Ci a ion: A belaiz, A.; Yu amendi, T.;
La uscain, A.; A izabalaga, A.;
Eceiza, A.; Peña-Rod iguez, C.
P epa a ion and Cha ac e iza ion o
No el Poly(Lac ic Acid) Composi es
Rein o ced wi h “La xa” Sheep Wool
Fibe s: The E ec o Pe oxide Su ace
T ea men s and Fibe Con en .
Ma e ials 2024,17, 4912. h ps://
doi.o g/10.3390/ma17194912
Academic Edi o : Sukhoon Pyo
Recei ed: 11 Sep embe 2024
Re ised: 2 Oc obe 2024
Accep ed: 4 Oc obe 2024
Published: 8 Oc obe 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/).
ma e ials
A icle
P epa a ion and Cha ac e iza ion o No el Poly(Lac ic Acid)
Composi es Rein o ced wi h “La xa” Sheep Wool Fibe s: The
E ec o Pe oxide Su ace T ea men s and Fibe Con en
Ai o A belaiz *, Telmo Yu amendi, Ande La uscain, Ane A izabalaga , A an xa Eceiza and
C is ina Peña-Rod iguez *
‘Ma e ials + Technologies’ G oup (GMT), 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, 20018 Donos ia-San Sebas ian, Spain;
[email p o ec ed] (T.Y.); ande [email p o ec ed] (A.L.); [email p o ec ed] (A.A.);
[email p o ec ed] (A.E.)
*Co espondence: ai o [email p o ec ed] (A.A.); [email p o ec ed] (C.P.-R.)
Abs ac : “La xa” sheep wool is ough, and i is no used in he ex ile indus y because he ibe
diame e is high compa ed wi h o he wool ibe s. Nowadays, his wool is conside ed as disposal
and, wi h he aim o gi e i alue, new uses mus be explo ed. In he cu en wo k, he “La xa” sheep
wool ibe was e alua ed as poly(lac ic acid) (PLA) polyme ein o cemen . Wi h he objec i e o
op imize ibe /ma ix adhesion, ibe s we e su ace modi ied wi h pe oxide. Oxida ion ea men
wi h pe oxide led o chemical modi ica ions o he wool ibe s ha imp o ed he ibe /PLA adhesion,
bu he s eng h alues achie ed o he composi es we e lowe compa ed o he nea PLA ones.
The mechanical p ope ies ob ained in he cu en wo k we e compa ed wi h he li e a u e da a o
he PLA composi es ein o ced wi h ege able ibe s. The wool ibe s showed in e io mechanical
p ope ies compa ed o he ege able ibe coun e pa s. Howe e , he p elimina y esul s indica ed
ha he inco po a ion o wool ibe s o PLA educed he lammabili y o composi es.
Keywo ds: wool ibe ; poly(lac ic acid); composi e; mechanical p ope ies; i e esis ance
1. In oduc ion
The La xa sheep b eed, o iginally om Basque Coun y and Na a a, in he no h-
eas o Spain, is used o p oduce milk, om which di e en dai y p oduc s a e p oduced
such as Idiazabal Cheese, among o he s. A e he sheep a e shea ed, abou 2000 ons o
wool a e ob ained pe yea , being his wool conside ed as disposable. La xa sheep wool
canno be used in ex ile because i is ough and he ibe diame e is e y high compa ed
wi h o he wool ibe s used in ex ile indus y [
1
,
2
], and consequen ly o he uses mus
disco e ed o add alue o “La xa” sheep wool. The alue o he wool depends on ibe
coa seness, because ibe s wi h 35
µ
m diame e o highe ha dly ha e applica ions [
3
]. In
he li e a u e, he e a e epo ed wo ks whe e wool ibe s a e used o he p epa a ion
o
biocomposi es [4–15]
. Wool ibe shows in e es ing mechanical p ope ies and can be
used as ein o cemen in composi es. Fo example, Alzee and MacKenzie [
4
] inco po a ed
long wool ibe s in o a geopolyme ma ix. One limi a ion o geopolyme when used in
he cons uc ion and building sec o s is i s b i le ailu e mode unde applied o ce. A e
he inco po a ion o long wool ibe s he p epa ed composi es showed, in addi ion o an
imp o emen in lexu al s eng h, a ound a 40% mo e s able ac u e mode.
Fio e e al. [5]
p epa ed and s udied cemen mo a ein o ced wi h wool ibe s. They concluded ha
a e he inco po a ion o wool ibe s in o mo a s, he he mal insula ion p ope ies we e
enhanced bu he comp essi e s eng h dec eased. Salama e al. [
6
] p epa ed and cha -
ac e ized polyme composi es based on polyp opylene and ecycled wool mic o-powde .
They concluded ha he composi e based on polyp opylene (PP) and wool powde showed
Ma e ials 2024,17, 4912. h ps://doi.o g/10.3390/ma17194912 h ps://www.mdpi.com/jou nal/ma e ials
Ma e ials 2024,17, 4912 2 o 17
imp o ed p ope ies. Gama e al. [
7
] p epa ed composi es using polyu e hane esidues
and ex ile ibe esidues, among hem being wool ibe s. They obse ed ha a e he
inco po a ion o ex ile ibe esidues in o polyu e hane, he modulus alue inc eased
conside ably, al hough he s eng h and de o ma ion a b eak diminished. They sugges ed
ha hese composi es can ind many applica ions, in cons uc ion o au omo i e sec o s,
wi h he ad an age o being p oduced om 100% ecycled aw ma e ials. Alka eb e al. [
8
]
used wool as an ene gy abso be . They ab ica ed ellip ical ubes wi h composi es based on
wool wo en and epoxy esin. They obse ed ha he wool composi e wi h 30 w % ibe
con en abso bed he highes speci ic ene gy.
