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This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution
and sharing with colleagues.
Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party
websites are prohibited.
In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information
regarding Elsevier’s archiving and manuscript policies areencouraged to visit:
Adhesive properties of polyester treated by cold plasma in oxygen andnitrogen atmospheres
I. Novák a,⁎, A. Popelka a,b, A.S. Luyt c, M.M. Chehimi d, M. Špírková e, I. Janigová a, A. Kleinová a, P. Stopka f,M. Šlouf e, V. Vanko g, I. Chodák a, M. Valentin a
a Polymer Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 84541 Bratislava 45, Slovakiab Center for Advanced Materials, Qatar University, PO Box 2713, Doha, Qatarc University of the Free State (Qwaqwa Campus), Private Bag X13, Phuthaditjhaba 9866, South Africad Interfaces, Traitements, Organisation et Dynamique des Systèmes (ITODYS), Université Paris Diderot, 15 rue Jean-Antoine de Baïf, 75013 Paris, Francee Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovského nám. 2, 162 06 Praha 6, Czech Republicf Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, Řež, Czech Republicg VIPO, Gen. Svobodu 1069/4, 958 01 Partizánske, Slovakia
a b s t r a c ta r t i c l e i n f o
Article history:Received 9 January 2013Accepted in revised form 25 July 2013Available online 22 August 2013
Polyester foil was treated by the surface diffuse barrier discharge (SDBD) plasma at atmospheric pressure inoxygen and/or in nitrogen containing a small amount of oxygen to improve its surface and adhesive properties.Changes in a chemical structure of the polyester were analyzed by electron spin resonance (ESR) spectroscopy,attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy and by X-ray photoelectronspectroscopy (XPS). The changes in surface morphology were studied using scanning electron microscopy(SEM) and atomic force microscopy (AFM). A surface energy as well as an adhesion of polyester modified bythe SDBD plasma to polyacrylate significantly increased. The efficiency of the modification by the SDBD plasmadepends on the used processing gas as well as on the modification time. The decrease in the surface energy ofpolyester modified by the SDBD plasma due to hydrophobic recovery was also investigated. A correlationbetween adhesive properties of polyester modified by the SDBD plasma and its surface has been found.
Published by Elsevier B.V.
1. Introduction
Polyester foils are frequently used in many industrial applications,e.g. in the automotive industry for the car construction, due to its excel-lent properties. The surface energy of polyester is usually too low forcertain important applications, such as bonding, printing, lamination,etc. Several efficient methods have been used to improve the surfaceand adhesive properties of polymers [1,2]. One of these is based on ap-plication of cold plasma for pre-treatment of polymeric surface [3–9],which is a solvent-less, ecological method of modification, aimedmain-ly to the adjustment of the surface energy and consequently the adhe-sion to other materials. The most important feature consists in themodification of surface properties of the polymer without changingthe intrinsic bulk properties [10–16].
It is well known that the modification of polymers by cold plasmaleads to chemical and physical changes in surface parameters and adhe-sive properties. Depending on the processing gasses used, various func-tional groups are formed in a very thin surface layer of the polymer[17–19]. Polyester with a substantially higher surface energy than poly-olefin usually does not need to be pre-treated by plasma for printing,
because the surface energy in the untreated state is high enough forachieving permanent print [20,21]. However, the surface free energy isinsufficient for obtaining strong adhesive joints. The plasma treatmentof polyethylene terephthalate (PET) foils by surface diffuse barrier dis-charge (SDBD) plasma in air introduces oxygen containing groups onthe polymeric surface [22–25]. The unpurified nitrogen applied as a pro-cessing gas for the surface modification of polymers contains a verysmall amount of oxygen at around 30 ppm, and can affect the degreeof the modification of the plasma treated polymer. The oxygen tracespresent in nitrogen result in a formation of oxygenic functional groupsincreasing the polarity of plasma treated PET [25]. The modification ofpolyester in oxygen and nitrogen by the SDBD plasma was chosen inorder to compare the resulting effect on surface and adhesion propertiesof the plasma treated polymer in these processing gasses.Moreover, theapplication of nitrogen as the processing gas in terms of the modifica-tion of polyester is safer than oxygen also from safety of the process ifapplied in a large industrial scale. As stated above, for the preparationof strong adhesive joints, the modification of polyester by SDBD plasmais essential. This can be performed as a continuous technological pro-cess, which is optimal for plasma treatment of PET foils in an industrialscale. A significant problem with the cold plasma modification of poly-ester is that the effect of the surface modification is diminishing ratherquickly during storage, and surface and adhesive properties of the foil
0257-8972/$ – see front matter. Published by Elsevier B.V.http://dx.doi.org/10.1016/j.surfcoat.2013.07.057
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are deteriorating. Due to the changes during storage the plasma-treatedPET foils should be processed to final stage (printing, adhesion) within7 days. Themodification of PET by the SDBD plasma in oxygen or nitro-gen is scalable to a roll-to-roll processingmethod suitable for industrialproduction.
