Structural Changes Of CR Under Thermal Oxidative Aging And Its Effects On Tensile Crystallization And Mechanical Properties

Chloroprene rubber(CR)has excellent mechanical properties,good dynamic properties and certain weather resistance,abrasion resistance,and flame resistance.It is suitable for all areas of people's lives,such as auto parts,gaskets,rubber rollers,flame-retardant rubber products,oil-resistant and friction-resistant hoses,etc.As a common type of aging,thermal oxidative aging seriously affects the physical and mechanical properties and service life of CR materials.Therefore,the research on the aging resistance of CR is particularly important.In this paper,firstly,the synthetic CR material was aged at 100? with a heat aging tester,and the chemical structure,the tensile crystallization effect and mechanical properties of the CR during the high temperature aging process were studied.FTIR,GPC,EDS and the other methods were used to study the changes of chemical structure,sol molecular weight and chlorine content.XRD,tensile test,SEM and other instruments and methods were used to study and analyze the tensile crystallization and tensile cross-section morphology of unaged rubber and aged 192h rubber.The mechanical properties,oil resistance and abrasion resistance of the aged rubber without and with antioxidant 1010 were analyzed and systematically studied.The results were as follows(1)FTIR showed that during the high temperature aging process of rubber,the(CH=C)-Cl molecular structure combined with oxygen to form oxygen-containing groups and an oxidation reaction occurred.The cross-linking density test showed that the cross-linking density in the early stage of aging was mainly increased by double bond cross-linking,and the molecular chain was broken in the later stage,and the cross-linking value decreased.GPC characterization showed that the content of sol decreased and the molecular weight distribution narrowed at the early stage of aging,in the late,the content of sol increased and the molecular weight distribution became wider.XRD results showed that during the entire aging process,the "dechlorination" phenomenon causes the rubber's chlorine content decreased continuously.The tensile cycle test showed that: compared with the unaged rubber,the aged 192h rubber consumes more energy,and the permanent deformation after the tensile cycle test was smaller.(2)The tensile test results showed that the unaged rubber had obvious strain-induced crystallization during the stretching process,and the tensile crystallization point of the aged 192h rubber became obscure.The crystallinity test results of the unaged rubber and the aged 192h rubber showed that the rubber crystallinity increased slowly at lower elongation.As the rubber continues to stretch,the rate of crystallization increased.When the elongation reached 250%,the increased in crystallinity decreased.And the crystallinity value of aged rubber was lower than that of unaged rubber under the same elongation.The test results of the molecular chain orientation of the stretched rubber crystallite region showed that the molecular chain orientation changes little in the initial stage of stretching.As the rubber stretched,the degree of orientation value increased faster.When the elongation reached 200%,the increase rate of rubber molecular chain orientation value will slow down with the increase of elongation.And at lower elongation,the molecular chains of the aging rubber crystal region were more likely to be oriented.When the elongation is large,the molecular chain orientation degree of the aged rubber crystal region is lower than that of the unaged rubber.The SEM results showed that the tensile fracture surface of the unaged rubber was uneven,and the surface of the aged 192h rubber was relatively flat.(3)The vulcanization property test and crosslinking density test showed that the maximum torque value and crosslinking density of the rubber decreased after adding antioxidant 1010.In the early stage of aging,the double bond cross-linking reaction occurs,the cross-linking density increases and the hardness increases.At the later stage of aging,the molecular chain of rubber was oxidized seriously,the crosslinking density decreased and the hardness of rubber decreased.In the whole aging process,the structural reactions such as double bond crosslinking,dechlorination and molecular chain oxidation made the network chain orientation difficult,which was not conducive to stress dispersion.In addition,many network defects made the molecular chain more easily damaged,so the tensile strength and tensile strength continue to decline during the whole aging process.Regardless of whether antioxidants were added,the overall change trend of the mechanical properties during the aging process was the same,but compared to rubber materials without antioxidants,the mechanical properties of the rubber during the aging process after the addition of antioxidants have relatively low changes,so that in the later stages of aging The mechanical properties of the latter were higher than the former.(4)The results of high temperature oil resistance test showed that the aging rubber with antioxidant had better oil resistance than the aging rubber without antioxidant.The wear test results showed that the wear resistance increased with the increase of aging time.In the later,the wear resistance became worse.SEM showed that there was no obvious tear,tensile trace and wear deposit on the worn surface of unaged rubber.After aging for 192h,there were tongue like protrusions on the worn surface of rubber,and the tearing and stretching phenomena are obvious.