Acrylonitrile-butadiene-styrene plastic (ABS) has excellent comprehensive properties and is widely used in the fields of electronics, automobiles and construction, especially in the field of white goods. It is the most important material for the exterior parts of home appliances such as refrigerators, air conditioners and washing machines one. Since the rubber phase of ABS contains double bonds, it is prone to aging under the action of environmental factors such as light, heat, and oxygen, causing yellowing and deterioration of mechanical properties, which affect its appearance and use. Therefore, research on the aging behavior of ABS materials in different environments has attracted much attention. The test methods include artificial accelerated aging tests and outdoor natural aging tests. Compared with the indoor artificial accelerated aging test, the outdoor natural aging test can more truly reflect the influence of the specific climate environment on the material, but the outdoor natural aging test period is longer. Therefore, by discussing the correlation between the performance changes of ABS materials in the process of artificial accelerated aging and outdoor natural aging, using artificial accelerated aging tests to predict the weather resistance and service life of ABS materials in a natural environment in a short period of time has become a scientific research, It has become a common concern of production and use units, but there are not many related studies.
The author uses three kinds of artificial accelerated aging tests (UVA, UVB and xenon lamp) and outdoor natural aging tests to study the aging behavior of several ABS materials. The photooxidative reaction mechanism of ABS under different aging tests and the correlation between artificial accelerated aging test and outdoor natural aging test. On this basis, the time conversion equation of the color difference and mechanical properties of ABS materials in the process of artificial accelerated aging and outdoor natural aging is established, which provides a basis for choosing a fast and effective artificial accelerated aging test method.
1 Experimental part
1.1 Main raw materials
ABS: HI-121H, LG Ningbo Yongxing Co., Ltd.;
ABS color masterbatch: porcelain white color (inorganic content is 60%, of which rutile TiO 2 accounts for 57%, BaSO 4 accounts for 43%), Guangzhou Bosi Plastic Pigment Co., Ltd.;
Antioxidant 1010, Antioxidant 168, Ultraviolet light absorber UV–
327 and HALS UV-770: Ciba Specialty Chemicals.
1.2 Instruments and equipment
Co-rotating twin-screw extruder: CTE20 type, Coperion Keya (Nanjing) Machinery Co., Ltd.;
Vertical injection molding machine: KSU-250ST type, Jinsu (Hong Kong) Precision Machinery Co., Ltd.;
Xenon lamp aging Test Chamber (Q-Sun), ultraviolet aging Test Chamber (QUV): American Q-Panel Company;
Fourier Transform Infrared Spectroscopy (FTIR) Analyzer: TENSOR27, German BrukeR Company;
Colorimeter: CR400, Minolta, Japan;
Multifunctional Tensile Testing Machine: Model GT–AI 7000M, Taiwan, CHINA High Speed Rail Corporation.
1.3 Sample preparation
The samples include pure ABS, ABS with 4% color masterbatch, 1‰ antioxidant (antioxidant 1010 and antioxidant 168 each account for 50%) and 1% weather resistance agent (UV-327 and UV-770 each account for 50%) %) of weather-resistant ABS. Before preparing the samples, both the ABS raw material and the color masterbatch were dried at 60°C for 12h. The components are uniformly mixed according to a certain mass ratio, melted and extruded by an extruder, water-cooled and pelletized, and the extrusion temperature is 200°C. According to the GB/T 1040.2-2006 standard, the pellets are injection-molded on an injection molding machine into standard specimens for mechanical testing and swatches for color testing. The injection molding pressure is 6 MPa, and the injection molding temperature is 220°C. The pellet sample was hot-pressed into a 60 μm thick film, and infrared analysis was carried out after the light aging test.
1.4 Outdoor natural aging test
The outdoor natural aging test was carried out in Shunde, Foshan. Samples were submitted in April 2010 and the test was completed in March 2011. Shunde, Foshan has a subtropical climate, with an annual average temperature of 21.7°C, an average humidity of 75.3%, and an average daily irradiance of 3.82 kWh/m 2 . Film samples, swatches and test strips for mechanical properties were tested on self-made sample racks. The front of the sample racks faced south and the inclination angle was 22.5°.
1.5 Artificial accelerated aging test
(1) Xenon lamp aging test: according to GB/T 16422.2-1999, the light aging test is carried out, the irradiance is 0.50 W/m 2 (340 nm), the black standard temperature is 65 ℃, the relative humidity is 65%, and each exposure cycle is 120 minutes, of which the first 102 minutes are not sprayed with water, and the next 18 minutes are sprayed with water, and the test is uninterrupted and sampled.
(2) UVA and UVB aging test: Carry out light aging test according to GB/T 16422.3-1997, UVA type light source is selected for UVA test, and the irradiance is 0.55 W/m 2 (340 nm), UVB type light source is selected for UVB test, irradiance 0.55 W/m 2 (313 nm). The relative humidity is 65%, and each exposure cycle is 12 hours, including 8 hours of light at 60°C and 4 hours of non-radiation condensation at 50°C. Continuous cycle test and sampling test.