In this paper the surface and adhesive properties of polyester modi-fied by the SDBD plasma in either oxygen or unpurified nitrogen havebeen investigated using contact angle measurements, peel tests, aswell as ESR, ATR-FTIR, XPS, SEM and AFM analyses. The process of hy-drophobic recovery of modified polyester foil during storage has beeninvestigated and the correlation between adhesive properties of polyes-ter modified by the SDBD plasma and surface energy has been found.
2. Experimental
2.1. Materials
In our experiments polyester/PET foils (Tenolan OAN, Technoplast,Czech Republic) with a thickness of 0.12 mm, (melting enthalpyΔHm = 64.8 J/g, degree of crystallinity 47.2%) were used for the modi-fication by discharge plasma. PET foils were immersed in acetone inorder to remove additives influencing their surface properties. The solu-tion of poly (2-ethylhexyl acrylate) (PEHA) (Polysciences, USA) in ethylacetate was used for the preparation of PET–PEHA adhesive joints. Alayer with a thickness of 0.12 mm was deposited on supported biaxialoriented polypropylene (BOPP) Tatrafan ON (Chemosvit, Slovakia)with the aid of a coating ruler (Dioptra, Czech Republic). The processinggasses of technical purity— oxygen and unpurified nitrogen containing30 ppm of oxygen (SIAD, Slovakia) have been used for the preparationof plasma-treated polymer.
2.2. SDBD set-up
Themodification of the PET foils by the SDBD plasmawas performedon a laboratory plasma equipment at atmospheric pressure in nitrogenor oxygen gasses of technical purity. The nitrogen contained 30 ppm ofoxygen. The SDBD plasma generator consists of electrodes separated byan alumina dielectric plate. The discharge electrodes having an area of80 × 80 mm, and consisting of 1-mm wide and 80-mm long tungstenstrips, are fixed to the upper surface of the alumina. The voltage of theSDBD source was 100 V, current intensity 1 A, and frequency 6 kHz.The power 100 W was used for the modification by SDBD plasma inN2 or O2 at atmospheric pressure. The power applied for thewhole elec-tric circuit of the SDBDplasma source has beenmeasured using a specialdevice Voltcraft Plus Energy Logger 3500 (Voltcraft, Germany).
2.3. Characterization methods
2.3.1. SEMThe morphology of the PET before and after the SDBD plasma
irradiation was investigated by scanning electron microscope JSM-6400 (JEOL, Japan). The foils were sputter-coated (SCD 050, BALTEC)with a Pt layer (4 nm).
2.3.2. AFMThe surface morphology and local surface heterogeneities of the
modified polymer were measured by AFM. All measurements wereperformed under ambient conditions using a commercial atomic forcemicroscope (NanoScope™ Dimension IIIa, MultiMode Digital Instr.,USA) equipped with a PPP-NCLR tapping-mode probe (Nanosensors™Switzerland; spring constant 39 N ∙m−1, resonance frequency ≈160 kHz). The surface properties of all the films were measured in xand y axis sizes between 2 and 25 μm on different positions of thefilms in order to find characteristic and significant surface features.The AFM analysis provides 2D or 3D information on both the height
and material heterogeneity contrast with high resolution when record-ing height and phase shifts simultaneously.
