Improvement of thermal properties of pigmented acrylic resin using

Improvement of thermal properties of pigmented acrylic resin using silica aerogel Sara Karami,1 Siamak Motahari

,1 Malihe Pishvaei,2 Navid Eskandari1

1

School of Chemical Engineering, Faculty of Engineering, University of Tehran, P.O. Box 11365/4563, Tehran, Iran Department of Resins and additives, Institute for Color Science and Technology, P.O. Box 16765/654, Tehran, Iran

2

Correspondence to: S. Motahari (E - mail: [email protected])

ABSTRACT: In this study, two different types of acrylic resins were synthesized through emulsion polymerization. The first category of acrylic resins contained methyl methacrylate, 2-ethylhexyl acrylate, and acrylic acid monomers. The second composition had an additional monomer named acrylamide (AAm). The kinetic behavior of polymerization reaction was investigated. The results showed that the presence of the AAm monomer increased the monomer conversion (>90% in the first 10 min) and the rate of polymerization. Furthermore, the latexes were characterized by Dynamic Light Scattering (DLS) Fourier Transform Infrared, and differential scanning calorimeter analysis. To study the effect of silica aerogel as a thermal barrier additive for acrylic resins, samples were mixed with silica aerogel using sodium dodecyl sulfate as a surfactant. In the next step, the above-mentioned resins were used to make white acrylic-based paints. The heat transfer measurements revealed that the thermal insulation properties were not affected by the composition of the resin. On the other hand, the use of AAm monomer increased the paint adhesion properties and helped the resin to receive more aerogel (up C 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45640. to 5 wt %) which in turn decreased the heat loss of the painted wall. V

KEYWORDS: acrylic resin; aerogel; emulsion polymerization; synthesis; thermal insulation

Received 11 March 2017; accepted 24 July 2017 DOI: 10.1002/app.45640 INTRODUCTION

Nowadays, many investigations are focused on the energy saving. Ceilings and roofs are the main sources of energy dissipation in the buildings; therefore, suitable thermal insulator coatings for roofs and ceilings are desired. A number of literature studies have included research and evaluation reports about the thermal insulation materials. Hun et al.1 worked on different lightweight roofing structures and its effect on the space cooling load. The dynamic modeling of the heat transfer is used, and the results showed that the efficiency of the polyurethane thermal insulator was more than the glass wool in the roofing systems. Furthermore, the brighter thermal insulator led to a lower space cooling load. Yu et al.2 focused on the optimum insulation thickness of residential roof with different insulation materials. Among the insulation materials, the optimum insulation thicknesses from high to low in turn were expanded polystyrene (0.141–0.187 m), extruded polystyrene (0.095–0.126 m), foamed polyurethane (0.072– 0.096 m), and foamed poly(vinyl chloride) (0.065–0.088 m). In another study, Yew et al.3 proved that applying a thermal insulation coating (TIC) on the roof of the attic could reduce the attic temperature of the building from 42.4 to 29.6 8C. The TIC was formulated using titanium dioxide pigment with chicken

eggshell waste as a bio-filler bound together by a polyurethane resin binder. Producing the thermal insulator paints with low thickness and high efficiency is a field of interest, especially in the construction industries. Generally, paints are made using the binders, pigments, solvents, and additives. Acrylic resins are well-known resins in the paint and coating industry. The pigmented acrylic resins have an important application in the surface-coating materials such as the gloss latex paints. Moreover, nitrocellulose in car finishes and alkyd-melamine systems in autobody enamels have been replaced by the acrylic resins.4 The solvent-based and water-based resins are two major groups of the acrylic resins. The volatile organic compounds in the paint’s formulation cause environmental contaminations,5 so currently, the water-based acrylic resins are desired due to their eco-friendly nature.6,7 The water-based resins are widely produced through emulsion polymerization.8 Many different parameters influence the mechanical and physical properties of the synthesized acrylic copolymers through emulsion polymerization. The type and proportion of the monomers,9,10 the presence of some special co-monomers,11,12 and also the use of polymeric and

