Silicon (2014) 6:35–43DOI 10.1007/s12633-013-9164-0

ORIGINAL PAPER

Injection Molding of Superhydrophobic Liquid SiliconeRubber Surfaces

Christian Hopmann · Clemens Behmenburg ·Ulf Recht · Katja Zeuner

Received: 27 May 2013 / Accepted: 17 July 2013 / Published online: 7 August 2013© Springer Science+Business Media Dordrecht 2013

Abstract Superhydrophobic functional surfaces havenumerous applications. Their self-cleaning ability and theassociated savings in energy, water and cleaning agentsenhance the sustainability of products and often make activecleaning of these surfaces unnecessary. Silicone surfaces,which aim to imitate the surface of the lotus plant, were pre-pared using a microstructured injection mold. The conicalmicrostructures were varied in diameter and height rangingfrom 5 to 20 μm as were the process parameters within theframework of a statistical experimental plan. The moldedstructures were evaluated by scanning electron microscopyand confocal laser microscopy, and the resulting contactangles were measured. In contrast to the structural dimen-sions, the process parameters had only a minor impact onthe contact angle. Smaller base diameters of the individualcones and the resulting smaller distances between the conetips produced larger contact angles. Larger aspect ratiosand increasing heights at equal intervals of the individualstructures led to smaller standard deviations from the meanmeasured contact angles. Subsequent mechanical load testsshowed the resistance of the functionalization. Our resultsreveal that it is possible to produce robust superhydrophobicsurfaces in a single-step liquid silicone injection moldingprocess.

Keywords Silicone rubber · Surface structure · Structuralanalyses · Micro structures · Superhydrophobia

C. Hopmann · C. Behmenburg (�) · U. Recht · K. ZeunerInstitute of Plastics Processing at RWTH Aachen University,Pontstr. 49, 52062 Aachen, Germanye-mail: behmenburg@ikv.rwth-aachen.de

1 Introduction

Ongoing new developments and the constant optimizationof existing products and processes are essential for any com-pany to be successful. In nature, over long periods of time,organisms continuously adapt to changing environmentalconditions and have, by evolution, developed outstandingsolutions to life’s challenges. Their efficacy can often not bematched by technical means [1, 2], yet technical and scien-tific progress allows a more efficient analysis and transfer ofbiological mechanisms to technical products [2]. Althoughalready employed for hundreds of years, biomimeticapproaches for “technical implementations of the principlesof nature” [3] are currently attracting considerable attentionagain [4–7]. For the manufacturing industry, the modifica-tion of surfaces is of major interest, as boundary layer func-tionalization is an important factor for many applications.Due to their structure, natural surfaces often incorporatean astonishing level of functionalization or even multi-functionalization. Physical effects caused by the dimensionand geometry of microscopic and nanoscopic structureson the boundary layer can result in self-cleaning capabil-ities, a reduction or increase of the frictional resistance,minimization of the refractive index, evaporation protec-tion, material and gas exchange, thermoregulation or isola-tion, communication, pathogen defense, UV resistance andadhesion [8, 9].

The self-cleaning capabilities of the lotus leaf havebeen observed and reported on many occasions [10, 11].A detailed analysis showed that the prerequisite for thisphenomenon is the combination of a knob-like microstruc-ture and a hydrophobic base material. A droplet of waterforms a very small contact area with the surface and cantherefore roll at the slightest angle of inclination of the sur-face, carrying dirt particles with it [12]. For technical use,

36 Silicon (2014) 6:35–43

self-cleaning surfaces could be applied to improve a varietyof existing products and could also lead to the developmentof new ones, e.g. in medical or in energy applications. Sofar, the production of suitable microstructured surfaces canonly be realized by expensive and time-consuming multi-step processes [13]. In addition, the generated structureshave insufficient resistance to mechanical stress, and merelytouching them can often destroy the functionalization [14].

