Indian Journal of Fibre & Textile Research Yo 1.3 I , March 2006, pp. 72-82
Microencapsulation of essential oils and phase change materials
for applications in textile products
Bojana Boha Department of Chemical Education and Informatics, Faculty of Natural Sciences and Engineering, University of Ljubljana,
Yegova 4, 1001 Ljubljana, Slovenia
and
Emil Knez Aero Chemical, Graphic and Paper Industries, Ipavceva 32, 3000 Celje, Slovenia
The paper reports the development and testing of three types of microcapsules for applications i n textile products, namely microencapsulation of antimicrobial essential oils of sage, lavender and rosemary for nonwoven textile shoe insoles; smell-based animal repellents for agricultural textiles, designed to protect plants against damage caused by deer and rabbits; and paraffinic phase change materials (PCMs) for active thermal control garments. III situ polymerisation of melamineformaldehyde prepolymers was used as the microencapsulation technology in all three cases, based on partly methylated trimethylolmelamine and hexamethoxymethylolmelamine resin as wall materials and a styrene-maleic acid anhydride copolymer as a modifying agent. The microencapsulation process was modified to achieve the desired characteristics of microcapsule wal ls (different permeabi l i ty and sensitivity/resistance to pressure). Bibl iometric trends in microencapsulation technology with special reference to textile industries have also been discussed with an overview of main application fields and uses of microencapsulated additives in textile products.
Keywords: Microencapsulation, Nonwoven, Phase change material
IPC Code: I nt. C I .8 C09K5/00, DO I F I I I O
1 Introduction
1.1 Microencapsulation Trends
Microencapsulation includes several technologies for coating micrometer sized particles of finely ground solids, drops of liquids, or gaseous components with protective spherical membranes - microcapsule walls . Microencapsulation can be considered as a typical example of a knowledge-intensive and dynamic research field with an increasing growth of publications (Fig. I ) . 1 Trends in patent vs. non-patent l i terature i llustrate the growth of basic research (non-patent literature), and a faster growth of industrial research represented in waves of patented inventions.
Microencapsulation technologies offer many opportunities to improve the properties of textiles or to give them new functions. B ibliometric analyses show that the vast majority of publications on microencapsulation for textile applications are the patents (Fig. 2), i l lustrating the importance of industrial property rights in this specialized field. The first wave of microcap"ule inventions for textiles broke out in 1 970s and in-
., all the correspondence should be addressed . . � .boh @ntf.uni-Ij .s i .
troduced microencapsulated dyes, pigments, softeners, antistatic agents and fire retardants for textiles, while the second wave of i nventions in the 1 990s brought thermochromic and photochromic materials , antimicrobials, insect repellents, cosmetic and medical textiles. The third wave, biggest of all, took rise after the year 2000 and covered primarily the microencapsulated phase change materials (PCMs).
2500 (ij Q) >- 2000 Q; 0.. C/) C Q) E :::J <..> 0 -0 0 Q; .0 E :::J Z
1970 1975
D "
1980 1985 Year
1990 1995 2000
Fig. I -Yearly growth in new publ ications on microencapsulation in the Chemical Abstracts Plus database
BOH & KNEZ: M ICROENCAPSULATION OF ESSENTIAL OILS & PHASE C HANGE MATERIALS 73
1.2 Microencapsulation Processes
I The selection of microencapsulation processes depends on the desired characteristics and uses of textile final products. The properties of microcapsules, their sizes, shapes, wall materials, active substance release mechanisms, method of application, and compatibil i ty with formulation additives have to be adapted to the requirements of textile processing methods and uses of final products . The preparation of microcapsules for textile finishing compositions was in i tially based on coacervation processes and later on polymerisation methods, such as interfacial polymerisation and in situ polymerisation of aminoaldehyde resins.
The coacervation phenomenon takes place in colloid systems, where coacervate droplets (a macromolecular colloid rich phase) surround dispersed microcapsule cores and form a viscous microcapsule wall ,
701
� w�---------------- -------------------n.
j 501' 0 Non-paten!