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J. APPL. POLYM. SCI. 2017, DOI: 10.1002/APP.45640

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Table I. Recipe used for Preparation of Acrylic Resins via Emulsion Method Components

Function

A1 (g)

A2 (g)

2-ethylhexyl acrylate

Monomer

120.63

120.63

Methyl methacrylate

Monomer

79.73

79.73

Acrylic acid

Monomer

4.10

4.10

Acrylamide

Monomer

—

2.03

Kenon 40

Nonionic emulsifier

6.14

6.14

SDS

Ionic emulsifier

3.07

3.07

Potassium persulphate

Initiator

0.50

0.50

Sodium bicarbonate

Buffer

0.40

0.40

Water

Solvent

275.00

275.00

nonpolymeric emulsifiers13,14 can be mentioned as influential parameters. Thus, the monomer and additive selection for polymerization are prominent. The acrylic polymers and copolymers possess unique combination of the properties such as excellent weather ability, great resistance to abrasion, water and cracks and also low thermal conductivity which is a key parameter for thermal insulators. Some investigations have focused on improving the mechanical properties of the acrylic resins with various additives such as silica nano-particles,15 fly ash,16 glass fiber,17 and woven electrical glass fibers (E-glass fiber).18 Delmonico19 fabricated an insulating paint for interior and exterior of buildings including hollow glass extender or glass microspheres. These particles were uniformly dispersed in an acrylic paint formulation by high speed mixing. This coating showed very good thermal insulation properties and enhanced energy saving in the buildings. Comite et al.20 prepared an inorganic–organic composite coating as a thermal barrier coating. In this article, an acrylic-based waterborne coating (base matrix) and alumina microparticles were used. The results revealed that Al2O3 as pigment improved the barrier properties and the thermal resistance of the waterborne coatings even at a low paint thickness. Aerogel ceramics present extraordinary properties such as high specific area, low density, low di-electric constant, and low thermal conductivity.21 These unique features have made silica aerogel an appealing material for a wide variety of applications, especially as thermal insulations.22 The native silica aerogels are fragile, brittle, and crumbling into many pieces even at low stresses.23 So that is enough to keep them out of the practical and structural applications like insulation fields. Mixing aerogels with polymers to make composites is a way to enhance the mechanical behavior of aerogels without sacrificing their porous structure.24,25 Perez26 investigated aerogel blankets and confirmed that these materials are good for insulating the outside walls of buildings. Preparation of silica aerogel/epoxy composite, as an efficient thermal insulation material, was investigated by Zhao et al.27 through doping silica aerogel of different sizes into epoxy resin followed by the thermo-curing process. The results showed that the thermal conductivity of the composite was reduced to 0.105 W m21 k21 in the presence of 60 wt % silica aerogel. Ge et al.28 investigated the fabrication of silica aerogel/ epoxy composite through dry mixing hydrophobic aerogels with


epoxy powders as a binder using the heat pressing method. The results showed the composites had low density (0.72–0.25 g cm23) and low thermal conductivity (0.11–0.044 W m21 k21) in a wide temperature range. Chang et al.29 prepared a thermal insulation composite by blending silica aerogel into polyurethane. The silica aerogel/polyurethane composites possessed a lower thermal conductivity with a suitable composition 35/65 (v/v) than polyurethane at room temperature. In this research, the water-based pigmented acrylic resins were mixed with silica aerogel to obtain an efficient compound with combined properties such as eco-friendly nature, excellent thermal insulation, and crack resistance, which are the main parameters for the roof coatings. In our work, utilization of 5% of silica aerogel in the formulation of the paints decreased the heat loss and the temperature of the painted wall by 25% and 7– 8 8C, respectively. EXPERIMENTAL

