Epoxy Resin Mixtures Containing Phosphonic/Phosphinic Acid Anhydride Adducts

This invention concerns epoxy resin mixtures for producing prepregs and composites and it also concerns the prepregs and composites produced from said epoxy resin mixtures.

Composites based on epoxy resins and organic or inorganic reinforcing materials have gained an important position in many areas of the industry and daily life. The reasons include first the relatively simple and reliable processing of epoxy resins and secondly the good mechanical and chemical properties of cured epoxy resin molding materials that make it possible to adapt to a variety of applications and permit advantageous utilization of the properties of all the materials that make up the composite.

Epoxy resins are preferably processed to composites by the intermediate step of producing prepregs first. For this purpose, organic or inorganic reinforcing materials or embedding components in the form of fibers, nonwovens and woven fabrics as well as other flat materials are impregnated with the resin. In most cases, this is accomplished with a solution of the resin in a volatile solvent. The resulting prepregs according to this process must not be sticky but they should not be fully cured either. Instead, the resin matrix should simply be prepolymerized. In addition, the prepregs must have sufficient storage stability. Thus, for example, a storage stability of at least three months is required for the production of circuit boards. In further processing to form composites, the prepregs must also melt at higher temperatures and they must form a strong and permanent bond under pressure with both the reinforcing materials or embedding materials and the materials intended for the composite--in other words, the cross-linked epoxy resin material must yield a high interlaminas adhesion with the reinforcing materials or the embedding components and with the materials to be bonded, such as metallic, ceramic, mineral and organic materials.

In general, a high mechanical and thermal strength, good mechanical and thermal stability, thermal undeformability, and good aging resistance are required of composites when cured. For electrotechnical and electronic applications, there is also the requirement that the electric insulation properties must be permanently high, and for many special applications there are numerous additional requirements. For example, good thermal undeformability over a wide temperature range, good adhesion to glass and copper, a high surface resistance, a low dielectric loss factor, good machining properties (punchability, drillability), a low water uptake and high corrosion resistance are required if these materials are to be used as circuit board materials.

With increasing loads and extensive use of the composites, the thermal undeformability requirement becomes especially important. This means that the materials must withstand high temperatures during processing and use without showing any deformation or damage to the composite--due to delamination, for example. In dip soldering, for example, circuit boards are exposed to a temperature of 270.degree. C. Likewise, local temperatures of more than 200.degree. C. may occur briefly during cutting and drilling operations. Materials with a high glass transition temperature T.sub.G have good properties in this regard. If the glass transition temperature is higher than these temperatures, thermal undeformability is assured in the entire temperature range covered during processing, and damage such as warpage and delamination can be largely ruled out. The epoxy resin currently used on a large scale worldwide for FR4 composites has a glass transition temperature of only 130.degree. C. after curing. However, this results in the damage described above and thus leads to rejects in production. Therefore, there has long been a demand for inexpensive materials that can be processed relatively well and have a glass transition temperature higher than about 180.degree. C.

Another requirement that has become increasingly important in recent times is that these materials must be flame retardant. In many areas, this requirement is a top priority because of the danger to humans and property--for example, in construction materials for aircraft and motor vehicles and for public transportation. Flame retardancy of circuit board materials is an indispensable requirement for electrotechnical applications and especially for electronic applications because of the high value of the electronic components mounted on the circuit boards.

Therefore, in order to evaluate flammability, a material must pass one of the most severe material test standards, namely the V-0 classification according to UL 94V. In this test, a test article is exposed to a deformed flame vertically at the lower edge. The total burning times in 10 tests must not exceed 50 sec. This requirement is difficult to satisfy, especially when the material has thin walls, as is customary in electronics. The epoxy resin used industrially throughout the world for FR4 composites meets this requirement only because it contains approx. 30-40% (based on the resin) ring-brominated aromatic epoxy components--in other words, it contains approx. 17-21% bromine. Comparably high concentrations of halogen compounds are used for other applications and are often combined with antimony trioxide as a synergist. The problem with these compounds is that although they are excellent flame retardants, they also have some highly objectionable properties. For example, antimony trioxide is on the list of carcinogenic chemicals, and aromatic bromine compounds not only release highly corrosive free bromine radicals and hydrogen bromide when they undergo thermal decomposition, but especially when decomposition occurs in the presence of oxygen, highly brominated aromatics can also lead to polybromo-dibenzofurans and polybromo-dibenzodioxins (PBDs) which are extremely toxic. Furthermore, disposal of waste materials containing bromine also poses considerable problems.

