A novel approach to obtain conductive tracks on PP/MWCNT nanocomposites by laser printing†

G. Colucci*^a, C. Beltrame^a, M. Giorcelli^a, A. Veca^b and C. Badini^a
aDipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129, Torino, Italy. E-mail: giovanna.colucci@polito.it
^bCRF, Centro Ricerche FIAT, Strada Torino 50, Orbassano, 10043, Torino, Italy

First published on 10th March 2016

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Introduction

In the last years CNTs-polymer nanocomposites have been widely investigated and have shown outstanding mechanical, thermal and electrical properties. By now, hundreds of publications have reported certain aspects of the mechanical and electrical enhancement of different polymer systems by CNTs. These studies have been discussed in some important reviews.^1–5

The variation of many parameters, such as CNT type, growth method, chemical pre-treatment as well as polymer type and processing strategy has given some encouraging results in fabricating relatively strong CNTs-polymer nanocomposites. The goal of these reviews was to examine a large collection of published data in order to extract general conclusions about the dependence of the above mentioned parameters on the mechanical, thermal and electrical behavior of nanocomposites.^1–5 The researchers have tried to translate the excellent properties of individual CNTs to larger assembled components. However, at the state of art, their full potential has not been reached when combined with polymer matrices in nanocomposites.

The interaction between CNTs and polymers is believed to play a key role in determining the overall properties of the final nanocomposites. The interfacial characteristics directly influence the efficiency of CNTs reinforcement in improving electrical, mechanical, and thermal properties of the polymer nanocomposites.⁵

In literature, the interactions of CNTs-based nanocomposites have been investigated at three different levels: between different shells of MWCNTs,^5,6 between different CNTs in a bundle,⁷ and between CNTs and the polymer matrix.⁸ The first two types of interaction are only related to the intrinsic CNTs characteristics, such as type of nanotubes or number of nanotubes in a bundle. However, the CNTs-polymer interaction depends on the characteristics of both constituents and how they interrelate with each other. CNTs possess extremely high thermal and electrical conductivity, a negative coefficient of thermal expansion, and superior mechanical properties. Incorporating CNTs into polymer matrices influences nanocomposite properties accordingly. The effect of the CNTs is more pronounced when the polymer-carbon nanotubes interaction is stronger.^8–10 It should be noted that the aforementioned interaction improvement techniques may be employed differently for thermoplastic^9–11 and thermoset¹² polymers. Thermoset polymers cure irreversibly by forming a three-dimensional network of cross-linked chains. On the other hand, the interactions between the polymer chains in thermoplastics occur mostly through van der Waals forces, dipole–dipole interactions or hydrogen bonds. Thus, thermoplastic polymers may undergo phase transition by applying or removing heat. The full potential of CNTs in polymer nanocomposites will be reached through realistic modeling techniques accompanied by innovative experimental procedures. Generally, the methods employed to prepare polymer-based nanocomposites filled with CNTs are the in situ polymerization,^1–4 solution mixing,^1,2 and melt blending.^1,13,14 Among them the melt processing provides less cost in a very short time and does not have limitations due to environmentally reasons, because it is solvent free. Therefore, this technique is useful to produce CNTs-polymer nanocomposites because the tendency of the CNTs to form aggregates and agglomerates may be minimized by appropriate applications of shear during the melt mixing.^13,14

The present work aims to introduce an attractive and useful method to obtain metal free conductive tracks on the surface of thermoplastic polymer nanocomposites, prepared by dispersing MWCNTs within a polypropylene matrix by melt blending, using the effective action of the laser printing.

