Papreg

Paper laminates treated with phenolic resins have been made for a number of years. They have been used chiefly for electrical insulating panels and for other nonstructural uses that do not require exceptional mechanical properties. The manufacturers, in developing these materials, have approached the problem primarily from the resin standpoint. We felt, therefore, that further development of paper-base laminates, from the standpoint of finding the most suitable paper for the purpose, was a promising field of research. This proved to be the case. Within 6 months after the research was started, a paper-base laminate was developed that Possessed several properties having more than double the mechanical values of those of the former laminates. For example, before the war the tensile strength of materials of this type rarely exceeded 14,000 pounds Per square inch. Parallel-laminated papreg is now being made with tensile-strength values from 35,000 to 50,000 pounds to the square inch. Papreg is stronger than fabric-base plastics, and can be molded at considerably lower pressures-75 pounds to the square inch in contrast to 1,000 to 2,000 pounds to the square inch. It is not equal to the cloth laminate in toughness, however.

Several commercial concerns make papreg. It was extensively used in airplane parts and accessories, such as gunner's seats, gunner's turrets, ammunition boxes, and the surface of a type of cargo aircraft flooring. It was tried to a limited extent for the skin surface of structural airplane parts, such as wing tips. The chief objection to it in this use is that it is more brittle than aluminum and requires special rather than conventional fittings. By modifying the fittings, we were able to fabricate a papreg fin for a trainer airplane that met laboratory tests as well as did the counterpart aluminum fin.

Papreg is not so readily molded to complex shapes as are fabric laminates, but can be made to take considerable double curvature. Normally it has a tan to amber color, but can be made from pigmented paper in a number of the darker colors. When the surface sheets are treated with a melamine rather than a phenolic resin, pastel surface colors are obtainable. Papreg shows considerable promise for use in a number of products.

Plastic Paper-Faced Plywood

During the war, plywood was faced with plastic paper laminates to increase its water resistance, hide plywood defects, minimize grain raising, and to produce a readily paintable surface. The paper surface can be molded directly to plywood under pressures as low as 75 pounds to the square inch without compressing the plywood. When it is desired only to hide plywood defects and avoid grain raising, high-strength grades of paper need not be used.

A considerable amount of paper-faced Douglas-fir plywood was made during the war for military use in storage lockers, table tops, and similar objects. New uses indicated for plastic paper-faced plywood are walls, floors, partitions, cabinets, showers, ramps, bins, sheathing, and concrete forms for buildings; boxcar, passenger car, and truck-trailer lining and siding; hulls, bulkhead, and cabins for small boats; airplane cabin linings; refrigerators; and boxes, trunks, and containers in general.

Low-Density Core Materials and Pulp Preforms

Wartime airplane construction needs produced a growing demand for a light core material to be used between plywood or metal faces to obtain skin surfaces less subject to flexing and buckling, without significantly increasing the weight of the airplane. A material was sought that was as light or lighter than balsa wood but more uniform in properties, readily available, and obtainable in larger sheets. These requirements were met with a light insulating type of board treated with phenolic resin to impart stability to the core. This and related cellular materials may find postwar use in prefabricated house construction, soundproof partitioning for boats, railroad passenger cars, doors, and other uses where lightweight and high rigidity are necessary or desirable.

Wartime research showed that plastics with a high percentage of longfibered filler possess general strength and impact properties far superior to those obtainable with ordinary molding powders. In many cases, moreover, the double curvatures needed are too great to permit molding preformed flat laminates to the desired shape. For such purposes, the Forest Products Laboratory devised a means of molding special ordnance items by the pulp preform method, which consists of forming a mat of resin-treated pulp in a suction box with screen surfaces having approximately the contour of the final item. After they are dried, these preforms are compressed in a conventional mold. For some articles uncontoured preforms can be used and flowed to shape in the mold.

Shaped preforms would appear to be especially suitable for large contoured objects where exceptional toughness is required. Refrigerator cabinets, theater seats, desk drawers, cafeteria trays, furniture, and motorcar and aircraft parts have been suggested. It might be used also for smaller objects such as ashtrays, hand wheels, instrument cases, and other purposes for which the usual molding-powder plastic is too brittle.

THE AUTHORS

Alfred J. Stamm has been a member of the Forest Products Laboratory staff since 1925 and chief of its Division of Derived Products since 1945. He has been largely responsible for the development of impreg, compreg, and staypak. A native of Los Angeles, Dr. Stamm is a graduate of California Institute of Technology and the University of Wisconsin. His research in the physical chemistry of wood won for him a Rockefeller Foundation fellowship, which enabled him to spend a year at the University of Upsala in Sweden. While there he represented the United States at the International Forestry Convention in Stockholm. Dr. Stamm has also investigated the electrical properties of wood, various aspects of the way moisture is held Within wood, and the molecular properties of wood constituents.

G. H. Chidester has spent the past 20 years at the Forest Products Laboratory doing research in pulp and paper manufacture. Since 1942, he has been chief of the Laboratory's Division of Pulp and Paper. He has specialized in the sulfite and semichemical processes of pulping woods, seeking to reduce costs and waste and to adapt the processes to a wider assortment of tree species, especially hardwoods little used for pulping. During the war he directed the laboratory's research in the field If paper plastic materials. A native of Hastings, Mich., Mr. Chidester is a chemical engineering graduate of the University of Michigan and worked for 5 years in pulp mills and on the staff of a paper-trade magazine before joining the laboratory staff.