What are the weaknesses of this paper (if any)?

There are many weaknesses and limitations. The results in most cases do not support the primary conclusion, i.e., the use of detergent pods leads to the formation of microplastics.

a)        Wrong assumption/wrong use of method: Film samples from three brands of detergent pods were dissolved in tap water or distilled water, and the weight of film was measured before and after dissolution. The weight remaining after drying (removing the water) (about 80% in distilled water and >100% in tap water) was equated to “incompletely dissolved” film. Dissolution does not mean the disappearance of a substance. For example, if the same method were used to measure the dissolution of sugar in water, one would have obtained similar results. In theory, 100% of a soluble substance should remain if measured with this method. Therefore, the fact that most of the film “mass” remained after dissolution in water cannot be used to support the conclusion of microplastic formation.

b)        FTIR analysis/wrong use of method: The FTIR analysis confirmed that the residues from the dissolution of the detergent pod films retained some of the original chemical structures. The outcome should be expected and cannot be used to support the conclusion that the residues were microplastics. If the same method were used to analyze sugar (or other similar substances), of course one would expect to see similar chemical fingerprints. 

c)        Missing details: Critical details are missing. The film samples were dissolved in water, but the authors did not provide information on the initial size/mass of the film samples, or how long the film samples were kept in the water. Dissolution of a substance depends on the contact time and other factors (e.g., movement of water as in a washing machine). Images of water samples collected from three cycles of washing machine use are presented in Results and Discussion, but this experiment was never mentioned in the Experimental section. What were the conditions such as length of each wash cycle, temperature of water, and amounts and nature of the laundered material? The omission of such details speaks to the poor quality of this publication.

The paper uses a very limited sets of experiments to come to the final conclusion. Especially for such a sensitive topic, this is not good. The authors should have used for instance GPC to study the MW of the "dissolved" polymers, DLS and or TEM for size analysis etc.
A scientific paper should not make statements like "... and they are
probably one of the worst ways to add detergent to washing
machines."

1) The experimental work is not of a high quality, 2) it is unclear if the experimental conditions were representative of actual conditions experienced during laundry applications; 3) the experimental procedure was not described fully to the reader; 4) the interpretation of the data is questionable; and 5) the the Introduction section of the paper contains several false statements. In addition, 6) the overall presentation and writing quality are low. (further detail on each can be provided.)

I am afraid that I have identified many weaknesses and overall I find this a very weak paper.
The main issue is related to the mere fact that the authors did not consider an important aspect of the ECHA definition of microplastics: polymers with a water solubility of >2 g/L are excluded from the definition. This basically takes away the basis of this study.
A second issue is that there is a disconnect between the title and the aims of the paper, and what was actually done and what can actually be concluded. Certainly, the study did not determine the release of microplastics as a consequence of the use of detergent pods. At best, only the FTIR measurements could be indicative of some kind of presence of soluble polymer, but the study does not determine the environmental release of microplastics.
A third general issue is that often general statements are made which are not supported by actual findings or actual facts. Commonly, in such cases terms are used like 'probably', 'some other', 'possibility', 'can be'.


