polyme s
A icle
Easy- o-Make Polyme Hyd ogels by UV-Cu ing o he
Cleaning o Ac ylic Emulsion Pain Films
I ene Cá daba 1,*, Luca Po ca elli 2, An onela Gallas egui 2, Da id Mece eyes 2and Mi en I xaso Magu egui 1
Ci a ion: Cá daba, I.; Po ca elli, L.;
Gallas egui, A.; Mece eyes, D.;
Magu egui, M.I. Easy- o-Make
Polyme Hyd ogels by UV-Cu ing o
he Cleaning o Ac ylic Emulsion
Pain Films. Polyme s 2021,13, 2108.
h ps://doi.o g/10.3390/
polym13132108
Academic Edi o : Albe o Figoli
Recei ed: 21 May 2021
Accep ed: 23 June 2021
Published: 26 June 2021
Publishe ’s No e: MDPI s ays neu al
wi h ega d o ju isdic ional claims in
published maps and ins i u ional a il-
ia ions.
Copy igh : © 2021 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/).
1Pain ing Depa men , Facul y o Fine A s, Uni e si y o he Basque Coun y (UPV/EHU), Ba io Sa iena
S/N, 48940 Leioa, Bizkaia, Spain; [email p o ec ed]
2POLYMAT, Basque Cen e Fo Mac omolecula Design and Enginee ing, Uni e si y o he Basque Coun y
UPV/EHU, A enida Tolosa 72, Donos ia, 20018 San Sebas ián, Gipuzkoa, Spain;
l.po ca [email p o ec ed] (L.P.); [email p o ec ed] (A.G.); [email p o ec ed] (D.M.)
*Co espondence: ica [email p o ec ed]
Abs ac :
The cleaning o ac ylic emulsion pain su aces poses a g ea challenge in he conse a ion
ield, due o hei high wa e sensi i i y. In his a icle, we p esen easy- o-make polyme hyd ogels,
made by UV-pho opolyme iza ion, ha show excellen cleaning p ope ies. The o mula ion o
hyd ogels ob ained by UV-cu ing and hei pe o mance as d y cleane s o ac ylic pain s was
in es iga ed. Fi s , di e en hyd ogel o mula ions based on unc ional ac ylic monome s we e
used o o mula e a se ies o UV c oss-linked hyd ogels by as UV pho opolyme iza ion. Thei
e ec i eness on su ace di emo al was in es iga ed by SEM mic oscopy and colo ime y. The
hyd ogels showed excellen cleaning p ope ies and con olled wa e elease, and hey s ill pe o med
sa is ac o ily a e se e al cleaning uses. The ob ained UV-hyd ogels we e compa ed o he well-
known aga gels, showing bene i s in e ms o educing excess wa e . This a icle shows ha easy- o-
make UV-cu ed hyd ogels a e an e icien ool o he cleaning o su ace di om wa e -sensi i e
pain ings, o e coming he limi s o adi ional cleaning me hods.
Keywo ds: ac ylic pain ; UV-cu ed hyd ogels; a conse a ion; cleaning
1. Resea ch Aim
The aim o his wo k is o make he i s app oach o imp o ing he cleaning me hod-
ology o wa e -sensi i e pain su aces by using as , easy- o-make, and unable UV-cu ed
hyd ogels. The main ad an age o he p oposed me hod is ha UV-cu ing is a as and
a ailable me hod [
1
–
4
] wi h high success in he coa ing indus y and 3D p in ing, as well
as o he daily applica ions in heal h and es he ics. Besides he chemical compounds, i only
needs a UV-lamp o ob aining he needed ma e ial in a apid manne . Thus, a oiding
complex labo a o y p ocesses while op imizing he p epa a ion imes is wha makes hese
gels mo e a o dable o he es o e han o he ypes o hyd ogels o cleaning ma e ials.
In his wo k, we assessed he cleaning pe o mance o se e al ac ylic hyd ogels. To
do his, we in es iga ed se e al o mula ions o UV-cu ed hyd ogels based on comme -
cially a ailable unc ional (me h)ac yla es, in o de o ind he mos e ec i e o mula ion
o emo ing su ace di while no ha ming he pain su ace. The second pa o he
wo k compa es he cleaning capabili y o UV hyd ogels wi h he well-known aga gel.
Aga is a commonly used and easy- o-make sys em in he conse a ion o a which has
shown se e al d awbacks in e ms o wa e e en ion capabili y [
5
]. This s udy shows
he imp o emen in e ms o wa e con ainmen when using he UV-cu ed hyd ogels
p esen ed he e.
