Manufacturing and consumption of any product generate wastes because complete elimination of waste could not be possible. The world is facing tremendous challenges to focus on the mitigation, removal, reuse, recycle the produced wastes during/after the production, and end of use wastes. However, the world is trending towards zero waste which demands sustainable production and consumption cycle. Footwear industry produces boots and other footwear consumed by the Military personnel and common people. After the use of boots and other footwear, End of Life (EoL) waste is generated because the end of the functional life of a boot forces it to be a waste.
The most widely used footwear for Military forces including the Army, Navy, Air force and other forces is the boot, designed to combat all fields of harsh work environments, training, and garrison. It is prominent to state that the purposes served by a Military/Army boot are of safety, and protection against cold/hot weather which eliminates the need of wearing different safety shoes, hot weather boots, and cold weather boots separately. Military boots are commonly made by a combination of leather upper, sole, insole, metals, linings, threads, shanks, eyelet, lace, textiles, foams, etc. The world’s total Military forces account for approximately 70 Million personnel who are professionally wearing boots (Firepower 2023). In Bangladesh, according to the Global Firepower (GFP) report 2023, the total personnel of Military Forces accounts for 69,65,000 people among which Army personnel 1,75,000, Navy Force 30,000, Air Force 21100 members, and paramilitary of 68,00,000 members (Firepower 2023). Besides, Bangladesh Police has 2,56,344 members, Border Guard Bangladesh (BGB) has 38,000 members, Coast Guard has 900 (2020). In addition, some private security guards also use a similar type of boot. Defense boots are used continuously on high duty (daily field exercise, physical training, garrison work, battlefield, etc.) and are considered to be used more than typical shoes which lead to relatively short useful life of a boot. Therefore, a defense boot’s tiny but worthy life drives defense personnel to use at least two pairs of boots every year. On average, two pairs of boots are used by one Army/Military person in Bangladesh which accounts for almost 14 million pair of defense boots is consumed in Bangladesh every year. This figure comes 140 million when the report is for total world Military usage of boots per year. Nevertheless, the most disciplined and trained personnel work in the Bangladesh Armed Forces and other forces. Therefore, collecting all the waste boots would be possible when their functional lives have ended. As far the author’s knowledge until now in Bangladesh, the only management of these used boots is landfilling which is not considered to be a sustainable and value added management scheme. Now it’s the prime time to manage or process the different materials obtained from the waste boot in a sustainable and environment-friendly way. Otherwise, this large waste stream generated in this sector at the end of its functional life is disposed of in landfills as practiced. Thus, this work investigated the options for utilizing the Army used boot of Bangladesh for reusability, recyclability, reducibility and energy recoverability of the waste generated after the EoL. The paper also mentioned framework for the critical aspects of various materials obtained from the post-consumer used boot through different mechanical, thermochemical, and chemical treatment processes.
In addition, the footwear business is a large area of manufacturing globally that uses various materials to make a wide range of products, from different types and designs of footwear to more specialized shoes like defense boot (Staikos, Heath et al. 2006). However, the typical shoe's material composition is shown in Table 1.
Table 1
Composition of a typical footwear (Staikos, Heath et al. 2006)
Footwear Materials
|
Percentage (%wt)
|
Leather
|
25
|
Polyurethane (PU)
|
17
|
Thermoplastic Rubber (TPR)
|
16
|
Ethylene Vinyl Acetate (EVA)
|
14
|
Poly (Vinyl Chloride) (PVC)
|
8
|
Rubber
|
7
|
Other (adhesives, metals, etc.)
|
7
|
Textiles and Fabrics
|
6
|
1.1 Collection and characterization of used Army boots
Four pairs of used and one pair of new boot sample (NBS) were collected from Savar cantonment area, Bangladesh, and weighed, as shown in Table 2. These used boot samples are identified as UBS1 (used boot sample 1), UBS2 (used boot sample 2), UBS3 (used boot sample 3), and UBS4 (used boot sample 4), respectively, as shown in Fig. 1. The used boots were used for approximately six months before sampling for this study. Used boot parts were separated carefully into their components with the help of an anti-cutter and scissors.
