INTRODUCTION:

Polyurethanes from renewable resources are having considerable importance in the recent years. Cardanol, a meta- substituted alkyl phenol which is the major component of cashew nut shell liquid obtained during the roasting of cashew nuts¹. Cardanol is a potential natural resource for biomonomers and finds many industrial applications like surface coatings², reinforcement of rubber^3,4, brake linings⁵, composites⁶, friction lining materials, lamination paints and polyurethanes^7-9. The phenolic nature of cardanol makes it possible to react under a variety of conditions to form both base catalyzed resoles and acid catalyzed novolacs and is also served as a source of polyols for making polyurethanes. The use of cardanol derivatives in polyurethane preparation by a few other methodologies has been reported^10-13, and products with better thermal, mechanical, and chemical characteristics were obtained. Other than formaldehyde, furfural a renewable resource, is produced from vegetable waste like cane sugar, bagasse, rice hulls, maize cobs, and other cellulosic waste materials can also be used in together with phenol, acetone or urea to make solid resins. Such resins are used in making fiberglass, some aircraft compounds and automotive brakes.

Phenol-furfural resins have an economic advantage over similar materials from phenol and formaldehyde, resulting from the high combining weight of furfural and the relatively high cost of phenol¹⁴. Because of the importance of furaldehydes in chemical, biological and industrial areas, these compounds have been studied intensively for many years^15-19, especially the determination of 2- furaldehyde (furfural) and its derivatives in polymer^20,21 is of critical importance. O-cresol-furfural resins were synthesized and characterized by thermal analysis²².The synthesis and characterizations of thermosetting resins derived from cashew nutshell liquid–furfural–substituted aromatic compounds were also reported²³. Thermal properties of the semi- interpenetrating networks composed of castor oil polyurethanes and cardanol-furfural resin were studied²⁴.In the present study, the bio-based monomers like cardanol and furfural were used for making the resins. The cardanol-furfural resin was synthesized using adipic acid catalyst which can be properly modified to a high molecular weight hydroxyalkylated derivative and used for the production of polyurethanes.

EXPERIMENTAL:

Materials:

Cardanol was obtained from M/s Sathya Cashew Pvt.Ltd., Chennai, India. Furfural(AR.grade) was received from M/s S.D.Fine chemicals, Adipic acid, epichlorohydrin, diphenylmethane diisocyanate and toluene diisocyanate were received from E.Merck (Germany), Methanol (BDH) was used to dissolve the catalyst.PPG-1200 was received from Aldrich chemicals(USA) and dibutyltin dilaurate was received from Fluka Chemie (Germany).The chemicals were used as received.

Methods:

Infrared spectra were taken in a Shimadzu 8400 S FT IR Spectrophotometer (4500-350cm^-1) by Neat/KBr method.¹H-NMR spectra was recorded using a Bruker Avance III 400MHz FT NMR spectrometer. Specific gravity, iodine value, hydroxyl value and intrinsic viscosity of the resin were determined according to the IS Standard 840-1964.Molecular weight of the resin was determined by gel permeation chromatography using µ-styragel columns, 100A⁰ and 500A⁰ UV detector and 280 nm filter. The polyurethanes were subjected to thermo gravimetric analysis (TGA) studies at a rate of 10⁰C/min in air using Perkin Elmer, Diamond TG/DTA instruments. Acid, alkali and solvent resistance were estimated according to ASTM Standard D 3137, C 267. The tensile properties of the soft polyurethane are measured on an Instron (Norwood, MA, USA) 4202 according to the ASTM D882-97 standard. Dumbbell shaped specimens were cut out from the soft polyurethane using an ASTM D 638 Type V cutter. At least five identical specimens prepared by cutting the material from a polymer sheet were tested and the results averaged. Shore A hardness of the specimens were tested by using Durometer type A Shore instrument (Newyork, USA) according to the ASTM. D 2240 standard.

