s 16 (2007) 630–636www.elsevier.com/locate/diamond

Diamond & Related Material

Development of new PVC materials. Characterization and feasibilityof diamond coatings on model PVC materials

H.-K. Koponen a, R. Kuisma b, J. Kasanen a, H.-R. Kymäläinen b, E. Pesonen-Leinonen b,M. Hautala b, M. Suvanto a, T.A. Pakkanen a,⁎, T.T. Pakkanen a, A.-M. Sjöberg b

a Department of Chemistry, University of Joensuu, PO Box 111, FI-80101 Joensuu, Finlandb Department of Agrotechnology, University of Helsinki, PO Box 28, FI-00014 University of Helsinki, Finland

Received 15 March 2006; received in revised form 23 September 2006; accepted 13 November 2006Available online 28 December 2006

Abstract

Diamond-like, tetrahedrally bonded amorphous carbon (ta-C) coating was used to enhance the wear resistance of laboratory-mademicrostructured poly(vinyl chloride) (PVC) surfaces. Formulation of PVC samples was kept simple; they contained only stabilizer and plasticizeradded to PVC. Two sizes of microstructures were imprinted on PVC. Both smooth and microstructured PVC surfaces were coated with amorphousdiamond and the surfaces were characterized by spectroscopic ellipsometry, AFM and contact profilometry. In addition, water contact angles weremeasured. The plasticizer, plasticizer concentration and amorphous carbon coating affected the topography of the microstructure and the watercontact angles on surfaces. The ta-C-coated microstructured surfaces were worn with a soiling and wearing drum tester to study their wearingbehavior. Surface topographies were measured with a profilometer after wearing studies.© 2006 Elsevier B.V. All rights reserved.

Keywords: Plasticized PVC; Wear resistance; Microstructure; Ta-C

1. Introduction

Poly(vinyl chloride) (PVC) is a commonly used material dueto its versatility. Plasticized PVC is a soft material with poorwear and soil resistance. In order to enhance their wear and soilresistance commercial PVC products, especially flooringmaterials are often coated with other materials. UV curingcoatings have good stain, scratch and abrasion resistance [1]. Inaddition, they can be used to photostabilize PVC surfaces. Otherpossible coatings to enhance stain and wear resistance of PVCinclude unplasticized PVC, polyurethanes, acrylics, acrylicblends and polyesters [2,3].

Wear is defined as damage to a solid surface, generallyinvolving progressive loss of material, due to relative motionbetween the surface and a contacting substance or substances[4]. There are several types of equipment and examination

⁎ Corresponding author.E-mail address: tapani.pakkanen@joensuu.fi (T.A. Pakkanen).

0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.diamond.2006.11.053

methods for wear [5–9]. One of those is the soiling and wearingdrum tester, which was found to be useful in the examination ofwearing of soft plastic surfaces [10].

Diamond is commonly known as the hardest material innature. Furthermore, it is a chemically inert and biocompatiblematerial with good wear and corrosion resistance. Amorphous,diamond-like carbon contains a high fraction of diamondbonding (sp3) and a very small amount of hydrogen bonded tothe carbon network. The Young's modulus and microhardnessof amorphous carbon are comparable with those of naturaldiamond and thus it has a great resistance to wear [11].

Diamond-like carbon coatings have been used to enhance thewear resistance of medical applications in severe environmentssuch as artificial hip and knee joints [12] and to improve thereproducibility of arthroscopic evaluation of articular cartilageby controlling the friction between cartilage and the arthro-scopic cartilage stiffness probe [11]. In addition to medicalapplications, amorphous carbon coatings have been shown toincrease the wear resistance of slurry pumps [13] and to enhancethe operation and lifetime of photoconductive switches [14].

Table 1Chemical compositions of the PVC model materials

Code ofthe sample

Composition of the material

Polymer [w%] Plasticizer [wt.%] Stabilizer [wt.%]

Benzo20 S 98 PVC(Dynea)

