-Unsaturated ( I 1) Ibid., 2,544,667, (12) Ibid., 2,544,668, (13) Grant, J. A., and Babcock, D. F. (to Owens-Corning Fiberglas Cory).), U. S. Patent 2,477,407 (July 26, 1949). (14) Hartman, S. 9., British Patent 607,035 (1948). (15) Hauserman, F. B., and Robb, R. XI., Reprint, Society of Plastics (16) (17) (18) (19) (20)
Industry, Inc., Reinforced Plastics Div., Eighth Ann. Tech. and Management Conf., Sec. 2 7 H , 1953. Hyde, J. F. (to Corning Glass Works), U. S. Patent 2,472,799 (June 14, 1949). Ibid., 2,567,110 (Sept. 4 , 1951). I b i d . , 2,582,215 (Jan. 15, 1952). Iler, R. K. (to E. I. du Pont de Nemours & Co.), U.S. Patent 2,356,161 (Aug. 22, 1944). Jellinek, M. R., Society of Plastics Industry, Inc., Reinforced Plastics Div., Proc. Seventh Ann. Tech. Session, See. 17,
1952. (21) Rlacllullen, C. W. (to Cowles Chemical Co.), U. S. Patent 2,587,636 (March 4 , 1952). (22) Nordberg, Rd. E. (to Corning Glass Works), U. S. Patent 2,494,259 (Jan. 10, 1950). (23) Rochow, E. G., “Chemistry of the Silicones,” pp. 83-4, New York, John Wiley & Sons, 1947. (24) Slayter, G., Ohio State Units. Eny. Espt. Sta. N e w s , 16 (4), 308 (1944). (25) Slayter, G., et al., M o d e m Plastics, 21, No. 9, 100 (1944). (26) Steinman, R., Ibid., 29, So. 3, 116 (1951). (27) Steinman, R., Society of the Plastics Industry, Inc., Reinforced
Polyesters-
Plastics Division, Proc. Seventh Ann. Tech. Session, Sec. 16, 1952. (28) Steinman, R. (to Owens-Corning Fiberglas Corp.), U. S. Patent 2,513,268 (June 27, 1950). (29) Ibid., 2,563,288 (Aug. 7, 1951). (30) Ibid., 2,563,289. (31) Ibid., 2,611,718 (Sept. 23, 1952). (32) Talet, P. A., and Cor, P. (to Societe Nobel Franpaise), U. S. Patent 2,572,407 (Oct. 23, 1951). (33) Torrey, J. V. P., Society of Plastics Industry, Inc., Reinforced Plastics Div., Proc. Seventh Ann. Tech. Session, See. 19, 1952. (34) White, E., Steinman, R., and Biefeld, L. P. (to Owens-Corning Fiberglas Corp.), U. S. Patent 2,446,119 (July 27, 1948). (35) Wier, J. E., Society of Plastics Industry, Inc., Reinforced Plastics Div., Proc. Seventh Ann. Tech. Session, See. 9, 1952. (36) Wier, J. E., Pons, D. C., and axilrod, B. S . , SPE Journal, 8 , 8-13, 27 (1952). (37) Yaeger, L. L., Society of Plastics Industry, Inc., Reinforced Plastics Div., Proc. Sixth Ann. Tech. Session, Sec. 12, 1951. (38) Yaeger, L. L., Henning, J. E., Marshall, S. V., and Cox, R. P.,
Air Materiel Command, Wright-Patterson Air Force Base, Ohio, Air Force Tech. Rept. 6220, 1950. RECEIVED for review October 19, 1953. ACCEPTED June 5, 1954. Experimental work was conducted and permission for publication was obtained under Contract KO.A F 33(038)-8902 for the Structural Test Branch, Materials Laboratory, Wright Bir Development Center, U. S. Air Force.
Effect of Filler Particle Size on Resins R . IC. WITT T h e Johns Hopkins Unicersity, Baltimore, Md.
E. P. CIZEK T h e Englander Co., Inc., Plastics Division, Baltimore, M d .
A study was made to determine the effects of particle size of a filler on the flexural strength of two different molding resins. Silica sand w a s chosen as the filler, since its shape is relatively constant over a wide size range. The resins were a melamine formaldehyde-type and an unsaturated polyester. The resin content of sample batches was varied from 5 to 3 0 q ~ .The data were examined in terms of a voids theory; consideration was given to the relation of the volume of resin present to the volume of the voids between closely packed particles of filler. Data obtained from the melamine resins were in contrast with those from the polyester mixes.
