Adhesive Properties of Neoprene-Phenolic-EPN Blends · 2006-12-07 · A variety of polymers, thermosets as well as thermoplastics, can be blended or co-reacted with epoxy resins to

253Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 4, 2006

Adhesive Properties of Neoprene-Phenolic-EPN Blends

©Rapra Technology, 2006


Lity Alen Varghese1, Beena T Abraham2 and Eby Thomas Thachil1*

1Department of Polymer Science & Rubber Technology, Cochin University of Science & Technology, Kochi- 682 022, Kerala, India2Department of Chemistry, S.N.M College, Maliankara, N.Paravur, 683516, Kerala, India

Received: 8 August 2005 Accepted: 6 January 2006

ABSTRACT

This study investigates the adhesive properties of neoprene-phenolic-EPN blends

for SBR-SBR and Al-Al bonding. The effect of varying the phenolic: EPN ratio on

adhesive properties was studied. The fi rst part (Case A) brings out the signifi cance

of blending a phenolic copolymer with epoxidized phenolic novolac (EPN) resin

and neoprene rubber. The phenolic resin was the product of condensation of a

mixture of phenol and cardanol with formaldehyde. Cardanol, a meta-substituted

phenol, is the main ingredient of cashew nut shell liquid (CNSL), a renewable

resource. In the second part (Case B), the phenolic resin used was a commercial

resin based on p-tertiary butyl phenol. For SBR-SBR bonds, 30 phr EPN in the

phenolic/EPN blend leads to optimal shear and peel properties for both Cases

A & B. For Al-Al bonds, on the other hand, 60-80 phr EPN in the phenolic/EPN

blend gives optimal properties for shear and peel properties for both cases. The

formulation containing cardanol performs better on Al-Al bonds while the t-butyl

phenol based formulation is better suited for SBR-SBR bonds. Morphological

studies using SEM indicate greater energy absorption and improved bond strength

on addition of EPN into neoprene-phenolic formulations for both for SBR-SBR

and Al-Al bonding. The successful application of cardanol, a renewable resource,

for adhesive formulation is of signifi cance.

1. INTRODUCTION

Epoxy resins are one of the most versatile materials used in industrial applications. They dominate the fi eld of structural adhesives due to excellent mechanical and chemical properties and comparatively high temperature

* To whom correspondence should be addressed. Phone: 0484-2575723 Fax: 0484-2577747 [email protected]

PRPRT 4 06.indb 253PRPRT 4 06.indb 253 20/10/06 11:02:21 am20/10/06 11:02:21 am

254 Progress in Rubber, Plastics and Recycling Technology, Vol. 22, No. 4, 2006

Lity Alen Varghese, Beena T Abraham and Eby Thomas Thachil

resistance(1). Epoxies offer very high shear strengths and can be modifi ed to meet a wide variety of bonding needs. Generally, epoxy bonds are rigid; they fi ll gaps well with little shrinkage. But fully cured epoxies have the limitation of high brittleness(2). Hence the main challenge of epoxy adhesive formulators is to improve the fl exibility of the adhesive system. This can be done by attaching chemical groups to the epoxy structure or by adding fl exibilizing resins or elastomers to the formulation to create a hybrid epoxy adhesive system.

A variety of polymers, thermosets as well as thermoplastics, can be blended or co-reacted with epoxy resins to provide desired specifi c properties. The most common of these are nitrile, phenolic, nylon, polysulfi de and polyurethane resins(3). At high levels of addition these additives result in hybrid or alloyed systems. These hybrid epoxy adhesives are generally used for demanding structural applications such as in the aerospace industry where the individual properties of each component are desired.

The blending of phenolic resins with epoxy resins improves the high temperature resistance of the adhesive system. Resistance to weathering, oil, solvents and moisture are also very good for these blends. Usually the phenolics used are the resole type and often the epoxy is a minor component(4). The phenolic:epoxy ratio is adjusted to give a cross-linked material with moderate fl exibility and excellent resistance to chemicals, heat and moisture. Phenolic resins with low molecular weights can also act as curing agents for epoxy resins. These resins can react with epoxy resins via the hydroxyl groups of the phenolic resin and the epoxide ring structures(5).

