Dalhousie University

Report of Summer Work Term ProjectMay 3 – August 28, 2015

How Moisture Absorption affects the Dimensional Stability of NRC’s Single-Piece Rim-Supported Composite Radio Telescope Reflector

Performed at:NRC DRAO

717 White Lake Rd.Kaleden, B.C.

Prepared by: Nicholas Driscoll B00600106

In partial fulfillment of the requirements of the Engineering Co-operative Education Program

August 28, 2015

Summary

The National Research Council of Canada (NRC) DRAO (Dominion Radio Astrophysical Observatory) has designed and is currently constructing a radio astronomy parabolic dish antenna that could be selected for use in the Square Kilometer Array (SKA) project. Previous designs have been met with skepticism focusing on the longevity of the dish surface, primarily because it is composed of fibre reinforced polymers (FRP’s) as opposed to metals or alloys. The purpose of this report is to investigate how moisture absorption affects the composite material being used to fabricate the reflector; more specifically how moisture absorption will or will not lead to dimensional instability of the SRC reflector over the 50 year project life.

A review and analysis of literature regarding moisture absorption and composites in combination with initial test results have led to the conclusion that moisture absorption will not lead to the dimensional instability of the SRC reflector. This being said, the final tests for this project are scheduled for September 21st, beyond the duration of this work term which concludes on the 28 th of August; thus a final analysis cannot be provided. As previously discussed with Dr. Jan B. Haelssig the initial results will suffice.

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TABLE OF CONTENTS

Summary...................................................................................................................................................... i

LIST OF FIGURES.......................................................................................................................................... iii

LIST OF TABLES............................................................................................................................................ iii

Acknowledgments...................................................................................................................................... iv

1 Introduction.........................................................................................................................................1

2 Background..........................................................................................................................................1

2.1 Radio Telescope...........................................................................................................................12.1.1 NRC’s Radio Telescope.........................................................................................................2

2.2 Composite Material.....................................................................................................................32.2.1 Polymers..............................................................................................................................3

2.2.2 Thermosets..........................................................................................................................3

2.2.3 Vacuum Infusion..................................................................................................................4

3 Moisture Absorption...........................................................................................................................5

3.1 Diffusivity.....................................................................................................................................53.1.1 Fickian Diffusion...................................................................................................................5

3.2 Properties of Hetron CL 90501....................................................................................................64 Determining Dimensional Change.......................................................................................................8

4.1 Strategy.......................................................................................................................................85 Testing...............................................................................................................................................10

5.1 Environmental Chamber............................................................................................................105.1.1 Coefficient of Moisture Expansion.....................................................................................10

5.1.2 Four Point Bending............................................................................................................12

5.1.3 Dynamic Mechanical Analysis............................................................................................15

6 Conclusions........................................................................................................................................15

7 Recommendations.............................................................................................................................15

8 References.........................................................................................................................................16

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LIST OF FIGURESFigure 1 DVA-1 stationed at DRAO [3]...................................................................................................2Figure 2 Styrene monomer [9]...............................................................................................................6Figure 3 The reaction to create a typical epoxy vinyl ester monomer [11]............................................7Figure 4 Properties of Bisphenol-A epoxy vinyl ester resin [13]............................................................7Figure 5 Example of FEA output for CTE with temperature difference of 25C.......................................9Figure 6 The Thermotron environmental chamber [16]......................................................................10Figure 7 Control sample failing on the compressive side of the 4 point bending test, the side at which the aluminum is ~1mm from the surface [16]...........................................................................................12Figure 8 Load-displacement curves of control samples 1-5, collected July 27th...................................13Figure 9 Load-displacement curves of exposure 1 samples A-E, collected August 10th.......................14

LIST OF TABLESTable 1 Average dimensions, mass, volume and S.G. for 5 samples being used to determine CME......11Table 2 Standard deviations for the averages shown in Table 1............................................................11Table 3 CME results calculated using equations 1, 5 and 6....................................................................12Table 4 Summary of data.......................................................................................................................14

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Acknowledgments

Fabrication of test samples was conducted at NRC’s Dominion Radio Telescope Observatory in a

vacuum infusion laboratory by Tyler Willis and the author. Testing strategy was developed by Dean

Chalmers of NRC DRAO, and Bryn Crawford and Dr. Abbas Milani of the Composites Research Network.

