Ethylene Propylene Diene Monomer (EPDM) is a synthetic rubber widely used in automotive, construction, electrical, and industrial applications.
- EPDM combines excellent weather resistance, flexibility, and durability, making it a preferred choice for molded components. Transfer molding, being precise and suitable for high-volume production, enhances the utility of EPDM.
Definition of EPDM
- Full form: Ethylene Propylene Diene Monomer.
- Definition:
EPDM is a terpolymer made from ethylene, propylene, and a small amount of non-conjugated diene monomers. - Key property: The saturated polymer backbone provides resistance against heat, oxidation, ozone, and weathering, unlike natural rubber.
- Nature:
- Amorphous or semi-crystalline.
- Cross-linked during vulcanization to achieve desired elasticity.
- Relation to transfer molding: EPDM compounds can be pre-formed and molded under pressure into complex shapes with excellent dimensional stability.
Chemical Composition of EPDM
Main ingredients:
- Ethylene (45–75%) – Provides crystallinity and tensile strength.
- Propylene (25–55%) – Adds flexibility and processability.
- Diene (2–12%) – Provides unsaturation sites for sulfur or peroxide vulcanization. Common dienes:
- Ethylidene Norbornene (ENB) – Most widely used.
- Dicyclopentadiene (DCPD).
- Vinyl Norbornene (VNB).
Additives in EPDM compounds:
- Fillers (Carbon black, silica, CaCO₃).
- Plasticizers (paraffinic/naphthenic oils).
- Processing aids.
- Cross-linking agents (Sulfur, Peroxides).
- Accelerators (MBT, CBS).
- Anti-oxidants and anti-ozonants.
Manufacturing Process of EPDM Material
Step 1: Polymerization
- EPDM is produced by Ziegler–Natta catalysis or metallocene catalysis.
- Ethylene, propylene, and diene are polymerized in a solution or gas phase.
Step 2: Recovery
- Polymer slurry is separated from the catalyst and solvent.
- Residual catalyst is deactivated and removed.
Step 3: Finishing
- Polymer is dried, pelletized, and packaged for shipment.
Step 4: Compounding
- Raw EPDM gum is blended with fillers, plasticizers, and curing agents.
- Banbury mixers or two-roll mills are used.
Step 5: Pre-forming
- Compound is sheeted or pelletized for use in molding operations.
Shelf Life of EPDM
Uncured EPDM compound:
- Shelf life: 6–12 months if stored below 25°C and away from direct sunlight.
- Susceptible to premature vulcanization if improperly stored.
Cured EPDM products:
- Shelf life: 5–15 years depending on formulation.
- Resistant to ozone, UV, and weathering, giving long service life in outdoor applications.
Storage guidelines:
- Store in cool, dark, dry conditions.
- Avoid exposure to copper, manganese, or other curing-sensitive metals.
Properties of EPDM
Mechanical Properties
- Tensile Strength: 7–21 MPa (depending on formulation).
- Elongation at break: 300–600%.
- Hardness: 30–90 Shore A.
Thermal Properties
- Service temperature range: –50°C to +150°C (peroxide cured EPDM can withstand up to 180°C).
Chemical Resistance
- Excellent resistance to:
- Water, steam.
- Alkalis, acids (dilute).
- Polar solvents (alcohols, ketones).
- Poor resistance to:
- Hydrocarbons (oils, fuels).
- Aromatic and halogenated solvents.
Electrical Properties
- Excellent insulation.
- Used in high-voltage cable insulation.
Weathering Properties
- Resistant to ozone cracking, UV degradation, and aging.
Transfer Molding Process for EPDM
Step 1: Pre-form Preparation
- EPDM compound is pre-mixed and formed into slugs or pellets.
- Preforms must match the cavity volume to reduce wastage.
- Compression density: 1.2–1.4 g/cm³ depending on filler content.
Step 2: Loading into Transfer Pot
- Preform is placed into the transfer pot of the molding press.
- Pot is heated to soften EPDM before injection.
Step 3: Transfer to Cavities
- A plunger pushes softened EPDM into runner channels.
- Material flows into cavities under pressure (20–70 MPa).
Step 4: Vulcanization
- Molds are heated (170–200°C).
- Vulcanization occurs inside the mold cavity.
- Time cycle: 2–10 minutes depending on part thickness.
Step 5: Demolding
- Mold opens, ejector pins release the cured EPDM part.
- Flash is trimmed manually or automatically.
Step 6: Post-Curing (Optional)
- For peroxide-cured EPDM: post-curing at 150–200°C improves thermal stability.
EPDM vs Other Rubbers
| Property | EPDM | NBR (Nitrile) | SBR (Styrene-Butadiene) | Silicone (VMQ) | FKM (Viton) | NR (Natural Rubber) |
|---|---|---|---|---|---|---|
| Oil Resistance | Poor | Excellent | Poor | Moderate | Excellent | Poor |
| Weather Resistance | Excellent | Fair | Poor | Excellent | Excellent | Poor |
| Temp. Range | –50°C to 150°C | –30°C to 120°C | –40°C to 100°C | –60°C to 230°C | –20°C to 250°C | –50°C to 80°C |
| Ozone Resistance | Excellent | Poor | Poor | Excellent | Excellent | Poor |
| Cost | Low | Medium | Low | High | Very High | Low |
| Common Use | Seals, roofs, cables | Fuel hoses, gaskets | Tires | Medical, food | Aerospace, fuel systems | Tires, general rubber goods |
Advantages & Limitations of Transfer Molding with EPDM
Advantages:
- Produces complex shapes with inserts (e.g., metal-rubber bonded seals).
