How to solve the problem of discolouration of produced plastic products?
● When colouring plastic products using methods such as colour powders or masterbatches, the phenomenon of colour change may occur, thus affecting the quality of the product.
Possible causes of colour change:
(1) Caused by oxidative degradation of the base resin during high-temperature moulding;
(2) Due to the chemical reaction between some components of the plastic products, such as the base and the auxiliary, or the base and the colouring pigment, or the auxiliary and the pigment;
(3) Caused by the colouring pigment or additives that are not resistant to high temperature, and so on.
By analyzing the mechanism of colour change caused by these factors, the following is to provide reference for plastic product manufacturers so that they can choose raw materials correctly and produce qualified plastic products.
-Colour change caused by plastic moulding process
1. Oxidative degradation and discolouration of base resin during high-temperature moulding.
When the heating ring or heating plate of the plastic moulding equipment is out of control and always in the heating state, it is easy to cause the local temperature to be too high, so that the resin oxidative decomposition occurs at high temperature, for those heat-sensitive plastics, such as PVC, etc., it is more likely to appear this phenomenon during the moulding process, and when it is serious, it will be burnt and turned yellow, or even blackened, and accompanied by a large number of low molecular volatiles escaping.
This degradation includes depolymerisation, random chain breaking, side group and low molecule removal and other reactions.
(1) Depolymerisation
Depolymerisation reaction is first in the macromolecule at the end of the break, and then according to the chain mechanism to quickly remove the monomer, in the polymerisation of the upper limit of temperature above the especially easy to carry out.
(2) Random chain breaking (degradation)
For polymers such as PE and other polymers in high temperature moulding, its main chain may be broken at any position, the molecular weight decreases rapidly, but the monomer yield is very small, this kind of reaction is called random chain breaking, sometimes also called degradation, polyethylene chain breakage of the formation of free radicals is very active, surrounded by a large number of secondary hydrogen, easy to chain transfer reaction, almost no monomer generation.
(3) Substituent removal
Polyvinyl chloride, polyvinyl acetate, polyacrylonitrile, polyfluoroethylene, etc. When heated, the substituent group will be removed. Polyvinyl chloride (PVC), for example, PVC processing molding in the temperature below 180 ~ 200 ℃, but at lower temperatures (such as 100 ~ 120 ℃), that is, the beginning of dehydrogenation (HCl), 200 ℃ around the loss of HCl very quickly, so that the polymer into a dark colour, the strength of the lower, the total reaction is briefly described as follows: ~ ~ CH2CHCICH2CHCl ~ → → ~ CH=CHCH=CH ～+2HCl
Free HCl has a catalytic effect on the removal of hydrogen chloride, metal chlorides, such as hydrogen chloride and processing equipment to form ferric chloride, to promote catalytic: 3HCl + Fe → FeCl3 + 3HCl
PVC in thermal processing shall be added a few per cent of the acid absorber, such as barium stearate, organotin, lead compounds, etc., in order to improve its stability.
When colouring municipal telecommunication cable lines with communication cables, the polyolefin layer on the copper conductor is not well stabilised, and green copper carboxylate will form at 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 rate of oxidative degradation of polyolefins, often add phenolic or aromatic amine antioxidants (AH), terminate the above reaction, the formation of inactive radical A-: ROO- + AH – → ROOH + A-
(4) Oxidative degradation
Polymers are exposed to oxygen in the air during processing and use, and oxidative degradation is accelerated when subjected to heat.
Thermal oxidation of polyolefins belongs to the free radical chain reaction mechanism with autocatalytic behaviour, which can be divided into three steps of initiation, growth and termination.
The breakage of the chain caused by the hydroperoxide group leads to a decrease in molecular weight, and the main products of its homolytic cleavage are alcohols, aldehydes, ketones, and finally oxidised to carboxylic acids. Carboxylic acid plays a major role in metal-catalysed oxidation.
2. When plastic moulding and processing, the colouring agent will decompose and discolour due to high temperature intolerance.
The pigments or dyes used for colouring plastics have the limit of temperature resistance, when the temperature reaches this limit, the pigments or dyes will undergo chemical changes, generating a variety of lower molecular weight compounds, whose reaction formula is more complicated; different pigments have different reactions and products, and the temperature resistance of different pigments can be detected through the weight loss and other analytical methods.
-Colour change caused by the reaction between colourant and resin
The reaction between colorant and resin is mainly manifested in some pigments or dyes and resin when processing and moulding, these chemical reactions will lead to the change of colour phase and degradation of polymer, so that the performance of products will be changed.
1.Reduction reaction
Certain polymers, such as nylon and amino plastics in the molten state, is a very strong acidic reducing agent, they can make in the processing temperature is very stable pigments or dyes are reduced to fade.
2. Alkali exchange effect
The alkaline earth metals in PVC emulsion polymer or some stabilized polypropylene can have “alkaline exchange” with alkaline earth metals in the colouring agent, thus making the colour change from blue-red to orange.
PVC emulsion polymer is VC in the emulsifier (such as sodium dodecyl sulfate C12H25SO3Na) aqueous solution with the aid of stirring polymerisation method, the reaction contains Na +; in order to improve the heat and oxygen performance of PP, often added 1010, DLTDP and other antioxidants, antioxidants 1010 is a tertiary-butyl by 3,5 a 4 hydroxypropanoic acid methyl ester and sodium pentaerythritol catalyzed ester exchange reaction. DLTDP is prepared by the reaction of Na2S aqueous solution and acrylonitrile to prepare thiodipropionitrile, hydrolysed to generate thiodipropionic acid, and finally esterified with lauryl alcohol, the reaction also contains Na+.
