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Progress in Organic Coatings 77 (2014) 247 256
Contents lists available at ScienceDirect
Progress in Organic Coatings
jou rn al hom ep age: www.elsev ier .com
Furan r ctivStructu riz
G. Riveroa Ecomateriales sidad 7600 Mar del Pb Ciencia e Inge s
(INTEJuan B. Justo 43c Electroqumic EMA), Justo 4302, 760
a r t i c l
Article history:Received 5 JunReceived in reAccepted 27
SAvailable onlin
Keywords:Furan resinsPhenolic resinsOrganic protective
coatingsNanoindentation
ient agous s wors, toxform
traditional phenolic resin. Physicochemical characteristics
including chemical structure, surface polarityand glass transition
temperature were evaluated by means of Fourier transform infrared
spectroscopy,contact angle measurements and dynamicmechanical
analysis, respectively. Nanomechanical and nan-otribological
properties were assessed by depth sensing indentation techniques.
As well, the corrosionresistance of the furan coating was
determined by potentiodynamic polarization tests. The
obtainedresults validate the furan resin as a feasible alternative
to phenolics to protect aluminum.
1. Introdu
Phenolichave excellreactivity wthey have btrial applicaadhesives
aapplicationtogether wsion. For exelectrical inthus prevenmedium
[4,regulationssince the 8applicationproduction,products co
CorresponE-mail add
[email protected].
0300-9440/$ http://dx.doi.o 2013 Elsevier B.V. All rights
reserved.
ction
resins, synthesized from phenol and formaldehyde,ent chemical
and thermal resistance, compatibility andith other resins. Due to
these desirable properties,een widely used for over 100 years in
many indus-tions that range from molding compounds to laminates,nd
protective coatings [13]. Particularly, the latter
takes advantage of their minimal thermal expansionith their high
resistance to abrasion, wear and corro-ample, they are used as
protective interior varnishes,sulators layers and anticorrosion
coatings for metals,ting the direct contact of metallic substrates
with the5]. However, due to its high toxicity, there are
rigorous
concerning the reduction of formaldehyde emissions0s [613]. The
main restriction is pointed to indoors, but formaldehyde release
can also occurs in every
usage, storage, transportation and deposition stage ofntaining
residual formaldehyde [7].
ding author. Tel.: +54 223 4816600; fax: +54 0223
4810046.resses: [email protected] (G. Rivero),edu.ar (L.A.
Fasce), [email protected] (L.B. Manfredi).
Following the current tendencies to a minimization
ofpetroleum-based materials, furfural is an appealing option
toreplace formaldehyde in the resins formulation. Furfural can
beeasily obtained from agriculture wastes; it is harmless to the
ozonelayer and is not as toxic as formaldehyde is. Novolak resins
preparedfrom furfural, phenol and formaldehyde have been used
duringdecades for molding compounds, and many commercial
patentshave been registered [1417]. Despite the great efforts to
elimi-nate formaldehyde from formulations, up to our knowledge,
thereis only one recent commercial register for molding resins
based onfurfuryl alcohol, glyoxal and urea, without formaldehyde
[18].
The reaction between phenol and furfural to give a furan
resinwithout using formaldehyde in the initial formulation has
beenrecently studied in our group [19,20]. In this work, the
formu-lated furan resin was proposed as a potential protective
coating foraluminum in replacement of the traditional phenolic
resins. Theanticorrosion function of the coating may be strongly
related to itsmechanical performance, thus directly inuenced by its
chemicalstructure. Hence, an extensive experimental analysis was
carriedout to evaluate the efciency of the proposed coating.
In order to characterize the mechanical performance of
thincoatings deposited onto substrates, nanoindentation
combinedwith nanoscratch tests appear as the most appropriate and
accuratetechniques [2124]. In a nanoindentation test, a tip of
well-dened
see front matter 2013 Elsevier B.V. All rights
reserved.rg/10.1016/j.porgcoat.2013.09.015esins as replacement of
phenolic proteral, mechanical and functional charactea, L.A.
