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EPDM sealing elements installed in detergent dosing pumps, washing machine dispenser valves, and commercial dishwasher manifold gaskets are subjected to alternating wet/dry cycling, alkaline surfactant solutions, and thermal excursions between 23 °C and 95 °C. Low compression set is required because the seal must maintain contact stress after prolonged compressive strain, particularly in static face seals and radial lip seals that experience repeated opening and closing under 25 % constant deflection. When the compound also contains a migratory plasticizer, repeated detergent immersion extracts the plasticizer from the non-polar ethylene-propylene matrix, causing volume shrinkage, surface tackiness, hardening, and the visual appearance of a bloomed surface film. This document addresses the compound design rules that permit low compression set without reliance on extractable ester plasticizers or high loadings of hydrocarbon oil. The formulation approach is built around high-molecular-weight EPDM with medium to high ethylene content, peroxide cure systems, high-structure carbon black reinforcement, and process aids that are either non-migratory or removed during post-cure. Testing under ASTM D395-18 Method B provides the primary compression set measurement, while liquid immersion is evaluated under ASTM D471-16a and ISO 1817:2022. Surface bloom is assessed by visual inspection at 10× magnification and by attenuated total reflectance Fourier transform infrared spectroscopy, where the ester carbonyl absorbance at 1735 cm⁻¹ is monitored after aging. In repeated detergent service, the absence of plasticizer bloom is not merely an aesthetic requirement; a migratory ester layer can retain surfactant residues, promote bacterial fouling, and alter the coefficient of friction at the seal interface. The design strategy described here eliminates the source of bloom by removing extractable low-molecular-weight species rather than attempting to mask their migration with surface treatments or bloom suppressants. This requires accepting narrower processing windows, higher compound viscosity, and more aggressive mixing than is typical for oil-extended EPDM. The following sections detail the crosslink chemistry, filler selection, processing constraints, and test methodology required to maintain both low compression set and clean surface behavior after repeated exposure to detergents.
Polymer selection for this service is governed by the interplay between ethylene content, diene content, molecular weight, and molecular weight distribution. EPDM grades with ethylene content between 55 % and 70 % by weight provide a useful balance of filler acceptance, tensile strength, and low compression set, while lower ethylene grades reduce low-temperature stiffness but sacrifice tensile properties and increase elastic strain under load. The diene termonomer, usually ethylidene norbornene, is maintained between 2 % and 9 % by weight to secure sufficient peroxide crosslink efficiency without leaving excessive residual unsaturation that can be attacked by oxidative detergent additives. Mooney viscosity ML(1+4) at 125 °C is typically selected in the 60 MU to 100 MU range because higher molecular weight improves compression set and reduces surface tack after exposure. Narrow molecular weight distribution reduces the concentration of low-molecular-weight chains that could act as extractable or bloom-forming species. When the formulation is entirely free of external plasticizer, the high gum viscosity must be managed through mixing intensity and filler morphology rather than by dilution with oil. This selection logic is supported by published EPDM supplier technical literature and by standard rubber compound design practice. The absence of ester plasticizer in the formulation automatically eliminates the principal source of detergent-induced bloom, but it also removes the traditional means of reducing compound viscosity, which alters downstream processing behavior. Therefore, the selection of a high-molecular-weight EPDM for low compression set must be coupled with an acceptance that the resulting compound will exhibit higher extrusion backpressure, increased shear heating, and shorter scorch time under the same processing conditions. Batch-to-batch viscosity variation of even ±5 MU can shift filler dispersion and extrudate surface quality, so incoming polymer lots are often tested for Mooney viscosity, ethylene content by FTIR, and residual transition metal catalyst content before release to mixing.
Compression set in an EPDM vulcanizate is not controlled solely by the polymer choice. The crosslink network must be sufficiently dense and thermally stable to resist permanent deformation, while the filler network must survive repeated compressive strain without accumulating fatigue damage. Under ASTM D395-18 Method B, the test specimen is compressed to 25 % deflection between parallel plates and aged for 22 h at 70 °C, 70 h at 100 °C, or 70 h at 125 °C, depending on the service temperature. The measured compression set is the percentage of original compression that is not recovered after the aging period. For EPDM compounds intended for hot detergent service, specification values below 20 % after 70 h at 125 °C are commonly targeted, although the exact limit depends on seal geometry and contact stress requirements. Achieving such values requires a high crosslink density and minimal chain scission. Peroxide cure systems that generate carbon-carbon crosslinks provide better compression set than sulfur donor systems because the carbon-carbon bond energy is higher and the network does not undergo the same reversible polysulfidic interchange under load. However, high crosslink density alone is not sufficient if the compound contains extractable plasticizer. As plasticizer is removed by detergents, the rubber matrix shrinks relative to the original molded dimensions, which increases internal stress and can lead to microcrack formation at the surface. This mechanically induced damage then degrades compression set, even when the initial dry compression set was acceptable. Therefore, the design rule for detergent service is to obtain low compression set from the polymer, crosslink, and filler network rather than from a softened, plasticized matrix that will lose mass during aging.
