Latent hardeners for epoxy resin compositions

A latent hardener material for epoxy resins comprising a combination of PA0 Hardener (A) which is a reaction product of phthalic anhydride and diethylenetriamine, and PA0 Hardener (B) which is a reaction product of a polyfunctional epoxy compound, an imidazole compound and a carboxylic acid anhydride. A combination of latent hardeners (A) and (B) can achieve cure of an epoxy resin composition in 15 minutes at 76.degree. C. A one part epoxy resin composition comprising an epoxy resin in admixture with a combination of hardeners (A) and (B) has a good shelf life.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to latent hardeners for epoxy resin compositions. In 
particular it relates to a synergistic combination of two classes of 
latent hardener. The invention is particularly suited for industrial use, 
especially in the electronics industry, for example in the manufacture of 
printed circuit boards and other electronic assemblies. 
An uncured epoxy resin to which a latent hardener has been added can be 
stored in a stable condition at ambient temperature. When the temperature 
is increased the hardener is activated to cure the resin. A one-part epoxy 
resin composition of this kind is advantageous but there is a need for 
such a composition which combines good shelf life with a short curing time 
at relatively low temperatures. 
2. Description of the Related Art 
British Patent Specification No. 1,121,196 of Ciba Limited describes a 
latent hardener for epoxy resins comprising a reaction product of phthalic 
anhydride and diethylenetriamine. However this hardener requires an 
elevated temperature of at least 100.degree. C. to cure epoxy resin 
compositions satisfactorily. 
U.S. Pat. No. 4,546,155 Hirose et al assigned to Ajinomoto Co., Inc. 
describes a latent curing agent for epoxy resins which is an adduct 
obtained by reacting (i) a polyfunctional epoxy compound, (ii) a compound 
having at least one OH, NH.sub.2, NH or SH group together with a tertiary 
amino group in the molecule and (iii) a carboxylic acid anhydride. The 
compound (b) may suitably be an imidazole compound such as 
2-ethyl-4-methylimidazole. The compound (c) may suitably be phthalic 
anhydride. With hardeners of this type, curing of an epoxy resin 
composition is possible at temperatures as low as 80.degree. C. but a cure 
time of at least 30 minutes is required. Such a long cure time is 
disadvantageous for industrial use. 
U.S. Pat. No. 4,546,155 indicates at column 4 lines 55-59 that the latent 
curing agent of that Patent can be used in combination with a known curing 
agent such as an acid anhydride, a dicyandiamide, a dibasic acid 
hydrazide, guanamine, or melamine. However there is no suggestion that any 
synergistic effect could be obtained. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an epoxy resin 
composition containing a latent hardener combination which has good shelf 
life but which exhibits a surprising effect in achieving cure in a short 
time at relatively low temperature. 
The present invention provides a latent hardener material for epoxy resins 
comprising a combination of 
Hardener (A) which is a reaction product of phthalic anhydride and an 
aliphatic polyamine, and 
Hardener (B) which is a reaction product of (i) a polyfunctional epoxy 
compound, (ii) a compound having at least one functional OH, NH.sub.2, NH 
or SH group together with a tertiary amino group in the molecule, and 
(iii) a carboxylic acid anhydride. 
The invention also provides curable one-part epoxy resin compositions 
comprising an epoxy resin in admixture with a combination of compounds (A) 
and (B) as defined above. 
It has surprisingly been found that a combination of latent hardeners (A) 
and (B) can achieve cure of an epoxy resin composition in 15 minutes at 
76.degree. C., while the composition containing the two hardeners has a 
good shelf life, e.g. of at least 1 month at 45.degree. C. Although this 
invention is not bound by any theory, it is believed that hardener (A) is 
activated at a lower temperature than hardener (B) i.e. about 
70.degree.-75.degree. C. However with hardener (A) alone the cure is very 
slow and no satisfactory cure is achieved in a reasonable time unless the 
temperature is increased to about 100.degree. C. When hardeners (A) and 
(B) are present in accordance with the invention, it is believed that the 
activation of hardener (A) at 70.degree.-75.degree. C. energizes hardener 
(B) and causes it to start up at lower temperature than usual (80.degree. 
C.) and to cure the composition more rapidly. 
Epoxy resin compositions are used industrially in very small portions e.g. 
for attaching semiconductor chips to printed circuit boards. In such small 
portions there is little or no exothermic effect on curing. Therefore a 
reduction in cure time from 30 minutes to 15 minutes and a reduction in 
cure temperature from 80.degree. C. to 76.degree. C. could be economically 
very significant in terms of production output and the heat input 
required. 
