| With the maturity of China’s auto market,customers are becoming more and more strict with the aesthetic requirements of automobiles,and the coating of exterior plastic parts has also put forward higher requirements.Through the tracking investigation of the color change of the market vehicles,this paper finds that the color of the coating of the automotive exterior plastic parts is gradually yellowing after aging in the outdoor environment,which is called “yellowing” phenomenon;this phenomenon will cause plastic exterior parts and bodywork.Inconsistent color of the ontology produces a significant sense of dissonance,which seriously affects customer satisfaction.Therefore,analyzing the causes of the "yellow change" and studying the methods of its prevention and control are urgent problems for major automobile manufacturers.In the car exterior plastic parts,because the bumper has the largest appearance area,its "yellow" problem is most visually obvious.Therefore,this thesis uses the acrylate coating for varnish of bumper as the research object,and studies the causes of yellowing phenomenon and the prevention and control measures through artificial ultraviolet accelerated aging.It is believed that this study can provide a theoretical basis for the development of automobile bumper coatings and the prevention and control of yellowing phenomena and the improvement of the appearance quality of automobiles.First,the effects of different process conditions on the yellowing phenomenon were studied.The mechanism of yellowing phenomenon was discussed and analyzed.Through the artificial ultraviolet accelerated aging experiment,it was found that the coating showed obvious yellowing after being irradiated with UV340 ultraviolet light source for 0.8h at a power of 0.83W/m2.Taking the b value in the CIE 1976 L*a*b* color space as a color representation,the b value after yellowing will increase by 1.5-2.5.Moreover,the thicker the varnish film thickness,the more severe the yellowing phenomenon,the △b of the coating having a varnish thickness of 10.2 μm is 1.39,and the △b of the thickness of 28.7 μm is 1.93.It shows that the yellowing phenomenon mainly occurs in the varnish layer.In addition,when the curing agent is excessive,the yellowing phenomenon is more serious.When the ratio of the varnish to the curing agent is 3:1(usually proportional),△b is 1.93,and when the ratio is 2:1(excessive curing agent),△b is 2.25.Total reflection Fourier transform infrared spectroscopy(ATR)study found that the CN bond in the urethane group is easily destroyed by ultraviolet rays,and the generated free radical reacts with oxygen in the air to form an oxygen-containing group.Caused a yellowing phenomenon.Then,in order to improve the yellowing phenomenon,antioxidants(Irganox1010,Irganox1098),UV absorbers(Tinuvin531,Tinuvin327,Tinuvin-P),and hindered amine stabilizers(HALS)were studied from the viewpoint of capturing free radicals or absorbing ultraviolet rays.770)The effect of anti-aging additives on the yellowing of varnish coatings on automotive plastic exterior parts.The results show that after aging for 300 h,the △b of the coating samples with the above anti-aging additives are 1.09,1.52,1.25,1.41,1.39,0.97,respectively;under the same conditions,the △b of the blank is 2.29;There is a clear improvement.The most effective of these was HALS 770(△b was 0.97),followed by Irganox 1010(△b was 1.09)and Tinuvin 531(△b was 1.25).By ATR infrared spectroscopy,the mechanism by which HALS 770 improves yellowing is to reduce the cleavage of C-N bonds.Finally,combined with the above research results,further studies on the effect of the improved UV absorber Tinuvin 531 and the hindered amine stabilizer HALS 770 on the yellowing.Through orthogonal experiments,it is well studied whether there is synergy between the two.The results showed that the improvement effect of Tinuvin 531 and HALS 770 at the significance level of 0.05 was obvious;the former p value was 0.049,the b value change was 0.14 compared with the blank sample;the latter p value was 0.0003,b value change The contrast blank dropped by 0.49.There is a synergy between the two,which is significant at a significance level of 0.10,with a p-value of 0.079. |
