Chapter 5. Summary an~ conclusion 5.1 INTRODUCTION 5.2 SUMMARY 5.3 CONCLUSION 5.4 FUTURE OUTLOOK

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Alaina Gardner
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1 Chapter 5 Summary an~ conclusion 5.1 INTRODUCTION 5.2 SUMMARY 5.3 CONCLUSION 5.4 FUTURE OUTLOOK

2 5.1 INTRODUCTION This chapter gives a glimpse of the summary and conclusions of the work carried out. The study reveals the effectiveness of neoprene phenolic adhesives for bonding different substrates and the role of various ingredients with special reference to polymeric resins. It also investigates the possibility of using cardanol as a starting material for the adhesive fonnulation. 5.2 SUMMARY The focus of the study is to develop new adhesive systems based on neoprene and phenolic blends for rubber-rubber, metal-metal and rubber-metal bonding applications. The contents of the various chapters in this thesis are summarized below. In Chapter 1, adhesive bonding, an overview of different types of adhesives, adhesive application methods, adhesive fonnulation, theories of adhesion, different surface preparation methods, joint stress and joint geometry are discussed. Renewable resources for adhesives and a discussion of neoprene and phenolic adhesives are also included. The optimisation of the basic components of the adhesive fonnulation like fillers, adhesion promoters, curing agents, solvents etc., the synthesis of phenol cardanol fonnaldehyde (PCF) resins, the optimisation of the ratio between phenol and cardanol on the basis of adhesive perfonnance and the optimisation of the stoichiometric ratio between total phenol and fonnaldehyde are included in Chapter 2. The adhesive performance is evaluated by conducting peel and lap shear strength tests on metal-metal, rubber-rubber and rubber-metal bonds. The synthesized resins are characterised by spectroscopic methods (FTIR and J H NMR). The thermal properties are studied using TGA and DSC techniques. The synthesised resins are also subjected to GPC. A detailed account of the effect of replacing the phenolic resins with epoxy resins is given in Chapter 3. Novolac type phenolic resins are prepared in different stoichiometric ratios and are subsequently epoxidized. These epoxy phenol

3 novolacs (EPN) are used along with PCF resin. Thennal properties are monitored using TGA. The synthesised EPNs are characterized by FTIR and IHNMR. The adhesive perfonnance is evaluated by estimating peel and shear strengths. The fractured surfaces of adhesive bonded substrates are examined using SEM. A... parallel study is also conducted using a blend of commercially available p-tertiary butyl phenolic resin and EPN. In the first part of Chapter 4, the effectiveness of the adhesive fonnulations for joining different rubber and metal substrates is discussed. Various rubbers, viz. natural rubber (NR), styrene butadiene rubber (SBR), chloroprene rubber (CR), acrylonitrile butadiene rubber (NBR) and ethylene propylene diene monomer (EPDM) are employed as substrates. The rubbers are compounded and strip specimens are moulded. The metals used in bonding studies are aluminium, stainless steel, titanium and galvanized iron. In the second part of Chapter 4, the effect of ageing on adhesive properties of rubber-rubber, metal-metal and rubbermetal substrates are evaluated. The bonded joints are subjected to ageing under ambient conditions, air oven ageing, hot water ageing and salt water ageing. The storage life of the adhesive is also evaluated. 5.3 CONCLUSIONS The conclusions from the investigation can be stated as follows. Neoprene solution and additives On the basis of SBR-SBR and AI-AI bonding, a CR AD/W of 15-20wt% solution in toluene gives the best adhesive strength. Addition of 6pbr acetylene black to the adhesive fonnulation improves SBR-SBR bonding while the incorporation of 4phr silica improves the AI Al bonding. Addition of curatives of polychloroprene rubber reduces the peel strength for both AI -AI and SBR-SBR bonding. Sulphur, si lane and zinc phosphate are effective adhesion promoters. IQ?

4 Addition of epoxidized natural rubber to chloroprene improves the bond strength. Cardanol and phenol based resins Mixtures of cardanol and phenol are better raw materials for resin preparation and subsequent adhesive formulation compared to either of pure phenol or cardanol In the case of AI-AI bonding, about 80phr of 60/40 phenovcardanol formaldehyde resin gives the best peel strength and 80phr of resin gives the best shear strength. For SBR-SBR bonds, a 60/40 phenollcardanol formaldehyde resin yields the best peel strength values. For AI-SBR bonding, the optimal concentration in the primer side is 80phr and on the adhesive side, 40phr of a copolymer resin containing 40% cardanol and 60% phenol A ratio of 1: 1.7 between total phenol and formaldehyde is the best suitable for SBR-SBR and AI-AI bonds. A ratio of I: 1.5 between total phenol and cardanol is ideal for peel strength ofsbr-ai bonds. A ratio of 1: 1.7 between total phenol and cardanol is optimal for shear strength of SBR-AI bonds. EPN formulations Replacing part of the substituted phenolic resin with epoxidized phenolic resins yields higher adhesive strength in both AI-AI and SBR-SBR bonding. Among the EPNs, EPN prepared with a stoichiometric ratio of 1 :0.8 between phenol and formaldehyde gives the best bond strength values.

5 Adhesion on different substrates Stainless steel-stainless steel bonds give better shear strength compared to titanium-titanium, aluminium-aluminium and galvanized iron-galvanized iron samples for the optimal adhesive formulation based on neoprene-pcf blends. In rubber-rubber bonding, SBR-SBR samples give the highest peel and shear strength values. Ageing studies The bond strength of these adhesive joints does not substantially decrease with time (3 months) under ambient conditions. EPN based systems show better retention of properties for AI-AI and SBR Al bonds when aged under ambient conditions. Formulations containing neoprene and PCF/EPN blends show initial high peel and shear strengths compared to those containing only neoprene and PCF. But resistance to high temperatures and hot water is lower for EPN containing foidlulations. Formulations containing ENR similarly show high initial strength but show substantial fall in properties when exposed to high temperature or hot water. Compared to SBR-SBR and SBR-AI bonds, AI-AI bonds show considerable reduction in properties on exposure to sea water for 3 months. In this respect peel strength shows greater deterioration compared to shear strength. 5.4 FUTURE OUTLOOK The mechanism of adhesion is influenced by many parameters. The present work covered a few of them. Some other areas where investigation can be done are suggested below.

6 )- The role of surface treatment of substrates on adhesive perfonnance has to be investigated further. );- The present study was mainly using vulcanized rubber as substrate in. rubber-metal bonding. The adhesive perfonnance on unvulcanized rubbermetal bonding is an area where further study can be made. y Study to improve the ageing resistance of adhesive joints under different environmental conditions to be further pursued.

'-3.1 INTRODUCTION. C6apter 3. Adhesives based on phenolic-epoxy blends 3.2 EXPERIMENTAL

'-3.1 INTRODUCTION. C6apter 3. Adhesives based on phenolic-epoxy blends 3.2 EXPERIMENTAL C6apter 3 Adhesives based on phenolic-epoxy blends '-3.1 INTRODUCTION 3.2 EXPERIMENTAL 3.2.1 Materials 3.2.2 Preparation of epoxidized phenolic novolac resins 3.2.3 Spectroscopic analysis 3.2.4 Epoxide