For the manufacture of products with self-cleaning capa-bilities, a one-step process without any preceding or sub-sequent structuring or coating of each product is desirable.The generated surfaces must also be resistant to mechanicalstress. A solution to these two requirements is the injectionmolding of liquid silicone rubber (LSR) in microstructuredmold cavities. It combines an automatable single-stage pro-cess with a material that has elastic properties during useand low viscosity during processing. In comparison to ther-moplastic materials, there is no need for dynamic moldheating to produce small-sized structures, and the curedmicrostructures are not brittle, which means that they do notbreak and undercuts can be demolded.

The purpose of our research was to examine the pos-sibility of producing superhydrophobic surfaces by LSRinjection molding. By comparing different microstructures,the influence of the structural dimensions on the attain-able functional quality of the surfaces was analyzed. Inaddition, the dependencies of the cast quality and the attain-able superhydrophobia—which is characterized by the con-tact angle between a water droplet and the functionalizedsurface—on the parameters of the injection molding pro-cess were evaluated. For this purpose, the contact angles ofthe structures were measured. The replication accuracy ofthe structures was determined by measuring their mean andmaximum heights by confocal laser microscopy. Finally,the abrasion resistance of the structures was analyzed byfriction tests.

2 Experimental Methods

2.1 Injection Molding of Microstructured Surfaces

A microstructured mold insert of non-hardened, honed toolsteel 1.2767 was used in the clamping side of the injectionmold. The insert was microstructured by Fraunhofer Insti-tute for Laser Technology ILT, Aachen, Germany, using apicosecond laser. There is no melt accumulation on the edgeof the microstructures due to the extremely short irradiatedpulses. The introduced radiation energy vaporized the steel[14]. Six different structured areas of 10 × 10 mm, differingin their combination of structure heights (10 and 20 μm) anddiameters (5, 10 and 15 μm), were prepared on the insert,see Table 1.

Table 1 Dimensions of the microstructures

Area Diameter [μm] Height [μm] Aspect ratio [–]

1 20 15 0,75

2 20 10 0,5

3 20 5 0,25

4 10 15 1,5

5 10 10 1

6 10 5 0,5

The molded discs had a wall thickness of 3 mm, a diam-eter of 80 mm and a gate length of 71 mm. The sampleswere molded with a combination of a TOP 3000 dosing sys-tem of ELMET GmbH, Oftering, Austria, and a Sealmaster969.300 Z LSR injection molding machine of KlocknerDESMA Elastomertechnik GmbH, Fridingen, Germany.The material selected for the samples was LSR2630 fromMomentive Performance Materials Inc., Albany, USA. Likemany LSR types, this material features a hydrophobicmaterial behavior, low processing viscosity, good elasticproperties and is fast-curing. The LSR is supplied in twocomponents that must be mixed in a specified ratio beforecuring. Both components consist of vinyl groups contain-ing polysiloxane and fumed silica. One component containsthe crosslinking agent, the other contains the catalyst. TheLSR is crosslinked in an addition reaction of polyfunc-tional hydrosilane compounds and the vinyl groups of thepolyvinylsiloxane, using hexachloroplatinic acid as the cat-alyst. The crosslinking agents are low molecular-weightsiloxanes with three Si-H groups per molecule [15, 16].

As part of the design of the experiments (DOE), theparameters of the injection molding process were varied asshown in Table 2. The variation of the parameters, knownas factors, occurred by means of a full-factorial, two-stageexperimental plan with a central point. To check the linear-ity, the central point was processed three times throughoutthe molding, which resulted in 23 + 3 = 11 process points.Each process point was molded 15 times.

Table 2 Parameters of the injection molding process

Process parameter Factor

−1 0 +1

Melt temperature [◦C] 20 25 30

Mold temperature [◦C] 140 160 180

Injection speed [mm3/s] 5 12.5 20

Silicon (2014) 6:35–43 37

2.2 Surface Analysis

The microstructured surfaces of the samples were investi-gated using a confocal laser microscope VK-X100K/X200Kfrom Keyence Corporation, Osaka, Japan, with an accu-racy of 0.01 μm. In contrast to light and scanning electronmicroscopy, no sample pretreatment was necessary. Usingthe appropriate software analysis of the acquired data, themolded microstructures were quantitatively analyzed asshown in Fig. 1. The microscopic examination was per-formed at three positions on every structured area. Thescanned sample area per measurement was 200 μm by283.5 μm.

