WO2009003847A1

WO2009003847A1 - Moldings with a superhydrophobic surface of high pressure and shear resistance

Info

Publication number: WO2009003847A1
Application number: PCT/EP2008/057796
Authority: WO
Inventors: Werner Michel
Original assignee: Evonik Degussa Gmbh
Priority date: 2007-07-04
Filing date: 2008-06-19
Publication date: 2009-01-08
Also published as: EP2011817A1; ATE449126T1; EP2011817B1; DE502007002052D1

Abstract

Process for producing a molding which comprises a substrate of a polymeric material and particles which are present on the substrate, are bonded to it in a fixed manner and form a superhydrophobic layer having elevations and depressions, in which the hydrophobic nanoscale particles applied to the polymeric material and the support material are subjected to beta and/or gamma radiation over a period of 0.1 second to 5 hours such that a radiation dose of 10 to 1000 kGy is absorbed.

Description

Moldings with a superhydrophobic surface of high pressure and shear resistance

The invention relates to a process for producing a molding with a superhydrophobic surface of high pressure and shear resistance and to the molding itself.

According to the prior art, the wetting behaviour of plastics with respect to water and other highly polar liquids can be modified by coating with hydrophobic particles such that contact angles of more than 120 degrees up to the theoretically possible maximum of 180 degrees can occur in water droplets placed onto them. Such surfaces are referred to as

"superhydrophobic", and, in those cases where values of 140 degrees or higher are obtained, according to a definition of the Deutsche Bundesstiftung Umwelt [German Environment Foundation] (Bonn, 24 October 2000), it is also possible to use the term "Lotus Effect® surface".

The outstanding superhydrophobicity of a natural lotus leaf surface is achieved at the cost of great sensitivity to compressive and shearing (frictional) forces .

The cause is that the micrometer-size raised structures which cover the surface consist of wax crystals which rest only loosely on one another and additionally do not have a high intrinsic strength.

A significant improvement in the pressure and shear resistance is achieved in industrially produced super- hydrophobic surfaces by, instead of the wax crystals formed by the natural route, embedding synthetic particles into a matrix material such that the particles project partly out of the matrix.

In spite of these measures, the shear resistance even of such surfaces is insufficient for many applications in which the superhydrophobic surfaces are, in an unforeseeable manner or in accordance with intended use, subjected to any wiping or frictional stresses.

It was therefore an object of the invention to provide moldings which have improved pressure and shear resistance over the prior art. It was a further object of the invention to provide a process for producing these moldings.

It has now been found that the superhydrophobicity of the surface of a molding has an enhanced resistance to pressure and shear forces once the molding has been irradiated with high-energy radiation.

The invention therefore provides a process for producing a molding which comprises a substrate of a polymeric material and particles which are present on the substrate, are bonded to it in a fixed manner and form a superhydrophobic layer having elevations and depressions, in which the hydrophobic nanoscale particles applied to the polymeric material and the support material are subjected to beta and/or gamma radiation over a period of 0.1 second to 5 hours such that a radiation dose of 10 to 1000 kGy, preferably 25 to 500 kGy is absorbed (1 Gy = 1 kJ/kg) .

In the case of irradiation with beta radiation, the process is preferably performed such that irradiation is effected first over short periods, for example 0.1 to 10 s, then the object is allowed to cool and irradiated again. The cooling times are guided by the material of the substrate, the particles and the type of radiation source. In the case of use of gamma radiation, it is possible to carry out prolonged irradiation times without interruption.

The polymeric material of the substrate is preferably selected from the group consisting of thermoplastics, thermosets and elastomers.

