silicone liner-free pressure-sensitive adhesive labels

Communication

Silicone Liner-Free Pressure-SensitiveAdhesive Labels

Johanne Empereur, Mohamed Naceur Belgacem,* Didier Chaussy

Pressure-sensitive adhesives (PSA) were microencapsulated using simple and complexcoacervation and aminoplaste. The microcapsules thus prepared were characterized by FTIRspectroscopy, particle size distribution, rheological behavior, and peeling tests. The micro-capsules were isolated and found to be out of sticky indicating that the PSAs were indeedencapsulated. The prepared suspensions weredeposited at the surface of a paper sheets andthe dried labels were then pressed against eachother. The ensuing complex was then character-ized in terms of peeling forces and showed thatthe encapsulation using aminoplaste techniqueof a commercial PSA yielded peel energy of170 J �m�2, which constitutes the recovering ofabout 68% of the adhesive power of the originalnonencapsulated PSA.

Introduction

The labeling of commercial and industrial products

constitutes a very wide market which follows the same

steady increase as the corresponding packaging market.

Presently, the annual world market production of the label

represents 57 billions of US dollars which corresponds to

about 30 billions of square meter.[1,2] The self-adhesive

labels constitute the two-third of this production. A

self-adhesive label is made up of four distinct layers,

namely: (i) the frontal (destined to transmit the informa-

tion of the future label), (ii) the pressure-sensitive adhesive

(PSA) layer,[3,4] (iii) a silicone film which guarantees

the easy release of the labels, and (iv) a paper substrate.

The material constituted of the two last layers (silicone

J. Empereur, M. N. Belgacem, D. ChaussyLGP2, INPG Grenoble University, 461 rue de la Papeterie, BP65 -38402 Saint Martin d’Heres, FranceFax: þ33 476 826 933; E-mail: [email protected]

Macromol. Mater. Eng. 2008, 293, 167–172

� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

liner) is used to protect the label before its gluing and

it is removed just before this operation. It constitutes,

generally, an undesired waste. In fact, the presence of

silicon (highly hydrophobic polymer) renders it hardly

recyclable, even in energy-recovery route. In fact, this

residuemust be treated as a special waste i.e., burned with

the obvious environmental impact in terms of pollution

due to fumes and solid residues in the recovery boilers. Of

course, some of silicone liner could escape to waste

collection and therefore, could not be recovered, thus

leaving such a hydrophobic nonbiodegradable material

in the nature, which induces obvious environmental

problems.

The main objective of this investigation is to prepare

innovative silicone liner-free PSA. The elimination of

the protective layer is achieved by the ‘‘self-protection’’ of

the adhesive, thanks to its microencapsulation,[5,6] thus

yielding ‘‘dry labels.’’ The gluing mechanism is then

ensured by the application of a pressure which induces

the breakage of the shell’s capsules, thus releasing the core

DOI: 10.1002/mame.200700292 167

J. Empereur, M. N. Belgacem, D. Chaussy

168

material, i.e., the pressure-sensitive adhesive. Thus, we

intend to study the scientific and technological break-

through in the field of labels and labeling, by taking into

account all the operation units relative to themanufacture

of silicone liner-free self adhesive material. This work will,

therefore, focus on the materials and the processes related

to the shell of the microcapsules. Moreover, the use of

renewable natural materials to encapsulate PSAs was

privileged, which constitutes additional breakthrough

eco-friendly solution as alternative to those existing

today in the field of self-adhesive labels manufacturing.

In fact, chitosan,[7] gelatine,[8] and carboxymethylcellulose

(CMC)[9] were used. The coacervation[10] and aminoplaste

microencapsulation techniques will be reported here.

The first one presents the advantage of using sustainable

materials to build the microcapsule shells, whereas the

second calls upon the use of a well-established technology.

Experimental Part

Materials

Two types of adhesives were microencapsulated: commercial and

laboratory-made water-based emulsions of PSA. The synthesis of

PSA in laboratory was carried out with the aim of controlling the

formulation components, the particle size, and the size distribution

of themicelles to be encapsulated. The commercial PSAwas A4MED

used in medical application and supplied by OMS Inc., to whomwe

are indebted. A mixture of methyl methacrylate and 2-ethyl-hexyl

acrylate (20:80 molar ratio) was emulsified and polymerized, in the

presence of AIBN, as a thermal free radical initiator.

