Patent Application
20220325136
Energy savings and safety are among many potential benefits expected for advanced icephobic coatings. For example, keeping wind turbine blades free from ice accumulation is a major concern for wind farm operators in cold climates. Applications with equally demanding requirements include aircraft wings, where thin coatings are required for weight savings. This application is challenging for elastomeric coatings described herein because adhesion of a rigid object (ice) is inversely related to thickness (˜t/2).
Coating development is challenging because anti-icing technologies are needed to prevent or retard formation for several forms of ice.
For icephobic coatings, desirable features include ease of application and cure, good adhesion to substrates and long-term durability and economy. Coatings based on poly(dimethylsiloxane) elastomers have received much attention due to well-known water repellency and ease of application. Combining repellency with micro-scale roughness (superhydrophobicity) has been employed for amplifying icephobic properties. However, frost accumulation is unimpeded by rugosity and high humidity can result in ice formation between asperities which are subject to damage. Another approach to improve icephobicity is coatings containing perfluorinated species but these are bioaccumulative with little chance for translation to applications.
Icephobic coatings with oil addition have heretofore been very promising. Low or even negligible ice adhesion can be achieved by oil adsorption to porous substrates such as aluminum/boehmite. Low levels of oil addition to polyurethanes has been reported to give tough icephobic “slippage” coatings. The same oil addition strategy has been used for silicone coatings to achieve good durability and ice removal shear strength˜14 kPa.
A laboratory method for determining ice adhesion to silicone coatings with a dynamic mechanical analyzer has been previously reported. Using this method, the dependence of ice adhesion on coating thickness was established for platinum cured Sylgard 184 coatings. Over a coating thickness range of 2-600 m ice adhesion strength (Tice, previously Ps) was found to decrease with increased thickness, leveling off at˜270 μm. For elastomeric coatings, thickness is important because thicker coatings facilitate larger displacements that build up interfacial stress.
Data points are marked with wt % MQ-R.
One embodiment provides a composition, comprising a polymerization product of a reactant composition, the reactant composition comprising:
(a) vinyldimethylsiloxy-terminated polydimethylsiloxane, as monomer (MviDMvi); (b) 45-55% poly(methylhydro-co-dimethylsiloxane), a, 2-trimethylsiloxy terminated, as crosslinker (MDHDM);
(c) platinum-divinyltetramethyl-disiloxane complex, as catalyst;
(d) 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane, as inhibitor; and
(e) a filler-like resin composition comprising one or more of dimethylvinylated silica, trimethylated silica, and tetra(trimethylsiloxy)silane, as active ingredient, and one or more of xylene, ethylbenzene and toluene (MQ-R resin).
In one aspect, the vinyldimethylsiloxy-terminated polydimethylsiloxane (MviDMvi) has a molecular weight of about 28 kDa. In another aspect, the vinyldimethylsiloxy-terminated polydimethylsiloxane (MviDMvi) is present in an amount of about 40-94 wt. % based on the weight of the reactant composition, which range includes 40, 45, 50, 55, 60, 70, 80, 90, and 94 wt. % or any subrange therein.
In one aspect, the 45-55% poly(methylhydro-co-dimethylsiloxane), a, 2-trimethylsiloxy terminated, has a molecular weight of about 900-1200 Da, which range includes about 900, 950, 1000, 1050, 1100, and 1200 Da or any subrange therein. In another aspect, the 45-55% poly(methylhydro-co-dimethylsiloxane), a, 2-trimethylsiloxy terminated, is present in an amount of about 1-15 wt. % based on the weight of the reactant composition, which range include about 1, 3, 5, 7, 9, 11, 13 and 15 wt. % or any subrange therein.
In one aspect, the platinum-divinyltetramethyl-disiloxane complex is present in an amount of about 0.01 wt. % or less (measured as Pt); from 0.0001 to 0.01 wt. %, from 0.0001 to 0.005 wt. %, or from 0.001 to 0.005 wt. % based on the weight of the reactant composition.
In one aspect, the 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane is present in amount of about 0.2-1 wt. %, based on the weight of the reactant composition, which range includes about 0.2, 0.4, 0.6, 0.8 and 1 wt. % or any subrange therein.
In one aspect, the active ingredient of the filler-like resin composition (MQ-R resin) is present in an amount of about 5-50 wt. %, based on the weight of the reactant composition, which range includes about 5, 7, 10, 15, 20, 25, 30, 35, 40, 45 and 50 wt. % or any subrange therein.
In one aspect, the reactant composition further comprises hexane as solvent.
In one aspect, the polymerization product is a product of polymerization at a temperature of about 25 to 160° C., which range includes about 25, 30, 35, 40, 50, 60, 70, 90, 100, 120, 140, and 160° C. or any subrange therein.
In one aspect, the composition of has a storage modulus (E′) of about 1-15 MPa at−10° C. In another aspect, the composition has a storage modulus (E′) of about 1-20 MPa at −30° C.
