. 

i Material Properties of B&Modal 

Epoxy Networks 

Stephen J. MacKenzie 

Thomas J. Mulkern 

Nora C. Beck Tan 

ARL-TR-2413 MARCH 2001 

20010426 062 

Approved for public release; distribution is unlimited. 
The findings in this report are not to be construed as an official Department of the Army position 

unless so designated by other authorized documents. 

Citation of manufacturer’s or trade names does not constitute an official endorsement or approval of 

the use thereof. 

Destroy this report when it is no longer needed. Do not return it to the originator. 
Material Properties of Bi-Modal Epoxy 

Networks 

Stephen J. MacKenzie 

University of Delaware 

Thomas J. Mulkern 

Nora C. Beck Tan 

Weapons & Materials Research Directorate, ARL. 

Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5066 

ARL-TR-24 13 March 200 1 

Approved for public release; distribution is unlimited. 

i 
Abstract 

The necessity for epoxies in new technologies is overwhelming. For 

these epoxies to perform at their maximum capabilities, both high 

strength and toughness must be in the list of mechanical properties. 

This is where bi-modal networks become important. Their 

combinations of both long- and short-chain cross linkers vary the 

cross-link density of the epoxy and thus improve the toughness. Here, 

a study was done to see if these bi-modal networks increase 

toughness without loss in modulus. Through the use of the differential 

scanning calorimeter to find the glass transition temperature, the 

dynamic mechanical analyzer to find the storage modulus, and the 

Izod impact machine to find the impact strength, we can make a 

conclusion whether these networks really work. In the beginning, the 

results seemed to be what we had hoped for, that is, the decrease in 

cross-link density did improve the impact strength; however, the 

storage modulus also decreased with a decrease in cross-link density. 

Finally, the conclusion can be stated that the bi-modal network does 

increase the toughness of this system but with a loss in modulus. 

ii 
ACKNOWLEDGMENTS 

This work was partially funded through a summer student fellowship for Stephen 

MacKenzie from the Center for Composite Materials at the University of Delaware. 

Portions of this work were completed as part of the undergraduate senior thesis 

work of Stephen MacKenzie in the Department of Chemical Engineering at the 

University of Delaware. The advice and collaboration of Dr. Roy McCullough, 

Department of Chemical Engineering, University of Delaware, is also gratefully 

acknowledged. 

. 

. 

. 

. . . 

111 
INTENTIOIVALLYLEFTBLANK 

. 

iv 
Contents 

1. Introduction and Literature Survey .................... 

2. Experimental Background ........................... 

3. Project Objectives .................................. 

4. Description of Equipment. ........................... 

5. Experimental Methods .............................. 

6. Results ........................................... 

7. Discussion ........................................ 

8. Conclusion ....................................... 

References ............................................. 

Appendix 

A. Sample Calculations. ............................... 

Distribution List ........................................ 

Report Documentation Page ............................... 

Figures 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

13. 

Sample DSC Curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

SampleDMACurve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

A Typical DSC Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

A Typical DMA Setup. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

The Notched Izod Impact Machine . . . . . . . . . . . . . . . . . . . . . . . . 

A 3-D Diagram of the Mold Used in Making the DMA and Izod 

Samples........................................... 

Glass Transition Versus the Percent D-230 in a Br-DGEBA Epoxy 

System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Glass Transition Versus Mole Percent D-230 With an EPON 828 

Epoxy System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Impact Strength Versus Mole Percent D-230 at Room Temperature . . 

Impact Strength Versus Mole Percent D-230 at -180” C . . . . . . . . . . 

Impact Strength Versus Mole Percent D-230 at T,-60’ C . . . . . . . . . . 

Average Impact Strength Versus Mole Percent D-230 at Room 

Temperature........................................ 

Average Impact Strength Versus Mole Percent D-230 at -180” C . . . . 

1 

4 

7 

7 

10 

15 

17 

30 

31 

33 

39 

41 

9 

16 

16 

18 

18 

19 

20 

21 

V 
14. 

15. 

16. 

17. 

Average Impact Strength Versus Mole Percent D-230 at T,-60” C . . . . 

Average Impact Strength Versus Mole.Percent D-230 With Error . . . . 

Average Impact Strength Versus Glass Transition Temperature at 

Room Temperature With Error . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Average Impact Strength Versus Glass Transition Temperature at 

-180” C With Error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

18. 

19. 

Average Impact Strength Versus Glass Transition Temperature at 

60”CBelowT,WithError . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Average Impact Strength Versus Experimental Temperature for 

100 Mole % D-230 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

20. Average Impact Strength Versus Experimental Temperature for 

93 Mole % D-230 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

21. Average Impact Strength Versus Experimental Temperature for 

90Mole%D-230 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

22. Average Impact Strength Versus Experimental Temperature for 

88 Mole % D-230 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

23. Average Impact Strength Versus Experimental Temperature for 

84Mole%D-230 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

24. Storage Modulus Versus Mole Percent D-230 at Room 

25. 

26. 

27. 

,28. 

Temperature........................................ 27 

Storage Modulus Versus Mole Percent D-230 at T,-60’ C . . . . . . . . . 27 

Storage Modulus Versus Glass Transition Temperature . . . . . . . . . . 28 

Storage Modulus Versus Impact Strength at Room Temperature. . . . . 29 

Storage Modulus Versus Impact Strength at T,60” C . . . . . . . . . . . . 29 

Tables 

1. 

2. 

3. 

4. 

Calculated Table of Epoxy and Cross Linker Weight Percents 

Used in Mixing the Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Sample Chart of Data Recorded for Each Specimen Before the 

Izod Experiment Was Performed. . . . . . . . . . . . . . . . . . . . . . . . . . 

Dimensions of the Five Samples Used in the DMA Experiments 

as Required by the Manufacturer . . . . . . . . . . . . . . . . . . . . . . . . . . 

Reference Table for Figures 7 Through 22 . . . . . . . . . . . . . . . . . . . . 

21 

22 

23 

23 

24 

24 

25 

25 

26 

26 

11 

13 

14 

17 

vi 
MATERIAL PROPERTIES OF BI-MODAL JPOXY NETWORKS 

1. Introduction and Literature Survey 

The use of epoxies in today’s technologies has become an important part of 

manufacturing quality goods. Epoxy resins were first offered in 1946 and are 

now used in a wide variety of industries. Nearly half of all epoxies sold in the 

United States go toward protective coatings, while the remainder is used in 

structural applications such as laminates and composites, tooling, molding, 

casting, electronics, construction, bonding and adhesives [l]. The characteristics 

of epoxy resins are high chemical and corrosion resistance, good mechanical and 

thermal properties, outstanding adhesion to various substrates, low shrinkage 

upon cure, flexibility, good electrical properties, and the ability to be processed 

in a variety of conditions [l]. These characteristics, along with a long service life, 

make epoxies a necessity in the future growth of new technologies. 

The identifying characteristics of epoxy resins are the presence of a three-member 

ring known as the epoxide group. A long aliphatic, cycle-aliphatic, or aromatic 

backbone connects two epoxide rings. The capability of these rings to react with 

curing agents yields insoluble and intractable thermoset network polymers. 

Epichlorohydrin and bisphenol A-derived resins are the most widely used epoxy 

resins. The outstanding performance characteristics of‘the resins are conveyed by 

the bisphenol A moiety (toughness, rigidity, and elevated temperature 

performance), the ether linkages (chemical resistance), and the hydroxyl and 

epoxy groups (adhesive properties and formulation latitude) 121. 

