Synthesis of cardanol based epoxy resin for UV curable coatings using bio reactive diluent: Review

Excerpt: The paper describes some imp routes of developing liquid epoxy resin from Cardanol which can be made UV curable by converting into acrylates using bio based reactive diluent based on ricinolic acid

Abstract

With the increase in demand for petrochemical resources resulting in continuous depletion and increased price it's a great challenge for coating industry to look down palliatives based on renewable and sustainable materials which are ecofriendly and non-hazardous. The paper describes some important routes of developing liquid epoxy resin from Cardanol which can be made UV curable by converting into acrylates using bio based reactive diluent based on ricinolic acid(castor oil). Hence the developed product is solvent free and used for high solids coating minimizing the environmental pollution. This dark brown-colored viscous liquid obtained from shells of the cashew nut can be utilized for a number of polymerization reactions due to its reactive phenolic structure and a meta-substituted unsaturated aliphatic chain. The product can be implemented in primer coats of metal and wood replacing the conventional Bisphenol-A based epoxy system. The negative impact of BPA on human health and the environment necessarily implies the elimination of BPA especially in food contact materials. Therefore there is an increasing interest within the chemical industry for non-harmful aromatic substituents to BPA, especially for the synthesis of epoxy polymers. Thus, cardanol could be an interesting substitute to BPA in some polymers such as epoxy polymers.

Introduction

With a large number of chemistries involved in the coating industry ranging from epoxy to polyurethane most of them are derived from the petroleum based products, there has been a spate of issues such as exponentially rising prices, high depletion rate, handling issues, toxicity and health hazards and the volatile organic components that are emitted during application of coatings. Considering these ecological and economical aspects it is necessary to explore sustainable, economical, nontoxic and nonhazardous alternatives. The solution to this is the use of bio-based materials for the synthesis of resin/ polymers for coatings. These bio-based materials are derived from natural resources which are abundantly available in the universe also their use does not present any problem in handling as they are nontoxic and nonhazardous unlike the petroleum products. Cardanol extracted from cashew nutshell liquid (CNSL) is a promising aromatic renewable source. In this paper we will discuss the ecofriendly epoxy coating binders which can be made using cardanol as raw material. CNSL is directly extracted from the shell of cashew nut, fruit from cashew tree, anacardium occidentale (shown in Fig.1). This tree which is native to tropical regions of Brazil is mostly grown in India, East Africa and Brazil. CNSL is major source of naturally occurring phenols and is regarded as good natural alternative to petrochemicals derived phenols.

CNSL constitutes nearly 25% of the total weight of the nut and is composed of anacardic acid (3-n- pentadecylsalicyclic acid) and cardanol (3-n-pentadecylphenol), cardol (5-n-pentadecylresorcinol), and methylcardol (2-methyl-5-n-pentadecylresorcinol), the long aliphatic side chain may be saturated, mono-olefinic (8), di-olefinic (8,11), tri-olefinic (8,11,14) with an average of two double bonds per molecule.Fig.2 mention the physical characteristics and Fig. 3 shows chemical composition of CNSL:

Cashew nut shell liquid (CNSL) constitutes nearly one-third of the total nut weight; thus, a large amount of CNSL is formed as a by-product of the mechanical processes used to render the cashew kernel edible and its total production approaches one million tons annually. Thermally treated CNSL, whose main component is cardanol, a phenol derivative mainly having a meta substituent of a C15 unsaturated hydrocarbon chain with one to three double bonds, has various potential industrial utilizations, such as resins, friction lining materials, and surface coatings; however, only a small part of the CNSL that is produced is used in the industrial field.

The phenols with meta-substituted long chain saturated/unsaturated hydrocarbons are suitable for a number of polymerization reactions through addition as well as condensation mechanisms. The combination of aromatic ring and long chain maintain a good balance between flexibility and hardness properties of the coatings. They are used in a number of industrial applications, including brake lining materials, laminating resins, adhesives, ion-exchange resins, paint and coating resins, foundry chemicals, water-proofing agents, surface active agents, synthetic rubber etc.

