US2710266A

US2710266A - Polytetrafluoroethylene coating compositions, method of application to substrates, coated substrates, and films

Info

Publication number: US2710266A
Application number: US399567A
Authority: US
Inventors: Hochberg Jerome
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1953-12-21
Filing date: 1953-12-21
Publication date: 1955-06-07
Anticipated expiration: 1972-06-07

Description

FGLYKETRAFLUORGETHYLENE CQATING CDl /H- MEES MGNS, MIETHUD ti)? APPUCATEGN 5U5- S'E'RA'EEE, QQATED SUBSTRATES, AND FELMS orne ilftochberg, Newburg'lt, N. Y., assignor to E. i. do ..."ont d lslenuours and Company. Wilmington, Del.

corporation of Delaware No iirawing. Application December 21, 1953, Serial No. 399,567

18 Claims. (Cl. li7-lll3) loidal polytetrafiuoroethylene in water as disclosed in S. Patent 2,478,229 to Berry and U. S. Patent 2,534,058 to Renfrew.

it has long been known that polytetrafiuoroethylene films which are deposited on a substrate by a single application of an aqueous suspension of polytetraffuoroeth-ylene tends to develop cracks during the drying and baking steps when the dry film thickness exceeds the order of l to 2.5 mils depending upon the particular lot of aqueous polytetrafiuoroethylene suspensoid used. Each different lot of suspensoid possess-:3 its own "critical cracking thickncss value which is the dry film thickness above which objectionable cracks develop during drying and/or fusing. The cracks formed during drying of the polytetrafiuoroethylene aqueous suspensoid vary in size from unmagni fied visual cracks to microscopic cracks. tends to develop not only in films of uniform thickness greater than about I to 2.5 mils but also where such a .lm thickness occurs accidentally because of poor control in application or because of the shape of the object being coated. Such cracks not only adversely affect the appearance of the article but they lead to premature failure of the coating.

For certain purposes, particularly in the coating of woven. glass fabrics and various metals, it is necessary or desirable to have polytctrafluoroethylene films greater than 3.5 mils thick and sometimes even as high as 5 to 10 mils thick. Under very closely controlled conditions, relatively thick crack-free polytetrafiuoroethylene films can be built up on a substrate by applying several consecutive thin coats of an aqueous suspension, i. e., of the order of .5 mil or less. each followed by a separate baking or fusing operation after each coat. It is obvious, however, that this method is laborious, cumbersome and costly; and, furthermore, it does not eliminate the need for extreme care in avoiding heavy beads or fatty edges when coating irregularly shaped surfaces.

U. S. Patent 2,539,329 which issued January 23, 195i. to P. F. Sanders discloses a mechanical method of eliminating cracks in polytetrafluoroethylene coatings applied from an aqueous suspension to inorganic fabrics. such as, e. g. woven glass fabrics by calendaring, but it is obvious that such a process involvesan additional manufacturing operation which increases the cost. Further, the pressure treatment required to close the cracks tends to crush and break at least some of the glass filaments of the woven l 55 fabric which reduces the tensile strength.

The principal object of this invention is to increase the This cracking t critical cracking thickness of a film deposiited from a single coat of an aqueous polytetrafiuoroethylene coating composition.

Another object of this invention is to provide modified aqueous polytetraliuoroethyleue coating compositions which yield in one coat crack-free films substantially.

thicker than those obtainable with polytctrafiuoroethylene alone in one coat.

A further object is to provide liquid modified polytetrafiuoroethylene coating compositions which do not crack when applied at dry film thicknesses above the order of l to 2.5 mils, this thickness being representative of dry film thickness at which films of polytetrafluoroethylene alone can be expected to crack when applied in a single coat.

A still further object is to provide a liquid modified poyltctralluoroethylcnc coating composition which can be applied to a substrate in one coat to yield dry crack free protective coating which is substantially thicker than the thickest dry crack-free film capable of being produced by a particular aqueous polytctrafluorocthylene suspension, the critical cracking thickness" of which is to be improved.

A still further object is to provide a substrate with thick crack-free modified polytetrafluoroethylene coatings.

