Effect of Mechanical Recycling on the Mechanical Properties of PLA-Based Natural Fiber-Reinforced Composites
, Noel M. Gately 1, Vlasta Chyzna 3, Alex Portela 3, Gilberto Silva Nunes Bezerra 2, Paul McDonald 4 and Declan Mary Colbert 2,*
Round 1
Reviewer 1 Report
The paper has been assessed. I have raised some points which require clarification and amendments are required. I am enclosing these comments below.
In this paper, in order to investigate the feasibility of utilizing PLA and PLA-based composites in a mechanical recycling process, the authors analysis the mechanical properties of PLA90/HNT10 and PLA90/BF10 with 1, 3 and 6 mechanical recycling steps. There are significant discoloration (“yellowing”) of the polymer and a common indication of degradation during the thermal processing of the polymer due to the formation of chromatophores in the structure.
1. In page 6 line 227, The reduction in tensile properties may be due to a combination of poor compatibility between fiber and matrix material, fiber fraction, thermal degradation of the PLA-matrix as well as hydrolysis of the PLA at elevated the elevated temperatures used for the reprocessing steps. Please check and correct it.
2. In page 6 line 257, The tensile stress increases from 76.14 MPa to 83.79MPa while the Youngs Modulus increases from 2292.31 MPa to 3176.44 MPa. Is it not found that where the author got “76.14 MPa”? Please give the resource of value.
3. In page 7 line 275, The impact strengths of the NFRCs are summarized in Table 2 and shown graphically in Figure 3. Please check and correct it.
4. In page 7 line 275, The impact strengths of the NFRCs are summarized in Table 2 and shown graphically in Figure 3. Please check and correct it.
5 Section 4 Conclusions should be improved, the research result should be given clearly and in detail.
6. Section 0 part should be remove in the paper.
7. Keywords is too much, please be concise the Keywords.
8. The author investigate the mechanical properties of 1, 3 and 6 mechanical recycling steps. Please explain why the author chose 1, 3 and 6 mechanical recycling steps to investigate.
Author Response
Point 1: In page 6 line 227, The reduction in tensile properties may be due to a combination of poor compatibility between fiber and matrix material, fiber fraction, thermal degradation of the PLA-matrix as well as hydrolysis of the PLA at elevated the elevated temperatures used for the reprocessing steps. Please check and correct it.
Response 1: Sentence has been deleted
Point 2: In page 6 line 257, The tensile stress increases from 76.14 MPa to 83.79MPa while the Youngs Modulus increases from 2292.31 MPa to 3176.44 MPa. Is it not found that where the author got “76.14 MPa”? Please give the resource of value.
Response 2: This sentence has been deleted to avoid confusion
Point 3: In page 7 line 275, The impact strengths of the NFRCs are summarized in Table 2 and shown graphically in Figure 3. Please check and correct it.
Response 3: Corrected
Point 4: In page 7 line 275, The impact strengths of the NFRCs are summarized in Table 2 and shown graphically in Figure 3. Please check and correct it.
Response 4: Same as comment 3
Point 5: Section 4 Conclusions should be improved, the research result should be given clearly and in detail.
Response 5: Conclusion has been revised
Point 6: Section 0 part should be remove in the paper.
Response 6: Deleted
Point 7: Keywords is too much, please be concise the Keywords.
Response 7: Keywords have been reduced to 5 keywords
Point 8: The author investigate the mechanical properties of 1, 3 and 6 mechanical recycling steps. Please explain why the author chose 1, 3 and 6 mechanical recycling steps to investigate.
Response 8: The authors agree that the choice to not perform the analysis on steps 2 and 4 of the mechanical recycling process is a limitation to the current study and this has been alluded to in the conclusion. In future studies we look like to perform a similar study to determine the upper limit of mechanical recycling and would aim to perform the analysis on each step of the process.
Reviewer 2 Report
The submitted manuscript cannot be accepted for publication in this form, but it has a chance of acceptance after a major revision. My comments and suggestions are as follows:
1- Abstract gives information on the main feature of the performed study, but a couple of sentences about the details of conducted tests must be added.
2- Authors must clarify necessity of the performed research. Objectives of the study, must be clearly mentioned in introduction.
3- Title of the manuscript is too long.
4- There are articles on reinforced composites which are relevant to discussion. It is recommended to cite the articles for completeness: (a) https://doi.org/10.1016/j.prostr.2022.01.064 and (b) https://doi.org/10.1016/j.tws.2022.110263 and other research works.
5- Authors must discuss limitations and strength of their study in details.
6- The current version of introduction is not comprehensive. Authors must add a short review on the fracture of PLA 3D-printed parts. To this aim, "fracture in 3D printed parts" can be searched in search engine like ScienceDirect to find and refer relevant research works.
7- Since this manuscript deals with experimental tests, it is necessary to add several real figures to illustrate details and conditions.
8- How the impact strength is determined. Details of calculation must be added. Also, there are some claims which need evidence.
