DEPARTMENT OF MATERIALS SCIENCE
PEDRO JOÃO NERES CABRAL
Bachelor in Materials Science and Engineering
INK AND FILMS APPLICATION FOR TEMPERATURE AND LIGHT
MANAGEMENT OF BUILDINGS
MASTER IN MATERIALS ENGINEERING
NOVA University Lisbon
DEPARTMENT OF MATERIALS SCIENCE
INK AND FILMS APPLICATION FOR TEMPERATURE AND LIGHT
MANAGEMENT OF BUILDINGS
PEDRO JOÃO NERES CABRAL
Bachelor in Materials Science and Engineering
Adviser: Dra.Isabel Ferreira
Associate Professor with tenure, NOVA University Lisbon
Co-adviser: Dra. Ana Marques
Senior Researcher, NOVA University Lisbon
MASTER IN MATERIALS ENGINEERING
Ink and films application for temperature and light management of build- ings
Copyright © Pedro João Neres Cabral, NOVA School of Science and Technology, NOVA University Lisbon.
The NOVA School of Science and Technology and the NOVA University Lisbon have the right, perpetual and without geographical boundaries, to file and publish this dissertation through printed copies reproduced on paper or on digital form, or by any other means known or that may be invented, and to disseminate through scientific repositories and admit its copying and distribution for non-commercial, educational or research purposes, as long as credit is given to the author and editor.
Para os meus.
A c k n o w l e d g e m e n t s
Em primeiro lugar, quero agradecer às duas pessoas que tornaram o caminho até aqui possível, José e Olimpia. Obrigado pela incansável determinação, ousadia e esforço.
À minha orientadora e coorientadora, professora Dra. Isabel Ferreira e Dra. Ana Marques, pela partilha de conhecimento, pelos conselhos e pela sensibilidade e objetividade com que me prestavam auxílio.
À minha avó, tia, irmã, Fábio, Gu e Estrela pelo apoio, incentivos, sabedoria transmitida e especialmente por tudo aquilo que representam.
À Beatriz, pelo amor e carinho incondicional e pela prova empírica de que é possível fazer sestas de manhã.
À minha segunda família, Sandra, Nelson, Rafael, Camila, Marina e Tiago, Fifi e Marilu pelos trails, white whales in the flamingo, apostas ganhas e sobremesas gaúchas, para enumerar alguns.
Aos Nabiças 2.0 por, para pôr em termos simples, serem os meus disfuncionais favoritos.
Aos meus companheiros de curso, Xue, Catatão, Duarte, Bento, estando, portanto, excluí- dos o Tony, o Buchas e o Filipe por terem ido a Roma sem aviso com antecedência decente, pelos jogos de cartas e matrecos no convívio, pelos almoços no Velho, pelos nites, pelos cafés e, essencialmente por existirem.
Ao Duarte, que ainda longe, permanece um perto.
Aos meus colegas de laboratório, Freire, Frazão, Margarida e Teresa por fazerem o tempo passar mais depressa e por me apresentarem os dilemas mais insignificantes e hipotéticos já alguma vez ponderados.
A toda a equipa do laboratório, Íris, Catarina, Miguéis, Mariana e David pela ajuda e conselhos fornecidos.
Ao Sporting por, após 19 anos, ter vencido o campeonato nacional, distorcendo assim o significado de impossível.
“When you have confidence, you can have a lot of fun. And when you have fun, you can do amazing things.” (Joe Namath)
A b s t r a c t
The reshaping of the construction industry has led to a progressive increase of glazed facades in commercial buildings not just for aesthetic reasons but also to enhance access to natural light. However, glass is a material unable to operate efficiently as a thermal insulator. Thus, it becomes necessary the use of air conditioning devices to sustain a pleasant temperature indoors contributing significantly to the increase in greenhouse gas emissions.
As a pioneer class of materials in thermal energy management and harvesting, phase change material (PCM) are an innovative approach in temperature regulation applica- tions.
The work seeks to investigate the production of polymeric films with microencapsu- lated PCM and metal oxide particles with a specular transmittance superior to 50% in the visible range of the spectrum, great adhesive properties and resistance to solar degra- dation. The cellulose Acetate (CA) and polymethyl meth-acrylate (PMMA) polymeric films had a transmittance in the visible range higher than 88%. The pure vaseline from Trade Medic (PVTM) was the PCM with the lowest melting point around 40 ºC. From the materials studied, the polymer, metal oxide, PCM and shell were cellulose acetate, vanadium oxide, pure vaseline from TradeMedic and silica, respectively. This composite films exhibited great adherence to glass and a specular transmittance of over 56% at a 500 nm wavelength.
R e s u m o
As novas tendências no setor de construção civil têm vindo, progressivamente, a recor- rer à incorporação de fachadas envidraçadas em edifícios comerciais não só por motivos estéticos, mas também pela exposição à luz solar. Não obstante, o vidro, por si só, é um material incapaz de proporcionar um isolamento térmico eficiente. Por esse motivo, para manter uma temperatura confortável no interior é necessário a utilização de ar condicio- nado cujo uso excessivo, contribui para um aumento de emissões de gases com efeito de estufa.
Os phase change materials (PCMs), apontados como pioneiros na área da conserva- ção e gestão da energia térmica, manifestam-se como uma abordagem inovadora para aplicações de regulação de temperatura.
Este trabalho visa a explorar a produção de filmes poliméricos com PCM micro en- capsulados e com partículas óxido metálicas com: uma transmitância especular na região do visível superior a 50%, boas propriedades de adesão e resistência à exposição solar superior a 6 meses. Os filmes poliméricos de CA e PMMA possuiam uma transmitân- cia no visível superior a 88%. A vaselina PVTM foi o PCM com o ponto de fusão mais baixo, com uma temperatura perto dos 40 ºC. Dos materiais estudados, o polímero, óxido metálico, PCM e revestimento selecionados foram, respetivamente: acetato de celulose, óxido vanádio, vaselina pura da marca TradeMedic e sílica. Os filmes destes compósitos demonstraram possuir boa adesão ao vidro e uma transmitância especular acima de 56%
para um comprimento de onda de 500 nm.