Detergent formulations are complex aqueous systems that combine anionic surfactants, nonionic alcohol ethoxylates, alkaline builders such as sodium tripolyphosphate or sodium carbonate, sequestrants, enzymes, and oxygen-based bleaching agents. The pH of concentrated detergent solutions can reach 11 to 12, and diluted washing liquors typically operate between 9 and 10.5 at temperatures from 20 °C to 95 °C. In repeated service, the EPDM component passes through wet immersion, drain, and dry recovery cycles that create concentration gradients for the migration of low-molecular-weight compound ingredients. The saturated ethylene-propylene backbone of EPDM is intrinsically resistant to hydrolysis and to polar solvent attack, but residual unsaturation in the diene termonomer remains vulnerable to oxidative cleavage when peroxide-based detergents generate reactive oxygen species in the hot alkaline medium. Detergent surfactants can also form micelles that solubilize hydrocarbon oils, fatty acid-derived process aids, and low-molecular-weight fragments from the rubber compound. The result of this combined action is progressive extraction of oil or plasticizer, oxidation of residual diene unsaturation, and surface deposition of extracted species as a visible bloom or sticky residue. The crosslink network itself may remain intact, but the loss of plasticizer shifts the dimensions and hardness of the seal. Hardness increases of 5 to 15 Shore A units are commonly observed in oil-extended compounds after prolonged detergent immersion, accompanied by volume shrinkage that can compromise sealing contact pressure. In plasticizer-free compounds, the absence of migratory diluents means that dimensional change and hardness shift are much smaller, but the compound must be designed so that the filler and curative residues are not extracted instead.
Crosslink stability in detergent service is influenced by cure chemistry. Peroxide-cured EPDM networks contain primarily carbon-carbon crosslinks and are more resistant to hydrolytic and oxidative attack than the polysulfidic linkages produced by many sulfur cure systems. However, peroxide vulcanizates may retain decomposition products from the peroxide and coagent, including acetophenone, tert-butanol, and unreacted coagent monomers. These low-molecular-weight residues can migrate to the surface and produce tackiness or bloom if not removed by a post-cure cycle. A post-cure of 2 h to 4 h at 150 °C to 180 °C in a convection oven or continuous hot air tunnel is therefore standard practice for peroxide-cured EPDM in sealing applications. The post-cure step drives off volatile peroxide decomposition residues and completes the coagent crosslinking reaction. Without post-cure, volatile residues can be released during the first hot detergent cycles and interact with surfactant films on the seal surface. Sulfur donor systems, in contrast, can leave residual accelerators such as tetramethylthiuram disulfide, zinc dialkyldithiocarbamates, and sulfur bloom on the surface. These residues can act as bloom nuclei or react with detergent components, and they are generally avoided when the product must remain clean under repeated hot detergent exposure. Published data for the interaction between specific accelerator residues and modern detergent formulations is limited, but the use of peroxide cure is the dominant industrial approach for EPDM seal materials requiring low extractables and low compression set.
The filler system plays a direct role in network stability because the filler surface can adsorb curatives, antioxidants, and low-molecular-weight species. Carbon black grades with high structure and moderate surface area, such as N550 and N774, provide good dispersion and lower compression set than larger-particle thermal blacks because the high bound-rubber network supports the crosslinked matrix. Excessively small-particle high-surface-area blacks such as N110 or N220 increase viscosity and make plasticizer-free processing difficult without providing a proportional improvement in compression set. The filler loading is typically kept between 50 phr and 90 phr in plasticizer-free detergent-contact compounds, with the exact level adjusted to hit the required hardness and modulus. Mineral fillers such as calcined clay can be used as partial replacements for carbon black, but they may introduce more bound moisture and require additional drying before mixing if the relative humidity exceeds 60 %. Calcium carbonate is not normally used in high-performance detergent seals because it can be attacked by acidic detergent residues and because its coarse particle size does not provide the same reinforcement. The selection of filler therefore influences not only dry compression set but also the later extraction behavior and surface cleanliness of the aged part.
