K.Muthu #1, S.Muthuvel #2,
N.Rajini #3
Department of Mechanical Engineering,
Kalasalingam University
Tamilnadu,
India
Abstract: Polymer
Phase change materials (PCM’s) are the materials which can be used to store the
thermal energy for a broad range of applications. The unlike conventional
storage materials phase change materials may absorbs or release heat at a
constant temperature. They store 6-12 times more amount of heat per unit volume
than sensible storage materials. Increasing demand in solar applications and
spacecraft thermal control applications requires continuous development of
phase change materials to improve its effectiveness and reliability. The energy
storage devices, which are as important as to developing the new sources of
energy. In this paper we can show the combination of low molecular weight
polyester resin and Calcium Chloride hexa hydrate (CaCl2.6 H2O)
is an inorganic PCM that can be used as PCM’s of this study. Differential
Scanning Calorimeter (DSC) is an instrument which can be used to measure the
thermal characteristics of the phase change materials, Melt Flow Index test
were used to characterizing the new developed Compound for storing the heat
energy. In this project the target was to reduce the melting point hence that
we could obtain more heat storage for energy saving applications.
Keywords: Phase
Change Material, Latent heat storage, Calcium Chloride hexahydrate,
Polyester resin, Differential Scanning Calorimeter
1. Introduction:
Thermal
energy storage is one of the technologies offer exciting opportunity to improve
the efficient and reliable of energy markets. Customers and suppliers of the
energy systems will benefit from understanding and using energy storage
methods. The latent heat of fusion of solid and liquid phase of heat of fusion
to absorb and release the heating or cooling during conversion of phase [1,2].
PCM s are mainly used in latent
heat thermal storage systems for buildings, micro encapsulation, solar systems,
composites etc. The applications in wall and floor boards to store heat to
provide comfortable temperature to the room[3] .The storage of heat
energy in correct forms which can conventionally to be converted into the
required form. Energy storage not only decrease the difference between supply
and demand and also increase the performance and reliability of energy storage
systems and plays an important role in energy conservation[4,5]. The
phase change material can melt and solidify at certain range of temperature,
making them effectively in a number of applications. In this project we have
analyzed, the non commercial PCMs hybrid composite of low molecular weight
materials contains low molecular weight polyester, Methyl ethyl ketone peroxide
(MEKP) and acetyl acetone peroxide (AAP) solutions were used as initiators. The development of such material
with low freezing and melting point and the latent heat of fusion will leads to
break through in formation of hybrid composites during phase conversion.
The
increase in fuel prices is the main driving forces are effectively using many
non conventional energy sources. This is the one of the part to save the
energy. Developing polymer phase change materials for thermal storage
applications is the today’s exciting research. This is more benefits to humans
and also for energy conservation.
The efforts that are being carried out
to a greater use of solar energy have been accompanied by the development of
thermal energy storage systems in order to surpass the problem of the mismatch
between energy demand and supply. In this context latent heat thermal energy
storage (LHTES) systems have received a great deal of attention in the last
years. Low energy buildings and water heating are important application areas
of such systems. Phase Change Materials (PCMs) which are the core of LHTS
systems are currently an area of investigation of increasing interest. Latent
heat storage by solid–liquid phase transition is a particularly attractive
technique, since it provides a high energy storage density and has the capacity
to store energy as latent heat of fusion at a constant temperature
corresponding to the phase transition temperature of the phase change materials
[1].
Several
substances, differing in physical and chemical characteristics as well as in
thermal behavior have been studied as potential PCMs [1-3]. In order to meet
the requisites of particular systems, auxiliary materials are often used with
specific functions such as encapsulation or shape stabilizers.
Polymeric
materials have been proposed to perform different functions in PCMs. Actually
polymers appear as promising materials in this context. Besides being possible
to choose among a diversity of chemical natures, they can be cross linked and
characteristics such as molecular weight, crystalline structure and free volume
can be designed in order to meet specific requirements.
In
this bibliographic survey we look at the application of polymeric materials as
PCMs or as auxiliary materials aiming at relating their particular function
with their physical, chemical and thermal characteristics.
2. Materials And Experiments
The unsaturated polyester is one
of the thermoset material used in
composites for the fabrication of molding compounds with good mechanical
properties. The PCMs used in this study is Calcium Chloride hexahydrate which
have excellent properties and the melting point of 29˚C. The unsaturated
polyester could be mixed with Calcium Chloride hexahydrate. The cross linking
reaction between polyester resin and Calcium Chloride hexahydrate allows one
polymer chain to connect with other and to produce a three dimensional
structure, which convert the resin from a viscous phase. The Calcium Chloride
hexahydrate acts as a cross linking agent and viscosity reducer hence the
polyester can be processed. The resulting material becomes good mechanical,
physical and chemical properties. The curing reaction is more complicated
process that is affected due to weather, humidity, uniformity, equipment
condition and various factors.
