Life Cycle Analysis of a Novel Process from the Automotive Industry in Mexico for Recycling Nylon 6,6 into Polymeric Coatings

Abstract

Sustainability has converted the topic of the humanity and life-cycle analysis (LCA) is one of the main methods for evaluating the impact of each product, process, and system. Polymers, especially nylon 6,6, have found substantial relevance in several areas such as automobiles, packaging, food, batteries, etc., in the last years and estimation of their impact on the environment as well as persons from their increased disposal is of intense importance. LCA procedures are being employed to investigate the same in terms of several ecosystems, resources, and human-based restrictions but there are still some limitations to the same. This paper presents an overview of using recycled nylon 6,6 coating as an alternative to the traditional way of final disposal of this polymer, focusing on its life cycle, production, coating characterization, data reproducibility, and limitations. OpenLCA software was used for the LCA of the recycled coating formation processes. EIA09 software was employed to estimate the environmental impact assessment. Results obtained using EIA09 software show that the recycled nylon 6,6 coating has a low environmental impact with respect to soil contamination. This result shows a clear advantage of plastic recycling compared to the traditional final disposal.

1. Introduction

Plastics have highly absorbent properties that allow them to meet more stringent safety standards. As such, the automotive industry has become the third-largest consumer of polymers after the packaging and construction sectors [1,2,3,4,5,6,7]. This is because engineering plastics make it possible to realize a greater degree of lightness in materials and greater design freedom compared to metals.

Most of these plastic wastes are deposited in landfills and are subject to long-term degradation. Plastic materials undergo significant changes in their chemical structure under specific environmental conditions, thereby resulting in the loss of some properties. Polymeric characteristics, such as the molecular weight, crystallinity, functional groups, mobility, substituents in structures, and additives, play an important role in degradation. The fate of these polymers in landfill and the time required for total CO₂ mineralization are not yet fully understood. Polymers can degrade via chemical degradation, photo degradation, and biological degradation, leading to secondary microplastic pollution [8].

Currently, the global perception of the rational use of plastics has led to the creation of minimum standards in certain countries for the sustainable recycling of plastics. This scenario has also allowed for developments with respect to reprocessing recycled materials to obtain products in the industrial sector or to use such products in the energy sector. In 2016, for example, with respect to the polymers collected for final disposal, 16% were recycled, 40% were sent to landfills, and 25% were incinerated. Similarly, according to a report released by the Organization for Economic Co-operation and Development, globally, more than 300 MT of plastic waste flows into the environment yearly [9].

The most-used plastics in the world currently include the following: polyethylene terephthalate, polyethylene, polypropylene, polyvinyl chloride, polystyrene, polylactic acid, polycarbonate, polymethyl methacrylate, polyoxymethylene, and nylons. Most of these plastics are nonbiodegradable and require recycling [10].

Novel materials such as coating layers that possess better resistance to erosion and corrosion have been proposed [6]. Further, such coatings can be mostly categorized into hard layers of oxides, carbides, nitrides, and soft nonmetallic layers (e.g., polyurethane, epoxy, and nylon) [6], and the composite cermet coating of hard reinforcements in tough matrix materials [11].

However, the manufacturing process of new products from recycled polymers can result in higher environmental burdens than the burden caused by the dumping of the polymer waste into the environment. As such, the study of the environmental impact of the proposed new processes is essential. Additionally, life cycle analysis (LCA) and management act as tools for aiding decision making. Indeed, certain authors conduct environmental studies on coatings via the use of waste polymers. Still, no detailed studies considered a global approach as most studies adopted coating processes with respect to obtaining a general idea of the factors that have a more significant impact on the environment.

Polyamide or nylon 6,6 is one of the most-used polymers in the automotive sector [1,2,3,4,5]. Although the final disposal of nylon 6,6 polymers is in open-air landfills, its final disposal data are unknown. Mexico is a plastic producer with no strict regulations. In 2010, around 2400 municipalities in the country, from which information was collected daily, reported—on average—approximately 86,357 tons of urban waste, of which 10% comprised plastic waste [12]. In particular, nylon 6,6 is an engineering thermoplastic that is difficult to degrade. Therefore, its presence in landfills generates a substantial ecological impact [13]. Moreover, several thousand additives—such as brominated flame retardants, phthalates, and lead compounds, all of which are considered harmful—are used to produce this plastic, thus resulting in disturbances with respect to endocrine functions and leading to carcinogenic and mutagenic effects in living organisms [14].

