Research On Preparation And Performance Of Polyurethane/Lignin Nanoparticles Composites

Polyurethane(PU)is a synthetic polymer material that has the dual advantages of rubber and plastic.Its products have been widely used in various fields of the national economy such as construction,home appliances,textiles,and transportation.However,the synthesis of polyurethane consumes a large amount of petroleum resources.Petroleum resources are nonrenewable resources and national strategic materials.Therefore,the development of biomass materials as raw materials to prepare high-performance bio-based polyurethane materials has broad development prospects.Lignin is a natural polymer material with unique aromatic structure and abundant reserves.However,most of the lignin is discharged as fuel or waste,which is definitely a waste of resources.The surface of lignin contains a variety of reactive functional groups(hydroxyl,carboxyl,etc.),which can be used in the synthesis of various polymer materials.The use of lignin in the synthesis of polyurethane can not only reduce the consumption of fossil resources,but also improve the performance of the product.In view of this,this project uses lignin as a raw material for bio-based polyols,partially replacing petroleum-based polyols,and prepares lignin-based polyurethane composites through reaction modification,and characterizes their structure and properties.It is expected to expand the highvalue utilization of lignin in the field of polymer composite materials.And it also provides a new idea for the modification of polyurethane synthetic materials,which has important application and research value.The main research contents of this paper are as follows:First of all,the preparation and performance research of highly reactive nano-lignin.Lignin nanoparticles(LNP)was prepared by acid hydrolysis,and lignin microparticles(LMP)obtained by grinding was used as a comparison to systematically study the changes of its microstructure and morphology before and after acid hydrolysis.The effect of acid hydrolysis process on the reactivity of lignin was systematically studied,and the activity synergistic mechanism was discussed from the structural aspect.The results are as follows: in the process of acidolysis,the ether bond between the lignin aromatic hydrocarbons is broken to form new phenolic and alcoholic hydroxyl groups,while the aromatic structural units remain unchanged.After acid hydrolysis,the lignin nanoparticles form a more uniform granular shape with a particle size of about 35 nm.Due to the increase of the specific surface area and the increase of surface phenolic hydroxyl content,the antioxidant activity of lignin nanoparticles is improved(The scavenging activity of 1,1-diphenyl-2-trinitrophenylhydrazine free radicals is better than commercial antioxidants(dibutylhydroxytoluene)).It eliminates the reactive oxygen species stimulated by hydrogen peroxide in He La cells,while maintaining the integrity of the cells.In addition,lignin nanoparticles show better antibacterial properties,and the number of colonies in the antibacterial test decreased 83%.This proves that the lignin nanoparticles obtained by acid hydrolysis have higher reactivity and are more suitable for the synthesis of polymer composite materials.On this basis,using polyethylene glycol(PEG)and hexamethylene diisocyanate(HDI)as raw materials,and using lignin nanoparticles as cross-linking agent and bio-based polyol,through two steps of low-temperature prepolymerization and high-temperature curing,the lignin-nanoparticle-based polyurethane composite films were successfully prepared.The effects of different lignin-nanoparticle content on the microscopic phase morphology of polyurethane composites were systematically studied,as well as the optimization effects on mechanical properties,thermal properties,hydrophobic properties,dielectric properties,UV resistance and reworkability.It was found that the neat PU produced by the reaction of PEG and HDI has a linear structure.After the multi-hydroxyl structure of LNP participates in the reaction,a crosslinked network is formed in the polyurethane composite.The crosslinked network and the rigidity enhancement of lignin form a synergistic effect,which improves the mechanical properties of polyurethane(Tensile strength reaches 12.4 MPa,elongation at break reaches more than 1000%).At the same time,thermal stability and hydrophobic properties have also been improved(After adding 5 wt% of LNP,the water contact angle of the polyurethane composite material increased from 46.4° to 107.8°).On the other hand,the lignin nanoparticles can be better dispersed in the PU matrix through reaction compatibilization,which improves the UV shielding ability of the PU composite(The addition of 5 wt% LNP makes the PU composite can shield more than 94% of the ultraviolet light).In addition,the polyurethane molecular chain will rearrange in the presence of lignin naoparticles,making it exhibit insulating properties at high frequencies.At the same time,the carbamate in the polyurethane will undergo a transcarbamylation reaction at high temperatures,leading to rearrangement of the cross-linked network,so the lignin-nanoparticle-based polyurethane composite material has certain reworkability.For example,after adding 7wt% of LNP,the tensile strength of the polyurethane composite after repeated processing is still maintained at 83%,and the storage modulus is maintained at 51%.Finally,in order to further improve the reworkability of polyurethane composites,UPymodified LNP(LNP-UPy)was prepared by using 2-amino-4-hydroxy-6-methylpyrimidine(UPy)and HDI to react with LNP.Polytetrahydrofuran(PTMG)and isophorone diisocyanate(IPDI)were used as raw materials to prepare polyurethane prepolymers.A series of polyurethane composites were prepared,by using bis(4-hydroxyphenyl)disulfide(SS)and LNP-UPy as chain extender and crosslinker,respectively.And the effect of the addition of LNP-UPy on the reworkability of the composites was systematically studied.The synergistic effect of the multiple hydrogen bonds between UPy groups and the dynamic disulfide bonds of SS strengthens the structural rearrangement of the polyurethane cross-linked network and improves the reworkability of the composite material(After re-heating,the storage modulus of the composite material with 3 wt% UPy-LNP is still maintained at 93%).Due to the conjugation of the benzene ring in the SS structure and the intrinsic UV shielding properties of LNP,the prepared polyurethane film has a good UV shielding ability with a shielding efficiency of more than 99%,and the transparency of the film will be sacrificed after adding lignin(When 1 wt%UPy-LNP is added,the transmittance of polyurethane at 550 nm is reduced by 21%).In summary,this topic takes polyurethane and lignin as the research objects,and prepares highly reactive lignin-nanoparticle by acid hydrolysis.By using nano-lignin to replace part of polyols to synthesize polyurethane nanocomposites,it can not only improve its mechanical properties,but also endue special UV shielding functions and reworkability.And the polyurethane composite material finally has excellent reworkability,by introducing reversible disulfide bonds and multiple hydrogen bonds in the polyurethane/lignin system.