N,N′-dimethylolurea (DMU) was prepared and condensed with phenol (P) in the presence of an alcoholic alkali catalyst using 1:1 mole ratio of DMU:P. The resultant DMUP resin was characterized by elemental analysis, IR spectral studies and number average molecular weight estimates by nonaqueous conductometric titration. Further reaction of DMUP resin was carried out with the three epoxy resins DGEBA, DGEBC and DGEBF. The curing of the prepared resins was monitored by differential scanning calorimeter (DSC) and their kinetic parameters have been evaluated. Based on DSC thermograms, glass fiber-reinforced composites have been laminated and characterized by chemical, mechanical and electrical properties. The unreinforced cured resins were subjected to thermogravimetric analysis (TGA).
Keywords: composites; differential scanning calorimeter (DSC); hexamethylenetetramine (HEXA); infrared spectra (IR); N,N′-dimethylolurea (DMU); N,N′-dimethylolurea-phenol (DMUP) resin; number average molecular weight (Mn); thermogravimetric analysis (TGA)
The adhesives often used in furniture industries are formaldehyde-condensation resins. These are urea-formaldehyde (UF), phenol-formaldehyde (PF), malemine-formaldehyde (MF) and phenol-resorcinol-formaldehyde (PRF) resins. UF resins are preferred by the wood-based panels industry due to their high reactivity and cost efficiency. Bonding with UF adhesive is cheaper than with PF adhesive, and it permits the formation of strong bonds under a wide variety of conditions. The process of urea-formaldehyde resinification and characterizations are well-established [[
Epoxy resins are versatile resins having a wide range of properties such as adhesion to substrate, corrosion resistance and high tensile, flexural and compressive strengths. Because of the versatile properties exhibited by epoxy resin, it has found many applications [[
Graph: Scheme 1 Synthesis steps.
The specifications of the epoxy resins are:
- Epoxy equivalent weight of Diglycidylether of bisphenol-A (DGEBA), 190
- Epoxy equivalent weight of Diglycidylether of bisphenol-C (DGEBC), 210
- Epoxy equivalent weight of Diglycidylether of bisphenol-F (DGEBF), 160
E-type glass-woven fabric (0.25 mm thick) was obtained from Unnati Chemicals, India. All other chemicals were of laboratory grade.
DMU was prepared by following a reported method [[
Table 1: Preliminary characterization of DMU and DMUP resin.
Designation of product DMU Molecular formula C3H8N2O3 Elemental analysis %C %H %N Calc. 30.00 6.71 23.32 Found 29.83 6.62 23.20 No. of ‒OH group 2.1 ≈ 2.0 Melting point 123°C Solubility Soluble in water (1.5 g in 10 ml). Miscible with highly polar solvents like alcohols, DMF, THF DMUP Resin Molecular formula C9H10N2O2 Elemental analysis %C %H %N Calc. 60.66 5.65 15.72 Found 60.53 5.60 15.61 Number average molecular weight 1645 Degree of polymerization (DP) 9.23
DMU and phenol in 1:1 mole ratio were refluxed in a methanolic solution of 3% NaOH of the total weight of the reactants for 2 h. The resulting solution was then poured immediately into distilled water to give a yellow thick resin, and washed several times with distilled water to remove unreacted reactants. Preliminary characterizations of DMUP resin are given in Table 1.
The DMUP-epoxy resin system has been prepared by mixing DMUP resin and epoxy resin (i.e., DGEBA) in different proportions (as shown in Table 2). To this mixture, the catalyst hexamethylenetetramine (HEXA), at 0.5% of the weight of the DMUP resin, was added under continuous stirring and stirred well for 15 min to form a homogeneous system.
Table 2: Curing characterization of DMUP-Epoxy resin systems.
Compositions Epoxy resins DMUP Epoxy resin Designation Kick-off temp. Ti (°C) Peak temp. Tp (°C) Final temp. Tf (°C) Activation energy (Ea) KJ/mol Order of reaction 'n' DGEBA 60 40 1a 111 139 151 196.3 1.8 50 50 1b 119 141 160 195.6 1.8 40 60 1c 126 148 168 194.7 1.9 DGEBC 60 40 1d 109 131 144 192.4 1.9 50 50 1e 115 134 150 191.6 1.9 40 60 1f 120 139 157 191.1 1.9 DGEBF 60 40 1g 106 129 141 189.6 2.0 50 50 1h 113 131 146 188.8 2.0 40 60 1i 119 135 151 188.1 2.1
Similarly, other DMUP-epoxy resin systems from the epoxy resins DGEBC and DGEBF were prepared by using the same method and conditions used for DGEBA.
Three different DMUP-epoxy resin systems were prepared by using different proportions of DMUP and DGEBA, DGEBC and DGEBF epoxy resins as shown in Table 2.
Suspensions of DMUP-epoxy resin (DGEBA, DGEBC and DGEBF) systems were prepared in tetrahydrofuran (THF) and stirred well for 10 min. Each suspension was applied with a brush to 250 mm × 250 mm phenolic-compatible fiberglass cloth and the solvent was allowed to evaporate. The dried ten prepregs so prepared were stacked one over another and pressed between steel plates coated with a Teflon release sheet, and compressed in a flat platen press under 70 psi pressure. The prepregs stacks were cured by heating at 150 ± 5°C for 4 h in an air-circulated oven. The composite so obtained was cooled to 50°C before the pressure was released.