Cardanol, which is separated by double vacuum distillation from CNSL contains a characteristic long aliphatic alkyl chain in the meta position of the phenolic ring (Fig. 4) that confers attractive properties such as :

  • Good processability.
  • High solubility in organic solvents.
  • Influence many chemical transformations, introducing novel functionalities.
  • It has a blend of good flexibility and hardness.

Cardanol is hydrophobic and remains flexible and liquid at very low temperatures; its freezing point is below −20 °C, it has a density of 0.930 g/mL, and boils at 225 °C under reduced pressure (10 mmHg).

Routes of Cardanol epoxy synthesis

A) Cardanol Trifunctional epoxy oligomer

Scheme 1: Malenisation of Cardanol

Malenisation is a common technique to introduce polarity and active hydrogen sites in a molecule. Reaction of maleic anhydride with Cardanol is shown in figure showing various mechanisms. As shown in Fig. 5, there are four types of possible reactions proceeding in the first step. These include grafting of maleic anhydride moiety on the aliphatic chain facilitated by hydrogen transfer. Further, it is known that the non-conjugated double bonds undergo thermal rearrangements resulting in formation of conjugated double bonds. These double bonds would then lead to Diels–Alder reaction with maleic anhydride forming a Chroman ring. Cardanol also contains the aliphatic chain with three double bonds (about 40% of total composition) which includes a vinylic unsaturation. This can undergo an addition reaction by proton transfer mechanism. The product on being analyzed showed broad molecular weight distribution which obtained confirms that there is a polymerization reaction taking place during the first step. The iodine value of the product will be lower than expected which justifies that Diels–Alder reaction and polymerization occurs.

Acetylated Cardanol is used which makes the phenolic proton prone to epoxidation. A little excess of Maleic anhydride is used which is washed and removed at the end of reaction. Cobalt octoate is used as a catalyst. The reaction is carried out at 180-190°C which is less than the boiling point of Cardanol-225°C. The temperature is then lowered and product is hydrolyzed with water developing carboxylic groups in molecule. The product can be characterized for its iodine and acid value.

Scheme 2: Epoxidation

The trifunctional epoxy Cardanol resin from Carboxylic functional cardanol is obtained by reaction with Ephichlorohydrin(ECH). Phase transfer catalyst process is used using phase tranfer coupling catalyst such as quaternary ammonium salts which are not strong enough bases to promote dehydrochlorination. Once the coupling reaction is completed, caustic is added to carry out the dehydrochlorination step. Higher yields of the (n=0) monomer (>90 %) are readily available via this method. The epoxidised product is shown in Fig 6.

B) Cardanol novolac epoxy (CNE) resin:

CNSL has cardanol as its major constituent, due to structural similarity of cardanol with phenol. Cardanol reacts with formaldehyde under a variety of conditions, to yield resole and novolac depending on the type of catalyst used. Fig.7 shows the structure of cross-linked cardanol formaldehyde resin (cardanol novolac resin) where R represents the side chain. The phenolic nature of constituents of CNSL along with varying degrees of unsaturation in the side chain makes it highly polymerisable substance amenable to a variety of polymerization reactions. The condensation polymerization is the most obvious and common method of obtaining polymeric material from CNSL.

Cardanol readily reacts with compounds having an aldehyde and reactive methylene group such as formaldehyde, acetaldehyde, furfuryldehyde, aerolein and hexamine etc. In a typical formulation one mole of Cardanol is reacted with 0.8 moles of formaldehyde under reflux in presence of acidic catalyst such as oxalic acid, hydrochloric acid, sulphuric acid etc.

Epoxidation of Cardanol Novolac resin: The epoxidation of the Cardanol Novolac resin is carried out by addition of excess epichlorohydrin ( more than six times for complete epoxidation) at temperatures of around 60 degree Celsius with continuous stirring along with addition of 40% aqueous NaOH, the aqueous NaOH is added continuously during the process. The scheme for varying this reaction is shown in Fig 8.

