WO2010083650A1

WO2010083650A1 - Method of changing the wettability of plastic surfaces by solvent-induced precipitation

Info

Publication number: WO2010083650A1
Application number: PCT/CN2009/070278
Authority: WO
Inventors: Yoong Kim; Szu Lui Tey; Andreas Fechtenkoetter; Alexander Traut; Liang Zhang; Jian Xu
Original assignee: Basf Se; Basf (China) Company Ltd.
Priority date: 2009-01-22
Filing date: 2009-01-22
Publication date: 2010-07-29
Also published as: CN102292165A; CN102292165B

Abstract

A method for making an article comprising the steps of i) providing a polymer substrate of an amorphous, non-crystallizable polymer; ii) contacting a selected surface region of the polymer substrate with a swelling agent, the swelling agent having a high solubility limit for the polymer substrate, whereby a surface part of the polymer substrate is swollen and a part underneath the surface of the polymer substrate is non-swollen; iii) contacting the polymer substrate of step (ii), wherein a part of the surface of the polymer substrate is swollen, with a coagulating agent, the coagulating agent a low solubility limit for the polymer and being miscible with the swelling agent, whereby the swollen part of the polymer is precipitated back onto the non-swollen part of the polymer substrate, whereby an article having a structured surface is obtained; an article prepared by the method mentioned before and the use of the article in indoor applications selected from furniture, household appliances, computer peripherals, clothing apparel, filters and membranes and outdoor applications selected from cooling equipment, drainage pipes and technical textiles.

Description

Method of changing the wettability of plastic surfaces by solvent-induced precipitation

Description

The present invention relates to a method for making an article from a polymer substrate, the article having a structured surface, as well as an article prepared by said method, and the use of said article for outdoor and indoor applications.

There are many methods known in the art to modify the wettability of surfaces. It is known in the art that by increasing the surface roughness of a hydrophobic material the surface water repellency can be dramatically enhanced. This strategy is impeccably exhibited by the Lotus leaf, of which the surface is structured on two length scales by micron- and nanosized waxed protrusions. Inspired by the Lotus effect, researchers attempt to mimic the behaviour of the Lotus effect. Techniques known in the art for sur- face modification include for example plasma treatment, lithography, physical deposition/adsorption, or grafting. However, these techniques are usually time-consuming, difficult to control, expensive or suffer from poor durability of the coated films obtained. Therefore, a simple and economical procedure for obtaining suitably structured surfaces still remains.

In N. Zhao et al., ChemPhysChem 2006, 7, 824-827, a Lotus- leaf- 1 ike superhydropho- bic surface prepared by solvent-induced crystallization (SINC) is disclosed. Said surface is based on crystallizable bisphenol A polycarbonate (PC). According to the experimental section in N. Zhao et al. an amorphous polycarbonate plate is coated with acetone and the thin layer of acetone coating the polycarbonate plate is allowed to evaporate under ambient conditions. To obtain a superhydrophobic surface from said swollen polycarbonate surface, either small amounts of coagulator, such as water or methanol, was sprayed on the acetone-swollen polycarbonate surface, or the swollen polycarbonate plate was immersed into the coagulator bath for several seconds and then air-dried, or the swollen polycarbonate plate was dried in humid air at 70% relative humidity. The water contact angle obtained by this process was about 160°, which is superhydrophobic.

US 2007/0009709 A1 relates to a method to modify the surface of an article so as to alter its wettability. The method according to US 2007/0009709 A1 comprises the steps of: (a) providing a substrate comprising a polymer; and (b) inducing a phase transformation at a selected surface region of the substrate, wherein the phase transformation forms a texture at the selected surface region; wherein the texture comprises a plurality of features having a largest characteristic dimension of up to about 50 microns. Accord- ing to US 2007/0009709 A1 several different methods for inducing the phase transfor- mation according to step (b) are disclosed. According to the examples in US 2007/0009709 A1 a polycarbonate or a copolymer of polycarbonate and siloxane is made superhydrophobic. The swelling agent used according to both examples mentioned in US 2007/0009709 A1 is acetone. To obtain the desired surface the acetone is evaporated whereby crystallization/phase separation on the surface is induced and a texture is created. It is mentioned in the specification of US 2007/0009709 A1 that a quenchant may be used to arrest the phase transformation. In this case a polycarbonate surface is contacted momentarily with tetrahydrofuran and is after partial dissolution of the surface exposed to a quenchant comprising ethanol, which is a non-solvent in this case. According to US 2007/0009709 A1 further substrates comprising a polymer are mentioned, for example polyolefins, polyacrylamides, polystyrenes, polyesters, polyurethanes, acrylics and blends thereof. However, suitable swelling agents or quenching agents for obtaining the desired surfaces on said polymers are not mentioned.