The objective of this study was to elaborate the different stages of LCA with respect to a recycled nylon 6,6 polymeric coating, specifically from the preparation of the polymeric solution to the formation of the coat, until its final disposal. Furthermore, the analysis consisted of various life-cycle inventory data sources, whereby the electrical energy consumption and the materials used were considered. As such, the results of this study are expected to provide evidence for a recommendation regarding the possible use of auto part materials for fabricating recycled coatings.

2. Materials and Methods

First, it was necessary to conduct a preliminary study regarding the implications of the disposal of polymeric waste in terms of its influence and normative-legal analysis to determine its environmental effects.

To this end, the final disposal of polymers, specifically nylon 6,6, must be optimized for realizing increased efficiency with respect to degradation time. However, the process must also be sufficiently viable from the cost–benefit perspective to be competitive with the current final disposal practices.

During our investigations, we assumed that the final disposal of polymers in landfills leads to long degradation times and has environmental and health impacts as the waste is not adequately treated at the sites where it is disposed of and left in the open air. Thus, the inefficient disposal of polymers has remarkable environmental impact. For example, of the 300 million tons of polymers produced each year in Mexico, 3% are recycled [15].

Moreover, Mexico has 2456 municipalities and delegations (1881 of these sites are the sites for the final disposal of solid waste), of which 238 and 1643 are classified as sanitary landfills and open dumps, respectively.

However, the state of Morelos includes 33 municipalities and 18 disposal sites. Of these, only four meet the criteria for sanitary landfills, where solid waste is not treated for final disposal.

In these areas, the lack of integrated management for proper handling of this waste leads to the proliferation of infection sources and increased environmental health risks.

Therefore, we limited this work to the state of Morelos and neighboring states, namely, Mexico City, the state of Mexico, Puebla, Tlaxcala, and Guerrero (Figure 1).

For the subsequent environmental impact analysis, it was necessary to identify the locations of the final disposal sites in the aforementioned states and to identify whether they were located near protected natural areas or water wells to determine their health risks, harmful effects on the environment, water, and degree of soil contamination (Figure 2).

In the five states studied, there were 265 waste disposal sites, of which 85% were open dumps, 15% were sanitary landfills, and 7.69% were controlled sites with treated waste.

Another limitation of research in this topic was the limited access to the companies and persons in charge of the final disposal of polymeric material. Notably, Mexico still has no standards on the manner of final disposal of polymeric engineering waste; only standards that specify how this waste should be classified and some points of the European standard that are applied for its final disposal are available.

Life cycle impact assessments on the formation of recycled nylon 6,6 coating were performed using OpenLCA software following the ISO 14040 [16] and ISO 14044 standards [17].

OpenLCA is a free and open-source web-based software for sustainability and LCA, and it has the following features:

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Enables rapid and reliable calculation for sustainability assessment and LCA

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Provides comprehensive information on calculation and analysis results to identify the main drivers in the life cycle by process category and impact flow and to visualize results and locate them on a map

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Seamlessly integrates the life cycle cost and social assessment into the life cycle model

Adverse environmental effects throughout the life cycle were identified by selecting identifiers for each impact category.

Finally, the environmental impact assessment of nylon 6,6 waste was performed considering three scenarios using EIA09 software [16], which is a free application for the environmental impact assessment of various engineering projects. In the first scenario, the results of the LCA interpretation from the analysis of the environmental impact assessment inventory were also obtained using EIA09 software.

2.1. Goal, Scope, and Boundary

The objective of the LCA is to evaluate the environmental burdens associated with the life cycle in relation to the production of recycled polymer coating from nylon 6,6 waste and to compare it with the polymers that are disposed of in landfills. To fulfill these objectives, it is necessary to perform calculations in OpenLCA to determine the inputs and outputs of the material used to form the coating. In scenario 1, the data obtained from research on application of recycled nylon for manufacturing plasma-treated polymer coatings and its environmental impact [18,19] and its final disposal (Figure 3) are used. In comparison, we consider the following two scenarios, namely, scenarios 2 and 3. In scenario 2, nylon 6,6 waste was associated with final disposal in open landfills. In scenario 3, nylon 6,6 wastes were associated with disposal in controlled landfills.

