Circular Economy Alternative Repurposing Textile Solid Discarded Materials from the Leather Goods Sector in Ubrique (Cádiz), Southern Spain

Abstract

The leather industry is experiencing a significant rise in production, resulting in an increase in discarded materials–often classified as urban waste—that end up in landfills or are incinerated. Given the geographical proximity of the leather goods sector in Ubrique to the cork stopper manufacturing industry in El Puerto de Santa María/Jerez and to the viticulture sector in Jerez/Sanlúcar/Chiclana, it is essential to explore synergies that can address environmental challenges by contemplating a circular economy approach. This study focuses on the existing potential of repurposing discarded materials, such as reinforcements and linings, by meticulously recording, classifying, and characterizing them. The aim is to valorize these by-products in line with the circular economy principles set out by the European Union. Specifically, the research investigates the feasibility of using these materials as raw inputs, in combination with resins, to produce bottle stoppers for the high-end spirits industry. Findings indicate that a single company generates over six tons of textile waste per month, which equates to approximately EUR 25,000 annually in landfill disposal costs. By implementing a circular alternative that uses just 8.75% of these materials, an economic saving of around EUR 750 per year was reached. In addition, a low-cost vacuum molding method was tested for producing stoppers, yielding aesthetically pleasing and durable prototypes. The application of a materials circularity index enabled the identification of optimal remnants for reintroduction into the production cycle. This re-integration not only reduces the reliance on virgin plastic materials, but also has the potential to significantly reduce the overall environmental impact across interconnected sectors. Ultimately, this study highlights the viability of adopting circular economy practices within the leather goods sector to promote sustainability and economic efficiency.

1. Introduction

In 1975, approximately 24 million metric tons of textile fibers were produced worldwide. By 2021, this figure had almost quintupled to over 113 million metric tons [1]. Annual global leather production is estimated at 2.14 million metric tons, with the leather footwear sector dominating the market with a 65% share, while other types of leather products account for the remaining 35% of leather produced [2]. There are 9000 tanneries worldwide, yet the end use of these materials is primarily within the leather goods sector [3]. According to the United Nations [4], every second, the equivalent of a garbage truck full of textiles is landfilled or incinerated.

The European leather and related products industry is a significant player, encompassing approximately 36,000 companies, employing around 435,000 individuals, and generating a revenue of EUR 48 billion in 2021. Key applications for EU-produced leather include footwear (41%), leather goods (19%), furniture (17%), and the automotive industry (13%) [5]. The EU Strategy for Sustainable and Circular Textiles [6] sets out a roadmap to guide the textile, leather, and footwear sectors towards a more modern, eco-friendly, and competitive production model. Beginning in 2025, European companies, particularly those involved in footwear, leather, and textile production, will be required to adhere to European directives on waste and the circular economy. These directives mandate the management of organic waste generated during production processes, rather than discarding it in landfills.

The leather industry contributes to the recycling of waste from the meat industry. Leather production is the primary method for utilizing collagen-based biomaterials, with over 20 million cattle hides processed annually [7]. This makes leather a sustainably sourced raw material. Raw hides or skins undergo a series of mechanical and chemical processes, including hair removal, trimming, soaking, liming, fat liquoring, and tanning, to create durable and sustainable finished leather products. However, the widespread use of chromium(III) salts in tanning processes, employed by over 85% of the industry, poses environmental concerns [8,9,10].

Within a circular economy framework, a recent work published [11] found that material substitution is used to a similar extent in both product design and manufacturing; in fact, the design strategies related to the most routine design, i.e., materials substitution, reducing resources and energy consumption, and structural optimization, were used equally by both students and professional designers, to a certain extent.

The leather goods sector, based mainly in the municipality of Ubrique (Sierra de Cádiz, southern Spain), is made up of a network of highly specialized companies specialized in meticulous artisanal tasks, crafting products that are in demand by some of the major international fashion brands. This concentration of artisans and leather goods manufacturers has a wealth of expertise passed down through generations over centuries, resulting in products recognized as ‘Made in Ubrique’, which is synonymous with quality, safety, and reliability. According to data from the Institute of Statistics and Cartography of Andalusia statistics of companies registered with the Social Security [12], the province is home to more than 300 companies or professionals dedicated to leather goods. However, they operate in three distinct categories, each contributing approximately: Workshops (84% of the total) (W), Medium-sized companies (15%) (Me), and Parent companies (1%) (Pa). W comprises small family businesses with 1 to 10 workers; Me conduct work for Pa but lack direct contact with leading brands, employing between 10 and 49 workers. Pa, with more than 50 workers, engage in projects for globally renowned brands, serving as the sector and region’s economic drivers. These Pa are often referred to as ‘Tractor Companies’ as they subcontract parts of their production processes to other firms within the Cadiz region. Additionally, there are companies termed Own Brands (OB), which can function as headquarters, Me, or even W, producing items sold under their own branding.

