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Laser-induced surface activation of biocomposites for electroless metallization
Surface & Coatings Technology 311 (2017) 104–112
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Laser-induced surface activation of biocomposites for electroless metallization Piotr Rytlewski a,⁎, Thomas Bahners b, Filip Polewski a, Beate Gebert b, Jochen S. Gutmann b,c,d, Nils Hartmann c,d,e, Ulrich Hagemann d,e, Krzysztof Moraczewski a a
Department of Materials Engineering, Kazimierz Wielki University, ul. Chodkiewicza 30, 85-064 Bydgoszcz, Poland Deutsches Textilforschungszentrum Nord-West gGmbH (DTNW), Adlerstr. 1, 47798 Krefeld, Germany Universität Duisburg-Essen, Fakultät für Chemie, Physikalische Chemie, Universitätsstr. 2, 45141 Essen, Germany d Universität Duisburg-Essen, Center for Nanointegration Duisburg-Essen (CENIDE), 47057 Duisburg, Germany e Universität Duisburg-Essen, Interdisciplinary Center for Analytics on the Nanoscale (ICAN), 47057 Duisburg, Germany b c
a r t i c l e
i n f o
Article history: Received 28 September 2016 Revised 22 November 2016 Accepted in revised form 15 December 2016 Available online 16 December 2016 Keywords: Biocomposites Polylactide Polycaprolactone Laser Electroless metallization Copper compounds
a b s t r a c t In this work biocomposites containing polylactide (PLA), polycaprolactone (PCL), copper(II) oxide and copper acetylacetonate were manufactured by an extrusion process. The extruded composites differed with respect to the PLA/PCL ratio whereas the content of mixed copper(II) oxide and copper acetylacetonate powders was held constant at 20 wt%. The main aims for the addition of PCL was to increase impact strength resistance, improve surface catalytic properties and reduce the temperature of extrusion, thus limiting degradation effects initiated by copper acetylacetonate. The composite samples were irradiated with an ArF excimer laser varying the number of laser pulses and then metalized by electroless plating. Based on optical microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) measurements, it was found that (i) PCL was dispersed in the form of droplets in all volume of PLA, (ii) the copper compounds were preferably located in the dispersed PCL phase, and (iii) composites with higher PCL content were more effectively metalized. © 2016 Elsevier B.V. All rights reserved.
1. Introduction In the last tens of years a huge interest in surface engineering of polymer materials resulted from numerous applications of these materials in automotive, electronic and medical equipment industry. Many of these applications require surface metallization [1–3]. Electroless metallization is a large-scale industrial process to coat plastic elements with a metallic layer [4]. In this technique, neat polymers cannot be directly metalized without prior surface activation. Conventionally, the activation of polymer surfaces imposes wet chemical modifications of the polymer substrate in order to roughen, oxidize and seed it with catalyst species. It can be realized in one- or two-step chemical procedures [5,6]. However, this type of surface activation imposes considerable environmental and occupational risks. Moreover, by using chemical treatments all immersed surface is modified, thus the subsequent metallization cannot be confined to selected areas of the polymer surface, i.e. for fabrication of metallic patterns. Therefore great efforts are undertaken to elaborate new physical methods of
⁎ Corresponding author. E-mail address: [email protected] (P. Rytlewski).
http://dx.doi.org/10.1016/j.surfcoat.2016.12.048 0257-8972/© 2016 Elsevier B.V. All rights reserved.
polymer surface activation, which are ecologically friendly on the one hand, and allow for patterning on the other [7,8]. Lasers enable precise modification of the polymer surface area, which is intended to be metalized [9–11]. Moreover, the application is rather limited to metallization of small surface areas. Therefore, this technique can find applications mainly in electronic industry where the interface of polymer dielectric and metallic layer plays a crucial role. In previous studies, excimer lasers were applied in order to activate polymer surface by depositing palladium (Pd) species onto the polyimide surface immersed in aqueous solution of PdSO4 or Pd(CH3CO2)2 [12, 13]. Laser-induced implementation of copper from CuSO4 solution or silver from solution containing AgNO3 on a polymer surface were also demonstrated [14–17]. A different approach for surface activation is to laser-irradiate polymer composite which contains some metal compounds [18]. Laser irradiation releases metal species, which constitute catalytic nuclei for reduction of metal ions from metallization bath. This surface activation technique is currently under intensive study in view of new material compositions [19,20]. In this work an attempt to evaluate effectiveness of laser-induced surface activation of polymer blended structures containing copper compounds was undertaken. Polylactide (PLA) and polycaprolactone
P. Rytlewski et al. / Surface & Coatings Technology 311 (2017) 104–112 Table 1 Comparison of selected PLA and PCL properties (general estimation) [21].
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Table 3 Sample identifier of composites A, B, C and D for different applied laser pulses.
Property
PLA
PCL
Number of laser pulses
Melting point Glass temperature Young modulus Tensile strength Impact strength Surface polarity Biodegradability
168 °C 60 °C 3.3 GPa 72 MPa 3.0 kJ/m2 Hydrophilic Couple of months
58 °C −60 °C 1.2 GPa 23 MPa 35 kJ/m2 Hydrophobic Couple of years
10 50 100 300
(PCL) were selected as biodegradable thermoplastics with significantly differing physical properties. Some selected properties of these polymers are listed in Table 1. Although their physical properties are very different, PLA and PCL are biodegradable. Various blends of PLA/PCL are currently intensively examined [22–25] given the interesting possibility to produce various materials with wide scope of complementary properties. The main focus is to balance the high tensile strength of PLA with high impact strength of PCL but also to control the rate of biodegradation in medical applications [26,27]. The scientific objective of this work was to demonstrate the influence of (i) the PCL content and (ii) the laser irradiation conditions on surface activation and metallization properties of PLA/PCL blends containing 15 wt% of copper oxide and 5 wt% of copper acetylacetonate. For this purpose various PLA/PCL ratios and energy doses of laser radiation were applied and surface activation effects evaluated.