The polymeric material used may more preferably be at least one selected from the group consisting of polycarbonates, polyoxymethylenes, poly (meth) acrylates, polyamides, polyvinyl chloride, polyethylenes, polypropylenes, aliphatic linear or branched polyalkenes, cyclic polyalkenes, polystyrenes, polyesters, polyether sulfones, polyacrylonitrile or polyalkylene terephthalates, poly (vinylidene fluoride), poly (hexafluoropropylene) , poly (perfluoropropylene oxide), poly (fluoroalkyl acrylate) , poly (fluoroalkyl methacrylate) , poly (vinyl perfluoroalkyl ether) or other polymers formed from perfluoroalkoxy compounds, poly (isobutene) , poly (4-methyl-l-pentene) and polynorbornene as a homo- or copolymer. With very particular preference, it is possible to use poly (ethylene) , poly (propylene) , polymethyl methacrylates, polystyrenes, polyesters, polyvinyl chloride, acrylonitrile-butadiene-sytrene terpolymers (ABS) , polyethylene terephthalate, polybutylene terephthalate or poly (vinylidene fluoride), material comprising a rubber, a synthetic rubber or a natural rubber .

The particles used in the process according to the invention are hydrophobic and nanoscale. "Nanoscale" is understood to mean particles having a mean diameter of 2 to 100 nm. In the case of aggregated particles, this term relates to the primary particles present in the aggregate.

Hydrophobic particles are understood to mean those whose hydrophobic properties are attributable to the material properties of the materials themselves present on the surfaces of the particles or whose hydrophobic properties can be obtained by a treatment of the particles with a suitable compound. Before or after the application or binding to the surface of the molding, the particles may have been equipped with hydrophobic properties. To hydrophobize the particles before or after the application to the surface, they may be treated with a compound suitable for hydrophobization, for example from the group of the alkylsilanes, the fluoroalkylsilanes or the disilazanes.

The hydrophobic nanoscale particles used may, for example, be silicates, minerals, metal oxides, metal powders, pigments and/or polymers.

It is possible with preference to use hydrophobic nanoscale metal oxide particles. More preferably, it is possible to use metal oxide particles selected from the group consisting of aluminum oxide, cerium oxide, iron oxide, silicon dioxide, titanium dioxide, zinc oxide, zirconium dioxide and mixed oxides of the aforementioned oxides. Mixed oxides may preferably be binary mixed oxides, for example silicon titanium mixed oxides or silicon aluminum mixed oxides.

Most preferably, it is possible to use pyrogenic metal oxide particles. These may have a BET surface area of 20 to 400 m²/g and especially of 35 to 300 m²/g. Pyrogenic metal oxide particles in the context of the invention include aluminum oxide, cerium oxide, iron oxide, silicon dioxide, titanium dioxide, zinc oxide, zirconium dioxide and mixed oxides of the aforementioned oxides.

"Pyrogenic" is understood to mean metal oxide particles obtained by flame oxidation and/or flame hydrolysis. In this case, oxidizable and/or hydrolyzable starting materials are generally oxidized and hydrolyzed respectively in a hydrogen-oxygen flame. The starting materials used for pyrogenic processes may be organic and inorganic substances. Particularly suitable examples are the readily available chlorides, such as silicon tetrachloride, aluminum chloride or titanium tetrachloride. Suitable organic starting compounds may, for example, be alkoxides, such as Si(OC2H₅)₄, Al(OiC₃Hv)₃ or Ti(OiPr)₄. The metal oxide particles thus obtained are very substantially pore-free and have free hydroxyl groups on the surface. In general, the pyrogenic metal oxide particles are present at least partly in the form of aggregated primary particles. In the present invention, metalloid oxides, for example silicon dioxide, are referred to as metal oxide.

The particles can be hydrophobized by reaction with surface-modifying reagents which react with active groups on the particle surface.

To this end, it is possible with preference to use the following silanes, individually or as a mixture:

Organosilanes (RO) ₃Si (C_nH_2n+I) and (RO) ₃Si (C_nH_2n-I) where R = alkyl such as methyl, ethyl, n-propyl, isopropyl, butyl and n = 1-20. Organosilanes R' _x (RO) _ySi (C_nH_2n+i) and R' _x (RO) _ySi (C_nH_2n-i) where R = alkyl such as methyl, ethyl, n-propyl, isopropyl, butyl; R' = alkyl such as methyl, ethyl, n-propyl, isopropyl, butyl; R' = cycloalkyl; n = 1-20; x+y = 3, x = l, 2; y = l, 2.

