Number of Times Recycled and Its Effect on the Recyclability, Fluidity and Tensile Properties of Polypropylene Injection Molded Parts

Round 1

Reviewer 1 Report

The work reviewed herein titled "Number of Times Recycled and Its Effect on the Recylability, Fluidity and Tensile Properties of Polypropylene Injection Molded Parts" is research based around the processability impacts of mechanically recycling polypropylene (PP). The authors studied and presented the well known viscosity reduction in PP after recycling via the multiple rheological measurements, both during injection molding and through stand-alone characterization. Although it is well known that PP's viscosity, and therefore average molecular weight characteristics, decrease upon multiple heat histories, the novelty in this work are the relationships shown regarding viscosity and process characteristics during injection molding. 

The data, results, and analysis were all described and supported well using typical engineering and scientific concepts and references. The results and figures are presented clearly and I appreciated the well formatted figures. However, many sections of the paper are difficult to read due to grammatical errors and English style/language choices. I highly recommend reviewing and editing this paper to ensure your ideas and concepts are clearly understood by the audience. 

Additional technical suggestions are as follows:

• In section 2.2 Molding Equipment, the authors describe the tensile bar mold used to manufacture specimens for viscosity and pressure analysis. The ASTM standard is cited, however the type of tensile bar utilized is not specifically stated. A statement on line 130 says that the "dumbbell shaped tensile specimen sample dimensions varied substantially depending on the requirements...", referring to the ASTM standard. However very specific dimensions are provided in ASTM D638 for each of the five tensile bar types. Please provide the specific dimensions of the cavity and tensile bar type used.
• The authors use melt flow index testing to show the increase in flow after recycling of PP five different times. However, no MFI parameters are given and it is not clear if a standard method was used to collect the MFI data. Please provide the parameters used, if a standard method was used, the number of samples collected, and addition information about the MFI data collection.
• DSC data was collected and described in terms of PP's crystallinity, however no data or information was provided as to how crystallinity was calculated. Please provide the standard ΔH of 100% PP used, how many samples were measured to obtained the average and error data, and include the equation used to calculate percent crystallinity.
• Figures 7, 8, and 9 are showing the interesting relationships between recycling time, viscosity, and other properties ((crystallization temp., processing parameters, tensile strength). However, these data do not include error bars within the figures and the author does not address this in the discussion. Please include reasoning as to not including error information or include error bars to represent the error associated with these processes/characterizations.
• This research measures fluidity using melt flow length ratio from a mold built in the shape of a, as the authors put it, paper clip. The melt length data obtained is used to support the MFI and pressure data collected. However, it is common in the injection molding research literature to use a "spiral flow length" mold to do as the authors in this work have done. The reason for this is the fact that there is an ASTM standard to base the mold design from and allow for easier process comparisons throughout the industry and literature base. In the future, I recommend using such a mold. A link to the standard is provided: https://www.astm.org/Standards/D3123.htm 

Author Response

This paper attempts to employ the different TRs to observe and improve the property and quality of Tr-PP injection molded parts. It is an interesting study for an important research topic. Therefore, it is worthy of being considered for publication in SUSTAINABILITY after careful editing and polishing. The following comments will help to further improve the readability and completeness of this paper. Thanks a lot to the Reviewer for providing us the priceless suggestions on our Manuscript (ID: sustainability-1368358). For all the questions, we have tried our best to answer one by one as an attached file. Please take the time to review all answers.

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper is an interesting experimental study and has a large practical application potential due to the growing problem of plastics accumulation in the environment. I might be willing to recommend the publication of the manuscript, if the Authors have addressed the following comments:
- The literature review could be extended to contain more state-of-the-art (preferably journal papers with the focus of the same/similar tests presented in the paper). The novelty of the manuscript should also be set more clearly.    
- For the sake of clarity "Introduction" should be devided into two paragraphs - one with the literature review and the other with the purpose of the work and etc.  
- The paper is an experimental study. Thus, it should contain information about the error analysis. Figures 5 and 6 contain data with some error bands - is it the standard deviation from 15 samples tested, or the error of the determination of a given property? It would be welcome if the the Authors could provide more information regarding measurement errors.
- In the discussion part (chapter 3), it would be welcome to provide some data on the similar test results of other researchers, how the current experimental results correlate with the findings of others, etc. 
- Equation 1 should be placed closer to there it is first mentioned in the paper
- The language of the paper is generally fine, however the Authors are asked to proofread the paper due to the presence of few mistakes, for example in line 13-14 the sentence "Where polypropylene....low cost." has no verb, in line 74 "investigate" should be used instead of "investigated"; or the sentence in line 73 should start with the capital letter instead of ";the purpose of this study...", etc.   

Author Response

This paper attempts to employ the different TRs to observe and improve the property and quality of Tr-PP injection molded parts. It is an interesting study for an important research topic. Therefore, it is worthy of being considered for publication in SUSTAINABILITY after careful editing and polishing. The following comments will help to further improve the readability and completeness of this paper. Thanks a lot to the Reviewer for providing us the priceless suggestions on our Manuscript (ID: sustainability-1368358). For all the questions, we have tried our best to answer one by one as an attached file. Please take the time to review all answers.

Author Response File: Author Response.pdf

Reviewer 3 Report

This is an interesting article. The article presents very important and current research trends concerning pollution with plastics (PP - Polypropylene) and ecological impact on the ecosystems. Reduce–recover-recycling of PP is very important in circular manufacturing of plastic injection molding and a circular economy. The analysis is interesting and the conclusions are clear. This article is highly recommended for plastics researchers.

Author Response

This paper attempts to employ the different TRs to observe and improve the property and quality of Tr-PP injection molded parts. It is an interesting study for an important research topic. Therefore, it is worthy of being considered for publication in SUSTAINABILITY after careful editing and polishing. The following comments will help to further improve the readability and completeness of this paper. Thanks a lot to the Reviewer for providing us the priceless suggestions on our Manuscript (ID: sustainability-1368358). For all the questions, we have tried our best to answer one by one as an attached file. Please take the time to review all answers.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The paper has been improved and my comments have been addressed by the Authors. I would now recommend to accept the paper. 

Author Response

Thanks to the Reviewer's comment and affirmative.

Author Response File: Author Response.pdf