preparation and incorporation of microcapsules in functional coatings for self-healing of packaging...

PACKAGING TECHNOLOGY AND SCIENCEPackag. Technol. Sci. 2009; 22: 275–291Published online 24 February 2009 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pts.853

Preparation and Incorporation of Microcapsules in Functional Coatings for Self-healing of Packaging Board

By Caisa Andersson,1* Lars Järnström,1 Andrew Fogden,2,5 Isabel Mira,2 Wolfgang Voit,3 Sebastian Zywicki4 and Artur Bartkowiak4

1 Department of Chemical Engineering, Karlstad University, 651 88 Karlstad, Sweden2 Institute for Surface Chemistry, 114 86 Stockholm, Sweden3 XaarJet AB, 175 26 Järfälla, Sweden4 Department of Food Packaging and Biopolymers, University of Agriculture, 71-450 Szczecin, Poland5 Department of Applied Mathematics, The Australian National University, Canberra, ACT 0200, Australia

The replacement of fl exible polyolefi n barrier layers with novel, thin, functional polymer coatings in the production of paperboard packaging involves the risk of deteriorated barrier and mechanical properties during the converting process. Local defects or cracks in the protective barrier layer can arise because of the stress induced in creasing and folding operations. In this study, the incorporation of microencapsulated self-healing agents in coating formulations applied both by spot- and uniform-coating techniques was studied. The preparation process of microcapsules with a hydrophobic core surrounded by a hydrophobically modifi ed polysaccharide membrane in aqueous suspension was developed to obtain capsules fulfi lling both the criteria of small capsule size and reasonably high solids content to match the requirements set on surface treatment of paperboard for enhancement of packaging functionality. The survival of the microcapsules during application and their effectiveness as self-healing agents were investigated. The results showed a reduced tendency for deteriorated barrier properties and local termination of cracks formed upon creasing. The self-healing mechanism involves the rupture of microcapsules local to the applied stress, with subsequent release of the core material. Crack propagation is hindered by plasticization of the underlying coating layer, while the increased hydrophobicity helps to maintain the barrier properties. Copyright © 2009 John Wiley & Sons, Ltd.Received 15 October 2008; Revised 23 January 2009; Accepted 23 January 2009

KEY WORDS: microcapsules; self-healing; spot-coating; creasing; cutting; barrier properties

* Correspondence to: C. Andersson, Department of Chemical Engineering, Karlstad University, SE-651 88 Karlstad, Sweden.E-mail: [email protected]

Copyright © 2009 John Wiley & Sons, Ltd.

INTRODUCTION

In recent years, considerable efforts have been made to replace petroleum-derived plastic barrier materials with thin, functional biobased or syn-thetic polymer coatings in the production of paper-

board materials for food packaging. While these alternative polymers typically provide good barrier properties with respect to gases and fats, they char-acteristically exhibit poorer mechanical properties and higher sensitivity to moisture, compared with e.g. polyolefi ns. Improvements of their ability to

C. ANDERSSON ET AL.Packaging Technology

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Copyright © 2009 John Wiley & Sons, Ltd. 276 Packag. Technol. Sci. 2009; 22: 275–291DOI: 10.1002/pts

withstand mechanical stresses and of their water vapour barrier properties are therefore a prerequi-site to further development of sustainable packag-ing materials.

Paperboard packaging converting involves creasing to reduce the folding resistance of the board and to prevent cracks from arising when the material is folded.1 Creasing and cutting opera-tions are carried out on fl atbed or rotary machines, using sharp cutting rules to cut the board edges, and rounded creasing rules to make marks for subsequent folding of the carton. The creasing lines are formed by pressing the creasing rule against the board, forcing it into grooves on a counter die.2 These operations involve complex, tensile, compressional and shear forces acting over a very small area on the board.2,3 Emerging barrier coating materials, such as dispersion coat-ings, form fi lms with low fl exibility, thus involving the risk of crack formation upon creasing.4 Such defects or weak spots in the packaging can pro-vide pathways for undesirable transportation of gases or liquids in or out of the package or can cause microbial contamination.5 Besides detrimen-tal effects on the barrier properties, cracks in the outer pigment coating (often the bearer of printed graphics) would detract from the aesthetics of the packaging. One way to address this issue involves the localized reinforcement of paperboard coatings to increase their functionality and perfor-mance. Candidate solutions must be economically viable, must contribute to overall sustainability, and should be easily incorporated into existing production lines.

Self-healing of cracks in polymeric materials using microencapsulated healing agents embed-ded in the composite has been demonstrated, e.g. by White et al.6 Microcapsules or hollow reservoirs simply consist of an active material (core) encap-sulated by a membrane (shell). The microcapsules can be designed to rupture at some appropriate applied force, resulting in the release of the encap-sulated material and its possible reaction with spe-cifi c reagents to serve the desired purpose.6 Release of active core agents can also occur continuously by diffusion through a semi-permeable shell mem-brane, or the diffusion can be initiated by degrada-tion, swelling or melting of the shell.7 One mechanism for self-healing of cracks involves polymerization of the released healing agent to

bind the crack faces together,6 triggered by the action of a catalyst present in the surrounding polymer matrix.