Materials Hydrophobic silica aerogel which was based on tetraethoxy methyl silane was purchased from Nanosav Co. (Iran) and used as received. The surface area, density, and pore size of the silica aerogel were analyzed and found to be 680 m2 g21, 60 kg m23, and 20 nm, respectively. To prepare acrylic resin, two kinds of emulsifiers were used; first one was ionic, sodium dodecyl sulfate (SDS) (288.4 g mol21, NaC12H25SO4) and the second one was poly(ethylene glycol) ether (1980 g mol21, Kenon 40); both were purchased from Kimiagarane-emruz Co. (Iran). Potassium persulphate (K2S2O8) was used as the initiator and supplied by Merck Co. (Germany). Acrylic acid and acrylamide (AAm) were obtained from Merck Co. (Germany), and 2-ethylhexyl acrylate and methyl methacrylate were supplied by Daejung Co. (Korea). To synthesize pigmented acrylic resin, BYK-190 (BYK Chemie GmbH, Germany), titanium dioxide (TiO2) (Huntsman Corporation, USA), BYK-022 (BYK Chemie GmbH, Germany), and TH 110 (Simab Resin manufacturing company, Iran) were used as wetting agent, pigment, de-foaming agent, and thickener, respectively. Sample Preparation Acrylic Resin Preparation. Two types of acrylic resin were prepared via emulsion polymerization employing different contents and types of the monomers. Table I presents the components of

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Table II. Recipe used for Preparation of Pigmented Acrylic Resins Components

Function

Weight (g)

Water

Solvent

52.00

BYK-190

Wetting agent

3.60

TiO2

Pigment

206.00

BYK-022

De-foaming agent

2.50

TH 110

Thickener

2.60

Synthesized resins

Binder

600.00

acrylic resins A1 and A2 and also their amounts in grams. The emulsion polymerization were carried out using a double glazed and five-necked glass reactor equipped with a stirrer, a condenser, a thermometer, a dropping funnel, and a pipe for purging the nitrogen into it to remove oxygen. The water was initially charged in to the reactor, which then purged with nitrogen for 15 min to remove oxygen. All components, but the initiator, were then charged in to the reactor through the dropping funnel. After reaching a predetermined temperature (75 8C), the initiator was added and the mixture was stirred to dissolve the initiator. The total conversion of polymerization was calculated by the gravimetric determination of the solid content as explained later in the Polymerization Conversion section. Acrylic Resin/Silica Aerogel Preparation. One of the influential part of this study was achieving a proper mixture of the waterbased acrylic resin and hydrophobic silica aerogel. In this investigation, two methods of mixing were tried. In the first method, the temperature was increased to decrease the viscosity of the resin, and then silica aerogels were added gradually to the resin. A magnetic stirrer was used to mix the mixture of resin and aerogel. Due to the hydrophobic characteristic of silica aerogels and poor wettability of them by the resin, the aerogel particles tend to move to the surface of the mixture. Therefore, by this method, a limited amount of silica aerogel (up to 1 wt %) could be added to the mixture. The second method which was tried to obtain a suitable compatibility between the resin and aerogel was using SDS as a surfactant. SDS (0.5 wt %) was initially added to the resin and agitated well. The silica aerogel was then gradually added to the mixture. The analysis of the end mixture showed that the use of the surfactant produced an appropriate and compatible compound without any phase separation, even at high amounts of silica aerogel. Using this method, up to 5 wt % of silica aerogel was added to the resin. Therefore, the second technique, that is, using surfactant, was selected in the next step. Pigmented Acrylic Resin Preparation. Table II shows the components used to produce the paints. Primarily, the wetting and thickener agents were mixed in water, and then the de-foaming agent was added. Finally, titanium dioxide was gradually charged into the mixture. To disperse the titanium dioxide pigments, the system was stirred vigorously (800 rpm) for 2 h. Finally, the resin was added to the mixture and was stirred at a lower rate (100 rpm) for 10 min.