N. Zhao et al. as well as US 2007/0009709 A1 mention the changing of the wettability of surfaces of crystallizable polycarbonate.

However, polycarbonate is a very expensive polymer. Therefore, it is desired to provide a process for changing the wettability of surfaces of cheaper polymers, especially of amorphous polymers which are non-crystallizable.

The present invention therefore provides a method as well as an article, wherein cheaper amorphous polymers can be used as substrates to create structured surfaces thereon.

The present invention therefore relates to a method for making an article comprising the steps of

(i) providing a polymer substrate of an amorphous, non-crystallizable polymer;

(ii) contacting a selected surface region of the polymer substrate with a swelling agent, the swelling agent having a high solubility limit for the polymer substrate, whereby a surface part of the polymer substrate is swollen and a part underneath the surface of the polymer substrate is non-swollen; (iii) contacting the polymer substrate of step (ii), wherein a part of the polymer surface substrate is swollen, with a coagulating agent, the coagulating agent having a low solubility limit for the polymer and being miscible with the swelling agent, whereby the swollen part of the polymer substrate is precipitated back onto the non-swollen part of the polymer substrate, whereby an article having a structured surface is obtained. In one embodiment the method of the present invention additionally comprises the incorporation of additives, especially nanoparticles, into the structured surface obtained in step (iii).

Step (i)

Types of polymer substrate

The polymer substrate is an amorphous polymer substrate, which is non-crystallizable under the conditions of the method of the present invention.

The amorphous polymer substrate is characterized by a missing long range order. The amorphous polymer is in form of an interpenetrating network. It is characterized by a glass transition (Tg) but does not have a defined melting point.

This is an important difference from the prior art. According to the prior art mentioned before, crystallizable polycarbonate is employed and a superhydrophobic surface is prepared by solvent-induced crystallization (SINC). According to N. Zhao et al., Chem- PhysChem 2006, 7, 824-827, the superhydrophobic surface is achieved by the formation of nano-sized crystal structures on the polycarbonate surface. It is further mentioned in Zhao et al. that the method may be applied to other crystallizable polymers. Therefore, according to the teaching of the prior art, the use of crystallizable polymers to achieve superhydrophobicity on polymer surfaces by SINC is essential.

However, the inventors of the present invention found a way to provide superhydrophobic surfaces on amorphous polymers. This is very surprising since the formation of nano-sized crystals on the polymer surface seemed essential for providing superhydrophobic polymer surfaces by SINC.

The amorphous, non-crystallizable polymer substrate may be any amorphous polymer substrate known in the art. Preferred amorphous polymer substrates are selected from polystyrenes, poly(meth)acrylate, polyacrylamides, polyurethanes, polysulfones, amorphous polyolefins, blends and copolymers comprising at least one of the polymers mentioned before. Preferred polymer substrates are polystyrenes or blends or copolymers of polystyrenes.

In the meaning of the present invention the term "non-crystallizable" means that the polymers are not crystallizable under the conditions of the method of the present inven- tion. In the meaning of the present invention the term "copolymers comprising at least one of the polymers mentioned before" has the meaning that suitable copolymers comprise the monomer units present in the polymers mentioned before.

Suitable polystyrenes or copolymers of polystyrenes are in general amorphous thermoplastic materials. The preferred polystyrene is atactic or syndiotactic, whereby atactic polystyrene is preferred. Copolymers of polystyrene or blends of polystyrene with other polymers suitable as polymer substrate in the present invention are SAN (sty- rene-acrylonitrile-copolymers), ABS (acrylonitrile-butadiene-styrene-copolymer), ASA (acrylonitrile-styrene-acrylate copolymer), SB (styrene-butadiene), as well as high impact polystyrene (HIPS). Preferred polystyrenes or copolymers or blends of polystyrenes are polystyrene itself (homopolymer, preferably atactic polystyrene homopoly- mer), as well as SAN.

Suitable poly(meth)acrylates are polyacrylates and/or polymethacrylates as well as copolymers of acrylic esters and/or methacrylic esters and - for example - acrylonitrile. Suitable poly(meth)acrylates are known by a person skilled in the art. A preferred poly(meth)acrylate is polymethyl methacrylate (PMMA).

