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CM May 2001 - Page 10
COATINGS AND THE ENVIRONMENT - UPDATE 2001
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INTRODUCTION
There are few industrial activities in which environmental considerations are
not looming large in both the way products are manufactured, and their long-term
environmental impact.
Occupational health and safety, management of effluents and residues and greenhouse
issues are becoming very important in driving specifications and the selection
of materials.
In areas such as OH&S and waste management, government involvement in
setting and administering standards has driven the agenda in most jurisdictions.
In the areas of material sustainability and greenhouse management, the participation
by industry has been largely voluntary.
The protective coatings industry is in an interesting position within the
environmental debate. On one hand, the coatings themselves may not score 10/10
on the environmental scale, but have a major impact in enhancing the environmental
credentials of the products on which they are used.
Topics covered:
Environmental issues for coatings | Toxicity
of paint pigments | Industry Action | Metallic
coatings | Zinc as an environmentally sustainable coating
material | Zinc in coatings - where does it go? | Zinc
as a sustainable material
ENVIRONMENTAL ISSUES FOR COATINGS
The fundamental environmental issues for coatings are:
- Is the coating itself environmentally acceptable - what does it contain?
- Is its manufacturing process environmentally acceptable?
- Is its application process environmentally acceptable?
- What are its long-term effects on the environment?
- Can it be recycled?
Paint coatings have long been subject to scrutiny in all of these areas, and
the paint industry has worked hard on its technology to provide coatings that
comply with increasingly stringent regulations.
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TOXICITY OF PAINT PIGMENTS.
Red lead pigment in paint has been used as a rust inhibitor for centuries and
is still used extensively in maintenance painting in industrial situations:
e.g. bridges and process plants. The reason for this is that oleoresinous red
lead materials have had an unrivalled anticorrosive performance on compromised
surfaces. Lead chromate pigments have been used particularly in bright yellows
and red colours.
The toxic effects of lead materials have been well known since ancient Greek
times when Hippocrates first recorded its effect on miners.
Intact dried paint containing lead, in an industrial environment, represents
little if any health hazard. Hazards arise during application, especially spray,
but the main hazard occurs during removal of lead based paints. In construction
projects, this removal may occur when welding, burning or abrasive blasting
is performed on painted steel.
During burning and welding processes the temperature generated is high enough
to vaporise the lead. Abrasive blasting of the coating generates very fine airborne
particles, which present a serious hazard to workers and the public in close
vacinity. Lead is a poison that serves no known useful function the body. In
large enough doses it can kill in a matter of days. Exposure to smaller doses
over long periods of time can cause severe damage to the blood forming, nervous,
urinary and reproductive systems. Children born to parents who have been exposed
to excessive lead levels have a greater chance of having birth defects, mental
retardation, behaviour disorders or dying during the first year of life.
Lead is a particularly insidious poison because severe, permanent damage can
be done before any symptoms are felt, unless very large doses are involved.
While there is a treatment for lead poisoning, the procedure requires hospitalisation,
it is lengthy and frequently painful.
Removal of lead paints represents a significant cost penalty. Disposal of
hazardous waste together with the problem of containment and capture is a difficult
and expensive proposition. It is estimated that this could increase cleaning
and painting costs between three and six times more than normal.
Isocyanates
Isocyanates, in monomeric form, are used to manufacture pre-polymers which
are in turn used in paint manufacture, specifically in polyurethane coatings
which have a variety of end uses, for example as durable finishes or elastomers.
All paints have health hazards, but isocyanates have particular hazards and
careful precautions must be taken by paint applicators to avoid these.
Isocyanate containing coatings are safe to handle provided all precautions
as specified on the Health and Safety Data Sheets are taken. In the case of
two pack isocyanates, as well as the protective clothing worn for all paints;
the precautions include the use of air fed hoods when spraying, as specifically
recommended by the Health and Safety Executive for this type of material. If
these precautions are ignored the diseases below may result. The risks associated
with application of polyurethane coatings by brush are lower.
Isocyanate monomers which are used to make pre-polymers (resins) are the largest
contributors to ill health effects because of the small molecular size and hence
volatility. Very small amounts remain together with larger less volatile pre-polymer
molecules in paint. All the unreacted ‘free’ isocyanate represents a hazard
particularly when sprayed.
Isocyanates are particularly hazardous to the respiratory system and atmospheric
over-exposure can lead to varying conditions of ill-health. In mild cases, the
afflicted may suffer irritation of the eyes, nose and throat. There may be a
tightening of the chest and coughing. In more severe cases the symptoms experienced
can be acute bronchial irritation and difficulty in breathing. The onset of
such respiratory effects may be delayed for several hours after exposure.
