Ci a ion: Asue a, A.; A naiz, S.;
Miguel-Fe nández, R.; Lei a , J.;
Amunda ain, I.; A ambu u, B.;
Gu ié ez-O iz, J.I.; López-Fonseca,
R. Viabili y o Glycolysis o he
Chemical Recycling o Highly
Colou ed and Mul i-Laye ed Ac ual
PET Was es. Polyme s 2023,15, 4196.
h ps://doi.o g/10.3390/
polym15204196
Academic Edi o s: Pa chiya
Phan hong and Shige u Yao
Recei ed: 11 Augus 2023
Re ised: 28 Sep embe 2023
Accep ed: 8 Oc obe 2023
Published: 23 Oc obe 2023
Copy igh : © 2023 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
Viabili y o Glycolysis o he Chemical Recycling o Highly
Colou ed and Mul i-Laye ed Ac ual PET Was es
Asie Asue a 1,*, Six o A naiz 1, Ra ael Miguel-Fe nández 1, Jon Lei a 1, Izo z Amunda ain 1,
Bo ja A ambu u 1, Jose Ignacio Gu ié ez-O iz 2and Rubén López-Fonseca 2
1GAIKER Technology Cen e, Basque Resea ch and Technology Alliance (BRTA), Pa que Cien í ico y
Tecnológico de Bizkaia, Edi icio 202, 48170 Zamudio, Bizkaia, Spain; a naiz@gaike .es (S.A.);
miguel@gaike .es (R.M.-F.); lei a @gaike .es (J.L.); amunda ain@gaike .es (I.A.); a ambu u@gaike .es (B.A.)
2
Chemical Technologies o En i onmen al Sus ainabili y G oup, Chemical Enginee ing Depa men , Facul y
o Science and Technology, Uni e si y o he Basque Coun y (UPV/EHU), 48940 Leioa, Bizkaia, Spain;
[email p o ec ed] (J.I.G.-O.); [email p o ec ed] (R.L.-F.)
*Co espondence: asue a@gaike .es; Tel.: +34-946002323 o +34-688623389
Abs ac :
The chemical ecycling o poly(e hylene e eph hala e) –PET– ac ions, de i ed om ac ual
household packaging was e s eams, using sol olysis, was in es iga ed. This ecycling s a egy was
applied a e a p e ious on-line au oma ic iden i ica ion, by nea -in a ed spec oscopy –NIR–, and
a subsequen selec i e so ing o he di e en PET ma e ials ha we e p esen in he packaging
was es. Using his echnology, i was possible o classi y ac ions exclusi ely including PET, i ually
a oiding he p esence o bo h o he plas ics and ma e ials, such as pape , ca dboa d and wood, ha
a e p esen in he packaging was es, as hey we e e icien ly ecognised and di e en ia ed. The simple
PET ac ions, including clea and monolaye ed ma e ials, we e adequa e o be ecycled by mechanical
means meanwhile he complex PET ac ions, con aining highly colou ed and mul i-laye ed ma e ials,
we e sui able candida es o be ecycled by chemical ou es. The depolyme isa ion capaci y o he
ca aly ic glycolysis, when applied o hose complex PET was es, was s udied by e alua ing he e ec
o he p ocess pa ame e s on he esul ing o ma ion and eco e y o he monome bis(2-hyd oxye hyl)
e eph hala e –BHET– and he achie ed quali y o his eac ion p oduc . Compa able and easonable
esul s, in e ms o monome yield and i s cha ac e is ics, we e ob ained independen ly o he ype o
complex PET was e ha was chemically ecycled.
Keywo ds:
household packaging was e; plas ic was e; highly colou ed PET; mul i-laye ed PET;
on-line iden i ica ion; NIR; sol olysis; ca aly ic glycolysis; BHET
1. In oduc ion
Nowadays, due o he global awa eness abou en i onmen al issues in he socie y, he
op imisa ion o echnologies o he ea men and ecycling o plas ic was es is c ucial [
1
].
