barrier properties of organic–inorganic hybrid coatings based on polyvinyl alcohol with improved...

Barrier Properties of Organic–Inorganic Hybrid CoatingsBased on Polyvinyl Alcohol With Improved WaterResistance

Matteo Minelli,1 Maria Grazia De Angelis,1 Ferruccio Doghieri,1 Marco Rocchetti,2 Angelo Montenero2

1 Dipartimento di Ingegneria Chimica, Mineraria e delle Tecnologie Ambientali, Alma Mater Studiorum,Universita di Bologna Via U. Terracini 28 - I 40131 Bologna

2 Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica Universita degli Studi diParma Viale G. P. Usberti 17/A - I 43100 Parma

A comprehensive study of the gas barrier properties ofhybrid organic–inorganic coatings formed by polyvinylalcohol (PVOH) and Si-SiO2, obtained via sol–gel tech-nique, was carried out. It has been shown that the oxy-gen, nitrogen and carbon dioxide transfer rates of bar-rier polymers such as poly(ethylene terephthalate) andoriented polypropylene can be further reduced, by upto two orders of magnitude, with a thin coating (1–2lm) of PVOH/Si-SiO2. More notably, it has beenobserved that the material maintains this feature evenafter a prolonged contact with water, which is a strongsolvent for PVOH. Direct moisture sorption measure-ments show that silica lowers the water uptake ofPVOH and inhibits sorption-induced swelling and plas-ticization of the polymer. Correlations between the gastransport properties of the hybrid coatings and factorssuch as the silica content, the type of polymeric sub-strate, the nature of penetrant and the temperaturehave been found, providing guidelines for the selectionand design of multilayer materials for packagingapplications. POLYM. ENG. SCI., 50:144–153, 2010. ª 2009Society of Plastics Engineers

INTRODUCTION

A new class of transparent organic–inorganic nano-composite materials can be prepared with the sol–gelmethodology, through the incorporation of oligomeric orpolymeric molecules into a solution formed by a precur-sor of the inorganic phase [1–7]. The hybrid materialsobtained with this route are sometimes referred to as

‘‘ceramers,’’ ‘‘ormocers’’ (organically modified ceramics),or ‘‘ormosils’’ (organically modified silicates), and theyhave a nanocomposite structure [6–7]. The hybrids havecombined characteristics of organic and inorganic sub-stances, and the final material properties can be tunedbetween those of the polymeric component and an inor-ganic glass. Usually, the organic phase guarantees tenac-ity, flexibility, and adhesion to the polymeric substrate,and the inorganic one gives toughness and thermal, chem-ical, and flame resistance, as well as improved gas barrierproperties. These materials are currently used as opticalwaveguides, abrasion-resistant coatings for plastics, func-tional coatings for glasses and antistatic films, and barriercoatings for metals and polymeric sheets [1]. Currently,the application of such materials in drug delivery and bio-medical applications [8–11] or in membrane separationsprocesses such as pervaporation [12] and fuel cells isunder study [13–14].

The sol–gel process is a well-established chemical syn-

thesis method, which involves hydrolysis and condensa-

tion reactions and leads to the formation of an inorganic

oxidic network [15]. This process was initially employed

for the preparation of glasses and ceramics, and lately

applied to the synthesis of hybrid organic–inorganic net-

works with controlled composition, thus providing unique

possibilities to tailor the mechanical, electrical, and opti-

cal properties. Indeed, in the presence of polymeric spe-

cies with suitable functional groups, the inorganic precur-

sor that may be an organo(alkoxy)silane forms glassy

Si/SiO2 nanoscopic domains with a certain number of

covalent bonds with the polymeric chains, as proved by

different characterization techniques [9–13]. The method

can be applied to several different systems as the final so-

lution that is obtained can be easily attached to polymeric

substrates by dip, spin, or roll coating. Another important

feature of the sol–gel technology is the low temperature

of operation, which is the background for the industrial

use in coating soft materials [1].

Correspondence to: M.G. De Angelis; e-mail: [email protected]

Contract grant sponsor: MIUR; contract grant number: Prot.

2004030304.

DOI 10.1002/pen.21440

Published online in Wiley InterScience (www.interscience.wiley.com).

VVC 2009 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—-2010

Many polymeric and oligomeric species have been suc-

cessfully incorporated within inorganic networks by dif-

ferent synthetic approaches: the chemical bond between

inorganic and organic phases can be attained by function-

alizing the organic species with silane or silanol groups

or by using the existing functional groups of the polymer

[1–3]. In this work, the precursor of the inorganic phase

is tetraethoxysilane (TEOS) and the organic component is

polyvinyl alcohol (PVOH), which is expected to form

hydrogen and covalent bonds with the inorganic groups

during the synthesis [8–13].

