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.
在这种情况下，我们将看三种不同的聚合物：丁腈橡胶（NBR），硅氧烷（SL），氟碳（FKM），以了解通过改变配方可以改善低温性能。
NBR橡胶
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).
对于NBR化合物，ACN越高，油在油中越低，但玻璃化转变温度越高。因此，为了满足较低的温度性能，复合机需要改变ACN。这是几个步骤完成的：
- 为了改善低温，我们使用23％ACN的基础聚合物。ACN的范围从19％到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.
2. Silicone
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.
为了提高硅氧烷的低温性能，我们推荐使用苯基的硅氧烷，其额定为-104℃。通常，聚合物可以在压缩模塑配方中互换，但是该聚合物变化几乎将橡胶成本增加了两倍。使用液体硅橡胶（LSR）时，需要创建新的工具，这意味着需要一种新的模具设计，因为传统上没有苯基的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.
3. FKM
传感器通常在油田管道中应用。在南部管道中，低温不是对传感器的广泛考虑，因为温度主要是温暖的。然而，对于想要分支并将市场增加到北部气候的公司来说，提高传感器的低温性能是关键。
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.
Additional Recommendations:
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.
我们已经经验丰富的客户使用EPDM橡胶将外壳密封至-40℃，但新的应用需要壳体密封抵抗油。在这种情况下，我们会将外壳密封件改为HNBR。这使得耐燃料阻力和保护免受臭氧。
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 engineertoday.