Fasceb, S.M. Cerc, L.B. Manfredia,
Instituto de Investigaciones en Ciencia y Tecnologa de
Materiales (INTEMA), Univerlata, Argentinaniera de Polmeros
Instituto de Investigaciones en Ciencia y Tecnologa de Materiale02,
7600 Mar del Plata, Argentinaa y Corrosin Instituto de
Investigaciones en Ciencia y Tecnologa de Materiales (INT0 Mar del
Plata, Argentina
e i n f o
e 2012vised form 29 April 2013eptember 2013e 27 October 2013
a b s t r a c t
Phenolic coatings are usually a convenand corrosion. Furan
resins are analoby furfural in their formulation. In thiand used as
an aluminum coating. Thuderivative was used instead. The per/
locate /porgcoat
e coatings:ation
Nacional de Mar del Plata, Juan B. Justo 4302,
MA), Universidad Nacional de Mar del Plata,
Universidad Nacional de Mar del Plata, Juan B.
nd economical way to protect metallic materials against wearto
phenolics, as they are obtained by replacing formaldehydek, a furan
resin based on furfural and phenol was synthesizedic emissions of
formaldehyde were avoided, while a biobasedance of the proposed
resin was compared with the one of a
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248 G. Rivero et al. / Progress in Organic Coatings 77 (2014)
247 256
geometry is driven into the material surface while the
appliedforce and tip displacement data are collected. From
nanoindenta-tion data, near surface elastic and plastic properties
can be easilydetermined using the widespread method of analysis
proposedby Oliver ato characterequires sutip is movedload is
applare recordemine the frcoatings an[28]. In
adding/substrainterface oc
The corrby potentiodensity genmetal sampor defects tcorrosion
dto evaluate systems [30
In this wicochemicaof furan andout. In addaluminum w
2. Experim
2.1. Materi
A resolformaldehynol and a fowere placedity stirrer, aat 9.0
withallowed to tralized witreached. A 1prepared.
A furan pfurfural (Flurefrigerant,reaction me40 wt/vol%.30 min
in aature was mstructural cviously repo110 C and as a
catalysprepared.
Differentypes of reously polishalumina sulms of diffusing the
pr(50 and 100
A two-scoated alumperature to
at this temperature to allow the release of water and oligomer
bub-bles. In the second step, the temperature was raised up to 180
C(for furan coatings) and 190 C (for phenolic coatings) at 1
C/minand kept for 30 min to complete the curing.
eaftefor fumbe
ethod
Visco viscntongurared a
Fouri evonal g
cyclen mod ev400
ith thing (
DynaA exing r. Pri
the r relea
glasorreerage
Contatic cod usete
re. Drfacewas r
Scann an
thick waryofrse thted ster inerat
pation etric
Nanooindted ng Prlysisof thdentteriand Pharr [25]. This method
has been already appliedrize different polymeric lms with success
[26,27] butitably designed experiments. In a nanoscracth test,
the
over the surface while a constant or progressive normalied. The
tangential force and the normal displacementd. Nanoscratch tests
have been effectively used to deter-iction coefcient and scratch
resistance of polymericd to study the near surface deformation
mechanismsition, the adhesion strength can be evaluated for coat-te
systems, provided that coating debonding along thecurs in response
to load [26,29].osion protective quality of coatings can be
characterizeddynamic polarization tests that measure the
currenterated after applying a cyclic potential to the coatedle. An
electrical response implies the existence of poreshat allow the
passage of ions, thus evidencing certainamage. This technique has
been successfully employedthe corrosion process of several
metal/polymer coating33].ork, a deep study concerning the
comparison of phys-l characteristics and surface nanomechanical
behavior
phenolic resins deposited onto aluminum was carriedition, the
corrosive protection of the furan coating toas evaluated.
ental
als and sample preparation
-type phenolic prepolymer was prepared with ade to phenol molar
ratio equal to 1.3. Samples of phe-rmaldehyde aqueous solution
(37%, wt/wt) (Cicarelli)
in a stainless steel reactor equipped with a low veloc-
thermometer and a reux condenser. The pH was kept
a solution of NaOH 40% (wt/wt) and the mixture wasreact for 2 h
at 90 C. Thereafter, the mixture was neu-h a solution of boric acid
until a pH value of 6.87.0 was0 wt% solution of phenolic prepolymer
in acetone was
repolymer was synthesized from phenol (Anedra) andka). Phenol
was molten into a reactor supplied with a
a thermometer and constant mechanical stirring. Thedia was
adjusted using an aqueous solution of K2CO3
After heating up to 135 C, furfural was dropped during furfural
to phenol molar ratio equal to 1. The temper-
aintained at 135 C for 4 h. A detailed chemical
andharacterization of the furan prepolymer has been pre-rted [20].
The furan prepolymer was then heated up to
12 wt% of hexamethylenetetramine (HMTA) was addedt. A 10 wt%
solution of furan prepolymer in acetone was
t samples of aluminum (6063-TG) coated with bothsins were made.
