Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B24—GRINDING; POLISHING
  • B24C—ABRASIVE OR RELATED BLASTING WITH PARTICULATE MATERIAL
  • B24C7/00—Equipment for feeding abrasive material; Controlling the flowability, constitution, or other physical characteristics of abrasive blasts
  • B24C7/0084—Equipment for feeding abrasive material; Controlling the flowability, constitution, or other physical characteristics of abrasive blasts the abrasive material being fed in a mixture of liquid and gas
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B24—GRINDING; POLISHING
  • B24C—ABRASIVE OR RELATED BLASTING WITH PARTICULATE MATERIAL
  • B24C1/00—Methods for use of abrasive blasting for producing particular effects; Use of auxiliary equipment in connection with such methods
  • B24C1/08—Methods for use of abrasive blasting for producing particular effects; Use of auxiliary equipment in connection with such methods for polishing surfaces, e.g. smoothing a surface by making use of liquid-borne abrasives
  • B24C1/086—Descaling; Removing coating films
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B24—GRINDING; POLISHING
  • B24C—ABRASIVE OR RELATED BLASTING WITH PARTICULATE MATERIAL
  • B24C11/00—Selection of abrasive materials or additives for abrasive blasts
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B24—GRINDING; POLISHING
  • B24C—ABRASIVE OR RELATED BLASTING WITH PARTICULATE MATERIAL
  • B24C11/00—Selection of abrasive materials or additives for abrasive blasts
  • B24C11/005—Selection of abrasive materials or additives for abrasive blasts of additives, e.g. anti-corrosive or disinfecting agents in solid, liquid or gaseous form
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B24—GRINDING; POLISHING
  • B24C—ABRASIVE OR RELATED BLASTING WITH PARTICULATE MATERIAL
  • B24C7/00—Equipment for feeding abrasive material; Controlling the flowability, constitution, or other physical characteristics of abrasive blasts
  • B24C7/0046—Equipment for feeding abrasive material; Controlling the flowability, constitution, or other physical characteristics of abrasive blasts the abrasive material being fed in a gaseous carrier
  • B24C7/0053—Equipment for feeding abrasive material; Controlling the flowability, constitution, or other physical characteristics of abrasive blasts the abrasive material being fed in a gaseous carrier with control of feed parameters, e.g. feed rate of abrasive material or carrier
  • B24C7/0061—Equipment for feeding abrasive material; Controlling the flowability, constitution, or other physical characteristics of abrasive blasts the abrasive material being fed in a gaseous carrier with control of feed parameters, e.g. feed rate of abrasive material or carrier of feed pressure