Haloorganosilanes XsSi (C_nH_2n+i) and XsSi (C_nH_2n-i) where X = Cl, Br; n = 1-20.

Haloorganosilanes X₂(R¹JSi(C_nH_2n+I) and X₂ (R' ) Si (C_nH_2n-!) where X = Cl, Br, R' = alkyl such as methyl, ethyl, n-propyl, isopropyl, butyl; R' = cycloalkyl; n = 1-20.

Haloorganosilanes X (R' ) ₂Si (C_nH_2n+1) and X (R' ) ₂Si (C_nH_2n-_!) where X = Cl, Br; R' = alkyl such as methyl, ethyl, n-propyl, isopropyl, butyl; R' = cycloalkyl; n = 1-20.

Organosilanes (RO) ₃Si (CH₂) _m-R' where R = alkyl such as methyl, ethyl, propyl; m = 0.1-20; R' = methyl, aryl such as -CeH₅, substituted phenyl radicals, C₄F₉, OCF₂-CHF-CF₃, C₆F₁₃, OCF₂CHF₂, S_x- (CH₂) ₃Si(OR)₃.

Organosilanes (R")_x(RO)_ySi (CH₂)_m-R' where R" = alkyl, x+y = 3; cycloalkyl, x = 1, 2, y = 1, 2; m = 0.1 to 20; R' = methyl, aryl such as C₆H₅, substituted phenyl radicals, C₄F₉, OCF₂-CHF-CF₃, C₆F₁₃, OCF₂CHF₂, S_x- (CH₂) ₃Si (OR)₃, SH, NR¹R^{1 1}R''' where R' = alkyl, aryl; R' ' = H, alkyl, aryl; R' ' ' = H, alkyl, aryl, benzyl, C₂H₄NR' ' ' 'R' ' ' ' ' where R' ' ' ' = H, alkyl and R' ' ' ' ' = H, alkyl. Haloorganosilanes XsSi (CH₂) _m-R'

X = Cl, Br; m = 0.1-20; R' = methyl, aryl such as CeH₅, substituted phenyl radicals, C₄F₉, OCF₂-CHF-CF₃, C₆F₁₃, 0-CF₂-CHF₂, S_x- (CH₂) ₃Si (OR)₃, where R = methyl, ethyl, propyl, butyl and x = 1 or 2, SH.

Haloorganosilanes RX₂Si (CH₂) _mR'

X = Cl, Br; m = 0.1-20; R' = methyl, aryl such as C₆H₅, substituted phenyl radicals, C₄F₉, OCF₂-CHF-CF₃, C₆F₁₃, 0-CF₂-CHF₂, -0OC(CH₃)C=CH₂, -S_x- (CH₂) ₃Si (OR) ₃, where

R = methyl, ethyl, propyl, butyl and x = 1 or 2, SH.

Haloorganosilanes R₂XSi (CH₂) _mR'

X = Cl, Br; m = 0.1-20; R' = methyl, aryl such as C₆H₅, substituted phenyl radicals, C₄F₉, OCF₂-CHF-CF₃, C₆F₁₃, 0-CF₂-CHF₂, -S_x- (CH₂) ₃Si (OR)₃, where R = methyl, ethyl, propyl, butyl and x = 1 or 2, SH.

Silazanes R¹R₂SiNHSiR₂R' where R, R' = alkyl, vinyl, aryl.

Cyclic polysiloxanes D3, D4, D5 and their homologs, where D3, D4 and D5 are each understood to mean cyclic polysiloxanes having 3, 4 or 5 units of the -O-Si (CH₃) ₂ type, e.g. octamethylcyclotetrasiloxane = D4. polysiloxanes or silicone oils of type Y-O- [ (RR' SiO) _m- (R" R' ' ' SiO)_nJ₁₁-Y,

m = 0,1,2,3,... oo, preferably 0,1,2,3,... 100000, n = 0,1,2,3,... oo, preferably 0,1,2,3,... 100000, u = 0, 1,2,3, ....oo, preferably 0,1,2,3,... 100000, Y = CH₃, H, C_nH_2n+I, n=2-20; Si (CH₃) 3, Si (CH₃) ₂H,

