Poly(hydroxyl butyrate-co-valerate) (PHBV) is a biopolymer synthesized by microorganisms that is fully biodegradable with improved thermal and tensile properties with respect to some commodity plastics. However, it presents an intrinsic brittleness that limits its potential application in replacing plastics in packaging applications. Films made of blends of PHBV with different contents of thermoplastic polyurethane (TPU) were prepared by single screw extruder and their fracture toughness behavior was assessed by means of the essential work of fracture (EWF) Method. As the crack propagation was not always stable, a partition method has been used to compare all formulations and to relate results with the morphology of the blends. Indeed, fully characterization of the different PHBV/TPU blends showed that PHBV was incompatible with TPU. The blends showed an improvement of the toughness fracture, finding a maximum with intermediate TPU contents.Financial support for this research from Ministerio de Economía y Competitivi dad (project AGL2015-63855-C2-2-R (MINECO/FEDER) and Pla de Promoció de la Investigació de la Universitat Jaume I (PREDOC/2012/32 and E-2015-22) is
gratefully acknowledged

Cabedo, Luis

Gamez-Perez, Jose

González Ausejo, Jennifer

Sánchez-Safont, Estefanía

Journal of Multiscale Modelling

Repositori Institucional de la Universitat Jaume I

Page ProofNovember 2, 2016 11:47 WSPC/245-JMM 1640008Journal of Multiscale Modelling1Vol. 7, No. 4 (2016) 1640008 (7 pages)2c© World Scientiﬁc Publishing Europe Ltd.3DOI: 10.1142/S175697371640008445Toughness Enhancement of Commercial Poly6(Hydroxybutyrate-co-Valerate) (PHBV) by Blending7with a Thermoplastic Polyurethane (TPU)8Jennifer Gonza´lez-Ausejo∗, Estefania Sa´nchez-Safont†,9Luis Cabedo‡ and Jose Gamez-Perez§10Polymer and Advanced Materials Group (PIMA)11Universidad Jaume I, 12071, Castellon, Spain12∗ausejo@uji.es13†ausejo@uji.es14‡cabedo@uji.es15§gamez@uji.es16Accepted 8 September 201617Published18Poly(hydroxyl butyrate-co-valerate) (PHBV) is a biopolymer synthesized by microor-19ganisms that is fully biodegradable with improved thermal and tensile properties with20respect to some commodity plastics. However, it presents an intrinsic brittleness that21limits its potential application in replacing plastics in packaging applications. Films made22of blends of PHBV with diﬀerent contents of thermoplastic polyurethane (TPU) were23prepared by single screw extruder and their fracture toughness behavior was assessed24by means of the essential work of fracture (EWF) Method. As the crack propagation25was not always stable, a partition method has been used to compare all formulations26and to relate results with the morphology of the blends. Indeed, fully characterization27of the diﬀerent PHBV/TPU blends showed that PHBV was incompatible with TPU.28The blends showed an improvement of the toughness fracture, ﬁnding a maximum with29intermediate TPU contents.30Keywords: Poly(3-hydroxybutyrate-co-3-hydroxyvalerate); polyurethane; biodegradable;31blends; essential work of fracture.321. Introduction33The fracture behavior of materials that present high plastic deformation can be34described by post-yielding fracture mechanics (PYFM).1 The essential work of frac-35ture (EWF) method provides a technique for obtaining toughness parameters for36the ductile fracture process in either tensile or tearing conﬁgurations. Deeply dou-37ble edge notched tensile (DDENT) specimens are the most used geometry in EWF38determinations in tensile mode.2,339The EWF concept initially states that the energy involved during a ductile40fracture (Wf ) can be partitioned into two components. One component, the essen-41tial work (We) is associated with the energy spent at the fractured surface and is421640008-1Page ProofNovember 2, 2016 11:47 WSPC/245-JMM 1640008J. Gonza´lez-Ausejo et al.therefore proportional to the fracture area ( · t), where  is the ligament length1and t is the specimen thickness. The second component is the non-essential work2of fracture or plastic work (Wp), which is related to the energy of the process that3takes place out of the fracture surface and involves extensive plastic deformation4and other dissipative energy processes. Wp is proportional to the volume of the5deformed region surrounding the crack process zone, that is proportional to 2 · t.6The relation between Wf , We, and Wp is described in Eq. (1):7Wf = We + Wp = we · t + β · wp2 · t, (1)where we and wp are the speciﬁc EWF and the speciﬁc non-essential work of frac-8ture, respectively, whereas β is a dimensionless shape factor for the plastic zone.9Dividing both terms of Eq. (1) by the ligament section,  · t, we obtain that the10speciﬁc