Synthesis , Characterization and Glass-Reinforced Composites of N , N '-Dimethylolthiourea – m-Aminophenol Resin

N,N’-Dimethylolthiourea (DMTU) resin having the methylol group ( – CH2OH ) has been prepared and characterized. The condensation of DMTU resin with m-aminophenol was carried out in the presence of alcoholic alkali catalyst at varying ratios of DMTU: mAP, namely 1:1, 1:1.5 and 1:2. The resultant DMTUmAP resin was characterized by elemental analysis, IR spectral studies, number average molecular weight ( ) estimated by non-aqueous conductometric titration, and thermogravimetry. The curing study of DMTUmAP resin with hexamethylenetetramine (HMTA) was monitored by differential scanning calorimetry (DSC) and kinetic parameters were evaluated. Glass-reinforced composites based on the DMTUmAP-HMTA system have also been prepared and characterized.

The present communication comprises the synthesis, characterization of DMTU resin and its post-reaction with m-aminophenol.The resulting resin was studied for their curing characteristic with hexamethylenetetramine (HMTA) on DSC.Glass reinforcement of DMTUmAP-HMTA has also been studied.The whole process is shown in Scheme 1.

Experimental
All the chemicals used were of laboratory grade.

Synthesis of Dimethylol thiourea (DMTU) resin
These were prepared by method reported. 1A mixture of thiourea (1 mole), formaldehyde (2 mole) and 2.5% alcoholic KOH was heated at 50 0 C with constant stirring for 75 minutes.The resulting mixture was cooled at room temperature.The so-called resin obtained was in the form of colourless thick liquid.It was neutralized by acetic acid and dried azeotropically.The yield was 90%.Analysis of DMTU resin is given in Table 1 and Table 2.

Synthesis of DMTU -m-Aminophenol (DMTUmAP) resin
The DMTU resin and m-aminophenol in a molar ratio of 1:1, 1:1.5 and 1:2 were refluxed in the aqueous solution of 2.5% KOH of the total weight for two hours.The resin prepared, as method reported in 10 .The supernatant aqueous layer was decanted and the resultant resin was washed with large amount of distilled water to remove unreacted material.The water was removed by vacuum.All three resin obtained were thick reddish brown liquids.The yields were 85 to 90%.Details of all three resins are given in Table 1 and Table 2.

Composite fabrication
A suspension of freshly prepared DMTUmAP resin (2) and HMTA in acetone was prepared and was stirred well for 3 minutes.The suspension mixture was applied with a brush to a 250mm x 250mm fibre-glass cloth and the solvent was allowed to evaporate.The 10 dried prepegs prepared in this way were stacked one on top of another and pressed between steel plates coated with a Teflon film release sheet and compressed in a flat platen press under about 70 psi pressure.The prepeg stacks were cured by heating at 1000 C for 4 hours and post-cured at 110 ± 5 0 C for 8 hours in air circulation oven.The composite so obtained was cooled to 50 0 C before the pressure was released.Test specimens were made by cutting the composites and machining them to final dimensions.

Measurements
The C and H contents were estimated by means of a Carlo Erba (Italy) elemental analyser.The IR spectra were taken in KBr pellets using Nicolet-400 DR FTIR spectrophotometer.Non-aqueous condutometric titration of DMTU and DMTUmAP resin was carried out in pyridine against standard sodium methnolate in pyridine as titrant.The value of each sample was calculated according to the methods reported 11 .Curing of the DMTUmAP-HMTA system was carried out by differential scanning calorimetry.A Du Point 900 DSC was for this study.The instrument was calibrated using standard materials with known heats of fusion.Curing was carried out using a single heating rate (10 K min -1 ).The sample weight for this investigation was in the 4 to 5 mg range; an empty cell was used as a reference.Cured samples were subjected to thermogravimetric analysis (Universal V2.6D TA instruments) in air at a heating rate of 10 0 C min -1 .
All the chemical, mechanical and electrical tests of the prepared composites were conducted according to ASTM methods following the method reported 12 .

