Stabilizers are one of the most important additives used in PVC formulations, so that in all formulations of this polymer, there are one or more types of stabilizers. Pure PVC is destroyed at high temperature or exposed to light or high cutting and loses its original physical and mechanical properties. It should be noted that the low temperature of PVC destruction is due to the presence of incomplete structures in this material. If it is possible to prepare PVC in such a way that there are no incomplete structures in it, in such a way that all the monomeric units are connected head to tail and saturated and unbranched structures are obtained, in This form of PVC produced is stable even up to 300 degrees Celsius. As a result, it does not get destroyed in the forming process, which has a temperature in the range of 170 to 220 degrees Celsius.
For example, due to the oxidation of PVC, a small number of carbonyl, carboxyl, hydroperoxide and peroxide groups are formed on it. In some of these defective structures, unstable chloride groups are also formed. The number of these unstable chlorides is less than 0.5% of PVC chlorides, but nevertheless, they have an important effect on the destruction of PVC, so that they reduce the temperature of PVC destruction to about 100 degrees Celsius. The destruction of PVC is done in three general ways: thermal destruction, destruction due to light radiation, and destruction due to cutting, all three types have similar mechanisms.
It should be noted that thermal destruction is the most common type of destruction.
Mechanism of thermal degradation of PVC
The thermal degradation mechanism of PVC includes three stages: initiation, growth, and termination.
Degradation of PVC begins with the release of unstable chlorine and the formation of a double bond in the chain and the production of HCL. In this way, a polyene molecule is formed, which changes the color of PVC. If the destruction of PVC is done completely, along with the production of HCL, some carbon black, benzene and aromatic substances will also be produced. The rate of PVC degradation increases in the presence of oxygen, HCL and some chloride salts.
PVC stabilization mechanism
Due to the wide variety of stabilizers used in PVC formulations, several mechanisms have been presented to reduce or delay the degradation of PVC. In general, the most important mechanisms presented can be categorized as follows:
Replacement of unstable chlorides with stable ligands
Neutralize the formed HCL by slowing down the degradation rate
Reducing the number of double bonds so that the physical and mechanical properties are reduced less
Inhibition of oxidation
Deactivating free radicals
Types of PVC stabilizers
As mentioned earlier, various stabilizers have been used for PVC formulations. In general, a stabilizer should be compatible with polymer, resistant to light and heat, processable, colorless and odorless, non-toxic, durable and cheap. In the following, the famous stabilizers in the PVC industry are explained.
Lead compounds are the oldest and cheapest PVC heat stabilizers, which are prepared from lead metal with a purity of 99.84% in several steps. In the first stage, lead metal is oxidized in the presence of air. Then the obtained oxide is subjected to several stages of separation to obtain a raw oxide which is a mixture of lead monooxide and about 5 to 20% of lead metal. Of course, the product obtained from this step will contain a small amount of Pb3o4 and Pb.
In the third stage, the product and the second stage react with sulfuric, phthalic, phosphoric or stearic acid in the presence of water at a temperature of about 100 degrees Celsius.
In the final stage, the stabilizer obtained from the third stage is mixed with a suitable lubricant, in order to achieve synergistic properties, in a dry or molten form, and in this way, a suitable stabilizer for PVC formulations is obtained. comes .
The most important commercial lead stabilizers are:
Three-game lead sulfate
Two-game lead stearate
Lead phthalate two games
Lead phosphite two games
Normal lead stearate
This group of PVC stabilizers are actually complex solid, liquid or paste mixtures of metal salts and soaps with a synergistic effect that contain three to eight main components and according to the metal in it are being named. For example, cadmium-barium, barium-cadmium-zinc, barium-zinc and calcium-zinc stabilizers are the most important members of this group.
The most important fatty acids used to produce metallic soaps are lauric acid and acetic acid. Soaps are also obtained from the direct neutralization of acids with metal oxides or metal hydroxides in the following reactions:
2RCOOH+MO ————————-> (RCOO)2M+H2O
2RCOOH+M(OH)2 —————————-> (RCOO)2M+2H2O
If metal mixtures are used in PVC formulations, due to the presence of metal soaps in the composition of these stabilizers, there will be a need for a smaller amount of lubricant.
The effectiveness of mixed metal stabilizers changes drastically when they are used together with special materials called stabilizers. Stabilizers are effective through various mechanisms such as HCL neutralization, replacement of unstable chlorides, etc. They increase the stabilizer of the metal mixture.
Epoxy esters are the most common stabilizers that are used in the amount of 1 to 5 parts in metal mixtures. The most important member of this group is epoxidized soybean oil.
Among stabilizers, phosphite esters are the second most important. The phosphites used for this purpose were mostly diphenyldecyl phosphite and triphenyldecyl phosphite, but today other members of this group such as trilauryl phosphite, tetradecyl bisphenol A diphosphate and tetradecyl pentaerythritol diphosphate are used. It shows the positive effect of increasing different amounts of phosphite composition on PVC stabilization.
The stabilization mechanism of phosphites depends on the presence or absence of alkaline earth metals in the formulation. In the presence of soluble calcium or barium salts, phosphite gives the free electron pair to the calcium or barium salt and forms a soluble complex.
Choosing a thermal stabilizer system
Choosing a PVC heat stabilizer system is not an easy task and it is done according to the type of forming process and the application of the final product. Below are some examples of PVC stabilizing and softening systems, based on the type of forming process and product application.