Surface modification of low density polyethylene (LDPE) film by low pressure O2 plasma treatment

EUROPEAN POLYMER JOURNAL

European Polymer Journal 42 (2006) 1558–1568

www.elsevier.com/locate/europolj

Surface modification of low density polyethylene (LDPE) film by low pressure O2 plasma treatment M.R. Sanchis a, V. Blanes a, M. Blanes a, D. Garcia b, R. Balart

b,*

a

b

Textile Research Institute–AITEX, Plaza Emilio Sala, 1, 03801, Alcoy, Alicante, Spain Department of Mechanical and Materials Engineering, Higher Polytechnic School of Alcoy, Polytechnic University of Valencia, Paseo del Viaducto, 1, 03801, Alcoy, Alicante, Spain

Received 25 October 2005; received in revised form 18 January 2006; accepted 2 February 2006 Available online 14 March 2006

Abstract In this work, low pressure glow discharge O2 plasma has been used to increase wettability in a LDPE film in order to improve adhesion properties and make it useful for technical applications. Surface energy values have been estimated using contact angle measurements for different exposure times and different test liquids. In addition, plasma-treated samples have been subjected to an aging process to determine the durability of the plasma treatment. Characterization of the surface changes due to the plasma treatment has been carried out by means of Fourier transformed infrared spectroscopy (FTIR) to determine the presence of polar species such as carbonyl, carboxyl and hydroxyl groups. In addition to this, atomic force microscopy (AFM) analysis has been used to evaluate changes in surface morphology and roughness. Furthermore, and considering the semicrystalline nature of the LDPE film, a calorimetric study using differential scanning calorimetry (DSC) has been carried out to determine changes in crystallinity and degradation temperatures induced by the plasma treatment. The results show that low pressure O2 plasma improves wettability in LDPE films and no significant changes can be observed at longer exposure times. Nevertheless, we can observe that short exposure times to low pressure O2 plasma promote the formation of some polar species on the exposed surface and longer exposure times cause slight abrasion on LDPE films as observed by the little increase in surface roughness.  2006 Elsevier Ltd. All rights reserved. Keywords: Surface modification; Contact angle; AFM (atomic force microscopy); Low pressure plasma; Polyethylene film

1. Introduction In recent years a remarkable growth in the polymeric films industry for engineering applications has been observed due to the excellent combination * Corresponding author. Tel.: +34 966528421; fax: +34 966528478. E-mail address: [email protected] (R. Balart).

of properties that can be achieved such as easy processability, surface finishing versatility, excellent barrier behavior, balanced properties (mechanical, thermal, electrical, etc.). All these have enabled the extensive use of the so-called ‘‘technical’’ and ‘‘high performance’’ polymers in applications of high technological content on a film form (packaging, automotive and aircraft industry, electric and electronic equipment, health care, etc.) [1]. Many of

0014-3057/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2006.02.001

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these applications require good adhesion properties [2–4] to enhance good mechanical performance and allow replacing aluminium and steel parts on secondary structures; in this sense, surface phenomena regarding adhesion behavior acquire special relevance on materials study and characterization. Some films exhibit good adhesion properties but the most generalized situation is that polymer films show poor adhesive properties since they are characterized by low surface energy values, chemical inertness and smooth surface. For this reason polymeric films need, in many cases, some additional treatment to raise surface activity, thus enhancing wettability and, consequently, the adhesive properties [5–8]. This situation is particularly remarkable in non-polar polymers such as polyethylene (PE); non-polar polymers need surface treatment to enhance good adhesion properties. If we consider that the different adhesion mechanisms are based mainly on chemical interaction and mechanical interlock, surface treatments must focus on the addition of active species and on the generation of certain surface roughness to improve adhesive performance. In the last years a great interest about material preparation and modification has occurred. The basic principle is that it is possible to change the surface properties without changing the bulk properties of the material. There are many different methods to modify the surface properties of polymeric films such as chemical, thermal, mechanical and electrical (plasma) treatments [2,9–12]. Recently, research on the use of plasma treatments has grown in interest [13–15] since they are environmentally efficient. Low pressure plasma (glow discharge) is a popular technique since it is a dry process and allows better uniformity in the modified surface, so it is widely used for industrial applications [3,16,17]. Depending on the gas used for plasma generation and on the general conditions it is possible to activate a polymeric surface by inserting active species (mainly polar groups), surface abrasion or etching, cross-linking processes [6] or, in many cases, combined effects can be obtained. The use of low pressure conditions allows plasma treatments at low or moderate temperatures; in this way, the aggressiveness of the plasma treatment is considerably reduced and consequently degradation occurs in a less extent. It is possible to generate glow discharge by either radio frequency (RF) or microwave (MW) but at industrial level RF excitation is preferred for surface modification [5,8,11,17]. The different species present in the plasma induce the

