US2808342A

US2808342A - Composition for, and method of applying coatings of polychlorotrifluoroethylene

Info

Publication number: US2808342A
Application number: US633516A
Authority: US
Inventors: Mortimer H Nickerson
Original assignee: Individual
Current assignee: Individual
Priority date: 1957-01-11
Filing date: 1957-01-11
Publication date: 1957-10-01
Anticipated expiration: 1974-10-01

Description

COIVIPOSITION FOR, AND METHOD OF APPLY- ING COATINGS OF POLYCHLOROTRIFLUORO- ETHYLENE Mortimer H. Nickerson, Hazardville, Conn.

No Drawing. Application January 11, 1957, Serial No. 633,516

14 Claims. (Cl. 117-65) This invention relates to the formation of coatings of polychlorotrifluoroethylene.

This application is a continuation in part of application Serial No. 330,350, filed January 8, 1953, now abandoned.

Polychlorotrifiuoroethylene in high molecular weight form has a remarkable resistance to attack by corrosive materials and also by organic solvents. ln addition, it possesses excellent electrical insulation properties which are enhanced by its ability to withstand higher temperatures than most known plastics, coupled with a negligible water absorption. This combination of properties makes polychlorotrifluoroethylene almost ideal as a protective coating to prevent chemical or solvent attack or to prevent electrical leakage on conductors. The use of this polymer in the above manner has, however, been severely handicapped by the difliculty of applying such coatings. Since there is no known solvent which will dissolve appreciable amounts of the polymer at room temperature, the material cannot be applied as a lacquer as is commonly done with most protective coats.

One approach to the problem is to make dispersions of the finely ground polymer in a suitable volatile medium and thus obtain a thin coat of the material by first applying a coat of the dispersion, evaporating the dispersing medium, followed by fusion at sufiiciently high temperature to cause the fine particles to melt and coalesce to a continuous film. Hitherto, work along this line has not been very successful, due largely to the fact that unless the dispersion is applied in an extremely thin coat the powder layer which is left on evaporation of the dispersing medium shrinks and cracks prior to fusion. This type of cracking is quite common in most dispersion work and is sometimes described as mud cracking. if this cracking takes place, the resulting film has many discontinuities and therefore has no value for protection or elec trical insulation. The extremely low value for the maximum thickness which can be applied by the above procedure in any one coating operation necessitates multiple coating operations in order to build up a substantial film. Prior to the present invention, it was necessary to perform three or four successive coating and fusing operations to build up as little as 0.001 inch thickness of protective polymer film. These operations are not only laborious, but also successive heating of the previously laid down layers of polymer tends to weaken them physically, leading in extreme cases to serious embrittlement of the resin.

It is the object of this invention to provide dispersions of polychlorotrifluoroethylene by means of which it is possible to apply in one operation a coating of the polymer having a final fused thickness up to 0.005 inch or higher, such coating consisting of a well-adhered, continuous film, free of cracks. A further object of this invention is to provide techniques and methods by means of which such coatings are obtained.

The first requisite in making a dispersion of any solid material is that the material be finely ground. Poly- States PatentO 2 chlorotrifluoroethylene suitably prepared in a finelydivided state by any of the presently known methods may be used but for obtaining the ultimate in glossiness and smoothness of a coat, it is necessary to reduce the basic particle size by conventional equipment such as a micropulverizer. *If a slightly granular coat will serve the purpose, it will usually be sufficient to take the polymer powder as commercially supplied and break up any aggregates, as can be done, for example, by suspending the material in a suitable liquid medium and passing it through a colloid mill or by ball-milling. It should be borne in mind that the particle size of the polymer mainly alfects the'glossiness of the product and does not affect the cracking of the film which it is the object of this invention to eliminate. :In general the finer the particle size, the glossier the product. As previously stated commercially available high molecular weight polychlorotrifluoroethylene can be used satisfactorily provided that it has been pulverized by any of the usual means to reduce it to' the fine particle size necessary for dispersions. In general this size is in the range of l to 20 microns.

My investigation and study of the process of dispersion coating using polymeric chlorotrifuloroethylene has led to the conclusion that the phenomenon of mud cracking takes place after the evaporation of the volatile dispersing medium and before fusion of the polymer, due to heat, takes place. I have found that the conventional choice of a low-boiling dispersing medium only aggravates the conditions which cause mud cracking; because of the greater spread in time and temperature between the disappearance of the last of the dispersing medium and the fusion of the polymer. According to the present invention the ideal dispersing medium should have a boiling point that will allow the simultaneous evaporation of the dispersing medium, fusion of the resin, and attainment of the fusion temperature by the article being coated. Under these conditions the intermediate dry condition where all of the dispersing medium has evaporated and the polymer exists as a weak layer never occurs and discontinuities due to shrinking of the weak layer will not result.