77 Benzoflex® 2088 20 Therm-Chek®7500L

3Benzo20 25 μmBenzo20 40 μmBenzo30 S 98 PVC

(Dynea)67 Benzoflex® 2088 30 Therm-Chek®

7500L3

Benzo30 25 μmBenzo30 40 μmDOP20 S 98 PVC

(Dynea)77 DOP 20 Therm-Chek®

7500L3

DOP20 25 μmDOP20 40 μmDOP30 S 98 PVC

(Dynea)67 DOP 30 Therm-Chek®

7500L3

DOP30 25 μmDOP30 40 μmHexa20 S 98 PVC

(Dynea)77 Hexamoll® DINCH 20 Therm-Chek®

7500L3

Hexa20 25 μmHexa20 40 μmHexa30 S 98 PVC

(Dynea)67 Hexamoll® DINCH 30 Therm-Chek®

7500L3

Hexa30 25 μmHexa30 40 μm

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The soil resistance and cleaning properties of surfaces can beenhanced bymimicking the surface structures of lotus leaves [15].Water contact angles on surfaces can be increased to almost 160°by applying nano- and micrometer-size structures onto hydro-phobic surfaces. Thus the water drops will roll off the surfaces asspheres, removing contaminating particles as they flow.

PVC surfaces can be patterned with regular microstructuresin order to affect their surface properties, as in the present study.In order to enhance soil resistance and cleaning properties ofPVC the structures must be sufficiently wear resistant tomaintain their properties during the product lifetime. The wearresistance of surface structures on plasticized PVC surfaces hasnot been studied earlier. However, the wear resistance ofplasticized PVC is typically weak and thus it is probable that thewear resistance of microstructures on PVC needs to beenhanced. Because amorphous carbon coatings can be appliedto almost any substrate at low temperatures [14], their use aswear-resistant layers on PVC surfaces is attractive. Kiuru andAlakoski [16] reported the hydrophobic and oleophobicproperties of diamond-like carbon–polymer hybrid surfaces.

In this study the effects of tetrahedrally bonded amorphouscarbon (ta-C) coatings on the wear resistance of plasticized PVCsurface were examined. Both smooth and microstructured PVCsurfaces were coated with commercial amorphous carboncoatings and the properties of coated surfaces were evaluated.The effect of wear on the ta-C-coated microstructured surfaceswas examined.

2. Experimental

2.1. Preparation of PVC samples

PVC (S 98, Dynea) powder was dry-blended with 3 wt.%liquid stabilizer (Therm Chek 7500L) in a Henschel mixer (FM10L, 800726). When temperature had been increased to 60 °C, aplasticizer (20 or 30 wt.%) was added gradually. Three differentplasticizers were used: Benzoflex 2088, Hexamoll DINCH orDOP (Tables 1 and 2). Mixing was continued until thetemperature increased to 80–90 °C, whereafter the blend wasallowed to cool down.

PVC–stabilizer–plasticizer blends were granulated with aDSM Midi 2000 extruder and a grainer. Granulated blends wereinjection molded by the Arburg Allrounder 270S 350–150injection molding system. The injection molded samples werecylindrical discswith a diameter of 2.5 cm and a thickness of 2mm.

PVC samples were microstructured with regular repeatingpatterns using steel meshes (ISO 3310-1 AISI314L, Cemic Ltd.)of 25 and 40 μm. The microstructure was imprinted on PVCusing a hydraulic press (COMPAC EP 40-1). Before imprinting,the mesh was leveled with a pressure of 100 MPa for 5 s. Theleveled mesh was then imprinted on PVC without heating with apressure of 143 MPa for 3 s and the mesh was removed. Themicrostructures were on an area of approximately 100 mm2 inthe centre of the disc.

The chemical compositions of the uncoated plasticized PVCmaterials with different microstructures are presented in Table 1and the chemical composition of PVC additives in Table 2.

PVC samples were coated with commercial ta-C coating byplasma coating method in the laboratory of DIARC-TechnologyInc. Two coatings with different thicknesses were applied onsmooth surfaces and the thicker coating was also applied onmicrostructured surfaces. On microstructured surfaces thecoating was added after imprinting the microstructure onto thePVC surface.

2.2. Testing of wear

Awearing study was carried out for the profiled ta-C-coatedsurfaces using the soiling and wearing drum tester [10,17]. Ninerandomly selected pieces of ta-C-coated PVC plastic modelsurfaces were embedded in the middle of the plastic surfaces(8 cm×23 cm), after which the surfaces were fastened to the wallof the inner circle of the drum (diameter 245 mm and height250 mm) with a tape. A hexapod and 100 g of sand (Belgiumsand, average size 200 μm) were put into the drum. The wearingtime was 40 min. The rotating speed of the drum was 50±2 rpmand it reversed the direction of rotation after every 5 min. Afterthe wearing, ta-C-coated PVC plastic model surfaces wereremoved from the drum and the drum and the hexapod werecleaned for the next wearing test. Loose material and sand wereremoved from ta-C-coated PVC plastic model surfaces accordingto the standard EN 14565 [18] using a household vacuum cleanerwith a soft brush (model Philips TCX 889).