C • Patent (1) 40 -- ----------------------I-H-E :::J U ' _____________________ � .g JOT '0 20-;--------Iii .0 � IO -----� �----���HH�� ..... Z
1965 1 970 1 975 1980 1985 1990 1995 2000 Year Fig, 2-Bibl iometric analysis of microencapsulation applications in textiles as per the Chemical Abstracts Plus database
which is then hardened with a cross-linking agent (Fig. 3a) . In polymerisation methods, monomers polymerise around emulsified or dispersed cores and form a solid polymeric wal l . In the i nterfacial polymerisation (Fig. 3b), one of the monomers is typically dissolved in the aqueous phase and the other in a hydrophobic solvent phase, while in the in situ polymerisation (Fig. 3c), monomers or precondensates are added only to the aqueous phase of the emulsion.
1.3 Microcapsule Formulations for Textile Applications
Patents describe different ways of incorporating microcapsules onto or into texti les, such as by spraying, by coating with an air knife or rod coater, by impregnation or immersion during the stage of chemical treatment, or by incorporation into plastic carriers, such as polymer foams, coatings and multi layer composites, fol lowed by insertion into selected parts of textile clothing. In rare cases, microcapsules are incorporated directly into textile fibres during the spinning process.
I n a typical example, a suspension of microcapsules has to be formulated for applications on woven or nonwoven texti les. Formulation additives usually consist of binders, organic or inorganic pigments and fi l l ers, antifoaming agents, and viscosity controll ing agents.
B inders play a crucial role in microcapsule coating formulations. They may be selected from the groups of water soluble polymers (e.g. starch and modified starches, carboxymethyl cellulose, polyvinyl alcohol, and xanthanes), synthetic latexes (e.g. styrene-
Fig. 3-(a) Microcapsules produced by complex coacervation of gelatin and carboxymethyl cel l ulose (transmission stereomicroscopy, x 1 00); (b) dispersion of cross-l inked protein microcapsules, prepared by interfacial polymerisation in water- in-oi l emulsion [transmission microscopy (x 1 00), photographic enlargement (x lO) ] ; and (c) coating of microcapsules, produced by ill situ polymerisation of melamine-formaldehyde resins (SEM, x 1 900)
74 INDIAN J. FIBRE TEXT. RES., MARCH 2006
butadiene, polyvinyl acetate or acrylate latexes with anionic andlor nonionic emulsifiers), aminoaldehyde resins (e.g. urea- and melamine-formaldehyde resins, dimethylol ethylene urea, dimethylol dihydroxy ethylene urea, and dimethylol propylene urea), and sil icones.
1 .4 Applications of Microcapsules in Textile Products
The idea of using microencapsulation technology in textile products was born soon after the first introduction of the industrial production of microcapsules for carbonless pressure-sensitive copying papers in the middle of the previous century. Possibil ities of using microencapsulation technologies in textile products are numerous, and have already been described in detai I .2
The main application fields and uses of microencapsulated additives i n textile products are summarised in Fig. 4 . The products include: dyes and pigments l - 1 4 ; i ngredients for transfer pri nting 1 5-20 ; thermochromic . I 2 1 -26 h h ' . I "7-35 I matena s ; p otoc romIc matena s· . ; cata ysts and enzymes36. 37; fire retardants38-47; sizing and bond-. 48 49 bl ' d d' Ing agents . ; OWIng an expan Ing agents; water-proofing agents50-53 ; detergent components, such as enzymes54-62; bleaching and whitening additives63-74;
8edroSl:aUc pOoling
Solvent dyeing
Dot dyeing and sped<1ed printing H Dyes ard pignents � SCreen printing
Trar'lsfer printing
Uquid c:rystals
� ThermoclYomic materials Thennochromic dyes
Re�rsib� photochromic dyes Pholochromic materials � VVrinkJe reoo�ry agents
� Surface modifying agents Catalysts ard Enzymes
antifoami ng agents 75-77 ; dry c1eani ng agents 78; soften-d . . 79-84 f d f ers an antistatIc agents ; ragrances an per urnes 85-95 . I I 96- 1 02 d d d d' . i' ; Insect repe ents ; eo orants an ISInleC-
tants 1 03- 1 1 0; active i ngredients for cosmetic textiles 1 1 1 - 1 1 5 ; adsorbents and decontaminants 1 1 6- 1 1 8 ; thermal regulation agents based on sunlight conversion"9 ; and heat storage materials based on phase h . I 1 20- 1 13 C ange matena s . -
This paper describes microencapsulation of antimicrobial essential oils for shoe insoles, animal repellents for agricultural texti les, and PCMs for active thermal control garments. In situ polymerisation of melamineformaldehyde resins was used as the microencapsulation technology in all three cases.
