Abstract:
Abrasive articles are disclosed that may be used in low speed dry sanding applications. The abrasive articles disclosed may be made entirely from abrasive materials or alternatively may be made by fastening abrasive surfaces to handles or tools. The abrasive articles of the present invention have abrasive surfaces with controlled wear rates that renew themselves during use. The self renewing abrasive surfaces of the present invention may be prepared by pressing a mixture of abrasive particles, hollow micro-particles and a minimal amount of binder together into a mold and subsequently allowing the binder to harden. The resulting abrasive articles are long lasting and may be made low in cost.

Description:
[0001]     This is a Continuation-in-Part of application Ser. No. 11/828,270 filed on Jul. 25, 2007 which is a Continuation-in-Part of Ser. No. 11/503,058 filed Aug. 3, 2006, which claimed priority to provisional application No. 60/764,110 filed on Feb. 1, 2006 and provisional application No. 60/818,571 filed on Jul. 5, 2006. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates to abrasive articles. More particularly this invention relates to abrasive articles having wearable abrasive surfaces. The wearable abrasive surfaces of the present invention may be comprised of abrasive materials such as aluminum oxide along with hollow micro-particles and at least one binding agent. A minimal amount of binding agent may be used along with added hollow micro-particles so that the usable rate of wear is sufficient to continuously renew working surfaces during use.  
         [0004]     2. Description of the Related Art  
         [0005]     There are numerous methods that may be employed to sand surfaces. One of the more common methods employs sand paper. Sand paper is a thin sheet material usually made of paper that has an abrasive material securely bonded onto one side. Despite its name, the abrasive is rarely if ever sand. Commonly used abrasives such as aluminum oxide and silicon carbide are significantly harder than sand and are therefore more effective. This may be especially true when sanding hard materials such as glass or steel.  
         [0006]     Sand paper may be used by hand. This process is often referred to as hand sanding. The process of hand sanding involves using manual labor to repeatedly slide the sand paper back and forth and/or in a circular motion over the surface until smooth. Numerous textures of abrasives are available. Often sanding starts out with a relatively course grade of sand paper of about 80 grit followed by finer grades of several hundred grit to finish the job.  
         [0007]     One drawback often associated with sand paper is the tendency of producing dust that clogs the sand paper. One way to alleviate this problem is by using wet or dry Emery cloth. Wet or dry Emery cloth is an abrasive coated cloth having a wide variety of grades. It is designed for use with water thereby reducing clogging effects.  
         [0008]     Another drawback with using sand paper is the tendency for the abrasive to become dull and fall off from the sand paper backing surface.  
         [0009]     Sanding by hand using sand paper is not always practical owing to the amount of labor required. This is especially true for large jobs that may take a long time resulting in fatigue.  
         [0010]     In order to alleviate the worker fatigue issue in hand sanding operations, numerous power sanding techniques and/or equipment have been developed. Drum sanding, belt sanding, disc sanding, and orbital sanding are commonplace. These standard power sanding tools often employ some form of sand paper and therefore often suffer from many of the previously mentioned drawbacks. In particular is the need to change the sanding surface at regular intervals.  
         [0011]     Numerous modifications to ordinary sand paper have been made in order to improve the overall process. For example, sand paper having a lowered surface density of abrasive particles is available. This particular sand paper is made by 3M Corporation of ST. Paul Minn. and is designed for use in sanding relatively soft materials that quickly gum up ordinary sand paper. Significant improvements in sand paper life may be realized by reducing the tendency of particulate matter to clog the needed spaces between adjacent abrasive particles.  
         [0012]     Another improvement that may be made to ordinary sand paper involves the use of flexible and conformable foam backing. Such backing materials allow the sand paper to conform to surface contours thereby more rapidly smoothing contoured surfaces. Individual pieces of sand paper may be applied to foam pads or conversely, foam pads having previously attached sand paper may be employed. For example, Finishing Buddies (Mona Lisa Products 10770 Moss Ridge Road Houston, Tex., 77043) is a complete sanding tool kit consisting of a steel wool pad, oval sanding disc, and coarse, medium, and fine sanding pads. The oval pad is relatively rigid, and the three other sanding pads have a softer foam backing that has a greater degree of flexibility. This sanding kit is designed for slow hand sanding and finishing operations.  
