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PA66 system: Effect of PA66 phase geometry and graphite nanoplatelets addition
Polymer Testing 85 (2020) 106452
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Material Properties
Creep resistance of HDPE/PA66 system: Effect of PA66 phase geometry and graphite nanoplatelets addition �lkov�a a, Sabina Krej�cíkova � a, Ivan Kelnar a, *, Aleksandra Uj�ci�c a, Ludmila Kapra a b b b Alexander Zhigunov , Ctirad Novotný , Zden�ek Padovec , Milan Rů�zi�cka a b
Institute of Macromolecular Chemistry, Czech Academy of Sciences, Heyrovsk�eho N� am. 2, 162 06, Praha, Czech Republic Czech Technical University, Faculty of Mechanical Engineering, Technick� a 4, 166 07, Praha, Czech Republic
A R T I C L E I N F O
A B S T R A C T
Keywords: Graphite nanoplatelets Interface Microfibrillar composite Creep resistance Finite element analysis
Microfibrillar composites (MFC) with in-situ generated short polymeric fibres feature, unlike composites con taining inorganic rigid fibres/particles, lower creep resistance in comparison with analogous blends containing spheres. Further attribute is unprecedented decrease in creep resistance of the blend by graphite nanoplatelets (GNP). Explanation of this behaviour of the HDPE/PA66/GNP system consists in characterization of structure and finite element analysis (FEA) „mapping“ the effect of reinforcement and interface parameters on creep behaviour. Lowering of reinforcement modulus and its viscoelasticity may lead to worse creep resistance of fibrous composites. FEA also indicates marked negative effect of the soft interface, i.e. GNP-reduced crystallinity of HDPE near the interface, on creep resistance of the spheres-reinforced system in contrast to MFC. Structural changes are indicated by polarized light microscopy, SEM and TEM. The results reveal so far unknown complexity of the performance of polymer/polymer composites which may cause unprecedented antagonistic effects.
1. Introduction Low resistance against long-term loading is an important limitation of polymeric materials for many engineering applications [1,2]. In the case of thermoplastics, the improvement is mostly achieved by rein forcement, i.e. by formation of composites. The reinforcement is ach ieved by addition of rigid, mostly inorganic (short) fibres [3,4], microand nano-sized elements [5–12]. In the case of nanofillers (NF) with high aspect ratio, their high effect at low contents has an advantage of practically unchanged processability [13]. However, in addition to difficult dispergation, also geometric limitations may lead to possible re-stacking at higher contents, which may cause even decrease in creep resistance [5–7]. As a result, the combination of micro- and nano-reinforcement seems to be beneficial in many cases [14–16]. Successful example is NF modified microfibrillar composites MFC, which are in fact short-fibre composites with in-situ formed reinforce ment [17]. This structure is achieved by melt or cold drawing of polymer blends with advantage of perfect fibre dispersion, interfacial bonding, etc. [18–20] The relatively low mechanical parameters can be improved by additional NF reinforcement [21–27]; of importance is
structure-directing effect [28,29] of NF which is even more complex in comparison with undrawn blends. Recently, important effect of NF migration between polymer components in the course of melt drawing has been found. This may affect parameters of the interface, which may also lead to antagonistic effects [25]. In the area of polymer-polymer composites, creep behaviour was studied in some single polymer com posites [30–33]. In all cases, marked improvement was found in com parison with the matrix polymer. The creep behaviour of MFC has so far been studied very rarely. In the case of two step extrusion-prepared HDPE/PA6 system containing relatively high amount of wood flour, the short-term creep behaviour in three-point bending mode indicates positive effect of both types of reinforcement, i.e. of both PA6 fibrils and additional wood flour-reinforcing on creep resistance, when compared with the undrawn system [26]. In our work dealing with creep resistance of HDPE/PA66 MFC modified by GNP, similar results were found in analogous testing mode [34]. On the other hand, in the case of tensile creep resistance, unex pected systematically lower resistance of fibrils-containing samples (MFC) was found in comparison with undrawn, i.e. PA66 particles containing samples. Whereas creep of MFC was improved by graphite
* Corresponding author. E-mail address: [email protected] (I. Kelnar). https://doi.org/10.1016/j.polymertesting.2020.106452 Received 7 January 2020; Received in revised form 18 February 2020; Accepted 20 February 2020 Available online 21 February 2020 0142-9418/© 2020 Elsevier Ltd. All rights reserved.
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Fig. 1. RVE of fibre-containing system (a) and with diagonal cut for looking inside (b).
nanoplatelets (GNP), further peculiarity is the fact that addition of GNP has a negative effect on creep resistance of the undrawn blend. This is also a strong difference from HDPE/GNP and PA66/GNP behaviour. The present work aims at explaining this unexpected behaviour by more thorough evaluation of structure and especially using the FEM analyzing (“mapping“) effect of the reinforcing phase geometry, modulus and viscoelasticity together with parameters of the interface.
2.4. Morphology characterization Structure of the fibrils was examined using scanning electron mi croscopy (SEM) with a Quanta 200 FEG microscope (FEI, Czech Re public). The HDPE matrix was removed using a Soxhlet extraction apparatus with boiling xylene for 10 h. Polarized light microscopy (PLM) Morphology of the samples was visualized in a light microscope Nikon Eclipse 80i (Nikon, Japan) equipped with a digital camera ProgRes CT3 (Jenoptic, Germany). The samples were observed using bright field imaging in polarized light. TEM micrographs were obtained with a transmission electron microscope Tecnai G2 Spirit Twin (FEI, Czech Republic) using bright field imaging mode at accelerating voltage 120 kV. The ultrathin sections (thickness 50 nm) were prepared with an ultramicrotome Ultracut EM UC7 (Leica, Wetzlar, Germany) equipped with a cryo-attachment Leica EM FC7.
2. Experimental 2.1. Materials High-density polyethylene (HDPE) HYA 800 (Exxon Mobil); poly amide 66 (PA66) Zytel E55 NC10 (DuPont); HDPE/15 wt% graphite nanoplatelets (GNP) masterbatch heXo-HDPE-15W; PA66/15 wt% GNP masterbatch heXo-PA66-15W (NanoXplore Inc.); GNP are few-layer graphene, heXo-G V (NanoXplore Inc.). All components were used as received.
2.5. Wide-angle X-ray diffraction (XRD) Diffraction patterns were obtained using high-resolution diffrac tometer Explorer (GNR Analytical Instruments, Italy). The instrument is equipped with one-dimensional silicon-strip detector Mythen 1K (Dectris, Switzerland). Samples were measured in the reflection mode. The radiation CuKα (wavelength λ ¼ 1.54 Å) monochromatized with Ni foil (β filter) was used for diffraction. The measurement was done in range 2θ ¼ 5–45� with step 0.1� . Exposure time at each step was 10 s.