The aluminum substrate was previ-ed with increasing grades of
silicon carbide paper and
spensions with particle sizes up to 0.05 m. Polymericerent
thickness were prepared by dip-coating processepolymer solutions
and two different withdrawal rates
cm/min). In all cases, the immersion time was 4 s.tep thermal
curing cycle was applied to both types ofinum plates. They were rst
heated from room tem-
120 C at 1 C/min and the samples were kept for 30 min
HerF-100 The nu
2.2. M
2.2.1. The
in an Aa conmeasu
2.2.2. The
functiocuringmissioscanneof 600ized wstretch[34].
2.2.3. DM
a heatof 1 Hzcuringsilicon
Theature cThe av
2.2.4. Sta
methoGoniomSoftwathe sudrops
2.2.5. SEM
actual SEM
were cto expo
Coadiamewas op(0.2 minspeca geom
2.2.6. Nan
on coaScanni
Anaerties nanoining mar coated aluminum samples are identied as
F-50 andran coatings and P-50 and P-100 for phenolic coatings.rs
indicate the dipping withdrawal rate.
s
sity measurementsosity of the 10 wt% prepolymer solutions was
measured
Paar Physica Rheometer (MCR 301-CC27) at 20 C, withtion of
concentric cones. The stationary viscosity wasmong 1 and 100 rpm
using 20 ml of solution sample.
er transform infrared analysis (FTIR)lution of characteristic
bands associated to differentroups was followed by FTIR analysis
through the whole. Spectra were obtained in a Mattson Genesis II,
trans-de, equipped with a heating furnace. Samples were
ery 10 C from room temperature to 180 C in the range0 cm1. For
comparison purposes, spectra were normal-e intensity of the band
assigned to the C C benzene ring1595 cm1) whose value is expected
to remain constant
mic mechanical analysis (DMA)periments were carried out in a
Perkin Elmer DMA 7 atate of 10 C/min from 15 to 300 C and at a
frequencysmatic specimens were cut from plaques obtained byesins
between two glass slides previously treated withse agent (Siliar
S.A., Argentina).s transition temperature (Tg) was taken as the
temper-sponding to the maximum displayed in the tan curve.
results of three replicas were reported.
ct angle measurementsntact angle determination was made by the
sessile droping a Ram Hart model 500 Advanced Contact
Angler/Tensiometer equipped with DROPimage Advancedrops of 5 l of
doubly distilled water were placed ontos of coated samples. The
average contact angle of sixeported for each coating.
ing electronic microscopy (SEM) and Calotest
d Calotest experiments were used to determine theness of furan
and phenolic coatings.s performed in a JEOL JSM-6460LV equipment.
Samplesactured and coated with a 300 A gold layer previouslyeir
lateral side. A typical micrograph is shown in Fig. 1.amples were
ground by a rotating steel ball of 30 mm
a Calotest Compact unit from CSM Instruments. Ited at 2500 rpm
for 2 min with an abrasive suspensionrticles). Coating thickness
was determined by opticalof the abraded coating and substrate
section applyingal relationship [35].
indentation and nanoscratch testsentation and nanoscratch
experiments were carried outsamples in a Triboindenter Hysitron
equipped with aobe Microscope module (SPM).
procedures for determining reliable mechanical prop-in polymeric
lms deposited onto metal substrates byation are not yet
standardized as for other engineer-ls. For the sake of simplicity
and aiming to compare
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G. Rivero et al. / Progress in Organic Coatings 77 (2014) 247
256 249
Fig. 1. Coating thickness determination by SEM analysis in F-100
sample.
nanomechanical properties of furan and phenolic lms,
experi-ments were accurately designed to apply the simple Oliver
andPharr approach [25,36].
Nanoindentation tests were performed under load control
con-ditions using a diamond Berkovich tip. Maximum loads (Pmax)
werevaried from 0.1 to 9 mN to obtain properties at different
indentationdepths aiming to analyze the possible inuence of the
aluminumsubstrate [37,38].
For eaching period oand unloading curve [3using the st
Using thtic modulusfunction of to contact sof the mateated
[27,41]slope of the
Two types of nanoscratch experiments were performed. In therst
one, a diamond Berkovich tip was used to obtain the
frictioncoefcient. A constant load of 100 N, a sliding speed of
0.33 m/s,and a lateral displacement of 10 m were applied. For each
sample,10 experiments were carried out. The apparent friction
coefcient() was calculated as the ratio of the measured tangential
(Fx) andnormal (Fz) forces. The second type was intended to analyze
the sur-face damage behavior during scratch deformation of furan
coatings.A diamond spherical tip with 2 m radius of curvature was
slid ata speed of 0.33 m/s while the load was linearly increased up
to9 mN, which was the maximum force experimentally available.
