Grafting of phosphonate groups onto PVA by acetalyzation. Evaluation of the anti-corrosive properties for the acetalyzed PVA coatings

Article abstract of DOI:10.1016/j.reactfunctpolym.2011.03.005

One novel aldehyde bearing a phosphonate group, ca. (3-diethylphosphonate) propanal, was successfully synthesized and characterized by means of NMR. Then phosphonic acid ester groups were chemically grafted onto poly(vinyl acetal) pre-made polymers, and especially poly(vinyl butyral) (PVB). Four acetalyzed PVA were then synthesized with different molar contents of either butyral or (3-diethylphosphonate)propanal groups. Only PVAP4 was composed of both groups, i.e. 80 mol% of butyral and 20 mol% of phosphonate. TGA of acetalyzed PVA showed a clear improvement of the PVA thermal stability. Furthermore, for PVAP3, having 50 mol% of phosphonate groups, 40% of weight remained after 600 °C under air. This high thermostability was attributed to stable anhydride bonds. Finally, the coatings were subject to the salt spray test for duration up to 120 h. The results indicate that PVAP3 coating shows an excellent resistance to corrosion, thus excellent adhesive properties, as no corrosion was observed until 100 h subject to the salt spray test.

Full text of DOI:10.1016/j.reactfunctpolym.2011.03.005

Reactive & Functional Polymers 71 (2011) 599–606
Contents lists available at ScienceDirect
Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
Grafting of phosphonate groups onto PVA by acetalyzation. Evaluation
of the anti-corrosive properties for the acetalyzed PVA coatings
⇑
Ghislain David , Esteban Ortega, Kamel Chougrani, Abdelatif Manseri, Bernard Boutevin
Institut Charles Gerhardt UMR CNRS 5253, Laboratoire Ingénierie et Architecture Macromoléculaire, Ecole Nationale Supérieure de Chimie de Montpellier,
8 rue de l’Ecole Normale, 34296 Montpellier Cedex 05, France
a r t i c l e i n f o
a b s t r a c t
Article history:
One novel aldehyde bearing a phosphonate group, ca. (3-diethylphosphonate)propanal, was successfully
synthesized and characterized by means of NMR. Then phosphonic acid ester groups were chemically
grafted onto poly(vinyl acetal) pre-made polymers, and especially poly(vinyl butyral) (PVB). Four aceta-
lyzed PVA were then synthesized with different molar contents of either butyral or (3-diethylphospho-
nate)propanal groups. Only PVAP4 was composed of both groups, i.e. 80 mol% of butyral and 20 mol%
of phosphonate. TGA of acetalyzed PVA showed a clear improvement of the PVA thermal stability. Fur-
thermore, for PVAP3, having 50 mol% of phosphonate groups, 40% of weight remained after 600 °C under
air. This high thermostability was attributed to stable anhydride bonds. Finally, the coatings were subject
to the salt spray test for duration up to 120 h. The results indicate that PVAP3 coating shows an excellent
resistance to corrosion, thus excellent adhesive properties, as no corrosion was observed until 100 h sub-
ject to the salt spray test.
Received 9 December 2010
Received in revised form 8 March 2011
Accepted 8 March 2011
Available online 16 March 2011
Keywords:
PVA
Poly(vinyl butyral)
Acetalization
Phosphonate bearing aldehyde
Corrosion
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
observed a resistance to corrosion, by salt spray test, depending on
the bis-phosphonic methacrylate monomer structure. For the best
The potential applications for polymer products containing
phosphorous are numerous; dental adhesives, ion-exchange resins
and adhesion promotors are just three of the more common appli-
cations [1–4]. Compounds containing phosphorous are excellent
promotors with respect to adhesion, and thus anti-corrosion. Com-
mercial anti-corrosion polymer compounds are generally formed
from SipomerÒ or PhosmerÒ monomers, which are phosphate-type
(meth)acrylates [5,6]. Polymers with some phosphonate function-
ality have long been established as excellent adhesives and
anti-corrosion compounds, [7–14] and recently there has been
investigation into the use of phosphonate-type methacrylates for
the same purpose [15]. For instance, Lam et al. [16] synthesized a
methacrylate terpolymer, bearing phosphonic acid and n-butyl
side groups, which was incorporated in a UV acrylate formulation.