General council was provided by Dr. Anand Rau, an expert in polymers and composites.

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1 Introduction

The Square Kilometer Array (SKA) project is a collaboration of around 100 organizations across

about 18 countries [1] whose end goal is to create an array of ~2500 15m radio dishes, in Madagascar

and South Africa, becoming the most sensitive radio telescope in the world. The core member countries

of the SKA project are Australia, Canada, China, Germany, Italy, the Netherlands, New Zealand, Republic

of South Africa, Sweden and the United Kingdom [1]; three of which are competing to design and

fabricate the best dish, one of the first deliverables for the SKA, including the 15m/49.2-ft diameter

primary reflector [2]. The competing countries are Canada, China and South Africa with radio telescopes

called DVA-1, DVA-C and MEERKAT 1 respectively. The DVA-1 was completed in 2013, but with changes

in surface accuracy requirements by the SKA and international speculation concerning the longevity of

the composite material used to fabricate many of the parts for the radio telescope, a further iteration of

the design was required. The mechanical team led by Gordon Lacy at DRAO has designed and is

currently constructing a radio telescope called the DVA-2 with a lower tolerance for the surface accuracy

of the primary reflector which will be presented to an international consortium in November 2015 as

part of the down selection process in the SKA project. If NRC’s dish type were selected it would give

Canada international recognition in radio telescope design and work for Canadian industry. As previously

mentioned one of the main concerns is that the reflector will degrade during the 50 year project life.

This report will analyze how moisture absorption will or will not lead to degradation of the radio

telescope reflector within the lifespan of the SKA project.

2 Background

2.1 Radio Telescope

There are two types of telescopes, the optical telescope and the radio telescope. Optical

telescopes use visible light emitted and reflected from stars, planets, moons and other celestial bodies

to map the universe. Radio telescopes detect radio waves that are emitted from the same sources as

well as ‘invisible’ aspects of the universe: gravity, magnetism, black holes and dark energy [2]. An

additional benefit is that radio waves can pass through objects such as clouds and gases which would

otherwise block visible light [2]. The downside is that radio signals have longer wavelengths than visible

light, so to produce photo-equivalent detail and resolution to that of an optical telescope, a radio

telescope must have a much larger collecting area, the basis of the SKA project. One issue that arises is

that the radio wave is received at a different time by each dish so an accurate picture of the universe is

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not depicted. This problem is solved by a technique in radio astronomy called interferometry. This

technique accounts for the time difference between each telescope and correlates the radio frequency

data received based on the position of each telescope in the array. The only requirement is that each

telescope must be positioned to observe the same segment of the sky.

2.1.1 NRC’s Radio Telescope

Both the DVA-1 and DVA-2 are offset Gregorian type radio telescopes, the feed legs do not meet

above the middle of the primary reflector instead a secondary reflector is used. This design opens up

collecting area in the middle of primary reflector.

Figure 1 DVA-1 stationed at DRAO [3]

As shown in Figure 1 the backing structure connects to the rim of the dish. Dish rim connectors

(DRC’s) and ball and socket joints are used to connect the backing structure to the rim of the dish. This

reflector type is called a single-piece rim-supported composite (SRC) reflector. The primary and

secondary reflector, the feed legs that support the secondary reflector, the feed support platform, the

DRC’s and the rim are made of composite materials.

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2.2 Composite Material

A composite is a material made of two or more materials with resulting properties that are better

than the individual properties of the original materials; for FRP’s the materials are called the

reinforcement and matrix. The DVA-2 is constructed of carbon fiber, fiber glass and an aluminum mesh

(for the reflective layer). The carbon fiber gives the composite its stiffness and strength, and is known as

the reinforcement. The matrix is what holds the reinforcement together, it acts as an adhesive, and for

the DVA-2 it is an epoxy vinyl ester resin.

