Paint and lacquer drying solutions
Radiants with metallic fabric
space heating equipment, alternative to both luminous, bright-wave radiation heating heating installation, focused industrial spaces.
There are two main types of drying paint and varnishes:
- Physical drying: it is the result of evaporation of the volatile components, embedded in the paint solvent or thinner at ambient temperatures in about 40 - 60 min. after application
- Physico-chemical drying: is the result of two interrelated processes: evaporation of volatile components and chemical reactions carried out by both at ambient temperatures and at high temperatures (80-170°C) in drying ovens.
Reactions that may occur are:
• Polymerization or copolymerization (unsaturated polyesters, etc.)
• Oxidative cure with atmospheric oxygen (drying alkyd resins)
• Polyaddition of two or more components at the time when they come into contact or whose reaction occurs only in warm (polyurethane resins)
• Through various polyadditions as: the formation of resole, strengthening the melamine alkyd systems and urea, forming polyimides, polybenzimidazoles, etc
Reactions can be catalyzed by the addition of various curing accelerators, the use of drying agents etc.There are three main types of chemical drying:
- Oxidative, after taking oxygen from air, oxygen molecules react with each other forming chemical bonds between molecule binders; this drying is slower than the physical drying and can be accelerated by adding catalytic active substances, called driers.
- Baked: the drying process occurs by chemical reaction between the binder molecules, without atmospheric oxygen; This reaction occurs at temperatures above 70°C;
- Drying with two component product (epoxy resins and polyurethane) to which the film is realized through reactions, these reactions may take place either at ambient temperature or at temperatures higher than 70°C.
Drying through ovens creates a certain amount of heat by conduction, convection or thermal radiation. The drying process takes place in three modes of heat transfer, but in each case, depending on the drying method used and the construction of the plant, generally only one way to transfer heat is used, the other two participating in a lesser extent.
The behavior of varnish or paint is the real basis for calculating industrial dryers. This can not be determined, only experimentally, because the factors that influence evaporation of moisture or solvent which varies in space and time and subjects drying film changes its characteristics during the process.
In the hot air drying process, heat is transmitted only to the surface paint or varnish film, because hot air can not penetrate in the mass of the body. The film surface is transmitted by thermal conduction heat.
Because of the low thermal conductivity of the layer of varnish or paint, the exterior and interior temperature equalization occurs slowly, on the surface ,while keeping the higher temperature and, in the area on the surface and around the material, the temperature is lower.
As a result of the temperature gradient created, the layer of varnish or paint starts to harden from the surface to the inside. On the free surface, a hardened film (crust) forms which prevents the exit of the solvent from inside of the varnish or paint.
If drying by convection (hot air or flue gas), inner layer temperature is lower than the surface temperature and the temperature can not exceed the heat carriers body temperature.
Moving humidity inside the surface material is only because of the existing moisture gradient between the inner and outer layers of the material. In this case the temperature inside the material is smaller than on its surface, so the temperature gradient has a negative influence and humidity slows the movement of material by creating a crust that prevents moisture from material output.
Temperature gradient will lead to uneven expansions and contractions of the film of varnish or paint on the upper layers will be subjected to stretching, and the lower ones will be subjected to compression in the initial phase of drying and vice versa in the final phase. The tension that arises in the process of material drying becomes even more greater in the case of more intense drying.
Following the formation of the crust is the forming of blisters and cracks on the surface of the lacquer layer, which is sometimes defective or unusable varnishing made.
To accelerate convective drying, temperature must rise as much as possible, the effect of crust formation is stronger as the temperature is stronger, so the gradient is higher. The temperature of the carrier (hot air or flue gas must be properly regulated and subject material can not exceed drying temperature). If productivity is low by convection drying, drying process is proceeding slowly and takes several hours because the drying speed is limited by the strength of materials to uneven efforts. If accelerated drying degrades the materials, develops pores, cracks, blistering etc.
Large scale convection dryers.
High cost of investment of convective dryers.
Factors affecting the drying through radiation
Comparative analysis: convection drying, hot air/gas burning or drying through radiation
Comparative analysis of drying with radiant electric heater or gas
2. Use dryers mixed radiation or radiation and convection
from drying on a continuous band for heavy duty parts in ovens or directly through mere exposure to mobile installations.
3. Supplementing the current in the drying oven with a convection mode radiation placed in front of convective oven
The process of drying of paints and varnishes through radiation is based on the layer of varnish or paint to sources of radiation more precisely arranged using metallic fabric at a distance from the track. A part of the incidental radiation on the surface that penetrate the mass of the layer.
The thinness of the film, much of the radiation passing through the layer and reaches the carrier coated with varnish or paint. In the mass of the paint or varnish layer a part of the absorbed radiations turn into heat. Radiated radiation heat arises in the interior for the irradiated layer. This occurs not only to produce a more uniform heating, but also the fastest in the entire mass of the layer of varnish or paint.
The process may be conducted, through a strong absorption of the support film (support from) to warm up faster and more powerful than the exterior. Thus the drying process is done from the inside out and the solvent can leave the lacquer layer without danger of making blisters and cracks that occur when convection drying is accelerated.
- High drying shares through oven tunnels, ensures the possibility of mixed and of convection drying (exhaust gas or hot air recovery) enables application on relatively complicated piece shapes.
- Easy setting and intensity modulation of the radiation
- Crust is not achieved at the surface, the drying is made from the inside out
- Drying time is 5-6 times lower, compared to drying with hot air or flue gases
- Higher energy efficiency and the possibility of recirculation and heat recovery from flue gases
- Emitted pollutants are much lower due to both to reduce the fuel consumption and the use of radiant burners with metallic fabric.
- Space occupied by equipment decreased by 300%, mainly due to reduced length of the dryer
- Fast commissioning and for very low input regime
- The possibility of setting the speed of the conveyor, the intensity of radiation and interior temperature of the dryer, cause them to be able to adapt to different types of paint and varnish, or various types and shapes of parts.
Request the optimal scaling and drying of the radiation system.
You will receive a questionnare with the require data as soon as possible.
Factori ce influenteaza uscarea prin radiatie
Analiza comparativa : uscare prin convectie, cu aer cald / cu gaze de ardere sau uscare prin radiatie
Analiza comparativa uscare cu radianti electrici sau cu radianti pe gaz