Results and Discussion
DMTU condensation was performed by previous reported method 1 .The resultant DMTU resin is a transparent viscous liquid .All the properties are given in Table 1 and Table 2.The resin having active -CH 2 OH group is used for condensation with m-aminophenol, bearing in mind that (commercially base catalysed A-stage P-F condensate) resols are functioning in situ post-curing.DMTU condensates were obtained in the form of reddish brown thick liquids.They (freshly prepared) are soluble in most polar organic solvents and also water.On longer storage they harden.Efflux viscometry study of only the liquid resin of DMTUmAP has been attempted and the data are included in Table 2.It was observed that in the absence of m-aminophenol, DMTUmAP remains relatively intact under similar conditions.However, it will be cross-linked only at elevated temperature (110-135 o C).Elemental analysis of all the resin is consistent with the structure predicted (Scheme 1).A typical IR spectrum of self cured DMTUmAP resin was observed.The IR spectra of all three resin comprise important bands due to the DMTUmAP moiety and aromatic moiety.The broad bands appeared from 3390 to 2740 cm -1 with inflections at 3360, 3030 and 3290 cm -1 is due to the polymeric groups.The first two inflections and the 1620 cm -1 may arise from the asymmetric and symmetric vibration of -NH and -HN -CS -NH-systems.substituted aromatic rings.The band at 1170 and 1180 cm -1 may be attributed to the group.The band at 2945 cm -1 may be attributed to -CH 3 stretching.Hence on the basis of these observations and the reactive site present in the monomer concerned (i.eThe bands at 775 and 845 cm -1 may be attributed to adjacent H-atoms or 1,2-or 1,4-.DMTU and m-cresols) the most probable structure of such DMTUmAP resin may be shown in Scheme 1.
The values of all three resin estimated by non-aqueous conductometric titration are shown in Table 2.The values indicate that decreases with increase in molar proportion of m-aminophenol.
As the DMTUmAP resin produced can react with curing agent like HMTA the cure reaction of DMTUmAP-HMTA was studied for DMTU: mAP : HMTA ratios of 1 : 1 : 1, 1: 1.5 : 1 and 1 : 2 : 1.The data obtained from DSC scans are given in Table 3. From the DSC thermogram , it is seen that all the DMTUmAP-HMTA curing systems gave a single exotherm peak in the range of 95 to 138 o C. The kick-off temperature (T i ), peak exothermic temperature (T p ) and temperature of completion ( T f ) were obtained from the thermograms.The activation energy (E a ) values for such a system (Table 3) do not vary widely, which indicates DMTUmAP is more reactive towards the HMTA.The kinetic parameter E a (activation energy) was calculated by assuming that the curing reaction obeys Arrhenius type kinetics and the peak maximum represents a point of constant conversion at a heating rate of 10 o C. To obtain information about the properties of unreinforced crosslinked materials, cured samples were prepared using the same proportions and temperatures as presented in Table 3 and Table 4.The unreinforced cured DMTUmAP-HMTA samples were prepared at 110 ± 5 o C for 8 hours, are dark brown in colour and are brittle materials that form powder under normal hand pressure.They are insoluble in all common organic solvents.IR spectra of all three cured products resemble in most aspects those of DMTUmAP resin, however the spectra comprise more intense bands due to the -CH 2 group and a weak band arround 1940 cm -1 due to the 1,2,3,5-tetra substituted benzene ring system 13- 14 .Typical TG curve of unreinforced crosslinked materials.The TG data are shown in Table 4.It shows that they all degrade in a single step, and their decomposition starts around 271 o C. The degradation becomes faster between 200 and 300 o C. The weight loss was about 74 -78 % at 750 o C in all cases.The degradation of all three systems in the range of 500 to 750 o C indicates that the weight loss decreases with increase in the molar ratio of DMTUmAP systems.This may due to in situ reaction of fragments degraded at an earlier stage.
Glass reinforced composites based on DMTUmAP-HMTA systems have also been prepared at 110 ± 5 o C, and are in the form of reddish brown sheets.The specific gravity of all composites is in the range 1.23 to 1.26 (Table 5).Chemical resistance tests reveal that all composites have remarkable resistance to organic solvents and concentrated acid (25% V/V).However, the concentrated alkali (25% W/V) causes changes in their thickness and weight of about 1.2 to 1.4%.The composites are found to have good mechanical properties (Table 5).The electrical resistivity of all the composites has been found to be in the range of 2.04 x 10 7 to 4.23 x 10 7 Ohms.cm - .It was also observed that the produced composites have better alkali resistivity than those of commercial novolac/resol based composites.

Table 2 .
Physical Conductometric titration and by -OH group determination for all resin samples were approximately equal to two.# Time measured by flow cup 'B'5 (BS 1733) * Degree of polymerization.

Table 3 .
Curing characteristics of the DMTUmC-HMTA systems at 10 K min-1