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Ind. Eng. Chem. Res. 2008, 47, 4142–4147
Microencapsulation of Limonene for Textile Application Sofia N. Rodrigues,† Isabel Fernandes,‡ Isabel M. Martins,† Vera G. Mata,† Filomena Barreiro,‡ and Alirio E. Rodrigues*,† LSRE, Laboratory of Separation and Reaction Engineering, Department of Chemical Engineering, Faculty of Engineering, UniVersity of Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal, and LSRE, Laboratory of Separation and Reaction Engineering, Braganca Polytechnic Institute, Campus Santa Apolo´nia Ap 1134, 5301-857 Braganc¸a, Portugal
Polyurethane-urea microcapsules with limonene oil as the active agent were produced by interfacial polymerization, and their suitability for textile applications was studied. Experimental conditions for the textile substrates impregnation were based on industrial requirements and set up at laboratory scale using a minifoulard. The success of the polymerization reaction leading to the formation of the polyurethane-urea shell was checked by Fourier transform infrared spectroscopy. Particle size distributions and morphology of the microcapsules were studied using a particle size analyzer (Coulter LS230), optical microscopy, and scanning electron microscopy. The effectiveness of the textiles impregnation and the durability of the impregnation effect were evaluated by scanning electron microscopy and by headspace/GC/FID. Under the present research, a product was developed and its performance, in regard to industrial requirements, was successfully tested. 1. Introduction Chemical product engineering (CPE) is becoming a new paradigm in chemical engineering in response to the changes occurring in job structure.1 Chemical product design (CPD) is just a part of CPE and evolves from the identification of customer needs, the generation and selection of ideas, and up to product manufacture satisfying target specifications and performance.1–5 Various books have been published recently1–3 to address the need of adapting ChE curricula to the new times in line with the excellent book of Product Design and DeVelopment by Ulrich and Eppinger6 used in management schools. Microcapsules are small particles with sizes between 1 and 100 µm that contain an active agent surrounded by a natural or synthetic polymeric membrane.7 The encapsulation is used to protect fragrances or other active agents from oxidation caused by heat, light, humidity, and exposure to other substances over their lifetime. It has been also used to prevent the evaporation of volatile compounds and to control the rate of release. The encapsulated agent can be released by many actions, such as by mechanical, temperature, diffusion, pH, biodegradation, and dissolution means. Microencapsulation techniques include coacervation,8,9 atomization,10 interfacial polymerization11–13 and “in situ” polymerization.14,15 The selection of the technique and shell material depends on the final application of the product, considering physical and chemical stability, concentration, desired particle size, release mechanism, and manufacturing costs. Fragrances and perfumes, in general, possess terminal groups such as -OH, -NH, -CdO, -CHO, or -COOH. Their partial solubility in water, as described in some patents,16–19 leads to great instability in the microencapsulation by interfacial reactions. These chemical groups surround the wall of the microcapsule, modifying the hydrolytic stability of the particle and destabilizing the polymerization reaction. Moreover, several of * To whom correspondence should be addressed. E-mail: arodrig@ fe.up.pt. † University of Porto. ‡ Braganca Polytechnic Institute.
these reactive groups can react with the monomers during the interfacial polymerization, leading to microcapsules formation that might modify the properties of the fragrances and perfumes. Microencapsulation systems using interfacial polymerization, despite its versatility and strengths, need to be designed to take into account the restrictions mentioned above. The chemical system might be defined trying to avoid or minimize these undesirable secondary reactions. There are various patents where coacervation9,20–26 is used for the microencapsulation of fragrances and perfumes; however, the polymeric structures generally used in coacervation have low molecular mass, and the resulting microcapsules have weak mechanical resistance. The majority of the microencapsulated fragrance systems commercially available for textile applications are based on formaldehyde systems (phenol-formaldehyde or melamineformaldehyde resins), which are facing several restrictions under the present environmental policies. In such context, polyurethane-urea systems appear as a more attractive environmental friendly solution.27–32 Moreover, they are known as versatile polymer systems which can be tailor-made from a wide range of raw materials in order to achieve the desired physical chemical and mechanical properties. The only drawback is that polyurethane-urea systems must be designed and optimized to take into consideration the particularities of the active principle to be encapsulated. The target of our research is the production of perfumed textiles which have to meet required specifications. Various steps are involved: fabrication of microcapsules with the active agent, impregnation of the textile with microcapsules and binder, and testing of the final product. Polyurethane-urea microcapsules for textile application have been produced using the interfacial polymerization technology and limonene oil, a widely used oil in the perfumery industry, as the test active principle. Examples of applications cover shirts, underwear, homewear, suits, etc. Considering the type of textile substrate used, the main requirements for industrial application were defined as (i) resistance to dry cleaning (5 cycles) and (ii) resistance to abrasion (9000 cycles).
