Secondary Treatment Facilitating the Mechanical Recycling of Film-Coated Waste Automobile Bumpers

Abstract

Concerns have been raised regarding the mechanical recycling rates of automotive parts, which appear to be low in comparison to those of other sectors. Addressing this issue demands the promotion of the mechanical recycling of waste automobile bumpers. This study investigates primer treatment effects on the mechanical properties of injection-molded waste bumpers. The primer treatment effects vary depending on the bumper coating. The flexural strength of coated bumpers is greater: 27.6 MPa vs. 29.2 MPa. The flexural modulus is also greater: 1667 MPa vs. 1761 MPa. By contrast, the notched Charpy impact strength is less: 25.4 kJ/m² vs. 23.4 kJ/m². The flexural strength and flexural modulus of the bumpers lacking a coating are also lower, respectively, at 25.0 MPa vs. 22.9 MPa and 1523 MPa vs. 1314 MPa. However, the notched Charpy impact strength is greater: 40.0 kJ/m² vs. 73.6 kJ/m². These findings suggest that primer treatment can control the mechanical properties of injection-molded parts produced from waste automobile bumpers, which is an important achievement for promoting the mechanical recycling of waste automobile bumpers.

1. Introduction

Thermoplastics can be softened and shaped by heating, and can then be cooled and solidified to alter their form. They are highly recyclable and convenient because they are producible at low cost. Moreover, once molded, they can be remolded by heating again. The most notable thermoplastics are polypropylene (PP) and polyethylene (PE), which are used in all aspects of daily life for containers and packaging, electrical and electronic equipment, household goods, and clothing. Because of these characteristics, the global production of plastics is increasing annually. In 2020, 367 million tons of plastics were produced, constituting approximately 24 times the 15 million tons produced in 1960. Furthermore, demand has continued to increase in recent years [1].

The environmental consequences of discarded plastic products are staggering, with particular concern related to marine pollution and its effects on animal life. A particularly salient issue is the presence of microplastics, mineral particles smaller than 5 mm, in aquatic environments, often resulting from illegal dumping and waste disposal facilities. This pervasive phenomenon is a rapidly mounting global concern. Research has indicated that aquatic organisms, after ingesting microplastics, exhibit impeded growth and reduced reproductive capacity [2]. Concerns about the potential adverse effects of microplastics on fish, shellfish, and on humans who consume them are particularly great. A notable example is a 2016 report that described the detection of microplastics in anchovies in Tokyo Bay, which prompted investigation into microplastic contamination in Japanese waters [3].

Because of the gravity of these circumstances, a global shift in attitudes has occurred related to plastics, with the 3Rs (Reduce, Reuse, Recycle) now being implemented rigorously. This movement is particularly pronounced in addressing the pressing issue of existing plastic waste. In Korea, ongoing efforts are aimed at quantifying the types and weight ratios of plastics used in waste generation, with a view to assessing the efficacy of plastic subdivision and segregation during recycling [4]. In Kuwait, an investigation has emphasized the potential of domestically produced recycled plastics and the extent of the natural weathering of plastics in arid regions [5]. In Lebanon, Poland, and Nigeria, emphases are on promoting recycling through the modification of bumpers and cables used in automobiles to assess their physical properties [6,7,8]. Additionally, broader considerations have been advanced, such as trends in European recycling processes and future projections of emissions [9,10].

To increase recycling opportunities in an economically realistic manner, considering associated CO₂ emissions, energy consumption, and profitability associated with such recycling is also important. An analysis of these perspectives of the recycling system in Japan indicated that some recycling methods excel in profitability but lack future potential in terms of CO₂ emissions and energy consumption [11]. Among Asian countries, Japan generates the largest amount of plastic waste after China and India. Therefore, in addition to strengthening current systems, considering appropriate recycling methods with future potential is important [12].

A crucially important aspect of recycling program assessment is the examination of the post-recycling fates of waste materials. The term “recycling” encompasses a spectrum of approaches, classified based on varying treatment methodologies and intended applications. A comprehensive understanding of these distinctions is paramount. The following three categories represent the most widely employed recycling methods.

(1)

Material recycling is the process of re-molding waste to produce molded products.

(2)

Chemical recycling is the process of breaking waste down into monomers to obtain raw materials.