The weight of the NBS was found to be 1.436 Kg, whereas the average weight of a pair of used boots was 1.319 Kg (Table 2). Usually, defense personnel use a pair of boots for about six months.
Table 2
Weight of four pair of used boots.
Sample
|
Size of boot
(European)
|
Left boot weight, Kg
|
Right boot weight, Kg
|
Total pair weight, Kg
|
Average weight of each pair, Kg
|
UBS1
|
44
|
0.765
|
0.779
|
1.543
|
1.319
|
UBS2
|
43
|
0.679
|
0.679
|
1.357
|
UBS3
|
41
|
0.579
|
0.570
|
1.150
|
UBS4
|
42
|
0.597
|
0.627
|
1.224
|
NBS
|
44
|
0.738
|
0.698
|
1.436
|
1.436
|
1.2 Physical damage observation of the used boots
The physical appearance and damages of the collected used defense boots have been checked carefully to determine the each and every changes occurred during the use of boots and listed in Table 3. It has been revealed that for the total four pair of used boots, 92–96% of damage was noticed as a whole where the different parts of the boots showed different degree of damages including moderately to extreme damages (Table 3). The vamp part of the boots was observed extremely cracked whereas other leather upper part was moderately damaged. Finish film of leather was found moderate to highly crack. The sole and insole, important part of a boot was noticed cracked and degraded. One of the most significant factors for considering the reuse of boots is the adhesion (seam, adhesion by adhesives etc.) between different adhered parts of the boots which was found moderately to highly damage between the lines of joints. Despite this, fading of the color was noted for the materials including leather, metals and fabrics shown in Table 3.
Table 3
Physical damage identification of used boot components
Sample ID and Damage %
|
Main Parts
|
Nature of Damage
|
Degree of damage (Right one)
|
Degree of damage
(Left one)
|
UBS1,
UBS2,
UBS3, and
UBS4
(92–96%)
|
Vamp
|
Crack
|
Extreme
|
Extreme
|
Toe
|
Toe finish film
|
High
|
High
|
Lace
|
Tip condition and Stretchiness
|
Damaged tips and highly stretched
|
Damaged tips and highly stretched
|
Stitch line
|
Stitch line condition
|
Damaged
|
Damaged
|
Upper and Sole adhesion
|
Adhesion Failure
|
High
|
High
|
Edge
|
Edge Damage
|
Moderate
|
Moderate
|
Sock
|
Condition
|
Damaged and bad odor
|
Damaged and bad odor
|
Metal accessories
|
Condition
|
Damaged and discolored
|
Damaged and discolored
|
Insole
|
Condition
|
Fractured
|
Fractured
|
Finish film
|
Finish film of leather
|
Cracked
|
Cracked
|
Tongue
|
Fiber and color
|
Loose fiber, color faded highly
|
Loose fiber, color faded highly
|
Seam and Toe cap
|
Condition
|
Moderately Damaged
|
Damaged
|
Sole
|
Condition
|
Damaged, cracked and degraded
|
Damaged, cracked and degraded
|
Main body, Leather
|
Texture and feel
|
Loose and hard, cranky, unsmooth
|
Loose and hard
|
Heel grip
|
Condition
|
Broken and damaged
|
Broken and damaged
|
1.3 Description of different parts of used boot
Footwear is usually manufactured by assembling approximately 40 different materials (Staikos, Heath et al. 2006). Defense boots are commonly made with upper (vamp, quarter, toe cap, tongue, and counter) generally made by leather, sole (TPR, leather, EVA, etc.), insole (Leather, EVA, etc.), steel shank, lining (Leather, fabric, etc.), socks (Leather, board, etc.), foams (PU, other natural and synthetic rubbers), interlining cotton, shank board (Cellulose board, waste paper board, etc.), metal eylet, lace, toe puff (TPR, metal, leather, paper board, etc.), stiffener (TPR, paper board, leather, etc.), threads, and other reinforcement materials. In general, unwearable, recyclable defense boots have rips, holes, or tears in the material, and the soles could be separated from the upper vamp. These issues compromise the integrity of the used boot, which makes them unwearable. Despite that, the materials aren’t entirely useless.
