Selective curing of multiple layers of polyarylene sulfide coatings
United States Patent [191 Edmonds, Jr. et al.  
 [ 4] SELECTIVE CURING 0F MULTIPLE LAYERS 0F POLYARYLENE SULFIDE COATINGS [75] Inventors: James T. Edmonds, Jr.; Jennings P. Blackwell, both of Bartlesville, Okla.  
 [73] Assignee: Phillips Petroleum Company,  
 Bartlesville, Okla.  
 [22] Filed: Sept. 26, 1973 [21] App]. No.: 400,906  
 [52] US. Cl. 427/379; 260/79.1; 260/900 [51] Int. Cl B44d l/14; C08g 43/02 [58] Field of Search 260/79.l, 900; 117/72, 1l7/75, 132 C, 132 CF, 123 D, 161 U Z, 161  
 [56] References Cited UNITED STATES PATENTS 3,487,454 l2/l969 Oates ct al 260/79.l X  
 [451 Apr. 15, 1975 -1/l970 Ray 426/523 ll/l97l Tieszen et al 260/79.l X  
 Primary Examiner-Ralph Husack [57] ABSTRACT 10 Claims, No Drawings SELECTIVE CURING OF MULTIPLE LAYERS OF POLYARYLENE SULFIDE COATINGS BACKGROUND OF THE INVENTION This invention relates to the application of coatings of arylene sulfide polymer to a substrate.  
  Arylene sulfide polymers are known for their desirable characteristics as coating compositions. It is also broadly known that these coatings like other coatings can be applied in multiple layers as shown for instance in Ray US. Pat. No. 3,492,125. It is also broadly known to incorporate a fluorocarbon polymer with an arylene sulfide polymer as shown in Oates et al., US. Pat. No. 3,487,454. However efforts to produce coatings using multiple layers of arylene sulfide polymer containing a fluorocarbon polymer have been found to result in a poor quality final product.  
 BRIEF SUMMARY OF THE INVENTION It is an object of this invention to provide a method of applying arylene sulfide coating compositions containing fluorocarbon polymer in multiple layers; and it is yet a further object of this invention to provide an im proved arylene sulfide polymer-coated substrate.  
  In accordance with this invention at least one base coat of arylene sulfide polymer containing a fluorocarbon polymer is cured under a time and temperature relationship such as to give incomplete curing; thereafter a top coat is applied and the composite cured under normal curing conditions to give a relatively complete cure.  
 DESCRIPTION OF THE PREFERRED EMBODIMENTS The instant invention is applicable to the preparation of multiple layer coatings of arylene sulfide polymer containing a relatively low molecular weight polytetrafluoroethylene. By relatively low molecular weight polymers is meant polymers which exhibit at least some flow in the ASTM D 1238 melt flow test&#39;at 380C using a 5000 g weight. Preferred polymers have a number average molecular weight of less than about 10 million. Polymers exhibiting a melt viscosity of l X 10 to l X 10 poises are particularly suitable.  
  A determination of melt flow of PTFE is made according to ASTM-1238, Condition E, modified to operate at 380C. The amount of polymer extruded in one minute is determined using a 5,000 g weight on the plunger.  
  The melt viscosity is related to melt flow by the following formula:  
 melt viscosity, 1 (K X wt. applied (in grams)/melt flow (g/minute)) K 10.63 and is a factor which has to be determined for each polymer. This value is used by FDA in its test to determine the suitability of PTFE in food coating acceptability.  
  No determination of melt viscosity can be made if, under the conditions of the melt flow test, no flow occurs, i.e., the melt flow is essentially 0.  
  Exemplary low molecular weight fluorocarbon materials include polytetrafluoroethylene sold under the trademark TL-126 by the LNP Corporation, having a melt viscosity of 4 X poises as calculated from meltflow determinations by said ASTM D 1238 test, and.  
 polytetrafluoroethylene sold under the trademark Whitcon (WC-5 Whitcon (WC-6), and Whitcon (WC-8) sold by Pennwalt Corporation having melt viscosities of 3.9 X 10 5.1 X 10&#39;, and 7.4 X 10 poises respectively as calculated in accordance with said ASTM test and formula given above. While high molecular weight fluorocarbon polymer such as Teflon T-S polymer can be present it is possible to obtain smooth coatings w ::h such high molecular weight fluorocarbon polymer without using the time and temperature relationships of the invention to effect the cure. Teflon T-5 polymer has a number average molecular weight of 9.55 X 10 Teflon dispersion 30-B polymer has a number average molecular weight of 2.63 X 10 and sometimes gives improved results using the invention while at other times a satisfactory coating can be obtained without the steps of the invention. The Teflon T-5 and 30-Bpolymers have no flow under the ASTM D 1238 test. These molecular weights are determined using the equation log M (2.612 S.S.G./0.058)  
 as set out by C. A. Sterati and H. W. Stackwether Advances in Polymer Science Vol. II 465 (1960-61 S.S.G. stands for Standard Specific Gravity.  
