Ci a ion: Ba bosa, K.T.; Ca doso, G.V.;
Pe ei a Acos a, A.; A ambu u, A.B.;
Delucis, R.d.A.; Ga o, D.A.; Labidi, J.;
Bel ame, R. Un eiling he Po en ial o
B azilian Eucalyp us o T anspa en
Wood Manu ac u ing ia he K a
Pulping P ocess as a Fu u e Building
Ma e ial. Fo es s 2024,15, 1544.
h ps://doi.o g/10.3390/ 15091544
Academic Edi o : Zeki Candan
Recei ed: 30 July 2024
Re ised: 28 Augus 2024
Accep ed: 29 Augus 2024
Published: 2 Sep embe 2024
Copy igh : © 2024 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
This a icle is an open access a icle
dis ibu ed unde he e ms and
condi ions o he C ea i e Commons
A ibu ion (CC BY) license (h ps://
c ea i ecommons.o g/licenses/by/
4.0/).
A icle
Un eiling he Po en ial o B azilian Eucalyp us o T anspa en
Wood Manu ac u ing ia he K a Pulping P ocess as a Fu u e
Building Ma e ial
Kel in Teche a Ba bosa 1,* , Gab iel Valim Ca doso 2, And ey Pe ei a Acos a 3, A hu Behenck A ambu u 1,
Ra ael de A ila Delucis 4, Da ci A. Ga o 4, Jalel Labidi 5,* and Ra ael Bel ame 4
1
Pos g adua e P og am in Mining, Me allu gical and Ma e ials Enginee ing, Fede al Uni e si y o Rio G ande
do Sul, Po o Aleg e 91501-970, RS, B azil; a hu a ambu [email p o ec ed]
2
Wood Indus ial Enginee ing, Enginee ing Cen e , Fede al Uni e si y o Pelo as, Pelo as 96010-900, RS, B azil;
[email p o ec ed]
3Pos g adua e P og am in Ma e ials Science and Enginee ing—PIPE, Fede al Uni e si y o Pa aná,
Cu i iba 80035-060, PR, B azil; and ey.acos a@u p .b
4Pos g adua e P og am in Ma e ials Science and Enginee ing, Fede al Uni e si y o Pelo as,
Pelo as 96010-900, RS, B azil; [email p o ec ed] (R.d.A.D.); [email p o ec ed] (D.A.G.)
5En i onmen al and Chemical Enginee ing Depa men , Uni e si y o he Basque Coun y UPV/EHU,
Plaza de Eu opa 1, 20018 San Sebas ián, Spain
*Co espondence: [email p o ec ed] (K.T.B.); [email p o ec ed] (J.L.)
Abs ac : The eme gence o anspa en wood as a iable al e na i e o adi ional glass has spa ked
conside able in e es in ecen esea ch endea o s. Despi e ad ancemen s, challenges pe sis in he
deligni ica ion me hods and wood species u ilized in p io s udies. The e o e, his s udy del es in o
he po en ial o B azilian eucalyp us wood o anspa en wood p oduc ion h ough he k a pulping
p ocess. Deligni ica ion was ca ied ou in a labo a o y se ing, eplica ing he k a p ocess wi h a y-
ing eac ion imes (15, 30, 45, and 60 min). The esul ing deligni ied wood enee s we e imp egna ed
wi h a p e-polyme ized PMMA solu ion. The s udy encompassed a ious analyses, including UV-Vis
spec oscopy, colo ime y, SEM, op ical mic oscopy, and mechanical p ope y e alua ions. The esul s
e ealed in iguing ends in e ms o anspa ency, colo changes, mic os uc u al modi ica ions, and
mechanical p ope ies as a unc ion o deligni ica ion ime. This wo k p esen s aluable insigh s in o
he ans o ma i e po en ial o eucalyp us wood, o e ing a deepe unde s anding o he in e play
be ween wood modi ica ion and PMMA imp egna ion.
Keywo ds: lignin emo al; deligni ica ion; op ical p ope ies; bio-composi e; polyme hyl me hac yla e;
wood modi ica ion
1. In oduc ion
T anspa en wood has a ac ed signi ican a en ion in ecen yea s due o i s po en ial
applica ions in a eas such as sma windows, building acades, and e en sola cells [
1
–
4
].
Compa ed o adi ional glass ma e ials, anspa en wood o e s se e al ad an ages as i is
de i ed om a enewable esou ce, which makes i a sus ainable al e na i e o con en ional
building ma e ials. Hu e al. [
5
] examined he ca bon dioxide (CO
2
) emissions associa ed
wi h he Chinese con aine glass indus y, ca ego izing hem based on combus ion, aw ma-
e ial decomposi ion, and he powe and hea u ilized by he en e p ise (indi ec emissions).
Thei simula ion indings e ealed an a e age o 0.69 onnes o CO
2
emissions pe onne o
glass p oduc due o combus ion, a no ably highe igu e compa ed o he Eu opean con-
aine glass indus y’s 0.46 onnes o CO
2
emissions pe onne o glass p oduc as epo ed
by Zie e al. [
6
]. I s ela i ely high anspa ency and low opaci y, achie ed by adap ing he
mic os uc u e and chemical composi ion o he wood (achie ed by emo ing o al e ing
Fo es s 2024,15, 1544. h ps://doi.o g/10.3390/ 15091544 h ps://www.mdpi.com/jou nal/ o es s
Fo es s 2024,15, 1544 2 o 19
he lignin and imp egna ing i wi h an amo phous polyme ), make i an a ac i e al e na-
i e o con en ional ma e ials such as glass in a chi ec u al applica ions, as well as being
mo e ene gy e icien , p o iding be e insula ion p ope ies, and po en ially educing a i-
icial ene gy consump ion [
7
]. Howe e , i is impo an o men ion ha ansmi ance and
haze ( o wa d-sca e ed ligh ) a e commonly epo ed in he li e a u e as cha ac e is ics o
be imp o ed in anspa en woods [
8
,
9
]. Mo eo e , i boas s high du abili y and esis ance
o sha e ing, making i a sa e choice o cons uc ion applica ions. Despi e he use o a
ossil-based polyme in he p oduc ion o anspa en wood, du ing i s wood g ow h, he
ee cap u es he CO
2
p oduced du ing he syn hesis o he polyme i sel , hus c ea ing a
closed cycle. Fully bio-based anspa en wood ma e ials ha e al eady been epo ed in he
li e a u e [
10
]. T anspa en wood is, he e o e, de i ed om a enewable esou ce, which
makes i a sus ainable al e na i e o con en ional building ma e ials [11].
