Laser transmission welding of poly(ethylene terephthalate) and biodegradable poly(ethylene terephthalate) – Based blends

Optics and Lasers in Engineering 90 (2017) 110–118

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Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng

Laser transmission welding of poly(ethylene terephthalate) and biodegradable poly(ethylene terephthalate) – Based blends

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Annamaria Gisarioa, Francesco Venialia, Massimiliano Barlettab, , Vincenzo Tagliaferrib, Silvia Vescob a b

Sapienza Università di Roma, Dipartimento di Ingegneria Meccanica e Aerospaziale, Via Eudossiana, 18, 00184 Roma, Italy Università degli Studi di Roma Tor Vergata, Dipartimento di Ingegneria dell’Impresa, Via del Politecnico, 1, 00133 Roma, Italy

A R T I C L E I N F O

A BS T RAC T

Keywords: Laser welding Poly(ethylene terephthalate) Biodegradable material Analytical modelling Thermal degradation

Joining of Poly(Ethylene Terephthalate) PET and its biodegradable derivatives is of high relevance to ensure good productive rate, low cost and operational safety for fabrication of medical and electronic devices, sport equipments as well as for manufacturing of food and drug packaging solutions. In the present investigation, granules of PET and PETs modified by organic additives, which promote biodegradation of the polymeric chains, were prepared by extrusion compounding. The achieved granules were subsequently re-extruded to shape thin (330 μm) flat sheets. Substrates cut from these sheets were joined by Laser Transmission Welding (LTW) with a continuous wave High Power Diode Laser (cw-HPDL). First, based on a qualitative evaluation of the welded joints, the most suitable operational windows for PETs laser joining were identified. Second, characterization of the mechanical properties of the welded joints was performed by tensile tests. Accordingly, Young's modulus of PET and biodegradable PET blends was studied by Takayanagi's model and, based on the experimental results, a novel predicting analytical model derived from the mixture rule was developed. Lastly, material degradation of the polymeric joints was evaluated by FT-IR analysis, thus allowing to identify the main routes to thermal degradation of PET and, especially, of biodegradable PET blends during laser processing.

1. Introduction Poly(ethylene terephthalate) (PET) belongs to the class of aromatic polyesters. It is widely used in the form of extruded foils or sheets in several applications including manufacturing of sport equipments, medical and electronic devices. After sheet extrusion, PET is commonly mechanically stretched at temperatures above its glass transition (normally, around 70 °C) to enhance its mechanical response by the orientation of the polymeric chains. Good thermal stability and dielectric properties, high chemical inertness, elevated resistance to water and impermeability also characterize commercially available PETs. PETs also feature good compatibility with most of aliments and medicaments, being therefore pivotal to the manufacturing of food and drug packaging items. PET can be also be recycled [1], but, unfortunately, it is not biodegradable or compostable [2]. The aromatic moieties of PETs make them substantially insensitive to hydrolytic degradation and to enzymatic or microbial attack [3]. The ever-stringent demands for biodegradable/compostable polymeric materials are pushing, however, polyesters and, specifically, PET market towards the development of many bio-alternatives [4]. Bio-

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Corresponding author. E-mail address: [email protected] (M. Barletta).

http://dx.doi.org/10.1016/j.optlaseng.2016.10.010 Received 6 February 2016; Received in revised form 18 July 2016; Accepted 6 October 2016 0143-8166/ © 2016 Elsevier Ltd. All rights reserved.

based PETs do not rely on fossil materials, but on non-depleting natural sources. Yet, they are still extremely persistent in the environment [5]. Oxo-biodegradable alternatives to PET consist of conventional PET reformulated by the addition of pro-oxidants. Pro-oxidants are known to be extremely effective to promote the fragmentation of non-degradable polymers, while the occurrence of the last step of polymer degradation by mineralization of the small fragments is still unknown [6]. Plastic fragments can therefore persist in the environment for very long time and be extremely subtle, being able to re-enter the food chain through several routes [7]. Blending of PET with biodegradable polyesters (mostly, with aliphatic polyesters) can pose a serious eco-friendly alternative to conventional PETs [8]. However, the resulting blends may often feature limited mechanical response while their manufacturing costs can be extremely elevated and processing routes are significantly worst [9]. An interesting alternative to the manufacturing of potentially biodegradable PET is by melt blending with immiscible organic phases [10], generally based on olefins, and by embedding potential nutrients for bacteria in these phases. This second phase, normally called dispersed phase, is designed to be incompatible with the primary

