Environmental assessment of an innovative adhesive for the footwear industry: road map for product development

The world footwear production in 2020 was 20.5 billion pairs, representing a relevant sector of the commercial market, even though the COVID-19 pandemic hit the footwear business severely, leading to a reduction in world’s production by almost 4 billion pairs compared to the previous year. The impact of the pandemic was transversal, and at an aggregate level, it did not significantly change the geographical distribution of footwear production. Asia continues to be responsible for nearly nine out of ten pairs of footwear produced and has even increased its share by 0.1% points. Africa and Europe also achieved slight increases in share, at the expense of North and South America, with Oceania playing a minor role in the sector (APICCAPS 2021a).

Portugal accounts for 0.3% points of the world’s production (66 million pairs), being the vigesimal footwear producer of the world. However, Portugal has a strong tendency to export, exporting 93.2% points of its production (61 million pairs) and charging an average export price of more than 20 dollars per pair (APICCAPS 2021a). Despite the numerous challenges faced in 2020, the Portuguese footwear industry managed to export nearly 1.5 billion euros. The following year, in 2021, the industry experienced significant growth of 12% in foreign markets (APICCAPS 2021b). Furthermore, this industry has been an example and a reference for the Portuguese economy (DGAE 2017), as it has established itself in markets worldwide such as France, German, Netherlands, Spain, and UK (APICCAPS 2021a), is responsible for a significant part of exports, and is one of the most dynamic in the business sector. In addition, as stated by the Portuguese Agency for Investment and Foreign Trade (AICEP), the footwear industry in Portugal is currently recognized as one of the most innovative and competitive sectors within the country's economy. More than 1500 Portuguese companies operate in the footwear, components, and leather goods sectors, employing 40,000 people (APICCAPS 2021b). For these reasons, it has become essential to increase the degree of sustainability of this sector.

Footwear manufacturing involves transformation and assembly of various components made up of several materials where different adhesives play a key role, because, without them, the shoe would lack shape and structure (Orgilés-Calpena et al. 2019; Paiva et al. 2016a, b). A single running shoe can contain 65 discrete parts that require 360 processing steps for assembly (Cheah et al. 2013). In spite of the important function of the adhesives, their actual content is normally very low in a bonded product and is in most cases less than 1% of the final product weight (Cheah et al. 2013; Industrieverband Klebstoffe e.V. 2014). Nevertheless, footwear workers are routinely exposed to complex mixtures of solvents in them, such as toluene, n-hexane, and acetone (Gargouri et al. 2016; Heuser et al. 2005; Mayan et al. 2010; Staikos et al. 2006).

Like most industrial processes, the production and use of adhesives generates pollutants which can have an adverse health and environmental effect. The European adhesive market in 2022 was 4 million tonnes and is forecasted that its demand will increase in volume between 2021 and 2028 (FEICA 2024). Adhesives are a major industrial source of volatile organic compounds (Gargouri et al. 2016; Metzger and Eissen 2004; Packham 2009; Staikos et al. 2006). Much attention has been given in recent years to reducing their impact. It is now generally recognized that the emission of any volatile organic compound to the atmosphere is undesirable, as they contribute to the formation of photochemical smog, absorb infra-red radiation, and therefore act as greenhouse gases, and many are implicated in the aggravation of lung diseases such as asthma (Packham 2009). Isocyanates are regarded as one of the main causes of occupational asthma (Baur et al. 1994; Ameille et al. 2003; Lefkowitz et al. 2015; Gomez-Lopez et al. 2021; Karlsson et al. 2022). The large number of workers who are exposed to these chemicals has a concentration-dependent risk of developing chronic airway disorders, especially bronchial asthma (Baur et al. 1994; Coureau et al. 2021). Several studies describe the isocyanate emission potential of polyurethane adhesives (Heuser et al. 2005; Wirts et al. 2002) that may spread in aerosolized or gaseous form when heating or when vapors escape to the workplace air from open vessels at room temperature (Coureau et al. 2021; Heuser et al. 2005; Paal et al. 2002; Zhong and Siegel 2000). Respiratory disorders associated with isocyanate exposure (Collins 2002; Heuser et al. 2005; Paal et al. 2002; Skarping et al. 1996), and toxicity and/or genotoxicity, even in polymerized form, are described in several publications (Andersen et al. 1980; Bilban 2004; Collins 2002; Heuser et al. 2005; Coureau et al. 2021;  Kligerman et al. 1987; Maki-paakkanen and Norppa 1987; Mori et al. 1988; Zhong and Siegel 2000) and occupational exposures (Heuser et al. 2005; Karlsson et al. 2022; Leng 2016; Marczynski et al. 1992). Isocyanate has been classified as a carcinogen in animals (Heuser et al. 2005; IARC 1999; NTP 1986; Senthilkumar et al. 2012) and is a suspected carcinogen in humans (Heuser et al. 2005; IARC 1999; Senthilkumar et al. 2012).