This significant leather cluster generates a considerable amount and variety of textile solid discarded materials that can be used within Circular Economy (CE) alternatives in the surrounding geographical area [13]. Adding value to these materials represents an opportunity to reintegrate them into the production cycle, thus promoting the emergence of interlinked sectors [14]. This approach reduces the consumption of virgin raw materials and minimizes the overall environmental footprint [15], enabling the industry to advance key Sustainable Development Goals while increasing economic and environmental benefits [3,16,17,18]. The valorization process happens when using finished leather discarded materials, commonly generated by the leather manufacturing process. These discarded materials mainly consist of finished leather offcuts generated exclusively during the processing of finished leather, as well as used or substandard quality leather products [19,20,21].

Figure 1a shows various metallic and non-metallic waste materials from the sector, all mixed together, as they are currently managed in Ubrique. However, by way of example, Figure 1b shows a limited selection of eight samples segregated from a multitude of materials that are generated daily in the companies associated with the sector, forming Textile Groups (TGs). In addition to leathers, these groups include reinforcements, linings, and canvas; or, roughly speaking, Discarded Materials from the Leather Goods Sector (DiMaL), whose quantity, quality, composition, and periodicity remain unknown to them.

The circular economy action plan outlined by the European Union [22] identifies seven critical sectors that pose sustainability challenges and require coordinated action across value chains. These seven sectors include Electronics and Information and Communication Technology, Batteries and Vehicles, Packaging, Plastics, Textiles, Construction and Buildings, and Food, Water, and Nutrients. The primary focus of this study is directly linked to three of these sectors: Textiles (as the primary supplier of hides and other fabrics), Packaging (as a secondary industry that receives outputs from the circular process applied to discarded materials from the primary industry), and Plastics (where packaging companies, as significant consumers of plastics globally, could see reduced consumption by using repurposed materials from certain textile groups currently destined for landfills or incineration) [23]. It is therefore plausible to consider the circular relationship between leading brands in the clothing and fashion industry (Textiles) and leather goods companies [24]. These leather goods manufacturers generate remnants that can serve as inputs for packaging products manufactured as part of this circular process.

In addition, the global production of bottle caps for beverages reaches approximately 20 billion units annually, with approximately 13 billion relying on cork as a sealing method for the so called “cabezudos” type caps (see Figure 1c). By separating the crown material from the cork in contact with the liquid, this cork design prevents any contamination that might occur through migration [25,26]. This type of highly exclusive stopper is typically produced in small quantities but with a high selling price https://www.byflou.com/es/ferm-living-cairn-wine-stoppers-set-of-2/set-of-2-dark-brown?ident=bdedda0079008c01ffe9dc0cf397683a3e4512fb) (https://bolisartesanalesmallorquines.es/posabotellas-y-tapon-de-vino/tapon-de-vino/ (accessed on 3 November 2024). Roughly 10% of the global cap production occurs in Spain alone [27]. Hence, to validate DiMaL as an additive or reinforcement in packaging components such as cork stoppers for fortified wines or spirits, which allows for the immediate recognition of the leather origin of the additive, the decision was made to employ the vacuum molding process (VMP). This method was chosen for its swiftness and cost effectiveness [28]. Relying solely on vacuum pressure to fill the mold cavity of the preform, the resulting crowns have a high fiber content (typically 60–70% of the total weight) and lower porosity (less than 1%), resulting in stronger parts compared to those manufactured with the same thickness through alternative processes [29]. What is more, the crowns do not come into contact with the wine; that is the purpose of the cork stem. Therefore, it is possible to use even scraps that may contain traces of chromium due to the tanning process.

This study focuses on the counting, classification, characterization, and selection of remnants with the potential to be used for manufacturing stoppers within a CE framework in the geographical area where the remnants are generated. Through the use of statistics and the application of a circularity index, as an example of a potential alternative for recycling these materials, the manufacturing of prototypes for exclusive stoppers tailored to the spirits line offered by some of the leading brands with which they collaborate was considered.

