A New Perspective on Hydrogen Chloride Scavenging at High Temperatures for Reducing the Smoke Acidity of PVC in Fires—III: EN 60754-2 and the Species in Solution Affecting pH and Conductivity

Abstract

In the European Union, Regulation (EU) No 305/2011, in force since 2017 as CPR, requires the classification of cables permanently installed in buildings for reaction to fire, smoke, flaming droplets, and acidity. The latter is an additional classification evaluated through EN 60754-2, involving pH and conductivity measurements. Acidity is the weak point of a PVC cable due to the release of HCl during the combustion. Low-smoke acidity compounds, containing potent acid scavengers at high temperatures, are developed to reduce the acidity of the smoke. In order to design proper HCl scavengers to be used in PVC low-smoke acidity compounds, it becomes essential to evaluate the main actors affecting acidity and conductivity. In this paper, different cable PVC compounds were tested carrying out EN 60754-2 at different temperatures and temperature regimes: measurements of pH and conductivity were compared with ions’ concentration determined by ion chromatography, according to ISO 10304-1 and ISO 14911 for anions and cations, and inductively coupled plasma–optical emission spectrometry, according to ISO 11885. The conclusive results emphasize that HCl from PVC compounds’ thermal decomposition is the primary driver of pH and conductivity, and the contribution from the evaporation and or decomposition of additives and by-products from combustion is found to be negligible in most of the tested PVC compounds for cables. The findings highlight the effectiveness of ion chromatography and inductively coupled plasma–optical emission spectrometry as powerful analytical tools for developing efficient acid scavengers capable of maintaining performance at elevated temperatures. A further outcome regards the experimental demonstration of the limits and incongruencies of EN 60754-2 as an instrument for assessing the additional classification for acidity for cables. Finally, a statistical method to understand through pH and conductivity measurements if the scavenging mechanism acts in the condensed phase is presented.

1. Introduction

1.1. The Context of the Research

1.1.1. General Overview of the Topic of Acid Scavenging of HCl and Regulatory Context in the European Union

In the European Union (EU), several standards for assessing acidity and corrosivity of the fumes in case of the combustion of cables are performed (Table 1). Some of them aim to classify the cable in terms of acidity, and their use will be linked to the specific locations depending on their fire risk assessment, like, among others, apartments, parking lots, tubes, galleries, airport terminals, hospitals, cinemas, or theatres, where low-smoke acidity is required for safety in case of fire (see Section General Overview).

Other standards are carried out for the definition of halogen-free compounds. An assessment is needed to establish in which cables these compounds must be utilized for acidity or corrosivity aspects linked to their application.

In this context, the research on low-smoke acidity PVC compounds plays a critical role in developing compounds for cables capable of reaching the best acidity classification.

This paper is divided into two parts. The introduction described the primary standards assessing acidity/corrosivity/halogen content, indicating their characteristics, weaknesses, status, and use.

The second part of the article, the experimental one, focuses exclusively on the species affecting pH and conductivity when EN 60754-2 [1] is carried out. EN 60754-2 is the standard used in the EU for assessing the additional classification for acidity of cables permanently installed in buildings according to Regulation (EU) No 305/2011 (the Construction of Product Regulation or CPR) [2], which rules the marketing of building and construction products in the EU. Analyzing the species in solutions and how much they affect pH and conductivity, obviously imparting a specific classification for acidity, is crucial in designing acid scavengers capable of trapping HCl and manufacturing low-smoke acidity PVC compounds. The research on low-smoke acidity PVC compounds for cables is inserted in the regulatory framework described in Ref [3].

This paper is Part III and the last one of the group of articles: “A New Perspective on Hydrogen Chloride Scavenging at High Temperatures for Reducing the Smoke Acidity of PVC in Fires”, focusing on the species affecting pH and conductivity in solution when a PVC compound for cables burns. The context of the entire research is arranged in the following papers, which integrate and are completed by the experimental part of this article.

Part I gives an overview of the theory of acid scavenging at high temperatures acting in the condensed phase. It describes the test methods used for assessing the acidity and lays down the EU Regulatory Status in terms of construction products and additional classification for acidity [4]. Part II describes the behavior of some acid scavengers acting at high temperatures in the condensed phase [5]. Part IV shows the impact of the acid scavengers on the flame-retardant properties and the smoke production, highlighting how the more the efficiency of an acid scavenger, the more the heat release and the smoke production [6]. Part V focuses on the impacts of temperature and temperature regimes on the emission of HCl in the gas phase when EN 60754-2 is performed [7].