In formulations intended for low compression set, the conventional use of 20 phr to 40 phr of paraffinic or naphthenic oil creates a direct conflict with long-term detergent resistance. Paraffinic oil is compatible with the EPDM matrix at low service temperatures, but the oil is not chemically bound and can be extracted by surfactant solutions and hot alkaline water. The extraction rate depends on oil viscosity, oil loading, surfactant concentration, and temperature. Low-viscosity oils below approximately 20 mm²/s at 100 °C are more readily extracted than high-viscosity oils above 100 mm²/s at 100 °C. However, even high-viscosity paraffinic oils can be lost gradually under repeated wet/dry cycling, leading to surface oil bloom on drying because the oil is carried to the surface by water vapor and deposited as a hydrophobic film. The least extractable approach for detergent service is to reduce external plasticizer loading to 0 phr or to a maximum of 5 phr of a very high-viscosity mineral oil that remains below the solubility limit of the matrix and is not expected to bloom under thermal cycling. But this reduction severely increases compound viscosity and requires the polymer/filler system to provide all processability. The resulting compounds commonly display Shore A hardness values from 65 to 85, whereas oil-extended versions may be 45 to 60 Shore A. The harder compound can still seal effectively in static gaskets and square cross-section O-rings, but it may not be suitable for dynamic lip seals that require low modulus to follow shaft runout. This operational boundary must be evaluated before removing plasticizer from an existing sealing part.
One alternative to external ester plasticizers is the use of a liquid EPDM or liquid polybutadiene co-curing diluent that participates in the peroxide crosslink reaction and becomes part of the network. If the liquid diluent is fully reacted into the matrix, it cannot bloom or be extracted as a distinct plasticizer phase. However, complete conversion is difficult to guarantee, and the use of liquid rubber can reduce thermal stability and increase compression set because the liquid oligomer introduces additional chain ends and may create a more heterogeneous network. Published data for this specific configuration in detergent service is limited, so the approach is normally validated by extraction tests before production release. Another alternative is the use of very high-viscosity paraffinic oil at 2 phr to 5 phr, not to plasticize the compound but to act as a bound-rubber wetting agent during mixing. At these low addition levels, the oil is largely adsorbed onto the carbon black surface and is less available for extraction, although it cannot be regarded as completely non-migratory. The formulator must verify by total extractables testing that the final vulcanizate does not lose more than 2 % mass after extraction in boiling water or in a detergent solution under ASTM D471-16a conditions. If the mass loss exceeds this threshold, adjustments to the cure system, post-cure cycle, or filler selection are required.
Antioxidant selection is similarly constrained in peroxide-cured EPDM. Hindered phenolic antioxidants and thioether synergists can provide oxidative stability without the strong radical-scavenging interference associated with aromatic amine antioxidants. Aromatic amines are known to interfere with peroxide crosslinking by consuming free radicals during cure, reducing crosslink density and degrading compression set. Therefore, the combination of amine-based antidegradants and peroxide cure systems is identified as an incompatibility in this compound design. Zinc oxide at 3 phr to 5 phr and stearic acid at 0.5 phr to 1.5 phr are often retained in peroxide-cured EPDM to act as acid acceptors and to improve filler dispersion, although zinc oxide can promote ionic crosslinking and should be evaluated for its effect on compression set. The use of fatty acid process aids must be limited because these amphiphilic molecules can migrate to the surface and are readily removed by alkaline detergents. When processing aids are necessary, high-molecular-weight hydrocarbon waxes or non-migratory internal lubricants are preferred, but their effect on surface cleanliness must be confirmed under repeated detergent cycling. The principle throughout is that any ingredient that is not chemically bound into the network is a potential extractable that can produce bloom, surface tack, or dimensional change.
