Polyurethane-modified epoxy resin, method for producing same, epoxy resin composition and cured product

1. A polyurethane-modified epoxy resin, which is obtained by modifying a bisphenol-based epoxy resin (a) comprising a diglycidyl ether of bisphenol A or bisphenol F having an epoxy equivalent of from 150 g/eq to 200 g/eq and a hydroxyl equivalent of from 2,000 g/eq to 2,600 g/eq with a medium to high molecular weight polyol compound (b) having a number-average molecular weight of 200 or more, a polyisocyanate compound (c), and a low molecular weight polyol compound (d) having a number-average molecular weight of less than 200 serving as a chain extender,

the polyurethane-modified epoxy resin comprising a polyurethane having functional groups derived from the epoxy resin (a) covalently bonded to each of both terminals thereof and/or one terminal thereof, the polyurethane being obtained by:

producing a urethane prepolymer (P) by causing the medium to high molecular weight polyol compound (b) and the polyisocyanate (c) to react with each other in a presence of the epoxy resin (a), wherein the epoxy resin (a) is reacted in an amount of from 20 wt % to 40 wt % with respect to a total amount of (a), (b), (c), and (d), and the components (b) and (c) are reacted in an amount such that a molar ratio between primary OH groups of the component (b) and NCO groups of the component (c) falls within a range of from 1:1.5 to 1.3; and

then subjecting the urethane prepolymer (P) to a polyurethanation reaction by adding the low molecular weight polyol compound (d) so that a molar ratio between NCO groups of the urethane prepolymer (P) and OH groups of the low molecular weight polyol compound (d) falls within a range of from 0.9:1 to 1:0.9.

2. A polyurethane-modified epoxy resin according to claim 1 , wherein the medium to high molecular weight polyol compound (b) comprises a compound represented by any one of the following formulae (2), (4) to (7), (10), and (11), the polyisocyanate compound (c) comprises a compound represented by the following formula (12), and the low molecular weight polyol compound (d) comprises a compound represented by the following formula (13):

where R 2 represents H or a methyl group, b1, b2, and b3 each independently represent a number of from 1 to 50, and c represents a number of 0 or 1;

where f's each independently represent a number of from 1 to 20, and g represents a number of from 1 to 50;

where h1 and h2 each independently represent a number of from 1 to 20, and i represents a number of from 1 to 50;

where j1, j2, and j3 each independently represent a number of from 1 to 20, and k1 and k2 each independently represent a number of from 1 to 50;

where l1, l2, l3, l4, and l5 each independently represent a number of from 1 to 20, and m1 and m2 each independently represent a number of from 1 to 50;

where q1, q2, q3, and q4 each independently represent a number of from 1 to 20;

where r, s, and t each independently represent a number of from 1 to 20, and n represents a number of from 1 to 50;

in the formula 12, R 4 represents a divalent group selected from the formulae 12a to 12f;

in the formula 13, R 5 represents an alkylene group represented by the formula 13a and g represents a number of from 1 to 10.

3. A polyurethane-modified epoxy resin according to claim 1 or 2 , wherein the epoxy resin (a) comprises a bisphenol A-type epoxy resin represented by the following formula (14) or a bisphenol F-type epoxy resin represented by the following formula (15), the medium to high molecular weight polyol compound (b) comprises a polypropylene glycol represented by the following formula (16), the low molecular weight polyol compound (d) comprises 1,4-butanediol represented by the following formula (17), and the polyisocyanate compound (c) comprises 4,4′-diphenylmethane diisocyanate represented by the following formula (18):

where a1 represents a number of from 0.148 to 0.198 as average value;

where a2 represents a number of from 0.134 to 0.149 as average value;

where b1 and b2 each independently represent a number of from 1 to 50.

4. A method of producing the polyurethane-modified epoxy resin of claim 1 , comprising:

producing the urethane prepolymer (P) by causing the medium to high molecular weight polyol compound (b) and the polyisocyanate compound (c) to react with each other in a presence of the epoxy resin (a), wherein the epoxy resin (a) is reacted in an amount of from 20 wt % to 40 wt % with respect to a total amount of the components (a), (b), (c), and (d), and the components (b) and (c) are reacted in an amount such that a molar ratio between primary OH groups of the component (b) and NCO groups of the component (c) falls within a range of from 1:1.5 to 1:3; and

then subjecting the urethane prepolymer (P) to a polyurethanation reaction by adding the low molecular weight polyol compound (d) so that a molar ratio between NCO groups of the urethane prepolymer (P) and OH groups of the low molecular weight polyol compound (d) falls within a range of from 0.9:1 to 1:0.9.

5. A method of producing the polyurethane-modified epoxy resin according to claim 4 , wherein the bisphenol-based epoxy resin (a) comprises a bisphenol A-type epoxy resin.

6. An epoxy resin composition, which is obtained by blending the polyurethane-modified epoxy resin of claim 1 with a polyurethane-unmodified epoxy resin (e), a curing agent (f), and a curing accelerator (g), wherein the composition has a weight concentration of polyurethane constituent components of from 10 wt % to 30 wt %.

7. An epoxy resin cured product, which is obtained by curing the epoxy resin composition of claim 6 .

8. A polyurethane-modified epoxy resin according to claim 1 , wherein the bisphenol-based epoxy resin (a) comprises a bisphenol A-type epoxy resin.