Use of Fluorinated Additives in Coatings

9 Use of Fluorinated Additives in Coatings 9.1 Introduction This chapter is about the use of fluorinated additives as components of coating compositions, which generally contain significant amounts of components that do not contain fluorine. The continuous phase of these coatings is often a nonfluoropolymer thermoplastic, a thermosetting resin, or a highviscosity liquid or wax. This chapter is not about the use of fluoropolymers as the sole or major coating component, such as that used for cookware. We have chosen to divide the different fluorinated additives into three groups: Perfluorinated semicrystalline polymers, partially fluorinated semicrystalline polymers, and amorphous fluorinated polymers. The perfluorinated resins are exemplified by the semi-crystalline homopolymer and copolymers of tetrafluoroethylene (TFE), whereas the partially fluorinated semi-crystalline resins are exemplified by poly-vinylidene fluoride (PVDF) and its copolymers. Amorphous resins may be either totally or partially fluorinated and may have glass transition temperatures below or above room temperature. Information on the various additive groups will be presented later. The author also recommends the various editions of technical encyclopedias such as Refs. [1,2] as general references regarding any of these materials.

9.2 Purposes of Addition of Fluoropolymer Additives to Coatings Fluoropolymer additives are incorporated into coatings for similar reasons to their addition to bulk plastics:

• • • •

Reduction in wear, mar, and scratching Reduction in friction Improved release properties Water and oil repellency

Coatings are, of course, typically much thinner than shapes of bulk plastics. The thickness of the coatings and their hardness can affect their friction and wear performance.3 Analytical abrasion or wear tests for coatings are often, but not always, different from those used for plastic shapes. Typical wear tests for coatings are Taber abrasion (see ASTM D 4060) and falling abrasive (see ASTM D 968) tests. Wear tests normally employed for bulk plastics such as “pin-on-disk” (ASTM G 99 and G 133) or “thrust-washer” (ASTM D 370294 (2004) are also sometimes used for coatings. The Taber abrasion test consists of measuring the change in dry-film thickness (DFT) or weight loss of a coating after contact with abrasive wheels rotating for a given number of cycles under a specified load. The falling abrasive test is carried out by pouring a specified weight of sand, gravel, or aluminium oxide powders from a given height onto coated panels positioned at a 45 degree angle to the falling abrasive. Weight loss or DFT change is measured. The pin-on-disk and thrust-washer tests have some similarity with the Taber abrasion test but are more heavily instrumented and can measure friction coefficients as well as abrasion. In the pin-on-disk test, the coating is held against a rotating pin (or ball). Alternatively, the pin may be composed of the coating test material and it is rubbed against another surface. The coating in the thrust-washer test is rotated against an uncoated ring. The uncoated ring may be stainless steel or it may be composed of the expected contact material for the actual coating application. The tests are often run until an abrupt change in friction occurs, indicating that the wear has reached the coating substrate. “Fretting” is a particular kind of wear that occurs when there are small oscillatory movements between two surfaces. “Mar” is a visual change in the surface appearance that is caused by scuffing or plastic deformation. The term “mar” is used much more with coatings than with plastic shapes. Another related test that is more common with coatings is a

Fluoropolymer Additives. DOI: https://doi.org/10.1016/B978-0-12-813784-0.00009-0 © 2019 Elsevier Inc. All rights reserved.

193

FLUOROPOLYMER ADDITIVES

194

“scratch test” which measures the load and rate of contact movement that causes scratches. Coating hardness is often measured since it is related to wear and scratching. A typical hardness test for coatings is the pencil hardness test (ASTM D 336300). Pencils come in a range of hardness from soft to hard. The pencil points are sanded flat so that an even cutting surface is provided around the circumference of the point. They are then moved across the surface of the coating at an angle of 45 degree. Magnification is then used to determine if the point has cut into the coating. This procedure is followed with pencils of increasing hardness until the pencil that has cut the coating is identified. Friction is the tangential force that resists motion when a coated surface is moved against another surface or point. The total friction force between two bodies is generally said to be the sum of adhesion (Fadh) and deformation forces (Fdef). F 5 Fadh 1 Fdef Adhesion forces increase with the compatibility of the two surfaces and the contact area. Both surfaces in contact have asperities that will deform and contribute to the friction force. Deformation of asperities is a function of the yield strength of the surface and also of the contact area. In the case of thin coatings, asperities on the underlying substrate can also affect friction (and wear). A hard surface resists deformation and reduces the contact area but results in increased shear forces (per unit area). It is recognized that a microfilm with low shear strength on a hard surface affords low friction.3 If the fluoropolymer added to a hard plastic coating diffuses to the coating surface, it can provide the low shear strength microfilm and the hard plastic supports the load. The commonly reported standard ASTM method for coefficient of friction is D 1894. However, coefficients of friction may also be determined during wear tests (such as ASTM D 3702; “thrust-washer” test). The two major types of coefficients of friction are static and dynamic. Static coefficient of friction is the force required to initiate motion divided by the tangential force and dynamic coefficient of friction is the force required to maintain motion divided by the tangential force. Comparisons of friction coefficient of different coatings should be carried out under the same conditions. It is well known that the friction coefficient is a function of other factors such as sliding velocity, load, and

temperature. Reference [4] provides interesting graphical representations of a variety of sliding contact pairs as a function of these factors. Good reviews of the mechanical and tribological property measurement techniques for polymeric surfaces are given in Refs. [5,6]. Coating release and water/oil repellency properties are related and are a function of the coating surface energy. This subject was covered in Chapter 7, Fluorinated Additives for Plastics and will not be reviewed extensively here. Evaluation of release and water/oil repellency is typically made by measuring the contact angle of the coating surface with various liquids for which their surface energy is known. A high contact angle indicates poor wetting and high surface energy and a low angle indicates good wetting and a low surface energy. Poor wetting indicates good release properties and good water repellency. More details on this subject (in addition to that in Chapter 7: Fluorinated Additives for Plastics) are available in other references such as Refs. [7,8]. Fluorinated resins and chemicals are known to have very low surface energies. Polytetrafluoroethylene (PTFE) has a surface energy of about 18 20 mN/m and very few liquids will truly wet it. The addition of fluoropolymers (or other fluorochemicals) to coatings can reduce the coating surface energy. The degree to which the coating surface is actually modified will depend on the amount of fluoropolymer (or fluorochemical) additive actually present at the coating surface. This will depend on the particular additive, its compatibility with the rest of the coating, the coating viscosity, and, if the additive is a particulate, the additive particle size relative to the coating thickness. In addition to surface energy considerations, a large particulate particle size relative to the coating thickness can reduce the actual contact area with another material and improve release. Additional discussion of this subject will be presented later when the individual types of fluoropolymers are covered. “Gloss” is a subject often important for coatings. Gloss is a luster or shininess of a coating or paint. Different coating applications will require a low or a high gloss. A high gloss surface generally will be quite smooth and will tend to reflect light in a single direction. The addition of particulate additive to a coating composition will tend to make a coating rougher, less glossy, and reflect light in multiple directions. Large particulate sizes are sometimes

9: USE

OF

FLUORINATED ADDITIVES

IN

COATINGS

used to deliberately create what is called a “textured” surface that is “rough” or grainy. Most fluoropolymer additives are particulate and will tend to reduce gloss. However, particle size is important and a particle size less than about 200 nm will have minimal effect on gloss. Gloss can be measured with a “gloss meter” that relates the amount of reflected light relative to a black glass standard with a defined reflective index. The amount of reflected light increases with an increase in the illumination angle. Measurements are made at a 20 degree angle if the coating is high gloss, at 60 degree if the coating is semi-gloss, and at 85 degree if the coating is low gloss. The ASTM methods for gloss of coatings are D-523 and D-2457 (DIN EN ISO 2813 and DIN 67530).

9.3 Thermoplastic Coatings Thermoplastic coatings are produced from various resin solutions, aqueous dispersions, and powders. Examples of thermoplastics used in coatings are poly-acrylates, PVDF, polyurethanes, polyphenylene sulfide (PPS), polyethersulfone (PES) (Fig. 9.1), polyarylsulfones (PAS), polyether ketone (PEK), polyether ether ketone (PEEK), and polyamideimide (PAI) (see Chapter 7: Fluorinated Additives for Plastics for the chemical structures of these plastics). Thermoplastic coatings are applied to a substrate, dried/devolatilized, and then heated above the melting point of the resin to allow the resin to flow out and fuse. A major difference from thermosetting coatings (see Section 9.4) is that the coatings require a relatively high baking temperature that is dependent on the resin melting point and melt viscosity rather than on a curing temperature. For resins with high melting temperatures, the process is sometimes called sintering. Some coatings that have a high-intended use temperature are prepared from thermoplastics such as PES or PPS that

195

have a high melting point. A good description of the various thermoplastics used as binders in coatings is given by Laurence McKeen.9 A number of patents have been published describing the use of PTFE or other fluoropolymers in PEEK coatings (see Ref. [10]. A study of the use of fluoropolymer addition in PEEK coatings11 employed commercial coating formulations from DuPont, Whitford, and Impreglon which were said to contain PTFE with PEEK, MoS2, and/or polyvinyl pyrrolidone. Tribological tests were carried out with both aggressive reciprocating and unidirectional motion to simulate application in air-conditioning and refrigeration compressors. Coatings of PTFE/ pyrrolidone and PTFE/MoS2 performed better than PEEK/PTFE or PEEK/ceramic coatings. It was noted that PEEK filled with PTFE had performed best in the evaluation of bulk plastics (see discussion in Chapter 7: Fluorinated Additives for Plastics). No details of the specific fluoropolymer used in these coatings were available. The study of PTFE use in PAIs was studied by Gedan-Smolka et al.12 These workers irradiated TF 2025 high-molecular weight (MW) dispersionpolymerized PTFE from Solvay and then blended it with PAI in an extruder. They then ground the extrudate, dispersed it in a solvent and applied it to steel surfaces for wear and friction testing. Various irradiation and curing conditions were evaluated. Irradiation will reduce the PTFE MW and create surface reactive species such as carboxylic acid and epoxides. The addition of PTFE to polysulfone coatings has primarily been reported in patents.13,14 The PTFE is generally low-MW PTFE (from irradiation). A gradient distribution of PTFE in PPS coatings was described by Luo et al.15 They found that lowMW PTFE (irradiated) tended to migrate toward the coating surface during curing. The optimum PTFE content for friction and wear was said to be 40%. Production of PPS coatings with superhydrophobic properties by addition of fluoropolymer additives have also been reported (see Section 9.6 later).

