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A new methodology for rapidly assessing interfacial bonding within fibre-reinforced thermoplastic composites
Author’s Accepted Manuscript A new methodology for rapidly assessing interfacial bonding within fibre-reinforced thermoplastic composites Marc Gaugler, Jan Luedtke, Warren J. Grigsby, Andreas Krause www.elsevier.com/locate/ijadhadh
PII: DOI: Reference:
S0143-7496(18)30269-0 https://doi.org/10.1016/j.ijadhadh.2018.11.010 JAAD2298
To appear in: International Journal of Adhesion and Adhesives Accepted date: 5 May 2018 Cite this article as: Marc Gaugler, Jan Luedtke, Warren J. Grigsby and Andreas Krause, A new methodology for rapidly assessing interfacial bonding within fibre-reinforced thermoplastic composites, International Journal of Adhesion and Adhesives, https://doi.org/10.1016/j.ijadhadh.2018.11.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A new methodology for rapidly assessing interfacial bonding within fibre-reinforced thermoplastic composites Marc Gaugler2, Jan Luedtke1,*, Warren J. Grigsby2, Andreas Krause3
1. Thünen Institute of Wood Research, Leuschnerstr. 91c, 21031 Hamburg, Germany 2. Scion, Rotorua 3010, New Zealand 3. University of Hamburg, Leuschnerstr. 91c, 21031 Hamburg, Germany * corresponding author: [email protected] Abstract A new testing methodology is presented for determining the adhesion strength and failure at composite interfaces. An automated bonding evaluation system (ABES) has been adapted to process and test thermoplastic sandwich composites formed with various natural solid phase materials such as wood. Composites can be rapidly formed and then tested in situ over a range of temperatures to provide information on interfacial adhesion and shear strength between the thermoplastic and the substrate. This methodology is demonstrated for HDPE, PP, PLA and TPU composites comprising wood solid phases. The interfacial properties depend on processing and testing temperatures, individual thermoplastic phase properties, and wood species. It is expected that this new test method will have applicability in thermoplastic composite design, manufacture and service life predictions for reinforced or filled thermoplastics.
Key Words: Thermoplastic resin; Adhesion; Fibre/matrix bond; Interface/interphase; Wood plastic composite
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1. Introduction Composite materials comprising a solid phase such as carbon fibres, glass, wood, or other substrates with thermoplastic matrixes are increasingly used in the construction and automotive sector due to their formability, light-weight, and design attributes. These reinforced or filled materials can be processed by a range of manufacturing techniques including compounding, extrusion, and injection and compression moulding. Inherently, the material properties and composite performance will be derived from the thermoplastic phase and from the solid (fibre or filler type) phase used, but also from the bonding mechanisms between these components [1]. Typically, qualitative assessments using scanning electron microscopy (SEM) and quantitative testing of mechanical properties, for example flexural, tensile or impact properties, are used to characterise composite performance and gain an understanding of the interface between solid and thermoplastic phase. These traditional analyses can be time consuming, both in test specimen preparation and, especially in the case of natural material, may involve extensive conditioning and moisture equilibration prior to testing. Moreover, such analysis of bulk composite properties only allow an indirect evaluation of the interfacial bonding quality; there are only a few techniques such as dynamic mechanical analysis (DMA) capable of routinely testing interfacial properties above ambient temperature. The Automated Bonding Evaluation System (ABES) is a material testing instrument allowing rapid screening of adhesion development [2, 3] and established as ASTM D7998 – 15 [4]. Typically, this instrumentation is used in adhesive performance testing and characterising thermoset adhesive cure profiles [5-7] including the influence of the substrate [8-11] and has proven to correlate well with thermomechanical analysis [12]. Adhesive bonds are formed and cured across a range of temperatures, pressures, substrates, or substrate treatments, and the development of the shear strength is 2
measured by directly testing the adhesive joint in situ. A similar test would be valuable for rapidly forming and testing the development of interfacial adhesion within reinforced or filled thermoplastic composites. The ABES test has the potential to define adhesion strength and failure within composites at a range of temperatures and to be applied to understand and predict what may occur at composite interfaces. Furthermore, complex structures and interfaces can exist within natural fibre composites [1]; potentially, this ABES technique can simplify interactions between solid and thermoplastic phase by reducing the inherent complexity within the composite to a defined single interface for adhesion testing, such as between wood veneer as solid phase and thermoplastic phase. A standard ABES instrument has been custom-modified to include a heat/cool-press unit (Figure 1). With this adaptation, thermoplastic sandwich composites cannot only be rapidly formed, but also tested over a wide temperature range without releasing applied pressures. This provides the ability to define adhesion developed within thermoplastic composites at specific temperatures relevant to polymer properties such as glass transition, crystallization or melt, or those conditions used during composite processing or product service life. Unlike other test methods, the ABES sample can then be cooled to the target test temperature while still being compressed. Such information would be complementary to polymer assessments such as dynamic mechanical thermal analysis (DMTA) or rheology methods and mechanical testing. This paper describes the development of a new testing methodology using the ABES technique and combinations of four different thermoplastic polymers with three wood substrates. This new interfacial adhesion test method will have application in the design, manufacture and service life prediction of any thermoplastic composite comprising interfaces, such as filled, reinforced, or laminated materials.