As mentioned above, the addition of impermeable inor-

ganic domains into polymeric matrices often leads to the

reduction of gas permeability and diffusivity values,

because the diffusive path of gas molecules inside the

polymer becomes more tortuous. This is the main mecha-

nism of permeation and diffusion hindrance in the case of

‘‘traditional’’ nanocomposite materials obtained by melt

compounding or similar techniques [16–20]. For hybrid

materials with an interconnected network of organic and

inorganic domains, such as the ones inspected here, the

gas permeation rates may be further reduced by the cross-

links between the polymeric chains and the inorganic

domains [21–26]. Such mechanism has hypothesized to

occur in coatings similar to the ones inspected here, in

which the organic phase of the hybrid was formed by

a copolymer of polyethylene and polyethylene glycol,

and the substrate was low density poly ethylene (LDPE)

[22–23].

The main difference between the coatings analyzed in

previous works [23–24] that were chosen based on their

good adhesion to the LDPE substrate, and the ones

inspected here that is represented by PVOH is the organic

phase of the hybrid. The latter polymer, due to crystallin-

ity and the stiffness of its hydrogen-bond-forming chains,

is an exceptional oxygen barrier material. The oxygen per-

meability of PVOH is, indeed, even two orders of magni-

tude lower than a good barrier material as poly(ethylene

terephthalate), PET [27–29]. However, the applicability of

PVOH as packaging material is limited by a significant

sensitivity to moisture, which softens and plasticizes the

matrix, lowering dramatically the barrier and mechanical

properties, and can lead to complete dissolution at high

values of humidity [28]. The aim of combining PVOH and

Si-SiO2 in a hybrid network is thus to enhance the stability

of the hydrophilic organic phase in the presence of water,

and keep low or, possibly, reduce the gas permeability

[30–33].

The hybrid coatings fabricated in this work were

applied to different commercial polymeric films: PET,

oriented polypropylene (oPP), cast poly(propylene) (cPP),

linear LDPE (LLDPE) and a blend of LLDPE and LDPE

(COEX). For the most promising samples, evaluated

based on their oxygen permeability at 658C, a more

extensive characterization was carried on with the aim to

evaluate: (i) the effect of organic–inorganic O/I ratio,

(defined as the ratio PVOH/(Si-SiO2)) on the oxygen

transport properties; (ii) the consequences of a prolonged

contact with water on the oxygen permeability; (iii) the

dependence of water vapor uptake on the inorganic

weight fraction in the coating; (iv) the role of the pene-

trant nature and the temperature on the coating perme-

ability, solubility, and diffusivity. The various aspects

were investigated by means of specific permeation and

sorption experiments, whose details are described in the

following.

EXPERIMENTAL

Materials

For the organic phase of the hybrid, PVOH, a fully

hydrolyzed polyvinyl acetate (PVAc) (97–100% of the ac-

etate groups substituted) or a partially hydrolyzed PVAc

(86–89% of the acetate groups substituted) can be used:

both grades are commercially available. The inorganic

Si-SiO2 groups were obtained by the addition of TEOS

[34].

An aqueous or hydro alcoholic solution comprising

PVOH and the alkoxide in variable concentrations,

depending on the desired final O/I ratio, was prepared. To

catalyze the hydrolysis and condensation reactions, the so-

lution was adjusted to slightly acidic values by adding a

small amount (0.03–1 wt%) of HCl. In these conditions,

the components reacted according to the classical sol–gel

route shown in Fig. 1 [15]. In the scheme it can be seen

that, after hydrolysis, the silanol groups Si-OH originated

from TEOS may react according to two competitive con-

densation reactions: (i) with another silanol group, to

form Si-O-Si; (ii) with the hydroxyl groups of PVOH,

allowing the formation of the hybrid network. In the liter-

ature, there is strong evidence that supports the hypothe-

sized reaction scheme and that a strong interconnection,

represented either by a covalent or a hydrogen bond, is

present between the organic and inorganic domains in the

final systems [9–14, 31].

The sol–gel solution was then deposited on the poly-

mer substrates by dip coating at different velocities (from

4.2 to 11.6 cm/min) obtaining similar results in terms of

adhesion and resistance. This technique allowed to obtain

FIG. 1. Sol–gel reaction scheme for the preparation of PVOH-Si/SiO2

hybrids.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2010 145

samples coated on both sides, while for mono-layered

laminates a roll coating technique was employed, that

produced good results in terms of adhesion. The hybrid

layers obtained in this way were colorless and perfectly

transparent.

