ELECTROSTATIC DEPOSITION FORMULATIONS

Abstract: Fine abrasive powders can be made more free-flowing and better adapted to UP deposition in the production of coated abrasives by the control of the volume resistivity of the powder to a level that is not greater than 10<14> ohms.cm by incorporation of a silica powder.

Description: ELECTROSTATIC DEPOSITION FORMULATIONS Background of the Invention This invention relates to the deposition of abrasive materials by an electrostatic technique and particularly to formulations that greatly facilitate such a technique. In the production of coated abrasives by a process in which an abrasive grain is deposited on an uncured or partially cured binder material the most common deposition technique involves electrostatic deposition in which the grain is projected upwards under the influence of an electrostatic field into contact with the binder. These are usually described as UP (for upward projection) processes. The grain is fed from a hopper to a moving belt which is passed through a deposition location, defined by a charged plate located below the moving belt and directly opposite and parallel to a grounded plate located above the moving belt. The substrate on to which the grain is to be deposited follows a path parallel to and above the moving belt as they both pass through the deposition location. The electrostatic field between the charged plate and the grounded plate causes the grain to be projected upwards towards the down-facing surface of the substrate where it adheres to an uncured or partially cured binder coated thereon. Providing the particle size is uniform this usually results in a very uniform deposition of the grain. However if the grain has a tendency to form clumps or if the flow to the surface from which it is projected is uneven, the uniformity of the deposition can be seriously impaired. This problem is particularly serious when very fine particle sizes are involved. The present invention provides means for promoting free flow of the particles to be electrostatically deposited, even when their size is extremely small. The invention can be used in the feed mechanisms for an UP abrasive grain deposition process or it can be used to deposit a functional powder comprising abrasive grain on the surface of a formulation comprising abrasive grain dispersed within a curable binder in a process such as is described for example in USP 5,833,724. Description of the Invention The present invention provides an electrostatically projectable powder formulation comprising abrasive particles with a grit size smaller than 320 grit and silica powder in an amount sufficient to raise the volume resistivity and the surface resistivity of the abrasive particles by at least fifty percent but to not more than a surface resistivity of 1 d 4 ohms/square and/or a volume resistivity of 10 14 ohms.cm. Preferably these maximum resistivity values are less than lCf 2 ohms/square and ohms.cm respectively. The surface and volume resistivities are measured using ASTM D4496 which is the standard test method for measuring the "DC Resistance of Conductance of Moderately Conductive Materials" and ASTM D2557 which is the standard test method for measuring the "DC Resistance of Insulating Materials". Achieving an acceptable level according to the invention in one of the parameters, (volume and surface resistivities), will imply that an acceptable level has also been attained in the other such that measurement of either parameter alone is sufficient in practical terms. It is found that resistivity values can be correlated to the flowability of the powder such that the treated powder is more readily adapted to UP deposition in coated abrasive applications. However resistivities that are too low or too high are both undesirable. It is therefore necessary to control the resistivity to secure optimum results. Addition of a silica powder is effective to increase the resistivity of the abrasive particles but too large a resistivity creates projectability problems. It is therefore important to control the resistivity such that the optimum performance is achieved. A salient characteristic of the powder formulations of the invention in which this is achieved is that they are electrostatically projectable and have enhanced flowability. The desired resistivity values can be obtained for example by adding to the abrasive particles a suitable silica powder additive the amount of which will vary with the additive. In general however it is possible to secure target resistivity properties for the powders of the invention by the addition of from 0.02 to 5% by weight based on the weight of the formulation. The preferred amount of silica is from 0.05 to 3%, such as from 0.1 to 2%, based on the formulation weight. The silica powder preferably has a particle size no greater than that of the abrasive particles. The silica can be any of the available powdered silica pϋducts such as fumed or precipitated silicas. While silica is inherently somewhat resistant to charge-driven clumping, some silicas such as fumed silica have highly porous particle