Isocyanate-free technology is now available to provide coatings with a performance
similar to that of polyurethanes, such as durability, weathering and colour
and gloss retention, without the degree of risk of respiratory disease and sensitisation
associated with 2-pack isocyanate paints. The recently completed Olympic stadiums
were finished in a 2-pack isocyanate free catalysed acrylic coating.
Other Pigments and Binders
Restrictions have been placed on a number of other paint technologies including
coal tar and chromate pigmented paints because of the negative OH&S implications
of these materials. From a performance point of view, these coating systems,
along with lead pigments, are excellent technology for their design applications,
but environmental criteria have overridden technical performance.
V.O.C.’s
The global problems attributed to Volatile Organic Compounds, VOCs, arise from
the use of solvents in many industrial processes with the paint industry playing
a very significant part. In Europe and the US paint and the painting industries
have been targeted as a key area to receive legislative attention. Estimates
in Australia alone are that in excess of 80,000 tonnes of VOC’s are released
into the atmosphere annually with the existing technology.
The environmental impact of any substance is related to how it is released
into its surroundings. In the case of volatile materials (including solvents),
they evaporate into the air and are then oxidised by photo degradation. This
results in the three main air pollution problems of ozone layer depletion, a
photochemical smog and global warming. A further environmental problem is that
of acid rain.
Ozone Layer Depletion
The ozone layer in the upper atmosphere absorbs harmful UV irradiation which
would otherwise promote skin cancers in human beings and animals and damage
agricultural crops and marine organisms. The depletion of ozone in this region
(18-40 km above ground), was first noted about 20 years ago but ozone levels
are difficult to measure and it is only more recently that the evidence has
been universally accepted.
Halogenated substances (including chlorinated solvents) are the main cause
of this effect. They have a longer life than hydrocarbon solvents once released
into the atmosphere, which allows them to reach the higher atmospheric layers.
They are then decomposed by UV light to form halogen reactive groups which act
as catalysts to the break down of ozone to form oxygen, so removing the ozone
protective layer.
Photochemical Smog
Although hydrocarbon solvents rarely reach the upper atmosphere, they are retained
in the lower atmosphere where they cause damage. They actually react under the
influence of UV light with nitrogen oxides which are particularly dominant in
the atmosphere of industrial and large built-up areas, to promote the concentration
of ozone otherwise known as ‘photochemical smog’. Whilst in the upper atmosphere,
ozone is protective, its presence in the lower atmosphere causes acute human
respiratory disorders such as asthma, especially in young children, and severe
irritation of the eyes. In addition, the ozone has a damaging effect on agricultural
crops.
Global Warming
The sun’s radiation warms the earth’s surface, which in turn heats the atmosphere
by the emission of infra-red radiation. Higher molecular weight gases, including
those attributable to solvent emissions into the atmosphere, absorb and trap
some of this infra-red radiation which would otherwise reach outer space. The
result is an increase in the earth’s atmospheric temperature, an effect known
as global warming or the ‘greenhouse effect’.
Acid Rain
This dangerous product is caused by the emission of sulfur dioxide and nitrogen
oxides into the atmosphere. Although the largest contributors of this phenomenon
are the products of the combustion of fossil fuels and the agricultural burning
of forests, solvents do also play their part.
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INDUSTRY ACTION
The international paint industry has been active in developing coating systems
that eliminate or reduce the environmental impact of the older technologies.
More water-based systems are now available that offer levels of performance
equivalent to some solvent-based systems.
Low VOC or solventless systems are now readily available, and other technologies
are emerging using inorganic or hybrid organic-inorganic coatings that offer
superior technical performance. Unfortunately, in many developing countries,
many older technologies such as lead-based paints are still used, as are many
high VOC materials. International protocols for greenhouse reduction will ultimately
have an impact and bring developing country standards up to those of the industrialised
countries.
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METALLIC COATINGS
Zinc is used almost exclusively as the metallic coating for protecting steel
from corrosion. It is applied by various technologies, ranging from electroplating
through continuous galvanizing to after fabrication galvanizing. All these processes
have differing environmental impacts and produce coatings with varying levels
of durability.
Zinc, being a reactive metal well up in the electrochemical series of metals,
does not exist in nature as a metal, but, like most metals, is present as a
mineral, frequently in concert with lead and other less abundant metals. As
a result, the production of zinc metal requires that its ores be mined, milled,
smelted and refined to produce zinc metal. Once in this form, zinc then takes
its place in the manufacturing process and ends up as a coating (e.g. galvanizing),
as an alloying metal (in brass), as a pigment (zinc dust), as a chemical, (e.g.
zinc oxide used in rubber manufacture, fertilizer and medicine) or as a product
itself (e.g. zinc die castings, zinc sheeting).