(3)

Thermal recycling is the process of incinerating waste to obtain heat energy.

Of these, thermal recycling is the most widely implemented method, accounting for 60% of all treatment methods. This inexpensive method is the last remaining viable option for plastic waste that cannot be recycled. For this reason, it is implemented widely worldwide [13]. However, the residues must then be disposed of in landfills, which puts pressure on limited landfill sites. Therefore, this recycling method has little future potential in terms of resource recycling.

Material recycling and chemical recycling, by contrast, use waste as a renewable resource while changing its form. Particularly, material recycling is applicable to most of the recycled products in the general market. Oblak et al. reported the evaluation of the processability and the degree of degradation progression caused by 100-times application of heat when extruding plastics used as containers and packaging materials. They then measured their resultant mechanical properties [14]. Additionally, reports have described studies of effective recycling methods for polymer blends and analyses of the odors of recycled resins in terms of product perfection after recycling [15,16]. Research investigating waste plastics sorting methods is underway. For instance, Yu et al. proposed a sorting technique using terahertz spectroscopy for waste plastics [17]. Additionally, Okubo et al. combined near-infrared and terahertz spectroscopy with machine learning techniques, particularly XGBoost and Bayesian optimization, for the accurate identification of transparent polyethylene terephthalate (PET), transparent polystyrene (PS), and black PS [18].

The material recycling of plastic waste derived from automobile parts has seen limited advancement. In 2020, approximately 3.15 million vehicles were scrapped in Japan, with total weight of approximately 150 kg of plastics used per vehicle. Given that the total weight of the recycled material was approximately 470,000 tons, the material recycling rate for automobile parts is estimated at approximately 8.5% [19]. This huge amount suggests that enhancing the material recycling of plastic waste from automobile parts can reduce the environmental impacts of plastics.

To date, most studies have emphasized the recycling of bumpers and interior parts. In addition to the recycling of these components into recycled resin, some studies have addressed the recycling of bumpers and interior parts into graphene, an organic material, and the recycling of waste tires [20,21,22]. The material recycling of plastic wastes derived from automobile parts into automobile parts is regarded as difficult. The reasons underlying this difficulty are twofold: first, the disassembly and sorting processes are challenging; secondly, the physical properties of the recycled resin are lower than those of the virgin material, making it impossible to meet the high requirements of the automotive field when reusing recycled materials in the same application [23].

Bumpers are particularly susceptible to damage from collisions and wear. As presented in Figure 1, the composition of the coating film on a bumper typically has three layers: primer, a base coat, and a topcoat. Primer, applied before application of the coating itself, serves as the base coat of coating. Its primary functions include enhancing the adhesion, durability, and color retention of the coating. Examples of suitable coatings include those composed of phenolic resin, acrylic resin, and silicone resin. The term “base coat” in the context of automobile coating refers to the initial layer of coating applied to the vehicle’s body. The primary function of a base coat is to provide adhesion and durability. It is typically applied before the application of topcoats, clear coats, and other upper layers of coating. Common base coat formulations include acrylic resin-based, epoxy resin-based, and polyester resin-based compositions. A topcoat is the final layer of coating applied to the vehicle’s body. Its primary purposes are the protection of the surface of the body and the enhancement of its aesthetic appeal. Examples of common topcoats include clear coats and metallic coat applications.

The coating film, comprising these layers, is not thermoplastic. Consequently, when a bumper with coating film is crushed and reused as a raw material for its production, the crushed coating film remains inside the molded product. Although this might act as filler, the interface between the topcoat and the resin becomes defective because of the low affinity between the topcoat and the resin. Therefore, the molded product’s mechanical properties exhibit instability, impeding the consistent attainment of optimal quality [24].

In light of the background presented above, techniques to remove coating films have been sought. Specifically, physical removal methods using laser irradiation and chemical removal methods using solutions are being studied [25]. It has been documented that the removal of the coating film from the bumper, with subsequent pulverization and injection molding, results in molded products that exhibit superior impact strength compared to that of a molded product containing the coating film [26]. However, it is noteworthy that the process of removing the coating film increases energy consumption and increases the number of necessary processes, consequently leading to increased costs, rendering this method impractical in the present context.

