Patent Application
20050209395
The present invention relates to a (per)fluorinated substance for the metal surface coating, capable to form low thickness films, having the following combination of properties:
Specifically the invention relates to PTFE or modified PTFE polymers which are used, under the form of their aqueous dispersions or latexes, to obtain films having the combination of the above properties and thicknesses from 0.3 to 10 micron, preferably from 0.5 to 5 micron, still more preferably from 1 to 3 micron.
It is known in the prior art that coatings of fluoropolymers, in particular PTFE, obtained from the corresponding aqueous dispersions or latexes, are used due to their low friction coefficient to obtain surfaces having self-lubricating, antiadhesive and self-cleaning properties, capable to operate at high temperatures having high thermal stability and low flammability. Said coatings are utilized in various application, such for example kitchen utensiles, the industrial antiadhesion and the moving mechanical parts, etc. However fluoropolymer-based coatings have the drawback to have a low adhesion towards steel and metal surfaces in general.
In the prior art to overcome said drawback, the fluoropolymer film adhesion to metal surfaces is obtained by mixing a fluoropolymer aqueous dispersion, for example PTFE, with hydrogenated resins capable to form films, for example polyamidic acid salts. In this way the composition applied on the support stratifies, and the hydrogenated resin concentrates at the interphase with the support to give adhesion, while the fluoropolymer remains on the surface, giving to the obtained coating the antiadhesion properties. To obtain good results with said kind of composition the metal support must be pretreated for example by sandblast and/or etching. Said process has the drawback to need a preliminary step to the film formation phase. Besides, the above pretreatments cannot be used on precision mechanical parts, as gears, print rolls, etc., since for said mechanical parts the finishing and shape must not be modified.
To overcome said drawback in the patent application WO 96/13,556 a primer is used, having a low content of a perfluorinated polymer, which can be applied also on untreated substrata. The used primer is formed by a mixture of PTFE, polyamidic acid salts and silicates or metal oxides. According to said patent application to obtain coatings having antiadhesion properties various steps are required, applying successive fluoropolymer layers. Said repeated steps result disadvantageous from the industrial point of view.
The need was therefore felt to have available (per)fluorinated polymers, in particular based on PTFE, for the coating of metal surfaces, capable to form films having a low thickness from 0.3 to 10 micron, preferably from 0.5 to 5 micron, still more preferably from 1 to 3 micron, with the following combination of properties:
It has been surprisingly and unexpectedly found by the Applicant that particular PTFE dispersions, formed by surfactants and organic or inorganic compounds belonging to the sequestrant class, can be directly applied on metal surfaces without pre-treating the support by sandblast or etching, or mixing the fluoropolymer dispersion with hydrogenated resins.
An object of the present invention is a composition based on PTFE, homopolymer or modified, comprising for 100 parts of the component 1) PTFE:
The homopolimer or modified PTFE latex, containing particles with a diameter from 5 to 100 nm, can be obtained by radical tetrafluoroethylene polymerization in the presence of a microemulsion as described in EP 969,027 in the name of the Applicant, herein incorporated by reference. The microemulsions used in polymerization are described in U.S. Pat. No. 4,864,006 and U.S. Pat. No. 4,990,283.
The preferred nanoemulsions of the component 1) PTFE for the compositions according to the present invention contain particles having an average diameter from 10 to 80 nm, and are selected from the following classes:
The comonomers which can be used to prepare the modified PTFE of the aqueous dispersions component 1), are both of hydrogenated and fluorinated type. Among the hydrogenated comonomers it can be mentioned: ethylene, propylene, acrylic monomers, for example methyl(meth)acrylate, (meth)acrylic acid, butylacrylate, hydroxyethylhexylacrylate, styrene monomers, such for example styrene.
Among the fluorinated comonomers it can be mentioned:
Fluorinated comonomers are preferred, preferably those which do not compromise the thermal PTFE stability, in particular perfluoromethoxydioxole (MDO), perflubropropylvinylether (PPVE), perfluoromethylvinylether (PMVE), perfluoroethylvinylether (PEVE) and perfluoropropene (PFP).
The surfactants component 2) can be anionic, cationic, non ionic and can be hydrogenated or fluorinated. Mixtures of surfactants, both ionic and nonionic, hydrogenated and fluorinated can be used.