The final configuration was then analyzed with SEM

microscopy to measure the coating thickness, which was

determined to be equal to 1 6 0.1 lm on each side of the

sample for bi-coated samples. Figure 2 reports a SEM

micrograph of a coated PET sample, where the coating

layer has been purposively detached to allow the determi-

nation of its absolute thickness.

The final weight percentage fraction of inorganic phase

in the hybrid layers inspected in this work was equal to

about 30% for the coating named Co1 and to about 50%

for coating Co2. Also coatings formed by the organic

phase only (PVOH) were obtained and named Co0. The

three different coatings were applied onto a 36 lm thick

PET substrate: due to the polar nature of this support,

they can be dipped into the hydrophilic sol–gel solution

without any special surface treatment, obtaining a good

adhesion. Other polymeric supports (oPP, cPP, LLDPE,

and COEX) were coated with the hybrid named Co1 and,

due to their hydrophobic nature, treated with cold plasma

before deposition to improve adhesion. The plasma

treatment was performed in air at 30 W; the duration of

treatment was varied from 10 to 30 s, obtaining similar

adhesion strength. The machine used is a ‘‘Colibrı’’ de-

vice manufactured by Gambetti Vacuum SrL (Binasco,

Mi, IT) that operates within the absolute pressure range

of 0.1–1 mbar.

A list of the multilayer samples prepared and charac-

terized in this work is provided in Table 1 where the

thickness and type of polymeric support is reported, as

well as the composition of the hybrid coatings in terms of

final polymer/Si-SiO2 ratio.

Permeation

Pure oxygen permeability (0% R.H.) in the films was

investigated by means of a closed-volume manometric

apparatus, especially designed to characterize ultra barrier

films (Fig. 3) [35, 36]. The gas Transfer Rate, T.R., or

permeance, through the sample, is equal to the molar flux

of gas at the steady state of permeation divided by the

pressure difference between the two sides of the film. In a

manometric device the gas molar flux is calculated, given

the membrane area, by the rate of increase of pressure at

constant temperature in the downstream volume through

the ideal gas equation of state.

In our apparatus the downstream volume was equal to

about 30 6 1.5 cm3, and the membrane area to 9.6 cm2,

as declared by the cell manufacturer (Millipore). The unit

used to express the transfer rate was equal to cm3(STP)/

(cm2�d�atm) where 1 mol ¼ 22414 cm3(STP). The single

transfer rate measurement was thus affected by an uncer-

tainty equal to 65%, given by the uncertainty on the vol-

ume value, whereas the pressure measurement is

extremely accurate. The transient stage of the permeation

process was also monitored and the characteristic time

to reach the steady state evaluated with the time-lag

method [37]. The properties of each layer composing the

coated samples were calculated using the series resistance

formula:

1

T:R:¼

Xi

1

ðT:R:Þi(1)

where (T.R.)i is the permeance of layer i and is related to

its thickness li and permeability Pi as follows:

Pi � ðT:R:Þi � li (2)

In this work, the permeability was numerically

expressed in Barrer (1 Barrer ¼ 10210 (cm3(STP)�cm)/

(cm2�s�cmHg)). The uncertainty on the permeability value

is given as the sum of that on the transfer rate and the

one given by the thickness: therefore for pure substrates

the uncertainty is equal to 65% and for pure coatings it

is equal to 15%.

FIG. 2. SEM micrograph of the edge view of PET film coated with

PVOH hybrid coating.

TABLE 1. List of the samples characterized.

Substrate Coating

Type

Thickness

(lm)

O/I ratio

(final)

Thickness

(lm)

PET-Co0 PET 36 100/0 1þ1

PET-Co1 PET 36 70/30 1þ1

PET-Co2 PET 36 50/50 1þ1

oPP-Co1 oPP 30 70/30 1þ1

oPP1-Co1 oPP 30 70/30 1

cPP-Co1 cPP 75 70/30 1þ1

LLDPE-Co1 LLDPE 110 70/30 1þ1

COEX-Co1 LLDPE/LDPE blend 65 70/30 1þ1

146 POLYMER ENGINEERING AND SCIENCE—-2010 DOI 10.1002/pen

The diffusivity of each layer was calculated, by using

the appropriate time-lag formula, derived using Laplace’s

transform and the proper initial and boundary conditions

in the case of a multilayer sample [38, 39]; the formula-

tion in the case of a bi-layer film takes the following

form:

y12 ¼l21

D1

l16P1

þ l22P2

� �þ l2

2

D2

l12P1

þ l26P2

� �h i

l1P1þ l2

P2

(3)

where y12 is the time-lag for the multilayer sample. In

the case of a tri-layer sample, with the coating, labeled

by pedix 1, placed on both sides of the substrate, labeled

by pedix 2, the proper expression for the time-lag is as

follows:

y121 ¼l21

D1

4l13P1

þ l22P2

� �þ l2

2

D2

l1P1þ l2

6P2

� �þ P2l2l

21

P21D2

h i

2l1P1

þ l2P2

(4)

The uncertainty on the single diffusivity value was due

mainly to the uncertainty on the thickness, therefore it

was negligible for pure supports and equal to 620% for

pure coatings. The solubility of the various layers was

evaluated by invoking a solution-diffusion mechanism for

the gas permeation inside the layers:

P ¼ D � S (5)

The uncertainty on the single solubility measurement,

according to the previous considerations, was equal to

65% for pure supports and to 635% for the coatings.