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ephthalate (PET) and polypropylene (PP), hence analyzing the relationships of process parameters, molten pool geometry (both width and depth) and shear strength (SS) in LTW process [32]. Mamuschkin et al. investigated LTW of absorber-free thermoplastic [33]. More recently, some investigations concerned both LTW of other materials [34–37] or modelling efforts of the process [38–40]. In contrast, joining and, specifically, self-joining of biodegradable polymers is still sparsely reported (i.e., if any) in the literature. This is therefore the framework in which the present manuscript investigates joining of PET and biodegradable PETs by LTW using a continuous wave High Power Diode Laser (cw-HPDL). PET and biodegradable PETs achieved by the incorporation of different grades of organic additives (5, 10 and 15 wt%) were designed, compounded by shear mixing in a twin-screw extruder and, subsequently, processed through a sheet single-screw extrusion process. Setting of the operational parameters of laser joining of the polyester polymers was first investigated and, accordingly, process maps were built-on. The polymeric joints were, then, characterized by tensile tests and mechanical properties, especially, Young's modulus, were analyzed by Takayanagi's model. Based on the experimental results, a novel analytical model, derived from the mixture rule, was elaborated and calibrated empirically, showing a good predicting capability. Finally, material degradation of the welded joints was evaluated by FT-IR analysis, thus allowing the identification of the main routes to thermal degradation of PET and, especially, of biodegradable PET blends during laser processing and how this can affect the functional properties of the material.

PET phase, leading to the formation of a binary system during melt blending [11]. During shear mixing, the second phase forms very small sized core-shell structures that are dispersed in the primary phases as typical for binary blends of immiscible constituents [11]. The core-shell structures result to be uniformly dispersed in the primary phase, thus creating a broad interfacial surface with the primary phase [12]. When the blend is disposed in landfill, the broad interface between the two different phases can host an increased number of bacteria, which find optimal conditions to proliferate and the nutrients initially entrapped in the secondary phase. Bacteria proliferation can therefore promote the degradation of the polymer at an accelerated rate. This route accelerates biodegradation rates of PETs, although achievable results can depend on both the features and chemical structures of the polymers involved as well as by the landfill environments and atmospheric conditions (especially, humidity and temperature). The growing interest in biodegradable polymers is also increasing the attention towards the definition of easy routes for their processing, especially for maximizing production rates and containing operative costs. In this respect, joining of polyesters and, especially of those biodegradable, is of utmost relevance, being they often used in combination with other materials as it happens in fabrication of sport equipments, medical and electronic devices. Indeed, self-joining of polyesters is also of great interest, specifically in packaging industry, where sealing is necessary for food and drug protection and preservation over a long time range. Scientific and technical literature about joining of plastics and hybrid joining of metals and plastics abounds [13–15]. Great emphasis in the description of joining process for thermoplastic materials is found in several monographs [16–19], while thermoset cannot be welded without the addition of tie-layers (i.e., often thermoplastic polymers). In the last decades, further research efforts allowed to extend knowledge on LTW on a broader range of materials and laser sources. In 2007, Coelho et al. applied a CO2 laser for transmission welding of thermoplastic films, emphasizing the role of the low conductivity and high transparency of high and low density polyolefins [20]. Ilie at al. investigated the weldability of a polymeric material couple according to their thermal and optical properties [21]. In 2008, Michaeli et al. proposed the method of the intermediate film for improving effectiveness in laser welding of transparent plastics [22]. In 2009, Ghorbel et al. analyzed the influence of process parameters in welding process of polypropylene with diode laser using the overlap joint method [23]. In 2010, Jaeschke et al. investigated the weldability of high-performance polymers and carbon fiber reinforced thermoplastics (CFRP) using laser transmission welding techniques for different material combinations [24]. In 2011, Torrisi et al. proposed a power nanoseconds pulsed laser of polyethylene in order to irradiate the joint through one part, while the light was absorbed in the vicinity of the other part [25]. Response surface methodology was also involved to determine the optimum conditions of laser transmission joint of 1 mm thick PET film and PC film [26]. In 2012, Rodriguez et al. introduced a high power diode laser optical fiber coupled system to study laser weldability of ABS (acrylonitrile/butadiene/styrene) filled with two different concentrations of carbon nanotubes (0.01% and 0.05% CNTs) [27]. Laser transmission welding of PMMA (Polymethyl Methacrylate) by YAG (1.06 µm) laser using the orthogonal experiment method was also reported [28]. Brown et al. described a laser, noncontact sealing technique for thin, polyester-based lidding films in PET containers for both aseptic and food packaging, using a beam-steered laser and thereby enabling virtually instant changeover among different product lines [29]. The determination of whether two dissimilar polymers are weldable was investigated by Juhl et al. [30]. They suggested a correlation between laser weld strength and the ratio of the equilibrium interpenetration depth of two immiscible polymers to the maximum entanglement tube diameter of the two polymers. Chen et al. described the energy transmission in laser transmission welding of light scattering polymers [31]. More recently, an additional work studied the laser transmission welding (LTW) of polyethylene ter-

