Quality Control of Hard PVC According to ISO 18373

Introduction

Polyvinylchloride (PVC) has been used for over 60 years for pipes, window frames, films, electrical insulation material, and countless other applications. Hard PVC (PVC-U) is especially suitable for the manufacture of long-lasting products such as water pipes. More than 28 million tons of PVC was produced worldwide in 2001 [1].

In Europe, about half of all the different types of pipes (e.g. water, wastewater, and gas pipes) are made of hard PVC. Arguments for the use of PVC, for example for pipes are [2]:

• PVC has very long-lasting material properties and is continuously being developed. 
• PVC pipes are lighter than pipes made of other materials but are nevertheless strong, durable and flexible enough to bend without breaking. They are easy to install and are compatible with other materials used in pipe networks.
• PVC is resistant to corrosion. Pipes have low flow resistance due to their smooth surface. 
• PVC products have a good price/performance ratio. 
• PVC can be recycled and used in new products; its material properties remain intact. Great efforts are being made to process used material directly to make new products. If this succeeds, the life cycle assessment is good. Thermal recycling (burning) however remains a difficult problem.
  

PVC is polymerized from vinyl chloride in an aqueous emulsion or suspension. The process produces a powder with a grain size of about 100 µm. The morphology of the powder is complex and consists of agglomerates of primary particles of which about 5−10% are crystalline [1]. Hard PVC is produced by mixing the particles with additives (e.g. stabilizers) at a temperature of about 200 °C followed by extrusion to products such as pipes or frames. In this process, the particles “coalesce” or gelate. On cooling, a network consisting of entangled molecules and newly arranged, small crystalline domains, is obtained (Figure 1).

The degree of gelation has a decisive influence on the mechanical stability and the fracture behavior of products. The higher the processing temperature (T_p) the greater the degree of gelation. The processing temperature must however not be too high because otherwise degradation occurs in which HCl is eliminated (from about 180 °C onward) [3]. 

During the production process, crystallites with melting points at or above Tp will be annealed, thus slightly raising their melting point. Crystallites that melt up to T_p will recrystallize to form small secondary crystallites during the cooling phase after extrusion. On heating in the DSC, these will melt at temperatures below the processing temperature Tp [4]. For this reason, there are relatively few crystallites with a melting point around _Tp. This is observed as a melting gap in the DSC curve.

The processing temperature and the degree of gelation are therefore important quality parameters. Various test methods such as those involving solubility, hardness and other mechanical characteristics as well as optical microscopy are used to determine them. These methods often require time-consuming sample preparation or need larger samples. For this reason, DSC has been used for many years to investigate the melting behavior of PVC [e.g. 5-8].

The degree of gelation is defined as the ratio of the amount of small, secondary crystallites (melting point below T_p) and the amount of large primary crystallites with melting points above T_p. The content of crystallites is determined from the size of the corresponding enthalpies of fusion using DSC [3]. In simple terms, the larger the endothermic DSC peak in the range between T_a (about 100 °C) and T_p (about 200 °C), the better the degree of gelation (Figure 2).

Conclusions

The maximum processing temperature and the degree of gelation of PVC-U in finished products are two important parameters that have a significant influence on the stability and the mechanical behavior of the particular product. To simplify the evaluation, the enthalpy of fusion of the small crystallites below the processing temperature was determined in this method instead of the degree of gelation. DSC measurements performed according to ISO 18373 provide a simple and rapid method to determine the two parameters for quality control. Particularly advantageous is the fact that only small samples are required for the test. This enables samples to be taken from different locations around the pipe circumference.

^{Quality Control of Hard PVC According to ISO 18373 | Thermal Analysis Application No. UC 303 | Application published in METTLER TOLEDO Thermal Analysis UserCom 30}