Peroxide cure systems are selected for detergent-contact EPDM because they generate stable carbon-carbon crosslinks and leave fewer extractable organic residues after post-cure. The most common peroxide for EPDM is dicumyl peroxide or di(tert-butylperoxyisopropyl)benzene, with typical active peroxide loadings from 2 phr to 8 phr of a 40 % active supported grade, depending on polymer diene content and filler type. The peroxide decomposition temperature must be reached during molding, usually 170 °C to 190 °C, to generate the free radicals that abstract allylic hydrogen from the diene termonomer and form crosslinks. Coagents such as trimethylolpropane trimethacrylate at 0.5 phr to 2.5 phr or triallyl isocyanurate at 0.3 phr to 1.5 phr are used to increase crosslink efficiency and to reduce the formation of chain-scission products from the propylene segments. The coagent level must be optimized because too little coagent leaves unreacted peroxide and reduces crosslink density, while too much coagent creates a tightly crosslinked but brittle network with poor elongation and fatigue resistance. Under-cure is particularly damaging in detergent service because residual unsaturation and unreacted low-molecular-weight species remain available for oxidation and extraction. The state of cure is confirmed by moving die rheometer torque values and by physical testing before release. A typical cure curve at 180 °C on a moving die rheometer should reach at least 90 % of maximum torque within 5 min to 12 min, depending on part thickness and heat transfer.
When a peroxide cure system is used, the compound must be mixed and processed below the peroxide decomposition temperature to prevent scorch. A typical mixing sequence in an internal mixer loads the EPDM gum and carbon black first, with ram pressure applied until the compound reaches 110 °C to 130 °C. The batch is then dropped and sheeted on a two-roll mill or fed to a second-stage internal mixer at a controlled temperature below 100 °C, where the peroxide and coagent are added. The mill or mixer temperature is maintained below 100 °C to ensure a safe scorch margin, but the temperature must be high enough to disperse the solid peroxide. If the compound temperature exceeds 110 °C during the second stage, partial peroxide decomposition can occur prematurely, creating scorched particles that act as flaws in the molded seal. Scorch is detected by an increase in compound viscosity and the appearance of small hard particles on the mill sheet. In production-scale mixing, batch-to-batch variation in rotor speed and ram pressure can cause local overheating even when the bulk discharge temperature is within limits. Therefore, the peroxide addition step is often performed on a two-roll mill with temperature-controlled rolls at 50 °C to 70 °C, and the peroxide is added slowly to prevent agglomeration. The compounded stock is then cooled rapidly to 25 °C to 35 °C and stored in a cool, dust-free environment. If the stock is stored for more than 24 h before molding, it is usually rechecked for scorch time and Mooney viscosity.
Molding conditions for peroxide-cured EPDM seals are governed by the need to reach full cure without generating excessive flash or voiding. Compression molding of rubber gaskets uses press temperatures from 170 °C to 190 °C and curing times from 3 min to 10 min for parts up to 6 mm thick. Thicker parts require long cure times because EPDM has low thermal conductivity and the peroxide cure reaction is temperature-dependent. Injection molding of EPDM uses cold runner systems to prevent scorching in the runner and sprue. Mold temperatures are typically 180 °C to 200 °C, with injection pressures sufficient to fill the cavity before scorch occurs. Clamp force requirements vary with projected area and cavity count, but production equipment for EPDM seal manufacturing commonly specifies clamp force capacities in the range of 0.5 kN/cm² to 0.8 kN/cm² of projected part area. The viscosity of plasticizer-free compounds requires higher injection pressures and larger gates than oil-extended compounds, and the increased shear heating raises the stock temperature near the gate. This limits the allowable injection speed and can reduce process windows to only ±10 °C on mold temperature and ±0.5 s on injection time for thin-walled parts. Such narrow process windows are a direct consequence of removing plasticizer, and they must be managed by maintaining strict control of mold temperature, screw speed, and back pressure. If the process drifts outside the allowed range, scorch or incomplete fill occurs, and the resulting parts may show internal flow lines that reduce compression set and create preferential paths for detergent ingress.
Post-cure is mandatory for peroxide-cured EPDM in this service. After demolding, the parts are placed in a convective oven at 150 °C to 180 °C for 2 h to 4 h, depending on cross-section thickness. The post-cure step removes residual peroxide decomposition products, completes coagent reactions, and stabilizes the compression set properties. Without post-cure, low-molecular-weight residues can migrate to the surface during the first hot detergent cycle and produce a visible film or tacky deposit. The post-cure temperature should not exceed the onset of severe autoxidation, typically above 180 °C for EPDM, and the oven must have adequate airflow to remove volatiles. Post-cure times that are too short leave extractable residues, while excessively long post-cure can cause surface oxidation and discoloration. The optimum cycle is confirmed by measuring the total organic extractables of the vulcanizate and by checking compression set after post-cure. A properly post-cured peroxide EPDM part should show less than 1.5 % total extractables in hot water or detergent solution, although the exact limit depends on the specification. This low extractable level distinguishes peroxide-cured EPDM from many sulfur-cured EPDM and from plasticized compounds, and it is a key requirement for clean service in repeated detergent exposure.