Si (CH₃) ₂OH, Si (CH₃) 2 (OCH₃) , Si (CH₃) ₂ (C_nH_2n+i) , n=2-20 R = alkyl such as C_nH_2n+I, n being 1 to 20, aryl such as phenyl radicals and substituted phenyl radicals, (CH₂) _n-NH₂, H R' = alkyl such as CnH2n+l, n being 1 to 20, aryl such as phenyl radicals and substituted phenyl radicals, (CH2)n-NH2, H

R''= alkyl such as C_nH_2n+I, n being 1 to 20, aryl such as phenyl radicals and substituted phenyl radicals, (CH₂) _n-NH₂, H

R'''= alkyl such as C_nH_2n+I, n being 1 to 20, aryl such as phenyl radicals and substituted phenyl radicals, (CH₂) _n-NH₂, H.

Commercially available products that can be used are: RHODORSIL^® OILS 47 V 50, 47 V 100, 47 V 300, 47 V 350, 47 V 500, 47 V 1000, Wacker Silicon Fluids AK 0,65, AK 10, AK 20, AK 35, AK 50, AK 100, AK 150, AK 200, AK 350, AK 500, AK 1000, AK 2000, AK 5000, AK 10000, AK 12500, AK 20000, AK 30000, AK 60000, AK 100000, AK 300000, AK 500000, AK 1000000 or Dow Corning^® 200 fluid.

As surface modifiers it is possible with preference to use the following compounds: octyltrimethoxysilane, octyltriethoxysilane, hexamethyldisilazane, 3-methacryloyloxypropyltrimethoxysilane, 3-methacryloyloxypropyltriethoxysilane, hexadecyltrimethoxysilane, hexadecyltriethoxysilane, dimethylpolysiloxane, nonafluorohexyltrimethoxysilane, tridecafluorooctyltrimethoxysilane, tridecafluorooctyl- triethoxysilane .

With particular preference it is possible to use hexamethyldisilazane, octyltriethoxysilane and dimethylpolysiloxanes .

Suitable hydrophobic, pyrogenic metal oxides can be selected for example from the table of stated AEROSIL^® and AEROXIDE^® products (all from Degussa) .

Table: Hydrophobic metal oxides

The nanoscale hydrophobic particles may be applied to the substrate as powder or in the form of a dispersion.

The liquid phase of the dispersion is preferably volatile. The solvents used may especially be alcohols such as ethanol or isopropanol, ketones such as acetone or methyl ethyl ketone, ethers such as diisopropyl ether, or else hydrocarbons such as cyclohexane. Particular preference is given to alcohols. It may be advantageous when the dispersion contains 0.1 to 10% by weight, preferably 0.25 to 7.5% by weight and most preferably 0.5 to 5% by weight of particles based on the total weight of the dispersion. As a result of suitable selection of the dispersion apparatus, it is possible to obtain defined particle sizes. Suitable dispersion units may, for example, be rotor-stator machines, high-energy mills in which the particles are ground by collision with one another, planetary kneaders, stirred ball mills, vibratory ball mills, vibratory plates, ultrasound units or combinations of the aforementioned units.

The dispersion is then applied to the polymeric material and the solvent is subsequently removed. The dispersion can be applied by means of spin-coating, dip-coating, painting, spraying or knife-coating.

A particular type of application is that of injection- molding, in which the particles, preferably in the form of a dispersion, are first introduced into an injection mold, the solvent is allowed to evaporate and then an injection molding operation with a thermoplastic polymer is performed, in the course of which the particles are impressed into the surface of the polymeric material only for part of their diameter.

A further type of application, which is suitable especially for producing flat products, is the calendering process. In this process, the finely divided hydrophobic particles are forced by means of two contrarotatory rolls partly into the surface of a polymer material which has been made plastic by supplying heat, so as to give rise to, on the one hand, mechanical securing to the polymer, such that, on the other hand, the particles still project out of the polymer layer for part of their diameter. After the cooling, the surface of the material in question coated with particles is superhydrophobic .