The recognition of the potential health-hazards and environmental impacts of solvent-based adhesives has led to the development of adhesives with no organic solvents. Many adhesive systems formerly based on organic solvents are now produced as aqueous emulsions. Polyurethanes (PU) have become one of the most widely used classes of polymers and today are found in many high-performance materials, such as adhesives (Nasar et al. 1998; Paiva et al. 2016b). Nonetheless, guided by environmental concerns and legal obligations, greener and safer alternatives to conventional PUs are now being sought to avoid the use of toxic isocyanates that are one of the primary components used in their formulation. Despite all these developments, volatile emissions associated with adhesive application remain high (Coureau et al. 2021; Metzger and Eissen 2004; Packham 2009). Furthermore, some studies emphasize that the performance of many newly developed products in this field is not benchmarked to those of commercial analogues, which makes direct comparison difficult (Gomez-Lopez et al. 2021).

Several authors already see the life cycle assessment (LCA) methodology as a very useful tool for evaluating the environmental impact of footwear, and in particular of adhesives (Eisen et al. 2020; Maciel et al. 2017; Packham 2009; Yang and Rosentrater 2019). According to Milà et al. (1998) the manufacturing stage showed the main environmental burdens that contribute to the environmental impact of the footwear product system life cycle. This occurs mainly due to energy requirements in many shoe manufacturing steps, including drying both adhesives and primers, which leads to another environmental burden: organic emissions (Borchardt et al. 2011; Cheah et al. 2013; Maciel et al. 2017). However, this study did not include the adhesive production because they considered the weight of this component to be negligible. Albers et al. (2008) analyzed 4 different models of shoes; however, they exclude the adhesives as they considered the weight of this component to be negligible in the analysis. The same can be said for the case of the study by Milà et al. (1998), which also excluded the adhesives from the analysis. Packham (2009) discussed the significant environmental impacts of adhesive technology based on a qualitative assessment. However, he stresses the need of the application of a holistic life-cycle analysis already during adhesive development. Also, it is pointed out that there are already improvements, but also in the future, there is still a great need for research on renewable raw material sources, energy savings and the avoidance of emissions. Cheah et al. (2013), performed a carbon footprint study from cradle to grave, or life cycle Global Warming Potential (GWP), of a pair of running shoes and suggested strategies to reduce the product’s impact. The results indicated that most of the emissions are released during shoes’ material processing (29%) and manufacturing phase (68%). Concluding that the polypropylene glycol (PPG) adhesive component contributes only 1% to the total environmental impact. Muñoz (2008) applied the LCA methodology (analyzing water consumption, energy consumption and GWP indicators) to a pair of leather shoes, considering the entire life cycle of the shoes including the bonding of the different components. This study considered that a pair of leather shoes uses 168 g of adhesive and that this contributes to the GWP by 0.13456 kg CO₂ eq/pair of leather shoes. According to Paiva et al. (2016b), the footwear industry has a close association with the adhesive industry, using bonding techniques to join the variety of materials employed in assembling shoes. However, this study did not perform a full life cycle assessment. Yang and Rosentrater (2019) also complain that there are insufficient comparisons from other studies between petrochemical-based adhesives and bio-based adhesives that could be used for comparisons, which is a limitation in the interpretation of LCA results. Maciel et al. (2017) studied three polyurethane adhesive technologies used in the footwear industry: a solvent-based adhesive (SBA), a water-based adhesive (WBA), and a powder-based adhesive (PBA), using the LCA methodology. The analysis showed that any actions that seek to minimize environmental impacts should begin in “the footwear industry,” more precisely, in the stage of adhesive use due to the electricity required during the adhesive application. Therefore, a better management of the energy expended during the application step is suggested from renewable energy sources, improvement of equipment energy efficiency, and development of new formulations are potential alternatives for solutions seeking to reduce environmental impacts involving all adhesive technologies and consequently shoe production. Also, the studies analyzed in the literature review by Eisen et al. (2020), in which adhesives are considered, show that adhesives in particular usually have a very large influence on the LCA of the product system. Nevertheless, the environmental impacts were often only examined in relation to the entire product system, which meant that the environmental impacts of the adhesive technologies were always in relation to the entire product system and the adhesive itself was therefore difficult to assess. On the whole, it is positive to note that the number of relevant studies has increased in recent years and thus indicates an emerging relevance for the topic.

Nowadays, the LCA methodology is a well-established and widespread, though still evolving, tool and the only internationally standardized environmental assessment method (ISO 2006a; b). These and other considerations led to an increasing number of LCA-related publications, namely, those referring to adhesive production and application (Eisen et al. 2020; Gonzalezet al. 2017; Liu et al. 2018; Maciel et al. 2017; Packham 2009; Yang and Rosentrater 2019).