In one aspect, the reactant composition comprises:
(a) 40-94 wt. % vinyldimethylsiloxy-terminated polydimethylsiloxane, as monomer (MviDMvi);
(b) 1-15 wt. % 45-55% poly(methylhydro-co-dimethylsiloxane), α, Ω-trimethylsiloxy terminated, as crosslinker (MDHDM);
(c) 0.001-0.005 wt. % (measured as Pt) platinum-divinyltetramethyl-disiloxane complex, as catalyst;
(d) 0.2-1 wt. % 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane, as inhibitor;
and
(e) 5-50 wt. % a filler-like resin composition comprising one or more of dimethylvinylated silica, trimethylated silica, and tetra(trimethylsiloxy)silane, as active ingredient, and one or more of xylene, ethylbenzene and toluene (MQ-R resin).
Another embodiment provides a coating, comprising the composition.
In one aspect, the coating has a thickness of about 1-1000 μm, which range includes 1, 5, 20, 50, 100, 250, 500, 700, 1000 μm, or any subrange therein.
In one aspect, the coating has an ice adhesion strength (τice) of about 15-40 kPa at−10° C. In another aspect, the coating has an ice adhesion strength (τice) of about 20-60 kPa at −30° C.
Another embodiment provides an article, comprising the coating on a surface thereof.
In one aspect, the article may be selected from the group consisting of an airfoil, wing, propeller, hull, superstructure, railing, intake, hatch, keel, rudder, deck, antenna, medical device, kitchen device, counter, pipe, wind turbine, wind turbine blade, aircraft, ship, rotor blade, transmission tower, transmission line, cable, cooling coil, refrigerator, freezer, wire, tape, adhesive tape, wrap, solar panel, window, wall, floor, siding, roofing, shingle, tower, train, train undercarriage, automobile, cowling, cover, evaporator, condenser, radiator, metal, plastic, or combination thereof, comprising any of the coatings or compositions on a surface thereon. In another aspect, one or more intervening layer, primer, adhesive, tape, other coating, or a combination thereof may be present between the coating and the surface of the article.
Another embodiment provides a method, comprising contacting:
(a) vinyldimethylsiloxy-terminated polydimethylsiloxane, as monomer (MviDMvi);
(b) 45-55% poly(methylhydro-co-dimethylsiloxane), α, Ω-trimethylsiloxy terminated, as crosslinker (MDHDM);
(c) platinum-divinyltetramethyl-disiloxane complex, as catalyst;
(d) 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane, as inhibitor; and
(e) a filler-like resin composition comprising one or more of dimethylvinylated silica, trimethylated silica, and tetra(trimethylsiloxy)silane, as active ingredient, and one or more of xylene, ethylbenzene and toluene (MQ-R resin), to produce a reactant composition;
and polymerizing, to produce a polymerization product.
In one aspect, the polymerizing is carried out at a temperature of about 25 to 160° C., which range includes about 25, 30, 35, 40, 50, 60, 70, 90, 100, 120, 140, and 160° C. or any subrange therein. In another aspect, the method further comprises contacting the reactant composition with a surface, and polymerizing, to produce a coating comprising the polymerization product on said surface.
In one aspect, the coating is ice-phobic. In another aspect, the coating has an ice adhesion strength (τice) of <30 kPa at −30° C.
Another embodiment provides a reactant composition, comprising:
(a) vinyldimethylsiloxy-terminated polydimethylsiloxane, as monomer (MviDMvi);
(b) 45-55% poly(methylhydro-co-dimethylsiloxane), α, Ω-trimethylsiloxy terminated, as crosslinker (MDHDM);
(c) platinum-divinyltetramethyl-disiloxane complex, as catalyst;
(d) 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane, as inhibitor; and
(e) a filler-like resin composition comprising one or more of dimethylvinylated silica, trimethylated silica, and tetra(trimethylsiloxy)silane, as active ingredient, and one or more of xylene, ethylbenzene and toluene (MQ-R resin).
Another embodiment provides a method, comprising contacting:
(a) vinyldimethylsiloxy-terminated polydimethylsiloxane, as monomer (MviDMvi);
(b) 45-55% poly(methylhydro-co-dimethylsiloxane), a, 2-trimethylsiloxy terminated, as crosslinker (MDHDM);
(c) platinum-divinyltetramethyl-disiloxane complex, as catalyst;
(d) 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane, as inhibitor; and
(e) a filler-like resin composition comprising one or more of dimethylvinylated silica, trimethylated silica, and tetra(trimethylsiloxy)silane, as active ingredient, and one or more of xylene, ethylbenzene and toluene (MQ-R resin).
to produce a reactant composition.
Another embodiment provides a kit, comprising:
a first composition, comprising:
(a) vinyldimethylsiloxy-terminated polydimethylsiloxane, as monomer (MviDMvi);
(c) platinum-divinyltetramethyl-disiloxane complex, as catalyst;
(d) 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane, as inhibitor; and
(e) a filler-like resin composition comprising one or more of dimethylvinylated silica, trimethylated silica, and tetra(trimethylsiloxy)silane, as active ingredient, and one or more of xylene, ethylbenzene and toluene (MQ-R resin);
and
a second composition, comprising:
(b) 45-55% poly(methylhydro-co-dimethylsiloxane), α, Ω-trimethylsiloxy terminated, as crosslinker (MDHDM).
