MODIFIED PHENALKAMINE CURING AGENT FOR EPOXY RESIN COMPOSITION AND USE THEREOF

Abstract

The present invention relates to new compounds based on polyetheramine modified phenalkamine, their use as curing agents, compositions comprising the novel compounds, the manufacture of such compounds and of such compositions, and the use of these compositions, in particular in a potting process in electrical and electronic components and devices.

Claims

1. A compound comprising: one or more of a substituted cardanol and a substituted bisphenol, wherein the one or more of a substituted cardanol and a substituted bisphenol includes an aromatic ring substituted with at least two different groups each comprising at least one active hydrogen linked to an amine group, and a group of the at least two different groups further comprises at least one ether group.

2. The compound of claim 1, wherein the group further comprising at least one ether group comprises a polyether segment.

3. The compound of claim 1, wherein a group of the at least two different groups comprises a secondary amine.

4. The compound of claim 1, wherein each group comprising at least one active hydrogen linked to an amine group further comprises an alkyl group, an aryl group, an heteroaryl group, an cycloalkyl group, or a heterocycloalkyl group.

5. The compound of claim 1, further comprising one or more of oxalic acid and a salt of oxalic acid.

6. A curable composition comprising the compound of claim 1 and an amine reactive resin.

7. The curable composition of claim 6, wherein the amine reactive resin is an epoxy resin.

8. The curable composition of claim 6, further comprising a dye or a fluorescent marker.

9. The curable composition of claim 6, wherein the composition has a peak exotherm temperature in the range of 20° C. to 60° C.

10. (canceled)

11. (canceled)

12. A process for preparing a compound, the process comprising: in the presence of formaldehyde and a catalyst, reacting one or more of a cardanol and bisphenol with at least a compound comprising one or more of a primary amine group and a secondary amine group and a compound comprising an ether group and one or more of a primary amine group and a secondary amine group.

13. The process of claim 12, wherein the ether group is comprised in a polyether segment.

14. The process of claim 12, wherein each compound comprising one or more of a primary amine group and a secondary amine group further comprises an alkyl group, an aryl group, an heteroaryl group, an cycloalkyl group, or an heterocycloalkyl group.

15. A process for preparing an article or a material, the process comprising: forming a mixture including an amine reactive resin and a compound comprising: one or more of a substituted cardanol and a substituted bisphenol, wherein the one or more of a substituted cardanol and a substituted bisphenol includes an aromatic ring substituted with at least two different groups each comprising at least one active hydrogen linked to an amine group, and a group of the at least two different groups further comprises at least one ether group; and curing the mixture.

16. (canceled)

17. A material obtained by the process of claim 15.

18. A process for preparing an article or a material, the process comprising: curing the curable composition of claim 6.

19. A material obtained by the process of claim 18.

20. An electrical device comprising a potted component including a cured form of the curable composition of claim 6.

21. An electronic device comprising a potted component including a cured form of the curable composition of claim 6.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0025] The present invention relates to a compound comprising a substituted cardanol or a substituted bisphenol, or mixtures thereof, wherein the aromatic ring of the cardanol and/or of the bisphenol is substituted with at least two different groups each having at least one active hydrogen linked to an amine group and one group further comprising at least one ether group.

[0026] In a preferred embodiment of the compound of the invention, the group further comprising at least one ether group comprises a polyether segment.

[0027] In another preferred embodiment of the compound of the invention, the group having at least one active hydrogen linked to an amine group comprises a secondary amine.

[0028] In another preferred embodiment of the compounds of the invention, each group comprising at least one active hydrogen linked to an amine group further comprises a group selected from alkyl, aryl, heteroaryl, cycloalkyl and heterocycloalkyl. More preferably, the compounds of the invention further comprise a group selected from alkyl and heterocycloalkyl.

[0029] In another preferred embodiment, the compounds of the invention further comprise oxalic acid and/or salts thereof.

[0030] Since the compound and/or the composition of the invention contain amines, the oxalic acid in the composition or in the compound may generally be present as a salt.

[0031] The compounds of the invention used as curing agents allow to obtain bright materials, which can be colored by addition of a dye.

[0032] Oxalic acid has been found to be an effective bleaching agent for obtaining brighter phenalkamines, in particular using commercially available cardanol.