work of fracture, wf is then:11Wf/( · t) = wf = we + β · wp. (2)According to this equation, we and β · wp can be obtained from linear regression12of a set of values represented in a diagram of speciﬁc total fracture energy versus13ligament length. It has been shown that the speciﬁc essential work, we is in theory14of a material constant dependent only on thickness and equivalent to JIC,4 which15has also been supported experimentally and compared with the CTOD values.5 It16is assumed that for the correct application of the EWF method, some experimental17constraints must be accomplished, including pure plane stress conditions, no border18eﬀect, full yielding of the ligament length prior to crack propagation, a geometrical19similarity between the fracture load versus. displacement (L–d) curves (Fig. 1(a))20of specimens with diﬀerent ligament lengths and steady crack propagation during21fracture.6 If these criteria are not accomplished, the results cannot be regarded as22true fracture toughness values.23However, there are some works7–9 in which the energy spent on the fracture24process is split into diﬀerent terms (initiation, necking, plastic work, viscoelastic25energy, etc.), so called “partition energy” approaches. The main terms are, gen-26erally the initiation process (mainly yielding of ligament section, wf,y) and crack27propagation process (i.e., ligament necking and tearing, wf,n), treated as if they28were independent phenomena. According to the approach described in Ref. 7 these29terms can be related with the fracture L–d curves, as shown in Fig. 1(b). Hence,30Eq. (2) can be rewritten as:31wf = wy + wn = (we,y + β · wp,y) + (we,n + β · wp,n) (3)where wy and wn can be calculated from L–d curves for each specimen and therefore32the speciﬁc initiation EWF parameters (we,y, β ·wp,y) and propagation ones (we,n,33β · wp,n) can be obtained.34From this approach, if the criteria previously exposed for applying the EWF35method applies to the initiation part of the fracture of DDENT specimens, the36EWF technique can be used to assess toughness and resistance to initiation of371640008-2Page ProofNovember 2, 2016 11:47 WSPC/245-JMM 1640008Toughness Enhancement of PHBV Films by Blending with TPUWyWnFig. 1. Schema showing the L–d curves where the work of fracture can be obtained, the partitionenergy based on yielding criterion and the wf versus  theoretical plots for assessment of fractureparameters.crack propagation in materials which show overall ductile fracture behavior, even1though the propagation of the crack does not fulﬁll the self-similarity or steady2crack growth conditions.3In this work Polyhydroxybutyrate-co-valerate) (PHBV) ﬁlms have been pre-4pared, with diﬀerent percentages of thermoplastic polyurethane (TPU) as an addi-5tive in order to improve the fracture toughness and brittleness of virgin PHBV.6PHBV is a biopolymer synthesized by microorganisms that is fully biodegradable7with improved thermal and tensile properties with respect to some commodity plas-8tics but too brittle to replace commodity plastics in day-to-day packaging appli-9cations. As the crack propagation was not always stable in those ﬁlms, in order10to optimize the TPU content in the ﬁlm formulations, the EWF energy partition11approach has been used, in combination with other techniques that provide infor-12mation about the morphology and tensile behavior.132. Experimental14PHBV with 3mol% hydroxyvalerate (HV) content was supplied by Tianan Bio-15logic Material Co. (Ningbo, P. R. China) in pellet form (ENMATTM Y1000P).16The TPU Elastollan r© 880a 13N000 was purchased from BASF. Both materials17were used as received. PHBV and the TPU used in this study were dried at 80◦C18for 2 h before use. The PHBV/TPU blends were obtained by a single screw extruder19(Haake Rheomex 252p) with a Maddock screw with an L/D ratio of 25. The tem-20perature proﬁle was set to 120◦C/160◦C/750◦C, a die temperature of 175◦C and21a typical residence time of 3min. Films of nominal thickness of 0.2mm with dif-22ferent TPU contents were obtained: 0% (referred as Neat PHBV), 15wt% TPU23(15-TPU), 20wt% TPU (20-TPU) and 25wt% TPU (25-TPU). The morphology241640008-3Page ProofNovember 2, 2016 11:47 WSPC/245-JMM 1640008J. Gonza´lez-Ausejo et al.of the cryofractured surface of the ﬁlms and post-mortem DDENT specimens was1observed by Scanning Electron Microscopy (SEM) using a JEOL 7001F.2DDENT and tensile dumbbell specimens (ASTM D638 Type IV) were cut from3the ﬁlms.7 For EWF tests, ﬁve ligament lengths between 5mm and 15mm with4a step of 2–3mm were prepared and for each ligament length, three replicas were5tested. All the experiments were conducted in a universal testing machine, Shi-6mazdu