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New developments in ACM fluid resistance technology.

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 and 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. However, in today's modern engines, not only are these higher operating temperatures of concern, but in combination with this, the continuing requirements placed on automotive manufacturers to reduce harmful emissions and comply with emission control legislation such as Euro 6 and other global standards has led to the refining of technologies such as exhaust gas recirculation (EGR), urea treatment (DEF: Adblue), selective catalytic reduction (SCR) and positive crankcase ventilation (PCV) systems which assist in meeting these emission targets. Other technologies such as belt in oil, three cylinder engines, hybrid and electric vehicles are all aimed at emission reduction combined with benefits in cost and economy.

The target of these control methods in gasoline (SI) and diesel engines is to reduce the emissions of the harmful by-products of combustion, principally nitrogen oxides (N[O.sub.x]), total hydrocarbon (THC), non-methane hydrocarbons (NMHC), and particulate matter, CO and C[O.sub.2].

Introduction

This article 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 existing standard ACM and HT-ACM elastomers, and the latest AEM acrylic sealing elastomer. The data demonstrate how the new technology has improved the properties of this ACM elastomer in applications where such aggressive contaminated engine oils are encountered.

Emission control systems in vehicles

In EGR, recirculation of hydrocarbon exhaust gases in gasoline engines displaces the amount of combustible matter. In diesel engines, where up to 50% recycle is possible, the hot recirculated exhaust gas replaces a proportion of oxygen in the precombustion mixture (ref. 1). Because N[O.sub.x] is formed when airborne nitrogen and oxygen are subjected to high temperature (>1,300[degrees]C), the presence of the exhaust gas initially dilutes the nitrogen gas present. This makes more of the gases inert to combustion, effectively reducing peak in-cylinder temperatures below the reaction threshold and limiting the amount of N[O.sub.x] generated (ref. 2). The complex chemical mixtures in the exhaust gas, including acidic exhaust gas condensates of varying composition, can be detrimental to many engine sealing elastomers. Zeon HT-ACM elastomers have demonstrated good performance in many of these acidic compositions, and data confirming this have been previously reported (ref. 3). Data for ACM-DP5238 in both acidic and basic condensates are shown later in figure 18.

In diesel engine technology, advanced emission control such as SCR uses catalyst technology to minimize NOx emissions. A gaseous reductant derived from ammonia based additives or urea is used in this system. Diesel exhaust fluid (DEF), a water based urea solution (ref. 4), commonly known as AdBlue, has been in use for some time. When injected into the exhaust flow, in combination with the catalyst, a series of chemical reactions results in the formation of harmless water vapor and nitrogen. Due to their excellent resistance to diesel fuels and their use in sealing applications, existing Zeon HT-ACM elastomer grades have been studied in contact with AdBlue, and are already present in several applications at typical operating temperatures of 80[degrees]C to 100[degrees]C, and offer good short term resistance up to 120[degrees]C.

PCV systems control and reduce the blow-by-gas (BBG) emissions generated during the engine combustion process. Combustion inevitably involves a small but continuous amount of blow-by being formed when some of the corrosive combustion gases leak past the piston rings and enter the oil filled crankcase, causing pressure to build up. The PCV valve controls the pressure by allowing these gases to vent back to the combustion chamber via the intake manifold. These gases are comprised mainly of mixtures of fuel and acid condensates with some contaminated engine oil vapor. Zeon HT-ACM elastomers perform as well as other acrylic based elastomers in BBG compositions generated by diesel fueled engines where they find use in seals, gaskets and ducts. ACM elastomers are more severely affected by direct gasoline contact, and their use here is limited.

Within the European Union, road transport is responsible for approximately 20% of all C[O.sub.2] emissions, with passenger cars contributing about 12% (ref. 5). Further reduction of carbon dioxide emissions is being made by advances in turbocharger technology. With the integration of air management, turbo design, and EGR, PCV and SCR emission control systems in the engine, the same complex chemical by-products as already described are found and can also be detrimental to the elastomers used in these charged air systems. HT-ACM elastomers capable of resisting these mainly acidic by-products and operating at the high service temperatures found in HT Turbo hose applications are well proven (ref. 6).