The resulting coatings showed a good resistance to the corrosion of
steel for only 3 wt.% of phosphonated methacrylate. Investigation
of electrochemical impedance for these coatings revealed a Rt resis-
coating, the corrosion started after 900 h; this interesting result
was explained by a migration of the phosphonic acid components
to the interface, leading to efficient adhesion towards steel. This
relatively good adhesion to the metallic surface is however limited
by hydrolysis of the methacrylate function [10,11]. As methacry-
lates undergo hydrolysis, some authors proposed the use of
(meth)acrylamide bearing phosphonic acid groups. The resulting
cross-linked polymers improved adhesion and were efficiently
used in dental application [19–22]. The post-phosphonation of
pre-made polymers, which are highly stable towards hydrolysis,
is another strategy to introduce phosphonic acid groups. In this
study we focus on the chemical grafting of phosphonic acid ester
groups onto poly(vinyl acetal) pre-made polymers, and especially
poly(vinyl butyral) (PVB). PVB is obtained by acetalisation of
poly(vinyl alcohol) (PVA) with butyraldehyde by using either a
solvent process or an aqueous process [23–27]. Recently, PVB
was produced in an industrial scale by Kuraray Specialities
[28–30] under MowitalÒ trademark with 65–88 wt.% of butyral
content, 11–27 wt.% of PVA content and 1–8 wt.% of acetate con-
tent. PVB is extensively used in the manufacture of textiles, safety
glass, adhesives, engineering plastics and binders [31–36]. The
acetalisation of PVA into PVB leads to a loss of biodegradation
properties whereas the thermal stability is increased. Interestingly,
PVB can be also used as metal primers and coatings due to its good
adhesion to metallic substrates [37].
tance higher than 244 k
X
cmÀ2, compared to untreated steel
cmÀ2. Recently, Chougrani et al. pro-
showing a Rt value of 55 k
X
posed new methacrylate coatings based on bis-phosphonic meth-
acrylates [17,18]. This new series of methacrylate monomers was
incorporated in a UV acrylate formulation (3 wt.%) and the authors
⇑
Corresponding author. Tel.: +33 0 4 67 14 43 07.
E-mail address: ghislain.david@enscm.fr (G. David).
1381-5148/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.reactfunctpolym.2011.03.005

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G. David et al. / Reactive & Functional Polymers 71 (2011) 599–606
The aim of this paper is to improve adhesion of PVB onto metal-
13C NMR (CDCl3, 101 MHz, 300 K): d 16.3 (d, JCP = 6.8 Hz, CH3),
19.4 (d, JCP = 144.0 Hz, Ca), 26.8 (d, JCP = 4.4 Hz, Cb), 61.5 (d,
JCP = 6.7 Hz, CH2), 65.0 (s, Cd), 103.2 (d, JCP = 19.4 Hz, Cc) ppm.
lic substrate, i.e. galvanized steel, by chemically grafting phospho-
nate groups. Acetalisation of PVA with both butyraldehyde and
phosphonate-containing aldehyde will produce PVB with phospho-
nate contents, which then will be used as new coating. Thermal
properties as well as resistance to corrosion of the resulting coat-
ings will be evaluated.
2.4. Synthesis of (3-diethylphosphonate)propanal (2)
2. Experimental
2.1. Materials
All manipulations involving air-sensitive compounds were per-
formed under a dry argon atmosphere using standard Schlenk
techniques. All solvents and reagents were of reagent grade, pur-
chased commercially and used without further purification. PVA
(Mw = 31,000–50,000, degree of hydrolysis 98–99 mol%) was used
as received. Dry chloroform was refluxed over CaH2 and distilled
under argon prior to use.
To a solution of protected aldehyde (1) (0.50 g; 2.1 mmol) in
8 mL of THF was added dropwise 8 mL of HCl (1 N). The mixture
was stirred 4 h at 40 °C then THF was eliminated under reduced
pressure. After extraction with dichloromethane (5 Â 10 mL), the
combined organic layers were dried over sodium sulfate and evap-
orated. Aldehyde (2) was obtained as a colorless oil (0.30 g; 75%
yield).
2.2. Instruments
1H NMR (400 MHz, CDCl3, 300 K): d 1.24 (t, J = 7.1 Hz, 6H, CH3),
2
3
Nuclear magnetic resonance (NMR) spectra were recorded on
Bruker 400 MHz spectrometer. Chemical shifts are reported in
1.95 (dt, 2H, JHP = 18.2 Hz, JHH = 7.7 Hz, Ha), 2.70 (dt, 2H,
3JHP = 12.4 Hz, JHH = 7.9 Hz, Hb), 3.96–4.06 (m, 4H, CH2), 9.71 (s,
3
ppm relative to SiMe4 (1H; 13C) or H3PO4 31P). 13C NMR assign-
(
1H, Hc) ppm.
ments were confirmed by inverse gradient d 1H–13C HMQC and
1H–13C HMBC experiments. Thermogravimetric analyses (TGA)
31P NMR (CDCl3, 162 MHz, 300 K): d 30.7 (s) ppm.
