Curing agent for epoxy resin laminating compositions comprising a mixture of dicyandiamide and a tetra-alkylguanidine

Curing agents for mixed di- and tetra-functional epoxy resin laminating compositions comprise a mixture of dicyandiamide and from 0.01 to 1 parts per part of dicyandiamide of a tetraalkylguanidine. B-staged prepregs for cores and for multi-layer bonding sheets having superior storage stability and adapted to provide reproducible multilayer laminates are made from laminating compositions employing the new curing agent combination.

The present invention relates to a curing composition for epoxy resins and 
to its use in preparing impregnated sheets for producing laminates for use 
in printed circuits, and the like. 
BACKGROUND OF THE INVENTION 
Glass fiber reinforced laminates prepared from cured epoxy resins are well 
known in the art. The use of difunctional brominated epoxy resins produces 
laminates having self-extinguishing properties. See, for example, Lapitz, 
Jr., U.S. Pat. No. 3,600,263 and Fujiwara et al, U.S. Pat. No. 3,741,858, 
the disclosures of which are incorporated herein by reference. A 
disadvantage noted is the tendency of such laminates to be swelled or 
attacked by solvents during fabrication of printed circuits. Solvent 
resistance has been improved by mixing the difunctional brominated epoxys 
with tetrafunctional epoxys prior to impregnation of the glass fibers; see 
Chellis, U.S. Pat. No. 3,523,037, the disclosure of which is incorporated 
herein by reference. In his mixed epoxy laminating systems Chellis uses a 
curing system comprising dicyandiamide catalyzed with a tertiary amine. It 
has been found that such a curing system suffers the disadvantage of 
producing B-staged saturated sheets (prepregs) which are not particularly 
consistent within a lot or from lot-to-lot. The prepregs produce laminates 
of the single and multilayer type which are not as consistent and 
reproducible in their properties as would be desired Lopez et al, U.S. 
Pat. No. 3,391,113, the disclosure of which is incorporated herein by 
reference, disclose that di-functional epoxy resin compositions can be 
rapidly cured at lower than expected temperatures with a curing system 
comprising curing amounts of dicyandiamide and from 0.01 to 1 part by 
weight of dicyandiamide of a tetramethyl- or tetraethyl-guanidine. The 
system is stated in Col. 2, lines 15-16, to have a relatively low shelf 
life. Lopex et al. have to overcome this and extended shelf life by adding 
acids such as benzoic acid in substantial quantities. 
It has now been discovered that superior laminating compositions and 
prepregs can be made by mixing di- and tetra-functional epoxides and 
curing them with a system comprising dicyandiamide and a 
tetraalkylguanidine. Unexpectedly, there is no need to introduce 
stabilizing additives to overcome the expected low shelf life because 
stable prepregs are produced in spite of the teachings in Lopez et al, 
when instead of difunctional epoxy resins, a mixture of di- and 
tetra-functional epoxy resins is used. In addition, it is preferred to use 
a difunctional epoxy resin with a lower molecular weight than that 
employed by Chellis thereby obtaining more consistent and more 
reproducible prepreg and bonding sheet materials. The present invention 
uses one with an epoxy equivalent weight (EEW) of 370-430, whereas Chellis 
states that an EEW or 455 to 500 is critical. 
DESCRIPTION OF THE INVENTION 
According to the present invention, there are provided curing agents for an 
epoxy resin mixture of (i) from 70 to 90 parts of a brominated epoxy resin 
comprising the reaction product of tetrabromobisphenol-A and 
epichlorohydrin having an epoxide equivalent weight in the range of 
370-430 and a bromide content in the range of 15-22% and (ii) from 10 to 
30 parts of a tetrafunctional epoxy resin comprising the reaction product 
of tetra-bis(hydroxyphenyl)ethane having an epoxide equivalent weight in 
the range of 210-240, said curing agent consisting essentially of a 
mixture of dicyandiamide and from 0.01 to 1.0 parts per part of said 
dicyandiamide of a tetraalkylguanidine. 
Among the features of the invention are an epoxy resin composition for 
preparing B-stages storage-stable, reproducible prepregs and multi-layer 
bonding sheets adapted to produce flame retardant laminates, said 
composition comprising 
(a) an epoxy resin mixture of (i) from 70 to 90 parts of a brominated epoxy 
resin comprising the reaction product of tetrabromobisphenol-A and 
epichlorohydrin having an epoxide equivalent weight in the range of 
370-430 and a bromine content in the range of 15-22% and (ii) from 10 to 
30 parts of a tetrafunctional epoxy resin comprising the reaction product 
of tetra-bis(hydroxyphenyl)ethane having an epoxide equivalent weight in 
the range of 210-240; 
(b) an effective amount of dicyandiamide as a curing agent; 
(c) from about 0.01 to 1.0 parts per part of said dicyandiamide of a 
tetraalkylguanidine as a catalyst; and 
(d) a suitable solvent. 