Among hydrogenated surfactants those non ionic can be mentioned obtained by reaction of ethylenoxide (EO), which represents the hydrophilic part, with compounds capable to give to the structure an hydrophobic character, such for example the following:
The compound obtained from the above reaction contains a polyoxyethylene chain having from 8 to 24 EO units.
Among non ionic hydrogenated surfactants those having the following structure are preferred:
Among anionic hydrogenated surfactants those having the following formula can for example be mentioned:
RIA—MAXA
wherein:
Among cationic hydrogenated surfactants, the tetraalkylammonium salts can be mentioned wherein at least an alkyl is C8-C24 and the other substituents are hydrogen or C1-C5, linear or branched, alkyl.
Among anionic fluorinated surfactants those selected from the following classes can be mentioned:.
T—O—Rf—CFX—COOM (IA)
wherein:
Among the formula (IA) compounds, as anionic surfactants, those having the following formula
T—O—(C3F6O)m(CF2O)n—CF2—COOM
wherein T, M, m and n are as above, are preferred.
The non-ionic fluorinated surfactants are preferably selected from the following:
CF3(CF2)y—L—Rh (IB)
wherein y is an integer from 3 to 20, L and Rh are as defined below;
T—O—Rf—L—Rh (IIB).
wherein:
Preferably the non ionic fluorinated surfactants are selected from the following:
The organic or inorganic sequestrants (complexants) component 3) usable in the invention are capable to give bi- or poly-coordination bands and form with metal ions soluble complexes and prevent from forming insoluble salts of said metals. Preferably the sequestrants have a functionality of acid type, for example of carboxylic or phosphonic type, in a number higher than or equal to two, preferably two or four. A test to verify if component 3) complies with the present invention conditions is the following: to an aqueous solution at pH 10 containing a Ca2+ soluble salt, preferably calcium nitrate, in an amount equal to 360 mg (as Ca2+), 1 g of the sequestrant is added, and subsequently a water-soluble salt of an anion, preferably carbonate, which precipitates the calcium as insoluble salt. When an insoluble salt forms, component 3) is not suitable to be used in the present invention.
When the sequestrants are of inorganic type, they are for example selected from polyphosphates,. for example sodium hexamethaphosphate Na6P6O18, potassium pyrophosphate K4P2O7, sodium tripolyphosphate Na5P3O10.
When the sequestrants are of organic type, they are for example amino-polycarboxylic acids such as EDTA, polyhydroxymonocarboxylic acids such as gluconic acid and phosphonates.
Among organic sequestrants the following are preferred:
The optional component 4) is well known in the prior art and belongs to the class of the compounds used to lower the friction coefficient. Preferably it is selected from the following: MoS2, WS2 and graphite.
The present invention compositions can optionally be further diluted with water, and added with adhesion promoters, for example colloidal silica, rheological modifiers to regulate the viscosity, for example water-soluble polymers.
Before applying the invention compositions, the metal or steel substratum to be treated can optionally be pretreated with functionalized perfluoropolyethers. Preferably the perfluoropolyethers are mono or bifunctional. Still more preferably the end groups are ionic and of phosphate type. It has been found by the Applicant that the thicknesses of said functionalized perfluoropolyethers on the metal surface or steel can also be very thin, even at a monomolecular level.
The preferred mono- or diphosphate perfluoropolyethers have the following formula:
RBf—[CF2CH2—O—LB—P(O)(OZ1)(OZ2)]1 (I)
wherein 1=1 or 2;
When RBf is monofunctional (1=1), one end group is of the perfluoroalkyl type such for example CF3O, C2F5O, C3F7O; optionally in the perfluoroalkyl end groups one fluorine atom can be substituted by one chlorine or hydrogen atom; examples of said fluoroalkyl end groups are ClC3F6O, HC3F6O;
when RBf is of the bifunctional (per)fluoropolyether type and it has preferably one of the following structures:
The preferred (per)fluoropolyether chain RBf is selected from the following structures:
The preferred compounds of formula (I) are those wherein LB=(CH2—CH2O)n″ with n″ integer from 1 to 3; Z1 equal to or different from Z2 is selected from H, NH4, or an alkaline metal cation; 1=2.