An epoxy network is formed when epoxide groups are reacted with a diamine 

cross-linking agent in the stoichiometric ratio of 2:l epoxies per cross linker. The 

mechanical properties of epoxies depend on their network structure and crosslink density. For example, in a cured epoxy resin of fixed chemistry, the strength 

and modulus of the resin increase as the cross-link density of the network 

increases, while the fracture toughness of the resin decreases with increasing 

cross-link density [3]. Toughness is related to how the material reacts to stress 

concentrations such as sharp cracks [4]. Another property is the glass transition 

temperature of the epoxy network, which has an effect on the strength and 

modulus as well as the toughness. The glass transition temperature, unique for 

each epoxy system, is the temperature where the material starts to exhibit 

rubber-like characteristics, for’ example, an increased toughness and loss of 

modulus 151. The glass transition temperature is also affected by the cross-link 

density; that is, as the cross-link density increases, so does the glass transition 

temperature; therefore, the modulus and the toughness of the material are 

directly affected by the glass transition temperature of the epoxy 151. 
Since both high modulus and high fracture toughness are required of structural 

materials, sacrifices in strength and toughness generally must be made in order to 

design useful epoxies. (The term toughness refers to a material with both high 

strength and high ductility in which strength is the amount of energy required to 

produce permanent deformation of the material [31). This is where the properties 

of bi-modal networks interact. A bi-modal network is an epoxy network that 

uses both short- and long-chain cross linkers instead of one or the other. This bimodal distribution of cross-linker chain length will vary the cross-link density 

and may produce a material with both strength and toughness [61. 

Previous studies of the mechanical properties of bi-modal networks have shown 

that the varying cross-link density of bi-modal networks leads to superior 

mechanical properties [71. A study by Madkour and Hamdi, done at the 

Department of Chemistry at Helwan University in Cairo, Egypt, shows that 

elastomers cured with sulfur and polyquinine cross linkers of greater lengths (a 

lower cross-link density) produce a bi-modal network with exceptional 

mechanical properties 171. These properties include high strength and toughness. 

When stress-strain measurements were performed on these elastomers with two 

cross-linking systems, the maximum Young’s modulus values were obtained after 

three days of continuing aging time, when the maximum bi-modal chain length 

distribution took place 171. This maximum bi-modal chain length distribution is 

when the polymer exhibits the most amount of varying cross-link densities and 

thus as an even distribution of both long- and short-chain cross linkers 

throughout the polymer system. 

Mechanical property studies done on single modal networks show .that an 

increase in cross-link density increases stiffness and decreases the elastic 

modulus 181. Thus, a decrease in cross-link density would decrease stiffness and 

increase the elastic modulus. At the lnstitut National des Sciences AppZiquees de 

Lyon in Villeurbanne Cedex, France, a study was done on the influence of chain 

flexibility and cross-link density on mechanical properties of epoxy-amine 

networks. The amount of chain flexibility is in direct proportion to the length 

between the epoxide rings, and the amount of cross-link density is based on the 

length of the cross-linking agents. The final conclusion was that two main factors 

affected the toughness of epoxy networks: chain flexibility and cross-link 

density 191. 

A joint study between the Department of Chemistry at Cairo University and the 

Department of Chemistry and Polymer Research Center at the University of 

Cincinnati shows that elastomers with high functionality cross links, that is, two 

distinct lengths, can be viewed as bi-modal networks. Studies done on these 

elastomers show that both short and long chains are present and this presence of 

a large number of short chains gives rise to a remarkably high modulus [lo]. 

2 
Another way of toughening epoxies is to induce elastic rubber particles into the 

epoxy matrix. This is a more conventional method than the use of bi-modal 

networks for toughening materials. J.F. Hwang and others from the Center for 

Polymer Science and Engineering at Lehigh University conducted a study of the 

structure-property relationships in rubber-toughened epoxies. They concluded 

that the structure was two phases consisting of spherical rubber particles 

dispersed in the epoxy matrix. This rubber toughening increased impact strength, 

plain strain fracture toughness, and fracture energy, while Young’s modulus and 

yield strength decreased slightly with rubber incorporation [ill. (Plain strain 

fracture toughness is a test done on a narrow strip of sample in which the sample 

is compressed by narrow plates until the sample fractures.) The result is a 

measure of the toughness of the material. 

At the Institute of Materials Science and Technology in Mar de1 Plata, Argentina, 

C.I. Callo and others performed a similar study using rubber-toughened hybrid 

epoxies. His study was conducted with diglycidyl ethers of bisphenol A 

(DGEBA) epoxy and carboxly terminated butadiene acrylo-nitrile (CTBN) rubber 

particles to form the network. Results were similar to those of Hwang and 

showed that introducing a rubber phase into the epoxy matrix increases fracture 

toughness and lowers flexural modulus and yield stress [12]. Rubber toughening 

sounds very promising, but there are some serious disadvantages to inducing 

rubber particles into an epoxy matrix. One is that as the material toughens, the 

modulus will decrease, which happens because more rubber particles that have 

lower glass transition temperature and modulus are added. Another problem is 

the bonding characteristics between the rubber and the epoxy; some epoxies will 

not bond with the particles, leaving a material with inherent weak spots. The 

final problem will occur mostly in industrial applications when ‘complex 

composite pieces need to be manufactured. Here, the rubber particles are 

excluded by the tightly woven fabrics during processing, leaving all the rubber 

particles on one side of the fabric and the epoxy on the other 1131. This is when 

the bi-modal epoxy networks become important; they do not exhibit the last two 

problems associated with rubber-toughened epoxies, and after extensive 

research, it is possible that they will not exhibit the first problem. 

In this study, we hypothesize that mixing two different lengths of cross linkers 

(each with the same chemistry) with the epoxy will directly induce the bi-modal 

network and thus toughen the material. This method is unique because in past 

studies, the bi-modal network was always obtained by varying the extent of 

curing by mixing two different cross linkers with varying chemistry together with 

the epoxy, or the epoxy was toughened by the introduction of rubber particles. 

The introduction of the long-chain cross linkers is in itself like rubber toughening 

because epoxy made with only the long-chain cross linkers is very rubber like. 

Therefore, one can conclude that the bi-modal epoxy network is like a pseudo 

rubber-toughened epoxy without any of the manufacturing problems. If 

successful, this study will show that a directly induced bi-modal network will 

3 
produce both strength and toughness in an epoxy network and produce better 

results than rubber-toughened epoxies. Past experiments show that bi-modal 

networks have enhanced mechanical properties on other polymer systems, but 

no one has proved that it will do the same for a directly induced epoxy system. 

2. Experimental Background 

In order to conduct this study of the mechanical properties of bi-modal 

networks, several experiments were performed in a specified order. The glass 

transition temperature was measured first with a differential scanning 

calorimeter (DSC); then the elasticity and modulus were measured with a 

dynamic mechanical analyzer (DMA). The final study was an impact study with 

an Izod impact machine. 