The manu ac u ing o anspa en wood in ol es emo ing lignin om he wood
enee h ough a chemical me hod ha p ese es he ibe s uc u e. Deligni ica ion is a
c i ical s ep as lignin con ibu es o 80%–95% o he ligh abso p ion in wood, impeding i s
anspa ency [
12
]. By selec i ely emo ing lignin, he esul ing wood e ains i s s uc u al
in eg i y while allowing ligh o pass h ough. The deligni ied wood is hen imp egna ed
wi h a anslucen polyme , enhancing i s anspa ency [
4
,
13
] and c ea ing a ma e ial ha
combines he s eng h and na u al es he ics o wood wi h he ligh - ansmi ing p ope ies
o glass. This inno a i e p ocess opens up new possibili ies o applica ions in a chi ec u e,
design, and enewable ene gy [14,15].
In addi ion, as he ch omopho e g oups p esen in lignin a e p edominan ly accoun -
able o i s b ownish colo and ligh -abso bing cha ac e is ics obse ed in na u al wood,
ecen s udies ha e highligh ed an al e na i e app oach o deligni ica ion by ocusing
on modi ying he ch omopho ic composi ion o lignin, a he han comple ely emo ing
lignin [
12
,
16
,
17
]. Fo ins ance, Li e al. [
12
] demons a ed he de elopmen o anspa en
wood by p ese ing he lignin con en and employing an alkaline H
2
O
2
hyd o he mal
solu ion o selec i ely elimina e lignin ch omopho es, he eby educing p ocessing ime.
Va ious me hods u ilizing di e en eagen s o lignin emo al ha e been s udied.
Zhu e al. [
18
] success ully emo ed lignin om a Tilia wood using a solu ion o sodium
hyd oxide (NaOH) and sodium sul i e (Na
2
SO
3
), esul ing in a lignin con en o 3%. Simi-
la ly, Qin e al. [
19
] and Wu e al. [
20
] in es iga ed he use o NaClO
2
, achie ing esidual
lignin con en s o 7% and 9%, espec i ely, o anspa en wood p oduc ion. Howe e ,
he use o chlo ine-based deligni ica ion p ocesses leads o he o ma ion o oxic e luen s,
inc eased ime and ene gy consump ion, and a mosphe ic pollu ion [19].
Based on his concep , using a consolida ed p ocess in he wood indus y such as he
k a p ocess and an abundan eucalyp us wood wi h a p oduc ion o 38.9 m
3
/ha/yea [
21
],
combined wi h a commonly used polyme , shows he possibili y o he mass p oduc ion
o anspa en wood on a pilo scale. The k a p ocess, widely u ilized in pulp and pape
indus ies, in ol es cooking wood wi h sodium hyd oxide (NaOH) and sodium sul ide
(Na
2
S) o dissol e lignin and disagg ega e ibe s wi h minimal ca bohyd a e deg ada ion [
22
].
Cu en ly, he k a p ocess is he p ima y me hod o pulp p oduc ion due o i s con inuous
cooking p ocess and he ad an age o he associa ed chemical p oduc eco e y sys em [
23
,
24
].
Cu en ly, he li e a u e on anspa en wood al eady encompasses s udies in ol ing
woods deligni ied h ough he k a p ocess. Fo ins ance, Zhu e al. [
25
] epo ed he use
o a p ocess in ol ing soaking wood blocks in a boiling solu ion con aining NaOH and
Na
2
SO
3
o dissol e pa o he lignin con en , ollowed by a ea men wi h H
2
O
2
o emo e
he emaining lignin. Hai e al. [
26
] conduc ed a simila s udy whe e ha dwood k a pulp
unde wen TEMPO-oxida ion ea men ollowed by nano ibe ab ica ion. These examples
highligh he exis ing esea ch bu emphasize he unique con ibu ion o ou s udy ocusing
on B azilian eucalyp us in he con ex o anspa en wood p oduc ion.
In B azil, he k a p ocess is employed wi h Eucalyp us sp., a as -g owing wood species
ha is c ucial o pulp indus ies while simul aneously mi iga ing de o es a ion [
27
]. Eucalyp-
us species ha e been gene ically manipula ed o pulp p oduc ion, esul ing in as -g owing
Fo es s 2024,15, 1544 3 o 19
wood wi h high p oduc i i y, cellulose con en , and low lignin con en and ex ac i es. This
makes i an ideal candida e o deligni ica ion p ocesses [
28
,
29
]. Gi en hese ac o s, he e
is a s a egic impo ance in in es iga ing he use o B azilian eucalyp us wood o he p o-
duc ion o anspa en wood. Thus, his s udy aims o apply he k a p ocess wi h sodium
hyd oxide (NaOH), sodium sul ide (Na
2
S), and hyd ogen pe oxide (H
2
O
2
) o he p oduc ion
o anspa en wood om a B azilian eucalyp us (Eucalyp us g andis).
2. Ma e ials and Me hods
2.1. Ma e ials
Eucalyp us g andis wood enee s we e ob ained om Eco olhas (São Paulo, B azil).
The densi y o he enee s was de e mined o be 0.44 g.cm
−3
using a p ecision analy i-
cal scale (
±
0.01 g) and a digi al calipe (
±
0.001 mm). Tes specimens wi h dimensions o
20 mm ×20 mm ×0.6 mm
(leng h
×
wid h
×
hickness, espec i ely) we e p epa ed o he
wood deligni ica ion p ocess. All he specimens we e s o ed in a clima ic chambe a 20
◦
C
and 65% ela i e humidi y un il eaching hyg oscopic equilib ium (12% mois u e con en ).
