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Thermal-regulation of nonwoven fabrics by microcapsules of n-eicosane coated with a polysiloxane elastomer
Materials Chemistry and Physics 226 (2019) 204–213
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Thermal-regulation of nonwoven fabrics by microcapsules of n-eicosane coated with a polysiloxane elastomer
T
Agnieszka Karaszewskaa,∗, Irena Kamińskaa, Alicja Nejmana, Bogumił Gajdzickia, Witold Fortuniakb, Julian Chojnowskib, Stanislaw Slomkowskib, Przemyslaw Sowinskib a b
Textile Research Institute, Brzezinska 5/15, 92-103, Lodz, Poland Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363, Lodz, Poland
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
of n-eicosane in • Microencapsulation cross-linked polysiloxsane was developed.
polysiloksanes have un• Cross-linked ique properties beneficial for textile materials.
maintain their thermal • Microcapsules properties during many heatingcooling cycles.
technique enable incorpora• Padding tion of the microcapsules in nonwoven.
nonwovens have • The regulating properties.
good thermo-
A R T I C LE I N FO
A B S T R A C T
Keywords: Phase change materials Chemical synthesis Encapsulation Padding nonwoven Thermal properties
Synthesis of microcapsules composed of a paraffin core coated with polysiloxane, which were developed in one of our laboratories, was adapted for preparation of the microcapsules for thermoregulation of textiles. n-Eicosane with melting temperature 37 °C was used as a phase change material (PCM). Shells of the microcapsules were made of a polysiloxane elastomer. The microcapsules were fabricated in an aqueous emulsion, which was prepared by mechanical co-emulsification of the paraffin with a reactive polysiloxane. The process yielded microcapsules consisting of n-eicosane coated with polysiloxane, which later was cross-linked in the emulsion. Chemical structure of the microcapsules was characterized by FT-IR, 29Si MAS NMR and EDX methods. SEM and TEM techniques were used for studies of microcapsule diameters, diameter distributions and thickness of the polysiloxane shells. Core-shell structure of the microcapsules and complete coating of the paraffin with polysiloxane were confirmed. Phase change enthalpy of the microcapsules was 146 J/g. Encapsulation coefficient (Xen) and energy storage coefficient (Xes) were close, which indicated that the absorbed and released heat was due to the melting and crystallization of n–eicosane, respectively. The PCM microcapsules were incorporated into the needled fabrics using a padding method. SEM, FT-IR and EDX determined their presence and location in the fabrics. It was found that content of the PCM microcapsules in the dry mass of the nonwovens was 37 wt percent. The high value of the coefficient energy storage capability (Ces) for the microcapsules alone, for the microcapsules with binder and for the modified nonwoven were close to 99% and indicated a tight encapsulation of the paraffin by polysiloxane shells. This ensured heat absorption and release during in many cycles. Indeed, the DSC results (over 100 cycles of phase changes) confirm high thermal stability of the microcapsules also in the
∗
Corresponding author. E-mail address: [email protected] (A. Karaszewska).
https://doi.org/10.1016/j.matchemphys.2019.01.029 Received 29 May 2018; Received in revised form 7 December 2018; Accepted 14 January 2019 Available online 17 January 2019 0254-0584/ © 2019 Published by Elsevier B.V.
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modified nonwoven. The heat effect corresponding to the phase change was equal 34 J per gram of the modified textile.
1. Introduction
2. Materials and methods
The phase change materials (PCMs) are widely used in many areas including solar energy harvesting and heat storage systems, waste heat recovery, building industry, premises thermoregulation, textile industry, food and medicaments transport, hot–cold therapies and many others [1]. Due to the high density of latent heat and nearly constant phase transition temperature, PCMs have been incorporated into textiles, to improve the thermal comfort. PCMs are used in clothing (sports, recreation and everyday use protective), lining and padding in shoes, bedding, gloves, pillows in the car seats and wheelchairs, building materials, military and home furnishings (curtains, floor and wall coverings, mattresses, foams, pillows) [2–4]. Clothing manufacturers are now seeking ways to control actively the clothing microclimate through heat emission and heat absorption by the garment. The most commonly paraffin used to provide thermal comfort of textiles are those whose phase transition occurs with melting point of 18–36 °C, such as nheptadecane, n-octadecane, n-nonadecane and n-eicosane [1,2]. Therefore, application of PCMs in the textile industry grows continuously, particularly in the developed countries. PCMs are incorporated into textiles in the form of microcapsules (MPCMs) [2] or without encapsulation [5–7]. Publications show that MPCMs are most frequently used [2,4,8–26]. The main advantage of MPCMs over naked phase change materials are as follows: larger surface of heat transfer; protection of PCMs from often unwanted interactions with environment; localization of phase change processes [2]. The MPCMs could be incorporated into the fibers surface by such known methods used in the textile industry as coating [4,8–11,13,14,18–23], padding [12,15,23–26], laminating [4] printing [21,23] or by addition to the dye bath [27]. Microcapsules PCMs can be also incorporated into the fibers during fiber spinning [28,29]. In publications, there are described many different materials for PCM encapsulation. Very often they are selected from homo- and copolymers as well as polymer containing composites. Melamine-formaldehyde [30–32] and urea-formaldehyde copolymers are most often used [33,34]. Their disadvantage is that their production utilizes harmful formaldehyde. Other polymers, such as cross-linked polystyrene [35–37] and polymethylmetacrylate [38], urea copolymers [39,40], polysilicates [41], polyurethanes [42] often fulfill needed requirements. Polysiloxanes containing properly chosen reactive groups are especially good candidate for paraffin containing MPCMs for application in textile industry. Polysiloxanes are chemically and thermally stable, biologically inert and friendly to environment, they are resistant to oxygen and UV light and provide a good barrier against water and water vapour. It should be noted that mechanical strength and elasticity of polysiloxanes could be controlled in a broad range by cross-linking. Microencapsulation of a paraffin with polysiloxane was tinvestigated earlier in one of our laboratories [43,44]. Microcapsules composed of n-eicosane embedded in polysiloxane were synthesized by the one pot aqueous emulsion process. During the above-mentioned process the molten paraffin was embedded in premodified polyhydromethylsiloxane, which in the same emulsion was cross-linked by hydrosilylation of divinyltetramethyldisiloxane. In this paper we present results of the studies of modification of the nonwoven polyester (needled) using the newly developed microcapsules containing n-eicosane covered with polysiloxane shells. The main objective of the studies was to evaluate the thermal properties of received microcapsules, microcapsules with binders and nonwoven fabric modified with microcapsules.
2.1. Materials 2.1.1. Microcapsules Polyhydromethylsiloxane (PHMS) with trimethylsiloxane groups on the end chains (viscosity 15–20 cSt, corresponding to Mn = 1.9·103 g/ mol) and 1,3-divinyltetramethyldisiloxane (DVTMDS) (purity 97%) were purchased from ABCR Gmbh.; polyvinylalcohol (PVA, Mn = 7.2·104 g/mol and isopropanol (purity 99.5%) were from POCH; tetrahydrofurane p.a. and 1,3-dioxane (purity 99.8%) were purchased from CHEMPUR; n-eicosane (purity 99.0%) was from Alfa Aesar Gmbbh. All chemicals were used without additional purification. Catalyst, platinium complex 20 wt % Pt(0) was received from Momentive Performance Materials Leverkusen. 2.1.2. Textile carrier and polymer binders Needle-punched nonwoven fabrics made of 100% polyester filaments were provided by a regional manufacturer. Parameters characterizing this nonwoven material were as follows: mass per unit area 155 ± 7.5 g/m2, thickness 4.6 ± 0.2 mm, bending stiffness for course along 6.0 ± 0.3 mNcm and air permeability for pressure 100 Pa 3900 ± 170 mm/s. For modification of nonwoven fabric the following agents were also used:
• binding agent - Helizarin Binder TX4738 (BASF, Germany) (HB), • cross-linker - Helizarin Fixing Agent TX4737 (BASF, Germany) (HA).