2.2.7. Potentiodynamic polarizationPotentiodynamic polarization
tests were carried out with a
standard three-electrode system to quantify the anticorrosive
pro-tection of the furan coating. The specimens were connected to
thealuminum either coated or uncoated as working electrode, a
pureplatinum wire was used as the counter electrode and
standardcalomel electrode (SCE) was used as the reference
electrode. Poten-tiodynamic scans were performed in NaCl solution
(0.15 mol/L)from 0.8 to 0.5 V at a scan rate of 1 mV/s and
backwards. The polar-ization curves were measured in a Gamry Ref
600 electrochemicalunit (Gamry Instrument, USA) initially and,
after 12 and 30 days ofimmersion in the solution at 20 C.
3. Results and discussion
3.1. Physicochemical characterization
syndehyion rs areatizeracteutedal stnds cross
rang loading condition, 5 indentations were made. A hold-f 15 s
was applied at maximum load between loading
ing stages to minimize the creep effect on the unload-9,40]. Tip
displacement was corrected by thermal driftandard Hysitron
routine.e approach outlined by Oliver and Pharr, reduced elas-
(Er) and indentation hardness (H) were calculated as
aindentation depth [25,36]. As well, the maximum
loadtiffness-squared parameter (P/S2), which is a measurerial
resistance to permanent deformation, was evalu-. The contact
stiffness (S) was calculated from the initial
unloading curve.
Theformaldensatbridgeschem
ChasubstitchemicThe bato the lengthFig. 2. Scheme showing the
condensation reactions that take placethesis of phenolic resins
starts with the addition ofde to the phenol ortho and para
positions. Then, con-eactions take place during curing in which
methylene
formed and water and formaldehyde are released, asd in Fig.
2.ristic infrared spectra wavelengths corresponding to
benzene rings as well as methylene bridges in theructure of
phenolic resins are well documented [4244].corresponding to
methylene bridges, directly relatedlinking density, appear in the
14001500 cm1 wave-e of the infrared spectra, and their specic
position
during curing of phenolic resins.
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250 G. Rivero et al. / Progress in Organic Coatings 77 (2014)
247 256
Fig. 3. Schem
can be relaparapara
lene bridgein furan rescrosslinked
As expespectra dueferences wein the furanbridges wassistently
wiof bridges fof op andnolic resin, However, thresulted sim
The valuulus in the rfrom DMA values of bsimilar cheto
crosslinkvery similar
The contples are incle showing the chemical structure of (a)
phenolic and (b) furan resins.
ted to a particular position in the phenol ring; i.e.(pp),
orthoortho(oo) and orthopara (op) methy-s, respectively [34].
Similarly, CH bridges are formedins during their curing reactions.
The resulting highly
structure of both resins are shown in Fig. 3 [4,45].cted, furan
and phenolic resins displayed similar FTIR
to their analogous chemical structure though slight dif-re
observed. The three types of bridges were assigned
resin spectra (Fig. 4a), but the band associated with oo
not detected in the phenolic resin spectra (Fig. 4b) con-th
previous reports [46,47]. The variation in the amountormed during
curing is compared in Fig. 5. The amount
pp bridges was larger in the completely cured phe-but additional
oo bridges appeared in the furan one.e nal amount of total bridges
contained in both resinsilar.
es of glass transition temperature (Tg) and storage mod-ubbery
state (Erubber) of phenolic and furan resins arisenexperiments are
reported in Table 1. Tg and modulusoth resins were practically
identical, evidencing theirmical structure. Moreover, as Erubber is
directly related
density [48], the total amount of bridges appeared to be in both
resins in agreement with FTIR results (Fig. 5).act angle
measurements for both types of coated sam-uded in Table 1. The
lower value measured for the furan
Fig. 4. Region of the FTIR spectra showing the characteristic
bands correspondingto bridges in (a) furan and (b) phenolic resin.
Some spectra obtained at differenttemperatures during curing are
shown.
Fig. 5. Evolution of the bridges amount in phenolic and furan
resins during curing.
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G. Rivero et al. / Progress in Organic Coatings 77 (2014) 247
256 251
Table 1Dynamic-mechanical properties and contact angle of the
resins.