For all these reasons, there is a need of increasing the use of technical ingenuity in order to develop ways of addressing all or most of these problems. In view of this conjuncture, an advanced technology (microencapsulation) was applied to develop a new adhesive, with microencapsulation of the isocyanate compounds. The microencapsulation of the isocyanate eliminates the risks associated with their direct handling, protects the isocyanate species from air moisture, increases the storage life, and at the same time offers control over its triggered release (Aguiar et al. 2023a; Loureiro et al. 2023). It must be emphasized that a broad view of environmental impact must be taken. For this reason, it has become essential to assess the life cycle environmental impact of the newly developed adhesive when compared to commercial adhesive for the footwear industry. The main objective was to evaluate the possible replacement of the traditional adhesive used in the footwear industry (PU adhesive) by other safely and technically more favorable (PUMC adhesive). It is important to state that a simultaneous technological validation study demonstrated the feasibility of replacing the traditional adhesive by the novel one, ensuring at least a similar technical performance of the adhesives.

The potential environmental impact of the adhesives (PU and PUMC adhesives) was evaluated using the LCA methodology, which includes all stages of a product’s life (Bauman and Tillman 2004; Finnveden et al. 2009; Guinée, 2002; Pennington et al. 2004; Rebitzer et al. 2004). The methodology was performed in accordance with the standards from the ISO 14040 series (ISO 2006a; b). It comprises four major stages, the definition of goal and scope, life cycle inventory analysis (LCI), life cycle impact assessment (LCIA), and results’ interpretation.

In this study, a LCA comparison was performed between a traditional adhesive used in the footwear industry (PU adhesive) with a novel microencapsulation approach which is safer and technically more favorable (PUMC adhesive). PUMC adhesive is a new adhesive prepared using a state-of-the-art technology (microencapsulation of isocyanates).

The findings indicate that the conventional PU adhesive’s environmental footprint stems from its production phase, involving the consumption of acetone and polyol, as well as the transportation of materials and energy (specifically, electricity) requirements. For the PUMC adhesive, the production stage is also the main contributor to the environmental impact. However, in this case, the acetone consumption and microcapsule production are the major factors responsible for the environmental impact. Within the microcapsule production stage, the polybutylene adipate terephthalate (PBAT), dichloromethane (DCM), and isophorone diisocyanate (IPDI) consumption are the main responsible for the environmental impact. In general, PUMC adhesive has the biggest impact in all categories, except in the Carcinogens environmental impact category.

A sensitivity analysis was conducted using a scenarios study to determine the response of the PUMC adhesive system to model, scenario, and parameter variability. Three cumulative alternative scenarios of PUMC adhesive systems were assessed: AS1—PUMC adhesive microcapsule production efficiency of 90%; AS2—plus PUMC adhesive microcapsule production with a 90% DCM recuperation; and AS3—plus alternative PUMC adhesive energy source (photovoltaic panels). As expected, the three alternative PUMC adhesive systems have a lower environmental impact than the PUMC adhesive; however, their environmental impact remains higher than that of the traditional PU adhesive, except for the Carcinogens environmental impact category. The alternative PUMC adhesive scenarios results show differences equal to or less than 10% in some environmental categories, and therefore should be considered equivalent to the PU adhesive system in these categories.

Thus, future studies that aim to decrease the environmental impact of the PUMC adhesive should focus on the optimization of the microcapsule production stage, seeking to increase the production efficiency in order to decrease material consumption. The results show that using an alternative renewable energy source does not significantly improve the environmental impact (compared to a decrease in material consumption).

Despite the fact that the PUMC adhesive has a slightly higher environmental impact compared to the traditional PU adhesive, the increased safety of the novel approach gives its merit. The production and use of adhesives generate pollutants which can have an adverse health and environmental effect (Coureau et al. 2021; Metzger and Eissen 2004; Packham 2009; Staikos et al. 2006), and footwear workers, in particular, are routinely exposed to isocyanates that are regarded as one of the main causes of occupational asthma (Baur et al. 1994; Ameille et al. 2003; Lefkowitz et al. 2015; Gomez-Lopez et al. 2021; Karlsson et al. 2022). With the proposed microencapsulation approach for the isocyanate compounds, there are zero emissions of isocyanate within the footwear production factory, isolating the factory workers from those compounds.

Another aspect that should be assessed in the future is the adhesive waste resulting from the industry footwear production process. The authors believe that the use of the PUMC adhesive will lead to a decrease in leftover adhesive (the traditional adhesive has a short life span once blended for use, and some of it often ends up curing from being exposed to air and is then wasted). In the PUMC adhesive, the isocyanate compounds are microencapsulated and the curing process is controlled by the footwear worker, thus increasing the lifetime of the adhesive blend and consequently decreasing adhesive waste.

In conclusion, this study describes and analyses the development of a new technology for footwear adhesives based on microencapsulation of isocyanate and its environmental advantages and disadvantages concerning the traditional adhesive that uses non-enclosed isocyanate. Finally, the PUMC adhesive also shows potential for mitigating some environmental impacts.