Enzymatic epoxidation and polymerization of Cardanol

Lipase is known to catalyze the epoxidation of unsaturated groups in the presence of a catalytic amount of carboxylic acids under mild reaction conditions. The conventional epoxidation process utilizes peracetic or performic acid to elicit oxygen transfer to double bonds, resulting in low yields due to side reactions such as the acid-catalyzed ring opening of oxiranes. In contrast, the enzymatic epoxidation provides a mild and simple alternative, especially for the production of sensitive epoxides.

The epoxide-containing polycardanols were synthesized using lipase catalyst via two routes One involves the synthesis of polycardanol from cardanol using peroxidase, followed by the epoxidation of the unsaturated groups in the side chain (route A, Fig.9). In the other route, epoxide-containing cardanol is prepared from cardanol in the presence of lipase and, subsequently, the epoxide-containing cardanol is polymerized with peroxidase (route B, Fig.9)

C) Polyol route conversion (directly UV curable polyol)

Polyol is one of the essential raw materials (monomers) in the preparation of any coating product. Depending upon the hydroxyl value and other characteristics of the polyol, it finds application in the development of adhesives, coatings, and flexible or rigid foams. When applied to the synthesis of polyols, cardanol-based polyols will have better hydrolytic stability compared to the triglyceride oil based polyols. The polyols are directly reacted with acrylate monomer to form epoxy acrylate for UV curing. The modification is done through the phenolic hydroxyl group for preparation of the polyols. The developed polyols were designated as diol and triol. In the second approach, glycerol was reacted with epichlorohydrin to give the monochlorohydrin. This, when reacted with cardanol under alkaline conditions, gave the polyol (glycard) Fig.10

UV Curing

Since the 1960's, radiation curing was introduced as a new technology in the field of coating and ink application. The coating industry was searching for alternative techniques which were energy saving, as well as coating types containing less organic solvents. In fact, the radiation curable coatings are essentially 100% solvent free, as the diluents used are reactive, and both the UV and EB curing technique are very energy efficient compared to e.g. the thermally curing counterparts. The real driving forces for UV and EB-curing were purely economic: improvement of production rate, savings in process costs and superior coating end properties, such as high gloss and hardness, excellent abrasion and scratch resistance and other resistance properties against solvents, staining and other chemicals). Since the last decade the environmental arguments are growing more and more as the industries are currently facing stringent legislation on the reduction of the VOC.

Currently, the majority of UV coatings which are commercially applied are based on acrylated binders. Generally, these binders (prepolymers) are acrylated derivatives of a wide variety of different resin types, e.g. epoxies, Polyesters, polyurethane etc. To adjust the coating's viscosity for application purposes, reactive diluents (monomers) are added. These are low molecular acrylates, e.g. tripropylene glycol diacrylate (TPGDA). The monomer used may also impart some special function like fire retardancy in coating in if nitrogen and phosphorus based monomers are used, hence they may impart a dual purpose. The basic principle of UV curing is shown in Fig.11

Currently, the majority of UV coatings which are commercially applied, are based on acrylated binders. Generally, these binders (prepolymers) are acrylated derivatives of a wide variety of different resin types, e.g. epoxies, Polyesters, polyurethane etc. To adjust the coating's viscosity for application purposes, reactive diluents (monomers) are added. These are low molecular acrylates, e.g. tripropylene glycol diacrylate (TPGDA). The monomer used may also impart some special function like fire retardancy in coating in if nitrogen and phosphorus based monomers are used, hence they may impart a dual purpose.

In case of UV curing , a photo-initiator which is added to the coating formulation which absorbs the UV energy and essentially effects the generation of free radicals. These reactive species initiate a polymerization reaction of the acrylated resin and the monomers ( see Fig.12 ). The complete curing reaction is very fast. It typically takes 0.2 - 0 . 5 seconds.