  The invention is applicable to the production of coatings of any normally solid, heat curable high molecular weight arylene sulfide polymer. The preferred arylene sulfide polymer is poly(phenylene sulfide) (PPS) having a melting or softening point of at least 300F, preferably 400 to 850F. Such polymer can be made in accordance with Edmonds et al., US. Pat. No. 3,354,129, the disclosure of which is hereby incorporated by reference.  
  The composition can contain other conventional additives such as stabilizers, and the like. The presence of pigments such as titanium dioxide or black iron oxide is preferred.  
  The initial coating is cured under a time and temperature relationship such as to give substantial undercure. The curing temperature is within the range of 225 to 610F for a time in minutes (I) no more than that represented by the formula I (-T/20) 62 wherein T is an integer corresponding to said temperature in F. The minimum temperature applicable is 225F because it is necessary to drive off water. The maximum temperature is 610F because at higher temperatures complete curing can occur. Preferably the temperature will be within the range of 250 to 600F. The maximum time thus can vary from a matter of about 32 minutes at 610F to approximately 51 minutes at 225F. Preferably the time in minutes (I) employed will be at least an amount represented by the formula I T/60 16 wherein T is said integer corresponding to said temperature. The first cure can quite advantageously be conducted at a temperature just below the melting point of the polymer. Times of 10 to 30 minutes at a temperature of 500 to 600F are particularly suitable. After application of a top coat the thus coated substrate is cured in a normal manner at a temperature within the range of 650 to 900F for a time in minutes (2) greater than that represented by the formula t (2T/25) +77 wherein T&#39; is an integer corresponding to the temperature of the second cure in F, preferably a temperature in the range of 700 to 800F for a time of at least 30, preferably 30 to 60 minutes. Again, obviously the coatings can be cured at the higher temperature with the shorter times.  
  The thickness of each coating can vary from 0.25 to 25 mils with the total thickness generally being in the range of 0.5 to 50 mils or more.  
 The coatings can be applied in the form ofa slurry in a liquid such as water or glycol or can be sprayed on in the form of a dry powder. A substrate can be hot enough on initial contact with the polymer to melt same or a coldsubstrate can be used with the polymer being melted during the cure.  
  The substrate may be any metal, glass, ceramic or plastic material which will form an adhesive bond with the arylene sulfide polymer coating formulation and which retains its rigidity and shape and does not decompose or deteriorate at the curing temperature for the required time.  
  The coated articles made in accordance with this invention can be used in any application where protective coating is required. They are of particular utility in the production of coated cookware.  
  Typical slurry compositions contain by weight: 100 parts arylene sulfide polymer: 5-60, preferably -40, more preferably lO-l 5, parts by weight of a fluorocarbon polymer; and preferably lO-50 parts by weight of pigment-filler such as titanium dioxide (TiO iron oxide (Fe=,O carbon black or a combination of these. The slurry is preferably an aqueous dispersion containing 0.5 2 wt percent of a non-ionic surfactant, such as octylphenoxypolyethoxyethanol. Other liquids may be used such as glycol, water-glycol, light hydrocarbons, such as pentane. hexane, benzene, toluene, or mixtures of these. Solids contents may vary from -60 percent by weight depending on the method of applying the coating such as by brushing, dipping, spraying, etc. For spraying the preferred solids contents is between -35 by weight percent.  
 EXAMPLE The following 25 runs were made by preparing the slurries as follows. The materials used which are not identified hereinabove were as follows.  
 Poly(phenylene sulfide), Ryton PPS, Grade V-l, virgin polymer, melt flow 2,000, ASTM Dl238-65T, Condition L, modified to operate at 343C TiO DuPont R-lOl Fe O,, Pfizer Black Oxide, BK-5099 Carbon Black (C), Channel Black, Monarch 74,  
 Cabot Corp.  
  Octylphenoxypolyethoxyethanol, Triton nonionic wetting agent, Rohm and Haas The fluorocarbon polymers used in preparing the coatings in accordance with the invention were of fine particle size, specifically 200 mesh (75 microns) or less. The WC-S, WC-6, and WC-8 polymers had a particle size of less than 1 micron. The TL-126 polymer had a particle size of 8 microns. The T-5 polymer had a particle size of 325 microns and finally the -B polymer had a particle size of less than 0.5 micron average particle size diameter.  
  In the preparation of the slurries containing low molecular weight polytetrafluoroethylene (products sold under the trademarks WC-5, WC-6, WC-8 and TL- l26), water and wetting agent were blended together in a Waring blender and then the dry ingredients, polyphenylene sulfide (PPS), pigment, and polytetrafluoroethylene (PTFE) were added and blended for about 5 minutes. The slurry was then poured into a ball mill 30 jar half full of one-half inch burundum grinding rods until the rods were just covered, and the slurry was then ball milled overnight or longer.  
