Composite materials comprising natural fibres with thermoplastic matrixes are increasingly used across many sectors due to their formability, light weight, and design attributes [1
]. It is common for wood plastic composites (WPCs) comprising wood-fibre-reinforced or -filled materials to be processed by compounding, extrusion, and injection and compression moulding for a range of product applications [2
]. In addition to wood or other natural fibres, these composite materials can also contain renewable or sustainable matrixes such as poly(lactic acid) (PLA), a biodegradable polyester. As a replacement for the polyolefin plastics typically used in composites, such materials also offer additional economic and environmental advantages with respect to their end of life [3
]. Poly(lactic acid) has been broadly considered for a range of applications including plastic components, films, packaging and paperboard products [3
]. This includes PLA being evaluated in WPC materials for a range of applications [6
]. Recently, the use of PLA in WPC materials has also been adapted to wood veneer to understand the adhesion developed at the wood–plastic interface [7
]. In this wood laminate form, significant adhesion can be developed between the wood substrate and PLA on composite consolidation. This adhesion is developed over a range of scales from molecular- to macro-scale [8
]. Initial evaluations of composite sandwiches formed with differing grades of PLA polymer and wood veneer species found a dependency between composite formation and performance with temperature [7
]. However, it was also revealed at the micro-scale that the PLA bondline thickness between veneer surfaces within laminates may also dictate composite performance. Therefore, to develop the processing and performance of these promising composites, further understanding of the PLA adhesion, interactions and interphase at a wood cellular level is required.
In order to further develop a fundamental understanding of the PLA adhesion interface at a wood cellular level, further investigative analysis is required. A range of techniques are available to determine interfacial properties and behaviours at a wood ultrastructure level and include X-ray fluorescence microscopy, nanoindentation, scanning thermal microscopy and UV microscopy [9
]. In the case of the latter, UV microscopy coupled with fluorescently tagged polyesters was initially applied to PLA-based WPCs to visualise interactions of PLA with wood fibre at microscale [10
]. However, direct application of this methodology to composite sandwiches at a wood cellular level requires more investigation due to the challenges of distinguishing the PLA fluorescent chromophore from natural fibre components [7
]. In the current paper a fluorescence microscopy methodology is further developed to quantify PLA polymer bondlines and interfacial adhesion developed between PLA and different wood species. By using two differing composite processing temperatures to alter PLA properties such as polymer flow and mobility, PLA migration into the ultrastructure of different wood species is characterised and compared with the composite performance. The outcomes of this study can be translated to the formation and integrity of wood interfaces with PLA or other polymers in a range of different WPCs, natural fibres and plastics applications.
2. Materials and Methods
Sycamore maple (Acer pseudoplatanus, hardwood) and spruce (Picea abies, softwood) sliced veneers (0.6 mm thickness) were used as wood substrates. Veneer specimens (100 × 20 mm2) were cut from veneer sheets and conditioned at 20 °C/65% relative humidity prior to sandwich composite formation. The amorphous grade of poly(lactic acid) (PLA) polymer was Ingeo 4060D and sourced from NatureWorks (Blair, NE, USA). This PLA was dried for 12 h under vacuum prior to any processing. Acriflavine, as acriflavine neutral chloride was obtained from Acros Organics (distributed by Thermo Fisher Scientific New Zealand Ltd.) and used as received. The wood fibres were generated by mechanically pulping Pinus radiata (softwood) chips using medium-density fibreboard processing conditions. These fibres were flash dried to <12% moisture content (mc) at the time of production and further dried (<8% mc) immediately prior to compounding with PLA.
2.1. Fluorescence Chromophore Label Grafting to PLA
Using a modified methodology [10
], PLA polymer was first dissolved in chloroform in a round-bottom flask. This flask was immersed in an oil bath and heated at 60–70 °C while maintaining stirring. In a separate flask, acriflavine (nominally 0.1% w
on PLA) was dissolved in water and then neutralised to ca. pH 8 with 1 M sodium hydroxide. The aqueous solution was poured into the chloroform solution to form a heterogeneous reaction mixture. Heating of this reaction mixture was continued at 60–70 °C under reflux for 17 h. After cooling, the solution was separated by centrifuging and washing the chloroform phase with water. Water washing was continued until the washings were deemed colourless. The residual PLA/chloroform solution was then poured onto an aluminium tray and allowed to evaporate at ambient temperature for at least 24 h. After evaporating, the fluorescently labelled PLA was recovered and further dried under vacuum at 45 °C for 24 h to produce an estimated 0.07% acriflavine loading on PLA as a PLA master batch (Supplementary Materials
2.2. Labelled PLA Compounding and Sheet (Foil) Preparation
A Haake Rheomix Rheomex 600 OS mixer equipped with Roller Rotors from Thermo Scientific (Germany) was employed to process the fluorescence-labelled PLA master batch with varying quantities of pure PLA. Mixing was performed at 170 °C for 1 min to obtain final fluorescence chromophore concentrations ranging from 0.005% to 0.015% acriflavine (w/w on PLA). These fluorescently labelled PLAs were produced on ca. 50 g scale.
