Investigation of toughening behavior of epoxy resin by reinforcement of depolymerized latex rubber

DOI 10.1515/secm-2013-0292 Sci Eng Compos Mater 2014; aop

Mayank Agarwal , Mohamand Arif , Ankita Bisht , Vinay K. Singh * and Sunanda Biswas

Investigation of toughening behavior of epoxy resin by reinforcement of depolymerized latex rubber Abstract: An epoxy resin (EP) matrix has been modi-

fied with depolymerized natural rubber (DNR). The 0.5,

1.0, 1.5, 2.0, and 2.5 wt% DNR-filled epoxy were used

for the present investigation. The primary aim of this

development is to scrutinize the mechanical properties

of such cured epoxy filled with DNR. When the rub-

ber content was low, the mechanical strength was low

and the free volume of DNR in epoxy matrix was less.

With the increase in rubber content, the free volume of

rubber in the composite increases and the mechanical

strength increases; however, after a specific weight per-

centage of rubber, if we increase the amount of rubber,

the mechanical strength decreases and the free volume

of rubber in the composite increases quickly, but with

the increase in DNR weight percentage in epoxy matrix,

the hardness decreases. The scanning electron micros-

copy (SEM) results justified the results obtained from

the mechanical tests.

Keywords: depolymerized natural rubber; epoxy resin;

mechanical properties.

*Corresponding author: Vinay K. Singh, College of Technology, G.B.

Pant University of Agriculture and Technology, Pantnagar-263 145,

Uttarakhand, India, e-mail: [email protected]

Mayank Agarwal, Mohamand Arif, Ankita Bisht and Sunanda Biswas: College of Technology, G.B. Pant University of Agriculture

and Technology, Pantnagar-263 145, Uttarakhand, India

1 Introduction Epoxy resins (EP) are used in a variety of applications

because they are a very important class of thermoset-

ting polymers that exhibit high tensile strength, excel-

lent chemical and corrosion resistance, good thermal

stability, low density and low creep, and reasonable

performance at elevated temperature. Hence, they are

widely used in structural adhesives, surface coatings, and

electrical laminates and as matrix resins for reinforced

composite materials. However, general epoxy systems

usually suffer the shortage of toughness due to the high

levels of cross-linking, which can and usually does result

in brittle behavior. Liquid rubber can be used as a tough-

ing material for EP because it is normally derived from

synthetic rubber. During the polymerization, the rubber

phase separates because it becomes less miscible with

the epoxy matrix, forming a sludge of rubber that is dis-

persed in the EP matrix. Several methods have been pro-

posed to increase the toughness of EP by the addition of

rubber in uncured EP and then controlling the polymeri-

zation reactions in order to restrict the phase separation

[1 – 6] . The rubbery materials that are added to the uncured

epoxy are types of copolymers with variable acrylonitrile

contents. The studies reported that, to modify EP, mostly

modified liquid rubber was used, such as liquid rubber

modified by divinylbenzene (DVB), hydroxyl terminated

butadiene (HTPB), carboxyl terminated butadiene-acry-

lonitrile (CTBN), or isocyanate terminated polybutadi-

ene (NCOPBER) [4, 7 – 9] . However, due to the increasing

awareness of environmental issues, natural latex rubber

has attracted great interest because it is a renewable

resource. In the present study, natural rubber latex is used

as a toughing material for EP. The properties of modified

epoxy are studied by tensile test. The dispersion of rubber

in the matrix of epoxy is verified by the scanning electron

microscopy (SEM) test.

2 Materials and methods

2.1 Epoxy resin

The bisphenol A-type EP (CY230) used for this study

was purchased from M/s Petro Araldite Pvt. Ltd.

(Chennai, India). Epoxy (CY230) is widely used in indus-

trial application because of its high strength and good

mechanical adhesiveness. It is also a good solvent and

has good chemical resistance over a wide range of

temperature.

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2 M. Agarwal et al.: Investigation of toughening behavior of epoxy resin

2.2 Hardener HY951

The hardener HY951 purchased from M/s Petro Araldite Pvt.

Ltd. was used as a curing agent. In the present investigation,

9 wt% has been used in all materials developed. The weight

percentage of hardener used in the present investigation is

as per the recommendation of Singh and Gope [10]

2.3 Natural rubber latex

Natural rubber latex (NR) was purchased from M/s Allied

Business (Pantnagar, India). It has 60% dry rubber content.

The NR has outstanding flexibility and high mechanical

strength. Moreover, it is a renewable resource, whereas its

synthetic counterparts are mostly manufactured from non-

renewable oil-based resources. Therefore, NR has created

a high level of interest regarding its use and its derivatives.

