Investigating UV light and ziram catalyst as alternative activators for sulfur-promoted benzoxazine curing

Abstract

Polymers have revolutionized nearly every aspect of our lives due to their remarkable range of properties. The desired physical and chemical properties can be easily obtained and used industrially. For example, they are used in packaging in the food industry, automotive, aviation, textile industry, etc. There are two main types of polymers, classified by their response to heat: thermoplastics and thermosets. Thermoplastics can be softened and reshaped when heated, while thermosets become permanently rigid due to their tightly linked molecular structure. This makes thermosets strong and resistant to solvents. Phenol-formaldehyde resins, the first synthetic thermosets, have many uses around the world. Thanks to its excellent thermal and mechanical properties, it is used in the coating, adhesion, and wood industry. However, in addition to these extraordinary features, it has some disadvantages, such as limited shelf life, by-product release during polymerization, and catalyst requirement. The synthesis of benzoxazine was first reported by Holy and Cope in 1944. Polybenzoxazines are also structurally similar to phenol-formaldehyde resins; additionally, they can overcome the disadvantages that phenol-formaldehyde resins face. Polybenzoxazines exhibit outstanding performance both thermally and mechanically. Their thermal stability is better than phenol-formaldehyde resins, they have high char yield, and they stand out both in industrial applications and academically with their features such as low moisture absorption, zero volumetric change during curing, and no toxic by-product formation, atom economy, and low cost. In addition, thanks to its functional groups, it can be easily derivatized, and the desired properties can be obtained. They can be easily processed and used with other polymers. Despite the many superior properties of polybenzoxazines, their most fundamental problem is the need for high temperatures during their polymerization. Polybenzoxazines are obtained by cationic ring-opening polymerization of 1,3-benzoxazine monomers. This reaction can occur with heat without requiring an extra curing agent. The curing temperatures required for this polymerization to occur are between 200-260 °C, depending on the structure of the monomer. However, curing at high temperatures can initiate degradation reactions with the network formation and ultimately destroy hydrogen bonds, which affect many properties of polybenzoxazines. Reducing these values, which are considered to be quite high, to more suitable temperatures will enable polybenzoxazines to be used more widely. It will make many applications easier. Many different catalyst systems have been tried to overcome this problem. According to the ring-opening polymerization reaction of benzoxazine, acidic compounds can trigger the ROP and lower the polymerization temperature. For example, Lewis acids PCl5, PCl3, POCl3, TiCl4, AlCl3, and MeOTf can lower the ring opening temperature. Although acidic compounds are successful in reducing the curing temperature, they bring other problems. The most important of these problems are: 1.Corrosion on equipment. 2.Early ring opening, that is, ring opening can be triggered when acid is added, but polymerization does not take place. Often, once acid is added, rapid curing is required. As time passes between acid addition and curing, instead of effective polymerization, oligomers and dimers and trimers are formed within the structure. 3.Acid catalysts, especially Lewis acids, form polyethers depending on the Lewis acid, as well as the typical polybenzoxazine structure in the obtained polybenzoxazines. 4.Acid catalysts cannot be placed in a same formula with benzoxazines. They do not show a latent character (catalyst that is activated when desired, dormant in the formulation under storage conditions). There is an increase in viscosity during storage. The catalyst and benzoxazine should be kept separate from each other and mixed only immediately before curing. 5.If Lewis acids would like to used, there should be little water in benzoxazine formulations, otherwise catalyst will not work. However, amines, amine salts, cyanuric chloride, lithium salts can be used as dormant catalysts. Apart from these approaches, it is also known that the ring opening temperature of benzoxazines can be reduced with sulfur, inspired by thiol-benzoxazine chemistry. In addition, elemental sulfur is the tenth most abundant element on Earth and is used in many different areas. For example, it is used in pharmaceutical production, fabric bleaching, gunpowder production and more recently in latex vulcanization. Being cheap and easy to produce makes sulfur preferable compared to alternatives. The largest source of sulfur production today is the refining of petroleum through hydrodesulfurization, producing approximately 70 million tonnes of by-product sulfur each year. Sulfur is used in many chemical reactions, but the vulcanization reaction is the most prominent. With this reaction, rubber or elastomeric materials are obtained. Thanks to this process, these materials return almost to their original shape after a significant mechanically imposed deformation. In addition, sulfur and benzoxazine also react and form copolymers. With the addition of sulfur, the ring opening polymerization temperatures of these copolymers decrease and soluble network structures are obtained. It is known that sulfur reacts with benzoxazines at around 160 °C and causes ring opening. Sulfur is an extremely cheap substance that is abundantly available. Above 160 °C, sulfur turns into polysulfide, but before it remains stable for a long time, it returns to elemental sulfur. When reacted with benzoxazines, the polysulfide binds to the polybenzoxazine and becomes stable. In other words, sulfur and benzoxazine transform into structures such as poly(benzoxazine-co-sulfide). This structure is cross-linked. This thesis investigated methods to reduce the ring-opening temperature of 1,3-benzoxazines. Sulfur/ziram and sulfur light systems were explored as potential catalysts. While traditional approaches utilize significant amounts of sulfur (around 50% by weight) to achieve a 30-40°C reduction. High sulfur content can lead to undermine the thermal and mechanical properties of the resulting polybenzoxazines. To address these limitations, this study utilized much lower sulfur content (1-3% by weight) by activating sulfur via catalyst or light. Thermal analysis (TGA) was used to evaluate the thermal properties of the cured polymers. Additionally, 1H NMR analysis confirmed the "latent catalyst" behavior of the systems at room temperature, indicating their potential for low-temperature activation.