In one aspect, the first composition further comprises hexane as solvent. In another aspect, the second composition further comprises vinyldimethylsiloxy-terminated polydimethylsiloxane as diluent.
Another embodiment provides a method, comprising contacting the first and second compositions, to produce a reactant composition.
In one aspect, the method further comprises contacting the reactant composition with a surface; and polymerizing, to produce a coating on said surface.
In one aspect, SYL-OFF™ 7210, available from the Dow Chemical Company, is suitable for use as MQ-R resin.
Low adhesion of ice (icephobic) coatings have been previously made by adding silicone oil and other oils to conventional coatings. The Pt-PDMS coatings of the present invention do not use oils to decrease ice adhesion. Nor do these coatings employ fluorinated components that are or degrade to bio-accumulative compounds. Furthermore, the coatings of the present invention are prepared from commercially available starting materials. Finally, the coatings of the present invention do not rely on fragile super-hydrophic wetting behavior that is generated by microfabrication or other expensive methods.
Cure at temperatures above 100° C. are required to generate maximum icephobic properties but other temperature cures, for example ambient temperature cures works well and are also contemplated.
To explore novel coatings with potential for icephobicity, characterization of a series of platinum-cured silicone coatings incorporating SYL-OFF™ 7210, designated MQ-R, was carried out. These optically transparent coatings are designated according to cure temperature and MQ-R wt %, e.g., Pt-PDMS(25)-20 represents 25° C. cure and 20 wt % MQ-R. Surface characterization included dynamic contact angles and morphology by atomic force microscopy. Bulk characterization utilized stress-strain measurements at ambient temperature and dynamic mechanical analysis from −110 to 150° C. Ice adhesion strength was determined by measuring the force, F, to remove ice for a given area A and defining ice adhesion strength as τice=F/A. Results show that at −10° C. storage modulus had a dominant effect in increasing τice. For −30° C. storage modulus was greater for coatings cured at 100° C. compared to 25° C., but ice removal tests at −30° C. (−22° F.) consistently showed τice for Pt-PDMS(100) MQ-R compositions was less than τice for Pt-PDMS(25). One possible reason could be that supercooled water at hydrophilic interfacial sites (−10° C.) does not impede ice adhesion but frozen water pins ice at −30° C. As a consequence of a relatively low surface density of frozen interfacial pinning sites, τice is less for Pt-PDMS(100) than that for Pt-PDMS(25) analogs at −30° C. despite the higher modulus for Pt-PDMS(100) coatings. Given the criterion that τice, <30 kPa at −30° C. constitutes “icephobicity”, Pt-PDMS(25)-5 and Pt-PDMS(100)-5 qualify for the icephobic designation. Interestingly, Pt-PDMS(100)-10 also meets this criterion (τice=28 kPa) but Pt-PDMS(25)-10 fails (τice=34 kPa). At 20 wt % MQ-R and above, τice increases to >30 kPa at −30° C. even for Pt-PDMS(100) compositions. Decreased hydrophilic site density is unable to overcome the effect of increased modulus.
Surprisingly, MQ-R was found to be a reactive filler that increased modulus depending on cure temperature, especially for Pt-PDMS(100)-30 (3 MPa) and Pt-PDMS(100)-40 (5 MPa).
In the present investigation, platinum cured PDMS coatings were prepared incorporating SYL7210, which is designated MQ-R for MQ-resin. Ice adhesion strength was determined by measuring the force, F, to remove ice for a given area A and defining ice adhesion strength as τice=F/A. An objective was to discern effects of increased modulus associated with increasing weight percent of MQ-R. Secondly, cure was carried out at 25 and 100° C. to confirm effects of water adhesion, namely low temperature cure results in adhesion of water drops (sticky) while cure at −100° C. greatly reduces adhesion of water drops (slippery). The difference can be expressed in terms of work of adhesion and receding contact angles (Eq 1), where wa is water drop adhesion and θR is the receding contact angle.
w
aα(1+cosθR) Eq 1
Without addition of oil, optically transparent “icephobic” coatings with nanoscale smoothness, good mechanical properties and τice less than 30 kPa have now been surprisingly found. For coatings cured at 100° C. the storage modulus was greater compared to cure at 25° C. For ice adhesion at −10° C. increased storage modulus had a dominant effect in increasing τice. However, ice removal tests at −30° C. (−22° F.) consistently showed τice for Pt-PDMS(100) MQ-R compositions was less than τice for the corresponding Pt-PDMS(25). A hypothesis is presented for origins for the opposite trends for τice at −10° C. and −30° C.
