The Theory Regarding Nitrogen Spray Painting Technology

The use of nitrogen as a fluid carrier in the spray painting industry and in other applications:

In the manufacturing industry inert gases are widely used for storing, conveying

and spraying paint.

However, the advantages of nitrogen over argon, whose use is not cost effective,

are its low density and its low fusion and boiling temperatures making it the

ideal fluid carrier.

Nitrogen is often used in the spray painting industry to seal polyurethane based

paints (isocyanic prepolymers in particular) which react when exposed to the

humidity present in the air we breathe, since vacuum sealing can not guarantee

the complete elimination of water molecules. This has proved to double its shelf

life.

The cost of obtaining nitrogen gas from its liquid state, though, offers no real

economic advantage. DERIVAIR, instead, has come up with a new method (recently

patented) for generating nitrogen through a process of selective permeation

(PSA) that is much more competitive.

Using nitrogen as a propellant in spray painting systems - a method and an

application technique covered by an international patent owned by EUROSIDER -

has produced outstanding results.

To date two spray painting techniques (excluding powder coating technology) have

been adopted:

1. Airless or rather the indirect use of air as a fluid carrier with equipment

that includes a special membrane pumping unit;

2. compressed air and a spray gun, where air itself is the propellant for the

paint.

The first is based on Pascal's principle, where the size of the membrane pumping

unit Determines the amount of increase in pressure; that is to say, from a

starting point of 8 bars The pressure, according to the capacity of the

membrane, can be increased to up to 200 Bar and more.

This technology is often used in industry in the application of solvent less or

solvent free Paint in a single coat (up to 2mm) where the paint is highly

viscous. These Types of paint, however, can not be used with spray guns or spray

painting systems which require the paint to be applied in much thinner coats.

In the body shop spray painting and woodworking sectors it is common practice to

use Paints with a high organic or inorganic solvent content (10% - 50%) with finished coats from just a few microns up to 200-300 microns, after which solvent free paint or paint with a high solvent content is applied.

In this case, however, the air even when it is dehumidified, dried and heated

contains impurities and has the potential to become charged with static

electricity, which can be transferred to the surface being sprayed with a

finished result that is less than satisfactory.

Moreover, with certain paints the presence of foreign bodies can even cause a

chemical reaction which alters the properties of the paint and its drying time.

Nitrogen, on the other hand, because it is anhydrous, removes humidity from the

surface being sprayed and because it has a low specific weight does not alter

the outlet flow pressure of the spray gun.

Furthermore, by heating the nitrogen to a temperature of 50°C and above, the

paint becomes more fluid reducing the need for solvents and in turn allowing for

a reduction of the outlet pressure, which means there is also a significant

reduction in the overspray effect.

Since nitrogen also has an extremely low dew point, it not only eliminates the

problem of humidity but also the age-old problem of blistering.

This new method classifies as 'surface tolerant', a term which indicates a type

of paint that can be applied to surfaces where humidity is present (pressure

pipes for example).

By first spraying the surface being treated with heated nitrogen, application

times are shortened and there is a considerable reduction in the amount of

contaminants released into the air, anticipating future standards regulations

for emissions.

In the case of wood and concrete which are more porous, application times are

reduced even further.

Of course certain paint products need to be slightly reformulated in order to

obtain the best results using a system such as that proposed by EUROSIDER, but

this is a minor consideration, since with the experience and know how acquired,

it is just a matter of simplifying the formulation itself.

There are in existence spray painting systems which use so-called reactive

coatings that harden in 5 to 6 seconds, where the surface being sprayed has to

be completely free of humidity. Such systems use various methods – hydralics for

the primary pump and compressed air for the secondary pump.

The use of nitrogen eliminates all the problems associated with the formation of

mositure during expansion, such as the malfunction of the main pumping unit and

the resulting imbalance of the system in general.

One of the most interesting characteristics of this new method is that it can charge the nitrogen (using a sophisticated, patented system) with static electricity. Since nitrogen has no polarity, this can be selected to prevent possible inverse polarity being generated in the paint and therefore on the surface being sprayed.