Using sieving and pretreatment to separate plastics during end-of-life vehicle recycling

Abstract

Plastics continue to be a challenge for recovering materials at the end-of-life for vehicles. However, it may be possible to improve the recovery of plastics by exploiting material characteristics, such as shape, or by altering their behavior, such as through temperature changes, in relation to recovery processes and handling. Samples of a 2009 Dodge Challenger front fascia were shredded in a laboratory-scale hammer mill shredder. A 2 × 2 factorial design study was performed to determine the effect of sample shape (flat versus curved) and sample temperature (room temperature versus cryogenic temperature) on the size of the particles exiting from the shredder. It was determined that sample shape does not affect the particle size; however, sample temperature does affect the particle size. At cryogenic temperatures, the distribution of particle sizes is much narrower than at room temperature. Having a more uniform particle size could make recovery of plastic particles, such as these more efficient during the recycling of end-of-life vehicles. Samples of Chrysler minivan headlights were also shredded at room temperature and at cryogenic temperatures. The size of the particles of the two different plastics in the headlights is statistically different both at room temperature and at cryogenic temperature, and the particles are distributed narrowly. The research suggests that incremental changes in end-of-life vehicle processing could be effective in aiding materials recovery.

Keywords

Shredder residue comminution end-of-life vehicle plastics cryogenic freezing waste management

Introduction

Modern technologies for recovery of materials from complex products, such as automobiles, involve a number of distinct steps. The typical steps involved in the recycling and recovery of end-of-life vehicles in North America include a dismantler receiving a vehicle from an owner, an auto dealer or an insurance company’s auction; the dismantler removing readily reusable or recyclable components, as well as those containing hazardous or valuable materials (i.e. catalytic converters); the remains being sent to a shredder to reduce the vehicle hulk into small pieces, of which (in a typical Canadian shredding operation) 18 wt% is greater than 26.5 mm and 37 wt% is less than 2 mm in size (Day et al., 1993); and ferrous and non-ferrous metals being separated out by mechanical and magnetic means, and being baled for sale as secondary material. As a last step, remaining shredder residue (SR), a heterogeneous mixture of materials, including many types of plastics, is usually sent to a landfill (de Marco et al., 2007; Ferrão et al., 2006). Various prototype methods for separating SR to recover valuable fractions have been developed; however, these presume that the post-SR fractions are easily identifiable, sufficiently valuable and amenable to recovery.

The European Union’s (EU) End-of-Life Vehicle (ELV) Directive has placed significant pressure on automotive manufacturers and automotive recyclers to develop more efficient and cost-effective methods to recover plastics from ELVs. As the Directive states, 95% by weight of vehicles produced after 1980 must be re-used and recovered, with 85% being recycled or re-used by the year 2015 (Gerrard and Kandlikar, 2007). Under this Directive, the current requirements are for 85% by weight of vehicles produced after 1980 to be re-used and recovered, with 80% being recycled or re-used. The majority of materials that have been re-used and recycled historically are the more dense and higher value ferrous and non-ferrous metals. The residual material remaining after vehicle shredding, SR, is a heterogeneous mixture of materials, of which 38–44% is comprised of various types of plastics (Ferrão et al., 2006). As stated previously, SR is usually sent to landfill. It has been reported (Duranceau and Sawyer-Beaulieu, 2011) that in 2007, 86.4% of the weight of an average vehicle was recycled, re-used and recovered. To put this in context, about 13 million vehicles are retired and recycled in Canada and the USA annually (Sawyer-Beaulieu et al., in press). Although a large amount of materials are being recovered, a significant amount of material continues to be landfilled each year. Assuming an average vehicle curb weight of 1469 kg (NHTSA, 2012), 2.6 million tonnes of SR are, potentially, sent to landfill annually in Canada and the USA. Current SR reduction efforts are not expected to diminish that volume significantly. While North American jurisdictions have not enacted legislation such as the ELV Directive, there is still growing awareness and concern over the recovery of such materials. Thus, enhanced methods for sorting and separating different plastics from SR can help companies and governments reach plastic waste reduction objectives.

Furthermore, the US Environmental Protection Agency has recently stated that it is acceptable to recycle plastics from shredder residue, as there had been some concern regarding polychlorinated biphenyl (PCB) concentrations in automotive shredder residue previously (Gerlat, 2013).

Energy recovery through segregation and incineration of the organics fraction is one means of meeting the recovery goals of the EU ELV Directive. Studies have shown that using SR (of which the organic matter has a heating value of 15,000–30,000 kJ/kg) as fuel in a blast furnace shows promising results (Mirabile et al., 2002). An advantage of energy recovery methods is that separation of the various plastics is not necessary. Disadvantages of energy recovery methods include the fact that SR is non-homogeneous and the energy content can vary, and these methods do not further the goals of companies to recycle the ELV materials. Incineration is also not accepted as a sustainable waste management process in all jurisdictions (Wagner and Arnold, 2008).