Experimental
Materials. Vinyldimethylsiloxy-terminated polydimethylsiloxane (MviDMvi, 28 kDa), poly(methylhydro-co-dimethylsiloxane), α, Ω-trimethylsiloxy terminated (MDHDM, crosslinker, 45-55 mol % DH, MW 900-1200 Da) and platinum-divinyltetramethyl-disiloxane complex, (3.0-3.5 wt. % platinum concentration in another divinyl polydimethylsiloxane as a separate solvent (catalyst)) were purchased from Gelest. In one embodiment, it is contemplated that the divinyl polydimethylsiloxane solvent in the platinum-divinyltetramethyl-disiloxane complex (catalyst) has a molecular weight the same as or similar to that of the vinyldimethylsiloxy-terminated polydimethylsiloxane MViDMVi, 28 kDa, or has a different molecular weight. The 2,4,6,8-tetramethyl-2,4,6,8-tetravinyl-cyclotetrasiloxane (inhibitor) was obtained from Tokyo Chemical Industry Co., Ltd. (TCI). Filler-like SYL-OFF™7210 resin, designated MQ-R was from Dow Corning. According to the MSDS, SYL-OFF™7210 contains xylene, ethylbenzene and toluene. SYL-OFF™7210 resin has 60% active ingredient. Without wishing to be bound, it is believed that this active ingredient includes one or more or all of dimethylvinylated silica, trimethylated silica, dimethylvinylated and trimethylated silica, and tetra(trimethylsiloxy)silane. Compositions reported herein are adjusted to wt % active ingredient or subject ingredient.
Coating Preparation. Platinum-divinyltetramethyl-disiloxane complex (catalyst) and 2,4,6,8-tetramethyl-2,4,6,8-tetravinyl-cylotetrasiloxane (inhibitor) were each diluted in hexane to 10 wt %. The amounts of materials, including MViDMVi, MDHDM crosslinker, inhibitor and catalyst for preparing a series of coatings with increasing MQ-R resin content are shown in Table 1.
aEach composition contained 1.5 g MDHDM crosslinker, 0.75 g inhibitor solution (10 wt % in hexane) and 150 μL catalyst solution (10 wt % in hexane).
In a typical coating preparation, MviDMvi, 0.75 g inhibitor solution (10 wt % in hexane), MQ-R resin and MDHDH crosslinker were placed in a 40 g Flacktek screw-top container. The container was put in a Speed Mixer-DAC 150FV (Flacktek Inc., Landrum SC) followed by high-speed mixing at 2700 rpm for 60 s. This process was repeated 3 times to obtain a homogeneous, bubble-free resin. The resin was cooled in a freezer (−20° C.) for 30 min to slow gelation in the next step. 150 μL catalyst solution (10 wt % in hexane) was added to the resin followed by three high-speed mixing sequences (2700 rpm/60 s). For each resin mixture, 8 dip-coated coverslips and 8 drip-coated microscope slides (1 g material on each slide) were prepared. Dip-coating refers to immersing the coverslips vertically in the resin followed by manipulation to give a reasonably smooth coating. Drip-coating refers to using a pipette to transfer the resin onto one side of a microscope slide and spreading evenly. Plaques were prepared by pouring resin into PTFE plates (1.5-2 g for each plate).
After gelation, one set of coatings was cured in air at ambient temperature for 48 h. A second set of coatings was cured in air at 100° C. by placing coated slides, coverslips and plaques in an oven for 48 h. Coatings are designated Pt-PDMS(25) and Pt-PDMS(100) according to cure temperature. Coatings and plaques were optically transparent and bubble-free.
Dynamic contact angles. Wetting characteristics were determined by dynamic contact angle measurements (DCA, Wilhelmy plate) with a First Ten Angstroms DCA instrument.
Briefly, beakers used for DCA analysis were cleaned by rinsing with distilled water, and treating with a gas/oxygen flame. Water surface tension was checked before each measurement by a contact angle measurement with a flamed glass cover slip.
Deionized water was used as the probe liquid with immersion/withdrawal rates of 100 μms−1. Force-distance curves (fdc's) were obtained and analyzed with instrument software to obtain advancing (θA) and receding (θR) contact angles. Three cycles in succession were obtained to study any change in wetting behavior on immersion into water. Reported contact angles are averages of several force-distance cycles. Surface tension of the test water used for analysis was examined with a flamed-glass coverslip to assess whether leaching of contaminants had occurred.
Atomic Force Microscopy. Morphological investigations were carried out with a Bruker Dimension FastScan atomic force microscope (AFM). Tapping-mode imaging was performed in air using microfabricated silicon cantilevers (40 N/m, Veeco, Santa Barbara, Calif.). Tapping force corresponded to set-point ratio rsp of 0.8, where rsp=Aexp/Ao, Ao is free oscillation amplitude and Aexp is experimental oscillation amplitude. Scanned images were analyzed and processed using NanoScope Analysis 1.8 software. Height, 3D height and phase images were obtained to assess morphological features and roughness.
Mechanical Testing. Dynamic mechanical analysis (DMA). A TA Instruments RSA-III dynamic mechanical analyzer was used for DMA on samples cut from plaques. Temperature was ramped from −120 to 150° C. at 5° C. min−1. Tension cycles were set at 1 Hz with maximum strain of 0.05%. Maximum autotension was set to 2 mm at a rate of 0.01 mm s−1.