Composites: A
B
C
D
A1 A2 A3 A4
B1 B2 B3 B4
C1 C2 C3 C4
D1 D2 D3 D4
Cu(acac)2. The compositions of composites selected for laser irradiation and electroless metallization are listed in Table 2. The temperatures of the barrel heating zones I, II, III were 140, 140 and 150, respectively and the temperature of the extruder die was 150 °C. Samples were cut-off from extruded polymer sheets for mechanical testing, laser irradiation and metallization. 2.3. Laser irradiation and electroless metallization
2. Experimental
The composite samples were irradiated with an ArF excimer laser type LPX 300 (Coherent, USA). Laser fluence was set to 300 mJ/cm2 while samples were exposed to 10, 50, 100 or 500 laser pulses, thus four different energy doses were applied. The laser pulse length was about 20 ns and the pulse frequency was 5 Hz. Sample identifier for different applied number of laser pulses are presented in Table 3. Metallization of the laser-irradiated samples was performed using a commercial metallization bath M-Copper 85 (MacDermid, USA) and formaldehyde as a reducing agent. The samples were immersed in the bath for 60 min at 46 °C to deposit copper layers.
2.1. Materials
2.4. Measurements
The following materials were applied to prepare composites:
A single-screw extruder of the type W25-30D (IPTSz “Metalchem”, Toruń, Poland), equipped with a folding screw, was used to obtain extruded sheets with different PLA/PCL ratios. After mechanical testing appropriate PLA/PCL compositions have been chosen to be mixed and extruded with a constant amount of 15 wt% of CuO and 5 wt% of
Determination of tensile strength and Young modulus was performed under quasi static tension, using a tensile testing machine, type Instron 3367 (Instron, USA), according to the PN-EN ISO 527-1 standard. Impact strength tests were performed using a Charpy apparatus (ATS-FAAR, Italy), according to the PN-EN ISO 179-1 standard. Fourier transform infrared spectroscopy (FTIR) was performed only to determine the chemical structure of the material dissolved in acetylacetonate. After chemical treatment the solution was evaporated whereas the remaining material was examined using a FTIR spectrometer Nicolet iS10 (Thermo Scientific, USA) operating in attenuated total reflection mode in the wavenumber range of 600–4000 cm−1 and at a resolution of 4 cm− 1. Average spectra from at least 16 scans were used for analysis. Thermal stability of copper compounds were performed using thermogravimetric apparatus Q500 (TA Instruments, USA). Copper compounds in the form of powder were heated at rate of 5 °C/min under nitrogen atmosphere. The catalytic properties were characterized based on photographic images of composites after laser irradiation and electroless metallization. The main qualitative measure of these properties was the apparent copper coverage of the laser irradiated surface of the composite. Surface morphology was examined using two different scanning electron microscopes (SEM) type SU8010 (Hitachi, Japan) and S-3400 Type II (Hitachi, Japan). Prior to the measurements, samples were
Table 2 Compositions of composites selected for laser irradiation and electroless metallization.
Table 4 Impact strength (KIC), tensile strength (σM) and Young modulus (E) for various PLA/PCL blends.
▪ Polylactide (PLA), type 2002D (NatureWorks®, USA). Its density was 1.24 g/cm3, melt flow rate 5 g/10 min (2.16 kg, 210 °C), and L-lactide content about 96.5%; ▪ Polycaprolactone (PCL), type CAPA™ 6800 (Solvay Caprolactones, UK). Its density was 1.1 g/cm3, melt flow rate 7.3 g/10 min (2.16 kg, 160 °C); ▪ Copper(II) acetylacetonate of the chemical formula: [CH3COCH = C(O-)CH3]2Cu, further in this article referred to as Cu(acac)2 (Sigma-Aldrich, USA), with molar mass 261.76 g/mol, melting point Tm = 245 °C, purity 97%; ▪ Copper(II) oxide, CuO (Sigma-Aldrich, USA) of molar mass 79.55 g/mol, Tm = 1326 °C, and purity 98%. ▪ Metallization bath, type M-Copper 85 (MacDermid, USA) with addition of formaldehyde (36 wt%) as reducing agent (POCH, Poland). 2.2. Sample preparation
Composites
PLA (wt%)
PCL (wt%)
CuO (wt%)
Cu(acac)2 (wt%)
A B C D
80 75 70 65
0 5 10 15
15 15 15 15
5 5 5 5
PLA/PCL (mass ratio)
KIC (kJ/m2)
σM (MPa)
E (MPa)
100/0 95/5 90/10 85/15 70/30
2.9 3.1 3.1 3.1 5.7
72.0 66.4 62.5 54.9 54.9
3299 3079 2961 2696 2534
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Fig. 1. Graphical representation of morphological evolution dependent on the ratio of PCL and PLA [22].
coated with thin gold layers. Chemical composition of the composite surface layers was analyzed by means of energy-dispersive X-ray spectroscopy (EDX). An EDX analyzer was coupled to a SEM microscope at a distance of about 15 mm from the composite surface. An average elemental composition of the composites was determined based on data collected from the scanned area of about 4.5 mm2 or from indicated local areas on the composites surface. Surface chemical properties of the treated samples were further determined by X-ray photoelectron spectroscopy (XPS) using a Versaprobe II™ instrument (Ulvac-Phi, Japan). This machine allows spatially resolved XPS measurements on a μm-scale, as the diameter of the X-ray beam can be focused to below 10 μm.