Synthesis of hydroxyalkylated cardanol-furfural resin:

Novolac resin with different mole ratios, i.e., 1:0.9, 1:0.8 and 1:0.7 of cardanol to furfural were prepared using adipic acid as catalyst and characterized²⁵.About 1.0 mole of cardanol-furfural resin was treated with 10 moles of epichlorohydrin at 75^°C for about 2 h. The reaction mixture was cooled to 5-10^°C and then 20% alcoholic sodium hydroxide was added drop wise to the above mixture for a period of 3 h with vigorous stirring. The reaction mixture was heated once again to 75^°±5^°C for about 4 h. Excess sodium chloride was removed by decantation. The epoxy resin was washed repeatedly with distilled water and extracted with ether. The resin was dried in vacuum. The epoxidised resin was hydrolyzed using 2N HCl and heated to 120^°C for about one hour. The resin was repeatedly washed with distilled water and dried using rotary evaporator.

Synthesis of polyurethanes:

The hydroxyalkylated cardanol-furfural resins were maintained under vacuum for 2 h before polymerization. The polyurethanes were prepared by mixing the hydroxyalkylated cardanol-furfural resins and the commercially available polyol, polypropylene glycol-1200,(PPG-1200) with different diisocyanates viz.,4,4^'-diphenyl methane diisocyanate and toluene diisocyanate, keeping the isocyanate index (NCO/OH mole ratio) constant at 1.4.The reaction was carried out at room temperature in the presence of dibutyltin dilaurate as catalyst. The polyurethanes formed were then allowed to cure for 48 h in a flat surface without any disturbance. The polyurethanes were again cured in a vacuum oven at 80^°C for 48 h and used to evaluate their thermal, mechanical and chemical properties. Table 1 summarizes the polyurethanes cure conditions.

RESULTS AND DISCUSSION:

The condensation of cardanol with furfural in all the mole ratios in the presence of adipic acid leads to ortho-para substituted products. The epoxy resins are synthesized by the reaction of cardanol-furfural resin with epichlorohydrin.The number of epoxy groups of a molecule in the resin is dependent upon the number of phenolic hydroxyl groups in the parent resin. Theoretically, all the phenolic hydroxyl groups might have reacted; but in practice all of them do not react because of steric hindrance²⁶. The epoxide group of epichlorohydrin reacted with phenolic hydroxyl groups under the basic medium and formed the chlorohydrin ether which on dehydrochlorination yielded glycidyl ether. Further, on treating the epoxy resin with hydrochloric acid, the epoxy rings get opened to yield the hydroxyalkylated cardanol-furfural resin. The synthesis of hydroxyalkylated cardanol-furfural resin is presented in Scheme 1.

All the synthesized resins are reddish brown in colour. The specific gravity and intrinsic viscosity of the cardanol-furfural resin and hydroxyalkylated cardanol-furfural resin are greater than that of cardanol. A slight decrease in the iodine value may be due to the crowding of the bulky groups during the addition of iodine monochloride. The hydroxyl content of the hydroxyalkylated cardanol-furfural resins were determined by the acetylation method and are presented in Table 1.

Spectral studies

The synthesized resins were characterized by FTIR and ¹H-NMR techniques. In the FTIR spectra of epoxidised resins, the characteristic band of the oxirane ring was observed at 854 cm^-1. It also exhibits a sharp peak at 1259 cm^-1 for aromatic C-O stretching of epoxide linkage and 1159 cm^-1for alkyl C-O stretching of the epoxide linkage. The peak at 1114 cm^-1 also indicates the formation of epoxide linkage.

The FTIR spectral data of the hydroxyalkylated cardanol-furfural resins reveals that the disappearance of peak at 854 cm^-1 and the significant reduction in epoxide group absorbance peak at 1114 cm^-1 clearly indicates that the epoxide ring has been hydrolyzed. The appearance of peak near 1700 cm^-1 is due to the formation of dihydroxy isopropyl group. The FT IR spectra of epoxidised resin and hydroxyalkylated cardanol- furfural resin are presented in Fig.1 and Fig. 2 respectively.

