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

  • Mayank Agarwal , Mohamand Arif , Ankita Bisht , Vinay K. Singh EMAIL logo and Sunanda Biswas
Published/Copyright: April 2, 2014
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Science and Engineering of Composite Materials
From the journal Science and Engineering of Composite Materials Volume 22 Issue 4

Abstract

An epoxy resin (EP) matrix has been modified 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 rubber 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 percentage 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 microscopy (SEM) results justified the results obtained from the mechanical tests.

Keywords: depolymerized natural rubber; epoxy resin; mechanical properties

1 Introduction

Epoxy resins (EP) are used in a variety of applications because they are a very important class of thermosetting polymers that exhibit high tensile strength, excellent 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 toughing 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 dispersed in the EP matrix. Several methods have been proposed to increase the toughness of EP by the addition of rubber in uncured EP and then controlling the polymerization 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-acrylonitrile (CTBN), or isocyanate terminated polybutadiene (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 industrial 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.

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 nonrenewable 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 peptizing 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 deproteinized natural rubber and then subjecting the resulting natural rubber or deproteinized natural rubber to air oxidation 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 concentration of 5 wt% based on rubber content followed by the addition of CH3CH2COCH3 and K2S2O8 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 CaCl2 solution. The coagulated substance was dissolved in n-hexane and stirred with magnetic bar for 12 h. Then, the resulting solution 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.

Table 1

Compositions of cured epoxy filled with DNR.

Designation of compositionEP (CY230) (wt%)Hardener (HY951) (wt%)DNR (wt%)
C010090.0
C110090.5
C210091.0
C310091.5
C410092.0
C510092.5

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 into the resin matrix plays a significant role on the improvement of the mechanical properties of the cured epoxy. It is seen from Figure 1A–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.

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.
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.

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 L0=5.65√A0, where A0 is the cross-sectional area of specimen (m2) and L0 is the standard gauge length of the specimen (m).

Figure 2 Specimen of tension test.
Figure 2

Specimen of tension test.

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.

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

Stress strain curve of different weight percentages of DNR.

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 bonding between the rubber particle to the resin particles. A further addition of DNR on the EP decreases the ultimate 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.

Table 2

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

Designation of compositionDNR (wt%)Ultimate strength (MPa)Elongation (%)Toughness (MPa)Modulus of elasticity (MPa)
C00.047.405.101.7371889.28
C10.552.289.503.0011067.73
C21.067.3312.585.229912.74
C31.541.3011.002.956671.76
C42.039.897.171.416526.75
C52.535.266.801.226513.89
Figure 4 Mechanical properties of cured epoxy filled with different weight percentages of DNR.
Figure 4

Mechanical properties of cured epoxy filled with different weight percentages of DNR.

The variation of the modulus of elasticity and toughness with the variation of rubber weight percentage is as shown in Figure 5. In Figure 5, it is seen that a nonlinear 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 toughness 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.

Figure 5 Modulus of elasticity and toughness of cured epoxy filled with different weight percentages of DNR.
Figure 5

Modulus of elasticity and toughness of cured epoxy filled with different weight percentages of DNR.

Up to 1.0 wt% DNR

(1)E/T=1101 (WR)2-2014 (WR)+1087, R2=1 (1)

More than 1.0 wt% DNR

(2)E/T=-195.1 (WR)2+972.5 (WR)-792.4, R2=1 (2)

where E and T are the modulus of elasticity in MPa and toughness in MPa, respectively. WR denotes the weight percentage of DNR. In the present case, toughness behavior 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.

Table 3

Hardness of cured epoxy filled with different weight percentages of DNR.

Designation of compositionDNR (wt%)Hardness
C00.044
C10.543
C21.041
C31.539
C42.037
C52.536
Figure 6 Hardness of cured epoxy filled with different weight percentages of DNR.
Figure 6

Hardness of cured epoxy filled with different weight percentages of DNR.

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 elasticity with hardness. The following correlation has been obtained:

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

E/H ratio of cured epoxy filled with different weight percentages of DNR.

(3)E/H=-6.950 (WR)5+49.76 (WR)4-129.8 (WR)3+151.8 (WR)2-85.44 (WR)+42.93, R2=1, (3)

where E and H are the modulus of elasticity in MPa and hardness in R-scale, respectively. WR denotes the weight percentage of DNR. Equation (3) indicates that the nonlinear 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 properties 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.

Corresponding author: Vinay K. Singh, College of Technology, G.B. Pant University of Agriculture and Technology, Pantnagar-263 145, Uttarakhand, India, e-mail:

Acknowledgment

The authors express their gratitude and sincere thanks to Department of Science & Technology, India for providing finance to carry out this research work smoothly.

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Received: 2013-11-17
Accepted: 2013-12-14
Published Online: 2014-4-2
Published in Print: 2015-7-1

©2015 by De Gruyter

This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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https://doi.org/10.1515/secm-2013-0292
Keywords for this article
depolymerized natural rubber; epoxy resin; mechanical properties
Creative Commons
BY-NC-ND 3.0

Articles in the same Issue

  1. Frontmatter
  2. Original articles
  3. Triticale straw and its thermoplastic biocomposites
  4. Morphological and mechanical properties of chemically treated municipal solid waste (MSW)/banana fiber and their reinforcement in polymer composites
  5. Structure-property responses of bio-inspired synthetic foams at low and high strain rates
  6. Water absorption and thickness swelling behavior of almond (Prunus amygdalus L.) shell particles and coconut (Cocos nucifera) fiber hybrid epoxy-based biocomposite
  7. Mechanical behavior of walnut (Juglans L.) shell particles reinforced bio-composite
  8. Melt grafting copolymerization of glycidyl methacrylate onto acrylonitrile-butadiene-styrene (ABS) terpolymer
  9. Investigation of toughening behavior of epoxy resin by reinforcement of depolymerized latex rubber
  10. Influence of tire-cord layers and arrangement direction on the physical properties of polyester tire cord reinforced with chloroprene rubber composite materials
  11. Biological effect of SAR on the human head due to variation of dielectric properties at 1800 and 2450 MHz with different antenna substrate materials
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  16. Influence of skin wrinkles and resin insertions in maximum stress of transitions of pure bending sandwich beams
  17. Development of a new method for design of stiffened composite pressure vessels using lattice structures
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