Bubbles can occur during the lining process. These become most evident after the curing process.
Why do bubbles occur?
There are many reasons this occurs. The most common reasons are the following. Bubbles occur as the air is heated and trapped. The air will expand and will show up after the cure. The vulcanization process gives the rubber the memory to keep the bubble in it’s final state after cure.
Trapped air in an autoclave is least likely to occur as a pressurized cure is more likely to keep the bubble small and the air from expanding. An atmospheric on the other hand is prone to bubbles. As there is no pressure to keep the bubbles small they are most likely to show up during this type of curing process.
Where does the air get trapped?
Often when the rubber get’s calendered, depending on the company doing the calendering, the bubble get trapped between the layers. Often if a rubber sheet supplier will realizes this and they will process their rubber sheet in a piercing roll, prickle roll. They will perforate the sheet to let the bubbles out. Often looking at the exterior of the sheet will not tell you if the bubbles are present.
Other places bubbles can form is between the steel, adhesive layers and the rubber layer. If the adhesive was not off gassed properly or there is a stitching issue, dirt in the adhesive, than rubber can not properly adhere to the sub straight. This can cause an area for the air to be trapped and expand.
Poorly welded seams that have trapped air or porosity, porous castings which get rubber lined are all prone to air bubbles. These types of porosity are better addressed before the adhesive process using a liquid metal epoxy to fill potential air pockets.
Can you repair this?
There are a few ways to properly repair these types of lining failures. The picture above is not repairable the liner should be stripped and the pipe relined. It would never pass a spark test or help in reduction of wear in this pipe.
If it is a problem within the rubber itself and fairly small a 10-12 gauge needle can be used with toluene or SC2000 to shrink and adhere the blister. After every repair a spark test must be performed in order to determine if the liner is compromised. For bubbles below the liner a more aggressive repair is required. the bubble must be cut out. The steel sub straight must be ground or blasted in order to give the steel a clean profile and a tooth. At this point there are two patching methods.A cold patch which can be done with an adhesive like SC2000 and a cured piece of rubber,stitched subsequently buffed flush and again a spark test performed to validate the patch quality. Or a localized hot patch be used. Uncured rubber with a Chemlok adhesive system, which is then stitched, cured with steam, buffed flush and then verified with a spark test.