As rubber ages, it hardens. This happens when the softening chemicals called "plasticizers" bleed out. The hardening process can take years, but it can be reversed before the rubber gets so brittle that it crumbles.
Plasticizers keep rubber in a pliable state for better utility. Silicone spray contains the plasticizers needed to restore hardened rubber to its original suppleness. You can also heat rubber to make it more flexible when cold temperatures have caused it to harden. Automobile window seals commonly suffer from rubber hardening.
What are plasticizers?
A plasticizer is a substance which when added to a material, usually a plastic, produces a product which is flexible, resilient and easier to handle. Since the dawn of civilization, water has been used to plasticize clay for the production of pottery and clay tablets. Also, lime has been used to promote the easier working of cements. Other early examples of plasticizers include the oils to plasticize pitch for caulking prehistoric boats and neatsfoot oil and sperm oil to soften leather.
In modern applications plasticizers are produced by reacting an alcohol (such as methanol or ethanol) with an acid such as phthalic anhydride, adipic acid, etc…These plasticizers must satisfy more demanding technical and economic requirements which are best met by esters such as phthalates, adipates, trimellitates, etc... Phthalates, due to their technical performance, versatility and cost effectiveness, are the most widely used
Two Ways to bring back rubber flexibility, Plus one additional way that can be used if time does not permit.
First Process (Silicone)
1. Spray the rubber with silicone spray. This spray can be found at any camping supply store. Spray until the rubber is soaked. Place the rubber inside an airtight, plastic bag. Use large garbage bags for big pieces of rubber.
2. Keep the rubber in the plastic bag for three to seven days.
3. Lift the bagged piece of rubber and bend it to check for pliability. Once the desired suppleness is attained, you can remove it from the bag for use. You might need to repeat this process if the rubber hasn't softened to the extent you desire.
Second Process (Heat)
1. Heat some water in a stew pot on the stove. Heat it until boiling.
2. Set the stew pot in the sink and submerge the rubber in the water to soften it.
3. Grip the rubber with a pair of salad or canning tongs and remove it from the water. If you need to touch the rubber at this point, put on a pair of oven mitts or leather work gloves to handle it.
Tips & Warnings
Use extreme caution when handling the stew pot containing boiling water to avoid potential burn injury.
An Additional Way to Soften Rubber if Time Does Not Permit the First two ways.
People have been known to use WD40 to soften rubber, but this is really not the best way to do it.
If you decide to use WD40, try these steps here.
1. Spray the old rubber with WD-40 or a comparable solution. If possible remove the rubber part from it's installation and allow it to soak in a small basin of the WD-40 or other product.
2. Allow the WD-40 or comparable product to penetrate the old rubber for at least 10 to 20 minutes.
3. Wipe the rubber part or area clean with a soft cloth until you can no longer see or smell the solution on it. The rubber will be noticeably softer and look newer.
Note: If you desire a more flexible and softer rubber, try repeating steps Number 2 & 3 above.