Tensile Testing. Rectangular samples were stamped from cast plaques. After thickness, width, and gauge (mm) measurements, samples were clamped into the holder of a TA Instruments RSA-III. The sample elongation rate was 0.05 mm/s with a data acquisition rate of 1 Hz (24° C.). The modulus was determined from the initial portion of the stress-strain curve.
Differential Scanning Calorimetry. Thermograms were obtained with a TA-Q1000 Series™ Instrument (TA Instruments). Samples (5-10 mg) were equilibrated at −90° C. followed by a heating ramp of 10° C./min to 100° C. A cooling ramp of 10° C./min was applied back to −90° C. and a second heating cycle of 10° C./min to 150° C. was employed. The consecutive heating cycles were followed to observe any changes due to heating.
Ice Adhesion Test. These measurements followed a previous report using a TA Instruments RSA-III and a sample holder for 1 in×1 in coated pieces cut from microscope slides. The force probe was fitted into the upper grip so that it passed 2 mm from the coating surface. Temperature control was achieved with liquid nitrogen boil-off. Tests were conducted at−10 or −30° C. (±0.5° C.).
Coating thickness was fixed at˜270 μm as our previous study showed that peak removal force plateaued at this thickness or greater. Molds to form ice cylinders on coatings were made by cutting 2 cm long pieces from the end (top) of plastic pipettes. The mold was filled with 200 μL of distilled water, placed on the coating and introduced into a freezer at −15° C. for 2-3 h. Ice cylinders were 7.5 mm in diameter (1000 μL pipet) or 5.2 mm in diameter (200 μL pipet).
A coated slide with an adherent ice cylinder was rapidly transferred from the freezer to the precooled chamber and sample holder. After allowing 2 min for thermal equilibration, the force probe was engaged and moved toward the ice cylinder at a selected speed. DMA software was modified to obtain a force vs. distance curves. Removal force is calculated according to Eq 2,
where τice is removal force (kPa), M is the normal force recorded by the load cell (g), g is the gravitational constant (9.8 m/s2), and d is the diameter of ice cylinder (mm).
Representative force-distance curves are shown in
Results
A study was initiated to evaluate SYL-OFF 7210 (MQ-R) as a filler on icephobic behavior and mechanical properties of a model silicone coating. SYL-OFF 7210 and similar compositions have been described as “release” resins for applications such as double sided tapes. From our AFM studies (vida infra) the nature of the “active ingredient” appears similar to treated fumed silica nanoparticles.
Designations for specific coatings include the MQ-R wt %, for example, Pt-PDMS(100)-30, where 30 is the MQ-R wt %.
Surface characterization. Dynamic contact angles. Contact angles for Pt-PDMS(25) and Pt-PDMS(100) are listed in Table 2.
aContact angle hysteresis = θΔ = θA − θR
bResults are excerpted from Table 5 (Run 1, cycle 1).
Force distance curves associated with contact angles in Table 2 are shown in
As illustrated in
W
a-25
/W
a-100 Eq 3
Pt-PDMS coatings and the ratio of the practical work of adhesion for 25 vs 100° C. cure which ranges from 1.5 to 2. The lower receding contact angle for 100° C. cure is important as Mueler, et al., found that ice adhesion (τice) followed the practical work of adhesion of water (Eq 1). By comparing icephobicity for cure at 25 and 100° C., trends based on mechanical properties and the presence of interfacial water (low θR) are described below.
Atomic Force Microscopy. AFM images for Pt-PDMS(100)-30 are shown in
To clarify near surface morphology, height images (50×50 μm) for Pt-PDMS(100) with 0, 5, 10, 20 and 30 wt % MQ-R are shown in
Insight into nanostructure is obtained from 5×5 m phase images for Pt-PDMS(100)-10 and Pt-PDMS(100)-30 shown in
Interestingly,
Bulk Characterization. Stress strain measurements. Representative stress-strain curves determined at ambient temperature for coatings with 20% MQ-R resin cured at 25 or 100° C. are presented in
Stress strain curves are characterized by modest strain hardening at ˜50%. Strain hardening is not observed for Pt-PDMS(25)-0, the base elastomer cured at 25° C. (
For a given composition, the modulus for Pt-PDMS(25) coatings is lower than the modulus of Pt-PDMS(100) analogs (
Unexpectedly, the modulus increased to much higher values for Pt-PDMS(100) at 30% and 40% MQ-R. This trend culminates in a modulus for Pt-PDMS(100)-40 that is double that for Pt-PDMS(25)-40. A higher modulus for Pt-PDMS(100) at 30% and 40% MQ-R is attributed to a chemical reaction of MQ-R with matrix Si—H and/or Si—OH that increases crosslink density via the formation of a third network. Additional evidence for this hypothesis comes from dynamic mechanical analysis in the next section.