2.2.1 Polymers

Polymer is a technical term for plastic, poly means many and mer represents the singular unit

molecule that makes up the polymer backbone (the main structure of the polymer). In composite

technology two types of polymers are commonly used; thermoplastics and thermosets. In general, once

a thermoplastic has cured it can be reheated and recycled. Thermosets do not exhibit this quality, when

a thermoset cures the polymer backbones form covalent bonds with each other; this is called

crosslinking. This means that when the thermoset is reheated it burns, it does not melt. Curing is a

process that the polymer undergoes to transition from its liquid to solid state; this can be induced by a

combination of increased heat and pressure and/or the addition of a catalyst known as a curing agent or

hardener [4]. One benefit of using a thermoset over a thermoplastic is the crosslinks that are formed

give the structure higher stiffness and strength properties.

2.2.2 Thermosets

The types of thermosets that will be reviewed in this report are; epoxy, polyester and epoxy vinyl

ester resin (EVER). As previously stated NRC uses an EVER for the construction of the primary reflector

for the DVA-2; to assist the reader with understanding the choice a brief background on each thermoset

will be provided.

2.2.2.1 Epoxy

Epoxies are commonly used in aerospace applications as they offer higher strength and stiffness

properties than epoxy vinyl ester and polyester, and are considered the highest grade of thermosets.

With this reputation comes a high cost, often 2 to 3 times than that of vinyl ester [5]. Epoxies also have

other drawbacks, such as high viscosity, which leads to long cure cycles and some possess highly toxic

curing agents (hardeners) [5].

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2.2.2.2 Polyester

Polyester is cheaper than epoxy and epoxy vinyl ester, but is known for to have weak chemical

resistance, especially to hydrolysis [5]. Polyester is not an appropriate choice considering the 50 year

project life.

2.2.2.3 Epoxy Vinyl Ester Resin

Epoxy vinyl ester resins fall between epoxies and polyesters in terms of cost. This resin was

created to combine the mechanical and thermal properties of epoxies with the low cure times of

polyester. Keeping the cure time low is a useful characteristic because if NRC’s radio telescope design is

selected thousands of reflectors will be manufactured, and having a low cure time will lower the cost of

manufacturing significantly. The epoxy vinyl ester resin selected by NRC is called Hetron CL 90501

synthesized by Ashland.

2.2.3 Vacuum Infusion

Vacuum infusion is the process used to fabricate the composite parts. The reinforcement

material is laid up dry and covered and sealed with a vacuum bag. Resin is then infused into the part,

being drawn through by a vacuum pump. It takes ~1 hour for Hetron CL 90501 to reach its gel point, the

point at which the resin will stop flowing and has hardened.

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3 Moisture Absorption

The chosen radio telescope design will be subjected to the climates of Madagascar and South

Africa. The bulk of the radio telescopes will be stationed in the Karoo region of South Africa which is

considered a semi-desert due to the lack of precipitation. Madagascar is a tropical environment; the

telescopes stationed there will be exposed to higher levels of humidity and precipitation. The most

extreme case will be Sainte Marie which is in northern Madagascar, where the average relative humidity

ranges from 81-83% year round, and the mean monthly precipitation ranges from 120-480mm [6].

Moisture absorption would cause the resin to expand while the fibers would absorb little to no

moisture, leading to a change in the mechanical properties of the composite material, and thus

dimensional instability. Another problem that could be caused by moisture absorption is the composite

delaminating.

When studying moisture absorption and composite materials two characteristics must be

determined, the % weight gain of the material at saturation, and the diffusivity of the material (the rate

at which the material absorbs moisture); to determine these characteristics the moisture uptake of the

material must be measured as a function of time.

Equation 1: Moisture uptake of the material as a function of time

M ( t )=Mw−M d

M dx100

Mw = mass of wet materialMd = mass of dry material

Once the plot of Equation 1 reaches an asymptote, Mm (mass of moisture at saturation) can be determined.