In light of the background described above, this study was conducted to establish a material recycling method for waste automobile bumper products that guarantees mechanical properties without compromising the coating film’s integrity. A simple secondary treatment was implemented as a prerequisite to facilitate the integration of the method into existing processes. This study specifically addressed a primer that adheres the coating film to the base material. The primer coating was applied to the recycled resin surface before injection molding. After the mechanical properties of the molded products were measured, the mechanical properties were compared with and without primer coating. The primer coating effects were then clarified.

2. Experiment Procedure

2.1. Materials

For this study, waste automobile bumper crushed materials were used (Yamagata Automobile Recycle Center Co., Ltd., Yamagata, Japan, Eco-R Co., Ltd., Tochigi, Japan, and Matec Co., Hokkaido, Japan). An overview of these materials is presented in Figure 2. The crushed materials had a size of approximately 10 μm. They contained coating films of various colors. Some materials were not coated. These were removed for this study by manual sorting. The removed crushed bumper material was considered uncoated and was examined separately from the coated material.

2.2. Primer Coating

Waste automobile bumper crushed materials of two types were primer-coated with a commercially available primer spray (acrylic primer, A-029; Asahipen Co., Ltd., Tokyo, Japan). The primer was applied using a spray gun at a distance of 20–30 cm. The materials were allowed to dry for 30 min. To ensure uniform coating of the crushed material with primer, the primer was stirred after drying and was then reapplied. Figure 3 shows the primer spray used for coating. It is noteworthy that the application of primer coating increased the weight of the crushed product by approximately 0.1 wt%.

2.3. FT-IR Measurement

To identify the composition of the polymers constituting waste automobile bumper crushed material and injection-molded parts, Fourier-transform infrared (FT-IR) measurements were taken using a Fourier-transform infrared spectrometer (Nicolet iS5; Thermo Fisher Scientific Inc., Tokyo, Japan). The measurement method employed for this study was the total reflection absorption (ATR) method, with 32 integrations, a wavenumber range of 400–4000 cm⁻¹, and a background of air. Matches for the obtained spectra were then sought in a database to identify the compositions of the measured products. The product with the highest identification rate in the search results was found to be the one with the highest identification rate among the search results.

2.4. Injection Molding

The waste automobile bumper crushed material was cut manually into pieces measuring 4–5 mm. It was subsequently filled into an ultra-compact electric injection molding machine (C, Mobile0813, 10 mm plunger diameter; Shinko Sellbic Co., Ltd., Tokyo, Japan) for injection molding. The molding temperature was set at 220 °C. The die temperature was 50 °C. The injection speed was 10 mm/s. The holding pressure was fixed at 42 MPa. The resulting shapes of the molded products are depicted in Figure 4. Moldings of two distinct types were prepared for this study: Type A, for which the flow path was adjusted to ensure resin aggregation in the center of the molded product; and Type B, in which the resin did not aggregate. For Type A, a weld, a molding defect, is formed in the center of the injection-molded product. It is understood that the solids dispersed in the melt at this weld are oriented as perpendicular to the flow direction. The solids considered in this study are the coating film formed on the bumper surface. To elucidate this point, two types were prepared for this study: Type A, which is oriented as perpendicular to the direction in which the coating film flows; and Type B, which is oriented parallel to the direction in which the coating film flows. Type A was used to assess the strength of the interface between the coating film and the matrix phase, whereas Type B was used to evaluate the strength and notched impact strength of the molded product.

2.5. Three-Point Bending Tests

In accordance with the standards outlined in ISO 178, three-point bending tests were performed using a tabletop tensile and compression testing machine (MCT-2150; A&D Co., Ltd., Tokyo, Japan) [27]. Type B injection-molded products were used for these tests. The test protocol entailed a loading speed of 10 mm/min, with a separation of 40 mm between the fulcrums. To derive the bending stress σ_f, the measured load P_f was analyzed, employing the following Equation (1):

$${\mathrm{\sigma }}_{\mathrm{f}}={\displaystyle \frac{3{\mathrm{P}}_{\mathrm{f}}\mathrm{S}}{2\mathrm{b}{\mathrm{h}}^2}}$$

