NR: Natural rubber

General Properties

Short Name:

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Natural rubber

NR is a natural product and is produced from the white milky juice (latex) of the rubber tree. The artificially manufactured kind is designated isoprene rubber (IR) and is made up of isoprene (2-methyl-1,3-butadiene) as a monomer. NR is an entirely 1,4 cis isomer. The 1,4 polyisoprene is also a natural product called gutta-percha. For more on cis-trans isomerism. Carbon black is often added in order to increase the tensile strength and abrasion resistance of the material, as well as for coloring.

Structural Formula

Properties

Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.Glass Transition Temperature	-72 to -55°C
Melting Temperatures and EnthalpiesThe enthalpy of fusion of a substance, also known as latent heat, is a measure of the energy input, typically heat, which is necessary to convert a substance from solid to liquid state. The melting point of a substance is the temperature at which it changes state from solid (crystalline) to liquid (isotropic melt).Melting Temperature	25 to 40°C
Melting Temperatures and EnthalpiesThe enthalpy of fusion of a substance, also known as latent heat, is a measure of the energy input, typically heat, which is necessary to convert a substance from solid to liquid state. The melting point of a substance is the temperature at which it changes state from solid (crystalline) to liquid (isotropic melt).Melting Enthalpy	67 J/g
Decomposition reactionA decomposition reaction is a thermally induced reaction of a chemical compound forming solid and/or gaseous products. Decomposition Temperature	375 to 400°C
Young's Modulus	1 to 5 MPa
Coefficient of Linear Thermal Expansion (CLTE/CTE)The coefficient of linear thermal expansion (CLTE) describes the length change of a material as a function of the temperature.Coefficient of Linear Thermal Expansion	180 to 260 *10^-6/K
Specific Heat Capacity	1.91 to 2.08 J/(g*K)
Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.Thermal Conductivity	0.13 to 0.15 W/(m*K)
DensityThe mass density is defined as the ratio between mass and volume. Density	0.91 to 0.93 g/cm³
Morphology	Amorphous rubber, partly with hard segments
General properties	High stability and high elasticity. High elongation at break. Good abrasion resistance
Processing	Cross-linking by means of sulfur incl. accelerator (e.g., mercaptobenzothiazole or sulfenamide) and activator (zinc oxide and stearic acid)
Applications	Tire industry. Technical rubber goods (e.g., rubber springs, band conveyors). Medical engineering. Toys. Shoe soles

NETZSCH Measurement

Instrument	DSC 204 `F1` `Phoenix^®`
Sample Mass	13.00 mg
IsothermalTests at controlled and constant temperature are called isothermal.Isothermal Phase	7 min
Heating/Colling Rates	10 K/min
Crucible	Al, pierced lid
Atmosphere	N₂ (40 ml/min)

Evaluation

Natural rubber (NR) is mainly amorphous with a small amount of crystalline (hard) segments. This can be seen from the Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition step at -63°C (2^nd heating, red, midpoint) which is overlapped by an enthalpy RelaxationWhen a constant strain is applied to a rubber compound, the force necessary to maintain that strain is not constant but decreases with time; this behavior is known as stress relaxation. The process responsible for stress relaxation can be physical or chemical, and under normal conditions, both will occur at the same time. relaxation, and from the shallow, broad melting transition between -10°C and 50°C (peak temperature: approx. 28°C, 2^nd heating) with a heat of fusion of only 1.5 J/g. An even smaller melting enthalpy of 0.5 J/g was observed in the 1^st heating (blue).