Polyacryl amide is the polymer of acrylic amide which is known in the art.

Suitable polyurethanes may be hard or soft or elastic materials depending on the properties of the isocyanate and diol or polyol component used for the preparation of the polyurethanes. Suitable isocyanates used as starting materials for the preparation of polyurethanes are for example diphenylmethanediisocyanate (MDI), polymeric di- phenylmethanediisocyanate (PMDI), toluenediisocyanate (TDI), naphthylenediisocy- anate (NDI), hexamethylenediisocyanate (HDI), isophoronediisocyanate (IPDI) and/or 4,4'-diisocyanatodicyclohexylmethane (H12MDI). Suitable polyols may for example be polyester polyols.

Polysulfones encompass polysulfones as well as polyethersulfones. Suitable polysul- fones and polyethersulfones are known by a person skilled in the art. Suitable polysulfones are for example polyarylsulfones. Examples are polysulfones, polyethersulfones and polyphenylsulfones, especially the Ultrason^®-types of BASF SE.

Amorphous polyolefins are well known in the art. Suitable amorphous polyolefins may be based on polyethylene or polypropylene or mixtures of polyethylene and polypropylene and may additionally comprise one or more further monomer units based on a- olefins with 4 to 20 carbon atoms, for example 1-butene, 1-pentene, 1-hexene, 1- octene, 1-decene, 1-dodecene, 1-octadecene, 3-methyl-1-butene, methylpentenes, for example 4-methyl-1-pentene, methylhexenes or methylheptenes.

As mentioned before, not only the homo- and copolymers of the polystyrenes, poly(meth)acrylates, polyamides, polyacrylamides, polyurethanes, polysulfones and amorphous polyolefins are useful as polymer substrates in the present application, but also blends comprising one or more of the homo- and copolymers mentioned before.

The molecular weight of the polymers and copolymers mentioned before is not specifi- cally limited. However, suitable molecular weights for specific applications are known by a person skilled in the art.

In a preferred embodiment, the polymer substrate is selected from the group consisting of polystyrenes, blends and copolymers comprising at least one of said polymers. More preferably, the polymer substrate is a polystyrene, whereby said polystyrene may be a homo- or copolymer or a blend comprising a homo- or copolymer of polystyrene. Most preferably, the polymer substrate is a homopolymer of polystyrene, especially a ho- mopolymer of atactic polystyrene or SAN.

The polymer substrate may be in the form of a fiber, film, sheet or in form of a bulk shape.

It is possible to produce highly hydrophobic articles by the method of the present invention. For the case of highly hydrophobic articles, the static water contact angle of the original flat, untreated polymer substrate should be 80 to 120°. In some preferred embodiments to achieve highly hydrophobic surfaces the static water contact angle of its original flat, untreated surface is 80 to 100°, more preferably 80 to 90°.

The static contact angle is the angle that a liquid droplet forms on the interphase of three phases, i.e. the angle made by the liquid/vapour interface from the liquid /solid interface. The contact angle of a liquid on a surface is governed by the Young relation and it is dependent on the interfacial tensions at a point on the three-phase contact line.

Generally, a hydrophobic material or surface is characterized by a static contact angle of water of 90° or above.

Some hydrophobic coatings are being referred to in the art as superhydrophobic coatings. Superhydrophobic coatings are generally defined by a static water contact angle above 150° (N. Zhao, J. Xu, Q. Xie, L. Weng, X. Guo, X. Zhang and L. Shi, Macromol. Rapid Commun. 26 (2005) 1075-1080 and W. Chen, A.Y. Fadeev, M.c. Hsieh, D. ner, J. Youngblood and TJ. McCarthy, Langmuir (1999), 15 (10), 3395-3399).

The static contact angles were measured as described in the example part of the pre- sent application.

According to the present invention, the polymer substrate is not crystallizable under the conditions of the present invention. It has been found that by the method of the present invention porous structures of globule-like polymer particles are formed by precipitation in step (iii), which are surprisingly able to form highly hydrophobic surfaces.