Demand o poly(e hylene e eph hala e) –PET– wo ldwide has almos doubled du ing
he las yea s, om 17 million me ic onnes in 2010 o nea 29 million me ic onnes in
2021 [
2
], being so -d inks (27.1%) and mine al wa e (34.6%) bo les he mos demanded
p oduc s [
3
]. In pa allel o his inc eased consump ion, he was e gene a ion and i s
collec ion, so ing and ecycling, ia he implemen ed was e managemen sys ems ha
esponds o he, by law, en o ced ex ended p oduce esponsibili y, ha e exhibi ed simila
g owing ends. The global ma ke o ecycled PET was es ima ed o be USD 8.9 billion in
2021 and is p ojec ed o each USD 11.7 billion by 2026. This g ow h is due o he ising
demand o manu ac u e bo les, shee s, ays, ib es, s aps and o he p oduc s h oughou
he wo ld. An inc easing end in e ms o collec ion, euse, ecycling, eco e y o PET
was es, and implemen a ion o legisla ion eques ing he use o ecycled ma e ials ha e
been wi nessed in he ma ke o ecycled PET – PET–. These ac i i ies ha e consequen ly
esul ed in a g owing demand o PET in a ious applica ions, such as ood and be e age
Polyme s 2023,15, 4196. h ps://doi.o g/10.3390/polym15204196 h ps://www.mdpi.com/jou nal/polyme s
Polyme s 2023,15, 4196 2 o 13
bo les, ex ile ib es and s apping [
4
]. As o Eu ope, PETCORE, he Eu opean associa ion
o o ganisa ions in ol ed in he PET alue chain, announced ha he amoun o so ed PET
bo les om collec ed was e s eams o e-p ocessing eached 2.0 million onnes in 2018,
an inc ease o abou 5% ela ed o p e ious yea ’s da a, and his end will con inue [5,6].
In addi ion o he demand o he ma e ial i sel , he need o he packaging sec o
o imp o ed con en p ese a ion has been esponsible o he de elopmen o ba ie
p ope ies by combining ma e ials in mul i-laye ed p oduc s. On he o he hand, he
ma ke ing s a egies ha e a ou ed he use o highly colou ed ma e ials o c ea e dis inc-
i e b and images and inc ease pe cei ed quali y o con ained p oduc s. One eme ging
challenge associa ed wi h PET ecycling is o p o ide a sui able ea men o hose highly
colou ed and complex mul i-laye ed ma e ials in pos -consume was e ac ions o PET.
Bo h ac ions a e no con enien o be mixed wi h clea o sligh ly colou ed PET, so ed o
be mechanically ecycled, and hence an al e na i e o downcycling o disposal is necessa y.
The complex PET ac ions a e less app op ia e o mechanical ecycling because de i ed
p oduc s p esen se ious limi a ions in ei he colou and anspa ency o in insic iscosi y,
limi ing hei alue and usage. In acco dance wi h his, by wo king wi h ac ual complex
PET was es, he p esen wo k aims o add ess he iabili y o chemical ecycling as an
al e na i e o he cu en managemen o he non-mechanically eco e able plas ics, e en
hough his op ion has no begun o be applied un il ecen da es only co e ed he ea men
o 0.4% o he o al plas ic was e collec ed in Eu ope [7].
In his wo k nea in a ed –NIR– spec oscopy, an on-line iden i ica ion echnique able
o ecognise ma e ials and used o so was e s eams, was applied o sepa a e was e PET
bo les and packages in o wo ac ions: one con aining sligh ly colou ed and monoma e ial
PET was e, o be mechanically ecycled, and ano he one including highly colou ed and
complex mul i-laye ed PET o be managed by chemical ecycling. NIR spec oscopy is
a sui able echnology o au oma ic iden i ica ion o polyme ic ma e ials such as wood,
plas ics, oils, pape , e c., wi hou any p e ious p epa a ion by jus eco ding and com-
pa ing hei spec a wi h known e e ences and has been selec ed because i combines
eal- ime analysis wi h quali a i e iden i ica ion [
8
,
9
]. Cu en ly, household packaging
was e so ing acili ies implemen NIR spec oscopy o sepa a ing only mechanically e-
co e able PET. Hence, he sepa a ion o chemically eco e able PET, which would be a
signi ican imp o emen and esul in a lowe amoun o e used ma e ial, needs o be
uned in e ms o bo h adjus ed so ing pa ame e s o deal wi h he speci ic cha ac e is ics
o he was e and ul illed equi emen s o a ma e ial o en e he chemical ecycling p ocess.