13C{1H} NMR (CDCl3, 101 MHz, 300 K): d 16.2 (d, JCP = 6.0 Hz,
CH3), 17.7 (d, JCP = 145.9 Hz, Ca), 36.7 (d, JCP = 4.1 Hz, Cb), 61.7 (d,
JCP = 6.5 Hz, CH2), 199.1 (d, JCP = 15.4 Hz, Cc) ppm.
d
were performed with a TGA/SDTA 851 thermobalance from Mettler
DAL 75965 and Lauda RC6 CS cryostat apparatus in static air with a
heating rate of 10 °C minÀ1 from room temperature up to a maxi-
mum of 600 °C. Differential scanning calorimetry (DSC) measure-
2.5. Acetalization of poly(vinyl alcohol) with (3-diethylphosphonate)-
propanal: PVAP4
ments were conducted using
a TA2920 analyser from TA
Instruments DA 73085, an RCS DA cooler and Sartorius MC5 bal-
ance. Scans were recorded at a heating/cooling rate of 20 °C minÀ1
from 0 °C to 250 °C. The salt spray test (DIN 50021) was used to
evaluate the adhesive/anticorrosive properties. A series of 10 Q
panels coated on each of the coatings were subject to the salt spray
test for 150 h. A solution of 0.5 mol/L of NaCl in purified water was
dispersed on the samples using a Ascott S120T salt spray machine,
at a constant temperature of 35 °C. At regular intervals (ca. every
20 h) a plate for each of the samples was removed from the ma-
chine and visually assessed for corrosion. The assessment was
achieved by dividing the plate into 10 Â 10 squares and giving a
value of one (for non corroded segment) or a value of zero (for cor-
roded segments), and finally reported as percentage of corrosion.
The salt spray test is widely accepted as the most intense of the
anticorrosive/adhesive test methods.
0.11 g of PVA (mol based on –CH2CH(OH)– as the repeat unit)
was dissolved in 10 ml of distilled water at 95 °C under stirring
for 30 min. Then, the solution was cooled to 30 °C and the 1.03 g
of (3-diethylphosphonate)propanal was added, followed by a pre-
scribed amount of hydrochloric acid (35% aqueous) to set the pH
of the solution below 1. The reaction mixture was stirred for 3 h
at 30 °C then at 60 °C for 12 h. The solution was then cooled to
30 °C followed by addition of prescribed amount of NaOH to set
the neutral pH. The polymer was purified by repeated precipita-
tions from a THF solution in a large amount of water and finally
dried under vacuum until constant weight.
2.6. Preparation of coatings
Acetalized PVA (10% w:w) was first solubilized in N-methyl
pyrrolidone (90% w:w) at 50 °C. This solution was deposited onto
2.3. Synthesis of diethyl(2-(1,3-dioxolane)ethylidene)phosphonate (1)
galvanized steel plates using a BarCoater (120 lm Braive instru-
ments). After the removal of the solvent (heating to 140 °C for
one night), the thickness of the films measured with byko-test
7500 apparatus was about 20 lm.
3. Results and discussion
A glass reactor was charged with bromoethyl-1,3-dioxolane
(5.00 g; 27.6 mmol) and triethylphosphite (9.18 g; 55.2 mmol).
Then the resulting mixture was heated at 155 °C during 24 h. Dis-
tillation under vacuum of the residue obtained (79 °C, 10À2 mbar)
afforded phosphonate (1) as a colorless oil (5.70 g; 87% yield).
1H NMR (400 MHz, CDCl3, 300 K): d 1.27 (t, J = 7.1 Hz, 6H, CH3),
1.66–1.94 (m, 4H, Ha + Hb), 3.80–3.94 (m, 4H, Hd), 4.00–4.10 (m,
4H, CH2), 4.90 (t, JHH = 7.1 Hz, 1H, Hc) ppm.