The chemicals used in he expe imen we e sodium hyd oxide (NaOH) wi h a mola
mass o 40.00 g/mol; sodium sul ide (Na
2
S) wi h a mola mass o 78.04 g/mol; e hanol
(C
2
H
6
O); hyd ogen pe oxide (H
2
O
2
) wi h a pu i y o 35%; me hyl me hac yla e (MMA)
wi h a pu i y o 99%, a liquid s a e, and a mola mass o 100.12 g/mol; and a benzoyl
pe oxide ca alys , all o which we e acqui ed om Sigma Ald ich (S . Louis, MO, USA).
2.2. Wood Deligni ica ion
The deligni ica ion p ocess o he eucalyp us wood enee s was conduc ed by epli-
ca ing he k a p ocess in he labo a o y. The liquo –wood a io was main ained a 4:1,
and he ac i e alkali condi ions o 18% and sul idi y o 20% we e achie ed using NaOH
(99.2 g/L) and Na
2
S (36.27 g/L). The samples we e ea ed wi h he deligni ica ion solu ion
using ou di e en deligni ica ion imes (c.a. 15, 30, 45, and 60 min) and a empe a u e o
90
◦
C. Fibe s s a ed sepa a ing om each o he a e 60 min o k a cooking; he e o e, no
es s we e conduc ed beyond his ime. The sepa a ion o he ibe s p obably occu ed due
o a signi ican b eakdown o lignin and hemicellulose, c ucial elemen s ha bind ibe s
oge he wi hin he wood s uc u e. As he deligni ica ion p ocess ad ances, hese binding
agen s g adually lose hei e ec i eness, causing ibe s o loosen and e en ually sepa a e
om each o he .
A e he deligni ica ion p ocess, he lignin-ex ac ed samples we e ca e ully insed
wi h dis illed wa e un il eaching a neu al pH. Subsequen ly, he samples we e imme sed
in a 35% hyd ogen pe oxide (H
2
O
2
) solu ion a 90
◦
C un il he yellowish colo o he
samples was comple ely elimina ed (a ound 2 h). Following he bleaching ea men , he
samples we e dehyd a ed by imme sing hem in pu e e hanol P.A. (C
2
H
6
O) o abou
60 min
o allow he imp egna ion o he wood by he PMMA polyme . The nomencla u es
assigned o he samples a e he deligni ica ion a e speci ied in Table 1.
Table 1. Nomencla u es used o deligni ied wood and hei espec i e deligni ica ion ime.
Deligni ied Woods Nomencla u es
Deligni ied p is ine wood o 15 min DW15min
Deligni ied p is ine wood o 30 min DW30min
Deligni ied p is ine wood o 45 min DW45min
Deligni ied p is ine wood o 60 min DW60min
2.3. MMA Imp egna ion
MMA and benzoyl pe oxide a a weigh a io o 98.5:1.5 we e mixed o p epa e he
polyme ic solu ion. The solu ion was hea ed a 70 ◦C o 15 min and hen cooled in an ice
ba h a 10
◦
C un il eaching oom empe a u e, esul ing in a p e-polyme iza ion o he
MMA solu ion. Fo wood imp egna ion, he deligni ied wood enee s we e placed in Pe i
Fo es s 2024,15, 1544 4 o 19
dishes and co e ed wi h he p e-polyme ized solu ion o 2 h. The polyme pe mea ed he
wood by capilla i y a a mosphe ic empe a u e and p essu e.
A e wa d, he samples we e w apped in aluminum oil, placed be ween wo me al
pla es o p ese e hei ana omical s uc u e, and hen placed in a o ced ai ci cula ion
o en a 75
◦
C o 4 h o comple e MMA polyme iza ion. A e he polyme iza ion, he
anspa en wood samples we e emo ed om he o en. The nomencla u es o he samples
a e he polyme iza ion a e speci ied in Table 2.
Table 2. Nomencla u es used o deligni ied and subsequen ly imp egna ed wood wi h hei espec-
i e deligni ica ion ime.
T anspa en Woods Nomencla u es
MMA imp egna ed DW15min TW15min
MMA imp egna ed DW30min TW30min
MMA imp egna ed DW45min TW45min
MMA imp egna ed DW60min TW60min
2.4. Chemical Cha ac e iza ion
We chemical analyses we e pe o med only o he p is ine wood and o he deligni-
ied wood g oups (DW15min, DW30min, DW45min, and DW60min) o analyze he e ec
o he k a p ocess in ela ion o he ea men ime. The specimens we e aken o a labo a-
o y o en hea ed a 50
◦
C un il eaching cons an mass. A e d ying, he samples we e
g inded in a Willey mill and sie ed in a 40-mesh sie e in acco dance wi h T257 cm-12 [
30
].
A e wa d, he e hanol– oluene ex ac i es (Tappi T204 om-97 [
31
]), acid-insoluble (Klason)
lignin (Tappi T222 om-98 [
32
]), ashes (T211 om-93 [
33
]), and holocellulose ( emaining mass
up o 100%) con en s we e de e mined.
Fou ie ans o m in a ed spec oscopy (FT-IR) was pe o med in o de o cha ac e ize
he s uc u al chemical g oups o he na u al and he highes deligni ica ion g oups. Thus,
p is ine wood, DW60min, and TW60min we e analyzed using a 4100 equipmen (Shimadzu
P es ige 21 spec ome e equipped wi h ATR-8200), in which each spec um esul ed om
32 scans pe o med in he wa eleng h ange o 4000 cm
−1
o 400 cm
−1
wi h a esolu ion o
4 cm−1, scanne speed o 2 mm/s, and il e o 30.000 Hz il e .
2.5. Mo phological Analyses
Mo phological analyses we e pe o med in he ans e se and angen ial planes using
su ace images ob ained by an op ical mic oscope (Rohs
®
ma k, 2.0 esolu ion), whe ein
he images o p is ine wood, deligni ied wood, and anspa en wood we e aken. The
p is ine wood and TW60min g oup we e submi ed o scanning elec on mic oscopy (SEM)
in a Phenon-Wo ld ma k (P o X model) wi h magni ica ions be ween 80 and 130,000 imes
( esolu ion
≤
14 nm) o e alua e he MMA imp egna ion in he anspa en woods. Finally,
he ligh ansmi ance spec a we e measu ed wi h a UV-Vis spec ome e (CARY 5000
Spec opho ome e ) and he wa eleng h ange was 400–800 nm. The es s we e ca ied ou
in all he anspa en wood g oups in acco dance wi h ASTM D1003 [34].