The contact angle measurements were performed usingan OCA20 from Dataphysics, Filderstadt, Germany, and thecorresponding SCA20 analysis software. The droplet vol-ume was a constant 7 μl throughout all the investigations.For dosing, a cannula with a diameter of 0.5 mm was used.A smaller drop was not possible because it was repelledby the functionalized surfaces and could not be separatedfrom the dispensing needle. Similar problems have beenencountered in previous studies [17]. All contact angle mea-surements were made at an ambient temperature of 22 ◦C(±1 ◦C). We define a surface as superhydrophobic when thecontact angle is larger than 140◦.

A verification of the abrasion resistance of the generatedstructures was done using a tensile testing machine Z010from Zwick Roell AG, Ulm, Germany. It was modified tohorizontally draw a slide on the sample table. A steel cuboidof approximately 1.4 kg with dimensions of 120 × 44 ×30 mm was used as the slide. The tester pulled the cuboidat a defined speed over the specimen, thereby stressing themicrostructures with uniform friction. A maximum test loadof 50 N was applied, which was analyzed by the TestEx-pert II software. The test speed varied between 100 and800 mm/min. The process was repeated for different sam-ples with different numbers of repetitions, ranging from 1to 5. Microscopic analysis and contact angle measurementsof the structured areas were performed before and after theabrasion resistance testing.

The statistical analysis of all the acquired data was per-formed with the Statgraphics 15 analysis software from

Fig. 1 Quantitative analysis of the molded microstructures

StatPoint Technologies Inc., Warrenton, USA. The influ-ence of the melt temperature, mold temperature and injec-tion speed on the contact angle, average structure height andmaximum height were analyzed. In addition, the resultingside-effects from interactions were examined.

3 Results

3.1 Structure Dimensioning

The averaged contact angle and the associated standarddeviations of the six structured areas are shown in Fig. 2.The difference between the measured contact angles of thestructured and unstructured regions of the part’s surface isclearly visible: while the contact angle in the unstructuredareas is approximately 115◦, the contact angle of the struc-tured ranges is up to 40◦ higher. An exemplary comparisonof a droplet of water placed on a structured and unstructuredarea is shown in Fig. 3.

The smaller contact angles in the unstructured areasconfirm that a superhydrophobic surface is based on twounderlying factors: a hydrophobic material and a suitablemicrostructure. In terms of the attainable contact angle,structures 4–6 are superior to structures 1–3. With thesestructures, superhydrophobic surfaces can be generated.Structure 1 also reaches values above 140◦, but like struc-ture 3, the high standard deviation of the mean value showsthe low consistency of the values. Large and uniform contactangles can be produced by molding the small base diame-ters of structures 4–6. Due to these smaller base diametersin comparison to structures 1–3, the distance from coneto cone is reduced by a factor of 1.7 from approximately12.9 to 7.6 μm, thereby also changing the distance betweenthe molded cone tips on which the water droplet rests. Alarger distance between those cone tips flattens the droplet,because not only is its weight carried by the cone tips, it alsoallows the droplet to partially emerge into the open spaces

Fig. 2 Averaged contact angle and standard deviation of the sixstructured areas

38 Silicon (2014) 6:35–43

Fig. 3 A droplet of water lyingon an unstructured LSR surface(contact angle 115◦) and on amicrostructured LSR surface(contact angle 150◦)

between the microstructures. A similar causal link betweenthe distances of the structures of the lotus leaf from eachother and the intrusion of a water droplet between thosestructures has already been established in the past [18]. Theincrease of the standard deviation of the mean contact anglesat decreasing aspect ratios of the structure is striking. Ineach case, the structures with the smallest aspect ratio withina group of the same base diameter can only show large con-tact angles by chance. In general, good replication of thestructures can be achieved, but in areas of poor replication,a microstructure hardly exists at all. The small height of thestructures with smaller aspect ratios leaves little scope forthe replication. The structures are either completely filledor not molded at all. Contact angle measurements in areaswith good replication show larger contact angles than thosein areas with poorer replication. By increasing the aspectratio, even in areas with poor replication, small structuresstill exist, resulting in less variation of the contact angle.There is no relationship between the aspect ratio and theresulting contact angle, although structures 1 and 4 showthe worst replication accuracy due to their large aspectratios. While there are no consistently unstructured areas,there are also no places where the structures are completelymolded. The areas consist almost entirely of truncated conesand a few molded peaks. The contact surface of a waterdroplet is higher on the stumps of structures 1 and 4 thanon the peaks of the other structures, which counteracts theformation of larger contact angles. Consequently, the con-tact angles could increase significantly in areas of ideallymolded structures with high aspect ratios.

