Original Article Studies on the effect of phenolic resin ...urpjournals.com/tocjnls/1_14v4i4_1.pdf · Original Article Studies on the effect of phenolic resin modifications on the

52 Advances in Polymer Science and Technology: An International Journal 2014; 4(4) : 52-59

ISSN 2277 – 7164

Original Article

Studies on the effect of phenolic resin modifications on the adhesive

properties of epoxy resin

Najuma Abdul Razack*, Lity Alen Varghese

Department of Chemical Engineering

National Institute of Technology, Calicut-673601, Kerala, India.

*E-mail: [email protected].

Received 30 October 2014; accepted 25 November 2014 Abstract

In the present study, various modifications for epoxy resin system were done for metal to metal bonding. Epoxy adhesives

were synthesized from epichlorohydrin and bisphenol-A and the mole ratio of reactants (BPA/ECH) were optimized to be

1:3 from various shear and peel strength experimentations. In the first part modification was done by blending epoxy with

phenol formaldehyde(PF) resole resin and later, the epoxy was further modified by replacing it partially with

hyperbranched structures like epoxidised phenolic novolacs (EPN) and epoxidised cresol novolacs (ECN) . Phenol and

cresol novolacs were prepared by the reaction of phenol and cresol respectively with formaldehyde. The results indicated

that ECN/PF blends gave better strength properties when compared to EPN/PF and Epoxy/PF blends. Morphological

analysis using SEM gives an idea on the energy absorption during failure for the Epoxy/PF blend.

© 2014 Universal Research Publications. All rights reserved

Key Words: adhesive, epoxy, novolac, blends, hyperbranched.

1. Introduction

Epoxies are probably the most versatile family of adhesives

because they bond well to many substrates and can be

easily modified to achieve widely varying properties

[1].The most common epoxies used in adhesives are

derived from bisphenol A and epichlorohydrin and are

usually cured with reactive hardeners containing primary

and/or secondary amine groups[2]. In general, epoxy resins

are prepared by the reaction of compounds containing an

active hydrogen group with epichlorohydrin followed by

dehydrohalogenation. Epoxies dominate the field of

structural adhesives due to their better wetting ability,

excellent mechanical properties, high chemical and thermal

resistance [3].Because of the normally brittle nature, epoxy

adhesives have been toughened with many different resins

including thermoplastic particles, nylon and various

elastomers [4].Incorporation of thermoplastic poly(vinyl

butyral) into an epoxy- novolak combination was found to

enhance adhesive shear and peel strengths [5].

Combinations of epoxy and phenolic resins

provide structural adhesives with superior high temperature

resistance. Both novolac and resole type phenolics may be

used. The blends cure through reaction of the epoxy groups

with the phenolic hydroxyl groups. When resoles are used,

the epoxy may also react with the methylol groups.

Adhesives based on epoxy-phenolic blends are good for

continuous high temperature service in the 3500F range or

intermittent service as high as 5000F. They retain their

properties over a very high temperature range [6].

Resistance to weathering, oil, solvents and moisture is very

good. Because of the rigid nature of the constituent

materials, epoxy-phenolic adhesives have low peel and

impact strength and limited thermal-shock resistance.

These were developed primarily for bonding metal joints in

high temperature applications. Usually the phenolics used

are the resole type and often the epoxy is a minor

component [7].

Epoxidised phenol novolac resins and cresol

novolac resins are made by glycidylation of phenol/cresol

formaldehyde condensates (novolacs) obtained from acid

catalysed condensation of phenol/cresol and formaldehyde.

Epoxidation of novolacs with an excess of epichlorohydrin

minimizes the reaction of phenolic hydroxyl groups with

glycidylated phenol groups and prevents branching. An

increase in the molecular weight of the novolac increases

the functionality of the resin. EPN resins range from a high

viscosity liquid to a solid. The major applications of epoxy

novolac resins have been in heat resistant structural

laminates, chemical resistant filament wound pipes and

high temperature adhesives. The epoxy functionality is

between 2.2 and 3.8. ECN resins derived from o-cresol

novolacs have even higher functionalities (2.5 to 6).