2.3.3. ESRSamples of PET foils were inserted into silica quartz tubes and placed
into the resonator of an ESR Bruker-Biospin, ElexSys E-540 spectrome-ter equipped with a Bruker high-sensitivity cavity ER 4119 HS-W1operating at 9.75 GHz (X-band) and room temperature. The ESR spec-trometer settings were as follows: modulation frequency, 100 kHz;modulation amplitude, 1.0 G; microwave power, 20 mW; receivergain, 60 dB; time constant, 1.28 ms; conversion time, 5.12 ms; andmagnetic field scan, 6 kG (center at 3480 G), number of scans, 30. AnXepr (Bruker) Linux-based package was used for acquisition of data,and the Origin (Origin Lab) software was used for the data analysisand presentation. The ESR measurements of PET were performed atroom temperature.
2.3.4. ATR-FTIRThe ATR-FTIR spectroscopy measurements of the PET foils [26,27]
were performed with a Nicolet Impact 400 FTIR spectrometer (Nicolet,USA) having a resolution of 4 cm−1, a scan range of 4000–400 cm−1,and a total of 1024 scans per analysis. The vertical ATR accessorycontained a KRS-5 (thallium–bromide–iodide) crystal and the angle ofincidence of the infrared beam was 45°.
2.3.5. XPSThe XPS spectra [28–31] were recorded using a VG Scientific
ESCALAB 250 system equipped with a micro-focused, monochromaticAl Kα X-ray source (1486.6 eV) and amagnetic lens to increase the sen-sitivity due to higher electron acceptance angle. An X-ray beam of650 nm size was used at a power of 20 mA × 15 kV. The spectra wereacquired in the constant analyzer energy mode, with pass energies of150 and 20 eV for the survey and narrow regions, respectively. Thecharge compensation was achieved with an electron flood gun coupledto an argon gun. The energy and emission currents of the electronswere4 eV and 0.35 mA, respectively. The argon partial pressure in the cham-ber was set at 2 × 10−6 Pa. Under these conditions, the surface chargewas negative but uniform. The Avantage software, version 2.2, wasused for digital acquisition and data processing. The spectral calibrationwas performed by setting the main C1s peak at 285 eV. The O/C atomicratios were determined by considering the integrated peak areas of C1sand O1s, and their respective Scofield sensitivity factors corrected forthe analyzer transmission function.
2.3.6. Surface energy determinationThe surface energy of PET was determined via measurements of the
contact angles (θ) [32,33] of a set of testing liquids (θ): re-distilledwater, ethylene glycol, formamide, methylene iodide, and 1-bromonaphthalene with a surface energy evaluation (SEE) system (Advex,Czech Republic). The drops of testing liquid (V = 3 μl) were placed onthe polymeric foil surface with a micropipette (Biohit, Finland), andthe dependence θ = f (t) was extrapolated to t = 0. The surface energyof the polymer as well as its polar and dispersive components wereevaluated by the Owens–Wendt–Rable–Kaelble (OWRK)methodmod-ified by a least squares method [34,35]:
1þ cosθð ÞγLV
2¼ γd
LVγds
� �1=2 þ γpLVγ
ps
� �1=2 ð1Þ
γLV ¼ γLVp þ γLV
d ð2Þ
γs ¼ γsp þ γs
d ð3Þ
where θ = contact angle (deg), γLV = surface free energy (SFE) of thetesting liquid (mJ ∙m−2), γLV
d , γLVp = dispersive component (DC), and
polar component (PC) of the SFE of the testing liquid (mJ ∙m−2), and
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γsd, γs
p = DC and PC of the SFE of the polymer (mJ ∙m−2). The values ofthe surface energies of the testing liquids at 23 °C used for the OWRKdetermination of the polymer surface energy are shown in Table 1.
