Sustainable Epoxy Composites with UV Resistance Based on New Kraft Lignin Coatings

Ci a ion: Seoane-Ri e o, R.;
A es-Elejos e, P.; Gond a, K.; Amini,
S.; de Hoyos, P.-L.; Gonzalez-Al iols,
M. Sus ainable Epoxy Composi es
wi h UV Resis ance Based on New
K a Lignin Coa ings. Molecules 2024,
29, 3697. h ps://doi.o g/10.3390/
molecules29153697
Academic Edi o : Jin Li
Recei ed: 12 July 2024
Re ised: 1 Augus 2024
Accep ed: 2 Augus 2024
Published: 5 Augus 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/).
molecules
A icle
Sus ainable Epoxy Composi es wi h UV Resis ance Based on
New K a Lignin Coa ings
Rubén Seoane-Ri e o 1,* , Pa icia A es-Elejos e 1, Koldo Gond a 1, Sa a Amini 2, Ped o-Luis de Hoyos 2
and Ma ia Gonzalez-Al iols 2
1GAIKER Technology Cen e, Basque Resea ch and Technology Alliance (BRTA), Pa que Tecnológico de
Bizkaia, Edi icio 202, 48170 Zamudio, Spain
2Bio e ine y P ocesses Resea ch G oup, Chemical & En i onmen al Enginee ing Depa men , Facul y o
Enginee ing, Gipuzkoa, Uni e si y o he Basque Coun y UPV/EHU, Plaza Eu opa 1,
20018 Donos ia-San Sebas ián, Spain
*Co espondence: seoane@gaike .es
Abs ac : Cu en ly, he composi e indus y is ocusing on mo e en i onmen ally iendly esou ces
in o de o gene a e a new ange o biobased ma e ials. In his manusc ip , we p esen a new wo k
using lignocellulosic was es om he pape indus y o inco po a e in o biobased epoxy sys ems. The
manu ac u ed ma e ials we e composed o k a lignin, glass ibe , and a sus ainable epoxy sys em,
ob aining a 40% biobased con en . Using a acuum in usion p ocess, we ab ica ed he composi es
and analyzed hei mechanical and UV esis ance p ope ies. The indings e eal a signi ican
co ela ion be ween he lignin con en and lexu al modulus and s eng h, showing an inc ease o
69% in he lexu al modulus and 134% in he lexu al s eng h wi h he p esence o 5% o lignin
con en . Mo eo e , i is necessa y o highligh ha he p esence o syn hesized lignin inhibi s he UV
deg ada ion o he biobased epoxy coa ing. We p opose ha he use o lignocellulosic-based was es
could imp o e he mechanical p ope ies and gene a e UV esis ance in he composi e ma e ials.
Keywo ds: sus ainable; composi e; epoxy; biobased; UV esis ance; k a lignin
1. In oduc ion
The need o de elop ma e ials in e ms o s eng h, s abili y, ligh ness, and cos
has been d i en by indus y and has gene a ed apid g ow h and s ong demand om
cus ome s in di e en sec o s, such as he au omo i e [
1
,
2
], wind [
3
,
4
], and ain sec o s [
5
].
Recen ly, he e has also been an inc eased ocus on he sus ainabili y a iable and ha is
why, nowadays, mo e and mo e esea che s a e wo king in biobased ma e ials [
6
–
8
]. In
addi ion o mo e sus ainable esin sys ems, he use o enewable addi i es in composi e
ma e ials has been conside ed in ecen yea s, in o de o imp o e p ope ies and inc ease
he biobased con en . Composi es a e ma e ials made om wo o mo e cons i uen
ma e ials wi h signi ican ly di e en physical o chemical p ope ies. When combined,
hese cons i uen s p oduce a ma e ial wi h cha ac e is ics di e en om he indi idual
componen s [
9
]. The impo ance o composi es in ma e ials science lies in hei abili y o
o e enhanced pe o mance p ope ies, such as highe s eng h, ligh e weigh , imp o ed
du abili y, and esis ance o en i onmen al deg ada ion. These enhanced p ope ies make
composi es c i ical in a ious applica ions, including he ae ospace, au omo i e, and
cons uc ion indus ies and o spo ing goods [10].
Epoxy composi es, a subse o his amily o ma e ials, a e pa icula ly signi ican due
o hei excellen mechanical p ope ies and s ong adhesi e capabili ies. They a e o med
by an epoxy esin ma ix, which is usually ein o ced wi h di e en ypes o addi i es [
11
].
The esul ing ma e ial gene ally exhibi s high s eng h- o-weigh a ios, excellen he mal
s abili y, and supe io esis ance o chemicals and mois u e [
12
]. Th oughou his wo k, we
ha e wo ked wi h biobased gelcoa and epoxy esins. Al hough hei p ice in compa ison
Molecules 2024,29, 3697. h ps://doi.o g/10.3390/molecules29153697 h ps://www.mdpi.com/jou nal/molecules
Molecules 2024,29, 3697 2 o 12
o o he polyme sys ems a e conside ably highe , i is impo an o no e hei no el y and
he enewable aw ma e ials used o hei syn hesis. Fo example, he biobased gelcoa is
o mula ed wi h 51% biobased ca bons, educing hei ca bon oo p in wi hou comp o-
mising pe o mance. Mo eo e , i should be no ed ha hey show e y good p ope ies
compa ed o o he esins, such as polyes e esins. T adi ionally, he addi i es inco po a ed
in o he epoxy ma ix a e de i ed om pe ochemical sou ces, which a e non- enewable
and con ibu e o ca bon emissions. Consequen ly, he in oduc ion o biobased p oduc s
in o epoxy composi es is essen ial o add ess he g owing need o sus ainable ma e ials
in esponse o en i onmen al conce ns [
13
]. In ac , by inco po a ing biobased ma e ials,
such as plan -de i ed oils, s a ches, o lignocellulosic componen s, he en i onmen al
impac can be signi ican ly educed. Mo eo e , hese biobased ma e ials o e se e al
ad an ages, namely, a educ ion in he ca bon oo p in , enhanced biodeg adabili y, and
esou ces enewabili y.
Lignocellulosic molecules, pa icula ly lignin, play a c ucial ole in he de elopmen
o sus ainable epoxy composi es. Lignin is a na u al polyme ound in he cell walls o
plan s and is a majo byp oduc o he pape and bioe hanol indus ies [
14
]. Fu he mo e,
i s in eg a ion in o epoxy composi es o e s se e al bene i s, ega ding he p oduc ion
p ocess and he ma e ial p ope ies. On he one hand, lignin is one o he mos abundan
o ganic polyme s on Ea h ensu ing a eadily a ailable and sus ainable aw ma e ial sou ce,
and i is ela i ely inexpensi e, po en ially educing he o e all cos o he composi e
p oduc ion [
15
]. On he o he hand, his na u al polyme can enhance he mechanical