2. Material and Methods

2.1. Methodology

To accurately measure the volumes of textile remnants generated and their timing, we conducted training for the staff responsible for the separation of the discarded materials in each Production Unit, where textiles were cut and sorted within each company’s production chain. Initially, training was given to 4 representative companies, excluding smaller workshops and medium-sized companies (Me), which manage the same textile groups (TG) as larger Parent Companies (Pa) and Own Brands (OB). In the second stage, the analysis focused on the company Artilab 2014 SL (Artilab, Ubrique (Cádiz), Spain), a single company that showed a strong commitment to implementing the DiMaL segregation and accounting method illustrated in Figure 2. The collection scheduling was divided into two time periods: the first period (March–July 2022) and the second period (October 2022–May 2023). The first DiMaL material is “vache new HD noir” leather, cowhide, chrome-tanned, with a matte finish and a grained surface. The second, which has the same chemical composition but a different appearance, is “vache plutone mate noir” leather, presenting a smooth surface rather than a grained one. Both volumes were combined for the same valorized purpose in the second period. The third material is Canva, exclusively generated by Artilab (Canva A). Finally, “doublure microfiber”, a microfiber used as lining for rigidity and surface enhancement in the final product, was included.

A. Statistical study for sustainable management of DiMaL.

The data were collected in a spreadsheet and the statistical analysis was performed using R software 4.4.1 [30]. A continuous variable based on the DiMaL collection plan was established; for this continuous variable “amount”, it was necessary to perform an analysis of variance (ANOVA). This ANOVA involved two discrete variables, “time” and “DiMaL”, in order to determine their influence on the “amount” variable, specifically:

• It showed significant changes depending on the “time” variable, i.e., the collection period.
• It showed significant changes depending on the “DiMaL” variable, i.e., the typology.

Based on the results, pairwise comparisons between DiMaL were studied using the Tukey test, with a p-value < 0.05. A descriptive study was conducted, emphasizing historical trends and regressions.

B. Evaluation of DiMaL management costs in proposed alternatives

To determine the current economic costs of the waste management carried out by the company and to estimate the potential savings associated with the proposed CE alternative, it was essential to understand the cost structure assigned to such management practices in Spain. Therefore, only the management of DiMaL, conducted by one of the companies participating in the study (Artilab), was considered. This is based on estimated costs presented in Figure 3 and compared with those from an authorized waste manager.

The total cost of the current management is subdivided into: (1) Intermediate waste management, including weekly collection of segregated fractions from the producer and temporary storage (approx. EUR 200/ton); (2) Transport from temporary storage to landfill (EUR 30/ton); (3) Landfill deposit tax (EUR 40/ton); and (4) Landfill management costs (EUR 45/ton). This alternative assumed that the cost of the current management option, specifically, landfill disposal (sum of 2 + 3 + 4), was a total of EUR 115/ton.

First, the quantities of DiMaL sent to the landfill “W_L” (DiMaL Landfill) by the authorized manager and those allocated to the proposed CE alternative “W_ce” (DiMaL CE) needed to be calculated. Second, the costs incurred in economic terms were computed, which included the incurred costs “C_i” (Costs incurred) and the estimated costs that could be saved if, instead of landfilling, they were valorized in a proposed CE alternative, excluding the costs incurred in the process of utilization and industrialization of the new products.

C. Application of a Circularity Index.

To assess the potential circularity associated with the analyzed DiMaL, we compared the current waste management model described in Figure 4, where the final destination for the textile waste generated by the sector was a landfill, against an alternative management approach shown in Section 3.2. This comparison was based on the Circular Transition Indicators derived from the methodology proposed by the World Business Council for Sustainable Development (WBCSD) in its updated 2023 version [31]. Taking into account the characteristics of the suggested alternative and the available information, an indicator concerning Material Circularity (MCI, % Material Circularity Index (%)) was employed [32].

D. Prototype production of stoppers for spirit wines using the Vacuum Molding Process (VMP) and verification through machining.

A flat Polymethyl methacrylate (PMMA) plate measuring 300 × 300 × 20 mm with a high-quality surface finish was prepared as the mold material. The schematic diagram of the entire experimental setup for the VMP is shown in Figure 4. Cylindrical holes matching the geometry of commercially available stopper crowns (<20 mm height, Ø30 mm outer diameter) were machined into this plate. To prevent composite adhesion to the surface, a 300 × 300 × 10 mm virgin politetrafluoroetileno flat plate with a similar surface finish was placed underneath. A layer of release agent was applied to the lateral surfaces of the mold’s cylindrical holes, enabling the easy release of the cured composite component without damage.

