Improving mechanical performance and functionality of birch veneer with mechano-enzymatic nanocellulose coating

Abstract

In this study, we investigated the coating of birch veneers (BVs) with mechano-enzymatically manufactured nanofibrillated cellulose (meNFC) to improve mechanical strength and functionality. The meNFC has a broad particle size distribution and similar chemistry to lignocellulose materials, which are both beneficial properties in the coating of wood products. The veneer coating trials were carried out with a spray coating system developed to coat controllable thin coating layers. The spray coating produced uniform layers, which smoothened the BV surface significantly and was verified with scanning electron microscope imaging and optical profilometer measurements. The surface energy measurements showed that the meNFC is like cellulose, whereas the BV is like lignin. This observation proposes pre-treatment methods to secure a good adhesion level between the meNFC and BVs. The adhesion and compatibility of meNFC with the BV surface were measured with pull-off tests and surface energy measurements. The adhesion on a native BV surface was limited, while pre-treatment with sanding or using a primer significantly enhanced the adhesion. The meNFC coating slightly improved the BV transverse tensile strength (perpendicular to the wood veneer grain direction). A thermochromic functionality was installed on the BVs using meNFC as a binder. The produced thermochromic BVs displayed thermochromic behaviour; the coating could control the warming of the BVs subjected to solar radiation. The activation of a photocatalytic reaction of a meNFC coating containing TiO 2 was studied on the wood surface under both ultraviolet and fluorescent light, indicating in a reduction of formaldehyde concentrations. The results also showed that wood discolouration was inhibited by meNFC with not only TiO 2 but also only the meNFC coating. This study presents a practical approach to surface-treating wood materials with the meNFC to improve the mechanical and functional properties of wood products.

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Improving mechanical performance and
functionality of birch veneer with mechano-
enzymatic nanocellulose coating
Hannes Orelma (  hannes.orelma@vtt. )
VTT Technical Research Centre of Finland Ltd
Vesa Kunnari
VTT Technical Research Centre of Finland Ltd
Akio Yamamoto
VTT Technical Research Centre of Finland Ltd
Mikko Valkonen
Aalto University
Lauri Rautkari
Aalto University
Antti Korpela
VTT Technical Research Centre of Finland Ltd
Research Article
Keywords: Mechano-enzymatic nanobrillated cellulose, birch veneer, coating, adhesion, compatibility,
thermochromic pigments
Posted Date: October 25th, 2022
DOI: https://doi.org/10.21203/rs.3.rs-2140955/v1
License:   This work is licensed under a Creative Commons Attribution 4.0 International License. 
Read Full License

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Abstract
In this study, we investigated the coating of birch veneers (BVs) with mechano-enzymatically
manufactured nanobrillated cellulose (meNFC) to improve mechanical strength and functionality. The
meNFC has a broad particle size distribution and similar chemistry to lignocellulose materials, which are
both benecial properties in the coating of wood products. The veneer coating trials were carried out with
a spray coating system developed to coat controllable thin coating layers. The spray coating produced
uniform layers, which smoothened the BV surface signicantly and was veried with scanning electron
microscope imaging and optical prolometer measurements. The surface energy measurements showed
that the meNFC is like cellulose, whereas the BV is like lignin. This observation proposes pre-treatment
methods to secure a good adhesion level between the meNFC and BVs. The adhesion and compatibility
of meNFC with the BV surface were measured with pull-off tests and surface energy measurements. The
adhesion on a native BV surface was limited, while pre-treatment with sanding or using a primer
signicantly enhanced the adhesion. The meNFC coating slightly improved the BV transverse tensile
strength (perpendicular to the wood veneer grain direction). A thermochromic functionality was installed
on the BVs using meNFC as a binder. The produced thermochromic BVs displayed thermochromic
behaviour; the coating could control the warming of the BVs subjected to solar radiation. The activation
of a photocatalytic reaction of a meNFC coating containing TiO2 was studied on the wood surface under
both ultraviolet and uorescent light, indicating in a reduction of formaldehyde concentrations. The
results also showed that wood discolouration was inhibited by meNFC with not only TiO2 but also only
the meNFC coating. This study presents a practical approach to surface-treating wood materials with the
meNFC to improve the mechanical and functional properties of wood products.
Introduction
Wood materials have already been used for thousands of years to produce dwellings, paper, and other
consumer goods(Ennos 2020). Wood-based materials have many unique inherent properties as they are
simultaneously of biobased origin, biodegradable, and recyclable. In recent years, the progressing climate
change has caused an immediate need to replace fossil-based materials with lignocellulosics to reduce
carbon emissions(Sathre and Gustavsson 2009, Bergman, Puettmann et al. 2014). In general, wood is
composed of chemical components that are rapidly biodegradable without forming microplastic-type
long-lasting residues(Higuchi 2012). The excellent enzymatic degradability of lignocellulose (LC) is
based on the accessible hydroxyl groups of cellulose and hemicelluloses. However, these functional polar
substituents simultaneously cause wood to swell in water, causing dimensional instability(Kocaefe,
Huang et al. 2015).
Additionally, the visual appearance of wood products, such as their smoothness, colour, and light
scattering, may need to be improved. Thus, wood materials are always post- or pre-treated to ensure their
use in the end applications(Ramage, Burridge et al. 2017). Unfortunately, many of the current treatment
formulations are based on synthetic compounds. Therefore, there is a need to develop more
environmentally-friendly protection methods to enhance the inherent properties of wood materials.