9.4 Thermosetting Coating Parameters Figure 9.1 Polyether sulfones (PES).

Thermosetting plastic coatings are converted to a crosslinked or networked structure after being

196

applied to a substrate. The coating material typically has a low-MW and a low-melt viscosity before crosslinking. Once crosslinked, the coating is quite tough and cannot be made to melt flow. The coating is typically treated thermally, sometimes with a catalyst, to develop the crosslinks after application to a substrate but it may be subjected to some sort of radiation. Thermosetting coatings are generally easy to apply and provide quite good performance for many applications. They are typically a preferred choice over thermoplastic coatings unless a high use temperature is needed. There are several ways in which thermosetting coating types may be characterized:

• Carrier • Resin binder or chemistry • Crosslinking approach

9.4.1 Carriers Coatings may be applied from solvent-based, water-reducible, or water-based compositions, and by powder coating. An organic solvent carrier typically has a low surface tension that assists wetting of a substrate and a high vapor pressure that allows rapid vaporization after coating application. Typical organic solvents are hydrocarbons (xylenes, toluene, and mineral spirits), ketones (acetone, methyl ethyl ketone or MEK), acetates (ethyl, propyl, and butyl acetate), dimethylformamide (DMF), N-methyl-2-pyrrolidone, and mixtures of solvents. A majority of early coating compositions were of this type but have become less favored due to concerns about volatile organic chemicals (VOC) and hazardous air pollutants (HAP) emissions. Aqueous-based coatings avoid most of the VOC and HAP concerns and usually simplify cleanups. Not all aqueous-based coatings may be pure waterbased systems. A water-miscible solvent such as a ketone may be used to dissolve a resin, which is then mixed with water. Emulsifiers are sometimes used with this type of carrier. These are called water-reducible coatings. Many water-based coatings start with polymers that have been polymerized via emulsion polymerization. The polymers in these emulsions are typically of submicron particle size. After application onto a substrate, the water is evaporated and the particles coalesce into a coating. Coating manufacturers sometimes add small

FLUOROPOLYMER ADDITIVES

amounts of a solvent or organic phase to achieve better coating properties. One crossover liquid system is called an organosol. It is produced from aqueous-polymerized fluoropolymer dispersions wherein the submicron particles are transferred into an organic solvent by one of several techniques. One technique is azeotropic distillation of water from a mixture of the aqueous dispersion and an organic solvent such as toluene.16 Another technique involves phase transfer wherein the aqueous PTFE dispersion is mixed with a water-soluble organic solvent or an aqueous solution of an electrolyte. This blend is then mixed with a water-insoluble organic solvent and the fluoropolymer transfers to the solvent.17 Once an organosol is prepared, it may be blended with other nonfluorinated resins to produce a coating composition. Some coatings such as liquid epoxies or twocomponent urethanes do not require a carrier or separate liquid phase. They have the advantages of no VOC or HAP issues and usually show reduced shrinkage of the coatings. Powder coatings are also systems that require no carrier. The powders are usually applied electrostatically and then heated to a temperature that will melt the polymer binder for a long enough time to form a smooth adherent coating.

9.4.2 Resin Binder or Chemistry Fluoropolymer additives are usually mixed with a resin binder or other chemical to produce a coating composition. There are many types of these binders and many variations within each type. Combinations of binders are sometimes used in a single coating composition. We do not here present an extensive discussion of each binder type. The reader is recommended to consult Refs. [1,2,9] for additional information on the different binder types. The incorporation of PTFE into thermosetting resin coatings goes back to at least 1958 as described in a patent assigned to the DuPont Company18 that covered phenolic coatings. This was expanded in 1961 to cover a variety of thermosetting resins.19 The PTFE used for these patents was in an aqueous dispersion form and would have had a high-MW. The use of both high- and lowMW PTFE powders as well as other fluoropolymers developed later.

9: USE

OF

FLUORINATED ADDITIVES

IN

COATINGS

Alkyds are the oldest type of binder or vehicle for coatings. Alkyds are prepared from polyols, dibasic acids, and fatty acids. The most commonly used polyol is glycerol and the most commonly used dibasic acid is phthalic anhydride. Soybean oil is a widely used fatty acid. The term “alkyd” is derived from alcohol and acid. Although alkyds are actually polyesters, that term is reserved for “oilfree” (no fatty acids) polyesters. Alkyds may be classified into oxidizing and nonoxidizing types and also by the ratio of monobasic fatty acid to dibasic acid. Coatings from oxidizing alkyds undergo auto-oxidative crosslinking when exposed to air. Nonoxidizing alkyds need to be crosslinked with melamine/formaldehyde, urea/formaldehyde, or isocyanates. Oxidizing alkyds may also be modified by reaction with a variety of other components such as styrene or one of the acrylates. They may also be modified with phenols, silicones, epoxides, or urethanes. The principal advantages of alkyds are low cost, low toxicity, and low surface tension that permits wetting of most surfaces. However, durability of alkyd coatings tends to be poorer than coatings of other binders such as acrylics, polyesters, and polyurethanes. An example of the use of PTFE in alkyd coating is found in Ref. [20] which describes a mixture of alkyd resin, PTFE, and microcrystalline wax used as wood coating. Acrylates may now be the largest category of binder resins for coatings. Thermosetting acrylic resins typically are copolymers of acrylic or methacrylate esters and a hydroxyl-functionalized acrylic ester. The hydroxyl-functionalized monomer provides a site for crosslinking. Other monomers such as styrene or vinyl acetate may also be incorporated into the resin. Acrylics with carboxylic acid functionality are used with epoxy resins as a crosslinking agent. Acrylic coating resins are available in solvents, in water-reducible form, and as emulsions. The most prevalent fluoropolymer used in acrylic coatings is probably PVDF or its copolymers. PVDF does not have good adhesion to most surfaces and the acrylic resin is added to improve adhesion and compatibility with pigments. The typical level of acrylic resin addition is 20% 30%.21 Separately, the addition of 5% 40% VF2/HFP fluoropolymer to a methacrylate polymer is said to improve its toughness.22 Polyesters have excellent heat and chemical resistance. The term polyester in the coatings field

197

is used to indicate a low-MW hydroxyl or carboxylic acid-terminated resin. Hydroxyl-terminated polyesters may be crosslinked with polyfunctional isocyanates, and carboxylic acid-terminated polyesters may be crosslinked with epoxy crosslinkers. Unsaturated di-carboxylic acids are used to make unsaturated polyesters that may be crosslinked by a free radical mechanism. Aromatic thermosetting polyesters (ATSP) were developed in the mid-1990s by curing a mixture of aromatic polyester oligomers with various MW configurations. The ASTP resins have excellent thermal stability and have been considered for surfacing air conditioner compressors. A study was made of a coating system based on ASTP/PTFE compositions of 25/75, 50/50, and 75/25 levels.23 A pin-on-disk tribometer was used to measure friction and wear. Friction dropped with increasing PTFE content and the coating with 25% PTFE had the best wear (B1 3 1026 mm3/Nm). However, no 100% ASTP control was shown. The PTFE source was not mentioned. Epoxies are used both as binders and as crosslinking agents. The key structure for the resins is the three-membered oxirane structure. Commercial epoxy resins contain two or more oxirane groups (Fig. 9.2). The most widely used epoxies are the glycidyl ether derivatives of bisphenol A (Fig. 9.3). The very reactive oxirane structure undergoes ringopening reaction with many other reagents such as amines and alcohols to form crosslinked structures. A good example is the reaction with melamineformaldehyde (MF) resins. Epoxy resins may also be reacted with acrylic acid to form an epoxyacrylate that can then be crosslinked with light or irradiation. Epoxies are noted for having good mechanical properties and resistance to heat and chemicals. However, they also have a high friction coefficient and relatively poor wear. Additional information on epoxies is given in Chapter 7, Fluorinated Additives for Plastics. The addition of PTFE to epoxy resin was described in a 1961 patent.19 The addition of a fluoropolymer to an epoxy-acrylate was described in a 1983 patent.24 The fluoropolymer was described as being a vinyl or vinylidene fluoride (VDF) polymer.

Figure 9.2 Oxirane.

198

FLUOROPOLYMER ADDITIVES

Figure 9.3 Glycidyl ether reaction product of bisphenol A with epichlorohydrin.

Figure 9.4 Fomblin® Y oil. https://Solvay.com/en/ markets-and-products/featured-products/FomblinPFPE-lubricants.html.

Other patents and Journal studies followed that described other epoxy thermosetting polymer systems. McCook et al.25 investigated the addition of ZnO and PTFE to epoxy resin as was discussed in Chapter 7, Fluorinated Additives for Plastics. That work found that the lowest friction coefficient (0.11) was obtained with 3.5 vol% ZnO and 14.5 vol% PTFE and the best wear performance (1.79 3 1027 mm3/Nm) with 1 vol% ZnO and 14.5 vol% PTFE. Interestingly, it was found that the latter composition gave better wear than 15 vol% PTFE alone. The authors attributed this to the ZnO providing toughness, arresting cracks, and reducing the effect of third body debris. A 100% epoxy composite had a friction coefficient of 0.70 and wear of 83.5 3 1027 mm3/Nm. A study of the addition of perfluoro-oil to epoxy resin was made by Kumar et al.26 The fluoro-oil, Fomblin Y, was compared to the use of graphite, graphene, and hydrocarbon oil (SN150). A ball-oncylinder tribometer (steel ball) was used to evaluate wear and friction. The SN150 afforded the lowest COF (0.045) but graphene/SN150 showed the lowest friction at high load. The best wear was shown by graphene/SN150 addition followed by graphite/ SN150. The composites with Fomblin addition failed very early. Thus, the fluoro-oil did not function well, at least under these test conditions, as an additive to epoxy Fig. 9.4. Urethane resins are the product of isocyanates and alcohols. Linear or crosslinked structures may be produced (Fig. 9.5). A variety of urethane resins are made commercially. Co-reactants may be hydroxylfunctional polyester or acrylic resins. Polymerization in the presence of excess isocyanate affords resin with isocyanate end-groups that are then reactive with water to form urea groups. These are called

moisture-cure polyurethanes. Polyisocyanates plus polyhydroxy resins can provide crosslinked systems. One important class of polyurethanes is based on blocked isocyanates. Blocked isocyanates are made by reaction with a blocking agent that affords a product that is stable in the presence of water and alcohols at ambient temperatures but reacts with amines and hydroxyl groups at elevated temperatures. Studies of composites of PTFE in polyurethane include Refs. [27,28]. Both references describe blending PTFE dispersion with polyurethane dispersion. Reference [27] was notable for its blending of aqueous urethane and aqueous PTFE dispersion with very small particle size (50 nm, Algoflon MD10 from Solvay). Composite surface contact angle increased with increasing amount of PTFE up to about a 20% loading where it leveled off at about 104 degree (contact angle with no PTFE was 59 degree). Spectroscopic and mechanical properties were also measured but no friction or wear data were obtained. Amino resins. Urea-formaldehyde and especially MF resins (Fig. 9.6) are widely used in coatings. Formaldehyde can react with each of the three pendant amine groups of melamine to form methylol groups, which are then converted into ethers with an alcohol. These MF resins are reactive with both hydroxyl and carboxylic acid groups and are used to crosslink resins having these functionalities. They also undergo self-condensation reaction. Similar reaction between amides and formaldehyde gives methyl-ol derivatives that, in turn, react with alcohols to give reactive ethers. Phenolic resins. These resins are condensation products of phenols and formaldehyde. The reaction affords CH2OH groups substituted on each phenol ring. Further heating of the mixture leads to methylene ( CH2 ) and ether ( CH2OCH2 ) bridges between phenol molecules. This technology may be blended with the MF chemistry described earlier. The addition of PTFE to phenolic resin was included in the earliest patents (Refs. [18,19]) that described thermosetting coatings with PTFE. More recently, Song and Zhang reported the tribological

9: USE

OF

FLUORINATED ADDITIVES

IN

COATINGS

199

Figure 9.5 Reaction of di-isocyanates with di-ols.

Figure 9.6 Melamine-formaldehyde resin.

properties of phenolic coatings that contained PTFE, fluorinated ethylene/propylene (FEP), and a mixture of PTFE and polyethylene (PE) that they call PFW.29 The PTFE and FEP, and PFW powders were said to have particle sizes of 5, 15, and 3 4 µm. The PTFE or FEP were provided by Shanghai 3F New Materials Co., LTD of China and the PTFE/PE mixture was said to be grade POLYFLUO 150 from Micro Powders Inc. of the United States. Addition levels ranged from 10% to 50%. A ring-on-block tester was used to evaluate friction and wear. Contact angles and friction dropped rapidly with addition level. A 10% addition of all of the additives was sufficient to reduce friction to nearly their lowest level although PTFE afforded the lowest friction. Wear was very similar with all additives until addition reached a 20% level when wear with PFW became significantly the lowest. Wear with PTFE addition was almost independent of addition level.

Polyimide (PI) resins. These resins are typically the product of a di-anhydride and a di-amine. Most commercial PIs are aromatic because of their higher thermostability. They are sometimes considered to be a pseudo thermoplastic and were discussed in Chapter 7 Fluorinated Additives for Plastics rethermoplastics. However, they are sometimes processed from solutions or as powders to produce coatings and may also be considered as a thermosetting resin. The reports of addition of PTFE to PI have given mixed results. The data in Table 7.13 (for Vespel®) in Chapter 7, Fluorinated Additives for Plastics showed lower friction and lower wear compared to PI with no additives. However, Ref. [30] reported higher wear with PTFE addition to their PI coating, although friction was lower. The details of the PI resins used for these studies are unknown. Nothing is known re the PTFE used in Vespel and the only data on the PTFE used in Ref. [30] were that it had a 30 µm particle size.

