Complete specification

One pot synthesis of novel skeletal modified diamine used as a curative and impact modifier for epoxy resins

Field of Invention:

The present invention relates to the synthesis of novel skeletal modified diamine, more particularly to one pot synthesis of skeletal modified diamine used as both curative and impact modifier for industrially valuable epoxy resins.

Background of the Invention:

Epoxy resin is one of the most important class of thermosetting polymeric materials widely used as high performance adhesives, coatings, sealants, matrices for composites and electronic encapsulating materials due to their outstanding chemical and mechanical properties including adhesion to various surfaces, light weight, high strength, good durability and high stability to UV exposure, excellent chemical resistance and easy processing. They play an increasingly important role in high-tech industries including aerospace, automobiles, land and marine transportation, chemical process industries and electrical and electronic industries due to the fact that their cured products have good electrical properties, lower coefficient of thermal expansion, superior thermal and thermo-oxidative stability and higher glass transition temperature and better mechanical strength.

However the brittle behavior and low impact resistance, which results from highly crosslinked structure of, epoxy resin restricts its utility for a number of high performance applications. Therefore, numerous
attempts have been made to improve their toughness. One of the successful modification routines is elastomer toughening done by incorporating soft rubber particles such as carboxyl- terminated butadiene-acrylonitrile rubber, amine- terminated butadiene-acrylonitrile rubber and silicone. This addition of soft rubbery particle is normally associated with reduced elastic modulus, yield strength and creep resistance of the epoxy resin and it also fails to produce a significant improvement in toughness in highly crosslinked epoxides and hence not suitable for aerospace applications. More recently, attempts have been made to toughen epoxy resins with high modulus, high glass transition temperature thermoplastics, especially to modify higher crosslinked epoxy resins. In case of increasing the toughness of epoxy resins using thermoplastics it is necessary to check the miscibility or solubility of thermoplastics with uncured epoxy resins and selection of most compatible polymer for the modification of epoxy resin is important.

One area of continued research towards improving the toughness of epoxy resin is to formulate a curing agent which is compatible with conventional epoxy resins at low curing temperatures, and which is sufficiently reactive with epoxy resins such that the system will cure over a wide range of temperatures. The reactivity of epoxy groups towards curatives depends on the electronic environment and steric factors. Number of amines, amides, anhydrides and polyamidoamine are used as curative for epoxy resins to make products in the form of coatings, adhesives, sealants, encapsulants, potting components, laminates, matrices and composites for wide range of industrial and engineering applications. Typical amine curing agent, which terminate with at least one primary amine group, often produce the undesired side effect of "blooming" or "hazing" in the cured product. To prevent the blooming and hazing to some extend the primary amine groups are converted to secondary amine groups which are less reactive and often require accelerators to obtain adequate cure times. Furthermore the converted secondary amine groups are poorly compatible with the epoxy resins.

In spite of various forms of modifications of epoxy resins carried out using different curatives to make them more suitable for advanced applications. Still it requires the development of better curatives and modifiers with increased toughness, improved curing speed, improved ability to cure at lower temperatures, high glass transition temperature and high flexural strength to make them most competitive and environmentally friendly materials to achieve epoxy coatings and matrices with high impact behavior.
The present invention provides a simple, novel, green and clean one pot synthetic route for the synthesis of skeletal modified diamine with minimum reaction cycle time. Further the skeletal modified diamine produced can effectively and efficiently acts as both curative and impact modifier for epoxy resins. Further this cost competitive methodology developed does not involve the use of any catalyst or solvent for the synthesis of skeletal modified diamine.

Objective of the Invention:

The main objective of the present invention is to provide a simple novel, green and clean one pot synthetic route for the synthesis of skeletal modified diamine.

The second objective of the present invention is to effectively and efficiently use the skeletal modified diamine as both curative and impact modifier for epoxy resins.

The third objective of the present invention is to reduce the cycle time and cost involved in the synthesis of skeletal modified diamine.

Another objective of the invention is to provide a simple one pot synthesis of skeletal modified diamine without using any catalyst or solvent.

Brief description of the prior art:

EP2094759B1 discloses a cycloaliphatic diamine as a curing agent for epoxy resins. The hardener composition for epoxy resins includes the combination of 1,3 bis(aminomethyl)cyclohexane; and 1,4-bis(aminomethyl)cyclohexane and their cis and trans isomers. The mixtures of 1-3, and l-4-bis(aminomethyl)cyclohexane provides very fast curing time as curing agent without sacrificing color/haze properties and chemical resistance. Further the patent discloses the use of several accelerators and catalyst to increase the curing rate of the epoxy resin curing agent.

US4162358A provides a polyaromatic amine curing agents for epoxy resins. The liquid methylene bridged polyaromatic amine curing agent is prepared by mixing and reacting a mixture of aniline with formaldehyde. The resulting methylene bridged polyaromatic amine curing agent produces a cured epoxy resin having a higher heat distortion temperature as compared to epoxy resins cured with a material prepared from aniline without the added diamine.