The lammabili y and low he momechanical s abili y o PLA limi s he expansion
o his polyme in many applica ions. In a p e ious wo k, he he momechanical s a-
bili y o PLA was imp o ed by he addi ion o ege able ibe s in combina ion wi h an
annealing p ocess [
16
]. Howe e , he pe o mance agains he i e o composi es based
on polyme ic ma ices and ege able ibe s is poo , and i e- e a dan and in umescen
sys ems should be inco po a ed [
17
–
20
]. The main d awbacks o mos o comme cial lame
e a dan s a e ha he p epa a ion p ocess in ol es heal hy isk and ha i is expensi e,
in addi ion o no being en i onmen ally sus ainable. Fu he mo e, usually, adding hem
o he polyme ic ma ix de e io a ed he mechanical p ope ies o he biocomposi es [
19
].
Shumao e al. [
17
] added ammonium polyphospha e lame e a dan o biocomposi es o
PLA and amie ibe s. They obse ed ha when lame- e a dan loading was 10.5 w %,
he s eng h dec eases conside ably. They sugges ed ha lame e a dan could hinde
he adhesion be ween PLA and amie ibe s. Bocz e al. [18] p epa ed lax ibe ein o ced
PLA/The moplas ic s a ch (TPS) biocomposi es wi h glyce ol phospha e plas icize . This
plas icize had a lame- e a dan e ec ; howe e , a e he addi ion o glyce ol phospha e
plas icize , he s eng h alue dec eased. They sugges ed ha he p esence o glyce ol
phospha e educed he compa ibili y be ween he cellulosic ibe s and he biopolyme ma-
ix. Shuko e al. [
20
] obse ed ha he lexu al s eng h was educed in kena ibe /PLA
biocomposi es a e he addi ion o ammonium polyphospha e lame e a dan . The e o e,
i is a challenge o p epa e no el biocomposi es wi h good mechanical p ope ies and
lame- e a dan p ope ies.
In he li e a u e, i was obse ed ha wool ibe s ha e be e i e esis ance han eg-
e able ones, and ha he p esence in wool ibe o sul u and ni ogen a oms, a ound 3 and
15 w %, espec i ely, leads o a highe i e esis ance han ege able ibe ones [
21
]. Wool
ibe o ms in umescen cha du ing he combus ion esul ing in low hea o combus ion
and also high limi ing oxygen index [
22
]. Wool showed he limi ing oxygen index (LOI) o
25, whe eas plan ibe s showed alues be ween 18 and 20 [
23
]. Najmah e al. [
9
] p epa ed
building blocks based on wool, sul u , and canola oil. They sugges ed ha he p esence o
wool gi es o he composi e a conside able lame esis ance as well as he abili y o esis
highe empe a u es. Mo eo e , a e he inco po a ion o wool, he modulus o elas ici y
inc eased compa ed o he un ein o ced coun e pa . On he o he hand, Kim e al. [
10
]
obse ed ha he i e- e a dan beha io was enhanced in he PP/wool ibe composi es
in compa ison wi h nea PP. Vasina e al.[
11
] p epa ed and cha ac e ized di e en poly-
me /sheep wool composi es. They obse ed ha an inc ease in he wool con en in he
composi es esul ed in he enhancemen o sound abso p ion p ope ies due o he highe
con e sion o acous ic ene gy in o hea . Tusnim e a al. [
12
] s udied he p ope ies o ju e
and sheep wool ibe - ein o ced hyb id polyp opylene composi es. They concluded ha
he mechanical p ope ies inc eased as he ibe loading was inc eased and ha he bes
esul s we e ob ained a 15% ibe loading wi h ju e and wool ibe a io o 3:1. In he
li e a u e, he e is one s udy whe e au ho s in es iga ed wool ibe embedded addi i e
manu ac u ing-based PLA s uc u es o biomedical applica ions [14]. E en hough some
ecen publica ions ea he wool ibe - ein o ced polyme composi es [
11
,
13
,
15
], o he
bes o ou knowledge no s udy has deal wi h PLA/wool ibe biocomposi es p epa ed by
injec ion molding.
Ma e ials 2024,17, 4912 3 o 17
In he cu en wo k, no el PLA/wool ibe composi es p epa ed by injec ion molding
we e cha ac e ized. On he o he hand, he mechanical esul s o p epa ed composi es we e
compa ed wi h a li e a u e su ey o PLA-based composi es ein o ced wi h ege able ibe s.
Finally, p elimina y lammabili y esul s o PLA/wool ibe composi es we e compa ed
wi h PLA composi es ein o ced wi h ege able ibe s.
2. Ma e ials and Me hods
2.1. Ma e ials
PLA used in he cu en wo k was IngeoTM 3051D (Plymou h, MN, USA) pu chased
by Na u eWo ks. Acco ding o he supplie , he mel low index is o 6 g/10 min a
210 ◦C
and i has a densi y o 1.24 g/cm
3
. “La xa” sheep wool ibe was p o ided by a local a me
om U nie a (Gipuzkoa, Spain). The diame e o wool ibe s can be highe han 100
µ
m [
2
]
and he densi y is a ound 1.24 g/cm3.
