Application of Borane Compounds in Heat-resistant Adhesives

Abstract : This paper briefly introduces the structure and synthesis method of borane, its application status at home and abroad, and its important position in the development of ultra-high temperature resistant adhesives.
Keywords: card decaborane; heat resistance; adhesive
1 Introduction
When people mentioned high-temperature polymers, the first thought was aromatic heterocyclic polymers such as polybenzimidazoles or triazines. However, due to their high curing temperature and pressure, low room temperature and low temperature strength, and poor water resistance, they were not Can get practical application. Since the middle of the last century, scientists have come up with many methods to develop high-temperature functional materials such as adhesives and sealants that are resistant to temperatures above 400°C. Among them, there is one element in organic compounds that is worthy of research and development by Chinese scientists and technicians. The subject is the application of borane-based organic compounds in heat-resistant adhesives.
Carborane is a type of borohydride containing carbon elements, of which the general formula of borane is BnC2Hn+2. For the sake of convenience, the name BnC2Hn+2 is referred to as card n borane or carborane-n, and n is the number of boron atoms, for example, B5C2H7 is card pentaborane. Currently used in the ceramic fiber and heat-resistant polymer materials and other areas of card Borane is mainly a cage-shaped card decaborane, molecular formula B10C2H12 (also known as card decaborane - [12]), English name is 1, 2-dicarba-clove-dodecarborane, "-clove-" is Greek, meaning "cage" (in the English name "cage" is written as "-closo-"), the Chinese translation of 1,2 two carbon cage 12 Borane is one of the most important raw materials for synthesizing heat-resistant polymer materials, often referred to simply as borane.
2 Boronane synthesis method
Card decaborane has several different methods of synthesis, commonly used methods are prepared by the decaborane (B10H14) and acetylene nested structure under the action of Lewis base catalysis, but this method requires good production equipment and operating experience, otherwise There will be danger of explosion and poisoning. However, there are reports in the literature that decaborane-14 and acetonitrile can be used first to produce diacetonitrile with decaborane B10H12.(CH3CN)2, and then refluxed with acetylene in benzene to obtain card decaborane. The reaction is as follows:
B10H14+2Cn3CN→B10H12 (CH3CN) 2+H2
B10H12(CH3CN) 2+C2H2→B10C2H12+H2+2CH3CN
This reaction can also be accomplished by the next step in the catalysis of Lewis bases:
B10H14+C2H2→B10C2H12+2H2
Useful Lewis bases are acetonitrile, amines, dialkyl oxime ethers, trialkyl phosphines, dimethylformamide, and the like. When the amount of catalyst is 1/10 of decaborane-14, the reaction can proceed, and when it reaches 9/10, the reaction can reach a higher yield within a short time.

Jordi Llop et al. of the Institute of Science and Materials, Barcelona, ​​Spain, confirmed and further plotted the cage structure of card decaborane and carbon decaborane with substituents at the carbon 1 and 2 positions by single crystal X-ray diffraction. . Published in the "Organic Metal Chemistry" magazine in September 2002.
According to the arrangement position of two carbon atoms on the cage structure, decaborane can be divided into ortho, meta and para isomers. The two hydrogen atoms carried on the carbon atom have a greater activity, and they can react with dimethyl dichlorosilane first to produce bis(dichlorosilane) borane, and then pass the chlorine carried on the silicon atom. The atomic reaction can enable the card decaborane clathrate compound to carry reactive groups such as bis-hydroxyl, bis-epoxy, bisisocyano or divinyl, respectively, as a binder or a curing agent, which can make caged card boron Alkane-embedded adhesives are used to increase the heat resistance of adhesives. The borane-polysiloxane polymer containing a vinyl group and a silyl group is prepared by polymerizing a borane with an organosiloxane to prepare a low-temperature vulcanized heat-resistant adhesive.
3 Boratane R&D in countries around the world
The United States, Russia, Japan, Germany, Canada and other countries have conducted research and application earlier.