The (per)fluoropolyethers of general formula (I) are obtainable by the well known processes of the prior art, see for example the following patents, herein incorporated by reference: U.S. Pat. No. 3,665,041, U.S. Pat. No. 2,242,218, U.S. Pat. No. 3,715,378, EP 239,123, EP 1,145,722.
The invention compositions are applied on metals, steels, with conventional methods such as casting, spraying, glazing, spin coating, deep coating also without pretreating the metal surface. For example by using the spin coating, a constant spinning rate in the range 2,000-5,000 rpm is preferably used; the time is generally comprised between 30 seconds and 5 minutes.
After the application on the metal surface the obtained film is sintered at a temperature higher than 320° C., preferably in the range 390° C.-410° C.; subsequently the sintered film is cooled by air exposure.
As said, the present invention compositions show the excellent combination of the above properties.
The films obtained by applying to metal substrata the compositions according to the present invention show an excellent adhesion to the support. This is surprising and unexpected on the basis of the prior art since tests carried out by the Applicant on compositions having the same composition as those of the present invention, but using a latex wherein the PTFE particles have sizes higher than those of the present invention component 1), give films which substantially do not adhere to metal supports. See the Examples.
The following aspects of the compositions according to the present invention result, on the basis of the prior art, quite unexpected:
The following Examples are given for illustrative purposes, but they are not limitative of the scope of the invention.
Characterization:
Particle Diameter
The particle diameter is measured by a laser light scattering instrument, in particular Photon Correlation Spectroscopy, equipped with Brookhaven correlator model 2030 AT and with an argon laser light source at a wave length of 514.5 nm by Spectra-Physics. The latex samples to be evaluated are previously diluted with water filtered at 0.2 μm on Millipore filter until the instrument signal is not within the scale. The scattering measure is carried out at room temperature (20° C.-25° C.) at a 90° angle. The latex particle diameter is obtained by the cumulant method.
Polymer Content in the Latex
About 3 grams of latex are weighed in a glass beaker and put in a stove to dry for 2 hours at 105° C. and then sintered at 380° C. for 15 minutes. The content of dry product of the latex is obtained from the formula:
dry product %=100×weight after sintering/latex initial weight.
Surfactant Content in the Latex
The difference between the solid weight after drying at 105° C. and the final weight after sintering gives the amount of the present surfactant.
First Melting Temperature Determination
The first melting temperature determination is carried out by the differential calorimetry technique by using a Perkin Elmer calorimeter of the DSC 7 type. About 10 mg of the dried polymer are heated from the temperature of 220° C. up to the temperature of 370° C. at a rate of 10° C./min. The temperature corresponding to the peak of the melting endothermic curve is indicated as the first melting temperature of the polymer.
Wetting Capability
The wetting capability of the formed latex, after application on the substratum (aluminum, copper, steel, stainless steel sheets), is visually evaluated by observing the behaviour of the film spread by casting on the substratum. The wetting capability is considered sufficient when the homogeneous substratum covering, without defects is obtained. The wetting capability is considered insufficient when “islands” of unwet substratum are formed.
Film Thickness Determination
The film thickness is measured according to ASTM B 499.
Presence Determination of Cracks in the Film
The crack presence is noticed by examining the film by a 40 magnification optical microscope.
Film Adhesion to the Substratum
The adhesion is evaluated with the following alternative methods:
The specimen passes the test if the film remains adherent to the substratum (case b)).
Film Scratch Resistance
The film scratch resistance is measured by observing with a reflecting optical microscope the scratch behaviour of the film adhering to the substratum. The test is an index of the film adhesion to the substratum and of the film mechanical resistance. The substratum covered by the polymer film is observed at the 40 magnification stereo microscope. With a steel spherical point having a diameter of 0.4 mm, with an applied load of 100 g, the surface is cut with circular trajectory counting about 5 rotatory movements (cycles) on a film surface having 2×2 mm sizes.
The film specimen is evaluated as follows:
The friction coefficient is measured by the ASTM D 1894 method.