Table 1. Inputs and outputs that were evaluated in each scenario to assess emissions.

Scenario	Inputs	Outputs
1. Nylon 6.6 coating	Nylon 6,6 waste polymer Formic acid Electricity Transport	Nylon solution waste Emissions
2. Open dumps	Transportation Emissions	Emissions
3. Controlled landfills	Treatments prior to removal from nylon 6,6 Transport Emissions	Emissions

lists the inputs and outputs that were evaluated in each of the scenarios. The OpenLCA program database was used for the outputs.

The SEMARNAT database from the INEGI 2012 register was used to calculate and compare the emissions generated with the final disposal of polymeric waste. The data used are listed in

Table 2. SEMARNAT database from the 2012 INEGI register to calculate and compare the emissions generated with the final disposal of polymeric waste.

Final Disposal Sites Reported as Destination of Municipal Solid Waste by State				Quantity Collected by Type of Material, Daily Average	Municipalities According to Availability of Studies on the Composition of Municipal Solid Waste by Federal Entity
Federal State	Final Disposal Sites	Type of Site		Plastic (kg)	With Studies on the Composition	No Studies on the Composition
		Landfill	Open Dump
Distrito Federal	0	0	0	3392	16	0
Guerrero	80	2	78	105	1	79
México	89	28	61	850	2	108
Morelos	18	4	14	24	0	27
Puebla	|	8	84	107	4	204

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Although many countries adopted different recycling practices, additional studies should still be conducted. A policy framework with recycling targets should also be established in developing countries. In addition, to collect data representing the magnitude of the problem, the following limitations were assumed: the cost of transporting the waste polymer to the laboratory, the use of one kilogram of waste polymer to make the nylon solution for subsequent coatings, and the amount of energy required.

Subsequently, we considered the type of action that generates sufficient impact to better determine the effects, including the intensity, extension, duration when the environmental changes were generated, reversibility of the damage, health risks, environment, and economic aspects. Notably, these impacts have already been reported in previous works.

Scenario one involves the recycling of polymeric coating from waste nylon 6,6. Figure 4 shows the coating formation process for the recycled nylon 6,6. By applying these processes, first, the material surface (316L steel) was prepared using 600 grit silicon carbide, and the residues of the sanding process were considered as the outputs. Subsequently, the surface was washed and dried with soapy water and deionized water and then subjected to ultrasonic vibration as inputs. Then, the material was placed in contact with a polymeric solution (30 s) to form a film (thickness: 5 mm). Here, while the polymer and solvents used for dissolution were considered inputs, the polymeric solution only covered an area of 2 m². Therefore, the remainder of the solution was considered as the output. Finally, the coating was dried in a naturally ventilated room where the volatile organic compounds were considered as the impact outputs to the environment.

Plasma treatment was also considered in LCA as the second process to improve the surface for successful applications. For example, atmospheric plasma treatment of nylon 6,6 films can modify the material roughness and improve the surface properties while improving the adhesion of the films. While the energy used for 15 s of plasma treatment was an input, the voltaic organic compound (VOC) particles were outputs.

The functional unit for LCAs was 1 kg of nylon 6,6.

2.2. Inventory Data

Information on the type and number of inputs and outputs were taken from [18,19] previous works and were related to the main processes of the recycled plastic coating process [20] (Figure 5).

After assigning the inputs and outputs to the LCA software, environmental loads were assigned to different process steps. Then, a vector-based methodology was used to balance the environmental loads. In particular, each process had a vector associated with information on the pollution generated throughout the life cycle. This ecovector was also a column vector in which each element corresponded to a specific pollutant [21].

2.3. Environmental Impact Evaluation

The following 13 environmental impact categories were selected for this study:

• Health
• Water pollution by leachate
• Rivers and streams
• Emissions to the atmosphere
• Smells
• Groundwater
• Soil erosion
• Social acceptance
• Impact on the landscape
• Fauna
• Solid contamination
• Investment
• Solid waste transportation

These environmental impact categories were chosen because we consider all scenarios regarding climate change, ecosystem quality, human health, and resources.

