Preparation Of Isocyanate Modified Sorbitol Composite Nucleating Agent And It’s Effect On Properties Of Polypropylene

The sorbitol nucleating agent tend to self-assembles into a three-dimensional network structure during the processing of polypropylene(PP).It would led the entire system to tern to gel state which will results in a sudden increase in the melt viscosity.This phenomenon can have a negative influences on the material processing,especially in the melt spinning process.The gelation caused by sorbitol nucleating agents would limit the material orientation and the draw ratio during melt spinning.In response to this problem,many scholars have found that a series of polyhedral oligomeric silsesquioxanes(POSS)with specific structure can interfere with the self-assembly behavior of sorbitol nucleating agent molecules,and the combination of the two can inhibit the gelation of nucleating agent molecules,however the high price of POSS and easily to agglomerate in non-polar PP has limited the wide application of this method.In this paper,we directly use dibenzylidene sorbitol(DBS),bis(p-methylbenzylidene)sorbitol(MDBS),bis(3,4-dimethylbenzylidene)sorbitol(DMDBS)as raw materials.Then we use n-butyl isocyanate(BI)to react with their free hydroxyl groups to synthesize three hydroxyl-free dibenzylidene sorbitol derivatives(DBS-BI,MDBS-BI,DMDBS-BI).These hydroxyl-free dibenzylidene sorbitol derivatives were used to replace POSS to compound with sorbitol nucleating agents.We have studied the optimal synthesis conditions of hydroxy-free dibenzylidene sorbitol derivatives.We also studied that how the hydroxy-free dibenzylidene sorbitol derivatives effect the gelation and the nucleation of the sorbitol nucleating agents.Finally,we analyzed the compounding mechanism of hydroxy-free dibenzylidene sorbitol derivatives and sorbitol nucleating agents.The main work of this paper are as follows:1.The non-hydroxy dibenzylidene sorbitol derivatives were synthesized by a one-time charging method using N-methylpyrrolidone as a solvent.The results show that three target products have been successfully synthesized,and their maximum thermal decomposition temperatures are all higher than 280°C,which can ensure that the non-hydroxy dibenzylidene sorbitol derivatives can stay thermally stable under the polypropylene processing conditions.The optimal synthesis conditions are as follows:dibutyltin dilaurate is used as a catalyst,the molar ratio of BI to sorbitol nucleating agent is 2:1,the amount of solvent is 1000% of the mass of the reaction substrate,the reaction temperature is 110 °C,and the reaction time is 180 min.2.We used isotactic polypropylene(iPP)as the matrix and the DBS as nucleating agent to prepare a series of three-component nucleation systems.The components of the nucleation systems were iPP/DBS/hydroxy-free dibenzylidene sorbitol derivative.The results showed that the gelation phenomenon caused by DBS in the cooling process was well suppressed when the amount of the non-hydroxydibenzylidene sorbitol derivative was only 0.4wt%.Moreover,it would only have little effect on the grain refining effect,the haze,the modulus and the elongation at break of the nucleated system,when the amount of the hydroxy-free dibenzylidene sorbitol derivative were less than 0.8% by weight.3.The effects of DBS-BI on DBS’s self-assembly were studied in two systems of N,N-dimethylformamide(DMF)and polypropylene melt.The results showed that theaverage size of the self-assembled aggregates of DBS/DBS-BI composite were decreasing with the increasing dosage of DBS-BI.which indicats that DBS-BI interfered with the self-assembly of DBS.The DBS/DBS-BI mixture was heated and melted and then cooled.When the second heating was carried out,it was found that the melting peaks of the two components were broadened,and the melting enthalpy became smaller,which indicats that there are interactions between DBS and DBS-BI after being heated.We uesed Materials Studio to simulate the interaction between DBS and DBS-BI,and we found that hydrogen bonds and π-π stacking could be formed between DBS-BI and DBS.We also found that the cohesive energy between DBS and DBS-BI were so close to DBS and DBS,Which indicated that the probability of forming DBS-DBS-BI structure was as high as forming DBS-DBS structure.So that DBS-BI could interfere with the self-assembly behavior of DBS.