The idea of fragranced textiles containing microencapsulated essential oils and aromas is more than thirty years old. Aromatic capsules have been developed, which either slowly release their contents through semipermeable walls or have impermeable walls which burst open by mechanical pressure and rubbi ng when the wearer moves. A new generation of aromatic textiles entered the market when better washfast bind-
Enzymes
Bleaches
r1 Detergents � Mtifoarring agents
Perfumes
Dyes '-i Dry deaning chemcalS
H Essenl�1 oils I Fragrarces ard perfumes I Aromas and perfumes
Mosquito repellents
H Insect. repellents Tk:k repellents
18 ���ij Molhproofflg agents
Fire retardingfe:dingushing agents Fire reta-dants � -{
Anlibaderial plant oils and I Deodorants & disinfectants Sizing agents
� Antifungal plant oilS and extracts I
Adhesives and activato� Sizirg and bonding � AntimaobiaJ agenlS Cross.-Wnk.ing agents
Antioxidants
Light-weight leather substitutes MOISturizilg agents
waterproofing coatings H � l.{ � Cosmetic textiles Sub-cutaneous fat controllers
BICMling ;n:! expandirg AntisWp materials I- Anti-cellulite agents
8Ipandable sewing threads Diuretics
Bkxxj circulation stimulants 8cpansion agents
� Wate(-proofmg agents WfiA..- proofirg
I H Water purification absorbent
Fillers ard deconta'l'linants Military decontamination agents I
Textile softeners in detergents
� Antiststtc; agents Laundry � J -1
Sunlight conversion agents Thermal agents
Fragrances in texliJe soneners Phase change materials I Fig. 4-Appl ications of microcapsules in texti le products
BOH & KNEZ: MICROENCAPSULATION OF ESSENTIAL OILS & PHASE CHANGE MATERIALS 75
ers were introduced. According to patent l i terature, these fragranced textile products retain the aroma over several years, keep the microcapsules over 25 washing cycles or resist dry cleaning. Typical products include fragranced garments, bed l inen, and textiles for furniture and car interiors.
In addition to the pleasant aroma, several essential oils also possess antimicrobial , fungicidal and deodorant properties. Based on some essential oils, antibacterial printing compositions for garments and antimicrobial nonwoven textile shoe insoles with prolonged activity, based on pressure-sensitive microcapsules which release the active ingredient during walking, have also been developed.
Textiles with prolonged insect repellent effects have been developed to protect the wearer against mosquitoes and ticks, or to protect textiles by applying microencapsulated mothproofing agents. Appl ications include insect repelling carpets and curtains, panty hoses with durable insect repel lence, sprays for mothproofing treatment, and animal repellent textiles for agricultural and horticultural uses: Similarly as in the case of microencapsulated fragrances, many improvements in microencapsulated repel lents on textile supports deal with binders for improved wash fastness.