structures leading to exaggerated surface areas and with such silicas a tendency to form clumps is sometimes encountered. Where such problems are encountered with fumed silica, it can be used effectively after treatment with an additive such as hexamethyldisilazane to increase the hydrophobicity of the silica surface and minimize the tendency to agglomerate. Such treatment is frequently used by commercial suppliers of fumed silica. Even if some agglomeration of commercial fumed silica powder does occur, the forces involved are much attenuated and can readily be broken down by shear stress. Suitable silicas which can be used with advantage include: FG-SP FLOW-GARD® with particle size of 25 microns and a BET surface area of 220 m 2 /gm; FG-AB with particle size of 20 microns and a BET surface area of 130 m 2 /gm; HI-SIL® T-600 with particle size of 2.0 microns and a BET surface area of 170 m 2 /gm; and HI-SIL ® T-152 with particle size of 1.4 micron and a BET surface area of 150 m 2 /gm; (all these are available from PPG Corporation); and CAB-O-SIL® TS-530 which has a particle size of 0.2 micron, a surface area of 220 m 2 /gm and has been given a surface treatment of hexamethyldisilazane. This product is available from Cabot Corporation. The abrasive particles can be for example fused or sintered alumina, silicon carbide, cubic boron nitride, diamond or fused alumina/zirconia. The most commonly used abrasives are however based on alumina or silicon carbide. The abrasive particle size that can be used corresponds to 320 grit or finer but the problem is usually encountered in greatest severity at grit sizes of PI 200 and finer. This corresponds to average particle sizes of about 25 microns and finer. The formulation can also comprise, in addition to the abrasive particles and silica powder, functional additives that convey specific properties to the abrasive product such as surface lubrication, anti-static properties, enhanced grinding capabilities and so on. Such additives are included along with and in intimate mixture with the abrasive particles. These too preferably have particle sizes equal to or smaller than the abrasive particle with which they are mixed. The amount of functional additive that can be present can be for example from 5 to 75% , and preferably from 25 to 60% and most preferably from 30 to 50% of the total weight of abrasive plus additive. Besides having resistivity levels consistent with the invention it is also found that the abrasive powders of the invention are in general much less susceptible to variations in moisture in the atmosphere or on the grain. With some grains notable alumina-based grains, the relative humidity surrounding the UP deposition apparatus very significantly affects the efficiency by which the abrasive particles are projected. The abrasive particle powders of the invention are however much more resistant to humidity variations, thereby providing a significant extra benefit from the practice of the invention. The invention therefore further comprises a process for the UP deposition of a formulation comprising abrasive particles having a grit size smaller than 320, and more preferably 400, grit and a silica powder in an amount sufficient to raise the surface resistivity and the volume resistivity of the abrasive particles by at least fifty percent but to no greater than a surface resistivity of 10 14 , and preferably no greater than 10 12 , ohms/square and/or a volume resistivity of no greater than id 4 , and preferably no greater than 10 12 , ohms.cm. The invention also comprises a process for the UP deposition of a such an abrasive formulation which comprises incorporating with the abrasive particles from 0.02 to 5% by weight based on the formulation weight of a silica with a particle size no larger than that of the abrasive particles. When referring to the abrasive particles the size is expressed in terms of a CAMI grading process defines an average particle size which corresponds to a specific number of microns. When referring to silica or other powdered additives the particle size is expressed in microns and refers to a volume average particle size as determined by, for example, a Horiba particle size analyzer. Drawings Figure 1 is perspective sketch drawing of a test bed apparatus used to evaluate the additives. Figure 2 is a graph showing the grain flow rate in the flow evaluation apparatus used in Example 1 at three sets of specified conditions. Figure 3 is a bar graph indicating flow improvements as a result of the use of formulations according to the invention. Figure 4 is a bar graph showing the grinding results obtained in Example 2. Figure 5 is a graph showing the effect of additive weight on volume resistivity of the resultant abrasive powder for three different aluminum oxide abrasive grains under two different relative humidity conditions. Figure 6 presents the data from Figure 5 in bar chart form. Figures 7 and 8 are similar to Figures 5 and 6 with the difference that the data collected refers to silicon carbide abrasive grain. Description of Preferred Embodiments The invention