Hardeners (A) and (B) may suitably be present in a ratio ranging from 12:1 
to 3:1. The quantity of total hardeners may suitably vary from 50 to 100 
parts per 100 parts of epoxy resin. 
Hardener (B) is preferably a reaction product of (i) a polyfunctional epoxy 
compound, (ii) an imidazole compound, and (iii) phthalic anhydride. The 
polyfunctional epoxy compound may be any compound having two or more epoxy 
groups in the molecule as described in U.S. Pat. No. 4,546,155. the 
contents of which are incorporated by reference. More preferably the 
imidazole compound is 2-ethyl-4-methylimidazole. 
Examples of these hardeners are disclosed in U.S. Pat. No. 4,546,155 
especially in Table 1 at column 6 thereof, where sample 13 in particular 
is an adduct of Epon 828 (Bisphenol Type epoxy resin epoxy equivalent 
184-194, a product of Shell Chemical Co.), 2-ethyl-4-methylimidazole and 
phthalic anhydride. A hardener of this type is commercially available from 
Ajinomoto Co., Inc. under the Trade Mark AJICURE PN-23. 
Hardener (A) is suitably a reaction product of phthalic anhydride and an 
aliphatic polyamine having at least 3 active hydrogens per molecule, more 
particularly diethylene triamine, triethylene tetraamine, tetraethylene 
pentamine or pentaethylene hexamine. Diamines such as ethylene diamine, 
tetramethylene diamine, or hexamethylene diamine may also be useful. 
The hardener (A) most suitably is a reaction product of approximately 
equimolecular proportions of phthalic anhydride and diethylenetriamine as 
described in British Patent 1,121,196, the contents of which are 
incorporated by reference. A hardener of this type is commercially 
available from Ciba Geigy A.G. under the designation HT-9506. 
The epoxy resin compositions may be any of such compositions known in the 
art and as described for example, in British Patent 1,121,196 and/or U.S. 
Pat. No. 4,546,155.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention will be further described and illustrated in the following 
Examples. 
EXAMPLES 
Formulations were made up using the following hardeners: 
Hardener (A): Reaction product of phthalic anhydride and diethylenetriamine 
commercially available as HT-9506 from Ciba Geigy A.G. 
Hardener (B): Reaction product of a polyfunctional epoxy compound, an 
imidazole compound and a carboxylic acid anhydride commercially available 
as AJICURE PN-23 from Ajinomoto Co., Inc. 
The basic formulation was as follows: 
______________________________________ 
Weight % 
______________________________________ 
Quatrex 1010 56.14 
DER 736 13.75 
Rocket Red dye 1.69 
Pylam Yellow dye 0.10 
Hardener (A) - HT 9506 
0 or 60 phr 
Hardener (B) - PN-23 
5, 10, 15, or 20 phr. 
______________________________________ 
Quatrex 1010 is an epoxy resin based on the reaction product of 
epichlorohydrin and bisphenol A, available from Dow Chemicals. Quatrex is 
a Trade Mark. 
DER 736 is a short chain polyglycol diepoxide available from Dow Chemicals. 
Der 736 acts as a reactive diluent for the Quatrex 1010 resin. 
phr=parts per hundred parts of the total composition. 
The exemplary formulations were numbered as follows: 
Formulation 1: 60 phr HT 9506 
Formulation 2: 60 phr HT 9506+5 phr PN-23 
Formulation 3: 60 phr HT 9506+10 phr PN-23 
Formulation 4: 60 phr HT 9506+15 phr PN-23 
Formulation 5: 60 phr HT 9506+20 phr PN-23 
Formulation 6: 5 phr PN-23 
Formulation 7: 10 phr PN-23 
Formulation 8: 15 phr PN-23 
Formulation 9: 20 phr PN-23 
CHIP-PUSH-OFF STRENGTH TESTS 
Formulations 1-4, with addition of 13 phr 9-65 hydrophobic silica TS-720, 
were subjected to Chip push-off strength tests by standard procedure which 
is described as follows: 
Appropriate sized drops of adhesive (sufficient for 80% cured coverage of 
device) are dispensed using a Siemens adhesive dispenser onto a FR 4 epoxy 
glass reinforced printed wiring board laminate. Surface mount device 
components are placed manually onto the adhesive drops using a tweezers. 