The fastest growing microencapsulation application in textiles has become sportswear and special technical apparel, based on micr.oencapsulate'd PCMs with heat and cold absorbing capacity . Several firms compete for the intellectual property rights and market shares in this field. Sportswear shops are offering coats, j ackets, boots, socks and gloves with the active thermal control technology.
PCMs are a sub-group of heat storage materials with a dynamic heat exchange process taking place at the melting point temperature. When a PCM undergoes a phase change transition from solid to liquid, energy is stored in the form of latent heat at a constant temperature. Accumulated latent thermal energy is then released when the PCM solidifies again. In general, the higher the PCM's latent heat of phase change is, the more thermal energy a material can store. The transi-' , , tion process is complet�ly reversible. To overcome' practical problems of'.. solid-liguid phase , transitions, , PCMs are microencapsulated and turned into solid formulations or suspensions for applications in various thermal management applications. In textile applications, microcapsules with PCMs are incorporated into fabrics with enhanced thermal properties, functioning as heat absorbers or as barriers against cold. In heating ,
PCMs, the temperature gradient flows from the PCM into the body, and the phase transition temperature is above the body's normal skin temperature. The cooling PCMs have a phase transition temperature below the body's normal skin temperature. When chilled below their transition temperature, the temperature gradient flows from the body into the PCM.
For garments that could better keep a constant temperature, microencapsulated PCMs were first incorporated in the fibres. The idea was further developed by several researchers, but it was difficult to incorporate enough PCM in the fibre structure to get a thermal result while retaining the fibre' s mechanical characteristics. To overcome this problem, PCM microcapsules were , incorporated into textile products in different ways, such as coated on fibres and fabrics with a binder or built into polymer foam inserts and composites with two or more layers. Several texti le applications were then developed, such as improved diving suits, fire wear, special working clothes, military uniforms, gloves and shoes, leather products and special textile products containing electric heating circuits.
2 Materi}lIs and Methods .
2.1 Materials l' Partly methylated trimetnylolrl1elamine and a hex
amethoxymethylolmelamine resin (both procured from Melamin;< Slovenia) were used as ;prepolymers for microcapsule wal�s. Styrene�maleic ' acid anhydride copolymer with' average mol. weight 350,000 (Hercules) was used as a modifying agent and emulsifier for ill situ polymerisation. Analytical grade sodium hydroxide (Kemika, Croatia) and sodium metabisulphite Na2S20S (BASF) were used for the termination of polymerisation reaction and removal of free formaldehyde from the suspension of microcapsules.
Essential oils of lavender (Lavandula hybrida), rosemary (Rosmarinus officinalis) and sage (Salvia officinalis) in mixtures with isopropylmyristate as a solvent were used as antimicrobial active agents. Con-
, centrations of 1 0%, 25% and 40% essential oil in the solvent were microencapsulated.
Daphne ' (Dragoco, Austr-ia) was used as an animal repellent, based on a mixtu� essential oils and other volatile co�pounds. Its main c'�mponents are vanillin, heliotropin (3 ,4-methylenedioxybenzaldehyde), cyclamaldehyde . 3-( 4-isopropy l-phenyl)-2-methy lpropanal] , methionyl acetaldehyde, citronellol (3 ,7-dimethyl-6-octen- l -ol) and dimethylphthalate as a solvent. Four paraffinic hydrocarbons (AGS, Turkey and Rubitherm,
76 INDIAN 1. FIBRE TEXT. RES., MARCH 2006
Germany) with melting points 25°C, 28°C, 40°C and 50°C were used as PCMs for microencapsulation.
Polyester, polypropylene and cellulose-polypropylene nonwoven textiles (30, 40, 45 and 250 g/m2) were used as textile carriers for the shoe i nsoles with antimicrobial activity. Polypropylene nonwoven textiles (20 and 30 g/m2) , and cellulose-polypropylene nonwoven textile (40g/m2) were used as carriers for Daphne animal repel lent microcapsules. The impregnated agricultural textiles were dried and cut into strips (length 3 m, width 5 cm).