C o n t e n t s
List of Figures xi
List of Tables xiv
Acronyms xv
1 Introduction 1
1.1 Motivation . . . 1
1.2 State-of-the-art . . . 1
1.3 Polymers . . . 2
1.4 Phase change materials . . . 2
1.4.1 Phase change materials encapsulation . . . 4
1.4.2 Metal oxides . . . 5
2 Materials and Methods 6 2.1 Materials . . . 6
2.2 Polymeric films . . . 6
2.3 PCM encapsulation . . . 6
2.4 Characterization techniques . . . 7
2.4.1 UV-Visible-NIR optical properties . . . 7
2.4.2 Calorimetry and thermogravimetry analysis . . . 7
2.4.3 Morphology . . . 7
2.4.4 Raman Spectroscopy . . . 7
2.4.5 Thermal Emissivity . . . 7
3 Results and Discussion 8 3.1 Optical analysis; experimental considerations . . . 8
3.2 Optical properties of polymeric films . . . 9
3.3 Materials costs . . . 10
3.4 Degradation tests . . . 11
C O N T E N T S
3.5 Phase change materials . . . 12
3.5.1 Encapsulation . . . 12
3.5.2 Phase change material study . . . 17
3.6 Phase change materials and polymer composite . . . 21
3.7 Metal oxides . . . 23
3.7.1 Visual aspect . . . 23
3.7.2 Set thickness . . . 23
3.7.3 Concentration . . . 24
3.7.4 Functional properties . . . 25
3.8 PCM and metal oxide polymer composite . . . 25
3.9 Thermal emissivity . . . 27
4 Conclusion and future perspectives 28 4.1 Conclusion . . . 28
4.2 Future perspectives . . . 29
Bibliography 30
Appendices
A PCM types 38
B Materials 39
C Film applicator optimization 40
D DSC-TGA 41
E Thermal storage study setup 47
F Films dried at room temperature 49
G CA encapsulation schematic diagram 50
H Raman analysis of PVTM micro-encapsulated in silica (P V T M@SiO2) and PVW micro-encapsulated in silica (P V W@SiO2) 51
I EDS report ofP V W@SiO2 52
J Set thickness effect 53
Annexes
I PCM encapsulation in silica procedure 54
II metal oxides (MO) deposits in the CA 12 wt% polymeric solution 55
L i s t o f F i g u r e s
1.1 Effect of phase change in latent heat storage (LHS) systems [29]. . . 3 3.1 Example of direct and diffuse light transmission: a) Schematic representa-
tion [58] and b) through clear and frosted glass. . . 9 3.2 Specular transmittance curves for CA, PCL, PVDF and PMMA with a) 1000
µm and 100 mm/s; b) 1000µm and 10 mm/s; c) 100µm and 10 mm/s; d) 100 µm and 100 mm/s. . . 9 3.3 Specular transmittance curves before and after the degradation tests for a) CA;
b) PMMA; c) polycaprolactone (PCL); d) poly(vinylidene fluoride) (PVDF) 11 3.4 Result of pure vaseline from Wells (PVW) in CA encapsulation attempt a)
before drying and b) after drying . . . 12 3.5 Raman characteristics signal for a) PVW, TEOS andP V W@SiO2; b) PVTM,
TEOS andP V T M@SiO2; c) PVW, CA and microencapsulated PVW in CA . 13 3.6 Raman spectra of micro-encapsulated PCM in different shells (CA orSiO2)
after the washing process . . . 14 3.7 Scanning electronic microscopy analysis of theP V W@SiO2 . . . 16 3.8 Energy dispersive X-ray spectroscopy of theP V W@SiO2. . . 16 3.9 thermal gravimetry analysis (TGA) curves on a 20-400 ºC temperature range
forSiO2shells, PVTM,P V T M@SiO2, andP V W@SiO2 . . . 17 3.10 differential scanning calorimetry (DSC) and TGA curves on a 20-140 ºC tem-
perature range for a) PVW, PVTM and vaseline Vasenol; b)P V T M@SiO2and P V W@SiO2 . . . 18 3.11 Heat storage study a) temperature curves for four different masses of PVTM;
b) temperature curves for PVW, PVTM, glycerin and ricin oil; c) voltage curves for four different masses of PVTM; d) voltage curves for PVW, PVTM, glycerin and ricin oil . . . 20 3.12 a) Effect ofP V W@SiO2 concentration in CA film specular transmittance; b)
Specular transmittance and reflectance of CA films with P V W@SiO2 and P V T M@SiO2; c) Absorptance of CA films withP V W@SiO2andP V T M@SiO2 and d) Diffused transmittance of CA films withP V W@SiO2andP V T M@SiO2 22
L I S T O F F I G U R E S
3.13 Specular transmittance at 500 nm for a) CA films withV O2different set thick- ness and b) MO particles with different concentrations (1, 3.5 and 6 wt%) in CA films . . . 24 3.14 Specular transmittance and reflectance spectra for CA films with different
concentrations (1, 3.5 and 6 wt%) of a)V O2and b)W O3 . . . 25 3.15 Specular transmittance and reflectance a) and absorptance b) forV P V T MSiO2,
3.5 wt%V O2and 10 wt%P V T M@SiO2 . . . 26 A.1 Different types of PCM . . . 38 D.1 DSC and TGA curves on a 20-140 ºC temperature range for a) PVW and
P V W@SiO2; b) PVTM andP V T M@SiO2 . . . 42 D.2 Weight loss vs time for PVTM andP V T M@SiO2samples . . . 42 D.3 DSC-TGA measurements in the 20-400 ºC temperature range for PVTM . . 43 D.4 DSC-TGA measurements in the 20-600 ºC temperature range forSiO2. . . 43 D.5 DSC-TGA measurements in the 20-400 ºC temperature range forP V W@SiO2
. . . 44 D.6 DSC-TGA measurements in the 20-400 ºC temperature rangeP V T M@SiO2 44 D.7 Three cycles of DSC-TGA measurements in the 20-140 ºC range for PVTM 45 D.8 Three cycles of DSC-TGA measurements in the 20-140 ºC range forP V T M@SiO2 45 D.9 One cycles of DSC-TGA measurements in the 20-140 ºC range for PVW . . 46 D.10 Three cycles of DSC-TGA measurements in the 20-140 ºC range for PVW . 46 E.1 Photos of the setup used to perform the thermal storage study . . . 47 E.2 Proportional relation between voltage generated by the Peltier and PCM mass 48 F.1 Photos of the films dried at room temperature a) CA film 200µm and b) PMMA
film 200µm . . . . 49 G.1 Schematic diagram of the process followed to encapsulate pure vaseline (PV)
inside a CA polymeric shell. . . 50 H.1 Single point Raman spectra taken at different regions of the washedP V W@SiO2
andP V T M@SiO2samples . . . 51 I.1 energy dispersive X-ray spectroscopy (EDS) report forP V W@SiO2analysis 52 J.1 Photos of CA films with the 3.5 wt%V O2made with different thicknesses 53 J.2 Photos of CA films with the 3.5 wt%W O3made with different thicknesses 53 I.1 Schematic diagram for the formation of the microencapsulated PV in silica via
in situ emulsion interfacial hydrolysis and polycondensation, adapted from [46]. . . 54
L I S T O F F I G U R E S
II.1 Sedimentation of the tungsten trioxide (W O3) particles in the cellulose acetate 12%wt solution . . . 55
L i s t o f T a b l e s
3.1 Estimated price per square meter of each polymer film . . . 10 3.2 Measured heat emissivity values for films with and without additives . . . 27 B.1 Materials used along the development of this work and their respective speci-
fications . . . 39 C.1 Optimization of CA, PVDF, PCL and PMMA films direct transmittance by
varying film applicator parameters (Set thickness, spreading rate and deposi- tion substrate temperature) and drying temperature . . . 40 D.1 DSC-TGA experimental parameters . . . 41
A c r o n y m s
P V T M@SiO2 PVTM micro-encapsulated in silica x, xi, xii, xv, 13, 15, 17, 18, 19, 22, 25, 26, 27, 28, 29, 42, 44, 45, 51
P V W@SiO2 PVW micro-encapsulated in silica x, xi, xii, 13, 14, 15, 16, 17, 18, 19, 21, 22, 28, 29, 42, 44, 51, 52
V O2 Vanadium dioxide xv, 23, 24, 25, 26, 29
V P V T M@SiO2 10 wt% ofP V T M@SiO2and 3.5 wt% ofV O225, 26, 29 W O3 tungsten trioxide xiii, 23, 24, 25, 55
CA cellulose Acetate vii, viii, x, xi, xii, xiv, 2, 6, 7, 10, 11, 12, 13, 14, 15, 21, 22, 23, 24, 25, 26, 27, 28, 29, 40, 49, 50, 53, 55
DSC differential scanning calorimetry xi, xii, 7, 18, 19, 29, 42, 43, 44, 45, 46 EDS energy dispersive X-ray spectroscopy xii, 7, 15, 28, 52
IR infrared 1, 2
LHS latent heat storage xi, 3, 4
MO metal oxides x, xii, 5, 22, 23, 24, 25, 27, 28, 29, 55 NIR near infrared 5, 7, 25
PCL polycaprolactone xi, xiv, 6, 10, 11, 27, 28, 29, 40
PCM phase change material vii, viii, x, xi, xii, 2, 3, 4, 5, 6, 7, 12, 13, 14, 17, 19, 20, 21, 22, 27, 28, 29, 38, 48, 54
PCMs phase change materials viii
PMMA polymethyl meth-acrylate vii, viii, xi, xii, xiv, 2, 6, 10, 11, 40, 49
AC R O N Y M S
PRC passive radiative cooling 2
PV pure vaseline xii, 6, 7, 12, 13, 14, 17, 19, 21, 28, 50, 54 PVDF poly(vinylidene fluoride) xi, xiv, 6, 10, 11, 27, 29, 40
PVTM pure vaseline from Trade Medic vii, viii, xi, xii, 12, 13, 15, 17, 18, 19, 21, 22, 28, 29, 42, 43, 45
PVW pure vaseline from Wells xi, xii, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 28, 29, 42, 46
SDS sodium dodecyl sulfate 6
SEM scanning electron microscope 7, 15, 16, 18, 28 SHS sensible heat storage 3, 21
ST specular transmittance 8, 10, 11, 21, 22, 24, 25 TCO transparent oxide conductor 1
TCS thermo-chemical storage 3
TEOS tetraethyl orthosilicate 12, 13, 15, 16, 28 TES thermal energy storage 2, 3, 5
TGA thermal gravimetry analysis xi, xii, 7, 17, 18, 19, 28, 42, 43, 44, 45, 46
UV ultraviolet 1, 7
1
I n t r o d u c t i o n
1.1 Motivation
Building sector is responsible for almost 40% of total energy consumption globally and, as a results, considerableCO2 emissions. Modern buildings are currently engaging in a new architectural trend where windows are larger and in higher number than ever before, enhancing aesthetic appeal and providing greater access to natural sunlight [1].
