The Study On The Preparation And Performances Of Thermoplastic Elastomer PA6-MDI-PA6/PTMG

As an important member in the thermoplastic elastomers (TPE) family, the polyamide-based thermoplastic elastomers (TPAE) are widely used in automotive interiors, building materials,high-grade sporting goods, medical equipments and other fields since they have good performances including high tensile strength, high elastic recovery, good thermal stability and so on. At present these elstomers are prepared via batch polymerization and melt-extrusion process.The process is difficultly controlled for the use of tetramethylene sulfone and other organic solvents. The poisonous solvents would overflow and pollute the environment, which restrict the large-scale production and application of TPAE, thus it becomes urgent to look for a new green process technology. In this study, a new synthesis route of preparing polyamide-6 based thermoplastic elastomer via continuous polymerization process in non-solvent condition has been introduced: In step 1 binary carboxyl PA6 by hydrolytic ring-opening polymerization of caprolactam was produced. In step 2 chain extension of PA6 with 4, 4'-diphenylmethane diisocyanate(MDI) occurred. Step 3 gave PA6-MDI-PA6/PTMG(TPMP) by esterification of PA6-MDI-PA6 hard segment with PTMG. In this study, the effect of process conditions on the caprolactam hydrolytic ring-opening polymerization, the chain-extention reaction and esterification were investigated. The structure and morphology, the thermal and mechanical performances of PA6-MDI-PA6 hard segment and TPMP were studied. The mechanical simulation on strain-stress has been examined briefly.In order to prepare binary carboxylated PA6 with number-average molecular weight about 2100, so that they could form PA6-MDI-PA6 hard segment with amide structure by inserting MDI,series of PA6 with different number-average molecular weight were prepared for analysis. The effect of terminator adipic acid quantity on the physical properties of PA6 has been investigated.The results indicated in due course of caprolactam hydrolytic ring-opening polymerization process,PA6 would contain equivalent number (43mmol/kg) of carboxyl and amino end-groups and its number-average molecular weight exceeded 15000 if terminator was not used. The use of adipic acid could not only terminate parts or all of the amino end-groups in PA6 but also restrict its chain extending. When the adding content of adipic acid reached 15wt.%, the number-average molecular weight of PA6 decreased to less than 2100, the content of amino end-groups was ?1mmol/kg, the content of carboxyl end-groups increased to 928mmol/kg.The effect of reaction temperature, reaction time on the number-average molecular and low-molecular weight extractables has been checked. It showed that it needed 12 hours more to get to equilibrium for caprolactam polymerization when reaction temperature was lower than 240?, the content of extractable was more than 20wt.%. The elevating of reaction temperature could speed the polymerization and thus shorten the reaction time, as a result the polymerization could reach equilibrium in 12 hours, the content of extractables in PA6 was close to the theoretic value of 10wt.%. If the reaction temperature was over 270?, polymer was easy to decompose and caused the number-average molecular weight of PA6 to go down,meanwhile the extractables content increased. The optimized polymerization process at normal pressure should be let caprolactam react at 255 ? for 12 hours.Water, amide bonds and other active bonds could react with MDI and produce by-products in chain-extention reaction. These side effects must be avoided in the preparation of PA6-MDI-PA6 hard segment with unified structure, therefore the reaction mechanism and kinetics of MDI have been studied. The reaction speed of PA6 reacts MDI needed to be controlled, otherwise the great number of carbon dioxide released in very short time would leave some small holes in the polymer.The fracture surface of hard segment was improved significantly when the adding speed of MDI decreased to 0.4g/min.The thermal properties of PA6-MDI-PA6 hard segment have been measured by using DSC and TG methods. It proved that after chain-extending, the thermal stability of PA6-MDI-PA6 was improved obviously. The melt point of PA6-MDI-PA6 was elevated more than 40?, and its initial decomposing temperature was increased too.In the esterification process, the catalytic mechanism of esterification was studied. It indicated that the transesterification activity of PTMG was very low without catalyst and could only obtain TPMP products which relative viscosity was lower than 1.6. However, the using of catalyst tetrabutyl titanate could improve both transesterification activity and relative viscosity of TPMP.In this study, the relative viscosity and extractables of TPMP were analyzed to investigate the effect of reaction time, reaction temperature and reaction pressure on the properties of TPMP. It indicated that TPMP with relative viscosity of 1.7 and extractables less than 3wt.