2.4 Preparation of the material

2.4.1 Depolymerization of NR

Depolymerization means opening the active linkage in the

polymer backbone by the reaction of a reagent with reactive

polar group. Depolymerization can reduce the chain length

of natural rubber. In general, a depolymerized natural

rubber (DNR) can be obtained by mastication, photolysis,

chemical decomposition, or the like of the natural rubber.

Mastication is a method for accelerating reduction in the

molecular weight by breaking the rubber molecular chains

of the raw rubber through a mechanical action and heating

in a roller mill or internal mixer and then adding a peptiz-

ing agent such as a mercaptan [11] . Vitaly and Eduardo [12]

used the photolysis method for breaking the molecular

chains with light energy, that is, ultraviolet light. Another

approach that has been used to reduce the molecular

weight of natural rubber is chemical decomposition. This

method is the degradation of molecular chains by chemical

reagents. In 1996, Tanaka et al. [13] proposed the process for

depolymerizing natural rubber, which comprised adding

a carbonyl compound to natural rubber latex or depro-

teinized natural rubber and then subjecting the resulting

natural rubber or deproteinized natural rubber to air oxida-

tion in the presence of a radical-forming agent. The results

showed that the DNR having a narrow molecular weight

distribution can be obtained at high reaction efficiency.

In the present method, 60% total dry content natural

rubber latex was diluted by distilled water to a concen-

tration of 5 wt% based on rubber content followed by the

Table 1 Compositions of cured epoxy filled with DNR.

Designation of composition

EP (CY230) (wt%)

Hardener (HY951) (wt%)

DNR (wt%)

C0 100 9 0.0

C1 100 9 0.5

C2 100 9 1.0

C3 100 9 1.5

C4 100 9 2.0

C5 100 9 2.5

addition of CH 3 CH

2 COCH

3 and K

2 S

2 O

8 in an amount of 4 – 6

vol% of total volume and 2 wt% based on the rubber content,

respectively. The pH of latex was adjusted to about 9 – 10 with

10 wt% aqueous KOH solution. Then, the reaction mixture

was mechanically stirred at a speed of 200 – 300 rpm at 60 ° C

on the sand (for the equal distribution of heat) for 24 h in

the presence of air. At the end of the reaction, the reaction

mixture was coagulated by 1 wt% aqueous CaCl 2 solution.

The coagulated substance was dissolved in n -hexane and

stirred with magnetic bar for 12 h. Then, the resulting solu-

tion was allowed to stand overnight and filtered. The filtrate

mixture was bathed with methanol followed by vacuum

drying at 40 ° C until the weight is made constant.

2.4.2 Preparation of rubber-filled EP

The DNR was dissolved completely in EP (CY230) at 100 ° C

using a mechanical stirrer at a speed of 500 rpm for 2 h. After

2 h, the whole solution is taken out and allowed to cool to a

temperature of 80 ° C. When a temperature of 80 ° C has been

attained, a 9 wt% hardener is mixed immediately. After the

addition of the hardener, viscous solution was again mixed

mechanically by a high-speed mechanical stirrer. The

viscous solution so obtained is poured into different moulds

for sample preparation for tensile testing. The compositions

of cured epoxy filled with DNR are given in Table 1 .

3 Results

3.1 Characterization of cured epoxy filled with DNR

The specimens were gold coated and examined by SEM

using a LEO435V6 instrument. The accelerating voltage

was kept at 10 kV and magnification factor of × 250. The

SEM test was conducted on the fractured surface to see the

mechanism of the fracture. The state of dispersion of DNR



M. Agarwal et al.: Investigation of toughening behavior of epoxy resin 3

A B

C D

Figure 1 SEM images of fracture surface for several cured epoxy: (A) pure epoxy, (B) 0.5 wt% rubber, (C) 1.0 wt% rubber, (D) 1.5 wt% rubber.

into the resin matrix plays a significant role on the improve-

ment of the mechanical properties of the cured epoxy. It is

seen from Figure 1 A – D that DNR are well dispersed in the EP

matrix and sharp surface failure occurs in all the tests. The

absence of any voids indicates a good adhesion between

the DNR and epoxy matrix. Figure 1A – C shows that the DNR

added to the epoxy matrix has completely bonded with it

and there is no free volume of DNR in the epoxy matrix.

From Figure 1D, it is evident that there is some free volume

of DNR that has not chemically bonded with the matrix, as

estimated from the observations. Due to this reason, the

mechanical strength increases up to 1 wt% rubber and then

decreases with further increase in the weight percentage of

rubber due to the excess or free volume of rubber.

3.2 Mechanical properties

The tensile test specimen ( Figure 2 ) prepared for each

weight percentage of DNR was loaded in uniaxial tension

on a 100 kN servohydraulic universal testing machine

(ADMET, Norwood, MA, USA) at 0.1 mm/s crosshead speed

according to ISO 1608:1972. The standard gauge length of

the specimen should be given by L 0 = 5.65 √ A

0 , where A

0

is the cross-sectional area of specimen (m 2 ) and L 0 is the

standard gauge length of the specimen (m).