The glass transition temperature is the temperature at which the material begins 

to exhibit liquid properties. This transformation occurs when the molecules’ 

translational motion starts to occur and the thermal motion starts to increase 

1141. The increase in thermal motion can be measured with a DSC in the form of 

heat capacity. A DSC is an isoperibol calorimeter, which means that the 

environment is kept at a constant temperature, while the temperature changes 

with time (heating rate) are governed by the thermal resistance between 

calorimeter and surroundings 1141. The difference in measured heats is between 

the sample and a reference, which are both present inside the DSC and are 

heated at the same rate. A reference is just an empty aluminum sample holder 

with no sample present. The precision of a DSC is quite good. A typical sample 

weighs 20 mg and shows a heat capacity of 50 mJ/“K, that is, the precision is 

fO.OO1 J, or a measured precision of 20%. This is an acceptable value for heat 

capacity that changes slowly with temperature 1141. The glass transition 

temperature will be measured for all samples, and those that present a glass 

transition temperature of 30” or more above room temperature will be studied 

further. These samples will be used because they are all “glassy” at room 

temperature and if samples that have glass transition temperatures below room 

temperature were used, then biased results would occur. Figure 1 is a sample 

curve of the results obtained from the DSC; notice the onset lines used to find the 

glass transition temperature. 

A Dh4A will be used on the glassy materials to study the modulus and the 

elasticity. The modulus is a constant for each material that relates stress to strain, 

in which the stress is directly proportional to the strain and is independent of the 

strain rate, that is, perfect elasticity. This perfect elasticity is an ideal behavior 

described by Newton’s law. The idealized laws cannot describe the behavior of 

matter if the ratios of stress to strain or of stress to rate of strain are not constant. 

4 
Plastic deformation is a common example of such non-ideal behavior. It occurs 

for solids if the elastic limit is exceeded and irreversible deformation takes place 

[15]. The DMA is not the only piece of equipment that will analyze a material 

and calculate the modulus and the elasticity. One such common method is the 

tensile test, in which the material is subjected to a continually increasing uniaxial 

tensile force while observations are made of the elongation of the specimen. 

From these data, a curve of the stress-strain relationship is produced. The only 

problem with this method is that a large amount of material is required to do 

these tests, compared to the few milligrams of material needed to run a DSC 

experiment. The DMA senses and quantifies all changes in the degree of 

molecular motion in a material and gives the elastic storage modulus (log of 

Young’s modulus) and damping (tangent of the deformation angle) over its full 

temperature and frequency range. A damping peak occurs in sympathy with 

each motional change that signals an increase the dissipation of energy [141. 

Figure 2 is an example of a typical DMA result curve. Notice how similar it is to 

a DSC curve except that modulus is recorded as a function of time. 

-3 On Set Curves 

Temperature 

Figure 1. Sample DSC Curve. (Notice how the onset curves are used to find the T, 

glass transition temperature.) 

. 

The final study will involve the use of an Izod impact tester (American Standard 

for Testing Materials [ASTM] D256) in order to find the impact strength of the 

material. The impact strength or impact energy is the energy necessary to 

fracture a standard test piece with an impact load [3]. This type of experiment is 

used to determine the tendency of a material to behave in a brittle manner and 

will also detect differences between materials, which are not observable in a 

tension experiment. Three experiments must be done with the Izod, one at room 

temperature, one at a temperature that is 200°C below the lowest glass transition 

5 
temperature, and one at set incremented temperature from each of the samples’ 

respective glass transition temperatures. The reason behind the latter two 

experiments is to remove any effects that the different sample glass transition 

temperatures have on the toughness. As the samples’ glass transition 

temperature decreases, the material becomes more rubbery, and thus, its 

toughness increases. By testing each sample at different temperatures, the effects 

of the glass transition are removed, and each sample can be compared to the 

others without biased results. 

Temperature 

Figure 2. Sample DMA Curve (very similar to the DSC curve). 

There are three factors that contribute to a brittle fracture: a triaxial state of 

stress, a low temperature, and a high strain rate of rapid rate of loading. All 

three factors do not have to be present at the same time to produce brittle failure 

[HI. The Izod experiment uses the principle of turning kinetic energy into 

potential energy through the use of a swinging pendulum. The principal 

measurement from the impact experiment is the energy absorbed in fracturing 

the specimen and is frequently used for quality control and material acceptance 

purposes 141. 

6 
3. Project Objectives 

The purpose of this research is to investigate whether a bi-modal epoxy network 

will have improved strength and toughness relative to a similar regular epoxy 

network. An epoxy resin and two cross linkers of identical chemistry but 

I different molecular weight were chosen and a series of bi-modal networks was 

prepared from them. In the series of bi-modal networks, the cross-link density of 

the epoxy was varied by changing the mole ratios of the two cross linkers, while 1 keeping the total mole ratio of epoxy to cross linker at 2:l. The experiments 

included DSC to find the glass transition, followed by DMA and Izod 

experiments to find the modulus and impact strength, respectively. The 

toughness of the material is characterized by the impact strength, since a direct 

correlation can be drawn that relates impact strength to toughness. That is, as the 

impact strength increases, so does the toughness. 

4. Description of Equipment 

To complete any research project, different types of equipment are employed to 

perform the tedious tasks of running experiments. In this particular project, three 

major pieces of equipment were used to acquire knowledge of the material 

properties’ bi-modal networks: a DSC, a DMA, and an Izod impact machine. 

The DSC was the first piece of equipment used to analyze the glass transition 

temperature of each sample. This machine requires only a few milligrams of 

material, which are placed inside an aluminum pan and sealed. This pan is then 

placed inside the DSC, along with an empty reference pan. Here, the DSC 

measures the heat difference between the sample and a blank reference while 

increasing the temperature at a constant rate. The data are stored i.n a computer 

and plotted against temperature to find the glass transition. Figure 3 shows the 

DSC setup; notice the nitrogen and helium tanks that are used to purge the DSC 

cell to ensure a constant convection of heat inside the cell. Notice the RCS 

* (refrigeration cooling system), which allows one to cool a sample to -80” C. 

The next piece of equipment is the DMA, which is used to find the storage and 

loss modulus of a material. The DMA is very similar to the DSC in that it 

measures mechanical properties as a function of temperature, but the DSC does 

not impose stress. The sample used is a 21-mm by 5-mm by l-mm strip of 

material that is placed between a pair of vice grips. The sample is subjected to a 

cyclical pull-relax force as a function of temperature; that is, the material is 

stretched with a specified force. The result is the storage modulus of the material 

as a function of temperature. Unlike the DSC, the DMA is capable of cooling a 

7 
sample to -180” C with liquid nitrogen. Figure 4 shows a DMA setup; notice that 

there is no need for the purging gas. 

I In 

I _ (RCS 1 

Sample 

= Cooling 

System 

Computer DSC machine 

Figure 3. A Typical DSC Setup. 

A 

5 

C F 

Z 

-7 DMA Liquid 

Nitrogen 

Figure 4. A Typical DMA Setup. 

8 
. 

The Izod impact machine was used to determine the impact strength of a piece 

of material. The sample is struck with a 2-pound hammer and the amount of 

energy lost is determined. The principle of turning potential energy into kinetic 

energy makes this piece of equipment easy to operate. The most difficult part is 

cutting (i.e., “machining”) the sample to the correct specifications required by 

the ASTM D256 standard; this required the aid of an expert machinist. The 

readings from the experiment must be converted to impact strengths and 

corrected for friction losses with the equations provided in the ASTM D256 

standard. Figure 5 shows a schematic of the Izod machine; notice its simplicity. 

Impact Dial 

lzod Machine 

Specimen 

Striker 

/ 

I I 

Figure 5. The Notched Izod Impact Machine. 

