und Good W a t e r Repellency
POST-WAR PERIOD has seen a major expansion in the utilization of synthetic resinous products as binders for cheap, particulate mineral solidsparticularly in the manufacture of flooring, roofing, and decorative wallcovering-which are widely used in the building construction and automotive trades. Application of such compositions to the fabrication of structural elements in buildings has been very limited, however, primarily because of the high cost of such compositions relative to conventional inorganicbonded compositions such as concrete, plaster, and gypsum-, silicate-, or limebonded fibrous or granular materials. This prohibitive cost arises in large part from the high concentrations of binder required to produce a composition with suitable mechanical properties 20 to 40Y0 of resin on a -typically weight basis. Were a technique to be found for preparation of durable, resinbonded particulate structures where resin requirements were about 0.1 these quantities, the economic picture would be drastically altered, and the potentialities for large scale use of these compositions in construction greatly enhanced. From elementary geometric considerations, it is a simple matter to show that a consolidated mass of granular particles will be optimally bonded together when the bonding adhesive is concentrated a t the contact points between the particles; adhesive in excess of that necessary to occupy the contact points, while contributing to the cohesion or strength of the mass, will have far less influence than that reT H E
quired to establish the interparticle bonds. Thus, if it were possible to distribute an adhesive in this highly idealized fashion throughout a granular solid matrix, the volume fraction of adhesive required for satisfactory strength development would be far less than the void fraction in the porous solid. In any kind of random mixing procedure, however, the probability that an element of adhesive will deposit a t an interparticle contact point is very small for low adhesive concentrations, but rises rapidly with volume concentration of adhesive. This to a large degree explains why resin concentrations of nearly 40% by volume are required to develop adequate strength in conventional filled-plastic formulations. While the attainment of this optimum resin distribution would offhand appear to be an unrealizable goal, under certain circumstances concentration of adhesive a t particle contacts can actually be thermodynamically favored. For example, if solid particles are uniformly coated with thin layers of (fluid) resin, and then brought together in a compacted mass, capillary forces favor accumulation of the resin in lenticular elements at the points of contact as shown in the diagram below. This principle is utilized in the fabrication of certain types of synthetic fiber felts, and to a degree in the fabrication of shell moldings for foundry use. An indispensible requirement for successful use of this principle, however, is exceedingly uniform initial distribution of adhesive over all particle surfaces; with low adhesive concentrations, this can be accomplished only by employing large
volumes of resin solvent and/or protracted mixing, both of which are costly and time-consuming. Treatment of a mineral solid in water suspension with a surfactant which renders the solid oil-wettable, followed by treatment with emulsified resin, has previously been demonstrated (7) to yield compositions with superior mechanical properties at low resin contents. In particular, high-strength, kaolinepoxy resin compositions were prepared by this procedure, using dimethylaminomethyl phenol (Rohm and Haas “DMP-10”) as a combination surfactantcuring agent. The proposed mechanism of surfactant action is illustrated in the diagram below. As resin requirements for effective bonding would be expected to decrease with increasing mineral particle size, evaluation of this resin-surfactant system with coarsegrained silica sand seemed in order. By aqueous dispersion blending techniques, using dimethylaminomethyl phenol to promote resin adhesion, it is indeed possible to produce strong, durable structures from sand-epoxy resin compositions containing very little resin component. Resin requirements for successful bonding decrease with increasing grain size-for narrow-sized distribution sand-and can be further reduced by combining relatively large with very small particle-sized solids. With 3.570 of resin by weight-4.2% total organic component-a fine-coarse sand blend composition with a tensile strength of about 1100 p.s.i. can be prepared. Application of these materials to foundry operations-shell, core molds--would appear to merit VOL. 52, NO. 9
e
SEPTEMBER 1960
785
UINERIL
PARTICLE
1
MINERAL PART/CLE
A
Surfactant promotes uniform resin coating of minerals in water
evaluation. At 60 cents per pound for resin and curing agent, a materials cost of less than $60.00 per ton of composition can be anticipated; this may also place such formulations in a competitive position with respect to conventional prefabricated structural materials used in the building trades. Experimental
Materials. Epon 828 (Shell Chemical Co.), a liquid, bisphenol-A-based epoxy resin, having a melting point of 8" to 12" C., a viscosity (25" C.) of 50 to 150 poises, and an epoxide equivalent of 175 to 210 grams per equivalent. Dimethyl aminomethyl phenol [ (CH3)2N-CH&HsOH, Rohm and Haas, "DMP-lo"], has a boiling point range of 80" to 130' C. at 2 mm. of mercury, and a specific gravity of 1.020 at 25" C.