2.2. Wool Fibe T ea men s
The aw ibe s we e cleaned wi h a neu al soap in wa e a he empe a u e o 55
◦
C.
A e d ying he cleaned wool ibe s, hey we e dipped in hyd ogen pe oxide solu ion wi h
a concen a ion o 33%. A ound 35 g o wool ibe was dipped in 1 L o hyd ogen pe oxide
solu ion and di e en ea ing imes we e selec ed, 30 min and 24 h. A e he pe oxide
ea men , ibe s we e washed wi h abundan wa e and inally hey we e d ied.
2.3. Compounding and P ocessing o Ma e ials
PLA and wool ibe s we e d ied in an o en a 100
◦
C o 12 h. The ibe loading
a ied in he composi es om 5 o 30 w %. Fi s , d ied PLA pelle s we e mol en in a
HAAKE Rheomix 600 in e nal mixe (The mo Scien i ic, Ka ls uhe, Ge many) a 185
◦
C.
Once he polyme was mol en, he d ied wool ibe s we e inco po a ed, and he mix u e
was p ocessed o 10 min a 80 pm. The ob ained blends we e pelle ized and d ied in an
o en p io o p ocess by injec ion molding echnique using a HAAKE Minije II machine.
Injec ion was ca ied ou a 185
◦
C applying a p essu e o 650 ba . Tensile es specimens
(ASTM-D638-10, ype V) we e ob ained.
2.4. Cha ac e iza ion Techniques
2.4.1. Fou ie T ans o m In a ed Cha ac e iza ion
Di e ences in chemical composi ion be ween aw wool ibe and ea ed ibe s we e
obse ed by Fou ie ans o m in a ed spec oscopy (FTIR). FTIR spec a we e ob ained
in a Nexus 670 spec ome e (Nicole , Ma kham, ON, Canada) equipped wi h a MKII
Golden Ga e accesso y (Specac, O ping on, UK). The measu emen s we e aken in he
ange be ween 4000 and 650 cm−1wi h a esolu ion o 4 cm−1.
2.4.2. The mog a ime ic Analysis
The mog a ime ic analysis (TGA) was pe o med using a TGA/SDTA 851 analyze
(Me le Toledo, G ei ensee, Swi ze land). Samples wi h weigh s be ween 5 and 10 mg
we e hea ed om 25 o 800 ◦C a a hea ing a e o 10 ◦C/min in ni ogen a mosphe e.
2.4.3. Con ac Angle Measu emen s
Con ac angle (CA) alues o aw wool ibe s and ea ed sisal ibe s we e measu ed
wi h OCA 20 (Da a Physics Ins umen s, Filde s ad , Ge many) using HPLC wa e as es
liquid. Samples o CA we e ob ained by comp essing sho wool ibe s in a mold. The
wa e con ac angle o a wa e d ople deposi ed on he sample su ace was measu ed.
2.4.4. Di e en ial Scanning Calo ime y
The he mal p ope ies o nea PLA and composi es wi h 30 w .% o wool ibe we e
de e mined by di e en ial scanning calo ime y (DSC). A Me le Toledo DSC 3+ equipmen
Ma e ials 2024,17, 4912 4 o 17
was used and samples (5–10 mg) we e hea ed om oom empe a u e o 170
◦
C a a
scanning a e o 10 ◦C/min in ni ogen a mosphe e.
2.4.5. Tensile Tes
To de e mine he ensile p ope ies o wool ibe s, he c oss-sec ion o ibe mus be
de e mined. E en hough wool ibe s show i egula c oss-sec ion, o simplici y, ibe s
wi h cylind ical shape we e conside ed. The c oss-sec ion a ea was calcula ed measu ing
he wool ibe diame e by op ical mic oscopy. Fo each ibe sys em, ensile es s we e
pe o med using 10 mm gauge leng h and he a e o 1 mm/min. Fi een wool ibe s we e
es ed o ensile p ope ies calcula ions. On he o he hand, o composi es, a minimum o
i e specimens we e es ed a he a e o 1 mm/min, and ensile s eng h, modulus, and he
de o ma ion a b eak we e calcula ed.
2.4.6. Scanning Elec on Mic oscopy
SEM mic og aphs o he ac u ed su ace o composi es we e pe o med by JEOL
JSM-6400 (Tokyo, Japan) equipmen . F ac u ed su aces we e p e iously coa ed wi h gold
using Q150TES me allize (Lewes, UK).
2.4.7. Ve ical Bu n Tes s
As p elimina y esul s and o compa ison pu poses, using he ensile specimens,
he lammabili y and sel -ex inguishing pe o mance o p epa ed sys ems we e e alua ed
using e ical bu n es s. Composi es wi h 30 w % ibe con en we e bu ned and, also, he
nea PLA specimens we e es ed o compa ison pu poses. A Bunsen bu ne WLD-TEC
GmbH (A enshausen, Ge many) was used, he spacing be ween he op o he bu ne and
he lowe end o he ensile specimens was se 70 mm. The lame was applied o 10 s and,
a e emo ing i , he pe o mance o samples was obse ed.