The final piece of equipment used was the tensile mold that was used in the 

preparation of the Izod impact samples and the DMA samples. The mold was 

machined from a solid piece of aluminum that had a removable back in order to 

free the cured samples. The dimensions of the mold are 3.5 inches by 2.5 inches 

by 1.0 inch; Figure 6 shows a diagram of the mold. 

/ 

L 

/ 

/ 

Figure 6. A 3-D Diagram of the Mold Used in Making the DMA and Izod 

Samples. 

9 
Other, less significant pieces of equipment were also used: a vacuum oven to 

remove the bubbles from the molds. A slow speed saw used to cut the very thin 

samples necessary for the DMA; a hack saw and miter box to cut larger samples 

for the Izod, The combination of these lesser pieces of equipment with the three 

main pieces led to the successful completion of this project. A list of each piece of 

equipment, the model number, and manufacturer follows: 

1. DSC - TA Instruments Model 2920 modulated 

2. DMA - TA Instruments Model 2980 

3. Izod impact tester - Testing Machines, Inc., Model 43-1 

4. Vacuum oven - VWR’ Scientific 

5. Slow speed saw - Buehler Isomet 

5. Experimental Methods 

The raw materials used in these experiments were EPON’ 828 epoxies supplied 

by Shell Chemical Corporation, and the cross linkers used were D-230 and 

D-2000 Jeffamines from Huntsman Chemical Corporation. The 230 and 2000 

refer to the average molecular weight of the cross linkers. Three experiments will 

be performed on the final bi-modal networks: a DSC experiment to find the 

glass transition temperature of each sample, a DMA experiment to find the 

modulus, and an Izod experiment to determine the impact strength. 

To complete this research in a timely and orderly fashion, fixed procedures were 

followed. These methods will not only accomplish the desired goals but will also 

promote success and bring about quality work. Before doing any research on 

epoxy networks, one must first examine all the different epoxy systems that can 

be used to form the network. The characteristics that one looks for are the 

following: the epoxy and cross linker must be reactive with each other, they 

must be readily available, and when the network is formed, an even distribution 

of cross-link density must be observed. The determination of an even 

distribution of cross-link density .is commonly done by small angle X-ray 

diffraction. These characteristics were found for a number of systems, including 

the Br-DGEBA epoxy/Jeffamine system, which was too difficult to study 

because of high melting point of the epoxy, and the EPON 828/Jeffamine 

system; the latter system was chosen because it was easier to study. 

To ensure that every epoxy reacts with every cross linker, one must mix the two 

chemicals together in a 2:l mole ratio of epoxy to cross linker. These mole ratios 

‘Not an acronym 

10 
I - 

were maintained for each cross-linker mixture, and their subsequent weight 

fractions were calculated (see Appendix A): Then, 5- to lo-gram mixtures were 

made for each of the cross linker ratios (see Appendix B) in 0.5-cm aluminum 

pans. These pans were then placed in a vacuum oven drawing (i.e., “pulling”) 

15 inches mercury for 1 hour to remove any bubbles. A final vacuum of 

29.5 inches mercury was pulled for 3 hours to remove the few remaining bubbles 

before curing. The cure cycle for this system consists of a pre-cure for 2 hours at 

80” C, followed by a post-cure at 125” C for 8 hours. After curing, the pans were 

removed and a lo- to 20-mg sample was taken from the center of each sample. 

Table 1 shows the calculated epoxy and cross-linker weight percents used in the 

mixing of the samples. The numbers represent the amount of epoxy and cross 

linkers that must be added to ensure the correct mole ratio of epoxy to short to 

long cross linkers. 

Table 1. Calculated Table of Epoxy and Cross Linker 

Weight Percents Used in Mixing the Samples 

(The epoxy to cross linker mole ratio was always kept at a 2:l ratio.) 

Sample Ratio Wt % Epoxy Wt % D-230 Wt % D-2000 

m/o 77 23 0 

32/15/l 69 20 11 

22/10/l 66 18 16 

18/8/l 63 18 19 

12/5/l 59 15 26 

6/2/l 55 13 32 

9/3.5/l 48 10 42 

4/l/l 40 6 54 

9/l/3.5 35 4 61 

6/W 32 2 66 

12/l/5 31 2 67 

18/l/8 29 1 70 

2/O/l 27 0 73 

The samples were placed in aluminum pans and crimped for use in the DSC to 

find their glass transition temperature. Each sample was scanned twice, starting 

at -50” C and gradually increasing to 150” C at 10” C/minute. This temperature 

range was chosen to ensure that the glass transition temperatures for all the 

materials would be found. The first run in the DSC is to remove any nonequilibrium effects that remain from the cure cycle. All the samples were 

analyzed, and those samples that exhibited a glass transition of 20” above room 

temperature or greater were chosen for further study in the DMA and Izod 

experiments. The reason for choosing these samples is that one can only perform 

11 
experiments on materials that exhibit the same distinct phase-in this case, the 

glassy phase. All the networks that had glass transition temperatures of 40” C 

and below exhibited rubber-like characteristics at room temperature and would 

not be suitable for structural applications. 

The chosen systems were then cast into molds with the dimensions of 3.5 inches 

by 2.5 inches by 1.0 inch for further experiments. The preparation of the molds 

involved significantly more time and patience than with the pans. The total 

weight of each mold was around 160 grams, and with this much material, 

vigorous mixing for 15 minutes was required to ensure full reaction. The reaction 

was completed when the mixture changed from cloudy to clear. The dry mold 

was then coated with “Frekote,” a silicone-releasing agent, and filled with the 

sample. The mold was placed into the vacuum oven for 2 hours operating at 

15 inches mercury. Careful attention was made to ensure that the bubbles did 

not form too fast, which would result in an eruption of epoxy similar to that of a 

volcano. After 2 hours, a 25-inch mercury vacuum was pulled, again for 2 hours 

under careful supervision. Finally, a 29.5-inch mercury vacuum was pulled for 

24 hours to ensure complete bubble removal. The reason for removing the 

bubbles is that even the smallest bubble will act as a crack initiation site resulting 

in premature failure and inaccurate results from the Izod experiment. The molds 

were then heated for 24 hours at 125” C to ensure complete cure. The curing time 

was obtained from the supplier of the epoxy and past studies with similar 

systems 1161. These epoxies were then removed from the molds and cut into 

2.5-inch by 0.5-inch by 0.25-inch samples for future DMA and Izod experiments. 

The samples for Izod experimentation were labeled according to their mole 

ratios and mailed to DESS’ Machine, Smyrna, Delaware, to be precisely cut to 

match the requirements for the ASTM D-256 standard for Izod experimentation. 

This standard requires the samples to have a length of 2.5 inches, a width of 

0.5 inch, and the thickness can vary among the samples as long as each sample 

has even thickness throughout. The width from the base of the notch to the other 

side must be 0.4 inch; this will ensure the correct notch radius. Once returned, 

the samples’ length, width, and thickness were carefully measured with digital 

calipers to ensure that they matched the strict specifications of the ASTM 

standard. The dimensions of the machined samples are tabulated in Table 2. 

