Rubber is an incredibly important material that has numerous applications, many of which have vastly improved our quality of life. The majority of rubber is used by the automobile industry for tires, however, it has other applications including waterproof clothing, cleaning supplies, and medical products. The number of uses that we have for rubber today is thanks to the vulcanization process, which improves the quality of uncured rubber by creating new atomic bridges between the rubber polymers. This gives the rubber increased elasticity, and the ability to revert to its original shape after undergoing stress. It is able to maintain a more stable form instead of becoming soft and sticky when heated and brittle when cold. The rate of cure (vulcanization) has a large influence on the physical properties of the resulting rubber product. Cheheb et al. (2012) used a controlled and homogenous curing process to study the impact of different curing rates on the thermal conductivity of the resulting rubber. They then applied what they had learned about the thermal conductivity to a model that predicts the vulcanization rate during the curing process.
Figure 1. Tire manufacturing is the single largest user of rubber on the planet.1
Cheheb et al. (2012) manipulated the curing process to produce rubber samples that had been produced with different vulcanization rate. An uncured sample was also obtained for use as a reference. The thermal conductivity of each of the samples was measured using the Hot Disk Transient Plane Source 2500s. The Thermal Conductivity Analyzer Hot Disk TPS 2500s is state of the art thermal conductivity equipment which measures values between 0.005 and 1000 W/mK. It is capable of working with solids, liquids, pastes and powders, and can perform both low and high temperature readings. The researchers carefully selected a sensor size whose probing depth would stay within the bounds of the sample. Thermal conductivity is measured using the two-sided sensor by sandwiching identical sample pieces on either side (Figure 2). Sufficient pressure to ensure good contact between the samples and sensor is applied by the sample holder (Figure 2). Five measurements were performed on each sample. A single sided sensor is also available for use with the Hot Disk TPS system; this sensor can perform the same measurements as the two-sided sensor but does not require two pieces of the same sample, an excellent option for clients who cannot or do not want to cut samples.
Figure 2. Two test set-ups to measure the thermal conductivity of rubber are shown in the Thermtest lab. The set-up on the left is using the two-sided Hot Disk sensor, two identical rubber samples are placed on either side. The stand used can add and monitor the amount of pressure needed. On the right is a set-up with a single-sided sensor, which requires just a single rubber sample to be placed on top.
The sensitivity with which the Hot Disk TPS 2500s determines thermal conductivity enabled the researchers to ensure that there were no air bubbles present in the samples. This is an excellent reminder that the Hot Disk TPS equipment can be highly useful for quality control purposes. In this case, Cheheb et al. (2012) were able to establish that no bubbles were present by assessing the residual plot produced by the system during measurement. An accurate measurement produces a plot containing randomly scattered dots.
Rubber Samples Thermal Conductivity Results
Cheheb et al. (2012) determined that the vulcanization rate had a large influence on the resulting thermal conductivity of the rubber produced. The thermal conductivity of the samples increased alongside the vulcanization rate. Once they had established that the vulcanization rate affected the thermal conductivity of the rubber produced, Cheheb et al. (2012) used their results to understand the effect that the thermal conductivity variation would have on the predicted vulcanization kinetic rate.
This work was made possible by the accurate, reliable measurements that researchers expect from the Hot Disk TPS system. The ability of this equipment to detect small changes in the thermal conductivity of samples is crucial to furthering understanding on how manufacturing processes can influence the properties of the resulting product.
Note: For comprehensive results and an in-depth discussion, please follow the link in the reference section to the scientific paper.