Preparation of Two-Component Polyurethane Coatings from Bleached Liquefied Wood

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Liquefied wood-based polyurethane wood coatings of an aesthetically acceptable light colour were prepared and characterised.Liquefied black poplar wood was obtained by solvolysis in a polyethylene glycol/glycerol mixture, and it was bleached with hydrogen peroxide.The bleaching treatment converted liquefied wood from a dark brown to a yellowish product.Polyurethane films were prepared by the curing of liquefied wood with polymeric diphenylmethane diisocyanate or trimethylolpropane toluene diisocyanate prepolymer (TMP/TDI) isocyanate-type hardeners.It was found that the selected properties of the films prepared from the bleached liquefied wood were, in general, equivalent to those prepared from unbleached liquefied wood.The mechanical properties of the films obtained with the TMP/TDI curing agent were acceptable for wood coating applications.The initial poor resistance of the films to water and ethanol was substantially improved by the addition of n-octyltriethoxysilane to the liquefied wood prior to the preparation of the polyurethane coatings; the hardness of the films also increased.

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## Preparation of Two-Component Polyurethane Coatings from Bleached Liquefied Wood

Arnaud M. Cheumani-Yona, a,b Franc Budija, a David Hrastnik, b Andreja Kutnar, a,c
Matjaž Pavlič, b Pavel Pori, d Črtomir Tavzes, a and Marko Petrič b,

Liquefied wood-based polyurethane wood coatings of an aesthetically
acceptable light colour were prepared and characterised. Liquefied black
poplar wood was obtained by solvolysis in a polyethylene glycol/glycerol
mixture, and it was bleached with hydrogen peroxide. The bleaching
treatment converted liquefied wood from a dark brown to a yellowish
product. Polyurethane films were prepared by the curing of liquefied wood
with polymeric diphenylmethane diisocyanate or trimethylolpropane
toluene diisocyanate prepolymer (TMP/TDI) isocyanate-type hardeners. It
was found that the selected properties of the films prepared from the
bleached liquefied wood were, in general, equivalent to those prepared
from unbleached liquefied wood. The mechanical properties of the films
obtained with the TMP/TDI curing agent were acceptable for wood coating
applications. The initial poor resistance of the films to water and ethanol
was substantially improved by the addition of n-octyltriethoxysilane to the
liquefied wood prior to the preparation of the polyurethane coatings; the
hardness of the films also increased.

_Keywords: Liquefied wood; Bleaching; Polyurethane wood coating; N-octyltriethoxysilane_

_Contact information: a: Institute of Wood Science and Technology and Sustainable Development, Celovška_
_cesta 268, 1000 Ljubljana, Slovenia; b: Department of Wood Science and Technology, Biotechnical Faculty,_
_University of Ljubljana, Jamnikarjeva 101, 1000 Ljubljana, Slovenia; c: Andrej Marušič Institute, University_
_of Primorska, Muzejski trg 2, 6000 Koper, Slovenia; d: Chemcolor Sevnica d.o.o., Dolnje Brezovo 35, 8290_
_[Sevnica, Slovenia; *Corresponding author: marko.petric@bf.uni-lj.si](mailto:marko.petric@bf.uni-lj.si)_

INTRODUCTION

Polyurethanes (PUs) are versatile materials used in practically all fields of polymer
applications: foams, elastomers, sealants, fibers, adhesives, and coatings (Ionescu 2005).
Because of their excellent properties (abrasion resistance, toughness, flexibility, and
chemical and corrosion resistance), the applications of PUs in the coating industry are
increasing ( _e.g._, wood furniture and floorings, the automotive industry, and chemicalresistant coatings) (Wicks _et al._ 2007). PUs are obtained by the reaction of a polyol (a lowmolecular weight polymer with terminal hydroxyl groups) and a polyisocyanate (a
molecule containing at least two terminal isocyanate groups). These components are
traditionally oil-derived. The depletion of fossil resources ( _i.e._, crude oil) and
environmental concerns related to their exploitation have led to extensive research into
alternative sources of raw materials for the PU industry, especially for the polyol
component. Various hydroxyl-containing liquid products from different kinds of renewable
biomass resources, such as vegetable oils, sucrose, and lignocellulosic biomass, have been
developed and tested as biopolyols for the preparation of PUs (Ionescu 2005; Petrovic
2008).

