Industrial pressure sensitive adhesives suitable for physicochemical ...

International Journal of Adhesion & Adhesives 31 (2011) 629–633
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International Journal of Adhesion & Adhesives
journal homepage: www.elsevier.com/locate/ijadhadh
Industrial pressure sensitive adhesives suitable for
physicochemical microencapsulation
R. Abderrahmen a,n , C. Gavory b , D. Chaussy a , S. Brianc-on b , H. Fessi b , M.N. Belgacem a
a LGP2—Laboratoire de Génie des Procédés Papetiers, Grenoble INP-Pagora, CNRS UMR 5518, 461, Rue de la Papeterie BP 65, 38402 Saint Martin d’Heres Cedex, France
b LAGEP, UMR CNRS 5007, 43 Bd du 11 Novembre, Université Lyon 1, 69622 Villeurbanne, France
article info
Article history:
Accepted 9 June 2011
Available online 25 June 2011
Keywords:
Pressure-sensitive
Water-based
Peel
Microencapsulation
abstract
The main objective of this investigation is to characterize several industrial pressure sensitive adhesives
(PSA) for their microencapsulation. The adhesive selected must remain stable within the variations of
the microencapsulation conditions. Thanks to its encapsulation, the adhesive will be self protected into
the microcapsule’s shell, and its release will be then controlled applying a light pressure. First,
physicochemical microencapsulation processes were selected, and three industrial water-based acrylic
adhesives were consequently chosen. The adhesive main characteristics (composition, particle size
distribution and peel strength) were determined. Then, particle size stability was tested under ranges of
pH (from 2 to 9), temperature (from 5 to 85 1C) and electrical conductivity variations (up to 12 mS/cm),
within an interval of values encountered in the physicochemical processes of microencapsulation.
Finally, the compatibility with various solvents (acetone, ethanol and ethyl acetate) was also tested.
& 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Typical labels are divided into 3 parts: a coated paper, an
adhesive layer and a silicon liner. This anti-adherent liner cannot
be recycled and costs more than 50% of the label price. The
objective of this study is the elimination of this protective layer,
to solve both environmental and economical issues. It can be
achieved by the adhesive ‘‘self protection’’, thanks to its incorporation
into microcapsules [1]. This allows the preparation of
‘‘Dry labels’’ gluing under the application of a pressure, which
induces the rupture of the microcapsules, thus releasing the core
material, a pressure sensitive adhesive (PSA). To reach this
objective, a pressure sensitive adhesive must possess physicochemical
properties in agreement with the encapsulation technology.
In particular, the PSA should remain stable when external
variations (pH, temperature, etc.) are imposed. In fact, in case of a
PSA emulsion, the microencapsulation occurs at the micelle/water
interface, which requires the perfect stability of these micelles.
Any flocculation, precipitation or collapsing of these particles
should be avoided.
Microencapsulation is a technology in which a ‘‘core’’ material
is coated with or entrapped within a ‘‘shell’’ (or ‘‘wall’’) material.
The core material can be solid particles, liquid droplets or gas
bubbles. The particles obtained, called microcapsules, have
usually diameters ranging from 1 to 100 mm [2–6]. This technique
n Corresponding author. Tel.: þ33 4 76 82 69 84; fax: þ33 4 76 82 69 33.
E-mail address: Robin.Abderrahmen@lgp2.grenoble-inp.fr (R. Abderrahmen).
is used to protect a core material into a shell material, and allows
the controlled release of the core during the application, by
rupture of the shell material. Microencapsulation processes can
be divided into three groups [7,8]:
– Chemical techniques: the wall material is formed in situ by
polymerisation between monomers at the core material’s
surface (e.g., interfacial polymerisation, in situ polymerisation)
– Physicochemical techniques: based on phase separation in a
colloidal system: a soluble shell material aggregates around
the core material to form a solid wall (e.g., coacervation,
solvent evaporation)
– Mechanical techniques: the wall material is mechanically
applied or condensed around the core material (e.g., spray
drying, extrusion)
Physicochemical encapsulation techniques were selected to
encapsulate PSA, and more specifically coacervation. This process
has a major advantage that is to occur in an aqueous media,
allowing the microencapsulation of emulsions (a wide form of
PSA). It consists in dispersing or suspending a non miscible core
material in a solution containing a solvated wall material. Several
methods are available in order to induce a separation of the wall
polymer from the solution as microdroplets, called coacervate
(viscous phase). Among the main parameters that can be switched
in order to induce the separation of the shell, one could mention
the modification of pH and temperature, the addition of a non
solvent for the polymer, or enhancing the electrical conductivity by
0143-7496/$ - see front matter & 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijadhadh.2011.06.003

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the addition of a salt. The coacervate will then deposit on the core’s
surface to form its shell. Finally, the wall is hardened by crosslinking
[9–12].
