Microencapsulation in textile finishing

Gordon Nelson

Introduction Microencapsulation is a micropackaging technique that has traditionally involved the deposition of thin polymeric coatings on small particles of solids, droplets of liquids or dispersions of solids in liquids. This process was primarily established as the basis of the carbonless copy paper industry and is now used widely in a number of industries including pharmaceutical, agricultural, bulk chemical, food processing, cosmetics and toiletries. A wide variety of materials are encapsulated and a number of different types of capsules are available [l-31. Most people will be familiar with the benefits of microcapsules in the form of over-the-counter medicines such as encapsulated analgesics. Encapsulation makes the formulations more palatable and less irritant to the digestive track.

The textile industry has been slow to react to the possibilities of microencapsulation, although by the beginning of the 1990s a few commercial applications were appearing with many more at the research and development stage [4]. As the industry moves into the 21st century the number of commercial applications of microencapsulation in the textile industry continues to grow, particularly in Western Europe, Japan and North America. The move by the more developed countries into textiles with new properties and added value, into medical textiles and technical textiles for example, has encouraged the industry to use microencapsulation processes as a means of imparting finishes and properties on textiles that were not possible or cost-effective using other technology.

The question that must be asked is ‘why use microencapsulation technology?’, and indeed there are many reasons, depending on the end use. Encapsulation of active ingredients for a wide range of industries is carried out for one or more of the following purposes: (a) Rendering liquids into powders, to prevent

clumping and improve mixing (b) Protecting active ingredients from oxidation, heat,

acidity, alkalinity, moisture or evaporation (c) Preventing ingredients from interacting with other

compounds in the system, which may result in degradation or polymerisation

(d) Masking the taste of unpleasant flavours or odours (e) Improving handling of an ingredient before

processing (f) Release active chemicals in a controlled or targeted

fashion

(g) Protecting workers or end users from exposure to hazardous substances.

Many different techniques are available for encapsulation, the choice of which depends on factors such as: (a) What functionality must the capsule provide to the

finished product? (b) What coating material will not react with either the

ingredient to be encapsulated and the formulation in which the encapsulate will be added?

(c) What processing conditions must the encapsulant survive before releasing the contents?

(d) What is the optimum concentration of core material in the microcapsule?

(e) What will be the mechanism of release of the active agent from the microcapsule, e.g., agitation, pH, pressure, solubility, time?

(f) Is a sustained, targeted or controlled release profile required?

(g) What are the particle size, density and stability requirements for the encapsulated ingredient?

(h) What is the cost of the capsules and the cost of formulation or application into or onto the final product?

Not all of the above are relevant to the textile industry. Nevertheless, with the breadth of modern textile end uses, such as apparel, sports and outdoor wear, medical and engineering textiles, most microcapsule properties are utilised in one form or another.

In practice, most textile companies, particularly those manufacturing textiles for apparel, would not normally enter into development projects to produce microencapsulated formulations. A more likely scenario would be a company with a background in encapsulation technology, perhaps involved in flavourings or textile auxiliaries, approaching the textile manufacturer. Such a company could present the potential benefits of its formulations and demonstrate that the microcapsules could be applied to textiles using equipment already being used by the textile manufacturer. Companies with a strong representation in the microencapsulation business are shown in Table 1.

In textiles the major interest in microencapsulation is currently in the application of durable fragrances and skin softeners. Other applications include, insect repellants, dyes, vitamins, antimicrobial agents, phase-

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Table 1 Some companies involved in microencapsulation

Company Location

3M Canada Celessence International Eurand Brace Gmbh Coating Place Ronald T Dodge Company Thies Technology Ciba Specialty Chemicals Karma1 L1 Specialities

London, Ontario, Canada Pinner. Middlesex, UK Vandalia, Ohio, USA and Milan, Italy Alzenau. Germany Verona, Wisconsin, USA Dayton. Ohio, USA St Louis, Missouri, USA Bradford, West Yorkshire, UK Kibbutz Ramot Menashe. Israel Chesterfield. UK

change materials and medical applications, such as antibiotics, hormones and other drugs.

Phase-change materials Microencapsulation 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) 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, the Triangle Research and Development Co., to Outlast Technologies, based in Boulder, Colorado. Outlast has exploited the technology in textile fibres and fabric coatings, and PCM capsules are now applied to all manner of materials [5-91.