         [0013]     There are numerous flexible sanding surfaces, components, and articles comprised of abrasive materials fixedly attached to flexible foam backings. Of particular interest is a sanding system employing a relatively thin foam backing disclosed in U.S. Pat. No. 6,923,840 and assigned to 3M Innovative Properties Company, St. Paul Minn. (US). U.S. Pat. No. 6,923,840 discloses a flexible abrasive product comprised of an open cell foam backing, a foraminous barrier coating, and a shaped foraminous abrasive coating. The top abrasive coating is discontinuous and allows for holding lubricants such as water as well as spaces for removal of debris.  
         [0014]     U.S. Pat. No. 6,949,128 also assigned to 3M, discloses a method for making a foam backed abrasive article having embossed raised areas.  
         [0015]     U.S. Pat. No. 3,401,490 discloses a method for forming an abrasive article having a resiliently yielding open cell meltable base which is passed under a heated roll to melt the surface to a desired depth followed by application of abrasive particles to the melted surface. The result is a flexible foam based abrasive article capable of following irregular, uneven, or sunken surfaces.  
         [0016]     U.S. Pat. No. 6,997,794 by James Matthew Pontieri discloses a disposable sanding device fabricated as a continuous rope like article adapted for selective segmentation. This device may employ a foam central portion along with an abrasive outer portion. In particular the flexible cylindrical geometry illustrated in several embodiments of the invention lends itself to the hand sanding of difficult to reach contours and may prove especially useful in woodworking applications.  
         [0017]     There are numerous flexible foam based cleansing and scouring pads having added abrasive materials. An example of this can be found in U.S. Pat. No. 3,377,151. U.S. Pat. No. 3,377,151 discloses a method for making flexible resilient cleansing and scouring pads having an abrasive surface. A thermoplastic foam web material is hot laminated to abrasive web material. In addition, one or more cleansing materials may be added.  
         [0018]     U.S. Pat. No. 3,619,843 discloses sponges having dry impregnated materials. In this invention, impregnated sponges are prepared by a process that deposits particulate material on one surface of the sponge and subsequently pierces the sponge with spikes to form crevices followed by drawing particulate material into the crevices. The result is a modified sponge suitable for surgical and sanitizing applications.  
         [0019]     Also of interest are flexible open cell foam scouring and cleaning pads having numerous protrusions. These pads are disclosed in U.S. Pat. No. 4,055,029 by Heinz Kalbow, Lichgasse. The flexible pad has numerous protrusions on the working surface having an abrasive layer. U.S. Pat. No. 4,111,666 also by Heinz Kalbow discloses a method of manufacturing flexible abrasive cleaning pads along with improvements in tear resistance.  
         [0020]     U.S. Pat. No. 4,421,526 discloses polyurethane foam cleaning pads composed of a densified flexible sponge like polyurethane foam material impregnated with various cleansing additives. Excessive mixing of the freshly blended polymers inhibits foam formation long enough to add the cleansing ingredients. The resulting pads have added strength due to collapsed, ruptured, and distorted cells along with fibers that result from the specific mixing process employed. The result is an unusually strong dense flexible cleaning pad capable of absorbing substantial amounts of water that releases additives along with absorbed water on gentle squeezing.  
         [0021]     U.S. Pat. No. 4,594,362 discloses a dry type textile cleaning article comprised of a friable hydrophilic polyurethane foam with incorporated abrasive particles as well as other additives. The abrasive particles are chemically bonded to the foam using silane coupling agents thereby reducing their tendency to separate from the mass and subsequently damage cloth material.  
