Production of PVC composites containing halloysite: Investigation of thermal and mechanical properties

Polyvinylchloride (PVC) is one of the mostly used thermoplastic materials in the industry. It is used in many engineering applications due to its fire resistance, chemical resistance, and mechanical properties. The fire resistance of PVC can be improved by adding various fillers. In this study, halloysite nanotube (HNT) was used to improve the thermal and mechanical properties of PVC/Mg(OH)2 composites. The composite compounds were produced by melt-mixing of PVC with plasticizer, magnesium hydroxide, HNT and other processing additives in an internal mixer. The test samples were prepared by spraying them with a laboratory-type extruder in the form of strips. The DSC and TGA analyzes were performed to determine thermal properties of the composites. The limiting oxygen index (LOI) values and tensile mechanical properties of the composites were also investigated. From the high magnification SEM images, it was seen that the Mg(OH)2 filler was dispersed in the matrix as large sheet blocks. In addition, it was observed that silica nanotubes exhibited dispersion in the form of agglomerates/aggregates as well as individual separation in the composite. In the composites in which Mg(OH)2 and HNT filler were used, glass transition temperature of around 85 ºC was not observed due to the increased limitations in the segmental mobility of the polymer molecules with the increase of HNT. Compared to the HNT-free Mg(OH)2 filled composite, the percentage of PVC crystallization in HNT filled PVC/Mg(OH)2 composites increased with the increase in the amount of HNT. This result was based on the HNT filler acting as the nucleating agent for PVC crystallization. According to the DSC analysis results, the PVC gelation percentage tended to decrease as the HNT content in the composite increased. When 100% HNT was used as the filler, this reduction was at the maximum level. The elastic modulus value increased as the HNT content in the PVC composite increased. It was concluded that the expected decrease in elastic modulus due to the decreasing gelation ratio with the increase of HNT in PVC composites is compensated by the hardening HNT filler. A significant decrease was found in the tensile strength and toughness values of the composite containing only HNT without using Mg(OH)2. As a result of TGA analyses, decreases in degradation rates were observed in HNT-containing composites compared to the reference composite containing only Mg(OH)2 filler, and this result was thought to be an indicator of increase in thermal stability since it was compatible with increasing Td50 temperature values in the same compositions. As a result of the LOI analyzes of the composites containing both fillers, it was observed that the required oxygen concentration value increased as the HNT ratio increased compared to the composite containing only Mg(OH)2. On the other hand, it was determined that the LOI value of the composite containing only HNT decreased compared to the composite containing only Mg(OH)2. As a result, when PVC polymer and Mg(OH)2 and HNT fillers were used together, a synergistic effect was created and improved mechanical and thermal properties were obtained when 10%, 15 and 20% HNT was used in the total filler amount.