Almost all of the thermoset elastomer compounds contain granular fillers to increase the strength and modulus of the vulcanized rubber product.Carbon black and silica are the most common reinforcing fillers to achieve a wide range of particle size, particle aggregation and surface activity . However, the choice of elastomer packing is only a starting point; the vulcanized rubber performance also depends on the dispersion of the filler. In fact, the progress of the rubber technology over the past century has a considerable portion involving the filler, and the dispersion filler also includes Its related equipment and process conditions.
Despite the continuous improvement of filler and mixing techniques, traditional fillers in some elastomers and applications can cause undesirable side effects. A well-known example is the strain induced softening that is called the Payne effect, which leads to an increase in the rolling resistance of the tire The current focus is on the little - known and equally unfavorable filler - mediated consequences of the presence of elastomers in the application of automotive under the hood of hot air.
The elastomer selected for this study is Vamac (AEM), an amorphous random copolymer (ethylene, methyl acrylate and an amine reaction vulcanization crosslink). AEM has a wide variety of attractive Of the properties, including heat and oil resistance, low temperature flexibility, compression resistance and good extrusion performance.Therefore, in the automotive hood application AEM has been steady growth.
These AEM pre-compounds are commercially available and are manufactured by DuPont High Performance Materials Inc. through proprietary processes. Once the thermoplastic droplets are dispersed, the typical vulcanization of AEM and The use of this new method, the subsequent mixing and processing will not affect the dispersion of the filler, thus eliminating the important source of product variation.More importantly, in the diffusion limit conditions of the oxidation process , For example, where the temperature in the air is greater than 150 ° C, the thermoplastic filler significantly increases the life of the AEM component.
material
Table 1 lists the materials used in this study.
Table 1, the materials used in this study
Summary of polymer filled AEM preforms
The AEM pre-compound (e.g., AEM 5015) contains about 45% by weight (or about 82% by weight) of the thermoplastic filler when externally supplied. Figure 1 shows that the thermoplastic filler disperse spherical droplets with droplet sizes ranging from Microns to about 2 microns.) The droplets are completely independent and free of polymerization. Traditionally, such large and unstructured fillers will only be used for space filling and are not capable of providing reinforcement. However, the mixture of AEM and fillers , Can lead to excellent phase adhesion and strength in vulcanization.
AEM pre-compounds may be diluted by standard AEM grades to reduce hardness and packing levels. The non-diluted pre-vulcanized product has a hardness of about 75. Table 2 compares conventional N550 carbon black at a target hardness of about 65 Enhance the properties of AEM and AEM 5015. Please note that AEM 5015 (118%) is diluted with AEM HT (36%), the total amount of AEM is increased to 100%, and the polymer packing level reaches 53%.
Several of the details that are worth noting with the compounds in Table 2 are:
Table 2, N550 and polymer enhanced AEM physical performance comparison
• Compound 3 (containing AEM 5015) has a small amount of carbon black used as a colorant. This low content of carbon black does not affect its physical or aging properties.
Compared with conventional AO-1, AEM compound AO-2 containing polymer filler is more effective.
• Compounds containing polymer fillers often require less vulcanizing agents, which are slower to cure than conventional compounds. These two phenomena are due to graft polymerization of AEM and fillers, some of which are replaced by AEM-AEM cross- And to prevent direct mixing of the vulcanizing agent with the AEM molecules of the impregnated filler particles.
The results in Table 2 show that the polymer-enhanced AEM 5015 compound maintains the good strength and elongation at break properties of conventional AEM compounds at 23 ° C and 175 ° C. Short-term anti-compression properties are also similar to conventional compounds, ℃ / 1008 hours of oil aging, its strength and elongation retention is also improved.However, AEM 5015 and the biggest difference between conventional compounds, is reflected in the hot air after 175 ℃ aging is still a great improvement. After aging for 1008 hours, the conventional compound became hard and brittle, while the AEM 5015 compound remained soft and remained at 75% of the initial elongation.
Standard for thermal aging in the air
A key element in the selection of an AEM compound is the resistance to its hot air aging, where typical carbon black and silica enhanced AEM compounds are located between the hydrogenated nitrile rubber and the fluororubber compound.
Table 3, carbon black enhanced AEM in 200 ℃ nitrogen or air after 1 week of thermal aging performance
Although there are different ways to assess the hot air aging properties of elastomeric compounds, this study uses the following three criteria:
1, less than 15 points of hardness changes
2, less than 50% of the tensile strength changes
3, less than 50% elongation change
These standards include specifications for many automotive OEMs, and any violation of any of the criteria will be considered unacceptable at a given time temperature.
In the air with the nitrogen control of aging
The presence of oxygen greatly accelerates the degradation of AEM polymers and compounds, and the degradation of the compound under nitrogen also results in thermal degradation, but the degradation temperature is higher than that of oxidative degradation.
Table 4, compounds for hot air aging
Table 3 compares the physical properties of the two standard AEM compounds, and under these conditions, the thermal aging of the compound under nitrogen is better when heated at 800 ° C in nitrogen or air for 168 hours, but is shown in the air according to the above criteria failure.