2.3.7. Peel strength of the adhesive jointThe layer of poly (ethylhexyl acrylate) solution in ethyl acetate hav-
ing a thickness of 0.12 mm was deposited on supported BOPP using acoating ruler (Dioptra, Czech Republic) and dried in an oven at 60 °C.The adhesive joints of PET–poly (ethylhexyl acrylate) were preparedbymeans of the rubber roller after pressing of biaxial oriented polypro-pylene with the adhesive layer to the PET surface. The peel strength ofthe adhesive joint (Ppeel) of the plasma-treated polyester foil topolyacrylate was determined by peel tests consisting in the peeling ofthe adhesive joint at a 90° angle using a universal testing machine,Instron 4301 (Instron, England). The adhesive joints were fixed in thealuminum peeling circle jaws. The width and length of the adhesivejoints were 20 mm and 140 mm, respectively. The speed of 90° peelingof the adhesive jointwas 1.5 mm∙min−1. Themechanicalwork of adhe-sive joints by peeling (Am (J ∙m−2)) was calculated by the equation:
Am ¼ Fs=b ð4Þ
where Fs is the medium peeling force (N), and b is the width of theadhesive joint (m) [36,37].
3. Results and discussion
The changes in the surface topography of the polyester modified bylow-temperature plasma were investigated by SEM [38,39]. The SEMmicrophotographs for the original PET film (Fig. 1A) and the film afterthe SDBD plasma irradiation in N2 (Fig. 1B), and in O2 (Fig. 1C) arepresented in Fig. 1. The surface of the pristine PET foil is relativelysmooth, but some irregularities are seen in the microphotographformed during the foil production (Fig. 1A). Numerous small blisterscan be seen on the polymeric surfaces modified by the SDBD plasmain N2 (Fig. 1B) or O2 (Fig. 1C).
The changes in the surface roughness of the plasmamodified polymerwere investigated by AFM [40–44]. A first plasma-induced process is sur-face etching of PET, evidenced by AFM analysis. The removal of thepolymericmaterial towards gas-phase reasonably occurs through the for-mation of lowmolecular weight compounds, formed as by-products dur-ing oxidation of the polymer [44]. After plasma-treatment the crystallineregionswill remain intact, while the amorphous regions are etched away,thus leading to a rougher surface of PET [23]. The AFM results of the PETsurface topography and local heterogeneities (high and phase images)after the SDBD plasma modification are shown in Fig. 2B and C. The sur-face properties of each foil were measured at three different places, butno extensive differences were found between these places. The highand phase images of the unmodified polymer film (Fig. 2A) show no sig-nificant differences in the high and phase profiles (local heterogeneities)for pristine PET, except for localized melting marks on the surface [44].The high and phase images of pristine PET are rather rough, but withoutobservable inhomogeneities. The Ra value of pristine PET foil (Fig. 2A)representing roughness was 10.2 nm. Plasma treatment resulted in anappearance of very fine heterogeneous regions, and the polymer surfacebecame more irregular. The local heterogeneities after modification by
the SDBD plasma are more pronounced for O2 (Fig. 2C) than for N2
(Fig. 2B). The roughness of plasma-treated PET foils after modificationin N2 and in O2 was two times and four times higher, respectively, com-pared to the pristine polymer. The surface roughness Ra for PETmodifiedin N2 plasma was 21.6 nm (Fig. 2B), and 39.5 nm (Fig. 2C) for PET modi-fied in O2 plasma.
The PET foil treated by plasma was exposed to excited and unstableparticles, which could transfer their energy to the polymeric surface.The modification of PET by the SDBD plasma led to a breakdown ofthe C\H and/or C\C bonds creating free radicals on the PET surface.The relative free-radical concentration in the plasma-treated PET wasmeasured by means of ESR spectroscopy [43]. The ESR spectra of PETmodified by the SDBD plasma in N2 and in O2 are shown in Fig. 3A
Table 1Surface energy and polar ratio of the testing liquids at 23 °C.
and B. By EPR measurements two main singlet signals were obtainedfrom radical breakdown of the bonds in the polymer.