These operations produce a stream of secondary zinc waste products (e.g. zinc
drosses and oxides, scrap, zinc rich waste acid solutions) which are then subject
to secondary processing operations to recover zinc and other components of commercial
value for subsequent return to the material cycle.
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ZINC AS AN ENVIRONMENTALLY SUSTAINABLE COATING MATERIAL
The issue of environmental sustainability is becoming increasingly significant
at all levels of our society. It is not only on the political agenda as ‘green’
candidates represent an increasing proportion of the political landscape at
local, state and federal level, but is also a high priority for the design professions
and their clients in the 21st Century.
A simple method of rating materials is to compare them on the basis of their
Gross Energy Requirements (GER.). This accounts for all the energy used in mining,
smelting, refining and forming the material. For metals in particular, another
factor called Gibbs Free Energy (GFE) is a measure of the energy required to
convert the ores to the metal. Nature always seeks equilibrium at the lowest
energy levels and the GFE makes all metals intrinsically unstable. Their stored
energy constantly seeks an opportunity to get out. The GER and the GFE are not
necessarily related. Some metals like copper have high GER requirements because
of the nature of their ores, and low GFE requirements because of the nature
of the material.
The following table illustrates this relationship:
|
Table 1.
| Material |
Mineral |
Gross Energy Requirement (MJ/kg)
|
Gibbs Free Energy (MJ/kg)
|
|
|
| Aluminium |
AI203 |
270 |
29.00 |
| Copper |
Cu2S |
115 |
0.70 |
| Zinc |
ZnS |
70 |
3.00 |
| Steel |
Fe203 |
35 |
6.60 |
| Lead |
PbS |
30 |
0.45 |
|
It can be seen from this table that in the context of protective coatings for
steel, zinc has double the GER of steel but has less than half the GFE.
Zinc, when used as a component in a protective coating for steel is by its
nature, sacrificial. All zinc used as a protective coating for steel will be
returned to the environment as it oxidises or corrodes sacrificially to prevent
corrosion of the steel. Protective coatings of all kinds work on the principle
that a small amount of coating can protect a large amount of steel.
On hot dip galvanized products, for example, the galvanized coating mass is
typically less than 5% of the mass of the steel that it is protecting. If unprotected,
the steel would corrode at rates typically 20 times faster than zinc. Using
adequate protective coatings systems on steel to delay the escape of its Gibbs
Free Energy as long as possible is thus a major factor in determining environmental
sustainability.
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ZINC IN COATINGS - WHERE DOES IT GO?
Zinc is the most widely used metal for the protection of steel from corrosion
as well as being present in a range of other manufactured products and in the
natural environment. About 7 million tonnes of zinc metal are produce annually,
of which about 20% is from recycled material and the rest from mining and refining.
From this, about 70% goes into coatings for corrosion prevention, 15% goes
into brass, about 5% goes into rubber tyres (as zinc oxide) while the balance
is used for zinc chemicals, dry cell batteries and diecasting.
As coatings and tyres are consumable products, it is logical to assume that
the zinc in these products will eventually end up dispersed into the environment.
Is this a health hazard? Does it represent a risk to the natural environment?
The important issue from an environmental point of view is that of where the
zinc ends up after it becomes a corrosion product. Does it migrate far from
its original source? Does it accumulate? Research on these subjects is starting
to provide a better understanding of the behavior of zinc leached from coatings.
The International Lead-Zinc Research Organization (ILZRO) has undertaken research
on the leaching of zinc from galvanized coatings on transmission
towers. This research concluded that even in aggressive
acid rainfall zones, zinc concentrations in soil were at
background levels within 6-9 metres of the tower base. Research
continues in this area.
While zinc is often classified with the so called ‘heavy metals’ such as lead
and cadmium when environmental standards are discussed, zinc is in fact one
of the most beneficial metals. There would be ‘no life without zinc’ to quote
Prof. Heinrich Vahrenkamp from the Institute of Inorganic and Analytical Chemistry,
University of Freiburg, Germany from a paper of the same title presented at
International Zinc Day, 1994.
The human body contains about 2.5g of zinc and more than 200 enzymes are known
that require zinc to function correctly. This is a far higher number than any
of the other metals essential to healthy body functions. (e.g. iron, magnesium,
calcium, sodium and trace metals such as copper). Zinc has been identified as
essential in wound healing, digestion, reproduction, kidney function, breathing,
diabetes control, inheritance functions, tasting and skin health.