Sorption

Moisture sorption tests were carried out in a fixed-vol-

ume manometric apparatus (Fig. 4); the amount of vapor

absorbed by the sample was calculated by measuring the

pressure decrease within the known volume of the appara-

tus [35]. A certain amount of water vapor was fed into

the sample chamber and the equilibrium water uptake was

evaluated from the final asymptotic pressure value of the

gaseous phase, which also determines the equilibrium

vapor activity, calculated as pressure/vapor pressure.

For the solubility of multilayer samples, a mass addi-

tive rule was used to identify the individual contributions

of each layer to the water uptake:

ceq;tot ¼X

xi � ceq;i (6)

where ceq,tot is the equilibrium water concentration in the

sample, ceq,i is the equilibrium water concentration in

each layer i and xi the corresponding mass fraction.

RESULTS AND DISCUSSION

Pure oxygen (0% R.H.) permeation experiments were

carried out at 658C on the various samples inspected and

listed in Table 1. All tests were carried out after a proper

evacuation of samples to fully remove moisture and gases

absorbed from air for 4–5 days. At least two experiments

were carried out on each sample and the confidence inter-

val was determined considering both the uncertainty on

single measurements, as explained in the previous section,

and the repeatability of the data. The value reported in

the table is the arithmetic mean between the minimum

and maximum value obtained.

The hybrid coating with an O/I ratio of 70/30 (Co1)

was applied on several different polymeric supports to test

the effect of the substrate on the properties of the multi-

layer film. The most interesting samples, i.e. the ones that

showed the lowest permeation rates, were selected and

characterized more thoroughly: the effect of the O/I ratio

on the oxygen permeability and water vapor uptake was

tested on samples supported on PET by varying the O/I

FIG. 3. Layout of the permeation apparatus.

FIG. 4. Layout of the sorption apparatus.


ratio of the coating from 100/0 (Coating Co0) to 50/50

(Coating Co2). The effect of water ageing on the oxygen

permeability values of the coatings was also studied and

determined on PET-based laminates. The permeability of

O2, N2, and CO2 at 35 and 658C was determined on a

multilayer film of oPP and Co1 coated on one side

(oPP1-Co1).

Oxygen Transport in Hybrid Layer With O/I Ratio 70/30Coated on Different Substrates

In Table 2 (lines 1-9, 11), the transport properties, in

terms of T.R., time-lag, permeability, diffusivity and solu-

bility, are reported for different polymeric substrates

coated by the hybrid coating Co1 (O/I ratio 70/30), at

658C. Lower temperatures were also inspected but, for

this specific laminated material, the transport rates were

too small to be efficiently detected in reasonable time.

In all cases, the T.R. of coated samples is lower than

that of the neat substrate, by two to three orders of mag-

nitude, and the time-lag is increased after addition of the

coating to a similar extent. A general improvement of the

oxygen barrier properties is thus observed, that makes

these materials certainly suitable for packaging applica-

tions. The size of the variations of the transfer rate and

time-lag values depends strongly on the transport proper-

ties of the substrate, as it is obvious: for instance, the rel-

ative reduction of transfer rate in the case of the most

permeable polymer (COEX) is more significant than in

the case of the less permeable one (PET). However, to

compare the absolute performance of the coatings one has

to look at the values of the material properties P, D, andS, as evaluated from Eqs. 1–5, that are also listed in Table

2. It can be seen that, for coating Co1 applied on cPP,

LLDPE, and COEX, the permeability is of the order of

1023 Barrer, whereas for the same coating applied on

oPP the permeability is equal to about 1024 Barrer and

for those applied on PET the permeability is one order of

magnitude lower (1025 Barrer). For coatings applied on

cPP, LLDPE, COEX only the order of magnitude of dif-

fusivity could be assessed, that is equal to 10211 cm2/s in

all cases. The value of oxygen diffusivity in oPP and

PET-based coatings could be evaluated more accurately

and is equal to 1.3 3 10212 and 6.3 3 10213 cm2/s for

coatings applied on oPP and PET, respectively.