Specifically, the laser technique promotes an action of ablation on the polymer surface, leading to the formation of conductive tracks by means of the matrix removal, guaranteeing minimal mechanical and thermal deformation.^15–17 The main purpose is to develop conductive tracks on polypropylene surface that could be used as integrated wiring, replacing the commonly used metal wires for electrical transmission, actually obtained by lithography.^18–20 Although the laser process is considered very promising for obtaining conductive tracks on polymers from industrial point of view, as the patents present in literature reveal,^21–23 for our knowledge, it is not yet well-investigated from scientific point of view. In fact, only one paper it was found where metal free conductive tracks were obtained by laser on polyethylene with the aim to improve the electrical properties of the polymer matrix by adding CNTs.²⁴

For our study, the choice to work with polypropylene was made considering that it is the most widely used polymer in the automotive field, because of its low cost, high processability, light weight, relatively low environmental impact with respect to all the other polymers, and also because it can be reprocessed several times guaranteeing very good final properties.²⁵

The paper describes how to obtain surface conductive tracks on polypropylene-based nanocomposites by the presence of MWCNTs using the laser processing and also investigates the electrical and thermal behavior and the effect of the laser irradiation on the microstructure of the CNTs-based nanocomposites obtained using this approach. The main purpose was to obtain in a very simple way electrical wirings directly embedded to the polymer avoiding the use of the common metal wires for the electrical transmission.^18–20 A complete investigation was carried out in order to study the effect of the laser processing on the polymer matrix and the influence of the MWCNTs on the electrical, thermal and morphological properties of the final polymer-based nanocomposites.

Experimental

Materials and methods

The starting materials were polypropylene Hostacom CR 1171 G with a melt flow index of 11 g/10 min at 230 °C, filled with 10 weight percent of inorganic mineral and few percentages of carbon black used as pigment, purchased from Lyondellbasell (Netherland) and thin multi-walled carbon nanotubes NC7000, produced via the catalytic carbon vapor deposition process from Nanocyl (Belgium).

A masterbatch of PP containing 15 wt% of MWCNTs were prepared by melt blending. The specifications of the MWCNTs in the masterbatch are as follows: average diameter of 9.5 nm, average length of 1.5 μm and purity >90%. Then, nanocomposites with different wt% of MWCNTs were prepared by diluting the masterbatch with the neat PP matrix by an internal mixer HAAKE PolyLab QC at 190 °C for 8 minutes with a speed of 50 rpm. After pelletizing, the nanocomposite granules were re-melted and the final specimens obtained by injection molding by using a Sandretto MICRO 65 machine for thermoplastics.

The basic parameters included an injection time of 1.8 s, injection speed of 30 cm³ s⁻¹, pressure of 411 bar, cold time of 35 s and total time cycle of 54 s. To avoid thermal degradation phenomena, the temperatures in the three main zones of the equipment were carefully selected at 210, 205, and 200 °C, respectively.

Laser printing

The laser machine consists in a CO₂ laser Towermark XL (Lasit) emitting in the IR range (λ = 10600 nm) equipped with scanning heads and focusing optics with a maximum power of 100 W operating in pulsed emissions. The laser irradiation leads to the formation of tracks of different length, width, and geometry depending on the parameters set up on the rectangular polymeric plates obtained by injection molding. Specifically, the laser beam acts an action of ablation on the polymer surface, leading to the formation of a conductive track by means of the matrix removal with minimal mechanical and thermal deformation, as sketched in Fig. 1. These advantages derive from the ability of the laser ablation to deposit high energy onto a polymer in a very short time, before thermal diffusion can occur, as reported in literature.^14–16 As a result, the heat-affected spot of material, where melting and solidification can occur, is significantly reduced, leading to structured features smaller in size, with high aspect ratio and greater spatial precision. The laser treatment leads to the thermal degradation of the polymer matrix because of the high energy absorbed on the polymer surface and the high temperatures generated.^14–16 These two important effects, induced to the laser printing process, can be explained considering the intrinsic nature of the polymer matrix.

In fact, polymers are characterized by long macromolecular chains consisting of sequences of monomers, molecular groups of the same nature, strictly bonded with strong covalent bonds with adjacent molecular groups of the same chain and interconnected by weak bonds with neighboring groups that belong to different polymer chains. In this way, the removal of molecular groups from the surface of the polymer matrix during the laser ablation can involve the breaking of intra-chain covalent bonds or the breaking of the weak bonds which connect a single chain with the surrounding material, as well known from literature.^14–16

Characterization techniques

The electrical resistance of the PP2CNTs nanocomposites were investigated on the surface of ten parallel tracks for each sample. Their electrical resistance was measured by changing each time the laser parameters in terms of frequency (F in kHz), power (P in %), writing speed (v in mm s⁻¹), number of repetitions (N), and defocusing (D in mm) by using a two-point probe technique using a digital Multimeter Keithley 2700E R2W equipped with standard steel probes and a full-scale of 120 MΩ.