For the purposes of this article, contaminated engine oil will be further referred to under the acronym CEO.

The refinement of emission control systems which generate many complex chemical by-products, the use of gasoline fuels blended with alcohols, diesel fuel alone and in blends with biodiesel, has effectively resulted in the modern combustion engine resembling an efficient reflux condenser for these chemical cocktails! One consequence of this is that modern fully synthetic engine oils, already aggressively protected with additive packages, are becoming more and more contaminated with fuel, alcohols and acidic condensates. In recent times, the aggressive nature of such contaminated engine oils has been found to have a significant effect on the properties and performance of sealing elastomers, including ACM.

The result can be severe hardening and associated changes in physical properties with the potential to reduce sealing performance leading to seal failure. Following several incidents of actual seal failure, car producers in general are becoming more concerned about the potential impact of contaminated engine oil on the performance of elastomeric sealing components. One European car manufacturer (further referred to in this article under the acronym ECM) has been particularly proactive regarding this issue, and has developed and now specifies testing of sealing elastomers in what they refer to as a worst case oil (WCO) test fluid. This is designed to assess the ability of elastomers used in seals and gaskets to perform where such contaminated engine oils will be encountered at typical service temperatures of 150[degrees]C. Alone and in combination with ethanol and water, this test fluid is said to represent both typical hot and cold start driving conditions with such contaminated oils. The WCO fluid itself is a combination of engine oil, light fuel components and acid. Other global car manufacturers experiencing similar issues are also considering such test fluids. This article will present data for ACM-DP5238 in both test fluids, demonstrating how the new technology used in its manufacture leads to its superior performance compared to standard ACM, existing FIT-ACM grades and comparable to the latest AEM acrylic sealing elastomer. In addition, a sealing component must provide acceptable resistance to "clean" engine oils such as those used for first fill and subsequent replacement at service intervals. This article also presents data for the ACM-DP5238 in a European reference oil designed to simulate such oils, Lubrizol OS 206304. Additionally, data are presented for aging in the North American reference oil designated SF105G.

Investigation

As previously described, several European car manufacturers (ECMs) 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 and 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. In more detail, attack of the ester groups in the polymer by the acetic and other acids would create alcohols which, with abstraction of water, would create a hydride group leading to further 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 further 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 program 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.

The first step in this process was to conduct a screening evaluation of both existing standard ACM and HT-ACM grades in the CEO defined by one ECM. Data from this initial screening indicated that, although performance in some cases could be considered as satisfactory, it was clear that improvement in resistance to this CEO was necessary.

Experimental

There are four distinct families of HyTemp ACM elastomers characterized by the type of cure site monomer incorporated. These cure site families are chlorine, dual chlorine/carboxyl and epoxy types for the standard ACM elastomers, and a proprietary cure site type for the HT-ACM elastomer range which utilizes amide crosslinking technology for improved heat resistance. The least common types are the older epoxy cure site types.

ACM elastomer screening--CEO

In order to assess the performance of both standard ACM and HT-ACM grades, the whole range of available types in each family, excluding the epoxy types, was assessed in contact with the CEO under the ECM specified test conditions of one week aging at 150[degrees]C. For the purpose of demonstrating the performance of the different polymer families in the CEO fluid under the condition cited, compounds based on a selection of these polymers for each type are included in this article. These are identified as HT-X1 and HT-X2 for the HT-ACM grades, AR70-1 and AR70-2 for the chlorine cure site standard ACM types, and H4050 for the chlorine/carboxyl cure site standard type. All compounds were mixed and tested using standard laboratory methods and procedures. The ECM specifies limits for both hardness and volume change after immersion in the CEO fluid. Figures 1 and 2 demonstrate the characteristics of the evaluated compounds in the CEO. As can be seen, the lowest property changes are observed with the HT-ACM based compounds, while the standard ACM compounds show severe hardening, severe loss in elongation at break, and large tensile strength changes. Volume change is lowest for the HT-ACM grades, while generally highest for the standard ACM grades. This screening evaluation confirmed that the basis for development of a new polymer should be considered using the HT-ACM technology.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

