Articles
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Articles
Under sustained exposure at 150–175°C, AEM undergoes oxidative crosslinking and limited chain scission at the ethylene-acrylic backbone, with additional degradation at the ester carbonyl side groups in the presence of acidic intercooler condensate. Field measurements from intercooler drain samples in heavy-duty diesel engines have recorded pH values in the range 2.8–4.2, which is sufficient to accelerate hydrolysis of the acrylic ester linkages and generate a harder, less extensible sealing surface. The resulting failure mode is characterized by loss of elongation at break, an increase in low-strain storage modulus, and development of surface microcracks at the seal lip during thermal contraction after engine shutdown. Heat ageing per ASTM D573-04(2019) or ISO 188:2023 typically shows AEM tensile and elongation changes of −25% to −45% after 168 h at 150°C, while the same ageing condition tends to produce smaller elongation loss in addition-cure LSR because the siloxane backbone does not contain hydrolysable ester groups. In hot-side intercooler joints, this degradation is accelerated by simultaneous exposure to boost-pressure pulsation, aluminium flange fretting, and oil mist carryover from the turbocharger compressor seal; the AEM seal therefore transitions from elastic sealing to semi-brittle load bearing, leading to low-cycle fatigue cracks at the clamp-band edge. The degradation pattern is not uniform across the seal cross-section because the outer surface is in contact with hot aluminium and the inner surface is exposed to lower-temperature charged air; this produces a through-thickness hardness gradient that can be detected as a 2–5 Shore A increase at the outer skin after 500 h of engine testing.
| Property | Test method | Typical AEM range | Typical LSR range |
|---|---|---|---|
| Hardness, Shore A | ISO 48-4:2018 | 60–75 | 40–70 |
| Tensile strength | ISO 37:2017 | 10–16 MPa | 5–10 MPa |
| Elongation at break | ISO 37:2017 | 200–350% | 300–700% |
| Tear strength, Die C | ISO 34-1:2022 | 30–60 kN/m | 10–40 kN/m |
| Compression set, 22 h at 150°C | ISO 815-1:2019 | 20–40% | 8–20% |
| Low-temperature TR10 | ISO 2921:2019 | −25°C to −35°C | −50°C to −70°C |
| Continuous upper service temperature, 1000 h | ISO 188:2023 reference condition | 150–170°C | 200–230°C |
| Volume change after 72 h in IRM 903 at 150°C | ASTM D471-16a | +10% to +25% | +20% to +45% |
Metering, mixing, and mold filling behavior of addition-cure LSR differ fundamentally from compression or transfer molding of AEM. The LSR compound is supplied as two pumpable components with a viscosity of 100–700 Pa·s at 10 s⁻¹ and 25°C; these are transferred from pails or drums using servo-driven piston metering units, combined in a static mixer of 24–32 elements, and injected through a water-cooled runner block maintained at 15–20°C to prevent premature cure before the hot cavity. The mold itself is held at 150–200°C; at 175°C, a 2 mm thick LSR seal typically reaches a degree of cure sufficient for ejection in 60–120 s depending on part mass and thermal boundary conditions. Production-scale experience indicates that feed line temperature excursions above 25°C produce progressive viscosity increase and mixing-ratio drift because the platinum catalyst is thermally activated prior to the static mixer; this creates batch-to-batch variance in flash thickness and seal crosslink density. Vacuum-assisted mold venting at −0.8 bar to −0.95 bar relative is standard for intercooler seal geometries with peripheral o-ring profiles, because entrained air from static mixing otherwise accumulates at the parting line and creates porosity at the seal surface. The low injection pressure required for LSR, commonly 5–20 MPa, reduces clamp force demand compared with solid AEM compression molding, but the need for a closed cold-runner system and precision mixing makes the conversion capital-intensive despite lower material waste.
The polydimethylsiloxane backbone has a silicon-oxygen bond energy of approximately 444 kJ/mol, compared with 348 kJ/mol for a carbon-carbon single bond, giving it inherent resistance to oxidative chain scission at intercooler service temperatures. Addition-cure LSR is crosslinked by hydrosilylation between vinyl-functional polydimethylsiloxane and hydride-functional crosslinker, catalyzed by a platinum complex; this reaction produces no volatile byproducts and creates ethylene-bridge crosslinks that are thermally stable. The resulting network has a glass transition temperature below −120°C, so chain mobility is retained at engine cold-start temperatures, while the high crosslink density limits permanent set under compressive load. When tested per ISO 815-1:2019 for 22 h at 150°C, typical addition-cure LSR grades report compression set values of 8–20%, whereas AEM under identical conditions frequently measures 25–40%, particularly after prior heat ageing. This compression set advantage is temperature-dependent: at 175°C the LSR advantage narrows but remains measurable, and at continuous exposure above 200°C the ranking may reverse if the AEM is a high-heat-resistant grade and the LSR is not post-cured sufficiently. The lower compression set of LSR does not mean superior abrasion resistance; its tear strength and cut-growth resistance remain lower than those of AEM, so seal lip geometries must be redesigned to avoid sharp edges, high shear strain at the contact point, and installation damage.
