- November 01, 2001, David J. Bentley Jr., RBS Technologies
A friend talking about orange peel recently said he preferred his with a dark chocolate coating! Sounds delicious, but orange peel is actually a phenomenon that can occur during a coating operation.
It is, in fact, a very accurate description of the appearance of certain coated products after they have been applied to a substrate and dried, if they contain water or solvents. The surface looks exactly like the uniform, dimpled outer surface of an orange.
Note that the type of citrus fruit and the variety are important factors to this description. Grapefruit do not usually have a dimpled skin; neither do some varieties of lemons and limes. Some orange varieties also do not have a dimpled skin. Orange peel used as a descriptive term in the converting industry specifically refers to those varieties of oranges with a dimpled skin.
What causes an orange peel appearance in converting? In all cases, it is due to improper or insufficient flow of a coating on the surface to which it is applied. Such poor flow conditions can result from various causes.
Excessive coating thickness can result in orange peel. Application of too much coating may not result in sufficient flow of the material into a smooth and uniform surface. Although the coating tries to distribute itself evenly on the surface of a substrate, excessive material may result simply in too much mass or volume for proper distribution. The resulting appearance will look exactly like the surface of an orange peel.
In some cases, the orange peel appearance may not be readily apparent to an untrained eye. It may require very close examination or holding so light falls on the surface in such a way to make the dimpled surface more visible. To avoid this cause of orange peel, one must always use the proper coating deposition.
Improper drying is another cause of orange peel. If the temperature in an oven is too hot during evaporation of water or solvents from a coating, the coating can dry prematurely before it has sufficient time to flow adequately to a smooth surface. If the drying in an oven is too rapid when evaporating water or solvents, orange peel can occur. In this case, the rapid drying of the water or solvents causes the coating viscosity to increase to a level at which the coating will not flow uniformly. Avoiding those causes of orange peel requires very careful control of oven temperature and drying time.
The final cause of orange peel discussed here will be an unsatisfactory or improper surface. The discussion so far has emphasized that elimination of orange peel requires excellent wetting or flow of a surface by a coating. The coating itself is not the sole factor that determines proper wetting; the substrate onto which the coating is applied can be equally important. Sometimes the substrate requires proper preparation, such as removal of a contaminated or dirty surface. In other cases, the coating may require treatment to change its character.
A more common manifestation of poor surface wetting is the appearance of fish eyes. This is more dramatic than the poor surface wetting due to orange peel. As with fish eyes, orange peel due to improper substrate surface characteristics may be more common with aqueous systems than with solvent systems.
Adding a surfactant to an aqueous system to make it flow better often can eliminate orange peel. To remedy an orange peel appearance when using coatings in solvents or coatings that are 100% solids, certain silicone surfactants can be very effective.
Laminations also can exhibit orange peel. The phenomenon is due to improper flow of a laminating adhesive before the combination of the substrates in the nip or combining station.
Again, close examination of the laminate may be necessary to see the orange peel. Elimination of orange peel in a lamination is identical to its elimination from a coating. Simply implement the necessary remedy from the possibilities listed above to ensure proper flow and wetting.
David J. Bentley Jr. is a recognized industry expert in polymers, laminations, and coatings with more than 30 years of experience in R&D and technical service. Contact him at e-mail: firstname.lastname@example.org.