Solid-State Shear Milling for Recycling Aluminum–Plastic Packaging Waste: A Sustainable Solution for Mixed Plastic Waste

Round 1

Reviewer 1 Report

This paper demonstrates that mixed plastics from waste aluminum plastic packaging can be recycled by being milled using S3M technology. There is little doubt about the results. I think this paper is industrially useful.

The following are my comments. Please consider them.

 1.          As a result, the advantage of applying the S3M technology to the milling of waste mixed plastics seems to be the ability to mill them uniformly and finely. If waste mixed plastics can be ground uniformly and finely, it is likely that other methods will yield recycled products of good quality.

2.          Page 4, line 142 ~146; Where are Fig. S1, Fig. S2 and Table S1 shown?

3.          In the stress-strain relationship shown in Fig. 10, the rate of increase in stress is decreasing at a strain of approximately 3. This may suggest that a specific component of the plastic is fractured. This phenomenon should be explained.

4.          Other points to be corrected

 Caption Figure 8 and Figure 9 should be bolded.

Page 4, line 157, S3M

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

Title: Recycling of mixed plastic waste from Aluminum-Plastic Packaging Waste by solid state shear milling technology

Manuscript ID: sustainability-2163890

Authors: Wei et al.

 

Dear Authors,

 

Thank you for the opportunity to read your article. I found the topic is interesting and fundamental. Generally speaking, there are some results presented in order to capture some trends but the methods and results need more clear explanation and detail discussion with fair point of view. Also there are so many flaws and missing data/results (i.e., Figures, Tables) in this article while many of the presented results in the article are neither described nor discussed, and thus I cannot properly judge the quality of the article. I suggest that this article will be revised throughout before its re-submission for another review process if applicable. As a conclusion, I recommend its rejection/major revision at this state.

 

I hope my comments are helpful.

Good luck,

A reviewer

 

Major concerns:

“Article title”

-It is too long. Please consider removing unnecessary word(s) from the title. For example, you may remove “technology” since one in recycling can easily understand milling is a technology (or process/method).

 

“Abstract”

-Lines 15 and 16“…realize the pulverizing and excellent dispersion of multi-component polymer.”->Please consider citing your key results with some numbers/values if any.

 

“Keywords”

->Please consider providing keywords that are not used in the article title.

 

“1. Introduction”

-Lines 39-41: “According to Tetra Pak international, most of abandoned Tetra Pak is burned or dumped as landfills…”->Please consider citing a reference.

-Lines 56-57: “The recycling of post-consumer multi-plastic waste (PMW) is extremely hard…”->Please consider explaining the reason why it is hard.

-Lines 67: “…solid-state shear milling (S³M)…”->Please consider describing the unique feature of S³M over other conventional mills, such as hammer mill. Also, if you have results obtained by using other mill(s), please consider showing and comparing them with the ones obtained by using S³M in the results and discussion.

https://doi.org/10.1163/156855206775123467

https://doi.org/10.3390/met9080899

 

“2. Materials and Methods”

“2.1. Materials”

-In this section, please consider providing the particle size distribution and elemental assay of the feed of your milling experiment.

“2.2. Preparation of samples”

-In this section, please consider stating (a) whether your milling was performed in dry or wet, and (b) how many times you performed the milling test at each condition.

-Lines 81-83 (and elsewhere): “…S³M technology…self-designed pan-mill type equipment.”->If S³M is equal to self-designed pan-mill type equipment, please use either one and be consistent throughout the article. Otherwise, a reader will be confused.

-Lines 89-91: “…the crushed sample was uniformly fed into the feeding funnel of this equipment.”->Please consider providing the feed rate (e.g., g/s, kg/min) in this statement.

-Figure 1(a): Please consider adding a scale in each image.

-Figure 1(b): Please consider providing the dimension of your equipment.

 

“2.3. Characterization”

-Lines 104-105: “Particle size after grinding was determined by…”->Please consider introducing the particle size distribution of your feed since the results of milling operation can be strongly affected by the feed particle size distribution.

-Lines 105-106: “A small number of milled particles were dispersed…”->Please consider stating how you sampled the small number of milled particles. Otherwise the representativity of your “small number of milled particles” is questioned.

-Lines 128-130: “Rheological properties were conducted…”->(a) Viscosity of milled particles was measured…? (b) Please consider stating whether your measurement was performed in dry or wet. In either case, please consider justifying the representativity of your measurements since your sample can be highly heterogenous with your coarse size of particles after the milling (>250 micron meters as shown in Fig.3) and thus can be easily segregated during the shear force application in a viscosity measurement.

-In this section, please consider explaining your methods detail enough for someone else to perform similar experiment in the future. For example, for SEM imaging, please consider stating the way you deposited your sample(s) in an SEM chamber, detector type (SE? BSE?), accelerating voltage, and working distance. Also, please consider explaining how you analyzed your images. Those information would be helpful for future researchers. This comment also applied to all the characterization methods introduced in this section.

doi.org/10.1016/j.actamat.2005.12.014

doi:10.3390/electronics8101202

 

“3. Results and discussion”

-In general, please describe and discuss the results shown in the article. Please find the details comments below.

 

“3.1. Characterization of raw material”

-Lines 140-141: “…we selected and classified them, and then determined their components.”->This phrase is vague. Please consider stating what you actually did instead of such a vague phrase.

-Lines 142-146 (and elsewhere applicable): “…Fig.S1…Fig.S2…Table S1.”->Please provide those Figures and Table in the appendices of the article. There is no appendix in the article and in the MDPI system. Thus, one cannot evaluate the validity of your statements and results.

 

“3.2. Morphology of treated PMW”

-Lines 150-152: “In this experiment, we used S³M technology to treat PMW…”->This statement is confusing and does not match with the section title. Please consider (a) stating what you did to study the morphology of milled PMW, and (b) sending the milling procedure to the method section.

-Lines 155-157: “…ball-milling and air jet comminution failed to pulverize ductile polymer materials effectively, while PMW can be milled to powder through S³M technology.”->(1) In Figs.3(e), (f), please consider plotting the average particle size and particle size distribution generated by using ball mill and air jet comminution for the comparison with S³M. (2) Please consider discussing why S³M works but the other two do not, and citing relevant reference(s) to compare with your results.