         [0022]     While the above described examples of foam based abrasive articles provide a wide variety of uses, there exists a need in the art for semi-rigid or rigid abrasive articles having a wearable surface that renews itself during use that may be employed in hand and/or low speed dry abrasive operations including dry sanding.  
         [0023]     Many of the above described examples outline the use of foam with abrasive materials in order to achieve certain advantageous and desirable properties. Still others outline some of the more simple methods and materials commonly employed in dry sanding. While generally effective for dry sanding, there exists a need in the industry for further improvements in low speed dry abrasive operations.  
         [0024]     For example, improvements in low speed dry abrasive operations may be realized in the area of sanding cloths. Flexible abrasive cloth materials such as emery rapidly become dull and shed abrasive particles. Emery cloth only has one layer of abrasive particles. Once this layer of abrasive wears out, becomes clogged, or becomes dull, the emery cloth is discarded. Because of this, sanding operations often require several pieces of emery cloth to complete. While making discrete zones of attached abrasive may serve to reduce the tendency of debris to build up in the sanding surface, the issue of rapid dulling and shedding of surface abrasive particles still remains a major issue to be resolved.  
         [0025]     Finally, flexible abrasive surfaces employing foam have certain added benefits that may be realized in numerous applications. Many of the earlier patents referenced in this application fall under this class of abrasive surfaces.  
         [0026]     Despite numerous advancements in the field of abrasives there is a need for abrasive articles having a wearable surface that renews itself during use that may be employed in dry abrasive operations.  
         [0027]     It is an object of this invention to provide abrasive surfaces for low speed dry sanding applications.  
         [0028]     It is a further object of this invention to provide numerous grades of abrasive surfaces for low speed dry sanding applications.  
         [0029]     It is a further object of this invention to provide abrasive surfaces for low speed dry sanding applications resistant to excess build up of debris.  
         [0030]     It is a further object of this invention to provide abrasive surfaces for low speed dry sanding applications in both rigid and flexible forms.  
         [0031]     It is a further object of this invention to provide abrasive surfaces for low speed dry sanding applications that are low in cost.  
         [0032]     It is a further object of this invention to provide simple methods for producing abrasive surfaces for low speed dry sanding applications.  
         [0033]     It is a further object of this invention to provide abrasive surfaces for low speed dry sanding applications that may be used for extended periods of time without wearing out.  
         [0034]     Finally, it is an object of this invention to provide abrasive surfaces for low speed dry sanding applications that have wear rates that result in continuous renewal of working surfaces during use.  
       SUMMARY OF THE INVENTION  
       [0035]     This invention proposes articles for dry sanding applications employing wearable abrasive surfaces that renew themselves on continued use. The dry sanding articles of the present invention may have wearable abrasive surfaces comprised of abrasive particles along with hollow micro-spheres and at least one binding agent. Wearable abrasive surfaces may be formed by compressing a mixture of abrasive particles and hollow micro-spheres together with a binding agent. Compression of the above described wearable abrasive surfaces may be take place in a mold thereby forming them into specific shapes. Numerous binding agents may be employed including condensation polymer resins such as epoxies, polyurethanes, and polyamides. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0036]     A more complete appreciation of the invention and many of the advantages thereof will be readily obtained as the same becomes better understood by reference to the detailed description when considered in connection with the accompanying drawings, wherein:  
         [0037]      FIG. 1  shows an abrasive surface suitable for low speed dry sanding applications.  
         [0038]      FIG. 2  shows a cross sectional view of an abrasive composition suitable for low speed dry sanding applications.  
         [0039]      FIG. 3  shows an abrasive article suitable for low speed dry sanding applications.  
         [0040]      FIG. 4  shows a cross sectional view of an abrasive surface suitable for low speed dry sanding applications comprising a plurality of protrusions extending in an outward direction.  
         [0041]      FIG. 5  shows a cross sectional view of a sanding disk for a rotary tool suitable for low speed dry sanding applications.  
         [0042]      FIG. 6  shows a hand held abrasive article suitable for low speed dry sanding applications comprised of a handle portion fixedly attached to a wearable abrasive surface.  