s eng h and igidi y o epoxy composi e ma e ials. Fu he mo e, i s a oma ic s uc u e
con ibu es o be e he mal s abili y o his ype o composi es, making hem sui able o
high- empe a u e applica ions [
16
]. Acco dingly, in ecen yea s, he inco po a ion o lignin
in o epoxy ma ix-de i ed composi es has a ac ed a g ea deal o a en ion. Thus, he
syn hesis o epoxy esins by eac ing epichlo ohyd in wi h lignin has been widely epo ed
in he li e a u e [
17
,
18
]. In his case, he lignin is an en i onmen ally iendly sou ce o
hyd oxyl g oups, which can eplace he use o o he syn he ic polyme s such as bis phenol
A (BPA). Howe e , a common issue in he subs i u ion o BPA wi h lignin can be i s high
polydispe si y index, high molecula weigh , and low solubili y in some o ganic sol en s
and wa e [
19
]. To o e come his challenge, some s udies ha e ocused on he u iliza ion
o low-molecula weigh lignin o lignin-de i ed oligome s [
20
,
21
]. In addi ion o ha ,
ano he app oach employed is he p e ea men o he lignin, i.e., i s chemical modi ica ion.
In his ega d, a ious lignin modi ica ion eac ions a e implemen ed, e.g., phenola ion [
22
],
deme hyla ion [23], es e i ica ion [24], glycida ion [25], and amina ion [26].
Conce ning he p ope ies o he esul ing epoxy–lignin composi es, once he p e i-
ously men ioned hu dles a e sol ed, i can be con i med in gene al ha he inco po a ion
o lignin esul s in epoxy composi es wi h enhanced mechanical and he mal p ope ies.
In a ecen s udy by Xue e al. 2021, he elabo a ion o epoxy composi es wi h di e en
deg ees o subs i u ion o BPA wi h lignin was p esen ed, and hey showed he men ioned
imp o ed endency ega ding he mechanical pe o mance o hese ma e ials. Thus, hey
ound ha he composi es wi h 100% o lignin we e he ones showing he highes alues
o ensile s eng h [
27
]. Ano he wo k by Zhen e al. 2021 p o ed ha he in oduc ion o
lignin no only achie ed highe alues o lexu al s eng h and s o age modulus bu also
p omo ed a ce ain deg ee o lame e a dancy in he esins [
28
]. In addi ion o yielding
some le el o i ep oo ing, lignin added in o he epoxy composi es p oduced an inc e-
men in he he mal p ope ies as well. The eby, i has ecen ly been epo ed by X. Wang
e al. [
29
] ha an inc ease in he amoun o lignin in di e en epoxy esin o mula ions
leads o a highe pe cen age o esidue a e he mog a ime ic analysis. Likewise, hey
showed ha he o mula ion wi h he highe amoun o lignin was he one wi h he highes
alue o T50%. In his wo k, a biobased epoxy gelcoa was combined wi h lignin in o de
o ab ica e new sus ainable composi es wi h UV esis ance capabili ies. We syn hesized
new k a lignin om he Zicuñaga pulp and pape indus y (Gipuzkoa, Spain), which was
in eg a ed in o he coa ing o he composi e. Glass ibe was used as ein o cemen and
Molecules 2024,29, 3697 3 o 12
epoxy esin wi h 38% o biobased con en was used in o de o manu ac u e he composi e
by an in usion p ocess.
2. Resul s
The KL sample’s cha ac e iza ion by a GPC analysis indica ed ha he lignin p esen ed
a molecula weigh (Mw) o 4936 g/mol and a polydispe si y alue (Mw/Mn) o 4102,
which means ha he lignin p esen ed a medium molecula weigh and polydispe si y
alue, sui able o being used in a wide a ie y o o mula ions [30].
The cha ac e iza ion o he KL samples in e ms o he hyd oxyl g oups’ con en ,
conduc ed wi h a 31P NMR spec oscopy, p esen ed he esul s included in Table 1. The
concen a ions o he OH g oups in alipha ic, C5-subs i u ed, guaiacyl, p-hyd oxyphenyl,
and ca boxylic acid o ms we e measu ed (mmol OH/g lignin uni s). The sy ingyl/guaiacyl
(S/G) monome s’ a io was also calcula ed by di iding he OH g oups o C5 subs i u es
(conside ed as he sy ingyl OH g oups as a esul o signal o e lapping) in o guayacyl
OH g oups. This a io p o ides in o ma ion abou he numbe o
β
-O-4 bonds p esen
in he lignin ne wo k. The o ma ion o a monome ic uni equi es he clea age o wo
β
-O-4 bonds, one on each side o he a oma ic moie y. This means ha , du ing he lignin
biosyn hesis, G uni s end o o m C–C bonds, esul ing in a dec ease in he o ma ion o
he e he bonds ha gene a e monome s.
Table 1. Con en o di e en hyd oxyl g oups in KL samples (mmol/g lignin) ob ained a e
RNM analysis.
Alipha ic OH C5-Subs i u ed OH Guaiacyl OH
p-Hyd oxyphenyl OH Ca boxylic Acid OH
S/G Ra io
2.04 4.63 0.28 1.56 0.28 16.38
The ob ained esul s a e in good conco dance wi h he biomass sou ce (ha dwood),
due o he low con en o p-hyd oxyphenyl uni s, which appea mainly in coni e ous wood
ypes and he baceous species. The alue o he S/G a io, much bigge han one, also
co esponded o a ha dwood species [
31
]. The esul s o he elemen al analysis o he KL
sample and SGi128 + KL (5% w .) coa ing sample a e included in Table 2.
Table 2. Resul s o he elemen al analysis.
Sample % C % H % N % S % O (by Di e ence)
KL (5% w .) 61.63 5.63 0.13 6.60 26.01
KL + SGi128 41.11 6.16 14.15 2.56 36.02
The esul s o he elemen al composi ion e eal a high ni ogen pe cen age p o ided
by he coa ing o mula ion. The FTIR spec a o he KL (5% w .) sample, SGi128 coa ing
sample, and KL (5% w .) + SGi128 coa ing sample a e p esen ed in Figu e 1.
The main bands iden i ied in he inge p in egion a e classi ied in Table 3.
The bands ela ed o he G and S s uc u es a e cohe en wi h he aw ma e ial ype
(ha dwood), and he spec um o he lignin plus gelcoa shows a good in eg a ion o lignin
in he ma ix.
DSC es s we e ca ied ou o cha ac e ize he he mal ansi ions o he polyme ic
sys em and he e ec o he lignin in he gelcoa . The ob ained esul s can be ound in
Table 4. In he analyzed sys ems, in he i s un om an ambien empe a u e o 280
◦
C, an
exo he mic peak wi h a maximum exo he mic empe a u e was ob ained, due o he cu ing
eac ion o bo h epoxy sys ems.
Molecules 2024,29, 3697 4 o 12
Molecules2024,29,xFORPEERREVIEW4o 12



