Chapter 1 Introduction
1.3 Thermoset PU coatings
Thermoplastic PU coatings possess major drawbacks such as poor resistance against mechanical deformations and high temperature degradation. In thermoset coatings, the presence of chemical cross-linking points in thermoset coatings provides them with enhanced tensile strength, abrasion resistance and chemical resistance lacking in thermoplastic PU coatings which are essential for most industrial coatings. Cross-links are occurring by reaction of isocyanate groups as mentioned earlier. Coatings may therefore contain polyether or polyester soft segments with high functionality17-21, isocyanates with functionality greater than two22, 23, NCO/OH ratios greater than one19-21, 24. The increase in functionality increases cross-linking concentration which, in general, promotes phase mixing25-28. The introduction of such chemical cross-linking points reduces the mobility of the hard segments and thereby their ability to form hydrogen bonds18, 29. For high performance applications, a calculated amount of cross-linker is needed to adjust the properties of the PU coating. At last, the material, obtained with cross-links deliberately added or created in-situ, exhibits both phase-separated and phase-mixed structures, depending on the concentration of cross-links.
1.3.1 High solids content
For solventborne coatings, the main challenge since 1980s is to improve the solids content. For this purpose, quantities of organic solvents have been reduced leading to the so-called “high solids content” paints. Many efforts have also been made to lower the general viscosity of the formulation like the addition of reactive diluents or the reduction of the viscosity of the binder or of the polyisocyanate cross-linker30. In such a high solid content formulation, most common binders are terminated polyesters or hydroxy-functionalized acrylic resins. For polyester-urethane 2K coatings, controlling of molecular
weight and distribution, selecting the number of functional groups, using hydrogen bond acceptor solvents have been efficient ways to obtain low volatile organic compounds (VOC) paints31. Polyesters usually achieve higher solids content and better adhesion to metal than acrylic resins32.
1.3.2 Acetoacetylation
The acetoacetylation of part of the hydroxyl groups contained in polyesters or acrylic polyols leads to the formation of the less polar acetoacetate groups. It allows a higher solid content at the application viscosity as well as better adhesion due to chelate effects. For the coating application their production is preferably achieved by transesterification33-36. The keto-enol equilibrium of these species allows the presence of two potential cross-linking sites: the active methylene group and the ketone carbonyl group. The cross-linking of the methylene groups with diisocyanates yields additional cross-links with better weathering stability and superior properties37-40. Furthermore, the β-ketoester groups are amphoteric and can be used to modify or cross-link polymers.
1.3.3 Introduction of specific functional groups
Imide
Chemical cross-linking of thermoset PU provides them with thermal stability or thermomechanical properties. In order to improve further such behavior, the introduction of heterocyclic structures, like imide functions, in the PU backbone has proven to be efficient. Isocyanate-capped PU prepolymers are usually reacted with acid dianhydride to produce PU containing imide groups41-44.
Glycidyl carbamate
The introduction of glycidyl carbamate groups can provide PU with the reactivity of epoxides. These functions are generally incorporated by functional oligomers such as biuret glycidyl carbamate or isocyanurate glycidyl carbamate which are synthesized from different polyfunctional isocyanate oligomers and glycidol45.
Chapter 1 Introduction 1.3.4 Polyurea
Within PU coatings, one can distinguish polyurea coatings in which the hydroxyl precursors are replaced by aminofunctional ones. The reaction between isocyanate and amine is significantly quicker than that occurring between isocyanate and alcohol. They are, therefore, ideal for aggressive environment where high speed curing is required (e.g. oil pipeline). However, their high reactivity implies a short pot life. The use of secondary amines instead of primary ones can, for example, increase this storage time. If the amine is bulky and sterically hindered, kinetics will be altered and the reactivity will greatly be reduced.
1.3.5 Moisture-cured PU
Moisture-cured PU contain isocyanate-terminated prepolymers and lead to highly cross-linked coatings. The diffusion and reaction of moisture produces primary amines that further react into urea groups. The drawback of such coating is their storage instability.
Several side-products such as allophanate or isocyanurate are usually generated while stored. The introduction of those additional hard segments changes their volume fraction within the coating and ultimately alter adhesion or thermal properties46-48. However, moisture scavengers can be used to improve shelf life and pot stability. On the other hand, moisture-cured PU produce coatings with superior hardness, strength and stiffness. Since moisture is consumed, the risk of blisters or the formation of a weak boundary layer caused by water trapped under the coating is also greatly reduced.
1.3.6 UV-cured PU
UV-curable PU coatings present no or very low VOC. Their principle is based on the polymerization of unsaturated species induced by UV-radiation to lead a three-dimensional network. The main components of UV curable formulation are oligomers, reactive diluent and photoinitiator. This technique possesses many advantages: low energy requirement, fast and efficient polymerization, selective cure limited to irradiated areas and environmentally friendly with its low VOC. The major disadvantage lies in the inhibition of the reaction caused by the presence of oxygen. On the surface of the coating, oxygen
terminates the polymerization resulting in low molecular weights which leads to tacky films.
To overcome such phenomenon, oxygen scavengers (tannin, carbohydrazide), high radiation intensity or high initiator concentration are applied49. The nature and properties of the cured film depend on the properties of the component but also on the kinetics of the photo-polymerization (rate and final conversion). The irradiation flux, sample thickness, temperature, photo-initiator concentration and reactive diluents content affect these kinetics and, therefore, the physical and mechanical properties of the final films.
1.3.7 Waterborne coatings
The constant demand in lowering VOC contents has conducted researchers to focus on waterborne coatings. They are dispersions of PU particles in continuous water phase. The particles are about 20-200 nm and have high surface energy which is responsible for the film formation after water evaporation. This technology requires new type of binder and additives to fulfill high quality requirements.
PU is usually not soluble in water and the degree of hydrophilicity is, therefore, a key parameter. The PU polymer backbone is generally modified by the introduction of hydrophilic groups (PU ionomer) or surfactant is added to obtain aqueous PU dispersion. PU ionomer exhibit pendant acid or tertiary nitrogen groups which are completely or partially neutralized or quaternized respectively, to form salts.
In all processes to prepare aqueous PU dispersion, prepolymers are formed from suitable polyols with a molar excess of polyisocyanates in the presence of an emulsifier which allows the dispersion of the polymer. The emulsifier is usually a diol with an ionic (carboxylate, sulfonate, quaternary ammonium salt) or non ionic (polyethylene oxide) group. The dispersion of the prepolymer and the molecular weight build up differ from one process to another50-52.
Depending on the type of hydrophilic group present in the PU backbone, the dispersion can be defined as cationic, anionic and non-ionic. For each species, a minimum ionic content is required for the formation of a stable PU ionomer. Interactions between ions and their counter ions are then responsible for the formation of stable dispersion.
Chapter 1 Introduction