         [0043]      FIG. 7  shows an abrasive fabric suitable for low speed dry sanding applications employing protrusions comprised of the composition of the present invention.  
         [0044]      FIG. 8  shows an abrasive loaded polymeric resin protrusion for low speed dry sanding applications having a top surface portion containing a pattern of grooves and ridges.  
         [0045]      FIG. 9  shows an abrasive fabric glove suitable for low speed dry sanding applications having numerous abrasive protrusions attached to working surfaces comprised of the composition of the present invention.  
         [0046]      FIG. 10  shows a sectional view of a rotary tool sanding disk suitable for low speed dry sanding applications having a layer of surface protrusions.  
         [0047]      FIG. 11  shows an abrasive article having a discontinuous surface topography comprising a pattern of cavities and raised portions suitable for low speed dry sanding applications. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0048]      FIG. 1  shows an abrasive surface suitable for low speed dry sanding applications. Abrasive surface  2  is comprised of exposed abrasive particles  4  along with hollow micro-spheres  6  and material matrix binding agent  8 .  
         [0049]     Material matrix binding agent  8  is comprised of a polymer resin and therefore is significantly softer than abrasive particles  4 . Materials used for abrasive particles  4  include silicon carbide, zirconia, diamond, ceria, cubic boron nitride, garnet, ground glass, quartz, and combinations thereof.  
         [0050]      FIG. 2  shows a cross sectional view of an abrasive composition suitable for low speed dry sanding applications. Abrasive composition  10  is shown in cross sectional view. Abrasive composition  10  is shown having abrasive particles  12  embedded within polymeric matrix material  14 . Abrasive particles  12  may comprise a material selected from the group consisting of aluminum oxide, silicon carbide, zirconia, diamond, ceria, cubic boron nitride, garnet, ground glass, quartz, and combinations thereof. Also shown are hollow micro-particles, shown in the form of hollow micro-spheres  16 . Hollow micro-particles are hollow particles, such as hollow micro-spheres, which allow for the addition of voids to the abrasive composition  10 . Hollow micro-spheres are hollow spherical particles having diameters ranging from a few microns to several hundred microns. Micro-spheres may be made from numerous materials including polymers and glass. Hollow micro-spheres having a diameter range of 50-100 microns may be employed to impart increased wear properties to abrasive composition  10 .  
         [0051]     The rate of wear at major abrasive surface  18  of the abrasive composition  10  determines how fast abrasive particles  12  become exposed and shed during use. If abrasive composition  10  has too slow a wear rate the major abrasive surface  18  may become dull and filled with debris. This condition renders the surface  18  non-useable. If the rate of wear is too rapid, sanding of rough surfaces may tear up the major abrasive surface  18  resulting in limited product life and inconsistent sanding properties. It is important to find a usable rate of wear, which is the rate of wear at the major abrasive surface  18  of the abrasive composition  10 , sufficient for renewal at the major abrasive surface  18  to allow continuous use of the abrasive composition  10 . Micro-sphere loading densities ranging from about 25% by volume to about 75% volume of the mix often exhibits a usable rate of wear at the major abrasive surface  18 .  
         [0052]     Other factors affecting rate of wear of the abrasive composition  10  includes hardness of matrix material  14 , loading density of abrasive particles  12 , and pressure used in forming operations. These factors, in addition to the volume of micro-particles, may be modified accordingly to achieve a usable rate of wear.  
         [0053]     Major abrasive surface  18  is shown having numerous cavities  20 . Cavities  20  may result from wearing open hollow micro-spheres  16  during use. Alternatively, cavities  20  may be formed during manufacture by cutting or wearing of the surface prior to use. Cavities  20  are shown providing discontinuous surface topography to major abrasive surface  18 .  
         [0054]     Matrix material  20  may be made from polymer resins selected from the group of condensation polymers including epoxy resins, polyamide resins and polyurethane resins. Under certain circumstances, condensation polymers such as polyurethane compositions may be produced having a reduced molecular weight.  
