The fundamentals of barrier protection of both porous materials and of intact polymer films and coatings are outlined. The importance of the elimination of even small pinholes is demonstrated and comparisons made under various conditions of protection such as changes in humidity and the presence of vacuum or air at the low concentration side.
The types of paper, board or glassine substrates is discussed, together with their influence on the protective properties of the barrier. The design of the polymeric coating on its permeability properties when free from pinholes is discussed in detail and the molecular and other factors are related to the performance of the barrier. Crystallinity, the glass transition temperature, orientation and cross linking and their effect on gas and vapour barrier properties are also reviewed. In addition, the possible differences between coatings deposited from latex, melt and solution are considered.
Finally, it is pointed out that the nature of the substance against which protection is needed is highly important and a polymer which is an excellent water vapour barrier might be a poor grease or organic vapour barrier and vice versa. Some newer developments in the barrier coating field are also presented.