Silicone O-Ring Hardness Increase After 200°C Aging: Check Test Conditions, Cure, Volatiles or Filler First?

A marked hardness increase after 200°C hot-air aging cannot be assigned to silicone gum, silica or the cure system as a single cause. First determine whether elongation and rebound fall, mass is lost, compression set rises, shrinkage occurs or cracks form. Then investigate in the order: test conditions, cure state, volatile components, filler system, gum structure and actual service medium.

Under-cured parts may continue crosslinking during aging. Loss of low-molecular species or processing additives may cause shrinkage and hardening. Silica surface condition, dispersion and polymer interaction can further change modulus. Reducing silica alone is therefore not a sufficient solution.

Which changes commonly accompany increasing hardness?

Hardness measures resistance to indentation. Sealing also depends on rebound, compression set, tensile and tear properties, dimensional stability and medium resistance. Use Table 1 to interpret combined changes.

Why verify the aging conditions first?

· Two tests both described as “200°C aging” are not comparable if time, medium, specimen, oven or measurement conditions differ (Table 2).

· ASTM D573 evaluates the effect of elevated air temperature on vulcanized rubber, but accelerated results cannot be directly converted into service life. Hardness scale, specimen thickness, test temperature and reading time must also be fixed.

How can silicone gum affect aged hardness?

· The gum defines the backbone, side groups, vinyl or other reactive groups and initial molecular weight, but must be evaluated in the full compound. Molecular weight and distribution affect mixing and network formation; reactive groups affect cure; low molecules affect mass loss; batch variation affects filler bonding and cure response.

· A 200°C requirement does not automatically require phenyl silicone rubber. Suitability also depends on low temperature, medium, rebound, cost and processing. More phenyl content cannot replace full-compound validation.

· IOTA can assist with comparisons among methyl vinyl silicone rubber, phenyl gum and phenyl silicone rubber. The route depends on continuous and peak temperature, medium, hardness and seal design.

Why is silica loading not the only factor?

· Silica affects hardness, tensile, tear, rheology, rebound and processing. Check grade, surface area, silanol/treatment, dispersion, moisture and impurities, structure-control agent, mixing temperature and addition order.

· Lower loading may lower hardness but may also reduce reinforcement, tear strength or dimensional stability.

Why can the cure system cause further hardening?

· Insufficient primary cure can continue at 200°C.

· Inconsistent post-cure temperature, time, ventilation or cooling changes starting properties and subsequent drift.

· Peroxide type, amount, decomposition and by-product removal affect odor, mass and mechanics.

· Addition-cure systems require control of vinyl/Si-H ratio, catalyst, inhibitor and contamination; a formed surface does not prove complete cure.

· Excessive crosslink density raises hardness and modulus while reducing elongation and elastic reserve.

How can gum, silica and cure effects be separated?

Use small, single-variable comparisons. Keep raw-material batch, mixing and forming constant, include replicates, and compare the groups in Table 3.

Which additional seal properties should be tested?

When the field failure is leakage, compression set and a sealing test in the actual assembled state are usually more relevant than hardness alone. See Table 4.

Which service conditions must be confirmed?

Confirm continuous/peak temperature and duration; air, enclosure, vacuum, steam or other environment; oils, fuels, solvents, acids, alkalis and cleaners; O-ring section, compression and groove; static/dynamic sealing; allowable hardness drift; tensile, elongation and compression-set requirements; primary/post-cure; failure mode; customer and industry standards. Do not select only from the label “200°C silicone rubber.”

Recommended validation steps

· Record initial hardness, tensile, elongation, mass and dimensions for the same batch. Review mixing batch, raw-material batches, cure curve and post-cure record. Age in standardized 200°C hot air, condition specimens consistently and repeat all measurements.

· Test compression set or aging under actual compression; inspect surface and cross-section for cracks, bubbles and local hardening; run single-variable gum, silica and cure comparisons; retest in the actual medium and temperature cycles; confirm multiple production batches before setting purchasing and process limits.

Common misconceptions

· Higher aged hardness means better heat resistance.

· Hardness increase always means too much silica.

· Reducing silica always solves the problem.

· Passing 200°C hot-air aging proves suitability for every 200°C seal.

· Compounds with the same initial hardness age the same way.

· Phenyl silicone rubber is automatically superior to methyl vinyl silicone at 200°C. None of these statements is valid without the other properties, medium, process and seal design.

Recommended diagnostic order

· Standardize aging conditions; relate hardness to mass, dimensions, elongation and compression set; verify primary and post-cure; review gum batch, reactive groups and low molecules; review silica type, loading, treatment and dispersion; check process additives and curatives; use single-variable controls; validate sealing under the actual medium, compression and temperature cycles.

· IOTA Silicone Oil (Anhui) Co., Ltd. can support screening of silicone gums, phenyl materials, silica and related compounding materials. Changing one raw material without supporting test data is not recommended.

Related product information

· For projects with defined requirements for hardness, elongation and compression set after 200°C thermal aging, review the following IOTA heat-resistant silicone rubber materials and validate them with the actual medium, cure system and seal design:

· IOTA-34 Phenyl Silicone Gum HTV

· IOTA HCR 2980 U

· IOTA HCR 2950 U

FAQ

Is a hardness increase after 200°C aging normal?

Some change may occur, but acceptance depends on aging time, method and product specification. Check elongation, mass and compression set at the same time.

Why does under-cure raise aged hardness?

The part may continue crosslinking during subsequent 200°C aging, increasing hardness and modulus.

Does more silica always cause a greater hardness increase?

No. Silica type, surface treatment, dispersion, structure-control agent, gum and cure network also matter.

Can Shore A hardness alone determine seal failure?

No. Compression set, rebound, dimensions, cracks, medium swelling and assembly design also control sealing.

Can 200°C hot-air aging replace hot-oil aging?

No. Oxidation, swelling, extraction and medium interactions differ; test the actual contact medium.

Does a longer post-cure always improve stability?

No. Follow the compound and process requirements; excessive temperature or time can also change properties.

Must a 200°C O-ring use phenyl silicone rubber?

No. Low-temperature needs, medium, mechanics and processing also determine the material route.