High-Performance UV-Cured Acrylic Coatings - American Chemical

Chapter 12

High-Performance UV-Cured Acrylic Coatings Christian Decker Downloaded by PENNSYLVANIA STATE UNIV on July 12, 2012 | http://pubs.acs.org Publication Date: August 30, 2007 | doi: 10.1021/bk-2007-0962.ch012

Département de Photochimie Générale (UMR 7525 - CNRS), Ecole Nationale Supérieure de Chimie de Mulhouse, 3 rue Werner 68200 Mulhouse, France

Highly crosslinked polymers have been rapidly produced at ambient temperature by light-induced polymerization of multifunctionalized monomers and oligomers. The reaction was monitored in real time by infrared spectroscopy and shown to proceed extensively within a fraction of a second for acrylate-based resins containing highly efficient photoinitiators (diphenoxybenzophenone and α-aminophenylketone) and a very reactive monoacrylate carbamate mono mer. Dual-cure systems that combine photopolymerization and thermal curing have been developed to achieve an effective hardening of resins in areas receiving no U V light (shadows areas, thick pigmented coatings). The physico-chemical properties of the photocured polymer were modulated in a large range by a proper selection of the chemical structure of the oligomer and of the monomer functionality. High-modulus crosslinked polymers have been produced for coatings applications to improve the surface properties of various substrates, in particular their resistance to scratching, moisture and weathering. The UV-curing technology proved to be an environment-friendly process which transforms rapidly a solvent-free resin into a highly resistant coating with minimum energy consumption and no V O C emission.

Light-induced polymerization of multifunctional monomers, also called UV-radiation curing, is commonly recognized as the most effective process to transform quasi-instantly a solvent-free liquid resin into a highly resistant polymeric material. The subject has been extensively investigated in the past

176

© 2007 American Chemical Society

In New Developments in Coatings Technology; Zarras, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

177 few years with respect to both the polymerization kinetics and mechanism, and the properties of different types of UV-cured polymers (acrylates, epoxides, vinyl ethers, thiol/polyene,... ) . Because of its distinct advantages regarding processing and product performance, this environment-friendly technology has found a wide range of applications, in particular in the coating industry for the surface protection of a variety of materials (plastics, wood, paper, metals). Remarkable progress has been made during the past decade regarding the processing stage, with the design of highly reactive UV-curable formulations and of well-suited light sources, as well as the service properties of the final product, which can be adjusted to fit the requirements imposed for the considered application (coatings, adhesives, composites, e t c . ). Such U V curable resins usually consist of acrylate monomers and oligomers, associated to radical-type photoinitiators. The viscoelastic properties of the cured coatings can be finely controlled through the chemical structure of the oligomer and the monomer functionality, depending on the type of substrate used (flexible or rigid). Most of the research efforts in UV-curing chemistry have been devoted to the development of new photoinitiators, monomers and functionalized oligomers, specially designed for improving the performance of both the processing stage (speed and cure extent) and the coating properties (abrasion and scratch resistance). In this contribution, we report some of the progress recently made in the development of UV-curable acrylic resins with respect to both the resin formulation and the characteristics of the final product. Special attention will be given to the kinetic aspect because the properties of UV-cured coatings are largely depending on a good understanding and control of the manifold reactions occurring during such ultrafast molecule to material transformation.

Downloaded by PENNSYLVANIA STATE UNIV on July 12, 2012 | http://pubs.acs.org Publication Date: August 30, 2007 | doi: 10.1021/bk-2007-0962.ch012

