Eby Thomas Thachil

Cashew nut shell liquid (CNSL) - a versatile monomer for polymer synthesis

Cashew nut shell liquid (CNSL) is a by-product of the cashew industry. It is a naturally occurring substituted phenol which can take part in a variety of reactions. It is a cheap and renewable substance and can be employed for the manufacture of a multitude of useful products. It can replace phenol in many applications with equivalent or better results. CNSL by itself is useful for insecticidal, fungicidal, anti-termite, and medicinal applications, and as an additive, in many plastic formulations. Resins derived from CNSL are employed widely in the fields of friction materials, automobiles, surface coatings, adhesives, laminates, rubber compounding, and have several miscellaneous applications. Greater utilization of CNSL as a monomer for indusrial polymer products can be an attractive proposal in view of its low cost, abundant availability, and chemically reactive nature, amongst other attributes. This review gives an accounts of the composition, extraction, reactions, and applications of CNSL based on the available literature.

Toughening of epoxy resins

Epoxy resins constitute an important class of thermosets which are used extensively in the field of composites, adhesives surface coatings, etc. They are of aliphatic, cycloaliphatic or aromatic backbones. Epoxy resins based on bisphenol A are commercially available and they have good thermal and mechanical properties. They are cured by a variety of curatives, such as amines and anhydrides, and the mechanism of curing largely depends on the type of curing agent used. They show comparatively low cure shrinkage. Cured epoxies are brittle with poor resistance to crack initiation and growth. Their mechanical properties, in particular toughness, can be modified by incorporating a rubbery phase into the resin matrix. The epoxy matrix can also be modified by various other methods such as incorporation of thermoplastics and particulate fillers. The toughening process leads to improvement in fracture toughness and impact resistance. This review examines the various options and the state-of-the-art vis-à-vis epoxy modification.

Studies on the Modification of Commercial Epoxy Resin using Cardanol-based Phenolic Resins

Bisphenol A epoxy resin is blended with different phenolic novolac resins for property modification. The phenolic resins employed are (a) novolac resins derived from phenol/formaldehyde (PN) and cardanol/ formaldehyde (CDN) condensation and (b) condensation products of cardanol—phenol mixtures with formaldehyde (CPNs). Cardanol is the main constituent of cashew nut shell liquid (CNSL), a renewable resource. Epoxy resins containing 2.5—20 wt% of the above novolacs are prepared and tested for mechanical properties, water absorption, etc. Blends containing 5—10 wt% of novolacs show substantial improvement in properties. These properties show a declining trend with higher novolac loading. The stoichiometric ratio between phenolic material and formaldehyde in the novolacs is optimized (1:0.8) for maximum property enhancement. Among the novolacs studied, those prepared from cardanol—phenol mixtures of different molar ratios (CPNs) yield comparatively higher energy absorption at break and this property increases with increase in cardanol content in the modifier resin up to 40%. The property profiles of these epoxy—novolac blends imply a toughening action by novolac resins, especially phenol—cardanol—formaldehyde resins.

Synthesis and Characterization of Cardanol-Based Epoxy Systems

The use of cardanol, a renewable natural resource, in place of a phenol or diphenol, in the synthesis of two different epoxy systems has been studied in this work. Cardanol, prepared by the distillation of cashew nut shell liquid (CNSL), was epoxidised using epichlorohydrin in presence of caustic soda catalyst at different reaction intervals and the products were characterized by spectroscopy, gel-permeation chromatography and by determining the epoxide equivalent. The epoxide equivalent increased sharply with time of epoxidation and reached almost a limiting value around 9 h. Cardanol, being less reactive, undergoes epoxidation to a smaller extent than phenol or bisphenol. Although these epoxy compounds do not form cross-linked networks due to being mono-functional, they are found to be effective in producing relatively flexible systems when reactive blended with commercial DGEBA. Co-epoxides containing bisphenol A and cardanol in varying molar compositions were also synthesised and characterised. Introduction of 20 mol% cardanol into bisphenol A resulted in a resin having reduced tensile, impact and compressive strengths upon curing by a polyamine hardener. However, the resin showed considerable improvement in elongation-at-break without much decrease in energy absorption.

Studies on the adhesive properties of neoprene–phenolic blends

Polychloroprene (neoprene) rubber in combination with phenolic resins is a versatile adhesive formulation. The phenolic resin used in this case was derived from a mixture of cardanol, a meta-substituted naturally-occurring substance, and phenol. Cardanol is the main ingredient of cashew nut shell liquid (CNSL), a renewable resource. This study aims to investigate the adhesive properties of cardanol-based resin when used in combination with two grades of polychloroprene rubber. The effects of varying the solid content and resin content, choice of resin, fillers, crosslinking agents, adhesion promoters, solvents, etc. in the adhesive formulations were also studied. Moreover, relative proportions of rubber and resin that give optimum adhesion performance were identified. The results show that cardanol-phenol-formaldehyde resin is an effective ingredient in adhesives for bonding aluminium to aluminium and SBR to SBR. The addition of 3-aminopropyltriethoxysilane to the formulation improves the bond strength of metal-to-metal specimens.