Available online at http://www.urpjournals.com

Advances in Polymer Science and Technology: An International Journal

Universal Research Publications. All rights reserved

http://www.urpjournals.com/


Novolac epoxy resin, being multifunctional, can

produce a more tightly crosslinked three dimensional

network and hence give better adhesive strength. Also, they

combine the reactivity of the epoxy group and the thermal

resistance of the phenolic backbone.In this paper, we are

reporting the adhesive properties of epoxy resin, EPN and

ECN in blend with phenol formaldehyde for metal to metal

(Al-Al) bonding.

2. Materials and methods

2.1. Materials

Phenol, m-cresol, formaldehyde (40% solution),

sodium hydroxide (All from Merck India Ltd.),

Epichlorohydrin (Sisco Research laboratories Pvt. Ltd.,

Mumbai, India) and Bisphenol A (Loba Chemie Pvt. Ltd.,

Mumbai, India) was used in this investigation.

2.2. Synthesis of Epoxy resin

Bisphenol A (1 mole) was dissolved in a mixture

of an excess of epichlorohydrin (6 moles) and 50 cc water

in a one litre three necked flask. The flask was equipped

with a mechanical stirrer, thermometer and a Liebig's

condenser. The mixture was heated gently over a water

bath till the epichlorohydrin began to boil. Heating was

stopped and caustic soda (2 moles) was added in two

pellets at a time down the condenser. The reaction was

allowed to subside before more alkali was added. When all

the caustic soda pellets had been added, the reaction

mixture was heated strongly for 45 minutes. Heating was

stopped as the reaction mixture turned viscous. The excess

epichlorohydrin was removed by vacuum distillation. The

remaining mixture was extracted with benzene to

precipitate sodium chloride which was removed by

filtration under vacuum. The filtrate was distilled in

vacuum to remove benzene and dried in vacuum for about

3hours.

2.3. Synthesis of novolac resins

The novolacs were prepared by reacting phenol

with formaldehyde in the molar ratio 1:0.8 in presence of

oxalic acid catalyst in a 3-necked flask fitted with a

mechanical stirrer, water condenser and thermometer. The

reaction mixture was heated and allowed to reflux at about

100°C for 2-3 hours. When the resin is separated from the

aqueous phase the reaction was stopped. The resin was

neutralised with sodium hydroxide, filtered, washed with

water and vacuum dried. The novolac resin contains 4-6

benzene rings per molecule. The same procedure was used

to synthesise novolac resin from m-cresol (ECN) [8].

2.4. Synthesis of Epoxidised phenolic novolacs (EPNs)

1 mole of the novolac resin (1:0.8) was dissolved

in 6 moles of epichlorohydrin and the mixture heated in a

boiling water bath. The reaction mixture was stirred

continuously for 4 hours while 1mole of sodium hydroxide

in the form of 30 %aqueous solution was added drop wise.

The rate of addition was maintained such that the reaction

mixture remains at a pH insufficient to colour

phenolphthalein. The resulting organic layer was separated

and then fractionally distilled under vacuum. Epoxidised

novolac resin was similarly prepared from m-cresol

novolac (ECN) using the same procedure.

2.5. Synthesis of Phenol formaldehyde (PF) resole resins

The resole resin were prepared by reacting phenol

with formaldehyde in the molar ratio 1:1.7 in presence of

33% NaOH solution in a 3-necked flask fitted with a

mechanical stirrer, condenser and thermometer. The

reaction mixture was heated and allowed to reflux at about

100°C for 1 hour. The resin was further neutralised with

oxalic acid in minimum amount of water, washed with

water and vacuum dried.

2.6. Bonding and adhesive performance

Aluminium strips of size 100x25 mm were

machined from 0.8 mm thick sheets to serve as metal

substrates for peel studies on metal-to-metal bonds. Strips

of 100x25x1 mm were used for shear strength. Surface

preparation is necessary before the application of adhesive.