A still further object is to provide a relatively thick crack-free unsupported film of modified polytetrafiuoroethylene.

A still further object is the provision of a polytetrafiuoroethylene coating composition with improved adhesion to metal.

These objects are accomplished by incorporating into an aqueous dispersion of colloidal polytetralluoroethylene a substantial amount of an aqueous solution of an alkali metal silicate and applying the resulting composition to a substrate.

Any aqueous solution of an alkali metal silicate may be used to blend with the polytetrafluoroethylene aqueous dispersion. The molar ratio of the alkali metal oxide to SiOz in the silicate solutions may vary over a wide range, c. g. from lzl to 1:4 or higher as long as the silicates are water soluble. Silicate solutions having such widely varying ratios are commercially available.

'l'hc term critical cracking thickness-- as used throughout the specification and claims means the dry film thickness above which cracks develop in a film deposited from one coat of an aqueous dispersion of polytetralluoroethylone. Throughout the specification and claims the terms dispersionf suspensoid" and suspension are used synonymously to denote a composition which contains discrete particles distributed uniformly throughout a liquid medium. The term crack means a linear rupture or fissure which extends from the outer surface of a film partially or completely through the film to the substrate beneath, such fissure being at least large enough to be visible to the naked eye or when magnified one hundred times under good illumination. The terms crack-free" and free of cracks" mean that the film in question contains an average ot not more than one of the above defined cracks for each 100 sq. centimeters of film surface area.

A method of determining the critical cracking thickness of a polytctrafluoroethylene suspcnsoid consists of (1) pouring an aqueous suspensoid of polytetrafiuoroethylene in a container having 11 slightly tiltcd bottom in an amount that when the water is evaporated the thickness of the dry film at one end of the pan will be about .l mil thick and about if) mils thick at the opposite end, (2) drying the film at about lflt)" C. until substantially all of the water has evaporated, (3) baking or fusing at the required temperature and (-l) examining the coating by the naked eye, or under magnification if necessary, to determine the greatest thickness at which no cracks are present.

in a copending application, Serial No. 24l.l.70, filed August 9. i951, now U. S. Patent 2.681.324, there is disclosed a method of improving the critical cracking thickness of aqueous polytetralluoroethylene compositions by incorporating organic polymeric materials which are film forming and produce crack-free films thicker than the thickest film capable of being produced from an aqueous 1. spensoid of the polytetrafluoroethylene. The invention described in this application differs from the earlier mentioned above in that the modifying material for etrafinoroethylene is an aqueous solution of an tal siiicate. following detailed description is given by way of illustration and not limitation. The parts and percentages are on a weight basis.

EXAMPLE l Per cent by wt. Aqueous polytetrafiuoroethylene suspensoid:

Poiytetrafiuoroethylene 46.0 9} 0 Gctyl phenyl polyglycol ether 2.8

Water 51.2

2 7.69? aqueous solution of sodium silicate 9.0

he 37.6% sodium silicate solution contained 10.6% 7 and 2 .0% SiOz, which is equivalent to a molar ratio (NaQO to SiOzl of l to 2.5.

The above composition was prepared by adding the sodium silicate solution to the polytetrafluoroethylene ensoid with moderate stirring. The above composiwas cast into a film'on a chromium plate at room temperature, dried at 230 F. and fused by heating above the fusion temperature of polytetrafiuoroethylene which corresponds "to 621 F. A crack-free film was formed h was approximately 25 mils thick. The film was rawn to a thickness of 1 mil. The tensile strength .1 sample i"x.001x% registered 2.5 lbs. on at Scott which is equivalent to 10.000 lbs. per sq. inch of cro. -sectional area. The dielectric strength of the drawn film corresponded to 2500 volts per mil.