A strain at break of ˜100% is typically found for silicone nanocomposites. For Pt-PDMS compositions with lower MQ-R content a relatively high (>150%) strain at break is found. The high modulus for high MQ-R content is accompanied by a decreased strain at break to˜100% for both Pt-PDMS(25) and Pt-PDMS(100) elastomers (
Toughness was determined using the area under the stress strain curves (
DMA and DSC analysis. Studies aimed at understanding low temperature thermal transitions have been carried out for PDMS, PDMS segmented and block copolymers and PDMS nanocomposites. Rates and/or extents of polydimethylsiloxane crystallization are affected by parameters including crosslink density, molecular weight and the presence of nanoparticles. For example, a prior DMA and DSC study on a PDMS nanocomposite containing 10% silica nanoparticles showed that transitions are sensitive to heating and cooling rates.
For Pt-PDMS coatings reported herein the importance of DMA data rests on the determination of mechanical properties at −10 or −30° C. because these are test temperatures for ice adhesion measurements. DSC was used as a complementary method to identify phase transitions accompanied by thermomechanical changes.
DMA for Pt-PDMS(25) and Pt-PDMS(100) films is shown in
Cold crystallization is evidenced by an E′ peak at −75° C. for Pt-PDMS(25)-20 and Pt-PDMS(100)-20. This thermomechanical transition results in a ×10 decrease in modulus for Pt-PDMS(25)-20 while the decrease for Pt-PDMS(100)-20 is ×5. Cold crystallization occurred after cooling at 5° C. min−1 but not for slow cooling (1° C. min−1). The absence of an exotherm in DSC for crystallization for Pt-PDMS(25)-20 and Pt-PDMS(100)-20 is attributed to differences in PDMS molecular weights and instrument limitations for cooling below about −85° C. (
Nanocomposites with 30 and 40% MQ-R have a small change in slope for E′ at about−100° C. near the instrumentation limit. An endotherm at −50° C. is absent for Pt-PDMS(25) and Pt-PDMS(100) with 30 and 40% MQ-R. Rather, a rapid decrease in E′ occurs to about −50° C. where E′ levels out. The glass-like state for Pt-PDMS(100) with 30 and 40 wt % MQ-R results in a higher modulus at ice release test temperatures compared to Pt-PDMS(25) analogs (
Ice adhesion measurements. Modes for ice release. Coating thickness was fixed at ˜270 μm as our previous study showed that peak removal force leveled off at this thickness.
By keeping the probe distance at 2 mm, which is a practical minimum, non-shear contributions are minimized and/or approximately constant. Thus, by not changing the probe distance, coating thickness and contact area, the peak removal force for ice at two test temperatures can be examined for correlations with bulk and surface characterization data.
Representative ice release tests at −10 and −30° C. for Pt-PDMS(25)-20 and Pt-PDMS(100)-20 are shown in
“Mode 2” ice release was characterized by decreases in shear stress at τice to low values, but stress did not reach zero for the remainder of the measurement.
Mode 2 was common in a previous study of thickness dependence of τice for Sylgard 184 but not as common for tests carried out in the present work. About 34% (59 out of 174) tests for ice adhesion resulted in Mode 2 (sliding) release.
For “Mode 3” ice release shear stress oscillated after τice. After one or more oscillations zero force was reached. Mode 3 ice release is illustrated for unfilled Pt-PDMS(100)-0 in
The area under the force-distance curve corresponds to removal energy. Qualitatively, Mode 2 and Mode 3 ice release involve a higher removal energy compared to Mode 1. The absence of a pattern for the different Modes precludes an explanation and requires further study. Below, data for specific compositional ranges are presented.
Low MQ-R (5, 10, 20 wt %). Whether cured at ambient temperature or 100° C., Pt-PDMS nanocomposites with 5 and 10 wt % MQ-R are noteworthy for having τice at or below 20 kPa for tests at −10° C. These compositions have the lowest storage moduli at the −10° C. test temperature: 1.05 and 1.24 mPa, respectively, (Table 3). At −10° C., τice follows compositions with higher modulus in accord with Eq. 2. It is important to note that τice for Pt-PDMS(25)-5 (17 kPa) and Pt-PDMS(100)-5 (19 kPa) was achieved without employing solvents or addition of oils and did not employ fluorous moieties that are likely PFOA-containing precursors.
An effect of increased MQ-R wt % is seen with an increase in storage modulus at −10° C. from 1.3 MPa for Pt-PDMS(25)-10 to 1.89 MPa for Pt-PDMS(100)-10. Within experimental error the effect of increased modulus does not affect τice at −10° C. for Pt-PDMS(25)-10 (21 kPa) compared to Pt-PDMS(25)-5 (19 kPa) (Table 3). For τice at −10° C., Pt-PDMS(25)-10 (21 kPa) is about the same as that for Pt-PDMS(100)-10 (22 kPa). Compared to Pt-PDMS(25) and Pt-PDMS(100) with 5 and 10 wt % MQ-R increasing MQ-R to 20 wt % results in an increase in τice. At −10° C. τice is 36 kPa for Pt-PDMS(25)-20 and 38 kPa for Pt-PDMS(100)-20 which correlates with higher storage moduli (1.71 and 2.76 MPa, respectively).
