As with adhesives, synthetic binders can be divided into thermoplastic and thermosetting products. Thermosetting products are often called synthetic resins and are usually based on epoxy, polyurethane or polyesters. Alkyd oil is a polyester product and is the dominant binder in most resin based coatings sold on the consumer market. Important thermoplastic paints are based on polyvinyl-acetate PVAC and acry-lates. The binders can be dissolved in water or organic solvents or a combination of these. A wide range of other additives are also needed, often including softeners, dispersion agents, pH-regulating agents, fungicides, drying retardants, film-forming agents, foam reducers and perfumes.
Synthetic paints and varnishes are based on fossil oils. The production is intensive in energy and a wide range of serious pollutants can be emitted. This includes compounds used in the production of polymers as well as different additives. Products containing organic solvents, epoxy and polyurethane are known to create a problematic working environment for painters.
Emissions to the indoor environment can be expected. Organic solvents can be emitted up to six months after application. Many water-based paints also contain other volatile additives which can be released over an even longer period, such as fungicides and softeners. Some synthetic paints and varnishes also emit residual monomers. Most synthetic paints, especially the thermosetting products, have a low vapour permeability, which means that moisture buffering properties in the underlayer will be less accessible. They are also able to induce considerable electrostatic charging, especially when used on floors.
Waste paint should be treated as special waste, even if the pigments are inert. Painted products have little re-use value and normally have a decreased value for material recycling. Flammable materials can be burned for energy recovery in incinerators with high-graded filters.
Epoxy is one of the commercial materials most commonly known to trigger allergies and other problems. At workplaces where people are exposed to epoxy, many of them develop eczema. This is mainly due to the ingredient epichlorohydrin which is also a potent carcinogen. As with another basic ingredient, bisphenol A, it is also a suspected environmental estrogen. Epoxy, even in low concentrations, has a toxic and corrosive effect on water organisms. Ready-cured epoxy products are probably chemically stable, although a certain amount of organic solvent is emitted first.
Polyurethane products contain isocyanates that can easily cause skin allergies and asthma. Sensitization causing permanent damage to the mucous membranes can develop, and asthma attacks can then occur practically independent of the level of exposure. The most exposed places are industrial and building sites, but unreacted residues can also be released within buildings.
Alkyde oil is based on polyester derived from the reaction of an alcohol and an acid anhydride such as phthalic anhydride, modified with vegetable oils to give the mixture drying properties. Paints and varnishes based on alkyde oil came into widespread use during the 1950s and contain large quantities of organic solvents, usually aromates like toluene and xylene. Alkyde oil paint does not penetrate material as well as pure linseed oil paints, but it adheres well to wood even if the surface is not completely dry. Alkyde oil paint is also considered hard wearing, and is used on concrete, plaster and galvanized iron. With no pigment the paint can be used as a varnish.
Alkydeoil is very thick, and because of the high percentage of organic solvent required (between 50-70%), it carries a large risk for the working environment. The emission of solvents in the building can continue for a few days or several months, depending upon the climate of the room, how the paint has been applied and the type of solvent. In certain alkyde oil products, mainly the varnishes, alkylphenols, including non-ylphenol, are present in the binder in a proportion of about 1% by weight. Several alkylphenols are seen as environmental estrogens. As corrosion inhibitors in alkyd paint for metal surfaces, additives of lead and zinc are used. These can seep out and pollute the earth, and ground water and should be avoided. A less damaging alternative is aluminium tripolyphosphate.
Thermoplastic products are mostly dissolved as dispersions in water, but small amounts of organic solvents are often added as film-forming agents. They are frequently referred to as latex paints although they do not contain any latex derived from the rubber tree. As binders, polyvinyl-acetate (PVAC), vinyl acetate and acrylates are much used. These move freely about in the water in the form of microscopic plastic pellets. To make the mixture work as a paint, a wide range of additives have to be mixed in. Whilst the binders form about 30% of the product, pigments will make up of 30-35%, fillers 16-20%, water 20-25% and the remaining additives about 5%. Different binders are often mixed to improve the technical properties and reduce the need for additives. Whilst a homopolymer PVAC-paint must have softeners added to make it suitable as a paint, a co-polymer of PVAC and acrylate can do without.
Additives used in PVAC products often include sulphonamides, which can damage the immune system, and alkylphenol ethoxylates such as nonylphenol which are thought to be environmental estrogens. Acrylate monomers can cause eczema through contact with wet paint. Many of the paints can emit volatile compounds for long periods after painting is complete, such as excess monomers of acrylates, styrene, softeners (e.g. phthlates) and fungicides. Several of these can stimulate hypersensitive reactions and lead to allergies. The emissions diminish with time, depending upon the temperature, the moisture situation and the thickness of the paint. After a year most of the emissions cease.
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