[0033] Accordingly, the presence of oxalic acid or its salts further improves the possibility to obtain colorable final materials.

[0034] The invention further relates to a curable composition comprising anyone of the compounds of the invention and an amine reactive resin.

[0035] The amine reactive resin of the curable composition might also react with —OH groups.

[0036] Preferably, the amine active resin is an epoxy resin. More preferably, the epoxy resin has on average at least one epoxy group per molecule, preferably at least one glycidyl group per molecule.

[0037] More preferred epoxy resins are those epoxy resins having 1.8 epoxy groups per molecule.

[0038] Commercially available epoxy resins are suitable for being cured with the compounds of the invention. Preferably, the commercially available epoxy resins suitable for the invention are based on bisphenol A, bisphenol F or mixtures thereof.

[0039] In a further preferred embodiment, the composition of the invention has a peak exotherm temperature of the composition in the range of 20° C. to 60° C., preferably, in the range of 55° C. to 25° C., more preferably in the range of 30° C. to 50° C.

[0040] In a preferred embodiment, the composition of the invention further comprises a dye or a fluorescent marker.

[0041] A dye is a colored substance that has an affinity to the substrate to which it is being applied. Dyes and pigments are colored, because they absorb only some wavelengths of visible light. Dyes are usually soluble in water whereas pigments are insoluble. Some dyes can be rendered insoluble with the addition of salt to produce a lake pigment.

[0042] Dye can be natural, synthetic, organic or inorganic based.

[0043] The dye is used as a coloring agent, meaning that the coloring effect is given to the final product obtained from resin and curing agent mixture.

[0044] Preferred dye of the inventions are blue dyes.

[0045] Accordingly, anyone of the compounds of the invention can be used in any curable composition in the described embodiments comprising an amine reactive resin.

[0046] In a further embodiment, the invention relates to the use of a compound of the invention for curing a composition comprising an amine reactive resin. Preferably an epoxy resin. More preferably the epoxy resin has on average at least one epoxy group per molecule, preferably at least one glycidyl group per molecule.

[0047] In a preferred embodiment, anyone of the compounds of the invention can be used in any curable composition in the described embodiments comprising an amine reactive resin.

[0048] In another embodiment, the invention relates to the use of the composition comprising the compound of the invention for potting electrical and electronic components or devices.

[0049] In another embodiment, the invention relates to the use of the compound of the invention in a composition for potting electrical and electronic components or devices.

[0050] In another embodiment, the invention relates to a process for preparing the compound of the invention comprising the step of reacting cardanol and/or bisphenol with at least one compound having a primary or secondary amine group and one compound having at least one primary or secondary amine group and further comprising an ether group in the presence of formaldehyde and a catalyst.

[0051] In another preferred embodiment, the primary and/or secondary amine group further comprises a group selected from alkyl, aryl, heteroaryl, cycloalkyl and heterocycloalkyl, preferably from alkyl and heterocycloalkyl.

[0052] In a preferred embodiment, the primary or secondary amine further comprises an ether group, preferably a polyether segment.

[0053] In a preferred embodiment, the primary or secondary amine further comprises an ether group is a polyetheramine.

[0054] In another preferred embodiment, the catalyst is an acid compound, preferably oxalic acid.

[0055] In a further embodiment, the invention relates to a process for preparing an article or a material, comprising the steps of: [0056] a. mixing an epoxy resin having on average at least one epoxy resin per molecule, preferably at least one glycidyl group per molecule, with any compound of the invention; [0057] b. curing the mixture of step a.

[0058] In a further embodiment, the invention relates to an article obtained by a process comprising the steps of: [0059] a. mixing an epoxy resin having on average at least one epoxy resin per molecule, preferably at least one glycidyl group per molecule, with any compound of the invention; [0060] b. curing the mixture of step a.

[0061] In a further embodiment, the invention relates to a material obtained by a process comprising the steps of: [0062] a. mixing an epoxy resin having on average at least one epoxy resin per molecule, preferably at least one glycidyl group per molecule, with any compound of the invention; [0063] b. curing the mixture of step a.

[0064] Preferred compounds having primary and/or secondary amine groups used in the process of the invention are aliphatic amines, cycloaliphatic amines, heterocyclic amines, aromatic amines.

[0065] Accordingly, the compound comprising at least a primary or a secondary amine may further comprise a group selected from alkyl, aryl, heteroaryl, cycloalkyl and heterocycloalkyl.