At the test temperature of −30° C., the storage moduli for Pt-PDMS(25)-5 (1.08 kPa) and Pt-PDMS(100)-5 (1.16 kPa) follow an increase in tensile modulus with increasing cure temperature reported by others. However, τice is lower for Pt-PDMS(100)-5 (24 kPa) compared to Pt-PDMS(25)-5 (29 kPa). Similarly, at the −30° C. test temperature Pt-PDMS(100)-10 with a higher modulus (E′ 1.74 MPa) than Pt-PDMS(25)-10 (E′ 1.22 MPa) has a lower τice (28 kPa) than Pt-PDMS(25)-10 with τice 34 kPa.
aWeight percent MQ-R.
bτice are average values that include all Modes of ice adhesion.
At −30° C. the storage modulus for Pt-PDMS(100)-20 (2.65 kPa) is substantially higher than Pt-PDMS(25)-20 (1.69 kPa). Again, however, at −30° C., τice is clearly lower for Pt-PDMS(100)-20 (46 kPa) compared to Pt-PDMS(25)-20 (61 kPa).
High MQ-R (30, 40 wt %). At 30 and 40% MQ-R storage modulus (E′) increases dramatically (
The storage moduli for 40 wt % MQ-R are remarkably high ranging from 4.15 MPa at −10° C. for Pt-PDMS(25)-40 to 18.4 MPa for Pt-PDMS(100)-40. The amplification is attributed to a chemical reaction of MQ-R with the PDMS matrix and a change in morphology to a glass like solid without a detectable PDMS Tm (
Ice adhesion strength for Pt-PDMS MQ-R coatings.
At −30° C. the elastic modulus is also higher for Pt-PDMS(100) compared to Pt-PDMS(25) (Table 3). Surprisingly, however, tests at −30° C. show a systematic trend of lower τice for cure at 100° C. compared to −10° C. That is, blue bars are higher than orange bars (
Theory and Ice adhesion. Kendall developed a theory for removing a rigid object from an elastomeric substrate. When adapted for adhesion of ice to an elastomer, Eq 4 is derived that correlates the force required to remove a rigid cylinder (τice) with work of adhesion (wa), modulus (K), thickness (t), and area of contact (a). A similar relationship was derived by Ghasemi.
This relationship gave a good fit to the thickness dependence of ice adhesion for a model elastomer (Sylgard 184) over a coating range of 20-300 μm. The prediction that τice is proportional to modulus is borne out by increasing τice at −10° C. with increasing MQ-R wt % (
This finding supports the notion that MQ-R is a reactive filler that increases the modulus by forming a third network depending on temperature and time.
A surprise was lower τice at −30° C. for Pt-PDMS-(100) MQ-R compositions compared with their Pt-PDMS(25) counterparts that have lower storage moduli (
Prior work on poly(PDMS-b-polycarbonate) included adding silicone oil for enhanced ice release. The temperature dependence of ice adhesion (−5 to −20° C.) was reported for a copolymer containing 35 wt % PDMS (dp ˜20). Interestingly, a maximum in ice adhesion was found at −10° C., with much lower adhesion at −20° C. Oil addition resulted in a tenfold reduction of adhesion along with a flattening of temperature dependence.
A model to explain lower Tice for Pt-PDMS(100) at −30° C. begins with considering the theory of wetting behavior for heterogeneous surfaces developed by Pease (Pease, D. M., J. Phys. Chem. 1945, 49, 107-110.) and later by Johnson and Dettre (Johnson, R. E., Jr.; Dettre, R. H., J. Phys. Chem. 1964, 68 (7), 1744-1750). These pioneers showed that a small area fraction of polar moieties had little effect on θA but a strong effect on θR due to water-pinning by hydrogen bonding. This model is adapted herein with consideration of studies of supercooled water and/or mixtures.
Others have reduced ice adhesion by creating hydrophilic “anti-freeze” surfaces. With a test temperature of −15° C., others reduced τice from −320 kPa for Sylgard 184 to −120 kPa by adding a minor amount of PDMS-PEG copolymer. This finding was attributed to interfacial lubrication by nonfrozen water. Grafting polyacrylic acid to a silicon wafer and crosslinking with water-soluble polyethylene glycol diacrylate gave a film that absorbed water and decreased τice to˜55 kPa at test temperatures of −15 to −22° C. At lower temperatures τice rapidly increased to more than 1000 kPa as supercooled water froze and pinned ice. The hydrophilic polymer film thus decreased the freezing point of water to about −20° C. An even lower freezing point depression was obtained by the synthesis of polyurethane nanoparticles having a hydrophilic corona and a polyurethane core. An aqueous dispersion was used for coating and subsequent curing. For optimum compositions the freezing point of water was reduced to about −50° C. Ice adhesion strength was maintained at −27 kPa from −15 to −50° C. Others have also shown a freezing point depression for surfaces functionalized with hydrophilic polymers. From these studies the freezing point depression of surface water depends on surface chemistry and morphology.