The C, H, N contents were estimated by means of a Carlo Earba elemental analyzer (Italy). The IR spectra of all the samples were taken in KBr pellets on a Nicolet 760 D spectrophotometer. The number average molecular weight was estimated by using nonaqueous conductometric titration method [[
A Du Pont 900 DSC was used for the curing study of DMUP-epoxy resin systems. The instrument was calibrated using a standard indium metal with a known heat of fusion (ΔH = 28.45 J/g). Curing was carried out using a single heating rate of 10°C/min in air. The sample weights for this investigation were in the range of 4–5 mg, along with an empty reference cell.
Thermogravimetric analysis (TGA) of DMUP-epoxy resin systems have been carried out using a Du Pont 950 thermogravimetric analyzer at a heating rate of 10°C/min in air. The sample weights for this investigation were in the range of 4–5 mg.
The chemical, mechanical and electrical tests on composites were all conducted according to the ASTM methods listed below using five specimens for each test.
ASTM D 543-67 was used to measure the chemical resistance of the composites towards sodium hydroxide, organic solvents and mineral acids.
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1 ) The flexural strength was measured according to ASTM D 790. - (
2 ) The compressive strength was measured according to ASTM D 695. - (
3 ) The impact strength was measured according to ASTM D 256. - (
4 ) The Rockwell hardness was measured according to ASTM D 785. - (
5 ) The electrical strength was measured according to ASTM D 149.
The DMU, having two active ‒CH
The elemental analysis of the DMUP resins was found to be consistent with their predicted structure. The number average molecular weight of DMUP resins was estimated by nonaqueous conductometric titration [[
The curing study of DMUP-epoxy cured product was carried out on DSC. The data obtained from DSC thermograms show that all the cured DMUP-epoxy resin systems give a single exothermic peak in the range 137 to 196°C. The values of activation energy (Ea) for such systems, furnished in Table 2, did not vary widely. The results of curing temperature with activation energy (Ea) and order of reaction (n) are given in Table 2.
The unreinforced cured DMUP-epoxy resin (DGEBA, DGEBC and DGEBF) samples were prepared at 150 ± 5°C for 4 h. They formed a powder under normal hand pressure, and were insoluble in all common organic solvents. TG data are shown in Table 3 for unreinforced cured resin samples and show that they all degrade in a single step and their decomposition starts around 200°C. The rate of decomposition becomes faster in the range of 300 to 600°C temperature. The glass-reinforced composites based on DMUP-epoxy resins were also prepared at 150 ± 5°C for 4 h. The density of all the composites was in the range of 1.31 to 1.34 g/cm
Table 3: TGA of unreinforced cured DMUP-Epoxy resin systems.
% Weight loss at various temperature °C from TGA Designation 200 300 400 500 600 700 1a 2.0 9.3 17.3 28.1 53.9 56.1 1b 1.9 8.9 16.8 27.8 53.2 55.4 1c 1.7 8.4 15.7 27.6 52.4 54.3 1d 2.2 9.8 17.9 28.9 54.2 57.0 1e 2.1 9.4 17.3 28.6 53.8 56.8 1f 1.9 8.9 16.2 27.9 53.1 56.0 1g 2.4 9.8 18.4 29.1 55.1 57.7 1h 2.3 9.6 17.8 28.8 54.7 57.1 1i 2.2 9.1 9.1 28.3 54.0 56.2
Table 4: Chemical, mechanical and electrical properties of glass fiber-reinforced composites prepared from DMUP-Epoxy resin systems.
% Change on exposure to 25% (W/V) NaOH Glass fiber-reinforced composites Thickness Weight Density g/cm3 Flexural strength (MPa) Compressive strength (MPa) Impact strength (MPa) Rockwell hardness (R) Electrical strength (in air) (kV/mm) 2a 1.16 1.18 1.33 316 318 316 132 18.3 2b 1.14 1.17 1.33 321 319 320 135 18.4 2c 1.11 1.14 1.34 326 322 324 139 18.5 2d 1.18 1.19 1.32 310 312 314 130 18.1 2e 1.17 1.18 1.32 317 314 318 133 18.1 2f 1.14 1.16 1.33 320 319 321 135 18.3 2g 1.19 1.21 1.31 307 306 310 128 17.8 2h 1.18 1.20 1.31 311 309 312 130 18.0 2i 1.16 1.18 1.32 315 314 317 131 18.2
The DMUP and DMUP-epoxy resin (DGEBA, DGEBC and DGEBF) systems can be prepared easily. The glass-reinforced composites of DMUP-epoxy resin systems have good chemical, mechanical and electrical properties. There was not much variation in the mechanical properties with molar ratio. The improved properties of DMUP-epoxy resin-based composites might be due to the presence of aliphatic ketonic segments and strong H-bonds between the phenolic ‒OH and keto (C˭O) groups of neighboring polymeric chains and, of course, due to the presence of epoxy resin. The properties of DMUP-epoxy resin systems are better than individual PF, UF, dimethylolurea-phenol (DMUP) and epoxy resins.
We are thankful to the Head of the Chemistry Department for providing research facilities.
By HasmukhS. Patel; BhavdeepK. Patel and ManishM. Morekar
Reported by Author; Author; Author