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It has been reported that liquefied biomass products obtained by the solvolysis of
biomass in reactive solvents such as cyclic carbonates or polyhydric alcohols are suitable
for use in the preparation of PU foams (Alma _et al._ 2003; Hu _et al._ 2012; D’Souza _et al._
2014; Ertaş _et al._ 2014; Hu and Li 2014; Hu _et al._ 2014; Čuk _et al._ 2015), resins (Patel _et_
_al._ 2009), and plasticizers (Yin and Hakkarainen 2014). The film-forming ability of such
PU resins has also been reported (Hu _et al._ 2014). For instance, Kurimoto _et al._ (2000,
2001, 2002) studied the mechanical and thermal properties and durability of PU films
prepared from liquefied sugi ( _Cryptomeria japonica_ ) with polymeric diphenylmethane
diisocyanate (PMDI) as a function of the isocyanate/hydroxyl group (-NCO/-OH) ratio and
of the dissolved wood content in the PU film. The primary objective of these investigations
was to elucidate the role of the dissolved woody components in PU network structures. It
was shown that the co-polymerisation of liquefied wood (LW) and PMDI contributed to
the formation of the three-dimensional networks of the polymers. The mechanical
properties (tensile strength and maximum elongation) of the PU films prepared from LW
were more resistant to hydrolysis than those without woody components. Lee and Lin
(2008) compared PU films prepared from liquefied Taiwan acacia ( _Acacia confuse_ ) or
liquefied China fir ( _Cunninghamia lanceolata_ ) wood and a polyisocyanate based on
hexamethylene diisocyanate (HDI) (Desmodur N) or trimethylol propane toluene
diisocyanate (TMP/TDI) prepolymer (Desmodur L). The cured films had high degrees of
gelation, and their mechanical properties were dependent on the wood species and the
isocyanate used. Hrastnik and co-authors (2014) published a paper on preparation of
polyurethane wood coatings based on copper-, chromium-, and boron (CCB) containing
liquefied wood, as an alternative way to manage postconsumed preservative-contaminated
wood. Recently, it was shown that liquefied wood could be utilized for preparation of
polyurethane films also by Lee _et al._ (2014). Budija _et al._ (2009a) studied PU coatings
made from liquefied black poplar and polyisocyanate resins based on hexamethylene
diisocyanate (HDI) (YCM402, Renner). The authors found that, except for the water
resistance, the properties of the bio-based PU coatings investigated ( _e.g._, resistance to dry
and wet environments, flexibility, or resistance to scratching) were comparable to those of
a commercial two-component PU made from non-renewable resources. Excluding
numerous reports on silane treated (nano) particles in PU coatings, it is known that the
resistance of coatings to water can be improved also by treatment of resins or formulations
with silanes (Materne _et al_ . 2012). For example, Zhai _et al._ (2013) observed that PU endcapped by alkoxysilane (tetraethyl orthosilicate) via a sol-gel process increased the water
contact angle and water resistance of coating films. Similarly, approach with silane
endcappers was applied in the case of polyurethane based adhesives, resulting in their
improved properties (Nomura _et al._ 2007). Fu _et al._ (2014) found that bio-based PU
prepared from castor oil modified with a silane (mercaptopropyl trimethoxysilane)
exhibited reduced surface energy, and moreover, better mechanical and thermal properties.
It is also known that silane-urethane hybrid crosslinkers can improve mechanical properties
of PU films (Unkelhäußer _et al._ 2014). Further on, environmentally friendly waterborne
polyurethane-silane (WBPUSi) resins were prepared through a prepolymer process using
N-[3-(trimethoxysilyl)propyl]-ethylenediamine (TMSiP-EDA), which acted both as a
chain extender and crosslinker (Rahman _et al._ 2010). WBPUSi showed improved
protection against marine fouling. Just recently, Mori (2015) published a paper on adding
of tetraethoxysilane into liquefied wood-derived polyurethane. Si was introduced in PU at
a molecular level, while maintaining the urethane structure. The mechanical strength of the
material was improved.

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Jeng _et al._ (2009) also showed that PU films prepared from liquefied _Cryptomeria_
_japonica_ wood (Japanese cedar) and Desmodur L could be used as coatings. The films
exhibited good mechanical properties and resistance to weathering, but their attempts to
lighten the colour of the liquefied product by performing liquefaction in an atmosphere of
nitrogen were unsuccessful. Specifically, the colour of the liquefied biomass products was
dark brown or almost black, which could be a major drawback for their application in fields
with high aesthetic requirements, such as the coating industry. Recently, it was found that
LW could be lightened to a considerable extent by treatment with activated hydrogen
peroxide without drastic degradation of the material (Cheumani-Yona _et al._ 2012).

Considering the state-of-the-art in the field of preparation of polyurethane wood
coatings from liquefied wood as the starting point of our research, by far the most important
goal of this work was to prepare LW-based PU wood finish that is aesthetically appealing
and light-coloured, not a dark brown or black wood finish. We could not find reports on
light-coloured LW-based finishes in the literature, and the solution of this issue was
expected to be the most important novelty of the paper. Therefore, the influence of
hydrogen peroxide bleaching of LW on the properties of LW-based PU wood coatings was
investigated. Secondly, the novel PU finish made from bleached LW should have
acceptable mechanical and water-resistance properties, hopefully close to those of
commercial PU wood coatings. In order to achieve satisfactory mechanical properties of
the novel finish, the authors searched for the most appropriate isocyanate/hydroxyl group
(-NCO/-OH) ratio. In addition, treatment of LW with n-octyltriethoxysilane was also
studied, in order to improve mechanical properties and especially the resistance against
water, as it was shown in literature that silanes could exhibit positive influence on the
properties of PU films. However, we could not find such data on n-octyltriethoxysilane.