In addition to having a high boiling point I have found that the dispersing medium must possess other properties. One of these properties -I describe as paste forming. By this I mean that it possesses the ability to form smooth relatively thick, continuous, sustained films of the dispersed polymer at below fusion temperature. Other requirements are that the medium must be chemically stable at rather high temperatures so that it will volatilize cleanly from the polymer film as it fuses. Preferably, it should exert some solvent action on the polymer, at least at high temperature, to facilitate fusing of the film, and necessarily it must be at least partially soluble in the polymer in order that it may escape from the fusing film without bubbling. The medium must not chemically interact with the polymer to change its properties or promote decomposition. Obviously if the dispersion is to be applied at room temperature, the dispersing medium must be a liquid.

' I have found that the chlorinated diphenyls containing 18% to 62% of combined chlorine and boiling in the range 225 C. to 430 C. meet the unique combination of properties described above and when used as the dispersing medium for polychlorotrifluoroethylene make possible the production of continuous, crack-freefilms of fused polymer of a thickness hitherto impossible to attain.

Chlorinated diphenyls suitable for this work are available under the trade name Aroclor from the Monsanto Chemical Company. There is an extensive series of these materials, and they are available in a variety of chlorine contents, molecular weights, and boiling points The particular chlorinated diphenyl to be used will depend upon the cross-sectional dimensions of the article to be coated. That member of the series designated as Aroclor 1221, which has a distillation range of 275 C. to 320 C., is preferred as the dispersing medium for work on thin cross-sections, particularly on thickness'less' than 0.125 inch. The higher boiling members of the series are preferred for thicker cross-section articles. It was found that a steel bar of 1 inch thickness could be successfully coated with-polychlorotrifluoroethylene using that member of the series designated as Aroclor 1262, which has a distillation range of 400 C. to 430- C. It is not necessary that one definite chlorinated diphenyl be the sole constituent of the dispersing medium, but rather, mixtures of chlorinated diphenyls can be used. Similarly, other more volatile liquids may be used as diluentsin conjunction with the chlorinated diphenyls in order to reduce viscosity. In such cases the essential feature is that sufficient chlorinated diphenyl be used so that after evaporation of the more volatile constituents, there is present enough chlorinated diphenyl to hold the resin in a continuous film by means of its paste-forming ability as described above. I have found that in general an amount of chlorinated diphenyl equal to 50% of the dispersed polymer is adequate to maintain this continuity of film prior to and during fusing, although the use of a larger amount is preferable.

Requirements for the diluent are much less stringent since its only function is to provide fluidity to the dispersion. However, it should be miscible with the chlorinated diphenyl and should also act as a good dispersant for the polymer. I have found that a mixture of solvents is the best diluent and that for best results it should consist of three parts of an aromatic hydrocarbon of the class benzene, toluene, xylene and one part of an aliphatic ketone of the class acetone, methyl ethyl ketone, methyl isobutyl ketone, di-isobutyl ketone. For general use the mixture of xylene and methyl isobutyl ketone is preferred, but other combinations may be used depending upon what volatility is most suited for the application at hand.

The following examples are ofiered by way of illustrating how the teachings of this invention may be used;

Example l.--Polychlorotrifiuoroethylene, micropulverized to a particle size of about three microns is added gradually with stirring to a chlorinated diphenyl having a chlorine content of about 21% and boiling in the range 225 C. to 350" C. S'ufficient polymer is so added that the final weight of polymer is equal to the weight of chlorinated diphenyl. The resulting-product is a smooth, soft paste which can readily be spread and leveled to a smooth coat.

A portion of this paste was spread on a flat steel plate toa depth of approximately 12 mils and the plate then heated rapidly to a temperature of 250- C. and kept at that temperature for three to five minutes or until the fumes of chlorinated diphenyl ceased to come olf thefilm'. n cooling the plate, the polymer was found to bein the form of a clear, transparent film, free of cracks, and tightly adhered to the plate. The thickness was 5 mils.

Example HLA portion of the paste prepared as in Example I was diluted with a mixture of three parts xyleneand one part methyl isobutylketone, using one part of the mixture to onepart of the paste. The paste readily stirred into the diluent to produce a free-flowing, low-viscosity dispersion. A puddle of this dispersion was poured? onto a flat steel plate and then flowed over the surface by tilting the plate. The coated plate" was then dried briefly, at a temperature of 1200" C'., to-ridi'tat least partially of the lower boiling diluent. The plate" was then heated rapidly to'a temperatureof' 250 C. and" kept at that temperature" for three to fiveminutes' or' until fumes-of the chlorinated diph'enyl ceased to'come off the film. On cooling the plate, the polymer'wasifound' to be in: the form of a clear, transparent film, free of cracks and tightlyadhered to the-plate: The thickness was 2.5 mils.