[0066] Preferably the compounds comprising at least a primary or a secondary amine are selected from the group consisting of N-aminoethylpiperazine; diethylenetriamine; triethylenetetramine (TETA); tetraethylenepentamine; 2-methylpen tamethylene; 1,3-pentanediamine; trimethylhexamethylene diamine; a polyamide; a polyamidoamine and combination of thereoftrimethylolpropane tris[poly(propylene glycol), amine terminated] ether, 2-(1-piperazinyl)ethylamine, 3-aminomethyl-3,5,5-trimethylcyclohexylamine, 3,6,9,12-tetraazatetradecane-1,14-diamine, tetraethylenepentamine and triethylenetetramine, 3,6-diazoctanethylenediamine, 3,6,9-triazaundecamethylenediamine, 1,3-bis(aminomethyl)benzene, 1,3-bis(aminomethyl)cyclohexane, 5-diethyl toluene-2,4-diamine and 3,5-diethyl toluene-2,6-diamine (mixture of the two isomers), 2,2′-(ethylenedioxy)bis(ethylamine), 1,6-diamino-2,2,4(2,4,4)-trimethylhexane, N′-(3-aminopropyl)-N,N-dimethylpropane-1,3-diamine, 4,4′-diaminodicyclohexylmethane, 1,2-cyclohexanediamine and mixtures thereof.

[0067] Preferred compounds having at least a primary or a secondary amine group further comprising at least one ether group are polyetheramines. Preferred polyetheramines of the invention preferably comprise at least one oxyethylene diamine segment, oxyethylene triamine segment or polyoxyethylene diamine segment. Polyetheramines of the invention preferably comprise at least one propylene oxide segment, ethylene oxide segment or a mixture of both.

[0068] Preferred polyetheramine used in the present invention are commercially available under the trademark JEFFAMINE from Huntsman Corporation, Houston, Tex. such as, JEFFAMINE D-230, JEFFAMINE D-400, JEFFAMINE D-2000, JEFFAMINE T-403, JEFFAMINE ED-600, JEFFAMINE ED-900, JEFFAMINE ED-2001, JEFFAMINE EDR-148, JEFFAMINE XTJ-509, JEFFAMINE T-3000, JEFFAMINE T-5000, and combinations thereof. Preferred polyetheramines of the invention are JEFFAMINE® D-230.

[0069] Within the meaning of the invention, the terms curing agent and hardener have the same meaning.

[0070] Cured composition in this invention may further comprise inorganic and/or organic fillers. Suitable inorganic fillers include barium sulfate, silica, aluminum oxide, calcium oxide, magnesium oxide, sodium oxide, potassium oxide, iron oxide, titanium oxide, other oxides or mixtures thereof. Other suitable fillers include silica-gels, calcium silicates, calcium nitrate, calcium phosphates, calcium molybdates, calcium carbonate, calcium hydroxide, fumed silica, clays such as bentonite, aluminium trihydrates, magnesium dihydrates, glass microspheres, hollow glass microspheres, polymeric microspheres, and hollow polymeric microspheres. Organic fillers such as aramid fibers and polyolefin fibers such as polyethylene fibers can be used in the curable composition.

[0071] The curable composition can contain any suitable amount of the filler. It may contains 0.01 to 50 weight percent filler based on a total weight of the curable composition.

[0072] The compounds of the inventions can be used for curing a composition comprising an amine reactive resin, preferably an epoxy resin, more preferably, the epoxy resin has on average at least one epoxy resin per molecule, preferably at least one glycidyl group per molecule. Preferably, the compounds of the invention can be used in a composition further comprising a dye or a fluorescent marker.

[0073] The invention further relates to a process for preparing the compounds of the invention. The process comprises the steps of reacting the cardanol and/or bisphenol with at least one compound having at least one primary or secondary amine group and one compound having at least one primary or secondary amine group further comprising an ether group in the presence of formaldehyde and a catalyst.

[0074] The invention further relates to a process for preparing an article or material comprising the step of mixing an amine reactive resin, preferably an epoxy resin, with the compound of the invention and curing the mixture.

