Operating temperatures for engine oil seals (see Fig. 14.11 and cross-section of lip seal with garter spring in Fig. 14.22) vary widely, depending on engine design and location within the engine. Typically, the rear crankshaft seal is subjected to much higher temperatures than the front seal. Oil sump temperatures vary considerably, depending on provisions for oil cooling. This allows use of hydrogenated nitrile (HNBR), silicone, or acrylic elastomers for some seals in relatively low-temperature environments (120–140°C or 250–284°F). Standard fluoroelastomers (FKM), bisphenol-cured VDF/HFP/TFE terpolymers with 68–69% fluorine content, perform well in oil service up to about 160°C (320°F). More resistant fluoroelastomers are necessary for reliable long-term performance in more severe environments.
As can be seen from the seal cross-section shown in Fig. 14.2, shaft seals are complex shapes that require advanced mold design and molding techniques (see Section 7.3 for discussion of fluoroelastomer molding). For some time, most shaft seals were made in the United States by compression molding. Injection molding of shaft seals is prevalent in Europe, and is being used increasingly in the United States. An advantage of compression molding is that preforms (usually rings cut from extruded tubing) are used that closely approximate the amount of stock required for the final parts, so compound waste is minimized. For injection molding, the amount of cured stock in the central sprue and runner (actually a thin sheet leading to the seal lip) is often large compared to the stock required for the final part, so the waste of high-cost fluoroelastomer may be high. Such waste is reduced in modern injection molding designs.
The seal shown in Fig. 14.2 is a relatively simple design; most automotive seals are more complex. Dust lips are often used to keep outside contaminants away from the oil lip seals; such seals thus have undercuts that make demolding more difficult. Fluoroelastomer compounds used for such undercut shapes must have reasonably high elongation at break at molding temperatures to avoid tearing the part during demolding. The metal insert is often U-shaped, and stock may be molded to form a thin layer over the outside of the insert. Since both compression and injection molding methods are used, suppliers of fluoroelastomers for shaft-seal applications often must provide different versions of the same polymer composition-medium to high viscosity for compression molding, and low to medium viscosity for injection molding. Different precompounds may be necessary to accommodate relatively long compression-molding times at low temperature and very short injection-molding times at high temperature.
Obtaining adequate adhesion of fluoroelastomer compounds to metal inserts is a major consideration in fabrication of shaft seals. Adhesive systems worked out for bisphenol-cured VDF/HFP/TFE elastomers often do not perform adequately for peroxide-curable fluoroelastomers and more base-resistant polymers that contain little or no VDF. The trend toward use of more resistant fluoroelastomers in shaft seals has necessitated considerable effort on compounding and adhesive system development to get adequate bonding of the new materials. Silane-type primers are often used to coat metal inserts; these contain residual active groups such as amine functions that interact with the fluoroelastomer compound to attain good adhesion, especially for VDF/HFP/TFE elastomers. Other adhesive systems, using epoxy compounds or tie-coats, may be necessary for difficult bonding situations.3
Metal inserts must be carefully prepared in operations involving cleaning and roughening surfaces (grit-blasting or phosphatizing), stamping out parts, application of primer (usually by dipping), and curing of the primer (often by baking for a short time at moderate temperature).3 Primer curing minimizes the possibility of wiping primer off portions of the insert by stock flow during molding. The treated metal inserts must be used within a relatively short time (usually a day or less), so that functionality necessary for bonding is not lost by reaction with moisture in the air. Freshness of the primer surface is particularly important for peroxide-cured and base-resistant fluoroelastomer compounds. Compound formulation should be adjusted to attain good adhesion.
