Swelling of rubber seal
Oct 07, 2021
Based on the knowledge of organic chemistry, DSH Seals concluded that the general mechanical properties of rubber seals will be greatly reduced after swelling. However, organic solvents can be compatible with high-molecular organics, but some high-molecular organics are not easy to dissolve. They will adsorb solvent molecules and expand the volume; hydrophilic polymer materials will also absorb water molecules and expand in volume. The so-called swelling properties of polar substances.
It can be seen that the swelling property can also be explained by the principle of similar compatibility. They will have mutual solubility when they are in contact or under certain pressure and temperature, but it has nothing to do with the intermolecular gravitational force. The swelling property is one of the common characteristics of rubber or polymer. one.
In some solvents, cross-linked rubber or other polymers generally will not dissolve, but the solvent molecules will enter the voids of the polymer chain, increasing the volume between the segments, so the volume of the polymer will expand due to swelling.
Swelling is a very important property of rubber, so rubber should try to avoid contact with solvents of similar polarity.
The swelling properties of rubber seals are shown in two types:
(1) Infinite swelling: The linear polymer is dissolved in a benign solvent and can absorb the solvent without restriction until it dissolves into a homogeneous solution. So dissolution can also be seen as the result of unlimited swelling of the polymer. For example, natural rubber swells in gasoline.
(2) For cross-linked polymers and linear polymers in poor solvents, swelling can only be carried out to a certain extent. No matter how long the contact with the solvent is, the amount of inhaled solvent will no longer increase and reach equilibrium, and the system will always Maintain a two-phase state. For example, nitrile butadiene rubber (a synthetic rubber) can swell in an organic solution of liquefied dimethyl ether.
The swelling mechanism of nitrile nitrile rubber in the organic solution of liquefied dimethyl ether can be roughly understood as: when the valve of the cylinder is opened, the liquefied dimethyl ether in the bottle contacts the nitrile rubber seal ring in the valve, and the nitrile rubber will swell; When the cylinder valve is closed, the inside of the valve is gradually "dried", the swelling property of nitrile butadiene rubber gradually declines, and the volume of the rubber shrinks. As the number of valve openings increases and the content of dimethyl ether blended in the liquefied petroleum gas increases After many times of stress cycles of "swelling and shrinking", the rubber stress decreases and the aging speeds up. In the end, the rubber elasticity fails and the performance of the seal decreases, which causes the valve to leak.
