Silane Crosslinking Agents: Basic Concepts and Classification

Silane Crosslinking Agents: Basic Concepts and Classification

1.1 Concept of Silane Crosslinking Agents Silane crosslinking agents refer to silanes or siloxanes containing three or more easily hydrolyzable groups in their molecules, which can condense with the Si-OH groups at the ends of α,ω-dihydroxypolydimethylsiloxane molecular chains to form a crosslinked structure. They are one of the most important types of compounding agents in condensation-curing liquid silicone rubber. The crosslinking agent, along with α,ω-dihydroxypolydimethylsiloxane, catalyst, filler, plasticizer, and other additives, work together to prepare condensation-curing liquid silicone rubber. 1.2 Classification of Silane Crosslinking Agents The general chemical formula of silane crosslinking agents 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. Based on the different byproducts of the condensation reaction in one-component room-temperature vulcanizing silicone rubber, they can be classified into deacidification type, deketoxime type, dealcohol type, deamino type, and deamide type silane crosslinking agents.

1.2.1 Deacidification Type

The general formula of acid-type silane crosslinking agents is RnSi(OCOR')4-n, where R is methyl, ethyl, propyl, vinyl, phenyl, etc., R' is hydrogen, methyl, etc.; n is 0, 1. Methyltriacetoxysilane is the most widely used, and is extensively used as a crosslinking agent in one-component room-temperature vulcanizing silicone rubber.

Acid-type crosslinking agents mainly include methyltriacetoxysilane, ethyltriacetoxysilane, propyltriacetoxysilane, and vinyltriacetoxysilane, plus their blended products, forming a group of deacetic acid type crosslinking agents with acyloxy groups as active groups. Organic acyloxysilane crosslinking agents have a carboxylic acid odor, and some are liquid and some are solid at room temperature. Their thermal stability is inferior to that of the corresponding alkoxysilanes, and they are more easily decomposed into the corresponding carboxylic anhydrides and silane polymers; they have high hydrolysis activity, but their hydrolysis activity is not as high as that of chlorosilanes. Hydrolysis reactions can occur at room temperature without a catalyst, releasing a large amount of heat.

1.2.2 Ketoxime-type silane crosslinking agents are mainly composed of methyl tributanone oxime silane, vinyl tributanone oxime silane, tetrabutanone oxime silane, phenyl tributanone oxime silane, and methyl triacetone oxime silane, plus blended products of the above crosslinking agents, forming a group of ketoxime-type crosslinking agents with ketoxime groups as active groups. In room temperature vulcanized silicone rubber, ketoxime-type adhesives have become the mainstream in the market.

1.2.3 Alcohol-type

The general formula of alcohol-type silane crosslinking agents is RnSi(OR＇)4-n, where R is methyl, ethyl, propyl, phenyl, etc., R＇ is hydrogen, methyl, ethyl, propyl, etc.; n is 0 or 1.

Organic alcohol-type crosslinking agents are mainly composed of methyl trimethoxysilane, tetramethoxysilane, and tetraethoxysilane, forming a group of alcohol-type crosslinking agents with alkoxy groups as active groups. 1.2.4 Amino-type Prepared by the reaction of organic halosilanes with ammonia or amines, the reaction is usually carried out in an inert solvent. Common R2Si(NH2)2 is very unstable and easily undergoes intermolecular deamination condensation reactions to obtain linear or cyclic polysilanes. The steric hindrance of organic halosilanes and the size of the organic group on the amino group both affect the reaction; the greater the steric hindrance and the larger the organic group, the more difficult the reaction. Due to the large difference in electronegativity between silicon and nitrogen, the Si-N bond has very high thermal stability. The Si-N bond can be hydrolyzed, but it is far less sensitive than the Si-Cl and Si-OR bonds. The extent to which acids and organic solvents promote hydrolysis is influenced by the number and steric hindrance of organic groups attached to the silicon and nitrogen atoms; greater steric hindrance and fewer electrophilic organic groups result in slower hydrolysis. It is commonly used in the preparation of high-temperature resistant, high-modulus fibers or fine ceramics, and in small amounts as a crosslinking agent for one-component room-temperature vulcanizing silicone rubber. It can also be used as a silylation reagent.

1.2.5 Deamidation Type

This type is prepared by the reaction of organohalosilanes with amides through a de-HX reaction. To ensure the reaction proceeds completely, a basic compound is sometimes added to absorb HX, thereby increasing the yield of the target product. The most common example is the reaction of N-methylacetamide with organochlorosilanes to produce amide-functionalized silane crosslinking agents. Organoamide-functionalized silane crosslinking agents are neutral compounds. Due to the formation of hydrogen bonds between molecules, they have high boiling points and thermal stability, and are solid at room temperature. Because organoamide-functionalized silane crosslinking agents contain a carbonyl group, they are susceptible to nucleophilic attack, undergoing substitution reactions to form the corresponding silyl derivatives and amides. They are commonly used as crosslinking agents or chain extenders for room-temperature vulcanizing silicone rubber, hydroxyl scavengers, etc., and can also be used as silylation reagents in the pharmaceutical industry.