2. Experimental 2.1. Material Neopet 84 Poly(Ethylene Terephthalate) co-polyester was supplied by Neogroup (UAB Neo Group, Rimkai, Lithuania). Neopet 84 is supplied in the form of loose pellets, with nearly spherical shape. It is food grade, suitable for a wide range of applications, including the fabrication of thermoformable sheets for cosmetics and household packaging. Neopet 84 was chosen for its high intrinsic viscosity (0.84 ± 0.02 dl/g) and melting temperature close to 250 °C, that makes this polymer grade suitable for reprocessing by the addition of other constituents with lower properties. Additivation of Neopet 84 to make it biodegradable was performed by melt mixing with a specialty compound constituted of a carrier resin (that is, a blend of Ethylene-Vinyl Acetate Copolymer Mixture, namely PEVA, and, eventually, a polyolefin with melting temperature of approximately 105 °C) and other proprietary ingredients, which can include organoleptic species, esters, proteins and fatty acids (that are nutrients for bacteria). When the additive is melt blended by shear mixing with PET granules, the resulting material forms a binary system, in which the immiscible additive is uniformly dispersed in the form of core-shell structures inside the primary phase composed by PET. Neopet 84 is characterized by a melting temperature of ~250 °C. The carrier resin of the additive is characterized by a melting temperature of approximately 105 °C. When the additive starts to solidify, the PET is already completely solidified and shrunk. For this reason, merging between the two immiscible materials is only partial after melt blending and some discontinuities at the interface may arise. Finally, when the material, after its lifespan, is disposed in landfill, the environmental conditions can promote the formation of narrow interfacial gaps between the two immiscible phases (i.e., PET and the additive dispersed in), favor the proliferation of bacteria and accelerate the biodegradation of the polyester resin. 2.2. Melt processing and sheet extrusion Four different types of materials were formulated by compounding in a twin-screw extruder. The first material was achieved by compound111

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distribution along the fast axis in the process. Since emission from a single diode laser is well known to be confined to the narrow junction region (~1–2 μm), diffraction of the light should be expected to result in a large beam divergence of ~35–45° half angle in the direction perpendicular to the emission line (‘Gaussian’ or fast axis) and ~5–10° half angle in the direction parallel to the emission line (‘slow’ axis). The focal distance of the lens, which was used during the experimental tests, is 63 mm, while the working distance is 32 mm. Table 1 summarizes the experimental plan. Five different laser powers, in the range of 50–90 W (step 10 W), were chosen. Scan speeds in the range of 6–10 mm/s were set. Including the four different type of materials, this experimental schedule gave rise to 60 different tests (3×4×5 tests). The tests were replicated, at least, three times to ensure data reliability and repeatability.

ing the as-received Neopet 84. Other materials were achieved by melt blending the Neopet 84 with 5, 10 and 15 wt% of the additive. Melt blending was performed by a co-rotating twin-screw extruder (Micro 27, Point Plastic, Colleferro, Italy) with a screw diameter of 27 mm and a cylinder diameter of 27.2 mm. The ratio of length to diameter L/D of the extruder was 36. The extruder was composed of eight heating zones. Compounding extrusion was performed by setting the screw speed at 320 rpm and the temperature in the range of 255–270 °C. The material flow rate was set at 22 kg/h. After compounding extrusion, the resulting materials were quenched in an in-line water bath and, subsequently, pelletized in the form of quasi-spherical granules. The granules were subsequently re-extruded with a single-screw extruder (52 mm screw diameter, Point Plastic, Colleferro, Italy) to manufacture flat sheets, 350 mm wide and 330 μm thick, starting by the different material grades. Sheet extrusion was performed by setting the operative temperature in the same range of 255–270 °C, using a three-roll hauling off device to calibrate the sheet thickness. The roll temperature was always kept constant at approximately 60 °C, during sheet extrusion. No annealing or additional stretching treatments were performed to improve the mechanical properties of the extruded sheets. This resulted in tensile modulus of the materials ranging from ~1000 to ~1300 MPa.

2.4. Characterization tests After laser welding, the substrates were tested for their mechanical response by a static testing machine (MTS Insight 5, MTS, Eden Praire, MN, United States of America). Tensile tests were performed setting the deformation speed at 2 mm/min. Strength at break of the substrates was evaluated together with Young's Modulus and maximum elongation. Chemical structure (i.e., functional groups and chemical bonds) and thermal degradation of the polymeric materials after welding were analyzed by Fourier Transform Infrared Spectroscopy (FT-IR, FT-IR 4000 Jasco, Hachioji, Tokyo, Japan). FTIR spectra were taken in ATR mode in a spectral range of 4000– 600 cm−1 with a resolution of 2 cm−1 and a sampling rate of 64 scans min−1. Lastly, differential scanning calorimetry (Netzsch, DSC200PC, Selb, Germany) was performed. A heating rate of 5 °C/min was set to measure both the glass transition and melting temperature of the polymeric materials. The peak melting points of PET and PETs modified by the additive is of approximately 248 °C. The carrier polymer of the additive does not show any significant peak, despite the manufacturer declares a small melting peak at approximately 104 °C.

