Study On The Synergetic Modification Of Lignin-based Phenolic Foamed Plastics By Silicon-containing Compounds

As the third generation of thermal insulation materials,phenolic foam has the characteristics of good flame retardancy,non-toxicity,low smoke and good thermal stability.It is widely used in construction,petrochemical equipment,electrical pipelines,transportation and other fields.However,the phenolic foam has the disadvantages of high brittleness and low toughness,which seriously restricts its application.Therefore,it is the current development trend to modify the phenolic foam to improve its performance.As the world's second-order natural organic matter,lignin is a kind of aromatic polymer which is widely distributed in plants and can replace some phenols with aldehydes to prepare modified phenolic resin,that can reduce the free formaldehyde and free phenol content of the resin.The main component of the active whisker silicon is silica,which does not decompose at high temperatures,and can improve the mechanical properties such as flame retardancy,heat distortion temperature and bending strength of the product.There is a large amount of active silanol on the surface of nano-silica,which can be combined with phenolic resin to form a cross-linked network structure,but its activity is high,thermodynamically unstable,poor dispersion in polymer,easy to agglomerate.The introduction of silicon element and phenolic resin to form Si-O-C bond,improve the interface properties between the resin and the bonding material,and the mechanical strength and thermal stability of the material.The Si-O bond length in the siloxane chain is longer.The flexibility of the resin has been further improved.The main research contents are as follows:The active whisker silicon was used as the silicon source,and the silicon element was introduced into the phenolic resin structure.The free phenol,free aldehyde and methylol content test showed that the free phenol and free aldehyde content of the common phenolic resin PR1 were 6.9%and 8.3%,respectively.When the content of lignin and whisker silicon increased,the content of free phenol and free aldehyde decreased first and then increased.The change trend of methylol and free aldehyde content was opposite.When the lignin substitution amount was 30%?based on phenol?,and content of whisker silicon was 0.5%?based on resin??PR4?,the free phenol content was 4.5%,and the free aldehyde content was 4.7%,then the methylol content was 41.5%,and the resin activity was improved.Silanized nano-SiO₂modified lignin-based phenolic resin when the substitution of lignin is 30%.The free phenol free aldehyde content decreased first and then increased with the increase of nano-SiO₂.The methylol content increased first and then decreased with the increase of nano-SiO₂content.When the content of nano-SiO₂was 0.7%and the silane coupling agent was 2 g,the effect of PR4-4 was better,the free phenol,the free aldehydecontent and the methylol content respectively was 4.5%,4.9%and 40.7%.FT-IR analysis showed that the two types of modified resins have stable Si-O-C bonds in their molecular structure.The modified phenolic foam is prepared by foaming and curing the modified phenolic resin with foaming agent,surfactant and curing agent.The thermal stability,flame retardant,thermal insulation and mechanical properties of the foam were characterized.TGA/DTG showed that the carbon residue rate of foam increased with the increase of whisker silicon and lignin content.The residual carbon ratio of PF4 at 800?was 54.5%,and the residual carbon rate of PF8 increased the most,which was 22.6%higher than ordinary resin.Thermal conductivity,closed cell ratio and scanning electron microscopy analysis of foam materials showed that the closed cell ratio decreased with the increase of lignin and whisker silicon content,and the thermal conductivity increased accordingly.Among them,PF4 had the best cell uniformity,small pore size and thermal conductivity which is 0.033 W/m·K.The higher the content of whisker silicon and lignin,the higher the LOI value and the better the flame retardancy.The LOI value of PF4 is 49.6%,which is 39.3%higher than that of PF1.When the content of lignin and whisker silicon is too high,it will affect the mechanical properties of the foam and the distribution of cells.The compressive strength of PF4 is 0.311 MPa,the flexural strength is 0.412 MPa,and the performance is the best.The lignin and whisker silicon content continues to increase and the mechanical properties are reduced.Phenolic foams PF4-1?PF4-6 were prepared from modified resins PR4-1?PR4-6,and the effects of the amount of nano-SiO₂and silane coupling agent on the properties were discussed.TGA,DTG and TGA/FT-IR analyzed the products of three stages of thermal degradation of phenolic foam.Each foam follows the same thermal degradation mechanism.The content of silanized nano-SiO₂increases,and the thermal stability of foam increases first and then decreases.The heat resistance is greatly improved,and the carbon residue rate is 56.3%.The flame retardancy of the foam increases with the content of nano-SiO₂,first increasing and then decreasing;flame retardant PF4-4>PF4-5>PF4-6>PF4-3>PF4-2>PF4-1.When the nano-SiO₂is excessive,the agglomeration affects the cell structure.When the lignin substitution rate is30%and the nano-SiO₂content is 0.7%,the foam has the best cell distribution and the best flame retardancy,and the LOI value is 54.8%.When the silane coupling agent was 2 g at this time,the compressive strength and the flexural strength were 0.442 MPa and 0.552 MPa,respectively,and the effect was the best.In summary,by comparing the properties of whisker silicon and silanized nano-SiO₂ modified lignin-based phenolic foam,the latter effect is better than the former.