3.1 Diffusivity

This is the rate at which the composite material absorbs moisture, usually measured in cm 2/s. The

diffusivity of a material is dependent on a number of factors; diffusivity of the matrix, diffusivity of the

fibers, the volume fraction of the fiber and the orientation of the fibers with respect to the exposed

surface [7]. Generally the diffusivity of the fiber is small when compared to the diffusivity of the matrix

and negligible when determining the diffusivity of the entire composite material [7].

3.1.1 Fickian Diffusion

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The second law of Fickian diffusion is used to model the behaviour of polymers when exposed to

high relative humidity or immersed in water. Fick’s second law of 3-dimension diffusion holds true for

relatively large and thin composite plates [8]. This equation describes moisture diffusion in the z-

direction, through thickness of the plate, theoretically could be applied to a reflector dish given its size,

and will be applied to test samples to determine the rate of moisture absorption.

Equation 2: Second law of Fickian diffusion

M t

Mm= 4h √ Dtπ

Mt = amount of moisture diffused after time t, (%)Mm = amount of moisture absorbed at saturation, (%)h = sample thickness, (cm)D = diffusion coefficient in the z-direction, (cm2/s)

3.2 Properties of Hetron CL 90501

Hetron CL 90501 contains 35-40% styrene by weight. Styrene is commonly used as a diluent in

epoxy vinyl ester resins to lower the viscosity, allowing for the resin to flow more quickly and cover the

surface of the composite material before the resin gels (crosslinks) during the vacuum infusion process.

Styrene is a hydrocarbon, an ethyl attached to a benzene ring as shown in Figure 2, and therefore

nonpolar. This makes styrene hydrophobic; it does not have an affinity for water since there are no

dipoles to attract the dipoles of a H2O molecule. Epoxy vinyl ester resin does have regions of polarity

which could lead to some moisture absorption.

Figure 2 Styrene monomer [9]

The specific reactants used to create Hetron CL 90501 will not be released by Ashland due to the

fact that it is proprietary information. This being said, the typical reactants used to create an epoxy vinyl

ester resin can be examined. In general an epoxy vinyl ester resin is produced by the esterification of an

epoxy resin with an unsaturated monocarboxylic acid; leaving terminal reactive double bonds derived

from the carboxylic acid [10]. Clues to whether or not the polymer backbone of Hetron CL 90501 is

susceptible to moisture absorption would come from the epoxy used to synthesize Hetron CL 90501.

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Figure 3 The reaction to create a typical epoxy vinyl ester monomer [11]

As shown in Figure 3 the functional groups at the end of the epoxy vinyl ester monomer are esters.

The functional ester groups have a higher level of polarity than styrene molecules as they can participate

in hydrogen bonds as hydrogen acceptors [12]. However, they are still considered moderate in polarity

and likely not susceptible to hydrolysis.

The epoxy being added to the methacylic acid (MAA) in Figure 3 is diglycidyl ether of Bisphenol-A

(DGEBA), resulting in Bis-A EVER shown in Figure 4.

Figure 4 Properties of Bisphenol-A epoxy vinyl ester resin [13]

Figure 4 illustrates some of the properties that Hetron CL 90501 possesses because it is a Bis-A

EVER. The spatial arrangement of the methyl group acts as a shield, making the polymer less susceptible

to moisture. Another characteristic of Bis-A EVER that increases the hydrolytic stability is lower ester

content relative to other thermosets like Bis-A fumaric polyester and isophthalic polyester, 5-10%

compared to 10-15% and 20-30% respectively [13].

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4 Determining Dimensional Change

One of the SKA specifications for the SRC reflector being developed by NRC is to have a surface

accuracy of ±0.5 mm root mean square (RMS). Technically, it is not the surface accuracy which is

important, as this is the layer of paint, but the reflective layer of aluminum reflective material. The

accuracy of the reflective layer is paramount because the slightest changes in the parabolic dish will shift

the reflected waves from hitting the focal point or antenna of the radio telescope, and in turn affect the

collection and interpretation of data. The RMS is measured by taking the square root of the arithmetic

mean of the squares of the deviations, as shown in equation 3.

