Ci a ion: Acos a, A.; A ambu u, A.B.;
Bel ame, R.; Ga o, D.A.; Amico, S.;
Labidi, J.; Delucis, R.d.A. Wood Flou
Modi ied by Poly( u u yl alcohol) as
a Fille in Rigid Polyu e hane Foams:
E ec on Wa e Up ake. Polyme s
2022,14, 5510. h ps://
doi.o g/10.3390/polym14245510
Academic Edi o : Na hanael Guigo
Recei ed: 9 No embe 2022
Accep ed: 13 Decembe 2022
Published: 16 Decembe 2022
Publishe ’s No e: MDPI s ays neu al
wi h ega d o ju isdic ional claims in
published maps and ins i u ional a il-
ia ions.
Copy igh : © 2022 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/).
polyme s
A icle
Wood Flou Modi ied by Poly( u u yl alcohol) as a Fille in
Rigid Polyu e hane Foams: E ec on Wa e Up ake
And ey Acos a 1,*, A hu B. A ambu u 1, Ra ael Bel ame 2,3, Da ci A. Ga o 2,3, Sand o Amico 1,
Jalel Labidi 4,* and Ra ael de A ila Delucis 2,3
1Pos -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 91509-900, B azil
2Pos -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 96010-610, B azil
3Pos -G adua e P og am in En i onmen al Sciences, Fede al Uni e si y o Pelo as, Pelo as 96010-610, B azil
4
En i onmen al and Chemical Enginee ing Depa men , Uni e si y o he Basque Coun y, Plaza de Eu opa 1,
20018 San Sebas ián, Spain
*Co espondence: and ey.acos a@u gs.b (A.A.); [email p o ec ed] (J.L.)
Abs ac :
The use o lignocellulosic ille s in igid polyu e hane oams (RPUFs) has been ecei ing
g ea a en ion due o hei good mechanical and insula ion p ope ies and he high sus ainable
appeal o he ob ained cellula polyme s, al hough high wa e up akes a e ound in mos o hese
sys ems. To mi iga e his de imen al e ec , RPUFs illed wi h wood lou (2.5% w ) we e ab ica ed
wi h he addi ion o u u yl alcohol (FA) o c ea e a polyme g a ed wi h he wood ille . Two
concen a ions o FA (10 w % and 15 w %) we e in es iga ed in ela ion o he wood lou , and he
RPUFs we e cha ac e ized o cell mo phology, densi y, comp essi e p ope ies, he mal s abili y,
and wa e up ake. The in oduc ion o wood lou as a ille dec eased he cell size and inc eased
he aniso opy index o he RPUFs and, in addi ion o ha , he FA g a ing inc eased hese e ec s
e en mo e. In gene al, he e we e no signi ican changes in bo h mechanical and he mal p ope ies
asc ibed o he inco po a ion o he ille s. On he o he hand, a educ ion o up o 200% in wa e
up ake was asc ibed o he FA- ea ed ille s.
Keywo ds: polyu e hane oam; u u yl alcohol; wa e up ake; wood lou
1. In oduc ion
Rigid polyu e hane oams (RPUFs) a e h ee-dimensional, highly c oss-linked po ous
ma e ials wi h low densi y, high dimensional s abili y, and low he mal conduc i i y [
1
].
These desi able p ope ies make RPUFs a e sa ile class o polyme ma e ials wi h a la ge
ange o applica ions, including insula ion and manu ac u e o u ni u e and spo ing
goods [
2
]. Howe e , RPUFs a e p epa ed h ough he eac ion be ween an isocyana e and a
polyol, which a e o en de i ed om pe oleum-based esou ces [
3
]. In his sense, he use
o enewable esou ces in RPUFs as subs i u es o pe oleum-based compounds has been
ecei ing g ea a en ion, especially due o he use o bio-based oils as polyols, dis illed
wa e as blowing agen s, and chemical addi i es based on glyce ol, among o he s [4–6].
Fu he mo e, he use o na u al ille s also plays an impo an ole in his ield due o
hei posi i e in luence on se e al p ope ies o RPUFs. In his sense, o es and ag icul u al
esidues s and ou due o hei high a ailabili y, low p ice, and high enewabili y [
7
], as
well as good he mal [
8
] and acous ic [
9
] insula ing p ope ies. Fu he mo e, ee hyd oxyl
g oups (–OH) on he su ace o hese lignocellulosic ma ixes gi e a high hos chemical
compa ibili y wi h RPUFs, since hey a e p one o binding o isocyana e g oups (–NCO)
om he polyme ma ix [10].