9.4.3 Crosslinking or Curing Crosslinking or curing of coating compositions can be accomplished by a number of primary mechanisms. The first is through free radical or oxidizing reactions assisted by an increased temperature and the presence of air. UV light or other radiation sources may also promote crosslinking. Another mechanism is a chemical reaction such as the condensation of an alcohol or an amine with a carboxylic acid. Epoxides undergo ring opening. Some two-component systems may be cured at room temperature but other systems require an elevated temperature. Some additional information on crosslinking was provided above under binder types.

FLUOROPOLYMER ADDITIVES

200

9.5 Coating Processes The total coating process involves: 1. Preparing the coating formation 2. Application of the coating 3. Drying/baking of the resulting coating film

9.5.1 Preparing the Coating Formulation The coating formulation may be solvent or water-based or may be dry powder. In addition to the coating material, it is common to add to the formulation wetting agents, film-forming aids, and curing agents if the coating is thermosetting. Mixtures of solvents and solvents with water are also common. One component of the formulation may be soluble in one solvent and another component may be soluble in a different solvent. The use of two different solvents (or water) in a single formulation is not an issue if they are compatible. One interesting concept is latent solvents, that is, solvents for components that are not soluble at ambient temperature but are soluble at an elevated temperature at which the coating may be applied. Either type of liquid formulation is usually easy to prepare if there are no suspended solids. However, suspended solids are quite common and milling of formulations to disperse solids is often carried out. Whether solvent or water-based, any suspended solid must have a relatively small particle size in order to remain suspended. Particle suspension is an issue even with a small particle size. Most people are familiar with the concept that a new can of paint should be shaken or stirred before use. The same considerations about suspended solids exist for both solvent- and aqueous-based formulations. Mill types that are used include roller mills, ball mills, attritors, media mills, and rotor/stator mills. Powder coatings also require some preparation. Many powders must be ground to the desired particle size and screening or other classification techniques may be applied. Mixtures of different resins, pigments, film-forming aids, and wetting agents may be combined in a separate process before grinding.

9.5.2 Application of Coating There are many variations on coating application techniques but the major ones might be covered under the general headings of:

• • • •

Dipping Transfer (typically from a roll) Spraying Electrolytic

Each of these headings is generally descriptive of the processes but there are many equipment designs and hardware arrangements and the primary objective of this chapter is to discuss fluorinated additives, not the coating process. Limited details are presented herein with the various coating studies but the reader is referred to general Refs. [1,2] for more details on coating application.

9.5.3 Drying/Baking of Coating After liquid coatings have been applied, the solvent or water must be removed and the dried coating heated for fusing or curing (if necessary). Two- or even three-stage heating is often applied. The first and optionally the second stages are to remove solvent, water, and other volatiles. This may then be followed by a higher temperature stage during which the coating is fused/sintered and/or cured. Many drying/ baking processes are carried out on a belt or conveyor moving continuously through an oven. Powdered coatings must also be heated for fusing or curing.

9.6 Superhydrophobic Coatings “Superhydrophobic” coating is a term that was inspired by surfaces in nature (such as “lotus leaves”) which tend to repel water. Two factors are typically involved, the coating surface energies and the surface roughness. The superhydrophobic term is usually defined as meaning a contact angle greater than 150 degree and a droplet sliding angle of less than 10 degree (the angle of inclination of a surface when a droplet completely rolls off the surface due solely to gravity).31,32 Surface roughness assists a high contact angle by developing small air pockets

9: USE

OF

FLUORINATED ADDITIVES

IN

COATINGS

at the contact surface which causes droplets to be suspended on the surface asperities (this is called the Cassie-Baxter state). Fluoropolymer or silicone/ fluorosilicone coatings may have high contact angles but may not have the needed surface roughness. They also sometimes do not have the mechanical properties that may be needed. Fluorinated additives may be used to afford a low surface energy and accomplish a high contact angle for mechanically strong coatings that have surface roughness. The importance of the curing conditions of PPS coatings has been stressed by Luo et al.33 to achieve the optimum surface roughness and surface energy. Wu et al.34 reported that even with the use of fluorinated polymers, the maximum achievable contact angles on flat surfaces for water, diiodomethane, and hexadecane droplets with respective surface tensions of 72.1, 50.8, and 27.5 mN/m were only B120, 100, and B80 degree, respectively. These data demonstrated a low surface energy is not sufficient and that the coating surface roughness is critical for achieving the “superhydrophobic” state. Wang et al reported high contact angles for PPS coatings that contained PTFE and were spray coated onto a surface. Especially high angles were achieved when PDMS (polydimethylsiloxane) was added to the coatings (see Table 9.1).35 The contact angle of pure PPS coating was said to be 96 degree. The friction and wear performance of the PPS/PTFE coatings were also determined with a pin-on-disk tribometer (see Chapter 7: Fluorinated Additives for Plastics for details on friction/wear testing). Friction was fairly consistent for all the coatings at less than 0.3 but wear decreased with

201

increasing PTFE content. Although not emphasized in the report, the spray-coating process apparently afforded the needed rough surface. The PDMS undoubtedly was helpful but it would have been useful to know if the effect was long-lasting since it could weather away. A separate report by Wang et al.36 continued the study of PPS/PTFE coatings and evaluated contact angle in water, crude oil, and “oilfield water” as well as friction and wear. The measured contact angles of the PPS/45%PTFE/1% PDMS coating with water, crude oil, and oilfield water were 172 degree, 151 degree, and 168 degree. The corresponding angles for pure PPS coating with water, crude oil, and oilfield water were 92 degree, 90 degree, and 89 degree. Thus, the addition of PTFE/ PDMS increased the contact angles sufficiently for the coatings to be classified as “superhydrophobic.” Friction was lowest with higher loads and wear (0.4 MPa load) decreased with increasing velocity. With a load of 0.7 MPa and a sliding velocity of 0.47 m/s, the wear was 49.1 times lower with the PPS/45%PTFE/1% PDMS coating than with a PPS coating. The PTFE for both of the earlier studies was said to be from DuPont and have a 45 µm particle size. This somewhat large particle size might have assisted the requirement for a rough surface. Kim et al.37 reported the preparation of superhydrophobic coatings onto glass and stainless steel mesh by supersonic-spraying mixtures of dry PTFE and titania nanoparticles. It was stated that the high kinetic energy of the particles from the supersonic spraying caused the particles to flatten out and adhere strongly to the surface. The coating surface

Table 9.1 Effect of % PTFE/PDMS Levels on Contact Angle of PPS Coatings % PTFE in PPS/PTFE Coating

Contact Angle With PDMS

Contact Angle Without PDMS

8

151 degree

115 degree

15

159

136

21

164

144

31

168

152

39

171

158

45

172

162

Data from Wang H, Zhao J, Zhu Y, Meng Y, and Zhu Y. The fabrication, nano/micro-structure, heat- and wear-resistance of the superhydrophobic PPS/PTFE composite coatings. J Colloid Interface Sci 2013; 402:253 8.

202

contact angle could be adjusted by the ratio of PTFE to titania. The highest contact angle (163 degree) was achieved with 100% PTFE but increases in the titania content allowed adjustment of the coating to make it less superhydrophobic. Since 100% PTFE coatings typically have a much lower contact angle (108 degree 130 degree), the data suggest that the super-sonic spraying technique created a rough surface that was responsible for the low angle. The PTFE source was Sigma-Aldrich no little is known about its manufacturing. It was said to have a 1 µm particle size so it most likely was a low-MW dispersion-polymerized resin. Other examples of fluorinated additives affording superhydrophobic coatings have been reported. The process of electrodeposition probably has some advantages since it produces a rough surface texture. Daniel Iacovetta, Jason Tam, and Uwe Erb produced superhydrophobic nickel/PTFE coatings directly by electrodeposition using a very high PTFE content in the plating bath.38 The PTFE was Fluon FL1710 (low-MW irradiated granular PTFE) from AGC Chemicals America. The AGC literature states that FL1710 has a particle size of 9 µm. A 69% PTFE content in the electro-deposited coating afforded a contact angle of 152 degree. Wang, Arai, and Endo reported producing superhydrophobic nickel/PTFE coatings by electroless deposition using a cationic surfactant and a high PTFE content in their bath.39 They achieved a contact angle of 154.9 degree with a PTFE level in their coating of 47.4 vol%. The PTFE used by Wang et al. was type L-2 from Daikin and had a 0.3 µm particle size. The small particle size may have assisted dispersion in their coating bath. A patent reference from 3M40 described a coating of fluoropolymer particles, such as PTFE, dispersed in a low-melting partially fluorinated polymer such as THV [ter-polymer of TFE, hexafluoropropylene (HFP), and VDF]. The coatings were said to be highly water repellent. Although the term “superhydrophobic” was not used, some of the patent examples had contact angles greater than 150 degree.

9.7 Fluoropolymer Types Used in Coatings There is a wide variety of fluoropolymers used in coatings but they generally consist of several general types. There are perfluoropolymer and

FLUOROPOLYMER ADDITIVES

fluoropolymer (partially fluorinated) resins. The perfluoropolymers, as indicated by their name, contain no C H bonds, whereas the fluoropolymer resins contain both C F and C H bonds. The resins may be further divided into classes of semicrystalline and amorphous resins. For our discussion, we divide the perfluororesins into semi-crystalline perfluoropolymers, semi-crystalline fluoropolymer resins, and amorphous resins. The partially fluorinated resins include poly-vinyl-fluoride (PVF) and poly-vinylidene fluoride (PVF2), and polychlorotrifluoroethylene (PCTFE).

9.7.1 Perfluorinated SemiCrystalline Resins The perfluoro semi-crystalline resins are TFE homopolymer (PTFE) and its copolymers with HFP or with perfluoroalkylvinyl ethers (PAVE). The TFE/HFP and TFE/PMVE resins are generally called FEP (for fluorinated ethylene/propylene) and PFA (perfluoroalkoxy), respectively. Although “PFA” generally refers to copolymers produced with all of the various fluorinated alkyl vinyl ethers, the term PFA sometimes is used specifically to refer to copolymers of TFE with the most common ether, perfluoropropylvinylether (PPVE). The commercial copolymer of TFE and perfluoromethyl vinyl ether (PMVE) is sometimes called MFA. Recommended references regarding these resins are.41 43 Perfluoro resins are widely used in many applications, including coatings, wherein they are the sole or major resin. The perfluorinated resins have excellent friction and release properties and are thermally stable at high temperatures, which makes them very useful for cookware and wire coating. Our subject is their use as additives in nonfluoropolymer coatings to which they afford some of the advantages of fluoropolymers.

9.7.1.1 High-MW PTFE High-MW PTFE is available from both suspension and dispersion polymerization, and the particle morphology of powders from these two polymerization procedures is quite different. Powder isolated from dispersion polymerization consists of several hundred micrometer agglomerates of 180 270 nm spherical primary particles. If the resin has a highMW, it has a tendency to easily form a fibrous structure under very mild shear conditions.