Step (ii)

Types of swelling agent (solvent)

The swelling agent used according to the method of the present invention has a high solubility limit for the polymer substrate. The suitable swelling agent therefore depends on the polymer substrate employed. Suitable swelling agents may comprise of liquid and/or gaseous fluids. A liquid fluid may be an acid, base, water, organic solvent or a suitable mixture thereof. Organic solvents that may be used include, but are not limited to formamides like dimethylformamide, cyclic ethers like tetrahydrofuran, halogenated solvents like chloroform or methylene chloride, acetates like ethyl acetate or butyl acetate, aromatic solvents like toluene or xylene, ketones like acetone, alcohols like etha- nol, isopropanol, ethyleneglycol as well as N-methylpyrrolidone, dimethylacetamide. The most generally used gaseous fluid may be moisture or humidity in the environment. If the polymer is polystyrene (homopolymer), the swelling agent is preferably dimethylformamide. If the polymer is SAN, the swelling agent is preferably ethyl acetate.

According to the process of the present invention a part of the polymer substrate sur- face is swollen and a part of the polymer substrate underneath the surface is non- swollen. The part of the polymer substrate which is swollen is usually the top surface of the polymer substrate. The polymer surface which is preferably swollen is usually at the top 50 to 200 μm of the substrate surface. Preferred effective swelling of the polymer substrate is indicated by softening of the top 50 to 200 μm of the substrate surface, which is usually achieved within at most 10 minutes, preferably at most 5 minutes, more preferably at most 1 minute.

According to step ii) of the method of the present invention a selected surface region of the substrate is contacted with the swelling agent. The term selected surface region as used in the present invention includes the surface and a region preferably from 50 to 200 μm below the surface of the polymer substrate.

The part of the polymer substrate which is swollen is the selected surface region, which is the region comprising the surface of the polymer substrate and the region from 50 to 200 micrometers below said surface. The part of the substrate which is non-swollen is the part of the substrate underneath the swollen surface region.

Method of introducing the swelling agent

If a liquid swelling agent is used, step (ii) of the method of the present invention is carried out by introducing the liquid swelling agent onto the surface of the polymer substrate by drop cast application, spray application, doctor blade application or dipping the substrate into the liquid swelling agent, or by any other methods, depending on the requirements of the intended application.

If a gaseous swelling agent is used, step (ii) of the method of the present invention is carried out by placing the polymer substrate inside an enclosed environment that is filled in with the vapour of the particular gaseous fluid, which may be moisture or the humidity in the environment. Said fluid is usually fed to specific levels of vaporisation.

The choice of the introduction of the swelling agent onto the surface of the polymer substrate depends on the intended application, especially on whether the process can fit into the current forming process for manufacturing of plastic components specific to the intended application.

Step (iii)

Types of coagulating agent (non solvent)

The coagulating agent has a low solubility limit for the polymer substrate. According to the present invention the coagulating agent is miscible with the swelling agent in order to facilitate mass transfer, and therefore phase separation, of the polymer from the swelling agent into the coagulating agent.

The term "miscible with the swelling agent" means that at least 20 wt.-% of the coagulation agent are soluble in the swelling agent, preferably, 50 to 100 wt-% are soluble in the swelling agent, more preferably 80 to 100 wt.-% are soluble in the swelling agent. Suitable coagulating agents depend on the polymer substrate employed. The coagulating agent used may comprise of liquids and/or gaseous fluids. A liquid fluid may be water, an organic solvent or a suitable mixture thereof. Organic solvents that may be used include but are not limited to alcohols like methanol, ethanol, isopropanol, ke- tones like acetone, methyl ethyl ketone (MEK) and/or cyclohexanone, whereby ethanol is preferred in the case of polystyrene homopolymers and SAN. The most generally used gaseous fluid may be moisture or humidity in the environment.

MEK (methyl ethyl ketone) and cyclohexanone coagulating agents for polyolefins like polypropylene.

If the polymer substrate is polystyrene (homopolymer) or SAN, the coagulating agent is preferably ethanol.

In step (iii) of the method of the present invention the coagulation is carried out, whereby the swollen polymer substrate is precipitated back onto the non-swollen part of the polymer substrate, whereby an article having a structured surface is obtained.

The structured surface is not in form of nano-sized crystal structures, since the polymer substrate is amorphous. The structured surface obtained is a porous structure of globule-like polymer particles

The coagulation of the swollen part of the polymer substrate, i.e. the coagulation of the swollen surface polymer chain of the substrate polymer, onto the non-swollen part of the polymer substrate occurs via precipitation. This means, once the swollen part of the polymer substrate surface is in contact with a coagulating agent in step (iii) of the method of the present invention, phase separation will involve precipitation of the swollen part of the polymer substrate, i.e. the swollen polymer chains, back onto the non- swollen part of the polymer substrate.