The significant energy consumption in this sector relates with glass poor thermal in- sulation (1.05W m−1K−1) which demands the use of temperature management devices (fans, heaters) to sustain comfortable conditions indoors for longer periods of time [2–4].
Therefore, the development of new technologies for glazing facades is key to reduceCO2 emissions.
This chapter aims to briefly summarize the current state-of-the-art of windows heat shielding technologies, as well as the role of polymers, phase change materials and metal oxides in novel approaches.
1.2 State-of-the-art
Radiative heat transfer from the sun extends across ultraviolet (UV) (below 400 nm), visible (400-700 nm) and infrared (IR) radiation. However, about 95% of total heat gen- eration happens in UV (45%), and IR (55%) wavelength ranges [5]. Thus, to enhance windows thermal performance without harming its optical properties, both UV and IR radiation should be blocked while visible light transparency is sustained [6].
To this date, all commercial and conceptual technologies developed to address this issue can be grouped in two main categories: coatings and fillers. Coating technologies are defined as the deposition of thin layers of materials with radiative heat shielding properties on windows. Highlighting approaches include multilayer coating for glasses, ultra-thin metals, transparent oxide conductor (TCO), TCO composites, thermochromic materials or smart windows and multifunctional heat insulation solar glasses. From the above, the most commonly used are multilayer glasses or low-emissivity coatings which consist in Dielectric/Metal/Dielectric based structures where metal provides heat mirroring properties while dielectrics maximize visible light transmission [7–9].
C H A P T E R 1 . I N T R O D U C T I O N
Fillers are materials enclosed in traditional double-glazing windows that reduce heat transfer due to their poor thermal conductivity or ability to harvest heat. [10] Double glazing windows are the most commercially successful technology, they take advantage of air’s lack of thermal conductivity to dissipate the heat. However, when compared to other alternatives like vacuum glasses, gases or aerogel, this approach is rather in- efficient. [10, 11]. As previously said, heat harvesting materials such as PCM are also strong contenders to face this issue [12] Although the considerable number of alternatives, most of them fail to accomplish simultaneously optimal thermal performance, optical properties, durability, cost, chemical inertness while also being environmental-friendly.
The work aims to produce a low-cost alternative for windows thermal performance enhancement through incorporating microencapsulated PCM and metal oxides in poly- meric films.
1.3 Polymers
One of the first methods for solar energy saving purposes consisted of passive radiative cooling (PRC) systems [13]. This technique exploits Earth’s atmosphere transparency in the IR radiation wavelength range (8 and 13 µm) for releasing heat into outer space and minimize the absorption of incoming atmospheric radiation [14]. Detailed theo- retical studies about the PRC fundamentals can be found in multiple works [15–17].
In the first experiment regarding PRC, Catalanotti et al. [18] investigated polyvinyl- fluoride polymer film as PRC material and although it absorbed IR radiation there were also reported absorption peaks outside the IR radiation wavelength range. Since then, several polymers such as polydimethylsiloxane [19], polyethylene [20], polyvinylchlo- ride [17], poly(4methylpentene) [21], PMMA [22] and polyphenylenoxid [23] were tested as PRC systems. Recently, Mandal et al. [24] tested a porous poly(vinylidene fluoride- cohexafluoropropene) for passive day-time radiative cooling which revealed a sub-ambient cooling of 6 °C and average cooling power of 96W .m−2under solar intensities of 890 and 750 W .m−2, respectively. However, the polymers with the highest cooling perfor- mance usually also have the lowest visible light transmittance, i.e., poor optical per- formance. Nonetheless, since polymers are suitable host materials, particle-embedded polymer structures could well be produced by carefully matching transparent or semi- transparent polymers with solar radiation shielding materials. These composites make use of synergistic effects between the two materials which enable enhanced chemical and mechanical stability and easy application on substrate [25, 26].
Among the promising options are amorphous polymers such as PMMA and CA which produce films with high transparency in the visible light wavelengths [27, 28].
1.4 Phase change materials
thermal energy storage (TES) is a thermal energy storing technology used in heating or cooling applications and power generation [29]. TES systems simultaneously allow
1 . 4 . P H A S E C H A N G E M AT E R I A L S
minimal thermal energy loss and high efficiency in stored thermal energy extraction [30].
Often used in the building sector, this low-cost technology effectively diminishes energy consumption andCO2 emissions. TES systems are grouped into three different types thermal energy storage: sensible heat storage (SHS) which consists in heat storage through heating or cooling a liquid or solid capable of storing energy like water; LHS relies on PCM phase transition temperature to release or absorb heat and thermo-chemical storage (TCS) uses chemical energy to store and release thermal energy [30].
Due to the phase transition, LHS systems, have higher storage capacity, as shown in figure 1.1.
Figure 1.1: Effect of phase change in LHS systems [29].
The working principle consists in two stages: discharging and charging. Thermal energy is absorbed during the discharging process where PCM store heat by transitioning to the liquid state, and thermal energy is released during the charging process as PCM transition back to solid-state, releasing the accumulated heat during the discharging stage [31]. As illustrated in appendix A figure A.1, PCM can be categorized into three distinct groups [16, 17]. For energy harvesting purposes, these are the properties that influence PCM performance: Melting temperature; Thermal conductivity, which should be high to assist in charging of PCM within the limited period; High latent heat per unit volume; High specific heat capacity; Volume changes during phase change process;
Long-term chemical stability; Resistance to corrosion with the container or enclosure;
Non-toxic, non-flammable and non-explosive; No super-cooling or sub-cooling should occur during phase change transition [32–35].
