Study On Cotton Fiber/Poly(lactic Acid)Composites

With the increasingly serious problems of resource shortage and environment pollution,the development of environmentally friendly biodegradable materials has been paid attention in recent years.Poly(lactic acid)(PLA)is considered to be one prominent alternative for the replacement of conventional petroleum-based polymers because of the better availability of raw materials,low toxicity and completely biodegradability.However,the application of PLA is still restricted because of its low heat distortion temperature and non-satisfying impact resistance.As one of the four natural fibers,cotton fibers have some obvious advantages,such as higher cellulose content,higher degree of polymerization,the extensive source and the low price.To sum up,it has a good potential to reinforce PLA with cotton fibers.Therefore,in this thesis,cotton fibers were used as the reinforcing fibers,and a series of cotton fibers reinforced PLA composites were prepared by compounding and injection molding.And the effects of compounding process,cotton fiber content and length,interface modification treatment methods(alkali treatment,plasma etching,and coupling agent treatment)and heat treatment conditions on the mechanical properties,crystalline behavior and interface morphology of the composites were studied,respectively.Firstly,the effects of the twin-screw mixing process and banburying technology on the properties and structure of cotton fiber/PLA composite were investigated.The results showed that compared with the twin-screw mixing process,the mechanical properties and structure of cotton fiber/PLA composite prepared by banburying mixing process were better and the interfacial bonding between cotton fiber and PLA was stronger.On this basis,the effects of cotton fiber content and fiber length on the properties and structure of cotton fiber/PLA composites were further investigated.The results showed that the crystallinity of the composites increased gradually with the increase of cotton fiber content,whereas the mechanical properties of the composites increased firstly and then decreased with the further addition of cotter fiber.The composite with 20wt%cotton fiber had an optimal mechanical performance.Moreover,cotton fiber length had a similar effect as cotton fiber content and too long or too short fiber could't achieve a better reinforcing effect.The composite with good interfacial bonding between cotter fiber and PLA and the better mechanical properties could be obtained when the mass fraction of cotton fiber was 20%and the length of the fiber was 4 mm.Compared with pure PLA material,the tensile strength and modulus,flexural modulus and impact strength of the resultant composite were increased by 35.7%,44.9%,45.0%and 5.6%,respectively.In order to improve the interface compatibility and thereby improve the mechanical properties of cotton fiber/PLA composite,two kinds of physical modification methods including alkali treatment and plasma etching,and one chemical modification method using coupling agent were used to improve the interfacial adhesion between cotton fiber and PLA based on the composites with 20wt%cotton fiber.The results showed that(1)after alkali treatment,the number of hydroxyl in cellulose molecular chain of cotton fiber decreased,and the compatibility between cotton fiber and PLA improved slightly.However,alkali treated fibers entangled together and led to partial agglomeration.The formation of above structure defects reduced the reinforcement efficiency of cotton fiber in the matrix.Moreover,the internal structure of modified fiber was damaged and its strength reduced sharply.Thus,the mechanical properties of composites reduced by a certain degree.(2)After the plasma discharge etching,the surface of the cotton fiber became rough and its specific surface area increased,but the mechanical properties of the corresponding composites had no obvious change.(3)With the incorporation of the coupling agent HMDI(hexamethylene diisocyanate),the interfacial bonding between cotton fiber and PLA was improved obviously,and the mechanical properties of the composites increased significantly.The cotton fiber/PLA composite had better mechanical performance when the coupling agent content was 1 wt%.Compared with the composite without coupling agent,the tensile strength,flexural strength and impact strength of composite with 1wt%coupling agent were increased by 22.3%,28.0%and 51.9%,respectively.In addition,after interface modification by coupling agent,the storage modulus,loss modulus and thermal stability of the composites were also improved to some extentBased on the above results,the heat treatment was used in order to improve the thermal performance of composite,and the effects of heat treatment conditions on the thermal and mechanical properties of cotton fiber/PLA composite were investigated.The results showed that in the experiment range,the crystallinity and heat distortion temperature(HDT)of both pure PLA and composites were significantly increased after heat treatment with different times and different temperatures.Compared with pure PLA,the addition of cotton fibers accelerated the crystallization during the heat treatment,and the composite showed higher crystallinity and HDT.Taking the coupling agent modified composite as an example,after being heat-treated at 110 ? for 10 min,the HDT of the composite could reach 128.0 ?,which was 62.8 ?higher than that of the composite without heat treatment.However,with the extension of heat treatment time,tensile strength and flexural strength of the composites were both decreased,the impact strength changed little,while the flexural modulus increased due to the increase of crystallinity.Based on the existing experimental results,the optimal heat treatment conditions for preparing cotton fiber/PLA composite with better comprehensive properties were as follows:heat treatment temperature was 110?and heat treatment time was 10 min.