Analysis of the chemical behavior at the molecular level of lined pipes with fluoropolymers in a sodium hypochlorite production line/bibliographic review

This case study is about finding the best fluoropolymer coating for pipes that resists the sodium hypochlorite continue production, which is one of the most aggressive chemical processes that can lead to molecular attack in reactors made by lined pipes. There are several types of coatings pipe such as fiberglass, polymers and elastomers, but the fluoropolymers which have unique properties that make them resistant to chemical attack. In this production process, the premature deterioration of coating pipes is common, due to the expansion of chlorine at the inlet of the reactor, caused by the reaction of chlorine–sodium hydroxide, this is the critical point of the process. Some problems that we find is the chemical attack in that some fluoropolymers coating suffer premature degradation caused by the chemical compatibility, in this case, we explain in detail the chemical and molecular composition of each of the fluoropolymers and how this change occurs at the molecular level. While the lined pipes are the best economical option for chemicals applications, however, it is important to know the correct coating to ensure a long lifetime and avoid piping changes due to premature degradation. Based on the findings of the chemical resistance of each fluoropolymer under study, it is determined which is the best fluoropolymer that resists continuous production of sodium hypochlorite. Results were obtained by a systematic review of the literature. Why is the dehydrofluorination process generated in fluoropolymers, how does it affect it and when does it start. Chemical compatibility of each material under review such as Polyvinylidene Fluoride, Ethylene Tetrafluoroethylene, Polytetrafluoroethylene and Perfluoroalkoxy and the reaction or molecular behavior when they come into contact with the raw materials used for NaClO production. The importance of the high manufacturing standards of coated pipe, permeability and crystallinity of the fluoropolymers under study. Why is the dehydrofluorination process generated in fluoropolymers, how does it affect it and when does it start. Chemical compatibility of each material under review such as Polyvinylidene Fluoride, Ethylene Tetrafluoroethylene, Polytetrafluoroethylene and Perfluoroalkoxy and the reaction or molecular behavior when they come into contact with the raw materials used for NaClO production. The importance of the high manufacturing standards of coated pipe, permeability and crystallinity of the fluoropolymers under study.