In this paper we are reporting the use of mixtures of epoxidized phenolic novolac (EPN) and phenol-cardanol formaldehyde (PCF) copolymer resins with neoprene rubber for rubber-to-rubber (SBR-SBR) and metal-to-metal (Al-Al) bonding. Cardanol, the main ingredient of cashew nut shell liquid (CNSL), a renewable resource, is a meta substituted phenol. It has a side chain with 15 carbon atoms at the meta position of the benzene ring(6,7). The presence of cardanol in the PCF copolymer resin improves the fl exibility of the system(8). Novolac epoxy resin, being multifunctional, can produce a more tightly crosslinked three-dimensional network and hence give better adhesive strength retention at elevated temperatures. The performance of the PCF copolymer resin based formulation (Case A) is compared with similar blends made from a commercially available resin, viz. p-tertiary butyl phenolic resin (PTBP) (Case B). The effects of adding EPN on the morphology have been highlighted by SEM studies.




2. EXPERIMENTAL

2.1 Materials

Phenol, formalin (40% formaldehyde) and solvents (toluene and methyl ethyl ketone) were obtained from Merck India Ltd. All chemicals were of analytical grade. The neoprene rubbers (AD and W grades) were obtained from Dupont, Akron, Ohio. Styrene butadiene rubber (SBR 1502) used was manufactured by Japan Synthetic Rubber Co. Ltd., Tokyo. CNSL was obtained from Vijayalakshmi Cashew Exports, Kollam, India. Carbon black was supplied by Phillips Carbon Black Ltd., Cochin, India. Precipitated silica was procured from Sameera Chemicals, Kottayam, India. 3-aminopropyl triethoxysilane was supplied by Sigma Aldrich, Bangalore, India. The novolac epoxy resin (EPN 1138) having epoxy functionality 3.6 and epoxy value 5.4 eq/kg was obtained from Hindustan Ciba Geigy, Mumbai, India. P-tertiary butyl phenolic resin (DAP 1001) was supplied by Dujodwala Chemicals, Mumbai.

2.2 Preparation of copolymer (PCF) resin

Cardanol was separated from commercial CNSL by distillation under reduced pressure (1 mm Hg). The pale yellow fraction collected at 206-208 °C is cardanol(9). It’s formula weight varies from 304 to 298 corresponding to n=0 to n=6 in the C15H31-n structure of the side chain. PCF resin was synthesized by reacting a mixture of phenol and cardanol with formaldehyde at 90-95 °C for about one hour(10,11). Two different phenolic resins with varying phenol/cardanol ratio (wt%), viz. 60/40 (PCF-I) and 80/20 (PCF-II), were synthesized. The resins were prepared with a stoichiometric ratio of 1:1.7 between total phenols and formaldehyde in an alkaline medium. The resole type resin obtained was neutralized and dried.

2.3 Metal-to-metal bonding

Aluminium strips of size 100 x 25 mm were machined from 0.8 cm thick sheets to serve as metal substrates for peel studies on metal-to-metal bonds. Strips of 100 x 25 x 1.8 mm were used for testing shear strength. Mechanical cleaning (surface roughening) was done with a No. 100 emery paper. Solvent degreasing with trichloroethylene followed mechanical cleaning.

Neoprene rubber for the adhesive formulation was masticated on a two roll mill along with other ingredients and dissolved in toluene so as to get a 15 wt% solution. The formulations are given in Table 1. From earlier studies(12), it was found that an optimum resin concentration for metal-to-metal bonding was 80 phr. Keeping the total resin content at 80 phr, a part of the phenolic resin




(PCF or PTBP) was progressively replaced with EPN. The resulting adhesive solution was applied on each substrate on an area of 2.5 x 2.5 cm so as to get an approximate coating thickness of 0.1 mm. Different phenolic/EPN ratios, viz. 100/0, 80/20, 60/40, 40/60, 20/80 and 0/100 were employed. PCF-I resin was employed for peel tests and PCF-II, for shear tests. This is in accordance with fi ndings of an earlier study(12) where PCF-I and PCF-II resins were compared for peel and shear performance.

On applying the adhesive, the substrates were kept aside so as to aid the evaporation of solvent. They were subsequently bonded together and the adhesive cured for 30 minutes at 150 °C and 12.5 kg/cm2.