EXAMPLE ll A crack-tree film, was prepared from a composition similar to that described in Example I by casting a film on a polished chromium plate followed by drying at about 23 F. and then'heating above 62l F. to fuse the polytetrafiuoroethylene. A second coat was cast on the d film attached to the chromium plate. followed by ing and fu ing under the same conditions. The fused was tripped from the chromium plate. It had a appea ance. was free of cracks and had an averthickness of 6.2 mils. he dielectric strength of the fused film as stripped from the chromium plate was 500 volts per mil of'thickness. After cold drawing the film to three times its original length to a thickness of approximately 2 mils the average dielectric strength was increased to 1120 volts per mil.

EXAMPLE in The unstretched fused film of Example ll as stripped fr m the chromium plate was subjected to hot working or rolling by passing between heated (270 F.) even speed calender rolls, twice in one direction and then twice in the opposite direction.

The calendering or rolling operation reduced the average thickness of the film from 6.2 mils to 3.] mils and the average dielectric strength was increased from 500 \Oils per mil to l l90 volts per mil.

EXAMPLE IV standard square-weave glass fabric, identified as Iii) lit)

lib

Owens-Corning Fibcrglas Corporations ECC-llZ and having the following specification:

Mil thickness 3.0 Yarn, size 450% Thread count 40x 39 Ounces per sq. yd 2.09

was given three dip coats of the following dispersion:

Per centby wt.

The coated fabric was dried by passing through a heated tower in which the air temperature was about Z50-300 F. The thickness of the dry coated glass fabric at this stage was approximately 4.7 mils and there were microscopic cracks in the coating. The cracked coating was calendered to close the cracks and then subjected to an air temperature of 700-800 F. to sinter the coating. The coated fabric was further coated by dipping in the sodium silicate-polytetrafluoroethylene composition of Example 1 and then dried and fused by subjecting to an air temperatureof 700-800 F. There were no cracks in the dried sodium silicatepolytetrafiuoroethylene coating. The dry thickness after the second composition was applied corresponded to 6.7 mils.

in this example it is preferred to coat the glass fabric with unmodified polytctrafiuoroethylene before coating with the aqueous sodium silicate containing composition, since the latter has a solvent action on the glass filaments. a

The dry film pickup of the second composition amounted to 2 mils thickness. i. e., approximately 1 mil on each side. In another experiment in which the second composition was the same as the first. i. e.. an unmodified aqueous suspensoid of polytctrailuoroethylcne, the pick-up of the dry film was only .2 mil or .1 mil on each side and after drying there were cracks in the surface of the coating.

The polytetrafiuoroethylene-sodium silicate coated glass fabric sample of this example was divided into four portions designated A, B, C and D. The A sample was not processed further and served as the control. The B sample was given a heat and pressure treatment by passing it four times between heated (270 F.) smooth, even speed. calender rolls under pressure. Sample C was calendered between unheated uneven speed steel rolls operating under pressure, such as used for milling rubber. Sample D was cold worked on both sides by scraping each side with a knife blade While the sample was supported on a hard smooth surface. After the me chanical surface treatment of samples 8, C and D they were rendered more transparent. Each sample was then tested for dielectric strength with the following results:

The above data indicate that hot or cold working of the coating increases the dielectric strength.

The coated glass fabric sample of this example showed no loss in weight after soaking three weeks in water. The coating was not affected in any manner as a result of the three week water soak. This was surprising since the sodium silicate introduced into the coating composition is water sensitive.

5 EXAMPLE V A crackdree unsupported film 4.0 mils thick was pared from the following composition in a single coat:

For cent by wt.

entire surface of each copper and steel. panel, coating both the primed and unprirned surface:

Per cent by wt. Polytetrafluoroethylcne susgacnsoid (some as used Aqueous polytetrafiuoroethylene susponsoldz l Compobmon) Polytotrailuoroethylene d. 46.0 g cm pbenyl poly/glycol Ether 2 8 3.0 sodium salt or a rncnure or long .iain alconols, watp'r A M predominantly lauryl alcohol u 1.9