[0075] Preferably, the material or article prepared using the composition of the invention has a resistance to thermal shock of at least 10 thermal cycles passed at −20° C./+120° C. for one hour duration, preferably at least 15 thermal cycles passed at −20° C./+120° C. for one hour duration, more preferably at least 20 thermal cycles passed at −20° C./+120° C. for one hour duration.

[0076] Another object of the invention are the articles or materials produced by the process comprising the step of mixing an amine reactive resin with the compound of the invention and curing the mixture.

[0077] The present invention is explained in more details by the following non-limiting examples.

EXAMPLES

[0078] The phenalkamine of the invention were synthetized analogously to the synthesis procedure given in the U.S. Pat. No. 6,262,148 B1 and Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 52, pp, 472-480, [2014], Wiley Periodicals, Inc.

Example 1

[0079] Preparation of Polyetheramine Modified Phenalkamine Using Bisphenol a as a Source of Phenolic Structure

[0080] A three-liter reaction flask was charged with the following ingredients to prepare a polyetheramine modified phenalkamine based compound: [0081] 264.55 g Bisphenol A [0082] 1582.01 g Polyetheramine (Jeffamine D 230) [0083] 63.50 g Paraformaldehyde [0084] 88.62 g TETA (Triethylenetetramine) [0085] 2.00 g oxalic acid

[0086] In a three liter reaction flask with a water cooled reflux condenser, 264.55 g bisphenol A, 1582.11 g polyetheramine and 88.62 g TETA were charged and stirred for 20 minutes. Afterwards, 63.50 g paraformaldehyde along with 2 g of oxalic acid were added in above mixture under stirring. After complete addition of above ingredient, the temperature was slowly increased to 120° C. under constant stirring for 2.5 hour. At this temperature, a clear mixture was obtained which indicated a complete reaction took place. The residual water present in mixture was removed by applying vacuum of 650 mm/Hg for 1 hours duration to the system.

Example 2

[0087] Preparation of Polyetheramine Modified Phenalkamine Using Cardanol as a Source of Phenolic Structure

[0088] In three-liter reaction flask following ingredients were charged to prepare a polyetheramine modified phenalkamine based curing agent: [0089] 450.71 g Cardanol [0090] 1383.65 g Polyetheramine (Jeffamine D 230) [0091] 81.45 g Paraformaldehyde [0092] 81.45 g TETA (Triethylenetetramine) [0093] 2.00 g Oxalic acid (Catalyst)

[0094] All the above ingredients were charged in three liter reaction flask with a water cooled reflux condenser under continuous stirring and heated slowly to 120° C. maintaining the continuous stirring for the duration of 2.5 hours. After completion of reaction, the residual water present in mixture was removed by applying vacuum of 650 mm/Hg to the system for 1 hour.

Example 3

[0095] Preparation of Polyetheramine Modified Phenalkamine Using Bisphenol a and Cardanol as a Source of Phenolic Structure

[0096] The below ingredient were added in three-liter reaction flask to prepare a polyetheramine modified phenalkamine based curing agent: [0097] 211.03 g Bisphenol A [0098] 211.03 g Cardanol [0099] 1295.70 g Polyetheramine (Jeffamine D 230) [0100] 127.12 g Paraformaldehyde [0101] 152.55 g TETA (Triethylenetetramine) [0102] 2.00 g Oxalic acid (Catalyst)

[0103] 50:50 ratios of cardanol with bisphenol were charged in the three liter reaction flask along with other additives under continuous stirring and heated slowly to 120° C. with maintaining the continuous stirring for 2.5 hours duration. After completion of reaction, residual water present in mixture were removed by applying vacuum of 650 mm/Hg for 1 hour duration to the system.

Example 4—Comparative Example

[0104] Preparation of Unmodified Phenalkamine Based Curing Agent

[0105] The following ingredient were added in three liter reaction flask for preparation of unmodified phenalkamine based curing agent. [0106] 1206.70 g Cardanol [0107] 157.80 g Paraformaldehyde [0108] 621.30 g Aminoethyl piperazine [0109] 2.00 g Oxalic acid (catalyst)

[0110] Firstly, cardanol and paraformaldehyde were charged in three liter reaction flask with a water cooled reflux condenser under continuous stirring and heated slowly to 40° C. Aminoethyl piperazine was added under stirring along with oxalic acid in above mixture. This mixture was stirred at 120° C. for 2.5 hours. After completion of reaction, residual water present in mixture was removed by applying to the system vacuum of 650 mm/Hg for 1 hour.

