|Title of Invention||
A PROCESS FOR PREPARING ADDITION CURABLE PHENOLIC RESIN
|Abstract||A process for preparing addition curable phenolic resin having ethynyl phenyl azo groups comprising the steps of coupling ethynyl phenyl diazonium salt with a phenol formaldehyde resin, separating, purifying and drying the precipitated phenolic resin having ethynyl phenyl azo groups.|
This invention relates to a process for preparing addition curable phenolic resins particularly with improved char residue.
Phenolic resin possessing ethynyl phenyl azo groups belong to a new
family that can be thermally polymerised by an addition mechanism to a
crosslinked polymer whose thermal stability and anaerobic char residue are
superior to those of conventional condensation phenolic resins and hitherto
known addition-curable phenolics. Phenolic resins are work horse matrices in
composites for structural and thermostructural applications in aerospace
industry. This is an important area witnessing rapid evolution in process and
technology. The detailed chemistry, processes and applications of phenolic
resins have been compiled by Knop and others in the book. Chemistry and
Applications of Phenolic Resin, publisher Verlag, 1979. Some of the salient
features of phenolic resins include good thermal stability and high char-yield at
higher temperature and slow rate of thermal erosion. Conventional phenolic
resins based on resole and novolac undergo curing by condensation mechanism,
with concomitant evolution of volatile by-products. This feature of such resins
necessitate application of counter pressure during moulding to get void-free
components either from the neat resin or from their composites. A solution to
this problem is to synthesise phenolic resins that cure by addition mechanism.
Allyl and vinyl functional phenolic resin cannot be moulded as such, since the
polymerisation of these functional groups proceed with great difficulty and is
generally incomplete. Such polymers also need a cure catalyst like peroxides to
content estimation and by its molecular characteristics of gel permeation chromatography (GPC). The degree of substitution is varied by regulating the quantity of ethynyl phenyl diazonium chloride used for coupling with the phenol formaldehyde resin. The substitution by ethynyl phenyl azo groups varies from 50 to 70% by weight. The number average molecular weight (Mn) of the resultant polymer varies in the range 300 to 400 and the polydispersity in the range 3 to 6. The prepolymer is then thermally cured at 200 to 250°C to the crosslinked network and the thermal stability of the cured polymer is assessed by thermogravimetric analysis in nitrogen atmosphere. The reaction scheme for the synthesis and thermal polymerisation are shown in scheme 1. The characterisations are described in detail later.
Accordingly the present invention provides a process for preparing addition curable phenolic resin having ethynyl phenyl azo groups comprising the steps of coupling ethynyl phenyl diazonium salt with a phenol formaldehyde resin, separating, purifying and drying the precipitated phenolic resin having ethynyl phenyl azo groups.
Ethynyl phenyl diazonium salt is prepared by diazotising amino phenyl acetylene with sodium nitrite in the presence of sulphuric acid.
in 80 ml 1 normal sodium hydroxide) at 0°C and stirred well. After the addition, the solution is kept for one to three hours by occasional stirring at the same temperature. The reaction mixture is then added to distilled water and the pH is adjusted to 7 by adding dilute sulphuric acid. The precipitate formed is filtered. It is dissolved in 100 ml acetone and precipitated to one litre distilled water containing 100 ml of methanol. The product is filtered and then dried in vacuum at 40-60°C. Yield obtained is 13 to 15g (70-80%).
Example 2 :
In this examples all the reaction parameters and steps are the same as in example 1, except that the quantity of meta amino phenyl acetylene and sodium nitrite taken are 12 g. each and the latter is dissolved in 50 ml distilled water.
Example 3 :
In this examples all the reaction parameters and steps are the same as in example 1, except that the quantity of m-amino phenyl acetylene and sodium nitrite taken are 15g. each and the latter is dissolved in 60 ml distilled water.
The following are the methods by which the polymer described in examples 1, 2 and 3 are characterised.
Differential Scanning Calorimetry (DSC) analysis of the resin at a heating
rate of 10°C/minute shows the initiation of
the curing at 150°C. The cure reaction is completed at 230°C. The infra red spectrum of the product shows a strong abosrption at 3290cm" due to the terminal acetylene group. The hydroxyl group absorptions are located at 3300-3800cm . The spectrum shows other characteristic absorptions due to the aromatic groups at 1550 and 1600cm . The molecular weight of the polymer is found by Gel Permeation Chromatography (GPC) of the polymer, done in tetrahydrofuran (THF) and is in the range of peak moelcular weight (Mp), 1100-2500, number average moleuclar weight (Mn), 300-400, weight average molecular weight (Mw), 1000-2100. The composition of the polymer is found from the nitrogen-content of the polymer as estimated by the micro elemental analysis. The composition can be varied by regulating the amount of diazonium salt used for the coupling reaction as in examples 2 and 3. The characteristics of polymers with different degrees of ethynyl phenyl a2o substitution are given in table 1. A maximum of 70 mol% of ethynyl phenyl azo substitution with respect to the phenol function is possible.
The curing of the resin is done by heating at 200-250°C for 2 to 4 hrs and is followed by infra red spectrum which shows the complete disappearance of the absorption at 3290cm" due to the acetylene groups. The crosslinking is evidenced also from the insolubility of the cured polymer in all solvents in which the precursor is soluble. These solvents include, acetone,
tetrahydrofuran, dimethyl formamide and methyl ethyl ketone. The cured polymer on analysis for thermal stability by thermogravimetric analysis (TGA) done under nitrogen atmosphere at a heating rate of 10°C/minute shows mass-loss starting from 350°C. The slow rate of thermal erosion is evidenced from good char residue at higher temperature. A char residue of 69-73% is obtained at 700°C whereas the cured conventional resole gives only 60-63% char under idential thermal analysis conditions.
Isothermal pyrolysis of the resin at 700°C for 2 hours in arogen
shows 70-73% char residue for these polymers as against 55-60% for cured resole under identical conditions.
Table 1. Composition and thermal characteristics of the ethynyl phenyl aso phenolic (EPAP) polymers in comparison with resoles.
The major advantage of the present process is that it gives a method for the synthesis of a new family of addition-cure phenolic bearing ethynyl phenyl azo groups.
Reaction scheme for synthesising and curing the addition curable phenolic resins of the invention.
1. A process for preparing addition curable phenolic resin having ethynyl phenyl azo groups comprising the steps of coupling ethynyl phenyl diazonium salt with a phenol formaldehyde resin, separating, purifying and drying the precipitated phenolic resin having ethynyl phenyl azo groups.
2. The process as claimed in claim 1, wherein the phenol formaldehyde resin is prepared by condensing phenol with formaldehyde in the presence of an acid catalyst, said phenol formaldehyde resin having a number average molecular weight (Mn) of 550-600, weight average molecular weight of 2200-2300 and a peak molecular weight (Mp) of 1800-1900.
3. The process as claimed in claim 1 and 2 wherein said ethynyl phenyl diazonium salt is obtained by reacting amino phenyl acetylene and sodium nitrite in the presence of sulphuric acid.
4. The process as claimed in claim 3, wherein the aminophenylacetylene is selected from, para, meta or ortho aminophenyl acetylene.
5. The process as claimed in claim 1, wherein said phenolic resin having
ethylnyl phenyl azo groups has a number average molecular weight (Mn) of
300-400, a weight average molecular weight (Mw) of 1000 to 2100 and a peak
molecular weight (Mp) of 1100-2500.
6. The process as claimed in claim 1, wherein said ethynyl phenyl azo
substituted resin has 50 to 70% by weight of ethynyl phenyl azo substitution.
7. The process as claimed in any of the preceding claims wherein said precipitated phenolic resin is capable of being cured by heating at a temperature of200to250°C.
8. A process for preparing addition curable phenolic resin, substantially as herein described and exemplified.
|Indian Patent Application Number||449/MAS/1999|
|PG Journal Number||26/2007|
|Date of Filing||21-Apr-1999|
|Name of Patentee||INDIAN SPACE RESEARCH ORGANISATION|
|Applicant Address||ANTARIKSH BHAVAN, NEW BEL ROAD, BANGALORE-560 094.|
|PCT International Classification Number||C08G8/04|
|PCT International Application Number||N/A|
|PCT International Filing date|