From the comparison of these values, one can conclude

that the properties of the hybrid coating depend rather

markedly on the properties of the substrate film, which is

a result that deserves some further discussion. First of all,

it can be noticed that coatings deposited onto PET exhibit

the lowest oxygen permeability, whereas those applied

onto the poly-olefinic substrates have worse barrier per-

formance. As far as adhesion is concerned, PET is the

optimal support for the present coatings, as verified

directly through pull-off and scratch experiments. Indeed,

PET has several sites that can interact with the organic

phase of the coating, whereas the poly-olefinic substrates

are less compatible to PVOH-based hybrids [31]. Previous

studies show that the coating permeability is affected, to

some extent, by the surface treatment of the support

TABLE 2. Oxygen transfer rate and time-lag values in the multilayer samples at 658C Permeability, diffusivity and solubility of the pure polymers

and pure coatings at 658C.

Multilayer material Pure material

T.R. cm3(STP)/

(cm2�d�atm) Time-lag s P Barrer D cm2/s

S cm3(STP)/

(cm3�atm)

1 oPP 1.7 6 6% 3.7 6 15% oPP 7.5 6 6% 4.1 3 1027 6 15% 0.15 6 21%

2 oPP-Co1 1.3 3 1023 6 6% 1.2 3 104 6 1.5% Co1 on oPP 3.8 3 1024 6 16% 1.3 3 10212 6 22% 2.2 6 38%

3 cPP 0.94 6 7% 10 6 18% cPP 11 6 7% 1 3 1026 6 18% 9 3 1022 6 25%

4 cPP-Co1 1.1 3 1022 6 9% �102 Co1 on cPP 34 3 1024 6 19% �10211 �100

5 LLDPE 1.0 6 8% 8.2 6 5% LLDPE 17 6 8% 2.5 3 1026 6 5% 5.3 3 1022 6 13%

6 LLDPE-Co1 1.38 3 1022 6 6% �103 Co1 on LLDPE 43 3 1024 6 16% �10211 �100

7 COEX 2.3 6 7% 2 6 25% COEX 23 6 7% 3 3 1026 6 25% 6 3 1022 6 32%

8 COEX-Co1 8.8 3 1023 6 8% �102 Co1 on COEX 27 3 1024 618% �10211 �100

9 PET 1.8 3 1022 6 6% 95 6 10% PET 0.10 6 6% 2.3 3 1028 6 10% 3.4 3 1022 6 16%

10 PET-Co0 2.0 3 1024 6 7% 1.40 3 104 6 2% Co0 on PET 0.6 3 1024 6 17% 6.5 3 10212 6 22% 7 3 1022 6 39%

11 PET-Co1 1.7 3 1024 6 13% 2.6 3 104 6 5% Co1 on PET 0.5 3 1024 6 23% 6.3 3 10213 6 25% 0.6 6 48%

12 PET-Co2 6 3 1024 6 18% 4.7 3 104 6 31% Co2 on PET 1.9 3 1024 6 28% 1.6 3 10213 6 51% 9.1 6 79%

13 PET-Co0a –b –b Co0 on PET – – –

14 PET-Co1a 8.3 3 1024 6 8% �103 Co1 on PET 2.7 3 1024 6 18% �10212 �1021

15 PET-Co1c 7.7 3 1024 6 6% –d Co1 on PET 2.5 3 1024 6 16% – –

16 PET-Co2c 2.0 3 1023 6 8% �103 Co2 on PET 6.7 3 1024 6 18% �10211 �1021

a Immersion in liquid water for 3 days and evacuation.b After the treatment the coating was dissolved: the transport properties of this sample can be assumed equal to those of neat PET.c Immersion in saturated water vapor for 4 days and evacuation.d The permeation curve exhibited an anomalous transient behavior that did not allow to interpret it with the time-lag method.


before deposition [24]. These results seem to indicate that

the stronger the adhesion, the best the barrier effect of the

coating: such behavior can partly be attributed to the pres-

ence of an interfacial layer whose transport properties

depend on the interactions between the support and the

coating and that was neglected in the present approach.

However, the magnitude of the deviations between the

properties of similar coatings on different substrates is so

large that other factors might be involved, that need to be

further investigated.

In the following, the study of the effect of O/I ratio

and other factors on the transport properties of the coating

will be performed on the multilayer materials based on

the most effective substrates, namely PET and oPP.

Effect of O/I Ratio on the Oxygen Transport inPET-Supported Hybrid Coatings

The T.R. and time-lag values obtained from the

experimental tests on the coated PET systems at 658C,without any previous treatment, are listed in Table 2

(lines 9–12).

