What is the color change mechanism of plastic colored products?

May 21, 2018

When the plastic products are colored by dry powder or color masterbatch, the color change may occur, thus affecting the quality of the product.

 

Discoloration may be caused by the following reasons:

 

1) oxidative degradation of matrix resin during high temperature molding;

 

(2) due to chemical reactions between certain components of plastic products, such as matrix and auxiliaries, or matrix and pigments, or between auxiliaries and pigments;

 

Due to coloring pigments or auxiliaries such as insensitive to high temperature caused, and so on. By analyzing the mechanism of color change caused by these factors, we provide reference for many plastic manufacturers in order to select raw materials correctly and produce qualified plastic products.

 

Color change caused by plastic forming

 

1. Oxidation degradation and discoloration of matrix resin at high temperature

 

When the heating ring or the heating plate of the plastic molding equipment is always in the heating state because it is out of control, it is easy to cause the local temperature to be too high, so that the resin is oxidized and decomposed at high temperature. For those thermosensitive plastics, such as PVC, This phenomenon is more likely to occur in the molding process. In severe cases, the scorching will turn yellow or even black, and accompanied by a large number of low molecular volatile escape.

 

Such degradation includes depolymerization, random chain breakage, side group and low molecular weight removal.

 

Depolymerization

 

The depolymerization reaction was carried out at the unend of the macromolecule, and then the monomer was removed rapidly according to the linkage mechanism, especially when the polymerization temperature was above the upper limit.

 

Random chain breaking (degradation)

 

When a polymer such as PE is formed at high temperature, its main chain may break at any position and its molecular weight will decrease rapidly, but the yield of monomer is very low. This kind of reaction is called random broken chain, sometimes also called degradation. The free radical activity of polyethylene is very high after breaking the chain, and there is more secondary hydrogen around it. The chain transfer reaction is easy to occur and almost no monomer is produced.

 

Removal of substituents

 

When polyvinyl chloride, polyvinyl acetate, polyacrylonitrile, polyfluoroethylene are heated, the substituents will be removed. Taking PVC as an example, PVC was formed at a temperature below 180 ~ 200 ℃, but at a lower temperature (such as 100 ~ 120 ℃), the HCl was lost quickly at about 200 ℃, and the polymer became dark and the strength became lower. The total reaction can be summarized as follows: CH2CHCIH2CHClCHClCHClCHCH ~ 2HCl

 

Free HCl catalyzes dehydrochlorination. Metal chlorides, such as ferric chloride formed by the interaction of hydrogen chloride with processing equipment, promote the catalysis of .3HCl Fe FeCl3 3HCl.

 

In order to improve the stability of PVC, several percent acid absorbents, such as barium stearate, organotin and lead compounds, should be added during hot processing.

 

If the polyolefin layer on the copper wire is not stable enough, the green copper carboxylate will be formed on the interface between polymer and copper. 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 aromatic amine antioxidants are often added to terminate the above reaction and form an inactive free radical A: Roo AH- ROOH A

 

Oxidation degradation

 

Polymers are processed and used in contact with oxygen in the air, and accelerated oxidative degradation when heated.

 

The thermal oxidation of polyolefin belongs to the mechanism of free radical chain reaction and can be divided into three steps: initiation, growth and termination.

 

The cleavage of the chain induced by hydrogen peroxide leads to the decrease of molecular weight. The main products of the cleavage are alcohols, aldehydes, ketones, and finally oxidized to carboxylic acids. Carboxylic acids play a major role in the catalytic oxidation of metals.

 

2. When plastics are formed, the coloring agent dissolves, discoloration, discoloration and discoloration due to the inendurance of high temperature.

 

The pigments or dyestuffs used in plastics coloring all have temperature resistance limit, reaching this limit temperature, the pigments or dyestuffs will have chemical changes and produce all kinds of lower molecular weight compounds, their reaction formulas are all more complex. Different pigments have different reactions and products. In general: color changes caused by the reaction of colorants with resins. The reaction between coloring agents and resins occurs mainly during the processing of certain pigments or dyes with resins, and these chemical reactions will lead to changes in hue. The polymer is degraded and the properties of the products are changed.

 

Reduction

 

Some polymers, such as nylon and amino plastics, are highly acidic reductants in molten state, which can reduce or fade pigments or dyes that are stable at processing temperature.

 

2, Alkali exchange

 

Alkali earth metals in polyvinyl chloride emulsion polymers or some stabilized polypropylene can be "alkali exchanged" with alkali earth metals in colorants, thus changing the color from blue to red to orange.

 

PVC emulsion polymer is prepared by agitation polymerization of VC in aqueous solution of emulsifier such as sodium dodecyl sulfonate C12H25SO3Na.The reaction contains Na, and in order to improve the heat and oxygen resistance of PP, the antioxidant such as 1010m DLTDP is often added. Antioxidant 1010 is transesteric exchange reaction between methyl ester 3- (5-di-tert) Ding Ji -4-hydroxypropionate and sodium pentaerythritol, while DLTDP is prepared from Na2S aqueous solution and acrylonitrile to thiodipropionitrile, which is hydrolyzed to form thiodipropionic acid. Finally, sodium was obtained by esterification of lauryl alcohol, and Na was also included in the reaction.