High levels of zinc are not required in humans or plants and they do not accumulate
zinc, with one or two notable exceptions. Oysters have 10 times as much zinc
as the next highest source (red meat), and a small flower, silene vulgaris can
accumulate up to 3% of its dry weight of zinc. Some plant species can tolerate
very high levels of zinc, and vulgar knotgrass (polygonum) has been found to
extract over 300 kg of zinc per hectare per year from zinc contaminated soils.
In humans, zinc is found in the highest concentrations in the reproductive
system and lowest in the nerves and brain. Mother’s milk, sperm and ova have
very high concentrations of zinc and humans require around 20 mg/day of zinc,
which is available in a normal balanced diet with a supply of fruit, vegetables,
cereals, red meat and seafood.
The most dramatic effect of zinc deficiency is on the reproductive system in
humans, particularly in the Middle East. Extreme growth retardation in adolescents
and the skin disease, acroder-matitis enteropathica, which is a very painful
and potentially lethal condition, have both been immediately cured by simply
adding zinc supplements to severely zinc deficient diets.
Zinc in soil is vital for cereal crops and the well being of a wide range of
vegetation. For this reason, zinc compounds are widely used as additives in
fertilizers. Excess levels of zinc in soils can produce reduction in yields
and unhealthy plant growth just as zinc deficiency will detrimentally affect
similar plants.
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ZINC AS A SUSTAINABLE MATERIAL
Compared to other base metals zinc occupies a favorable position as an environmentally
sustainable material. Energy consumption for primary zinc production is 25-50%
higher than that of steel and only about 20% of aluminium.
About 20% of zinc used is recovered as scrap and this is likely to increase
to over 60% as recovery process technology improves.
The galvanizing of steel as sheet, wire, tube and fabrications offers very
good corrosion resistance on steel and greatly increases its life. On average,
about 70 kg of zinc (which consumes 250 kWh of energy to produce) is consumed
to prolong the service life of 1 tonne of steel as sheet, which consumes about
2900 kWh of energy to produce, by a factor of between 3x and 5x. At the end
of its service life, the galvanized material can still be recycled, except for
the zinc lost through corrosion and run-off.
As weathering occurs with these zinc-based coatings, the zinc is consumed in
two ways. These are:
1. Oxidation of the zinc and physical removal of the zinc oxide products by
washing or erosion.
2. Electrochemical dissolution of the zinc adjacent to exposed steel when an
electrolyte (water) is present.
These zinc corrosion products are transported into the surrounding environment.
It is their impact in this context that determines their viability as coatings
into the foreseeable future. The rate at which zinc moves into its surrounding
environment from the weathering of coatings is obviously determined by coating
life.
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CONCLUSION
The protective coatings industry internationally is well aware of the major
environmental challenges facing the industry into the 21st Century. These are:
- Minimising energy usage in the mining, processing and production of coatings
raw materials and in the processes that utilise coatings.
- Minimising effluents and contamination entering the
ecosystem (rivers, groundwater, and the atmosphere) from
the weathering and rehabilitation of coatings.
- Responsible planning and operation of mining and processing
sites, including rehabilitation of mine-sites.
- Optimising the recycling of zinc coated materials,
including galvanized scrap steel.
- Development of by-products to ensure maximum usage
of available zinc in a form that meets international standards
of occupational health and safety and are free of long-term
toxicity risks.
- Technological improvement to enhance the durability
of coatings and extend the service like of the material
to which they are applied.
REFERENCES
Green D.H. 1993 Zinc: Environmental Constraints & Opportunities
for a Base Metal, World Zinc ’93 Conference. Australian Institute of Mining
and Metallurgy, Hobart 1993. Plenary Address pp 5.
Vahrenkamp H 1994 No Life without zinc. Zinc Day 94. Zinc Association Conference
Papers pp ZA3/1-8.
Wyatt K. 1996 The Material Cycle. BCME Technical Feature June/July 1995 pp 15-18
Szekely I. 1996 Steel making and Industrial Ecology - Is it Green Material.
ISIJ Journal Vol 36 No 1 pp 121-132.
Lars Landner and Lennart Lindstr6m. Zinc in Society and in the Environment -
an Account of the Facts on Fluxes, Amounts and Effects of Zinc in Sweden, by.
Published by Swedish Environmental Research Group (MFG), Fryskta, Sweden 1998
160 pp.
Robinson J 1997 The future of zinc coatings. Corrosion Management Vol. 6 No.
2 June 1997 pp 7-10
Mackay J 1994 Factors affecting paint coating development. Corrosion Management
Vol.3 No.4 November 1994 pp 15 -17.
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