1.6 Performance test
Infrared test was carried out on the thin film samples with FTIR instrument, the scanning range was 4 000-400 cm –1 , the resolution was 4 cm –1 , and the scans were 16 times;
Use the color difference meter to test the color difference of the color plate during the aging process;
According to GB/T 1040.1-2006, the tensile properties of the unaged and aged samples were tested using a multifunctional Tensile Testing Machine, and the tensile rate was 50mm/min.
2 Results and Discussion
2.1 Analysis of ABS photooxidation reaction
The use of artificial accelerated aging test to predict the service life of materials needs to be established on the basis that artificial accelerated aging and natural aging follow the same photooxidation reaction mechanism, otherwise there will be large deviations. Studies have shown that after ABS is exposed to light, the oxidative degradation mainly occurs in the rubber (PB) phase, while the styrene-acrylonitrile phase affects the penetration of oxygen and inhibits the oxidative degradation of the PB phase. The photooxidation of ABS is essentially a process in which hydroperoxides undergo degradation reactions to generate carbonyl products. Using infrared spectroscopy to study carbonyl products can analyze whether the photooxidative reaction mechanism of artificial accelerated aging is consistent with outdoor natural aging. Figure 1 shows the FTIR spectra of pure ABS carbonyl products during different aging tests. It can be seen from Figure 1 that with the increase of aging time, the carbonyl product absorption peak of pure ABS in the range of 1 600 to 1 800 cm -1 gradually increases, and photooxidation occurs around 1685, 1718, 1735, 1755, and 1775 cm -1 , respectively. The resulting unsaturated double bonds, associated carboxylic acids, esters, free carboxylic acids and γ-lactones have characteristic absorption peaks. In different aging tests, the peak shapes and changes of the absorption bands of pure ABS carbonyl products are similar, indicating that pure ABS follows the same photooxidation reaction mechanism during artificial accelerated aging and natural aging. Therefore, the three accelerated aging methods of UVB, UVA and xenon lamp can be used to simulate outdoor natural aging, so as to predict the weather resistance and service life of materials.
Figure 2 is the FTIR spectra of carbonyl products of weathering ABS during different aging tests. It can be seen from Figure 2 that in different aging tests, the peak shape and changing law of the infrared spectrum of weather-resistant ABS are similar to those of pure ABS, indicating that the addition of antioxidants and light stabilizers did not change the photooxidation reaction mechanism of pure ABS.
2.2 Analysis of correlation of photooxidative degradation rate
ABS 光氧化降解为生成羰基产物的过程，通过检测老化过程中羰基产物的生成速率可表征不同老化试验下 ABS 的光氧化降解速率，从而建立起人工加速老化试验和户外自然老化的相关性。常用羰基指数 ( 指羰基吸收峰的相对面积与内标峰的相对面积的比值，其可定量表征聚合物光氧化降解后所产生的含羰基产物 ) 来表征聚合物的光氧化降解程度。对于 ABS，由于氰基在光氧化降解过程中基本不发生反应，故可选择氰基吸收峰作为内标峰。图 3 为耐候 ABS 在不同老化试验下羰基指数随老化时间变化的规律曲线。可见，除老化后期部分数据外 ( 此时聚合物已完全降解，羰基不再增加，导致数据偏离原来规律 )，在大部分老化时间内，羰基指数随着老化时间的增加呈线性增长，因此，羰基指数与老化时间的函数关系可用式 (1) 表示。
Y =A +kt (1)
式中：Y 为羰基指数；A 为拟合常数，与样品初始状态有关；k 为一常数，由不同老化条件下的反应速率的常数决定，可将其定义为“反应速率常数”；t 为老化时间，单位为 h。
对羰基指数与老化时间进行线性拟合，结果见图 4，可得出不同老化试验下的 A 和 k 值，结果见表1。为了对比不同老化试验的光氧化降解速率，引入氧化反应加速倍率 R (R 为人工加速老化的反应速率常数 k 与户外自然老化的反应速率常数 k 之间的比值 )。通过 R，可建立人工加速老化与户外自然老化之间的相关性。
从表 1 可见，对于耐候 ABS，UVB，UVA 和氙灯的氧化反应加速倍率 R 分别为 25.99，3.84 和10.76，即 UVB 光源对耐候 ABS 的破坏作用最大，其次为氙灯，而 UVA 最低。对于纯 ABS，UVB，UVA 和氙灯的加速倍率分别为 4.30，4.23 和 6.92，即氙灯光源对纯 ABS 起到更明显的光老化降解作用；对于添加色母的 ABS，UVB，UVA 和氙灯的加速倍率分别为 11.49，3.57 和 12.07，即 UVB 和氙灯对添加色母的 ABS 的老化作用相差不大，但大于UVA。以上结果表明，不同 ABS 材料，利用同种光源，其加速倍率是不一样的。同样，同一种 ABS 材料，不同光源，其加倍速率也是不一样的。这可能与ABS 材料中基体、填料和耐候剂对不同波长光的作用不一样所致。
