stretching-controlled micromolding process with etched metal surfaces as templates towards...

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Stretching-Controlled Micromolding Processwith Etched Metal Surfaces as TemplatesTowards Mass-Producing SuperhydrophobicPolymer Films

Jie Feng,* Feiyun Lin, Mingqiang Zhong

A novel technique is described that uses stretching-controlled thermal micromolding withetched metal surfaces as templates for the mass-production of superhydrophobic polymerfilms. First, the metal surface is etched and then used as a template to thermally replica-moldthe polymer (e.g., polyethylene). The resulting film surfaces exhibited stable superhydropho-bicity with water contact angles >1508 and sliding angles �78. SEM imaging demonstratesthat the microstructure on the superhy-drophobic surface is formed by stretch-ing from the microholes of the templateduring separation. This technique can beeasily combined with melt-flow castingfor manufacturing superhydrophobicpolymer surfaces on a large scale.

Introduction

Superhydrophobic surfaces have attracted continuous

attentions for their wide applications in the fields of self-

cleaning, fluidic drag reduction, anti-fog, anti-biofouling,

and humidity proofing.[1–4] Numerous studies have con-

firmed that a certain extent of surface roughing was the

precondition for afilmexhibiting superhydrophobicity.[4–6]

A very large number of clever ways to produce rough

surfaces that exhibit superhydrophobicity have been

reported. The representative ways include phase separa-

tion,[7,8] films extending,[9] surface sanding,[10] surface

frothing,[11–13] and template-based replica molding.[14–18]

Among them, template replica molding may be the most

J. Feng, F. Y. Lin, M. Q. ZhongDepartment of Materials Science and Engineering, ZhejiangUniversity of Technology, Hangzhou 310014, ChinaE-mail: [email protected]

Macromol. Mater. Eng. 2010, 295, 859–864

� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlin

promising and straightforward technique for manufactur-

ing polymer superhydrophobic films in a large scale.

Melt-flow casting is a most popular technique for

manufacturing polymer films on a large scale. If we could

combine it with template replica molding, polymer

surfaces with hierarchical structure and superhydropho-

bicity could be manufactured on a large scale. However,

fabricatingmm- and nm-scale 2D ordered holes on the

surface of an industrial rollermold,which is typicallymade

of stainless steel, is not aneasy task.[19,20]Moreover,weand

other researchers have found that it was difficult to get

superhydrophobic polymer surfaces by simply thermal

pressing melt polymers onto the stamp replicated from

lotus leaf.[21,22] This may be caused by the poor replication

quality on account of the high melt viscosity: nanostruc-

tures can not be replicated successfully due to incomplete

melt filling.

However, in the same early study, we also found that

superhydrophobic polyethylene (PE) films could be pro-

duced by a stretching-controlled micromolding process

elibrary.com DOI: 10.1002/mame.201000084 859

J. Feng, F. Y. Lin, M. Q. Zhong

860

using a poly(dimethylsiloxane) (PDMS) template replicated

from a lotus leaf.[22] This implies that it is possible to

produce polymer superhydrophobic films on a large scale if

only the PDMS stamp was changed into a roller mold that

can be used for flow casting, even if the holes on the mold

surface were not deep enough and their distribution was

not regular. Inspired by this analysis, metal surfaces

including that of stainless steel were etched in acidic

solutions. Using these etched metal surfaces as templates,

thermoplastic polymers such as PE were thermally replica-

molded and the resulting films show a good super-

hydrophobicity.

Experimental Part

Materials

Stainless steel (GB0Cr18Ni9, Ni: 8.00–11.00, Cr: 18.00–19.00, other

mainly isFe), brass (GBH59,Cu:57.0–60.0%, other ismainlyZn), and

aluminum (GB6063) were purchased from Shanghai AofengMetal

Products Co. Ltd.; Low-density polyethylene (LDPE,N220) and high-

density polyethylene (HDPE, SH1200) were both purchased from

Sinopec Shanghai Petrochemical Co. Ltd.; FeCl3 �6H2 O and H3PO4

were purchased from SinopharmChemical Reagent Co., Ltd, China;

hydrochloricacid (HCl) andH2NCSNH2waspurchasedfromNingbo

Chemical Reagent Co., Ltd, China; all the reagents were chemical

pure grade and used without any further treatment.

Fabrication of Metal Template

The stainless steel plates with area 2�2 cm2 and thickness 2mm

weremechanicallypolishedand cleaned in acetoneultrasonic bath

for 30min. Then they were placed in an acidic etching solution

composed of 600 g � L�1 FeCl3, 20 g � L�1 H3PO4, 80 g � L�1 HCl, and

4 g � L�1 H2NCSNH2 in water. After being incubated in stirred

solution (60mL in 100mL beaker, 240 rpm) at 20 8C for 20min, the

stainless steel plates were rinsed with deionized water and dried

by N2 for application in the following micromolding process. The

brass andaluminumplateswereetchedwith the similarprocedure

except that the solution was composed of 400g � L�1 FeCl3 and

80g � L�1 HCl and the times were 1.5h and 5min, respectively.