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formation of free radicals in the polymeric chains and in this way it is possible to insert or interlock certain functional groups on the polymeric surface; this will have a positive effect on the functionalization/activation of the polymer surface [16] and will improve surface adhesion properties. The present study uses RF oxygen plasma to improve the intrinsic low wettability of a low density polyethylene (LDPE) used for technical applications on a film form. The use of O2 plasma promotes surface modification by both surface activation and slight etching as the main plasma mechanisms. The functionalization of the LDPE surface is characterized by FTIR–ATR analysis and the evolution of the surface contact angles is observed as a function of the exposure time. The evolution of the polar and dispersive contributions to LDPE solid surface energy is determined in terms of the treatment time. Moreover, the durability of the plasma treatment is evaluated by subjecting the sample-treated to an aging process. The etching effects of plasma are analyzed using SEM and AFM analysis to characterize morphology changes caused by the plasma treatment. 2. Experimental 2.1. Materials and sample preparation The film used for the study was a transparent low density polyethylene (LDPE) film supplied by Logoplast (Logoplast SL, Alicante, Spain) for automotive applications with a density q = 0,92 g cm3 and 50 lm thickness. Double distilled water and glycerol with AR grade were used as test liquids for contact angle measurements. Samples of 20 · 20 cm in size were prepared for the plasma treatment and after this, samples of different dimensions were cut for different measurements. For the aging studies, plasma-treated samples were stored in a vacuum desiccator for extended periods of time. 2.2. Low pressure RF plasma treatment Low density polyethylene (LDPE) films were exposed to a radio frequency (RF) low pressure oxygen plasma to change wettability properties. It was used a glow discharge RF generator (operating at 13.56 MHz with a maximum power of 600 W) type CD 400 MC option PC (Europlasma, Oudenaarde, Belgium) which is specially appropriate for surface modification of small size technical parts;

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the plasma chamber consists of a four aluminium shelves for sample holders and a volume of 64 l. The gas used for the plasma generation was oxygen and the working power was fixed to 300 W since higher power values promote important overheating that can perform changes on structure and unnecessary surface degradation. All samples were placed on the upper shelf to compare results because small changes in surface functionalization, attributed to the sample position, were observed. Samples were exposed to O2 low pressure plasma for different times (1, 2, 5, 10, 15, 20, 25 and 30 min). It was used an oxygen flow rate of 100 cm3 min1 and the working pressure varied in the range of 31–32 Pa. All posterior measurements were carried out as soon as possible to avoid aging processes. 2.3. Contact angle measurements and surface energies estimation Static contact angle measurements of the plasmatreated samples were carried out at room temperature on a KSV CAM 200 goniometer (KSV Instruments, Helsinki, Finland) using two different test liquids: water and glycerol. Al least six different measurements on the plasma treated surfaces were obtained and the average values for contact angles were calculated. The maximum error in the contact angle measurement did not exceed ±3%. Surface energies were estimated from the use of theory of adhesion work among solid and liquid phases on which polar and non-polar (dispersive) contributions on interaction processes are considered. The selected method to evaluate surface energy values was Owens–Wendt; this method takes into account dispersive and polar components of the surface energy and using two different test liquids it is possible to determine the solid surface energy (c) as the sum of polar (cp) and dispersive (cd) contribution. The relationship between the equilibrium contact angle of the liquid phase deposited onto a solid phase is derived from the general Fowkes expression which considers the polar and dispersive contributions for both solid and liquid designed as cl and cs with a superscript ‘‘d’’ or ‘‘p’’ for the dispersive and polar contribution, respectively. 1

1

W sl ¼ cl  ð1 þ cos hÞ ¼ 2  ðcdl  cds Þ2 þ 2  ðcpl  cps Þ2 Constant values for the two test liquids used for contact angle measurements are as follows:

