NITROCELLULOSE SOLUTION

Ideally, Nitrocellulose lacquers are prepared with modern high speed stirrers which are first charged with non-active solvents (or diluents) and alcohols (or co-solvents).

SOLVENT TYPES AND RELATION TO SYSTEMS SELECTION

The solvent system used for preparation of the solution has a very important role in the final property of the solution including its viscosity.

The choice of the solvent depends upon:

• Viscosity
• Solids content that gives a particular viscosity
• Speed of evaporation
• Cost

Therefore the properties of solvent of relevance to the nitrocellulose solution are:

• Solubility of nitrocellulose in the given solvent
• Boiling point and evaporation rate of the solvent
• The solvents are categorized into the following types-

Based on solubility of nitrocellulose they are categorized as:                                                     

a) True solvents that can dissolve nitrocellulose on their own e.g.:

b) Co- solvents or latent solvents that act as solvents but only in the presence of true solvents      e.g.:  

c) Non-solvent or diluents or extenders that are used primarily on account of low cost but in     which the nitrocellulose is not soluble e.g.
    Aliphatic & aromatic hydrocarbons e.g.- benzene, xylenetoluene, white spirit etc.

Based on their boiling point ranges they are categorized as follows:

Low boilers yield solutions of low viscosity and quick evaporation. The medium boilers check the rate of evaporation, impart good flow properties and reduce the defects caused by too rapid an evaporation. High boilers also act similarly and impart high gloss and brushing properties. Evaporation rates of pure solvents and their mixtures give only an approximate guide to the rate of evaporation from lacquer film.

Taking the evaporation of butyl acetate equal to one as standard, the following are the relative rates of evaporation at 50% loss of some of the common solvents used in lacquers.

COMMON PROBLEMS ENCOUNTERED WITH IMPROPER SOLVENT SELECTION

It is necessary to control evaporation of various volatile components of the lacquer in order to prevent ‘Chilling’, ‘Gum Blush’ and ‘Cotton Blush’. These features are described below.

‘Chilling' refers to the formation of an opaque film and is attributable to the lowering of the temperature of the film and air near the film to below the dew point. It is caused by the high rate of evaporation of a highly solvent from the solution. The problem can be remedied by increasing the proportion of the ‘medium boiler’ in the solvent mix and butanol is particularly useful for this.

‘Gum Blush’ and ‘Cotton Blush’ occur when either resins or nitrocellulose respectively are precipitated from the lacquer. ‘Gum Blush’ is caused by too rapid an evaporation of solvent for the resin which eventually is precipitated out and similarly ‘Cotton Blush’ is caused if at any point during evaporation of solvents the dilution ratio is exceeded. ‘Blushing' can be avoided by using a small proportion of high boiling solvent which dissolves both nitrocellulose and resin.

PLASTICIZERS

Plasticizers are substances of low volatility which are added to lacquers to improve flexibility, tensile strength, gloss, adhesion, thermoplasticity, evaporation of volatile solvents, and the susceptibility to ‘Gum Blush’. These are classified as follows:

Choice of plasticizer type depends on characteristics desired in the film i.e. durability, colour retention, flammability and temperature flexibility

RESINS

Solutions of nitrocellulose require resins which enable formulators to increase solids content significantly without any increase in viscosity. In this way thickness of films is increased in addition to gloss, adhesion, chemical resistance, and durability.

Common resins are alkyds, ketone, acrylic, polyvinylacetate and sulphonamide types.

PIGMENTS

Most coloured lacquer films are pigmented although for some purposes, such as coatings on aluminum foil, dyes are used.

SELECTION OF NITROCELLULOSE

Choice of the correct grade of nitrocellulose for formulating lacquers is governed mainly by solution characteristics and also the properties of the final coating. A knowledge of the factors that govern the solubility of nitrocellulose, the viscosity of solution and the properties of lacquers and films made from it is therefore of considerable practical value to lacquer manufacture.

SOLUBILITY

The solubility of nitrocellulose depends on the nitrogen content and to a lesser extent on viscosity. All industrial nitrocellulose grades with nitrogen content of 10.7 to 12.2% are soluble in esters, ketones, glycol ethers (active solvents) or in suitably formulated mixtures of these with hydrocarbons (diluents) and/ or alcohols of low molecular weight. Low nitrogen grades are used where high alcohol or spirit solubility is desired.

