FIELD OF THE INVENTION

The present invention generally relates to low-friction and low-adhesion materials, coatings, and systems.

BACKGROUND OF THE INVENTION

Coatings and materials can become soiled from debris (particles, insects, oils, etc.) impacting the surface. The debris affects airflow over the surface as well as aesthetics and normally is removed by washing. Insect impact residue affects vehicle fuel economy, aesthetics, and operator vision. On aircraft, insect residue interferes with airflow over a surface, increasing drag and thus fuel consumption. On automobiles, the light dispersion of headlights, operator vision through the windshield, and aesthetic appeal are degraded from insect remains.

Many solutions to reduce insect debris, such as mechanical scrapers, sacrificial continually released liquid layers, and passive coatings with engineered topology have been flight tested. However, the best-performing liquid layer release systems add a large size and weight penalty while the durability of nanostructured surfaces on aircraft or automobile surfaces is unproven. Attempts to mitigate insect accumulation during the early days of aircraft development included mechanical scrapers, deflectors, traps, in-flight detachable surfaces, in-flight dissolvable surfaces, viscous surface fluids, continuous washing fluids, and suction slots. The results of most of these trials were determined ineffective or impractical for commercial use.

One approach to this problem is to create a passive, self-cleaning surface that removes debris from itself by controlling chemical interactions between the debris and the surface. Passive coatings that reduce insect debris are desirable because they have less parasitic mass and do not require the wiring and energy of active systems. No commercial coating provides sufficient residue reduction. While superhydrophobic surfaces perform well in laboratory testing, their limited durability due to fragile asperities and high solid filling fractions are barriers to adoption. In contrast, currently used highly durable aircraft and automotive coatings are lightly filled polymer systems.

Recently, Wohl et al., “Evaluation of commercially available materials to mitigate insect residue adhesion on wing leading edge surfaces,”Progress in Organic Coatings76 (2013) 42-50 describe work at NASA to create anti-insect adhesion or “bugphobic” surfaces. Wohl et al. tested the effect of organic-based coatings on insect adhesion to surfaces, but the coatings did not fully mitigate the issue. Wohl et al. also describe previously used approaches to reduce bug adhesion such as mechanical scrapers, deflectors, paper and/or other coverings, elastic surfaces, soluble films, and washing the surface continually with fluid.

Superhydrophobic and superoleophobic surfaces create very high contact angles (>150°) between the surface and drops of water and oil, respectively. The high contact angles result in the drops rolling off the surface rather than remaining on the surface. These surfaces do not repel solid foreign matter or vapors of contaminants. Once soiled by impact, debris will remain on the surface and render it ineffective. Also, these surfaces lose function if the nanostructured top surface is scratched.

Enzyme-filled coatings leech out enzymes that dissolve debris on the surface. However, enzymes are quickly depleted and cannot be refilled, rendering this approach impractical.

Kok et al., “Influence of surface characteristics on insect residue adhesion to aircraft leading edge surfaces,”Progress in Organic Coatings76 (2013) 1567-1575, describe various polymer, sol-gel, and superhydrophobic coatings tested for reduced insect adhesion after impact. The best-performing materials were high-roughness, superhydrophobic surfaces. However, they did not show that debris could be removed from the superhydrophobic surfaces once insects broke on the surface.

Polymeric materials having low surface energies are widely used for non-stick coatings. These materials are tailored with careful control of their chemical composition (thus surface energy) and mechanical properties. Polymers containing low-energy perfluoropolyethers and perfluoroalkyl groups have been explored for low adhesion and solvent repellency applications. While these low-energy polymers facilitate release of materials adhering to them in both air and water, they do not necessarily provide a lubricated surface to promote clearance of foreign substances. See Vaidya and Chaudhury, “Synthesis and Surface Properties of Environmentally Responsive Segmented Polyurethanes,”Journal of Colloid and Interface Science249, 235-245 (2002). A fluorinated polyurethane is described in U.S. Pat. No. 5,332,798 issued Jul. 26, 1994 to Ferreri et al.