Example 5

[0111] Preparation of Colored Polyetheramine Modified Phenalkamine

[0112] Different types of colored polyetheramine modified phenalkamine were developed for various applications.

[0113] The compound resulting in Example 1 was mixed with the quantity of blue dye summarized in Table 1.

[0114] Epoxy resin was cured with the mixture of compound obtained from Example 1 and blue dye. The colors of the resulting cured resins are also summarized in Table 1.

Example 6

[0115] Preparation of Cured Material by Using Color Polyetheramine Modified Phenalkamine with Epoxy Resin

[0116] The above mixture are degases at 650 mm/Hg for 15 minutes duration. Poured this mixture in suitable container at room temperature for 3 hours to form a thermoset.

Example 7

[0117] Composition and process for the preparation of filled epoxy system.

[0118] The following ingredient were added in three kg container for preparation of filled epoxy system. [0119] 129 g Epoxy Resin (DGEBA) [0120] 143 g Epoxy Resin (Blend of DGEBA and DGEBF) [0121] 94 g Epoxy Diluent [0122] 0.5 g Black Dye (Coloring agent) [0123] 3 g Antiterra U (Wetting and dispersing additive) [0124] 2 g BYK W 969 (Wetting and dispersing additive) [0125] 2.5 g Hydrophilic Fumed Silica (Rheology and thixotropy controlling agent) [0126] 605 g Aluminum Hydrate (Inorganic Filler) [0127] 21 g Barium Sulphate (Inorganic Filler)

[0128] In a three KG capacity container, added Epoxy Resin (DGEBA), Epoxy Resin (Blend of DGEBA and DGEBF), Epoxy Diluent and Black Dye and stirred for 30 minutes at 65° C. Later on added Antiterra U, BYK W 969, Hydrophilic Fumed Silica under stirring at 65° C. Finally, added Aluminum Hydrate, Barium Sulphate and stirred the mixture for 130 minutes at 65° C. The above mixture is degased at 650 mm/Hg for 15 minutes duration.

TABLE-US-00001 TABLE 1 Concen- tration of colored dye Color of the Hardener Epoxy Resin [ppm] cured resin Conventional 0 Dark yellow polyamide Unmodified DGEBA 0 Dark Brown Phenalkamine (Diglycidyl Ether without oxalic acid of Bisphenol A) Unmodified Mixing ratio of Brown Phenalkamine of resin and Example 4 hardener: 100:35 wt/wt Polyetheramine 500 Dark blue Modified Phenalkamine of 100 Bright blue Example 1 10 Bright yellow color 0 Very light yellow color

[0129] Comparison of physical properties and final performance properties of polyetheramine modified phenalkamine as compared to unmodified phenalkamine.

TABLE-US-00002 TABLE 2 Properties Ex 4 Ex 1 Ex 2 Ex 3 Viscosity at 2000 120 50 615 25° C. in cP Amine 285 430 362 405 Value in mg KOH/g Gel Time in 40 minutes 180 182 190 without Minutes without without without accelerator (Mixing ratio catalyst accele- catalyst and of resin and rator 110 minutes hardener: with 65:35) accelerator Final Performance Properties Glass 60 74 75 77 Transition Temp. (Tg) in ° C. Hardness 80 70 78 82 (Shore D)

[0130] Table 2 shows that the material obtained using the compositions comprising polyetheramine modified phenalkamine have superior physical properties as compared to the material using the composition comprising unmodified phenalkamine curing agent. Generally, in potting application in electrical and electronics industry need longer gel time which help them for used the mixture and resin and hardener system for longer pot life.

[0131] Viscosity and amine value data of example number 3 shows the use of combination of cardanol and bisphenol A allows lower viscosity and higher amine value and yet providing longer pot life as compared to example 4.

[0132] Additionally, polyetheramine modified phenalkamine hardener system give characteristic properties to the final material such as lower viscosity, higher amine value, higher glass transition temperature and higher hardness value (shore D) as compared to the unmodified phenalkamine system.

