Modified Stainless Steel Surfaces Targeted to Reduce Fouling––Surface

Journal of Food Engineering 64 (2004) 63–79 www.elsevier.com/locate/jfoodeng

Modiﬁed stainless steel surfaces targeted to reduce fouling––surface characterization Olga Santos a,*, Tommy Nylander b, Roxane Rosmaninho c, Gerhard Rizzo d, Stergios Yiantsios e, Nikolaos Andritsos e, Anastasios Karabelas e, Hans Muller-Steinhagen€ d, Luis Melo c, Laurence Boulange-Petermann f, Christelle Gabet f, Alan Braem b, Christian Trag€ ardh a, Marie Paulsson a a Department of Food Engineering, University of Lund, P.O. Box 124, S-221 00 Lund, Sweden b Department of Physical Chemistry II, University of Lund, P.O. Box 124, S-221 00 Lund, Sweden c Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, s/n, 4200-465 Porto, Portugal d Institute for Thermodynamics and Thermal Engineering, University of Stuttgart, Pfaﬀenwaldring 6, 70550 Stuttgart, Germany e Chemical Process Engineering Research Institute––C.E.R.T.H., 6 km Charilaou––Thermi Road, P.O. Box 361, GR-57001 Thermi, Thessaloniki, Greece f Ugine and Alz––Groupe Arcelor, Centre de Recherches d’Isbergues, BP 15, F-62330 Isbergues, France Received 30 May 2003; accepted 13 September 2003

This paper is dedicated to the memory of Dr. Hans Visser

Abstract The surface properties of several modified stainless steel samples were characterized according to their chemical composition, þ 2þ roughness, topography and wettability. The modifications tested were SiF3 and MoS2 ion implantation; diamond-like carbon (DLC) sputtering; DLC, DLC–Si–O and SiOx plasma enhanced chemical vapor deposition (PECVD); autocatalytic Ni–P–PTFE and silica coating. X-ray photoelectron spectroscopy (XPS) and X-ray microanalysis were applied to determine the surface chemical composition. Atomic force microscopy (AFM) and stylus-type instruments were used for roughness determination, and the surface topography was imaged with AFM and scanning electron microscopy (SEM). The contact angle and surface tension were measured with the Wilhelmy plate method and the sessile drop method. For thick modified layers, only the elements of the coating were detected at the surface, whereas for thin layers the surface composition determined was that of the stainless steel substrate. The roughness of the 2R (cold rolled and annealed in a protective atmosphere) surfaces was not altered by the modification techniques (except for the Ni–P–PTFE coating), while for the 2B (cold rolled, heat treated, pickled and skinpassed) surfaces an increase in roughness was observed. The silica coating produced surfaces with consistent roughness, independent of which steel substrate was used. DLC sputtering and Ni–P–PTFE coating produced surfaces with the highest roughness. All modified surfaces revealed a similar surface topography with the exception of the Ni–P–

PTFE coating, for which the coating masked the underlying steel topography. In terms of wettability, the SiOx-plasmaCVD and Ni– P–PTFE coating techniques produced the most hydrophilic and hydrophobic surfaces, respectively. 2003 Elsevier Ltd. All rights reserved.

Keywords: Modiﬁed stainless steel; Fouling; Chemical composition; Roughness; Surface energy

1. Introduction Harold Burton in the 60’s (Burton, 1968). Already, these early studies established that protein deposition is one of Fouling of process equipment in the dairy industry the key factors in fouling of heat exchanger surfaces in has been one of the main challenges for researchers the dairy industry. Protein adsorption onto solid sur- within food engineering, starting with the early work of faces results from a competition between different types of interactions involving the protein, the surface, the * Corresponding author. Tel.: +46-46-222-98-08; fax: +46-46-222- solvent and any other solute present in the system 46-22. (Haynes & Norde, 1994). These interactions are signi- E-mail address: [email protected] (O. Santos). ficantly influenced by the surface properties. The