Equation 3: RMS

xrms=√ 1n (x12+x2

2+…+xn2)

The deviations in the surface are measured using a laser tracker which is manually controlled and

takes approximately 30,000 measurements. These measurements are compared to a model created

using computer aided design (CAD) software, and then the RMS is calculated. As previously mentioned,

it is not the surface accuracy which is important, but the reflective layer. The reflective layer is ~1mm

beneath the surface; a technique called holography is used in radio astronomy to measure the

dimensional change in this layer. This technique was used by NRC to determine the dimensional change

in the reflective layer of two parabolic telescope reflectors (the MARK 1 & 2) both designed for the SKA

project, and it was confirmed that the difference between the surface and reflective layer is negligible.

For information on radio holography, refer to [14].

4.1 Strategy

The finite element analysis (FEA) software used to model the stresses and dimensional change in

the reflector has an input for the coefficient of thermal expansion (CTE) of the composite materials used

in the SRC reflector.

Equation 4: Coefficient of thermal expansion

CTE= ∆ xl∗∆T

Δx = change in length, (mm)l = original length, (mm)ΔT = change in temperature, (°C or K)

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According to [15] there is also a coefficient of moisture expansion (CME) that can be determined for

composite materials. For [15] the CME was determined for carbon-epoxy for unidirectional, quasi-

isotropic and woven laminates which can be compared to NRC’s results. Equations 5 and 6 are the

equations used to determine CME.

Equation 5: Strain due to moisture absorption

ϵm=∆xl

Equation 6: Coefficient of moisture expansion

CME= ϵm

Mm

Mm = mass of moisture at saturation

The CME is comparable to the coefficient of thermal expansion, but measures strain over % of

moisture uptake as opposed to change in temperature.

The Composites Research Network (CRN) will determine the CME for the composite material

being used in the DVA-2. Once the CME is determined it will be inputted into the FEA software, replacing

the CTE, and the dimensional change due to both the CME and CTE analyzed to determine the overall

dimensional change. If the RMS is within the tolerance of ±0.5 mm then, theoretically, the reflector

should meet the SKA specification. The FEA output for the CME would be similar to the one shown for

the DVA-1 in Figure 5.

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Figure 5 Example of FEA output for CTE with temperature difference of 25C

5 Testing

Testing will be conducted by CRN at the University of British Columbia’s (UBC) Okanogan campus.

The testing will be led by Bryn Crawford and Dr. Abbas Milani along with a team of students.

5.1 Environmental Chamber

Samples in an environmental chamber, called the Thermotron shown in The Thermotron

environmental chamberFigure 6, will be exposed to 60C and 85% relative humidity. Samples will be

removed every 2 weeks to be tested and measured.

Figure 6 The Thermotron environmental chamber [16]

5.1.1 Coefficient of Moisture Expansion

Five samples with dimensions 100mm x 80mm (length by width) were placed in the Thermotron

on July 27th, 2015. On the 10th of August the samples were removed and measured according to ASTM

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standard D5229. The mass, density and specific gravity were measured using a densimeter and the

length, width and thickness were measured using a digital calliper to a 100 th of a millimetre or 10

microns. These measurements will continue to be made every 2 weeks until September 21 st; Mm is

expected to be reached in the first 25-30 days of exposure.

Table 1 Average dimensions, mass, volume and S.G. for 5 samples being used to determine CME

Batch Dimension (mm) Densimeter

x y z Mass (g) Volume (cm3) S.G.

Control

(July 27th)

100.08 80.03 5.26 62.705 39.142 1.602

Exposure 1

(August 10th)

100.02 80.07 5.22 62.807 39.128 1.605

Exposure 2

(August 24th)

- - - - - -

Exposure 3

(September 7th)

- - - - - -

Exposure 4

(September 21st)

- - - - - -

Table 2 Standard deviations for the averages shown in Table 1

Batch Dimension (mm) Densimeter

x y z Mass (g) Volume (cm3) S.G.