2.2. Methods 2.2.1. Synthesis of microencapsulated n-eicosane with polysiloxane shell The solution of 100 g of PHMS with of 18.5 g of DVTMDS in 218 mL 1,4-dioxane was heated at 45 °C and 0.111 g Karstedt catalyst in 0.75 g 1,4-dioxane was introduced. Evolution of gas was observed. The solution was stirred for 20 min at 45 °C. During this time viscosity of the mixture increased. Thereafter the solution of 240 g n-eicosane in 218 mL of isopropanol was introduced to the reactor. Obtained solution was stirred 5 min at 45 °C and then quickly introduced to 5000 mL of water containing 1.87 g of dissolved PVA. The mixture was homogenized for 25 s using a high-speed MPW-120 homogenizer set to 4.000 rpm. Obtained emulsion was added to the 20 L reactor equipped with an anchor stirrer. The reactor was filled with 11.5 L of water containing 43.13 g of PVA. During reaction the mixture was stirred at 45 °C during 70 h. Thereafter, the microcapsules were separated by sedimentation. The microcapsules were washed with water to remove remaining surfactant. The microcapsules of spherical shape (or distorted spheres) were obtained. They could be well dispersed in water by simple agitation and the dispersion was stable for about 5 h. The suspension of 46 wt % of microcapsules in water was used for the nonwoven fabric treatment. 2.2.2. Binding PCM microcapsules with textile 2.2.2.1. Mixture of MPCM and binder. To compare and examine the influence of binding and cross-linking agents (HB – Helizarin Binder TX4738 and HA – Helizarin Fixing Agent TX4737) on MPCM thermal properties and to determine the content of MPCM in the microcapsules/ binder/cross-linker mixture was prepared. The mixture contained 125% (wt) of microcapsules with respect to the binder. 2.2.2.2. Modification of nonwovens. Modification was performed using 205
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evaluated using a differential scanning calorimeter DSC 204 F1 Phoenix (Netzsch, Germany). Samples placed in aluminum pans with a pierced lid were scanned in the temperature range 0–60 °C with the scanning rate 5 °C/min. During measurements samples were kept in the nitrogen atmosphere. Three samples of each studied material (5–15 mg) were tested. The onset (TOnset) and end (TEnd) temperatures of melting and crystallization process, melting (Tm) and crystallization (Tc) temperatures in peak maxima and enthalpies of melting (ΔHm) and crystallization (ΔHc) were determined for microcapsules, microcapsules with binding agents and microcapsules modified nonwoven fabric. The coefficients of encapsulation (Χen) and the coefficient of energy storage (Xes) were estimated from the phase transitions enthalpies. The values of Χen (%) and Xes (%) were calculated from Eqs. (1) and (2) [45]:
a two-roller padding machine from BENZ- Switzerland. Conditions of padding: down force roller 35 kG/cm, padding rate 2 m/min, wring degree of 200%. After padding the nonwoven fabric was dried for 3 min in air at 80 °C, and then reheated for 10 min in air at 100 °C. The last step was responsible for the binder cross-linking. The modifying bath contained the following agents: Microcapsules PCM (aqueous suspension) - 50 g/100 mL, HB - 20 g/ 100 mL, distilled water - 28 g/100 mL, HA – 2 g/100 mL. Optimal composition of modifying bath was determined in our earlier studies [23,24]. 2.3. Characterization 2.3.1. Microscopic analysis SEM/EDX Microscopic analysis SEM/EDS was carried out using a scanning electron microscope VEGA 3 (Tescan, Czech Republic) equipped with e X-ray microanalyzer EDS INCA Energy (Oxford Instrument Analytical, UK). Diameters of the MPCM were determined by a digital image analysis of the SEM microphotographs. EDS surface analysis was performed under 10 Pa pressure, using electron beam energy of 20 keV, without sputtering a conductive substance on the sample. Studies were carried out by a point method in microareas with diameter of 0.70 μm.
X en (%) =
ΔHm,K x100%, where K= A, A1, A2. ΔHm, n − eicosane
(1)
X es (%) =
ΔHm,K + ΔHc,K x100%, K= A, A1, A2 ΔHm, n − eicosane + ΔHc, n − eicosane
(2)
where: ΔHm,A, ΔHm,A1, ΔHm,A2 denote the melting and ΔHc,A, ΔHc,A1, ΔHc,A2 crystallization enthalpies of microcapsules (A), mixture microcapsules with binding agent (A1) and microcapsules in modified nonwoven fabric (A2), respectively. ΔHm,n-eicosane and ΔHc,n-eicosane denote the melting and crystallization enthalpy of n-eicosane. The coefficient of energy storage capability (Ces) of microcapsules A, mixture A1 and microcapsules on modified nonwoven fabric A2 was calculated using Eq. (3) [45–47]:
2.3.2. Microscopic analysis TEM Microstructure of MPCM was determined using Tesla BS-500 (Tesla, Brno Czech Republic) transmission electron microscope (TEM) operating at 90 kV. Samples were prepared by placing a drop of an aqueous suspension of the microcapsules on the carbon coated copper grid, and then the solvent was evaporated at room temperature.
Ces (%) =
2.3.3. Fourier transform infrared (FTIR-ATR) spectroscopy FTIR spectroscopy was used for characterization of chemical compositions of the n-eicosane, polysiloxane, polysiloxane microcapsules containing n-eicosane, nonwoven and modified nonwoven fabric, using a Vertex 70 spectrophotometer (Bruker, Germany) in the range 4000–600 cm−1 with a resolution of 8 cm−1.
X es x100% X en
(3)
The supercooling degree (ΔTs) is an important parameter reflecting the crystallization-melting hysteresis of a PCM, and is calculated using Eq. (4) [46,47]:
ΔTs = Tm − Tc
(4)
where: Tm and Tc are the melting and crystallization peak temperatures, respectively. Based on the melting enthalpies of the microcapsules (A) in powder, microcapsules with binding agents (A1) and microcapsules on modified nonwoven fabric (A2). The microcapsules A content in the mixture A1 and in the modified nonwoven (DA), was calculated using Eq. (5):
2.3.4. 29Si CP NMR spectroscopy analysis Solid state 29Si CP NMR spectra were registered with a DSX 400 Bruker spectrometer. The spectra were acquired with cross-polarization, at 59.627 MHz, applying 90-μs pulses, 6-s pulse delay, and 3-ms contact time, with samples placed into a 4.0-mm zirconia rotors, with spinning at 8 kHz. The peak positions were referenced to the signal of Q8M8.
DA (wt%) =
ΔHm,L Dm,L , where L= A1, A2 ΔHm,A
(5)
where: Dm,A1 and Dm,A2 denote the content of dry substance (wt.% of paste with microcapsules and of modified nonwoven fabric for L = A1, A2), DA is the microcapsules A content in dry substance (wt.%), ΔHm,A, ΔHm,A1 and ΔHm,A2 defined in Eq. (2), respectively.
2.3.5. Selected properties of nonwovens The values of mass per unit area of the fabrics were determined according to the PN-EN 29073-1: 1994 Textiles – Test methods for nonwovens – Determination of mass per unit area. The thickness of the studied nonwovens was determined according to the PN-EN ISO 9073–2:2002 Textiles - Test methods for nonwovens Part 2: Determination of thickness. The bending stiffness of nonwoven fabrics was measured according to the PN-ISO 9073-7: 2011 Textiles - Test methods for nonwovens- Part 7: Determination of bending length. The air permeability of nonwovens fabrics was measured according to the PN-EN ISO 9237:1998 Textiles - Determination of permeability of fabrics to air. Air permeability – the velocity of air passing perpendicularly through a given area of fabric measured at a given pressure difference across the fabric test area (100 Pa, 200 Pa) over a given period of time.
2.3.6.2. Temperature regulating factor (TRF). To assess the transient thermal properties of the nonwoven, the temperature regulating factor (TRF) was determined in accordance with the ASTM standard test method [48]. The measurements were carried out using a test stand constructed in the Textile Research Institute. The TRF was defined by Hittle and Andrè [49] as the quotient of the amplitude of the temperature variation and the amplitude of the heat flux variation divided by the value of the steady state thermal resistance of the fabric (see Eq. (6)):
TRF =
Tmax − Tmin 1 × qmax − qmin R
where:
2.3.6. Thermal characteristics 2.3.6.1. Differential scanning calorimetry (DSC). Thermoregulating properties of the microcapsules, microcapsules with binding agents, and of the nonwoven fabric modified with microcapsules were
Tmax, Tmin –maximum and minimum temperature, qmax, qmin - maximum and minimum flux 206
(6)
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R – thermal resistance
3.1.3. FTIR-ATR and 29Si CP NMR spectroscopy analysis of microstructure FTIR-ATR spectra of microcapsules displayed in Fig. 7c give important information on the capsule structure. Intensive bands at 10001100 cm−1 are due to the vibration of siloxane groups. Bands at 28003000 cm−1 come from C-H stretching vibration in n-eicosane and in polysiloxane. Broad band at about 3300 cm−1 is due to stretching vibration of O-H in polysiloxane, while the band present at about 900 cm−1 is attributed to the stretching vibration of Si-OH groups. The stretching vibration bond appears at 2100 cm−1, Si-CH3 group vibrations contribute to absorptions at 1200 and 1350 cm−1. Other intensive bonds at 750-850 cm−1 are assigned to Si-CH3 rocking vibrations and to vibrations of CH2 groups in paraffin. 29 Si CP MAS NMR spectrum of microcapsules (Fig. 8) provides information about chemical structure of the polysiloxane shell. Signals of the Si-OH and Si-H reactive groups appear at −59 and −37 ppm, respectively. Signals at −22 and + 8 ppm are due to Si-CH2 groups of cross-links formed by hydrosilylation while that at −67 ppm comes from Si-O-Si cross-links formed by the Si-H + HO-Si dehydrocondensation.