According to the analysis of θR described above, Pt-PDMS(25) has a higher density of interfacial water than Pt-PDMS(100) analogs as Wa-25/Wa-100 ranges from 1.5 to 2. Lower adhesion of ice for Pt-PDMS(100) MQ-R compositions at −30° is then explained by the hypothesis that interfacial water is frozen at −30° C. but liquid/supercooled at −10° C. This is illustrated in
Following the hypothesis that water pinning sites are in a liquid/supercooled state at −10° C., the removal of ice is not impeded and modulus controls ice release (Eq 2). That is, Pt-PDMS(100) coatings have a higher modulus than Pt-PDMS(25) analogs resulting in τice at −10° C. for Pt-PDMS(25) coatings being lower than Pt-PDMS(100).
In contrast to hydrophilic “anti-freeze” surfaces described above, the surface concentration of hydrophilic pinning sites defined by θR is low as PDMS surfaces are otherwise hydrophobic as defined by θA. Considering the theory of Johnson & Dettre, a solid with a receding contact angle of ˜40° would have a surface fraction of hydrophilic sites on the order of 0.05 or 5%. Thus, the effect of pinning ice by frozen interfacial water is moderate. At −30° C. τice is about 15% less for Pt-PDMS(100) compared to Pt-PDMS(25).
Icephobicity is characterized by low adhesion strength for ice but tests and standards are not yet in place. Furthermore, icephobicity will depend on the form of ice such as frost or glaze. For the stress test used herein a value of τice<30 kPa at −30° C. may be reasonably taken as a threshold for icephobicity. This threshold was reached for an “antifreeze” coating described as “self-lubricating”. The peak removal force (27 kPa) was retained until the freezing point was reached (˜−50° C.). A similar peak removal force range is found for PDMS blended with oils such as “slippage” coatings prepared by blending 40 wt % silicone oil with a commercial silicone elastomer.
Given the criterion of τice, <30 kPa at −30° C., Pt-PDMS(25)-5 and Pt-PDMS(100)-5 qualify for the icephobic designation (Table 3). Interestingly, Pt-PDMS(100)-10 also meets this criterion (τice=28 kPa) but Pt-PDMS(25)-10 fails (τice=34 kPa). At 20 wt % MQ-R and above, τice increases above 30 kPa at −30° C. even for Pt-PDMS(100) compositions. Decreased interfacial water density is unable to overcome the effect of increased modulus.
The impact of MQ-R in increasing the modulus of Pt-PDMS is surprising and unexpected. Provided herein is a comparison of Pt-PDMS with 30 and 40 wt % MQ-R with a report of mechanical properties for six commercial platinum cured systems. These systems are meant for injection molding applications with cure carried out at elevated temperatures (−200° C.) for less than one minute. Dynamic mechanical analysis (frequency sweep) showed that E′ at ambient temperature was between 2 and 3×106 MPa for all six products. Pt-PDMS(100)-30 and Pt-PDMS(100)-40 have ambient temperature E′ in the same range, namely 2-4×106 MPa (
Equation 4 predicts a correlation of (E′)1/2 with τice. As discussed above, attempts to correlate (E′)1/2 with τice gave nonlinear relationship (
Leaching, contact angles and ice adhesion. Dynamic contact angles were obtained by the Wilhelmy plate method that involves coating a coverslip and immersion in water.
Details are in the Experimental section. The characterization focused on Pt-PDMS(25) and Pt-PDMS(100) MQ-R coatings (1) to obtain contact angles, especially the receding contact angle (θR) which is connected to adhesion of water to silicone coatings and (2) to determine whether or not leaching (oils, surfactants, processing materials) affected the adhesion of ice.
Systematic changes in work of adhesion and the connection to adhesion of ice have been presented and discussed in the main paper. In the section below, evidence for leaching is presented. In summary, we were not able to connect changes in leaching into water during Wilhelmy plate measurements to ice adhesion.
Table 4 lists DCA data taken for repeated runs on Pt-PDMS(25) and Pt-PDMS(100) MQ-R coatings. Receding contact angles for Pt-PDMS(100) coatings are reasonably stable while variations are observed for coatings prepared at ambient temperature.
Minimal water contamination is found for Pt-PDMS(100) coatings. Condensation cure at 100° C. forms a second network that may provide a contorted pathway for leaching or low molar mass species may be incorporated in the network. In addition, volatile species are simply removed.
aRun 1 and Run 2 refer to different coated coverslips. Multiple runs were carried
aContact angle hysteresis = θΔ = θA − θR
The entire contents of each reference mentioned herein are hereby incorporated by reference for all purposes.
The entire contents of Safety Data Sheet for Product name SYL-OFF™ 7210 Release Modifier, published by The Dow Chemical Company, Issue Date 10/31/2018, 17 pp, are hereby incorporated by reference for all purposes.
This application claims the benefit of U.S. Application No. 62/892,686, filed Aug. 28, 2019, the entire contents of which are hereby incorporated by reference, the same as if set forth at length.
This invention was made with government support under Grant Number 1608022 awarded by the National Science Foundation (NSF). The government has certain rights in the invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2020/058072 | 8/28/2020 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 62892686 | Aug 2019 | US |
