OCCUPATIONAL EXPOSURE AS A PAINTER
1.2 Production and use of paint products
1.2.1 Production
(a) Production processes
The modern manufacture of paints, which are produced mostly in batches, sometimes also continuously, involves three major steps: (i) mixing and grinding of raw materials; (ii) tinting (shading) and thinning; and (iii) filling operations.
Manufacturers first load an appropriate amount of pigment, resin and various liquid chemicals for homogenizing into a stirrer. The homogenized mixture is then transferred to a roller mill, which is a large rotor-stator steel cylinder. Mills for grinding primers or dark pigments are partly filled with steel balls or ceramic pearls of about 0.1–3 mm in diameter.
Mills for grinding light colours usually contain flattened ceramic or zirconium dioxide spheres (pebbles) of about 0.1–2 mm in diameter. Depending on the type of mill used, the grinding process lasts for about 0.5–2 hours or until the pigment has been ground to a sufficiently fine paste. After that stage, the grinding pearls are removed and more resin and solvent are added to the paste, and the process repeated. The paste is then pumped out of the mill through a strainer to a holding tank (Brock et al.. 2000; Goldschmidt & Streitberger, 2002).
68 IARC MONOGRAPHS VOLUME 98
The ‘tinting’ step involves comparing samples in the holding tank with colour standards. Small amounts of shading pastes, which are highly concentrated blends of ground pigments, and a vehicle are added as required to match the standard. After the batch has been shaded to specifications, it is thinned to the desired viscosity by the addition of solvent, filtered and poured into containers for shipment (Brock et al., 2000; Goldschmidt &
Streitberger, 2002).
The complexity of paint technology is indicated by the numerous types of raw material required. A plant that produces a broad line of trade, maintenance, and industrial paints requires over 1000 different raw materials as well as intermediates including oils, pigments, extenders, resins, solvents, plasticizers, surfactants, metallic driers, and other materials.
The modern manufacture of unpigmented lacquers is generally a cold-cutting or simple mixing operation. For example, cellulose nitrate solutions are made by adding the nitrated cellulose from alcohol-wet cotton to the solvent mixture and agitating for 1–2 hours in a paddle or turbine blade mixer. Alkyd resins, which are supplied in solution, can be added directly to the cotton-based solution. Hard resins may be dissolved separately and added as solutions, or the lumps may be dissolved directly in the cotton-based solution by stirring.
The new paint systems are usually produced in the same sequence and with similar equipment as solvent-borne paints. Coating powders, by contrast, need other machineries, such as the extruders used in the plastics industry.
A general trend in the production is the reduction of exposure of workers to the material. Pigment, fillers and solvents (and solvent-based binder solutions) are increasingly removed by ventilation exhausts or by totally-closed automatized production lines.
However, many small manufacturers, especially in low-resource countries, still produce paints with a technology without exhausts.
(b) Production volume
Traditionally, two types of coatings are produced: trade sale paints and industrial product finishes.
Trade sale paints are primarily for exterior and interior coatings for houses and buildings, although sizeable amounts of automobile and machinery refinishes, traffic paints and marine shelf-goods are also dispensed through trade sales outlets.
Industrial product finishes or chemical coatings are produced to user specification and sold to manufacturers for factory applications on such items as automobiles, aircraft, appliances, furniture, plastic parts, and metal containers. They also include the category of industrial maintenance coatings, which are specially formulated and are used to maintain industrial plants and equipment (e.g. as resistance to corrosion).
World production of surface coatings in 2005 by selected countries or regions is given in Table 1.3. In 2005, North America produced 6.3 million tonnes (23.1%), western Europe 6.6 million tonnes (24.1%), and eastern Europe 2.5 million tonnes (9.1%). China produced 3 million tonnes (10.9%), with a strong trend in increasing production.