As previously noted, investigation of novel new monomers was already ongoing, and on the basis of that work, the subsequent research resulted in the development of the improved ACM-DP5238 elastomer.

New development elastomer

In order to demonstrate the improved performance seen with the ACM-5238 polymer after immersion in the CEO, a range of compounds with differing filler and plasticizer levels was evaluated. In addition, a compound based on Vamac IP (AEM-IP) was used for comparison, and the recipe for this competitive polymer was taken from the manufacturer's literature (ref. 7), where studies had been conducted in the same CEO. Three ACM-DP5238 compounds are illustrated in this article, identified respectively as: DA FEF 1 (low ester plasticizer level), DA FEF 2 (increased carbon black and increased plasticizer level), and DA SRF 1 (less reinforcing carbon black at higher level and high ester plasticizer level), together with the representative AEM-IP sealing compound. These four compounds can be viewed in table 1. Processing characteristics of each compound, as represented by rheological and Mooney viscosity/Mooney scorch characteristics (figures 3 and 4, respectively) indicate similar properties, which are typical for HT-ACM elastomers, including somewhat faster scorch and typically higher viscosity than AEM-IP. HT-ACM based compounds with similar characteristics have been used successfully for many years to produce sealing components by injection molding.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Thermal stability

The automotive standard SAE J 2236 defines an elastomeric compound's upper thermal aging limit as measured at 23[degrees]C by retention of at least 50% original elongation and tensile at break after 1,008 hours of heat aging. Using this definition, all three ACM-DP5238 compounds easily meet this definition at 150[degrees]C, as seen in figure 5, with minimal property change, including change in hardness. Similar performance is seen with the AEM-IP compound. Generally speaking, both polymer types show comparable properties, with ACM-DP5238, showing benefits in lower elongation at break change, while AEM-IP shows a lower change in tensile strength, and compound DA FEF 1 the lowest hardness increase. Also in figure 5, similar compression set performance in air is observed after 1,008 hours heat aging at 150[degrees]C, with compound DA FEF 1 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 and gaskets will also encounter these contaminated engine oils (CEO) during their service life.

Nevertheless, resistance to non-contaminated engine oil, for example the first fill and 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-DP5238, long term testing was conducted in European engine reference oil, Lubrizol OS 206304. Figure 6 shows minimal property change for the ACM-DP5238 compounds in this oil. With respect to swelling behavior, both compounds DA FEF 2 and DA SRF 1 contain more ester plasticizer than DA FEF 1. Therefore, this allows for a greater oil exchange during immersion, giving lower swelling than DA FEF 1, which was principally designed for optimum performance in the CEO. The AEM-IP compound contained a significantly higher level of ester plasticizer than the three ACM-DP5238 compounds; however, it still showed much higher swelling behavior in the Lubrizol oil.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

Both compression set and compressive stress relaxation (CSR) performance are well-established indicators of acceptable sealing performance for elastomeric components. Figure 5 demonstrates typical compression set performance in air at an elevated temperature of 150[degrees]C for the ACM-DP5238 and AEMIP based compounds. Figures 7 and 8 demonstrate the performance of these same compounds following immersion in the Lubrizol oil. Figure 7 shows the compressive stress relaxation performance at intervals over a period of up to 1,512 hours at 150[degrees]C. Compound DA FEF 1, principally optimized for best performance in the CEO, shows directly comparable performance to AEM-IP, with the more highly extended/plasticized ACM-DP5238 based compounds also giving excellent retention of sealing force. For these compounds, values approaching and exceeding 40% sealing force retention more than meet the >10% requirement specified by many automotive manufacturers in their sealing specifications. Compression set performance follows a similar trend, as shown in figure 8.