Resistance of the Film to Wear
To the specimen a steel spherical point having a diameter of 0.95 mm is applied with a load of 50 or 100 g. The point is let drag on the film at a rate of 800 mm/min. By using a 40 magnification optical microscope one notices if at the edges of the incision groove, shavings of removed material are present. The presence of shavings shows that the film in the test conditions is subject to wear.
By the microscope, using a millesimal comparator, one determines the groove width, which is taken as film hardness index.
In a glass reactor there are introduced:
The resulting microemulsion results perfectly limpid.
3810 grams of the obtained microemulsion are added to 29 litres of carefully degassed water into a 50 litre autoclave equipped with a mechanical stirrer and previously put under vacuum. Also 140 grams of paraffin with softening point in the range 52° C.-54° C. had been previously introduced into the reactor. 900 mbar (9.104 Pa) of ethane are fed to the autoclave. The autoclave is kept under mechanical stirring and is pressurized with tetrafluoroethylene (TFE) up to a pressure of 20 bar (2 MPa) at a temperature of 88° C. At this point 500 cc of solution of (NH4)2S2O8 (APS) corresponding to 5,000 mg of APS are fed to the autoclave as polymerization initiator.
When the pressure in the reactor has decreased of 0.5 bar (5.104 Pa), one starts to feed TFE by means of a compressor so as to maintain a constant pressure of 20 bar inside the reactor. In the meantime the reactor internal temperature is increased up to 100° C. at a rate equal to 0.7° C./min. After 32 minutes, the TFE feeding is stopped, the reactor evacuated and cooled.
The polymer primary particle diameter measured by Laser Light Scattering (LLS) is equal to 17 nm. The discharged latex has a concentration of 312 g PTFE/kg dispersion. The PTFE latex contains a concentration by weight of anionic perfluorinated surfactant equal to 8.65% based on the PTFE.
By the DSC analysis it results that the obtained polymer has a first melting temperature of 323.6° C.
To 10 kg of said dispersion, put in a thermostatable Pyrex glass reactor, 6 kg of a solution at 25% by weight of Triton X-100 are added. The mixture, initially at room temperature, is carefully homogenized and the temperature increased up to 71° C. At this temperature stirring is stopped and an upper clear aqueous phase and a lower phase having a high content of fluorinated polymer particles separate. The lower phase contains an amount of fluorinated polymer equal to 43% by weight and of Triton X-100 equal to 6.4% by weight.
To prepare the film the latex is formed as follows (percentages by weight referred to the weight of the total dispersion):
The so formed latex is deposited by casting, with a slope of 40°-50°, on a steel plate previously treated in an oven at 400° C. for 20 minutes. Then it is dried at 120° C. in a ventilated stove and the so formed film is treated in an oven at 400° C. for 4 min. to allow the polymer sintering. The characterization is reported in Table I.
1503 grams of the microemulsion obtained as in Example 1 are added to 29 litres of carefully degassed water in a 50 litre autoclave equipped with a mechanical stirrer and previously put under vacuum. Also 210 grams of paraffin with softening point in the range 52° C.-54° C. had been previously introduced into the reactor. 125 grams of perfluoromethylvinylether (MVE) are fed to the autoclave. The autoclave is kept under mechanical stirring and is pressurized with tetrafluoroethylene (TFE) up to a pressure of 20 bar (2.106 Pa) at a temperature of 80° C. At this point 85 ml of solution of (NH4)2S2O8 (APS) corresponding to 850 mg of APS are fed to the autoclave as polymerization initiator.
When the pressure in the reactor has decreased of 0.5 bar (5.104 Pa), one starts to feed TFE by means of a compressor so as to maintain a constant pressure of 20 bar inside the reactor. In the meantime the reactor internal temperature is increased up to 85° C. with a gradient of 0.25° C./min. After 120 minutes, the TFE feeding is stopped, the reactor evacuated and cooled.
The polymer primary particle diameter measured by Laser Light Scattering (LLS) results equal to 72 nm. The discharged latex has a concentration of 330 g PTFE/kg dispersion. The modified PTFE latex contains a concentration by weight of anionic perfluorinated surfactant equal to 3.16% based on the PTFE.