Among he mos s udied bio-based esidues, wood lou is one o he mos widely
used ille s as ein o cemen o polyme composi es. Fo ins ance, Chanle and Ruam-
cha oen [
9
] achie ed imp o ed sound abso p ion p ope ies in RPUFs using a ubbe
Polyme s 2022,14, 5510. h ps://doi.o g/10.3390/polym14245510 h ps://www.mdpi.com/jou nal/polyme s
Polyme s 2022,14, 5510 2 o 12
wood-based sawdus as a ille . In a mo e ecen s udy, De Luca Bossa and co-wo ke s [
7
]
epo ed inc eases in he mal s abili y and some mechanical p ope ies, which we e a -
ibu ed o he inco po a ion o ege able ille s, namely powde ed cellulose and walnu
shells. In he same way, Delucis and co-wo ke s [
11
–
13
] epo ed he use o wood, wood
ba k, pine cones, pine needles, powde ed k a lignin, and pape sludge as ille s in cas o
oil-based RPUFs. These au ho s epo ed inc eases in he mal and mechanical p ope ies,
accompanied by dec eases in cell sizes. On he o he hand, hese same au ho s epo ed
losses in hyg oscopic p ope ies.
The high wa e up ake o RPUFs ein o ced wi h na u al ille s is mos ly ela ed o
bo h he dis up ion o cell edges and he hyd ophilic na u e o he inse ed ille s [
11
].
Du ing he RPUF ising, he p esence o mois ai in cellula po es can also inc ease he
cu e a e h ough he eac ion o NCO g oups wi h wa e , which a o s he g ow h o he
polyme ne wo k h ough polyu ea o ma ion and gene a ion o encapsula ed CO
2
[
14
].
Howe e , he abso p ion o mois u e in cu ed RPUFs may b ing de imen al e ec s o he
whole cellula polyme since he NCO g oups may eac wi h wa e molecules in he ai a
oom empe a u e, eleasing CO
2
and educing he RPUF pe o mance [
15
]. The con ac
o wa e wi h RPUF sys ems may also cause he hyd olysis o es e g oups de i ed om
he polyol [
16
]. The e o e, low wa e up ake is linked o high du abili y and high he mal
conduc i i y in RPUFs. Thus, inc easing he hyg oscopic pe o mance o a RPUF is a way
o minimizing i s long- e m pe o mance a ia ion. In addi ion, mois u e abso bed by he
ille om a RPUF may b ing a de imen al e ec on i s he mal conduc i i y since liquid
wa e has 10 imes highe he mal conduc i i y han a con en ional RPUF [11].
In con en ional PU-based composi e ma e ials, su ace ea men s o lignocellulosic
ma e ials a e widely applied in o de o mi iga e hei hyd ophilic cha ac e since hese
p ocesses may enhance in e acial adhesion o na u al ille s o he polyme ma ix, lead-
ing o inc eases in physical pe o mances [
17
,
18
]. In his sense, many au ho s epo ed
solu ions o inc ease he compa ibili y o lignocellulosic ille s and PU sys ems. Kılınç
and co-wo ke s [
17
] used alkaline and silane modi ica ions o imp o e he compa ibili y
o wood lou and a PU elas ome ic composi e. Członka and co-wo ke s [
2
] applied a
silaniza ion p ocess in walnu shells o eaching imp o emen s in he mechanical and
he mal p ope ies o hei illed RPUFs. Finally, B adai and co-wo ke s [
19
] applied ace y-
la ion as a p e- ea men on di e en wood-based esou ces, namely milled wood sie ed
a di e en pa icle sizes (<0.106 mm, 0.1–0.3 mm and 0.3–0.5 mm), k a chemical pulp,
and mic oc ys alline cellulose. These ma e ials we e hen inse ed in o he RPUFs, yielding
di e en imp o emen s in mechanical and he mal p ope ies.
In his sense, u u al is a pla o m chemical (i.e., his is a eeds ock o p oducing
a ious high alue-added p oduc s), being one o he main de i a i es om hemicelluloses
and a na u al p ecu so o u an-based chemicals, such as u u yl alcohol [
20
–
22
]. u u yl
alcohol (FA) g a ing is a widely used p ocess o wood ea men (wood u u yla ion),
ha consis s o imp egna ing FA in o he wood s uc u e and inducing an in si u polyme -
iza ion in o poly( u u yl alcohol). In his s udy, wood lou was ea ed a di e en FA
concen a ions and hen added o RPUF sys ems o imp o e hei hyg oscopic pe o mance.