Various composite samples were prepared by mixing bio-based resin percentages (Sikabiresin CR75 “A” + Sikabiresin CR75-1 “B” at a ratio of 2.5:1) [33] and additives (such as canvas linings and reinforcements) ground to 2 mm granules. The ratios of additive to resin ratios used in the composite samples are detailed in

Table 1. Composite samples subjected to Vacuum Molding Process (VMP) analysis.

	Ww * [g]	WR ** [g]	Ww [%]	WR [%]	Grading [mm]	Total Weight [g]
R100	0	50	0	100	-	50
C20R80	10	50	20	80	<1	60
HD25R75	15	50	25	75	<1	65
Pl20R80	10	50	20	80	2–10	60

R: Resin; C: Canva A; HD; Pl: Plutone; * W_w: DiMaL weight in grams; ** W_R: Resin weight in gram.

and were replicated three times (n = 3).

The methodology was consistent with reference [28]: the peel ply layer was left on the mold. A double-sided sealant tape was applied along the mold’s perimeter to create a leak-proof assembly. Following this, a purported plastic layer and subsequent molding mesh layer were placed over the peel ply. A vacuum bag was placed over the entire setup to establish vacuum pressure within the system. A polyvinyl chloride hose pipe was connected to the pump inlet. Once an airtight setup was confirmed, the inlet valve was opened, and vacuum pressure was applied using the vacuum pump.

The curing process was carried out by continuously applying vacuum pressure for up to 12 h to ensure a high-quality product. The resulting composites in each hole were individually demolded from the final PMMA flat plate.

E. Analysis of Relative Humidity and Apparent Density.

Analysis was performed on VMP test samples to measure these physical parameters, with particular emphasis on the weight of each sample. The aim was to validate both the resin used and the composite material produced. The test method used to determine the weight is based on the Spanish Standards UNE 56917-88 and UNE 56922-2004 [34,35]. To complete the analysis of the samples listed in Table 1, three replicates (n = 3) of each batch were conducted. A drying oven was used for 24 h at 100 °C until reaching a constant weight. Humidity (H) was calculated as the loss of weight (m₁–m₂) over the mass after drying, m₂, with m₁ being the mass before drying, using the Equation (1):

$$\mathrm{H}={\displaystyle \frac{{\mathrm{m}}_1-{\mathrm{m}}_2}{{\mathrm{m}}_2}}·100$$

(1)

H in Equation (1) is rounded to the nearest tenth of a percentage unit.

The apparent density (n = 3) was determined according to the Spanish standard UNE 56922-2004 (UNE-EN 12726-2019) using a single sample from each batch. Mass measurements were taken on a precision balance accurate to 0.001 g, and their dimensional values, previously measured with a Vernier caliper, were calculated. The calculation was carried out by using Equation (2).

$$\mathrm{D}={\displaystyle \frac{4·{10}^6·\mathrm{m}}{\mathsf{\pi }·{\mathrm{d}}^2·\mathrm{l}}}$$

(2)

where:

• D: apparent density in kg/m³, rounded to the nearest unit [36].
• m: mass in grams, rounded to 0.01 g
• d: calculated mean diameter
• l: length in mm, rounded to 0.1 mm

F. Verification Process of Machining VMP Samples

Before inserting cork spindles, 14 VMP specimens, including a virgin resin sample, were machined on a Computer Numerical Control lathe using an 8 mm three-lip end mill. The machining process involved linear cuts at 500 rpm and a 262 mm/min feed rate, followed by a deeper cut at 1200 rpm, and a final frontal penetration at 1200 rpm and 142 mm/min feed rate.

Once the material’s suitability for machining was confirmed, a final machining procedure was carried out to create the cylindrical slot into which the cork spindles were inserted, using a solid adhesive bond. Only the 14 initial samples composed of HD leather and Lona lining, both from the manufacturer Artilab, were selected. The former was tested at a particle size of <0.5 mm and the latter at >2 mm. The following tools were used:

• TNGG160404R-UM triangular plate, spindle conditions: 2000 rpm and feed rate of 0.1 mm/rev.
• Ø17 LCMX 0303 08-53 plate drill, spindle conditions: 1100 rpm and feed rate of 0.15/rev.
• DCMT070204-PM-T6130 internal boring bar, spindle conditions: 2000 rpm and feed rate of 0.1 mm/rev.