2.3. Laser processing 28×120×0.33 mm3 flat substrates were cut by fine blanking from the 350 mm wide extruded sheets of the four different materials (as-is Neopet 84, Neopet 84 with 5, 10 and 15 wt% additive). After cleaning in a bath of a bland detergent and rinsing with bi-distilled water, the substrates were clamped in a custom-built holder. Two substrates were therefore partially (for a length of 40 mm) superimposed (Fig. 1) and hold firmly each other by a screw and bolt system. The underlying substrate was pre-painted with a thin black acrylate layer (6–8 g/m2) to increase and make uniform the absorption coefficient of the polymeric materials and allow the implementation of the LTW. LTW was performed by a continuous wave High Power Diode Laser (cw-HPDL, ROFIN-SINAR DL 015, Plymouth, Michigan) with a 940 ± 10 nm wavelength. Fig. 1(i.e., the dark grey zone under the red laser beam in the middle of the clamping system) shows the laser scanning pattern, with the laser beam irradiating the short side (28 mm width) of the overlying substrate. Actually, the scanning pattern is 40 mm that is longer than the short side of the substrate. The laser starts irradiating 6 mm before the first edge of the superimposed substrates and stops the irradiation 6 mm after the second edge of the substrate (i.e., 6 mm +28 mm+6 mm=40 mm). In this way, utmost uniformity of the laser treatment on the substrate surface is ensured. During laser irradiation, heat flux is thus expected to get through the overlying substrate, being it basically transparent in the wavelength range typical of the laser beam. Then, the heat flux comes across the overlying substrate and approaches the black layer on the surface of the underlying substrate, where it is absorbed. Heat absorption increases the local temperature and triggers the status change in the polymeric material and, accordingly, the formation of the welded joints (Fig. 2). During laser welding, an argon flux was flushed on the substrate surface for protection and insulation purposes. The beam profile of a high power diode laser shows typically a rectangular shape with a top hat profile in one direction (slow axis) and a Gaussian profile in the other axis (fast axis). If the intersection of the beam profile with the focal plane is considered, this will result in the formation of an ellipse. The ellipse is characterized in the focal plane by a fast axis 1.2 mm and a slow axis 3.8 mm. During the present investigation, the slow axis is the one travelling over the welding line. This should ensure the maximum uniformity of the laser energy distribution across the bent zone. The fast axis, that is, the one characterized by the Gaussian energy distribution is chosen parallel to the welding line, so as to minimize the influence of the lack of uniformity of the laser power

3. Results and discussion 3.1. Analysis of the operational parameters LTW of as-received Poly(Ethylene Terephthalate) (PET) and of Poly(Ethylene Terephthalate) (PET) modified by the 5, 10 and 15 wt% additive, which promotes the biodegradation of the polymeric chains, is extremely sensitive to setting of operational parameters, specifically to laser power and scanning speed. The mechanism by which the substrates join each other forming the welded joints is by the wellknown Through Transmission Infrared (TTIr) welding in agreement with [13]. The continuous wave High Power Diode Laser (cw-HPDL) of

Fig. 1. Clamping device during LTW.

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parts to be welded. Occurrence of melting phenomena can be, therefore, essentially ascribed to sufficient intensity of laser irradiation delivered to the surface, where intensity is defined as the ratio of the power (energy per unit of time) delivered by the laser beam to the surface area of the irradiated zone. Intensity of laser radiation is hereby depending on the selection of the laser parameters, namely, laser power and scanning speed, being focus conditions set constant for all the experimental tests. Fig. 3 reports the qualitative evaluation of the welded joints by varying the laser operational parameters for the as-received PET and of PETs modified by the 5, 10 and 15 wt% additive. Fig. 4 depicts some samples of welded joints for the different materials under investigation. Four different scenarios were therefore identified: (i) no melting (low intensity of the laser radiation, especially for PET), where local heating of the faying surface is not enough to cause sufficient melting of the polymeric materials and adhesion of the two superimposed layers; (ii) melting/no welding (low and moderate intensity), where local heating of the faying surface causes local melting of the polymeric materials, but without forming a stable welded joint; (iii) welding (intermediate intensity), where local heating of the faying surface causes sufficient melting of the polymeric materials to generate a firm welded joint; (iv) welding with degradation/degradation (high intensity), where laser irradiation causes a local overheating of the faying surface with a significant thermal degradation of the material. In the latter case, welding can also occur (i.e., welding with degradation), but it is always associated to polymeric materials, which are severely impaired by excessive heating and it is, therefore, not acceptable for practical purposes. In the case of widespread thermal degradation, the polymeric material is compromised and sufficient welding cannot be achieved. The continuous wave High Power Diode Laser (cw-HPDL) was found to boast operational parameters to weld each of the investigated materials on a wide enough processing window (i.e., green zone of the maps in Fig. 3). As-received PET and PETs modified by the additive perform rather similarly, with as-received PET presenting wider zone where no melting or severe thermal alteration of the material take place. This result can be ascribed to the composition of the additive. It is composed of material (Poly(Ethylene Vinyl Acetate) and, especially, Polyolefins) with higher absorbance in the IR wavelength rather than the highly transparent as-received PET [13,41]. For

