- October 31, 2004, David J. Bentley Jr., Contributing Editor
People have different ideas about the best time to purchase a new automobile to replace their present one. Some people wait a specific number of years; others drive their vehicle for a specific number of miles. Still others become intrigued with a new model because of its looks or features.
Oops, this is the wrong kind of vehicle retention. This column normally addresses items concerning the converting and packaging industries and does not concentrate on subjects associated with the automotive industry, although vehicle retention is a term that is common to both fields.
In the converting and packaging arena, “vehicle retention” refers to the amount of carrier, such as water or solvent supplied with an adhesive, coating, or ink, that remains in a construction after drying.
The adhesives, coatings, or inks used in a typical construction often come as solutions, dispersions, suspensions, or the like in water, ethyl acetate, methyl ethyl ketone, etc. After its application to a moving web, the functional material — adhesive, coating, or ink — requires complete removal of the liquid carrier for it to contribute its ultimate properties to the final product.
The drying process an adhesive, coating, or ink undergoes after its application is, therefore, an extremely important component of the entire converting operation. During the time the coated web spends after it enters the oven or drying chamber until it exits at the other end, all its water or solvent must be removed. (Note that 100% solids materials that contain no vehicle can, by definition, have no vehicle retention.)
Vehicle retention is extremely important because the presence of any vehicle often detracts from the final properties of an adhesive, coating, or ink.
Water or solvent that remains trapped acts as a softening or plasticizing agent. Adhesives or coatings that should cure to a non-tacky state usually will be very soft as a result of retaining vehicle, and they will not provide the bonds or other properties for which the converter is using them. Inks on or in a construction likewise will be soft, and this can cause smearing or similar characteristics.
One time when vehicle retention may not be a serious problem involves a construction that contains a porous surface through which any retained vehicle eventually can evaporate. Obviously, paper is such a surface. A coating in solvent applied to paper, dried incompletely, and then rewound will have some retained solvent. As the roll ages, the retained vehicle will dissipate into the atmosphere.
While this action can result in a dried coating, some problems may arise during the time the coating retains the solvent, because it is soft. It may be so tacky that it will act as an adhesive and cause the roll to block.
At the other extreme, retained solvent in a lamination in very small amounts can be a particularly vexing problem.
For example, in a food packaging application, a level as low as a few parts per million can cause a problem in a laminate using an adhesive. This is because over time, the very low levels of solvent retained from the adhesive may migrate through a plastic film used to make the construction and enter the food. This contaminates the food and can cause it to taste or smell badly. An extremely small amount of vehicle retention then can result in an extremely large problem.
Thus, whenever a converter uses an adhesive, coating, or ink supplied in a liquid vehicle, drying to evaporate all solvent or water is an extremely important operation. Testing of the final construction is necessary to ensure the level of any vehicle retained after the drying step is sufficiently low that no problems will result during additional processing or use.
Vehicle retention with an automobile usually is a matter of personal preference. Vehicle retention when using an adhesive, coating, or ink is a very different matter, and it's one that can have dire consequences. Try to avoid this potential problem at all costs.
David J. Bentley Jr. is a recognized industry expert inpolymers, laminations, and coatings with more than 30years of experience in R&D and technical service. Contact him at firstname.lastname@example.org.