A pressure sensitive adhesive (PSA) is a material that adheres
instantaneously to solid surfaces, with the application of a light
pressure [13–15]. They are polymers based, with usually a
complex formulation (additives, as plasticizers or resins) [16].
They are widely used in many industries, as tapes, labels or
protective films. The three main properties determining PSA
performances are the tack, the peel strength and the shear
resistance [17–19]. Tack is the ability to bond rapidly, when it is
pressed against the surface of a substrate. Peel strength is the
force required to peel away a tape from a substrate, under
standards conditions (angle test 901 or 1801). Shear resistance is
the internal cohesive strength (ability of a PSA to withstand
creep). PSAs can be divided into four classes: solvent-based,
water-based, hot melt or radiation-cured PSA. Two of them are
mainly used in the labelling industry, because of good adhesive
performances and low prices: hot-melt [20,21] and water-based
PSAs [22]. A hot-melt PSA is composed of thermoplastic polymers.
It has the advantage to be solvent free, i.e., 100% solid. However, it
involves high viscosity processing conditions (viscosity from 1000
to 3000 mPa s) [23]. This thermoplastic adhesive requires heating
to be processed. A water-based PSA is composed of polymers in
emulsion: acrylic, latex, rubber, etc. Its solid content is usually
from 40 to 60% wt. It has the advantage to be in aqueous media,
with a lower viscosity (viscosity from 200 to 600 mPa s [23]),
whereas a solvent is required (e.g., heptane) to reduce hot-melt
viscosity (which causes some organic volatile compounds (OVC)
drawbacks, with respect to environmental issues). Thus, waterbased
PSAs were selected for this investigation.
2. Experimental
2.1. Adhesives main characteristics:
Three water-based PSA emulsions from SEALOCK were selected.
They are used in the labelling industry. They are all acrylic-based
emulsions, with anionic surfactant. Indeed, PSA 1 and PSA 3 are
composed of acrylic co-polymer and rosin, whereas in PSA 2 main
compound is acrylic resin. For each adhesive, pH and electrical
conductivity were first measured. Dry content was also determined,
after drying at 100 1Cinanovenfor48h.Thechemicalcompositions
were carried out by fourier transform infrared spectroscopy, with
attenuated total reflectance system (ATR–FTIR), using an infrared
spectrometer (Perkin Elmer, Paragon 1000). Their glass transition
temperature (Tg) was estimated on 10 mg dry adhesive films by
differential scanning calorimetry (TA instrument, DSC Q100), at a
heating rate of 3 1C/min. Rheological behaviour was measured using
arotaryrheometer,withaconeandplanconfiguration(AntonPaar
MCR 301) at 20 1C. Particle size distributions were determined by
laser diffraction with a Mastersizer 2000 Malvern.
2.2. Adhesive property—peel strength
The adhesive property of the PSAs investigated was characterised
by peeling tests at 1801 (Peel tester Twing—Albert
company (Model 225-1)) [24]. The test consisted of coating a
tracing paper with the chosen PSA, using a thin film applicator
(film applicator M360, ERICHSEN). This substrate was selected
because of its high internal cohesion and its low stiffness. Another
tracing paper was used as the substrate from which the sample
was peeled. The sample tested was 25 mm wide and 120 mm
long. The 20 first mm of the first substrate were uncoated to
initiate the peeling of the assembly. The substrate was rolled
three times with a 4 kg roller over the uncoated substrate to bond
them together. Then the sample was adhered to the surface of the
peel tester by means of a double-coated tape and the upper part
was peeled at a rate of 100 mm/min through a distance of
100 mm at a peel angle of 1801. Trials were made at 23 1C and
50% relative humidity.