Low
molecular weight of pure polyester resin and calcium chloride hexahydrate with
density (0.xxxx) g/cm3 was obtained from BORNEO INDAH SDN BHD with the
properties such as appearance (white), density (1.12) g/cm3, and stability in
the dark below 30ºC. The unsaturated polyester resin contains 50% calcium chloride hexahydrate was prepared by
adding 10g of styrene to 100g from unsaturated polyester resin.(80%up + 20%st).
A
various combinations of initiators were used to cure an unsaturated polyester
resin. The compressive properties and the curing behavior of the polyester
resins in the presence of Vinyl Ester resin were investigated. The results
shows that maximize the cure temperature and vinyl ester content lead to
improvement in the compression strength and Young’s modulus. The unsaturated
polyester resin properties were dependent on calcium chloride hexahydrate
content including the thermal stability and mechanical properties of the
polyester resin reflect the extension of phase segregation. Gel time and peak
temperatures were studied in terms of wt% of MEKP, accelerator, filler and
glass fibers. They found that the gel time improved with quantity of filler and
glass fiber while reduced with amount of catalyst and accelerator. This paper
exhibits the thermal properties of the polyester resin containing different
concentrations of calcium chloride hexahydrate via measuring viscosity, gel
time, maximum temperature and mechanical properties. In addition, the curing
reaction process of polyester resin by
adding different weight fraction of calcium chloride hexahydrate. This is a
very important stage in processing of polyester resin for producing a composite
product with high quality and reliability.
Methyl
ethyl ketone Peroxide (MEKP) is used as catalyst and Dimethyl Aniline (DMA)
acts as accelerator were obtained from the same supplier for polyester resin.
The properties of DMA were as follows: the density at (21-25) ºC temperature -
(0.944-0.959) g/cm3, molecular weight - 122.28 g/mol, and the chemical symbols
- C8H12N.
2.2 Differential Scanning
Calorimeter
The
differential scanning calorimeter was carried out in a TA Instruments DSC 2961,
which can calculates the heat flow rate to and from the specimen with respect
to the temperature and time, sample weight of (6-12) mg at heating rate of 10
ºC/min. There is a reference material introduced in this method to compare the
rate of heat exchange irrespective of whether it’s an exothermic such as
melting property, crystallization and curing of mixtures with respect to
temperature. Change in heat capacity is also found using this test. The testing
temperature is usually ranges from 150-200˚C.
2.3 Mould Design and Fabrication
Specimens
have been prepared for four different salt hydrate PCM content 1%, 2%, 3% and
5% concentrations with pure polyester resin. The accelerator and catalyst were
added (1, 2, 3 and 4)% by volume of 100 ml of polyester resin, mixing with
mechanical stirrer for 20-30 minutes and
laid up in the mould for 24 hours. The mould should be well cleaned and dry,
for this reason, a release agent (wax) is laid up on the mould before powering
mixture. The same process is applied to prepare the resin specimens with fixed
MEKP concentration of 1% and change the DMA volume fraction stared from 1%, 2%
,3% and 4% by volume of pure polyester resin. The mould consists of three
parts, the base part and the upper part made form glass plate while the
intermediate gasket made from foam board with 3mm thickness.
3.Results And
Discussions
3.1 Measurement
of viscosity
LVDV-II+Pro
Viscometer were used to measure the
viscosity of the polyester resin for
different calcium chloride hexahydrate weight
concentrations at different temperatures. A standard glass beaker with 500
ml was used in this test. The experiments are start at 31 ºC and raise until 60
ºC. As expected with rise of temperature
the viscosity may drops significantly. It has been found that the viscosity decreses with increase of
calcium chloride hexahydrate concentration ratio, the results shows that the
viscosity was 215.3 cP at 31 ºC and 62.5cP at 60 ºC and the percentage
decreasing was 28.56 % for the first ratio of
(80% polyester +20% Cacl2.6H2O), while the
percentage decreasing for the ratio of (70% polyester +30% Cacl2.6H2O
) is 64.83%, and for the last calcium chloride hexahydrate ratio of (60%
polyester +40% Cacl2.6H2O) is 61.59%
3.2 Density Measurement
The
calcium chloride hexahydrate ratio controls the reactivity of the pure
polyester resin and also the cross linking density of the final result. The
density
variations
with the increase in calcium chloride hexahydrate concentration at 29 ºC. It is found that the density of the
polymer matrix is decreasing while the calcium
chloride hexahydrate ratio was increased.
3.3 Cure Characteristics
3.3.1 Curing Time and Peak
Exothermic Temperature
The
cross linking reaction in the processing of unsaturated polyester resin into a
composite product, also does the exothermic temperature of cure after end of
the processing. The cross linking reaction is a high exothermic reaction, and
the temperature can raise up to 110-220 ºC.. The maximum exothermic temperature
was about 160 ºC for the ratio 80% of
calcium chloride hexahydrate followed
20% at about 140 ºC and 30% at about 125 ºC.