10.1021/ie800090c CCC: $40.75 2008 American Chemical Society Published on Web 05/24/2008
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Figure 2. Microencapsulation process of limonene by interfacial polymerization.
Figure 1. Reaction scheme for polyurethane-urea microcapsules production.
2. Materials and Methods 2.1. Materials. Hexamethylene diisocyanate (HMDI, trade name Desmodur W) from Bayer, polyethylene glycol with molecular weight of 400 (PEG 400) from Merck, ethylenediamine (EDA) from Panreac, and hydrazine (HYD) from Fluka were used to prepare the polyurethane-urea shell. Dibuthyltindilaurate (DBTDL) from Fluka was used as a catalyst, and polyvinylalcohol (PVA, trade name Celvol 840) from Celanese was used as a surfactant. Triton CA, a nonionic specialty alkoxylate (HLB 10-12), from Dow was used as a wetting agent in the final microcapsule suspension. Limonene from Aldrich was used as the core material. 2.2. Microcapsules Preparation. Polyurethane-urea microcapsules were prepared by interfacial polymerization. Figure 1 shows the reaction scheme for the polycondensation reactions. The polymer shell is formed, in a first step by the reaction of HMDI with PEG 400, followed by the reaction of HMDI with EDA and HMDI with HYD. The microencapsulation process was carried out in a 1000 mL polymerization reactor with controlled temperature and equipped with a mechanical stirrer using the general procedure described in Figure 2. Prior to microencapsulation, the oil phase (limonene comprising the isocyanate reactant) was mixed with an aqueous phase containing the PVA (aqueous phase 1). The mixed solution was then mechanically emulsified with an IKA mixer (model T25 Basic) at 11 000 rpm for 3 min at room temperature. The originated oil-in-water emulsion (o/w) was thereafter transferred into the polymerization reactor and heated to 80 °C. With stirring (100 rpm), the following aqueous phases were sequentially added after 1 h intervals: (1) an aqueous phase containing PEG 400 and catalyst DBTDL (aqueous phase 2), (2) an aqueous phase containing EDA (aqueous phase 3), and
(3) an aqueous phase containing HYD (aqueous phase 4). The total reaction time was 3 h. The resulting microcapsules were collected by centrifugation and washed first with an ethanol solution (30% v/v) and thereafter twice with distilled water to remove the remaining reactants. Finally, they were suspended in a solution of water containing the wetting agent (Triton CA). Table 1 shows the chemical system and the composition of oil and aqueous phases used to prepare the polyurethane-urea microcapsules. 2.3. Impregnation of Microcapsules on Textile Substrate. Microcapsules were applied to textiles at a laboratory scale, reproducing the industrial application conditions. The microcapsules were applied in the finishing process of textiles fabrication using a foulard, in which the textile to be treated is impregnated using a finishing bath containing (i) microcapsules, (ii) a softener (Perisoft Nano, Bayer), and (iii) a self-crosslinking agent (Baypret USV, Bayer). The conditions required for this procedure are shown in Table 2. The pick-up (expressed in %) is just the ratio: (mass of bath solution taken by the textile /mass of dry textile)×100. 2.4. Characterization Techniques. 2.4.1. Microcapsules. Fourier Transform Infrared Spectroscopy (FTIR). FTIR spectra were collected on a FTIR BOMEN (model MB 104) by preparing KBr pellets using a sample concentration of 1% (w/w). Spectra were recorded between 650 and 4000 cm-1 at a resolution of 4 cm-1 and by coadding 48 scans. Mean Particle Size and Particle Size Distribution (Laser Dispersion). The particle size distribution of produced microcapsules was analyzed by laser dispersion using a Coulter LS230 instrument. Optical Microscopy. Microcapsules in solution form were analyzed by optical microscopy using Leica DM 2000 microscopy equipped with the Leica Application Suite InteractiVe measurement software. The samples were analyzed using the transmitted light mode. 2.4.2. Textile Impregnated with Microcapsules. Scanning Electron Microscopy (SEM). The analyzed samples correspond to the impregnated textiles with microcapsules. Impregnated textile samples were coated with a thin layer of sputtered gold prior to examination, using an ion sputter JEOL JFC 1100 device. Samples were observed using a scanning electron microscope JEOL JSM-6301S at 15 kV.
