Bubbles have been observed on finished liners. They are localized swellings which appear to involve only the coating, but underlying felt may also be involved.
Bubbles have been recorded in a number of shapes and sizes. The Figures listed below are in order of bubble size. Some are in a state of inflation while others are deflated or damaged in some way.
Causes: The formation of bubbles on the internal surfaces of liners may be due to a porous composite which allows ground water pressure to act directly onto the coating, disbonding it with a peeling mechanism. This explains the bubbles that are full of water.
Others which are deflated could be due to styrene or water vapor during cure, or by reduced pressure when hot brought about by the partial collapse of steam after steam curing.
Figure 1: Gas-filled bubble. (Click photo to view video.)
Figure 2: Gas-filled bubble. (Click photo to view video.)
Figure 3: Collapsed bubble. (Click photo to view video.)
Figure 4: Bubble torn off. (Click photo to view video.)
Figure 5: Partially water-filled bubble. (Click photo to view video.)
Figure 6: Water-filled and highly extended bubble. (Click photo to view video.)
Solutions: Porosity can be avoided by ensuring that impregnation is carried out under sufficient vacuum and at the correct nip roller gap.
Insufficient head during cure may allow resin drain down. Any air present in the resin will expand on heating, displacing resin resulting in a degree of porosity.
Especially in thick liners it is important to avoid excessive exotherm which can cause the monomer, usually stryrene, to boil thus resulting in a degree of porosity.
When steam curing it is important to avoid negative pressure while the coating is still hot.