Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From phenol – phenol ether – or inorganic phenolate
Reexamination Certificate
2000-03-20
2002-03-05
Lovering, Richard D. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From phenol, phenol ether, or inorganic phenolate
C523S435000, C525S526000, C528S094000, C528S108000
Reexamination Certificate
active
06353080
ABSTRACT:
The present invention relates to curable epoxy-resin-containing formulations, and particularly to formulations useful for making laminates for printed wiring boards.
It is known to make electrical laminates and other composites from a fibrous reinforcement and an epoxy-containing matrix resin. Examples of suitable processes usually contain the following steps:
(1) an epoxy-containing formulation is applied to a substrate by rolling, dipping, spraying, other known techniques and/or combinations thereof. The substrate is typically a woven or nonwoven fiber mat containing, for instance, glass fibers.
(2) The impregnated substrate is “B-staged” by heating at a temperature sufficient to draw off solvent in the epoxy formulation and optionally to partially cure the epoxy formulation, so that the impregnated substrate can be handled easily. The “B-staging” step is usually carried out at a temperature of from 90° C. to 210° C. and for a time of from 1 minute to 15 minutes. The impregnated substrate that results from B-staging is called a prepreg. The temperature is most commonly 100° C. for composites and 130° C. to 200° C. for electrical laminates.
(3) One or more sheets of prepreg are stacked in alternating layers with one or more sheets of a conductive material, such as copper foil, if an electrical laminate is desired.
(4) The laid-up sheets are pressed at high temperature and pressure for a time sufficient to cure the resin and form a laminate. The temperature of lamination is usually between 100° C. and 230° C., and is most often between 165° C. and 190° C. The lamination step may also be carried out in two or more stages, such as a first stage between 100° C. and 150° C. and a second stage at between 165° C. and 190° C. The pressure is usually between 50 N/cm
2
and 500 N/cm
2
. The lamination step is usually carried on for from 1 to 200 minutes, and most often for 45 to 90 minutes. The lamination step may optionally be carried out at higher temperatures for shorter times (such as in continuous lamination processes) or for longer times at lower temperatures (such as in low energy press processes).
(5) Optionally, the resulting copper-clad laminate may be post-treated by heating for a time at high temperature and ambient pressure. The temperatures of post-treatment are usually between 120° C. and 250° C. The post-treatment time usually is between 30 minutes and 12 hours.
It is conventional in the preparation of epoxy-containing laminates to incorporate into the epoxy resin composition various additives to improve the flame-retardancy of the resulting laminate. Many types of flame retardant additives have been suggested, but the additives which are most widely used commercially are halogen containing additives, such as tetrabromodiphenylolpropane, or epoxy resins prepared by reacting diglycidyl ether of bisphenol-A with tetrabromodiphenylolpropane. Typically, in order to reach the desired fire retardancy level (V-0 in the standard “Underwriters Laboratory” test method UL 94) levels of such compounds are required which provide a bromine content of from 10 to 25 weight percent based on the total polymer weight in the product.
Although halogen containing fire-retardant additives are effective, they are considered by some to be undesirable from an environmental standpoint, and in recent years there has been increasing interest in the formulation of halogen-free epoxy resins, which are able to meet the fire retardancy requirements.
Proposals have been made to use phosphorus based flame retardants instead of halogenated fire retardants (see, for example EP-A-0384939, EP-A-0384940, EP-A-0408990, DE-A-4308184, DE-A-4308185, DE-A-4308187, WO-A-96/07685, and WO-A-96/07686. In these formulations the phosphorus flame retardant was pre-reacted to form a di or multifunctional epoxy resin. This makes the epoxy resin relatively expensive.
The phosphonic acid esters are commercially available fire retardant materials (e.g., Amgard™ V19 and Amgard™ P45—supplied by Albright and Wilson Ltd, United Kingdom). These phosphonic acid esters, may be solids or liquids. Alkyl and aryl substituted phosphonic acid esters are compatible with epoxy resins. In particular lower (i.e., C
1
-C
4
) alkyl esters of phosphonic acid are of value because they contain a high proportion of phosphorus, and are thus able to impart good fire retardant properties upon resins in which they are incorporated. However, it has been found by the present inventors that they are not satisfactory as a substitute for halogenated flame retardants in epoxy resins for the production of electrical laminates, because their use in amounts sufficient to provide the necessary flame retardancy increases the tendency of the resulting cured epoxy resin to absorb moisture. The moisture absorbency of the cured laminate board is very significant, because laminates containing high levels of moisture tend to blister and fail, when introduced to a bath of liquid solder at temperatures around 260° C., a typical step in the manufacture of printed wiring boards.
EP-A-0754728 describes the production of flame retardant epoxy resin systems by blending epoxy resins with phosphonic acid esters and incorporating them into the cured resin. This reference indicates that large quantities (in excess of 18 weight percent) of the phosphorus additive are needed in order for the resin system to meet UL 94 V-0.
We have now discovered that it is possible to produce epoxy resins which meet the desirable standards of fire retardancy without the need for halogen-containing flame retardants, or at least employing significantly lower levels of such halogen-containing flame retardants than is conventional in the art, by employing relatively low levels of phosphonic acid ester flame retardant (to give 0.2 to 5 weight percent phosphorus in the solid resin), together with particular combinations of accelerator and catalyst, and, in preferred embodiments, particular types of epoxy resin. The accelerators and catalysts are generally known per se, but their use in conjunction with low levels of phosphonic acid ester fire retardants to obtain compositions which have both good fire retardancy, and yet sufficiently low water absorption has not hitherto been described.
According to the invention, there is provided a flame retardant epoxy resin composition containing no more than 10% by weight of halogen, comprising
a) an epoxy resin,
b) a phosphonic acid ester in an amount such as to provide from 0.2 to 5 weight percent phosphorus in the composition,
c) a nitrogen-containing crosslinking agent having an amine functionally of at least 2, in an amount of from 10 to 80 percent of the stoichiometric amount needed to cure the epoxy resin,
d) from 0.1 to 3 weight percent of a catalyst capable of promoting the reaction of the phosphonic acid ester with the epoxy resin and promoting the curing of the epoxy resin with the crosslinker, and optionally,
e) a Lewis acid in an amount of up to 2 moles, per mole of catalyst.
The phosphonic acid ester employed in the present invention is preferably an ester of the formula
wherein R
1
is C
1
to C
3
alkyl,
R
4
is C
1
to C
3
alkylene,
R
2
, and R
3
, are each C
1
to C
3
alkyl, or C
6
to C
10
aryl, or R
2
and R
3
taken together represent the residue of a glycol or a polyol.
Preferred phosphonic acid esters are for example esters of methanephosphonic acid with polyhydroxy compounds such as glycols, and polyols. Such phosphonic acid esters of polyhydroxy compounds can have polymeric and/or cyclic structures.
Specific preferred examples are polymers with repeating units such as:
and/or cyclic structures such as
n is 2 to 10, R
5
is a C
1
to C
3
alkylene group or the residue of a glycol or polyol,
R
6
is the residue of a triol, for example glycerol or trimethylol propane.
The phosphonic acid ester is preferably an ester having methyl or methylene adjacent to phosphorous. Preferred phosphonic acid esters are those of the formula,
In order to obtain satisfactory resistance to water absorption, it is important that the amount of the phosphon
Everett John P.
Gan Joseph
Goodson Alan
Koenig Raymond A.
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