Materials that meet the demand for a good thermal stability include, for example, molding materials based on bismaleinimid/triazine (BT) with a T.sub.G of approx. 200.degree. C. or polyimide (PI) with a T.sub.G of 260.degree.-270.degree. C. Recently, BT-epoxy blends with a T.sub.G of 180.degree. C. have also been, available, but laminates produced with these resin systems cannot be processed or machined as well as laminates based on epoxy resin. Thus, for example, production of laminates based on polyimide requires a compression molding temperature of approx. 230.degree. C. and a much longer post-curing time (approx. 8 hrs) at temperatures of 230.degree. C. Another serious disadvantage of these resin systems is that the price of materials is 6 to 10 times higher.

A comparatively inexpensive resin system is obtained when aromatic and/or heterocyclic polyepoxy resins, i.e., polyglycidyl compounds, are combined with aromatic polyamines acting as the hardener. Such polyamines, which are disclosed in German patent 2,743,680, for example, lead to cross-linked polymers that have an especially good thermal undeformability under heat and good resistance to aging. According to European patent 274,646, when 1,3,5-tris-(3-amino-4-alkylphenyl)-2,4,6-trioxo-hexahydrotriazines are used as hardeners, the resulting laminates have a glass transition temperature of up to 245.degree. C. and are characterized by good processing and machining properties.

Although the flammability properties of these resin systems may vary greatly, they all have the disadvantage that their flame retardancy is inherently inadequate. Thus, in order to pass the flammability test according to UL 94V with a classification of V-0, which is essential for many applications, it has not yet been possible to do without the use of the highly effective brominated flame retardants. First, this results in the potential hazards associated with bromine compounds, and secondly, there are some unavoidable negative effects on the thermal and mechanical properties due to the use of bromine compounds.

For these reasons, there has been no lack of attempts to replace brominated flame retardants with less problematical substances. For example, fillers with a quenching gas effect have been proposed, such as aluminum oxide hydrates (see: J. Fire and Flammability, vol. 3 (1972), pages 51 ff.), basic aluminum carbonates (see: Plast. Engng., vol. 32 (1976), pages 41 ff.) and magnesium hydroxides (European patent 243,201) as well as vitrifying fillers such as borates (see: Modern Plastics, vol. 47 (1970), no. 6, pages 140 ff.) and phosphates (U.S. Pat. Nos. 2,766,139 and 3,398,019). However, all these fillers have the disadvantage that they often have an extremely negative effect on the mechanical, chemical and electrical properties of the composites. In addition, they require special processing techniques which are usually more expensive because they increase the viscosity of the filled resin system and they also have a tendency toward sedimentation.

The flame-retardant effect of red phosphorus has also been described previously (British-patent 1,112,139), optionally in combination with very finely divided silicon dioxide or aluminum oxide hydrate (U.S. Pat. No. 3,373,135). This yields materials that restrict the possible uses for electrotechnical and electronic applications because of the phosphoric acid that is released in the presence of moisture and the resulting corrosion problems. In addition, organic phosphorus compounds such as phosphoric acid esters, phosphonic acid esters and phosphines have already been proposed as flame-retardant additives (see: W. C. Kuryla and A. J. Papa Flame Retardancy of Polymeric Materials, vol. 1, pages 24-38 and 52-61, Marcel Dekker Inc., New York, 1973). Since these compounds are known for their "plasticizing" properties and are in fact used internationally as plasticizers for polymers on a large scale (British patent 10,794), this alternative is not very promising either.

Organic phosphorus compounds such as phosphorus compounds that contain epoxy groups and can be anchored in the epoxy resin network can also be used to make epoxy resins flame-retardant. Thus, for example, European patent 384,940 discloses epoxy resin blends that contain commercially available epoxy resin, the above-mentioned aromatic polyamine 1,3,5-tris-(3-amino-4-alkylphenyl)-2,4,6-trioxo-hexahydrotriazine and a phosphorus compound that contains an epoxy group and is based on glycidyl phosphate, glycidyl phosphonate or glycidyl phosphinate. Without adding halogen, flame-retardant laminates or composites that can be classified in the UL 94 V-0 class and have a glass transition temperature of >200.degree. C. can be produced with such epoxy resin blends. In addition, these epoxy resin blends can be processed by methods comparable to those used with the current epoxy resins.

