With the continued trend for ever decreasing space in the engine bay, temperatures continue to rise, placing ever more demanding requirements on the components operating in this environment. One consequence of this is the need for Elastomers capable of ever-greater long-term high temperature resistance. The superiority of high-temperature ACM (HT-ACM) elastomers over traditional ACM & other Acrylic elastomers with respect to heat and aggressive automotive lubricating oil resistance has been demonstrated in many applications including a variety of engine seals, oil pan and valve cover gaskets, hose and air ducts.
This paper describes the development of a new Zeon HyTemp ACM elastomer, HyTemp DP5238 (ACM-DP5238), which utilizes novel monomer technology giving improved sealing performance compared to existing ACM elastomers in aggressive contaminated engine lubricants. Included are comparisons of the ACM-DP5238 elastomer with the latest AEM acrylic sealing elastomer. The data demonstrates how the new technology has improved the properties of this ACM elastomer in applications where such aggressive contaminated engine oils (CEO) are encountered and cause traditional acrylic gaskets to severely harden & loose their elastomeric properties.
European Car Manufacturers (ECM’s) have reported a hardening phenomenon with Acrylic gaskets, some of which have led to field failures. Analysis of the gaskets from these vehicles confirmed this hardening and further analysis of some of the used engine oils indicated contamination with acids, principally acetic, formic & oxalic acid together with alcohol, fuel and water. It was postulated that the most probable cause of the hardening of the gasket was due to hydrolysis reactions initiated by the acids in the oil generated from alcohol containing fuels in combination with water. Carboxylic acids identified via IR spectroscopy in the failed gaskets further supported this theory, a likely mechanism involving hydrogen abstraction creating free radicals which in turn led to further crosslinking of the rubber gasket resulting in its hardening.
To counteract this, Zeon Scientists considered that protection of the tertiary carbon atom in the polymer, through the use of novel monomers already under investigation, would more protect the polymer from the influence of attack by the complex chemical mixtures present in contaminated engine oil (CEO). This initiated a global Zeon research programme to develop a new ACM elastomer with improved performance in such lubricating fluids and other aggressive automotive fluids as they become more and more contaminated during service life.
New Development Elastomer
In order to demonstrate the improved performance seen with the ACM-5238 polymer after immersion in CEO, a range of compounds with differing filler and plasticiser levels were evaluated. In addition, a compound based on Vamac IP (AEM-IP) was used for comparison purposes. The recipe for this polymer was taken from the manufacturer’s literature where studies had been conducted in the same CEO. Three ACM-5238 compounds are illustrated in this paper, identified respectively as: DA FEF1 (low ester plasticiser level); DA FEF2 (increased carbon black & increased plasticiser level); DA SRF1 (less reinforcing carbon black at higher level & high ester plasticiser level), together with the representative AEM-IP sealing compound. Processing characteristics of each compound indicate similar properties, which are typical for HT-ACM elastomers.
The automotive standard SAE J 2236 defines an elastomer compound’s upper thermal aging limit as measured at 23ºC by retention of at least 50% original elongation and tensile at break after 1008 hours of heat ageing. Using this definition, all three ACM-5238 compounds easily meet the requirements for 150°C. Similar performance is seen with the AEM-IP compound. Generally, both polymer types show comparable properties with the ACM-5238 showing benefits in lower elongation at break change, whilst AEM-IP shows a lower change in tensile strength and compound DA FEF1, the lowest hardness increase. Similar compression set performance in air is observed after 1008 hours heat ageing at 150°C, with compound DA FEF1 showing marginally improved performance versus the other ACM compounds and being comparative in performance to AEM-IP.
Engine Oil Resistance
ACM elastomers are well proven in engine sealing having been the predominant elastomer of choice for engine sealing for many years, with lower fluid swell than other competitive Acrylic sealing elastomers, such as AEM. Polyacrylate materials are generally more polar than ethylene acrylate materials due to the ethylene content within the AEM polymer backbone structure. This gives ACM materials greater resistance to non-polar fluids such as engine oils, diesel fuels, automatic transmission fluids and other common oils and greases. As already described, improvements in emission reduction technologies are leading to the increased likelihood of contaminated engine oils being generated during the lifetime of automotive vehicles and so it is reasonable to conclude that seals & gaskets will also encounter these contaminated engine oils (CEO) during their service life.