2.6. The mog a ime ic Analysis
A he mog a ime ic analysis (TGA) was pe o med in he p is ine wood, DW60, and
TW60 g oups in o de o analyze he he mal beha io o he samples a he end o he
ea men . A TGA-1000 equipmen (Na as b and) was used, and he analysis was ca ied
ou wi h a hea ing a e o 10
◦
C-min
−1
unde an ine a mosphe e and a ni ogen gas low
o 2 L-min−1.
2.7. Colo Measu emen s
Colo ime y was ca ied ou o analyze he colo change in he wood enee s due o
he deligni ica ion ime. P is ine wood and anspa en wood g oups colo pa ame e s
Fo es s 2024,15, 1544 5 o 19
we e measu ed wi h a UV-Vis spec ome e (CARY 5000 Spec opho ome e ) ollowing he
CIEL*a*b* me hod used o measu e he luminosi y index (L*) and ch oma ici y pa ame e s
(coo dina e (a*), coo dina e (b*), and o al colo a ia ion (
∆
E)). Fi e eadings we e acqui ed
by each g oup.
2.8. Su ace Analysis
The su ace oughness o p is ine wood and anspa en wood g oups was e alua ed
using an Ambios XP-2 Mechanical P o ilome e . Roughness (Su ace Topog aphy), 5 eadings o
a e age oughness (Rz, a e age dis ance be ween peak and alley), and (Ra, a e age oughness)
pe g oup we e aken in he angen ial plane. Fi e eadings we e acqui ed by each g oup.
Fu he mo e, su ace hyd ophobici y analysis was pe o med in o de o e alua e
su ace epellence o wa e in he na u al and anspa en wood g oups. Fi e samples pe
g oup we e used, measu ing he appa en con ac angle a six di e en imes, om 5 s o
60 s a e wa e (a d op o 10
µ
L)/slide con ac , using a K uss DSA25 goniome e acco ding
o he sessile d op me hod. Fi e eadings we e acqui ed by each g oup.
2.9. Wa e Abso p ion
The hyd ophobici y o he ma e ial was e alua ed o he anspa en wood g oup by
he wa e epellency e iciency (WRE). Ini ially, he samples we e o en-d ied a 100
◦
C ( o
cons an mass) o s anda dize he ini ial mois u e condi ion. Fi e eadings we e acqui ed
by each g oup.
The le el o WRE o he anspa en wood a di e en deligni ica ion imes was
measu ed du ing ou imme sion-d ying cycles. In each cycle, he mass and olume o
he samples we e measu ed a ze o ime ( he samples d ied a 100
◦
C) and a e 96 h o
imme sion. Wi h he da a ob ained, he WRE was calcula ed by Equa ion (1).
WRE = ((∆M n −∆M )/(∆M n )) ×100% (1)
whe e
∆
M = (Mu
−
Ms)/Ms whe e
∆
M = mass change; Mu = we sample mass (g);
Ms = d y
sample mass (g); = ea ed; and n = un ea ed.
2.10. Mechanical P ope ies
Tensile es s we e ca ied ou acco ding o he ASTM D 638-10 [
35
] wi h adap a ions
o he dimensions o he es specimens, which we e 70 mm
×
20 mm
×
0.6 mm. Fo ha ,
5 eadings we e acqui ed by each g oup using a Uni e sal Mechanical Tes ing machine
(DL500 Emic), wi h a load cell o 5 kN a a con olled speed o 5 mm/min. Tensile s eng h
(
σ
) was calcula ed by di iding he maximum load sus ained by he specimen by he
a e age o iginal c oss-sec ional a ea in he gage leng h. Pe cen elonga ion a b eak was
de e mined by he change in gage leng h a he poin o specimen up u e. The modulus
o elas ici y (E
) was calcula ed by he linea po ion o he load s. displacemen cu e.
Finally, he oughness was calcula ed by he a ea unde he load s. displacemen cu e.
2.11. S a is ical Analysis
All he da a we e submi ed o he one-way ANOVA analysis o a iance, excep o
he con ac angle. The homogenei y o he a iances and no mali y o da a we e e i ied
using he Shapi o–Wilk es s. Whene e he null hypo hesis was ejec ed, Tukey’s es s
we e used o compa e he means, and all he s a is ical analyses we e implemen ed a he
5% signi icance le el.
3. Resul s and Discussion
3.1. Chemical Composi ion
Table 3shows a signi ican inc ease in he ash con en a e he deligni ica ion p ocesses,
which can be a ibu ed o he ino ganic sal s used in hese ea men s. These sal s, such
as NaOH and Na
2
S, con ibu e o he emo al o lignin om he wood, bu hey can also
Fo es s 2024,15, 1544 6 o 19
in oduce addi ional mine al con en in o he wood s uc u e [
36
]. As a esul , compa ed o
he p is ine wood, he o he chemical quan i ies we e all educed in he deligni ied ones.
Table 3. Chemical composi ion o he eucalyp us wood p io o and pos he deligni ica ion p ocesses.
Holocellulose (%) Ex ac i es (%) Ashes (%) Acid-Soluble Lignin (%) Acid-Insoluble Lignin (%)
P is ine wood 69.52 ±0.54 c 2.63 ±0.25 ab 0.08 ±0.05 a 1.16 ±0.18 c 26.60 ±0.27 e
DW15min 43.37 ±1.53 a 2.05 ±1.65 a 30.25 ±0.08 b 0.34 ±0.24 ab 23.98 ±0.50 d
DW30min 44.36 ±0.48 ab 2.55 ±0.41 ab 30.96 ±0.51 bc 0.23 ±0.07 a 21.86 ±0.89 c
DW45min 44.90 ±2.37 ab 3.96 ±0.77 bc 31.66 ±0.64 c 0.69 ±0.13 b 18.76 ±1.72 b
DW60min 46.05 ±1.22 b 5.06 ±0.45 c 32.84 ±0.37 d 0.58 ±0.28 ab 15.46 ±0.45 a
whe e di e en le e s ep esen s a is ically di e en means.