The TRF is a dimensionless quantity varying in a range (0, 1). The TRF shows how well a fabric containing microcapsules PCMs moderates the hot plate temperature. A TRF value of 1 means the fabric has no heat capacitance and poor temperature regulation. A TRF equal to zero means that fabric has an infinite heat capacitance and that a body in contact with it will remain at a constant temperature. While determining the TRF value, the following measurement parameters were taken: a medium heat flux of 70 W/m2 and an amplitude of the heat flux change of 30 W/m2. The temperature of the cold plate was selected in such a range that the temperature changes of the hot plate might fluctuate within the phase change temperature range of the microcapsules PCMs applied. The TRF was determined for 6 selected frequencies of heat flux changes (t): 240, 480, 600, 720, 900 and 1200 s. The results of TRF measurements are shown in diagrams presenting the relationship TRF = f (t). 3. Results and discussion 3.1. Synthesis and characterization of PCM microcapsules
3.1.4. Energy dispersive X-ray spectrometry (EDX) studies of the MPCM microcapsules with cross-linked polysiloxane shells EDX spectrometry is a technique providing information on element content in the probed interfacial layer of investigated sample. However, one has to keep in mind that the depth of probing depends on energy of the beam and on the density of the probed interfacial layer. Very often it is in the range from a few tenths to several hundreds of nanometers, however, for samples with low density it may be even larger. Electron microscopy studies (Subsection 3.1.2) revealed that thickness of the cross-linked polysiloxane shells ranges from 0.26 to 0.48 μm. Thus, one should take into consideration that the depth of probing by EDX may exceed thickness of the shell. In pure polyhydromethylsiloxane the proportion of silicon, carbon and oxygen is 1:1:1 (without taking into account contribution of the end-groups). Cross-linking could change this proportion. In extreme example, when in reaction of polyhydromethylsiloxane with 1,3-divinyltetramethyldisiloxane all –Si(H) (CH3)O– groups of the polymer would be converted to the cross-link points, the proportion of silicon, carbon and oxygen should be 1:2.5:0.75. Results of EDX analysis of randomly selected five areas of microcapsules (Fig. 9) gave the percentage of silicon, carbon and oxygen elements equal 17.4%, 59.4% and 23.3%, respectively, what corresponds to proportion 1:3.4:1.4 of these elements. The content of carbon, which was higher than in the case of even fully cross-linked polyhydromethylsiloxane indicates that also n-eicosane contributes to the EDX spectrum. It should be noted that our earlier, independent studies revealed not only that n-eicosane is encapsulated in cross-linked polysiloxane but that it does not leak from the microcapsules during many
3.1.1. Preparation of microcapsules The first step of the microcapsule synthesis proceeds in solution. During this process part of the cross-linker, 1,3-divinyltetramethyldisiloxane (ca 60%) is grafted on the PHMS polymer by hydrosilylation (Fig. 1). The presented above preliminary modification facilitates crosslinking of the polymer because after this step both Si-H and Si-vinyl groups are localized on polymer chains, which are going to form a network. The reaction mixture produced in the first step is used in the main process of synthesis, which proceeds in emulsion. The emulsion is easily formed mechanically from the solution in a water miscible solvent containing premodified polymer/unreacted cross-linker/catalyst/ paraffin and water containing with added surfactant (PVA). The emulsion contains droplets of polysiloxane with n-eicosane and the catalyst. Two important reactions occur in the emulsion: hydrosilylation forming bridges between macromolecules (Fig. 2) and hydrolysis of Si-H groups producing a large numbers of Si-OH groups (Fig. 3). Due to formation of silanol groups the polysiloxane becomes it more hydrophilic than the paraffin. Therefore, the polymer does not mix with hydrophobic n-eicosane forming the particles shells. The cross-linking occurs also by heterocondensation shown in Fig. 4. The diameters and shapes of synthesized microparticles depend significantly on time and intensity of the stirring during emulsification [43]. Usually, the microcapsules have shape very close to spheres (some distorted). However, the majority of ≡Si-OH groups formed by hydrolysis of ≡Si-H remains, what makes surface of microcapsules hydrophilic and well dispersible in water. 3.1.2. Diameters of microcapsules and thickness of their shells Morphology of the fabricated microcapsules was investigated by analysis of their SEM and TEM micrographs. Examples are shown in Fig. 5a–b. TEM images displayed in Fig. 5b show a sharp contrast between paraffin and polysiloxane shells of microcapsules, which confirms their core-shell structure and the complete covering of n-eicosane by the cross-linked polysiloxane layer. The SEM studies allowed to determination of the average diameter of microcapsules, which was 10.0 μm with the standard deviation of 0.8 μm (Fig. 6a). The thickness shell of the majority of microcapsules was in the range of 0.26–0.46 μm as shown in Fig. 6b. The mean shell thickness of the population studied was 0.38 μm. The average shell to radius of microcapsules was 0.03.
Fig. 1. Scheme of grafting of vinyl group containing moieties onto PHMS. 207
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Fig. 2. Scheme of cross-linking PHMS by hydrosilylation.
Fig. 3. Scheme hydrolysis of SiH groups.
Fig. 4. Scheme of heterocondensation involving ≡SiH and ≡SiOH groups.
Fig. 6. SEM microphotograph and distribution histograme: (a) diameter of microcapsules, (b) shell thickness.
Fig. 7. The FT-IR spectra of: (a) n-eicosane, (b) polysiloxane and (c) microcapsules containing n-eicosane.
Fig. 5. Images (a) SEM and (b) TEM of microcapsules and cross-section revealing the microcapsule core–shell structure.
phase transfer cycles.
3.2. Incorporation of the microcapsules PCM in nonwoven structure 3.2.1. Morphology of the dried padding mixture, pristine and modified nonwoven The SEM images of the surface of dried padding mixture containing binder and the microcapsules and of the unmodified and modified nonwoven are shown in Fig. 10. The binder-microcapsule suspension used for modification the nonwoven contained 38% of dry mass (determined gravimetrically). After drying it was milky, but macroscopically homogeneous. Any traces of the released paraffin were visible on the surface. SEM microphotographs displays presence of some round objects on the surface.
Fig. 8. 29Si CP MAS NMR spectrum of microcapsules containing n-eicosane coated with polysiloxane shell.
Many of them are open forming cavities with the microcapsules sticking to their walls (Fig. 10 b and c). It is clear that the microcapsules interact with the binder. The protruding round bubbles, some with small 208
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3.2.2. Studies of the not modified and modified nonwoven by energy dispersive X-ray (EDX) spectrometry SEM microscope with EDX spectrometer could be used not only for obtaining standard SEM microphotograms but also for detection of various elements in the sample and for visualization of element distribution in the samples’ interfacial layer. For microcapsules containing silicon in the shells distribution of silicon was used as microsphere indicator. Fig. 11 shows the standard SEM microphotograph and the microphotograph, showing map of silicon distribution in the same sample area. These microphotographs prove that silicon atoms are present exclusively where the microcapsules. This observation supports conclusion that if any microcapsules are broken into small pieces not seen in the SEM microphotogram, their amount is very low. Fig. 12 displays EDX spectra for five different microareas of the pristine and modified nonwovens. As expected, according to EDX spectrum (Fig. 12a) the elemental composition of the polyester nonwoven unveiled exclusively the presence of carbon and oxygen atoms. The EDX spectrum (Fig. 12b) confirmed the presence of Si element on the surface of the test nonwoven level of about 4%.
Fig. 9. EDX spectra and elemental compositions of the microencapsulated neicosane with a polysiloxane shell.
3.2.3. FTIR-ATR analysis of the modified nonwoven Fig. 13 shows the FTIR-ATR spectra of the pristine (Fig. 13a) and modified polyester nonwoven (Fig. 13b). Signals were assigned according to the literature [14,49]. The FTIR-ATR spectrum of the pristine nonwoven is typical for aliphatic-aromatic polyester copolymer and contains the following signals: 721 cm−1 (bending CH2); 1014 cm−1, 1091 cm−1 and 1241 cm−1 (stretching C-O-C);1712 cm−1 (stretching C=O); 2966-2854 cm−1 (C-H stretching in aliphatic groups); 15041450 cm−1 (aromatic ring C=C-C stretching); 970-794 cm−1 (aromatic C-H bending); 3298 cm−1 and 3425 cm−1 (weak bands: O-H stretching in carboxylic acid and hydroxyl groups). After modification, the nonwoven showed specific bands for both components, i.e. for polyester and for PCM microcapsules (see also Fig. 7c). The FTIR-ATR spectrum of the modified nonwoven shows peak at 1018 cm−1. This signal is typical for bending vibration of Si-O-Si. The peaks at 1249 end 794 cm−1 are due to the bending vibration of Si-CH3 and the peak at 2163 cm−1 is to the bending vibration of Si-H [50]. The peak 906 cm−1 corresponds to the stretching vibration of Si-OH. Bands at 2800-3000 cm-1 come from C-H stretching vibration in polysiloxane and in n-eicosane. 3.2.4. Selected properties of nonwoven According to SEM microphotogram shown in Fig. 5d, the structure of nonwoven is very loose. Thus, the padding mixture could easily penetrate the fabric, reach spaces between fibers and enable their efficient modification. The modification resulted in higher surface mass of the fabric (SM), which increased from 155 g/m2 (SMN) to 267 g/m2 (SMMN) reflecting the high degree of padding (Dm = [(SMMN – SMN)/ SMN]·100%). Taking into account the surface mass of nonwoven before and after modification, the degree of padding of obtained fabric was equal 72%. Because the degree of the dry mass of the microcapsules in the dry mass of the padding mixture (DM-PCM) was 37% the mass content of the microcapsules in the modified fabric was 26.2%. The binder bonded the loose fibers in the nonwoven fabric, making it more compact. In result, thickness of the nonwoven slightly decreased, from 4.60 mm to 4.39 mm. Moreover, stiffness of the fabric significantly increased, from 5.99 mNcm to 35.22 mNcm. More compact structure of the fabric caused reduction of air permeability by about 15%; from 3890 mm/s to 3290 mm/s).
Fig. 10. Macroscopic (a) and SEM microphotographs (b)–(e): (a) macroscopic photo of the dry microcapsules-binder mixture, (b) and (c) – surface of dry binding agent containing microcapsules, (d) – nonwoven before modification, (e) – modified nonwoven coated with binder and containing attached microcapsules.
openings contain the microcapsules almost completely or completely covered with the binder. This means that the microcapsules are not perfectly dispersed, but form clusters. SEM microphotographs of not modified nonwovens (Fig. 10,d) indicate that before modification the surface of fibers was smooth. After modification the clusters of microcapsules are attached to the fibers. Isolated microcapsules are rarely seen. It should be noted that SEM pictures of modified nonwovens indicate that at both sides they are modified similarly.