The impact categories selected were taken from the literature [22].

3. Results

In this phase, the environmental impact of each of the analyzed processes was evaluated after the collection and subsequent introduction of the inventory data in OpenLCA version 1.7 [23] and EIA09 version 1.0 [24]. The OpenLCA program is free software intended for the LCA of a product and the carbon and water footprint, but it can also provide the possibility to develop, among others, economic models.

It is a modular system to which plugins can be added to extend its features.

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It has an extremely wide selection of databases: any database in EcoSpold or ILCD data format can be imported and used in OpenLCA. In addition to many free databases, they also offer nonfree GaBi and Ecoinvent databases on a pay-per-use license basis. This is currently the widest selection of data available worldwide in any LCA software.

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It has interesting compatibility features with other programs, as it has an extension that allows exporting and importing analyses between the most important and widely used applications in this field.

Meanwhile, the EIA09 program allows us to extract the ecological profile of a process or product, thus allowing us to identify the materials and processes with the worst environmental performance via databases, both proprietary and bibliographic.

All products damage the environment in one way or another. Raw materials have to be extracted and the product has to be manufactured, distributed, packaged, and finally disposed of. The use of products also involves an environmental impact as energy or materials are usually consumed at this stage of the life cycle. Hence, to assess the environmental damage of a product, we must study all stages of its life cycle. The environmental analysis of all stages of the life cycle is called the LCA.

The LCA method was developed to include a method for weighting coefficients. This allows us to calculate a single value for the total environmental impact based on the calculated effects. This figure is called the eco-indicator. To obtain the data, a search for data and information was carried out in different official documents and secretariats [8,12,13,14], on most of the materials and processes that are involved in the different final disposal sites. These data were used to calculate the eco-indicator. The materials and processes were defined in such a way that they fit together like pieces of a puzzle.

Thus, the eco-indicator of a material or process consists of a number that indicates the environmental impact of that material or process based on the LCA data. The higher the indicator of a material or process, the greater is its environmental impact.

The eco-indicator 99 standard values are obtained for materials and processes as follows:

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Materials: for the analysis of the input and output data, the calculation was made based on 1 kg of nylon 6,6 polymeric material.

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Production processes: they involve the treatment and processing of various materials. The standard value for each treatment is expressed in a unit appropriate to the particular process (square meters, kilograms, and welded meters).

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Transportation processes: usually expressed in tons per kilometer.

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Energy generation processes: units are determined for electricity and heat.

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Disposal scenarios: the standard values are expressed in kilograms of materials, and subdivided considering types of materials and disposal methods. The European average figures are used to obtain these estimates using a particular definition of the terms “material” and “process.”

Inventory data obtained from literature and recognized institution sources were analyzed to quantify the environmental and health effects [13] for specific landfill areas, then classified into categories and modeled in the programs to obtain the indicators and later carry out the environmental impact. To study the recycling process of nylon 6,6 in the form of polymeric films, LCA was modeled in the OpenLCA program. Modeling was performed after obtaining the indicator values for each category. The LCA inventory analysis was then combined with the environmental impact assessment implemented in the EIA09 program.

The environmental impact categories were evaluated considering the three aforementioned scenarios.

The environmental impact categories for the three scenarios are summarized in Table 1 and described in Figure 6.

The environmental impact categories for the three scenarios are summarized in

Table 3. Environmental impact assessment of the final disposal of nylon 6,6 in open-air dumps, controlled landfills, and polymeric films (negative signs of the values represent the negative impacts on the environment).

	Open Air Dumps	Controlled Landfills	Polymeric Films
Solid waste transportation	0	0	0
Investment	0	0	−3.92
Soil contamination	3.49	−8.65	1.08
Fauna	−32.76	11.52	36.18
Impact on the landscape	−20.16	−6.48	6.35
Social acceptance	−5.20	2.73	4.56
Soil erosion	−5.76	3.52	2.98
Groundwater	−3.31	3.31	7.80
Smells	−6.12	−2.88	−3.97
Emissions to the atmosphere	−14.58	−6.982	−6.67
Rivers and streams	−1.44	6.016	4.72
Water pollution by leachate	−6.63	5.12	4.61
Health	−6.66	−4.59	−4.56

and shown in Figure 6. A negative environmental impact on the health category can be seen in all three scenarios. However, polymer films had the lowest environmental impact. This finding is important because polymeric films may pose less health risks, such as respiratory diseases and heart disease. Therefore, this scenario can reduce different types of pollution and can help prevent numerous serious health problems and deaths because only a minimal amount of particulates enters the atmosphere, and unlike landfills, they do not cause short-term damage. Inorganic pollutants in landfills enter the atmosphere and cause short-term damage to the atmosphere. Regarding the category of leachate water contamination, the environmental impact was positive in the scenarios with controlled landfills and polymeric films.