When compa ing he deligni ied woods, i is obse ed ha he acid-soluble lignin
con en emained s able, while he ex ac i es and holocellulose con en s inc eased, and
acid-insoluble lignin con en dec eased wi h he deligni ica ion ime. These ends can
be explained by he ac ha du ing he deligni ica ion p ocess, he ea men p ima ily
a ge s he acid-insoluble lignin and, as a esul , ha acid-soluble lignin ac ion is less
a ec ed and emains ela i ely s able h oughou he p ocess [
37
]. Acco ding o Jiang [
38
],
he acid-insoluble po ion o lignin is mo e easily dissol ed and emo ed in an alkaline
en i onmen , as i eac s eadily wi h he hyd oxide ions (OH-) om NaOH, o ming
soluble alkali lignin compounds. These compounds can be washed away du ing he insing
p ocess, and he lignin becomes soluble in he alkaline medium. Addi ionally, he inc ease
in he ex ac i es and holocellulose con en s can be a ibu ed o he emo al o lignin,
which ac s as a ba ie , making hese componen s mo e inaccessible o ex ac ion [39,40].
Figu e 1displays he a ia ion in bo h he pe cen age o emo ed lignin and he o al
lignin con en (c.a. he sum o he soluble and insoluble lignin in acid). I can be obse ed
ha he lignin emo al inc eased g adually wi h he deligni ica ion ime, eaching 42.22%
a e 60 min. The achie ed lignin con en s can be conside ed highe han mos o hose
ound in he li e a u e ela ed o enee s in ended o anspa en wood manu ac u ing.
A e he hyd ogen pe oxide ea men , based on he li e a u e, i is expec ed ha lignin
emo al inc ease by app oxima ely 15%, which may depend on some in luencing ac o s,
such as he concen a ion o hyd ogen pe oxide, ea men ime, empe a u e, and he
ini ial lignin con en in he wood [41–43].
Fo es s 2024, 15, x FOR PEER REVIEW 7 o 20
Figu e 1. Pe cen age o lignin emo al o he deligni ied wood enee s whe e diffe en le e s abo e
he ba s ep esen s a is ically diffe en means.
Howe e , i is essen ial o no e ha he inal lignin con en o he deligni ied wood
does no singula ly dic a e he inal anspa en wood p ope ies. The o he a ibu es o
he deligni ied wood, such as he cellulose s uc u e, po e cha ac e is ics, chemical com-
posi ion, densi y, and he imp egna ion p ocess [45,46], subs an ially impac he p ope -
ies o in e es in anspa en wood. The e o e, he inal p ope ies o anspa en wood
may depend on a ious ac o s, including he lignin con en , mic os uc u e, chemical
composi ion, p esence o de ec s, p ocessing condi ions, impu i ies, and mois u e con en
[20,47,48]. Despi e his, he s udy conduc ed by Wu e al. [20] epo ed a ia ion in he
inal lignin con en s anging om 9 o 24% o deligni ied Basswood (Tilia) enee s. This
speci ic s udy employed a mix u e o 2 w % NaClO
2
, 0.1 w % glacial ace ic acid, and 97.9
w % ul apu e wa e . The d ied samples unde wen deligni ica ion in a wa e ba h oscil-
la o a 80 °C, wi h an oscilla ion equency o 40 pm, o diffe en ea men du a ions
spanning om 30 o 150 min.
3.2. FTIR Spec oscopy
Figu e 2 displays he FTIR spec a o he o iginal eucalyp us enee , as well as he
samples subjec ed o he 60 min deligni ica ion ea men (DW60min) and he subsequen
imp egna ion wi h PMMA (TW60min). In addi ion o he peaks associa ed wi h he p es-
ence o ca bohyd a es a 1030 cm
−1
(C-O-C s e ching in cellulose) and 1730 cm
−1
(ace yl
g oups in hemicelluloses) [46], he p is ine wood exhibi s abso p ion bands ela ed o lig-
nin, such as hose nea 1700 cm
−1
(C=O s e ching in lignin) [40] and 1200–1100 cm
−1
(C-O
s e ching in lignin) [49].
In compa ison o he p is ine wood, he modi ied samples exhibi ed a educ ion a
1700 cm
−1
and a disappea ance o lignin- ela ed abso p ion bands a 1500–1600 cm
−1
as a
esul o he deligni ica ion p ocess. The deligni ied sample (DW60 min) also displayed
al e a ions in abso bance a a ious wa enumbe s co esponding o wood componen s
due o he deligni ica ion ea men , such as a 1510–1550 cm
−1
(associa ed wi h C-H bend-
ing ib a ions in lipid componen s and linked o C=C s e ching ib a ions in phenolic
ings p esen in annins and o he phenolic compounds) [50].
Wi hin he anspa en wood (TW60 min), he e was an indica ion o PMMA- ela ed
abso p ion bands, including ca bonyl s e ching a app oxima ely 1720–1730 cm
−1
[47].
Addi ionally, newly eme ged abso p ion bands associa ed wi h PMMA polyme iza ion
Figu e 1. Pe cen age o lignin emo al o he deligni ied wood enee s whe e di e en le e s abo e
he ba s ep esen s a is ically di e en means.
Fo es s 2024,15, 1544 7 o 19
I ’s impo an o highligh ha eucalyp us wood equi es ca e ul conside a ion. As
epo ed by Pin o e al. [
44
], a empe a u es below 70
◦
C, abou 20% o he lignin dissol es
wi hou majo s uc u al changes. Howe e , when he empe a u e exceeds 70
◦
C, he lignin
in he cell wall unde goes signi ican deg ada ion and agmen a ion be o e dissol ing.