Figu e1.In a edspec aob ainedbyFTIRanalysis.
Themainbandsiden i iedin he inge p in  egiona eclassi iedinTable3.
Table3.Iden i ica iono  hebandsin he inge p in  egion om hek a ligninFTIR[32,33].
Wa enumbe (cm−1)BandAssignmen 
3450S e chingo  healipha icO–Hbond
2934,2831C–Hs e chinme hylandme hyleneg oups
1600A oma icskele on ib a ionsplusC–Os e ching
1510A oma ic ib a ionso Guni s
1574A oma icskele on ib a ionsplusC–Os e ching
1458C–Hde o ma ions(asymme icin–CH3and–CH2–)
1423A oma icskele on ib a ionscombinedwi hC–Hinplanede o -
ma ions
1383Alipha icC–Hs e chinginCH3andphenolicOH
1363Alipha icC–Hs e chinginCH3andphenolicOH
1299G ingplusC+Os e ching
1245C–C
,
C–O
,
andC=Os e ching
1183Typical o HGSligninsamples;C–Oines e g oups
1099A oma icC–Hde o ma iono Suni s
1038A oma icC–Hinplanede o ma ionplusC–Ode o ma ioninp i-
ma yalcoholsplusC–Hs e ching(unconjuga ed)
911C–Hou o plane(a oma ic ings)
827C–Hou o planeinposi ions2,5,and6(Guni s)
Thebands ela ed o heGandSs uc u esa ecohe en wi h he awma e ial ype
(ha dwood),and hespec umo  heligninplusgelcoa showsagoodin eg a iono lignin
in hema ix.
DSC es swe eca iedou  ocha ac e ize he he mal ansi ionso  hepolyme ic
sys emand heeffec o  heligninin hegelcoa .Theob ained esul scanbe oundin
Table4.In heanalyzedsys ems,in he i s  un omanambien  empe a u e o280°C,
anexo he micpeakwi hamaximumexo he mic empe a u ewasob ained,due o he
cu ing eac iono bo hepoxysys ems.
Figu e 1. In a ed spec a ob ained by FTIR analysis.
Table 3. Iden i ica ion o he bands in he inge p in egion om he k a lignin FTIR [32,33].
Wa enumbe (cm−1) Band Assignmen
3450 S e ching o he alipha ic O–H bond
2934, 2831 C–H s e ch in me hyl and me hylene g oups
1600 A oma ic skele on ib a ions plus C–O s e ching
1510 A oma ic ib a ions o G uni s
1574 A oma ic skele on ib a ions plus C–O s e ching
1458 C–H de o ma ions (asymme ic in –CH3and –CH2–)
1423 A oma ic skele on ib a ions combined wi h C–H in plane
de o ma ions
1383 Alipha ic C–H s e ching in CH3and phenolic OH
1363 Alipha ic C–H s e ching in CH3and phenolic OH
1299 G ing plus C+O s e ching
1245 C–C, C–O, and C=O s e ching
1183 Typical o HGS lignin samples; C–O in es e g oups
1099 A oma ic C–H de o ma ion o S uni s
1038 A oma ic C–H in plane de o ma ion plus C–O de o ma ion in
p ima y alcohols plus C–H s e ching (unconjuga ed)
911 C–H ou o plane (a oma ic ings)
827 C–H ou o plane in posi ions 2, 5, and 6 (G uni s)
Table 4. The mal p ope ies o di e en epoxy sys ems.
Fo mula ion Tm (◦C) ∆H (J/g)
In ug een 810 esin + SD8822 ha dene 122 496
SGi128 coa ing + SD228 ha dene 99 225
SGi128 coa ing + SD228 ha dene + lignin (5% w .)
97 210
All he polyme sys ems showed a single T
max
on he DSC measu emen s, as can be
seen in Figu e 2. I is impo an o no e ha hese exo he mic peaks we e be ween app ox-
ima ely 97
◦
C and 120
◦
C. Once he lignin was in oduced, he exo he mic empe a u e
dec eased by a ound 20
◦
C. The e o e, i can be conside ed ha lignin has no e ec on ei he
he exo he mic empe a u e o he cu ing p ocess. This esul is expec ed, because he e a e
some s udies ha demons a e ha lignin has li le impac on he mal p ope ies [34,35].
Molecules 2024,29, 3697 5 o 12
Molecules2024,29,xFORPEERREVIEW5o 12


