Research Article pubs.acs.org/journal/ascecg
Comparative Life Cycle Assessment among Three Polyurethane Adhesive Technologies for the Footwear Industry Vinícius Gonçalves Maciel,*,† Geovana Bockorny,‡ Nei Domingues,‡ Moara Britz Scherer,‡ Rafael Batista Zortea,§,∥ and Marcus Seferin† †
Post-Graduation Program in Materials Engineering and Technology, School of Chemistry, PUCRS−Pontifical Catholic University of Rio Grande do Sul, Av. Ipiranga, 6681, 90619-900 Porto Alegre, RS, Brazil ‡ Artecola Química, R&D Adhesives, highway RS 239, no. 5801, 93900-000 Campo Bom, Brazil § IFSUL−Sul Rio Grandense Federal Institute, Av. Piratini, 100, 93216-120 Sapucaia do Sul, RS, Brazil ∥ 3BL do Brasil−Sustentabilidade e Resultado, Av. Otto Klein, 682, 93900-000 Ivoti, RS, Brazil ABSTRACT: This study presents an environmental assessment involving 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). SBA is a versatile adhesive, but the presence of volatile organic compounds has some disadvantages regarding both environmental and workers’ welfare issues. On the other hand, PBA and WBA require more complex processing than SBA. PBA is a hot-melt adhesive in powder form and a solvent free thermoplastic material, presenting lower risks to workers’ health and flammability, but its application requires electric energy because it is carried out by machine. Thus, a comparative study among these three polyurethane adhesive technologies using a life cycle assessment methodology was conducted from “cradle-to-gate”. Primary data for environmental emissions, wastewater, chemical components, and technical specifications were collected during visits in local. Based on the results, PBA decreased environmental impacts in all categories evaluated except in the respiratory organic category. Therefore, if the purpose of this analysis is to look at the footwear workers’ health, WBA would be considered the best technology. This work shows that any actions that seek to minimize these impacts should begin in “the footwear industry”, more specifically; in the stage of use due to the electricity required during the adhesive application. Besides that, all three technologies offer possibilities to minimize some of the environmental impacts. Therefore, it is suggested that better management of the energy expended during the application step from renewable energy sources, improvement of equipment energy efficiency, and development of new formulations are potential alternatives for solutions seeking to reduce impacts involving all adhesive technologies and consequently shoe production. KEYWORDS: Life cycle assessment, Adhesive, Polyurethane, Footwear industry, Hot-melt
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INTRODUCTION The Brazilian footwear production in 2013 was 899 million pairs, representing a major participation in the National Trade Balance.1 In this case, the supply chain diversity formed with several actors in a wide geographical location increased concerns regarding the working conditions in that industrial sector, raising questions about potential environmental and social impacts in the shoe industry.2,3 It is important to highlight that the footwear in the state of Rio Grande do Sul, specifically in the Sinos Valley region, is responsible for the major production and export trade in Brazil. Therefore, it is very important to find alternatives to minimize environmental impacts on sustainability in the Brazilian shoe industry. 4−6 Moreover, according to Milà et al. 4 the manufacturing stage showed the main impacts that contribute to the environmental profile for the footwear product system © 2017 American Chemical Society
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.5,6 According to Paiva et al.7 the footwear industry has a close association with the adhesives industry, using bonding techniques to join the variety of materials employed in assembling shoes. However, these studies did not perform a full life cycle assessment. According to Packham,8 it is not just the energy involved during the adhesives production and processing that would be relevant. This author highlights that it is important to consider the environmental impact by a life cycle approach from the initial Received: July 24, 2017 Revised: August 8, 2017 Published: August 17, 2017 8464
DOI: 10.1021/acssuschemeng.7b02516 ACS Sustainable Chem. Eng. 2017, 5, 8464−8472
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ACS Sustainable Chemistry & Engineering
Figure 1. Illustration of the difference in the application step among these three technologies: (a) application step of the water and solvent based adhesives and (b) application step of the powder-based adhesive.