3.1. Acetalyzation of PVA from functionalized aldehydes
First of all, the synthesis of aldehyde precursor bearing phos-
phonic acid diethyl ester was required to be further grafted onto
neat PVA. Usually, aldehydes are obtained by selective oxidation
of primary alcohols. The oxidation of alcohols to corresponding
carbonyl compounds is a very important and frequently used
transformation in organic synthesis and a vast number of methods
have been developed for this transformation. One of the most
31P NMR (CDCl3, 162 MHz, 300 K): d 32.1 (s) ppm.

G. David et al. / Reactive & Functional Polymers 71 (2011) 599–606
601
common method is the Swern oxidation which proceeds via DMSO
activated by electrophilic reagents such as oxalyl chloride, acid
anhydrous, phosgene [38,39]. This method has some drawbacks
such as high exothermy during the synthesis associated to the
use of toxic reagents. For these reasons several authors developed
the oxidation of primary alcohol groups by using highly selective
nitroxyl radical-mediated compound (such as TEMPO) and hypo-
chlorite mixture [40–42]. TEMPO/hypochlorite has been reported
to be a selective, efficient and convenient catalyst system for a
wide variety of primary alcohols and compatible under aerobic
and aqueous conditions [43–45]. Following this procedure, we
used a primary alcohol bearing a phosphonate group, the synthesis
of which was described elsewhere [46]. This alcohol was oxidized
in the presence of TEMPO. However, the alcohol oxidation re-
mained unsuccessful since the phosphonate group might complex
nitroxyl radical compound, which then reduced its catalytic effi-
ciency. We then decided to use dioxolane precursor as these com-
pounds are readily used in organic chemistry to easily generate the
corresponding aldehydes in acid medium [47]. The suggested strat-
egy, which ends up to aldehyde bearing a phosphonic acid diethy-
lester group, is the following (Scheme 1): (i) Arbusov reaction of
bromoethyl-1,3-dioxolane in the presence of triethylphosphite,
(ii) deprotection of dioxolane group with HCl (1 N).
The Arbusov reaction, i.e. nucleophilic substitution of halogen
atom, is a well-known reaction and usually proceeds quantita-
tively.[48] Deprotection of dioxolane group was then carried out
at 40 °C and after 4 h of reaction about 80% was converted into
Scheme 1. Two-steps synthesis of (3-phosphonic acid)propanal.
Fig. 1. 1H spectrum of (3-diethylphosphonate)propanal, recorded in CDCl3.
Fig. 2. 31P spectrum of (3-diethylphosphonate)propanal, recorded in CDCl3.

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G. David et al. / Reactive & Functional Polymers 71 (2011) 599–606
Scheme 2. Acetalization of PVA with both (3-diethylphosphonate)propanal and butyraldehyde.
Table 1
Composition, yield and thermal characteristics of acetalized PVA in the presence of both butyraldehyde and (3-diethylphosphonate) propanal.
Polymer
Polymer compositiona
Yield (%)a
Tg (°C)b
TGA under air (°C)c
x1a’ (mol%)
ba’ (mol%)
za’ (mol%)
Td10
Td50
d
e
PVA
99
51
26
19
49
0
48
73
60
0
0
0
0
20
50
/
85
72
70
68
18
290
390
385
340
250
300
410
410
380
380
PVB1
PVB2
PVAP3
PVAP4
88
86
92
89
a
Calculated from 1H NMR of purified polymer.
a’
x1, b and z stand for the molar content of vinyl alcohol, butyral and (3-diethylphosphonate)acetal, respectively.
b
c
Recorded from DSC at a heating/cooling rate of 20 °C minÀ1
.
Recorded at 10 °C minÀ1
.
d
e
Temperature at 10% weight loss.
Temperature at 50% weight loss.
Fig. 3. 31P spectrum of PVA acetalized with (3-diethylphosphonate)propanal (PVAP4), recorded in CDCl3.
aldehyde. Figs. 1 and 2 show both 1H and 31P NMR spectra of
unprotected (3-diethylphosphonate)propanal obtained after
purification. 31P NMR especially shows two peaks, corresponding
to (3-diethylphosphonate)propanal and diethyl(2-(1,3-dioxo-
lane)ethylidene)phosphonate at 30.8 and 32.1 ppm, respectively.
Both spectra very well characterize (3-diethylphosphonate)prop-
anal with more than 98% purity.
Acetalization of PVA (Mn of about 50 000 g/mol), which has an
acetate content of y = 1 mol%, was then carried out with both (3-
diethylphosphonate)propanal and butyraldehyde, according to
the procedure described by Kuraray specialities, [49] i.e. 30 °C in
distilled water with HCl 1 N (Scheme 2).
