Mechanical, chemical, and curing characteristics of cardanolfurfural-based novolac resin for application in green coatings.
Keywords Renewable resource, Cardanol, Furfural, FTIR, NMR, DSC, Coatings
Increasing environmental concerns have opened significant opportunities for polymers from renewable resources. The utilization of renewable resources in polymers and coating applications is receiving increasing attention and has been the subject of keen interest among both academic and industrial researchers. (1-3) The need to reduce the use of petrochemical-derived monomers in the manufacture of polymers is evident as a result of spiraling cost and the high rate of depletion of petrochemical-derived stocks. This requires the investigation and use of renewable resources which can serve as alternative feedstocks of monomers for the polymer industry. (4) Among the renewable resources, cashew nut shell liquid (CNSL), an agricultural renewable resource material obtained as a byproduct of the cashew processing industry, is unique in that it possesses phenolic moiety with an unsaturated 15-carbon side chain, as shown in Scheme 1. (5) Cardanol, a natural metasubstituted alkyl phenol from CNSL, can be regarded as a versatile and valuable raw material for polymer production, (6-8) and like phenol, it can be condensed with active hydrogen-containing compounds to yield a series of phenolic resins, for instance, base-catalyzed resoles and acid-catalyzed novolacs. (9) Resins derived from CNSL/cardanol are widely employed in the field of surface coatings, adhesives, and laminates, and have several miscellaneous applications. (10)
Furfural, the heteroaryl aldehyde, is obtained as an agricultural waste product which has an extensive application in the formation of resins. (11) Several resins have been prepared by using cardanyl acrylate and furfural in the presence of an acid catalyst and a selective organic compound, and their thermal properties have been studied. (12)
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Novolac resins based on phenol-furfural have already been discussed in previous publications. (9,10) The synthesis of cardanol-formaldehyde resins have been reported earlier. (13-15) However, these have some limitations, like weak chemical stability, low flexibility, and weak impact resistance. (16) Therefore, the cardanolformaldehyde resins may be further modified by replacing formaldehyde with an aromatic aldehyde such as furfural in the presence of a suitable catalyst to improve the chemical and mechanical stability of such cardanol-novolac resins. Since the study of mechanical, chemical, and curing properties of cardanol-furfural resins has hardly been investigated so far, the present work is concerned with the mechanical, chemical, and curing characteristics of cardanol-furfural resin for application in green coatings, as these coatings are derived from renewable resources (cardanol and furfural).
Cardanol was procured from M/s Dheer Gramodyog Ltd., Kanpur, India. Furfural (A.R. grade) was obtained from Qualikems Fine Chemicals Pvt. Ltd., New Delhi, India, and was used for formylation. Succinic acid and hexamethylenetetramine (HMTA) were received from Central Drug House Ltd. (CDH), Mumbai and New Delhi, India, respectively. Methanol was used to dissolve the free catalyst and was received from M/s Thomas Baker Chemicals Ltd., Mumbai, India. Powdered soap was procured from the local market.
Analysis of cardanol
Cardanol was subjected to extensive analysis for the determination of iodine value, viscosity, and specific gravity as per the procedures mentioned in IS-Standard 840-1964 (refer to Table 1).
Synthesis of cardanol-furfural-based novolac resin
Cardanol-furfural-based novolac resins were synthesized from cardanol and furfural in the mole ratios 1:0.5, 1:0.6,1:0.7, and 1:0.8 using oxalic acid as a catalyst by a method published in the literature for cardanol-formaldehyde resin. (16) Four samples of cardanol-furfural novolacs were prepared using four different mole ratios as mentioned earlier (refer to Table 2). Under warm conditions, the catalyst (1% based on cardanol) was dissolved in 5 mL methanol. In a three-necked round-bottomed flask containing cardanol, with a mechanical stirrer and distillation unit attached, furfural was added dropwise through a dropping funnel along with the catalyst solution. The formation of multinuclear cardanol-furfural resin (Scheme 2) might occur when the reaction mixture was heated under constant stirring at a temperature of 120[degrees]C. The reaction mixture was withdrawn after every 45 min to determine the free formaldehyde content (as per ASTM standard D1312-56) and free phenol content (as per ISO standard 9397) for checking the completion of the reaction. The pH of the reaction mixture was found to be 2 at the end of the reaction. The final resinous product was collected and dried under vacuum at 60[degrees]C overnight. Finally, the resin was purified by column chromatography. A resin solution prepared with n-hexane, charged to the silica gel column chromatographic purification, was adopted mainly to remove the unreacted components, impurities, etc., from the methylolated cardanol. Purification was accomplished by using the eluent mixture of ethyl acetate-benzene (60:40).
