JPH0669668B2 - Improved method for removing coatings from easily scratchable substrates and jetting media useful therefor - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to methods for removing coatings from surfaces of fragile metals and composites or similar substrates and jetting media useful therein.

BACKGROUND OF THE INVENTION It is often desirable to wash or remove coatings from the surfaces of various structures and devices that vary from buildings to industrial equipment. A number of techniques are known for this purpose, ranging from mechanical polishing to chemical applications for cleaning or removing surface coatings such as paints, sealants, lacquers and the like. Hard and durable surfaces such as granite walls and heavy steel plating can be cleaned or stripped with vigorous polishing techniques such as sandblasting. More delicate surfaces require less aggressive treatment to prevent substrate damage.

Civil aviation and military authorities spend a lot of money routinely stripping or polishing paint and other coatings from the exterior of modern aircraft. These surfaces are made of lightweight aluminum or other metal alloys, or composites, which are relatively soft, and paints and other coatings need to be carefully removed to avoid excessive polishing and chemical damage. There is. Such damage
In extreme cases mechanical damage can occur.

Various improved stripping techniques similar to sandblasting have been proposed for the removal of coatings from vulnerable metal and composite surfaces such as aircraft. A jetting medium useful for such purposes (bl
asting media) preferably meets the following criteria:

1. Be relatively non-invasive (Mohs hardness is about 2.0 ~
3.0), 2. Various particle size distributions are available for the treatment of different substrates, 3. Free-flowing under high humidity conditions over a wide range of air pressures and medium flow rates, 4. Exfoliated insoluble paint Water-soluble and non-polluting, which facilitates the separation of resin and resin and facilitates waste disposal.

Car US Patent No. 4,731,125 (Granted March 15, 1988)
Describes the use of plastic media for jet cleaning of easily scratchable metal, composite surfaces. However, such materials are relatively expensive and their use can cause waste disposal problems.

Sodium bicarbonate has also been proposed as a spray medium for removing coatings from vulnerable surfaces such as aircraft parts. Baking soda easily meets the criteria 1, 2 and 4 above, making it an ideal choice for such media. Thus,
It is relatively non-invasive (Mohs hardness about 2.5), is available in various particle sizes, is water soluble, and is suitable for treating wastewater treatment plants for alkalinity and pH adjustment. It is usually used. The mild abrasive properties of sodium bicarbonate are already utilized in tooth abrasives, for example. For example, U.S. Pat.Nos. 3,882,638, 3,972,123,
No. 4,174,571, No. 4,412,402, No. 4,214,871, No. 4,46
No. 2,803, No. 4,482,322, No. 4,487,582, No. 4,492,575
See Nos. 4,494,932 and 4,522,597.

The major drawback with using sodium bicarbonate as the propellant medium is its tendency to cake, either by compression or, more importantly, by exposure to high humidity conditions. This is violent in commercial jet finishing operations, the compressed air stream is substantially saturated with moisture, that is, it has a relative humidity of 90% or more and contains oily debris from the air compressor. In addition, commercially available sodium bicarbonate inherently has poor flow properties due to the usual particle size distribution and crystalline form.

It is known to add flow aids to improve the flow and anti-caking properties of sodium bicarbonate. Thus, it has previously been proposed to incorporate sodium bicarbonate with calcium phosphate (TCP) in a baking recipe or a dental air jet prophylaxis medium. The addition of such materials improves the bicarbonate flow and anti-caking properties. However, TCP treated sodium bicarbonate is storable at room temperature for 3 to 6 months because TCP absorbs moisture to saturation and then the product sets.

Accordingly, it is an object of the present invention to provide a blasting media containing sodium bicarbonate and a method of using such media to remove coatings from the surface of sensitive metals and composites. To do. Bicarbonate jetting media available are free-flowing, have a long shelf life under poor commercial jetting conditions, and are humid, jetting media under a wide range of well-controlled high flow rates and air pressures. Available as Other objects and advantages of the invention will be apparent from the following description of the preferred embodiments.

SUMMARY OF THE INVENTION According to the present invention, crystalline water-soluble sodium bicarbonate particles having an average particle size in the range of about 100 to 500 microns, preferably 250 to 300 microns are used as the jetting medium in the weight of the bicarbonate salt. A high flow rate stream under pressure of about 10 to 150 psi, containing at least about 0.2%, preferably about 0.2 to 3% of hydrophobic silica mixed with a flow / anti-caking agent,
Desirably, there is provided a method of coating removal from a vulnerable substrate which comprises jet finishing the vulnerable surface with a substantially saturated stream of compressed air.

It has been found that the relatively large particle size of bicarbonate is quite effective in removing corrosion by-products as well as other coatings from the substrate without damaging the substrate. this is,
Contradictory with the common knowledge that small particles with a larger effective surface area as a jetting medium are more effective cleaning agents than large particles. Moreover, in most known injection applications,
The faster the feed rate of the spray material, the higher the production rate, ie the faster the removal of the coating. However, the use of crystalline bicarbonate within the above particle size effectively results in a conventional sandblasting machine with a 0.5 inch diameter nozzle.
Economic production rates can be reached with a supply of about 3-4 pounds per minute. Below this speed, the production rate hardly increases even if the supply rate of the bicarbonate injection medium is significantly increased.