However, KIC significantly increased at 30 wt% of PCL content at which PCL transforms from a dispersed to more continuous phase which is graphically represented by the middle image in Fig. 1. For further examination compositions with up to 15 wt% of PCL were chosen in order to mix them with CuO and Cu(acac)2 filler. These compositions provided more densely dispersed PCL, and thus more densely dispersed copper compounds. As a general rule it is considered that in polymer blends the low viscosity phase has better affinity to entrap filler particles than the high viscosity phase. Therefore, it was expected that copper compounds will be located preferably in the PCL phase. In the examined composites with 15 wt% of CuO and 5 wt% of Cu(acac)2, KIC increased by 50% with increasing PCL content, whereas
3. Results and discussion 3.1. Characterization of PLA/PCL biocomposites As preliminary research mechanical testing of PLA/PCL blends at various ratio was done in order to select appropriate blend ratio for further examination. Impact strength (KIC), tensile strength (σM) and Young modulus (E) were determined. It was found that with increasing content of PCL σM and E decreased (Table 4).
Table 5 Impact strength (KIC), tensile strength (σM) and Young modulus (E) for composites A, B, C and D. Composite
KIC (kJ/m2)
σM (MPa)
E (MPa)
A B C D
2.1 1.9 2.4 3.2
47.8 46.1 45.6 43.0
2950 2737 2688 2468
Fig. 2. Mass loss and its temperature derivative for powder Cu(acac)2.
Fig. 3. SEM images of samples A and D after etching with acetylacetonate.
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Fig. 4. EDX measurements: Cu content in A, B, C and D samples etched with acetylacetonate.
Fig. 6. EDX measurements: Cu content in metallized layers on the composites with various PCL content (samples A4–D4 modified with 300 laser pulses).
σM and E decreased by 10% and 16%, respectively, at the same time (Table 5). Due to the potential application of these composites in electronic devices, resistance to dynamic stress is expected to be more important than the tensile strength or Young modulus. However, the application of PLA/PCL blends to produce these composites is justified by one more important effect. By introducing PCL the temperature of extrusion could be reduced to 150 °C due to the low melting temperature of PCL of approximately 58 °C. In contrast, PLA melts at 160 °C. This reduction in processing temperature allows to limit degradation of Cu(acac)2. As presented in Fig. 2 Cu(acac)2 is thermally stable up to about 150 °C. Degradation of Cu(acac)2 during processing is undesirable because it can initiate degradation of the polymer matrix.
In order to evaluate the dispersion of PCL and copper compounds, the composite samples were immersed in acetylacetonate, which is a good solvent for PCL but poor for PLA. After 24 h immersion, the composite samples and the dissolved phase were examined. FTIR analysis revealed that the dissolved polymer was PCL and black powder of CuO and blue powder of Cu(acac)2 were clearly visible after evaporation of the solvent. The treated composite samples were analyzed by means of SEM and EDX. As could be expected from the analysis of the dissolved phase, SEM micrographs clearly show increasing surface etching with increasing PCL content of the composite (Fig. 3). SEM analysis also confirmed that the PCL phase was not continuous but dispersed in PLA in the form of droplets. These composites were also examined using EDX analysis. For the samples with higher PCL content lower Cu content was detected after etching (Fig. 4).
Fig. 5. Metallization effects on laser irradiated composites.
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Fig. 7. SEM images of laser irradiated and metalized composites: A, B, C and D. Laser irradiation has been carried out at 300 laser pulses (samples A4–D4).
Fig. 8. SEM images of composites A, B, C and D irradiated with 300 laser pulses (samples A4–D4, no metallization).
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Fig. 9. SEM cross-section images of composites A, B, C and D irradiated with 300 laser pulses (samples A4–D4, no metallization).
It is assumed that this results from heterogeneous distribution of copper compounds within the matrix of blended PLA/PCL because these compounds are preferably located in the PCL phase. Therefore after chemical etching, they are removed with the dissolved PCL phase. In effect, post-treated composites with higher PCL content contained less copper compounds. 3.2. Effects of laser irradiation and metallization The composites were irradiated with various numbers of laser pulses and then metalized in an electroless process. Effects of metallization are shown in Fig. 5. With increasing number of laser pulses better copper coverage was observed for all studied samples. Moreover, the higher was the content of PCL the better were metallization effects. Sample D was well visibly coated with copper even after 50 laser pulses. These effects are well reflected by the results of EDX analysis (Fig. 6). As follows from Fig. 6 deposited copper constituted a maximum of 50 at.% for sample D, which indicated incomplete coverage of the composite surfaces. This is well confirmed by SEM images where only locally deposited copper agglomerates are visible (Fig. 7).
Copper aggregates are identified as the bright areas in the SEM images. It can be taken from the micrographs that not only the overall copper content, but also the deposition morphology varies with the PLA/ PCL blend ratio. Samples with high PCL content exhibit a more coarse copper distribution after metallization. In order to elucidate this effect of PCL on quantitative and qualitative copper deposition, laser irradiated composites were analyzed by SEM and EDX analysis before metallization. Fig. 8 presents surface morphologies of the composites A, B, C and D irradiated with the same number of laser pulses. In general, the SEM analysis does not reveal significant differences between the composites with different PCL content. The SEM micrographs shown in Fig. 8 indicate that local agglomeration of copper occurs upon laser radiation, which has also been confirmed by EDX analysis (EDX-mapping). However, differences between the irradiated composites were visible in SEM images taken from cross-sections of irradiated samples (Fig. 9). As follows from Fig. 9 with increasing PCL content the composite surface showed an increased roughness/waviness. To summarize this part of SEM analysis, distribution of copper agglomerates on the composite surface is similar while the surface roughness increased with
Fig. 10. Laser-induced surface alteration in composites A, B, C and D.
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Fig. 11. Cu content detected by EDX in the composites varied with PCL content irradiated with 300 laser pulses (samples A4–D4, no metallization).
