Why does the color of plastic products change insensibly?

September 2, 2019

Why does the color of plastic products change insensibly?
 

When coloring plastic products by means of toner or masterbatch, color change may occur, which may affect product quality. Possible causes of discoloration:


(1) oxidative degradation of the matrix resin during high temperature molding;
(2) due to certain components of plastic products, such as matrix and additives, or matrix and coloring pigments, or chemical reactions between additives and pigments;
(3) Due to the inability to withstand high temperatures such as coloring pigments or auxiliaries, and the like.

 

The following is a description of the mechanism of color change caused by these factors, providing a reference for plastics manufacturers to correctly select raw materials to produce qualified plastic products.

 

Color change caused by plastic molding

1. When the high temperature molding, the matrix resin oxidatively degrades and discolors

 

When the heating ring or heating plate of the plastic molding processing equipment is always in a heated state due to loss of control, it is easy to cause the local temperature to be too high, so that the resin is oxidized and decomposed at a high temperature, and it is easier for the heat-sensitive plastics such as PVC to be formed during molding. This phenomenon occurs. In severe cases, the char is turned yellow and even blackened, and a large amount of low molecular volatiles escape.

 

Such degradation includes depolymerization, random chain scission, removal of pendant groups and low molecular species, and the like.

(1) Depolymerization
The depolymerization reaction first breaks at the end of the macromolecule, and then rapidly removes the monomer according to the linkage mechanism, which is particularly easy to proceed above the upper limit of polymerization temperature.

(2) Random chain breaking (degradation)
When a polymer such as PE is molded at a high temperature, its main chain may be broken at any position, the molecular weight decreases rapidly, but the monomer yield is small. This type of reaction is called random chain breaking, sometimes called degradation, polyethylene. The free radicals formed after chain scission have high activity, and there are more secondary hydrogens in the surrounding area. The chain transfer reaction is easy to occur, and almost no monomer is produced.

(3) Removal of substituents
When polyvinyl chloride, polyvinyl acetate, polyacrylonitrile, polyvinyl fluoride or the like is heated, the substituent is removed.

 

Taking polyvinyl chloride (PVC) as an example, PVC is processed at a temperature below 180-200 ° C, but at a lower temperature (such as 100-120 ° C), dehydrogenation (HCl) begins, and HCl is lost at about 200 ° C. Fast, the polymer becomes dark, the intensity is low, the total reaction is as follows: _CH2CHClH2CHCl~~→~~CH=CHCH=CH~~+2HCl

Free HCl catalyzes dehydrochlorination, and metal chlorides, such as hydrogen chloride, and ferric chloride formed by processing equipment, promote catalysis. 3HCl + Fe→FeCl3 + 3HCl

PVC should be added with a few percent of acid absorbent, such as barium stearate, organotin, lead compounds, etc. during thermal processing to improve its stability.

 

When the local communication communication cable is colored by the communication cable, the polyolefin layer on the copper wire is not well stabilized, and a green copper carboxylate is formed at the interface of the polymer-copper. These reactions promote the diffusion of copper into the polymer, accelerating the catalytic oxidation of copper.

 

Therefore, in order to reduce the oxidative degradation rate of polyolefin, a phenol or aromatic amine antioxidant (AH) is often added to terminate the above reaction to form an inactive free radical 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 polyolefin is a free radical chain reaction mechanism with autocatalytic behavior, which can be divided into three steps of initiation, growth and termination.

The cleavage of the chain caused by the hydroperoxide group leads to a decrease in molecular weight, and the main products of the homogenization are alcohols, aldehydes, ketones, and finally oxidized to carboxylic acids. Carboxylic acid plays a major role in the catalytic oxidation of metals.