Characterization and Instrumentation The Fourier Transform Infrared (FTIR) spectrometry (Thermo Nicolet) was adopted to investigate the functional groups of the acrylic resins. A differential scanning calorimeter (DSC, Maia200 F3, Netzsch, Germany) was used to determine the glass transition temperature (Tg) of the synthesized acrylic resins. The samples were heated to 100 8C and left at that temperature for 3 min then cooled to 250 8C under the nitrogen atmosphere and reheated to 100 8C. The rate of heating was 10 8C min21. The particle size and particle size distribution of synthesized resins were measured by DLS Mastersizer 2000 (Malvern, UK). The Stormer-Type Viscometer (Wingtai, China) (ASTM D562) was used to examine the viscosities of samples. The morphology of specimens was observed by Philips XL30 scanning electronic microscopy (Eindhoven, Netherlands). To evaluate the thermal degradation of compounds, thermogravimetric analysis [TGA, Perkin Elmer-diamond (SII) Pyrif] was carried out on the samples. To investigate the thermal barrier property of the paint, a home-made setup was used. The setup, as presented in Figure 1, was a cubic aluminum container 20 3 20 3 20 cm. The external surface of the bottom of the cube was covered by 1 in. of polyurethane foam with a thermal conductivity of 0.119 W m21 C21 as presented in the producer data sheet. Thus, the heat loss from the bottom of the cube could be neglected so it is assumed that the heat was only transferred from the walls of cubic container. The tank was also equipped with an electric heater. The acrylic resin paints with a thickness of 120 lm were coated on one of the external surfaces of the cube and were dried at 25 8C for 7 days. The thicknesses of the films were measured by Quanix 8500 Bisec modular system (Automation Koln Co.) based on ASTM D3363–00. Then the tank was filled with water and the electric heater and a thermocouple was placed in it. The water temperature inside the tank was fixed at 65 8C, and the temperature of the external surface of the tank, with and without the paint, was measured. The difference between the temperature of the painted and not painted surfaces could indicate the thermal barrier property of the paint based on the calculations presented in the Heat Transfer section. To study the mechanical properties of the paint, a 120-lm layer of the paint was coated on a galvanized iron foil. The hardness

Figure 1. Schematic diagram of home-made setup for heat transfer analysis.

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Table III. Rate of Polymerization Reaction for A1 and A2 Samples Sample

dX dt

A1 A2

(min21)

[M]0 (mol L21)

Rp (mol min–1 L21)

1.607

5.480

8.810

9.018

5.600

50.050

Solid content weight of dried acrylic resin 2 weight of hydroquinone 5 (2) weight of wet acrylic resin

Overall conversion of polymerization reaction was calculated using Overall conversion 5

Figure 2. Monomer conversion–time curves for A1 and A2 samples.

of the film was measured according to ASTM D3366 by a set of special pencils. The water resistance was examined according to ASTM D870–02. The abrasion resistance was checked based on ASTM D4060 at 22 8C. The Pull-off test was also conducted according to ASTM D4541. Furthermore, the cracks formation on the surfaces of the paint film was inspected according ASTM D522. THEORETICAL PROCEDURES

Monomer Fraction Estimation To ensure the film formation of the paint in an atmospheric condition, the final glass transition temperature of the resin was designed to be 25 8C, so the monomer compositions were calculated based on that. The fox equation was used to calculate the monomer fractions30: 1 X wi 5 (1) Tgco Tgi where Tgco is the glass transition temperature of composite which has been considered to be 25 8C, Tgi is the glass transition temperature of monomer i, and wi the is the weight fraction of monomer i.

experimental solid fraction theoretical solid fraction

(3)

where the theoretical solid content was considered 45% and was measured from Theoretical solid content weight of solids in the system 5 weight of all solids in the system1weight of water

(4)