Introduction
The decade of the 1990s brought great advances and introduced new materials to daily life to optimize the production of many elements both daily and for industrial use. Although there have already been data on the use of fluoropolymers such as polytetrafluoroethylene (PTFE) since 1938, it is towards this decade that the greatest commercial appearance of this type of polymers occurs [1]. Consequently, there was a notorious advance at an industrial level due to the reduction of production costs, the improvements in industrial safety, operational efficiency and the reuse of elements considered polluting at the environmental level [1,2]. For example, there are applications from fields as basic as personal hygiene items such as toothbrushes or dental floss, kitchen utensils such as pans or knife handles, to the aerospace or energy industry.
Currently, the technological application of polymers is much broader and covers fields as complex as those related to the matrices used for the transport of energy, sensors, or data information. For example, Fluorinated ethylene propylene (FEP) provides excellent dielectric properties for the fast and stable transfer of information and electrical pulses [1,3]. Along the same lines, in energy and chemical industries, the effectiveness of fluoropolymers lies in their chemical and thermal resistance as well as their high resistance to corrosion [1,3,4].
Thus, in the chemical industry, an essential point to consider is the corrosion of materials. In this way, pipe's corrosion is an inevitable fact regardless of the material used and fluoropolymers are no exception in the process of sodium hypochlorite (NaCIO) production. Pipes and equipment with fluoropolymers linings fail despite having excellent chemical, thermal and mechanical properties [5]. These linings are used in equipment and pipes that are exposed to highly aggressive and corrosive fluids. Therefore, the lined pipes are a good alternative compared with the expensive metal pipes, such as highquality stainless steel, Hastelloy, titanium, and Monel. The continuous exposure of fluoropolymers to chemicals, high temperatures and the manufacturing method, among other factors, affect the integrity and quality of the internal surface of pipes and equipment lined [6].
The limitations of fluoropolymers come from their molecular behavior, this depends on the compatibility of the fluoropolymer in contact with the chemicals and the operating conditions. In addition, chemical resistance of fluoropolymers depends on the fluorine content in its molecular structure and manufacturing method [7]. Furthermore, it should be pointed out fluorine is a highly electronegative element that has a low coefficient of friction, higher thermal stability, ductility with good mechanical and chemical properties [8,9]. Few investigations explain the non-compatibility of fluoropolymers with specific chemicals or processes. It is important to understand the molecular behavior as well as the results of chemical tests explained from an engineering approach. This bibliographic review explains the chemical behavior, at the molecular level, of fluoropolymers like poly-vinyl fluoride (PVDF), ethylene-tetrafluoroethylene (ETFE), polytetrafluoroethylene (PTFE), perfluoroalkoxy alkanes (PFA).
In this way, the first goal of this review is to identify the fluoropolymer with the highest chemical resistance that supports the continuous production of NaClO, constant chemical attacks of bases like sodium hydroxide (NaOH) and halogens like chlorine (Cl 2 ) in a reactor made with lined pipes that produce NaClO.
The second objective is to go in depth the rule of Zaitsev and Hofmann's compliance that explains the dehydrofluorination effect of polymers partially fluorinated [10][11][12]. As we well know, the NaClO transportation in lined pipe is not the problem because it is a fluid that can be easily handled with others simpler polymers like polyvinylchloride (PVC), chlorinated polyvinyl chloride (CPVC) or polypropylene (PP). The critical point in the production of NaClO is when the Cl 2 enters the reactor, it expands (its expansion power is 480 times) [13], it immediately binds with the 14% NaOH and they begin to react. So, it is essential to make a correct selection of the fluoropolymer to be used in the lined pipe that will be in continuous contact with the chemical reaction of these two components.
Nowadays, the factories in Latin America (LATAM) that produce NaCIO conserve the technology from the 1970s, this one is an inefficient and very manual technology that takes up a lot of space and has very long reaction times between the Cl 2 and NaOH. For this reason, it is proposed the development of a new system which enables said production the way automated and continuous by means of reactors made of lined pipes that support the production of NaClO massively. This system would improve the efficiency, safety and cost of production. This research was born from the idea of guaranteeing a long lifetime by choosing the right coating for reactors and lined pipes used in this automated production system from NaCIO.
In the next section of this literature review, we consider the methods and strategies used to select and filter the most relevant and important articles and case studies. In the following section are the results obtained from the search in which the cases of how, when and why chemical attacks occur in the coatings and also the molecular composition of each fluoropolymer are explained in detail, in Sect. 4 we present the discussion in which a case of its own is mentioned in which the degradation of the liner (2022) 4:238 | https://doi.org/10.1007/s42452-022-05119-4 Review Paper of a pipe that was erroneously exposed to the reaction of hypochlorite vs. the liner suitable for the process is evidenced, finally the conclusions of the study are presented.

Search strategy
The bibliographic reviewing processes was conducted according to the PRISMA checklists [14], improving the results obtained through previously established research strategies. Also were included all studies of fluoropolymer degradation, dehydrofluorination of partially fluorinated polymers, manufacture of fluoropolymers, nucleophilic attack by bases and chemicals applications with fluoropolymers. The literature review was obtained from the Cochrane Database of Systematic Reviews, Scopus and Springer Nature, delimiting the searching to the last 10 years, including keywords such as fluoropolymers, dehydrofluorination, chemical attacks, chemical compatibility and uses of lined pipes in chemical applications. Finally, the articles were separated by research journals and verified in SCJ ranking.

Study selection with the inclusion and exclusion criteria
In this research only were considered articles published in English. It was obtained 398 reviews between articles and books which talk about fluoropolymers in general, fluoropolymers applications in the industry, fluoropolymer manufacturing methods, quantitative studies supported by a laboratory or specialized organization, degradation of polymers in contact with alkaline fluids, acids or halogens, dehydrofluorination effect, thermal stability of fluoropolymers and crystallinity of fluoropolymers. Studies excluded were thermoplastics, polymers without fluorine in their molecular structure, empirical quantitative studies not supported by a laboratory or organization and degradation or dehydrofluorination of fluoropolymers attacked by compounds different to bases, acids and halogens.

Data extraction
In the pre-selection stage, only were considered the articles and books titles and abstracts. Subsequently a detailed review of the article was performed and the final selection was done according to the inclusion and exclusion criteria. The articles were uploaded to the bibliographic manager Mendeley and were separated by journals and books by authors.

Bias risk assessment
Researchers used the Newcastle-Ottawa tool to assess the quality of each article and minimize the bias risk, also a review of each journal in the SCJ ranking was also carried out to identify the quartiles of each study.

Search results
According to the aforementioned inclusion and exclusion parameters, in Fig. 1 it can be seen that 354 articles were obtained. Due to this, a new exclusion was carried out verifying the quantitative studies supported specifically by specialized laboratories or organizations. Thus, 48 articles were found, 30% books of interest, 47% of the articles are Q1, 14% articles Q2 and 9% articles Q3, finally 8 reviews were considered for meta-analysis.