(1)

where b stands for the molded product width, h represents the molded product thickness, and S expresses the distance between the fulcrums. The bending strain ε_f was obtained from the obtained deflection δ using the following Formula (2):

$${\mathrm{\epsilon }}_{\mathrm{f}}={\displaystyle \frac{6\mathrm{\delta }\mathrm{h}}{{\mathrm{S}}^2}}$$

(2)

The σ_f–ε_f curve was then delineated. The maximum value of σ_f was ascertained as the flexural strength, denoted by σ_F. The slope at the onset of loading was determined to be the flexural modulus, E_F. This test was repeated ten times. The mean values of σ_F and E_F were used as the characteristic values. The standard deviation was also determined. Significant differences between samples were assessed using analysis of variance.

2.6. Notched Charpy Impact Tests

Notched Charpy impact tests were performed using a Charpy impact tester (Mys-shikenki Co., Ltd., Osaka, Japan) in accordance with ISO 179-1 [28]. The specimen, which featured a 1 mm notch machined in the width direction at the center of a Type B injection-molded product, was used for testing. The loading speed was 2.91 m/s. The inter-fulcrum distance was 40 mm. Absorbed energy U was obtained from the obtained swing angle. Also, the notched Charpy impact strength a_iN was calculated using the following Equation (3):

$${\mathrm{a}}_{\mathrm{i}\mathrm{N}}={\displaystyle \frac{\mathrm{U}}{\mathrm{h}(\mathrm{b}-\mathrm{a})}}$$

(3)

For this experiment, a denotes the notch depth. The test was repeated 30 times. The mean value of a_iN was calculated to ascertain the characteristic value. The standard deviation was also determined. Significant differences between samples were found using analysis of variance.

2.7. Tensile Tests

Tensile tests were conducted on a small universal mechanical testing machine (FSA-1KE-1000N-L; Imada Co., Ltd., Tokyo, Japan) in accordance with ISO 527 [29]. Type A injection-molded products were obtained through injection molding at 10 mm/min loading speed. The distance between the chucks was 22 mm. The nominal tensile stress σ_t was obtained using the following Equation (4) from the obtained load P_t:

$${\mathrm{\sigma }}_{\mathrm{t}}={\displaystyle \frac{{\mathrm{P}}_{\mathrm{t}}}{\mathrm{b}\mathrm{h}}}$$

(4)

In addition, the following Equation (5) was used to calculate the true tensile strain ε_t from the measured elongation, shown as δ_t:

$${\mathrm{\epsilon }}_{\mathrm{t}}=\ln \left(1+{\displaystyle \frac{{\mathrm{\delta }}_{\mathrm{t}}}{\mathrm{H}}}\right)$$

(5)

where H denotes the inter-chuck distance. The σ_t-ε_t curve was calculated. The tensile strength of the welding zone was identified as the extreme value. The tensile strength of the welding zone is designated as the weld strength, which, as reported by the authors, is equivalent to the interfacial strength between the fiber and the base material in short-fiber-reinforced thermoplastic composites [30]. The tensile strength evaluated by this test was regarded as the interfacial strength between the coating and the base material. The fracture surface observation after the test revealed that numerous coating film sides were observed. This test was conducted ten times. The mean value of weld strength was used as the characteristic value. The standard deviation was also determined. Significant differences between samples were found using analysis of variance.

2.8. Microfocus X-Ray CT Observation

The distribution of coating films within the type B injection-molded products was examined using a microfocus X-ray computed tomography (CT) apparatus (ScanXmate-D225RSS270; CosmoScan Techno Co., Ltd., Tokyo, Japan). The CT scan parameters included 70 kV tube voltage, 100 μA tube current, 7.0 W tube power, and 15× magnification.

2.9. Scanning Electron Microscope Observation

Using a scanning electron microscope (Tiny-SEM 510; Technex Lab Co., Ltd., Tokyo, Japan), the fracture surface was examined after the notched Charpy impact test to assess the fracture morphology.