2.3. Characterization of the surfaces

2.3.1. Thickness of the ta-C coatingThe thickness of the ta-C coating was determined by

spectroscopic ellipsometry (J.A. Woollam Co., Inc. VASEHS-190). The wavelength range for measurements was from400 nm to 800 nm and angles of incidence were 65°, 70° and75°. A three layer model (PVC, ta-C, surface roughness) wasapplied to fit the model to experimental data obtained from the

Table 3Water contact angles and their standard errors on smooth plasticized PVCsurfaces, uncoated and with ta-C coatings of different thicknesses

Sample Ta-C coating [nm] Water contact angle [°]

Benzo20 – 79.0±1.037.8±1.3 63.8±0.487.9±1.0 63.5±0.2

Benzo30 – 66.4±0.936.9±1.0 72.2±1.181.3±0.4 72.2±0.6

DOP20 – 81.2±0.838.4±1.3 73.1±0.384.9±1.1 70.6±0.2

DOP30 – 81.8±0.639.2±1.6 74.7±0.589.1±2.4 77.1±0.5

Hexa20 – 81.3±0.540.2±1.7 75.5±0.284.9±2.3 79.6±0.1

Hexa30 – 82.8±0.540.2±1.2 80.5±0.790.9±1.1 85.1±0.3

Table 2Chemical compositions of the additives

Additive Composition Manufacturer

Benzoflex 2088 61–69% diethylene glycol dibenzoate,16–24% dipropylene glycol dibenzoate,11–19% triethylene glycol dibenzoate

Velsicol

DOP Dioctyl phthalate BorealisHexamoll DINCH Di-isononyl-cyclohexane-1,

2-dicarboxylateBASF

Therm-Chek® 7500L Calcium/zinc complex mixture Ferro

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samples. The Cauchy dispersion model was used for polymers,tabulated data for the ta-C layer and effective mediumapproximation for ta-C surface roughness. Tabulated layershave a wavelength-by-wavelength list of optical constantsassociated with them. These values were found in theWVASE32®-program but they were originally acquired fromPalik's Handbook of Optical Constants of Solids [19]. Filmthickness of the ta-C layer for each sample was calculated usingthe WVASE32®-program with tabulated values (Cubic Carbon(Diamond) from Palik: pp. 671–672). For calculations,WVASE32® assumed that the standard tabulated layer ishomogeneous and isotropic, and that the interfaces are perfectand parallel.

2.3.2. TopographyThe topographies of the uncoated and ta-C-coated smooth

surfaces were measured by atomic force microscopy (AFM,Thermo Microscopes EXPLORER 4400-11) in non-contactmode. The topography of microstructured samples with andwithout ta-C coating was studied using a contact stylusprofilometer (Kla-Tencor P-15). Two- and three-dimensionalroughness profiles of surfaces were measured. Three replicatemeasurements were made both for AFM and three-dimensionalprofilometer measurements. For two-dimensional profilometermeasurements five replicate scans were performed. The lengthof the two-dimensional scan was 10000 μm and the area of thethree-dimensional scan was 500 μm×500 μm. Detaileddescriptions of the measurement methods were presented byKuisma et al. [10,17,20].

2.3.3. Water contact anglesStatic water contact angles on uncoated and ta-C-coated

smooth PVC surfaces were measured with a KSV CAM200contact angle meter. A deionized water drop (5 μl) was placedon the surface and imaged for 30 s. Images were taken at 1-second intervals. The contact angles were obtained as anaverage of values recorded between 15 and 29 s of eachmeasurement. For smooth PVC surfaces without ta-C coating16 replicate measurements and for smooth surfaces with ta-Ccoating 4 replicate measurements were made.

For ta-C-coated microstructured PVC samples the staticwater contact angles were measured with the KSV CAM100meter. A water drop was placed on the surface and imaged for30 s. The contact angles were obtained as an average of valuesrecorded between 5 and 25 s. For microstructured ta-C-coatedsurfaces 12 replicate measurements were made.

3. Results and discussion

3.1. Thickness of the ta-C coating

Spectroscopic ellipsometry can be used to determine thethickness and optical properties of diamond-like carbon coat-ings [21–23]. The measured thicknesses of the ta-C coatings onsmooth PVC surfaces are presented in Table 3. It was assumedduring the measurement that the ta-C coating was homogenousand isotropic, and that the interfaces were perfect and parallel.However, the ta-C-coated surfaces had a slightly wavelikeroughness, which is later introduced in the AFM results (Fig. 1).The observed surface roughness may have caused uncertainty inthe thickness values. However, the thickness values of ta-Ccoatings are of the correct order of magnitude.