Definitions

• TITLE IMPROVED PROCESS FOR REMOVING COATINGS FROM SENSITIVE SUBSTRATES, AND BLASTING MEDIA USEFUL THEREIN
• This invention relates to a process for removing coatings from sensitive metal and composite surfaces or like substrates, and to blasting media useful therein.
• Blasting media useful for such purposes should, preferably, meet the following criteria:
• Sodium bicarbonate has also been proposed as a blasting medium for removing coatings from sensitive substrates such as aircraft parts.
• Bicarbonate is an ideal choice for such a medium since it readily meets criteria 1, 2 and 4 above.
• it is relatively non-aggressive (Mohs hardness of about 2.5), is available in a variety of particle sizes, and is both water soluble and commonly utilized to treat sewage treatment facilities for the control of alkalinity and pH.
• the mild abrasive characteristics of sodium bicarbonate have previously been utilized, for example, in polishing media for teeth. See, for example, U.S. Patents Nos.
• TCP tricalciu phosphate
• Bicarbonate blasting media which may be so utilized are free flowing and have long storage lives under adverse commercial blasting conditions, and may be utilized as blasting media at high humidities and under a broad range of finely controlled, high flow rates and air pressures.
• a process for removing coatings from sensitive substrates comprising blasting such surfaces with a high velocity fluid stream, desirably a substantially saturated compressed air stream under a pressure of about 10-150 psi, containing as a blasting medium crystalline water-soluble bicarbonate particles having average particle sizes within the range of about 100-500, preferably about 250-300, microns, desirably in admixture with at least about 0.2%, preferably about 0.2-3%, of a hydrophobic silica flow/anti-caking agent, by weight of the bicarbonate.
• hydrophobic silica to the blasting medium imparts significantly better flow characteristics than comparable media which are either free of flow aids or which contain other conventional flow aids such as TCP or hydrophilic silica.
• bicarbonate blasting media incorporating the hydrophobic silica flow aid have significantly longer, substantially indefinite shelf lives, and exhibit superior resistance to the high relative humidities of commercial compressed air streams.
• crystalline sodium bicarbonate As the abrasive material in the blasting media of this invention. It is, however, intended that other crystalline water-soluble bicarbonates, e.g., alkali metal bicarbonates such as potassium bicarbonate, or ammonium bicarbonate may similarly be employed. Accordingly, while the following description principally refers to the preferred crystalline sodium bicarbonate-containing blasting media, it will be understood that the invention embraces blasting media incorporating other water-soluble crystalline bicarbonate abrasives as well.
• Hydrophobic silica has previously been utilized in admixture with hydrophilic silica as a flow aid in polishing media for dental prophylaxis. Such media are applied under conditions which differ dramatically from commercial blasting media.
• dental prophylaxis media contain bicarbonate particles having particle sizes of about 65-70 microns, and are applied at rates of about 3 grams per minute through 1/16-1/32 inch nozzles under the pressure of clean, laboratory compressed air supplies under pressures of about 50-100 psi. Most important, such air supplies are not substantially saturated with moisture, and present quite different flow and caking problems from those inherent in the application of commercial blasting media at high thruputs in saturated compressed air blast streams.
• the blasting media of the invention consist essentially of the crystalline water-soluble bicarbonate, e.g., sodium bicarbonate, in admixture with hydrophobic silica particles.
• Hydrophobic silica unlike known hydrophilic silicas, is substantially free of non-hydrogen bonded silanol groups and absorbed water.
• Aerosil R 972 a product which is available from Degussa AG.
• This material is a pure coagulated silicon dioxide aerosol, in which about 75% of the silanol groups on the surface thereof are chemically reacted with dimethyldichlorosilane, the resulting product having about 0.7 mmol of chemically combined methyl groups per 100m 2 of surface area and contai.ni.ng about 1% carbon. Its particles vary in diameter between about 10-40 nanometers and have a specific
• the hydrophobic silica particles are admixed with the crystalline sodium bicarbonate blasting agent in the proportion of at least about 0.2%, and up to about 3% by weight thereof.
• the crystalline bicarbonate particles may be of any desired average particle size, within the range of from about 100 to 500 microns.
• Blasting media thus constituted are useful for cleaning or de-coating sensitive metals, e.g., aluminum or aluminum alloys, or composite substrates, such as utilized on exterior aircraft surfaces, without abrading or otherwise damaging the substrates.
• Composites which can be treated with the blasting media hereof comprise matrices, e.g., epoxy resins, which may contain fibers such as glass strands, graphite or the like for reinforcement.
• the blasting medium thus constituted is preferably applied in commercial compressed air streams, i.e., streams which are substantially saturated with moisture (90% or higher relative humidities) and contain oil contaminants from compressors.
• a stream of water may be added to the media/air stream to cool the workpiece and control dust formation, although the addition of water may reduce the coating removal rate somewhat.
• the bicarbonate/hydrophobic silica blasting medium may be applied at flow rates of about 1-10, desirably about three to four, pounds per minute and under air pressures of about 10-150 psi, from 1/4 inch or larger blasting nozzles.
• blasting media so constituted and employed do not cake, have excellent shelf lives, and are free-flowing. They may thus be readily employed in commercial blasting operations for removing coatings from sensitive metal and composite surfaces.
• Figure 1 is an electron scanning photomicrograph of a clad aluminum test panel which has visible corrosion at the surface, penetrating the cladding, prior to blasting;
• Figure 2 is an electron scanning photomicrograph of a clad aluminum test panel which has been cleaned according to the process of the invention. It is apparent that the surface corrosion by-product materials have been removed without damaging the metal surface;
• Figure 3 is a graph comparing production rates obtained employing bicarbonate blasting media having different average particle sizes, at varying feed rates.
• DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples illustrate the free-flowing characteristics of (Example 1), and the high production rates obtained by (Examples 2 and 3), the blasting media of the present invention. In the examples, all parts and percentages are given by weight and all temperatures in °F unless otherwise indicated.
• EXAMPLE 1 Two crystalline sodium bicarbonate samples, each having an average particle size of about 250-300 microns, and one in admixture with 0.5% Aerosil R-972 hydrophobic silica, were used as media in a standard sand blasting apparatus.
• the apparatus was a Schmidt Accustrip System, manufactured by Schmidt Manufacturing, Inc. of Houston, Texas, having a 0.5 inch nozzle diameter and a Thompson valve connected to a 6 cubic foot blast pot and an 800 cfm compressor. The blast pot was mounted on a scale so that media flow rates could be determined.
• the crystalline sodium bicarbonate particles absent the hydrophobic silica flow aid flowed intermittently through the system, rapidly clogging the nozzle and preventing further flow. Sustained flow could not be maintained.
• the crystalline bicarbonate-hydrophobic silica formulation flowed continuously through the system for more than 65 hours at controllable rates varying from 1 pound per minute to 5 pounds per minute. No flow problems were encountered therewith.
• Test panels of aluminum, two feet by two feet by 0.032 inch thick were cleaned by treating with water, washing with phosphoric acid solution, rinsing with water, applying Alodyne corrosion inhibitor, and rinsing with a final water rinse.
• An epoxide primer was applied and the panels were dried in air for eight hours.
• a polyurethane paint was then applied and the panels were dried for seven days.
• the panels were then aged for 96 hours in an air oven at 210°F.
• the target thickness for the paint and primer was 2 mils.
• the blasting media consisted of 99.5% sodium bicarbonate (in differing particle sizes) and 0.5% Aerosil E972 hydrophobic silica.
• the nozzle pressure was set at 60 psi and -li ⁇
• the media were placed under differential pressure sufficient to give the desired feed rate.
• the water pressure was set at a pressure of 200 psi, yielding a flow of 0.5 gal/min.
• the angle blast nozzle made with the workpiece was set at 60°.
• the media feed rates were varied from two to four lbs/min.
• the production rates and average particle sizes of the bicarbonates in the respective media were as follows:
• Example 2 The blasting medium described in Example 1, incorporating a crystalline sodium bicarbonate having an average particle size of about 250-300 microns, in admixture with 0.5% Aerosil R-972 hydrophobic silica, was compared with a blasting medium incorporating a compacted bicarbonate, having particles in the range of -20 mesh to +40 mesh, in admixture with a like amount (0.5%) of the same hydrophobic silica flow aid.
• the respective media were utilized to depaint aluminum test panels in the manner described in Example 2.
• the maximum production rates obtained with the respective media were determined, and are tabulated below.
• the tabulation shows the improved production rate achieved using the blasting medium incorporating the crystalline sodium bicarbonate rather than the medium incorporating the compacted sodium bicarbonate:
• the present invention provides an improved process for cleaning or removing paint or other coatings from the surfaces of sensitive substrates, and to crystalline sodium bicarbonate-containing blasting media useful therein. It will be understood that various changes may be made in the blasting process and blasting media exemplified in the preferred embodiments described hereinabove without departing from the scope of the invention. Accordingly, the preceding description should be construed as illustrative and not in a limiting sense.

