PVC is a rigid polymer at room temperature with low thermal stability. Hence, it requires heat stabilisers to aid in processing at high temperatures. The main functions of a heat stabiliser are to prevent degradation during processing. This is done by:
· Reacting with HCl liberated while processing.
· Reacting with the labile chlorine atom present in PVC.
Since the beginning of the stabilisers market, lead-based stabilisers have played an important role. For most of the PVC processors in the market, the thumb rule of ‘More lead equals more stability has been a renowned notion for a very long time. However, the toxicity of lead is one of the major disadvantages of these types of stabilisers. Thus, many industries have switched to alternative heat stabilisers to make their products lead-free.
One such class of heat stabilisers which have gained popularity in the PVC market are Mixed Metal stabilisers.
Composition of Mixed Metal Stabilisers
· In a mixed metal blend, one component must have appropriate polarity to interact with the potential degradative site of PVC i.e., the labile Chlorine atom.
· This provides evidence for their classification as primary stabilisers.
· The primary metals chosen react with the labile chlorine atoms, next the secondary metals are chosen such that they form very weak chloride Lewis acid, but are quite stable thermally.
· Primary metals cations include Cadmium and Zinc, or occasionally Stannous tin.
· Secondary metals include Barium and Calcium, and occasionally Strontium or magnesium.
· Primary metals(Zn/Cd) provide for a good early colour as they quickly react with the chlorine atom present in PVC.
· The findings show that the mechanism followed is an ionic reaction mechanism. This leads to the formation of Zinc Chloride or Cadmium Chloride.
· These two are Lewis acids, and can also bind more than two chlorine atoms by forming complexes. Due to the formation of the chlorides, an unwanted catalysation of degradation may happen which may affect the long-term thermal stability.
· The secondary alkaline earth metal soaps(Ba/Ca) react with the HCl liberated to form respective chlorides.
Factors affecting the properties of PVC
· The properties like long term thermal stability and early colour requirements can be easily manipulated by changing the ratios of the primary metals and secondary metals in the stabiliser.
· The higher percentage of the secondary metals(Ba/Ca), ensures good thermostability with moderate early colour.
· On the other hand, if the primary metals dominate in composition, then it ensures an early colour with acceptable levels of thermostability.
· They provide an alternative to the traditional toxic lead stabilisers.
· They are non-toxic and thus can be used in fields of application where lead stabilisers can’t be used like food-grade products.
· Zinc and Cadmium salts act as a kicker in foamed applications by activating the decomposition of blowing agents. Thus, they are preferred in products that require f foaming.
· The market of mixed metal stabilisers has seen a trend in the sale of stabilisers in the form of one-pack systems with lubricants. These provide for the customers the necessary customization according to their produce.
· The market is ever-evolving with a potential for prospects with further development in research and study.
· The evolution of weak Lewis acids in the mechanism, calls for the use of suitable co- stabilisers, which interfere with some of the physical properties like water absorption and odour; and also incur an additional cost.
· Metal salts interfere with many of the properties of the product like transparency, gloss, weathering properties, etc.
· Metal salts also interfere with the flow behaviour of the polymeric melt and also influence the rheology of the melt. Thus, they might sometimes create complexion while processing.
· Liquid Barium-Zinc salts are used along with co-stabilisers in PVC plastisol. PVC-plastisol is used for the production of PVC wallpaper, artificial leathers, foils, and tarpaulins, and calendared flexible and semirigid films.
· For cable sheathing and insulation with lower requirements, calcium-zinc stabilisers can substitute for lead stabilisers at no extra cost.
· For foamed products, Zinc and cadmium salts are advantageous to use. Thus, there is increased use of mixed metal stabilisers in this particular sector.
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