A METHOD FOR APPLYING A NON-STICK EMBOSSER ROLL COATING

Abstract: This invention provides a method for applying a coating on an embosser roll used for embossing polyolefin film-fibril sheets. The coating comprises a primer layer formed from an aqueous suspension of polytetrafluoroethylene with chromic acid and phosphoric acid and a topcoat layer of polytetrafluoroethylene or perfluorinated polyvinyl ether copolymers of tetrafluoroethylene and poly(propyl vinyl ether).

Description: TITLE A Method for Applying A Non-stick Embosser Roll Coating BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention provides a method for applying a non-stick embosser roll coating on rolls used for embossing polyolefin film-fibril sheets. The coating comprises a primer layer formed from an aqueous suspension of polytetrafluoroethylene with chromic acid and phosphoric acid and at leaεt one topcoat layer of polytetrafluoroethylene or fluoropolymer blend compositions containing copolymers of perfluorinated polyvinyl ether. DESCRIPTION OF THE PRIOR ART The production of nonwoven sheets from continuous networks of film-fibril elements is described in U.S. Patent No. 3,169,899 wherein a solution of polymer iε flash-spun at a temperature above the boiling point of the solvent and at high pressure into a low pressure region, whereupon a three-dimensional network of film-fibrils forms at the spinneret. The network is spread laterally by meanε of a baffle and is then collected in multidirectional, overlapping, and intersecting arrangement on a moving belt. For a soft drapable nonwoven sheet retaining good delamination and abrasion resistance useful in disposable garments etc., the film-fibril sheet iε point bonded. Various methods have been disclosed for point bonding polyethylene film-fibril sheets. For example, sheets of flash-spun polyethylene film-fibril strands of the type disclosed in U.S. Patent 3,169,899 have been bonded (a) over 3 to 25% of the εurface area of the sheet by passage through a loaded nip formed by a heated metal roll having 50 to 1000 hard bosses per square inch which extend from the surface of the roll to a height of at least 1.2 times the thickness of the sheet and a hard back-up roll having a Shore Durometer D hardness of at least 70, as disclosed by Miller, United States Patent 4,152,389 and (b) over 1 to 5% of the area of the sheet by passage of the sheet through in a loaded nip formed by a heated, embossed metal roll having bosses and a soft back-up roll of a 60 to 90 Shore Durometer B hardness, as disclosed by Dempsey and Lee, United States Patent 3,478,141. At higher production speeds, higher temperatures are required for the embosεing rollε because the heat transfer load on the embosser alεo increaεeε. At these higher temperatureε, the likelihood of the molten sheet sticking to the hot rollε increases. In addition, any stoppage in production can cause the embossing roll to be covered with molten polymer, which can adhere to the patterned rollε and can not be completely removed by, for example, bruεhing. Normally, non-εtick coatings are applied to εuch rolls to eliminate thiε adheεion, however, the application of n n-εtick coatingε affect heat tranεfer through the rollε, thus the application muεt be very precise and uniform. Alεo, emboεεing patterns with small points cause additional concerns because the small points provide moεt of the effective contact area between the emboεsing roll and the sheet in the nip. Correct coating thickness is crucial in these areas, however, the coating thickness in these small contact areaε iε difficult to measure accurately. Coating compoεitions of εilicones and epoxieε are disclosed in U.S. Pat. No. 4,252,702. Similar coating compositions but also containing fluorocarbon resins are disclosed in U.S. Patents 4,121,000 and 4,369,279. The prior art coatings could not handle the sustained heat tranεfer or loading without prematurely flaking off and losing the required non-εtick property. Thiε flaking causes the polyolefin sheet to stick to the exposed surface of the roll. The sheet must be manually removed from the area and discarded as waste. The worn coated roller muεt be replaced resulting in costly downtime. There is provided by this invention a process for applying a non-stick coating comprising of a primer layer of polytetrafluoroethylene with chromic acid and phosphoric acid and at least one topcoat layer of polytetrafluoroethylene or fluoropolymer blend compoεitionε containing copolymerε of perfluorinated polyvinyl ether for rolls used for embossing polyolefin film-fibril εheetε. The coating of the present invention provides uniform heat transfer through the roll, optimizes anti-stick properties, allows for increased throughput and extends the roll life over the coatings of the prior art. SUMMARY OF THE INVENTION There iε provided by this invention a method for applying an embosser roll coating composition for use in the manufacture of polyolefin film-fibril sheets