Landscapes

• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Application Of Or Painting With Fluid Materials (AREA)
• Paints Or Removers (AREA)
• Detergent Compositions (AREA)
• Cleaning And De-Greasing Of Metallic Materials By Chemical Methods (AREA)

Priority Applications (3)

Applications Claiming Priority (2)

Publications (1)

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Family Applications (1)

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Cited By (10)

Families Citing this family (2)

Citations (9)

Family Cites Families (3)

• 1991
  • 1991-03-26 NZ NZ237596A patent/NZ237596A/xx unknown
  • 1991-03-30 MY MYPI91000529A patent/MY109653A/en unknown
  • 1991-04-04 WO PCT/US1991/002338 patent/WO1991015308A1/en not_active Application Discontinuation
  • 1991-04-04 CA CA002079892A patent/CA2079892C/en not_active Expired - Lifetime
  • 1991-04-04 JP JP3510591A patent/JPH0669668B2/ja not_active Expired - Fee Related
  • 1991-04-04 KR KR1019920702462A patent/KR960003113B1/ko not_active IP Right Cessation
  • 1991-04-04 EP EP19910911117 patent/EP0525122A4/en not_active Ceased
  • 1991-04-04 AU AU79828/91A patent/AU643039B2/en not_active Ceased
  • 1991-04-05 PL PL91289763A patent/PL165174B1/pl unknown
  • 1991-04-05 TR TR91/0360A patent/TR25589A/xx unknown
  • 1991-04-06 CN CN91103072.7A patent/CN1029599C/zh not_active Expired - Fee Related
• 1992
  • 1992-10-01 NO NO923823A patent/NO176751C/no unknown

Patent Citations (9)

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