Silicone is a synthetic polymer composed of silicon, oxygen, carbon, and hydrogen atoms. The definitive answer to whether silicone absorbs water is no; silicone is highly resistant to water absorption, classifying it as an extremely hydrophobic material.
Silicone’s Water Repellency
Silicone’s interaction with water is characterized by hydrophobicity, which literally translates to “water-fearing.” This means the material actively repels water molecules. The surface of silicone has a low surface energy, which determines how a liquid will interact with a solid.
Water molecules are highly cohesive. Because the cohesive forces within the water droplet are stronger than the adhesive forces between the water and the low-energy silicone surface, the water beads up. Instead of penetrating the material, water forms distinct spherical droplets that roll off easily.
This resistance is measured by the contact angle of a water droplet on the surface; a material is considered hydrophobic if this angle is greater than 90 degrees. The material prevents water from penetrating its structure, but it can be relatively permeable to gases, which allows for water-resistant yet breathable applications.
Molecular Structure and Hydrophobicity
The scientific explanation for silicone’s water resistance lies directly in its molecular architecture. The backbone of the silicone molecule is an inorganic chain of alternating silicon and oxygen atoms, known as a siloxane chain. This silicon-oxygen (Si-O) backbone is exceptionally stable and flexible.
Attached to the silicon atoms are organic side groups, most commonly methyl groups, which are composed of carbon and hydrogen atoms (CH3). These methyl groups are non-polar. Water, on the other hand, is a highly polar molecule, having a positive and negative end.
The principle of “like dissolves like” dictates that polar substances mix well with other polar substances, and non-polar with non-polar. Since the outward-facing methyl groups on the silicone surface are non-polar, they are fundamentally incompatible with polar water molecules. These non-polar methyl groups arrange themselves outward, presenting a surface that repels the approaching water.
This outward orientation of the non-polar organic groups hinders the water molecules from interacting with the underlying, more polar Si-O bonds. The resulting structure is a polymer with an inorganic, stable skeleton shielded by a layer of non-polar, water-repelling organic groups.
Everyday Uses Due to Water Resistance
Silicone’s water-repelling nature is utilized across everyday products and industrial applications. One of the most common applications is in construction and plumbing as a sealant or caulk. Its ability to form a durable, watertight seal around windows, doors, bathtubs, and sinks prevents water penetration and subsequent damage or mold growth.
The medical industry relies on silicone’s water resistance and biocompatibility for items like tubing, medical adhesives, and various implants. In these applications, the material’s stability ensures that it will not degrade or absorb bodily fluids.
Its use in protective coatings is widespread, such as in waterproof fabrics and windshield treatments, where it forces water to bead up and roll away, improving visibility and protecting the underlying material. Silicone is also a preferred material for outdoor gear, including swim goggles, diving masks, and certain footwear components, because it maintains its integrity and sealing properties even when submerged. Its stability across a wide temperature range, from extremely cold to very hot, complements its water resistance, making it reliable in diverse environments.