9: USE

OF

FLUORINATED ADDITIVES

IN

COATINGS

203

Suspension-polymerized PTFE has a rather large irregular particle size directly from polymerization and is usually “cut” or ground before sale or use. Suspension-polymerized PTFE has only a slight tendency to form a fibrous structure when sheared. Although there has been some use of high-MW PTFE from suspension polymerization as components of coatings, the use of polymer from dispersion polymerization is much more common. The early patent Refs. [44 49] that described use of PTFE as components in nonfluoropolymer coatings generally employed high-MW PTFE aqueous dispersions. The use of the aqueous dispersions probably avoided some of the tendency of this type of PTFE to fibrillate. Modified- or low-MW PTFE was not then yet available. The references described use of PTFE (or PCTFE) with thermoplastic and thermosetting resins such as polyamide, phenol/formaldehyde, polyacrylates, copolymers of styrene and butadiene, and cellulose ethers. Various procedural differences and other additives explain the number of patents. Reference [48] was interesting in that it emphasized that its cellulose ester coating with PTFE allowed room temperature drying—no baking needed. Reference [49] described a mixture of PTFE with PE for the coating of razor blades. Eventually, the dispersion polymerization of some high-MW PTFE resins was modified so that the particle shell had a different composition that was more difficult to fibrillate.17,50 52 Several of these patent references referred to conversion of the aqueous dispersions to organosols. Various fluorinated and partially fluorinated comonomers, ethylene (E), hexafluoroisobutene (HFiB), VDF, chlorotrifluoroethylene (CTFE), HFP, perfluoroalkylvinyl ethers (PAVEs), and others were incorporated into the particle shells. This allowed the use of these resins as additives in applications, including coatings, where fibrillation was a

problem. The presence of a high-MW PTFE core with a copolymer shell affords better abrasion resistance to coatings with no increase in friction (COF or coefficient of friction) compared to the use of the copolymer alone (see the data in Table 9.2 from Ref. [17]). The coatings referred to in Table 9.2 were approximately 53% PAI and 47% fluoropolymer. Patent Ref. [50] refers to a PTFE core with a shell incorporating a fluoroalkyl acrylate (CH25 CXCO2RF, wherein X is CH3, F, or a fluoroalkyl group and RF is a fluoroalkyl group). Reference [51] is similar but refers to a core/shell resin wherein the shell is modified with PVDF. Patent Ref. [52] did not actually refer to coatings but the author is aware that the commercial product (Zonyl® MP1500 from the DuPontt Company) represented by this patent has been used in coatings. Although high-MW PTFE has been replaced by low-MW PTFE (see Section 9.7.1.2) in many coatings, high-MW resin is still preferred when coatings with high wear resistance are wanted. Recent patent activity regarding high-MW PTFE use with nonfluoropolymer coatings has centered on the effort to replace the perfluoro eight-carbon carboxylic acid surfactants (PFOA) for which there have been concerns regarding its environmental and biological persistence.53

9.7.1.2 Low-MW PTFE A preference eventually developed to employ lowMW PTFE as the additive in many coatings. LowMW PTFE may be ground into a small particle size that is easier to disperse in a coating formulation and provides adequate performance in most cases. The production of low-MW PTFE is usually accomplished by thermal treatment or irradiation of high-MW PTFE resins from either dispersion or suspension polymerization but it may also be directly

Table 9.2 Addition of Fluoropolymer to Polyamideimide Resin Coatings: Effect of Fluoropolymer Resin Type Resin

Taber Abrasion Loss, mg

COF

PTFE core TFE/E shell

3.0

0.06

PTFE core VDF/HFiB shell

5.0

0.05

TFE/E

11.5

0.07

VDF/HFiB

31.5

0.06

204

polymerized. The low-MW PTFE “micropowders” or “fluoroadditives” can be ground to whatever particle size is preferred for particular coatings. The use of thermally degraded PTFE in thermoplastic and thermosetting resin coatings was described in patent Ref. [54] in 1966. Subsequently, manufacture of low-MW PTFE via irradiation of high-MW resin was found to be simpler and more consistent and has become the more general production method (see Chapter 4: Manufacturing and Properties of Low-Molecular Weight Fluoropolymer Additives for details on production of micropowder by irradiation of high-MW PTFE). Examples of the use of low-MW PTFE from the irradiation process may be found in numerous patent Refs. [55 63]. It should be noted that not all of these patents mention that the PTFE has a low-MW. In some cases, this is the interpretation of this author based on the PTFE grade or other information presented in the patents. The applications range from blends with PESs56,57 for coating cookware and glass cloth, blends with silicone resin and a polyurethane for paint that can be cleaned with strong solvents,58 solvent-free coatings with an epoxy and a fluorinated curing agent,59 and mixtures with molybdenum or tungsten disulfide in oil.60 Reference [61] describes blends of a low-MW PTFE/PE mixture, chromium trioxide, and epoxy resin. The patent states that the mixture of PTFE and PE is easier to blend with other components than PTFE alone. The use of PE wax blends with fluoropolymer wax is also described in Ref. [64]. A PTFE/PE mixture is sold by several commercial producers (see later discussion of commercial products). Reference [62] describes a mixture of a fluoropolymer with a noncrosslinked product of an amine plus a multifunctional carboxylic acid as an additive to a thermosetting resin as giving improved friction and wear performance. A variety of low-MW PTFE products were mentioned. References [65 67] describe the coating of fluoropolymer particles with a “macromolecule” followed by a high energy treatment such as plasma or electron beam irradiation. The fluoropolymer could be PVDF, ECTFE, PTFE, or any of the TFE copolymers. Most patent examples were of lowMW PTFE (“micropowder”) but the high energy treatment would be expected to reduce MW even further. The macromolecule could be a variety of resins such as polyvinyl alcohol, poly vinyl pyrrilidone, PE glycol, poly acrylic acid, a silane, or even

FLUOROPOLYMER ADDITIVES

a thermoplastic such as poly-etheretherketone (PEEK) or polyetherimide. The process could be carried out on dry powder or powder dispersed in a solvent or water. The high energy treatment was believed to graft the resin onto the fluoropolymer. Coatings prepared from this powder had better performance than nonirradiated powders. Coatings that include fluoropolymers can be applied by electrodeposition from an aqueous bath.68 Reference [68] describes the use of a variety of fluoropolymers, including low-MW PTFE. A range of thermosetting resins including epoxy and amino (melamine) resins were described as components of the coatings. Some patents refer to specific applications for coatings that contain low-MW PTFE. Examples are: Use in ski wax,63 use in a catheter tube,69 and as a protective coating for firearms70 or wood products.20 Antifouling paints are used for objects such as ship bottoms that are exposed to a marine environment. This type of environment leads to significant buildup of marine life, which can be a particular drag problem for ships. Ships routinely have to be removed from the water for cleaning, and cleaning is difficult. The DuPont Company website71 in 2012 for PTFE micropowders described the use of PTFE additives in antifouling paints. The DuPont Company study added several different low-MW irradiated PTFE additives at various concentrations into a commercial antifouling paint. The paints were then applied to clear acrylic plaques, dried for 24 hours, and then the plaques were suspended from a frame in the ocean at Lewes, DE. After 140 days, the plaques were removed from the ocean for evaluation via ASTM D 2486 scrub resistance testing. The results clearly showed that the addition of any of the PTFE additives reduced the number of scrubbings required for cleaning. Fluoropolymers have a fairly high-MW, even those that we call “low MW.” It is clear that thermoplastics that have been compounded with low-MW PTFE do not contain much fluoropolymer on their surfaces (see Chapter 7: Fluorinated Additives for Plastics). This leads to initial high wear and friction before the surface is worn a little. This is due to several factors including transfer of PTFE to the metal processing surface (extruder, injection molding machine), the high viscosity of thermoplastic melts, and the minimal time at elevated temperature after plastic shapes are formed.

9: USE

OF

FLUORINATED ADDITIVES

IN

COATINGS

Coatings are formed differently from compounded thermoplastics. The coating surface to be used is normally not in contact with a metal surface during formation and the coating formulations have a much lower viscosity, at least during the early stages of their formation. They also are often exposed to elevated temperature for some time after manufacture (for drying or curing) during which time diffusion of fluoroadditive to the air surface may occur. There has been little reported study of the effect of coating formulation parameters and heating of the coating after formation on the amount of fluoroadditive at the surface. However, a study of PPS compounded with irradiated PTFE indicated that the fluoropolymer chains shifted gradually from the interior to the surface of the coating during curing.15 Low-MW PTFE has also been produced by direct polymerization in a fluorocarbon liquid72 and in aqueous dispersions.73,74 This low-MW PTFE does not have a tendency to fibrillate and is easy to deagglomerate back toward its primary particle size. The very low-MW (2 10,000) PTFE resin polymerized as described in Ref. [72] appears to have been used in the nonfluoropolymer coatings described in Ref. [75]. The diffusion of low-MW PTFE to a coating surface can result in a chalky appearance. Patent Ref. [74] claims that certain additives during polymerization prevents chalking of coatings. It describes coatings of PES and PPS with low MW PTFE produced by its process. A comparison is made of the tendency for these coatings to chalk per ASTM D-4214 versus very similar resins polymerized without the additives. The coating produced with low-MW PTFE polymerized by the process of Ref. [74] did not generate chalking but the coating produced with control PTFE resin did show chalking.

9.7.1.3 Copolymers of Tetrafluoroethylene The melt-processible fluorinated copolymers of TFE such as FEP and PFA do not fibrillate and may be used in coating compositions. These copolymers have a significantly lower MW and contain more than 1%, usually more than 3%, comonomer. They are not commonly used as additives for coatings but some use has been reported. A blend of PTFE and FEP dispersions has been converted into an organosol and used successfully to produce coatings with

205

other nonfluorinated resins.16 Other patent references also describe use of FEP or PFA in thermosetting76 and thermoplastic coatings.77,78 Reference [79] describes the preparation of composite powders of melt fabricable fluoropolymers (FEP and PFA exemplified) and a high-temperature nonfluoropolymer resin binder that are to be used as powder coatings. Most of the earlier references to coatings involved aqueous or solvent-based formulations. Powder coating formulations also exist. In addition to the process described earlier in Ref. [79], Ref. [80] describes a process wherein a blend of fluoropolymers with a thermoplastic is melt-blended at high temperature (250°C 400°C), ground to a particle size of 5 100 µm, and then blended with additional fluoropolymer particles (5 100 µm) and inorganic filler. A substrate was then powder coated with the blend by electrostatic application. The preferred fluoropolymers were PTFE or its copolymers with HFP, PAVE, CTFE, or ethylene. The preferred thermoplastic resins were PES, PAS, PPS, PAI, and PEEK. Fluoropolymers may also be incorporated into flexible rubber-containing coatings.76 The fluoropolymers exemplified in Ref. [76] were FEP, the FEP/PTFE organosol of Ref. [16], and low-MW PTFE. The fluoropolymers were used in combination with a thermosetting resin (epoxy, alkylphenol, or polyurethane) and a rubber (nitrile, chloroprene, and urethane types). Many coatings are prepared from simple blends of fluoropolymer and another thermoplastic. Patent Ref. [81] describes a process for adhering particles of a thermoplastic to the surface of fluoropolymer particles in a high energy mixing device. The fluoropolymer was melt processable resin such as FEP, PFA, or ETFE. The thermoplastic was any of several thermoplastics such as polyamide, polysulfide, PES, etc. Coatings from the product particles were said to have excellent uniform adherence to metal surfaces. A preference was stated in the patent for fluoropolymer from suspension polymerization to avoid the use of a fluorosurfactant.

