NR GUM
Grade · default condition
GUM = gum / unfilled NR vulcanizate
Baseline unfilled sulfur-cured NR — highest elongation and resilience, used for rubber bands, gum sheeting, tubing and vibration springs.
Values for the GUM condition
Other NR grades and fills
NR is listed in 14 conditions; see the grade overview at NR.
Compare all 14 grades and fillsHide
| Grade · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness |
|---|---|---|---|---|---|
NR sponge / closed-cell foam | Blown NR for cushioning, sealing strips and anti-vibration pads where low density beats strength. | — | 2 | 200 | 25 Shore A |
TSR20 / SCR20 raw block rubber | Standard technically specified raw bale traded on SHFE/SICOM — the feedstock grade compounders buy. | — | 3 | 900 | 30 Shore A |
RSS3 ribbed smoked sheet | Visually graded smoked-sheet raw rubber, preferred for truck tire and high-green-strength compounds. | — | 3 | 900 | 32 Shore A |
Flame-retardant NR compound | ATH/halogen-loaded NR for mining conveyor belts, cable jackets and transit parts needing flame/smoke ratings. | — | 12 | 350 | 20 Shore D |
Shore A 90 | Near-rigid rubber for wear plates, mill liners and metal-bonded stiff mounts. | — | 17 | 230 | 38 Shore D |
Shore A 80 | High-modulus stock for bridge bearings, heavy-duty bushings and roll covers. | — | 20 | 350 | 30 Shore D |
Peroxide-cured NR | Choose when low compression set and better heat ageing matter more than tensile/tear. | — | 20 | 500 | 55 Shore A |
Shore A 40 | Soft, highly resilient stock for low-stiffness mounts, diaphragms and soft rollers. | — | 22 | 700 | 40 Shore A |
Synthetic cis-polyisoprene | Protein-free, consistent synthetic equivalent of NR for medical/latex-allergy and clean applications; slightly lower green strength and tear. | — | 22 | 700 | 50 Shore A |
Shore A 70 | Stiffer black stock for load-bearing pads, wheel treads and abrasion liners. | — | 23 | 450 | 22 Shore D |
GUMDefaultThis page Gum / unfilled NR vulcanizate | Baseline unfilled sulfur-cured NR — highest elongation and resilience, used for rubber bands, gum sheeting, tubing and vibration springs. | — | 24 | 750 | 40 Shore A |
Shore A 50 | Most common sheet/gasket hardness; best balance of tear, resilience and handling. | — | 25 | 620 | 50 Shore A |
Shore A 60 | Standard black-filled engineering grade for mounts, bushings and conveyor covers. | — | 26 | 550 | 15 Shore D |
Ebonite / hard rubber | 30–40 phr sulfur cure giving a rigid, chemically resistant lining material for tank and pipe linings and battery cases. | — | 50 | 5 | 85 Shore D |
What is NR GUM?
NR GUM is NR in the GUM grade — baseline unfilled sulfur-cured NR — highest elongation and resilience, used for rubber bands, gum sheeting, tubing and vibration springs. NR GUM has a tensile strength of 24 MPa (3.48 ksi), weaker than 68% of polymer grades. Elongation at break is 750% and the elastic modulus is 0 GPa (2.18×10⁻⁴ Msi). NR GUM has a density of 0.97 g/cm³ (0.035 lb/in³), lighter than 73% of polymer grades. Its glass-transition temperature is -70°C (-94 °F).
Advantages
- Ductile — tolerates forming and impact — 750%, better than 99% of polymer grades
- Forms and bends easily — 95/100, better than 99% of polymer grades
- Low embodied carbon — 1.9 kg CO₂/kg, better than 98% of polymer grades
Limitations
- Difficult to machine — 20/100, worse than 97% of polymer grades
- Low stiffness — 0 GPa (2.18×10⁻⁴ Msi), worse than 96% of polymer grades
- Degrades in UV without stabilisers — 20/100, worse than 88% of polymer grades
- High mold shrinkage — 2.2%, worse than 87% of polymer grades
- Absorbs moisture — dry before molding — 1%, worse than 84% of polymer grades
NR GUM properties
Typical room-temperature values for NR GUM; 17 are specific to this condition. Other listed values are NR reference data and may not represent this condition. Bars are the position among polymer grades.
Physical5
NR GUM has a density of 0.97 g/cm³ (0.035 lb/in³), lighter than 73% of polymer grades. Its glass-transition temperature is -70°C (-94 °F).
Mechanical10
NR GUM has a tensile strength of 24 MPa (3.48 ksi), weaker than 68% of polymer grades. Elongation at break is 750% and the elastic modulus is 0 GPa (2.18×10⁻⁴ Msi).
Thermal8
NR GUM is rated for continuous service to 80°C (176 °F). It conducts heat at 0.14 W/m·K (0.0809 BTU/hr·ft·°F), worse than 81% of polymer grades. Thermal expansion is 220 µm/m·K (122.2 µin/in·°F).
Electrical6
NR GUM is an electrical insulator with a dielectric strength of 22 kV/mm (558.8 V/mil) and a resistivity of 1×10¹⁴ Ω·m.
