Measuring the Thermal Conductivity of Chlorinated Polyvinyl Chloride (CPVC)
A Correlation with ASTM C177
Chlorinated polyvinyl chloride (CPVC, Figure 1) is a thermoplastic polymer derived from polyvinyl chloride (PVC). It offers improved corrosion and thermal resistance relative to PVC, and exhibits flame retardant properties. Recently, we were asked by a client to provide correlative data in characterizing the thermal conductivity of the material via C-Therm’s Modified Transient Plane Source (MTPS) method with traditional ASTM C-177 (guarded hot plate) test data. Motivating our client’s interest is the understanding that while C-177 is a well-trusted method for measuring thermal conductivity – its sample restrictions and long test times make it challenging to work with. Comparatively, the MTPS’ one-sided convenient interface offers greater flexibility and they wish to understand the correlation between the methods.
First, we will provide a little further background on CPVC as a material. One substantial benefit to using CPVC in piping is it will self-extinguish if not in direct contact with a flame. CPVC also offers significantly improved ductility and crush resistance relative to PVC. These properties contribute to this material’s great popularity for plumbing applications. In plumbing applications, the thermal conductivity of the material is important. Any piping used to transmit hot water should ideally have low thermal conductivity (or high thermal resistance). Thermal conductivity of the piping material is an important aspect in maximizing efficiencies of the overall plumbing system.
Figure 1. CPVC is used primarily in plumbing, where CPVC is the material of choice for fittings, valves and pipes in most applications.
CPVC includes a broad class of polymeric compounds, with chlorination percentages which vary manufacturer to manufacturer. The material’s thermal properties, including specific heat capacity, glass transition temperature, and thermal conductivity, are highly dependent upon the polymer’s composition.
In characterizing the thermal conductivity, researchers have traditionally leaned towards steady-state techniques such as ASTM C-177. However, over the past twenty years, innovations in transient methods, such as the MTPS, have opened up opportunities for faster, easier, and more versatile test methods. This offers the potential for improved quality control testing of actual parts and accelerated R&D characterization. The Modified Transient Plane Source technique is widely considered the most consistent and accurate of the transient techniques. A comparison between these techniques is presented below.
ASTM C177 (Guarded Hot Plate) is known to be an absolute, accurate method of thermal conductivity analysis – it is the method to which many other standards, such as ASTM C518, are calibrated. To be valid, analyses require the achievement of thermal steady state, defined as follows:
8.8.1 Thermal steady state for the purpose of this test method is defined analytically as:
220.127.116.11 The temperatures of the hot and cold surfaces are stable within the capability of the equipment at the test conditions. Ideally an error analysis will determine the magnitude of the allowable differences, however the difference is usually less than 0.1 % of the temperature difference.
18.104.22.168 The power to the metering area is stable within the capability of the equipment. Ideally an error analysis will determine the magnitude of the allowable differences, however the difference is usually less than 0.2 % of the average result expected.
22.214.171.124 The required conditions above exist during at least four intervals 30 min in duration or four system time constants, whichever is longer. (ASTM C177)
Following achievement of steady state, three data acquisition runs are completed, each taking a minimum of 30 minutes, for a total minimum testing time of at least 3.5 hours, often much longer for thick, microporous, or especially dense samples. It is not uncommon for single samples to take a day to run. Testing rigid samples – including glasses, ceramics, and polymers below their glass transition temperature – via C177 requires extensive sample preparation to ensure that the sample planes are parallel and flat to the same extent as the plates, which results in extensive and highly precise machining requirements. As well, C177 requires special precautions be taken for materials with thermal conductivity over 0.1 W/mK, and for loose-fill samples (see sections 7.2.2 and 7.2.4 in ASTM C177 as well as ASTM C687 for more information).
In contrast, the Modified Transient Plane Source(MTPS) method of thermal conductivity analysis is a one-sided transient method. A test consists of a transient heat pulse which is applied to the sample surface through the sensor’s platinum coil. At the same time as the sensor coil heats, the guard ring is heated as well, ensuring heat flow into the sample is one-dimensional (for the short test time employed). Because of the transient nature of the measurement, a single measurement is obtainable in under a few seconds, allowing collection of a statistically significant number of data points in a matter of minutes. The MTPS method is a far more convenient method of collecting thermal conductivity data. For further detail on the points of difference between the methods a further detailed comparison is available here.
Comparing Thermal Conductivity Results
A piece of CPVC was characterized both by ASTM C177 and the MTPS method of thermal conductivity analysis at room temperature conditions (approximately 25 ◦ Celsius). The results are shown in Figure 2.
Figure 2. Thermal conductivity of CPVC, measured by ASTM C177 and by MTPS.
The thermal conductivity of CPVC as measured by ASTM C177 was 0.136 W/mK. The Modified Transient Plane Source method was employed to also test CPVC, and the observed thermal conductivity was 0.139 W/mK. The results for the two test methods agree within better than 2.5%. Similar studies testing NIST-source Standard Reference Material (SRM) of Expanded PolyStyrene (EPS) confirmed similar performance accuracy.
In conclusion, both methods provide accurate measurement of thermal conductivity – but the Modified Transient Plane Source offers several added advantages in characterizing representative samples in much shorter time.