The s udy o he PET glycolysis p ocess was conduc ed a e comple ing he condi ioning
o he eac o eed, he ac ual complex PET was es, ha comp ises di iness emo al and
comminu ion s eps. The eac ion consis s o he deg ada ion o polyme and usually is
conduc ed a empe a u es in he ange o 180–240
◦
C, a mosphe ic p essu e, wi h an excess
o e hylene glycol –EG– and he p esence o a ca alys , ypically zinc ace a e, which leads o
he o ma ion o bis(2-hyd oxye hyl) e eph hala e –BHET– [
10
,
11
]. In p inciple, glycolysis
condi ions allow o he use o educed amoun s o eagen s as well as lowe ope a ion
empe a u es and p essu es when is compa ed wi h o he chemical ecycling me hods such
as me hanolysis o he mal deg ada ion [
12
–
15
]. Likewise, hyd olysis unde alkaline o
acid condi ions in ol e se ious echnical and en i onmen al issues associa ed wi h co o-
sion and managemen o he liquid e luen s [
16
,
17
]. BHET, a e an adequa e pu i ica ion,
can be used ei he o PET syn hesis o he p oduc ion o o he polyes e s, polyu e hanes,
plas icise s, epoxy esins, and addi i es o ex iles and biocompa ible ma e ials [
18
–
23
].
These nume ous applica ions, in u n, p o ide an economical lexibili y when using he
aw ecycled monome o he mos p o i able op ion upon demand.
2. Expe imen al
2.1. Ma e ials
In his wo k a e e ence clea PET, g ade A acco ding o in e nal s anda ds o ecycled
PET de ined by ECOEMBALAJES ESPAÑA, S.A. (ECOEMBES), Mad id, Spain, he Spanish
Polyme s 2023,15, 4196 3 o 13
managemen sys em o ligh packaging was e, and samples o highly colou ed and mul i-
laye ed pos -consume complex PET was es p o ided, also by ECOEMBES, we e used.
Da a ela ed o hese PET ma e ials a e included in Table 1.
Table 1. PET ma e ials es ed in glycolysis eac ions.
PET Ma e ial Type Colou O igin P oduc Con en
PET R-CL Re e ence Clea Mechanical ecycling Bo les Wa e
PET C-BL Highly colou ed Blue Pos -consume was e Bo les Wa e
PET C-GY Highly colou ed G ey Pos -consume was e Bo les So d ink
PET C-GN Highly colou ed G een Pos -consume was e Bo les So d ink
PET C-WH Highly colou ed Whi e Pos -consume was e Bo les Va ious
PET C-AM Highly colou ed Ambe Pos -consume was e Packages Miscellaneous
PET M-AM Mul i-laye ed Ambe Pos -consume was e Bo les Bee
All pos -consume PET was e om bo les and packages we e g ound o 5 mm lakes
and in o de o a oid he impac o di iness on eac ion es s, u he cleaned wi h a
dilu ed aqueous sodium hyd oxide solu ion (1 w %), insed wi h wa e and d ied a 80
◦
C
o e nigh in o de o mimic he indus ial ea men unde gone by he e e ence clea PET.
Clean PET lakes we e again milled wi h a c yogenic mill o adjus he pa icle diame e
o 250
µ
m in o de o ensu e a la ge a ailable su ace a ea and minimise mass and hea
ans e limi a ions on he measu ed eac ion a es. The sol en , EG (e hylene glycol 99%),
was pu chased om Ald ich and he ca alys , Zn(OAc)
2·
2 H
2
O (zinc ace a e dihyd a e,
99.5%), was p o ided by P obus. In o de o simula e eac ion condi ions expec ed o be
ound in indus ial chemical ecycling plan s, eagen s we e di ec ly used wi hou any
u he pu i ica ion.