1.1.2. Main Acidity and Corrosivity Standards for Cables in the European Union (EU) and the Application Field

General Overview

In the EU, several standards assess the corrosivity and acidity of fumes released from cables in case of fire. Table 1 and Table 2 show the main ones and their current status. Appendix B provides a detailed description of the application scope, and the procedures employed.

Table 1. Standards used for detecting halogen and acidity/corrosivity of the smokes in the EU, their status, the product standard impacted by them, their current status, and test apparatus.

EU Standard	Use	Product Standard	Current Status	Test Apparatus
EN 60754-1:2014+A1:2020 [8]	Assessment of halogens 1	EN 50525-1:2011+A1:2022 Annex B [9], EN 50620:2017+A1:2021 [10], EN 50618:2014 [11]	Active	Furnace tube, titration
EN 60754-2:2014+A1:2020	Assessment of halogens 1	EN 50525-1:2011+A1:2022 Annex B, EN 50620:2017+A1:2021, EN 50618:2014	Active	Furnace tube, pH, conductivity
EN 60754-2:2014+A1:2020	Additional classification for acidity	EN 50575:2014+A1:2016 [12]	Active	Furnace tube, pH, conductivity
EN IEC 60754-3:2019 [13]	Additional classification for acidity	EN 50575:2014+A1:2016	Under evaluation	Furnace tube, ion chromatography
EN 50267-2-1:1998 [14]	Assessment of halogens 1	EN 50525-1:2011 Annex B [15]	Withdrawn in 2022 1	Furnace tube, titration
EN 50267-2-2:1998 [16]	Assessment of halogens 1	EN 50525-1:2011 Annex B	Withdrawn in 2022 1	Furnace tube, pH, conductivity
IEC 60684-2:2011 [17]	Assessment of halogens 1	EN 50525-1:2011+A1:2022 Annex B	Active	F− selective electrode
EN 50267-2-3:1998 [18]	Additional classification for acidity before 2014	EN 50575:2014+A1:2016	Withdrawn in 2022 1	Furnace tube, pH, conductivity

¹ EN 50267-2-1 and EN 50267-2-2 were referenced in EN 50525-1:2011, annex B. However, they were withdrawn in 2022. In the integration of EN 50525-1:2011, EN 50525-1:2011/A1:2022, EN 60754-1, and EN 60754-2 are mentioned instead.

Table 1. Standards used for detecting halogen and acidity/corrosivity of the smokes in the EU, their status, the product standard impacted by them, their current status, and test apparatus.

EU Standard	Use	Product Standard	Current Status	Test Apparatus
EN 60754-1:2014+A1:2020 [8]	Assessment of halogens 1	EN 50525-1:2011+A1:2022 Annex B [9], EN 50620:2017+A1:2021 [10], EN 50618:2014 [11]	Active	Furnace tube, titration
EN 60754-2:2014+A1:2020	Assessment of halogens 1	EN 50525-1:2011+A1:2022 Annex B, EN 50620:2017+A1:2021, EN 50618:2014	Active	Furnace tube, pH, conductivity
EN 60754-2:2014+A1:2020	Additional classification for acidity	EN 50575:2014+A1:2016 [12]	Active	Furnace tube, pH, conductivity
EN IEC 60754-3:2019 [13]	Additional classification for acidity	EN 50575:2014+A1:2016	Under evaluation	Furnace tube, ion chromatography
EN 50267-2-1:1998 [14]	Assessment of halogens 1	EN 50525-1:2011 Annex B [15]	Withdrawn in 2022 1	Furnace tube, titration
EN 50267-2-2:1998 [16]	Assessment of halogens 1	EN 50525-1:2011 Annex B	Withdrawn in 2022 1	Furnace tube, pH, conductivity
IEC 60684-2:2011 [17]	Assessment of halogens 1	EN 50525-1:2011+A1:2022 Annex B	Active	F− selective electrode
EN 50267-2-3:1998 [18]	Additional classification for acidity before 2014	EN 50575:2014+A1:2016	Withdrawn in 2022 1	Furnace tube, pH, conductivity

¹ EN 50267-2-1 and EN 50267-2-2 were referenced in EN 50525-1:2011, annex B. However, they were withdrawn in 2022. In the integration of EN 50525-1:2011, EN 50525-1:2011/A1:2022, EN 60754-1, and EN 60754-2 are mentioned instead.

Table 2. Some standards used for detecting the smoke’s halogen and acidity/corrosivity in the EU and the corresponding IEC standards.

