896
INDUSTRIAL AND ENGINEERING CHEMISTRY
These data represent tests made on an insulated run-ofthe-mill latex wire obtained from daily production. No attempt was made to pick a wire of particularly high insulation resistance. Here again the insulation resistance rises until a constant high value is reached-in this case, around 20,000 megohms per 1000 feet. Also, during the period of aging only a small decrease in tensile strength occurred. In order to give a clear picture of the usual results obtained on aging latex-insulated wire, Figure 1 gives results with compound 784 to show its extremely good aging qualities. At present a direct comparison of latex-insulated wire and wire obtained by the extrusion process is not possible. This is primarily due to the fact that it is difficult t o make a centered wire by the extrusion process with the same wall thickness as that obtained by the latex process. At the present time any such direct comparisons would be vitiated by the unevenness obtained by the extrusion process. However, it is reasonable to suppose that, if such a comparison
Fiber Building Boards Their Manufacture and Use CHARLES G. WEBER National Bureau of Standards, Washington, D. C.
Fiber building boards constitute an important article of building construction. The boards are made of crudely refined vegetable fibers obtained principally from agricultural crop plant wastes, and sawmill wood wastes. The raw materials are softened for pulping by digesting in dilute alkali or acid or water under pressure. Boards are made on modified paper-making machines in continuous sheets and dried over hot rolls or between heated platens. Moisture resistance is obtained by incorporating sodium resinate or by means of waterproof surface coatings. Heat- and sound-insulating properties are based on the low-density and high-porosity characteristics of loosely felted fibrous structures. HE two classes of fiber building boards are based on their general characteristics: Wall boards are designed primarily as finishing covers for interior walls and ceilings; insulating boards are designed particularly to reduce the passage of heat or to absorb sound. While these products are of comparatively recent development, they have attained a position of real importance as building materials, and their manufacture is classed as a large industry. Wall board became an article of commerce about 1906, but it was not extensively used until
VOL. 27, NO. 8
were possible and were carried out, electrical tests would be certainly as good and probably superior in the case of the latex process than in the case of the extruded material. In order to give a clear picture of the really vital improvement in insulating conductors by the use of latex, Figure 2 gives cross sections of wire insulated by the latex method and by the ordinary extrusion method. The samples of extruded wire were obtained a t random from different manufacturers. The latex samples were obtained from factory runs in this factory; no special precautions were taken. The picture represents the actual type of results which can be expected from the two processes. All of the above information indicates that perfect centering combined with outstanding electrical properties has thus been made possible by the modern development of latex insulating methods. RBCEIVED April 27, 1935.
the World War; the insulating boards are of still more recent development. The growth of the industry and its importance are indicated by the value of the products of the industry for recent years. The total value (1) of fiber wall board and insulating board and of flexible fiber insulation produced in the United States during 1929 was $29,337,909, and the total value for the depression year 1931 was $19,296,169. Fiber building boards are, in a sense, synthetic lumber designed for specific purposes and made in forms that can be handled and applied rapidly with a minimum of skilled labor.
Fibrous Raw Materials Unbleached ground-wood pulp was formerly used almost exclusively in the manufacture of fiber building boards, and some of the well-known brands of wall boards are still made wholly or principally from this material. A variety of crudely refined vegetable fibers from other sources are now utilized, particularly in the insulating types of boards. These fibers are obtained from extracted sugar cane, sawmill waste, straws, cornstalks, grasses, extracted licorice roots, tobacco stems, waste papers, bark, and other similar vegetable materials. This list of fibrous raw materials is comprised largely of farm crop and industrial wastes, the utilization of which is of great economic interest. The position of increasing importance assumed by the waste crop materials is largely a result of recent researches on the utilization of farm wastes (4, 5 , 7 , 8 ) .
Manufacture There are three important steps in the manufacture of the typical fiber building boards from a fibrous raw material. The raw material is pulped; a mat is formed from a water suspension of the pulp on a modified form of paper machine; and the mat is pressed and dried t o form the finished board. There are two general types of boards made by different methods. One type is a laminated product consisting of several plies of thin pulp board pasted together with a suitable adhesive, usually sodium silicate; the other is a homogeneous product built up to thickness in the forming operation. The manufacture of laminated boards consists essentially of making pulp board similar to ordinary box board on a conventional multicylinder paper-making machine, and pasting several (usually four) plies together to make up the finished board. Ground-wood pulp, made by reducing wood t o a coarse pulp by contact with a revolving abrasive stone
I m n J is leis wweptible to uidd growtll tlian untreated wimd tiy t,reat,irrgwith soliitiuns of zinc i4ihlride, incrcitric cliloridt:, ( t r
other strong fiingicides.