2.4 Rubber-to-rubber bonding

The mixing and homogenization of SBR and other compounding ingredients for the substrates were done on a laboratory size (15 x 30 cm) two-roll mill as per ASTM D3186 prior to compression moulding in a hydraulic press. The compound was moulded into 150 mm x 150 mm x 2 mm sheets employing a pressure of 200 kg/cm2 and a temperature 150 °C. The time of cure was in accordance with the results of cure studies initially done on a Rubber Process Analyser (RPA 2000, supplied by Alpha Technologies, USA). The moulding was cut into strips of 100 mm x 25 mm x 2 mm size to serve as substrates. Surface roughening of rubber substrates was done with a No. 100 emery paper.

A 20 wt% solution of neoprene rubber in toluene along with other ingredients was prepared. The total resin concentration in the adhesive was fi xed at

Table 1. Adhesive formulation

Ingredients Al-to-Al bonding (Formulation-1)

SBR-to-SBR bonding (Formulation-2)

Neoprene ADNeoprene WSilicaAcetylene blackSulfurZinc phosphateSilanePhenolic resin/EPNSolid content %

9010

4 phr*-

1 phr1.5 phr2 phr

100/0 - 0/100 phr15-17

9010-

6 phr0.5 phr

--

100/0 - 50/50 phr20-22

* phr here refers to parts per hundred rubber by weight




30 phr(12). Various proportions of PCF-I/EPN (Case A) and PTBP/EPN (Case B) were employed. The amount of EPN was varied from 0 to 50 phr in steps of 10. The resulting adhesive solution was applied on each rubber substrate. After drying for about 30minutes the substrates were joined and curing was done at 150 °C

2.5 Adhesive performance

Peel strength and lap shear strength of rubber-to-rubber and metal-to-metal specimens were determined as per ASTM D903 and ASTM D 1002 respectively. These tests were performed on a Shimadzu Universal Testing Machine (UTM) (50KN) with a grip separation rate of 50 mm/min.

Thermogravimetric analysis was carried out using TA Instruments Model TQ 50 TGA, at a heating rate of 20 °C in nitrogen atmosphere. The fracture surface morphology was investigated by environmental scanning electronic microscopy (ESEM, Philips XL30 FEG operated at 1-10 kV).

3. RESULTS AND DISCUSSION

3.1 Shear and peel properties

Figure 1 illustrates the change in peel strength with the addition of EPN for Al-Al bonds. The highest peel strength is obtained on addition of 60 parts EPN into the system for both Cases A and B. Case A (PCF-I) gives somewhat better values than Case B. The addition of epoxidized resins improves the polarity of the system. EPN molecules are large in size because of their polyphenolic nature. Their presence can also contribute to higher energy absorption and hence better peel properties. A higher degree of crosslinking can also result from the multifunctional nature of EPN. This can lead to improvement in shear strength. But as the amount of EPN goes beyond 60 phr, the system becomes more and more brittle due to excessive crosslinking and the peel strength decreases. The effect of addition of EPN on shear strength of Al-Al bonds is shown in Figure 2. In this case, it is found that the addition of about 60 phr EPN gives the best performance for the PCF-II resin system (Case A). But with PTBP resin system (Case B) the addition of about 80 phr EPN gives the maximum value.

Figure 3 shows the effect of varying EPN content on the peel strength of SBR-SBR substrates. Both PCF-I and PTBP resin systems show the same pattern of behaviour although the PTBP/EPN system (Case B) has a clear superiority. The




higher reactivity of PTBP ensuing from a shorter chain (compared to C15H31-n chain at the meta position in cardanol) substitution in the para position may be responsible for this. Moreover, cardanol molecule is much larger (formula weight 208-304) than para tertiary butyl phenol (molecular weight 150), which is conducive to higher molecular weights in the condensate and lower peel strength. The highest peel strength is observed for formulations containing

Figure 1. Effect of varying the EPN content in the adhesive on peel strength of Al-Al bonds

Figure 2. Effect of varying the EPN content in the adhesive on shear strength of Al-Al bonds




a minimum of 30 parts EPN for Case A and 10 parts for Case B. Figure 4 shows the effect of incorporation of EPN on the shear strength. In all the cases substrate failure occurred by snapping of the rubber. The highest values are indicated for formulations containing 30 parts EPN for both cases.