2.2. Blend and MFC preparation Prior to mixing, PA66 was dried in a vacuum oven at 85 � C for 12 h. The mixing proceeded in a co-rotating segmented twin-screw extruder (L/D 40) Brabender TSE 20 at 400 rpm, and temperatures of the respective zones of 240, 260, 260, 275, 275, and 270 � C. The extruded bristle was melt-drawn using an adjustable take-up device. The draw ratio is the ratio between the velocity of the take-up rolls and the initial velocity of the extruded bristle. Dog-bone specimens (gauge length 40 mm) were prepared in a laboratory micro-injection moulding machine (DSM). The barrel and the mould temperatures were 200 � C and 70 � C, respectively. GNP addition protocols used: a) application of pre-made PA66/GNP nanocomposite b) application of the analogous HDPE/GNP nano composite c) combination of a) and b). Example of sample composition: “HDPEþ2/PA66 þ 2” means that MFC consists of HDPE pre-blended with 2 phr GNP and PA66 pre-blended with 2 phr GNP. The ratio of HDPE/PA66 was 80/20 w/w in all systems studied. Dog-bone specimens for creep evaluation (gauge length 100 mm) were prepared by injection moulding in Engel Victory 200/50 machine. Analogous smaller speci mens with gauge length of 40 mm were prepared using micro-injection moulding machine (DSM). The barrel and mould temperatures were 200 � C and 70 � C, respectively.
2.6. Tensile creep measurements Tensile creep was measured using an apparatus equipped with a mechanical stress amplifier (lever) 10:1. A digital strain gauge (with accuracy of about 1 μm) was connected with the upper clamp of the specimen to indicate the displacement. The samples were prepared from injected dumb-bell specimens with initial distance between grips 90 mm and cross-section 3.9 mm � 10.1 mm. Short-term measurements in the interval of 100 min were carried out by application of stress level of ~5 MPa. Specimens were stored and creep tests were implemented at (22 � 2)� C. 3. Numerical analysis – sensitivity study Two types of Finite Element (FE) models were created to assess the effect of the reinforcement shape and the interphase thickness. The models were created and analyzed by commercial software Abaqus/ Standart [36]. In all cases, the FE-mesh consists of 8-nodes linear brick elements with reduced integration (C3D8R Abaqus-type).
2.3. Mechanical testing
3.1. Influence of reinforcement shape on effective modulus – elastic analysis
Tensile tests were carried out at 22 � C using an Instron 5800 appa ratus at crosshead speed of 50 mm/min. At least 10 specimens were tested for each sample. Young modulus (E), maximum stress (σ m) and elongation at break (εb) were evaluated; the corresponding variation coefficients did not exceed 10%, 2% and 20%, respectively.
The first type of models is used for estimation of homogenized effective elastic properties, namely Young’s modulus. A periodic 2
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Fig. 2. RVE of sphere-containing system (a) and with diagonal cut for looking inside (b).
Fig. 3. Model for viscoelastic-analysis of fibre-containing system (a) and with diagonal cut for looking inside (b).
representative volume element (RVE) was created for both fibre and sphere-containing systems. Fibre containing model has a hexahedron shape. The fibres are oriented parallel to the longer edge. The RVE contains one whole fibre and eight eighths of the fibre (Fig. 1). The sphere-containing model has the shape of a cube. RVE contains one whole sphere and eight eighths of sphere (Fig. 2). The space between the fibres or the spheres corresponds to the matrix. A thin layer modelling the interphase is considered between the fibres or the spheres and the matrix. The diameter of the fibres or spheres was chosen as a unit. Other dimensions of RVE are given by the requirement of equal volume frac tion of fibres or balls in both models - 0.2%. Both shorter sides of the hexahedron with fibres are 2.238 mm, the longer edge is 45 mm long. The distance between nodes on the shorter edges ranges from 0.038 to 0.04 mm, on the longer edge 0.5 mm. The cube edge with spheres is 1.736 mm in size. The distance between the nodes at the edges ranges from 0.033 to 0.038 mm. Node-to-node periodic conditions [37] were introduced for estimation of homogenized effective Young’s modulus in the direction of fibre orientation or in the direction perpendicular to the selected wall in the case of the sphere-containing model. For practical application, plug-in of Abaqus software developed by the authors of the article [37] was used. In the homogenization calculations, elastic
materials with the following Young’s modulus were considered: fibre or sphere 3000 MPa, matrix 1200 MPa. The Young’s modulus of the interphase was chosen as 10 or 500 or 1200 MPa. The last case means that the interphase has the same properties as the matrix, it is a model without an interphase. The Thickness ratio (ratio of interphase thickness to fibre or sphere diameter) was 0.001, 0.002, 0.005, 0.01 and 0.02. The calculated values of homogenized effective Young’s modulus are given in Table 2. In the case of fibre-containing RVE, the effect of the interphase is relatively small. The difference of effective Young’s modulus for a model with no interphase and with a Young’s modulus of 10 MPa is 2 MPa (thickness ratio 0.001) or 30 MPa (thickness ratio 0.02). The interphase and its thickness in the sphere-containing RVE have greater influence. The difference of effective Young’s modulus for a model with and without an interphase with Young’s modulus of 10 MPa is 127 MPa (thickness ratio 0.001) or 476 MPa (thickness ratio 0.02). 3.2. Influence of reinforcement shape on effective modulus – viscoelastic analysis The second type of models serves to assess the influence of shape of the reinforcement and thickness of the interphase, considering the 3
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Fig. 4. Model for viscoelastic-analysis of sphere-containing system (a) and with diagonal cut for looking inside (b).
Fig. 5. SEM images of PA6 reinforcement a) undrawn and b) melt drawn HDPE/PA66 b) c) undrawn HDPEþ2GNP/PA66þ2GNP d) drawn HDPE/PA66þ2GNP
viscoelastic creep of all components. The RVE models are kinematicloaded and therefore were not used for creep analysis with force loading. New FE-models composed of 27 RVEs were used. These models
were created by copying RVE three times in each perpendicular direc tion (Figs. 3 and 4). Thus, the models are 6.865 � 6.865 � 135 mm (fibre-containing) or 5.209 � 5.209 � 5.209 mm (sphere-containing). 4
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Fig. 6. TEM images of undrawn blends a) HDPEþ2GNP/PA66, b)HDPE/PA66þ2GNP
Fig. 7. PLM of a) HDPE/PA66, b)HDPE/PA66þ2GNP
Fig. 8. X-ray diffraction pattern of the undrawn samples containing PA66 spheres.
Fig. 9. Effect of increasing rigidity of PA66 phase on ratio of stiffness of MFC and undrawn blend.
The distance between nodes on the shorter edges of the fibre-containing model ranges from 0.167 to 0.214 mm, on the longer edge 0.938 mm.
The distance between the nodes on the shorter edges of the spherecontaining model ranges from 0.122 to 0.125 mm. The nodes lying on one face are prescribed zero shifts (in the case of 5
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Unlike MFC based on HDPE/PA66 modified with organophilized montmorillonite (oMMT) leading to markedky thicker fibres (up to 10 μm) in comparison with original inclusions [23], the effect of GNP on the coalescence in the case of melt drawing was lower [27]. Because of expected importance of parameters of the interface due to variation in HDPE crystallinity in this area induced by interfacially localized GNP [25], the morphological observations were focused on highlighting possible differences in parameters of the interface and/or surface of fi bres and spheres. The high-magnification SEM images (Fig. 5) of sepa rated inclusions and fibres also indicate some difference between neat and GNP-modified systems, most probably caused by GNP localized in this area. However, we must bear in mind that there is a risk of artefacts due to extraction of the matrix by hot xylene. TEM images of the un drawn blend in Fig. 6 confirm dominant presence of GNP in PA66 in clusions at the interface. Therefore, we can anticipate GNP-induced changes in HDPE crystallinity at the interface, i.e. formation of a low-modulus interlayer [25]. Similar localization was found in the case of melt–drawing formed fibres in MFC [24]. Although the Polarized light microscopy and especially digitalized images are practically on limit of resolution in the system studied, the images (Fig. 7) indicate different (lower) amount of HDPE spherulites (small light-grey domains) around the PA66 inclusions (larger more light domains without GNP or darker domains due to GNP inside and at the interface). From Fig. 7b follows lower amount of HDPE sperulites around PA66 particles in GNP-modified system, which may lead to change in parameters of the HDPE matrix in the vicinity of the PA66 fibres/particles. Similar, slightly more marked changes in structure have been observed in related PA66 fibres containing MFC [24].