 

2. When the plastic molding process, the colorant is decomposed and discolored due to high temperature resistance.

Pigments or dyes used for coloring plastics have temperature extremes. At this extreme temperature, pigments or dyes will undergo chemical changes, resulting in a variety of lower molecular weight compounds, the reaction formula is more complex; different pigments have different reactions And the product, the temperature resistance of different pigments can be detected by analytical methods such as weightlessness, generally:
For example, ←- heat resistance-→ poor example
Black dark light orange, pink
Everlasting purple, single color, color matching, yellow, red, purple
Cadmium red, cadmium yellow, titanium yellow, cobalt blue, cobalt green inorganic pigment organic pigment, dye lake red, azo yellow, transparent red
Quinacridone red macromolecular pigment small molecule pigment lake pigment azo 2B red
Injection, blow molding, short-term heating, long-term heating, sheet extrusion
Blown film, low temperature, high temperature coating
See Table 1 for the heat resistance of some pigments.
Table 1 Heat resistance of different pigments
Structure Type Pigment Color Change Temperature (°C)

 


Pigment name and index number (C.I.Pigment) Weight loss (%)
150°C 200°C 250°C 300°C
Benzidine yellow G P.Y.12 0 0 0.12 15
Benzidine yellow GG P. Y.17 0 4 6.2 7.7
Benzidine yellow HR P.Y.83 0.7 6.3 14 18.2
Macromolecular Yellow 2GL 0 0 0 1.3
Light fast red BBC P.R.48:2 5.5 7 8.2 11.4
Light fast red BBN P.R.48:1 2.1 3.8 10.2 18.1
Jin Guanghong C P.R.53:1 0.35 1.5 5.4 8.9
Macromolecular red R 0 0 0 0
Permanent Solid Purple RL P.V.23 0 0 0.7 1.1
Phthalocyanine Blue B P.B.15 0 0 0.1 3
Turnip Green G P.G.7 0 0 0 0

 

Therefore, when selecting masterbatch or coloring pigment, we must select the corresponding heat-resistant and economical coloring agent according to the requirements of the molding process of plastic products.

 

Color change caused by reaction of colorant with resin

The reaction of the colorant with the resin is mainly caused by the fact that certain pigments or dyes and resins are formed during processing. These chemical reactions will cause changes in the hue and degradation of the polymer, thereby changing the properties of the article.

 

Reduction reaction

Certain high polymers, such as nylon and aminoplasts, are strong acidic reducing agents in the molten state, which render pigments or dyes that are stable at processing temperatures reduced and faded.

 

2. Alkali exchange

Polyvinyl chloride emulsion polymers or alkaline earth metals in certain stabilized polypropylenes can "alkali exchange" with alkaline earth metals in colorants, thereby changing the color from blue-red to orange.

 

The PVC emulsion polymer is a method in which VC is stirred and stirred in an aqueous solution of an emulsifier (such as sodium dodecylsulfonate C12H25SO3Na), and Na+ is contained in the reaction; in order to improve the heat-resistant oxygen property of PP, an anti-1010, DLTDP, etc. resistance is often added. Oxygen agent, antioxidant 1010 is a transesterification reaction catalyzed by methyl 3,5-di-tert-butyl-4-hydroxypropionate and sodium pentaerythritol, and DLTDP is prepared by reacting an aqueous solution of Na2S with acrylonitrile to prepare thiodi Propionitrile is hydrolyzed to form thiodipropionic acid, and finally esterified with lauryl alcohol. Na+ is also contained in the reaction.

 

During the molding of plastic products, the residual Na+ in the resin will react with a lakeside-containing lake pigment such as C.I.Pigment·Red48:2 (BBC or 2BP): XCa2++ 2Na+→ XNa2+ + Ca2+

3. Reaction between pigment and hydrogen halide (HX)

The PVC is desorbed to a conjugated double bond at a temperature of 170 ° C or by light.

 

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

(1) Ultramarine and HX reaction:
It is widely used in plastics to color or eliminate yellow light. It is a sulfur-containing compound.

(2) Copper powder pigment accelerates the oxidative decomposition of PVC resin:
Copper pigments can be oxidized to form Cu+ and Cu2+ at high temperatures, which will accelerate the decomposition of PVC.

(3) Destructive effect of metal ions on the polymer:
Some pigments have a destructive effect on the polymer. For example, the manganese lake pigment CIPigment Red 48:4 is not suitable for PP plastic products because the variable metal manganese ions are used in the thermal oxidation or photooxidation of PP to catalyze hydrogen peroxidation by electron transfer.