The process involved mounting the samples on a 3-jaw chuck for machining, carrying out a pass on each one of 0.2 mm on the front. Machining operations of 0.5 mm radius were performed, leaving a Ø29 mm. Drilling operations were performed at Ø17 mm and a length of 7 mm. Internal machining was performed to achieve Ø19.7 mm for the cork housing. Finally, a protrusion was created at the rear to facilitate its handling for proper insertion and removal from the bottle.

2.2. Materials

The commercial name of the TG and the companies where they were utilized are depicted in

Table 2. Textile Groups and associated companies in Ubrique.

	Leather	Booster	Lining	Canva
Artilab (Ar)	Cow leather with chrome tanning (smooth and grain) (common material in all companies) HD and Plutone	Lyliane	Doublure microfibre	Canva A
Betangible (Be)		Talyn Microfiber lining	Recycled lining (PL)
Dimopel (Di)		Talyn Microfiber lining
Invercumbre (In)		Talyn Microfiber lining

.

3. Results and Discussion

This section presents a statistical analysis of the management of discarded textile materials waste in Ubrique’s leather goods sector (DiMaL). Initially, it examined the typology and quantities of discarded materials generated. One company was selected as representative of the sector, focusing specifically on a fraction with specific characteristics. Subsequently, the section delves into the potential use of the selected DiMaL for the production of large-headed stoppers intended for the wine industry.

3.1. Statistical Analysis of Discarded Materials Management

The proposed approach began with a statistical analysis that provided insight into the most suitable frequency of DiMaL collection and the types of materials that might have been more promising, based on their generation quantities and periodicity.

Table 3. ANOVA statistical test results.

	Df	Sum Sq	Mean Sq	F Value	Pr(>F)
PERIOD	7	2449.9	350.0	1.6	1.8 × 10−1
DiMaL	4	18402.5	6134.2	28.7	1.3 × 10−7
RESIDUALS	21	4489.9	213.8

shows the results of the ANOVA applied to the collection periods of the quantities in Artilab. The type of DiMaL significantly affected the amount collected. However, the collection time or period did not result in a statistically significant change in the amount collected. This suggests that the designated collection period may not be significant, whereas the type of collected material is.

Additionally,

Table 4. Statistical analysis results using Tukey’s test.

	Difference	Lower Bound	Upper Bound	Adjusted p-Value
HD-lining (SIGNIFICANT)	63.84	43.47	84.22	1.0 × 10−7
Canva-lining	12.76	−7.62	33.14	3.3 × 10−1
PLUTONE- lining (SIGNIFICANT)	20.72	0.34	41.09	4.5 × 10−2
Canva-HD	−51.08	−71.46	−30.71	3.8 × 10−6
PLUTONE-HD (SIGNIFICANT)	−43.13	−63.51	−22.75	4.1 × 10−5
PLUTONE-Canva	7.96	−12.42	28.33	7.0 × 10−1

presents the results obtained from the pairwise comparisons using Tukey’s Test. This analysis provides information on the relative significance of different DiMaL types based on the collected amounts of each type.

The results are conclusive and emphasize the significance of collecting HD and Plutone leathers, as well as Canva, compared to lining in the Artilab company. This is in line with the historical trend shown by the material collection throughout the study, as shown in Figure 5 and Figure 6. If we consider the entire study period (2022 and 2023), an average of 6227.10 kg/month (±10.22%) of DiMaL was collected. Figure 6 shows the prevalence of HD leather over other DiMaL, with a maximum collection in 2022 of 322.9 and a minimum of 301.9 kg/month, and in 2023, a maximum of 338.7 and a minimum of 138.3 kg/month. Over the entire study (22–23), it reaches an average value of 251.8 kg/month.

