In the manufacture of foundry molds and cores from sand which is bonded with a curable binder, recycling the sand is an important economic consideration. Foundry sand can normally be used, then reused, repeatedly, when most resin binders are used. Normally there is a small loss of sand on reuse, and this loss is made up by adding pristine sand to the used sand. These conditions are experienced with most resin binders.
In recent years, however, alkaline phenolic resins have gone into widespread use. These resins can be ester cured at ambient temperature. Such resin binder systems are disclosed, for example, in patents: U.S. Pat. No. 4,426,467, in which lactones are used as the curing agents; U.S. Pat. No. 4,474,904, in which carboxylic acid esters are so used; and U.S. Pat. No. 4,468,359, in which the esters are in the gaseous or vapor phase. These patents are expressly incorporated herein by reference.
While these binding systems offer many advantages, there are offsetting disadvantages that are sometimes observed. For example when the sand is reused, the tensile strength of the molds or cores drops off. This may have a serious effect on the economics of the foundry.
The extent to which previously used sand is able to be reused is often determined by the tensile properties that can be achieved. One factor which determines the tensile strength is the ability of the binder to bond to the surfaces of the reclaimed sand. Higher rebonding strengths allow higher usage levels of reclaimed sand.
To reclaim sand from a foundry mold or core, the used mold or core is shaken, vibrated or dismantled mechanically subsequent to removal of the casting, to loosen the sand and break up any lumps or agglomerates. The bonding properties of reclaimed sand are generally poorer than those of pristine sand, requiring further processing for effective use. There are three generally recognized treatment methods for reclaiming sand, mechanical, wet and thermal.
The mechanical treatment processes typically involve subjecting the used sand to grinding, scrubbing or other mechanical attrition to free up the individual grains of sand, remove binder residues, provide clean sand surfaces, and remove fines. The wet treatment processes involve washing the sand with water, draining, and drying the washed sand to the moisture levels necessary for subsequent use. In thermal treatment processes, the sand is heated to a temperature of about 120.degree. C. or above, so that the binder residue is decomposed or burned.
One common objective of these treatment processes is to remove binder residues. The binder residue level may be determined by a loss-on-ignition (L.O.I.) test. New sand has an L.O.I. of about 0.1%, while untreated used sand has an L.O.I. of about 0.5% to 3%.
Where the reclaimed sand is recovered from foundry molds or cores in which the binder was an ester cured alkaline phenolic resin, wet treatment processes have been found to be very effective. Wet treatment processes can produce sand which exhibits high bonding strength substantially equivalent to that of pristine sand. However the thermal and mechanical treatment processes do not produce treated used sand with such high bonding strength. It is believed the wet treatment processes "cleanse" the surfaces of the reclaimed sand of any harmful residues. The mechanical and thermal treatment processes apparently are not as effective in removing these harmful residues.
However, mechanical treatment processes are the most commonly employed by the foundry industry because they are the most economical. Thermal treatment processes are undesirable in comparison because of their high energy costs. Wet treatment processes are undesirable in comparison because of disposal problems associated with the wash water and energy costs for drying the sand.
Because of limitations in the mechanical and thermal treatment processes, reclaimed sands so treated, particularly those recovered from foundry molds or cores wherein the cured binder was an ester cured alkaline phenolic resin, have bonding properties inferior to those of pristine sand. These inferior bonding properties limit the usage level of reclaimed sand in many cases to about 50 weight percent of the total sand when forming foundry cores and molds. Reclaimed sand levels of up to 90 weight percent are needed to achieve the desired economics and minimize disposal costs.
In comparison, the used, reclaimed sand obtained from foundry molds and cores which employ an acid cured binder, such as an acid cured phenolic resin, an acid cured furan resin, or a phenolic-urethane resin, do not suffer significant losses in bonding strength after thermal or mechanical treatment. However, wet treatment processes are generally not effective for such used sand.
Often, the usage levels of mechanically and thermally treated reclaimed sand in the foundry is as high as 80% to 90% by weight of the total sand usage. Theoretically, the usage levels of such reclaimed sand could be as high as 100%; however, there are handling losses and it is usually necessary to replenish the stock of sand with a makeup amount of pristine sand.
It is desirable to enhance the bonding ability of used sand reclaimed from foundry molds and cores made with ester cured alkaline phenolic resins, to the extent that usage levels of such used reclaimed sand as high as 80% to 90% by weight can be achieved.
In U.S. Pat. No. 3,487,043, treatment of inorganic filler material for filler may be, for example, fiberglass, titania, ceramic fibers or powders, carbon black, silica, alumina, silica flour, asbestos, clays, and the like. The silane may be applied directly to this filler--reinforcing material, col. 3, lines 55-58. The reinforced or filled resin is used to make pipes, tanks, and other such objects. There is no suggestion of usefulness in the foundry industry.
The treatment of reinforcing inorganic fillers (glass mats) for polymers with a silane solution has been disclosed in U.S. Pat. No. 4,118,540 and UK Patent Specification 882058. In such treatment processes, the filler is immersed in a silane solution, then dried. The silane is believed to improve the affinity or receptivity of the glass mat for the polymer. There is no suggestion of using such a treatment in a foundry application, and the drying step consumes energy.
J. Robins and others have several patents relating to the use of isocyanate binder systems in the foundry industry. In one such U.S. Pat. No. 3,403,721, there is a disclosure of precoating the sand with a silane, col. 4, lines 32-39, but as that patent points out, pre-coating in this system offers no advantage over adding the silane at some other point.
The addition of silane to resin binder solutions to increase bonding strength is well known. Methods and compositions wherein a silane is added to the resin solution have been disclosed in the two patents just discussed and also in U.S. Pat. Nos. 4,256,623, 4,111,253 and 3,234,259, and UK Patent Specification 876,033.