Curing of cardanol-furfural-based novolac resin
A process mentioned elsewhere (17) was adopted for curing of the cardanol-furfural-based novolac resin by using HMTA. HMTA is the most widely used agent for curing processes of novolac resins. (18,19) Novolac resins made from cardanol and furfural at four different mole ratios, viz. 1:0.5, 1:0.6, 1:0.7, and 1:0.8, were cured by the addition of 15% HMTA. The mixtures of novolac resin and curing agent were taken in small glass vials and mixed uniformly with the help of a glass rod at room temperature. Thereafter, the glass vials were kept in a preheated air oven.
Coating of panels
The prepared samples of cardanol-furfural novolac resins were mixed with 15% HMTA and stirred well to get a homogenous coating mixture. For the mechanical properties of the films, the mild steel panels were cleaned well with mineral turpentine oil (MTO) and detergent such as powdered soap and finally with methanol, and dried in an oven at 100[degrees]C for 20 min. Panels were then uniformly coated with the prepared homogenous mixture by using a Bird Film Applicator (M/s Sheen Instruments Ltd.). Resin-coated panels were then subjected to the hot air oven for curing the applied films. For chemical resistance of the films, glass panels were used instead of mild steel panels. All of the coating films were cured at 160[degrees]C but at different time intervals, as mentioned in Table 3.
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Evaluation of mechanical properties of films: All the resin-coated panels were used for the evaluation of cured film properties such as scratch hardness, adhesion, flexibility, gloss, and impact resistance.
Evaluation of chemical resistance properties of films: Each resin-coated panel was used for the evaluation of chemical resistance properties of the cured films such as acids, alkalis, deionized water, and solvents.
Characterization of cardanol-furfural based novolac resin
Fourier-transform infrared (FTIR) spectroscopic analysis
The purified resin was subjected to Fourier transform infrared (FTIR) spectroscopic analysis to monitor the formation or disappearance of various functional groups using a Perkin-Elmer (Model 843) infrared spectrophotometer in the wavelength range of 5004000 [cm.sup.-1]. Potassium bromide (KBr) pellets were used to get the spectra of uncured material.
[sup.1]H-NMR spectroscopic analysis
[sup.1]H-NMR (nuclear magnetic resonance) of the purified cardanol-furfural-novolac resin was recorded using a Jeol-LA 500 NMR spectrophotometer. About 20 mg of the sample, in a 10 mm diameter sample tube, was dissolved in about 5 mL of chloroform-dl (CD[Cl.sub.3]), which was used a solvent along with tetramethylsilane (TMS) as an internal standard. Finally, the spectra were recorded on a computer.
Differential scanning calorimetry (DSC)
Differential scanning calorimetric (DSC) analysis of the prepared samples were carried out to investigate the curing behavior of the cardanol-furfural novolac resins. Cure temperatures of the prepared samples were observed by taking a small amount of sample into a shallow aluminium pan sealed by the aluminium cover of a differential scanning calorimeter (TA instrument, USA; modulated DSC-2920). This was placed in a sample cell of the instrument; the starting temperature, programmed rate, and final temperature were taken at a heating rate of 10[degrees]C/min. Dynamic scans were obtained which were used for assuming the cure temperature.
Mechanical properties of the prepared films
Scratch hardness tester
The scratch hardness of the films of cardanol-furfural novolac resins were checked by an "automatic scratch hardness tester" (M/s Sheen Instruments Ltd., UK).
Impact hardness tester
The impact hardness of the films of the prepared novolac resins were tested using a "tubular impact hardness tester" (M/s Khusboo Scientific, Mumbai).