1.3.1 Leather part
Leather is the most suitable material for uppers (vamp and quarter), insoles, toecap, counter, socks, and the tongue for its high tensile strength, durability and comfortability (Venkatappaiah 1988). A total of seven leather parts, including insoles, were separated from the used boot, as shown in Fig. 2 (a-e, g and h). The average weight of these parts was about 514 g, and approximately 39% of the total used boot, as listed in Table 4. Compared to the standard footwear composition stated in Table 1 in this study, more usage of leather was found for defense boot manufacture than a typical shoe (25%) which conforms to the durability, comfortability, and strength factors of the boots. Leather is a complicated material made primarily of a collagen matrix that is chemically stabilized by different tannins and chemicals. About 29% of raw bovine skin is made up of collagen, 2% is keratin, and 3% is elastin, with tiny amounts of lipids and other inorganic compounds (Parisi, Nanni et al. 2021). Currently, chromium salt is used in over 90% of the world's leather industry to turn hides and skins into non-putrescible leather to attain high thermal stability and other strength properties (Swarnalatha, Ganesh Kumar et al. 2009). The remaining skins are tanned using vegetable tannins, or a combination with Aluminium, Chromium and vegetable tannins. Chromium and Aluminium were discovered to make up 98% of the total concentration of heavy metals in finished leather tanned using chromium and aluminium salts, while zirconium was the only heavy metal found in certain vegetable-tanned leather as a combination technology (Aslan 2009). In Cr tanning, only 60–70% of chromium salts react with the hide and skin (Mia, Sayid et al. 2020). Different types of dyestuffs are used to color the leather. In the industry, more than 90% of the hides are dyed with azo dyes (Gomes, Piccin et al. 2016). Black is the indispensable color for elegant shoes and boots (MUTLU, ORK et al. 2017).
1.3.2 Outsole
Outsole is the very bottom part of a boot and always be contacted with the ground. Generally, two types of soles are used in defense boots. The most popular one is the thermoplastic rubber (TPR) sole, which is composed of styrene-butadiene-styrene (SBS). The SBS consists of various ingredients such as naphthalene oil, and crystal-grade polystyrene. Another is polyvinyl chloride (PVC) sole made by polymerizing vinyl chloride monomer. Generally, solid and microcellular PVC blends and PVC/nitrile rubber blends are used to manufacture this type of sole (Venkatappaiah 1988). From the anatomy of four defense used boot samples shown in Fig. 2 (f), it was found that all soles were thermoplastic rubber (TPR). The average weight of the separated parts was about 634 g which accounts for 47% of the total used boots, as depicted in Table 4. Commonly, a rubber sole is composed of the raw materials as follows (parts by weight): natural rubber (60–80 parts), styrene-butadiene-styrene block copolymer (SBS) (15–25 parts), white carbon black (10–20 parts), ethylene-vinyl acetate copolymer (EVA) (10–15 parts), polybutadiene (10–15 parts), Si-69 (0.5-1 part), diethylene glycol (0.3–0.5 part), vulcanizing agent (0.8–1.6) and antioxidant softener (5–15 parts) (Ce 2015).
1.3.3 Midsole
The midsole sits between the outsole and insole to facilitate the absorption of shocks during the use of boots and is typically made of leather, ethylene vinyl acetate (EVA) etc. EVA is a polymer extracted from ethylene and vinyl acetate monomers. Midsoles are covered by polyurethane-coated fabrics (Venkatappaiah 1988). In this study, part of leather midsole was made by buffalo leather shown in Fig. 2 (g). The average weight was nearly 122 g and accounted for about 9.15% of the total used boots, as delineated in Table 4.
