Dynamic Mechanical Behavior of Resin/Hardener Ratio Based Epoxy Variants

Arpit Sharma, N. Rajesh, Vijay Kumar Surla, Rajesh Kitey

1

Indian Institute of Technology Kanpur

I. Introduction

Fiber reinforced polymer composites are widely used in aerospace applications due to

their superior specific strength and elastic modulus. Since these composite structures are often

subjected to repeated thermo-mechanical loading, their damping characteristics should also be

considered as one of the major design parameters. In general a polymer composite’s strength and

stiffness decrease with increasing damping, therefore the choice of the combination of strength,

stiffness and damping is inevitably guided by the composite’s end usage. It is shown in the past

that the damping properties of polymer composite can be tailored by controlling its constituent

properties and the laminate geometry [1]. The key motivation behind current study is to develop

application oriented polymer matrix based on their dynamic mechanical and thermal

characteristics.

The epoxy is chosen for this study because the availability of wide variety of stable

chemical compositions of its constituents (epoxy resin and curing agent) makes it possible to

tailor the cured material properties. Besides the epoxy also have favorable matrix material

characteristics such as easy processibility, excellent mechanical, fracture and adhesive properties,

and better resistance to moisture ingression. The chemical reaction between resin and hardener

(curing agent) during curing process develops three dimensional cross linked polymeric chains.

The crosslink density and the flexibility of chains between crosslinks control the physical and

mechanical characteristics of the epoxy [2]. In this study the crosslink densities are varied by

choosing several resin/hardener ratios. Thus prepared epoxy variants’ dynamic mechanical

properties are investigated for a range of temperature and loading frequencies.

1 Corresponding author, email id: kitey@iitk.ac.in

II. Literature Review

Few researchers have studied the effect of curing agent’s functionality, chemical content

and its ratio in the resin/hardener (R/H) mixture on the material properties of neat epoxy, however

their conflicting conclusions, especially with regards to the mechanical properties, demands a

systematic study in this field. Also, the effect of resin/hardener ratio on the damping properties of

cured epoxy is not apparent from the literature. Bignotti et al. [3] investigated the effect of

hardener content in epoxy resin/hardener mixture on the curing process, material structure and the

glass transition temperature. They observed that near stoichiometric composition the crosslink

density and glass transition temperature reaches maximum. Garcia et al. [4] have investigated the

effect of amine based curing agent’s functionality (mixed with DGEBA resin) on the mechanical

and fracture properties of the epoxy material. Both tensile and flexural moduli were shown to be

decreasing with increasing crosslink density (or the increasing number of functional groups

present in the curing agent). On the other hand the glass transition temperature and the fracture

toughness were reported to be higher for the epoxy of higher crosslink densities. Crawford and

Lesser [5] have studied the effect of molecular weight on the mechanical and fracture behavior of

epoxy system. They observed that the elastic properties are independent of the molecular weight

whereas the glass transition temperature and the yield strength decrease with increasing molecular

weight. They also showed that the fracture toughness increases with a change in fracture mode

from brittle to ductile as the molecular weight of the epoxy increases. d’Almeida and Monteirob

[6] reported that both elastic modulus and yield stress of epoxy decrease with increasing hardener

content whereas the ductility increases with increasing amount of hardener in resin/hardener

mixture. Aziz [7] conducted a detailed study to note the effect of resin/hardener ratio on several

mechanical properties of epoxy material. He showed that the elastic modulus of cured epoxy

attains maximum value when the hardener content is slightly larger than that recommended for

stoichiometric ratio. Similar conclusions were drawn about other mechanical properties such as

impact strength, flexural strength, compressive strength and ultimate tensile strength.

III. Material and Test Specimens

The epoxy resin (LY556) diglycidyl ether of bisphenol-A (DGEBA) and the curing agent

(HY951) triethylenetetramine (TETA/HY951) of densities, 1.15 g/cm3 and 0.98 g/cm

3,

respectively, were carefully mixed. Four different resin/hardener ratios, 7.5:1, 10:1

(stoichiometric ratio), 12.5:1 and 15:1, by weight were used. The mixture was stirred for ~20

minutes in humidity and temperature control environment before being poured into a mold. The

room temperature curing was chosen to minimize residual stresses. The material was kept in the

mold for approximately 48 hours followed by free atmosphere curing for about a month. The cast

sheets were machined into samples of dimension, 50 mm x 10 mm x 4 mm, and tested using

Dynamic Mechanical Analyzer (DMA) at several temperatures and frequencies. The flat beam

specimens with end taps were also prepared to conduct uniaxial tension tests.