3. Resul s
Wool ibe is ib ous p o ein, ke a in, wi h a high abundance o cys eine amino acid. Be-
ween p o ein chains, he cys eine amino acid c ea es disulphide linkages. These disulphide
bonds can be in e - and in amolecula and, consequen ly, a compac h ee-dimensional
s uc u e is c ea ed ha s abilizes and insolubilizes he ke a in p o eins [
4
,
24
]. FTIR spec a
o di e en wool ibe sys ems a e shown in Figu e 1. Wool ibe s showed a s ong b oad
band a 3275 cm
−1
ela ed o he N-H and O-H s e ching ib a ions. The bands a 1635,
1508, and 1228 cm
−1
co espond o amide I, II, and III bands, espec i ely, ela ed wi h
amino acid g oups o wool. The amide I band is a ibu ed o he ib a ion o C=O g oups
and he amide II band is ela ed wi h N-H bending and C-H s e ching ib a ions. The
amide III band is de i ed om a combina ion o C-N s e ching and N-H bending wi h
con ibu ion om C-C s e ching and C=O bending ib a ions [
25
]. The sys ems ea ed
wi h pe oxide showed a band a ound 1036 cm
−1
assigned o he S-O symme ic s e ching
ib a ion o cys eine-S-sulphona e o cys eine sul onic acid [
21
,
26
–
28
]. In addi ion, a new
band appea ed a ound 1169 cm−1due o oxida ion eac ions.
In Figu e 2, he mechanism o he clea age eac ion o in amolecula disulphide
bonds due o oxida ion ea men wi h pe oxide was p oposed. The in ensi y o hese
bands, 1036 and 1169 cm
−1
, inc eased as he pe oxide ea men ime was inc eased.
Bha sa e al. [
29
] sugges ed ha he a ia ions obse ed in he egion 1000–1300 cm
−1
a e a ibu ed o di e en sulphu -con aining chemical g oups o wool ha comp ise he
oxida i e disulphide in e media es and he amide III band.
Ma e ials 2024,17, 4912 5 o 17
Ma e ials 2024, 17, x FOR PEER REVIEW 5 o 17
Figu e 1. FTIR spec a o s udied wool ibe s.
In Figu e 2, he mechanism o he clea age eac ion o in amolecula disulphide
bonds due o oxida ion ea men wi h pe oxide was p oposed. The in ensi y o hese
bands, 1036 and 1169 cm−1, inc eased as he pe oxide ea men ime was inc eased.
Bha sa e al. [29] sugges ed ha he a ia ions obse ed in he egion 1000–1300 cm−1 a e
a ibu ed o di e en sulphu -con aining chemical g oups o wool ha comp ise he oxi-
da i e disulphide in e media es and he amide III band.
(a)
(b)
Figu e 2. The possible clea age o in amolecula disulphide bonds due o oxida ion ea men wi h
pe oxide: (a) In amolecula ; scission in wo molecules wi h he su ace chemically modi ied and
(b) In amolecula ; he su ace chemical modi ica ion.
Figu e 1. FTIR spec a o s udied wool ibe s.
Ma e ials 2024, 17, x FOR PEER REVIEW 5 o 17
Figu e 1. FTIR spec a o s udied wool ibe s.
In Figu e 2, he mechanism o he clea age eac ion o in amolecula disulphide
bonds due o oxida ion ea men wi h pe oxide was p oposed. The in ensi y o hese
bands, 1036 and 1169 cm−1, inc eased as he pe oxide ea men ime was inc eased.
Bha sa e al. [29] sugges ed ha he a ia ions obse ed in he egion 1000–1300 cm−1 a e
a ibu ed o di e en sulphu -con aining chemical g oups o wool ha comp ise he oxi-
da i e disulphide in e media es and he amide III band.
(a)
(b)
Figu e 2. The possible clea age o in amolecula disulphide bonds due o oxida ion ea men wi h
pe oxide: (a) In amolecula ; scission in wo molecules wi h he su ace chemically modi ied and
(b) In amolecula ; he su ace chemical modi ica ion.
Figu e 2. The possible clea age o in amolecula disulphide bonds due o oxida ion ea men wi h
pe oxide: (a) In amolecula ; scission in wo molecules wi h he su ace chemically modi ied and
(b) In amolecula ; he su ace chemical modi ica ion.
The ou e mos laye o a woolen ibe , epicu icle, is made o o e lapping scales.
These o e lapped scales ac as liquid wa e epellen , as can be obse ed in con ac angle
pho og aphs o p essed wool ibe disc wi h a d ople o wa e (Figu e 3). E en hough
he oughness o he p epa ed su aces makes i di icul o de e mine he con ac angle
accu a ely, bo h sys ems showed con ac angle alues be ween 110 and 120
◦
. Theo e ically,
he su ace o he con ac angle should be smoo h, bu in p ac ice his assump ion does
Ma e ials 2024,17, 4912 6 o 17
no pe ain. The lack o smoo hness is mo e e iden in he pe oxide- ea ed sample whe e
some ibe s can be obse ed inside he wa e d op. Gama e al. [
7
] epo ed a con ac angle
o 133◦when a d op o wa e was deposi ed on he su ace o wool.