The main benefit is in the areas of the body where extremes of temperature have the greatest impact, the head, hands and feet. Gloves, socks, hats, helmets, etc. have all been produced incorporating materials based on Outlast’s work. The technology has also been used in 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, microencapsulation for textiles also helps to combat overheating. The PCMs have a cooling as well as an insulating effect, so overall the effect can be described as thermoregulation. In Outlasts’s words ‘the material behaves like a thermal shock absorber by slowing the rate of temperature change within a wearer’s personal microclimate. It reduces the rate at which a person will overheat or get cold. It is not a superior insulator and does not have the ability to endlessly absorb all of the heat a body can generate. I t will provide greatly enhanced comfort in the products that incorporate it, but it is not magic and will only perform within the limits of the science upon which it is based.’

The microcapsules have walls less than 1 pm thick and are typically 20-40 pm 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 temp- erature changes is very rapid [lo].

The nature of the capsules embedded within a fibre or applied to the surface of a fabric allows the effect to operate irrespective of the ambient conditions, even when the material is compressed or crushed (Figure 1) . Well known brands have been licensed to use Outlast technology, for example, Timberland, Keebok, Eddie Bauer and many more. As many as 150 companies use microencapsulated PCMs under licence.

PCMs such as nonadecane [C,9H4J and other medium chain-length alkanes function by changing their physical state from a solid to a liquid, or vice versa, in response to changes in the external temp- erature. An increase in the external temperature provides the energy required to cleave the chemical bonds, transforming a solid into a liquid. The usual increase in temperature of a garment is interrupted when the local temperature reaches the PCM melting point (32.1 “C for nonadecane) and overall there is no change in the temperature of the PCMs on the fibre or fabric substrate. If the ambient temperature increases further then the temperature of the material will also

Figure 1 PCM microcapsules coated on the surface of fabric (left) and embedded within fibre (right)

58 0 Rev. Prog. Color.. 31 (2001)

increase and a large amount of latent heat will be absorbed. Once the ambient temperature reduces and the PCMs solidify (crystallisation temperature for nonadecane is 26.4 “C) the latent heat is released, and as before the temperature of the PCM and the sur- rounding material remains constant. If the environmental cooling continues then the material temperature will continue to cool.

Outlast controls the production of the microcapsules and the processes for incorporation into fibre and other materials. I t frequently licenses other companies to produce the microcapsules and separately incorporate them into the fibre. For example, Ciba Specialty Chemicals, based in Bradford, UK, encapsulate PCMs using in-house microencapsulation systems. The capsules are then sold on to fibre manufacturers who will co-extrude the capsules during the fibre-forming process. A number of factors must be taken into account when carrying out this process, which are covered to some degree in the checklist in the Introduction section. Is the wall material strong enough to maintain the integrity of the capsule during the fibre processing’? Can the capsules be freely dispersed within the unpolymerised fibre mix? Are the capsules of the correct size and shape to operate effectively within a given fibre? Failure to answer these questions can lead to products with poor strength, or release of the capsule contents during processing.

Accordis, formerly Courtaulds Fibres, in Bradford, UK, developed the technology of in-fibre incorporation of the Outlast microcapsules, loading the fibre with 5- 10% of microcapsules 1111. The process utilises late injection technology that was also used to produce the antimicrobial fibre Amicor. In this way the PCM is permanently locked within the fibre: there is no change necessary in subsequent fibre processing (spinning, knitting, dyeing, etc.) and the fibre exhibits its normal properties of drape, softness and strength. Overloading of the microcapsules can cause a reduction in fibre performance, and currently the finest fibre available is 2.2 dtex. Work is in progress to produce smaller micro- capsules that are compatible with the fibre-forming chemicals.

In the medical field Outlast’s technology is begin- ning to gain a foothold [12]. For example, micro- encapsulation of fibres used in surgeons’ gowns helps to improve their comfort in use over long periods of time. The garments are inherently uncomfortable, as they have to provide effective barriers against particles and liquids that can carry infection. Microclimate control in hospital bedding may aid healing, and a number of products such as acrylic blankets and mattress covers are already available. Potentially the technology may help with the thermoregulation of patents undergoing operations or recovering within intensive care units, although this possibility requires further research.