Several methods for encapsulating materials in hollow reservoirs exist, e.g. emulsion polymeriza-tion, coacervation or solvent evaporation.8–11 One of the major diffi culties in microcapsule manufac-ture is to obtain capsules small enough for many practical applications and with a size distribution narrow enough for controlled release.12

Microcapsules have been used in the paper industry for a range of different purposes,13 e.g. in self-copying carbonless copy paper,14 and in the food and packaging industries for applications such as control of aroma release7 and as tempera-ture or humidity indicators.10,15 Other possible applications might include encapsulation of anti-microbial agents or scavengers in active packag-ing. This paper reports on the potential of a particular class of microcapsules as self-healing agents in paperboard coatings. In this case, unde-sirable damage to the coating layer during con-verting or handling is expected to be reduced by a localized rupture of the capsules under the extreme stresses at a growing crack tip, immediately releas-ing their liquid core contents to plasticize the sur-rounding coating matrix and to hinder further propagation of the crack. The use of a hydrophobic core healing agent may also further contribute to maintaining barrier properties in the damaged region of the coat by increasing its hydrophobicity and thereby minimizing the permeation of water and water vapour.16 Two application concepts have been identifi ed and studied in detail. The fi rst entails a uniform application of microcapsules by incorporation in the coating layer formulation during coated board manufacture, having the advantages that standard coating applicators may be used and that overall self-healing protection is provided no matter where on the surface extreme stresses are subsequently applied. The commercial viability of this concept of bulk incorporation of microcapsules is, however, limited if the cost of the microcapsules is relatively high. The second alter-native concept involves local or spot application of a microcapsule topcoat during converting at the positions where creasing or cutting is subsequently performed. This approach places far lower demands on microcapsule material usage, but may involve the risk of a bottleneck in converting because of the


MICROCAPSULES IN FUNCTIONAL COATINGS Packaging Technology

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extra process step required. The investigation of the cost–benefi ts that result from the different ways of coating application is not the subject of this paper; however, the relative performance of the two application methods is considered. The main contributions of this paper is to develop and prepare microcapsules with appropriate size for self-healing effi ciency, and to investigate the per-formance of functional microcapsule-based coat-ings on packaging board in terms of application strategies. As a further part of the study, this inves-tigation evaluates the performance of microcap-sules containing different types of core materials and with different core/shell ratios.

MATERIALS AND METHODS

Microcapsule preparation and characterization

Core-shell microcapsules were prepared by a two-stage solvent evaporation technique using ethyl-cellulose (Ethocel® Standard Industrial 10, Dow Europe GmbH, Bomlitz, Germany) as encapsulant (shell material) and trichloromethane as solvent. Two different hydrophobic core materials were used: rapeseed oil (ZT Kruszwica, Kruszwica, Poland) and porcine lard (MAT, Swiecie, Poland). The melting points of these core materials are −18.6 and 32.6°C, respectively, as measured by differen-tial scanning calorimetry (DSC Q10, TA Instru-ments, New Castle, DE, USA). The core material viscosities were measured at three fi xed tempera-tures by means of a controlled shear stress rheom-eter (MCR 300, Physica Messtechnik GmbH, Ostfi ldern, Germany) operated in a cone-plate geometry. Data at low shear rate (ca. 10/s) are given in Table 1.

In the fi rst microcapsule preparation stage, emulsifi cation by sonifi cation in an ultrasound homogenizer (Sonotrode S14, Hielscher GmbH, Teltow, Germany) was carried out at room tem-perature. In the second stage, dispersion of the emulsion droplets in the vehicle phase was carried out using a mechanical stirrer (Heidolph RZR 2021, Heidolph GmbH, Schwabach, Germany). The solvent was evaporated in a rotary vacuum evapo-rator (RVO 200A, Ingos, Czech Republic) or in open-top agitator at a processing temperature of 54–62°C for 140 min (or longer if needed). The microcapsules were concentrated by reversible fl occulation, and the concentrated phase was sepa-rated by a reverse decantation process. This micro-encapsulation method was used to prepare a series of capsules of differing size and core/shell ratios by varying the processing parameters and raw material feed amounts. The process development is described in more detail elsewhere.16

As is evident from Table 1, the porcine lard vis-cosity decreases dramatically with temperature, and at 60°C, its value is close to that of the rape-seed oil. This implies that the two different core materials have similar fl ow properties at the criti-cal solvent evaporation stage during the encapsu-lation process, suggesting that the fi nal morphology of the capsules should be quite similar.The particle size distributions of all microcapsule types were determined by means of laser light scattering using a Malvern Mastersizer 2000 (Malvern Instruments Ltd., Malvern, UK). Average distributions from three independent measurements are reported for each sample. For the estimation of the size distri-butions, the refractive index of ethylcellulose (n = 1.458) was used.

The thermal resistance of microcapsules as a function of the encapsulated substance was investigated by applying microcapsule suspen-sions with either rapeseed oil or porcine lard cores onto small pieces of heat-resistant OH-fi lm (designed for laser printing). A few drops of the aqueous suspensions, with a solids content of approximately 28%, were spread evenly over the surface by the pipette tip. Samples were dried at 23°C for 24 h or alternatively in an air oven at 50, 80, 105, 140 or 200°C, each for 5 min. All the samples were stored at room temperature for at least 48 h prior to further microscopic analysis (described below).