For bisphenol-cured VDF/HFP/TFE polymers, calcium hydroxide level should be low and magnesium oxide level should be high to promote adhesion to metal inserts. Thermal black or mineral fillers generally give good adhesion.3 For most adhesive systems, it is necessary to limit postcure temperatures to about 200°C (392°F).3
Modern engine oils, such as the current SG classification for gasoline engines, contain a large fraction of additives, many of which are detrimental to fluoroelastomers. The primary functions of oil-additive packages are to protect metal parts, avoid deposits in the engine, minimize oil degradation, and adjust fluid viscosity. Little attention has been paid to avoiding damage to rubber seals. Instead, elastomer producers have been expected to provide new, higher-performing products at no increased cost to auto manufacturers. Among the additives with moieties that may attack fluoroelastomers at high temperature are detergents (phenolates), dispersants (succinimides, alkylphenol amines), and antioxidants (amines, sulfides, hindered phenols).4 Many of these components are multifunctional, containing phenol or amine groups that can dehydrofluorinate and crosslink VDF-containing fluoroelastomers, leading to loss of elongation and eventual embrittlement. However, the rate and extent of reactions with seals are affected by many factors, including whether air is present in the system. When oil is exposed to air at high temperature, additives may undergo considerable changes. For example, a significant fraction of amines may be oxidized to amides, which have little effect on fluoroelastomers.5
Vulcanizates of several fluoroelastomers, listed in Table 14.1, were exposed to a standard 5W-30 engine oil, ASTM Service Fluid 105, for up to 6 weeks at 150°C (302°F).5 The oil was changed weekly, but was not aerated. Retained elongation was measured after exposure for 1, 2, 3, and 6 weeks; data are shown in Fig. 14.3. The results indicate that bisphenol-cured FKM-A500 VDF/HFP copolymer, FKM-B600 VDF/HFP/TFE terpolymer, and peroxide-cured FEPM-7456 TFE/P/VDF terpolymer lost most of their original elongation over the course of the test exposure, indicating considerable additional cross-linking occurred by reaction with amine- and phenol-containing oil additives. The other fluoroelastomers showed better retention of elongation, being much less susceptible to additional crosslinking. Note that FEPM-7456 contains a high level of VDF (about 30%), while FEPM-7506 contains a relatively low VDF level (10–15%) to serve as cure site for bisphenol curing. The other FEPM types contain no VDF.Polymer DesignationCompositionCure System% FMonomersFKM-A50066VDF/HFPBisphenolFKM-B60069VDF/HFP/TFEBisphenolFKM-GFLT67VDF/PMVE/TFEPeroxideFEPM-745658TFE/P/VDFPeroxideFEPM-750657TFE/P/(VDF)BisphenolFEPM-746355TFE/PPeroxideFEPM-ETP67E/TFE/PMVEPeroxide
From this kind of standard immersion testing, one would expect that bisphenol-cured VDF/HFP/TFE fluoroelastomers would not give good service life as oil seals. Similar tests with other elastomers, such as HNBR, silicone, and acrylic rubbers, show less loss of elongation. However, it is found that, in actual service, FKM shaft seals6 have much longer service life than seals of the other elastomers. In a Japanese study of FKM lip seals, rear crankshaft seals from high-mileage automobiles (70,000–280,000 mi ie, 110,000–450,000 km) were collected and examined. No serious oil leakage was found when the seals were removed from the engines. Some deposits were found around the seal lip and on the garter spring holding the lip against the shaft. No surface cracks were found on the seal lip, and only minor crazing on the crankcase side of the flexure portion of the seal in some samples. The seal compositions were not noted, but most were probably VDF/HFP/TFE elastomers with 68–69% fluorine content.
Bauerle and Bruhnke7 found that aeration reduces the effect of oil additives on fluoroelastomer properties. Some of their data is reproduced in Fig. 14.4,5 showing the effect of aeration of an SF-grade 5W30 oil on the retention of elongation of a VDF/HFP copolymer (FKM-E430), a VDF/HFP/TFE terpolymer (FKM-B600), and a VDF/PMVE/TFE fluoroelastomer (FKM-GFLT). The HFP-containing polymers show much better retention of properties with aeration.
A more comprehensive study of aeration by Dinzburg8 showed that even a minimal level of aeration of an aggressive European SF oil led to protection of a VDF/HFP/TFE compound, but to severe deterioration of an HNBR compound. He notes that aeration increases the severity of aging in oil for silicone and acrylic elastomers, while decreasing the severity for FKM elastomers.
For more severe oil-seal service at temperatures of 160°C (320°F) or higher for extended periods, more resistant fluoroelastomer compositions are required for long service life. High-fluorine VDF/PMVE/TFE elastomers, along with TFE/olefin FEPM elastomers, are much less susceptible to attack by oil additives. TFE/P fluoroelastomers have the requisite chemical resistance, but have low fluorine content, leading to relatively high swell and to soft vulcanizates with lower wear resistance than desired.
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