Updated: Aug 21, 2021
Polyvinyl Chloride(PVC) is the world’s third most widely used plastic in the world. About 40 million tonnes of PVC is produced each year. PVC is a versatile and resource-efficient thermoplastic with a wide range of applications, making it useful in virtually all areas of human activity.
However, without the use of additives in PVC processing, it does not yield much use; but its high compatibility with different additives to make it soft, to impart colour provides it with a broad range of applications.
One such important additive is heat stabilizers.
Pure PVC is a rigid polymer at room temperature with low thermal stability. Hence, PVC requires heat stabilisers during processing at high temperatures. The main function of stabilisers is to;
Prevent degradation during processing.
Reacts with HCl when it is liberated in PVC.
Replace with labile chlorine atoms
One of the recent studies done in the field is regarding the manufacture of Zeolites as secondary heat stabilisers. Secondary heat stabilizers are capable of reacting with the HCl that is liberated while processing, to avoid any further degradation.
Zeolites are Aluminosilicates with a general formula of [(M+,M2+0.5)AlO2]x.[SiO2]y.[H2O]z, where M+ is an alkali metal cation and M2+ is an alkali earth metal cation. Due to the presence of these cations, Zeolites are alkaline. Apart from these, they have a microporous structure that contains these cations. This structure accounts for their property as excellent ion-exchangers. The porous structures also account for the adsorption of various chemicals; therefore, they are widely used as catalysts in the commercial sector.
The different factors affecting the properties of Zeolites as Heat Stabilizers are:
1. Alkaline nature of Zeolite
One of the reasons Zeolites are being used as Heat stabilisers is because of their alkaline nature.
The extent of basic nature in different types of Zeolites is determined by their 3-D dimensional structure.
In a typical zeolite, the three-dimensional tetrahedral structure framework is such that the oxygen atom is shared with two tetrahedra.
If all tetrahedra contained Silicone, the overall structure would be neutral. However, the presence of Aluminium creates a charge imbalance and requires other metal ions to be present in relatively large quantities.
Thus, there is a significant increase in the thermal stability and acid resistance as the ratio of Al/Si increases in the Zeolite.
2. Presence of water of Crystallization
The presence of water of crystallisation is like a double-edged sword for Zeolite.
When Hydrochloric Acid is released while processing PVC, the reactive hydroxide in hydrous Zeolites reacts with the chloride as follows:
However, the presence of a higher amount of water of crystallisation may lead to the formation of bubble on the surface of PVC while processing
3. Microporous structure
The highly porous structure helps the HCl scavenging in two ways.
Firstly, the pores allow for the adsorption of HCl on the surface of Zeolites.
Secondly, the pores allow for ease of movement within the complex tetrahedral structure of Zeolites. These help in facilitating ion exchange. It also helps in the mobility of the reactive hydroxide; this helps in increasing the efficiency of minimising the production of released HCl.
2 parts per hundred of a typical zeolite with a chemical formula of Mg0.76Al0.24(OH)2(CO3)0.12.0.5H2O was added to 100g of PVC. The resultant formulation showed an effective heat stabilisation at 180oC, with an induction time of 33 mins, and resistance to HCl evolution for 337 mins. Thus, it displayed a 4x times better heat stabilisation as compared to blank PVC.
The information and research that are currently recorded regarding the use of Zeolites as heat stabilisers are currently limited as compared to the other traditional heat stabilizers. Nonetheless, they are effective in their usage and show a high potential in their growth in the coming years.
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