The acetalization by using only butyraldehyde was first carried
out, resulting to poly(vinyl butyral) (PVB). The acetalization with
both butyraldehyde and (3-diethylphosphonate)propanal led to
poly(vinyl alcohol) phosphonate (PVAP) and required a two-steps
reaction: PVA was first acetalized with (3-diethylphosphonate)-
propanal to keep the resulting polymer soluble in water, the sec-
ond acetalization proceeded with butyraldehyde by reaction with
the remaining alcohol groups. Interestingly, acetalization reactions
were achieved at high yield in homogeneous phase, independently
of both the amount and the type of aldehyde (Table 1). Four differ-
ent acetalized PVAs were thus synthesized, bearing different molar
contents of both butyral and diethyl-phosphonate groups, noted b

G. David et al. / Reactive & Functional Polymers 71 (2011) 599–606
603
Fig. 4. 1H spectrum of PVA acetalized with (3-diethylphosphonate)propanal (PVAP4), recorded in CDCl3.
and z, respectively. Structural characterization of PVB was previ-
ously performed from mono and two-dimensional proton NMR
[26,50]. The interpretation of NMR spectra was rather complex
accordingly to the occurrence of two stereostructures of meso
and racemic butyral rings. These model studies allowed us charac-
terizing acetalized PVA with (3-diethylphosphonate)propanal from
both phosphorus and proton NMR (Figs. 3 and 4).
b-position of phosphonate group. NMR analysis confirms the ex-
pected PVAP structure. Hence, two PVAP were synthesized with
different contents of phosphonate groups, i.e. 20 and 50 mol% for
PVAP3 and PVAP4, respectively. PVAP3 also contains 60 mol% of
butyral group (Table 1). Since all acetalized PVA remained soluble
in NMP, no intermolecular condensation occurred.
From 31P NMR, a main peak was observed at 33.5 ppm that can
be attributed to the phosphonate group of the resulting PVAP. A
peak of very low intensity was also observed at 27.5 ppm, ascribed
to partial hydrolysis of phosphonate groups into phosphonic acid
groups [17]. The 1H NMR spectrum of purified PVAP4, i.e. without
any butyral unit, shows characteristic signals of phosphonate
methyl ester groups at 4.2 and 1.2 ppm for O–CH2 and O–CH2–
CH3, respectively. The signal observed at 4.6 ppm (noted He) char-
acterizes dioxymethyne (O–CH–O) proton in the acetal ring, hence
confirming the efficient reaction between aldehyde and hydroxyl
groups. Interestingly, Fernandez et al. [26] observed two signals
for dioxymethyne protons, from 1H NMR of PVB, at 4.6 and
4.9 ppm for the meso and the racemic stereostructures, respec-
tively. From Fig. 4, no dioxymethyne proton, arising from racemic
stereostructure (obtained from hydroxyl groups being at opposite
sites), was observed. Indeed, the acetalysation between (3-diet-
hylphosphonate)propanal and hydroxyl groups being at opposite
sites is not favored due to the steric hindrance of phosphonate es-
ter groups. The region between 3.6 and 4.2 ppm evidences the
methine protons of both acetal ring and vinyl alcohol (noted Hf
and Hi, respectively). The intensity of signals He, Hb, Hf and Hi al-
low the quantification of the extent of acetalization, ranging from
50 to 80 mol% (z + b as reported in Table 1). Finally, the signals of
methylene protons for both acetal ring and vinyl alcohol are ob-
served at 1.7 and 1.4 ppm, respectively. These signals are over-
3.2. Thermal analysis
The polymer composition for each polymer is given in Table 1 as
well as their thermal behavior, especially their glass transition
temperature and thermal stability.
Fig. 5. TGA curves for PVA, PVB1, PVB2, PVAP3 and PVAP4 recorded with heating
rate of 10 °C/min under air.
lapped due to the presence of signals of methylene both in
a and

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G. David et al. / Reactive & Functional Polymers 71 (2011) 599–606
Since the content of acetal groups increases (i.e. b + z), the Tg
decomposition temperatures at 10 and 50% weight loss are given
in Table 1.
values decrease linearly, as previously observed [26]. This can be
easily implied to the plasticizing effect of acetal groups, which is
however lowered by hydrogen bonding in PVB polymers. We can
also note that the content of phosphonate group into PVAP3, i.e.