Water: cl = 72.8 mJ m2, cdl = 21.8 mJ m2, cpl = 51.0 mJ m2. Glycerol: cl = 64.0 mJ m2, cdl = 34.0 mJ m2, cpl = 30.0 mJ m2. 2.4. Differential scanning calorimetry (DSC) characterization A Mettler–Toledo 821 DSC (Mettler–Toledo Inc., Schwarzenbach, Switzerland) was used for thermal analysis of plasma-treated samples. Samples of 2–3 mg were subjected to a first heating (30–140 C at 10 C min1) followed by a slow cooling and then were heated again (40–250 C at 10 C min1) until degradation, both them under nitrogen atmosphere (with a flow rate of 40 ml min1). Melting temperature was registered as the minimum of the endothermic melting peak in the first heating. In addition, sample crystallinity was calculated by comparing the normalized fusion heat of the sample to the fusion heat of a fully crystalline polyethylene (290 J g1) [9]. Furthermore, degradation induction temperature was determined on the second heating as the onset of the irreversible degradation process. 2.5. FTIR–ATR surface analysis This technique is very useful to obtain the chemical changes induced by the plasma treatment. The infrared analysis was performed on a Perkin–Elmer spectrum BX spectrometer (Perkin–Elmer Espan˜a SL, Madrid, Spain) equipped with attenuated total reflexion (ATR) accessory. 150 scans with a resolution of 4 cm1 were carried out for each one of the plasma-treated samples. 2.6. Surface morphology study Morphology analysis of the plasma treated surfaces for different exposure times was carried out by means of a scanning electron microscope Jeol JSM-6300 (Jeol USA Inc., Peabody, USA) using secondary electrons. Samples were covered with a 5–7 nm Au layer in vacuum conditions prior to each measurement. Atomic force microscopy (AFM) was used to determine surface topography and roughness of the plasma-treated samples. AFM analysis was performed on a Multimode AFM microscope with a Nanoscope IIIa ADCS controller (Veeco Metrology Group, Cambridge, United Kingdom).

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A monolithic silicon cantilever (NanoWorld Pointprobe NCH) with a force constant of 42 N/m and a resonance frequency of 320 kHz was used to work on tapping mode. From the analysis of the images, the root-mean-squared roughness (Rrms) and the maximum height for the topographic profiles measured on 5 lm · 5 lm images, were evaluated. 3. Results and discussion 3.1. Changes in polyethylene surface wettability Low pressure O2 plasma treatment on LDPE film improves surface wettability with a marked hydrophilic nature. Fig. 1 shows variation in the surface contact angles of the polyethylene film for different treatment times in the range of 1–30 min and different test liquids. As it can be observed, the contact angles of the untreated surface (89.3 and 77.9 for water an glycerol, respectively) are considerably reduced after the oxygen plasma treatment even for short exposure times (1–5 min) [18–20] shifting them to lower values, in the 40–42 and 49–51 range for water and glycerol, respectively. This situation is quite interesting since longer exposure times do not cause significant changes in contact angle values but slight abrasion/degradation may occur at exposure times in the 20–30 min range. It is also important to emphasize that in plasmatreated films, the contact angle values of both test liquids are higher than the values corresponding to total wettability (contact angles near 0); this situation indicates that the plasma treatment with oxygen considerably enhances the formation of polar

Fig. 1. Variation of surface contact angles for different exposure times under low pressure O2 plasma for different test liquids.

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Table 1 Surface energies (its polar and dispersive component) for low pressure O2 plasma-treated LDPE surfaces as a function of the exposure time Exposure time (min)

cps (mJ m2)

cds (mJ m2)

cs (mJ m2)