The MWCNTs nanocomposites microstructure was investigated, inside and outside the track, focusing the attention on the PP2CNTs by scanning electron microscopy (SEM) using a FEI Quanta 200i 3D Dual Beam Instrument with a field emission tungsten filament. Before the analysis, the specimens were cryo-fractured and all the surfaces of the nanocomposites metallized with chromium.

The morphology of the 3D surface of the conductive tracks obtained after laser irradiation was also investigated by means of a Profilometer confocal microscope Leica DCM8, which offers ultra-fast XY topography stitching. In this mode, acquired 3D models are automatically joined together to form a topographic image larger than a single field of view.

The final surface data shows a seamless, highly precise model of a large surface area including perfectly in focus texture, while keeping the original optical properties of the smaller section.

The XPS studies were also carried out by PHI 5000 Versa probe scanning X-ray photoelectron spectrometer (monochromatic Al k-alpha, 1486.6 eV energy, 15 kV voltage and 1 mA anode current) in order to investigate the surface chemical modification on-track and outside the track. A spot size of 100 μm was used to collect the photoelectron signal for both the survey and the high resolution spectra. Different pass energy values were also employed: 187.85 eV for survey and 23.5 eV for high resolution peaks.

The XPS spectra were collected before and after 2 minutes of argon ion sputtering at 2 kV, made to remove surface contaminations eventually present because of the air exposure. The spectra were all analyzed using a Multipack 9.6 software.

Raman spectra were recorded by means of a Raman Renishaw with a green laser (514 nm) source on the neat PP and on the PP2CNTs nanocomposites on-track and outside the track.

Thermal gravimetrical analysis (TGA) was performed, in air and in argon, with a Mettler-Toledo TGA/SDTA 851e instrument in the temperature range between 25–700 °C with a heating rate of 10 °C min⁻¹ in order to evaluate the thermal behavior of the PP2CNTs nanocomposites. All curves were normalized to the unit weight of the samples.

Differential scanning calorimetry analyses (DSC) were performed with a Netzsch DSC 204 F1 Phoenix System in the temperature range from −50 to +250 °C with a heating and cooling rate of 10 °C min⁻¹ under nitrogen flow (50 ml min⁻¹). For each experiment the sample was heated from −50 to 250 °C, then cooled from 250 to −50 °C for two times. The first heating/cooling cycle was recorded in order to eliminate the thermal history of the samples. The thermal transitions of the nanocomposites were then measured on the second heating/cooling cycle, and discussed.

Results and discussion

Effect of laser printing on the electrical properties

As well-known from literature,^26–28 the electrical properties of the PP-CNTs nanocomposites are strongly dependent on the CNTs content and on their dispersion within the polymer.