3.3. Thermal properties of modified nonwovens 3.3.1. Phase change properties Phase change properties of microcapsules containing n-eicosane, dry microcapsules-binder mixture and modified nonwovens were 209
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Fig. 11. Standard SEM microphotogram (a) and (b) map of Si distribution on the surface of modified nonwoven.
Fig. 13. FT-IR spectra of: (a) pristine polyester nonwoven, (b) modified polyester nonwoven fabric.
investigated monitoring many cycles of the heating and cooling covering temperature range of melting and crystallization of n-eicosane. Fig. 14 shows DSC traces for n-eicosane, microcapsules containing neicosane, mixture of microcapsules containing n-eicosane (A), with the binder (A1) (dry mass content of microcapsules 37%) and nonwoven modified by attachment of n-eicosane containing microcapsules (A2). Whereas the thermogram for n-eicosane is a typical one the DSC traces for the microcapsules, microcapsules with binder and modified nonwovens are complicated. Crystallization traces, for these materials are evidently bimodal. Plausible explanation of this observation may be related to different quality (defects) of n-eicosane crystals in the center of the microcapsules and next to the polysiloxane shells. Parameters evaluated from DSC traces are given in Table 1. The temperature corresponding to maximum of the heat flow during melting (heating of the sample) is highest for the microcapsules A. The temperature at peak during crystallization (heat flow during cooling) is the lowest one for the microcapsules A and the highest for nonwoven fabric A2. Thus, the supercooling degree (ΔTs) is the largest one for pure microcapsules (A) and the smallest for the modified fabrics. Evidently, the binder and nonwovens delay transfer of heat to n-eicosane.
Fig. 12. EDX spectrum and elemental compositions: (a) of the nonwoven, (b) modified nonwoven. Content of each element is given in atom percent.
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Table 2 Encapsulation coefficient of n-eicosane into PCM microcapsules – data from literature and from this work. Shell
n-eicosan/symbol in the reference (weight ratio wt./wt.)
TiO2 [46]
n-eicosane/titanium butoxide 60/40 50/50 40/60
77.97 65.59 73.73
n-eicosane/Zn(CH3COO)2∙ 2H2O
ZnO [47]
Fe3O4/SiO2 [51]
60/40 50/50
70.00 50.00
40/60
35.00
n-eicosane/tetraethoxy silane 70/30 50/50 40/60
P(MMA-co-AA) [52]
71.78 64.95 52.97
n-eicosane/poly(methyl methacrylate-coacrylic acid) 55/45
PMMA [53]
32,90
n-eicosane/poly(methyl methacrylate) 50/50
PMMA [54]
polysiloxane
Fig. 14. DSC thermograms of: (a) n-eicosane, (b) A – microcapsules, A1 – dry microcapsules-binding agent mixture, A2 – nonwoven fabric containing microcapsules.
Xen, %
35,00
n-eicosane/poly(methyl methacrylate) 50/50
62.00
n-eicosane/polysiloxane
This study
67/33
81.4
modified nonwoven it is still high (19.2%). It is worth to mention also that for each investigated material, the energy storage (Xes) is very close to the coefficient of encapsulation. In result, values of the coefficient of energy storage (Ces) exceed 98.9%, i.e. are close to the perfect energy storage. Suitability of any phase change material formulation for application as a temperature-controlling additive strongly depends on coefficient of encapsulation of the PCM active substance (Xen). Table 2 provides some data on coefficient of encapsulation of n-eicosane in various formulations (inorganic and organic) described in literature and in our work. It is evident that the PCM microcapsules developed in this work have higher encapsulation coefficient for n-eicosane than the described in the literature microcapsules with inorganic and organic shells. Stability of controlling thermal properties of nonwovens with bound PCM microcapsules containing n-eicosane is maintenance of high reversibility of the system exposed to the heating and cooling cycles. Fig. 15 shows plots of the enthalpies and temperatures of melting and crystallization for nonwovens containing n-eicosane loaded polysiloxane microcapsules (ΔHm, ΔHc, Tm and Tc, respectively) as function
Such behaviour may be due also to amount of microcapsules in each specimens. The smallest amount of microcapsules (A) was in the modified nonwoven fabric and the range of phase transitions temperatures is the narrowest. The onset temperature for nonwoven fabric A2 is lower than for microcapsules A and dry microcapsules-binding agent mixture A1 what may result from amount of microcapsules in the fabric and better heat availability and its propagation in the sample than for A and A1 where the amount of microcapsules is higher and the possibility of reaching the heat inside the sample is slower. The modified nonwoven fabric A2 has the lowest values of melting and crystallization enthalpies than samples A and A1. This is due to the lowest amount of microcapsules on fabric A2. It is obvious that the coefficient of encapsulation of n-eicosane (neicosane content – Xen) is the highest one for the microcapsules and the lowest for the modified fabrics. However, it is worth noting that for
Table 1 Parameters characterizing thermal properties of investigated materials. Sample
n-eicosane A A1 A2
Melting process
Crystallization process
TOnset (°C)
TEnd (°C)
Tm (°C)
ΔHm (J/g)
TOnset (°C)
TEnd (°C)
Tc (°C)
ΔHc (J/g)
32.7 35.3 34.9 32.9
39.5 48.2 42.2 39.6
37.0 41.8 38.4 37.3
180.3 146.8 49.1 34.6
32.6 34.6 34.7 34.1
24.0 17.4 23.0 23.0
31.1 22.8 30.0 31.4
181.4 146.9 48.7 34.2
where: A -microcapsules, A1-microcapsules with binding agents, A2-microcapsules on modified nonwoven fabric. 211
Xen (%)
Xes (%)
Ces (%)
ΔTs (°C)
81.4 27.3 19.2
81.2 27.0 19.0
99.7 98.9 98.9
6.0 18.0 8.4 5.9
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microcapsules PCM exhibit a lower TRF value in the whole range of frequencies of heat flux changes than the reference nonwovens. This is due to the increase in nonwoven thermal capacitance resulting from the incorporation of microcapsules PCM. Similar dependence was also noticed for nonwovens modified with commercial PCM microcapsules (neicosane) [11,55]. 4. Conclusions Developed process of fabrication of PCM microcapsules consisting of n-eicosane cores coated with polysiloxane shells ensures very effective encapsulation of the above-mentioned active phase-change paraffin substance (n-eicosane), higher than for other systems described in the literature. Cross-linked polysiloksanes used for shell preparation have unique properties beneficial for textile materials: softness, inertness for many chemicals, resistance for high temperatures, elasticity at low temperatures, biocompatibility, permeability for gases and resistance for oxidation, and nonflamability. Very simple, standard methods (padding) could be used for introduction of developed n-eicosane core, polysiloxane shell microcapsules into nonwoven. Energy storage coefficient of modified nonwovens is very high and, exceeds 98.9%. Thermoregulating properties of the modified nonwoven vary randomly within a narrow range, during the heating and cooling cycles. Beside the clothing, the developed n-eicosane loaded microcapsules could be considered as good candidates also for other applications, like packaging requiring thermoregulation and construction.
Fig. 15. Stability of phase change enthalpies (a) and crystallization and melting temperatures (b) during the heating-cooling cycles.
Acknowledgements This work was supported by the Key Project POIG.01.03.01-00-004/ 08 Functional nano- and micro textile materials – NANOMITEX co-financed by the European Union with financial resources of the European Regional Development Fund and National Centre for Research and Development within the framework of the Innovative Economy Operational Programme, 2007–2013, Priority 1. Research and development of modern technologies, Activity 1.3. Supporting R&D projects for enterprises undertaken by science establishments, Subactivity 1.3.1. Development projects. Przemyslaw Sowinski made SEM and TEM studies which were financial supported from the CMMS PAS Statutory Fund.
Fig. 16. The transient thermal performance: (a) view of the instrument to determine temperature regulating factor (TRF); A sample nonwoven, B cold plate, C hot plate, D cooling water supply to cool plates, E) guide bars, F) thermostat; (b) TRF as a function of cycle time.
of the number of the heating and cooling cycles (in the temperature range from 0 to 60 °C). Plots in Fig. 15 reveal that during 101 cycles of heating and cooling the above-mentioned parameters varied in a narrow range: from 34.2 to 34.65 J/g (ΔHm), from 34.15 to 34.5 J/g (ΔHc), from 37.3 to 37.6 °C (ΔTm) and from 31.25 to 31.38 °C (ΔTc). These observations prove that n-eicosane does not leak during the heating-cooling cycles and that thermal properties of the modified nonwoven are retained.
