Why choosing Calcium Zinc stabilizers?
PVC is a versatile plastic that requires the help of additives in order to fulfill a certain function and to acquire additional properties, since it is useless on its own. In this article we will talk about Calcium Zinc thermal stabilizers, a type of additive whose function is to delay PVC thermal degradation caused when it is subjected to high temperatures and mechanical stress.
As with any organic compound, PVC tends to char when exposed to high temperatures. The thermal degradation mechanism basically consists of the emission of hydrochloric acid, an irritating gas for smell, which causes the gradual yellowing of the plastic and the loss of its mechanical properties. As PVC must go through a transformation process where stress and temperature are applied, it is important to postpone this decomposition mechanism as much as possible. This is where the thermal stabilizers help to form intermediate compounds and thus prolong this effect. There are two main classes of heat stabilizers: those based on tin and metallic salts.
As the evolution of PVC towards products that are friendly to the environment and human health, thermal stabilizers have presented changes. Originally, we can point to Barium/Cadmium/Zinc (BaCdZn) and tin stabilizers as the pioneers in the industry. Although tin-based stabilizer is still widely used today, BaCdZn, which belongs to the group of metal salts, did not have the same fate. This is due to the growing pressure to displace Cadmium as a carcinogenic element.
All these changes and new developments brought calcium zinc thermal stabilizers (CaZn) as a new proposal in the face of pressure from organizations to use non-toxic additives in PVC. Failure to do this would displace PVC from industries such as medical, food and toy, markets that are constantly under FDA supervision in the United States.
The first obstacle presented to calcium zinc thermal stabilizers was obtaining a product with similar performance to the grades accepted in the market at a competitive price. It is worth mentioning that calcium zinc thermal stabilizers can have different compositions. In the first place, there are metal salt derivatives in percentages that depend on the performance desired by the manufacturer in relation to its final application.
CaZn is usually accompanied by antioxidants and "boosters" that have a synergystic effect to improve the performance of the product. Also they come with a vehicle that supports the dispersion of the additive in the PVC matrix. There are usually two types of calcium zinc heat stabilizers: powder or liquid.
Depending on the process, application, and method of addition, the manufacturer can determine what works best. Those powder materials have the advantage of not presenting many volatiles, having low odor, and good mold release in the injection process, however, they can give opacity in high concentrations and increase the viscosity of its formula. On the other hand, the liquids have good transparency and brightness, although they may have more odor and volatiles.
This is not the only thing that is important when choosing the correct thermal stabilizer for your PVC formulation. You may need some light or weather resistance, some initial color retention, some long-term stability and/or lubrication. These are important factors in determining when to use the heat stabilizer.
Nor can we forget those toxicity requirements established by organizations such as California's Prop 65, where the use of certain excellent antioxidants in the performance of stabilizers is prohibited. This, in turn, causes them to be replaced by other, more expensive materials that complicate obtaining the same performance. An excellent case is Bisphenol A, a type of antioxidant that is being displaced in various medical, food and toy applications.
Latest Calcium Zinc trends
The latest developments of calcium zinc thermal stabilizers seek to enter those markets where there is still more resistance: rigid PVC. Currently, it is more common to use these materials in flexible, semi-rigid and Plastisol applications. This is because long thermal stability is not needed, due to low friction and torque when they are processed. You can measure the thermal stability in a rheometer to verify this.
These new developments of calcium zinc thermal stabilizers have presented the first complexity: because they are packages with a lubricating nature, their mechanism depends on a correct lubrication system balanced with the formula to which it will be applied so as not to have a surplus that may have negative implications in the fusion of the product to be manufactured. This is mainly because Calcium and Zinc Chloride are intermediate compounds that do not resist heat for a long time, a situation that does not occur with other compounds in other thermal stabilizers. They are usually powder grades due to the negative effect that liquids could have on the heat distortion temperature.
That is why for rigid PVC, a specific development must be carried out so that each client, for each formula and process, preventing a general use grade from being marketed. Another relatively new application is calcium zinc thermal stabilizers for electrical cables, which partially replace the lead-based thermal stabilizer, which has been banned due to human health issues. These materials are packages whose ingredients seek to offer hygroscopic properties and help maintain the volumetric resistivity of the cable made of PVC.
According to the temperature of use of the cable to be manufactured, there are different CaZn grades that prevent the plastic from losing elongation and tension properties throughout its use. This is very important to comply with market regulations.
In Chemical Additives we offer PVC compounds and additives for your formulas.
You can visit our mixed metal salts section HERE.
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