Fig. 13. X-ray spectra from points 1 and 2 of composite A shown in Fig. 12.
increased PCL content. A graphical model for these surface alterations is presented in Fig. 10. When assuming the model of surface changes in Fig. 10 the ratio of surface area with copper to the surface area without copper decreases with increasing surface roughness. Thus using the EDX technique and scanning with the electron beam large surface areas (4.5 mm2) of the composites it was possible to detect a decreased copper content for the composites with higher PCL content (Fig. 11). The trend in the content of Cu well fits the change in geometrical structure. The biggest change occurred between sample A and B, then the changes were less significant (compare Figs. 9 and 11). In view of these observations, namely referring to the apparent dependence of copper content in irradiated domains on PCL content, it had to be clarified, whether differences in the structure and composition of copper agglomerates were the reason that the composites with higher PCL content were more effectively metalized. EDX analysis at specified positions in areas of copper agglomerates and between them was performed in order to answer this question. Results of these analyses are shown in Figs. 12–15. Figs. 12 and 14 show high-resolution SEM micrographs of composites A and D, i.e. at 0 and 15 wt% PCL content, respectively. Brighter local areas represent rather confined copper domains, whereas between
these domains practically no copper is detected. This is clearly confirmed by EDX analysis at the indicated points 1 and 2. The relevant spectra are shown in Figs. 13 and 15. The important difference in the elementary composition of the copper domains in composites A and D according to EDX are the strong signals attributed to the carbon and oxygen elements in the case of composite A (Fig. 13). It has to be concluded that polymer macromolecules are still present in the copper domains. However, in the case of composite D these emission bands were drastically reduced in comparison to the Cu signal. This indicates that bright agglomerates are composed mainly of copper with only negligible content of polymer macromolecules. This fundamental difference between the composites can be assumed to be responsible for better coverage with copper composites with higher PCL content. Using EDX analysis it is not possible to determine whether detected copper occurs in metallic form or in oxidized binding states (e.g. CuO and CuO2). From the viewpoint of metallization the outermost surface layer of the composite is most crucial, which is in contact with copper ions present in metallization bath. If this outermost composite surface has copper in metallic form, the surface will have autocatalytic properties. Therefore, XPS analysis was performed in addition to EDX. By means of this method information of the Cu bonding states in a very thin nanometric surface layer of the composites could be attained,
Fig. 12. Specified points (1, 2) for EDX analysis on the surface of composite A irradiated with 50 laser pulses (samples A2, no metallization).
Fig. 14. Specified points for EDX analysis on the surface of composite D irradiated with 50 laser pulses (samples D2, no metallization).
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4. Conclusions
Fig. 15. X-ray spectra from points 1 and 2 of composite D shown in Fig. 14.
whereas EDX analysis relates to a surface layer having a thickness of several microns and is unspecific to the bonding states of an element. Using a 20 μm X-ray beam it is possible to perform XPSmeasurements of only the copper domains, as identified by the EDX analysis. Fig. 16 on the left shows the photoelectron Cu 2p3/2 signal from such a domain for sample A4 (irradiated with 300 laser pulses) whereas the beam position and its diameter are depicted on the right in Fig. 16, which presents a scanning X-ray induced secondary electron image (SXI). Based on this measurement it was clearly stated that copper occurs largely in metallic form because no oxide satellite structures around 940 eV were observed in the XPS signal [28]. An estimation on the basis of these data suggests a maximum oxide concentration of 5 at.% copper oxide. Thus, it was concluded that under laser irradiation the copper compounds are reduced to copper in metallic form. Aside from copper, only silicon, oxygen and carbon are detected on the surface of the samples. Because of significant local variations in geometrical and chemical surface structure, no clear trend in the atomic concentration of copper on the sample surface in dependence of the surface structure could be identified.
PCL as a biodegradable thermoplastic was added at various contents to PLA and mixed with constant content of 15 wt% of CuO and 5 wt% of Cu(acac)2. Using PCL the processing temperature was reduced to 150 °C, whereas impact strength increased by more than 50% with increasing content of PCL up to 15 wt%. The reduced temperature during composite extrusion minimized the effects of Cu(acac)2 degradation, which starts above 150 °C, as proved by thermogravimetric measurement. It was found based on SEM/EDX analysis that copper compounds were preferably located in PCL than PLA phase of the extruded composites. Moreover, PCL was dispersed in the form of droplets in a continuous PLA volume. The droplets had similar sizes. Probably for that reasons copper agglomerates formed by laser radiation had also similar sizes and differ only slightly with increasing number of radiation pulses. It was found that the same laser irradiation caused higher surface roughness in composites with higher PCL content. This is assumed to explain the decrease of Cu coverage with increasing PCL content as detected by EDX because the surface area covered with copper domains decreased relatively to the effective surface area of uncovered regions developed during laser radiation. The positive effect of increasing PCL content on metallization effects was determined based on general optical assessment and EDX analysis of the copper coated layers. The origin of this positive effect is attributed to the difference determined in the chemical structure of the laserformed copper agglomerates prior to metallization. The higher was the PCL content the more copper content in bright agglomerates was detected. This difference between the composites of varying PCL content can be responsible for better coverage with copper in the case of composites with higher PCL content. XPS analysis of laser-processed samples prior to metallization proved that laser irradiation cause the formation of copper in metallic form, which generally is expected to be most suitable to initiate reduction of copper ions from metallization bath. Layers of metalized copper on the biocomposites surface were not continuous, thus their applications appears more suitable as electromagnetic shielding for biodegradable casing of modern electronic devices. Further work is required in order to fabricate continuous layers for use as conductive layers in integrated circuits. The two-polymersystem and various copper compounds were applied to manufacture
Fig. 16. Left: Cu 2p3/2 signal of a copper domain on sample A4 (no metallization). Right: 500 μm × 500 μm SXI of sample A4 with the position of the 20 μm X-ray beam during the measurement.