Properties and Lses The r i m s t ilnpurtaiit of tlie fiber building Iii,ards at present are those designed Six iient atid sound iusolatiou. These film prducts owe their lieat-insulating prqierties to iiinurneralile minute air spaces d i i e i i
ha- lieen fwind quite effective; Iiowerer such treatmip,in tlic acuiatir, properties of the boards. It is 1x,*,siih* that ti!^ lv~iirdseiin be made slow-hurning Iiy impr?guating with me or niore of the salt.$ used for flairre~pr,x~!ingtext,ili+ :ual ~ J > L ~ I C Y(0) S witliout serious1 affcct,iug tlic 1w:~t. or .~,iiriii-iir.ointiirg properties. It is al powililii to r w h w sii+i:qitil,ility to nrolding to n point where tin, f i l m
they cintain arid to t h e infrequency d fibers pirrallel to the liue of heat flow to act a.5 conductors; hence, the insuhting stslue US a board depends on it,s density and oil the arraugernent of its fibers. The results of lieat t r a n sni i s L: io ii te R o c k (3) indicated that, regetable fiber insulation, tioat transfer ttirougli tlie fiber insulating uiaterials i,. pract,ically all by conductiou in tlie fillers and ttirough the enclosed air. Other thingbeing ?qual, boards haviuy tile greatest proportion of fibers parallel to the surfacri of tlie hoard. and . ueruendicular to tlw . l i n e of heat flow, have the lowest conductivity. However. present methods of nianufacture tend to orient the fiberin this posit.ion, leaving deusity as the important factor. Above a minimum MJt attairied in comniercia~ product% the tlierriral conductivity of sheet insulating materials habeen found tu iucrease with density at an approxiinately uniforrn rate.
INDUSTRIAL AND ENGINEERING CHEMISTRY
898
VOL. 27, NO. 8
edges and often with special surface modifications in the way of regularly spaced holes or grooves t o increase sound absorption. Common wall boards are designed only for use as a finish cover for interior walls and ceilings. They differ from the insulating boards in that they are usually of much higher density, with greater strength per unit of thickness, and have B surface either with decorated finish or designed for such finish. In view of the present emphasis on the value of heat and sound insulation, it would appear that the insulating type combines the essential utility of the wall boards with heat- and soundinsulating properties, and superior surface finishes and greater strength are the properties that prevent the displacement of the wall boards by the insulating types. The properties of a typical wall board follow (9): Thickness, inch Density, gram/cc. Flexural breaking strength (12-inch span, 3 inches wide) pounds
CANE-FIBER(CELOTEX) BOARDEMERGIXG FROM DRYINGOVEN
TAE
While all structural work usually involves the question of strength, few uses of insulating boards require that they contribute materially to the strength of the structure, and sufficent strength for economic handling and erection appears to be all that is needed. Since resistance to flexural breaking is the property most essential to economical handling of such boards, the flexural breaking strength appears t o be the strength property most closely related t o use requirements. The boards are commonly made with the lowest density consistent with sufficient resistance to flexural breaking for satisfactory handling. The essential properties of a typical fiber insulating board follow (9) : Thickness, inch Density. gram/cc Thermal conductivity, B t u /(sq ft ) (hour) (inch)' F Flexural breaking strength (12-inch span, 3 inches wide) pounds
0 0 0 9
44 26
34 8
Some of the properties that make a fiber board effective as a heat insulator tend to make it sound-absorptive. The loose fiber arrangement and high poroqity characteristic of boards of low density result in high absorption of sound and make the ordinary insulating boards valuable for acoustical correction. With rapidly expanding use of boards in this field, special types have been developed for sound insulation These boards are essentially heat-insulating boards, an inch or more in thickness, uwally made in small panels with beveled
0.19 0.55
15.3
Fiber boards are subject to dimensional changes with changes in the relative humidity of the surrounding atmosphere. However, with modern methods of manufacture the dimensional changes are minimized, and many of the boards purchased by the Federal Government are now required to show a lineal expansion of not over 0.5 per cent for a relative humidity change of from 50 to 96 per cent (8). Fiber board is used widely in home construction in walls, roofs, attic floors, and basement ceiling as heat insulation. It is also used in place of lath and plaster because of greater ease and speed of application. It does not, however, make a surface suitable for papering because expansion and contraction are often sufficient to crack the paper a t the joints. Most pleasing effects are obtained in panel effects with a decorated surface. It is well adapted for use as sheathing under brick or stone veneer, and is used in place of lath for inside plaster or outside stucco. If kept well painted, it will give good service as outside sheathing for summer cottages and temporary construction. In factories and office buildings it is used extensively for roof insulation. The acoustic boards are being used increasingly as inside wall covering for theaters, auditoriums, hotels, restaurants, schools, and churches for sound-absorbing purposes. Ordinary wall boards now find their chief applications ab wall coverings in cottages, temporary buildings and partitions. and similar construction where low cost and ease and speed of application are the principal factors. However, boards designed for special finishes or with decorated finishes are used for interior finish covers in building construction of the better classes. Literature Cited (1) Bur. of Census, Census of Manufacturers, 1931. (2) Federal Specification Board, Spec LLL-F-321. (3) Finck, J. L., Bur. Standards, Research Paper243 (1930). (4) Gibson, 8 .G., IXD. ENQ.CHEM.,22,223 (1930). (5) Hind, R. R., Sugar News, 7, No. 1 (1926). (6) Ingberg, S. H., Bur. Standards Letter Circ., 1931. (7) Sweeney, 0. R., and Arnold, L. K., Iowa State Coll. Agr. Mech. Arts, Bull. 98 (1930). (8) Sweeney, 0. R., Hartford, C. E., Richardson, R . W., and Whittemore, E. R., Ibid., 102 (1931). (9) Weber. C. G., Carson, F. T., and Snyder, L. W., Bur. Standards, Miscellaneous Pub. 132 (1931). RECEIVED April 27, 1935. Publication approved by t h e Director of the National Bureau of Standards.
ROD MILLS USED IS REFININGTHE FIBERAFTER IT Is E X P L O D E D FROM THE
STEAM
GUSS (.v.iSOXITE
PROCESS)