In both of the processes outlined, a layer of hydrophobic particles forms on the substrate in such a thickness that the substrate has been covered completely.

The invention further provides a molding which is obtainable by the process according to the invention.

The layer thickness of the superhydrophobic surface may vary within wide limits. It may preferably be 0.05 to 100 μm, in which case several layers of particles are present one on top of another which adhere to one another through the van der Waals forces for mechanical securing.

The molding also has elevations on its surface, caused by the irregular shape of the hydrophobic nanoscale particles. These may preferably have a mean height in the range of 20 nm to 25 μm and a mean separation of 20 nm to 25 μm.

The height and separation of the elevations can be estimated from TEM images. Owing to the distribution and the superimposition of the particle agglomerates formed, a specification is possible only within rough limits. It can be seen that the mean height is about equal to the mean separation between two corresponding agglomerates and that these dimensions are predominantly between 20 nanometers and 250 nanometers. In calendered plates or powder-coated areas, the abovementioned dimensions may be significantly greater and may be up to 25 micrometers, in which case smaller fine structures occur on the surface of these coarse structures .

The molding may, for example, be a film, a plate, a tube, a lampshade, a bucket, a vat, a dish, a measuring cup, a funnel, a bath or a casing part.

Examples

Feedstocks : D-I: Dispersion of trimethylsilyl-coated silicon dioxide (AEROXIDE®LE2) , 1% by weight in ethanol;

P-I: HDPE, high molecular weight polyethylene

P-2: Vestodur RS1777 polybutylene terephthalate

P-3: Vestodur CL2030 polybutylene terephthalate, filled with 30% glass fibers

P-4: V-PTS Createc B3NM800/25 polybutylene terephthalate

P-5: Polypropylene with 2.5% triallyl isocyanurate

P-6: Polypropylene with 3% PTS AOlOPO aging protectant

Radiation source: The radiation sources used may be customary electron accelerators. The accelerator voltage is generally 100 keV to 3 MeV. In the examples, an electron accelerator with a total power of

150 kilowatts and an accelerator voltage of 2.8 MeV was used.

Determination of the adhesion: Dry rubbing: dry felt, load: 1 N/cm²

Wet rubbing: water-wetted felt, load 1 N/cm²

Number of strokes: refers to the number of rubbing operations of the loaded felt on the surface of the specimen Contact angle measurement: with water by the known methodology of measurement on a droplet at rest (Colloid & Polymer Science, Volume 55, Number 2, 169- 171) .

Example 1 : An aerosol of D-I is applied to an injection mold by means of a spray apparatus such that the surface of the mold cavity has been moistened uniformly. The evaporation of the carrier liquid (ethanol) is awaited for a few seconds. The injection mold thus prepared is used, with a mold surface temperature of 60⁰C and a pressure of 55 bar, by means of a standard injection-molding machine (Engel 150/50 S), to injection-mold rectangular platelets of dimensions 50><30^χ2 mm of roughness level 33 according to VDI from HDPE. The melting point is 300⁰C and the hold pressure is 50 bar.

Example 2: As Example 1, except that the injection molding is irradiated with a radiation intensity of 25 kGy over a period of 0.25 second.

Examples 3 to 5 : Analogous to Example 2, except that the specimens pass through the radiation beam in a plurality of passes staggered in time and interrupted by cooling phases until the desired dose has been absorbed.

Feedstocks and experimental conditions are reproduced in Table 1.

Examples 6 to 10: Analogous to Example 2, except with variation of the polymer and of the radiation intensity. The index D relates to the evaluation by means of dry rubbing, the index W to the evaluation by means of wet rubbing. Feedstocks and experimental conditions are reproduced in Table 2 and 3.

Table 1 : Dry rubbing

* = Comparative Table 2 : Dry rubbing

Table 3: Wet rubbing

Figure 1 reproduces the values from Table 1. In this plot, the contact angle in [°] is plotted against the number of strokes. It is clearly evident from the values measured for the contact angle that the superhydrophobic surfaces from inventive Examples 2-5 are more stable toward shearing forces than the surface which was obtained without irradiation (Example 1) .