2. Ma e ials and Me hods
2.1. Fille s P epa a ion and Fu u yla ion
Wood lake le o e s om he p ocessing o pinewood logs we e collec ed in a sawmill
in Sou he n B azil. This o es -based esou ce was o en d ied a 50
◦
C un il eaching a
cons an weigh , milled using a Wiley mill, and hen sie ed using a 100-mesh sc een. The
esul ing wood lou was cha ac e ized ia we -chemical analysis o ob ain e hanol- oluene
ex ac i es [
23
], acid-insoluble lignin [
24
], ashes [
25
], and holocellulose ( emaining mass
pe cen age) con en s.
The u u yla ion p ocess occu ed by he addi ion o a high-pu i y (98 w %) u u yl
alcohol (FA) acqui ed om Sigma Ald ich a wo di e en concen a ions (c.a. 10 and 15
w % in ela ion o he wood lou weigh ). A high-pu i y (98 w %) maleic anhyd ide (MA)
Polyme s 2022,14, 5510 3 o 12
acqui ed om Sigma Ald ich was added as a ca alys a a concen a ion o 1.5 w % in
ela ion o he FA weigh . The FA, MA, and wood lou ac ions we e mixed wi h 500 mL
o wa e and hen magne ically homogenized o 15 min. A e wa d, he ea ed wood lou
was o en hea ed a 60
◦
C o 24 h. Appa en con ac angle measu emen s we e pe o med
using a CREVIS goniome e . Fo ha , a 5-mL deionized wa e d ople was placed on
uni o m pelle s (diame e : 10 mm) manu ac u ed by a comp essi e load o 80 kN applied
o 15 s.
2.2. P epa a ion o he RPUFs
Cas o oil and glyce in we e used a a 3:1 weigh a io as a bio-based polyol. Iso ane
DM, which is a polyme me hylene diphenyl diisocyana e (p-MDI), was used as an NCO
sou ce. Poly-e hylene glycol (PEG-400), silicon oil, and dime hylbenzylamine we e used as
a chain ex ende , a su ac an , and a ca alys , espec i ely.
The RPUF pa s we e p oduced by he single-sho me hod using wo componen s
(A and B), keeping a cons an NCO/OH a io o 1.2. Componen A (polyol mix u e)
consis ed o a simple mix u e o cas o oil (67.5 pa s/g), dis illed wa e (4 pa s/g), glyce ol
(22.5 pa s/g), PEG-400 (10 pa s/g), silicon oil (2.5 pa s/g), and ille , which we e s i ed
oge he a 1000 pm o 60 s and hen wi h componen B (p-MDI and amine (1 pa /g)) o
an ex a 60 s. All RPUFs eely ose inside opened wooden boxes and we e le o cu e a
60
◦
C o 2 h and pos -cu e o wo weeks a oom empe a u e. Table 1shows he adop ed
nomencla u e o he s udied RPUFs.
Table 1. Nomencla u e o he s udied RPUFs.
G oup Fille Weigh F ac ion (%) FA Weigh F ac ion (%)
PU 0 0
RPUFW2.5 0
RPUFW10 2.5 10
RPUFW15 2.5 15
2.3. Cha ac e iza ion o he RPUFs
All ab ica ed RPUFs we e milled and analyzed using a Fou ie - ans o m in a ed
(FTIR) spec oscopy coupled wi h an a enua ed o al e lec ion de ice (ATR) in an IRSpi i
equipmen (Shimadzu
®
b and). The epo ed spec um o each sample is he a e age o
32 scans wi hin he 600–4000cm
−1
ange a a scan in e al o 4 cm
−1
. The FTIR da a we e
smoo hed by applying a Sa i zky–Golay il e (second deg ee polynomial a an in e al o
12). The mal s abili y was e alua ed using a he mog a ime ic analyze (TG) a a hea ing
a e o 10
◦
C.min
−1
om oom empe a u e (c.a. 20
◦
C) o 800
◦
C, which was ca ied ou
using a TGA-1000 Na as equipmen unde a ni ogen a mosphe e. Su ace mo phology
was analyzed pe pendicula o he ise di ec ion using a scanning elec on mic oscopy
(SEM) in a MA10 equipmen (Zeiss E o b and) ope a ing a 3 kV. A e age cell size and
aniso opy index we e measu ed using he ImageJ so wa e, as desc ibed by Delucis and
his co-wo ke s [11].
Se en samples (5.0
×
5.0
×
2.5 cm
3
) o each g oup we e aken o de e mine appa en
densi y (ASTM D1622) and mechanical p ope ies unde comp ession pa allel o he ise
di ec ion (ASTM D 1621). The comp essi e es s we e ca ied ou a a 2.5 mm.min
−1
speed
using a 23-5D Emic uni e sal es ing machine. Finally, wa e up ake measu emen s we e
pe o med in 10 samples pe g oup, which had he dimensions o 5.0
×
5.0
×
2.5 cm
3
(smalle dimension o ien ed in he ise di ec ion). The specimens we e o ally imme sed in
dis illed wa e a oom empe a u e (a ound 20
◦
C) in acco dance wi h he ASTM D570.