The content and functions of the main components of such formulations are described in Table 1. Typically, formulations for radiation-curable coatings contain about 25–90% oligomeric resins, 0–60% reactive diluents, 0.5–5% photoinitiators, 1–3% additives, like leveling agents, defoamers, and optionally pigments, fillers, and matting agents. Since the application fields are very broad, the requirements on the coating properties differ very much. The 'reactive diluents' are, on the one hand, used to adjust the viscosity of the formulations, so that the paint can be casted, rolled, or sprayed without use of solvents. On the other hand, the cross-linking density can be varied in a wide range by the functionality of the reactive diluents. Increasing the functionality results in increased rate of polymerization, glass transition temperature, hardness, and chemical and scratch resistance, but decreased double bond conversion, flexibility, and elongation.

Synthesis of Cardanol epoxy acrylate (Oligomer resin)

Epoxy acrylate was prepared by treatment of epoxy/polyol functianl cardanol resin with acrylic acid or its derivatives. Known amount resin is reacted with acrylic acid (1 mol per epoxy groups with 0.02%(w/w) hydroquinone as an indicator, with stirring at 80 C for 5 hrs in a flask equipped with a condenser. The product obtained was washed with water to remove excess acrylic acid and hydroquinone. The product was characterized for its iodine value. Figure 13,14 15 demonstrate the acrylates of epoxy Cardanol resin.

The reaction of resin with acrylic acid results in unsaturation in the molecule which are the active site for polymerization on UV curing in presence of photoinitator. Generally freeradical imitators are used in acrylate system. Photoinitiators are molecules that absorb photons upon irradiation with light and form reactive species from their excited state, which initiate consecutive reactions. Almost all radical photoinitiators contain the benzoyl(phenyl–CO–) structure element. The two most important classes are the α- cleavable (Norrish type I) and the non-cleavable (“hydrogen abstraction” type II) photoinitiators (Figure 16). The photoinitiators also have to be selected in order to match with the output spectrum of the UV light source. The α-cleavage-type photoinitiators are very versatile, exhibiting higher efficiency compared to hydrogen abstraction types due to the unimolecular cleavage reaction and consequently they are the most widely used.

Reactive diluent from ricinoleic acid

The most common formulations for UV curable coating contain three important components: unsaturated acrylic oligomer, photointiator and reactive diluent .The oligomers used for formulations is epoxy acrylate. Reactive diluents are incorporated into UV curing system to reduce the viscosity of oligomers Reactive diluent is a low viscosity and compatible material act as a solvent in UV curing systems, is chemically copolymerizes with oligomer in presence of photointiator and converted into the cross linked film during photo-polymerization.

The petroleum based reactive diluents have excellent properties but several of their restrictions are skin irritancy, poor pigment wetting and film shrinkage often in poor adhesion. Therefore, biobased derived reactive diluent would be promising alternative for petroleum based reactive diluent. Vegetables oils are interesting alternative to non-renewable resources that have been used for coating applications. They are inexpensive and widely available from different resources. Among vegetable oils, castor oil has attracted more interest for preparation of different biodegradable polymer. It contains about 85–90% of ricinoleic acid. Ricinoleic acid obtained from the hydrolysis of castor oil and has secondary hydroxyl groups in the 12th position, a double bond in the 9th position and a carboxyl.

Castor oil based di and tri-acrylate has good properties and dilution with improved viscoelastic properties. The coating shows improved water resistance, and glass transition temperature with increasing castor oil content. The ricinamide triacrylate is synthesis via two step route using ricinoleic acid, diethanolamine and glycidyl methacrylate as raw materials. The synthesis route is shown in Fig 17.

Conclusion

The paper discussed that bio based coating requiring no solvent in in composition with excellent properties such as adhesion, corrosion resistance, water resistance, chemical resistance. The product is versatile and can be incorporated in automotive industry, wood coatings, packaging coatings, can coatings etc. Various modifications are possible in the formulation to get desirable and functional properties. Lots many routes have already been discussed in the paper. Hence the paper holds a wide scope in the field of Green-Paint Technology.

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Author Details

Rupanshu Rastogi, Rishabh Dwivedi

Department of Paint Technology, Harcourt Butler Technical University Kanpur-208 002