Solvent degreasing was done with trichloroethylene, to

remove dust and traces of oily impurities on the surface.

Following the solvent wiping, the surface was abraded with

emery paper (P 100) and is again wiped with solvent to

remove completely the debris of abrasion. An even contact

pressure is applied throughout the joining surface after the

application of the adhesive and it is then kept in the oven at

the required temperature for the specified time.

Peel strength and lap shear strength of metal-to-

metal specimens were determined on a universal testing

machine as per ASTM D903 [9] and ASTM D1002 [10]

respectively with a grip separation rate of 50 mm/min at

room temperature.

2.7. Formulation of adhesive blends

Epoxy resin were synthesized in varying molar

ratios of Epichlorohydrin(ECH) to Bisphenol-A(BPA) viz,

1.57:1, 3:1, 3.5:1 and 4:1 and the mole ratio were optimised

based on lap shear and peel strength experimentations. The

epoxy was further modified by replacing it with epoxy/PF

blend in varying ratios. The same procedure is repeated for

EPN/PF and ECN/PF. All the blends were tested for lap

shear and peel strength in the Universal testing machine.

3. Resin Characterisation

3.1. Epoxide Equivalent weight (EEW)

The epoxy content of epoxy resins is an important variable

in determining their reactivity and the properties of

coatings made from them. Knowing the EEW, the required

amount of the crosslinking agent can be calculated [11].

EEW is usually defined as the weight of resin containing

one gram equivalent of epoxide. The epoxy content of

liquid resins, is frequently expressed as weight per epoxide

(wpe) or equivalent/kg of the resin. A common method of

analysis of epoxide content of liquid resins involves the

opening of the epoxy ring by hydrogen halides

(hydrohalogenation) [12]. Weight per epoxide values of the

synthesised epoxy, EPN and ECN resins were determined

by the pyridinium chloride method as per ASTM D 1652-

73 [13].

0.1 to 0.2 gm of the epoxy resin was mixed with

2ml HCl in 25 ml pyridine. The mixture was heated to

reflux on a water bath for 45 minutes. The solution was

cooled to room temperature and the unreacted acid present

was estimated by back titration with standard NaOH

solution (0.1 N) using Phenolphthalein indicator. A blank

was also carried out under same reaction conditions.

Epoxide equivalent = N x V/w, where N is the

strength of alkali, V is the volume of alkali used up and w


is the weight of the resin. Epoxide equivalent can be

obtained as eq/kg from which wpe value of the resin can be

calculated.

3.2. Gel time determination of blends

This method covers the determination of the time from the

initial mixing of the reactants of a thermoset composition to

the time taken when soldification commences, under

conditions approximating the conditions of use.

About 2 gm of the sample is filled into a test tube

and dipped into a water bath at 1000C. The increase in

viscosity is monitored with a glass rod used roughly like a

reciprocating piston. As soon as the gel point is reached the

glass rod gets trapped in the resin. This period of time is

recorded as the gel time. The test is done as per DIN 16945

[14].

3.3. Spectroscopic analysis Fourier transform infra-red (FTIR) spectra are generated by

the absorption of electromagnetic radiation in the frequency

range 400 to 4000 cm-1 by organic molecules. Different

functional groups and structural features in the molecule

absorb at characteristic frequencies. The frequency and

intensity of absorption are indicative of the bond strengths

and structural geometry in the molecule. FTIR spectra of

the samples were recorded on a Shimadzu, IR Prestige

21Spectrometer by KBr pellet method. Nuclear magnetic

resonance (NMR) spectroscopic method has been used to

good effect by authors investigating the structure of phenol-

formaldehyde resins [15]. Here 1HNMR spectroscopy is

used for the characterization of the synthesized

resins(Epoxy, EPN, ECN and PF).The spectra were

recorded on a 500MHz Bruker Avance DPX spectrometer

using acetone as solvent.