3.2 Process Parameters

To study the influence of the process, its parameters werevaried within the limits of Table 2, and area 5 was evaluatedas an example. Considering the target dimension “contactangle”, a Pareto analysis showed that only the injectionspeed has a significant effect, which is positive. Raisingthe injection speed results in higher contact angles dueto the high curing reaction rate of the silicone rubber ofabout 3 to 7 mm/s in wall thickness [19, 20]. In order to

achieve a high molding quality of the microstructures, thesestructures must be completely penetrated by the siliconerubber before any premature scorching. Therefore, the injec-tion speed has a significant impact on the quality of themicrostructure molding, as it defines the filling time in theinjection phase [21]. Only a high injection speed ensures ashort filling time without premature scorching. Longer fill-ing times result in premature scorching. The silicone rubbercrosslinks in contact with the hot cavity wall and forms acured skin that hinders further filling of the microcavities.Conversely, when the melt is injected rapidly, there is higherinternal pressure in the cavity, resulting in faster penetrationof the silicone rubber into the microcavities with a reducedrisk of overflowing or premature scorching. The pre-curedskin in each microcavity is formed by comprehensive radialheating. The base diameter of the microstructures is max.20 μm. Neglecting the cooling by the adjacent melt, the bot-toms of these structures with radii of 10 μm are cured in0.07 s, making material movements impossible. Even whentaking the cooling of an adjacent melt into account, only afew tenths of a second remain until a crosslinked surfacelayer is formed. Thus, the microstructures need to be filledin this short period of time in order to avoid an obstructionof the filling by premature curing, which cannot be real-ized at low injection rates. Even a high injection rate cannotguarantee complete filling of the microstructure tips, andresults instead in a more uniform molding of the structures.This has a positive effect on the contact angle. The oppositeeffect, which could be caused by high injection speeds, is adissipative heating of the material by shearing. This couldlead to a higher melt temperature. This effect is negligible inour studies, as a gate with a large diameter and thus a verylow flow resistance is used.

It should be noted that the effect of the injection speed onthe contact angle depends to a certain extent on the adjust-ment of the melt temperature, as shown in the interactionplot in Fig. 4.

At a high melt temperature setting, an increase of theinjection rate has a positive effect on the contact angle.The reason may be the improved fluidity of the LSR atslightly elevated temperatures, which need to be below the

Silicon (2014) 6:35–43 39

Fig. 4 Standardized Pareto chart of the contact angle

processing limits. An injection speed of 20 mm3/s isapparently high enough to ensure sufficient filling of themicrostructures, which is supported by an increased fluid-ity. At an injection speed of 5 mm3/s, however, the filling ofthe structures may be impaired, and the initial crosslinkingis further accelerated by a higher melt temperature, whichis close to the processing limit. This significant interactiondoes not mean, however, that a high melt temperature isgenerally advantageous for the formation of large contactangles. Varying the melt temperature, in fact, has no signif-icant effect. Changing the mold temperature does not haveany major influence on the contact angle either, but a signif-icant side-effect can be seen in the interaction of the moldand the melt temperature. Although, when considered sepa-rately, neither parameter affects the contact angle, the rightcombination of both factors results in larger contact angles.At a high melt temperature, an increase of the mold temper-ature has a negative effect on the contact angle. On the otherhand, at a low melt temperature, the effect is positive. Toproduce large contact angles, the combination of a low meltand a high mold temperature is therefore slightly beneficial.