Results
Fluoropolymers in which the fluorine atom (F) is replaced by hydrogen atoms (H) in their molecular structure suffer several impacts. This change is the main cause of chemical attacks caused by strong bases such as NaOH with a pH of 14 [5,9,15], accompanied by one of the strongest and most reactive halogens such as Cl 2 . These two are used as raw materials to manufacture NaClO in the reactor which is a strong base with a pH of 12 approximately. All fluoropolymers studied in this systematic literature review are semi-crystalline, therefore they have a crystalline phase and an amorphous phase [16]; this crystallinity of partially fluorinated polymers is directly related to chemical resistance [17]. Whenever the molecular structure of the PVDF and ETFE is attacked, its degradation comes from in the dehydrofluorination [18] caused by deprotonation suffered by the hydrogen atoms causing a rearrangement of the carbon atoms (C) [12]; the dehydrofluorination fully complies with Zaitsev's rule and partially Hofmann's rule due to the elimination reaction E2 that will be explained later [9][10][11].
However, it is explained how a chemical attack is evidenced in pipe coatings, either because of molecular permeation or low manufacturing standards, the role that crystallinity has in partially fluorinated polymers is also explained. Subsequently, it is explained in detail the chemical compatibility of each material under review such as PVDF, ETFE, PTFE and PFA and the reaction or molecular behavior when they come into contact with the raw materials used for NaClO production.

Chemical attack
A chemical attack is visually evidenced by blistering, swelling of the lined surface, color change of the fluoropolymer [19], localized polymerization, environmental stress cracking (ESC), oxidation and general degradation [5]. The fluoropolymers degradation starts always under an alkaline environment (generally NaOH) [7]; it is important to mention that all polymers are permeable, and this permeation is defined as the transit of a gas or a liquid through a material (solid). In this point the first Fick's law (derived from Fick's first law for the masses) is fulfilled considered the following factors such as chemical concentration, temperature, pressure, nature of the polymer, polymer thickness and crystallinity obtaining a diffusion coefficient as a result [20]. The permeation speed slows down as the thickness of the lining increases, it is very fast in very thin thicknesses, but with increasing thickness it is evident that the permeation speed is much lower [20,21], their higher or lower value of degradation depends on the manufacturing method, pressure, temperature and time of contact with the chemical.
Lined pipes with PTFE and PFA must have a venting at the end required by ASTM F1545 [22], due to their high permeability compared to other polymers. Therefore, these pipes have those venting that allow gases to escape at the molecular level [23], otherwise it increases the risk of collapse or internal detachment of the lined, as higher temperature and pressure lead to greater permeation [24].
The crystallinity has a very important role in partially fluorinated polymers: a high percentage of crystallinity increases chemical resistance and thermal stability. The crystalline phase in the molecular structure avoids that the fluid penetrates easily and attacks carbon atoms due to the fluorine of its molecular structure does not cover the carbon atoms entirety. It should be considered that the higher degree of crystallinity the less permeable is the fluoropolymer [25] but at the same time it increases the probability of rupture caused by stress, that is to say, less resistance to impact [23,26]. It must be considered the stress the fluoropolymer will be subjected to avoid premature failure due to ESC [27], if the process has temperature changes or if the internal (fluid) and external (ambient) temperature differential is very large, a polymer with a high percentage of crystallinity is not recommended. In addition to the pressure, the pipe is subjected to any water hammer that may occur in there. Unlike perfluoropolymers, the higher chemical resistance is a result of the higher fluorine content in its molecular structure, considering that it has lower degree of crystallinity and greater molecular permeation [5,20,28].