To 10 kg of this dispersion, put in a thermostatable Pyrex glass reactor, 6 kg of a solution at 25% by weight of Triton X-100 are added. The mixture, initially at room temperature, is carefully homogenized and the temperature increased up to 70° C. At this temperature stirring is stopped and an upper clear aqueous phase and a lower phase having a high content of fluorinated polymer particles separate. The lower phase contains an amount of fluorinated polymer equal to 49.3% by weight and of Triton X-100 equal to 7.3% by weight.
By the DSC analysis it results that the obtained polymer has a first melting temperature of 321.6° C. By the TGA analysis it is determined a weight loss of 0.98% by weight at 425° C. in 2 hours, which is an index of good thermal stability.
The latex obtained in Example 2 is formed as follows
The substratum (steel plate) is previously treated with an hydroalcoholic emulsion (water/isopropyl alcohol 20/80) at 2% by weight of Fluorolinko F10 under the form of ammonium salt, chemically defined as a bifunctional PFPE phosphate, having number average molecular weight of about 1,500, corresponding to the following formula:
Q—CH2CF2—O—(CF20)a—(CF2CF2O)b—CF2CH2—Q
wherein:
The hydroalcoholic emulsion is spread on the plate, which is then dried at 60° C. After drying the possible compound excess is removed for example by hand, by rubbing with a cloth.
The so formed latex is deposited by casting, with a slope of 400-500, on a steel plate previously treated in an oven at 400° C. for 20 minutes. Then it is dried at 120° C. in a ventilated stove and the so formed film is treated in an oven at 400° C. for 4 min. to allow the polymer sintering. The characterization is reported in Table I.
11 grams of the aqueous solution of ammonium perfluorooctanoate and 31 litres of carefully degassed water are fed to a 50 litre autoclave equipped with a mechanical stirrer and previously put under vacuum. Also 140 grams of paraffin with softening point in the range 52° C.-54° C. had been previously introduced into the reactor. The autoclave is kept under mechanical stirring and is pressurized with tetrafluoroethylene (TFE) up to a pressure of 20 bar (2.106 Pa) at a temperature of 62° C. Successively 500 ml of a solution of (NH4)2S2O8 (APS) and disuccinic peroxide (DSAP) corresponding to 150 mg of APS and 3,000 mg of DSAP are fed.
When the pressure in the reactor has decreased of 0.5 bar (5.104 Pa), one starts to feed TFE by means of a compressor so as to maintain a constant pressure of 20 bar inside the reactor. In the meantime the reactor internal temperature is increased up to 80° C. at a rate equal to 0.5° C./min. During the reaction 50.5 grams of the aqueous solution at 100 grams/litre of ammonium perfluorooctanoate (PFOA) are fed to the autoclave. After 90 minutes, the TFE feeding is stopped, when 15,800 grams of TFE have reacted, the reactor is vented and cooled. The latex discharged from the reactor has a concentration of 510 g PTFE/litre of water.
The polymer primary particle diameter measured by Laser Light Scattering (LLS) is equal to 230 nm. The obtained PTFE latex is concentrated up to a PTFE concentration equal to 60% by weight. The PTFE latex contains a concentration by weight of anionic perfluorinated surfactant PFOA equal to 0.08% b<sfd on the PTFE.
To 10 kg of this dispersion, put in a thermostatable Pyrex glass reactor, 2 kg of a solution at 25% by weight of Triton X-100 are added. The mixture, initially at room temperature, is carefully homogenized and the temperature increased up to 69° C. At this temperature stirring is stopped and an upper clear aqueous phase and a lower phase having a high content of fluorinated polymer particles separate. The lower phase contains an amount of fluorinated polymer equal to 66% by weight and of Triton X-100 equal to 1.49% by weight.
The DSC analysis shows that the polymer has a first melting temperature of 343° C.
The substratum (steel plate) is previously treated with Fluorolink® F10 as described in Example 2A.
The latex formed as in Example 1A is deposited by casting, with a slope of 40°-50°, on a steel plate previously treated in an oven at 400° C. for 20 minutes. Then it is dried at 120° C. in a ventilated stove and the so formed film is treated in an oven at 400° C. for 4 min. to allow the polymer sintering. The characterization is reported in Table I.
