| Since the massive use of petroleum-based plastics has caused great damage to the environment,the application of bio-based polymer materials has gradually increased,among which polylactic acid(PLA)has gained widespread attention due to its excellent biocompatibility,processing properties.However,the slow degradation rate of PLA and the slow crystallization rate during the molding process have limited its application in industry.Many scholars have tried to add other components to improve the crystallization rate and degradation rate of PLA.In this thesis,we used two biomass materials,starch and lignin,as the matrix of PLA,and took advantage of the low cost and high yield of starch and the high strength of lignin due to the three-dimensional mesh structure to prepare all-biomaterial composites by compositing them with PLA separately.structural properties and evaluated the degradation behavior of the composites.The main studies are as follows:1.PLA/starch composites were prepared using a dense mixing method,and the effects of starch addition in different proportions(mass fraction of 1-40%)on the crystallization properties of PLA,mechanical properties and hydrophilicity of the composites were investigated.Firstly,the effects of starch addition and content on the mechanical properties of PLA composites were investigated.It was found that the tensile strength of the composites showed a small increase only when the starch content was 1% after the addition of starch,and the tensile strength of the composites decreased when the starch content continued to increase,and the elongation at break of the composites decreased after the addition of starch,and the elongation at break decreased with the increase of starch content,from which we We observed the SEM of the tensile section and found that when the starch content was 5%,a more obvious gap appeared at the interface between starch and PLA.We also studied the effect of starch addition on the crystallization performance of PLA,and the DSC results confirmed that the starch addition promoted the crystallization of PLA,and the time required for the complete crystallization of PLA was first shortened with the increase of starch content and then remained unchanged.At the same time,the crystallinity of PLA was calculated,and it was found that the crystallinity increased slightly after adding starch,from 42%to 46%.We also did XRD characterization of the composites and found that the addition of starch resulted in a small angle shift and a sharp crystallization peak,indicating a higher degree of crystallization and larger grains.The hydrophilicity of the composites was characterized and it was found that the hydrophilicity of the composites increased gradually with the increase of starch content due to the strong hydrophilicity of starch,and the contact angle of pure PLA was 70°,which decreased to 55.7° when the starch content was 40%.2.In view of the fact that the addition of starch did not improve the crystalline properties of PLA much and the mechanical properties decreased more obviously,we chose another biomass material,lignin,and investigated the effect of lignin addition on the crystalline properties of PLA,mechanical properties,thermal stability and hydrophilicity of the composites.Different contents of lignin(1-40%)and PLA were mixed by dense refining,and the effects of lignin and content on the mechanical properties of PLA were firstly investigated.It was found that the tensile strength of the composites increased and then decreased after the addition of lignin,and the mechanical properties of the composites increased to a smaller extent when the lignin content was less,only 1-2%,with a maximum increase of 3.7%,and the elongation at break kept the same Therefore,we studied the SEM of the tensile section,from which we found that the interfacial gap between lignin and PLA was more obvious when the lignin content reached 10%,and the gap between the two gradually became obvious as the lignin content increased.We also studied the effect of lignin addition on the crystallization performance of PLA,and the DSC results confirmed that the addition of lignin promoted the crystallization of PLA,and the time required for the complete crystallization of PLA was shortened with the increase of lignin content,and it took 60 min for the complete crystallization of pure PLA at 115°C,but only 15 min for the complete crystallization at 40% lignin content.The crystallinity of PLA was also calculated,and it was found that the addition of lignin increased the crystallinity of PLA from 42% of pure PLA to 52% at the highest.We also did the POM test on the composites and found that the number of spherical crystals increased after the addition of lignin,but the regularity of birefringence became worse and the size of spherical crystals did not change significantly.We also investigated the effect of lignin on the thermal stability of the composites.We found that the thermal stability of the composites improved after the addition of lignin by TG and DTG,but the more the lignin content,the worse the thermal stability,and the best thermal stability of the composites was achieved when the lignin content was only 1%,and the initial decomposition temperature increased from 209°C to 301°C.Finally,we also studied the effect of lignin addition on the hydrophilicity of the composites and found that the contact angle did not change significantly after the addition of lignin.The contact angle of pure PLA was 70° and decreased to 65.3° at 40% lignin content.3.Finally we investigated the effect of lignin and starch addition on the degradation properties of the composites,we studied two phosphoric acid(PBS)buffers and soil degradation in two systems.We selected pure PLA,PLA/lignin composites with 1% and 40% lignin content respectively and starch composites with 1% and 40%starch content respectively for degradation.We found that the intensity of the absorption peaks of the PLA ester bonds gradually decreased in the IR image,indicating that the degradation mainly occurred in the breakage of the PLA ester bonds,and the intensity of the crystalline peaks increased and then decreased in the XRD image,indicating that the degradation occurred first in the amorphous region and then in the crystalline region.Comparing the mass loss and sample photos,the moderate loss of the material with the addition of starch and lignin is greater and the degree of fragmentation is stronger,which indicates that the degradation rate is accelerated with the addition of starch and lignin,and the more the content of lignin and starch,the faster the degradation rate.Comparing the mass loss and sample photos by adding the same content of starch and lignin,we found that the promotion effect of adding starch on the degradation of composites was more obvious.We also did soil burial experiments,and the same samples with the same ratios mentioned above were placed in soil with 50%soil content and 35°C for 2,4,6 and 8 weeks,and then characterized their mass loss and surface SEM images,and found that the degradation was accelerated by adding starch and lignin,where the degradation was greatly accelerated by adding starch,and the number of cracks appeared very high in the SEM images.It was hypothesized that this was because starch provided nutrients to the microorganisms in the soil,which led to a higher microbial content in the soil,thus promoting degradation. |
