Jos Huybrechts

Star oligomers and hyperbranched polymers in low VOC polyurethane coatings : Part III

ConclusionIn this paper the effects of the functionality of hydroxyl functional star oligomers and dendritic polys have been demonstrated in a low VOC clear 2-component clear coat crosslinked with a polysocyanate. At a fixed initial composition of the overall system and with the same reactivity of hydroxyl and isocyanate functional groups, the molecular weight increase as a function of time and the conversion can be calculated using the theory of branching processes. The study has shown that the practical results agree quite well with theory and that the potlife is shortened in case of higher functionality as well as in the case of a broad functionality distribution. The hardness in such a system is not only the effect of functionality, but also of solvent evaporation during crosslinking. If the molecular weight increase upon drying is too fast due to a high functionality (and/or distribution), early gel formation can result in a low film glass transition temperature and hardness due to solvent entrapment.

Star oligomers with different reactivity in low VOC polyurethane coatings: Part II

ConclusionsIn this paper the effects of the reactivity of hydroxyl functional oligomers with a polyisocyanate crosslinking agent have been demonstrated by comparing the potlife/hardness of a 3.5 VOC two-component polyurethane clear coat based on a low TG, high reactive against a high TG, low reactive oligomer.The reaction rate of two polymers forming a crosslinked network becomes diffusion controlled when, during the reaction, the increasing TG comes close to the reaction temperature.4 If this reaction takes place in the presence of a solvent which evaporates during film formation and crosslinking, the speed of evaporation will affect both the reaction rate and the film TG. The results of this study have shown that the reactivity determines the potlife whilst the TG controls the hardness. The reactivity of blends of such oligomers against pure oligomers is different as can be calculated using the theory of branching processes and as shown from practical measurements. Blends of oligomers or a pure oligomer with up to 75% of the properties of the high TG, low reactive oligomer do show a hardness close to the properties of the low TG, high reactive oligomer which further proves that the chemical reactivity controls the overall drying performance. A fast chemical reactivity shortens the potlife with a risk of solvent entrapment near to and at the gel point which results in low hardness.

New applications of catalytic chain transfer polymerization to waterborne binders for automotive paint systems

Abstract Catalytic chain transfer polymerization (CCTP) is a conventional free radical polymerization technique that allows the preparation of macromonomers in a one step process. Acid functional macromonomers can be copolymerized with acrylate backbone monomers to form graft copolymers that, after neutralization with a base, are water dispersible. Low molecular weight oligomers from CCTP act as chain transfer agents themselves for methacrylate monomers via addition–fragmentation mechanism and lead to (semi) AB-block copolymers. Group transfer polymerization (GTP) is another polymerization technique to make AB-block copolymers but economically less attractive for functional comonomers since they interfere with the initiation mechanism. Graft and AB-block copolymer dispersions offer advantages in waterborne coatings compared to linear polymers of the same overall composition and molecular weight. Examples discussed in this paper are pigment dispersants and dispersion resins in which the backbone or A-segment of the copolymer has specific groups to anchor to the pigment surface and the side chains or B-segment give both charge and steric stabilization. AB-block copolymers with one block water soluble or dispersible also function as copolymerizable surfactants in an emulsion polymerization process and allow the synthesis of surfactant-free emulsions with low amounts of hydrophilic groups. The catalytic chain transfer agents (CCTAs) used in CCTP do have a high chain transfer activity for methacrylate monomers at very low concentrations so that low molecular weight oligomers (e.g. dimers) can be made. This chain transfer activity is lost in an emulsion polymerization process if the CCTA is (partly) water soluble. The paper will further demonstrate the use of dimers in the control of molecular weight in emulsion copolymerization.

Role of Distributions in Binders and Curatives and Their Effect on Network Evolution and Structure

Precursors of cross-linked polymer systems are blends of many compounds, some of them forming series (distributions) of compounds of increasing molecular weights, type and number of functional groups, or some other property. The distributions may arise from impurities in the raw materials, or a result of side reactions. In most cases, the distributions are generated intentionally; functional copolymers, hyperbranched polymers, off-stoichiometric highly branched polymers, chain extended systems, or precursors prepared in several stages can serve as examples. The distributions affect processing and materials properties. We show ways to generate these distributions from bond formation kinetics and reaction mechanisms. Statistical branching theories based on assemblage of branched molecules and gel structure from building units in different reaction states are used to model the evolution of the cross-linked system.