2.2. Au oma ic Iden i ica ion Tes s
A p og ammable comme cial au oma ic iden i ica ion and so ing sys em, he UniSo
®
(RTT Sys em echnik GmbH, Zi au, Ge many), was used o he pos -consume was e PET
packages iden i ica ion and so ing es s. The sys em includes an accele a ion bel , an
iden i ica ion bel , a senso head wi h 32 p obes, wo nozzle g oups o classi ica ion by
blowing ai , a collec ing hood, and an ope a ion and con ol uni . Iden i ica ion es s
we e ca ied ou wi h ep esen a i e samples o ligh packaging pos -consume was es in
o de o e alua e bo h sepa a ion and so ing a es. The in a ed senso lines measu e he
comple e spec al egion, allowing he as iden i ica ion o polyme ic ma e ials. E en so,
as con ac - ee measu emen unde e lec ion, analysis wi hou sample p epa a ion, and
he abili y o measu e as mo ing samples a e he main ad an ages o his sys em [
24
].
NIR spec a anging om 4000 o 10,000 cm−1(1000–2500 nm) a e cha ac e ised by b oad
bands and low mola abso p ion coe icien s. The abso p ion in his ange esul s om
ha monics o he undamen al equencies in he middle in a ed –MIR– egion, ac i a ed
because o hei anha monici y. Hence, NIR echnology is especially sui ed o he de ec ion
o samples wi h –C-H, -N-H and -O-H bonds.
2.3. Glycolysis o PET
Glycolysis consis s o a depolyme isa ion eac ion o he PET polyme and mo e
speci ically a anses e i ica ion be ween PET es e g oups and a diol, usually e hylene
glycol –EG–, o ob ain he monome , in ha case, he bis(2-hyd oxye hyl e eph hala e)
–BHET– ollowing he Equa ion (1) and as shows Figu e 1:
(PET)n+ (n −1) EG ⇔n (BHET) (1)
As a o emen ioned, he eac ion is ope a ed a , ela i ely, mild condi ions, empe a u e
below 200
◦
C and a mosphe ic p essu e, wi h he p esence o me al ca ions, as Zn
2+
o Na
+
,
dispe sed in he media, ha a e added in he o m o me allic sal s and ac as ca alys s [
25
].
Polyme s 2023,15, 4196 4 o 13
The ca aly ic glycolysis eac ion is accep ed o occu h ough he nucleophilic a ack o he
hyd oxyl g oup o he diol on he ca boxylic g oup p esen in he polyes e s uc u e. The
a ack is ac i a ed by he me al ca ion, ia o ma ion o a coo dina ion complex be ween he
ca boxylic g oup and he me al ha educes he elec onic densi y o he ca boxylic g oup
and acili a es he nucleophilic a ack on he posi i ely pola ised ca bon a om. The s eng h
o he me al-oxygen bond o he ca boxylic g oup de e mines he speci ic ca aly ic ac i i y
o a gi en ca ion. The sho e he bond, he s onge he in e ac ion, which is consis en
wi h he highe le el o ac i i y o he Zn
2+
when compa ed wi h he Na
+
since i can cause
a g ea e cha ge pola isa ion ha imp o es he glycolic a ack.
Figu e 1. Depolyme iza ion o PET o p oduce BHET using EG as glycolysis agen .