Resin Erubber (GPa) Tg (C) Contact angle (degree)
Phenolic 0.25 255 74.9 0.8Furan 0.24 252 66.8 2.5
Table 2Coating thickness values of furan and phenolic
samples.
Sample P-50 P-100 F-50 F-100
Coating thickness(m)
0.24 0.08 0.29 0.08 2.14 0.10 2.62 0.04
coating displayed its greater hydrophilic character. The
higherpolarity is a result of the polar groups in the furan coating
surface,but it is also related with the interaction between these
groupsand the hydroxyls of the aluminum substrate, as it will be
laterdiscussed.
3.2. Coating thickness
Actual thickness values of furan and phenolic coatings mea-sured
by SEM and veried by Calotest experiments are reported inTable 2.
Phenolic lms were notably thinner than furan ones whenprepared
uwithdrawalthe withdra
It is welseveral forcdrag [49,50the substratdrawal speelead to
thic
The visclic prepolymrespectivelynesses can prepolymer
The withto control tis usually ewith x valuplotted acco
Fig. 6. Relatioand furan resin
oad vs. indentation depth curves for different maximum applied
loads for (a)d (b) P-100 systems. The indentation response of the
aluminum substrateed for comparison.
of x about 0.3 was observed, evidencing the same thicknessence
with withdrawal speed. Hence, as differences in thick-e mainly due
to the viscosity, similar values for both coatingse achieved by
adjusting the solution concentration.
nomechanical properties
ical loaddepth curves obtained at different maximum loads for
two of the studied systems are shown together
he substrate response in Fig. 7. The mechanical responsecoated
systems was completely different from the one ofbstrate. While
aluminum deformed plastically, the coat-s able to elastically
recover deformation after the tip wased from the surface.
Properties were determined as a func-
indentation depth and analyzed considering the inuenceunderlying
substrate. The time-dependence of the indenta-sponse of the systems
was clearly evident by the increase insing the same prepolymer
solution concentration and rate. As expected, the coating thickness
decreased aswal rate diminished for each type of resin.l known that
the dip coating process is controlled byes such as gravitational,
inertia, capillary and viscous]. The latter force moves the liquid
solution upward withe and it is proportional to the liquid
viscosity and with-d. So, larger solution viscosities and
withdrawal speedsker coatings.osities of the 10% (w/w) solutions of
furan and pheno-ers in acetone were 17.93 0.24 cP and 3.95 0.12
cP,. So, the great difference in the resulting coating thick-be
directly related to the dissimilar viscosity of their
solutions.drawal speed, u0, is the most common parameter usedhe
lm thickness, t. The relationship between t and u0xpressed as a
power law mathematical form (t u0x)es ranging from 0 to 1 [51].
Fig. 6 shows thickness datarding to the mentioned correlation. For
both coatings
Fig. 7. LF-100 anis includ
a valuedependness arcould b
3.3. Na
Typappliedwith tof the the suing waremovtion ofof the tion
renship between coating thickness and withdrawal rate for
phenolics. Additional data at 25 cm/min are also included.
indentationtion creep).curves pracchanged wcoatings (i.minum
resploaddeptherally attribTherefore, aluminum s depth during the
holding period at peak load (indenta- For the thickest coatings
(i.e. F-100 in Fig. 7a), loadingtically overlapped while the shape
of unloading curvesith increasing maximum applied load. For the
thinnere. P-100 in Fig. 7b), the curves resembled the alu-onse
especially at high indentation loads. For all cases,
curves did not exhibit discontinuities, which are gen-uted to
failure events like coating detachment [5254].the adhesion of both
furan and phenolic lms to theubstrate appeared to be very good.
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252 G. Rivero et al. / Progress in Organic Coatings 77 (2014)
247 256
Fig. 8. Contaccoatings on alu
The conpenetrationexpected totic modulusdeviation frtation
deptmeasured iminum sub
Reduceddentation thickness-npendently dthe Er valuetation
deptcombines thlying substrcoating [22iments for difference i
For furavery low insic elastic mdeterminedcompliant cthan 1 m,at
depths lovalues of futhickness a
On the ccase of phe(about 0.2 range resulreliable indgenerally
dtact area esvibrations. phenolic coant coatingconsiders thapparent
m
E = Ec + (E
where E* reEs are the i
Reduced elastic modulus values as a function of thickness
normalizedion depth for (a) furan coatings and (b) phenolic
coatings deposited ontom substrate.
actual coating thickness and is a tting parameter. If Esn, the
model tting provides the values of Ec and . The
matical form of Eq. (1) predicts a plateau value for low
con-pths. Fitted model parameters are shown in Table 3. The eter
increased while decreasing the coating thickness indi-a not
straightforward dependence of apparent modulus witht stiffness vs.
indentation depth for furan coatings and phenolicminum.
tact stiffness values, S, are plotted against maximum depth in
Fig. 8. For a homogeneous material, S is
increase linearly with indentation depth since elas- does not
vary with depth [38]. Data showed a positiveom linearity, which
increased with increasing inden-h or reducing coating thickness.