Fluoropolymer sheets or treated surfaces have low surface energies and thus low adhesion force between foreign matter and the surface. However, friction between impacting debris and the surface results in the sticking of contaminants.

Fluorofluid-filled surfaces have very low adhesion between impacting debris and the surface. However, if any of the fluid is lost, the surface cannot be refilled/renewed once applied on the vehicle, and thus loses its properties (see Wong et al., “Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity,”Nature477, 443-447, 2011).

In view of the shortcomings in the art, improved coating materials and systems, and compositions suitable for these systems, are needed. In particular, what is desired commercially is a highly durable, low-friction coating to reduce drag and improve visibility. Passive coatings that reduce insect debris are desirable because they have less parasitic mass and do not require the wiring and energy of active systems.

SUMMARY OF THE INVENTION

The present invention addresses the aforementioned needs in the art, as will now be summarized and then further described in detail below.

Some variations provide a copolymer composition comprising:(a) fluoropolymers having an average molecular weight from about 500 g/mol to about 20,000 g/mol, wherein the fluoropolymers are (α,ω)-hydroxyl-terminated and/or (α,ω)-amine-terminated, and wherein the fluoropolymers are present in the triblock structure:


wherein:
X, Y=CH2—(O—CH2—CH2)p-T, and X and Y are independently selected;
p=1 to 50;
T is a hydroxyl or amine terminal group;
m=1 to 100; and
n=1 to 100;(b) one or more isocyanate species possessing an average isocyanate functionality of about 2 or greater, or a reacted form thereof; and(c) one or more polyol or polyamine chain extenders or crosslinkers possessing an average functionality of about 3 or greater, or a reacted form thereof.

In some embodiments, the fluoropolymers have an average molecular weight from about 1,000 g/mol to about 10,000 g/mol.

X and Y may be the same or different. That is, the end groups X and Y may differ in terminal group or in the p value. In some embodiments, p is selected from 6 to 50, independently for each of X and Y. In certain embodiments, p is selected from 10 to 20, independently for each of X and Y.

The isocyanate species may be selected from the group consisting of 4,4′-methylenebis(cyclohexyl isocyanate), hexamethylene diisocyanate, cycloalkyl-based diisocyanates, tolylene-2,4-diisocyanate, 4,4′-methylenebis(phenyl isocyanate), isophorone diisocyanate, and combinations or derivatives thereof.

The polyol or polyamine chain extenders or crosslinkers may be selected from the group consisting of glycerol, trimethylolpropane, 1,2,6-hexanetriol, triethanolamine, pentaerythritol, N,N,N′,N′-tetrakis(2-hydroxypropyl) ethylenediamine, and homologues, derivatives, or combinations thereof.

In some embodiments, the composition contains, in a hard segment, the reacted form of the one or more isocyanate species, combined with the reacted form of the one or more polyol or polyamine chain extenders or crosslinkers.

The copolymer composition may be present in a coating. In some embodiments, the coating is characterized by a coefficient of friction less than 0.5. In these or other embodiments, the coating is optically transparent.

Other variations of the invention provide a low-friction material comprising:a substantially continuous matrix containing a first component; anda plurality of inclusions containing a second component, wherein the inclusions are dispersed within the matrix;wherein one of the first component or the second component is a low-surface-energy fluoropolymer having a surface energy between about 5 mJ/m2to about 50 mJ/m2, and the other of the first component or the second component is a hygroscopic material that is covalently connected to the fluoropolymer in a triblock copolymer,and wherein the material forms a lubricating surface layer in the presence of humidity.

The fluoropolymer may be selected from the group consisting of polyfluoroethers, perfluoropolyethers, polyfluoroacrylates, polyfluorosiloxanes, and combinations thereof.

The hygroscopic material may be selected from the group consisting of poly(acrylic acid), poly(ethylene glycol), poly(2-hydroxyethyl methacrylate), poly(vinyl imidazole), poly(2-methyl-2-oxazoline), poly(2-ethyl-2-oxazoline), poly(vinylpyrolidone), cellulose, modified cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, hydrogels, PEG diacryalate, monoacrylate, and combinations thereof.