The encapsulation with coacervation method involved the use

of two positive polyelectrolytes: chitosan (from Fluka-BioChemika

medium molecular weight of about 750 000 and a deacetylation

degree of 75%) and gelatin (referenced as 3723 SKWby Biosystems

with pI¼5 and a melting temperature of 50 8C). Sodium salt CMC

from Fluka-BioChemika was used as a negative polyelectrolyte,

in order to achieve the complex coacervation with gelatin and

chitosan.

Other products, such as formaldehyde (cross-linking agent),

acetic acid, and sodium hydroxide (pH regulators) were high

purity commercial product used without further purification.

Preparation Procedures

Simple Coacervation

This procedure was previously optimized in our laboratory.[11]

First, the corematerial was prepared by diluting 74 g of the chosen

adhesive (50% of dry matters) in 75 ml of water and stirred before

adding acetic acid solution (30% w/w) to reach a pH of 5. In

parallel, the shell material was formulated by adding 0.75 g of

chitosan to 100 ml of acetic acid solution (1% w/w) and stirring

until total solubilization. The core and shell formulations were

then mixed during 15 min. Then, 200 ml of NaOH solution



(4.5% w/w) was added to increase the pH to 12, in order to induce

the precipitation of chitosan, thus forming the microcapsules.

The shell/core ratio of the simple coacervation route was 0.02. The

resulting mixture was then left for 2 h. The capsules were

recovered by precipitation and filtration.

Complex Coacervation

Two systems were studied: coacervation between gelatine and

CMC and that concerning gelatine/chitosan and CMC. Only the

last system will be described. A mixture of 120 ml of acetic acid

aqueous solution (1%) and 0.2 g of chitosan was prepared (pH of

the medium 5) and kept under stirring till the chitosan totally

solubilized. Then, 3 g of gelatin were let swelling in the prepared

solution for about 30 min. The ensuing mixture was then heated

under continuous stirring at 60 8C in order to dissolve the gelatin.

80 g of the chosen adhesive (50% of drymatters) were dilutedwith

80 ml of water, in order to avoid the formation of agglomerates,

thus giving a shell/core ratio of 0.1. To initiate the complex

coacervation, 80 ml of 1% CMC aqueous solution were added

under stirring for 15 min and the resulting mixture was cooled

down to 10 8C, under vigorous stirring in order to avoid the gel

formation in the mixture. After 30 min, the pH was increased to

10 with concentrated NaOH solution, followed by the addition of

7.5 ml of formaldehyde (30% v/v in water) as a cross-linking agent

and the resulting mixture left for 10 min, before stopping the

reaction. The objective of the final step is to leave the capsules to

harden, in order to make easy their isolation by precipitation

and filtration.[12] When gelatine is used, the temperature of the

reaction medium was kept higher than that of its gel point, i.e.,

above 50 8C. The mixing was ensured by mechanical stirrer

(Dispermat).

Aminoplaste Route

This approach was applied according to BASF’s patent[13] and

consisted on preparing a diluted emulsion of the chosen PSA in

300 ml of water to which 35 and 32 g of the two component

(Luracoll and Lupasol from BASF) of melamine-formaldehyde

commercial resin was added. The pH of the reaction medium was

maintained at 3.6–3.9 thanks to formic acid. Themixture is heated

at 45 8C, for 60 min and under mixing. The temperature is then

increased up to 85 8C for 30 min and kept for 120 min, before

adding 3 g of melamine solution each 10 min and for 1 h,

maintaining the acidic pH by the addition of formic acid. Then,

triethanoamine is added to reach a pH of 8 followed by ammonia

addition to reach a pH of 10. The reactionmediumwas then left to

cool down and the isolation of the microcapsules was carried out

as described above.

Coating

The microcapsule suspensions were formulated with a binder

(starch in the case of coacervation encapsulation and styrene-

butadiene latex or polyvinyl alcohol in the case of aminoplaste

approach). The prepared suspensions were then deposited at

the surface of paper using, both laboratory (Endupap) and pilot

scale roll-to-roll coaters (Diproma). The metering size device was a

Meyer bar in both cases. The basis weight of the deposited

microcapsules varied from 9 to 22 g �m�2, after drying.