PCM have been targeted as a potential solution for windows lack of efficient thermal performance [36] Hence, in recent years, some works have successfully integrated PCM in glazing units and exhibited promising results regarding optical, thermal and, energy performance [12, 37–42]. PCM incorporation in windows allows solar energy absorption without fully compromising visible light transparency [37].
Usually, PCM are enclosed between the double or multiple glass panes [36]. Li et al. [43] studied the thermal performance of a double-glazing unit containing PCM and realized that when the melting point of the PCM was 32 ºC, the peak temperature and en- ergy consumption reduced respectively by 16.3 ºC and 47.5%. In another work, Shuhong
C H A P T E R 1 . I N T R O D U C T I O N
et al. [44] compared a hollow window and PCM-filled glass window, where PCM was N a2SO4·10H2O, thermal behavior throughout different seasons. On a sunny summer day, peak temperature registered was 10.2 ºC lower than the hollow window which cor- responded to a 39.5% reduction in heat transfer. Furthermore, regarding PCM optical performance, Gowreesunker et al. [37] studied the optical properties of glazing units filled with paraffin RT27 and reported that 90% of the visible light was being transmitted when the PCM was in the liquid state and 40% when in the solid state. Thus, it was transparent in the liquid phase and translucent in the solid state.
Among all PCM, fatty acids outperform in several properties, including suitable melt- ing temperature range, congruent melting, non-toxic, good chemical and thermal stability and low cost. Therefore, this work seeks to exploit the large number of advantageous features that this type of PCM offers.
1.4.1 Phase change materials encapsulation
PCM transition to the liquid phase during the discharging stage is a requirement for LHS systems. This process occurs several times over the course of the PCM lifespan, thus this materials need to be encapsulated to prevent leakage and, as results, material loss. As seen previously stated, PCM are frequently integrated between glass windows in double glazing units. Nevertheless, in order to merge them with polymeric films, another method of containment is required.
Encapsulation is a procedure used for enclosing PCM in a shell. Usually, PCM are micro or nano encapsulated (< 1000µm) to enable larger surface areas and lower leak- ages [45]. The material of the shell provides structural integrity, stability, and controls volume changes during phase transitions [46]. Polymers, silica, metal oxides, and hy- droxides have all been investigated as coating materials, and multiple studies have re- ported several techniques for achieving PCM encapsulation using these shell materials.
The techniques can be grouped in: physical and chemical methods. The primary differ- ence between the two categories is the size of the encapsulated particles, with chemical techniques being able to produce smaller ones [47]. As for shell materials, polymers are cheap, lightweight, mechanically stable, easily processed, flexible, and compatible with PCM [45]. However, they have some disadvantages mainly related with the lack of thermal stability and thermal conductivity [48]. The use of inorganic shells is an at- tractive approach to tackle thermal performance drawbacks as well-performing shells can increase heat transfer rate, thermal stability and be produced with cheaper equip- ment and more eco and user-friendly techniques like oil in water emulsions [46]. Fang et al. [49] and Li et al. [46], for example, successfully microencapsulated paraffin withSiO2 using different techniques, resulting in enhanced thermal stability. However, they are frequently associated with difficulties in processing and overall energy storage capacity reduction [47].
Thus, when choosing the shell material all sorts of variables should be consider and the last decision must be the one that best meets the end goal. Following the selection
1 . 4 . P H A S E C H A N G E M AT E R I A L S
of the shell material it is important to properly adjust the masses of the core and shell materials to obtain the desired thermophysical properties and loading content [45].
1.4.2 Metal oxides
Although all PCM are excellent candidates for TES systems,most of them have low ther- mal conductivity [50, 51]. This implies longer recovery times for TES systems due to insufficient heat transfer with longer discharging/charging rates [51, 52]. Therefore, to increase PCM heat storage efficiency it is required a thermal responsiveness improvement.
Even though heat transfer enhancement is possible due to various types of additives, ad- ditives inherently increase PCM weight, especially certain denser ones, which ends-up in latent heat storage capacity depletion. Thus, lighter materials are more suitable to efficiently optimize PCM thermal storage capacity [53].
MO are a good type of material to act as an additive because besides having a much greater thermal conductivity than PCM they have been extensively used in several in- dustry fields as near infrared (NIR) shielding materials. Especially in the construction sector, whereas the application of these materials in external walls and roofs results in a large reduction on energy consumption due to indoors cooling [54]. MO convert the absorbed NIR light in local heat so, if combined with PCM, the generated heat would be harvested by the PCM which then would enable a faster discharging stage. In addition, MO powders can be easily dispersed in a polymeric film [55]. The synergistic effect be- tween PCM and metal oxides is well evidenced in the work of Zhu et al. [56] where a paraffin wax–Cs0.33W O3composite outperformed a normal glass window by reducing 18 ºC more a model house indoors temperature after one hour of irradiation.
The aim of this work is to develop a stable polymeric film with microencapsulated PCM and MO particles. The films’ composite must transmit at least 50% of the visible light across the spectrum, have an uniform particle distribution over the film’s surface, and a durability longer than 6 months.
2
M a t e r i a l s a n d M e t h o d s
2.1 Materials
The information regarding suppliers, CAS/Product Number and observations of the ma- terials used throughout this work are displayed in appendix B table B.1.
2.2 Polymeric films
Four polymers (cellulose Acetate (CA), polycaprolactone (PCL), poly(vinylidene fluoride) (PVDF) and polymethyl meth-acrylate (PMMA)) were studied to prepare films as follows:
spread of a polymer/solvent solution through the use of a Standard Automatic Film Applicator and Micrometer Film Applicator from TQC Sheen over a glass substrate.These four polymers were chosen based on previous works reporting great optical properties (CA and PMMA) and radiation blocking (PCL and PVDF). While polymeric films without additives solutions were prepared by mixing each selected polymer with a compatible solvent, for polymeric films with additives (metal oxides and/or composites) solutions were prepared as follows: polymer was mixed with 66.7% of total solvent while the remaining portion of the solvent was added into a glass container alongside with the additives. For 30 minutes, the polymer/solvent solution was placed in a stirring plate at 300 rpm and the other in an ultrasonic bath. After that, additive/solvent solution was added drop wise with a plastic pipette into the polymer/solvent one, and then the mixture was stirred overnight.
2.3 PCM encapsulation
Two different encapsulation methods were tested. The first one followed the experimental procedure using silica shells reported by Li et al. [46] Although, n-amyl alcohol was not added and surfactant (cetyltrimethylammonium bromide) and phase change material (PCM) (paraffin) were replaced for sodium dodecyl sulfate (SDS) and pure vaseline (PV), respectively. Apart from this, a not yet tried technique for attempting the encapsulation of PV in CA consisting in a oil/water emulsion was also tested:
2 . 4 . C H A R AC T E R I Z AT I O N T E C H N I Q U E S
1. Added 4 mL of 12 wt% CA, 40 mL of acetone and 2 g of PV into a goblet at 70ºC and waited until the PV was completely melted
2. The mixture was mixed vigorously with a kitchen mixer for 5 minutes
3. While continuously mixing, added 1 mL of deionized water drop wise every 5 minutes until a total of 10 mL
4. The system was stirred for an extra 10 minutes before allowing it to dry overnight at room temperature.
2.4 Characterization techniques
2.4.1 UV-Visible-NIR optical properties
Optical measurements were performed with Jasco V-770 ultraviolet (UV)-Visible-near infrared (NIR) spectrophotometer. A FLH-740 film holder was used to acquire film’s specular transmittance while total transmittance and reflectance were measured using a 60 mm UV-Visible-NIR Integrating Sphere.
