongs to the free radical chain reaction mechanism, which has autocatalytic behavior and can be divided into initiation, growth, and termination of three-step reactions.
The breakage of the chain caused by the hydroperoxide group leads to the decrease of the molecular weight, the main products of the homogenization are alcohol, aldehyde, ketone, and finally oxidized to carboxylic acid. The carboxylic acid plays a major role in metal catalytic oxidation.
2. When the plastic molding process, the colorant decomposes and discolors due to high temperature resistance.
The pigments or dyes used for plastic coloring all have a temperature limit. When this limit temperature is reached, the pigments or dyes will undergo chemical changes, resulting in a variety of lower molecular weight compounds. The reaction formulas are more complex; different pigments have different reactions. And products, can measure the temperature resistance of different pigments by weight loss and other analytical methods. Generally:
Color change caused by reaction of colorant with resin
The reaction between the colorant and the resin mainly occurs during the processing and molding of certain pigments or dyes and resins. These chemical reactions will cause the change of the hue and cause the degradation of the polymer, thereby changing the properties of the product.
1. Reduction reaction
Certain polymers, such as nylon and aminoplasts, are strong acidic reducing agents in the molten state. They can make pigments or dyes that are very stable at the processing temperature be reduced and discolored.
2 alkali exchange
Alkaline earth metals in polyvinyl chloride emulsion polymers or certain stabilized polypropylenes can undergo 'alkaline exchange' with the alkaline earth metals in colorants, thereby turning the color from blue to red to orange.
PVC emulsion polymer is a method in which VC is polymerized by stirring in an aqueous solution of an emulsifier (such as sodium dodecyl sulfate C12H25SO3Na), and the reaction contains Na+; in order to improve the heat-resistant oxygen performance of PP, 1010, DLTDP, and the like are often added. Oxygen, Antioxidant 1010 is a transesterification reaction catalyzed by methyl 3,5-di-tert-butyl-4-hydroxypropionate and pentaerythritol sodium, while DLTDP is produced by the reaction of Na2S aqueous solution and acrylonitrile to produce thiobis. Propionitrile is hydrolyzed to produce thiodipropionic acid, which is finally obtained by esterification of lauryl alcohol. The reaction also contains Na+.
In the molding process of plastic products, the residual Na+ in the resin will react with a lake pigment containing metal ions such as C.I. Pigment Red 48:2 (BBC or 2BP): XCa2++2Na+→XNa2+ +Ca2+
3. Reaction between pigment and hydrogen halide (HX)
PVC detaches HCI at a temperature of 170°C or under light to form a conjugated double bond.
Halogen-containing flame-retardant polyolefin or colored flame-retardant plastic products are also dehydrohalogenated HX at high temperature.
(1) Reaction of Ultramarine with HX
5. Ultramarine blue pigment widely used in plastic coloring or eliminating yellow light, is a sulfur-containing compound.
(2) Copper gold pigment accelerates the oxidative decomposition of PVC resin
Copper pigments can be oxidized at high temperatures to produce Cu+, Cu2+, which will accelerate the decomposition of PVC
(3) Destruction of Polymers by Metal Ions
Some pigments have a destructive effect on polymers. For example, manganese lake pigment CIPigmentRed 48:4 is not suitable for molding PP plastic products. The reason is that metathesis of manganese ions in the thermal oxidation or photooxidation of PP catalyzes the hydroperoxidation through the transfer of electrons. Decomposition of the material leads to accelerated aging of the PP; the ester bond in the polycarbonate is easily hydrolyzed upon heating and decomposes with alkali, and once metal ions are present in the pigment, decomposition is more easily promoted; metal ions also promote thermal oxygen decomposition of the resin such as PVC. And lead to color changes.
In summary, when producing plastic products, we should avoid the use of coloring pigments that react with the resin is the most feasible and effective way.
Reaction between colorant and auxiliary agent
1, reaction between sulfur-containing pigments and additives
Sulfur-containing pigments, such as cadmium yellow (solid solutions of CdS and CdSe), due to poor acid resistance, should not be used in PVC and should not be used with lead-containing additives.
2, Lead compounds react with sulfur-containing stabilizers
Chromium yellow pigment or molybdenum red lead component reacts with antioxidants such as thiodistearate DSTDP.
3, reaction between pigment and antioxidant
Resins with antioxidants, such as PP, in addition to the above-mentioned '3.2', certain pigments and antioxidants also react, thus weakening the function of antioxidants, and deteriorating the thermal oxygen stability of the resin. For example, phenolic antioxidants are easily absorbed by carbon black or react with them and lose their activity; in white or light-colored plastic products, phenolic antioxidants and titanium ions form phenolic aromatic complexes that cause yellowing of the product. Select suitable antioxidants or add supplementary additives such as zinc citrate (zinc stearate) or P2 phosphite to prevent discoloration of white pigment (TiO2).
4, The reaction between pigment and light stabilizer
The effects of pigments and light stabilizers, in addition to the previously described sulfur-containing pigments that react with the nickel-containing light stabilizers, generally reduce the effectiveness of light stabilizers, in particular hindered amine light stabilizers and azo yellow, red pigments, their light The effect of stabilizing descent is even more pronounced. It is not as stable as uncolored. There is no clear explanation for this phenomenon.
Reaction between auxiliaries
Inappropriate use of many auxiliaries may cause unexpected reactions that may cause artifacts in the product. For example, flame retardant Sb2O3 reacts with sulfur to generate Sb2S3: Sb2O3+–S–→Sb2S3+–O–
Therefore, when considering the production formula, it is necessary to carefully select additives.
Color change due to automatic oxidation of additives
The auto-oxidation of phenolic stabilizers is an important factor in promoting the discoloration of white or light-colored products. This discoloration is often referred to as 'Pinking' in foreign countries. It is composed of antioxidants such as BHT (2-6). - di-tert-butyl-4-methylphenol) oxidation products are coupled and form a pale red reaction product such as 3,3',5,5'-homo-stilbene, this discoloration is only in aerobic and water and none In the case of light, exposure to ultraviolet light, the reddish stilbene quinone rapidly decomposes into a yellow monocyclic product.
Tautomerism Caused by Coloring Pigments Caused by Photothermal Reaction
Some of the coloring pigments under the action of photothermal, tautomeric isomerism, such as the use of CIPig.R2 (BBC) pigments from azo type to quinoid type, changing the original conjugate effect, causing conjugated bonds The decrease causes the color to change from a dark blue red light to a light orange red; at the same time, under the catalysis of light, it decomposes with water, causing co-crystal water to change and cause discoloration.
The color change caused by atmospheric pollutants
When the plastic products are stored or used, some reactive groups, whether they are resins or additives, or color pigments, will react with atmospheric moisture or chemical contaminants such as acids and alkalis under light and heat. Induced by a variety of complex chemical reactions, over time, will lead to discoloration or discoloration, by adding appropriate thermal oxygen stabilizers, light stabilizers, or use of high-quality weatherproofing additives and pigments can avoid or ease the occurrence of this situation.
in conclusion
(1) The oxidative degradation of the matrix resin may cause discoloration during high-temperature molding;
(2) Discoloration of colorants at high temperatures can cause discoloration of plastic products;
(3) The chemical reaction of the colorant with the matrix resin or auxiliary agent will cause discoloration;
(4) The reaction between auxiliary agents and the automatic oxidation of auxiliary agents will cause color changes;
(5) Tautomerism of coloring pigments under photothermal effects can cause color changes in the article;
(6) Air pollutants may cause changes in plastic products.