IEC Standard	EN Standard	Amendments	Amendments
IEC 60754-1:2011 [19]	EN 60754-1:2014 [20]	IEC 60754-1/AMD1:2019 [21]	EN 60754-1+A1:2020
IEC 60754-2:2011 [22]	EN 60754-2:2014 [23]	IEC 60754-2/AMD1:2019 [24]	EN 60754-2+A1:2020
IEC 60754-3:2018 [25]	EN IEC 60754-3:2019

Table 2. Some standards used for detecting the smoke’s halogen and acidity/corrosivity in the EU and the corresponding IEC standards.

IEC Standard	EN Standard	Amendments	Amendments
IEC 60754-1:2011 [19]	EN 60754-1:2014 [20]	IEC 60754-1/AMD1:2019 [21]	EN 60754-1+A1:2020
IEC 60754-2:2011 [22]	EN 60754-2:2014 [23]	IEC 60754-2/AMD1:2019 [24]	EN 60754-2+A1:2020
IEC 60754-3:2018 [25]	EN IEC 60754-3:2019

The standards in Table 1 and Table 2 are mainly used in the EU applying CPR for building and construction products marketing and in the halogen assessment according to EN 50525-1 [9], which lays down the procedure described in Annex B for assessing if a cable can be defined “halogen-free”.

Construction Product Regulation and the Additional Classification for Acidity

CPR, in force since 2017, lays down harmonized conditions for marketing construction products in the EU. Cables permanently installed in buildings are construction products that must be classified in terms of burning behavior, smoke production, flaming droplets, and acidity, according to the second basic requirement for construction works of CPR. Several classes and additional classifications are defined according to EN 13501-6 [26] and, depending on the fire risk assessment of the location, cables with a specific class and additional classifications must be chosen through the following rule: the higher the fire risk, the more performant the cable. The additional classification for acidity requires three classes: a₁, a_2, and a₃ (the worst). Their assessment is determined by EN 60754-2, which uses a tube furnace where 1 g of compound is burnt between 935 °C and 965 °C. Fumes are collected in bubblers where the pH and conductivity of the solution are measured.

Table 3. Additional classification for acidity. Requirements from EN 13501-6. pH and conductivity must be measured through EN 60754-2.

Additional Classification for Acidity	pH	Conductivity [μS/mm]
a1	>4.3	<2.5
a2	>4.3	<10
a3	≤4.3	≥10

shows the requirements for meeting the specific additional classification for acidity.

In the EU, the additional classification for acidity is required only for cables and not for other building and construction products such as, among others, flooring, panels, wall covering, and linear insulation for pipes, according to EN 13501-1 [27].

The Halogen Assessment According to EN 50525-1

According to Annex B of EN 50525-1, the halogen assessment was developed to evaluate if a compound can be defined as “halogen-free” and, therefore, utilized in some applications where low halogen content/acidity/corrosivity smokes are required. The layout is described in

Table 4. Stages in the scheme of Annex B-2 in EN 50525-1.

Stages	Test Method	Measurement	Result	Outcome
Stage 0	EN 50525-1	Halogen: fluorine,		If negative: stop test
	Annex C	chlorine and bromine		No further test is needed. Accept material
				If positive: continue with stage 1
Stage 1	EN 60754-2	pH	<4.3	Reject material
			>4.3	Evaluate conductivity
		Conductivity	<2.5 microS/mm	Accept material. No further testing is needed
		Conductivity	>10 microS/mm	Reject material
		Conductivity	>2.5 microS/mm but <10	Test with EN 60754-1
Stage 2	EN 60754-1	Chlorine and bromine Content expressed as HCI	>0.5%	Reject material
			<0.5%	Test to EN 60684-2
Stage 3	EN 60684-2 [28]	Fluorine content	>0.1%	Reject material
			<0.1%	Accept material

. Annex B is recalled from other standards which require halogen-free compounds in their cables, such as EN 50620 (charger for electrical vehicles) [10] and EN 50618 (solar cables) [11].