Figure 3. Effect of varying the EPN content in the adhesive on peel strength of SBR-SBR bonds

Figure 4. Effect of varying the EPN content in the adhesive on shear strength of SBR-SBR bonds




In general, PTBP/EPN system is superior to PCF/EPN system for rubber-to-rubber bonding while PCF/EPN system is better for Al-Al bonding.

3.2 Thermal studies

Thermal stability of the adhesive system was studied in nitrogen atmosphere by thermogravimetric analysis. The thermogravimetric (TG) as well as the differential thermogravimetric (DTG) plots were obtained for a) unmodifi ed neoprene, b) phenolic resin modifi ed neoprene and c) the optimum EPN- phenolic modifi ed adhesive system for both Cases A and B and for both Formulations 1 and 2 (Figures 5-8). Table 2 shows comparative fi gures for T50% decomposition. The weight loss at around 100 °C is due to the loss of water generated by the crosslinking of phenolic resin. The next two steps of mass loss in TG trace and the shoulders observed in DTG in all the three cases indicate thermal degradation of neoprene-phenolics. The residual weights as well as the peak degradation temperature are greater when the formulation contains EPN. This can be attributed to higher thermal stability due to the presence of EPN, which has a high phenolic content and functionality. Formulation 1 which contains more resin gives higher residue and T50%.

Figure 5. TGA and DTG traces of a) unmodifi ed neoprene (formulation 1), b) PCF resin modifi ed neoprene, and c) the optimum EPN-PCF modifi ed adhesive




Figure 6. TGA and DTG traces of a) unmodifi ed neoprene (formulation 1), b) PTBP resin modifi ed neoprene, and c) the optimum EPN-PTBP modifi ed adhesive

Figure 7. TGA and DTG traces of a) unmodifi ed neoprene (formulation 2), b) PCF resin modifi ed neoprene, and c) the optimum EPN-PCF modifi ed adhesive




Table 2. Results of thermogravimetric studies

Component T50% (°C) Residue wt (%)

Formulation 1Unmodifi edPCF modifi edPCF-EPN modifi edPTBP modifi edPTBP-EPN modifi ed

436432449463446

2624292829

Formulation 2Unmodifi edPCF modifi edPCF-EPN modifi edPTBP modifi edPTBP-EPN modifi ed

412418407399388

1819192121

3.3 Morphological studies

Fracture surfaces of the failed adhesive bonds were subjected to scanning electron microscopy (SEM) to observe morphological features. Figures 9-16 show fracture surfaces after peel test. The micrographs give clear indications of greater energy absorption during failure on addition of EPN.

Figure 8. TGA and DTG traces of a) unmodifi ed neoprene (formulation 2), b) PTBP resin modifi ed neoprene, and c) the optimum EPN-PTBP modifi ed adhesive




Figure 9. Scanning electron micrographs of SBR substrate bonded with neoprene-PCF blends

Figure 10. Scanning electron micrographs of SBR substrate bonded with neoprene-PCF-EPN blends




Figure 11. Scanning electron micrographs of SBR substrate bonded with neoprene- PTBP blends

Figure 12. Scanning electron micrographs of SBR substrate bonded with neoprene-PTBP-EPN blends




Figure 13. Scanning electron micrographs of Al substrate bonded with neoprene-PCF blends

Figure 14. Scanning electron micrographs of Al substrate bonded with neoprene-PCF-EPN blends




Figure 15. Scanning electron micrographs of Al substrate bonded with neoprene-PTBP blends

Figure 16. Scanning electron micrographs of Al substrate bonded with neoprene-PTBP-EPN blends




Figure 9 is a view of the fractured surface of a SBR substrate when an adhesive consisting of only neoprene and PCF-I is employed. The fractured surface although rough, has relatively shallow cavities. Figure 10 shows the fracture pattern when the formulation contains 30 phr EPN. The striations on the surface caused by layer-by-layer failure in Figure 10 as well as more prominent valleys and peaks indicate a more emphatic cohesive failure of the substrate. Similarly Figures 11 and 12 of the fractured SBR surfaces indicate the effect of addition of EPN to neoprene-PTBP blends. A deepening of the crevices is seen in Figure 12. This is an indication of greater energy absorption or toughness. Here again the positive effect of adding EPN is indicated. EPN is multifunctional in nature by virtue of a large number of epoxy groups per molecule. It is possible that these epoxy groups when crosslinked give rise to an anchoring effect leading to higher energy absorption and peel strength.

