Construction And Application Of Chitin-based Polymeric Colorants

Polymeric colorants refer to the colored polymer formed by introducing small molecular chromophores into the main or side chains of polymer through certain chemical reactions.Polymeric colorants are not only endowed with the same excellent coloration ability as small molecular colorants,but also have the characteristics of high strength,processability,migration resistance,heat resistance and solvent resistance.However,most known polymeric colorants,such as polyether type polymeric colorants,polyamine type polymeric colorants and polyurethane type polymeric colorants,are prepared from coal,oil,natural gas and other non-renewable fossil resources.Therefore,it is desirable to develop greener colorants from more environmentally friendly natural polymer materials.In this paper,we used chitin as the polymeric backbone to construct polymeric colorants.To improve the accessibility of chitin to reactive dyes,it was deacetylated,dissolved in aqueous phosphoric acid,and regenerated using various coagulation baths.The pretreated chitin was then reacted with commercially available reactive dyes R195,B194 and Y145 in the binary non-aqueous solvent system containing dimethyl sulfoxide(DMSO)and ethyl octoate(EO)system to obtain the chitin-based colorants,which was shown to be effective in coloring polylactic acid(PLA).The main experimental works involved and key findings are summarized as follows:For deacetylation,chitin(5.0 g)was treated by a 35 wt%aqueous sodium hydroxide solution at 90°C for 1,3,and 5 hours.The deacetylation ratio was determined by titration with a standard Na OH solution.The results showed that the degree of deacetylation reached 39.7%at 5 hours,and further elongation of treatment time would resulted in significant decrease in yield due to the increased solubility of highly deacetylated chitin.The chitin samples were dissolved in 85.0%aqueous phosphoric acid at 30°C to obtain viscous solutions of chitin concentration(2.0%),which was then regenerated in coagulation baths of water.To obtain regenerated chitin(RCh)of varied degree of polymerization and crystallinity,the chitin solutions in phosphoric acid was kept at 30°C and different times(0.5 h,1 h,2 h,3 h,5 h,7 h).The results showed that with increasing dissolution time,DP of chitin decreased notably,and it became hard to recover chitin.The yield of the dissolution/regeneration process ranged from 82.0%to 38.0%.and the crystallinity of the recovered sample all decreased from 79.0%of the original chitin to the range of 50.0%-57.0%,except for RCh-7h(crystallinity=67.0%).Dissolution of chitin in phosphoric acid was also carried out at-18°C and 50°C,respectively.And it was shown that at-18°C,viscosity of chitin remained mostly constant within 24 h and the crystallinity of the corresponding RChs were similar to samples prepared at 30°C,being 57.5%,55.5%,53.7%,and 52.0%respectively.RCh prepared from the solution kept at-18°C for 24 h was lowered to 46.7%.On the other hand,at 50°C,serious degradation of chitin via hydrolysis of glycosidic linkages occurred within 40 min of dissolution,resulting in significantly lower yields.Finally,RChs were obtained using water,ethanol,acetone,and aqueous solution of sodium hydroxide(p H=10)as the coagulation baths.The results showed The particles of regenerated chitin obtained in ethanol and acetone bath were larger than those obtained in water and aqueous solution of sodium hydroxide(p H=10).Regenerated chitin is easier to precipitate in ethanol and acetone coagulation bath than in water and sodium hydroxide solution coagulation bath.The results show that more dyes were grafted on samples with higher the degree of deacetylation and up to347.4 mg/g of RR195 could be fixed on the RCh that was pretreated with Na OH for 5 h.The optimum time of dissolution was 1 h.The coagulation bath was found to have limited effect on dye fixation.PLA was colored with regenerated chitin-based polymeric colorant RCh₅-B.The results showed that the PLA dyed with polymer colorant was not easy to exudate compared with the PLA dyed with small molecular dye after solvent extraction.When the content of RCh₅-B was4 wt%,the fracture strength of PLA was enhanced by 29.7%,suggesting the RCh-based polymeric colorants could be effectively used not only to impart color to PLA but also as a reinforcing filler.