How to distinguish the type and material of the seal? (2)


2.The oil resistance of nitrile rubber O-rings NBR rubb […]

2.The oil resistance of nitrile rubber O-rings
NBR rubber O-rings are resistant to most oils, especially to alkane oils. There are two types of oils: alkane oils and aromatic oils. Alkane oils refer to the molecular structure of the chain. Aromatic oils refer to the molecular chain containing benzene rings. Oils with an isocyclic structure, such as common gasoline, motor oil, diesel, peanut oil, soybean oil, etc., are alkane oils; pine tar and coal tar, etc., are aromatic oils. This is relatively easy to understand. The molecular chain of nitrile rubber contains -CN (cyano) groups, which is a polar rubber, while the molecular chain of alkane oil is straight and has no prepolar group. It is a non-polar structure. Aromatic oils contain cyclic structures such as benzene rings, which also exhibit strong polarity, which is the so-called "similar compatibility" principle.
NBR is short for NBR. A synthetic rubber made by copolymerizing butadiene and acrylic acid. NBR rubber O-rings are known for their excellent oil resistance. Their oil resistance is second only to polysulfide rubber, acrylic rubber and fluoro rubber. In addition, NBR rubber O-rings also have good abrasion resistance and aging resistance. And air tightness, but the ozone resistance, electrical insulation and cold resistance are relatively poor, and the electrical conductivity is better.
The production process of nitrile rubber O-ring has the following characteristics:
1. The strong polarity of the monomer acrylic acid makes the latex less stable during the polymerization. The larger the amount of acrylic acid, the worse the stability of the latex.
2. The alkalinity or acidity of the medium is too strong or the polymerization temperature is too high, which will cause the hydrolysis of the cyano group, that is, the generated acid will destroy the emulsifier, which is also one of the reasons for the instability of the latex.
3. The intermediate product of the hydrolysis reaction, the amide group and the cyano group in the polymer chain, may undergo a cross-linking reaction at a higher temperature, resulting in poor product quality.
4. The reactivity ratios of butadiene and acrylene are quite different (0.3 and 0.02 at 40 ° C, respectively). Therefore, the composition and distribution of the monomers in the copolymer are highly dependent on the conversion rate. The cyano group distribution can be improved by adding acrylic acid in batches. Butadiene rubber is more difficult to process due to the stronger inter-molecular force and greater hardness. Among them, the hard rubber produced at a polymerization temperature of 30 ° C is the hardest to process. It must be plasticized on a cold roll before operation. The industry often adopts a more effective method of adjusting molecular weight and lowering the polymerization temperature to 5 ° C to reduce side reactions and improve its processability. On the other hand, NBR can also be blended with a variety of rubbers such as neoprene, isoprene, butadiene, styrene-butadiene rubber, etc. and synthetic resins such as polyvinyl chloride, phenolic resin, etc. (see Polymer Blends) Material) to improve performance.