         [0055]     Such compositions may be prepared by using a slight excess of either reactant that may be on the order of an excess of a few percent to about 10 percent by weight of the resin mix. The mixture need not be modified very much from the ideal mixture quoted by the manufacturer. An excess of 10% of either component will drastically affect molecular weight. This has to do with the inherent properties of condensation polymers. In order for a two component reactive condensation polymer to achieve a high molecular weight, exact proportions need to be combined and subsequently allowed to react to completion. If the mixture is off by even a small amount, the reaction stops as soon as the first reactant runs out. The result is limited molecular weight with the polymeric chains terminated by the excess reactant.  
         [0056]     Abrasive compositions made with polymer resins having their molecular weight limited in the above described fashion may be softer and therefore have increased rate of wear. It is important to bear in mind that the abrasive compositions of the present invention may have abrasive properties resembling sand paper of a finer grit than what was employed in the mix. This may be advantageous owing to the lower cost of coarse grit abrasive materials when compared to finer grit abrasive materials.  
         [0057]     The abrasive surfaces suitable for dry sanding applications of the present invention may be prepared by mixing hard abrasive particles with hollow micro-spheres along with a minimal amount of uncured polymeric resin. The amount of polymeric resin binding agent may be present from 5% to 35% by weight of the mix. It should be noted that the density of hollow micro-particles tends to be quite low. Because of this, the remainder of the composition by weight is substantially the result of abrasive particles, i.e. abrasive particles may be present from 65% to 95% by weight of the mix. This limited quantity of binding agent may be used in order to achieve a useable rate of wear. The mixture may then be pressed into a suitable mold under controlled pressure. When the resin cures the part may be removed from the mold. A specific example of an abrasive surface having a useable rate of wear will now be given in further detail.  
         [0058]     Example: 2.0 grams of polyurethane resin 341 A available from Plastic Depot (2907 San Fernando Blvd. Burbank, Calif. 91504. Telephone number (818) 843-3030). were placed in a 4 ounce polyethylene container along with 2.0 grams of polyurethane resin 341 B. This mixture was then rapidly stirred until uniform. To this were added 18 grams of 80 grit silicon carbide abrasive powder along with 0.4 grams of West System 407 low density fairing filler. West systems INC. P.O. box 665. Bay City Mich. 48707 USA. Telephone number (866) 937-8797. The mixture was then rapidly stirred a second time until uniform. The resulting mixture was slightly damp to the touch indicating that only a minimal amount of polyurethane resin was present in the mix as a material matrix. A material matrix of the present invention may be regarded as an agent used to hold discrete particles together into a solid mass.  
         [0059]     The above described mixture was then transferred into a 1.5″×2″ rectangular silicone mold cavity. A 1.5″×2.0″ rectangular ceramic block was then placed on top of the mixture in the mold. A downward force of 40 pounds was then applied to the rectangular ceramic block for a period of two minutes. The mixture was then allowed to stand undisturbed for two hours to thoroughly cure. Once cured, the part was removed from the mold. Separation was then attempted between the ceramic block and abrasive surface. The two surfaces could not be separated without the risk of breakage and therefore were left in a joined condition.  
         [0060]     The above described abrasive composition sample was then used to dry sand paint off of numerous metal surfaces. Initial sanding was rapid with some build up of paint particles. The rate of wear of the major abrasive surface was sufficient to renew the surface fast enough to prevent excess dulling of abrasive particles, and therefore represents a usable rate of wear.  
         [0061]     The above experiment was repeated with increasing amounts of micro-spheres. This resulted in more rapid rate of wear of the abrasive surface.  
         [0062]     Polyurethane resin 341 A and 341 B represents a two part system It is designed to be blended into equal parts by volume. Because the two components of the polyurethane resin system have different densities, a slight excess of one reactant results when equal weights of each component are used in the mix. Reducing the molecular weight of the polymer matrix helps to increase wear properties in the final mix.  