Nevertheless, resistance to non-contaminated engine oil, example: first fill & new oil at service changes is also very important for effective overall long-term sealing performance. In order to demonstrate the performance of the ACM-5238, long term testing was conducted in European engine reference oil, Lubrizol OS 206304. It was found that the ACM-5238 compounds show minimal property change in this oil. With respect to swelling behavior, both compounds DA FEF2 and DA SRF1 contain more ester plasticiser than DA FEF1. Therefore, this allows for a greater oil exchange during immersion, giving lower swelling than DA FEF1, which was principally designed for optimum performance in the CEO. The AEM-IP compound contained a significantly higher level of ester plasticiser than the three ACM-5238 compounds, however, still showed much higher swelling behaviour in the Lubrizol oil.
Both compression set and compressive stress relaxation (CSR) performance are well-established indicators of acceptable sealing performance for elastomeric components. Tests were completed for both typical compression set performance in air at an elevated temperature of 150°C and for compressive stress relaxation performance at intervals over a period of up to 1512 hours at 150°C. Compound DA FEF1, principally optimised for best performance in CEO shows directly comparable performance to AEM-IP. For these compounds, values approaching and exceeding 40% sealing force retention more than meet the greater than 10% requirement specified by many automotive manufacturers in their sealing specifications.
Contaminated Engine Oil (CEO) Resistance
Having established via the initial screening evaluation that the HT-ACM polymer technology would provide the best opportunity for improvement in resistance to CEO, the resultant ACM-5238 polymer utilizing novel new monomers was compared with the existing standard ACM grades and AEM-IP. Figures 1 and 2 clearly demonstrate the significant improvements achieved with the new polymer in all three of the evaluated compounds, but especially in DA FEF1. Figure 1 shows the very low hardness & almost zero volume change with a similar trend in Figure 2 for tensile strength and elongation at break. In both cases, this compound shows better performance than the AEM-IP. Figures 3 and 4 show the performance of the evaluated compounds in the blend of CEO with Ethanol & Water in the ratio: 55/25/20% immersed for one week at 70°C. This fluid combination was developed to recreate “Cold start” driving conditions. Figure 3 shows that all ACM-5238 compounds are well within the specified limits for both hardness and volume change in this fluid. However, an important consideration in tests of this nature is to quantify any change in properties after re-drying the samples in order to assess any potential permanent influence of the test media on the elastomeric component during service conditions. As can be clearly seen in Figure 3, following sequential dry out periods of 24 hours at 60°C, followed immediately by 24 hours at 80°C and finally 24 hours at 120°C, all three ACM-5238 based compounds are well within the +5 points (hardness) and -5% (volume) change requirements. In contrast, the AEM-IP compound is right on the limit of acceptance under these conditions, whilst the optimised DA FEF1 compound clearly shows the least effect after this dry out sequence, with almost no change in properties.
Figure 4 shows the effect both with and without the dry out sequence on both tensile strength and elongation at break. Once again, the best overall performance is seen for the ACM-5238 based compounds, properties under both conditions being well within the specified limits, with compound DA FEF1 again showing best performance.
Conclusion and Summary
The Automotive Industry is continuously refining emission control and other engine technologies. These in turn, are leading to ever more aggressive vehicle engine conditions, including already heavily protected engine lubricants becoming contaminated during service. These new and refined technologies are proving extremely challenging for elastomeric seals, hoses and other components. Recent scientific developments in polyacrylate chemistry have improved HT-ACM performance and this has been demonstrated in this paper with the introduction of a new generation of HT-ACM elastomers meeting this challenge. HyTemp DP5238 (ACM-DP5238) has been shown to possess excellent resistance to the contaminated engine oil (CEO) developed by one major European car manufacturer to recreate these severe environments. It has been shown to be a viable alternative to other acrylic elastomers available in the market, specifically AEM-IP in this case. The ACM-5238 is now available for sampling and customer evaluation. As noted earlier in this paper, the name HyTemp DP5238 is the current development designation for this polymer and the final, commercialized product name will be announced following the customer evaluation period.
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