4144 Ind. Eng. Chem. Res., Vol. 47, No. 12, 2008 Table 1. Chemical System and Composition of Oil and Aqueous Phases composition limonene (mL)
phase oil aqueous aqueous aqueous aqueous
water (mL)
100 1 2 3 4
HMDI (mL)
PVA (g)
PEG 400 (mL)
DBTDL (mL)
46
1.0
EDA (mL)
HYD (mL)
30 500 200 40 20
8.4 8 2.9
Table 2. Conditions of Textile Impregnation Process finishing conditions
scent (limonene microcapsules)
substrate process pressure bath volume limonene microcapsules Perisoftal Nano (softener) Baypret USV (self-cross-linking agent) drying conditions thermofixing conditions pick-up
fabric wool/polyester foulard 3 bar 1L 50 g/L 10 g/L 50 g/L 100 °C, 3 min 140 °C, 3 min 70–80%
Headspace/GC/FID of Impregnated Fabrics with Microcapsules. Gas chromatography using the GC/FID mode was carried out, using a Varian CP-3800 instrument equipped with a split/splitless injector, a CP-Wax 52 CB bonded fused silica polar column (50 m × 0.25 mm, 0.2 µm film thickness), and a Varian FID detector controlled by the Saturn 2000 WS software. The following GC oven temperature protocol was used: 50 °C for 5 min, a ramp of 2 °C/min up to 200 °C, and then held at 200 °C for 25 min. The injector was set at 240 °C, with a split ratio of 1/50 for FID. The FID detector was maintained at 250 °C. In the case of headspace analysis, the injection mode used was splitless. A 4 cm × 4 cm sample of impregnated fabric was placed in a plastic bag 12 cm × 2.3 cm; the microcapsules were broken and after equilibration overnight of the gas phase, a volume of 0.5 mL of gas phase was taken from the bag and analyzed by headspace/GC/FID for the limonene present in the sample. The carrier gas was helium He N60, at a constant flow rate of 1 mL/min. 3. Results and Discussion 3.1. Microcapsules Solution. FTIR Analysis. Figure 3 shows the FTIR spectrum of the produced polyurethane-urea
Figure 3. FTIR spectrum of polyurethane-urea microcapsules.
microcapsules. Several infrared vibrations, typical of polyurethane-urea systems, could be assigned: (1) the N-H stretching mode assigned at 3365 cm-1 and (2) the carbonyl zone assigned between 1780 and 1600 cm-1. The strong intensity of the vibration at 1643 cm-1 (urea carbonyl) compared with that of the overlapped vibration at 1732 cm-1 (urethane carbonyl) indicates that, under the synthesis conditions used, urea formation is favored over urethane formation and (3) the amide II (1556 cm-1) and amide III (1244 cm-1) vibration modes. The vibration assigned at 1106 cm-1 corresponds to the ether group C-O-C stretching mode. The low intensity associated to this vibration also confirms the low urethane character of the produced microcapsules, since it corresponds to the PEG 400 incorporation. C-H stretching vibrations are assigned at 2926 and 2856 cm-1. The absence of the peak at 2270 cm-1, due to the isocyanate stretching vibration, confirms that all the HMDI was consumed completely through the reaction with PEG 400, EDA, and HYD.Particle Size Distribution. Figure 4 shows the particle size distribution of the prepared polyurethane-urea microcapsules containing limonene oil. The microcapsules mean size (based on volume distribution) is 10 µm.Optical Microscopy. The analysis by optical microscopy had the objective to study the microcapsules morphology right after the production. Figure 5 parts a and b show the aspect of the microcapsules in the bright field option at different magnifications (20× and 100×, respectively). Microcapsules have spherical shapes, with different sizes, and one can notice also the absence of agglomerates.Solid Content and pH. The solid content was measured using a procedure adapted from the European Standard EN 827:1996 (“Adhesives: Determination of conventional solid content and constant mass solid content”). The washing phase was monitored by measuring the pH after each washing step. As the number of washing steps increases, the free reactants diminish (including the free amines) and the pH becomes lower. Our objective is to obtain a final microcapsule solution with a pH of around 7. pH and solid content results (before and after the washing phase) are shown in Table 3.
Figure 4. Particle size distribution of polyurethane-urea microcapsules with limonene oil.
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Figure 5. Optical microscopy of microcapsules solution. Magnification: (a) 20×; (b) 100×. Table 3. Values of Solid Content and pH Determined for Produced Microcapsules microcapsules solution
solid content (%, m/m)
pH
with washing without washing
41.2 41.7
7.18 8.60
3.2. Textile Impregnated with Microcapsules Characterization. SEM Analysis. Textile substrates have been analyzed by SEM after being impregnated with microcapsule solutions. SEM micrographs confirmed that the adhesion between textile fibber and microcapsules was effective, as can be observed in Figure 6. Also in this figure, surface morphologies of the microcapsules can be of two types: (i) microcapsules with a soft and smooth surface and (ii) microcapsules with rough surface. These differences on surface morphology of the microcapsules wall are favorable to the release of fragrance on the textile substrate, thus increasing the durability of fragrant textiles. It is observed that the microcapsules are individually distributed without excessive agglomeration.GC-Headspace. The amount of limonene mass presented on impregnated textiles was determined using the GC-headspace technique; the limonene peak appears at 12 min. The GC-headspace analysis allowed verifying and quantifying the limonene present inside the microcapsules, as well as the fragrance present on the impregnated textile. The effect of dry cleaning cycles on impregnated textile substrates was studied, in order to improve the lifetime of scent textiles. Dry cleaning is a process which uses perchloroethylene (perc) as solvent; according to the European Norm EN ISO 3175-2, the washing time was for 15 min and the final rinse was for 5 min.