-Lines 158-159: “…from the initial millimeter level to the micron level…”->…from several millimeters to several hundred micron meters…

-Figure 3 (e), (f): Please consider plotting the particle size or particle size distribution of the feed (without milling), and adding error bars in the both figures. Also, please consider renaming the title of Fig.3(e) to “average particle size” if it is what is shown in Fig.3(e).

-Lines 159-162: “…particle size experienced a prompt decrease in the initial milling cycles. Then with the increasing of milling cycles, the particle size decreased gradually to 200 µm…”->Please be more specific (e.g., cite specific milling cycle) and well describe the results. From Fig. 3(e), one can see that the average particle size decreases till the 4^th milling cycle and no further size reduction after the 6^th milling cycle that reaches the average particle size to about 250 µm.

-Lines 162-164 and Fig.2: “…as shown in Fig.2(g).”->Please properly name Fig.2 since there is no Fig.2(g).

 

“3.3. Chemical characterization of treated PMW”

-Lines 180-181: “…no new functional groups were formed during milling.”->Please consider removing “mechanochemical” from the line 67 since in this study there is no clear evidence that your milling equipment works as a mechanochemical reactor.

-Lines 185-190: “…powerful extrusion and shearing force of milling disc made particle pulverize, and the surface structure of particle became severely disturbed and irregular surface structure was formed after repeated milling…the decrease of crystallinity.”->Please consider citing your characterization results shown in Figure 4 to support your discussion. From your current statements, there is no strong connection between your results and discussion/statement.

 

“3.4. Morphology of PMW samples”

-Lines 216-218: “Experimental results revealed that untreated sample had a disorderly morphology due to…observed. However, after S³M treatment, the surface was more uniform…”->Please consider citing your results and figures in order to support your statements.

-Line 229: “…as shown in Fig.7(d)…”-> …as shown in Fig.6(d)…?

-Figure 6: Please describe and discuss the images and results.

-Figure 6(c),(f): In the method section, please consider defining the “domain” and describing the procedure of measuring “domain size”.

-Lines 227-228: ”The average size of dispersed phase was 9.64 µm…”->The average size of domain was 9.64 µm…? Please be consistent between the main text and Fig.6(c). This comment applies to the other statement on lines 231-232 describing Fig.6(f).

-Lines 239-240: “Ca in sample mainly came from sand and stone components…”->Is this true that your aluminum-plastic packages contain sand and stone? Or do they contaminate the aluminum-plastic packages after their end of life?

-Line 241: “…Table 1.”->Where is it? I cannot find it in the article.

 

-Figure 7 title: “EDS of samples…”->EDS of hot pressed sample…?

-Figures 7(a),(b): In the method section or this section, please consider explaining your intention to show the five elements selected. Especially C and O should be explained.

 

“3.5. Rheological properties”

-Lines 259-260: “Owing to the large particle size of PMW after recycling, it was easy to block processing equipment during the processing process.”->Please consider revising this statement since it is not clear. What is your definition of “recycling”? Do you mean milling operation? What is the “processing process”? Do you mean milling process?

-Line 261: “…accessed a barrier to reprocess.”->Please consider rephrasing this since it is not clear what you wish to say.

-Lines 262-263 and Figure 9 title: “rheological properties…”->Shear viscosity of…

-Figure 9 title (and Figure 5 title): “…waste Tetra packaging…”->Please be consistent with what you wrote in the other places of the article. One can find “Tetra packaging” only in Figures 5 and 9.  

-Figure 9: Please consider adding error bars and discussing the difference between the results obtained with the milled products after different cycles. In other words, the difference in the viscosity with the milled products after different cycles is beyond the error range or not? It seems that the difference is minor. These comments also apply to Figs.10(b)-(d).

 

“4. Conclusions”

-Lines 302-303: “…200 µm.”-> …250 µm.

-Lines 306-307: “…recycled material can be reprocessed…”->…milled material can be extruded…? Please be more specific.

-You may state future perspectives in Conclusions.

 

Minor concerns:

-Please consider polishing English more. You may use some of my comments above for this purpose.

 

 

 

 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

The manuscript, entitled "Recycling of mixed plastic waste from Aluminum-Plastic Packaging Waste by solid state shear milling technology", presents the study of recycling mixed plastic wastes using a new shear milling technology, showing comprehensive analysis on the material characteristics after different milling cycles. This work aligns with the scope of MDPI Sustainability very well, and it possesses overall good quality. I would recommend the acceptance of this manuscript after minor revisions.

1. The authors described the sample preparation methods, but the geometries, the treating histories of different samples for different characterization methods are still not very clear. For example, was DMA sample prepared using the same method as other mechanical tests (e.g. tensile)? How many repeats were done for tensile experiments? I would recommend that the authors can include a figure showing the photographs of different testing specimens (shape, size, etc).

2. In Figure 4 (b), please add some arrows and words to point out different phenomena during heating (melting, Tg, etc). In Figure 4 (d), please include tan delta curves as well. 

3. The authors interpreted Figure S1 and S2. I think the raw material information is very important for understanding the output of the milling process. I would recommend that these figures can be presented in the main context rather than only showing in Supporting Information.

4. The authors mentioned Table 1 in Line 241, but without including an actual Table 1. Is it in Supporting Information? Please clarify.

5. There are some grammatical errors that need to be corrected. Please proof-read carefully.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Dear Authors,

 

Can you kindly send us your responses to each question one by one? As you tried to answer many of my comments in groups, it is very difficult for me to understand your responses. Thank you so much for your cooperation.

Best regards,

A reviewer

 

Author Response

Response to Reviewer 2 Comments

Point 1: “Article title”-It is too long. Please consider removing unnecessary word(s) from the title. For example, you may remove “technology” since one in recycling can easily understand milling is a technology (or process/method).

Response: we think this is a nice suggestion. We have changed our title to “Recycling of Mixed Plastic Waste from Aluminum-Plastic Packaging Waste by Solid State Shear Milling”

 

Point 2: “Abstract”-Lines 15 and 16“…realize the pulverizing and excellent dispersion of multi-component polymer.”->Please consider citing your key results with some numbers/values if any.