Howe e , i is essen ial o no e ha he inal lignin con en o he deligni ied wood
does no singula ly dic a e he inal anspa en wood p ope ies. The o he a ibu es o he
deligni ied wood, such as he cellulose s uc u e, po e cha ac e is ics, chemical composi ion,
densi y, and he imp egna ion p ocess [
45
,
46
], subs an ially impac he p ope ies o in e es
in anspa en wood. The e o e, he inal p ope ies o anspa en wood may depend on
a ious ac o s, including he lignin con en , mic os uc u e, chemical composi ion, p esence
o de ec s, p ocessing condi ions, impu i ies, and mois u e con en [
20
,
47
,
48
]. Despi e his, he
s udy conduc ed by Wu e al. [
20
] epo ed a ia ion in he inal lignin con en s anging om
9 o 24% o deligni ied Basswood (Tilia) enee s. This speci ic s udy employed a mix u e o
2 w % NaClO
2
, 0.1 w % glacial ace ic acid, and 97.9 w % ul apu e wa e . The d ied samples
unde wen deligni ica ion in a wa e ba h oscilla o a 80
◦
C, wi h an oscilla ion equency o
40 pm, o di e en ea men du a ions spanning om 30 o 150 min.
3.2. FTIR Spec oscopy
Figu e 2displays he FTIR spec a o he o iginal eucalyp us enee , as well as he
samples subjec ed o he 60 min deligni ica ion ea men (DW60min) and he subsequen
imp egna ion wi h PMMA (TW60min). In addi ion o he peaks associa ed wi h he
p esence o ca bohyd a es a 1030 cm
−1
(C-O-C s e ching in cellulose) and 1730 cm
−1
(ace yl g oups in hemicelluloses) [
46
], he p is ine wood exhibi s abso p ion bands ela ed
o lignin, such as hose nea 1700 cm
−1
(C=O s e ching in lignin) [
40
] and 1200–1100 cm
−1
(C-O s e ching in lignin) [49].
Fo es s 2024, 15, x FOR PEER REVIEW 8 o 20
we e de ec ed a 2990 cm−1 (an inc ease in CH3 s e ching ib a ions signi ying he p es-
ence o me hyl g oups in he polyme s uc u e) [47]. Fu he mo e, bo h he DW60 min
and TW60 min samples displayed an enhanced p ominence o a cellulose- and hemicellu-
lose- ela ed peak (1730 cm−1), indica ing he emo al o lignin.
Figu e 2. FT-IR spec a o a p is ine wood, a deligni ied wood, and a anspa en wood.
3.3. The mal S abili y
The p is ine wood p esen ed a ypical g adual weigh loss in he empe a u e ange
co esponding o he decomposi ion o wood componen s, wi h majo weigh loss a -
ibu ed o he b eakdown o cellulose, hemicellulose, and lignin (Figu e 3). Besides, he
DTG cu e displays sha p peaks co esponding o he decomposi ion o diffe en wood
componen s, wi h dis inc peaks o cellulose, hemicellulose, and lignin deg ada ion. This
he mal decomposi ion p o ile is no mally di ided in o h ee s ages. The ini ial phase
akes place wi hin a lowe empe a u e span (20–100 °C), p ima ily linked o he expulsion
o adso bed and abso bed wa e molecules alongside ola ile subs ances. The subsequen
phase demons a es a subs an ial mass educ ion (app oxima ely 70%), co esponding o
he de ola iliza ion phenomena ela ed o wood polysaccha ides mani es ing be ween 300
and 400 °C. The ul ima e s age in ol es a g adual mass dec emen , occu ing wi hin he
empe a u e ange o 250–500 °C, a ibu ed o he g adual decomposi ion o lignin. A
comp ehensi e elucida ion o his he mal p og ession was pu o h by [51].
The deligni ied sample exhibi ed a dec eased weigh loss wi hin he empe a u e
ange o 120–350 °C, a ibu ed o he lignin emo al. This led o a smoo he and ea lie
decomposi ion p o ile compa ed o he p is ine wood. Addi ionally, he DTG peak ob-
se ed a 340 °C o he p is ine sample shi ed o 353 °C o he deligni ied one, sugges ing
he absence o signi ican lignin- ela ed peaks. These indings a e consis en wi h p e ious
esea ch on deligni ica ion’s impac on he mal p ope ies [8,52].
Mo ing o he anspa en wood, dis inc weigh loss s ages associa ed wi h he de-
composi ion o wood componen s and PMMA imp egna ion we e no disce nible, possi-
bly due o o e lapping effec s. This aligns wi h he indings in he li e a u e ha no e he
challenge o dis inguishing be ween indi idual weigh loss s ages when mul iple compo-
nen s a e in ol ed. The absence o a clea diffe en ia ion migh s em om he complex
Figu e 2. FT-IR spec a o a p is ine wood, a deligni ied wood, and a anspa en wood.
In compa ison o he p is ine wood, he modi ied samples exhibi ed a educ ion a
1700 cm
−1
and a disappea ance o lignin- ela ed abso p ion bands a 1500–1600 cm
−1
as
a esul o he deligni ica ion p ocess. The deligni ied sample (DW60min) also displayed
al e a ions in abso bance a a ious wa enumbe s co esponding o wood componen s due
Fo es s 2024,15, 1544 8 o 19
o he deligni ica ion ea men , such as a 1510–1550 cm
−1
(associa ed wi h C-H bending
ib a ions in lipid componen s and linked o C=C s e ching ib a ions in phenolic ings
p esen in annins and o he phenolic compounds) [50].
Wi hin he anspa en wood (TW60min), he e was an indica ion o PMMA- ela ed
abso p ion bands, including ca bonyl s e ching a app oxima ely 1720–1730 cm
−1
[
47
].
Addi ionally, newly eme ged abso p ion bands associa ed wi h PMMA polyme iza ion
we e de ec ed a 2990 cm
−1
(an inc ease in CH
3
s e ching ib a ions signi ying he p esence
o me hyl g oups in he polyme s uc u e) [
47
]. Fu he mo e, bo h he DW60min and
TW60min samples displayed an enhanced p ominence o a cellulose- and hemicellulose-
ela ed peak (1730 cm−1), indica ing he emo al o lignin.