9- There are sentences which have to be rewritten.
10- The current version of conclusion is too short. The conclusion must be more than just a summary of the manuscript. List of references must be updated based on the proposed papers. Please provide all changes by red color in the revised version.
Author Response
Point 1: Abstract gives information on the main feature of the performed study, but a couple of sentences about the details of conducted tests must be added.
Response 1: Further information has been added to the manuscript abstract…
Point 2: Authors must clarify necessity of the performed research. Objectives of the study, must be clearly mentioned in introduction.
Response 2: The necessity of the research has been further expanded upon at the end of the introductory section with the addition of “It is hoped that this work will lead to increased focus on the recyclability of PLA-based materials as opposed to solely composting the material, thus improving the waste management options available.”
Point 3: Title of the manuscript is too long.
Response 3: Manuscript title has been shortened to “Effect of mechanical recycling on the mechanical properties of PLA-based natural fiber reinforced composites” though any further suggestions would be more than welcome.
Point 4: There are articles on reinforced composites which are relevant to discussion. It is recommended to cite the articles for completeness: (a) https://doi.org/10.1016/j.prostr.2022.01.064 and (b) https://doi.org/10.1016/j.tws.2022.110263 and other research works.
Response 4: The authors feel that the aforementioned papers which deal with the fracture of 3D printed PLA parts is not relevant to the research study at hand which deals solely with extrusion and injection molding of fiber reinforced composites, similar to how the mentioned papers to not discuss the injection molding of PLA-based composites. The authors have added the following to the introduction “. Pilla et al. (2009) manufactured molded natural composites of PLA and recycled wood fibers (RWF) [15]. It was shown that though the addition of RWF increased the tensile modulus of samples, the RWF lead to a significant decrease in the toughness of samples compared to that of neat PLA. Bajpal et al. (2012) showed that the incorporation of a 20 wt.% loading of sisal fibers improved the tensile, impact and flexural properties by 80.6 MPa, 106 kJ/m2 and 249.8 MPa respectively [16]. The incorporation of various volume fractions of kenaf, hemp and jute fibers have shown to also increase the mechanical properties of PLA-based composites [17, 18, 19] with a 30 vol.% of jute fibers was shown to be the optimal volume fraction for improvements in tensile and flexural properties [19].”
Point 5: Authors must discuss limitations and strength of their study in details.
Response 5: Discussion of limitations of the research added to Conclusion
Point 6: The current version of introduction is not comprehensive. Authors must add a short review on the fracture of PLA 3D-printed parts. To this aim, "fracture in 3D printed parts" can be searched in search engine like ScienceDirect to find and refer relevant research works.
Response 6: See response to comment 4
Point 7: Since this manuscript deals with experimental tests, it is necessary to add several real figures to illustrate details and conditions.
Response 7: “Figure 2 Injection molded tensile and impact test specimens” has been added to illustrate test specimens.
Point 8: How the impact strength is determined. Details of calculation must be added. Also, there are some claims which need evidence.
Response 8: The calculation for impact strength has been included
Point 9: There are sentences which have to be rewritten.
Response 9: Several sentences have been revised and rewritten
Point 10: The current version of conclusion is too short. The conclusion must be more than just a summary of the manuscript. List of references must be updated based on the proposed papers. Please provide all changes by red color in the revised version.
Response 10: The conclusion has been revised
Reviewer 3 Report
The paper entitled "An initial investigation into the effect of mechanical recycling via conical twin screw extrusion on the mechanical properties of PLA and PLA-based natural composites" focused on the evaluation of the mechanical performance of multi-processed materials. From my point of view, the research subject is interesting since PLA has the potential for recycling rather than composting; however, some additional work should be done before publication.
1. In its current form, the text resembles a test report, presenting only the results of mechanical tests. It is necessary to conduct additional tests to identify the reasons for the property change.
Please provide the microscopic (SEM) analysis of the fractured surface.
2. Figure 3 - error bars should be provided
3. Since the color change (yellowing) is one factor that influences the material properties, it is necessary to conduct the property analysis, for example, L *a *b or CIELab.
Author Response
Point 1: In its current form, the text resembles a test report, presenting only the results of mechanical tests. It is necessary to conduct additional tests to identify the reasons for the property change. Please provide the microscopic (SEM) analysis of the fractured surface.
Response 1: It is noted that additional tests are required to fully “flesh out” this work, though this study was aimed to be an initial investigation into the potential recycling of PLA-based composites. Additional characterization in the form of DSC, Colorimetry and SEM have been performed and discussed throughout the document.
Point 2: Figure 3 - error bars should be provided
Response 2: Error bars have been provided. Additionally, the graphs included in the document have been regenerated using GraphPad for additional clarity of images.
Point 3: Since the color change (yellowing) is one factor that influences the material properties, it is necessary to conduct the property analysis, for example, L *a *b or CIELab.