EXPERIMENTAL

Materials
Black poplar ( _Populus nigra_ L.) wood was ground, sieved using a 0.24-mm screen,
and oven dried at 103 °C for 24 h prior to the liquefaction reaction. Hydrogen peroxide
(30% aqueous solution) used for the treatment of LW was obtained from Belinka Perkemija
(Helios Group, Slovenia). PMDI (isocyanate, 15.2%) was obtained from Chemcolor
(Slovenia). TMP/TDI prepolymer was prepared from TMP (Merck) and TDI (SigmaAldrich) using toluene as a solvent. Toluene and TMP were mixed in a three-neck, 0.5-L
round-bottom flask and heated to 70 °C under reflux and continuous mechanical stirring
until the complete dissolution of TMP was achieved. TDI was slowly added to the mixture
for 5 min. The temperature was then increased to 90 °C and maintained for 3 h. After this
reaction time, the temperature was decreased to 50 °C, and the viscosity was adjusted by
the addition of ethyl acetate (EA). The theoretical -NCO content of the TMP/TDI
prepolymer was decreased from 13.2% to 10.6% with the addition of EA. All of the other
chemicals used were of reagent grade and used without further purification.

Methods
_Preparation of liquefied wood_
Liquefaction of wood was conducted in a 1-L reactor using a mixture of
polyethylene glycol #400 (PEG) and glycerol (gly) at a 9:1 mass ratio as the reactive
solvent and sulphuric acid as the catalyst. In a similar study of liquefaction of wood, it was

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also suggested that the 9:1 PEG/gly ratio was optimal for some wood species (Kurimoto _et_
_al._ 1999). The wood-to-solvent mass ratio was 1:3, and the catalyst-to-solvent mass ratio
was 3:100. Glycerol, PEG #400, sulphuric acid, and wood sawdust were charged into the
reactor and refluxed under continuous mechanical stirring for 120 min at 150 °C. After this
time, the liquefied mixture was cooled down and diluted with a 1,4-dioxane/water mixture
(4:1, v/v). The residue from liquefaction was removed by filtration under vacuum through
a filter disk (Sartorius 388 grade, 12 to 15 μm particles retention). The liquefaction yield
(LY), as determined by Eq. 1, was 99.2 % ( _i.e._, a residue content of 0.8%),

??

?? (%) = (1 − ?? ) ? 100 (1)

where _Wr_ and _Wo_ are the masses (g) of the liquefaction residue and initial dry wood sample,
respectively. LW was obtained after evaporation of dioxane and water.

_Treatment of liquefied wood with hydrogen peroxide_
It has been shown that the colour of LW can be lightened by treatment with
activated hydrogen peroxide. Various activators, such as potassium carbonate, magnesium
sulphate, and sodium bicarbonate, have been used (Cheumani-Yona _et al._ 2012). In the
current research, a two-stage process was chosen. First, 100 g of LW was diluted with 300
g of a 1,4-dioxane/water binary mixture (4:1, v/v) in a 2-L reactor. Then, 33.33 g of a 30%
aqueous hydrogen peroxide solution (corresponding to a hydrogen peroxide-to-LW mass
ratio of 10:100) was diluted three times in a 1,4-dioxane/water binary mixture (4:1, v/v),
then added dropwise to the diluted LW for 24 h at room temperature. The decomposition
of hydrogen peroxide, in this case, was certainly controlled by pH, _i.e._, by sulphuric acid
(the residue of the catalyst used for liquefaction) and carboxylic acids formed during the
liquefaction reaction. After the first 24 h, the pH of the reaction mixture was adjusted to
8.5 with a 40% aqueous sodium hydroxide solution to allow the decomposition of the
residual peroxide. The reaction mixture was kept at room temperature for another 24 h.
This step was important because of the high stability of hydrogen peroxide in acidic media.
The direct evaporation of the solvent under reduced pressure after this step could be
especially dangerous because of the (explosive) decomposition of the residual hydrogen
peroxide during the process. After the second 24 h of treatment, the residual sodium salts
were removed by filtration under vacuum through a filter disk (Sartorius 388 grade, 12 to
15 μm particles retention), because the presence of these salts could have a negative impact
on the properties of cured polyurethane films on the basis of liquefied wood. The solvents
in the filtrate were evaporated at 80 °C under reduced pressure. The discoloured LW
samples were re-dissolved in a fresh 1,4-dioxane/water mixture; the pH was adjusted to 4
to 4.5 with a sulphuric acid solution (5 M) for further precipitation of salts, then filtrated.
The solvent was once again evaporated under reduced pressure. It is necessary to note that
this treatment can be carried out in water, but the major problem after the treatment would
then be the separation of the bleached LW and the salts resulting from the activators. The
LW treated with hydrogen peroxide was labelled LWB.

It should be noted that the removal ratio of the inorganic salts was not controlled.
As stressed in the Introduction, the main novelties of this work, beyond the state-of-theart, were bleaching of liquefied wood and the feasibility study of utilisation of bleached
liquefied wood to prepare a light-coloured wood coating of the polyurethane type. As a
consequence of the pioneering nature of this research and in order to accomplish the main

?? (%) = (1 −

??

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goal, some experiments were not foreseen to be carried out in detail. But certainly, during
the future process optimisation studies, also the removal ratio of the inorganic salts should
be determined.