Blends of Unsaturated Polyester Resin with Functional Elastomers

Unsaturated polyester resins are widely used in the fiber-reinforced plastic industry. The fracture toughness and impact resistance of rigid unsaturated polyester can be improved by blending with elastomers bearing reactive functional groups. In this study, blends of unsaturated polyesters are prepared with functional rubbers such as hydroxy terminated polybutadiene, epoxidized natural rubber, hydroxy terminated natural rubber, and maleated nitrile rubber. The mechanical properties of the cured blends are compared with the control resin. The elastomers bearing reactive functional groups show better compatibility with the resin and improve the toughness and impact resistance of the cured resin substantially, compared to unmodified elastomers. The performance of maleic anhydride-grafted-nitrile rubber is found to be far superior to all other rubbers considered in this investigation. An improvement in toughness, impact resistance, and tensile strength has been achieved without seriously affecting any other property.

Aging and Thermal Studies on Epoxy Resin Modified by Epoxidized Novolacs

ABSTRACT Epoxy resins are thermosetting polymers widely used for polymer composites, adhesives, high performance coatings, potting and encapsulation, and numerous other applications. These resins have excellent mechanical and electrical properties, low cure shrinkage, and good adhesion to most substrates. This study is an attempt to improve the thermal and aging characteristics of epoxy resin by blending with other multifunctional epoxies such as EPN and ECN. Bis-phenol A epoxy resins containing 2.5 to 20 wt% of epoxy novolac were cured in the presence of a polyamide hardener and tested for thermal and mechanical properties, hardness, water absorption, etc. Blends containing 10 to 15 wt% of epoxy novolac show substantial improvement in properties such as tensile strength, elongation, and energy absorbed to break. The novolac derived from p-cresol was better than that based on phenol in enhancing the properties. TGA, DSC, and DMA were employed for studying the thermal properties of the modified resin. The study reveals that modification using epoxy phenol and p-cresol novolac resins (EPN and ECN, respectively) improves the aging characteristics of the epoxy resin in addition to overall improvement of the mechanical properties.

Blends of Epoxy and Epoxidized Novolac Resins

The effect of the addition of two types of epoxidized novolac resins separately to cold-set bisphenol-A-based epoxy resin has been investigated in this study. Bisphenol A epoxy resins containing 2.5-20 wt% of epoxy novolac are cured in the presence of a polyamide hardener and tested for thermal and mechanical properties, hardness, water absorption, etc. Blends containing 10-15 wt% of epoxy novolac show substantial improvement in properties, such as tensile strength, elongation, and energy absorbed to break. The novolac derived from p-cresol is better than that based on phenol in enhancing the properties. The property profiles of these blends imply a toughening action by the novolac epoxies. The blends also exhibit improved thermal stability compared to the neat resin.

Modification of Unsaturated Polyester Resin Using Elastomers

Unsaturated polyester resin is widely used in the fiber reinforced plastic industry It suffers from lack of toughness, which makes it susceptible to breakage in the absence of reinforcement. This problem can be solved, at least partly, by the incorporation of elastomers. In this study a simple technique of dissolving various masticated elastomers in styrene and subsequent solution blending with unsaturated polyester resin has been done. The results point to the suitability of nitrile rubber for this purpose compared to other elastomers. An increase in toughness has been achieved without sacrificing any other property substantially.

The Modification of Commercial Epoxy Resin Using Cardanol - Formaldehyde Copolymers

ABSTRACT Unmodified epoxy resins based on bis-phenol A exhibit brittleness and low elongation after cure. Hybrid polymer networks of DGEBA resin and phenolic resol type resins were prepared from phenol (PF) and cardanol (CF), the main constituent of cashew nut shell liquid (CNSL), a renewable resource. The modified epoxy resins were found to exhibit improved elongation and energy absorption at break. Bisphenol A-epoxy resins containing 2.5 to 20 wt% of phenolic resols were prepared and tested for mechanical properties, water absorption, and so on. Blends containing 5 to 10 wt% of PF resin show substantial improvement in properties such as tensile strength, elongation, and energy at break, and these properties show a decline with higher resol loading. However, epoxy-CF blends show decreased tensile and compressive strengths. Resol resins made from cardanol–phenol mixtures (CPF) have improved energy absorption at break and this property increases with increase in CF content in the modifier resin. DGEBA/CPF blend containing cardanol and phenol in the mole ratio 3:2 is found to be superior in energy absorption at break. The property profiles of these blends imply a toughening action by resol type phenolic resins, especially cardanol–formaldehyde resins. The blends, however, show higher water absorption with only marginal improvement in certain other mechanical properties and surface hardness.