US4855386A provides a composition of curing agents for epoxy resins comprising diamines with di(p-aminophenyl)-diisopropyl benzene structure such as a,a'-bis(3,5-diemyl-4-aminophenyl)-p-
diisopropylbenzene and a,a'-bis(3-ethyl-5-methyl-4-aminophenyl)-p-diisopropylbenzene. Curing of the epoxy resin with the preferred curing agents is conducted by heating the resin to a temperature within the range of about 125° C to 225° C, and more preferably about 170° C to 200° C for about 1 to 3 hours. The described composition has good physical properties, high-temperature properties, and good property retention in a moist environment and has a relatively long working life.

US4874833 relates to a curable epoxy resin composition comprising: a low viscosity liquid epoxy resin; an alkyl hindered aromatic primary diamine curing agent having at least two aromatic rings; a trialkylsulfonium salt amine cure accelerator; a liquid monoaromatic primary amine curing agent; at least one reactive monomer; and a peroxide polymerization initiator. This composition provides a curable epoxy resin system for pultrusion of a B-stage composite such that the composite would exit the pultrusion die in a postformable condition having the ability of flowing to form a slightly different shape, such as a slight taper or twist. The composition is applicable to any low viscosity liquid epoxy resin.

US5508373A discloses a method of curing a epoxy resin comprising mixing an epoxy resin with a curing agent which is selected from mixture essentially consisting of 1,2-diaminocyclohexane and an aliphatic polyamine in a molar ratio of from 3:1 to 5:1. In another embodiment the curing agent is adduct of mixture of amines with an epoxy component having an epoxy functionality greater than one this increases the viscosity of a mixture of adduct with an epoxy resin to be cured. The aliphatic polyamine is selected from the group consisting of 1,4-butylenediamine, 1,5-pentylenediamine, 1,6-hexylenediamine, and 1,7-heptylenediamine. The epoxy resin and curing agent composition can be used in any of a variety of applications including laminating, filament winding, pultrusion, casting, or molding.

US7008555B2 discloses a curing agent for epoxy resins comprising N-phenyl-p-phenylenediamme(4-aminodiphenylamine), further comprising one or more additional aromatic amines, an anhydride or boron trifluoride piperazine. One embodiment the curing agent for epoxy resins comprises N-phenyl-p-phenylenediamme(4-ammodiphenylamine). In another embodiment the epoxy resin combination comprises a main agent (i.e epoxy resin) and a curing agent (N-phenyl-p-phenylenediamme(4-aminodiphenylamine)). This aromatic amine, used alone or in combination with other amines, curing agents, or compatible modifiers, may be useful as a curing agent for epoxy resins for resin transfer molding (RTM) resins, vacuum assisted resin transfer molding (VARTM) resins, pultrusion resins, film adhesives, prepreg resins, and other composite manufacturing type applications.

All of the above said methods for the production of epoxy resin curatives involve time consuming multi step process involving numerous chemical reagents such as solvents, catalysts, accelerators, etc. The present invention eliminates the drawbacks of existing methods by presenting a simple, green and clean one pot synthesis of skeletal modified diamine which acts as both curative and impact modifier for commercially valuable and industrially useful epoxy resin. Further the present invention does not use any catalyst, accelerators or solvents for the synthesis of skeletal modified diamine and it also involves less reaction cycle time with reasonable cost competitive methodology. The present invention further provides high product yields with high impact strength and increased toughness.

Summary of the invention:
The present invention relates to a simple, novel, green and clean, one pot synthesis of skeletal modified diamine which functions as effective curative and efficient impact modifier for industrially valuable epoxy resins. Skeletal modified diamine of the present invention is prepared using appropriate derivatives of amines and condensate of amino acid through melt mixing addition process without the use of any catalyst, accelerators and solvents. Single step synthetic route for the synthesis of skeletal modified diamine comprises of following steps: reacting the stoichiometric quantities of amine derivatives and amino acid condensate through melt mixing addition process; maintaining the reaction mixture at a desired temperature and time to complete the addition reaction; and finally the resulting product is cooled to room temperature to obtain the skeletal modified diamine. Further the present invention proved to be a cost competitive methodology for the production of skeletal modified diamine with excellent purity, high yield and less cycle time. The skeletal modified diamine curative influences low temperature curing of epoxy resins that produces cured epoxy matrix with increased toughness and improved impact behavior which is useful in the form of coatings, adhesives, sealants, encapsulants, potting components, caulks, laminates, matrices and composites for high performance industrial, engineering and aerospace applications.