1-9

Experimental The UV-curable resins contained three basic components: a photoinitiator which generates free radicals upon UV-exposure, an acrylate end-capped oligomer having different chemical structures (polyurethane, polyether, polyester,...) which will determine the properties of the UV-cured polymer, and an acrylate monomer used as reactive diluent to adjust the resin viscosity. The following compounds were used for the formulation of UV-curable resins: Irgacure 2959, Irgacure 651, Irgacure 819, Irgacure 369 (Ciba SC), benzophenone (BP), and 4,4'diphenoxybenzophenone (DPBP) as photoinitiators, Ebecryl 600 (UCB), Ebecryl 284 (UCB), Laromer 8987 (BASF) and Laromer 8949 (BASF) as telechelic acrylate oligomers, hexanedioldiacrylate (HDDA from U C B ) and a carbamate (Acticryl CL-960 from SNPE) as reactive diluents. The chemical formulas of the compounds used are given in Chart 1. The formulation was coated on a BaF crystal or on a glass plate at a typical thickness of 25 μπι or 50 μπι, respectively, by means of a calibrated wire2


178

Photoinitiators -OH

HcT^


Irgacure 651

Irgacure 2969 ο ο ο

ο irgacure 369

irgacure 819

ο DPBP Monomers

or

Or^YY Carbamate-acrylate

Monoacryiate

or

Ο

Diacrylate Teiecheiic Oligomers

or

3

Tnacrylate aromatio polyether

Ebecryl 600

Aliphatic polyurethane

or:Ebecryl 284 - Laromero8987r Waterborne aliphatic polyurethane diacrylate

Laromer8949

Chart 1. Chemical formulas of the compounds used


179 wound bar. The sample was exposed for a short time to the UV-radiation of a medium pressure mercury lamp, at a light intensity ranging from 15 to 600 mW cm' . The polymerization was followed in real time by infrared spectroscopy through the decrease of the IR bands characteristic of the acrylate double bond (810 cm" and 1410 cm" ). Monomer conversion was determined from the relative decrease of the IR band after a given exposure. The Persoz hardness of the UV-cured coating was evaluated by monitoring the damping of the oscillations of a pendulum placed onto a coated glass plate. Persoz hardness values are ranging from 50 s for soft elastomers up to 400 s for very hard and glassy materials. The scratch resistance, expressed in grams, was evaluated according to the Taber procedure. The weathering resistance of some UV-cured samples coated onto a BaF crystal was tested in an accelerated Q U V - A weatherometer operated under wet cycle conditions (8 h U V exposure at 70°C, 4 h in the dark at 50°C with water condensation). 2


1

1

2

Results and Discussion Photocuring of acrylate-based resins Acrylate-based resins are widely used today for producing UV-cured coatings because of the great reactivity of the acrylate double bond and the large choice of monomers and oligomers available. The latter differ by their chemical structures, which can be polyesters, polyurethanes, polyethers or polysiloxanes, and by their molecular weight, which ranges typically between 500 and 2000 g. During the light-induced liquid to solid phase change, the viscosity increases rapidly and will cause the polymerization to slow down progressively, up to a complete stop when vitrification occurs. Replacing the di- or triacrylate reactive diluent by a monoacrylate will increase the molecular mobility and lead to a more complete but slower polymerization, as shown by the conversion versus time profiles recorded by real-time infrared spectroscopy for a polyurethaneacrylate (PUA) coating (Figure 1). Fast and complete curing was achieved by using as reactive diluent a monoacrylate carbamate (Acticryl CL-960). The reason of this much-enhanced reactivity is still unknown, but it seems to be related to a lower termination rate constant, as well as to the presence of labile hydrogens which favors chain transfer reactions. This would account for the fact that this monomer bearing a single acrylate function becomes completely insoluble upon U V exposure. Similar results were obtained by introducing a cyclic carbonate in the structure of a monoacrylate monomer, with a 5 fold decrease of the ratio of the termination and propagation rate constant (k /k ). Another distinct advantage of this monomer used as reactive diluent is that it imparts hardness and flexibility to the UV-cured polymers, thus making them more resistant to abrasion, scratching and shocks. t


p

180 Acrylate conversion (%)

Conversion (%)

High-Performance UV-Cured Acrylic Coatings - American Chemical

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