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2. Experimental 2.1. Materials The thermoplastics used in this investigation were high density polyethylene (HDPE), polypropylene (PP), poly(lactic acid) (PLA), and thermoplastic polyurethane (TPU). Both semi-crystalline PLA (PLA3052D) and amorphous PLA (PLA 4060D) grades were sourced from Natureworks, USA. The PP and HDPE were obtained from Sabic, Saudi Arabia (PP 575P, HDPE 0863F); TPU was obtained from Covestro, Germany (Desmopan 9370A). Table 1 shows typical properties of these polymers. Table 1 Typical properties of polymers used within this study Polymer type
Density
Melt Flow Rate (MFR)
Melting
Tensile Yield
Temperature
Strength
kg/m³
g/10 min
°C
MPa
HDPE 0863F [13]
964
8 (at 190 °C, 2.16 kg)
134
N/A
PP 575P [14]
905
10,5 (at 230 °C, 2.16 kg)
N/A
35
PLA 4060 D [15-17]
1240
10 (at 210 °C, 2.16 kg)
150-180
59
PLA 3052D [18]
1240
14 (at 210 °C, 2.16 kg))
145-160
62
TPU 9370A [19]
1060
N/A
N/A
26
The wood substrates were maple (Acer pseudoplatanus), beech (Fagus sylvatica), and spruce (Picea abies). To assure consistency in surface quality and roughness, sliced veneers were used for all substrates without any further surface treatment. The veneers (0.6 mm thickness) were cut to 20 mm width (radial-tangential mix) and 100 mm length, and were (i) conditioned at 20 °C/65 % relative humidity or (ii) oven dried at 103 °C for 14 h prior to testing. Thermoplastic foils were used as a film adhesive, which were prepared by film extrusion or flat pressing each thermoplastic to ca. 0.5 mm thickness. Pre-testing showed a good gap-fill with minimal squeeze-out when the foil samples used in the bondline (5 mm veneer overlap) were cut to 20 x 2.7 mm. 2.2. Microscopy
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To obtain qualitative micro-morphology information, images were obtained from representative areas using a field emission scanning electron microscope (FESEM, Quanta FEG 250, USA) at an acceleration voltage of 5 kV. After preparing the samples by microtome and gluing them on stubs, the surfaces were coated with gold prior to the microscopy characterization. To obtain distribution of the polymer after penetration into the wood tissue PLA was labelled with acriflavine following a published procedure [20]. The samples were visualized under a confocal laser scanning microscopy (CLSM, Leica TCS SP5 II, Germany) at excitation wavelengths of 488 and 561 nm and sequential scanning of wavelengths between 515 – 545 and 566 – 643 nm to ensure distinctive imaging of labelled PLA and wood tissue. The projected image were obtained from zstacks at a resolution of 1024x1024 pixels and contained 15 – 47 sections at 1 µmintervals.
2.3. ABES Methodology Set Up Protocols The tested bond area, defined by the width of the two substrates and the overlap was 20 x 5 mm (Figure 2). All samples were tested parallel to the wood grain. An advantage of the ABES method is that this overlap (bond area) can be changed to ensure the strength of the tested bondline is significantly less than the tensile strength of the substrate, i.e. the solid phase (veneer). The thickness of the bondline depends on temperature and pressure used during the bonding/compression step. As thermoplastics rather than thermosetting adhesives are used, melt viscosity of the polymer is considered important. However, foils which are too thick may result in excessive squeeze-out as mentioned above, while foils which are too thin could lead to thermoplastic bonds with insufficient anchoring of substrates. Foils used in this study had a nominal thickness of ca. 0.5 mm. The resulting bondline was sufficiently thick, so that a full polymer bondline between the substrates was achieved. Prestudies showed 5
that film thicknesses of 0.2-0.5 mm work well, but depending on the interface properties other film thicknesses are possible. The statistical analysis of the ABES results was done using two tailed t-test. The significance threshold was at a p-value of .05.
3. Methodology Development and Results 3.1. Instrument Temperature Profiles The understanding and control of instrument temperatures and the heating and cooling rates is important to process and test fibre-reinforced thermoplastic composites (Figure 2). The heating and cooling ramps applied to the bondline need to be reproducible during ABES testing to ensure consistency in test specimen bond formation, thermal history of the tested samples and, ultimately, quality test results. The heat conduction and transfer rates from the press head (Figure 1) through the substrate and the thermoplastic material were determined for several press head temperatures and cooling air pressures by placing a thermocouple in the bondline. This revealed heating profiles in which the time to heat the specimens (meaning the thermal lag) was generally less than 10 s until the target temperatures (up to 200 °C) were achieved and an isothermal state in the bondline was established (Figure 3). A visual inspection also confirmed that the bonded area between the veneers was filled completely with the thermoplastic material which was also evident with microscopy (Figure 4). Therefore setting a common pressing time of 20 s was considered satisfactory to achieve the set point temperature and the heat transfer to the thermoplastic phase and to induce melt flow and consolidation of the sandwich across a broad temperature range (140 to 200 °C). In comparison, when assessing more traditional thermoset adhesives, lower press temperatures but a much broader range of press times (30 seconds up to several minutes) are often used with ABES [12]. 6
Generally, thermosets cured by ABES are directly tested after bond formation and cure [10]. However, unlike most thermoset systems, the mechanical properties of thermoplastic materials depend on their testing temperature and, importantly, any differential cooling can also affect properties such as crystallinity in semi-crystalline polymers like PP [21]. In the modified ABES, cooling of the test specimen is achieved via compressed air flow into the heat/cool-press heads to rapidly reduce the plate surface temperatures while maintaining an applied force (Figures 1 and 2). The cooling ramps were evaluated using varying infeed air pressures to cool the press heads and to achieve differing cooling rates (Figure 5). This enabled defined and reproducible cooling profiles of the thermoplastic phase. Cooling rates could be relatively rapid using high pressure (4.0 MPa) or slow using low pressure (0.5 MPa), respectively. The cooling speed is a function of Newton’s cooling function:
(1) where T is the shear test temperature (°C), Te is the environmental temperature (°C), i.e. 28 °C in the particular setup, Ti is the initial temperature before cooling (°C), k is the cooling constant depending from pressure, and t is the time in (s). The cooling behaviour of 12 replicates for both cooling air pressures at various initial temperatures were fitted to this function. The cooling constant for 0.5 bar air pressure is 0.007 (R2 = 0.99) and for 4.0 bar 0.019 (R2 = 0.99), respectively. Using these constants the temperature of the bondline can be predicted for any cooling time or the required cooling time for any shear test temperature. Additionally, this revealed that the cooling at a particular air pressure and sample temperature was independent of the initial press head temperature. This confirmed that specimens pressed at different press head temperatures were not subjected to varying 7
cooling within the instrument temperature range. Cooling from 200˚C to ambient temperatures can be achieved in less than 150 s. Arguably, the cooling rates capable of this ABES set up are comparable to those used for example with injection moulding, so generally they are unlikely to cause differences in crystallization of some thermoplastic polymers. Development of press head cooling regimes and temperature set points were developed from the common cooling profile (Figure 5) and incorporated any differential thermal lag between pressed thermoplastic phase and the press head. A modest hold time (10 s) was also introduced to ensure stable set point temperatures prior to specimen testing and, while minimal, to also avoid any temperature differential between the thermoplastic phase and press head, as noted above.