In some embodiments, the fluoropolymer is perfluoropolyether and the hygroscopic material is poly(ethylene glycol). The triblock copolymer may be


wherein:
X, Y=CH2—(O—CH2—CH2)p-T, and X and Y are independently selected;
p=6 to 50;
T is a hydroxyl or amine terminal group;
m=1 to 100; and
n=1 to 100.

In some embodiments, the continuous matrix further contains (i) one or more isocyanate species possessing an average isocyanate functionality of about 2 or greater, or a reacted form thereof; and (ii) one or more polyol or polyamine chain extenders or crosslinkers possessing an average functionality of about 2 or greater, or a reacted form thereof.

The isocyanate species may be selected from the group consisting of 4,4′-methylenebis(cyclohexyl isocyanate), hexamethylene diisocyanate, cycloalkyl-based diisocyanates, tolylene-2,4-diisocyanate, 4,4′-methylenebis(phenyl isocyanate), isophorone diisocyanate, and combinations or derivatives thereof.

The polyol or polyamine chain extenders or crosslinkers may be selected from the group consisting of 1,3-butanediol, 1,4-butanediol, 1,3-propanediol, 1,2-ethanediol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, neopentyl glycol, 1,6-hexanediol, 1,4-cyclohexanedimethanol, ethanol amine, diethanol amine, methyldiethanolamine, phenyldiethanolamine, glycerol, trimethylolpropane, 1,2,6-hexanetriol, triethanolamine, pentaerythritol, ethylenediamine, 1,3-propanediamine, 1,4-butanediamine, diethyltoluenediamine, dimethylthiotoluenediamine, isophoronediamine, diaminocyclohexane, N,N,N′,N′-tetrakis(2-hydroxypropyl) ethylenediamine, and homologues, derivatives, or combinations thereof.

In preferred embodiments, the one or more polyol or polyamine chain extenders or crosslinkers possess an average functionality of about 3 or greater, and the triblock copolymer is


wherein:
X, Y=CH2—(O—CH2—CH2)p-T, and X and Y are independently selected;
p=1 to 50;
T is a hydroxyl or amine terminal group;
m=1 to 100; and
n=1 to 100.

The low-surface-energy polymer and/or the hygroscopic material may be surface-treated. In some embodiments, the material further comprises one or more additional components selected from the group consisting of a particulate filler, a pigment, a dye, a plasticizer, a flame retardant, a flattening agent, and a substrate adhesion promoter. The particulate filler (when present) may be selected from the group consisting of silica, alumina, silicates, talc, aluminosilicates, barium sulfate, mica, diatomite, calcium carbonate, calcium sulfate, carbon, wollastonite, and combinations thereof and wherein the particulate filler is optionally surface-modified with a compound selected from the group consisting of fatty acids, silanes, alkylsilanes, fluoroalkylsilanes, silicones, alkyl phosphonates, alkyl phosphonic acids, alkyl carboxylates, alkyldisilazanes, and combinations thereof.

The low-friction material may be characterized by a water absorption capacity of at least 10 wt % water based on total weight of the material. The low-friction material may further be characterized by a surface contact angle of water of greater than 90°. In these or other embodiments, the low-friction material is characterized by a coefficient of friction less than 0.5, such as less than 0.2. In any of these embodiments, the low-friction material may be optically transparent.

Certain variations of the invention provide a fluoropolymer having an average molecular weight from about 500 g/mol to about 20,000 g/mol, wherein the fluoropolymer is present in the triblock structure:


wherein:
X=CH2—(O—CH2—CH2)p1-T;
Y=CH2—(O—CH2—CH2)p2-T;
X and Y are independently selected and p1 and p2 are independently selected;
either {p1=6 to 50 and p2=0 to 50}, or {p1=0 to 50 and p2=6 to 50};
T is a hydroxyl or amine terminal group;
m=1 to 100; and
n=1 to 100.