HYDROCARBON TOLERANCE: DILUTION RATIO

A hydrocarbon, non solvent is added normally to a solution of nitrocellulose for achieving economy and dilution. In this addition a point is eventually reached when the nitrocellulose no longer remains in solution and is precipitated out. The volume ratio of hydrocarbon non solvent or diluent to solvent at this point is called dilution ratio.

Hydrocarbon tolerance of any given Nitrocellulose varies with the solvent used and the type of diluent. For a given solvent mixture it depends on

Variation of dilution ratio with type of solvent and dilution is illustrated in Table 1. In this table the value of dilution ratio is shown to vary with type of solvent i.e. butylacetate or MIBK or ethyl alcohol and the type of diluent i.e. toluene, SBP petroleum solvents and n-butanol for the same concentration and grade of HX 30/50. In Table 2 the value of dilution ratio has been shown to be varying for different grades of nitrocellulose in the same solvent

Dilution Ratio is useful as a guide to the quantity of one diluent that may be used as a substitute for another in an established formulation. The actual ratio of diluent / hydrocarbon non-solvent to active solvent in practice should always be appreciably less than quoted dilution ratio for two reasons.

EFFECT OF SOLVENT COMPOSITION TYPE ON NITROCELLULOSE LACQUER VISCOSITY

The viscosity of a solution containing a given concentration of nitrocellulose depends on the chemical type of the solvent i.e. whether esters or ketones etc. within a particular chemical type, the solvent with the lowest boiling point give the lowest viscosity. As an example ketones of same boiling point as esters would give lower viscosity.

In mixed solvents, moderate amount of alcohols reduce viscosity. Minor amounts of hydrocarbon diluents have little effect, but as the proportion of hydrocarbon non solvent increases, the viscosity rises until finally the nitrocellulose becomes insoluble.

AGEING EFFECTS

The viscosity of nitrocellulose solutions drops somewhat on storage. This fall is marked during first week and varies according to composition and storage temperature. The effect is attributable entirely to solvation and implies no chemical degradation of the nitrocellulose. There is no known way of preventing it and users are not recommended to try additives as a short cut to attaining equilibrium.

FILM PROPERTIES

As with all polymers, the strength and durability of nitrocellulose lacquer films depends on the molecular weight, which in turn varies with viscosity. Thus the lowest viscosity grades e.g. HX 8/13 which have low molecular weight would the weakest, least durable film. These are recommended for high solid lacquers and inks where high gloss and build up are more important than durability and high film strength.

The latter properties improve as we move up the viscosity scale and HX30/50 represents the best compromise between good durability and reasonable solids. Where more flexibility is needed as in leather lacquers, HL 25/45 is recommended. Grades with viscosity higher than this offer no significant improvement in film properties and are used only in specialized outlets e.g. where a minimum of film is required or for blending with lower viscosity grades to make lacquers with a particular combination of solids and viscosity.

STABILITY

All industrial Nitrocellulose supplied passes the Bergmann and Junk stability test. However the following points should be noted:

1. The addition of alkaline and strongly acidic material is harmful
2. Certain amines (diethylamine, monoethanolamine, morpholine etc.) if allowed to come in contact with industrial Nitrocellulose can cause spontaneous ignition or charring. There is a risk of fire particularly with dry nitrocellulose. Any spillages should be cleaned up immediately.

These amines also cause severe discolouration of nitrocellulose films and solutions. The use of alcohol denatured with pyridine should be rigorously avoided.
Avoid storing nitrocellulose or nitrocellulose lacquers at high temperature or in direct sunlight.
Avoid using pigments which give an alkaline reaction.

BLENDING

With the wide range of viscosity grades supplied, it is not normally necessary to blend different grades of nitrocellulose. Occasionally, however, stocks of a particular grade may be exhausted, or there may be a special requirement that cannot be met by available materials. In such cases mixtures of suitable grades will give the desired viscosity.

BLENDING LIMITATIONS

It is not advisable to blend different nitrogen grades or any grades which are far apart in viscosity, since either procedures can cause rough, granular solutions or resin incompatibility problems.