Rethinking PE-HD Bottle Recycling—Impacts of Reducing Design Variety
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsDear authors,
Thanks for sharing your manuscript. In general it is well-comprehensible, the results are as expected for recycling experts and are probably undesired and counter-intuitive for brand-owners. There are only a few minor issues that need to be resolved as far as I am concerned.
Statement of the objective.
On lines 59-60 you clearly describe the research gap. On line 60-67 you describe what you have studied, but you do not clearly stipulate the objective of this study. Please add such a statement.
Deposit-refund system for milk bottles in the UK? No, there is not. On lines 71-72 you mention for the first time that there would be a deposit-refund system for milk bottles in the UK. This is wrong because it doesn’t exist. In the UK milk bottles are co-collected with all other “recyclables” and sorted in a MRF. Because milk bottles in the UK have fluorescent labels of Interket, they are selectively extracted at the MRF. Director Tim Pattison is so proud of his labels that he can talk for hours about them… So please remove every mention of ”deposit refund” in your text.
Lines 77-78. The origin of PP contamination in sorted HDPE bales (DKR 329) is not only sorting mistakes but also non-targeted components such as labels and caps that are insufficiently removed during the sorting and recycling processes [DOI: 10.1002/pts.2551].
Table 1: what is the nature of the PP-h? Is this a PP homopolymer?
Figure 1. From this figure it is not clear whether or not opaque and coloured feedstocks are treated as mixtures from granulation on or separately. Please clarify.
Section 2.5. Were the test specimen conditioned prior to the tensile tests? Virgin polyolefins are normally not-conditioned, but recycled polyolefins contain polar contaminants (ink residues, PSA’s, pigments and the lot) and become slightly sensitive for moisture, read the work of Garofalo in Italy [for example, DOI: 10.1016/j.jclepro.2021.126379].
Section 3.1 You start this section with a lot of interesting compositional data. What is the error in this data? This could be either a standard deviation from replicate measurements or a methodical standard error related to the manual sorting itself (one object of 20 grams in a sample of 20 kg, causes a standard error of at least 0.1%, etc.).
Line 264-265. This doesn’t happen only in the UK. Both at the industrial facilities of Remondis Plano in Lünen, Germany and at Morssinkhof in Heerenveen, Netherlands contact-sensitive HDPE is produced from DKR 329, by AI-sorting of bottles to obtain only “natural” (read: transparent / opaque) HDPE bottles and an elaborate recycling process in which all remaining flakes of differently coloured labels and caps are removed a various flake-sorters. This material is used in personal care packages, such as for example blue bottles of sun screen of NIVEA, Beiersdorf. (Yes, it is a bit absurd that they use transparent/opaque rHDPE for making blue bottles). This came on the market in 2024.
Line 318: “Inadequate product quality”, this is too generic and can be written more precisely.
Line 332: What does the abbreviation ND mean? This abbreviation is used everywhere in figures and I have not clue what it stands for: “not determined”, “not dark”?
Line 393. I agree that EBM test specimen will probably have higher degrees of crystallinity, but did you check it with DSC?
Line 452-453. Indeed the elements of Aluminium and Silicium have previously been reported to be present in rHDPE, see Sani [DOI: 10.1002/pen.26441]. Probably this is dust and ordinary sand.
Line 463-464: This opaque rHDPE from British milk bottles doesn’t only consist of transparent and white HDPE flakes, the grade of HDPE for the milk bottles is also specific. As a supplier of HDPE resin once explained me, the grades used for milk bottles have a distinct molecular weight distribution that mimics a camel bump (two peaks in the GPC). This is required for stretch blow moulding and avoiding environmental stretch cracking
Line 469: The results still “indicate” the same, so do not use the past tense.
Line 490: I would refrain from using the term “reuse potential”, as “reuse” is connotated with refill systems and multiple rotations, I would rather write: “closed loop HDPE recycling potential”.
Good luck!
Author Response
We thank Reviewer #1 for their appreciation of our manuscript and for expressing their
appreciation and its value to experts and brand owners. We are grateful for the provided
suggestions for improvement and have implemented them all without exception. All queries
are addressed below point by point.
*** The full response is attached as PDF ***
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThis study investigates the impact of reducing design variety in PE-HD packaging on the quality and performance of post-consumer recycled PE-HD (rPE-HD). Generally speaking, this paper was well designed and written. In order to improve the paper’s quality, please consider the following suggestions:
1. In the introduction part, please supplement the latest research progress on PE-HD recycling technology in the past 3 years (e.g., AI screening technology, etc.) .
Also, the unique advantages of reducing design diversity in recycling progress, as well as the cost-effectiveness and environmental benefits compared to other recycling methods, need to be further emphasized to enhance the innovative and practical significance of the study.
2. in section 2.1, it’s mentioned that “Both flake batches were hot washed, dried and re-extruded at industrial scale”, please Supplement industrial-scale process parameters (e.g., temperature, pressure, residence time, etc.) to enhance repeatability.
Author Response
We sincerely thank Reviewer #2 for openly expressing their appreciation for the presented
manuscript and addressed each of the provided queries below.
*** The full response is attached as PDF ***
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for Authors- “melt blending with suitable virgin material is likely one of the most effective options to maximise regranulate utilisation in hollow bodies ” How to adress the compatibility among them?
- " The impacts of packaging variety on regranulate properties were studied, including the effects of PP contamination by preparing a blend of opaque rPE-HD and PP homopolymer (PP-h)." It is better to clearly express the novelty.
- How about the cost of the post-consumer PE-HD compared to the pure PE?
- The number of reference is too much, 40 is better.
- The abstract needs to be rewritten with more data.
Author Response
We thank Reviewer #3 for their time, effort and suggestions for the improvement of our
manuscript.
*** The full response is attached as PDF ***
Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report
Comments and Suggestions for AuthorsThe revision is good.
Author Response
We thank Reviewer #3 for expressing their appreciation about the implemented changes.