% could be obtained after reacting 2.5?3.5 hours. The results also showed that reaction temperature would affect the properties and colour of TPMP. When reaction temperature was lower than 235 ?,the prepared TPMP looked grey and easy to break, its relative viscosity was lower than 1.5 and its extractables content was higher than 3wt.%. In the range of 235?245?, the obtained TPMP was white and presented good elasticity. However, when reaction temperature increased to 250?, the relative viscosity of obtained yellow TPMP decreased and in the same time the content of extractables increased too, moreover, the higher reaction temperature could lead to phase separation. The volatiles could be removed by elevating the vacuum in due course of esterification and push the reaction to continue. When the reaction pressure decreased to 500?100Pa (a), the extractables contents was <3wt.%. The optimized esterification process should be: let PTMG and PA6-MDI-PA6 react for 3.5 hours at 245? and 100Pa(a).To investigate the effect of the number-average molecular weight and content of PTMG on the properties of TPMP, series of TPMP contains PTMG with different number-average molecular weight and content were prepared in 5L stainless steel reactor by using optimized esterification process. The composition of TPMP was characterized by 'H-NMR and IR test. The results proved that PA6-MDI-PA6 hard segment and PTMG altered in TPMP and ester bonds could be determined. SEM results also proved that TPMP with lower content of soft segment was easy to break, scale-like debris could be observed on the fracture surface. By increasing the content of soft segment, the elasticity of TPMP was improved.The melting and crystallization behaviors of TPMP have been examined by scanning calorimetry, hot stage polarized light microscopy analysis. It indicated that the melting point and crystallization temperature of TPMP were high, but the crystallization region was very small. By increasing the high crystallinity hard segment content, the melting point and crystallization temperature of TPMP could be elevated.The thermal stability of TPMP was studied by TG and DTG tests. The results showed that TPMP presented good thermal stability. The weight loss of TPMP was <3wt.% in due course of heating from room temperature to the initial decomposing temperature. By increasing the content of hard segment or soft segment number-average molecular weight, the initial decomposing temperature could be elevated. TPMP still remained high breaking strength after being aged at high temperature for 5?15 days by using light and heat stabilizer Nylostab SEED.The mechanical performances of TPMP were measured by strain-stress, DMA and compression set tests. It showed that TPMP had dual characteristics of plastic and rubber, their glass transition temperature was lower than 0?. The increasing of soft segment content could make the glass transition temperature of TPMP lower and in the mean time their elasticity was improved. When the content of soft segment PTMG (1000) increased from lOwt.% to 30wt.%, the elongation rate of prepared TPMP increased 100%. However, the PTMG (2000) would be part of crystallization at room temperature and thus weakened the effect of soft segment content on the glass transition temperature. Moreover the elongation rate of TPMP prepared by PTMG (2000)decreased since they were induced to crystallize in drawing process. By increasing the content of hard segment and molecular weight of soft segment, the mechanical performances of TPMP was enhanced significantly, their maximum tensile strength was improved 55%, and in the same time their Shore hardness and storage modulus were enhanced. The performance of TPMP was enhanced very little after extractables had been removed by hot water, which proved that TPMP could be used directly without being extracted. At temperature of 20 ?, the permanent deformation rate of TPMP was only 6% after being compressed 25%. However the permanent deformation rate of TPMP increased obviously when compressed at 50 ?, the maximum deformation rate of TPMP was close to 55%.Compared to TPU,TPEE and other TPAE, TPMP has more excellent mechanical performances, less compressed deformation rate at low temperature and can be used in a wider temperature range.In the final, a four-component model made by a Kelvin model and a Maxwell model in parallel was used to simulate the tensile mechanical properties of TPMP. The unknown parameters of simulation equation were determined by using a comprehensive mathematical optimization and analysis tools package-1stOpt. Through reverse fitting, the theoretical stress-strain curves of materials could be obtained. By comparing the fitted and experimental stress-strain curves, the unknown parameters were further corrected and thus the optimized model had been obtained. As a result the relationship between the content of soft segment in TPMP and the tensile mechanical performance has been established, and the mechanical properties of TPMP with different soft segment content can be predicted successfully.