Response: we have included the values of the experimental results in the abstract.

“The results of particle size analyzer and SEM showed that the average particle size of PMW was reduced from 700 μm to 226 μm after 10 milling cycles. The morphology observation indicated a good dispersion of PMW domains achieved by S³M processing with size reduction from 9.64 μm to 2.65 μm.”

 

Point 3: “Keywords”->Please consider providing keywords that are not used in the article title.

Response: Thank you for your suggestion. We think the choice of keywords used in the manuscript better fits the key research content of this article.

 

Point 4: “1. Introduction”

-Lines 39-41: “According to Tetra Pak international, most of abandoned Tetra Pak is burned or dumped as landfills…”->Please consider citing a reference.

-Lines 56-57: “The recycling of post-consumer multi-plastic waste (PMW) is extremely hard…”->Please consider explaining the reason why it is hard.

-Lines 67: “…solid-state shear milling (S³M)…”->Please consider describing the unique feature of S³M over other conventional mills, such as hammer mill. Also, if you have results obtained by using other mill(s), please consider showing and comparing them with the ones obtained by using S³M in the results and discussion.

https://doi.org/10.1163/156855206775123467

https://doi.org/10.3390/met9080899

Response: We have added the corresponding references in the manuscript: “According to Tetra Pak international, the global output of Tetra Pak was 190 billion, while its recovery rate is only 26%. In many countries, most of abandoned APPW is burned or dumped as landfills, resulting in the secondary environmental pollution and waste of resources (Geogiopoulou et al., 2021).”

In general, the recycling of APPW is focused on the recovery of high value Al and cellulose, The residual multi-plastic waste (PMW) usually contains many impurities, and the separation process is very complicated. Due to the high economic cost and complex process, there is no industrial technology to effectively recycle the waste mixed plastics. So, the recycling of PMW is extremely hard

Compared with other conventional pulverizing technology, S³M technology can provide fairy strong shear force field, which has multiple functions of pulverizing, dispersing and mixing materials. As shown in Figure 2, frequently used technique like ball-milling and air jet comminution failed to pulverize ductile polymer materials effectively, while PMW can be milled to powder through S³M technology.

 

Point 5: “2.1. Materials”

-In this section, please consider providing the particle size distribution and elemental assay of the feed of your milling experiment.

Response: The raw material for milling experiment is shown in the Figure 2. In order to compare the milling effect, we put the particle size distribution and element assay in “Results and discussion” section, as shown in Figure 6 and Figure 7.

 

Point 6: “2.2. Preparation of samples”

-In this section, please consider stating (a) whether your milling was performed in dry or wet, and (b) how many times you performed the milling test at each condition.

-Lines 81-83 (and elsewhere): “…S³M technology…self-designed pan-mill type equipment.”->If S³M is equal to self-designed pan-mill type equipment, please use either one and be consistent throughout the article. Otherwise, a reader will be confused.

-Lines 89-91: “…the crushed sample was uniformly fed into the feeding funnel of this equipment.”->Please consider providing the feed rate (e.g., g/s, kg/min) in this statement.

-Figure 1(a): Please consider adding a scale in each image.

-Figure 1(b): Please consider providing the dimension of your equipment.

Response: the whole milling process was performed in dry.

The crushed sample was uniformly fed into the feeding funnel of S³M equipment. The material was milled along the spiral track in the milling disc and discharged from the edge of the milling disc. The milling powder was collected from the discharging port and recorded as finishing one cycle of milling. The S³M equipment is based on the concept and structure of the traditional Chinese stone mill, and is designed to be used in the processing of polymers such as pulverizing, mixing and force-chemical reactions.

S³M equipment is designed for handling polymers with some unique structure. To avoid confusion, S³M equipment is used uniformly in the manuscript.

The feed rate of S³M equipment depends on the nature of the material. For example, polymer pellets are fed at a high speed. Then polymer film materials, especially light weight, are relatively slow to be fed.

Figure 1a is a schematic diagram of the operation process. The specific particle size and morphology are shown in Figure 6 and Figure 7.

The specific details and dimensions of the equipment are described in detail in the previous literature (Xu X, Wang Q, Kong X, et al. Pan mill type equipment designed for polymer stress reactions: theoretical analysis of structure and milling process of equipment[J]. Plastics rubber and composites processing and applications, 1996, 25(3):152-158).

 

Point 7: “2.3. Characterization”

-Lines 104-105: “Particle size after grinding was determined by…”->Please consider introducing the particle size distribution of your feed since the results of milling operation can be strongly affected by the feed particle size distribution.

-Lines 105-106: “A small number of milled particles were dispersed…”->Please consider stating how you sampled the small number of milled particles. Otherwise the representativity of your “small number of milled particles” is questioned.

-Lines 128-130: “Rheological properties were conducted…”->(a) Viscosity of milled particles was measured…? (b) Please consider stating whether your measurement was performed in dry or wet. In either case, please consider justifying the representativity of your measurements since your sample can be highly heterogenous with your coarse size of particles after the milling (>250 micron meters as shown in Fig.3) and thus can be easily segregated during the shear force application in a viscosity measurement.

-In this section, please consider explaining your methods detail enough for someone else to perform similar experiment in the future. For example, for SEM imaging, please consider stating the way you deposited your sample(s) in an SEM chamber, detector type (SE? BSE?), accelerating voltage, and working distance. Also, please consider explaining how you analyzed your images. Those information would be helpful for future researchers. This comment also applied to all the characterization methods introduced in this section.

doi.org/10.1016/j.actamat.2005.12.014

doi:10.3390/electronics8101202

Response: The PMW composition is more complex, and the size varies widely between different components. In this experiment, as the material is repeatedly added to the equipment for crushing, the final size of the obtained material is more uniform.

In S³M process, the material was milled along the spiral track in the milling disc and discharged from the edge of the milling disc, which was a long path. So, after the initial grinding, the material can be better crushed. When sampling the “small number of milled particles”, the powder was first mixed well and then randomly sampled. Little variability between samples.

Viscosity of milled particles was measured by a capillary rheometer, which can keep consistent with the shear rate of polymer materials in the actual processing process.

The measurement was performed in dry.