0260-8774/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2003.09.013 64 O. Santos et al. / Journal of Food Engineering 64 (2004) 63–79 challenge is to determine which property is the impor- duction and maintenance costs. In spite of the many tant factor. Surface free energy of the solid–liquid in- studies devoted to finding ways to avoid or reduce terfaces, as defined by the Young equation, is one factor fouling, no major break-through has been reported that that can give some indication of the degree of foulant can provide the food industry with a sustainable and adsorption. However, a quantitative prediction relating economic anti-fouling coating. The challenge is that surface energy and adsorbed amount is still lacking. stainless steel, the preferred material in food, pharma- Several reports can be found in the literature dealing ceutical and biotechnology industry due to its resistance with the influence of the substrate surface energy on the to corrosion, cleanability, strength and durability, is protein adsorption. For instance, Janocha et al. (2001) hard to modify. However, recent developments in sur- observed that the adsorbed amount of albumin de- face treatment technology in material science have creased with increasing surface energy. Yoon and Lund opened up new possibilities. These possibilities are ex- (1994) also found higher milk fouling rates for the plored in the European MODSTEEL project to produce PTFE-coated plates in plate heat exchangers than for stainless steel surfaces that are less prone to milk fouling, the 304 stainless steel plates with a high surface energy. and in this paper we will present and discuss some of our However, in a study by Addesso and Lund (1997), a major achievements regarding surface modification. similar rate and extent of protein (a-La, b-Lg) adsorp- We have applied and tested four different approaches tion was reported for surfaces with varying surface en- for surface treatment of stainless steel surfaces: ergies. Regarding the influence of surface energy on adhesion strength, Britten, Gree, Boulet, and Paquin (1) Ion implanted surfaces, obtained by introducing þ 2þ (1988) observed weaker protein (raw milk) and phos- SiF3 and MoS2 ions. The modifying material is di- phate adhesion forces on the lower surface energy sur- rectly implanted into the base material to form a faces. Bornhorst, Steinhagen, and Zhao (1999) found a surface alloy. This approach is expected to lower similar correlation between CaSO4 deposits and solid the surface energy. surfaces. A critical surface tension of wetting, approxi- (2) Diamond-like carbon (DLC) surfaces obtained by mately 30 mN m1, at which protein adsorption/depo- sputtering and plasma enhanced chemical vapor de- sition is minimal, was reported by Baier and Meyer position (PECVD). Using this method, the aim is to (1992) and later by Zhao and Muller-Steinhagen€ (2003). produce a thin film on the surface. Also, properties like surface roughness, charge and (3) Silica surfaces obtained by charge density play an important role for protein ad- (a) SiOx also manufactured by PECVD. This will sorption. Wahlgren and Arnebrant (1990) observed give a hard glass-like surface. higher adsorbed amounts of b-Lg on polysulfone than (b) Silica coating by the sol–gel process. This ap- on methylated silica surfaces, which they attributed to proach is applied to produce a hydrophilic and the higher surface roughness of the polysulfone surface. hydrated anionic surface. The same trend was found in other studies for b-Lg (4) Creating a Teflon surface by autocatalytic Ni–P– adsorbed on silicon surfaces and on steel surfaces with PTFE deposition to obtain a hydrophobic ‘‘non- increased hydrophobicity (Krisdhasima, McGuire, & sticky’’ surface. Sproull, 1992; Santos, Nylander, Paulsson, & Trag€ ardh, þ 2003). The effect of surface charge and charge density Some of these modifications, namely SiF3 implanta- was studied by Norde, Arai, and Shirahama (1991), who tion and DLC sputter coating, have previously proved found that on hydrophilic surfaces, structurally stable to be efficient in reducing CaSO4 scale formation during proteins adsorb only if electrostatic interaction is fa- pool boiling and bacteria attachment (Bornhorst et al., vourable, e.g. if the net charge of the proteins is opposite 1999; Muller-Steinhagen€ & Zhao, 1997; Muller-Stein-€ to the surface. However, on hydrophobic surfaces they hagen, Zhao, & Reiss, 1997). are also adsorbed on charged surfaces that carry the Bornhorst et al. (1999) obtained on these two modified same (net) charge, but to a lesser extent. For less stable surfaces final heat transfer coefficients that were 2–3 proteins, where structural re-arrangements contribute to times higher as compared to the unmodified surface. In the tendency to adsorb, the adsorption on surfaces with addition, the deposit layer formed on the modified sur- the same charge can occur. Surface topography has also faces was thinner and easier to remove. These modified þ been found to affect bacterial adhesion. Boyd, Verran, surfaces, SiF3 implanted and DLC sputtering, have also Jones, and Bhakoo (2002) observed higher bacterial shown promising results regarding reduction of b-Lg adhesion forces and cell retention after scanning with an adsorption (Santos et al., 2003). A 19% reduction of the AFM tip on abraded surfaces (unidirectional topogra- adsorbed amount of b-Lg on the DLC sputtering surface phy) as compared with either polished or unpolished was determined in a previous study (Santos et al., 2003). (grain structure) surfaces. The focus of this work is on the interactions involving Fouling does not only impair the safety of the final surface and foulant (protein, salt, bacteria). In order to product but also accounts for higher investment, pro- understand these interactions, it is important to have a O. Santos et al. / Journal of Food Engineering 64 (2004) 63–79 65 good knowledge of the different properties of the modified mersed in a 2.0% w/v detergent (RBS35) solution in surfaces. Therefore, the aim of the work presented in this distilled water at 50 C for 10 min; (2) rinsed with dis- paper was to characterize the modified surfaces men- tilled water at 50 C for 10 min and (3) rinsed with tioned above in terms of surface energy, chemical com- distilled water at 20 C. position, roughness, and topography, for subsequent studies on adsorption/deposition of organic and inor- 2.3. Surface modification techniques ganic material. Several techniques were used for this purpose. X-ray photoelectron spectroscopy (XPS) and 2.3.1. Sputter technique X-ray microanalysis gave information about the chemical Sputter coating is a physical vapor deposition (PVD) composition of the samples. The measurement of the process, which take place at sub-atmospheric pressures. surface roughness was performed by atomic force mi- The surface to be coated (the ‘‘substrate’’) and the croscopy (AFM) and with stylus-type instruments. The coating material (the ‘‘target’’) are arranged in the vac- topography of the surfaces was recorded by the use of uum, as depicted in Fig. 1, or they may be moving past microscopic techniques, such as AFM and scanning each other in case of larger substrates. electron microscope (SEM). The surface energy (contact The high voltage applied between substrate and tar- angle and solid/liquid interfacial tension) was determined get ionises the gas (usually argon, but other gases are using either the sessile drop or the Wilhelmy plate method. possible) in the chamber until a plasma is ignited. In Fig. 1, the reactive gas is indicated by big black circles and 2. Materials and methods the ionised gas by small black circles. In practice, both direct current (DC) and radio frequency (RF) voltages 2.1. Materials are used, with the target usually being the cathode. Due to their positive charge, the plasma ions are accelerated Alkaline detergent RBS35 (NFT 72151–72190), con- towards the cathode. On hitting the cathode surface, the taining a mixture of non-ionic and anionic surfactants, plasma ions dislocate atoms from the target surface was obtained from RBS Chemical Products, Brussels, (indicated by grey circles in Fig. 1). These atoms can in Belgium. The water used was deionised and passed turn be deposited on other parts of the surface. through a Milli-Q Gradient water purification system If reactive gases, e.g. N2, are available in the vacuum (Millipore S.A., Molsheim, France). chamber in addition to the inert plasma gas, they may The unmodified surfaces tested were of type 316L react with the available ions and atoms. In this case, the with surface finish 2R (cold rolled, bright annealed) and technique is referred to as ‘‘reactive sputtering’’. If the 2B (cold rolled, heat treated, pickled and skinpassed), desired final surface film does not adhere well, the sub- received from a European manufacturer of stainless strate can be precoated with another material. DLC thin steel. Their chemical compositions (in percentages), films are produced in this way with a TiN and then a given by the manufacturer, are presented in Table 1. The þ 2þ surface modifications consisted of SiF3 and MoS2 ion implantation; a DLC sputter coating; DLC, SiOx, and DLC–Si–O plasmaCVD thin films; an autocatalytic Ni– P–PTFE coating, provided by the University of Stutt- gart (Germany) and Silica coating provided by CPERI þ (Greece). Two different batches of SiF3 implanted surfaces, SiF and SiF FZ, prepared in different plants under identical conditions, were used.

2.2. Cleaning procedure

All stainless steel samples were cleaned with the commercial detergent RBS35 before each experiment. The procedure was as follows: samples were (1) im- Fig. 1. Schematic representation of sputtering equipment.

Table 1 Chemical composition of the stainless steel 316L ﬁnishes used in this work Steel Elements (wt.%) Fe Cr Ni Mo Mn Si Cu C Others 316L 2R 66.5 17.7 11.1 2.1 1.7 0.52 0.14 0.03 0.24 316L 2B 66.8 17.5 11.1 2.1 1.6 0.55 0.13 0.02 0.20 66 O. Santos et al. / Journal of Food Engineering 64 (2004) 63–79

TiC film formed in presence of mixed N2 gas and acet- the substrate material is considerably improved through ylene on top of a metal substrate before the final DLC the implanted ions and/or the implantation process layer is added. (Zettler & Muller-Steinhagen,€ 2000). The drawback is The DLC coating itself can be produced by sputtering the high capital and operating costs of directed ion im- of graphite targets or through a plasma CVD process. plantation processes. The advantages of sputter coatings are the hard and The investigated sample surfaces have been implanted þ generally very adhesive films. with SiF3 ions with an implantation energy of 200 keV. The ion concentration was 5 · 1016 ions/cm2. In order to 2.3.2. Plasma enhanced chemical vapor deposition achieve homogenous modification, the ion beam scan- Plasma CVD––or plasma enhanced CVD––is a ned the surface in a regular pattern. chemical vapor deposition process where the chemical reaction is not initiated through thermal energy (as in 2.3.4. Turbulent ion implantation normal CVD), but through formation of a plasma. This Similar to sputtering, a plasma is ignited in the vac- plasma is usually induced through RF or microwaves. uum chamber during turbulent ion implantation, but in Contrary to sputter techniques, where the coating ma- this case the substrate is a cathode. However, the atoms terial is introduced into the process as a solid target, impacting on the surface of the substrate have such a CVD processes use gaseous or vaporous precursors. high energy that they can penetrate into the interior of Typical gases are CxHx for DLC films or Hexamethyl- the material as in direct ion implantation. In fact, atoms disiloxan (HMDSO) for SiOx films. As with other film hitting the surface can push already deposited atoms techniques, argon and other process gases are added to further inside, leading to implantation depths of up to the vacuum chamber. In addition to the easier handling, 100 lm, for initial surface layers thickness of 2–5 lm. Plasma CVD also offers the possibility to implement Similarly to reactive sputtering, it is possible to intro- additional atoms into the coating. By adding a silicon- duce additional reactive particles to the vacuum cham- containing precursor and oxygen to the working gases, it ber by turbulent ion implantation. This leads to a high is, for example, possible to obtain DLC–Si–O coatings. flexibility in the selection of the coating materials. MoS2 Disadvantages of plasma CVD coatings are their reduced particles were chosen in this study. Another advantage density and weaker adherence to the substrate material. with this technique is that it is possible to implant on the inside of an object, e.g. tubes or valves. 2.3.3. Directed ion implantation During directed ion implantation (or ion beam im- 2.3.5. Autocatalytic Ni–P–PTFE coating plantation), the surface is bombarded with highly ac- The Ni–P–PTFE coating is produced by an autocat- celerated ions, with an average energy of several 100 alytic plating process (Nasser, 2000). The process con- keV. The modifying material is directly implanted into tains five steps, as can been seen in Fig. 2. The first step the base material to form a surface alloy. Hence, there is an alkaline cleaning bath, followed by a pickling are no adhesion problems such as in case of chemical process. After this, the substrate must be activated. This vapor deposition (CVD) or galvanic coatings. Further- takes place in a third step, the galvanic deposition of more, it has been found that the corrosion resistance of nickel. The fourth step is the beginning of an autocat-