RESULTS AND DISCUSSION

Acrylic Resin Tests Kinetics of Emulsion Polymerization. Figure 2 presents the monomer conversion of A1 and A2 samples versus time. For A2 sample, in the first 10 min, 90% monomer conversion occurs but A1 sample needs more time (300 min) to reach the same level of conversion. The polymerization rate31 can be calculated from Rp 5½M 0

dX dt

(5)

where Rp is the rate of polymerization, [M]0 is the initial monomer concentration based on the water, X is the monomer conversion which is calculated according to eq. (3), dx/dt is the least squares-best-fitted slope of the linear portion of the monomer conversion (X) versus time curve.32

Finally, the theoretical and experimental glass transition temperatures, which was measured by DSC test, were compared. To calculate the percentage of the other materials used in the formula, the theoretical solid content was considered 45%. Table I shows the materials and their quantities in each formula. Polymerization Conversion The solid content of synthesized resins was evaluated by sampling in different time periods of the polymerization reaction. To stop the polymerization reaction, samples were placed in an aluminum container which contained hydroquinone. The containers were weighed and put in an oven at 95 8C for 24 h. To remove the unreacted reactants and oligomers which were formed during the synthesis, samples were washed with distilled water and were placed again in the oven in the same condition. The solid content computation was provided using


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Figure 3. Particle size distributions of A1 and A2 samples.


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Figure 4. FTIR spectrometry of A1 sample. [Color figure can be viewed at wileyonlinelibrary.com]

Based on the results in Table III, it can be concluded that the presence of AAm monomer in the resin’s composition increases the polymerization rate due to its high activity.33 The ANH2 groups in the AAm’s structure accelerates the free radical production and consequently the rate of the polymerization reaction increases. DLS. The particle size and the particle size distribution of the resins are two key parameters in the rheological, optical, and mechanical properties and also the viscosity of resins during the film formation.34 For a precise comparison between the samples, the particle size distributions of the samples should be the same. These data can be provided by the DLS test. Figure 3 shows the particle size distributions of A1 and A2 samples. It can be seen that the particle size distributions of both samples are very close together. FTIR. The FTIR spectrometry of A1 and A2 samples are displayed in Figures 4 and 5. The absorption peaks of A1 functional groups; OAH, CAH (stretch), C@O, CAH (bend), and

Figure 6. Viscosities of acrylic resins (A1 and A2) with different aerogel contents.

CAO are seen at 3200–3700, 2933, 1733.1 cm21, 1452.9, and 1138.6 cm21, respectively. For sample A2, the absorption peaks of NAH (stretch), CAH (stretch), C@O, NAH (bend), CAH (bend), and CAO are taken place at 3445.7, 2957.7, 1660.6, 1509, 1458.2, and 1139 cm21, respectively.35–40 Furthermore, a similar spectrum of AAm can be seen in the work of Jamshidi and Rabiee41 According to the FTIR results, it is obvious that all the functional groups of the monomers are participated in the polymerization reaction. DSC. The minimum film forming temperature (MFFT) is the lowest temperature at which the moisture of a latex, emulsion, or adhesive vaporizes and the mixture uniformly coalesces when applied on a substrate as a thin film. It is shown that the film forms when the operational temperature is above the MFFT.42 The MFFT of a resin is usually a few degrees lower than the glass transition temperature (Tg) of the polymer.34 Used as the roofing cover, it is very important for the paint film to be a thermoplastic. It is experimented that thermoplastic coatings are ideal for roofing systems due to their elasticity and resistance to the expansion and contraction at the ambient conditions. Therefore, the DSC tests were carried out to estimate the MFFT and to find out the usability of the synthesized resins in the roofing systems. The glass transition temperatures of A1 and A2 are quantified via the DSC method. The Tg’s are found to be 25.66 and 22.73 8C for A1 and A2 samples, respectively. Considering the fact that the monomer contents for the both samples have been calculated in a way to obtain a Tg equal to 25 8C, the experimental and theoretical results show a very good match. Since the MFFT is always below the Tg, it can be concluded that the MFFT is always below the operational temperature (ambient temperature).