Fig. 10. Tensile creep resistance of HDPE/PA66/GNP blend and related MFC, DR ¼ draw ratio.
4.2. XRD As it is seen from the diffractograms in Fig. 8, all the peaks remain in their positions. The only traceable change is observed at 2Θ ¼ 26.44� , which corresponds to graphite (002) plane. Obviously, the peak is not present in the sample of the GNP-free system and intensity is growing up with increasing GNP content in the composite. These results indicate that, within the resolution of XRD applied, there are no changes in crystallinity of both semicrystalline polymer components. As mentioned previously, the possible changes in crystallinity of the HDPE matrix in the vicinity of the PA6 inclusions represent extremely small volume. Therefore, their contribution to bulk crystallinity is practically unde tectable by XRD.
Fig. 11. FEA: Effect of reinforcement geometry and interface on creep resis tance of HDPE/inorganic rigid (glass, 74 000 MPa) reinforcement.
the fibre-containing model it is the face perpendicular to the fibre di rection). The 10 N load is applied to the reference point from which it is distributed to the nodes lying on the opposite face. Creep properties were considered the same for all components (fibre or sphere, matrix and interphase) for simplicity and missing data. The starting point is the creep tensile curve of HDPE from which creep compliance was evalu ated. The dependence of relaxation modulus was calculated using nu merical interconversion [38] and subsequently used for FE linear viscoelastic simulation. The calculation was performed at 0–6000 s time. The displacement of the reference point is analyzed and the effective modulus Eeffective is calculated. This is defined as Eeffective ¼
4.3. Effect of composition and drawing on properties Table 2 shows mechanical behaviour of blends and related MFC without GNP and with GNP added using 3 mixing protocols, i.e. GNP premixed solely in PA66, HDPE and in both components. The results indicate that tensile strength and modulus in some cases do not fully correspond to dual reinforcement with PA66 particles/fibrils and GNP. This was attributed to a complex influence of GNP [24] producing both antagonistic and synergistic effects in spite of similar main structural parameters (composition, geometry of reinforcement and crystallinity). The plausible explanation seems be GNP-induced changes in HDPE crystallinity at the interface [25], see above. The presence of these ef fects is simply indicated by markedly different ratios of modulus of MFC and analogous undrawn blend (Table 2), which is in contrast with only slight change in EMFC/EBlend in the expected range of components pa rameters with increasing modulus of reinforcement predicted using Halpin-Tsai model [35] in Fig. 9.
F⋅l A⋅u
where is the loading force F, dimension of model in the direction of loading l, area of loaded face A and displacement of reference point u. 4. Results and discussion 4.1. Structure of HDPE/PA66 systems and GNP localization
4.4. Effect of PA66 phase geometry and interface on creep resistance
The discussion dealing with the effect of GNP and the mixing pro tocol on fibrils dimensions is presented in our previous works [24,34]. An important fact is that fibre diameter (~2–3 μm) and length in all systems were comparable, aspect ratio (AR) exceeded the value of 15.
Fig. 10 shows example of the unexpected behaviour of MFC, i.e. lower creep resistance of the fibre-contaning system in comparison with the spheres-reinforcement. This is a great difference e.g. from glass 6
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Fig. 12. FEA: Effect of reinforcement geometry and interface on creep resistance of HDPE/polymer-based reinforcement, interface thickness/fibre diameter ratio 0.1 a) creep of matrix only b)creep of all components.
beads- and fibres-based systems with the same HDPE matrix with markedly higher creep resistance of the fibre-reinforced composite, as shown in the preceding study [34]. At the same time, within MFC, the only „expected“ behaviour consists in increase of creep resistance of MFC by the GNP reinforcement. Fig. 10 also shows minor affecting of
creep resistance of MFC by the GNP addition method (see above) with the best creep resistance achieved in the case of pre-blending GNP in PA66, i.e. with elimination of the negative effect of GNP migration in the course of melt drawing [24] (leading also to high E, see Table 2.). From Fig. 10, another unexpected finding also follows, i.e. the negative effect 7
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Fig. 13. FEA: Effect of reinforcement geometry and interface on creep resistance of HDPE/polymer-based reinforcement, interface thickness/fibre diameter ratio 0.1 a) creep of matrix only b)creep of both components.
of GNP addition on creep resistance of the undrawn blend, which is in contrast to MFC and also to the positive effect of GNP on the blend constituents, i.e., PA66 and HDPE.
4.5. Finite element analysis To understand the peculiar creep behaviour of MFC and the related blend, FEA was applied to highlight the role of expected crucial effects, i. e. reinforcing component geometry, modulus and viscoelasticity (creeping/non-creeping reinforcement) together with interphase 8
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Fig. 14. FEA: Effect of reinforcement geometry and interphase on creep resistance of HDPE/polymer-based reinforcement, interface thickness/fibre diameter ratio 0.02 a) creep of matrix, b) creep of all components.
parameters (thickness, modulus, creeping). An important advantage of FEA is especially good description of effects of the interface parameters on performance of composites [25,39]. Figs. 11–13 demonstrate the effect of reinforcement and interface modulus in the case of interface thickness/fibre diameter ratio 0.1
(relatively thick layer corresponding to dimensions of spherulites [25]) on creep modulus. From the creep curves in Fig. 11, marked difference follows between the particulate and fibrous composites in the case of high modulus (74 000 MPa) glass reinforcement, while this difference is reduced in polymer blend and related MFC with significantly lower 9
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Fig. 15. FEA: Effect of reinforcement geometry and interphase on creep resistance of HDPE/polymer-based reinforcement, interface thickness/fibre diameter ratio 0.002 a)creep of matrix b) creep of all components.
(3000 MPa) modulus of reinforcement (Fig. 12a), especially if it creeps (Fig. 12b). This difference is the lowest for the polymer-based rein forcement with E 2000 MPa (Fig. 13a and b). If we consider the effect of „soft“ (10 MPa) interface on creep resistance, FEA indicates an inter esting trend. With the rigid glass-based reinforcement, where the matrix
creep is considered only, the soft interface layer reduces creep of the fibrous composite more significantly than that of the spheres-system (Fig. 11). This negative effect on creep of the fibrous system (differ ence between fibres and spheres) is reduced with decreasing Ereinf (Figs. 12 a, 13a) and, more markedly, if we consider also viscoelasticity 10
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5. Conclusions
Table 1 The homogenized effective Young’s modulus for fibre and sphere-containing RVE. Thickness ratio [1]
Fibre-containing RVE 0.001 0.002 0.005 0.010 0.020
The results based on FEA „mapping“ of the effect of parameters and geometry of the components and the interface in combination with structural analysis indicate that, unlike rigid glass-reinforcement, the difference between creep resistance of the PA66-fibres and the HDPE matrix reinforced with spherical inclusions is less marked or may be even eliminated. This originates especially from low (EReinf/EMatrix) ratio and viscoelasticity (creeping) of the reinforcement. Lower creep resis tance of MFC in real systems corresponds to further specific features of polymeric fibres, such as tangling. The quite unexpected negative effect of GNP on creep of the PA66 spheres-reinforced system corresponds to the FEA-indicated more marked negative effect of the soft interphase on creep of the spheres-containing system in comparison to fibrous one. As a result, the GNP-induced changes in interface rigidity may have crucial impact on creep resistance and can eliminate the reinforcing effect of GNP on the components, whereas this effect dominates in MFC. The results indicate the unknown complexity of the performance of multi component polymer systems in long-term loading.