uncoated Ni-P-PTFE coated sample sample

5 ... 10 min 30 s ... 1 min 1 ... 3 min 1 h PTFE particles

alkaline pickling galvanic autocatalytic autocatalytic cleaning Ni plating Ni-P plating Ni-P-PTFE (2 ... 3 A/dm2) plating

Fig. 2. The autocatalytic Ni–P–PTFE coating process. O. Santos et al. / Journal of Food Engineering 64 (2004) 63–79 67 alytic reaction in which a Ni–P plating is deposited on techniques for physical surface analysis are required. the sample. After the Ni–P plating has reached a certain The 316 2R and 2B surfaces, as received and after the thickness, PTFE particles are added to the coating bath, cleaning procedure, were analysed by XPS. The chemi- to be incorporated in the Ni–P matrix. cal composition of the modified and unmodified surfaces To be effective in reducing fouling, the outer surface was measured quantitatively by X-ray microanalyses. of the Ni–P–PTFE plating should contain a high per- centage of homogenously distributed PTFE particles. 2.4.1. X-ray photoelectron spectroscopy Up to now, the maximum achievable amount of PTFE All samples were analysed after a plasma cleaning in the outer surface is 20%. treatment in order to reduce the surface contamination and to exhibit photo-peaks coming from inner layers. 2.3.6. Silica coating The argon plasma treatments were carried out in a The procedures adopted for silica coating are based March I Instrument Plasmod unit. The radio-frequency on the sol–gel process. Commonly, the sol–gel method power was 50 W and the treatment time was 10 min. uses metal alkoxide compounds as raw ingredients, After any surface treatment, the samples were trans- which are hydrolysed in the presence of water and ferred within 3 min to the XPS instruments. condensed to form M–O–M bonds (M ¼ Si, Ti, Zr, etc). XPS analysis was performed using a Vacuum Gen- After hydrolysis and condensation have proceeded to erator XR3E2 spectrometer employing a Mg Ka (1253.6 some degree (sol formation), a coating can be applied on eV) achromatic X-ray source operated at 15 kV with a the substrates by, for example, dip- or spin-coating. power of 300 W. Survey scan spectra were obtained with Finally, the coating structure is consolidated by high a pass energy of 30 eV for all samples, to determine what temperature annealing, where residual organic mole- elements were present in the top 5 nm of the surface. The cules are oxidized and removed. value of the take off angle between the surface and di- Sols were prepared from methyl–triethoxysilane rection of electron detection was 90. Typical operating 10 (MTES):H2O (molar ratio 1:3) solutions in EtOH at pressures were 10 torr and the analysed area was a various dilutions (1:10–1:4), and annealed at tempera- 10 · 4mm2. All binding energies were referenced to the tures in the range 200–500 C. Under these conditions, carbon C–H photo-peak at 285.0 eV. crack-free coatings on both surface finishes, 2R and 2B, The atomic sensitivity factors used for quantitative could be obtained. determination were obtained from Scofield data. The The mechanical properties of the films, as measured background in the peak area computation was assumed by advanced nano-indentation techniques, are superior to follow a Shirley behavior. to those of the stainless steel substrate, in terms of The structure of the passive film is expressed by the elasticity modulus (415 vs. 215 GPa for the substrate) thickness, the hydroxides/oxides ratio and the chro- and yield stress (270 vs. 65 GPA for the substrate). mium/iron ratio. The thicknesses of the various modified films are presented in Table 2. 2.4.2. X-ray microanalysis In SEM, an electron beam is scanned across a sam- 2.4. Chemical composition techniques ple’s surface. When the electrons strike the sample, a variety of signals are generated and the detection of The corrosion resistance of stainless steels depends on specific signals produces an image of the surface or a the protective properties of their passive layer, which are sample’s elemental analysis. Interaction of the electron determined by their physical and chemical characteris- beam with atoms in the sample causes shell transitions tics. Since the thickness of the passive layer is of the that result in the emission of an X-ray. The emitted order of 1–10 nm, depending on the case, sensitive X-ray has the energy characteristic of the parent element. Detection and measurement of that energy permits a quantitative analysis of the elemental com- Table 2 position of the sample with a sampling depth of 1–2 lm. Thickness of the modified layers Analysis of the constitutive elements on the sample Modifications Thickness (lm) surfaces was measured by energy dispersive spectroscopy SiF implantation 0.2 (EDS) using an OXFORD ISIS 300 system. Quantitative SiF FZ implantation 0.2 microanalysis was performed at least at three different MoS2 implantation 0.2 positions and the average composition was recorded. DLC sputtering 2 DLC-plasmaCVD 1 DLC–Si–O-plasmaCVD 2 2.5. Surface roughness techniques

