This invention relates to phenolic resole resins, particularly to stabilized phenolic resole resin compositions and to the cured compositions prepared therefrom.
Thermoset phenolic resins are known to exhibit excellent physical properties, e.g., strength and adhesion, and excellent chemical properties, such as resistance to attack by various solvents and acids. Due to these properties, phenolic resins are used extensively in coating applications, particularly as protective coatings for containers, air conditioning equipment, water tanks and the like.
It is well known in the art that the condensation reaction of an aldehyde with a phenol provides materials curable to thermoset phenolic resins. Base-catalyzed condensation reactions of at least a stoichiometric amount of an aldehyde with a phenol provide a condensate known as a phenolic resole resin or A-stage resin. Alternatively, acid catalyzed condensation of a phenol with less than a stoichiometric amount of aldehyde provides a phenolic novolak resin. Characteristically, phenolic resole resins can be heat cured to fully cross-linked, infusible resins (commonly referred to as C-stage resins) without the need for added cross-linking agent. From this standpoint, they are more descriptively referred to as one-step resins in contrast to phenolic novolaks (two-step resins) which require the addition of a cross-linking agent for curing.
Unfortunately, resole resins are unstable. Frequently, upon standing, this instability is evidenced by the formation of a distinct water rich phase, which phase generally separates from the remainder of the resole resin. Often, under normal storage and shipping conditions, this instability imparts to the resin undesirable handling characteristics and renders the resin unusable.
Heretofore, several methods have been proposed for improving the stability of the resole resins. For example, it is known that reducing the mole ratio of the reactants, i.e., moles of aldehyde per mole of phenol, provides a more stable resin. See, for example, Polymer Processes, edited by Calvin E. Schildknecht, published in 1956 by Interscience Publishers, Inc., New York, Chapter VIII. Unfortunately, the reduced aldehyde content in the resin increases the curing temperature required to prepare the C-stage resin.
Alternatively, U.S. Pat. No. 1,802,390 discloses that maintaining the resole resin in an alcohol will increase the resin's stability. Unfortunately, the volatile material, i.e., the alcohol, promotes the formation of pinholes and similar defects in coatings prepared therefrom.
Similarly, U.S. Pat. No. 2,937,159 teaches that the addition of a small amount of certain aliphatic polyamines to the resole resin will increase the stability thereof. Unfortunately, coatings and other products formed therefrom exhibit a reduced resistance to chemical attack.
In view of the stated deficiencies of the prior art compositions, it remains highly desirable to furnish a stabilized phenolic resole resin which when applied to a suitable substrate will cure at relatively low temperatures to a chemically resistant coating substantially devoid of imperfections.