Detailed description of the invention:
The present invention relates to the one pot synthesis of skeletal modified diamine which functions as both curative and impact modifier for industrially valuable epoxy resins in order to make them useful for high performance industrial and engineering applications. Epoxy resins are thermosetting polymeric materials widely used in the form of adhesives, coatings, sealants, encapsulants and matrices for number of industrial applications. The present invention provides a simple one pot melt mixing addition mechanism for the production of skeletal modified diamine with reduced reaction cycle time and high yield under favorable experimental conditions. The present invention further provides simple, pollution free, clean and green, cost competitive methodology for the production of skeletal modified diamine which is used to produce high performance adhesives, sealants and matrices for the fabrication of high temperature composite components.

Synthesis of skeletal modified diamine:

A new and novel skeletal modified diamine which effectively acts as efficient curative and potential impact modifier for epoxy resin is synthesized using appropriate derivatives of amines and condensate of amino acid through melt mixing addition process without the use of any catalyst, accelerators and solvents. This simple, clean, green, novel one pot synthesis of skeletal modified diamine comprises of following steps:

a) reacting the stoichiometric quantities of amine derivatives and condensate of amino acid through melt mixing addition process;

b) maintaining the reaction mixture at a desired temperature and time to complete the addition reaction; and

c) finally the resulting product is cooled to room temperature.

In the preferred embodiment the diamine derivative used is azepan-2-one.

In the preferred embodiment the condensate of amino acid used is 4-(4-ammobenzyl)benzenamine.

In the preferred embodiment the temperature of the reaction mixture is preferably maintained at a range of 80 to 120°C and more preferably at 100°C.

In the preferred embodiment the reaction mixture is preferably maintained at the desired temperature for 60 to 180 minutes and more preferably for 120 minutes with efficient agitation to facilitate the completion of the addition reaction.

In the preferred embodiment the resulting skeletal modified diamine is N-(4-(4(6-aminohexylamido) benzyl) phenyl)-6-aminohexanamide.

In the preferred embodiment the yield of the skeletal modified diamine ranges from 94% to 98% and more preferably is 96%.

In the preferred embodiment the purity of the skeletal modified diamine ranges from 92% to 97% and more preferably is 95%.

In another embodiment the stoichiometric quantities of the skeletal modified diamine and epoxy resin are mixed and cured to obtain epoxy matrix which possesses increased toughness and improved impact behavior over the conventionally cured epoxy matrix.

In the preferred embodiment the resulting skeletal modified diamine of the present invention influences low temperature curing of epoxy resins over the conventional curatives.

In the preferred embodiment the skeletal modified diamine cured epoxy resins with increased toughness and improved impact behavior can be conveniently used in the form of coatings, adhesives, sealants, encapsulants, potting components, caulks, laminates, matrices and composites for high performance industrial, engineering and aerospace applications.

Example 1
Stoichiometric quantities of azepan-2-one and 4-(4-aminobenzyl)benzenamine is charged into the two necked round bottomed flask equipped with reflux condenser. The reactants are gradually heated to 100°C and maintained for 2 hours with efficient agitation to facilitate the completion of melt mixing addition reaction by the reactants. The resulting skeletal modified diamine product N-(4-(4(6-aminohexylamido) benzyl) phenyl)-6-aminohexanamide is cooled to room temperature. The skeletal modified diamine product is stiochiometrically mixed with DGEBA epoxy resin and cured to obtain epoxy matrix which possessed increased toughness and improved impact behavior over the conventionally cured epoxy matrix.

We Claim:
1. A simple novel, green and clean one pot synthetic route for the synthesis of skeletal modified diamine with high yield and
excellent purity comprises of:

d) reacting the stoichiometric quantities of amine derivatives and condensate of amino acid through melt mixing addition process;

e) maintaining the reaction mixture at desired temperature and time to complete the addition reaction; and

f) finally the resulting skeletal modified diamine product is cooled to room temperature.

2. As claimed in claim 1, wherein the said amine derivative is azepan-2-one.

3. As claimed in claim 1, wherein the said amino acid condensate is 4-(4-ammotenzyl)beiizenamine.

4. As claimed in claiml, wherein the said skeletal modified diamine product is N-(4-(4(6-aminohexylamido) benzyl) phenyl)-6-aminohexanamide.

5. As claimed in claiml, wherein the reaction mixture is maintained at a desired temperature range of 80 to 120°C and more preferably atlOO°C.

6. As claimed in claiml, wherein the reaction mixture is maintained at the desired temperature for 60 to 180 minutes and more preferably for 120 minutes with efficient agitation.

7. As claimed in claiml, wherein the yield of the skeletal modified diamine obtained through the said process ranges from 94% to 98% and more preferably is 96%.

8. As claimed in claiml, wherein the purity of the skeletal modified diamine obtained through the said process ranges from 92% to 97% and more preferably is 95%.

9. As claimed in claiml, wherein the said skeletal modified diamine is used as both curative and impact modifier for the epoxy resins.