| Qualification property | Test method | Condition | Acceptance range |
|---|---|---|---|
| Hardness change after ageing | ISO 48-4:2018 | 168 h at 175°C | ≤ +10 Shore A |
| Tensile strength change after ageing | ISO 37:2017 | 168 h at 175°C | ≤ ±30% |
| Elongation at break change after ageing | ISO 37:2017 | 168 h at 175°C | ≤ −30% |
| Compression set | ISO 815-1:2019 | 22 h at 150°C | ≤ 20% |
| Low-temperature brittleness | ISO 812:2017 | 3 min at −50°C | No crack |
| Volume change in IRM 903 | ASTM D471-16a | 72 h at 150°C | ≤ +30% for validation gate |
Although LSR has better thermal stability and compression set than AEM, its resistance to liquid hydrocarbon and blow-by oil mist is inferior. In continuous contact with turbocharger blow-by condensate, addition-cure LSR seals exhibit volume swell up to 20–45% after 72 h in IRM 903 at 150°C when tested per ASTM D471-16a; this swelling reduces contact stress and increases friction against the duct wall. Diesel oil droplets combined with soot can form carbonaceous deposits in the seal groove that erode the low-hardness LSR surface; this is particularly apparent on seals with Shore A hardness below 50. Water vapor permeability of LSR is also higher than that of AEM, so moisture can permeate through the seal body and condense on the aluminium flange, promoting corrosion under cyclic thermal conditions. The acid resistance of the siloxane backbone is generally adequate for intercooler condensate, but published data for direct AEM-to-LSR intercooler seal substitution under high-sulfur diesel condensate is limited, and qualification testing should include synthetic condensate with nitrate and sulfate concentrations representative of the specific engine platform. If continuous oil exposure produces volume swell above 10–15%, the seal maintenance interval must be reduced or a fluorosilicone grade with lower swell should be evaluated.
Addition-cure LSR is based on polydimethylsiloxane with a siloxane backbone that is inherently resistant to hydrolysis at low pH; laboratory immersion in synthetic intercooler condensate with pH 2.5 and nitrate/nitrite content of 50–200 ppm has shown no significant hydrolysis of siloxane bonds after 168 h at 90°C. The dominant LSR failure mode in this environment is not acid chain scission but swelling by oil mist and dynamic abrasion at the duct joint. Because LSR has a linear thermal expansion coefficient of 220–300 µm/m·K, a seal that occupies 70–80% of the gland at room temperature can lose 10–15% of its initial contact pressure when the flange reaches 150°C if the aluminium duct expands faster than the silicone. This condition is worsened by stick-slip movement at the seal lip during boost-pressure transients, which generates friction heat and can locally exceed the bulk flange temperature. Gland fill ratios and squeeze levels for LSR intercooler seals therefore require finite element analysis using hyperelastic material models calibrated to uniaxial, planar, and equibiaxial tensile data; the reduced modulus of a 40–50 Shore A LSR relative to a 65–75 Shore A AEM means that the LSR seal often needs 15–25% initial compression to achieve the same contact stress as an AEM seal at 10–15% compression. The lower hardness also influences clamp band torque because excessive clamping can extrude the silicone into the flange gap at high temperature, while insufficient clamping produces leak paths during cold-start boost spikes.
Tooling for LSR intercooler seals must be designed for tight flash control, vacuum venting, and low clamp force. A 4–16 cavity cold-runner mold with needle shut-off nozzles is typical for production volumes of 20,000–100,000 seals per year; the cold runner block is maintained at 15–20°C and the cavities are heated to 165–185°C with electric cartridge heaters and oil or water temperature control units. Vacuum channels at the parting line are specified at 0.02–0.05 mm depth to extract air without allowing silicone flash to fill the vent and block evacuation. Post-curing of demolded LSR seals at 200°C for 4 h is commonly required to complete addition cure, remove residual volatile cyclic siloxanes, and stabilize compression set; this operation is performed in forced-air ovens with stainless steel trays and must avoid contact with nitrile or PVC mats because plasticizer or sulfur contamination can inhibit the platinum catalyst and produce a tacky surface. In-process control for LSR intercooler seal production requires monitoring mix ratio by volume at 1:1 with an accuracy of ±1%, recording dynamic mixer pressure drop, and testing cure speed by moving-die rheometer per ISO 6502:2016 to verify that the curing profile remains within the validated envelope for each heat lot.