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: all are designed to impart a fresh aroma to the textile. However, no matter the quality of the technology used to impart the fragrance, the effect is relatively short-lived. Numerous attempts have been made at adding fragrances directly to fibre and fabrics but all fail to survive one or two wash cycles. Only through microencapsulation are fra- grances able to remain on a garment during a signifi- cant part of its lifetime. Microencapsulation of flavours has led to many novelty applications, particularly for children’s garments, but it has also allowed exposure at home and in the work place to the beneficial effects of aromatherapy. In future, fashion garments may carry the smell of branded perfumes, particularly as many perfume houses have entered the world of haute couture.

Since 1979 R T Dodge of Dayton, Ohio, has been involved in the development and manufacture of microcapsules for a wide range of industries. In recent years the company has gained much experience in the provision of microcapsules for textiles 113). The majority of the work has been in microencapsulated ‘scratch and sniff’ T-shirts and in women’s hosiery. The nature of the microcapsules have not been revealed but i t is claimed that the shirts survive washing (typically 8-20 cycles), depending on the active agent encap- sulated, and the hosiery up to ten 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 micro- encapsulated fragrant-smelling compounds for a number of years. In the early days the applications included drawer liners, paper handkerchiefs, gift wrapping, stationary, 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, oxidisation and contamination [ 141. The capsules range in size from 1 to 20 pm. 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 such as bus tickets for promotional activities and now in textiles.

Celessence launched its TXT capsules in the UK with a major retail group in October 1999. The company allows a textile manufacturer to add a

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fragrance, vitamin, moisturiser or even an insect repellent to all types of textile substrates, including hosiery. Depending on application weights and the wash cycle used, up to 30 washes can be achieved without complete loss of fragrance. 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 (or feel) 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, and as such pose no health hazards.

For screen-printed application the encapsulates are simply mixed with water-based, solvent-free inks or binders. The capsule printing must be the last pass under a screen to avoid damage to the walls by further screens. Once printed, the fabric is then cured as with standard textile inks to achieve a good bond to the fibres.

In late 1999 Celessence formed an alliance with Brookstone Chemicals (part of the Croda International Group), a textile auxiliary and dyestuff supplier. Brook- stone will act as the sole distributor of Celessence’s TXT capsules. The products claim encapsulation of skin moisturisers, vitamins and insect repellents as well as fragrances [15]. The capsules can be formed from a number of polymeric materials, for example urea-formaldehyde resins. In this case, although some trace levels of formaldehyde are present, the level is less than that found in the industrial guidelines for cosmetics, or even oral hygiene products [16].

Kanebo Gohsen of Osaka, Japan, has continued its interest in microencapsulated fragrances derived from its perfumery division. The products continue to sell well in Japan, particularly in hosiery, scarves, handkerchiefs and other products. The Matsui Shikiso Chemical Co. of Kyoto has developed a way of fixing aroma compounds to fabric using microcapsules [ 171. 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 pm and are made using interfacial or in situ polymerisation techniques. Vpical compounds encapsulated include perfumes such as musk, civet, ambergris, pine and citrus oils.

LJ Specialities has also introduced microencap- sulated fragrance products for the textile industry. LJ is part of he Itochu group and hence much of the technology has been sourced in Japan. LJ Specialities has worked with some of the larger textile manu- facturers in the UK and elsewhere to produce fresh- smelling sheets, towels and garments with a wide range of perfumes such as eau de Cologne and fruity smells such as apple and orange [16]. More unusually fragrances such as cola and pizza have also been

encapsulated and applied to textiles. The contents of the capsules are released with light abrasion as would take place during day-to-day wear. The capsules survive repeated washing and can be applied to fibres such as cotton as a dispersion with a binder, using padding, exhaust or screen-printing techniques. Usually a softener is also required, as unsoftened fabric containing microcapsules can sometimes appear to be stiffened. The capsules are colourless and can be applied over coloured fabric or printed patterns without any adverse visible effects.

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 [ 181. 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 three and five years. So far the company has applied the technology to curtains, sofas, cushions and sheets, as well as some toys. Like many of these pro- ducts, the manufacturers claim aromatherapy effects such as ability to help with insomnia. The most frequently applied aromas include peppermint, lemon, jasmine, pine and orange. Silk ties have also been produced that release fragrant oils during normal wear, and if rubbed they produce a large burst of fragrance. The fragrant effect can last for a year and a half. Gloves and socks are also available that have fragrance-release properties and some antibacterial effects, which the manufacturers claim to last for up to 25 wash cycles.