According to another characteristic of the invention, the addition of hydrophobic silica to the propellant medium does not include flow aids or TCP
Alternatively, it provides significantly better flow properties than comparative media containing flow aids such as hydrophilic silica. In addition, bicarbonate jet media containing hydrophobic silica flow aids have significantly longer, substantially indefinite shelf life and excellent resistance to high relative humidity of commercial compressed air streams. Play.

It is preferred to use crystalline sodium bicarbonate as the abrasive in the jetting medium of this invention. However, it is contemplated that other water soluble bicarbonates, such as alkali metal bicarbonates such as potassium bicarbonate and ammonium bicarbonate may be used as well. Therefore, while the following description refers to preferred crystalline sodium bicarbonate-containing jetting media, it is understood that this invention also includes jetting media containing other water-soluble crystalline bicarbonate abrasives as well. Will

Hydrophobic silica has been used in admixture with hydrophilic silica as a flow aid in dental prophylactic polishing media. Such media are applied under conditions that are dramatically different than commercial jet media. Thus, the dental prophylactic medium contains bicarbonate particles having a particle size of about 65-75 microns and contains about 50-
Approximately 3g per minute through a 1/16 to 1/32 inch nozzle under the pressure of a clean laboratory compressed air supply under 100psi pressure
Applied in proportion.

Most importantly, such air feeders are not substantially saturated with moisture and differ from those inherent in applying commercial jet media at high extrusion rates in saturated compressed air jets. There are problems of fluidity and cake.

The jetting medium of the present invention consists essentially of a crystalline water-soluble bicarbonate salt, such as sodium bicarbonate, with a mixture of hydrophobic silica particles. Unlike known hydrophilic silica, hydrophobic silica does not substantially contain non-hydrogen-bonded silanol groups and absorbed water.

One preferred hydrophobic silica that can be used in the jetting medium is Aerosil R972, a product available from Degussa AG. It is a pure coagulated silicone dioxide aerosol with about 75% of its surface silanol groups chemically reacting with dimethyldichlorosilane and the product is about 0.7 mmol of chemical per 100 m ² surface area. It has a methyl group attached to and contains about 1% carbon.
The particles range in diameter between about 10-40 nanometers and have a specific surface area of about 110 m ² / g. It can be made by flame hydrolysis of hydrophilic silica. More specifically Angew.Chem. 72, 744 (1960) , F-PS1,368,765 and DT-
AS 1,163,784. Further details regarding these materials are contained in the August 1986 Degussa AC Technical Report, "Basic Properties and Applications of Elosil".

The hydrophobic silica particles are admixed with the crystalline sodium bicarbonate propellant at a rate of at least about 0.2% and up to about 3% (by weight). The crystalline bicarbonate particles can be any desired average particle size in the range of about 100 to 500 microns. When the jetting medium is utilized to remove paint on the exterior of an aircraft, it is preferred to utilize crystalline bicarbonate particles having an average particle size of about 250-300 microns.

Jet media constructed in this manner are useful for cleaning or decoating fragile metals, such as aluminum or aluminum metal, or composite substrates, such as aircraft exterior surfaces. The composite material which can be treated with this jetting medium consists of a matrix, for example glass strands for reinforcement with epoxy resins, which may contain fibers such as graphite.

Such a propellant medium is a commercial compressed air stream, i.e.
It is preferably applied to streams that are substantially saturated with moisture (90% or higher relative humidity) and that contain oil fouling from the compressor. A water stream can be added to the media / air stream to cool the work piece and control dust formation, but the coating removal rate is somewhat reduced. The bicarbonate / hydrophobic silica jet medium is about 1-10 pounds per minute, preferably about 3-4.
Applicable at a flow rate of lbs and air pressure of about 10-150 psi from a 1/4 inch or larger injection nozzle.

According to the invention, as will be shown in more detail above and below, the jetting medium configured and used as described above is not caked, has an excellent shelf life and is free-flowing.
Therefore, it is easy to use in commercial spraying operations to remove coatings from the surface of vulnerable metals and composites.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an electron scanning micrograph of a clad aluminum test panel having visible corrosion on the surface and infiltrating the clad before spray finishing.

FIG. 2 is an electron scanning micrograph of a clad aluminum test panel cleaned by the method of the present invention.
It is clear that the corrosion by-products were removed without damaging the metal surface.

FIG. 3 is a graph comparing production rates obtained using bicarbonate jetting media having different average particle sizes at varying feed rates.

Detailed Description of the Preferred Embodiments The following examples illustrate the free-flowing properties (Example 1), high production rates (Examples 2 and 3) for the jetting media of the present invention. In the examples, all parts and percentages are by weight and all humidity are degrees Fahrenheit unless otherwise noted.

Example 1 Two crystalline sodium bicarbonate samples (each about 250-300)
It had an average particle size of micron), one of which was a 0.5% blend of Elosil R-972 hydrophobic silica used as medium in a standard sandblasting machine. The equipment is
The Schmidt Acustrip system from Schmidt, Houston, Tex., Has a 6-way foot blast pot and a 0.5 inch nozzle diameter and a Thompson valve connected to an 800 cfm compressor. The blast pot was equipped with a scale such that the average flow rate could be measured.