Control

(July 27th)

0.0050 0.0191 0.1274 0.278 0.146 0.0054

Exposure 1

(August 10th)

0.0057 0.0719 0.0823 0.293 0.135 0.0045

Exposure 2

(August 24th)

- - - - - -

Exposure 3

(September 7th)

- - - - - -

Exposure 4

(September 21st)

- - - - - -

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Table 3 CME results calculated using equations 1, 5 and 6

Batch Strain M(t) CME (/ %)

x y z x y z

Exposure 1

(August 10th)

-0.00060 0.00050 -0.0076 0.163 -0.00369 0.00307 -0.0466

Exposure 2

(August 24th)

- - - - - - -

Exposure 3

(September 7th)

- - - - - - -

Exposure 4

(September 21st)

- - - - - - -

5.1.2 Four Point Bending

Twenty samples with dimensions 140mm x 15mm (length by width) were placed in the

Thermotron on July 27th, 2015. These samples were tested according to ASTM standard D6272; initial

results from these tests show that the aluminum and 30gsm fibre glass veil seem to buckle first. The

support span length is 119.4mm and the loading span is 62.17mm for all 4-point bending tests.

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Figure 7 Control sample failing on the compressive side of the 4 point bending test, the side at which the aluminum is ~1mm from the surface [16]

Five control samples were tested for load vs. displacement; from which flexural strain, stress and

modulus were calculated using equations 7-9.

0 1 2 3 4 5 6 7 80

200

400

600

800

1000

1200

1400

1600

Control 1Control 2Control 3Control 4Control 5

Displacement (mm)

Load

(N)

Figure 8 Load-displacement curves of control samples 1-5, collected July 27th

As shown in Figure 8 the control samples failed between 1179-1338N when subjected to the 4-point

bending test, the failure always occurring on the compressed side, the side with the aluminum.

Equation 7: Flexural strain

ε f=6dδ

(Ss−Ls )2

d = sample depth, (mm)δ = displacement, (mm)Ss = support span, (mm)Ls = loading span, (mm)

Equation 8: Flexural stress (MPa)

σ f=3 F (Ss−L s)4bd2

F = load, (N)b = sample width, (mm)

Equation 9: Chord modulus (GPa)

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E f=σ fi+1−σ fiε fi+1−ε fi

Chord modulus is one of 3 ways to calculate Young’s modulus of elasticity as outlined in ASTM D790-10.

Local stiffness is calculated between points as shown in equation 9.

0 1 2 3 4 5 6 70

200

400

600

800

1000

1200

1400

Exposure 1AExposure 1BExposure 1CExposure 1DExposure 1E

Displacement (mm)

Load

(N)

Figure 9 Load-displacement curves of exposure 1 samples A-E, collected August 10th

As shown in Figure 9 the control samples failed between 1242-1309N when subjected to the 4-point

bending test.

Table 4 Summary of data

Statistic July 27th

(Controls)

August 10th

2 weeks

August 24th

4 weeks

September 7th

6 weeks

September 21st

8 weeks

Flexural

strength (MPa)

Average 2.586 259.6 - - -

Standard dev. 13.17 6.278 - - -

Modulus (GPa) Average 10.04 9.744 - - -

Standard dev. 0.1257 0.1565 - - -

As shown in Table 4 after 2 weeks there was a 0.3% increase in maximum stress on average and a 3%

decrease in modulus.

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5.1.3 Dynamic Mechanical Analysis

The Dynamic Mechanical Analyser (DMA) will be used to perform creep tests at elevated

temperature, ~80C. They will be performed in a TA Instruments Q-800 DMA, using a 3-point-bending

fixture [16].