3.4. Gel Permeation Chromatography (GPC)

GPC, also called size exclusion chromatography, makes

use of a chromatographic column filled with a gel or porous

solid beads having a pore size similar to that of the polymer

molecules [16]. A dilute solution of the polymer is

introduced into a solvent stream flowing through the

column. Smaller molecules of the polymer will enter the

beads while the larger ones will pass on. Thus the larger

molecules have a shorter retention time than the smaller

molecules.

The synthesized epoxy resin was subjected to

GPC analysis with a view to estimate the molecular weight

of the resin. The analysis was carried out using Waters

GPC equipped with an autoinjector with an injection

volume of 50 µl and Tetrahydrofuran (THF) as the eluent.

3.5. Morphological studies

Scanning electron microscope (SEM) is a very useful tool

in polymer research for studying morphology and

microstructure [17]. Hitachi SU6600 Variable pressure

field emission SEM (FESEM) was used to investigate the

morphology of the failed adhesive surfaces. In this

technique, an electron beam is scanned across the specimen

resulting in back scattering of electrons of high energy,

secondary electrons of low energy and X-rays. These

signals are monitored by detectors (photo multiplier tube)

and magnified. An image of the investigated microscopic

region of the specimen is thus observed in a cathode ray

tube and photographed using black and white film. The

SEM observations reported in the present study were made

on the fracture surfaces of failed peel specimens.

4. Results and discussion

4.1. Mechanical properties

The average lap shear and peel strengths of epoxy

resins synthesized in varying molar ratios of BPA/ECH is

given in table 1.

Table 1 Variation of lap shear and peel strengths with

BPA/ECH molar ratio

Sample

(BPA/ECH)

Average lap

shear strength

(Nmm-2)

Average Peel

strength

(Nmm-1)

1: 1.57 - -

1: 3 4.402 0.550

1: 3.5 1.412 0.513

1: 4 0.627 0.497

In the case of 1:1.57, the glycidyl ether formation is less

than 10% and the resin formed is in solid form which is

uncoatable. On increasing the ratio beyond 1:4, the

viscosity of the resin formed was found to decrease

considerably. Consequently, the more viscous liquid

epoxides are usually preferred to the less viscous members

of the series for adhesive applications[18].According to the

experimental data, a relatively good adhesion property is

achieved at the molar ratio of BPA/ECH 1:3 and hence we

optimised the resin for further studies. This epoxy is further

blended with PF resin. The PF content in the blend is varied

from 10 phr to 100 phr. Similarly EPN and ECN resin

systems were blended with PF in varying ratios from 10 phr

to 100 phr. The variations in lap shear strengths and peel

strengths of these systems can be observed from Figure 1

and Figure 2 respectively.

From the plot, it was observed that the lap shear

and peel strengths of almost all the systems increased with

PF content. The results indicated that ECN/PF blends gave

better strength characteristics when compared to EPN/PF

and Epoxy/PF. Phenol formaldehyde, due to its brittle

nature, increases the cross linking sites in the system,

thereby increasing the flexibility and lap shear strength. m-

cresol, has 3 reactive positions, which can react with PF, to

give a cross linked structure, resulting in an increase in

strength.

Figure 1: Variations of Lap shear strengths of various

systems.


Figure 2: Variations of Peel strengths of various systems.

4.2. Resin Characterisation

4.2.1. Epoxide Equivalent weight (EEW)

The epoxide equivalents of the synthesized resins were

determined by the titration method. The epoxy equivalents

for Epoxy, EPN and ECN resins were found to be 4.2, 2.6

and 3.1 eq/kg respectively. Epoxidised cresol novolac was

found to have greater epoxide content than epoxidised

phenol novolac. The high epoxide functionality, results in a

higher density of crosslinks.

4.2.2. Gel time determination of blends

From the experiment, it was found that the gel time for

Epoxy/PF blend was 40 min, while the EPN/PF and

ECN/PF blends were stable after 1 hr. ie, the gel time was

found to increase when the epoxy was replaced with

novolac resin in the blend. Also, it is evident that Epoxy/PF

blend have a short gel time and often a short shelf life

compared to the other blends. This is attributed to the low

reactivity of novolac based blends.