The adhesion and flexibility of the cured films of cardanol-furfural resins were tested using a cylindrical mandrel (M/s Sheen Instruments Ltd., Model: 809), with the mandrel of the diameters 1/12 to 1.3 of an inch.
Gloss was measured using a triglossometer (M/s Sheen Instruments Ltd., UK). Watching the films from 60[degrees] angles, it was observed that all the coating films had good gloss.
Chemical resistance of the prepared films
For chemical resistance testing, the panels were prepared by applying the resin mixture to 150 x 50 x 1.25 mm glass panels by using a Bird Film Applicator (Sheen Instruments Ltd., UK). A dry film thickness of about 100 [micro]m was maintained throughout on all the panels.
Results and discussion
Analysis of cardanol
An analysis of cardanol is necessary to understand and control the synthesis of phenolic resins. In this relation, the iodine value is required to determine the presence of a degree of unsaturation in the cardanol molecule. Moreover, the viscosity and specific gravity is also required for the synthesis of novolac resin. The analytical data of cardanol was compared with standard technical CNSL. The lesser ash content and nonself-polymerized fractions were indicated by the relatively lesser viscosity (41.3 cP) of the cardanol, in comparison with that of standard technical CNSL (552 cP). The specific gravity of cardanol (0.9 g/[cm.sup.3]) was also relatively lower than standard CNSL (0.960 g/[cm.sup.3]), due to lower ash content and self-polymerized fraction. The degree of unsaturation in cardanol is determined experimentally by the iodine value, which was found to be 278.8 Wijs. Wijs is the standard unit of iodine value. These observations provided evidence that cardanol is a monoene metasubstituted phenol and its empirical formula is written as [C.sub.21][H.sub.34]O. This was also given in our earlier studies (20,21) for different systems. The structure of cardanol may be proposed as shown in Scheme 1.
Synthesis of cardanol-furfural-based novolac resin
The methylolation of cardanol was carried out with furfural in the presence of dicarboxylic acid, viz. succinic acid using four different mole ratios. The completion of the methylolation reaction was checked by the periodic withdrawal of the reaction mixture to analyze free formaldehyde content and free phenol content.
The polymerization of cardanol can be accomplished in two ways: firstly, by the condensation of furfural, and secondly, through the unsaturation present in the side chain. The side chain of cardanol remained unaffected, because it is clear from the measure of the iodine value. (22) The iodine value of cardanol before polymerization was 278.8 Wijs and after polymerization the iodine value of the reaction product was found to be 278.2 Wijs. Therefore, it was concluded that the polymerization proceeded by the first way, i.e., by the complicated step-growth polymerization reaction mechanism. (23) The proposed mechanism of the reaction between cardanol and furfural was based on the literature published earlier. (24) The mechanism of formation of novolac oligomers in acidic media, using an excess of cardanol over furfural, might proceed through the following steps. First, a furansubstituted methylene glycol is protonated by an acid from the reaction medium, which then releases water to form a furan-substituted hydroxyl methylene carbonium ion. This ion acts as a hydroxyalkylating agent by reacting with the cardanol via electrophilic aromatic substitution. A pair of electrons from the benzene ring attacks the electrophilic element, forming a carbon anion intermediate, followed by deprotonation. The methylol group of the hydroxylmethylolated cardanol, being unstable under acidic conditions, would lose water readily to form a benzylic carbonium ion. The products formed would react with another cardanol molecule to form a methylene bridge in another electrophilic aromatic substitution. This process would repeat until all of the furfural has been exhausted. (25) The related reactions have been represented in Scheme 3.
Evaluation of coating films for their curing characteristics
The curing of all the prepared novolac resin samples (FFNR51, FFNR61, FFNR71, and FFNR81) on mild steel panels was completed in an air oven using HMTA as the curing agent. The observations related to curing conditions are presented in Table 3. The curing characteristics of the resin films show that the resin FFNR81 prepared with mole ratio 1:0.8 has good curing characteristics, as the films were cured in 90 min. However, the films of resins designated as FFNR7F FFNR61, and FFNR51 with mole ratios 1:0.7, 1:0.6, and 1:0.5 were cured in 150, 120, and 100 min, respectively, at 160[degrees]C.
