•
Introduction
Chromium
compounds have been used in the formulation of water-borne wood preservatives
for about a century. They function as essential chemical fixatives
for the copper and other fungicidal and insecticidal components in
the widely used chromated-copper preservatives, of which the best
known example is CCA (copper-chrome-arsenic). Studies have shown that
chromium in these preservatives has little or no direct action against
decay fungi. However, stabilisation of the other preservative components
achieved through valency state reduction of chromium makes them resistant
to leaching and provides the treated wood with long-term durability
against fungal and insect attack, even in high risk environments.
Recent research and development efforts have led to the introduction
of controlled and accelerated fixation systems which provide CCA treated
timber products having very high degrees of fixation (>99%) of
the chromium, copper and arsenic components. The development, functional
role, safety and environmental aspects of chromium containing wood
preservatives are summarised in this article.
•
Historical development and current status
The first
major application of chromium compounds in wood preservation took
place in the early part of this century with the development of “Wolman”
salts based on sodium fluoride and dinitrophenol with sodium or potassium
dichromate(1). This
was followed in 1926 by the development of copper-chromate (CC) preservative
by Gilbert Gunn of the Celcure Company. The CC preservative was a
mixture of equal parts of potassium dichromate and copper sulphate
which was dissolved in water to a concentration of about 10%w/v(1,2).
This solution was then applied to dried timbers in large-scale impregnation
cylinders using a combination of vacuum to remove air from the wood
and hydraulic pressures of 8 to 12 bar to inject the preservative
liquid deep into the wood. Although CC preservatives are effective
against a wide range of wood destroying organisms, a number of copper
tolerant species are able to attack even well-treated wood.
The most
significant chromium based wood preservatives were invented in 1933
with the patent by Kamesam of a water diluted copper-chrome-arsenic
(CCA) formulation which gave very good all round fungal and termite
resistance to the treated wood. This first CCA formulation was commercially
marketed as “Ascu” and has led to the development of a range
of formulations differing to a greater or lesser degree in the relative
proportions of the main components and the source chemicals (see
Table 1). The CCA preservatives are applied to timber by
the same methods as used for the CC systems. Since the late 1930s
several variants have been evolved on the chromated copper formulations
e.g. copper-chrome-boron (CCB), copper-chrome-fluoride (CCF), copper-chrome-phosphate
(CCP) but the most widespread and commercially dominant system have
been the CCAs.
CCAs
are now the market leaders internationally with commercial tonnages
of about 100,000 per annum traded internationally (compared with about
15,000 tonnes per year for all other waterbornes, 1990 figures)(3).
The growth of markets for CCA preservatives has been dramatic over
the last 25 years. CCA formulations are registered for use throughout
the world including the European Union, USA, Australia, New Zealand,
Canada, India, Brazil, South Africa, Malaysia, Indonesia and Japan.
•
Functional role of chromium compounds in wood preservatives
Chromium
compounds fulfil two principal functions in wood preservatives: as
a chemical “fixative” to prevent or reduce loss by leaching
of other components of the preservative, and/or as an anti-corrosion
agent.
The first
function can be regarded as the most significant and was the main
reason for the incorporation of potassium or sodium dichromate into
preservatives such as CC, CCA and “Wolman” sodium fluoride/dinitrophenol
salts in the 1920s and 30s. Of these, the CCA and related preservatives
will be considered in more detail below. The work of Brüning
was the first to recognise that the addition of chromates could impart
leaching resistance to other metal salts in the formulation (H. Brüning
1912, British Patent 2972) and the ability to fix copper sulphate
was then exploited by Gunn (G. Gunn 1926 British Patent 273007) in
the Celcure copper-chrome based preservatives. In 1933 Kamesam (S.
Kamesam 1933, British Patent 404855) developed the first CCA formulation
in which copper sulphate and arsenic pentoxide are fixed by dichromate
after he had spent a period working with Falck in Germany where they
developed the “Falkamesam” chrome-arsenate preservative(4,5).
Other preservatives to make use of the fixing properties of chromates
have been the “Boliden” salts, chromated zinc chloride and
chromated mercury chloride in the 1930s and 40s and the boron-fluorine-chrome-arsenic
(BFCA) diffusion treatment formulation in the 1950s and 1960s(1,
6, 7).
The
second function of imparting corrosion resistance was recognised early
and was used by Wolman in 1913 to improve the corrosivity properties
against iron and steel (in treating tanks and metal fastenings) of
his sodium fluoride and dinitrophenol “Trioloth” preservative
(often called “Wolman salts”). The content of chromate as
either sodium or potassium dichromate in these formulations was relatively
low (NaF 85%, Dinitrophenol 10%, Na2Cr2O7 or K2Cr2O7 5%). In the 1920s,
arsenic salts were added to Triolith to form a new preservative “Tanalith”
and in the 1930s the dichromate content of both preservatives was
increased considerably to about 35% in recognition of its role not
only as a corrosion inhibitor, but also as a fixative for the arsenic
component. These high Cr and Cr/As formulations were identified respectively
with a U or UA designation (e.g. Tanalith U, Trioloth UA). Chromates
do not always improve the corrosion resistance of preservatives and
have been reported to increase the corrosivity of zinc sulphate or
chloride, whereas the corrosivity of copper sulphate and other components
can be neutralised(1).