The Izod experiment was performed at room temperature for all the samples. Six 

attempts were made on the 2/1/O sample to obtain more accurate results, since 

the toughness of this sample will be used as a basis for comparison to the other 

four samples to see if the toughness increased. Three attempts at room 

temperature were performed on the other four samples, and the Izod reading 

was recorded. Since the glass transition temperatures of each of the five samples 

differ so much, three specimens from each of the five samples were placed into 

‘Not an acronym 

12 
liquid nitrogen for 7 to 8 minutes to ensure evaluation of each sample in glassy 

conditions. By doing this, one removes any biased results from the toughness, 

which are only increased by a difference in glass transition. This allows one to 

see if the bi-modal distribution really does increase the toughness. The final 

three specimens from each of the samples were used in the final Izod 

experiment. Each sample was cooled to 60” below its respective glass transition 

for 30 minutes; this again will ensure unbiased results. The cooling of the 

32/15/l samples was done with ice water which reached a temperature of 5” C. 

The cooling of the 22/10/l samples was done with an ice water and alcohol 

solution to achieve 0” C. The final two samples required temperatures below 

0” C; in order to do this, ethanol was cooled to its desired temperature with 

liquid nitrogen. The chilled solutions were placed into dewers to maintain a 

constant temperature. The 18/8/l samples were cooled to -8” C, and the 12/5/l 

samples chilled to -20” C. 

Table 2. Sample Chart of Data Recorded for Each Specimen 

Before the Izod Experiment Was Performed 

(The after-notch width indicates the width of the specimen from the base of the notch to 

the end of the sample. For this particular sample, two different types of Izod experiments 

were performed: one at room temperature and one with the specimens cooled with liquid 

nitrogen. Three Izod experiments were performed on the remaining four samples.) 

After-Notch 

Sample Number Length (in.) Width (in.) Thickness (in.) Width (in.) 

2/1/o 2-1 

2-2 

Room 2-3 

Temp 

--e-v 2-4 

----- 2-5 

----- 2-6 

Liquid 2-7 

Nitrogen 2-8 

----- 2-9 

Liquid 2-10 

Nitrogen 2-11 

30 min. 2-12 

2.502 0.502 0.236 0.408 

2.502 0.504 0.231 0.408 

2.502 0.503 0.243 0.407 

2.504 0.503 0.227 0.408 

2.504 0.502 0.226 0.409 

2.503 0.502 0.229 0.410 

2.504 0.502 0.261 0.407 

2.504 0.502 0.216 0.405 

2.508 0.500 0.272 0.408 

2.505 0.501 0.253 0.404 

2.502 0.500 0.214 0.404 

2.504 0.502 0.225 0.407 

The recorded data were corrected to account for windage and friction of the 

pendulum. This calculation was especially important because of the low impact 

strength of the samples. The correction factor was subtracted from the recorded 

13 
dial data, and this quantity was divided by the thickness of the sample. This 

final result is the impact strength of the material in foot-pounds per inch 

(ft-lb/in.). 

Finally, the DMA experiments were performed on the five samples used in the 

Izod experiments. The results obtained from the DMA experiments will be 

storage modulus or stiffness of the material as a function of temperature. The 

purpose of these experiments is to determine whether the toughness of the 

material can be enhanced without loss of modulus or stiffness. The experiments 

were performed with a TA Instruments DMA with a film tension clamp. Each 

sample was machined to the specifications required by the experiment by a slow 

speed saw. Table 3 gives the length, width, and thickness of the five samples 

used. 

Table 3. Dimensions of the Five Samples Used in the DMA 

Experiments as Required by the Manufacturer 

Sample Length (in.) Width (in.) Thickness (in.) 

2/1/o 24.68 4.74 0.96 

32/15/l 21.74 4.77 1.29 

22.10/l 22.74 4.77 1.04 

18/8/l 21.06 4.73 0.91 

12/5/l 20.95 4.83 1.04 

After cutting, each sample was placed in the clamping device lengthwise and the 

screws were hand tightened on both top and bottom. Then the screws were 

given a quarter turn with a torque wrench that applied 1 to 2 ft-lb of torque. 

Even though this amount of torque was less than recommended,, any more 

resulted in the compression of the specimen and ultimate failure during the 

experiment. The DMA cooled each sample to 75“ C below its respective glass 

transition temperature via liquid nitrogen. This will ensure an even baseline 

before any transition will begin. Then each sample was gradually increased to 

120” C at 5 ’ C per minute with 0.01 Newton as an initial static force with a l-Hz 

oscillation of 1.5 Newtons. Finally, the storage modulus at various temperatures 

could be read from the graph and compared to the impact strength for each 

specific sample. 

14 
6. Results 

The results show that as the cross-link density decreases, the glass transition 

temperature decreases, as expected. The Br-DGEBA system, which was first used 

to conduct the research, has been abandoned, even though the glass transition 

temperatures have been found. The abandonment was because of the extreme 

difficulty of removing the bubbles from the molds with a vacuum oven. The 

reason the bubbles did not want to come out is because the Br-DGEBA system is 

a solid at room temperature. Any attempts to increase the temperature to the 

system’s melting point and still pull a 29.5-inch mercury vacuum resulted in a 

bubbly mess of epoxy because the bubbles were being removed too fast. 

Therefore, a smaller vacuum was pulled while remaining at the melting point of 

the system. The only problem with this approach is that when the temperature 

was increased, the cure cycle started and the epoxy cured before all the bubbles 

could get out. This bubble removal problem led to the conclusion that this 

system could not be used to conduct further research. 

These results showed that an epoxy system, which was liquid at room 

temperature, must be chosen if bubble removal was to take place. This led to the 

choice of the EPON 828 epoxy system, which is very similar to the Br-DGEBA 

system, except that the EPON system is not brominated. The same experiments 

were run with the EPON system to find the glass transition temperature of each 

of the mole ratios. The results were just as expected: the lower the cross-link 

density, the lower the glass transition. Figures 7 and 8 show two graphs (one for 

the Br-DGEBA system and one for the EPON 828 system) which demonstrate 

how the increase of the mole percent of D-2000 (and thus the decrease in crosslink density) affects the glass transition. The sample number, weight-percent 

D-230, and corresponding glass transition temperatures are compiled in Table 4. 

These two graphs show that the cross-link density has a direct impact on the 

glass transition temperature and therefore on the toughness of the material. 

Notice how much the glass transition temperature in Figure 2 falls with just the 

slight addition of D-2000. Also, the glass transition temperature of the EPON 828 

system is affected much more by the addition of D-2000 because of the low 

molecular weight of the EPON epoxy. The results of Figure 8 show that a 5:l 

ratio of D-230 to D-2000 is the lowest system that can be used in further 

experiments because its Tg glass transition is the last system to be 20” or more 

above room temperature. 
100 _ 

c 

80 1 -- / 

60 _-- 

0 20 40 60 

Mole Percent of D-230 

80 100 

Figure 7. Glass Transition Versus the Percent D-230 in a Br-DGEBA Epoxy 

System. (Notice how much the temperature drops with just a small 

percent of D-2000. As the mole percent of D-230 decreases, the mole 

percent of D-2000 increases, so at 80 mole % D-230, there is 20 mole % 

D-2000 present.) 

100 

-20 _ -----_-. .-I-. -...-L-- 

-40 _ -- -.--- 

-60 _ -- _- -- 

-80 _ --_ 

-100 ' I ( I 

0 40 60 

Mole Percent of D-230 

80 100 

b 

. 

Figure 8. Glass Transition Versus Mole Percent D-230 With an EPON 828 Epoxy 

System. 

16 
Table 4. Reference Table for Figures 7 Through 22 

Sample Wt % Epoxy Wt % D-230 Wt % D-2000 Tg (C) , 

2/1/o 77 23 0 80 

32/15/l 69 20 11 65 

22/10/l 66 18 16 60 

18/8/l 63 18 19 57 

12/5/l 59 15 26 43 

7. Discussion 

Each of the following figures shows how the toughness is correlated with the 

mole percent of D-230, the glass transition temperature, or the storage modulus. 