2. In oduc ion
The cleaning o wa e -sensi i e pain ings such as ac ylic emulsion pain ilms is a e y
common p ac ice in he conse a ion o con empo a y a . The g adual accumula ion o
Polyme s 2021,13, 2108. h ps://doi.o g/10.3390/polym13132108 h ps://www.mdpi.com/jou nal/polyme s
Polyme s 2021,13, 2108 2 o 13
su ace g ime on pain ings is a e y common conce n e e ywhe e ha a is p esen [
6
].
E en in a p esumably benign museum en i onmen , he slow accumula ion o su ace
g ime leads o he disce nible isual al e a ion o a pain ed su ace a e app oxima ely
50 yea s [
6
,
7
]. The e o e, we can assume ha mos o he a wo ks made ou o ac ylic
pain , which was i s in oduced in 1953 by Rohm and Haas [
8
], a e now p one o needing
su ace cleaning. In ac , many con empo a y a is s employed ac ylic pain s as hei main
media o c ea e a wo ks [
9
]. Mo eo e , he as p oduc ion o con empo a y pain e s
has illed museum collec ions wi h hese kinds o wo ks [
10
]. Howe e , his appa en ly
simple conse a i e in e en ion is a delica e and challenging ope a ion, due o, on he one
hand, he complexi y o he pain sys em i sel [
6
], and on he o he hand, i s high wa e
sensi i i y [11–16].
Ac ylic emulsion pain s a e made om wa e , ac ylic monome s, s abilize s, hicken-
e s, coalescing agen s, pigmen s, and a wide ange o addi i es ha imp o e he pain ’s
p ope ies [
6
,
17
]. The esul ing ilm can become acky a oom empe a u e due o a low
glass ansi ion empe a u e (Tg), which igge s he adhesion o ai bo ne dus [
11
,
17
].
Mo eo e , s a ic cha ges and hyd ophilic addi i es accumula ed in he su ace, such as
su ac an s, can a ac and embed en i onmen al soil [
11
,
18
,
19
]. Resea ch ca ied ou
by a numbe o conse a ion scien is s [
13
,
14
,
16
–
27
], iden i ied he ac o s ha in luence
he sensi i i y o ac ylic pain ilms o o ganic sol en s and wa e -based cleaning sys-
ems [
6
,
11
]: addi i e mig a ion, swelling o he pain ilm, and e- anspo o solubilized
ma e h ough he po ous ma ix o he a i ac s du ing he cleaning p ocedu e [
12
–
15
]
a e, among o he ac o s, only a ew o he obs acles ahead.
Acco ding o some au ho s, he swelling o he pain co esponds o he sensi i i y o
he hickene s ha a e pa o he composi ion [
6
]; o he s s a ed ha wa e sensi i i y is
ela ed o he quan i y o non-ionic su ac an s ha a e commonly p esen in ac ylic pain
su aces [18,22].
Se e al au ho s ag ee ha he modi ica ion o he pH and he conduc i i y in a cleaning
solu ion can help o con ol wa e abso p ion and ma e ial ex ac ion [
6
,
18
,
28
–
34
]. In an
alkaline o neu al en i onmen , ac ylic ilm hyd a es and swells, so ening and causing
dis up ion [
31
]. The e o e, o minimize he swelling, he pH o he cleaning sys em mus be
kep as low as possible [
6
]. Fu he mo e, he ionic s eng h needs o be con olled. Bo h he
cleaning solu ion and he pain laye ha e o be iso onic, in o de o a oid he mo emen
o ions be ween laye s [
6
]. All his leads o la ge p oblems in inding accu a e and sa e
cleaning sys ems, and he subsequen need o in es iga e new cleaning me hods [20,32].
The con olled elease o wa e needs o be conside ed as a ool o imp o e e ec-
i i y, especially when non-con ined sol en s and solu ions a e employed [
15
,
16
]. The e-
o e, he use o con ined cleaning luids wi hin a gel ma ix ha eleases hem in a con-
olled way on o he su ace o he wo k o a has been p oposed [
15
,
16
]. In ecen
yea s, se e al me hods ha e been es ed o imp o e he con ol o cleaning luids. Some
esea che s p oposed hickene s, such as cellulose de i a i es and polyac ylic-based o -
mula ions [
33
], o enhance he con ol o sol en di usion by inc easing he iscosi y o he
liquid [15,16,33,34]
. Howe e , he esidue in hickened sys ems is s ill a challenge ha has
no been esol ed [
17
,
35
,
36
]. Rigid gels, which can be classi ied as physical o chemical,
acco ding o hei bonding na u e [
15
,
37
], we e p oposed as esidue- ee con ining sys ems.
Polysaccha ides such as gellan gum and aga a e he mos popula gelling agen s employed
o ob ain he igid physical gels used by conse a o s/ es o e s [
15
,
38
], due o hei as
and easy p epa a ion and handling. Howe e , hey e ain high amoun s o wa e , making
con ol o e he liquid elease p oblema ic [39–41].
