MICRO-ENCAPSULATION

1. Introduction:

“Small is beautiful” would be an appropriate slogan for the many people studying micro-encapsulation, a process in which tiny particles or droplets are surrounded by a coating to give small capsules with many useful properties. The material inside the microcapsule is referred to as the core, internal phase or fill, whereas the wall is sometimes called a shell, coating or membrane. Most microcapsules have diameters of few micrometres.

The reasons for micro-encapsulation are countless. In some cases, the core must be separated from its surroundings, as in isolating vitamins from the deteriorating effects of oxygen, retarding evaporation of a volatile core, improving the handling properties of a sticky material, or isolating a reactive core from chemical attack. In other cases the objective is not to separate the core completely but to control the rate at which it leaves the microcapsules as in the controlled release of drugs or pesticides [7]. Manufacturing costs are based on coating material, solvent, equipment and labour. Coating-material prices vary greatly, and as a rule, the cheapest acceptable material is used. Coatings that can be applied without solvent or water are preferred. Environmental and safety regulations greatly increase the cost of process that use volatile organic solvents.

2. Classification:

A classification of encapsulation processes is given below:

Category I

Liquid suspending media

Water-in-oil (W/O) Oil-in-water (O/W)

Complex coacervation

Polymer-polymer incompatibility

Interfacial and in situ polymerization

Desolvation

Solvent evaporation from emulsion

Gelation

Pressure extrusion

In Category-I, a liquid is used as the suspending medium throughout. Emulsification or dispersion of two or more immiscible phases is a key step in these processes. In Category-II, a gas is the suspending medium in which the capsules are formed, and atomization of a liquid phase is the key step. Some authors refer to Category-I processes as chemical processes and Category-II processes as physical or mechanical processes. According to this classification, processes such as solvent evaporation, gelation and polymer-polymer incompatibility are termed chemical processes even if no chemical reaction occurs. A spray drying process in which reactive components are polymerized during the drying step to form microcapsules would be called a physical process, even though a chemical reaction clearly occurs during capsule formation. Many Category-I and Category-II processes are similar. For example, solvent evaporation is a key step in spray-dry encapsulation and in processes involving solvent evaporation from an emulsion. The only difference is that evaporation in the former case occurs directly from the liquid phase to the gas phase. In the latter case, evaporation involves transfer of a volatile liquid to an immiscible liquid from which it is subsequently removed. Another example is gelation encapsulation. The droplets, which are gelled to form capsules, can be formed by emulsification or atomization [1].                       Figure 1: Microencapsulation process

Figure 2: Schematic description of the microencapsulation system

3. Application of Microcapsules in Textiles:

          The consumer demands for textiles with new characteristics and added value into medical and technical fields have encouraged the industry to use micro-encapsulation processes as a means of imparting finishes and properties to fabrics which were not possible or cost-effective using other technology. Textile manufacturers are demonstrating increasing interest in the application of durable fragrances to textile as well as skin softeners; other potential applications include, for example, insect repellents, dyes, antimicrobials, phase change materials.

3.1 Phase-change materials

Micro-encapsulation technology was utilised in the early 1980s by the US National Aeronautics and Space Administration (NASA) with the aim of managing the thermal barrier properties of garments, in particular for use in space suits. They encapsulated phase-change materials (PCMs) (e.g.nonadecane) with the hope of reducing the impact of extreme variations in temperature encountered by astronauts during their missions in space. Ultimately the technology was not taken up within the space programme. However, the potential was recognised and after further development the work was licensed by the inventor. Outlast Technologies has exploited the technology in textile fibres and fabric coatings [Fig.1]. PCM capsules are now applied to all manner of materials [2, 3], particularly outdoor wear (parkas, vests, thermals, snowsuits and trousers) and in the house in blankets, duvets, mattresses and pillowcases. As well as being designed to combat cold, textiles containing PCMs also helps to combat overheating, so overall the effect can be described as thermoregulation. The microcapsules have walls less than 1 μm thick and are typically 20–40 μm in diameter, with a PCM loading of 80–85%. The small capsule size provides a relatively large surface area for heat transfer. Thus the rate at which the PCM reacts to an external temperature changes is very rapid [4].