Glycolysis expe imen s we e ca ied ou in a 500 cm
3
, a h ee-necked, la -bo om
glass eac o ha was elec ically hea ed and equipped wi h a he mome e and a e lux
condense ope a ed a a mosphe ic p essu e. A magne ic s i e is inco po a ed o he
ank o ensu e p ope mixing and u bulence. In all uns, 30.0 g o PET oge he wi h 1%
ca alys , Zn(OAc)
2
, by weigh basis we e loaded in o he eac o . The eac ion essel and
he mix u e o EG eac an and he ca alys we e p ehea ed o he selec ed empe a u e
(165–196
◦
C) p io he addi ion o he PET pa icles in o de o minimise he ime equi ed
by he solid o each he eac ion empe a u e. The ex en o he eac ion was s udied by
de e mining he amoun o BHET ob ained in he p oduc mix u e a e quenching he
eac o essel and i s con en . Fi s ly, a se o eac ions wi h e e ence clea PET was ca ied
ou o analyse he e ec s o eac ion ime, empe a u e and EG/PET a io and selec he
app op ia e condi ions o he depolyme isa ion o PET ac ions including pos -consume
highly colou ed o mul i-laye ed complex PET was es. Table 2summa ises he expe imen al
condi ions o he uns ca ied ou in his wo k. The eac ion mix u e was allowed o eac
o ime in e als anging om 0.33 up o 8 h. The ollowing p ocedu e was employed
o eco e and quan i y he BHET yield. Ho dis illed wa e was hen added in excess o
he eac ion mix u e wi h igo ous agi a ion since BHET is known o be qui e soluble in
boiling wa e . Ho wa e also dissol es he ca alys employed, and p obably o some ex en
highe oligome s such as dime s and ime s. While s ill ho , he suspension was quickly
il e ed. The p oduc was sepa a ed in o solid and aqueous phases using a sin e ed glass
il e (Wha man glass mic o ibe binde ee, g ade GF/C-1.2
µ
m) unde acuum. The
glycolysed p oduc was ob ained as a esidue a e il a ion. The il a e con ained EG,
BHET, and small quan i ies o ew wa e -soluble oligome s. This was hea ed un il a clea
mix u e was ob ained. A e wa ds, i was il a ed wi h a sin e ed glass il e (Wha man
glass mic o ibe binde ee, g ade GF/C-0.7 µm) unde acuum. This second il a e was
collec ed and s o ed in a e ige a o a 5
◦
C o 16 h o p o oke he p ecipi a ion o whi e
c ys alline BHET lakes. A e il a ion, BHET was d ied in an o en a 60
◦
C o 30 h, and
weighed on an analy ical balance o es ima e he yield acco ding o he ollowing equa ion:
Y(%) = WBHET, /MBHET
WPET,0/MPET
×100 (2)
In Equa ion (2) W
PET,0
and W
BHET,
e e o he ini ial weigh o PET and he weigh
o BHET a a speci ic eac ion ime, espec i ely. M
BHET
and M
PET
a e he molecula mass
o BHET (254.24 g mol
−1
) and he PET (192.17 g mol
−1
) epea ing uni , espec i ely. This
pu i ied p oduc was hen subjec ed o a ious cha ac e isa ion echniques.
Polyme s 2023,15, 4196 5 o 13
Table 2. Glycolysis eac ions wi h e e ence PET and pos -consume was e PET.
Run (h) T (◦C) PET
Ma e ial PET (g) EG/PET
(mol/mol)
Zn(OAc)2/PET
(w/w)BHET Yield (%)
#1 8 196 PET R-CL 30 7.6/1 1% 67.1
#2 8 196 PET R-CL 30 7.6/1 --- 56.8
#3 1 165 PET R-CL 30 7.6/1 1% 39.4
#4 1 180 PET R-CL 30 7.6/1 1% 66.1
#5 1 196 PET R-CL 30 3.8/1 1% 54.9
#6 1 196 PET R-CL 30 5.7/1 1% 66.1
#1 2 196 PET R-CL 30 7.6/1 1% 67.1
#7 2 196 PET C-BL 30 7.6/1 1% 63.9
#8 2 196 PET C-GY 30 7.6/1 1% 67.3
#9 2 196 PET C-GN 30 7.6/1 1% 65.8
#10 2 196 PET C-WH 30 7.6/1 1% 61.2
#11 2 196 PET C-AM 30 7.6/1 1% 67.5
#12 2 196 PET M-AM 30 7.6/1 1% 57.2
2.4. Analysis o Glycolysis P oduc s/Cha ac e isa ion o BHET
Elemen al analysis was ca ied ou wi h an Eu oVec o Elemen al Analyze appa a us
(Eu oVec o , he Elemen al Analysis Company, Reda alle, I aly). Di e en ial scanning
calo ime y –DSC– was pe o med on a Me le Toledo DSC822e ins umen (Me le -
Toledo L d., Po Melbou ne, Aus alia), wi h 3–5 mg samples, unde a pu ge gas low o
15 cm
3
min
−1
wi h ni ogen, and wi h a hea ing a e o 10
◦
C min
−1
in he ange
25–350 ◦C
.
A FTIR Nicole P o egé460 spec ome e (The mo Fishe Scien i ic Inc., Wal ham, MA,
USA) was used in ansmission mode wi h a esolu ion o 2 cm
−1
o eco d spec a om de-
polyme isa ion p oduc s on KB discs (100 mg wi h a dilu ion o 1/50, 13 mm in diame e ).