Table 1. Viscosity data for microcapsule core material at various temperatures

Temperature (C)

Viscosity (mPas)

Rapeseed oil Porcine lard

25 45 12 60040 34 32860 17 20


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Coating formulations and application techniques

Substrates and pre-coatings. A commercial three-ply, hard-sized, solid bleached sulphate packaging board (Cupforma Classic, Stora Enso Imatra Mill, Imatra, Finland), with a basis weight of 230 g/m2, was used as an uncoated substrate for both uniform coating and spot-coating. A commercial barrier latex product, Rebarco RB 736 (Ciba Specialty Chemicals, Basel, Switzerland), was applied uni-formly as a barrier pre-coating on this board for subsequent spot-coating of microcapsules on top of this, using the ink-jet application technique. Rebarco RB 736 is a ready-formulated suspension of styrene-butadiene (SB)-latex comprising talc fi ller. This barrier coating provides low water vapour transmission rates (WVTRs), but is rela-tively brittle because of its high fi ller content. The coating is thus susceptible to loss of barrier proper-ties by fi lm cracking upon creasing. The coating was applied by means of a laboratory bench coater (K-coater, RK Print-Coat Instruments Ltd., Royston, UK) in two layers to give a fi nal dry coat weight of approximately 20 g/m2.

A commercial packaging board pre-coated with a double clay coating on the top side (Performa Natura 255 g/m2, Stora Enso Imatra Mill) was used as a substrate to study the effect of top-coated microcapsules on the cutting performance of the board. The pre-coated side constitutes a basis for further printing of packages for products such as detergent and household cleaners, chocolate and confectionery, and various food packaging. The backside is normally polymer coated to meet the requirements for the target product. In this case, the microcapsules were applied on top of the pre-existing clay-based coating as a thin pigment-free layer by means of fl exography.

Spot-coating by ink-jet. The coating vehicle com-prised an approximately 20 : 80 wt% mixture of 1,3-propanediol in water, into which a low-molec-ular-weight poly(vinyl alcohol) (PVOH), (Mowiol 4-98, Kuraray Specialties Europe GmbH, Frankfurt a.M., Germany) was dissolved at 4.5 wt% to serve as a binder. The propanediol co-solvent acts as a humectant to prevent premature drying and fi lm formation of the PVOH at the nozzle plate of the ink-jet printhead, which would otherwise block

the printer. A rapeseed oil-fi lled microcapsule type with a core/shell ratio of 57/43 by weight and a dispersive phase volume ratio of 0.19 was then added in an amount corresponding (on a dry basis) to 42 g microcapsules per 100 g PVOH. A non-ionic surfactant (Dynol 604, Air Products, Allentown, PA, USA) was then added at 0.05 wt% on the total solvent amount to reach an equilibrium surface tension of 30 mN/m. The viscosity of the formula-tion was determined to be 10.6 mPas using a low-shear Ubbelohde capillary viscometer (Schott Geräte GmbH, Hofheim, Germany). The surface tension and viscosity values were tailored to fall within the windows recommended for the ink-jet printhead operation.

Ink-jet application was performed on a drum rig, using a Xaar 126/80 printhead (Xaar Jet AB, Järfälla, Sweden) with a nominal drop volume of 80 pl. Sheets of the Rebarco pre-coated board were mounted on the drum and were rotated vertically at a separation distance of 1 mm past the printhead at a surface speed of 0.2 m/s. The static printhead was angled to give a physical resolution of 360 dpi transverse to the rotation direction and 200 dpi in the paper feed direction (i.e. along the line to be creased). The width of the printed line was 6 mm, thus giving appropriate margins for the placement of the creasing line of width about 1 mm. The sub-strate was printed either in the machine direction (MD), i.e. in the direction of the coated paper web, or in the cross direction (CD), i.e. across the coated paper web, by rotating the sheet 90 . Multiple printing passes were performed in order to provide progressively increasing spot-coat coverage and thickness. The printed sheets were dried under ambient conditions.

Uniform coating. A conventional pigment coating formulation was used for investigation of the effi -ciency of microcapsules in hindering the propaga-tion of cracks when distributed uniformly in a bulky coating layer fully covering the uncoated substrate surface. The formulation was deliber-ately chosen to give a coating with relatively high brittleness, consisting (on a dry basis) of 100 g ground calcium carbonate pigment (Hydrocarb 90, Omya AG, Oftringen, Switzerland), 12 g SB-latex binder (DL 950, Dow Chemical, Horgen, Switzerland) with a glass transition temperature (Tg) of 7°C and 0.5 g carboxymethyl cellulose (FF10,



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CP Kelco, Äänekoski, Finland). The two different types of microcapsules, fi lled with rapeseed oil or porcine lard, were investigated, both having a core/shell weight ratio of 60/40. The microcapsule aqueous suspensions, of solids content approxi-mately 28%, were added to the pre-mixed coating suspension in an amount corresponding to 2 g dry microcapsules, and the formulation was stirred carefully. An identical coating mix without micro-capsules was also tested as reference. The overall solids content in the fi nal formulations was adjusted to 63%, giving a viscosity around 350 mPas at a shear rate of 100/s in all cases. The coatings were applied by the laboratory bench coater on the uncoated board substrate using rod no. 5 desig-nated to give a wet fi lm deposit of 50 mm. The coated sheets were dried at 105°C for 90 s to give fi nal dry coat weights of approximately 36 g/m2 corresponding to an average thickness of about 32 mm.