Response 3: See response to reviewer comment 2
Reviewer 4 Report
Up to now, PLA is mainly used as biobased plastic for packaging. However, there is a trend to extent its field of application to engineering plastics. In this case, reinforcement is often necessary, preferably by natural fibres. Due to the increasing use of such biobased composites, the recycling of these materials presently attracting growing attention.
The study of Colbert et al. is an interesting contribution to this field of research. The authors investigated the influence of the recycling process on crucial material properties of PLA composites reinforced with basalt fibres or halloysite nanotubes, respectively. The composites were six times extruded using a conical twin screw extruder. Testing of specimens manufactured by injection moulding was performed after 1, 3 and 6 recycling steps. These investigations revealed that the recycling moderate deteriorates crucial material properties. That means the study indicates, that repeated recycling of such-like composites could be practicable.
The manuscript should be published after performing a few supplementations and corrections. First of all, please remove section 0 (how to use this template) from the manuscript. The introduction part seems to me incomplete because the authors mention four categories of recycling (primary, secondary, tertiary and quaternary), but only the primary and secondary ones have been explained in the following text. Please supplement the introduction accordingly. Some parameters of the extrusion process are given in subsection 2.3. (temperature profile, rotation speed …). There is no information on the throughput and the residence time of the material in the extruder. However, these parameters strongly influence the thermal and mechanical stress exerted on the material when extruded. Subsection 2.4 (Injection moulding) is incomplete too. The residence time of the material is also not given. More important, it is not clear, whether injection moulding was performed after each extrusion or only after the first, third and sixth one.
The authors argue that the changes of material properties of the composites after extrusions are mainly caused by reduction of molecular weight, increase of crystallinity and decrease of fibre length. However, this argumentation is not supported by experimental results. It is a pity, that whether molecular weight determinations (GPC) were carried out nor microscopic investigations of the morphology were conducted.
Thus, to confirm the results additional studies are necessary in future.
Author Response
Point 1: First of all, please remove section 0 (how to use this template) from the manuscript.
Response 1: Section 0 has been removed
Point 2: The introduction part seems to me incomplete because the authors mention four categories of recycling (primary, secondary, tertiary and quaternary), but only the primary and secondary ones have been explained in the following text. Please supplement the introduction accordingly.
Response 2: The additional recycling steps have been explained as “3° recycling, also known as chemical recycling, refers to the chemical breakdown of polymer waste into its monomers which may be subsequently re-polymerised, thereby reproducing virgin polymer. 4° recycling is by far the least environmentally friendly option as it refers to the transformation of plastic waste into energy through process such as pyrolysis.”
Point 3: Some parameters of the extrusion process are given in subsection 2.3. (temperature profile, rotation speed …). There is no information on the throughput and the residence time of the material in the extruder. However, these parameters strongly influence the thermal and mechanical stress exerted on the material when extruded.
Response 3: Further information was added regarding the extrusion throughput
Point 4: Subsection 2.4 (Injection moulding) is incomplete too. The residence time of the material is also not given. More important, it is not clear, whether injection moulding was performed after each extrusion or only after the first, third and sixth one.
Response 4: Further molding conditions have been added. This has been further explained that only material recycled 1, 3 or 6 times was injection molded (i.e. recycled 2 and 4 times did not undergo injection molding). The authors note that not performing steps 2, and 4 is a limitation of the study and tis has been noted in the conclusion
Point 5: The authors argue that the changes of material properties of the composites after extrusions are mainly caused by reduction of molecular weight, increase of crystallinity and decrease of fibre length. However, this argumentation is not supported by experimental results. It is a pity, that whether molecular weight determinations (GPC) were carried out nor microscopic investigations of the morphology were conducted.
Response 5: Thank you for your comment, the paper has been further expanded with experimental results of DSC, SEM and Colorimetry
Thus, to confirm the results additional studies are necessary in future.
Round 2
Reviewer 1 Report
Dear editors:
The revised manuscript has been assessed carefully. The authors have solved the problems that I have proposed. However, the abstract should be further revise and give the result of investigation.
Reviewer 2 Report
The paper has been improved and corresponding modifications have been conducted. In my opinion, the current version can be considered for publication.
Reviewer 3 Report
Since most of my suggestions are reflected in the newest version of the paper I have no other comments. In my opinion, The paper can be published in its present form.
Reviewer 4 Report
The manuscript has been altered and supplemented according to the reviewers demands. The revision significantly improved the quality of the article. Therefore, I recommend the second version of the manuscript for publishing.
The authors inserted the requested supplementations into the introduction part. Most important, additional experiments were carried out: the morphology of fracture surface was investigated by scanning electron microscopy and colour measurements were conducted. Experimental details of the extrusion process were given too.
In addition, the discussion of the results including the conclusion part is considerably improved now.
Before publishing, please remove the following small errors from the manuscript: Line 116: Please replace SiO2 by SiO2 and replace Al2O3 by Al2O3.
