By David Gecic
Originally published in Canadian Coatings 1997 Finishing Handbook
A lot of emphasis is placed on the application of coatings to surfaces and the associated pretreatment. But coatings also need to be removed from surfaces.
Material which needs to be stripped can come from several sources — parts which have been rejected due to quality problems and fixtures (paint hooks) which are used to support material during the painting process, are the most common. Refinishing or refurbishing operations require the use of paint strippers. It is also sometimes necessary to strip off masking used on parts. Some electronic, printing, and etching processes also require specialty stripping operations.
With electrocoating or electrostatically applied coatings, the residual coating on the fixtures becomes very important to remove because good contact between the fixture and the part becomes essential.
It is important to consider whether an in-line stripping process or a batch process is needed.
In a controlled situation where the paints and substrates used have a high degree of consistency, a paint stripping stage can be added to an existing processing line. In a process where there are large or expensive fixtures being used in a high volume application,
an in-line process may be desirable. This consists of a tank in which the painting fixtures are immersed for a given period of time before pretreatment. The process should be followed by a spray rinse. to remove the chemical residues and loose paint. The amount of time the parts need to be immersed determines the size of the tank needed.
More commonly, a batch process is used. Fixtures or parts are put into a chemical stripping tank at intervals. This can be done very effectively if there are numerous sets of fixtures, so that the time required to strip does not cause any “down time” on line.
Many times it is economically feasible to send objects such as paint fixtures to a vendor for stripping. However , a good quality vendor must be selected. Some vendors may use processes that can damage the surface of the work or may use excessive temperatures in stripping.
Each type of stripping technology has its advantages.
Some coatings are difficult to remove by plastic media blasting
because of the durability or thickness of the coating.
Burn off processes also have their limitations. Some thin gauge work may be warped or distorted by heat. Some metals, such as zinc, have melting points which are below the temperatures required to strip paint, and other metals such as magnesium can burn when exposed to high
Some coatings, for example polyvinyl chloride and fluoropolymers, can create toxic gases when heat stripped. In these cases, it is more desirable to use chemical stripping methods.
Chemical paint removers operate in several ways. Breaking the bond between the surfaces and the coating, dissolving the coating, chemically breaking down the paint molecules, and softening the paint to allow the chemicals to penetrate into the surface of the coating.
There are basically two types of paint strippers: hot and cold. There are numerous products available for each process. It is important to know the uses and limitations of each. Important aspects to consider in choosing a paint stripping process are safety, cost, processing speed, type of metal to be processed, temperature and ease of disposal.
Hot Strippers
Hot strippers operate at elevated temperatures to soften and remove paint. They are almost exclusively used in immersion operations. They generally consist of mixtures of alkaline or acidic builders, plasticizers, surfactants, and solvents such as glycols, polyglycols,
and amines.
Work is immersed in the stripping solution at 70 to 100 degrees centigrade from 10 minutes to several hours. After the paints have been stripped, parts are generally washed by a water spray to remove chemical residues and any paint which may remain clinging to the surface.
Most hot strippers are alkaline in nature. Polymers such as alkyd, nitrocellulose, ureas, and some polyesters can easily be dissolved by using a hot alkaline solution, since the molecular bonds break down with alkalinity. However, this process is slow without acceleration by plasticizers or temperature.
Plasticizers allow the coating to soften at lower temperatures, allowing the stripping materials to penetrate the coating and dissolve the bonds between the molecules.
The technology of traditional hot alkaline strippers applies very well to stripping modern coatings from steel; however, they are too aggressive to be used on such metals as aluminum. Some products contain inhibitors to prevent attack on the surface of these metals.
Hot strippers have their limitations because the increased demand for high quality coatings has created paints which have a high degree of chemical resistance.
In particular, epoxies and fluoropolymers can be difficult to remove. Most of the products are corrosive in nature.
Equipment is usually made of mild steel, with conventional heating elements such as electric, steam, or gas fired heaters.
Stripping times vary with the type of coating, the amount of coating,
and the mass and shape of the part being stripped.
Cold Strippers
Cold strippers operate at or near room temperature. Traditionally cold strippers were based on a solvent called methylene chloride. Recently there has been great concern about the health hazards associated with this material.
The traditional methylene chloride paint stripper softened the coating through plasticizer action and penetrated below the surface to break the bond between the surface and the paint, lifting the coating off the surface.
Many replacement products have tried to duplicate the characteristics of methylene chloride.
Most of these products match the solvent characteristics, but do not have the penetrating power of methylene chloride and will not strip higher tech, more heavily cured coatings. Most of these products do not work on a wide range of paints or need exceptionally long times to strip surfaces.
There have been cold stripping products developed recently which have some of the penetrating power and versatility of methylene chloride and can strip paints in an economically feasible amount of time. These still have limited use and do not penetrate into thicker deposits of paint.
Many cold stripping products are limited by slow stripping action, presence of chlorinated solvents, flammability, or general health concerns.