 

During the molding of plastic products, the residual Na in the resin will react with the pigment containing metal ions, such as C.I.Pigment Red48:2(BBC or 2BP XCa 2 2Na XNa2 Ca2.

 

3, Reaction between pigment and HX

 

The conjugated double bond of PVC is formed by removing HCI from HCI at 170C or under the action of light.

 

Halogen-containing flame retardant polyolefin or colored flame retardant plastic products are also dehalogenated HX at high temperature.

 

HX reaction

 

Ultramarine pigment, widely used in plastic coloring or yellow light elimination, is a sulfur compound.

 

Accelerated Oxidation decomposition of PVC Resin by Copper Gold Powder pigment

 

Copper pigments can be oxidized to Cu2 at high temperature, which will accelerate the decomposition of PVC.

 

(3) Destruction of polymer by metal ions

 

Some pigments have destructive effect on polymers, such as manganese pigment C.I.PigmentRed48:4 is not suitable for forming PP plastics, because of the variable value metal manganese ion in the thermal oxidation or photooxidation of PP, The decomposition of hydrogen peroxide catalyzed by electron transfer leads 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 the decomposition once metal ions exist in the pigment. Metal ions also promote thermo-oxygen decomposition of resins such as PVC and cause color changes.

 

To sum up, the most feasible and effective way to produce plastic products is to avoid coloring pigments that react with resins.

 

Reaction between colorants and auxiliaries

 

1. Reaction between sulfur pigments and auxiliaries

 

Sulfur pigments, such as cadmium yellow CDs and CdSe solid solution, are not suitable for use in PVCs or lead additives because of their poor acid resistance.

 

2. Reaction of lead compounds with sulfur stabilizers

 

Lead in chrome yellow pigment or molybdenum chrome red reacts with antioxidant such as thiodicarboxylate DSTDP.

 

3, Reaction between pigment and antioxidant

 

Resins with antioxidants, such as PPPs, react with antioxidants in addition to the aforementioned "3.2", which weakens the antioxidant's function and degrades the thermal oxygen stability of the resin. For example, phenolic antioxidants are easily absorbed by or reacted with carbon black and become inactive; phenolic antioxidants in white or light-colored plastic products form phenolic aromatic complexes with titanium ions and yellowing the products. We choose suitable antioxidant or add auxiliary additives such as zinc stearate (zinc stearate) or P2 type phosphite to prevent the color change of white pigment TiO2.

 

4. Reaction between pigment and light stabilizer

 

The action of pigment and light stabilizer, except for the reaction between sulfur pigment and nickel light stabilizer, generally reduces the effect of light stabilizer, especially hindering the action of amine light stabilizer and azo yellow and red pigment. The effect of light stability decline is more obvious than that of uncolored stability, which has not been explained exactly.

 

Reaction between promoters

 

If many auxiliaries are not used properly, unexpected reactions may occur and the products will be colored. For example, the reaction of flame retardant Sb2O3 with sulfur-containing anti-reaction to form Sb2S3:Sb2O3 -S- Sb2S3 -O-

 

Therefore, when considering the production formula, we must carefully select additives.

 

Color change caused by Auto-Oxidation of Additives

 

The automatic oxidation of phenolic stabilizers is an important factor in promoting the color change of white or light-colored products. This color change is often called "Pinking" abroad, which is coupled with oxidation products such as BHT antioxidant 2-6-di-tertiary Ding Ji -4-methylphenol. The reaction product is similar to that of 3 ~ (3) C ~ (3) C ~ (3) C ~ (3 +) -5 '-iso-stilbene quinone. This discoloration occurs only in oxygen and water and without light. When exposed to ultraviolet light, the reddish iso-stilbene quinone is rapidly decomposed into yellow monocyclic product.

 

In addition , under the catalysis of light , the color changes from dark blue light red to light orange ; meanwhile , under the catalysis of light , water and water are decomposed to cause the cocrystallization water to change to cause fading .

 

Color change caused by atmospheric pollutants

 

When plastic products are stored or used, some reactive groups, whether they are resins or auxiliaries, or coloring pigments, will interact with atmospheric water or chemical pollutants, such as acids and bases, under the action of light and heat. A variety of complex chemical reactions will cause discoloration or discoloration over time, which can be avoided or mitigated by the addition of suitable thermal oxygen stabilizers, light stabilizers, or the selection of high quality weathering aids and pigments.

 

conclusion

 

(1)Oxidative degradation of matrix resin may lead to color change during high temperature molding;

 

(2) Discoloration of coloring agent at high temperature will cause discoloration of plastic products;

 

(3) The coloration will be caused by the chemical reaction between the coloring agent and the matrix resin or auxiliary agent.

 

(4) The reaction between auxiliaries and the automatic oxidation of auxiliaries will cause color changes.

 

(5) The tautomerism of colored pigments under photothermal action will cause the color change of the products.

 

Atmospheric pollutants may cause changes in plastic products.

 

 

 

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