Figure 9 shows the relationship between the impact strength and the mole 

percent of D-230 at room temperature. Here, the results are to be expected; the 

impact strength increases with a decrease in mole percent D-230, showing that 

there is a direct relationship between the resin composition and the impact 

strength. Figure 10 shows the same relationship, but this time at -180 “C, with 

liquid nitrogen. Here, each sample is evaluated well below its glass transition, so 

now a true comparison can be made between the samples. The results for the 

2/1/O and the 32/15/l samples seem to be a little high, suggesting that the 

impact strength may stabilize at high D-230 content. 

Figure 11 shows the same relationship again for impact strength measured at 

60” C below each sample’s respective glass transition temperature. These 

temperature conditions will normalize the experiment so that unbiased results 

can be obtained which compare the impact strength and the cross-link density. 

The results here differ greatly from those in the previous two studies. The impact 

strengths for the 2/1/O and 32/15/l samples stayed the same from the room 

temperature results, while the other three did not. Again, this should be the case 

since their respective glass transition temperatures are already well above room 

temperature, and cooling them any further should have no effect on the 

toughness. The other three samples’ glass transition temperatures are in the 

region of instability, f20”C of the reported glass transition temperature, at room 

temperature. 

’ 17 
0.75 _ I I I 

0.7 : Y I -- Legend 

82 M -- 86 ^- 88 -- YO 92 94 96 98 100 

Percent D-230 

Figure 9. Impact Strength Versus Mole Percent D-230 at Room Temperature. 

(Notice how the impact strength increases as the mole percent 

decreases. This directly shows that the impact strength increases with 

decreasing cross-link density.) 

0.75 

I I 

1 I Ll 

0.7 

s 0.65 

c, 

7 0.6 

& 

E nrr t- I I I I I I I I I = “.J.J 

$ 

g 0.5 

cd 

82 a4 ^- 86 -- 88 -- 90 92 94 96 98 100 

Mole Percent of D-230 

Figure 10. Impact Strength Versus Mole Percent D-230 at -180’ C. (Notice the 

decrease in impact strength from those at room temperature, except 

for the 12/5/l sample, which did not change.) 
0.42 

0.38 

0.36 

0.32 

0.28 

0.26 

82 84 86 88 90 92 94 96 98 100 

Mole Percent D-230 

Figure 11. Impact Strength Versus Mole Percent D-230 at T,-60” C. (Here, the 

results are similar to those taken at 180’ C except now the 12/5/l 

sample falls on a linear line with the rest.) 

The instability refers to when the sample is not completely in the glassy state or 

the rubbery state but more like a hybrid state. We would therefore expect their 

impact strengths to fall when measured at 60” C below their respective glass 

transition temperature (Ts-60 O C) because the room temperature experiment was 

done in that region of instability where the epoxies contained some rubber-like 

character. The 22/10/l and the 18/8/l samples’ impact strengths fell only about 

0.05 ft-lb/in, which is to be expected since we are far away from their glass 

transition temperatures. The 12/5/l sample fell about 0.30 ft-lb/in, which is a 

large drop compared to the others. This shows us that the true impact strength 

for the 12/5/l sample is not really that high when compared to the others. These 

results were to be expected because at a lower temperature, a material becomes 

brittle and its toughness decreases. Now we can compare each sample and draw 

a conclusion that at a normalized temperature, there is only a small 

improvement in the toughness of the epoxy from the bi-modal network. When 

comparing these results to those in Figure 10, we would expect the results to be 

similar because both studies are done at a temperature well below the glass 

transition temperatures, but this is not the case. We can only conclude that the 

experiments done at the liquid nitrogen temperature may have been tainted. The 

linear relationship between impact strength and cross-link density was 

maintained for all the experiments. 

19 
Figures 12 through 14 are the average results for the three impact strength 

measurements conducted for each sample at the three different temperatures 

chosen, with error. Notice that the error is very small for some of the results but 

large for others, and this is consistent for each experiment. Notice that the 

Ts-60” C experiments have the greatest error overall among all five different 

D-230 ratio samples. Thus, one should use caution to include the error range 

when comparing exact values of the impact strength measured during different 

conditions. Figure 15 compares the results of each experiment. One can see that 

the liquid nitrogen results are similar to the room temperature results, within 

experimental error, and that the liquid nitrogen values are higher than the 

T,-60” C values. This also suggests that the liquid nitrogen values are unusually 

high and may be tainted. 

0.25 ’ I I I ” 

82 84 86 88 90 92 94 

Mole Percent of D-230 

96 98 100 

Figure 12. Average Impact Strength Versus Mole Percent D-230 at Room 

Temperature. (Here are the combined results from the room 

temperature experiments with error. Notice how the 32/15/l 

sample is lower than the 2/1/O sample, contrary to expectations.) 

20 
g 0.7 

P 

g 0.65 

V 

86 88 90 92 94 96 98 100 

Mole Percent D-230 

Figure 13. Average Impact Strength Versus Mole Percent D-230 at -180” C. (Here 

are the combined liquid nitrogen results with error. Again notice the 

32/15/l ratio is lower than the 2/1/O.) 

0.42 

2 0.4 

r 

3 0.38 

5 

A 

sil 

0.36 

b % 0.34 

CA 

3 0.32 

$ 0.3 

%b ep 0.28 

3 

1 4 0.26 

0.24 

82 84 86 88 90 92 94 

Mole Percent D-230 

96 98 100 

Figure 14. Average Impact Strength Versus Mole Percent D-230 at T,60” C. (Here 

are the compiled results with error. Again the 32/15/l sample is 

lower than the 2/1/O.) 

21 
0.7 

2 

5 0.65 

2 0.6 

8 

g 0.55 

& m 0.5 

'; 

& 0.45 

kz &I 0.4 

E $ 0.35 

4 

0.3 

88 90 92 94 96 98 100 

Mole Percent D-230 

Figure 15. Average Impact Strength Versus Mole Percent D-230 With Error. 

The next three graphs, Figures 16 through 19, show the impact strength versus 

the glass transition temperature. These results show the same trend when 

compared to Figures 12 through 14, which should be the case since the mole 

percent of D-230 is directly related to the glass transition temperature. Here, you 

can also see that the impact strength increases with a decrease in the glass 

transition temperature, again proving that there is an inverse relationship 

between impact strength and cross-link density. Figures 19 through 23 show the 

average impact strength versus experimental temperature for each of the five 

samples. Here, one can see the different results obtained with each of the three 

experiments. The 2/1/O, 32/15/l, and 22/10/l samples show very little 

difference in the impact strength among the three results, while the 18/8/l and 

the 12/5/l samples’ impact strengths vary considerably among the three 

experiments. The results for the 18/8/l and the 12/5/l samples in Figures 22 

and 23 were expected. Their glass transition temperatures are very close to room 

temperature, so their toughness is very dependent on the experimental 

temperature. The other three samples displayed the logical result since their 

glass transition temperatures are farther from room temperature. 

22 
I2 0.7 

$ 0.65 

Y 

ct: 

E 0.6 

SJ E 0.55 

g 

m 0.5 

50 55 60 65 70 

Glass Transition Temperature (C) 

75 80 85 

Figure 16. Average Impact Strength Versus Glass Transition Temperature at 

Room Temperature With Error. (Notice how it follows the same 

trend as Figures 8 through 13.) 