Currently, no PVC compound can overcome the sequential stages in Table 4. When the request for halogen assessment is mentioned in a specific standard, PVC compounds are automatically excluded.

pH and Conductivity Measures and Their Incongruences

If the solutions’ pH is directly linked to the acidity or basicity of the dissolved substances, the conductivity is only related to the concentration of electrolytes in there. Therefore, the conductivity increases only as the electrolytes in solution, basic, acidic, and neutral, rise in concentration, while pH decreases if the species are acidic and increases if they are basic. PVC compounds release hydrogen chloride (HCl) during combustion. HCl reaches bubbling devices, reducing pH and increasing the conductivity as its concentration rises. Theoretically, even neutral by-products of the PVC compound combustion, like MgCl₂ and ZnCl₂, if they do not hydrolyze before, can evaporate at the test temperature of the standard. If so, they cannot affect pH but can impact the conductivity. Therefore, in a PVC compound, pH and conductivity can be influenced by several reactions if their by-products can reach the bubblers. However, the PVC thermal decomposition through zip-elimination is crucial, bringing a massive quantity of HCl to the solutions. In other polymers, different things happen, depending on the nature of the polymer and its additives. For example, thermoplastic polyurethanes (TPUs) are not inherently flame retarded as with PVC, and they need flame retardants to improve fire performance to meet the requirements of specific standards in terms of reaction to fire. Melamine used in some intumescent systems as a component of flame retardant decomposes, emitting ammonia, which evolves during the combustion. Ammonia affects pH but simultaneously increases the conductivity of the solutions in which fumes are dissolved. Contrary to PVC compounds, in some flame-retarded TPUs, pH and conductivity increase simultaneously as the concentration of intumescent flame retardant increases. The paradox in some technical standards requiring halogen assessment is that some halogen-free compounds could bring basic solutions, but, having a conductivity of more than 10 μS/mm, they would not pass the halogen-free status, according to Annex B of EN 50525-1. Sometimes, these paradoxes have been resolved in other standards with special derogation decided in the technical committees and the specific working groups. For example, halogen-free TPUs not capable of reaching a conductivity of lower than 10 μS/mm would be non-halogen-free compounds, according to Annex B of EN 50525-1. That is why in EN 50620, which regulates charging cables for electric vehicles and where mainly flame-retarded TPU compounds are utilized in jackets, the limits of conductivity were increased from 10 μS/mm to 40 μS/mm to pass the assessment. In this case, the standard itself has been sized on the best technical achievement of the compound as the best compromise between fire performances, pH, and conductivity.

1.2. Thermal Decomposition and Combustion of PVC Compounds

PVC compounds have a complicated pattern of thermal decomposition depending on the specific additives used to characterize them. For instance, a PVC compound utilized for producing CPR cables, depending on their classification and additional classifications, may incorporate varying proportions of suspension PVC K70, a general-purpose plasticizer (GPP) like DINP, a 70 °C thermal stabilizer, filler like calcium carbonate, flame retardants such as antimony trioxide (ATO) and zinc borate (ZnBO), and flame-retardant fillers like magnesium hydroxide (MDH) or aluminum trihydroxide (ATH). Ref. [6] describes the thermal decomposition and combustion of such PVC compounds in detail. Running through, two main stages are found. The first stage starts around 220 °C and ends at 350 °C. Here, HCl is released following the zip elimination of HCl, and polyene sequences are formed. They can arrange intramolecularly, yielding benzene or intermolecularly bringing the matrix’s crosslink [29,30,31,32,33]. In this stage, besides benzene, water from ATH and MDH, CO₂ from the reaction of CaCO₃ with HCl, and plasticizers are released in the gas phase. All HCl is released in the first stage. The second stage starts over 450 °C, where the crosslinked matrix releases moieties (mainly aliphatic hydrocarbons) and yields a black charred mass in the condensed phase. If the thermal decomposition and combustion pattern depends on the specific ingredients in the PVC compound, HCl will be released mainly between 220 °C and 350 °C.

Therefore, a good HCl scavenger must be efficient in the first stage to minimize the release of HCl in the gas phase from the PVC compound. However, also the “stability” of the reaction products coming from the acid scavenger must be high, avoiding decompositions or evaporation capable of affecting the pH and conductivity of the solutions in the bubblers according to EN 60754-2. Specifically, the reaction products must be stable up to 965 °C, i.e., the maximum temperature of that standard, and the species capable of affecting pH and conductivity should be carefully evaluated.

1.3. Experimental Part and the Objectives of the Research

Exploring the concentrations of anions and cations in solution is crucial in researching and developing novel acid scavengers at high temperatures capable of capturing HCl evolving during the combustion of PVC compounds to meet the best additional classifications for acidity, according to CPR.

The standards based on the analysis of anions are IEC EN 60754-3 and ISO 10304-1 [34], which are based on the ion chromatography technique (IC). However, they are used only at low-level halogen concentrations (below 0.5%).

In this paper, we adopted IC to verify if it can be a quick and precise tool used for PVC compounds to detect halogens at higher concentrations than 0.5% and even to dose other ions, potentially affecting pH and conductivity as halogens do.