The present work was focused on the study of the effect of the laser printing on the electrical conductivity of PP2CNTs nanocomposites. In fact, it was found that an amount just above 2 wt% of MWCNTs represents the percolation threshold (see Fig. S1 in ESI†), guaranteeing good values of electrical resistance for PP-based nanocomposites on-track but relatively low inter-track resistance values which can lead to short circuits. On the contrary, a percentage of CNTs of 2 wt% gives rise to good values of electrical resistance on-track and very high values of resistance inter-track (see Table S1 and Fig. S2 in ESI†). In this frame, a systematic study was carried out in order to evaluate the changes on the electrical resistance of the PP2CNTs tracks obtained by tuning the laser parameters in terms of power, frequency, writing speed, number of repetitions and defocusing (see Table S2 in ESI†). Here, we report only the results of laser treatments which give rise to the formation of conductive tracks on the PP2CNTs surface without any sort of deformation on the sample, in order to preserve its structural integrity, and also without inter-track conductivity, in order to avoid short circuits. For this reason, different set of laser parameters were changed taking into account firstly different laser defocusing (0, 50, 100 and 150 mm) which is considered to play a crucial role on the final electrical properties. The width and depth of the laser tracks significantly change by varying the defocusing of the laser beam, and thus the sample deformation. Fig. 2(a) reports examples of laser printing process. Here the behavior of the electrical resistance of the PP2CNTs tracks is described as a function of the number of repetitions, by changing the laser defocusing from 0 up to 150 mm, and leaving constant the power of the laser (20%), the frequency (10 kHz) and the writing speed (150 mm s⁻¹). As it is possible to see, the electrical resistance of the tracks varies depending on the defocusing; when the number of repetitions exceeds 30, the values of resistance are higher for higher values of defocusing. However, after 40 repetitions for defocusing of 50 and 100 mm the electrical resistance values are lower than 1 kΩ cm⁻¹ and the tracks show only little deformations. On the contrary the specimens tested with zero defocusing does not give good results of electrical resistance nor in terms of shape, depth and deformation of the track.

The best results, in terms of compromise between electrical resistance values and lack of deformation, seem to be obtained by using a defocusing of 150 mm. Fig. 2(b) describes the role of the laser power on the final resistance of the PP2CNTs tracks. The curves report the trend of the track electrical resistance as function of the number of repetitions, obtained by fixing the frequency at 10 kHz, the writing speed at 150 mm s⁻¹, and the defocusing at 150 mm and changing the power of the laser beam up to 50%. Among the laser treatments carried out, the tracks obtained at lower values of power (20 and 30%) reveal good electrical conductivity and do not show deformation; on the contrary higher values of power lead to serious damages of the PP2CNTs plates.

Changing the laser parameters and working with a fixed defocusing of 50 mm, power of 15% and frequency of 0.1 kHz, it is also possible to follow the trend of the electrical resistance by varying the number of repetitions depending on the writing speed, as shown in Fig. 2(c). The resistance values decrease by increasing the number of repetitions and at low writing speed. Unfortunately, a speed of 100 and 150 mm s⁻¹ lead to the sample deformation; only laser treatments at higher speed (200 mm s⁻¹) give rise to tracks with good electrical conductivity without any sort of deformation.

The electrical resistance of the tracks as a function of the writing speed was also studied varying the number of repetitions, fixing the defocusing at 50 mm, the power at 15%, and the frequency 15 kHz. Fig. 2(d) shows that, at relatively lower writing speed (150 mm s⁻¹), it is possible to repeat the laser writing more than 20 times because this treatment gives rise to a useful functionalization process without sample damage. In these conditions the PP2CNTs conductive tracks appear whole and without any visible changes, and show resistance values lower than 2 kΩ cm⁻¹. By increasing the writing speed, up to 250 mm s⁻¹, the conductive resistance trend completely changes by varying the number of repetitions. Lower values of electrical resistance are obtained at high number of repetitions (up to 30); in the other cases the resistance shifts towards higher values.

Moreover, the trend of the electrical resistance was evaluated as function of the writing speed by changing the frequency from 0.1 up to 20 kHz. The curves reported in Fig. 2(e) show that good values of conductivity can be obtained for laser treatments involving higher values of frequency and high writing speed. However, the frequency seems not to influence significantly the final electrical conductivity of the nanocomposites if compared with the other laser parameters. Finally, the trend of the electrical resistance as function of the power of the laser was studied by changing the defocusing. As reported in Fig. 2(f), the electrical resistance of the tracks decreases for higher values of laser power (from 30 up to 40%) independently from the defocusing. In this frame, three different set of laser parameters (T1, T2, and T3) were chosen as best solutions for developing PP2CNTs conductive tracks on the basis of their final electrical resistance values and samples characteristics, as summarized in Table 1.