3.2. Influence of press temperature and test temperature To establish the new testing methodology, a range of thermoplastics and wood substrates have been initially surveyed to establish testing protocols and any inherent variabilities in composite performance due to differing polymer properties or wood species used as test substrates. Three thermoplastics, high density polyethylene (HDPE), polypropylene (PP), and poly(lactic acid) (PLA) representing polymers with increasing polarity, and one elastomeric thermoplastic type, polyurethane (TPU), were assessed. The semi-crystalline PLA grade (PLA3052D) was used to evaluate conditions above and below the melting (Tm, 140 °C), crystallization (Tc, ca. 100 °C) and glass transition (Tg, ca. 57 °C) temperatures [22]. Poly(lactic acid) sandwich composites were formed using maple veneers (Acer pseudoplatanus) at 140 °C and 200 °C press head temperature. The higher press temperature is comparable to compounding and injection moulding processing temperatures expected for PLA/natural filler composites. After
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pressing and cooling, the composite sandwich specimens were tested at 45, 60, 80 and 100 °C bondline temperature to promote different modes of adhesion failure. Table 2 Average shear strength of composite sandwiches formed with the semi-crystalline PLA3052D and maple veneer substrate and pressed and tested at differing thermoplastic phase temperatures Substrate Conditioning Press temperature Cooling rate (Cooling air pressure) 1 Average shear strength °C °C 20 °C, 65 % RH 140 fast (4 bar) 45 60 80 100 20 °C, 65 % RH 200 fast (4 bar) 45 60 80 100 20 °C, 65 % RH 200 slow (0.5 bar) 45 80 103 °C oven 140 fast (4 bar) 45 80 103 °C oven 200 fast (4 bar) 45 80 1
Test temperature MPa (Std. Dev.) 5.66 (0.43) 5.70 (0.58) 2.32 (0.26) 1.35 (0.13) 8.23 (0.73) 8.69 (0.90) 3.14 (0.34) 1.82 (0.16) 8.63 (1.01) 3.01 (0.47) 5.74 (0.57) 2.21 (0.29) 7.90 (0.66) 3.39 (0.14)
Average and standard deviation for at least 5 replicates
Table 2 indicates that the higher press temperature (200 °C) gave higher shear strengths than 140 °C; this may be attributable to the lower PLA melt viscosity at higher temperature [23], hence promoting greater polymer mobility into the wood, better anchoring of the PLA and the wood and higher shear strength. Across the different testing temperatures, the average shear strength at 45 °C was ca. 8.5 MPa when pressed at 200 °C with typically cohesive or substrate failure (Table 3). These failure modes were defined as “Adhesive” (bond line polymer residue only on one of the veneers), “Cohesive” (bond line residue on both veneers) and “Substrate” (veneer shear in at least one veneer; one half of this veneer is still attached to the bondline). Table 3 Examples of different bondline failure modes achieved at different pressing and testing temperatures for composite sandwiches formed with the semi-crystalline PLA3052D and maple veneer substrate Press temperature (°C)
Test temperature (°C)
Tested samples
140
45 60 80 100
15 11 15 10
Adhesive (%) 30 25 40 30
9
Failure mode Cohesive Substrate (%) (%) 50 20 50 25 60 0 70 0
200
45 60 80 100
21 10 20 10
0 0 5 0
35 20 95 80
65 80 0 20
Testing near the PLA Tg (60 °C) gave comparable shear strength as for 45 °C. At 80 °C bond temperature, the shear strength was significantly reduced (3.1 MPa), because this temperature is above Tg of PLA. The shear strength decreased at 100 °C (1.8 MPa) due to further weakening of the amporphous sections in the polymer (Table 2). To understand any influence of the cooling rate on the semi-crystalline PLA performance, the test specimens were prepared with both, rapid and slow cooling rates (Table 2). This revealed no significant difference between shear strength results when tested at two different temperatures (45 °C and 80 °C). This finding has practical implications as small differences in cooling speed can be considered insignificant for the test results. It also indicates that the degree of PLA crystallization was unaffected by the cooling rate.
3.3. Differing Substrates In establishing this new test methodology, the testing protocols have been set for fast evaluation in which the composite sandwiches are rapidly heated and pressed above 100 °C and then cooled between the press heads before opening and testing. In this time frame (< 3 min), specimens are unlikely to re-absorb moisture prior to the testing step. Therefore, moisture conditioning prior to testing, as required for other tests, is not needed. However, to establish any susceptibility of moisture, testing of oven dried and conditioned maple substrates (10 % moisture content, equilibrated to 20 °C and 65 % RH) was undertaken. Oven-drying of the wood substrate prior to pressing and testing had no significant effect on the shear strength compared to the conditioned wood veneer for both press temperatures and at 45 and 80 °C bond temperature during testing (Table 2). Another benefit of this rapid testing methodology is minimisation of any susceptibility of specimens to moisture-induced stress and strain caused by the 10
substrate reabsorbing moisture before testing [1]. Moreover, in more traditional static testing of wood composites, larger dimensioned samples can undergo delamination or sample warping due to moisture change during their pre-test conditioning. Polyethylene, PP and TPU are examples of commodity plastics and an elastomer, respectively. The shear strengths achieved with these thermoplastics has been evaluated at varying press temperatures typical of those used to process these plastics (Figure 6). Together with an amorphous PLA grade (PLA4060D), this series of polymers offered a range of different polymer polarity which may also impact shear strength formed with three different wood species used as substrates. The substrates, maple (Acer pseudoplatanus), beech (Fagus sylvatica), and spruce (Picea abies) by their nature provide varying wood chemistry and surface structure including fibre size, vessels and strength differences. Figure 6 shows a range of shear strength values which vary with the thermoplastic material, wood species and pressing temperature. Evident for PP was an influence of wood species on shear strength. Bonding with beech provided a higher strength (4.9 MPa) than using maple (4.2 MPa; p = .002) whereas spruce produced a shear strength intermediate of these species. Pressing at 170 or 200 °C did not provide any significant difference in strength for the same wood species compared to the press temperatures as shown in Figure 6 (detailed data are shown in [24]). HDPE bondlines showed significantly lower shear strength (< 3 MPa) than PP (p < .001). Additionally, as with PP, the wood species used as substrate also impacted the bond performance. The composites formed with spruce produced significantly higher shear strength (2.4 MPa) than either of the HDPE/beech (2.2 MPa; p = .03) and HDPE/maple composites (2.0 MPa; p < .001 ). In the case of the TPU, pressing at 180 °C was found to provide significantly higher shear strengths than 160 °C (p < .001) [24]. The wood substrate also has an effect on TPU composite performance with spruce 11
producing a lower shear strength (3.3 MPa) than beech (3.8 MPa; p < .001) and maple (. 3.7 MPa; p < .001). As with the semi-crystalline PLA (Table 2), pressing an amorphous grade of PLA at higher temperature (160 °C) also produced higher shear strength than at lower temperature (140 °C). As outlined above, the lower PLA melt viscosity at higher pressing temperature might have led to increased penetration of the polymer into the wood substrate. The amorphous PLA also produced the highest (8 - 9 MPa) strength of all four plastics in this series which may be a consequence of the relatively higher polarity of this polymer providing greater effect of attraction forces between the polymer and the wood surface. There was little difference in shear strength (8 - 9 MPa) between beech, maple and spruce when pressed at 160 °C; at these higher strength values, however, a greater variability in the measured strength values was given (Figure 6). In developing this rapid assessment methodology, careful consideration should be given to the strength of the used wood material to ensure the substrate’s tensile strength match with the bonding polymer being evaluated. However, proportion of wood failure have not yet been assessed comprehensively at this stage of research reported here. Higher shear strengths such as those observed for PLA (Table 2 and Figure 6) led to increased variability as wood failures became more pronounced in testing data. Figure 7 shows the variation coefficients obtained for test data sets for the four different plastics as given in Figure 6. It was evident as shear strength values increased, higher variation in measured shear strength manifested as testing approached the margins of the wood (solid phase) strength. A recommendation for the methodology protocols is that selection of stronger substrates, while considering any possible influence on thermoplastic penetration and adhesion, might reduce variability evident with higher performing adhesion processes and produce less test result scatter due to wood
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failures. Using thicker wood veneer, a higher shear load that can be applied to the polymer-wood interfaces while avoiding wood failure outside the bondline.