_Treatment of liquefied wood with n-octyltriethoxisilane_
The hydrophobisation agent n-octyltriethoxisilane was added to LW at mass ratios
of 2%, 5%, or 10% (based on LW). The mixtures were kept at room temperature for 24 h
to ensure the hydrolysis of the alkoxysilane. The hydrolysis of the silane was favoured by
the acidity of the LW. It has been reported that acidic conditions enhance the hydrolysis of
alkoxysilanes (Witucki 1993) but stabilise the ensuing silanols towards condensation at
room temperature (Salon _et al._ 2008). The LW/silane mixtures were then heated at 100 °C
for 30 min to increase the condensation rate. The silanol groups could undergo selfcondensation (a reaction between two silanol groups) or condensation with the hydroxyl
groups of the LW. The heat treatment of the LW/silanol mixtures also caused the
evaporation of residual ethanol (a by-product of the hydrolysis of the alkoxysilane).
Homogeneous LW mixtures were obtained and used in the preparation of PU films.

_FT-IR analyses_
FT-IR analyses were performed with a PerkinElmer (USA) Spectrum One FT-IR
spectrometer using the horizontal attenuated total reflection (HATR) technique (with a
HATR ZnSe 45° flat plate). All of the spectra were recorded at a 4 cm -1 resolution, and
each was the average of 40 scans. The samples were oven dried at 80 °C for 2 h before the
measurements.

_Colour measurements_
The CIE _L_    - _a_    - _b_    - parameters ( _L*_, _a*_, and _b*_ ) and the colour change (Δ _E*_ ) (as
defined in Brock _et al._ 2000) of the LW after the discolouration reaction were measured
with an X-Rite (USA) SP62 spectrophotometer using a D65-type light source. The liquid
samples were placed between two glass plates. The thickness between the plates was 140
μm.

_Determination of mass loss due to the discolouration reaction_
The mass loss during the discolouration process was determined by Eq. 2,

(??−??)

???? ???? (%) = [ ~~]~~ ? 100 (2)

where _wi_ and _wf_ are the masses (g) of LW before and after the discolouration reaction,
respectively.

_Determination of the hydroxyl number_
The hydroxyl number of the LW before and after bleaching was determined
according to the procedure described in the literature (Kunaver _et al._ 2010). Thus, 0.5 to
1.0 g of sample was dissolved in 25 cm 3 of a phthalation reagent and heated at 115°C for
1 h under reflux. This was followed by the addition of 50 cm 3 of pyridine through the
condenser. The mixture was back-titrated with a 0.5 M sodium hydroxide solution. The
indicator was a 1% phenolphthalein solution in pyridine. The phthalation reagent consisted

???? ???? (%) = [

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of 115 g of phthalic anhydride, dissolved in 700 cm 3 of pyridine. The hydroxyl number,
defined as mg KOH/g of sample, was calculated as follows (Eq. 3),
###  B  A  M  561.

_Hydroxylnumber_   _acid_ _number_ (3)

_w_

where _A_ is the volume of the 0.5 M NaOH solution required for titration of the sample
(cm 3 ), _B_ is the volume of the NaOH solution required for the titration of the blank solution
(cm 3 ), _M_ is the molarity of the NaOH solution, and _w_ is the amount of sample. The acid
number was determined as follows: 0.4 g of sample was weighed into a 400 cm 3
Erlenmeyer flask and dissolved in 50 cm 3 of solvent mixture. The solvent mixture consisted
of dioxane and water (4:1 v/v). Then, 0.5 cm 3 of phenolphthalein indicator solution (1% in
ethanol) was added and titrated with 0.1 M KOH solution in ethanol to the equivalent point.
The acid number (mg KOH/g of sample) was calculated using the following equation (Eq.
4),

###  C  B 

###  C  B  M  561.

_Acid_ _number_  (4)

_w_

where _C_ is the titration volume of the KOH solution (cm 3 ), _B_ is the titration volume of
blank solution (cm 3 ), _M_ is the molarity of the KOH solution, and _w_ is the amount of the
sample (g).

Polyurethane Films and their Properties
_Preparation of PU films_
Pure LW, bleached LW (LWB), or LW-treated with silane (5 g) were dosed with
an appropriate amount of polyisocyanate (given by the -NCO/-OH ratio) and ethyl acetate
(EA), then thoroughly mixed in a polyethylene cup with a glass rod. The mass of EA added
in each run was 10% of the overall mass of LW and polyisocyanate. The mixtures were
applied to a glass plate and to a beech wood substrate using a film applicator. The thickness
of the PU films was 120 µm. Curing of the films was performed at room temperature. All
of the films were characterised after seven days of curing.

_Determination of hardness by the pendulum damping test method_
The hardness of the cured films was determined by a König pendulum according to
the method of damped oscillations (EN ISO 1522:2001). The hardness value corresponded
to the damping time of the pendulum oscillating on the film surface from 6° to 3°.

_Resistance to scratching_
The resistance of the films to scratching was determined according to the standard
EN ISO 1518-1:2011 (2011). The scratching needle, with a hard hemispherical tip 1 mm
in diameter, was drawn across the surface of the coated test specimen, perpendicular to the
grain direction, at a constant speed (30 to 40 mm/s). Scratching was performed on different
parts of the test panels, using an increasing load on the scratch needle, until the coating
cracked or the scratch was wider than 0.5 mm. The force level in N, which produced such
damage, was defined as a critical scratch, exhibiting resistance to scratching.