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Currently, three major treatment strategies for wood have been used: a) chemical substitution of the
hydroxyls of LC; b) impregnating the wood with preservatives that can eliminate microorganisms or with
re retardants that lower the likelihood of incineration; c) blocking the surface of the wood to prevent
moisture, microorganisms, re, or light penetrating the cellular structure of the material(Ramage, Burridge
et al. 2017). Painting is the easiest route to protect wood since it can also be applied on a construction
site by the end user. A typical paint consists of pigments, binder polymers, extenders, solvents, and
additives. The paint composition is always application- and substrate-dependent. Most commercial
paints use binders that are synthetic acrylic, alkyd, and epoxy polymers(Stoye, Funke et al.). Wood-based
materials have already been used in paints as cellulose derivatives for binders and particle llers for
improving paint structure and coverage(Croll and Kleinlein 1986, Bulian and Graystone 2009). The role of
llers in paints is crucial since the paint polymers cannot ll the surface roughness (SR) induced pores of
the painted surface. Therefore, it is conceivable that an alternative compound between a polymer and
microsized ller could be helpful in painting applications of wood to adhere and smoothen the wood
surface simultaneously. One potential option for this could be LC-based nanocellulose (NC).
Nanocellulose is a general name for nanoscale materials covering cellulose nanocrystals, cellulose
nanobrils (CNFs), and bacterial cellulose(Moon, Martini et al. 2011). Cellulose nanomaterials have a
strong tendency to form hydrogen-bonded networks when water is evacuated from the material during
drying(Österberg, Vartiainen et al. 2013). Thus, a NC lm is a good oxygen barrier that concurrently
hinders water transportation through the lm to some degree(Lavoine, Desloges et al. 2012, Hubbe, Ferrer
et al. 2017). Nanocellulose materials have also been shown to tightly bind almost any particles, which is
useful when using the NCs as binders for other particles(Mattos, Tardy et al. 2020). The surface hydroxyl
groups enable the surface modication of the NC with varying chemistries(Klemm, Philipp et al. 1998).
Control of the surface charge is one widely-used chemical tool. The control can be achieved by using
carboxymethylation(Siró, Plackett et al. 2011)and 2,2,6,6-tetramethyl-piperidinyl-1-oxyl-mediated
oxidation(Saito, Nishiyama et al. 2006). An installed anionic charge enhances the brillation of wood
bres into NC and increases the lm-forming ability of the NC. The cationisation of NC has been shown
to make it antimicrobial(Uddin, Orelma et al. 2017)and to enhance the retention of minerals in cellulose
bres in paper-making applications(Liu, Liu et al. 2019). The mechano-enzymatic method is one
environmentally-friendly approach to producing NC(Pere, Tammelin et al. 2020). This meNFC has a
polydisperse size distribution and a tendency to form a paste in high solids content(Pere, Tammelin et al.
2020). Moreover, the given material differs from conventional mechanically made NC because it does not
shrink much when water is removed due to its higher solids content(Klar, Pere et al. 2019).
Wood construction materials face external stresses during service time, such as humidity and
temperature changes caused by uctuations in solar radiation. Darkly-tinted exterior surfaces absorb
more solar radiation, which causes rapid temperature and humidity changes in wood materials, leading to
cracking(Taha, Sailor et al. 1992). However, white exterior surfaces absorb less solar radiation, leading to
fewer surface temperature changes and a reduced need for cooling down the buildings. Then again, the
white surfaces may expose the materials to microbial action in wet conditions. For example, it is reported
that black and white exterior wood panels may reach surface temperatures of 80 and 35°C under direct

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sunlight(Ahonen 1995). In the literature, an idea has been presented to make houses reactive to
temperature changes by using thermoactivated coatings that change in colour as a function of external
temperature. The thermoactive response would be based on chemical compounds with thermochromism
behaviour(Abdellaoui, Raji et al. 2020). These coatings can control the microclimate of buildings by
controlling the energy-absorbing or -reecting behaviour of building surfaces(Karlessi, Santamouris et al.
2009, Zheng, Xu et al. 2015). These studies reported that it is possible to achieve signicant energy
savings in warm areas or regions with a cold winter and warm summer with thermoactive coatings
compared to thermally inactive ones.
In addition, little is known about the potential of meNFC on wood surfaces as a coating material. Few
studies have attempted to dene the potential of CNFs as a coating material to add functionality to the
wood surface(Yuan, Guo et al. 2021).Since a high anity exists between the CNFs and wood surface, a
certain degree of adhesion occurs due to the strong hydrogen bonds at the interface when the CNFs are
applied to a wood surface(Cheng, Wen et al. 2016, Yano, Omura et al. 2018). The main advantage of the
CNFs is their high specic surface area(Yano, Omura et al. 2018), which provides ecient functionality.
Titanium dioxide (TiO2) is a photocatalyst that could decompose contaminants, including volatile
organic compounds (VOCs), when exposed to light irradiation at a specic wavelength range(Ichiura,
Kitaoka et al. 2003). The effects and mechanism of the photocatalytic (PC) reaction of TiO2 have been
widely studied since the seminal
Nature
publication in 1972(Fujishima and Honda 1972). The research
on wood surface-treatment with TiO2 has attracted considerable critical attention in recent years, aiming
to improve ultraviolet (UV) protection(Pori, Vilčnik et al. 2016, Pánek, Šimůnková et al. 2020),
hydrophobicity and self-cleaning(Rao, Liu et al. 2016, Pandit, Tudu et al. 2020, Xing, Zhang et al. 2020),
and VOC removal(Zhu, Liu et al. 2016)and anti-fungal capabilities(Chen, Yang et al. 2009).
In this study, the use of meNFC(Pere, Tammelin et al. 2020)was investigated for the surface coating of
BVs (Figure 1). The study investigated how the used meNFC coats adhered to a BV surface. The coating
was carried out using a simple spray-coating setup that enabled reasonable coating weight control. It is
conceivable that the prepared meNFC with broad size distribution and a high solids content could help
protect the wood surface. In general, the small polymeric materials of the meNFC enhance the adhesion,
while the larger particles ll the rough surfaces, and the high solids content prevents drying-based
shrinkage that may break the formed lm. The chemical and physical properties of the meNFC
inuencing its adhesion to wood surfaces were investigated. The mechanical strength of the BVs was
also studied regarding the effect of the coating. The meNFC was also used as a binding matrix for the
thermochromic (TC) pigments. The TC activity of the meNFC was studied in laboratory and exterior
conditions to demonstrate the usability of the TC meNFC coating. Additionally, the effects of the meNFC
containing TiO2 were studied, focusing specically on UV protection and VOC removal. The PC reaction
was activated with light in the UV and visible wavelength range. The degree of the VOC removal was
measured using formaldehyde (HCHO), a major indoor-air pollutant in modernhousing. The reectance
and scattering of UV light in TiO2-doped meNFC were determined by the degree of discolouration of the

