Characteristics of additives used in the processing of PVC.
PVC processing requires the use of many additives, thanks to which it is possible to obtain products for different purposes and with different levels of flexibility.PVC is very easily modified and therefore its properties can be freely customized to very different requirements depending on the intended use.Modification is accomplished through the appropriate selection of PVC blend components.A variety of stabilizers, plasticizers, impact and flow modifiers, fillers, and other additives are used to produce PVC plastics.
A feature that distinguishes PVC from other thermoplastic materials is the ability to modify its physical and mechanical properties via plasticizing, which has long been a widely used means of modification.Plasticizers are added to PVC in order to achieve the required level of flexibility of the end product.Their main task is to decrease the Tg (the glass transition temperature) of the PVC, which in turn leads to a lowering of the gelling and moulding temperature during processing.This also leads to a permanent improvement of the properties of the polymer, including itsflexibility, impact resistance and resistance to low temperatures.Plasticizers, which this article provides a general overview of, are one of the wide array of additives used in PVC processing. They will be discussed in more detail in subsequent articles.Plasticizers can be classified based on different criteria, such as:
- process characteristics (high-temperature, low-temperature plasticizers),
- type of chemical (phthalates, phosphates, polyesters etc.),
- molecular weight (monomeric, polymeric).
The most common plasticizer groups include:
adipic acid esters (DINA), (DIDA) – added to products used in low temperatures,
phosphate softeners-used in PVC plastificates with reduced flammability,
trimelliates – used in PVC plastificates resistant to high temperatures.
blends of metal-based stabilisers – Ca, Ca-Zn, Zn-Mg, Ba-Ca and Ba-Zn-Ca (generally stearates).They are suitable for stabilization of non-plasticized, semi-hard, plasticized PVCs and plastisols.Zn-Mg stabilisers are used in hard and flexible blends in place of obsolete
Ba-Cd stabilisers, whereas Ba-Zn stabilisers are a substitute of cadmium-based stabilisers.
In turn, Ba-Zn and Ca-Zn stabilisers are used with softened PVCs as a substitute for lead stabilisers.Ca-Zn stabilisers are used primarily for products made of plasticized poly(vinyl chloride) (PVC-P), intended for use in products which come into contact with food and in those used by children.
organic stabilisers – including organotin compounds, organic phosphites, epoxy compounds, aminocrotonates and a-phenylindoles.
This part of the article will present 2 of the largest groups of additives, while part II will deal with lubricants, impact modifiers, fillers and dyes used in the processing of PVC.
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