Nitrocellulose as coating

Christian Schönbein discovered Cellulose nitrate in 1845, like many things in science, it happened by chance, when he went to dry a piece cotton (cellulose fiber) piece cloth in the heater, with the remains of  mixture of sulfuric and nitric acid, he observed that cloth inflamed. He deduced, that nitric acid, sulfuric and cotton cloth had reacted forming: nitrocellulose (cellulose nitrate).

If the nitrocellulose applied to a surface letting it air dry letting it air dry, we can see, that formes a thin, tough and very resistant film. For this reason, it can be used as coatings and care of surfaces.

The nitrocellulose was the first semi-synthetic stable resin. with these characteristics, we can be achieved. texture rigid and impact resistant , they were used as coating and finishing of wood, due to its texture rigid and impact resistant.

If the cellulose nitrate is mixed with solvents and plasticizers, improve their conditions of flexibility and faster drying, while achieving better application viscosity for both brush and spray.

In 1919, Nobel company introduced the first decorative varnish in UK, based in nitrocellulose.

In 1920,  New Explosives Company improved formula about nitrocellulose, giving finished product of high quality to metals and woods.

In 1923, DuPont company developed a method for producing a coating (lacquer) of low viscosity and high content of nitrocellulose.

In 1924, DuPont company creates the trade name: Duco for the production line of Ford cars, improving the drying time and introducing a very important thing: the color.

Glidden and Sherwin-Williams  compañies introduced into the domestic market a range of nitrocellulose for brush application.

In 1925, these coatings are improved by the introduction of alkyd resins.

This type of formulation had a rather short life, due to a number of disadvantages  such as high solvent content, high flammability (due to solvent) and the problem of the speed of evaporation, making it difficult to application.

The nitrocellulose compositions were improved over the years, through solids, controlling the degree of nitration (nitrogen content) and molecular weight (viscosity), through mechanical with plasticizer and through synthetic resin to give better texture. Through these variables we can have nitrocellulose taylor made.

Characteristics of the nitrocellulose are:

  1. Wide solubility in organic solvents.
  2. Compatibility with different resins and plasticizers.
  3. Odourless transparent films.
  4. Easy processing.
  5. Quick drying
  6. Rapid removal of the solvents in the application.
  7. Low retention of residual solvents.Non toxic

The nitrocellulose is easy to use due to features: auto lacquers, surface coatings, inks, enamels, etc.

The nitrocellulose

Cellulose is a polysaccharide of high molecular weight and linear structure. It has as monomer the beta-glucose, having three free hydroxyl per glucose unit. These will be responsible for the nitration, by grade level we will get different viscosities of nitrocelluloses.

The polymerization degree of the nitrocellulose is measured by the number of bonds beta glycolysis in the molecule. This index indicates the viscosity.

Depending on the degree of nitration, we will have different types of nitrocellulose. If we get the nitration of three hydroxyls of each molecule, we will obtain the maximum poncertage  14.40%.

nitrocelulosa1 (2)

Cellulose is a polymer composed of between 1,500 to 10,000 bonds beta-Glucose. When it is nitrated, partially broken chains, producing bonds between 70 to 300 units. The mechanical nitrocellulose properties are better against longer the chain. The viscosity indicates the length of the chain. The rheological behavior of the polymer is non-Newtonian.

 nitrocelulosa2
 

The nitrocellulose by nitration can be classified.

N2  content
% Solids Viscosity (solvent)
High (Solubility esters, ketones glycols) 11,80 – 12,20 % 10 – 1000 cp
Low (Solubility alcohols) 10,80 – 11,20 % 18 – 60 cp

 

As we know, the nitrocellulose are very soluble in organic solvents, therefore can be formulated products with a wide range of them to satisfy COV.

 

The nitrocellulose can be formulated as follows:

Type of composition Components
Binary Nitro / plastificizer
Tertiary Nitro / plasticizer / resin
Quaternary
Nitro / plasticizer /resina A / resin B

 

Plasticizers for nitrocellulose

Product Features
18 p blown Caster oil Flexibility
Epoxydized Soybean oil
Thermal and light stability
Fatty acids epoxy ester Thermal and light stability
Acetyl tributyl Citrate Light fastness

Resins for nitrocellulose

Resin
Characteristics Dosage about resins
Alkyd Elasticity / adhesion, /durability 50 %
Maleic Hardness / gloss / adhesion / drying
30 – 45 %
Polyamide Flexibility/gloss/heat and light resistance 10 %
Formaldehyde Flexibility/gloss/heat and light resistance 10 %

 

Nitrocellulose1


Problems of nitrocellulose

The nitrocellulose fogged is the whitening of film or non-leveling and gloss loss, as resulting from rapid evaporation of solvents, which produces a cooling surface of film, resulting in condensing into drops of water vapor, that gives appearance fagged.

The nitrocelluloses are very sensitive to moisture, for this reason drying is very important. We must use appropriate solvents with high boiling point (100-140 ° C), due to slow evaporation, achieving perfect drying depth and surface in these conditions, removing the problems of fogged produced by the absorption of water retention.

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