Applications

Triple Detection of Chlorinated Polyvinyl Chloride

Introduction

Usage: Chlorinated polyvinyl chloride (CPVC) is a popular polymer used on a large scale for piping. CPVC is also found in sprinkler systems due to its fire retardant properties. Being a thermoplastic, it can be easily molded into a desired shape at high temperatures which makes it recyclable since the molding is reversible. In some cases, the remolded CPVC product may not contain the required physical properties due to structural or chemical changes which can be directly linked to structural deficiencies of the final product. These changes are detectable by Triple detection GPC analysis which can determine the concentration of CPVC present in the sample product, making it possible to predict pipe failure and therefore prevent further processing of defective material. Ideally, a final CPVC product with desired physical properties contains CPVC as the primary component with a small quantity of additives (~15%).

Manufacturing: CPVC is formed from polyvinyl chloride (PVC) by chlorination as shown in Figure 1. After chlorination chlorine atoms displace hydrogen on the carbon backbone, increasing the chlorine weight percent of commercial CPVC to approximately 63% to 69% compared to 57% in PVC. The increased chlorine content of CPVC makes it more stable at higher temperatures than PVC; hence it is more commonly used in industrial applications of high temperature liquids such as hot water heaters. Control of molecular weight (MW) and MW distribution is essential for quality control of properties such as elasticity, strength and electrical resistance.

Chemical Formula: (C2HCl3)n

Chemical Structure

Conversion of Polyvinyl Chloride to Chlorinated Polyvinyl Chloride by Chlorination

Figure 1. Structure of polyvinyl chloride.

Instrumentation

One virgin and one cracked CPVC samples were dissolved in tetrahydrofuran (THF) at a concentration of 1 mg/mL. The sample were shaken at 200 rpm for 48 hours then passed through a 0.2 µm teflon filter. Analysis was completed on a Viscotek Triple Detector Array (TDA) 302 equipped with a Refractive Index (RI) detector, Viscometer, Right Angle (90o) Light Scattering (RALS) and Low Angle (7o) Light Scattering (LALS). Separation was performed using two PolyAnalytik SupeResTM columns (1 x PAS 105L and 1 x PAS 104L). The sample was injected at a volume of 100 µL and eluted through the system at flow rate of 1 mL/min in THF. A temperature of 30oC was maintained during separation and detection.

Results and Discussion

Analysis of PVC and the higher molecular weight CPVC using conventional techniques will give similar results since separation in GPC is based on size and the hydrodynamic radius of the two will be the same. Triple detection GPC employs a concentration detector, a viscometer, and a light scattering detector that allows differentiation between PVC and CPVC. A good chromatogram has a clean baseline and high resolution peaks that come back to baseline, all of which can be seen in an RI overlay of one virgin CPVC sample and one cracked CPVC sample shown in Figure 2. Both samples have a good signal to noise ratio. Using this data and that from the light scattering detector and viscometer, the molecular weight and physical parameters presented in Table 1 are an average of three injections of each sample. Sample A visually gives the lowest RI signal of the two samples but when analyzed with all detectors it gives the higher molecular weight of the two samples. While the two samples have the same retention volume, all detectors are required to detect small differences in the molecular weight of the samples.

Overlay Plot of Refractive Index of Polyvinyl Chloride

Figure 2. RI overlay of CPVC samples.

Overlay Plot of the Refractive Index Chromatograms Versus Retention Volume of Polyvinyl Chloride

Figure 3. Triple detection chromatogram of sample A.

Using data from the light scattering detector and viscometer, the molecular weight and physical parameters can be seen in Table 1.

Table 1. Molecular weight and size derived from triple detection method.

Table 1

Using the viscometer, along with the other detectors, information on the polymer backbone can be determined. Based on the densities of the samples the degree of chlorination of the different CPVC sample can be determined which predict their respective properties. The Mark Houwink plot of the two samples, as seen in Figure 4, completely overlay indicating that there are no differences in the backbone. Downward curvature of the plot at high molecular weight is indicative of branching, however, in order to determine the degree of branching a known linear reference is required.

Mark Houwink Plot of Polyvinyl Chloride

Figure 4. Mark Houwink plot of CPVC samples.

Conclusion

Triple detection of CPVC samples with different molecular weights but the same retention volume was successfully achieved. Using conventional techniques the same molecular weight would be obtained for each sample. Triple detection technique is therefore essential in order to predict the physical performance of the polymer in such applications as drinking water piping or fire sprinkler systems.

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