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wood surface. The results show that the meNFC is a potential coating material for BV products and can
potentially be used to install active pigments on the surfaces of other wood products.
Experimental Section
Materials
Never-dried birch veneer (BVs) with nominal thicknesses of 0.6, 1.0, and 1.5 mm were obtained from
Koskisen Oy. The specimens were stored in a freezer room at ca. –20°C prior to thawing just before the
coating trials. The cellulose source of the meNFC was bleached softwood kraft pulp obtained from Metsä
Fibre (Äänekoski bioproduct mill, Finland). Black TC pigment (Chameleon Slurry Black 31 °C,
5102CPT03102EKGL, Batch A9-031121-1T) from LCR Hallcrest with a colour transition temperature in the
warming direction of 25.0 – 32.5 °C and in the cooling direction 29.0 – 21.0 °C. Orange TC pigment
(ChromaZone®Slurry Orange 31 °C, 5300CPT03106EKG, Batch A9-012320-1) from LCR Hallcrest with
colour transition in the warming direction 21.5 – 33.0 °C and in the cooling direction 29.5 – 16.0 °C. The
TiO2 used in the study was analytical grade in powder form and purchased from Merck (Darmstadt,
Germany). All other chemicals used were laboratory-grade. All water used was puried using a Milli-Q
device.
Methods
The preparation of the mechanoenzymatic nanocellulose (meNFC). The meNFC used in the study was
produced following the procedure reported in (Pere, Tammelin et al. 2020). The process utilises a
simultaneous enzymatic action and bre-to-bre friction caused by the gentle mixing. The cellulosic pulp
was treated at ca. 25.0 wt.-% for ca. 8 h at ca. 70 °C using a two-shaft sigma mixer (Jaygo Incorporated,
NJ, USA) running at ca. 25 rpm up to ca. 8 h. The size distribution of the prepared meNFC is shown in
Figure 2. First, the average length-weighted length and width of the pulp bres were determined using a
Metso FiberLab bre image analyser. Then, the present soluble substances were measured
gravimetrically by dialysing the meNFC vs Milli-Q water in a 3.5 kDa dialysis tube. The produced material
was stored in a refrigerator (4°C) before its use in the coating studies.
Birch veneer coating with the meNFC. With the help of a self-made laboratory spray-coating unit, the
meNFC was applied to the BV specimens (Figure 3). The coating station included a motorised vacuum
sledge that xed the specimens horizontally. The diameter of the spray nozzle and used pressure were ca.
254 µm and 3.5 bar, respectively. The width of the spray was ca. 260 mm. The specimens were placed on
the sledge and coated with meNFC at a ca. 10.0 wt.-% dry matter content using a constant speed (1
m/min). The coating thickness was adjusted by preparing specimens with multiple coating layers (CLs)
layer-by-layer. The coating amount was measured gravimetrically by balancing the specimen before and
after meNFC coating (dry specimens). The coating trials were carried out on dry and wet BV specimens
with and without sanding.

