INVESTIGATION OF A RADICAL INDUCED CATIONIC FRONTAL PHOTOPOLYMERIZATION (RICFP) IN HETEROGENEOUS SOFT POROUS MEDIA

In recent years, photopolymerization process has gained more and more importance according to its economical and ecological value. A wide range of application are based on photopolymers such as adhesives, coatings, inks and microelectronics covering a huge number of fields such as biomedical and industrial sectors. The main drawback related to UV-curing processes is the inability to reach deep layers and creating three-dimensional thick crosslinked structure principally due to the light absorption phenomena. RICFP process is defined as the combination of a photo-induced surface curing of the formulation and the subsequent thermal-activated reaction front which will lead to the polymerization of thick parts in a short time. Implementing one mechanism that induces the acquisition of a specific shape in a very fast way can be very appealing. In many applications, holes, cavities, or, breaks in continuity represents a huge issue. Structural and mechanical instabilities linked to these agents will lead inexorably to catastrophic failures. In these conditions, reaching a high mechanical stability filling in a controlled way the break in continuity is of paramount importance. Another example can be the ear cavity and the requirement of producing a personalized earbud. Lots of benefits can be envisaged: first of all a better user experience mainly due to the perfect adaptation of the earbud to the ear cavity. As a result, the device won’t hurt when a prolonged use is required, won’t fall frequently and will ensure a better insulation from external noises. The system should be composed by two important elements, one (soft foam) must have the ability to reach the desired shape and the other (UV-curable formulation) must induce its solidification when an external stimulus occurs. The main problem related to this procedure is the high exothermicity and the large production of gasses. If structural applications are considered, degradation phenomena related to the studied device may endanger the mechanical stability. In addition, temperature is a severe factor when a medical device is considered, such as the earbud, considering that it must not be higher than 38 °C due to the high heat sensitivity linked to the ear cavity. In the present study, a screening of the main properties and phenomena involved in an RICFP process were investigated using thermographic, thermogravimetric, DSC, Photo-DSC and FTIR analysis. In addition, the focus has been set in kinetic, structure, degradation and conversion behaviour changing the amount of used initiators. Best result in terms of structure, has been ascribed to the formulation in which the amount of both initiators is minimum (0,5%wt for both of them). Samples related to this composition showed better conditions in terms of structure retention, and exothermicity. Temperature values recorded are extremely high if compared with the temperature threshold imposed for skin-contact devices. TGA measurements showed the presence of degradation in all studied samples. Photo-DSC measurements confirmed the skin effect present at higher amount of photoinitiator as well as shield effect at higher amount of thermal initiator, leading to a possible delay of the starting time of the front. Finally, another porous media, an aluminium foam, was cured taking advantage of the studied process. This could open a wide new chapter on ultralightweight composites field maintaining all the advantages related to UV curing process.