Swellability Of Rubber Sealing Products

According to the knowledge of organic chemistry, organic solvents can affinity macromolecular organics, but some macromolecules are not easy to dissolve, they will absorb solvent molecules and expand their volume; hydrophilic macromolecules will also absorb water molecules and Volume expansion, which is the so-called swelling of polar substances. Swelling can also be explained from the principle of similar solubility, they will have mutual solubility when they are in contact or under certain pressure and temperature, but have nothing to do with the attraction between molecules.

Swelling properties are one of the common properties of rubbers or polymers. In some solvents, cross-linked rubber or other polymers generally do not dissolve, but the solvent molecules will enter the gaps of the polymer chain and increase the volume between the segments, so the volume of the polymer will expand due to the expansion of the polymer. and swelling. After the rubber is swollen, the general mechanical properties will be greatly reduced. Swelling is a very important property of rubber, so rubber should try to avoid contact with solvents with similar polarity.

There are two types of rubber swelling properties: (1) Infinite swelling: The linear polymer dissolves in a benign solvent and can absorb the solvent without limit until it dissolves into a homogeneous solution. So dissolution can also be seen as the result of infinite swelling of the polymer. For example, natural rubber swells in gasoline. (2) For cross-linked polymers and linear polymers in poor solvents, the swelling can only be carried out to a certain extent. No matter how long it is in contact with the solvent, the amount of inhaled solvent will not increase, but will reach equilibrium, and the system will always be Maintain the two-phase state. For example, nitrile rubber (which is a synthetic rubber) can swell in liquefied dimethyl ether organic solutions.

The swelling mechanism of nitrile rubber in liquefied dimethyl ether organic solution can be roughly understood as: when the valve of the cylinder is opened, the liquefied dimethyl ether in the bottle contacts the nitrile rubber sealing ring in the valve, and the nitrile rubber will swell; When the cylinder valve is closed, the inside of the valve is gradually "dry", the swelling property of NBR gradually declines, and the volume of the rubber shrinks. , Nitrile rubber has undergone several "swelling-shrinking" stress cycles, the rubber stress is reduced, the aging is accelerated, and finally the rubber elasticity fails and the sealing performance is reduced, resulting in air leakage of the valve.