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The BASF Epotal ECO 3675X primer (a CL area density of ca. 2.1 ± 0.5 g/m2) was also tested to improve
the adhesion between the meNFC coating and BVs. The specimen size in the coating studies was kept
constant at ca. 15×15 cm2. The meNFC coating was always applied on the less dense side of the BV
specimens. The drying of the coated BVs was carried out with two different methods: a) hot-air-ow
drying (referred to as blower drying, BD) with a blow-air dryer (Harju BG-E3A 3 kW) and b) in a standard
condition (SC) at ca. 20 °C and 65 % relative humidity (RH).
Scanning electron microscopy. The scanning electron microscope (SEM) imaging of the coated
specimens with an uncoated native BV reference specimen was carried out with a Merlin Field Emission
(FE)-SEM (Carl Zeiss NTS GmbH, Germany) with the applied gold sputter coatings (ca. 30 mA, 30 s). The
electron gun voltage was constant at ca. 3.0 kV with a grid current of ca. 60 pA. The pixel resolution of
the SEM was 2 048×1 536.
Surface free energy measurements with a water contact angle goniometer. Water contact angle (WCA)
measurements were conducted with an Attention Theta Optical Tensiometer (Biolin Scientic, Sweden).
The measurements were performed at ca. 23 °C and 50 % RH with Milli-Q water as the probe liquid with a
droplet volume of ca. 4.0 µl, and at least ve locations were measured from each specimen for statistical
accuracy and repeatability. Uncoated native BV specimens were used as references. The surface free
energy (SFE) measurements were carried out with four test liquids: water, ethylene glycol, formamide, and
3,3’-diindolylmethane. The SFEs were calculated based on the Lewis acid-base model.
The determination of the strength, adhesion, and SR properties.The transverse tensile strength (TTS) of
the BV specimens was measured using a Zwick 1475 universal material testing machine (ZwickRoell
Group, Ulm, Germany) equipped with a MTS control system (MTS Systems Corp., Eden Prairie, MN, USA)
and a 1 000 N load cell. The specimens were prepared from coated BV sheets with dimensions of ca.
50×150 mm (
w
×
l
, the specimen length direction wasperpendicular to the wood grain). Uncoated native
BV specimens were used as references. Before the test, the specimens were conditioned at ca. 20 °C and
65 % RH for at least 1 wk. The loading direction was along the specimen length with a 2.0 mm/min
nominal loading speed. The test was initiated with a crosshead clearance of ca. 50 mm. The standard EN
314-1(Standardization 2004)was used for calculating the TTS of each specimen. Specically, the
effects of the drying methods were determined by conducting the TTS test on BVs at three different BV
thicknesses and for three different CLs. The details of the sample matrix are summarised inTable 1. The
TTS properties were analysed from the sample matrix of the BV specimens (
N
= 9).
Table 1
The sample matrix of the tested BV specimens.

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The adhesion between the meNFC coating and BVs was investigated using pull-off measurements with a
Lloyd LS5 tensile tester equipped with a 100 N sensor (AMETEK Measurement & Calibration
Technologies, Largo, FL, USA) in standardised laboratory conditions (ca. 50 % RH and 23 °C). Aluminium
pull stubs (a diameter of ca. 11 mm) were glued to the BV surfaces with epoxy glue. Then, the stub was
pulled off from the surface under the tensile tester in a vertical direction from the specimen. The cut-out
diameter of the coating under the stub was used to calculate the adhesion strength. At least ve
replicates were used, while the uncoated native BV specimens functioned as references. Furthermore, the
SR of the BV specimens were measured using a Veeco Wyko NT 9100 optical prolometer (Bruker, USA).
At least ve spots on each specimen were investigated for statistical accuracy and repeatability. The SR
values represent the average values calculated from the repetitions.
A thermoresponsive meNFC coating and solar warming studies. The TC meNFC coatings were prepared
by mixing a TC pigment (black and orange colours were tested) in a ca. 10.0 wt.-% meNFC dispersion
using a blade mixer. The added pigment contents in the meNFCs were ca. 2.5, 8.0, and 20.0 wt.-% to the
dry matter content of the meNFC. The coated specimens were prepared by masking a constant area of
the BV specimens and adding one CL of the TC meNFC coating per the masked area. Uncoated native BV
specimens acted as references. The prepared specimens were dried in room conditions. A colour change
test was carried out under a heat blower (Harju BG-E3A 3 kW) by controlling the heating with a power
switch. The cycling was carried out by visually following when the specimen changed in colour. The eld
tests were conducted from 8/11-12/2021 at ca. 21 °C. The specimens were placed in direct sunlight, and
the surface temperature was monitored continuously with a heat camera (Flir C5, Teledyne Flir systems,
USA).
The preparation of the BV specimens coated with the meNFC containing TiO2. The TiO2-doped meNFC
was prepared by mixing the TiO2 in a meNFC suspension at a ca. 10.0 wt.-% of the solids content. The
preparation involved stirring the mixture manually with a spatula for over 3 min in a 200 ml beaker. Next,
the suspension was applied gap-free to the specimen surface with a paintbrush. After that, the specimens
were dried in a conditioning room (20 °C and 65 % RH) for at least 24 h before the experiments. Uncoated
native BV specimens functioned as references.
The weathering treatment of the BV specimens.The BV specimens were tested using a Suntest CPS+
Xenon Weathering instrument (AMETEK Atlas / AMETEK, Inc.; Mount Prospect;IL; USA) which was used
to accelerate the degree of discolouration on the BV surfaces with or without the meNFC and its potential