As an al e na i e, xPVOH-bo ax gels [
42
,
43
], eeze- haw PVA gels [
44
], and pHEMA/
PVP [
5
,
39
] chemical hyd ogels ha e eme ged, p o ing o be mo e e icien , sa e, e en i e
and unable o he cleaning o wa e -sensi i e a i ac s.
Howe e , he manu ac u e o hese chemical gels usually equi es complex chemical
p ocesses, wi h he excep ion o he bo ax gels, and a e he e o e less in eg a ed (ex ended)
in o he daily wo k o he es o e .
Polyme s 2021,13, 2108 3 o 13
This pape aims o be he i s app oach o in oducing UV-cu ed hyd ogels using
comme cially a ailable ac ylic monome s, in o de o achie e a sys em wi h good cleaning
p ope ies and wi h he ease o being able o p epa e hem in you own wo kspace.
3. Ma e ials and Me hods
3.1. Ma e ials
2-(Hyd oxye hyl)me hac yla e (HEMA) (assay 97%-) inylpy olidone (VP), [2-
(me hac yloyloxy)e hyl] ime hylammonium chlo ide (METAC) 75 w % in H
2
O, 2-
ac ylamido-2-me hyl-1-p opanesul onic acid (AMPS) (99%), poly(e hylene glycol)
dime hylme hac yla e (PEGDMA, MW 550 g mol
−1
), 2-hyd oxy-4
0
-(2-hyd oxye hoxy)-2-
me hylp opiophenone (IRGAcu e 2956), glacial ace ic acid (99.99%), and ammonium hy-
d oxide solu ion (28% NH
3
in H
2
O), we e all supplied by Sigma Ald ich (S . Louis, MO,
USA) and used as ecei ed o he syn hesis o he UV-cu ed hyd ogels. Aga -aga was
p o ided by C.T.S. España. S.L. Ac ylic pain mock-ups we e p epa ed using b own PB 7
and i anium whi e PW6 pain s om Liqui ex®(Cincinna i, OH, USA).
3.2. P epa a ion o Hyd ogels by UV-Pho opolyme iza ion
Chemically c oss-linked UV-cu ed hyd ogels we e syn hesized by pho o-polyme iza ion
acco ding o he eac ion shown in Figu e 1. In a ypical eac ion, o 4 g o hyd ogel so-
lu ion, 1.79 g o HEMA, 0.2 g o VP, 0.01 g o PEGDMA and 0.04 g o he pho oini ia o
IRGAcu e 2956 we e dissol ed in 2 g o pH-adjus ed wa e . All he componen s we e
s i ed oge he excep he pho oini ia o , which was added jus be o e he UV exposu e.
Di e en hyd ogels we e designed by a ying he p opo ions o monome /c oss-linke
a io, pho oini ia o pe cen ages, and he pH o he adjus ed wa e . P elimina y es s we e
pe o med o ind he mos adequa e copolyme composi ions and monome /c osslinke
a io. The composi ions o he hyd ogels selec ed o he cleaning ials a e p esen ed in
Table 1. Componen s we e chosen o hei a ini y wi h wa e o c ea e hyd ogels. Di e en
monome s we e es ed in o de o check he cleaning capaci y o hei di e en unc ional
g oups. HEMA and VP we e chosen o ob ain a wa e -insoluble ne wo k, as hey comp ise
wo eac i e g oups capable o c oss-linking he hyd ogel ac yla e backbone [
5
]. METAC
and AMPS we e used as monome s in o de o imp o e cleaning e iciency while main ain-
ing he good mechanical p ope ies o he s uc u e (s ickiness and lexibili y). PEGDMA
was added as a c osslinke in o de o ensu e mechanical s abili y wi hou s i ening he hy-
d ogel [
3
]. Two wa e bu e s o pH 5.0 and 6.5 we e p epa ed o compa ison. The highes
and lowes pH alues we e selec ed acco ding o he sa e y ange p oposed by S a oudis
in he modula cleaning p og am (MCP) ecipes o he cleaning o ac ylic pain s [
6
,
45
].
Bo h bu e s we e adjus ed in conduc i i y o 6.0 mS/cm2, as ad ised elsewhe e [17,45].
The solu ion was i adia ed wi h UV ligh o 15 min o polyme iza ion. A UV
chambe om Boekel Scien i ic was used o hyd ogel c osslinking, model 234100. C ys al
ials we e used as molds, ob aining 2.5-cm diame e and 1-cm hick gels.