OCCUPATIONAL EXPOSURE AS A PAINTER 69 Table 1.3. World production of surface coatings by selected country or
regions in 2005
Region Production (in thousands of tonnes)
%
USA NAFTA Western Europe Germany Eastern Europe Russia Asia Japan China Latin America Brasil Rest Total
5373 6330 6600 1810 2500 750 8700 1512 3000 1540 674 1750 27430
19.6 23.1 24.1 6.6 9.1 2.7 31.7 5.5 10.9 5.6 2.5 6.4 100.0 NAFTA, North American Free Trade Agreement
From CHEM Research GmbH (2006)
The worldwide production of industrially applied paints grew from 6.3 million tonnes in 1980 to 10.5 million tonnes in 2006. In contrast, for the same period, solvent consumption barely increased from 4.1 to 4.2 million tonnes – a consequence of the increasing use of solvent-reduced paints (Streitberger, 2007). Table 1.4 gives details of the production of paints and coatings in Germany by type of resin.
Table 1.4. Production of paints and coatings
Type of resin Production (in thousands of tonnes) by type of resin Solvent-borne
Alkyd 110.4
Acryl 50.9
Natural oils 6.1
Vinyl, styrene 26.2
Epoxy 68.4
Urethane 68.2
Cellulosic 26.7
Polyester 55.7
Phenolic, melamine, urea 4.2
Bitumen, tar 28.5
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Table 1.4 (contd)
Type of resin Production (in thousands of tonnes) by type of resin
Shellac etc. 5.7
High Solids 32.9
Other resins 42.9
Total 526.9
Powder coatings 73.5
Waterborne Dispersions (interior) 639.5
Dispersions (exterior) 174.7
Primers 80.3
Synthetic resin plasters 199.2
Glue paints etc. 22.7
Silicate wall paints 30.4 Silicate plasters 27.1 Dispersion laquers 91.7 Electrodip coatings 36.7 Phenolic, melamine, urea 1.0
Putties 189.4
Silicon resin paints 10.6 Silicon resin plasters 19.3 Other resins, synthetic 121.3 Other resins, natural 2.2
Total 1646.1
Thinners (organic solvents) 231.3 From Verband Der Deutschen Lackindustrie (2007)
1.2.2 Application methods
The uses and properties of polymer systems in industrial coatings are described in Table 1.5 and 1.6. The various methods of paint application are presented in Table 1.7.
Most paints are applied by simple methods such as brushing or rolling, yielding high transfer efficiency, and with no spray dust formation. Electrodeposition of paint, introduced during the 1960s, was an important milestone in industrial painting and has proven especially advantageous for painting automobile bodywork and other parts thanks to its superior corrosion resistance. In this technique, the coating is an aqueous dispersion of low solid content. The binder particles carry ionized functional groups which may be positive or negative, thus having either anodic or cathodic deposition.
OCCUPATIONAL EXPOSURE AS A PAINTER 71 Table 1.5. Uses of polymer systems in industrial coatings
Polymer systems Coil Metal Appli- ance
Furni- ture
Hard- board
Lumber and plywood
Marine Main-tenance
Auto- mobile OEM
Auto- mobile refinish
Tins
Natural and modified polymers
Drying oils (+) (+) + + + (+)
Cellulose esters + + + + +
Cellulose ethers + +
Condensation systems
Alkyd resins + + + + + + + + (+) (+) (+)
Polyesters, high molecular weight + + + + +
Amino resins + + + + + + + +
Phenolic resins + + + + + + +
Polyamides + + + +
Polyurethanes + + + + + + + + + +
Epoxy resins + + + + + + + + + +
Silicones + + + + + + +
Vinyl polymers and copolymers based on:
Butadiene + +
Acrylic or methacrylic ester + + + + + + + + + +
Vinyl acetate + + + + +
72IARC MONOGRAPHS VOLUME 98 Table 1.5 (contd)
Polymer systems Coil Metal Appli- ance
Furni- ture
Hard- board
Lumber and plywood
Marine Main-tenance
Auto- mobile OEM
Auto- mobile refinish
Tins
Vinyl chloride + + + + + + + + + +
Vinylidene chloride + +
