Preparation Of Copolyamide-modified PA6 And Exploration Of The Ring-opening Mechanism Of Caprolactam

Due to its excellent comprehensive properties,polyamide 6 is widely used in various fields,such as civil fiber,machine parts,electronic components,etc.However,pure PA6 has poor transparency,heat resistance and dimensional stability.Therefore,the second monomer and the third monomer were copolymerized with caprolactam to modify PA6 to compensate for the lack of performance.In this work,series of amino acid or dibasic acid/diamine combination was copolymerized with caprolactam and further explored the mechanism of copolymerization.Then on the base of these,the best polymerization conditions were determined for a copolymer with better comprehensive performance and a batch of copolymers were prepared in a reaction kettle.Further,comparison of the mechanical properties between this copolymer and pure PA6 was done.Firstly,a total of 17 reaction systems were designed with caprolactam,amino acid or dibasic acid/diamine(molar ratio 1:1)as raw materials.In this work,caprolactam also plays an important role as the solvent for high melting dibasic acid,hence,melt copolymerization can be carried out without using other solvents.The structure of the products was characterized,and the water-soluble extractables content,relative viscosity and relative molecular weight of products were also tested.The experimental results show that after the introduction of the dibasic acid and the diamine,the caprolactam can be subjected to ring-opening polymerization under anhydrous conditions.Therefore,the copolymerization mechanism can be explained by that when the caprolactam coexisting with the dibasic acid and the diamine in the system and in a molten state,the protons on the carboxyl group of the dibasic acid would transfer to the carbonyl oxygen atom of caprolactam to form an active center,and then the diamine attacks the carbocation,hence initiating the ring opening and poly-addition reaction and generating linear low molecular weight pre-polymer,then followed by the poly-condensation reaction with the dibasic acid,a long chain copolymer was produced.Subsequently,after analyzing the properties of the copolyamide prepared by the above 17 groups of reaction systems,biphenyl-4,4-dicarboxylic acid/4,4-methylenedianiline system was selected as the additive to copolymerize with caprolactam because of its good modification effect.The effects of addition amount,time and reaction temperature on product performance were studied in detail and the best copolymerization conditions were obtained.The experimental results reveal that the thermal stability of the copolyamide was effectively improved with the increase of the amount of biphenyl-4,4-dicarboxylic acid,and the low molecular weight extractables content were reduced to 3.14% and 3.07% respectively,when the addition amount reached 5% and 7%.In addition,the experimental results also demonstrate that the low temperature reaction can inhibit the generation of low molecular extractables.Moreover,raising the temperature appropriately and prolonging reaction time properly are proved to be beneficial to increase the molecular weight of the product.Therefore,considering the influence of various factors,the best polymerization conditions are as follow: the addition amount of 4,4-diphenyl phthalate is 5%,the reaction temperature is 260 °C,and the reaction time is 12 h.The copolymer obtained has a viscosity of 2.54 and endows excellent thermal performance.The thermal decomposition temperature reaches 448 ? which is 40 ? higher than that of pure PA6,and the melting point even lowers to 209 ?,which is advantageous to the molding process of the product.Finally,under the optimal copolymerization conditions,batches of samples were prepared by using reaction kettle.After pelletizing,extraction,drying and injection molding was carried out to test,the mechanical properties compare with pure PA6 with the same molecular weight.It's found that 4,4-BPDA and DDM improves the tensile strength,flexural strength and modulus of the material.In addition,affected by the structure of the benzene ring,the copolymer tends to form a ? crystal form during crystallization,and has a higher glass transition temperature of 81.1 ?,which is nearly 10 ? higher than that of pure PA6.