4. Summary Using the ABES technique, the key attributes of this interfacial adhesion (shear strength) test method include:
Testing of shear strength of thermoplastic-based composites, transferred to a methodology with defined surface contact areas.
Composite preparation and testing is in situ using one instrument
Fast, easy and reliable testing, being independent of any moisture susceptibility of substrate materials.
Undertaken at various processing and testing temperatures, pressures and different solid phases.
The new interfacial adhesion test methodology has been developed to allow the rapid preparation and testing of thermoplastic-based composites over a range of temperatures. The methodology is based on a modified ABES instrument to allow the rapid heating and formation of thermoplastic-based sandwich composites followed by rapid cooling and in situ testing at specified temperatures. Press temperatures may be related to polymer properties such as melt or typical processing temperatures used in manufacturing environments. Similarly, shear strength assessments can be undertaken over a wide range of temperatures which may relate to polymer properties or postprocessing or in service temperatures. Composite sandwich formation and testing using this methodology has been demonstrated with a range of thermoplastic materials and three different wood substrate phases. Although the strength properties may not be directly transferred one-to-one to all industrially-manufactured composite (due to the 13
more complicated production processes of various composites giving more complex interfaces, as well as differences in production conditions between lab and industry), results reveal a range of interfacial adhesion. The composites’ shear strength results depend on pressing and testing temperatures, individual thermoplastic phase properties, and different wood species. In particular, for PLA-based composites the shear strength of the composites was affected by the pressing temperature. Higher pressing temperatures led to higher shear strength. While this is likely to be caused by different PLA penetration into the wood substrate due to differences in melt viscosity, polymer degradation or different crystallisation cannot be ruled out. The effect of pressing conditions and polymer properties on the structure of the composites discussed in this paper will need to be further assessed. The concept of tailoring the adhesive penetration by controlling its melt viscosity, is applicable to other thermoplastics.Nevertheless, a high potential is given for application of this new interfacial adhesion test for design and manufacture of composites based on thermoplastic adhesives.
Acknowledgements The authors are grateful to the following organisations for assistance with funding for the New Zealand-Germany Science and Technology Programme. This was contributed to by the New Zealand Royal Society and Catalyst Seed funding [FRG-FRI1402] and the Bundesministerium für Landwirtschaft und Ernährung (Federal Ministry of Food and Agriculture) [02/14-15-NZL], together with the Thünen Institute of Wood Research, Scion and the University of Hamburg.
References
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[12] M. Lecourt, A. Pizzi, P. Humphrey, Comparison of TMA and ABES as forecasting systems of wood bonding effectiveness, Holz Roh-Werkst, 61 (2003) 75-76. [13] SABIC (2017), SABIC® HDPE F0863 High Density Polyethylene, Global Technical Data Sheet, Revision 20171013 [14] SABIC (2012), SABIC® PP 575P PP homopolymer for Injection moulding, Technical Data Sheet, Revision 20121203 [15] NatureWorks (2015), Safety Data Sheet Ingeo™ biopolymer 4060D, Rev. Number 20, Revision 20150903 [16] NatureWorks (2011), Ingeo™ resin product guide brochure, https://www.natureworksllc.com/~/media/Files/NatureWorks/Technical-Documents/OnePagers/ingeo-resin-grades-brochure_pdf.pdf, (accessed January 19, 2018) [17] Jamshidian, M., Tehrany, E. A., Imran, M., Jacquot, M. and Desobry, S. (2010), Poly-Lactic Acid: Production, Applications, Nanocomposites, and Release Studies. Comprehensive Reviews in Food Science and Food Safety, 9: 552–571. DOI: 10.1111/j.1541-4337.2010.00126.x [18] NatureWorks (2011), Ingeo Biopolymer 3052D Technical Data Sheet, Revision NW3052D_051915V1 [19] Covestro AG (2016), Desmopan 9370A, ISO Data Sheet, Revision 20160502 [20] W. Grigsby, A. Thumm, H. Klepser, Assessing Interfacial behaviours of natural fiber-plastic composites by fluorescent microscopy, 8th International Conference on Woodfiber-Plastic Composites, Madison, WI, USA, 2005. [21] J.L. White, H. Shan, Deformation-induced structural changes in crystalline polyolefins, Polym. Plast. Technol. Eng., 45 (2006) 317-328. [22] F. Carrasco, P. Pagès, J. Gámez-Pérez, O.O. Santana, M.L. Maspoch, Processing of poly(lactic acid): Characterization of chemical structure, thermal stability and mechanical properties, Polym. Degrad. Stab., 95 (2010) 116-125. [23] D. Garlotta, A literature review of poly(lactic acid), J. Polym. Environ., 9 (2001) 6384. [24] C. Schulz, Zugscherversuche mit dem Automated Bonding Evaluation System (ABES) unter Variation verschiedener Holzarten und Polymere (Tensile Shear Tests With The Automated Bonding Evaluation System Under Variation Of Different Wood Species And Polymers), Seminar Paper, University of Hamburg, Center of Wood Science, (2016) (unpublished)
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Figures
Heat/cool press head Gripping device
Moving direction
Thermoplastic phase
Solid phase (veneer)
Figure 1 Scheme for the modified Automated Bonding Evaluation System equipment adapted for the interfacial adhesion testing methodology: For details of modification see [10].
