Use of curcumin and glycerol as an effective photoinitiating system in the polymerization of urethane dimethacrylate

Abstract

The objectives of this study were to verify the viability of the photoinitiating system using curcumin as a photoinitiator and glycerol as a coinitiator, to analyze the thermal and morphological features of the polymers produced, and finally evaluate the possibility of removing toxic Ethyl-p-dimethylaminobenzoate from the photoinitiating system. Curcumin, an orange-yellow dye extracted from the rhizoids of the plant Curcuma longa, is known to be a common photosensitizer, and the type II photoinitiating system consisting of a dye and an amine is long known to be effective. In recent years, the production of biodiesel has increased, and consequently, so has the generation of its main by-product, glycerol. Thus, it becomes necessary to study ways to incorporate glycerol into products. Using thermoanalytical methods (Thermogravimetry, Differential Thermal Analysis, and Differential Scanning Calorimetry), Middle Infrared Spectroscopy, and Scanning Electronic Microscopy, it was possible to assess the thermal and morphological characteristics of the polymers produced, and compare whether the presence or lack of tertiary amine, as well as the addition of different molar ratios of glycerol, had any significant impact on these characteristics. Results suggested that neither the removal of tertiary amine nor the addition (in different proportions) of glycerol affected the thermal stability of the polymers. Also, removing the tertiary amine enhanced the total conversion degree of the polymers.

Dimethacrylate polymers with different glycerol content

“…Data were collected every 10 s, and 40 measurements were made for all samples. Equation (1) was used to quantify the degree of conversion for each sample [19,20,27,28].…”

“…The reaction was initiated with free radical photoinitiators. The CQ absorbed visible light in the spectrum of 400-550 nm, reaching an excited state and withdrawing a hydrogen atom (H.) from the tertiary amine or glycerol [17][18][19][20], to produce a primary radical. Figure 1 shows the reaction of initiator formation by light.…”

“…The radical triggers the homolytic cleavage in p bonds (C = C) on dimethacrylate monomers, which produces new radicals that propagate the reaction to form the polymer [19,20,27,29]; this mechanism can be viewed in Fig. 2a.…”

“…The following monomers are used: diurethane dimethacrylate (UDMA), triethylene glycol dimethacrylate (TEGDMA), bisphenol a glycerolate (Bis-GMA), and bisphenol a ethoxylate (Bis-EMA); these are the most used monomers to produce biomaterials and in industry. The two-component (Type II) method is the most widely used in photopolymerization (polymerization by UV or visible light), applying camphorquinone as initiator and coinitiators such as ethyl-p-dimethylaminobenzoate, which can be combined with glycerol [17][18][19][20], which is a non-toxic compound and is considered as ''generally recognized as safe'' (GRAS) [21].…”

“…Thermoanalytical techniques such as simultaneous thermogravimetry-differential thermal analysis (TG-DTA) and differential scanning calorimetry (DSC) are used in order to determine their thermal stability, degradation steps, glass transition, melt, and polymer temperature cure in other works [22][23][24]. To verify the extent to which polymerization occurs when using different ratios of glycerol in the dimethacrylate systems, middle infrared spectroscopy (MIR) could be used [17][18][19][20]. Scanning electron microscope (SEM) is widely used to analyze the morphological aspects of the polymers [25,26].…”