However, landfill leachate is a major source of environmental pollution. As expected, the open-air landfill scenario had a negative environmental impact. However, the controlled landfill and polymeric film scenarios had positive environmental impacts with respect to rivers and streams, while open dumps had a negative environmental impact. This result is important because the two positive scenarios for the rivers and stream category will help maintain the quality of water which is essential for human life. In addition, rivers, and streams provide drinking water, offer flood protection, and are used to irrigate agricultural areas. In addition, rivers are a source of nutrients for agriculture and play a major role in moderating the climate. The three scenarios have negative environmental impacts with respect to emissions into the atmosphere, but notably, there was less negative impact in the polymeric film scenario. This result means that the polymeric film scenario is a better option to reduce the emission of pollutants into the air, which can also cause changes to the climate. Reducing the emission of pollutants, including greenhouse gases, and preventing the destruction of the ozone layer in the atmosphere will help to ameliorate global warming. For all three scenarios, a negative environmental impact was observed in terms of the smell, i.e., odor category. However, the negative impact was less in the case of the controlled landfill and polymeric film scenarios. In these scenarios, the irritation (skin, eyes, or nasal tract) problem, which has a wide range of adverse health effects (from none to mild discomfort, to more serious symptoms), is observed because certain chemicals with strong odors may cause irritation of the eyes, nose, throat, or lungs. Further, the polymeric film scenario had the greatest positive environmental impact with respect to the groundwater category. This is an excellent result attributed to the reduction in the contamination of groundwater. Groundwater contamination can result in poor quality of drinking water, losses of water supplies, degraded surface water systems, high cleanup costs, high costs for alternative water supplies, and/or potential health problems. Additionally, this is an important result because these Mexican states have numerous underground water sources that are used for agriculture and occasionally for human consumption. The controlled landfill and polymeric film scenarios had positive environmental impacts with respect to soil erosion category. This result implies that there is a considerable reduction in the effects of soil erosion with regard to agriculture. Soil erosion can include loss of fertile land to floods or water pollution, among other concerns. Soil also filters and purifies water, reduces flooding, regulates the atmosphere, and plays a crucial role in driving the carbon and nitrogen cycles. In addition, it is the key to reducing the extent of climate change as it captures and stores vast amounts of carbon. Meanwhile, with regard to the social acceptance category, the polymeric film scenario has the greatest positive environmental impact. This is a vital result because it can help reduce the frequent common effects in this category, such as food shortages, water insecurity, respiratory illness, disease, mental distress emotional health problems, family separation, social network loss, housing damage, unemployment, income disruption, and asset depletion. Environmental change is also a social justice issue. The impact on the landscape category included a positive environmental impact with respect to the polymeric film scenario, while the other scenarios had negative impacts. Landscaping had both negative and positive impacts on the environment. This is a good result from the perspective that negative impacts, which range from deforestation to pollution (air, water, and land), and modification of the ecosystem, in this category can be reduced. Some of these impacts can result in irreversible effects on the environment, such as environmental pollution, extinction of species, depletion of resources, and habitat destruction. Moreover, as with the growth in human population, natural resources are being depleted. The fauna category shows two positive environmental impacts with respect to the controlled landfill and polymeric film scenarios; however, the polymeric film scenario showed the greatest positive impact. This is also a crucial result because the Mexican states that are considered in the present study have rich wildlife, and the flora and fauna hugely contribute toward human existence as they play an important role in realizing the exchange of oxygen and carbon dioxide in the atmosphere. Additionally, plants are responsible for producing oxygen and releasing carbon dioxide, and most the fauna do the opposite. The open-air dump and polymeric film scenarios were found to show positive environmental impacts. These scenarios reduced the instances of poor waste management, which contributes to climate change and air pollution and directly affects many ecosystems and species. Additionally, these scenarios also aid in reducing the release of methane, which is usually considered last in the waste management hierarchy but is a very powerful greenhouse gas linked to landfills.