Ma e ials 2024, 17, x FOR PEER REVIEW 6 o 17
The ou e mos laye o a woolen ibe , epicu icle, is made o o e lapping scales. These
o e lapped scales ac as liquid wa e epellen , as can be obse ed in con ac angle pho o-
g aphs o p essed wool ibe disc wi h a d ople o wa e (Figu e 3). E en hough he
oughness o he p epa ed su aces makes i di icul o de e mine he con ac angle accu-
a ely, bo h sys ems showed con ac angle alues be ween 110 and 120°. Theo e ically, he
su ace o he con ac angle should be smoo h, bu in p ac ice his assump ion does no
pe ain. The lack o smoo hness is mo e e iden in he pe oxide- ea ed sample whe e
some ibe s can be obse ed inside he wa e d op. Gama e al. [7] epo ed a con ac angle
o 133° when a d op o wa e was deposi ed on he su ace o wool.
(a)
(b)
Figu e 3. Pho og aphs used o con ac angle alues measu emen s: (a) soap-cleaned wool ibe s
and (b) pe oxide- ea ed ibe s o 24 h.
E en hough he wool su ace ac s as liquid wa e epellen , he e a e small spaces
be ween he scales om which wa e apo can slowly en e he ibe . The TGA cu es
(Figu e 4) showed ha all ibe s showed a ound 100 °C, a weigh loss ela ed o he wa e
e apo a ion in ag eemen wi h o he wo ks [4,21,30]. The weigh loss due o wa e e ap-
o a ion in pe oxide- ea ed wool sys ems is highe han o he soap-cleaned coun e pa ,
shown in Table 1. The esul s sugges ed ha he epicu icle o wool ibe seemed o be
damaged by he pe oxide ea men and would acili a e he di usion o wa e inside o
wool ibe . Fu he mo e, a e pe oxide ea men cys eine sul onic acid was c ea ed, as
obse ed in FTIR spec a, which led o mo e possibili ies o he c ea ion o hyd ogen
bonds wi h wa e molecules compa ed o he un ea ed coun e pa , and consequen ly he
wa e abso p ion capaci y o he ea ed wool ibe s was inc eased.
(a)
(b)
Figu e 4. (a) The mog a ime ic analysis and (b) de i a i e he mog a ime y cu es o wool i-
be s.
200 400 600 800
0
20
40
60
80
100
Weigh (%)
Tempe a u e (ºC)
Soap cleaned
Soap + 30 min H2O2
Soap + 24 h H2O2
Figu e 3. Pho og aphs used o con ac angle alues measu emen s: (a) soap-cleaned wool ibe s and
(b) pe oxide- ea ed ibe s o 24 h.
E en hough he wool su ace ac s as liquid wa e epellen , he e a e small spaces
be ween he scales om which wa e apo can slowly en e he ibe . The TGA cu es
(Figu e 4) showed ha all ibe s showed a ound 100 ◦C, a weigh loss ela ed o he wa e
e apo a ion in ag eemen wi h o he wo ks [
4
,
21
,
30
]. The weigh loss due o wa e e apo-
a ion in pe oxide- ea ed wool sys ems is highe han o he soap-cleaned coun e pa ,
shown in Table 1. The esul s sugges ed ha he epicu icle o wool ibe seemed o be
damaged by he pe oxide ea men and would acili a e he di usion o wa e inside o
wool ibe . Fu he mo e, a e pe oxide ea men cys eine sul onic acid was c ea ed, as
obse ed in FTIR spec a, which led o mo e possibili ies o he c ea ion o hyd ogen bonds
wi h wa e molecules compa ed o he un ea ed coun e pa , and consequen ly he wa e
abso p ion capaci y o he ea ed wool ibe s was inc eased.
Ma e ials 2024, 17, x FOR PEER REVIEW 6 o 17
The ou e mos laye o a woolen ibe , epicu icle, is made o o e lapping scales. These
o e lapped scales ac as liquid wa e epellen , as can be obse ed in con ac angle pho o-
g aphs o p essed wool ibe disc wi h a d ople o wa e (Figu e 3). E en hough he
oughness o he p epa ed su aces makes i di icul o de e mine he con ac angle accu-
a ely, bo h sys ems showed con ac angle alues be ween 110 and 120°. Theo e ically, he
su ace o he con ac angle should be smoo h, bu in p ac ice his assump ion does no
pe ain. The lack o smoo hness is mo e e iden in he pe oxide- ea ed sample whe e
some ibe s can be obse ed inside he wa e d op. Gama e al. [7] epo ed a con ac angle
o 133° when a d op o wa e was deposi ed on he su ace o wool.
(a)
(b)
Figu e 3. Pho og aphs used o con ac angle alues measu emen s: (a) soap-cleaned wool ibe s
and (b) pe oxide- ea ed ibe s o 24 h.
E en hough he wool su ace ac s as liquid wa e epellen , he e a e small spaces
be ween he scales om which wa e apo can slowly en e he ibe . The TGA cu es
(Figu e 4) showed ha all ibe s showed a ound 100 °C, a weigh loss ela ed o he wa e
e apo a ion in ag eemen wi h o he wo ks [4,21,30]. The weigh loss due o wa e e ap-
o a ion in pe oxide- ea ed wool sys ems is highe han o he soap-cleaned coun e pa ,
shown in Table 1. The esul s sugges ed ha he epicu icle o wool ibe seemed o be
damaged by he pe oxide ea men and would acili a e he di usion o wa e inside o
wool ibe . Fu he mo e, a e pe oxide ea men cys eine sul onic acid was c ea ed, as
obse ed in FTIR spec a, which led o mo e possibili ies o he c ea ion o hyd ogen
bonds wi h wa e molecules compa ed o he un ea ed coun e pa , and consequen ly he
wa e abso p ion capaci y o he ea ed wool ibe s was inc eased.