3.3. The mal S abili y
The p is ine wood p esen ed a ypical g adual weigh loss in he empe a u e ange
co esponding o he decomposi ion o wood componen s, wi h majo weigh loss a -
ibu ed o he b eakdown o cellulose, hemicellulose, and lignin (Figu e 3). Besides, he
DTG cu e displays sha p peaks co esponding o he decomposi ion o di e en wood
componen s, wi h dis inc peaks o cellulose, hemicellulose, and lignin deg ada ion. This
he mal decomposi ion p o ile is no mally di ided in o h ee s ages. The ini ial phase akes
place wi hin a lowe empe a u e span (20–100
◦
C), p ima ily linked o he expulsion o
adso bed and abso bed wa e molecules alongside ola ile subs ances. The subsequen
phase demons a es a subs an ial mass educ ion (app oxima ely 70%), co esponding o
he de ola iliza ion phenomena ela ed o wood polysaccha ides mani es ing be ween
300 and 400
◦
C. The ul ima e s age in ol es a g adual mass dec emen , occu ing wi hin
he empe a u e ange o 250–500
◦
C, a ibu ed o he g adual decomposi ion o lignin. A
comp ehensi e elucida ion o his he mal p og ession was pu o h by [51].
Fo es s 2024, 15, x FOR PEER REVIEW 9 o 20
in e ac ion be ween he wood componen s and imp egna ed polyme s, in luencing he
he mal deg ada ion p ocess.
The p ima y he mal e en indica ed by he shi o he DTG peak om 353 °C o 429
°C in he deligni ied wood upon PMMA imp egna ion con i ms he p esence o PMMA.
This shi can be a ibu ed o he in e ac ions be ween PMMA and he wood s uc u e,
affec ing he he mal beha io . P e ious s udies sugges ha he p esence o polyme s can
lead o shi s in he he mal deg ada ion peaks, emphasizing he in luence o he imp eg-
na ion ma e ials on he he mal p ope ies o wood [8].
Figu e 3. (A) and (B) show, espec i ely, he loss o mass in ela ion o empe a u e and he de i a-
i e o he loss o mass in ela ion o ime o p is ine wood, deligni ied wood o 60 min and ans-
pa en wood ha was deligni ied o 60 min.
3.4. Hyg oscopic Cha ac e is ics
Figu e 4 illus a es he appa en con ac angle dynamics o e a pe iod o 60 s. In he
case o he p is ine wood, he ini ial con ac angle was ela i ely high, measu ing a ound
120°. Howe e , his angle consis en ly dec eased o e ime, eaching nea ly 0° a app ox-
ima ely 55 s. This dec ease in con ac angle can be a ibu ed o he wood’s su ace becom-
ing mo e hyd ophilic as wa e molecules we e p og essi ely abso bed in o he po ous
s uc u e. This phenomenon aligns wi h p io esea ch ha highligh s he we abili y o
wood su aces due o capilla y ac ion and he in e ac ion o wa e wi h cell wall compo-
nen s [53].
Figu e 3. (A) and (B) show, espec i ely, he loss o mass in ela ion o empe a u e and he de i a i e
o he loss o mass in ela ion o ime o p is ine wood, deligni ied wood o 60 min and anspa en
wood ha was deligni ied o 60 min.
The deligni ied sample exhibi ed a dec eased weigh loss wi hin he empe a u e
ange o 120–350
◦
C, a ibu ed o he lignin emo al. This led o a smoo he and ea lie
decomposi ion p o ile compa ed o he p is ine wood. Addi ionally, he DTG peak obse ed
a 340
◦
C o he p is ine sample shi ed o 353
◦
C o he deligni ied one, sugges ing he
absence o signi ican lignin- ela ed peaks. These indings a e consis en wi h p e ious
esea ch on deligni ica ion’s impac on he mal p ope ies [8,52].
Mo ing o he anspa en wood, dis inc weigh loss s ages associa ed wi h he de-
composi ion o wood componen s and PMMA imp egna ion we e no disce nible, possibly
due o o e lapping e ec s. This aligns wi h he indings in he li e a u e ha no e he
challenge o dis inguishing be ween indi idual weigh loss s ages when mul iple compo-
nen s a e in ol ed. The absence o a clea di e en ia ion migh s em om he complex
Fo es s 2024,15, 1544 9 o 19
in e ac ion be ween he wood componen s and imp egna ed polyme s, in luencing he
he mal deg ada ion p ocess.
The p ima y he mal e en indica ed by he shi o he DTG peak om 353
◦
C o
429 ◦C
in he deligni ied wood upon PMMA imp egna ion con i ms he p esence o PMMA.
This shi can be a ibu ed o he in e ac ions be ween PMMA and he wood s uc u e,
a ec ing he he mal beha io . P e ious s udies sugges ha he p esence o polyme s
can lead o shi s in he he mal deg ada ion peaks, emphasizing he in luence o he
imp egna ion ma e ials on he he mal p ope ies o wood [8].
3.4. Hyg oscopic Cha ac e is ics
Figu e 4illus a es he appa en con ac angle dynamics o e a pe iod o 60 s. In
he case o he p is ine wood, he ini ial con ac angle was ela i ely high, measu ing
a ound 120
◦
. Howe e , his angle consis en ly dec eased o e ime, eaching nea ly 0
◦
a app oxima ely 55 s. This dec ease in con ac angle can be a ibu ed o he wood’s
su ace becoming mo e hyd ophilic as wa e molecules we e p og essi ely abso bed in o
he po ous s uc u e. This phenomenon aligns wi h p io esea ch ha highligh s he
we abili y o wood su aces due o capilla y ac ion and he in e ac ion o wa e wi h cell
wall componen s [53].
Fo es s 2024, 15, x FOR PEER REVIEW 10 o 20
Figu e 4. Con ac angle esul s o he p is ine and anspa en woods.
Con e sely, he anspa en woods, p oduced h ough a ious deligni ica ion du a-
ions, exhibi ed minimal a ia ions in he e olu ion o con ac angles. Despi e diffe ences
in he deligni ica ion imes, he con ac angles emained wi hin a ela i ely na ow ange
o 75–68° ac oss all he samples. Such s abili y can be a ibu ed o he uni o m dis ibu ion
o he imp egna ing polyme , which can ac as a p o ec i e laye on he wood’s su ace.
S udies sugges ha polyme imp egna ion can al e he su ace chemis y o wood, a -
ec ing i s in e ac ion wi h liquids and consequen ly s abilizing con ac angles [9].