AGS-X 500N. The crosshead speed for mechanical and fracture characteri-7zation was 5 mm/min and tests were conducted at room temperature (22± 1◦C).83. Results and discussion9Blends showed a continuous PHBV matrix with evenly distributed TPU ﬁbers10oriented along the melt ﬂow axis during ﬁlm processing in all case, as shown in11Fig. 2. The size of the ﬁbrils did not vary signiﬁcantly with the TPU content.12However, in post-mortem DDENT specimens, the PHBV containing 25% TPU13showed some necking with extensive plastic deformation and a close look revealed14formation of ﬁbrils along the crack propagation direction.15With respect to tensile performance, all ﬁlms showed strong anisotropy between16the extrusion direction (MD) and the transverse one, as observed in the representa-17tive stress versus strain curves in Fig. 3. This behavior is quite typical for extruded18ﬁlms, being especially enhanced in highly crystalline systems, such as PHBV, where19crystals grow in a preferred orientation [REF]. Neat PHBV showed brittle behavior20without yielding, whereas blends with TPU showed in all cases a yielding point with21some plastic deformation. The ﬁlms blended with TPU showed fast crack propa-22gation after yielding, always at higher deformation values than those obtained by23Neat PHBV. Table 1 summarizes the main values obtained from tensile tests; it24Morphology Fractography15-TPU (MD) 15-TPU (TD) 15-TPU DDENT (MD)15-TPU DDENT (MD)25-TPU (MD) 25-TPU (TD) 25-TPU DDENT (MD)25-TPU DDENT (MD)Fig. 2. Morphological and fractographic SEM micrographs.1640008-4Page ProofNovember 2, 2016 11:47 WSPC/245-JMM 1640008Toughness Enhancement of PHBV Films by Blending with TPUFig. 3. Representative stress versus strain curves of all compositions studied.Table 1. Summary of tensile and fracture properties of PHBV and PHBV-TPU ﬁlms.Tensile Properties Fracture ParametersE σy εr we βwp we,y βwp,y(MPa) (MPa) (mm/mm) (kJ/m2) (J/m3) (kJ/m2) (J/m3)Neat PHBV TD 2200± 200 33 ± 1 0.021± 0.00315-TPU TD 1800± 100 31.6± 0.5 0.034± 0.002 3.4 0.19 2.9 0.1620-TPU TD 1800± 100 32.5± 0.7 0.032± 0.002 2.0 0.45 0.7 0.4525-TPU TD 1500± 100 27 ± 1 0.034± 0.002 5.1 0.75 0.9 0.83Neat PHBV MD 2500± 100 42 ± 1 0.027± 0.00315-TPU MD 2000± 200 41 ± 1 0.047± 0.002 8.8 0.23 6.3 0.1820-TPU MD 2000± 100 41 ± 1 0.047± 0.004 4.8 1.38 2.4 0.9325-TPU MD 1700± 100 36.0± 0.8 0.09± 0.02 4.3 0.82 0.8 0.84can be seen as a trend where there is an increase in deformation at rupture as more1TPU is added.2Even though adding TPU reduced the Young Modulus in all cases by at least320%, the values obtained for tensile strength did not vary that much in formulations4with 15% and 20% TPU content. The reason for such small diﬀerences is the fact5that Neat PHBV ﬁlms break before reaching plastic yielding by spontaneous crack6generation and propagation.7In terms of fracture behavior, Load versus displacement curves, like the ones8shown in Fig. 4, were self-similar up to yielding in all PHBV-TPU systems. After9maximum load, some sort of disagreement in the tails of the curves was observed.10Generally, this type of behavior would prevent from applying the EWF method11or, at least, the values obtained should be taken carefully. However, by using the12partition approach with the energy values corresponding to the yielding of the13DDENT samples, some certitude can be obtained in terms of energy absorbed to14crack initiation.151640008-5Page ProofNovember 2, 2016 11:47 WSPC/245-JMM 1640008J. Gonza´lez-Ausejo et al.wywfFig. 4. L–d curves and w versus  plots for determination of EWF parameters for ﬁlms 15-TPUMD.Therefore, the speciﬁc work of fracture as well as the speciﬁc initiation work of1fracture were determined and plotted as a function of the ligament length to assess2the EWF parameters and the parameters corresponding to the proposed energy3partition procedure. The values obtained for all ﬁlms are summarized in Table 1.4As Neat PHBV did not show yielding at all, the EWF method did not provide any5valid parameter.6From the EWF values, some general trends can be appreciated, like the7anisotropy found in tensile behavior with higher we and βwp values in MD than in8TD, or the fact that as TPU content increases, there is higher deviation of we values9with respect to we,y . A close look to these values show that the we,y decreases as10TPU content increases. This indicates that the contribution of generation of two11new surfaces at the initiation of the crack propagation decreases by adding TPU.12However, during this process there is also some plastic deformation with the liga-13ment yielding, which also contributes to energy absorption, represented by the term14β ·wp,y. This term, however, is tricky to evaluate, since