Figure 5. FTIR spectrometry of A2 sample. [Color figure can be viewed at wileyonlinelibrary.com]


Acrylic Resin/Silica Aerogel Tests Viscosity. A lower viscous resin allows the use of more aerogel content that causes a higher level of the thermal barrier. Different amounts of silica aerogel are added to the resins along with

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TGA. The TGA are run on A1 and A2 samples with 5% silica aerogel to study the effect of silica aerogel on the thermal degradation of the acrylic resins. The weight reduction percentage of A1 and A1S5 (A1 sample with 5% silica aerogel) samples versus temperature are presented in Figure 7(a). In accord with TGA curve, the residual weight percentage of A1 sample which is an organic matter is 0.7% but on the other hand, thermogram graph of sample A1S5 shows 5.2% as the ultimate residual weight. The presence of silica aerogel can be verified by comparison of these two numbers. The decomposition temperatures (Td) of the samples44 are presented in Figure 7(b). The Td of A1 and A2 samples are very similar to the acrylic resins reported in the other articles.44,45 It can be deduced from the

Figure 7. (a) TGA graph and (b) DTA graph of A1 and A1 with 5% silica aerogel.

the surfactant and then the viscosity of each sample is measured. Figure 6 presents the viscosity against silica aerogel content of the acrylic resins. The viscosities of A1 and A2 resins including no aerogel are very close together and similar to that is reported by Slinckx et al.43 It can also be seen that the viscosity of sample A2S, which includes AAm monomer, is less than the other sample. This can be attributed to the formation of the hydrogen bonds among the nitrogen atoms of AAm monomer and the hydrogen of the residual OH groups at the surface of the silica aerogel. Although the silica aerogel used in this study has been hydrophobic, it should be reminded that the silylation process which is used to eliminate the OH groups from the surface of the silica aerogel cannot totally do this due to the high surface area of the silica aerogel. The formation of the hydrogen bonds allows a higher level of silica aerogel dispersion in the resin including AAm monomer. This, in turn, results in a lower viscosity. It also worth noting that the resins include 10 wt % of the aerogel that have been too viscous to be used to make any paint.


Figure 8. Scanning electron microscopy of (a) pure silica aerogel, (b) A1S5 (resin without AAm monomer 1 5 wt % aerogel), and (c) A2S5 (resin with AAm monomer 1 5 wt %aerogel).

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Table IV. Heat Transfer Results of Pigmented Acrylic Resins with Different Amount of Silica Aerogel

Sample

External wall temperature (8C)

Heat transfera (W)

Heat loss savingb (%)

Not coated

60.8

15.2

NA

AP1

57.2

13.3

12.1

AP1S1

56.4

12.9

14.8

AP1S3

54.9

12.2

19.7

AP1S5

53.2

11.4

25.2

AP2

56.9

13.2

13.1

AP2S1

56.3

12.9

15.1

AP2S3

54.7

12.1

20.4

AP2S5

53.1

11.3

25.6

a b

plate temperature, Ta is the ambient temperature, and e is the emissivity of air at 20 8C (0.85). Heat loss saving5 heat loss of noncoated plate2heat loss of coated plate with paint heat loss of noncoated plate

(9)

3100

The quantities of the external wall temperature of the tank, heat transfer through the wall, and heat loss saving are presented in Table IV. Based on the data, increasing the silica aerogel content in the formulation leads to a decrease in the heat transfer and heat dissipation up to 25.6%. The findings also reveal that the presence of AAm co-monomer in the resin has no significant effect on the thermal barrier property of the paint.