used for chemical treatment to change the polarity, minimizing the flow of the adhesive into the pores and improving the compatibility of the adhesive substrate.7 Figure 1 presents an illustration with these different compatibilities in the application step among these three technologies. It is important to highlight that water- and solvent-based adhesives are applied similarly (Figure 1a); it is necessary to apply in two parts, and powder adhesive is applied in only one part (Figure 1b), see Table 2. Thus, an environmental assessment could be necessary to consider which adhesive technology will be the best choice from the environmental point of view in a life cycle approach. In this context, life cycle assessment (LCA) is a robust tool to answer these questions, and due to its solid credibility and scientific recognition it is being explored to investigate industrial processes.14 Therefore, this work presents a comparative life cycle assessment among three different adhesive technologies: powder-based (hot-melt in powder form), solvent-based, and water-based using the same polymeric base. This study was performed based on understanding, diagnosis, and collection of data related to adhesive production and application processes.
extraction of raw materials through to manufacture and the use stage. In this context, solvent-based adhesive (SBA) in the shoe industry is a technology used for a long lasting product. However, the use of heat is a great disadvantage for the environment and footwear workers.9 These workers have been routinely exposed to complex mixtures of solvents in degreasers, cleaners, primers, and adhesives.10 Furthermore, organic solvents can be correlated with increasing the risk of some cancers.2 In this context, recognition of the potential health-hazards of SBA has led to the development of adhesives with no organic solvents, for instance, the water-based adhesives. Water-based adhesive (WBA) is a chemical material with good performance in shoe bonding that is easy and fast to apply. The WBA can be manually applied to the joining surfaces, but heating equipment is required during bonding processes in order to evaporate water, similar to solvent-based adhesive.7,11 Recent technology such as powder-based adhesives has been explored. This technology is based on the hot-melt adhesive as a thermoplastic material, free of solvents and heated for melting and application. In this work, the hot-melt used in the powder form may offer a versatile and economic technology. This technology requires a small amount of adhesives, since it utilizes an automatic equipment that reuses all powder material (recycling) and requires a low temperature to melt the adhesive in the shoe assembly.12 The processes using powder- and water-based adhesives are delicate and complex compared to those for solvent-based adhesive. However, they might cause less environmental impact, because they reduce volatile organic emissions (VOCs), for example, reducing risks to workers’ health and even flammability.12,13 Summarizing, besides differences in manufacturing, water- and solvent-based adhesives differ from the powder-based adhesive (PBA) mainly in relation to application steps. First of all, water and solvent based adhesive do not require the use of energy, because they are usually a manual application,7 whereas PBA demands electric energy to be used, because it is carried out by machine, and it is a standard industrial process.12 On the other hand, water- and solvent-based adhesives require a significant amount to be bonded whether compared with PBA. However, the substrate that receives the PBA requires additional substances (primers). These substances are
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METHODOLOGY
In this study an LCA methodology was employed. It is an essential tool when the aim is to evaluate the environmental impacts throughout the process system. The LCA is also a methodology that encompasses all processes and stages related to a product or process and their corresponding inputs and output amounts resulting in a certain environmental burden of products at all stages.15 Therefore, this study was based on Standards ISO 1404016 and 14044,17 and all primary data used in this work operated with 2015 values as baseline. The Impact 2002+ baseline method was applied to assess the midpoint impacts.18 Also, the WSI baseline method by Boulay et al.19 was employed to measure the water footprint. The impact categories were defined in several discussion rounds and based on studies concerned with footwear manufacturing. During these rounds information was exchanged regarding the meaning of life cycle impacts and the main output flows and potential production hotspots associated with adhesive production and their raw materials. Lastly, the main flows that integrate the most used impact methodologies were analyzed. These discussions resulted in the following impacts for this study: global warming (GW), ozone layer depletion (OL), nonrenewable energy sources (NR), respiratory organics (RO), and water footprint (WF). Although Milà et al.4 had said that potentially toxic substances emitted during footwear manufacturing come mainly from adhesives used, this work opted for respiratory organics impact in place of human toxicity potential impact. This choice was made, because the inventory done presented acetone as the main emission 8465