4.2.3. Spectroscopic data

The FTIR spectrum of epoxy resin is given in Figure3. The

C-H stretching in epoxies is at 2964cm-1.Symmetrical

stretching or ring breathing frequency is observed at

1242cm-1and this is characteristic of the epoxy ring. The

band at 915 cm-1(asymmetric ring stretching in which C-C

stretches during contraction of C-O bond), 829cm-1and 750

cm-1are typical of the epoxy group [19].

Figure 3: FTIR spectra of Epoxy resin.

In Figure 4, which shows the FTIR of PF resin,

characteristic absorptions were found around 3250 cm-1

(hydroxyl group, broad band), 1504cm-1(phenyl ring) and

1479cm-1(CH2 bending).The bands around 1220cm-1 and

1019cm-1are characteristic of C-O stretching in phenol and

methylol groups respectively. The bands at 881.48cm-1 and

831.10 cm-1 are characteristic of ortho-para substitution.

Figure 4: FTIR spectra of PF resin.

Figure 5: FTIR spectra of EPN resin.

The Figure 5 shows the FTIR spectra of

synthesised EPN resin. Similar to Figure 3, here also we

can see peaks which show the epoxide functionality.The

band at 1242 cm-1 denotes symmetrical C-O stretching

(ring breathing frequency) in epoxides. Further, the bands

at 2931 cm-1, 815cm-1 and 748 cm-1 are also typical of

epoxides. The less intense broad band at 3250-3500 cm-1

range indicates the phenolic hydroxyl group, which also

has a considerable involvement in epoxidation. FTIR

spectra of ECN in Figure 6 also shows peaks similar to

EPN, which confirms the epoxide functionality.

Figures 7, 8, 9 and 10 shows the 1HNMR spectra

of Epoxy, PF, EPN and ECN resins respectively. In the

proton NMR for PF resin, the spectra displayed peaks

of aromatic protons (δ 7.2), methoxy protons (δ 3.7),

-CH=CH- protons (δ 5.35), -CH2-Ar protons (δ 2.55) and

-CH2-CH=CH- protons (δ 2.01).The proton NMR of EPN,


Figure 6: FTIR spectra of ECN resin.

Figure 7: NMR spectra of Epoxy resin.

+

Figure 8: NMR spectra of PF resin.


Figure 9: NMR spectra of EPN resin.

Figure 10: NMR spectra of ECN resin.

Figure 11: Gel Permeation Chromatogram of Epoxy resin.


Figure 12: Scanning electron micrograph of Epoxy/PF blend.

displayed strong peaks of Aryl protons (δ 2.1,δ 2.45) and

–CH3-C (δ 0.98). Peaks for aromatic protons (δ 6.8, δ 7.3)

and –CH2 protons (δ 3.7) are also evident.

In proton NMR of ECN, the doublet signal at 6.8 ppm is for

aromatic protons. The spectra also displayed peaks of Aryl

protons (δ 2.3), CH-X protons (δ 2.9), Alkenyl H (δ 4.1)

and -CH3-C protons (δ 0.95). The weak singlet at 3.8 ppm

is for –CH2 protons.

4.2.4. Gel Permeation chromatography This is a reliable, precise, and fast method to measure the

molar mass averages, the polydispersity index, PDI and the

complete molar mass distribution of polymers. Figure 11

shows the Gel Permeation Chromatogram of the

synthesized epoxy resin. The observed number average

molecular weight and the polydispersity index were found

to be 400 and 2.375 respectively.

4.2.5. Morphological studies

Fracture surfaces of the failed adhesive bonds were

subjected to Scanning electron microscopy (SEM) to

observe morphological features.

Figure 12 is a view of the fracture surface of an

Epoxy/PF blend after the peel test. A deepening of the

crevices is seen in the figure. This is an indication of

greater energy absorption or toughness. The striations on

the surface caused by layer by layer failure as well as

more prominent valleys and peaks indicate a more

emphatic cohesive failure of the substrate.