When moulding plastic products, the residual Na+ in the resin will react with the colour precipitation pigment containing metal ions, such as C.I.Pigment-Red48:2(BBC or 2BP): XCa2++2Na+→2XNa++Ca2+.
3. Reaction between pigment and hydrogen halide (HX)
PVC is deconjugated to HCI and forms conjugated double bonds when the temperature rises to 170°C or under the action of light.
Halogen-containing flame-retardant polyolefin or coloured flame-retardant plastic products are also dehalogenated hydrogen HX in high temperature moulding.
(1) Reaction between Ultramarine and HX
Ultramarine pigment, which is widely used for colouring plastics or eliminating yellow light, is a sulphur-containing complex.
(2) Copper pigment accelerates the oxidative decomposition of PVC resin.
Copper pigment can be oxidised at high temperature to form Cu+ and Cu2+, which will accelerate the decomposition of PVC.
(3) Destructive effect of metal ions on polymer
Some pigments have destructive effect on polymers, such as manganese pigment C.I.PigmentRed48:4 is not suitable for PP plastic products moulding, the reason lies in that manganese ions of variable valence metal catalyze the decomposition of hydroperoxides through the transfer of electrons in the thermal oxidation or photo-oxidation of PP, which leads to the accelerated aging of PP; ester bond of polycarbonate is easy to hydrolyze when heated and decompose in the face of alkali, and the metal ions will promote the decomposition more easily when there are in pigment; metal ions will promote the decomposition more easily. Once there are metal ions in the pigment, it is easier to promote the decomposition; metal ions will also promote the thermo-oxidative decomposition of PVC and other resins, and lead to colour changes.
To sum up, it is the most feasible and effective way that we should avoid using the colouring pigment which reacts with resin when producing plastic products.
-Reaction between colourants and auxiliaries
1、Reaction between sulphur-containing pigment and auxiliaries
Sulphur-containing pigments, such as cadmium yellow (solid solution of CdS and CdSe), should not be used in PVC because of poor acid resistance, and should not be used together with lead-containing auxiliaries.
2、Reaction between lead-containing compounds and sulphur-containing stabilisers
Chrome yellow pigment or molybdenum chrome red in the lead component and antioxidant such as thiodistearate DSTDP reaction.
3、Reaction between pigment and antioxidant
In the resin with antioxidant, such as PP, some pigment and antioxidant will also react, thus weakening the function of antioxidant and making the thermal and oxygen stability of resin inferior.
For example, phenolic antioxidant can be easily absorbed by carbon black or react with it and lose its activity; phenolic antioxidant and titanium ion form phenolic aromatic complex in white or light-coloured plastic products to make the products yellowing phenomenon, we can prevent the white pigment (TiO2) from colour change by choosing suitable antioxidant or adding auxiliary additives such as zinc antacid salt (zinc stearate) or P2-type phosphite.
4、Reaction between pigment and light stabiliser
When pigment and light stabilizer react, in addition to the reaction between sulfur-containing pigment and nickel-containing light stabilizer mentioned before, it will reduce the effect of light stabilizer in general. Especially when it is affected by the role of hindering amine light stabilizer and azo yellow and red pigment, the light stabilizing effect decreases even more obviously, and it is not as good as the uncoloured stabilizing effect, and there is no exact explanation for this phenomenon at present.
-Reaction between auxiliaries
If many auxiliaries are used improperly, unexpected reaction may occur and make the products change colour. For example, the flame retardant Sb2O3 reacts with sulphur to form Sb2S3: Sb2O3+-S-→Sb2S3+-O-.
Therefore, when considering the production formula, the auxiliaries must be carefully selected.
-Colour change due to auto-oxidation of additives
Auto oxidation of phenolic stabilizers is an important factor in promoting the colour change of white or light-coloured products, which is often referred to as “Pinking” (reddish) in foreign countries.
It is coupled by the oxidation products such as BHT antioxidant (2-6-di-tert-butyl-4-methylphenol), and shaped as 3,3′,5,5′ a homostilbene quinone light red reaction product, this kind of discolouration only occurs in the presence of oxygen and water and the absence of light, exposure to ultraviolet light, light red homostilbene quinone is decomposed rapidly into a yellow monocyclic product.
-Colour change of colouring pigments under the action of light and heat
Under the action of light and heat, the molecular configuration of some coloring pigments changes into isomerism, for example, the use of C.I.Pig.R2(BBC) pigment changes into quinone type from azo type, which changes the original conjugation effect and causes the reduction of conjugate bond, and causes the colour to change from dark blue-red to light orange-red.
At the same time, under the catalytic effect of light, it decomposes with water, causing changes in the co-crystalline water and fading.
-Colour change caused by atmospheric pollutants
When plastic products are stored or used, some reactive groups, no matter resin or additives, or colouring pigments, under the action of light and heat, will interact with moisture or chemical pollutants in the atmosphere such as acid and alkali, causing various complex chemical reactions, which will lead to discolouration or discolouration in the long run.
By adding suitable heat and oxygen stabilizers, light stabilizers, or choosing high-quality weathering additives and pigments, this situation can be avoided or moderated.