The second most generated DiMaL was Plutone. In 2022, it reached a maximum of 161.7 and a minimum of 118.1 kg/month. In 2023, it reached a maximum of 114.3 and a minimum of 91.9 kg/month. In the overall study, it had an average value of 111.9 kg/month. As for the other two DiMaLs, Canva and Lining, they were less frequent, with average quantities in the study set of 101.3 and 60.8 kg/month, respectively. In the months considered, as shown in Figure 6, Plutone leather accounted for around 48.1%, while Canva accounted for 18.6% and Lining for 11.3%. If we add the two types of leather (HD + Plutone), they reach 70.1% of the total studied; as was predictable, it was the most strategic TG in terms of quantity. Conversely, the obtained data were cross-referenced with the authorized waste manager responsible for handling the transfer from the selected company (Artilab) to the final landfill in the area. From the information gathered by the waste manager, the total quantities collected were accounted for, out of which those selected for this study were segregated, focusing solely on the four types of DiMaL. In Figure 6, graphs illustrating the work carried out can be observed, showing that the DiMaLs studied represent an average of 8.75% of the total discarded material generated by that company, along with the percentage quantities of each.

Figure 7a shows the quantities of DiMaL sent to the landfill (W_L) by the authorized waste manager and those allocated to the proposed CE alternative, W_ce. In Figure 7b, the incurred costs (C_i) in managing the discarded materials over the study months are shown, alongside the average cost in the study set, approximately EUR 1950.80/month (±10.1%). This monthly average cost was calculated according to the management scheme shown in Figure 3. If the cost of sending them to the landfill, EUR 115, is separated, it represents 36.5% of the total costs (EUR 315). The annual cost of wasting the discarded materials, based on the monthly average cost, would amount to EUR 23,409.60/year. Therefore, the cost associated with the studied quantity, 8.75% of the total, is EUR 2048.34/year, of which the landfill disposal accounts for EUR 747.64/year. Although the amount of discarded materials at Artilab depends on the start date of a new campaign initiated by the subcontracted commercial brand, what the study seems to reveal is a consistent trend over time. This trend is noteworthy because the cost savings in EUR represent approximately one-third of the amount of discarded material used as a by-product/raw material for another production process (C_i = 3 W_ce − 1). The data in Figure 7a,b, corresponding to the month of March, confirm the start of a new handbag campaign for the Canvas brand, in which larger quantities of the four studied DiMaLs were consumed.

An estimation of the material and labor costs for the production of one of the DiMaL crowns was calculated, resulting in EUR 2.17 and EUR 3.16, respectively, totaling EUR 5.33 per unit. Compared to the two references cited in the Introduction section, both valued at EUR 19 per unit, the manufactured ‘cabezudo’ stopper was almost four times less expensive.

3.2. Application of a Circularity Index

Starting from the generated discarded materials, the possible circularity of the output flows (% circular outflow) was evaluated. The % circular outflow was determined by the % recovery potential (focused on design or alternative management) and the actual recovery. The study tested a potential alternative management approach for the selected DiMaL to determine the potential valorization (% recovery potential) for the alternative management, considering the current waste management model, where the discarded materials are sent to landfills without any valorization, which means a 0% recovery potential, contrasted with the proposed alternative model. The alternative model introduces selected DiMaL back into the production cycle, marking an 8.75% recovery potential. If a potential alternative valorization could be found for the remaining fraction of DiMaL (91.25%), which was not studied, it could increase the recovery potential, subsequently enhancing the circularity of materials used within the company; the potential economic benefits of such measures would need evaluation.

3.3. Manufacturing of “Cabezudos” Caps Using VMP

According to the compositions detailed in Table 1, samples of solid big-headed cap crowns were manufactured. A test was conducted to evaluate their performance during machining. As can be seen in the images in Figure 8, they presented no problems when cut with the tool, showing no breakage, burrs, or burns. They also showed no deformations and their vertices remained entirely defined.

Once the machining of the cork housings was carried out, the physical appearance of the samples is presented in Figure 9. Irregularities can be seen on the surface of the stoppers made with Canva A. Nevertheless, the uniqueness of the product obtained is visibly distinctive, and allows for the identification of the material specific to the Canva A commercial brand. For the determination of relative humidity and apparent density, the samples were cut to the same size (see

Table 5. Relative Humidity Analysis of Composite Samples via VMP Testing.

		m1 [g]	m2 [g]	m1–m2	H [%]	v [mm3]	D [kg.m−3]
	R100	8.73	8.51	0.22 ± 1.54 × 10−4	0.15 ± 0.03	6833	1278.71
	C20R80	6.83	6.77	0.06 ± 1.15 × 10−4	0.43 ± 0.21	6833 *	1003.44
	HD25R75	7.06	6.94	0.12 ± 5.77 × 10−5	0.83 ± 0.26	6833	1034.37
	P20R80	7.17	7.15	0.02 ± 1.73 × 10−4	0.2 ± 0.08	6833	1049.5
	T20R80	6.62	6.58	0.04 ± 1.15 × 10−4	0.26 ± 0.04	6833 *	969.35

m₁: average mass in grams before; m₂: average mass in grams after drying; H: average humidity; v: volume; D: bulk density; * Estimated volume.