DOI: 10.1002/mame.200700292

Silicone Liner-Free Pressure-Sensitive Adhesive Labels

Characterization

The FTIR spectroscopy was used to ascertain the chemical

composition of the capsules. The FTIR spectra were obtained

from KBr pellets with a Perkin Elmer spectrometer Paragon 1000

spectrophotometer used in transmission mode with a resolution

of 2 cm�1 in the range of 4 000–400 cm�1. Optical microscope

(Olympus) coupled with a CCD camera and equipped with

appropriate image analysis software (Optimas) was used to

examine the shape and the size of the microcapsules. The size and

size distribution of the microcapsules were also determined by a

Malvern Mastersizer. SEM micrographs were collected using an

Environmental SEM from FEI Inc. The rheological properties of

these dispersions were studied using a cone/plate CSL2500 TA

rheometer, working in a flow mode at room temperature. Peeling

tests were also carried out at 180 8 at a rate of 1 mm � s�1,

using paper strips of 15 mm width. The apparatus used was from

Twing Albert Co., and gave a peeling force in N or a specific energy

in J �m�2.[14]

All the data collected in this work were the average of 10 trials

and the mean relative error was always less than 5%.

Results and Discussions

The FTIR spectra of themicrocapsules showed the presence

of the peaks characterizing both the shell and core

material. In fact, on the one hand, in the case of chitosan

or gelatin (as shell material), band at 3 445 cm�1,

associated with –OH or –NH groups, were detected. On

the other hand, the presence of the adhesive (acrylic-based

PSA) was established by the detection of peaks at 2 959

and 1 735 cm�1, attributed to (–CH) and (–C––O) groups,

Figure 1. FTIR spectrum of chitosan-based microcapsules of the press



respectively. Figure 1 presents the FTIR spectrum of

isolated and dried microcapsules made with commercial

PSA encapsulated by chitosan, calling upon the use of

coacervation method. The microcapsules prepared using

other shell materials were also characterized by FTIR and

gave rise to the same conclusions as those described for

chitosan-based microencapsulation. These data confirm

the well-known mechanism of complex coacervation

which is based on the adsorption of the cationic poly-

electrolytes (chitosan and gelatine) at the surface of the

emulsion’smicelles. The consolidation of the shellmaterial

is guaranteed by the use of CMC (anionic polyelectrolyte),

which complex with chitosan and gelatine. Formaldehyde

is used to cross-link the macromolecule chains of the used

polymers yielding a chemically stable shell.

The microcapsules were observed under optical and

scanning electron microscopes, as shown from Figure 2,

which illustrates the SEM micrographs of microcapsules,

obtained by coacervation and aminoplaste routes and

from which it can be deduced that the capsules are

spherical and mononuclear.

The size distribution of the prepared samples

showed that the average-number size of the particles

was close to that of the original micelles, i.e., about 1 and

6 mm for the commercial and laboratory-made PSA,

respectively. Nevertheless, as shown from Figure 3, when

calculating the average-volume size of the dispersions, one

can deduce that the prepared particle has the tendency to

form aggregates, with an interval of average-volume size

between 50 and 150mm. Different trials were conducted in

order to minimize this effect (concentration, surfactant

ure-sensitive adhesive prepared in the laboratory.

www.mme-journal.de 169


Figure 2. SEM micrographs of the microcapsules obtained by complex coacervation (left) (micrograph’s size is 150� 100 mm2) and byaminoplaste route (right).

170

amount and nature, mixing parameters, temperature etc)

and yielded the preparation of microcapsule suspensions

with similar average-number size, but with reduced

average-volume particle size, i.e., about 25 mm. It is worth

noting that the most relevant parameters allowing the

preparation aggregate-free suspension were the dilution

of the initial emulsion, as well as the addition formalde-

hyde, which played the role of rheomodifier and cross-

linking agent.

Simple coacervation method based on chitosan was the

simplest technique to handle, but it gave rise to micro-

capsule suspensions with a tendency of sedimentation

and formation of agglomerates, because the surface of the

particles has some residual gluing ability. Instead, the

suspensions made with gelatine (complex coacervation)

are more time-consuming technique, but gave more stable

systems and allowed an easier isolation procedure. The

aminoplaste yielded very similar results to compare with

those obtained by simple coacervation counterpart.