All hepolyme sys emsshowedasingleTmaxon heDSCmeasu emen s,ascanbe
seeninFigu e2.I isimpo an  ono e ha  heseexo he micpeakswe ebe weenapp ox-
ima ely97°Cand120°C.Once heligninwasin oduced, heexo he mic empe a u e
dec easedbya ound20°C.The e o e,i canbeconside ed ha ligninhasnoeffec on
ei he  heexo he mic empe a u eo  hecu ingp ocess.This esul isexpec ed,because
he ea esomes udies ha demons a e ha ligninhasli leimpac on he malp ope ies
[34,35].

Figu e2.The malp ope ieso polyme sys ems.
Table4.The malp ope ieso diffe en epoxysys ems.
Fo mula ionTm(°C)ΔH(J/g)
In ug een810 esin+SD8822ha dene 122496
SGi128coa ing+SD228ha dene 99225
SGi128coa ing+SD228ha dene +lignin(5%w .)97210
Themechanicalp ope ieso  hemanu ac u edcomposi esa eshowninFigu e3.
Ascanbeseenin hea o emen ioned igu e,ligningi esbe e mechanicalp ope ies o
hecomposi es’s uc u es.Mo eo e ,as helignincon en inc eases, he lexu als eng h
andmodulusalsoimp o esigni ican ly.The esul scon i m ha  he lexu almodulus
inc easesa ound31%wi h hep esenceo 2.5%w .lignincon en and69%wi h5%w .
Rega ding he lexu als eng h, hiseffec ismo eno iceable,asi inc easesa ound34%
wi h hep esenceo 2.5%w .lignincon en and134%wi h5%w .This ac shows ha 
hein oduc iono  hisbiomassaddi i ehasasigni ican imp o emen on he lexu al
p ope ies.Thiscanbeanindica o o  he ein o cemen effec o ligninin hebiobased
gelcoa .No muchin o ma ionis oundin heli e a u eon hemechanicalp ope ieso 
he mose composi escon aininglignin,bu , o example,WuZhee .al.analyzedhow
headdi iono lignin ibe changed hecomp essioncha ac e is icso  hema ixma e ial,
esul ingin heappea anceo as eng henings age[36].
Figu e 2. The mal p ope ies o polyme sys ems.
The mechanical p ope ies o he manu ac u ed composi es a e shown in Figu e 3.
As can be seen in he a o emen ioned igu e, lignin gi es be e mechanical p ope ies o
he composi es’ s uc u es. Mo eo e , as he lignin con en inc eases, he lexu al s eng h
and modulus also imp o e signi ican ly. The esul s con i m ha he lexu al modulus
inc eases a ound 31% wi h he p esence o 2.5% w . lignin con en and 69% wi h 5% w .
Rega ding he lexu al s eng h, his e ec is mo e no iceable, as i inc eases a ound 34%
wi h he p esence o 2.5% w . lignin con en and 134% wi h 5% w . This ac shows ha
he in oduc ion o his biomass addi i e has a signi ican imp o emen on he lexu al
p ope ies. This can be an indica o o he ein o cemen e ec o lignin in he biobased
gelcoa . No much in o ma ion is ound in he li e a u e on he mechanical p ope ies o
he mose composi es con aining lignin, bu , o example, Wu Zhe e . al. analyzed how
he addi ion o lignin ibe changed he comp ession cha ac e is ics o he ma ix ma e ial,
esul ing in he appea ance o a s eng hening s age [36].
Molecules2024,29,xFORPEERREVIEW6o 12



Figu e3.Mechanicalp ope ieso composi es.
Rega ding heUVeffec on hemechanicalp ope ies,i canbesaid ha  hesamples
didno suffe highchangesa e deg ada ion,because hisa ackis ocusedon hegelcoa .
I wasobse ed ha  hesampleswi hou lignininc eased hei modulusands eng h,
whe eas hesampleswi h hisbio ille a a5%w .con en showedhighe  alueso  hese
pa ame e sa e UVi adia ion,asobse edinp e iouss udies[37].I is ema kable ha 
hemechanicalp ope ieso  heKL(5%w .)samples epo edbe e  alues han heones
ha didno con ainligninanddidno suffe  heUVa ack.Wi h hegoalo cha ac e izing
heUV esis ancecapabili yo  hemanu ac u edcomposi esin hep esenceo lignincon-
en , heb igh nessandcolo we eanalyzed.Figu e4shows heb igh ness a ia ionand
colo changes.
Ascanbeseen, heeffec o UVa ackandwa e condensa iona 50°Cismo esig-
ni ican in hecolo  hanin heb igh nessp ope ies.Thismaybebecause hesamples
we ema andwhi eincolo .

