Douglas A. Wicks

Blocked isocyanates III : Part A. Mechanisms and chemistry

As the third installment in the series of reviews of the literature on blocked isocyanates, this review brings together the most important developments documented in over 3000 patents and publications that have been printed through the end of 1998. The mechanisms of the different blocking methods are discussed, as are examples of blocking groups that are cited. Particular emphasis is placed on the analytical techniques used to evaluate deblocking temperatures. Part B, Uses and applications of blocked isocyanates will be covered in a forthcoming paper of this journal.

Blocked isocyanates III: Part B: Uses and applications of blocked isocyanates

Abstract As the completion of the third installment in the series of reviews of the literature on blocked isocyanates, Parts A and B of this review bring together the most important developments documented in over 1700 patents and publications that have been printed between 1980 and mid-2000. For references before 1980, see earlier reviews in this series [Prog. Org. Coat. 3 (1975) 73; Prog. Org. Coat. 9 (1981) 3]. The uses and applications of blocked isocyanates in coatings and non-coatings fields are reviewed. Part A: Mechanisms and Chemistry was published earlier in this journal [Prog. Org. Coat. 36 (1999) 148].

Two package waterborne urethane systems

Abstract This article is an extensive review of the literature on two component urethane systems that use water as a carrier. It generally covers the period from 1985 through 2000 with special emphasis on patent references since 1993. It includes both ambient temperature cured and baked systems containing unmodified and modified isocyanate building blocks. The main criterion for inclusion into this paper was that the finished urethane group containing polymer reaches its ultimate molecular weight after it is applied to a substrate, so that traditional latex-type materials are not emphasized. The paper describes the raw materials and chemistry used in this field and presents some of the factors that must be considered to make this unique chemistry industrially feasible. The major concern is formulating, mixing, and applying the systems in a manner that limits the water reacting with the highly active isocyanates to avoid defects from generated carbon dioxide. The review also discusses some of the end use areas where two component urethanes have been commercially applied.

Multistep chemistry in thin films; the challenges of blocked isocyanates

Abstract The paper provides an overview of the history and progress of blocked isocyanates in coatings. Typically blocked isocyanate systems are used to obtain the performance of two-component polyurethanes in a one-component thermally cured system. Two-component polyurethanes have an established position as high performance coatings, but may be inappropriate for a given application because of equipment costs or the need to be mixed just prior to use with a limited potlife. Blocked isocyanate systems overcome these difficulties by chemically masking the isocyanate, allowing it to be mixed with coreactants in a one-package delivery form. Formation of the polyurethane coating is then accomplished by thermally decomposing the isocyanate-blocking agent bond. There are many factors that affect this reaction: • Structures of the isocyanate and blocking agent. • Thermal stability of the isocyanate-blocking agent bond. • Polarity of the reaction media (i.e. coreactant). • The nucleophylicity of the blocking agent vs. the coreactant. • Ability of the blocking agent to diffuse out of the film. • Catalysis. However, higher temperatures are required for curing, in some cases, toxic hazard of the escaping blocking agent must be considered, and in some cases the blocking agent can cause discoloration. Major efforts in the last few years have been centered on reducing these difficulties. The complexity of the deblocking reaction requires a full understanding of these factors and how they interact.

Amine chemistries for isocyanate-based coatings

The inherent high reactivity and color instability of primary amines has limited their applicability in polyisocyanate-based coatings. However, recent developments in the use of modified amine functional coreactants in such formulations shows significant progress towards having two-component systems with excellent application and durability properties. The systems reviewed here use amine functional groups which are either blocked by addition of carbonyl compounds, or are of reduced reactivity through steric hindrance and intramolecular hydrogen bonding. The relative benefits and drawbacks of the various systems are detailed along with their applicability to high solids coatings.