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Development of synthetic Hydrotalcite as a secondary stabilizer for PVC
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Need to develop Synthetic Hydrotalcite (SHT):
Natural Hydrotalcite contains other minerals like Penninite, Muscovite & potentially undesirable elements like heavy metals that are difficult to remove
Attempts are made to manufacture synthetic Hydrotalcites (SHT) without impurities, especially for pharmaceutical industries, but also found effective for PVC processing using Ca-Organic stabilizer system.
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Advantages of SHT:
The main advantage of SHT, also known as Anionic clay is that it can be a perfect substitute to those toxic heavy metal containing stabilizers used in PVC compound containing lead, cadmium, barium, etc.
With the advent of Ca-Zn stabilizer, SHT grades with varying MgO/Al2O3 molar ratios at pH level 8-9.5 & bulk density are available in the growing market.
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Formula of Hydrotalcite:
Formula for natural hydrotalcite that is expressed as Mg6 Al2 (OH)16CO3· 4(H2O) is altered effectively.
Many modifications of this structure are made by changing MgO/Al2O3 ratio between 4-5.
A typical hydrotalcite may be represented by formula such as Mg4.5 Al2(OH)13· CO3 3(H2O).
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Water of crystallization:
Hydrotalcites contain water of crystallization that gets liberated on heating.
If, the processing temperature is more than the temperature of commencing of dehydration, then the bubbles may appear on the surface.
Notice 4(H2O) in natural and 3(H2O) in synthetic Hydrotalcite.
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Processing of PVC & CPVC:
Different grades having different temperature of commencing dehydration are available in the market.
UPVC is generally processed at Tg + 100° C and the extrudate temperature is 180 – 190° C.
While, CPVC is processed at Tg + 100° C and the extrudate temperature will be 215-225° C.
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Effect on processing & properties:
A grade of Hydrotalcite that commences dehydration at 185 ᵒ C, will not be suitable for CPVC. The surface will have bubbles.
Where transparency is required, SHT will be suitable. A grade having R. I.≈ 1.54 matches the R.I of PVC and the product will be transparent. Up to 10 Phr is reported to be used as functional filler.
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