Figure 6. Scanning electron microscopy photos of microcapsules.
In Figure 7, it is possible to observe the chromatograms that represent textiles up to five cycles of dry cleaning. In order to quantify the analysis of GC-headspace, the ratio between of the quantity of limonene present in the impregnated textiles substrates subjected to dry cleaning (x_lim) and the amount of limonene present in the impregnated textiles substrates but without dry cleaning (x_lim_0) was determined. Figure 8 shows a striking decrease between the amount of aroma of limonene present in the textiles substrates before dry cleaning (x_lim/x_lim_0)1) and the amount of aroma present in the textile after five dry cleaning cycles (x_lim/x_lim_0)0.027). For the analysis of the results of GC-headspace, a loss of 24% of limonene after the first dry cleaning cycle was found; the loss of limonene was 57% after the third cycle and 97% after the fifth cycle. The GC-headspace analysis of textile substrates impregnated with microcapsules and subjected to different abrasion cycles, namely, 3000, 6000, and 9000 abrasion cycles were performed. By GC-headspace, the quantity of limonene as a function of abrasion cycles was determined . It should be noted that the abrasion test is intended to evaluate the resistance of textile substrate when subjected to flexing, rubbing, shock, compression, stretching, and other mechanical forces. The sample is placed between two discs that are driven by a rotor along a zigzag course in a circular orbit (cycle). The test was carried out in a Martindale apparatus according to the norm of ISO 12947-2. In order to quantify the analysis of headspace, the amount of limonene in the impregnated textiles substrates subject to cycles
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Figure 7. Chromatograms obtained by GC-FID-headspace of textile substrates impregnated with microcapsules and subject to dry cleaning as follows: (1) with one dry cleaning; (2) with two dry cleanings; (3) with three dry cleanings; (4) with four dry cleanings; and (5) with five dry cleanings.
The completion of the isocyanate reaction was checked by FTIR analysis. The absence of the peak at 2270 cm-1 confirmed that HMDI was fully consumed through reaction with PEG 400, EDA, and HYD. The particle size distribution of microcapsules measured using a laser dispersion technique was of bimodal size distribution in volume, with a mean particle size of 10 µm. The observation of microcapsules by optical microscopy confirmed the spherical shape, the different sizes of the microcapsules with a bimodal distribution, and more importantly, the absence of agglomerates. SEM micrographs have shown an effective adhesion between microcapsules and textile fibers and also confirmed the spherical morphology and size. The solid content achieved through this formulation is about 41% and pH of 7.18 for microcapsules solution with washing steps. Through GC-headspace analysis, the presence of limonene on textile substrate was quantified. During dry cleaning, the loss of limonene is of 24% in the first cycle and up to 97% with five dry cleaning cycles. When impregnated textile substrates are subjected to abrasion cycles, the loss of limonene is of 40% for 3000 cycles and up to 60% for 9000 cycles. Acknowledgment This work was part of the project SCENTFASHION, Contract ADI/2004/M2.3/0015POCI funded by Agencia de Inovac¸a˜o (AdI) in the framework of POCI 2010-Medida 2.3-IDEIA. We acknowledge support of our partners CITEVE and A PENTEADORA, S.A.
Figure 8. Quantity of limonene present in the headspace according to the number of dry cleaning cycles.
Figure 9. Relative amount of limonene present in the headspace according to the number of abrasion cycles.
of abrasion (x_lim), relative to the amount of limonene present in the impregnated textiles before abrasion cycles (x_lim_0) was determined. As can be seen in Figure 9, the relative amount of limonene in the impregnated textiles substrates with 3000 cycles is x_lim/ x_lim_0)0.59 and a further decline is observed for impregnated textiles substrates with 6000 cycles of abrasion and with 9000 cycles of abrasion (x_lim/x_lim_0)0.40), corresponding to a loss of aroma of 60%. 4. Conclusions In the present study we have successfully produced polyurethane-urea microcapsules by using the interfacial polymerization technique, with limonene as the active agent for textile applications.
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ReceiVed for reView January 18, 2008 ReVised manuscript receiVed March 4, 2008 Accepted March 11, 2008 IE800090C
