The Waterborne Symposium

Solvents continue to be a large part of the surface coating industry, making it important to know how the chosen solvents and their quantities affect the final product and performance.  One of the defects seen in solventbased systems is solvent trapping.  Rapid solvent loss from a freshly applied coating could result in the surface layer forming a skin, which significantly decreases the solvent diffusion coefficient at the air interface and traps the solvent remaining in the film.  The residual solvent has significant influence on the film's properties. Primarily, residual solvent induces film plasticization, which is manifest as reduction in modulus and glass transition temperature.  Due to skin formation, these changes are noted more towards the substrate interface.  
The effects of residual solvent on interfacial properties such as adhesion are virtually unexplored.  In the absence of residual solvent, increasing film thickness leads to increased internal stress and predictably lower adhesion.  As a consequence, there is a critical thickness for any formulation above which the coating is unable to be applied because it will spontaneously delaminate.
This solvent trapping process can be modeled to predict the amount of residual solvent over a range of time and temperatures.  The present work demonstrates a reversal of the relationship in the presence of residual solvent leading to better adhesion with increased thickness and elimination of the spontaneous delamination thickness.  Numerous factors contribute to this outcome and it is the goal of this work to investigate internal stress, modulus and interfacial interactions to explain what factors contribute in what way to the solvent trapping effect on adhesion.
The coatings in this study are model epoxy-amine networks formulated to cover a range of crosslink densities and glass transition temperatures.  Material properties, cure temperature, solvent boiling point and solvent-resin affinity encompass the processing knobs available to control the residual solvent content and its distribution.  Solvents that were used include ethanol, isopropanol, butanol, di(propylene glycol) propyl ether, xylene, tetrahydrofuran and p-chlorobenzotrifluoride. At each film thickness, the films were evaluated for residual solvent content, adhesion, modulus, internal stress and glass transition temperature. Residual solvent was determined by thermal gravimetric analysis. Adhesion was quantified using the pull-off method on a MTS load frame, while tensile testing was performed on the same load frame to collect modulus data. Internal stress of formation was calculated based on substrate deflection. The glass transition temperature was quantified from the tan d peak measured via dynamic mechanical analysis. These studies will help with the prediction and planning of interfacial adhesion properties of coatings and better formulation skills.

In an effort to correlate oil industry characterization parameters with properties commonly used in the polymer/coatings industry, sixteen process oils were screened for use in a two-component aromatic polyurethane coating formulation.  A combination of viscosity and proton NMR testing was employed to successfully predict aniline point (oil industry) and cloud point (ink varnish industry) for the various oils.  Aniline/cloud point values were used to interpret oil migration behavior as well as tensile and adhesion properties of the coating films.  Cured film properties of the coatings will be presented.  These results provide a useful framework for the formulator to incorporate process oils in coating formulations.

Two-component epoxy systems have proven track record for providing excellent mechanical properties, chemical resistance, and adhesion to a wide range of substrates. They are frequently chosen over other technologies especially when chemical resistance is an important attribute. Chemical resistance is required in a variety of applications from construction and infrastructure to marine and protective metal coatings, and involves various substrate surfaces in flooring, bridges, wastewater treatment plants, power plants, metal equipment, and marine ships. These assets are constantly under attacks from various chemicals such as salt water, acids, and organic solvents. Chemical resistance is critical to provide protection to maximize the service life of the assets.

A number of factors determine the type of epoxy system required for chemical resistance. Key aspects influencing the chemical resistance of an epoxy system include crosslinking density, and degree of cure. Crosslinking density largely depends on the structure and functionality of epoxy resin and curing agent, while system mobility impacts the degree of cure. Molecular modelling and analytical techniques of differential scanning calorimetry, Raman spectroscopy, and dynamic mechanical analysis shed light on the fundamental understanding of the influence of crosslinking density and degree of cure on chemical resistance. This paper will detail the results of such fundamental study.

Abstract:
Conventional polyurethane coatings are crosslinked by reaction of a polyisocyanate prepolymer with a polyol. Recently there has been interest in the design of polyurethane polymers that can be crosslinked by alternative chemistries. The Storey Research Group at The University of Southern Mississippi has developed a series of these polymers for coatings, for which the final curing step involves azide-alkyne cycloaddition “click” chemistry. However, the conversion of a polyisocyanate prepolymer into a poly(alkynyl carbamate) results in dramatic viscosity increases due to hydrogen bonding of the carbamate structure, which is undesirable for coating application. To overcome this problem, a series of poly(alkynyl carbamate) prepolymers with lower viscosities was designed by chemically incorporating various glycol ether plasticizing moieties into the prepolymer chains. The glycol ether moieties were either of the conventional type, e.g. 2-ethoxyethanol, resulting in a non-load-bearing end group and a sacrifice of functionality, or they were specialty compounds containing both a hydroxyl group and an alkyne group, e.g. triethylene glycol monopropargyl ether, resulting in a load-bearing moiety and no sacrifice of functionality. This synthetic strategy, whether sacrificing the average functionality of the poly(alkynyl carbamate) or not, has proven to be effective in reducing the viscosity of the poly(alkynyl carbamate) prepolymer without compromising the final polyurethane coating performance.

Abstract
This tutorial will discuss current and emerging strategies aimed at developing new, practical chemistries, polymers, processes, and technologies with potential to improve the sustainability, and where applicable, the degradability of coatings and coatings precursors.