It is general knowledge that the interlaminar adhesion and the adhesion to copper of laminates with a high glass transition temperature, such as those based on polyimide or BT resins, is lower than that of the halogenated FR4 laminates used predominantly today. This is also true of the laminates described in European patent 384,940. Many of the circuit boards produced today are so-called multi-layer circuit boards (ML) that have several conducting layers that are kept at a distance from each other and are insulated by epoxy resin compounds. However, the trend in ML technology has been toward an ever increasing number of conducting layers. For example, today MLs with more than 20 conducting levels are being produced. An excessive thickness of the ML must be avoided for technical reasons, so the distance between the conducting levels becomes smaller and smaller and thus the interlaminar adhesion and the adhesion to copper become increasingly problematical with ML laminates that have a high glass transition temperature.

In circuit board technology, interlaminar adhesion is usually determined indirectly. Circuit boards must pass a measling test that is widely used for this purpose. In this test, a laminate without the copper lamination is treated with a tin chloride solution and then with water at a high temperature and then is immersed for 20 sec in a hot soldering bath at 260.degree. C. The laminate is then inspected visually for delamination. With regard to lamination resins with a high glass transition temperature (180.degree. C. or higher), most of the increasingly thin core components used in ML technology today will not pass this test because their interlaminar adhesion is inadequate for such thin laminates. Other problems caused by inadequate interlaminar adhesion are encountered in further processing of laminates for the electronics industry by such methods as drilling and milling. Therefore, the drilling and milling speeds must be reduced in comparison with those used with FR4 material.

Therefore, there has been a great demand for electronics laminates that first of all meet the required level of flame retardancy without using halogen, as indicated above, and secondly have a high glass transition temperature with good interlaminar adhesion at the same time--even when extremely thin core components are produced. No satisfactory combination of these properties has previously been achieved, especially not for extremely thin laminates such as those use in ML technology.

The object of this invention is to provide epoxy resin blends that are available by simple methods industrially and are thus inexpensive, that can be processed by methods comparable to those used with the conventional industrial epoxy resins and that are suitable for production of prepregs and laminates for the multi-layer technology. These new epoxy resin blends should also yield molding materials that are flame-retardant (in other words, classified according to UL 94 V-0) without the use of any halogen and they must have the highest possible glass transition temperature .gtoreq.180.degree. C.) while also having an improved interlaminar adhesion and adhesion to copper that are adequate for producing ML cores.

This is accomplished according to this invention by having the epoxy resin blends contain the following components:

a phosphorus-modified epoxy resin with an epoxy value of 0.02 to 1 mol/100 g, composed of structural units derived from

(A) polyepoxy compounds having at least two epoxy groups per molecule and

(B) phosphinic acid and/or phosphonic acid anhydrides, and

an aromatic polyamine as a hardener having the following structure: ##STR1## where one of the R.sup.1 and R.sup.2 groups on each of the three aromatic partial structures denotes a hydrogen and the other is an alkyl.

The phosphorus-modified epoxy resins are synthesized by reacting conventional commercial polyepoxy resins (polyglycidyl resins) with the following phosphorus compounds:

phosphinic acid anhydrides: anhydrides of phosphinic acids with alkyl, alkenyl, cycloalkyl, aryl or aralkyl groups;

bis-phosphinic acid anhydrides: anhydrides of bis-phosphinic acids, especially alkane-bis-phosphinic acids with 1 to 10 carbons in the alkane group;

phosphonic acid anhydrides: anhydrides of phosphonic acids with alkyl, alkenyl, cycloalkyl, aryl or aralkyl groups.

Phosphorus-modified epoxy resins of the type defined above as well as a process for synthesis, thereof are the object of the international patent application (application no. PCT/EP94/00748 "Phosphorus-modified epoxy resins, process for producing same and use thereof" (GR 93 P 8506 P)) submitted concurrently.

Phosphorus-modified epoxy resins can also be synthesized by reacting polyepoxy compounds--instead of phosphonic acid anhydrides--with phosphonic acid monoesters and splitting off the alcohol from the resulting products at temperatures of at least 80.degree. C.