This research tested different kinds of PVC compounds for cables, applying EN 60754-2 at 950 °C and with the ramp of EN 60754-1 (20 min from room temperature to 800 °C and 20 min at 800 °C). The smoke collected in the bubblers was not only characterized by pH and conductivity, as assessed by the reference standards. The smoke so collected was also analyzed employing IC. Inductively coupled plasma–optical emission spectrometry (ICP-OES) was also carried out to complete the elemental analysis focusing on those elements (not detected by IC) capable of reaching the bubbling devices through evaporation of the molecules containing them. Such an analysis of anions and cations allows the understanding of the main drivers of pH and conductivity, helping to understand the contribution of the additives involved in each compound.

In addition, these different kinds of cable PVC compounds are compared regarding pH and conductivity with some common cable compounds, some PVC-based, and one halogen-free-based (TPU-FR compound).

Finally, the article proposes a method, described in Annex B, to understand if the scavenging acts in the condensed phase without the contribution of substances evaporating in the gas phase and conveyed in the bubblers. From pH and conductivity measurements, there is the opportunity to understand whether the mechanism is in the condensed phase or if gas-phase reactions are activated, resulting in deviations from the Debye–Hückel–Onsager (DHO) equation for HCl solutions. Using the DHO equation, the theoretical conductivity can be mathematically correlated with the HCl’s pH in water solutions. This is the DHO model for the ideal aqueous HCl solutions. Deviations between the measured pH and conductivity, as determined by EN 60754-2, and the DHO model, suggest the involvement of additional electrolytes. The correlation between pH and conductivity observed experimentally in tested PVC cable compounds indicates if the mechanism primarily occurs in the condensed phase with negligible contributions from volatile species transported into the bubblers. Some PVC compounds, including a PVC compound jacket for plenum spaces, and a TPU jacket compound flame retarded through an intumescent system were compared.

2. Materials and Methods

2.1. Materials

Table 5. DINP means Di Iso Nonyl Phthalate. ESBO stands for Epoxidized Soy Bean Oil. The used antioxidant is Arenox A10, which is Pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate), CAS number 6683-19-8. COS stands for calcium organic stabilizer. PCC means precipitated calcium carbonate. AS-1B and AS-6B are potent acid scavengers at high temperatures.

Raw Materials	Trade Name	Producer	F50.0 [phr]	F50.1 [phr]	F50.2 [phr]	F50.3 [phr]	F50.4 [phr]	F50.5 [phr]
PVC	Inovyn 271 PC	Inovyn	100	100	100	100	100	100
DINP	Diplast N	Polynt	50	50	50	50	50	50
ESBO	Reaflex EP/6	Reagens	2	2	2	2	2	2
Antioxidant	Arenox A10	Reagens	0.1	0.1	0.1	0.1	0.1	0.1
COS	RPK B-CV/3037	Reagens	3	3	3	3	3	3
CaCO3	Riochim	Umbria Filler	90	0	0	0	0	0
Al(OH)3	Apyral 40 CD	Nabaltec	0	90	0	0	0	0
Mg(OH)2	Ecopyren 3.5	Europiren	0	0	90	0	0	0
PCC	Winnofl S	Imerys	0	0	0	90	0	0
HTAS 1	AS-1B	Reagens	0	0	0	0	90	0
HTAS 2	AS-6B	Reagens	0	0	0	0	0	90

shows the first set of formulations. F50.0 represents the typical formulation for the PVC jacket compound used in low-voltage cables. All the other formulations in Table 5 are derived from F50.0, replacing CaCO₃ with different quantities of acid scavengers at high temperatures acting in the condensed phase. These formulations put in evidence the effects of the individual acid scavenger on smoke acidity and its impacts on cations and anions in the bubblers. The formulations reported in Table 5 are the same as in Ref. [5].

Table 6. DINP means Di Iso Nonyl Phthalate. ESBO stands for Epoxidized Soy Bean Oil. The used antioxidant is Arenox A10, which is Pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate), CAS number 6683-19-8. COS stands for calcium organic stabilizer. PCC means precipitated calcium carbonate. AS-6B is a potent acid scavenger at high temperatures. Riochim and Omya 95 T are calcium carbonates with a standard and fine particle size. RI004 is an ATO from Quimialmel with a median diameter D50 around 0.7–0.9 (µm).