3.3. Ac ylic Pain Mock-Ups
Ac ylic pain was applied on Myla shee s as desc ibed elsewhe e [
17
]. Two comme -
cial pain s (Liqui ex whi e PW6 and b own PB 7) we e selec ed o e alua e he cleaning
e iciency o he me hod. Bo h colo s we e chosen in o de o de ec he a iable esul s
ha may be ob ained acco ding o he di e en na u e o he pigmen s. Whi e samples
we e employed o es he cleaning e iciency, and b own pain was used o es he sensi-
i i y owa d he cleaning me hod. Some samples we e kep as e e ence and o he s we e
a i icially soiled acco ding o published p o ocols [
24
], using a mix u e o ca bon black,
i on oxide, silica, kaolin, gela in powde , soluble s a ch, cemen , oli e oil, mine al oil and
pe oleum spi i , in he s a ed p opo ions [
24
]. Cleaning es s we e conduc ed a e h ee
yea s [
24
] o na u al aging. The excess loose soil was emo ed wi h a b ush and he gel
cleaning es s we e pe o med on he embedded soiling laye . All mock-ups we e s o ed in
da k condi ions and a dus - ee en i onmen .
Polyme s 2021,13, 2108 4 o 13
Polyme s 2021, 13, x FOR PEER REVIEW 4 o 14
Table 1. Weigh pe cen composi ion o he UV-cu ed hyd ogels; 2% weigh based on monome
(wbm) o IRGACURE 2956 was added o each solu ion.
Name Monome 1
(w %)
Monome 2
(w %)
C osslinke
(w %)
Aqueous Solu ion
(w %)
HEMA_5 HEMA (49.75) - PEGDMA (0.25) pH 5.0 bu e (50)
HEMA_VP5 HEMA (39.8) VP (10) PEGDMA (0.25) pH 5.0 bu e (50)
HEMA_METAC 5 HEMA (39.8) METAC (10) PEGDMA (0.25) pH 5.0 bu e (50)
HEMA_AMPS5 HEMA (39.8) AMPS (10) PEGDMA (0.25) pH 5.0 bu e (50)
HEMA_6.5 HEMA (49.75) - PEGDMA (0.25) pH 6.5 bu e (50)
HEMA_VP6.5 HEMA (39.8) VP (10) PEGDMA (0.25) pH 6.5 bu e (50)
HEMA_METAC6.5 HEMA (39.8) METAC (10) PEGDMA (0.25) pH 6.5 bu e (50)
HEMA_AMPS6.5 HEMA (39.8) AMPS (10) PEGDMA (0.25) pH 6.5 bu e (50)
Figu e 1. UV-cu ed hyd ogels o he cleaning o wa e -sensi i e pain ings: eac ion scheme and chemical s uc u e o he
monome s used and ep esen a ion o a UV-cu ed hyd ogel.
3.3. Ac ylic Pain Mock-Ups
Ac ylic pain was applied on Myla shee s as desc ibed elsewhe e [17]. Two
comme cial pain s (Liqui ex whi e PW6 and b own PB 7) we e selec ed o e alua e he
cleaning e iciency o he me hod. Bo h colo s we e chosen in o de o de ec he a iable
esul s ha may be ob ained acco ding o he di e en na u e o he pigmen s. Whi e
samples we e employed o es he cleaning e iciency, and b own pain was used o es
he sensi i i y owa d he cleaning me hod. Some samples we e kep as e e ence and
o he s we e a i icially soiled acco ding o published p o ocols [24], using a mix u e o
ca bon black, i on oxide, silica, kaolin, gela in powde , soluble s a ch, cemen , oli e oil,
mine al oil and pe oleum spi i , in he s a ed p opo ions [24]. Cleaning es s we e
conduc ed a e h ee yea s [24] o na u al aging. The excess loose soil was emo ed wi h
Figu e 1.
UV-cu ed hyd ogels o he cleaning o wa e -sensi i e pain ings: eac ion scheme and chemical s uc u e o he
monome s used and ep esen a ion o a UV-cu ed hyd ogel.
Table 1.
Weigh pe cen composi ion o he UV-cu ed hyd ogels; 2% weigh based on monome (wbm) o IRGACURE 2956
was added o each solu ion.
Name Monome 1
(w %)
Monome 2
(w %)
C osslinke
(w %)
Aqueous Solu ion
(w %)
HEMA_5 HEMA (49.75) - PEGDMA (0.25) pH 5.0 bu e (50)
HEMA_VP5 HEMA (39.8) VP (10) PEGDMA (0.25) pH 5.0 bu e (50)
HEMA_METAC 5 HEMA (39.8) METAC (10) PEGDMA (0.25) pH 5.0 bu e (50)
HEMA_AMPS5 HEMA (39.8) AMPS (10) PEGDMA (0.25) pH 5.0 bu e (50)
HEMA_6.5 HEMA (49.75) - PEGDMA (0.25) pH 6.5 bu e (50)
HEMA_VP6.5 HEMA (39.8) VP (10) PEGDMA (0.25) pH 6.5 bu e (50)
HEMA_METAC6.5 HEMA (39.8) METAC (10) PEGDMA (0.25) pH 6.5 bu e (50)
HEMA_AMPS6.5 HEMA (39.8) AMPS (10) PEGDMA (0.25) pH 6.5 bu e (50)
3.4. FTIR
A B ueke ALPHA FTIR spec ome e was used o eco d IR spec a o he syn hesized
hyd ogel. The spec a we e collec ed using 32 scans wi h a esolu ion o 2 cm
−1
in he ATR
mode. An a e age spec um was ob ained ou o se e al measu emen s aken om he
su ace o he hyd ogels, in o de o p obe he comple e cu ing o he hyd ogels.