- I Non-ion c dis-lensing o ent 1.3

17.-* a ueo s solution 0 sodium silicate 35.0 i b r r q u L 10 Toluene 10,9

4 Polyethylene glycol other of an nllryinterl phenol.

line above composition was thoroughly mixed and 1 H 03 a Chmmium g The fil was (Mad at 230 .Eaon successive coat of the above composition was F. and then further heated to at least 62l F. to sinler Cured y heating f" 5 l fli 250 F. The resisthp fig "[1 d m was m' f 1 once of the cured nlms to stripping Irorn the copper and Chmmium plate h was i]k h and Tcndercd steel panels was measured by 21 Scott tensile tester. The

x n I, r. more transparent when the surface was cold worked by rccoldt'd *JMQW m Ldble Tai ie 3 Pounds ltvquirt-d to Pull o1" Strip From Panel I I Ratio of Polytotralluosrtfi thyl-l l l I l t Potass'un i eat l id rrimeruniufl. 01 9 1 sax/10.7 Ion 23.4 l 11 4 25.6 ti6.7, 33.3 mas 31.5 sun/41.2 c 0.s* 1.0 3.5a 3.3} 3.31 2.5 2.8

1 Control no primer.

romping with a knife blade when supported by a hard H] "Ehc examples illustrate blending the aqueous sodium Suriaca or potassium silicate solutions with the polytetrarluoro- EXAMPLE Vi ethylene. It is to be understood that it is within the x fii 3 0 i1 2, was m scope of this invention to use a mixture of s dium sili- A f uxnsup?9nec m i A 5;: cats and potassium silicate solutions for modifying the Band we wnowmg comgmsmon m a critical cracking thickness of the polytctralluoroelhylenc Per cent by wt. aqueous dispersions.

Aqueous polytetrafluoroethylene susponsoid: 1 Additional tests on the coatings and films derivcd from Polytcirallnoroethylene 46-51 90 tho compositions of this invention show that the cracL- Uctyl phcnyl polyglycol ethcr 2.3 i" free coatings also possess ll'c dcsirablo propcrlics of coal- Wgter M 50.7} v lugs dcrivcd from polytctrulluorocthylcno alone, namely,

IZill /Q aqueous solution of potassium SiilCfl'tC 10 good olcclricul propcrlics, chemical and solvent resistonce, corrosion rcsistzmcc, and rcsisiuncc to the adhesion 160 or sticking of extrinsic substunccs :lpplicd thereto. Q P 't n comaned in the preferred examples tho coatings are applied by f? T y gi??? 6 9 Co n milky} dipping the substrate to be coated in tho cornposilion or I a u r t l Z'Lb/O 2 p by casting the composition on the surface by well lznown was thorougnly mixed and cast on a chromium platc. h v v I I 5 "r mm (mm at about F and then further rec niques. in ad 1.1011, the coatings may be upplien healed to at least 621 F. to sintcr or fuse the film. 'luc by rdocwr (1061013 436 migkywmte film was Stripped from the Chmmium other well nnown tccnniqucs in the urt of couting surolztte. it was crack-free. 3.0 mils thick and the diclecn films with llquld -g COIllPCtSlUOnS- Th6 (303151135 {m giy-entrf was 200 l per mil thickness By d may be applied to one or both surfaces of the substrate "5 the film down to 1.5 mils thickness the dielectric i b8 o ted.

was increased lo 1000 volts pcr mil thickness. An important advantage of thc products of this invenre tr-ns arenc of the milk -while film was increased tion is an economic one. The alkali metal silicate costs 1 Q P Y Y I I 14 lli "-3 but a small fraction 01 the cost or polytetralluorocthylcnc EXAMPLE Vii and provides a blending material which does not detract gteel and comer 9511615 X were prepared f from many of the desirable properties or" polyletrniluoro coating by sanding with #240 sandpaper. Each primer ly I I composition illustrated in Table 2 below was sprayed my [bus ll. 15 seen that the problem of cracking of reluon one-half of the prepared surface of a copper and steel lively thick polytctruiluoroeihylcne coatings deposited panel and cured for 3 minutes at 750 F. from aqueous dispersions which heretofore has restricted Table 2 6 Composition of lrlmt-r P I I l t. "ilu roethyltnc l: lotls ium i i l i l A .lo o'mrno" '--r ;.s 3

