GB2055842A - Fire resistant epoxy resin composition - Google Patents

Abstract

A resinous composition suitable for impregnation of glass fiber sheets contains a fire-retardant halogenated epoxy resin, up to an equal amount of a non-halogenated epoxy resin, a curing agent for the epoxide(s) and from 10 to 40 parts of hydrated alumina per 100 parts of epoxide(s) plus alumina. Heating the impregnated sheets to advance the resin to the B-stage produces prepregs which can then be consolidated under heat and pressure into fire-resistant unitary laminates having high electrical and physical properties and improved drilling characteristics together with outstanding capability for conversion to printed circuits.

Description

SPECIFICATION Fire-resistant glass-base epoxy resin laminates The present invention relates to electrical grade self-extinguishing glass-base epoxy resin laminates. More particularly, the invention relates to a class of epoxy resin compositions adapted for the production of improved fire-resistant glass-base laminates. The invention includes the epoxy resin compositions, electrical grade laminates prepared therewith, and the method of producing the laminates from B-stage prepregs.

Fire-resistant and self-extinguising reinforcing resinous laminates have long been known in the art.

Many of the prior art laminates have employed glass fiber or cloth, mica, or other inorganic reinforcing materials in conjunction with resinous impregnants as binding agents. In some instances, the resinous binder was one of the flame retardant chlorinated or brominated epoxy resins in conjunction with one or more curing agents. See by way of illustration, Harrington, U.S. 3,378,434; Chellis, U.S. 3,523,037; Lapitz, Jr., U.S. 3,600,263; and Fujiwara, U.S. 3,741 ,858, the disclosures of which are incorporated herein by reference. While such laminates are fire-resistant, they are usually deficient in some physical property, such as machinability, especially drilling performance. In particular, glass appears to be a major contributor to drill wear.

It has now been discovered that some of the abrasive glass can be replaced with hydrated alumina, a much softer material, with unexpectedly beneficial improvement in properties. It is believed that hydrated alumina serves as a "bridging agent" between secondary hydroxy groups on the macromolecular epoxy backbone and silanol groups on the glass (or silyl ether groups on silane-treated glass). This epoxy glass interaction is evidenced by improved "wetting" the glass fibers in drilled thrn holes made with the product, and by superior insulation resistance and volume resistance.The improved machinability of the laminates produced by the use of hydrated alumina in the epoxy to replace part of the glass is demonstrated by decreased drill wear, improved hole quality and increased productivity -- all of which are of essential importance to the printed circuit board industry.

Although the use of hydrated alumina in combination with epoxy resins has been previously known, there is no suggestion in such prior teachings that superior laminated products could be produced. For example, the Encyclopedia of Polymer Science and Techology, Volume 6, New York, John Wiley 8 Sons, pages 241 and 242, lists hydrated alumina as a typical commercial filler for epoxy resins, among other fillers; and commonly assigned Kessel et al, U.S. 2,997,527 discloses that disposing epoxy resin containing 20 to 70% by weight of hydrated alumina between two electrical conductors having a large potential difference provides an insulation with superior resistance to arc tracking.In neither of these publications, both of which are incorporated herein by reference, is there any suggestion that hydrated alumina will improve resin to glass wetting, inter-molecular bonding and permit the partial replacement of glass with hydrated alumina, in laminates. Hydrated alumina is also unique with respect to other conventional additives or fillers in the amount used herein because after forming laminates from epoxy resins containing hydrated alumina there is no reduction in translucency, and selfextinguishing properties show improvement in flame resistance and greatly decreased smoke levels and noxious fumes.

According to the present invention, in its broadest aspects, there are provided epoxy resin compositions for preparing prepregs adapted for consolidation under heat and pressure into insulating substrates for flame-retardant glass-reinforced printed circuit boards having improved drilling characteristics, said compositions comprising: (a) a flame-retardant amount of a chlorinated or brominated epoxy resin; (b) an effective amount of (i) a curing agent for (a), alone, or (ii) a curing agent for (a) in combination with a curing catalyst; (c) from about 10 to about 40 percent of hydrated alumina based on the weight of (a) and (c); and (d) a suitable solvent.

The invention also contemplates, as articles of manufacture, glass fiber sheets impregnated with a B-staged epoxy resin composition as above-defined.

Also among the principal features of this invention are insulating substrates for flame-retardant printed circuit boards comprising a plurality of the foregoing epoxy resin composition-saturated glass fiber sheets consolidated into a unitary structure under heat and pressure.