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3.3.2. The thermo –regulating properties of nonwovens modified with PCM microcapsules Determination of the temperature regulating factor (TRF) of pristine nonwoven and nonwoven with microcapsules was done by means of an instrument with a dynamic heat source, presented in Fig. 16a. This instrument simulates the following arrangement: skin – apparel – environment. The fabric sample is sandwiched between a hot plate and two cold plates, one on either side of the hot plate. These cold plates at a constant temperature simulate the environment outside the apparel. The sinusoidally varying heat input to the hot plate simulates heat produced by human activity. With purpose to measure the steady state thermal resistance of the fabric, the controlled heat flux is maintained constant, and the test proceeds until a steady state is reached. To assess the temperature regulating ability, the heat flux is sinusoidal and the temperature regulating factor (TRF) is determined. The results of measurements are presented as TRF function of the heat flux frequency. (Fig. 16b). Analyzing these plots one could notice that nonwoven with 212
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Thermal-regulation of nonwoven fabrics by microcapsules of n-eicosane coated with a polysiloxane elastomer
T
Agnieszka Karaszewskaa,∗, Irena Kamińskaa, Alicja Nejmana, Bogumił Gajdzickia, Witold Fortuniakb, Julian Chojnowskib, Stanislaw Slomkowskib, Przemyslaw Sowinskib a b
Textile Research Institute, Brzezinska 5/15, 92-103, Lodz, Poland Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363, Lodz, Poland
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
of n-eicosane in • Microencapsulation cross-linked polysiloxsane was developed.
polysiloksanes have un• Cross-linked ique properties beneficial for textile materials.
maintain their thermal • Microcapsules properties during many heatingcooling cycles.
technique enable incorpora• Padding tion of the microcapsules in nonwoven.
nonwovens have • The regulating properties.
good thermo-
A R T I C LE I N FO
A B S T R A C T
Keywords: Phase change materials Chemical synthesis Encapsulation Padding nonwoven Thermal properties
Synthesis of microcapsules composed of a paraffin core coated with polysiloxane, which were developed in one of our laboratories, was adapted for preparation of the microcapsules for thermoregulation of textiles. n-Eicosane with melting temperature 37 °C was used as a phase change material (PCM). Shells of the microcapsules were made of a polysiloxane elastomer. The microcapsules were fabricated in an aqueous emulsion, which was prepared by mechanical co-emulsification of the paraffin with a reactive polysiloxane. The process yielded microcapsules consisting of n-eicosane coated with polysiloxane, which later was cross-linked in the emulsion. Chemical structure of the microcapsules was characterized by FT-IR, 29Si MAS NMR and EDX methods. SEM and TEM techniques were used for studies of microcapsule diameters, diameter distributions and thickness of the polysiloxane shells. Core-shell structure of the microcapsules and complete coating of the paraffin with polysiloxane were confirmed. Phase change enthalpy of the microcapsules was 146 J/g. Encapsulation coefficient (Xen) and energy storage coefficient (Xes) were close, which indicated that the absorbed and released heat was due to the melting and crystallization of n–eicosane, respectively. The PCM microcapsules were incorporated into the needled fabrics using a padding method. SEM, FT-IR and EDX determined their presence and location in the fabrics. It was found that content of the PCM microcapsules in the dry mass of the nonwovens was 37 wt percent. The high value of the coefficient energy storage capability (Ces) for the microcapsules alone, for the microcapsules with binder and for the modified nonwoven were close to 99% and indicated a tight encapsulation of the paraffin by polysiloxane shells. This ensured heat absorption and release during in many cycles. Indeed, the DSC results (over 100 cycles of phase changes) confirm high thermal stability of the microcapsules also in the
∗
Corresponding author. E-mail address: [email protected] (A. Karaszewska).
https://doi.org/10.1016/j.matchemphys.2019.01.029 Received 29 May 2018; Received in revised form 7 December 2018; Accepted 14 January 2019 Available online 17 January 2019 0254-0584/ © 2019 Published by Elsevier B.V.
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modified nonwoven. The heat effect corresponding to the phase change was equal 34 J per gram of the modified textile.
1. Introduction
2. Materials and methods
The phase change materials (PCMs) are widely used in many areas including solar energy harvesting and heat storage systems, waste heat recovery, building industry, premises thermoregulation, textile industry, food and medicaments transport, hot–cold therapies and many others [1]. Due to the high density of latent heat and nearly constant phase transition temperature, PCMs have been incorporated into textiles, to improve the thermal comfort. PCMs are used in clothing (sports, recreation and everyday use protective), lining and padding in shoes, bedding, gloves, pillows in the car seats and wheelchairs, building materials, military and home furnishings (curtains, floor and wall coverings, mattresses, foams, pillows) [2–4]. Clothing manufacturers are now seeking ways to control actively the clothing microclimate through heat emission and heat absorption by the garment. The most commonly paraffin used to provide thermal comfort of textiles are those whose phase transition occurs with melting point of 18–36 °C, such as nheptadecane, n-octadecane, n-nonadecane and n-eicosane [1,2]. Therefore, application of PCMs in the textile industry grows continuously, particularly in the developed countries. PCMs are incorporated into textiles in the form of microcapsules (MPCMs) [2] or without encapsulation [5–7]. Publications show that MPCMs are most frequently used [2,4,8–26]. The main advantage of MPCMs over naked phase change materials are as follows: larger surface of heat transfer; protection of PCMs from often unwanted interactions with environment; localization of phase change processes [2]. The MPCMs could be incorporated into the fibers surface by such known methods used in the textile industry as coating [4,8–11,13,14,18–23], padding [12,15,23–26], laminating [4] printing [21,23] or by addition to the dye bath [27]. Microcapsules PCMs can be also incorporated into the fibers during fiber spinning [28,29]. In publications, there are described many different materials for PCM encapsulation. Very often they are selected from homo- and copolymers as well as polymer containing composites. Melamine-formaldehyde [30–32] and urea-formaldehyde copolymers are most often used [33,34]. Their disadvantage is that their production utilizes harmful formaldehyde. Other polymers, such as cross-linked polystyrene [35–37] and polymethylmetacrylate [38], urea copolymers [39,40], polysilicates [41], polyurethanes [42] often fulfill needed requirements. Polysiloxanes containing properly chosen reactive groups are especially good candidate for paraffin containing MPCMs for application in textile industry. Polysiloxanes are chemically and thermally stable, biologically inert and friendly to environment, they are resistant to oxygen and UV light and provide a good barrier against water and water vapour. It should be noted that mechanical strength and elasticity of polysiloxanes could be controlled in a broad range by cross-linking. Microencapsulation of a paraffin with polysiloxane was tinvestigated earlier in one of our laboratories [43,44]. Microcapsules composed of n-eicosane embedded in polysiloxane were synthesized by the one pot aqueous emulsion process. During the above-mentioned process the molten paraffin was embedded in premodified polyhydromethylsiloxane, which in the same emulsion was cross-linked by hydrosilylation of divinyltetramethyldisiloxane. In this paper we present results of the studies of modification of the nonwoven polyester (needled) using the newly developed microcapsules containing n-eicosane covered with polysiloxane shells. The main objective of the studies was to evaluate the thermal properties of received microcapsules, microcapsules with binders and nonwoven fabric modified with microcapsules.
2.1. Materials 2.1.1. Microcapsules Polyhydromethylsiloxane (PHMS) with trimethylsiloxane groups on the end chains (viscosity 15–20 cSt, corresponding to Mn = 1.9·103 g/ mol) and 1,3-divinyltetramethyldisiloxane (DVTMDS) (purity 97%) were purchased from ABCR Gmbh.; polyvinylalcohol (PVA, Mn = 7.2·104 g/mol and isopropanol (purity 99.5%) were from POCH; tetrahydrofurane p.a. and 1,3-dioxane (purity 99.8%) were purchased from CHEMPUR; n-eicosane (purity 99.0%) was from Alfa Aesar Gmbbh. All chemicals were used without additional purification. Catalyst, platinium complex 20 wt % Pt(0) was received from Momentive Performance Materials Leverkusen. 2.1.2. Textile carrier and polymer binders Needle-punched nonwoven fabrics made of 100% polyester filaments were provided by a regional manufacturer. Parameters characterizing this nonwoven material were as follows: mass per unit area 155 ± 7.5 g/m2, thickness 4.6 ± 0.2 mm, bending stiffness for course along 6.0 ± 0.3 mNcm and air permeability for pressure 100 Pa 3900 ± 170 mm/s. For modification of nonwoven fabric the following agents were also used:
• binding agent - Helizarin Binder TX4738 (BASF, Germany) (HB), • cross-linker - Helizarin Fixing Agent TX4737 (BASF, Germany) (HA).