SiOx-plasmaCVD 2.5 Ni–P–PTFE coating 10 The surface roughness of the different samples was Silica-sol gel 0.1–0.2 measured by AFM and by the stylus instruments, 68 O. Santos et al. / Journal of Food Engineering 64 (2004) 63–79 Dektak3ST profiler and Perthometer (micrometer scale). direction of the instrument placement on the sample These techniques differ in probe sizes and consequently must be chosen carefully. Because of the rolling process in their capability in resolving surface defects, with used, the plates as well as the samples show a significant AFM being the one having the highest resolution straight texture. In the present study the measurements (nanometer scale). were always performed perpendicular to the texture lines. For all surface texture measurements, the same 2.5.1. Atomic force microscopy tracing length of 5.6 mm with a cutoff of 0.80 mm was The most commonly used roughness parameter, chosen. The radius of the diamond tip of the used mean roughness Ra, was measured by AFM (Binning, perthometer is 5 lm and the stylus force was for all Quate, & Gerber, 1986) for the different steel surfaces. measurements 0.8 mN. The profile resolution is equal to Ra is defined as the mean value of the surface relative to 12 nm. the center plane and is calculated by Z Z 2.6. Topographic techniques 1 Ly Lx Ra ¼ jf ðx; yÞjdxdy LxLy 0 0 2.6.1. AFM and SEM where f ðx; yÞ is the surface relative to the center plane The AFM (Nanoscope III) and SEM (NORAN- and Lx and Ly are the dimensions of the surface. VOYAGER and JEOL 6300) were also used to image The surfaces 316 2R and 2B had the dimensions 1 · 1 the topography of the unmodified and modified stainless cm2 and 0.6 and 1 mm thickness, respectively. The steel surfaces. equipment was operated in contact mode in air. At least five readings were taken for each surface tested. The 2.7. Contact angle measurement techniques Nanoscope III AFM was equipped with an ultrasharp silicon cantilever (MikroMasch). The cantilever chip The contact angles of the different surfaces were includes two triangular springs. The one used for most measured by dynamic and static methods. The dynamic of the surfaces has a length of 290 lm, a width of 40 lm, measurements were performed with the Wilhelmy plate a resonant frequency of 15 kHz and a force constant of technique and the static measurements by using the 0.12 N/m. The reflective side is coated with aluminium. sessile drop technique. In the Wilhelmy plate method the The tip has a radius of curvature less than 10 nm and a measured thermodynamic property is the wetting ten- height between 15 and 20 lm. The force applied to the sion, s, which is related to the contact angle, H,by cantilever was kept constant at 6 nN (50 nm cantilever s ¼ cl cos H, where cl is the liquid surface tension. In the deflection). sessile drop technique the contact angle is directly obtained. The total interfacial surface tension between a TOT 2.5.2. Stylus-type instruments solid s and a liquid l ðcsl Þ can be expressed in terms of 3 3 TOT TOT 2.5.2.1. Dektak ST profiler. The Dektak ST (VEECO, the total surface tension of the solid ðcsv Þ, liquid ðclv Þ Santa Barbara, USA) profiler was used to characterize and the contact angle by the use of Young’s equation, TOT TOT TOT the surface roughness of steel samples. Measurements csl ¼ csv clv cos H. It should be noted that the are made electromechanically by moving the sample (by phases must be in mutual equilibrium, and thus the solid moving the stage on which the sample is placed) beneath surface must be in equilibrium with the saturation vapor TOT a diamond-tipped stylus, which is coupled with a dif- pressure of the liquid. The value of csv therefore con- ferential transformer. The radius of the diamond stylus tains the pressure of the already adsorbed film of the is 2.5 lm and the stylus force was selected at 0.3 mN liquid on the surface (Adamson & Gast, 1997). In the (30 mgf) for most surfaces. This is the force or effective case of low energy materials, this pressure has been weight of the stylus suitable for hard surfaces, whereas suggested to be negligible. On the contrary, for high for soft surfaces this force should be significantly lower. energy surfaces like metals or oxides, the spreading Measurements were also carried out with a stylus force pressure is important. In these cases, the use of the of 10 mgf, without noticing any roughness increase. The solid–liquid–vapor method can lead to errors in the Ra values reported are the mean value of at least five calculation of surface tensions. The solid–liquid–liquid different traces of a scanning distance of 400 mm. The method offers an alternative way to evaluate the surface measurements were made with a scanning direction tension of high energy surfaces. The contact angle of the perpendicular to the texture lines. liquid drop is measured in a hydrocarbon medium. Also, in this case the liquids must be in equilibrium with each 2.5.2.2. Perthometer. A stylus instrument Perthometer other. Several semi-empirical models to extend the rk5 (MAHR, Germany) was also used to measure the Young’s equation have been proposed (Adamson & surface roughness of the different samples. Although the Gast, 1997). Two different approaches, the van Oss method is simple to apply, the roughness is registered approach (van Oss, 1994) in which the total surface only along one direction in the surface plane. Hence the tension is expressed as the sum of an apolar Lifshitz–van O. Santos et al. / Journal of Food Engineering 64 (2004) 63–79 69 der Waals component ðcLWÞ and an acid–base polar Table 4 component ðcABÞ, and Owens and Wendt approach Surface tension values (in mN/m), for the liquids used in the different (Owens & Wendt, 1969) that considers the total surface models d TOT LW d þ AB p tension as the sum of a dispersive component ðc Þ and a cl cl =cl cl cl cl =cl non-dispersive, acid–base or polar component ðcpÞ, were Water 72.8 21.8 25.5 25.5 51 used in this work. In both approaches, the solid–liquid– Formamide 58 39 2.28 39.6 19 vapor method was employed in which the contact angle a-Bro- 44.4 44.4 0 0 0 monaphtalene of the liquid drop is measured under a vapor medium. In Diiodomethane 50.8 50.8 0 the Owens and Wendt approach for the solid–liquid– liquid contact angle, the adsorption of the vapor phase on the solid surface is controlled by replacing this phase Table 5 by a neutral liquid (commonly an alkane, a) with only Surface tension values (in mN/m), of the alkanes used in the Owens dispersive contribution. and Wendt model for the solid–liquid–liquid method TOT TOT The relevant equations used in each approach and ca cla methods are summarized in Table 3. Heptane 20.6 51.6 The surface tension data of the liquids used are Decane 23.4 51.0 shown in Tables 4 and 5. Hexadecane 26.7 51.2

2.7.1. Wilhelmy plate method In the Wilhelmy plate method, the sample is held by 2.7.2. Sessile drop method an electrobalance and is then immersed and retracted, at 2.7.2.1. Solid–liquid–vapor method––approach of van a constant speed, into and out of the liquid contained in Oss. In this approach, as mentioned above, the surface a beaker. During these cycles the force acting on the tension is expressed as the sum of an apolar Lifshitz–van plate vs. depth of immersion are recorded. The meniscus der Waals component ðcLWÞ and an acid–base polar formed at the solid–liquid interface is characterized by component ðcABÞ. The Lifshitz–van der Waals interac- the contact angle. With this technique, two contact an- tions arise due to three distinct interactions: induction gles are measured. As the surface moves down, ad- (Debye), orientation (Keesom) and dispersion (Lon- vancing into the liquid, an advancing contact angle is don), the last one being the most signiﬁcant term (van obtained, and as the surface moves up, receding from Oss, 1994). The acid base component ðcABÞ consists of the liquid, a receding contact angle is obtained. The two non-additive parameters, one for the electron donor hysteresis is related to the roughness and heterogeneity ðcÞ and one for the electron acceptor ðcþÞ contribution of the surface. Neglecting the weight of the plate and the (van Oss, 1994). viscous force, the contact angle is obtained from the equation presented in Table 3, where the wetting ten- cAB ¼ 2ðcþcÞ1=2 ð1Þ sion, s, vs. immersion depth, x, is obtained from F ðxÞ=P, where F ðxÞ is the total force measured on the sample and Combining the Young’s equation with the total in- P is the sample’s perimeter. terfacial tension equation (Table 3), a relation between The sample surfaces 316 2R and 2B had the dimen- the measured contact angle and the solid and liquid sions of 3 · 1cm2 and thickness of 0.6 and 1 mm, re- surface tension terms can be obtained: h i spectively, and were suspended from the microbalance TOT LW LW 1=2 þ 1=2 þ 1=2 of a DST 9005 Dynamic Surface Tensiometer (Nima ð1 þ cosHÞcl ¼ 2 cs cl þ cs cl þ cs cl Technology, Coventry, UK). The liquid used was Milli- ð2Þ Q water. The plate was moved at a constant speed of 2 mm/min and three immersion–retraction cycles at a The contact angle values for water, formamide and a- temperature of approximately 20 C were performed. bromonaphtalene (a-BR) on the diﬀerent surfaces were