Acrylic latex, styrene-butadiene latex, and watersoluble binders (polyvinyl alcohol and carboxymethyl cellu lose) were added to microcapsule suspensions prior to textile impregnation.
2.2 Methods
A d'f' d " I . . h d b 1 34 135 mo I Ie In sltu po ymensatlOn met 0 y . was used as the basic microencapsulation process for the preparation of microcapsules with melamineformaldehyde prepolymers as a wall material, and a
- -
Water + aminoaldehyde resin + anionic polyelectrolyte
modifying agent
Emulsification of a Jipophylic core material
(active substance) -
styrene-maleic acid anhydride copolymer as a modifying agent. The latter served both as an emulsifier and as a polycondensation i nitiator, which enabled the poIymerisation to develop only at the surface of the emulsified oil droplets (future microcapsule cores), and not throughout the whole water phase (Fig. 5) .
The melting points of PCMs were determined by differential scanning calorimetry (Perkin Elmer Pyris- l ) . The microcapsule size and size distribution were measured by Coulter Counter TA I l apparatus and by Alkatel Ci lias Laser Granulometer 7 1 5 . Aqueous suspensions of microencapsulated PCMs were dried by a B lichi B290 spray dryer. Scanning electron microscopy was performed by JEOL JSM-6060LV microscope at accelerating voltage 1 5 kV w;th microcapsule coating C + Au/Pd.
For the impregnation of nonwoven textiles with microcapsule formulations, a technique for the transport
. of the textile carrier through the impregnation basin was used (Fig. 6). Headspace gas chromatography was
Polycondensation (change of
temperature and pH)
Microcapsule hardening, removal of
residual monomers
Fig. 5-Microencapsulation by in situ polymerisatioll of amino-aldehyde resins l34
Plain non- Suspension of woven microcapsules in a texti le binder solution
Drying . oven
Curing oven
Non-woven textile irnpregnated with
microencapsulated scents
Fig. 6--- A process for preparing nonwoven textile carriers saturated with microencapsulated active agents l3S
BOH & KNEZ: MICROENCAPSULATION OF ESSENTIAL OILS & PHASE CHANGE MATERIALS 77
used for the determination of quantities of essential oil in new and worn textile shoe insoles. Textile insole samples containing microencapsulated essential oils were put i nto measuring flasks and exposed to elevated temperature at 1 60"C for 45 min. Gaseous phases were then analysed by gas chromatography using internal standards (flame-ionisation detector, capillary column HP Ultra 1 and HP Carbowax M20, ini tial temperature 60°C, temperature gradient 2SClmin, and final temperature 1 90"C).
Mechanical testing of shoe insoles was performed by walking (experimental person 80 kg) on average 3 km per day for a total distance of 50 km.
The antimicrobial activity of shoe insoles made of nonwoven textiles impregnated with microencapsulated antimicrobial essential oils was tested by a standard method for the determination of antibiotic activi ty. Clinical isolates of Trichophyton mentag rophytes, Candida albicalls, and Staphylococcus aureus were used as reference microorganisms. The tests were performed with 1 0% and 40% concentrations of essential oils in isopropylmiristate, and with the further dilutions to 1 :2 , 1 :4, 1 :8 , 1 : 1 6, 1 :32, 1 :64, 1 : 1 28, 1 :256, 1 :5 1 2 and 1 : 1 024.
Field testing of microencapsulated animal repellents on textile strips was carried out in the winter season on two agricultural fields near the forest. The pressure of wild animals on agricultural surfaces was strong in snow because the availabi l ity of natural food was very limited. The efficacy of the repellent agricultural textiles was studied on deer and rabbits. Textile carriers (3 m long and 5 cm wide) impregnated with microencapsulated repellen t Daphne were tested using baits with 10 one-year-old apple branches in each bait ( l m long branches, 10 cm distance between branches, I SO m distance between baits); the methodology as described earlierl 36 was used. The number of damaged branches per bait ( 1 - 1 0) was used as the criterion to evaluate damage caused by animals .