This melt mixing procedure was also applied to mix the labelled PLA with wood fibre. One extrusion run was sufficient to mix the fluorescently labelled PLA master batch, pure PLA and wood fibres with compounding also undertaken at 170 °C for 1 min to minimise any fluorescence degradation. The final PLA/wood fibre contents were 95/5 and 75/25 on an oven dry basis.
PLA sheets/foils were formed on a hot press (300 × 300 mm2) (Weverk, Sweden) using a platen temperature of 180 °C. A small amount of labelled PLA (1–5 g) was placed between two 0.8 mm aluminium plates covered with Teflon sheets and separated by 0.3 mm spacer bars. A pressure of 50 kN was applied for ca. 45 s (<1 min total pressing time). The recovered PLA foils were typically 200–300 µm thickness.
2.3. Composite Sandwich Formation and Automated Bond Evaluation System (ABES) Testing
A modified ABES instrument from Adhesive Evaluation Systems (Corvallis, OR, USA) was used to prepare and test composite sandwiches in situ [8
]. The wood veneer strips (20 × 100 mm2
) were mounted with a 5 mm overlap and a PLA foil section (2.7 × 20 mm2
) was positioned between the overlapped veneer strips. The press heads were then closed and composite sandwich assembly hot-pressed at either 140 or 200 °C for 20 s with a constant pressing force (190 N) for each variation. At the completion of the hot-pressing step, the press heads were rapidly cooled to 45 °C with the press heads still closed. After achieving 45 °C for 10 s, the press heads were opened and the composite sandwich then tested (pulled) in tension recording the break load. At least 8 replicates were prepared and tested for each sample set. Samples for microscopy-only evaluation were prepared as above, but after cooling to 45 °C each specimen was removed from the ABES equipment untested.
2.4. Confocal Microscopy Analysis
A confocal TCS SP5 microscope (Leica, Germany) was used for all assessments of PLA fluorescence, PLA/wood fibres combinations (as foils) and composite sandwich bondlines. Samples from foils or sectioning bondlines from composite sandwiches were prepared by either cutting 2–3 mm sections or microtoming 20–30 µm sections of bonded veneers, respectively. Bondline images were acquired with a 10× or 60× lens at a resolution of 1024 × 1024 pixels and a stepwise depth of 2–3.5 µm. Excitation wavelengths were 476 nm and 561 nm (Argon laser) and fluorescence was recorded at 488–507 nm and 570–707 nm. Photomultiplier gain settings were adjusted for every image to avoid overexposure. Typically, 10 images of each bondline were obtained per sample. The image stacks were projected to a single image using maximum intensity projection (Leica Application Suite Advanced Fluorescence Lite (LAS AF Lite software, Germany). Emissions in the 488–507 nm region are represented in green and emissions in the 570–707 nm region are represented in red in microscopy images (Figure 1
). Additional, selected images are also available in Supplementary Materials
Composite sandwich bondline thickness was also measured using the LAS AF Lite software. The bondline thickness was determined as the distance between the two wood veneer surfaces, perpendicular to the direction of the bondline (Figure 1
, and also see Supplementary Materials
). Regions where PLA was present in vesicular structures or surface defects were not included in these measurements.
Across the microscopy images it was evident that PLA polymer flow and mobility at temperature dictated the PLA bondline thicknesses observed previously in composite sandwich formation [7
]. The initial PLA foil thickness (ca. 300 µm) did not define the bondline thickness or the extent of migration into the wood substrate as PLA was additionally squeezed out on pressing; this can also occur when gluing wood with traditional adhesives [16
]. While flow of PLA into wood vesicular structures was generally observed (Figure 1
, Figure 2
, Figure 3
, Figure 4
, Figure 5
, Figure 6
and Figure 7
), there was no defined PLA infiltration into wood cell walls of the veneer surface or individual softwood fibres. This is distinct from liquid wood adhesive behaviours where cell wall infiltration is commonly observed [12
]. This suggests that the greater molecular weight of PLA [14
], together with polymer hydrophobicity [9
] and/or high viscosity, may restrict PLA mobility into the wood cell wall compared to that of the water-soluble condensation polymers of typical wood adhesive systems [12
]. Moreover with liquid wood adhesives, adhesive flow can be parameterised to measure adhesive penetration depth from bondlines. However, this can be difficult given the randomness of the wood structure, particularly in hardwoods [14
], which was also apparent across the various bondline images in this study. Nonetheless, both the qualitative observations and the quantitative information obtained in this study can be combined to describe how bondline strength is developed during composite sandwich formation and the effect of wood fibre inclusion with PLA.