Also in Korea, workers at h s a n National University were able to prepare microcapsules using melamine- formaldehyde systems containing fragrant oil [ 19). When attached to cotton these capsules were able to survive over 15 wash cycles. The capsules up to 10 pm diameter were produced. Scanning electron micro- scopy 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.

Euracli, a company based in Chasse-sur-RhBne 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. Many fibre types have been produced containing microcapsules with many successful products such as: (a) The Hermes scarf with Caleche perfume (b) Neyret lingerie (c) Small silk squares by LancBme perfumed with

(d) Perfumed dresses for the Olivier Lapidus summer Poeme to celebrate the Chinese New Year

show (July 1998)

60 0 Rev. h o g . Color., 31 (2001)

(e) Playtex bras (fl Dim moisturising and energising tights, on the

market since December 1998 [ Z O ] .

Paper-like products have been produced containing microencapsulated essential oils such as lavender, sage and rosemary for odour control applications in shoe liners and insoles by Aero of Celje, Slovenia [21]. Paper and other nonwoven products, no matter the method of manufacture, lend themselves very well to entrapment of microecapsules, producing long-lasting effects.

In Germany Hako-Werke Gmbh has produced a microencapsulated, fragrance-coated floor cloth (221. A fresh fragrance is released during normal use reducing the requirement for aerosol fragrance application.

Polychromic and thermochromic microcapsules Colour-changing technology has been around for a number of years, generally applied to novelty application such as stress testers, forehead ther- mometers and battery testers. These applications are now becoming more mainstream: they are beginning to be seen in textiles applications such as product labelling, and medical and security applications. In addition there is continued interest in novelty textiles for purposes such as swimware 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 microencapsulation 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.

Physiochemical 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 poly- merisation techniques are nearly always adopted; these are the same techniques used to produce textile fibres and films such as polyester, nylon and polyurethane. The most widely used system for microencapsulation of thermochromic and photochromic inks involves urea or melamine-formaldehyde systems [ 23).

Microencapsulated colour-changing dyes can be applied to textiles and other materials using a variety of printing processes, including screen printing, offset lithography, flexography and gravure printing. Liquid

crystals give precise colour modifications in response to specific temperature changes, while leuco dyes respond over a more general range of temperatures (e.g. 3-6 "C). Usually the colour change is from coloured to clear when the temperature is raised, but with careful formulation other changes can be made. For example, if a blue colour is prepared by mixing a yellow ink with a blue thermochromatic dye then in the cool state the material is coloured blue. When heated the blue colour clears and a yellow colour is revealed. Currently it is possible to produce colour change formulations in the range -25 "C up to 66 "C. The microcapsules used range widely in size but typically would be around 3-5 pm in diameter. Microencapsulation does not protect the dyes com- pletely from the elements and eventually the properties are lost, usually after around 6 months. Particular care must be taken with the solvent and other components within the ink mix.

Some efforts are being made to improve the shelf- life of thermochromatic dyes and companies such as Chromatic Technologies Ltd have recently demon- strated methods that substantially improve perform- ance by careful choice of microcapsules and dispersing solvents [24]. Microencapsulated thermochromatic dyes generally survive up to 20 laundering cycles, although excessive drying at elevated temperatures or use of bleach can reduce the longevity of the finish.

Both forms of microencapsulated system are avail- able from a number of suppliers, including PPG Industries, Pittsburg [25], Color Change Corp., Chicago [26], Solar Active, Tarzana, California and Second Story Concepts, Cleveland. In Japan Matsui Shikiso Chemical Co., Kiroku Sozai Sozo Kenkyusho, and Dainichiseika Color and Chemicals Co. are among the leading companies.

PPG produces a range of photochromic dyes with trade names such as Photosol 0265 [25]. The H W Sands Corp. Jupiter, Florida, claims to have worldwide rights to distribute the dyes in the imaging industry, including textiles. Photochromic dyes in PPGs Photosol range alters colour on exposure to UV light in the 300-360 nm waveband. In high-intensity light the colour change takes place in milliseconds while in sunlight this effect takes between 20 and 60 s. A full range of colours can be produced, including blues, yellows, purples and orangeheds. When the light source is removed the colour returns to its original state. In processing, the Photosol dyes can survive temperatures of 180 to 240 "C for a few minutes without degradation [27].