As it can be seen, the permeance (T.R.) of PET is

reduced by about two orders of magnitude after addition

of the coatings Co0, Co1, Co2, having increasing silica

contents. From the comparison of these values, one can

notice that the permeance reduction caused by the hybrid

coating Co1 is of the same order of magnitude than that

obtained with pure PVOH (Co0). It can also be observed

that the trend of permeance reduction versus silica content

in the coating is not monotonic: such behavior will be

discussed more thoroughly in the following paragraphs.

The time-lag increases by two orders of magnitude with

respect to the PET support after addition of organic and

hybrid coatings and it increases with a monotonic trend

with increasing silica content.

The permeability varies from 0.6 3 1024 Barrer, for

pure PVOH, to 0.5 3 1024 Barrer for Co1 and 1.9 31024 Barrer for Co2. Clearly, all these coatings have a

permeability which is 3–4 orders of magnitude lower than

that of the PET support. The great barrier performance

given by the PVOH coating on PET is not surprising, due

to the fact that the oxygen fed to the film is completely

dry; the permeability value obtained from Eqs. 1–2 is

in good agreement with literature data for pure PVOH

[28, 40].

The values of permeability, diffusivity and solubility

variation caused by the addition of silica to pure PVOH

are reported in Fig. 5, where they are expressed as P/P0,

D/D0, and S/S0 with P, D, and S being the permeability

and diffusivity of the hybrid coating and P0, D0, and S0the respective quantities for pure PVOH. In the x-axis we

reported the volume fraction of silica, as estimated from

the mass fraction by assuming volume additivity. It is

interesting to notice that, when the inorganic phase is

added to PVOH, the oxygen permeability decreases by a

factor of about 20%, and, by increasing further the silica

content (O/I ratio 50/50) the permeability does not further

decrease but it rather reaches a value higher than that of

pure PVOH. On the contrary, the diffusivity values

decrease with increasing inorganic content.

The transport behavior of the hybrid materials can be

interpreted by considering, as reference, the behavior pre-

dicted by the Maxwell model for noninteracting compos-

ite materials with impermeable, spherical particles [41].

Several studies indicate that silica domains into hybrid

networks such as the one inspected here can be approxi-

mately considered nanospheres [13, 32, 42–44].

The Maxwell model provides a formula for the

increased tortuosity conferred by the impermeable phase,

that affects the diffusivity and permeability, and for the

FIG. 5. Permeability and diffusivity ratio of the coating versus filler

fraction and comparison with the Maxwell model.

FIG. 6. Permeability of pure PET and coatings at different O/I ratios

before and after water treatment.


reduction of available transport area, that affects the solu-

bility and the permeability [41]:

D

D0

¼ 1

1þ f=2S

S0¼ 1� f

P

P0

¼ 1� f1þ f=2

(7)

In the above equation, / is the volumetric fraction of

the inorganic phase, which is assumed to be impermeable

to the gas and to have negligible oxygen sorption

capacity. Clearly, the Maxwell model neglects the interac-

tions between the two components.

The diffusivity decreases, as Maxwell model predicts,

with decreasing silica content, but the two data points

inspected lie much below the predicted trend: such behav-

ior can indicate that there is an interaction between the

two phases, and in particular that the diffusivity of the

polymeric phase is further reduced by contact with silica.

On the other hand, the permeability of coating Co1 lies

close to the Maxwell curve, whereas the behavior of the

coating with the higher inorganic content is higher than

the predicted behavior. One has to remember that the per-

meability, in the solution-diffusion framework, is the

product of the diffusivity D and the solubility coefficient

S. In the panel in Fig. 5, it can be seen that the oxygen

solubility S increases with filler content, which is opposite

to what predicted by the Maxwell model according to

Eq. 7. This behavior can be attributed to the fact that

impermeable silica domains adsorb oxygen onto their sur-

face. After noting this, one may conclude that for the

coating with lower inorganic content the permeability

behavior is governed by the diffusion process, which is

affected by the increased tortuosity induced by silica. For

a higher inorganic content, the increase of tortuosity

seems to be less significant than other factors that contrib-

ute to enhance the solubility and permeability, such as the

adsorption of gas by silica.

Effect of Water Ageing on the Oxygen Transportin PET-Co1

It is known that, as in several hydrophilic polymers,

the oxygen permeability of PVOH under humid condi-

tions increases with respect to the value measured in a

dry environment; but, most significantly, PVOH may dis-

solve when exposed to liquid water or high activity water

vapor [28, 45]. For packaging applications the materials

have to be stable and maintain the intrinsic properties also

in humid environments.