6 Conclusions

Based on review and analysis of literature it is unlikely that moisture absorption will lead to

consequential dimensional change of the SRC reflector. Initial test results of CME between |0.00307 and

0.0468| will have to be input into the FEA software to determine what the effect is on the 18m x 15m

parabolic dish. There was a 0.3% increase in the maximum stress and a 3% decreases in modulus for the

4 point bending test. Further conclusions are dependent on the completion of testing, scheduled for the

21st of September.

7 Recommendations

Analysis of the completed test results

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8 References

[1] Jodrell Bank Observatory, Exploring the Universe with the World’s Largest Radio Telescope. Cheshire, UK: SKA Organisation, 2013.

[2] J. Sloan, “Composites Steady Radio Telescope Reflector,” High-Performance Composites, vol. 22, no. 4, pp. 36-41, 2014.

[3] D. Crabtree. (2013, September). “NRC Herzberg News,” Canadian Astronomical Society [Online]. Available: http://casca.ca/?p=4716

[4] F. L. Matthews and R. D. Rawlings, “Polymer matrix composites,” in Composite Material: Engineering and Science, London, UK: Chapman & Hall, 1994.

[5] L. L. Sobrinho et al., “The Effects of Water Absorption on an Ester Vinyl Resin System,” Depart. of Metall. and Mat. Eng., Rio de Janeiro, Brazil, No. 3, 353-361, 2009, vol. 12.

[6] Average Weather in Sainte Marie, Madagascar. (2010). World Weather & Climate Information [Online]. Available: http://www.weather-and-climate.com/average-monthly-Rainfall-Temperature-Sunshine,sainte-marie,Madagascar [July 31, 2015]

[7] C. H. Shen and G. S. Springer, “Moisture Absorption and Desorption of Composite Materials,” Depart. of Mech. Eng. The Uni. of Mich., Ann Arbor, Mich., January 1976, vol. 10.

[8] A. Naceri, “An Analysis of Moisture Diffusion According to Fick’s Law and the Tensile Mechanical Behaviour of a Glass-Fabric-Reinforced Composite,” Mechanics of Comp. Mat., 2009, vol. 45.

[9] Styrene (Styrene Monomer). (September 2014). Look for Diagnosis [Online]. Available: https://lookfordiagnosis.com/mesh_info.php?term=styrene&lang=1 [August 18, 2015).

[10]J. Chaudhary, “Synthesis, Characterization and Curing of Vinyl Ester Resin,” Depart. of Polymer Sci. Uni. Col. of Sci. MLS Uni., J. Environ Nanotechnol., (42-45), 2013, vol. 2.

[11]Multi-modal vinyl ester resins, by S. J. J. La, G. R. Palmese, and J. M. Sands. (2005, May 6). WO 2005118657 A2 [Online]. Available: http://www.google.com/patents/ WO2005118657A2?cl=en [August 18, 2015].

[12]Ester. (2015, July 29). Wikipedia [Online]. Available: https://en.wikipedia.org/wiki/Ester [July 31, 2015]

[13]Dr. A. V. Rau, Vinyl Ester Resins, June, 2015.

[14]J.W.M. Baars, R. Lucas, J.G. Mangum, and J.A. Lopez-Perez. (2007). “Near-Field Radio Holography of Large Reflector Antennas,” IEEE Antennas and Propagation Magazine [Online], vol. 49, no. 5. Available: http://arxiv.org/pdf/0710.4244.pdf

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[15]Y. Arao, J. Koyanagi, H. Hatta, Y. Aoki, and H. Kawada. (2007, July). “Effect of Moisture Absorption on Dimensional Stability in Carbon/Epoxy Composites,” in 16 th International Conference on Composite Materials [Online], Kyoto, Japan. Available: http://www.iccm-central.org/Proceedings/ICCM16proceedings/contents/pdf/FriJ/FrJA1-02ge_araoy224568p.pdf [July 31, 2015]

[16]B. Crawford and Dr. A Milani, “Environmental Exposure and Dimensional Stability of the SKA composite Telescope Laminate,” Project Report, August, 2015.

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