Figure  3:  (a) PCM microcapsules coated on the surface of fabric and (b) embedded within fibres

The late injection technology processes allow the in-fiber incorporation of Outlast microcapsules, loading the fiber with 5–10% of microcapsules. In this way the PCM is permanently locked within the fiber; there is no change necessary in subsequent fiber processing (spinning, knitting, dyeing, etc.) and the fiber exhibits its normal properties of drape, softness and strength.

3.2 Fragrance finishes:

The addition of fragrances to textiles has been carried out for many years in the form of fabric conditioners in the wash and during tumble-drying to impart a fresh aroma. However, no matter the quality of the technology used, the effect is relatively short-lived. Numerous attempts have been made at adding fragrances directly to fiber and fabrics but all fail to survive one or two wash cycles. Only through micro-encapsulation are fragrances able to remain on a garment during a significant part of its lifetime. Micro-encapsulation of essential oil flavours has led to many novelty applications, particularly for children’s garments, but it has also allowed utilization at home and in the work place to the beneficial effects of aromatherapy.

In recent years several companies have gained much experience in the provision of microcapsules for textiles. The majority of the work has been in microencapsulated “scratch and sniff” T-shirts and in women’s hosiery: it is claimed that the shirts survive washing (typically 8–20 cycles), depending on the active agent encapsulated, and the hosiery up to 10 washes. The capsules also survive drying in conventional tumble-dryers. Well-established techniques such as in situ and interfacial polymerisation are used to manufacture the capsules.

Celessence International of Hatch End, Middlesex, has been investigating and manufacturing microencapsulated fragrant-smelling compounds for a number of years.

In the early days the applications included paper handkerchiefs, gift wrapping, ornaments, greeting cards, advertising brochures, books, cartons and labels. The company has now turned its attention to textiles, using its basic technology of encapsulating fragrances in gelatin or synthetic capsules, which protects the contents from evaporation, oxidization and contamination. The capsules range in size from 1 to 20 μm. In practice, the smaller the capsules the greater the covering of the product and the longer the fragrance will last, as it takes longer for the capsules to be ruptured by physical pressure. Larger capsules release more fragrance when ruptured. Traditionally the “scratch and sniff” application of microcapsules used screen-printing, but now litho and web printing techniques have been adopted, initially in paper products and now in textiles.

Celessence TXT capsule systems comprise aqueous dispersions of encapsulates, which can be applied by pad, exhaustion or hydroextraction techniques to a wide variety of textile substrates. Durability to washing and handle may be further improved by incorporating suitable formaldehyde free binders and softeners. All applied products are blended from natural and synthetic materials that conform to legislative guidelines for cosmetic products[5]. For screen-printed application the encapsulates are simply mixed with water-based, solvent-free inks or binders. The capsule printing must be the last step to avoid damage of microcapsules walls, once printed; the fabric is then cured as with standard textile inks to achieve a good bond to the fibres.

The Matsui Shikiso Chemical Co of Kyoto has also developed a way of fixing aroma compounds to fabric using microcapsules. The fabric is first treated with a nitrogenous cationic compound and the microcapsule wall is manufactured to adhere to this layer. The capsules can range in size from 0.1 to 100 μm and are made using interfacial or in situ polymerisation techniques.

In Korea the Eldorado International Co of Seoul and a number of other companies offer new fabrics that emit the natural aroma of flowers, fruit, herbs and perfumes. Emulsified microcapsules containing a natural aroma or essential oil are attached to the fabric after dyeing. The capsules break on movement of the wearer, releasing the aroma. In general the capsules continue to emit aroma for up to 25 wash cycles and on the shelf the finish will remain ready for action for between 3 and 5 years. So far the company has applied the technology to curtains, sofas, cushions and sheets, as well as some toys. Silk ties have also been produced that release fragrant oils during normal wear, and if rubbed they produce a large burst of fragrance.

Also in Korea, workers at Pusan National University were able to prepare microcapsules using melamine-formaldehyde systems containing fragrant oil [6]. When attached to cotton these capsules were able to survive over 15 wash cycles. Scanning electron microscopy indicated that the smaller of the capsules in the range survived more effectively after laundering. This phenomenon may simply be due to the relative thickness of a capsule within an adhesive film binding the capsules to the textile substrate [Fig.2].

