1 Sealing strip is one of the important parts of passenger cars. It integrates coordination, function and appearance, and takes into account both dynamic and static. In addition to sealing functions such as dustproof and waterproof, it can also effectively prevent the glass from shaking when the body vibrates slightly. At the same time, the body parts are buffered, effectively improving the service life of the body, and providing passengers with a clean and quiet riding environment. Sealing strips of passenger cars are prone to aging due to exposure to the natural environment, sunlight (ultraviolet rays), high heat (high temperature above 80 ℃ in summer), oxygen, ozone, damp heat and other factors. If the sealing strip is aged and damaged, it will not only directly cause water leakage in the compartment, but also increase wind noise, which will seriously cause corrosion of the body. Therefore, in the actual application environment and performance requirements, the weather resistance and light aging resistance of the sealing strip are higher. At present, in the process of developing new models, sealing strip enterprises and major main engine factories have gradually increased their investment in the test of sealing strips. In addition to the necessary indoor artificial accelerated xenon lamp aging test, they have also invested heavily in the atmospheric exposure test of the whole vehicle.

The main failure modes of automobile sealing strip light aging are surface cracking, frost spraying, pulverization and color change, etc. These failures may lead to customer complaints and even recalls. Therefore, it is necessary to explore the factors that affect the light aging performance of the sealing strip.

2

Production Process of Sealing Strip for Passenger Car

The main production process of the sealing strip is: batching → mixing → extrusion → vulcanization → post-processing. Among them, the formula of ingredients is the basis for the survival of sealing strip suppliers. Even if it is the same sealing strip, the ingredients of each supplier are different, and the composition and dosage are different. The boiling point of paraffin oil in the ingredients and the content of calcium carbonate largely determine the light aging resistance of the finished sealing strip. Vulcanization is another important process in the production of sealing strips. The vulcanization process is: preheating → microwave vulcanization → baking → cooling. In the vulcanization process, the need to control the temperature, speed and pressure and other three factors, because the vulcanization temperature can accelerate the vulcanization speed, improve production efficiency, but also cause the rubber molecular chain cracking and vulcanization reduction, resulting in the decline in the mechanical properties of the rubber; in addition, the thick products inside and outside the temperature difference increases will also lead to uneven vulcanization. These will affect the light aging resistance of the sealing strip.

The production process of the sealing strip is very complex, there are many uncontrollable factors, the research of this paper is carried out under the premise of the same glue, extrusion and post-processing process, by exploring the effect of ingredients and vulcanization on the light aging resistance of the finished product, looking for some factors that affect the light aging resistance of the sealing strip.

3

Test

3.1 test equipment and instruments

The equipment and instruments used in the light aging resistance test of sealing strip products include xenon lamp aging test box ATLASCi4000, spectral photometer UltrascanPRO, standard light source box SpectralightIII and AATCC standard gray scale card.

3.2 test sample

Taking the sealing strip on the top of the glass run channel of a passenger car (with a large exposed area) as an example, the following 5 samples of formula or vulcanization process are selected for research. Sample A is a fixed formula sample of a supplier; sample B is a sample with paraffin oil flash point increased based on the formula of sample A; sample C is a sample with peroxide dosage increased based on the formula of sample B; sample D is a sample with vulcanization maturity reduced based on the formula of sample B; sample E is a sample with vulcanization maturity increased based on the formula of sample B.

3.3 Test Standard

Because the xenon lamp energy spectrum defined in the xenon lamp aging standard SAEJ2527 is very close to the solar energy spectrum (Figure 1)[1], and the parts that meet the standard have good after-sales feedback, this standard is used as the test standard. The collected test data include color difference △E (instrument collected data) and color fastness to light (visual evaluation data), which can more comprehensively reflect the state of the sample during the test. In addition, according to the GMW14169-2010 enterprise standard requirements, the acquisition period is 221, 442, 663, 1240, 2000kJ/m2 and 2500kJ/m2. The standard evaluation requirements for the test sample are as follows: △E≤ 3, color fastness to light ≥ 4; The maximum irradiation energy is 2500kJ/m2.

3.4. Test results

The light aging resistance test was carried out for the selected 5 samples. The test results are shown in Tables 1 and 2.