DOI: 10.1021/acssuschemeng.7b02516 ACS Sustainable Chem. Eng. 2017, 5, 8464−8472
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Figure 2. System boundary of three polyurethane adhesive technologies (process and/or flow in orange color: product system of SBA; process and/ or flow in blue color: product system of WBA; process and/or flow in green color: product system of PBA; process and/or flow in black color process and/or flow are common to SBA and WBA systems; gray color process and/or flow are common to all product systems). for all technologies. However, the impact method selected to calculate the human toxicity potential impact did not contemplate acetone as one of the emissions; therefore, this work changed for a respiratory organics impact that encompasses acetone as an emission. Goal and Scope. The study considered flow boundaries from the cradle to the customers’ gate, in other words, beginning with the extraction of resources to produce raw materials until the phase when adhesives were used. In relation to upstream processes, the work is based on Ecoinvent v3.0 database and the Simapro Software v 8.003 which were used for process modeling and impact characterization. Figure 2 presents the elementary flows related to three technologies. Moreover, the functional unit (FU) defined in this study was one square meter of bonded surface following the Standard ABNT NBR:2012 − Adhesives for Footwear and Resistance Bonding.20 This FU was defined, because it was considered suitable to compare adhesives performance.15 In the present work, the same polymer base, polyurethane, was used. Considering each technology, it is important to highlight that while in the hot-melt technology thermoplastic polyurethane in powder form was used, the solvent-based works with polyurethane dissolved in organic solvents and the water-based adhesive operates with polyurethane in emulsion. This is a versatile and flexible polymer for use in the footwear industry, because it offers good compatibility with a diversity of materials used in shoe manufacturing presenting desirable properties of bonding resistance.12,21 The production of SBA is simple. The process begins with a solvent mixture and after that polymers and additives are introduced. Then all of these components are blended. Lastly, the final product is packed, usually into metal containers. The WBA manufacturing process is more complex compared to SBA, because it involves a chemical reaction related to polyurethane polymerization. Following the polymerization, some specific additives are introduced with the intention of controlling viscosity and solid contents. In addition, for each batch it is necessary to clean the mixing
reactor after the manufacture. This is done with water and the wastewater generated is sent to an industrial waste treatment plant. Finally the powder technology is 100% solid material, and it is composed only of solid raw material in particulate form. The adhesive works in a specific size being responsible for a reduced and low cost bonding process. The WBA and SBA present a similar use process (application). First, it is necessary to clean the surface that will be bonded, and then a specific primer is applied after which an adhesive is applied manually. Once these stages are completed, the adhesives are reactivated by heat, and finally the surfaces that must be bonded are pressed. Regarding PBA, the two first steps are similar to SBA and WBA; however, the third step uses a specific machine (automated system) for the adhesive application. It is important to highlight that this system has advantages as, for instance, it uses adhesive on only one shoe surface and the composition is solvent free. Last but not least the PBA technology is made at another manufacturing plant located around 1000 km from the main factory. Data Collected. The life cycle inventory was made using primary data collected in an adhesive production plant related to two technologies: WBA and SBA at the adhesive factory. Referring to PBA technology the adhesive factory provided data collected from their suppliers.12 All flows associated with equipment manufacturing employed during the adhesive technology life cycle were left out of this study. Thus, the infrastructure is not considered in this work for two reasons: (i) there is a lack of available LCA data on the all of the infrastructure related to the life cycle of all adhesives assessed in this work and (ii) it is not usual to consider the infrastructure in environmental LCA studies.15 Flows related to packaging production and its transport were excluded because they represented less than 1% of the total mass or energy of the system. Therefore, based on this FU and the elementary flows presented in Figure 1, it was available to quantify all necessary flows and process to bond one square meter of female footwear. This quantification 8466
DOI: 10.1021/acssuschemeng.7b02516 ACS Sustainable Chem. Eng. 2017, 5, 8464−8472
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ACS Sustainable Chemistry & Engineering resulted in the life cycle inventory (LCI) of three technologies. The inventory is presented in Tables 1−3.