Fourier-transforni infrared (FTIR) spectroscopic analysis
The FTIR spectra of sample FFNR81 (Fig. 1) are discussed here. IR spectral analysis of cardanol-furfural-based novolac resins reveals not only the condensation of methylolated cardanol, but also the degree of ortho- and pura-substitution. The band observed at 3472 [cm.sup.-1] might be due to the presence of the hydroxyl group in the methylolated cardanol. The peaks that appear near 3011 and 2926 [cm.sup.-1] in Fig. 1 might be due to the presence of aromatic CH stretching and aliphatic CH stretching, respectively, present in the side chain of cardanol. The sharp band observed at 2854 [cm.sup.-1] might be due to the C-H structure in a methylene bridge. The methylene bridge might form due to the condensation reaction between cardanol and furfural. The stretching vibrations near 2926 and 2854 cm 1 and deformative vibrations near 1464-1486 [cm.sup.-1] indicate the presence of C[H.sub.2] and C[H.sub.3] groups, respectively. The sharp peaks near 722 and 884 [cm.sup.-1] represent the ortho- and para-substitution in benzene nuclei, respectively. A peak near 1266 [cm.sup.-1] might correspond to phenol C-O stretching. The preceding spectral data were found to be identical to those given in the literature. (25)
[sup.1]H-NMR spectroscopic analysis
Figure 2 shows the [sup.1]H-NMR spectrum of cardanolfurfural resins of samples FFNR81. The appearance of a peak at 6.6-7.4 ppm is due to aromatic protons of benzene and the furan ring. The peak around the region 6.6 ppm might be due to the presence of the phenolic hydroxyl group. The peak at 4.7-5.4 ppm indicates the methylene (C=C[H.sub.2]) proton of a long alkyl side chain originally present in cardanol, and the peak at 0.8-2.9 ppm is due to a long aliphatic side chain. The peak at 0.9 ppm might be due to a terminal methyl group of the chain. The strong peak at 1.3 ppm is attributed to the long chain (more than five methylene groups) of the side chain. The peak at 2.8 ppm shows the methane proton of [([C.sub.6][H.sub.5]).sub.2]-CH-[C.sub.4][H.sub.3]O for the bridge between two phenyl rings and one furan ring. All these spectral data indicate that condensation of methylolated cardanol with furfural has been completed under experimental conditions and was fully consistent with the proposed structure (Scheme 3) due to the reaction mechanism as discussed in our previous publication. (23)
Differential scanning calorimetric analysis for curing of FFNR
The temperature of onset ([T.sub.i]), peak temperature ([T.sub.p]), and the temperature of completion of the exotherm (Tstop) are noted in Fig. 3 (sample FFNR81), and the data related to the dynamic DSC scans of FFNR51, FFNR61, FFNR71, and FFNR81, respectively, are summarized in Table 4. It is evident from the table that the initiation of the crosslinking reaction lies in the range of 64.1-121[degrees]C with peak maximum temperature 129, 140, 146, and 153[degrees]C for samples FFNR51, FFNR61, FFNR71, and FFNR81, respectively. The completion of the exotherm was observed in the range of 185-165[degrees]C. The AH values related to the cure process were determined from the area of the exotherm peak obtained from DSC analysis taken in dynamic mode. The preceding data and results of the DSC scans were found to be in close agreement with the values given in the literature for phenol formaldehyde resin. (26) The peak exotherm ([T.sub.p]) is shifted to a higher temperature due to the increased reaction rate with the higher molar ratio of cardanol and furfural. Such a trend was observed by various authors during the studies of the curing of epoxy and phenolic resins. The resins prepared by cardanol-furfural resin were found to show high thermal stability, unlike phenol- or cardanol-formaldehyde resins, which were thermally less stable. (16)
Mechanical properties of the prepared films
The results on the mechanical properties of the cured films are tabulated in Table 5. The table indicates that films of resin FFNR81 were the hardest of all the films, with a maximum value of 1500 g. The hardness of the resins was indicative of the [C.sub.15] long side chain of cardanol that induced softness in the films, resulting in an increase in flexibilty. Thus, it is clear now that all of the films were found to possess good flexibility, which balances the rigid nature of the cured films by introducing this chain in the crosslinked structure, thus increasing the intermolecular spacing. Adhesion of the films was measured by a crosshatch tape test, and all the fims showed 100% adhesion, i.e., no square was lifted by the crosshatch test. The presence of