2.2. Methods 2.2.1. Synthesis of microencapsulated n-eicosane with polysiloxane shell The solution of 100 g of PHMS with of 18.5 g of DVTMDS in 218 mL 1,4-dioxane was heated at 45 °C and 0.111 g Karstedt catalyst in 0.75 g 1,4-dioxane was introduced. Evolution of gas was observed. The solution was stirred for 20 min at 45 °C. During this time viscosity of the mixture increased. Thereafter the solution of 240 g n-eicosane in 218 mL of isopropanol was introduced to the reactor. Obtained solution was stirred 5 min at 45 °C and then quickly introduced to 5000 mL of water containing 1.87 g of dissolved PVA. The mixture was homogenized for 25 s using a high-speed MPW-120 homogenizer set to 4.000 rpm. Obtained emulsion was added to the 20 L reactor equipped with an anchor stirrer. The reactor was filled with 11.5 L of water containing 43.13 g of PVA. During reaction the mixture was stirred at 45 °C during 70 h. Thereafter, the microcapsules were separated by sedimentation. The microcapsules were washed with water to remove remaining surfactant. The microcapsules of spherical shape (or distorted spheres) were obtained. They could be well dispersed in water by simple agitation and the dispersion was stable for about 5 h. The suspension of 46 wt % of microcapsules in water was used for the nonwoven fabric treatment. 2.2.2. Binding PCM microcapsules with textile 2.2.2.1. Mixture of MPCM and binder. To compare and examine the influence of binding and cross-linking agents (HB – Helizarin Binder TX4738 and HA – Helizarin Fixing Agent TX4737) on MPCM thermal properties and to determine the content of MPCM in the microcapsules/ binder/cross-linker mixture was prepared. The mixture contained 125% (wt) of microcapsules with respect to the binder. 2.2.2.2. Modification of nonwovens. Modification was performed using 205
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evaluated using a differential scanning calorimeter DSC 204 F1 Phoenix (Netzsch, Germany). Samples placed in aluminum pans with a pierced lid were scanned in the temperature range 0–60 °C with the scanning rate 5 °C/min. During measurements samples were kept in the nitrogen atmosphere. Three samples of each studied material (5–15 mg) were tested. The onset (TOnset) and end (TEnd) temperatures of melting and crystallization process, melting (Tm) and crystallization (Tc) temperatures in peak maxima and enthalpies of melting (ΔHm) and crystallization (ΔHc) were determined for microcapsules, microcapsules with binding agents and microcapsules modified nonwoven fabric. The coefficients of encapsulation (Χen) and the coefficient of energy storage (Xes) were estimated from the phase transitions enthalpies. The values of Χen (%) and Xes (%) were calculated from Eqs. (1) and (2) [45]:
a two-roller padding machine from BENZ- Switzerland. Conditions of padding: down force roller 35 kG/cm, padding rate 2 m/min, wring degree of 200%. After padding the nonwoven fabric was dried for 3 min in air at 80 °C, and then reheated for 10 min in air at 100 °C. The last step was responsible for the binder cross-linking. The modifying bath contained the following agents: Microcapsules PCM (aqueous suspension) - 50 g/100 mL, HB - 20 g/ 100 mL, distilled water - 28 g/100 mL, HA – 2 g/100 mL. Optimal composition of modifying bath was determined in our earlier studies [23,24]. 2.3. Characterization 2.3.1. Microscopic analysis SEM/EDX Microscopic analysis SEM/EDS was carried out using a scanning electron microscope VEGA 3 (Tescan, Czech Republic) equipped with e X-ray microanalyzer EDS INCA Energy (Oxford Instrument Analytical, UK). Diameters of the MPCM were determined by a digital image analysis of the SEM microphotographs. EDS surface analysis was performed under 10 Pa pressure, using electron beam energy of 20 keV, without sputtering a conductive substance on the sample. Studies were carried out by a point method in microareas with diameter of 0.70 μm.
X en (%) =
ΔHm,K x100%, where K= A, A1, A2. ΔHm, n − eicosane
(1)
X es (%) =
ΔHm,K + ΔHc,K x100%, K= A, A1, A2 ΔHm, n − eicosane + ΔHc, n − eicosane
(2)
where: ΔHm,A, ΔHm,A1, ΔHm,A2 denote the melting and ΔHc,A, ΔHc,A1, ΔHc,A2 crystallization enthalpies of microcapsules (A), mixture microcapsules with binding agent (A1) and microcapsules in modified nonwoven fabric (A2), respectively. ΔHm,n-eicosane and ΔHc,n-eicosane denote the melting and crystallization enthalpy of n-eicosane. The coefficient of energy storage capability (Ces) of microcapsules A, mixture A1 and microcapsules on modified nonwoven fabric A2 was calculated using Eq. (3) [45–47]:
2.3.2. Microscopic analysis TEM Microstructure of MPCM was determined using Tesla BS-500 (Tesla, Brno Czech Republic) transmission electron microscope (TEM) operating at 90 kV. Samples were prepared by placing a drop of an aqueous suspension of the microcapsules on the carbon coated copper grid, and then the solvent was evaporated at room temperature.
Ces (%) =
2.3.3. Fourier transform infrared (FTIR-ATR) spectroscopy FTIR spectroscopy was used for characterization of chemical compositions of the n-eicosane, polysiloxane, polysiloxane microcapsules containing n-eicosane, nonwoven and modified nonwoven fabric, using a Vertex 70 spectrophotometer (Bruker, Germany) in the range 4000–600 cm−1 with a resolution of 8 cm−1.
X es x100% X en
(3)
The supercooling degree (ΔTs) is an important parameter reflecting the crystallization-melting hysteresis of a PCM, and is calculated using Eq. (4) [46,47]:
ΔTs = Tm − Tc
(4)
where: Tm and Tc are the melting and crystallization peak temperatures, respectively. Based on the melting enthalpies of the microcapsules (A) in powder, microcapsules with binding agents (A1) and microcapsules on modified nonwoven fabric (A2). The microcapsules A content in the mixture A1 and in the modified nonwoven (DA), was calculated using Eq. (5):
2.3.4. 29Si CP NMR spectroscopy analysis Solid state 29Si CP NMR spectra were registered with a DSX 400 Bruker spectrometer. The spectra were acquired with cross-polarization, at 59.627 MHz, applying 90-μs pulses, 6-s pulse delay, and 3-ms contact time, with samples placed into a 4.0-mm zirconia rotors, with spinning at 8 kHz. The peak positions were referenced to the signal of Q8M8.
DA (wt%) =
ΔHm,L Dm,L , where L= A1, A2 ΔHm,A
(5)
where: Dm,A1 and Dm,A2 denote the content of dry substance (wt.% of paste with microcapsules and of modified nonwoven fabric for L = A1, A2), DA is the microcapsules A content in dry substance (wt.%), ΔHm,A, ΔHm,A1 and ΔHm,A2 defined in Eq. (2), respectively.
2.3.5. Selected properties of nonwovens The values of mass per unit area of the fabrics were determined according to the PN-EN 29073-1: 1994 Textiles – Test methods for nonwovens – Determination of mass per unit area. The thickness of the studied nonwovens was determined according to the PN-EN ISO 9073–2:2002 Textiles - Test methods for nonwovens Part 2: Determination of thickness. The bending stiffness of nonwoven fabrics was measured according to the PN-ISO 9073-7: 2011 Textiles - Test methods for nonwovens- Part 7: Determination of bending length. The air permeability of nonwovens fabrics was measured according to the PN-EN ISO 9237:1998 Textiles - Determination of permeability of fabrics to air. Air permeability – the velocity of air passing perpendicularly through a given area of fabric measured at a given pressure difference across the fabric test area (100 Pa, 200 Pa) over a given period of time.
2.3.6.2. Temperature regulating factor (TRF). To assess the transient thermal properties of the nonwoven, the temperature regulating factor (TRF) was determined in accordance with the ASTM standard test method [48]. The measurements were carried out using a test stand constructed in the Textile Research Institute. The TRF was defined by Hittle and Andrè [49] as the quotient of the amplitude of the temperature variation and the amplitude of the heat flux variation divided by the value of the steady state thermal resistance of the fabric (see Eq. (6)):
TRF =
Tmax − Tmin 1 × qmax − qmin R
where:
2.3.6. Thermal characteristics 2.3.6.1. Differential scanning calorimetry (DSC). Thermoregulating properties of the microcapsules, microcapsules with binding agents, and of the nonwoven fabric modified with microcapsules were
Tmax, Tmin –maximum and minimum temperature, qmax, qmin - maximum and minimum flux 206
(6)
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R – thermal resistance
3.1.3. FTIR-ATR and 29Si CP NMR spectroscopy analysis of microstructure FTIR-ATR spectra of microcapsules displayed in Fig. 7c give important information on the capsule structure. Intensive bands at 10001100 cm−1 are due to the vibration of siloxane groups. Bands at 28003000 cm−1 come from C-H stretching vibration in n-eicosane and in polysiloxane. Broad band at about 3300 cm−1 is due to stretching vibration of O-H in polysiloxane, while the band present at about 900 cm−1 is attributed to the stretching vibration of Si-OH groups. The stretching vibration bond appears at 2100 cm−1, Si-CH3 group vibrations contribute to absorptions at 1200 and 1350 cm−1. Other intensive bonds at 750-850 cm−1 are assigned to Si-CH3 rocking vibrations and to vibrations of CH2 groups in paraffin. 29 Si CP MAS NMR spectrum of microcapsules (Fig. 8) provides information about chemical structure of the polysiloxane shell. Signals of the Si-OH and Si-H reactive groups appear at −59 and −37 ppm, respectively. Signals at −22 and + 8 ppm are due to Si-CH2 groups of cross-links formed by hydrosilylation while that at −67 ppm comes from Si-O-Si cross-links formed by the Si-H + HO-Si dehydrocondensation.