The test of rheological properties was to analyze the influence of particle size and interaction between different polymer on the processing properties of materials after S³M milling. Highly heterogenous particles can be easily segregated during the shear force application in a viscosity measurement, which also confirmed the processing capacity of the equipment. The material without S³M processing was easy to block processing equipment, while with the increase of milling cycles, the viscosity of the sample decreased, showing that S³M treatment realized the transformation of processability of materials.

Scanning electron microscope (SEM) (FEI Instrument, USA) with an accelerating voltage of 20 kV was conducted to study the morphology of samples. Before SEM observation, samples were quenched in liquid nitrogen, and the fractured surfaces of the samples were coated with a thin layer of gold by ion sputtering prior to visualization. Dispersion of elements was obtained by Mapping (OXFORD Instruments Co., Ltd., Britain).

 

Point 8: “3.1. Characterization of raw material”

-Lines 140-141: “…we selected and classified them, and then determined their components.”->This phrase is vague. Please consider stating what you actually did instead of such a vague phrase.

-Lines 142-146 (and elsewhere applicable): “…Fig.S1…Fig.S2…Table S1.”->Please provide those Figures and Table in the appendices of the article. There is no appendix in the article and in the MDPI system. Thus, one cannot evaluate the validity of your statements and results.

Response: we agree with the reviewer’s assessment. Here we would like to make a general classification of this mixed material to understand more clearly the complexity of the PMW. The related content has been presented in the revised manuscript.

 

Point 9: “3.2. Morphology of treated PMW”

-Lines 150-152: “In this experiment, we used S³M technology to treat PMW…”->This statement is confusing and does not match with the section title. Please consider (a) stating what you did to study the morphology of milled PMW, and (b) sending the milling procedure to the method section.

-Lines 155-157: “…ball-milling and air jet comminution failed to pulverize ductile polymer materials effectively, while PMW can be milled to powder through S³M technology.”->(1) In Figs.3(e), (f), please consider plotting the average particle size and particle size distribution generated by using ball mill and air jet comminution for the comparison with S³M. (2) Please consider discussing why S³M works but the other two do not, and citing relevant reference(s) to compare with your results.

-Lines 158-159: “…from the initial millimeter level to the micron level…”->…from several millimeters to several hundred micron meters…

-Figure 3 (e), (f): Please consider plotting the particle size or particle size distribution of the feed (without milling), and adding error bars in the both figures. Also, please consider renaming the title of Fig.3(e) to “average particle size” if it is what is shown in Fig.3(e).

-Lines 159-162: “…particle size experienced a prompt decrease in the initial milling cycles. Then with the increasing of milling cycles, the particle size decreased gradually to 200 µm…”->Please be more specific (e.g., cite specific milling cycle) and well describe the results. From Fig. 3(e), one can see that the average particle size decreases till the 4^th milling cycle and no further size reduction after the 6^th milling cycle that reaches the average particle size to about 250 µm.

-Lines 162-164 and Fig.2: “…as shown in Fig.2(g).”->Please properly name Fig.2 since there is no Fig.2(g).

Response: Thanks for your suggestion. We have sent the milling procedure to the method section.

We acknowledge the reviewer’s comments very much. The structure of the self-designed pan-mill type equipment is very different from that of most conventional pulverizing equipment. The pair of milling pans are placed face to face. The ridges and bevels on both pans form many unit cells. The materials being milled undergo strong pressure and shear forces owing to the space changes of the unit cell and the complex motion in both the circular and radial directions. Particles being milled move in a spiral path from the feed opening to the exit. During the process, the materials mainly endure pressure and shear force instead of impact force as in most other pulverizing equipment, which is very favorable for the pulverization of polymers. Most polymers are tougher than inorganic materials and easier to be broken down by pressure and shear force rather than by impact force. Additionally, the regular spiral movement is also different from the random movement of particles in most other pulverizing equipment. The regular movement reduces the possibility of friction between particles, which is one of the main sources of waste heat generated during milling. As a result, the energy input is mainly consumed in pulverizing instead of random movement of materials or milling media.

Compared with other conventional pulverizing technology, S³M technology can provide fairy strong shear force field, which has multiple functions of pulverizing, dispersing and mixing materials. As shown in the manuscript, frequently used technique like ball-milling and air jet comminution failed to pulverize ductile polymer materials effectively, while PMW can be milled to powder through S³M technology. The pulverizing effect varied greatly between different methods, with significant differences.

Unmilled particle size is too large, which will block the particle size analyzer.

Other details have been revised in the revised manuscript.

 

Point 10: “3.3. Chemical characterization of treated PMW”

-Lines 180-181: “…no new functional groups were formed during milling.”->Please consider removing “mechanochemical” from the line 67 since in this study there is no clear evidence that your milling equipment works as a mechanochemical reactor.

-Lines 185-190: “…powerful extrusion and shearing force of milling disc made particle pulverize, and the surface structure of particle became severely disturbed and irregular surface structure was formed after repeated milling…the decrease of crystallinity.”->Please consider citing your characterization results shown in Figure 4 to support your discussion. From your current statements, there is no strong connection between your results and discussion/statement.

Response: Thanks for your suggestion. We have revised the manuscript.

“The effect of S³M treatment on the crystallization behavior of PMW was studied by DSC. As shown in Figure 5b, there were five characteristic melting peaks, which was mainly LDPE, HDPE, PP and PET. HDPE have different melting peaks (129 ℃ and 134 ℃) because of different molecular weight. During the milling process, the molecular chain broken and the molecular weight decreased, so melting temperature was decreased after milling. With increase of milling recycles, the melting enthalpy at 108 ℃ of the milled PE decreased from 15.13J/g of the unmilled sample to 5.8 J/g, indicating that the crystallization was destroyed. This was mainly due to the decrease of particles size after S³M process, which could restrict the movement of polymer molecular chain and hinder the orderly arrangement, resulting in the decrease of crystallinity [21].”

 

Point 11: “3.4. Morphology of PMW samples”

-Lines 216-218: “Experimental results revealed that untreated sample had a disorderly morphology due to…observed. However, after S³M treatment, the surface was more uniform…”->Please consider citing your results and figures in order to support your statements.

-Line 229: “…as shown in Fig.7(d)…”-> …as shown in Fig.6(d)…?