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These PP2CNTs conductive tracks show resistance values between 3–4 kΩ cm⁻¹ for 50 mm of defocusing, 2–3 kΩ cm⁻¹ for defocusing of 100 mm, and 1.5–2.5 kΩ cm⁻¹ for 150 mm of defocusing. Lower values of resistance (<1 kΩ cm⁻¹) were found after more drastic laser treatments but the samples evidence clear signs of deformation (see Fig. S3 in ESI†), and for this reason these conditions were not considered useful for the final purpose of the work. The best conductive tracks obtained by laser printing treatments were then characterized from morphological and thermal point of view and by XPS and Raman spectroscopy.

Nanocomposites morphology before and after laser printing

To evaluate the effect of the laser processing on the morphology of the PP2CNTs nanocomposites containing MWCNTs, FESEM images were collected on a not irradiated sample and on the tracks generated by laser printing. Fig. 3 shows a comparison between the same PP2CNT nanocomposite before (a) and after (b) the laser treatment. As it is possible to see from the micrographs at 25k×, the untreated specimen does not evidence any trace of the presence of the CNTs. This can be due to the tendency of the MWCNTs to move towards the bulk and because the polymer matrix forms a sort of coating on the CNTs network, known as “skin effect”.²⁹

This is a clear indication that the addition of 2 wt% of MWCNTs to a polypropylene matrix is not a sufficient condition for the formation of a conductive nanocomposite. On the contrary, after laser irradiation, it is possible to observe the changes occurring on the nanocomposite morphology by the analysis of the track surface. The thermal effect of the laser action leads to the formation of a melted polymer phase in the region interested by the laser printing, which is partially stored at the borders of the track (see Fig. S3(a) in ESI†).

In this way, it is possible to see a higher number of MWCNTs emerging from the bulk, after the polypropylene matrix removal.²⁴

Fig. 3(b) clearly shows the presence of CNTs finely and uniformly dispersed within the polypropylene with small amount of agglomerates. At higher magnification (50 and 100k×), the FESEM micrographs (c) and (d) of Fig. 3 put in evidence how the MWCNTs are strictly interconnected among themselves and within the polymer network. This result clearly indicates that the masterbatch dilution process using melt blending is a valuable method to obtain a homogeneous dispersion of MWCNTs in a polymer matrix, as already found previously by other authors.¹³

Surface topography of the PP2CNTs conductive tracks

In addition to the classical microscopic analysis performed to study the morphology of the PP2CNTs nanocomposites before and after the laser ablation, the crater surface profile of the conductive tracks was also investigated by 3D Surface Metrology.

Topographic measurements were carried out in order to evaluate the physical changes occurred on the PP-based nanocomposites containing 2 wt% of MWCNTs after the laser irradiation.

Fig. 4 reports a FESEM image of a generic conductive track, reported as an example, collected at very low magnification (200×) and the 3D micrographs (a–c), measured by 3D topography, on three different tracks obtained after the best laser printing treatments. This analysis was performed in order to compare the morphology of the PP2CNTs conductive tracks prepared by laser processing obtaining accurately three dimension measurements of the track surfaces.

Fig. 4 shows the chromatic scales of the depth (in micrometers) for three different tracks (T1, T2, and T3, as reported in Table 1), taking as reference the value zero for the untreated sample. Fig. 4 also reports the 3D images, obtained by profilometry, of the conductive tracks obtained by changing the laser parameters.

Fig. 4(a) shows the first track (T1) obtained by setting up 50 mm of defocusing, 15% of laser power, with a frequency of 15 kHz, speed of 250 mm s⁻¹ and number of repetition of 25.