In the ¹H-NMR spectra of hydroxyalkylated cardanol-furfural resins, the phenolic OH group at 6.6 δ observed in cardanol-furfural resin disappeared in the hydroxyalkylated cardanol-furfural resin. The peak observed at 5.30 δ represents methylene protons of C=CH₂ of the long unsaturated side chain. The new peak at 3.7-4.09 δ represents the methylene protons of dihydroxypropyl chain ends. The ¹H-NMR spectrum of hydroxyalkylated cardanol-furfural resin is presented in Fig. 3.

Fig 1.FT IR spectrum of cardanol-furfural epoxy resin

Fig 2.FT IR spectrum of hydroxyalkylated cardanol-furfural resin

Fig 3. ¹H- NMR spectrum of hydroxyalkylated cardanol-furfural resin

The polyurethanes were characterized by FTIR spectroscopy, which indicates the absence of free OH groups and hence complete conversion of –OH groups of cardanol to the urethane moiety. The FT IR spectra of the synthesized MDI and TDI derived polyurethanes are presented in Fig. 4a and Fig.4b respectively. The characteristic absorption at 3301 cm^-1 corresponding to urethane linkage (-NH stretching, bonded), which is broadened due to the formation of hydrogen bond with a carbonyl group²⁷.The absorbance at 1718 cm^-1 corresponding to C=O stretching (free) in urethane,1645 cm^-1 corresponding to C=O stretching (bonded) in urethane and 1537 cm^-1 corresponding to N-H bending in urethane.

Fig 4a.FT IR spectrum of MDI derived polyurethane

Fig 4b.FTIR spectrum of TDI derived polyurethane

Thermal studies:

Thermal stability is an important property of polyurethanes. The thermal stabilities of the polyurethanes are influenced by the presence of long alkyl side chains in the phenyl ring, unreacted dihydroxypropyl chain ends, the geometry of the molecule and molecular weight between crosslinks, the degree of segments in the flexible sequence, the flexible polyether polyol segments and elastically active branch points. The flexible soft segments are polyol segments that start either from urethane or allophanate linkages. The elastically active branch points comprise biuret and allophanate.

The TGA thermograms of cured polyurethanes obtained from MDI and TDI are presented in Fig. 5a, 5b, 5c and Fig.6a, 6b, 6c respectively. In these polyurethanes, a very small weight loss of 1-2 % occurs at 200°C due to the loss of associated water molecules as the molecule exhibits strong intramolecular hydrogen bonding. The decomposition of MDI treated polyurethanes starts at about 335°C whereas the decomposition starts at about 290°C for TDI treated polyurethanes. Around 300-400°C decrosslinking starts which is a slow process initially. Above 400°C, both the polyurethanes start to decompose at a faster rate and 75-95 % weight loss occurs in the temperature range of 400^°- 500°C.

Fig 5a TGA curve of PU1

Fig 5b TGA curve of PU2

Fig 5c TGA curve of PU3

Fig 6a TGA curve of PU4

Fig 6b TGA curve of PU5

Fig 6c TGA curve of PU6

In the case of diphenylmethane diisocyanate treated polyurethanes, the percentage of weight loss are almost same in the mole ratios of cardanol: furfural (1:0.8 and 1:0.7) derived polyurethanes are comparatively less than that of higher mole ratio of cardanol: furfural (1:0.9) derived polyurethanes. But in the case of toluene diisocyanate treated polyurethanes, the percentage of weight loss, even at higher temperature are comparatively less in the case of lower mole ratio derived polyurethanes. Hence the lower mole ratio of cardanol: furfural (1:0.7) derived polyurethanes are comparatively more stable than that of higher mole ratio of cardanol: furfural (1:0.9 and 1:0.8) derived polyurethanes.