Raw Materials	Trade name	Producer	REA1 [phr]	REA2 [phr]	REA3 [phr]	REA4 [phr]	REA5 [phr]	REA6 [phr]	REA7 [phr]	REA8 [phr]	REA9 [phr]	REA10 [phr]
PVC	Inovyn 271 PC	Inovyn	100	100	100	100	100	100	100	100	100	100
DINP	Diplast N	Polynt	50	50	50	50	50	50	50	50	50	50
ESBO	Reaflex EP/6	Reagens	2	2	2	2	2	2	2	2	2	2
Antioxidant	Arenox A10	Reagens	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1
Antimony Trioxide	RI004	Quimialmel	0	5	5	5	5	5	5	5	5	5
COS	RPK B-CV/3037	Reagens	3	3	3	3	3	3	3	3	3	3
CaCO3 standard	Riochim	Umbria Filler	0	0	90	0	0	0	0	0	0	0
CaCO3 fine	Omya 95 T	Omya	0	0	0	90	0	0	0	0	0	0
PCC	Winnofil S	Imerys	0	0	0	0	90	0	0	0	90	90
HTAS 2	AS-6B	Reagens	0	0	0	0	0	90	0	0	0	0
Al(OH)3	Apyral 40 CD	Nabaltec	0	0	0	0	0	0	90	0	30	0
Mg(OH)2	Ecopyren 3.5	Europiren	0	0	0	0	0	0	0	90	0	30

gives the formulations REA01—10. They represent PVC formulations containing ATO as a flame retardant. They also contain different acid scavengers at high temperatures acting singularly and in combination with ATH and MDH. ATO volatilizes in the gas phase as SbCl₃, which could theoretically affect the conductivity of the solution in bubblers. To investigate this, we employed ICP analysis to quantify the Sb content in the bubblers and evaluate whether this phenomenon significantly influences conductivity.

The ingredients of the formulations in Table 5 and Table 6 were chosen because they are the typical additives found in PVC compounds for cables in the EU and compounds used in cables for fixed installation in buildings. Some of these additives are acid scavengers, flame retardants, smoke suppressants, and components of stabilizers capable of generating chlorides, potentially affecting the conductivity of the solution in the bubbling devices.

Some acid scavengers and their combinations in Table 6 are less effective than others in terms of efficiency. Riochim [35] and Omya hydrocarb 95 T [36] are two coated ground calcium carbonates that are commonly less performant than ultrafine coated precipitated calcium carbonate, Winnofil S (PCC) [37]. AS-1B and AS-6B are potent acid scavengers at high temperatures from Reagens S.p.A. Two different synergistic mixtures are efficient at different levels: the combination of PCC and MDH is more efficient in HCl scavenging than that involving PCC and ATH. The difference in the efficiency of HCl scavenging will leave various amounts of HCl available for ATO in yielding SbCl₃, which is the leading actor that poisons the flame [6]. In this optics, this second series of formulations becomes very interesting, allowing us to verify if the bubbling devices contain Sb³⁺ and if its concentration changes depending on the presence of acid scavengers with different efficiency.

The amount of ingredients is expressed per hundred resin (phr). The compounds were tested with test apparatuses in

Table 7. Main test apparatuses utilized.

Test Apparatus	Producer	Model	Additional Info’s
Plasticorder	Brabender, Duisburg, Germany,	Plastograph EC	50 CC, chamber
Halogen acid gas test apparatus	SA Associates, Delhi, India	Standard model	According to IEC 60754-1, IEC 60754-2, Porcelain combustion boats
Multimeter	Mettler Toledo, Columbus, OH, USA	S213 standard kit
Conductivity electrode	Mettler Toledo, Columbus, OH, USA	S213 standard kit	Reference thermocouple adjusting temperature fluctuation
pH electrode	Mettler Toledo, Columbus, OH, USA	S213 standard kit	Reference thermocouple adjusting temperature fluctuation
Ion chromatography system	Thermo Fisher Scientific, Waltham, MA, USA	Dionex IonPacTM AS22 4, ×250 mm
Anion exchange column	Thermo Fisher Scientific, Waltham, MA, USA	Dionex IonPacTM CS12A 4, ×250 mm
Cation exchange column	Thermo Fisher Scientific, Waltham, MA, USA	Aqueon
ICP-OES	Thermo Fisher Scientific, Waltham, MA, USA	iCAP 7000 series

according to internal methods 4 and 5, as indicated in

Table 8. Tests for acidity assessment.