2.4.2 Calorimetry and thermogravimetry analysis
The measurements settings for each sample are displayed in appendix E table D.1. ther- mal gravimetry analysis (TGA) was used to study thermal stability and to calculate PCM/shell ratio in silica microcapsules. The melting and solidification temperature and specific heat of PV and micro-encapsulated composites were determined from differential scanning calorimetry (DSC) results.
2.4.3 Morphology
The scanning electron microscope (SEM) measurements were performed to determine the morphology of the composite materials with the Analytical JEOL 7001F FEG-SEM field emission SEM of high resolution while the equipment used for chemical analysis by energy dispersive X-ray spectroscopy (EDS) was Hitachi H8100 TEM equipped with a ThermoNoran light elements EDS detector.
2.4.4 Raman Spectroscopy
Confocal Raman spectrophotometer (Witec Alpha 300 RAS) using a laser of 532 nm and 0.5 mW of power was used to demonstrate the chemical composition of the PCM, microcapsules shells and PCM-shell composites before and after the washing process.
2.4.5 Thermal Emissivity
The films emissivity was determined through a model Frontier FTIR spectrophotometer Perkin Elmer with a PIKE integrated sphere. Emissivity of each sample was determined by the average of three spectra measured in different spots of the film for a 0-50 µm wavelength range.
3
R e s u l t s a n d D i s c u s s i o n
The films deposition on glass windows can be accomplished through several methods (e.g.
drop casting, electrospinning and electrospray). However, our approach aimed testing a more industrially scalable technique for films production by means of a film applicator.
Since the composite films functional properties were to be provided by metal oxides and PCM, the polymeric matrix requirements were excellent transmittance in the visible light wavelength range, great adherence to glass and adequate mechanical properties. Before selecting the polymer for composite films production, a study was performed to assess the influence of the film applicator parameters on the polymeric films optical properties. The optimized parameters were thickness set in the film applicator (set thickness), spreading rate, substrate temperature and drying temperature. In the film applicator parameters optimization study, the films specular transmittance at 500 nm wavelength was used as selection criteria (as shown in appendix C table C.1).
3.1 Optical analysis; experimental considerations
Total transmittance is the amount of incident light that is transmitted across the film. It splits into two modules, specular transmittance (ST) and diffuse transmittance, as given by the following equation [57]:
τtot=τdir+τdif f
The two concepts are well illustrated in figure 3.1 a). ST is the proportion of light for which the exit angle can be predicted from the incident beam entrance angle. Diffuse transmittance occurs when light is scattered after passing through a sample and then further transmitted. As observed in figure 3.1 b), because of the way light travels across diffuse glass (glass on the right-side), they do not allow a clear view. However, on standard glass windows (glass on the left-side) the outlines of the object are accurately detected.
Consequently, films’ specular transmittance results were used in this study when assess- ing light transmitted across the films.
3 . 2 . O P T I C A L P R O P E R T I E S O F P O LY M E R I C F I L M S
Figure 3.1: Example of direct and diffuse light transmission: a) Schematic representa- tion [58] and b) through clear and frosted glass.
3.2 Optical properties of polymeric films
The study of the film applicator optimization exhibited that by drying films at room tem- perature, selecting smaller set thickness, slower spreading rates and higher substrate tem- peratures lead to the production of high transparency films (results found in table C.1). In these circumstances, thickness has great influence on the material’s transparency as thin- ner films have a lower number of defects which means that light diffusion is reduced [59].
Figure 3.2: Specular transmittance curves for CA, PCL, PVDF and PMMA with a) 1000 µm and 100 mm/s; b) 1000µm and 10 mm/s; c) 100 µm and 10 mm/s; d) 100µm and 100 mm/s.
C H A P T E R 3 . R E S U LT S A N D D I S C U S S I O N
However, if set thickness is lower than 100µm, films detachment from the substrate becomes challenging. Therefore, using films with such small thickness would require an on site application on the final substrate. After drying at room temperature, both CA and PMMA films were brittle and had a milky aspect (appendix F figure F.1). Considering that films drying was carried out under uncontrolled ambient conditions, water absorption during drying followed by respective evaporation may have occurred, which led to the formation of air voids in the polymer structure [24].
Figure 3.2 shows the specular transmittance curves in the 200-2500 nm wavelength range for the four polymeric films at a constant drying (60 ºC) and substrate temperature (40 ºC) for different set thicknesses (100 and 1000µm) and spreading rates (10 and 100 mm/s).
One concluded that drying the films in the muffle at 60ºC, under controlled ambient con- ditions leads to a faster and more homogeneous solvent evaporation which is highlighted by the dependence of CA and PMMA films transparency, in the visible range of the spec- trum, on the films drying process. On the other hand, instead of an amorphous structure as CA and PMMA, PCL and PVDF are known to have a semi-crystalline structure [27, 28, 60, 61]. The semi-crystalline polymers have higher crystallinity levels therefore if some of the formed crystallites have a size larger or equal to the visible light wavelength a reduction in film’s transparency is induced due to light scattering phenomena. The specular transmittance for CA and PMMA is, in each graph, higher than 85% at 500 nm whereas those for PCL and PVDF are less than 50% and thus, unfulfilling optical properties requirements, as illustrated in figure 3.2.
3.3 Materials costs
As it was selected an industrially scalable method to manufacture the films, the materials cost is a key parameter to consider for low-cost coatings, susceptible of being installed in glass windows. The material expenses for spreading a square meter of film were estimated and presented in Table 3.1 based on the amount of solution needed to obtain 5x5cm2 films and the supplier’s price tag of each polymer and solvent utilized.
Table 3.1: Estimated price per square meter of each polymer film Polymers PCL 12 wt% CA 12 wt% PMMA 10 wt% PVDF 6.7 wt%
Price (=C/m2) 123.55 136.54 176.51 218.8
In terms of optical properties, the two worst performing polymers, PCL and PVDF are the cheaper and the more expensive, respectively. Within the price range for the four polymers, the production cost for the polymers that attain optical properties requirements (CA and PMMA) is average. Considering that the ST values of CA and PMMA are quite similar, as shown in figure 3.2, it can be argued that CA, which is 30 =C/m2 cheaper than PMMA, is the polymer with the best cost-to-transparency ratio from the entire
3 . 4 . D E G R A DAT I O N T E S T S
set. Overall, the price is slightly higher than the regular commercialized low emissivity glasses for windows applications [62]. Nevertheless, the study did not consider the costs reduction on purchasing and processing large quantities of material.
3.4 Degradation tests
Polymer’s optical properties degradation due to solar radiation was studied during the 3 summer months (June-September) by placing four polymers in touch with the inner side of a glass window oriented south, which means that samples were exposed to the highest solar radiation intensity.
As shown in figures 3.3 a), b) and d), the exposure to sunlight had a negligible effect on CA, PMMA and PVDF optical properties. In the visible region of the spectrum (500 nm), the percentages of specular transmittance for CA, PMMA and PVDF before and after the degradation tests were 90.7%, 90.6%; 91.0%, 91.3%; 66.0%, 67.3%, respectively.
Nonetheless, as observed in figure 3.3 c), PCL ST curve shifted downwards after the exposure to sunlight, suggesting a slight deterioration of the film optical properties. PCL produces films with a low homogeneity, thus unmatched measured regions, before and after the degradation, may also have been a contributing factor for the results seen.