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TiO2 doping due to the high radiation levels of the UV light. The colour coordinates of the BV surfaces
were measured with a spectrophotometer (Spectrolino, Gretag-Macbeth AG, Regensdorf, Switzerland).
Earlywood was chosen for the measurement to reduce the measurement variation (Yamamoto, Rohumaa
et al. 2017). The colour differences were calculated as the parameter ∆
Eab
, commonly used for the colour
measurement of the wood surface(Yamamoto, Rohumaa et al. 2015).
The evaluation of the VOC removal on the coated BVs with gas chromatography with ame-ionisation
detection.In the gas chromatography with ame-ionisation detection (FID) experiment, a headspace
technique combined with a gas chromatography analysis was performed to verify the activation of the
photocatalyst in the meNFC by the light of different wavelengths following the Shimadzu’s application
note No. G327 with slight modications. It is important to note that the FID detector cannot see small
concentrations of formaldehyde. In this study the formaldehyde levels measured were high that made
them visible in the FID detector. Due to these limitation, the results shown here can be used only
qualitatively. The BV specimen was prepared from the coated BVs in ca. 1×5 cm dimensions and placed
in an 18-mm headspace screw-top vial with a nominal usable volume of 20 ml. Approximately a 1.0 μl
volume of 36 % HCHO solution in analytical grade (Avantor® / VWR International, LLC; Philadelphia; PA;
USA) was added to the inner wall of the vial to prepare the sample. The vial was sealed with a metal
screw cap with a septum. During the experiment, the vials were placed under either regular uorescent
lights (480 nm, 730 lx) or a UV light (350 nm, 450 lx). The headspace sample was collected with a syringe
and immediately injected into a gas chromatograph (GC)-2010 Plus with a ame ionisation detector
(FID)-2010 Plus (Shimadzu Corp., Kyoto, Japan) using hydrogen as the carrier gas (ca. 1.0 ml/min). The
injector temperature was set to ca. 250 °C, and a split injection mode was applied as required. The oven
temperature programme was set as constant at ca. 40 °C. The FID signal sampling rate was ca. 50 s−1.
Results And Discussion
The properties of the meNFC. The particle size distribution and morphology of the meNFC were studied
with a bre image analyser and SEM. The bre image analyses showed that the meNFC had a broad
particle size distribution (Fig.2). The material was principally disintegrated at the microscale, which is
observable in the no. of bre nes in the sample. The microscale (< 200 µm) contained up to 98% of the
material. The material also contained up to 5% of a water-soluble fraction containing water-soluble
sugars (enzymatic hydrolysis products) and water-soluble polymers (incompletely hydrolysed cellulose
and hemicellulose). In the SEM images, the microscale bre fragments and nanoscale CNFs were visible
(Fig.4a and b). The largest bre fragments had a diameter of several micrometres. The disintegrated
material exhibited a visual appearance typical to coarse mechanically disintegrated microbrillated
cellulose (Nakagaito and Yano 2004). The water-soluble polymers and sugars were not visible in the SEM
images due to the resolution limits of the techniques. These results are similar to the original study with
the meNFC by Pere et al. (Pere, Tammelin et al. 2020).
The meNFC coating of the BVs. The meNFC was coated on the wet BV specimens using a spray-coating
technique. The meNFC coating made the surface of the BVs look whiter (Fig.5a). The used meNFC did

Page 9/25
not contain lignin (Pere, Tammelin et al. 2020), i.e., the brownish component in wood (Zhang, Fu et al.
2020). To the naked eye, the meNFC-coated specimens looked more natural than the uncoated
specimens. The coated specimens were imaged with the SEM. The native BV surface was very rough,
exhibiting open lumens and pores with sizes ranging from tens to hundreds of micrometres (Fig.5b). The
reason for the high SR of the BV surface was the rotary cutting method used. This method peeled the
wood cell wall so that some of the bres were cut in the grain direction, revealing bre lumens (Bekhta,
Proszyk et al. 2014). The meNFC coating signicantly smoothened the BV surface, as seen in the SEM
image (Fig.5c). The morphology of the meNFC observed earlier (Fig.4) was replicated on the BV surface
(Fig.5c). It is conceivable that the coating mechanism allowed the bre fragments to ll the large pores
while the microsized meNFC smoothened the surface, and the water-soluble fraction penetrated the
surface pores.
The effect of the applied meNFC coating amount on the roughness (SR) of the BVs was measured using
an optical prolometer. The root mean squared SR of the native BVs was ca. 4.7 ± 0.8 µm and decreased
to ca. 3.7 ± 0.7 µm when one CL of the meNFC was applied to the BVs (Fig.5d). The SR decreased
linearly as a function of the applied CLs, and the smallest SR of ca. 1.8 ± 0.5 µm was observed with three
CLs. The coating area density of one meNFC CL was ca. 29.0 ± 0.6 g/m2, and the process parameters can
be set in the spray-coating technology (Luangkularb, Prombanpong et al. 2014). The multiple CLs
increased the coating area density on the surface linearly. The highest coating area density was ca. 96.5
± 4.8 g/m2 with the three applied CLs. The sample standard deviation of the coating area density was
slight and shows that the spray-coating technique has a reasonable weight control for preparing thin CLs.
The compatibility, adhesion, and strength development of the meNFC-coated BVs. The compatibility of
the meNFC and BVs was estimated using the WCA and SFE measurements. The WCA of the BVs and
meNFC were ca. 50° (5 s) and below 10° (5 s), respectively (Fig.6a). The reason for this WCA difference is
the lignin on the wood surface, which is not present in the meNFC (Pere, Tammelin et al. 2020). Lignin is
the less hydrophilic component in LC. Pure mill wood lignin has a reported WCA of ca. 55° (Notley and
Norgren 2010), and native cellulose has a reported WCA of ca. 25° (Mohan, Kargl et al. 2011). These
WCAs indicate that the native BV surface was chemically akin to the lignin surface, and the meNFC
coating resembled pure cellulose. Earlier, for enzymatically-manufactured softwood NC, a WCA of ca. 18°
was presented (Pere, Tammelin et al. 2020), which correlates with our results. The WCA value decreased
slowly in the native BV specimens as the water droplets penetrated the wood pores. As similarly observed
with the meNFC, this also indicates that the meNFC specimen was smoother and lacked suitable pores,
so the effect was that the water could not evaporate rapidly. After 30 s, the WCA of the native BVs was ca.
30°. The pre-treatment of the BV surfaces with sanding reduced the measured WCA to ca. 25° (5 s) and
12° (30 s). On wood materials, this can be explained by the accumulation of extractives on the wood
surface that increases the WCAs (Hse and Kuo 1988). A similar effect is observed with thermomechanical
pulp, where the behaviour is called “self-sizing”, which explains why the hydrophobicity of mechanical
pulp increases as the pulp ages (Bialczak, Holmbom et al. 2011).