Young’s modulus of interphase [MPa] Sphere-containing RVE
10
500
1200
10
500
1200
1566 1564 1559 1551 1538
1567 1567 1565 1562 1556
1568
1321 1260 1162 1073 972
1445 1443 1435 1424 1403
1448
Table 2 Composition and mechanical properties of HDPE/PA66 80/20 blends and related MFC. Composition
DR
E (MPa)
σm (MPa)
(EF-ES)a
(EF/ES)
HDPE/PA66 HDPE/PA66 HDPEþ2/PA66 þ 2 HDPEþ2/PA66 þ 2 HDPEþ2/PA66 HDPEþ2/PA66 HDPE/PA66 þ 2 HDPE/PA66 þ 2 HDPE/PA66 þ 3 HDPE/PA66 þ 3
1 6 1 6.5 1 6.5 1 6.5 1d 6
1240 � 74 1405 � 82 1470 � 26 1565 � 63 1295 � 36 1475 � 45 1494 � 60 1595 � 56 1353 � 50 1613 � 55
33.1 � 2.1 44 � 2.4 32.5 � 0.8 42.2 � 0.4 31.2 � 0.7 43.3 � 1.3 35.7 � 0.5 46.6 � 0.8 34.2 � 1.3 45.5 � 1.3
165 – 95 – 180 – 101 – 260 –
1.133 – 1.064 – 1.139 – 1.067 – 1.192 –
Data availability statement The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Declaration of competing interest
a
EF, ES moduli of fibre-reinforced MFC and spheres-reinforced undrawn blend, resp.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[40–42] and thus creep of reinforcement and interface (Figs. 12b and 13b). Another important fact is that, in polymer/polymer systems, the negative effect of the soft interface on creep resistance is always more marked for spheres (Figs. 12–15). In the case of the HDPE/PA66 systems, the effect of thinner interface (thickness/diameter ratio 0.02 and 0.002) is also analyzed (Figs. 14 and 15). As expected, the negative effect on the creep is lower with smaller interphase thickness (see Figs. 13 and 15), which also leads to decreasing difference between fibres and spheres. Figs. 14 and 15 show even negligible affecting of the fibrous composites (both creeping and non-creeping) by the soft interface. Finally, in the case of the thin (ratio 0.02 and 0.002) rigid (1200 MPa) creeping interface (and reinforce ment), the creep resistance of the fibrous composite is even slightly worse than of that containing spheres (Figs. 14b and 15b). The above analysis indicates that peculiar behaviour of polymer/ polymer systems is above all caused by low modulus of the reinforce ment with relatively important contribution of viscoelasticity and the interface. The effect of the last two parameters diminishes with the interface thickness. Although the FEA indicates that polymeric rein forcement can only lead to merging of the fibre- and sphere-containing systems, other unfavourable features of polymer fibres, especially tangling, can undoubtedly further decrease creep resistance, as found in real systems. At the same time, the more marked negative effect of the „soft“ [43–46] interface on creep of the blend (spherical inclusions) in com parison with MFC (Figs. 12–15, Table 1) may be a basis for explanation of unexpected deterioration of creep resistance of such system with GNP addition (Fig. 10). This is based on the fact that the expected effect of interface-localized GNP (Fig. 6) on lowering of the interface rigidity (see above) may thus cause more marked worsening of creep resistance of the blend, eliminating contribution of the GNP-reinforcement. Due to a much lower or negligible negative effect of the interphase on creep resistance of MFC (Figs. 12–15), the reinforcing effect dominates and creep resistance is improved in this case.
CRediT authorship contribution statement �: Data Ivan Kelnar: Writing - original draft. Aleksandra Ujc�ic � lkova �: Data curation. Sabina Krej� �: curation. Ludmila Kapra cíkova Formal analysis. Alexander Zhigunov: Formal analysis. Ctirad �k Padovec: Investigation. Milan Novotný: Conceptualization. Zdene Rů� zi� cka: Supervision. Acknowledgments This work was supported by Czech Science Foundation (Grant No 1906065S). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.polymertesting.2020.106452. References [1] M.A. Meyers, K.K. Chawla, Mechanical Behavior of Materials, Cambridge University Press, 1999, p. 573. [2] N.G. McCrum, C.P. Buckley, C.B. Bucknall, Principles of Polymer Engineering, Oxford Science Publications, 2003. [3] M. Basso, L. Pupure, M. Simonato, R. Furlanetto, L. De Nardo, R. Joffe, Nonlinear creep behaviour of glass fiber reinforced polypropylene: impact of aging on stiffness degradation, Compos. B Eng. 163 (2019) 702–709. [4] S.T. Mileiko, Steady state creep of a composite material with short fibres, J. Mater. Sci. 5 (1970) 254–271. [5] M.K. Hassanzadeh-Aghdam, R. Ansari, M.J. Mahmoodi, A. Darvizeh, Effect of nanoparticles aggregation on the creep behavior of polymer nanocomposite, Compos. Sci. Technol. 162 (2018) 93–100. [6] C.-M. Liu, F.-F. Ma, Z.-X. Zhang, J.-H. Yang, Y. Wong, Z.-W. Zhou, Selective localization of organic montmorillonite in poly(L-lactide)/poly(ethylene vinyl acetate) blends and the resultant proprieties, Compos. B Eng. 123 (2017) 1–9. [7] L. Tang, Y. Li, Y. Chen, P. Ji, C. Wong, H. Wong, Q. Huang, Preparation and characterization of graphene reinforced PA6 fibre, J. Appl. Polym. Sci. 135 (2018) 45834.