Figu e4.Colo (le )andb igh ness( igh ) a ia iono  ab ica edcomposi es.
Mo eo e ,i isimpo an  ono e ha  hecolo  a ia iondec easeswi h helignin
con en ,soi canbeconcluded ha  heeffec o ligninisconside ableinUV esis ance,
dec easing heΔE(colo )bymo e han10be ween he e e enceand hecomposi ewi h
KL(5%w .).
Ascanbeobse edinFigu e5, hecolo changesa e isible o henakedeye,due o
heyellowingcausedbyUV adia ion.I shouldbe akenin oaccoun  ha  he eisano-
ablediffe encein hecolo be ween hesamplewi hou lignincon en and hesample
ha con ains5%w .Ino de  ocha ac e ize heeffec o UV ea men onsu ace
Figu e 3. Mechanical p ope ies o composi es.
Rega ding he UV e ec on he mechanical p ope ies, i can be said ha he samples
did no su e high changes a e deg ada ion, because his a ack is ocused on he gelcoa .
I was obse ed ha he samples wi hou lignin inc eased hei modulus and s eng h,
whe eas he samples wi h his bio ille a a 5% w . con en showed highe alues o hese
pa ame e s a e UV i adia ion, as obse ed in p e ious s udies [
37
]. I is ema kable
ha he mechanical p ope ies o he KL (5% w .) samples epo ed be e alues han
he ones ha did no con ain lignin and did no su e he UV a ack. Wi h he goal o

Molecules 2024,29, 3697 6 o 12
cha ac e izing he UV esis ance capabili y o he manu ac u ed composi es in he p esence
o lignin con en , he b igh ness and colo we e analyzed. Figu e 4shows he b igh ness
a ia ion and colo changes.
Molecules2024,29,xFORPEERREVIEW6o 12



Figu e3.Mechanicalp ope ieso composi es.
Rega ding heUVeffec on hemechanicalp ope ies,i canbesaid ha  hesamples
didno suffe highchangesa e deg ada ion,because hisa ackis ocusedon hegelcoa .
I wasobse ed ha  hesampleswi hou lignininc eased hei modulusands eng h,
whe eas hesampleswi h hisbio ille a a5%w .con en showedhighe  alueso  hese
pa ame e sa e UVi adia ion,asobse edinp e iouss udies[37].I is ema kable ha 
hemechanicalp ope ieso  heKL(5%w .)samples epo edbe e  alues han heones
ha didno con ainligninanddidno suffe  heUVa ack.Wi h hegoalo cha ac e izing
heUV esis ancecapabili yo  hemanu ac u edcomposi esin hep esenceo lignincon-
en , heb igh nessandcolo we eanalyzed.Figu e4shows heb igh ness a ia ionand
colo changes.
Ascanbeseen, heeffec o UVa ackandwa e condensa iona 50°Cismo esig-
ni ican in hecolo  hanin heb igh nessp ope ies.Thismaybebecause hesamples
we ema andwhi eincolo .

Figu e4.Colo (le )andb igh ness( igh ) a ia iono  ab ica edcomposi es.
Mo eo e ,i isimpo an  ono e ha  hecolo  a ia iondec easeswi h helignin
con en ,soi canbeconcluded ha  heeffec o ligninisconside ableinUV esis ance,
dec easing heΔE(colo )bymo e han10be ween he e e enceand hecomposi ewi h
KL(5%w .).
Ascanbeobse edinFigu e5, hecolo changesa e isible o henakedeye,due o
heyellowingcausedbyUV adia ion.I shouldbe akenin oaccoun  ha  he eisano-
ablediffe encein hecolo be ween hesamplewi hou lignincon en and hesample
ha con ains5%w .Ino de  ocha ac e ize heeffec o UV ea men onsu ace
Figu e 4. Colo (le ) and b igh ness ( igh ) a ia ion o ab ica ed composi es.
As can be seen, he e ec o UV a ack and wa e condensa ion a 50
◦
C is mo e
signi ican in he colo han in he b igh ness p ope ies. This may be because he samples
we e ma and whi e in colo .
Mo eo e , i is impo an o no e ha he colo a ia ion dec eases wi h he lignin
con en , so i can be concluded ha he e ec o lignin is conside able in UV esis ance,
dec easing he
∆
E (colo ) by mo e han 10 be ween he e e ence and he composi e wi h
KL (5% w .).
As can be obse ed in Figu e 5, he colo changes a e isible o he naked eye, due o
he yellowing caused by UV adia ion. I should be aken in o accoun ha he e is a no able
di e ence in he colo be ween he sample wi hou lignin con en and he sample ha
con ains 5% w . In o de o cha ac e ize he e ec o UV ea men on su ace oughness, a
con ocal mic oscopy was also used. The su ace opog aphy measu emen s we e conduc ed
be o e and a e he UV a ack.
Molecules2024,29,xFORPEERREVIEW7o 12


oughness,acon ocalmic oscopywasalsoused.Thesu ace opog aphymeasu emen s
we econduc edbe o eanda e  heUVa ack.