0.75 

2 0.7 

$ 0.65 

tt: 2 0.6 

% 8 0.55 

b 1/(1 0.5 

Y 

g 0.45 

8 0.4 

& 

E 0.35 

22 

4 0.3 

0.25 

I I 

-- Le end 

d B 2/1/O Sample 

--__ e 32/15/l Sample 

lscl 22/10/l Sample 

* 181811 Sample 

r A 12lS~l Sample 

40 45 50 55 60 65 70 75 80 85 

Glass Transition Temperature (C) 

Figure 17. Average Impact Strength Versus Glass Transition Temperature at 

-180” C, With Error. (Again it follows the same trend as Figures 8 

through 13.) 

23 
0.42 I I I 

a * I 3 v.4 

$ 0.38 

5 

s 0.36 

sh 

40 45 50 55 60 65 70 75 80 85 

Figure 18. 

Glass Transition Temperature (C) 

Average Impact Strength Versus Glass Transition Temperature 

60°C Below Ts With Error. (Again it follows the same trend 

Figures 9 through 14.) 

-200 -150 -100 -50 

Test Temperature (C) 

0 50 k, 

Figure 19. Average Impact Strength Versus Experimental Temperature for 100 

mole % D-230. (Notice how the room temperature results and the T, 

60” C are very similar, which is not as expected. Also the impact 

strength at -180” C was expected to be lower than those at room 

temperature.) 

at 

as 

. 

24 
I 

$ 0.66 -.-__-_-.-. -- 

$ 0.61 

s 

a 0.56 

EJ 

B 

1 - / ~----, 

I 

I: 0.51 

rA 

‘j 0.46 

zd 

3 0.41 

fj 0.36 

3 

$ 0.31 

0.26 

-200 

Figure 20. Average Impact Strength Versus Experimental Temperature for 93 

mole % D-230. (This graph displays the same characteristics as 

Figure 19.) 

g 0.65 

P 

k 0.6 

s 

E 

!f 

0.55 

& 0.5 

m 

3 0.45 

l& $ 0.4 ----- 

% E 0.35 

$ 4 0.3 

0.25 

-200 -150 -100 -50 0 50 

Test Temperature (C) 

-100 -50 

Test Temperature (C) 

Figure 21. Average Impact Strength Versus Experimental Temperature for 90 

mole % D-230. (These results are consistent with the expected results 

that the impact strength at -180” C should be less than those at room 

temperature.) 

25 
g 

0.66 

g 0.61 

Figure 22. 

$ 0.66 

5 0.61 Y 8 

E 0.56 

2 s b 0.51 

m 

z 0.46 

g 

a 0.41 

' 0.36 E 

5 0.31 

0.26 

-200 

Test Temperature (C) 

Average Impact Strength Versus Experimental Temperature for 88 

mole % D-230. (This figure follows the same trend as Figure 21.) 

-150 -100 -50 

Test Temperature (C) 

0 50 

Figure 23. Average Impact Strength Versus Experimental Temperature for 84 

mole % D-230. (Notice how this figure follows the same trend as in 

Figures 21 and 22, but there is more scatter present in the results.) 

26 
2000 

1800 

4 

E 3 1600 

2 % 1400 

53 

I: I I I I I I ! I 

1000 

iIii-I-- 

800 

82 84 86 88 90 92 94 96 98 100 

Mole Percent D-230 

Figure 24. Storage Modulus Versus Mole Percent D-230 at Room Temperature. 

(Notice the linear trend that is very similar to the DSC results, 

2000 

1800 

Figure 8.) 

82 84 86 88 90 92 94 96 98 

Mole Percent D-230 

----- 

100 

Figure 25. Storage Modulus Versus Mole Percent D-230 at T,- 60” C. (Notice the 

linear relationship that is similar to Figure 24, except for sample 

12/5/l whose modulus actually increased with a decrease in 

temperature.) 

27 I 
Next, the storage modulus versus mole percent D-230 relationship is shown at 

room temperature and at T,60° C. The results primarily follow a linear 

relationship between the storage modulus and the mole percent D-230, but we 

can see that the 12/5/l sample does not follow this pattern. Here, the storage 

modulus at Ts-60” C is increased by 500 MPa beyond its room temperature value, 

which should be the case since a decrease in temperature stiffens the material 

and increases the modulus. However, this increase is about 5 times greater than 

the increase in the 32/15/l and 18/8/l samples and shows that something is 

definitely different with the 12/5/l sample. One may conclude that far away 

from the glass transition temperature and with a low cross-link density, the bimodal nature of the network is having some effect. Figure 26 shows the effect of 

the glass transition on the storage modulus; the relationship is linear as to be 

expected from previous discussions. Figure 27 shows how the storage modulus 

is related to impact strength at room temperature. Here, one can see that when 

the impact strength increases, the storage modulus decreases. This is the usual 

result but is not what we wanted to happen; we hoped to increase toughness 

without a loss in ,modulus. Here, we can see that the bi-modal network does 

decrease the modulus of the epoxy, which may be undesirable. When this study 

is done at -60’ C from the glass transition, the 12/5/l sample with the highest 

impact strength actually has a larger storage modulus than the 18/8/l and 

22/10/l samples, which have lower impact strengths. Even though Figure 28 

displays the results we wanted for the 12/5/l sample, an increase in strength 

without a great loss in modulus, the results were obtained at -18” C. Thus, the 

results may be of limited use since most design is for applications at room 

temperature. 

1800 

1000 -- .--.- -.-. __- 

.- 

40 45 50 55 60 6.5 70 7.5 80 85 

Glass Transition Temperature (C) 

Figure 26. Storage Modulus Versus Glass Transition Temperature. (Again this 

graph displays the same trend as Figure 24, which is to be expected 

since there is a direct relationship between cross-ink density and 

glass transition.) 

28 
1800 

. 

I I Lq3amp*e 2lllO 

‘--ISample 32fEl1 

H Sample 22/10/I 

* Sample 18lWI 

- A Sample 12/S/1 

800 

0.4 0.45 0.5 0.55 

Impact Strength (ft*lb/in) 

0.6 0.65 0.7 

Figure 27. Storage Modulus Versus Impact Strength at Room Temperature. (This 

figure shows that there is a loss in modulus as the impact strength 

increases.) 

0.26 0.28 0.3 0.32 0.34 0.36 0.38 0.4 

Impact Strength at Tg - 60 (ft*lb/in) 

Figure 28. Storage Modulus Versus Impact Strength at T, - 60” C. (Notice the 

scatter in this figure. The storage modulus should decrease with 

increasing impact strength; apparently this is not the case for the 

12/5/l sample.) 

29 
8. Conclusion 

A study of the relationship between a bi-modal cross-link distribution in an 

epoxy matrix and the mechanical properties of the material has been completed. 

We can conclude that there is a linear relationship between the cross-link density 

and the glass transition temperature, not only for the EPON 828 system but also 

for the Br-DGEBA system. This shows us that this relationship should hold for 

all epoxy systems with varying cross-link densities. From this relationship, one 

can choose a set of samples to study, all of which have glass transition 

temperatures of 20°C or more above room temperature. 