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[8] A.T. Seyhan, A. Tuna, O. Durukan, Y. G€ oncü, S. Turan, N. Ay, Creep and recovery behaviors of chemically induced grafted low density polyethylene films containing silanized hexagonal boron nitride nanosheets, Mater. Expr. 8 (2018) 55–67. [9] Y.V. Suvorova, S.I. Alekseeva, M.A. Fronya, I.V. Viktorova, Investigations of physical and mechanical proprieties of polymeric nanocomposites, Inorg. Mater. 49 (2013) 1357–1368. [10] Z. Dai, Y. Gao, L. Liu, P. P€ otschke, J. Yang, Z. Zhang, Creep-resistance behavior of MWCNT-polycarbonate melt spun nanocomposite fibres at elevated temperature, Polymer 54 (2013) 3723–3729. [11] A. Dorigato, A. Pegoretti, J. Kola�rík, Nonlinear tensile creep of linear low density polyethylene/fumed silica nanocomposites time-strain superposition and creep prediction, Polym. Compos. 31 (2010) 1947–1955. [12] M. Li, D. Li, L.J. Wang, B. Adhikari, Creep behavior of starch-based nanocomposite films with cellulose nanofibrils, Carbohydr. Polym. 117 (2015) 957–963. [13] S.N. Bhattacharya, R.K. Gupta, M.R. Kamal, Polymeric Nanocomposites, Hanser, Munich, 2008. [14] N. Saba, P.M. Tahir, M. Jawaid, A review on potentiality of nano filler/natural fibre filled polymer hybrid composites, Polymers 6 (2014) 2247–2273. [15] B. Suresha, B.N. Ravi Kumar, M. Venkataramareddy, T. Jayaraju, Role of micro/ nano fillers on mechanical and tribological properties of polyamide66/ polypropylene composites, Mater. Des. 31 (2010) 1993–2000. [16] H. Varela-Rizo, M. Weisenberger, D.R. Bortz, I. Martin-Gullon, Fracture toughness and creep performance of PMMA composites containing micro and nanosized carbon filaments, Compos. Sci. Technol. 70 (2010) 1189–1195. [17] K. Friedrich, M. Evstatiev, S. Fakirov, O. Evstatiev, M. Ishii, M. Harrass, Microfibrillar reinforced composites from PET/PP blends: processing, morphology and mechanical properties, Compos. Sci. Technol. 65 (2005) 107–116. [18] K. Jayanarayan, S. Thomas, K. Joseph, Morphology, static and dynamic mechanical properties of in situ microfibrillar composites based on polypropylene/poly (ethylene terephthalate) blends, Compos. A Appl. Sci. Manuf. 39 (2008) 164–175. [19] S. Fakirov, D. Bhattacharyya, R.J.T. Lin, C. Fuchs, K. Friedrich, Contribution of coalescence to microfibril formation in polymer blends during cold drawing, J. Macromol. Sci. Part B-Physics 46 (2007) 183–193. [20] I. Kelnar, I. Fortelný, L. Kapr� alkov� a, J. Kratochvíl, M. Nevoralov� a, Effect of layered silicates on fibril formation and properties of PCL/PLA microfibrillar composites, J. Appl. Polym. Sci. 133 (2016) 43061. [21] R.J.T. Lin, D. Bhattacharyya, S. Fakirov, Innovative manufacturing of carbon nanotube-loaded fibrillar polymer composites, Int. J. Mod. Phys. B 24 (2010) 2459–2465. [22] S. Yesil, G. Bayram, Effect of carbon nanotube surface treatment on the morphology, electrical, and mechanical properties of the microfibre-reinforced polyethylene/poly(ethylene terephthalate)/carbon nanotubecomposites, J. Appl. Polym. Sci. 127 (2013) 982–991. [23] I. Kelnar, L. Kapr� alkov� a, J. Kratochvíl, J. Kotek, L. Kobera, J. Rotrekl, J. Hrom� adkov� a, Effect of nanofiller on the behaviour of a melt-drawn HDPE/PA6 microfibrillar composite, J. Appl. Polym. Sci. 132 (2015) 41868. [24] I. Kelnar, Ü. Bal, A. Zhigunov, L. Kapr� alkov� a, I. Fortelný, S. Krej�cíkov� a, J. Kredatusov� a, Complex effect of graphite nanoplatelets on performance of HDPE/ PA66 microfibrillar composites, Compos. Part. B, Eng. 144 (2018) 220–228. [25] I. Kelnar, J. Kratochvíl, L. Kapr� alkov� a, Z. Padovec, M. Rů�zi�cka, A. Zhigunov, M. Nevoralov� a, Antagonistic effects on mechanical properties of microfibrillar composites with dual reinforcement: explanation by FEA model of soft interface, J. Appl. Polym. Sci. 134 (2017) 44712.
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Material Properties
Creep resistance of HDPE/PA66 system: Effect of PA66 phase geometry and graphite nanoplatelets addition �lkov�a a, Sabina Krej�cíkova � a, Ivan Kelnar a, *, Aleksandra Uj�ci�c a, Ludmila Kapra a b b b Alexander Zhigunov , Ctirad Novotný , Zden�ek Padovec , Milan Rů�zi�cka a b
Institute of Macromolecular Chemistry, Czech Academy of Sciences, Heyrovsk�eho N� am. 2, 162 06, Praha, Czech Republic Czech Technical University, Faculty of Mechanical Engineering, Technick� a 4, 166 07, Praha, Czech Republic
A R T I C L E I N F O
A B S T R A C T
Keywords: Graphite nanoplatelets Interface Microfibrillar composite Creep resistance Finite element analysis
Microfibrillar composites (MFC) with in-situ generated short polymeric fibres feature, unlike composites con taining inorganic rigid fibres/particles, lower creep resistance in comparison with analogous blends containing spheres. Further attribute is unprecedented decrease in creep resistance of the blend by graphite nanoplatelets (GNP). Explanation of this behaviour of the HDPE/PA66/GNP system consists in characterization of structure and finite element analysis (FEA) „mapping“ the effect of reinforcement and interface parameters on creep behaviour. Lowering of reinforcement modulus and its viscoelasticity may lead to worse creep resistance of fibrous composites. FEA also indicates marked negative effect of the soft interface, i.e. GNP-reduced crystallinity of HDPE near the interface, on creep resistance of the spheres-reinforced system in contrast to MFC. Structural changes are indicated by polarized light microscopy, SEM and TEM. The results reveal so far unknown complexity of the performance of polymer/polymer composites which may cause unprecedented antagonistic effects.
1. Introduction Low resistance against long-term loading is an important limitation of polymeric materials for many engineering applications [1,2]. In the case of thermoplastics, the improvement is mostly achieved by rein forcement, i.e. by formation of composites. The reinforcement is ach ieved by addition of rigid, mostly inorganic (short) fibres [3,4], microand nano-sized elements [5–12]. In the case of nanofillers (NF) with high aspect ratio, their high effect at low contents has an advantage of practically unchanged processability [13]. However, in addition to difficult dispergation, also geometric limitations may lead to possible re-stacking at higher contents, which may cause even decrease in creep resistance [5–7]. As a result, the combination of micro- and nano-reinforcement seems to be beneficial in many cases [14–16]. Successful example is NF modified microfibrillar composites MFC, which are in fact short-fibre composites with in-situ formed reinforce ment [17]. This structure is achieved by melt or cold drawing of polymer blends with advantage of perfect fibre dispersion, interfacial bonding, etc. [18–20] The relatively low mechanical parameters can be improved by additional NF reinforcement [21–27]; of importance is
structure-directing effect [28,29] of NF which is even more complex in comparison with undrawn blends. Recently, important effect of NF migration between polymer components in the course of melt drawing has been found. This may affect parameters of the interface, which may also lead to antagonistic effects [25]. In the area of polymer-polymer composites, creep behaviour was studied in some single polymer com posites [30–33]. In all cases, marked improvement was found in com parison with the matrix polymer. The creep behaviour of MFC has so far been studied very rarely. In the case of two step extrusion-prepared HDPE/PA6 system containing relatively high amount of wood flour, the short-term creep behaviour in three-point bending mode indicates positive effect of both types of reinforcement, i.e. of both PA6 fibrils and additional wood flour-reinforcing on creep resistance, when compared with the undrawn system [26]. In our work dealing with creep resistance of HDPE/PA66 MFC modified by GNP, similar results were found in analogous testing mode [34]. On the other hand, in the case of tensile creep resistance, unex pected systematically lower resistance of fibrils-containing samples (MFC) was found in comparison with undrawn, i.e. PA66 particles containing samples. Whereas creep of MFC was improved by graphite
* Corresponding author. E-mail address: [email protected] (I. Kelnar). https://doi.org/10.1016/j.polymertesting.2020.106452 Received 7 January 2020; Received in revised form 18 February 2020; Accepted 20 February 2020 Available online 21 February 2020 0142-9418/© 2020 Elsevier Ltd. All rights reserved.