3 Results and Discussion
3.1 Microencapsulated Antimicrobial Essential Oils for Textile Shoe Insoles
Impermeable pressure-sensitive microcapsules contain ing antimicrobial mixtures of sage, lavender and rosemary essential oils were successfully produced by the process described earlierl 34. 1 35 and then applied onto nonwoven textiles by immersion impregnation. Shoe i nsoles were produced from dried impregnated textiles. An example of essential oil compo-
nents present in non-ruptured microcapsules in a new polypropylene shoe insole, detected by the headspace gas chromatography, is given in Table I .
Chromatographic analyses, performed prior to and after the mechanical testing of shoe insoles, proved that the antimicrobial essential oils were kept in the microcapsule core until its wall was broken by a mechanical pressure during walking. Tests confirmed that the microencapsulation of volatile essential oil s enabled a sustained and prolonged release of essential oils from microcapsules during wearing of shoes. The results of headspace gas chromatograpy showed that after 50 km of walking, shoe insoles still contained 62-72% of microencapsulated active ingredients. The release was more intense on insole parts exposed to higher mechanical pressure.
In in vitro antimicrobial activity tests against Staphylococcus aureus, a non-encapsulated mixture of 40% essential oil and 60% isopropyl miristate (solvent) was proven to have bactericidal activity. Bacte-
Table I -Composition analysis of new shoe insoles made of a nonwoven polypropylene texti le impregnated with microencapsulated essential oil (Lavalldlila hybrida. Rosmarinus officilla/is
and Salvia (�fficillalis) mixture in isopropylmyristate using gas chromatography
Essential oil com- Conc. of Weight of AmountJpair of ponent in micro- component, % nonwoven shoe insoles capsule g I m" (450 cm
2)
mg
a-pinene 1 . l0 1 .54 69.30
Camphene 0.85 1 . 1 9 53.55
Sabinene 0.02 0.03 1 .26
p-pinene 0.37 0.52 23.3 1
1 ,8-cineol 3.60 5.04 226.80
Limonene 0.8 1 1 . 1 3 5 1 .03
a-thujone 0.93 1 .30 58.59
p-thujone 0.24 0.34 1 5 . 1 2
Campherol 1 . 1 8 1 .65 74.34
Borneol 1 .36 1 .90 85.68
Linalool 5.00 7.00 3 15 .00
Terpi nene-4-o1 0.94 1 .32 59.22
Linalyl acetate 0.74 1 .04 46.62
Cariophylene 0.25 0.35 1 5 .75
Isopropyl my- 74.00 1 03 .60 4662.00 ristate (solvent)
Other components 8.6 1 1 2.05 542.43
Total 1 00.00 1 40.00 6300.00
78 INDIAN J . FIBRE TEXT. RES., MARCH 2006
riostatic activities of above 40:60 mixtures were also observed using di lutions from I : 1 to 1 : 1 024 (0.42 mg/ml). For mixtures of 1 0% essential oil and 90% of isopropyl miristate, the bacteriostatic activity was evident in dilutions up to 1 : 1 28 .
In vitro tests against Candida albicans showed a fungicide activi ty of 40:60 mixtures up to the dilution 1 : 1 28 (3.36 mg/ml) , while the fungicide activity of 1 0:90 mixture was proven at the minimum concentration of 0.42 mg/ml .
For the c linical isolate of dermatophyte Trichophyton mentagrophytes, a higher concentration was needed for the fungicidal effect; the 40:60 mixture was fungicidal in minimal concentration of 2 1 5 mg/ml . Isopropylmiristate, which was used as a solvent for dilution of essential oils, did not exhibit any growth inhibition effects on tested microorganisms.