_Elasticity_
Elasticity was determined according to the standard DIN 53 155 (1971). Part of the
cured film on a glass substrate was cut off with a specially designed blade, and the elasticity

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was then assessed on the basis of the shape and the length of the obtained continuous,
unbroken paint chips. Soft paint chips that are at least 30 mm long indicate excellent
elasticity, assessed as 1.1. The marks 1.2 and 1.3 mean a bit less elastic films that are rolled
up, 2 is the assessment mark when the chip falls apart into particles shorter than 30 mm,
and 3 is the worst result for non-elastic films, with chips that are shorter than 3 mm or even
in powder form.

_Adhesion strength, determined by the pull-off test method_
Pull-off tests were used to measure the adhesion strength between PU films and a
wood substrate (beech wood). The procedure described in EN ISO 4624:2003 (2003) was
carried out: the test aluminium dolly was glued onto the coating film with an epoxy resin.
After 24 h of drying under room conditions, the dolly was isolated from the surrounding
film with a cutting tool. The adhesion was measured using a DeFelsko(USA) PosiTest AT
adhesion tester that applied a tensile load perpendicular to the test surface.

_Resistance to cold liquids_
The resistance to water (distilled water) and to alcohol (48% aqueous solution of
ethanol) was determined according to the standard EN 12720:2009 (2009). Paper disks (25
mm in diameter, 480 g m -2 ) were dipped into the test liquid (at a temperature of 23 °C ± 2
°C) for 30 s, then placed onto the coated surface and covered with a standardised glass cup
for the test period (1 h). After the test period, the glass cups and paper disks were removed,
and, after 20 h, the surfaces were cleaned and assessed according to the numerical rating
codes defined in the standard (from 1 to 5), where 5 represents the best assessment (no
visible damage) and 1 the poorest.

RESULTS AND DISCUSSION

Bleaching of Liquefied Wood

The bleaching treatment converted LW, originally dark brown, to a yellowish
product, as shown by the CIE _L_ - _a_ - _b_ - parameters (Table 1). In the CIE _L_ - _a_ - _b_ - colour
system, _L_ - represents the brightness of the sample, and it varies from 0 (black) to 100
(white); _a_ - represents the green (- _a_ *) to red (+ _a_ *) axis, and _b_ - is the blue (– _b_ *) to yellow
(+ _b_ *) axis (Brock _et al._ 2000). A considerable colour difference (Δ _E_ - = 48.9) was
measured, signifying that a change from the dark brown colour of LW to a light yellow
colour was achieved.

Compared to the bleaching methods investigated (one-stage alkali bleaching) in
previous work (Cheumani-Yona _et al._ 2012), the treatment applied here led to a similar
level of discolouration but with a lower consumption of hydrogen peroxide (as already
mentioned, the hydrogen peroxide/LW mass ratio of 10:100 was sufficient), which is
considered to be an important improvement of the bleaching process. A substantial colour
change in the LW was observed after only the first stage (bleaching under acidic
conditions). It is likely that bleaching during the first acidic stage not only activated the
chromophoric units to render them more susceptible to elimination under alkaline
conditions, as was reported for the acid-alkali two-stage bleaching of wood pulp (Wojciak
_et al._ 2007), but also facilitated several other processes, such as electrophilic reactions that
initiate lignin-degrading bleaching or nucleophilic reactions complementing electrophilic
lignin breakdown or removing colour without lignin degradation.

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Table 1. Change of Colour and Hydroxyl Number of Liquefied Wood after
Hydrogen Peroxide Treatment

Colour parameter Hydroxyl

Sample

number 2
_L*_ _a*_ _b*_ _ΔE*_ _1_
(mg KOH/g)

Liquefied
28.3 0.2 0.1 0.0 285.8 (5.8)
wood (LW)

Bleached

liquefied
wood (LWB)

54.3 8.5 40.7 48.9 305.0 (9.4)

1Δ _E_ *: colour difference between the liquefied wood (LW) and of the bleached liquefied wood
(LWB)
2 Standard deviation is shown in brackets

The mass loss of the LW samples exposed to the bleaching process approached a
maximum of 1.2%; therefore, it is believed that the discolouration of the LW was achieved
without a drastic degradation of the product. FT-IR analyses revealed only minor changes
between the spectra of LW and LWB (Fig. 1), suggesting that the chemical structure of the
LW was not greatly modified. The change was observed mostly in terms of the relative
intensity of the band at 1720 cm -1, assigned to the absorption vibrations of carbonyl (C=O)
groups. The increase in the intensity of this band could be due to the oxidation of some of
the functional groups (alcohols, phenols, or carbon-carbon double bonds) in the LW to
ketones, aldehydes, or carboxylic acids. In particular, as reported in the literature (Araujo
_et al._ 2002; Wojciak _et al._ 2007), hydrogen peroxide can cause various chemical reactions,
depending on its concentration and the nature of the activators. Such reactions include the
epoxidation of carbon-carbon double bonds (and the subsequent hydrolysis of the epoxy
groups, forming hydroxyl groups); the oxidation of carbon-carbon double bonds into two
carboxylic acid groups; the oxidation of hydroxyl or phenolic groups to ketones, aldehydes,
and carboxylic acid groups; or the severe degradation of organic compounds, forming
carbon dioxide and water. Obviously, some changes occurred in the liquefied wood
mixture because of bleaching. These changes could maybe influence on the properties of
polyurethane films, and so, one of the goals of our research was to check this possibility.
The other vibration bands were not significantly modified. These bands were assigned,
respectively, to the asymmetric and symmetric stretching of C-H bonds in the methylene
(-CH2-) and methyl (CH3-) groups (Budija _et al._ 2009a) at 2850 cm -1 and 2955 cm -1 ; C-H
bond bending in methylene and methyl groups at 1462 cm -1, 1425 cm -1, and 1330 cm -1 ; the
C-O-C bond stretching at 1160 cm -1 ; and C-O bond stretching at 1268 cm -1 and 1000 cm -1