The rheological properties of the prepared suspension

were carried out in order to ascertain their behavior during

the coating process and to determine the microcapsules’

Figure 3. Size distribution of the microcapsules obtained by com-plex coacervation method.



resistance to shear rate. The coating formulations dis-

played rheo-thinning behavior and obeyed to a power law

equation with a consistency of 0.6 and 350 and flow index

of 0.8 and 0.3 (Figure 4), for aminoplaste microcapsules

arising from commercial and laboratory-made PSA,

respectively. The viscosities were also quite different since

for these suspensions, the apparent viscosity at 100 s�1

was 0.5 and 14 Pa � s, respectively. Both the intrinsic values

of the viscosity and the rheo-thinning behavior of the

suspensions prepared fall within the requirement of

Figure 4. Rheological properties of aminoplaste capsules contain-ing (a) commercial and (b) laboratory adhesive.

DOI: 10.1002/mame.200700292

Silicone Liner-Free Pressure-Sensitive Adhesive Labels

Figure 5. Micrographs of a paper coated by aminoplaste capsules containing laboratory adhesive.

the coating processes, particularly that which involves a

Meyerbar as a metering device. As mentioned below, the

average-number particle size is also different for both

suspensions, i.e., about 1 and 6 mm for commercial and

laboratory-made adhesive, respectively.

Then, the rheological behavior of the suspensions was

studied under shearing, in order to establish the value of

shear rate under which the dispersed microcapsules start

to break. Figure 4(a) shows the rheograms of suspensions

containing aminoplaste-commercial PSA microcapsules.

As mentioned above, the average particle size of these

suspensions is small (i.e., about 1 mm). This figure shows

that the particles present in these suspensions resisted to a

shear rate up to 2 000 s�1. This behavior could be ascribed

to the fact that the particle size of the microcapsules was

small, which limits their breaking under shearing.

Figure 4(b) presents the rheograms relative to laboratory-

made PSA encapsulated with aminoplaste route and

shows that the microcapsules were broken at 260 s�1,

which results, most probably, from their much bigger

size (6 mm). This negative result should be taken into

account when proceeding with the coating of particle

sizing around 6 mm and higher. Thus, less drastic metering

devices should be used (metering-bar, air knife, curtain

coating, etc.).

In all cases, the coating formulations were successfully

deposited at the surface of a paper substrate with a large

interval of basis weight, i.e., from 5 to 22 g �m�2. In fact,

Table 1. The most relevant microcapsules’ formulation and their adh

Encapsulation method Aminopl

Coating method Laborato

Adhesive Commercial

Coated microcapsules basic weight g �mS2 9

Binder SBR lat

Peel energy (J �mS2) 100



independently from the coating process (laboratory or

pilot scale), as well as coating parameters, namely: very

low shear rate (with laboratory equipment) or higher

speed of coating (a shear rate of about 103 s�1), neither

significant breaks of the capsules nor pitch or adhesive

were observed, even after 1 h of coating. For example,

Figure 5 shows SEM micrographs of a paper coated by

about 6 g of capsules by square meter of substrate.

Finally, the adhesive property of the obtained ‘‘dry’’ PSA

papers was performed. The first set of experiments was

qualitative and similar to that used for carbonless paper. It

consisted on signing the investigated paper against

another substrate and separating the two sheets in order

to check if they were glued to each other. This test was

successful for the optimized systems. Then, the quantita-

tive peel test was carried out by measuring the force

needed to separate a complex of a pressed ‘‘dry’’ PSA paper

against standard counterpart. In fact, different coating

formulationswere prepared and deposited, as summarized

in Table 1, which also gives the coated weight and the

peel energy needed to separate two sheets after gluing

by pressing. The peel energy of a pure PSA coated paper

(9 g �m�2), used as a reference, was about 250 J �m�2. Then,

different other samples were tested and the most relevant

ones can be given, namely:

(i) C

esive

aste

ry

A4M

ex

ommercial PSA encapsulated via complex coacerva-

tion method and deposited using laboratory equip-

power.

Aminoplaste Complex coacervation

Pilot Laboratory

ED Commercial A4MED Commercial A4MED

22 9

SBR latex Starch

170 70

www.mme-journal.de 171


172Macr

� 200

ment at a basis weight of 9 g �m�2 using starch as a

binder gave peel energy of 70 J �m�2, i.e., recovering

about 28% of the adhesive power of the pristine

nonencapsulated PSA.

(ii) E
ncapsulation using the aminoplaste technique of a
commercial PSA, deposited using laboratory equip-

ment at a basis weight of 9 g �m�2 in combination

with SBR latex as a binder yielded peel energy of

100 J �m�2, i.e., recovering about 40% of the adhesive

power of the virgin nonencapsulated PSA.