5 0 0 1 0 0 0 1 5 0 0 2 0 0 0 2 5 0 0
0
2 0 4 0 6 0 8 0 1 0 0
B e f o r e e x p o s u r e A f t e r e x p o s u r e
c ) d )
b )
5 0 0 1 0 0 0 1 5 0 0 2 0 0 0 2 5 0 0
0
2 0 4 0 6 0 8 0 1 0 0
S p e c u la r tr a n s m it ta n c e ( % ) W a v e l e n g t h ( n m )
a )
5 0 0 1 0 0 0 1 5 0 0 2 0 0 0 2 5 0 0
0
2 0 4 0 6 0 8 0 1 0 0
5 0 0 1 0 0 0 1 5 0 0 2 0 0 0 2 5 0 0
0
2 0 4 0 6 0 8 0 1 0 0
Figure 3.3: Specular transmittance curves before and after the degradation tests for a) CA; b) PMMA; c) PCL; d) PVDF
C H A P T E R 3 . R E S U LT S A N D D I S C U S S I O N
3.5 Phase change materials
The following section contains the characterizations used to select the shell for the PCM encapsulation. Two fatty acids, pure vaseline from Wells (PVW) and pure vaseline from Trade Medic (PVTM), were studied in-depth as PCM, and two different types of shell materials, CA and silica.
3.5.1 Encapsulation
The encapsulation method study was performed by using PVW as a PCM. The approach to attempt encapsulation in silica shells was based on the work of Li et.al [46] via in situ emulsion interfacial hydrolysis and poly-condensation process of tetraethyl orthosilicate (TEOS), the schematic illustration of the pure vaseline (PV) encapsulation in silica process is displayed in annex I figure I.1.
Figure 3.4: Result of PVW in CA encapsulation attempt a) before drying and b) after drying
PV encapsulation in CA was tested through a simple procedure involving oil/water emulsion (appendix G figure G.1). In both figures 3.4 a) and b) are observed clusters of multiple small white particles.
3.5.1.1 Composition
Raman spectroscopy was performed to look for evidence(s) that could help understanding whether the encapsulation was well succeeded. The strategy consisted first in acquiring the spectral signature of raw materials and then investigate if it was possible to detect PCM in the produced composites. Evidently, this analysis simply demonstrates if the PCM is somewhere within the composites, i.e., not necessarily inside and encapsulated.
3 . 5 . P H A S E C H A N G E M AT E R I A L S
However, if the PCM was outside, washing and filtering would lead to its removal and, in that case, the PCM signal would not be detected by Raman spectroscopy.
Figure 3.5: Raman characteristics signal for a) PVW, TEOS and P V W@SiO2; b) PVTM, TEOS andP V T M@SiO2; c) PVW, CA and microencapsulated PVW in CA
Figure 3.5 a) shows the Raman spectra for PVW, TEOS and PVW micro-encapsulated in silica (P V W@SiO2). According to the literature, the vibrational band at 1086cm−1 can be assigned to C-C stretching of theCH2group, the 1298cm−1band is due to C− H2 twisting, at around 1442 cm−1 it corresponds to CH2 deformation vibrations, the 2849cm−1band belongs to symmetricCH2stretching and the band located at 2883cm−1 is ascribed to asymmetricCH2 stretching mode [63]. In the TEOS spectrum the peaks centered at 561, 782, 1086 and 2927 cm−1 are attributed to Si–O–Si ring vibrational modes, Si–O–Si stretching, Si–O–Si asymmetric vibration andCH3vibrational stretching mode, respectively [64, 65]. In the spectrum ofP V W@SiO2, the peaks assigned to TEOS at 561, 782 and 1091cm−1, and the peaks ascribed to PVW at 1297, 1442 and 2880cm−1 were still detected, which proves thatP V W@SiO2 were formed simply by a physical interaction between PVW andSiO2from TEOS.
Figure 3.5 b) displays the Raman spectra for PVTM, TEOS and PVTM micro-encapsulated in silica (P V T M@SiO2). The PVTM and PVW Raman spectra are nearly equivalent. Con- trary toP V W@SiO2, in theP V T M@SiO2Raman spectra, the shell corresponding peaks are not so intensely exhibited. However, the band at 831 cm−1 can be assigned to the vibrational modes of TEOSSiO4 units and, since the PVTM Raman characteristic sig- nals are well-depicted in theP V T M@SiO2spectrum, one can argue assumed that both materials exist in the composite [64].
The Raman spectra of PVW, CA and the PV micro-encapsulated in CA are displayed
C H A P T E R 3 . R E S U LT S A N D D I S C U S S I O N
in figure 3.5 c). From the CA Raman spectrum, the 652 and 905cm−1peaks are related to C-OH and C-O bonds vibrations. The corresponding Raman signals for the acetyl group at 1372 and 1736 cm−1 are ascribed to the vibration of the carbonyl group (C=O) and symmetric vibrations of the C-H bond while the 2941 cm−1 peak is attributed to C-H stretching due to the presence of cellulose [66, 67]. In the composite spectra, the peaks assigned to CA at 652, 913, 1736, and 2941cm−1, and the characteristic Raman signals for PVW at 1298, 1442 and 2721 cm−1were also detected. Thus, similar to the results seen before,it seems to be a physical bonding between CA and PVW as intended.
As previously stated, the composite samples were subjected to a washing process and subsequently re-analyzed. The procedure was carried out as follows:
1. A small mass (300 mg) of the samples was mixed with 5 mL of water and two drops of detergent at 70 ºC and stirred for 30 minutes;
2. The solution was then filtered using a filter with 20µm pore size;
3. The steps 1 and 2 were repeated 2 times with the not filtered material;
4. The remaining material was left to dry overnight at room temperature.
At a 70 ºC temperature, PV is completely melted, so this procedure intended to deter- mine if the encapsulation was achieved, that is discard the hypothesis of PCM hanging outside of the shell, i.e., not encapsulated.
Figure 3.6: Raman spectra of micro-encapsulated PCM in different shells (CA orSiO2) after the washing process
The Raman spectra obtained after the washing process are shown in figure 3.6. Figure 3.6 a)-I and a)-II forP V W@SiO2particles, figure 3.6 b) for CA and PVW micro-capsules
3 . 5 . P H A S E C H A N G E M AT E R I A L S
and figure 3.6 c)-I and c)-II forP V T M@SiO2particles. The washed CA microcapsules, as seen in figure 3.6 b), do not exhibit any of the PVW characteristic vibrational bands leading to the conclusion that it was probably removed. By acknowledging that the encapsulation of the PVW in CA was unsuccessful in the first place, it can be stated that the results measured in figure 3.5 c) were just PVW and CA clusters.
The lack of composition homogeneity in bothP V W@SiO2andP V T M@SiO2follow- ing the washing process required two different Raman spectra to entirely represent the samples composition (consult appendix H figure H.1). As seen figure 3.6 a)-II and figure 3.6 c)-I, at least for some particles, the Raman measurements were similar to the ones per- formed before the washing process thus, the effect on the composition was insignificant.
Nevertheless, figures 3.6 a)-I and c)-II showed that Raman spectra of both composites changed in some locations. As figure 3.6 c)-II displays,SiO2vibrational bands are more pronounced over the PVTM ones which is the exact opposite situation detected in figure 3.5 b). One could argue that a few portion of vaseline attached to the shell external re- gions might have been shadowing the detection ofSiO2Raman signals and, the washing process led to its removal. However, vaseline was not fully eliminated from the composite as the 1444cm−1and 2882cm−1peaks illustrate suggesting that some vaseline resisted to the washing process perhaps due to successful encapsulation. TEOS vibrational bands seem to have disappeared from theP V W@SiO2spectrum as depicted in figure 3.6 a)-I.
