High Solid Polyurethane Research Articles

Cycloaliphatic polyester-based high-solids polyurethane coatings: II. The effect of difunctional acid

Two series of polyesters were synthesized with isomeric cyclohexane diacids. The first series of polyesters was synthesized with 1,4-cyclohexanedimethanol (CHDM) and three cycloaliphatic difunctional acids, 1,4-cyclohexanedicarboxylic acid (1,4-CHDA), 1,3-cyclohexanedicarboxylic acid (1,3-CHDA), or hexahydrophthalic anhydride (HHPA). The second series was prepared with 1,4-CHDA and 1,3-CHDA with CHDM. Control polyesters with adipic, azelaic, and isophthalic acid (AA, AZA, IPA) were prepared for comparison. The solubility and viscosity of polyesters were investigated using the common solvent methyl ethyl ketone (MEK). All the polyesters were cross-linked with hexamethylene diisocyanate (HDI) isocyanurate, forming polyurethane films. General coatings, tensile, and viscoelastic properties were evaluated for the cured polyurethane films. In addition, fracture toughness and the mode of energy dissipation were investigated. The polyesters based on cycloaliphatic diacids have better solubility in MEK compared to the polyesters based on the aromatic or linear aliphatic diacids. The cycloaliphatic diacids based polyurethane coatings had intermediate mechanical and viscoelastic properties compared to polyurethane based on aromatic and linear aliphatic diacids. In addition, the cycloaliphatic diacids afforded polyurethane with a greater adhesion on aluminum substrate compared to the aromatic diacid, IPA.

Cycloaliphatic polyester based high solids polyurethane coatings: I. The effect of difunctional alcohols

High-solids polyesters were synthesized with two cycloaliphatic diacids, 1,4- cyclohexanedicarboxylic acid (1,4-CHDA) and 1,3-cyclohexanedicarboxylic acid (1,3-CHDA); and with five diols, 1,4-cyclohexanedimethanol (CHDM), neopentyl glycol (NPG), hydroxypivalyl hydroxypivalate (HPHP), 2-butyl-2-ethyl-1, 3-propanediol (BEPD), and 1,6-hexanediol (HD). The viscosity of the polyesters was dependent on the structures of diols. The viscosity of polyesters is lower with the diol HD, intermediate with BEPD and HPHP, and higher with the diols CHDM and NPG. The polyesters were crosslinked with hexamethylene diisocyanate isocyanurate (HDI isocyanurate) affording polyurethane coatings. The mechanical properties, tensile properties, fracture toughness, and viscoelastic properties were investigated for the polyurethane films with five different diols. The cyclohexyl structure of the CHDM provides the polyurethane with rigidity which is manifested in high tensile modulus, hardness, and fracture toughness. In contrast, the linear diol, 1,6-hexanediol provides polyurethane with very high flexibility, but these coatings suffer with respect to low hardness and tensile modulus.

Bicyclic orthoesters as binders with latent hydroxyl functionality

A big problem, encountered during the formulation of ultra high solids polyurethane coatings, is the gap between potlife in the can and cure speed on the substrate. To compete with the partly physically drying products from the medium solids generation, (ultra) high solids polyol–polyisocyanate based coatings have to be catalyzed so much that a potlife of only some minutes remains. Bicyclic orthoesters (BOEs) are a class of compounds that give an excellent solution to this dilemma. The BOE ring is closed in the can. The isocyanate crosslinker and a high dose of catalyst can be added without major consequences for the potlife. After application, e.g. as a car refinish coating, the ring opens by reaction with atmospheric moisture, the reactive hydroxyl groups are formed and a very fast curing occurs. Compounds with a BOE group can be synthesized by reacting polyalcohols with acyclic orthoesters or by the rearrangement of oxetanes. The latter route is preferred. Strong acids act as catalysts for the ring opening reaction with atmospheric moisture and the through cure of the polyurethane coatings is then catalyzed by metal compounds. The influence of the application conditions on the cure speed of these polyurethane coatings is very low. The crosslinking of this binder with latent hydroxyl functionality with melamine resins is catalyzed by the same strong acid as used for the hydrolysis. Polyurethane coatings and stoving enamels with a very high solids content and an excellent physical performance can be formulated using BOEs as binder component.

Processes and states during polymer film formation by simultaneous crosslinking and solvent evaporation

Coating film formation with simultaneous crosslinking and solvent evaporation, accompanied by passage of the polymer film through glass transition region, is a complex process by which temporary or permanent anisotropic and gradient network structures can be formed. Evaporation and crosslinking are processes that are interdependent. The changes in structure (growth of branched molecules and network evolution) are a function of reaction kinetics, which gets diffusion controlled when the system passes through the glass transition region. Structural changes are determined by branching, gelation, and network build-up and depend on the architecture of network precursors. Thermodynamic interactions of polymer with solvents affect the solvent activity which determines the vapor pressure of the solvent over the film and thus the evaporation rate. The glass transition temperature increases as a result of both the decreasing solvent content and conversion of functional groups into bonds. By interplay of these two factors more or less solvent can be locked in by vitrification. The roles and intensity of these basic processes and interrelations are discussed. Some older results are reviewed and new experimental evidence is added. The interrelations are illustrated by time dependences of solvent evaporation and conversion of functional groups for solvent-based high-solids polyurethane systems composed of a hydroxyfunctional star oligomer and triisocyanate and by the role of the ratio of evaporation to crosslinking rates. Evidence was obtained of gradient formation in which appearance of a glassy surface layer is an important event in the history of film formation that determines solvent retention and other film characteristics.

Legislation‐compliant polyurethane and epoxy coatings

Legislation is currently the key driver in the coatings industry. Reduction of the volatile organic content (VOC) of coating systems is a main aim. This may be achieved by designing the resin and coating with water as the carrying solvent. Reduction of resin viscosity to enable high solids coatings production is another route. The design and development of legislation‐compliant paint formulations based on water‐based epoxy resin systems and high solids polyurethane topcoats is discussed. A short review and discussion of the weaknesses of traditional accelerated test methods including salt‐spray, QUVTM and ProhesionTM used to check coating performance is presented. Some simple reaction and degradation mechanisms are shown.

The synergy between water-borne epoxy and high solids polyurethane legislation compliant coatings

The coatings industry is under pressure to reduce the Volatile Organic Content (VOC) of coating systems. One way of doing this, is to design the resin and coating with water as the liquid phase. In addition reducing the resin viscosity to make high solids coatings is another method. A company that offers both methods of VOC reduction is in a good position commercially. This paper discusses the design and development of a number of legislation compliant paint formulations based on a new water-based epoxy resin curing agent. Also high-solids single-component oxazolidine-cured aliphatic finising coats are discussed briefly. A short review and discussion of traditional accelerated test methods including salt-spray, QUV’ and Prohesion’ used to assess coating performance is presented. Some simple reaction and degradation mechanisms are shown.

Regular features

Additions for use in polyurethane coatings. Angus Chemie GmbH has announced the introduction of two new additions to its product line for polyurethane coatings. Zoldine RD‐20 Reactive Diluent is designed to replace higher viscosity polyols in high solids polyurethane coatings. Zoldine MS‐Plus Moisture Scavenger eliminates bubbles, pinholes, downglossing and hazing in polyurethane coatings to allow for fast cure times in all types of weather.