Polyvinylidene fluoride (PVDF)
Polyvinylidene Fluoride-(C2H2F2)n Fig. 2, is a partially fluorinated homopolymer with a fluorine content of approximately 59.4% [29]. This one is manufactured from the polymerization of vinylidene fluoride forming an extended zigzag molecular structure. If the percentage of crystallinity is about 52% [30], the maximum operating temperature is between − 28 and 120 °C [6,30]. The fluorine content allows it to be a fluoropolymer with good chemical resistance, thermal stability, and a relatively low coefficient of friction [31,32], but usually unstable at high temperatures, and they should be used under moderate conditions [32]. The hydrogen content makes it vulnerable to chemical attacks allowing to the elimination of fluorine of its molecular structure [29]. This process is called dehydrofluorination and it is an autocatalytic process [12], which means that once one fluorine ion is removed, the second one is easily removed and is replaced by a proton [19,33].
The Polyvinylidene Fluoride suffer chemical attack by strong bases such as NaOH [15], allowing electrons to transfer to another receptor hydrogen atom. The hydroxide is inserted into the nucleus of the hydrogen giving an electron to the hydrogen proton that forms a bond with the carbon β (CH2) [27,34]. Through this bond, the hydroxide makes a nucleophilic attack [35] to the β-carbon, causing the formation of double bonds between the carbon β and the carbon α (CH=CF2) that contains fluorine (CF2) [12]. In this way, the carbon α no longer requires the electron of the fluorine and thus breaks the bond with the fluorine (CH=CF), fully complying with the Zaitsev rule [9][10][11]. In other words, the E2 elimination reaction happens when, one of the hydrogens on the carbons adjacent to carbon α lose its nucleus to form a double bond with carbon α (C=C) [7,15,36]. Hydrogen is the weakest atom prone to a nucleophilic attack [34], due to that its electronegativity is 2.1 in the Pauling scale. This hydrogen bonds with an atom carbon of 99.5 kcal/mol [37], which has an electronegativity of 2.5 Pauling. This is compared with a stronger bond between the fluorine and carbon [38] equivalent to 116 kcal/mol due to the fluorine's electronegativity of 4 Pauling, resulting in higher electron density [25,37].
Hydroxide attacks hydrogen because its electronegativity is equivalent to 3.44 Pauling [39,40]. It should be noted that to higher concentration of NaOH and higher temperature loses hydrogen atoms more easily [39] and leads to the elimination of fluorine due to the formation of double bonds (C=C) [7,23,40]. Hoffman's rule is partially fulfilled when the oxidation reaction is initiated after dehydrofluorination in partially fluorinated polymers. Oxidation occurs under conditions of high temperature or alkali concentration (sodium hydroxide at 14%). The oxidation effect also happens in some cases by direct exposure of the UV polymer converting some carbon double bonds into hydroxyl groups [10,12], this reaction is also bimolecular (E2) [11].
Polyvinylidene Fluoride resists chlorine if protected from UV exposure [41], conversely, if a PVDF pipe is directly exposed to UV with fluid of chlorine, the chlorine reacts to free radicals due to high reactivity and inserts into the molecular structure and replacing the hydrogen [42]. This elimination causes polymer oxidation represented by the color change and the pipe becomes highly crystalline [32], susceptible to ESC due to the oxidation reaction of carbons [27,30]. In this case, there are no problems because the PVDF is used like coating and it is protected by the external pipe, avoiding the PVDF has oxidation problems.

Ethylene tetrafluoroethylene (ETFE)
Ethylene Tetrafluoroethylene-(C4H4F4)n- Fig. 3, is a modified copolymer of tetrafluoroethylene with a fluorine content of 59.4% and 55% crystallinity [29], the maximum operating temperature is between − 28 and 150 °C [24]. The polymer ETFE has excellent properties such as weather resistance, high thermal stability, chemical resistance, and special dielectric properties [43]. Its high degree of crystallinity and polymerization allows ETFE to be chemically more resistant than PVDF, however, it is susceptible to strong oxidants, chlorinated solvents, sulfonic acids, and some amines [26,35,44]. Ethylene Tetrafluoroethylene is also susceptible to thermal oxidation for its hydrogen content in its molecular structure [34]. Also, it is susceptible to chemical attack which causes slow dehydrofluorination but impedes autocatalytic processes because crystalline parts are more difficult to be penetrated. In addition, to being a polymer derived from tetrafluoroethylene, makes it stronger for being a powerful dienophile compared to vinylidene fluoride, which is a thermoplastic [33].