3.5. Analysis of test results

When the irradiation energy is 221kJ/m2, the △E value of sample A is as high as 7.49. With the increase of xenon lamp irradiation time, △E gradually decreased, and the color fastness to light gradually decreased. When the irradiation energy is increased to 2500kJ/m2, the appearance of the finished sealing strip becomes dull from initially very bright (Figure 2). This phenomenon may be due to the formulation of paraffin oil in the flash point is too low and the precipitation in the test process, at the same time, because the test is carried out in high temperature and rain environment, the precipitation of paraffin oil part is evaporated, so the ΔE gradually decreased.

In order to verify whether there is any problem with paraffin oil in sample A, paraffin oil flash point test and fog test are added. In the paraffin oil flash point test, it is found that the paraffin oil flash point in the sample A formula is only 185 ℃, which is relatively low. The measurement data of fog test exceeds 10mg, which is far greater than 2mg required by the standard. The test results verify that the paraffin oil has a low flash point and volatilization, resulting in the test results can not meet the requirements.

According to the above test results, paraffin oil with a flash point greater than 300°C was used in the formulation of sample B. After the test, the test result of sample B (Fig. 3) meets the standard requirements. It can be seen that the use of paraffin oil with high flash point can significantly improve the light aging performance of the sealing strip.

Sample C is based on the formula of sample B with 20% peroxide vulcanizing agent added. However, it can be seen from Table 1 and Table 2 that the △E value of sample C continues to increase, the light fastness gradually decreases, and there are many white precipitates that are difficult to remove on the surface, as shown in Figure 4. According to the chemical composition analysis, the white precipitate is peroxide, which is caused by excessive peroxide vulcanizing agent in sample C. Because the appropriate amount of vulcanizing agent can form a cross-linked structure between the rubber molecular chains (Figure 5), increasing the strength of the product, but the excess vulcanizing agent free from the molecular chain will gradually precipitate from the surface of the sample and gradually accumulate when it is exposed to light, heat, oxygen and other energy. Therefore, the excess vulcanizing agent is also a factor affecting the light aging resistance of the sealing strip.

For sample D and sample E, the vulcanization process is adjusted based on the formula of sample B. The vulcanization temperature of sample D is 20 ℃ lower than that of sample B, and the line speed is increased from 13 m/min to 15 m/min. The vulcanization temperature of sample E is 20 ℃ higher than that of sample B, and the line speed is reduced from 13 m/min to 10 m/min. Compared with the compound Mooney, vulcanization and scorch tests, sample D is in the state of under-sulfur and sample E is in the state of over-sulfur. The xenon lamp aging test showed that when the irradiation energy was 442kJ/m2, white precipitates began to appear on the surface of sample D, and peroxide was precipitated by chemical analysis. This is because sulfidation is not sufficient, the catalyst in the formulation is not completely integrated into the molecular chain, and these catalysts slowly precipitate under photosynthesis (Figure 6). The test results of sample D can prove that sulfur deficiency is a factor leading to the degradation of the light aging resistance of the product.

The test data of sample E is relatively poor. Shortly after the test, the surface of the product began to turn white and powder. When the irradiation energy reached 2500kJ/m2, the surface powder was serious (Figure 7), and the performance and strength of the parts also decreased seriously. This is due to the excessive sulfur of the product. Because the vulcanization temperature is too high and the time is too long, some molecular chains have been interrupted during the vulcanization process (Figure 8). The interrupted molecular chains split faster under the action of ultraviolet rays, etc., resulting in powdering of the surface of the product. It can be seen that over-sulfur is a factor in the decline of the light aging resistance of the product, and over-vulcanization of the product is the most necessary to avoid.

4

Concluding remarks

According to the production process of passenger car sealing strip, some factors affecting the light aging resistance of the sealing strip were studied. The test shows that the paraffin oil flash point and the amount of vulcanizing agent in the ingredients are the factors that affect the light aging resistance of the sealing strip, and the formula must be designed reasonably. Over-sulfur or under-sulfur is also an important factor that leads to the decline of the light aging resistance of the sealing strip, so the occurrence of over-sulfur or under-sulfur should be avoided in the production process.