Table 1 Polyurethanes cure conditions

Polyurethane	Cardanol: furfural ratio	Hydroxyalkylatedcardanol-furfural resin		Polypropylene glycol-1200 (Functionality)	Isocyanate	Isocyan ate index (NCO/OH ratio)	Cure temperature (°C)
Polyurethane	Cardanol: furfural ratio	OH Number mgKOH/g	Functionality	Polypropylene glycol-1200 (Functionality)	Isocyanate	Isocyan ate index (NCO/OH ratio)	Cure temperature (°C)
PU1	1:0.9	186	10	2	MDI	1.4	80
PU2	1:0.8	198	9	2	MDI	1.4	80
PU3	1:0.7	195	8	2	MDI	1.4	80
PU4	1:0.9	186	10	2	TDI	1.4	80
PU5	1:0.8	198	9	2	TDI	1.4	80
PU6	1:0.7	195	8	2	TDI	1.4	80

Table 2 Thermal properties of polyurethanes

Polyurethane	Percentage of weight loss at different temperatures					Oxygen index
Polyurethane	100°C	200°C	300°C	400°C	500°C	Oxygen index
PU1	-	0.5	8.5	42.5	80.5	0.1755
PU2	-	1.0	8.5	38.1	75.0	0.1750
PU3	-	0.9	8.7	43.4	75.1	0.1751
PU4	-	5.7	20.0	47.0	97.0	0.1761
PU5	-	1.9	23.0	41.1	96.1	0.1750
PU6	-	2.3	24.1	41.8	94.4	0.1756

A comparative picture of the thermal data characteristics, including the oxygen index value (O.I) are furnished in Table 2. The oxygen index of a material can be taken as a measure of its nonflammability. The oxygen index values were calculated based on carbonaceous char (C.R) as related by the empirical equation

O.I × 100 =17.5 +0.4 × C.R

The MDI treated polyurethanes are more thermally stable than TDI treated polyurethanes. In the former one, only 35% weight loss occurs but in the later about 55% weight loss occurs at a relatively high temperature. This may be due to the fact that MDI treated polyurethanes are stereo chemically more stable than TDI treated polyurethanes.

The kinetics of thermal degradation of the synthesized polyurethanes is analyzed on the basis of the Madusdanan-Krishnan-Ninan method^28,29 to the main stage, which can be expressed by the following equation:

ln G (α)/T^1.92 = ln AEa/φR +3.77 −1.92lnEa – Ea/RT

where A is the pre-exponential factor in the Arrhenius equation, Ea is the activation energy, R is the universal gas constant, φ is the heating rate, T is absolute temperature, and G(α) is the integral form of the conversion dependence function. The correct form of G (α) depends on the proper mechanism of degradation reaction. Different expression of G (α) for some solid-state reaction mechanisms can be described as follows: in first order (n=1), G(α) is –ln(1-α) ; in second order (n=2), G(α) is 1/(1-α) ; in third order (n=3), G(α) is 1/(1-α)².

Where W₀ – Wt

α = -------------

W₀-W∞

where W₀ is the initial weight, Wt is the residual weight at temperature T, and W∞ is the final weight. The plots of ln G (α)/T^1.92vs 1/T for different mechanism functions, the activation energy, Ea (kCal/mol) can be calculated.

From Table 3, in the main stage (360^° -490°C) of degradation of all polyurethanes, the linear correlation coefficient for the first order mechanism is greater than that of the second and third order mechanism. Hence the main thermal degradation reaction of polyurethanes follows first order kinetics.

