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POWER AT ITS COREUsing microencapsulation to produce smart coatings. By Claus Jurisch, Koehler Innovative Solutions.

Core-shell microencapsulation technology offers a number of advantages for the paints and coatings industry. It helps over-come incompatibilities in coating systems and can be used with a variety of release mechanisms to suit different applica-tions. Accelerating the development phase and understanding the key parameters will pave the way for future growth in this field.

S ince its humble beginnings in carbonless paper some 60 years ago, chemical microencapsulation has made great strides in the

past decade to enable new industrial chemical benefits. Early appli-cations following capsules for carbonless paper included agricultural pesticides and perfume oils in scratch and sniff magazine inserts, which benefited from the controlled-release properties (environ-mental decay/diffusion and mechanical shear breakage) that these core-shell microcapsules offered. These applications paved the way for a more widespread use of chemical microencapsulation in various functions and different branches. Whether it be microencapsulation of perfume oils in homecare products, additives in paints and coatings or one-component adhesives, the increase in patents over the years (Figure 1) highlights the great interest of industry in this technology.

MICROENCAPSULATION STATE OF THE ART

Microencapsulation is an umbrella term for different technologies that encapsulate liquids, gases or micronised solids with a shell, with

the aim of protecting and releasing these ingredients. [1] In principle, there are two types of microcapsules: ą a) capsules in which the content is dispersed in a basic mass (ma-trix capsules)

ą b) microcapsules, in which the ingredient is uniformly surrounded by a defined sheath (core-shell capsules (Figure 2).

The focus in this paper is exclusively on microcapsules with a core-shell structure containing a lipophilic active compound. For the en-capsulation of lipophilic core materials the most common process is the preparation of an oil-in-water emulsion. Based on this principle, various capsule types have been developed to envelop the finely dis-persed oil droplets with a solid shell. Wall formers that are complete-ly soluble in water are consequently deposited from the continuous water phase onto the oil droplets and then crosslinked (see Figure 3). This mechanism is used to produce aminoplast and phenoplast microcapsules (in situ polymerisation) [2] and to produce water-solu-ble hydrocolloid capsules (coacervation) [3, 4]. However, the reverse case is also possible, for example in the case of wall formation by free-radical polymerisation of oil-soluble acrylate monomers [5]. In addition, methods are used in which water-soluble and oil-soluble starting materials react at the phase boundary of the emulsion drop-lets to form a solid shell. Examples are the reaction of isocyanates and amines or alcohols to give polyurea or polyurethanes (interfacial polymerisation) [6], but also the hydrolysis of silicate precursors with subsequent condensation to form an inorganic capsule wall (sol-gel method) [7].

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RESULTS AT A GLANCE

ű A lipophilic core material is encapsulated in a solid shell and released on demand to provide additional functions

ű Microcapsules help overcome incompatibilities within coat-ing systems, for example, biocides with binders.

ű Microencapsulating anti-corrosives and combining them with corrosion indicators produces smart, self-healing coatings that prolong the service life of equipment

ű The release mechanism is the key to functionality and anal-ysis must be swift to accelerate development

ű Microencapsulation technology offers great potential for more complex applications using different active compounds to offer even smarter coatings

TARGETED RELEASE TRIGGERS

The choice of polymer and the degree of crosslinking makes it pos-sible to select individual capsule properties. The capsule size and the wall thickness are also important control variables. In most applica-tions, the encapsulated core material should be released on demand or within a defined period. In some cases, such as the encapsulation of phase change materials (PCM), a permanent inclusion of the core material is desired [8]. A prolonged release of the ingredient can be

Figure 1: Development of patents in 5-year inter vals (source: database CAPLUS 17.01.2017; Key words; microcapsules in paints, varnishes and coatings).

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Figure 2: SEM core-shell capsule.

achieved by a defined degree of crosslinking and the construction of the polymer microcapsule shell. This diffusion-controlled delivery of active substances by microencapsulation can be found in many tech-nical applications such as for biocides [9], inhibitors and insecticides. Targeted release of the core material requires a trigger, which opens the shell. According to the capsule structure this mechanism can be triggered by temperature [10], UV light [11], enzymatic activity [12] or also by a change in the pH value [13]. Alternatively, mechanical forces in the form of pressure, friction and shear can also destroy the cap-sule shell and release the contents abruptly. This release mechanism is used for the sudden burst of perfume oil in fabric softeners and laundry detergents or self-healing agents in protective coatings.

MICROCAPSULES IN ACTION

The paints & coatings industry has embarked on using microencap-sulation to increase product functionality and to offer smart coatings for different applications such as antifouling, corrosion protection and self-healing. The microencapsulated active ingredient within the paint or coating matrix provides a controlled delivery system of the active compound via a release method anticipated by the capsule designer for the particular application.

