2-Ethyl-4-methylimidazole (EMI). CH. C. N. CH3. CH3. CH2. N. C. H. 110. Liquid at 25oC. (77°F). 4-8 Pa.sec,. (4000-8000). Lewis base used as the sole.
How do we calculate the amount of amine hardener require to cure epoxy resin? Epoxy resins: The most common matrix for advanced composites and for a variety of demanding applications is epoxy. Epoxies have taken this major role because of their:
Excellent adhesion, Good strength, Low shrinkage, Good corrosion resistance, Processing versatility, Many other properties.
Epoxies may be more expensive than polyesters and may not perform as well in high temperature applications as polyimides, but overall their properties are excellent. Generally epoxies do a fine job in transferring loads to the reinforcement and in protecting the reinforcement from adverse environmental effects. Epoxy chemical structure and polymer formation: Epoxies are characterized by the presence of an epoxy group-a three-membered ring with two carbons and oxygen as shown in Figure 1. This epoxy group is the site of the crosslinking reaction in roughly the same way as the carbon-carbon double bond in the site of polyesters, although the crosslinking reactions are quite different. The properties of the crosslinked polymer are more dependent upon the choice of curing systems in epoxies than in polyesters, so both the nature of the epoxy and of the curing system must be understood. Many epoxies use the slightly modified epoxy group with one additional carbon. This group is called the glycidyl group and is illustrated in Figures 1 and 2. In addition for being the site that is used for crosslinking, the epoxy group provides for good adhesion with the reinforcement or with the “surface” of another material. The aromatic (benzene) groups provide the stiffness, temperature stability and other physical properties characteristic of epoxies. Dyglycidyl ether of bisphenol A (DGEBPA): The most common of the epoxy resin systems is based upon the condensation polymer formed from epichlorohydrin and bisphenol A and is called diglycidyl ether of bisphenol A (DGEBPA) (see Figures). The reaction to form the resin from the reactants is as follows:
1
CH O Epoxy Group CH
GROUPS
CH O Glycidyl
CH
REACTANTS
CH3
CH CH2 O
CH2
Cl + H
O
Bisphenol-A
EPOXY PRODUCT CH
OH CH3
Epichlorohydrin
CH2
CH2
CH3 CH2 O
O
O
CH2
CH2 CH O
CH3
CH3 CH2 CH CH2 O
Cl + H
O
OH CH3 Bisphenol-A
Epichlorohydrin
CH3 CH
CH2
CH2 O
O
O
H
CH3
Add another epichlohydrin CH2 CH CH2 O
Cl
CH3
CH3 HO
Epichlorohydrin
OH CH3
CH
CH2 O
CH2 O
O CH3
CH2
CH2 CH O
Above is the basic unit of DGEBPA Add another Bisphenol A:
2
CH3 HO
OH
Bisphenol-A
CH3 CH3
CH3 CH CH2 O
CH2 O
CH2 CH
O
CH2 O
OH
OH
CH3
CH3
Add another Epichlohydrin: CH CH2 O
CH2 Cl
Epichlorohydrin
CH3
CH3 CH CH2 O
CH2 O
O CH3
CH2 CH OH
CH2 O
O CH3
CH2
CH2 CH O
Figure 1. Epoxy groups and reactants for epoxy polymers. This resin is available at various chain lengths depending upon the amounts of reactants present and other reaction conditions. Specialty epoxy resins: The other epoxies listed in Figure 2 are generally used for special property consideration where DGEBPA is not appropriate. For instance:
The epoxidized phenolic novolacs react more rapidly than do DGEBPA systems and added strength and chemical resistance at high temperatures. Example: DOW DEN 438. The tetraglycidyl ether of tetrakis (hydroxyphenyl) ether has excellent adhesive properties and greater strength, rigidity, resistance to moisture and improved electrical properties at elevated temperatures. Example SHELL EPON 103 Tetraglycidylmethylene dianiline (TGMDA) also exhibits improved properties at elevated temperatures. Example: CIBA My-720.
3
CH3
CH3 CH CH2 O
CH2 O
O
CH2 O n
CH2 CH OH
CH3
O
CH2
CH2 CH O
CH3
Diglycidyl Ether of Bisphenol A (DGEBPA)
O
CH2
O
CH
CH2
O
CH
CH2 CH
CH2
CH2
CH2
O
O
O
CH2
CH2
n
Epoxy Novolac (Epoxidized Phenolic Resin) Example: DOW DEN 438 H CH
CH2
CH2 O
C
O
O
O CH
CH2
CH2
CH2 CH
CH2 O
O
C
O
CH2
CH2 CH O
H
Tetraglycidylether of Tetrakis (Hydroxyphenyl) Ether Example: SHELL EPON 103 O CH2
CH
CH2
CH2
N CH CH2 O
CH2
CH2
O CH2 CH
N CH2
CH2 CH O
Tetraglycidylmethylene Dianiline (TGMDA) Example: CIBA MY-720 Figure 2. Major epoxy resin systems
Curing systems for epoxies (hardeners): Three cure systems are of primary importance for epoxies: amines, anhydrides and Lewis acids (or catalytic curing). Amines:
4
The most common curing system is the amine, in which one of the amine hydrogen reacts with the epoxy ring to form a hydroxyl group which can then react with another group to crosslink the chains. These reactions are addition reactions, and, therefore, no byproduct is formed as would be formed in a condensation reaction. Primary amines (RNH2), which contain two active amine hydrogens, are therefore capable of reacting with two epoxy groups and achieving a greater crosslink density (number of crosslinks per polymer) than single functional hardeners. This greater crosslink density generally improves physical properties, although at the expense of flexibility. If the amine is aromatic, the overall stiffness, low shrinkage, and temperature capability is improved, although toughness is sacrificed. The chemical structures, and viscosities of the commonly used amine curing agents are shown in Table 1. Table 1. Structures and Characteristics of Commonly Used Amine Curing Agents
AMINE CURING AGENTS
Diethylenetriamine (DETA) H2N
CH2
CH2
NH
CH2 NH2
CH2
Triethylenetetramine (TETA) H2N
((CH2)2 NH)2 CH2 CH2
NH2
AMINE HYDROGEN EQUIVALENT WEIGHT - AHEW (g/eq)
VISCOSITY @ 25°C (77°F), Pa.sec (cP)
20
0.00550.0085 (5.5-8.5)
24
0.020-0.023 (20-23)
65
5.0
COMMENTS
Available from Dow Chemical Company as DEH 20 and also from Union Carbide Corporation. Available from Dow Chemical Company as DEH 24 and also from Union Carbide Corporation.