Processability constraints become acute when plasticizer content is reduced below 5 phr. The compound viscosity under low shear can be two to three times higher than that of a conventional oil-extended EPDM, requiring internal mixers with higher torque capacity and more aggressive rotor geometries. Tangential internal mixers with fill factors of 0.70 to 0.80 and rotor speeds of 40 min⁻¹ to 60 min⁻¹ are common for first-stage mixing of high-viscosity EPDM. Intermeshing internal mixers provide better temperature control and dispersive mixing but may require longer cycles. The ram pressure must be sufficient to seat the batch and promote filler incorporation, but excessive ram pressure can force material into the ram seals and generate localized heating. Production experience indicates that high-molecular-weight EPDM batches can exhibit over-temperature alarms if the mixer speed is raised too quickly, particularly when the carbon black loading exceeds 75 phr. The resulting batch may show an acceptable Mooney viscosity but poor filler dispersion, which later compromises compression set and surface finish. To avoid this, the first-stage mixing is usually performed with the carbon black split into two additions, and the mixer temperature is allowed to rise to only 120 °C to 140 °C before discharge. Batch-to-batch variation of ±5 MU in the raw gum Mooney viscosity can require adjustment of the filler or oil addition, which is one reason why incoming EPDM polymers are often specified within a narrower viscosity window than standard grades.
Extrusion of plasticizer-free EPDM requires short extruder screws with low compression ratios and controlled feed-section temperatures. A cold-feed extruder with a barrel length-to-diameter ratio of 16:1 to 20:1 and a screw compression ratio of 1.2:1 to 1.6:1 is generally used for rubber profiles and gasket blanks. The feed zone is maintained at 40 °C to 60 °C, and the die temperature is set at 80 °C to 100 °C. Because the compound is stiff, the die swell is lower than that of oil-extended EPDM, but the extrusion backpressure can exceed 20 MPa at normal throughput. If the stock temperature rises above 110 °C during extrusion, the material may scorch and produce hard particles or surface roughness. In continuous profiles, vacuum venting is used to remove trapped air and moisture because plasticizer-free high-filler compounds have a tendency to entrain air during mixing. Without vacuum venting, porosity can appear in the extrudate and lower the tensile strength and compression set of the final seal. The extrudate is cooled rapidly in a water bath or on a cooling conveyor to prevent scorch during storage. Production-scale extrusions of high-hardness EPDM often show a rough surface if the die temperature is too low or the screw speed is too high, so die design and temperature control are critical. Surface roughness can create microchannels that retain detergent residues, so it is not merely a cosmetic issue.
Storage conditions for plasticizer-free EPDM compounds are less sensitive to oil migration but remain sensitive to moisture absorption and pre-scorch. Carbon black and mineral fillers can absorb moisture when stored in an environment exceeding 60 % relative humidity, and this moisture can cause porosity during molding and reduce tensile properties. The polymer itself does not hydrolyze, but moisture in the filler or on the pellets can generate steam at cure temperatures and create internal voids. Therefore, fillers are preferably stored in closed silos or airtight containers, and the compound is kept in polyethylene film to limit moisture uptake. If the compound has been exposed to high humidity, predrying at 50 °C to 60 °C for 2 h to 4 h is used before molding, but the temperature must remain below the peroxide decomposition threshold. The compound should not be stored near sources of amines or sulfur because cross-contamination can alter peroxide cure behavior. Even small amounts of sulfur can modify the crosslink network and reduce high-temperature compression set, while amines can consume peroxide radicals. These incompatibilities are often overlooked in facilities where both EPDM and sulfur-cured compounds are processed on the same equipment, and they must be managed by thorough equipment cleaning and by using dedicated storage areas for peroxide-cured stock.