It should
be noted that chromium compounds have virtually no fungicidal (or
insecticidal) activity in wood. This has been demonstrated clearly
in work by Gray and Dickinson on the decay inhibiting activity of
various combinations of copper, chromium and arsenic salts in treated
timber(8).
•
Chromium in CCA preservatives
CCAs
have fulfilled an essential role in providing durable, safe, treated
timber products for nearly 50 years. In addition to providing added
durability, they have only minimal effects on wood strength properties
and are compatible with most metal fastenings, glues and coatings
for timber. CCA preservatives are sold only for professional application
to timber and the preservative is usually diluted with water at industrial
wood treatment plants to give working solution strengths varying from
about 2 % w/v to 5% w/v depending upon the type of wood to be treated
and its intended use. In general, the amount of preservative formulation
delivered to the wood varies between about 4 and 24 kg/m3 of wood
and is termed the preservative “retention”.
Research
and development efforts have focused on understanding the fundamental
mechanisms of action of CCA (principally of the Cu and As components)
against wood destroying organisms, preservative treatment mechanics,
the chemistry of “fixation” in wood, leaching behaviour
in different environments and health and safety and environmental
impact issues. The published literature is extensive and useful texts
and reviews have been presented by Nicholas(9),
Wilkinson(2), Placket(10),
Barnes(11), Anderson
et al.(12), Cooper
et al.(13), Gray(14),
Eaton and Hale(15),
Barnes and Murphy(16),
Hillier et al.(17),
Murphy and Dickinson(18).
In addition to publication in the scientific literature, research
on CCA and other wood preservatives has been reported to conferences
of the International Research Group on Wood Preservation which has
provided a significant international forum for discussion since 1969*.
The fixation
reactions begin when aqueous solutions of CCA preservative are injected
into timber and may go on for some considerable time after the impregnation
process is completed. A number of researchers, notably Wilson(19),
Henry and Jerowski(20),
Dahlgren and Hartford(21-27),
Smith and Williams(28,29),
Pizzi(30-35), Ostmeyer
et al.(36,37) have
studied the chemistry of reactions between CCA preservatives and wood
and although several questions remain, a general scheme is available
(Table 2). The reactions
can be divided into three phases according to their rapidity. Initially,
there is a rapid (minutes) adsorption of hexavalent chromium Cr042–
and Cu2+ to wood components which removes
about 50% of the total Cr6+ from the solution absorbed by the wood.
This
is followed by the principal reaction involving chromium which is
a reduction of the free Cr6+ to Cr3+ over a time period of several
hours. This is accompanied by a rise in pH of the wood/preservative
system and the formation of several low solubility compounds (CrAsO4,
CuCrO4, Cu(OH)CuAsO4) involving the copper and arsenic components.
It is unclear at present what happens to the initially adsorbed Cr6+
but it is clear from Electron Paramagnetic Resonance (EPR) and other
studies that this is eventually reduced to Cr3+, possibly after passage
through intermediate Cr5+ and Cr4+ steps which have been observed
in treated wood(38,39).
These reactions are followed by possible “long-term” reactions
involving fluctuating pH and possible interconversions and additional
reactions over several months(22)
although reports of these reactions are limited.
The importance
of the fixation reactions is critical to the efficacy and acceptance
of CCApreservatives. Fixation and the resistance to leaching of the
copper and arsenic components is essential to guarantee long-term
performance (e.g. decades) of the treated wood against fungal decay
and insect and marine borer attack in environments ranging from house
foundations to permanent soil contact to exposure in sea water. Completion
of fixation also means that the chromium in treated wood is converted
to the considerably less toxic trivalent form(40,
41) which also has lower aquatic and soil ecotoxicity.
Until
recently most fixation of the preservative took place under ambient
conditions at treatment plants, usually for periods of between 48
and 72 hours under various types of cover (e.g. roofing, tarpaulins,
cover boards). However, whilst this has proved satisfactory in warmer
climates, it is now known that fixation is highly temperature dependent
and can take from 1 to 10 hours at 70°C to give complete reduction
of Cr6+ to Cr3+, up to 30 hours at 50°C, and between 48 and 100
hours at 20°C(13,42,43)
at moderate relative humidities. Since pioneering work by Peek and
Willeitner on the use of accelerated fixation on freshly treated wood
by elevated temperatures such as steaming, a number of commercial,
controlled fixation systems have been developed(44-46).
