preface
PVC plastic is one of commonly used plastics at present, the product variety is more.But some products give the impression that they are not durable.PVC raincoats, films, sandals, etc. will become brittle or even cracked after three or two years of use.Life span is shorter if used outdoors.There are two reasons for this: one is the current township enterprises in the processing process, such as cutting corners, a large number of fillers caused by the plastic performance decline;Structure that is the PVC itself has many weaknesses such as branched, terminal double bond, head to head structure (1, 2 ~ = chloride structure link), initiator, the funds belong to impurity, structure of oxygen, allyl chloride, poly and degree, molecular weight distribution, etc., these unstable factors in the external causes (such as light, heat, oxygen, solvents, additives, and other factors, such as HCl) under the action of aging, the degradation or crosslinking, make its physical and mechanical properties worse, losing application value.
PVC resin oxidation resistance requirements are not very high, than polyolefin is much smaller.In some cases, however, antioxidants are also needed, and small amounts of bpa can be added to outdoor products, such as agricultural film formulations, to slow down their degradation rate.Since the degradation process of most PVC is an ionization process, the main antioxidant is only used when considering the free radical degradation process.Because oxygen can intensify the heat of PVC, photodegradation process, high temperature plasticizer oxidation is also very fast.After oxidizing plasticizer can stone compatibility decline, "volatility" increase.All of these damage effect, make PVC product performance rapidly decline, and there will be a smell.Once a double bond is formed during the oxidation process of PVC, the subsequent oxidation process is the same as other unsaturated polymers.In order to prevent and alleviate the aging of PVC in the process of processing and use, and improve the application quality of PVC products, a certain amount of antioxidant should be added in some formulas.
The action of antioxidant
Antioxidants can be divided into two categories according to their action mechanism.Namely: chain termination and prevention.
Chain termination type antioxidant is also known as the main antioxidant.The antioxidants in this category include blocked phenols and sec-arylamines;Preventive antioxidants can eliminate free radical sources, inhibit or slow down the generation of free radicals in the induced peroxide, also known as auxiliary antioxidants, such as peroxide decomposition agents and metal ion passivation agents.
The antioxidants used in PVC are phosphite and blocked phenols.The mechanism of action of the two antioxidants is described below.
1. Phosphite
Phosphite belongs to the auxiliary antioxidant namely hydrogen peroxide decomposition agent.
The hydroperoxides generated by the free group chain reaction in the aging process of polymer materials can further initiate the free group chain reaction.The function of hydroperoxides decomposition agents is to decompose macromolecules of hydroperoxides to make them become stable hydroxyl compounds and be stabilized, thus ending the chain reaction.
2. Blocked phenols
Blocked phenols belong to the main antioxidant, namely, chain stop antioxidant.It belongs to the hydrogen donor in the form of action.
Hindered phenol molecules containing O -- - H has a lively hydrogen reaction functional group, as a result of the hydrogen transfer and the heat, light and oxygen, produce the macromolecular free radical R * and oxygen free radical ROO *, to generate a stable macromolecular, which itself into a low activity cannot spread a chain reaction of free radical Ar *, so that the chain reaction stops.The mechanism of "trapping" macromolecular free radicals to inhibit aging was illustrated by an example of 2,6~ ditert-butyl ~4~ methoxyphenol (264).
In addition, the addition of antioxidants, so that the plasticizer oxidation of hydrogen peroxide produced by the same inhibition and relief, to prevent the plasticizer further oxidation.
The antioxidant used in PVC should have good compatibility with PVC and be fully integrated into PVC plastic to give full play to its effectiveness.The compatibility of antioxidants can be altered.With the change of the internal composition of PVC in plastics, different compatibility is produced.
In addition, the main factors that influence the effectiveness of antioxygens include:
1. Structure of antioxidant.Different structure determines different mechanism and efficiency.
2. Antioxidant concentration.The application of antioxidant in PVC should be an appropriate concentration, too high or too low will have a negative impact on its effectiveness.
3. Volatility of antioxidant.The volatility has a great influence on the performance of antioxidant during processing and application.High volatility is not conducive to the performance.
4. Use of antioxidants.There may be additive, synergistic or antagonistic effects between antioxidants.When antioxidant is matched, it is better to choose antioxidant system with additive effect.
Application of antioxidant in PVC
PVC antioxidant requirements are not very high, so most of the PVC formula does not have antioxidant.But for a long time in the outdoor application, the application of high temperature environment, high weathering requirements of PVC products.Especially the plasticized PVC products, such as cable materials, which are prone to oxidation cracking and potential degradation.The formula is generally in addition to heat stabilizer, add a certain amount of antioxidant, in order to ensure the internal stability and appearance quality of PVC products.
In addition, in the reprocessing of recycling and utilization of waste PVC products, not only the lost heat stabilizer should be added, but also a certain amount of antioxidant should be added to reduce or lose the activity of free radicals caused by aging.Avoid chain reaction, enhance the stability of new products and prolong their service life.
Antioxidants are generally used in combination with primary and secondary antioxidants.Considering the comprehensive performance, source and cost, bisphenol A is the most used in PVC.The combined effect of bisphenol A and the antioxidant phosphite is significant. The combined use of bisphenol A and phosphite not only has antioxidant effect on PVC, but also has inhibition effect on preventing the plasticizer from volatilization and oxidative decomposition.The antioxidant BISphenol A is A low-efficiency antioxidant, but it can completely meet the use requirements of PVC, the dosage is generally 0.3~0.5%.The antioxidant phosphite (TPP) is widely used as PVC chelating agent. Especially when metal soap is added as the main stabilizer, the combined use can reduce the harm of metal chloride and protect the transparency and pigmentation of the products. Generally, the added amount is 0.3~1%.
In addition, the application of antioxidant in PVC mainly include: antioxidant CA, antioxidant 264, antioxidant 2246, antioxidant 1076 and so on.The addition amount is generally 0.01~0.5%.Epoxy lipids, such as epoxy soybean oil, have a certain inhibitory effect on the thermal oxidation of PVC and synergistic effect with antioxidant.
In the actual production of antioxidants, the basic use of mixed in resin or mixed in plasticizer added method.When producing soft PVC material, antioxidant with the help of plasticizer to add better, the production of hard PVC material, antioxidant through the conventional method of dry mixture.
conclusion
1. Antioxidant can delay or inhibit the oxidative degradation of PVC, enhance the stability of PVC, improve the color and weather resistance of products, and extend the life of products.
2. Antioxidants are not commonly used in PVC, but a certain amount of antioxidants can be added to the formulation of outdoor products, cable materials, waste PVC recycling, etc., to enhance its performance and improve its appearance quality.
3. Antioxidant can effectively prevent plasticizer oxidation in PVC products.
4. To improve the performance of PVC, it is necessary to further study the application of antioxidant in PVC.