Top coating by fl exography. A set of three ethyl-cellulose microcapsules having a rapeseed core with core/shell ratios of 57/43, 71/29 and 86/14, respectively, were dispersed in a medium-carboxy-lated styrene-acrylate latex (Dow xz 95085.01, Dow Europe GmbH) at a concentration of 5 wt% based on the dry amount of latex. The fi nal viscosity was around 40 mPas, and the surface tension was approximately 38 mN/m for all spot-coating for-mulations. The latex was chosen to have a rela-tively high Tg, 22 C, in order to give a brittle coating layer allowing a clearer demonstration of the plas-ticizing/self-repairing effi ciency of any oil released from broken microcapsules.

The aqueous dispersions were applied in full tone in a thin layer of 45 mm width on the surface of the pre-coated Performa Natura board by an IGT-F1 Printability Tester (IGT Testing Systems, Amsterdam, The Netherlands). The machine set-tings were chosen to transfer a relatively high amount of coat to the substrate and thus accentu-ate any effects of the microcapsules. The ink trans-fer roll was fi tted with a medium hardness (57° Shore A) photopolymer (Nylofl ex ART-D II, BASF, Stuttgart, Germany) with a thickness of 1.70 mm. An anilox roll with a screen ruling of 180 lines/cm and a volume of 8 ml/m2 was used. The anilox force was kept constant at 15 N, and the roll-board nip pressure was held at the lowest possible level

(10 N) since pre-studies showed the risk of prema-ture microcapsule rupture when exposed to higher nip pressures. The speed and number of rotations of the roll were set to 0.30 m/s and 5, respectively. The coated strips were allowed to dry in an atmos-phere of 23°C and 50% relative humidity (RH). The average dry coat weight was measured at 9 g/m2, and the average coating thickness was approxi-mately 5 mm.

Creasing and cutting. Creasing of the uniform- or spot-coated board sheets was carried out on a cutting board (KASEMAKE KM 300, AG/CAD Ltd., Cheshire, UK) fi tted with a software tool for control of the placement of crease lines. Machine settings were justifi ed by visual assessment to give smooth creases without damage of external regions.6 Creasing was then performed in both the MD and CD of the board to give a crease line width of around 1 mm with an average depth of around 18 mm. Microscopic investigation of cross-sectioned board confi rmed that the creasing ditches were undulatory in shape with smooth edges.

Board cutting was carried out manually by means of a sharp knife on a fl atbed board cutting device. The cutting lines were drawn in the MD along the centre of the fl exography top-coated strips. The cutting performance was judged quali-tatively from micrographs of the cut edges.

Characterization of coated substrates. The surface morphology of the coated paperboard samples was analysed by means of an environ-mental scanning electron microscope (ESEM) (XL30 ESEM TMP, FEI/Philips, Eindhoven, The Netherlands). The samples were mounted onto a double-sided carbon tape on aluminum stubs and were lightly coated with gold in a sputter coater (SCD 050, BAL-TEC AG, Balzers, Liechtenstein). The ESEM micrographs were obtained at various magnifi cations under a high vacuum mode with the acceleration voltage set to 20 kV. Images of creased samples were recorded close to the crease lines, with uncreased coated board as a reference.

The WVTR was measured on creased samples, with uncreased references, to detect any loss of barrier properties due to induced cracking and to determine the effects on this loss due to the pres-ence of microcapsules. The WVTR was measured by the gravimetric cup method (ASTM E96) at


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23°C and 50% RH, using silica gel as desiccant and the surface-treated side exposed to the humid air. The results presented are averages of three mea-surements. In order to compare the achieved results with market requirements, the WVTR for a set of existing packages for various food types was mea-sured. Based on these measurements, a general critical level of 10 g/m2 d was set. In terms of the self-healing properties of the creased samples, the goal would be to attain WVTR values identical to, or lower than, those prevailing in the uncreased state. Thus, a quantitative measure of healing effi -ciency defi ned as WVTRcreased (with microcapsules) divided by WVTRuncreased (without microcapsules) was calculated.

RESULTS AND DISCUSSION

The practical utility of microcapsules as self-healing agents in coated paperboard requires that they remain intact and colloidally stable in the coating suspension without adverse reaction to the other ingredients or pH, and that they are able to survive the external forces involved in mixing, pumping and coat application, the capillary and mechanical forces acting during the consolidation of the coating layer, as well as the thermal stresses induced by drying the coating at elevated tem-perature. Colloidal stability considerations limit the solids content of the starting microcapsule aqueous suspension to the range of 5–30 wt%.16

The fi nal solids content of a coating formulation should, in practice, be as high as possible, and the introduction of a diluted additive, even at low amounts, may have strong negative consequences for coater runnability, dryer capacity and coat quality. In the ideal case, the microcapsules should rupture and release their repairing core material only at the right moment, when the paperboard is exposed to extreme mechanical stresses upon creasing or when the package is prone to unpre-dictable damage during fi lling, transportation or handling. Engineering the appropriate level of microcapsule strength to satisfy these requirements necessitates the appropriate choice of thickness and mechanical properties of its shell. Further-more, the level of addition of microcapsules to the coat must be chosen judiciously, e.g. too high

amounts causing extensive release of microcapsule core content could result in exaggerated plasti-cization, having a negative impact on coat strength and barrier properties.