(a)
(b)
Figu e 4. (a) The mog a ime ic analysis and (b) de i a i e he mog a ime y cu es o wool i-
be s.
200 400 600 800
0
20
40
60
80
100
Weigh (%)
Tempe a u e (ºC)
Soap cleaned
Soap + 30 min H2O2
Soap + 24 h H2O2
Figu e 4. (a) The mog a ime ic analysis and (b) de i a i e he mog a ime y cu es o wool ibe s.
Table 1. The i s mass loss pe cen ages, he onse and maximum deg ada ion empe a u es o second
mass loss and he cha pe cen ages o wool ibe s.
Wool Fibe 1s Weig h Loss 2nd Weig h Loss Cha a 800 ◦C
(%) Tonse (◦C) Tmax (◦C) (%)
Soap cleaned 2.5 195.7 272.7 30.5
Soap + H2O230 min 5.9 203.5 275.6 25.1
Soap + H2O224 h 6.9 209.3 275.2 24.4
The second weigh loss is ela ed o he he mal deg ada ion o he wool ibe s. The
deg ada ion cu e o he soap-cleaned wool ibe is sligh ly di e en compa ed o pe oxide-
ea ed ones. In he soap-cleaned wool ibe , di e en small shoulde s can be obse ed a
Ma e ials 2024,17, 4912 7 o 17
he beginning o his deg ada ion s ep. Those shoulde s could be ela ed wi h he he mal
deg ada ion o low molecula weigh compound such as lanolin. In he li e a u e [
31
,
32
] i
was obse ed ha he lanolin s a ed o deg ade a ound 200
◦
C and showed a mul i-s ep
deg ada ion. Due o ha shoulde , he soap-cleaned wool ibe s showed a sligh ly lowe
onse deg ada ion empe a u e han he pe oxide- ea ed ones, as shown in Table 1. Wool
ibe ea ed wi h pe oxide o 30 min seemed o show a small shoulde ; howe e , a e
24 h ea men his shoulde was missing in he he mog am. The TGA esul s sugges ed
ha a e soap cleaning o he ibe s, some lanolin is p esen in he wool ibe s, bu he
pe oxide ea men seemed o be e ec i e o emo e his esidual lanolin. In ag eemen
wi h he TGA esul s, he in ensi y o he abso p ion band obse ed by FTIR echnique a
2926 cm
−1
, a ibu ed o –CH
2
s e ching [
29
], seemed o diminish a e pe oxide ea men s
(Figu e 1). This band educ ion could be ela ed wi h lanolin emo al om wool ibe s since
chemically, lanolin consis s o a mix u e o se e al s e ols, a y acids, and hei es e s [
33
].
The onse empe a u es o he pe oxide- ea ed ibe s we e supe io compa ed o
he soap-cleaned ibe ones, indica ing ha he pe oxide ea men imp o ed he he mal
s abili y o wool ibe s. Kim e al. [
21
] obse ed ha wool ibe s a ed o deg ade a
a ound 250
◦
C due o up u es o he helical s uc u e, and a e wa ds cys ine disulphide
bonds we e b oken a a ound 320
◦
C. A high empe a u es, all wool sys ems showed a
signi ica i e esidue amoun , being highe o wool ibe s wi hou pe oxide ea men , as
shown in Table 1. In he li e a u e, he cha ing abili y o wool was obse ed p e iously [
21
].
In Table 2, he ensile p ope ies o wool ibe s a e epo ed and compa ed wi h
li e a u e da a o ege able ibe s. The soap-cleaned wool ibe showed s eng h, modulus,
and de o ma ion a he b eak alues o 163 MPa, 6.2 GPa and 16.1%, espec i ely. I mus
highligh ed ha he s anda d de ia ion alues we e high, indica ing a high a iabili y in
he ensile p ope ies. Mechanical p ope ies di e ed om one wool ibe o ano he due
o se e al ac o s [
24
]. We no iced ha he a e age diame e alues a ied a e pe oxide
ea men s. The soap-cleaned ibe was a ound 63
µ
m, bu a e he oxida ion ea men
wi h pe oxide he diame e was educed o a ound 50
µ
m. Based on he p oposed clea age
mechanism o Figu e 2a, his eac ion could be he eason o educing he ibe diame e .
The s eng h alue epo ed in he cu en s udy is in he ange o he alues epo ed in
he li e a u e [
24
,
34
,
35
]. Zhang e al. [
34
] epo ed o me ino wool ibe s a s eng h o
151 MPa
and an elonga ion a he b eak alue o 43.5%; un o una ely, hey did no epo
he modulus alue. Kim e al. [
35
] cha ac e ized he ensile p ope ies o wool ibe s as he
a e age s eng h, modulus, and s ain a he b eak alues o 160.9 MPa, 4.8 GPa, and 27.7%,
espec i ely. Bouagga e al. [
24
] s udied he physico-chemical, he mal, and mechanical
p ope ies o Tunisian wool. They epo ed ha wool ibe s showed an a e age ensile
b eaking o ce o 16.89 cN, wi h he a e age diame e o 28.33
µ
m. Based on hese da a, he
es ima ed ensile s eng h o Tunisian wool is o 268 MPa. The s eng h da a epo ed in he
cu en s udy a e lowe han hose es ima ed o Tunisian wool. Rega ding he elonga ion
alue, Bouagga e al. [
24
] epo ed an a e age elonga ion alue o 32.5%, being he alue
epo ed in he cu en s udy lowe . Howe e , he young modulus epo ed by hem,
907 MPa, is signi ican ly lowe compa ed o wha we de e mined in he cu en wo k.
Table 2. Tensile s eng h, modulus, and de o ma ion a b eak alues o di e en na u al ibe s.
