Introduction: In the production of plastic products, adding toner or masterbatch to color the plastic products may cause color changes, which will affect the quality of the products. This article shares the reasons that cause the color of plastic products to change, and the content is for your reference:
Possible causes of color changes
· During high temperature molding, the oxidative degradation of raw materials may cause discoloration;
· Fading of the colorant at high temperature will cause discoloration of plastic products;
· Chemical reaction between colorant and raw materials or additives will cause discoloration;
· The reaction between the additives and the automatic oxidation of the additives will cause color changes;
· Tautomerism of colored pigments under the action of light and heat will cause the color of the product to change;
· Air pollutants may cause changes in plastic products
Plastic molding processing
1) When molding at high temperature, the oxidative degradation of raw materials causes discoloration
When the heating ring or heating plate of the plastic molding processing equipment is always heated due to out of control, it is easy to cause the local temperature to be too high, causing the raw material to oxidize and decompose at high temperatures. For those heat-sensitive plastics, such as PVC, it is easier to mold and process When this phenomenon occurs, when it is severe, it will be scorched to yellow or even black, accompanied by a large amount of low-molecular volatiles overflow.
This degradation includes reactions such as depolymerization, random chain scission, removal of side groups and low-molecular substances.
The cleavage reaction occurs on the terminal chain links, causing the chain links to fall off one by one, and the resulting monomers evaporate quickly. At this time, the molecular weight changes very slowly, just like the reverse process of chain polymerization. Such as the thermal depolymerization of methyl methacrylate.
· Random chain scission (degradation)
Also known as random break or random chain break. Under the action of mechanical force, high-energy radiation, ultrasonic waves or chemical reagents, the polymer chain breaks without a fixed point to produce low molecular weight polymer. It is one of the ways of polymer degradation. When the polymer chain is randomly degraded, the molecular weight drops rapidly, and the weight loss of the polymer is small. For example, the degradation mechanism of polyethylene, poly internal ene and polystyrene is mainly random degradation.
For polymers such as PE, when they are molded at high temperatures, their main chain may be broken at any position, and the molecular weight drops rapidly, but the monomer yield is low. This type of reaction is called random chain scission, sometimes called degradation, polyethylene The free radicals formed after chain scission are very active, and there are more secondary hydrogens around, which are prone to chain transfer reactions, and almost no monomers are produced.
· Removal of Substituents
When PVC, PVAc, etc. are heated, the substituent removal reaction can occur, so a plateau often appears on the thermal weight loss curve. When polyvinyl chloride, polyvinyl acetate, polyacrylonitrile, polyvinyl fluoride, etc. are heated, the substituents will be removed. Take polyvinyl chloride (PVC) as an example. PVC is processed and formed at temperatures below 180~200℃, but at lower temperatures (such as 100~120℃), dehydrogenation (HCl) begins, and it is very difficult to lose HCl around 200℃. It is fast, so during processing (180-200℃), the color of the polymer will become darker and the strength will decrease.
Free HCl has a catalytic effect on dehydrochlorination, and metal chlorides, such as ferric chloride formed by the action of hydrogen chloride and processing equipment, promote catalysis.
A few percent of acid absorbent, such as barium stearate, organotin, lead compound, etc., must be added to PVC during thermal processing to improve its stability.
When the communication cable is used to color the communication cable, if the stability of the polyolefin layer on the copper wire is not good, the green copper carboxylate will be formed on the polymer-copper interface. 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 polyolefins, phenolic or aromatic amine antioxidants (AH) are often added to terminate the above reaction and form inactive free radicals A·: ROO·+AH-→ROOH+A·
· Oxidative degradation
The polymer product exposed to the air absorbs oxygen and oxidizes to form hydroperoxide, which further decomposes to generate active centers to form free radicals, and then a free radical chain reaction (ie, auto-oxidation process) occurs. Polymers are exposed to oxygen in the air during processing and use, and accelerate oxidative degradation when heated.
The thermal oxidation of polyolefins belongs to the free radical chain reaction mechanism and has autocatalytic behavior, which can be divided into three steps of initiation, growth, and termination.
The scission of the chain caused by the hydroperoxide group leads to a decrease in molecular weight. The main products of the scission are alcohols, aldehydes, and ketones, and finally oxidized to carboxylic acids. Carboxylic acid plays a major role in the catalytic oxidation of metals. Oxidative degradation is the main reason for the deterioration of the physical and mechanical properties of polymer products. The oxidative degradation varies according to the molecular structure of the polymer. The presence of oxygen can also aggravate the destructive effects of light, heat, radiation and mechanical forces on the polymer, causing it to undergo more complex degradation reactions. Adding antioxidants to the polymer can slow down oxidative degradation.
2) During plastic processing and molding, the colorant decomposes, fades and becomes discolored due to high temperature resistance
The pigments or dyes used for plastic coloring all have a temperature resistance limit. When this limit temperature is reached, the pigment or dye will undergo chemical changes to generate various lower molecular weight compounds. The reaction formulas are more complicated; different pigments have different reactions And products, the temperature resistance of different pigments can be tested by analysis methods such as weight loss.
Colorant reacts with raw materials
The reaction between colorants and raw materials is mainly manifested in the processing and molding of certain pigments or dyes and raw materials. These chemical reactions will lead to changes in hue and degradation of polymers, thereby changing the properties of plastic products.
· Reduction reaction
Certain polymers, such as nylon and aminoplasts, are very acidic reducing agents in the molten state. They can reduce and fade pigments or dyes that are stable at processing temperatures.
· Alkaline Exchange
The alkaline earth metals in polyvinyl chloride emulsion polymers or some stabilized polypropylene can undergo “alkali exchange” with the alkaline earth metals in the colorant, thereby changing the color from blue-red to orange.