Polytetrafluoroethylene (PTFE) and perfluoroalkoxy (PFA)
Polytetrafluoroethylene (C2F4)n Fig. 4, is a Perfluoropolymer with no risk of dehydrofluorination. It is formed with fluorine and carbon atoms, obtained through the tetrafluoroethylene monomer by vinyl polymerization of free radicals. The maximum operating temperature is between − 28 and 260 °C [6,45]. Polytetrafluoroethylene has a crystallinity of approximately 41% with a content of 76% fluorine [29]. Due to the size of the fluorine atom is larger than hydrogen atoms, it can completely cover the carbon atom and avoid chemical attack because it is immiscible with the protonated material [37,46]. However, Perfluoro polymers are susceptible to high permeability [20], they are susceptible to fluorine gas, liquid sodium, molten alkali metals, causing electrolytic attack [24], due to their lack of double bonds and, unlike polymers partially fluorinated that receive a nucleophilic attack. The Polytetrafluoroethylene or Perfluoroalkoxy color change indicates degradation of the polymer by defluorination that incurs the oxidation of molecules under extreme temperature conditions and long exposure periods [37]. Polytetrafluoroethylene is the Perfluoropolymer with greater thermal stability, excellent resistance to chemical attack, high melting temperature and low surface energy since its coefficient of friction is very low [6,9,37]. Polytetrafluoroethylene is heterogeneous for having crystalline domains in an amorphous matrix; therefore, here the velocity permeation depends on the temperature to which it is exposed, at a higher temperature the helical structure is separated [35]. Polytetrafluoroethylene has different changes at the molecular level as a function of temperature. The temperature makes its helical structure to have a rearrangement of the molecules, these movements are divided into IV phases, but only until the IV phase are observe significant movement in the amorphous matrix of its molecular structure [16]. The phase II reports that up to a temperature of 19 °C the molecular structure of the PTFE is ordered [47] and contains 13 units of CF2 for every 180° to complete turn its molecular structure, the helixes of which have a rotation angle of 13.8° and a distance between molecules of 1.69 nm [23,24,31]. But when the temperature increases to 30 °C change to phase IV begins and a crystalline disorder is formed, increasing to 15 units of CF2 for every 180° C, the distance between the molecules increases to 1.95 nm [20,37].
This molecular movement is generated in the amorphous matrix within the crystal and the dependence of the cooling rate on the manufacturing method. If the cooling time in the manufacturing of PTFE decreases, the molecular structure will be more separated [48].
Perfluoropolymer PFA, Fig. 5, is a copolymer manufactured from the polymerization of TFE with perfluoroalkyl vinyl ether (PPVE). This PFA contains about 1% of mol of PPVE to introduce an additional chain through the union of 1 oxygen atom relieving the tension of tertiary carbon [24]. The result is a perfluoro polymer with greater mechanical resistance and chemical resistance like PTFE [45]. The maximum operating temperature is between − 28 and 260 °C [23,24], due to its oxygen atom. The fluorine content is 73.4% and has a crystallinity of approximately 40% [17,29] and it is characterized for having better mechanical resistance than PTFE [49], due to its lower degree of crystallinity produced by ether [37]. Perfluoroalkoxy has a lower molecular weight than PTFE [35] and is also susceptible to fluorine gas, liquid sodium, and molten alkali metals. It is important to understand that its high permeability does not cause any damage to the overall structure of the polymer, for example a chlorine atom, can takes the place of a fluorine atom on a PTFE or PFA polymer chain on the surface of lined. It can then jump from there to a PTFE or PFA molecule further into the structure, and so on through the entire thickness of the material. It should be noted that this occurs at a molecular level, this transfer of individual atoms between molecules is normal and occurs in the amorphous phase of the lined. In the case the polymer meets some substance to which it is vulnerable, it is chemically attacked and generated cracks and voids in the bulk polymer [26,37].