Na u al Fibe s S eng h
(MPa)
Modulus
(GPa)
De o ma ion a B eak
(%) Re e ence
Soap cleaned 163 ±23 6.2 ±2.0 16.1 ±7.1 Cu en wo k
Soap + H2O2
30 min 160 ±33 6.7 ±2.3 10.0 ±7.5 Cu en wo k
Soap + H2O2
24 h 170 ±33 8.2 ±3.6 19.0 ±12.0 Cu en wo k
Flax ibe 802 46.9 1.5 [36]
Sisal ibe 366 9.5 ±3.4 3.9 ±1.3 [37]
Ma e ials 2024,17, 4912 8 o 17
A e he pe oxide ea men , he s eng h alue ha dly changed. E en hough he
oxida ion eac ion wi h pe oxide esul ed in chemical modi ica ions o he wool ibe s, as
obse ed by FTIR, hese modi ica ions did no al e he ensile s eng h signi ican ly. While
he oxida ion ea men wi h pe oxide in oduced a clea age o he co alen in amolecula
disulphide bonds, i seems ha hese bonds we e no mainly esponsible o he s eng h
o he wool ibe s [
38
,
39
]. Zahn and Blankenbu g [
40
] sugges ed ha he hyd ophobic
in e ac ions be ween he chains we e esponsible o e aining he s eng h o wool e en a
high mois u e egains. I is p obable ha he pe oxide ea men did no al e ha dly he
hyd ophobic in e ac ions be ween chains, and consequen ly he mechanical p ope ies did
no al e signi ican ly.
Rega ding he modulus, he wool ibe ea ed wi h pe oxide showed highe modulus
alues han un ea ed one. Fu he mo e, as he ea men ime was inc eased, he modulus
alue was inc eased. One possible explana ion o his inc ease would be he emo al
o lanolin om wool ibe s ha can ac as plas icize . On he o he hand, he modulus
inc emen could be also due o he c ea ion o new H-bonds among di e en pep ide chains
hanks o cys eine sul onic acid g oups c ea ed in pe oxide ea men . As he ea men
ime was inc eased, he in ensi y o FTIR bands ela ed wi h cys eine sul onic acid g oups
was inc eased, sugges ing ha he numbe o cys eine sul onic acid g oups we e inc eased.
I should be indica ed ha he inc emen obse ed should be aken wi h cau ion due o he
high s anda d de ia ions.
Rega ding he compa ison wi h ege able ibe s, he wool ibe s showed highe de o -
ma ion a he b eak alue compa ed o he ege able coun e pa s. Howe e , he s eng h
and modulus alues o ege able ibe s we e highe han wool ibe ones, sugges ing ha
he ein o cing e ec o wool ibe s was, heo e ically, lowe han he ege able ibe s.
The lack o c ys allini y o wool ibe s [
7
,
21
,
30
], among o he easons, led o a ma e ial
wi h lowe mechanical p ope ies compa ed o he ege able ibe ones. Usually, highly
c ys alline ibe s show highe s eng h and can inc ease he s i ness o composi es [7].
Figu e 5shows he injec ion molded specimens o PLA/wool ibe wi h di e en
ibe loading.
Ma e ials 2024, 17, x FOR PEER REVIEW 9 o 17
Figu e 5. The ibe loading inc eased om 5 w .% (le ) up o 30 w .% ( igh ).
Figu e 6 shows he s eng h alues o nea PLA and PLA/wool ibe composi es wi h
di e en ibe loadings. The s eng h alues o he composi es we e lowe han he nea
PLA one, indica ing ha he wool ibe s we e no ein o cing he polyme ma ix. In gen-
e al, as he con en o ibe was inc eased in he composi es, he s eng h alue was de-
c eased. As he s eng h alue o ibe was conside ably supe io o nea PLA, he esul s
indica ed ha he wool ibe /PLA adhesion was poo and he e was a de iciency o s ess
ans e om he ma ix o he ibe .
(a)
(b)
(c)
Figu e 6. Tensile p ope ies as a unc ion o wool ibe loading and ibe ea men : (a) s eng h; (b)
modulus and (c) de o ma ion a b eak.
The composi es ha we e ein o ced wi h wool ibe washed wi h soap showed lowe
s eng h alues han he composi es ein o ced wi h he pe oxide- ea ed ones. P obably,
he p esence o lanolin could, in addi ion o hampe ing he ibe /ma ix adhesion, educe
he ibe ic ion du ing he ibe pull-ou . E en hough he composi es con aining he
510 15 20 25 30
0
20
40
60 Soap cleaned
Soap + 30 min H2O2
Soap + 24 h H2O2
S eng h (MPa)
Fibe con en (w %)
PLA
510 15 20 25 30
0
2000
4000
6000
PLA
Soap
Soap + 30 min H2O2
Soap + 24 h H2O2
Modulus (MPa)
Fibe con en (w %)
510 15 20 25 30
0
1
2
3
PLA
Soap
Soap + 30 min H2O2
Soap + 24 h H2O2
De o ma ion a b eak (%)
Fibe con en (w %)
Figu e 5. The ibe loading inc eased om 5 w .% (le ) up o 30 w .% ( igh ).
