Curable epoxy composition and its use in prepregs and core filling

A curable composition comprising:

• i) a glycidyl ether of a novolac, comprising moieties having the formula (I):
• wherein
• Ra is either always hydrogen or always methyl;
• B is either always *—CH2—** or always
• a fraction of 0.8 to 0.99 of the Y moieties are essentially —O-glycidyl, this fraction being designated as x, and the remainder of the Y moieties, this fraction being designated as (1-x), are divalent bridging spacers of the structure *—O—CH2—CH(OH)—CH2—O—** connecting two moieties according to above formula (I);
• and
• n is a number in the range of 0.1 to 3.0;
• and wherein said novolac glycidyl ether has an epoxy equivalent weight EEW in the range of 160 to 270 g/eq. and the average number of epoxy groups per molecule of novolac glycidyl ether (I), designated as f, is in the range of 2.1 to 5.0;
• ii) dicyandiamide;
• iii) an urea derivative of the formula (II):
• wherein
• R1a and R1b may be equal or different and are individually selected from the group of hydrogen, C1-C6-alkyl and C3-C6-cycloalkyl;
• or R1a and R1b taken together with the nitrogen atom to which they are attached form a heterocyclic substituent selected from the group consisting of aziridinyl, azetidinyl, pyrrolidinyl and piperidinyl;
• R2a and R2b are equal or different and are individually selected from the groups consisting of hydrogen and C1-C6-alkyl;
• R3a, R3b, and R4 are equal or different and are individually selected from the group consisting of hydrogen, halogen, C1-C6-alkyl, C1-C6-alkoxy, —C(O)R5, —N(H)—C(O)—NR1aR1b, —CN, NO2, trifluoromethyl, —SOR6 and SO2R6;
• wherein R5 is selected from the group consisting of hydrogen, C1-C6-alkyl, C1-C6-alkoxy, C3-C6-cycloalkyl and NR1aR1b, and R6 is selected from the group consisting of C1-C6-alkyl, C1-C6-alkoxy, and C3-C6-cycloalkyl;
• (iv) a powdered SiO2-based frit comprising 50-70% by weight of SiO2, Na2O and K2O, in combined amounts of 10 to 30% by weight;
• and Al2O3, B2O3, CaO, ZnO and fluorine in amounts of 0 to 5% by weight, 0 to 40% by weight, 0 to 10% by weight, 0 to 10% by weight, and 0 to 5% by weight, respectively;
• wherein all amounts are based on the frit, with the proviso that the amounts of the indicated components add to at least 95% by weight, of the frit;
• and the amount of the powdered SiO2-based frit is 10 to 30% by weight, based on the composition;
• and
• v) optionally, a reactive diluent being selected from the group consisting of glycidyl ethers of monohydric phenols, glycidyl ethers of cresols and glycidyl ethers of aliphatic or cycloaliphatic C4-8-alcohols.