Plasticizer migration from an EPDM matrix is governed by the solubility parameter difference between the plasticizer and the polymer, the molecular weight of the plasticizer, the temperature, and the presence of surfactant micelles in the contacting liquid. EPDM has a Hildebrand solubility parameter in the range of 16 MPa1/2 to 17 MPa1/2, while common ester plasticizers such as dioctyl phthalate have solubility parameters above 18 MPa1/2 to 19 MPa1/2. This partial incompatibility promotes migration of ester plasticizers to the surface even in dry aging. In aqueous detergent solutions, the situation is more severe because the external phase contains amphiphilic surfactants that can swell the rubber surface and solubilize the plasticizer into micelles. Once the plasticizer reaches the surface, it is emulsified and removed, creating a concentration gradient that drives further diffusion from the bulk. The apparent diffusion coefficient increases with temperature, and repeated wet/dry cycling accelerates the migration because the drying phase deposits extracted plasticizer as a surface film that is then re-emulsified on the next wet cycle. Paraffinic oils are more compatible with EPDM but are also susceptible to micellar extraction, particularly if the oil viscosity is low and the detergent concentration is high. The extracted oil can form a hydrophobic bloom on the air-exposed surface during drying, leaving a tacky residue that attracts dust and organic debris. Plasticizer-free EPDM compounds avoid these transport mechanisms entirely because they contain no separate plasticizer phase. However, low-molecular-weight fractions of the polymer, such as oligomers or unreacted processing aids, can still migrate and produce a thin film if the compound is not properly selected and post-cured.
Alkaline detergent service also introduces the possibility of chemical transformation of plasticizer and processing aid residues. Ester plasticizers are subject to saponification in hot alkaline solution, producing alcohol and carboxylic acid salt products that are water-soluble or surface-active. This reaction can accelerate deterioration of the rubber surface and create ionic species that alter the seal interface. Phthalate and adipate esters are particularly sensitive to alkaline hydrolysis. Paraffinic oil is not hydrolyzable, but it can undergo autoxidation at elevated temperature when exposed to oxygen and metal ions, leading to polar oxidation products that are more water-sensitive. In EPDM compounds that use fatty acid processing aids, the alkaline detergent neutralizes the fatty acid to form soap, which is an efficient blooming and foaming species. These chemical transformations explain why removing plasticizers and migratory processing aids is more effective than simply increasing polymer molecular weight or adding bloom suppressants. The clean surface requirement also applies to sulfur-cured EPDM because zinc accelerator residues and sulfur bloom can react with detergent components and produce visible films. Peroxide-cured, plasticizer-free EPDM minimizes the chemical species available for such reactions, but it is still necessary to verify that the peroxide and coagent residues have been removed by post-cure. For this reason, total extractables testing is part of the quality control system for detergent-contact compounds.
Hardening under repeated detergent immersion is the direct result of plasticizer loss, oxidation of residual unsaturation, and continued crosslinking or chain scission in the rubber network. A plasticizer-free peroxide-cured EPDM may show a hardness increase of only 1 to 3 Shore A units after hundreds of cycles, whereas an oil-extended compound can increase by 8 to 15 Shore A units under the same conditions, based on published industry data for hot water and detergent aging. The low-hardness shift is important for maintaining seal contact pressure because the seal interface relies on the initial compression modulus. If the rubber hardens, the sealing force may increase but the ability of the seal to conform to surface roughness decreases, potentially causing leakage under low-temperature or low-pressure service. The absence of plasticizer also reduces the risk of volume shrinkage, which can create a gap between the seal and the gland. In a static seal, volume shrinkage of 2 % to 5 % is often sufficient to produce a leak path. Therefore, the performance requirement in repeated detergent service is not only low compression set but also low volume change and minimal hardness drift after exposure. These three properties are measured together in a formal test plan, because a compound that passes compression set alone may still fail in service if it shrinks or hardens excessively after plasticizer extraction.
The compliance matrix for plasticizer-free, low-compression-set EPDM in detergent service combines dry compression set testing with liquid immersion aging and surface analysis. Dry compression set is measured according to ASTM D395-18 Method B with 25 % constant deflection and aging at 70 h at 125 °C for hot service approvals. Medium-temperature applications may use 22 h at 70 °C or 70 h at 100 °C as specified in ISO 815-1:2019. Liquid immersion is performed according to ASTM D471-16a or ISO 1817:2022 using a reference detergent solution maintained at 60 °C to 80 °C for 70 h to 168 h. Volume change, mass change, hardness change, and tensile property retention are recorded at the end of the exposure period. For repeated service simulation, the immersion is often followed by a drying step at 50 °C for 24 h to create a wet/dry cycle that promotes surface migration. After 10 such cycles, the specimen is inspected for surface bloom and re-tested for compression set to detect any degradation that did not appear in the single-cycle immersion test. The repeated-cycle protocol is not a single universal standard but is built from the immersion conditions of ASTM D471-16a and the compression set procedure of ASTM D395-18 Method B, and it is often used in OEM approval programs for detergent-contact rubber parts. Published data for this specific multi-cycle sequence are limited, so acceptance criteria are usually derived from field requirements rather than from a standard limit.