Styrene + + + + + + +
Vinyl acetal or butyral + + + + + +
Fluorocarbons + +
Resin combinations
Acrylic and amino + + + + + + +
Acrylic and epoxy + + + +
Acrylic and silicone + + +
Alkyd and amino + + + + + + + +
Alkyd and acrylic + + + + + + +
Alkyd and epoxy + + + +
Alkyd and silicone + + +
Polyester and epoxy + + + + +
Polyester and silicone + + +
OCCUPATIONAL EXPOSURE AS A PAINTER 73 Table 1.5 (contd)
Polymer systems Coil Metal Appli- ance
Furni- ture
Hard- board
Lumber and plywood
Marine Main-tenance
Auto- mobile OEM
Auto- mobile refinish
Tins
Cellulose ester and urethane +
Alkyd, acrylic and amino +
Polyester and amino +
Phenolic and epoxy + + +
Epoxy and amino +
Phenolic and amino +
Alkyd and vinyl chloride polymers + +
Updated from IARC (1989a) by Working Group OEM, original equipment manufacturer
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Table 1.6. Properties of paint systems for different uses
System Use for Advantages Limitations Use trends (Nitro)cellulosics
physical drying
Furniture, small mass articles Fast drying, scratch resistant, alcohol resistant, thin layers possible
Poor light and solvent stability, poor solids content, high solvent content
Declining
2K-PUR (‘DD-paints’) and 1K-PUR-heat curing paints
Furniture (kitchen), high performance exterior, e.g.
vehicles, ships, metal, aircraft
Mechanically and chemically very stable, elastic
Expensive, time in which the material must be used before hardening in the can
Increasing
Acid-curing enamels, urea–formaldehyde resin +Alkyd resin
Similar to 2K-PUR Similar to 2K-PUR Formaldehyde emission Declining
Oil-based resins (alkyd resins)
Exteriors, wall paints, DIY Good gloss and surface, resistant against weather and chemicals
Slow hardening, often brittling, sensitive against alkali
Declining Unsaturated Polyesters
(UP-paints)
High performance, glossy surfaces e.g. pianos. Putties
Scratch resistant high gloss, low solvent content, high thickness possible
Not light stable,
short processing time (2K), poor storage stability and adhesion
Constant for special uses
Waterborne:
Acrylate- and PUR- dispersions
Interior furniture, DIY Low VOC content, light stable Slow drying, expensive, strong roughening at wood surfaces
Increasing
UV-curing paints, also:
electron beam coating
Furniture, specially in schools, parquet
Extremely fast and resilient curing, very low VOC
UV: limited pigmentability Increasing
Powder coatings (esp. acryl-, polyester-based)
Metal, appliance,
machineries, automotive parts Solvent-free, fast hardening Heatability of object (plastics
and wood very difficult) Increasing
OCCUPATIONAL EXPOSURE AS A PAINTER 75
Table 1.6 (contd)
System Use for Advantages Limitations Use trends Polyester (1K, 2K) Primers, various interior uses Cheap, mechanically resistant Poor weather and chemical
stability Declining
Thermoplastic acrylates Industrial metal coating, low performance appliances
Weather stability, flexible, chemically stable
Poor hardness, heat-softening, expensive
Declining
Melamin resins (oven-curing with OH-polyester, -acrylate,- alkyd)
Weather-stable top coats:
vehicles, machinery, appliances, coil coating
Hardness, resistance, adhesion to substrate
Properties not really at level of 2K-PUR
Declining
Epoxy resins (esp. 2K) Vehicle primers, corrosion and construction protection, Tank interior
Excellent adhesion (especially on zinc, a difficult substrate), resistance, flexibility
Poor weather stability:
yellowing, chalking
Constantly high level
Phenolic resins and similars, different hardeners
Primers, electro insulation, tin interiors
Temperature- and chemical-resistant
Yellowing Special uses
Polyvinylbutyral Metal primer, e.g.
