Effect Of Hot Stretching On The Structure And Properties Of Nylon 6 Films

With the increasing demand for high-performance films in packaging,electronics,agriculture,and other fields,It is important to effectively further improve the mechanical,barrier,optical and other properties of the film.Because the stretching method has the advantages of high production efficiency,excellent product performance,and small thickness difference,it has become the mainstream processing method in the polymer film industry.Nylon 6(PA6)is a semi-crystalline polymer with typical viscoelasticity.Because of its good processability and excellent comprehensive properties,it is usually used as the main raw material to produce biaxially stretched nylon film.In the process of biaxial stretching,nylon 6 is extruded,cast,stretched,heat set,cooled and other steps.In the process of stretching,processing parameters need to be adjusted,including temperature,stretching rate,stretching ratio and so on.In essence,this process is the evolution process of the multi-scale structure under the action of multiple external fields such as stretching and temperature.The properties of the products are strongly dependent on the conditions of stress,temperature,stretching ratio,and stretching rate.In recent years,although some domestic enterprises have realized the large-scale production of biaxially stretched nylon 6 film in packaging and other fields by introducing equipment,raw materials,technology,and other conditions,there is still a lack of in-depth study on the microstructure and deformation mechanism of nylon 6 film in the drawing process.In this paper,the microstructure evolution and deformation mechanism of nylon 6 film in the process of stretching were studied by means of the combination of structural characterization and mechanical property test,and the dynamic relationship among the process-structure-property of biaxially stretched nylon 6 was established.A new rheological model was introduced to characterize the tensile deformation of nylon 6films.The main work of this paper is summarized as follows:(1)The effect of the hot stretching process on the microstructure and mechanical properties of nylon 6(PA6)films was studied.The PA6 films were treated by uniaxial hot stretching in the temperature range of glass transition temperature(T_g)and melting point(T_m),and microstructure and mechanical properties of the films were characterized.The results show that with the increasing of the tensile temperature and the tensile ratio,the degree of transformation from?crystal form to?crystal form in PA6 is increasing,which promotes the crystallization and orientation of the molecular chain in PA6 along the stretched direction,thus significantly improved the crystallization degree of PA6.Moreover,it is found that the amorphous region decreases and the glass transition temperature(T_g)increase.The increase of tensile temperature is beneficial to the growth of?crystal plane in PA6,and the more complete?crystal is formed at high temperatures.With the increase of the tensile ratio,the integrity degree of?crystal increases at first and then decreases.After hot stretching,the tensile strength and storage modulus of PA6films increased,yet the elongation at break decreased.Compared with unstretched PA6 films the tensile strength of PA6 films at the tensile temperature of 160?and the tensile ratio of 3 was increased by 371%,and the elongation at break decreased by 235%.(2)The crystal structure,mechanical properties and barrier properties of PA6 films under different drawing conditions were studied.The results showed that with the increase of drawing ratio,the transformation degree of?to?-crystal in PA6 films increased,without apparent crystal orientation,the film had good equilibrium,the oxygen barrier property,increase in tensile strength and decrease in elongation at break.With the increase of tensile temperature,the content of?-crystal(002)in biaxially stretched PA6 film increased,the more complete the?-crystal formed by tensile induction,the higher the film's equilibrium and oxygen barrier property,the higher the tensile strength first and then the lower the elongation at break.With the increase of tensile rate,the integrity of?crystal in biaxially stretched PA6 film decreased,the equilibrium and oxygen barrier properties of the film decreased,the tensile strength increased and the elongation at break decreased.(3)A fractional Burgers model was proposed to investigate the tensile rheological mechanism of PA6 film.The creep tests at various conditions were conducted on dynamic mechanical analysis(DMA)tester,and the strain-time(?-t)curves of deformation were analyzed by the derived model.The results indicated that the fractional Burgers model had a better performance than the counterpart integer-order model in simulating the deformation of PA6 film.Parameters analysis showed that the stress facilitated the transformation from viscoelastic deformation to the viscoplastic one under ambient condition.While at the state of higher temperature,the deformation could be directly activated into viscoplastic deformation.The softening effect of PA6 film was found to be related to the expansion of viscoplastic units during stretching.Also,the fluidity of the viscoelastic units was greater than that of the viscoplastic units.Through the analysis of dynamic rheological parameters,we speculated that the variation of loss factor may come from the evolution of viscoelastic units.