3.5. Colo ime y
Colo measu emen s we e ca ied ou using a Konica-Minol a spec o-colo ime e ,
model CM700d. I was equipped wi h an in eg a ing sphe e, d/8
◦
measu emen geome y,
and wo ked in he 400–700 nm spec al ange wi h a 10 nm acquisi ion s ep. The ligh
sou ce and de ec o we e, espec i ely, a pulsed xenon lamp wi h a UV cu il e and a
silicon pho odiode a ay. The ins umen was p o ided wi h i s own whi e e e ence (100%
e lec i e) and a ze o-calib a ion box (0% e e ence). Colo measu emen s we e acqui ed
using D65 illuminan , a 10
◦
supplemen a y s anda d obse e , and excluded he specula
Polyme s 2021,13, 2108 5 o 13
componen o ligh . The da a epo ed we e based on an a e age o h ee measu emen s
and we e calcula ed o he CIE L*a*b* 1976 colo space. Fo each measu emen , he spec o-
colo ime e was posi ioned in exac ly he same spo (Ø 8 mm) o each sample. Colo
di e ences, ∆(L*a*b*) and ∆E [46], we e calcula ed on a e age alues.
3.6. Scanning Elec on Mic oscope
Scanning elec on mic oscope (SEM) pic u es we e acqui ed using a
Σ
IGMA (Ca l
Zeiss Mic oscopy GmbH, München, Ge many) scanning elec on mic oscope wi h an
accele a ion po en ial o 5 kV. Ac ylic pain ilm mock-ups we e gold-me alized using an
Aga Scien i ic au o spu e coa e .
4. Resul s and Discussion
4.1. Cha ac e iza ion o UV-Cu ed Hyd ogels
Eigh di e en hyd ogels we e syn hesized by UV-pho opolyme iza ion acco ding o
he eac ion scheme shown in Figu e 1. By his me hod, a eady- o-use hyd ogel is di ec ly
ob ained in less han 30 min. As shown in Table 1, each hyd ogel was named acco ding o
he cons i uen monome s, ollowed by he pH alue o he aqueous pa .
The UV-cu ing deg ee o he hyd ogels was cha ac e ized by using FTIR-ATR, as
shown in Figu e 2a, which depic s a ypical FTIR-ATR spec um o a hyd ogel. The spec-
um shows he dis inc i e s e ch o he es e ca bonyl g oup C=O a 1729 cm
−1
and
he dis inc i e CO e he s e ches a 1096 cm
−1
associa ed wi h he ac ylic polyme ne -
wo k. O he bands, co esponding o he CH s e ch a 2866 cm
−1
, CH2 bending s e ch
a 1450 cm
−1
, and CH3 bending s e ch a 1350 cm
−1
, we e also obse ed. In e es ingly,
quan i a i e monome polyme iza ion was demons a ed by he disappea ance o he
cha ac e is ic C=C s e ch band o he ac ylic g oups in he inge p in egion a 1640 and
660 cm
−1
. All in all, he hyd ogel o ma ion h ough he e ec o he adical pho opolyme -
iza ion p ocess induced by he UV-cu ing is con i med. Nex , scanning elec on mic oscopy
(SEM) images we e aken in o de o cha ac e ize he su ace mo phological ea u es o he
UV-cu ed hyd ogels. Figu e 2b shows one ypical image showing a homogeneous smoo h
su ace o a ep esen a i e HEMA-VP pH 5 hyd ogel. All he hyd ogels showed simila
SEM images, and none o he samples showed ele an mo phological ea u es ha could
a ec hei cleaning pe o mance.
G a ime ic measu emen s de e mined he wa e e en ion/ elease capabili y o he
hyd ogel. A e gen ly d ying he hyd ogel wi h blo ing pape , a ully swollen hyd ogel
was weighed and placed in a Pe i dish o e abso ben pape and co e ed wi h a myla
shee o a oid wa e e apo a ion. A e 30 min, wa e elease was de e mined by he weigh
di e ence o he hyd ogel be o e and a e i s applica ion (Table 2). The measu emen s
we e epea ed h ee imes.