Example III.-A paste prepared as described in Example I was diluted with an equal part by weight of chlorinated diphenyl of the same type used in making the paste. There resulted a low-viscosity fluid dispersion which flowed readily over the surface of a metal plate to produce a uniform coating. This coating was then fused by the same procedures as used in Examples I and II without the necessity of an intermediate drying step. The resulting coating was clear and transparent, tightly adhered to the metal, and had a thickness of. 2 mils.

Example IV.-A dispersion was prepared by mixing the following ingredients in a ball-mill, colloid mill, or other homogenizing device:

parts finely divided polychlorotrifluoroethylene 55 parts Aroclor 1221 75 parts xylene 25 parts methyl isobutyl ketone (By weight) The resulting product is a smooth white liquid mass exhibiting some thixot'ropic characteristics. A portion of this dipersion was spread on aflat metal plate to a depth of about 15 mils and then warmed to 100 C. for live minutes to volatilize most of the diluent. The plate was then rapidly heated to 250 C. and fused as described in the preceding examples. The resulting coating was clear, transparent, free of cracks,' and tightly adhered to the metal plate. Its thickness was 4 mils.

Example V.--A dispersion was prepared from the following ingredients as described in Example IV.

133 parts finely divided polychl'orotrifiuoroethylene 122.5 parts Aroclor' 1262 4l parts Aroclor 1221 37 parts methyl is'obutyl ketone The resulting product is a smooth white pasteexhibiting thixotropic properties. A portion of this dispersion was spread on a clean steel-bar of the dimensions 1 inch by 3 inches by 10 inches to a depth of. about 7 mils. The bar was rapidly heated to 250 C- for 50 minutes followed by a cold water quench; The resulting. coating was clear, transparent, free of cracks, and tightly adhered to the metalbar. Its thickness was 2.5 mils-.1

It will be readily seen that these dispersions are extremely versatile andcan be used in: a number ofways. The heavier pastes are suitable for knife coating. and, by dilution with the indicated thinners or additional chlorinated diphenyl, are suitable for spraying, casting} flow coating, or dipping. The dispersions can be used to coat metal, glass, ceramics. wire, smooth: or irregularly shaped objects. The only basic requirement of the objectto be coated is that it must withstand without objectionable decomposition the temperatures necessary to fuse the polymer.

Within the general classification of high molecular weight polymers; there are available" commercially a number of grades differing to some extentinmolecular weight and correspondingly in softening point. Aroclor 1221 has: a boiling range making it useful for a fairly large grouping of these grades of polychlorotrifluoroeth- 'ylene, but when higher or lower molecular weight polymers are used, itmay be preferable to use a chlorinated d'iphenyl having a correspondingly higher or lower boilingrange in order to preserve the proper relationship between boiling range of' the Aro'clor and the softening point of the resin. The choice of a chlorinated diphenyl is therefore not restricted to Aroclor 1221', and the use of any other such material according to the above teachings is within the scope of this invention;

What is claimedis: 7

1. A coating composition a dispersion of finely divided polychlorotrifluoroethyl'ene in a chlorinated diphenyl containing 20% to 40% of combined chlorine having a boiling range essentially including the fusion temperature of the polymer and present in a quantity at least approximately equal to 50% by weight of the dispersed dry polymer.

2. A coating composition comprising a dispersion of finely divided polychlorotrifluoroethylene in a dispersion medium the essential constituent of which is a chlorinated diphenyl containing 20% to 40% of combined chlorine, boiling in the range 225 C. to 350 C. and present in a quantity at least approximately equal to 50% by weight of the dispersed dry polymer to maintain the continuity of the coating film prior to and during fusion of the polymer.

3. A coating composition as in claim 2 in which the chlorinated diphenyl is present in an amount in excess of 50% by weight of the dispersed polymer.

4. A coating composition comprising a dispersion of finely divided polychlorotrifluoroethylene in a dispersion medium the essential constituent of which is a chlorinated diphenyl containing 20% to 40% of combined chlorine, having a boiling point in the range 225 C. to 350 C. and present in a quantity at least approximately equal to 50% by weight of the dispersed dry polymer, the balance of the dispersion medium comprising a diluent miscible with the chlorinated diphenyl and evaporating without decomposition at a temperature below the fusion temperature of the polymer.