(1) Substrates
(2) Pressure
(3) Pressure
(4) Pressure
mounted in ABES
and heating
and cooling
released
Substrate and thermoplastic polymer combined
Composite sandwich formed at pressing temperature
Composite sandwich cooled to test temperature
Composite sandwich tested in shear
Figure 2 Overview of the procedure to process and test thermoplastic composite sandwich materials in situ using the adapted ABES equipment.
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Figure 3 Typical thermoplastic composite sandwich heating profiles using differing press head temperatures (140, 160 and 200 °C).
Figure 4 Scanning electron microscopy (left) and fluorescence confocal laser scanning microscopy (right) images of a PLA composite sandwich bond line (pressed at 140 °C). 18
Figure 5 Cooling profiles of the thermoplastic phase of composite sandwich using press head cooling regimes with either 4.0 bar and 0.5 bar air pressure using different initial press plate temperatures of 200, 180, 160 and 140 °C.
19
Figure 6 Average shear strength and standard deviation for thermoplastic composite sandwiches formed with HDPE (at 160 °C), PP (at 200 °C), TPU (at 180 °C), and the amorphous PLA (at 160 °C) with beech, maple, and spruce wood veneers tested at 45 °C.
20
Figure 7 Comparison of average shear strength values with variation coefficients (ratio of standard deviation to the mean) for composite sandwiches formed with the four thermoplastics HDPE, PP, TPU and amorphous PLA.
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PII: DOI: Reference:
S0143-7496(18)30269-0 https://doi.org/10.1016/j.ijadhadh.2018.11.010 JAAD2298
To appear in: International Journal of Adhesion and Adhesives Accepted date: 5 May 2018 Cite this article as: Marc Gaugler, Jan Luedtke, Warren J. Grigsby and Andreas Krause, A new methodology for rapidly assessing interfacial bonding within fibre-reinforced thermoplastic composites, International Journal of Adhesion and Adhesives, https://doi.org/10.1016/j.ijadhadh.2018.11.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A new methodology for rapidly assessing interfacial bonding within fibre-reinforced thermoplastic composites Marc Gaugler2, Jan Luedtke1,*, Warren J. Grigsby2, Andreas Krause3
1. Thünen Institute of Wood Research, Leuschnerstr. 91c, 21031 Hamburg, Germany 2. Scion, Rotorua 3010, New Zealand 3. University of Hamburg, Leuschnerstr. 91c, 21031 Hamburg, Germany * corresponding author: [email protected] Abstract A new testing methodology is presented for determining the adhesion strength and failure at composite interfaces. An automated bonding evaluation system (ABES) has been adapted to process and test thermoplastic sandwich composites formed with various natural solid phase materials such as wood. Composites can be rapidly formed and then tested in situ over a range of temperatures to provide information on interfacial adhesion and shear strength between the thermoplastic and the substrate. This methodology is demonstrated for HDPE, PP, PLA and TPU composites comprising wood solid phases. The interfacial properties depend on processing and testing temperatures, individual thermoplastic phase properties, and wood species. It is expected that this new test method will have applicability in thermoplastic composite design, manufacture and service life predictions for reinforced or filled thermoplastics.
Key Words: Thermoplastic resin; Adhesion; Fibre/matrix bond; Interface/interphase; Wood plastic composite
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1. Introduction Composite materials comprising a solid phase such as carbon fibres, glass, wood, or other substrates with thermoplastic matrixes are increasingly used in the construction and automotive sector due to their formability, light-weight, and design attributes. These reinforced or filled materials can be processed by a range of manufacturing techniques including compounding, extrusion, and injection and compression moulding. Inherently, the material properties and composite performance will be derived from the thermoplastic phase and from the solid (fibre or filler type) phase used, but also from the bonding mechanisms between these components [1]. Typically, qualitative assessments using scanning electron microscopy (SEM) and quantitative testing of mechanical properties, for example flexural, tensile or impact properties, are used to characterise composite performance and gain an understanding of the interface between solid and thermoplastic phase. These traditional analyses can be time consuming, both in test specimen preparation and, especially in the case of natural material, may involve extensive conditioning and moisture equilibration prior to testing. Moreover, such analysis of bulk composite properties only allow an indirect evaluation of the interfacial bonding quality; there are only a few techniques such as dynamic mechanical analysis (DMA) capable of routinely testing interfacial properties above ambient temperature. The Automated Bonding Evaluation System (ABES) is a material testing instrument allowing rapid screening of adhesion development [2, 3] and established as ASTM D7998 – 15 [4]. Typically, this instrumentation is used in adhesive performance testing and characterising thermoset adhesive cure profiles [5-7] including the influence of the substrate [8-11] and has proven to correlate well with thermomechanical analysis [12]. Adhesive bonds are formed and cured across a range of temperatures, pressures, substrates, or substrate treatments, and the development of the shear strength is 2
measured by directly testing the adhesive joint in situ. A similar test would be valuable for rapidly forming and testing the development of interfacial adhesion within reinforced or filled thermoplastic composites. The ABES test has the potential to define adhesion strength and failure within composites at a range of temperatures and to be applied to understand and predict what may occur at composite interfaces. Furthermore, complex structures and interfaces can exist within natural fibre composites [1]; potentially, this ABES technique can simplify interactions between solid and thermoplastic phase by reducing the inherent complexity within the composite to a defined single interface for adhesion testing, such as between wood veneer as solid phase and thermoplastic phase. A standard ABES instrument has been custom-modified to include a heat/cool-press unit (Figure 1). With this adaptation, thermoplastic sandwich composites cannot only be rapidly formed, but also tested over a wide temperature range without releasing applied pressures. This provides the ability to define adhesion developed within thermoplastic composites at specific temperatures relevant to polymer properties such as glass transition, crystallization or melt, or those conditions used during composite processing or product service life. Unlike other test methods, the ABES sample can then be cooled to the target test temperature while still being compressed. Such information would be complementary to polymer assessments such as dynamic mechanical thermal analysis (DMTA) or rheology methods and mechanical testing. This paper describes the development of a new testing methodology using the ABES technique and combinations of four different thermoplastic polymers with three wood substrates. This new interfacial adhesion test method will have application in the design, manufacture and service life prediction of any thermoplastic composite comprising interfaces, such as filled, reinforced, or laminated materials.