Fig. 2. Scanning pattern, faying surface and formation of the melting interface during LTW of the as-received PET and of PETs modified by the 5, 10 and 15 wt% additive.

Table 1 Experimental schedule. Laser power, W

50

60

70

80

90

Material

Neopet 84

6

Neopet 84 +10 wt% organic additive 10

Neopet 84 +15 wt% organic additive –

–

Scanning speed, mm/s

Neopet 84 +5 wt% organic additive 8

–

the present work emits a radiation with a wavelength of approximately 940 nm that falls in the IR range of 800–1100 nm, where PET is highly transparent. As reported in previous Section, the experimental set-up involves two partially superimposed substrates hold firmly together by a custom-built holder. Basically, the overlying substrate is transparent to laser radiation, while the surface of the underlying substrate can absorb, being it preventively blackened with an acrylic paint. Heat absorption on the underlying faying surface causes the rapid heating of the irradiated zone and, consequently, the local melting of the polymeric material. Therefore, melting at the interface between the two substrates and solidification by the subsequent cooling allows the

Fig. 3. Process maps of LTW varying laser operational parameters (i.e., laser power and scanning speed): (a) as-received PET; (b) PET modified by the 5 wt% additive; (c) PET modified by the 10 wt% additive; (d) PET modified by the 15 wt% additive.

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Fig. 4. Laser joints: (a) as-received PET; (b) PET modified by the 5 wt% additive; (c) PET modified by the 10 wt% additive; (d) PET modified by the 15 wt% additive.

superior to the strength of similar systems. For example, Wang et al. found strengths at break of the welded joints of poly(ethylene terephthalate) and polypropylene by LTW in the range of 5–11 MPa [38]. In addition, the rupture at break of the welded joints herein achieved are definitely higher than those achieved by Brown et al. on thin polyesters film after laser sealing [29]. Previous results about PET and PET/PEVA welded joints are also in good agreement with the experimental findings reported by Abolhasani et al. [11,12] in their earlier studies on binary and ternary blends of poly(ethylene terephthalate) blended with poly(vinyl ethyl acetate) (PEVA) and polypropylene (PP). Abolhasani et al. showed a nearly linear decrease in Young's modulus of binary and ternary blends by increasing the concentration of the second (and third) dispersed phase in the polymeric material. For binary blends of poly(ethylene terephthalate) with poly(vinyl ethyl acetate), Abolhasani et al. found a remarkable decrease in elastic modulus of more than 20%, already for very low concentration of the dispersed PEVA phase in PET. Indeed, poly(ethylene terephthalate) and poly(vinyl ethyl acetate) or polyolefins form a binary system after melt blending. According to [42,43] and as confirmed by experimental findings reported in [11,12], the second phase (in this investigation, the PEVA-based additive) can be expected to be almost uniformly dispersed in the form of small spherical droplets inside the primary phase (i.e., the polyethylene terephthalate). The herein reported organic additive should therefore forms the so-called