The xylene was reagent grade, and Ottawa sand-esentially pure silicawas graded by dry-sieving into two narrow cuts: 40- to 60- and 100- to 120mesh. Potter's Flint-essentially pure silica-an extremely fine pulverized silica-10070 finer than 270-meshhad the following size analysis: 90% finer 50y0 finer than than 70 microns; 34 microns; 10% finer than 6.5 microns. The sand was first cleaned of surface contaminants by treatment with warm aqua regia, followed by exhaustive washing with tap water until the filtrate was neutral, and then dried a t room temperature. This elaborate cleaning procedure was subsequently found to be unnecessary to achieve satisfactory results. A weighed quantity of Epon 828 resin was thoroughly mixed with an amount of DMP-10 equal to 2070 of the resin weight, and this mixture was diluted to pourable consistency with a small 1 part to amount of xylene-roughly 3 parts of resin. Two hundred grams of clean, dry sand were added to 1 liter of water, a t room temperature, with agitation provided by a high-speed, propeller-type stirrer, thereby maintaining an essentially uniform suspension. The resin mixture was then added slowly to the slurry with continued agitation. After a few seconds of mixing, the sand underwent detectable agglomeration, at which point the mixing was stopped and the slurry was transferred to a Buchner vacuum filter where most of the water was removed. The relatively dry, but cohesive, filter cake was broken up and promptly transferred to small, wooden tensile test ("dog-bone" shaped) bar molds, and then compacted under a static load of about 500 p.s.i. The compacted samples, without removal from the molds, were placed in a hot air oven to cure a t 110' C. for 2.5 hours. After curing, the samples were removed from the molds, and trimmed to remove
flash and rough spots. Five or six specimens were prepared from each batch. Sample densities were determined by weighing each test specimen, and measuring its volume by mercury displacement. Samples were then stressed to failure in tension in a low-strain rate. tensile test machine. Wet-strengths of the compositions were determined by immersing cured specimens in water for 24 hours and testing as above. Where necessary, resin contents of the compositions were determined by ignition of fragments of the tensile test specimens to constant weight. Where mixtures of fine and coarse sand were used, variations of mixing and drying procedures were also studied. In some runs, both fine and coarse sands were slurried in water, and the resin mixture then added; in others, resin was added to the coarse sand slurry, followed by addition ol the fine sand. Certain batches were allowed to dry under vacuum before compaction, while others were merely filtered free of excess water, and compacted damp. Samples compacted while wet were allowed to dry at room temperature for a day, and then were oven cured. Results a n d Discussion Uniform-Sized Sands: Effect of Particle Size and Resin Content. Figures 1 and 2 present the variations of as-cured and soaked tensile strengths with resin content for the 40- to GO-, 100-to 120-: and < 270-mesh sands. As expected, strength first increases rather rapidly with increasing resin content, and then becomes relatively insensitive to this variable. Peak strengths reach surprisingly high levelsover 1000 p.s.i. in some instances-~even for the coarsest sand. Optimum resin concentration increases \vith decreasing particle size: roughly 6Yo by weight for 40-to 60-mesh sand, 8% €or 100-to 120-mesh, and 11 to 12y0 for
IO00
4'
aoo
BOO
. i . 9
2u 5
2 Y 2
600
600
400
400
zoo
200
0
0
Figure 1. Tensile strength increases with grain size at low resin contents
786
INDUSTRIAL AND ENGINEERING CHEMISTRY
0
4
6
8
Figure 2. Water-soaked strengths are about two thirds of the dry values
CONSTRUCTION MATERIAL Table 1.
Sand Size, Mesh 40-60 100-120
