Criterios y Especificaciones del Proveedor de Rosas y Claveles

Changes in moisture content up to fibre saturation point invariably involve movement, shrinkage with drying and swelling with wetting. Although it is normal to dry wood to a moisture content equivalent to the average atmospheric relative humidity anticipated in use, it is common to encounter movement problems. Faults such as gaps appearing between floor blocks or boards are due to the wood drying after installation, either through inadequate kilning or perhaps re-wetting between kilning and installation. A door or drawer jammed in humid weather may be exceedingly

slack under drier conditions. Frames which introduce an end-grain surface in contact with side-grain will inevitably result in cracking of any surface-coating system. In other situations the cross-sectional movement may become apparent as warping through twisted grain effects. The obvious solution to all these problems is to use only wood with low movement but this is not always realistic.

The alternative is to impregnate the wood with chemicals which induce stabilization.

Unfortunately these processes are also frequently unrealistic because of the difficulty of achieving complete impregnation, a problem that has already been discussed in connection with normal wood preservation.

Paint and varnish

One obvious solution is to enclose the wood within a protective film to stabilize the moisture content. Paint and varnish coatings will act in this way, provided they completely cover the wood and remain completely undamaged.

Unfortunately, whilst these coatings give good protection against rainfall, they are unable to prevent moisture content changes resulting from slow seasonal fluctuations in atmospheric relative humidity. As a result the painted wood will shrink or swell with changes in relative humidity, causing the surface coating to fracture wherever a joint involves stable side-grain in contact with unstable end-grain. Rain is absorbed by capillarity into the crack, yet the remaining paint coating restricts evaporation, so that the moisture content steadily increases until fungal decay is sure to occur if the wood is non-durable. It is frequently suggested that preservation provides a simple solution to this problem, but this ignores the fact that water also damages the paint coating. It is explained in Chapter 2 that wood is an hygroscopic material, covered with hydroxyl groups which have a strong affinity with water so that penetrating water will tend to coat the wood elements, displacing paint and varnish coatings. This failure is known as preferential wetting and is

Water repellents, stabilizers and decorative systems

responsible for blistering and peeling in paintwork and the loss of transparency in varnishes.

Water repellent preservatives

The best solution to both the decay and preferential wetting is treatment of external joinery (millwork) and cladding before painting, with a formulation that is both a preservative and a water repellant. A water-repellent treatment coats the pores of a structural material, reversing the angle of contact so that capillary absorption of water is prevented; water repellency is associated with water globulation on the surface but absence of globulation on a weathered surface does not necessarily mean that a treatment is no longer effective as the pores may still be water repellent.

Waxes and resins

Various waxes, particularly paraffin waxes, are the most commonly used water-repellent components in wood preservative formulations, although a treatment based on a wax is generally as susceptible to preferential wetting failure as the surface coating that it is designed to protect.

Treatments of this type are reliable only if they penetrate deeply and are applied at sufficient retentions to ensure that the wood elements are entirely inaccessible to even changes in the relative humidity of the atmosphere. High-wax retentions cannot be used if wood is to be finished with a paint or varnish coating as the adhesion is seriously affected—the coating cannot adhere to the wax deposit and is unable to penetrate if the wax retention is too high, but in addition, the wax may migrate into the coating solvents, affecting both solvent loss and the ability to absorb the oxygen required for drying so that the coating may remain tacky. The wax will continue to migrate through subsequent coatings, affecting inter-coat adhesion, perhaps even causing cissing, the situation when a coat is unable to wet a surface and tends to

concentrate in globules, leaving other areas uncoated. Yet another problem with migrating wax is the tendency to prevent the development of gloss in the final top coat. For these various reasons waxes are generally used at low retentions and the desired pore-sealing action is achieved by the addition of resins.

Resin selection is critical in terms of water and water vapour resistance as well as paintability. The aliphatic and aromatic hydrocarbon resins are inexpensive and efficient but they do not dry; they solidify only by loss of solvent and may be re-dissolved by coating solvents, perhaps interfering with the drying and durability of the coating system. Natural drying oils such as boiled linseed oil can also be used but paintability problems may arise through slow drying. The use of suitable modern alkyd resins can avoid this difficulty but they are also expensive. The most realistic systems therefore tend to be based on mixtures of waxes, hydrocarbon resins and alkyd resins to avoid these problems, and there are therefore distinct differences between proprietary products.

It is particularly important to appreciate that unsaturated or drying resins are likely to significantly reduce the activity of some cationic preservatives such as zinc, and particularly tributyltin oxide. In addition, alkyd resins will solidify only in the presence of driers or catalysts such as metal naphthenates, and these catalysts may be inactivated by tributyltin oxide which is a base and will thus absorb their acids. Such problems can be avoided by using other toxicants, yet tributyltin oxide is particularly suitable as it tends to improve the resistance of the formulation to preferential wetting, and it is therefore better to use tributyltin compounds other than the oxide such as the naphthenate or o-phenylphenate which do not suffer from these disadvantages.

Silanes (silicones)

There have been several attempts to develop more suitable water repellent components in

view of the difficulties associated with the use of waxes. Tributyltin oxide orientates onto the wood fibres, giving a water-repellent surface through the presence of the hydrophobic butyl groups. However, this compound is expensive, and toxic at high retentions, so that use as a water repellant is unrealistic, but other Group IV organometal compounds can be used. The organosilicon compounds, the silanes or silicones, are the best known water repellents in this group but the very stable silicone oils tend to possess many of the disadvantages associated with heavy organic oils and waxes. The only silicones suitable are those which have a high degree of functionality so that they are able to attach themselves to the wood components in the same way as tributyltin oxide, thus giving good resistance to preferential wetting failure.