this reason, during laser irradiation, part of the energy is absorbed by the overlying substrates in the case of PETs modified by the additive. Therefore, as-received PET features a sharper response to laser irradiation where all the energy can pass through the overlying substrate and get to the faying surface. In contrast, PETs modified by the additive features a smoother response to laser irradiation, as part of the energy is absorbed by the overlying substrate and does not reach the faying surface. In this respect, PET modified by the 15 wt% additive boasts the broader green zone on the map, this being probably related to the increased capability of this material to absorb part of the incoming energy during laser welding, thus screening partially the faying surface during welding. 3.2. Mechanical response of the welded joints: an application of Takayanagi's model Mechanical response of welded joints was evaluated by tensile tests. Fig. 5 reports the generic trends of the load vs. elongation of the welded joints of the as-received PET substrates. During tensile tests, rupture always takes place along the welded joints. The trend of the load is sharply and linearly increasing. Rupture takes place when the polymeric material is still in the elastic field. The average values of the load at break of the welded joints of the as-received Poly(Ethylene Terephthalate) (PET) and of Poly(Ethylene Terephthalate) (PET) modified by the 5, 10 and 15 wt% additive are rather similar. As-received PET boasts the higher average load at break of 110.4 N. PETs modified by the 5, 10 and 15 wt% additive boast average load at break of 90.4, 98.6 and 102.8, respectively. This means the average strength of the welded joints varies in the range of approximately 10–12 MPa. Table 3 summarizes the experimental results in terms of maximum performance achievable in a specific experimental condition. Accordingly, PETs modified by the additive feature an improved average value of elongation at break (up to 10%) if compared with the average value of the as-received PET. The asreceived PET boasts the highest Young's modulus of the welded joints by varying laser operational parameters (Fig. 6). Increasing the concentration of the additive in the PET causes a remarkable decrease in Young's modulus of the welded joints over the whole range of laser parameters. On average, as-received PET boasts Young's modulus of approximately 1100 MPa. PETs modified by the 5, 10 and 15 wt% additive boast average Young's moduli of 1030, 950 and 860, respectively. A comparison with data available in literature confirms that the strength of the welded joints achieved in the present work is equal or

Fig. 5. Trends of the load vs. elongation for a set of tensile tests performed on welded joints of the as-received PET substrates (laser power from 50 to 90 W, scanning speed of 6 mm/s).

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Young's modulus of their blends. The decrease in Young's modulus of binary systems can be, however, predicted with a great deal of accuracy by the application of Takayanagi's model [42,43]:

Table 2 FT-IR analysis: interpretation of the peaks. Wavenumber (cm−1)

Assignment

730

Out of plane deformation of the two carbonyl (C=O) substituents on the aromatic ring C–H deformation of three adjacent coupled hydrogens on an aromatic ring C–H deformation of two adjacent coupled hydrogens on an aromatic ring Out of plane of benzene group C–H rocking of glycol (O–CH2) Normally not present in PET, but associable to C–O stretching of di-ethylene -glycol (DEG) end groups C–O stretching of glycol (O–CH2) In-plane vibration of benzene (1,4-substitution in the skeletal ring) Mainly due to C–O stretching of ester Mainly due to C–O stretching of ester CH2 wagging of glycol (O–CH2) CH2 wagging of glycol (O–CH2) Benzyl Ring in-plane stretching deformation –CH2– deformation band Skeletal ring breathing (especially, in case of degradation, even during polymerization) C=O stretching band Aromatic summation band Aliphatic C–H stretching Aromatic C–H stretching O–H stretching of di-ethylene-glycol (DEG) end groups Moisture

820 850 870 897 940 970, 980 1020, 1070, 1120, 1175 ~1090 (broad band) ~1230 (broad band) 1340 1370 1410, 1430, 1450, 1615 1465 1580 1720 1950 2880, 2960 3060 3440 3535

⎤ 1 ⎡ 1−λ 1 =⎢ + ⎥ E ⎣ E1 (1−φ) E1 + φE2 ⎦

where E is the elastic modulus of the binary system, E1 and E2 the elastic moduli of PET and additive, respectively. φ and λ are the mixing parameters, that are related to the earlier mentioned stress transfer capability between the two different phases of the investigated polymeric blend when submitted to external loads (that is, to the degree of series-parallel coupling in the binary polymeric system [45]). Their product φ·λ equals the volume fraction of the dispersed phase in the primary phase. According to Cohen and Ramos [45], φ and λ for spherical droplets depend on the dispersed phase concentration and degree of parallel coupling between the two different phases of a binary system and they can be estimated through Eq. (2):

%parralel =[φ (1 − λ )/(1 − V )]×100

Maximum load at break, N

Maximum Young's modulus, (MPa)

Maximum elongation, mm

PET PET – PEVA (5 wt%) PET – PEVA (10 wt%) PET – PEVA (5 wt%)

110.4 102.8

~1100 ~1030

0.97 1.04

98.6

~950

1.09

90.4

~860

1.10

(2)

where V=φ·λ is always an estimate of the volume fraction of the dispersed phase. By the combined application of Eqs. (1) and (2) to experimental data, the empirical constants of the model can be calibrated, allowing to estimate the elastic moduli of the investigated blends. Fig. 8 reports the trends of Young's modulus of PET and PETs modified by the PEVA-based additive. It also includes the comparison of the available experimental results with analytical modelling. Good matching can be observed between experimental and numerical results over the whole range of the additive concentration, revealing that a simple and first approximation analytical model is suitable to predict the mechanical response of the binary blends formed by the dispersion of the PEVA-based additive inside the primary PET phase. In this case, a linear simplified model can be therefore used to predict the values of Young's moduli of the welded joints according to the concentration of the additive inside PET:

Table 3 Summary of the mechanical response of the welded joints. Material

(1)

w Ecw=EPET (1 − W ) + kEADD W

(3)

Ecw

is the elastic modulus of the laser welded joints of the where w investigated materials, EPET is the elastic modulus of the laser welded joints of the investigated PET, EADD is an estimate of Young's modulus of the carrier polymer of the additive, W is the weight fraction of the additive in the PET formulation and, lastly, k is the calibration constant. A mono-parametric model is thus sufficient to achieve a highly reliable prediction of Young's moduli (i.e. R-square > 0.95) of the welded joints for the different PET formulations herein investigated. The parameter k corrects the model for the non-perfect merging at the interface between the two immiscible phases of the binary systems and, furthermore, accounts for the stress transfer capability between the aforementioned immiscible phases when the material is submitted to external loads. It stands to reason that, in case of an ideal binary system, Eq. (3) gives back the mixture rule and k=1. Any deviation and, specifically, negative deviation from the mixture rule is modelled by the calibration constant k. Such a parameter can be assumed constant and calculated empirically (that is, by simple fitting procedure) or, in turn, remodeled through a number of different equations, among which the aforementioned Cohen and Ramos's equation [45].

core shell structures in the primary PET phase (i.e., the visible droplets sticking out from surface in Fig. 7), giving rise to the expected binary system. Although Young's modulus of these binary systems could be expected to follow the mixture rule, many experimental results on a variety of polymeric blends often show a negative deviation from that [11,12,44]. This worse mechanical behavior of the binary systems can be attributed to a number of reasons. However, non-perfect joining at the interface between the two immiscible phases (in this case, the poly(ethylene terephthalate matrix) and the dispersed core shell structures formed by the additive) is the root of the problem. Poor bridging at interface can reduce the stress transfer capability between the two different phases, when the blend is submitted to external loading conditions. This phenomenon leads usually to a decrease in Young's modulus of the resulting blends. Moreover, in this case, the additive mostly relies on a carrier polymer based on PEVA. PEVA is notoriously a softer polymer and it is definitely less stiff than PET, this being ascribable to the lack of aromatic groups in the polymeric chains of PEVA, which, oppositely, make PET much more rigid and inert [8]. As known, aromatic groups feature a high steric hindrance, by which they make the polymeric chains less prone to deform when submitted to external loads. Therefore, the addition of the softer PEVA-based dispersed phase in the primary PET phase is expected to reduce further

3.3. Fourier transform infrared spectroscopy: analysis of thermal degradation of welded joints Fig. 9 reports the Fourier Transform Infrared (FT-IR) spectra of PET and PETs modified by the 5, 10 and 15 wt% additive, which promotes the biodegradation of the polymeric chains in the blends. Table 2 summarizes the most significant peaks of the FT-IR analysis of the starting materials (that is, before LTW) in agreement with some 115

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Fig. 6. Trends of Young's modulus of the welded joints by varying laser operational parameters (i.e., laser power and scanning speed): (a) as-received PET; (b) PET modified by the 5 wt % additive; (c) PET modified by the 10 wt% additive; (d) PET modified by the 15 wt% additive.

Fig. 7. Core shell structures formed by the immiscible additives inside the PET matrix.

Fig. 8. Trends of the average value of Young's modulus of the welded joints of the asreceived PET and of PETs modified by the 5, 10 and 15 wt% additive and comparison with analytical model.

data reported in [46]. Starting from high wavenumbers, at about 3440 cm−1, the O–H stretching mode of the di-ethylene-glycol end groups in PET (also featured by PEVA) can be observed. In the range of 2880–3060 cm−1, the C–H stretching mode of the aliphatic chains can be clearly seen. These groups are featured by both PET and PEVA (and, polyolefins, as well) of the carrier polymer of the additive. However, the relative amounts of these groups in PEVA and, even more, in polyolefins is higher than in PET, as confirmed by the sharpest corresponding peaks in Fig. 9a for the formulations of PET modified by the addition of the different percentages of the PEVA-based additive. FT-IR spectra in Fig. 9a are not able to discriminate the quantity of additive (the spectrum of PET+5 wt% additive features the sharpest peaks in the range of 2880–3060 cm−1). However, the FT-IR spectra in Fig. 9a are sufficiently sensitive to discern the increased intensity of

these groups for all the cases in which the additive is present in the PET formulation. Accordingly, in the case of the as-received PET, these peaks are rather flat. The sharpest peak at 1720 cm−1 is attributed to the C=O stretching mode of the carbonyl group of PET (also, featured by PEVA). The peaks at 1410, 1430, 1450 and 1615 cm−1 can be ascribed to the in-plane stretching deformation of the benzyl ring of PET. At 1340 and 1370 cm−1, the peaks of the CH2 wagging of the glycol are found. The broad bands at approximately 1090 and 1230 cm−1 are attributed to the C–O stretching mode of the ester groups. The peaks at 1020, 1070, 1120 and 1175 cm−1 are ascribable to the in-plane vibration of benzene and, specifically, to the 1,4-substitution in the skeletal ring. The peaks at 970 and 980 cm−1 are attributed

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Fig. 9. FT-IR spectra of PET and PETs modified by the 5, 10 and 15 wt% additive: (a) peaks found at higher wavenumbers; (b) peaks found at lower wavenumbers.