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Fig. 1. RVE of fibre-containing system (a) and with diagonal cut for looking inside (b).
nanoplatelets (GNP), further peculiarity is the fact that addition of GNP has a negative effect on creep resistance of the undrawn blend. This is also a strong difference from HDPE/GNP and PA66/GNP behaviour. The present work aims at explaining this unexpected behaviour by more thorough evaluation of structure and especially using the FEM analyzing (“mapping“) effect of the reinforcing phase geometry, modulus and viscoelasticity together with parameters of the interface.
2.4. Morphology characterization Structure of the fibrils was examined using scanning electron mi croscopy (SEM) with a Quanta 200 FEG microscope (FEI, Czech Re public). The HDPE matrix was removed using a Soxhlet extraction apparatus with boiling xylene for 10 h. Polarized light microscopy (PLM) Morphology of the samples was visualized in a light microscope Nikon Eclipse 80i (Nikon, Japan) equipped with a digital camera ProgRes CT3 (Jenoptic, Germany). The samples were observed using bright field imaging in polarized light. TEM micrographs were obtained with a transmission electron microscope Tecnai G2 Spirit Twin (FEI, Czech Republic) using bright field imaging mode at accelerating voltage 120 kV. The ultrathin sections (thickness 50 nm) were prepared with an ultramicrotome Ultracut EM UC7 (Leica, Wetzlar, Germany) equipped with a cryo-attachment Leica EM FC7.
2. Experimental 2.1. Materials High-density polyethylene (HDPE) HYA 800 (Exxon Mobil); poly amide 66 (PA66) Zytel E55 NC10 (DuPont); HDPE/15 wt% graphite nanoplatelets (GNP) masterbatch heXo-HDPE-15W; PA66/15 wt% GNP masterbatch heXo-PA66-15W (NanoXplore Inc.); GNP are few-layer graphene, heXo-G V (NanoXplore Inc.). All components were used as received.
2.5. Wide-angle X-ray diffraction (XRD) Diffraction patterns were obtained using high-resolution diffrac tometer Explorer (GNR Analytical Instruments, Italy). The instrument is equipped with one-dimensional silicon-strip detector Mythen 1K (Dectris, Switzerland). Samples were measured in the reflection mode. The radiation CuKα (wavelength λ ¼ 1.54 Å) monochromatized with Ni foil (β filter) was used for diffraction. The measurement was done in range 2θ ¼ 5–45� with step 0.1� . Exposure time at each step was 10 s.
2.2. Blend and MFC preparation Prior to mixing, PA66 was dried in a vacuum oven at 85 � C for 12 h. The mixing proceeded in a co-rotating segmented twin-screw extruder (L/D 40) Brabender TSE 20 at 400 rpm, and temperatures of the respective zones of 240, 260, 260, 275, 275, and 270 � C. The extruded bristle was melt-drawn using an adjustable take-up device. The draw ratio is the ratio between the velocity of the take-up rolls and the initial velocity of the extruded bristle. Dog-bone specimens (gauge length 40 mm) were prepared in a laboratory micro-injection moulding machine (DSM). The barrel and the mould temperatures were 200 � C and 70 � C, respectively. GNP addition protocols used: a) application of pre-made PA66/GNP nanocomposite b) application of the analogous HDPE/GNP nano composite c) combination of a) and b). Example of sample composition: “HDPEþ2/PA66 þ 2” means that MFC consists of HDPE pre-blended with 2 phr GNP and PA66 pre-blended with 2 phr GNP. The ratio of HDPE/PA66 was 80/20 w/w in all systems studied. Dog-bone specimens for creep evaluation (gauge length 100 mm) were prepared by injection moulding in Engel Victory 200/50 machine. Analogous smaller speci mens with gauge length of 40 mm were prepared using micro-injection moulding machine (DSM). The barrel and mould temperatures were 200 � C and 70 � C, respectively.
2.6. Tensile creep measurements Tensile creep was measured using an apparatus equipped with a mechanical stress amplifier (lever) 10:1. A digital strain gauge (with accuracy of about 1 μm) was connected with the upper clamp of the specimen to indicate the displacement. The samples were prepared from injected dumb-bell specimens with initial distance between grips 90 mm and cross-section 3.9 mm � 10.1 mm. Short-term measurements in the interval of 100 min were carried out by application of stress level of ~5 MPa. Specimens were stored and creep tests were implemented at (22 � 2)� C. 3. Numerical analysis – sensitivity study Two types of Finite Element (FE) models were created to assess the effect of the reinforcement shape and the interphase thickness. The models were created and analyzed by commercial software Abaqus/ Standart [36]. In all cases, the FE-mesh consists of 8-nodes linear brick elements with reduced integration (C3D8R Abaqus-type).
2.3. Mechanical testing
3.1. Influence of reinforcement shape on effective modulus – elastic analysis
Tensile tests were carried out at 22 � C using an Instron 5800 appa ratus at crosshead speed of 50 mm/min. At least 10 specimens were tested for each sample. Young modulus (E), maximum stress (σ m) and elongation at break (εb) were evaluated; the corresponding variation coefficients did not exceed 10%, 2% and 20%, respectively.
The first type of models is used for estimation of homogenized effective elastic properties, namely Young’s modulus. A periodic 2
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Fig. 2. RVE of sphere-containing system (a) and with diagonal cut for looking inside (b).
Fig. 3. Model for viscoelastic-analysis of fibre-containing system (a) and with diagonal cut for looking inside (b).
representative volume element (RVE) was created for both fibre and sphere-containing systems. Fibre containing model has a hexahedron shape. The fibres are oriented parallel to the longer edge. The RVE contains one whole fibre and eight eighths of the fibre (Fig. 1). The sphere-containing model has the shape of a cube. RVE contains one whole sphere and eight eighths of sphere (Fig. 2). The space between the fibres or the spheres corresponds to the matrix. A thin layer modelling the interphase is considered between the fibres or the spheres and the matrix. The diameter of the fibres or spheres was chosen as a unit. Other dimensions of RVE are given by the requirement of equal volume frac tion of fibres or balls in both models - 0.2%. Both shorter sides of the hexahedron with fibres are 2.238 mm, the longer edge is 45 mm long. The distance between nodes on the shorter edges ranges from 0.038 to 0.04 mm, on the longer edge 0.5 mm. The cube edge with spheres is 1.736 mm in size. The distance between the nodes at the edges ranges from 0.033 to 0.038 mm. Node-to-node periodic conditions [37] were introduced for estimation of homogenized effective Young’s modulus in the direction of fibre orientation or in the direction perpendicular to the selected wall in the case of the sphere-containing model. For practical application, plug-in of Abaqus software developed by the authors of the article [37] was used. In the homogenization calculations, elastic
materials with the following Young’s modulus were considered: fibre or sphere 3000 MPa, matrix 1200 MPa. The Young’s modulus of the interphase was chosen as 10 or 500 or 1200 MPa. The last case means that the interphase has the same properties as the matrix, it is a model without an interphase. The Thickness ratio (ratio of interphase thickness to fibre or sphere diameter) was 0.001, 0.002, 0.005, 0.01 and 0.02. The calculated values of homogenized effective Young’s modulus are given in Table 2. In the case of fibre-containing RVE, the effect of the interphase is relatively small. The difference of effective Young’s modulus for a model with no interphase and with a Young’s modulus of 10 MPa is 2 MPa (thickness ratio 0.001) or 30 MPa (thickness ratio 0.02). The interphase and its thickness in the sphere-containing RVE have greater influence. The difference of effective Young’s modulus for a model with and without an interphase with Young’s modulus of 10 MPa is 127 MPa (thickness ratio 0.001) or 476 MPa (thickness ratio 0.02). 3.2. Influence of reinforcement shape on effective modulus – viscoelastic analysis The second type of models serves to assess the influence of shape of the reinforcement and thickness of the interphase, considering the 3
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Fig. 4. Model for viscoelastic-analysis of sphere-containing system (a) and with diagonal cut for looking inside (b).