In some cases, the most pronounced effects contradicteach other, and not all influences can be taken into accountwhen selecting the optimum operating point. There is nosignificant effect of the relationship between the mold tem-perature and the injection speed on the contact angle. Theoptimum setting to maximize the contact angle is an oper-ating point with a high injection rate in conjunction with ahigh melt temperature and a low mold temperature. How-ever, the absolute differences between the contact anglesobtained at all operating points are very low. For example,the average contact angle of area 5, Table 2, varies in ourexamination by 3◦, between 148◦ and 151◦. Even the abso-lute maximum values of all the measurements are only 8◦apart, between 145◦ and 153◦, Fig. 5.

Regardless of the settings, contact angles larger than 145◦(and thus in the superhydrophobic range) can be produced.The process is very stable.

Fig. 5 Maximum and minimal contact angle

The averaged structure height is significantly influencedby the mold and melt temperature (see Fig. 6). The rela-tionship between the mold temperature and the resultingaveraged height is negative. Thus, higher structures are onaverage obtained at low mold temperatures. The durationof the filling phase and the curing time in LSR injectionmolding are influenced by the mold temperature [20, 21],because a low mold temperature can delay the crosslink-ing reaction, and thus a longer fluidity of the LSR canbe realized. At constant wall thickness, the vulcanizationtime decreases linearly with increasing mold temperatures[19]. The penetration of the LSR into the structural peaksis improved as preliminary curing happens slightly later. Athigher mold temperatures, the scorch indices rise faster. Itis therefore likely that the LSR partially crosslinks beforeit penetrates the microcavities, and therefore cannot com-pletely fill the precise structures.

The time for crosslinking is reduced by higher melt tem-peratures, as the temperature compensation is carried outfaster [21]. Premature curing before the melt reaches thetips of the structures is encouraged by the earlier start ofthe crosslinking, resulting in a negative effect. Additionally,with lower melt temperatures, a higher internal pressure inthe mold can be realized, which favors the penetration of theLSR into the structures transversely to the direction of flow[19].

Fig. 6 Standardized Pareto chart of the averaged and maximummicrostructure height

40 Silicon (2014) 6:35–43

To maximize the average height of the microstructures,a large number of these structures need to be entirelymolded. In general, the average height does not say any-thing about the uniformity of the molding. High valuescan be achieved if only a limited number of structures arecompletely molded while others are not molded at all. Thestandard deviation of the mean height shows a relation-ship between the average height and the contact angle. Asshown before, there is not necessarily a correlation betweena higher average height and a large contact angle. If only afew tips are molded, the probability increases of a dropletrolling off these structures into a rather poor molded area,where it stops. As shown in Fig. 7, the contact angle is neg-atively dependent on the standard deviation of the averagestructure height. The general presence of regular structuresis crucial for the contact angle, as it enables a stable contactof the droplet on a small contact area.

A negative correlation between the average structureheight and the standard deviation of the contact angle inthe evaluation of the different structural areas has beenobserved in the past (see Fig. 2). This correlation is notconfirmed in the evaluation of the influence of the processparameters on the molding of the structures of area 5. Sum-marizing both statements, the contact angle fluctuates moreat low altitudes of the structures, and the contact angle islarger in areas where the structure height fluctuates less.With respect to all analyses however, the extremely smalldifferences between the averaged heights and the con-tact angles of the different tests and the resulting processdependence need to be considered.

In addition to the average heights of the structures, ananalysis of the influence of the process parameters on themaximum attainable height of the microstructures was per-formed. As shown in Fig. 6, the Pareto diagram shows onlyone significant effect: Maximum levels are molded predom-inantly at low mold temperatures. This illustrates that, as

previously explained, sufficient flowability of the LSR forthe complete filling of the microcavities can be obtainedat low mold temperatures. The comparatively low moldtemperature of 140 ◦C is particularly suitable for moldingthe tips of the microstructures, but on the other hand it alsoextends the necessary heating time.

3.3 Resistance

A dependency between the structure’s resistance to mechan-ical stress and the process parameters is not to be expected.The injection molding parameters do not affect the hardnessof the material to a measurable degree. Additionally, differ-ent degrees of curing can be excluded due to post-curingduring storage. The investigations are therefore limited tovariations of the load level and the number of repetitions.The analysis is based on randomly chosen samples. Theaverage heights as well as the resulting contact angles beforeand after the frictional stress are examined. Table 3 showsthe test parameters for the abrasion tests.

