Paint may be defined as a liquid applied in layers to protect, decorate or finish a solid surface and which hardens to form a solid coating. The change to a solid state may result through solvent evaporation, by chemical reaction, or by a combination of these processes. Paints usually comprise a pigment to provide colour, opacity, or body; a binder (generally a resin); a solvent to regulate viscosity; and quite often one or more of a variety of additives to provide special characteristics. The majority of printing inks consist of three main components. These are additives consisting of dryers, waxes, and wetting aids, pigments with carbon black being by far the most common; and so-called varnishes, consisting of resins and solvents with various waxes and plasticisers, and which are vehicles used to carry and transfer pigment through the printing machines .and on to the print stock.
The manufacture of paints involves the weighing and mixing of dry pigments and the grinding of these with the appropriate resin binder in a suitable mill which may be a ball mill, a roller mill or a continuous flow head mill. The shearing or crushing action of the mill ensures that each pigment particle is wetted by the resin. The milled pigment is transferred to a mixer where solvents and additives are introduced to adjust consistency, viscosity, colour and drying time. After mixing, the paint is sieved or filtered, packaged and labelled.
Printing Ink Manufacture
The manufacturing process is similar to that for paints. Pigments are weighed, mixed with the appropriate varnish, and the whole is milled to ensure that the pigment particles are thoroughly wetted by the dispersion vehicle. Additives are introduced and the finished material can then be packed.
Resins, added as the binder or varnish to paint or printing ink formulations, are either purchased from outside suppliers or manufactured in a section of the paint or printing ink premises. The process typically employs a large (1,000 litres or more) heated vessel known as a kettle, which is effectively an enclosed pot fitted with a stirrer. The reactant materials are added to the kettle and continuous temperature measurement is used to monitor the reaction, which may be exothermic.
As already discussed, the process uses flammable organic compounds and hence large volumes of such vapours could be generated in the process area. This needs to be dissipated using a proer ventilation system. The primary aim of both general ventilation and specific local exhaust ventilation around process vessels is to ensure that the airborne concentrations of contaminants and flammable vapours are maintained at the lowest practicable level. Atmospheric contaminants in the workplace often derive from a number of sources, rather than one point source. The preferred solution in these circumstances, after minimising the emissions from each source is to provide sufficient general dilution ventilation throughout the area. This can be achieved by natural or mechanical ventilation, as long as clean air is supplied in sufficient quantity, and well distributed throughout the process area. Where specific process vessels in an area are the prime contributors to atmospheric contaminants in the workplace, these sources are best controlled by local exhaust or extraction ventilation. Process vessels should be enclosed as far as practicable, and be provided with adequate mechanical exhaust ventilation.
Some paint and printing ink premises have installed bulk storage tanks, either below or above ground, to hold large quantities of commonly used solvents such as toluene and isopropanol. There are many advantages in providing bulk storage facilities but such storage also introduces additional hazards. Smaller quantities of commonly used solvents are dispensed as required from drums. Some firms have found it convenient to place these drums on racks in the open air; this means that spillage and/or drips of solvents can readily evaporate in safety, and access to individual drums is easy. The positioning of vats, tanks, and processing steps within the factory layout should be such that maximum use can be made of natural ventilation.
Static charges are generated within liquids and powders when movement occurs in such operations as the passage of liquids through pipes, mixing, pumping, agitation filtering or by pouring from one container to another. In general, the greater the velocity of the liquid, the greater the charge which is generated. Constrictions such as valves and filters also increase the rate of charge generation, and the free fall of liquids into vessels results in the liquid acquiring a quite considerable charge. In all of these instances of static charge generation, a serious hazard of ignition exists if the liquid or powder is flammable and air or oxygen is present. This is the case with many of the liquid solvents used in the manufacture of paint and printing inks.
The degree of this hazard depends on many factors including the static generating ability of the substance, the amount of time available for the build-up of charge, and the resistivity. This last quantity is a measure of the ability of the substance to hold an acquired charge; the higher the resistivity (the lower the conductivity) the greater the hazard.
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