From the stress strain curves as shown in Figure 3 ,

the ultimate strength, modulus of elasticity, and percent

elongations were determined. The room temperature and

humidity during testing were 32 ° C and 88%, respectively.

Remarkable differences have been observed in the stress

strain behavior due to the addition of DNR in the EP matrix.

3.3 Tensile properties

From the results, remarkable differences can be seen on

the ultimate tensile strength of DNR-filled cured epoxy

having different weight percentages of DNR tested at

0.1 mm/s crosshead speeds given in Table 2 . It can be seen

from the results that, for all specimens containing 1.0 wt%

DNR, the ultimate tensile strength is highest from among

the other compositions reported. About 42% increase in

ultimate tensile strength due to the addition of 1.0 wt%

DNR has been noticed compared to pure epoxy. This

increase in strength is observed due to the intermolecular




Grip section

Gagelength

Width ofgrip section

Dia. or width

“Reduced” section

Figure 2 Specimen of tension test.

80

70

60

50

40

Str

ess

(MP

a)

30

20

10

00 0.02 0.04 0.06 0.08 0.10

0 wt% of R

1.0 wt% of R

2.0 wt% of R1.5 wt% of R

2.5 wt% of R

0.5 wt% of R

0.12 0.14

Strain

Figure 3 Stress strain curve of different weight percentages of DNR.

Table 2 Tensile properties of cured epoxy filled with different weight percentages of DNR.


DNR (wt%) Ultimate strength (MPa)

Elongation (%) Toughness (MPa)

Modulus of elasticity (MPa)

C0 0.0 47.40 5.10 1.737 1889.28

C1 0.5 52.28 9.50 3.001 1067.73

C2 1.0 67.33 12.58 5.229 912.74

C3 1.5 41.30 11.00 2.956 671.76

C4 2.0 39.89 7.17 1.416 526.75

C5 2.5 35.26 6.80 1.226 513.89

1480

13

12

11

10

9

8

7

6

5

4

Ulti

mat

e st

reng

th (

MP

a)

70

60

50

40

0

30

200.5 1.0 1.5 2.52.0

Ultimate strength (MPa)

% Elongation

Elo

ngat

ion

(%)

Rubber (wt%)

Figure 4 Mechanical properties of cured epoxy filled with different

weight percentages of DNR.

bonding between the rubber particle to the resin particles.

A further addition of DNR on the EP decreases the ulti-

mate tensile strength of the DNR-filled cured epoxy due

to excess rubber particles, which is present free without

bonding. Similar observations have been noticed for

percent elongation as shown in Figure 4 . About 2.47 times

increase in the modulus of elasticity has been observed

due to the addition of 1.0 wt% DNR at 0.1 mm/s crosshead

speed. A further addition of the DNR decreases the percent

elongation but is higher than the neat epoxy material.

About 1.4 times increase in the modulus of elasticity is

noticed for the 2.0 wt% DNR-filled cured epoxy.

The variation of the modulus of elasticity and tough-

ness with the variation of rubber weight percentage is

as shown in Figure 5 . In Figure 5, it is seen that a non-

linear relation exists between the modulus of elasticity

and weight percentage of filler materials. The maximum

modulus of elasticity is found for neat resin. It has been

noticed that toughness is found to be maximum at the

addition of 1.0 wt% DNR compared to pure epoxy. This

increase in strength is due to the proper intermolecular

bonding between rubber particle to the resin particles. A

further addition of DNR on the EP decreases the tough-

ness of the DNR-filled cured epoxy due to excess rubber

particles, which is present unbonded. Keeping in view the

importance of the modulus of elasticity and toughness in

design and analysis, an attempt has been made to model

an empirical relation of the following type to interpret the

filler polymer interaction.

Up to 1.0 wt% DNR

2 2

R RE/T=1101 (W ) -2014 (W )+1087, R =1 (1)

More than 1.0 wt% DNR

2 2

R RE/T=-195.1 (W ) +972.5 (W )-792.4, R =1

(2)



M. Agarwal et al.: Investigation of toughening behavior of epoxy resin 5

where E and T are the modulus of elasticity in MPa and

toughness in MPa, respectively. W R denotes the weight

percentage of DNR. In the present case, toughness behav-

ior is the opposite after the addition of more than 1 wt%

DNR.

3.4 Hardness

All hardness tests are conducted on a Rockwell hardness

testing machine supplied by P.S.I. Pvt. Ltd. (New Delhi,

India) on R scale. The effect of the weight percentage of

DNR on the hardness values of DNR-filled cured epoxy is

shown in Figure 6 . It is found that the hardness of neat EP

is 44 HRR. The hardnesses of the fabricated cured epoxy

filled with 0.5, 1.0, 1.5, 2.0, and 2.5 wt% DNR are 43, 41, 39,

37, and 36 HRR, respectively, as given in Table 3 .