The WRE da a (shown in Figu e 5) exhibi he same pa e n as he con ac angle, in-
dica ing no signi ican diffe ences in he compa ison be ween he means ob ained om
diffe en cycles, no e en in a compa ison among he diffe en ypes o anspa en wood.
The cong uence be ween he WRE and con ac angle da a unde sco es he eliabili y and
ep oducibili y o anspa en wood’s su ace modi ica ions. This consis ency opens doo s
o u ilizing anspa en wood in di e se echnological applica ions, whe e con olled su -
ace in e ac ions play a c ucial ole in ma e ial pe o mance.
Figu e 4. Con ac angle esul s o he p is ine and anspa en woods.
Con e sely, he anspa en woods, p oduced h ough a ious deligni ica ion du a-
ions, exhibi ed minimal a ia ions in he e olu ion o con ac angles. Despi e di e ences
in he deligni ica ion imes, he con ac angles emained wi hin a ela i ely na ow ange o
75–68
◦
ac oss all he samples. Such s abili y can be a ibu ed o he uni o m dis ibu ion o
he imp egna ing polyme , which can ac as a p o ec i e laye on he wood’s su ace. S ud-
ies sugges ha polyme imp egna ion can al e he su ace chemis y o wood, a ec ing i s
in e ac ion wi h liquids and consequen ly s abilizing con ac angles [9].
The WRE da a (shown in Figu e 5) exhibi he same pa e n as he con ac angle,
indica ing no signi ican di e ences in he compa ison be ween he means ob ained om
di e en cycles, no e en in a compa ison among he di e en ypes o anspa en wood.
The cong uence be ween he WRE and con ac angle da a unde sco es he eliabili y and
ep oducibili y o anspa en wood’s su ace modi ica ions. This consis ency opens doo s
o u ilizing anspa en wood in di e se echnological applica ions, whe e con olled
su ace in e ac ions play a c ucial ole in ma e ial pe o mance.
Fo es s 2024,15, 1544 16 o 19
emo al om he sample, esul ing in a deligni ica ion pe cen age o 42.22% a 60 min in o
he p ocess. Al hough some s udies in he li e a u e ha e achie ed a highe deligni ica ion
con en han ha ound in his wo k, he esul s ob ained wi h he TW60min g oup ha e
al eady p o en o be sa is ac o y o ob aining wood wi h anspa ency cha ac e is ics.
Fu he mo e, he use o his k a p ocess wi h eucalyp us wood is inno a i e, and u he
esea ch should be conduc ed o imp o e his me hodology. In his con ex , he UV-Vis
spec oscopy e ealed he ansi ion om lignin- ich wood o anspa en wood, wi h
dis inc abso p ion bands indica ing success ul PMMA imp egna ion. The colo ime ic
analyses aced he colo shi s and anspa ency, while he mic os uc u al s udies depic ed
he g adual changes in he su ace ex u e and cellula s uc u e. The mechanical p ope ies
showed a delica e balance be ween deligni ica ion and mechanical in eg i y, wi h c i ical
p ese a ion obse ed a 60 min o ea men . The su ace oughness and we abili y
analyses indica ed changing physical a ibu es, and isual obse a ions aligned wi h
analy ical indings, po aying he p og essi e na u e o ans o ma ion. These combined
esul s enhance ou unde s anding o he po en ial o anspa en wood, laying a solid
ounda ion o inno a i e applica ions in a chi ec u e, design, and sus ainable ma e ials.
O e all, he k a pulping p ocess has he po en ial o s imula e economic g ow h, c ea e
alue-added p oduc s, and d i e sus ainable de elopmen in B azil and beyond. These
indings pa e he way o u u e esea ch in op imizing p ocess pa ame e s and explo ing
he e ec s o a ying he chemical composi ion o aw ma e ials o enhance bo h he op ical
p ope ies and o e all pe o mance o anspa en wood.
Au ho Con ibu ions: Concep ualiza ion, K.T.B., G.V.C., J.L. and A.P.A.; me hodology, K.T.B., G.V.C.
and A.P.A.; so wa e, K.T.B., A.P.A. and A.B.A.; alida ion, K.T.B., A.P.A. and A.B.A.; o mal analysis,
K.T.B., A.P.A., A.B.A. and R.d.A.D.; in es iga ion, K.T.B., A.P.A. and A.B.A.; esou ces, R.d.A.D.,
R.B. and D.A.G.; da a cu a ion, K.T.B., A.P.A. and A.B.A.; w i ing—o iginal d a p epa a ion, K.T.B.,
A.B.A., J.L. and R.d.A.D.; w i ing— e iew and edi ing, K.T.B., J.L. and R.d.A.D.; isualiza ion,
K.T.B. and R.d.A.D.; supe ision, R.d.A.D. and R.B.; p ojec adminis a ion, R.d.A.D. and R.B.;
unding acquisi ion, D.A.G. and J.L. All au ho s ha e ead and ag eed o he published e sion o
he manusc ip .
Funding: This wo k was suppo ed by Coo dina ion o he Imp o emen o Highe Educa ion—
CAPES (code 001).
Da a A ailabili y S a emen : Da a a e con ained wi hin he a icle.
Con lic s o In e es : The au ho s decla e no con lic s o in e es .
Re e ences
1.
Wu, X.; Kong, Z.; Yao, X.; Gan, J.; Zhan, X.; Wu, Y. T anspa en Wood wi h Sel -Cleaning P ope ies o nex -Gene a ion Sma
Pho o ol aic Panels. Appl. Su . Sci. 2023,613, 155927. [C ossRe ]
2.
Mi, R.; Li, T.; Dalgo, D.; Chen, C.; Kuang, Y.; He, S.; Zhao, X.; Xie, W.; Gan, W.; Zhu, J.; e al. A Clea , S ong, and The mally
Insula ed T anspa en Wood o Ene gy E icien Windows. Ad . Func . Ma e . 2020,30, 1907511. [C ossRe ]
3.
Yaddanapudi, H.S.; Hicke son, N.; Saini, S.; Tiwa i, A. Fab ica ion and Cha ac e iza ion o T anspa en Wood o nex Gene a ion
Sma Building Applica ions. Vacuum 2017,146, 649–654. [C ossRe ]
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