Table 3 Summary of the equations used for contact angle and surface tension evaluation, according to the different methodsa Technique Method Approach Equation Wilhelmy plate s ¼ c cos H l hi 1=2 1=2 1=2 Sessile drop Solid–liquid–vapor Van Oss cTOT ¼ cTOT þ cTOT 2 cLWcLW 2 cþc þ ccþ sl s l s l s l s l 1=2 1=2 Owens and Wendt cTOT ¼ cTOT þ cTOT 2 cdcd 2 cpcp sl s l s l s l 1=2 1=2 Solid–liquid–liquid Owens and Wendt cTOT ¼ cTOT þ cTOT 2 cdcd 2 cpcp sl s l s l s l TOT TOT TOT d d 1=2 p p 1=2 csa ¼ cs þ ca 2 cs ca 2 cs ca a See the text for details. 70 O. Santos et al. / Journal of Food Engineering 64 (2004) 63–79 measured in a contact angle meter (DataPhysics OCA15 handling. Both 2B and 2R finish surfaces are therefore Plus) using an image analysing system. contaminated, as determined from the carbon signal from XPS analyses. These analyses could detect the 2.7.2.2. Solid–liquid–vapor method––approach of Owens presence of carbon down to an apparent depth of about and Wendt. In the Owens and Wendt model, the polar 2.1 nm for both surface finishes. Here it is noted that the contribution is expressed differently and the equivalent roughness of these surfaces are 30 and 70 nm for 2R and of Eq. (2) becomes 2B finish respectively (see Section 3.2). Therefore it is 1=2 cTOTð1 þ cos HÞ 2ðcdÞ1=2 ¼ðcpÞ1=2 cp=cd þðcdÞ1=2 difficult to determine the extent of the contamination. l l s l l s The alkaline detergent cleaning leads to a decrease in ð3Þ both the iron/chromium ratio and the passive film and For contact angle measurements using several refer- the apparent contamination film thickness (for instance, TOT d 1=2 ence liquids, a plot of cl ð1 þ cos HÞ =2ðcl Þ against the apparent thickness of the contamination film on the p d 1=2 cl =cl according to this model should give a straight 316 2B sample is 2.1 nm before cleaning and 1.4 nm p 1=2 d 1=2 after cleaning). line with slope ðcs Þ and intercept ðcs Þ . X-ray microanalyses were performed on both un- 2.7.2.3. Solid–liquid–liquid method––approach of Owens modified and modified samples and the EDS spectra and Wendt. In this method, the total interfacial surface obtained are presented in Figs. 3 and 4. The layer tension between the solid and the alkane ðaÞ must also analysed by this technique is about 1 lm deep, which be considered. Combining the equations in Table 3 with allows some conclusions to be made about the thickness Young’s equation, and assuming that the polar contri- of the modified layer. For thick coatings (of the order of bution to the surface tension of the alkane is negligible, 1 lm and thicker) only the elemental composition of the p ðca ¼ 0Þ gives coating layers was measured (Figs. 3(d, g and h) and cTOT cTOT þ cTOT cos H 4(d, e and f)). The elemental composition of 316L l a h la i stainless steel as well as the surface layer composition is d 1=2 d 1=2 TOT 1=2 p p 1=2 ¼ 2ðcs Þ ðcl Þ ðca Þ þ 2 cs cl ð4Þ apparent in the spectra for thin coated or ion implanted surfaces (Figs. 3(a–f) and 4(a–c)). For contact angle measurements using several n-alk- TOT TOT TOT The presence of argon was observed on the spectrum of anes, a plot of cl ca þ cla cos HðyÞ against d 1=2 TOT 1=2 the 2R DLC–Si–O–plasmaCVD surface (Fig. 3(h)), ap- ðcl Þ ðca Þ ðxÞ gives according to this model a d 1=2 parently due to the presence of this gas in the coating straight line, y ¼ ax þ b, with slope a ¼ 2ðcs Þ and in- 1=2 process, although its content was rather high (Table 7). tercept b ¼ 2 cpcp . s l The analysis on both DLC sputtered surfaces, 2R and 2B, Consequently, cd ¼ a2=4, cp ¼ b2=4cp and cTOT ¼ cd þ s s l s s revealed only the presence of titanium (Figs. 3(g) and 4(f); cp can be calculated. s Table 7). This can be related to the sampling depth of the technique (about 1–2 lm) and the composition of this 3. Results and discussion modification, which consists of several layers (total thickness of 2 lm), the first one being titanium. The molybde- 2þ 3.1. Surface chemical composition num content of the 2R and 2B MoS2 surfaces was similar to the content of the unmodified surface. This can be ex- The results of XPS analyses of the unmodified sur- plained by the low concentration of molybdenum used in faces are reported in Table 6, and show that the passive the modification process together with the low resolution film on the 2B stainless steel surface is thicker and of the technique making its measurement difficult. contains more chromium than the passive film on the 2R finish. This can be caused by the pickling operation on 3.2. Surface roughness the 2B surface. It should be born in mind that a pure metal/metaloxide surface is very susceptible to contam- The average roughness for the different modified ination from the atmospheric air during processing and surfaces was measured by AFM and by the stylus in-

Table 6 Characteristics of the passive and contamination films, before and after the cleaning procedure, on 316 2B and 2R finishes, as determined by XPS Samples Passive film Contamination film Thickness (nm) Fe/Cr Hydroxide/oxide Apparent thickness (nm) 316 2B 3.2 0.7 1.2 2.1 316 2B + cleaning 2.8 0.1 1.3 1.4 316 2R 2.4 1.1 1.2 2.1 316 2R + cleaning 1.6 0.6 0.55 1.4 O. Santos et al. / Journal of Food Engineering 64 (2004) 63–79 71

Fig. 3. EDS spectra of modified and unmodified 2R surfaces: (a) unmodified; (b) SiF ion implanted; (c) MoS2 ion implanted; (d) Ni–P–PTFE coated; (e) SiOx-plasmaCVD; (f) silica coated; (g) DLC sputtered; and (h) DLC–Si–O-plasmaCVD. struments, Dektak3ST and Perthometer. The results are with a higher spring constant (2 N/m) was therefore used presented in Table 8. As these methods operate on dif- for these surfaces. No such problem occurred with the ferent length scales, it is interesting to note that the Ra Dektak3ST profiler, where the dimension of the stylus is values give similar trends although they are quantita- about 100 times larger than the AFM tip. tively different. The Ra values (in the nanometer scale) Modified and unmodified 316 2R surfaces have a very obtained from AFM, having the highest resolution, are similar Ra, approximately 30 nm, when measured by also the lowest. Due to strong adhesive forces, arising AFM and Dektak3ST profiler, or 40 nm when using the from van der Waals attractive force between the silicon stylus instrument Perthometer. The exception is the tip and surface, measurements of surfaces modified with Ni–P–PTFE coating, which gave a significantly rougher silicon ions posed some problems. A smaller cantilever surface. The modified 316 2B surfaces exhibit a higher 72 O. Santos et al. / Journal of Food Engineering 64 (2004) 63–79

Fig. 4. EDS spectra of modified and unmodified 2B surfaces: (a) unmodified; (b) SiF ion implanted; (c) MoS2 ion implanted; (d) Ni–P–PTFE coated; (e) SiOx-plasmaCVD; (f) silica coated; and (g) DLC sputtered.

roughness as compared to the unmodified one. The ex- modified surfaces, 2B has a roughness of one order of ceptions are the silica surface, which had a significantly magnitude higher than the corresponding 2R surfaces. 2þ lower roughness, and the MoS2 surface, with a similar This is consistent with the pickling treatment that the 2B roughness. surface had been exposed to, which is expected to give a All investigated surface modifications prepared on 2B rougher surface. finish stainless steel samples were rougher than the ones The Ra values obtained by AFM and Dektak3ST produced on the 2R finish. Depending on the technique profiler for the 2B surfaces (Table 8) show that the used, the 2B unmodified surface has a roughness 2–4 modification increase the roughness of the surface 316 times higher than the 2R unmodified surface. For the 2B by a factor of 3, while 2R is unaffected in this respect. O. Santos et al. / Journal of Food Engineering 64 (2004) 63–79 73

Table 7 Chemical composition of the unmodified and modified steel surfaces 316 2R and 2B, as determined by energy dispersive spectroscopy Samples Elements (wt.%) Fe Cr Ni Mo Si Ti P Ar Others 2R Unmodified 67.4 ± 0.7 18.4 ± 0.3 11.2 ± 0.3 2.4 ± 0.2 0.6 ± 0.1 000C SiF ion implanted 68.2 ± 0.6 17.6 ± 0.3 10.8 ± 0.3 2.2 ± 0.2 1.2 ± 0.2 000C,F