The thicknesses of the ta-C coatings on the microstructuredPVC surfaces were not determined with the ellipsometerbecause rough microstructures scatter light, thus preventingmeasurements. The ta-C coatings were applied onto themicrostructured surfaces using the same process parameters asthose used for the thicker coatings on the smooth surfaces. It canthus be assumed that the thicknesses of the coatings on themicrostructured surfaces were of the same order of magnitudeas the thicknesses of thicker coatings on the smooth surfaces.

3.2. Surface topography

3.2.1. Smooth surfacesThe structural features of uncoated smooth PVC surfaces

were characterized by AFM, which revealed height differencesin the area of 0.01 mm2. Differences were between 60 and90 nm on Benzo, DOP or Hexa20 surfaces. On Hexa30 surfacethe height difference was 420 nm. Thus the Hexa30 surface wasthe roughest, although according to profilometer measurementsHexa20 and Benzo30 surfaces were rougher than Hexa30. OnHexa20 and Benzo30 surfaces the roughness parameter Ra was

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about 100 nm over an area of 0.25 mm2. Ra varied between 20and 40 nm on Benzo20, Hexa30 and on both DOP surfaces. TheAFM images of smooth Benzo surfaces are presented in Fig. 1aand d.

Profilometry provided a more accurate picture of surfaceroughness than AFM because the area measured in profilometryis greater. The location of a measurement point in AFM imagingaffected the observed results if the surface structure was nothomogeneous at all points.

Ta-C coating resulted in a wavelike structure on PVCsurfaces containing 20% plasticizer. The structure was similarregardless of whether the coating was thin or thick. With thethicker coatings the wavelike structure was larger in area and itsheight differences were more notable than with the thinner

Fig. 1. a) The structure of smooth Benzo20 surface, b) thin and c) thick ta-C coatingcoatings on Benzo30 surface imaged by non-contact AFM.

coatings. The structures of ta-C coatings on smooth Benzosurfaces are shown in Fig. 1b and c. In fact, wavelike structureson amorphous carbon surfaces can be predicted from computersimulation [24]. The roughness of the surface increased whenthe thickness of the ta-C coating increased. The shape of the ta-C structure was similar in Benzo20 and DOP20 surfaces. OnHexa20 surfaces the carbon structure was similar to theaforementioned surfaces but the bulges were more scattered.

The ta-C coatings on surfaces containing 30% plasticizer hada structure similar to those with 20% plasticizer, but the increasein the amount of plasticizer caused the coating to break. Thecracks in DOP surfaces were similar to those of ta-C coatings onBenzo30 surfaces, as shown in Fig. 1e and f. On DOP30surfaces there were clumps at the intersection of two cracks. On

on Benzo20 surface and d) smooth Benzo30 surface and e) thin and f) thick ta-C

Fig. 2. Examples of three-dimensional contact profilometric images and Ra values of PVC surfaces with 25 μm structure before and after ta-C coating and after wearing. a) Benzo20 with 25 μm structure, b) ta-C-coatedBenzo20 with 25 μm structure and c) ta-C-coated Benzo20 with 25 μm structure after wearing. d) DOP20 with 25 μm structure, e) ta-C-coated DOP20 with 25 μm structure, f) ta-C-coated DOP20 with 25 μm structureafter wearing, g) Hexa20 with 25 μm structure, h) ta-C-coated Hexa20 with 25 μm structure and i) ta-C-coated Hexa20 with 25 μm structure after wearing.

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Fig. 3. Three-dimensional contact profilometric roughness Ra values of new and worn ta-C-coated laboratory-made PVC model surfaces. Columns are means of threereplicates, bars in columns are standard deviations of means of the replicates (±SD).

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Hexa30 surfaces the structure of the thin ta-C coating wassimilar to that on the uncoated surface. The thick coating onHexa30 resembled the ta-C structure on Benzo30 and DOP30surfaces but it contained more cracks.