The epoxy resin component (a) will have at least a considerable proportion of halogen, e.g., chlorine or bromine, especially bromine, to impart flame-resistance. The halogenated epoxide can also he used with a halogenated curing agent component (b), e.g., dichloromaleic, chlorendic, tetrachlorophthalic anhydride, etc., but as will be shown later, this is not essential. It is also to be understood that non-halogenated (or standard) epoxy resins can be employed in the above compositions in desired proportions up to 1 00% by weight of the flame retardant halogenated epoxy herein.

It has been found that, in the resinous components of the system, from about 12% to about 50%, or more, by weight must consist of halogen in order to obtain the flame retardant benefits of the invention. Preferably, with bromine, the range will be from about 12 to about 19%. These amounts are conventional and well known.

The epoxy resins contemplated in the invention are those resulting from the reaction of at least one polyhydric phenol and at least one epihalohydrin in an alkaline medium.

Phenols which are suitable for use include those which contain at least two phenolic hydroxyl groups per molecule. Polynuclear phenols which have been found to be particularly suitable include those wherein the phenol nuclei are joined by carbon bridges, such as, for example, 2,2-bis(4hydroxyphenyl)propane (also known as bisphenol-A), 4,4'-dihydroxydiphenyl methane, tetra bis(hydoxyphenyl)ethane, mixtures of the foregoing, and the like.

While it is usual to employ epichlorohydrin as the epihalohydrin in the preparation of polymeric epoxides, analogs thereof as, for example, epibromohydrin may be substituted.

In the preparation of the highly halogenated epoxides of the invention, desirably epichlorohydrin is reacted with tetrachlorobisphenol-A or tetrabromobisphenol-A, etc. In this reaction the halogen content of the polymer may be regulated to produce the halogen contents above specified.

Illustratively, resinous polymeric epoxides (or glycidyl polyethers) suitable for use in accordance with the invention may be prepared by mixing and reacting from 1 to 2 mole proportions of epihalohydrin, preferably epichlorohydrin, with about 1 mol proportion of tetrabromobisphenol-A in the presence of at least a stoichiometric excess of alkali base on the amount of replaceable halogen present.

The polymeric epoxides may be prepared in either solid or liquid form. The commercially available glycidyl polyethers (epoxides) which are solids are less expensive than the liquid grades, thus the use of the solid material affords a substantial cost savings. The epoxy resin may, however, be used in the invention in either the solid or the liquid form. In any event, it is preferred that a solvent, such as toluene, or one of the ketones, or the like be present in an amount sufficient to prepare an impregnating varnish.

Preferably the solvent will comprise an alkylene glycol mono- or di-ester or -ether, an N-alkylformamide, a di(lower C,~C6) alkyl ketone, or a mixture of any of the foregoing. A specific such solvent comprises a mixture of ethylene glycol monomethyl ether, dimethyl formamide and acetone.

Conventional curing agents can be employed as component (b) in conventional amounts as anhydride curing agents such as the halogenated anhydrides mentioned above or phthalic anhydride or other acid polydrids can be used generally in an amount of 20~30% of the -.

weight of epoxy resin component (a). On the other#hand, amine type curing agents, e.g., diethylene triamine, triethanolamine, etc., in an amount of from 5 to 25% by weight can be used.

The preferred curing agent is a dicyan-diamide, alone, or combined further with a tertiary amine or tetralkyl guanidine catalyst. These are described in some of the above-mentioned patents and especially in Lopez et al., U.S. 3,391,113, the disclosure of which is incorporated herein by reference. Generally, the dicyandiamide is used in amounts of about 1 to about 20 parts, preferably from 1 to about 5 per 100 parts of resin component (a), and the tertiary amine or tretraalkylguanidine from about 0.01 to about 0.5 parts by weight per 100 parts by weight of epoxy resin component (a).

The hydrated alumina is a reinforcing filler whose function is to assist in improving machinability.

The action may be physical or chemical although its precise nature is unknown. The amount employed is between 10 and 40, based on the weight of (a) and (c), preferably from 25 to 30 percent. It is believed that the combined water in the hydrated alumina, Al2O3.3H20, serves to from bonds between the epoxy resin secondary hydroxyl groups and hydroxyl groups on the glass reinforcement. Unhydrated aluminum oxide is not effective in the present invention. A preferred commercially available hydrated alumina is Grade C--331, available from Aluminum Company of America.

Obviously, the compositions can include additional, conventional additives in conventional amounts. Chlorinated phenyls, aromatic phosphates, antimony trioxide, etc. can be used to minimize afterglow, to upgrade dielectric properties, and to smother flame, and the like.