Fig. 5. SEM images of PA6 reinforcement a) undrawn and b) melt drawn HDPE/PA66 b) c) undrawn HDPEþ2GNP/PA66þ2GNP d) drawn HDPE/PA66þ2GNP
viscoelastic creep of all components. The RVE models are kinematicloaded and therefore were not used for creep analysis with force loading. New FE-models composed of 27 RVEs were used. These models
were created by copying RVE three times in each perpendicular direc tion (Figs. 3 and 4). Thus, the models are 6.865 � 6.865 � 135 mm (fibre-containing) or 5.209 � 5.209 � 5.209 mm (sphere-containing). 4
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Fig. 6. TEM images of undrawn blends a) HDPEþ2GNP/PA66, b)HDPE/PA66þ2GNP
Fig. 7. PLM of a) HDPE/PA66, b)HDPE/PA66þ2GNP
Fig. 8. X-ray diffraction pattern of the undrawn samples containing PA66 spheres.
Fig. 9. Effect of increasing rigidity of PA66 phase on ratio of stiffness of MFC and undrawn blend.
The distance between nodes on the shorter edges of the fibre-containing model ranges from 0.167 to 0.214 mm, on the longer edge 0.938 mm.
The distance between the nodes on the shorter edges of the spherecontaining model ranges from 0.122 to 0.125 mm. The nodes lying on one face are prescribed zero shifts (in the case of 5
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Unlike MFC based on HDPE/PA66 modified with organophilized montmorillonite (oMMT) leading to markedky thicker fibres (up to 10 μm) in comparison with original inclusions [23], the effect of GNP on the coalescence in the case of melt drawing was lower [27]. Because of expected importance of parameters of the interface due to variation in HDPE crystallinity in this area induced by interfacially localized GNP [25], the morphological observations were focused on highlighting possible differences in parameters of the interface and/or surface of fi bres and spheres. The high-magnification SEM images (Fig. 5) of sepa rated inclusions and fibres also indicate some difference between neat and GNP-modified systems, most probably caused by GNP localized in this area. However, we must bear in mind that there is a risk of artefacts due to extraction of the matrix by hot xylene. TEM images of the un drawn blend in Fig. 6 confirm dominant presence of GNP in PA66 in clusions at the interface. Therefore, we can anticipate GNP-induced changes in HDPE crystallinity at the interface, i.e. formation of a low-modulus interlayer [25]. Similar localization was found in the case of melt–drawing formed fibres in MFC [24]. Although the Polarized light microscopy and especially digitalized images are practically on limit of resolution in the system studied, the images (Fig. 7) indicate different (lower) amount of HDPE spherulites (small light-grey domains) around the PA66 inclusions (larger more light domains without GNP or darker domains due to GNP inside and at the interface). From Fig. 7b follows lower amount of HDPE sperulites around PA66 particles in GNP-modified system, which may lead to change in parameters of the HDPE matrix in the vicinity of the PA66 fibres/particles. Similar, slightly more marked changes in structure have been observed in related PA66 fibres containing MFC [24].
Fig. 10. Tensile creep resistance of HDPE/PA66/GNP blend and related MFC, DR ¼ draw ratio.
4.2. XRD As it is seen from the diffractograms in Fig. 8, all the peaks remain in their positions. The only traceable change is observed at 2Θ ¼ 26.44� , which corresponds to graphite (002) plane. Obviously, the peak is not present in the sample of the GNP-free system and intensity is growing up with increasing GNP content in the composite. These results indicate that, within the resolution of XRD applied, there are no changes in crystallinity of both semicrystalline polymer components. As mentioned previously, the possible changes in crystallinity of the HDPE matrix in the vicinity of the PA6 inclusions represent extremely small volume. Therefore, their contribution to bulk crystallinity is practically unde tectable by XRD.
Fig. 11. FEA: Effect of reinforcement geometry and interface on creep resis tance of HDPE/inorganic rigid (glass, 74 000 MPa) reinforcement.
the fibre-containing model it is the face perpendicular to the fibre di rection). The 10 N load is applied to the reference point from which it is distributed to the nodes lying on the opposite face. Creep properties were considered the same for all components (fibre or sphere, matrix and interphase) for simplicity and missing data. The starting point is the creep tensile curve of HDPE from which creep compliance was evalu ated. The dependence of relaxation modulus was calculated using nu merical interconversion [38] and subsequently used for FE linear viscoelastic simulation. The calculation was performed at 0–6000 s time. The displacement of the reference point is analyzed and the effective modulus Eeffective is calculated. This is defined as Eeffective ¼
4.3. Effect of composition and drawing on properties Table 2 shows mechanical behaviour of blends and related MFC without GNP and with GNP added using 3 mixing protocols, i.e. GNP premixed solely in PA66, HDPE and in both components. The results indicate that tensile strength and modulus in some cases do not fully correspond to dual reinforcement with PA66 particles/fibrils and GNP. This was attributed to a complex influence of GNP [24] producing both antagonistic and synergistic effects in spite of similar main structural parameters (composition, geometry of reinforcement and crystallinity). The plausible explanation seems be GNP-induced changes in HDPE crystallinity at the interface [25], see above. The presence of these ef fects is simply indicated by markedly different ratios of modulus of MFC and analogous undrawn blend (Table 2), which is in contrast with only slight change in EMFC/EBlend in the expected range of components pa rameters with increasing modulus of reinforcement predicted using Halpin-Tsai model [35] in Fig. 9.
F⋅l A⋅u
where is the loading force F, dimension of model in the direction of loading l, area of loaded face A and displacement of reference point u. 4. Results and discussion 4.1. Structure of HDPE/PA66 systems and GNP localization
4.4. Effect of PA66 phase geometry and interface on creep resistance
The discussion dealing with the effect of GNP and the mixing pro tocol on fibrils dimensions is presented in our previous works [24,34]. An important fact is that fibre diameter (~2–3 μm) and length in all systems were comparable, aspect ratio (AR) exceeded the value of 15.