TABLE-US-00003 TABLE 3 Thermal shock resistance data of materials obtained using the composition comprising polyetheramine modified phenalkamine of example 1 and materials obtained using the composition comprising unmodified phenalkamine Number of Thermal Cycle Passed (−20° C./ +120° C.) for 1 Hour Hardener Epoxy Resin duration each Unmodied Filled Epoxy  6 phenalkamine of Resin example 4 Mixing ratio of resin and hardener: 100:12 wt/wt Polyetheramine 50 Modified Phenalkamine of Example 1 Polyetheramine 22 Modified Phenalkamine of Example 3

[0133] The number of thermal cycle passed by thermoset prepared using polyetheramine modified phenalkamine curing agents with filled epoxy system is higher than the thermoset prepared from unmodified phenalkamine with filled epoxy resin (Table 3). This types advance properties of thermoset obtained due to the unique structure of hardener system.

TABLE-US-00004 TABLE 4 Comparison of peak exotherm temperature data of unmodified phenalkamine with polyetheramine modified phenalkamine: Peak Exotherm Temp- erature Type of Hardener Used Type of Epoxy Resin Used in ° C. Unmodified Filled Epoxy Resin Mixing 70 Phenalkamine of Ratio of Resin and Hardener: Example 4 100:12 wt/wt Polyetheramine Modified (224 g level) 30 Phenalkamine of Example 1 Polyetheramine Modified 30 Phenalkamine of Example 3

[0134] Peak exotherm temperature observed for the polyetheramine modified phenalkamine system is only 30° C., whereas unmodified phenalkamine system shows higher peak exotherm (70° C.). The ether linkage available in the polyetheramine modified phenalkamine system most probably helps for the low peak exotherm.

[0135] The value indicated as “g level” in the measurement of the peak exotherm temperature means the quantity of mass of the reaction mixture.

[0136] Methods

[0137] The properties summarized in the Tables above were measured applying the following methods.

[0138] (1) The viscosity was measured at a temperature of 25° C. [in cP] using a BROOKFIELD viscometer, spindle number 27 at share rate of 10 rpm.

[0139] (2) The Amine Value in mg KOH/g was measured using the following method.

[0140] Reagents: [0141] Glacial acetic acid [0142] Perchloric acid in glacial acetic acid 0.1 N standardised. [0143] Crystal violet indicator solution. (1.0 g crystal violet in 100 ml glacial acetic acid.)

[0144] Apparatus: [0145] Idometric flack—capacity 250 ml [0146] Graduated glass cylinder capacity 50 ml [0147] Burette—capacity 25 ml (min dev.0.1 ml) [0148] Precision balance.

[0149] Procedure: [0150] 1. Weigh accurately 0.3-0.5 g sample in a clean & dry idometric flask. [0151] 2. Dissolve in about 50 ml glacial acetic acid worm if necessary, cool to room temp. [0152] 3. Titrate against standardised perchloric acid using crystal violet indicator solution till colour change from violet to green. [0153] 4. Note the reading (R ml)

[0154] Calculation:

[00001]$.Math.\mathrm{Amine}\mathrm{value}=\frac{56.1\times N\times R}{\mathrm{Weight}\mathrm{of}\mathrm{sample}}$ [0155] N=Normality of perchloric acid [0156] R=ml of perchloric acid required.

[0157] (3) The Gel Time was measured at a temperature of in Minutes (Mixing ratio of resin and hardener: 65:35) using gelation timer (TECHNE instrument).

[0158] (4) The Glass Transition Temperature (Tg/° C.)) was measured using the Metter Toledo at a rate of 10° C./min in air (25° C. to 600° C.).

[0159] (5) The Hardness (Shore D) was measured using the DUROMETER FROM HIROMETER at 25° C.

[0160] (6) The thermal shock cycles were performed using the CM Envirosystem, model Kinetic 25 B2 at −40/100° C. No. of Thermal Cycle Passed at −40/100° C. for 1 hour duration each without any visual cracks was reported above.

[0161] (7) Measurement of peak exotherm temperature was carried as below method.

[0162] Required amount of resin and hardener are weighed in a container and thoroughly mixed at 25° C. Then a thermometer is placed in the mixture of resin and hardener and the temperature is noted. The rise in temperature is measured at regular time interval. The rise in temperature is observed due to the reaction between epoxy groups of the resin and amine groups present in the hardener. When the reaction reaches gelation stage, mass becomes very viscous and no further rise in temperature is observed as the reaction rate is reduced and consequently a decline of temperature starts. The maximum temperature observed is noted as the peak exotherm temperature of the system.