-Figure 6: Please describe and discuss the images and results.

-Figure 6(c),(f): In the method section, please consider defining the “domain” and describing the procedure of measuring “domain size”.

-Lines 227-228: ”The average size of dispersed phase was 9.64 µm…”->The average size of domain was 9.64 µm…? Please be consistent between the main text and Fig.6(c). This comment applies to the other statement on lines 231-232 describing Fig.6(f).

-Lines 239-240: “Ca in sample mainly came from sand and stone components…”->Is this true that your aluminum-plastic packages contain sand and stone? Or do they contaminate the aluminum-plastic packages after their end of life?

-Line 241: “…Table 1.”->Where is it? I cannot find it in the article.

-Figure 7 title: “EDS of samples…”->EDS of hot pressed sample…?

-Figures 7(a),(b): In the method section or this section, please consider explaining your intention to show the five elements selected. Especially C and O should be explained.

Response: Figure 6 was optical microscope picture, which was only qualitative analysis without specific value.

We feel sorry for our carelessness. In our resubmitted manuscript, the type is revised. Thanks for your correction.

"Domain size" describes the size and shape of dispersed phase of polymer blends. In this experiment, because there are many impurities in PMW, the domain size is obtained by statistics on SEM images, and only as an attachment result to qualitatively verify the effectiveness of this method.

After their end of life of APPW, impurities such as sand and stone will inevitably be brought into the recycle process.

EDS of hot-pressed sample was performed in this experiment.

C and O elements were chosen to verify the milling effect. S³M technology can realize the ultra-fine powder extraction and uniform dispersion of PMW at room temperature.

Other details have been revised in the revised manuscript.

 

Point 12: “3.5. Rheological properties”

-Lines 259-260: “Owing to the large particle size of PMW after recycling, it was easy to block processing equipment during the processing process.”->Please consider revising this statement since it is not clear. What is your definition of “recycling”? Do you mean milling operation? What is the “processing process”? Do you mean milling process?

-Line 261: “…accessed a barrier to reprocess.”->Please consider rephrasing this since it is not clear what you wish to say.

-Lines 262-263 and Figure 9 title: “rheological properties…”->Shear viscosity of…

-Figure 9 title (and Figure 5 title): “…waste Tetra packaging…”->Please be consistent with what you wrote in the other places of the article. One can find “Tetra packaging” only in Figures 5 and 9.  

-Figure 9: Please consider adding error bars and discussing the difference between the results obtained with the milled products after different cycles. In other words, the difference in the viscosity with the milled products after different cycles is beyond the error range or not? It seems that the difference is minor. These comments also apply to Figs.10(b)-(d).

Response: Thanks for your suggestion. We feel sorry for our poor writings. “PMW after recycling” is really not clear. We have changed it to “PMW without S³M process”.

“This phenomenon still existed when the temperature rose to 250 ^oC, which accessed a barrier to reprocess”has changed to “This phenomenon still existed when the temperature rose to 250 ^oC, which is not conducive to reprocessing into useful products.”

Other details have been revised in the revised manuscript.

 

Point 13: “4. Conclusions”

-Lines 302-303: “…200 µm.”-> …250 µm.

-Lines 306-307: “…recycled material can be reprocessed…”->…milled material can be extruded…? Please be more specific.

-You may state future perspectives in Conclusions.

Response: We have made corresponding modifications：“Through S³M process, the recovered multi-component (PE, PET, PVC and other mixtures) material was milled from from 700 μm to 226 μm and the distribution range of particles tended to be smaller and narrower with the milling cycles increasing.”

The first step of effective recycle of polymer materials is to have reprocessing ability. In this experiment, the material can be extruded normally through S³M process, which becomes the key to effective utilization in the later stage.

 

Point 13: Minor concerns:

-Please consider polishing English more. You may use some of my comments above for this purpose.

Response: Thanks for your suggestion. We have tired our best to polish the language in revised manuscript.

 

Author Response File: Author Response.docx

Round 3

Reviewer 2 Report

Dear Authors,

As there is no change in your responses, I cannot make changes in my recommendation.

 

Best regards,

A reviewer

Author Response

Response to Reviewer 2 Comments

Point 1: “Article title”-It is too long. Please consider removing unnecessary word(s) from the title. For example, you may remove “technology” since one in recycling can easily understand milling is a technology (or process/method).

Response: we think this is a nice suggestion. We have changed our title to “Solid State Shear Milling for Recycling Aluminum-Plastic Packaging Waste: A Sustainable Solution for Mixed Plastic Waste”

 

Point 2: “Abstract”-Lines 15 and 16“…realize the pulverizing and excellent dispersion of multi-component polymer.”->Please consider citing your key results with some numbers/values if any.

Response: we have included the values of the experimental results in the abstract.

“The results indicate that a significant reduction in the the average particle size of PMW from 700 μm to 226 μm after 10 milling cycles, as evidenced by both particle size analyzer and Scanning Electron Microscopy (SEM). Furthermore, S3M processing leads to a good dispersion of PMW domains, as confirmed by the reduction in domain size from 9.64 μm to 2.65 μm.”

 

Point 3: “Keywords”->Please consider providing keywords that are not used in the article title.

Response: Thank you for your suggestion. We think the choice of keywords used in the manuscript better fits the key research content of this article.

 

Point 4: “1. Introduction”

• -Lines 39-41: “According to Tetra Pak international, most of abandoned Tetra Pak is burned or dumped as landfills…”->Please consider citing a reference.
• -Lines 56-57: “The recycling of post-consumer multi-plastic waste (PMW) is extremely hard…”->Please consider explaining the reason why it is hard.
• -Lines 67: “…solid-state shear milling (S³M)…”->Please consider describing the unique feature of S³M over other conventional mills, such as hammer mill. Also, if you have results obtained by using other mill(s), please consider showing and comparing them with the ones obtained by using S³M in the results and discussion.

https://doi.org/10.1163/156855206775123467

https://doi.org/10.3390/met9080899

Response: Thank you for your suggestion, we have made the following revisions to the relevant content.