 

Decomposition of the material leads to accelerated aging of PP; the ester bond in polycarbonate is easily hydrolyzed and decomposed by alkali when heated, and metal ions are more likely to promote decomposition once the pigment is present; metal ions also promote thermal oxygen decomposition of PVC and other resins. And cause color changes.

 

In summary, when producing plastic products, we should avoid using colored pigments that react with the resin is the most feasible and effective method.

 

Reaction between colorants and auxiliaries

1. Reaction between sulfur-containing pigments and auxiliaries

Sulfur-containing pigments, such as cadmium yellow (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 compounds with sulfur-containing stabilizers

The lead component of the chrome yellow pigment or molybdenum chrome red is reacted with an antioxidant such as thiobis stearate DSTDP.

3. Reaction between pigment and antioxidant

Antioxidant-added resins, such as PP, in addition to the aforementioned "3.2", some pigments react with antioxidants, thereby weakening the function of the antioxidant and deteriorating the thermal oxygen stability of the resin. For example, phenolic antioxidants are easily absorbed by or react with carbon black to lose activity; phenolic antioxidants in titanium or light-colored plastics form phenolic aromatic hydrocarbon complexes with titanium ions to cause yellowing of the product, we pass The selection of a suitable antioxidant or the addition of an auxiliary additive such as an acid-resistant zinc salt (zinc stearate) or a P2 type phosphite prevents the white pigment (TiO2) from being discolored.

4. Reaction between pigment and light stabilizer

The pigment and the light stabilizer act, in addition to the reaction of the sulfur-containing pigment described above with the nickel-containing light stabilizer, generally the effect of the light stabilizer is lowered, especially the hindered amine light stabilizer and the azo yellow and red pigments, and the light thereof The effect of steady decline is more obvious, and it is not as stable as uncolored. There is no exact explanation for this phenomenon.

Reaction between auxiliaries

Many auxiliaries, if used improperly, will likely cause unexpected reactions that can cause discoloration of the product. For example, the flame retardant Sb2O3 reacts with sulfur to form Sb2S3:Sb2O3+ –S–→Sb2S3+ – O –

Therefore, when considering the production formula, the auxiliaries 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. This color change is often referred to as "Pinking" in foreign countries.

It is coupled by an oxidation product such as a BHT antioxidant (2-6-di-tert-butyl-4-methylphenol) and is shaped like a 3,3',5,5'-stilbene quinone reddish reaction product. This discoloration occurs only in the presence of oxygen and water as well as in the absence of light. Upon exposure to ultraviolet light, the reddish stilbene oxime rapidly decomposes into a yellow monocyclic product.

Coloration of pigments by tautomerism under photothermal heating

Partially colored pigments undergo tautomerization under the action of photothermal heat, such as the use of CIPig.R2 (BBC) pigments from azo type to 醌 type, changing the original conjugate effect, causing conjugated bonds Decrease, causing the color to change from dark blue to red to light orange.

 

At the same time, under the catalysis of light, it decomposes with water, causing the co-crystallized water to change and causing fading.

 

Color change caused by atmospheric pollutants

When the plastic products are stored or used, some reactive groups, whether they are resins or additives, or colored pigments, will react with the moisture or chemical pollutants in the atmosphere, such as acid and alkali, under the action of light and heat. Causes a variety of complex chemical reactions that, over time, can cause discoloration or discoloration.


This can be avoided or mitigated by the addition of suitable thermal oxygen stabilizers, light stabilizers, or the use of high quality weathering aids and pigments.


In conclusion  

(1) Oxidative degradation of the matrix resin may cause color change during high temperature molding;
(2) Fading of the colorant at high temperatures causes discoloration of the plastic product;
(3) The chemical reaction of the colorant with the matrix resin or auxiliary agent will cause color change;
(4) The reaction between the auxiliary agent and the automatic oxidation of the auxiliary agent will cause color change;
(5) The tautomerization of the colored pigment under the action of photothermal causes the color change of the product;
(6) Air pollutants may cause changes in plastic products.

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