4. Conclusions

to C–O stretching of glycol (O–CH2). C–H rocking mode of glycol (O– CH2) is found at 897 cm−1. Out of plane of benzene groups is found at 870 cm−1. The C–H peaks at 820 and 850 cm−1 are attributed to C–H deformation of three and two adjoining coupled hydrogen on the aromatic rings, respectively. Lastly, the peak at 730 cm−1 is ascribable to the out of plane deformation of the two carbonyl (C=O) substituents on the aromatic group. Additional IR peaks of minor relevance are, however, summarized in Table 2. Fig. 10 shows the change in the FT-IR peaks after laser welding and refers to the substrates made from the PET modified by the 15 wt% additive. The most important changes after laser welding regard the spectral range of 2880–3060 cm−1. The peaks attributed to the C–H stretching mode of the aliphatic chains become significantly sharper. In addition, the peaks attributed to the presence of aromatic groups (for example, the in-plane vibration of benzene and the C–H deformation of the adjoining coupled hydrogens on the aromatic rings) grows slightly. In contrast, the peaks attributed to the C–O stretching mode of the ester groups at 1230 and 1090 cm−1 decreases visibly after laser welding. The mechanism of thermal degradation of modified PET can be therefore ascribed mainly to the breaking of the C–O single bonds of the ester groups (i.e., typical breaking mechanisms of polyesters by hydrolysis reaction [47]), featured by both PET and PEVA-based additive. In contrast, C–H bonds belonging to both aliphatic and aromatic groups show a better endurance to high temperature during laser welding, thus revealing an increased thermal stability of these moieties of the polymeric chains at high temperature.

The present investigation deals with the LTW of Poly(Ethylene Terephthalate) and Poly(Ethylene Terephthalate) modified by an additive that promotes the biodegradation of the primary polymeric phase. A continuous wave High Power Diode Laser (cw-HPDL) was chosen, as it emits in the infrared wavelengths, thus allowing to benefit from the high transparency of PET in that range. Transmission Welding in Infrared of the polymers was, therefore, achieved by superimposing two substrates and blackening the faying surface of the underlying substrates. The following pointwise conclusions can be drawn:

• • • • •

Local heating of the absorbing surface is found to cause local melting of the polymers and formation of the welded joint. Analysis of the operational parameters of laser welding allowed the identification of working windows, sufficiently broad for practical purposes. Tensile testing of the welded joints allowed determining the rupture and elongation at break of the welded joints, stating their good mechanical response. Young's moduli of the welded joints are related to the concentration of the additive, as it is dispersed in the primary phase forming small spherical droplets (i.e., the so-called core shell structure) which are immiscible. An increase in the additive concentration causes a decrease in Young's modulus of the blends due to the scarce bridging at the

Fig. 10. FT-IR spectra of the PET modified by the 15 wt% additive after laser welding and comparison with as-received PET and PET modified by the 15 wt% additive: (a) peaks found at higher wavenumbers; (b) peaks found at lower wavenumbers.

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• • • •

interface between the primary (i.e., PET) and dispersed phase (i.e., the additive) as well as to the mediocre mechanical properties of PEVA. Achieved trends of Young's modulus can be modelled with accuracy by Takayanagi's model, particularly if modified by Cohen and Ramos's equation. Simple mono-parametric model derived from mixture rule can allow predicting with good reliability Young's moduli of the welded joints and accounting for the common negative deviation from mixture rule of the non-ideal binary polymeric systems. Thermal degradation can be attributed to the breaking of the C–O single bonds of the ester groups, featured by both PET and PEVA in the additive, similarly to what happens during typical hydrolytic degradation of polyesters. In contrast, C–H bonds belonging to both aliphatic and aromatic groups show a better endurance to high temperature during laser welding, thus revealing an increased thermal stability of these moieties of the polymeric chains at high temperature.

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Acknowledgements

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The authors wish to thank you Mrs. Patrizia Moretti for some support in the FT-IR testing of the polymeric materials. The authors also wish to thank Mr. Antonio Severini and the company Point Plastic Srl (Colleferro, Italy) for the continuous support during compounding and sheet extrusion.

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