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VARNISH CHARACTERISTICS   The data reflected in this table should be taken in general terms since each varnish has specific characteristics
Kind	Drying	Flexibility Y accession	Resistance to the sulfuration	Resistance chemistry	Use	Applications
Oleoresinins “R”	205ºC/12′	Good	Bad	Resistant to acids	Varnishes interiors	Acid fruits Vegetables
Oleoresinsins “C”	205ºC/12′	Good	Good	Does not resist to acids	Varnishes interiors	Rich foods in proteins
Phenolics	200ºC/15′	Good	Very good	Good	Finished varnish B. interior	Meats and fish
Epoxy-phenolics	200ºC/15′	Good	Regular	Good	B. interior B. hook	Rich foods in proteins
Epoxy-phenolic + Al	200ºC/15′	Good	Very good	Good	B. interior	Rich foods in proteins
Epoxy-phenolic + OZn	200ºC/15′	Good	Good	Not fit for acids	B. interior	Rich foods in proteins
Epoxy-amines	195ºC/12′	Good	Regular	Good	B. hook B. outside	Decor
Epoxi-ester	180ºC/12′	Good	Bad	Regular	B. finished B. outside	Decor
Epoxi-modified	190ºC/15	Good	Good	Good	B. white inter B. hook B. outside	Aggressive products Decor
Vinyl	180ºC/10′	Very good	Bad	RegularB. interior White enamel B. hook	B. interior White enamel B. hook	Drinks Decor
Acrylics	190ºC/15′	Very good	Very good pigmented	Very good	White enamel B. finished	Vegetables Vegetables little pigment. Decor
Polyester	200ºC12′	Variable	Regular	Good	B. white inter B. interior B. hook White enamel	Env. embut and tapas in 2 passes Drinks Decor
Organosols	195ºC/15′	Very good	Good	Good	B. white inter B. interior	Easy tops opening Tapas Env, embut.

FILM WEIGHT.

The load or weight of dry film is the amount of dry extract remaining on the metal after applying the wet and baked varnish. It is measured in g / m2 (or mg / pg2)

Orientatively it can have the following values:

Interior systems:

– Varnishes in general Between 5 to 7 g / m2

– Varnishes + Al “6 to 9”

– Varnishes + OZn “7 to 10”

– White “9 to 15”

– Organosols “13 to 21”

External systems:

– Hooks Between 2 to 5 g / m2

– White enamels “14 to 17”

– Finishing varnishes “5 to 7”

 

VARNISH APPLICATION TECHNIQUES

 

There are three basic procedures for varnishes applied in liquid state:

• Application on sheets on one side thereof by means of rolls
• Application on both sides coils
• Application by spray.

Roller sheets

It is the oldest and most extended varnish procedure. In it, each sheet of metal is passed through a pair of rollers, one of them – elastic – is impregnated with the varnish to be applied. The material of said roller is gelatin or synthetic rubber, not attackable by solvents. With the appropriate adjustments the thickness of the applied coating is controlled.

The set of devices that allow each sheet to be fed from one package, moved and passed through the applicator system, receives on the market the name of the coating machine. Afterwards, a continuous furnace is mounted, which dries the varnish, eliminating solvents and allowing the film of solid residues adhering to the sheet to reach the degree of polymerization and crosslinking suitable for its protective function.

The baking conditions are variable according to the type of resin used. With a very general character, it can be around 200º C and a time of 12 minutes. The temperature will never be higher than 232º C – tin melting – to avoid the damaging effects of remelting it.

This technique of varnishing is the most common in the metal industry, since it is very flexible adapting to all the needs of varnishes as to the different parts of the container. Thus making the appropriate “reserves” on the elastic roller, can be varnished bodies for containers of three pieces, where the areas to be welded have to be free of varnish, sausages, lids, etc.

Coils

It is possible to varnish coils in a process similar to the previous one but continuously. It is much more complex and has more limitations, since it is not possible to make varnish reservations. Its use is limited to covers and bodies embedded in large runs.

Spray

There are some uses of containers that due to the aggressiveness or delicacy of the product – for example: carbonated drinks – require that its interior surface be totally free of exposed metal – pores, scratches, abrasions, etc. – and it is required to apply the varnish – usually in several layers of different type – once the finished container is in place. In these cases, spray application is used, using a fixed or mobile gun while the container is rotating at high speed. Subsequently the drying process is similar to the first technique. This application is typical of “two-piece” DWI containers.

Also a similar manner of spray coating can be applied to cover the side seam zone of the “three piece” container bodies. Said area has been reserved for varnish to make possible the welding and later it is necessary to protect it, resorting for it to the application by atomization and subsequent drying. This procedure was in force for a long time but nowadays it has been displaced by another powder coating application system.

These powder varnishes are of the thermoplastic or thermostable type, are applied by an electrostatic deposition technique on the surface to be protected and subsequently subjected to heat to cause their fusion. They have the advantage of reducing the emission of solvents to the atmosphere since 100% of them are solids.

 

PARAMETERS OF THE VARNISHES

The most important to control are the following:

Liquid varnish :

• Viscosity
• Specific weight
• Type of resin
• Solvent
• Proof of assignment or migration

Dry applied varnish :

• Load or weight of film
• Curing control
• Adherence
• Porosity
• Resistance to autoclave
• Scratch resistance
• Resistance to sulfuration

The description of these tests would make this work excessively long. As they are common to all varnishes and in a certain way suppose a separate chapter on them, it will be treated in an independent theme.