The sustainable use of resources in the production, use and disposal of a product is one of the current challenges that the European industry faces, which demands innovative developments in materials and joining technologies implementing the circular economy model (1) (2).. Due to the continuous development of raw materials, adhesives and adhesive bonding products, adhesive bonded technologies have gained a high level of acceptance in recent decades. In view of the steadily increasing regulatory requirements, a wide range of innovative solutions are already being successfully implemented through more environmentally friendly adhesive developments (3).
In several industrial sectors, the use of polyurethanes is key to the development of innovative and sustainable products. At present, the raw materials used to produce these polyurethanes come mainly from non-renewable fossil resources, which means that they need to be replaced by other low-carbon materials of renewable origin. Therefore, in the polymer industry, and specifically, in the polyurethanes sector, there is a ocurrent towards sustainable products that minimise the use of petroleum resources, without worsening their performance (4).
There are various alternatives for obtaining polyurethane from renewable raw materials such as vegetable oils (soya, sunflower, etc). In recent years, they are one of the main alternatives to end the use of petroleum-derived materials in the production of polyols, these being the basic raw material for the development of polyurethane (5). Their use as a reagent for the synthesis of polymers represents numerous advantages, for instance their biodegradability, low toxicity, sustainability, industrial viability, cost competitiveness and design of final polymer properties. Soybean, sunflower, rapeseed, linseed and castor oils are the most commonly used in polymer synthesis through chemical modification (6) (7) (8).
The major components of vegetable oils are triglycerides, which are esters of glycerol with three long-chain fatty acids (9). Fatty acids can be obtained from these vegetable oils by hydrolysis and subsequently transformed into polyols through different production routes (epoxidation, ozonolysis...) (10) (11) (12).
Another alternative to produce polyurethane is the use of carbon dioxide (CO2) as a reagent base for polymer synthesis. CO2 is one of the greenhouse gases that contributes more than 60% to global warming due to its large number of emissions into the atmosphere, which are uncontrolled emissions that could be a source of raw materials (13) (14).
Although it is true that the uncontrolled increase of this gas poses a threat to our planet, CO2 is part of our life, for example, through the breathing process of living beings. At industrial scale, it is also used as an additive in food preservation and in the carbonation of beverages such as sparkling water, soft drinks, beer, etc. (15) (16)
This greenhouse gas is an interesting alternative to oil, as CO2 is a useful, versatile, non-flammable product and is abundant in the atmosphere. In particular, carbon dioxide can be used for the production of polyols, as in the case of vegetable oils. This is a significant innovation, as this greenhouse gas leads to a future sustainable raw material for the chemical industry (17) (18).
Polyurethanes, whose history is relatively short at just over 80 years, constitute the most versatile family of polymers currently in existence, due to their properties being suitable for a wide range of applications, such as foams, elastomers, thermoplastics, thermosets, adhesives, coatings, sealants, fibres, paints, etc (19) (20).
Specifically, in the footwear sector, polyurethanes (PU) are one of the most used materials, in insoles, coatings, elastomers, adhesives, etc., thanks to their great versatility (21). Indeed, TPUs are a relevant class of thermoplastic elastomers with a wide variety of compositions and properties. Similarly, PU adhesives are of great importance in the footwear sector, fulfilling technical requirements across the wide range of materials used in footwear manufacturing (22).
Polyurethane adhesives are highly versatile and can be designed with tailor-made and improved properties, including sustainable ones, through the selection of their reagents (23) (24). The substitution of reagents from fossil resources with those from biological sources or CO2 as a reagent base for polymer synthesis provides them with additional beneficial non-toxic and ecological characteristics, without detriment to their performance for application in the footwear industry (25) (26) (27) (28) (29) (30).
In this work, INESCOP focuses on the synthesis and characterisation of polyurethane reactive hotmelt bioadhesives based on polyols from renewable sources or from CO2 captured in industrial environments, allowing the reduction or removing fossil-based raw materials, and opting for eco-friendly ones. These sustainable raw materials provide polyurethane adhesives with additional beneficial non-toxicity and sustainable characteristics, without harming their properties during their useful life.