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Laser-induced surface activation of biocomposites for electroless metallization Piotr Rytlewski a,⁎, Thomas Bahners b, Filip Polewski a, Beate Gebert b, Jochen S. Gutmann b,c,d, Nils Hartmann c,d,e, Ulrich Hagemann d,e, Krzysztof Moraczewski a a
Department of Materials Engineering, Kazimierz Wielki University, ul. Chodkiewicza 30, 85-064 Bydgoszcz, Poland Deutsches Textilforschungszentrum Nord-West gGmbH (DTNW), Adlerstr. 1, 47798 Krefeld, Germany Universität Duisburg-Essen, Fakultät für Chemie, Physikalische Chemie, Universitätsstr. 2, 45141 Essen, Germany d Universität Duisburg-Essen, Center for Nanointegration Duisburg-Essen (CENIDE), 47057 Duisburg, Germany e Universität Duisburg-Essen, Interdisciplinary Center for Analytics on the Nanoscale (ICAN), 47057 Duisburg, Germany b c
a r t i c l e
i n f o
Article history: Received 28 September 2016 Revised 22 November 2016 Accepted in revised form 15 December 2016 Available online 16 December 2016 Keywords: Biocomposites Polylactide Polycaprolactone Laser Electroless metallization Copper compounds
a b s t r a c t In this work biocomposites containing polylactide (PLA), polycaprolactone (PCL), copper(II) oxide and copper acetylacetonate were manufactured by an extrusion process. The extruded composites differed with respect to the PLA/PCL ratio whereas the content of mixed copper(II) oxide and copper acetylacetonate powders was held constant at 20 wt%. The main aims for the addition of PCL was to increase impact strength resistance, improve surface catalytic properties and reduce the temperature of extrusion, thus limiting degradation effects initiated by copper acetylacetonate. The composite samples were irradiated with an ArF excimer laser varying the number of laser pulses and then metalized by electroless plating. Based on optical microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) measurements, it was found that (i) PCL was dispersed in the form of droplets in all volume of PLA, (ii) the copper compounds were preferably located in the dispersed PCL phase, and (iii) composites with higher PCL content were more effectively metalized. © 2016 Elsevier B.V. All rights reserved.
1. Introduction In the last tens of years a huge interest in surface engineering of polymer materials resulted from numerous applications of these materials in automotive, electronic and medical equipment industry. Many of these applications require surface metallization [1–3]. Electroless metallization is a large-scale industrial process to coat plastic elements with a metallic layer [4]. In this technique, neat polymers cannot be directly metalized without prior surface activation. Conventionally, the activation of polymer surfaces imposes wet chemical modifications of the polymer substrate in order to roughen, oxidize and seed it with catalyst species. It can be realized in one- or two-step chemical procedures [5,6]. However, this type of surface activation imposes considerable environmental and occupational risks. Moreover, by using chemical treatments all immersed surface is modified, thus the subsequent metallization cannot be confined to selected areas of the polymer surface, i.e. for fabrication of metallic patterns. Therefore great efforts are undertaken to elaborate new physical methods of
⁎ Corresponding author. E-mail address: [email protected] (P. Rytlewski).
http://dx.doi.org/10.1016/j.surfcoat.2016.12.048 0257-8972/© 2016 Elsevier B.V. All rights reserved.
polymer surface activation, which are ecologically friendly on the one hand, and allow for patterning on the other [7,8]. Lasers enable precise modification of the polymer surface area, which is intended to be metalized [9–11]. Moreover, the application is rather limited to metallization of small surface areas. Therefore, this technique can find applications mainly in electronic industry where the interface of polymer dielectric and metallic layer plays a crucial role. In previous studies, excimer lasers were applied in order to activate polymer surface by depositing palladium (Pd) species onto the polyimide surface immersed in aqueous solution of PdSO4 or Pd(CH3CO2)2 [12, 13]. Laser-induced implementation of copper from CuSO4 solution or silver from solution containing AgNO3 on a polymer surface were also demonstrated [14–17]. A different approach for surface activation is to laser-irradiate polymer composite which contains some metal compounds [18]. Laser irradiation releases metal species, which constitute catalytic nuclei for reduction of metal ions from metallization bath. This surface activation technique is currently under intensive study in view of new material compositions [19,20]. In this work an attempt to evaluate effectiveness of laser-induced surface activation of polymer blended structures containing copper compounds was undertaken. Polylactide (PLA) and polycaprolactone
P. Rytlewski et al. / Surface & Coatings Technology 311 (2017) 104–112 Table 1 Comparison of selected PLA and PCL properties (general estimation) [21].
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Table 3 Sample identifier of composites A, B, C and D for different applied laser pulses.
Property
PLA
PCL
Number of laser pulses
Melting point Glass temperature Young modulus Tensile strength Impact strength Surface polarity Biodegradability
168 °C 60 °C 3.3 GPa 72 MPa 3.0 kJ/m2 Hydrophilic Couple of months
58 °C −60 °C 1.2 GPa 23 MPa 35 kJ/m2 Hydrophobic Couple of years
10 50 100 300
(PCL) were selected as biodegradable thermoplastics with significantly differing physical properties. Some selected properties of these polymers are listed in Table 1. Although their physical properties are very different, PLA and PCL are biodegradable. Various blends of PLA/PCL are currently intensively examined [22–25] given the interesting possibility to produce various materials with wide scope of complementary properties. The main focus is to balance the high tensile strength of PLA with high impact strength of PCL but also to control the rate of biodegradation in medical applications [26,27]. The scientific objective of this work was to demonstrate the influence of (i) the PCL content and (ii) the laser irradiation conditions on surface activation and metallization properties of PLA/PCL blends containing 15 wt% of copper oxide and 5 wt% of copper acetylacetonate. For this purpose various PLA/PCL ratios and energy doses of laser radiation were applied and surface activation effects evaluated.