Technical Standard	Measurement	Temperature [°C]	Note
Internal method 4	Multimeter pH and conductivity	Isothermal at 950 °C	DDW, pH, and conductivity The general method, according to EN 60754-2 2014 version
	IC Anions and cations	Isothermal at 950 °C	Li+, Na+, NH4+, K+, Mg2+, Ca2+, Cl−, F−, NO2−, Br−, NO3−, PO43−, SO42−
	ICP-OES Elements	Isothermal at 950 °C	Mg, Al, Ca, Zn, Sb
Internal method 5	Multimeter pH and conductivity	Thermal profile of EN 60754-1	DDW, pH, and conductivity The general method, according to EN 60754-2 2014 version
	IC Anions and cations	Thermal profile of EN 60754-1	Li+, Na+, NH4+, K+, Mg2+, Ca2+, Cl−, F−, NO2−, Br−, NO3−, PO43−, SO42−
	ICP-OES Elements	Thermal profile of EN 60754-1	Mg, Al, Ca, Zn, Sb

.

Internal methods 4 and 5 are based on EN 60754-2. The samples are combusted in a tube furnace and fumes collected in bubbling devices. These devices allow for the assessment of pH and conductivity as well as the analysis of electrolytes in the bubblers through IC and ICP-OES. Internal method 4 is conducted at 950 °C, while 5 follows a temperature regime of heating for 40 min up to 800 °C followed by an additional 20 min held isothermally at 800 ± 10 °C. Both use the following materials: double deionized water (DDW) is internally produced by an ion exchange deionizer. The pH of DDW must be between 5.50 and 7.50, and conductivity must be less than 0.5 μS/mm. Buffer and conductivity standard solutions come from VWR international (pH: 2.00, 4.01, 7.00, 10.00, conductivity: 2.0, 8.4, 14.7, 141.3 μS/mm). Dionex™ Combined Seven Anion Standard II and Dionex™ Combined Six Cation Standard-I have been used to measure the concentration of the analytes.

2.2. Test Apparatus

Table 7 describes the utilized test apparatuses.

2.3. Sample Preparation

Ref. [5] describe apparatuses and the procedures for preparing PVC compound test specimens in detail. In summary, the formulations in Table 5 and Table 6 are mixed in a turbo mixer up to 105 °C, producing the dry blends, then processed into kneaders by a plasticorder for 10 min. The kneaders are pressed at 160 °C for 4 min in 0.5 mm plaques in a hydraulic press, and the test specimens for the methods indicated in Table 7 are obtained from them. Appendix A gives a schematic diagram of the sample preparation and testing process.

2.4. Internal Methods for Evaluating the Species in Solutions

Table 8 recalls the internal methods, measures, and determined analytes.

The initial procedures of internal method 4, which recall EN 60754-2, are as follows: an empty combustion boat is carried from the sample carrier into the central part of the quartz tube. The probe of a calibrated thermocouple is introduced in the central part of the quartz glass tube, and the temperature is adjusted to 950 ± 5 °C and maintained for at least one hour. After, the solution in bubbling devices is checked to see if pH and conductivities are between 5.50 and 7.50 and less than 0.5 μS/mm, respectively, to confirm that the combustion boat, quartz tube, bubblers, and connections are clean. If not, it is necessary to repeat the cleaning procedures and adopt the precautions mentioned in Part I of this article and EN 60754-2. The tube furnace is ready for the first run when the temperature is stable. Then, a 1.000 ± 0.001 g sample is weighed in a porcelain combustion boat. It is rapidly shifted into the quartz glass tube, moving the magnet along the sample carrier, and then the countdown starts. The fume is collected into the bubblers containing DDW for 30 min by a normalized air flux set according to the standards. After 30 min, the connectors are opened, and the magnet takes the combustion boat back from the quartz glass tube. The water from the bubbling devices and washing procedures is collected in a 1 L volumetric flask filled to the mark. All precautions mentioned in Parts I and II of the series were taken to prevent errors leading to poor repeatability and reproducibility, which are commonly found in furnace tube tests. Despite this, most of the smoke acidity measurements were repeated more than 3 times for each sample, and the statistical method for analyzing the measurements is indicated in the standard EN 60754-2.

The initial procedures of internal method 5 are as follows: the thermal profile of EN 60754-1 has been applied: 40 ± 5 min to 800 °C and 800 ± 10 °C for a further 20 ± 1 min. The calibrated thermocouple and the countdown are used to precisely check the heating regime in terms of time and temperature. This test is run by processing an empty combustion boat and checking the water quality in bubbling devices, stating the cleaning status of the test apparatus.

The detection procedures of the above-indicated methods are the following. Internal methods 4 and 5 utilize the multimeter and electrodes for measuring pH and conductivity. The general method in EN 60754-2:2014 was applied to analyze the precision of pH and conductivity data. It requires the measurement of the coefficient of variation (CV) of pH and conductivity, which implies three or more replicates for each sample.

