The accumulation of fat in a rubber liner occurs mainly in two regions, the mouthpiece and the portion of the barrel in contact with the teat. It is suggested that the mouthpiece is attacked by body fats and udder salves mainly, whereas the barrel absorbs milkfats from milk trapped between the rubber and teat. The accumulation of fat increases linearly with the number of milkings performed.
The loss of milking efficiency is connected with the stiffness of the rubber compound, and this latter property decreases linearly with the number of milkings. The length of service of a liner on the farm is shown to be largely dependent on the individual farmer and his standards of efficiency and cleanliness.
The presence of fat in rubber leads to an enhanced rate of oxidation and consequent loss of physical properties. This has been confirmed in practice. An increase in the tension of a liner increases the rate of swelling and the souring of milk on rubber gives a similar result. The use of detergents has no direct deleterious effect on the physical properties of rubber. They are not effective in removing fat from below the surface layers. The occurrence of ozone-cracking of parts of milk liners or tubing, which are under tension, is discussed and the use of waxes in the compound as a remedy is described.