Hardened rubber loses its soft, flexible qualities, becoming stiff, brittle, and prone to cracking. This degradation makes the material less functional, particularly in items like seals, gaskets, or hoses that require elasticity. The change is a chemical process that affects the material’s molecular structure. This article focuses on the chemical agents used to reverse this hardening, restoring pliability and extending the useful life of rubber items.
Why Rubber Hardens
Rubber loses elasticity due to irreversible chemical changes at the molecular level, often driven by environmental exposure. One major process is chain hardening, where oxygen, heat, and ultraviolet (UV) light trigger the formation of free radicals within the material. These reactive molecules combine to create new cross-links, which are additional chemical bonds between the long polymer chains that make up the rubber.
This increased density of cross-links restricts the movement of the polymer chains, resulting in the material becoming rigid and brittle. Heat aging and oxidation accelerate this process significantly over time. Another factor is the gradual loss of internal compounds called plasticizers that were added during manufacturing. These oily molecules sit between the polymer chains, allowing them to slide easily; as they evaporate or leach out, the rubber stiffens.
Practical Chemicals Used for Restoration
Chemical restoration relies on introducing new compounds into the rubber matrix to counteract hardening. One effective agent is Methyl Salicylate, commonly known as oil of wintergreen, which is often found in topical muscle-relief creams. This specialized plasticizer penetrates the rubber deeply, effectively replacing lost softening compounds. For application, it is mixed with isopropyl alcohol (rubbing alcohol), often in a 1:3 ratio of methyl salicylate to alcohol, to help carry the active agent into the material.
Silicone-based compounds, such as silicone oil or spray, are recognized as safe for nearly all types of rubber. Silicone does not react negatively with rubber polymers and acts as a lubricant and protective barrier, helping maintain flexibility and inhibit further drying. For general conditioning, mineral oil (a petroleum-based oil) can be used, but caution is necessary as it can cause swelling or degradation in certain types of natural rubber. Petroleum-based solvents like naphtha are generally limited to specific rubber types due to their potential to excessively swell or dissolve the material.
How These Chemicals Restore Flexibility
Chemicals restore flexibility through two actions: re-plasticizing and swelling. Re-plasticizing occurs when agents like methyl salicylate or silicone oil are absorbed into the polymer network. Once inside, they act as new plasticizers, reducing the attractive forces between polymer chains and allowing them to move freely, restoring elasticity.
Certain solvents and plasticizers can also cause a temporary swelling of the rubber material. This swelling forces the tightly packed polymer chains further apart, which temporarily increases flexibility. When the solvent part of the mixture, such as the isopropyl alcohol, evaporates, the rubber may return to its original size, but the absorbed plasticizer remains to maintain the improved flexibility. This combination of initial swelling and permanent re-plasticizing is what makes the wintergreen oil and alcohol solution effective for hardened rubber items.
Safe Handling and Application Methods
Proper safety and application techniques are necessary when using these chemical restoratives. When working with methyl salicylate, wear chemical-resistant gloves and safety glasses to prevent skin or eye contact. The process should be conducted in a well-ventilated area because the vapors from the wintergreen oil and alcohol can be strong.
A patch test is a necessary first step for any restoration chemical, involving applying the agent to a small, inconspicuous area. This test confirms that the specific rubber compound does not suffer adverse effects, such as becoming excessively sticky or disintegrating. Application methods vary; for the methyl salicylate mixture, the rubber part is usually soaked completely for a period ranging from 24 hours to several days, depending on the severity of the hardening. Oils and silicone sprays are typically wiped or sprayed onto the surface, often followed by sealing the item in an airtight bag for several days to allow maximum penetration. After treatment, any excess residue should be wiped clean to prevent the material from attracting dirt and debris.