The TRF is a dimensionless quantity varying in a range (0, 1). The TRF shows how well a fabric containing microcapsules PCMs moderates the hot plate temperature. A TRF value of 1 means the fabric has no heat capacitance and poor temperature regulation. A TRF equal to zero means that fabric has an infinite heat capacitance and that a body in contact with it will remain at a constant temperature. While determining the TRF value, the following measurement parameters were taken: a medium heat flux of 70 W/m2 and an amplitude of the heat flux change of 30 W/m2. The temperature of the cold plate was selected in such a range that the temperature changes of the hot plate might fluctuate within the phase change temperature range of the microcapsules PCMs applied. The TRF was determined for 6 selected frequencies of heat flux changes (t): 240, 480, 600, 720, 900 and 1200 s. The results of TRF measurements are shown in diagrams presenting the relationship TRF = f (t). 3. Results and discussion 3.1. Synthesis and characterization of PCM microcapsules
3.1.4. Energy dispersive X-ray spectrometry (EDX) studies of the MPCM microcapsules with cross-linked polysiloxane shells EDX spectrometry is a technique providing information on element content in the probed interfacial layer of investigated sample. However, one has to keep in mind that the depth of probing depends on energy of the beam and on the density of the probed interfacial layer. Very often it is in the range from a few tenths to several hundreds of nanometers, however, for samples with low density it may be even larger. Electron microscopy studies (Subsection 3.1.2) revealed that thickness of the cross-linked polysiloxane shells ranges from 0.26 to 0.48 μm. Thus, one should take into consideration that the depth of probing by EDX may exceed thickness of the shell. In pure polyhydromethylsiloxane the proportion of silicon, carbon and oxygen is 1:1:1 (without taking into account contribution of the end-groups). Cross-linking could change this proportion. In extreme example, when in reaction of polyhydromethylsiloxane with 1,3-divinyltetramethyldisiloxane all –Si(H) (CH3)O– groups of the polymer would be converted to the cross-link points, the proportion of silicon, carbon and oxygen should be 1:2.5:0.75. Results of EDX analysis of randomly selected five areas of microcapsules (Fig. 9) gave the percentage of silicon, carbon and oxygen elements equal 17.4%, 59.4% and 23.3%, respectively, what corresponds to proportion 1:3.4:1.4 of these elements. The content of carbon, which was higher than in the case of even fully cross-linked polyhydromethylsiloxane indicates that also n-eicosane contributes to the EDX spectrum. It should be noted that our earlier, independent studies revealed not only that n-eicosane is encapsulated in cross-linked polysiloxane but that it does not leak from the microcapsules during many
3.1.1. Preparation of microcapsules The first step of the microcapsule synthesis proceeds in solution. During this process part of the cross-linker, 1,3-divinyltetramethyldisiloxane (ca 60%) is grafted on the PHMS polymer by hydrosilylation (Fig. 1). The presented above preliminary modification facilitates crosslinking of the polymer because after this step both Si-H and Si-vinyl groups are localized on polymer chains, which are going to form a network. The reaction mixture produced in the first step is used in the main process of synthesis, which proceeds in emulsion. The emulsion is easily formed mechanically from the solution in a water miscible solvent containing premodified polymer/unreacted cross-linker/catalyst/ paraffin and water containing with added surfactant (PVA). The emulsion contains droplets of polysiloxane with n-eicosane and the catalyst. Two important reactions occur in the emulsion: hydrosilylation forming bridges between macromolecules (Fig. 2) and hydrolysis of Si-H groups producing a large numbers of Si-OH groups (Fig. 3). Due to formation of silanol groups the polysiloxane becomes it more hydrophilic than the paraffin. Therefore, the polymer does not mix with hydrophobic n-eicosane forming the particles shells. The cross-linking occurs also by heterocondensation shown in Fig. 4. The diameters and shapes of synthesized microparticles depend significantly on time and intensity of the stirring during emulsification [43]. Usually, the microcapsules have shape very close to spheres (some distorted). However, the majority of ≡Si-OH groups formed by hydrolysis of ≡Si-H remains, what makes surface of microcapsules hydrophilic and well dispersible in water. 3.1.2. Diameters of microcapsules and thickness of their shells Morphology of the fabricated microcapsules was investigated by analysis of their SEM and TEM micrographs. Examples are shown in Fig. 5a–b. TEM images displayed in Fig. 5b show a sharp contrast between paraffin and polysiloxane shells of microcapsules, which confirms their core-shell structure and the complete covering of n-eicosane by the cross-linked polysiloxane layer. The SEM studies allowed to determination of the average diameter of microcapsules, which was 10.0 μm with the standard deviation of 0.8 μm (Fig. 6a). The thickness shell of the majority of microcapsules was in the range of 0.26–0.46 μm as shown in Fig. 6b. The mean shell thickness of the population studied was 0.38 μm. The average shell to radius of microcapsules was 0.03.
Fig. 1. Scheme of grafting of vinyl group containing moieties onto PHMS. 207
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Fig. 2. Scheme of cross-linking PHMS by hydrosilylation.
Fig. 3. Scheme hydrolysis of SiH groups.
Fig. 4. Scheme of heterocondensation involving ≡SiH and ≡SiOH groups.
Fig. 6. SEM microphotograph and distribution histograme: (a) diameter of microcapsules, (b) shell thickness.
Fig. 7. The FT-IR spectra of: (a) n-eicosane, (b) polysiloxane and (c) microcapsules containing n-eicosane.
Fig. 5. Images (a) SEM and (b) TEM of microcapsules and cross-section revealing the microcapsule core–shell structure.
phase transfer cycles.
3.2. Incorporation of the microcapsules PCM in nonwoven structure 3.2.1. Morphology of the dried padding mixture, pristine and modified nonwoven The SEM images of the surface of dried padding mixture containing binder and the microcapsules and of the unmodified and modified nonwoven are shown in Fig. 10. The binder-microcapsule suspension used for modification the nonwoven contained 38% of dry mass (determined gravimetrically). After drying it was milky, but macroscopically homogeneous. Any traces of the released paraffin were visible on the surface. SEM microphotographs displays presence of some round objects on the surface.
Fig. 8. 29Si CP MAS NMR spectrum of microcapsules containing n-eicosane coated with polysiloxane shell.
Many of them are open forming cavities with the microcapsules sticking to their walls (Fig. 10 b and c). It is clear that the microcapsules interact with the binder. The protruding round bubbles, some with small 208
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3.2.2. Studies of the not modified and modified nonwoven by energy dispersive X-ray (EDX) spectrometry SEM microscope with EDX spectrometer could be used not only for obtaining standard SEM microphotograms but also for detection of various elements in the sample and for visualization of element distribution in the samples’ interfacial layer. For microcapsules containing silicon in the shells distribution of silicon was used as microsphere indicator. Fig. 11 shows the standard SEM microphotograph and the microphotograph, showing map of silicon distribution in the same sample area. These microphotographs prove that silicon atoms are present exclusively where the microcapsules. This observation supports conclusion that if any microcapsules are broken into small pieces not seen in the SEM microphotogram, their amount is very low. Fig. 12 displays EDX spectra for five different microareas of the pristine and modified nonwovens. As expected, according to EDX spectrum (Fig. 12a) the elemental composition of the polyester nonwoven unveiled exclusively the presence of carbon and oxygen atoms. The EDX spectrum (Fig. 12b) confirmed the presence of Si element on the surface of the test nonwoven level of about 4%.
Fig. 9. EDX spectra and elemental compositions of the microencapsulated neicosane with a polysiloxane shell.
3.2.3. FTIR-ATR analysis of the modified nonwoven Fig. 13 shows the FTIR-ATR spectra of the pristine (Fig. 13a) and modified polyester nonwoven (Fig. 13b). Signals were assigned according to the literature [14,49]. The FTIR-ATR spectrum of the pristine nonwoven is typical for aliphatic-aromatic polyester copolymer and contains the following signals: 721 cm−1 (bending CH2); 1014 cm−1, 1091 cm−1 and 1241 cm−1 (stretching C-O-C);1712 cm−1 (stretching C=O); 2966-2854 cm−1 (C-H stretching in aliphatic groups); 15041450 cm−1 (aromatic ring C=C-C stretching); 970-794 cm−1 (aromatic C-H bending); 3298 cm−1 and 3425 cm−1 (weak bands: O-H stretching in carboxylic acid and hydroxyl groups). After modification, the nonwoven showed specific bands for both components, i.e. for polyester and for PCM microcapsules (see also Fig. 7c). The FTIR-ATR spectrum of the modified nonwoven shows peak at 1018 cm−1. This signal is typical for bending vibration of Si-O-Si. The peaks at 1249 end 794 cm−1 are due to the bending vibration of Si-CH3 and the peak at 2163 cm−1 is to the bending vibration of Si-H [50]. The peak 906 cm−1 corresponds to the stretching vibration of Si-OH. Bands at 2800-3000 cm-1 come from C-H stretching vibration in polysiloxane and in n-eicosane. 3.2.4. Selected properties of nonwoven According to SEM microphotogram shown in Fig. 5d, the structure of nonwoven is very loose. Thus, the padding mixture could easily penetrate the fabric, reach spaces between fibers and enable their efficient modification. The modification resulted in higher surface mass of the fabric (SM), which increased from 155 g/m2 (SMN) to 267 g/m2 (SMMN) reflecting the high degree of padding (Dm = [(SMMN – SMN)/ SMN]·100%). Taking into account the surface mass of nonwoven before and after modification, the degree of padding of obtained fabric was equal 72%. Because the degree of the dry mass of the microcapsules in the dry mass of the padding mixture (DM-PCM) was 37% the mass content of the microcapsules in the modified fabric was 26.2%. The binder bonded the loose fibers in the nonwoven fabric, making it more compact. In result, thickness of the nonwoven slightly decreased, from 4.60 mm to 4.39 mm. Moreover, stiffness of the fabric significantly increased, from 5.99 mNcm to 35.22 mNcm. More compact structure of the fabric caused reduction of air permeability by about 15%; from 3890 mm/s to 3290 mm/s).
Fig. 10. Macroscopic (a) and SEM microphotographs (b)–(e): (a) macroscopic photo of the dry microcapsules-binder mixture, (b) and (c) – surface of dry binding agent containing microcapsules, (d) – nonwoven before modification, (e) – modified nonwoven coated with binder and containing attached microcapsules.
openings contain the microcapsules almost completely or completely covered with the binder. This means that the microcapsules are not perfectly dispersed, but form clusters. SEM microphotographs of not modified nonwovens (Fig. 10,d) indicate that before modification the surface of fibers was smooth. After modification the clusters of microcapsules are attached to the fibers. Isolated microcapsules are rarely seen. It should be noted that SEM pictures of modified nonwovens indicate that at both sides they are modified similarly.