The created free radicals can react with either nitrogen or oxygenions in the SDBD plasma. The concentration of oxygen depends on thepresence of oxidizing species (traces of oxygen, water) in the plasmagaseous medium. When subsequently exposed to air, these radicalsreact with oxygen in air to form peroxides and hydroperoxides.
The concentrations of free radicals present in PET modified by theSDBD plasma in N2 (Fig. 4a) or in O2 (Fig. 4b), decreased by a factor of5 over a period of 41 days, as determined from the intensities of thetwo main singlet signals (Table 3).
Fig. 5 shows the FTIR-ATR spectra of pristine PET (Fig. 5C), as well asPET modified by the SDBD plasma in N2 (plot b) and O2 (plot a) atmo-spheres after treatment for 10 s. For the unmodified PET foil (plot c),the characteristic IR bands at 1710, 1505 and 1173 cm−1were observedfor the C_O stretch, benzene ring \C\C\ and \CH stretching vibra-tions, as well as for the ring \C\H in plane bending. The IR band at1358 cm−1 belongs to the \CH2 wagging; the benzene ring in-planeC\H bending and the\C\C\ stretching vibration bands, respectively,appear around/at/close to 1173 and 1037 cm−1. The spectra of theSDBD plasma treated PET in N2 plasma (Fig. 5, plot b) and in O2 plasma(Fig. 5, plot a) show important differences from those of unmodified
Fig. 2. AFM phase images of PET: untreated PET (A), PET treated by SDBD plasma in N2, 10 s (B), PET modified by SDBD plasma in O2, 10 s (C).
Fig. 3. ESR spectra of PET modified by SDBD plasma: in N2 (A) and O2 (B).
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PET. The C_O stretch at 1710 cm−1 broadened due to the creation ofoxygen-containing sites. The C_O content is higher for the PET modi-fied by the oxygen SDBD plasma (Fig. 5, plot a) than those for the PETmodified by the nitrogen plasma (Fig. 5, plot b), and consequently alarger broadening of the peak at 1710 cm−1 was observed for the oxy-gen plasma modified PET.
The XPS survey scans and C1s and O1s peaks of the pristine PET, aswell as the survey scans and C1s, O1s and N1s peaks of the modifiedPET are shown in Table 3 and in Figs. 6–8.
The main peaks of pristine PET C1s and O1s are located at 284.7 and531.9 eV, respectively (Fig. 6). The C1s peak of untreated PET afterdeconvolution is composed of three peaks: peak A belonging to theC\C andC\Hbonds, peak B belonging to the C\Obond, and peakC be-longing to the O_C\O bond [9].
For the PET treated by the SDBDplasma in nitrogen (Fig. 7) the dom-inating feature of the survey scan is a sharp C1s peak at 284.7 eV, and anO1s peak at 531.8 eV, which significantly increases due to the surfaceoxidation of PET by the effect of the SDBD plasma. The oxygen contentincreased from 24.4 to 30.1 at.% after the SDBD plasma treatment in ni-trogen (5 s), and/while the concentration of carbon decreased from75.6 to 68.2 at.%. Nitrogen is also present on the PET surface (N1s at
399.7 eV) of the PET modified by the SDBD plasma in nitrogen (5 s),and its content was at 1.5 at.%. For a longer time (10 s) of PET modifica-tion by the SDBD plasma in nitrogen its concentration increased to 4.0at.%. In the case of the treatment of the PET by the SDBD plasma in nitro-gen an effective attachment of the nitrogen-containing groups was ob-served. The nitrogen content in the PET as determined by XPS analysisincreased with time of the SDBD plasmamodification (cSDBD, N2, 5 s b c-SDBD, N2, 10 s).
The XPS results of the PET modified by the SDBD plasma in oxygenare shown in Fig. 8. The C1s and O1s peaks of the PET treated by theSDBD plasma in oxygen show almost no effect of pre-treatment on thesurface composition of the polymer, but the treated sample exhibitshigher proportion of oxidized carbon type.