Properties of microcapsules

Particle size distribution. In order for the micro-capsules to minimize impact with abrasive pigment particles in the coating suspension and during application, to reside in thin coating layers of micrometer scale thickness, and to be able to achieve a relatively high density of surface cover-age at low addition levels for optimal self-healing, the size of the particles should ideally be well below 1 mm. Figure 1 shows the particle size distri-bution of the microcapsule ‘batch a’ (mean diam-eter, 0.20 mm), which was incorporated into an ink-jet formulation (described above). Figure 1 also displays the distributions for the microcapsule batches b, c and d, which were incorporated into fl exographic spot-coats. These three microcapsule batches had a mean diameter of 0.34, 0.45 and 0.48 mm for the low (batch b), intermediate (batch c) and high (batch d) core/shell ratios, respectively, and are all of the appropriate size for thin top coat-ings (in particular with no particles larger than several micrometres). The rapeseed- and lard-type microcapsules used in the uniform pigment coating and thermal resistance studies were larger than targeted, with average diameters around 5 mm. However, given the thickness of these coating layers (~30 mm), the microcapsules would still

Figure 1. Particle size distribution of microcapsule batches a, b, c and d.



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encapsulation method, it was found that a lower processing temperature in the solvent evaporation step resulted in a reduction in the amount and size of shell macropores.16 A higher processing tem-perature furthermore increased the number of capsule agglomerates.

Thermal resistance. The thin fi lms of microcap-sules applied on heat-resistant OH-fi lm for analy-sis of thermal resistance appeared white when dried up to 80°C, but became transparent when dried at 105°C and above. This implies loss of inter- and intraparticle porosity due to the collapse of the shells and the release of the core materials at the highest temperatures. Figure 3 displays the ESEM images of microcapsules fi lled with (a) lard and (b) rapeseed dried at 80°C for 5 min. The cap-sules in Figure 3a are close-packed with a slightly irregular spherical shape and appear intact. The image also reveals a broad range in particle size (3–10 mm).

The oil-fi lled capsules are clearly weaker and started to deform to some extent already at room temperature. In Figure 3b, they appear to be

Figure 2. ESEM micrograph showing the porous morphology of microcapsules (here illustrated by some

oversized capsules).16 Scale bar is 10 µm.

Figure 3. ESEM images of microcapsules fi lled with porcine lard (left column) and rapeseed oil (right column) applied on heat resistant OH-fi lm and dried at (a,b) 80, (c) 200 and (d) 105°C. Scale bar is 50 µm.

be expected to serve their intended purpose.The particle morphology investigated by ESEM revealed a characteristic porous shell structure (Figure 2). During the development of the micro-


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semi-molten, and the capsules have started to merge and collapse. At 105°C (Figure 3d), no sign of spherically shaped particles could be identifi ed at the highest magnifi cation possible. Instead, the texture appeared as molten fi laments of the shell polymer surrounded by the expelled oil. Images recorded on the lard-fi lled microcapsules dried at 200°C (Figure 3c) displayed, on the other hand, some white features that appeared to be residuals from broken microcapsule shells. Clearly, this highest drying temperature has induced substan-tial collapse of the capsules, leaving remnants and larger aggregates from phase separation of the shell polymer in a sea of expelled core material, consistent also with the observed transparency of the fi lms.

Morphology of coated substrates

Ink-jet application. The ink-jet technique has gained increased attention in the packaging chain in recent years due to the growth in short-run lengths and the demand for customized products. This application technique, however, puts consid-erable limitations on solids content and viscosity of the liquids for controlled drop ejection. For a liquid to be jettable using piezoelectric drop-on-demand printheads of the Xaar type, its viscosity should be in the range of 8–12 mPas and with a surface tension of 25–40 mN/m. Furthermore, the size of suspended particles should not exceed 1 mm since larger particles or agglomerates risk clogging the printhead nozzles. Thus, the ink-jet technique is not suited for deposition of thick barriers with high solid load, but rather to the application of thin protective layers on an underlying barrier coating. Even though water-based ink-jet formulations con-taining microcapsules require some polymeric binder for particle adhesion, this binder will typi-cally not be present in suffi ciently high amounts to give a truly continuous fi lm matrix, nor at suffi -ciently high molecular weights to give a high degree of strength. The binder mainly serves the role of gluing microcapsules to each other and to the substrate over the short time period immedi-ately prior to creasing and folding.

The ESEM images of the Rebarco pre-coated board surfaces before and after the ink-jet applica-tion of the microcapsule spot-coats show that one

pass (dry coat weight of approximately 0.6 g/m2, resulting in an average coating thickness of about 0.5 mm) is insuffi cient for complete coverage within the spot-coated line, while four passes (dry coat weight about 2.2 g/m2, average coating thickness about 1.9 mm) is apparently necessary for full cov-erage (Figure 4), seen as the disappearance of fea-tures of the pre-coat (in particular, protruding talc particles). The microcapsules on or near the surface become more apparent, although for thicker layers, the microcapsules are embedded under the surface due to the increasing amount of polyvinyl alcohol binder accompanying the spheres.

Uniform coating. Images of the uncreased pigment coat with and without incorporated microcapsules are shown in Figure 5. The ground calcium carbon-ate (GCC) pigment particles are clearly visible, while the microcapsule spheres cannot be easily distinguished, presumably due to the low micro-capsule-to-pigment ratio. Further increase in the dosage of microcapsules in the formulation is, however, limited by the low solids content in the starting microcapsule suspensions, leading to a decrease of the overall solids content and detri-mental effects on the coating performance. The experiments, however, demonstrated that the microcapsules were readily compatible with the other coating components and that, with respect to rheological properties, similar formulations can be applied using conventional coating techniques.