). The Talyn composite samples exhibit favorable machining characteristics, which are best illustrated by the image “T20R80” provided in Table 5. An excellent interior housing to accommodate the cork can be observed.

The results obtained, both aesthetically and in terms of cork adhesion to the composite crown made from textile waste, demonstrate a manufacturing process requiring minimal effort and energy resources. A comparison of the finish of these crowns can be made by employing compression molding and injection molding, as described in [25,37,38], but in both processes, identical results were achieved using the same proportion of textile waste.

3.4. Relative Humidity and Bulk Density

The DiMaL samples obtained from the VMP process were analyzed for relative humidity and bulk density, and the results are shown in Table 5.

Humidity increased as the percentage of leather in the composite rose, which was expected. This phenomenon was observed in the sample composed of HD leather. However, although HD and P leathers were similar and the former could accommodate a higher loading percentage, the density of the latter was higher. The density of the Talyn composite sample was particularly relevant, resembling polypropylene, the thermoplastic currently used in spirit bottle caps (see Figure 10). The resin, used as a binder, is the main contributor to the compound’s mass. Nevertheless, the choice of this resin was motivated by two aspects that ultimately proved decisive: its high level of transparency and the fact that, according to its manufacturer, it contains 20% natural oils.

As observed among the collected cork samples (Figure 10) from various wineries within the Jerez wines region (Cádiz, southern Spain), despite design changes over more than 100 years, the size and shape of the cork has remained unchanged over time. Although the use of internal reinforcements in the housing to improve cork-plastic adhesion seems prevalent, there was only one case where this was not used (bottom sample in red). Similarly, these reinforcements were not used in the samples manufactured in this study, yet the adhesion was successful. In reference [38], tensile tests were performed to verify the adhesive strength of the bond, and the results showed that in most samples, the cork broke before detaching. Additionally, a study of the actual adhesive consumption in these types of crown-cork joints was conducted in [25].

4. Conclusions

In the leather sector of Ubrique, a wide variety of discarded materials (DiMaL) are generated from the manufacturing of different products brought to market, not necessarily based on tanned leather. Overall, most of these textile discarded materials are managed alongside urban waste, being sent to landfills without any form of valorization, which is increasingly costly under the current framework.

A quantitative and qualitative temporal characterization of the DiMaL was conducted, initially across various companies in the sector, and subsequently focusing on a specific company, Artilab. During the period under study, an average monthly quantity of these DiMaL generated was obtained, totaling more than six tons per month. In the qualitative and quantitative classification of the set of DiMaL analyzed, four stand out, representing only 8.75% of the total. These are HD, P, Canva A, and Lining, with a generation percentage of 48.1, 22.0, 18.6, and 11.3%, respectively. For these selected DiMaL, an alternative management option was proposed in the form of bottle caps for spirits. This alternative approach represents a commitment to the circularity of the sector, with a potential material recovery of 8.75% based on the current waste management practices. Additionally, there is a potential cost-saving of 36.5% in the management of the studied discarded materials fraction, compared to the alternative of sending it to landfills. The manufacturing cost of these crowns was also estimated to be four times cheaper than the two commercial references found.

However, the most revealing aspect was that the other 91.25% could also be valorized/reused to reach 100% circularity. A successful final alternative for DiMaL circularity was the production of cap crowns using VMP, using only 20% textile fibers. The increase of this percentage is hoped for, which would not only reduce the weight of the final product, but also bring more discarded materials back into the business. It is essential to continue exploring various mixtures and alternative manufacturing processes, such as infusion molding or compression molding, to find more efficient and sustainable solutions in cap production. Additionally, plastic injection could be a viable option for mass-produced items, although in the luxury sector, exclusivity and personalization remain crucial aspects for customers. Nevertheless, although exclusivity and customization within the presented textile discarded materials management approach are still crucial for the scenario proposed, the authors envision that the approach could and should also be studied and applied within more popular and general frameworks in other sectors (such as composting, anaerobic digestion, and construction materials).