3. Results

3.1. Coating and Primer Composition

Figure 5 presents an illustration of the FT-IR measurements of the crushed waste automobile bumpers. Spectroscopic analysis of the coating film revealed the presence of polymethyl methacrylate (PMMA) on its surface. The infrared spectrum exhibited several specific peaks: at 1725–1730 cm⁻¹, corresponding to the C=O bond stretching vibration; at 2950 and 2870 cm⁻¹, indicative of C–H bond stretching vibration; and at 1450 and 1380 cm⁻¹, representing the C–H bond angular displacement vibration [31]. These are believed to be components of the coating film. In contrast, IR spectra derived from polypropylene were detected on the surface devoid of the coating film. Specifically, C–H bond stretching vibrations were detected at 2950, 2915, and 2835 cm⁻¹; C–H bond angular displacement vibrations were detected at 1455 and 1375 cm⁻¹ [32]. By applying primer treatment to the crushed products, an IR spectrum derived from PMMA similar to that of the surface on the coating side was detected, which is regarded as a primer component.

Figure 6 presents an example of the FT-IR measurement results obtained for a type B injection-molded product. IR spectra derived from polypropylene were detected in the injection-molded products, irrespective of whether they were coated with primer or not. The C–H bond stretching vibrations detected at 2950, 2915, and 2835 cm⁻¹ and the C–H bond angular displacement vibrations detected at 1455 and 1375 cm⁻¹ were specific [32]. These observations suggest that the components comprising the coating film and primer are dispersed throughout the molded product’s interior during the injection molding process. These findings demonstrate that a PMMA layer forms on the surface of crushed waste automobile bumpers through the application of the primer coating, and that the dispersed components of the coating film and primer are present within the molded product’s interior after the injection molding process.

3.2. Dispersion State of the Coating Film Inside the Molded Product

Figure 7 presents the findings derived from the X-ray CT analysis of a specimen of a Type B injection-molded product, wherein the coating film exhibited a dispersion state within the specimen’s interior, oriented in a manner parallel to the flow direction. This dispersion state remained unaltered by the application of the primer coating. By contrast, in products of injection molding made from materials selected for their uncoated state, the coating film demonstrated minimal dispersion within the specimen’s interior. This outcome supports the validity of the sorting criteria used for this study. Furthermore, this observation indicates that items classified as bumper crushed with coating film might include instances of crushed items devoid of a coating film.

3.3. Effects of Primer Coating on Bending Properties and Impact Strength

Table 1. Mechanical properties of injection-molded bumper. Values following ± in the table are standard deviations.

Materials	Coating	Primer	σF [MPa]	EF [MPa]	aiN [kJ/m2]	Weld [MPa]
Bumper	Yes	No	27.6 ± 0.1	1667 ± 27	25.4 ± 4.5	9.0 ± 0.5
	Yes	Yes	29.2 ± 0.2	1761 ± 24	23.4 ± 4.0	10.1 ± 0.4
	No	No	25.0 ± 0.1	1523 ± 14	40.0 ± 4.4	-
	No	Yes	22.9 ± 0.4	1314 ± 45	73.6 ± 8.5	-

presents the flexural strength and flexural modulus, respectively, as σ_F and E_F, as obtained from the results of a three-point bending test using Type A injection molding products. The values following ± in the table denote the standard deviation. The values obtained for these parameters differed depending on the presence or absence of a coating film. Specifically, the flexural strength and flexural modulus of the crushed product with a coating film dispersed internally were, respectively, approximately 2.6 MPa and 140 MPa higher than those of the other samples. This trend is consistent with the results reported by Sover et al. [25].

The flexural strength exhibited an increase from 27.6 MPa to 29.2 MPa, whereas the flexural modulus increased from 1667 MPa to 1761 MPa upon the application of primer to the bumper that had been crushed with a coating film. However, when the primer was applied to the crushed bumper without coating, the flexural strength and flexural modulus decreased, respectively, from 25.0 MPa to 22.9 MPa and from 1523 MPa to 1314 MPa. The analysis of variance applied to the primer application effect yielded p < 0.05, irrespective of the presence or absence of a coating, indicating a significant difference.

As Table 1 shows, the notched Charpy impact strength, designated as a_iN, is presented as a function of the results obtained from notched Charpy impact tests employing Type B injection molding products. The values following ± in the table represent the standard deviation. It is noteworthy that this value is contingent upon the presence or absence of a coating film, with the internal dispersion of the coating film leading to reduction in the value by approximately 15 kJ/m². This trend is consistent with the results reported by Sover et al. [25].