Thermal Micromolding Processes

Polyethylenewas selected in this study due to its wide application

and lowmelting temperature (Tm�140 8C). In the thermal replica-

molding process, a piece of PE with area of 1.5�1.5 cm and

thickness 2mm was first placed onto the above metal templates.

Then a piece of glass plate (4�4�0.3 cm3) was put on the PE piece

and two pinchcocks were used to apply a pressure (�3.5 kPa)

between the PE and themetal template. Then the whole construct

was incubated in an oven (200 8C) for 10min. After being cooled to

�120 8C at ambient conditions, the PE filmwas peeled off from the

template in 2 s manually with the help of pinchers. The thermal


� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

micromolding was repeated ten times on each template to ensure

the repeatability of the peeling-off process.

Characterization of Microstructure and Wettability

Surfacemorphologies of themetal templates, including those after

peeling off process and the micromolded polymer films were

imaged by field emission scanning electron microscopy (FE-SEM,

Hitachi S-4700, Japan). The water contact angles (WCAs) were

measured with a sessile drop method using a WCA measuring

system (DSA100, Kruss, Germany). The droplet volume was 4mL.

The sliding angle was defined as the surface inclining angle at

which the droplets began to roll down and measured by inclining

the superhydrophobic surfaces gradually from 08 to higher angles

till thedroplets start rolling.All themeasurementswere carriedout

at roomtemperatureandrelativehumidity�75%andthefinaldata

were averaged over six measurements. The stability of super-

hydrophobicitywasevaluatedby studying the impactdynamicsof

continuous 30mL size water droplets free falling from 100 cm

height upon the surface.

Results and Discussion

Metal, in particular stainless steel, is difficult to be etched in

a tunable way. In experiment procedure here, we tried

many different etching conditions such as formulation and

concentration of the etching solutions, the etching tem-

perature, the etching time, and if the etching solution was

stirred or not. Different combination of these etching

parameters brought different etching effect, e.g., the

topographical structure of the metal surface. Figure 1

shows the scanning electron microscopy (SEM) images of

the stainless steel surface etched by the optimized

procedure described in experimental part. It can be seen

that the etching brought large amount ofmm, sub-mm

and nm structures on the metal surface. Their distribution

was homogeneous and their depths were �2mm.

However, just using above structured stainless steel

surface as template, HDPE surfaces with high and homo-

geneous spine like structures were micromolded success-

fully (Figure 2). Such structures with high aspect ratio

should be formed by the adhesion of the polymermelt onto

the above sub-mm and nm structures of the template and

subsequent stretching from the microstructures during

separating operation. The high extent of melt filling in the

sub-mm and nm structures of the metal template and

the interaction at the PE/template interfaces should be

responsible for such stretching. The peeling-off operation

also demonstrated this analysis. It is found that it was

easier to peel off the HDPE films from the control smooth

metal surface than from the etched rough metal surface.

The template after ten times of micromolding operation

was also examined by SEM imaging. No obvious HDPE

residue was found on the template except for some very

DOI: 10.1002/mame.201000084

Stretching-Controlled Micromolding Process with Etched Metal Surfaces as Templates . . .

Figure 1. SEM images of the stainless-steel template fabricated byetching in stirred water solution (240 rpm) composed of600g � L�1 FeCl3, 20 g � L�1 H3PO4, 80g � L�1 HCl, and4 g � L�1 H2NCSNH2 at 20 8C for 20min. (b) Partially magnifiedimage of (a).

Figure 2. SEM images of HDPE film surface thermally replicamolded from the stainless steel template shown in Figure 1.The peeling off temperature was �120 8C. (b) Partially magnifiedimage of (a).

small dots (Figure 3). Factually, even the residue is visible,

the templates could still be applied because new batch of

polymermelt wouldmelt and bring the residue away from

the templates.

In addition to stainless steel and HDPE, other metal and

polymer types were also tried to fabricate superhydropho-

bic films. Figure 4 showed the SEM images of the etched

brass plates after peeling off fromLDPE and the correspond-

ing LDPE replica. It can be seen that a little different from

stainless steel surface, brass surface was endowed with

homogeneous but much rougher microstructures by

corresponding acid etching. There were many sub-mm

and nm structures on the microstructures. However,

compared with high aspect ratio of spine like structures

formed on HDPE replica by stainless steel template, grass

like structures with much higher aspect ratio were formed

on LDPE surface after thermally replicating LDPE on herein

brass template. It was obvious that a similar stretching

phenomenon was also occurred during LDPE/brass tem-



plate separating process. Similar resultswere also obtained

by using aluminum plates and LDPE as studying objects.

However, in contrast to the other two templates, the brass

template adhered some obvious polymer residues after

peeling off operation (see the circles in Figure 4a). This could

be causedby themuch roughermicrostructures of thebrass

template.

Surface hydrophobicity, especially superhydrophobicity,

is a very straightforward property of most solid surfaces.

Stream fromawater bottle or even awater pipe canbeused

to qualitatively estimate surface superhydrophobicity.

