In rubber compounding, more compression permanent deformation tests are done than tensile permanent deformation tests. As will be discussed below, many aspects of a rubber compound affect its deformation properties. It should be noted here that compressive permanent deformation and tensile permanent deformation are two different properties. Therefore, what improves compression permanent deformation does not necessarily improve tensile permanent deformation, and vice versa. In addition, for rubber sealing products, compressive permanent deformation is not a good predictor of sealing pressure or sealing performance. Usually, the harder the compressive stress relaxation experiment is to be performed, the better the sealing performance of the product is predicted.
The following experimental protocols are used to improve the permanent deformation performance of the rubber. Note: These experimental protocols may not be applicable in all cases. In addition, any variable that can reduce the permanent deformation in compression or tension can affect other properties and will not be addressed in the text.
1. vulcanization system
Consider the use of peroxides as vulcanizing agents, which can form C-C cross-linked bonds and thus improve the permanent deformation of the rubber. Vulcanization of ethylene propylene rubber with peroxide can reduce the compression permanent deformation of the rubber. The advantages of peroxide over sulfur are the simplicity of handling peroxide and the low compressive permanent deformation of the rubber.
2. Vulcanization time and temperature
Higher vulcanization temperature and longer vulcanization time can increase the degree of vulcanization and therefore reduce the compression set of the rubber.
3. Cross-linking density
Increasing the crosslinking density of the rubber can effectively reduce the compression permanent deformation of the rubber.
4. Sulfur vulcanization system
In order to reduce the compressive permanent deformation of EPDM compound and improve the heat resistance, we can consider this "low deformation" vulcanization system (mass): sulfur 0.5PHR, ZDBC 3PHR, ZMDC 3PHR, DTDM 2PHR, TMTD3PHR.
In W type neoprene, the use of diphenylthiourea accelerator can make the rubber with low compression permanent deformation, but avoid using CTP as anti-coke agent, although it can prolong the scorching time, but it has more damage to compression permanent deformation.
For NBR rubber, in the selected vulcanization system, the amount of sulfur should be reduced, try to use sulfur to give body such as TMTD or DTDM to replace part of the sulfur, less sulfur elements will improve the compression permanent deformation performance of the rubber. The vulcanization system with HVA-2 and hyposulfuramide can make the rubber with lower compression permanent deformation.
5. peroxide vulcanization system
The choice of BBPIB peroxide will give the rubber a better permanent deformation in compression. In peroxide vulcanization systems, the use of co-crosslinkers increases the unsaturation in the system, which in turn leads to a high crosslink density, because crosslinking of free radicals with unsaturated bonds occurs more easily than taking hydrogen from saturated chains. The use of co-crosslinkers changes the type of crosslinking network and thus improves the compression permanent deformation properties of the adhesive.
6. post-vulcanization
There are vulcanization by-products during the vulcanization process, and the post-vulcanization process at atmospheric pressure allows these by-products to be released, thus giving the rubber a lower compression set.
7. Fluoroelastomer FKM/Bisphenol AF Vulcanization
For fluoroelastomers, the use of bisphenol vulcanizing agent instead of peroxide vulcanizing agent can give the rubber a lower permanent deformation in compression.
8. Effect of molecular weight
In a rubber formula, the choice of rubber with large average molecular weight can effectively reduce the compression permanent deformation of the rubber.
For NBR rubber, the rubber with high Mooney viscosity should be used, which can make the rubber with small compression permanent deformation.
9. Neoprene
W type neoprene has lower compression permanent deformation than G type neoprene.
10. EPDM
To make the rubber with low compression permanent deformation, try to avoid using EPDM rubber with high crystallinity.
11. NBR
NBR, which is emulsion polymerized with calcium chloride as coagulant, usually has a low compression set.
For NBR rubber, if you want to focus on its compression permanent deformation performance, then try to choose varieties with high branching and high chain entanglement or varieties with low acrylonitrile content.
12. Ethylene-acrylate rubber
For AEM rubbers, peroxide vulcanizing agents can give a lower compression set than diamine vulcanizing agents.
13. Resin-based homogenizers
Avoid the use of resin-based homogenizers in rubber compounds, as this increases the compression set of the compound.
14. Fillers
Reducing the filling, structure and specific surface area of the filler (increasing the particle size) will usually reduce the compression set. At the same time, increasing the activity of the filler surface can also improve the compression set resistance of the compound.
15. Silica
Lower silica filler in the compound will reduce the compression set. In order to have a low compression set, it is necessary to avoid a high filling of silica. If the filling quantity is higher than 25 parts (by mass), the compressive permanent deformation of the compound becomes large.
16. Silane coupling agent
Considering the use of silane coupling agent in the high filling amount of precipitated silica, the compression permanent deformation of the adhesive can be reduced. Silane coupling agent can reduce the compression permanent deformation of silica filled rubber, and also reduce the compression permanent deformation of silicate type filler such as clay, talcum powder and other filled rubber.
17. Plasticizers
Reducing the filling amount of plasticizer in the rubber will usually reduce the compression permanent deformation of the rubber.
