Epoxy resin compositions that contain epoxy resin and curing agents for epoxy resin as essential components have excellent physical properties such as high heat resistance and moisture resistance and are thus widely used in electronic parts such as semiconductor encapsulating materials and printed circuit boards, conductive adhesives such as conductive paste, other adhesives, matrices for composite materials, paints, photoresist materials, and developer materials.
In recent years, these various usages and advanced material usages in particular require further improvements in performance such as heat resistance, moisture resistance, and solder resistance. In particular, on-vehicle electronic devices required to achieve particularly high reliability are now being installed more in engine rooms where the temperature is high than in cabins and the reflow temperature is increasing due to use of lead-free solder. Thus, a highly heat resistant material that has a higher glass transition point and is capable of withstanding a heat peeling resistance test (hereinafter referred to as “T288 test”) is desired.
When epoxy resin compositions are used as printed circuit board materials, halogen-based flame retardants such as bromine are blended together with antimony compounds in order to impart flame retardancy. However, due to recent environmental and safety concerns, an environmentally friendly and safety-oriented approach of achieving flame retardancy without using halogen-based flame retardants that would generate dioxin and antimony compounds that are possibly carcinogenic has been strongly demanded. In the field of printed circuit board materials, use of halogen-based flame retardants has become a factor that harms high temperature exposure reliability. Thus there is high expectation for halogen-free materials.
To satisfy the required properties, an epoxy resin composition having flame retardancy and heat resistance has been disclosed in PTL 1. According to this technology, a reaction product obtained by reacting 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (hereinafter simply referred to as “HCA”) and p-hydroxybenzaldehyde is reacted with phenol to obtain a phosphorus-atom-containing bisphenol and this phosphorus-atom-containing bisphenol is used as an epoxy resin raw material or an epoxy resin curing agent.
However, phosphorus-atom-containing bisphenols have very high crystallinity and little or no solvent solubility and thus they cannot be prepared into varnish for printed circuit board materials. Moreover, the flame retardancy of cured products made by using phosphorus-atom-containing bisphenols as an epoxy resin curing agent has been insufficient. Since the melting point of the phosphorus-atom-containing bisphenols is 200° C. or higher, they are extremely difficult to produce industrially.
NPL 1 below discloses a technology of obtaining an intermediate product by reacting HCA with p-hydroxybenzaldehyde and oligomerizing the intermediate product in THF.
However, according to the technology disclosed in NPL 1, the crystallinity of the reaction product from the intermediate HCA and p-hydroxybenzaldehyde is very high and the solvent solubility is poor. Thus, as described in NPL 1, THF which has a low flash point and is thus highly unsafe needs to be used in the subsequent reactions, which makes industrial production impossible. Moreover, the solvent solubility of the resulting oligomer itself is low and the oligomer is difficult to be prepared into varnish for printed circuit board materials.
PTL 2 below discloses a technology of producing a phosphorus-atom-containing phenol compound by reacting HCA with hydroxybenzaldehyde. However, the phenol compound described in PTL 2 is a monofunctional phenol compound and has very high crystallinity and low solvent solubility. When this compound is used as a curing agent for epoxy resin, sufficient flame retardancy is not obtained.