Then, he mass gain was moni o ed e e y 30 min un il eaching a o al soaking ime o
300 min.
Polyme s 2022,14, 5510 4 o 12
2.4. S a is ical Analyses
All da a, excep he chemical and wa e up ake esul s, we e subjec ed o homogenei y
o a iance and da a no mali y es s. La e , ANOVA es s we e ca ied ou and, whene e
he null hypo hesis was ejec ed, Tukey es s we e used o compa e he means. All s a is ical
analyses we e implemen ed a a signi icance le el o 5%.
3. Resul s and Discussion
3.1. Fille ’s P ope ies
The high holocellulose con en (abo e 70 w %) ob ained o he s udied wood lou
(shown in Figu e 1) is compa able o o he holocellulose- ich biomasses, such as co ncob
and co k [
26
]. This high holocellulose con en may impac se e al con e sion p ocesses
applied o wood p oduc s due o he high numbe o s ong in a-mac omolecula co alen
bonds be ween suga uni s om cellulose and hemicelluloses, equi ing a high ene gy
amoun o each homoly ic b eaks [
11
]. In addi ion, he high holocellulose con en may be o
in e es due o ee OH g oups om he amo phous polysaccha ides, such has amo phous
cellulose and hemicelluloses, which a e p one o o m u e hane linkages wi h NCO g oups
om he p-MDI.
Polyme s 2022, 14, x FOR PEER REVIEW 4 o 13
di ec ion (ASTM D 1621). The comp essi e es s we e ca ied ou a a 2.5 mm.min−1 speed
using a 23-5D Emic uni e sal es ing machine. Finally, wa e up ake measu emen s we e
pe o med in 10 samples pe g oup, which had he dimensions o 5.0 × 5.0 × 2.5 cm3
(smalle dimension o ien ed in he ise di ec ion). The specimens we e o ally imme sed
in dis illed wa e a oom empe a u e (a ound 20 °C) in acco dance wi h he ASTM D570.
Then, he mass gain was moni o ed e e y 30 min un il eaching a o al soaking ime o
300 min.
2.4. S a is ical Analyses
All da a, excep he chemical and wa e up ake esul s, we e subjec ed o homogene-
i y o a iance and da a no mali y es s. La e , ANOVA es s we e ca ied ou and, when-
e e he null hypo hesis was ejec ed, Tukey es s we e used o compa e he means. All
s a is ical analyses we e implemen ed a a signi icance le el o 5%.
3. Resul s and Discussion
3.1. Fille ’s P ope ies
The high holocellulose con en (abo e 70 w %) ob ained o he s udied wood lou
(shown in Figu e 1) is compa able o o he holocellulose- ich biomasses, such as co ncob
and co k [26]. This high holocellulose con en may impac se e al con e sion p ocesses
applied o wood p oduc s due o he high numbe o s ong in a-mac omolecula co a-
len bonds be ween suga uni s om cellulose and hemicelluloses, equi ing a high ene gy
amoun o each homoly ic b eaks [11]. In addi ion, he high holocellulose con en may be
o in e es due o ee OH g oups om he amo phous polysaccha ides, such has amo -
phous cellulose and hemicelluloses, which a e p one o o m u e hane linkages wi h NCO
g oups om he p-MDI.
Figu e 1. Chemical cons i u ion o he wood lou .
The con ac angle kine ics obse ed o each ille a e shown in Figu e 2. Bo h u u-
yla ed woods p esen ed simila p o iles o d op abso p ion, in which a sha p dec ease
occu ed along he i s 4 s. Compa ed o he p is ine wood lou , he FA- ea ed ones
p esen ed mo e uns able p o iles, which indica es ha hey p esen ed a smalle su ace
hyd ophobici y. The chemical a ini y o milled lignocellulosic p oduc s wi h wa e is
mos ly a ibu ed o highly pola chemical g oups, such as hyd oxyls and me hoxyls,
which may in e ac wi h wa e ia hyd ogen bonds. The e o e, he changes in su ace hy-
d ophobici y impa ed by he ea men s a e p obably associa ed wi h modi ica ions in
pola chemical g oups p esen ed in cellulose, lignin, and hemicelluloses om he wood
Figu e 1. Chemical cons i u ion o he wood lou .
