Der Einfluß des Photoinitiators auf die radikalische Härtung von Siliconacrylaten, 9

Abstract

ZUSAMMENFASSUNGDie photoinduzierte radikalische Vernetzung von Siliconacrylaten wurde mit verschiedenen Photoinitiatoren vom Benzoyltyp untersucht. Die Photovernetzung derartiger Produkte kann mit Hilfe eines radikalischen Kettenprozesses beschrieben werden. Aus kalorimetrischen Messungen folgt, da8 Sauerstoff den KettenprozeB inhibiert und terminiert, wobei die Vernetzungsgeschwindigkeit und die Inhibierungszeit Funktionen des Luftdrucks, der Lichtintensitat und der Quantenausbeute der Initiatorphotolyse sind.Der VernetzungsprozeB kann nur beginnen, wenn der Sauerstoff in der Schicht verbraucht ist. Mit Hilfe des 1. Fickschen Diffusionsgesetzes kann gezeigt werden, da8 der in das System eindiffundierende Sauerstoff auch den Grenzumsatz bestimmt.Obwohl Sauerstoff in der Schicht die Reaktionsgeschwindigkeit, die Inhibierungszeit und den Grenzumsatz bestimmt, wurde jedoch uberraschenderweise festgestellt, daR Sauerstoff auf die Netzwerkdichte keinen EinfluB besitzt.Der negative Effekt des Sauerstoffs auf den VernetzungsprozeR kann durch eine hohe Startgeschwindigkeit (Funktion der Lichtintensitat, der Absorption und der Quantenausbeute der Initiatorphotolyse) reduziert werden.Mit Hilfe von zwei kinetischen Methoden wurde ermittelt, daB die Initiierungseffizienzen der verwendeten Initiatoren verschieden sind, wobei relative Abstufungen bestimmt werden konnten. SUMMARYThe photo-induced, free-radical network formation of silicone acrylates has been investigated in the presence of several photoinitiators containing benzoyl groups. The crosslinking process can be described by means of a free-radical chain process. From calorimetrical measurements it is inferred that oxygen inhibits and terminates the chain propagation process. Both the reaction rate and the inhibition time of the crosslinking are proportional to the air pressure, to the intensity of the incident light, and to the quantum yield of the initiator photolysis. The reaction starts when all oxygen is consumed in the silicone layer. Using Fick's 1st diffusion law it can be show that only oxygen, which diffused into the system, determines the final conversion.Contrary, 0, influences the reaction rate, the inhibition time, and the final conversion. The network density does not depend on the oxygen concentration.The negative influence of oxygen on the reaction rate, on the inhibition time, and on the final conversion can be reduced by means of a high rate of the start reaction which depends on the incident light intensity, light absorption, and the quantum yield of the initiator photolysis.By means of two kinetical methods it is possible to show that any initiator used exhibits a different initiating efficiency; their relative order has been estimated.

Convection Patterns Trapped in the Solid State by UV-Induced Polymerization

“…The initiator 1-hydroxycyclohexyl phenyl ketone (HPK) (Aldrich) was used as received. HPK has a quantum yield of photolysis of Φ = 1, and it yields clear cured films of poly(dimethyl acrylates) and polyesters . The concentration of the initiator, c HPK , in the MAOP-DMS films varied from 0.25 to 5.0 wt %.…”

Temperature Dependence of the Oxygen Solubility in Acrylates and its Effect on the Induction Period in UV Photopolymerization

“…The initiation quantum yield of BDK was reported to be 0.52. [22][23][24][25] In Figure 6, the values of t i which were calculated from the oxygen solubilities are plotted vs. the experimental data from RT-FTIR spectroscopy which were taken from Figure 4. From the close correlation between the both sets of data it can be concluded that the decay of the induction period with increasing temperature is solely due to the decreasing oxygen solubility in the acrylate.…”

Catalytic Systems for the Cross‐Linking of Organosilicon Polymers

“…[17a] Acrylates are arguably the most popular cross-linkers for free-radical photo-curingo fs ilicone formulations. [32] Their main advantages are high reaction rates, solvent-free conditions, ambient temperature operation, and high spatial resolution, whereas their biggest disadvantage is oxygen inhibition, which may lead to tackiness and poor structural proper-ties. [33] The oxygen level in at ypical industrialc oating line shouldb eb elow 50 ppm.…”

“…By irradiation with UV‐light or low‐energy electron beams (EB), polysiloxane chains containing reactive functionalities such as epoxide ( 7 ), acrylate ( 8 ), thiol ( 9 ), and vinyl ( 10 ; Scheme ) can be cross‐linked by free‐radical or cationic polymerization mechanisms . Acrylates are arguably the most popular cross‐linkers for free‐radical photo‐curing of silicone formulations . Their main advantages are high reaction rates, solvent‐free conditions, ambient temperature operation, and high spatial resolution, whereas their biggest disadvantage is oxygen inhibition, which may lead to tackiness and poor structural properties .…”