5. Conclusion

Adhesive behaviour of different epoxy/phenolic

blends has been investigated by lap shear and peel test

using aluminium substrates. It is observed from the results

that, the modifications by phenolic resin systems are very

effective for improving the strength properties of epoxy

resins. The 3:1 ratio of ECH/BPA is found to give optimum

lap shear and peel strengths. The strengths of various

systems were found to increase by increasing the

incorporation of PF. It is also observed that ECN blending

gave better mechanical strength properties compared to

others. Further modifications for this system to obtain

comparable shear strengths with the other systems are

possible by trying with various adhesion promoters. The

fracture mechanisms were also studied using FESEM.

References

1. E. M. Petrie, Handbook of adhesives and Sealants.

McGraw Hill., New York, 2000, pp.355-373.

2. Manilal Savla, Epoxy resin adhesives, in: Handbook of

Adhesives, I. Skiest (Ed.), Van Nostrand Reinhold

Company, New York, 1977, pp.434-445.

3. Silvia G. Prolongo, Gilberto del Rosario and Alejandro

Urena, Comparative study on the adhesive properties

of different epoxy resins, Int. J. Adhes. Adhes. 26

(2006) 125-132.

4. Sun.Luyi, L.G.Warren, D. Davis, and H.J.Sue, Nylon

Toughened Epoxy/SWCNT Composites, J. Mater. Sci.

46 (2011) 207-214.

5. A. Salimi, H. Omidian and M. J. Zohuriaan-Mehr,

Mechanical and thermal behavior of modified epoxy–

novolak film adhesives, J. Adhes. Sci. Technol., 17

(2003) 1847-1861.

6. Lity Alen Varghese, Beena T Abraham and Eby

Thomas Thachil, Adhesive properties of neoprene-

phenolic-epn blends, Progress in Rubber, Plastics and

recycling technology. 22 (2006) 225-240.

7. E.M. Petrie, Epoxy hybrid adhesives, in: Special Chem

Adhesives and Sealants (2005).

8. K.P. Unnikrishnan, Studies on the toughening of epoxy

resins. Ph.D thesis, Cochin University of Science and

Technology (2006).

9. Annual Book of ASTM standards: D 903-98.


11. G. Bianchini , R. Dyer, A. Fream, P. Heffer, S.P.

McOwan and P. Oldring, Waterborne and solvent

based epoxies and their end user application. Wiley,

New York, 1997.

12. H. R. Kricheldorf, O. Nuyken and G. Swift, Handbook

of Polymer Synthesis.Marcel Dekker, NewYork, 2005.



14. DIN 16945 standard.

15. N. A. Novikov, A. P. Mozoleva, A.V. Uvarov, S. G.

Alekseyeva and I. Y.A. Slonim, Analysis of the 13C

and 1H NMR spectra of phenol-formaldehyde resole

oligomers, Polymer Science. 28 (1986) 1187-1196.

16. V. R. Gowariker, N. V. Viswanathan and Jayadev

Sreedhar, Polymer Science, First ed., New age

international Publishers, New Delhi, 1986.

17. L. Engel, H. Klinglee, G.W. Ehrenstein and H.

Schaper, An atlas of polymer damage, Wolfe Science

Books, Munich, 1981.

18. Manilal Savla, Epoxy resin adhesives, in: Handbook of

Adhesives, I. Skiest (Ed.), Van Nostrand Reinhold

Company, New York, 1977, pp.434-445.

19. R.M. Silverstein and F. X. Webster, Spectroscopic

identification of organic compounds. John Wiley and

Sons Inc., New York, 2002.

Source of support: Nil; Conflict of interest: None declared

http://www.sciencedirect.com/science/article/pii/0032395086902704



Documents

Original Article Studies on the effect of phenolic resin ...urpjournals.com/tocjnls/1_14v4i4_1.pdf · Original Article Studies on the effect of phenolic resin modifications on the