20 mol%, does not affect the linear behavior since the content of
butyral group remains high. In the other hands, the high content
of phosphonate groups for PVAP4 (z = 50 mol%) has a detrimental
effect onto the Tg value that was measured at 18 °C for PVAP4
against 72 °C for PVB1. This result may be explained by either
existing hydrogen bonding from PVB since PVAP4 is only made
of vinyl alcohol and (3-diethylphosphonate)propanal units, or high
plasticizing effect from phosphonate groups.
After loosing water, PVA starts its degradation from 280 °C
(T10 = 290 °C) to 320 °C, which is slow down due to hydrogen
bonding. Acetalization of PVA with butyraldehyde significantly in-
creases the thermal stability even at 48 mol% of butyral group.
Only one degradation was obtained for PVB1 and PVB2, starting
at 390 °C until complete degradation at about 490 °C, as previously
observed [26]. For PVB1 and PVB2, hydrogen bonding poorly oc-
curs and no slowing down of the thermal degradation was noted
for both curves. Then, when butyral groups are totally replaced
by phosphonate ones, i.e. PVAP4, thermal degradation clearly oc-
curs in two steps: the first one at about 120 °C corresponds to
phosphonate ester groups cleavage, leading to the corresponding
phosphonic acid groups [51]. Then degradation of vinyl alcohol
units occurs until about 300 °C, temperature at which a plateau
was reached due to condensation of phosphonic acid to thermally
stable anhydride bonds: at 600 °C less than 60% of weight loss was
observed. PVAP3 degradation curve shows an intermediate behav-
ior between PVA and PVB despite the low content of phosphonate
group (20 mol%). The phosphonate ester group cleavage was also
observed at the same temperature, and similarly to PVB, PVAP3
shows a main degradation starting at 340 °C, corresponding to vi-
nyl butyral units. Another degradation step was observed at 390 °C
(approximately 70% of weight loss) characterizing the effect of
phosphonate groups, and finally complete degradation was
reached at about 470 °C. Since no plateau was obtained for PVAP3,
(3-diethylphosphonate)propanal units may be randomly distrib-
uted along the PVB backbone.
The thermal stability of modified PVA was evaluated from ther-
mogravimetric analysis (TGA) under air, recorded at 10 °C/min. The
TGA curves were plotted against temperature (Fig. 5) and the
3.3. Resistance to corrosion
As mentioned in the introduction part, PVB are currently used
as adhesion promotors for metallic surfaces [37]. In the other
hands, we have already demonstrated the high capacity of phos-
phonate group, grafted to a polyacrylate backbone, for promoting
adhesion towards metallic surface and hence preventing such
Fig. 6. Graphical result of the resistance to corrosion after being subject to the salt
spray test (0.5 mol/L NaCl in water at 35 °C) for PVA, PVB1, PVB2, PVAP3 and PVAP4.
Fig. 7. Q-plates coated with a coating based on PVB1 (left) PVB2 (middle) and PVAP3 (right) after being subject to the salt spray test (0.5 mol/L NaCl in water at 35 °C). The
plates were removed at 28 h for PVB1 and 100 h for PVB2 and PVAP3.

G. David et al. / Reactive & Functional Polymers 71 (2011) 599–606
605
surface against corrosion [15,18]. In this paper we have chemically
grafted phosphonate group onto PVB in order to increase efficiency
of PVB coatings against corrosion. To this aim, PVAP3 and PVAP4
were coated onto metallic surface and the resulting films were
subject to the salt spray test (0.5 mol/L NaCl in water at 35 °C),
which allows the most direct comparison between compositions,
i.e. PVA, PVB1 and PVB2 were also coated. Fig. 6 shows a direct
comparison of the resistance to corrosion for the coatings, obtained
from visual assessment (details are given in the experimental part),
while some examples (Fig. 7) of the plates are given after being ex-
posed to the test.
ings, however their resistance to corrosion remained low, due to
their high molar content of vinyl alcohol groups (i.e. 50 mol%).
The results obtained with PVAP3 coating were excellent, even
better than PVB1 (having 80 mol% of butyral groups) with little
corrosion observed after ca. 100 h. This indicates that (3-diet-
hylphosphonate)propanal and butyraldehyde both grafted onto
PVA provide highly efficient adhesion promoters and could be
further optimized by cleavage of the phosphonate ester groups.
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sion since the whole surface remained corroded after 20 h of expo-
sure. The resistance to corrosion for PVB1 is poorly improved as
compared to the pristine PVA. As an example, Fig. 7 shows that
the most entire surface of PVB1 coating was corroded after 28 h
subject to the salt spray test. This result can be attributed either
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