The components used are 1206 resistors and capacitors, small outline 
transistor (SOT) 23's and cylindrical MELF components. A minimum of five 
of each component are required per test. 
Adhesive curing is carried out in a conventional convection oven fitted 
with a metallic heat sink. A thermocouple with direct temperature readout 
is affixed to the FR 4 epoxy glass laminate bonding surface in order to 
monitor the actual temperature of cure accurately. When the laminate 
attains the desired curing temperature a timing device is started and the 
curing process allowed to continue for the desired period. The assembly is 
removed from the oven and allowed to equilibrate to ambient conditions. 
Bond strengths are determined by measuring a push off force using a 
Chatillon gauge fitted with a suitable gripping apendage. The strength 
measured is expressed as an average plus or minus a standard deviation in 
kilograms. 
In these tests, each of the formulations 1-4 was cured at 75.degree. C. for 
15 minutes. The results of the tests are set out in Table 1. 
TABLE 1 
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Chip push-off strengths 
Formulation 
Resistor Capacitor SOT. 23 
Melf 
______________________________________ 
#1 -- -- -- -- 
#2 -- -- -- -- 
#3 1.3 .+-. 0.1 
1.2 .+-. 0.2 
2.2 .+-. 0.1 
0.8 .+-. 0.2 
#4 1.3 .+-. 0.3 
2.0 .+-. 0.3 
1.8 .+-. 0.2 
1.6 .+-. 0.8 
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Strengths in KGS. 
It will be seen that formulation 1 containing 60 phr hardener (A) alone and 
formulation 2 containing only 5 phr of hardener (B) in combination with 60 
phr hardener (A) showed no appreciable bond strengths after cure under 
these conditions, whereas formulations (3) and (4) containing 10 phr and 
15 phr of hardener (B) with 60 phr hardener (A) showed acceptable bond 
strengths. The measurements were all made at ambient temperature. 
LAP SHEAR STRENGTH TESTS 
Formulations 1-9 were subjected to Lap Shear Strength Tests using grit 
blasted mild steel lap shears. Each formulation was applied to one lap 
shear, a second lap shear was placed in overlapping relationship with it, 
and the formulation between the two lap shears was cured at 75.degree. C. 
for 15 minutes. Strengths required to pull the lap shears apart were then 
measured. The results are shown in Table 2. 
TABLE 2 
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Lap shear strengths at 75.degree./15 Minutes 
Formulation Strength 
______________________________________ 
#1 69.0 .+-. 8.1 
#2 73.0 .+-. 5.0 
#3 102.3 .+-. 6.5 
#4 111.0 .+-. 5.0 
#5 106.0 .+-. 5.7 
#6 
#7 No strength 
#8 
#9 No strength Hand Fixtured 
______________________________________ 
It will be seen by comparison of formulations 3, 4 and 5 with formulation 1 
that addition of 10 phr or 20 phr Hardener (B) to Hardener (A) achieves an 
approximate 54% increase in lap shear strength. However the results for 
formulations 7-9 show that no strength is achieved with Hardener (B) at 
these concentrations. The measurements were all made at ambient 
temperatures. 
DIFFERENTIAL SCANNING CALORIMETRY 
Formulations 1-8 were subjected to an isothermal scan at 80.degree. C. The 
results are shown in Table 3. 
TABLE 3 
______________________________________ 
DSC - Isothermal (80.degree. C.) Information 
Formulation .DELTA.H (Jlg) 
Peak Time (Minutes) 
______________________________________ 
#1 73.3 3.0 
#2 90.5 11.7 
#3 1383.8 2.7 
#4 233.4 9.6 
#6 No response 
#7 No response 
#8 No response 
______________________________________ 
It will be seen that Hardener (A) kicks at lower time than Hardener (B) but 
the .DELTA.H value of Hardener (A) alone is too low to give any acceptable 
strength, whereas acceptable .DELTA.H strengths are achieved with 
combination of Hardeners (A) and (B). 
STABILITY 
Formulation 1, 3 and 5 were subjected to accelerated elevated stability 
tests in 5 ml. polypropylene syringes. The results are shown in Table 4. 
TABLE 4 
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Gel times versus temperature in 5 ml. polypropylene syringes 
T(.degree.C.) 
#1 #3 #5 
______________________________________ 
55 5 2 0.75 
50 6 3 2 
45 408 72 20 
35 &gt;408 &gt;408 408 
______________________________________ 
Gel times in hours unless otherwise shown.