5. A coating composition as in claim 4 in which the diluent constituent of the dispersion medium comprises a mixture of 3 parts by weight of an aromatic hydrocarbon of the class benzene, toluene, xylene, and 1 part by weight of an aliphatic ketone of the class acetone, methyl ethyl ketone, methyl isobutyl ketone, and diisobutyl ketone.

6. A coating composition as in claim 4 in which the diluent constituent consists of 3 parts by weight Xylene and -1 part by Weight methyl isobutyl ketone.

7. A coating composition comprising a dispersion of finely divided polychlorotrifluoroethylene in a chlorinated diphenyl containing 20% to 40% of combined chlorine, boiling in the range 225 C. to 350 C. and present in an amount not substantially less than the dry polymer by weight.

8. A coating composition comprising a dispersion of finely divided polychlorotrifluoroethylene in a chlorinated diphenyl containing 18% to 62% of combined chlorine, boiling in the range 225 C. to 430 C. and present in an amount not substantially less than the dry polymer by weight.

9. A coating composition comprising a dispersion of finely divided polychlorotrifluoroethylene in a dispersion medium the essential constituent of which is a chlorinated diphenyl containing 18% to 62% of combined chlorine, boiling in the range 225 C. to 430 C. and present in a quantity at least approximately equal to 50% by weight of the dispersed dry polymer, the balance of the dispersion medium comprising a diluent miscible with the chlorinated diphenyl and evaporating without decomposition at a temperature below the fusion temperature of the polymer.

10. The method of providing articles with a continuous crack-free coating of polychlorotrifiuoroethylene which comprises dispersing finely divided polychlorotrifluoroethylene in a paste forming medium having a boiling range within a range including the fusion temperature of the polymer, both chemically stable and at least partially soluble in the polymer at said temperature, applying said paste dispersion over the surface of the article to be coated, heating the coated surface to the fusion temperature of the polymer to substantially simultaneously evaporate the dispersing medium completely and fuse the polymer, and cooling the article.

11. The method of providing articles with a continuous crack-free coating of polychlorotrifluoroethylene which comprises dispersing finely divided polychlorotrifluoroethylene in a dispersion medium essential constituent of which is a chlorinated diphenyl containing 20% to 40% of combined-chlorine having a boiling point in the range 225 C. to 350 C. and present in a quantity at least equal to 50% by weight of the dispersed dry polymer, the balance of the dispersion medium comprising a diluent miscible with the chlorinated diphenyl and evaporating without decomposition at a temperature below the fusion temperature of the polymer, applying said dispersion over the surface of the article to be coated, raising the temperature of the surface of the article to a point to at least partially evaporate the di: luent, then rapidly raising the temperature of the surface of the article to the fusion temperature of the polymer to substantially simultaneously evaporate the chlorinated diphenyl and fuse the polymer, and finally cooling the article.

12. The method recited in claim 11 in which the diluentconstituent of the dispersion medium comprises a mixture of 3 parts by weight of an aromatic hydrocarbon of the class benzene, toluene, xylene, and 1 part by weight of an aliphatic ketone of the class acetone, methyl ethyl ketone, methyl isobutyl ketone, and di-isobutyl ketone.

13. The method recited in claim 11 in which the diluent constituent consists of 3 parts by weight xylene and 1 part by weight methyl isobutyl ketone.

14. The method recited in claim 11 in which the essential constituent of the dispersion medium is a chlorinated diphenyl containing 18% to 62% of combined chlorine having a boiling point in the range 225 C. to 430 C.

2,542,069 Young Feb. 20, 1951 Compton June 6, 1950

Claims

10.THE METHOD OF PROVIDING ARTICLES WITH A CONTINUOUS CRACK-FREE COATING OF POLYCHLOROTRIFLUOROETHYLENE WHICH COMPRISES DISPERSING FINELY DIVIDED POLYCHLOROTRIFLUOROETHYLENE IN A PASTE FORMING MEDIUM HAVING A BOILING RANGE WITHIN A RANGE INCLUDING THE FUSION TEMPERATURE OF THE POLYMER, BOTH CHEMICALLY STABLE AND AT LEAST PARTIALLY SOLUBLE IN THE POLYMER AT SAID TEMPERATURE, APPLYING SAID PASTE DISPERSISON OVER THE SURFACE OF THE ARTICLE TO BE COATED, HEATING THE COATED SURFACE TO THE FUSION TEMPERATURE OF THE POLYMER TO SUBSTANTIALLY SIMULTANEOUSLY EVAPORATE THE DISPERSING MEDIUM COMPLETELY AND FUSE THE POLYMER, AND COOLING THE ARTICLE.