Also contemplated by the invention are, as an article of manufacture, a 
glass fabric impregnated with an epoxy resin composition comprising: 
(a) an epoxy resin mixture of (i) from 70 to 90 parts of a brominated epoxy 
resin comprising the reaction product of tetrabromobisphenol-A and 
epichlorohydrin having an epoxide equivalent weight in the range of 
370-430 and a bromine content in the range of 15-22% and (ii) from 10 to 
30 parts of a tetrafunctional epoxy resin comprising the reaction product 
of tetra-bis(hydroxyphenyl)ethane having an epoxide equivalent weight in 
the range of 210-240; 
(b) an effective amount of dicyandiamide as a curing agent; 
(c) from about 0.01 to 1.0 parts per part of said dicyandiamide of a 
tetraalkylguanidine. 
The brominated epoxy resin component is a polyglycidyl ether of 
tetrabromobisphenol-A or a mixture of tetrabromobisphenol-A and 
bisphenol-A. It has a functionality of about 2, a bromine content of 
15-22%, preferably 17-20%, and has an epoxide equivalent (solids) of 
370-430. A procedure for preparing a suitable brominated epoxy resin will 
be exemplified hereinafter. 
The tetrafunctional epoxy resin component is a polyglicydyl ether of 
tetraphenylethane, and is prepared from tetrabis(hydroxyphenyl)ethane and 
epichlorohydrin. This epoxy resin has an average functionality of about 4 
and an epoxide equivalent weight of about 210 to 240. It is described in 
Encyclopedia of Polymer Science and Technology, John Wiley and Sons, New 
York, N. Y., Vol. 6, page 214, formula (13), incorporated herein by 
reference and in the above-mentioned Chellis patent. Shell Chemical 
supplies a suitable resin under the trade name Epon 1031; and Union 
Carbide under the tradename Bakelite ERR-0153. 
Dicyandiamide component (b) is well known as a latent curing agent for 
epoxy resins; See U.S. Pat. No. 2,637,715, which is incorporated herein by 
reference, and the above-mentioned Lopez et al patent. This is 
commercially available from American Cyanamide Co. 
The term "tetraalkylguanidine" is intended to include guanidines bearing 
alkyl groups, independently, of from 1 to 6 carbon atoms. As in U.S. Pat. 
No. 3,391,113, the preferred tetraalkyl guanidines are tetraethylguanidine 
and tetramethylguanidine. These are commercially available from American 
Cyanamide Co. 
Any conventional solvent will be suitable for the present impregnants. See, 
for example, the illustrative patents cited above. Preferably, the 
solvents will comprise aromatic hydrocarbons, e.g., toluene; alkylene 
glycol mono- or di-ethers or -esters, e.g., methyl cellosolve, di(lower 
alkyl C.sub.1 -C.sub.6) ketones, e.g., acetone, methyl ethyl ketone, alkly 
formamides, e.g., dimethyl formamide, mixtures of any of the foregoing, 
and the like. 
The relative proportions of components making up the compositions for 
impregnating glass cloth, glass paper, glass mat, and the like, can vary 
widely. For example, for each part of resin solids, one can use from 1 to 
about 10, and preferably from about 2 to 5 parts of dicyandiamide, from 
about 0.01 to about 1 part of tetraalkyl guanidine, preferably from 0.02 
to 0.1 parts; and sufficient solvent to provide a concentration of resin 
components giving the proper resin loading. The amount of solvent is 
easily determinable by those skilled in the art; and will be exemplified 
hereinafter. 
In carrying out the invention to produce a B-stage prepreg, the glass web 
is passed through the varnish comprising the resins and solvent. The 
impregnating apparatus may be of any commercial type such as a two zone 
treater of the dip-squeeze type. Typical treating conditions include a 
squeeze roll setting of about 0.010 inch, a wet zone temperature of about 
270.degree.-300.degree. F., a dry zone temperature of about 
310.degree.-330.degree. F., and a web speed of 100-200 inches per minute. 
Dwell time in the oven is about 3-7 minutes. The resin varnish is usually 
about 40-70% resin solids. The above treating conditions result in a resin 
composition content in the treated glass web of about 40-70% by weight 
based on the glass weight. The impregnated, B-staged and dried glass cloth 
is cut to the desired size and stacked to obtain the desired laminate 
thickness. To make multilayers, bonding sheets can be used. Foil cladding, 
e.g., copper foil, can also be included in conventional lay-up procedures. 
To consolidate the stack or stacks into unitary laminates, the pressures of 
about 200-2000 psi and temperatures of about 130.degree. C.-175.degree. C. 
are employed depending on the press cycle used and the desired density of 
the laminate. The pressing cycle may vary from a few minutes up to about 
75 minutes depending on the laminate thickness and the number of stocks 
pressed in each sheet opening. The stacked sheets are loaded into a cold 
press, consolidated under the above pressing conditions, and unloaded when 
cooled to about 150.degree. F.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention will be further particularized by the following examples. It 
is to be understood that the Examples are given solely for the purpose of 
illustration. 