0 1 2 5 10 15 20 25 30

3.7 59.8 59.6 63.3 62.9 66.7 67.0 68.1 67.0

24.2 2.6 3.3 2.6 2.2 2.0 1.6 1.3 1.8

27.9 62.4 62.9 65.9 65.1 68.7 68.6 69.4 68.8

species on the LDPE surface as corroborated by the estimations of the polar and dispersive contributions to the global solid surface energy value [21]. To estimate LDPE solid surface energy the Owens–Wendt method was used; this method is based on a linear approximation of the general Fowkes expression using contact angle values of at least two different liquids (Table 1). The surface energy of untreated LDPE surface is about 28 mJ m2 like in other polyolefins [22,23] (with low polar contribution due to the non-polar nature of the polyethylene structure). Low pressure plasma treatment increases surface energies up to 65 mJ m2 for short exposure times; these values do not change significantly with longer exposure times although slightly higher values are observed for times in the 20–30 min range with surface energies up to 70 mJ m2 with a remarkable increase in polar contribution and a decrease in dispersive contribution [14,24,25]. It is possible to know the different polar groups obtained after the plasma treatment. As it has been described by other authors [19], in a first stage, hydroperoxides are formed on the polyethylene surface; this situation slightly increases polymer surface energy. As we used a medium-power plasma (300 W) in our study, this stage was not observed since the use of short exposure times already led to important surface energy increase. In the next stage, the decomposition of the hydroperoxides promotes the formation of hydroxyl and alkyl radicals, which are characterized by a high reactivity. Finally, some polar groups are formed on the polyethylene surface due to the removal of some hydrogen atoms from the polymer chain. These species are mainly responsible for an increase in the polar contributions to the surface energy values. The FTIR–ATR spectra of the LDPE

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Fig. 2. FTIR–ATR spectra of O2 plasma-treated LDPE surfaces for different exposure times.

plasma-treated samples at different exposure times clearly show the evolution of the polar groups on the solid surface (Fig. 2). These polar groups consist mainly of carboxyl, carbonyl and hydroxyl groups [19–21] resulting from the interaction between the LDPE surface and the O2 plasma. The presence of these polar groups strongly contributes to increase the hydrophilic nature of the LDPE surface. The presence of carbonyl groups is evident from the observation of the FTIR spectra since the C@O stretch absorbs in the range 1750–1600 cm1. The presence of carboxyl groups is confirmed by the presence of a strong peak around 1650–1560 cm1, which corresponds to anti-symmetrical deformation of COO groups and a weak peak in the range 1400– 1310 cm1 corresponding to the symmetrical deformation of those groups. Furthermore, the presence of a strong peak in the 680–580 cm1 and 3700– 3600 cm1 ranges can be indicative of the presence of hydroxyl groups as a result of the O2 plasma treatment. Fig. 2 shows an increasing contribution of oxygen containing groups to the FTIR spectra as the exposure time increases. The plasma treatment produces a slight increase on surface roughness which can affect FTIR spectra but the main contribution is attributed to surface functionalization. It is important to note that plasma functionalization occurs by insertion of active species (oxygen containing species) in the free radicals generated during the plasma treatment and some functionalization can be achieved after plasma treatment; when plasma-treated samples are exposed to ambient atmosphere,

Fig. 3. Evolution of the aging effect on plasma treated LDPE surface for different test liquids: (a) water contact angles, (b) glycerol contact angles.

oxygen from the air can rapidly react with some free radicals and this contributes to intensify surface functionalization phenomenon [26]. This phenomenon is more intense on samples exposed to long treatment times since greater amounts of free radicals are generated. Similar behaviour is observed in other polyolefins [27]. If we consider that low pressure O2 plasma treatment considerably enhances polar contributions, then it is important to determine the durability of this plasma treatment and to study the effect of the aging process on surface wettability. As it can be observed in Fig. 3(a) and (b) for the two test liquids, water and glycerol respectively, the surface activation induced by the plasma treatment is not permanent; this indicates that the plasma-treated surface undergoes an aging process that can be attributed to a re-arrangement of the hydrophilic

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groups (most of them with low stability) obtained during the O2 plasma exposure [19]. Most of these polar groups change their orientation towards the bulk thus reducing the hydrophilic nature of the treated surface. It is also important to note that not all the hydrophilic nature achieved by the plasma treatment is lost during the aging process. This is because the plasma treatment not only promotes the formation of polar groups but also increases surface roughness which improves wettability and, what is more important, this component remains constant during aging. The aging process can be clearly observed by the evolution of the contact angles with the store time. As it has been described, contact angles values for the two test liquids tend to reverse towards their original values. Oxidation reactions start at the surface but the polar groups (oxygen containing species) generated during the plasma treatment tend to rotate and bury themselves below the surface (re-arrangement process) and this phenomenon is mainly responsible of the hydrophobic recovery since most of the hydrophilic properties are lost [28]. Although other external factors such as contamination may contribute to the aging process, the re-arrangement of the polar groups plays a major role in the hydrophobic recovery. The aging process occurs in an exponential form [6] but we must make the following consideration: samples treated for short times experience more severe aging since the contact angle values obtained after one-week aging are higher than those obtained in samples treated for longer times, as shown in

