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Collaborative Study With HASETRI Shows How to Reduce Heat Build-Up Testing Time from Days to Minutes

Alpha-Engineer-with-arms-crossed-near-Premier-RPA-dilip

Author

Dilip Dhupia

Application Engineer, India

Alpha’s engineers conducted a study in collaboration with the Hari Shankar Singhania Elastomer & Tyre Research Institute (HASETRI) in India to develop methods that accelerated testing of heat build-up properties of elastomer compounds from as much as 18 hours to about 30 minutes.  Download the full technical report or read the summary below to learn more.


Background On Heat Build-Up

We all know why we use fillers, especially carbon black, in our rubber compounds. Carbon black fillers enhance the ultimate performance of rubber products such as good wear resistance, damping behavior and the ability to undergo very large deformation with nearly no permanent deformation. They can affect not only the dynamic modulus, both viscous modulus and elastic modulus, but also their ratio, i.e., loss factor, which is related to the portion of the energy dissipated during dynamic deformation.

In practice, the energy loss in rubber products during dynamic strain is of great importance, for example, in vibration mounts and tires.  Energy loss acts as service performance predictors of these products. Specifically, energy loss is a predictor of heat generation and fatigue life for the vibration mounts, and rolling resistance, traction, and skid resistance for tires.  When rubber is deformed an energy input is involved which is released in part when the rubber returns to its original shape.  The part which is not returned as mechanical energy is dissipated as heat. 

Rubber products under dynamic conditions and subjected to repeated deformations with sufficient magnitude and frequency, will result in the generation of considerable heat (depending on the rate of heat dissipation) and significant rise in temperature.  The product’s temperature reaches steady state after running for a certain period, when the rate of heat generation equals with the rate of heat dissipation.  The extent of heat generation governs the change in viscoelastic properties of the product, which may lead to premature failure.  The process is more severe in thick rubber blocks and causes the interior part of the block to become so hot that the rubber explodes.  This is an important mode of failure in thick rubber products such as tire treads and tank track pads. 

Accurate, repeatable testing of rubber compounds to assess their heat build-up properties is essential for product manufacturers to safeguard the people using their products. Typically, tests for heat build-up characteristics involve multiple instruments such as those used in a this study.  Specifically:  

    • Rheometric properties were studied by using a Premier™ MDR (Alpha Technologies, USA) in line with ASTM D5289 
    • Shore A hardness was determined on Multi Unit Hardness Tester in line with ASTM D2240.  
    • Stress strain and Hysteresis properties were ascertained by using Universal Testing Machine in line with ASTM D412 using a die C dumbbell.  
    • Density was carried out in line with ISO 2781.  
    • Rebound Resilience was tested in line with ISO 4662 at 100ºC.  
    • Heat buildup (HBU) test was carried out on a Goodrich Flexometer in line with ASTM D623 
    • Dynamic Mechanical Analysis (DMA) of cured samples was carried out in line with ASTM D5992. 

Parallel testing was performed on identical samples on Alpha’s Premier RPA. 

The study was performed in two phases: phase 1 establishes that filler is the primary reason for HBU in the rubber compound; phase 2 studies the effect of carbon black particle size on HBU.

 

Why Use a Premier RPA to Measure Heat Build-Up?

The Premier RPA is an ideal instrument for measurement of heat build-up in a cured rubber sample, because of its favorable sample surface area to sample volume ratio and ability to apply very high strains (± 90° {± 1256%}).  This was found to be an ideal strain range for measurement of heat build-up. The RPA die design has a high surface area in relation to the mass of the sample itself. This design was deliberate to assure rapid temperature recovery for the sample, once the dies close and the test starts. Premier RPA maintains the temperature of rubber sample within ± 0.2°C of set temperature, utilizing combination of its custom designed heating control system and forced air cooling system (across the dies). Every effort is applied to keep the temperature constant to obtain precise dynamic properties of the rubber sample.  Dynamic property measurements such as G’, G’’, h‘, and h* would not have much validity if the sample temperature was allowed to rise from viscous heating due to test conditions.

However, a special modification of the Premier RPA disabled the forced air-cooling system to allow the temperature of the upper and lower dies to freely rise from viscous heating.  Thus, the heat from the sample was a result of carefully controlled high sinusoidal strain movement of the lower die.  As a result, this unique die cavity geometry also provides effective measurements of heat build-up when the test conditions in Premier RPA are properly optimized. 

 

The Takeaway: Excellent Correlation. Faster Results.

This study demonstrates excellent correlation between Premier RPA tan δ and the ΔT of Goodrich Flexometer, UTM hysteresis (Mullin’s effect), and DMA tan δ with R² value above 0.900.  Also, inverse correlation of RPA tan δ with rebound resilience is observed. The data indicates that the defined test method utilizing a Premier RPA is both effective and efficient in measuring HBU properties of cured rubber sample. 

This test procedure utilizing the Premier RPA shows it is efficient and effective in measuring HBU in a rubber compound. The test sequence in the Premier RPA involves sample curing and subsequent testing in single sample placement. Total test duration for the rubber compound which cures at 16°C/20min is 30 minutes. The Premier RPA can estimate the HBU properties in about half an hour as compared to other laboratory tests which involve sample conditioning time for 18 hours after sample curing. 

To read the Technical Report detailing this test of Premier RPA for measuring HBU, or if you have any questions about this topic, please do not hesitate to contact us. 

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