EXAMPLE 1 
The following resin varnish is applied to woven glass fabric in a two zone 
treater. After being dried the prepreg contains about 60% by weight of the 
impregnant. 
______________________________________ 
Material Parts by Weight 
______________________________________ 
Difunctional brominated epoxy resin.sup.a 
100 
Tetrafunctional epoxy resin.sup.b 
22.0 
Dicyandiamide 2.7 
Tetramethylguanidine 0.1 
Solvent methyl cellosolve 
13.6 
Solvent dimethyl formamide 
9.0 
Solvent acetone 3.4 
______________________________________ 
.sup.a Epoxide equivalent (solids) 370-430; bromine content 17-20%; 
.sup.b Epoxide equivalent 200-240; 80% solids in methyl ethyl (ketone; 
Shell EPON 1031. 
The brominated epoxy resin component is prepared by the following 
procedure: 
Equal parts by weight of a bisphenol-A-epichlorohydrin reaction product and 
tetrabromobisphenol-A are heated to above 180.degree. C. until homogeneous 
and reacted, then to the hot liquid is added enough of a blend of methyl 
cellusolve and acetone to cool it to about 75.degree. C. Then a mixture 
comprising 1 part of tetrabromobisphenol-A to 14 parts of 
bisphenol-A-epichlorohydrin-reaction product to 6 parts of acetone is 
added. Final solids content is about 70% by weight. 
The treated and dried prepreg is cut and is storage stable for a year or 
more. 
Laminates prepared therefrom are flame-resistant and have remarkably 
reproducible properties: 
EXAMPLE 2 
The general procedure of Example 1 is repeated with the following resin 
varnish: 
______________________________________ 
Material Parts by Weight 
______________________________________ 
Difunctional brominated epoxy resin.sup.a 
100 
Tetrafunctional epoxy resin.sup.b 
22.0 
Dicyandiamide 3.0 
Tetramethylguanidine 0.2 
Solvent methyl cellosolve 
14.7 
Solvent dimethyl cellosolve 
9.7 
Solvent acetone 3.4 
______________________________________ 
.sup.a,b See footnotes to Table 1 
The treated and dried prepreg is storage stable. 
Laminates therefrom are flame-resistant and have remarkably reproducible 
properties. 
EXAMPLE 3 
Using the formulation described in Example 2, a study of consistency and 
reproducibility of varnishes and B-stage materials is carried out by 
comparing 0.2 parts by weight tetramethylguanidine versus 0.2 parts by 
weight benzyldimethylamine, an example of a tertiary amine catalyst used 
in the prior art (Chellis, U.S. Pat. No. 3,523,037). The 
tetramethylguanidine catalyst is found to provide more consistent and more 
reproducible varnish than the benzyldimethylamine catalyst does. For the 
benzyldimethylamine catalyzed varnish, the gel time at 170.degree. C. 
changes from 600 to 200 seconds during the first 18 hours after mixing. On 
the other hand the tetramethylguanidine varnish exhibits a constant gel 
time of 200 seconds at 170.degree. C. during the same 18 hour period, 
which is typical of time required to manufacture B-staged prepreg or 
bonding sheet. The tetramethylguanidine catalyzed varnish is also more 
reproducible when additional varnishes are made and measured after 18 
hours, having gel times of 180, 200, 180 and 190 seconds. Typical values 
for the benzyldimethylamine catalyzed varnishes are 300, 240, 180, and 280 
seconds. 
B-staged bonding sheets for the manufacture of multilayer are found to be 
more consistent when using tetramethylguanidine than when using 
benzyldimethylamine. Gel times at 170.degree. C. are measured during an 18 
hour period of manufacture (one lot). The tetramethylguanidine materials 
exhibits gel times ranging 120-130 seconds whereas the benzyldimethylamine 
materials exhibit a broad range of 120-400 seconds. This improved 
within-lot consistency is also demonstrated by laminating multilayers with 
B-stage from the beginning, middle, and end of a lot. The 
tetramethylguanidine catalyzed B-stage displays constant flow properties 
throughout the lot whereas the benzyldimethylamine catalyzed B-stages 
exhibits changing flow throughout the lot, ranging from excessive flow 
(dielectric too thin) to minimal flow (dielectric too thick). 
The above description will be appreciated by those skilled in the art as 
illustrative of a class of curing systems, varnishes and prepregs for 
fire-resistant laminates possessing excellent physical and electrical 
properties. The method of their preparation is simple and may be carried 
out in conventional apparatus. All obvious variations in light of the 
above specific teachings are within the full intended scope of the 
appended claims.