Similar to the preceding European Lubrizol reference oil tests, an oil more familiar to the North American domestic automotive industry, ASTM/SAE reference oil SF105G, was also used to quantify the compounds. As can be seen in figure 9, after 504 hours at 150[degrees]C, both the DA FEF 2 and the DA SRF 1 compounds show the least changes in hardness and tensile. With respect to elongation change, the DA SRF 1 compound exceeds the elongation retention of the other compounds, although there are not huge deltas overall between any of the compounds. In volume changes, the AEM IP stock has the highest swelling at 18%, whereas the new HT-ACM DP5238 compounds are clustered around 13% change. (Note: As of March 2016, the 1,008 hour data are not yet completed. They will be added to subsequent editions.)

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

Compressive stress relaxation data (Dyneon fixture) for the SF105G oil is shown in figure 10. It can be seen based upon this chart that the DA FEF 1 (optimized DP5238 CEO polymer compound) shows far superior resistance to compressive stress forces over time, especially when compared with the AEM IP based compound. This is maintained to 1,512 hours, where the test was stopped.

Compression set testing in SF 105G oil is illustrated in figure 11, and clearly shows that after 1,008 hours at 150[degrees]C, the DA FEF 1 (optimized compound) is virtually identical to the AEM IP stock. This indicates that robust performance of the new HT-ACM can be expected.

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-DP5238 polymer utilizing novel new monomers was compared with the existing standard ACM grades and AEM-IP. Figures 12 and 13 clearly demonstrate the significant improvements achieved with the new polymer in all three of the evaluated compounds. Once again, it is worth noting that during the compound design phase of this development, compound DA FEF I was considered likely to be the most optimized compound for resistance to the CEO. This is clearly seen in both figures 12 and 13. Figure 12 shows the very low hardness and volume change, both well within the ECM's specified limits, with a similar trend in figure 13 for tensile strength and elongation at break. In both cases, this compound shows better performance than the AEM-IP.

[FIGURE 10 OMITTED]

This excellent performance is further seen in figure 14, where the retained sealing force measured via compressive stress relaxation is greater than 40%, exactly matching that of the AEM-IP and exceeding that of the two other ACM-DP5238 compounds. However, as with the CSR results in Lubrizol oil, it should again be noted that even for these compounds, values approaching and exceeding 20% sealing force retention are possible after 1,512 hours immersion at 150[degrees]C, which more than meets the >10% requirement previously cited and is certainly sufficient to maintain function as a sealing component. Similarly, compression set performance in the CEO is excellent for all ACM-5238 based compounds, with DA FEF 1 giving very similar performance to the AEM-IP based compound (figure 15).

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

[FIGURE 13 OMITTED]

Reference to figures 16 and 17 shows the performance of the evaluated compounds in the blend of CEO with ethanol and water in the ratio 55/25/20%, immersed for one week at 70[degrees]C. This fluid combination was developed by the ECM in order to recreate cold start driving conditions. Figure 16 shows that all ACM-DP5238 compounds are well within the specified limits for both hardness and volume change in this fluid, with slightly better performance for the AEM-IP. 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 also be clearly seen in figure 16, following sequential dry out periods of 24 hours at 60[degrees]C, followed immediately by 24 hours at 80[degrees]C and finally 24 hours at 120[degrees]C, all three ACM-DP5238 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, while the optimized DA FEF 1 compound clearly shows the least effect after this dry out sequence, with almost no change in properties. Figure 17 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-DP5238 based compounds, properties under both conditions being well within the limits specified by the ECM, with compound DA FEF 1 again showing best performance.