Mechanical recycling of plastic solid waste is a method in which post-consumer plastics are used as a material in the manufacture of new plastic components. With mechanical recycling, it is necessary to separate plastics so that each type of plastic is liberated. Contamination, degradation and non-homogeneity of mechanically recycled plastics are issues faced by this method (Al-Salem et al., 2009). With respect to SR, there are not only various plastics to liberate and separate from each—composed mainly of polypropylene (35%), polyurethane (14%), polyethylene (10%) and polyvinylchloride (7%) (Mirabile et al., 2002)—there are also glass, textiles and other materials present (de Marco et al., 2007). However, it has been reported (Ferrão et al., 2006) that 88–90% of ELVs may be re-used and recycled if SR mechanical separation technologies are utilized. Furthermore, considering environmental impacts, in the case of polyethylene terephthalate bottles, recycling results in reduced overall environmental impacts of the recovery process, versus the collection and burning of the bottles along with non-recyclable waste in an energy-from-waste plant (Chilton et al., 2010).

Some studies have been performed to evaluate the potential of separating plastics from metals or other non-plastics during the recycling of ELVs using cryogenics (Dom et al., 1997; Gente et al., 2004). Results show that there is the potential of using differences in the behavior of plastics at cryogenic temperatures to aid in the separation of various plastics during ELV recycling. Previous studies using a granulator show little difference in the size distributions of acrylonitrile butadiene styrene (ABS) and polyvinylchloride plastics when specimens were comminuted at room temperature (Tam and Jekel, 2004). However, using a granulator with cryogenic pretreatment resulted in differences in the size distribution of various plastics from automotive components (Barsha, 2008). Although a granulator does result in comminution of components, it is not the same type of comminution process as experienced within a hammer mill shredder. Granulation comminutes feed materials owing to the cutting action of rotating blades, while shredding uses shearing force and slow cutting action (Barsha and Tam, 2009). A shredder better simulates the behavior of automobile components during ELV shredding.

This study examines material characteristics that may be used after vehicle shredding to sort and separate various plastics from SR and to suggest how those involved with ELV recycling may take advantage of this knowledge.

Materials and methods

A front fascia material (that covers the front bumper) from a mid-size coupé was donated and cut with a band saw to provide test specimens. Used headlights from three minivans were obtained and cut in half with a band saw to provide two test specimens from each light. Specimens of the fascia and headlights were tested using Bruker Fourier transform infrared spectroscopy equipment to determine the types of polymers used. The fascia was composed of polypropylene and ethylene propylene diene monomer (EPDM) thermoplastic polyolefin with 20% talc filler. The headlight was composed of clear polycarbonate (PC)/ABS blend and black polyamide-6,6.

For the fascia testing, the researchers conducted a 2 × 2 factorial design experiment. The factors of study in the factorial design include temperature and shape of the test specimens. For temperature, the levels include specimens shredded at room temperature and specimens that have been pretreated with liquid nitrogen prior to shredding. For shape, the levels include flat and curved specimens. Five specimens were tested for each temperature and shape configuration. For the headlight testing, five specimens were shredded at room temperature, and five specimens were pretreated with liquid nitrogen and then shredded.

The main problem when processing discarded manufactured products is that the various materials exhibit different physico-chemical properties, resulting in different breakage behaviors (Gente et al., 2004). Complex consumer products, such as automobiles, consist of many dissimilar materials. Also, different types of fastening methods are used to join these materials together. These multi-component materials should be liberated from one another as much as possible during comminution for more satisfactory separation. While the different plastics may behave similarly at room temperature, they may behave differently when subjected to cryogenic temperatures. It may be possible to take advantage of these increased differences (e.g. one plastic preferentially breaks compared to the other) to better facilitate liberation of one material from another.

A Schutte-Buffalo (Schutte-Buffalo Hammermill, Buffalo, NY, USA) hammer mill (model WA-8-H) was used to shred the specimens. Exit screens with 76.2 mm diameter openings were placed in the shredder to ensure that all material would at least have a minor axis dimension of less than 76.2 mm. To test the specimens, each specimen was placed in the feed chute, entered the shredder as a result of gravity, was discharged from the unit through the exit screens and then through the discharge chute, and finally collected in a drum lined with a plastic bag. The plastic bag containing the shredded specimen was then removed from the drum for weighing and analysis.