Composites: A
B
C
D
A1 A2 A3 A4
B1 B2 B3 B4
C1 C2 C3 C4
D1 D2 D3 D4
Cu(acac)2. The compositions of composites selected for laser irradiation and electroless metallization are listed in Table 2. The temperatures of the barrel heating zones I, II, III were 140, 140 and 150, respectively and the temperature of the extruder die was 150 °C. Samples were cut-off from extruded polymer sheets for mechanical testing, laser irradiation and metallization. 2.3. Laser irradiation and electroless metallization
2. Experimental
The composite samples were irradiated with an ArF excimer laser type LPX 300 (Coherent, USA). Laser fluence was set to 300 mJ/cm2 while samples were exposed to 10, 50, 100 or 500 laser pulses, thus four different energy doses were applied. The laser pulse length was about 20 ns and the pulse frequency was 5 Hz. Sample identifier for different applied number of laser pulses are presented in Table 3. Metallization of the laser-irradiated samples was performed using a commercial metallization bath M-Copper 85 (MacDermid, USA) and formaldehyde as a reducing agent. The samples were immersed in the bath for 60 min at 46 °C to deposit copper layers.
2.1. Materials
2.4. Measurements
The following materials were applied to prepare composites:
A single-screw extruder of the type W25-30D (IPTSz “Metalchem”, Toruń, Poland), equipped with a folding screw, was used to obtain extruded sheets with different PLA/PCL ratios. After mechanical testing appropriate PLA/PCL compositions have been chosen to be mixed and extruded with a constant amount of 15 wt% of CuO and 5 wt% of
Determination of tensile strength and Young modulus was performed under quasi static tension, using a tensile testing machine, type Instron 3367 (Instron, USA), according to the PN-EN ISO 527-1 standard. Impact strength tests were performed using a Charpy apparatus (ATS-FAAR, Italy), according to the PN-EN ISO 179-1 standard. Fourier transform infrared spectroscopy (FTIR) was performed only to determine the chemical structure of the material dissolved in acetylacetonate. After chemical treatment the solution was evaporated whereas the remaining material was examined using a FTIR spectrometer Nicolet iS10 (Thermo Scientific, USA) operating in attenuated total reflection mode in the wavenumber range of 600–4000 cm−1 and at a resolution of 4 cm− 1. Average spectra from at least 16 scans were used for analysis. Thermal stability of copper compounds were performed using thermogravimetric apparatus Q500 (TA Instruments, USA). Copper compounds in the form of powder were heated at rate of 5 °C/min under nitrogen atmosphere. The catalytic properties were characterized based on photographic images of composites after laser irradiation and electroless metallization. The main qualitative measure of these properties was the apparent copper coverage of the laser irradiated surface of the composite. Surface morphology was examined using two different scanning electron microscopes (SEM) type SU8010 (Hitachi, Japan) and S-3400 Type II (Hitachi, Japan). Prior to the measurements, samples were
Table 2 Compositions of composites selected for laser irradiation and electroless metallization.
Table 4 Impact strength (KIC), tensile strength (σM) and Young modulus (E) for various PLA/PCL blends.
▪ Polylactide (PLA), type 2002D (NatureWorks®, USA). Its density was 1.24 g/cm3, melt flow rate 5 g/10 min (2.16 kg, 210 °C), and L-lactide content about 96.5%; ▪ Polycaprolactone (PCL), type CAPA™ 6800 (Solvay Caprolactones, UK). Its density was 1.1 g/cm3, melt flow rate 7.3 g/10 min (2.16 kg, 160 °C); ▪ Copper(II) acetylacetonate of the chemical formula: [CH3COCH = C(O-)CH3]2Cu, further in this article referred to as Cu(acac)2 (Sigma-Aldrich, USA), with molar mass 261.76 g/mol, melting point Tm = 245 °C, purity 97%; ▪ Copper(II) oxide, CuO (Sigma-Aldrich, USA) of molar mass 79.55 g/mol, Tm = 1326 °C, and purity 98%. ▪ Metallization bath, type M-Copper 85 (MacDermid, USA) with addition of formaldehyde (36 wt%) as reducing agent (POCH, Poland). 2.2. Sample preparation
Composites
PLA (wt%)
PCL (wt%)
CuO (wt%)
Cu(acac)2 (wt%)
A B C D
80 75 70 65
0 5 10 15
15 15 15 15
5 5 5 5
PLA/PCL (mass ratio)
KIC (kJ/m2)
σM (MPa)
E (MPa)
100/0 95/5 90/10 85/15 70/30
2.9 3.1 3.1 3.1 5.7
72.0 66.4 62.5 54.9 54.9
3299 3079 2961 2696 2534
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Fig. 1. Graphical representation of morphological evolution dependent on the ratio of PCL and PLA [22].
coated with thin gold layers. Chemical composition of the composite surface layers was analyzed by means of energy-dispersive X-ray spectroscopy (EDX). An EDX analyzer was coupled to a SEM microscope at a distance of about 15 mm from the composite surface. An average elemental composition of the composites was determined based on data collected from the scanned area of about 4.5 mm2 or from indicated local areas on the composites surface. Surface chemical properties of the treated samples were further determined by X-ray photoelectron spectroscopy (XPS) using a Versaprobe II™ instrument (Ulvac-Phi, Japan). This machine allows spatially resolved XPS measurements on a μm-scale, as the diameter of the X-ray beam can be focused to below 10 μm.