In carrying out the invention, the selected glass fiber web (glass mat, glass paper, glass cloth, etc.) is passed although a varnish containing the resin/hydrated alumina impregnant. While any type of glass can be used, an electrical grade can be best selected and preferably the glass will be pretreated with a conventional coupling agent, e.g., a silane. 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 100 C. for two minutes; a dry zone temperature of about 1 6000. for about 4 minutes (web speed of about 100-200 inches).The resin varnish is usually about 50~70% resin and filler solids and 50~30% solvent. These conditions usually result in a resin composition in the treated glass web of about 40% to about 70% by weight. The impregnated, B-staged and dried prepregs are cut to the desired size and then stacked to obtain the desired thickness. Multilayer constructions are specifically contemplated, and metal foils, e.g., aluminum, copper and the like, can be applied to either or both major faces without, or with, a conventional adhesive prior to consolidation.

In consolidating the stack or stacks of prepregs, pressures of about 200-2000 psi and temperatures of about 13000. to 18500. 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 stacks pressed in each press opening. The stacked sheets are loaded in a cold press, consolidated under the above pressing conditions, and unloaded cooled to about 1 500 F.

The invention will be further particularized by the following examples. It is to be understood that the examples are given solely for the purposes of illustrations.

EXAMPLE 1 A resin varnish having the following composition is applied to silane-treated woven glass cloth in a 4-zone vertical treater. After being dried, the prepreg contains about 60% by weight of the impregnant.

The treated and dried prepregs are cut to size and stacked, six to the stack. Copper foil is placed on the outer faces of each stack. Eight stacks are consolidated in each opening. Pressing conditions are 1000 psi. pressure; temperature 1 750C., and pressing cycle 50 minutes. A standard FR-4 laminated containing no hydrated alumina is similarly prepared, but using seven prepreg sheets to each stack. The laminates are tested for physical, electrical and flammability properties. The compositions used and the results obtained are set forth in Table 1: TABLE 1.

Flame-Retardant Laminates from Brominated Epoxy Resin Containing Hydrated Aluminum Oxide Example Composition (parts by weight) 1 lA* Example Composition (parts by weight) 1 lA* Brominated Epoxy resin (19% Br)a 100 100 Epichlorohydrin-bisphenol-A resinb 50 50 Dicyandiamide 4 4 Benzyldimethylamine 0.3 0.3 Hydrated aluminac 46.5 0 Solvent (methylene glycol mono methl ether/dimethyl formamide/ acetone, to make 69% solids) 2.5 3.4 Properties Water absorption 0.09 0.09 Flexural strength, psi L 78,400 77,300 C 61,100 62,400 Flexural modulus, psi L 2,785,000 2,785,000 C 2,715,000 2,460,000 Izod impact, ft. Ibs/in. L 11.27 11.92 C 8.28 9.46 Rockwell Hardness 106 105 Ash 55.34 51.73 Flammability (UL-94) V-O V--O Max Burn time (UL-94, sec.) 1 7 TABLE 1 (Cont'dJ.

Flame-Retardant Laminates from Brominated Epoxy Resin Containing Hydrated Aluminum Oxide.

Example Dielectric Constant (A) 4.21 422 Dissipation Factor at 1 MHz 0.014 0.016 Dielectric Constant (D-24/23) 4.27 4.38 {Dissipation Factor at 1MHz 0.017 0.016 Breakdown S"T 80 76 S/S 70 58 Strength S"'T 603 603 Strength S/S 474 483 Insulation Resistance 10,100,000 3,200,000 Surface Resistance 1,300,000 1,100,000 Volume Resistance 40,400,000 3,200,000 Peel Strength 9.9 9.4 Example 1 1A* Peel After Solder 11.0 10.2 Seconds to Blister (5000) 60+ 60+ Arc Resistance 125 123 Seconds to Blister (5500 C.) 60+ 60+ Solvent Resistance OK OK Tug(+5) 125 126 % Expansion at 1900C. 3.06 3.14 Stress Normal Normal a Epoxide equivalent (solids) 1280-1360, bromine content 19%; 70% solids in acetone and methylene glycol monomethyl ether.

b Epoxide equivalent 180-190, 100% solids.

c Aluminum Company of America, Al203.3H2O, Type C331.

When drilled the 6 ply material exhibits substantially less drill wear than the seven ply control (1A). The quality of the hole produced in laminate (1) is much superior to that produced in (1A).