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The rst requirement for good adhesion between an adhesive and a substrate is that the adhesive evenly
wets the substrate (Kinloch and Kinloch 1987). In surface tension terms, this means that the SFE of the
substrate needs to be higher than that of the adhesive material. In water-based coating systems, the bond
formation occurs when the water is evaporated from the spread CL, followed by the capillary force based
action that pulls the coated material into contact with the surface of the substrate. The SFE of the meNFC
was slightly smaller than that of the native BVs (γtot,meNFC ≈ 50 mN/m vs γtot,veneer ≈ 54 mN/m) (Fig.6b).
The native BVs had a somewhat higher dispersive component and smaller polar component compared to
the results of the meNFC. The reason for these observations was the presence of the more hydrophobic
lignin and extractive components on the surfaces of the native BVs that do not exist on the meNFC. The
slight differences between the SFEs indicate that the SFE matching between the meNFC and native BVs
was not perfect, suggesting the use of pre-treatment techniques.
The adhesion of the meNFC onto the BVs and the mechanical strength of the coated BVs. The adhesion
of the coated meNFC CLs on the BVs was measured using the pull-off measurements from the
specimens coated on either dry or wet BVs (Fig.7a). The pull-off forces of the meNFC CLs (both coated
on dry and wet BVs) were ca. 10 N (0.10 MPa). The pre-treatment with the sanding increased the pull-off
force of the meNFC coating to ca. 14 N (0.15 MPa, coated on wet BVs) and ca. 24 N (0.25 MPa, coated on
dry BVs), respectively. The sanding is reported to inuence the SR and chemical structure of the wood
surfaces, enhancing coating adhesion (Sinn, Gindl et al. 2004, Thoma, Peri et al. 2015, Söğütlü, Nzokou et
al. 2016). As a reference, we tested if a commercial biodegradable water-based dispersion (BASF Epotal
ECO 3675X) primer could further enhance the adhesion of the meNFC on the BVs. The adhesion of the
meNFC on the BVs was increased to above ca. 40 N (0.44 MPa), which indicates that the adhesion was
doubled compared to the value measured on the sanded BV surfaces. The adhesion strengths were
calculated above to correlate with the literature values. The adhesion values were smaller compared to
the results obtained with commercial waterborne acrylic paints (adhesion strength up to ca. 5.0 MPa) and
polyurethane coatings (up to ca. 1.0 MPa) (Turkulin, Richter et al. 2002). The most conceivable reason for
the lower adhesion strengths is that the meNFC coating could not penetrate the pores of the wood
surface to form the mechanical interlocks that polymeric paints can form.
The effects of the coating amount, BV thickness and drying methods were also investigated via a TTS
test performed on the BV specimens coated with the meNFC (Fig.7b and c). The results showed that all
the coated BV specimens, except those with a 1.5 mm thickness, had higher TTSs than the reference,
indicating that the coating with the meNFC could increase the TTSs of the BV specimens. The coated BV
specimens that had been dried using a blower dryer (BD) tended to have better TTSs than those dried in a
SC. It is possible that the active drying (BD) of the CLs promoted the attachment of the meNFC onto the
BV wood surfaces, resulting in higher TTSs. Although specic apparent differences were observed from
the results of the BV specimens in 0.6 mm and 1.0 mm thickness, no signicant differences were
observed from the ones in 1.5 mm. The thick BV specimen was probably less exible than the others,
resulting in less elasticity and a higher likelihood of a weak point in the same specimen area, easily
breakable during the test. The coating amount also affected the mechanical property performance in the