Ei , n = (Pi , nTi , n/Efi , n)Ad i , n
where Ei,n is the energy consumed to produce a necessary amount of adhesive “i” to bond one square meter of two substrates using the machine and/or equipment “n” (kWh/FU); Pi,n is the equipment power “n” used in adhesive manufacturing “i” (kW; see Table 1); Ti,n is the manufacturing time per batch “i” of adhesive (h/batch; see Table 1); Efi,n is the equipment efficiency per batch to produce adhesive “i” (kg/batch; see Table 1); Adi,n is the amount of adhesive “i” necessary to bond one square meter of two substrates (kg/m2; see Table 2); i is the adhesive technology (e.g., PBA, WBA, and SBA); n is the type of machine and/or equipment used in adhesive manufacturing; FU is the functional unit was defined as one square meter of bonded surface (two substrates).
Table 1. Machinery and Equipment Used for Manufacturing and Application (Use Phase) per FUa machinery and/or equipment
product system
efficiency
Adhesive Manufacturing mixing reactor: electric motor WBA 940 kg/batchb 25 cv mixing reactor: electric motor SBA 3942 kg/batchc 40 Hp mixing PBA 40 kg/batchd powdered process PBA 40 kg/h Adhesive Application (Use Phase) hot wind all systems 120 pairs/h flash activator all systems 120 pairs/h applicator powder adhesive PBA 560 pairs/h press all systems 120 pairs/h
(1)
power (kW) 18.38 29.82 148.9 114.94
Ei , n = (Pi , n/Efi , n)/(2A)
(2)
where Ei,n is the energy consumed to apply the adhesive “i” to bond one square meter of two substrates using the machine and/or equipment “n” (kWh/FU); Pi,n is the equipment power “n” used in adhesive application “i” (kW; see Table 1); Efi,n is the equipment efficiency “n” in pairs per hours to adhesive application “i” (pairs/h; see Table 1); A is the area of standard average Brazilian female shoe (in m2; amount considered equal to 0.0099 m2; the factor “2” was considered to calculate the total area per pair); i is the type of adhesive technology (e.g., PBA, WBA, and SBA); n is the type of machine and/ or equipment used to apply the adhesive “i” (e.g., hot wind, flash activator, applicator powder adhesive (only PBA), and press); FU is the functional unit was defined as one square meter of bonded surface (two substrates). Assumptions and Limitations. Formulation. All substance formulations used in the FU studied were supplied based on evidence collected locally by the researchers with the exception of raw materials used in BPA manufacturing. Manufacture Emissions. The emissions that come from manufacturing water and solvent technologies were estimated by mass flow analysis based on the difference between raw material input and the final product obtained in the mixing batch. The mixing yield (the ratio of mass of inputs to amount of adhesive produced) was estimated as 96.71% for SBA and 96.99% for WBA technology. Using this information, the waste generated was estimated taking into account the substance proportionality in the adhesive formulation. Therefore, 3.29% of SBA and 3.01% of WBA in the case of supplies are considered production losses. The powder manufacturing process emissions were disregarded due to lack of available information. Application Emissions. During the application stage the total solvent fraction in formulation of adhesive applied was considered as emissions. Those emissions were considered as indoor air. Transport. The local transport of supplies was considered as done by trucks with 16 tons capacity and Euro III engines using an average distance of 1000 km. The same assumptions were made for the transport to the adhesive and primer, but using a distance of 300 km. These distances are applied to the modal value of raw materials and products transported to the footwear industry. As to industrial waste, it was considered that the distance between the adhesive manufacturing factory and the waste treatment site was 300 km. Upstream Database. The Ecoinvent database was applied for all processes that are outside the elementary flow boundaries. Moreover, Ecoinvent supported enough data to model emission amounts with reference to supply processes, transport truck, and power grid. Due to the work complexity and chemical characteristics of some substances listed in the inventory, some of them were not available in the Ecoinvent v.3 database, mainly minor substances such as some additives. Aiming to overcome this limitation, general and similar data identified with the substances listed in the inventory were sought. When one of these alternatives could not be satisfied, then the associated substance amount was added together with another substance that presents a potential impact; that is, a worst case approach was used. Therefore, data on the following products were not available in the database: biocides, organic additives, and organic solvent. Thus, the “organic unspecified” generic data set by Ecoinvent
18 5 5 2.2
a Source: Primary data. b8 h of operation per batch. c6 h of operation per batch. d1 h of operation per batch; FU: Functional unit was defined as one square meter of bonded surface (two substrates).