MoS2 implanted 67.5 ± 0.6 18.1 ± 0.2 11.2 ± 0.2 2.5 ± 0.3 0.7 ± 0.1 000C,S Ni–P–PTFE coated 0 0 90.5 ± 0.5 0 0 0 9.5 ± 0.5 0 C, F

SiOx-plasmaCVD 46.6 ± 0.4 13.2 ± 0.4 7.3 ± 0.3 1.5 ± 0.2 31.4 ± 0.6 000O Silica coated 69.3 ± 0.7 15.7 ± 0.3 11.5 ± 0.5 2.4 ± 0.2 1.1 ± 0.2 000C,O DLC sputtered 0 0 0 0 0 100 0 0 C DLC–Si–O-plasmaCVD 54.9 ± 0.4 16.4 ± 0.2 8.8 ± 0.3 2.2 ± 0.2 14.9 ± 0.2 0 0 2.8 ± 0.1 2B Unmodiﬁed 69.7 ± 0.6 16 ± 0.5 11.5 ± 0.2 2.4 ± 0.1 0.4 ± 0.05 000C SiF ion implanted 68.3 ± 0.7 17.5 ± 0.3 10.7 ± 0.4 2.1 ± 0.2 1.4 ± 0.2 000C,F

MoS2 implanted 68.5 ± 0.3 17.5 ± 0.3 11.0 ± 0.2 2.4 ± 0.2 0.6 ± 0.1 000C,O,S Ni–P–PTFE coated 0 0 90.9 ± 0.7 0 0 0 9.1 ± 0.5 0 C, F

SiOx-plasmaCVD 47.1 ± 0.3 13.1 ± 0.3 7.5 ± 0.2 1.8 ± 0.1 30.6 ± 0.3 0 0 0 Silica coated 69.2 ± 0.6 15.6 ± 0.3 11.2 ± 0.5 2.3 ± 0.2 1.7 ± 0.3 000C,O DLC sputtered 0 0 0 0 0 100 0 0 C

Table 8 the 2B as for the 2R stainless steel surface treatment. Ra values (in nm) of the unmodified and modified steel surfaces 316 2R DLC sputtering and autocatalytic Ni–P–PTFE coating and 2B, as determined by AFM and by the stylus instruments, Dektak3 ST profiler and Perthometer led to the roughest. This can be explained by the modification procedure. The DLC sputter coating requires a 3 Samples AFM Dektak ST Perthometer multilayer adhesive film, which can cause an increase of 2R the roughness. For the Ni–P–PTFE coating the surface Unmodified 30 ± 2 30 ± 5 40 roughness is influenced almost only by the process pa- SiF implanted 24 ± 3 30 ± 5 70 rameters of the autocatalytic baths. MoS2 implanted 25 ± 3 45 ± 5 50 Ni–P–PTFE coated 57 ± 4 75 ± 25 200 SiOx-plasmaCVD 23 ± 3 58 ± 23 50 3.3. Topography Silica-sol gel 35 ± 18 30 ± 5 – DLC sputtered 30 ± 1 45 ± 10 90 DLC-plasmaCVD 28 ± 5 – 50 The unmodified 2R sample exhibit a unidirectional DLC–Si–O- 27 ± 2 26 ± 4 – surface topography in which some holes (defaults) are plasmaCVD observed, while the 2B sample reveals grain boundaries SiF Fz implanted 26 ± 4 – – that appear as valleys or cracks (Figs. 5–8(a)). This 2B different topography results from the different produc- Unmodified 67 ± 9 88 ± 13 160 tion method. 2R stainless steel type is produced in a SiF implanted 247 ± 18 225 ± 25 210 non-oxidizing annealing atmosphere, which gives a MoS2 implanted 75 ± 4 175 ± 25 160 Ni–P-PTFE coated 150 ± 8 230 ± 50 220 bright surface with a more homogeneous appearance.

SiOx-plasmaCVD 206 ± 48 200 ± 20 150 On the other hand, the 2B type is produced in an an- Silica-sol gel 37 ± 6 75 ± 5 – nealing oxidizing atmosphere, which gives thick oxide DLC sputtered 267 ± 23 450 ± 100 480 layer. This layer is removed in the pickling operation, which attacks in the grain boundaries and in turn gives a heterogeneous grainy superficial structure. The Ra values obtained for the unmodified and modified The DLC-plasmaCVD; DLC–Si–O-plasmaCVD and þ 2þ surfaces with the stylus instrument Perthometer were SiF3 , FZ and MoS2 implantation techniques did not similar, except for the 2R Ni–P–PTFE coated and 2B affect the steel topography for either surface finish. On DLC sputtered surfaces, which were rougher. the other hand, the Ni–P–PTFE coating masked all Film thickness measurements were also performed by surface characteristics of the steel support (Figs. 5–7 and profilometry on silica coated microscope glass slides, 8(c)). þ prepared under identical conditions as the steel samples The images obtained for the 2R SiF3 , SiOx-plasma- coated with the silica sol–gel technique. CVD and DLC sputtered surfaces (Figs. 5(b, d, f) and In general the silica sol–gel coating technique seemed 7(d)), revealed a more heterogeneous topography to give similar roughness and type of modifications for with some cracks present along the surface, as well as 74 O. Santos et al. / Journal of Food Engineering 64 (2004) 63–79

Fig. 5. AFM height images and proﬁles of the surfaces 316 2R: (a) unmodiﬁed; (b) SiF ion implantation; (c) Ni–P–PTFE coated; (d) SiOx- plasmaCVD; (e) silica coated; and (f) DLC sputtered.

particles with a diameter larger than 100 nm. The pro- than 500 nm (Fig. 6(d, f)). The SEM images acquired for files obtained for the silica coated surface (Figs. 5 and the 2B surfaces (Fig. 8) reveal that with the exception of 6(e)), indicate a much smoother surface than the un- the Ni–P–PTFE coating, all other modifications left the treated surface. This is in accordance with the roughness topography unchanged, although the grain boundaries values obtained in Section 3.2. The topography of 2B seemed to be more pronounced. SiOx-plasmaCVD and DLC sputtered surfaces, as im- In conclusion, the only modification that significantly aged by AFM, was different compared to the unmodi- changes the morphology of the steel surface is the Ni–P– fied surface and featured particles with a diameter larger PTFE coating. O. Santos et al. / Journal of Food Engineering 64 (2004) 63–79 75

Fig. 6. AFM height images and proﬁles of the surfaces 316 2B: (a) unmodiﬁed; (b) SiF ion implanted; (c) Ni–P–PTFE coated; (d) SiOx-plasmaCVD; (e) silica coated; and (f) DLC sputtered.

þ 3.4. Wettability SiOx plasmaCVD < SiF3 DLC-plasmaCVD < Silica-sol gel DLC sputtered 3.4.1. Contact angle The contact angle values obtained with the Wilhelmy DLC–Si–O-plasmaCVD plate and sessile drop techniques are presented in Tables < Ni–P–PTFE 9 and 10, respectively. In general, the hydrophobicity of 2þ the diﬀerent modiﬁed surfaces increases according to The water contact angle of the MoS2 ion implanted their water contact angle in the order: surface was dependent on the techniques used. When 76 O. Santos et al. / Journal of Food Engineering 64 (2004) 63–79

Fig. 7. SEM images of some modified and unmodified 316 2R samples: (a) unmodified; (b) SiF ion implanted; (c) Ni–P–PTFE coated; (d) SiOx- plasmaCVD; and (e) silica coated.