is now illustrated by evaluating the properties of a number of formulations and the abrasive performance of a coated abrasive incorporating one such formulation. The Examples are therefore for demonstrative purposes and do not imply any essential limitation on the invention or the scope of its utility in all circumstances. Example 1 To demonstrate the flow properties of the formulations according to the present invention, a test apparatus was set up as shown in Figure 1. The apparatus comprised a hopper, 1 , adapted to feed grain on to the outer surface of a roll, 2, rotating at a controllable speed about an axis parallel to the hopper axis, wherein the gap, 3, between the hopper and the roll surface is controllable. The grain passes over a wire, 4, and falls into a pan, 5. In the operation of the above testing apparatus, powder was fed into the hopper, the gap between the slot and the drum was fixed and the drum was rotated at a given rate. The grain flow rate, in grams of grain deposited in the pan per 15 seconds period, was measured. This was repeated at a number of gap settings, drum speeds and powder compositions. The powder run through the system comprised PI 200 grit aluminum oxide particles in a 2:1 parts by weight ratio with potassium fluoroborate and with varying amounts of a precipitated silica available from PPG Corporation under the trademark FLOW-GARD® AB. Both the silica and the abrasive particles have an average particle size of about 20 microns. Depending on the amount of silica added the surface resistivity of the formulations ranged from about 10 9 to about 10 u ohms/square and the volume resistivities from about l(f to about 10 11 ohms.cm. The amount of grain deposited in a collector pan in a 15 second period was measured at three different settings for the gap between the hopper and the roll surface. The results are presented in graphical form in Figure 2. As will be seen, the results for this system indicate that maximum flow is achieved at a concentration of the silica additive of about 1% regardless of the gap. Repetition at a different roll speed yielded the same result. A similar series of tests using the same test rig was performed to evaluate the improvement obtained with the same components but using a constant roll speed of 40 inches/minute, (about 1 meter/minute) and three different gap settings of 0.03", 0.04" and 0.06", (0.76mm, 1.02mm and 1.52mm respectively). Each formulation according to the invention comprised 2% by weight of the same silica mixed with the same PI 200 alumina and potassium fluoroborate particles. Each was compared against the same composition but without the silica and before conducting the test, the powder mixture had been stored at 1 OCPF for three weeks to ensure optimum free-flowing properties. No such drying precautions were taken with the products according to the invention. The results are shown in Figure 3 in which the percentage improvement (in terms of grams of powder deposited in a 15 second period) over the silica- free composition under the same conditions is shown in bar chart form. The length of the bar indicates the percentage improvement over the non-silica-containing product. An error bar on each bar indicates the degree of variability in the results that are summarized in the chart. As might be expected, where the gap is larger the improvement displayed is smaller because the wider gap allows through some of the agglomerated particles. It is however still significant. Evaluations at alumina particle sizes larger than PI 200 but with the other parameters held constant showed a significant improvement at PI 000 alumina particle size but below that level the test conditions were not sensitive enough to display a clear improvement. In addition as indicated above the problem of particle agglomeration becomes less acute with increasing particle size. To examine such larger particle sizes a funnel with an outflow hole with a 0.1", (2.54mm), diameter was used to evaluate flow rates. Here it was found that where flow is impeded, the addition of 2% of the silica was effective at increasing the flow rate of alumina with 320 grit and smaller particle sizes. Example 2 In this Example the grinding performance of a coated abrasive with an engineered surface was evaluated with and without the silica additive; The coated abrasive substrate was obtained by depositing a formulation comprising an alumina abrasive grit dispersed in a UV-curable acrylate binder Resin, and then applying to the surface of the deposited formulation a layer of a powder comprising a 2:1 weight ratio of PI 200 alumina abrasive particles and potassium fluoroborate particles. A repeating pattern was embossed on the formulation and the resin binder component was cured. Three samples were prepared that were identical except that the powder layer deposited on the uncured surface of two of the abrasive/binder resin formulations contained silica and the third had none. The samples were then tested for abrasive performance on a ring of 304 stainless steel with a pressure of 16 psi, (applied to the back of the