Discussion
The image presented in Fig. 6 shows the physical changes in a Polyvinylidene Fluoride pipe without external protection. It is observed some changes because of the reaction occurred after 5 batches during the production of sodium hypochlorite (NaClO): the pipe swelling and the color change of the fluoropolymer, both are signs of the chemical attack produced by NaOH in a PVDF pipe. The effect of the NaOH solution permeated the pipe is the premature degradation and it is evidence that the material could not resist completely the counterproductive reaction. The blisters in a coating pipe like PVDF means chemical attacks produced by dehydrofluorination, due to in the molecular structure of PVDF (Fig. 2) has 2 H atoms, which makes PVDF vulnerable to chemical attacks from NaCIO, because the electronegativity of hydroxide is greater than that of hydrogen, the electronegativity of hydroxide is equivalent to 3.44 Pauling vs 2.5 Pauling of hydrogen, this is the reason for allowing elimination of fluorine from its molecular structure, the NaOH hydroxide inserts into the hydrogen nucleus by giving an electron to the hydrogen proton which forms a bond with the β-carbon. In this way, the α-carbon no longer requires the fluorine electron and therefore breaks the bond with the fluorine and the blisters seen in Fig. 6 are formed, because fluorine is removed from the molecular chain. In this way it is evident that this pipe does not support the mentioned chemical attack.
On the other hand, Fig. 7 shows a pipe covered with Polytetrafluoroethylene liner from a reactor that produces NaClO, this section of pipe has been producing NaClO for more than 25 years, the Polytetrafluoroethylene pipe does not have a physical change in the liner or detachment internal. In addition, the brown color is given by the oxidation of chlorine during the reaction, however, this does not molecularly affect the polymeric chain of PTFE, Excellent results have been obtained with the use of fully fluorinated fluoropolymers in critical corrosive applications.
Polytetrafluoroethylene over time has shown that it has high durability compared to other coatings pipes in different production processes when exposed to strong chemical attack. The reason why PTFE or PFA is immiscible to suffer chemical attacks by dehydrofluoration or nucleophilic attacks by NaCIO or NaOH is because its molecular structure is formed only by fluorine and carbon atoms. The fluorine atom completely covers and protects the carbon atom making it immiscible to the protonated material.
Fluorine is the strongest atom because the electronegativity is 4 Pauling, when the fluorine atom forms a molecular structure with the carbon atom which has an electronegativity of 2.5 Pauling, it is form a very strong bond equivalent to 116 kcal/mol which is difficult to break. However, it is important to consider that in order to obtain a long lifetime for these accessories and pipes, it is not only a matter of choosing the appropriate coating, but also necessary to incorporate the technology used by the manufacturers, the type of anchorage used between the liner and the metal pipe, the manufacture and molding of the resin, the heating and cooling times of the coating. In addition, careful material processing, coating thickness, and rigorous quality testing must be considered, all of which provide confidence that the coating will have a good performance under all process conditions. To mention an example, it has been shown that in different production processes a pipe with an isostatically anchored liner does not have the same performance as a pipe with a floating liner. Also, consider that pipes and accessories that are going to be acquired for any production process must comply with the certification ASTM F1545, this certification guarantees that the pipe has the minimum quality standards that a coated pipe must have to guarantee high performance, system reliability, and high lifetime.

Conclusion
Corrosion in fluoropolymers is generated mainly in the following ways: degradation by nature, permeation, premature degradation because of the manufacturing method with low-quality standards, oxidation from dehydrofluorination in partially fluorinated polymers [50], defluorination in perfluorinated polymers generated by chemical attack and radiation and thermal degradation that implies depolymerization [27,28].
A partially fluorinated polymer degradation is generated in the following order: phase I, dehydrofluorination (hydrogen elimination), phase II, nucleophilic attack (formation of double bonds between carbons eliminating fluorine) [7,34] and in some cases occurs the phase III, oxidation (degradation of the polymer making it highly crystalline) [32,36]. These steps of degradations would be with PVDF-lined pipe if is selected as lined pipe to produce NaClO in the reactor. In the other hand, the ETFE-lined pipe can support the reaction; however, it suffers deterioration before the pipe with liner in PTFE or PFA due to the lower degree fluorine content in its molecular structure.
Consequently, Teflon PTFE is the pipelining selected to support the continuous production of NaClO in a reactor without molecular changes. Its lifetime is not affected due to its chemical and mechanical resistance, compared with all the partially fluorinated polymers. The Polytetrafluoroethylene lined pipe is recommended for highly corrosive applications [17,48].
Perfluoroalkoxy has better mechanical resistance compared to Polytetrafluoroethylene and similar chemical resistance [17,29,43], it is also a good choice to be used as a lined pipe in the manufacture of NaClO. It should be noted that the ETFE lined pipe is compatible with NaClO, liquid chlorine and sodium hydroxide, but it is evident that deteriorates faster than PTFE due to its hydrogen content in the molecular structure [23,38,43,51].
Finally, the Polyvinylidene Fluoride lined pipe is not recommended for strong bases such as NaOH because it suffers a chemical attack caused by dehydrofluorination [7,29,52]. It should be noted that it does not have a compatibility problem with liquid chlorine [15,[30][31][32][33]52].
Nowadays, science is constantly evolving on Polytetrafluoroethylene, trying to find the best molecular modification of polytetrafluoroethylene that can overcome its few limitations which aim to improve permeation trying to exceed the performance in general of Teflon PTFE.
Author contributions All the authors contributed to the study conception and writing the work, the idea for the article was presented by AABR, the literature search and data analysis were performed by AABR and LPR, this manuscript was drafted by AABR, corrected LFAF and AABR and critically revised the work LPR and LFAF.
Funding The authors have not disclosed any funding.

Conflict of interest
The authors declare that they have no conflict of interest.
Ethical approval Not applicable.

Consent to publish Not applicable.
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