Plastic Polypropylene vs. Rubber

Compare Plastic Polypropylene vs. Rubber strength, stiffness, weight and thermal properties.

Plastic PolypropyleneBulk polymer (molded or machined)

RubberBulk polymer (molded or machined)

Mechanical	Plastic Polypropylene	Rubber
Yield strength	33MPa1	not given (tensile used)
Ultimate tensile strength	not stated (yield given)	23MPa2
Elongation at break	not sourced	830%2
Young's modulus (stiffness)	1.45GPa1	not sourced
Density	900kg/m³1	960kg/m³2
Strength to weight	36.7kN·m/kg1.53x higher	24kN·m/kg
Stiffness to weight	1.61MN·m/kg	not sourced
Poisson's ratio	base material0.423	government0.54
Shear modulus	0.511GPa	not sourced
Bulk modulus	3.02GPa	not sourced
Speed of sound	1,269m/s	not sourced

Stiffness against densityHigher and further left: stiffer for its weight

Strength against densityHigher and further left: stronger for its weight

Thermal	Plastic Polypropylene	Rubber
Thermal conductivity	base material0.17–0.22W/m·K3	0.151W/m·K5
Thermal expansion	base material80–100µm/m·K3	not sourced
100 mm part over a 50 °C swing	450µm growth	not sourced
Specific heat	not sourced	government1,881J/kg·K4
Heats up and cools (diffusivity)	not sourced	0.0836mm²/s
Thermal shock resistance	28.6W/m	not sourced
Max service temperature	HDT 0.45 MPa90°C1	70°C2

Thermal conductivity against densityHigher and further left: spreads more heat for its weight

Thermal expansion against thermal conductivityLower and further right: warps less when heated

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Sources5 documents: LyondellBasell, Weir Minerals, INEOS Olefins & Polymers USA, National Institute of Standards and Technology, Defense Technical Information Center

Plastic Polypropylene: LyondellBasell Moplen HP501H polypropylene homopolymer (general purpose), typical ISO values
Rubber: Weir Minerals Linatex Premium rubber (95 % natural rubber), typical physical properties, spec sheet WMD0118/202305

1
Moplen HP501H Product DataLyondellBasellPlastic Polypropylene · Yield strength: typical, tensile stress at yield ISO 527Plastic Polypropylene · Young's modulus (stiffness): typical, tensile modulus ISO 527Plastic Polypropylene · Density: typical, 23 °C ISO 1183-1Plastic Polypropylene · Max service temperature: HDT 0.45 MPa (ISO 75B), unannealed
2
Linatex Premium Rubber Spec Sheet (WMD0118, 2023)Weir MineralsRubber · Ultimate tensile strength: typicalRubber · Elongation at break: typicalRubber · Density: typical (specific gravity)Rubber · Max service temperature: continuous use operating range
3
Typical Engineering Properties of PolypropyleneINEOS Olefins & Polymers USAarchived copyPlastic Polypropylene · Poisson's ratio: base material: polypropylene homopolymer family (INEOS Olefins & Polymers USA, Typical Engineering Properties of Polypropylene)Plastic Polypropylene · Thermal conductivity: base material: polypropylene homopolymer family (INEOS Olefins & Polymers USA, Typical Engineering Properties of Polypropylene)Plastic Polypropylene · Thermal expansion: base material: polypropylene homopolymer family (INEOS Olefins & Polymers USA, Typical Engineering Properties of Polypropylene)
4
Compressibility of Natural Rubber at Pressures Below 500 kg/cm², J. Res. NBS 68A (1964)National Institute of Standards and Technologyarchived copyRubber · Poisson's ratio: government (NBS J. Res. 68A, 1964): computed by NBS for peroxide-cured gum NR vulcanizate, 25 °C, infinitesimal deformation (0.49 in the solver: the linear element locks as the ratio nears 0.5 (nearly incompressible).)Rubber · Specific heat: government (NBS J. Res. 68A, Table 7; values observed for unvulcanized NR by Bekkedahl and Matheson), 25 °C
5
Thermal Conductivity of Soft Vulcanized Natural Rubber: Selected Values (AD 643325)Defense Technical Information Centerarchived copyRubber · Thermal conductivity: government selected value (US Army Natick Laboratories TR 66-49-PR, 1966), soft vulcanized natural rubber, 25 °C

Each number points to the document that states it, with the exact basis. Values without a number are computed from the sourced ones. "Not sourced" means no citable source states it (or only a specification minimum); figures computed from it are left out too.

Questions

Mechanical

Is Plastic Polypropylene stronger than Rubber?

Plastic Polypropylene is stronger: its yield strength is 33 MPa against tensile strength 23 MPa for Rubber (43%).

Which is lighter, Plastic Polypropylene or Rubber?

Plastic Polypropylene is lighter: 900 kg/m³ against 960 kg/m³ for Rubber.

Which is lighter for the same job, Plastic Polypropylene or Rubber?

For the same stiffness or strength: Plastic Polypropylene is lighter for a strong rod or tie, strong beam, strong panel or plate.

Part that must be	Plastic Polypropylene	Rubber
Strong rod or tie	lighter	1.53x heavier
Strong beam	lighter	36% heavier
Strong panel or plate	lighter	28% heavier

For the same stiffness or strength, mass scales with density over stiffness (or strength) raised to a power set by how the part is loaded: 1 for a rod in tension, 1/2 for a beam in bending, 1/3 for a panel (the standard material-selection indices).

Thermal

Which conducts heat better, Plastic Polypropylene or Rubber?

Plastic Polypropylene conducts heat better: 0.195 W/m·K against 0.151 W/m·K for Rubber.