The con ac angle kine ics obse ed o each ille a e shown in Figu e 2. Bo h u -
u yla ed woods p esen ed simila p o iles o d op abso p ion, in which a sha p dec ease
occu ed along he i s 4 s. Compa ed o he p is ine wood lou , he FA- ea ed ones
p esen ed mo e uns able p o iles, which indica es ha hey p esen ed a smalle su ace hy-
d ophobici y. The chemical a ini y o milled lignocellulosic p oduc s wi h wa e is mos ly
a ibu ed o highly pola chemical g oups, such as hyd oxyls and me hoxyls, which may
in e ac wi h wa e ia hyd ogen bonds. The e o e, he changes in su ace hyd ophobici y
impa ed by he ea men s a e p obably associa ed wi h modi ica ions in pola chemical
g oups p esen ed in cellulose, lignin, and hemicelluloses om he wood caused by he FA
polyme iza ion. Al hough some ecen s udies ha e p oposed some explana ions o he in
si u polyme iza ion o FA inside wood [
27
], his mechanism has no been ully cla i ied ye .
Polyme s 2022,14, 5510 5 o 12
Polyme s 2022, 14, x FOR PEER REVIEW 5 o 13
caused by he FA polyme iza ion. Al hough some ecen s udies ha e p oposed some ex-
plana ions o he in si u polyme iza ion o FA inside wood [27], his mechanism has no
been ully cla i ied ye .
Figu e 2. Appa en con ac angle e olu ion in e ms o mean alues (A) and images (B) o he s ud-
ied ille s.
Figu e 3 shows he in a ed spec a o he ea ed and un ea ed wood lou s and
hei espec i e RPUFs. The peaks a 1060 cm−1, 2900 cm−1, and 3360 cm−1, which a e asso-
cia ed wi h he s e ching ib a ion o C-O-C [28], CH3 [29], and N-H [29] g oups, espec-
i ely, a e ound o all he s udied RPUFs and indica e a good polyme o ma ion in all
cases. The FA ea men is e idenced by he a enua ion in he peak a 1700 cm−1 [30],
which is ound o bo h FA- ea ed wood lou s (RPUFW10 and RPUFW15). Fu he mo e, he
Figu e 2.
Appa en con ac angle e olu ion in e ms o mean alues (
A
) and images (
B
) o he
s udied ille s.
Figu e 3shows he in a ed spec a o he ea ed and un ea ed wood lou s and
hei espec i e RPUFs. The peaks a 1060 cm
−1
, 2900 cm
−1
, and 3360 cm
−1
, which a e
associa ed wi h he s e ching ib a ion o C-O-C [
28
], CH
3
[
29
], and N-H [
29
] g oups,
espec i ely, a e ound o all he s udied RPUFs and indica e a good polyme o ma ion in
all cases. The FA ea men is e idenced by he a enua ion in he peak a 1700 cm
−1
[
30
],
which is ound o bo h FA- ea ed wood lou s (RPUF
W10
and RPUF
W15
). Fu he mo e,
he p ominen peak a 2280 cm
−1
, ound in bo h RPUF
W10
and RPUF
W15
, a e ela ed o
un eac ed NCO g oups. This indica es ha he FA blocks some OH g oups om he wood
lou , p e en ing he o ma ion o u e hane linkages wi h NCO g oups om he p-MDI.
Polyme s 2022,14, 5510 6 o 12
Polyme s 2022, 14, x FOR PEER REVIEW 6 o 13
p ominen peak a 2280 cm−1, ound in bo h RPUFW10 and RPUFW15, a e ela ed o un e-
ac ed NCO g oups. This indica es ha he FA blocks some OH g oups om he wood
lou , p e en ing he o ma ion o u e hane linkages wi h NCO g oups om he p-MDI.
Figu e 3. FTIR spec a o he s udied ille s and RPUFs.
3.2. Foam Mo phology and Densi y
The SEM images shown in Figu e 4 indica e ha all ille s a e well dispe sed in o he
RPUF cell s uc u e since no agglome a es a e ound. In addi ion, he e a e no no iceable
signal o cell dis up ions associa ed wi h he ille inse ion, which indica es a good
ille /ma ix in e ac ion. Only he ille ea ed wi h 15% o FA induces a signi ican in-
c ease in bo h RPUF cell leng h and aniso opy index. The elonga ion o polyme cells is
associa ed wi h a igo ous cell g ow h, which is no mally caused by chemically compa i-
ble ille s [31]. This indica es ha he ea men wi h 15% o FA p obably yields a ille
wi h a highe hos compa ibili y in he RPUF sys em when compa ed o he o he ille s.
Figu e 3. FTIR spec a o he s udied ille s and RPUFs.