Effective coagulation in step (iii) is usually indicated by the appearance of a white precipitate within up to 30 minutes, preferably up to 5 minutes, more preferably up to 1 minute of contact between the part of the polymer substrate which is swollen (polymer substrate surface) and the coagulating agent. Thereafter, the entire polymer substrate surface usually turns completely white and opaque within up to 60 minutes, preferably up to 30 minutes, more preferably up to 10 minutes of contact between the swollen polymer substrate surface (swollen part of the polymer substrate) and the coagulating agent. Preferably, a white precipitate is observed on the swollen polymer substrate surface within 5 minutes and the entire substrate turns completely white and opaque within 30 minutes. More preferably, a white precipitate is observed on the swollen poly- mer substrate surface within one minute and the entire substrate surface turns completely white and opaque within 10 minutes.

Method of introducing the coagulating agent

If a liquid coagulating agent is used in step (iii) of the method of the present invention, the liquid coagulating agent may be introduced onto the swollen part of the surface region of the polymer substrate for example by immersion application, drop cast application, spray application or any other method, depending on the requirements of the intended application.

If a gaseous coagulation is used in step (iii) of the method of the present invention, the polymer substrate is usually placed inside an enclosed environment that is filled with the vapour of a particular gaseous fluid. The gaseous fluid is usually fed to specific levels of vaporisation.

The choice of the method used for introducing the coagulating agent depends on the intended application, especially on whether the method can fit into the current forming process for manufacturing of plastic components specific to the intended application.

In a preferred embodiment of the present invention the polymer substrate is polystyrene homopolymer, especially atactic polystyrene homopolymer, the swelling agent is dimethylformamide (DMF) and the coagulating agent is ethanol, methanol, water or mixtures of two or more of the coagulating agents mentioned before, preferably ethanol.

In a further preferred embodiment the polymer substrate is SAN, the swelling agent is ethyl acetate and the coagulating agent is water, ethanol, methanol, or mixtures of two or more of the coagulating agents mentioned before, preferably ethanol.

Most preferably, the polymer substrate is polystyrene homopolymer, especially atactic polystyrene homopolymer, the swelling agent is DMF and the coagulating agent is ethanol, methanol, water or mixtures of two of the coagulating agents mentioned before, preferably ethanol.

Environment temperature during the process

The temperature during the process of the present invention may be the same or different in steps (i), (ii), and (iii). In general, the temperature is dependent on the speed at which the surface polymer chains of the substrate may be swollen or coagulated in respect to the swelling agent or coagulating agent used. Heating may for example be required to accelerate the swelling and/or coagulation step. In one embodiment the polymer substrate alone is preheated to temperatures of 90 to 120⁰C, preferably 90 to 100⁰C. Preferably, the polymer substrate is heated 5 to 20°C above its glass transition temperature (T₉). In a further embodiment (which may be combined with the first em- bodiment) the swelling agent and/or the coagulating agent having a flash point below 20⁰C, for example ethanol, are maintained at temperatures of at most 25°C; the swelling agent and/or coagulating agent having a flash point below 60°C are maintained at temperatures of 30 to 80⁰C, preferably 40 to 60°C. More preferably, the swelling agent and/or coagulation agent is maintained at not more than 10°C below its flash point dur- ing use. In a further preferred embodiment, no heating is required and the entire process comprising steps (i), (ii) and (iii) of the present invention is conducted at ambient temperature.

Environment relative humidity during the process

The relative humidity of the environment during the method of the present invention may be critical when water or moisture can serve as a swelling agent or as a coagulating agent for the polymer substrate. Under such circumstances, the uniformity of surface structuring is greatly influenced by the environment relative humidity, which may be between 10 to 70 % RH (relative humidity), preferably between 20 to 40% RH.

Incorporation of particles onto the structured surface

In a further embodiment of the present invention additional performance benefits, for example ultraviolet- or infrared-absorbents or anti-microbial effects, may be introduced onto the structured polymer substrate surface by incorporating additives, especially particles, onto the structured surface obtained in step (iii). Preferably, the additional performance benefits are introduced via the incorporation of additives, which may be present in the form of particles, preferably organic or inorganic nanoparticles or organic small molecules. Examples for suitable additives are silica nanoparticles which may be non, partially or fully hydrophobized. The additives may be deposited in either an ordered or disordered manner, leading to partial or complete coverage of the polymer substrate surface with said additives. Deposition methods may include, but are not limited to, drop cast application, spray application or doctor blade application. The incor- poration of the additives may take place as an additional process step or one or more additive may be dispersed into the swelling agent and/or coagulation agent used. In a preferred embodiment of the method of the present invention the additives are homogeneously dispersed into the swelling agent and/or a coagulation agent. In a further preferred embodiment, the additives are homogenously dispersed into the swelling agent, where the additives can act as seeds for controlled precipitation to occur during phase separation with the coagulating agent. The amount of additives in the swelling agent and/or coagulating agent is in general of from 0.5 to 30 wt.%, preferably 1 to 20 wt.%, more preferably 1 to 10 wt.%.