Euracli, a company based in Chasse-sur-Rhone in France, has produced microcapsules containing perfumes or cosmetic moisturisers that can be padded, coated or sprayed onto a textile and held in place using an acrylic or polyurethane binder.

Figure 4:  example of microcapsules application on fabrics

3.3 Polychromic and thermo-chromic microcapsules

Colour-changing technology has been for a number of years generally applied to stress testers, forehead thermometers and battery testers. New applications are now beginning to be seen in textiles, such as product labelling, and medical and security flexible displays. In addition there is continued interest in novelty textiles for purposes such as swimwear and T-shirts. There are two major types of colour-changing systems: thermochromatic which alter colour in response to temperature, and photochromatic which alter colour in response to UV light. Both forms of colour-change material are produced in an encapsulated form as micro-encapsulation helps to protect these sensitive chemicals from the external environment.

Today manufacturers are able to make dyes that change colour at specific temperatures for a given application, e.g. colour changes can be initiated from the heat generated in response to human contact. Physico-chemical and chemical processes such as coacervation and interfacial polymerisation have been used to microencapsulate photochromic and thermo-chromic systems. However, to obtain satisfactory shelf-life and durability on textiles, interfacial polymerisation techniques are nearly always adopted. The most widely used system for micro-encapsulation of thermochromic and photochromic inks involves urea or melamine formaldehyde systems [7].

4. Micro-encapsulation: the future

The ideal feature for most textile applications using microcapsules would be a system that is easy to apply, does not effect the existing textile properties and has a shelf-life on a garment that allows normal fabric-care processes to take place. Currently, although capsules can survive 25–30 wash cycles, conventional ironing and other heat-input processes such as tumble-drying can cause a dramatic reduction in the desired effect. The micro-encapsulation industry must take more notice of the possibilities within the textile industry and specifically design microcapsules that overcome these problems. For the future, the consumers desire that novel and unique effects will always be present. But more importantly, in an ever-increasing desire for convenience, the consumer will require that fabric properties are inherent in the garment, e.g. fresh odour and softness. Consumers will expect these properties to last the lifetime of the garment, and not involve routine intervention in the form of the never-ending addition of washing aids and fabric conditioners. Micro-encapsulation may deliver these long-term goals. The desire for a healthier and more productive lifestyle will continue to generate a market for textiles that promote “well-being”. Textiles that “interact” with the consumer, reducing stress, promoting comfort and relaxation, are possible through active delivery from microcapsules. In the last decade the textile industries have concentrated on developing performance fabrics with added value for sports and outdoor application, as well as novel medical textiles. Micro-encapsulation can play a part in this continued development, for example by allowing sensing chemicals to be attached to sports clothing and medical products; these will be able towarn of damage or hazard to the wearer. Systems can also be developed that deliver measured dosages of chemicals to combat muscle pain or other more serious injuries.

The potential applications of micro-encapsulation in textiles are as wide as the imagination of textile designers and manufacturers. Early success for some companies in producing microencapsulated finishes for textiles have come about from collaboration and adaptation of technology from other industrial sectors [8].

6. References:

1. H. F. Mark, D. F. Othmer, C. G. Overberger, G. T. Seaborg, Micro-encapsulation, Encyclopedia of Chemical Technology (III ed.), vol.15, 470-493, Wiley Interscience publication
2. H. F. Mark, N. M. Bikales, C. G. Overberger, G. Menges, Microencapsulaion, Encyclopedia of Polymer Science and Engineering (II ed.), vol.9, 724-745, Wiley Interscience publication
3. N. S. Zubkova, Thermal Insulation, Knit. Int. 102 (1216), 1995a, 50
4. N. S. Zubkova, Phase change technology outlasts lofted fabrics, Tech. Text. Int. 4 (7), 1995b, 28-29
5. D. P. Colvin and G. Y. Bryant, Protective clothing containing phase change materials, Advances in heat and mass transfer in biotechnology (HTD), New York: ASME 362, 1998, 123-132
6. B. Pause, Measuring the thermal barrier function of phase change materials in textiles, Tech. Text.. Int. 9 (3), 2000, 20-21
7. K. Yamada and Y. Yamada, A scent of the Unusual, Int. Dyer, 2000 (june), 26
8. K. Hong and S. Park, Melamine resin microcapsules containing fragrant oil: synthesisand characterization, Mat. Chem. Phys.,1999, 58, 128-131