Effects of a DBD plasma discharge on bond strength

Surfaces and Interfaces 18 (2020) 100461

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Effects of a DBD plasma discharge on bond strength a,⁎

Thomas Schuman (PhD) , Rory A. Wolf a b

b

T

Tetra Pak Packaging Solutions AB, SE-221 86, Lund, Sweden Pillar Technologies, Hartland, WI 53029, USA

ARTICLE INFO

ABSTRACT

Keywords: Corona Plasma DBD Surface energy Surface chemistry Adhesion

DBD plasma treatment of polymers for adhesion properties in a printing and lamination application is explored here. The ink-substrate adhesion is dependent on the plasma dosage (watt density). At too low treatment level, lower oxidation and inferior wetting behaviour, caused a delamination failure between the water-based ink and the polymeric substrate surface. The adhesion properties, measured in a manual peel test, were clearly improved at higher plasma dosage as a consequence of the greater wetting and oxidation. There was a clear relationship between treatment level (surface chemistry and surface energy) and adhesion properties discerned, which suggests that it is the Debye interaction forces between the polar functional groups that are contributing to the bond strength between the ink and the plasma-treated substrate.

1. Introduction Corona treatment is extensively used in the industry to modify surfaces [1–7], e.g. to improve the adhesion between two components [8]. Corona discharge treatment of a surface involves the application of a high-energy electromagnetic field that, in air, produces highly energetic species (e.g. oxygen radicals, ozone, ions, excited species and electrons) which change and affect the surface characteristics [9]. These radicals lead to the formation of the oxygen-based functional groups (e.g. hydroxyl, ether, ketone, carboxylic and ester groups) on the substrate surface. At atmospheric pressure, the voltage required for the initiation of the discharge in plasma treatment is lower than for the corona technology and this is claimed to result in a more uniform treatment without the formation of streamers or electrical filaments [10]. The frequency of the electromagnetic field used in corona and plasma discharge technologies are typically in the range of 10–40 kHz where vibration of atoms and gas particles between the electrode and ground roller result in ionization, dissociation and excitation. The plasma treatment modifies only the outermost atomic layers [11–13] of treated substrates. One advantage of plasma is that it may be possible to change the surface composition by altering the gas composition and therefore selectively produce the desired functionality. The purpose of corona and plasma techniques for printing applications is to improve both wettability and the printability of the substrate that in turn may enhance the adhesion properties [11–14]. The substrate (e.g. polymer films, aluminium foils, paper or paper

⁎

laminates, and fabrics) is conveyed through the gap between the electrode and ground roller and bombarded with high-speed electrons. When electrons with higher energy than the binding energy of the molecules bombard the substrate surface, the molecular bonds break, and a modification event occurs [15]. For some polymeric materials, low-molecular weight oxidized materials (LMWOM) can form at the surface and, if not bound to the substrate, may cause delamination problems after printing or lamination. Strobel et al. [16] and Żenkiewicz [17] have reported on the presence of LMWOM for corona treated polypropylene (PP) and bi-oriented polypropylene (BOPP). However, for BOPP subjected to flame treatment, no formation of LMWOM was observed despite both surface treatment technologies causing an oxidization event on the BOPP surface [16]. An activation energy of a corona discharge in the range of 0.5–2 kJ/m2 corresponding to a corona dosage of 8-33 Wmin/m2 (watt density) was found to significantly improve wettability and raise surface energy. Higher activation energy/corona dosage causes structural changes due to ablation of the amorphous phase where a granular structure was formed [17]. The corona discharge results in a low-temperature plasma consisting of ionized air comprising an equal number of electrons and ions, and neutral atoms and particles of oxygen and nitrogen, but also photons of electromagnetic radiation [18]. The mechanism of plasma treatment is a very complex process, accounting for the plasma nature and plasma-substrate surface interaction. The combined surface modification effect is typically surface cleaning, activation, and cross-linking [19,20].

Corresponding author. E-mail address: [email protected] (T. Schuman).

https://doi.org/10.1016/j.surfin.2020.100461 Received 8 May 2019; Received in revised form 21 January 2020; Accepted 29 January 2020 Available online 01 February 2020 2468-0230/ © 2020 Elsevier B.V. All rights reserved.