1.3.4 Toe puff and counter stiffener
The toe puff is inserted in between the toe portion of the boot upper and the lining to reinforce the toe portion of boots and also to protect the toes of feet from any falling heavy matters on the feet. For general shoes, toe puff is incorporated for maintaining the shoe format but for defense boots this is used for the protection as well. Contrary, counter stiffener is used to maintain the proper shape retention of a boot’s back part as well as whole boots. Toe puffs and stiffeners are produced from thermoplastic rubber, vegetable-tanned leather, leather board, polystyrene-impregnated fabric, rubber-impregnated fabric, and metals. Thermoplastic toe puffs and stiffeners are made from un-vulcanized rubber fabric, synthetic fibres impregnated with styrene butadiene rubber lattices, polychloroprene rubber lattices, and styrene or acrylate copolymers (Venkatappaiah 1988). The main difference between toe puffs and a counter stiffener is in their thickness. Generally, toe puffs are thicker than the counter stiffener. This study found that the toe puff and stiffener was made of thermoplastic (TPR) material, as shown in Fig. 2(k) and 2(p). This has an average weight of 48 g and accounts for nearly 3.59% of the total used boots, according to Table 4.
1.3.5 Metal shank and eyelet
Shank is a strip of material kept between the insole and outsole as a reinforcement component. Generally, metal shanks are produced from high-grade carbon steel (Venkatappaiah 1988). Shank can also be made from steel (Mangum 1966). The shanks are placed in a shank board (usually made by cellulose board or paper waste) which is depicted in Fig. 2(i). Eyelets are made of metal, such as copper, stainless steel, aluminium, brass, and lacquered aluminium. In this study, the dissected boot metal shank and eyelet, as shown in Fig. 2 (l and m), have an average weight of about 71.3 g and is about 5.32% of the total used boots, as listed in Table 4.
1.3.6 Lace
Laces provide a wide range of attachments at the facings and are a common fastener through eyelets. Laces are made of cotton and nylon. Eyelets enable the lace to pass through, providing minimum friction between the lace and the eyelet (Venkatappaiah 1988). Separated laces, as depicted in Fig. 2(o), had an average weight of 9.75 g and account for less than 1% of the total used boots shown in Table 4.
1.3.7 Other materials (Foams, linings, threads, cotton fabric, etc.)
In the defense boots, foams are used for cushioning purposes mainly and made by PU polymers. Generally, a varied thickness of foams is used in the collar, upper, socks, and tongue part of the boot. Lining in a boot are used to conceal the raw edges, sewn parts, interlinings etc. and generally composed of leather or cotton fabrics. Here in the collected samples, leather was found to be used as linings (Fig. 2j) and cotton fabrics was noticed to be as interlinings. In addition, different types of sewing threads (e.g., Nylon, polyester, etc.) are used for sewing the boots. The amount of these materials is less in amount compared to the other materials used to manufacture boots.
Table 4
Composition of different parts of used boot on weight basis
Name of materials
|
UBS1
(g)
|
UBS2
(g)
|
UBS3
(g)
|
UBS4
(g)
|
Average
(g)
|
Percentage
(% wt)
|
Leather
|
420.00
|
355.00
|
303.00
|
380.00
|
364.50
|
27.27
|
Insole (leather)
|
38.00
|
34.00
|
24.00
|
14.00
|
27.50
|
2.05
|
Midsole
|
127.00
|
111.00
|
123.00
|
128.00
|
122.25
|
9.15
|
Outsole
|
743.00
|
691.00
|
531.00
|
531.00
|
624.00
|
46.69
|
Polyurethane (PU)
|
30.00
|
--
|
--
|
--
|
30.00
|
2.24
|
Toe puff
|
59.00
|
51.00
|
42.00
|
40.00
|
48.00
|
3.59
|
Metal shank
|
20.00
|
25.00
|
18.00
|
14.00
|
19.25
|
1.44
|
Metal eyelet
|
34.00
|
36.00
|
68.00
|
70.00
|
52.00
|
3.89
|
Shank board
|
18.00
|
14.00
|
--
|
14.00
|
15.33
|
1.15
|
Lining
|
19.00
|
14.00
|
18.00
|
12.00
|
15.75
|
1.18
|
Lace
|
5.00
|
10.00
|
11.00
|
10.50
|
9.13
|
0.68
|
Foam
|
1.68
|
1.68
|
1.00
|
0.60
|
1.24
|
0.09
|
Thread
|
1.48
|
1.49
|
1.50
|
1.51
|
1.50
|
0.11
|
Fabric oven tape
|
--
|
--
|
--
|
6.00
|
6.00
|
0.45
|
Total
|
1516.16
|
1344.17
|
1140.50
|
1231.15
|
1338.83
|
100
|