The application of primer to bumper crushed products with a coating film led to a reduction in notched Charpy impact strength from 25.4 kJ/m² to 23.4 kJ/m². The notched Charpy impact strength of the injection-molded bumper crushed without coating exhibited an increase from 40.0 kJ/m² to 73.6 kJ/m² upon primer application. The analysis of variance applied to the primer application effect yielded a result of p < 0.05, irrespective of the presence or absence of a coating, indicating a significant difference.

These study findings suggest that the application of a coating film to bumpers through primer treatment enhances their flexural strength and flexural modulus. However, this treatment also engenders a reduction in notched Charpy impact strength. By contrast, the absence of primer treatment to bumpers devoid of coating film led to a reduction in flexural strength and flexural modulus, while concurrently augmenting the notched Charpy impact strength.

3.4. Primer Coating Effect on Interfacial Strength

Table 1 presents the weld strength found from the results of tensile tests using Type B injection molding products. The values following ± in the table represent standard deviations. The application of primer to the bumper, subsequent to its crushing with coating film, augmented the weld strength, raising it from 9.0 MPa to 10.1 MPa. The analysis of variance for the primer application effect yielded p < 0.05, indicating a significant difference. These findings suggest positive correlation between the primer treatment and weld strength, indicating an inclination toward increased strength with primer application. The filler material orientation has previously been reported as perpendicular to the flow direction within the welding zone, defined as the resin-joining portion of an injection-molded product that incorporates dispersed inorganic filler [30]. Applying uniaxial tensile loading to this condition causes the interface between the filler and base material to delaminate, reducing the yield of the molded product. Observations using X-ray CT show that the coating film disperses throughout the injection-molded product without melting, indicating that the coating film is oriented as perpendicular to the flow direction at the welding zone. Applying a tensile load in this state leads to the delamination of the interface between the coating film and the base material, thereby causing the molded product to yield. Therefore, the weld strength found as a result of this study can be regarded as the strength of the interface between the coating film and the base material. The findings obtained from this study indicate that the weld strength of bumper injection-molded parts with coating film increases with primer treatment, suggesting that primer treatment strengthens the coating film–base material interface.

When considered in conjunction with the evaluation results outlined in Section 3.3, the primer treatment applied to the coated bumpers demonstrated a strengthening effect on the coating–base material interface. This treatment enhanced the flexural strength and flexural modulus, but it also reduced the notched Charpy impact strength.

4. Discussion

4.1. Primer Coating Effects on Bending Properties of Injection-Molded Products

As presented in Section 3.3, the flexural strength of the injection-molded bumper crushed with a coating film exhibited a tendency to increase following the application of the primer coating. Furthermore, the findings presented in Section 3.4 indicate that the coating film–base material interface strength increases with primer coating. Integrating these results suggests that positive correlation exists between flexural strength and interfacial strength. Because the coating film can be designated as a filler with a large aspect ratio, as shown in Figure 7, the coating film can be regarded as a fibrous filler.

As Takayama et al. reported, the flexural yield of short-fiber-reinforced thermoplastic injection-molded products is attributable to fiber pullout or interfacial debonding [33]. The orientation angle of the fibers is the primary factor determining this yield. Fiber pullout is applied when the fibers are oriented parallel to the loading direction. Interfacial debonding is applied when the fibers are oriented perpendicularly. The X-ray CT observations presented herein indicate that the coating film is oriented as nearly parallel to the flow direction. Therefore, one can infer that the bending and yielding of the injection-molded bumper with a coating film results from the pullout of the coating film. In this instance, the stress σ_fy at the initiation of the bending and yielding of an injection-molded product employing a crushed bumper with a coating can be calculated using Equation (6), presented below.

$${\mathrm{\sigma }}_{\mathrm{f}\mathrm{y}}={\displaystyle \frac{2{\mathrm{\tau }}_{\mathrm{I}}\left(1+\mathrm{\upsilon }\right)}{\cos \mathrm{\phi }}}{\displaystyle \frac{{\mathrm{l}}_{\mathrm{p}}{\mathrm{V}}_{\mathrm{f}}}{\mathrm{d}{\mathrm{l}}_{\mathrm{f}}}}$$

(6)