Figu e5.(a)Re e ence;(b) e e encea e UV ea men ;(c)KL(2.5%w .)samplea e UV ea -
men ;and(d)KL(5%w .)samplea e UV ea men .
Thesamples ha didno unde go heUVa ackshowedasimila  oughness,wi h
Sa alueso a ound1.6µm,ascanbeseeninFigu e6.Incon as ,a e suffe ingwa e 
condensa iona 50°CandUV adia ion, hesamplesinc eased hei  oughnessconside -
ably;in hecaseo  he e e ence, he oughnessinc easeda ound171%,while hesamples
ha con ainedligninshowedape cen agelowe :157% o  heKL(2.5%w .)sampleand
132%in hecaseo  heKL(5%w .)sample,ascanbeseeninTable5.Thepho odeg ada-
ioneffec washighe in hesampleswi hlesslignincon en .

Figu e6.(a)Re e ence;(b) e e encea e UV ea men ;(c)KL(2.5%w .)sample;(d)KL(2.5%w .)
samplea e UV ea men ;(e)KL(5%w .)sample;and( )KL(5%w .)samplea e UV ea men .
Figu e 5. (a) Re e ence; (b) e e ence a e UV ea men ; (c) KL (2.5% w .) sample a e UV ea men ;
and (d) KL (5% w .) sample a e UV ea men .
The samples ha did no unde go he UV a ack showed a simila oughness, wi h Sa
alues o a ound 1.6
µ
m, as can be seen in Figu e 6. In con as , a e su e ing wa e con-
densa ion a 50 ◦C and UV adia ion, he samples inc eased hei oughness conside ably;
in he case o he e e ence, he oughness inc eased a ound 171%, while he samples ha
con ained lignin showed a pe cen age lowe : 157% o he KL (2.5% w .) sample and 132%
in he case o he KL (5% w .) sample, as can be seen in Table 5. The pho odeg ada ion
e ec was highe in he samples wi h less lignin con en .
Molecules 2024,29, 3697 7 o 12
Molecules2024,29,xFORPEERREVIEW7o 12


oughness,acon ocalmic oscopywasalsoused.Thesu ace opog aphymeasu emen s
we econduc edbe o eanda e  heUVa ack.

Figu e5.(a)Re e ence;(b) e e encea e UV ea men ;(c)KL(2.5%w .)samplea e UV ea -
men ;and(d)KL(5%w .)samplea e UV ea men .
Thesamples ha didno unde go heUVa ackshowedasimila  oughness,wi h
Sa alueso a ound1.6µm,ascanbeseeninFigu e6.Incon as ,a e suffe ingwa e 
condensa iona 50°CandUV adia ion, hesamplesinc eased hei  oughnessconside -
ably;in hecaseo  he e e ence, he oughnessinc easeda ound171%,while hesamples
ha con ainedligninshowedape cen agelowe :157% o  heKL(2.5%w .)sampleand
132%in hecaseo  heKL(5%w .)sample,ascanbeseeninTable5.Thepho odeg ada-
ioneffec washighe in hesampleswi hlesslignincon en .

