Лако-красочные материалы — производство

I INTRODUCTION

Waterborne paints have been used since earliest times, often as solutions of clays in water, dyed with natural materials such as barks, berries, etc. Later pigments were dispersed in gums, glue, egg whites, gelatine or casein. Waterborne materials continued along this line of development until the 1940’s when emulsified drying oils and alkyds were being introduced. Other developments included polyvinyl acetate and butadiene/styrene latices produced in Germany and the United States. These were often referred to as emulsion polymers. Waterborne alkyds in solubilised and dispersed forms were introduced in the early fifties.

Water reducible polymers are often described as waterborne, water based or water soluble materials although only a few polymers may be regarded as truly water soluble. For coatings, the term waterborne can be regarded as genetically correct for coatings containing water in their volatile components, which includes emulsion resins.

The growth of waterborne coatings and hence the need for water containing resins started to gain impetus in the period 1955-1960. These requirements came mainly from the United States, because they were a means of

• reducing the fire risk from flammable solvents

• reducing the need for hydrocarbon solvents which were becoming expensive and in short supply

• complying with environmental legislation which was intended to reduce emissions to the atmosphere.

The major concern of this legislation was the reduction of air pollution caused by emission of toxic or irritating volatile organic compounds or breakdown products, into the atmosphere. The original regulations commenced with the Californian Rule 66, which attempted to control the emission levels of photochemically reactive solvents used in resins and coatings. It became obvious that non-photochemically reactive solvents can be irritants and that the answer was to limit the total amount of solvent emissions. The direct result of this was to encourage the development and use of waterborne, high solids, electrocoating (waterborne), radiation curable and powder coating materials.

1. Volatile Organic Compound Content — VOC’s

One of the ways of expressing and limiting the amount of volatile emissions was to invoke the concept of volatile organic compound content, often represented as VOC, of a coating system. Frequently volatile organic compound content and volatile organic content are used interchangably, with both being represented by the abbreviation VOC. Strictly speaking VOC represents volatile organic compound. It does not matter which is referred to, because in practice the legislation and people refer to VOC limits or the VOC of a coating, when they are really referring to the VOC content.

Europe is now actively involved in reducing the emission of organic materials to the atmosphere. Whilst both Europe and the USA have encompassed the concept of VOC’s, the definitions for calculation differ, so that European and American values are not interchangeable and cannot be correlated easily and directly. The USA uses mass/volume and Europe uses mass/mass, thus the density must be known for American VOC’s.

In the United States the Environment Protection Agency ( EPA ) adopted the concept of VOC’s with vapour pressures higher than 0.1 mmHg at 25°C. This is simply the number of grams of solvents present per litre ( or more commonly lbs/gal) of paint or resin. The EPA has presented limits for stoving and for air drying types, but many states have reduced these levels.

There are many ways of expressing VOC’s in the USA; some relate to the VOC of the liquid paint or coating, whilst others relate to the VOC with respect to the dry film weight. The calculations are further complicated in that VOC’s for coatings “as supplied” and “as applied” are required in many states. This requires knowledge of dilution factors. Yet another complication is the absence or presence of water in the coating, with different equations being required in some cases. The following equations can be used for the calculation of the VOC of a coating as supplied.

The reader must refer to the relevant ASTM test manuals(la) for fuller details on the methods of measuring these quantities, their definitions and other equations which can be used.

If the coating contains no water then the VOC as mass per unit volume of coating as supplied is;

vor _ wP/o organic volatile content x density 100%

If the coating contains water the equation becomes;

УДЄ wt% organic volatile content x density 100-vol% water

It is also possible to express the VOC as mass per unit volume of solids. In this case the same equation is used for solvent and waterborne coatings, namely:

wt% organic volatile content x density
vol% solids

All of the above terms relate to measurements of the coating as supplied (in the can). There are a similar set of equations for VOC’s as applied, with appropriate changes to the definitions. There is also a method of calculating the VOC as applied from knowledge of the VOC as supplied and the dilution solvent ratio. Consult the ASTM test manualla) for further details.

In Europe, VOC limits are currently under re-evaluation but the EEC is committed to reducing the total solvent emission by at least 60% for surface coatings, from the level which they would have obtained if coatings throughout the EEC were all solvent based. Some companies incinerate or reclaim solvents, whilst others use solvent systems containing sufficient water to comply with these requirements. The next implementation date is about 2002. European VOC’s are essentially calculated as follows:

(100 — wt %. water-% solid)

% solid X 000

This gives measurement for units in g/kg.

The water content is determined by either Karl Fischer or chromatography with a flame ionisation detector. The ASTM solids at 110°C for 1 hour are used for the non-volatile content.

Consider two examples with the same solvent content (14%) but differing solids (20% and 40%).

i) 20% solids

(100-66-20)

20

This is the current compliant limit for internal sprays for DWI cans, which are in essence 20% solids (non-volatile content).

ii) 40% solids

When one compares the emission levels obtained with waterborne materials containing relatively small amounts of organic solvents, with conventional solvent borne materials, the selection of water as a solvent becomes an obvious choice.