From a report of the US military’s AD, the United States began the research and development of borane from 1964 to Robert E. The researchers led by Williams synthesized card decaborane in the laboratory and published the Direct Process Makes Carboranes Rapidly method in the C&EN magazine in August 1968. The development of borane-resistant ultra-high temperature adhesives lays the foundation for a solid theoretical and raw material source. More than a hundred patents and articles have revealed the progress of U.S. in cold-cure ultra-high temperature adhesives, such as:
U.S. Patent No. 3,669,993 issued a simple boranane epoxy heat-resisting adhesive in 1972, which was prepared by first reacting with butyllithium, 4-bromo-1-butene and decaborane 1,2 - Bis(3-butene)borane, followed by oxidation of the two double bonds with trifluoroperacetic acid to give 1,2-bis(epoxybutyl)borane. The obtained double-epoxy boranes were cured with 3% boron trifluoride ethylamine complex at room temperature. The adhesive sheared specimens obtained by this method were under 400~C. Shear strength up to 980psi (approximately 7MPa).
In another U.S. Patent No. 5,290,897, an adhesive capable of curing at a low temperature (100 to 140C) and having a temperature resistance of at least 600C is described. Its formulation consists of the following three components: a) A number average molecular weight Over 120,000 polyborane-organosiloxane compounds with a vinyl group attached directly to the backbone, with respect to the vinyl groups attached to the backbone in terms of the carborane segments contained in the backbone The content must reach 3%~30%; b) The curing agent is a compound with two terminal groups -SiH group; c) A complex containing a transition metal is used as hydrosilylation catalyst.
In Russia (former Soviet Union), the study of borane heat-resistant adhesives was carried out very early and many effective ultra-high temperature adhesives were developed. The BK-20 and BK-20M developed in the 1970s were polyurethane heat-resistant adhesives. The compounds containing modified borane were used in the adhesive. The curing temperature was 80~150°C and room temperature respectively. The curing time was 3h and 5d. Temperature resistance up to 500 ~ 800 °C, instantaneous temperature resistant to 1000 °C adhesive, the main components of polyurethane, containing the modified card borane compound curing agent and curing catalyst. The ultra-high temperature adhesives UMK-7, UMK-8, and UMK-9 that were later developed have a long pot life, all of which use borane and no solvent in the adhesive. The use temperature is -253 to 800°C.
The former Soviet Union had many unique features in the development of high-temperature adhesives using borane. They not only used epoxy resins and polyorganosiloxanes for the manufacture of high-temperature adhesives, but in a patent published in June 2003. , Borane (SU2203917) is also used in the phenolic-carboxybutyl rubber binder, so that the adhesive can withstand up to 50 hours at 400 °C;
From the literature search, it can be seen that some countries in Japan, Germany and Europe have also done a lot of work in the research and use of borane. Japanese scientists have done a lot of research on the synthesis of boranes, organosilicon polymers containing boranes, and boranane organosilicon polymers with flame retardance, and low-temperature curable kaborane adhesives. Work; Germany has done a lot of work on the purification of boranes, the borane-based polyorganosiloxane adhesives, and the performance of heat-resistant boranes-organosiloxane elastomer polymer intermediates and their manufacturing processes; Europe’s Some patents indicate that they have made outstanding achievements in the preparation of boranes-silicone or silicone unsaturated polymers and their use as borate-containing adhesives, low-temperature-curing boranes, and others. Work; scientists in Canada and other countries in high-temperature borane - silicone elastomer intermediate polymerization products and manufacturing process (high temperature carborane - siloxane elastomers intermediate polymeric products), double epoxy alkyl borane high temperature adhesives, etc. A lot of work has been done.
As early as the 1960s in the last century, China began to carry out research on borohydride compounds. Unfortunately, due to the thin foundation of basic scientific research and inadequate funding support, China has so far not seen the development of barriers other than Hong Kong. In the study of alkanes, the application of borane in ultrahigh temperature adhesives has not been reported. In order to further meet the needs of China's aviation, aerospace, and other high-tech equipment manufacturing fields, it is time to start research in this area as soon as possible.