III. Results and Discussions

Effect of R/H ratio on E’ and Tan – Temperature scan

The effect of epoxy R/H ratio on storage modulus (E’) for varying temperature is

illustrated in Fig. 1. The temperature is gradually increased at 30C/min and the experiments were

conducted at 10 Hz frequency in tensile mode by applying 10 m dynamic displacement. The

plots show glassy state, glass transition region and rubbery plateau, a typical temperature

dependent dynamic mechanical behavior of a polymer. The E’ decreases linearly with increasing

temperature for all R/H ratios in glassy state (temperature less than 600 C). The rate of decrease in

E’ with temperature (E’/0C) in this region is nearly the same for all specimens. For a given

temperature the 10:1 R/H ratio specimens show maximum storage modulus. The specimens’

storage moduli at room temperature (250 C) are plotted in Fig. 2. The increase or decrease in R/H

ratio from 10:1 monotonically decreases E’. A steep drop in E’ is observed in glass transition

zone (see the temperature range of 600 - 90

0 C in Fig. 1). Nearly parallel curves for all specimens

suggest that the hardener content does not affect the decreasing rate of storage modulus (E’/0C).

However, rapid drop in E’ at different time instants in this zone suggests that the glass transition

temperature is affected by R/H ratio. Beyond 900 C (rubbery plateau) all specimens show

negligible E’.

The Tan variation with temperature is plotted next in Fig. 3. The curves depict rapid

rise in Tan after ~ 600 C, followed by a steep drop upon reaching the peak values. This behavior

corresponds to the sharp decrease in E’ in glass transition zone. The temperature corresponding to

the peak of Tan curve is referred as glass transition temperature (Tg) and plotted in Fig. 4 for

considered R/H ratios. A monotonic increase in Tg is observed for any deviation in R/H ratio

from 10:1.

Effect of R/H ratio on E’ – Frequency scan

The effect of epoxy R/H ratio on frequency dependent dynamic mechanical behavior is

investigated by conducting DMA experiments at varying frequency in tensile mode. The

frequency ranging from 1 to 20 Hz is applied with 10 m dynamic displacement at room

temperature. A moderate linear increase in E’ with increasing loading frequency is observed for

all R/H ratios (see Fig. 5). For a given frequency the maximum and minimum E’ correspond to

the specimens prepared with 10:1 and 15:1 R/H ratios, respectively. To compare E’ with the

elastic modulus (E) of the material the storage modulus curves are extrapolated to zero Hz

frequency and plotted in Fig. 6. The 10:1 emerges to be the optimum R/H ratio to attain

maximum storage modulus. Any change in epoxy constituents’ proportion monotonically

decreases E’. The elastic modulus of the specimens, obtained by conducting uniaxial tension tests

in displacement control mode at the loading rate of 0.1 mm/min, are also plotted in Fig. 6 along

with E’ values. An excellent match between E’ and E is evident from the plots, especially for

10:1 and 12.5:1 ratio specimens.

Figure 1: Variation of Storage Modulus with temperature in epoxy specimens using DMA at 10 Hz

frequency

Figure 2: Effect of epoxy resin/hardener ratio on tanδ. Measurements performed using DMA at 10

Hz frequency.

Ratio (Resin:Hardener)

Tg Tan delta @Tg Tan delta

@25deg

7.5:1 83.18 1.345205 0.009107775

10:1 81.383 1.134763 0.013277

12.5:1 82.325 1.287715 0.0146896

15:1 84.9 1.38883 0.01495065

Table.2 Values of Glass Transition temperature(Tg),Tan delta at Tg and 25 for

corresponding ratios.

Frequency dependent Mechanical behavior of Epoxy system observed on DMA:

Mechanical Properties of Epoxy system observed on UTM:

It is observed that the Hardener rich systems have high Ultimate tensile strength and

Resin rich systems have low ultimate tensile strenghs. The Table.3 Shows the Ratios of systems

and the loads at which the specimens broken(Ultimate Tensile loads) for the specimens having

dimensions approximately 85*18*8mm.As said by J.R.M. d’Almeida” & S.N. Monteiro The

hardener rich systems have large deformation capacity.

Ratio (Resin:Hardener) UltimateTensile

load(KN)

Elastic

modulus(Gpa)

7.5:1

10:1

12.5:1

15:1

Figure 3: Effect of Epoxy Resin/Hardener ratio on Storage Modulus at varying frequency. DMA

measurements are performed at 25°C (room temperature)

Figure 4: Variation of Storage Modulus at room temperature (32 °C). DMA measurements for

temperature-sweep are done at 10 Hz frequency.

Figure 5: Variation of Glass Transition Temperature, Tg. DMA measurements for temperature-

sweep are done at 10 Hz frequency.

Figure 6: Variation of Storage Modulus in static analysis. The Static tests (blue squares) are

performed with 20 ton hydraulic UTM machine at room temperature and red dots are the values

which are obtained by extrapolating the data to zero Hz, obtained in DMA frequency test at room

temperature.

Figure 7: Variation of tanδ measured a Glass Transition Temperature, at Tg, correspondingly. DMA

measurements for temperature-sweep are done at 10 Hz frequency.

IV. Conclusions

The dynamic mechanical behavior of resin/hardener ratio based epoxy variants is

investigated. The experiments were conducted using DMA-100 in tensile mode with a static

displacement of 50 m. The temperature range of -200 C to 160

0 C and the loading frequency

varying from 1 Hz to 200 Hz were chosen to study the effect of hardener content on viscoelastic

properties of epoxy. Results indicate significant effect of resin/hardener ratio on elastic and

damping properties as well as on thermal characteristics. The storage modulus at 1 Hz frequency

was compared to the elastic modulus obtained from uniaxial tension test. A detailed analysis and

discussion is presented in full paper.

