Visible light cure packages for improved drying kinetics in alkyd coatings

Highlights

• Solventborne alkyd coatings were cured quickly without cobalt drier.

• Visible light was used to augment the curing process.

• While photoinitiators helped surface curing, curing in depth seemed to suffer.

Abstract

A solventborne linseed oil alkyd was formulated with two different free radical photo-initiators and a photosensitizer to enhance autoxidative crosslinking. Irgacure 784 and Irgacure 819 significantly decreased the dry-to-touch and through-cure times of alkyd systems at ambient conditions using indoor lighting. Real-time Fourier transform infrared spectroscopy, and solid state ¹³C NMR were used to monitor the curing process and two mechanisms are proposed to explain the increase in crosslinking. Films were evaluated for tensile, viscoelastic, and thermal properties, indicating an increase in crosslink density. Coating performance was evaluated, significantly increasing film hardness, while maintaining appearance. However, the adhesion and chemical resistance decreased. Visible-light curable alkyds offer excellent appearance and fast curing rates, without the use of cobalt driers.

Introduction

Alkyd resins are a common seed oil-based binder used in the coatings industry. Their excellent appearance and low cost sustain their use as architectural and wood coatings, but their use as high performance coatings has been replaced by more durable petroleum-based coatings which cure at significantly faster rates [1]. Alkyds are typically synthesized in a two-step “Monoglyceride process” by first forming a monoglyceride via transesterification of the triglyceride seed oil with glycerol. The monoglyceride undergoes a step-growth polymerizes with dibasic acids to form a polyester backbone of the alkyd [1]. The pendent fatty acid can participate in an autoxidative crosslinking slowly converting the alkyd from a liquid to a solid film. This autoxidative process is typically catalyzed by heavy metal driers. Cobalt, the most common drier, is being phased out due to toxicity [2] and genotoxic concerns [3]. This slow process can take several hours to days to cure, which is undesirable for many applications. Work by Bartolozzi showed chemical activity in alkyd coatings up to 10 weeks after application [4]. This research will use photocuring to accelerate autoxidation without alkyd modification or the use of cobalt driers.

Ultraviolet (UV) curable resins are an important class of coating technology that allow zero to low-VOC formulations and can cure-on-demand. The volume of radiation-cured coatings is relatively small, but the economic impact is considerable due to its high value usage in the medical, dental, optical, communication, and computer fields [[5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]]. UV-curable resins are limited by the line of sight of the light source, governed by Beer’s law, and suffer from adhesion and film defects due to the shrinkage and internal stress of a coating cured over short time scales. Additionally, UV-exposure is hazardous, and expensive lamps are not practical for many applications, such as architectural coatings [1].

Many studies have been reported that attempt to modifying seed oils for use as UV curable resins or reactive diluents for alkyds [[16], [17], [18], [19], [20]]. A UV-curable tung oil-based alkyd was reported by Soucek et al. by modifying the fatty acid of the alkyd via Diels-Alder cycloaddition of trimethylolpropane trimethacrylate [21]. These coatings were further tested using pigmented systems and curing kinetics were characterized by photo differential scanning calorimetry [22]. Ang et al. synthesized UV curable non-drying palm stearin alkyds by using maleic anhydride as the dibasic acid forming unsaturation in the polyester backbone [23,24].

There are two classes of photo-initiators (PI): Type I unimolecular photo-initiators and Type II bimolecular photo-initiators. In Type I, molecules undergo bond cleavage to form two radicals when separated. Type I photo-initiators can break down into more than two free radicals, such as bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide (BAPO), which generates four different free radicals [1]. Allonas et al. [25] studied the effect of BAPO on autoxidative crosslinking of model alkyd resins, showing an increase in conversion and a decrease in drying time. The system required a photosensitizer to produce singlet oxygen leading to the cobalt drier to decompose the peroxyl radicals leading to crosslinking. Two commercially available PIs were chosen for this research; Irgacure 784 (IRG 784) and Irgacure 819 (IRG 819). IRG 784 is a fluorinated diaryl-bis-cyclopentadienyl titanium complex, or titacocene that photo dissociates at the bonding sites between the titanium and its four ligands. This forms a variety of free radicals and absorbs between 400 to 500 nm. IRG 784 has been studied to understand the reaction pathway with various findings [[26], [27], [28], [29], [30]]. The decomposition of the PI requires isomerization followed by photocleaving into unstable radical intermediates. It is used for imaging applications activated at 488 nm by argon ion laser [31]. Irgacure 819 undergoes type I homolytic cleavage, which excites at 405 nm forming two free radicals.

Photosensitizers (PS) change electronic states when excited by a specific wavelength. Aromatic ketones, such as benzil, are activated at 480 nm and the electron is promoted from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). This creates an electron hole as both the LUMO and HOMO contain unpaired electrons. This state allows high oxidizing potential in which one of two electron transfers can occur with the neighboring PI. If the HOMO of the PI is higher than the PS, the PI will act as an electron donor to achieve the lowest energy state. Conversely, if the HOMO of the PI is lower than the PS, the PI will become and electron acceptor. In both situations, both the PI and PS form free radicals that can participate in polymerization [32]. Ye et al. proposed that PS Rose Bengal dye acts as a singlet oxygen sensitizer, forming hydroperoxides via “ene” reactions and 1,2-cycloaddition of oxygen on an alkene, or 1,4-cycloaddition on conjugated double bonds of alkyd resins [25].

Herein, a method is demonstrated to reduce curing times of alkyd coatings using photoinitiators activated in the visible light spectrum to enhance the autoxidative curing process. Specifically, a crosslinking mechanism between epoxides and alkenes is proposed and the subsequent coating performance was analyzed.