The present invention further relates to an article prepared by the method according to the present invention. Said article has a surface which is highly hydrophobic in the case of a hydrophobic polymer substrate, and a surface which is highly hydrophphilic in the case of a hydrophiliic polymer substrate. The static water contact angle of the article prepared according to the method of the present invention is - in the case of hydro- phobic polymers - in general by at least 10°, preferably by at least 20°, more preferably by at least 30° higher than the static water contact angle of the polymer substrate used as starting material. Preferably, the the static water contact angle of the hydrophobic article prepared according to the method of the present invention is between 120 to 180°, preferably 135 to 175°, more preferably 150 to 170°. The static water contact angle of the article prepared according to the method of the present invention is - in the case of hydrophilic polymers - in general by at least 10°, preferably by at least 20°, more preferably by at least 30° lower than the static water contact angle of the polymer substrate used as starting material. The surface of the articles prepared according to the method of the present invention comprises a precipitate of the amorphous polymer substrate employed. In general, the precipitates of said amorphous polymers exist as particles or globules of different sizes.

Suitable applications

The articles of the present invention are useful in indoor as well as in outdoor applications. The surfaces of the articles according to the present invention are preferably self- cleaning surfaces.

Such self-cleaning surfaces can be used for indoor applications to produce articles with controlled surface wettability, where highly hydrophobic surfaces are preferred, including, but not limited to, furniture, household appliances, computer peripherals, clothing apparel, filters and membranes.

The articles of the present invention may also be useful for outdoor applications. Also in outdoor applications controlled surface wettability is desired. Certain outdoor applications can favour highly hydrophobic surfaces and may include, but are not limited to, cooling equipment, drainage pipes or technical textiles.

The following examples serve to illustrate the features and advantages offered by the present invention, and are not intended to limit the invention thereto. Examples

1. Test methods for characterization

A Static water contact angle measurement

The static contact angles were measured according to the present invention using a Krϋss FM 40 easy drop equipment. This piece of equipment consists of a manually adjustable syringe, an adjustable sample stage and a video camera. Before any measurements were carried out the surface tension of the water in the syringe was measured and was found to be between the values 70 - 74 mNm^"1. The static contact angle of a coating was measured by dispensing a 5 μl droplet of de-ionized water onto 5 different locations of each sample surface. From the form of the droplet, the contact angle was calculated. The average of these measurements is quoted as the static water contact angle..

B Field emission secondary electron microscopy (FE-SEM)

The surface morphology of the final solvent-treated sample was analyzed using field emission secondary electron microscopy on a JEOL JSM 6700 F equipment, where an accelerating voltage of 5 kV was used (see X. Li, G. Chen, Y. Ma, L. Feng, H. Zhao, L. Jiang and F. Wang, Polymer 47 (2006) 506-509, for reference).

2. Examples

i. Solvent-Induced Precipitation (SINP) on an Amorphous Homopolymer Plastic Substrate

A piece of 3cm x 3cm transparent Polystyrol^® substrate was first cleaned with isopropyl alcohol and left to dry under ambient conditions (23°C, 60%RH) for 5 minutes. A thin layer of dimethyl formamide (DMF) solvent (0.5 - 1 mL) was uniformly spread across the entire Polystyrol^® surface and swelling of the substrate surface was allowed to take place under low humidity conditions (23°C, 24%RH) in a dessicator for 30 seconds. After which, the surface swollen Polystyrol^® substrate was removed from the dessicator and immersed into an ethanol bath for 3 minutes. Ethanol acts as a nonsolvent, which served to phase separate and precipitate the mobile swollen Polystyrol^® polymer chains on the surface. At the end of 3 minutes, the Polystyrol^® substrate surface appeared completely white and opaque, leaving the unswollen bulk Polystyrol^® still transparent. Finally, the surface treated Polystyrol^® substrate was placed inside a Petri dish and covered with perforated polyolefin Parafilm^®, then dried under ambient conditions in the fumehood for 24 hours.