On pressing composite sandwiches at 200 °C, the PLA has a lower viscosity more comparable to that of liquid adhesives [7
]; this contributes to greater PLA mobility into the wood and correspondingly thinner bondlines due to the extent of this ingress (Figure 4
and Figure 7
). This presence of PLA in the wood matrix provides a greater interface between the PLA polymer and wood matrix, contributing to better interfacial adhesion as well as the benefit of physical interlocking of PLA within the wood ultrastructure to reinforce the wood–PLA interface. At the same time, the thickness of bondlines being generally less than 100 µm may challenge liquid adhesives’ requirements for thicker (100–800 µm), gap-filling bondlines to achieve good adhesion between wood surfaces [16
]. In contrast, at 140 °C, greater PLA viscosity reduces flow and ingress into the wood matrix and was associated with the retention of a thicker bondline. In assessing the tensile strength of bondlines, the relative thickness of the PLA matrix may impact the stress transfer across PLA bondlines, particularly given that PLA is a hard, rigid polymer below its glass transition when tested [17
]. However, with tensile testing, a thinner PLA bond appeared able to transfer stress and, in combination with greater interfacial adhesion and mechanical interlocking, provided higher performance of composites formed at 200 °C.
With wood fibre incorporation, bondline thickness variously increased by some 100% at 140 °C and ca. 240% at 200 °C pressing temperature. The variable strength of these composite sandwiches suggests that the deposition of wood fibres within the X–Y plane of testing did not readily act to transfer stress across the bondline as expected for fibre-reinforced plastics [6
]. Instead, wood fibre alignment and overlap in the heterogeneous bondline contributed to greater bondline thickness and potentially a lower degree of PLA ingress. A resulting impact was lower strength of the maple–PLA composite sample with 25% wood fibre pressed at 140 °C. However, thinner bondlines and sufficient PLA ingress were evident for the corresponding 200 °C/25% wood fibre sample which had comparable performance to pure PLA (Figure 2
) or other polymers [8
]. For spruce samples, the presence of wood fibre in PLA does not appear to significantly impact composite performance.
With spruce veneer, the contrasting cellular structures of spruce and maple led to distinctions in bondline thickness, with spruce–PLA bondlines being generally thicker (Figure 5
); this was attributable to lower PLA mobility and ingress away from the bondline. A greater bondline thickness together with lower PLA ingress arguably contributed to reduced interfacial adhesion, resulting in lower bond strength in spruce samples than in those of maple. However, along with surface morphology, differences in wood chemistry and surface polarity may also contribute to this weaker PLA–spruce wood interface. The effect of polymer polarity has previously been demonstrated by significantly lower composite sandwich bond strengths for polyethylene or polypropylene polymer foils which have relatively similar melting points and melt flow to PLA [8
]. Any impact of wood chemistry and polymer surface polarity may also extend to using softwood wood fibre in PLA, which potentially may have impacted interfacial adhesion, reinforcement and stress transfer within the PLA matrix. However, as above, wood fibre inclusion with PLA did not significantly impact spruce composite performance. Furthermore, polymer crystallinity around wood–polymer interfaces and within the plastic/polymer matrix will also impact performance, so must also be considered in the context of interfacial properties [6
]. In related adhesion studies, nano- and molecular-scale interactions have also been found to be important [14
], and these will also need to be considered in sandwich composite formation and performance. Further studies will be required to understand these complexities across differing scales in the formation and performance of plastic–wood interfaces.
Overall, we find bond strength increases with temperature and, through the promotion of PLA flow into the wood vesicular structure, provides improved mechanical adhesion and reinforcement of bondlines. At the same time, any need to maintain a minimum bondline thickness may not be a requirement as wood–wood contacts increase due to a lack of gap filling [16
]. The addition of wood fibres may present a way to achieve both greater polymer flow at higher temperature and, acting as a “spacer”, the impediment of excessive flow into the wood vesicular structure, which may ensure a sufficient adhesive layer between the veneer substrates. Nonetheless, composite strength requirements will be dependent on applications, and study findings suggest that if higher strength is needed, higher processing temperatures should be employed.