## 1 Introduction  
Materials weathering behavior and prevention have become one important field in automobile performance research. To systematically study the weathering performance and thermal stability of engineering materials, many weathering test standards have been established according to indoor and outdoor conditions, mainly including atmospheric exposure standard [1], exposure standard under glass [2], black-box exposure standard [3] and artificial accelerated weathering standards under fluorescent ultraviolet lamp [4] and xenon lamp [5]. Hereby, scientific researchers have designed many test methods to characterize and discuss the material weathering behavior in detail from multiple perspectives of appearance, mechanical and electrical properties. The yellowing index and color difference of various engineering plastics have been investigated [6; 7; 8]. The experimental results indicate that the weathering of PC and poly(butyleneerephthalate) increased linearly with the enhancement of irradiance, while the degradations of both SAN and ABS presented tendencies to grow non-linearly. The materials also tend to become hard and brittle in the weathering process, which deteriorates the mechanical properties [9; 10; 11]. The tensile strength, bending strength and breaking elongation of polyamide 6 (PA6) exposed to nature were characterized in detail [12]. The long-term results prove that the retention rate of mechanical properties decreased significantly with seasonal fluctuations. In terms of electrical performance, weathering induces the decrease in insulation efficiency of epoxy resin, which increases electrical energy loss in automotive instrumentation [13]. The dielectric properties of polypropylene film and polyimide nanocomposite film before and after weathering were evaluated by surface discharge method [14; 15]. The results reveal that the residual breakdown strength decreased significantly after weathering, while the partial discharge resistance and breakdown strength of the composite film could be improved by adding nano-fillers. Weathering is a very complex physicochemical reaction process, which is essentially the breaking and branching of macromolecular chains, as well as the formation of carbonyl groups, hydrogen peroxide groups and low molecular weight substances [16; 17; 18]. For the sake of further investigating and verifying the internal mechanism of material weathering behavior, researchers have established and developed a variety of detection techniques and experimental methods in recent years. The degradation products and their contents among six types of automotive coatings were quantitatively analyzed by infrared spectroscopy. The alterations of the position and intensity of the characteristic absorption peaks in the spectra suggested that the carbonyl group and hydrogen peroxide products increased gradually. The transformations of molecular structure and functional groups changed the absorption and reflection of the material to the spectrum, which led to the discoloration and cracking of the coating [19]. The variation on microscopic morphology and molecular weight of HDPE samples were studied and compared by differential scanning calorimetry (DSC) and crystallization fractionation [20]. The results indicate that molecular chain breakage and molecular weight reduction appeared in samples with exposure time. The above research results reveal the internal factors and mechanisms of material weathering from the perspective of microscopic variations and verify the impacts of alterations in molecular structure, morphology and composition on the weathering performances of materials. The characterization of weathering behavior and the application of advanced non-destructive experimental methods have rapidly promoted the development of weathering-resistant material screening technology based on weathering internal factors.