In general, the results summarized in Table 1 and Figure 6 show that polymer films were the alternatives that led to the lowest impacts on the environment. Furthermore, while the negative signs of the values reflect the negative impacts on the environment, the final disposal with respect to open-air dumps had a greater environmental impact compared to the other two scenarios. In the first case, the greater accumulation of solid waste in open spaces, VOC emissions, and leachate were the reasons for the high environmental load, unlike in the case of the controlled landfill scenario. Therefore, considering the value of the categories, the one with the greatest impact was that of harmful fauna, mainly due to the proliferation of harmful fauna responsible for diseases. This category has an essential impact that must be given due weight considering the fact that the Mexican states considered in this work are rich in wildlife and flora.

4. Conclusions

In this work, the environmental impact of the final disposal practices for nylon 6,6 in a region in Mexico was evaluated with respect to three scenarios. These scenarios were as follows: open-air dumps, controlled landfills, and the proposed nylon 6,6 recycled coating−formation processes. The main conclusions drawn from the investigation of these scenarios are as follows:

• The results obtained show that there are notable differences between the eco-profiles of the compared alternatives. The final disposal of polymeric waste with respect to coatings had a lower impact than disposal in landfills, most likely because degradation takes less time.
• The final disposal of nylon in open-air dumps had a negative environmental impact assessment of 10 out of a total of 13, while the controlled landfills scenario had 5, and the polymeric films scenario only had 4. As the proliferation of harmful fauna is an important category, improvement strategies can be proposed to reduce environmental impact, and its impact on other associated categories such as emissions into the atmosphere should also be reduced. As can be seen in the polymeric films scenario, the impact on harmful fauna is a 60%-higher positive environmental impact assessment than the controlled landfills and the negative impact of the open air dumps scenario. This means that the process proposed in the present work considerably reduces the environmental impact and it is a suitable alternative.
• Controlled landfills is the second scenario with low environmental impact with a positive environmental impact assessment of 5. However, the high cost assumed for acquiring the necessary infrastructure for adequately operating landfills implies that polymeric films can help to reduce the environmental impact with respect to the final disposal of nylon.
• The polymer films were found to be the alternative with the least impact on the environment, that is, a negative impact of 4 and a positive impact of 8 with a positive environmental impact assessment average of 25% higher than the other scenarios. When considering the values obtained for the categories investigated in the present work, the present proposal of nylon 6,6 recycled coating formation processes had lower impact with respect to the fauna, groundwater, emission to the atmosphere, and health categories, which are crucial for the study region in Mexico. This is because these areas are rich in wildlife, flora, underground rivers, and population settlements. Hence, nylon 6,6 recycled coating formation processes is an excellent alternative for reducing the environmental contamination by the automotive industry in Mexico.
• This work showed one way to support sustainable replacement of commercial products to reduce its environmental contaminations. Even though this problem was solved in the case study, further research is required to find better ways to publicly disclose sensitive information in a manner that prevents intellectual property infringement.

Note that the LCA methodology effectively determines the environmental impacts of products, processes, or services, thereby proposing that the philosophy of life cycles be disseminated in scientific, business, and industrial fields by designers and managers who consider the degree of decision making and the environmental/health impacts associated with the life cycle.

Likewise, it is important to determine the impacts right from the collection of raw materials to the final disposal of a product or service for better decision making. In the case of the evaluation of pollutants that are generated in the transport of polymeric material to the recycling point, the impact on the environment must be considered. In this case, the main pollutant is CO₂, which is the main pollutant generated during the recycling of polymeric material.

To the best of our knowledge, this is the first study and proposal to investigate the reduction of nylon 6,6 products for the automotive industry in Mexico. As such, based on this research study, we presented a starting point for future research to determine the indicators based on the eco-profiles described above; in turn, we can construct a comparative starting point. Meanwhile, this research study can help us compare the recycling alternatives for future research. Further, the culture in Mexico should be considered when conducting studies of environmental impacts and searching for alternatives that can reduce environmental deterioration.