Fig. 8. SEM images of some modified and unmodified 316 2B samples: (a) unmodified; (b) SiF ion implanted; (c) Ni–P–PTFE coated; (d) SiOx- plasmaCVD; and (e) silica coated. O. Santos et al. / Journal of Food Engineering 64 (2004) 63–79 77

Table 9 Water contact angle (advancing and receding) and hysteresis values (in degrees), for the different steel samples as measured by the Wilhelmy plate technique Modifications 316 2R 316 2B Adv. Rec. Hyst. Adv. Rec. Hyst. Unmodified 69 ± 1 14 ± 1 55 99 ± 2 21 ± 7 78 SiF implantation 61 ± 10 22 ± 5 39 79 ± 6 12 ± 1 67 DLC sputtering 84 ± 3 19 ± 5 65 78 ± 3 0 78 DLC-plasmaCVD 65 ± 1 17 ± 1 48 – – – Ni–P–PTFE coating 107 ± 5 31 ± 1 76 104 ± 4 34 ± 2 70

SiOx-plasmaCVD 42 ± 3 13 ± 1 29 45 ± 5 13 32 MoS2 implantation 49 ± 9 12 ± 5 37 46 ± 9 6 ± 1 40

Table 10 unmodiﬁed surface (Table 10). For the 2B based sur- Contact angle values (in degrees), for the diﬀerent steel samples as faces, all techniques, except the Ni–P–PTFE coating, measured by the sessile drop technique resulted in a decrease of the water contact angle. For the

Samples HH2O HFormamide Ha-BrNa Ni–P–PTFE coated surfaces both techniques gave simi- 2R lar water contact angle values, but for the other modi- Unmodiﬁed 24 ± 4 20 ± 5 13 ± 3 ﬁed surfaces, the values obtained with the sessile drop SiOx-plasmaCVD 37 ± 3 25 ± 8 22 ± 3 2þ technique (Table 10) are higher (or lower for the MoS2 SiF implanted 31 ± 7 25 ± 6 14 ± 5 surface) than the ones obtained with the Wilhelmy plate DLC sputtered 67 ± 4 50 ± 2 9 ± 3 Ni–P–PTFE coated 114 ± 1 92 ± 2 61 ± 1 technique (Table 9). Silica-sol gel 65 ± 6 49 ± 2 39 ± 1 In conclusion, it can be stated that, independent of

MoS2 Implanted 65 ± 6 53 ± 4 37 ± 1 the steel substrate, the most hydrophilic and hydro- DLC–Si–O-plasmaCVD 70 ± 1 44 ± 3 16 ± 1 phobic surfaces were, respectively, the SiOx-plasmaCVD SiF FZ implanted 42 ± 4 31 ± 2 29 ± 2 and Ni–P–PTFE coated. 2B Unmodified 83 ± 1 74 ± 1 15 ± 2 3.4.2. Contact angle hysteresis SiOx plasma-CVD 15 ± 3 12 ± 1 14 ± 2 SiF implanted 49 ± 4 39 ± 2 21 ± 4 As mentioned in Section 2.4.1, the Wilhelmy plate DLC sputtered 59 ± 4 33 ± 5 12 ± 1 technique measured both advancing and receding con- Ni–P-PTFE coated 111 ± 3 89 ± 7 69 ± 2 tact angle. Chemical heterogeneity and roughness of the Silica-sol gel 61 ± 1 44 ± 3 37 ± 1 surfaces are believed to contribute to the contact angle MoS implanted 64 ± 3 50 ± 3 18 ± 2 2 hysteresis (Adamson & Gast, 1997). Heterogeneity may arise from impurities concentrated at the surface, from crystal imperfections, or from differences in the prop- measured with the Wilhelmy plate its water contact erties of different crystals faces. Almost all of the modi- angle was similar to the one obtained for the SiO - x fications led to a lower hysteresis (Table 9). The higher plasmaCVD surface, and when using the sessile drop it values found for the 2R DLC sputtered and Ni–P– was similar to the one obtained for the silica surface. PTFE samples can be related to the more heterogeneous It is noteworthy that the same modification technique topography observed on the DLC surfaces (Section 3.3) produces different surface characteristics depending on and the increase in roughness on the Ni–P–PTFE sam- the type of stainless steel used (Tables 9 and 10), where ples (Section 3.2) compared to the untreated surface. the unmodified 2R finish surface is more hydrophilic than the unmodified 2B finish surface. The fact that the same modification technique gives rise to different sur- 3.4.3. Surface energy face properties depending on the base substrate was al- The total surface energy value as well as the separa- ready observed in terms of surface roughness (Section tion in corresponding non-polar and polar contributions 3.2). For this reason, to analyse the effect of the modi- as determined by the van Oss and Owens approaches are fication treatments, the samples were distinguished ac- presented in Tables 11 and 12. cording to their stainless steel type substrate. For the 2R The apolar component ðcLWÞ has approximately the based surfaces, all modification techniques, except DLC same value for all samples, except for the Ni–P–PTFE sputtering and Ni–P–PTFE coating, gave a decrease in coated, being around 40 mN/m for the 2R based sam- the advancing water contact angle values (Table 9). This ples and around 41 mN/m for the 2B (Table 11). The is in contrast with what was determined by the sessile Owens method for solid–liquid–vapor interface also drop technique where all modified 2R surfaces exhibited gives constant but lower values for the apolar contri- a higher water contact angle value compared to the bution of about 30 mN/m, with the exception of the 78 O. Santos et al. / Journal of Food Engineering 64 (2004) 63–79 Table 11 most apolar surface, independent of the steel type, was Surface tension values (in mN/m) for the different steel samples ac- the Ni–P–PTFE coated sample. cording to the approach of Van Oss Samples cLW cþ c cTOT s s s s 4. Conclusions 2R Unmodified 43.1 ± 0.7 0.6 ± 0.2 48.2 ± 4.2 53.3 ± 2.4 The surface properties of different modified surfaces SiOx-plasmaCVD 41.3 ± 1.3 0.9 ± 0.4 37.2 ± 1.6 52.3 ± 3.6 SiF implanted 43.3 ± 0.7 0.5 ± 0.2 44.6 ± 4.9 52.7 ± 2.6 were assessed in terms of chemical composition, DLC sputtered 43.9 ± 0.3 0.1 ± 0.0 14.3 ± 4.9 45.4 ± 0.7 roughness, topography and wettability. Analyses of the Ni–P-PTFE 24.6 ± 0.4 0.3 ± 0.2 0.1 ± 0.2 25.0 ± 0.6 chemical composition on surfaces where the modified coated layer had a thickness higher than 1 lm (see Table 2) Silica-sol gel 34.9 ± 1.3 0.7 ± 0.3 15.3 ± 3.4 41.6 ± 1.0 revealed mainly the chemical elements of the coating. MoS2 implanted 35.9 ± 0.5 0.2 ± 0.2 18.4 ± 5.0 39.8 ± 1.7 DLC–Si–O-plasma- 42.7 ± 0.2 0.6 ± 0.3 8.1 ± 0.8 47.0 ± 0.7 For surfaces with a thinner coating and the ion im- CVD planted surfaces, the implanted ions as well as the SiF FZ implanted 38.8 ± 1.0 1.0 ± 0.7 33.8 ± 0.6 50.1 ± 3.5 stainless steel composition could be determined. 2B The modification techniques affected the roughness of Unmodified 42.8 ± 0.4 2.0 ± 0.2 11.5 ± 1.0 52.5 ± 0.3 the rougher 2B surfaces more than the 2R surfaces, SiOx-plasmaCVD 43.0 ± 0.3 0.8 ± 0.1 52.1 ± 1.6 55.6 ± 0.1 which were largely unaffected in this respect. The silica SiF implanted 41.5 ± 1.1 0.3 ± 0.1 29.6 ± 3.9 47.0 ± 0.8 coating, using the sol–gel technique, gave surfaces with DLC sputtered 43.4 ± 0.2 1.1 ± 0.5 14.5 ± 3.4 51.0 ± 1.9 Ni–P-PTFE coated 20.5 ± 0.6 0.0 ± 0.0 0.2 ± 0.1 20.6 ± 0.6 similar roughness independently of the steel substrate. Silica-sol gel 35.8 ± 0.7 0.9 ± 0.1 17.7 ± 1.7 44.0 ± 1.4 The DLC sputtering and Ni–P–PTFE coating produced