T
HE use of fillers or inert substances such as asbestos, glass
cloth, wood, flour, clay, and others in order to increase the strength characteristics of various resins as well as to lower the cost of the article is quite universal. The selection of the filler is based generally on the resin, the desired and final properties the article must have, and the process by which the article is made. I n the plastics industry today, some of these processes are known as compression molding, injection molding, and high and low pressure laminating. I n high and low pressure laminating, widespread use is made of polyester resins for the production of plastic parts, both large and small in size. These resins when uncured are in the liquid state. I n the fabrication of a part, layers of glass cloth or mat are shaped around or in the mold to be used, then wetted with the catalyzed resin, and cured. I n order to study the effect of size of filler and filler concentration on the strength of parts made by this process, i t was necessary t o choose a filler that could be easily sized by screening and also to
August 1954
choose one that could be measured readily and was compatible chemically with the resin. Silica sand was selected for these reasons. Silica-polyester formulations were made using different sizes of silica particles and varying the silica concentration. Relatively low pressures were used in the curing of these forniulations. References are given on the use of fillers in previous literature (6, 7 ) . Silica particles of about the same size and concentration were used with a different resin, a melamine-formaldehyde type. This composition was processed by cold molding or shaping the parts in a cold mold and curing them, free of the mold, a t atmospheric pressure and moderate temperatures. The data obtained from these two different materials are presented here. THEORETICAL BASIS
Flexure Test. As a measure of the effect of variation in pnrticle size of the filler and resin content on physical strength the
INDUSTRIAL AND ENGINEERING CHEMISTRY
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flexural strength test ,1STM Specification D 750-45T ( 1 ) was chosen. Cold molded articles are notoriously weak in flexure compared with articles made by other means, and an improvement in this property is desired. A flexural test applies a compressive stress on the load-bearing surface and a tensile stress on the opposite side. Therefore, it can be ascertained whether the sample failed in tension or compression first. Concept of Film Thickness. I n mixing a resin with filler, the particles become coated mith a film or layer of resin if the resin is in the fluid state. However, the film thickness of resin surrounding a filler particle is quite different for particles of different size even if the resin percentage is constant. If 20% by weight of resin is used with a filler particle of 20 microns, the number of particles in a given volume is quite different than if particles of 50 microns are used, and the thickness of the resin film changes accordingly. I n order to calculate the film thickness, the average particle dimensions in the filler must be determined. I n this investigation, the most convenient diameter was that one based on the volume-surface diameter, which was calculated thus:
METHOD OF INVESTIGATIOE
Preparation of Silica. Commercial grade crushed silica was classified with standard sieves and a Ro-Tap shaking machine, Approximately 200 grams of crushed silica mas placed in the coarse sieve in a set of six, and the sieves placed in the Ro-Tap machine and shaken for 10 minutes. T h e classified fractions were collected and used in the preparation of the sample bars, after the particle diameter had been determined. Particle Size Determinations. After grading the silica, a sample was placed on a microscope slide, magnified, and the particles measured. The particle diameter value is its measurement on a horizontal plane, so that the area is approximately halved, regardless of whether this dimension is the short axis or the long one. This ensures randomness in the measurement of particle size, Various diameters could have been chosen for consideration in the correlations. A diameter based upon the preponderance of the average particle would be calculated
di where d = particle of diameter n = number of particles 2 = sum of variable involved Concept of Voids. If spheres are placed in a container and packed rhombohedrally or in the closest fashion, the spheres occupy 74.1% of the volume and the free space accounts for the remaining 25.9% (4). On a weight basis, this relation or percentage changes depending on the specific gravity of the filler and of the resin. ;\laximum stresses may be expected from resin-filler formulations where the resin is present in sufficient quantity to completely fill the voids otherwise present, but not in excess so that the filler particles are separated from each other. If the voids are filled sufficiently, there will be no space in the structure that does not contribute to the over-all strength. If an excess of resin is present, the particles are prevented from contacting each other. Where compressive stresses are involved, failure occurs a t lower stress values, since the resin cannot carry so great a compressive stress as the silica filler. If an excess of filler is present, the voids will not be completely filled, and premature tensile stress failure map be expected. Thus, optimum strengths in flexure may be expected where, on a volume basis, the resin is present to the extent of 25.5%, and the filler is present to the extent of 74.1%. Then the structure will have sufficient resin to fill the voids, yet not too much to cause the filler particles to be separated appreciably. Use of Silica as Filler. Silica is used in quite a few commercial formulations, because it has a relatively constant shape factor (ii). Under magnification, the small particles can be seen to resemble spheres but with flat faces. The use of fibrous-type fillers in this study would have introduced the complicated problem of choosing the correct fiber dimension for measurement and correlation. However, data on glass fibers are included to show that length of the fiber in the macro sense of fiber length does affect the strength developed. Choice of Resins. A melamine-type resin, Melmac 405, manufactured by American Cyanamid Corp., was used in the initial study because of its high resistance to an electric arc, making i t a good choice as a cold molding material. T h e high temperature properties of silica should make the combination useful in the manufacture of electrical parts. A typical polyester resin, Rohm & Haas Co.'s Paraplex P-43, was selected to observe whether or not the effects were the same using a material that is processed quite differently. Selectron 5070, made by Pittsburgh Plate Glass Co., vias used in the glass fiber-resin formulations.
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=
Znd
A diameter based upon the area effect of the particle diameter would be calculated
A diameter based upon the volume-surface effect of the particle diameter vould be calculated d, =
Znd3 p 2
2nd
It can be shorn statistically that for the eame particle size The diameter d3 is often called the distribution curve dl