• We have added the corresponding references in the manuscript: “Despite the increasing global output of Tetra Pak, which was estimated at 190 billion bags, its recovery rate is merely 26%. Most abandoned Tetra Pak is either burned or dumped in landfills, leading to secondary environmental pollution and resources wastage in numerous countries (Geogiopoulou et al., 2021).”
• In general, the recycling of APPW is focused on the recovery of high value Al and cellulose. The residual multi-plastic waste (PMW) is a complex mixture of different plastic materials with varying chemical structures, densities, and melting points, which makes recycling a challenging task. Moreover, PMW may contain other types of waste, such as sawdust and sand, which can further complicate the recycling process. Due to the high economic cost and complex process, there is no industrial technology to effectively recycle the waste mixed plastics. So, the recycling of PMW is extremely hard. This has been added to the introduction
• Compared with other conventional pulverizing technology, S³M technology can provide fairy strong shear force field, which has multiple functions of pulverizing, dispersing and mixing materials. As shown in Figure 3, frequently used technique like ball-milling and air jet comminution failed to pulverize ductile polymer materials effectively, while PMW can be milled to powder through S³M technology. The specific details of the S³M technology are as follows：
• The structure of the self-designed pan-mill type equipment is very different from that of most conventional pulverizing equipment. The pair of milling pans are placed face to face. The ridges and bevels on both pans form many unit cells. The materials being milled undergo strong pressure and shear forces owing to the space changes of the unit cell and the complex motion in both the circular and radial directions. Particles being milled move in a spiral path from the feed opening to the exit. During the process, the materials mainly endure pressure and shear force instead of impact force as in most other pulverizing equipment, which is very favorable for the pulverization of polymers. Most polymers are tougher than inorganic materials and easier to be broken down by pressure and shear force rather than by impact force. Additionally, the regular spiral movement is also different from the random movement of particles in most other pulverizing equipment. The regular movement reduces the possibility of friction between particles, which is one of the main sources of waste heat generated during milling. As a result, the energy input is mainly consumed in pulverizing instead of random movement of materials or milling media.

 

 

Point 5: “2.1. Materials”

-In this section, please consider providing the particle size distribution and elemental assay of the feed of your milling experiment.

Response: The raw material for milling experiment is shown in the Figure 2. In order to compare the milling effect, we put the particle size distribution and element assay in “Results and discussion” section, as shown in Figure 6 and Figure 7.

 

Point 6: “2.2. Preparation of samples”

• -In this section, please consider stating (a) whether your milling was performed in dry or wet, and (b) how many times you performed the milling test at each condition.
• -Lines 81-83 (and elsewhere): “…S³M technology…self-designed pan-mill type equipment.”->If S³M is equal to self-designed pan-mill type equipment, please use either one and be consistent throughout the article. Otherwise, a reader will be confused.
• -Lines 89-91: “…the crushed sample was uniformly fed into the feeding funnel of this equipment.”->Please consider providing the feed rate (e.g., g/s, kg/min) in this statement.
• -Figure 1(a): Please consider adding a scale in each image.
• -Figure 1(b): Please consider providing the dimension of your equipment.

Response:

• the whole milling process was performed in dry. “The whole milling process is carried out under dry conditions”. The crushed sample was uniformly fed into the feeding funnel of S³M equipment. The material was milled along the spiral track in the milling disc and discharged from the edge of the milling disc. The milling powder was collected from the discharging port and recorded as finishing one cycle of milling. The milling times directly determine the particle size and affect the product performance. The milling times were analyzed in the discussion section of this paper
• S³M equipment is designed for handling polymers with some unique structure. To avoid confusion, S³M equipment is used uniformly in the manuscript.
• The feed rate of S³M equipment depends on the nature of the material, like size, density. For example, polymer pellets are fed at a high speed. Then polymer film materials, especially light weight, are relatively slow to be fed.
• Figure 1a is a schematic diagram of the operation process. The specific particle size and morphology are shown in Figure 6 and Figure 7.
• The specific details and dimensions of the equipment are described in detail in the previous literature (Xu X, Wang Q, Kong X, et al. Pan mill type equipment designed for polymer stress reactions: theoretical analysis of structure and milling process of equipment[J]. Plastics rubber and composites processing and applications, 1996, 25(3):152-158).

 

Point 7: “2.3. Characterization”

• -Lines 104-105: “Particle size after grinding was determined by…”->Please consider introducing the particle size distribution of your feed since the results of milling operation can be strongly affected by the feed particle size distribution.
• -Lines 105-106: “A small number of milled particles were dispersed…”->Please consider stating how you sampled the small number of milled particles. Otherwise the representativity of your “small number of milled particles” is questioned.
• -Lines 128-130: “Rheological properties were conducted…”->(a) Viscosity of milled particles was measured…? (b) Please consider stating whether your measurement was performed in dry or wet. In either case, please consider justifying the representativity of your measurements since your sample can be highly heterogenous with your coarse size of particles after the milling (>250 micron meters as shown in Fig.3) and thus can be easily segregated during the shear force application in a viscosity measurement.
• -In this section, please consider explaining your methods detail enough for someone else to perform similar experiment in the future. For example, for SEM imaging, please consider stating the way you deposited your sample(s) in an SEM chamber, detector type (SE? BSE?), accelerating voltage, and working distance. Also, please consider explaining how you analyzed your images. Those information would be helpful for future researchers. This comment also applied to all the characterization methods introduced in this section.

doi.org/10.1016/j.actamat.2005.12.014

doi:10.3390/electronics8101202

Response:

• The PMW composition is more complex, and the size varies widely between different components. In this experiment, as the material is repeatedly added to the equipment for crushing, the final size of the obtained material is more uniform. We would like to clarify that the particle size of the raw material is irrelevant to our processing since it is dependent on the powerful plastic shredder. After crushing, our S³M process produces a uniform, ultrafine powder. Therefore, the particle size in the earlier stage is not critical as long as it can enter the equipment properly.
• In S³M process, the material was milled along the spiral track in the milling disc and discharged from the edge of the milling disc, which was a long path. So, after the initial grinding, the material can be better crushed. When sampling the “small number of milled particles”, the powder was first mixed well and then randomly sampled. Little variability between samples.
• (a) Viscosity of milled particles was measured by a capillary rheometer, which can keep consistent with the shear rate of polymer materials in the actual processing process. (b) The measurement was performed in dry. The test of rheological properties was to analyze the influence of particle size and interaction between different polymer on the processing properties of materials after S³M milling. Highly heterogenous particles can be easily segregated during the shear force application in a viscosity measurement, which also confirmed the processing capacity of the equipment. The material without S³M processing was easy to block processing equipment, while with the increase of milling cycles, the viscosity of the sample decreased, showing that S³M treatment realized the transformation of processability of materials.
• Scanning electron microscope (SEM) (FEI Instrument, USA) with an accelerating voltage of 20 kV was conducted to study the morphology of samples. Before SEM observation, samples were quenched in liquid nitrogen, and the fractured surfaces of the samples were coated with a thin layer of gold by ion sputtering prior to visualization. Dispersion of elements was obtained by Mapping (OXFORD Instruments Co., Ltd., Britain).

