New applications push the limits of low-temperature performance of standard rubber compounds. For most rubber compounds, as the temperature drops, the material begins to reach its glass transition temperature, and the hardness of the rubber increases. We also see higher modulus or increase resistance to deformation. Higher cold temperature modulus can cause leakage for seal applications as the material is not deforming into the seal gland. Additionally, higher force deformation is a real problem for low-pressure applications. To meet changing criteria and ensure proper sealing, formulation changes must be made to the rubber compounds.
In this case study, we will look at three different polymers: Nitrile Rubber (NBR), Silicone (SL), Fluorocarbon (FKM) to find out how low-temperature performance can be improved through changing formulation.
1. NBR Rubber
In an air braking system application for a rail car, the industry requirements unexpectedly lowered from -30℃ to -50℃. However, the current formulation of NBR used in the o-ring did not offer enough resistance to temperatures this low. The current compound used a 33% acrylonitrile content (ACN).
For NBR compounds, the higher the ACN, the lower the volume swell in oil, but the higher the glass transition temperature. Therefore, to meet lower temperature performance, compounders need to change the ACN. This was done in a few steps:
- To improve the low temperature, we used a base polymer of 23% ACN. ACN can range from 19% to 50%
- To compensate for the higher volume swell, we changed out the plasticizer that resists extraction. Lowering the volume swell allowed our customer’s air valve to operate at a lower temperature while maintaining the same lubrication
While formulation of NBR changed to match the application requirements, the polymer price was not affected.
In an aerospace application, a standard dimethyl silicone, which is rated to -65℃ was applied. However, during performance testing, they noticed the flex strength was still too high at the -65℃. High flex force required the customer to replace the current motor with a higher force capability. Additionally, high flex strength was observed in different durometers as well.
To improve low temperature performance of the silicone, we recommended using a phenyl based silicone, which is rated down to -104℃. Typically, the polymer can be interchanged in compression molded formulation, but this polymer change would nearly double the cost of the rubber. When using Liquid Silicone Rubber (LSR), new tooling would need to be created, meaning a new mold design was required because traditionally there is no phenyl-based LSR.
Using the phenyl-based silicone in this application resulted in a lower torsional stiffness at -65℃ and allowed the device to operate at lower forces.
Sensors are often applied in oilfield pipelines. In southern pipelines, low temperature is not an extensive consideration for sensors since temperatures remain primarily warm. However, for companies wanting to branch out and increase their market to northern climates, improving the low-temperature performance of sensors is key.
In one example, a company typically applied a sensor using an o-ring made from a bisphenol cured standard FKM. These compounds are typically rated to -15℃ for static applications. Moving to a low-temperature performance FKM base would change the polymer and cure system of the formulation. This change would then rate the compound, to -40℃.
This new compound would be cured with peroxide and have a slightly higher compression set. While the price of the rubber compound would triple in price, the small o-rings size does not require a lot of material.
In some cases, a base polymer change will not meet the requirements for applications. To combat this, a completely different class of rubber may need to be used. For example, if a customer needs to meet -65℃ temperature resistance but now requires more oil and fuel resistance, a silicone seal would need to be shifted to fluorosilicone.
We have experienced customers using EPDM rubber to seal a housing down to -40℃, but a new application requires a housing seal to resist oil. In this case, we would change the housing seal to an HNBR. This gives fuel resistance and also protection from ozone.
When improving low-temperature performance and meeting additional sealing requirements, it is common for additional formulation changes to be made in order to provide a strong enough seal. If you have questions about your application requirements or need help finding the right material for your application, contact an Apple Rubber engineer today.