Surfaces and Interfaces 18 (2020) 100461

T. Schuman and R.A. Wolf

Fig. 1. Contact angle (distilled water) as a function of plasma dosage.

The choice of gas and settings of the plasma treatment parameters can also imply topographical changes due to an etching/abrasion event that, in turn, can impact wettability. The typical reactive gases used in plasma treatment are oxygen and nitrogen. They provide the similar consequences of increasing surface energy, but the substrate being treated also plays a role [21]. Mixtures of nitrogen gases into the discharge have been attempted, to substitute air, where grafting amino and amido groups to the surface were observed [5]. The application of plasmas which have very low ion density and high density of neutral-reactive particles can functionalize substrate surfaces with polar groups, without modification of bulk properties. The flux of neutral atoms suitable for functionalization of a variety of polymers may exceed 1024 m−3, which implies that a smooth polymer surface will become saturated with polar functional groups [22]. Plasma chemistry can also be used to synthesize gaseous compounds at atmospheric pressure. As in the case of solid substrates, collision processes between molecules and highly energetic electrons cause radicals to be created. By controlling the electron energies, by varying energies, it is possible to specifically promote certain reactions and to suppress others [23]. Application of atmospheric plasma to finished films has been theorized and practiced in order to provide specific functionality to the base film substrate adequate for improved adhesion [24]. Since atmospheric plasma contains highly reactive species within the high-density plasma at atmospheric pressure, it is proven to significantly increase surface area and to create polar groups on the surface of polymers so that strong covalent bonding between the substrate and its interface (i.e. inks, coatings, and adhesives) takes place [25]. For example, an argon/oxygen-containing plasma treatment causes the breakage of surface bonds, leading to the formation of the carbon radicals and crosslinking effects. The surface cross-linking can compete with the oxidation process, influencing the number of covalently bond groups introduced by plasma treatment [26]. In the present work, the focus has been on the influence of an

atmospheric DBD plasma discharge in nitrogen atmosphere on the surface, ink adhesion (ink-substrate adhesion), and adhesion properties between the substrate and a low-density polyethylene (LDPE) coating. The substrate comprises a polymer-film laminated to a paperboard where the plasma treatment was conducted on the polymer film side. The plasma dosage was in the same order of magnitude, and higher, as reported by Żenkiewicz [17]. 2. Experimental 2.1. Materials The polymer film used in this work is based on bi-oriented polypropylene (BOPP) supplied by Polyplex, India. The film thickness of the BOPP is 18 µm. The polymer film was laminated to a paperboard and referred to as “lami material” prior to the printing and lamination processes. The web width of the lami material was 1574 mm. The paperboard grade and supplier are not disclosed. The printed side of the lami material was then extrusion-coated with a low-density polyethylene (LDPE) polymer. 2.2. Plasma treatment The plasma treatment tests were conducted in an atmospheric DBDplasma treater in nitrogen atmosphere, Protean1™ from Pillar Technologies, Hartland, WI, USA. The plasma treater, comprising ceramic electrodes and a ceramic ground roller, was operated at a frequency of 16 kHz and at dosages ranging from 0–33 Wmin/m2. At the highest dosage, the voltage and current on the power supply were 360 V and 65 A, respectively. The gap between the electrodes and the substrate was constant at 1.5 mm. The web speed was 10 m/s in the plasma trial. The dosage (D), in unit Wmin/m2, describes the energy put into one m2 according to Eq. 1, where P is the power output (W), v is the web speed (m/min), and L is the electrode length (m). The dosage is 2

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T. Schuman and R.A. Wolf

Table 1 Surface energy components of plasma-treated BOPP, flame-treated BOPP, and untreated BOPP. Variants

θA(○)

θB(○)