I was also necessa y o assess he op ical cha ac e is ics o he hyd ogels acco ding o
he needs o he a conse a o s. The e o e, he selec ed hyd ogels we e hose wi h he
mos adequa e physical p ope ies ( anspa ency, s ickiness, malleabili y, lexibili y) o
pe o ming he cleaning. T anspa en hyd ogels we e p e e ed, since hey allow be e
con ol o soil emo al om he wo k o a . HEMA_5 and HEMA_6.5 we e oo opaque,
while HEMA_METAC5 and HEMA_METAC6.5 we e mo e igid and less s icky. On he
con a y, bo h HEMA_VP and HEMA_AMPS gels a e mo e malleable.
Polyme s 2021,13, 2108 6 o 13
Polyme s 2021, 13, x FOR PEER REVIEW 6 o 14
Figu e 2. (a) FTIR-ATR spec a o HEMA_5. (b) SEM image o HEMA_5 gel su ace.
G a ime ic measu emen s de e mined he wa e e en ion/ elease capabili y o he
hyd ogel. A e gen ly d ying he hyd ogel wi h blo ing pape , a ully swollen hyd ogel
was weighed and placed in a Pe i dish o e abso ben pape and co e ed wi h a myla
shee o a oid wa e e apo a ion. A e 30 min, wa e elease was de e mined by he
weigh di e ence o he hyd ogel be o e and a e i s applica ion (Table 2). The
measu emen s we e epea ed h ee imes.
Table 2. G a ime ic esul s o wa e elease om UV-cu ed hyd ogels.
Name g /cm2
HEMA_METAC 0.03
HEMA_AMPS 0.02
HEMA_VP 0.026
I was also necessa y o assess he op ical cha ac e is ics o he hyd ogels acco ding
o he needs o he a conse a o s. The e o e, he selec ed hyd ogels we e hose wi h he
mos adequa e physical p ope ies ( anspa ency, s ickiness, malleabili y, lexibili y) o
pe o ming he cleaning. T anspa en hyd ogels we e p e e ed, since hey allow be e
con ol o soil emo al om he wo k o a . HEMA_5 and HEMA_6.5 we e oo opaque,
while HEMA_METAC5 and HEMA_METAC6.5 we e mo e igid and less s icky. On he
con a y, bo h HEMA_VP and HEMA_AMPS gels a e mo e malleable.
Figu e 2. (a) FTIR-ATR spec a o HEMA_5. (b) SEM image o HEMA_5 gel su ace.
Table 2. G a ime ic esul s o wa e elease om UV-cu ed hyd ogels.
Name g /cm2
HEMA_METAC 0.03
HEMA_AMPS 0.02
HEMA_VP 0.026
The mechanical p ope ies o he hyd ogels we e analyzed by ensile es s and dynamic
mechanical analysis (DMA) es s. In Figu e 3a,b, he esul s o con olled ension e sus
elonga ion o he hyd ogels p epa ed in bu e s 5 and 6.5 can be obse ed. A pH 5, he
hyd ogels based on poly(HEMA) homopolyme show maximum elonga ion a a b eak
o a ound 50%. The hyd ogels based on he HEMA, including o he comonome s such
as VP, METAC and AMPS, show highe elonga ion a b eak alues be ween 200% and
400% bu a e sho e . A simila end is obse ed o he hyd ogels p epa ed wi h a
bu e o a pH o 6.5. In addi ion, DMA es s we e ca ied ou o s udy he beha io o
hese hyd ogels agains a equency sweep (Figu e 3c,d). The s o age modulus (E
0
) o he
hyd ogels syn hesized wi h bo h bu e s showed alues be ween 1 ×109and 1 ×1010 Pa
a 1 Hz o equency, high alues ha allow hyd ogels o be conside ed as igid ma e ials.
These esul s, oge he wi h he ensile es s, a e indica i e ha hese hyd ogels a e so ,
elas ic and duc ile ma e ials wi h su icien igidi y o be used wi h li le need o ca e.
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Polyme s 2021, 13, x FOR PEER REVIEW 7 o 14
The mechanical p ope ies o he hyd ogels we e analyzed by ensile es s and
dynamic mechanical analysis (DMA) es s. In Figu e 3a,b, he esul s o con olled ension
e sus elonga ion o he hyd ogels p epa ed in bu e s 5 and 6.5 can be obse ed. A pH
5, he hyd ogels based on poly(HEMA) homopolyme show maximum elonga ion a a
b eak o a ound 50%. The hyd ogels based on he HEMA, including o he comonome s
such as VP, METAC and AMPS, show highe elonga ion a b eak alues be ween 200%
and 400% bu a e sho e . A simila end is obse ed o he hyd ogels p epa ed wi h a
bu e o a pH o 6.5. In addi ion, DMA es s we e ca ied ou o s udy he beha io o
hese hyd ogels agains a equency sweep (Figu e 3c,d). The s o age modulus (E′) o he
hyd ogels syn hesized wi h bo h bu e s showed alues be ween 1E109 and 1E1010 Pa a 1
Hz o equency, high alues ha allow hyd ogels o be conside ed as igid ma e ials.