the hydroxyl groups (-OH) in the cardanol-furfural resin might be responsible for the adhesion property of cardanol-furfural novolac resins and this could also be attributed to the high crosslink density and increased crystallinity due to the aromatic backbone of the furfural moiety used. All of the films were found to possess good flexibility due to the presence of [C.sub.15] long side chain in the cardanol, and they passed the 1/8-in. mandrel bend test successfully. All the cured films were found to be smooth and uniform with good gloss. Impact resistance of all the films was found to be excellent. Overall, the mechanical properties of the cured films of the prepared novolac resins showed promising results for nearly all the samples of novolac resin for application in surface coatings, as opposed to phenol- or cardanol-formaldehyde resins that were found to have low flexiblity and weak impact resistance. (16)
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Chemical resistance of the prepared films
Table 6 shows the comparative acids, alkalies, and solvents resistance of the cured films of cardanol-furfural-based novolac resin. A quick perusal of Table 6 clearly illustrates that the films of coatings prepared from resins FFNR71 and FFNR81 (i.e., mole ratios 1:0.7 and 1:0.8) have offered maximum resistance towards different solvents, deionized water, and different concentrations of alkalis and acids as compared to the films from other resin samples. The chemical resistance was evaluated by solvents such as methyl ethyl ketone (MEK), xylene, and methanol, and showed no visible effect on cured films in all cases, irrespective of the hardener system employed. This could be due to the increasing polarity of the cured backbone, as discussed above. This was further supported by increasing the aromatic content from the furfural moiety and highly crosslinked structure of prepared novolac resin, as opposed to phenol-formaldehyde or cardanol-formaldehyde resin, which were found to exhibit weak chemical stability. (16) The deionized water resistance of the cured films showed no effect, and maintained homogeneity even after 6 months of dipping. This could be attributed to the hydrophobic nature of the novolac resin backbone, which resisted any possible interaction between the crosslinked backbone and water. The novolac resin backbone was further responsible for giving excellent alkali (5% sodium carbonate) and weak acid (5% acetic acid) resistance. But the films immersed in strong acid (2% HC1) and strong base (2% NaOH) were affected to a large extent. These coated films showed either dissolution or blistering during the first 3 months and were further affected afterward. This was evident from the fact that the resultant novolac resin structure was found to be highly crosslinked, which would lead to increasing polarity of the system, which is one of the deciding parameters for adhesion of coating on the metallic panels and its solvent compatibility. Thus, the prepared novolac resin system possessed poor strong acid and strong base resistance, irrespective of excellent solvent, weak acid, alkali, and deionized water resistance. Moreover, the cured film of FFNR81 was found to exhibit excellent and remarkable chemical resistance properties compared to the other film samples (see Table 6), except for the strong acid and the strong base. The results obtained from chemical resistance after 6 months are given in Table 6.
The following conclusions seem to be warranted from the present findings of the study on the performance properties of cardanol-furfural-based novolac resins and their films.
(1) The proposed research might prove to be a milestone in the field of resins based on renew able resources. The synthesized cardanol-furfural-based novolac resin has the potential to minimize the use of phenol resin based on petrochemical derivatives like phenol-formaldehyde resin because, unlike phenol-formaldehyde resin, these cured cardanol-furfural resins have better mechanical properties, heat resistance, and especially chemical resistance, particularly to alkalies, solvents, and water. Moreover, furfural, being a product of vegetable origin and available in virtually unlimited quantities, is a much more economical aldehyde than formaldehyde. In these many respects, cardanol-furfural resins are similar to, but superior to, phenol-formaldehyde resins as well as cardanol-formaldehyde resins. Hence, the work is novel. The mechanical and thermal properties of cardanol-furfural resins were found to be better when compared with the literature (28,29) on phenol-formaldehyde resins.