to1100 cm -1 . The band at around 3400 cm -1 from the stretching of hydroxyl groups (Helsen
_et al._ 2007) signified a greater presence of alcohol (or phenol) compounds containing –OH
groups than carboxylic acids with –OH groups, suggesting a large portion of the hydroxyl
groups in the LW remained non-oxidised. The vibration band of –OH groups in carboxylic
acid compounds has been reported to be very intensive and broad (at 3500 cm -1 to 2400
cm -1 ) (Coates 2000).

The –OH number of a product designed to be used as a polyol component in the
synthesis of PU is an important parameter. The –OH numbers of LW and LWB are reported
in Table 1. This value for LWB was only slightly higher than for LW. This slight increase
could be due to the formation of carboxylic acid groups (although, as mentioned
previously, it is believed that a large portion of the –OH groups in the LW remained non

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oxidised) and/or the formation of –OH groups from the epoxidation of carbon-carbon
double bonds and hydrolysis of epoxy groups.

(b)

(a)

4000

3200 2400 1800 1400 1000 650

Wavenumber (cm -1 )

Fig. 1. FT-IR spectra of (a) original liquefied wood (LW) and (b) bleached liquefied wood (LWB)

PU Films Prepared from LW and LWB

The mechanical properties and resistance to cold liquids (water and alcohol) of PU
films prepared from LW and LWB are presented in Table 2. It has to be noted that there
are only few reports on liquefied wood based polyurethane films in literature (for instance
the recent ones: Lee _et al._ 2014; Mori 2015) and up to our best knowledge, this is the first
report on polyurethane films made from bleached liquefied wood. The -NCO/-OH ratios
were either 1 or 1.25. The bleaching treatment on the LW seemed to have only a minor
effect on the performance of the films. The properties of the LWB-based PU films were all
comparable to the properties obtained with LW-based films. Generally, these results
suggest that the bleaching pretreatment could be applied to LW before its use in the
preparation of PU without any harmful effects on the properties of the cured PU coatings
based on LWB, what is believed to be an important new finding.

The mechanical properties of the TMP/TDI-based PU films were acceptable for
coating applications, especially the films prepared with the -NCO/-OH ratio of 1.25. Quite
high pendulum hardness values (> 78 s) were measured for these films. The scratching
resistance values of the films were above 16 N. For TMP/TDI (at a ratio of 1.25) LW- and
LWB-based PU films (Table 2), the elasticity was optimal, and, at the same time, the
adhesion strength on the beech substrate was higher than 3 MPa. The failures in the pulloff test occurred predominantly at the interface between the film and the substrate;
nevertheless, they were in many cases accompanied by the fracture of the beech wood
substrate. A possible explanation for the observed fracture of the substrate might be similar
to the one described by Ugovsek _et al._ (2013), who studied the wood bond line when
liquefied wood was used as an adhesive. They proposed at least partial _in situ_ liquefaction
of the substrate after application of the liquefied wood-based adhesive, in the period until
the end of the curing process, forming a weak boundary layer of delignified cells.

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Table 2. Mechanical Properties and Resistance to Liquids of the PU Films
Prepared from Liquefied Wood (LW) and Bleached Liquefied Wood (LWB)

(MPa)

Resistance

to
scratching

Elasticity 2

Resistance to

liquid 4
Water Alcohol

Sample 1

Pendulum

hardness

Adhesion

strength 3

(s)

(N)

(1 h)

(N) (1 h) (1 h)

LW/PMDI 1 8.4 3 1.1 2.27A 3 3
LW/PMDI 1.25 14.0 14 1.1 3.77A 2 2
LW/TMP/TDI 1 37.7 18 1.1 3.97A 3 3
LW/TMP/TDI 1.25 78.1 20 1.1 3.90A 3 3
LWB/PMDI 1 12.7 9 1.1 2.8A 3 3
LWB/PMDI 1.25 12.6 14 1.1 4.14A 3 3
LWB/TMP/TDI 1 38.1 14 1.1 4.42 (A) 3 3
LWB/TMP/TDI 1.25 85.9 16 1.1 3.38A 3 4
1 LW:liquefied wood based film; LWB: bleached liquefied wood based film; PMDI or TMP/TDI:
isocyanate-type curing agent; 1 or 1.25: the NCO/OH ratio
2 Assessment of elasticity from 1.1 (the best - large elasticity) to 3 (no elasticity - high
hardness)
3 A: adhesive type of failure between the coating and the substrate occurred
4 Assessment of the resistance to liquids from 1 (no resistance) to 4 (acceptable resistance)
and 5 (the best resistance)