As shown in Fig. 8, there is a strong relationship betweenthe intensity and number of repetitions on the one hand andthe resulting abrasion on the other. A comparison was madebetween the loaded samples at test speeds of 200 mm/minand 500 mm/min, which were loaded once, three and fivetimes, and a test speed of 400 mm/min, loaded once to fivetimes. The abrasion increases by the number of repetitions.This dependency can be detected at various speeds. A slightsusceptibility of the structures after single or short-termheavy use was determined. The microstructures respondedmore sensitively to repeated or prolonged friction.

The extent of the remaining superhydrophobicity of theexamined functional areas after the abrasion test was deter-mined by additional contact angle measurements. Figure 9shows the results of the measurements before and after thefrictional stress. The absolute values of the contact angles

Fig. 7 Relationship betweenthe contact angle and thestandard deviation of theaveraged structure height

Silicon (2014) 6:35–43 41

Table 3 Parameters of the abrasion test

Sample Test speed [mm/min] Number of repetitions

1 100 3

2 200 1

3 200 3

4 200 5

5 300 3

6 400 1

7 400 2

8 400 3

9 400 4

10 400 5

11 500 1

12 500 3

13 500 5

14 800 3

of all samples are slightly lower, but are consistently above147◦. The difference of the contact angles before and aftertesting is just 0.42 to 2.18◦, which corresponds to a decreaseof 0.28 to 1.45 %. This means that the effective self-cleaningcapabilities of the structures are ensured even after severemechanical stress.

4 Discussion

We propose the following hypotheses with regard to thedimensioning of the structures:

• Lower base diameters of the individual cones and theresulting lower distances between the cone tips lead tolarger contact angles.

• Larger aspect ratios and rising heights at equal inter-vals of the individual structures result in lower standarddeviations from the mean measured contact angle.

Fig. 9 Resulting contact angle before and after the abrasion tests

• Small aspect ratios can easily be molded. For moldingof larger aspect ratios, the curing should be delayed asmuch as possible to avoid early crosslinking.

• Moldable structures with large aspect ratios could resultin even larger contact angles.

There seems to be no direct relationship between thesize of the contact angle and either the height or the aspectratio of the evaluated microstructures. Instead, the targetedcontact angle depends on the peak spacing. A smallerpeak-to-peak distance leads to higher contact angles. Itis assumed, however, that optimal peak spacing exists asa function of the droplet size. The peak distance can-not be reduced arbitrarily. If the peaks are too closetogether, they form a compact surface on which the dropspreads. Nevertheless, the established hypotheses are accu-rate in the analyzed micron scale sizes. A correlationbetween the mean structural height and the contact anglecan only be made with the standard deviation of the con-tact angle. The standard deviation of the contact anglesincreases with decreasing averaged heights. There is, how-ever, an obvious mutual linear relationship between themean and maximum heights of the structures as larger

Fig. 8 Height decrease withdifferent number of repetitions

R² = 0,9255

0,5

1

1,5

2

2,5

3

Test speed [mm/min]/ Repetitions [-]

I500/3

I500/1

I500/5

R² = 0,9868

0,5

1

1,5

2

2,5

3

Test speed [mm/min]/ Repetitions [-]

I400/1

I400/3

I400/5

R² = 0,9878

0,5

1

1,5

2

2,5

3

Abr

asio

n [

µm]

Test speed [mm/min]/ Repetitions [-]

I200/3

I200/1

I200/5

42 Silicon (2014) 6:35–43

maximum heights increase the average heights of the struc-tures and larger averaged heights are also generally asso-ciated with higher values for the maximum heights. Thepeak-to-peak distance is substantially determined by thespacing of the cones base diameters and only slightlyaffected by the structural heights. Different heights ofindividual cones result only in marginal changes of theactual distance between the cone peaks and the surfaceof the truncated cones. The result of this dependency isnegligible.