it represents the plastic work15developed by initial volume unit, and this value depends on the stress required to16produce plastic work and the extension at which the plastic deformation has been17carried out. An increase in TPU makes on one hand to decrease the stress required18to produce plastic deformation and on the other hand, to absorb more energy19because more plastic deformation is promoted, in agreement with the tensile char-20acterization.21So the global balance in the fracture process initiation is either to ease the plastic22deformation at lower energy levels, which decreases we but increases the extension23of plastic work or diﬃcult the plastic deformation with higher we values and a raise24in the stress needed to produce plastic deformation. In any case, the EWF method25and its partition energy approach allow to determine parameters that can be used26to tune the amount of TPU to be used and predict the diﬀerent fracture behavior27of the ﬁlms.281640008-6Page ProofNovember 2, 2016 11:47 WSPC/245-JMM 1640008Toughness Enhancement of PHBV Films by Blending with TPUThere is also another observation that is worth to comment with respect to the1energy partition analysis. By looking at the diﬀerences between we and we,y and2β · wp and β · wp,y, it can be argued that most of the fracture energy spent in315-TPU ﬁlms is produced during initiation of the crack (similar values of initiation4and overall fracture parameters), whereas the TPU content increases, more energy5is dissipated during the crack growth and hence initiation values diﬀer more from6the overall fracture ones.74. Conclusions8As the PHBV-TPU ﬁlms show ductile behavior, EWF approach is the only one that9can be used to assess fracture parameters. It has been shown that increasing TPU10content decreases stiﬀness and yield strength, but allows higher plastic deformation11in tensile tests. By using the partition energy approach of the EWF method, it has12been shown and quantiﬁed the inﬂuence of the aforementioned phenomena on the13initiation of the crack propagation of the ﬁlms.14Acknowledgments15Financial support for this research from Ministerio de Economı´a y Competitivi-16dad (project AGL2015-63855-C2-2-R (MINECO/FEDER) and Pla de Promocio´ de17la Investigacio´ de la Universitat Jaume I (PREDOC/2012/32 and E-2015-22) is18gratefully acknowledged.19References201. K. B. Broberg, J. Mech. Phys. Solids 23 (1975) 215–237.212. A. B. Martinez, J. Gamez-Perez, M. Sanchez-Soto, J. I. Velasco, O. O. Santana and22M. Ll Maspoch, Eng. Fail. Anal. 16(8) (2009) 2604–2617, doi:10.1016/j.engfailanal.232009.04.027.243. J. G. Williams and M. Rink, Eng. Fract. Mech. 74(7) (2007) 1009–1017,25doi:10.1016/j.engfracmech.2006.12.017.264. Y.-W. Mai, B. Cotterell, R. Horlyck and G. Vigna, Polymer Eng. Sci. 27(11) (1987)27804–809, doi:10.1002/pen.760271106.285. A. B. Mart´ınez, A. Segovia, J. Gamez-Perez and M. L. Maspoch, Eng. Fract. Mech.2977(14) (2010) 2654–2661, doi:10.1016/j.engfracmech.2010.07.017.306. ESIS-TC4. (n.d.). Testing protocol for essential work of fracture (revised by Clutton,311997).327. M. L. Maspoch, J. Gamez-Perez and J. Karger-Kocsis, Polymer Bull. 50(4) (2003)33279–286.348. M. M. Hossain, C.-F. Lee, D. M. Fiscus and H.-J. Sue, Polymer 96 (2016) 104–111,35doi:10.1016/j.polymer.2016.04.070.369. P.-Y. B. Jar and W. Cao, Eng. Fract. Mech. 96 (2012) 179–191, doi:10.1016/37j.engfracmech.2012.07.019.381640008-7

Toughness Enhancement of Commercial Poly (Hydroxybutyrate-co-Valerate) (PHBV) by Blending with a Thermoplastic Polyurethane (TPU)

Poly(hydroxyl butyrate-co-valerate) (PHBV) is a biopolymer synthesized by microorganisms that is fully biodegradable with improved thermal and tensile properties with respect to some commodity plastics. However, it presents an intrinsic brittleness that limits its potential application in replacing plastics in packaging applications. Films made of blends of PHBV with different contents of thermoplastic polyurethane (TPU) were prepared by single screw extruder and their fracture toughness behavior was assessed by means of the essential work of fracture (EWF) Method. As the crack propagation was not always stable, a partition method has been used to compare all formulations and to relate results with the morphology of the blends. Indeed, fully characterization of the different PHBV/TPU blends showed that PHBV was incompatible with TPU. The blends showed an improvement of the toughness fracture, finding a maximum with intermediate TPU contents.</jats:p

Toughness Enhancement of Commercial Poly (Hydroxybutyrate-co-Valerate) (PHBV) by Blending with a Thermoplastic Polyurethane (TPU)

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