Self-healing properties of microcapsules

Effect of creasing on barrier properties. As expected, the single-pass ink-jet spot-coating con-taining microcapsules did not contribute to the water vapour barrier of the pre-coated board sub-strate in its uncreased state (Figure 6). Creasing in either MD or CD of the reference substrate without spot-coating signifi cantly impaired its barrier properties, with a nearly doubled WVTR. With respect to creasing in the MD, the spot-coating in one pass gave a small positive effect in reducing this increase in WVTR, with a further slight improvement using four passes (Figure 6). However, these absolute values of WVTR would still be unacceptable for most food packaging



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applications. In contrast, a single-pass spot-coating in the CD far more effectively hinders the detri-mental impact of the (CD) creasing on the WVTR, giving values signifi cantly below those for the uncreased reference sample without spot-coating. Notably, the average WVTR of 16 g/m2 d obtained for this single-pass coating is close to the critical level of 10 g/m2 d set as an upper bound based on measurements made on existing packages for various food types. Further application of the spot-coating in four passes did not lead to further improvement; indeed, the WVTR for CD creasing

Figure 4. ESEM images of uncreased board substrate (a) without spot-coating ink-jet spot coated, (b) in one pass

and (c) in four passes. Scale bar is (a) 50 and (b,c) 5 µm.

Figure 5. ESEM images of uncreased board substrate uniformly coated (a) with a pigment coating (b) without

microcapsules, with rapeseed oil-fi lled microcapsules added at 2 g/100 g pigment (dry on dry). Scale bar is 5 µm.


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increased somewhat relative to the single pass. This is most likely due to the over-abundance of the PVOH binder submerging the microcapsules (recall Figure 4), reducing their tendency to rupture and providing a polar barrier against passage of released oil to self-heal the cracking pre-coat below.

For the board uniformly coated with a thick pigment-based layer, this layer itself is not designed as a water vapour barrier, thus leading to only a very slight reduction in WVTR com-pared with the uncoated board (Figure 7). The

measured WVTR values thus remain more than one-order-of-magnitude higher than the target level. To exaggerate any evidence of self-repair, measurements of WVTR were performed on samples creased in both the MD and CD, having the crossed creasing lines in the centre of the area exposed to the humidity gradient during testing. All coated samples displayed a slight increase in WVTR after creasing, and both rapeseed oil- and porcine lard-fi lled microcapsules incorporated in the top coat provided a small reduction in the loss of barrier pro perties (Figure 7). Given the highly

Figure 6. Effect of creasing on WVTR for pre-coated paperboard top-coated by ink-jet with a microcapsule-containing formulation in one or four passes. Board without spot-coating is shown for reference.

Figure 7. Effect of creasing on WVTR for paperboard uniformly coated with and without microcapsules in the pigment coating formulation. Data for the uncoated board are shown for reference (the creased area constitutes ca. 1/63 of the

total area used for the determination of WVTR).



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permeable nature of the porous coating (intended for ink vehicle absorption), small quantities of microcapsules clearly though in this case cannot instil a functioning water vapour barrier for food packaging, but rather may serve to reduce coat cracking to aid print aesthetics.The dimensionless number of healing effi ciency, defi ned above, should ideally be ≤1 for effective self-healing of creased surfaces. The calculated healing effi ciency is pre-sented in Table 2 for both spot-coated and uni-formly coated samples. In the former case, creasing in the CD fulfi ls the goal for effective self-healing, with spot-coating in one pass being the most effec-tive, as explained above. In the case of uniform coating, both types of core materials succeed in maintaining the WVTR as in the uncreased state.

With regard to the microcapsule-containing spot-coats applied by fl exography, the latex used for dispersion of the microcapsules itself exhibits intermediate effi ciency as a water vapour barrier coating when applied at suffi ciently high coat weights. The target with the relatively thin layer printed by fl exography was, however, to reinforce the upper surface prior to cutting, rather than enhancing the barrier properties. The WVTR was thus not measured on these samples coated for analysis of cutting performance.

Surface morphology along creasing lines. Micro-graphs of the ink-jet spot-coated Rebarco pre-coated board samples recorded after creasing are displayed in Figure 8, focusing on cracks in the vicinity of the crease lines. These cracks arise as a consequence of the stresses induced by the creas-ing procedure, since no such defects were visible in the uncreased counterparts (Figure 4).

In the images from the pre-coated board sub-strate without ink-jet spot-coating (Figure 8a,b),

the fl aky talc particles are clearly visible, as are some deep and wide cracks. The cracks formed on substrates spot-coated in one or four passes (Figure 8c–f) appear, however, to be narrower but more distinct, with sharper edges than those formed in the unprotected coating layer. This observation supports the WVTR data in Figure 6. The cracks in the vicinity of the creasing line for board creased in the MD (left column) are clearly more frequent, broader and longer than their coun-terparts in the CD (right column). It is also possible to discern the underlying structure of the pre-coating layer, i.e. talc particles, at the bottom of the cracks in these MD-creased samples, indicating that they also extend further down than for CD creasing. These observations are also consistent with Figure 6. Furthermore, these cracks exhibit a tendency to orient themselves along the creasing line in the case of spot-coating and creasing in the MD. In contrast, the cracks do not propagate forc-ibly in the direction of the creasing line when creasing in the CD, but tend to spread more evenly out from it, giving a network of smaller and thinner cracks (shown in Figure 8g,h for coating in four passes). This agrees with expectations based on cracks more easily propagating along the underly-ing fi bres than across them. Lower resolution images (not shown) suggest that the frequency of cracks was less after four passes than a single pass, most likely due to improved coverage with increas-ing number of printhead passes. The fact that this does not translate to reduced WVTR again implies that the increased PVOH binder presence for four passes, while itself providing some crack protec-tion, is inhibiting the release of microcapsule oil to fi ll the cracks below.