However, KIC significantly increased at 30 wt% of PCL content at which PCL transforms from a dispersed to more continuous phase which is graphically represented by the middle image in Fig. 1. For further examination compositions with up to 15 wt% of PCL were chosen in order to mix them with CuO and Cu(acac)2 filler. These compositions provided more densely dispersed PCL, and thus more densely dispersed copper compounds. As a general rule it is considered that in polymer blends the low viscosity phase has better affinity to entrap filler particles than the high viscosity phase. Therefore, it was expected that copper compounds will be located preferably in the PCL phase. In the examined composites with 15 wt% of CuO and 5 wt% of Cu(acac)2, KIC increased by 50% with increasing PCL content, whereas
3. Results and discussion 3.1. Characterization of PLA/PCL biocomposites As preliminary research mechanical testing of PLA/PCL blends at various ratio was done in order to select appropriate blend ratio for further examination. Impact strength (KIC), tensile strength (σM) and Young modulus (E) were determined. It was found that with increasing content of PCL σM and E decreased (Table 4).
Table 5 Impact strength (KIC), tensile strength (σM) and Young modulus (E) for composites A, B, C and D. Composite
KIC (kJ/m2)
σM (MPa)
E (MPa)
A B C D
2.1 1.9 2.4 3.2
47.8 46.1 45.6 43.0
2950 2737 2688 2468
Fig. 2. Mass loss and its temperature derivative for powder Cu(acac)2.
Fig. 3. SEM images of samples A and D after etching with acetylacetonate.
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Fig. 4. EDX measurements: Cu content in A, B, C and D samples etched with acetylacetonate.
Fig. 6. EDX measurements: Cu content in metallized layers on the composites with various PCL content (samples A4–D4 modified with 300 laser pulses).
σM and E decreased by 10% and 16%, respectively, at the same time (Table 5). Due to the potential application of these composites in electronic devices, resistance to dynamic stress is expected to be more important than the tensile strength or Young modulus. However, the application of PLA/PCL blends to produce these composites is justified by one more important effect. By introducing PCL the temperature of extrusion could be reduced to 150 °C due to the low melting temperature of PCL of approximately 58 °C. In contrast, PLA melts at 160 °C. This reduction in processing temperature allows to limit degradation of Cu(acac)2. As presented in Fig. 2 Cu(acac)2 is thermally stable up to about 150 °C. Degradation of Cu(acac)2 during processing is undesirable because it can initiate degradation of the polymer matrix.
In order to evaluate the dispersion of PCL and copper compounds, the composite samples were immersed in acetylacetonate, which is a good solvent for PCL but poor for PLA. After 24 h immersion, the composite samples and the dissolved phase were examined. FTIR analysis revealed that the dissolved polymer was PCL and black powder of CuO and blue powder of Cu(acac)2 were clearly visible after evaporation of the solvent. The treated composite samples were analyzed by means of SEM and EDX. As could be expected from the analysis of the dissolved phase, SEM micrographs clearly show increasing surface etching with increasing PCL content of the composite (Fig. 3). SEM analysis also confirmed that the PCL phase was not continuous but dispersed in PLA in the form of droplets. These composites were also examined using EDX analysis. For the samples with higher PCL content lower Cu content was detected after etching (Fig. 4).
Fig. 5. Metallization effects on laser irradiated composites.
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Fig. 7. SEM images of laser irradiated and metalized composites: A, B, C and D. Laser irradiation has been carried out at 300 laser pulses (samples A4–D4).
Fig. 8. SEM images of composites A, B, C and D irradiated with 300 laser pulses (samples A4–D4, no metallization).
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Fig. 9. SEM cross-section images of composites A, B, C and D irradiated with 300 laser pulses (samples A4–D4, no metallization).
It is assumed that this results from heterogeneous distribution of copper compounds within the matrix of blended PLA/PCL because these compounds are preferably located in the PCL phase. Therefore after chemical etching, they are removed with the dissolved PCL phase. In effect, post-treated composites with higher PCL content contained less copper compounds. 3.2. Effects of laser irradiation and metallization The composites were irradiated with various numbers of laser pulses and then metalized in an electroless process. Effects of metallization are shown in Fig. 5. With increasing number of laser pulses better copper coverage was observed for all studied samples. Moreover, the higher was the content of PCL the better were metallization effects. Sample D was well visibly coated with copper even after 50 laser pulses. These effects are well reflected by the results of EDX analysis (Fig. 6). As follows from Fig. 6 deposited copper constituted a maximum of 50 at.% for sample D, which indicated incomplete coverage of the composite surfaces. This is well confirmed by SEM images where only locally deposited copper agglomerates are visible (Fig. 7).
Copper aggregates are identified as the bright areas in the SEM images. It can be taken from the micrographs that not only the overall copper content, but also the deposition morphology varies with the PLA/ PCL blend ratio. Samples with high PCL content exhibit a more coarse copper distribution after metallization. In order to elucidate this effect of PCL on quantitative and qualitative copper deposition, laser irradiated composites were analyzed by SEM and EDX analysis before metallization. Fig. 8 presents surface morphologies of the composites A, B, C and D irradiated with the same number of laser pulses. In general, the SEM analysis does not reveal significant differences between the composites with different PCL content. The SEM micrographs shown in Fig. 8 indicate that local agglomeration of copper occurs upon laser radiation, which has also been confirmed by EDX analysis (EDX-mapping). However, differences between the irradiated composites were visible in SEM images taken from cross-sections of irradiated samples (Fig. 9). As follows from Fig. 9 with increasing PCL content the composite surface showed an increased roughness/waviness. To summarize this part of SEM analysis, distribution of copper agglomerates on the composite surface is similar while the surface roughness increased with
Fig. 10. Laser-induced surface alteration in composites A, B, C and D.
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Fig. 11. Cu content detected by EDX in the composites varied with PCL content irradiated with 300 laser pulses (samples A4–D4, no metallization).