These esul s, oge he wi h he ensile es s, a e indica i e ha hese hyd ogels a e so ,
elas ic and duc ile ma e ials wi h su icien igidi y o be used wi h li le need o ca e.
Figu e 3. Tensile es (a,b) and dynamic mechanical analysis (DMA) (c,d), employing he hyd ogels
p epa ed wi h bu e pH 5 and bu e pH 6.5.
4.2. Applica ion and Main enance o he Hyd ogels
UV-cu ed hyd ogels can be applied bo h d y (di ec ly a e syn hesis) o loaded (a e
24-h imme sion in a wa e -based cleaning solu ion). I he gel is used wa e -loaded, i is
ecommended o blo he gel wi h abso ben pape be o e applica ion in o de o emo e
he excess liquid.
The swelling capabili y o he hyd ogels and hei wa e up ake depends on hei
chemical composi ion. A e 24-h imme sion in wa e , HEMA hyd ogels showed 0%
wa e up ake, HEMA-VP a 11%, HEMA-METAC a 43% and HEMA-AMPS up o 52%,
espec i ely.
Gels can be manipula ed using weeze s and he conse a o ’s own hands, always
wi h clean glo es. Fo he applica ion, place he gel on he su ace ha needs o be cleaned
Figu e 3.
Tensile es (
a
,
b
) and dynamic mechanical analysis (DMA) (
c
,
d
), employing he hyd ogels
p epa ed wi h bu e pH 5 and bu e pH 6.5.
4.2. Applica ion and Main enance o he Hyd ogels
UV-cu ed hyd ogels can be applied bo h d y (di ec ly a e syn hesis) o loaded (a e
24-h imme sion in a wa e -based cleaning solu ion). I he gel is used wa e -loaded, i is
ecommended o blo he gel wi h abso ben pape be o e applica ion in o de o emo e
he excess liquid.
The swelling capabili y o he hyd ogels and hei wa e up ake depends on hei
chemical composi ion. A e 24-h imme sion in wa e , HEMA hyd ogels showed 0%
wa e up ake, HEMA-VP a 11%, HEMA-METAC a 43% and HEMA-AMPS up o 52%,
espec i ely.
Gels can be manipula ed using weeze s and he conse a o ’s own hands, always
wi h clean glo es. Fo he applica ion, place he gel on he su ace ha needs o be cleaned
and apply a li le p essu e o enhance he adhesion be ween he gel and he su ace. A
plas ic ilm can be placed o e he gel o educe he e apo a ion o wa e du ing he
cleaning p ocess.
Gels should be s o ed, whe he d y o imme sed in wa e , in closed con aine s. They
can be kep in da k condi ions a oom empe a u e o se e al mon hs a e he syn hesis.
I is ad isable o check gels a e a long s o age ime, in o de o e i y ha no al e a ion
has occu ed.
4.3. Cleaning E iciency o UV-Cu ed Hyd ogels
All he hyd ogel o mula ions desc ibed in Table 1we e es ed in o de o assess
he cleaning pe o mance when applying he hyd ogels on a i icially soiled mock-ups.
Cleaning es s we e ca ied ou by di ec applica ion o he UV-cu ed hyd ogels on o he
soiled pain laye s o 20 s. Applica ion ime can be adjus ed acco ding o he speci ic needs
o he su ace. Good mechanical p ope ies allow easy handling and i is wo h no ing ha
hey can be di ec ly applied o he su ace o he pain ing. UV-cu ed hyd ogels we e used
Polyme s 2021,13, 2108 8 o 13
as p epa ed, wi hou he need o any addi ional loading o bu e ed aqueous solu ions,
hus educing he wa e con en .
SEM images we e aken be o e and a e cleaning o he soiled su aces o compa ison
wi h he cleaned a eas. The mos ep esen a i e images o he esul ing su aces a e clean-
ing a e shown in Figu e 4a. Cleaning wi h HEMA_VP5, HEMA_VP6.5, HEMA_AMPS5
and HEMA_AMPS6.5 gels showed he bes esul s ega ding e icien soil emo al, no
we ing he su ace and inducing no colo change o he su ace. As shown in Figu e 4e,
hyd ogels wi h he bes cleaning capabili y we e also hose ha showed he bes op ical
pe o mance. Acco ding o UNE EN 15886:2011, he
∆
E* alue should be below 2 ( ed line
in Figu e 5) o he colo a ia ion o no be pe cep ible o he human eye.
