ASTM C509

Type I: Standard Ozone Resistance

Type I gaskets resist ozone at 100 pphm (parts per hundred million). Use for interior applications, protected exterior locations, or areas with minimal air pollution and UV exposure.

• Ozone test: 100 pphm for 70 hours at 40°C, no cracking
• Low-temp brittleness: No failure at -26°C
• Compression set: ≤35% after 22 hours at 70°C
• Water absorption: ≤20% by volume
• Suitable for: Protected facades, interior glazing, low-UV areas

Type II: Enhanced Ozone Resistance

Type II gaskets withstand ozone at 300 pphm - three times Type I resistance. Specify for direct sun exposure, coastal environments, high-altitude locations, or areas with elevated smog levels.

• Ozone test: 300 pphm for 70 hours at 40°C, no cracking
• Low-temp brittleness: No failure at -40°C (14°F colder than Type I)
• Same compression set and water absorption as Type I
• Required for: Direct UV exposure, coastal salt air, urban pollution
• Longer service life in harsh environments (15-20 years vs 10-15 for Type I)

Physical Properties (Both Types)

• Hardness range: 15-60 Shore A durometer
• Tensile strength: Varies by formulation, typically 200-400 psi
• Elongation: Minimum 100% typical
• Heat aging: 70 hours at 70°C, properties retained within limits
• Density: Cellular structure provides flexibility and recovery

Option I: Flame Propagation

Optional requirement per ASTM C1166 for projects with heightened fire safety needs. Testing measures flame spread and heat contribution. Specify when required by local fire codes or for high-rise construction, hospitals, schools, and assembly occupancies.

Surface Preparation

• Clean all surfaces: Remove dust, oil, old gasket residue, and release agents
• Dry thoroughly: Moisture prevents proper adhesion and gasket seating
• Inspect for damage: Burrs, sharp edges, or corrosion compromise the seal
• Prime if required: Some substrates need primer for gasket adhesion

Compression Guidelines

Proper compression is critical. Too little compression leaks. Too much causes excessive permanent set and reduces service life.

• Target compression: 25-50% of original gasket thickness
• Minimum compression: 25% to ensure weathertight seal
• Maximum compression: 50% to prevent over-stressing the material
• Example: 1/4" gasket compressed to 3/16" (25%) or 1/8" (50%)
• Verify with manufacturer: Some formulations have specific limits

Corner Details

• Miter joints: Cut at 45° for clean corners, apply adhesive to joint faces
• Vulcanized corners: Factory-molded corners eliminate field joints
• Overlap joints: Not recommended - creates stress concentration points
• Gap at corners: Leave 1/16" gap, fill with compatible sealant if needed

Common Installation Errors

• Stretching gasket during installation: Causes pull-back from corners
• Contaminated surfaces: Oil or silicone residue prevents adhesion
• Inconsistent compression: Wavy glass-to-frame gaps create leaks
• Wrong gasket size: Oversized gaskets buckle, undersized gaskets leak
• Incompatible adhesive: Some adhesives attack rubber, causing deterioration

When to Specify Type II

Type II costs 15-30% more than Type I but lasts significantly longer in harsh environments. Specify Type II for:

• Exterior facades with direct sun exposure (south and west orientations)
• Coastal locations: Salt air accelerates ozone attack
• High-altitude sites: UV intensity increases 10-12% per 1,000 meters
• Urban environments: Smog, exhaust fumes, industrial emissions
• Hot climates: Arizona, Nevada, Texas - extreme heat + UV combination
• Critical facilities: Hospitals, data centers, cleanrooms where replacement is costly

Type I is Adequate For

• Interior glazing: Atriums, interior partitions, interior lite doors
• Protected exterior: North-facing facades, deep overhangs, covered walkways
• Moderate climates: Pacific Northwest, Great Lakes region
• Budget-sensitive projects: Where upfront cost is critical factor
• Short-term installations: Temporary structures, exhibits

Hardness Selection

Softer gaskets (15-30 Shore A) compress easily but have less recovery force. Use for irregular surfaces or where compression force is limited. Harder gaskets (40-60 Shore A) provide better long-term compression set but require more clamping force. Match hardness to frame stiffness and surface flatness.

Compression Set

Measures permanent deformation after compression. Test specimen compressed to 50% of original thickness, held at 70°C for 22 hours, then released. After 30 minutes recovery, compression set calculated:

Compression Set % = [(Original thickness - Final thickness) / (Original thickness - Compressed thickness)] × 100

Maximum allowable: 35%. Lower is better. Values above 35% indicate the gasket won't recover sufficiently between thermal cycles or building movements.

Ozone Resistance

• Type I: 100 pphm ozone concentration, 40°C chamber temperature, 70 hours exposure
• Type II: 300 pphm ozone concentration (3× higher), same temperature and duration
• Pass criteria: No cracks visible at 7× magnification
• Why it matters: Ozone attacks carbon-carbon double bonds in rubber, causing surface cracking
• Real-world correlation: Type II simulates 15-20 years coastal or urban exposure

Low-Temperature Brittleness

Test specimen bent around a mandrel at specified low temperature. Material must not crack or break:

• Type I: -26°C (-15°F) test temperature
• Type II: -40°C (-40°F) test temperature
• Critical for: Cold climates, ski resorts, Canadian projects
• Failure mode: Brittle fracture during thermal shock or wind-induced movement
• Design margin: Test 14°C below expected minimum service temperature

Heat Aging

Specimens aged 70 hours at 70°C (158°F). Tensile strength and elongation measured before and after aging. Properties must remain within acceptable limits, typically no more than 25% reduction. Simulates years of elevated temperature exposure in attics, south-facing walls, or hot climates.

