What is claimed is
1. A liquid composition capable of drying to form a brittle film responsive to strains by cracking and adapted for use in measuring the surface strain in a rigid article, said composition comprising brittle resin selected from the class consisting of limed rosin, phenol formaldehyde, gum rosin, dammar, copal, zinc resinate and zinc-calcuim resinate; low boiling organic solvent present in an amount between about 2 and about 10 times the volume of the resin; plasticizer present in an amount between about 2 and about 20 percent, based on the weight of the resin; wetting agent present in an amount between 0.01 and 2 percent, based on the weight of the resin; and water present in an amount between 0.01 and 2 percent, based on the weight of the resin.
2. The liquid composition of claim 1 wherein the wetting agent is present in an amount between about 0.01 percent and about 0.5 percent, based on the weight of the resin.
3. The liquid composition of claim 1 wherein the water is present in an amount between about 0.01 percent and about 0.5 percent, based on the weight of the resin.
4. A liquid composition capable of drying to form a brittle film responsive to strains by cracking and adapted for use in measuring the surface strain in a substantially rigid article, said composition comprising limed rosin; methylene chloride, present in an amount between about 2 and about 10 times the volume of the limed rosin; alkyl stearate or oleate, wherein the alkyl group contains one to four carbon atoms, present in an amount between 2 and about 20 percent, based on the weight of the rosin; wetting agent, present in an amount between about 0.01 percent and about 0.5 percent, based on the weight of the rosin; and water present in an amount between about 0.01 percent and about 0.5 percent, based on the weight of the rosin.
5. The liquid composition of claim 4 which further contains dye present in an amount from about 0.2 to about 1 percent, based on the weight of the rosin.
6. A liquid composition capable of drying by evaporation to form a brittle film responsive to strains by cracking and adapted for use in measuring surface strain in a rigid article, said composition consisting essentially of zinc-calcium resinate, methylene chloride present in an amount equivalent to about three times the volume of the resinate, butyl stearate present in an amount about 5 percent by weight based on the resinate, linear alkyl sulfonate present in an amount about 0.1 percent by weight based on the resinate, and water present in an amount about 0.2 percent by weight based on the weight of the resinate.
7. The composition of claim 6 in which oil soluble dye is also incorporated in an amount less than about 1 percent by weight, based on the resinate.
8. The composition of claim 7 in which the dye is incorporated in an amount less than about 1/4 of 1 percent by weight, based on the resinate.
FIELD OF THE INVENTION
The present invention relates to brittle coating compositions and more particularly, to unique brittle coating compositions which can be applied to structural materials in order to reveal the location of maximum strain and the directions of principal strains.
BACKGROUND OF THE INVENTION
In the early 1940's patents were issued to Greer Ellis in connection with his invention of the brittle coating method for experimental stress analysis. The various patents issued to Ellis, including U.S. Pat. Nos. 2,186,014; 2,294,897; 2,310,845, 2,325,116; and 2,428,559, disclose that brittle lacquer compositions composed of a combination of resin, solvent and plasticizer can be applied as a coating, by spraying or brushing methods, onto structural parts of rigid materials of relatively high elastic limit. The resulting coating becomes brittle upon drying and under suitable conditions it is possible to detect flaws and cracks in the coated materials which are not apparent to the naked eye. Fine cracks in the brittle coating reveal the location of maximum strain and the directions of principal strains. This permits small strain gages to be positioned at the locations which have been pinpointed by the brittle coating. Moreover, by calibration approximate values of strain magnitude can be predicted.
The two solvents which have conventionally been employed in brittle lacquers are carbon disulfide and methylene chloride. Since carbon disulfide is not only highly explosive but also poisonous, methylene chloride has increasingly become adopted as the standard in the industry. Use of methylene chloride, however, has the drawback of causing a rather troublesome bubble problem to occur in brittle lacquers. The bubbles formed when methylene chloride is used tend to continue to grow and disrupt the final coatings. The bubbles consequently interfere with the use of the coatings in determining crack patterns for location of maximum strain and the directions of principal strains.
In addition to bubble formation, coatings of conventional brittle coatings have developed two types of crazed finishes upon drying. The so-called temperature craze, resembling the crazed finish of an old piano, occurs when a coated structural part is subjected to a variation in temperature. Differences in the expansion characteristics between the brittle coating and the coated structure causes internal tension to build up resulting in a crazed finish. The other type of crazed finish, the so-called drying craze resembling a baked mud flat, is a more serious problem and is normally a function of the coating formulation. Rounded cracks form early in the drying process before the coating is completely hard. The cracks appear first at the surface in this type of crazed finish and then continue into the interior of the coating. These cracks, unlike the cracks developed by temperature craze, will not close up when the temperature is changed.
SUMMARY OF THE INVENTION
An object of the present invention is to provide improved brittle coating compositions.
Another object of the present invention is to provide brittle coating compositions for use in experimental stress analysis which are highly sensitive, non-flammable and substantially odorless.
Still another object of the present invention is to provide brittle coating compositions which minimize the formation of bubbles when methylene chloride is employed as a solvent.