In general, both aliphatic and aromatic glycidyl compounds as well as the mixtures thereof can be used to synthesize phosphorus-modified epoxy resins. Preferred glycidyl compounds include bisphenol A diglycidyl ether, bisphenol F diglycidyl ether, polyglycidyl ethers of phenol-formaldehyde novolacs and of cresol-formaldehyde novolacs, diglycidyl esters of phthalic acid, tetrahydrophthalic acid, isophthalic acid and terephthalic acid, as well as blends of these epoxy resins. Other polyepoxies that can be used include, for example, hydrogenated bisphenol A and bisphenol F diglycidyl ether, hydantoin-epoxy resins, triglycidyl isocyanurate, triglycidyl-p-aminophenol, tetraglycidyl-diaminodiphenylmethane, tetraglycidyl-diaminodiphenyl ether, tetrakis-(4-glycidoxyphenyl)-ethane, uracil-epoxy resins, oxazolidinone-modified epoxy resins and epoxies described in the Handbook of Epoxy Resins by Henry Lee and Kris Neville, McGraw-Hill Book Company 1967, and in the monograph Epoxy Resins by Henry Lee, American Chemical Society, 1970. The average molecular weight M.sub.n of the polyepoxy compounds is in general 150 to 4000, preferably 300 to 1800.

Depending on the choice of the starting molecular ratio of the phosphorus compound to the epoxy resin, it is possible to produce phosphorus-modified epoxy resins with a varying epoxy content and also with a varying phosphorus content. Phosphorus-modified epoxy resins with an average of one epoxy group per molecule, but preferably with two epoxy groups, are used to produce the laminates. In general, the starting molar ratio is selected so the reaction products will contain 0.5 to 13 wt % phosphorus. The phosphorus content is preferably 1 to 8%, especially 2 to 5%. The total phosphorus content of the epoxy resin mixture should be 0.5 to 6 wt %, preferably 1 to 4 wt %.

The epoxy resin mixtures according to this invention may also contain a phosphorus-free aromatic and/or heterocyclic epoxy resin. A cycloaliphatic epoxy resin may also be added to this epoxy resin. In general, up to 80 wt % phosphorus-modified epoxy resin may be replaced by phosphorus-free epoxy resin.

Adding the phosphorus-free epoxy resin serves to achieve certain properties in the laminates produced from the epoxy resin mixtures according to this invention. The mixing ratio of the two components is determined by the flame-retardancy requirement according to UL 94 V-0 for a layer thickness of 1.6 mm. This means that the phosphorus-free component can be added only to the extent that the total mixture still contains enough phosphorus that the above requirement is met. Therefore, in the case of epoxy resins with a high phosphorus content, it is possible to add more of the phosphorus-free epoxy resin than in the case of epoxy resins with a low phosphorus content.

The following polyglycidyl compounds are especially suitable as the additional phosphorus-free polyepoxy resin or to produce the phosphorus-modified epoxy resins: aromatic polyglycidyl ethers such as bisphenol A diglycidyl ether, bisphenol F diglycidyl ether and bisphenol S diglycidyl ether, polyglycidyl ethers of phenol-formaldehyde and cresol-formaldehyde resins, resorcinol diglycidyl ether, tetrakis-(p-glycidylphenyl)-ethane, di- and polyglycidyl esters of phthalic acid, isophthalic acid and terephthalic acid, as well as trimellitic acid, N-glycidyl compounds of aromatic amines and heterocyclic nitrogen-containing bases such as N,N-diglycidylaniline, N,N,O-triglycidyl-p-aminophenol, triglycidyl isocyanurate and N,N,N',N'-tetraglycidyl-bis-(p-aminophenyl)-methane, hydantoin-epoxy resins and uracil-epoxy resins as well as di- and polyglycidyl compounds of polyhydric aliphatic alcohols such as 1,4-butanediol, trimethylolpropane and polyalkylene glycols. In addition, oxazolidinone-modified epoxy resins are also suitable. Such compounds are already known (see: Angew. Makromol. Chem., vol. 44 (1975), pages 151-163, and U.S. Pat. No. 3,334,110). The reaction product of bisphenol-A diglycidyl ether with diphenylmethane diisocyanate (in the presence of a suitable accelerator) can be mentioned as an example. The polyepoxy resins may be used individually or in the form of a mixture to produce the phosphorus-modified epoxy resin. An epoxidized novolac is the preferred type of polyepoxy resin.

