Forced drying of finishing materials

A very important part of the finishing process is drying the materials. About their drying in ambient environment we've already talked. But in large factories (and not only) there are installations for the forced drying of finishing materials, generically called dryers. These reduce drying times and thus increase productivity. They are usually inserted in the finishing lines, following the natural flow of the technological process, but they can also be separated for manually finished parts or to serve several finishing lines. More about these dryers in the following material.

But first I want to tell you that, depending on how the film formation takes place, drying is of 3 kinds:

• Physical drying -the products dry out as a result of solvent evaporation. After drying, these materials can be re-dissolved in the same solvent (reversible process). The time for physical drying can be significantly reduced by increasing the temperature. This is the case for nitrocellulose products
• Chemical drying -drying of materials in two components. In reality it is the chemical reaction between the 2 liquid components that results in a third product, the varnish film, which is solid. This is how polyurethane, polyester and polyacrylic products dry. Chemical drying can also be considered as the drying of single-component products such as water-soluble products (acrylic dispersions), in which case the chemical reaction is triggered when the molecules come very close together following the evaporation of water. Unlike nitrocellulose varnish, in this case the process is irreversible, i.e. the film does not become liquid if water is added.
• Oxidative drying - is how synthetic alkyd resins are dried and cured. In this case drying is the result of evaporation of solvents and a reaction between resin and oxygen in the air. It is the type of drying that is not greatly influenced by temperature rise.

Let's get back to the dryers used in industry. The most common are:

 Convection dryers. Such a dryer has 3 distinct portions: the de-drying space, the actual drying space and the cooling space. The de-waxing area is immediately after the application of the finishing material and is necessary for the varnish film to settle as well as possible. The temperature in this area is close to ambient temperature. In the drying zone the temperature rises to 45-50 grd.C. This is where the drying and hardening of the film takes place. In the cooling zone the objects are brought to room temperature before being removed from the dryer. Cooling the objects is very important because most coatings are thermoplastic, which means they are soft at high temperatures. Air exchange in the cooling zone must be intense.

These dryers are most commonly used and they can be built vertically or horizontally. The materials move inside them by means of plates that raise and lower in the case of vertical dryers, or conveyor belts or chains (chains) in the case of horizontal dryers. The speed of travel is calculated so that the material is dry on leaving the tunnel.

IR infrared radiation dryers. In such dryers the heating power of IR radiation is used. Electromagnetic radiation with a wavelength of 0.76 µm - 1 mm is called infrared. Depending on the wavelength of the radiation there can be IRS dryers (short IR radiation), IRM dryers (medium radiation) and IRL dryers (long radiation). IR dryers are often combined with convectional dryers.

IR dryers are more economical than convectional dryers in terms of energy consumption.

Ultraviolet UV dryers. In these dryers the material is cured by UV radiation. Varnishes and paints specially formulated for this type of drying (acrylic, polyester, combined) are used. Hardening takes place very quickly (5-15 sec).

Two types of radiation sources are used: gallium (Ga) lamps and mercury (Hg) lamps. Gallium lamps have a wavelength greater than 420 nm and a lifetime of about 2000 hours. They are used to harden pigmented varnishes and are often combined with mercury lamps. Mercury lamps have a wavelength of 200-380 nm and a lifetime of about 3000 hours. They are used for curing clear varnishes. UV lamps need to be checked regularly because their power decreases over time. Energy consumption is significantly lower than for convective drying.

When drying in UV the solvent must be removed until the product goes under the lamp. Remaining traces of solvent make the film opalescent. Therefore, before the lamps, there is a convectional or IR drying tunnel to help remove the solvent. The length of this tunnel depends on how quickly the solvent can be removed. For drying water soluble products the tunnel is longer. The tunnel can only be missing for products that are applied with a valve and have body 100%.

Dual-cure products, i.e. products that have a combined UV + chemical drying, can be used for pigmented finishing. Such products are mixed with hardeners and when they pass through the lamps the drying takes place on the surface, but not in the depth of the coating. The film hardening process is continued after the objects come out from under the lamps thanks to the added catalyst. This is necessary because, due to the pigment, UV radiation cannot penetrate the film to harden it instantly. So why not use normal drying without the UV? The instant surface drying when passing through the lamps means that the film no longer catches dust from the atmosphere when it is put on the squeegee for total drying, thus increasing the quality of the film (especially for glossy films).

Drying time is a very important element in the working technology as well as in the economics of the production process, especially when dealing with large volumes of identical products. A dryer greatly shortens the time by increasing productivity. If, however, your field of activity involves artistic, unique, labour-intensive products that add value, a high-performance dryer is not the best investment.