Yet another object of the present invention is to provide brittle coating compositions which will resist the formation of drying craze.
A further object of the invention is to provide coating formulations capable of yielding a coating film characterized by uniform brittleness which is substantially unaffected by local variations in thickness.
In accordance with the present invention, highly sensitive non-flammable, substantially odorless brittle coating compositions for use in experimental stress analysis applications are obtained by combining a resin, a solvent, a plasticizer, a wetting agent and water. In a preferred embodiment, the resin is a limed rosin, the solvent is methylene chloride, the plasticizer is an alkyl stearate or oleate, and the wetting agent is a linear alkylate sulfonate. The wetting agent must be incorporated into the brittle coating composition in an amount of less than 2 percent of wetting agent, based on the weight of the resin. The amount of water which is incorporated must be limited to an amount less than about 2 percent, based on the weight of the resin. Normally, a pigment or a suitable oil soluble dye is also incorporated into the coating compositions.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The brittle coating compositions of the present invention provide a quick and economical way of determining exact strain gage location and orientation thereby permitting precise measurement of strain magnitude at points of maximum interest.
Typically, articles to be tested for possible flaws are coated with a continuous adherent film which is brittle enough to crack at a strain produced by a stress below the elastic limit of the material which is being tested. Using loading techniques which are now standardized, preferential cracking of the film about defects in rigid material can be achieved. The coating composition can be applied to effect either a partial or complete coating by dipping, brushing, spraying or melting the coating onto the surface to be tested. The resulting coating is then allowed to dry for a sufficient time to acquire its brittle properties.
As indicated hereinabove, brittle coating compositions of the present invention include a resin, a solvent, a plasticizer, a wetting agent and water. A pigment or suitable dye can also be included in the brittle coating compositions to provide some color which will give good contrast in observing cracks.
Transparent, hardened natural resins are preferable for use in the compositions according to this invention, although hard brittle synthetic resins, such as thermosetting, phenol-formaldehyde condensation products, can also be used. Among the natural resins are gum rosin, wood rosin, dammer and various copals such as Manila, Kauri, Congo and the like. Hardening of the natural resin can be effected by treatment with lime, either by fusion or in the presence of water, by a treatment with other calcium compounds, such as calcium acetate, or treatment with basic hydroxides or oxides such as magnesium or zinc oxide, by oxidation with a current of hot air, and in other manners known in the art relating to natural resins. The hardened resins can thus comprise a metallic resinate or oxidation product of a natural resin or both. Various types of lime-hardened resins, especially wood rosin treated with large amounts of lime or calcium acetate, or both, have been found particularly useful. Reaction products of a resin or rosin with zinc oxide are also well suited for the purposes of this invention. Included in the various types of lime and zinc oxide hardened resins, produced by reacting gum or wood rosin with lime and/or zinc oxide, in the presence of a catalyst, to form a limed rosin, zinc resinate, or zinc-calcium resinate, are those set forth and described in U.S. Pat. Nos. 2,346,992 and 2,346,995. Such hardened resins are available commercially as uniform commodities at relatively low cost.
Since unplasticized brittle resin film tends to craze or crack spontaneously, it is necessary to incorporate a small amount of plasticizer with the resinous material in an amount sufficient to prevent crazing under testing conditions, but insufficient to remove all internal strains in the film. The plasticizer used should have a very low evaporation rate, to insure that the film, once formed, retains its mechanical properties unchanged for considerable periods of time. It should also be compatible with the resin and solvent used, both in the film forming composition and in the film. Numerous substances of the type commonly used as plasticizers have been found useful for purposes of the present invention. Such plasticizers include dibutyl phthalate and normal butyl stearate. The latter is often particularly preferred in conjunction with the use of methylene chloride (dichloromethane). In addition, the alkyl oleates, having an alkyl group of one to about four carbon atoms, can be advantageously used.
The solvent which is employed must be capable of dissolving the resin and plasticizer at all stages during formation of the film. In addition, the solvent should not have a tendency to absorb moisture from the air, should evaporate rapidly without leaving a residue, and no appreciable fraction of the solvent should be retained by the dried film. In general "nonsolvating" solvents, or those solvents which do not tend to be absorbed by or to form loose compounds or complexes with the dissolved resin are desired for purposes of the present invention. In particular, low boiling solvents are desirable in order to effect rapid evaporation and drying. A pure compound is also preferable since it tends to evaporate uniformly without leaving behind a slow drying higher boiling fraction. Non-polar, low boiling liquid compounds of carbon are particularly suitable and these include carbon disulfide, methylene chloride and 1,2(cis) dichloroethylene. As indicated above, there are good reasons for avoiding the use of carbon disulfide. The compositions of the present invention overcome a particularly serious problem involved when the preferred solvent, methylene chloride, has heretofore been employed in brittle coating compositions. While all of the solvents tend to cause some degree of bubble formation, the desire is that the bubbles be kept to a minute size too small to be discernible to the naked eye.