The Matsui Shikiso Chemical Co. in Japan has produced microencapsulated polychromatic dyes that are claimed to show good light-fastness properties and give a very efficient polychromatic effect [28,29]. In the UK Matsui products are sold via The Cornelius Chemical Co. Matsui developed its colour-change

0 Rev. Prog. Color., 31 (2001) 61

technology in the 1980s specifically for the toy market. Its products marketed under the Chromicolor name have been used in a whole range of novelty appli- cations (toothbrushes, pens, drink and food cartons) and many others as well as textiles such as polyester, cotton and in polyurethane foam applications.

Companies such as Solar Active offer photochromic inks for embroidery thread and T-shirts. The threads have been used in a range of products from handkerchiefs and scarves to designs on conventional apparel 1301. T-shirts, for example, are available from Del Sol of Santa Rosa, California [31]. The colour change in other photochromic and thermochromic dyes can be made to be irreversible, but these dyes are usually only used as indicators and their application in textiles is limited.

The cost of photochromic inks can be as much as four to eight times that of a traditional screen-printing ink. However, although this may add an extra $0.20 to a T-shirt, the added value of the product is easily recovered, with prices generally $2.00 to $4.00 more for the equivalent photochromic garment.

New colour-changing technologies are on the way, including microencapsulated hydrochromic dyes, which colour change in response to water, and piezochromic dyes, which change colour in response to pressure [32].

Fire retardants Fire retardants have been applied to many textile products, but in certain cases they can affect the overall handle, reducing softness and adversely affecting drape. Fabrics used in military applications must be strongly fire retardant. For example, tentage which must be light, easily deployable, strong, durable, non- irritating to skin or eyes, cost-effective, and available in commercial quantities, as well as being fire resistant. Currently the existing finishes used for tentage are very complex, costly and may produce toxic smoke on contact with flames. Workers in the US Army have investigated microencapsulation as a means of over- coming these inherent problems, encapsulating both fire-retardant chemicals and intumescents that expand in volume on exposure to fire, insulating the textile from its effects [33). They encapsulated these chemi- cals using a poly(viny1 acetate) resin, which also acted as the adhesive for attachment of the capsules to the fabric, usually cotton alone or in blends with nylon or polyester. The products were effective at retarding fire but further work needs to be directed at producing a more flexible treated fabric.

In Moscow N S Zubkova has also been incorporating a microencapsulated T-2 fire retardant during spinning of a polyester fibre for blending with cotton [34]. It was found that microencapsulation in silicon-containing shells, in particular, vinyltriethoxysilane, produced

significant advantages in decreasing combustibility in poly(ethy1ene terephthalate). The new fibre exhibits a lower maximum rate of transformation and the amount of carbon monoxide released is also reduced. Overall in the presence of external flame the fibre does not drip on melting and the nature of the capsule shell wall (silicon) also helps by reducing the thermal conductivity of the fibre.

Counterfeiting In high added value textiles, and in branded and designer goods there is great pressure to protect from illegal copying within the marketplace. Micro- encapsulation can be used to help with this problem by offering a covert yet distinctive marking system. One example of this technology is that developed by Gundjian and Kuruvilla of Nocopi Technologies [ 351. This system for combating textile counterfeiting utilises microcapsules containing a colour former or an activator applied to, for example, a thread or a label. The microcapsules adhere to the textile and, depending on the type of chemical within the capsules, can be detected at a later date to check authenticity. Detection may be achieved directly using UV light or more likely by using a solvent to break open the capsules, releasing the contents and allowing a colour to develop. Using printing techniques the capsules can be applied onto a label as a logo or another printed message. The capsules used are 2-7 pm in diameter and are made of synthetic materials that are physically and chemically resistant to the processes normally used to process textiles as well as to any subsequent washing process to which the textile may be subjected. To obtain this robustness in certain cases the capsules had to be doubly or triply coated.