The following compounds in particular are used as the phosphorus component to produce the phosphorus-modified epoxy resins:

Phosphinic acid anhydrides: dimethyl phosphinic acid anhydride, methylethyl phosphinic acid anhydride, diethyl phosphinic acid anhydride, dipropyl phosphinic acid anhydride, ethylphenyl phosphinic acid anhydride and diphenyl phosphinic acid anhydride;

alkane-bis-phosphinic acid arthydrides: methane-1, 1-bis-methyl-phosphinic acid anhydride, ethane-1,2-bis-methyl phosphinic acid anhydride, ethane-1,2-bis-phenyl phosphinic acid anhydride and butane-1,4-bis-methyl phosphinic acid anhydride;

phosphonic acid anhydrides: methanephosphonic acid anhydride, ethanephosphonic acid anhydride, propanephosphonic acid anhydride, hexanephosphonic acid anhydride and benzenephosphonic acid anhydride.

The phosphorus-modified epoxy resins can be produced by reacting the polyepoxy compounds with phosphinic acid or phosphonic acid anhydrides, preferably in an inert solvent or diluent, but also in substance, assuming the reaction process is suitably modified. The phosphorus-modified epoxy resins have an average molecular weight M.sub.n of up to 10,000, preferably 200 to 5000, and especially 400 to 2000.

Some of the aromatic polyamines that serve as hardeners in the epoxy resin mixtures according to this invention are already known. Polyamines having the stated structure where R.sup.1 =alkyl and R.sup.2 =H are described in European patent 274,646. They are synthesized by trimerization of 2,4-diisocyanato-alkylbenzenes and subsequent hydrolysis of the remaining isocyanate groups. Compounds where R.sup.1 =H and R.sup.2 =alkyl are similarly obtained by trimerization of 2,6-diisocyanato-alkylbenzenes and subsequent hydrolysis. Both polyamines of the two types mentioned above as well as mixtures of these two compounds can be used as hardeners in the epoxy resin mixtures according to this invention. In addition, polyamines that are obtained by trimerization of mixtures of 2,4-and 2,6-diisocyanato-alkylbenzenes and subsequent hydrolysis of the trimers can also be used. Such mixtures are available on a large scale commercially and make it possible to produce the hardener component inexpensively.

In hydrolysis of the trimerization products that contain isocyanate groups, a reaction may also take place between the isocyanate groups and the amino groups. Heterocyclic polyamines with urea groups are obtained as a by-product of the hydrolysis reaction. Such polyamines can also be used as additive hardener components in the epoxy resin mixtures according to this invention--in other words, they may be used in mixture with the actual hardener. In addition to containing the actual hardener or hardener mixtures of the type described above, the epoxy resin mixtures according to this invention may also contain aromatic polyamines of another type such as 4,4'-diaminodiphenylmethane and 4,4'-diaminodiphenylsulfone and/or other heterocyclic polyamines. The maximum amount of such polyamines in the hardener mixture is generally 30 wt %.

The equivalent ratio of the starting epoxy function to the starting amine hydrogen function in the epoxy resin mixtures according to this invention may vary between 1:0.5 and 1:1.1, preferably 1:0.7 to 1:0.9.

The epoxy resin mixtures according to this invention may also contain accelerators, which are known to play an important role in curing epoxy resins. Tertiary amines or imidazoles are generally used. Suitable amines include, for example, tetramethylethylenediamine, dimethyloctylamine, dimethylaminoethanol, dimethylbenzylamine, 2,4,6-tris-(dimethylaminomethyl)-phenol, N,N'-tetramethyl-diaminodiphenylmethane, N,N'- dimethylpiperazine, N-methylmorpholine, N-methylpiperidine, N-ethylpyrrolidine, 1,4-diazabicyclo(2,2,2)-octane and quinolines. Suitable imidazoles include, for example, 1-methylimidazole, 2-methylimidazole, 1,2-dimethylimidazole, 1,2,4,5-tetramethylimidazole, 2-ethyl-4-methylimidazole, 1-cyanoethyl-2-phenylimidazole and 1-(4,6-diamino-s-triazinyl-2-ethyl)-2-phenylimidazole. The accelerators are added in a concentration of 0.01 to 2 wt %, preferably 0.05 to 1%, each based on the epoxy resin mixture.