Calculated for surface area of the tank. Calculated in comparison with uncoated surface.

results that adding 5% silica aerogel increases the residual weight and the decomposition temperature up to 4.5% and 20 8C, respectively. The peaks at 100 and 220 8C are related to the vaporization of water and emulsifiers, respectively. Even though the samples were completely dried, little amount of humidity is inevitable. SEM. The SEM technique is used to observe the morphology of the silica aerogel particles in the acrylic resin bulk. The similar structure of the aerogel particles can be observed in the research studies carried out by Chetty et al.46 and Gupta and Ricci.47 Figure 8(b,c) shows A1S5 and A2S5 samples. It seems that the dispersion of the silica aerogel particles is more homogenous in A2S5 sample comparing A1S5. This can be attributed to the hydrogen bonds as explained in the Viscosity section. Acrylic Resin Paint/Silica Aerogel Tests Heat Transfer. To examine the thermal barrier properties of the paints the home-made setup, as explained in the Characterization and Instrumentation section, is used. Since the temperature of the water inside the tank is kept constant, in a steady-state heat transfer situation, the heat transferred through the wall by conduction has to be equal to the heat transferred from the wall into the air by free convection and radiation. Having the external wall temperature of the tank, the ambient temperature and the geometrical characteristics of the tank wall the heat transfer from the wall to the surrounding air can be calculated,48 which is equal to the heat transferred through the wall plus the paint layer.

Moreover, the thermal barrier results of the current study can be compared to the findings of the other researchers. Azemati et al.49 synthesized thermal-insulating paints using inorganic particles. They showed that the use of the microceramic particles in the acrylic paints and coatings decreased the energy loss by 16–17%. Moretti et al.50 investigated a new basalt fiberbased insulating panel. According to them, the thermal transmittance of walls was reduced by 20–40%. In another study, Guo et al.51 proved that the heat reflective insulation coating (a commercial grade) could reduce the exterior wall surface temperature effectively, and the maximum temperature change was about 8–10 8C; hence insulation coating can affect the annual air-conditioning electricity saving. In this article, utilization of 5% of silica aerogel in the formulation of the paints decreases the heat loss and wall temperature of the apparatus by 25% and 7.7 8C, respectively. Mechanical Properties. Hardness. The hardness of pigmented acrylic resins is analyzed using a set of pencils with known hardness values of 6B (the softest) to 6H (the hardest) according to the ASTM D3366. The test is started with the hardest pencil and precedes down the scale of hardness to end up at the point that the pencil would not cut in to the film. Based on the results in Table V, the hardness of AP2 sample which includes AAm monomer in its composition is more than that of AP1. The main reason of the hardness differences between the paint with and without AAm in its formulation is a higher level of dispersion and a strong interfacial interaction of the silica aerogel particles in the paint with AAm. It is clear that a higher level of dispersion and also a strong interfacial interaction improve the mechanical properties and increase the hardness.52 Abrasion resistance analysis. The abrasion tests evaluate the abrasion resistance of the films and coatings by the

Heat transfer through either coated or uncoated wall 5 convective heat transfer from wall into air ðqconv Þ 1 radiative heat transfer from wall to surroundings ðqrad Þ

Table V. Mechanical Properties of Pigmented Acrylic Resins with and without 5% Silica Aerogel

(6)

Sample

qconv 5 h3W 3H3ðTp 2Ta Þ

(7)

qrad 5 Sigma3W 3H3e3ðTp4 2Ta4 Þ

(8)

where h is the average heat transfer coefficient, W is the plate width (across flow), H is the plate height (with flow), Tp is the


Pull-off (MPa)

Abrasion (%)