Figure 6 indicates that the hardness decreases with

the DNR content, reflecting the reinforcement formed in

the DNR-filled cured epoxy. The variation of the ratio of

the modulus of elasticity with the hardness of DNR-filled

cured epoxy is shown in Figure 7 . Figure 7 illustrates that

the hardness value follows a nonlinear relation with the

modulus of elasticity of the DNR-filled cured epoxy. An

attempt has been made to correlate the modulus of elas-

ticity with hardness. The following correlation has been

obtained:

5 4 3

R R R

2 2

R R

E/H=-6.950 (W ) +49.76 (W ) -129.8 (W )

+151.8 (W ) -85.44 (W )+42.93, R =1,

(3)

where E and H are the modulus of elasticity in MPa and

hardness in R-scale, respectively. W R denotes the weight

6

5

4

3

2

1

0

Mod

ulus

of e

last

icity

(M

Pa)

2000

1800

1600

1400

1200

1000

800

600

4000 0.5 1.0 1.5 2.52.0

Modulus of elasticity

Toughness

Toug

hnes

s (M

Pa)

Rubber (wt%)

Figure 5 Modulus of elasticity and toughness of cured epoxy filled

with different weight percentages of DNR.

Har

dnes

s (H

RR

)

45

44

43

42

41

40

39

38

37

36

350 0.5 1.0 1.5 2.52.0

Rubber (wt%)

Figure 6 Hardness of cured epoxy filled with different weight

percentages of DNR.

Table 3 Hardness of cured epoxy filled with different weight

percentages of DNR.


DNR (wt%) Hardness

C0 0.0 44

C1 0.5 43

C2 1.0 41

C3 1.5 39

C4 2.0 37

C5 2.5 36

Mod

ulus

of e

last

icity

(M

Pa)

/har

dnes

s (H

RR

)

45

50

35

40

25

30

15

20

5

0

10

0 0.5 1.0 1.5 2.52.0

Rubber (wt%)

E/H

Poly. (E/H)

Figure 7 E/H ratio of cured epoxy filled with different weight

percentages of DNR.

percentage of DNR. Equation (3) indicates that the non-

linear relationship between the modulus of elasticity

and hardness has a good correlation. It shows that the




modulus of elasticity is directly related to the hardness of

this type of cured epoxy filled with DNR.

4 Conclusions A DNR-filled cured epoxy was prepared. Such DNR-filled

cured epoxy was experimentally characterized by means

of microscopy, tensile testing, and hardness testing.

Remarkable improvements in the mechanical proper-

ties have been noticed due to the addition of DNR in

EP. Regression models were developed to simulate the

mechanical behavior of such materials from the volume

content of the DNR.

Acknowledgment: The authors express their gratitude

and sincere thanks to Department of Science & Technol-

ogy, India for providing finance to carry out this research

work smoothly.

Received November 17 , 2013 ; accepted December 14 , 2013

References [1] Huang J, Kinloch AJ. J. Mater. Sci. 1992, 27, 2753 – 2762.

[2] Huang J, Kinloch AJ. J. Mater. Sci. 1992, 27, 2763 – 2769.

[3] Guild FJ, Kinloch AJ. J. Mater. Sci. 1995, 30, 1689 – 1697.

[4] Barcia FL, Amaral TP, Soares BG. Polymer 2003, 44,

5811 – 5819.

[5] Shukla SK, Srivastava D. Polym. Sci. 2006, 100, 1802 – 1806.

[6] Chikhi N, Fellahi S, Bakar M. Eur. Polym. J. 2002, 38,

251 – 264.

[7] Thomas R, Ding Y, He Y, Yang L, Moldenaers P, Yang W, Czigany T,

Thomas S. Polymer 2008, 49, 278 – 294.

[8] Saadati P, Baharvand H, Rahimi A, Morshedian J. Iranian Polym. 2005, 14, 637 – 646.

[9] Seluga U, Kurzeja L, Galina H. Polym. Bull. 2008, 60, 555 – 567.

[10] Singh VK, Gope PC. J. Reinforced Plast. Compos. 2010, 29,

2450 – 2468.

[11] Okwu UN, Akinlabi AK. J. Appl. Polym. Sci. 2007, 106,

1291 – 1293.

[12] Vitaly R, Eduardo A. Food Chem. 2005, 92, 235 – 254.

[13] Tanaka Y, Sakaki T, Kawasaki A, Hayashi M, Kanamaru K,

Shibata K. US Patent 5,1999,856,600.



Investigation of toughening behavior of epoxy resin by reinforcement of depolymerized latex rubber

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