Fig. 13. X-ray spectra from points 1 and 2 of composite A shown in Fig. 12.
increased PCL content. A graphical model for these surface alterations is presented in Fig. 10. When assuming the model of surface changes in Fig. 10 the ratio of surface area with copper to the surface area without copper decreases with increasing surface roughness. Thus using the EDX technique and scanning with the electron beam large surface areas (4.5 mm2) of the composites it was possible to detect a decreased copper content for the composites with higher PCL content (Fig. 11). The trend in the content of Cu well fits the change in geometrical structure. The biggest change occurred between sample A and B, then the changes were less significant (compare Figs. 9 and 11). In view of these observations, namely referring to the apparent dependence of copper content in irradiated domains on PCL content, it had to be clarified, whether differences in the structure and composition of copper agglomerates were the reason that the composites with higher PCL content were more effectively metalized. EDX analysis at specified positions in areas of copper agglomerates and between them was performed in order to answer this question. Results of these analyses are shown in Figs. 12–15. Figs. 12 and 14 show high-resolution SEM micrographs of composites A and D, i.e. at 0 and 15 wt% PCL content, respectively. Brighter local areas represent rather confined copper domains, whereas between
these domains practically no copper is detected. This is clearly confirmed by EDX analysis at the indicated points 1 and 2. The relevant spectra are shown in Figs. 13 and 15. The important difference in the elementary composition of the copper domains in composites A and D according to EDX are the strong signals attributed to the carbon and oxygen elements in the case of composite A (Fig. 13). It has to be concluded that polymer macromolecules are still present in the copper domains. However, in the case of composite D these emission bands were drastically reduced in comparison to the Cu signal. This indicates that bright agglomerates are composed mainly of copper with only negligible content of polymer macromolecules. This fundamental difference between the composites can be assumed to be responsible for better coverage with copper composites with higher PCL content. Using EDX analysis it is not possible to determine whether detected copper occurs in metallic form or in oxidized binding states (e.g. CuO and CuO2). From the viewpoint of metallization the outermost surface layer of the composite is most crucial, which is in contact with copper ions present in metallization bath. If this outermost composite surface has copper in metallic form, the surface will have autocatalytic properties. Therefore, XPS analysis was performed in addition to EDX. By means of this method information of the Cu bonding states in a very thin nanometric surface layer of the composites could be attained,
Fig. 12. Specified points (1, 2) for EDX analysis on the surface of composite A irradiated with 50 laser pulses (samples A2, no metallization).
Fig. 14. Specified points for EDX analysis on the surface of composite D irradiated with 50 laser pulses (samples D2, no metallization).
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4. Conclusions
Fig. 15. X-ray spectra from points 1 and 2 of composite D shown in Fig. 14.
whereas EDX analysis relates to a surface layer having a thickness of several microns and is unspecific to the bonding states of an element. Using a 20 μm X-ray beam it is possible to perform XPSmeasurements of only the copper domains, as identified by the EDX analysis. Fig. 16 on the left shows the photoelectron Cu 2p3/2 signal from such a domain for sample A4 (irradiated with 300 laser pulses) whereas the beam position and its diameter are depicted on the right in Fig. 16, which presents a scanning X-ray induced secondary electron image (SXI). Based on this measurement it was clearly stated that copper occurs largely in metallic form because no oxide satellite structures around 940 eV were observed in the XPS signal [28]. An estimation on the basis of these data suggests a maximum oxide concentration of 5 at.% copper oxide. Thus, it was concluded that under laser irradiation the copper compounds are reduced to copper in metallic form. Aside from copper, only silicon, oxygen and carbon are detected on the surface of the samples. Because of significant local variations in geometrical and chemical surface structure, no clear trend in the atomic concentration of copper on the sample surface in dependence of the surface structure could be identified.
PCL as a biodegradable thermoplastic was added at various contents to PLA and mixed with constant content of 15 wt% of CuO and 5 wt% of Cu(acac)2. Using PCL the processing temperature was reduced to 150 °C, whereas impact strength increased by more than 50% with increasing content of PCL up to 15 wt%. The reduced temperature during composite extrusion minimized the effects of Cu(acac)2 degradation, which starts above 150 °C, as proved by thermogravimetric measurement. It was found based on SEM/EDX analysis that copper compounds were preferably located in PCL than PLA phase of the extruded composites. Moreover, PCL was dispersed in the form of droplets in a continuous PLA volume. The droplets had similar sizes. Probably for that reasons copper agglomerates formed by laser radiation had also similar sizes and differ only slightly with increasing number of radiation pulses. It was found that the same laser irradiation caused higher surface roughness in composites with higher PCL content. This is assumed to explain the decrease of Cu coverage with increasing PCL content as detected by EDX because the surface area covered with copper domains decreased relatively to the effective surface area of uncovered regions developed during laser radiation. The positive effect of increasing PCL content on metallization effects was determined based on general optical assessment and EDX analysis of the copper coated layers. The origin of this positive effect is attributed to the difference determined in the chemical structure of the laserformed copper agglomerates prior to metallization. The higher was the PCL content the more copper content in bright agglomerates was detected. This difference between the composites of varying PCL content can be responsible for better coverage with copper in the case of composites with higher PCL content. XPS analysis of laser-processed samples prior to metallization proved that laser irradiation cause the formation of copper in metallic form, which generally is expected to be most suitable to initiate reduction of copper ions from metallization bath. Layers of metalized copper on the biocomposites surface were not continuous, thus their applications appears more suitable as electromagnetic shielding for biodegradable casing of modern electronic devices. Further work is required in order to fabricate continuous layers for use as conductive layers in integrated circuits. The two-polymersystem and various copper compounds were applied to manufacture
Fig. 16. Left: Cu 2p3/2 signal of a copper domain on sample A4 (no metallization). Right: 500 μm × 500 μm SXI of sample A4 with the position of the 20 μm X-ray beam during the measurement.
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