Polyme s 2021, 13, x FOR PEER REVIEW 9 o 14
Figu e 4. (a–d) SEM images be o e and a e cleaning o whi e Liqui ex mock-ups using
HEMA_VP5.0 and HEMA_AMPS5.0 gels. (e) Colo ime y esul s be o e and a e cleaning using
he s udied gel o mula ions. The ed line ep esen s ∆E* limi o isual colo -change pe cep ion.
( ) Cleaning wi h HEMA_VP5.0 on whi e mock-up.
Figu e 5. Cleaning es wi h HEMA_VP6.5 and HEMA_VP5.0 (le ). Colo ime y esul s o soiled and cleaned su aces
wi h HEMA_VP5.0 and HEMA_VP6.5 gels.
Figu e 4.
(
a
–
d
) SEM images be o e and a e cleaning o whi e Liqui ex mock-ups using HEMA_VP5.0
and HEMA_AMPS5.0 gels. (
e
) Colo ime y esul s be o e and a e cleaning using he s udied gel
o mula ions. The ed line ep esen s
∆
E* limi o isual colo -change pe cep ion. (
) Cleaning wi h
HEMA_VP5.0 on whi e mock-up.
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Figu e 4. (a–d) SEM images be o e and a e cleaning o whi e Liqui ex mock-ups using
HEMA_VP5.0 and HEMA_AMPS5.0 gels. (e) Colo ime y esul s be o e and a e cleaning using
he s udied gel o mula ions. The ed line ep esen s ∆E* limi o isual colo -change pe cep ion.
( ) Cleaning wi h HEMA_VP5.0 on whi e mock-up.
Figu e 5. Cleaning es wi h HEMA_VP6.5 and HEMA_VP5.0 (le ). Colo ime y esul s o soiled and cleaned su aces
wi h HEMA_VP5.0 and HEMA_VP6.5 gels.
Figu e 5.
Cleaning es wi h HEMA_VP6.5 and HEMA_VP5.0 (le ). Colo ime y esul s o soiled and cleaned su aces
wi h HEMA_VP5.0 and HEMA_VP6.5 gels.
4.4. Tes ing he Cleaning E iciency Acco ding o pH
To check he e ec o he pH on he cleaning e iciency [
6
], hyd ogel o mula ions we e
cu ed by using wo di e en pH-adjus ed wa e solu ions, wi h pHs a he uppe and lowe
secu i y ange o ac ylic emulsion pain s (pH 5 and pH 6.5) [
6
,
18
]). Acco ding o p e ious
wo ks, gels wi h highe pH we e expec ed o pe o m a deepe albei mo e agg essi e
cleaning [
27
,
29
]. Howe e , as shown in Figu e 5, no signi ican di e ences we e obse ed.
The cleaning esul s o HEMA_VP gels a e shown in Figu e 5, which a e consis en o all
he o mula ions. Mac oscopic esul s showed a simila cleaning e iciency o bo h pH
alues, suppo ed by colo ime y da a. Thus, pH 5.0 hyd ogels we e chosen since hey
achie ed good cleaning esul s while main aining he sys em in lowe (sa e ) alues o pH.
4.5. Aga Gels s. UV-Cu ed Hyd ogels
The cleaning pe o mance o UV-cu ed hyd ogels was compa ed wi h a e e ence
aga hyd ogel loaded wi h a pH 5.0 bu e ed aqueous solu ion. The aga hyd ogel was
p epa ed in a 3% aqueous bu e solu ion, as desc ibed elsewhe e [
47
]. Aga gel is e y
popula among conse a o s since i is easy o p epa e ou o labo a o y condi ions, which
is mo e a ac i e o use s. Howe e , he highe wa e abso p ion and he syne esis
p ope y [
48
] o aga gels led o a less con ollable elease o liquid ha emains on he
su ace a e emo ing he gel (see Figu e 5) [
40
]. This ca ies se e al d awbacks o
wa e -sensi i e pain s [
49
]. In con as , UV-cu ed hyd ogels ha e p o en o be highly
e en i e, allowing he con olled ac ion o he aqueous solu ions e en on highly sensi i e
subs a es. The wa e elease capaci y o bo h UV-cu ed hyd ogels and aga gels was
measu ed g a ime ically, o compa ison. Fully swollen gels we e placed on a Pe i dish
o e abso ben pape and co e ed wi h a myla shee o a oid wa e e apo a ion. A e
30 min, wa e elease was de e mined by weighing he il e pape be o e and a e he
applica ion. The measu emen s we e epea ed h ee imes.
The wa e elease a e o he UV-cu ed hyd ogel and o he aga gel is epo ed in
Table 3. The excess we ing o he su ace when using aga is e idenced by he wa e
emaining on he su ace oge he wi h he g a ime ic esul s.