Hardness

AP1

3.62 6 0.05

4.78 6 0.02

6B

AP1S5

3.36 6 0.05

7.79 6 0.02

5B

AP2

3.82 6 0.05

3.51 6 0.02

5B

AP2S5

3.54 6 0.05

4.54 6 0.02

3B

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measurement of the amount of the separated matter from the surface. The abrasion of the samples is noted in Table V. According to the data, it can be inferred that the silica aerogel decreases the abrasion resistance of the samples because of its high porosity. Conversely, the presence of the AAm monomer in AP2 sample increases the abrasion resistance in comparison with AP1. The abrasion resistance of AP2S5 is about 42% more than that of AP1S5 because of the interaction which takes place between the functional groups of AAm monomers and silica aerogel. In fact, the abrasion resistance of the samples is affected by the hardness of the coatings.15 Pull-off strength. The pull-off strength measurements which highly depend on the geometry and physical properties of the substrates and coatings can be evaluated using the adhesion tests. The adhesion is one of the important parameters influencing the durability of any surface coating.53,54 The determination of the compatibility of the coatings and the substrates is the aim of this experiment that is tested according to ASTM D4541. This experiment evaluates the required force to come off the film. The tests are carried out on the acrylic resin paints to find out the failure mode of the paint. Failure occurs along the weakest plane within the system comprised of the test fixture, adhesive, coating system, and substrate and is exposed by the fracture surface. This test determines the greatest perpendicular force (in tension) that a surface area of a sample can bear before a plug of material is detached. More force leads to more acceptable adhesion property of the system. Referring to the results in Table V, it is clearly observed that the pull-off results veer in the same direction as the abrasion analysis. The presence of silica aerogel reduces the adhesion properties; on the other hand, AAm in the formulation improves the adhesion attributes. The substrate failure and the coating cohesive failure of the samples are 70 and 15%, respectively. Therefore, the result indicates a proper degree of adhesion of the coatings to the substrate according to ASTM D4541. Bending. The thermal and mechanical expansions and contractions of the roofs due to the different atmospheric conditions are the origins of the cracks. The bending test is used to inspect the cracks formation on the surfaces of the synthesized samples. The specimens are bent over a conical mandrel for 1 s, and then the bent surfaces of the samples are examined immediately with the naked eye. All samples are intact after the test which proves they are crackresistant material that is a key factor in the roof coating. The result of bending test is similar to what is reported by Athawale and Bailkeri.54 Athawale and Bailkeri54 focused on the synthesis and characterization of the acrylic copolymers as well as the liquid crystalline acrylic copolymers. They have reported no cracking on the films of the either acrylic copolymer or LC acrylic copolymers. Water resistance. The water resistance tests are run on AP1 and AP2 samples and also the samples including 5% silica aerogel based on ASTM D870-02. Initially, the samples are immersed in the distilled water at 25 8C for 48 h. Then, they are taken out of water and remained at the ambient temperature for 12 h. The


samples are observed carefully, and no evidence of color change, blistering, loss of adhesion, and softening was seen.14 To check the paint stability, the wet paint was inspected after 1 month. No agglomeration or coagulation was observed. The viscosity of the wet paint and the pull-off strength of the paint film were also measured and the thermal analyses were repeated again after a month and the results were similar to what are presented in Figure 6 and Tables IV and V. CONCLUSIONS

In this article, the presence of silica aerogel and AAm monomer in the formulation of acrylic resin are studied on the thermal and mechanical properties of the pigmented acrylic resins. The major findings and conclusions derived from this study are summarized as follows: 1. The presence of AAm monomer in the formulation of acrylic resin causes the hydrogen bonding of the resin and the silica aerogel that results in a better distribution of the aerogel in the resin. This, in turn, causes the reduction of the viscosity of the resin. 2. The TGA results showed that adding 5 wt % aerogel to the resin increases the thermal degradation temperature of samples by 20 8C. 3. Application of a 120-lm layer of the paint included 5% of silica aerogel on a metallic surface decreases the heat loss and temperature of the surface by 25—26% and 7–8 8C, respectively. 4. The mechanical properties of the pigmented acrylic resins including aerogel are less, compare to those of the paint without aerogel, due to the porosity and brittleness of the silica aerogels. The presence of AAm monomer in the composition of the resin, on the other hand, improves the mechanical properties of the paint. 5. This study indicates that the acrylic resin modified with AAm monomer and silica aerogel can be used as roof coating film due to the remarkable thermal insulating and proper mechanical properties.

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