  In the preparation of the slurries containing high molecular weight molding grade PTFE a wet milling pro- 25 cedure was used wherein a blend of the molding grade PTFE (a product by DuPont sold under the trademark Teflon T-5) in water and wetting agent was ball milled for 24 hours or longer. The PPS and pigment were then added and ball milling continued overnight or longer.  
 30 To incorporate the dispersion grade fluorocarbon added to the Teflon 30-B polymer dispersion with gentle stirring. The weights of the PPS slurry and Teflon 30-B polymer dispersion were calculated to give the desired proportion of ingredients in the final slurry. Ball milling, or rapid stirring of mixtures with Teflon 30-B polymer caused coagulation of the polytetrafluoroethylene dispersion and thus had to be avoided.  
  A Binks model 18 spray gun with siphon feed was used for coating. Coatings were tested by spraying aluminum test coupons 3X6 inches and curing in an air vented oven at the desired temperature.  
  Coatings were evaluated by feel, visual observation, and appearance under 20X magnification.  
 TABLE I DOUBLE COATS WITH PPS/PIGMENT/PTFE SLURRIES PRIMER COAT TREAT- TOP COAT POLYTET- MENT POLYTET- AP- AD- RA- RA- FLUORO- TRITON OF FLUORO- TRITON PEAR- HES- RUN PIGMENT ETHYLENE X- I00 CURE PRIMER PIGMENT ETHYLENE X-l00 CURE ANCE ION No. (a) (a) (h) (min/F) COAT (a) (a) (b) (min/F) (c) (d) I 33 TiO l0 TL- l 26 1 30/700 None 33 TiO. I0 TL-I&#39;lb I 30/700 Poor Good 2 33 TiO: I0 TL-Ilo l 30/500 do. 33 TiO l0 TL-I26 I 30/700 Even Good 3 33 TiO- I0 TL-I 26 l 30/700 Rinsed 33 TiO I0 TL-I26 1 30/700 Poor Good with IV! X-I00 4 33 TiO l0 TL-l 26 l 30/500 Rinscd 33 TiO 10 TL-] 26 I 30/700 Even Good with I7! X-l00 5 33 TiO I0 TL-l26 l 30/700 Grit- 33 TiO. l0 TL-l26 I 30/700 Poor Good blasted (1 33 Fe O l0 TL-lZb I 30/500 Rinsed 33 Fe O l0 TL-l26 l 30/700 Even Good with 1% X-I00 7 33 Fe O, I0 TL-I26 l 30/700 Rinsed 33 F13 0 I0 TL-l26 I 30/700 Uneven Good with I7r TABLE I Continued DOL&#39;BLE COATS WITH PPS/PIGMENT/PTFE SLURRIES PRIMER COAT TREAT- TOP COAT POLYTET- I MENT POLYTET- AP- AD- RA- RA FLUORO- TRITON OF FLUORO- TRITON PEAR- HES- RUN PIGMENT ETHYLENE X-l CL&#39;RE PRIMER PIGMENT ETHYLENE X-l00 CURE ANCE ION No. (a) (a) (h) (min/F) COAT (a) (a) (h) (min/F) (c) (d) S 33 TiO- l0 WC- I 30/500 33 TiO: l0 WC-5 1 30/700 Even Good 9 33 Ti(). W\&#39;-5 1 30/700 33 TIO I0 WC-5 1 30/700 Poor Fair l0 Fe;,(). 15 WC-S I 30/500 I5 Fe;,() 15 W05 1 30/700 Even Good 10a 1 Fe O, l5 WC-S I 30/6()() I5 mo, 15 WC-S I 30/725 Even Good I I 3 TiO- I0 \NC-o 1 30/500 33 T10 10 WC-fi I 30/700 Even Good 12 3 Ti() 10 WC-(i I 30/700 33 TIO l0 WG-6 I 30/700 Poor Four I3 R 0 10 WC-(i I /500 4 20 Fe. ,O 20 WG-6 I 30/700 Even Good 14 33 Ti(). 10 WC-X I 30/500 33 TiO l0 WC-Ii I 30/700 Even Good 15 33 TiO l0 WC-X I 30/700 33 TiO l0 WC-X I 30/700 Poor Poor lb 1.25 Ti(). 12.5 WCX I 30/500 12.5 TiO 12.5 WC-li I 30/700 Even Good 17 12.5 TiO. 12.5 WC-S I 30/700 12.5 TiO: 11.5 WC-li I 30/700 Poor Poor 11 15 R3 0 l5 WC-X I 30/500 15.0 Fe o l WC-li I 30/700 Even Good I) 33 F12 0, 10 T-. I 30/700 33 Pia o. 10 T-S I 30/700 Even Good 20 TiO. l0 30-B 60/600 33 TiO- l0 30-B 60/650 Rough Poor 2] TiO- [0 30-B 30/500 33 TiO 10 30B 60/650 Even Good 32 -10 TIO, 50 30-B 2 30/250 TiO- 50 30-H 2 30/700 Even Fair 10C 10C 23 33 TiO, l0 30-B I 10/500 33 TiO- I0 3043 I 30/700 Even Good 24 40 TiO- 50 30-B 2 30/700 40 TiO 50 30-B 1 30/700 Even Poor (a) Paris by weight per 100 parts by weight PPS. (h) &#39;4 .\1-|00 in water used in making slurry.  