3.3.2 DSC Results for Different
weight Concentrations of PCM
The
output of a DSC measurement is a plot of the difference of heat delivered to
the sample and to the reference as a function of the sample temperature for
(80% polyester-20% pcm) polyester resin with 1%MEKP. The figure shows that the
significant changes in TG of unsaturated polyester resin for different weight
concentrations of pcm with different MEKP and DMA concentration ratios. In the
first ratio of pcm 20% there insignificant changes with increasing MEKP, while
Tg reduces with adding 0.1vol% DMA then started to increases with the
increasing of DMA. While for the second ratio of pcm the maximum transition
temperature is at 2vol% of MEKP and after that stared to decrease with
increasing of MEKP. The glass transition temperature has the equal trend for DMA
and the maximum is at 0.2 vol%. It is found that Tg measured by DSC increases
as the MEKP level increases for certain level and then decreases.
3.3.3 The Effect of Moisture
Content
The
effect of varying the level of water content on the exothermic behavior of the
unsaturated polyester resin for 80% calcium chloride hexahydrate as shown in
figure. The reaction of water with unsaturated polyester resin the rate of
temperature increases is almost equal, but gel time gets increased. Because of the water slows the reaction by
absorbing the heat from the storage system. Gel time was 40min for the 0%
water, 43min for the 1vol% water, and 46 min for the 2vol% water. However, when
the concentration of water was 3vol% water the gel time has decreased and
reached 34min (2min) only above the 0% water ratio. That means after the 3%
water concentration ratio there was no action affecting the gel time. Otherwise
it may affect the mechanical and chemical properties. In addition, the water
affected the action of initiator and accelerator system. Water Bubbles may appear in the polyester resin because the
water can increase reactivity in some peroxide system.
3.4 Mechanical Properties
3.4.1 Tensile Test
The
tensile test was performed in accordance to ASTM (D638) specification. Four
dumbbell shape (Type I) specimens from each matrix were tested in Universal
Testing Machine (UTM), with load cell of 50 KN. The specimen was loaded in
tension at a speed of 6.5 mm/min. An extensometer of 8 mm gage length was
mounted on the specimen for measurement of the strain. The cross section area
of specimen was determined using a digital micrometer. The average results from
four specimens were taken.
3.4.2 The Effect of MEKP and PCM
concentrations on Tensile Strength
The results of
mechanical properties for the polyester resin were determined. The results of
the effect of MEKP concentration on the tensile strength of cured samples at a
fixed (80%up + 20%pcm). It can be found that the load increases to the maximum
value and then suddenly decline as a brittle fracture was placed in the
material. The tensile strength does not significantly change with the rising of
MEKP ratio. In fact, the percentage of increase between the strength for 5%
MEKP and the other ratio is less than 10% while the other ratios are (1, 2, 3)
% about (2-3) %. The mechanical properties of the cured samples are varied from
soft to hard, depending on the molar mass of the end grouping. A high molecular
mass will give more hardness, tensile and flexural strength of the final cured
material. If the molecular mass is low,
the mechanical properties of the cured resin will be less. Calcium chloride hexahydrate
will generally more brittleness to the specimen, therefore the embrittlement
causes reducing the internal stresses in the part because of the increase of
part shrinkage that occur with higher degrees of cross linking. In another
expression the tensile strength increases with decreasing of PCM weight
concentration ratio at constant MEKP concentration.
Fig.8 Gel time
for unsaturated polyester resin containing different concentrations of PCM and
MEKP ratios
4. Conclusions
The conducted
project have revealed the following:
1. The physical parameters such as viscosity
and density is decreases with the increase of environmental temperature and
different weight concentration of PCMs used.
2.
The PCM ratio controls the reactivity of the unsaturated polyester resin and
also the cross linking density of the final network structure by increasing the
gel time, time to peak, and exothermic temperature.
3.
Maximum strength and modulus of elasticity have the same trend for different
weight concentration ratios of PCM and for certain concentration of polyester
resin, different ratio MEKP and DMA have a significant effect on the maximum
tensile strength, while DMA has insignificant effect young’s modulus.
4.
The presence of polymer matrix is
usually inanimate the cross linking reactions, by added mass which is
absorb the heat that occurs and delaying the gel time interval.
5.
The moisture content in the resin affected the curing reaction by increase of
the gel time and time to peak, while there insignificant effect on the
exothermic temperature for the equal MEKP concentration at an operating temperature.
6. The glass
transition temperature TG decreases with increase of calcium chloride
hexahydrate concentration for 1vol% MEKP, beyond that there is insignificant
effect on TG with the increasing of MEKP and DMA.
5. Acknowledgement
The authors would like to thank everyone
contributed to this research; and Kalasalingam University for financial support.
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