Table 3 The kinetic parameters of thermal degradation of polyurethanes

(360^° –490°C) at 10°C/min heating rate.

polyurethane

Reaction order

Correlation coefficient

Activation energy(E_a) Kcal/mole

PU1

0.9419

0.6374

0.6676

16.389

18.537

39.720

PU2

0.977

0.8069

0.8674

10.163

4.911

12.466

PU3

0.9472

0.8645

0.906

9.383

5.3536

13.351

PU4

0.9729

0.859

0.9003

10.418

12.183

32.917

PU5

0.9884

0.9144

0.943

6.497

4.8751

12.347

PU6

0.9307

0.725

0.7762

7.776

8.642

19.934

Table 4 Mechanical properties of polyurethanes

Polyurethanes	Tensile strength (MPa)	Elongation (%)	Tensile modulus (MPa)	Hardness (Shore A)
PU1	15.71	115	13.66	71
PU2	16.99	110	11.54	75
PU3	16.01	112	14.30	74
PU4	10.73	105	10.22	63
PU5	12.30	104	11.86	65
PU6	13.70	102	13.43	67

Table 5 Chemical reactivity of polyurethanes: Percentage of weight loss

Polyurethane	1N HCl	1N H₂SO₄	1N CH₃COOH	1N NaOH	30% H₂O₂	1N NaCl	Toluene	Diethyl ether	Ethanol	Water
PU1	1.48	1.54	0.02	1.9	0.35	0	1.8	0	0	0
PU2	1.34	1.58	0.025	1.85	0.25	0	1.78	0	0	0
PU3	1.35	1.38	0.015	1.78	0.38	0	1.68	0	0	0
PU4	1.21	1.32	0.01	1.65	0.61	0	2.0	0	0	0
PU5	1.05	1.23	0.015	1.59	0.52	0	1.94	0	0	0
PU6	1.15	1.27	0.015	1.60	0.50	0	2.0	0	0	0

Mechanical properties:

One of the most extensively used mechanical tests for polymers is the tensile or stress-strain test. The tensile strength of the polyurethanes is largely influenced by the presence of aromatic groups, ether groups, long alkyl chain, dangling chains, branching and cross linking and also degree of hydrogen bonding. The mechanical properties of the synthesized polyurethanes are presented in Table 4.The MDI derived polyurethanes are having more tensile strength than the TDI derived polyurethanes. The lower strength and modulus of polyurethanes is a result of the large amount of dangling chains present and also the stereo chemical factors, which are imperfections in the final polymer network and do not support stress when the network is under load. Hardness is the resistance of a (polymer) surface to deformation. The hardness data reflects the resistance to local deformation, which is a complex property, related to crosslink density, modulus, strength, elasticity, plasticity and porosity of the polymer matrix. The shore A hardness of the synthesized polyurethanes are found to be more for MDI treated polyurethanes than TDI treated polyurethanes which are also included in Table 4.

Reactivity of polyurethanes towards chemical reagents:

The chemical reactivity of polyurethanes is studied using 1N hydrochloric acid,1N sulphuric acid,1N acetic acid,1N sodium hydroxide, 30% hydrogen peroxide, 1N sodium chloride, diethyl ether, toluene ,ethanol and water and the percentage weight loss of the MDI and TDI derived polyurethanes are furnished in Table 5.From the results, it has been found that in both MDI and TDI treated polyurethanes from hydroxyalkylated cardanol-furfural resin have shown a slight percentage of degradation is within 2% towards acids, alkali and oxidizing agent and toluene. There was no change in weight when these polyurethanes are treated with 1N sodium chloride, diethyl ether, ethanol and water.

CONCLUSION:

The development of novel polyurethanes from hydroxyalkylated cardanol-furfural resin is reported. The MDI treated polyurethanes are thermally more stable and possess better mechanical properties than the TDI treated polyurethanes. Both the polyurethanes are stable towards the chemical reagents.

ACKNOWLEDGEMENTS:

One of the authors(RS) wish to thank the University Grant Commission, New Delhi, the director of Collegiate Education, Chennai and the Principal, Sarah Tucker

College,Tirunelveli for selecting her under FDP Programme.

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Received on 19.11.2010 Modified on 28.11.2010

Asian J. Research Chem. 4(2): February 2011; Page 322-328