A possible explanation is the formation of PVW agglomerates due to vaseline cooling and further solidification during filtration. This is a process that does not happen immedi- ately thus, PVW may have had enough time to transition to the solid state and to form agglomerates larger than the filter’s pore size.
Among all encapsulation attempts, different unexpected scenarios can be consider such as undetected PVW in the exterior regions of the shells, porous shells and general lack of encapsulation efficiency.
3.5.1.2 Morphology
The SEM and EDS measurements performed to theP V W@SiO2are displayed in figures 3.7 and 3.8, respectively. The as preparedP V W@SiO2 are shown in figure 3.7 a), the white color of the samples is given by theSiO2shell [49]. An amplified top view of the P V W@SiO2is displayed in figure 3.7 b), the composites seem to be composed of quasi- spherical particles with diameters ranging from 100 to 1000 µm. The coarse surfaces of the composite observed in figure 3.7 c) can be a result of a fastN H3·H2Omolecules diffusion and poly-condensation of TEOS in the surface of the PVW that preventedSiO2 particles from forming a complete and smooth shell [46]. Furthermore, from the close- up shown in figure 3.7 d), the samples appear to have a porous aspect. This lack of cohesiveness may reinforce the approach developed in the Raman results that vaseline could be removed, after the washing process, due to shell defects. The EDS measurement support the Raman spectroscopy results by confirming the presence of both Si and O leading to the underlying conclusion thatSiO2was formed during the poly-condensation
C H A P T E R 3 . R E S U LT S A N D D I S C U S S I O N
process of TEOS. Although a small concentration of carbon was also detected (consult appendix I figure I.1), it cannot be safely assumed that it was on account of PVW since the substrate used for SEM measurements is carbon tape.
Figure 3.7: Scanning electronic microscopy analysis of theP V W@SiO2
Figure 3.8: Energy dispersive X-ray spectroscopy of theP V W@SiO2
3 . 5 . P H A S E C H A N G E M AT E R I A L S
3.5.2 Phase change material study
The weight loss ofSiO2, PVTM,P V T M@SiO2andP V W@SiO2were studied in the 20- 400 ºC temperature range through TGA measurements, the results are displayed in figure 3.9.
1 0 0 2 0 0 3 0 0 4 0 0
0
2 0 4 0 6 0 8 0 1 0 0
M a s s ( % )
T e m p e r a t u r e ( º C )
P V T M
P V W @ S i O 2 P V T M @ S i O 2 S i O 2
Figure 3.9: TGA curves on a 20-400 ºC temperature range for SiO2 shells, PVTM, P V T M@SiO2, andP V W@SiO2
ForSiO2, there is a small weight loss of 3% before temperature reaches to 100 ºC, which might be related with evaporation of residual solvent or absorbed water. Further- more, after reaching 400 ºC the total weight loss is nearly 10%. In PVTM TGA measure- ment, the large step in the 210-400 ºC range corresponds to a 86% mass loss and it may be ascribed to the pure vaseline (PV) evaporation. Additionally, the mass loss seems neg- ligible for both PV up to the maximum temperature (50 ºC) reached in windows coatings applications. In the TGA curve forP V W@SiO2, the main weight loss also takes place in the 200-400 ºC temperature range, and it may be assigned to the evaporation of the PVW, that is incorporated in the composite, contributing to roughly a 60% loss of total weight. Likewise, theP V T M@SiO2curve also displays one significant step occuring in the 200-400 ºC range, possibly related to the removal of PVTM, resulting in a 45% weight loss. In theP V T M@SiO2sample, PVTM evaporation begins a few degrees earlier than the evaporation of the only PVTM sample. This might be correlated to an increase in PCM thermal conductivity due to silica shells [47].
Although, the lack of solid evidence to yet confirm the encapsulation of vaseline in
C H A P T E R 3 . R E S U LT S A N D D I S C U S S I O N
SiO2 shells, these results are in accordance with the ones obtained from Raman spec- troscopy as the presence of vaseline in the composite systems is almost undeniable. Other detail worth considering is the speed at which the vaseline is evaporated. In appendix D figure D.2, it is displayed the weight loss during time curves for PVTM andP V T M@SiO2. Here, the slopes highlight that evaporation of the only PVTM sample is much faster than the one incorporated in the P V T M@SiO2 sample. One could be argued that if PVTM was just on the out parts of theSiO2shells the slopes of the two sample should be similar.
Therefore, theSiO2shell is somehow slowing down the removal of PVTM. Furthermore, hypothetically assuming that vaselines are encapsulated, the weight loss due to evapora- tion may be linked to the departure of vaseline through existing pores in theSiO2shells which agrees with the obtained SEM images.
In addition, TGA measurements also allow estimating the load content of pure vase- line on the composites. Loading content is the total weight percentage of pure vaseline in the composite thus, considering that during the TGA measurements the loss due to pure vaseline was 60% and 45% forP V W@SiO2andP V T M@SiO2respectively, those values represent the loading contents of each sample.
In figure 3.10 a) are shown the DSC and TGA curves for PVTM and PVW samples for the 20-140ºC temperature interval.
Figure 3.10: DSC and TGA curves on a 20-140 ºC temperature range for a) PVW, PVTM and vaseline Vasenol; b)P V T M@SiO2andP V W@SiO2
3 . 5 . P H A S E C H A N G E M AT E R I A L S
As the TGA measurements highlighted, weight loss in both vaselines within that spe- cific temperature range is negligible. The thermal stability of the PVTM is evidenced by the TGA measurements thus, even after two further cycles (dash curves), the mass percentage remained close to 100% and consistency in the curves within the cycles. Re- garding the DSC results, PVTM has one energetic endothermic peak at 40 ºC which can be ascribed to its melting point. Although discrete, a small exothermic peak at around 55 ºC may be attributed to the PVTM solidification. Likewise, for PVW the endothermic and exothermic peak also appear to be attributable to melting temperature at 48 ºC and solidification temperature at 47 ºC, respectively. In addition, it was also depicted the DSC curve for vaseline Vasenol, a fatty acid from a previous study. In comparison to the other two PCM, Vasenol melting (47 ºC) and solidification (46.5 ºC) peaks are both well distinguished from the plateaus. The three materials have in general a similar thermal behavior, the differences between melting temperatures, specially seen in PVTM, may be related to the length of carbon chains within the chemical composition of each fatty acid [68].
The DSC and TGA curves forP V T M@SiO2andP V W@SiO2are displayed in figure 3.10 b), three cycles were performed for both samples, with the dash curves representing the last two cycles for each material. The TGA measurements show that initially there was a small weight loss of 2% forP V T M@SiO2and 4% forP V W@SiO2suggesting that the removal of residual solvent and/or evaporation of adsorbed water. Besides, as subse- quent cycles demonstrate, the mass percentage remains similar advocating that samples remain thermally stable in the 20-140 ºC range. TheP V T M@SiO2DSC curves have two endothermic peaks and one exothermic, the more intense peak occurs at 39 ºC and it might correspond to melting of the pure vaseline incorporated in the composite. None of the peaks could be assigned to the expected PV solidification temperature, and this is partially in line with the absence of a well-pronounced peak for the equivalent phase tran- sition in the PVTM DSC curve. Furthermore, the remaining peaks seem to be related to aSiO2reversible solid-solid phase transition [69]. In the case ofP V W@SiO2, the peaks found were on account of the shell material thus, none matched the melting or solidifi- cation temperature of PVW. The large difference in DSC signal intensity between PVW andP V W@SiO2 may be one of the root causes for the missing phase transition peaks as well as the lack of adjustment in the selected settings for the experimental procedure (appendix D figure D.1 b)).