Each specimen tested at cryogenic temperatures was immersed in a chamber of liquid nitrogen for 30 mins, reaching a cryogenic temperature of approximately −194^oC. It was then quickly transferred to the feed chute of the shredder to minimize warming and tested in the same manner as described above. During initial testing, using thermocouples inserted into the core of the material, it was determined that immersion in liquid nitrogen for 30 mins was a sufficient duration to allow the entire specimen to reach −194^oC.

The collected pieces from each specimen were placed in a sieve pack with sieve sizes 3.0”, 2.5”, 2.0”, 1.75”, 1.5”, 1.25”, 1.0”, 7/8”, ¾”, ½”, 3/8”, 5/16”, 4, 6, and 8 (USA Standard Test Sieve, conforming to ASTM E-11 specification). To accommodate the sieve shaker (CSC Scientific Sieve Shaker, Fairfax, VA, USA), the sieve pack was divided into two. The first pack (from 3.0” to 1.0”) was shaken for 10 mins at a setting of 5. The contents of the pan were then placed in the second pack (from 7/8” to 8) and shaken for 10 mins at a setting of 5. After shaking, each sieve was removed and weighed to determine the weight of the specimen retained in each sieve.

Analysis of variance and Tukey mean comparison tests (P ≤ 0.05) were performed using Minitab 15 (Minitab Inc., State College, PA, USA) software.

Results and discussion

Upon sieving and weighing the shredded specimens, the percentage of material under the nominal aperture size of the sieves was graphed for each specimen. Using these graphs, the nominal aperture sizes for which 25%, 50% and 75% of the material passes through (d₂₅, d₅₀, and d₇₅ respectively) were determined. The size distribution of particles can be expressed by an equation describing the distribution of various size fractions. Although particles usually span a range of sizes after comminution, characterizing the size distribution allows for analysis and comparison of one output set against another. It is usually depicted as sigmoidal curves plotting mass fraction smaller than a particle size as a logarithmic function of particle size (Vesilind et al., 2002). The d₅₀ value is frequently used as a reference point for the size distribution of a comminuted material, while the probable distribution, E_p, is used as an indication of the precision of the shredding process: the larger the E_p value, the greater the range of sizes from comminution. The value, E_p, is calculated as:

E_{p} = ​ \frac{d_{75} - d_{25}}{2}

The average and SD of the d₅₀ and E_p values for the various temperature and shape configurations of the fascia samples are shown in Table 1. Also, Figure 1 shows a graph of the average values for the cumulative amount of material under the nominal aperture size of the sieves for the four fascia samples.

Table 1.

d₅₀ and E_p values for the shredded fascia samples.

Sample	d₅₀ (mm)	E_p (mm)
RT-flat	45.9 ± 13.76^a	13.5 ± 1.87^a
RT-curved	42.8 ± 14.25^a	15.5 ± 4.46^a
CT-flat	6.2 ± 1.58^b	3.2 ± 1.21^b
CT-curved	6.0 ± 0.45^b	3.1 ± 0.27^b

RT: room temperature; CT: cryogenic temperature; d₅₀: the nominal aperture size for which 50% of the material passes through;

$E_{p} = \frac{d_{75} - d_{25}}{2}$ , where d₂₅ and d₇₅ refer to the nominal aperture sizes for which 25% and 75% of the material passes through respectively.

a,b

Means in same column with different letters are significantly different (P ≤0.05).

At room temperature, the flat and curved samples have average E_p values of 13.5 mm and 15.5 mm respectively (not statistically significant); however, at the cryogenic temperature, the flat and curved samples have average E_p values of 3.2 mm and 3.1 mm respectively. These values are not statistically significant in comparison with each other, but they are statistically significant when compared with the room temperature samples. The samples that were pretreated in liquid nitrogen exhibit a decreased E_p value, representing a narrower particle size distribution of the specimens. This is likely due to the preconditioning with liquid nitrogen.

Realizing that the shape of a component is not significant is beneficial to those tasked with separating plastics from a waste stream because the shapes of plastic automobile components vary tremendously and is not a factor that is easily controlled. In this regard, the conclusions potentially mean that one factor in the design of automobile components for materials—the resulting shape—can be eliminated in designing for improved recovery. Conversely, it is possible to control the temperature of the components during the recovery process. Should recovery operators wish to exploit the temperature effects, then pretreating components with liquid nitrogen is an option. When pretreatment with liquid nitrogen is used, the size of post-shredded particles is fairly constant, with only a small amount of variation compared to the outcomes from shredding at room temperature. This has the potential of allowing for simpler sieving techniques to be employed to separate various types of plastics from each other, rather than more elaborate chemically-based separation operations [such as those described by Jody and Daniels (2006)]. However, prior to implementing any separation operation, the economic trade-off between the costs of the separation operation versus the value of the recovered materials would need to be studied.