They have not been extensively employed, probably through disappointing results following the use of unsuitable silicone oils and resins.

Organoaluminium compounds—Manalox Organic compounds of aluminium, titanium and zirconium can also be used but the water-repellent groups in typical available commercial products are usually long-chain fatty acids such as stearate which give a waxy treatment and are more susceptible to oxidation when applied at low retentions than the short-chain alkyl groups on typical silicone resins. However, aluminium compounds can incorporate unsaturated chains and, when used for preservative, water-repellent or priming treatments, they can provide excellent adhesive bonding between the wood elements and alkyd systems, giving resistance to preferential wetting. Even toxic groups such as pentachlorophenate can be incorporated, thus avoiding the need for special co-solvent or anti-blooming systems. These principles are most highly developed in various Manalox products which can be described as polyoxoaluminium compounds.

These advantages of organoaluminium compounds

are not apparent in the normal aluminium stearate, which performs only in the same way as a wax.

Stabilizers

If tributyltin oxide is applied at retentions in excess of the toxic limits required to protect wood against fungal decay, the treatment eventually saturates all the free hydroxyl groups on the cellulose chains which are responsible for hygroscopic movement and the wood becomes completely stabilized. Such treatments are uneconomic but there are other possible systems for chemically reacting these troublesome hydroxyl groups. Formaldehyde treatment in the presence of an acid catalyst will cross-link hydroxyl groups on adjacent chains, reducing the dimensions of the wood in the process but also reducing the movement to less that 10% of normal. Acetylation involves the treatment of wood with acetic anhydride in the presence of a strong acid catalyst, a process that considerably reduces the hygroscopicity of wood and also increases its resistance to fungal attack.

However, all these chemical modification treatments suffer from the severe disadvantage that they are effective only if the wood is completely impregnated and they can therefore be used realistically only on permeable species;

acetylation is being used in this way to an increasing extent on radiata pine.

Bulking—Impreg—PEG—Carbowax—

MoDo

In bulking, the wood is impregnated with a very high retention of material which will physically restrain movement. Several resin systems have been employed in this way such as the phenolic resin in Impreg and a styrene/polyester co-polymer system used for the impregnation of floor blocks, in Finland. These systems rely on physical restraint and are reliable if deep penetration is achieved, although complete penetration is not essential. The polyethylene glycol waxes are also

Water repellents, stabilizers and decorative systems

bulking treatments but they are applied in water and retain the wood in the expanded wet state.

Treatments of this type such as PEG, Carbowax and MoDo are generally applied by prolonged diffusion. The compounds with low molecular weight of 200–600 are readily soluble in water and diffuse reasonably quickly but 1000 is less soluble and gives slower diffusion, although it is less hygroscopic so that after drying the wood is not so tacky as with the lower molecular weight treatments. These systems are used particularly for the stabilization of archaeological specimens, the largest to be treated so far being the warships Wasa in Stockholm and Mary Rose in Portsmouth which were spray-treated with a mixture of polyethylene glycol and borate whilst the atmospheric relative humidity was maintained at a high level to prevent drying. Although this system has been used successfully for the stabilization of gun stocks, the relationship between molecular weight, treatment time and hygroscopicity is a distinct disadvantage. In one system for the treatment of floor blocks the low molecular weight compounds are employed, followed by complete drying, and the introduction of isocyanate vapour which reacts with the glycol to form a polyurethane resin, avoiding all the disadvantages and giving a treatment which is stable and resistant to heavy floor wear. There are many other polymer systems that have been or could be used but they are generally unrealistic, combining the need for high retentions with expensive chemical compounds.

Decorative preservatives—Madison formula

While many water-repellent preservatives are designed specifically for use as pretreatments prior to painting or varnishing, perhaps in place of conventional priming treatments, other systems are designed as complete maintenance treatments, frequently serving a decorative as well as a protective function; these decorative preservatives are particularly popular in the Nordic countries.

The two types of water-repellent preservative are not necessarily similar; the first type must be compatible with subsequent paint or varnish coatings whilst the second type must clearly have good resistance to weathering. The Madison formula, developed in the United States as a maintenance treatment for western red cedar cladding, is perhaps the best known. It consists of paraffin wax, pigments and boiled linseed oil binder with pentachlorophenol as the preservative and zinc stearate to give water repellency, colour retention and freedom from stain. It has now been largely replaced by various improved proprietary products, those containing trihalomethylthio-compounds being much more efficient in controlling stain, as explained in Section 4.9.

Royal process

Weather resistance is poor with systems that are simple deposits of hydrophobic components such as waxes which are susceptible to preferential wetting, but can be improved by using a binder as in the Madison formula or by fixation to the wood as with silicone resins, although deep penetration will improve the performance of most systems. In the Royal process developed by Häger for the treatment of external joinery (millwork) a water-borne preservative treatment is followed by deep treatment with a drying oil. This is a very effective process but involves a complex multi-stage treatment and the need for a multiple oil-storage system to provide finishes in different colours. Whilst the Royal process gives an exceptionally durable decorative finish it is also very expensive. The main problem is that treatment is carried out in two stages, the first introducing large quantities of water which must be removed before the second oil impregnation stage can be satisfactorily achieved. There is no reason why similar reliability could not be achieved by single impregnation with an organic system, designed to achieve both the preservative and the decorative functions, but commercial companies are reluctant to invest in systems that,

because of the need for different colours, involve multiple storage tanks and a danger of contamination in the impregnation cylinder.