Suitable preservative treatments seem likely for the protection of wood to be used in cooling towers; also the use of diffusion treatments now widely applied to untreated wood in place is being studied for cooling towers.

Continuous research keeps enlarging the number of new or improved water- and oil-carried preservatives that insure longer service from a variety of wood products that need to be painted or finished, including building lumber and plywood, signs, and highway posts.

Improved selection of woods that are resistant to mild gases and chemicals, for uses in which these properties are important and wood is superior to other materials of moderate cost, will be undertaken. References

1. Baechler, R. H. Determining Preservative Retention in Piling by Assay of Borings. American Society for Testing and Materials, Special Technical Publication No. 275, 1959.

2. U.S. Forest Products Laboratory. Wood Handbook. U.S. Department of Agriculture, Agriculture Handbook No. 72, 1955.

FOREST PRODUCTS LABORATORY NATURAL FINISH

Description and use

A durable natural finish that can be used for wood outdoors without excessive maintenance has been sought for many years. Film-forming finishes such as varnishes and lacquers have not given good performance when used outdoors. Yet there are many Government-owned buildings and structures, particularly in recreation areas on national parks and national forests, where the natural beauty of wood and the rustic appearance are considered important.

The Forest Products Laboratory has developed a natural finish, comparable to a modified semitransparent oil-base stain, that effectively retains much of the natural beauty of wood when exposed to the weather. It is intended primarily for siding on buildings, but has been used to advantage on signs, picnic tables, and similar applications. The finish does not lie on the surface in a film, but soaks into the wood, imparting a certain amount of water repellency as well as attractive color and appearance. It can be applied in a single coating, and refinishing is required only every 3 to 5 years. It is available commercially.

Future potential

It is anticipated that the Forest Products Laboratory natural finish will be used extensively on structures maintained by Government agencies, particularly where the natural beauty of wood and its rustic appearance are considered desirable. As a result, a Federal specification for the finish has already been prepared. The finish is now manufactured by a number of paint manufacturers and is available in many parts of the country.

References

1. General Services Administration. Federal Specification TT-S708, Stain, Oil; Semitransparent, Wood, Exterior.

2. U.S. Forest Products Laboratory. Forest Products Laboratory Natural Finish. Forest Products Laboratory Report No. 2096, 1961.

WATER-REPELLENT PRESERVATIVES FOR MILLWORK

Description and use

Water-repellent preservatives for wood are thin liquids that may be used in a dip or brush treatment to impart water repellency to the surface of the wood and also incorporate a certain amount of preservative that will prevent stain, mildew, and to a limited extent, decay. These chemicals have been found particularly useful as a dip treatment for millwork, wood siding, and other wood materials that are used well above ground level in structures. In such applications, the decay hazard comes primarily from liquid water that reaches the wood surface in the form of rain or condensation. The treatments prevent this water from soaking into the wood and thus improve the performance of wood products. These treatments are particularly useful for window sash and frame and other millwork that is exposed on the exterior of houses, because of the many joints in such members, which tend to entrap water.

Research at the Forest Products Laboratory has shown that treatment with water-repellent preservatives greatly enhances the performance of such wood products in preventing stain and decay at joints, the rapid pickup of water, the swelling of wood at joints that is associated with such water pickup, and the resulting opening of joints that would otherwise occur. Paint performance over surfaces is also greatly enhanced. Already a good share of the exterior millwork on the market receives water-repellent preservative treatments.

Future potential

Treatments with water-repellent preservatives are encouraged by the National Woodwork Manufacturers Association, and members of that association have developed quality control standards for customer protection. A certain amount of lumber siding for structures is also treated. The Forest Products Laboratory is continuing research on water-repellent preservatives to study the effectiveness of such treatments on performance of wood products under exterior exposure. Through such treatment it is anticipated that many species that are not now considered suitable for exterior use in buildings may be made comparable in performance to some of our very best siding and millwork species.

References

1. Miniutti, V. P., Mraz, E. A., and Black, J. M. Measuring the Effectiveness of Water-Repellent Preservatives. Forest Products Journal, volume 11, No. 10, 1961.

FIRE-RETARDANT TREATMENTS FOR WOOD

Description and use

Because of the low heat conductivity of wood and charcoal, fire penetrates into heavy wood members at the rate of only about 11⁄2 inches per hour under standard time-temperature fire conditions. A beam or arch of large cross section, therefore, loses section very slowly. Since the temperature even a short distance from the point of char is relatively low, a large wood beam loses strength essentially only in proportion to its loss in cross section. A large beam or arch, there

fore, even if untreated, can withstand fire exposure for extended periods without collapse. In addition, considerable research has gone into developing methods of making wood resistant to the harmful effects of fire. Research has resulted in the development of two major methods: One through impregnating treatments that leave certain salts such as monoammonium phosphate, ammonium sulfate, borax, and the like in the wood; and two, through the application of surface coatings of paints of oil, resin, or latex-base preparations. Both of these methods result in a great reduction in the rate at which fire spreads over the surface of wood. They are available commercially, are widely used, and are gaining more general acceptance in building codes.

