Natural rubber is an elastomer that was originally derived from milky latex found in the sap of some plants. The purified form of natural rubber is the chemical polyisoprene, which can also be produced synthetically. Natural rubber is used extensively in many applications and products, as is synthetic rubber. Natural rubber is an ideal polymer for dynamic or static engineering applications. Natural rubber features:
- Excellent dynamic properties with a low hysteresis loss
- Good low temperature properties
- Strong bonding to metal parts
- High resistance to tear and abrasion
- Ease of processing
Natural rubber cannot compare to many synthetics for heat aging and resistance to sunlight, oxygen, ozone, solvents or oils. However, it performs well in many applications such as:
- Rubber gaskets, rubber seals and rolls
- Hose and tubing
- Vibration isolators
- Shock mounts
- Electrical components
|Common Names||Natural Rubber|
|ASTM D-2000 Classification||AA|
|Military (MIL-STD 417)||RN|
|Durometer Range (Shore A)||20-100|
|Tensile Range (P.S.I)||500-3500|
|Elongation (Max %)||700|
|Resilience — Rebound||Excellent|
|Low Temperature Usage (F°)||-20° to -60°|
|High Temperature Usage (F°)||Up to 175°|
|Aging Weather — Sunlight||Poor|
|Adhesion to Metals||Excellent|
Comment — Natural Rubber has a variety of positive characteristics. It has high resilience, good compression set, food roll building behavior, and molding properties. Additionally, Natural Rubber displays high tear strength and low crack growth. It is usable for ketones and alcohol and also low temperature properties. Natural Rubber is not recommended for applications where the rubber part well be exposed to ozone, oils or solvents, all of which attack the rubber.
Properties of Natural Rubber
- Natural rubber combines high tensile and tear strength with an outstanding resistance to fatigue
- Excellent green strength and tack — has the ability to stick to itself and other materials, making bonded natural rubber products easier to fabricate
- Exhibits excellent dynamic properties with a low hysteresis which leads to low heat generation
Natural rubber has been successfully used as an engineering material for many years. The following range of properties shows just how versatile of a material natural rubber has proven to be to engineers:
- Adjustable hardness from very soft to very hard (ebonite)
- Appearance and color ranges from translucent (soft) to black (hard)
- Compounded to meet nearly any mechanical requirement
- The ability to be electrically insulating or fully conductive
- Protect, insulate and sealing properties
- Absorb vibration and silence noise
- Available in any surface roughness and shape
To achieve the specific properties required for a given product, raw natural rubber has to be compounded using ingredients such as carbon black, anti-degradants, softeners and a vulcanization system. Some of the compounding rules to achieve the properties desired include:
Hardness of natural rubber products is determined by the amount and type of filler, as well as the degree of dispersion and the cross-link density. The most common rubber filler in engineering applications is carbon black. For maximum tensile properties approx. 25% volume carbon black is needed.
To achieve low creep properties, the level of carbon black used should be kept to the minimal acceptable level. If the application in question requires a high abrasion resistance, a small particle size type carbon black should be used. The overall stiffness of a natural rubber product is a result of the modulus of the rubber, the shape of the product and its dimensions, and the mode of deformation.
The modulus of a rubber compound is also determined by the amount of filler used. To obtain a low modulus product, low reinforcing blacks or non-reinforcing white fillers are normally used. Modulus can also be influenced by the rubber grade, or viscosity, and by the vulcanization system used.
Low modulus properties can be achieved through the use of a soluble efficient vulcanization (EV)-system, based on soluble accelerators, activators, and also low sulfur levels.
As filler level increases in natural rubber compounds, resilience decreases. To achieve high resilience, the filler level must be kept to an acceptable level to maintain physical properties.
High damping can be reached in natural rubber compounds through filler/oil extensions using highly viscous oil. This effect can also be achieved by blending natural rubber with synthetic rubbers such as SBR or EPDM. The damping occurs as a consequence of the synthetic polymers having a Tg not far under the normal service temperatures.
Low Compression Set
Vulcanization with organic peroxides can be used to achieve a low compression set for natural rubber products with outdoor applications. However, since peroxide vulcanized products have poor tear strength and are incompatible with anti-ozonants, the common practice is to use conventional vulcanization (CV) or semi-efficient vulcanization (SEV)-systems.
For good creep/relaxation resistance the filler content should be kept to a minimum level and make use of medium particle sized blacks. For a low creep/relaxation rate, a soluble EV-system is also advised.
Cross Link Density
The degree of cross-linking in natural rubber compounds affects various properties. For maximum tear strength, the cross-link density is slightly lower than for maximum tensile strength and abrasion. Compression set, resilience, creep and relaxation resistances are best at relatively higher levels of cross-linking.