The spot profile measured along the dotted line reveals the depth of the track, which has a length of 876 μm, a width of 659 μm and a depth at 404 μm. Fig. 4(b) reports the 3D topographic image of the track T2 obtained by setting up 100 mm of defocusing, 20% of laser power, with a frequency of 15 kHz, speed of 200 mm s⁻¹ and number of repetition of 25. As it is possible to see, the track profile is completely different from the previous one because the crater created by laser with a higher defocusing is larger than the other is. In this condition, the profile of the area investigated shows a length of 873 μm and a width of 1273 μm; the depth has a maximum at 304 μm. For the last sample, two different measurements were carried out because of the bigger dimensions of the track due to the higher defocusing of the laser beam, then the two acquisitions were collected and elaborated by the LeicaMAP software in order to obtain the final 3D image of the track. In Fig. 4(c) it is reported the 3D reconstruction of the morphology of the track T3 obtained by varying again the set-up of the laser process: 150 mm of defocusing, 40% of laser power, with a frequency of 15 kHz, speed of 200 mm s⁻¹ and number of repetition of 25. In this case, the track profile shows an area of 884 × 1630 μm². The 3D topography of the depth, obtained by collecting four consecutive images, reveals a maximum of depth track of 304 μm. The profile appears more irregular, evidencing the presence of humps and cracks, probably because of the effect of the higher defocusing of the laser processing. These results confirm that the reduction of the conductive track depth and width is strictly dependent on the defocusing value of the laser.

PP2CNTs nanocomposites structure: XPS and Raman investigation

XPS and Raman analyses were also carried out in order to investigate the elemental surface composition of PP-based nanocomposites containing 2 wt% of MWCNTs, outside and inside the track obtained by laser printing, taking as a reference the XPS and Raman spectrum of the polypropylene. The XPS survey patterns of the PP and PP2CNTs spectra result almost the same and show the presence of the C1s and O1s main bands, with different intensity, as result of the different atomic concentration of the elements (see Fig. S4 in ESI†). The track analysis of the PP2CNTs nanocomposites, after laser irradiation, shows a lower amount of carbon and, consequently, a higher oxygen concentration with respect to the PP matrix, due to the ablation action of the laser, which leads to the degradation of the polymer because of the high energy absorbed on its surface and to the oxidation occurring at the high temperatures generated. The deconvolutions of the high resolution peaks, after 2 minutes of argon ion sputtering at 2 kV, of the PP and of the nanocomposites in the presence of MWCNTs clarify this aspect. Fig. 5(a) shows the deconvolution of the HR peaks of the PP, revealing that the C1s peak is only due to single C–C and C–H bonds characteristics of the polymer structure, with a small amount of impurities. The O1s and Si2p peaks are mainly due to the presence of SiOx species in the composition of the polymer matrix and used as inorganic filler.

The XPS spectra of PP2CNTs nanocomposites outside the track reported in Fig. 5(b) are comparable with those related to the polymer, evidencing that no trace of the presence of MWCNTs is evident on the nanocomposites surface without the laser irradiation. On the contrary, completely different XPS spectra can be observed from Fig. 5(c) for the PP2CNTs sample after laser printing. Looking inside the conductive track, it is possible to find a broad C1s band, made up by four different peaks, attributed to the CC double bonds (8.9%), the single C–C and C–H bonds (83.6%), the carbon atom singly bound to oxygen in carbonyl groups CO (6.1%), and the carbon doubly bound to two oxygen atoms in carboxyl COO–H, carboxylic anhydrides, and esters COO–O, (1.4%).

This result remarks the crucial role played from the laser printing on the final properties of the PP2CNTs.

The deconvolution of the O1s spectrum results in two peaks due to oxygen atom singly bounded to carbon in C–O bonds and metal oxides. On the basis of the XPS results (summarized in Table S3 in ESI†) for the PP2CNTs nanocomposites showing conductive tracks, it can be underlined that the C1s peak has the typical shape resulting from the presence of graphitic carbon materials, such as CNTs, as also reported in literature.^30–32

Raman spectroscopy is another important tool used to study and characterize the polymer-based nanocomposites containing MWCNTs, because it is suitable to identify the presence of CNTs. In addition, it can give information about MWCNTs degree of dispersion within the polymer and allows to evaluate the interactions between the CNTs and the polypropylene matrix.^5,6,11

Fig. 6 shows the Raman spectra of PP2CNTs nanocomposites before (black curve a) and after (red curve b) the laser printing of a conductive track surface. Two characteristic peaks of the MWCNTs are evident along the electrically conductive track obtained after laser irradiation: the first peak at 1360 cm⁻¹ is attributed to the D band related to the presence of defects on the CNTs graphitic structure. The second peak at around 1590 cm⁻¹ is assigned to the G band and it is due to the sp²-hybridized carbon atoms, and strictly connected with the degree of graphitization of the MWCNTs network.