However, a more quantitative understanding of the

superhydrophobic surface should bemadeby contact angle

measurements. Figure 5a showed the representative CCD

camera image of 4mLwater droplets on the HDPE filmwith

thesurface topographyshowninFigure2.TheaverageWCA

was 152.4� 1.78. Figure 5b shows the snapshot of a 4mL

water droplet that just started rolling down at the inclined

HDPE surface, showing the sliding angle was �78. Figure 6

www.mme-journal.de 861


Figure 3. SEM images of the stainless steel template after tentimes of micromolding operation. (b) Partially magnifiedimage of (a).

Figure 4. SEM images of the brass template fabricated by etchingin static water solution composed of 400g � L�1 FeCl3 and80g � L�1 HCl at 20 8C for 1.5 h (a) and the corresponding LDPEreplica (b).

862

showed the dynamic contact angle of a 4mL water droplet

on HDPE surface. It can be seen that theWCAswere always

higher than 1508. All these results confirmed the HDPE

surface was do superhydrophobic. In addition to HDPE

Figure 5. The CCD camera images of a 4mL water droplet that stayed on the aclinic HDPEsurface (a) and the snapshot of a 4mL water droplet that just started rolling down on theinclined HDPE surface (b).

surface replicated by stainless steel

template, the LDPE surfaces replicated

by the brass and aluminum templates

also showed superhydrophobicity, with

WCA at 152.7� 2.1 and 153.1� 1.88,respectively, and the sliding angles were

both �78.In addition to static WCA and sliding

angle, the stability to water impact was

also crucial to a superhydrophobic sur-

face especially thinking of practical

application. For evaluating such stability,

continuous 30mL size water droplets

from a plastic bottle were released at

different heights to impact upon the



HDPE surface replica-molded by the stainless steel tem-

plate. The dynamics of drop impact was recorded by the

camera equipped inDSA 100. From the snapshots (Figure 7)

captured from the video, we can see that even when the

DOI: 10.1002/mame.201000084

Stretching-Controlled Micromolding Process with Etched Metal Surfaces as Templates . . .

Figure 6. The dynamic contact angle of 4mL water droplets on theHDPE films with the surface topography shown in Figure 2.

Figure 7. The snapshot of continuous 30mL water droplets drop-ping onto the HDPE surface with static WCA of 152.4� 1.78 andsliding angle �78 from 100 cm height.

water droplets were released at 100 cm height, they could

also be bounced off from the surface, either formed

vibrating water balls and finally rolled away from the

surface, or formed several satellite droplets with WCA

above1508. This implied thatatpresent impactvelocity, the

microstructures on the surface could not puncture the

water droplets, but still remain the droplets at Cassie state,

e.g.,with theair trappedunderneath thedrop. The repellent

interactionbetweenthedroplets and theair trappedwithin

the rough structure brings such high anti-drop impact

ability.

In fact, the etching ofmetal surfaces by immersing them

in oxidation-reducing agent/acid mixed solution has been

studied bymany other researchers. However,most of them

focused on the direct fabrication of superhydrophobic

metal surface by treating the etched metal surface with



fluorine-containing substances. Rarely was the etched

surface used to replica-mold polymer superhydrophobic

films. This may be caused by the intuitive expectation that

the shallow concave pits on the etched metal surface

generally endow the replicated polymer films with little

structure. In this study, however, we obtained a micro-

structure with a much higher aspect ratio on replica-

molded polymer films by using shallow-structured metal

templates.Althoughvery smallmetal templates (2� 2 cm2)

were used here, it is easy to expand to the templates with

large area in the future, e.g., the steel roller mold used in

flow casting technique. This is a meaningful finding

because it means PE and other polymer superhydrophobic

films could be produced on a large scale easily if only the

etching process, the thermal pressure and the strength for

peeling off were all homogeneous over the surface of the

roller mold.

Conclusion

This paper describes the preparation of superhydrophobic

surfaces by combining thermal replica molding and

stretching procedure to achieve elongated polymer surface

structures with increased hydrophobic property. The core

ideaof this technique is that it applied the interactionat the

polymer-template interface to form stretched features. To

our knowledge, this is the first time to demonstrate the

feasibility of fabrication of superhydrophobic surfaces by

using facile metal template and simple thermal pressing

process. Because this technique can be easily transferred

onto the surface of the stainless steel roller mold that

generally used inflowcasting technique formanufacturing

polymer films, it may be a very promising technique for

producing polymer superhydrophobic films in a large scale.

Acknowledgements: Financial support from theNational NaturalScience Foundation of China (grant no. 50703037), the ZhejiangNatural Science Foundation (Y407256), and the Zhejiang QianjiangProject (2007R10005) is gratefully acknowledged.

Received: March 5, 2010; Revised: May 22, 2010; Published online:July 21, 2010; DOI: 10.1002/mame.201000084

Keywords: metal templates; polyethylene; stretching; superhy-drophobic; thermal replica molding

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DOI: 10.1002/mame.201000084

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