Thermostability PVC
Thermostabilität PVC | A service provided by |
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Thermostability PVC
General
Plastics are mostly exposed to heat, radiation and oxygen during their application, which can start processes of ageing. These processes affect the chemical and physical properties of the material.
Shear during processing, thermal energy, high-energy radiation or the influence of metal ions, e.g. from catalyst residues, can start an autocatalytic oxidation (reaction of the polymer with oxygen) in the polyvinyl chloride ( Abbreviation: PVC) material. The result of the chain reaction taking place is a change in molecular weight, rapid uptake of oxygen and the formation of hydroxy peroxide ( Abbreviation: ROOH), which further accelerates the chain reaction and can ultimately lead to technical failure of the material. The addition of stabilizers is an approach to slow down the ageing process.
Determination of the stability time
According to ISO 182-3 [1], the thermal stability time of PVC materials can be determined. High stability times can be equated with a longer service life. In the conductivity process, the PVC material is brought to a temperature of, for example, 200 °C and held there (Figure 1). Over time, this process releases hydrogen chloride and other decomposition products, which are transferred and absorbed in a measuring cell containing demineralized water. This leads to a change in the electrical conductivity of the water. The stability time is determined at a conductivity change of 50 µS/cm. The stability time is given in industrial minutes. A conversion into SI units is useful for better manageability.
The induction time indicates the time after which the change in electrical conductivity begins. It is determined at the intersection of two tangents applied to the conductivity curve.
| Fig. 1: | Stability time (green line) and induction time (red line) determined at 200 °C for a PVC material |
Factors influencing the stability time
The stability time determined depends on various factors, such as:
- used temperature,
- Specimen mass,
- Particle diameter and
- Particle thickness.
Figure 2 shows the influence of temperature and specimen properties on the thermostability of PVC window profile materials. As a result of increasing the test temperature (Figure 2 a), the ageing process of the material is accelerated, which is reflected in a decrease in the stability time. If, on the other hand, the specimen mass (Figure 2 b) is increased, this leads to a decrease in the stability time. If more material is available, the proportion of available hydrogen chloride also increases, which causes the change in the conductivity of the water in the measuring cell to occur more quickly.
According to the standard ISO/DIS 182-3, the particle diameter should be ≤ 2.0 mm. How this affects the stability time on PVC window profile materials is shown in Figure 2 c. A reduction and an increase by this value lead to an increase in the stability time. In both cases, more PVC and thus more dissolved hydrogen chloride is available. The same effect can be observed if the particle thicknesses varies, but the diameter of the grain is kept constant at 2 mm. The thicker the particle, the higher the stability time (compare Figure 2 c).
| Fig. 2: | Influence of temperature (a), sample mass (b), particle diameter (c) and particle thickness (d) on the thermostability of PVC window profiles [2] |
References
| [1] | ISO 182-3 (2023-12): Plastics – Determination of the Tendency of Compounds and Products Based on Vinyl Chloride Homopolymers and Copolymers to evolve Hydrogen Chloride and any other Acidic Products at Elevated Temperatures – Part 3: Conductometric Method (Draft) |
| [2] | Oßwald, K.: Own Studies, Polymer Service GmbH Merseburg (2023) |