Synthesis and characterization of hyperbranched polyurethane acrylates used as UV curable oligomers for coatings

doi:10.1016/j.porgcoat.2004.10.001

Progress in Organic Coatings

Volume 52, Issue 2, 1 February 2005, Pages 110-117

https://doi.org/10.1016/j.porgcoat.2004.10.001 Get rights and content

Abstract

A series of UV curable hyperbranched polyurethane acrylates (HUAs) were synthesized by modifying the hyperbranched polyester end-capped with hydroxyl groups (HBPE-OH) using urethane acrylate prepared from toluene diisocyanate and 2-hydroxylethyl acrylate, and characterized by infrared spectroscopy and nuclear magnetic resonance spectroscopy. These materials, which are present in white powder at room temperature and have high melting viscosity. However, their viscosity could be reduced rapidly by adding co-monomer or raising the temperature. Their maximum photopolymerization rates (R_P^max) and final unsaturation conversion (P^f) in the presence of a photoinitiator were investigated. The results showed that adding an excess of 1,6-hexanediol diacrylate (HDDA) as a co-monomer resulted in the decreases in R_P^max and P^f. The higher R_P^max and P^f were achieved with the HUA-S obtained from the modification of HBPE-OH by succinic anhydride compared with that by phthalic anhydride. From the data of dynamic mechanical thermal analysis, these hyperbranched prepolymers have good miscibility with HDDA. Moreover, The HUA based resin with 20 wt.% HDDA addition has the highest softening point and grass transition temperature because of its high cross-linking density.

Introduction

During the latest decades polymer chemists have created a number of non-linear variations on linear polymeric structures. A new family of the materials with three-dimensional molecular architectures are dendritic macromolecules including dendrimers [1] and hyperbranched polymers [2]. Due to their special molecular structures, such as, manifold of end-groups, compact molecular shape, and decreasing chain entanglement, the physical and chemical properties of dendritic polymers are rather different from their conventional linear counterparts. It has been recognized that highly branched dendritic polymers offer very intriguing properties of great potential value of applications, such as, high solubility and reactivity, and good rheological behavior. Over the last decade, dozens of patents have been filed on the synthesis of dendritic polymers [3], [4]. The applications of this technology are being proposed in some fields, such as coatings, rheology modifiers, medicine, catalysts and so on [5]. However, the actual exploitation for preparing dendritic polymers and their practical applications can be counted on one's fingers, largely due to the expense of their synthesis and the limited availability of these new materials.

Hyperbranched polymers have a lower degree of branching than dendrimers but possess many similar properties. Moreover, hyperbranched polymers are less costly to produce and much easier to process than dendrimers. Therefore, hyperbranched polymers have more potential applications than dendrimers in a variety of fields. To explore their application possibilities for UV coatings, adhesives and laminates, a series of hyperbranched polyesters as oligomers with different structures were prepared using divergent approach, modified with (meth)acrylate to obtain the reactivity under UV irradiation in our laboratory [6], [7], [8]. Their photopolymerization characters and rheological behaviors, and the mechanical properties of UV-cured films were also investigated.

As well known, polyurethane acrylates are widely used as oligomers for UV coatings, which provide excellent physical and mechanical properties, such as adhesion on substrates, flexibility and impact strength. However, little work has been performed to prepare hyperbranched urethane acrylates for UV curing systems.

In this work, three types of hyperbranched polyurethane acrylates (HUAs) were synthesized. Infrared spectroscopy, vapour phase osmometry (VPO) and nuclear magnetic resonance spectroscopy (¹H NMR) were used to characterize their molecular structures. The photopolymerization kinetics and dynamic mechanical thermal properties were also investigated.

Section snippets

Materials

1,2,4-Benzenetricarboxylic anhydride (BTCA) was supplied by Aldrich, Germany. Pentaerythritol, toluene diisocyanate (TDI), epoxy propane, succinic anhydride and phthalic anhydride were purchased from Shanghai First Reagent Co., China. Hydroxylethyl acrylate (HEA) was supplied by Shanghai Shanhu Chemistry Co. and dried over a 4 Å molecular sieve before use. Other chemicals were used as received without further purification. 2-Hydroxy-2-methyl-1-phenyl-1-propanone (Darocur 1173), as a

Characterization of HUAs

The yields of 84, 80 and 78% for HUA, HUA-S and HUA-P were obtained, respectively. The number average molecular weight (M_n) of HUA, HUA-S and HUA-P were experimentally measured by VPO to be 2984, 3932 and 4542 g mol⁻¹, respectively. During the synthesis of TDI-HEA, there is an inevitable side reaction, though the reactivity of para -NCO group is seven times higher than ortho -NCO group. Two HEA molecules reacted with one TDI molecule, leading to the formation of urethane diacrylate. Therefore,

Conclusions

Three UV curable hyperbranched polyurethane acrylates were synthesized, possessing different double bond density and partially chain structures, which made the difference in their photopolymerization behaviors and dynamic mechanical thermal properties. Their photopolymerization kinetics, characterized by R_P^max and P^f, were intensively affected by double bond density and co-monomer HDDA content. The highest R_P^max as well as P^f was obtained in HUA-S based resin with 30 wt.% HDDA content. However,

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^☆: Contract/grant sponsor: National Natural Science Foundation of China, no. 50233030, and China NKBRSF, no. 2001CB049600.

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Synthesis and characterization of hyperbranched polyurethane acrylates used as UV curable oligomers for coatings☆