Fig. 4. Content of free radicals in PET modified by SDBD plasma vs. aging time in:O2 (a), N2 (b).
Table 2ESRmeasurement of free radical concentration onPET surfacemodifiedby SDBDplasma inN2 or O2 during aging.
Agingtime(days)
Processinggas
App (arbitrary units)(signal intensity)
ΔHpp (Gauss)(signal width)
Free radicalsconcentration(relative units)
0 N2 1.457 589.5 5.062 × 105
O2 1.461 597.7 5.219 × 105
4 N2 1.268 557.8 3.944 × 105
O2 1.418 548.8 4.271 × 105
6 N2 1.492 568.4 4.820 × 105
O2 1.475 557.8 4.590 × 105
14 N2 1.403 578.9 4.701 × 105
O2 1.550 538.2 4.490 × 105
41 N2 0.701 370.2 0.960 × 105
O2 0.746 388.6 1.126 × 105
Table 3XPS measurements of PET modified by SDBD plasma (conc. in at.%).
Table 2 shows the XPS results of the surface chemical composition ofpristine PET and PET modified by SDBD plasma, as well as the O/C, N/Cand (N + O)/C ratios of the investigated PET samples.
The O/C ratio of pristine PET (0.32) modified for 10 s by the SDBDplasma in N2 increased to 0.37, and in O2 to 0.43. Higher values of theO/C ratio were found for PET modified by the SDBD plasma in oxygencompared to the modification in nitrogen. An increase in the N/C ratiowas found for SDBD plasma modification of PET in nitrogen (0.06,10 s), while (N + O)/C ratio increased from 0.32 for pristine polymerup to 0.43 for the PET sample modified by the SDBD plasma for 10 s,irrespective of whether the modification proceeded in nitrogen or in
oxygen atmosphere. Our XPS data are similar to the results of De Geyteret al. [24], wherein the PET filmwas treated by the dielectric barrier dis-charge plasma in air, argon and helium at a medium pressure of 5 kPa[24]. The reason for a remarkable increase of hydrophilicity of plasma-treated polyester film consists in the formation of oxygen-containinggroups on the surface in a form of C\O, and O\C_O, and during theplasma-treatment in argon, and also cross-linking has been occurredin helium. For the PET film the O/C ratio increases from 0.35 for theuntreated polyester sample to 0.48 for the saturated polyester samplemodified by barrier plasma in air. These O/C ratios for the pristine andplasma-treated PET samples [24] are very similar to our results
Fig. 6. XPS of pristine PET: A — survey scan, C1s, O1s; B — deconvolution of C1s peak.
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summarized in Table 2. The increase in O/C ratio suggests that newoxygen-containing groups are formed on the polyester film after treat-ment with the air plasma.
The surface energy of PET foils modified by the SDBD plasma in ox-ygen and nitrogen at atmospheric pressure vs. the activation time isshown in Fig. 9.
The surface energy of PET during themodification by the SDBD plas-ma in oxygen andnitrogen (Fig. 9) significantly increased in comparisonwith the untreated polymer. Fig. 9, plot a shows that the surface energyof PET modified by the SDBD plasma in oxygen increases from an initialvalue of 47.8 mJ ∙m−2 for pristine PET to 82.6 J ∙m−2 for PETmodified byplasma for 20 s. Fig. 9, plot b shows the surface energy of PET modifiedby the SDBD plasma in nitrogen. These values are lower than those ofPET modified by the same method in oxygen.