The ins umen was equipped wi h a DTGS de ec o a e aging 50 scans. P o on nuclea
magne ic esonance –
1
H NMR– spec oscopy analysis was eco ded wi h a B ucke AV500
spec ome e (B ucke Co p., Bille ica, MA, USA) ope a ing a 500 MHz. The spec a we e
ob ained in d6-ace one solu ion.
3. Resul s and Discussion
3.1. Au oma ic Iden i ica ion Tes s
Rep esen a i e samples o he plas ic ac ion coming om ac ual household packaging
was e s eams collec ed by ECOEMBES we e s udied by means o a NIR spec ome e . The
NIR spec a o bo h plas ic (PET, high-densi y polye hylene –HDPE–, poly inyl chlo ide
–PVC–, ac yloni ile bu adiene s y ene –ABS–, polyp opylene –PP– and polys y ene –PS–)
and non-plas ic ma e ials (wood, pape and ca dboa d), usually ound due o hei appli-
ca ion o packaging, we e eco ded and examined. As shown in Figu es 2and 3, PET
spec um p esen s a dis inc i e peak a 1657 nm, which made i possible o di e en ia e
packaging ma e ials, including his polyme , om he o he s.
On-line iden i ica ion o he PET ac ion and subsequen au oma ic so ing, wi h
high yield and pu i y, was success ul using he UniSo
®
sys em. Recogni ion o highly
colou ed and mul i-laye ed PET ma e ials was no possible by NIR alone. Image analysis
and colou so ing echnology is p oposed o he seg ega ion o hose ac ions in a second
sepa a ion s ep.
Polyme s 2023,15, 4196 6 o 13
Figu e 2. NIR spec a o PET and o he polyme s.
Figu e 3. NIR spec a o PET and o he no-polyme s was es.
3.2. Glycolysis Reac ions
Time, empe a u e and EG/PET mola a io we e chosen as he independen a iables
o be analysed o he glycolysis p ocess, keeping cons an he ca alys /PET a io a 1% by
weigh . Run #1 was pe o med using e e ence PET o s udying he e olu ion o BHET
yield wi h ime. Reac ion p og ess a 0.33, 0.66, 1, 2, 3, 4, 5 and 8 h a 196
◦
C and wi h an
EG/PET mola a io o 7.6/1 is shown in Figu e 4. Run #2 was used o compa e he eac ion
Polyme s 2023,15, 4196 7 o 13
a e in he absence o any ca alys , which was ound o be a he slowe . Al hough Run
#1 e ealed ha he ca alysed eac ion eached equilib ium a e 2 h, in o de o sho en
he du a ion o p elimina y wo k, a eac ion in e al o 1 h was selec ed o de e mine
he in luence o he independen a iables, such empe a u e and EG/PET mola a io.
Runs #1, #3 and #4 we e pe o med a 165, 180 and 196
◦
C, espec i ely, and con i med he
need o ope a ing a he maximum eachable empe a u e ha , in p ac ice, co esponds
o he boiling poin o EG a a mosphe ic p essu e. The achie ed BHET yields a a gi en
ime in e al (1 h) wi h empe a u e a e included in Figu e 5. Finally, Runs #5 and #6
co esponded o uns wi h a lowe EG/PET mola a io (3.8/1 and 5.7/1). I was ound
ha a la ge excess o EG (7.6/1) was equi ed o a aining a easonable BHET yield as
plo ed in Figu e 6.
Figu e 4.
E ec o eac ion ime on BHET yield ( e e ence PET, 196
◦
C, EG/PET = 7.6/1, Runs
#1 and #2—squa es co espond o expe imen al da a om ca aly ic uns; iangles co espond o
expe imen al da a om non-ca aly ic glycolysis).