This finding is surprising, since it was expected that the energy-rich radiation would lead to a loss of hydrophobicity, as observed, for example, in other experiments under the influence of UV radiation.

Figure 2 shows a TEM image (transmission electron microscopy) of the surface of the injection molding from Example 2. The surface of the HDPE plate has impressed particles of hydrophobized silicon dioxide which form a tightly packed, irregularly structured layer. At the surface of the injection molding, the contact angle for a water droplet was determined. For a 40 μl water droplet, a contact angle of 157° was found. Figure 3 shows the result of an XPS analysis (X-ray photoelectron spectroscopy) of the injection molding from Example 2. 0 0 = Si, + + = 0, x x = C.

Thereafter, the carbon concentration increases with increasing distance from the sample surface; the oxygen and silicon concentration decreases (X axis: t in 10³*s; Y axis: intensity in 10³*cps) . The time t on the x axis corresponds to a particular penetration depth which, however, as a result of the system, cannot be converted to a length. The intensity is a measure of the concentration of the element in question.

This result supplements the finding from the TEM image; namely, it is found that the surface of the polymeric substrates is covered by Siθ2. The carbon content shown in the left-hand part of the diagram originates from the hydrophobized surface of the Siθ2 particles (up to an ion abrasion time of about 35 seconds) and, after this time, is converted to the significantly higher carbon level of the organic support polymer, while the silicon signal simultaneously decreases, since the support polymer does not contain any silicon in significant amounts.

Claims

1. Process for producing a molding which comprises a substrate of a polymeric material and particles which are present on the substrate, are bonded to it in a fixed manner and form a superhydrophobic layer having elevations and depressions, in which the hydrophobic nanoscale particles applied to the polymeric material and the support material are subjected to beta and/or gamma radiation over a period of 0.1 second to 5 hours such that a radiation dose of 10 to 1000 kGy is absorbed.

2. Process according to claim 1, characterized in that the polymeric material is selected from the group consisting of thermoplastics, thermosets and elastomers .

3. Process according to claim 2, characterized in that the polymeric material used is at least one selected from the group consisting of polycarbonates, polyoxymethylenes, poly(meth)- acrylates, polyamides, polyvinyl chloride, polyethylenes, polypropylenes, aliphatic linear or branched polyalkenes, cyclic polyalkenes, polystyrenes, polyesters, polyether sulfones, polyacrylonitrile or polyalkylene terephthalates, poly (vinylidene fluoride), poly (hexafluoro- propylene) , poly (perfluoropropylene oxide), poly- (fluoroalkyl acrylate) , poly (fluoroalkyl methacrylate) , poly (vinyl perfluoroalkyl ether) or other polymers formed from perfluoroalkoxy compounds, poly (isobutene) , poly ( 4-methyl-1- pentene) and polynorbornene as a homo- or copolymer.

4. Process according to claims 1 to 3, characterized in that the particles used are hydrophobic nanoscale metal oxide particles.

5. Process according to claim 4, characterized in that the metal oxide particles used are pyrogenic hydrophobized metal oxide particles having a BET surface area of 20 to 400 m²/g.

6. Process according to claims 1 to 5, characterized in that the particles are applied to the polymeric material in the form of a dispersion and the solvent is then removed.

7. Process according to claim 1 to 6, characterized in that the particles are introduced into an injection mold and then an injection molding operation is performed, in which the particles are impressed into the surface of the polymeric material .

8. Process according to claims 1 to 7, characterized in that the particles are applied to a polymeric material by means of a calendering apparatus such that the particles are impressed into the surface of the polymeric material.

9. Molding obtainable by the process according to claims 1 to 8.

10. Molding according to claim 9, characterized in that the layer thickness of the superhydrophobic surface is 0.05 to 100 μm.

11. Molding according to claim 9 or 10, characterized in that the elevations have a mean height of 20 nm to 25 μm.

12. Molding according to claims 9 to 11, characterized in that the elevations have a mean separation of 20 nm to 25 μm.