The surface morphology of such Polystyrol^® substrate, treated via SINP, was investi- gated using FE-SEM. Representative images (Fig. 1a and b) show loose clusters of irregular shaped particles (300nm - 2μm) aggregated with one another, producing micropores that allow pockets of air to be trapped to facilitate the desired superhydrophobic effect. The as-obtained superhydrophobic surface was confirmed with static water contact angle measurements performed on the dried substrate surface, where the inset of Fig. 1a exhibits the corresponding static water contact angle that is larger than 150° (157.1 ° +/- 2.8°) (criteria for superhydrophobic surface). In comparison, the original untreated Polystyrol^® substrate exhibits a static water contact angle of only 88°.

Compared to the prior art of US2007009709 patent application and N. Zhao et al., ChemPhysChem, 2006, 7, 824-827, this example clearly demonstrates that superhydrophobic surfaces on typical amorphous plastics like Polystyrol^® can be successfully produced using a solvent-induced surface treatment process. This greatly broadens the range of plastics that could be adopted into the process technology, since the above- mentioned prior art provided examples on Polycarbonate that produced crystalline sur- face structures i.e. crystallization is not the only route to structuring surfaces for superhy- drophobicity. In contrast, with exceptions like Polycarbonate, most amorphous plastics should produce surface structures via precipitation as a phase separation step.

ii. Solvent-Induced Precipitation (SINP) on an Amorphous Copolymer Plastic Substrate

A piece of 3cm x 3cm transparent styrene-acrylonitrile (SAN; VLR grade with 35wt.% acrylonitrile) substrate was first cleaned with isopropyl alcohol and left to dry under ambient conditions (23°C, 60%RH) for 5 minutes. A thin layer of ethyl acetate solvent (0.5 - 1 mL) was uniformly spread across the entire SAN surface and swelling of the substrate surface was allowed to take place under low humidity conditions (23°C, 24%RH) in a dessicator for 10 seconds. After which, the surface swollen SAN substrate was removed from the dessicator and immersed into an ethanol bath for 10 minutes. Ethanol acts as a nonsolvent, which served to phase separate and precipitate the mobile swollen SAN polymer chains on the surface. At the end of 10 minutes, the SAN substrate surface appeared completely white and opaque, leaving the unswollen bulk SAN still transparent. Finally, the surface treated SAN substrate was placed inside a Petri dish and covered with perforated polyolefin Parafilm^®, then dried under ambient conditions in the fumehood for 24 hours. The surface morphology of such SAN substrate, treated via SINP, was investigated using FE-SEM. Representative images (Fig. 2a and b) show loose clusters of irregular shaped particles (100 - 800nm) aggregated with one another, producing micro-pores that allow pockets of air to be trapped to facilitate the desired superhydrophobic effect. The as-obtained superhydrophobic surface was confirmed with static water contact angle measurements performed on the dried substrate surface, where the inset of Fig. 2a exhibits the corresponding static water contact angle that is larger than 150° (154.6° +/- 6.7°) (criteria for superhydrophobic surface). In comparison, the original untreated Luran^® VLR SAN substrate exhibits a static water contact angle of only 84°.

Compared to the prior art of US2007009709 patent application and N. Zhao et al., Chem-

PhysChem, 2006, 7, 824-827, this example clearly demonstrates that superhydrophobic surfaces on plastics of random copolymers like SAN can be successfully produced using such solvent-induced surface treatment process. Again, this greatly broadens the range of plastics that could be adopted into the process technology; despite the high hydrophilic acrylonitrile content (35 wt.%), the porous surface morphology was sufficient for a superhydrophobic surface to be obtained.

iii. Incorporation of Nanoparticles into Solvent-Induced Precipitation (SINP) Proc- ess on an Amorphous Homopolymer Plastic Substrate