To produce the prepregs, the individual components are dissolved separately or together in inexpensive solvents such as acetone, methylethyl ketone, ethyl acetate, methoxyethanol, dimethylformamide and toluene or in a mixture of such solvents, then they are optionally combined to form one solution and are processed on conventional impregnation systems for impregnating fibers of organic or inorganic materials such as glass, metal, minerals, carbon, aramid, polyphenylene sulfide and cellulose as well as wovens or nonwovens produced from these materials or for coating other flat materials such as metal or plastic sheets. The impregnating solutions may optionally also contain other halogen-free additives to improve the flame retardancy, some of which may be in the form of a homogeneous solution or dispersion. Such additives include, for example, melamine cyanurates, melamine phosphates, pulverized. polyether imide, polyether sulfone and polyimide.

Mainly glass cloth is used to produce prepregs for circuit board technology. Especially prepregs made of glass cloth grades having a weight of 25 to 200 g/m.sup.2 are used for multilayer circuit boards. Prepregs with a lower weight may also be produced as required using impregnating solutions of the type described above. The impregnated or coated reinforcing materials or embedding components are dried at an elevated temperature, which removes the solvent while also inducing prepolymerization of the impregnating resin. On the whole, this yields an extraordinarily good cost/benefit ratio.

The resulting coatings and prepregs are not sticky and they are stable when stored at room temperature for a period of three months or more--in other words, their storage stability is adequate. They can be pressed at temperatures up to 220.degree. C. to form laminates that are characterized by a high glass transition temperature of .gtoreq.180.degree. C. and an inherent flame retardancy. When glass cloth is used in the amount of 60-62 wt %, based on the laminate, the material will pass the bum test according to UL 94 V with a definite V-0 classification without the addition of halogen compounds or any other flame-retardant additives, even when the test object has a wall thickness of 1.6 mm or even 0.8 mm. It has proven to be especially advantageous that no corrosive or highly toxic by-products are formed and the amount of smoke released is greatly reduced in comparison with other polymer materials, especially brominated epoxy resin molding materials.

The cured laminates are also characterized by a low thermal expansion coefficient that is constant over a wide temperature range and by a high chemical resistance and corrosion resistance, a low water absorbency :and very good electric properties. Adhesion to the reinforcing materials and to the sandwich layers is excellent. When using reinforcing materials of the aforementioned type, prepregs for construction materials that are exposed to high stresses can be produced. These construction materials are suitable for applications in mechanical engineering, automotive engineering, aeronautical engineering and electronic engineering--for example, in the form of prepregs for producing circuit boards, especially multi-layer circuits.

Special advantages of using this material for circuit boards include the high adhesion strength to printed conductors made of copper, a high delamination strength and excellent machining properties, as manifested by the fact that satisfactory holes can be produced with little wear on the drill bits when drilling for plated-through holes. Materials produced using the epoxy resin mixtures according to this invention, especially multi-layer circuit boards with thin cores of .ltoreq.100 .mu.m, can be produced more reliably and less expensively.

This invention will now be illustrated in detail on the basis of the following embodiments.

Mix a solution of A parts by weight (WT) of a reaction product (epoxy value 0.34 mol/100 g) of an epoxidized novolac (epoxy value 0.56 mol/100 g, average functionality 3.6) and propanephosphonic acid anhydride in G parts by weight methylethyl ketone and I parts by weight ethyl acetate with a solution of E parts by weight of a polyamine prepared by trimerization of a 4:1 mixture of toluene 2,4-diisocyanate and toluene 2,6-diisocyanate and subsequent hydrolysis (yielding a product with an NH.sub.2 value of 9.35%) in G parts by weight methylethyl ketone and H parts by weight dimethylformamide. The resulting solution is next used to impregnate glass cloth (weight 106 g/m.sup.2) continuously on a laboratory impregnation system and dried in a vertical drying system at temperatures of 50.degree. C. to 160.degree. C. Prepregs produced in this way are non-sticky and are stable when stored at room temperature (max. 21.degree. C. and max. 50% rel. humidity). The composition of the impregnating resin solution and the properties of the prepregs are summarized in Table 1.