 

Point 8: “3.1. Characterization of raw material”

• -Lines 140-141: “…we selected and classified them, and then determined their components.”->This phrase is vague. Please consider stating what you actually did instead of such a vague phrase.
• -Lines 142-146 (and elsewhere applicable): “…Fig.S1…Fig.S2…Table S1.”->Please provide those Figures and Table in the appendices of the article. There is no appendix in the article and in the MDPI system. Thus, one cannot evaluate the validity of your statements and results.

Response:

• We agree with the reviewer’s assessment. Here we would like to make a general classification of this mixed material to understand more clearly the complexity of the PMW. The related content has been presented in the revised manuscript.
• We were really sorry for our careless mistakes. Thank you for your reminder. The content of Fig. S1, Fig. S2 and Table S1 was placed in Figure 2 and Table 1 in the revised manuscript.

 

Point 9: “3.2. Morphology of treated PMW”

• -Lines 150-152: “In this experiment, we used S³M technology to treat PMW…”->This statement is confusing and does not match with the section title. Please consider (a) stating what you did to study the morphology of milled PMW, and (b) sending the milling procedure to the method section.
• -Lines 155-157: “…ball-milling and air jet comminution failed to pulverize ductile polymer materials effectively, while PMW can be milled to powder through S³M technology.”->(1) In Figs.3(e), (f), please consider plotting the average particle size and particle size distribution generated by using ball mill and air jet comminution for the comparison with S³ (2) Please consider discussing why S³M works but the other two do not, and citing relevant reference(s) to compare with your results.
• -Lines 158-159: “…from the initial millimeter level to the micron level…”->…from several millimeters to several hundred micron meters…
• -Figure 3 (e), (f): Please consider plotting the particle size or particle size distribution of the feed (without milling), and adding error bars in the both figures. Also, please consider renaming the title of Fig.3(e) to “average particle size” if it is what is shown in Fig.3(e).
• -Lines 159-162: “…particle size experienced a prompt decrease in the initial milling cycles. Then with the increasing of milling cycles, the particle size decreased gradually to 200 µm…”->Please be more specific (e.g., cite specific milling cycle) and well describe the results. From Fig. 3(e), one can see that the average particle size decreases till the 4^thmilling cycle and no further size reduction after the 6^th milling cycle that reaches the average particle size to about 250 µm.
• -Lines 162-164 and Fig.2: “…as shown in Fig.2(g).”->Please properly name Fig.2 since there is no Fig.2(g).

Response: Thanks for your suggestion.

1. We have made the necessary revisions accordingly. “morphology of PMW after S³M treatment was studied by SEM and Particle Size Analyzer”. We have sent the specific milling procedure to the method section: “Figure 1b shows the schematic drawing of the self-designed pan-mill type equipment. The left part of Figure 1bFig.1(b) is the inlaid pans of the equipment, and its main structural parameters are radius R, the division number of milling pan n, their corresponding slot number m, slot top width δ and bevel angle α. Two pans with similar structure are placed in opposite directions, and one of the pans is stationary while the other is movable. In one milling cycle, the angel between two pans gradually decreases to 0, so that the material between them is crushed by strong extrusion and shearing. This shear force is three-dimensional. During sample preparation, the crushed sample was uniformly fed into the feeding funnel of this equipment. The material was milled along the spiral track in the milling disc and discharged from the edge of the milling disc. The milling powder was collected from the discharging port and recorded as finishing one cycle of milling. The equipment was running at ambient temperature with cooling water, and the speed of milling disc was controlled at 30 r/min. The whole milling process is carried out under dry conditions The materials could be milled repeatedly by continuous operation.”

• (1) The ball mill and air jet comminution were essentially ineffective in pulverizing the PMW, with no change in particle size, which could not be tested in Particle Size Analyzer, so the effect of pulverizing might be more visual with an optical photograph. (2) We have included a more detailed explanation about why S³M works in the manuscript. “The process of ball milling involves gradually reducing materials to a powdered state by means of the impact, friction, and grinding of milling balls. Conversely, air jet milling involves pulverizing materials into small particles by utilizing the impact and shear force of high-speed rotating blades and air flows. However, due to the inherent flexibility and extensibility of plastic films, the generation of sufficient collision and shear forces during ball milling or air jet milling presents a challenge in breaking them into particles of a sufficiently small size [20]”
• “…from the initial millimeter level to the micron level…” have been revised to “Following milling treatment, the average size of PMW was reduced from its initial value of 700 μm to the 226 μm, as demonstrated in Figure 4a-d” in the revised manuscript.
• Particle size after milling was determined by a Particle Size Analyzer (S3500-SⅠ, Microtrac Instrument Co., Ltd., USA). The particle size of PMW without milling was large, which was not suitable for testing. In order to compare unity, no sample (without milling) was placed in the figure. We have added error bars in Figure 4c, and the title has been revised to “Average particle size (e) and particle size distribution (f) of PMW after different milling cycles” in the manuscript.
• This sentence “Figure 4 indicates that the average particle size continued to decrease untill the 4th milling cycle, with no further size reduction observed beyond the 6th milling cycle, at which the average particle size reached approximately 250 µm”, is a well description of the change in particle size during S³M treatment.
• We have made related modifications in the manuscript. “Additionally, particle size distribution maps from various milling cycles were obtained and presented in Figure 4f,”

 

Point 10: “3.3. Chemical characterization of treated PMW”

• -Lines 180-181: “…no new functional groups were formed during milling.”->Please consider removing “mechanochemical” from the line 67 since in this study there is no clear evidence that your milling equipment works as a mechanochemical reactor.
• -Lines 185-190: “…powerful extrusion and shearing force of milling disc made particle pulverize, and the surface structure of particle became severely disturbed and irregular surface structure was formed after repeated milling…the decrease of crystallinity.”->Please consider citing your characterization results shown in Figure 4 to support your discussion. From your current statements, there is no strong connection between your results and discussion/statement.