2.3. Compatibility with physicochemical microencapsulation
processes
As previously mentioned, experimental conditions are submitted
to several changes during physicochemical encapsulation
processes. Indeed, phase separation could be induced by temperature
or pH modification, or by the addition of a salt. In simple
coacervation, solvents can be required to promote coacervation,
e.g., ethanol with gelatine, or acetone with methylcellulose.
Particle size stability is therefore a major parameter in controlling
the encapsulation process and in preventing the adhesive aggregation
or precipitation, thus conditioning the final size of the
microcapsules. PSA stability was consequently tested under the
following different conditions:
For temperature change, 100 ml of each adhesive was cooled,
under mechanical stirring, with a cooling bath at 5 1C, and then
heated at 85 1C with a heating plate. Concerning pH modification,
0.5 M sulphuric acid solution was added slowly to 100 ml of each
adhesive, under mechanical stirring, until the pH reached 2. Then,
0.5 M sodium hydroxide solution was added until the pH turned
to 9. Electrical conductivity was modified by adding slowly
sodium chloride at 3% wt to 100 ml of each adhesive, under
mechanical stirring, until the electrical conductivity reached
12 mS/cm. Finally, for solvent addition, 30 ml of 3 different
organic solvents (acetone, ethanol and ethyl acetate) were added
slowly with 100 ml of each adhesive, under mechanical stirring.
Compatibility between solvent and adhesive was validated if no
aggregation was observed.
All samples were kept at 23 1C and 50% HR for 24 h, before
performing the particle size analysis. Particle size distributions
were measured and the results were compared to those obtained
for the former original adhesive emulsions. Particle size distributions
were determined by laser diffraction with a Mastersizer
2000 Malvern. Particle size variations were correlated to zeta
potential, deduced from electrophoretic mobility measurements
with a Zetasizer Malvern.
3. Results and discussion
3.1. Adhesive main characteristics
The dry content, pH and electrical conductivity of the PSAs
studied are presented in Table 1. PSA 1 and 2 are both neutral,
with similar dry contents, whereas PSA 3 is an acidic emulsion
containing higher dry content. The electrical conductivity was
between 4 and 7 mS/cm for all the PSAs.
The FTIR spectra of each PSA were recorded and compared
(Fig. 1). The main peaks are summarised in Table 2. In each
spectrum, the presence of the strong bands at 1730 cm 1
Table 1
Main characteristics of industrial water-based PSAs.
Reference PSA 1 PSA 2 PSA 3
Solid content (wt%) 40 45 60
pH 7.1 6.8 4.6
Electrical conductivity (mS/cm) 4.9 6.2 4.1

R. Abderrahmen et al. / International Journal of Adhesion & Adhesives 31 (2011) 629–633 631
PSA 3
15
PSA 2
Volume (%)
10
5
PSA 1
PSA 2
PSA 3
PSA 1
T %
0
0,1
1 10 100 1000
Particle size (μm)
Fig. 2. Particle size distribution of three PSAs.
1.4
1.2
4000
3000 2000 1500 1000 600
Wavelength (cm -1 )
Fig. 1. Comparative ATR–FTIR spectra of three PSAs.
σ - viscosity (Pa.s)
1
0.8
0.6
0.4
0.2
PSA 1
PSA 2
PSA 3
Table 2
FTIR absoprtions present in the three PSAs spectra.
Wavelength (cm 1 )
Group
0
0
500 1000 1500 2000 2500
γ - shear rate (s -1 )
2956; 1457; 1380 –CH3
2930 –CH2
2860 –CH
1730 C¼O
1240 O–C¼O
1160 C–O–C
confirms that these adhesives are from acrylic family. Peaks at
2956, 2930, 2860, 1457 and 1380 cm 1 are assigned to the
aliphatic carbons of the polymer chains backbone and those
appended to it. Signals at 1240 and 1160 cm 1 were attributed
to C–O ether linkages and gave further evidence about the acrylic
structure of the PSAs.