$216.. if ..i no 1; Mil/Hi7 70. [211 1 71. 1/23.; l3ti.T'll2$.Il (on, i, 5 l {trill/41,1: lolytetrslluurot-thylcnc Sus'pcnwul l l l 1 1 pt- "ant. 2320 1 09.0 i 59.5 3 f" 8 47.2 i 12.8 l 38 27% Potassium SllicutcSolulion... ...(lo t 18.0 I 3L0 i 40.5 i 4 z I 52.8 57.2 G1

The polytetrafluorocthylcnc susut-nsnitl consistctl of long choln alcohols,predominantly luurylulc lolund 37% unti The potassium silicate solution u'us u 27, corresponds to 7.7% K20 and 19.3%, $101 on a. weight bu. s.

coats of the following poiytetrafluoroethylcne suspensoid coating composition were sprayed over the of polyll-Lruiluoroctllylenc, 3C}, of tho sodium still of a znlrrturu oi aqueous solution in which the molar rutio oi K10 to rl'lol wus 1 tolil l, which finishes, has been solved in an u expected way by modiamazes fying the aqueous dispersion of polytetrafiuoroethylertc with another chemically dissimilar material.

In the examples the ratio of dry sodium or potassium silicate to dry polytetrafiuoroethylene varies between about 6 to 94 and 41 to 59. It is to be emphasized that these ratios illustrate the preferred embodiment. In certain applications where film strength is not important. the ratio of silicate to polytetrafiuoroethylene may be as high as 50 to 50, and useful products may be produced with less silicate than the preferred ranges.

The polytetrafiuoroethylene dispersion and/or the mixfare of the silicate solution and polytetrafiuoroethylene dispersion may contain modifying agents, such as pigments. dyes, soluble chemical substances, inhibitors, dispersing agents, and other modifiers well known in the coating composition art, to color, stabilize or otherwise modify the chemical or physical properties of the codispersions or the films derived therefrom, provided any such modifying agent employed is innocuous to the comosition and its components.

The products of this invention are particularly useful in coating heat resistant surfaces. such as. metals, ceramics, glass fabrics, asbestos fabrics, woven wire fabrics and heat treated polyacrylonitrile fabrics. The compositions of this invention adhere to copper better than the aqueous dispersion of polytetrafiuoroethylene alone. The coating compositions of this invention are also useful for coating substrates which have a decomposition temperature below the fusion temperature of polytetrafiuoroethylene. such as, cotton, nylon, rayon, and various synthetic resins, in which case the baking temperature must be below the fusion temperature of the substrate.

Specific uses for the products of this invention include, in addition to coated glass fabrics and unsupported films described in the specific examples, anti-sticking coatings for muffin tins or cookie sheets, electrically insulated coatings for wire, spark plugs, condensers and corrosion istant interior coatings for metal tanks containing corrosive chemical materi l in coating wire and other metal surfaces with the crmipositions of this invention, where the maximum adhesion of the coating to metal is desired, the metal surface may be primed with the polytetrailuoroethylene-chromic acid compositions disclosed in U. S. Patent 2,562,117 or the polytetrailuoroethylene-chromic acid-phosphoric acid compositions disclosed in U. S. Patent 2,562,118.

it is apparent that many widely different embodiments of this invention can be made without departing from the spirit and scope thereof and, therefore, it is not intended to be limited except as defined in the appended claims.

1 claim:

l. A. liquid coating composition comprising an aqueous dispersion of polytetrafluoroethylene and an aqueous soluof at least one water-soluble alkali metal silicate from the group consisting of sodium silicate and potassium silicate, said alkali metal silicate being present in an amount of about 6.0% to 50.0% of the combined weight of polytetrafiuoroethylene and said alkali metal silicate, the molar ratio of alkali metal oxide to silicon dioxide in the alkali metal silicate being at least 1:1.

2. The product of claim 1 in which the alkali metal silicate is sodium silicate.

3. The product of claim 1 in which the alkali metal silicate is potassium silicate.

t. A substrate having a surface coating comprising polytetrafiuoroethylene and at least one water-soluble alkali metal silicate from the group consisting of sodium silicate and potassium silicate, said alkali metal silicate being present in an amount of about 6.0% to 50.0% of the combined weight of polytetratiuoroethylene and said alkali metal silicate, the molar ratio of alkali metal 1 to silicon dioxide in the alkali metal silicate being at lzl.