(iii) E
ncapsulation using the aminoplaste technique of a
commercial PSA, deposited using pilot equipment at a

basis weight of 22 g �m�2 and SBR latex as a binder

yielded peel energy of 170 J �m�2, i.e., recovering

about 68% of the adhesive power of the original

nonencapsulated PSA.

Conclusion

This paper shows that ‘‘dry’’ pressure-sensitive labels can

be prepared, avoiding the use of silicone-layer protective

layer. Simple and complex coacervation techniques as well

as aminoplaste approach were found to be successful

processes to encapsulate PSA. In fact, the adhesive was out

of sticky after its incorporation into microcapsules. The

resulting microcapsules were spherical, mononuclear and

their diameter was close to that of the original emulsion’s

micelles. However, they have a tendency to agglomerate,

which could constitute a drawback during their deposition

and metering at the substrate’s surface. The only limita-

tion of this operation is the stability of the adhesive within

the pH range during the encapsulation. In fact, since in all

cases, a switch of pH is operated, the colloidal stability of

the emulsion between the two extreme pH values is

crucial. An additional advantage of this strategy is that the

microcapsules isolation is not necessary to carry out. In

fact, the prepared microcapsule-containing suspensions

can be used as such in order to prepare the formulations to

be coated at the substrate’s surface. Thus, it was

shown that the addition of the adequate binder into the

suspensions of the microcapsules and the coating of the

resulting formulation gave ‘‘dry’’ PSA substrate without

technological difficulties. The pressing of these ‘‘innova-

omol. Mater. Eng. 2008, 293, 167–172

8 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

tive’’ silicone liner-free PSA papers induced their gluing to

each other and gave an assembly with an acceptable

joint-strength i.e., about 70% of the nonencapsulated PSA.

Other encapsulation techniques (interfacial polycondensa-

tion) are under study in our laboratory and will be the

subject of upcoming communications.

Acknowledgements: The authors wish to thank the EuropeanCommunity for its financial support in the frame of CRAFT-STARproject.

Received: October 1, 2007; Revised: December 3, 2007; Accepted:December 12, 2007; DOI: 10.1002/mame.200700292

Keywords: chitosan; gelatine; labels; microencapsulation;pressure-sensitive adhesives

[1] D. Bouvron, Etiquette plus 2006, 9, 10.[2] A. Fritsch, Emballage Magazine 2006, 789, 9.[3] I. Bendek, ‘‘Pressure Sensitive Adhesive Technology’’, Marcel

Dekker, New York, USA 1997.[4] R. Jovanic, M. A. Dube, J. Macromol. Sci., C: Polym. Rev. 2004,

C44, 1.[5] S. Benita, ‘‘Micro-encapsulation: Methods and Industrial

Application’’, Marcel Dekker, New York, USA 1996.[6] H. Q. Mao, K. Roy, V. L. Troung-Lee, K. A. Janes, K. Y. Lin, Y.

Wang, J. T. August, K.W. Leong, J. Controlled Release 2001, 70,399.

[7] M. N. V. Ravi Kumar, S. M. Hudson, ‘‘Chitosan - Encyclopediaof Biomaterials and Biomedical Engineering’’, Marcel Decker,New York 2004.

[8] R. Schrieber, H. Gareis, ‘‘Gelatine Handbook: Theory andIndustrial Practice’’, John Wiley and Sons, New York 2007.

[9] D. Klemm, B. Philipp, T. Heinze, U. Heinze,W. Wagenknecht,‘‘Comprehensive Cellulose Chemistry’’, Vol. 1,Wiley-VCH, NewYork 1998.

[10] B. Gander, M. J. Blanco-Prıeto, C. Thomasin, Ch. Wandrey, D.Hunkeler, ‘‘Coacervation and Phase Separation’’, Encyclope-dia of Pharmaceutical Technology, Marcel, Decker, New York2006.

[11] B. Truffi, Ph. D Thesis of National Polytechnic Institute ofGrenoble, France 2000.

[12] J. Empereur, Ph. D Thesis of National Polytechnic Institute ofGrenoble, France 2006.

[13] US 4406816 (1983), inv.: S. Wolfgang.[14] L. Bradeley, R. Venditti, H. Jameel, Tappi J. 2001, 84, 70.

DOI: 10.1002/mame.200700292

silicone liner-free pressure-sensitive adhesive labels

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