Translated Abstract
Phenolic resin (PR) and its composites are the most widely used ablation resistant material in industral practice for its outstanding performance. But the application of PR in some specific fields is limited by the poor structure stability and high coefficient of thermal expansion (CTE), which can’t meet the demands of materials for the rapid development of modern science and technology. In order to develop PR and PR matrix composites (PRMC) with good thermal stability, the relationship between the structure and properties of PR, in particular for the Novolac resin is studied in my dissertation. Based on the full understanding on the relationship between structure and thermal stability, a novel boron-containing novolac phenolic resin (BNPR) with good thermal stability is synthesized and the influence of the introduction of boron on the thermal stablity of the resin is further studied. The following results and conclusions are obtained.
(1) By characterizing the structures of novolac and the cured resin, as well as the curing process in depth, the curing mechanism of novolac cross-linked by hexamethylenetetramine (HMTA) is proposed. Besides typical functional groups of phenolic hydroxyl and methylene, some additional oxygen-containing functional groups are contained in Novolac resin, like phenylate, carbonyl and methoxyl. During the curing process, the formaldehyde and dimethylolamine generated by the decomposition of HMTA could react with Novolac, results in the formation of the three-dimensional cross-linked network. With small amount of HMTA, the reaction between formaldehyde and Novolac is the dominated one, and methylene is formed. When the amount of HMTA is lager, more dimethylolamine molecules particpate in the curing reaction and nitrogen-containing methylene is generated.
(2) By the examination of pyrolysis process, the pyrolysis mechanism and the influence of structure on the heat resistance of PR are studied in depth. The pyrolysis of PR involves three stages and the oxidative cleavage reactions occur in each stage, and the thermal stability of PR is determined by the substituted degree of phenol. The influence of cross-link density on the heat resistance of PR is that cross-link can reduce the release of phenol and its derivatives, and inhibit the activity of functional groups simultaneously. Based on the cross-link density, a novel pyrolysis mechanism and a quantitative relationship between the cross-link density and residual weight of PR are proposed.
(3) From the view point of molecular design, the synthesis of a novel boron-containing Novolac phenolic resin (BNPR) from the precursors of hydroxymethyl phenylboronic acid and Novolac resin by a hydrothermal synthesis strategy has been reported. The structure and relationship between structure and properties are investigated in depth. The synthesized BNPR retains the skeleton structure of Novolac resin and grafts the boric acid structure via B-C bond, simultaneously. BNPR shows the same processability as Novolac but much better thermal stability. The remaining boron hydroxyl in BNPR could react with phenolic hydroxyl or boron hydroxyl groups to form B-O-B or B-O-C structures, which offers the possibility of recycling by means of hydrolysis or solvolysis. The generated B-O-B or B-O-C structures are helpful to the improvement the thermal stability of resin through increasing the cross-link density and reducing the emissions of small molecules during the pyrolysis and carbonization process.
(4) The relationship between CTE and the structure of cured PR, as well as the influence of composition of PRMC on CTE is studied. By modifying the state equation of linear amorphous polymer, the equation that can be used to quantitatively describe the temperature dependence of CTE of cross-linked polymer below glass transition temperature (Tg) is established, which enriches viewpoint of the free volume theory. The emergence of the inorganic particles seems to have little influence on the temperature dependence of the CTE of polymer matrix, and the CTE of PRMC shows the similar temperature dependence as the matrix. By analysis the temperature dependence of CTE and fully considering the role of interface in composites, an equation is proposed to predict the CTE of PRMC with different filler loadings at a specific temperature, which proved to be more accurate than the existing theoretical models.
(5) The structural change of Novolac resin under the oxidation condition is studied by treating PR in the oxidation solution, which could provide guidance to the surface treatment of PR and PRMC. Phenolic hydroxyl and methylene are the active sites in PR, which could be easily oxidized to ketone and quinone groups. The generated ketone group could be spoiled by the further oxidation, results in the destruction of the PR chains. By examining the treatment effect of Novolac under different conditions, the optimized treatment condition for PR and its composite is obtained. The hydrophilicity and the roughness of the PRMC surface are increased after oxidation treatment, which may be attributed to the generation of more hydrophilic functional groups and the exposition of inorganic fillers.
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