ENCAPSULATED BIOCIDES OVERCOME INCOMPATIBILITIES

The regulations for the use of biocides are becoming increasingly re-strictive, specifying the lowest active concentration possible. Typically, biocides should be released slowly over time to achieve the maximum service time of the protective coating. However, controlling the bio-logical growth of microorganisms and marine crustaceans depends mainly on the optimal concentration of the biocide. Too low a quan-tity of the active ingredient will not successfully prevent fouling, but if the concentration is too high, this will have a negative impact on the environment and might end up in the food chain. Adding biocides in stable core-shell microcapsules controls the leaching of the active ingredient and thus reduces the overall concentration [14].Furthermore, by using microencapsulation Rohm & Haas [15] could overcome incompatibilities of the biocide with the binder systems in the paint. Using the active compound in encapsulated form prevented any impact on viscosity. By using a balanced mix of free and encapsu-lated biocide it was possible to easily achieve the required antifouling effect.

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SMART COATINGS FOR SELF-HEALING AND CORROSION PROTECTION

Microcapsules, added to coatings, can fulfil additional functions and open new perspectives in industrial applications. Microcracks and corrosion often remain undetected until severe damage becomes ap-parent. This then proves costly, time consuming in reconditioning, and might bear significant safety risks. Smart coatings comprise corrosion indicators and self-healing agents, which make these deficiencies vis-ible and instantly repair cracks and scratches when they arise. The key component for self-healing applications is the encapsulated self-healing agent, which can be crosslinked using temperature, oxy-gen or light. More reliable coatings contain both a self-healing agent and a crosslinking catalyst (Figure 5). Furthermore, additional function-ality can be added using indicators such as colours or malodours. The combination of the healing agent and a corrosion inhibitor is a further extension of functionality for paints and coatings. Jadhav et al. [16] researched the encapsulation of linseed oil together with a drying agent and corrosion inhibitor, and the protective per-formance of a coating with and without capsules. The core materials were encapsulated in phenol formaldehyde microcapsules via an in situ polymerisation process. The self-healing properties as well as the anti-corrosive performance of the encapsulated microcapsules were studied in a polyurethane coating. As soon as cracks occurred in the paint film they could be successfully repaired by the release of the linseed oil from the microcapsules that ruptured under simulated me-chanical action. The corrosion inhibitor played an important role in preventing corrosion in the scribed line region.

Figure 3: Schematic image of microencapsulation by in situ polymerisation or coacer vation.

1 Core material isemulsified in anaqueous phase

2 Condensationand phaseseparationby pH reduction

3 Deposition ofwall materialon the emulsiondroplets

4 Hardening ofthe capsule wallby increasingthe temperature

Capsule shellCore material

CAPSULES RELEASE ANTI-CORROSIVE COATING TO PROLONG SERVICE LIFE

Further examples to increase the service time of military equipment include combining primers with self-healing microcapsules. This system is called “polyfibroblast” [17], and was developed at Johns Hopkins University’s (JHU) for the US Office of Naval Research for zinc-rich military and commercial paints. Their development began in 2008 to create a dry powder consisting of microscopic liquid-filled polymer spheres that can be combined with standard military primers. The innovative formulation allows paint scratches to heal by breaking cap-sules and forming a coating over the exposed steel before corrosion takes place, while cutting maintenance costs and enabling vehicles to operate in the field for a longer time.

DUAL FUNCTION – INDICATION AND INHIBITION

In addition to the healing effect, NASA [18] further developed pH-sensitive microcapsules and microparticles to incorporate the au-tonomous corrosion indication and inhibition function into a smart coating. The microcapsules and particles are designed specifically to detect the pH changes that are associated with the onset of corro-sion. The formulation responds autonomously to indicate early-stage corrosion. The release of corrosion inhibitors and self-healing or film-forming agents repairs the mechanical damage of the coating. The use of microcapsules for self-healing applications provides many options and inspires numerous research projects in universities and in research companies. But a successful development is proven in the practical use in industry. There are several ways to reach this goal whether the development is carried out internally or external part-ners are chosen. Some approaches are crucial for developing applica-tion-driven solutions.

KEY MICROCAPSULE DESIGN CRITERIA

Microcapsules have many advantages but also certain restraints, which must be considered during the development phase. It is there-fore essential to provide a detailed specification of the required func-tionality. When developing microcapsules for self-healing coatings, their design depends on the following parameters.Capsule size is primarily determined by the amount of active ingredi-ent in the formulation and the substrate coating thickness. Microcap-sules can be produced with an average diameter of between 1 and 100 µm (Figure 4). As capsules for self-healing purposes must release enough liquid to cover the damaged surface, the capsules should be

Figure 4: Particle size distr ibution – coulter particle sizer and light microscopic image.