PVC emulsion polymer is a method of VC in the emulsifier (such as sodium dodecyl sulfonate C12H25SO3Na) aqueous solution with the aid of stirring polymerization method, the reaction contains Na+; in order to improve the heat-resistant oxygen performance of PP, often add 1010, DLTDP and other resistance Oxygen, antioxidant 1010 is a transesterification reaction catalyzed by methyl 3,5-di-tert-butyl-4-hydroxypropionate and sodium pentaerythritol, while DLTDP is prepared by reacting Na2S aqueous solution with acrylonitrile to prepare thiodi Propionitrile is hydrolyzed to produce thiodipropionic acid, which is finally obtained by esterification with lauryl alcohol. The reaction also contains Na+.
During the molding and processing of plastic products, the residual Na+ in the raw materials will react with lake pigments containing metal ions such as C.I.Pigment·Red48:2 (BBC or 2BP): XCa2++2Na+→XNa2+ +Ca2+
· Reaction between pigment and hydrogen halide (HX)
When the temperature rises to 170°C or under the action of light, PVC removes HCI to form conjugated double bonds.
Halogen-containing flame-retardant polyolefin or colored flame-retardant plastic products are also dehydrohalogenated HX at high temperature.
1) Ultramarine and HX reaction
Ultramarine blue pigment widely used for coloring plastics or eliminating yellow light. It is a sulfur-containing compound.
2) Copper and gold powder pigments accelerate the oxidative decomposition of PVC raw materials
Copper pigments can be oxidized to produce Cu+ and Cu2+ at high temperatures, which will accelerate the decomposition of PVC
3) The destructive effect of metal ions on polymers
Some pigments have a destructive effect on polymers. For example, the manganese lake pigment CIPigmentRed 48:4 is not suitable for the molding of PP plastic products. The reason is that the variable-valence metal manganese ion catalyzes the hydroperoxide through the transfer of electrons in the thermal oxidation or photooxidation of PP. The decomposition of PP leads to accelerated aging of PP; the ester bond in polycarbonate is easily hydrolyzed and decomposed in contact with alkali when heated. Once metal ions exist in the pigment, it is easier to promote decomposition; metal ions will also promote the thermal oxygen decomposition of PVC and other raw materials, and Causes color changes.
In summary, when producing plastic products, we should avoid the use of colored pigments that react with the raw materials is the most feasible and effective method.
Reaction between colorants and additives
1) Reaction between sulfur-containing pigments and additives
Sulfur-containing pigments, such as cadmium yellow (a solid solution of CdS and CdSe), are not suitable for PVC due to poor acid resistance, and should not be used with lead-containing additives.
2) Reaction of lead-containing compound with sulfur-containing stabilizer
The lead component of chrome yellow pigment or molybdenum chrome red reacts with antioxidants such as thiodistearate DSTDP.
3) Reaction between pigment and antioxidant
For raw materials with antioxidants, such as PP, some pigments and antioxidants will also react, thus weakening the function of antioxidants and making the thermal and oxygen stability of the raw materials worse. For example, phenolic antioxidants are easily absorbed by carbon black or react with carbon black to lose their activity; in white or light-colored plastic products, phenolic antioxidants and titanium ions form phenolic aromatic complexes to cause yellowing of the products. Choose a suitable antioxidant or add auxiliary additives, such as acid-resistant zinc salt (zinc stearate) or P2 type phosphite to prevent the white pigment (TiO2) from discoloring.
4) Reaction between pigment and light stabilizer
The effect of pigments and light stabilizers. In addition to the reaction between sulfur-containing pigments and nickel-containing light stabilizers as described above, the effectiveness of light stabilizers is generally reduced, especially hindered amine light stabilizers and azo yellow and red pigments. The stabilization effect is more obvious, and it is not as stable as uncolored. There is no exact explanation for this phenomenon.
Reaction between additives
If many additives are used improperly, unexpected reactions may occur, which may cause discoloration of the product. For example, the flame retardant Sb2O3 reacts with sulfur-containing anti-reaction to produce Sb2S3: Sb2O3+–S–→Sb2S3+–O–
Therefore, when considering the production formula, the additives must be carefully selected.
Caused by auto-oxidation of additives
The automatic oxidation of phenolic stabilizers is an important factor in promoting the discoloration of white or light-colored products. This discoloration is often called “Pinking” in foreign countries.
It is coupled by oxidation products such as BHT antioxidant (2-6-di-tert-butyl-4-methylphenol), and is shaped like a light red reaction product of 3,3′,5,5′-stilbene quinone This discoloration only occurs in the presence of oxygen, water and no light. When exposed to ultraviolet light, the reddish stilbene quinone quickly decomposes into a yellow monocyclic product.
Colored pigments are tautomerized under the action of light and heat
Part of the colored pigments undergoes tautomerism in molecular configuration under the action of light and heat. For example, the use of CIPig.R2 (BBC) pigments changes from azo-type to quinone-type, which changes the original conjugation effect and causes the conjugate bond. The reduction causes the color to change from a dark blue light red to a light orange red.
At the same time, under the catalysis of light, it decomposes with water, changing the co-crystal water and causing fading.
Caused by air pollutants
When plastic products are stored or used, some reactive materials, whether raw materials or additives, or coloring pigments, will interact with moisture or chemical pollutants in the atmosphere, such as acids and alkalis, under the action of light and heat. Cause various complex chemical reactions, over time, will lead to fading or discoloration.
This situation can be avoided or alleviated by adding suitable thermal oxygen stabilizers, light stabilizers, or selecting high-quality weathering additives and pigments.