Each bicarbonate formulation utilized compressed air at room temperature, which was saturated with moisture, and was sprayed through a nozzle at 60 psig pressure through the spray media of the blast pot.

Crystalline sodium bicarbonate particles without hydrophobic silica flow aid intermittently flowed through the system, causing rapid nozzle clogging and impeded further flow. The replenished flow could not be maintained.

The crystalline bicarbonate-hydrophobic silica formulation flowed through the system continuously for over 65 hours at an adjustable rate varying from 1 pound to 5 pounds per minute. There was no flow problem due to this.

Example 2 A 2 foot x 2 foot x 0.032 inch thick aluminum test panel was treated with water and washed with phosphoric acid solution,
Rinse with water, clean with alloden corrosion inhibitor, final water rinse. The epoxide primer was applied and the panel was air dried for 8 hours. The polyurethane paint was then applied and the panel dried for 7 days. Then
Aged in a 210 ° F. air oven for 96 hours. The target thickness of the paint and primer was 2 mm.

Various sodium bicarbonate propellants were placed in a plasto pot equipped with a suitable orifice plate to regulate its flow rate. The jetting medium consists of 99.5% sodium bicarbonate (different particle size) and 0.5% Elosil E972 hydrophobic silica. The nozzle pressure was set to 60 psi and the medium was at a pressure differential sufficient to give the desired feed rate. The water pressure was 200 psi and the flow rate was 0.5 gal / min. The blast nozzle angle made of the work piece was set to 60 °. Medium supply from 2 to 4
changed to bs / min.

The time for the panel to be completely depainted was recorded.

The bicarbonate production rate and average particle size for each medium were as follows:

This data is summarized in Figure 3, curve 1 at 2 bs / min,
Curve 2 is 3 b / min and curve 3 is 4 b / min.

This data shows that media using large average particle sizes of sodium bicarbonate (150 and 250 microns) are more effective at all feed rates than those containing small particles.

It is also clear that economic production is achieved with an average flow rate of about 3 pounds per minute, with little improvement in production rate at high feed rates.

Example 3 An injection medium as described in Example 1 is prepared by mixing crystalline sodium bicarbonate having an average particle size of about 250 to 300 microns and 0.5% of Elosil R-972 hydrophobic silica.
Compressed bicarbonate, in the range of 40 mesh, was compared to a jetting medium mixed with a similar amount (0.5%) of the same hydrophobic silica aid. Each media was used to depaint aluminum test panels as described in Example 2. The maximum production rate obtained for each medium is measured and is given in the table below. The table shows that there is an improvement in production rate with the injection media containing crystalline sodium bicarbonate over the media containing compressed sodium bicarbonate.

From the above disclosure, it is believed that the present invention provides a method of cleaning or removing paint or other coatings from the surface of a vulnerable (sensitized) substrate and a crystalline sodium bicarbonate-containing jetting medium useful therein. Will be understood. It will be appreciated that the plast process and jetting medium illustrated in the preferred embodiment above may be modified in various ways without departing from the scope of the invention. Therefore, the preceding description is illustrative,
It should be understood that it is not limited.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventors Speyers, William, E. United States, Texas 77043, Houston, Triway 1927

Claims (9)

[Claims] 1. Water-soluble crystalline bicarbonate particles selected from the group consisting of alkali metal bicarbonate and ammonium bicarbonate as the jetting medium, the bicarbonate particles having an average particle size of 100 to 500 microns. For things and the bicarbonate
A vulnerable metal comprising blasting a vulnerable surface with a high velocity flow stream containing 0.2 to 3% by weight of hydrophobic silica,
A method for removing a coating from the surface of a composite material. 2. The method of claim 1 wherein the flow stream is a substantially saturated compressed air stream under a pressure of 10 to 150 psi. 3. The jet stream has 1 to 10 bicarbonate particles per minute.
The method of claim 1 applied to the treated surface at a rate of pounds. 4. The method of claim 1 wherein the bicarbonate particles have an average particle size in the range of 250-300 microns. 5. The method of claim 1 wherein the bicarbonate is sodium bicarbonate. 6. The method of claim 1 wherein the hydrophobic silica is substantially free of non-hydrogen bonded silanol groups and absorbed water. 7. Water-soluble crystalline bicarbonate particles selected from the group consisting of alkali metal bicarbonate and ammonium bicarbonate, having a particle size of 100-500 microns, and the flow / condensation of hydrophobic silica. 0.2 to 3 against bicarbonate as an inhibitor
% Fragile metal which becomes essentially by mixing with (weight), a jetting medium for removing a coating from the surface of a composite material. 8. The jet medium of claim 7, wherein the bicarbonate is sodium bicarbonate and the hydrophobic silica is substantially free of non-hydrogen bonded silanol groups and absorbed water. 9. The jet medium of claim 7, wherein the bicarbonate salt has an average particle size in the range of 250 to 300 microns.