- Better dimensional accuracy than compression molding.
- Faster cycle times than compression.
- Suitable for medium to high volume production.
Limitations:
- More material wastage than injection molding (flash & transfer pad).
- Higher tooling cost compared to compression molding.
Application Areas of EPDM in Transfer Molding
Automotive Industry
- Door seals, weatherstrips, gaskets.
- Radiator hoses, brake components.
- Wire & cable insulation.
Construction Industry
- Roofing membranes.
- Window and door seals.
- Expansion joints.
Electrical & Electronics
- Cable jacketing.
- High-voltage insulation parts.
Industrial Applications
- O-rings, gaskets.
- Vibration dampers.
- Conveyor belts.
Consumer Goods
- Household appliance seals.
- Sports equipment grips.
Detailed EPDM Manufacturing & Compounding
1 Polymerization Techniques
- Solution Polymerization (most common): Hydrocarbon solvent, Ziegler-Natta catalyst.
- Slurry Process: Ethylene-propylene polymerization in hexane, then diene addition.
- Gas-Phase Polymerization: Advanced, eco-friendly, eliminates solvents.
2 Catalysts Used
- Ziegler-Natta (TiCl₄ + AlEt₃).
- Metallocene catalysts (better control of molecular weight).
3 Compounding Recipe Example (Automotive Seal)
- 100 phr EPDM gum.
- 80 phr Carbon black N550.
- 40 phr Paraffinic oil.
- 5 phr ZnO (activator).
- 1 phr Stearic acid.
- 2 phr Sulfur (crosslinking).
- 1.5 phr MBT accelerator.
- 1 phr TMQ antioxidant.
phr = parts per hundred rubber.
4 Key Factors in Compounding
- Ethylene content (low = flexible, high = stiff).
- Diene type (ENB gives faster cure rate).
- Filler loading controls hardness, strength, cost.
Pros and Cons of EPDM
Pros
- Excellent weather and ozone resistance.
- Long service life.
- Flexible at low temperatures.
- Good electrical insulation.
- Cost-effective compared to fluorocarbon rubbers.
- Compatible with peroxide curing for high-temperature stability.
Cons
- Poor resistance to petroleum-based oils and fuels.
- Lower tensile strength compared to some rubbers (e.g., NBR, FKM).
- Cannot be used in applications with direct hydrocarbon exposure.
- Processing challenges: requires precise mixing and vulcanization.
Conclusion
- EPDM is best for weather, ozone, and outdoor applications.
- It is not suitable for oil/fuel environments (NBR/FKM are better).
- Silicone beats EPDM in extreme high/low temperatures but at higher cost.
FAQ
What does EPDM stand for?
Ethylene Propylene Diene Monomer.
Is EPDM natural or synthetic rubber?
EPDM is a synthetic rubber.
Why is diene added in EPDM?
Diene provides unsaturation sites for vulcanization.
What is the typical hardness range of EPDM?
30–90 Shore A.
Can EPDM withstand oil and fuel?
No, EPDM has poor oil and fuel resistance.
What is the temperature range of EPDM?
–50°C to +150°C (up to 180°C with peroxide curing).
How long does EPDM last outdoors?
10–20 years depending on conditions.
Can EPDM be used for potable water applications?
Yes, with proper formulation and certification.
Is EPDM recyclable?
Yes, via mechanical and chemical recycling methods.
Why is EPDM used in roofing membranes?
Because of its excellent UV and weather resistance.
What is the difference between EPDM and NBR?
EPDM resists weathering but not oil; NBR resists oil but not weathering.
Can EPDM be colored?
Yes, pigments can be added during compounding.
Which curing system is better for EPDM – sulfur or peroxide?
Peroxide curing offers better heat resistance; sulfur is more economical.
Does EPDM absorb water?
Very low water absorption (<0.1%).
Is EPDM suitable for medical applications?
Not typically, but can be modified for specific uses.
Can EPDM be injection molded?
Yes, EPDM can also be processed via injection molding, but transfer molding is preferred for inserts.
Does EPDM float in water?
No, density is typically 0.9–1.3 g/cm³, so it usually sinks.
Is EPDM food-safe?
With FDA-approved formulations, EPDM can be food-grade.
Can EPDM be bonded to metals?
Yes, with appropriate primers and adhesives in transfer molding.
Why is ENB preferred over DCPD in EPDM?
ENB provides faster cure, better heat resistance.
Can EPDM be vulcanized without sulfur?
Yes, peroxide curing is common.
What gases can EPDM resist?
Steam, CO₂, nitrogen, but not hydrocarbons.
Is EPDM biodegradable?
No, but recycling is possible.
What colors are available in EPDM?
Black is common, but other colors possible with pigments.
Why does EPDM have long outdoor life?
Its saturated backbone resists ozone attack.
Can EPDM be used in brake fluid systems?
Yes, it is compatible with glycol-based fluids.
What is EPDM’s compression set?
Typically 10–40% depending on cure system.
Does EPDM resist acids?
Yes, dilute acids; not concentrated ones.
What happens if EPDM is exposed to oil?
It swells and degrades.
Can EPDM withstand freezing temperatures?
Yes, down to –50°C without cracking.
Is EPDM flammable?
Yes, burns slowly; flame-retardant grades available.
Can EPDM be painted?
Surface treatment needed for adhesion.
How does EPDM compare with Neoprene (CR)?
EPDM is better for weather; CR is better for oil.
Why is EPDM cheaper than Silicone?
Simpler polymerization, cheaper raw materials.
Does EPDM conduct electricity?
No, it is an insulator.