3.2 Microencapsulation of Animal Repellents for Agricultural Textiles
The main purpose of microencapsulating volatile animal repel lents was to prolong their activity by slow diffusion through microcapsule wal ls . To achieve partly permeable microcapsule walls, process parameters had to be modified. An example of in situ polymerisation microencapsulation parameters for the preparation of Daphne repellent microcapsules in a 1 0 L reactor is given in Table 2 .
Agricultural field testing showed a good repel ling effect against deer and rabbits for both formulations of nonwoven textile strips i mpregnated with microencapsulated repellent Daphne. The results are given i n Table 3 and Fig. 7 .
1 0 .. _ . __ .. _.-... _ . ..••. ..•.• .• .•• _ .•..• _- ·-7--·-·- ·------- - Control ( F I ) :
� _ 20glm2 l F I ) .;;, � " ---co _ 30glm2 ( F I )
ti 7 � - - - - Control (F2) c .. 6 +------------+----.-'-.---+--� .;;, _ 20glm2 (1'2) � 5 .. __ 30glm2 (F2) � 4 ... � = � " .0 E 2 = Z 1 ""
0
o 6 10 12 14 In 1 8 20 22 Oays of uP' rim e n I
Fig. 7-Test results of nonwoven textiles i mpregnated with micro· encapsulated deer and rabbit repel lent Daphne on two agricultural fields ( F I and F2) with two textile formulations (impregnation 20g/m2 and 30g/m2), compared to control baits with no repellent
Table 2- Main parameters of in situ polymerisation microencapsulation of Daphne repellent in a 10 L reactor
Parameter Value
Melamine-formaldehyde 1 1 gl I �Og of core material prepolymer
Modifying agent/microcapsule core 6.5 g/ l OOg of core material
Diameter of dissolver plate
Mixing speed
Emulsification t ime
Share of dispersed phase in emulsion
Polymerisation time
Polymerisation temperature
80 cm
1 500 rpm
20 min
35 vol. %
90 min
75°C
Table 3-Animal repellent efficacy of nonwoven textiles impregnated with microencapsulated repel lent Daphne-Field testing against deer and rabbit
[Test location: Field 1 and Field 2; 1 0 apple branches per bait; Test sample: Nonwoven polypropylene strips (3 m long. 5 cm wide); and Impregnation with microcapsule suspension: 20g/m
2 and 30g/m
2)
Evaluation day of experiment
o 2
4
8
10
13
18
22
Control (Untreated)
o
2
3
1 0
1 0
1 0
Number of damaged branches per bait (maximum 1 0) Field 1
Impregnated textile 20g/m2 30g/m2
0 0
0 0
0 0
2 0
2 0
4 0
5 0
1 0 8
Control (Untreated)
0
0
0
3
5
5
7
8
Field 2 Impregnated textile
20g/m2
30g/m2
0 0
0 0
0 0
0 0
0 0
0 0
0 0
0
BOH & KNEZ: MICROENCAPSULATION OF ESSENTIAL OILS & PHASE CHANGE MATERIALS 79
Table 4--- Main process parameters for the preparation of PCM microcapsules in a JO L reactor
Parameter
Fil l ing
Amount of PCM in emulsion (microcapsule core)
Value
7 - 1 0 L
25 - 40%
Conc. of modify ing agent 4 - 6.5%
Conc. of prepolymer (micro- 1 8 .5 - 40 gi l 00 mL of microcap-capsule wall material) sule core material
Stirrer diameter
Mixing speed
90 mm
1 000 - 2000 rpm
Table 5- Characteristics of four bathes of m icrocapsules containing PCMs with the melting point of 25 "C, 28 "C, 40 "c
and 50 "c
Parameter Value
Melting point of PCM i n 2 5 2 8 40 50 microcapsule core, °C
Mix ing speed during emul- 1 500 1 500 1 900 1 500 sification, rpm
Av. d iameter of microcap- 9. 1 0 8.30 7 .86 8.63 sules, llm
Av. thickness of microcap- 0. 1 4 0.22 0. 1 5 0. 1 7 sule wal l, Ilm
M icrocapsule suspension 37.0 38.2 39.5 36.4 dry matter, %
M icrocapsule suspension 300 425 300 470 viscosity at 25 °C (Brookfield), mPas
Microcapsule suspension 7.0 7.6 7 .7 7 .3
pH
3.3 Microencapsulation of PCMs for Textile Applications
To remain functional over numerous phase transition cycles, microencapsulated PCMs have to remain encapsulated within the impermeable microcapsule walls for the whole product l ife. PCM microcapsules needed to be highly resistant to mechanical and thermal stress, which was achieved by the modifications of microencapsulation process .