Page 11/25
BV specimens. The results indicated that the double-coated BV specimens tended to have lower TTSs
than the other coated specimens. This nding suggests that excessive coating on the BVs reduces the
adhesion between the meNFC and the wood surfaces.
Thermoresponsive meNFC coating with TC pigments. The meNFC was a binder for the two TC pigments
to prepare a thermoresponsive (temperature-responsive) wood surface. The meNFC binds spherical TC
pigments well and attaches them to the wood surface, as seen in the SEM image (Fig.8a). The ability of
NC materials to tightly bind various particles has also been reported earlier (Mattos, Tardy et al. 2020).
The TC pigments change their colour reversibly as the temperature rises above the transition temperature
(Fig.8b). In this study, the pigments used were black and orange with a transition temperature of ca.
28°C. The pigment-doping inuenced the deepness of the coating. It was observed that the specimen
prepared with the ca. 8.0 wt.-% consistency produced a deep colour. The colour disappeared quickly when
the specimen was placed under a heat source. The colour change of the black pigments was more
remarkable than the orange ones. It was observed that after 10 s of heating, the black pigment-doped
specimen with a pigment loading below 8.0 wt.-% reached the initial colour of the pure meNFC (Fig.8c
and d). The 20.0 wt.-% black pigment loading did not produce as strong a colour transition, and some
black pigment was visible after the heating. The probable reason was that the heat blower was not strong
enough to heat the specimen thoroughly to activate all the TC pigments. This lack of heating capability
was also visible with the orange TC pigment-doped specimen, where the orange colour was more evident
after the heating trial. A cycling test was performed by switching the heater on and off frequently.
The TC meNFC coating changed in colour over tens of heating cycles, and no irreversible changes were
observed in the specimens. The maximum surface temperatures of the TC specimens in the laboratory
test were monitored with a thermal camera (Fig.9a). The black TC pigment specimens warmed more
than the specimens made with the orange TC pigment. Overall, the shapes of all the warming curves were
similar. The black specimens reached a surface temperature above 55°C, whereas the orange specimens
were ca. 10°C lower in surface temperature. Studies on TC pigments in the wood coating eld have been
reported earlier. Li et al. (Li, Hui et al. 2017) used 2′-chloro-6′-(diethylamino) pigments to produce a colour-
changing wood coating that displayed a reversible action when the temperature was cycled.
The warming of the TC meNFC coatings under the sun was tested in outdoor eld tests. The functionality
of the black TC pigment meNFC coating was compared to the black paint and a pure meNFC coating.
Figure9b presents the surface temperatures of all specimens as a function of the exposure time. The
black surface warmed more quickly than the black TC meNFC surface, which can be seen in the higher
slope value at ca. 0–1 min. All the specimens reached the rst plateau during the rst 5 min, indicating
that the coating had become warm. The TC pigment-containing specimens changed their colour from
black to white after a 1 min exposure. After 5 min, all measured surface temperatures continued to rise,
and after 15 min, all specimens reached their nal plateau levels. The black reference surface warmed to
ca. 45°C, whereas the surface temperature of the pure meNFC was ca. 10°C lower.

Page 12/25
The black TC pigment specimens followed a warming curve similar to the pure meNFC coating.
Furthermore, there was no signicant effect concerning the amount of TC pigment in the meNFC. These
results indicate that the black TC meNFC coating limits the warming of the wood surface under solar
radiation when the temperature of the wood plate rises over 30°C. In the literature, Zheng et al. (Zheng, Xu
et al. 2015) reported that a TC coating worked best when the outside temperature rose over 25°C during
the year. The approach used in this study offers a simple method to attach functionalities to the BV
surfaces.
The optical properties and PC activity of the meNFC coating doped with the TiO 2. The BV specimens
were treated in a weathering device for 48 h, and the colour coordinates of each specimen were recorded
over the period (Fig.10). It is commonly known that a chromophore is formed on the surface of the wood
when exposed to UV radiation. All the BV specimens turned darker during the weathering. A clear trend of
the remaining lightness on the specimens coated with the TiO2 exists.
Moreover, the degree of the effect seems to correlate with the concentration of the TiO2 in the meNFC.
Surprisingly, the UV-light-reected effect was indicated in the specimen coated with only the meNFC.
Hypothetically, the meNFC can be used solely for wood surfaces as a coating agent to prevent wood
surface discolouration. A comparison of the colour differences (∆
Eab
) values in the specimens showed
that the meNFC coatings markedly stabilised the wood surface colour during the weathering. The ∆
Eab
refers to the degree of the colour changes over time compared to the measurement at zero. These
ndings suggest that the meNFC with the TiO2 effectively prevents wood discolouration and stabilises
the wood surface colour vs UV radiation. As a further observation, the pure meNFC provides some of
these effects without the TiO2 doping.
A UV and uorescent light tested the PC activity of the TiO2 + meNFC vs the air chemicals. The degree of
the PC functionality was measured via the HCHO gas concentration reduction rate in the glass vials. The
HCHO is a well-known VOC, a common indoor-air pollutant released from wood products like plywood
(Uchiyama, Matsushima et al. 2007). Figure10c and d show the summary of the photoactivation tests.
Under both light sources, the concentrations of the HCHO have reduced in the TiO2 + meNFC BV vials, and
the reduction rate was higher in the vial under UV light. The HCHO concentrations in the meNFC BV vial
and the blank were also reduced under UV light. This reduction could probably be due to the
photodegradation of the HCHO by the UV light itself (Shie, Lee et al. 2008). In all the vials, the initial gas
concentrations varied. This variance could originate from differences in the gas dispersion rates, or some
of the gas might have been absorbed or trapped in the wood pores at 0 h. Under the uorescent light, no
signicant reduction in the concentration was observed in the blank and meNFC BVs, unlike the ones
under UV light. Similar approach has earlier utilized to prepare an active surface that can remove indoor
pollutans as formaldehyde (Ichiura, Kitaoka et al. 2003).
Conclusion