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sized appropriately with a very narrow size distribution. Considering the thickness of the coating layer (about 100 µm), the diameter of the incorporated capsules should be adequate to avoid a rough surface. The capsule size is determined during the emulsifying step and is pre-defined basically by droplet size. As the viscosity of the core material has a major impact on the droplet formation it is necessary to define the range of acceptable viscosities during the development stage. The release mechanism is the key to functionality. Application tests can be very time consuming and therefore it is important to create a rapid analytical method that closely reflects the capsule performance. Modifications of the formulation and process parameters, to optimise the functional properties, can then be quickly verified and accelerate the development phase. The formulation of functional capsules re-quires the following development steps to evaluate core material and capsule performance: ą Encapsulate several oils to determine the best core material for the application

ą Ascertain the appropriate emulsifying conditions for each liquid core material to obtain the desired narrow particle size distribution (Figure 4).

ą Determine the optimum amount of wall material and the right reac-tion conditions to minimise by-products.

Oversize microcapsules, agglomerations or other by-products need to be filtered out. If the level of by-products is too high, this hinders the final purification of the coating formulation by filtering. Fines in the capsule slurry will cause turbidity of the coating and disturb the rheological behaviour of the coating formulation.Once the application tests have proven the required quality in the coating, the next challenge is to scale up from lab scale (1–2 kg) to

Figure 5: Schematic illustration of self-healing.

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pilot scale (several hundred kg). There are two factors to scale up: the emulsifying process and the chemical wall formation. As the viscosity has a great impact on droplet size it is important to determine the ad-equate kinetics in the emulsifying step. Parameters such as the type of stirrer, stirring speed and time must be adapted and optimised. Using the right mechanical treatment results in only minor differences in the wall-forming reaction when changing the production scale.

POTENTIAL FOR MORE COMPLEX APPLICATIONS

Self-healing agents and corrosion additives can be encapsulated eco-nomically. However, special requirements regarding particle size dis-tribution, purity and polymer by-products are higher than for other commercial applications such as fragrance capsules for homecare products. When encapsulating active ingredients, the most important factor is the evaluation of the right core material and its compatibility to the wall material. The required performance determines the cap-sule size whereas the required release mechanism defines the wall structure and thickness. The possibility of adding different active com-pounds encapsulated and separated by a stable shell opens up new potentials for complex applications. Microencapsulation technology is one approach that can be used in paints and varnishes to produce smart coatings with characteristics such as self-healing and anticor-rosion properties.

“What is needed are not the microcapsules, but their desired core material.“

Dr Claus JurischProduct and Process DevelopmentTE-PVE Teamleader Carbonless Copy Papers and MicroencapsulationPapier f abr ik August Koehler SEclaus.jur isch@koehlerpaper.com

2 questions to Claus Jurisch

The loading of microcapsules is limited in the coating film. Do you expect a technical so-lution to increase the microcapsules proportion in the future? The more microcapsules are incorporated in a coating film, the more the functionality of the film is disturbed. What is needed are not the microcapsules, but their desired core material. I believe there is still some scope for reducing the wall material, which will automatically lead to higher contents of core material in the coating. Another advantage of this approach is less remainder of wall material after fracture of the capsules. The formulation of the coating, i. e. the adaptation/the tuning of the coating formulation towards a good compatibility to the incorporated capsules is another playing field, of course in combination with the design of the surface of the microcapsules.

Self-healing of damages in the coatings film is limited. How will the design of microcap-sules increase the self-healing performance in the future? Cost effective methods to produce very uniformous microcapsules with a narrow particle size distribution, which allow high release of core material in case of rupture, have to be developed. But I think more room for improving the self-healing systems is in searching for effective self-healing agents and trigger systems. Self-healing coatings suffer from not satisfying efficient homogeneous catalysts, which are able to cure the spe-cific unsaturated loading of microcapsules. The future demands for microcapsules will be less per-meability and thinner walls by using new multi-layered shell systems.

REFERENCES

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Energy Rev. 2014, 31, 531.[9] Nunn M. B., US5164096, 1991.[10] Bergueiro J., Calderón M., Macromol. Biosci. 2015, 15, 183.[11] Huang Y., Dong R., et al., Soft Matter. 2014, 10, 6121.[12] Santinho A .J., Uet a J.M., Freit as O., Pereir a N.L ., J. Microencapsul.

2002, 19, 549.[13] Hu Y., Cheng G., et al., Colloid Poly. Sci. 2014, 292, 2611.[14] Nikkola J. , European Coatings Journal 2014, 4, 36.[15] Reybuck S.E., Schwar tz C. US7377968, 2008.[16] Jadhav R.S., Mane V., Bagle A .V. et al. Int. J. Ind. Chem. 2013, 4, 31.[17] Benkoski J.J., Sr inivasan R., Maranchi J.P., US20130017405, 2013.[18] L i W., Buhrow J.W., Jolley S.T., Calle L .M., US 8859288, 2014.