Curing agents for cationically curable compositions

Abstract

Novel catalyst systems in cationically curable compositions provide colorless cationically cured compositions with improved degree of cure and thermal stability. The curable composition comprises at least one cationically curable monomer and a curing agent comprising Photocurative A or Photocurative B. Photocurative A comprises an effective amount of an accelerating photochemically active salt comprising a photochemically active cation and an accelerating anion, and an effective amount of an inhibiting non-photochemically active salt comprising a non-photochemically active cation and an inhibiting anion. Photocurative B comprises an effective amount of an inhibiting photochemically active salt comprising a photochemically active cation and an inhibiting anion, and an effective amount of an accelerating non-photochemically active salt comprising a non-photochemically active cation and an accelerating anion. The inhibiting and accelerating salts in photocuratives A and B are defined by photo differential scanning calorimetry (pDSC) and thermal differential scanning calorimetry (tDSC). The compositions are useful, for example, as optically colorless or low color coatings, adhesives, encapsulants, and sealants for optical and electronic applications.

Description

FIELD OF THE INVENTION Novel catalyst systems provide cationically photocured compositions with reduced color, improved degree of cure, and increased thermal stability. Such compositions are useful as optically colorless or low color coatings, adhesives, encapsulants, sealants and abrasives for optical and electronic applications. BACKGROUND OF THE INVENTION Salts that have an organic, inorganic or organometallic cation and a nonnucleophilic counteranion are widely used in industry as initiators, particularly photoinitiators, for cationic addition polymerization reactions. Common initiator salts include onium salts such as diaryliodonium, triarylsulfonium, and (cyclopentadienyl)(arene)iron+ salts of the anions PF6−, AsF6−, or SbF6−. It has been well documented that the identity of the anion associated with the onium cation can significantly affect the rate of polymerization as well as the color and thermal stability of the resulting cured polymer. For example when a sulfonium SbF6...

AI Technical Summary

Benefits of technology

In yet another aspect, the present invention provides a cured composition comprising the curable composition described above that has been subjected to curing conditions. These compositions preferably have one or more of the following characteristics: increased degree of cure, increased Tg, increased thermal stability, and reduced color or colorless cured compositions compared to conventional cationically cured compositions.

We have found that the addition of additional anions to a conventional photochemically active initiator salt can improve the properties of a cationically cured composition depending upon the nature of the added anion and the anion of the photochemically active salt. It is the added anion that produces the effect. The anion can have any cation associated with it that does not interfere with the photochemical activity of the initiator salt.

The present invention provides advantages in cationically cured compositions not heretofore seen in the art. Higher performance compositions (higher Tg values, higher degree of cure, better thermal stability, etc.) that can be colorless or low color can be obtained that represent in improvement in the art compared to conventional single initiator systems that impart a deep yellow / brown color to the cured composition or have poor curing characteristics.

Catalysts for cationic addition photopolymerization reactions that produce compositions with long dark stability (before light exposure) and result in highly cured, colorless or low color, thermally stable products can be very useful in optically colorless coatings, adhesives, encapsulants, and sealants for optical and electronic applications. Improved dark stability can allow greater latitude in thermal processing of curable / thermoplastic blends, allowing the use of higher temperature thermoplastics or higher processing temperatures for lower viscosity extrusion conditions. Improved thermal stability of the cured compositions can allow higher operating temperatures of these materials and lower levels of outgassing that can be important to critical applications such as electronics (hard disk drives, for example) or in places that require stability in extreme environments (such as automotive applications). Improved thermal stability of the cured compositions can also be important to products used under stressful conditions, such as abrasives, where the use of the product generates heat that can contribute to product failure.

Problems solved by technology

Measurement of the degree of cure of these two systems, however reveals that while the sulfonium PF6− catalyst produces a colorless product there is a high level of unreacted epoxy monomer present in it, producing thermal instability in the cured product.

Although methide-containing initiators seem to be a direct replacement for SbF6− anion-containing initiators in cationic addition polymerization reaction systems as far as cure rate is concerned, these systems impart significantly reduced thermal stability to the cured material.

Images