From Fig. 6, it is also possible to see the intensity of the classical band of the polypropylene matrix in the range 2800–2950 cm⁻¹ due to various types of C–H bonds. This band becomes very weak after laser printing, as also found by other authors.^18,32

The presence of the D and G peaks in several points of samples under study, inside and outside the tracks, have been used to evaluate the uniform degree of dispersion of CNTs in the nanocomposite, as supported by FESEM analysis. The interactions between CNTs and polypropylene matrix has been evaluated as ratio intensities between D and G peaks compare with the characteristic polymer Raman peaks. In the presence of CNTs on the external composite surface, the Raman signal was more comparable with the one obtained inside the track. In the opposite, when CNTs are not well exposed on the external surface, D and G peaks are weak if compared with the characteristic polypropylene matrix Raman peaks.

Thermal properties: TGA and DSC analyses

Thermogravimetrical analysis of the PP-based nanocomposites containing MWCNTs was also carried out, both in oxidizing and inert atmosphere, in order to get a complete thermal characterization. The TGA and DTG curves of the PP matrix and of the PP2CNTs nanocomposites are compared in Fig. 7(a) and (b) for analyses in air and argon respectively. The temperatures at which there is 5 and 50 weight percent of weight loss (T₅ and T₅₀) and the maximum degradation rate temperatures (T_max) were extracted from the TGA and DTG curves, and the results summarized in Table 2. The thermal degradation of the PP matrix and the nanocomposites obtained by incorporating 2 wt% of MWCNTs in air occurs with a maximum decomposition rate temperature at around 414 °C for the thermoplastic matrix and 415 °C for the nanocomposites, as visible in Fig. 7(a). Moreover, the values at which there is a weight loss of 5 and 50 wt% for PP were found at 334 and 400 °C, respectively, while the values of T_max remain almost the same. The addition of CNTs, as already found by other authors,^2,4,25 leads to an enhancement of the thermal stability of the nanocomposites in inert atmosphere, as visible from the thermograms of Fig. 7(b). Under argon flow, the thermal stability of the PP2CNTs increases, by around 12 °C for T₅ and 28 °C for T₅₀, with respect to the pure PP matrix (see Table 2). This improvement in the resistance to thermal degradation for nanocomposites incorporating MWCNTs can be attributed to a protective action of the MWCNTs with respect to the PP matrix and to the ability of the MWCNTs to dissipate heat. As already reported in previous papers,³³ such behavior can be due to a barrier effect implying a hindered diffusion of the polymer degradation products from the condensed to the gas phase, strongly dependent on the interactions between the CNTs and the polymer matrix.

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TGA measurements were also performed to determine the amount of MWCNTs dispersed within the PP-based nanocomposites. After the degradation of the polymer matrix, it was possible to detect at 700 °C the residue values for the composites studied, both in air and in argon (see Table 2). Specifically, the amount percentage of MWCNTs in the nanocomposites, experimentally determined in air, is in good agreement with the theoretical content (2 wt%) introduced within the polymer system, taking into account that the pristine PP matrix contains 10 wt% of inorganic filler and around 3 wt% of carbon black as pigment, as also reported in the supplier data-sheets.

Higher values of the residue at 700 °C are achieved for PP and for PP2CNTs after TGA measurements carried out in argon (14 and 17% respectively) due to the presence of traces of not totally degraded PP matrix in inert conditions.

Finally, DSC analyses were carried out to complete the investigation of the thermal behavior of the PP-based composites in the presence of MWCNTs. The DSC thermograms of the PP matrix, related to the second heating/cooling cycle, show a single crystallization peak, at around 121 °C, and the presence of a little shoulder, at around 112 °C, which can be due to the presence of the thermally less stable β crystalline form of the PP matrix.

The DSC thermograms of the PP2CNTs nanocomposites show only one crystallization peak at 122 °C; the shoulder in this case is less visible (see Fig. S5 in ESI†).