A high -NCO/-OH ratio generally increases the cross-linking density of the PU
polymers and consequently the mechanical properties (strength, hardness) of the final
products. The influence of the -NCO/-OH ratio on the mechanical properties of the PU
films can be seen from the data in Table 2, best illustrated by the difference in the pendulum
hardness between the films prepared with -NCO/-OH ratios of 1 and 1.25. The elasticity of
the films was not modified, but the adhesion strength in some cases decreased with the
increase in the –NCO/-OH ratio. The low hardness (pendulum and scratching) values
measured for the PMDI LW- or LWB-based films were probably due to the surface
roughness of those films. As observed, PMDI was very reactive with LW, and the
LW/PMDI mixtures showed a high tendency to foam. This result could have been expected
because PMDI is widely and efficiently used for the production of PU foams from LW
(Alma _et al._ 2003; Hu _et al._ 2011). On the other hand, a pendulum hardness of about 80 s
was achieved with the TMP/TDI (at a ratio of 1.25) LW- or LWB-based coatings. This
value was close to the reported values of between 70 s and 90 s for some wood coatings,
on the basis of acrylic and acrylic/polyurethane wood finish dispersions (Vielhauer _et al._
2013), but still less than the 110 s to 120 s reported for commercial two-component
polyurethane based coatings (Bock 2001).
One of the major requirements of coating films is their performance against
chemicals. The resistance of the films was tested only against water and alcohol, and the
results were quite poor. The resistance to distilled water and to a 48% aqueous solution of
ethanol was assessed at a value of 3. According to the standard EN 12720 (2009), the
minimal assessment of the resistance acceptable for a coating application is 4, and the best
resistance has a value of 5. Research on similar LW-based coating systems (Budija _et al._
2009b) suggested that the sensitivity of the films to polar solvents (water and ethanol) arose
from the presence of some residual hydroxyl groups in the film network structure. The
reaction between the hydroxyl groups in the liquefied product and the isocyanate groups
might have been incomplete, in spite of the relatively high -NCO/-OH ratio of 1.25.

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Similarly, low resistance to water was proposed to be the consequence of a low crosslinking rate in self-crosslinked LW-based coatings (Budija _et al._ 2009a).
Analyses of the FT-IR spectra of the PU films were performed after seven days of
curing. The spectra obtained with the TMP/TDI LW-based PUs, at -NCO/-OH ratios of 1
and 1.25, respectively, are shown in Fig. 2 and Fig. 3 (other spectra containing similar
results are not shown). The -NCO groups of the polyisocyanate components (PDMI or
TMP/TDI) were almost completely consumed, even in the films with -NCO/-OH ratios of
1.25. The band at around 2270 cm -1, representing the asymmetrical stretching vibration of
the -NCO group, was not observed. Similar observation was presented by Mori (2015);
however, it is interesting to note that in his case, the –NCO/-OH ratio was only 0.21, but
he liquefied another wood species ( _Prunus cerasus_ ) and the curing agent was PMDI, so in
fact, the system investigated by Mori (2015) was quite different than ours. The -NCO
groups can react with the -OH groups of LW to form urethane bonds, or they can react with
water (residual water in LW and water vapour in ambient air) to form urea links through
the formation of amine groups (with carbon dioxide as a by-product). The absorption band
at 1707 cm -1 was assigned to carbonyl groups (C=O) initially present in LW and hydrogenbonded carbonyl groups (Furer 2000; Mishra _et al._ 2006) in urethane bonds, resulting from
the reaction between hydroxyl groups in LW and the isocyanate hardener. The absorption
bands at 3297 cm -1, 1532 cm -1,and 1216 cm -1, assigned to the stretching of N-H, bending
of N-H with stretching of C-O-N (amide II)(Jena _et al._ 2007), and stretching of C-N with
bending of N-H (amide III), respectively, were also characteristic of urethane linkages
(Hummel _et al._ 1979; Kurimoto _et al._ 2000). The formation of urea linkages was not seen.
The bands characteristic of urea were not observed: 1650 cm -1 to 1620 cm -1 (amide I), 1580
cm -1 to 1550 cm -1 (amide II), and 1240 to 1220 cm -1 (amide III) (Hummel _et al._ 1979;
Yilgör _et al._ 2002).

(b)

(a)

4000 3200 2400 1800 1400 1000 650

Wavenumber (cm -1 )

Fig. 2. FT-IR spectra of the PU films prepared from original liquefied wood and TMP/TDI at
-NCO/-OH ratios of (a) 1.00 and (b) 1.25

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(b)

(a)

4000 3200 2400 1800 1400 1000 650

Wavenumbers (cm -1 )

Fig. 3. FT-IR spectra of the PU films prepared from bleached liquefied wood and TMP/TDI a -NCO/OH ratios of (a) 1.00 and (b) 1.25

The absorption band at 3450 cm -1 could be assigned to hydroxyl groups in the cured
films, and, as already mentioned, the films’ poor resistance to polar liquids could be
connected to the presence of these groups. Some hydroxyl groups could have remained
unreacted in the films, perhaps because of steric hindrances in the compounds of liquefied
wood. Adsorbed water molecules on the surfaces of the films could have also contributed
to this band because the films were not dried in a desiccator before the FT-IR analyses. The
presence of some substances that were not incorporated into the film network (as reported
by Kurimoto _et al_ . (2002)) could also have contributed to the polar solvent-induced
degradation of the films. Such substances could be removed from the films during exposure
to the liquids (distilled water, ethanol solution), causing visible damage to the cured films’
surfaces.