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Fig. 3(b) which illustrates the contact angle evolution of glycerol as test liquid with aging time at different plasma exposure times. The initial plasma treatment reduces surface contact angle from 77.8 to 51. Once this value is obtained, aging starts and after a week, the contact angle raises again to 73 and 67 for 2 and 20 min plasma-treated samples respectively. This situation can be explained if we take into account that short-time plasma treatment does not lead to significant increases in surface roughness its main effect being the formation of polar species on the film surface; many of these polar species are re-arranged during aging thus reducing surface wettability. On the other hand, long-time plasma treatments are more aggressive, causing some changes in surface morphology by slightly increasing surface roughness and softening surface aging. 3.2. Changes in surface morphology 3.2.1. Changes in the thermal behavior of the surface In addition to the changes in the polyethylene surface described above, low pressure oxygen plasma presents a similar effect to that observed by exposure to light or other types of radiation [29]. The aging process occurs as a consequence of some breaks on the polymer chain; this situation allows the occurrence of some phenomena such as cross-linking, free radical formation, etc., with dramatic effects on the final performance of polymeric materials. Aging is much more severe in semicrystalline polymers like low density polyethylene since the breakage of some

Fig. 4. DSC curves of O2 plasma-treated LDPE surfaces for different exposure times.

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Table 2 Calorymetric parameters of the melting peak and degradation onset for low pressure O2 plasma-treated LDPE surfaces as a function of the exposure time Exposure time (min)

1st Heating T peak (C)

Enthalpic content (J g1)

Crystallinity (%)

0 1 2 5 10 15 20 25 30

111.1 112.4 112.2 112.2 111.0 107.9 111.3 111.5 111.2

64.5 65.5 67.1 74.8 90.3 91.8 97.4 93.8 87.3

22.2 22.6 23.1 25.8 31.1 31.6 33.6 32.3 30.1

2nd Heating degradation onset (C)

216.7 215.3 213.0 215.8 215.0 212.6 210.9 207.8 208.5

polymer chains can induce an increase in crystallinity and a subsequent increase in brittleness. As aforementioned, plasma treatment promotes different interaction phenomena between the treated gas and the polymer surface. The breakage of some polymer chains causes the re-arrangement of the polymer chains present on the polymer surface; this phenomenon can be appreciated by calorimetric analysis (Fig. 4). Untreated samples and short-time plasmatreated samples show great similarity in their calorimetric curve shape, and no important changes in crystallinity are observed (Table 2). These calorimetric curves are characterized by the presence of a melting peak around 111 C. Nevertheless as the O2 plasma exposure time increases, the shape of the calorimetric curve dramatically changes. Samples with more than 10 min exposure time to O2 plasma show two different melting peaks [30]; the main melting peak around 111 C and an additional peak that varies its position in the range of 65–95 C. The appearance of this additional peak can be due to changes in the crystalline structure of the polyethylene, and this phenomenon is responsible for a slight increase in crystallinity (Table 2) [9,30]. This twopeak curve disappears after a slow cooling process under controlled conditions, and then, the second peak does not appear again after a second heating process. This fact confirms that the shape of the curve is related to the structural changes caused by the plasma treatment. Also note that the O2 plasma treatment, in addition to the changes in crystallinity described above, causes changes in the surface degradation temperatures as a function of the exposure time. Plasma treatment favours the formation of a great number

of active species on the polymer surface; these active species accelerate degradation processes at high temperatures [31]. We can observe a slight decrease in the onset degradation temperature as a function of the exposure time with values of 216 C (for untreated or short-time treated samples) up to 208 C (for long exposure time). 3.2.2. Changes in surface morphology One of the plasma-acting mechanisms is based on etching as a consequence of the impact of the gas plasma species on the polymer surface. This mechanism increases surface roughness and contributes to better surface wettability. It is possible to quantify the extent of the etching caused by the plasma treatment by simple weight loss analysis. Weight loss highly depends on the polymer structure and the reactivity of the gas used for the plasma treatment [6]. Fig. 5 shows the weight loss of the polyethylene film as a function of the exposure time to O2 plasma. It can be observed a linear behavior similar to what happens in other polymers [32,33]; the weight loss rate has been calculated from the resulting slope value of 22 lm cm2 min1. This rate is usual in polyolefins treated with O2 plasma [34]. Degradation of the polyethylene surface leads to a small modification of the surface morphology by increasing surface roughness [32], as it can be observed in Fig. 6 that shows SEM micrographs for different exposure times. This change in surface roughness is evident from the observation of Fig. 6, in which a small increase in surface roughness can be appreciated in the log-time treated samples.