3.2. Foam Mo phology and Densi y
The SEM images shown in Figu e 4indica e ha all ille s a e well dispe sed in o
he RPUF cell s uc u e since no agglome a es a e ound. In addi ion, he e a e no no-
iceable signal o cell dis up ions associa ed wi h he ille inse ion, which indica es a
good ille /ma ix in e ac ion. Only he ille ea ed wi h 15% o FA induces a signi ican
inc ease in bo h RPUF cell leng h and aniso opy index. The elonga ion o polyme cells is
associa ed wi h a igo ous cell g ow h, which is no mally caused by chemically compa ible
ille s [
31
]. This indica es ha he ea men wi h 15% o FA p obably yields a ille wi h a
highe hos compa ibili y in he RPUF sys em when compa ed o he o he ille s.
Fu he mo e, he changes in cell size and cell shape do no cause changes in he RPUF
densi y, as shown in Figu e 4. Densi y plays an essen ial ole in many applica ions since
RPUFs a e widely known by hei high speci ic mechanical p ope ies. Mo eo e , he
appa en densi y (Figu e 5) is an indi ec measu emen o oid con en , and he highe
he oid con en , he highe he gas con en and he highe he insula ion p ope ies [
32
].
Conside ing ha some ecen s udies asc ibed inc eases in RPUF densi y o simila ille s
being inco po a ed in o he RPUFs [
32
], in his s udy, he inse ed ille s p obably did no
change he RPUF densi y due o hei low con en (c.a. 2.5 w %).
3.3. Comp essi e P ope ies
Rega ding he comp essi e s ess, he use o ille s in he RPUFs did no modi y
he shape o he s ess
×
s ain cu es ob ained by comp essi e es s (Figu e 6a), which
also explains why he inse ion o he ille s changed nei he he comp essi e modulus
no he comp essi e s eng h (Figu e 6b). On he o he hand, he comp essi e s eng h
o he RPUF ein o ced by wood lou o e came hose o he RPUFs inco po a ed wi h
ea ed ille s (RPUF
W10
and RPUF
W15
). This p obably occu ed due o he OH-blocking
Polyme s 2022,14, 5510 7 o 12
caused by he u u yla ion, which p obably led o a smalle amoun o u e hane linkages
be ween he ille and he ma ix [
33
]. In addi ion, he ounded cell shape o he RPUF
illed wi h p is ine wood lou may be a mo e mechanically s able con igu a ion han he
ellip ical shape o he RPUFs inco po a ed wi h ea ed wood lou . These esul s a e in
ag eemen wi h hose epo ed in he li e a u e wi h ille pe cen ages close o he p esen
s udy. S akowska and co-wo ke s [
34
] manu ac u ed RPUFs ein o ced wi h a suga bee
pulp a h ee di e en concen a ions (1, 2, and 5 w .%). These au ho s also epo ed simila
comp essi e p ope ies in a compa ison be ween he illed and un illed RPUFs.
Polyme s 2022, 14, x FOR PEER REVIEW 7 o 13
Figu e 4. SEM images and mo phological p ope ies o he s udied RPUFs. Di e en le e s ep e-
sen s a is ically di e en means. Di e en le e s (e.g., a, b and c) ep esen s a is ically di e en
means and equal le e s ep esen no s a is ically di e en means (e.g., a and ab).
Fu he mo e, he changes in cell size and cell shape do no cause changes in he RPUF
densi y, as shown in Figu e 4. Densi y plays an essen ial ole in many applica ions since
RPUFs a e widely known by hei high speci ic mechanical p ope ies. Mo eo e , he ap-
pa en densi y (Figu e 5) is an indi ec measu emen o oid con en , and he highe he
oid con en , he highe he gas con en and he highe he insula ion p ope ies [32]. Con-
side ing ha some ecen s udies asc ibed inc eases in RPUF densi y o simila ille s be-
ing inco po a ed in o he RPUFs [32], in his s udy, he inse ed ille s p obably did no
change he RPUF densi y due o hei low con en (c.a. 2.5 w %).
Figu e 4.
SEM images and mo phological p ope ies o he s udied RPUFs. Di e en le e s ep esen
s a is ically di e en means. Di e en le e s (e.g., a, b and c) ep esen s a is ically di e en means
and equal le e s ep esen no s a is ically di e en means (e.g., a and ab).
Polyme s 2022,14, 5510 8 o 12
Polyme s 2022, 14, x FOR PEER REVIEW 8 o 13
Figu e 5. Appa en densi y o he s udied RPUFs. RPUF is pu e igid polyu e hane oam, RPUFw
is wood lou ein o ced oam, RPUFW10 is wood lou ein o ced oam ea ed wi h 10% u u yl
alcohol, and RPUFW15 is wood lou ein o ced oam ea ed wi h 15% u u yl alcohol. Di e en
le e s ep esen s a is ically di e en means.