AEM Compounds Antioxidants
The addition of AEM compound antioxidants significantly improved the aging properties of the heat-resistant air. Table 3 Two hundred parts of AO-1 was used in the formulation to help reduce the oxygen-induced degradation at high temperatures. Many of the candidate antioxidants used in AEM compounds The product is being evaluated; however, none of the conventional fillers can exceed the performance of AO-1.
The use of polymer fillers instead of carbon black or mineral fillers
It is interesting to note that the filler-free AEM compound has better resistance to hot air aging than the filler-filled compound (using carbon black or mineral filler), but the filler-free compound exhibits poor physical Performance, such as difficult to process and there is no commercial application.
Table 5, Life expectancy prediction in different thermal aging curves
The AEM / carbon black compound has a Shore A hardness of 55 to 80. The hardness of the AEM / carbon black compound is from 55 to 80. The hardness of the AEM / carbon black compound is from 55 to 80. The hardness of AEM / The compound, the N550 carbon black level can be as low as about 30% (medium surface area of carbon black) or N990 is about 60% (low surface area carbon black). Even at these lower levels of carbon black, the filler heats its hot air Have an adverse effect.
Table 6, Compressive stress relaxation test of seal compounds
Recent studies have shown that the conventional packing can affect the aging of the hot air of the AEM compound by changing the oxidation configuration, and the AEM compound is not uniformly oxidized at a temperature of 150 ° C or higher, but is highly oxidized on the surface layer and oxidized at the central portion This phenomenon is limited by the rate of oxidation diffusion; most oxygen has been consumed in the outer layer of the sample before oxygen can diffuse into the interior of the sample, since carbon black and mineral fillers are oxygen free, Oxygen diffusion to the center is very slow.Therefore, relative to the non-filler parts, with low oxygen diffusion rate of filler components will lead to parts of the surface oxygen concentration is relatively high, causing the surface oxidation faster.Thus, in the filler and polymerization The surface of the material produces a crack, and oxygen is further attacked by the internal AEM. The oxidation of the 'wave front' propagates relatively quickly to the entire filler sample, resulting in catastrophic loss of sample performance.
Table 7, Low hardness compounds made with AEM and polymer fillers
These findings have led to the idea of using polymer fillers to replace conventional fillers, which allow oxygen to diffuse, resulting in a decrease in oxygen content on the surface because the polymer fillers are completely wetted (without particle-particle contact) and tight Linked to AEM, the elastomer surface is still intact and no cracking time is much longer than the conventional compound. Eventually, the diffused polymer filler sacrifices itself to protect AEM by its own consumption of oxygen.
Table 8, laboratory test methods
Figure 2 provides a series of pictures to illustrate the oxidized appearance formed during the hot air aging process. The control compound is a carbon black filled AEM compound and is selected to allow color changes to be observed. Molded pushbutton (ISO) at 190 ° C for different time periods After aging, the button was cut and photographed in its cross section.Although the white carbon reinforced AEM was originally transparent and colorless, but only one week after 190 ° C, the sample had become completely black. This result shows that, High levels of oxygen penetrate into the interior, causing catastrophic degradation.
Figure 1, AEM 5015 micrograph; thermoplastic filler is tainted black
Figure 3 shows the results of the same experiment with the AEM 5015 compound, which is mostly white (the color of the polymer nanoparticles), even at 190 ° C for three weeks.
Figure 2 shows the cross section of the AEM HT compound containing the silica filler after thermal aging
Figure 3 shows the cross section of the AEM 5015 compound after heat aging
Comparison of hot air aging between carbon black and polymer filler
The compounds in Table 4 were subjected to an aging study in a large amount of hot air to compare the carbon black filled AEM with the polymer filled AEM compound, and the data were then used to plot the Arrhenius diagram.
Based on the failure criteria described above, the Alenius time exhibits a compound against the temperature map (Fig. 4) that violates one or more failure criteria.
Figure 4, hot air aging Alenius time on the temperature chart
The AEM compounds containing N550 carbon black and polymer filler type, the hot air aging properties can be compared at a holding time or at a constant temperature.The improvement in the aging properties of the hot air caused by the polymer filler may have the following characteristics:
Hold time constant can significantly increase the temperature level:
● 6 weeks - rating from 167 ° C to 182 ° C (15 ° C)
● 3 weeks - from 175 ℃ to 190 ℃ rating (15 ℃)
● 1 week - from 185 ° C to 205 ° C rating (20 ° C)
Maintaining a constant temperature indicates a significant increase in time until failure occurs:
● 160 ° C - from 1800 hours to 3600 hours (2 times)
● 175 ° C - from 504 hours to 1680 hours (3.3 times)
● 185 ° C - from 168 hours to 750 hours (4.5 times)
It is further stated that the Arrhenius model can be used to estimate the service life of AEM components (eg, turbocharger hoses or engine gaskets) exposed to different temperature environments. In most cases the temperature may be relatively low Occasionally high temperatures can cause damage.