To test the potential benefits of cryogenic freezing, a component made of two types of plastic was also tested to determine whether the separation of plastics based on particle size is possible. The headlights were tested at room temperature and at cryogenic temperature. The average and standard deviation of the d₅₀ and E_p values for the two types of plastics at the two different temperatures for the headlight samples are shown in Table 2. A graph of the average values for the cumulative amount of material under the nominal aperture size of the sieves for the four headlight samples is shown in Figure 2.

Table 2.

d₅₀ and E_p values for the shredded headlight samples.

Sample	d₅₀ (mm)	E_p (mm)
RT-clear	9.2 ± 0.05^a	2.6 ± 0.32^a
RT-black	5.1 ± 0.13^b	1.9 ± 0.09^b
CT-clear	7.4 ± 0.26^c	2.3 ± 0.29^a,c
CT-black	3.8 ± 0.19^d	2.1 ± 0.12^b,c

RT: room temperature; CT: cryogenic temperature; d₅₀: the nominal aperture size for which 50% of the material passes through;

$E_{p} = \frac{d_{75} - d_{25}}{2}$ , where d₂₅ and d₇₅ refer to the nominal aperture sizes for which 25% and 75% of the material passes through respectively.

a,b,c,d

Means in same column with different letters are significantly different (P ≤0.05).

The results of the headlight testing show that the sizes of the particles of the two different plastics in the headlights are statistically different both at room temperature and at cryogenic temperature. The average d₅₀ values for the clear and black plastics at room temperature are 9.2 mm and 5.1 mm, respectively, and 7.4 mm and 3.8 mm, respectively, for the cryogenic temperature samples. In addition, the size distribution of the particles has a narrow range for all samples. The average E_p values for the clear and black plastics at room temperature are 2.6 mm and 1.9 mm, respectively, and 2.3 mm and 2.1 mm, respectively, for the cryogenic temperature samples. Both at room temperature and cryogenic temperature, the two plastics show a statistical difference in d₅₀ values and have small E_p values, thus enabling sieving to more easily separate the plastics. From these results, it appears that pretreatment with liquid nitrogen does not provide any additional, notable benefit when sieving and separating these particular plastics.

Finally, the two types of plastics in the headlights were joined using an adhesive. For the samples at cryogenic temperature, the clear plastic (PC/ABS) and the black plastic (Nylon-6,6) were completely liberated from each other; however, the adhesive remained attached to both types of plastics. For the samples at room temperature, there was 99.4% liberation of the plastics. Although this is impressive in terms of liberation, two specimens contained at least one particle that contained both types of plastic still held together by the adhesive. In other words, under controlled conditions, some cross contamination is still present, and under realistic operating conditions with significantly greater volumes of plastic, the amount of impurities within the sorted fractions could be considerably worse. Furthermore, all of the specimens had adhesive attached to both types of plastics.

Conclusions

In this study, liquid nitrogen was used to cryogenically freeze a plastic automobile fascia prior to being shredded in a hammer mill. The shape of the specimen that is shredded does not affect the resulting size distribution and precision of the particles produced; however, the initial temperature of the specimen does. Pretreating the specimens with liquid nitrogen resulted in particle sizes with less variability, which would be desirable in using sieving as a method for plastics recovery.

The potential of using sieving to aid in the recycling of plastics from ELVs was further studied by cryogenically freezing automobile headlights prior to being shredded in a hammer mill. In this case, pretreatment with liquid nitrogen does not provide a significant benefit when sieving and separating these particular plastics. The size of the particles of the two different plastics in the headlights is statistically different both at room temperature and at cryogenic temperature, and the distribution of the particles is narrow for all samples.

It was observed that adhesive remained attached to both plastics comprising the headlights. The issue of separating plastics and adhesives would not be a factor if different fastening techniques were used to attach different plastics together that did not inherently bond or weld one material to another, for example using clips or screws.

These results demonstrate that cryogenic temperatures may not be necessary for certain plastic or adhesive combinations. Shredding and sieving may be sufficient. However, cryogenic temperatures may be useful in other combinations not yet tested (e.g. for materials such as rubbers or foams). Further studies into using shredding, pretreatment and sieving as the initial steps in the recycling of plastic automobile components may elucidate such instances.

Finally, prior to implementing any separation operation, the economic trade-off between the costs of the separation operation versus the value of the recovered materials would need to be studied.

Footnotes

Conflict of interest

The authors declare no conflict of interest.

Funding

This work was supported by the AUTO21 network [Project #EB302-ELC].

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