This means that thendentation response was inuenced by the stiff
alu-strate.
elastic modulus values, Er, obtained from nanoin-experiments are
shown in Fig. 9 as a function oformalized depth. A value of 72.5
2.8 GPa was inde-etermined for the aluminum substrate. As
expected,
s of the coating/substrate systems increased with inden-h. These
values represent an apparent property whiche mechanical properties
of the coating and the under-ate, and are not an elastic modulus
prole through the]. The dissimilar hmax/t range achieved in the
exper-furan and phenolic coatings was due to the large
n their thicknesses (Table 2).n coatings, a plateau value was
clearly observed atdentation depths (Fig. 9a). This value was the
intrin-odulus of the furan coating. It coincided with the value
by applying the 10% rule, which establishes that foroating/stiff
substrate systems and for coatings thicker
the elastic modulus of the coating can be evaluatedwer than 1/10
of the coating thickness [25,55]. The Er
Fig. 9. indentataluminu
is the is knowmathetact deparamcating ran coatings turned out to
be independent on coatingnd near 10 GPa (Table 3).ontrary, a
plateau value could not be observed for thenolic coatings (Fig.
9b). Because coatings were too thinm), the minimum experimentally
achievable hmax/t
ted higher than 1/10 of the coating thickness. Obtainingentation
curves at very low displacements (
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G. Rivero et al. / Progress in Organic Coatings 77 (2014) 247
256 253
Table 3Intrinsic nanomechanical properties of furan and phenolic
lms.
Property Method F-50 F-100 P-50 P-100
Coating elasticmodulus (GPa)
10% rule 9.8 0.7 9.5 0.6 (Ec) Eq. (1) 10.2 0.1 (R2 = 0.91) 9.7
0.1 (R2 = 0.87) 12.4 1.2 (R2 = 0.93) 10.5 1.2 (R2 = 0.92)
Coating hardness (GPa) Average value 0.63 0.03 0.62 0.05 (Hc)
Eq. (2) 0.57 0.04 (R2 = 0.82) 0.59 0.02 (R2 = 0.89)
hardness (Table 3). The intrinsic hardness of the furan coating
wasabout a half of the aluminum hardness.
On the other hand, H values increased with increasing
thickness-normalized depth for the case of phenolic coatings (Fig.
11b).Moreover, it is observed that for hmax/t larger than 1, H
dataapproached the value of the aluminum (1.15 0.05 GPa). Aiming
toobtain the intrinsic hardness of phenolic coatings, a simple
modelproposed by Bhattacharya and Nix [59] was applied. The
modelconsiders the combined effect of substrate and coating
propertieson the apparent hardness as a function of indentation
depth, as:
H = Hs + (Hc Hs) exp(
n (
h
t
)n)(2)
where H* represents the apparent hardness, Hc and Hs are
theintrinsic properties of coating and substrate, is a tting
parameterand n is equal to 2 for the case of soft coatings/hard
substrates sys-tems [23]. Tof the phenthe furan co
The valuPmax/S2, i.e. plotted as aboth types oparameter ever, it
cleamore resistThis result elastic and aluminum. ulus but ona given
inda larger amto a lower ptypes of coaing that the
Fig. 10. ReducSolid lines sho
similar. Consistently, the H/Er2 ratio which is proportional to
thePmax/S2 parameter, was almost identical for both types of
coatings(Table 3).
3.4. Nanoscratch behavior
The average values of the apparent friction coefcient,
app,measured at low loads were 0.35 and 0.38 for furan and
phenoliccoatings, respectively. In these experiments, the scratch
grooveswere imperceptible indicating that deformation was
completelyrecovered after load removal, i.e. an elastic deformation
modeprevailed. The app values reected the adhesion
characteristicsbetween the coating and the diamond tip since the
principal fric-tion mechanism under elastic contact conditions is
adhesion. Thehe tted Hc values are reported in Table 3. The Hc
valueolic coating was very close to the H value displayed
byatings.es of the ratio of maximum load to stiffness squared,the
materials resistance to plastic deformation [54], are
function of thickness-normalized depth in Fig. 12 forf coatings.