9.7.2 Partially Fluorinated Semi-Crystalline Resins The second major type of fluoropolymer used in coatings is partially fluorinated and is usually based on VDF (CF2 5 CH2 or VDF). The homopolymer,

206

PVDF, has a significantly lower melting point (155 192°C) than PTFE (317 337°C), FEP (260 282°C), or PFA (302 310°C) and this presents some advantage for the preparation of coatings (but is a disadvantage for some end-use applications). PVDF is a thermoplastic and must be baked in an oven at high temperature for fusing of its coatings to take place. The common comonomers with VDF are TFE, HFP, and CTFE. The addition of comonomers reduces crystallinity, increases flexibility, and reduces the resin melting point (and end-use temperature). The term “PVDF” as used later refers to semi-crystalline copolymers of VDF as well as the homopolymer. General information about these resins can be found in Refs. [1,2]. More details on the VDF copolymers are presented in Refs. [82,83]. Resins of VDF with high levels of comonomers are amorphous with a low glass transition temperature (Tg) and are important elastomer products. Polymerization of PVDF polymers is carried out by emulsion (dispersion) and suspension processes. MW and distribution are functions of the polymerization procedures and are important determinates of resin properties. The mechanical properties of the PVDF resins are generally superior to those of the perfluorinated resins. However, the friction and release properties of the PVDF resins, although good, are poorer than those of the perfluorinated resins. Major reasons for the use of PVDF in coatings are their solubility in certain solvents (especially ketones), their good weatherability, and good release properties. As with TFE copolymers, thermoplastic copolymer resins of VDF with other fluorinated and partially fluorinated monomers are also used as coating resins. There are many uses for unblended PVDF polymers in wire and cable products, electronic devices, and in chemical processing fields as 100% PVDF coatings but our interest is their use as components of coatings in blends with something else. PVDF polymers are used much more as components of coatings than they are for the other additive applications discussed in this book. One reason for their wide use in coatings is that they are thermodynamically compatible with other polymers. In particular, they are particularly miscible with methyl methacrylate resins.84 Subsequent discussions later that refer to polyacrylates will include the normal range of acrylate monomers but especially methyl methacrylate. Thermoplastic blends of PVDF resins with acrylics

FLUOROPOLYMER ADDITIVES

were originally developed in the 1960s85 and have become commercially important for coatings. Blends of PVDF with thermosetting acrylates eventually were also developed.86 A typical PVDF/ acrylate coating blend has a weight ratio of 70:30 or a volume ratio of 50:50. There has been a lot of patent activity regarding PVDF coating compositions. The following gives just examples of those patents. Aqueous coating compositions of PVDF with emulsified liquid epoxy resins were claimed by Ref. [87] in the late 1970s, and solvent blends of PVDF, epoxy resin, and acrylic resin were claimed by Ref. [24] in the 1980s. A blend of PVDF, acrylic resin, and a flow modifier, which could be a benzoguanamine resin, a blocked isocyanate, or a polymeric urethane was claimed by Ref. [88]. Reference [89] described the use of an aminoplast curing agent with a solvent mixture of PVDF and a polyacrylate that contained 3% 8% 2-hydroxyethyl acrylate. A coating for glass was described by Ref. [90] that was composed of PVDF, a functional organosilane, and a thermosetting epoxy of polyester resin. Reference [91] described organosols of PVDF in a solution of acrylic resins wherein the acrylic resin contained some amino-alkylacrylate. Reference [51] described a core/shell particle structure wherein the core is high-MW PTFE and the shell is PVDF. It was said to allow good dispersion into other coating materials. Reference [92] described the addition of particulate PVDF to a coating composition of milled fluoropolymer and acrylic resin. The purpose of the particulate PVDF was to create a textured surface to the coating. Powder coatings that contain PVDF have seen the most patent activity in the past few years, probably due to concerns over VOCs and HAPs from solvent and even aqueous formulations. Examples are Refs. [93 99]. All coating formulations employed polyacrylates with the PVDF or VDF copolymers. It was generally found that PVDF homopolymer gave poor cracking performance and surface defects called “orange peel” so copolymer resins were preferred.93,97,98 The addition of a small amount (0.1% 5%) of a VDF/TFE/HFP terpolymer with PVDF homopolymer was also mentioned by Refs. [94,95] to afford good coatings. References [93 95] all employed melt blending of the coating ingredients followed by cryogenic grinding to achieve the small particle size wanted for powder coating. It was then found that cryogenic grinding could be avoided by polymerizing the acrylic resin in the presence of

9: USE

OF

FLUORINATED ADDITIVES

IN

COATINGS

a PVDF emulsion.96,97 Powders suitable for coatings could also be obtained by coagulation of mixtures of PVDF latex and water-reducible acrylate resins.99

9.7.3 Amorphous Fluoropolymers A less common fluoropolymer coating resin type is amorphous fluoropolymer. Amorphous resins have no crystalline melting points but do have glass transitions (Tg) that have a significant effect on their mechanical properties. In general, amorphous resins are harder and stiffer below their glass transition temperatures. Amorphous fluoropolymers with glass transition temperatures above room temperature are unusual but do exist. A prime example is the copolymer of TFE and perfluorodioxole that is sold by the Chemourst Company under the trade name Teflon® AF (Fig. 9.7). The rigid dioxole ring structure inhibits molecular chain rotation and affords the high Tg. Patent Ref. [100] described optically clear, low friction, low wear coatings that are produced from solutions of amorphous fluoropolymers with partially fluorinated acrylates that can then be crosslinked. The patent describes the use of Teflon AF but says nothing about Tg. All commercial Teflon AF resins have a Tg well above room temperature. Most amorphous fluoropolymer resins have a Tg below room temperature. The category is dominated by fluoroelastomer resins. Fluoroelastomers are usually based on VDF, TFE, HFP, PAVE, TFE, ethylene, and a few other partially fluorinated monomers. One key to their lack of crystallinity is a relatively high level of branched comonomers such as HFP and PAVE. The inclusion of VDF is primarily to serve as a crosslink point. Reference [101] refers to a coating composition of an amorphous elastomeric copolymer of TFE, VDF, and HFP with a PVDF or polyacrylate resin that is

Figure 9.7 Structure of Teflon AF. File: Teflon af. png - CleanEnergyWIKI.

207

designed to consolidate and protect stone materials from atmospheric pollutants. Other types of low-TG fluoropolymers used in coatings are the copolymers of TFE or CTFE with vinyl ether or ester (PFEVE). The original resin of this type that was developed contained vinyl ether and is sold under the trade name Lumiflon® by Asahi Glass Company (AGC). A similar product with the trade name Cefral® contains vinyl ester units and was developed by Central Glass Company. In the structure shown (Fig. 9.8), “Y” is typically either F or Cl, indicating that it arises from either TFE or CTFE. The methyl ( CH3) group in the structure may be replaced by a group that contains a functional group such as an alcohol that permits crosslinking with melamines or isocyanates. The resin may also be blended with crosslinkable acrylate resins. Lumiflon and Cefral are soluble in some organic solvents, which simplifies their use, but the required amounts of solvent are rather high which raises concerns about VOCs. The resins weather very well and, although costly, are widely used in architectural paint, especially in Asia. Partially fluorinated polyurethanes and polyacrylates are also amorphous and are used in coatings102,103 both as the base resin and as additives. They can be crosslinked with polyols or isocyanates. Few, if any, applications of amorphous fluoropolymer as additives to other materials have been developed. The very high cost of these products is probably a hindrance to their use.

9.7.4 Particle Size of Fluoropolymer Additives As noted in Chapter 6, Applications of Fluorinated Additives for Lubricants, it is important

Figure 9.8 Structure of TFE(CTFE)-vinyl ester copolymer.

208

to understand particle size analysis of fluoropolymer powders before comparing products, especially those from different providers. Very fine powders, especially the agglomerates from dispersion polymerization, are difficult to measure accurately. Agglomerates from coagulation or other isolations techniques must be broken up before analysis. The very high surface areas that correspond to very small particle sizes mean that a great deal of energy must be provided for de-agglomeration However, excessive energy input can cause smearing of primary particles together, affording a broader, larger particle size. Particle size data from different sources must be compared with care. Analysis of PTFE primary particle size while polymer is still in dispersion form is much easier and more accurate. Those suppliers of PTFE that describe (some don’t) a primary particle size are either reporting size from dispersion analysis or from SEM analysis. For many years, the primary particle size of dispersion-polymerized PTFE and its copolymers was 180 280 nm. This was primarily the result of the wide-spread use of ammonium perfluoro-octanoate (PFOA or C-8) as the polymerization surfactant and applied to both high and low MW products. The evaluation of surfactant alternatives that began in the 1980s eventually allowed much smaller fluoropolymer primary particle sizes (see for example Refs. [104,105]). This effort expanded when it was recognized that PFOA was an environmental problem. The general interest in “nano”-sized materials also arose at about the same time. Many studies of fluoropolymers and some PTFE products now include the term “nano” in their titles, sometimes even if the polymer primary particle size is still 180 280 nm. However, today there are a few commercial products with much smaller primary particle sizes and some of these have been studied as additives to coatings. However, the importance of primary particle size for coatings may not yet be clear. The use of PTFE nanoparticles with a particle size less than 100 nm as an additive to polysulfone was claimed by patent Ref. [106]. One goal of the use of nanoparticles was apparently to achieve a more transparent composite. It was noted that the process of patent Ref. [105] was a suitable route to prepare such nanoparticles. Most directly polymerized low-MW PTFE has a primary particle size of 200 250 nm. However, there are some commercial products with much

FLUOROPOLYMER ADDITIVES

smaller primary particle sizes. The use of one of those products, Zonyl TE5069AN from the DuPont Company (now Chemours Company), was used for a study of a coating system based on ATSP.107 The coating friction coefficient was 0.265 and the wear rate was 7.36 3 1027 mm3 Nm21. The authors of this study made no mention of the primary particle size of the TE5069AN PTFE powder and they may not have realized that the size was unusually low. Particle size of fluoropolymer agglomerates from dispersion polymerization or the irregular particles from thermal degradation or cut particles from granular polymerization can be important for coating performance. Larger particles may afford a roughness (“texture” to coating surfaces that improves release properties.

9.8 Perfluoropolymer/Metal Coatings by Electrolytic or Electroless Processes It was found in the late 1960s108 that fluoropolymers could be co-deposited with metals into coatings by either electrolytic or electroless processes. Several metals, including nickel, copper, silver, and zinc, may be used but nickel is the most common. A combination of nickel and phosphorus (NiP coatings) is often used because phosphorus increases the hardness of the coating. The coatings that are produced have superior wear and corrosion resistance, low friction, and water/oil repellency. They can be applied in very uniform thickness even onto irregular shapes. Co-deposition of PTFE with nickel is the most common. Electrolytic coatings are applied by passing an electric current through a bath containing metal salts, fluoropolymer, surfactants, buffers, the object to be coated, and, optionally, other ingredients.109,110 Electroless coatings are applied similarly except that no current is applied but metal salts and reducing agents for those metal salts are incorporated into the bath (Fig. 9.9). Electroless processes have become more generally used as the achievable PTFE incorporation is higher111 (typically 20% or more). Several reviews of electroless coatings have been written.112,113 Surfactant choice for these coating formulations is particularly important and cationic surfactants

9: USE

OF

FLUORINATED ADDITIVES

IN

COATINGS

209

Specimen rotator

Teflon cap

Thermometer (90°C)

Sample

Electroless nickel bath Water area

Temperature control Thermostat

Figure 9.9 Basic diagram of the apparatus usually used in electroless experiments.113

are normally used so that the PTFE particles have a negative charge. Process details can be complicated and some coating producers still keep much of their processes confidential. However, good descriptions of the electroless process can be found in patent Refs. [114 116]. In addition to PTFE, other additives such as phosphorus, silicon carbide (SiC), boron nitride (BN), graphite, and molybdenum disulfide (MoS2) are sometimes incorporated into these coatings.117 Reference [117] concluded that NiP with PTFE and MoS2 displayed the best sliding performance. Comparisons of NiP and NiP/PTFE coatings and the effect of their heat treatment showed that the addition of PTFE significantly improved wear resistance.118 The specific wear rate of a coating of NiP plus PTFE was 4.76 3 1028 mm3/Nm compared to a value of 1.52 3 1025 mm3/Nm for a coating of just NiP. Heat treatment of the coatings improved the NiP coating but not the NiP/PTFE coating. A separate comparison of various coatings concluded that a Ni/P/PTFE/SiC composite afforded the optimum of coating hardness, low friction, and low surface energy.119 Although the fluoropolymer does not have to be PTFE, low-MW PTFE is generally employed in either electrolytic or electroless processes. Reference [120] claims that the highest lubricity and water and oil repellency can be obtained when the MW of the PTFE is 10,000 or less and the resin end groups are perfluorinated ( CF3). Rahmati and Mahboobi121 have stated that the addition of PTFE to NiP coatings decreases their hardness and strength. They studied the addition of both PTFE nanoparticles and carbon nanotubes into NiP coatings to achieve both low friction and higher strength. The PTFE was said to have a particle

size of 100 200 nm and the carbon nanotubes were said to have a length of 30 50 µm and 5 15 µm diameter. Wear and friction tests were carried out with a pin-on-disk tribometer on NiP coatings with PTFE and with PTFE plus carbon nanotube addition. The ratio of PTFE to carbon nanotubes in the bath was 5:1. The coefficient of friction of a NiP coating was reduced from about 0.64 to 0.20 by the addition of PTFE alone but the addition of carbon nanotubes with the PTFE reduced the COF to about 0.11. No values were provided for the wear losses but SEM images were shown which seemed to show less wear when both PTFE and carbon nanotubes were present. However, the writers also commented that the wear of the NiP coating with only PTFE addition also had better wear resistance than the NiP coating with no additions. Pazderova et al.122 studied the electroless composite coatings of zinc plus PTFE. The PTFE source was described as 60% dispersion but no source or description of MW was given. The PTFE concentration in the coating bath was varied at 1, 5, 10, and 23 vol% and bath stirring was varied between continuous and intermittent. The zinc concentration was not described. Friction was measured with a linear reciprocating tribometer. The conclusion from the study was that 10% PTFE level afforded the lowest friction and both static and dynamic friction were reduced compared to a zinc deposit without PTFE. No wear data were reported. The electroless silver (Ag)-PTFE coating onto silicone rubber was reported by Ref. [123]. A silane coupling agent was first grafted onto a silicone surface to anchor the silver atoms during the electroless coating. The mixture of silver nitrate and PTFE (60% aqueous dispersion from Aldrich) was