MoS2 implanted 42.1 ± 0.5 0.2 ± 0.2 17.9 ± 2.0 45.1 ± 2.1 the highest increase in surface roughness. All the modified surfaces presented have similar topography as the unmodified samples, with the exception Table 12 Surface tension values (in mN/m) for some 2R modified surfaces, of the surface coated with Ni–P–PTFE where the to- calculated by the approach of Owens and Wendt with both solid– pography of the unmodified steel was no longer visible. liquid–vapor (s-l-v) and solid–liquid–liquid (s-l-l) methods In general, the surface modification produced more d p TOT hydrophilic surfaces, with the exception of the Ni–P– Samples cs cs cs PTFE, which gave hydrophobic surfaces. The SiO - s-l-v s-l-l s-l-v s-l-l s-l-v s-l-l x plasmaCVD coating produced the most hydrophilic Ni–P-PTFE coated 15 13 2 0 17 13 surface. DLC sputtered 33 64 8 5 41 69 SiO -plasmaCVD 31 38 34 26 65 64 The great potential of some of the surface modifica- x þ SiF implanted 27 18 26 36 53 54 tion (SiF3 implanted and DLC sputtering) studied in this work has been shown in previous studies (Bornhorst et al., 1999; Muller-Steinhagen€ & Zhao, 1997; Muller-€ Ni–P–PTFE surface. The same holds for the electron Steinhagen et al., 1997; Santos et al., 2003) which re- þ acceptor component ðc Þ, which is in all cases close to vealed that surface modification can lead to reduction in: zero. The biggest differences among samples were found (1) CaSO scale formation during pool boiling, (2) bac- 4 in the electron donor component ðc Þ. The values for teria attachment and (3) protein adsorption. Presently we the c component vary between 0.1 (Ni–P–PTFE coat- are investigating the milk protein and mineral fouling on ing) and 48.2 mN/m (unmodified surface) for the 2R all the modified surfaces presented in this paper. based samples and between 0.2 (Ni–P–PTFE coating) and 52.1 mN/m (SiOx-CVD coating) for the 2B samples. TOT Acknowledgements Similar total surface tension values, csl , were obtained by the Van Oss and Owens and Wendt approaches for the 2R samples. The authors would like to deeply acknowledge Dr. Regarding both methods used, solid–liquid–vapor Hans Visser, the initiator and co-ordinator of this work and solid–liquid–liquid, the calculated surface tension within the MODSTEEL project. by the Owens and Wendt approach (Table 12) were Financial support was obtained from the European quite similar, except for the DLC coating. The reason Community under the ‘‘Competitive and Sustainable might be that the spreading pressure is high. DLC and Growth’’ Programme (MODSTEEL, Contract No. Ni–P–PTFE coatings are nearly apolar, unlike the sur- G5RD-CT-1999-00066, Project No. GRD1-1999-10856). þ faces coated with SiOx and implanted with SiF3 , which are very polar, like glass. References In conclusion, all techniques reduce the apolar com- LW ponent ðc Þ, increase the c component for the 2R Adamson, A. W., & Gast, A. P. (1997). Physical chemistry of surfaces. samples and decrease this parameter for the 2B. The New York: John Willey and Sons. O. Santos et al. / Journal of Food Engineering 64 (2004) 63–79 79

Addesso, A., & Lund, D. B. (1997). Influence of solid surface energy Muller-Steinhagen,€ H., & Zhao, Q. (1997). Investigation of low fouling on protein adsorption. Journal of Food Processing and Preservation, surface alloys made by ion implantation technology. Chemical 21, 319–333. Engineering Science, 52(19), 3321–3332. Baier, R. E., & Meyer, A. E. (1992). Surface analysis of fouling- Muller-Steinhagen,€ H., Zhao, Q., & Reiss, M. (1997). A novel low resistant marine coatings. Biofouling, 6, 165–180. fouling metal heat transfer surface. In Proceedings of the 5th UK Binning, G., Quate, C. F., & Gerber, C. (1986). Atomic force National Heat Transfer Conference. microscopy. Physical Review Letters, 56, 930–933. Nasser, K. (2000). Galvanotechnik. Hanser Munchen.€ Bornhorst, A., Steinhagen, H. M., & Zhao, Q. (1999). Reduction of Norde, W., Arai, T., & Shirahama, H. (1991). Protein adsorption in scale formation under pool boiling conditions by ion implantation model systems. Biofouling, 4, 37–51. and magnetron sputtering on heat transfer surfaces. Heat Transfer Owens, D. K., & Wendt, R. C. (1969). Estimation of the surface free en- Engineering, 20(2), 6–14. ergy of polymers. Journal of Applied Polymer Science, 13, 1741–1747. Boyd, R. D., Verran, J., Jones, M. V., & Bhakoo, M. (2002). Use of Santos, O., Nylander, T., Paulsson, M., & Trag€ ardh, C. (2003). atomic force microscope to determine the effect of substratum Adsorption behaviour of b-Lactoglobulin on modified stainless surface topography on bacterial adhesion. Langmuir, 18, 2343– steel surfaces. In Proceedings of the heat exchanger fouling, 2346. fundamental approaches and technical solutions conference, Britten, M., Gree, M. L., Boulet, M., & Paquin, P. (1988). Deposit Davos, Switzerland. formation on heated surfaces––effect of interface energetics. van Oss, C. J. (1994). Interfacial forces in aqueous media. New York: Journal of Dairy Research, 55(4), 551–562. Marcel Dekker. Burton, H. (1968). Reviews of the progress of dairy science. Section G. Wahlgren, M., & Arnebrant, T. (1990). Adsorption of b-Lactoglobulin Deposits from whole milk in heat treatment plant––a review and onto silica, methylated silica and polysulfone. Journal of Colloid discussion. Journal of Dairy Research, 35, 317–330. and Interface Science, 136(1), 259–265. Haynes, A., & Norde, W. (1994). Globular protein at solid/liquid Yoon, J., & Lund, D. B. (1994). Magnetic treatment of milk and interfaces. Colloids and Surfaces B: Biointerfaces, 2(6), 517–566. surface treatment of plate heat exchangers: effects on milk fouling. Janocha, A., Hegemann, D., Oehr, C., Brunner, H., Rupp, F., & Geis- Journal of Food Science, 59(5), 964–980. Gerstorfer, J. (2001). Adsorption of protein on plasma-polysilox- Zettler, H., & Muller-Steinhagen,€ H. (2000). Confidential Report for ane layers of different surface energies. Surface and Coatings BP Exploration. Technology, 142–144, 1051–1055. Zhao, Q., & Muller-Steinhagen,€ H. (2003). Intermolecular and Krisdhasima, V., McGuire, J., & Sproull, R. (1992). Surface hydro- adhesion forces of deposits on modified heat transfer surfaces. In phobic influences on b-Lactoglobulin adsorption kinetics. Journal Proceedings of the heat exchanger fouling, fundamental ap- of Colloid and Interface Science, 154(2), 337–350. proaches and technical solutions conference, Davos, Switzerland.