3.3. Comp essi e P ope ies
Rega ding he comp essi e s ess, he use o ille s in he RPUFs did no modi y he
shape o he s ess × s ain cu es ob ained by comp essi e es s (Figu e 6a), which also
explains why he inse ion o he ille s changed nei he he comp essi e modulus no he
comp essi e s eng h (Figu e 6b). On he o he hand, he comp essi e s eng h o he
RPUF ein o ced by wood lou o e came hose o he RPUFs inco po a ed wi h ea ed
ille s (RPUFW10 and RPUFW15). This p obably occu ed due o he OH-blocking caused by
he u u yla ion, which p obably led o a smalle amoun o u e hane linkages be ween
he ille and he ma ix [33]. In addi ion, he ounded cell shape o he RPUF illed wi h
p is ine wood lou may be a mo e mechanically s able con igu a ion han he ellip ical
shape o he RPUFs inco po a ed wi h ea ed wood lou . These esul s a e in ag eemen
wi h hose epo ed in he li e a u e wi h ille pe cen ages close o he p esen s udy.
S akowska and co-wo ke s [34] manu ac u ed RPUFs ein o ced wi h a suga bee pulp
a h ee di e en concen a ions (1, 2, and 5 w .%). These au ho s also epo ed simila
comp essi e p ope ies in a compa ison be ween he illed and un illed RPUFs.
Figu e 5.
Appa en densi y o he s udied RPUFs. RPUF is pu e igid polyu e hane oam, RPUFw
is wood lou ein o ced oam, RPUF
W10
is wood lou ein o ced oam ea ed wi h 10% u u yl
alcohol, and RPUF
W15
is wood lou ein o ced oam ea ed wi h 15% u u yl alcohol. Di e en
le e s ep esen s a is ically di e en means.
Polyme s 2022, 14, x FOR PEER REVIEW 9 o 13
Figu e 6. Rep esen a i e s ess s. s ain cu es (A) and comp essi e p ope ies (B, C) o he s udied
RPUFs. Di e en le e s ep esen s a is ically di e en means.
On he o he hand, some s udies on illed RPUFs epo ed losses in mechanical p op-
e ies asc ibed o dis up ions in cell edges caused by he ille s. In his sense, Ju e al. [18]
and B adai e al. [19] epo ed dec eases in comp essi e s eng h in RPUFs illed wi h
s eam-exploded peanu shell ibe s and wood lou s, espec i ely. These changes in me-
chanical p ope ies we e also a ibu ed o he changes in densi y, which did no occu in
he p esen s udy.
3.4. The mal S abili y
The TG esul s a e p esen ed in Figu e 7 and Table 2. The s udied ille s p esen sim-
ila TG cu es, which means ha his analysis did no de ec e ec s a ibu ed o he wood
u u yla ion. In his ega d, he i s he mo-decomposi ion s age ended a 100 °C and
can be a ibu ed o a small amoun o mois u e e ained in he wood s uc u e. The sec-
ond s age, om 100 °C o 250 °C, is a ibu ed o he decomposi ion o amo phous egions
om cellulose and hemicellulose [35]. In his empe a u e ange, he he mal deg ada ion
o AF also begins wi h he scission o me hylene bonds and he o ma ion o ce ain com-
pounds, such as 2-me hyl u an and 2- u u yl-5-me hyl u an [36]. Howe e , since he
addi ion o u u yl alcohol was ela i ely low, no signi ican di e ences we e ound in a
compa ison be ween he wood samples. The hi d TG s age, o e 250 °C, can be a ibu ed
o he decomposi ion o c ys alline cellulose and lignin uni s [12].
Table 2. Main TG e en s o he s udied ille s and RPUFs.
G oup
T
2%
(°C)
T
5%
(°C)
T
50%
(°C)
Residue a 600 °C (%)
RPUF
84.88
114.81
500.58
21.26
RPUF
W
91.38
125.24
504.69
25.94
RPUF
W10
89.32
187.31
514.94
33.88
RPUF
W15
67.27
94.96
494.59
29.23
W
91.55
108.65
342.40
15.40
W
10
97.70
112.76
355.23
16.77
W
15
108.65
113.10
351.98
17.85
Figu e 6.
Rep esen a i e s ess s. s ain cu es (
A
) and comp essi e p ope ies (
B
,
C
) o he s udied
RPUFs. Di e en le e s ep esen s a is ically di e en means.
On he o he hand, some s udies on illed RPUFs epo ed losses in mechanical p op-
e ies asc ibed o dis up ions in cell edges caused by he ille s. In his sense, Ju e al. [
18
]
and B adai e al. [
19
] epo ed dec eases in comp essi e s eng h in RPUFs illed wi h
s eam-exploded peanu shell ibe s and wood lou s, espec i ely. These changes in me-
chanical p ope ies we e also a ibu ed o he changes in densi y, which did no occu in
he p esen s udy.