The LCIs of technologies assessed were divided into two stages as follows: (i) manufacturing: raw material mixing stage and adhesive manufacturing and (ii) adhesive application: identified with the necessary stages during adhesive use when the shoe is being assembled. The manufacturing data related to WBA and SBA technologies were collected by personal visits to the industrial plant during August to November 2015. Meanwhile, the data on the PBA manufacturing technology was supplied by the adhesive factory during the same period; therefore, they are considered data from a secondary source. The processes identified with the application stage in all technologies cited in this work were monitored by personal visits to the adhesive factory researchers, and the quantitative data related to the application step (adhesive and primers used) were measured in laboratory tests, as shown in Table 1. The laboratory procedures considered two general substrates being bonded as demonstrated in the description below: (i) The size of the test specimens was measured and cleaned using an appropriate substance. (ii) All test specimens were weighted. (iii) Considering the powder technology, a stabilizing primer was used, and after a few minutes the amount evaporated was discounted. (iv) In the other technologies the adhesive was applied directly. (v) Regarding the primers used for solvent and water technologies, it was necessary to follow steps “i” and “ii”, and after that the primers were applied. After a few minutes the amount evaporated was discounted. (vi) In the end, the dry mass was multiplied by the solvent fraction indicated in the official formulation, and then the quantity of each substance necessary for the adhesion was calculated, based on Table 1. Energy Consumed for Manufacturing and Application. The flows related to energy consumption during the manufacturing and application stages were estimated using eqs 1 and 2, respectively. In this calculation the machinery power and efficiency were considered, as presented in Table 1. All necessary data to obtain this evaluation were collected by an adhesive factory research group; therefore, they can be considered primary data. However, the manufacturing data of PBA were supplied by the adhesive factory. In this case, these data were considered secondary data. The shoe surface area for this work was estimated as 99 cm2 (0.0099 2 m ) and matches that of standard average Brazilian female shoe size. The results of energy consumption per FU evaluated are shown in Table 3 together with the aggregate inventory of this work. 8467
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Table 3. LCI results of SBA to 1 m2 of Bonded Surface (FU)
was employed for biocides and organic additives. For organic solvent that was assumed with the “methyl ethyl ketone” data set, it was considered a worst case approach. Power Grid Energy. In order to provide energy flows with good representativeness and appropriate to Brazilian reality, in other words, adapted according to the economic and geographic conditions,22 this study considered the 2015 Brazilian power grid.23
Manufacturing: SBA Product System acetone inorganic additive polyurethane thermoplastic organic additive local raw materials transport manufacturing energy
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RESULTS AND DISCUSSION Data Collected Results. Using the methodology described, it was feasible to identify the supplies necessary to perform a one square meter bonding between two substrates. The results reported regarding this stage are presented in Table 2, and it was possible to note the difference between product systems to perform the same FU. The LCI was conducted based on this information.
