Basic concepts and classification of silane crosslinkers
---【CAFSI】
1.1 Concept of silane crosslinkers Silane crosslinkers refer to silanes or siloxanes that contain three or more easily hydrolyzed groups in the molecule and can condense with the Si-OH group at the end of the α,ω-dihydroxypolydimethylsiloxane molecular chain to form a crosslinked structure. They are the most important type of compounding agent in condensation-type liquid silicone rubber. The crosslinker works together (combines) with α,ω-dihydroxypolydimethylsiloxane, catalysts, fillers, plasticizers, additives and other components to prepare condensation-type liquid silicone rubber.
1.2 Classification of silane crosslinkers The chemical formula of silane crosslinkers is RnSiX(4-n), where n=0,1, R is Me, Et, phenyl, etc.; X is OMe, OEt, OAc, ON=CMeEt, OC(Me)2, etc. According to the different byproducts of the condensation reaction in the single-component room temperature vulcanized silicone rubber, it can be divided into deacidification type, deketoxime type, dealcoholization type, deamination type, and deamidation type silane crosslinking agent.
1.2.1 Deacidification type
The general formula of acid-type silane crosslinker is RnSi(OCOR＇)4-n, where R is methyl, ethyl, propyl, vinyl, phenyl, etc., R＇ is hydrogen, methyl, etc.; n is 0, 1. Among them, methyltriacetoxysilane is used in the largest amount and is widely used as a crosslinker in single-component room temperature vulcanized silicone rubber.
The acid-type crosslinker is mainly composed of methyltriacetoxysilane, ethyltriacetoxysilane, propyltriacetoxysilane, and vinyltriacetoxy. The compound products of the above crosslinkers form a deacetation type crosslinker group with acyloxy as the active group. Organic acyloxysilane crosslinkers have the smell of carboxylic acid. Some are liquid and some are solid at room temperature. Their thermal stability is not as good as that of the corresponding alkoxysilanes, and they are easier to decompose into corresponding carboxylic anhydrides and silane polymers; they have high hydrolysis activity, but not as good as chlorosilanes, and can undergo hydrolysis without a catalyst at room temperature and release a large amount of heat.
1.2.2 Deketoxime type Ketoxime type silane crosslinkers are mainly methyl tributylacetone oxime silane, vinyl tributylacetone oxime silane, tetrabutyl tributylacetone oxime silane, phenyl tributylacetone oxime silane, and methyl triacetone oxime silane. The compound products of the above crosslinkers form a deketoxime type crosslinker group with ketoxime as the active group. In room temperature vulcanized silicone rubber, deketoxime type rubber has become the mainstream of the market.
1.2.3 Dealcohol type
The general formula of alcohol type silane crosslinker is RnSi(OR＇)4-n, where R is methyl, ethyl, propyl, phenyl, etc., R＇ is hydrogen, methyl, ethyl, propyl, etc.; n is 0, 1.
Organic alcohol type crosslinkers are mainly methyltrimethoxysilane, tetramethoxysilane, and tetraethoxysilane, forming a dealcohol type crosslinker group with alkoxy as the active group. 1.2.4 Deamination type Organohalosilane is prepared by reaction with ammonia or amine, and the reaction is usually carried out in an inert solvent. Common R2Si(NH2)2 is very unstable and is prone to intermolecular deamination condensation reaction to obtain linear or cyclic polysilanes. The steric hindrance of organohalosilane and the size of the organic group on the amine group affect the reaction. The greater the steric hindrance and the larger the organic group, the more difficult the reaction is. Since the electronegativity of silicon and nitrogen is very different, the Si-N bond has high thermal stability. Si-N bonds can be hydrolyzed, but they are far less sensitive than Si-Cl bonds and Si-OR bonds. The degree to which acids and organic solvents promote hydrolysis is affected by the number of organic groups linked to silicon and nitrogen atoms and steric hindrance. The greater the steric hindrance and the fewer electrophilic groups in the organic group, the slower the hydrolysis. It is often used to prepare high-temperature resistant, high-modulus fibers or fine ceramics. A small amount is used as a crosslinker for single-component room temperature vulcanized silicone rubber, and it can also be used as a silicon-based reagent.
1.2.5 Deamidation type
Organohalosilanes and amides are prepared by de-HX reaction. In order to ensure the thoroughness of the reaction, it is sometimes necessary to add alkaline compounds to absorb HX, thereby increasing the yield of the target product. The most common is the reaction of N-methylacetamide and organochlorosilane to obtain amide silane crosslinkers. Organic amide silane crosslinkers are neutral compounds. Since hydrogen bonds can be formed between molecules, their boiling points and heat stability are relatively high, and they are solid at room temperature. Since organic amide silane crosslinkers contain ketone groups in their molecules, they are easily attacked by nucleophilic reagents, undergoing substitution reactions and generating corresponding silyl derivatives and amides. It is often used as a crosslinking agent or chain extender, hydroxyl scavenger, etc. for room temperature vulcanized silicone rubber. It can also be used as a silicon-based reagent in the pharmaceutical industry.