5. The product of claim 4 in which the glass fabric.

6. The product of claim 4 in which the metal.

7. The product of claim 4 in which the copper.

8. The process of preparing crack-free polytetrafiuoroethylene films which comprises blending an aqueous solution of at least one water-soluble alkali metal silicate l'rom the group consisting of. sodium silicate and potassium silicate with an aqueous dispersion of polytetrafiuoroet'nylene, spreading the resulting blend in the form of a film on a substrate, heating to remove the aqueous medium, further heating above 621 F. to sinter the film, cooling the sintered film and stripping the sintered film from said substrate. said alkali metal silicate being present in an amount of about 6.0% to 50.0% of the combined weight of polytctratluoroethylene and said alkali metal silicate, the molar ratio of alkali metal oxide to silicon dioxide in the alkali metal silicate being at least lzl.

9. The process of claim 8 in which the alkali metal silicate is sodium silicate.

10. The process of claim 8 silicate is potassium silicate.

I l. The process of preparing crack-free polytetrafiuoroethylene coatings on a substrate which comprises blending an aqueous dispersion of polytetrafiuoroethylene with an aqueous solution of at least one water-soluble alkali metal silicate from the group consisting of sodium silicate and potassium silicate, applying said blend to a substrate, heating to remove the aqueous medium, further heating above 621 F. to sinter the coating, and cooling the sintered coating, said alkali metal silicate being present in an amount of about 6.0% to about 50.0% of the combined weight of polytetrafluoroethylene and said alkali metal silicate, the molar ratio of alkali metal oxide to silicon dioxide in the alkali metal silicate being at least 1:1.

12. The process of claim ll in which the alkali metal silicate is sodium silicate.

13. The process of claim ll in which the alkali metal silicate is potassium silicate.

l4. The process of claim ll in which the substrate is glass fabric.

15. The process of claim 11 in which the substrate is metal.

l6. The process of claim ll in which the substrate is copper.

17. A film comprising polytetrafiuoroethylene and uniformly distributed throughout at least one water-soluble alkali metal silicate from the group consisting of sodium silicate and potassium silicate, said alkali metal silicate being present in an amount of about 6.0% to 50.0% of the combined weight of polytetrafiuoroethylene and alkali metal silicate. the molar ratio of alkali metal oxide to silicon dioxide in the alkali metal silicate being at least 1:1.

18. The process of preparing crack-free shaped articles which comprises blending an aqueous dispersion of polytetratluoroethylene with an aqueous solution of at least one water-soluble alkali metal silicate from the group consisting of sodium silicate and potassium silicate, shaping the blend, heating to remove the aqueous, medium, further heating to at least 621 F. to sinter the composition and cooling the sintered composition, said alkali metal silicate being present in an amount of about 6.0% to 50.0% of the combined weight of polytctrafiuoroethylene and alkali metal silicate, the molar ration of alkali metal oxide to silicon dioxide in the alkali metal silicate being at least lzl.

substrate is substrate is substrate is in which the alkali metal Sanders Aug. 29, i950 Skeda Apr. 3, 1952

Claims

4. A SUBSTRATE HAVING A SURFACE COATING COMPRISING POLYETRAFLUOROETHYLENE AND AT LEAST ONE WATER-SOLUBLE ALKALI METAL SILICATE FROM THE GROUP CONSISTING OF SODIUM SILICATE AND POTASSIUM SILICATE, SAID ALKALI METAL SILICATE BEING PRESENT IN AN AMOUNT OF ABOUT 6.0% TO 50.0% OF THE COMBINED WEIGHT OF POLYTETRAFLUOROETHYLENE AND SAID ALKALI METAL SILICATE, THE MOLAR RATIO OF ALKALI METAL OXIDE TO SILICON DIOXIDE IN THE ALKALI METAL SILICATE BEING AT LEAST 1:1.