3.3. Thermal properties of modified nonwovens 3.3.1. Phase change properties Phase change properties of microcapsules containing n-eicosane, dry microcapsules-binder mixture and modified nonwovens were 209
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Fig. 11. Standard SEM microphotogram (a) and (b) map of Si distribution on the surface of modified nonwoven.
Fig. 13. FT-IR spectra of: (a) pristine polyester nonwoven, (b) modified polyester nonwoven fabric.
investigated monitoring many cycles of the heating and cooling covering temperature range of melting and crystallization of n-eicosane. Fig. 14 shows DSC traces for n-eicosane, microcapsules containing neicosane, mixture of microcapsules containing n-eicosane (A), with the binder (A1) (dry mass content of microcapsules 37%) and nonwoven modified by attachment of n-eicosane containing microcapsules (A2). Whereas the thermogram for n-eicosane is a typical one the DSC traces for the microcapsules, microcapsules with binder and modified nonwovens are complicated. Crystallization traces, for these materials are evidently bimodal. Plausible explanation of this observation may be related to different quality (defects) of n-eicosane crystals in the center of the microcapsules and next to the polysiloxane shells. Parameters evaluated from DSC traces are given in Table 1. The temperature corresponding to maximum of the heat flow during melting (heating of the sample) is highest for the microcapsules A. The temperature at peak during crystallization (heat flow during cooling) is the lowest one for the microcapsules A and the highest for nonwoven fabric A2. Thus, the supercooling degree (ΔTs) is the largest one for pure microcapsules (A) and the smallest for the modified fabrics. Evidently, the binder and nonwovens delay transfer of heat to n-eicosane.
Fig. 12. EDX spectrum and elemental compositions: (a) of the nonwoven, (b) modified nonwoven. Content of each element is given in atom percent.
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Table 2 Encapsulation coefficient of n-eicosane into PCM microcapsules – data from literature and from this work. Shell
n-eicosan/symbol in the reference (weight ratio wt./wt.)
TiO2 [46]
n-eicosane/titanium butoxide 60/40 50/50 40/60
77.97 65.59 73.73
n-eicosane/Zn(CH3COO)2∙ 2H2O
ZnO [47]
Fe3O4/SiO2 [51]
60/40 50/50
70.00 50.00
40/60
35.00
n-eicosane/tetraethoxy silane 70/30 50/50 40/60
P(MMA-co-AA) [52]
71.78 64.95 52.97
n-eicosane/poly(methyl methacrylate-coacrylic acid) 55/45
PMMA [53]
32,90
n-eicosane/poly(methyl methacrylate) 50/50
PMMA [54]
polysiloxane
Fig. 14. DSC thermograms of: (a) n-eicosane, (b) A – microcapsules, A1 – dry microcapsules-binding agent mixture, A2 – nonwoven fabric containing microcapsules.
Xen, %
35,00
n-eicosane/poly(methyl methacrylate) 50/50
62.00
n-eicosane/polysiloxane
This study
67/33
81.4
modified nonwoven it is still high (19.2%). It is worth to mention also that for each investigated material, the energy storage (Xes) is very close to the coefficient of encapsulation. In result, values of the coefficient of energy storage (Ces) exceed 98.9%, i.e. are close to the perfect energy storage. Suitability of any phase change material formulation for application as a temperature-controlling additive strongly depends on coefficient of encapsulation of the PCM active substance (Xen). Table 2 provides some data on coefficient of encapsulation of n-eicosane in various formulations (inorganic and organic) described in literature and in our work. It is evident that the PCM microcapsules developed in this work have higher encapsulation coefficient for n-eicosane than the described in the literature microcapsules with inorganic and organic shells. Stability of controlling thermal properties of nonwovens with bound PCM microcapsules containing n-eicosane is maintenance of high reversibility of the system exposed to the heating and cooling cycles. Fig. 15 shows plots of the enthalpies and temperatures of melting and crystallization for nonwovens containing n-eicosane loaded polysiloxane microcapsules (ΔHm, ΔHc, Tm and Tc, respectively) as function
Such behaviour may be due also to amount of microcapsules in each specimens. The smallest amount of microcapsules (A) was in the modified nonwoven fabric and the range of phase transitions temperatures is the narrowest. The onset temperature for nonwoven fabric A2 is lower than for microcapsules A and dry microcapsules-binding agent mixture A1 what may result from amount of microcapsules in the fabric and better heat availability and its propagation in the sample than for A and A1 where the amount of microcapsules is higher and the possibility of reaching the heat inside the sample is slower. The modified nonwoven fabric A2 has the lowest values of melting and crystallization enthalpies than samples A and A1. This is due to the lowest amount of microcapsules on fabric A2. It is obvious that the coefficient of encapsulation of n-eicosane (neicosane content – Xen) is the highest one for the microcapsules and the lowest for the modified fabrics. However, it is worth noting that for
Table 1 Parameters characterizing thermal properties of investigated materials. Sample
n-eicosane A A1 A2
Melting process
Crystallization process
TOnset (°C)
TEnd (°C)
Tm (°C)
ΔHm (J/g)
TOnset (°C)
TEnd (°C)
Tc (°C)
ΔHc (J/g)
32.7 35.3 34.9 32.9
39.5 48.2 42.2 39.6
37.0 41.8 38.4 37.3
180.3 146.8 49.1 34.6
32.6 34.6 34.7 34.1
24.0 17.4 23.0 23.0
31.1 22.8 30.0 31.4
181.4 146.9 48.7 34.2
where: A -microcapsules, A1-microcapsules with binding agents, A2-microcapsules on modified nonwoven fabric. 211
Xen (%)
Xes (%)
Ces (%)
ΔTs (°C)
81.4 27.3 19.2
81.2 27.0 19.0
99.7 98.9 98.9
6.0 18.0 8.4 5.9
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microcapsules PCM exhibit a lower TRF value in the whole range of frequencies of heat flux changes than the reference nonwovens. This is due to the increase in nonwoven thermal capacitance resulting from the incorporation of microcapsules PCM. Similar dependence was also noticed for nonwovens modified with commercial PCM microcapsules (neicosane) [11,55]. 4. Conclusions Developed process of fabrication of PCM microcapsules consisting of n-eicosane cores coated with polysiloxane shells ensures very effective encapsulation of the above-mentioned active phase-change paraffin substance (n-eicosane), higher than for other systems described in the literature. Cross-linked polysiloksanes used for shell preparation have unique properties beneficial for textile materials: softness, inertness for many chemicals, resistance for high temperatures, elasticity at low temperatures, biocompatibility, permeability for gases and resistance for oxidation, and nonflamability. Very simple, standard methods (padding) could be used for introduction of developed n-eicosane core, polysiloxane shell microcapsules into nonwoven. Energy storage coefficient of modified nonwovens is very high and, exceeds 98.9%. Thermoregulating properties of the modified nonwoven vary randomly within a narrow range, during the heating and cooling cycles. Beside the clothing, the developed n-eicosane loaded microcapsules could be considered as good candidates also for other applications, like packaging requiring thermoregulation and construction.
Fig. 15. Stability of phase change enthalpies (a) and crystallization and melting temperatures (b) during the heating-cooling cycles.
Acknowledgements This work was supported by the Key Project POIG.01.03.01-00-004/ 08 Functional nano- and micro textile materials – NANOMITEX co-financed by the European Union with financial resources of the European Regional Development Fund and National Centre for Research and Development within the framework of the Innovative Economy Operational Programme, 2007–2013, Priority 1. Research and development of modern technologies, Activity 1.3. Supporting R&D projects for enterprises undertaken by science establishments, Subactivity 1.3.1. Development projects. Przemyslaw Sowinski made SEM and TEM studies which were financial supported from the CMMS PAS Statutory Fund.
Fig. 16. The transient thermal performance: (a) view of the instrument to determine temperature regulating factor (TRF); A sample nonwoven, B cold plate, C hot plate, D cooling water supply to cool plates, E) guide bars, F) thermostat; (b) TRF as a function of cycle time.
of the number of the heating and cooling cycles (in the temperature range from 0 to 60 °C). Plots in Fig. 15 reveal that during 101 cycles of heating and cooling the above-mentioned parameters varied in a narrow range: from 34.2 to 34.65 J/g (ΔHm), from 34.15 to 34.5 J/g (ΔHc), from 37.3 to 37.6 °C (ΔTm) and from 31.25 to 31.38 °C (ΔTc). These observations prove that n-eicosane does not leak during the heating-cooling cycles and that thermal properties of the modified nonwoven are retained.
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3.3.2. The thermo –regulating properties of nonwovens modified with PCM microcapsules Determination of the temperature regulating factor (TRF) of pristine nonwoven and nonwoven with microcapsules was done by means of an instrument with a dynamic heat source, presented in Fig. 16a. This instrument simulates the following arrangement: skin – apparel – environment. The fabric sample is sandwiched between a hot plate and two cold plates, one on either side of the hot plate. These cold plates at a constant temperature simulate the environment outside the apparel. The sinusoidally varying heat input to the hot plate simulates heat produced by human activity. With purpose to measure the steady state thermal resistance of the fabric, the controlled heat flux is maintained constant, and the test proceeds until a steady state is reached. To assess the temperature regulating ability, the heat flux is sinusoidal and the temperature regulating factor (TRF) is determined. The results of measurements are presented as TRF function of the heat flux frequency. (Fig. 16b). Analyzing these plots one could notice that nonwoven with 212
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