The only difference observed in the three spectra concerns the
peak at 875 cm 1 , which corresponds to aromatic rings, which
could originates from the presence of a resin added only to PSA
2 and 3. It could also be the fingerprint of residual aromatic
initiator, such as benzoyl peroxide.
The glass transition temperature was found to be around
64 1C for PSA 1, 50 1CforPSA2and 35 1C for PSA 3. Typically,
the Tg of raw materials for PSAs is between 25 and 70 1C [14].
The Tg difference between theses PSA can be explained by the
presence of additives in the formulations, especially plasticizers.
3.2. Particle size distribution
The average particle diameters of the micelles were comprised
between 0.1 and 3 mm, which are typical for this kind of
adhesives, as shown in Fig. 2.
Nevertheless some differences exist: PSA 1 was mainly composed
of very small particles with a diameter around 0.16 mm and
with very narrow distribution. PSA 3 had higher particle size, i.e.,
between 0.3 and 3 mm, and also a wider distribution. Finally, PSA
2 showed three different particle populations, with sizes close to
Table 3
Rheological behaviour of three PSAs.
0.4, 5 and 200 mm, respectively. Such heterogeneity could be
ascribed to the tendency of flocculation of such an emulsion
(occurring during the different steps of the process, e.g., mixing,
transport, storage, etc.) and/or to the presence of different
additives used for its formulation (thickening, antifoam, etc.).
3.3. Rheological behaviour
Fig. 3. Rheological behaviour of three PSAs.
Reference k Consistency factor n Flow index
PSA 1 0.5 0.68
PSA 2 3.4 0.47
PSA 3 1.7 0.72
All the PSA emulsions displayed rheo-thinning behaviours, as
presented in Fig. 3. PSA 3 has a higher apparent viscosity, because
of its higher dry content. The behaviour of these fluids was found
to fit with Ostwald’s model:
s ¼ kg n
ð1Þ
s is the apparent viscosity (Pa s), g is the shear rate (s 1 ).
k and n, are two coefficients reflecting the consistency and the
flow index. The values of the flow index (n) were found to be
lower than 1 for all the PSAs (with coefficient of determination,
R 2 , very close to 1), as pointed out Table 3, thus confirming their
rheo-thinning behaviour. PSA 2 had the lowest flow index
indicating that it was more shear-rate sensitive than the two
other PSAs (Table 4).

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R. Abderrahmen et al. / International Journal of Adhesion & Adhesives 31 (2011) 629–633
Table 4
Peel strength of three PSAs.
Reference
Peel
strength
(N/2.5 cm)
Coat
weight
(g/m2)
Solid content
(wt%)
Adhesive strength
(J/dry weight
adhesive)
8
6
PSA 3
PSA 3 - pH change
PSA 3 - Temperature change
PSA 3 - conductivity change
PSA 1 1.7 20 40 8.5
PSA 2 3.5 16 45 19
PSA 3 13 18 60 47
10
Volume (%)
4
2
Volume (%)
8
6
4
2
0
0.1
PSA 2
PSA 2 - pH change
PSA 2 - temperature change
PSA 2 - conductivity change
1 10 100 1000
Particle size (μm)
Fig. 4. PSA 2 particle size distribution before and after changes.
Volume (%)
0
0.1
16
12
8
1 10 100
Particle size (μm)
Fig. 5. PSA 3 particle size distribution before and after changes.
PSA 1
PSA 1 with ethyl acetate
3.4. Adhesive property—peel strength
Peel strength results (Table 4) indicated that PSA 3, with peel
strength of 12.7 N, had the highest adhesive strength. On the
other hand, PSA 1 and 2, with peel strength lower than 4 N, were
easily removable. Yet, the adhesive dry weight coated was not
exactly the same, because of the difference in the initial dry
contents. This was the reason why the adhesive strength, represented
here by the specific energy needed to remove it divided by
the adhesive dry weight, was more relevant for comparison
purposes. PSA 3 was much more adhesive than PSA 2. PSA
1 adhesion was very low and can be removed very easily.