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2. Experimental 2.1. Materials The thermoplastics used in this investigation were high density polyethylene (HDPE), polypropylene (PP), poly(lactic acid) (PLA), and thermoplastic polyurethane (TPU). Both semi-crystalline PLA (PLA3052D) and amorphous PLA (PLA 4060D) grades were sourced from Natureworks, USA. The PP and HDPE were obtained from Sabic, Saudi Arabia (PP 575P, HDPE 0863F); TPU was obtained from Covestro, Germany (Desmopan 9370A). Table 1 shows typical properties of these polymers. Table 1 Typical properties of polymers used within this study Polymer type
Density
Melt Flow Rate (MFR)
Melting
Tensile Yield
Temperature
Strength
kg/m³
g/10 min
°C
MPa
HDPE 0863F [13]
964
8 (at 190 °C, 2.16 kg)
134
N/A
PP 575P [14]
905
10,5 (at 230 °C, 2.16 kg)
N/A
35
PLA 4060 D [15-17]
1240
10 (at 210 °C, 2.16 kg)
150-180
59
PLA 3052D [18]
1240
14 (at 210 °C, 2.16 kg))
145-160
62
TPU 9370A [19]
1060
N/A
N/A
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The wood substrates were maple (Acer pseudoplatanus), beech (Fagus sylvatica), and spruce (Picea abies). To assure consistency in surface quality and roughness, sliced veneers were used for all substrates without any further surface treatment. The veneers (0.6 mm thickness) were cut to 20 mm width (radial-tangential mix) and 100 mm length, and were (i) conditioned at 20 °C/65 % relative humidity or (ii) oven dried at 103 °C for 14 h prior to testing. Thermoplastic foils were used as a film adhesive, which were prepared by film extrusion or flat pressing each thermoplastic to ca. 0.5 mm thickness. Pre-testing showed a good gap-fill with minimal squeeze-out when the foil samples used in the bondline (5 mm veneer overlap) were cut to 20 x 2.7 mm. 2.2. Microscopy
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To obtain qualitative micro-morphology information, images were obtained from representative areas using a field emission scanning electron microscope (FESEM, Quanta FEG 250, USA) at an acceleration voltage of 5 kV. After preparing the samples by microtome and gluing them on stubs, the surfaces were coated with gold prior to the microscopy characterization. To obtain distribution of the polymer after penetration into the wood tissue PLA was labelled with acriflavine following a published procedure [20]. The samples were visualized under a confocal laser scanning microscopy (CLSM, Leica TCS SP5 II, Germany) at excitation wavelengths of 488 and 561 nm and sequential scanning of wavelengths between 515 – 545 and 566 – 643 nm to ensure distinctive imaging of labelled PLA and wood tissue. The projected image were obtained from zstacks at a resolution of 1024x1024 pixels and contained 15 – 47 sections at 1 µmintervals.
2.3. ABES Methodology Set Up Protocols The tested bond area, defined by the width of the two substrates and the overlap was 20 x 5 mm (Figure 2). All samples were tested parallel to the wood grain. An advantage of the ABES method is that this overlap (bond area) can be changed to ensure the strength of the tested bondline is significantly less than the tensile strength of the substrate, i.e. the solid phase (veneer). The thickness of the bondline depends on temperature and pressure used during the bonding/compression step. As thermoplastics rather than thermosetting adhesives are used, melt viscosity of the polymer is considered important. However, foils which are too thick may result in excessive squeeze-out as mentioned above, while foils which are too thin could lead to thermoplastic bonds with insufficient anchoring of substrates. Foils used in this study had a nominal thickness of ca. 0.5 mm. The resulting bondline was sufficiently thick, so that a full polymer bondline between the substrates was achieved. Prestudies showed 5
that film thicknesses of 0.2-0.5 mm work well, but depending on the interface properties other film thicknesses are possible. The statistical analysis of the ABES results was done using two tailed t-test. The significance threshold was at a p-value of .05.
3. Methodology Development and Results 3.1. Instrument Temperature Profiles The understanding and control of instrument temperatures and the heating and cooling rates is important to process and test fibre-reinforced thermoplastic composites (Figure 2). The heating and cooling ramps applied to the bondline need to be reproducible during ABES testing to ensure consistency in test specimen bond formation, thermal history of the tested samples and, ultimately, quality test results. The heat conduction and transfer rates from the press head (Figure 1) through the substrate and the thermoplastic material were determined for several press head temperatures and cooling air pressures by placing a thermocouple in the bondline. This revealed heating profiles in which the time to heat the specimens (meaning the thermal lag) was generally less than 10 s until the target temperatures (up to 200 °C) were achieved and an isothermal state in the bondline was established (Figure 3). A visual inspection also confirmed that the bonded area between the veneers was filled completely with the thermoplastic material which was also evident with microscopy (Figure 4). Therefore setting a common pressing time of 20 s was considered satisfactory to achieve the set point temperature and the heat transfer to the thermoplastic phase and to induce melt flow and consolidation of the sandwich across a broad temperature range (140 to 200 °C). In comparison, when assessing more traditional thermoset adhesives, lower press temperatures but a much broader range of press times (30 seconds up to several minutes) are often used with ABES [12]. 6
Generally, thermosets cured by ABES are directly tested after bond formation and cure [10]. However, unlike most thermoset systems, the mechanical properties of thermoplastic materials depend on their testing temperature and, importantly, any differential cooling can also affect properties such as crystallinity in semi-crystalline polymers like PP [21]. In the modified ABES, cooling of the test specimen is achieved via compressed air flow into the heat/cool-press heads to rapidly reduce the plate surface temperatures while maintaining an applied force (Figures 1 and 2). The cooling ramps were evaluated using varying infeed air pressures to cool the press heads and to achieve differing cooling rates (Figure 5). This enabled defined and reproducible cooling profiles of the thermoplastic phase. Cooling rates could be relatively rapid using high pressure (4.0 MPa) or slow using low pressure (0.5 MPa), respectively. The cooling speed is a function of Newton’s cooling function:
(1) where T is the shear test temperature (°C), Te is the environmental temperature (°C), i.e. 28 °C in the particular setup, Ti is the initial temperature before cooling (°C), k is the cooling constant depending from pressure, and t is the time in (s). The cooling behaviour of 12 replicates for both cooling air pressures at various initial temperatures were fitted to this function. The cooling constant for 0.5 bar air pressure is 0.007 (R2 = 0.99) and for 4.0 bar 0.019 (R2 = 0.99), respectively. Using these constants the temperature of the bondline can be predicted for any cooling time or the required cooling time for any shear test temperature. Additionally, this revealed that the cooling at a particular air pressure and sample temperature was independent of the initial press head temperature. This confirmed that specimens pressed at different press head temperatures were not subjected to varying 7
cooling within the instrument temperature range. Cooling from 200˚C to ambient temperatures can be achieved in less than 150 s. Arguably, the cooling rates capable of this ABES set up are comparable to those used for example with injection moulding, so generally they are unlikely to cause differences in crystallization of some thermoplastic polymers. Development of press head cooling regimes and temperature set points were developed from the common cooling profile (Figure 5) and incorporated any differential thermal lag between pressed thermoplastic phase and the press head. A modest hold time (10 s) was also introduced to ensure stable set point temperatures prior to specimen testing and, while minimal, to also avoid any temperature differential between the thermoplastic phase and press head, as noted above.