In general, the impregnating treatments lend themselves best to the initial treatment of wood before it is incorporated into the structure. Such treatment can be given to lumber, plywood, laminated members, and even to such newer products as particle board and fiberboard. Where the building is already erected and a certain degree of fire protection is required, the use of fire-retardant paints is recommended.

A very active research program is underway at the Forest Products Laboratory and other laboratories to study the fire performance of wood from both the practical and the theoretical standpoints. Through basic research on pyrolysis and combustion of wood, it is hoped that major breakthroughs will be made that will result in the development of radically new and different ways of imparting fire resistance to wood.

Future potential

Even today the use of fire-retardant treatments and finishes has many applications in Government-owned buildings and structures. There are, however, many applications where complete answers are not yet available. For example, in bridges and trestles preservative treatments are required that may not always be compatible with the incorporation of fire-retardant chemicals. No good treatments are yet available for combining with oilborne preservatives that may be used to treat bridge timbers for example. Further research is needed on problems of this type, and when successful it will result in great savings through the improved performance of wood products in

service.

References

1. U.S. Forest Products Laboratory. Fire-Retardant Treatments for Wood. Forest Products Laboratory Report No. 2081, 1957.

BUILDING FIBERBOARDS AND PARTICLE BOARDS

Description and use

A building fiberboard is a sheet material manufactured of refined or partially refined vegetable fiber, principally from wood. Binding agents and other materials may be added to increase strength or other properties. Building fiberboards are manufactured primarily for use as panels, insulation, and cover materials in buildings, for components of cabinets, cupboards, doors for millwork, and furniture, and other constructions where flat sheets of moderate strength are required.

A particle board is a sheet material of wood particles or chips bonded ith various resins and consolidated under heat and pressure. They re used for many of the same uses as fiberboards.

The production of wood-base fiber and particle panel materials like sulation boards, hardboards, and particle boards is about 6 billion quare feet per year in the United States. This is about the equivalent f one-seventh of the total lumber production. The indications are hat production and consumption of these panel materials by 1975 ill approximate 12 billion square feet

In the manufacture of wood-base fiber and particle panel materials, ood is broken down into smaller components and then reassembled o make the large-size sheets. Hence, the particles and fibers can be reated during manufacture to improve their fire resistance over the ntreated material. At present, one manufacturer of particle board 3 treating the particles to make them very fire resistant. The nished board is being used for cores for solid-core doors where high re resistance is desired. A door of this construction has a fireesistance rating equivalent to that of a door with a foamed-glass ore. A manufacturer of insulation board treats the fiber with fireetardant chemicals to improve the fire resistance and reduce the glow in times of fire. This enhances the use of the insulation board n the form of acoustical tile for interiors of public buildings.

Future potential

The use of fiber and particle boards is expanding rapidly and has a great potential. Current research, as illustrated by the following projects, will expand the use of these materials:

The Forest Products Laboratory has recently started an extensive study of the behavior of these materials under stress and is determining the magnitude of effects of heat and moisture on these materials. When the research is completed, it will be possible to assign design stresses for the various qualities of these products which will permit them to be used in engineering construction.

The American Hardboard Association recently completed a severalyear study on the requirements for hardboard for use in large signs. Special boards and methods of construction have been developed which permit the use of this material with conventional paint and highly reflective plastic films in large traffic signs where lack of distortion is of major importance.

The insulation board industry has developed a product called insulating formboard for use in roof structures with lightweight poured decks. In this construction, the special-quality insulation board serves as the bottom of the form. It is left in place after the lightweight deck is poured and contributes to the structural strength and thermal insulation. It also may be finished so that no additional ceiling treatment is required. A standard specification for this product is nearly complete.

References

1. U.S. Forest Products Laboratory. Wood Handbook. U.S. Department of Agriculture, Agriculture Handbook No. 72, 1955.

Description and use

MODIFIED WOODS

The tendency for wood to swell and shrink, which is objectionable for certain uses, is a result of wood's affinity for moisture; moisture is given off in periods of low relative humidity and taken on in periods of high relative humidity. Changes in volume, of course, are undesirable where a close fit or tolerance between members must be maintained.

Where the problem is due to short-time changes in relative humidity, temporary protection is adequate. This is one of the functions of a finish or paint coating. This temporary barrier is effective, of course, only when the pronounced changes in relative humidity are of relatively short duration, and only providing the barrier retains its integrity under severe conditions of outdoor weathering.

Where changes in average relative humidity are seasonal, and the exposed wood may undergo appreciable changes in volume, the most successful approach is by reducing the affinity that wood has for moisture. This requires the deep penetration of a stabilizing chemical into wood. Because of the slowness with which chemicals move into wood, these processes are time consuming and therefore expensive. The Forest Products Laboratory has developed the principle of reducing moisture affinity by impregnating wood with certain resins. A proper treatment will reduce the natural swelling or shrinking tendency of wood by 60 to 70 percent. The resulting treated wood, which may contain 30-percent resin, is called "impreg." "This product is manufactured by three companies in the United States. The total volume of about 11⁄2 million board feet is used almost exclusively in the automobile industry for making patterns and die models for tooling the industry.

Improved dimensional stability, together with improved mechanical properties, can be obtained by compressing the resin-treated wood still further to almost twice the density of impreg. The resulting ma.