Plasma-treated BOPP Flame-treated BOPP Untreated BOPP

66.4 87.0 98.0

46.9 51.4 56.3

P S

10.5 2.5 0.6

d s

36.0 33.5 30.7

γS

xp

46.3 36.0 31.3

0.22 0.07 0.02

A=Distilled water B=Methylene iodide

2.3. Flame treatment Flame treatment was conducted on one variant prior to printing to compare to the plasma treatment technology. A triple slot burner was used at a burner gap of 32 mm. Natural gas having a heat value of 37 kJ/l was used in the test. The air/gas ratio was 10.5:1. The flame dosage (specific energy) was 16 kJ/m2 at a web speed of 6.7 m/s. 2.4. Flexographic printing Flexographic printing was performed in a printing press, comprising seven print units, supplied by Tresu, Denmark. The printing speed was conducted at 6.7 m/s for the flame-treated variant whereas 10 m/s was used for the plasma-treated variants. The surface treatment operations were performed in the printing press with subsequent printing and drying. The drying settings were kept constant. The process settings of the printing are not disclosed. The print design and the water-based inks (Siegwerk, France) were the same in the tests. Cyan, White, Yellow and Dark Blue colours (100 % full-tone areas) were employed. 2.5. Extrusion coating The extrusion coating operation was performed in a laminator line supplied by Davis-Standard LLC, US, at a web speed of 10 m/s. The coat weight of low-density polyethylene (LDPE) was 12 g/m2 in all tests. The extrusion coating process settings are not disclosed. 2.6. Methods All the coated paperboard specimens were conditioned for 24 hours at 23 °C and 50 % relative humidity before any measurements were made. The surface energy of the polymer surfaces, before and after plasma treatment, was evaluated from contact angle measurements at room temperature. The contact angle (θ) between a drop of liquid and the surface of the film was measured as a function of time using a Mobile Surface Analyzer, contact angle tester, Krüss GmbH, Germany. The surface energy was assessed by the Owens, Wendt, Rabel, and Kaelble (OWRK) method referring to the geometric-mean equation, Eq. 2,[27]:

(1 + cos )

s

+

p p s l ,

(2)

=

d s

+

p s .

(3)

Distilled water and methylene iodide were used as the test liquids and their surface tension components in the software for computing the surface energy were γl=72.8 mN/m, γld=21.8 mN/m, and γlp=51.0 mN/m (distilled water), and γl=50.8 mN/m, and γld=50.8 mN/m, and γlp=0.0 mN/m (methylene iodide) [28]. The polarity, xp, was obtained as the ratio of the polar component to the

often denominated energy density, power density or, most commonly, watt density in the industry.

P v· L

d d s l

where γl is the surface tension of the test liquid and γs the surface energy of the substrate. The superscripts d and p denote the dispersive and polar (non-dispersive) contributions to the surface energy, respectively. The total surface energy is given by Eq. 3

Fig. 2. Contact angle (distilled water) of a) Untreated BOPP, b) Flame-treated BOPP, and c) Plasma-treated BOPP at 33 Wmin/m2.

D=

l = 2

(1) 3

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T. Schuman and R.A. Wolf

Fig. 3. ESCA survey spectra of a) Untreated BOPP and b) Plasma-treated BOPP at 33 Wmin/m2.

total surface energy [29]. The portable contact angle device PGX+ from Fibro System, Sweden, was used to assess the wettability (distilled water) directly after the surface treatment trials. The drop volume was 4.0 µl and a contact time of 0.1 s was used for the contact angle. The average value from five measurements was used for the contact angle. Electron spectroscopy for chemical analysis (ESCA) was used to assess the chemical composition of the surface of the coatings. The instrument employed was a PHI 5000 VersaProbe III, Perkin Elmer, USA, and operated with a monochromated aluminium (Al Kα) X-ray source (photon energy of 0-1486.6 eV). The take-off angle was 45°. The

instrument was calibrated against gold (84.0 eV). Infrared spectroscopy (FTIR-ATR technique) comprising a Germanium crystal was used to characterize functional groups. The Nicolet 6700 instrument from Thermo Scientific was used at the following settings: 32 scan accumulations for each sample and a spectral resolution of 4 cm−1. The surface structure was characterized with an AFM instrument. The instrument employed was Nanowizard II from JPK Instruments. The cantilever PPP-NCHR-20, silicon-based probe, from Nanosensors was used. The scan rate was 0.9 Hz. Surface topographies of the samples 4