For the uniform thick pigment-coated samples incorporating microcapsules, since the WVTR measurement is too insensitive to quantify any effects of self-repair (Figure 7), the appearance of creased surfaces was thoroughly investigated by microscopic analysis. The ESEM images were recorded to display the region local to the crossing of the MD–CD creases (Figure 9a), to provide a representative view of the occurrence of cracks due to the tensile stresses acting there. The images were subsequently recorded with progressively higher magnifi cation focusing on details lying outside these crease lines, in particular on crack tips, to judge whether or not microcapsules have been

Table 2. Healing effi ciency for spot-coated and uniformly coated samples

Coating/creasing Healing effi ciency

Spot-coated 1× 4×Creased MD 1.63 1.50Creased CD 0.67 0.96Uniformly coated Rapeseed oil Porcine lardCreased MD + CD 1.00 0.99


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Figure 8. ESEM images of board samples creased in MD (left column) and CD (right column). Without (a,b) spot-coating, (c,d) ink-jet spot-coated in one pass and (e–h) in four passes. Scale bar is (a–f) 50 µm and (g,h) 1 mm.



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Figure 9. ESEM images of creased pigment-coated board without microcapsules, showing the (a) crossing crease lines, with a scale bar of 1 mm, and the (b) close-up appearance of cracks, with a scale bar of 100 µm.

responsible for halting their progress. In the case of the reference coating without microcapsules, a relatively large number of long, sharp and angular (as lightning bolts) cracks were found, suggesting a high brittleness of the coating layer (Figure 9b).

Figure 10 compares coat cracking without and with microcapsules. Note that cracks appear to be frequently occurring even in the presence of micro-capsules, although not as long, sharp and deep as in the reference coating (Figure 10a). Signifi cant distinctions between the different types of micro-

capsules were observed. The rapeseed oil-fi lled microcapsules successfully reduced cracking, but in a non-targeted manner. Higher magnifi cation ESEM images revealed craters or caverns of spher-ical form, presumably the remnants of microcap-sules. Many cracks terminate at such features, which could be interpreted as hindrance of propa-gation due to capsule rupture to leave the empty cavern remnant (Figure 10b). However, a more likely explanation is that microcapsule rupture occurs during drying of the coating, with expan-sion of the core oil upon heating causing the cap-sules to explode to give the observed empty caverns. Cracks subsequently caused by creasing would then be stopped by these already-drained bubbles. Rupture during consolidation or drying presumably would yield a fi ne distribution of (emulsifi ed) oil buried in the interior of the coating rather than being released over the surface, thereby resulting in unwanted potholes, which can detract from product appearance and act as crack sources in their own right.

On the other hand, the microcapsules fi lled with porcine lard showed a tendency to achieve self-healing of the coating, with shorter, thinner and distinctly shallower cracks (Figure 10c). These smaller cracks would presumably detract less from appearance and dust creation than their longer, harder and deeper counterparts. Stains apparent in their vicinity (appearing dark in Figure 10c) are microcapsule core material expelled by capsule rupture. The fact that these stains are always present in the vicinity of cracks and are not found in regions far from the crease lines, implies that they were formed during crack propagation, thus providing in situ rupture aiding crack termination.

Despite identical ratios of core-to-shell thick-nesses in these two microcapsule types, the higher viscosity of the lard core makes these microcap-sules more robust under the conditions prevailing during storage, mixing, coating application and coat consolidation, and its higher thermal resis-tance guards against premature rupture during coat drying. The release of its core material thus occurs in a more controlled manner only when needed, i.e. as a consequence of the stress induced by creasing. Although bulk application of micro-capsules by incorporation into a thick coating layer may be of less commercial interest due to their relatively high cost, these results, however, provide


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proof of principal of the ability of microcapsules to rupture in a controlled manner to prevent and heal cracks. The importance of an in situ healing rate faster than the crack-growth rate for maintenance of functionality was discussed extensively by Brown et al.17

The effi ciency of microcapsules is thought to be greatest when used together with an already-good barrier pre-coating able to resist serious cracking. Achieving a barrier coating fulfi lling the target levels of e.g. WVTR would probably require the application of multiple coating layers. Such a mul-tilayer structure could consist of e.g. a pigment pre-coating to reduce substrate porosity, one or two layers of fi lled latex to provide barrier proper-ties, and a top protective self-healing layer to coun-teract loss of barrier properties of the coating underneath.

Effect on cutting performance. A thin layer con-taining microencapsulated material applied over the pre-coated substrate may provide protection to the underlying coating when exposed to mechani-cal stresses during cutting operations, thereby improving cutting performance. The advantage with application of spot-coats via fl exography is that it is already integrated into the packaging chain as a conventional printing and lacquering technique and involves less material use than incorporation of microcapsules throughout a thick barrier coating.