A piece of 3cm x 3cm transparent Polystyrol^® substrate was first cleaned with isopropyl alcohol and left to dry under ambient conditions (23°C, 60%RH) for 5 minutes. A thin layer of dimethyl formamide (DMF) solvent (0.5 - 1mL), containing 10wt.% of 30nm silica nanoparticles, was uniformly spread across the entire Polystyrol^® surface and swelling of the substrate surface was allowed to take place under low humidity conditions (23°C, 25%RH) in a dessicator for 30 seconds. Based on theoretical calculations for 100% hy- drophobization, the silica nanoparticles were surface modified with isobutyl-triethoxy si- lane to facilitate its dispersion into DMF and to reduce the hydrophilicity of the particle surface. After which, the surface swollen Polystyrol^® substrate was removed from the dessicator and immersed into an ethanol bath for 3 minutes. Ethanol acts as a nonsol- vent, which served to phase separate and precipitate the mobile swollen Polystyrol^® polymer chains on the surface. At the end of 3 minutes, the Polystyrol^® substrate surface appeared completely white and opaque, leaving the unswollen bulk Polystyrol^® still transparent. Finally, the surface treated Polystyrol^® substrate was placed inside a Petri dish and covered with perforated polyolefin Parafilm^®, then dried under ambient conditions in the fumehood for 24 hours.

The surface morphology of such Polystyrol^® substrate, treated via SINP and incorporated with silica nanoparticles, was investigated using FE-SEM. Representative images (Fig.

3a and b) show loose clusters of irregular shaped particles (300nm - 2μm) aggregated with one another, with nanoparticles (less than 50nm) deposited on top of such particle surfaces. When static water contact angle measurements were performed on the dried substrate surface, the water droplet had spread rather quickly, indicating that a highly hydrophilic surface had instead been obtained. These observations from contact angle measurements and FE-SEM images lead one to conclude that silica nanoparticles were indeed deposited on top of the precipitated Polystyrol^® particles, as it is not possible to fully hydrophobize silica particles and unreacted silanol groups remaining on the surface will contribute to the hydrophilic effect.

Prior art US2007009709 patent application and N. Zhao et al., ChemPhysChem, 2006, 7, 824-827, do not mention the incorporation of nanoparticles into their invention. The SINP of Polystyrol^® alone is sufficient to generate the desired surface roughness and the successful incorporation of silica nanoparticles provides a proof-of-concept for other types of nanoparticles to be incorporated for additional performance benefits like IR-absorbance, anti-microbial effect, etc.

Claims

1. A method for making an article comprising the steps of

i) providing a polymer substrate of an amorphous, non-crystallizable polymer; ii) contacting a selected surface region of the polymer substrate with a swelling agent, the swelling agent having a high solubility limit for the polymer substrate, whereby a surface part of the polymer substrate is swollen and a part underneath the surface of the polymer substrate is non-swollen; iii) contacting the polymer substrate of step (ii), wherein a part of the surface of the polymer substrate is swollen, with a coagulating agent, the coagulating agent having a low solubility limit for the polymer and being miscible with the swelling agent, whereby the swollen part of the polymer is precipitated back onto the non-swollen part of the polymer substrate, whereby an article having a structured surface is obtained.

2. The method of claim 1 , wherein additives are incorporated into the structured surface obtained in step (iii).

3. The method according to claim 1 or 2, wherein the polymer substrate is selected from the group consisting of polystyrenes, poly(meth)acrylates, polyacrylamides, polyurethanes, polysulfones, amorphous polyolefins, blends and copolymers comprising at least one of said polymers.

4. The method according to claim 3, wherein the polymer substrate is selected from polystyrene, blends and copolymers comprising polystyrene.

5. The method according to any of claims 1 to 4, wherein the swelling agent is selected from dimethylformamide, tetrahydrofuran, chloroform, methylene chloride, ethyl acetate, butyl acetate, toluene, xylene, acetone, ethanol, isopropylalcohol, ethyleneglycol, N-methylpyrrolidone and dimethylacetamide.

6. The method according to any of claims 1 to 5, wherein the coagulating agent is selected from the group consisting of water, methanol, ethanol, isopropylalcohol and mixtures of two or more of the coagulating agents mentioned.

7. The method as claimed in any of claims 1 to 6, wherein the polymer substrate is selected from the group consisting of polystyrene homopolymer or SAN, the swelling agent is selected from dimethylformamide or ethyl acetate and the co- agulating agent is ethanol, methanol, water or mixtures of two or more of the coagulating agents.

8. An article prepared by the method as claimed in any of claims 1 to 7.

9. The article of claim 8 comprising a structured surface having a static water contact angle of from 120 to 180°, preferably 135 to 175°, more preferably 150 to 170°.

10. Use of an article according to claim 8 or 9 in indoor applications selected from furniture, household appliances, computer peripherals, clothing apparel, filters and membranes and outdoor applications selected from cooling equipment, drainage pipes and technical textiles.