Acknowledgement

Authors thank Indian Institute of Technology Kanpur for supporting this research through

IITK initiation grant. Authors also acknowledge summer intern Shivani Ghai’s enthusiastic

contribution in this study.

References

1. Saravanos, D. A., Chamis, C. C., “Unified Micromechanics of Damping for Unidirectional

and Off Axis Fiber Composites”, Journal of Composites Technology and Research, Vol. 12,

No. 1, Spring 1990, pp. 31-40.

2. Hwang J. S., Yim M. J., Paik K. W., “Effect of epoxy functionality on the properties and

reliability of the anisotropic conductive films for flip chips on organic substrates”, Journal of

Electronic Materials, Vol. 35, No. 9, 2006, pp. 1722-1727.

3. Bignotti F., Pandini S., Baldi F., Santis R. D., “Effect of the resin/hardener ratio on curing,

structure and glass transition temperature of nanofilled epoxies”, Polymer Composite, Vol.

32, 2011, pp. 1034-1048.

4. Garcia F. G., Soares B. G., Pita V. J. R. R., Sánchez R., Rieumont J., “Mechanical properties

of epoxy networks based on DGEBA and aliphatic amines” Journal of Applied Polymer

Science, Vol. 106, No. 3, 2007, pp. 2047-2055.

5. Crawford E., Lesser A. J., “Brittle to ductile: Fracture toughness mapping on controlled

epoxy networks”, Polymer Engineering and Science, Vol. 39, No. 2, 1999, pp. 385-392.

6. d’Almeida J. R. M., Monteirob S. N., “The Effect of the resin/hardener ratio on the

compressive behavior of an epoxy system”, Polymer Testing, Vol. 15, 1996, pp. 329-339.

7. Aziz M. E., “A Study on the effect of hardener on the mechanical properties of epoxy resin”,

M. S. Thesis, University of Technology, Republic of Iraq, 2010.

The stoichiometric ratio for DGEBA/TETA Epoxy system is 12.8phr (7.8125:1).At this

ratio all the molecules of Resin and Hardener are equally bonded. If we increase the ratio beyond

the stoichiometric means we are increasing the unreacted resin molecules similarly decreasing the

ratio means we are increasing the unreacted Hardener molecules in the epoxy system. The present

work is based on increasing the the ratios of epoxy system (DGEBA/TETA) from 7.5:1 to 15:1 at

an increment of 2.5 and finding out the variation of Dynamic mechanical properties as a function

of Temperature(-20deg to 160deg) at 10Hz Frequency, as a function of frequency(1Hz to 200Hz)

at room Temperature and Finding out the Mechanical properties of the Epoxy system from the

UTM as well.

Temperature dependent Mechanical behavior of Epoxy system observed on DMA:

The Fig1.shows Graphs of The Storage modulus as a function of Temperature for all the

Ratios. It is observed that The 10:1 has got high Elastic modulus before It starts to fall

down.From 10:1 as we increasing the Resin content The E’ slightly decreasing. Similarly From

10:1 If we increase the hardener content the E’ slightly decreasing. This can be attributed to the

unreacted resin or hardener molecules in the system which has the tendency to move when the

load is applied. The E’ values for corresponding ratios at a certain temperature (25 degrees) are

shown in Table.1. Of all the plots the E’ for 10:1 is dropping first just before Tg and then 12.5:1

and then 15:1.

Stoichiometric calculations:

Resin: Diglycidyl ether of bisphenol-A (DGEBA) (LY556)

Molecular weight :380gm

Hardener: triethylene tetramine (TETA) (HY951)

H2N(CH2)2NH(CH2)22NH(CH2)2 CH2

6 carbons =6*12= 72

4 nitrogens =4*14= 56

18 hydrogens =18*1= 18

Molecular weight=72+56+18=146

There are 6 amine hydrogens functionally reactive with Epoxy group

Therefore Equivalent weight =146/6=24.3

The equivalent weight of Epoxy resin=390/2=190 (since it has two epoxide groups

participating in reaction)

24.3/190=12.8/100=12.8phr (12.8 gms of hardener to 100 gms of resin) or 7.8125:1

(7.8125 gms of resin to 1 gm of hardener).

Reference : Handbook of thermoset plastics By Sidney H. Goodman page:210

The Fig.2 shows the graphs of Tan delta as a function of Temperature for all the ratios.

From 10:1 as we increase the resin content or hardener content the glass transition temperature

slightly increases having the low value for 10:1.. Damping at glass transition temperature is also

following the same pattern.The Table.2 Shows the Ratios of Epoxy systems and their

corresponding Tg’s and Damping(Tan delta) at Tg, 25deg .From Table.2 Tan delta is increasing

gradually from 7.5:1 to 15:1.we can say that from the stoichiometric ratio as we increase the resin

content means we are increasing damping capacity of a material.

Results and Discussions :