Due to the inuence of the substrate, Pmax/S2
decreased as the indentation depth increased. How-rly emerged
that furan and phenolic coatings resultedant to plastic deformation
than aluminum substrate.could be explained by the benecial
combination ofplastic properties of polymeric coatings in relation
toPolymeric coatings showed about a 7-fold lower mod-ly 2-fold
lower hardness than the substrate. Hence, atentation depth, the
coating was able to accommodateount of elastic deformation than the
substrate, leadingermanent damage. In addition, Pmax/S2 values of
bothtings overlapped for the lowest hmax/t range, indicat-
mechanical protection conferred by both coatings wased elastic
modulus as a function of thickness to contact depth ratio.w model
ttings according to Eq. (1). The parameter is reported.
Fig. 11. Hardnfor (a) furan coThe dashed liness values as a
function of thickness normalized indentation depthatings and (b)
phenolic coatings deposited onto aluminum substrate.e represents
the hardness of the aluminum substrate.
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254 G. Rivero et al. / Progress in Organic Coatings 77 (2014)
247 256
Fig. 12. Maximum load to the stiffness squared values as a
function of thicknessnormalized indentation depth for furan and
phenolic coatings. The dashed linerepresents the value determined
for the aluminum substrate alone.
slightly lower app of the furan resin was consistent with its
higherpolarity.
The damexperimentobtained foslope in thwas observform. The athe
normalmation mescratch grocoating deftrack as wecess causedthe tip,
thuwell, in SPMnation, demcoating andof a compliaif the shear of
the syst
Fig. 13. Normplacement meloading.
Fig. 14. SPM image of the scratch groove left on the furan
coating surface of F-100sample after ramping normal loading.
[62oatinesumnismss, wlic reiffers off
rrosi
ults sion um r imd a she coat reponsnces. For the coated sample,
the current density decreasedage mechanism was investigated through
nanoscratchs under increasing normal load. A typical responser
furan coatings is shown in Fig. 13. An almost constante normal
displacement vs. lateral displacement curveed indicating that the
mechanical response was uni-pparent friction coefcient increased
with increasing
force (or scratch distance) consistently with a defor-chanism of
ductile ploughing [60]. SPM images of theoves revealed that during
the sliding process the furanormed plastically and accumulated at
both sides of thell as ahead of the crack tip (Fig. 14). The
scratch pro-
shear-dominant stresses in the immediate vicinity ofs promoting
shear yielding of the furan resin [61]. As
images there were no evidences of coating delami-onstrating that
the adhesion strength between furan
aluminum was excellent. It is known that for the casent coating
on a stiff substrate, delamination could occurstress along the
interface exceeds the adhesion strengthem because the shear stress
tends to dislocate the
coatingnolic c
In rmechahardnephenois no dcoating
3.5. Co
Resimmeralumin
Afteshoweity of tlm thcal resdiffereal displacement and friction
coefcient as a function of lateral dis-asured for F-100 sample in
Nanoscratch tests under ramping normal Fig. 15. Typic
aluminum sam]. A similar scratch behavior was observed for the
phe-g.e, nanoscratch tests showed that coating deformation
was dominated by the yield stress. As the indentationhich is
proportional to the yield stress, of furan and
sins was practically the same, it can be stated that thereence
in the mechanical protection that both types ofered to the aluminum
substrate.
on behavior
of potentiodynamic polarization tests at differenttimes are
shown in Fig. 15 for uncoated and furan coatedspecimens.mersion in
the NaCl solution, the uncoated aluminumtrong diminution of the
current density in the vicin-rrosion potential, due to the
development of a passivemained stable in the assayed period. The
electrochemi-e of uncoated and furan coated specimens showed
clearal potentiodynamic polarization curves of uncoated and furan
coatedples.
-
G. Rivero et al. / Progress in Organic Coatings 77 (2014) 247
256 255
Fig. 16. Changted aluminum(longitudinal sAl OH), with time is
also inc
approximataluminum. substrate efollowed thformation oimproved
w
As expectiodynamicnor furan cosuch defect
The gooaluminum ity among hydroxylateminum is ispontaneoulayer
causinThe presencsamples imThere was oistics bandsexposure timthat
the chtered durin
4. Conclus
A deep ccal and alumcompare thpolymer an
The coaconcentratithe large di
Both respractically tAs well, it wsurface nanequivalent.