Figu e6.(a)Re e ence;(b) e e encea e UV ea men ;(c)KL(2.5%w .)sample;(d)KL(2.5%w .)
samplea e UV ea men ;(e)KL(5%w .)sample;and( )KL(5%w .)samplea e UV ea men .
Figu e 6. (a) Re e ence; (b) e e ence a e UV ea men ; (c) KL (2.5% w .) sample; (d) KL (2.5% w .)
sample a e UV ea men ; (e) KL (5% w .) sample; and ( ) KL (5% w .) sample a e UV ea men .
Table 5. Su ace oughness alues o e e ence, KL (2.5% w .) sample, and KL (5% w .) sample be o e
and a e he UV adia ion.
Sa (µm)
Re e ence KL (2.5% w .) KL (5% w .)
Ini ial 1.6 ±0.1 1.8 ±0.2 1.4 ±0.1
500 h UV 4.4 ±0.3 4.6 ±0.3 3.4 ±0.2
3. Ma e ials and Me hods
3.1. Epoxy Resin Sys em
A comme cial epoxy esin, In uG een 810, wi h 38% o biobased con en (a ca bon
o plan o igin) was used. The ha dene ha was used was SD 822, supplied by Sicomin
Epoxy Sys ems (F ance). This sys em shows a e y low iscosi y a oom empe a u e, and
i can be mixed wi h di e en ha dene s. The mos ele an physicochemical p ope ies o
he abo e-men ioned sys em a e shown in Table 6.
Table 6. Physicochemical p ope ies o he epoxy esin used.
P ope y Uni s Value
Viscosi y a 20 ◦C mPas 215.0
Densi y a 20 ◦Cg/cm31.2
Po li e a 20 ◦C 500 g. h 4.7
Molecules 2024,29, 3697 8 o 12
The applied coa ing was also supplied by Sicomn Epoxy Sys ems and was o mula ed
wi h 51% o he biobased gelcoa called SGi 128 combined wi h he SD 228 ha dene . This
is a halogen- ee and i e- esis an epoxy coa ing wi h low le els o smoke densi y and
gas oxici y. The ein o cemen used was a quad iaxial glass ibe wi h a g ammage o
450 g/m2.
3.2. K a Lignin
K a lignin (KL) was p ecipi a ed om black liquo s p o ided by he Zicuñaga pulp
and pape indus y (Gipuzkoa, Spain). This indus y p ocesses eucalyp us wood. K a
lignin is conside ed a medium–high-molecula weigh lignin wi h alues anging om
200 o 200,000 g/mol, high ca bohyd a e con en s due o condensa ion eac ions du ing
he k a p ocess, and 1–3% sul u con en s [
38
]. KL is p ecipi a ed by he addi ion o
acid, p o ona ing he phenolic compounds p esen in lignin, lowe ing hei hyd o ilia ion,
and p ecipi a ing as a esul . In his case, p ecipi a ion was ca ied ou by acidi ying he
liquo o a pH o 4 using glacial ace ic acid. Once he lignin p ecipi a ion was comple ed,
he solu ion was le o s and o 24 h. Then, a acuum il a ion was pe o med, and he
lignin sample eco e ed in he il a e, which was washed wi h dis illed wa e un il i was
neu alized. Finally, he eco e ed lignin was d ied o 2 h a 28 ◦C and s o ed.
3.3. K a Lignin Cha ac e iza ion
The lignin samples we e cha ac e ized by se e al echniques. A gel pe mea ion
ch oma og aphy/size exclusion ch oma og aphy (GPC) was used o de e mine he a e age
molecula weigh and size dis ibu ion o he lignins. A Jasco Inc. ch oma og aph wi h
an LC-Ne II/ACD in e ace, a CO-2065Plus o en column, and an RI-2031PlusIn elligen
Re ac i e Index de ec o was used. In addi ion, he ch oma og aph was equipped wi h a
gua d column and wo Pola Gel-M columns (Va ian Inc., Palo Al o, CA, USA) in se ies.
DMF was used as he mobile phase a a low a e o 0.7 mL/min and a empe a u e o 40
◦
C.
The ins umen was calib a ed wi h polys y ene s anda ds in he molecula weigh ange
o 250–70,000 g/mol. Fo he analysis, 25–50 mg o he sample was aken and dissol ed in
5 mL o he mobile phase.
One o he mos impo an p ope ies o lignin is he con en o i s hyd oxyl com-
pounds, as i ac s on solubili y and eac i i y. Consequen ly, he quan i ica ion o he
di e en OH g oups o he molecules is o g ea impo ance.
31
P NMR is a quan i a i e
de e mina ion by spec oscopy. Phospho yla ion was ca ied ou ollowed by phospho us
nuclea magne ic esonance (
31
P NMR) spec oscopy in B uke AVANCE II 400 equipmen
o analyze he con en o hyd oxyl g oups in he lignin samples.
An elemen al analysis was pe o med using a Leco T uSpec mic oanalyze a 1050
◦
C.
Bo h he ca ie gas (3X pu e helium) and es gas (4X ex a-pu e oxygen) we e supplied
by Nippon Gases. The calib a ion was pe o med using Leco Sul ame hanize (C = 51.78%;
H = 5.07%; and S = 11.5%). The samples we e analyzed in iplica e.
To e alua e he main s uc u al le el cha ac e is ics and unc ional g oups o lignin,
he cha ac e iza ion was pe o med wi h Fou ie ans o m in a ed (FTIR) spec oscopy
using a Pe kinElme Spec um Two FT-IR ins umen equipped wi h an a enua ed o al
e lec ance (ATR) accesso y and an in e nally e lec ing diamond glass lens. The samples
we e analyzed using 64 scans wi h a esolu ion o 4 cm
−1
and a wa e numbe ange o
4000–400 cm−1.
3.4. Biobased Resin Sys em and Gelcoa Fo mula ion Cha ac e iza ion
To de e mine he eac i i y o he esin sys em be o e using i in he in usion p ocess,
a he mal analysis was pe o med. In he same manne , he in luence o he lignin on he
eac i i y o he gelcoa sys em was s udied in o de o p e en i om inhibi ing he cu ing
a a concen a ion o 5% w .
The he mal ansi ions o he esin and gelcoa we e analyzed by a di e en ial scan-
ning calo ime y (DSC) on a Me le Toledo (DSC 822e) calo ime e equipped wi h a liquid
Molecules 2024,29, 3697 9 o 12
ni ogen accesso y. The samples we e weighed on a METTLER TG 50 4-digi analy ical
balance in 40
µ
L aluminum pans. These samples we e weighed in he liquid s a e (app oxi-
ma ely 10 mg), in o de o ob ain he maximum exo he mic empe a u e peak, T
max
, and
he hea o he eac ion. Wi h ega d o he ope a ing condi ions, he samples we e hea ed
om 25
◦
C o 280
◦
C a a a e o 10
◦
C
·
min
−1
and unde an ine ni ogen a mosphe e. The
da a in Table 4we e aken om he i s scan.
Once he o mula ion was cu ed, o obse e he e ec o lignin on he sys ems, es-
pecially a he le el o he elemen al composi ion and mo phology, a scanning elec on
mic oscopy (SEM), using a Zeis EVO 50 mic oscope a 20 kV and an ene gy dispe si e
X- ay (EDX) (INCA, Ox o d Ins umen s, Ox o d, Uni ed Kingdom), was conduc ed. The
cu ed o mula ions we e coa ed by a spu e Leica EM SCD005, be o e he measu emen s
we e conduc ed wi h gold palladium. Fu he , a 3D op ical p o ile PL
µ
NEOX (Senso a )
was used o he su ace opog aphy measu emen s.
3.5. Composi e De elopmen and Cha ac e iza ion
The biobased epoxy composi es we e de eloped using a acuum in usion p ocess.
This p ocess p o ides high-pe o mance composi es o di e en sec o s such as he wind,
ae onau ic, au omo i e, and ailway sec o s. The acuum in usion echnique is as ep e-
sen ed in Figu e 7.
Molecules2024,29,xFORPEERREVIEW10o 12


sys em, he esinwasinjec edun il he ibe wasp ope lyimp egna edandcu ed,acco d-
ing o hemanu ac u e ’scu ingandpos -cu ingcondi ions.