As he las s ep o his s udy, he glycolysis p ocess o clea , colou ed (g een, ambe ,
blue, whi e and sil e ), and mul i-laye ed ecycled PET we e p elimina ily compa ed
in Figu e 7(Runs #7 o #12). In he li e a u e, ew epo s can be ound ega ding he
managemen o hese ypes o was es by glycolysis [
26
]. The expe imen al condi ions used
we e 196
◦
C, EG/PET mola a io 7.6/1 and 1 w % Zn(OAc)
2
a e a eac ion ime in e al
o 2 h. I could be no iced ha , wi hin he expe imen al e o , hese se en ypes o ecycled
PET yield p ac ically simila glycolysed p oduc s wi h con e sion anging be ween 59–68%.
The e o e, he addi i es p esen in he ecycled colou ed PET appa en ly did no a ec he
ex en o depolyme isa ion, al hough i ends o discolou he glycolysed p oduc s, unless
an ex a ex ac ion s ep is aken o isola e i .
Polyme s 2023,15, 4196 8 o 13
Figu e 5.
E ec o empe a u e on BHET yield ( e e ence PET, EG/PET = 7.6/1, 1 h, 1 w % Zn(OAc)
2
,
Runs #1, #3 and #4).
Figu e 6.
E ec o PET/EG mol a io on BHET yield ( e e ence PET, 196
◦
C, 1 h, 1 w % Zn(OAc)
2
,
Runs #1, #5 and #6).
Polyme s 2023,15, 4196 9 o 13
Figu e 7.
E ec o he na u e o he PET was e on BHET yield (196
◦
C, EG/PET = 7.6/1, 2 h, 1 w %
Zn(OAc)2, Runs #7–12).
3.3. Analysis o Glycolysis P oduc s
In o de o check he pu i y o he eco e ed BHET a e glycolysis, his p oduc was
analysed by means o a numbe o analy ical echniques, and esul s we e compa ed wi h
hose co esponding o a comme cial BHET p o ided by he Ald ich Chemical Co. The
esul s o he elemen al analysis, exp essed as he mola pe cen ages o C, H and O, o he
pu i ied BHET p oduc we e 56.4% C, 5.4% H and 38.2% O, in ai ly good ag eemen wi h
57.1% C, 5.9% H and 37.0% O ound o he comme cial sample. As o
1
H NMR analysis,
signals a
δ
8.1, 4.4, 4.2 and 3.9 ppm we e no iced. The signal a
δ
8.1 ppm indica ed he
p esence o he ou a oma ic p o ons o he benzene ing. Signals a
δ
4.4 and 3.9 we e
cha ac e is ic o he me hylene p o ons o COO-CH
2
and CH
2
OH, espec i ely (Figu e 8).
On he o he hand, he iple a
δ
4.1 ppm was a ibu ed o he p o ons o he hyd oxyl
g oup [
27
]. In addi ion, he FTIR spec a (Figu e 9) o he pu i ied monome clea ly showed
-OH band a 3450 cm
−1
and 1135 cm
−1
, C=O s e ching a 1715 cm
−1
, alkyl C-H a 2879 and
2954 cm
−1
and a oma ic C-H a 1411–1504 cm
−1
, p esen in BHET [
28
]. Fu he , he DSC
scan also showed a easonably sha p endo he mic peak a 110
◦
C in ag eemen wi h he
known mel ing poin o BHET (Figu e 10). F om all hese obse a ions, i was concluded
ha he pu i ied monome was BHET, i espec i e o he PET was e glycolysed.
On he o he hand, DSC analysis o he glycolysed p oduc ( he esidue ob ained
a e il a ion o he ho eac ion mix u e) was pe o med. As an example, he p o ile o
he sample co esponding o Run #1 is included in Figu e 11. Two dis inc peaks we e
clea ly no iced a abou 160 and 220
◦
C. They we e associa ed wi h he p esence o he
oligome s in he sample. Pa icula ly, he peak loca ed a 160
◦
C was a ibu ed o he
mel ing poin o he dime while he peak a 220
◦
C was easonably assigned o he mel ing
poin o a mix u e o oligome s ha ing be ween 3–5 epea ing uni s [
29
]. The absence o
oligome s ha ing a highe molecula weigh was con i med. This obse a ion was in line
wi h he possibili y o con olling he selec i i y o he p ocess owa ds BHET by using a
la ge excess o EG, his is o say, ope a ing wi h a high EG/PET mola a io [
30
]. The peak
a 110
◦
C co esponds o aces o BHET, which we e no p ope ly emo ed du ing he