[FIGURE 14 OMITTED]

[FIGURE 15 OMITTED]

[FIGURE 16 OMITTED]

Acidic and basic condensates

As noted, analysis of used engine oils has confirmed the presence of acid condensates originating mainly from the complex chemical mixtures in the exhaust gas. In SCR (selective catalytic reduction), basic condensates can also be formed and have the potential to be detrimental to many engine sealing elastomers. Various automotive manufacturers have developed their own unique mixtures of corrosive chemicals and test methods used to quantify performance of elastomeric materials. As noted previously, Zeon HT-ACM elastomers have demonstrated good performance in many of these acidic compositions, and data confirming this have been previously reported (ref. 3). It was considered important to assess the behavior of the ACM-DP5238 polymer in both acidic and basic condensates, and figure 18 shows moderate property changes in both Fiat and VW defined compositions for the ACM-DP5238 based compounds, all being comparable to the AEM-IP.

[FIGURE 17 OMITTED]

[FIGURE 18 OMITTED]

Conclusions and summary

In the automotive industry, environmental protection is leading, along with other issues, to the continuous refinement of 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 article 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-DP5238 is now available for sampling and customer evaluation. As noted earlier in this article, 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.

Zeon will continue to innovate and add to its range of HTACM elastomers in the lliture in order to provide further possibilities for automotive engineers to select elastomers which have already proved to be ideally suited to applications requiring long-term performance in severe temperature and fluid environments.

This article is based on a paper presented at the 188th Technical Meeting of the Rubber Division, ACS, October 2015.

References

(1.) Exhaust Emissions & Drivability--Chrysler Corporation, 1973, [http://www.imperialclub.com/Repair/Lit/Master/08/page08.htm].

(2.) "What is the EGR valve and what does it do?" Engine Technology, 2009.

(3.) J.R. Kelley, Zeon Chemicals L.P., "An advanced alternative to ethylene acrylic (AEM) elastomers for high-temperature, oil resistant hoses, "American Chemical Society, 182nd Fall Meeting, Cincinnati, OH, October 2012.

(4.) ISO Specification 22241.

(5.) "European Commission plans legislative framework to ensure the EU meets its target for cutting C[O.sub.2] emissions from cars, "Ref. IP/07/155; 07/02/2007.

(6.) P.J. Abraham, Zeon Chemicals Europe Ltd., "HT-ACM- the winning combination of processing and performance for high temperature oil resistant hose applications, "presented at IRC, Paris, March 20, 2013.

(7.) E. McBride and T. Dobel, DuPont Performance Polymers, "Advancements in AEM polymers for improved processing and improved properties, " Rubber World, April 2014.

by Peter J. Abraham, Ivan C. Burczak, David Tao, Aaron Bressler, Samuel C. Harber and Kazuhiro Ejiri, Zeon
Table 1--experimental compounds and
original properties

Identification     DA FEF 1   DA FEF 2   DA SRF 1   AEM-IP
Hytemp
  DP5238               100        100        100       --
AEM IP                  --         --         --      100
N550 FEF                55         60         --       --
N772 SRF                --         --         75       45
Thermax MT
  N990                  --         --         --       20
Edenol 181               3          6          6       --
Edenol T810T            --         --         --       15
Naugard 445              2          2          2        2
Stearic acid             1          1          1        2
Armeen 18D              --         --         --      0.5
Vanfre VAM             0.5        0.5        0.5        1
Vulcofac HDC
  75                  0.66       0.66       0.66     1.73
Rhenogran
  XLA 60                 2          2          2       --
Vulcofac Act
  55                    --         --         --        3
Hardness
  (durometer A)         57         60         59       57
Tensile (MPa)          9.9        9.2        9.7     15.1
Elongation at
  break (%)            220        230        225      335
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Author:Abraham, Peter J.; Burczak, Ivan C.; Tao, David; Bressler, Aaron; Harber, Samuel C.; Ejiri, Kazuhiro
Publication:Rubber World
Date:Apr 1, 2016
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