         [0063]      FIG. 3  shows an abrasive article suitable for low speed dry sanding applications. Abrasive article  22  consists of top exposed abrasive surface portion  24  (abrasive surface  2  of  FIG. 1 ). Exposed abrasive particle releasing abrasive surface portion  24  is shown to be larger in area than side surface portion  26  and therefore may be considered a first major surface of abrasive article  22 . Abrasive article  22  has a second major surface (not shown) oppositely facing first major surface  24 .  
         [0064]     Abrasive article  22  is suitable for dry sanding numerous surfaces including painted metal. The overall hardness of abrasive surface portion  24  may be adjusted for use in specific applications. For example, automotive body repair shops often require dry sanding of damaged panels under numerous stages of repair. It is often desirable to sand away paint, rust, and body filler. Abrasive surface  24  for automotive body repair shop uses requires enough wear resistance and hardness to provide a useful working life while at the same time having a rate of wear fast enough to renew the working surface before becoming dull.  
         [0065]      FIG. 4  shows a cross sectional view of an abrasive surface suitable for low speed dry sanding applications comprising a plurality of protrusions. Abrasive surface  28  is shown having surface protrusions  30  in discontinuous arrangement extending in an outward direction from laminate backing portion  32 . Also shown are abrasive particles  34  along with hollow micro-spheres  36 . Binding agent  38  is also shown. Binding agent  38  makes up the softer material matrix that holds abrasive particles  34  and micro-spheres  36  together in a controlled manner.  
         [0066]      FIG. 5  shows a cross sectional view of a sanding disk for a rotary tool suitable for low speed dry sanding applications. Abrasive dry sanding disc  40  is shown having abrasive first major top surface portion  42  along with rigid backing portion  44  and central hole  46  for mounting to a shaft (not shown). Top abrasive first major surface portion  42  is comprised of numerous abrasive particles along with micro-spheres embedded into softer material matrix material  50 . Top abrasive first major surface portion  42  is shown attached to rigid backing portion  44 . Bottom surface portion  48  of rigid backing portion  44  may be regarded as a second major surface portion.  
         [0067]      FIG. 6  shows a hand held abrasive article suitable for low speed dry sanding applications comprised of a handle portion fixedly attached to wearable abrasive surface.  
         [0068]      FIG. 6  shows a hand held abrasive article that may be used to dry sand automotive surfaces. Hand held abrasive article  52  is shown comprising a main handle portion  54  and a major abrasive surface working portion  56 . Also shown is side groove  58 . Side groove  58  provides an ergonomic fit to the hand for easier use. Hand held abrasive article  52  is shown having major abrasive surface portion  56  fixedly attached to main handle portion  54 . Major abrasive surface portion  56  may be comprised of the abrasive dry sanding composition of  FIG. 2 .  
         [0069]      FIG. 7  shows an abrasive fabric suitable for low speed dry sanding applications employing protrusions comprised of the composition of the present invention. Abrasive fabric  60  is comprised of a flexible water absorbent fabric such as cloth backing layer  62  along with attached abrasive loaded dry sanding protrusions  64 . Abrasive fabric  60  is shown as a discontinuous surface that releases hard abrasive particles during low speed dry sanding operations. The separation of individual abrasive loaded dry sanding protrusions  64  between each other forms a discontinuous surface. Also shown is material matrix portion  66  in the form of a polymer resin such as polyurethane (shown in further detail in  FIGS. 2 and 8 ) embedded with coarse abrasive particles  68  having a hardness significantly greater than polymer matrix portion  66 . Also present are hollow micro-spheres (not shown). Abrasive fabric  60  may be used for machine driven sanding surfaces such as belts and discs as well as hand sanding applications.  
         [0070]     It should be noted that the protrusions themselves provide points of high pressure that facilitate dry sanding operations. It should also be noted flexible fabric  62  allows individual abrasive loaded polymeric resin protrusions to follow surface contours during dry sanding operations. Individual protrusions  68  may have grooves like those shown in  FIG. 8 .  