In view of this requirement, we performed oxygen per-

meability tests on PET-based samples before and after

immersion in both liquid water and saturated vapor of

water at 658C. In particular, the oxygen permeability was

measured on samples previously immersed for 3 days in

liquid water (Treatment A) and for 4 days in saturated

water vapor (Treatment B) and the results are reported in

Table 2 (lines 13–16) in comparison with those relative to

the ‘‘as-received’’ films (lines 10–12). The same test was

carried on the sample coated with pure PVOH, but after

the treatment the coating was practically dissolved and

could not be used for permeation tests.

The oxygen permeability of the sample PET-Co1

increases by a factor of 5 after immersion in liquid water;

the effect is similar after immersion in water vapor. For

coatings with higher inorganic content (Co2) the increase

of permeability is equal to 3 after treatment with vapor,

which shows that samples with higher inorganic content

are less sensitive to water degradation, as shown in

Fig. 6.

The behavior observed on the hybrid coatings can be

explained by the fact that the hybrid structure allows to

depress water-induced plasticization, preventing the disso-

lution. To further investigate this aspect we performed

direct moisture sorption experiments on the samples, as

explained in the following section.

FIG. 7. Water solubility isotherm of the coatings at 358C and compari-

son with literature data for pure PVOH at 258C [45].

FIG. 8. Water solubility isotherm of the pure coatings at 658C.


Effect of O/I Ratio on the Water Vapor Uptake inPET-Supported Hybrid Coatings

Due to the highly hydrophilic behavior of the organic

phase of the coating, the samples were tested with respect

to water vapor sorption to evaluate the properties of the

hybrid materials in comparison to pure PVOH. The water

vapor solubility isotherms were measured on pure PET

samples and on samples coated with layers of PVOH,

Co1, and Co2. The results have been manipulated to

obtain the water uptake in the coating, by means of Eq. 6,that assumes a mass-additive behavior of water solubility.

The results are shown in Figs. 7 and 8, which are relative

to the temperature of 35 and 658C, respectively, and dis-

play the grams of water absorbed per total mass of solid

phase versus water activity. For pure PVOH, the water

uptake isotherm shows a marked positive concavity, that

is in good agreement with literature data [45, 46]. The

hybrid coating Co1 sorbs more water than pure PVOH at

low activity (below 0.35), but at higher activity the water

uptake in PVOH becomes higher than that in the hybrid,

due to swelling of the polymeric matrix. Interestingly, the

concavity of the solubility isotherm of Co1 shows a

glassy behavior which indicates the absence of any rele-

vant plasticization phenomenon. The same behavior is

observed in the coating with a higher content of silica

(Co2), for which the absolute values of water uptake are

extremely low and comparable to those of pure PET, not

reported in the plot for the sake of clarity. Even if we

neglect the silica sorption capacity (Maxwell’s hypothe-

sis), by referring the water uptake to the mass of PVOH

rather than to the total mass, its value is still lower than

that measured in the pure PVOH coating. This result indi-

cates that a synergy takes place between the phases of

the hybrid material: the water sorption of PVOH is a

swelling-enhanced process and silica lowers the ability

to swell of the polymeric phase and consequently, its

sorption capacity.TABLE3.

Transfer

rate

andtime-lagvalues

inoPP1-Co1sampleswithdifferentpenetrantgases

(O2,N2,CO2)at

35and658C

.Permeability,diffusivityandsolubilityofthevariousgases

inCo1hybrids,

coated

onoPP,at

35and658C

.

O2

N2

CO2

ToPP

oPP1-Co1

oPP

oPP1-Co1

oPP

oPP1-Co1

T.R.cm

3(STP)/(cm

2�d�

atm)

358C

0.476

5%

6.9

3102465%

0.0996

6%

1.9

310246

6%

1.6

65%

2.5

310236

5%

658C

1.7

66%

2.5

3102366%

0.566

6%

8.8

310246

6%

5.2

66%

7.8

310236

5%

Tim

e-lags

358C

11

6.6

31036

2%

21.2

62%

1.3

310461.5%

166

4%

9.373

1036

0.5%

658C

3.7

615%

1.3

31036

2%

4.6

630%

2.8

310363%

4.1

67%

2.103

1036

0.5%

oPP

Co1

oPP

Co1

oPP

Co1

PBarrer

358C

2.1

65%

1.0

31024615%

0.456

6%

2.9

310256

16%

7.4

65%

3.8

310246

15%

658C

7.5

66%

3.8

31024616%

2.6

66%

1.3

310246

16%

246

6%

1.2

310236

15%

Dcm

2/s

358C

1.4

31027

2.6

310213622%

7.1

310286

2%

1.3

3102136

22%

9.5

310286

4%

1.8

3102136

21%

658C

4.1

310276

15%

1.3

3102126

22%

3.3

310276

30%

5.9

3102136

23%

3.7

310276

7%

83

102136

21%

Scm

3(STP)/(cm

3�atm)

358C

0.126

5%

3.1

637%

4.9

310226

8%

1.7

638%

0.596

9%

166

36%

658C

0.146

21%

2.2

638%

6.0

310226

36%

1.7

639%

0.506

13%

116

36%

658C

4.1

310276

15%

1.3

3102126

22%

3.3

310276

30%

5.9

3102136

23%

3.7

310276

7%

83

102136

21%

FIG. 9. Gas solubility in pure oPP and in Co1 hybrids as function of

the penetrant critical temperature.


