Technical Requirements for Inner Layer Rubber of Rubber Flexible Connectors

Let's Start with the Core Job of Inner Layer Rubber

The inner rubber layer is the first line of defense—it touches the medium directly, needing to:

Seal tightly without leaks
Resist corrosion from the medium (acid, oil, etc.)
Stand up to wear from flowing materials
Block medium from seeping through to the reinforcement layer

Key Physical Properties (Based on GB/T 26121-2010)

Tensile strength: At least 10MPa at room temp (needs to be 15MPa+ for high-temp jobs over 120℃)
Elongation at break: Over 300% normally (400%+ for vibrating pipes to prevent cracking)
Hardness (Shore A): 60±5 for standard use (70±5 for wear-resistant, 50±5 for cold -40℃)
Permanent deformation: Less than 20% after 100℃×24h (15% max for steam at 150℃)
Adhesion to reinforcement layer: Over 3N/mm (5N/mm+ in wet/hot environments)

Chemical Resistance by Medium Type

1. Oil-resistant inner rubber (for fuel/lubricants)

Material: NBR (nitrile rubber) + oil-resistant additives
Must-haves:
- After soaking in diesel at 23℃ for 70h: volume change ≤+10%, hardness change ≤±15
- With hydraulic oil (ISO VG46) at 100℃×70h: keeps 80%+ of tensile strength

2. Acid/alkali-resistant inner rubber (chemical plants)

Material: EPDM or FKM (fluororubber)
Key tests:
- 30% sulfuric acid at 23℃×72h: volume change ≤+5%, no cracks
- 40% sodium hydroxide at 70℃×72h: hardness change ≤±10

3. High-temp steam inner rubber (heating/food industry)

Material: Silicone rubber (VMQ) with peroxide curing
Must pass:
- 150℃ saturated steam for 100h: keeps 70%+ tensile strength, no bubbles
- Meets food safety standard GB 4806.11-2016

Production & Design Tips

Vulcanization control: Crosslink density needs to hit 2.5×10⁻⁴mol/cm³ (tested by DSC)
Thickness rules: For DN≤100mm, thickness ≥3mm (tolerance ±0.3mm)
Surface quality: Small bubbles (≤1mm) allowed, max 2 per 10cm²; inner surface smooth (Ra≤12.5μm)

Extra Requirements for Tough Conditions

Scenario	Special test	How to test
Drinking water	CJ/T 170-2018 certification	Heavy metals ≤0.1mg/L
Gas pipelines	Gas permeability ≤10⁻⁷cm³/(cm²·s·Pa)	GB/T 1038-2000
LNG ( -196℃)	No cracks after -196℃×24h	GB/T 1682-2014
Coal powder	Abrasion loss ≤150mm³ (Akron test)	GB/T 9867-2008

Quick Material Selection Guide

Medium	Best inner rubber	Temp range	Where it's used
Water/cold liquids	EPDM	-30℃~120℃	AC systems, fire pipes
Oil/ gasoline	NBR	-20℃~100℃	Engine oil lines, hydraulics
Strong acids/alkalis	FKM	-10℃~200℃	Chemical reactor connections
Food-grade fluids	Silicone rubber	-60℃~200℃	Beverage lines, pharma equipment
Compressed air	CR (neoprene)	-20℃~80℃	Pneumatic systems, air compressors

Bottom line: The inner rubber makes or breaks the connector's lifespan. When choosing, check it matches your medium, temp, and pressure. Not sure which material fits? Just ask—we're here to help without the tech jargon!