Patent GB-952762-A (GB)
Title: Method for making grinding wheels and the like
IPC: B29C44/04
Filed: 1961-02-22
Abstract: A method of producing a polishing or abrasive article comprises differentially centrifuging a mixture comprising hardenable liquid synthetic resin composition, granular polishing or abrasive material and relatively fine particulate filling material to concentrate the granular material in a local region in the body of the resin while leaving the particulate filling material substantially unaffected and substantially uniformly distributed throughout the mixture, and hardening the resin composition. The preferred method uses a foamable resin composition and produces a foamed product.  Suitable resins are polyurethanes, preferably used as prepolymers. Suitable fillers are finely ground diatomaceous earth, limestone, aluminium oxide, magnesium oxide and silicon carbide flour. Flakes of graphite and metallic powder or flakes can also be added to the mixture. Aluminium oxide is a specified abrasive.  A suitable polyurethane may be made by blending a polyester (e.g. from trimethylol propane, adipic acid, phthalic anhydride and dimer acids) with metatoluene diisocyanate containing ethyl cellulose. Heat resistance may be improved by adding polymethylol phenyls.  Zinc stearate may be added to improve the cell structure. Water for the foaming can be carried by the filler which may be finely ground mica.  Muscovy glass, aluminium silicate, silicate aerogel, calcium carbonate and attapulgite. Other resins than polyurethanes can be used.  These include phenol-formaldehyde resins and also epoxy resins. A mixture of epoxy resin, polyoxyethylene sorbitan monolaurate, p, p1-oxybis (benzene-sulphonyl hydrazide or N, N1-dimethyl- N, N1-dinitroso-terephthalamide blowing agent and diethylenetriamine.ALSO:<PICT:0952762/C3/1> A method of producing a polishing or abrasive article comprises differentially centrifuging a mixture comprising hardenable liquid synthetic resin composition, granular polishing or abrasive material and relatively fine particulate filling material to concentrate the granular material in a local region in the body of the resin while leaving the particulate filling material substantially unaffected and substantially uniformly distributed throughout the mixture and hardening the resin composition.  The preferred method uses a foamable resin composition and produces a foamed product.  Suitable fillers are finely ground diatomaceous earth, limestone, aluminium oxide, magnesium oxide and silicon carbide flour.  Flakes of graphite and metallic flakes or powder can also be added to the mixture. Aluminium oxide is a specified abrasive.  When a polyurethane is the synthetic resin, the added water (to produce a foamed resin) may be carried on a filler such as finely ground mica, Muscovy glass, aluminium silicate, silicate aerogel, calcium carbonate and attapulgite. Heat resistance may be improved by adding polymethylol phenyls and zinc stearate may be added to improve the cell structure. Instead of polyurethanes phenol-formaldehyde and epoxy resins can be used.  The foamable mixture, less in amount than required to fill the mould and just prior to initiation of foaming is introduced into an annular, rotatable mould, which is then rotated until gelling of the resin has taken place and may optionally be subjected to cooling during this stage.  For final setting the wheel, still in the mould, is subjected to heating.  The resulting product has an integument a3 of substantially non-cellular resin, a central portion in which the cells a are numerous and minute and an outer portion in which the minute cells have coalesced to form bubbles a1 and the intervening web structure a2 is thickened, and the abrasive grains g have collected and are carried by webs a2 and spaced by bubbles a1.  By using an intermediate grade of abrasive it is possible to produce a wheel having three annuli, viz. the outer annulus, in which the larger and/or heavier granular abrasive is collected, an intermediate annulus in which is retained the abrasive less susceptible to the action of centrifugal force and an inner annulus substantially free of granular abrasive.