From the Izod and DMA studies of the five chosen systems, we can draw several 

conclusions. The cross-link density has a direct relationship with the impact 

strength; as the cross-link density decreases, the impact strength increases. We 

can also conclude that the toughness of the material is increased with a decrease 

in cross-link density. The studies performed at 60” C below the glass transition 

temperature were found to give true, normalized results. These results showed 

us that the bi-modal distribution did not increase toughness as much as the room 

temperature results suggested for the 12/5/l sample. When the study was 

performed at -180” C, the results were very different from what could reasonably 

be expected and no logical conclusion could be made from them. 

The DMA results proved to be the most revealing in this study. Here, we could 

see that the bi-modal distribution did increase the impact strength but not 

without a loss in modulus. This is what usually occurs but not what we wanted 

to happen. A new material that has increased toughness but loss of stiffness has 

little value over what is currently available. Even though the results obtained at 

60” C below the glass transition temperatures showed some promise in giving us 

strength plus toughness in EPON 828/Jeffamine systems, these results are not 

very useful because most applications do not require materials designed for use 

in a -18” C environment. However, it is possible that with a different choice of 

cross linkers and epoxies, a bi-modal cross-link distribution may offer some 

improved properties. Finally, one can conclude that the bi-modal distribution 

did increase the toughness of the material but with a loss in modulus. 

30 
/ I 

References 

1. 

. 

2. 

.a 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

I 

I 

10. 

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31 
11. Hwang, J.-F:, J.A. Manson, R.H. Hertzberg, G.A. Miller, and L.H. Sperling, 

“Structure-Property Relationships in Rubber-Toughened Epoxies,” Polymer 

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14. Wunderlich, B., “Thermal Analysis,” pp. 30, 71, 222, 244, 352, and 355, 

Academic Press, Inc., San Diego, CA., 1990. 

15. Cox, C.T., M.R. Ottaway, and R.E. Wetton, “Dynamic Mechanical Thermal 

Analyzer; A New Instrument and its Applications to Polymeric Materials,” 

taken from Proceedings of the Second European Svmposium on Thermal 

Analvsis, edited by Dollimore, David, Heyden, and Son Ltd., London, 1981. 

16. Wu, Wen-Li, and B.J. Bauer, “Epoxy Network Structures 3. Neutron - 

Scattering Study of Epoxies Containing Monomers of Different Molecular 

Weights,” Macromolecules, Vol. 19, No. 6, p. 1613,1986. 

32 
. 

, - 

APPENDIX A 

SAMPLE CALCULATIONS 

33 
’ I 

34 
SAMPLE CALCULATIONS 

Sample Calculations: Wind Correction Factor for lzod Tests 

h ,:=2.ft 

Maximum height of 

center of gravity of 

pendulum at start of 

L := d- .( 1.36s)’ 

i 4.x2 1 

E, :=0.03.ftdb 

Distance from fulcrum Energy correction for 

to center of gravity windage and friction. 

of pendulum 

test 

B mm:=acos[ l-[ (p).(1-?J]] 

E s := O.OPft-lb 

E m := 2.ft.lb 

Full scale reading 

from pendulum 

used 

Taken from the lzod reading of sample 

2 - 4. 

Maximum angle pendulum will travel with one swing of the pendulum. 

B :=acos[ l- [ (?).[l-z]]] Eb :=O.Ol.ft.lb 

Second correction for windage 

Angle traveled for a given specimen 

t := 0.223in 

Thickness of sample 2 - 4. 

%=[%- (;)]f-J+(;) 

Total energy correction for the breaking energy of 

a specimen. 

l *- CEs- Etc) 

s -- t 

ft,lb I,= - 

in 
Sample Calcualtions 

Here is a sample calcualtion for the weight fractions of epoxy and cross linkers. 

MW epoxy := 371 2 

32 

MW D230:= 230 22 

18 

MW D2000:=2000 12 

6 

Moles epoxy := 9 

4 

6 

9 

12 

18 

2 

Moles ~230 := 

1 

15 

10 

8 

5 

2 

3.5 

1 

1 

1 

1 

1 

0 

Moles D2000:= 

Total, := MW epoxy .Moles epoxyof MW D23UMoles D2300f MW D2000M01es D20000 

Total0 = 

MW epoxy ‘Mo1es 

Wt %.epoxy ‘= 

epoxy0 .lOO 

Total0 Wt %.epoxy = 

MW D230Moles D230 

Wt %D230:= O.100 

Total0 Wt %D230= 

Wt %D2000:= 

MW D2000M01es D20000 ,loo 

Total0 Wt %D2000= 

0 

1 

1 

1 

1 

1 

1 

1 

2 

1.5 

5 

8 

1 

36 
Sample Calculations: Error Analysis for the lzod lmapct Testing 

In this method we will use the standard t-test for finding the 80% confidence interval. The 80% 

confidence interval is commonly used for engineering materials. 

Part a. Room Temperature Study 80 % confidence t := 1.476 

The Impact data for the 2/1/O sample 

A% := 0.298277599 StD := 0.0467935 

Emor := t.?!!? 

& 

Error = 0.03 1 

Impact data for the rest of the samples 

Avg := 

n Z5 

n :=2 t := 1.886 

Error :=t.StD 

& Error = 

Part b: -180 OC Study 

All samples 

0.376528044 0.019867714 0.026 

0.380550491 0.02887 0.039 

Avg := 0.434780393 StD := 0.019798 Emor = 0.026 

0.412384705 

0.0065898 

Error i-t.7 

n 

-0.657987539 

8.788’10-3 

_1 0.05288 _ _ _ 0.071 

37 
Part c: Tg - 60 Study 

All the samples except the 2/1/O because the results are the same as in part a 

Avg := 

Il.oos10-3 7 

Error:=t.E Error= “03 

d- n 0.019 

0.015 

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4. TITLE AND SUBTITLE 5. FUNDING NUMBERS 

Material Properties of Bi-Modal Epoxy Networks 

PR: lL162618AHSO 

6. AUTHOR(S) 

MacKenzie, S.J. (U of DE); Mulkem, T.J.; Beck Tan, N.C. (both of ARL) 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS 

U.S. Army Research Laboratory 

Weapons & Materials Research Directorate 

Aberdeen Proving Ground, MD 2 1005-5066 

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REPORT NUMBER 

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U.S. Army Research Laboratory 

Weapons & Materials Research Directorate 

Aberdeen Proving Ground, MD 2 1005-5066 

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Approved for public release; distribution is unlimited. 

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13. ABSTRACT (Maximum 200 words) 

The necessity for epoxies in new technologies is overwhelming. For these epoxies to perform at their maximum capabilities, 

both high strength and toughness must be in the list of mechanical properties. This is where bi-modal networks become 

important. Their combinations of both Ion g- and short-chain cross linkers vary the cross-link density of the epoxy and thus 

improve the toughness. Here, a study was done to see if these bi-modal networks increase toughness without loss in modulus. 

Through the use of the differential scanning calorimeter to find the glass transition temperature, the dynamic mechanical 

analyzer to find the storage modulus, and the Izod impact machine to find the impact strength, we can make a conclusion 

whether these networks really work. In the beginning, the results seemed to be what we had hoped for, that is, the decrease in 

cross-link density did improve the impact strength; however, the storage modulus also decreased with a decrease in cross-link 

density. Finally, the conclusion can be stated that the bi-modal network does increase the toughness of this system but with a 

loss in modulus. 

4. SUBJECT TERMS 15. NUMBER OF PAGES 

bi-modal network impact strength 47 

epoxy 16. PRICE CODE 

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