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instrument) at a jaw speed of 50 mm/min and a sample width of 15 mm. From the peel force measurements, the adhesion is calculated and given in N/m. Sample preparations and procedures of the test method are not disclosed. The adhesion results of all variants are relatively compared. The Flame S-UV-Vis-ES spectrometer from Ocean Optics, Sweden, was used to monitor the excited species in the plasma discharge. The spectral range and spectral resolution were 200–870 nm and 1.4 nm, respectively. The spectrometer was calibrated at the supplier prior to the analysis. An optical fibre (2 m Premium Fibre UV/VIS 400 µm core diameter from Ocean Optics, Sweden) was connected to the spectrometer. The optical fibre end for emission measurements was directed perpendicular into the discharge between the electrode and the ground roller. The distance between the optical fibre end and the discharge was 150 mm. The software Ocean View, Ocean Optics, Sweden, was used to record the emission spectra at an integration time of 100 ms.

Fig. 4. O/C ratio as a function of plasma dosage.

3. Results and discussion

Table 2 Surface composition of untreated and treated BOPP. Treatment

Dosage [Wmin/m²]

C1 [%]

C2 [%]

C3 [%]

C4 [%]

Untreated Plasma Plasma Plasma Plasma Flame

0 6.5 12.5 25 33 267*

100 100 100 98.6 97.4 97.8

0 0 0 1.7 2.7 2.2

0 0 0 0 0 0

0 0 0 0 0 0

⁎

3.1. Contact angle The contact angle measurements, distilled water, in unprinted areas was performed directly after printing and the results are shown in Fig. 1. Even at the lowest plasma dosage attempted, the contact angle is significantly reduced compared to the untreated sample. At higher plasma dosages, some further improvement in the wetting behaviour is noted. The images from the contact angle device for untreated, plasmatreated and flame-treated BOPP are shown in Fig. 2. The plasma-treated BOPP exhibits greater wettability than the flame-treated variant.

Converted from 16 kJ/m²

before and after treatment were obtained in tapping mode with Nanowizard control software. The roughness parameters Ra and Rq being the arithmetical mean roughness and root-mean squared roughness (RMS), respectively, were obtained by employing Gwyddion software 2.55. The ink adhesion after printing is visually determined by an internal peel test method. The adhesive tape was supplied by Tesa, UK, material code 04122-0000500 having a tape width of 19 mm. The tape is applied on the substrate and pressed to avoid air entrapments. The tape is manually pulled from the substrate in a zig-zag manner. If the ink is not adhering to the substrate, ink delamination is visible on the substrate and then also present on the adhesive tape. This failure mode implies non-approved ink adhesion. The adhesion between the PE-layer and the substrate is measured by employing an internal peel test method using a tensile tester (Instron

3.2. Surface energy The greater wetting by the plasma treatment implies higher surface energy. The surface energy and polarity results are given in Table 1 for the plasma-treated variant at highest dosage attempted. The flametreated BOPP and the untreated BOPP have lower surface energy. The surface energy measurements were conducted four weeks after the surface treatment and there is a decay in the contact angle (distilled water) for the treated variants. It is known that the surface energy when subjected to flame, corona or plasma treatment can decline with time [5,30–32]. The surface energy level and the polar component reported here are in the same order of magnitude as those for BOPP subjected to corona discharge at similar dosage but at lower web speed (7.5 m/s), cf. [33].

Fig. 5. FTIR-ATR spectra for: a) Flame-treated BOPP, b) Plasma-treated BOPP (33 Wmin/m²), and c) Untreated BOPP. 5