210

then electrolessly coated onto the silicone. The primary objective of a silicone with strong and stable antibacterial activity was achieved with good adhesion. A Standard has been issued by SAE International establishing the requirements for a plating of electroless nickel plating codeposited with PTFE. It is AMS2454, “Plating, Electroless Nickel, Codeposited with Polytetrafluoroethylene (PTFE).” Companies such as Surface Technology (www. MacDermid Enthone surfacetechnology.com), (www.enthone.com), General Magnaplate Corp. (www.magnaplate.com), Coating Technologies Inc. (www.coatingtechnologiesinc.com), and ElectroCoatings (www.electro-coatings.com) carry out electrolytic and/or electroless coating operations with PTFE.

FLUOROPOLYMER ADDITIVES

9.9 Prevention of Staining of a Coating The tendency of a liquid to wet or stain a coating involves the ability of the liquid to spread over the coating and is a function of the surface energies/ tensions of the coating and the staining liquid. The term surface energy is usually used with solids and the term surface tension is usually used for liquids. However, the values are numerically equivalent for pure materials and no distinctions between these terms will be made here. A liquid with a high surface tension will not wet a coating with a low surface energy very well. Fluorinated additives reduce the surface energy of coatings and make them difficult to wet and thus to stain. The surface energies of selected polymers used as coating resins and also fluoropolymers are presented in Table 9.3.

Table 9.3 Selected Surface Energies of Polymers That Might Be Used in Coatings Name

CAS Ref. No.

Surface Free Energy at 20°C, mN/m

Polyethylene—linear (PE)

9002-88-4

35.7

Polyethylene—branched (PE)

9002-88-4

35.3

Polypropylene—isotactic (PP)

25085-53-4

30.1

Poly-vinyl-fluoride (PVF)

24981-14-4

36.7

Polyvinylidene fluoride (PVDF)

24937-79-9

30.3

Polytrifluoroethylene (P3FEt/PTrFE)

24980-67-4

23.9

Polytetrafluoroethylene (PTFE)

9002-84-0

20

Polyvinylchloride (PVC)

9002-86-2

41.5

Polyvinylidene chloride (PVDC)

9002-85-1

45

Poly-chlorotrifluoroethylene (PCTFE)

25101-45-5

30.9

Polyvinylacetate (PVA)

9003-20-7

36.5

Polymethylacrylate (Polymethacrylic acid) (PMAA)

25087-26-7

41.0

Polyethylacrylate (PEA)

9003-32-1

37.0

Polymethylmethacrylate (PMMA)

87210-32-0

41.1

Polyethylmethacrylate (PEMA)

9003-42-3

35.9

Polyethyleneoxide (PEO)

25322-68-3

42.9

Polyethyleneterephthalate (PET)

25038-59-9

44.6

Polyamide-6,6 (PA-66)

32131-17-2

46.5

Polyamide-12 (PA-12)

24937-16-4

40.7

Polydimethylsiloxane (PDMS)

9016-00-6

19.8

Data obtained from DataPhysics GmbH at www.surface-tension.de.

9: USE

OF

FLUORINATED ADDITIVES

IN

COATINGS

Some of the comonomers incorporated into TFE and VDF copolymers will change the values somewhat. A rough estimate is that the surface energy will drop with increasing fluorine content but this is not a perfect rule since the molecular structure will also affect surface energy. A list of surface tension data for selected solvents is shown in Table 9.4. The table includes solvents that are often used for coating formulations. The addition of fluoropolymers to coating formulations that include nonfluoropolymers will tend to reduce their surface energies and make them more difficult to stain. However, the ability for fluoropolymers to have this effect will depend on their tendency to diffuse to the coating surface. The relatively high-MW of fluoropolymers hinders their diffusion. Other factors that affect diffusion are the amount of added fluoropolymer, its compatibility with the formulation, and the viscosity of the coating. The concept of deliberately providing an incompatible formulation was explored by Yoshimura et al.124 This reference describes a blend of PVDF with two resins: The major one having poor compatibility and the minor one having good compatibility. It reported that after baking the coating, the surface side was richer in PVDF than the substrate side. The method of manufacture of low-MW PTFE can have an effect on its end group type. The degradation processes used on high-MW PTFE, especially the irradiation process, generate polar carboxylic acid and acid fluoride end groups that may increase surface energy and wetting. Some investigation of the effect of irradiation on PTFE surface energy has been carried out.125,126 Higher levels of irradiation (and carboxylic acid end

211

groups) afforded a higher surface energy. However, surface energies are still low enough to provide repellent surfaces. Direct polymerization generates far fewer carboxylic end groups. Other, probably less polar, end groups are produced during direct polymerization that are dependent on the chaintransfer agent used in polymerization. Low-MW fluorinated aliphatic alcohols have been proposed127 as a coating additive that can diffuse to the coating surface more readily. Other commercial low-MW fluorochemicals have been commercialized (see Section 9.10) as additives. Fluorinated surfactants have also been added to coating formulations and these could remain in a coating and affect staining if the coating does not experience a high temperature during curing.

9.10 Commercial Fluoroadditive Products All of the major fluoropolymer producers include resins and/or fluorochemicals for use as coating additives in their product lines. High-MW PTFE and the melt-processible perfluoro-copolymer resins (FEP, PFA) are widely used in nonadditive applications and are therefore available. However, the authors know of little use of these resins in nonfluoropolymer coatings with the exception of some cookware products. The fluoropolymer producers are usually receptive to any requests of the product for coatings and applications may exist of which the author is unaware. The largest volume sales as

Table 9.4 Surface Tensions of Selected Liquids Liquid

Surface Tension, mN/m

Water

72.1

Ethanol

22.1

Isopropanol

23.0

Methyl ethyl ketone (MEK)

24.6

Acetone

25.4

Toluene

28.4

Dimethylformamide (DMF)

37.1

N-Methyl-2-pyrrolidone

40.8

Data obtained from DataPhysics GmbH at www.surface-tension.de.

212

perfluorinated coating additives are the low-MW PTFE resins. The companies that provide low-MW PTFE powder additives are shown in Table 9.5 with the trade names for these products. More information on these products is presented in Chapter 4, Manufacturing and Properties of LowMolecular Weight Fluoropolymer Additives. The first six of the companies listed in Table 9.5 are all actual producers of fluoropolymers. The other companies obtain fluoropolymers (generally PTFE) from secondary sources and convert them into “micropowder” products. The fluoropolymer producers include directly polymerized powders (and some aqueous dispersions) in their product lines. Those companies that do not actually polymerize fluoropolymers do not sell directly polymerized products but some do have aqueous dispersions produced by re-dispersion of irradiated powders. The resin described by Ref. [52] that has a highMW PTFE core and a comonomer-modified shell was previously available from the Chemours Company as Zonyl MP1500. However, it is not now listed on their website. The authors are unaware of the commercial availability of it or any of the other high-MW core/modified shell resins that have been patented. Some of the earlier companies sell dry-film lubricants or release agents based on water, alcohol, or oil suspensions of the low-MW PTFE powders in addition to the dry powders themselves. There

FLUOROPOLYMER ADDITIVES

are also other companies such as MicroCare Corporation and the McLube Division of McGee Industries who do not sell powders but do sell dry-film lubricant and/or release agents based on fluoropolymers or fluorochemicals. As noted in Refs. [61,64], blends of PTFE and hydrocarbon waxes are easier to disperse into coating formulations than PTFE alone, especially formulations with a high surface tension. Several of the providers (Clariant, Micro Powders Inc., Shamrock Technologies) of low-MW PTFE powders also produce and sell blends of those powders with hydrocarbon waxes. The major fluoropolymer producers also sell low-MW fluorochemicals that may be used in coatings. These include surface modifiers from Solvay (Fluorolink®) and Chemours (Fluoroguardt, Zonyl and Capstone®), Unidyne® Fluororepellent and Daifreet mold release agent from Daikin, and AsahiGuard E Seriest stain repellent from Asahi Glass. These commercial products are most often used as a surface treatment but some are used as additives in coating formulations. A variety of chemical structures is included in this category and includes fluorinated acrylates, urethanes, polyethers, and fluoroalkyl structures with a range of hydrophilic end groups. A list of PVDF producers is shown in Table 9.6. At least some of these producers sell coating products through licensees, which may label their offerings with a different trade name. Two trade

Table 9.5 PTFE Micropowder Providers and Trade Names Micropowder Producer

Trade Names

Asahi Glass

Fluon®

Central Glass

Cefral Lube

Daikin

Lubront, Polyflont

Dyneont

Dyneont

Chemours Company

Zonyl®, Vydax®

Solvay Solexis

Algoflon®, Polymist®

Clariant, HuntsmanClariant

Ceridust®

Laurel Products

Ultraflont

Micropowders Inc.

Fluo, Polyfluo®, Synfluo, and others

Shamrock Technologies®

Fluorot, SSTt, FluoroSLIPt, and others

Heroflont

Herolube

Maflon®

Lineplus

9: USE

OF

FLUORINATED ADDITIVES

IN

COATINGS

213

Table 9.6 Manufacturers of PVDF Resins/Coatings Manufacturer

Trade Names

Arkema Inc.

Kynar®

Daikin Industries Ltd

Neoflon®

Dyneont

Dyneont

Kureha Corporation

KF Polymer®

Solvay Solexis

Solef®, Halar®

names are shown for Solvay PVDF. The Solef® trademark refers to polymer from suspension polymerization and Halar® refers to polymer from dispersion polymerization. Regarding amorphous fluoroadditive polymers, the Chemours Company produces and sells copolymers of TFE and perfluorodioxole under the Teflon AF name but it is used mostly as a standalone resin rather than as an additive. Asahi Glass and Central Glass companies produce copolymers of TFE or CTFE with nonfluorinated vinyl ether or vinyl ester monomers under the respective trade names Lumiflon and Cefral that are used as coating resins with and without other components and crosslinking agents.

References 1. Kirk-Othmer Encyclopedia of Chemical Technology, Wiley-Interscience Publication, John Wiley & Sons. 2. Encyclopedia of Polymer Science and Technology, Wiley-Interscience Publication, John Wiley & Sons. 3. Holmberg K. A concept for friction mechanisms of coated surfaces. Surf Coat Technol 1992;56:1 10. 4. Myshkin NK, Petrokovets MI, Kovalev AV. Tribology of polymers: adhesion, friction, wear, and mass-transfer. Tribol Int 2005;38:910 21. 5. Randall NX. Characterization of the surface mechanical properties of paints and polymeric surface coatings. Paint & Coating Industry Magazine September 2010;28 38. 6. McKeen LW. Fluorinated coatings and finishes handbook, the definitive user’s guide and databook. Norwich, NY: William Andrew; 2006.