In the context of the given parameters, the interfacial shear strength τ_l is theoretically equivalent to one-third of the weld strength. Also, V_f represents the fiber volume content, whereas parameter υ denotes the Poisson’s ratio of the molded product. The variable φ specifies the fiber orientation angle. The variables l_p and l_f, respectively, denote the pullout fiber length and the remaining fiber length. Also, d represents the fiber diameter. Although the application of the previously described equation is complicated by the disparity in morphology between the fiber and the coating film, a direct quantitative evaluation is possible. The results of the analysis indicate a positive correlation between the flexural yield initiation stress and the interfacial strength. In this context, the interfacial strength is equivalent to the weld strength described in Section 3.3. As presented in Table 1, the weld strength demonstrates a tendency to increase following the application of primer to the coated bumper crushed material. From Equation (6), it can be inferred that the primer treatment of the bumper crushed with a coating film augmented the interfacial strength, which consequently caused an increase in flexural strength.

However, the bending strength tended to decrease when the primer treatment was applied to the bumper crushed without coating. Figure 7 demonstrates that the presence of large inclusions, such as coating film, was not observed in the coating-free bumper that was subjected to the primer treatment. This finding indicates that the flexural yield of injection-molded bumps subjected to compression without a coating film is attributable to the shear yield of the base material. Poisson’s ratio and the longitudinal elastic modulus of the molded product can be estimated using the method proposed by Takayama et al. based on flexural strength and flexural modulus [34].

Table 2. Elastic properties and stress at yield initiation of injection-molded bumper without coating films. Values following ± in the table are standard deviations.

Materials	Coating	Primer	υ [-]	E [MPa]	YS [MPa]
Bumper	No	No	0.443 ± 0.001	451 ± 5	11.6 ± 0.1
	No	Yes	0.444 ± 0.002	384 ± 13	10.6 ± 0.2

presents the estimated Poisson’s ratio, longitudinal elastic modulus, and stress at yield initiation. The stress at yield initiation σ_y is determined by the following Equation (7):

$${\mathrm{\sigma }}_{\mathrm{y}}={\displaystyle \frac{{\mathrm{\sigma }}_{\mathrm{f}\mathrm{y}}}{1+\mathrm{\upsilon }}}$$

(7)

The data presented in Table 2 show that the change in Poisson’s ratio is negligible after primer treatment. The data suggest that the reduction in stress at yield initiation is predominantly attributable to the alteration in the longitudinal modulus of elasticity E. In essence, the observed effect of primer treatment on this system is a decrease in the longitudinal elastic modulus, which is associated with a decline in flexural strength.

Primers are typically designed to have a low molecular weight, even when applied and cured, because of their role as base materials. Because a low molecular weight tends to lead to more elastomer-like behavior, adhesion is greater. Moreover, the elastic modulus decreases when the primer component is dispersed. For bumpers without coating, the primer is expected to be dispersed uniformly in the matrix phase because no areas exist in which it can be localized easily. In this scenario, the primer can be regarded as being dispersed as an elastomer within the matrix phase. Given that the longitudinal elastic modulus diminishes when the elastomer is dispersed, it can be inferred that the primer treatment led to a reduction in the longitudinal elastic modulus. By contrast, enhancement in the flexural modulus of the bumpers with coating film following primer treatment is expected to be attributable to the improved adhesion between the coating film and the matrix phase, facilitated by the primer treatment, thereby facilitating stress transfer to the coating film.

4.2. Primer Treatment Effects on Impact Fracture Properties of Injection-Molded Products

This section presents a discussion of the effects of primer treatment on the notched Charpy impact strength of the coated bumper injection-molded products. As presented in Table 1, a decline in notch Charpy impact strength can be observed for coated bumper injection-molded products following primer treatment. This phenomenon is explored further using a mechanical model, contingent on the confirmation of fracture mode and the identification of energy dissipation mechanisms. Figure 8 portrays the morphology of the molded parts following the Charpy impact test. The molded parts containing the coating film exhibited a propensity for crack propagation, resulting in complete failure, with no whitening occurring near the crack. This observation signifies that the impact energy dissipation of the molded parts containing the coating film is governed predominantly by crack initiation. Consequently, the energy dissipated during the crack propagation phase is considered negligible. Moldings to which the primer was applied exhibited comparable crack propagation behavior to those to which no primer had been applied. By contrast, the molding without a coating film exhibited partial fracture, as characterized by the cessation of crack propagation at the midpoint of the specimen, concomitant with the occurrence of whitening near the crack. These observations imply that the dissipation of impact energy in the molding without a coating film transpires during crack propagation, extending up to its initiation point. This phenomenon is believed to account for the elevated a_iN found in the present study. By contrast, the primer-applied moldings exhibited diminished crack propagation and an augmented whitened area. This finding suggests that the dissipation of energy during crack propagation increased because of the primer dispersion inside the molding without the coating film.