Figu e7.Schemeo  he acuumin usion echnique.
A e  hecomposi ewasmanu ac u ed,se e al es s,suchasmechanical es s,aUV
a ack,andcolo andb igh nesscha ac e iza ions,we eca iedou .
Themechanicalp ope ieswe e ocusedon he lexu al es s.These es swe epe -
o medinacco dancewi hISO14125:1998/A1:2011[39].Asuppo spacingo 50mmand
a es speedo 1mm/minwe eused.Sampleso dimensions15×2.9mmwe ep epa ed
andloaded o ailu e.Themeasu emen swe eca iedou wi haloadcello 5kN.Fo 
eachcomposi e, h eesampleswe e es edand hea e age aluewas epo ed.
TheUVa ackwaspe o medacco ding oUNEENISO4892-3:2016(Me hodA)[40],
which equi esa o al imeo 500h.Thesampleswe esubjec ed ocycleso 8ho UV
exposu ea 70°Cwi h340nmUVAlamps, ollowedby4ho wa e condensa iona 50
°C.
Thecolo measu emen swe epe o medacco ding oUNEENISO11664-1:2011
[41].A e  hemeasu emen , hecolo ime e  eco ded heL,a,andbpa ame e s.Toob-
ain hecolo  a ia ionasexp essedin he esul ssec ion,ase ieso  ans o ma ionsde-
ailedin hes anda dswe eused.Mo eo e ,glossmeasu emen swe epe o medacco d-
ing o heUNE-ENISO2813:1999s anda d[42].Thisme hoduses h eeangleso inci-
denceo  heligh beam:20°,60°,and85°.Fo simpli ica ion, hemeasu emen sob ained
wi h he60°anglewe ecompa ed.Anelemen alanalysiso  hecomposi ewaspe o med
using heequipmen andcondi ionsindica edin helignincha ac e iza ionsec ion.
4.Conclusions
Nowadays,i canbesaid ha  he eisin e es  om hecomposi esindus yinob-
ainingnewsus ainableal e na i es odec ease heen i onmen alimpac o composi e
ma e ials.Thismanusc ip in oduces heconcep o usinglignocellulosic-basedwas es
om hepape indus yand,on heo he hand, heinco po a iono biobasedepoxysys-
ems( esinandgelcoa ),ino de  ogene a enewsus ainableepoxycomposi es.
In hismanusc ip ,wep esen awo kusinganewk a lignin om heZicuñaga
pulpandpape indus y(Gipuzkoa,Spain),whichwasinco po a edin oabiobased
epoxysys em.Fi s ly, h oughaGPC,elemen alanalysis,andRMNcha ac e iza ion,we
ensu ed ha  hesyn hesiswasingoodconco dancewi h hebiomasssou ce.Secondly,i 
isimpo an  ono e hesigni ican imp o emen o  hemechanicalp ope iesa e in o-
ducingsyn he izedligninin hecoa ingo  hecomposi e.These esul sshow ha  he
p esenceo 5%o lignincon en inc eased he lexu almodulusby69%and he lexu al
s eng hby134%.TheUV esis ancewasalsocha ac e izedbycolo andb igh ness es s
andacon ocalmic oscopy.While heb igh ness es didno showanydiffe encebe ween
he e e enceandsampleswi h2.5and5%o lignincon en , hecolo and oughnesspa-
ame e sdemons a ed ha  hep esenceo lignininhibi ed heUVdeg ada iono  he
biobasedepoxycoa ing.Conside ing hep esen eda gumen sand esea ch indings,i 
canbeconcluded ha wedemons a e ha  heuseo lignocellulosic-basedwas escould
Figu e 7. Scheme o he acuum in usion echnique.
Fi s ly, he elease agen was applied on he subs a e ( he molding pla e), in o de
o acili a e he subsequen demolding o he composi e. Then, a laye o gelcoa (SGi 128
combined wi h he SD228 ha dene ) and di e en pe cen ages o lignin (2.5 and 5% w .)
we e deposi ed on he glass subs a e and, a e 4 h, when he gelcoa was in a acky s a e,
i e laye s o ibe glass we e s acked. Once s acked, a peel ply was placed o acili a e he
demolding o he ma e ial a e he p ocess. A dis ibu ion mesh was hen placed o help o
achie e a co ec and homogeneous imp egna ion o he ibe . Nex , wo ubes we e placed
in he sys em, one o he acuum and he o he o he esin injec ion, and hen he sys em
was sealed wi h a ilm (comp ising he coun e -mold). A e assembling he sys em, he
esin was injec ed un il he ibe was p ope ly imp egna ed and cu ed, acco ding o he
manu ac u e ’s cu ing and pos -cu ing condi ions.
A e he composi e was manu ac u ed, se e al es s, such as mechanical es s, a UV
a ack, and colo and b igh ness cha ac e iza ions, we e ca ied ou .
The mechanical p ope ies we e ocused on he lexu al es s. These es s we e pe -
o med in acco dance wi h ISO 14125:1998/A1:2011 [
39
]. A suppo spacing o 50 mm and
a es speed o 1 mm/min we e used. Samples o dimensions 15
×
2.9 mm we e p epa ed
and loaded o ailu e. The measu emen s we e ca ied ou wi h a load cell o 5 kN. Fo
each composi e, h ee samples we e es ed and he a e age alue was epo ed.
The UV a ack was pe o med acco ding o UNE EN ISO 4892-3:2016 (Me hod A) [
40
],
which equi es a o al ime o 500 h. The samples we e subjec ed o cycles o 8 h o UV
exposu e a 70
◦
C wi h 340 nm UVA lamps, ollowed by 4 h o wa e condensa ion a 50
◦
C.
The colo measu emen s we e pe o med acco ding o UNE EN ISO 11664-1:2011 [
41
].
A e he measu emen , he colo ime e eco ded he L, a, and b pa ame e s. To ob ain he