Diethylaminepropylamine (DEAPA) CH3
CH2 N
CH3
CH2
CH2
3
NH2
2-Ethyl-4-methylimidazole (EMI) N
C CH
C CH3
CH2
N H
CH3
110
Liquid at 25oC (77°F)
4-8 Pa.sec, (4000-8000)
Available from Union Carbide Corporation. Lewis base used as the sole curing agent in small nonstoichiometric amount to provide long pot life and good elevated-temperature properties. Available from Houdry Process & Chemical Company as EMI-24
5
An amine is a substance that contains nitrogen. There are three types of amines. When a molecule containing a reactive group consisting of nitrogen is attached to two hydrogens, this is called primary amine. When the reactive group consists of nitrogen attached to one hydrogen, this is called secondary amine. When there is no hydrogen attached to the nitrogen, this is called tertiary amine. It can be seen that many of the structures given in Table 1 include reactive groups at each end. These permit the formation of crosslinks between epoxy molecules. For example, an amine end group with two hydrogens on the nitrogen (a primary amine) reacts with the epoxy molecule as follows:
OH NH2
NH2
+
CH2 CH O
NH2
NH
CH2 CH
When another amine hydrogen combines with a second epoxy molecule, a crosslink is formed. The curing agents in Table 1 that contain secondary amine end groups (one hydrogen on the nitrogen) react in much the same way. OH
OH NH2
NH
CH2 CH
+
CH2 CH O
NH2
N
CH2 CH
CH2 CH
OH
For thorough crosslinking, the hydrogens of the primary and secondary amines should be matched 1:1 with the epoxy groups. The amounts of curing agent and epoxy resin needed in order to obtain the 1:1 stoichiometric quantity are calculated as follows: Molecular weight of amine Parts by weight of amine to be used with 100 parts by weight of epoxy resin (phr)
Number of available hydrogens per molecule =
X 100 Epoxy equivalent weight
Tertiary amines (no hydrogen on the nitrogen) are Lewis bases that cure epoxy resins in an entirely different manner than the aromatic and the aliphatic amines. They are added to an epoxy resin in small nonstoichiometric amounts that have been empirically determined
6
to give the best properties. The curing agent operates as a true catalyst by initiating a self perpetuating anionic polymerization. R3 N + CH2 O
+
R3 N
CH2
CH
CH O
CH2 R
+
R3 N
CH2
CH
CH2 R
O
CH2 R + CH2 O
CH
CH2 R'
+
R3 N
CH2
CH
CH2 R
O CH2
CH
CH2 R'
O CH CH2 O
CH2R''
and so on.
This homopolymerisation of epoxy molecule to epoxy molecule results in a polyether. The ether linkages (C-O-C) are fairly stable against most acids (both organic and inorganic) and alkalis. Further, like ester linkages, they are more thermally stable than the carbon-nitrogen linkages formed by an amine cure. Of the Lewis bases listed in Table 1, EMI is the most efficient, that is, it produces the highest degree of crosslinking and the highest heat distortion temperature. Example: Question: It is desired to cross link a DGEBPA epoxy resin using an amine curing agent called DETA. The formula for the two materials are as shown below. How many grams of DETA should be used if 100g of the epoxy resin is used. Solution: The epoxy formula has 21 carbon atoms, 24 Hydrogen atoms, and 4 oxygen atoms. The mass of the epoxy unit is therefore: m1 = 21 (12) + 24 (1) + 4 (16) = 340 g/mole The DETA formula has 3 Nitrogen atoms and 13 Hydrogen atoms. The mass of DETA is: m2 = 3 (14) + 13 (1) =55 g/mole. In the epoxy molecule, there are two epoxy groups, the epoxy equivalent weight is therefore:
7
m3 = 340/2 = 170 g/mole. There are 4 available hydrogens in the amine molecule. The value of molecular weight of amine over the number of available hydrogens per molecule is: m4 = 55/4 = 13.75 g/mole Parts by weight of amine to be used with 100 parts by weight of epoxy resin are: % = 13.75/170 x 100 = 8.1% For 100g of epoxy, the amount of amine curing agent to be used is: 8.1 g.
8
