Packaging Material Knowledge — What Causes the Color Change of Plastic Products?

• The oxidative degradation of raw materials may cause discoloration when molding at high temperature;
• Discoloration of colorant at high temperature will cause discoloration of plastic products;
• The chemical reaction between the colorant and raw materials or additives will cause discoloration;
• The reaction between additives and the automatic oxidation of additives will cause color changes;
• Tautomerization of coloring pigments under the action of light and heat will cause color changes of products;
• Air pollutants may cause changes in plastic products.

1. Caused by Plastic Molding

1)  The oxidative degradation of raw materials may cause discoloration when molding at high temperature

When the heating ring or heating plate of the plastic molding processing equipment is always in a heating state due to out of control, it is easy to cause the local temperature to be too high, which makes the raw material oxidize and decompose at high temperature. For those heat-sensitive plastics, such as PVC, it is easier to When this phenomenon occurs, when it is serious, it will burn and turn yellow, or even black, accompanied by a large amount of low molecular volatiles overflowing.

This degradation includes reactions such as depolymerization, random chain scission, removal of side groups and low molecular weight substances.

• Depolymerization

The cleavage reaction occurs on the terminal chain link, causing the chain link to fall off one by one, and the generated monomer is rapidly volatilized. 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 breaks or random broken chains. Under the action of mechanical force, high-energy radiation, ultrasonic waves or chemical reagents, the polymer chain breaks without a fixed point to produce a low-molecular-weight polymer. It is one of the ways of polymer degradation. When the polymer chain degrades randomly, the molecular weight drops rapidly, and the weight loss of the polymer is very small. For example, the degradation mechanism of polyethylene, polyene and polystyrene is mainly random degradation.

When polymers such as PE are molded at high temperatures, any position of the main chain may be broken, and the molecular weight drops rapidly, but the monomer yield is very small. This type of reaction is called random chain scission, sometimes called degradation, polyethylene The free radicals formed after chain scission are very active, surrounded by more secondary hydrogen, prone to chain transfer reactions, and almost no monomers are produced.

• Removal of substituents

PVC, PVAc, etc. can undergo substituent removal reaction when heated, so a plateau often appears on the thermogravimetric curve. When polyvinyl chloride, polyvinyl acetate, polyacrylonitrile, polyvinyl fluoride, etc. are heated, the substituents will be 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), it starts to dehydrogenate (HCl), and loses HCl very quickly at around 200°C. Therefore, during processing (180-200°C), the polymer tends to become darker in color and lower in strength.

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 absorbents, such as barium stearate, organotin, lead compounds, 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 polyolefin layer on the copper wire is not stable, green copper carboxylate will be formed on the polymer-copper interface. 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 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

Polymer products exposed to the air absorb oxygen and undergo oxidation to form hydroperoxides, further decompose to generate active centers, form free radicals, and then undergo free radical chain reactions (ie, auto-oxidation process). Polymers are exposed to oxygen in the air during processing and use, and when heated, oxidative degradation is accelerated.

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

The chain scission caused by the hydroperoxide group leads to a decrease in molecular weight, and the main products of the scission are alcohols, aldehydes, and ketones, which are finally oxidized to carboxylic acids. Carboxylic acids play 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. Oxidative degradation varies with the molecular structure of the polymer. The presence of oxygen can also intensify the damage of light, heat, radiation and mechanical force on polymers, causing more complex degradation reactions. Antioxidants are added to polymers to slow down oxidative degradation.

2) When the plastic is processed and molded, the colorant decomposes, fades and changes color due to its inability to withstand high temperatures

The pigments or dyes used for plastic coloring have a temperature limit. When this limit temperature is reached, the pigments or dyes will undergo chemical changes to produce various lower molecular weight compounds, and their reaction formulas are relatively complex; different pigments have different reactions. And products, the temperature resistance of different pigments can be tested by analytical methods such as weight loss.

2. Colorants React with Raw Materials

The reaction between colorants and raw materials is mainly manifested in the processing 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 high polymers, such as nylon and aminoplasts, are strong acid reducing agents in the molten state, which can reduce and fade pigments or dyes that are stable at processing temperatures.

• Alkaline Exchange

Alkaline earth metals in PVC emulsion polymers or certain stabilized polypropylenes can “base exchange” with alkaline earth metals in colorants to change the color from blue-red to orange.