The influence of aging time on the surface energy of PET foil modi-fied by the SDBD plasma in oxygen (plot a) and in nitrogen (plot b)for 20 s is shown in Fig. 10. The surface energy of PET modified by theSDBD plasma in oxygen after 7 days of aging decreased from 82.6 to78.1 mJ ∙m−2 (5.4% decrease), and after 30 days of aging it decreasedto 75.8 mJ ∙m−2 (8.2% decrease). After the surface energy of PET treatedin nitrogen plasmaduring the same time of aging decreased after 7 daysfrom 80.2 to 73.5 mJ ∙m−2 (6.7% decrease) and after 30 days agingdown to 67.6 mJ ∙m−2 (15.7% decrease). Decrease in the surface energyduring aging of PET modified by the SDBD plasma was higher in nitro-gen (15.7%) compared to oxygen (8.2%). This process is called hydro-phobic recovery, and it leads to a reduced value of the surface energyof the modified polymer. The results of Morent et al. [23,25] show the
more intensive decrease (the loss of treatment efficiency by plasmawas 47%) of the surface energy of PET modified by barrier dischargeplasma in air during a shorter time (7 h) of aging or 10 days [23] ofaging. These results have been explained by the relation of the agingeffect of plasma-treated PET to both the crystallinity of the polymer aswell as to the type of barrier plasma used for the modification of thepolymer. We have used biaxial oriented PET with higher crystallinityand very efficient surface barrier discharge plasma treatment at atmo-spheric pressure. According Borcia et al. [45] most of the hydrophobicrecovery occur during the first 2–3 days after the plasma treatment.During the storage in air, the chemical groups created during plasmatreatment will re-orientate compared to the bulk resulting in reductionof the surface energy of the PET samples. If the crystallinity of the sam-ple is higher, chemical groups possess higher resistance towards agingbecause of the ordered crystalline structure, and this leads to a sloweraging process [23]. The aging effect is explained by the reorientationof induced polymer polar chemical groups into the bulk of the polymer.The aging of plasma-treated biaxial oriented PET depends on its highercrystallinity degree as well as on the type of barrier discharge plasmaused for the modification of the polymer. The effect of plasma-treatedpolymer aging depends on crystallinity of the polymermodified by bar-rier plasma. The slower course of aging during 1 monthwe observed forbiaxial oriented polypropylene with higher crystallinity in our previousstudy [46,47].
The hydrophobic recovery can be explained by the inter-reactionsof chemical groups on the plasma-treated surface of the polymerhaving many functionalities available for specific interactions, and the
Fig. 7. XPS of PET modified by SDBD plasma in N2, 10 s: survey scan, C1s, O1s, N1s peaks.
migration of the low-molecular mass oxidized material, e.g. the polarfragments, into the polymeric film to reach a thermodynamicallymore stable state with a lower value of the surface free energy of low-molecular mass compounds, which are incompatible with polymer tothe surface [47]. In the case of plasma-treated PET the aging effect is re-lated to themovement of C_O groups in the ester residues towards thesurface layer, and to the migration of C\O groups from the surface tothe bulk. Such migration of the C_O groups may occur within at least
3 nm from the surface [28]. We have achieved good stability of the PETmodification by the SDBD plasma, because the surface energy of thepolymer was diminished after 30 days of aging only by 8.2% (O2 plasma)and 15.7% (N2 plasma), respectively. The results of aging of plasma treat-ed PET show that the bestway for industrial production (e.g. for printing,bonding, lamination, etc.) consists in further processing of the plasma-treated PET foil performed immediately after treatment by the continu-ous technology.
Fig. 8. XPS of PET modified by SDBD plasma in O2, 10 s: survey scan, C1s, O1s peaks.
Fig. 9. Surface energy of PET foil modified by SDBD plasma in O2 (a) and in N2 (b) vs. ac-tivation time.
Fig. 10. Aging of PET foil treated by SDBD plasma (20 s) in O2 (plot a), and N2 (plot b) vs.aging time.
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The significance of the changes in topography (roughness) ofplasma-treated PET foils is very important for an increase of the adhe-sive properties of PET (higher PET roughness leads to higher peelstrength in adhesive joint PET–polyacrylate), namely if the chemicalchanges are not very different for the two used processing gasses, i.e.oxygen and nitrogen.