To prepare the film the latex is formed as follows (percentages by weight referred to the weight of the total dispersion):
The so formed latex is deposited by casting, with a slope of 40°-50°, on a steel plate previously treated in an oven at 400° C. for 20 minutes. Then it is dried at 120° C. in a ventilated stove and the so formed film is treated in an oven at 400° C. for 4 min. to allow the polymer sintering. The characterization is reported in Table I.
To prepare the film the latex is formed by omitting the surfactant PFOA, as follows (percentages by weight referred to the weight of the total dispersion):
The so formed latex is deposited by casting, with a slope of 40° -50°, on a steel plate previously treated in an oven at 400° C. for 20 minutes. Then it is dried at 120° C. in a ventilated stove and the so formed film is treated in an oven at 400° C. for 4 min. to allow the polymer sintering. The characterization is reported in Table I.
Example 1A is repeated but by applying the latex to a not pretreated plate of aluminum at the place of steel. The characterization is reported in Table I.
Example 1A is repeated but by applying the latex to a not pretreated plate of copper at the place of steel. The characterization is reported in Table I.
To prepare the film the latex is formed as follows (percentages by weight referred to the. ′weight of the total dispersion):
The so formed latex is deposited by casting, with a slope of 40°-50°, on a steel plate previously treated in an oven at 400° C. for 20 minutes. Then it is dried at 120° C. in a ventilated stove and the so formed film is treated in an oven at 400° C. for 4 min. to allow the polymer sintering. The characterization is reported in Table I.
For the application (film preparation) the latex is formed as follows (percentages by weight referred to the weight of the total dispersion):
The so prepared latex is deposited by casting, with a slope of 40°-50°, on a steel plate previously treated in an oven at 400° C. for 20 minutes. Then it is dried at 120° C. in a ventilated stove and the so formed film is treated in an oven at 400° C. for 4 min. to allow the polymer sintering. The characterization is reported in Table II.
The latex obtained in Example 2 is formed as follows:
The so prepared latex is deposited by casting, with a slope of 40°-50°, on a steel plate previously treated in an oven at 400° C. for 20 minutes. Then it is dried at 120° C. in a ventilated stove and the so formed film is treated in an oven at 400° C. for 4 min. to allow the polymer sintering. The characterization is reported in Table II.
To prepare the film the latex is formed as follows (percentages by weight referred to the weight of the total dispersion):
The so formed latex is deposited by casting, with a slope of 40°-50°, on a steel plate previously treated in an oven at 400° C. for 20 minutes. Then it is dried at 120° C. in a ventilated stove and the so formed film is treated in an oven at 400° C. for 4 min. to allow the polymer sintering. The characterization is reported in Table II.
The latex mixture is prepared using an amount of the latex of Example 3 (Comparative) and respectively of Example 2 such that the ratio between the PTFE from emulsion polymerization and the modified PTFE from microemulsion polymerization is 95:5 by weight based on the solid.
To prepare the film the latex mixture is formed as follows (percentages by weight referred to the weight of the total dispersion):
The so formed latex is deposited by casting, with a slope of 40°-50°, on a steel plate previously treated in an oven at 400° C. for 20 minutes. Then it is dried at 120° C. in a ventilated stove and the so formed film is treated in an oven at 400° C. for 4 min. to allow the polymer sintering. The characterization is reported in Table II.
To prepare the film the latex is formed as follows (percentages by weight referred to the weight of the total dispersion):
The so formed latex is deposited by casting, with a slope of 40°-50°, on a steel plate previously treated in an oven at 400° C. for 20 minutes. Then it is dried at 120° C. in a ventilated stove and the so formed film is treated in an oven at 400° C. for 4 min. to allow the polymer sintering. The characterization is reported in Table II.
To prepare the film the latex is formed by omitting the PFOA surfactant, as follows (percentages by weight referred to the weight of the total dispersion):
The so formed latex is deposited by casting, with a slope of 40°-50°, on a steel plate previously treated in an oven at 400° C. for 20 minutes. Then it is dried at 120° C. in a ventilated stove and the so formed film is treated in an oven at 400° C. for 4 min. to allow the polymer sintering. The characterization is reported in Table II.
| Number | Date | Country | Kind |
|---|---|---|---|
| MI2002 A 000403 | Feb 2002 | IT | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10372077 | Feb 2003 | US |
| Child | 11121079 | May 2005 | US |