There furthermore are various specific issues:
1 - Various references are irrelevant. Example: reference 4 is not associated with the topic of discussion, the same is true for references 6 and 7 and especially 8 and 9.
2 - Most of the paragraph on page 365, starting with 'Due to improper....." is incomplete or simply not true. With regard to the first sentence it is for instance ignored that plastics are also applied on purpose outdoor. Also, microplastics are not a threat because we cannot see them with the naked eye.
3 - Page 365: aim of the study 'to quantitatively and qualitatively evaluate microplastics from commonly used household products such as laundry capsules'. As already stated, soluble polymers (as is the case here) are not defined as microplastics. Furthermore, the experimental design was not suited to achieve this aim. One would expect a relative simple design in which one way or the other the presence of soluble polymers was quantified. This simple basic experiment was not performed and it is not clear to me why an experimental design was selected in which washing water was examined in general terms without focus on (soluble) microplastics.
4 - The methods used lack a description of the washing procedures (minor issue).
5 - It is not clear what is the intention of the determination of mass change. I have the impression that this is a experiment without any meaning and especially determining the mass change after evaporation of tap water, does not make any sense: all that is done is to concentrate the minerals present in tap water. The determination of mass change after evaporating a sample of detergent pods in distilled water has no clear purpose to me and the results were not linked to any of the other activities within the study. Most importantly, it is not clear what is actually happening during evaporation of a solution/suspension of the film of a detergent pod. Hydrolysis of possible present polyvinyl acetate is unlikely to occur unless the distilled water did still contain some minerals which might catalyze this hydrolysis. This is, however, quite unlikely. The obvious experiment of dissolving the residue obtained after evaporation of distilled water and quantifying the degree of hydrolysis of the polymers present, was not performed.
Thus, the conclusion drawn on page 367 'This is due to the absence of mineral salts in the distilled water and the possibility of partial degradation of the capsule film under the influence of water' is thus speculative and not supported by the findings of the study.
6 - The next sentences, starting with 'It should also be noted........', are inconclusive and the conclusion ' This means that capsule F has the highest PVA content.' cannot be deduced from the fact that the weight of capsule F is highest as the actual PVA content was not quantified.
7 - The subsequent sentence is vague and inconclusive and should not be included in any scientific publication '..., which may due to the different chemical composition....'.
8 - Page 368, first part of the right-hand column '...probably based on polyvinyl alcohol...'. This is a non-conclusive and void statement with no scientific meaning. Also the term 'some other material' is too general and non-informative.
9 - Page 368 ' In contrast, dissolving the P and V material in distilled water also reduced the number of available hydroxyl groups, but to a greater extent in comparison to the V material.' I am not able to deduce what this conclusion is based on. I actually see in Figure 4 an increase instead of a reduction for sample P and only a marginal change for sample F.
10 - Optical microscopy. The images are nice but I cannot deduce what purpose they actually serve as related to the aims of the study, whilst there is a lot of speculation on the interpretation of the images. These images are exactly what one expects when examining washing water and there is, in my opinion, no linkage to the presence or impact of soluble polymers.
11 - I fully disagree on the conclusion (right-hand column of page 371) 'Despite claims of degradability of the films by the manufacturers, sediment can be observed in the pipes after usage of such pods, resulting in the formation of microplastics'.  I actually claim that this statement/conclusion is nonsense whilst it is for sure that no robust explanation for this statement is provided.
12 - I also disagree with the next conclusion 'The research conducted in this study confirmed that the studied laundry capsule films were not entirely soluble in water.'. In itself the conclusion might be correct. However, this cannot be deduced from the results generated within this study.
13 - I also disagree on the next conclusion 'It gives proof that modifications of PVA structure by producers of washing pods have an impact on their degradability in water.'. The main reason for this disagreement is that degradation was not studied whilst it is unlikely that during evaporation degradation of polyvinylacetate will take place.
14 - I fully disagree with the final conclusion. In itself, this conclusion might be true but for sure this cannot be deduced from 'Based on the results' as suggested here. Finally, the very last statement 'they are probably one of the worst ways to add detergent to washing machines.' is a private opinion of the authors and neither the experimental design nor the results obtained allow to draw this conclusion.

There are a number of weaknesses with this publication, and they have been divided into the following sections.

1. Summary of field/introduction:

Incomplete referencing. The aim of the publication is to evaluate the behaviour of the pouches of liquid detergent pods, the dissolvable wrappers that contain the liquid detergent, under laboratory conditions (water), and in real world conditions (washing machine). Do they form ‘sludge’ in pipes, and microplastics? Only 9 references are provided in the introduction, and only 4 deal specifically with the primary component of these pouches, PVA (the abbreviation for which the authors confusingly use for both polyvinyl alcohol, and polyvinyl acetate). Much of the chemistry of PVA (polyvinyl alcohol) in water, including chemical and biodegradation in wastewater and other conditions, has been studied, but none of that work is cited here. This present paper seems to ignore prior work and starts “from scratch”, which is not how to approach a scientific problem in any area. 

Poor descriptions and unsatisfactory analytical approaches. As will be described in detail, below, in the section specifically dedicated to methods, experimental details are scant and not clearly explained. The ‘grande finale’ of the papers is the optical microscopy, but taking any solution of dissolved polymer and/or salt and allowing it to evaporate on a slide will always result in visible solids - for these polymer, or polymer+dissolved mineral combinations, images of evaporated solids with a variety of shapes and morphologies due to coffee ring effects, nucleation events, and related phenomena are to be expected and tell us little about the nature of the precursor solution. 

The claims in the conclusions are not supported by the evidence, as will be described in the section below. We cannot conclude from the evidence whether the pouch films result in ‘sludge’ in pipes, or whether what we observe in the optical images are in fact microparticles. 

i) This journal is not indexed by Web of Science, meaning that it has not met the standards of publishing required by Clarivate, the owner of Web of Science. To quote Clarivate, “A robust editorial selection process, continuous curation, and meticulous indexing make Web of Science Core Collection™ the world’s most trusted publisher-independent global citation database.”
https://clarivate.com/academia-government/scientific-and-academic-research/research-discovery-and-referencing/web-of-science/web-of-science-core-collection/content-collection-and-indexing-process/

ii) The paper itself has only 11 citations. For a publication in materials science/environmental chemistry, one would expect a properly referenced publication to need 40+ citations to properly represent the state of the art, and to support claims, methods, and conclusions. Of these 11 references, the newest dates from 2021, which would not be considered up-to-date. 

iii) This publication has not been cited, as per Google Scholar.