3.2. Influence of press temperature and test temperature To establish the new testing methodology, a range of thermoplastics and wood substrates have been initially surveyed to establish testing protocols and any inherent variabilities in composite performance due to differing polymer properties or wood species used as test substrates. Three thermoplastics, high density polyethylene (HDPE), polypropylene (PP), and poly(lactic acid) (PLA) representing polymers with increasing polarity, and one elastomeric thermoplastic type, polyurethane (TPU), were assessed. The semi-crystalline PLA grade (PLA3052D) was used to evaluate conditions above and below the melting (Tm, 140 °C), crystallization (Tc, ca. 100 °C) and glass transition (Tg, ca. 57 °C) temperatures [22]. Poly(lactic acid) sandwich composites were formed using maple veneers (Acer pseudoplatanus) at 140 °C and 200 °C press head temperature. The higher press temperature is comparable to compounding and injection moulding processing temperatures expected for PLA/natural filler composites. After
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pressing and cooling, the composite sandwich specimens were tested at 45, 60, 80 and 100 °C bondline temperature to promote different modes of adhesion failure. Table 2 Average shear strength of composite sandwiches formed with the semi-crystalline PLA3052D and maple veneer substrate and pressed and tested at differing thermoplastic phase temperatures Substrate Conditioning Press temperature Cooling rate (Cooling air pressure) 1 Average shear strength °C °C 20 °C, 65 % RH 140 fast (4 bar) 45 60 80 100 20 °C, 65 % RH 200 fast (4 bar) 45 60 80 100 20 °C, 65 % RH 200 slow (0.5 bar) 45 80 103 °C oven 140 fast (4 bar) 45 80 103 °C oven 200 fast (4 bar) 45 80 1
Test temperature MPa (Std. Dev.) 5.66 (0.43) 5.70 (0.58) 2.32 (0.26) 1.35 (0.13) 8.23 (0.73) 8.69 (0.90) 3.14 (0.34) 1.82 (0.16) 8.63 (1.01) 3.01 (0.47) 5.74 (0.57) 2.21 (0.29) 7.90 (0.66) 3.39 (0.14)
Average and standard deviation for at least 5 replicates
Table 2 indicates that the higher press temperature (200 °C) gave higher shear strengths than 140 °C; this may be attributable to the lower PLA melt viscosity at higher temperature [23], hence promoting greater polymer mobility into the wood, better anchoring of the PLA and the wood and higher shear strength. Across the different testing temperatures, the average shear strength at 45 °C was ca. 8.5 MPa when pressed at 200 °C with typically cohesive or substrate failure (Table 3). These failure modes were defined as “Adhesive” (bond line polymer residue only on one of the veneers), “Cohesive” (bond line residue on both veneers) and “Substrate” (veneer shear in at least one veneer; one half of this veneer is still attached to the bondline). Table 3 Examples of different bondline failure modes achieved at different pressing and testing temperatures for composite sandwiches formed with the semi-crystalline PLA3052D and maple veneer substrate Press temperature (°C)
Test temperature (°C)
Tested samples
140
45 60 80 100
15 11 15 10
Adhesive (%) 30 25 40 30
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Failure mode Cohesive Substrate (%) (%) 50 20 50 25 60 0 70 0
200
45 60 80 100
21 10 20 10
0 0 5 0
35 20 95 80
65 80 0 20
Testing near the PLA Tg (60 °C) gave comparable shear strength as for 45 °C. At 80 °C bond temperature, the shear strength was significantly reduced (3.1 MPa), because this temperature is above Tg of PLA. The shear strength decreased at 100 °C (1.8 MPa) due to further weakening of the amporphous sections in the polymer (Table 2). To understand any influence of the cooling rate on the semi-crystalline PLA performance, the test specimens were prepared with both, rapid and slow cooling rates (Table 2). This revealed no significant difference between shear strength results when tested at two different temperatures (45 °C and 80 °C). This finding has practical implications as small differences in cooling speed can be considered insignificant for the test results. It also indicates that the degree of PLA crystallization was unaffected by the cooling rate.