Additionally, from DSC curves it was also possible to study the tendency of the nanocomposites to crystallize during the cooling in the presence of MWCNTs. The degree of crystallinity (X_c) of PP and PP2CNTs was calculated using a well-known equation from literature.^11,22 The X_c values were determined from the heats evolved during the cold crystallization (ΔH_c), taking as a reference the enthalpy of theoretically 100% crystalline PP (ΔH⁰_m) equal to 207 J g⁻¹ and taking into account the weight fraction of the CNTs and of the other fillers present in the composites.

The results, listed in Table 2, show a slight increase of the crystallization degree (from 34.3 to 40.9%) in the presence of MWCNTs, which probably act as nucleating agents, as also reported from the literature.^25,34

The enlargement of the melting peaks of the pure PP and the PP2CNTs nanocomposites reveals the presence of two different peaks, at around 131 and 169 °C for PP, and 128 and 157 °C for the PP2CNTs nanocomposites, attributed to the melting of α-crystals and to the presence of the less stable β-crystal form of the PP, independently from the presence of MWCNTs.

Development of a circuit prototype

To conclude the study, an example of circuit prototype was designed and developed in order to demonstrate that it is possible to obtain an integrated circuit on PP2CNTs nanocomposites by means of a flexible and relatively simple technique as the laser printing.

A circuit was firstly designed by means of the CAD software with the purpose to simulate a quite complex circuit, and then the laser printing process was carried out, as reported in Fig. 8(a) and (b) respectively.

The conductive tracks on PP2CNTs were prepared by setting up a treatment with 50 mm of defocusing, 15 W of power, 15 kHz of frequency, 250 mm s⁻¹ of writing speed and 25 number of repetitions. The prototype was obtained in a very short time, less than 1 minute, as the video reported in the ESI† shows.

Conclusions

A novel approach for obtaining metal free conductive tracks on polymeric nanocomposites based on PP reinforced with 2 wt% of MWCNTs by laser printing has been introduced. The effect of the laser processing on the final properties of the PP2CNTs nanocomposites has been widely investigated by means of the study of the electrical resistance of the tracks, obtained after the laser irradiation, the changes induced to the laser action on the microstructure and the thermal behavior of the nanocomposites. The results clearly indicate that the laser printing is a simple, fast, flexible, and relatively low cost approach to obtain conductive tracks on polymer surface in the presence of MWCNTs. Depending on the laser parameters set up, the electrical resistance varies even reaching very low values (less than 1 kΩ cm⁻¹). However, it is strictly necessary to reach a good compromise between the electrical resistance values and the final integrity of the samples, because the laser printing can induce severe deformation that could damage irreparably the material. Three selected laser treatments (T1, T2, and T3) show the best conditions for developing PP2CNTs conductive tracks on the basis of the final electrical resistance values and sample characteristics. The microstructure of the selected PP2CNTs tracks, investigated by FESEM, reveals a good dispersion of the CNTs within the PP matrix with the presence of few agglomerates of CNTs on the track surface obtained after laser printing. No trace of CNTs has been found outside the track profile because of the tendency of the CNTs to shift towards the bulk and because of the “skin effect” of the PP matrix. The 3D reconstruction of the conductive track surfaces, performed by 3D profilometry, has also allowed to determine the length, width and depth of the tracks obtained by the selected laser conditions.

The presence of the XPS signal related to the carbon–carbon double bound and to the Raman D and G peaks assigned to the CNTs carbon species, also confirm the key role plays from the laser printing technique on the final properties of the PP2CNTs.

Moreover, the results of TGA and DSC analyses indicate that the MWCNTs induce a significant increase of the thermal stability of the nanocomposites and a slight increase of the tendency to crystallize, acting as protective agents with respect to the PP matrix.

Finally, a circuit prototype has been designed and developed to underline how the laser printing is very simple, flexible and useful to obtain metal free conductive tracks embedded to a polymeric thermoplastic matrix in the presence of few amount of MWCNTs.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra02726a

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