As presented in Figure 9, the fracture surface of the crack propagation region was examined using a scanning electron microscope. The crack propagation direction is indicated by arrows in the figure. The fracture surface of the molded product containing the coating film exhibited impurities with a diameter of approximately 500 µm. These impurities are believed to be the coating film. The observation of this coating film at the fracture surface suggests that the crack traverses the interface between the coating film and the matrix as it propagates. The interface between the coating film and the matrix showed a weak bond, as determined by the weld strength evaluation results, suggesting minimal plastic deformation at the interface between the coating film and the matrix. Consequently, the molded product containing the coating film demonstrated negligible plastic deformation in the crack propagation region. The observation of the coating in the primer-applied moldings indicates that primer application did not meaningfully alter the amount of energy dissipation within the crack propagation region. Conversely, plastic deformation progressed along the crack propagation direction in the moldings devoid of coating. Figure 10 presents a magnified perspective of the crack propagation region in a mold without coating, with arrows indicating the crack propagation direction. Subsequent to the primer application, the presence of multiple voids of 10–20 µm was observed. These voids probably result from unstable expansion in regions where the primer, characterized by its low molecular weight, has undergone aggregation. The formation of these voids probably led to the dissipation of a greater amount of impact energy, consequently increasing a_iN.

The results and discussion presented above suggest that injection molding after the primer treatment of crushed bumpers can produce injection-molded products with high mechanical properties. The results presented herein suggest a useful method to promote the mechanical recycling of waste automobile bumpers. This study employs a spray application method, but this method provides poor coating efficiency. Considering the associated environmental impacts, it would be more desirable to adopt a different method. Consequently, further research is warranted to identify a method that exhibits a reduced environmental impact and which enhances the practicality of this technology.

The results of this study are presented in Figure 11. The effects of primer application on the mechanical properties of crushed waste automobile bumpers differed depending on the presence or absence of a coating film. Specifically, for the primer with coating, the primer adhered to the coated surface and improved the interfacial strength, thereby improving the flexural strength and flexural modulus. By contrast, in the absence of coating, the primer manifested as an elastomer within the molded product, thereby augmenting the notched Charpy impact strength through increased plastic deformation within the crack propagation region.

The outcomes of this study suggest a method for the mechanical recycling of waste automobile bumpers. The present study used a spray application technique. However, this method exhibits suboptimal coating efficiency. From an environmental perspective, an alternative method would be preferred. Consequently, our research endeavors specifically emphasize the identification of a method providing reduced environmental impact and a technology offering enhanced practicality.

5. Conclusions

As described herein, a primer treatment was applied to crushed waste automobile bumpers. Changes in the mechanical properties of injection-molded parts caused by this treatment were investigated. The obtained results are presented below.

The application of primer to the coated bumper before crushing of the product enhanced the adhesion between the coating and the resin, thereby augmenting the flexural strength of the injection-molded product. Concurrently, the notched Charpy impact strength exhibited a decline. This phenomenon can be attributed to the fact that the dispersed coating film was greater than the critical size.

The notched Charpy impact strength of the injection-molded products was enhanced by primer treatment on the crushed bumpers lacking a coating film. This enhancement was attributed to the aggregation of the primer components within the molded product, leading to elastomer-like behavior.

The findings obtained from this study suggest a viable approach to encourage the mechanical recycling of automotive bumpers. The study used a spray application technique. However, this method exhibits suboptimal coating efficiency. From an environmental perspective, it would be preferable to adopt an alternative method. Ongoing research is being conducted to identify a technique that can provide both reduced environmental impact and enhanced practicality when applied.