Synthesis Of Foamable Phenolic Resin And Study On Its Structure, Toughness And Flame Retardancy

Phenolic foam has the advantages of low smoke and low toxicity during combustion,high dimensional stability and low thermal conductivity,which not only meets the 75%energy saving standard requirements for green buildings and residential buildings,but also has a wide market demand in chemical industry,transportation,energy,aerospace and other fields.Phenolic foam is a closed-cell rigid foam plastic prepared by expansion and crosslinking curing at a certain temperature with foamable phenolic resin（F-PR）as the matrix,foaming agent and curing agent as additives,which has the disadvantages of high brittleness,easy pulverization and easy smoldering.Therefore,under the premise of stabilizing its original mechanical properties,using a relatively simple process to prepare high-toughness and high-flame retardant phenolic foam,while solving its two major problems of brittleness and easy smoldering,can not only provide high-quality and safe thermal insulation materials for the market to a large extent,but also has important significance for further broadening the application field of phenolic foam.In this thesis,the effects of F-PR molecular structure and phenolic foam cell structure on the toughness and flame retardancy of phenolic foam were studied.Triethylene glycol and low-melting point glass powder（LMG）were used as modifiers to prepare triethylene glycol intra-toughened phenolic foam,low temperature ceramifiable flame retardant phenolic foam（GC-PF）and high toughness and high flame retardant phenolic foam（T-GC-PF）by chemical structure modification,physical blending modification and physico-chemistry composite modification,respectively.The details are as follows.F-PRs with different formaldehyde to phenol molar ratio（F/P）were synthesized and used to prepare different F/P phenolic foams with similar cell sizes.The effects of F/P on the molecular structure of F-PR and the cell structure of phenolic foam were studied,and the effects of F/P on the toughness and flame retardancy of phenolic foam were further summarized.The study found that the rigid structure of F-PR was the basic reason of the brittleness of phenolic foam.When F/P=1.9,the unmodified phenolic foam showed both relatively good toughness and flame retardancy.The increase of F/P can promote the polycondensation reaction between hydroxymethyl-phenols,resulting in the viscosity and solid content of F-PR increasing with the increase of F/P,and the rigidity of F-PR also increases.The greater the rigidity of F-PR,the more serious the damage of phenolic foam cell wall.The higher the F/P,the higher the content of inflammables in the phenolic foam that promoted the formation of the char layer,the higher the limiting oxygen index（LOI）of phenolic foam.It is worth noting that the smoldering time and smoldering velocity of phenolic foam also increased with the increase of F/P.When F/P exceeded 2.3,the phenolic foam smoldered to burnout.The relationship between smoldering velocity and F/P can be calculated by an Arrhenius-type expression.Arrhenius-type expression calculation showed that when the F/P reached 2.3,the smoldering velocity increased sharply,reaching a maximum of 0.193 mm/s.According to the best F/P,triethylene glycol modified F-PRs with two different molecular structures were synthesized by in-situ polymerization modification process and pre-polymerization modification process with triethylene glycol as internal toughening agent,and triethylene glycol in-situ polymerization modified phenolic foam（T-In-PF）and triethylene glycol pre-polymerization modified phenolic foam（T-Pre-PF）were prepared with the modified resins as foaming matrix.The molecular weight distribution of triethylene glycol pre-polymerization modified phenolic resin was uniform and its basic properties were better than triethylene glycol in-situ polymerization modified phenolic resin and unmodified F-PR.The cell size and distribution of triethylene glycol modified phenolic foams were more concentrated and the level of damage of cell wall was low,among which the cell shapes of T-Pre-PF were uniform polygonal structures and formed a stable honeycomb-like structure.The maximum bending strength compressive strength of T-Pre-PF were 42.16%and 60.00%higher than those of unmodified phenolic foam（U-PF）,and both were higher than those of T-In-PF.The number of partial benzene ring linkage bridges in the T-Pre-PF structure increased from three to four,which maintained strength while improving toughness.When the addition of triethylene glycol was 20%,the toughness of T-Pre-PF was the best.The fracture displacement of T-Pre-PF was increased by 85.13%compared with U-PF,and the pulverization rate was reduced by 55.85%compared with U-PF.The findings demonstrate that pre-synthesis modification process is more suitable for the preparation of triethylene glycol internal toughening modified phenolic foam than the in-situ polymerization modification process.GC-PF was prepared by incorporation of LMG containing B₂O₃ and Na₂O as main components into a F-PR（F/P=1.9）matrix by physical blending.The cell diameter of GC-PF became smaller and the cell wall defects were reduced.Although the toughness of GC-PF decreased with the increase of LMG content,a glassy ceramic protective layer was successfully formed when GC-PF was exposed to flame or a high temperature environment when LMG content was 50%.The flame retardancy and heat resistance of the foam were significantly improved,and no smoldering occurred.The molecular structure and cell structure of GC-PF before and after ceramicization were characterized.The results showed that the glassy ceramic protective layer was a dense structure with a certain self-supporting strength formed by covering the surface of char layer with blends containing glassy borates generated by the molten B₂O₃ and Na₂O,unmelted inorganic particles,and aryl configuration char.When the LMG content was 50%,the LOI of GC-PF was as high as 45.6%,the remaining area of the raw material matrix of GC-PF after combustion for 60 s was about 2.8 times larger than that of U-PF,and the char residue at 900°C was 7 times greater than that for U-PF.T-GC-PF was prepared by blending triethylene glycol pre-polymerization modified phenolic resin with LMG using composite modification of chemical and physical,in which T-Pre-PR was the foaming matrix.The results showed that the T-GC-PF with 50%LMG content had the best comprehensive performance,the toughness and flame retardancy of the foam were significantly improved,and the problems of brittleness and smoldering of phenolic foam were solved.When the content of LMG was 50%,the cell diameter of T-GC-PF was greatly reduced,and the cell wall had no obvious defects.The compressive strength and bending strength of T-GC-PF were 37.30%and 123.00%higher than those of U-PF,respectively.At this time,the fracture displacement of T-GC-PF was increased by 38.98%compared with U-PF,the pulverization rate was reduced by 24.94%compared with U-PF,and the foam toughness was significantly improved.A glassy ceramic protective layer was successfully formed when T-GC-PF was exposed to flame or a high temperature environment without smoldering,the remaining area of the raw material matrix after combustion for 60 s was about 2.7 times larger than that of U-PF,and the degree of foam deformation after combustion was less than that of GC-PF.The LOI of T-GC-PF reached 43.5%,the maximum pyrolysis rate was reduced by 74.07%compared with U-PF,and the char residue at 900°C was6 times greater than that for U-PF.