In in situ polymerisation of amino-aldehyde resins, all materials for the microcapsule wall originate from the continuous (aqueous) phase, and therefore have to be water soluble. Under ideal conditions, by the change i n pH or temperature all mass of the wall material precipitates and distributes evenly over the surfaces of droplets in emulsion (future microcapsule cores). This results in a uniform microcapsule wall thickness, regardless of the microcapsule size.
Fig. 8-Scanning electron micrograph (magnification x 8000) of m icroencapsules (3-6 11m diameter) contain ing paraffin ic PCMs, obtained after the spray dry ing of the microcapsule suspension
Better process control and improved mechanical properties of PCM microcapsules were achieved primarily by the selection and optimisation of a combination of wall prepolymer (partly methylated trimethylolmelamine) and the modifying agent (styrenemaleic acid anhydride copolymer with the molecular weight of 350.00 g/mol) , which had a double function of being an emulsifier and a poly condensation i nitiator for melamine-formaldehyde precondensates. At optimum conditions, polymerisation evenly developed at the surface of the emulsified PCMs, thus forming an impermeable microcapsule wal l .
The process parameters for the microencapsulation of PCMs in a l OL reactor are given in Table 4, and the characteristics of microcapsules containing PCMs as the core material are l isted in Table 5. The process enabled the production of microencapsulated hydrocarbon PCMs with mechanically and thermally stable amino-aldehyde walls and narrow particle size distribution (Fig. 8). By regulating the ratio of entering raw materials, it was possible to change the properties of microcapsule walls as well as to regulate the dry matter content, pH and viscosity of the final microcapsule suspensions.
4 Conclusions Microencapsulation of antimicrobial essential oils,
animal repellents and PCMs for textile applications has been described. In all three cases, a modified in situ polymerisation method was used for the preparation of synthetic polymeric walls . However, process parameters had to be modified and adjusted to achieve different release mechanisms. I n the case of volatile
80 INDIAN J. FIBRE TEXT. RES., MARCH 2006
animal repellents, essential oi ls needed to be slowly released from the microcapsule core by diffusion/evaporation, to achieve a prolonged release; therefore the microcapsule wall had to be partially permeable. The main goal of microencapsulating antimicrobial essential oi l s for textile shoe insoles was to achieve a targeted release during walking and no release when the shoes were not worn . Microcapsule walls had to be impermeable and pressure-sensitive. In the case of microencapsulated PCMs, in addition to impermeabi lity, an improved mechanical resistance of microcapsule walls was needed to assure a sufficient mechanical strength to withstand solid-liquid trans itions of microcapsule core without leaking. Main process modifications to reach the desired microcapsule characteristics were based primari ly on the selection and ratio of melamine-aldehyde prepolymers and of a styrene-maleic acid anhydride copolymer modifying agent, as well as on the determination of emulsification and polymerisation parameters (rpm, temperature, duration) . Experiments in a t OL reactor showed that for each core material , process parameters had to be empirically optimised to ach ieve the desired characteristics.
Acknowledgement The authors are thankful to the Siovenian Ministry
of Education , Science and Sport; and Aero Chemical, Graphic and Printing I ndustries for financial support. They are also thankful to Mr Vojko Skerlavaj of Agricultural Institute of Slovenia for conducting part of agriculture testing.
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