Why Only Hollow LSR Parts Float on Water

The density, or its reciprocal, the specific volume, is a commonly used property for polymeric materials. Density is primarily used as a material control tool, as well as to compare the buoyancy of different materials, but it can also be used to calculate the mass of rubber required to produce a given volume of material. The density of compounds with fillers or different materials can be computed at any temperature using the rule of mixture. Density measurements are performed following the standard tests ISO 1183 and ASTM D792. The density of Liquid Silicone Rubber (LSR) is typically in the range of 1.10 – 1.50 g/cm3 (density of natural rubber: 0.92, EPDM: 0.86 g/cm3), so it will sink in water (density of water: 1 g/cm3).

As with other properties, the specific volume is greatly affected by the temperature and pressure.  Temperature and pressure are both important for processing and is described in the post “Expansion or Shrinkage.”

floating hollow LSR parts
These LSR parts are hollow and thus contain a pocket of air, allowing them to float. When compressed, the air escapes and they fill with water, causing them to sink.

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[1] ASTM D 792: Density and Specific Gravity (Relative Density) of Plastics by Displacement; ASTM International, 2008

[2] ISO 1183-1: Plastics – Methods for determining the density of non-cellular plastics – Part 1: Immersion method, liquid pyknometer method and titration method; International Organization for Standards, 2004

[3] Osswald, T. A.; Baur, E.; Brinkmann, S.; Oberbach, K.; Schmachtenberg, E.: International Plastics Handbook; 4th edition, Hanser Gardner Publications, Cincinnati, 2006

[4] Campo, E. A.: Industrial Polymers

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