Surface cleanliness is confirmed by visual inspection under 10× magnification and by reflectance or ATR-FTIR spectroscopy. A plasticizer bloom typically appears as a greasy film, haze, or white crystalline deposit that can be wiped off with a dry cloth. If the deposit reappears after wiping and further aging, it indicates continuous migration from the bulk. ATR-FTIR analysis of the surface before and after extraction can detect ester carbonyl absorbance near 1735 cm⁻¹ for phthalate or adipate plasticizer residues or carboxylate absorbance near 1560 cm⁻¹ to 1610 cm⁻¹ for soap formation from fatty acid process aids. The absence of these bands in the aged sample supports the claim of no plasticizer bloom. Hardness is measured with a Shore A durometer according to ASTM D2240-15e1 or ISO 48-4:2018, and tensile properties are measured according to ASTM D412-16 or ISO 37:2017. Compression set specimens are cut from molded plaques or O-ring cross-sections, and the testing laboratory must control specimen thickness, platen parallelism, and oven temperature uniformity because compression set is sensitive to these variables. The following table summarizes the core test matrix used to qualify EPDM compounds for repeated detergent service without plasticizer bloom.
| Property | Standard | Conditions | Typical acceptance criterion |
|---|---|---|---|
| Compression set | ASTM D395-18 Method B / ISO 815-1:2019 | 25 % constant deflection; 70 h at 125 °C | ≤ 20 % for hot detergent service |
| Immersion volume change | ASTM D471-16a / ISO 1817:2022 | 70 h at 80 °C in reference detergent solution | − 3 % to + 5 % |
| Immersion hardness change | ASTM D471-16a / ASTM D2240-15e1 | Post-immersion Shore A change | ≤ + 5 Shore A units |
| Tensile retention | ASTM D412-16 / ISO 37:2017 | Post-immersion tensile strength retention | ≥ 80 % |
| Elongation retention | ASTM D412-16 / ISO 37:2017 | Post-immersion elongation retention | ≥ 80 % |
| Surface bloom | Internal method / visual at 10× | 10 cycles of 24 h wet / 24 h dry at 50 °C | No visible film, haze, or crystalline deposit |
Although EPDM's saturated ethylene-propylene backbone resists hydrolysis in alkaline media, residual diene unsaturation remains susceptible to oxidative attack by oxygen-based detergents and chlorinated bleach. Continuous exposure to sodium hypochlorite at concentrations above 10 ppm free chlorine is generally not recommended for EPDM parts in this service because chlorination of the diene unsaturation leads to surface cracking and compression set degradation. Published data for the specific interaction between EPDM grades and modern detergent chlorine systems is limited, so each formulation must be tested under the actual end-use chemical environment. The operational pH range for plasticizer-free EPDM seals is typically 8 to 12 for intermittent service and 9 to 11 for continuous immersion, but the presence of strong oxidizers, solvents, or highly concentrated surfactant mixtures can reduce the service temperature and service life. If the detergent contains more than 2 % to 3 % of nonionic alcohol ethoxylates, extraction of any residual low-molecular-weight species is accelerated, and even plasticizer-free compounds may show surface tack if the post-cure is incomplete. Therefore, the post-cure cycle and total extractables limit should be more stringent when the seal is exposed to high-surfactant detergents or to repeated thermal cycling above 90 °C. The compound is not recommended for continuous exposure to strong mineral acids, chlorinated organic solvents, or aromatic hydrocarbons because these media can attack the EPDM backbone or swell the crosslinked network beyond acceptable limits. The use of ester plasticizers in this service is specifically excluded, and the combination of plasticizer-free EPDM with amine-based antidegradants in a peroxide cure system is an identified incompatibility. Mechanical design must also account for the higher modulus and lower elongation of plasticizer-free compounds. Sharp corners, excessive squeeze, and high strain concentrations can initiate cracking in dynamic seals, so the gland design should limit maximum compressive strain to 25 % and avoid tensile stresses at the seal surface. The recommendations in this document are limited to the stated operational boundaries and do not substitute for end-use validation.