         [0071]      FIG. 8  shows the abrasive loaded protrusion of  FIG. 7  in further detail for low speed dry sanding applications having a top surface portion containing a pattern of grooves and ridges. Abrasive loaded polymeric dry sanding protrusion  70  is shown having having a top surface portion  72  containing a pattern of grooves  74 . Grooves  74  provide means for holding and removal of debris in low speed dry sanding applications. Also shown are ridges  76 . Ridges  76  result from grooves  74  in top surface portion  72  and provide for increased pressure at the start of dry sanding operations. This added pressure may help to facilitate the initial process of dislodging abrasive particles  78 . Once ridges  76  wear, abrasive particles  78  will continue to be released during use. Also shown are hollow micro-spheres  80  and softer matrix material  82 .  
         [0072]      FIG. 9  shows an abrasive fabric glove suitable for low speed dry sanding applications having numerous abrasive protrusions attached to working surfaces comprised of the composition of the present invention. Sanding glove  84  is shown having abrasive loaded dry sanding protrusions  86  that are discontinuous from each other and attached to glove  88  with epoxy resin  90 . Also shown are ridges  92  on protrusions  86 . Ridges  92  on protrusions  86  provide points of high pressure. Protrusions  86  consist of hard abrasive particles along with hollow micro-spheres held together within a softer material matrix as shown in  FIGS. 2 and 8 ).  
         [0073]      FIG. 10  shows a sectional view of a rotary tool sanding disk suitable for low speed dry sanding applications having a layer of surface protrusions. Abrasive dry sanding disc  94  is shown having abrasive releasing first major top surface portion  98  along with rigid backing portion  96  and central hole  100  for mounting to a shaft (not shown). Abrasive releasing first major top surface portion  98  is comprised of numerous protrusions  102  fixedly attached to rigid backing portion  96 . Protrusions  102  are comprised of hard abrasive particles  104  dispersed within softer material matrix  108  along with hollow micro-spheres  106 . The composition of protrusions  102  is described in further detail in  FIG. 2 . Protrusions  102  are shown in greater detail in figure  2 . Bottom surface portion  110  of rigid backing portion  96  may be regarded as a second major surface portion.  
         [0074]     Abrasive dry sanding disc  94  is suitable for dry rotary sanding operations. A shaft may be attached using central hole  100  using a threaded screw. The shaft may be subsequently fitted into the chuck of a low speed rotary tool such as a drill. A relatively low speed of about 50 to 500 RPM may be employed to dry sand numerous surfaces. It should be noted that high RPM conditions of 1000 or more may result in rapid tool wear and possible throwing of pieces of abrasive material from the disk.  
         [0075]      FIG. 11  shows an abrasive article having a discontinuous surface topography comprising a pattern of cavities and raised portions suitable for low speed dry sanding applications. Abrasive article  112  is shown consisting of exposed abrasive top surface portion  116  and side surface portion  114 . Also shown are cavities  118  present in exposed abrasive top surface portion  116 . Exposed abrasive top surface portion  116  is shown to be larger in area than side surface portion  114  and therefore may be considered a first major surface of abrasive article  112 . Abrasive article  112  has a second major surface (not shown) oppositely facing exposed abrasive top surface portion  116 . Abrasive article  112  is suitable for dry hand sanding operations. Exposed abrasive top surface portion  116  of abrasive article  112  is shown having a relatively large percentage of the particle releasing surface exposed with a relatively small percentage of surface cavities. Exposed abrasive top surface  116  of abrasive article  112  has a discontinuous surface topography due to the presence of surface cavities  118 .  
         [0076]     Those skilled in the art will understand that the preceding exemplary embodiments of the present invention provide foundation for numerous alternatives and modifications. These other modifications are also within the scope of the limiting technology of the present invention. Accordingly, the present invention is not limited to that precisely shown and described herein but only to that outlined in the appended claims.