3.2.2. Microstructured surfacesThe regular microstructure of the laboratory-made PVC

model surfaces was clearly seen in the profilometric images(Fig. 2). Most of the microstructured PVC surfaces becamesmoother when the surfaces were coated with amorphouscarbon. The ta-C coating partially filled the microdepressions ofthe structure. The Ra value of the uncoated microstructuredsurfaces varied between 1.1 μm (Benzo30 and DOP30 with25 μm structures) and 6.8 μm (DOP20 with 40 μm structure).The surfaces containing 40 μm structures were rougher thansurfaces containing 25 μm structures. After the ta-C coating wasapplied, the Ra values were diminished by 28–69% dependingon the sample. Ra values of the new ta-C-coated surfaces variedbetween 0.6 μm (Benzo30 with 25 μm structure) and 3.8 μm(DOP20 with 40 μm structure) (Fig. 3). The surfaces with40 μm structures were still slightly rougher than surfaces with25 μm structures but the difference was smaller. On the DOP30surface the ta-C coating did not influence the Ra value.

3.3. Contact angle

Water contact angles on smooth PVC surfaces with andwithout ta-C coatings are presented in Table 3. The watercontact angles varied between 66° and 83° depending on theplasticizer, showing that plasticized PVC is slightly hydrophilic.PVC with Benzoflex 2088 plasticizer was the most hydrophilicmaterial. The water contact angles on DOP and Hexa surfaceswere almost equal.

The water contact angle on ta-C surface has been reported tolie in the range of 75–80° [25]. In the present study theplasticizer of PVC and the thickness of the ta-C coating both

affected the surface structure of the coating, which in turnaffected the contact angle. Water contact angles varied in therange of 64–85° on ta-C-coated PVC surfaces. Thus the ta-C-coated smooth surfaces were all slightly hydrophilic, like theuncoated PVC surfaces.

The microstructure increased the water contact angle onuncoated PVC surfaces, as shown in Fig. 4. The ta-C coating onmicrostructured PVC surface decreased the water contact anglein most cases compared to the uncoated microstructuredsurfaces caused by leveling of the structures. However, ifcompared to contact angles on ta-C-coated smooth surfaces, inmost cases the water contact angles on ta-C-coated micro-structured PVC increased.

3.4. Wearing studies

Wearing caused breakdown of the microstructure on the ta-C-coated PVC surfaces (Fig. 2). All ta-C-coated model surfacesbecame rougher after wearing, except for the DOP20 surfacewith 40 μm structure (Fig. 3), due to the formation of cracks andscratches during wearing. The exceptional behavior of rough-ness parameters on DOP20 surface with 40 μm structure may bedue to the unusually high roughness of the surface beforewearing, causing inaccuracy of comparison of parametersbefore and after wearing. Examples of three-dimensionalprofilometric measurements are presented in Fig. 2 to illustratedifferent types of changes in the ta-C-coated PVC modelsurfaces after wearing. The Ra values of the worn PVC modelsurfaces were higher than those of the new surfaces (Fig. 3). TheRa values of the worn surfaces were between 1.4 μm (DOP30with 25 μm structure) and 4.1 μm (Hexa30 with 25 μmstructure).

Ta-C coatings were created on PVC surfaces in order toimprove their wear resistance. The poor wear resistance of ta-C-coated PVC surfaces was a consequence of the softness ofplasticized PVC surfaces. The breakage of the PVC surface

Fig. 4. Water contact angles and their standard deviations on smooth and microstructured PVC surfaces. Contact angles were measured on surfaces without ta-Ccoating (−), and on surfaces with thick ta-C coating (+).

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under the coating also caused breakage of the thin ta-C coating.This could be prevented if thicker ta-C coatings could beapplied onto PVC, making the whole surface harder. How-ever, the surface structures should not be leveled by the coat-ing. Reducing the amount of plasticizer also leads to hardersurfaces.

4. Conclusions

The coating study of plasticized PVC surfaces withamorphous carbon demonstrated that the polymer surfaces canbe coated with ta-C and that the thickness of the coatings can beregulated. The water contact angle on ta-C-coated PVC surfacedid not differ significantly from that on uncoated PVC surface.Both surfaces were hydrophilic. On microstructured PVCsurface the ta-C coating decreased water contact angles due toleveling of the microstructure.

Plasticized PVC is a soft material and the ta-C coatingsapplied were thin. The coatings were thus not able to preventbreakage of surfaces during wearing tests. Breakage of the PVCsurfaces under the ta-C coating also caused breakage of thecoating.

Acknowledgements

Financial support from the Finnish Funding Agency forTechnology and Innovation (TEKES) programme CleanSurfaces 2002–2006 is gratefully acknowledged. The authorsthank DIARC-Technology Inc. for the ta-C coatings.

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