7. Adamson AW, Gast A. Physical chemistry of surfaces. sixth ed Wiley-Interscience Publishers; 1997. 8. Wettability. In: Berg JC Surfactant science series, vol. 49. Marcel Dekker; 1993. 9. Chapter “Binders” in fluorinated coatings and finishes handbook (Second Edition), 2016, Pages 59 82, William Andrews publisher, Waltham MA. 10. Eigenbrod V. “Coating containing pek and/or peek”, US Patent appl 20130157024, assigned to Rhenotherm Kunstsoffbeschichtungs Gmbh, Jun 20, 2013. 11. Yeo SM, Polycarpou Andreas. Tribological performance of PTFE- and PEEK-based coatings under oil-less compressor conditions. Wear 2012;296:638 47. 12. Gedan-Smolka M, Marschner A, Lehmann D, Franke R, Klemm M, Leidich E, et al. Formulation and characterization of PAIPTFE-cg antifriction coatings. Key Eng Mater 2017;721:362 7. 13. Attwood T, Farrant B. “Coating compositions comprising a polysulfone and a fluorocarbon polymer”, US Patent 3,981,945, assigned to Imperial Chemical Industries Limited, Sept 21, 1976. 14. Saito T, Asai K, Nakagawa I, Kobayashi T. “Aromatic polysulfone resin composition suitable for use in self-lubricating molding compositions”, US Patent 4,477,630, assigned to Sumitomo Chemical Company, Limited, Oct 16, 1984. 15. Luo Z, Zhang Z, Wang W, Liu W. Effect of polytetrafluoroethylene gradient-distribution on the hydrophobic and tribological properties of polyphenylene sulfide composite coating. Surf Coat Technol 2009;203:1516 22. 16. Satokawa T, Fujii T, Honda N, Asano K, Nakamura Y, Suzue S. “Fluorocarbon polymer

214

17.

18.

19.

20.

21.

22.

23.

24.

25.

26.

27.

FLUOROPOLYMER ADDITIVES

composition and production and use thereof”, US Patent 3,904,575, assigned to Daikin Kogyo Co., Ltd, September 9, 1975. Shimizu T, Yamaguchi S. “Organosol of fluorine-containing polymer”, US Patent 5,188,764, assigned to Daikin Industries Ltd, February 23, 1993. Sanders P. “Coating compositions comprising polytetrafluoroethylene and phenol aldehyde, and article coated therewith”, US Patent 2,825,706, assigned to the DuPont Company, Mar 4, 1958. Dawe HJ. “Re-dispersible Dispersion of Polytetrafluoroethylene”, US Patent 2,976,257, assigned to Acheson Industries, Inc., Mar 21, 1961. Hall R. “Wood coating composition from alkyd resin, PTFE and microcrystalline wax”, US Patent 4,857,578 A, assigned to Rust-Oleum Corporation, Aug 14, 1989. Iezzi RA, Gaboury S, Wood K. Acrylicfluoropolymer mixtures and their use in coatings. Prog Org Coat 2000;40:55 60. Chu S-C. “Blends and molded articles comprising methacrylate polymers and vinylidene fluoride - hexafluoropropylene copolymer”, US Patent 4,824,911, assigned to Mobil Oil Corporation, Apr 25, 1989. Demas N, Zhang J, Polycarpou A. Tribological characterization of aromatic thermosetting copolyester PTFE blends in air conditioning compressor environment. Tribol Lett 2008;29:253 8. Miller JD, Grunewalder VJ. “Fluorocarbon coating compositions”, US Patent 4,379,885, assigned to PPG Industries, Inc., April 12, 1983. McCook NL, Boesl B, Burris DL, Sawyer WG. Epoxy, ZnO, and PTFE nanocomposite: friction and wear optimization. Tribol Lett June, 2006;22(3):253 7. Kumar V, Sinhab SK, Agarwal AK. Tribological studies of epoxy composites with solid and liquid fillers. Tribol Int 2017;105:27 36. Anbinder PS, Peruzzo PJ, de Siervo A, Amalvy JI. Surface, thermal, and mechanical properties of composites and nanocomposites of polyurethane/PTFE nanoparticles. J Nanopart Res 2014;16(8):1 11 Available from: https://doi. org/10.1007/s11051-014-2529-5 16:2529.

28. Koti Reddy C, Shailaja D. Improving hydrophobicity of polyurethane by PTFE incorporation. J Appl Polym Sci Sept 2015;132:42779 https://doi.org/10.1002/ Available from: app.42779. 29. Song H-J, Zhang Z-Z. Study on the tribological and hydrophobic behaviors of phenolic coatings reinforced with PFW, PTFE, and FEP. Surf Coat Technol 2006;201:1037 44. 30. Demian C, Liao H, Lachat R, Costil S. Investigation of surface properties and mechanical and tribological behaviors of polyimide based composite coatings. Surf Coat Technol 2013;235:603 10. 31. Chu Z, Seeger S. Superamphiphobic surfaces. Chem Soc Rev 2014;43:2784 98. 32. Celia E, Darmanin T, Amigone E de Givenchy, S, Guittard F. Recent advances in designing superhydrophobic surfaces. J. Colloid Interface Sci 2013;402:1 18. 33. Luo Z, Zhang Z, Wang W, Liu W, Xue Q. Various curing conditions for controlling PTFE micro/nano-fiber texture of a bionic superhydrophobic coating surface. Mater Chem Phys 2010;119:40 7. 34. Wu X, Wyman I, Zhang G, Lin J, Liu Z, Wang Y, et al. Preparation of superamphiphobic polymer-based coatings via spray- and dipcoating strategies. Prog Org Coat Jan, 2016;90:463 71 Available from: https://doi. org/10.1016/j.porgcoat.2015.08.008. 35. Wang H, Zhao J, Zhu Y, Meng Y, Zhu Y. The fabrication, nano/micro-structure, heat- and wear-resistance of the superhydrophobic PPS/ PTFE composite coatings. J. Colloid Interface Sci 2013;402:253 8. 36. Wang H, Yan L, Gao D, Liu D, Wang C, Zhu Y. Tribological properties of superamphiphobic PPS/PTFE composite coating in the oilfield produced water. Wear 2014;319:62 8. 37. Kim D-Y, Lee J-G, Joshi BN, Latthe SS, Al-Deyab SS, Yoon SS. Self-cleaning superhydrophobic films by supersonic-spraying polytetrafluoroethylene titania nanoparticles. J Mater Chem A 2015;3:3975. 38. Iacovetta D, Tam J, Erb U. Synthesis, structure, and properties of superhydrophobic nickel PTFE nanocomposite coatings made by electrodeposition. Surf Coat Technol 2015;279:134 41.

9: USE

OF

FLUORINATED ADDITIVES

IN

COATINGS

39. Wang F, Arai S, Endo M. Electrochemical preparation and characterization of nickel/ultradispersed PTFE composite films from aqueous solution. Mater Trans 2004;45(4):1311 16. 40. Jing N, Chen LP. “Highly water repellent fluoropolymer coating”, US Patent 8,632,856, assigned to 3M Innovative Properties Company, Jan 21, 2014. 41. Ebnesajjad S. Fluoroplastics, vols. 1 and 2, “Melt Processible Fluoropolymers, The Definitive User’s Guide and Databook”, Plastic Design Library, Division of William Andrew. 42. Ebnesajjad S. Fluoroplastics, vol. 1, “Non-Melt Processible Fluoroplastics, The Definitive User’s Guide and Databook”, Plastic Design Library, Division of William Andrew, Norwich, NY. 43. Hintzer K, Lohr G. Melt processable tetrafluoroethylene-perfluoropropylvinyl ether copolymers (PFA). In: Scheirs John, editor. Modern fluoropolymers. John Wiley & Sons; 1997. p. 223 7. 44. Emig FJ, Muth MJ. “Abrasion resistant wire enamels”, US Patent 2,668,157, assigned to E. I. du Pont de Nemours & Company, February 2, 1954. 45. Hochberg S. “Polytetrafluoroethylene coating compositions”, US Patent 2,681,324, assigned to E. I. du Pont de Nemours & Company, June 15, 1954. 46. Welch PR. “Method of coating a surface with polyhalocarbon resin and article formed thereby”, US Patent 2,777,783, no assignment, January 15, 1957. 47. Dawe HJ, Youse EL. “Re-dispersible Dispersion of Polytetrafluoroethylene”, US Patent 2,976,257, assigned to Acheson Industries, Inc., March 21, 1961. 48. Janssens WD. “Polytetrafluoroethylene-water insoluble cellulosic ester dispersions and method of forming coatings therewith”, US Patent 3,154,506, assigned to Acheson Industries, Inc., October 27, 1964. 49. Esemplare PE. “Polyethylene coated blades and process for their production”, US Patent 3,224,094, assigned to Philip Morris, Inc., December 21, 1965. 50. Shimizu T, Yamaguchi S. “Aqueous dispersion, composite powder and organosol of fluorinecontaining polymer”, US Patent 5,164,426,

215

51.

52.

53.

54.

55.

56.

57.

58.

59.

60.

61.

62.

assigned to Daikin Industries, Ltd, November 17, 1992. Shimizu T, Yamaguchi S, Yamamoto Y. “Composite microparticle of fluorine containing resins”, US Patent 5,494,752, assigned to Daikin Industries, Ltd, February 27, 1996. Morgan R, Stewart CW. “Modified polytetrafluoroethylene resins and blends thereof”, US Patent 4,952,630, assigned to E. I. Du Pont de Nemours & Company, August 28, 1990. Hintzer K, Ju¨rgens M, Kaspar H, Koenigsmann H, Lochhaas KH, Maurer AR, et al., “Coating composition”, US Patent 7,754,795, assigned to 3M Innovative Properties Company, July 13, 2010. Reiling VG. “Low friction composition containing a resinous binder and degraded polytetrafluoroethylene particles”, US Patent 3,287,288, no assignment, November 22, 1966. Van Voorhees AJ. “Electrically conductive, low friction fluorocarbon polymer coating method”, US Patent 3,573,230, assigned to Acheson Industries, Inc., March 30, 1971. Attwood TE, Buckley RP. “Coating compositions containing a mixture of a tetrafluoroethylene polymer an aromatic polyethersulfone”, US Patent 4,090,993, assigned to Imperial Chemical Industries, Ltd, May 23, 1978. Attwood TE, Buckley RP. “Coating process using dispersions of tetrafluoroethylene polymers and polyethersulphones and article”, US Patent 4,131,711, assigned to Imperial Chemical Industries, Ltd, May 23, 1978. Riddle R. “Paint composition”, US Patent 5,039,745, assigned to Creative Urethane Concepts, Inc., August 13, 1991. Smierciak RC, Giordano PJ. “Coating of an epoxy resin, fluorocarbon polymer fluorinated curing agent”, US Patent 5,075,378, assigned to The Standard Oil Company, December 24, 1991. Huggins GE. “Surface coating compositions”, US Patent 5,403,882, assigned to Eeonyx Corp., April 4, 1995. Moyle RT, Anderson KP, Paczesny J, Pisapia J, Whitherup LE. “Epoxy corrosion-inhibiting coating composition”, US Patent 5,859,095, assigned to Morton International, Inc., January 12, 1999. Steckel TF. “Coating additive, coating composition containing said additive and method for

FLUOROPOLYMER ADDITIVES

216

63.

64.

65.

66.

67.

68.

69.

70.

71.

72.

73.