PVC emulsion polymer is a method in which VC is polymerized by stirring in an emulsifier (such as sodium dodecylsulfonate C12H25SO3Na) aqueous solution. The reaction contains Na+; in order to improve the heat and oxygen resistance of PP, 1010, DLTDP, etc. are often added. Oxygen, antioxidant 1010 is a transesterification reaction catalyzed by 3,5-di-tert-butyl-4-hydroxypropionate methyl ester and sodium pentaerythritol, and DLTDP is prepared by reacting Na2S aqueous solution with acrylonitrile Propionitrile is hydrolyzed to generate thiodipropionic acid, and 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 material will react with the lake pigment containing metal ions such as C.I.Pigment Red48:2 (BBC or 2BP): XCa2++2Na+→XNa2+ +Ca2+

• Reaction Between Pigments and Hydrogen Halides (HX)

When the temperature rises to 170°C or under the action of light, PVC removes HCI to form a conjugated double bond.

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

1) Ultramarine and HX reaction

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

2) Copper gold powder pigment accelerates the oxidative decomposition of PVC raw materials

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

3) Destruction of metal ions on polymers

Some pigments have a destructive effect on polymers. For example, the manganese lake pigment C.I.PigmentRed48:4 is not suitable for the molding of PP plastic products. The reason is that the variable price metal manganese ions catalyze hydroperoxide through the transfer of electrons in the thermal oxidation or photooxidation of PP. The decomposition of PP leads to the accelerated aging of PP; the ester bond in polycarbonate is easy to be hydrolyzed and decomposed when heated, and once there are metal ions in the pigment, it is easier to promote the decomposition; metal ions will also promote the thermo-oxygen decomposition of PVC and other raw materials, and cause a color change.

To sum up, when producing plastic products, it is the most feasible and effective way to avoid the use of colored pigments that react with raw materials.

3. Reaction between colorants and additives

1) The reaction between sulfur-containing pigments and additives

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 content in chrome yellow pigment or molybdenum red reacts with antioxidants such as thiodistearate DSTDP.

3) Reaction between pigment and antioxidant

For raw materials with antioxidants, such as PP, some pigments will also react with antioxidants, thus weakening the function of antioxidants and making the thermal oxygen stability of raw materials worse. For example, phenolic antioxidants are easily absorbed by carbon black or react with them to lose their activity; phenolic antioxidants and titanium ions in white or light-colored plastic products form phenolic aromatic hydrocarbon complexes to cause yellowing of products. Choose a suitable antioxidant or add auxiliary additives, such as anti-acid zinc salt (zinc stearate) or P2 type phosphite to prevent discoloration of white pigment (TiO2).

4) Reaction between pigment and light stabilizer

The effect of pigments and light stabilizers, except for the reaction of sulfur-containing pigments and nickel-containing light stabilizers as described above, generally reduces the effectiveness of light stabilizers, especially the effect of hindered amine light stabilizers and azo yellow and red pigments. The effect of stable decline is more obvious, and it is not as stable as uncolored. There is no definite explanation for this phenomenon.

4. The Reaction Between Additives

If many additives are used improperly, unexpected reactions may occur and the product will change color. For example, flame retardant Sb2O3 reacts with sulfur-containing anti-oxidant to generate Sb2S3: Sb2O3+–S–→Sb2S3+–O–

Therefore, care must be taken in the selection of additives when considering production formulations.

5. Auxiliary Auto-oxidation Causes

The automatic oxidation of phenolic stabilizers is an important factor to promote 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 antioxidants (2-6-di-tert-butyl-4-methylphenol), and is shaped like 3,3′,5,5′-stilbene quinone light red reaction product , This discoloration occurs only in the presence of oxygen and water and in the absence of light. When exposed to ultraviolet light, the light red stilbene quinone rapidly decomposes into a yellow single-ring product.

6. Tautomerization of Colored Pigments Under The Action of Light and Heat

Some colored pigments undergo tautomerization of molecular configuration under the action of light and heat, such as the use of C.I.Pig.R2 (BBC) pigments to change from azo type to quinone type, which changes the original conjugation effect and causes the formation of conjugated bonds. decrease, resulting in a color change from a dark blue-glow 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.

7. Caused by Air Pollutants

When plastic products are stored or used, some reactive materials, whether raw materials, additives, or coloring pigments, will react with moisture in the atmosphere or chemical pollutants such as acids and alkalis under the action of light and heat. Various complex chemical reactions are caused, which will lead to fading or discoloration over time.

This situation can be avoided or alleviated by adding suitable thermal oxygen stabilizers, light stabilizers, or selecting high-quality weather resistance additives and pigments.