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3.3. Electron spectroscopy for chemical analysis (ESCA)

stretching, CH3 asymmetric rocking and C-H wagging vibrations, while the peak at 997 cm−1 is due to CH3 asymmetric rocking vibrations. The peak at 974 cm−1 can be attributed to CH3 asymmetric rocking and C-C asymmetric stretching vibrations, while the peak at 901 cm−1 is due to CH3 asymmetric rocking and C-C asymmetric and symmetric stretching vibrations. The peaks at 844 and 810 cm−1 are due to CH2 rocking vibrations [37]. FTIR-ATR was used by Sawtell et al. [38] to assess the crystallinity of the BOPP, with or without subjection to plasma treatment, by employing the ratio of the rocking vibrations (997/ 974 cm−1). The higher the ratio, the higher the crystallinity. Furthermore, it was shown that longer treatment times increased the crystallinity of the surface layer of the BOPP [38]. In this work, there was not a significant difference in the ratio which can be explained by the significantly shorter residence time here during the surface modification event. The residence time considered in the plasma trial here is about 40 ms compared to 5–300 s, cf. [38]. Fig. 6 shows the spectra of the two surface-treated BOPP variants when subtracting the untreated BOPP spectra.

The chemical composition of the surfaces of the coatings was characterized with ESCA. The carbon(1s)-peak was separated into four components, C1-C4, assigned to different types of carbon-oxygen bonds in the usual manner [34]. Introduction of carbon-nitrogen functionalities such as C^N, N-C-O or N-C=O to the surface by means of plasma treatment has been reported elsewhere, cf. [35]. No such peaks in the spectra were detected because of no trace of nitrogen (N1s) ~400 eV was observed. Fig. 3 shows the survey for untreated and plasma-treated BOPP where carbonyl groups at the surface are present for the latter. Furthermore, the oxygen to carbon ratio (O/C), which relates to the surface oxidation, increase with increasing dosage, cf. Fig. 4. Moreover, at the higher dosages the C2-component, referring to hydroxyl groups, increased. Table 2 presents the surface composition. The incorporation of hydroxyl groups onto the surface (increased polarity), observed here for both flame and plasma treatment technologies, may explain the greater wetting and increased surface energy results. Hydroxyl radicals are highly reactive and have high oxidation potential [36]. 3.4. Fourier transform infrared spectroscopy-attenuated total reflection (FTIR-ATR)

3.5. Atomic force microscopy (AFM) The surface topography was somewhat rougher when subjecting the BOPP surface to either plasma or flame treatment technologies which is in-line with plasma treatment results on BOPP by Mirabedinia et al. [39]. However, the exposure time in the discharge is significantly shorter here than those (30–180 s) reported by Mirabedinia et al. [39]. The observed roughening effect was attributed to an etching event, as a consequence of the bombardment of energetic particles in the discharge, where aggregates at the surface are formed. Fig. 7 shows the AFM images for treated and untreated BOPP. Table 3 presents the Ra and Rq results. Moreover, the surface roughness of the BOPP may also be smoother when subjected to a plasma discharge or flame plasma [40,41].

The spectra shown in Fig. 5 do not exhibit any significant differences between the untreated and treated variants. Since the depth of analysis of the present ATR technique is larger than the depth of the surface treatment (a few tens of nm), the infrared signal coming from the modified chemical groups at the surface is small. The peaks present, however, are attributed to the aliphatic C-H stretching modes of the BOPP at 2950–2800 cm−1. Peaks near 1460 cm−1 and 1378 cm−1 are the CH2 and the CH3 deformation bands, respectively. Stretching modes for the C-O group are visible between 1000–1400 cm−1. Hydroxyl groups (OH), however, are not observed in the 3200–3600 cm−1 region. The peak at 1167 cm−1 can be attributed to C-C asymmetric

Fig. 6. Subtracted FTIR-ATR spectra for: a) Plasma-treated BOPP (33 Wmin/m²), and b) Flame-treated BOPP.

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T. Schuman and R.A. Wolf

Fig. 7. AFM graphs for: a) Flame-treated BOPP, b) Plasma-treated BOPP (33 Wmin/m², and c) Untreated BOPP.