Page 13/25
The meNFC was used to coat the BVs, and it was spread on a BV surface using a spray-coating device.
The spray coating controls the coat weight and makes it possible to coat thin CLs. The properties of the
meNFC coating deposited on the BV surfaces were studied. The meNFC smoothens the BV surfaces and
makes them whiter. The compatibility of the BVs and meNFC CLs was also studied. The adhesion of the
meNFC on the BVs was investigated using SFE measurements. In their chemical proles, the meNFC and
BVs are similar, but the lignin and extractives on the surfaces of the BVs slightly decreased their
hydrophilicity character. The adhesion of the meNFC to the BVs was investigated using pull-off
measurements. Limited compatibility on the native BVs was seen in the low pull-off force, but this
signicantly improved when the BVs were pre-sanded or primer-coated. The good binding properties of
NC were utilised to immobilise the TC pigments to the BV surfaces. The TC meNFC coating made the BVs
temperature-responsive. The BV surfaces changed in colour when the BVs were heated with a hot-air
blower. The warming of the TC meNFC-coated specimens in sunlight was measured using a heat camera.
It was shown that the TC meNFC coating could be used to control the warming of the BVs under sunlight.
The addition of the TiO2 particles made the BV surfaces photoactive. The meNFC itself protected the
wood surfaces against UV radiation. The added TiO2 enabled the photoactivity that degraded the HCHO
under UV light. The meNFC coating is a versatile binder to protect the wood and install functionalities to
the wood surfaces.
Declarations
Funding
This study was carried out in the Verycoat (Novel high-performance veneer products by effective drying
and nano-coating) project funded by Business Finland and supported by Finnish industrial companies
(Stora-Enso Ltd., Raute Ltd., Koskisen Oy, Teknos Oy, Puustelli Oy, A-Factory Oy).
Author contributions
All authors contributed to the study conception and design.The manuscript was written through
contributions from all authors. H.O. was responsible for preparing the project plan, instructing the
research work and writing the rst draft of the article. V.K. was responsible for preparing the meNFC and
carrying out coating trials. A.Y. was responsible for the tensile measurements of the coated BVs. M.V.
continued the work of A.Y. and proofread and reviewed this manuscript. L.R. developed the measurement
plan and steered the work. Finally, A.K. was responsible for steering the research. The manuscript was
written with contributions from all the authors. All the authors have approved the nal version of the
manuscript.
Acknowledgements
The authors would like to thank Panu Lahtinen for preparing the meNFC, Jaakko Pere for developing the
project idea, and Vuokko Liukkonen for carrying out the contact angle and optical prolometer

Page 14/25
measurements. This work was a part of the Flagship Programme of the Academy of Finland under
Project Nos. 318890 and 318891 (Competence Center for the Materials Bioeconomy, FinnCERES).
Competing Interests
The authors have no relevant nancial or non-nancial interests to disclose.
Ethics approval
Not applicable. This article does not contain any studies with human participants or anamials performed
by any of the authors.
Availability of data and materials
The corresponding author will provide the datasets generated for this study on request. The data is stored
in the VTT’s server.
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Figures
Figure 1
A schematic illustration of the manufacture of functional meNFC coatings onto rotary cut veneer. Three
functional surfaces presented: a) Thermoresponsive coating with thermochromic pigments, b) UV-
decauing blocking with meNFC, and c) photodegradation of chemicals with TiO2.

Page 19/25
Figure 2
The bre length-weighted distribution, average particle sizes, and water-soluble fraction of the prepared
meNFC.
Figure 3
a) A schematic illustration of the laboratory-scale coating station. b) A photograph of spray coating the
meNFC coating on the BV specimens.

Page 20/25
Figure 4
The SEM images of the used meNFC show the polydispersity of the micro- a) and nanoscale b) fractions.

Page 21/25
Figure 5
a) A photograph of a BV specimen with a meNFC coating. The dashed line has been placed in the image
to show the borders of the coated area. The SEM images of the surface of the BV b) before and c) after
applying one CL of the meNFC. d) The negative linear correlation between the coating weight (area
density) and SR of the applied meNFC CLs.

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Figure 6
a) The typical WCAs and b) SFEs of the native BVs and meNFC. In addition, the WCAs of the sanded BV
surfaces without the meNFC coating are also shown.

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Figure 7
a) The adhesion force of the meNFC coating onto the BVs. The effects of the coating on the wet and dry
surfaces and sanding are shown. The effect of a biodegradable primer is also shown. b–c) The TTS of
the meNFC-coated BV specimens with the three different coating levels and BV thicknesses, respectively.
The coating levels were 1) 29.0 g/m2, 2) 64.3 g/m2, and 3) 96.5 g/m2. The coated BV specimens were
dried in: a) a SC in the conditioning room or by b) BD, which was conducted with an articial hot-air dryer.
Figure 8
a) A SEM image of the TC pigment-doped meNFC coatings on the BVs. b) The colour transition as the
specimen surface temperature is switched. c–d) The BV specimens with coated areas of two different
colours (orange and black) of the meNFC TC coatings (the tested pigment consistencies were 0, ca. 2.5,
ca. 8.0, and ca. 20.0 wt.-%) c) before and d) after 10 s of warming under the hot-air blower.

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Figure 9
a) The surface temperatures of the TC meNFC-coated BV specimens under the heat blower as a function
of the heating time. The specimen coding: O = the orange pigment, B = the black pigment, and the wt.-%s
identify the amount of the pigment in the meNFC. b) The surface temperatures of the black TC meNFC-
coated BVs in eld testing. c) a photography and a heat camera image of TC specimens under sun light.

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Figure 10
The changes to the colour coordinates in the BV specimens during the weathering test: a) The
L*
scale
(lightness) and b) the ∆
Eab
(the colour differences). For the ∆
Eab
calculation, the colour coordinates at 0
h were used as references. The PC effects of the TiO2 in the meNFC coating on the BV specimens under
different light sources c) and d).