Based on the Arrhenius model, three life expectancy estimates are shown in Table 5. In case 1, 80% of the AEM part was subjected to an operating temperature of 150 ° C and the remaining 20% of the time would be at 175 ° C. In Case 2 and Case 3, the AEM part was 80% of the time at 150 ° C, but the remaining 20% of the time encountered a higher temperature of 190 ° C and 200 ° C, respectively.
Figure 5: Compressive stress relaxation of the compounds in Table 6 using air in 165 ° C; ISO standard compression set tested under driving conditions, measured at room temperature Residual stress
Conventional AEM / carbon black compounds lasted more than 1000 hours in Case 2 and Case 3. In contrast, the AEM 5015 had a useful life of more than 1000 hours in all three exposed environmental conditions with an average duration of approximately AEM / Black compound 3 times longer.
Compressive stress relaxation (CSR)
Better hot air aging is a key advantage of polymer packing, and the close correlation in the air test is that the compressive stress relaxation is improved. In general, AEM / carbon black compounds are not well behaved in this test , But the antioxidant properties of the polymer-filled AEM compounds are greatly improved. Table 6 shows the formulation of compounds based on conventional AEM and carbon black reinforced HT ACM, as well as polymer-filled AEM 5015 seals. The compressive stress relaxation in the air is shown in Figure 5. Assuming that a seal failure occurs at a residual stress of about 10%, the HT ACM fails after about 250 hours and the carbon black is filled with AEM for about 500 hours after failure and the AEM 5015 after 1000 hours Still can retain nearly 20% of the sealing force.
Figure 6: Compressive stress relaxation of the compounds in Table 6 using Mobil 5W30 oil at 150 ° C; ISO standard compression set tested under driving conditions to measure residual stress at room temperature
AEM compounds typically perform well in compressive stress relaxation tests in the engine oil or in the drive fluid. A good AEM / carbon black compound retains a seal strength of more than 10% for more than 3000 hours in an oil atmosphere at 150 ° C. 6 indicates that the filler (carbon black or polymer) in Mobil 1 5W30 oil has little effect on the compressive stress relaxation of AEM. The difference in compressive stress relaxation caused by oxidation in air and oil, respectively, indicates that the oil can significantly slow the oxidation of the elastomer The
Non-black or colored compounds
Almost all AEM compounds are black, because the addition of carbon black provides a good balance of performance. For some applications, the end user may wish a non-black compound to facilitate the assembly of the product. For example, the two gaskets may be similar But not exactly the same shape, the assembler may want two parts to have different colors.When the blue gasket on the right, the left side can be equipped with red gasket.
AEM compounds can be made in different colors by replacing carbon black with mineral fillers (silica, talc, etc.); but in general, mineral fill compounds can impair physical properties, such as anti-strain properties. However, the polymer is filled with AEM Compounds are easily colored by adding low amounts of pigment and titanium dioxide, because the coloring is only a slight change in the formulation of the compound, and the polymer-filled AEM can achieve minimal adverse effects on performance.
Figure 7 shows a carbon black sample made using AEM 5015
Figure 7 shows the color of the molded product of AEM 5015 compound and its thermal aging in air at 175 ° C for one week after the first compound (1843) is the 'own' color without the addition of pigments or titanium dioxide. The darkening after heat aging in the examples is due to aging discoloration of the antioxidant (AO-1). All other compounds contain 2 parts by weight of pigment and 5 parts by weight of titanium dioxide, which helps to overcome the contamination from AO-1 . AO-2 Antioxidants are not used for colored compounds because they are more strongly stained than AO-1. Although AO-1 is less effective than AO-2, AO-1 colored AEM 5015 compounds are more effective than conventional AEM / Have better resistance to heat aging air.
Low hardness AEM compounds
The hardness of the AEM / carbon black compound A is usually in the range of from 55 to 80. Some of the compounds have a hardness of 45 to 50 hardness, but this soft compound is usually poor in processing properties.
In contrast, in the laboratory, the hardness of the AEM 5015 compound with hardness range of 37 to 47 has good physical properties and is easy to process. The Mooney viscosity (ML 1 + 4, 100 ° C) tends to indicate the Processing performance, the application to avoid less than 30MU viscosity of the compound.
Table 7 shows the low hardness AEM 5015 compound of the green pigment formulation. The compound containing 7 parts by weight of the polymer filler has a hardness of 37 hardness, a Mooney viscosity of about 40, and good processing properties under laboratory conditions.
in conclusion
Compared to conventional AEM compounds, polymer-filled AEM pre-compounds, such as AEM 5015, are hot-aged at any given temperature, and their service life is three times higher.
In addition, the advantage of using the polymer filler AEM is greater than that of the conventional AEM compound at a temperature of about 15 ° C or less while maintaining the same period of use at the same time.
In hot air, AEM / polymer filled compounds have excellent compressive stress relaxation properties, and in the oil also has conventional AEM compounds similar to the compressive stress relaxation performance.
Polymeric fillers can be used to make AEM compounds of different colors without sacrificing compressive permanent deformation properties. It can also be used to make relatively high viscosity, lower hardness compounds. Low hardness with carbon black AEM compounds, these compounds are easier to process.
Laboratory testing method
The ASTM and ISO methods used in this study are shown in Table 8.