3.6. Optical emission spectroscopy (OES)

Table 3 Surface roughness parameters Ra and Rq of untreated and treated BOPP. Treatment

Ra [nm]

Rq [nm]

Flame-treated BOPP Plasma-treated BOPP Untreated BOPP

2.5 1.7 1.4

3.1 2.5 1.7

Fig. 8 shows the OES spectra for the plasma discharge. There are many nitrogen transitions in the ultraviolet (UV) regime detected for the DBD plasma discharge fed with nitrogen. The spectrum in the 200–450 nm wavelength region comprises the second positive system (SPS) of molecular nitrogen and first negative system (FNS) [42,43]. The reactive oxygen and nitrogen species (RONS) present are much less of oxygen type but more dominantly nitrogen based. Hydroxyl radicals were observed in the spectra and the presence of these reactive species in the discharge may react with the substrate surface.

Fig. 8. OES spectra for the plasma discharge at a plasma dosage of 33 Wmin/m2. 7

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T. Schuman and R.A. Wolf

3.7. Peel test after printing

3.8. Peel test after lamination

The higher the plasma dosage, the higher the adhesion properties between ink and the treated film surface. The adhesion properties, in some printed areas comprising Yellow and Dark Blue colours, are shown in Fig. 9. It is evident that the untreated BOPP film surface does not have sufficiently high surface energy (surface chemistry) for the water-based ink molecules to adhere to the substrate. At higher plasma dosage, the ink adhesion is improved and attributed to more polar groups (hydroxyl groups) being incorporated at the substrate surface.

The peel force recorded in a tensile test was used as a measure for the adhesion properties in printed areas comprising Cyan (process colour), Yellow (spot colour) and Dark Blue (spot colour). In the manual tape test after printing, cf. Fig. 9, the highest plasma dosage used in the test provided the best ink-film adhesion performance and the adhesion levels after the lamination process for this variant were evaluated. The adhesion levels, between the LDPE and the different printed areas, were sufficiently high with regards to our internal approval level. The

Fig. 9. Manual adhesion tape test after printing (peel test) at various plasma dosages: a) 0 Wmin/m², b) 6 Wmin/m², c) 12.5 Wmin/m² and d) 33 Wmin/m².

Fig. 10. Adhesion data (peel test using tensile tester) in printed areas (Cyan, Yellow and Dark blue) for plasma-treated BOPP (33 Wmin/m2) and flame-treated BOPP.

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delamination in the peel test occurred between the LDPE coating layer and the ink layer meaning that the bond strength between ink-substrate is higher than the LDPE-ink and thus indicating on good surface treatment level obtained by the plasma discharge. The adhesion properties of the plasma-treated variant subjected to a plasma dosage of 33 Wmin/m2 is shown in Fig. 10. Plasma treatment provided similar adhesion properties as the flame-treated variant but there is an advantage in the production speed during the printing operation employing the plasma technology.

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4. Conclusions The surface energy of the substrate is of considerable importance for the wettability and adhesion to the water-based ink. Furthermore, the higher the plasma dosage, the greater the wetting and the oxidation level in the range tested improved the adhesion properties. The incorporation of hydroxyl groups at the polymer surface observed with the ESCA-technique explains the greater wettability and increased surface energy. The oxidation level on the surface was more affected by the plasma dosage than the wetting properties. However, the adhesion properties were significantly improved at higher plasma dosages and this is linked to the introduction of more polar functional groups onto the substrate surface when subjected to the plasma discharge. At the highest plasma dosage, the adhesion both after printing and lamination was sufficiently high to reflect a satisfactory bond. Subjecting the polymer surface to a plasma discharge or flame treatment led to a small increase in the surface roughness that may also contribute to improvements in the ink adhesion properties. Despite managing a higher production rate with plasma treatment, lower operational costs and carbon footprint can be obtained compared to the flame treatment technology used in this work. Acknowledgements The authors thank Eric Tam, at the Department of Materials and Manufacturing Technology at Chalmers University of Technology, Gothenburg, Sweden, for conducting the ESCA-measurements. Estephania Lira Salazar, at the Department of Physics, Faculty of Engineering, Lund University, Lund, Sweden, is thanked for performing the AFM-measurements. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.surfin.2020.100461. References [1] A.J.F. de Carvalho, A.A.S. Curvelo, J.A.M. Agnelli, Carbohyd. Polym. 45 (2001) 189. [2] S. Bourbigot, D.L. Vanderhart, J.W. Gilman, W.H. Awad, R.D. Davis, A.B. Morgan,

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