3.3. Differing Substrates In establishing this new test methodology, the testing protocols have been set for fast evaluation in which the composite sandwiches are rapidly heated and pressed above 100 °C and then cooled between the press heads before opening and testing. In this time frame (< 3 min), specimens are unlikely to re-absorb moisture prior to the testing step. Therefore, moisture conditioning prior to testing, as required for other tests, is not needed. However, to establish any susceptibility of moisture, testing of oven dried and conditioned maple substrates (10 % moisture content, equilibrated to 20 °C and 65 % RH) was undertaken. Oven-drying of the wood substrate prior to pressing and testing had no significant effect on the shear strength compared to the conditioned wood veneer for both press temperatures and at 45 and 80 °C bond temperature during testing (Table 2). Another benefit of this rapid testing methodology is minimisation of any susceptibility of specimens to moisture-induced stress and strain caused by the 10
substrate reabsorbing moisture before testing [1]. Moreover, in more traditional static testing of wood composites, larger dimensioned samples can undergo delamination or sample warping due to moisture change during their pre-test conditioning. Polyethylene, PP and TPU are examples of commodity plastics and an elastomer, respectively. The shear strengths achieved with these thermoplastics has been evaluated at varying press temperatures typical of those used to process these plastics (Figure 6). Together with an amorphous PLA grade (PLA4060D), this series of polymers offered a range of different polymer polarity which may also impact shear strength formed with three different wood species used as substrates. The substrates, maple (Acer pseudoplatanus), beech (Fagus sylvatica), and spruce (Picea abies) by their nature provide varying wood chemistry and surface structure including fibre size, vessels and strength differences. Figure 6 shows a range of shear strength values which vary with the thermoplastic material, wood species and pressing temperature. Evident for PP was an influence of wood species on shear strength. Bonding with beech provided a higher strength (4.9 MPa) than using maple (4.2 MPa; p = .002) whereas spruce produced a shear strength intermediate of these species. Pressing at 170 or 200 °C did not provide any significant difference in strength for the same wood species compared to the press temperatures as shown in Figure 6 (detailed data are shown in [24]). HDPE bondlines showed significantly lower shear strength (< 3 MPa) than PP (p < .001). Additionally, as with PP, the wood species used as substrate also impacted the bond performance. The composites formed with spruce produced significantly higher shear strength (2.4 MPa) than either of the HDPE/beech (2.2 MPa; p = .03) and HDPE/maple composites (2.0 MPa; p < .001 ). In the case of the TPU, pressing at 180 °C was found to provide significantly higher shear strengths than 160 °C (p < .001) [24]. The wood substrate also has an effect on TPU composite performance with spruce 11
producing a lower shear strength (3.3 MPa) than beech (3.8 MPa; p < .001) and maple (. 3.7 MPa; p < .001). As with the semi-crystalline PLA (Table 2), pressing an amorphous grade of PLA at higher temperature (160 °C) also produced higher shear strength than at lower temperature (140 °C). As outlined above, the lower PLA melt viscosity at higher pressing temperature might have led to increased penetration of the polymer into the wood substrate. The amorphous PLA also produced the highest (8 - 9 MPa) strength of all four plastics in this series which may be a consequence of the relatively higher polarity of this polymer providing greater effect of attraction forces between the polymer and the wood surface. There was little difference in shear strength (8 - 9 MPa) between beech, maple and spruce when pressed at 160 °C; at these higher strength values, however, a greater variability in the measured strength values was given (Figure 6). In developing this rapid assessment methodology, careful consideration should be given to the strength of the used wood material to ensure the substrate’s tensile strength match with the bonding polymer being evaluated. However, proportion of wood failure have not yet been assessed comprehensively at this stage of research reported here. Higher shear strengths such as those observed for PLA (Table 2 and Figure 6) led to increased variability as wood failures became more pronounced in testing data. Figure 7 shows the variation coefficients obtained for test data sets for the four different plastics as given in Figure 6. It was evident as shear strength values increased, higher variation in measured shear strength manifested as testing approached the margins of the wood (solid phase) strength. A recommendation for the methodology protocols is that selection of stronger substrates, while considering any possible influence on thermoplastic penetration and adhesion, might reduce variability evident with higher performing adhesion processes and produce less test result scatter due to wood
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failures. Using thicker wood veneer, a higher shear load that can be applied to the polymer-wood interfaces while avoiding wood failure outside the bondline.
4. Summary Using the ABES technique, the key attributes of this interfacial adhesion (shear strength) test method include:
Testing of shear strength of thermoplastic-based composites, transferred to a methodology with defined surface contact areas.
Composite preparation and testing is in situ using one instrument
Fast, easy and reliable testing, being independent of any moisture susceptibility of substrate materials.
Undertaken at various processing and testing temperatures, pressures and different solid phases.
The new interfacial adhesion test methodology has been developed to allow the rapid preparation and testing of thermoplastic-based composites over a range of temperatures. The methodology is based on a modified ABES instrument to allow the rapid heating and formation of thermoplastic-based sandwich composites followed by rapid cooling and in situ testing at specified temperatures. Press temperatures may be related to polymer properties such as melt or typical processing temperatures used in manufacturing environments. Similarly, shear strength assessments can be undertaken over a wide range of temperatures which may relate to polymer properties or postprocessing or in service temperatures. Composite sandwich formation and testing using this methodology has been demonstrated with a range of thermoplastic materials and three different wood substrate phases. Although the strength properties may not be directly transferred one-to-one to all industrially-manufactured composite (due to the 13
more complicated production processes of various composites giving more complex interfaces, as well as differences in production conditions between lab and industry), results reveal a range of interfacial adhesion. The composites’ shear strength results depend on pressing and testing temperatures, individual thermoplastic phase properties, and different wood species. In particular, for PLA-based composites the shear strength of the composites was affected by the pressing temperature. Higher pressing temperatures led to higher shear strength. While this is likely to be caused by different PLA penetration into the wood substrate due to differences in melt viscosity, polymer degradation or different crystallisation cannot be ruled out. The effect of pressing conditions and polymer properties on the structure of the composites discussed in this paper will need to be further assessed. The concept of tailoring the adhesive penetration by controlling its melt viscosity, is applicable to other thermoplastics.Nevertheless, a high potential is given for application of this new interfacial adhesion test for design and manufacture of composites based on thermoplastic adhesives.
Acknowledgements The authors are grateful to the following organisations for assistance with funding for the New Zealand-Germany Science and Technology Programme. This was contributed to by the New Zealand Royal Society and Catalyst Seed funding [FRG-FRI1402] and the Bundesministerium für Landwirtschaft und Ernährung (Federal Ministry of Food and Agriculture) [02/14-15-NZL], together with the Thünen Institute of Wood Research, Scion and the University of Hamburg.
References
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Figures
Heat/cool press head Gripping device
Moving direction
Thermoplastic phase
Solid phase (veneer)
Figure 1 Scheme for the modified Automated Bonding Evaluation System equipment adapted for the interfacial adhesion testing methodology: For details of modification see [10].
(1) Substrates
(2) Pressure
(3) Pressure
(4) Pressure
mounted in ABES
and heating
and cooling
released
Substrate and thermoplastic polymer combined
Composite sandwich formed at pressing temperature
Composite sandwich cooled to test temperature
Composite sandwich tested in shear
Figure 2 Overview of the procedure to process and test thermoplastic composite sandwich materials in situ using the adapted ABES equipment.
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Figure 3 Typical thermoplastic composite sandwich heating profiles using differing press head temperatures (140, 160 and 200 °C).
Figure 4 Scanning electron microscopy (left) and fluorescence confocal laser scanning microscopy (right) images of a PLA composite sandwich bond line (pressed at 140 °C). 18
Figure 5 Cooling profiles of the thermoplastic phase of composite sandwich using press head cooling regimes with either 4.0 bar and 0.5 bar air pressure using different initial press plate temperatures of 200, 180, 160 and 140 °C.
19
Figure 6 Average shear strength and standard deviation for thermoplastic composite sandwiches formed with HDPE (at 160 °C), PP (at 200 °C), TPU (at 180 °C), and the amorphous PLA (at 160 °C) with beech, maple, and spruce wood veneers tested at 45 °C.
20
Figure 7 Comparison of average shear strength values with variation coefficients (ratio of standard deviation to the mean) for composite sandwiches formed with the four thermoplastics HDPE, PP, TPU and amorphous PLA.
21