2.3色差变化相关性的分析
ABS 材料受光氧等作用破坏时，优先体现在外观颜色上就是发生黄变。图 5 为添加色母的ABS 材料在不同老化试验下色差随老化时间变化的规律曲线。由图 5 可见，除老化后期的数据，在大部分老化时间内，色差随着老化时间的增加呈线性增长，因此，色差随老化时间变化可以用式(2)表示。
Y′=A′+k′t (2)
其中：Y′ 为色差；A′ 为拟合常数，与样品初始状态有关；k′ 为一常数，由不同老化试验下的色差变化速率的常数决定；t 为老化时间，单位为 h。
对色差与老化时间进行线性拟合，可得出不同方法老化时的 A′ 和 k′ 值，结果见表 2。通过式 (2)和表 2 中的数据，可以得到不同老化试验下色差与时间的关系方程。比如户外自然老化时，耐候 ABS的色差 Y′ 户外 与老化时间 t 户外 的关系可用式 (3) 表示：
Y′ 户外 ＝ –3.93 +1.16 ×10 –3 t 户外 (3)
利用 UVB 加速老化时，耐候 ABS 的色差 Y′ UVB与老化时间 t UVB 的关系式如下：
Y′ UVB ＝ –1.08 +5.49 ×10–2 t UVB (4)
当色差相同时，即 Y′户外 ＝ Y′UVB，将式 (4) 代入
式 (3) 得户外自然老化与 UVB 加速老化的时间换
算方程：
t 户外 ＝ 47.34t UVB +2 450.61 (5)
同样，可得到户外自然老化与 UVA 或氙灯加速老化的时间换算方程，见表 3。笔者使用加速转换因子法 (ASF) [16] 研究人工加速老化和户外自然老化过程中色差变化的相关性。ASF 表示材料的某项性能参数，经某个室内模拟加速试验的性能对应于某地区自然环境试验性能随时间变化的加速倍率。如利用式 (3) 和式 (4) 计算，当耐候 ABS 材料的色差达到 3.0 时，UVB 加速老化时间需要 74.33 h，户外自然老化时间则需要 5 969.94 h，那么加速倍率为 80.32。UVA 加速老化时间需要 517.47 h，加速倍率为 11.54 倍；氙灯加速老化时间需要 654 h，加速倍率为 9.13 倍。对于添加色母的 ABS，色差达到 3.0 时，UVB 人工加速老化时间需要 15.91 h，户外自然老化时间则需要 1 367.18 h，加速倍率为85.93 ；UVA 加速老化时间需要 70.03 h，加速倍率为 19.52 ；氙灯加速老化时间需要 166.11 h，加速倍率为 8.23。
由上可见，不同人工加速老化试验对色差变化的加速倍率不同，其中 UVB 的加速倍率最大，其次是 UVA，而氙灯的加速倍率最低。利用相同人工加速老化试验对不同 ABS 材料进行加速试验，所得色差变化的加速倍率有所差别。因此，表 3 中的方程只是给出特定材料在不同人工加速老化时色差变化与户外自然老化时色差变化的关系式，不具普适性。
2.4力学性能变化相关性的分析
In the artificial accelerated aging test, the elongation at break is usually selected as the mechanical performance index of the material after aging. Figure 6 is the regular curve of the retention rate of weather-resistant ABS elongation at break as a function of aging time. It can be seen from Figure 6 that the elongation at break of weather-resistant ABS decreases linearly with the increase of aging time. Therefore, the equation (6) can be used to linearly calculate the elongation at break retention (Y'') and aging time (t). Fitting parameters are listed in Table 4.
Y''=A''+k''t (6)
Through the parameters in formula (6) and table 4, the relational equation of elongation at break retention and time can be obtained, and then the time conversion between UVB artificial accelerated aging and outdoor natural aging can be obtained through formula (3) and formula (4) equation:
toutdoor = 6.78tUVB +463.08 (7)
Using formula (7) to calculate, when the elongation at break reaches 50%, the UVB artificial accelerated aging time needs 635.12 h, and the outdoor natural aging time needs 4 769.90 h, and the acceleration rate is 7.51.
3 Conclusion
(1) Infrared analysis shows that the photooxidation mechanism of ABS in three accelerated aging processes of UVB, UVA and xenon lamp is consistent with outdoor natural aging. The analysis of the photooxidation reaction rate shows that: different ABS materials are aged with the same artificial light source, and their acceleration rates are quite different; and the same ABS material is aged with different artificial light sources, the doubling rate is also different, which may be related to the different ABS materials. How materials react to light.
(2) The color difference analysis shows that for the same ABS material, the order of acceleration magnification of the artificial accelerated aging test on the color difference change from large to small is: UVB > UVA > xenon lamp. However, when using the same artificial accelerated aging test to carry out accelerated tests on different ABS materials, the acceleration magnifications of the obtained color difference changes are quite different. Through the study of the correlation between the color difference and elongation at break of ABS material in the process of artificial accelerated aging and outdoor natural aging, the time conversion equation of artificial accelerated aging and outdoor natural aging was fitted.