Proceed as described in Example 1, but mix the impregnating resin solution with F parts by weight 2-methylimidazole. The composition of the impregnating resin solution and the properties of the prepregs are summarized in Table 1.

Glass cloth (weight 48 g/m.sup.2) suitable for use in thin core components was impregnated with the impregnating resin solution in addition to the glass cloth with a weight of 106 g/m.sup.2. The properties of the resulting prepregs are also summarized in Table 1.

Prepare a solution of B parts by weight of a reaction product (epoxy value 0.30 mol/100 g) of an epoxidized novolac (epoxy value 0.50 mol/100 g, average functionality 3.8) and propanephosphonic acid anhydride in G parts by weight methylethyl ketone and I parts by weight ethyl acetate and mix with J parts by weight of bisphenol-F-diglyceride ether and with a solution of E parts by weight of a polyamine prepared according to Example 1 (NH.sub.2 value 9.35%) in G parts by weight methylethyl ketone and H parts by weight dimethylformamide. Add F parts by weight 2-methylimidazole to the resulting solution and then process to form prepregs as described in Example 1. After drying, these prepregs are non-sticky and are stable for more than three months when stored at room temperature. Table 1 shows the composition of the impregnating resin solution and the properties of the prepregs.

Prepare a solution of A parts by weight of a reaction product (epoxy value 0.34 mol/100 g, average functionality 3.6) from an epoxidized novolac (epoxy value 0.56 mol/100 g, average functionality 3.6) and propanephosphonic acid anhydride in G parts by weight methylethyl ketone and I parts by weight ethyl acetate and then add a 50% solution of E pans by weight of a polyamine prepared according to Example 1 (NH.sub.2 value 9.35%) in methylethyl ketone/dimethylformamide (2:1) and C parts by weight of an oxazolidine-modified epoxy resin (epoxy value 0.21 mol/100 g). Mix the resulting solution with F parts by weight 2-methylimidazole and then process to prepregs as described in Example 1. The prepregs are non-sticky after drying and are stable for three months when stored at room temperature. The composition of the impregnating resin solution and the properties of the prepregs are summarized in Table 1.

Prepare a solution of A pans by weight of a reaction product (epoxy value 0.34 mol/100 g) of an epoxidized novolac (epoxy value 0.56 mol/100 g, average functionality 3.6) and propanephosphonic acid anhydride in G parts by weight methylethyl ketone and I parts by weight ethyl acetate and D parts by weight of an epoxy resin oligomer based on bisphenol A diglycidyl ether (epoxy value 0.25 mol/100 g) and then add a 50% solution of E parts by weight of a polyamine (NH.sub.2 value 9.35%) prepared according to Example 1 in methylethyl ketone/dimethylformamide (2:1). Mix the resulting solution with F parts by weight 2-methylimidazole and then process to prepregs as described in Example 1. The prepregs are non-sticky after drying and are stable when stored for more than three months at room temperature. Table 1 shows the composition of the impregnating resin solution and the properties of the prepregs.

Thirteen Prepregs produced according to Examples 1 through 5 (glass cloth grade 2116, weight 106 g/m.sup.2) were pressed at 175.degree. C., 65 bar. After 40 minutes, the 1.5 to 1.6 mm thick laminates were removed from the press and then after-baked for 2 hours at 200.degree. C. The objects thus obtained were tested by dynamic-mechanical analysis (DMTA) for glass transition temperature (T.sub.G) and flame retardance according to UL 94 V. The results are summarized in Table 2.

Press prepregs produced according to Examples 1 through 5 (glass cloth grade 2116, weight 106 g/m.sup.2) to form laminates composed of two layers of prepregs laminated on each side to a 35 .mu.m film of copper (pressing parameters: 175.degree. C., 60 to 65 bar, 40 min) and then after-bake for 2 hours at 200.degree. C. The measling test, the adhesion of the copper film, the solder bath stability and interlaminar adhesion are determined on the 0.30 to 0.33 mm thick laminates. The results are summarized in Table 3.