3.5. Compatibility with physicochemical microencapsulation
processes
3.5.1. Particle size stability versus experimental conditions
The temperature and the electrical conductivity changes
increased slightly the percentage of PSA 1s small particles
(0.2 mm). pH variation resulted in PSA 1 small particles aggregation,
thus the mean particle size was around 20 mm. In the case of
PSA 2, the biggest particles (200 mm) disappeared and the number
of small ones (0.3 mm) increased when pH and electrical conductivity
were modified, as presented in Fig. 4. Temperature
variation decreased the percentage of big particles while the
percentage of mean size particles (6 mm) doubled. For all the
experimental conditions tested, PSA 3 particle size remained very
stable (Fig. 5). Indeed, a constant particle size was observed
between 0.3 and 3 mm. This adhesive could be considered as an
interesting candidate to be encapsulated in rather different
conditions of pH, temperature and salt concentration.
Particle size distributions were then correlated to zeta potential
measurements, to explain particle size variations. At constant
electrical conductivity, PSA 1 zeta potential sharply decreased
when the pH of the medium slowed down from 7 to 3, whereas,
within the same conditions, it decreased only very slightly for PSA
2 and 3. Such a phenomenon points out the presence of a pH
4
0
0.1
1 10 100 1000
Particle size (μm)
Fig. 6. Particle size distribution of PSA 1 mixed with ethyl acetate.
sensitive surfactant used to stabilise PSA 1 emulsion, e.g., carboxylic
fatty acid salt. This kind of surfactant will loose a part of
its anionic charges when the pKa (around 4) is reached (because
of protonation events), which decreases strongly the repulsion
between the micelles and induces their flocculation, thus creating
an increasing of the particle sizes. Concerning PSA 2, the salt
addition produced higher amounts of small size particles, which
could be attributed to higher repulsion strength between particles,
because of the screening effect of the monovalent salt ions.
PSA 3 displayed a perfect particle size stability, which can be
explained by the probable use of a surfactant with permanent
charge, insensitive to pH and electrical conductivity changes (for
example quaternary ammonium-bearing compounds).
3.5.2. Particle size stability versus solvent addition
The addition of ethyl acetate did not induce any significant
aggregation, whereas the three adhesives aggregated when mixed
with acetone. Only PSA 3 was compatible with ethanol.
PSA 1 particle size did not remain stable after the addition of
ethyl acetate. Indeed, this induced the aggregation of all small
particles, forming medium ones (1 and 5 mm) and big aggregates
centred on 80 mm. This result is illustrated by Fig. 6. Concerning
the PSA 2 emulsion, the addition of ethyl acetate also affected the
micelles particle size. Indeed, the mixing reduced the percentage
of big particles (200 mm) by a factor of 3, whereas the percentage
of the smallest one (0.4 mm) was simultaneously doubled. PSA
3 had the advantage to be compatible with two of the three

R. Abderrahmen et al. / International Journal of Adhesion & Adhesives 31 (2011) 629–633 633
solvents tested. Its size distribution remained the same after
addition of ethyl acetate or ethanol. The stability of this emulsion
was particularly impressive. Thus, this adhesive offered the
biggest range of parameter changes during the encapsulation
processes.
4. Conclusion
Three industrial water-based PSAs were analysed with the
objective of further encapsulation. The work consisted in testing
the compatibility between these industrial PSA emulsions and the
experimental conditions handled in the case of physicochemical
microencapsulation processes. Thus, the particle size stability was
tested under different microencapsulation conditions: pH, temperature
and electrical conductivity changes, and by the addition
of several organic solvents, commonly used in microencapsulation
processes. It can be concluded that only the PSA 3 remained
stable in all the conditions tested. Moreover, it possessed a very
good adhesive strength, ensuring a better adhesion after microcapsule
shell’s rupture. Therefore, it will be selected to be
encapsulated by coacervation (with pH and temperature
changes). Bio polymers such as gelatin or chitosan will be selected
as shell materials for encapsulation to design environmentally
friendly microcapsules. Yet, the emulsions of PSA 1 and 2 could
also be encapsulated, but under the experimental conditions in
which they were stable.
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