Novel Silicon-based Flame Fetardants And Their Modifications Of High Performance Thermosetting Resins

High performance thermosetting resins (HPTRs) are a class of polymers with cross-linked network structure, and has received great attentions owing to their outstanding integrated properties including high mechanical properties, outstanding thermal, thermal-oxidative and hot-wet resistance, excellent dielectric properties as well as corrosion and radiation resistance, showing great potential in many cutting-edge fields, especially aerospace, electric and electronic industries. However, being polymers, HPTRs generally have poor flammability which is also one major disadvantage to restrict further prosperity of HPTRs for applications in cutting-edge fields. In fact, improving flame retardancy has been the main subject associated with the investigations of HPTRs for many years. To date, many approaches have been developed to effectively improve the flame retardancy of HPTRs, however they tend to sacrifice the original outstanding properties of HPTRs, and thus can not meet the harsh requirements on HPTRs proposed by the rapid development of modern industries. Therefore, developing new flame retardants for HPTRs, which can not only significantly improve the flame retardancy, but also simultaneously endow modified resins with other key properties is an interesting subject to be addressed, this is the subject of this thesis.First, hollow silica tubes (HSTs) with uniform size and high aspect ratio were prepared by hydrolyzing tetraethyl orthosilicate using template self-assembly from D, L-tartaric acid. The effect of the incorporation of HST to the cyanate ester (CE) resin on the curing behavior and integrated properties were systematically evaluated. Results disclose that properties of HST/CE composites are dependant on the content of HST. Essentially, the content of HST has significant influence on the structure of polymer chain and that of aggregation state for the crosslinked networks. Compared with CE resin, the composites with suitable content of HST have not only obviously catalyzed curing reactivity, but also significantly improved integrated properties including mechanical, thermal and dielectric properties as well as water absorption and flame retardancy.Second, based on the molecular design, a novel hyperbranched phenyl silicone resin (HBPSi) containing a large amount of phenyl and silanol groups derived from the hydrolysis of phenyltrimethoxysilane was designed and synthesized, which is then employed to modify CE, bismaleimide-cyanate (BCE), and epoxy (EP) resins. It is found that modified thermosetting resins with suitable contents of HBPSi have effectively improved toughness, strength and stiffness, especially for the modified EP system. These attractive results can be attributed to the synergistic effect resulting from changes of both polymer chain and aggregation state structures including a large amount of flexible linear siloxane segments, rigid benzene rings, many unoccupied spaces,π-πconjugation interaction among benzene rings, and low cross-linking density. In addition, HBPSi modified thermosetting resins have significantly improved flame retardancy, showing decreased heat release rate (HRR), and improved limited oxygen index (LOI). Investigations on thermal-oxidative stability and residual char demonstrate that HBPSi can induce cross-linking reactions, and promote to form a char on the surface of the modified resin during combustion. The char acts as a good insulating barrier, which not only prevents the mass transport, but also protects the underlying polymer from flaming, ultimately improving the flame retardancy of the thermosetting resin.Third, according to the structural features of CE and 4,4’-bismaleimidodi-phenyl methane/2,2’-diallyl bisphenol A (BDM/DBA) resin, a novel organically functionalized ladderlike polyphenylsilsesquioxane with Si-OH groups (coded as PLS), and that with–NH2 groups (coded as N-PLS) were synthesized, and then used to develop new high performance hybrids based on CE or BDM/DBA resin. Compared with neat thermosetting resin, the hybrids show significantly improved flame retardancy. In the case of N-PLS10/BDM/DBA hybrid (with 10wt% N-PLS), its LOI increases from about 26.1 to 42.1 %, while its peak heat release rate (PHRR) and total heat release (THR) are only about 68 % and 58 % of that of BDM/DBA resin, respectively. In addition, all hybrids have outstanding dimensional stability. Specifically, its coefficient of thermal expansion (CTE) in glassy state and rubbery state of N-PLS10/BDM/DBA hybrid are only about 51 % and 58 % of that of BDM/DBA resin, respectively. These attractive improved properties are attributed to the outstanding thermal stability and flame retardancy of PLS and N-PLS, and their good dispersion with resin.Subsequently, a novel fully end-capped hyperbranched polysiloxane (Am-HPab, a and b represent the molar ratio of phenyltrimethoxysilane andγ-aminopropyl triethoxysilane, respectively) with controlled branching degree and amine-groups was successfully synthesized by a controlled hydrolysis of phenyltrimethoxysilane andγ-aminopropyl triethoxysilane, and then used to develop new modified CE and BDM/DBA resins. Results show that the nature of thermosetting resin as well as the structure and content of Am-HPab have significant influences on the integrated properties of modified resins. Through controlling the molar ratio of a and b, we can prepare modified resin with different characteristics of performance. It is found that Am-HP82 can successfully endow BDM/DBA resin with improved overall properties (including flame retardancy, mechanical, dielectric and thermal properties, etc.). Specifically, the cone calorimeter measurements clearly indicate that the average heat release rate (AHRR) and THR of modified BDM/DBA resin with 10 wt% Am-HP82 are only 20 % and 17 % of that of neat BDM/DBA resin, respectively, exhibiting outstanding flame retardancy. A synergistic flame retarding mechanism is believed to be attributed to these results, which includes improved thermal stability, producing non-combustible gas, acting in the condensed phase, and providing a barrier for heat and mass transfer owing to the introduction of Am-HPab to BDM/DBA.Finally, in order to overcome the poor curing processing and toughness of silicone resin, a reactive hyperbranched polysiloxane (B-HBPSi) with a large amount of epoxy groups was designed and synthesized through the hydrolyzation ofγ-aminopropyl triethoxysilane, and hexamethyldisiloxane, and then used to prepare a new silicone resin system with a commercial silicone resin (SLER). Results show that cured B-HBPSi/SLER resin possesses outstanding dielectric properties, toughness, water resistance, and high char yield at high temperature, these properties are better than most thermosetting resins, exhibiting great potential to be used as high performance electronic packaging materials, resin matrices of advanced composites, adhesives, and insulating varnish, etc.