74.

coating a substrate using said coating composition”, US Patent 5,863,875, assigned to The Lubrizol Corporation, January 26, 1999. Arnold CB. “Lubricant composition for use in snow sliders”, US Patent 6,465,398, assigned to E. I. du Pont de Nemours and Company, October 15, 2002. Krendlinger E, Heinrichs F-L, Nowicki D. “Use of wax mixtures for coatings”, US Patent 6,761,764, assigned to Clariant GmbH, July 13, 2004. Coates M, Demonde W, Davidson K. “Surface-treating fluoropolymer powders using atmospheric plasma”, US Patent 6,824,872, assigned to Laurel Products LLC, Nov 30, 2004. Coates M, Demonde W, Davidson K. “Method of treating fluoropolymer particles and the products thereof”, US Patent 7,220,483, assigned to Laurel Products, LLC, May 22, 2007. Coates M, Demonde W, Davidson K. “Method of treating fluoropolymer particles and the products thereof”, US Patent 7,811,631, assigned to Laurel Products LLC. Nayder DA, Blaha DA. “Aqueous coating compositions, process and coated substrates”, US Patent 5,218,031, assigned to Man-Gill Chemical Company, June 8, 1993. Ren BQ. “Lubricious surface extruded tubular members for medical devices”, US Patent 6,086,970, assigned to SciMed Life Systems, Inc., July 11, 2000. Runge HV. “Surface penetrating fluoropolymer lubricant”, US Patent 4,615,917, assigned to Fluorocarbon Technologies, Inc., October 7, 1986. www.dupont.com/teflon_industrial/en_us/products/ product_by_name/zonyl_ptfe/zonyl_ptfe/index. html. Brady JD. “Telomers of tetrafluoroethylene”, US Patent 3,067,262, assigned to E. I. DuPont de Nemours & Company, December 4, 1962. Kuhls J, Fitz H, Haasemann P. “Fluorocarbon waxes and process for producing them”, US Patent 3,956,000, assigned to Hoechst Aktiengesellschaft, May 11, 1976. Morgan RA, Luc GPJ. D’haenens, “Non-chalking release/wear coating”, US Patent 6,060,167, assigned to E. I. DuPont de Nemours & Company, May 9, 2000.

75. Paulus GF. “Thermosettable resin compositions and method for forming low friction surface coatings”, US Patent 3,293,203, assigned to Acheson Industries, Inc., December 20, 1966. 76. Yasuda T. “Compound for coating containing fluorocarbonpolymer and method for its manufacture”, US Patent 4,297,447, assigned to Elastoflon Inc., October 27, 1981. 77. Berghmans JML. “Coating of poly(arylene sulfide), fluoropolymer and aluminum flake”, US Patent 4,287,112, assigned to E. I. du Pont de Nemours & Company, September 1, 1981. 78. Rau SE, Roberts R, Pochopien KP, Paul CW, Butler RA, Mackinlay AJ, et al., “Polymer-metal bonded composite and method of producing same”, US Patent 4,897,439, assigned to Edlon Products, Inc., January 30, 1990. 79. Brothers PD, Kerbow DL, McKeen LW. “Multicomponent particles of fluoropolymer and high temperature resistant non-dispersed polymer binder”, US Patent 6,232,372, assigned to E. I. du Pont de Nemours and Company, May 15, 2001. 80. Cavero J, Huesmann PL. “Process for coating a substrate with a coating including a fluoropolymer, thermoplastic polymer, and filler”, US Patent 7,740,915, assigned to Whitford Worldwide Company, June 22, 2010. 81. Shigenai F, Natatani Y. “Composite Particles, Coating Powder, Coating Film, Laminate, and Method for Producing Composite Particles”, US Patent applc 2015/0030857, assigned to Daikin Industries, LTD., Jan 29, 2015. 82. Seiler DA. PVDF in the chemical process industry. In: Scheirs J, editor. Modern fluoropolymers. John Wiley & Sons; 1997. 83. Tournut C. Thermoplastic copolymers of vinylidene fluoride. In: Scheirs J, editor. Modern fluoropolymers. John Wiley & Sons; 1997. 84. Humphrey JS, Amin-Sanayei R. Vinylidene fluoride polymers. Encyclopedia of Polymer Science and Technology October 2001;22 Available https://doi.org/10.1002/0471440264. from: pst392.0471238961.161512250415081.a01. 85. Koblitz FF, and Petrella R. “Vinylidene fluoride polymer dispersions”, US Patent 3,324,069, assigned to Pennsalt Chemicals Corporation, June 6, 1967.

9: USE

OF

FLUORINATED ADDITIVES

IN

COATINGS

86. Stoneberg RL. “Fluorocarbon resin coated substrates and methods of making”, US Patent 4,314,004, assigned to PPG Industries, Inc., February 2, 1982. 87. Bartoszek EJ, Christofas A. “Aqueous vinylidene fluoride polymer coating composition”, US Patent 4,128,519, assigned to Pennwalt Corporation, December 5, 1978. 88. Memmer TI, Abel PT. “Polyvinylidene fluoride coatings”, US Patent 4,557,977, assigned to SCM Corporation, December 10, 1985. 89. Tortorello AJ, Higginbotham CA. “Thermosetting fluorocarbon polymer coatings”, US Patent 4,659,768, assigned to DeSoto, Inc., April 21, 1987. 90. Connelly BA, Wehrle ME, Greigger PP. “Fluoropolymer based coating composition for adhesion direct to glass”, US Patent 4,879,345, assigned to PPG Industries, Inc., November 7, 1989. 91. Nguyen D, Lauer AJ, Montague RA, Millero Jr. ER. “Method of coating coils using a high solids fluorocarbon coating composition”, US Patent 6,858,258, assigned to PPG Industries Ohio, Inc., February 22, 2005. 92. Stoneberg RL, Stec RR, “Textured fluorocarbon coating compositions”, US Patent 4,400,487, assigned to PPG Industries, Inc., August 23, 1983. 93. Polek MD. “Powder coatings of vinylidene fluoride/hexafluoropylene copolymers”, US Patent 5,177,150, assigned to Elf Atochem North America, Inc., January 5, 1993. 94. Simkin B. “Application of smooth PVOFbased powder coatings to substrates”, US Patent 5,346,727, assigned to Elf Atochem North America, Inc., September 13, 1994. 95. Simkin B. “Powder coating compositions containing VDF/TFE/HFP terpolymers”, US Patent 5,405,912, assigned to Elf Atochem North America, Inc., April 11, 1995. 96. Gaboury SR, Drujon XF. “Fluoropolymer powder coatings from modified thermoplastic vinylidene fluoride based resins”, US Patent 6,362,295, assigned to Atofina Chemicals, Inc., March 26, 2002. 97. Tsuda N, Iwakiri R. “Powder coating composition containing vinylidene fluoride copolymer and methyl methacrylate copolymer”, US Patent 6,551,708, assigned to Daikin Industries Ltd, April 22, 2003.

217

98. Tsuda N, Iwakiri R. “Method for preparing a powder coating composition”, US Patent 6,803,419, assigned to Daikin Industries, Ltd, October 12, 2004. 99. Lin S-C, Kelly M, Kent B. “Precipitation process for making polyvinylidene fluoride powder coatings and coatings made by the process”, US Patent 6,812,267, assigned to Solvay Solexis, Inc., November 2, 2004. 100. Aharoni SM, Nahata A, Yardley JT. “Fluoropolymer blends for coatings”, US Patent 5,118,579, assigned to Allied-Signal Inc., June 2, 1992. 101. Mascia L, Moggi G, Ingoglia D. “Process for protecting and consolidating stone materials”, US Patent 5,212,016, assigned to Ausimont S. p.A., May 18, 1993. 102. Williams H, Young C-I. “Compositions for providing abherent coatings”, US Patent 4,321,404, assigned to Minnesota Mining and Manufacturing Company, March 23, 1982. 103. Moore GGI, Kumar RC, Qiu Z-M, Clark JC, Jariwala CP, Klun TP. “Water- and oilrepellent fluorourethanes and fluoroureas”, US Patent 7,268,197, assigned to 3M Innovative Properties Company, September 11, 2007. 104. Morgan R, Jones C, Treat T, Hrivnak J. “Polymerization of fluoromonomers”, US Patent 6,395,848, assigned to E. I. Dupont de Nemours & company, May 28, 2002. 105. Marchese E, Kapeliouchko V, Colaianna P. “TFE polymerization process”, US Patent 6,297,334, assigned to Ausimont S.p.A., Oct 2, 2001. 106. Weinberg S, Schwab T, Poggio T, Kapeliouchko V, Caille J-R, Gloesener D. “Aromatic sulfone polymer composition comprising tetrafluoroethylene polymer particles”, US Patent Applic. 20090264594, assigned to Solvay Oct 22 2009. 107. Zhang J, Polycarpou AA, Economy J. An improved tribological polymer-coating system for metal surfaces. Tribol Lett 2010;38:355 65. 108. Brown H, Tomaszewski TW. “Codeposition of a metal and fluorocarbon resin particles”, US Patent 3,677,907, assigned to The Udylite Corporation, July 18, 1972.

218

109. Helle K, Groot RC. “Stable dispersion of positively charged polyfluorocarbon resin particles”, US Patent 4,302,374, assigned to AKZO N.V., November 24, 1981. 110. Assoul M, Pena-Munoz E, Roizard X. A tribological study of electrolytic nickel-PTFE composite coatings. J Synthetic Lubric 1998;15(2):107 16. 111. Pena-Munoz E, Bercot P, Grosjean A, Rezrazi M, Pagetti J. Electrolytic and electroless coatings of Ni-PTFE composites, study of some characteristics. Surf Coat Technol 1998;107:85 93. 112. Sahoo P, Das S. Tribology of electroless nickel coatings-a review. Mater Design 17601775;32. 113. Sudagar J, Lian J, Sha W. Electroless nickel, alloy, composite, and nano coatings a critical review. J Alloys Compoud 2013;571:183 204. 114. Henry JR, Summers EM. “Co-deposition of fluorinated carbon with electroless nickel”, US Patent 4,830.889, assigned to Wear-Cote International, Inc., May 16, 1989. 115. Feldstein N, Lindsay DJ. “Composite electroless plating-solutions, processes, and articles thereof”, US Patent 5,145,517, assigned to Surface Technology, Inc., September 8, 1992. 116. Crotty D, Bengston J. “Polytetrafluoroethylene dispersion for electroless nickel plating applications”, US Patent 6,837,923, no assignment, January 4, 2005. 117. Straffelini G, Columbo D, Molonari A. Surface durability of electroless Ni-P composite coatings. Wear 1999;236:179 88. 118. Ramalho A, Miranda JC. Friction and wear of electroless NiP and NiP 1 PTFE coatings. Wear 2005;259:828 34.

FLUOROPOLYMER ADDITIVES

119. Wu Y, Liu L, Shen B, Hu W. Study of selflubricant Ni-P-PTFE-SiC composite coating. J Mater Sci 2005;40(18):5057 9. 120. Watanabe N, Chong Y-B, Matsumura S. “Fluorine compound - containing composite material and method of preparing same”, US Patent 5,589,271, assigned to Nobuatsu Watanabe, Yong-Bo Chong, and C. Uyemura & Co., December 31, 1996. 121. Rahmati H, Mahboobi F. The effect of simultaneous incorporation of PTFE nanoparticles and carbon nanotubes on the tribological cehavior of Ni-P coating. Iranian J Oil Gas Sci Technol 2016;5(3):73 81 (http://ijogst. put.ac.ir). 122. Pazderova M, Bradac M, Vales M. Tribological behavior of composite coatings. Procedia Eng. 2011;10:472 7. 123. Yoshimura T, Tomihashi N, Chida A. “Vinylidene fluoride resin composition”, US Patent 5,130,201, assigned to Daikin Industries, Ltd, July 14, 1992. 124. Guo R, Yin G, Sha X, Wei L, Zhao Q. “Effect of surface modification on the adhesion enhancement of electrolessly deposited Ag-PTFE antibacterial composite coatings to polymer substrates. 125. Jun T, Qunji X. Surface modification of PTFE by 60Co gamma-ray irradiation. J. Appl Polym Sci 1998;69:435 41. 126. Lunkwitz K, Burger W, Lappan U, Brink HJ, Ferse A. Surface modification of fluoropolymers. J Adhes Sci Technol 1995;9 (3):297 310. 127. Goldberg NN, Hudock JS. “Oil and dirt repellent alkyd paint”, US Patent 4,600,441, assigned to Westinghouse Electric Corp., July 15, 1986.