There is a very small influence of the process parame-ters on the target values. Only minor changes on three offour target values by varying the process parameters canbe observed. Despite the low process response, optimal set-tings that consider all targeted values and all significanteffects by the main variables can be achieved by usingfast injection rates as well as low melt and mold tem-peratures. Side effects are only partially included, as theysometimes contradict the main effects. Taking into accountall the main and side effects of the statistical experimentaldesign on the three main targets of contact angle, averagedheight and maximum height results in a slightly differentcombination of the process parameters. The targeted peakspacing is independent of the investigated process param-eters and therefore not included in this optimization. Dueto the interaction between melt temperature and injectionspeed and their effect on the contact angle, the statisti-cally best operating point leads to a slightly higher melttemperature. Because of the occurring viscosity, those melttemperature result in a more uniform but not maximumreplication of the microstructures. The remaining incuba-tion time at a relatively high melt temperature is probablyinsufficient for complete replication of the microstruc-tures. Maximum heights can be molded at lower melttemperatures.

Despite slight variations, the process is very stable.Reproducible superhydrophobic properties can be gener-ated with all of the process parameters. The casting ofthe microcavities is satisfactory, although the rather largedifferences between mean and maximum heights sug-gest that further improvements of the microstructuring arepossible.

The material-specific material properties of liquid sili-cone rubber result in a greatly improved abrasion resistanceand thus in increased stability of the microstructures com-pared with thermoplastic structures. In particular, the highelasticity of the rubber favors the fractural behavior underpressure and mechanical stress. Critical to the level of abra-sion (which can be measured by the decrease of the averagedstructural heights after the frictional tests) is not primar-ily the velocity and thus indirectly the impact strength, butrather the number of repetitions and thereby the durationof exposure. Short and heavy loads can be tolerated more

easily by the structures than longer and lighter frictionalloads.

The aspect ratio of the structures is a crucial factor forthe amount of abrasion. Structures with larger aspect ratiostend to be more abrasive, as they are less resistant due totheir geometry.

The produced microstructures show good abrasion resis-tance, especially considering the fact that thermoplasticstructures can be destroyed by the touch of a finger. Thesilicone rubber surfaces are resistant to touch and to consid-erably higher mechanical loads. The superhydrophobicityof the microstructured surfaces in all evaluated areas isretained, as the new measurements of the contact anglesshow.

5 Conclusions

Our studies demonstrate the feasibility of producing robustfunctional surfaces by the injection molding of liquid sili-cone rubber microstructures. Due to the strong hydropho-bicity of the material, self-cleaning surface properties can beproduced. The analysis of the attainable contact angle showsits dependence on the structural dimensioning. By injectionmolding microstructures with small peak-to-peak distances,contact angles of 150◦ can be achieved almost regard-less of the parameters of the molding process. Because ofthe superhydrophobicity, the good molding quality and thedurability of the structures, the suitability of microstructuredliquid silicone rubber surfaces for technical applications isvery high. These surfaces can be used in medical appli-cations, for example to enable complete draining of liquidcontainers. In electrical engineering, they offer advantagesfor outdoor insulators. The water-repellent and self-cleaningproperties can reduce the danger of electrical flashovers.The application of the surfaces can also be useful for tools,especially handles, where the cleaning can be simplified.Injection molding offers the advantage of being able to pro-duce complex microstructured parts in a single-step process.In addition, inserts of materials with higher strength, forexample thermoplastics or metals, can be overmolded witha microstructured liquid silicone rubber coating.

Acknowledgments The research project 17724 N of theForschungsvereinigung Kunststoffverarbeitung has been sponsored aspart of the “industrielle Gemeinschaftsforschung und -entwicklung(IGF)” by the German Federal Ministry of Economics and Technol-ogy (BMWi) due to an enactment of the German Bundestag throughthe AiF. We would like to extend our thanks to all of the organizationsmentioned.

We would like to thank the following companies for supplying uswith machinery, equipment and test materials: Klockner DESMA Elas-tomertechnik GmbH, Fridingen, Germany, MAPLAN GmbH, Ternitz,Austria, Momentive Performance Materials, Leverkusen, Germany,ELMET GmbH, Oftering, Austria.

Silicon (2014) 6:35–43 43

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