Compression Set Failure (Permanent Deformation)

Symptoms: Gasket won't spring back to original thickness. Visible gap around glass. Water leaks during rain.

Causes:

• Over-compressed beyond 50% of original thickness
• Heat aging from prolonged sun exposure on dark-colored frames
• Wrong material specified (non-C509 material substituted)
• Incompatible glazing compound or sealant attacking rubber

Prevention: Limit compression to 50% maximum. Use Type II for hot climates. Verify material certification.

Ozone Cracking

Symptoms: Fine surface cracks perpendicular to stress direction. Cracks deepen over time, eventually penetrating full thickness. Black gasket turns gray or chalky at surface.

Causes:

• Type I material used in high-ozone environment
• Urban pollution, coastal exposure, high-altitude UV
• Gasket under constant tension (stretched during installation)
• Insufficient antioxidants or antiozonants in rubber formulation

Prevention: Specify Type II for exterior applications. Avoid stretching gasket during installation. Replace every 15-20 years as preventive maintenance.

Low-Temperature Failure

Symptoms: Gasket cracks or fractures during winter. Brittle texture in cold weather. Water leaks after freeze-thaw cycles.

Causes:

• Material brittleness point higher than ambient temperature
• Thermal shock from rapid temperature swings
• Building movement concentrated at stiff, cold gasket
• Wrong material for climate zone

Prevention: Use Type II for cold climates (-40°C rating vs -26°C for Type I). Allow for thermal movement in frame design.

Adhesion Failure

Symptoms: Gasket pulls away from frame. Corner joints separate. Wind-driven rain infiltrates.

Causes:

• Contaminated surfaces (oil, silicone, release agent)
• Incompatible or missing adhesive/primer
• Gasket stretched during installation, pulls back over time
• Inadequate compression force
• Frame movement exceeds gasket recovery capability

Prevention: Clean surfaces with isopropyl alcohol. Use manufacturer-recommended adhesive. Install without stretching. Maintain 25% minimum compression.

What's the difference between Type I and Type II?

Ozone resistance and low-temperature brittleness. Type I resists 100 pphm ozone and stays flexible to -26°C. Type II resists 300 pphm ozone (3× better) and works to -40°C. Type II costs 15-30% more but lasts longer in harsh UV, coastal, or high-altitude environments. Use Type II for direct sun exposure, urban smog, or coastal installations.

How much should I compress C509 gaskets?

25-50% of original thickness. Less than 25% may leak. More than 50% causes excessive permanent set and shortens service life. A 1/4" gasket should compress to between 3/16" (25% compression) and 1/8" (50% compression). Measure the glass-to-frame gap before selecting gasket size.

Can C509 gaskets be used in exterior applications?

Yes, but specify Type II for most exterior work. Type I degrades faster under direct UV and ozone exposure. Type II's 300 pphm ozone resistance matches 15-20 years of real-world exterior exposure in harsh environments. Protected exterior locations (north-facing, deep overhangs) can use Type I if budget is tight.

How long do C509 gaskets last?

Type I: 10-15 years in protected locations, 5-8 years with direct sun exposure. Type II: 15-20 years exterior, 20-25 years protected. Actual life depends on UV exposure, compression percentage, climate, and building movement. Coastal and high-altitude locations reduce life by 20-30%. Inspect gaskets during window maintenance - replace if surface cracking, permanent compression set, or hardening is visible.

What causes premature gasket failure?

Over-compression (beyond 50%), wrong type for exposure (Type I in harsh UV), contaminated installation surfaces, incompatible sealants or glazing compounds, stretching during installation, and excessive building movement. Failure shows as surface cracks, permanent deformation, loss of compression force, or separation from frame. Most failures occur within first 5 years if installation was improper or wrong material specified.

Are C509 gaskets compatible with silicone glazing?

Generally yes, but verify with manufacturer. Most cellular EPDM gaskets work with silicone structural glazing per ASTM C1193. However, some gasket adhesives or plasticizers can interfere with silicone cure. Request compatibility testing data when C509 gaskets contact structural silicone beads. Use barrier tape if compatibility is questionable.

Do I need flame-rated gaskets?

Only if required by local fire code or specified by architect. Flame propagation testing (Option I per ASTM C1166) is optional in C509. High-rise buildings, hospitals, schools, and assembly occupancies sometimes mandate flame-rated materials. Check with authority having jurisdiction. Flame-rated gaskets cost 10-20% more than standard material.

Can C509 gaskets be painted or coated?

Not recommended. Paint forms a stiff skin that cracks when gasket compresses and releases. Paint also may not adhere to cellular rubber surface. If color matching is critical, order gaskets in custom colors from manufacturer - many formulations available in black, gray, white, bronze, or custom colors. Field painting voids most warranties.

What's the difference between C509 and C864?

C509 is cellular (foam-like structure), C864 is dense solid rubber. C509 compresses easier, recovers better, and works well in glazing pockets with variable gaps. C864 resists extrusion under higher compression loads, used for expansion joints and applications requiring dense material. C509 is softer and more forgiving. C864 is tougher but requires more clamping force.

How do I specify C509 in construction documents?

"Glazing gaskets shall be preformed cellular elastomeric material conforming to ASTM C509, Type II, with optional flame propagation testing per ASTM C1166 (if required). Hardness: 30-45 Shore A. Color: black. Install per manufacturer instructions with 25-50% compression." Include submittal requirements for manufacturer certification and test reports. Specify adhesive/primer if required.