Influence of N-octyltriethoxysilane Treatment

Only LWB was used for this study. A total of 2%, 5%, or 10% noctyltriethoxysilane (based on the liquefied wood mass) was added to LWB. The treated
liquefied mixtures were subsequently mixed with PMDI or TMP/TDI at an -NCO/-OH
ratio of 1.00. The mechanical properties and resistance to cold liquids of LWB-based PU
films containing this silane are presented in Table 3.

The resistance of TMP/TDI-based PU films to cold liquids was improved. The
resistance of the films, prepared from LWB with 2% of the silane and LWB with 10% of
the silane, exhibited the optimal assessment value of 5 to distilled water and the acceptable
assessment value of 4 to alcohol. Surprisingly, for unknown reasons, such an improvement
was not observed using LWB-based PU films with 5% of the silane. The addition of the
silane also increased the hardness of the films but decreased the value of the adhesion
strength to the surface of the beech wood substrate. The elasticity of the films was not
changed. Improvement of the properties (strength and water resistance) of PU films
modified with various silanes were reported by many authors (Xia and Larock 2011; Zhai
_et al_ [. 2013; Allauddin](http://pubs.acs.org/action/doSearch?ContribStored=Allauddin%2C+S) _et al._ 2013, Fu _et al._ 2014), but application of n-octyltriethoxysilane

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for such a purpose has not been shown yet. In contrast to the TMP/TDI-based PU films,
after addition of the silane, the properties of the PMDI-based PU films generally remained
poor.

Table 3. Mechanical Properties and Resistance to Liquids of the PU Films
Prepared with n-octyltriethoxysilane-treated Bleached Liquefied Wood (LWB)

(MPa)

Resistance

to
scratching

Elasticity 2

Resistance to

liquid 4
Water Alcohol

Sample 1

Pendulum

hardness

Adhesion

strength 3

(s)

(N)

(1 h)

(N) (1 h) (1 h)

LWBSi 2% PMDI 7.0 1 1.1 2.92A 2 2
LWBSi 5% PMDI 9.4 2 1.1 2.78A 2 2
LWBSi 10% PMDI 10.8 5 1.1 3.48A 3 3
LWBSi 2% TMP/TDI 62.7 16 1.1 2.96A 5 4
LWBSi 5% TMP/TDI 59.0 19 1.1 3.13A 3 3
LWBSi 10% 80.8 18 1.1 3.34A 5 4
TMP/TDI

80.8 18 1.1 3.34A 5 4

1 LWB: bleached liquefied wood-based film; Si: silane was added to the formulation; 2%, 5%,
or 10%: mass portion of the silane; PMDI or TMP/TDI: isocyanate-type curing agent
2 Assessment of elasticity from 1.1 (the best - large elasticity) to 3 (no elasticity - high
hardness)
3 A: adhesive type of failure between the coating and the substrate occurred
4 Assessment of the resistance to liquids from 1 (no resistance) to 4 (acceptable resistance)
and 5 (the best resistance)

Analyses of the FT-IR spectra of the films with the silane, recorded after seven days
of curing, revealed that the silane did not affect the reactivity of the -NCO groups, which
is the same conclusion than that reported by Mori (2015), but for n-octyltriethoxysilane
and not for tetraethoxy silane as described in the mentioned reference. Specifically, the
spectra of the silane-treated liquefied wood-based PU films were comparable to the
equivalent spectra obtained from non-silane-modified liquefied wood (spectra not shown).

CONCLUSIONS

1. Polyurethane films were prepared from dark brown liquefied wood or from bleached,
light-coloured, yellowish liquefied wood and polymeric diphenylmethane diisocyanate
(PMDI) or trimethylolpropane toluene diisocyanate (TMP/TDI) prepolymer and
characterised for use as wood coatings.

2. The bleaching treatment did not influence the properties of the light-coloured films
made of liquefied wood.

3. The PMDI-based films were generally of a poorer quality compared to the properties
of the TMP/TDI-based films, probably because of the high tendency of the liquefied
wood-PMDI mixture to foam.

4. A high -NCO/-OH ratio generally increases the cross-linking density of the PU
polymers and consequently the mechanical properties (strength, hardness) of the final
products

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5. The low resistance of polyurethane-type films prepared from the bleached liquefied
wood and TMP/TDI to cold polar liquids (water, alcohol) was substantially improved
by the addition n-octyltriethoxysilane to liquefied wood before the preparation of
coatings. The hardness of the films increased as well.

6. It was globally shown in this research that the bleaching process could be applied to
liquefied wood without significantly affecting the properties the resulting liquefied
wood-based PU. PU coatings can be prepared from liquefied wood modified with
silanes with an appropriate polyisocyanate component (e.g. TMP/TDI prepolymer)

ACKNOWLEDGMENTS

The authors are grateful for the support of the Slovenian Research Agency (the
project L4-2144 and the program P4-0015) and the Slovene Human Resources
Development and Scholarship Fund (Grant No. 11013-6/2011-4).

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Article submitted: February 5, 2015; Peer review completed: March 23, 2015; Revisions
received and accepted: April 16, 2015; Published: April 20, 2015;
DOI: 10.15376/biores.10.2.3347-3363

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