The ESEM images representative of the cut edges of the pre-coated Performa Natura board with the microcapsule-containing fl exographic spot-coating on the top surface are shown in Figure 11. In con-trast to the creased samples discussed above, there is no indication of cracks in the coating layer close to the cutting line. This can be partly due to the absence of pigment or fi ller particles, which can themselves act as crack initiators. Healing of a composite material is far more complex than that of a pure polymer.18 Any activity of the microcap-sules is therefore supposed to be more effi cient in the pure latex matrix than in a pigmented system, or in other polymers (such as PVOH; see above) for which the released oil neither penetrates nor plasticizes. The images with the lowest magn-ifi cation (left column) show interesting features differing between the three rapeseed oil-fi lled microcapsules used in the spot-coats. The formula-

Figure 10. ESEM images of creased pigment-coated board, either (a) without microcapsules or containing

microcapsules (2 g/100 g pigment dry-on-dry) fi lled with (b) rapeseed oil or (c) porcine lard. Scale bar is

(a,b) 10 and (c) 100 µm.



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tion containing microcapsules with the lowest core/shell ratio (57/43) (Figure 11a,b) resulted in a smooth coating evenly spread over the pigment-coated board. On the other hand, the intermediate core/shell ratio (71/29) (Figure 11c,d) gave rise to a repeating dark/light banded fi lament struc-ture. Finally, the highest core/shell ratio (86/14) (Figure 11e,f) also gave an uneven spot-coat distri-bution, although the dark/light regions are not of the same repeating character. These observations suggest untargeted leakage of oil into the matrix

to give regions rich in either oil or latex. This leakage has presumably occurred during the appli-cation process, since the features are present over the whole imaged surface.

At higher magnifi cation (right column), it is evident that the surface region close to the cutting edge appears darker (Figure 11b) for the spot-coat with the lowest microcapsule core/shell ratio. This staining is the result of oil released from microcapsules broken upon cutting. Such a con-centration of released oil is not apparent in the

Figure 11. ESEM images of board samples spot-coated by fl exography using a formulation containing rapeseed oil-fi lled microcapsules of core/shell ratio (a,b) 57/43, (c,d) 71/29 and (e,f) 86/14. Scale bar is 500 µm (left column) and

100 mm (right column).


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other two samples (Figure 11d,f), implying that the microcapsules with the lowest core/shell ratio, being mechanically most robust, are best able to resist premature rupture on coat applica-tion and thus remain available to prevent the spread of coat damage during cutting. Note that the magnifi cation used, chosen to display a sig-nifi cant length of the cut edge, is not high enough for individual microcapsules to be identifi able. The high-magnifi cation images also show other quali-tative variations in cutting performance due to the different microcapsules. Separate fi bres mainly arranged in the cutting direction are clearly distinguishable in the sample coated with the for-mulation containing microcapsules of the lowest core/shell ratio (Figure 11b). However, the fi bres are very unevenly distributed, clustered together in lumps and apparently damaged in the case of the other two microcapsule-containing coatings, in which premature breakage apparently occurred. Again, the results strongly indicate that a suffi -ciently robust (thick) shell is critical to activating the release of the healing agent at the desired moment and not before this.

CONCLUSIONS

A novel set of microcapsules with well-defi ned properties was prepared, characterized and suc-cessfully coated onto paperboard by means of various spot- and uniform-coating techniques. The preparation process was developed to obtain microcapsules fulfi lling both the criteria of small capsule size and reasonably high solids content to match the requirements set on surface treatment of paperboard for enhancement of packaging functionality. The composition of microcapsules, with a biopolymer shell of ethylcellulose and a vegetable oil/animal fat-fi lled core, further con-tributes to the sustainability of barrier-coated packaging board. The choice of materials also minimizes the risks of food contamination since all of these substances are regarded as safe for food contact.

When applied by means of ink-jet, thin coating layers containing microcapsules proved to improve the performance of the underlying barrier-coated substrate in terms of WVTR after creasing. Spot-

coating in the cross-direction of this board was shown to be most effective in reducing cracking and aiding healing upon creasing. Application of the microcapsule formulation along the machine direction did not appear to aid the barrier proper-ties to the same extent. However, this result may be mainly due to the macro-scale bands of the pre-coating running in the MD (caused by its rod application), which might make the self-healing of cracks on a micro-scale level more diffi cult than when the spot-coating is applied perpendicular to these stripes. Thus, for similar, but more smooth, barrier-coated boards produced using commercial coaters, improvements due to microcapsule spot-coats may be obtained independent of creasing direction.With respect to uniform bulk application of microcapsules in thicker coating layers, the microcapsules fi lled with porcine lard provided the intended self-healing effect by in situ rupture and release of core material in the vicinity of cracks. Their counterparts fi lled with rapeseed oil were apparently less effective, presumably because of lower robustness and poorer thermal resistivity. The latter property is critical for coatings dried at elevated temperature.

Rapeseed oil from microcapsules was released over the coated board surface in the vicinity of cut edges, acting as a local reinforcement leading to less fi bre damage. A suffi ciently robust shell, i.e. a relatively low core/shell ratio, is critical for the prevention of premature rupture.

ACKNOWLEDGEMENTS

Financial support from the European 6th Framework Programme and the SUSTAINPACK project is gratefully acknowledged. The authors thank R. Robinson for skilful operation of the ESEM instrument. The authors also extend their gratitude to their former colleagues in the project for invaluable contribution to this work.

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