The furaphenolic on
reected in a slight difference in their apparent friction
coefcient(app).
Both types of coatings showed the same mechanical
protectioninum. The scratch deformation behavior was dominated
yieldo evian coion be effeminucts i
lackon of
bet/hydas p
onal ion.
wled
horsnes al de
nces
Biederlysedogy 39Biedersol/ph
39 (2J. Songhenohnoloardzidizatiturialt streic resimaldeited
Stmaldeanizaulatio002/9ent (hang,aldee of the relative amount of
the bands corresponding to the hydroxyla- species measured at the
wavelenghts: () 670 (stretch Al O), () 738tretch Al O), () 763
(vibrations Al O) y () 830 cm1(vibrationsimmersion time. Evolution
of furan bridges amount during immersionluded.
ely one order of magnitude with respect to the uncoatedThis fact
can be attributed to the diminution of thexposed area. However,
after a long-time immersion, ite same trend as the uncoated
material because of thef the passive lm, and the corrosion
protection wasith the immersion time.ted, local corrosion spots
were formed after the poten-
destructive tests. Nevertheless, neither delaminationating
detachment was evidenced in the surrounding ofs.d adhesion between
the organic furan coating and thesubstrate could be attributed to
the excellent afn-the hydrophilic groups of the polymer (Fig. 3)
and ad aluminum surface. It is well known that when alu-n contact
with air a thin layer of aluminum oxide issly formed and then,
ambient water is absorbed on thatg the hydroxylation of the
aluminum oxide surface [63].e of such hydroxylated aluminum species
in the coatedmersed in NaCl solution was veried by FTIR
analysis.bserved an increase in the intensity of the character-
to alumby thewith n
Furcorrospositivsic aluor defe
Theadhesiafnityoxides
It wtraditicorros
Ackno
AuttigacioNacion
Refere
[1] M. ananol
[2] M. creogy
[3] H.-of pTec
[4] A. Gdar
[5] A. Sjoinnol
[6] ForUn
[7] ForOrg
[8] Reg[9] D. 2
liam[10] J. C
form
associated with Al O, Al OH [6367] with increasinge as shown in
Fig. 16. On the other hand, it was probed
emical crosslinks of the furan coating remained unal-g the whole
corrosion test, as demonstrated in Fig. 16.
ions
haracterization including physico-chemical, mechani-inum
corrosion protection was performed aiming to
e performance of coatings based on a proposed furand a
traditional phenolic resin.tings thickness obtained using the same
prepolymeron solutions and dipping rates greatly differed due
tosparity in furan and phenolic solutions viscosities.ins exhibited
similar chemical structures and hencehe same dynamicalmechanical
properties (Tg and E).as proved that, despite the difference in
thickness, theiromechanical elastic (Ec) and plastic properties (H)
are
n resin showed a greater hydrophilic character than thee,
related to its larger content of polar groups. This was
1016.[11] G.E. Mye
Hamel (Ein ManufUSA, 198
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[13] M. SmidformaldeForestry
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[21] B. Bhushalms, Int stress of the polymers (ductile ploughing
mechanism)dences of coating delamination.atings were able to
efciently protect aluminum fromy conferring a stable barrier. In
addition, a combiningct was displayed under severe conditions, as
the intrin-m passive lm may be capable to block eventual poresn the
polymeric lm.
of signs of coating detachment conrmed the excellent the coating
with the substrate, probably due to the greatween hydrophilic
groups of the furan polymer and theroxides evidenced in the
aluminum surface.roved that the proposed furan resin could replace
thephenolic resin to protect aluminum against scratch and
gements
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Furan resins as replacement of phenolic protective coatings:
Structural, mechanical and functional characterization1
Introduction2 Experimental2.1 Materials and sample preparation2.2
Methods2.2.1 Viscosity measurements2.2.2 Fourier transform infrared
analysis (FTIR)2.2.3 Dynamic mechanical analysis (DMA)2.2.4 Contact
angle measurements2.2.5 Scanning electronic microscopy (SEM) and
Calotest2.2.6 Nanoindentation and nanoscratch tests2.2.7
Potentiodynamic polarization
3 Results and discussion3.1 Physicochemical characterization3.2
Coating thickness3.3 Nanomechanical properties3.4 Nanoscratch
behavior3.5 Corrosion behavior
4 ConclusionsAcknowledgementsReferences