washprimer, shop primer
Excellent adhesion, corrosion protection, also on aluminium
Not for top coats Constant
Silicon resins Construction protection Temperature and weather resistance
Expensive Increasing
Chlorine-, fluorine-containing polymers
Corrosion protection, coil coating, dirt repellent top coats
Good adhesion on plastics, temperature- and weather-stable
Halogen content (environment, waste disposal), expensive
Special uses, declining
1K, 1-component material; 2K, 2-components material (also two-pack material); 1K-PUR, 1-component polyurethane; 2K-PUR 2-components polyurethane; DIY, Do-It-Yourself
Compiled by the Working Group
76IARC MONOGRAPHS VOLUME 98 Table 1.7. Application methods
Limitations Application Surface
quality
Dimensions Geometry Others
Throughput Solvent emissions
Transfer efficiency
Brushing Medium to
good
Small areas - - Very low Low Very good
Rolling Good Accessibility - Medium Low Very good
Drawing (putty) - Small areas - - Low Low Very good
Wiping Poor Large parts - Low Low Very good
Conventional dipping Medium Limit in object volume
No scooping parts Edge covering High Low Very good
Coating in barrel Poor Small parts Pourable - High Low Very good
Centrifuging Poor Small parts Pourable - High Low Very good
Flooding Medium Limit in object
volume
No scooping parts Edge covering High Low Very good
Flow Coating Medium Working width No scooping parts Edge covering High Low Very good Curtain coating Very good Working width Nearly flat objects - High Low Very good Roller coating/Coil coating Medium Working width Flat surfaces - Very high Low Very good
Electrodipping Low Limit in object
volume
No scooping parts - High Low Very good
OCCUPATIONAL EXPOSURE AS A PAINTER 77 Table 1.7 (contd)
Limitations Application Surface
quality
Dimensions Geometry Others
Throughput Solvent emissions
Transfer efficiency
Air – low-pressure atomization
Good - - - Low High Poor
Air – high-pressure atomization
Excellent - - - Low-to-
medium
Very high Very poor
Air – high-pressure HVLP Very good - - - Low High Poor
Airless atomization Medium - - - High Medium Medium
Airmix atomization Good - - - Medium High Medium
Electrostatically aided air
atomization Very good - No Faraday cages Electricity- conducting substrate
Medium High Good
High speed rotation atomization
Very good - No Faraday cages Electricity-conducting substrate
Medium High Good
Electric powder coating Good - -
Electricity-conducting substrate
Medium No Good
Fluidized bed coating Poor - - Thick layers Medium Low Very good
HVLP, high-volume low-pressure Compiled by the Working Group
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The anodic type typically uses amino- or alkali-solubilized polycarboxylic resins and the cathodic type, salts of amine-treated resins, such as epoxy resins (Stoye & Freitag, 1998).
For high quality surface requirements (“appearance,” gloss, smoothness), paints are often applied by direct contact or by deposition by atomization processes.
Deposition by atomization processes includes conventional spray, hot spray, electrostatic spray, and powder coating (Brock et al., 2000).
Probably the greatest advance during the early 1900s in the field of paint technology was the introduction of the spray gun. Its advent helped the introduction of cellulose nitrate lacquers and their application to automobile assembly line production. Electrostatic spraying was first introduced in the USA in the 1940s, and then later in the United Kingdom.
The solvent-free electrospray powder spray application was introduced in 1965 in the coating industry. In this process, the powder is first fluidized in a closed container by compressed air. The so-formed aerosol is transported by an injector to the spray gun. There, the powder particles are charged electrostatically and sprayed onto the object, which is earthed. The electrostatic charge allows the transport of the particles to the object and their adhesion to it.
Since the early-to-mid 1990s, – initiated mainly by Rule 1151 in southern California – the high-volume low-pressure technique has allowed savings on paint material by reducing spray dust. The modified spray technique, which requires new nozzles and a new spray gun interior, allows a 10–20% reduction in paint consumption. This technique is applied worldwide.