coated abrasive using a Matchless-B contact wheel), and a relative movement speed of 5000 sfpm. The first formulation according to the invention comprised a precipitated silica with a BET surface area of 170 rrr/gm and an average particle size of about 2 microns in an amount of 0.08% by weight based on the powder weight. The second contained 0.25% by weight, based on the powder weight of the 20 micron silica used in Example 1. The third contained no silica. The results of the grinding test are shown in Figire 4. As will be seen, the effect of the grinding results of the incorporation of silica is very minor and at very low concentrations may even be beneficial. Example 3 In this Example the objective is to show the impact of the addition of a silica additive on the resistivity of the resulting abrasive grain powder. In all cases the additive was a fumed silica powder with a hexamethyldisilazane treatment This silica is available from Cabot Corporation under the designation "TS 530". Two sets of tests were performed. The first was carried out on aluminum oxide abrasive powders available from Treibacher AG under the designations: BFRPL P600 (FEPA P-grading with 25.8 micron average particle size); FRPL P800 (21.8 micron average particle size); and FRPL P1500 (12.6 micron average particle size). The second set of tests was carried on silicon carbide abrasive powders available from Saint-Gobain Industrial Ceramics Inc. under the designations: E299 400 (ANSI grading 22.1 microns); E299 600 (ANSI grading 14.5 micron); and E599 PI 500 (FEPA P-grading, 12.6 micron). In each case the powders were given silica powder additions at a number of levels and the volume resistivity was measured at relative humidity levels of 20% and 50%. The results of the evaluations are shown in Figures 5-8. In Figures 5 and 7 the variation of resistivity with percent additive is tracked for the aluminum oxide and silicon carbide grains respectively. Figures 6 and 8 present the data in Figures 5 and 7 respectively in bar chart form as a comparison with the resistivity measurement for the grain in the absence of any modifier. To do this the resistivity value reported in the graph is divided by the resistivity of the identical unmodified abrasive grain at the same relative humidity. This shows more clearly the dramatic improvement in resistivity upon addition of relatively modest amounts of the silica additive.

Claims: What is claimed is: 1. An electrostatically projectable powder formulation comprising abrasive particles with a grit size smaller than 320 grit and silica powder in an amount sufficient to raise the volume resistivity and the surface resistivity of the abrasive particles by at least fifty percent but to not more than a surface resistivity of id 4 ohms/square and/or a volume resistivity of 10 14 ohms.cm. 2. A powder formulation according to Claim 1 in which the surface and volume resistivity values are less than i 2 ohms/square and 10 12 ohms.cm respectively. 3. A powder formulation according to Claim 1 in which the amount of silica powder added represents from 0.02 to 5% by weight of the powder formulation. 4. A powder formulation according to Claim 1 in which the silica is selected from the group consisting of precipitated silica; fumed silica; fumed silica treated to provide the silica particles with a hydrophobic surface; and mixtures thereof. 5. A powder formulation according to Claim 1 in which the size of the silica particles is not larger than that of the abrasive particles. 6. A powder formulation according to Claim 1 in which the abrasive particles are PI 200 grit and smaller. 7. A process for the preparation of a coated abrasive which comprises a step in which a powder formulation is electrostatically deposited on a substrate and the formulation comprises abrasive particles having a particle size finer than 320 grit and a silica powder in an amount sufficient to raise the volume resistivity and the surface resistivity of the abrasive particles by at least fifty percent but to not more than a surface resistivity of 10 14 ohms/square and/or a volume resistivity of 10 14 ohms.cm. 8. A process according to Claim 7 in which the surface and volume resistivity values of the powder formulation are less than Id 2 ohms/square and lθ' 2 ohms.cm respectively. 9. A process according to Claim 7 in which the amount of silica powder added represents from 0.02 to 5% of the total formulation weight. 10. A process according to Claim 7 in which the silica is selected from the group consisting of precipitated silica; fumed silica; fumed silica treated to provide silica particles with a hydrophobic surface; and mixtures thereof. 11. A process according to Claim 7 in which the silica particles have an average size that is no larger than that of the abrasive particles. 12. A process according to Claim 7 in which the abrasive particles have a size that is not greater than P 1200 grit.

Inventors: PETIGNY SYLVAIN, SWEI GWO SHIN, PETIGNY, SYLVAIN, SWEI, GWO, SHIN
Applicants: NORTON CO, NORTON COMPANY
Application year: 2000
Main classification: C09K3
IPC codes: B24D3, C09K3