The peel strength of the adhesive joints of PET foils, modified by theSDBD plasma in nitrogen (plot a), and in oxygen (plot b) to polyacrylatevs. time of activation is shown in Fig. 11. The peel strengths of PET topolyacrylate after modification by the SDBD plasma significantly in-creased; the increase was higher for the samples modified in oxygen.The peel strength of the adhesive joint increased substantially from77 N ∙m−1 (pristine PET) to 180 N ∙m−1 (SDBD, 10 s, nitrogen), and237 N ∙m−1 (SDBD, 10 s, oxygen).
These results clearly indicate that the SDBD plasma modification ofPET considerably improves its adhesive properties. The completelynew surface with different physical properties and chemical propertiesis obtained via the SDBD plasma treatment in either O2 or N2. Thus, notonly surface chemical modifications, but also topographical changes, i.e.the increase of roughness, are contributing independently to the ob-served increased adhesive ability of PET foils [44]. The dependenciesof the peel strength of the adhesive joint of PET modified by the SDBDplasma–polyacrylate in O2 or N2 versus the surface free energy weretested for PET foils modified in either O2 plasma (Fig. 12A) or N2 plasma(Fig. 12B).
Fig. 12 shows the relation between the peel strength of the adhesivejoint of PET modified by the SDBD plasma in oxygen or nitrogen topolyacrylate and the surface free energy of the polymer.
A correlation has been found between the surface energy and adhe-sion of the PET foil treated by the SDBD plasma in oxygen (Fig. 12A)and in nitrogen (Fig. 12B) to polyacrylate and the surface energy hasbeen found. A 2nd order polynomial regression was used for boththe above given dependencies for PET plasma-treated in oxygen:Ppeel = −564 + 18.2 × SFE − 0.1 × (SFE)2, r2 = 0.99 (Fig. 12A), andfor PET plasma-treated in nitrogen: Ppeel = −562 + 19.1 × SFE −0.12 × (SFE)2, r2 = 0.99 (Fig. 12B). The peel strengths of the adhesivejoint of PET modified by plasma in oxygen to polyacrylate are substan-tially lower than those if nitrogen is used as a processing gas.
4. Conclusions
The modification of polyester by the SDBD plasma in either N2 or O2
results in an increase of roughness of the surface. AFM confirmed thecreation of some heterogeneities on the PET surface due to the localmelting. EPR measurements of the SDBD plasma modified polymershowed twomain singlet signals,which resulted from the radical break-down of the covalent bonds in the polymer. Aging of PET modified by
the SDBD plasma in N2 and O2 led to a substantial decrease in the freeradical concentration; SDBD plasma-treated PET showed a broadeningof the C\O stretch at 1710 cm−1 due to the creation of oxygen-containing sites. XPS analysis showed the increase in the oxygen and ni-trogen contents in the PET surface layer treated by the SDBD plasma.The surface energy as well as the peel strength of PET significantly in-creased after modification by the SDBD plasma, and this increase washigher for treatment in O2 compared to N2 atmosphere. The correlationbetween the peel strength of the PET adhesive joint modified by theSDBD plasma in O2 or N2 to polyacrylate to the surface energy of poly-mer has been found. The results of aging of the plasma-treated PETfoil show that the bestway for industrial production consists in process-ing of the plasma-treated PET foil performed immediately after treat-ment by continuous technology, e.g. for printing with printing inks,bonding using adhesives, and lamination with other polymers, etc.
Acknowledgments
This contribution was supported by the Ministry of Education of theSlovak Republic Project No. 26220220091 by the Research & Develop-ment Operational Program, as a part of the project “Application ofKnowledge-Based Methods in Designing Manufacturing Systems andMaterials”, Project No. MESRSSR 3933/2010-11, and by the Slovakgrant agency VEGA (Grant No. 2/0185/10).
Fig. 11. Peel strength of adhesive joint of PET treated by SDBD plasma in O2 (a), and N2
(b) to polyacrylate vs. activation time.
Fig. 12. A. Relation between peel strength of adhesive joint of PET modified by SDBD plas-ma in O2 to polyacrylate vs. surface energy of the polymer. B. Relation between peelstrength of adhesive joint of PETmodifiedby SDBDplasma inN2 to polyacrylate vs. surfaceenergy of the polymer.
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