Figures 13 and 14 show the effect of addition of EPN to PCF-I/neoprene mixtures used for Al-Al bonding. As Figure 14 indicates, the addition of EPN (60 phr) alters the fracture pattern. In Figure 13 the fracture surface is akin to that of a normal thermoset failure whereas Figure 14 shows higher energy absorption by the creation of deeper cavities and larger fracture areas. Figures 15 and 16 show the effect of addition of EPN into PTBP/neoprene mixtures for Al-Al bonding. Figure 16 is the case of 60 phr EPN. In comparison with Figure 15, the fracture surface shows extensive cavitation on addition of EPN. There is a dramatic increase in the fracture surface area. In general, the PTBP based formulation gives greater energy absorption and bonding on rubber-to-rubber specimens and PCF based formulation gives better bonding on metal-to-metal specimens.

4. CONCLUSIONS

Cardanol, a renewable substance, in combination with phenol is an effective starting material for adhesive formulations for Al-Al and SBR-SBR substrates. Cardanol based formulations are superior to t-butyl phenolic resin based formulations for Al-Al bonding. In both these cases, replacement of 30 phr of phenolic resin with EPN gives the best peel and shear strength values for SBR-SBR bonds. For Al-Al bonding, peel properties are maximum for both cardanol based and p-tertiary butyl based resin formulations when 60 phr of the phenolic resin is replaced by EPN. But for shear strength, 80 phr of EPN is optimum for p-tertiary butyl phenol based resin formulation as against 60 phr for cardanol-based formulation. The addition of EPN to phenolic/neoprene adhesive formulations has led to some or considerable improvement in peel and shear properties of both Al-Al and SBR-SBR bonds. Morphological




evidence also points to improved energy absorption on addition of EPN to the adhesive systems.

REFERENCES

1. Manilal Savla, Epoxy Resin Adhesives, in Handbook of Adhesives, Skeist, I., Ed., Van Nostrand Reinhold Company, New York, (1977).

2. Kinloch, A.J. Rubber toughened thermosetting polymers, in Structural adhesives developments in resins and primers, Kinloch, A.J., Ed., Elsevier Applied Science Publishers, London, (1986).

3. Petrie, E.M., in Handbook of Adhesives and Sealants, Mc Graw Hill, New York, (2000).

4. Petrie, E.M., Epoxy Hybrid Adhesives, in, Special Chem Adhesives & Sealants, (2005).

5. Milka, T.F. and Bauer, R.S., in Epoxy Resins, Chemistry and Technology, May, C.A., Ed., Marcel Dekker, New York, (1988).

6. Symes, W.F. and Dawson, C.R., Nature, 171, (1953), 841.

7. Cornelius, J.A., Tropical Sci., 8(2), (1996), 79.

8. Barth, B.P., in Handbook of Adhesives, Skeist, I., Ed., 2nd ed, Van Nostrand Reinhold, New York, (1977).

9. Durrani, A.A., Davis, G.L., Sood, S.K., Tychopoulus, V. and Tyman, J.H.P., J. Chem. Technol. Biotechnol., 32, (1982), 618.

10. Nieu, N.H., Tan, T.T.M. and Huong, N.L., J. Appl. Polym. Sci., 61, (1996), 2260.

11. Nair, C.P.R., Bindu, R.L. and Joseph, V.C., J. Appl. Polym. Sci., 33, (1995), 622.

12. Lity Alen Varghese and Eby Thomas Thachil J., Adhesion Sci.Technol., 18, (2004), 1221-1222.


Documents

Adhesive Properties of Neoprene-Phenolic-EPN Blends · 2006-12-07 · A variety of polymers, thermosets as well as thermoplastics, can be blended or co-reacted with epoxy resins to