comprising the εtepε of: (a) spraying a volume of primer sufficient to yield at least one primer layer of dry film thickness between 5 to 18 micrometers on the roll, said primer layer formed of an aqueous mixture of 10 to 98 weight% polytetrafluoroethylene, 1 to 45 weight% chromic acid and phosphoric acid based on the total weight of the polymer and acid; (b) baking the roll at least 15 minutes at 218 to 260°C; (c) cooling the roll to a temperature below 49°C; (d) spraying a volume of topcoat sufficient to yield at leaεt one topcoat layer on the primer layer of total dry film topcoat thickness between 18 to 61 micrometers, εaid topcoat layer formed of an aqueous mixture of 50 to 100% by weight of total polymer polytetrafluoroethylene and 0 to 50% by weignt. or cotai polymer perfluorinated vinyl ether copolymers of 99.5 to 92% tetrafluoroethylene and 0.5 to 8% poly(propyl vinyl ether) ; (e) baking the roll at least 15 minutes at 371 to 427°C. DETAILED DESCRIPTION OF THE INVENTION The embosser rolls to be coated are of the type generally disclosed in U.S. Patents 4,152,389; 3,478,141; and 4,091,137. The teachings of U.S. Patents 4,152,389; 3,478,141; and 4,091,137 are incorporated herein by reference. The roll to be coated is heat treated at a metal temperature of about 480°C for at least 4 hours to loosen the old coating and to remove any oils or substances which might be preεent on the surface of the roll, or might be contained inside the roll, either of which could contaminate the final coated εurface. During thiε initial heating, the roll iε mechanically rotated in the oven to insure any internal heating oils or liquids are allowed to either flow from the roll or be uniformly spread within the roll, where they will evaporate and decompose during the heating. The εurface iε then grit blaεted to remove the old coating and provide a clean surface for the coating. The roll is grit blasted using vertical, horizontal and diagonal sweeping motions. The grit blasting material muεt be gentle enough to remove the coating without damaging the patterned surface of the embossing roll. The grit and particulates from the coating are removed using compressed air before the primer coat iε applied to the roll. Two different layerε are applied to the roll to yield the non-stick coating of the present invention; (1) at least one primer layer and (2) at leaεt one topcoat layer. There iε a bake cycle after the application of the primer and the topcoat layer. The primer layer iε an aqueous mixture " f 10 to 98 weight! polytetrafluoroethylene, 1 to 45 weight% chromic acid, and 1 to 45 weight% phosphoric acid based on the combined weight of polymer and acid, as disclosed in U.S. Patent No. 2,562,118. The teachings of U.S. 2,562,118 are herein incorporated by reference. The separate liquids of the primer layer, the polymer of tetrafluoroethylene and the mixed acid constituent, are thoroughly mixed together at room temperature by rolling the containers. Shaking creates bubbles which are undesirable. During this rolling procedure the roll to be coated is heated to 29°C surface metal temperature and held for 10 minutes. Due to the patterned nature of the embossing rolls, a true measurement of the coating on the rolls iε difficult, especially on the tips of the patterns. To address thiε concern, a specific volume of primer can be applied to the roll. The primer solution is strained to remove any large particles before application. Conventional compressed air spray equipment iε used to spray the primer layer on the rolls at approximately 40-60 psig. The primer layer iε applied in two pasεeε, the first to apply a thicknesε of approximately 5 micrometers to the roll, and the second to fill in any areas missed by the first pass. Key to the procesε iε the lightneεs of the mist. The volume may be checked after the first pasε to determine if the correct quantity iε being applied to the roll. The volume must be checked after the second pass. The volume of primer should be that which indicates a final primer dry film layer thicknesε of 5 to 18 micrometers. If any other volume remains, the primer should be cleaned off with water and the roll should be lightly grit blasted before re-application of the primer. Correct primer application is essential to the application procesε. Traditional primer application will be very blotchy due to the minimal amount of material actually applied to the roll. The roll witn *cne appneα primer layer is placed in an oven and allowed to bake at 218 to 260°C metal temperature for at least 15 minutes. Uniformity of bake iε also important, with a maximum of 5°C difference from one end of the roll to the other. Thermocouples can be placed on the metal roll at the ends to ensure that the endε reach the required baking temperature. After baking, the roll is allowed to cool below 49°C before applying topcoat. The topcoat layer iε a fluoropolymer blend coating composition containing copolymers of perfluorinated polyvinyl ether. Fluoropolymer