Liposomes Liposomes (Figure 2) are fatty acid-based micro- capsules, and were first characterised in the 1960s by Bangham 1361. Initially developed as models of biological membranes, their potential for drug delivery was gradually recognised and in the last decade a number of companies have successfully applied the technology for controlled and targeted release of anti- cancer and anti-fungal drugs 137,381. From medical applications the technology moved into skin care and now many cosmetic products contain liposomes, which are claimed to deliver bioactive compounds such as those for anti-wrinkle or anti-ageing through the skin surface to the stratum corneum.

Liposomes are prepared using a lipid or a combination of lipids; most commonly phosholipids such as phosphatidylcholine (PC) are used. The overall properties of the liposomes can be controlled by tailoring the lipid mix, resulting in changes in permeability and specificity in targeting. There are

62 0 Rev. Prog. Color., 31 (2001)

Figure 2 Microscopic image of liposomes

several ways to prepare liposomes, depending 011 the type to be made: (a) Multilamellar vesicles (MLVs) (b] Small unilamellar vesicles (SUVs), which are

under 100 nm in diameter (c) Large unilamellar vesicles (LUVs), larger than

100 nm in diameter.

Commonly used procedures include thin film hydration, sonication, extrusion, use of a French press, ethanol injection, detergent dialysis or reverse-phase evaporation.

While the medical and cosmetic applications are now well established, many workers have begun to examine ways of using liposomes in other industries (391. I n recent years liposomes have been examined as a way of delivering dyes to textiles in a cost-effective and environmentally sensitive way (40-433. A European-funded project involving 13 countries com- pared the effectiveness of liposome technology with existing textile auxiliaries. The team used com- mercially available liposomes, primarily made from PC or containing lipids such as cholesterol found in wool lipid, and examined their interaction with raw wool, top, yarn, and woven and knitted fabrics. The con- clusion of the report (Ecotrans W8814) indicated that wool dyeing using liposomes was entirely feasible. The liposomes used (for example, commercially available PC liposomes from Transtechnics SL) were cost-effect- ive, and no specific equipment or skills were required to handle them within the dyehouse. The results were very good with pure wool and wool blends, and as the temperature of dyeing could be reduced there was much reduced fibre damage. In their studies using Lanaset Yellow R dye at liposome concentrations of 1% the dyebath exhaustion was greater than 90% at the low temperature (80 "C) used, and at the relatively high dye concentration of 3 wt%. There was a significant saving in energy costs due to the lower dyebath temperatures (85-90 "C instead of the usual 98 "C), and

the impact of the dyeing process on the environment was also much reduced with chemical oxygen demand (COD] being reduced by about 1000 units. Some problems were found in specific cases with poor dye leveling, where other auxiliaries were required to correct the problem. However, new liposome design should overcome this problem.

Miscellaneous applications Working on a range of fabrics, a team of workers in France has encapsulated glycerol stearate and silk protein moisturisers for application on bandages and support hosiery [44]. The material maintains comfort and skin quality through extensive medical treatment where textiles are in direct contact with the skin.

The Mitsubishi Paper Mills has produced a polypropylene nonwoven material for application as a cleanindwiping cloth containing microencapsulated octane, tung oil and paraffin oil as cleaning solvents [45]. The cloths feel good in the hand and have very good cleaning properties.

The application of insecticides and acaricides to textiles to combat dust mites and insects such a mosquitoes has been investigated by many workers. Microencapsulation has been considered as a mech- anism of retaining the effect for significant periods without exposing the user to excessive dosages of hazardous chemicals. The use of alternative insecti- cidal compounds such as those found in many essential oils and other plant extracts has made the production of long-lasting acaricide bed sheets possible [461.

Microencapsulation: the future The 'holy grail' 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 microencapsulation industry must take more notice of the possibilities within the textile industry and specifically design microcapsules that overcome these problems.

For the future, the consumer's desire for novel and unique effects will always be present. But more importantly, in an ever-increasing desire for con- venience, 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. Microencapsulation 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 of Western Europe, Japan and the US have concentrated on developing performance fabrics with added value for sports and outdoor application, as well as novel medical textiles. Microencapsulation 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 to warn 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 microencapsulation 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. The textile industry must continue to be outward looking and develop the textiles that consumers desire in the first decade of the new millennium.

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3. 4. 5. 6. 7. a.

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11. 12.

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