Several of these use conventional kiln drying chambers which are operated
for the initial part of the cycle under conditions appropriate for
high temperature fixation (e.g. high RH (low dry bulb/wet bulb depressions)
maintained for several hours at 70°C) prior to commencement of
conventional drying schedules. Most recently, a novel process developed
by the Forest Research Institute in New Zealand makes use of CCA solutions
heated to about 70°C (usually avoided to prevent sludging problems
in conventional treatments) to treat Radiata pine by a multiple-phase
pressure schedule (MPP) which delivers dry fixed treated wood at the
end of the process(47,48).
Controlled fixation systems now offer the ability to complete the
CCA fixation reactions quickly at treatment plants and allow treated
wood to meet increasingly stringent local and national regulations
concerning occupational, public exposure and environmental contamination
by components of all wood preservatives.
•
Safety and environmental impact of CCA preservatives
Since
the introduction of CCAs and related preservatives, considerable improvements
have been made in the industrial design of wood treating plants, delivery
systems for the preservatives from manufacture to treatment sites,
fixation of the treated wood as described above and disposal of wastes
and residues(49,50).
There are very few reports of health and safety studies on workers
exposed to Cr containing wood and no reported excess of occupational
illness due to the normal handling of CCA preservatives or treated
wood(51,52). A study
of CCA exposed workers in Sweden and Norway concluded that, within
the limited number of person-years under risk in the study, no increased
risk of any cancer sites was indicated(53).
A number
of studies have been conducted on soils contaminated at wood treating
plants through spillage and dripping from freshly treated wood(50).
These have demonstrated the importance of good plant design including
concrete drip pads and collection sumps, maintenance and operational
procedures to minimise environmental impacts. Additional factors influencing
the potential for contamination include the soil type and valency
state of the contaminants, particularly Cr and As(54).
Studies using lysimeters to simulate aspects of spillages of preservatives
are in progress at several laboratories(55,56).
Recent studies have demonstrated the possibilities for clean-up of
CCA treatment sites at industrial scale(57,58).
Recent
research has also been concerned with establishing the potential for
losses of CCA components during the service life of treated wood to
the local environment. This work has often been done with laboratory-based
water leaching systems or as soil depletion studies on small, treated
wood stakes and there are relatively few long-term field soil studies(59,60).
The work of Cooper and Ung(61)
on CCA treated poles and Hudson and Murphy(62)
on CCA treated wood exposed in field soils for several years has established
practical information on soil concentrations of CCA preservative components
surrounding treated wood in service. These studies have generally
shown steep gradients of concentration of copper, arsenic and chromium
compounds in soil surrounding CCA treated wood in soil and little
or no elevation of environmental levels of these elements beyond 100
to 250 mm distance from the treated wood. Chromium shows the lowest
level of leaching and dispersal into surrounding soil in all the studies
conducted to date(61-65).
Data such as these have been used by Hillier et al.(66)
in whole life-cycle assessments of the environmental impact of CCA
treated wood. They concluded that the relatively low levels of loss
of CCA components to soil over a 40 year service life in soil of treated
wood (increased levels in soil of 1.6 mg/kg Cr, 4.9 mg/kg Cu and 3.3
mg/kg As in 0.5m3 soil affected) were insignificant in relation to
regulatory levels. More important in relation to the LCA was the need
to close the life-cycle with environmentally benign means of post-service
disposal of CCA treated timber and, encouragingly, several such routes
are now available(67-69).
•
The future
Despite
the long and successful history of use of chromate containing wood
preservatives, a number of regulatory authorities have and are considering
restrictions or bans on their use for certain products(70).
In Sweden, a legislative ban on the use of arsenic- and chrome-containing
wood preservatives for above-ground timbers came into effect between
1992 and 1994 and led to immediate substitution with alternative formulations
and a reduction of about 50% in the market size for CCA(71).
Interestingly, CCA preservatives are still permitted for timber for
export and for local heavy duty applications and they retained about
a 35% market share in 1994. However, international pressure to reduce
the consumption of CCA and related preservatives will remain because
of the possible health and environmental effects. It should be stressed
that this pressure is based only on possible effects and that the
published evidence indicates that such preservatives have not caused
either health or environmental problems when used in accordance with
normal industry procedures. The wood preservation industry has developed
alternative arsenic- and chrome-free preservative systems although
it remains to be seen whether such treatments will provide the long-term
performance, robust industrial handling properties and economy of
the chromated-copper wood preservatives such as CCA and CCB.
•
Conclusions
Chromium
compounds have played an essential role in several wood preservative
formulations the most important of which have been the copper chrome
arsenic (CCA) and copper chrome boron (CCB) types. These preservatives
are currently the dominant international wood preservative systems.
They have established an outstanding reputation for imparting long-term
durability to treated timber. Systems are available to allow their
safe handling and containment at industrial treatment sites and rapid
and complete fixation of the components in freshly treated timber.
Research on the environmental impact of CCA treated wood in service
has indicated that they do not lead to significant local soil pollution.
Methods for the re-processing of CCA and other treated wood are being
actively developed and their adoption in practice will close the life-cycle
for timber treated with chromated-copper preservatives.
•
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