Fig. 5. Evolution of the weight loss as a function of the exposure time to low pressure O2 plasma.

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Fig. 6. SEM micrographs of O2 plasma-treated LDPE surfaces for different exposure times ·10.000: (a) untreated, (b) 10 min, (c) 20 min and (d) 30 min.

Nevertheless, quantification of the etching process can not be carried out by SEM analysis on the plasma-treated surfaces. The atomic force microscopy (AFM) technique is very useful to

obtain a 3D representation of the surface topography [21,35,36]. AFM analysis shows that the surface roughness slightly increases with the exposure time and this is indicative of the action of the etching

Fig. 7. AFM 2D roughness profiles of O2 plasma-treated LDPE surfaces for different exposure times.

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Fig. 8. AFM 3D topographic representations of O2 plasma-treated LDPE surfaces for different exposure times: (a) untreated, (b) 10 min, (c) 20 min and (d) 30 min.

process (Fig. 7). Untreated LDPE surface is characterized by low surface roughness values as a consequence of the transformation process (blow moulding). O2 plasma treatment causes a slight change in surface roughness as it promotes etching processes. In a first stage, short exposure times promotes some abrasion among the small peaks but also causes an increase in maximum peak height. As the exposure time increases, we can observe a localized abrasion which generates great number of small peaks, thus slightly increasing surface roughness. A 3D representation of the different plasma-treated sample films (Fig. 8) shows a slightly increase in surface roughness. Furthermore, Table 3 shows the root-mean-squared roughness (Rrms) values obtained from the AFM analysis of the recorded images together with the maximum height for different exposure times to O2 plasma. We can observe a Table 3 Morphology parameters as determined by AFM analysis of O2 plasma-treated polyethylene films Exposure time (min)

Rrms (5lm · 5 lm) (nm)

Rmax (nm)

0 10 20 30

21.05 20.85 24.58 29.89

170.42 168.12 180.26 198.24

slight increase in surface roughness with the exposure time; this phenomenon is related with the weight loss and the etching process induced by the plasma treatment. 4. Conclusions Low pressure O2 plasma treatment on a low density polyethylene film increases surface wettability and this can be attributed to the action of different mechanisms that cause a decrease in the contact angle values of the plasma-treated samples. The estimation of the surface energy provides some information on how plasma treatment acts; O2 plasma treatment greatly enhances polar contributions to solid surface energy values and it is indicative that one of the most important plasma mechanisms is surface activation due to the formation of polar groups (mainly carbonyl, carboxyl and hydroxyl groups) as it can be seen from the FTIR spectra observation. On the other hand, the aging process of the plasma treatment shows clearly that the low pressure O2 plasma treatment is not permanent and almost disappears in a few days. This corroborates that the main plasma mechanism is surface activation by the insertion of polar groups; these polar groups re-arrange with time and this reduces surface wettability. Nevertheless,

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not all improvement in surface wettability is lost during the aging process and this can indicate that some etching occurs during the plasma treatment, which is evident from the weight loss study. The degradation process of the surface causes a change in the surface topography that can be appreciated by AFM analysis. This revealed a slight increase in surface roughness attributable to surface degradation/etching. In addition to surface activation and increased roughness, some structural changes related to crystallinity occur. The breakage of some polymer chains on the surface affects the material thermal behavior by changing the DSC curve shape, increasing crystallinity and decreasing the onset degradation temperature. As a general conclusion, it is important to highlight that low pressure O2 plasma treatment is an interesting and environmentally efficient method to modify the surface of low surface energy polymers and to improve adhesion properties for technical applications; however plasma treatment is not permanent and this may affect its industrial applications. Acknowledgements Authors would like to thank the R + D + i Linguistic Assistance Office at the Polytechnic University of Valencia (UPV) for their help in revising this paper. Also, Microscopy Services at UPV are gratefully acknowledged for their assistance in using SEM and AFM techniques.

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