3.4. The mal S abili y
The TG esul s a e p esen ed in Figu e 7and Table 2. The s udied ille s p esen simila
TG cu es, which means ha his analysis did no de ec e ec s a ibu ed o he wood
Polyme s 2022,14, 5510 9 o 12
u u yla ion. In his ega d, he i s he mo-decomposi ion s age ended a 100
◦
C and can
be a ibu ed o a small amoun o mois u e e ained in he wood s uc u e. The second
s age, om 100
◦
C o 250
◦
C, is a ibu ed o he decomposi ion o amo phous egions om
cellulose and hemicellulose [
35
]. In his empe a u e ange, he he mal deg ada ion o AF
also begins wi h he scission o me hylene bonds and he o ma ion o ce ain compounds,
such as 2-me hyl u an and 2- u u yl-5-me hyl u an [
36
]. Howe e , since he addi ion o
u u yl alcohol was ela i ely low, no signi ican di e ences we e ound in a compa ison
be ween he wood samples. The hi d TG s age, o e 250
◦
C, can be a ibu ed o he
decomposi ion o c ys alline cellulose and lignin uni s [12].
Polyme s 2022, 14, x FOR PEER REVIEW 10 o 13
Figu e 7. TG cu es o he s udied ille s and RPUFs.
The TG esul s o he RPUFs con i m he FTIR esul s and again indica e ha all he
s udied RPUFs ha e a simila chemical composi ion, which also means ha he inco po-
a ed ille s did no inse a signi ican chemical modi ica ion in he RPUF cell s uc u e.
This can also be explained by a sligh une en dis ibu ion o he ille in he RPUF ma ix
since he sample p epa ed o his analysis is oo small. The e o e, he di e en c osslink-
ing densi ies indica ed by he we -chemical and FTIR esul s we e no de ec ed by he TG
analysis. The cha ac e is ic empe a u es we e de ined as T2% ( empe a u e a ibu ed o
2% o weigh loss), T5% ( empe a u e a ibu ed o 5% o weigh loss), and T50% ( empe a-
u e a ibu ed o 50% o weigh loss).
3.5. Wa e Up ake
Figu e 8 shows ha , compa ed o he nea RPUF, bo h wood lou s ea ed by u u-
yla ion yield a dec ease in wa e up ake beha io s, especially a e 100 min o wa e ex-
posu e. Acco ding o Gu e al. [33], when chemically compa ible ille s a e inco po a ed
in o he RPUF sys em, he e is a o ma ion o new small bubbles, which may encapsula e
blowing gas and impai wa e illing. Besides ha , Członka e al. [37] s a ed ha he mo -
phological cha ac e is ics o he cellula s uc u e o RPUFs a e he main in luen ial ac o
due o hei hyd ophobic/hyd ophilic cha ac e . In his sense, as discussed abo e, he an-
iso opic cells impa ed by he ille s may be in luenced by he ob ained wa e up ake
esul s. In all, e en a simila mois u e up ake can be conside ed a a o able esul o a
illed RPUF since abso bed wa e may ha e a de imen al e ec on he mechanical, he -
mal, and acous ic p ope ies o he RPUF.
Figu e 7. TG cu es o he s udied ille s and RPUFs.
Table 2. Main TG e en s o he s udied ille s and RPUFs.
G oup T2% (◦C) T5% (◦C) T50% (◦C) Residue a 600
◦C (%)
RPUF 84.88 114.81 500.58 21.26
RPUFW91.38 125.24 504.69 25.94
RPUFW10 89.32 187.31 514.94 33.88
RPUFW15 67.27 94.96 494.59 29.23
W 91.55 108.65 342.40 15.40
W10 97.70 112.76 355.23 16.77
W15 108.65 113.10 351.98 17.85
The TG esul s o he RPUFs con i m he FTIR esul s and again indica e ha all he
s udied RPUFs ha e a simila chemical composi ion, which also means ha he inco po-
a ed ille s did no inse a signi ican chemical modi ica ion in he RPUF cell s uc u e.
This can also be explained by a sligh une en dis ibu ion o he ille in he RPUF ma ix
since he sample p epa ed o his analysis is oo small. The e o e, he di e en c osslinking
densi ies indica ed by he we -chemical and FTIR esul s we e no de ec ed by he TG
analysis. The cha ac e is ic empe a u es we e de ined as T
2%
( empe a u e a ibu ed o 2%
o weigh loss), T
5%
( empe a u e a ibu ed o 5% o weigh loss), and T
50%
( empe a u e
a ibu ed o 50% o weigh loss).