The particular wetting agent selected is not critical provided its properties do not interfere with the brittle coating which is formed. Various nonionic surface-active agents can be employed. For example, Triton X-100, which is isooctyl phenoxy polyethoxy ethanol containing 10 moles of ethylene oxide manufactured by Rohm and Haas Co., can be employed. Other suitable materials include Ultrawet K, a linear alkylate sulfonate surfactant made by the Atlantic Refining Co.; and the Surfonic nonionic surface-active agents sold by Jefferson Chemical Co., which are of the polyoxyalkylene ether type.
Theoretically, there is no upper limit on the amount of solvent which is used but there is obviously a practical lower limit. In general, the solvent is employed in an amount which is between about 2 and about 10 times the volume of the resin. This range permits the development of optitimum characteristics. The plasticizer is generally employed in an amount between about 2 and about 20 percent by weight, based on the weight of the resin. Often a slightly larger percentage of plasticizer than normally used is employed with compositions containing a pigment or dye in order to compensate for the added pigment or dye. While, as indicated above, the wetting agent employed is not particularly critical, the amounts of wetting agent and water employed in the compositions should be carefully regulated. Generally, the amount of wetting agent should be less than about 2 percent by weight, based on the weight of the resin, and preferably the wetting agent should be present in an amount between about 0.01 percent and about 0.5 percent based on the weight of the resin. The amount of water present should be less than about 2 percent by weight, based on the weight of the resin, and preferably the amount of water present should be in the range of between about 0.01 percent and about 0.5 percent, based on the weight of the resin.
In order to increase the contrast of the patterns, it is generally desirable to incorporate a pigment or dye. The incorporation of a dye helps in the application of the coating to obtain optimum thickness of the coating. The use of dyes or pigmented films is of special interest where the surface of the article to be tested is either dull or has surface markings. In the case of a dull surface, cracks in a transparent film would not be visible due to absorption of light transmitted through the film, while in the case of surface having various surface markings cracks in the transparent film would be difficult to recognize. Pigmented films are also advantageous with respect to some painted and dirty surfaces.
Although the amount of dye can be varied, it generally is not necessary to incorporate more than about 1 percent by weight, based on the weight of the resin, and preferably the amount of dye employed is less than about 1/4 of 1 percent, based on the weight of the resin. Typically, yellow, green, red, or blue dyes or some combination of these dyes are employed. Dyes which provide these colors include the well known naphthol yellow, malachite green, toluene blue, methylene blue and quinoline yellow. Since the nature of the dye is not critical any conventional oil soluble dye can be employed.
An example of a preferred embodiment comprises zinc-calcium resinate as the resin, methylene chloride present in an amount equivalent to about three times the volume of the resin as the solvent, butyl stearate in an amount about 5 percent, based on the resin, as the plasticizer, linear alkylate sulfonate in an amount about 0.1 percent, based on the resin, as the wetting agent, and about 0.2 percent of water, based on the weight of the resin. About 1/10 of 1 percent, based on the weight of the resin, of an oil soluble dye was also incorporated to provide a light green color to the finished coating.
The compositions of the present invention tend to have the same operative characteristics under a variety of temperature and humidity conditions. The compositions are not only non-flammable, but are odorless and provide coatings with high sensitivity, having a crack threshold sensitivity at room temperature of about 500 microinches.
If the brittle film which is formed is transparent, and if the surface coated thereby is uniformly light reflecting, cracks in the brittle film can be made easily visible by rays of light oblique thereto and viewed from the same angle as the impinging light. The easy visibility of cracks in glass depends upon a similar phenomenon, the rays of light being reflected from the surfaces of the crack. When an opaque brittle coating is used, staining the cracks with a suitable stain, if necessary, proceeded by enlarging the cracks by etching with suitable solvent, is also a means used to render the cracks in the brittle film easily visible by reflected light. A suitable black stain for staining the cracks can be made up from one gram of carbon black, 0.01 gram of sodium lauryl sulfate and 10 grams of water.
From the foregoing it will be seen that this invention is adapted to obtain all of the ends and objects hereinabove set forth, together with other advantages which are obvious and which are inherent. Compositions described herein can be applied by any of the conventional procedures, including dipping, brushing or spraying. After an overnight drying period the resulting films are characterized by substantially uniform brittleness which is independent of local variations in thickness. Not only do the compositions set forth herein avoid the problem of severe bubble formation and the clouding caused by those bubbles but the coating compositions result in a brittle coating which resists the problems associated with drying craze. Overcoming the problem of the bubbles by the incorporation of wetting agent and water into the brittle coating compositions was totally unexpected since the addition of water to the brittle coating compositions of the prior art tends to cause bubble formation.
While coating compositions of the present invention are normally used in connection with rigid materials of relatively high elastic limit, i.e., structural metal pieces, it will be understood that the compositions can be used to detect hidden defects in articles composed of many other substances such as Bakelite, wood, hard rubber, porcelain, and the like.
Obviously, many modifications and variations of the invention as hereinbefore set forth can be made without departing from the spirit and scope thereof and therefore only such limitations should be imposed as are indicated by the appended claims.