Specific heat capacity for PVW, PVTM,P V W@SiO2 andP V T M@SiO2 were deter- mined to be 1.56, 1.41, 1.42 and 1.35J.K−1Kg−1, respectively. These results are coherent in terms of order of magnitude with the ones seen in literature [70]. SinceSiO2specific heat capacity is 0.680 J.K−1Kg−1 [71], the pure vaselines specific heat capacity was ex- pected to be higher than the respective composites, as confirmed by the obtained values.
Additionally, results also depicted that PVW is able to store more heat than PVTM which also applies toP V W@SiO2as compared toP V T M@SiO2.
C H A P T E R 3 . R E S U LT S A N D D I S C U S S I O N
3.5.2.1 Thermal storage study
PCM are well-known for their intrinsic ability to store energy when exposed to a heat source. To confirm this effect, different masses of PVW were placed on top of a Peltier’s surface and subjected to a radiative heat source, an incandescent lamp (JC Type G4 Halo- gen Bulb 12 V 20 W), for 30 minutes while collecting temperature and Peltier’s voltage using a Datalogger (see appendix E figure E.1). Furthermore, the measurements were performed under a covered setup to minimize outside interference. Thus, the Peltier’s attribute to convert thermal into electrical energy was used to test if voltage values were going to increase due to PCM ability to accumulate heat.
0 2 0 4 0 6 0
2 5 3 0 3 5 4 0 4 5 5 0 5 5
6 0 P e l t i e r
1 . 5 g 2 . 5 g 3 . 5 g 4 . 5 g
Voltage (V)
d ) c )
b )
0 2 0 4 0 6 0 8 0 1 0 0 1 2 0
2 5 3 0 3 5 4 0 4 5 5 0 5 5 6 0 6 5
P e l t i e r P V W P V T M G l i c e r i n a Ó l e o R i c i n o
Temperature (ºC)
T i m e ( m i n ) a )
0 2 0 4 0 6 0
0 . 0 0 0 . 0 2 0 . 0 4 0 . 0 6 0 . 0 8 0 . 1 0
P e l t i e r 1 . 5 g 2 . 5 g 3 . 5 g 4 . 5 g
0 2 0 4 0 6 0 8 0 1 0 0 1 2 0
- 0 . 0 2 0 . 0 0 0 . 0 2 0 . 0 4 0 . 0 6
P e l t i e r P V W P V T M G l i c e r i n a Ó l e o R i c i n o
Figure 3.11: Heat storage study a) temperature curves for four different masses of PVTM;
b) temperature curves for PVW, PVTM, glycerin and ricin oil; c) voltage curves for four different masses of PVTM; d) voltage curves for PVW, PVTM, glycerin and ricin oil
Temperature curves showed that PCM placed on top of the Peltier module had a significant impact temperature since after 30 minutes of heat exposure, a mass of 1.5 g was 6 ºC higher than that measured on top of the Peltier and without PCM. Although 2.5 g and 3.5 g samples seem to achieve similar temperatures, it could stated that PCM mass increase leads to temperature rise as depicted in figure 3.11 a). The voltage curves in figure 3.11 c) are in line with previous results, as samples with PCM conducted to higher
3 . 6 . P H A S E C H A N G E M AT E R I A L S A N D P O LY M E R C O M P O S I T E
temperature differences and thus, to higher voltage values, which means that a greater amount of heat was reaching Peltier’s surface. Additionally, one could argue the existence of a proportional relation between voltage and PCM mass (appendix E figure E.2).
Apart from PVW, ricin oil, glycerin, and PVTM, three other fatty acids, were also tested with the described setup but for 1 hour and then cooled for another 1 hour after turning off the heat supply. Accordingly with figure 3.11 b), after 60 minutes of heat exposure the temperature reached for the ricin oil sample was 61 ºC, 3 and 5 ºC higher than glycerin and both pure vaselines, respectively. Figure 3.11 d) shows the lack of significant differences within the fatty acids voltage curves. However, in comparison with the test with no PCM, the voltage measured in fatty acids samples was three times higher. In contrast with pure vaselines, ricin oil and glycerin are in liquid form at room temperature thus, the lack of phase transition in the temperature range investigated might explain why the temperatures were slightly higher for these two samples. During the experiment, both pure vaselines need to use part of the thermal energy that is being supplied to fuse and during this process they are unable to raise their temperature.
These characteristics categorize ricin oil and glycerin as sensible heat storage (SHS) systems where energy stored depends on specific heat, change in temperature and mass of the solid or liquid being heated. In addition to the above, pure vaselines, as LHS systems, are capable of storing heat at constant temperature allowing a higher energy density per unit mass per unit volume [29].
3.6 Phase change materials and polymer composite
Firstly, to analyze the influence of the PV composites on CA films optical properties and to reach the optimal concentration, it was prepared five CA 12 wt% polymeric solutions with differentP V W@SiO2concentrations (1, 2.5, 6, 10, 15 wt%). Furthermore, it was spread 3 mL of each solution using the automatic film applicator with a set thickness, spreading rate, substrate temperature and drying temperature set to the following values:
1000µm, 10 mm/s, 40 ºC and 60 ºC, respectively.
Figure 3.12 a) depicts an empiric relation between specular transmittance (ST) at a 500 nm wavelength and concentration ofP V W@SiO2in CA solution (conc) given by the following equation:
ST(%) =−2.17conc(wt%) + 93.16
As the negative slope suggests, increasing P V W@SiO2 concentration will reduce the amount of light being transmitted across the film. For the first three concentrations (1 to 6 wt%), although small, there is a slight influence of the microcapsules in the film’s optical properties with a 5% difference between the maximum (1 wt%) and minimum (6 wt%) concentration values in the considered interval. However, once the fourth value is reached CA film’s ST suffer a significant hit, dropping from 84% at a 6 wt% concentration to 74 wt% at 10 wt%. Further, an even more pronounced decrease in transparency is measured in the films with 15 wt% ofP V W@SiO2with only 58% of the light was getting
C H A P T E R 3 . R E S U LT S A N D D I S C U S S I O N
Figure 3.12: a) Effect ofP V W@SiO2concentration in CA film specular transmittance; b) Specular transmittance and reflectance of CA films withP V W@SiO2andP V T M@SiO2; c) Absorptance of CA films withP V W@SiO2andP V T M@SiO2and d) Diffused trans- mittance of CA films withP V W@SiO2andP V T M@SiO2
across the film. Even though that it is still above the 50% transparency threshold, the further addition of metal oxides powders will also significantly reduce the CA films ST. Therefore, metal oxides (MO) would have to be employed in smaller quantities, causing an even harder triggering of the desired synergistic interaction between the MO and micro-encapsulated PCM. Nonetheless, since PCM mass increases energy storage capacity, concentration should not be defined only based on optical characteristics. As a result, CA film with 10 wt% was selected for further testing given that it has the second highest amount of mass and a transparency well above the defined limit. In figure 3.12 b), it is displayed a comparison between the specular transmittance and reflectance curves in the 200-2000 nm wavelength interval for the CA films with 10 wt% of PVTM and PVW. Once could argue that, from an optical properties perspective, both films are rather equivalent. Figure 3.12 c) shows the absorptance of both composites. Absorptance corresponds to the fraction of the radiant light that is absorbed by the film and converted in other energy form, the curves displayed were calculated from the following equation:
α+ρ+τ= 1