Figu e 6shows he s eng h alues o nea PLA and PLA/wool ibe composi es wi h
di e en ibe loadings. The s eng h alues o he composi es we e lowe han he nea
PLA one, indica ing ha he wool ibe s we e no ein o cing he polyme ma ix. In
gene al, as he con en o ibe was inc eased in he composi es, he s eng h alue was
dec eased. As he s eng h alue o ibe was conside ably supe io o nea PLA, he esul s
indica ed ha he wool ibe /PLA adhesion was poo and he e was a de iciency o s ess
ans e om he ma ix o he ibe .
Ma e ials 2024,17, 4912 9 o 17
Ma e ials 2024, 17, x FOR PEER REVIEW 9 o 17
Figu e 5. The ibe loading inc eased om 5 w .% (le ) up o 30 w .% ( igh ).
Figu e 6 shows he s eng h alues o nea PLA and PLA/wool ibe composi es wi h
di e en ibe loadings. The s eng h alues o he composi es we e lowe han he nea
PLA one, indica ing ha he wool ibe s we e no ein o cing he polyme ma ix. In gen-
e al, as he con en o ibe was inc eased in he composi es, he s eng h alue was de-
c eased. As he s eng h alue o ibe was conside ably supe io o nea PLA, he esul s
indica ed ha he wool ibe /PLA adhesion was poo and he e was a de iciency o s ess
ans e om he ma ix o he ibe .
(a)
(b)
(c)
Figu e 6. Tensile p ope ies as a unc ion o wool ibe loading and ibe ea men : (a) s eng h; (b)
modulus and (c) de o ma ion a b eak.
The composi es ha we e ein o ced wi h wool ibe washed wi h soap showed lowe
s eng h alues han he composi es ein o ced wi h he pe oxide- ea ed ones. P obably,
he p esence o lanolin could, in addi ion o hampe ing he ibe /ma ix adhesion, educe
he ibe ic ion du ing he ibe pull-ou . E en hough he composi es con aining he
510 15 20 25 30
0
20
40
60 Soap cleaned
Soap + 30 min H2O2
Soap + 24 h H2O2
S eng h (MPa)
Fibe con en (w %)
PLA
510 15 20 25 30
0
2000
4000
6000
PLA
Soap
Soap + 30 min H2O2
Soap + 24 h H2O2
Modulus (MPa)
Fibe con en (w %)
510 15 20 25 30
0
1
2
3
PLA
Soap
Soap + 30 min H2O2
Soap + 24 h H2O2
De o ma ion a b eak (%)
Fibe con en (w %)
Figu e 6. Tensile p ope ies as a unc ion o wool ibe loading and ibe ea men : (a) s eng h;
(b) modulus and (c) de o ma ion a b eak.
The composi es ha we e ein o ced wi h wool ibe washed wi h soap showed lowe
s eng h alues han he composi es ein o ced wi h he pe oxide- ea ed ones. P obably,
he p esence o lanolin could, in addi ion o hampe ing he ibe /ma ix adhesion, educe
he ibe ic ion du ing he ibe pull-ou . E en hough he composi es con aining he
wool ibe ea ed wi h pe oxide showed supe io s eng h compa ed o he composi es
ein o ced wi h wool ibe only washed wi h soap, he s eng h alues achie ed we e
lowe han he nea PLA one. The aspec a io o he ein o ced ibe and he in e acial
adhesion ul ima ely de e mined he ensile s eng h o composi es. The ob ained esul s
indica ed ha , i espec i e o he ea men , he wool ibe s we e no able o imp o e he
s eng h in composi e due o he poo ibe /ma ix adhesion. Howe e , i is clea ha
he pe oxide ea men imp o ed o some ex en he ibe /ma ix adhesion, bu i was
no enough. Among he p epa ed composi e sys ems, he highes s eng h alues we e
obse ed in sys ems ea ed wi h pe oxide o 24 h. A possible explana ion o he s eng h
imp o emen could be he o al emo al o lanolin a e 24 h in pe oxide solu ion, which
is in ag eemen wi h TGA esul s ob ained. On he o he hand, he pe oxide ea men
c ea ed cys eine sul onic acid g oups ha could c ea e new in e ac ions wi h he PLA ma-
ix, imp o ing he ibe /ma ix adhesion. E en hough he pe oxide ea men imp o ed
he ibe /ma ix adhesion, he s eng h alues ob ained indica ed ha he c ea ed new
ibe /ma ix in e ac ions we e weak. Manga e al. in es iga ed wool ibe embedded addi-
i e manu ac u ing-based PLA s uc u es [
14
]. The di ec compa ison o da a o specimens
p epa ed by 3D p in ed pa s and injec ion molded specimens in he cu en s udy had
no ele ance. Due o he laye -by-laye cons uc ion, he 3D p in ed specimens showed
conside ably lowe mechanical s eng h compa ed o he injec ion molded specimens.
Ma e ials 2024,17, 4912 16 o 17
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