 (cl Even Top cont completely covers primer: L&#39;ncven Incomplete coverage or spreading by top coat; Poor Beading of top coat.  
 (d) Ruled by using scalpel \lndcr 20X magnification to try to pick off top coat.  
  Runs 1-7 were made with a single grade PTFE, all containing 10 parts by wt. PTFETL-l26, melt viscosity 4 X 10 poise. Two kinds of fillers were used, Ti0 and Fe O and the first coat was treated as indicated. At a first coat curing temperature of 700F for 30 minutes, the coating appearance was always poor. while if the first coat was cured at 500F, both appearance and adhesion were good. In runs 824, no treatment of the primer coat was made since it had been shown to be ineffective in improving the appearance or adhesion of the second coat. Runs 8-9 are comparable, again showing differences due to curing temperature difference. Run 10 is included to show that other levels of fillers and PTFE can be used. The rest of the table follows the same pattern. Run 19 was made with a molding grade PTFE T-5 which exhibits no flow when melt viscosity is measured. This resin is outside the scope of the invention because the special cure temperature did not appear to be required for this type of material. A comparison of runs 20 and 21 shows the advantage for the invention as opposed to curing the first coat at 600F for 60 minutes although as can be seen by comparing these runs with run 24, it is possible to achieve good results without the invention with this relatively high molecular weight fluorocarbon polymer. Run 22 shows that as the amount of fluorocarbon polymer reaches 50 parts per hundred PPS the adhesion is adversely affected.  
  While this invention has been described in detail for the purpose of illustration, it is not to be construed as limited thereby but is intended to cover all changes and modifications within the spirit and scope thereof.  
 What is claimed is:  
  l. A method for coating a substrate with an arylene sulfide polymer composition comprising: applying at least one base coat of said arylene sulfide polymer containing a fluorocarbon polymer of sufficiently low molecular weight to exhibit some flow under ASTM D 1238 melt flow test at 380C with a 5000 g weight, and curing said thus formed base coat at a first elevated temperature within the range of 225 to 610F for a time, tin minutes, no more than an amount represented by the equation t= (T/20) 62 wherein T is an integer corresponding to said first elevated temperature in F, so as to give an incomplete cure; and thereafter applying a top coat of arylene sulfide polymer containing a fluorocarbon polymer and subjecting the thus-coated substrate to a second elevated temperature within the range of 650 to 900F for a time in minutes, I, greater than that represented by the formula I (2T&#39;l25) 77 wherein T is an integer corresponding to said second elevated temperature in F, to give a relatively complete cure.  
  2. A method according to claim 1 wherein said first elevated temperature is within the range of 250 to 600F.  
  3. A method according to claim 1 wherein said first elevated temperature is below the melting point of said arylene sulfide polymer.  
  4. A method according to claim 1 wherein said fluorocarbon polymer is polytetrafluoroethylene having a melt viscosity within the range of l X 10 and 1 X 10 poise as calculated from flow obtained by said ASTM test.  
  5. A method according to claim 1 wherein said arylene sulfide polymer is polyphenylene sulfide.  
  6. A method according to claim 1 wherein said substrate is one of metal or ceramic.  
  7. A method according to claim 1 wherein said fluo rocarbon polymer in each of said coats is present in an amount within the range of 10 to 40 weight per cent based on the weight of said arylene sulfide polymer in each coat. I  
  8. A method according to claim 1 wherein said time, t in minutes, is at least an amount represented by the equation t= (T/60) 16 where T is said integer corresponding to said first elevated temperature.  
  9. A method according to claim 1 wherein each of said coats is applied in the form of an aqueous slurry containing a non&#39;ionic surfactant and wherein said fluorocarbon polymer is present in each of said slurries in an amount within the range of 10 to parts by weight&#39; based on 100 parts by weight of said arylene sulfide polymer in that slurry and wherein each of said slurries comprises in addition 10-50 parts by weight of a pig ment based on 100 parts by weight of said arylene sulfide polymer in that slurry.  
 utes.