Although generally referred by this name, it’s correct description is cellulose nitrate.

It is derived from the cotton plant linters (short fibres attached to the seed), or wood pulp from coniferous trees.

The nitration process consists of treatment with nitric acid. During the process the nitric acid replaces some of the hydrogen atoms of the hydroxyl groups, thus making the cellulose then thoroughly water washed. It finally remains with about 30% water content, being classed as explosive in the dry state.

Final preparation for industrial use consists of removal of the water (by centrifuge) and replacement with an alcohol – Industrial Methylated Spirit or Isopropanpol.

The solubility of the NC is determined by the nitrogen content. There are three classes.

10.5 – 11.2% Low                            Readily soluble in weak solvents.

                                                          Forms films only suitable for paper lacquer.

11.2 – 11.8% Medium   ) _               Need strong solvents, Esters and Ketones.

11.8 – 12.2% High        )                   Forms strong films for industrial use.

Viscosity of the NC (in a solution) depends upon the degree of polymerization resulting from the nitration process and falls into categories of High, Medium, Low and Extra Low, for each of the Nitrogen classes.

For Auto finishes a high nitrogen content with Medium to Low Viscosity usually gives a strong film at maximum solids.

To make a film former the NC is dissolved in esters, ketones or ether alcohols. Common solvents are:

Acetone, MEK, Ethyl Acetate (low boilers), MIBK, Butyl Acetate (medium boilers), Methoxypropoyl Acetate, Methyl Cyclo Hexanane (high boilers).

These are balanced to give an evaporation rate that provides good drying and flow characteristics. Also included are latent solvents (alcohols) and diluents (hydrocarbons). The latter help maintain the solubility of any additives and modifying resins that are present.

Used alone, an NC film has poor gloss and durability. The addition of a non-drying alkyd and a plasticizer such as Dibutyl Phthalate are essential to give adequate gloss, adhesion and flexibility.
The majority of pigments can be used in NC lacquers. These are either in the form of basics or dispersions for mixing in, or pigmented chips (themselves NC) which are dissolved in the lacquer.

The large molecules of the nitrated cellulose polymer impart a high viscosity to any solution. This is typical of non-convertible coatings, as opposed to the convertible coatings which have small molecules and make lower viscosity solutions with high solids.

The high viscosity lacquers therefore require large quantities of solvent to reduce to spraying viscosity – usually a volume equal to that of the lacquers. The result is low solids and more coats have to be applied to give an acceptable build.

     NC LACQUER                              DRYING BY EVAPORATION

                                                                                                                       15 - 30%

ALKYD AND TWO PACK                EVAPORATION AND X-LINKING

                                                                                                                      30 – 60%

Evaporation of the solvents accounts for the drying and through hardening of the NC lacquers.

The thinners can be tailored fast and slow to suit cold and warm application conditions.

The soluble nature of the non-convertible dry film enables it to be overcoated at any time interval with no danger of reaction (or lighting).

Good quality NC finishes still have a share of the UK market in the car repainting and furniture finishing.

General properties can be summed up as follows:

PLUS                Fast dry.

                          Non-critical application and recoat.

                          Hard durable film.

                          Polishes well.

                          Good water resistance.

MINUS             Low solids at the gun.

                          Low flash solvents.

                          Poor durability as clearcoats.

Other Polymers Derived from Cellulose

Ethyl Cellulose and Cellulose Acetate are used on a small scale for some specialized coatings.

Cellulose Aceto Butyrate (CAB) is known in refinish paints for its control of metal flake in basecoats.

                             NITROCELLULOSE FORMULATION

½ second Nitrocellulose (spirit damped)                                 9.7%

15-20 second Nitrocellulose (spirit damped)                 0.5%

Medium Oil length Non-drying Alkyd                                     12.6%         28% Solids

Rutile Titantium Dioxide                                                         4.8%

Solvents Blend                                                               72.4%

                                                ALKYDS

By definition these are condensation polymers of alcohols and acids. They are linear polymers and have two double bonds.

Made from a combination of oil and glycerol, reacted by heating (or cooking)

With phthalic anhydride to from the alkyd resin.