blend compositions containing copolymers of perfluorinated polyvinyl ether are described in U.S. Patent No. 4,252,859. U.S. 4,252,859 is incorporated herein by reference. The topcoat layer compriseε at leaεt one layer on the primer layer. The topcoat layer iε formed of an aqueouε mixture of 50 to 100% by weight of total polymer polytetrafluoroethylene and 0 to 50% by weight of total polymer perfluorinated vinyl ether copolymerε of 99.5 to 92% tetrafluoroethylene and 0.5 to 8% poly(propyl vinyl ether) baked on the embosεer roll at least 30 minutes at 371 to 427°C. The topcoat is brought to room temperature and is stirred gently by rolling the container. If foaming occurs, the rolling has been too severe and the material should not be used until the foam haε diεappeared. The topcoat is applied at a greater thickneεε than the primer, and the volume approach iε again uεed to control the thickneεs of the of the topcoat. The volume of topcoat coating is measured and εtrained through 200 by 200 micron openingε. A light εpray is set on the compresεed air paint gun, using 40 to 45 pεig εpray pressure. The roll is sprayed with at leaεt one layer εo that a final total dry film topcoat thickness of 18 to 61 micrometerε is reached. After a layer is applied, the volume of liquid remaining is again checked to confirm that the correct volume of coating has been applied to the roll. If more remains, additional layers of coating can be applied if uniformity of coating on the roll iε adequate. If too εmall an amount remainε to apply an additional layer, the roll muεt be grit blasted and the entire coating procedure repeated. Immediately after coating, while the roll is εtill wet, the roll εhould be placed in the oven for final curing. The roll is baked in the oven for 15 minutes at 399°C metal temperature. Minimum bake muεt be at least 30 minutes at 371°C; maximum bake must not exceed 427°C and should not exceed 371°C for more than 3 hours. The temperature at both ends of the roll should be within 5°C. Final inspection of the roll iε made uεing an inεtrument such aε a microscope to detect the coating thicknesε. Flat areas can be attached to the ends of the rolls and/or lands can be left on the edge of the roll surface to allow a end-to-end confirmation of correct coating thickneεε. Viεual inspection of the entire roll iε made to confirm correct coating application has been made . If desired, modifying agents, such as pigments, fillers, organic εolventε, and other film forming materials may be added to the material forming either layer provided they are compatible with and stable in the presence of the other ingredients. EXAMPLE An embosser roll coated in accordance with this invention had a coating comprising: a) a primer layer of dry film thickneεε between 5 to 18 micrometers formed of an aqueous suspension of 42.4 weight% polytetrafluoroethylene, 4.2 weight% chromic acid 3.6 weight! phosphoric acid and approximately 50 weight% water, baked on the embosser roll at 232°C for 15 minutes; b) a topcoat layer of dry film thickneεε between 18 to 61 micrometers formed of an aqueous mixture of 94.4 weight% of total polymer polytetrafluoro-ethylene and 15.6 weight% perfluorinated vinyl ether copolymers of 96% polytetroflouroethylene and 4% poly(propyl vinyl ether) baked on the embosser roll at (750°F) 399°C for 15 minuteε. The aqueous suspenεion was approximately 50 weight% water, 1 weight% pigment, 46 weight% polymer and 3% miscellaneous ingredients. A useful life for embosεing 5.0 million kilogramε of polyethylene film-fibril sheet has been demonstrated.

Claims: CLAIMS 1. A method for applying an emboεεer roll coating composition for uεe in the manufacture of polyolefin film-fibril sheetε compriεing the stepε of: (a) spraying a volume of primer sufficient to yield at least one primer layer of total dry film thickness between 5 to 18 micrometers on the roll, said primer layer formed of an aqueous mixture of 10 to 98 weight% polytetrafluoroethylene, 1 to 45 weight% chromic acid and 1 to 45% weight% phosphoric acid based on the total weight of the polymer and acid; (b) baking the roll at least 15 minutes at 218 to 260°C; (c) cooling the roll to a temperature below 49°C; (d) spraying a volume of topcoat sufficient to yield at least one topcoat layer on the primer layer of total dry film topcoat thickness between 18 to 61 micrometers, said topcoat layer formed of an aqueous mixture of 50 to 100% by weight of total polymer polytetrafluoroethylene and 0 to 50% by weight of total polymer perfluorinated vinyl ether copolymers of 99.5 to 92% tetrafluoroethylene and 0.5 to 8% poly(propyl vinyl ether); (e) baking the roll at least 15 minutes at 371 to 427°C. '

Inventors: MCHALE WILLIAM F, MONTGOMERY CATHERINE CAMPBELL, OBAL WALTER DOUGLAS, MCHALE, WILLIAM, F., MONTGOMERY, CATHERINE, CAMPBELL, OBAL, WALTER, DOUGLAS
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1989
Main classification: B05D7
IPC codes: B05D1, B05D3, B05D5, B05D7, B21B31