Chemical & Environmental3
Corrosion resistance is very good (70/100), worse than 78% of polymer grades. UV resistance is poor.
Sustainability4
Producing a kilogram of NR GUM takes about 65 MJ of energy and emits 1.9 kg of CO₂ — less than 95% of polymer grades. Typical recycled content is 5%.
Manufacturability6
NR GUM's machinability is poor (20/100, worse than 97% of polymer grades); weldability poor (15/100); formability excellent (95/100).
Common Calculations4
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with NR GUM
Is NR easy to machine?
Cured NR is die-cut, waterjet or abrasive-ground rather than turned; freeze-trim or cryogenic deflashing gives clean edges, and any turning needs razor-sharp tools with water cooling.
Can NR be welded?
Not weldable — parts are joined by uncured splicing plus press vulcanization, or by CR/cyanoacrylate adhesives and hot-bonded metal inserts (Chemlok primer systems).
Can NR be formed, bent or molded?
Mill-mixed or internally mixed then compression, transfer or injection molded and cured 5–20 min at 140–170 °C; watch reversion at high temperature and allow ~2 % mold shrink.
What surface finishes work on NR?
Surfaces replicate the mold; chlorination or plasma treatment reduces tack and friction, and rubber-to-metal bonding is done with two-coat phenolic primer/adhesive during cure.
NR chemical composition (wt %)
Limits by weight percent from the governing specification, written the way the spec states them — a single maximum for impurities, a range for alloying elements, and the base element as balance. Nominal is the typical mid-range value.
| Element | Spec limit (wt %) | Nominal |
|---|---|---|
| carbon black (N330/N550)0 phr in gum, 30–60 phr in engineering compounds | 0 – 40 | 20 |
| zinc oxidecure activator, ~5 phr | 1.5 – 4 | 3 |
| stearic acidactivator, ~2 phr | 0.5 – 2 | 1.2 |
| sulfur1.5–2.5 phr conventional cure; 30–40 phr for ebonite | 0.5 – 2.5 | 1.5 |
| accelerator (CBS/TBBS/MBTS) | 0.3 – 1.5 | 0.7 |
| antioxidant / antiozonant (TMQ, 6PPD, wa | 0.5 – 3 | 1.5 |
| process oil / plasticizer | 0 – 10 | 3 |
| non-rubber constituents in raw NR (proteTSR20: dirt ≤0.16 %, ash ≤0.8 %, N ≤0.6 %, volatiles ≤0.8 % | 0.2 – 6 | 5 |
| cis-1,4-polyisoprene (NR)≥99.9 % rubber hydrocarbon in raw bale; 45–70 wt% of a typical vulcanizate | ≥ 45 (balance) | — |
NR GUM — frequently asked
What is NR GUM used for?
NR GUM is typically used for tires, vibration mounts, seals, conveyor belts and shoe soles. In short: highest-resilience general elastomer; 750 % elongation at break; poor oil and ozone resistance.
What is the tensile strength of NR GUM?
NR GUM has a typical tensile strength of 24 MPa (3.48 ksi) — weaker than 68% of polymer grades. Strength varies by condition: see the 14 listed tempers.
Is NR GUM easy to machine?
Not especially — machinability is rated poor (20/100, worse than 97% of polymer grades). Cured NR is die-cut, waterjet or abrasive-ground rather than turned; freeze-trim or cryogenic deflashing gives clean edges, and any turning needs razor-sharp tools with water cooling.
Can NR GUM be welded?
Not readily — weldability is rated poor (15/100). Not weldable — parts are joined by uncured splicing plus press vulcanization, or by CR/cyanoacrylate adhesives and hot-bonded metal inserts (Chemlok primer systems).
What surface finishes work on NR GUM?
Surfaces replicate the mold; chlorination or plasma treatment reduces tack and friction, and rubber-to-metal bonding is done with two-coat phenolic primer/adhesive during cure.
Can NR GUM be formed, bent or molded?
Mill-mixed or internally mixed then compression, transfer or injection molded and cured 5–20 min at 140–170 °C; watch reversion at high temperature and allow ~2 % mold shrink.
What is the maximum service temperature of NR GUM?
NR GUM is rated for continuous use to about 80°C (176 °F), and briefly to 100°C. Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
Sources
- ASTM D2000 — Classification System for Rubber Products in Automotive Applications — ASTM (2020)
- ASTM D412 / D624 / D395 / D2240 — tensile, tear, compression set, durometer test methods — ASTM (2022)
- ISO 2000 — Rubber, raw natural: guidelines for the specification of technically specified rubber (TSR) — ISO (2020)
- Rubber Technology (Morton, ed.) — Springer (1999)
- Engineering with Rubber: How to Design Rubber Components (Gent, ed.) — Hanser (2012)
- The Vanderbilt Rubber Handbook, 14th ed. — R.T. Vanderbilt Co. (2010)
- Granta/CES elastomer property and eco-property records — natural rubber — Ansys Granta (2023)
Property data is compiled from published supplier and standards handbooks and normalised for comparison. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.