Fig. 10 shows example of the unexpected behaviour of MFC, i.e. lower creep resistance of the fibre-contaning system in comparison with the spheres-reinforcement. This is a great difference e.g. from glass 6
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Fig. 12. FEA: Effect of reinforcement geometry and interface on creep resistance of HDPE/polymer-based reinforcement, interface thickness/fibre diameter ratio 0.1 a) creep of matrix only b)creep of all components.
beads- and fibres-based systems with the same HDPE matrix with markedly higher creep resistance of the fibre-reinforced composite, as shown in the preceding study [34]. At the same time, within MFC, the only „expected“ behaviour consists in increase of creep resistance of MFC by the GNP reinforcement. Fig. 10 also shows minor affecting of
creep resistance of MFC by the GNP addition method (see above) with the best creep resistance achieved in the case of pre-blending GNP in PA66, i.e. with elimination of the negative effect of GNP migration in the course of melt drawing [24] (leading also to high E, see Table 2.). From Fig. 10, another unexpected finding also follows, i.e. the negative effect 7
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Fig. 13. FEA: Effect of reinforcement geometry and interface on creep resistance of HDPE/polymer-based reinforcement, interface thickness/fibre diameter ratio 0.1 a) creep of matrix only b)creep of both components.
of GNP addition on creep resistance of the undrawn blend, which is in contrast to MFC and also to the positive effect of GNP on the blend constituents, i.e., PA66 and HDPE.
4.5. Finite element analysis To understand the peculiar creep behaviour of MFC and the related blend, FEA was applied to highlight the role of expected crucial effects, i. e. reinforcing component geometry, modulus and viscoelasticity (creeping/non-creeping reinforcement) together with interphase 8
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Fig. 14. FEA: Effect of reinforcement geometry and interphase on creep resistance of HDPE/polymer-based reinforcement, interface thickness/fibre diameter ratio 0.02 a) creep of matrix, b) creep of all components.
parameters (thickness, modulus, creeping). An important advantage of FEA is especially good description of effects of the interface parameters on performance of composites [25,39]. Figs. 11–13 demonstrate the effect of reinforcement and interface modulus in the case of interface thickness/fibre diameter ratio 0.1
(relatively thick layer corresponding to dimensions of spherulites [25]) on creep modulus. From the creep curves in Fig. 11, marked difference follows between the particulate and fibrous composites in the case of high modulus (74 000 MPa) glass reinforcement, while this difference is reduced in polymer blend and related MFC with significantly lower 9
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Fig. 15. FEA: Effect of reinforcement geometry and interphase on creep resistance of HDPE/polymer-based reinforcement, interface thickness/fibre diameter ratio 0.002 a)creep of matrix b) creep of all components.
(3000 MPa) modulus of reinforcement (Fig. 12a), especially if it creeps (Fig. 12b). This difference is the lowest for the polymer-based rein forcement with E 2000 MPa (Fig. 13a and b). If we consider the effect of „soft“ (10 MPa) interface on creep resistance, FEA indicates an inter esting trend. With the rigid glass-based reinforcement, where the matrix
creep is considered only, the soft interface layer reduces creep of the fibrous composite more significantly than that of the spheres-system (Fig. 11). This negative effect on creep of the fibrous system (differ ence between fibres and spheres) is reduced with decreasing Ereinf (Figs. 12 a, 13a) and, more markedly, if we consider also viscoelasticity 10
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5. Conclusions
Table 1 The homogenized effective Young’s modulus for fibre and sphere-containing RVE. Thickness ratio [1]
Fibre-containing RVE 0.001 0.002 0.005 0.010 0.020
The results based on FEA „mapping“ of the effect of parameters and geometry of the components and the interface in combination with structural analysis indicate that, unlike rigid glass-reinforcement, the difference between creep resistance of the PA66-fibres and the HDPE matrix reinforced with spherical inclusions is less marked or may be even eliminated. This originates especially from low (EReinf/EMatrix) ratio and viscoelasticity (creeping) of the reinforcement. Lower creep resis tance of MFC in real systems corresponds to further specific features of polymeric fibres, such as tangling. The quite unexpected negative effect of GNP on creep of the PA66 spheres-reinforced system corresponds to the FEA-indicated more marked negative effect of the soft interphase on creep of the spheres-containing system in comparison to fibrous one. As a result, the GNP-induced changes in interface rigidity may have crucial impact on creep resistance and can eliminate the reinforcing effect of GNP on the components, whereas this effect dominates in MFC. The results indicate the unknown complexity of the performance of multi component polymer systems in long-term loading.
Young’s modulus of interphase [MPa] Sphere-containing RVE
10
500
1200
10
500
1200
1566 1564 1559 1551 1538
1567 1567 1565 1562 1556
1568
1321 1260 1162 1073 972
1445 1443 1435 1424 1403
1448
Table 2 Composition and mechanical properties of HDPE/PA66 80/20 blends and related MFC. Composition
DR
E (MPa)
σm (MPa)
(EF-ES)a
(EF/ES)
HDPE/PA66 HDPE/PA66 HDPEþ2/PA66 þ 2 HDPEþ2/PA66 þ 2 HDPEþ2/PA66 HDPEþ2/PA66 HDPE/PA66 þ 2 HDPE/PA66 þ 2 HDPE/PA66 þ 3 HDPE/PA66 þ 3
1 6 1 6.5 1 6.5 1 6.5 1d 6
1240 � 74 1405 � 82 1470 � 26 1565 � 63 1295 � 36 1475 � 45 1494 � 60 1595 � 56 1353 � 50 1613 � 55
33.1 � 2.1 44 � 2.4 32.5 � 0.8 42.2 � 0.4 31.2 � 0.7 43.3 � 1.3 35.7 � 0.5 46.6 � 0.8 34.2 � 1.3 45.5 � 1.3
165 – 95 – 180 – 101 – 260 –
1.133 – 1.064 – 1.139 – 1.067 – 1.192 –
Data availability statement The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Declaration of competing interest
a
EF, ES moduli of fibre-reinforced MFC and spheres-reinforced undrawn blend, resp.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[40–42] and thus creep of reinforcement and interface (Figs. 12b and 13b). Another important fact is that, in polymer/polymer systems, the negative effect of the soft interface on creep resistance is always more marked for spheres (Figs. 12–15). In the case of the HDPE/PA66 systems, the effect of thinner interface (thickness/diameter ratio 0.02 and 0.002) is also analyzed (Figs. 14 and 15). As expected, the negative effect on the creep is lower with smaller interphase thickness (see Figs. 13 and 15), which also leads to decreasing difference between fibres and spheres. Figs. 14 and 15 show even negligible affecting of the fibrous composites (both creeping and non-creeping) by the soft interface. Finally, in the case of the thin (ratio 0.02 and 0.002) rigid (1200 MPa) creeping interface (and reinforce ment), the creep resistance of the fibrous composite is even slightly worse than of that containing spheres (Figs. 14b and 15b). The above analysis indicates that peculiar behaviour of polymer/ polymer systems is above all caused by low modulus of the reinforce ment with relatively important contribution of viscoelasticity and the interface. The effect of the last two parameters diminishes with the interface thickness. Although the FEA indicates that polymeric rein forcement can only lead to merging of the fibre- and sphere-containing systems, other unfavourable features of polymer fibres, especially tangling, can undoubtedly further decrease creep resistance, as found in real systems. At the same time, the more marked negative effect of the „soft“ [43–46] interface on creep of the blend (spherical inclusions) in com parison with MFC (Figs. 12–15, Table 1) may be a basis for explanation of unexpected deterioration of creep resistance of such system with GNP addition (Fig. 10). This is based on the fact that the expected effect of interface-localized GNP (Fig. 6) on lowering of the interface rigidity (see above) may thus cause more marked worsening of creep resistance of the blend, eliminating contribution of the GNP-reinforcement. Due to a much lower or negligible negative effect of the interphase on creep resistance of MFC (Figs. 12–15), the reinforcing effect dominates and creep resistance is improved in this case.
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