(2) Utilization of the prepared cardanol-furfural-based novolac resin from renewable resources in the green coatings can thus contribute to sustainable development and will help in realizing the principles of "green chemistry." Thus, the use of cardanol and furfural in the synthesis of novolac resin is attractive in view of its low price and renewable nature.
(3) The mechanical properties of the films of cardanol-furfural novolac resin, such as adhesion, flexibility, gloss, and impact resistance, were found to be good, though these films lacked in hardness.
(4) The chemical resistance properties of the films of the cardanol-furfural novolacs were found to be excellent, except in resistance to acids, such as sulfuric acid in particular.
(5) DSC results showed that the synthesized novolac resins showed high thermal stability.
R. Srivastava, D. Srivastava ([mail]) Department of Plastic Technology, H. B. Technological Institute, Kanpur 208002, India
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Table 1: Physical characteristics of cardanol Properties Calculated Literature value value Viscosity (cP) 39.4 32.0 Specific gravity (g/[m.sup.3]) 0.90 0.87 Iodine value (Wijs) 278.8 279.8 Moisture content at 100[degrees]C (wt%) 2.91 2.6 Table 2: Sample designation and reaction conditions for synthesized novolac samples S. Cardanol Furfural Sample Reaction Reaction no. (mol) (mol) codes of time temperature novolac (min) ([degrees]C) resin 1 1.0 0.5 FFNR51 360 120 2 1.0 0.6 FFNR61 360 120 3 1.0 0.7 FFNR71 360 120 4 1.0 0.8 FFNR81 360 120 Table 3: Study of cure schedule S. no. Samples Cardanoldurfural Cure time Cure temperature (mole ratio) (min) ([degrees]C) 1 FFNR51 1.0:0.5 150 160 2 FFNR61 1.0:0.6 120 160 3 FFNR71 1.0:0.7 100 160 4 FFNR81 1.0:0.8 90 160 Table 4: Results obtained from dynamic DSC scan S. no. Samples [T.sub.i] [T.sub.p] ([degrees]C) (a) ([degrees]C) (b) 1 FFNR51 64.1 129 2 FFNR61 87.2 140 3 FFNR71 111 146 4 FFNR81 121 153 S. no. [T.sub.stop] [DELTA]H ([degrees]C) (c) (J/g) 1 165 96.2 2 174 107 3 181 121 4 185 83.8 [T.sub.i] represents temperature of onset, [T.sub.p] peak temperature, [T.sub.stop] temperature of completion of the exotherm (a) Temperature of cure initiation (b) Temperature of cure maximum (c) Temperature of end of cure Table 5: Mechanical properties of the cured films S. no. Properties of films FFNR51 FFNR61 FFNR71 FFNR81 1. Adhesion (crosshatch 100 100 100 100 test) (%) 2. Flexibility (mandrel Pass Pass Pass Pass bend test) 3. Scratch hardness (g) 900 1150 1400 1500 4. Gloss (60[degrees]C 80.9 86.8 92.5 94.9 angle) 5. Impact resistance (kg cm) 30 35 40 40 Table 6: Chemical resistance properties of the cured films after 6 months S. no. Chemicals FFNR51 FFNR61 FFNR71 FFNR81 1. Deionized water 1 1 1 1 2. Methyl ethyl ketone 2 2 2 1 3. Xylene 1 1 1 1 4. Methanol 2 2 1 1 5. Sulfuric acid 2% 6 6 5 5 6. Acetic acid 5% 4 4 3 3 7. Sodium hydroxide 2% 3 3 2 2 8. Sodium carbonate 5% 4 3 2 1 1 unaffected, 2 loss in gloss, 3 softening observed, 4 slight loss in adhesion, 5 film partially removed, 6 film completely removed
Please note: Some tables or figures were omitted from this article.
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|Author:||Srivastava, Riya; Srivastava, Deepak|
|Publication:||Journal of Coatings Technology and Research|
|Date:||Mar 1, 2015|
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