The seal company can summarize the general mechanical properties of the rubber sealing ring after swelling according to chemical knowledge.However, the solvent can affinity the polymer, but some polymer is not easy to dissolve, they will absorb the solvent molecules and expand the volume;The Dongsheng seal tells you that the hydrophilic polymer will also absorb water molecules and expand in volume. this is the swelling of the so-called polar substance.
It can be seen that swelling can also be explained from the principle of similar phase dissolution. they will have mutual dissolution at contact or at a certain pressure and temperature, but it has nothing to do with the gravity between molecules, swelling properties are one of the commonalities of rubber or polymer.
In some solvents, the cross-linked rubber seal material or polymer generally does not dissolve, but the solvent molecule will enter the gap of the seal polymer chain, increasing the volume between the chain segments, so the volume of the polymer is swollen due to expansion.
Swelling is a very important property of the rubber seal material, so the rubber should try to avoid contact with solvent with similar polarity.
The field analysis of material swelling of polymer seals shows two types:
(1)Infinite swelling: the linear polymer is dissolved in a benign solvent and can not be absorbed until it is dissolved into an homogeneous solution.Therefore, dissolution can also be regarded as the result of infinite swelling of polymer.For example, rubber is swollen in gasoline.
(2) for cross-linked polymers and linear polymers in solvents, swelling can only be carried out to a certain extent. in the future, no matter how long the contact with the solvent is, the amount of inhaled solvent will no longer increase, and the balance will be reached, the system always maintains a two-phase state. For example, Ding Qing rubber (a synthetic rubber) can swell in the solution of liquefied DME.
The swelling mechanism of Ding Qing rubber seal material in liquefied DME solution can be roughly understood as: when opening the valve of the cylinder, the liquefied DME in the bottle is in contact with the NBR seal in the valve, ding Qing rubber will swell;When the cylinder valve is closed, the inside of the valve is gradually 'dry', the swelling of Ding Qing rubber is gradually declining, and the volume of the rubber seal will shrink, with the increase of the number of valve opening and the increase of the content of DME mixed in liquefied petroleum gas, Ding Qing Oak has been 'swollen-The stress cycle of contraction ', the rubber stress decreases, the aging is accelerated, the rubber elasticity fails and the seal performance is reduced, resulting in air leakage of the valve.
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