Response: Thanks for your suggestion.

• We have removed “mechanochemical” in the manuscript.
• “The effect of S3M treatment on the crystallization behavior of PMW was studied by DSC. Figure 5b illustrates five distinct melting peaks, namely LDPE, HDPE, PP and PET. HDPE exhibits distinct melting peaks (129 ℃ and 134 ℃) due to differences molecular weight. The milling process induces molecular chain breakage and a decrease in molecular weight, thereby decreasing the melting temperature. As the milling recycles increase, the melting enthalpy of the milled PE at 108 ℃ reduced from 15.13J/g to 5.8 J/g in the unmilled sample, indicating the destroy of crystallization. The reduction in crystallinity is mainly attributable to the decreased particles size after S3M process, which hinders the orderly arrangement of polymer molecular chain, thereby restricting their movement [22]”

 

Point 11: “3.4. Morphology of PMW samples”

• -Lines 216-218: “Experimental results revealed that untreated sample had a disorderly morphology due to…observed. However, after S³M treatment, the surface was more uniform…”->Please consider citing your results and figures in order to support your statements.
• -Line 229: “…as shown in Fig.7(d)…”-> …as shown in Fig.6(d)…?

-Figure 6: Please describe and discuss the images and results.

• -Figure 6(c),(f): In the method section, please consider defining the “domain” and describing the procedure of measuring “domain size”.

-Lines 227-228: ”The average size of dispersed phase was 9.64 µm…”->The average size of domain was 9.64 µm…? Please be consistent between the main text and Fig.6(c). This comment applies to the other statement on lines 231-232 describing Fig.6(f).

• -Lines 239-240: “Ca in sample mainly came from sand and stone components…”->Is this true that your aluminum-plastic packages contain sand and stone? Or do they contaminate the aluminum-plastic packages after their end of life?
• -Line 241: “…Table 1.”->Where is it? I cannot find it in the article.
• -Figure 7 title: “EDS of samples…”->EDS of hot pressed sample…?
• -Figures 7(a),(b): In the method section or this section, please consider explaining your intention to show the five elements selected. Especially C and O should be explained.

Response:

• Optical microscopy images are used for further exam the dispersion and particle size distribution of PMW on the highest magnification of digital microscope, as shown in Figure 6.
• We feel sorry for our carelessness. In our resubmitted manuscript, the type is revised. Thanks for your correction.
• "Domain size" describes the size and shape of dispersed phase of polymer blends. In this experiment, because there are many impurities in PMW, the domain size is obtained by statistics on SEM images, and only as an attachment result to qualitatively verify the effectiveness of this method.
• After their end of life of APPW, impurities such as sand and stone will inevitably be brought into the recycle process.
• We feel sorry for our carelessness. In our resubmitted manuscript, we have added the Table 1.
• EDS of hot-pressed sample was performed in this experiment.
• C and O elements were chosen to verify the milling effect. S³M technology can realize the ultra-fine powder extraction and uniform dispersion of PMW at room temperature.

 

Point 12: “3.5. Rheological properties”

• -Lines 259-260: “Owing to the large particle size of PMW after recycling, it was easy to block processing equipment during the processing process.”->Please consider revising this statement since it is not clear. What is your definition of “recycling”? Do you mean milling operation? What is the “processing process”? Do you mean milling process?
• -Line 261: “…accessed a barrier to reprocess.”->Please consider rephrasing this since it is not clear what you wish to say.
• -Lines 262-263 and Figure 9 title: “rheological properties…”->Shear viscosity of…
• -Figure 9 title (and Figure 5 title): “…waste Tetra packaging…”->Please be consistent with what you wrote in the other places of the article. One can find “Tetra packaging” only in Figures 5 and 9.  
• -Figure 9: Please consider adding error bars and discussing the difference between the results obtained with the milled products after different cycles. In other words, the difference in the viscosity with the milled products after different cycles is beyond the error range or not? It seems that the difference is minor. These comments also apply to Figs.10(b)-(d).

Response: Thanks for your suggestion.

• We feel sorry for our poor writings. “PMW after recycling” is really not clear. We have changed it to “PMW without S³M process”.
• “The processing of PMW without S3M treatment is hindered by the large particle size, which causes blockages even at temperatures as high as 250 ℃, making reprocessing a significant challenge”
• We have revised “rheological properties…” to “Shear viscosity…”.
• We have changed the “waste Tetra packaging” to PMW.
• The main emphasis of Figure 9 is the transformation of the material from non-processable to processable after S³M treatment. We have added error bars in Figure 10.

 

Point 13: “4. Conclusions”

• -Lines 302-303: “…200 µm.”-> …250 µm.
• -Lines 306-307: “…recycled material can be reprocessed…”->…milled material can be extruded…? Please be more specific.
• -You may state future perspectives in Conclusions.

Response:

• We have made corresponding modifications：“Results show that S3M process can reduce the size of the recovered multi-component from 700 μm to 226 μm, with a smaller and narrower particle distribution range observed with the increased milling cycles.”
• Your advice is very valuable. We have changed it.
• We have added some future perspectives in conclusion: “The recycling of multilayer plastic films has significant environmental and resource utilization benefits, thus its future prospects are very promising.”

 

Point 13: Minor concerns:

-Please consider polishing English more. You may use some of my comments above for this purpose.

Response: Thanks for your suggestion. We have tired our best to polish the language in revised manuscript.

 

Author Response File: Author Response.pdf

Round 4

Reviewer 2 Report

Dear Authors,

As all the comments were addressed, I would suggest the journal accept this article for its publication.

Best regards,
A reviewer