Drying oils such as linseed, soya bean, sunflower seed or tung produce alkyds  that dry by oxidation. Non-drying oils such as castor, coconut or cotton seed give plasticizing alkyds used to give flexibility to lacquer films.

Alkyds have an oil length expressed as a percentage – from below 45% as short oil, soluble only in aromatic hydrocarbons e.g. Xylol to medium oil up to 60%, and long oil over 60%, soluble in aliphatic hydrocarbons e.g. White Spirit.

They can be modified with various other polymers to give a wide range of drying and hardness characteristics and can be used in air drying and stoving finishes.

Long oil alkyds feature in household paints , they are relatively soft, but remain flexible. Medium oil alkyds are used in fast air dry enamels. Short oil alkyds are used with nitrogen resins (melamine) in industrial stoving finishes and in the vehicle industry – amino/alkyds types.

All are classed as convertible coatings, depending mainly upon oxidation and, therefore, not re-soluble when dry.

Because of the low molecular weight ( compared with say nitrocellulose), less thinner is required to reduce them to application viscosity. They may, therefore, be applied at a highet total solids content, and with less shrinkage occurring during the drying, they exhibit better build and filling.

      Solvent

   Evaporation                         Oxidation

           │                                           │                                      30 – 60% Solids

           │                                           │                                      Spray Application

Wet film                                                                  Dry film

Alkyds dry by the action of the oxygen in the air, catalyzed by metal compounds called driers. These are additives, and present in quantities of less than 1%.

During the drying process there is a considerable uptake of oxygen, linseed oil can take up about 12% of its own weight. The oxygen up take produces free radicals within the resin. These free radicals bond to other chains, finally producing a cross-linked resin.

Cobalt and Manganese are surface driers or primary driers, and cause the film to set up. Lead is a well known through drier or secondary drier, but due now to health restrictions on its use, it is being replace by calcium or Zirconium.

The driers are balanced to allow through drying to occur without causing wrinkling of the surface. Recoating cannot take place until full through dry is obtained – usually overnight.

Vehicle paints such as Permobel (medium oil alkyd) and Transport Finish (long oil alkyd) have the following general properties:

PLUS                            Good build - high solids.

                                      High gloss.

                                      Good flow.

                                      Mild, non- aggressive solvents.

                                      Low cost.

MINUS                         Longer drying time (than lacquer and 2-pack).

                                      Critical recoat time.

                                      Careful application and good conditions. 

                                      Non-polishable.

                                      Clearcoats tend to yellow.

Special alkyds that may be encountered in the vehicle finishes are:

Styrenated

Alkyds modified with between 10-60% addition of styrene. The advantage of fast dry, due to the rapid solvent release of styrene, but a limitation on recoat, requiring a full oxidation period of up to 24 hours. Useful in QD primers in wet on wet systems.

Vinyl Toluene

Use to speed up the drying of alkyds used in brush applied paints.

CHEMICAL PROCESS FOR PRODUCTION OF ALKYDS RESIN

 Alkyd Gloss Finish

Ready for brusing at 72.5% solids.  

ACRYLICS

The lacquer dry type acrylic, better known as thermoplastic acrylic or TPA is a polyacrylate.

It is formed of methyl methacrylate blended with ethyl acrylate and the properties of the coating depend upon the blend ratio.

MMA          __________________          EA

    Hard, brittle                                             Soft, flexible

The copolymer is of large molecular structure and its high viscosity requires extensive thinning for application – at least volume for volume.

Soluble in esters, ketones and aromatic hydrocarbons, it has fast dry properties by evaporation only and, therefore, classed as a non-convertible coating.

It is compatible with NC, plasticisers and vinyl resins, but not with alkyds and drying oils.

The dry film will soften back when subjected to high temperature and at 160°C-180°C will reflow to give a good gloss.

The reflow technique has been employed by the motor industry as OE.

Known as bake, sand, bake, it required first a force dry, followed by a second bake to obtain reflow. It is now almost completely replaced by stoving acrylics requiring only one bake.

The reflow system is not applicable to the systems in refinish.

TPA characteristics are: