DESC:FIELD
The present disclosure relates to phenol crotonaldehyde based resins and its preparation.
DEFINITIONS
As used in the present disclosure, the following terms are generally intended to have the meaning as set forth below, except to the extent that the context in which they are used indicate otherwise.
The term “resin” as used herein refers to a solid or liquid synthetic organic polymer used as the basis of plastics, adhesives, varnishes, or other products.
The term “thermoset, or thermosetting plastics” as used herein refers to a synthetic material that strengthens on being heated, but cannot be successfully remolded or reheated after their initial heat-forming. This is in contrast to thermoplastics.
The term “thermoplastic” as used herein refers to synthetic materials which softens when heated; and harden and strengthen after cooling.
The term “softening temperature” as used herein refers to a temperature at which a material softens beyond some arbitrary softness.
The term “curing” as used herein refers to toughening or hardening of a polymer material by cross-linking of polymer chains, brought about by electron beams, heat or chemical additives.
The term “Resol type PF resins” as used herein refers to "one step" major polymeric resins as they cure without a cross linker and are widely used for gluing and bonding building materials.
The term “hexa” as used herein refers to Hexamethylenetetramine in accordance with the present disclosure.
The term “pycnometer” as used herein refers to a container used for determining the density of a liquid or powder, having a specific volume and often provided with a thermometer to indicate the temperature of the contained substance.
BACKGROUND
The background information herein below relates to the present disclosure but is not necessarily prior art.
Phenolic resins are synthetic polymers obtained by the reaction of phenol or substituted phenol with formaldehyde. Phenolic resins are the first synthetic resins which have acquired a significant commercial success. Phenolic resins are a family of polymers, and oligomers; and are composed of a wide range of structures based on the various reaction products of phenols, and aldehydes. Phenolic resins have a wide variety of end uses varying from commodity construction materials to high technology applications in electronics, and aerospace; especially in laminates, moulding compounds, abrasives, coating applications, air and oil filters, and as composites. Thus, there is a growing demand for a polymer material which meets the practical needs, particularly thermal stability in the various ranges of services.
Therefore, there is felt a need to provide a thermally stable phenol crotonaldehyde based resin and process for preparation of phenol crotonaldehyde based resins.
OBJECTS
Some of the objects of the present disclosure, which at least one embodiment herein satisfies, are as follows.
It is an object of the present disclosure to ameliorate one or more problems of the prior art or to at least provide a useful alternative.
An object of the present disclosure is to provide a phenol crotonaldehyde based resin.
Another object of the present disclosure is to provide a thermally stable phenol crotonaldehyde based resin.
Yet another object of the present disclosure is to provide phenol crotonaldehyde based resin having thermosetting properties.
Still another object of the present disclosure is to provide a process for preparation of phenol crotonaldehyde based resin.
Other objects and advantages of the present disclosure will be more apparent from the following description, which is not intended to limit the scope of the present disclosure.
SUMMARY
The present disclosure relates to phenol crotonaldehyde resin having the following general structure:

Formula I
wherein, illustrates the positions at which further polymerization occurs.

The present disclosure further provides a process for preparation of phenol crotonaldehyde resins. The process comprises condensation of phenol and crotonaldehyde at a first predetermined temperature for a predetermined time in a base catalyzed medium to obtain an intermediate; and polymerizing the intermediate using a cross-linking agent at a second predetermined temperature to obtain the phenol crotonaldehyde resins.
In one embodiment, the phenol crotonaldehyde resins have the following structure,

Formula Ia

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
The phenol crotonaldehyde based resin of the present disclosure will now be described with the help of the accompanying drawing, in which:
FIG. 1 illustrates a 1H -NMR of uncured/partially cured PC resin in accordance with the present disclosure;
FIG.2 illustrates an FTIR spectrum of the uncured/partially cured PC resins i.e. Intermediates (PC-1 to PC-8) without conventional cross-linking agent (hexamethylenetetramine) in accordance with the present disclosure;
FIG. 3 illustrates IR spectra of the cured PC resin with hexa (5%, 10% and 20% hexa) in accordance with the present disclosure;
FIG. 4 illustrates the proton NMR spectrum of the cured phenol crotonaldehyde (PC) resin;
FIG. 5 illustrates the effect of temperature on the cured phenol crotonaldehyde resin with hexa; and
FIG.6 illustrates superimposed plots of the rate of change in mass of the uncured and cured PC.

DETAILED DESCRIPTION
Embodiments are provided so as to thoroughly and fully convey the scope of the present disclosure to the person skilled in the art. Numerous details are set forth, relating to specific components, and methods, to provide a complete understanding of embodiments of the present disclosure. It will be apparent to the person skilled in the art that the details provided in the embodiments should not be construed to limit the scope of the present disclosure. In some embodiments, well-known processes, well-known apparatus structures, and well-known techniques are not described in detail.
The terminology used, in the present disclosure, is only for the purpose of explaining a particular embodiment and such terminology shall not be considered to limit the scope of the present disclosure. As used in the present disclosure, the forms "a,” "an," and "the" may be intended to include the plural forms as well, unless the context clearly suggests otherwise. The terms "comprises," "comprising," “including,” and “having,” are open ended transitional phrases and therefore specify the presence of stated features, integers, steps, operations, elements, modules, units and/or components, but do not forbid the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The particular order of steps disclosed in the method and process of the present disclosure is not to be construed as necessarily requiring their performance as described or illustrated. It is also to be understood that additional or alternative steps may be employed.
As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed elements.
The terms first, second, third, etc., should not be construed to limit the scope of the present disclosure as the aforementioned terms may be only used to distinguish one element, component, region, layer or section from another component, region, layer or section. Terms such as first, second, third etc., when used herein do not imply a specific sequence or order unless clearly suggested by the present disclosure.

Phenolic resins have a wide variety of end uses varying from commodity construction materials to high technology applications in electronics, and aerospace; especially in laminates, moulding compounds, abrasives, coating applications, air and oil filters, and as composites. Thus, there is a growing demand for a polymer material which meets the practical needs in the various ranges of services.
In an aspect of the present disclosure, there is provided thermally stable phenol crotonaldehyde resins.
The present disclosure relates to phenol crotonaldehyde resins having the following general structure:

Formula I (Cured PC resin)
wherein, illustrates the positions at which further polymerization occurs.
In one embodiment, the phenol crotonaldehyde resins have the following structure,

Formula Ia (Cured PC resin)
The phenol crotonaldehyde resin (Formula I and Ia) of the present disclosure is a crosslinked resin. Figure 4 illustrates 1H -NMR of the cured phenol crotonaldehyde resin of Formula I and Ia.
In another aspect of the present disclosure there is provided a process for preparation of phenol crotonaldehyde resin.
The process comprises condensing and polymerizing phenol and crotonaldehyde at a first predetermined temperature for a predetermined time in a base catalyzed medium to obtain an intermediate. The intermediate undergoes curing using a cross-linking agent at a second predetermined temperature to obtain said phenol crotonaldehyde resin.
The intermediate of the present disclosure is partially cured or uncured and has the following structure:

Formula II (Intermediate)
wherein, illustrates the positions at which further polymerization occurs.

In one embodiment, the partially cured intermediate of the present disclosure has the following structure:

Formula IIa (Intermediate)

In one embodiment, the process involves condensation followed by polymerization of phenol and crotonaldehyde in the presence of alkali metal hydroxide at a temperature in the range of 90 -120 oC to obtain an intermediate of Formula II as shown in Scheme 1.

Step a: Synthesis of uncured/partially cured Phenol and crotonaldehyde resins (Formula II- Intermediate) in presence of NaOH

In one embodiment, crotonaldehyde having a purity of 99% is used for preparing the Formula II (Intermediate) of the present disclosure. Typically, commercially available crotonaldehyde is distilled before use, to obtain crotonaldehyde having a purity of 99%. In accordance with the present disclosure, the phenol can be m-cresol, resorcinol and the like.
The mole ratio of crotonaldehyde to phenol is in the range of 1.2-3.0. In one embodiment the mole ratio of crotonaldehyde to phenol is 1:2. The base catalyzed medium can be alkali metal hydroxide. The alkali metal hydroxide can be at least one selected from sodium hydroxide and potassium hydroxide. In one embodiment, the alkali metal hydroxide is sodium hydroxide. In the process of the present disclosure the alkali metal hydroxide base can act as a catalyst having concentration in the range of 0.01-0.5 mole.
The so obtained intermediate (Formula II) is cured using a cross-linking agent at a second pre-determined temperature to obtain the phenol crotonaldehyde resin of Formula I.

In one embodiment, the intermediate having Formula II is cured using a hexamethylenetetramine as a cross-linking agent at a second pre-determined temperature to obtain the phenol crotonaldehyde resin of Formula Ia.
The polymerization reaction between phenol and crotonaldehyde is controlled by means of refractive index and percentage of free crotonaldehyde in the reaction mixture. After the completion of the curing of the intermediate (Formula II), using cross-linking agent, the reaction is stopped and the reaction mixture is cooled to a temperature in the range of 30 oC to 40 oC, to obtain a resinous product (cured phenol crotonaldehyde resin). Typically, the resinous product formed is red to dark brown and is soluble in polar organic solvents.
Step b: Synthesis of cured PC resin by curing the intermediate with HEXA
In accordance with the present disclosure, the cross-linking agent is selected from hexamethylenetetramine (hexa). In one embodiment the cross-linking agent is hexamethylenetetramine (hexa).
The second predetermined temperature is in the range of 160 °C to 170 °C, to obtain phenol crotonaldehyde resin having thermosetting properties.
The thermostability of the uncured resins is lower than that of the cured resins. The reason for the high thermal stability of the cured resins observed may be because of the formation of cyclic ring moiety with fewer aliphatic groups in the polymer (resin). Another reason may be the cross-linking reaction with hexa leading to a strong three dimensional network with imide group bridging. Conventional studies on the thermal degradation of phenolic resins have revealed that substitution in the phenol ring reduces the thermal stability. The drawback is overcome by the process for preparing the phenol crotonaldehyde resin in accordance with the present disclosure.
The foregoing description of the embodiments has been provided for purposes of illustration and not intended to limit the scope of the present disclosure. Individual components of a particular embodiment are generally not limited to that particular embodiment, but, are interchangeable. Such variations are not to be regarded as a departure from the present disclosure, and all such modifications are considered to be within the scope of the present disclosure.
The present disclosure is further described in light of the following laboratory scale experiments which are set forth for illustration purpose only and not to be construed for limiting the scope of the disclosure. These laboratory scale experiments can be scaled up to industrial/commercial scale and the results obtained can be extrapolated to industrial/commercial scale.
EXPERIMENTAL DETAILS
Experiment 1:
Phenol was procured from Hindustan Organic Chemicals. Crotonaldehyde was obtained from Somaya Organic and Chemicals. Crotonaldehyde was distilled before use and its purity was checked by GC and was found to be 99% pure. Analytical reagent grade sodium hydroxide and hexamethylenetetramine (hexa) were used. Formaldehyde of 37% strength was obtained from Western India Chemicals and was used as received.
Table-1 below summarizes the mole fraction of crotonaldehyde in the resinous product (intermediates).
TABLE-1: Crotonaldehyde mol fraction in the resinous product (Intermediates)
Sr. No. Resinous product Code Feed Mol ratio (P:C:S)* Crotonaldehyde in Feed Fraction Crotonaldehyde in Polymer Fraction
1 PC1 1:1.2:0.3 0.545 0.570
2 PC2 1:1.6:0.3 0.615 0.666
3 PC3 1:2.0:0.3 0.666 0.710
4 PC4 1:2.5:0.3 0.714 0.770
5 PC5 1:3.0:0.3 0.750 0.800
6 PC6 1:2.0:0.1 0.666 0.700
7 PC7 1:2.0:0.05 0.666 0.670
8 PC8 1:2.0:0.01 0.666 0.620
*P: Phenol; C: Crotonaldehyde; S: Sodium Hydroxide
PC is phenol crotonaldehyde resinous product i.e. partially cured/uncured PC resin

TABLE-2: Physical properties of the resinous product (Intermediates)
Sr. No. Resinous product Code pH of resinous product Refractive Index
at 250C Viscosity (Sec) at 250C Sp. Gravity (gm/cc) at 250C
1 PC1 11.30 1.530 90 1.127
2 PC2 11.35 1.535 125 1.121
3 PC3 11.38 1.535 180 1.115
4 PC4 11.31 1.533 260 1.106
5 PC5 11.36 1.534 370 1.078
6 PC6 10.90 1.528 63 1.084
7 PC7 10.15 1.520 40 1.137
8 PC8 6.85 1.485 12 0.768

It is seen from Table-1 that when the mole fraction of crotonaldehyde was increased in the feed, keeping the alkaline metal hydroxide concentration constant (catalyst concentration), an increase in the mole fraction of crotonaldehyde in the product was observed, indicating that the incorporation of crotonaldehyde in the condensation product was governed not only by the kinetic effect of the feed composition, but also by reactivity. The same is also indicated in Table-2, which summarizes the viscosity data. However, the specific gravity of these resins indicates that a lower feed mole fraction, crotonaldehyde adds to phenol to a marginally greater extent. At higher mole fractions, crotonaldehyde becomes less reactive due to more sterically hindered substitute phenols. The increase in viscosities, perhaps, accounts for the influence of reaction conditions such as strong alkaline reactions.
Table-1 further illustrates the effect of varying the catalyst concentration in the range of 0.01 to 0.3 moles. It is seen from Table-1 that at lower catalyst concentrations specifically at 0.01 mole crotonaldehyde does not react with phenol, which is evident from the viscosities, specific gravity, and mole fraction of crotonaldehyde in the polymer formation as illustrated in Table-1.
The reactivity of crotonaldehyde increases with the increase in catalyst mole fraction in the feed. However, when the catalyst feed is 0.05 moles, the viscosity of the polymer is lower however; the specific gravity is almost comparable to when the catalyst mole fraction is 0.3. It is observed that as the catalyst mole fraction in the feed increases, the specific gravity of the product decreases proportionally as summarized in Table 1.
The so obtained resinous product (intermediate) was cured/treated with hexamethylenetetramine to obtain the thermosetting phenolic resin (thermosetting behaviour is shown at 180 oC or greater than the temperature of 180 oC). The results are summarized in Table 5.
The synthesized resins were characterised before purification stage for properties such as viscosity, refractive index, specific gravity, and pH. The viscosity was measured with a B4 (BS1733) type ford cup as per method DIN59211. The refractive index was tested with an abbe-refractometer (DIN 53491) and specific gravity was determined with the pycnometer (DIN53217). The softening temperature of the resinous product (Formula II- Intermediate) and thermosetting phenolic resin (Formula I- Cured PC resin) were determined by the capillary method using melting point apparatus. The temperature range over which the opaque particles transform into a clear, mobile liquid was recorded as the softening temperature range. The temperature at which the partially cured resin starts to soften was also recorded.
The resinous samples are purified and dried using known techniques.
The purified and dried resinous samples were used for molecular weight, elemental analysis, I.R spectra, and proton NMR, and cure behaviour studies.
The infrared spectra were measured on Shimadzu IR-470 spectrophotometer using potassium bromide pellets. The proton NMR spectra were recorded on a Bruker AC 200 (200 MHz) using deuterated solvents. Elemental analysis was carried out on a Hosli rapid carbon and hydrogen analyser. The molecular weight of the resin was evaluated at 40 degrees Celsius using a Knaur-vapour pressure osmometer. Benzene was used as the standard and the solvent used was ethyl acetate. GC measurements were carried out on a Perkin-Elmer Auto system XL with PSS and column BPX5 was used to estimate unreacted monomer in the resin mixture. An empirical, but proven parameter: the cure time or gel time was used to study the relative cure behaviour of the resins.

Proton NMR analysis
Figure 1 depicts the proton NMR spectrum of uncured phenol crotonaldehyde (PC) resin. From the proton NMR spectrum of phenol crotonaldehyde (PC), it is evident that the signals situated in the range of 0.6-1.4 ppm, belongs to –CH3 groups, the signals between 1.5-2.5 ppm corresponds to -CH2 groups. The signals between 4.5- 6.0 belong to =CH and 5.4-5.6 ppm are related to >CH-OH or >CH-O groups.

Infra –red analysis
The IR spectra of the uncured/partially cured resins (Formula II- Intermediate) without curing with the cross-linking agent (hexa) are shown in Figure 2. The PC resins were taken PC-1 to PC08 based on the phenol, crotonaldehyde and base ratio. The peaks at 808 and 748 cm-1 represent the characteristic bands of the aromatic substitution at 1,2, 1,4 and 1,2,4 positions. The characteristic bands of the ether linkages were observed at 1162 cm-1. Peaks at 3420 and 1671 cm-1 are owing to the presence of phenolic hydroxyl and aldehyde groups respectively. It was observed that trans-crotonaldehyde absorbed at IR frequencies to C = C double bond at 1640, 1310 and 970 cm-1 whereas the resins do not absorb in the regions indicating that there is no unsaturation.
Whereas, the sharp peak at 1007 cm-1 was observed in Figure 3, representing the presence of methylol group which is characteristic of hexamethylenetetramine. All the cured phenol crotonaldehyde (cured PC) resins exhibited almost identical infra-red spectra irrespective of the mole ratio of crotonaldehyde and catalyst. The interpretation data of the IR peaks are summarised in table 3.

Table 3: Infra-red absorption bands observed in phenol-crotonaldehyde resins:
Band (Cm-1) Assignment Nature
3420 (OH) stretching Phenolic and methylol (broad)
3015-3065 (CH) stretching Aromatic
2925-2870 (CH2) stretching in plane and out of plane Aliphatic
1671-1679 (CHO) stretching Carbonyl
1611-1502 (C=C) stretching Benzene ring
1453-1443 (CH2) deformative vibrations Aliphatic
1367 (CH3) deformative vibrations Aliphatic
1232-1222 (C-O) stretching in plane Phenolic
1162 (C-O) stretching and (CH) stretching Aliphatic ether d deformative vibrations of C-H bond
1007 (C-O) stretching Methylol (characteristic peak of hexa) Fig 2
808-827 (CH) deformation out of plane Adjacent 2H, para substituted
748-749 (CH) deformation out of plane Adjacent 4H, ortho substituted
689 (CH) deformation out of plane Adjacent 5H, phenol

Elemental analysis and molecular weights
Elemental analysis and the number average molecular weight data are listed in table 4. The number average molecular weights estimated by VPO (Vapor Pressure Osmometry) were in the range of 1110-1245.
TABLE-4: Microanalysis (%) and Number average molecular weight (gmol-1) data for PC resins.

Sr.No. Resin Code Mole fraction of crotonaldehyde in polymer Microanalysis Data
C Microanalysis Data
H Number average molecular weight
( Mn )
1 PC1 0.590 72.22 7.56 1205
2 PC2 0.660 72.13 7.64 1222
3 PC3 0.710 72.18 7.70 1245
4 PC4 0.770 72.41 7.31 1140
5 PC5 0.800 72.38 7.46 1110
6 PC6 0.700 72.33 7.2 1165
7 PC7 0.690 72.12 7.74 1210

The results summarized in table 4 indicate that six benzene rings are bound together by crotonaldehyde. The polycondensation reaction probably proceeds by the addition of aldehyde (-CHO) across the C=C double bond and condensation across the aldehyde group.
Cure behaviour
The cure behaviour studies of PC (phenol crotonaldehyde resin) are presented in table 5.
TABLE-5: Effect of curing temperature on cure behaviour of PC resins

Sr.No. Aldehyde –phenol mol ratio Curing temp 0C Softening point
at 250C 10% Hexa 0C/ Acetone* solubility with 10% hexa
1
1.2 120 120-125 140-144 Soluble
140 150-154 175-179 Soluble
160 163-167 205-210 Partially Soluble
180 220-225 infusible insoluble
2
2.0 120 134-138 150-155 Soluble
140 154-158 185-190 Soluble
160 165-170 208-213 Soluble
180 235-240 infusible insoluble
3 2.5 120 136-140 158-163 Soluble
140 148-154 163-166 Soluble
160 158-163 202-207 Partially Soluble
180 215-220 infusible insoluble
All the samples in table 5 were cured for 30 min- 60 min
*All hexa mixed PC samples cured at 120 oC to 160 oC, have shown acetone soluble behaviour. However PC resins shown insoluble characteristic such as thermosetting properties when the uncured PC resin is cured with Hexa at temperature above 180 oC.

It is noted that uncured/partially cured PC resins (Intermediates) are thermoplastic in nature. The PC resins which are cured at a temperature in the range of 150 oC to 160 oC are soluble in acetone, whereas the PF resins (phenol formaldehyde resins) evaluated under similar conditions exhibited infusible and insoluble thermosetting characteristics.

The influence of the chemical reactivity of the curing agent with uncured/partially cured phenol crotonaldehyde (PC) resins at a low temperature was not apparently seen. However, it is evident from table 5 that, the cross-linking with the use of hexamethylenetetramine of phenol crotonaldehyde resin at a temperature ranging from 150 oC to 160 oC has exhibited thermoplastic characteristics and the same at =180 oC shows thermoset characteristics. This peculiar behaviour offers interesting processing possibilities. It may be possible to cure partially Resol resins with an addition of cross-linking agent up to 20% process on an injection moulding machine to attain thermoplastic product shapes, followed by post-curing at =180 oC of the moulded part in an oven to impart the desired thermosetting characteristics.

Fig 3 shows the IR spectra of partially cured PC resin (Intermediates) with hexa. The difference in the shapes of the peaks with and without hexa (Fig 2 and Fig 3), at 1600 cm-1 and in the range of 1400-1100 cm-1 are clearly seen. It is found that the change is attributed to three dimensional network formations as the cure rate increases. It is observed that the curing reaction produces a band at a 1007 cm-1,which is not present in the reactants and peak intensities decrease with hexa based cured resin sample (Fig 3) to an increase in cure rate and is assigned due to the stretching of the azomethane group (-C=N-).

Figure 4 shows the 1H NMR spectrum of the cured phenol crotonaldehyde resin. From Figure 4, it is clear that double bonds shown in figure 1 of the uncured are not shown and there is a clear peak in between 6.5 to 7.5 phenol ring. There are no peaks shown for an aldehyde functional group.

Fig 5 shows the effect of temperature on the phenol crotonaldehyde resin with hexa. From fig 5 it is evident that there is no change in the properties of the PC resin when there is rise in temperature. From figure 5, it is observed that the resinous product (PC resin) of the present disclosure is thermostable as the product is stable at 25 oC, 90 oC, 160 oC, 180 oC, and 180 oC for 1 hour as well.

Effect of amount of hexa on the cure behaviour and molecular weights of PC resin:
The effect of amount of hex on the cure behaviour and molecular weights of PC resins was investigated and the data is presented in table 6.

TABLE-6: Effect of amount of hexa on the cure behaviour and molecular weight of PC resin (aldehyde –phenol mol ratio 2.0)

Sr.No. % of hexa Softening Temperature (oC) after curing at 1400C to 1800C for 30 min* Number average molecular weight
(Mn)
1 5 170-178 1430
2 10 185-190 1470
3 15 204-208 1530
4 20 218-224 1580
*All PC samples mixed with hexa from 5% to 20% and cured at 1800C for 30 min have shown infusible thermoset behaviour.
Mn is molecular weight tested after curing at 140 0C for 30 min.

It is evident that the softening temperature and molecular weight increases with increasing hexa concentration, indicating that the cross-linking reaction with hexa is operative.

Curing behaviour:
The curing behaviour of mixed PC resin (phenol crotonaldehyde resin) and PF resin (phenol formaldehyde resin) having aldehyde (crotonaldehyde or formaldehyde) to phenol mole ratio 1.0: 2.0 are presented in table 7.

TABLE-7: Curing behaviour of mixed PC and PF resins (aldehyde –phenol mol ratio 2.0).
Sr.No. Resin composition curing PC:PF(%w/w) Gel time at 1300C
(seconds) Gelled / Solidified mass characteristics after post curing at temperature `1600C for 60 minutes
Softening temperature range(0C) / Acetone solubility
Before post curing / After post curing
1 100:0* 160 110-115 148-152 Soluble
2 80:20* 103 1220-130 160-165 Soluble
3 60:40* 85 138-143 260-265@ Soluble
4 40:60 61 240-250@ Infusible Insoluble
5 20:80 42 Infusible Infusible Insoluble
6 0:100 30 Infusible Infusible Insoluble
@ Resins softens but does not melt into liquid
*Resin samples from PC: PF composition 100:0% to 60:40% were all soluble in acetone

It is observed that mixing of the PF resin with the PC, enhanced the curing rate of PF (high gel time) and no phase separation is observed. Result type PF resin possesses a bounded free methylol group (reactive sites) and curing these resins, even at low temperatures exhibited fast reactive (low gel time) thermostat characteristics.

Due to lack of reactive sites, the uncured or partially cured PC resins (Formula II- Intermediates) are thermoplastic nature. It is also observed that curing of PF resin with higher PC loading (above 40% by weight) exhibits thermoplastic behaviour. It may be due to the steric hindrance of the bulkier moieties of the phenol ring of the PC resin, leading to the inhibition of the cross-linking reaction of PF and PC. Thus, PC resins require additional reactive sites and loading up to 60% by PF could only transform them into infusible thermostats, and/ or PC resin with hexa is observed as thermostat resinous products.

Softening temperature studies:
The effect of curing temperature and the addition of the cross-linking agent; hexa, on the softening point of phenol crotonaldehyde resin (PC) of the present disclosure and conventional phenol formaldehyde resin (PF) was studied and the results obtained are summarized in Table-8.
Table-8: Thermal behaviour of PC and PF resin
Sample Aldehyde-phenol mole Before curing Softening temperature range (°C)
After curing at 140 °C for 30 min Post-curing at 180 °C for 30 min
Without hexa With 10 % hexa Without hexa*
PC1R 1.2 96-100 150-154 175-179 220-225
PC2R 1.6 98-101 150-155 173-177 218-222
PC3R 2.0 103-108 154-158 185-190 235-240
PC4R 2.5 98-103 147-152 163-166 210-215
PC5R 3.0 97-102 143-148 158-163 220-225
PF3R 2.0 63-67 Infusible Infusible Infusible
*With 10 % hexa after post-curing at 180 °C for 30 min, all PC samples have shown infusible behaviour
It is clearly seen from Table-8 that the PC resins exhibited thermoplastic behaviour even when cured with hexa at low temperature. Further, the PC resin, without the addition of hexa, cured at elevated temperature (=180 °C) retains the thermoplastic characteristics. Post-curing of these resins with the addition of hexa at a temperature (=180 °C) yielded thermoset behaviour. PF resins of similar type cured faster and showed thermoset behaviour.
The softening point of the PC and PF resins were found to be independent of the phenol-crotonaldehyde ratio and there was no appreciable increase in the softening temperature for the PC resins prepared without hexa and cured at lower temperatures. However, the softening temperatures increase with the addition of hexa, indicating that the PC resins do react with hexa as in the case of conventional PF resins. The incorporation of the methyl group in the aromatic ring seems to increase the softening temperature.
Degradation studies
The degradation study was conducted by dynamic TGA methods using thermogravimetric analyser such as pre-exponential factor (A), and the energy of activation (E). Differential thermogravimetry (DTG) was used to estimate the temperature of maximum degradation rate (Tmax), the initial degradation temperature (Ti) and the final degradation temperature (Tf). The superimposed plots of the rate of change in mass of the uncured and cured PC are depicted in Figure 6. It is seen from Figure 6 that both resins exhibited two stage degradations. In uncured resins the initial degradation rate starts relatively rapidly and goes up to 380 °C. The weight loss is 45-58% and is a result of the volatilization of bound water and other low molecular weight species. The final degradation stage is observed from 400 °C and is completed at 900 °C. The degradation of the cured resins starts initially at a very low rate and increases after 350 °C and is continued to 500 °C. The final degradation starts at 500 °C and continue to completion at 800 °C. The final stage, being the most significant was selected for comparison in terms of the effect of composition and structure on the energy of activation.
As seen from Figure 6, the PC resins of the present disclosure exhibited a sharp peak which is observable in the temperature range 400-500 °C for the uncured or partially cured PC resin and 530-630 °C for the cured PC resins. The bridging group in the PC resins consists of four carbon atoms and results in the sharp peak. The resins degrade at methylene bridges and the polymers further degrade into smaller fragments which volatilize within a sharp temperature range. In this temperature range, chain scission occurs and no depolymerisation takes place. After the second stage of degradation, a steady rate of weight loss is observed which can be attributed to the burning off of a carbonaceous residue.
The initial and final degradation temperatures are summarized in Table 9.

Table-9: Thermal properties of PC and PF resin
Sample Nature of sample Temperature (K)
Ti Tf Tf - Ti 10 % DT 20 % DT
PC3R Uncured 331 847 516 421 458
PC3RH Uncured with hexa 331 846 515 425 458
PC3C Cured with hexa 448 1076 629 669 789
PF3C Cured without hexa 327 1009 682 510 606
PC4R* Uncured 331 832 501 493 544
*The aldehyde/phenol mole ratio was 2.5 and for all other samples, the ratio was 2.0.
The results shown in table 9 indicate that the cured PC resins have higher degradation temperatures than those observed for uncured or partially cured PC resins (without hexa i.e. Intermediate). However, no significant change is observed when hexa is added before curing. Comparable degradation behaviour is observed in the PC resins before the curing stage with or without the use of hexa. Similarly, the degradation temperature of cured PC resins is higher than that obtained with the cured PF resin samples synthesized under similar experimental conditions. Thus, the temperature range for the degradation is wider for both cured PF and PC resins.
The data based on qualitative methods, i.e., temperature at 10 and 20 % degradation is also summarized in Table 9. From the data, it is evident that the thermal stability of the uncured resins is lower than that of the cured resins and confirms that the PC resins are thermally more stable than PF resins. The higher thermal stability of the cured PC resins as compared to the PF resins is probably because of the formation of a cyclic ring moiety with fewer aliphatic groups in the polymer. Another reason is that the cross-linking reaction with hexa leads to a strong three dimensional network with imide group bridging.
Further, the temperature ranges and extent of degradation of the PC and PF resins are summarized in Table 10.
Table-10: Decomposition and activation energies in the final stage for PC and PF resins:
Sample Nature of sample Aldehyde-phenol mole ratio Analysis range
Temperature (°C) Decomposition (%) Energy of activation (kJ mol-1)
Start End Start End HM* CR**
PC3R Uncured 2.0 707.0 803.0 26.3 96.7 107.9 108.7
PC3RH Uncured with hexa 2.0 699.0 811.0 23.6 97.8 102.5 103.3
PC4R Uncured 2.5 683.0 783.0 18.5 94.2 111.7 109.6
PC3C Cured with hexa 2.0 823.0 915.0 29.2 92.0 114.6 115.5
PF3C Cured without hexa 2.0 763.0 995.0 20.5 96.4 57.3 55.2
HM* plots is a Horowitz-Metzger plots for phenol crotonaldehyde resin for investigating degradation kinetics of polymers
CR** plots is a Coats-redfern plots for phenol crotonaldehyde resin for investigating degradation kinetics of polymers; and

The extent of degradation pertains only to that observed in the second degradation stage. Kinetic parameters for five resins were analysed in the 23-98% degradation range. The initial 20% degradation zone was not considered to avoid errors due to an overlap from the previous degradation zone.
The activation energies were computed from the slope of the CR and HM plots and are tabulated in Table 10. It is seen from Table-10 that identical activation energy values are obtained by both the methods i.e. curing and uncuring. First order reactions have the activation energy of about 172.8 kJ mol-1, for cured and 100-109 kJ mol-1 for uncured PC resins. However, the value is lower (57.3 kJ mol-1) for cured PF.
It is seen from the above studies that PC resins i.e., intermediates of the present disclosure exhibit thermoplastic characteristics and an additional cross-linking agent is required to convert them into thermoset materials. However, PC resins react with cross-linking agent, only at higher temperatures, and transform into infusible thermosets. The thermal properties of the cured and uncured PC resins seem to be capable of being significantly improved through the introduction of an imide group as a bridging medium for cross-linking to achieve a three dimensional network. The cured PC resins show higher initial 10 and 20% decomposition temperature (DT) as well as higher activation energy than those found for uncured PC resins. The observed activation energy of cured resins indicates that PC resins are more thermally stable over a high temperature range than PF resins. Hence, the PC resins of the present disclosure can be used in the applications where heat resistant composite materials are required.
The reaction of phenol with crotonaldehyde under base catalysed medium is similar to those found with conventional formaldehyde however, due to the presence of double bond in crotonaldehyde, the PC reactions undergo addition as well as condensation reactions. The physico-chemical spectral analysis revealed that the reaction product formed has six benzene rings, bound together by bulky four carbon atom side chains. Because of the presence of such bulky side chain on the benzene ring, these resins exhibited thermoplastic behaviour. The curing agent, hexamethylenetetramine is found to influence the reaction only at elevated temperature by converting them into thermostat materials. This peculiar behaviour offers interesting processing possibilities and indicates that desired product shapes of thermostat materials can be obtained.
TECHNICAL ADVANCEMENTS
The present disclosure described herein above has several technical advantages including, but not limited to, the realization of a process that is:
• simple, economical; and
• environment friendly.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the invention to achieve one or more of the desired objects or results. While certain embodiments of the inventions have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Variations or modifications to the formulation of this invention, within the scope of the invention, may occur to those skilled in the art upon reviewing the disclosure herein. Such variations or modifications are well within the spirit of this invention.
The numerical values given for various physical parameters, dimensions, and quantities are only approximate values and it is envisaged that the values higher than the numerical value assigned to the physical parameters, dimensions and quantities fall within the scope of the invention unless there is a statement in the specification to the contrary.
While considerable emphasis has been placed herein on the specific features of the preferred embodiment, it will be appreciated that many additional features can be added and that many changes can be made in the preferred embodiment without departing from the principles of the disclosure. These and other changes in the preferred embodiment of the disclosure will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.
,CLAIMS:WE CLAIM:
1. Phenol crotonaldehyde resins having the following general structure,

Formula I
wherein, illustrates the positions at which further polymerization occurs.

2. The phenol crotonaldehyde resins as claimed in claim 1, wherein said phenol crotonaldehyde resins have the following structure,

Formula Ia

3. A process for preparing phenol crotonaldehyde resins as claimed in claim 1 or claim 2, said process comprising the following steps:
a. condensing and polymerizing phenol and crotonaldehyde at a first predetermined temperature for a predetermined time in a base catalyzed medium to obtain an intermediate;
b. curing said intermediate using a cross-linking agent at a second predetermined temperature to obtain said phenol crotonaldehyde resins.
4. The process as claimed in claim 3, wherein the molar ratio of crotonaldehyde to phenol is in the range of 1.2 to 3.
5. The process as claimed in claim 3, wherein said first pre-determined temperature is in the range of 90 oC – 120 oC; and said pre-determined time is in the range of 1 hour to 3 hours; and said second pre-determined temperature is in the range of 160 oC to 200 oC.
6. The process as claimed in claim 3, wherein said base catalyzed medium is alkali metal hydroxide.
7. The process as claimed in claim 3 or claim 6, wherein said base catalyzed medium is selected from sodium hydroxide, potassium hydroxide and calcium hydroxide.
8. The process as claimed in claim 3, wherein the concentration of said base catalyzed medium is in the range of 0.01-0.5mol per mole of phenol.
9. The process as claimed in claim 3, wherein said cross-linking agent is hexamethylenetetramine.
10. The process as claimed in claim 3, wherein said phenol crotonaldehyde resin further crosslinked with hexa at 180 oC to convert into thermoset behaviour.
11. The process as claimed in claim 3, wherein the amount of said cross-linking agent is in the range of 5 - 20 wt% with respect to the amount of the intermediate.