Additional resin varnishes can be prepared in which the types of additives are varied, with substantially the same improvements being provided. For example, the non-halogenated epoxy resin can be omitted. For example, instead of an epichlorohydrin bisphenol-A resin, a polyglycidyl ether of tetraphenylethane or an epoxylated novolac can be used. Instead of benzyldimethylamine, tetramethylguanidine can be used. Instead of glass cloth, glass mat or glass paper can be used. Instead of 30 percent by weight of hydrated aluminum based on epoxy and alumina, 10, 15,20,25,35 and 40 percent can be used.

The foregoing description will be appreciated by those skilled in the art as illustrative of a class of fire-retardant electrical grade-base laminates possessing machinability and other properties greatly improved over prior art laminates. The method of their preparation is simple and may be carried out in conventional apparatus. The novel class of laminates represents a distinct advance over previous glassbase electrical grade laminates and presents to those skilled in the art a means of producing fireresistant, easily machinable substrates for printed circuit use.

Claims (20)

1. An epoxy resin composition which comprises: (a) a flame-retardant amount of a chlorinated or brominated epoxy resin; (b) an effective amount of (i) a curing agent for (a), alone, or (ii) a curing agent for (á) in combination with a curing catalyst; and (c) from 10 to 40% by weight of hydrated alumina based on the weight of (a) and (c).

2. An epoxy resin composition as claimed in Claim 1 wherein component (a) comprises a difunctional reaction product of tetrabromobisphenol-A and epichlorohydrin.

3. An epoxy resin composition as claimed in Claim 1 or 2 which also includes (a) (i) up to an equal weight of a non-chlorinated or non-brominated epoxy resin, based on the weight of (a) alone.

4. An epoxy resin composition as claimed in Claim 3 wherein component (a) (i) comprises a difunctional reaction product of tetrabromobisphenol-A and epichlorohydrin in combination with a difunctional reaction product of bisphenol-A and epichlorohydrin, a tetrafunctional reaction product of tetrabis(hydroxyphenyl)ethane and epichlorohydrin, or a mixture of the foregoing.

5. An epoxy resin composition as claimed in any preceding Claim wherein component (b) (i) comprises dicyandiamide.

6. An epoxy resin composition as claimed in Claim 5 wherein component (b) (i) comprises from 1 to 5 parts by weight per 100 parts by weight of epoxy resin component (a).

7. An epoxy resin composition as claimed in any of Claims 1 to 4 wherein component (b) (ii) comprises dicyandiamide in combination with a tertiary amine or tetraalkylguanidine curing catalyst.

8. An epoxy resin composition as claimed in Claim 7 wherein, in component (b) (ii), the dicyandiamide comprises from 1 to 5 parts and the tertiary amine or tetraalkylguanidine comprises from 0.1 to 0.5 parts by weight, per 100 parts by weight of epoxy resin component (a).

9. An epoxy resin composition as claimed in any preceding Claim wherein said hydrated alumina component (c) is present in an amount of from 25 to 3Q% by weight, based on the weight of (a) and (c).

10. An epoxy resin composition as claimed in any preceding Claim which comprises (d) a solvent.

11. An epoxy resin composition as claimed in Claim 10 wherein said suitable solvent (d) comprises an alkylene glycol mono- or di- ester or -ether, an N-alkyl formamide, a di-(lower) alkyl ketone, or a mixture of any of the foregoing.

12. An epoxy resin composition as claimed in Claim 11 wherein said solvent comprises a mixture of ethylene glycol monomethyl ether, dimethylformamide and acetone.

13. An article of manufacture comprising a glass fiber sheet impregnated with a B-staged epoxy resin composition as claimed in any of the preceding Claims.

14. An article of manufacture as claimed in Claim 13 wherein said glass fiber sheet comprises glass cloth.

15. An insulating substrate for flame-retardant printed circuit boards comprising a plurality of epoxy resin composition-saturated glass fiber sheets as claimed in Claim 13 or 14 consolidated into a unitary structure under heat and pressure.

16. An insulating substrate as claimed in Claim 15 including a metal foil laminated on one or both major surfaces thereof.

17. An insulating substrate as claimed in Claim 16 wherein said metal is copper.

18. An epoxy resin composition as claimed in Claim 1 and substantially as hereinbefore described with reference to the Example.

19. An article of manufacture as claimed in Claim 13 and substantially as hereinbefore described with reference to the Example.

20. An insulating substrate as claimed in Claim 15 and substantially as hereinbefore described with reference to the Example.