The tests were performed on the laminates as follows:

Thermal stability in the solder bath:

Perform this test according to DIN IEC 249 part 1, section 3.7 using a solder bath according to section 3.7.2.3. Use test objects measuring 25 mm.times.100 mm and place them with the copper side against the solder bath. There must not be any delamination or measling, spots or bubbles under the lamination.

Adhesion of the copper laminate

Separate a copper fill measuring 25 mm.times.100 mm from the glass cloth over a 20 mm length and pull away the remaining glass cloth at the rate of 50 mm/min straight up using a suitable device. Measure the force F required to do so in N.

Testing interlaminar adhesion Separate a 20 mm length of the top layer of hard glass cloth from the next underlying layer of hard glass cloth on a strip measuring 25 mm.times.100 mm and pull away at the rate of 50 mm/min using a suitable device. Measure the force F required to do so in N.

Measling test

Perform this test on test objects measuring 20 mm.times.100 mm without any copper lamination. Immerse the test objects for 3 minutes in a hot LT26 solution (composition: 850 ml deionized H.sub.2 O, 50 ml HCl, analytical purity, 100 g SnCl.sub.2.2H.sub.2 O, 50 g thiourea) at 65.degree. C., rinse with running water and then place in boiling water for 20 minutes. After drying the object in air (2 to 3 min), immerse it in a solder bath at 260.degree. C. for 10 sec. The laminate must not show any signs of delamination.

Press prepregs (glass fiber grade 1080, weight 48 g/m.sup.2) produced according to Example 2 to form a laminate composed of two layers of prepregs and laminated on both sides with a 35 .mu.m copper film. The adhesion of the copper film to the 0.19 to 0.21 mm thick laminate was determined (at room temperature) and found to be 1.7 N/mm, and the interlaminar adhesion was found to be 1.09 N/mm. This laminate passed the measling test and had good solder bath stability.

Prepare a solution of A parts by weight of a phosphorus-modified epoxy resin (epoxy value 0.36 mol/100 g, phosphorus content 3.53%) obtained by reacting an epoxidized novolac (epoxy value 0.56 mol/100 g, average functionality 3.6) with monomethyl methanephosphonate and then splitting off the methanol under heat, in G parts by weight methyl ethyl ketone and I parts by weight ethyl acetate and mix with a solution of E parts by weight of a polyamine according to Example 1 (NH.sub.2 value 9.35%) in G parts by weight methyl ethyl ketone and H parts by weight dimethylformamide. Then add F parts by weight 2-methylimidazole. Use the resulting solution to continuously impregnate glass cloth (weight 197 g/m.sup.2) on a laboratory impregnating system and dry in a vertical drying installation at temperatures of 50.degree. C. to 160.degree. C. Prepregs produced in this way are nonsticky and are stable when stored at room temperature (max. 21 .degree. C. and max. 50% rel. humidity). Table 4 shows the composition of the impregnating resin solution and the properties of the prepregs.

Press eight samples of prepregs produced in this way (glass cloth grade 7628, weight 197 g/m.sup.2) in a press at 175.degree. C. and 65 bar. Remove the 1.5 to 1.6 mm thick laminates from the press after 40 minutes and then after-bake for 2 hours at 200.degree. C. Use dynamic mechanical analysis (DMTA) to determine the glass transition temperature T.sub.G and test the flammability according to UL 94 V on the objects produced in this way. Values obtained in this way are summarized in Table 5.

Prepare a solution of B parts by weight of a phosphorus-modified epoxy resin (epoxy value 0.35 mol/100 g, phosphorus content 3.57%) obtained by reacting an epoxidized novolac (epoxy value 0.56 mol/100 g, average functionality 3.6) with monomethyl propanephosphonate and then splitting off the methanol by heating, in G parts by weight methylethyl ketone and I parts by weight ethyl acetate, then mix this solution with a solution of E parts by weight of a polyamine according to Example 1 (NH.sub.2 value 9.35%) in G parts by weight methylethyl ketone and H parts by weight dimethylformamide. Then also add F parts by weight 2-methylimidazole. Use the resulting solution to produce prepregs as described in Example 9. These prepregs are nonsticky after drying and are stable when stored at room temperature. Table 4 summarizes the composition of the impregnating resin solution and the properties of the prepregs.

Eight samples of each type of prepreg produced were processed as described in Example 9 to form laminates which were then tested. The values measured on the baked laminates are shown in Table 5.