SBA
WBA
PBA
a
substances
amount (g/m2)
SBA
297.4
primer A
19.3
WBA
117.3
primer B
18.3
PBA primer C primer D
35.6 16.4 33.8
flows
outputs
amount/FU
unit
2.60 × 10−1 2.46 × 10−3 4.40 × 10−2
kg kg kg
primary data primary data primary data
9.23 × 10−4 3.08 × 10−1
kg tkm
primary data calculateda
1.40 × 10−2
kWh
amount/FU
unit
calculated (eq 1) data source
SBA 2.97 × 10−1 acetone 8.55 × 10−3 inorganic additive 8.09 × 10−5 organic additive 1.45 × 10−3 Adhesive Application: SBA Product
Table 2. Necessary Amounts for 1 m2 of Bonded Surface (FU)a product system
flows
inputs
flows
inputs
description thermoplastic polyurethane polymer solvent based adhesive solvent mix, low solid, good compatibility with shoe material thermoplastic polyurethane polymer dispersion solvent mix, low solid, good compatibility with shoe material thermoplastic polyurethane adhesive solvent mix with low solid solvent mix with low solid
kg kg kg kg System
primary data calculatedb calculatedb calculatedb
amount/FU
unit
SBA
2.97 × 10−1
kg
energy: hot wind
7.58
kWh
energy: flash activator
2.10
kWh
1.71 × 10−2 2.18 × 10−3
kg kg
Primary data(see Table 2) Calculated (eq 2) Calculated (eq 2) primary data primary data
9.50 × 10−2
tkm
calculatedc
acetone (primer A)e polyurethane thermoplastic (primer A)e transport to footwear industry output flows
Source: Primary data.
data source
acetone by primer and acetone by adhesive
data source
amount/FU
unit
data source
0.269
kg
calculatedd
a
The local transport of supplies was considered using an average distance of 1000 km. bThe inputs/outputs were calculated considering that 3.29% of SBA and 3.01% of WBA are production losses. cThe same assumptions were made for the transport to the adhesive and primers to footwear industry but using 300 km of distance. dDuring the application stage the total solvent fraction in formulation of adhesive applied was considered as indoor air emissions. eSubstances present in formulation of primer A.
The LCIs attributed to three technologies in this study are presented in Tables 3−6, and they were built following the methodology and considerations already discussed. The input and output flows for each technology are linked with enough amounts to produce adhesive for one square meter of bonded surface. Life Cycle Assessment Results. Once all input and output flows and their amounts had been determined, the next step was to determine the impacts attributed to each life cycle technology. The absolute impact results are shown in Table 6. The results of the comparison of these three adhesive technologies available are presented in Figure 3. Figure 3 shows the relative results of the comparison having as reference the adhesive technology with the biggest impact. In general, SBA had the biggest impact in four categories: ozone layer depletion, global warming, nonrenewable energy sources, and respiratory organics. However, the SBA shows a better performance in the water footprint impact category. The adhesives manufacturing step did not show a significant contribution to the final results (see Figure 4). It is possible to note that the categories nonrenewable energy sources and respiratory organics had the biggest differences among the technologies while ozone layer depletion and water footprint demonstrated minimum differences in their impact results among the technologies analyzed (see Figure 3 and Table 6). On the contrary, BPA decreases impacts in four of the five assessed impact categories compared to SBA: −17% to ozone layer depletion, −50% to global warming, −71% to nonrenewable energy sources, and about 1% to water footoprint.
BWA shows the lowest environmental burden in relation to respiratory organics. According to Giannetti et al.,22 Staikos et al.,24 and Staikos and Rahimifard,25 one of the causes of environmental mitigation could be the implementation of new technologies based on scientific research, proactive activities that seek innovative materials and technologies. Contribution Analysis. This analysis is used aiming to identify which substances or processes are responsible for substantial contributions to the environmental interventions. Moreover, it is possible to say that the purpose of this interpretation stage is to calculate the total contributions to several factors that took part in environmental burdens. These results are shown in percentages over total, in other words, the objective at this stage is to identify which process or substances present the biggest impact participation among all products assessed. Figure 4 shows the contribution analysis. Processes that showed a contribution ≥5% were considered a significant contribution. Contributions