It has to be noticed that while the oxygen solubility

increases with silica content, as discussed before, the

water solubility decreases with it. This behavior can be

explained by the fact that the processes of water and

oxygen sorption differ significantly from one another,

because water sorption into PVOH is presumably much

higher than that onto silica surface and involves a more

significant swelling.

Effect of Penetrant Type and of Temperature on theDry Gas Transport in oPP1-Co1

To investigate the permeability of different gases and

different temperatures, a multilayer material with interme-

diate barrier properties, namely oPP1-Co1, was used, to

get results in a reasonable amount of time. The sample

has an oPP substrate and a single layer of Co1 applied

with a roll coating technique (thickness of 1 6 0.1 lm).

The gases inspected were dry nitrogen, carbon dioxide

and oxygen, at the temperatures of 35 and 658C. The gas

transport properties are listed in Table 3 in terms of gas

T.R. and time-lag; the material properties, such as perme-

ability, diffusivity and gas solubility of the different

layers are also reported.

The hybrid material Co1 shows a great barrier toward

all gases, as indicated by the extremely low values of per-

meability; the diffusivity varies between 10213 and 10212

cm2/s for all penetrants and oxygen shows the highest dif-

fusivity value at both temperatures. The permeability

varies over two orders of magnitude (1025–1023 Barrer)

among the various penetrants. To explain this fact, a dif-

ferent solubility of the gases in the hybrid coating has to

be invoked: for instance, CO2 is remarkably more soluble

in the hybrid matrix than nitrogen. The gas solubility in

polymers is known to be correlated with measures of the

penetrant condensability such as the critical temperature

TC [47, 48]; also in the case of the polymer (oPP) and of

the hybrid material inspected here, there is a linear corre-

lation between ln S and TC, at both the temperatures

inspected (Fig. 9).

The temperature effect on diffusivity is practically the

same for all the penetrants inspected: one can estimate an

activation energy value of about 44 kJ/mol for all gases.

The activation energy of permeability, on the other hand,

varies between 33 kJ/mol, for CO2, and 44 kJ/mol,

for N2, due to a negligible sorption heat in the case of

nitrogen.

CONCLUSIONS

The transport properties of hybrid organic–inorganic

layers of PVOH and Si-SiO2 obtained via sol–gel tech-

nique were investigated extensively. Several polymeric

films were evaluated as supports and, in all cases, the

addition of the coating results in a remarkable improve-

ment of the oxygen barrier properties: the oxygen T.R.and the time-lag vary by 2–3 orders of magnitude. In par-

ticular, it has been seen that the best support for the pres-

ent coatings is represented by PET and, to a lesser extent,

by oPP.

The trend of oxygen permeability and diffusivity with

O/I ratio indicates that the diffusivity decreases with

increasing silica content, due to increased tortuosity. The

solubility, on the other hand, increases with increasing

silica content, due to physical adsorption onto silica. As a

result, the permeability decreases with silica weight frac-

tion for the hybrid coating with lower silica content

(30%), whereas for a content of silica of 50% the perme-

ability is higher than that measured on a pure PVOH

coating.

Interestingly, the oxygen permeability of the hybrid

coatings remains extremely good even after a prolonged

immersion in either liquid water or saturated water vapor,

and the stability of the transport properties is more evi-

dent at higher contents of silica in the coating. This is a

good indication that the present materials can offer the

exceptional barrier properties of the organic phase

(PVOH) without being plasticized and dissolved by water.

This behavior can be explained when considering the

water vapor solubility, that decreases with increasing

silica content, due to the suppression of plasticization

induced by the formation of a hybrid network.

The permeability of other penetrants, namely N2 and

CO2 was also inspected, at 35 and 658C: the coating per-

meability follows the solubility trend that, as it is often

observed in polymers, increases with the penetrant critical

temperature, which is a measure of its condensability.

ACKNOWLEDGMENTS

Part of the experimental work was performed by Ales-

sandro Andreoni and Alessandra Massoni during their

Laurea Thesis and by Greg Simmonds from the Univer-

sity of Loughborough (UK), whose stay in Bologna was

financed by the Socrates/Erasmus program.

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