Internal methods 4 and 5 also analyze the anions, including chlorine, bromine, and fluorine, through the IC anion exchange column (Table 7). They also detect the cations indicated through IC (cation exchange column, Table 7) and ICP-OES. The ISO 10304-1 framework performed IC for detecting seven anions. Without a pre-treatment of the sample, the standard has lower application limits: ≥0.05 mg/L for bromine and nitrite and ≥0.1 mg/L for chlorine, fluorine, nitrate, orthophosphate, and sulfate. The framework of ISO 14911 [38] was used for determining the cation concentration. The lower limits of application are the following: lithium ≥0.01 mg/L, sodium, ammonium, and potassium ≥0.1 mg/L, and magnesium and calcium ≥0.5 mg/L. ISO 10304-1 and ISO 14911 are designed to detect analytes below 10 mg/g, and therefore, we modified some of their procedures for dosing anions and cations at concentrations up to 200 mg/g. For the methods requiring IC measurements, the rules reported in paragraph 8 of IEC 60754-3:2011 have been adopted to evaluate the data’s precision: two measurements with a difference of less than 0.1 mg/g and the ratio between the mean and standard deviation of less than 0.25. ICP-OES has been performed according to ISO 11885 [39] to complete the elemental analysis focusing on those elements (not detected by IC) capable of reaching the bubbling devices through evaporation of the molecules containing them, coming from the stabilizer and flame retardant in the PVC compounds: Sb from antimony trioxide and Zn and Al from the stabilizer and flame-retardant fillers. The limits of quantification (LOQs) at the specific used wavelength are the following: antimony and calcium ≥0.1 mg/L, zinc and magnesium ≥0.0033 mg/L, and aluminum ≥0.001 mg/L.

Through internal methods 4 and 5, the first aliquots of the solution from bubbling devices are collected into polypropylene flasks and utilized for pH and conductivity measures. pH and conductivity are taken at 25 °C ± 1. Before each measurement, pH is calibrated at two points (4.01 and 7.00), and conductivity pH is calibrated at 1 point (141.3 μS/mm). The measured values are corrected by the correction standard closer to them. The second aliquots are used to detect anions and cations through IC. Iodine content has not been measured because its presence in PVC compounds is not expected. After all, no additive can be a potential source. The third aliquots are utilized for ICP-OES measurements.

Figure A1 in Appendix A gives a schematic diagram of the testing process. The Supplementary Materials provide additional details on commercial additives, test apparatus, technical standards, sample preparation, and the measurement procedure.

3. Results

Table 9. pH and conductivity of the compounds in Table 5 are shown performing internal method 4 (isothermal at 950 °C). The thermal profile of EN 60754-2 has been used. The mean values, coefficient of variation, and standard deviations are reported. The calculated conductivity utilizing the DHO equation is presented with the precision of experimental measurements and quantified as the percentage error relative to the theoretical DHO conductivity. The pH and conductivity measurements of the first run performed in Ref. [5] are reported in Appendix E, Table A3.

Formulation	F50.0	F50.1	F50.2	F50.3	F50.4	F50.5
pH	2.67	2.29	2.29	2.71	2.75	2.81
SDpH	0.09	0.11	0.09	0.03	0.12	0.12
CVpH [%]	3.4	4.9	4.1	1.1	4.5	4.3
Conductivity [μS/mm]	95.9	223.2	222.7	77.0	72.1	73.2
SDc	3.2	8.8	10.2	4.2	3.2	3.1
CVc [%]	3.3	3.9	4.6	5.5	4.4	4.2
Accuracy [%] from DHO model	5.7	5.1	4.8	−5.6	−4.9	11.1
Conductivity [μS/mm] (DHO)	90.7	212.4	212.4	81.6	75.8	65.9

contains the pH and conductivity of the formulations listed in Table 5, which were determined using internal method 4. The data reported in Table 9 were collected during a second trial, following the same formulations previously described in Ref. [5]. The results presented here fall within 5.0% of the coefficient of variation (CV) of the earlier data. Additionally, the conductivity calculated from the pH using the DHO equation (provided in Appendix C and Appendix D,

Table A1. Comparison between coefficients a and b from regression of samples, standard solution of HCl and DHO model.

	a	b
Samples	39,117.15	2.2772
Standard solutions HCl	40,476.18	2.2897
DHO model	37,262.15	2.2555

and

Table A2. Condictivity and pH relationship according to DHO model, and related [H⁺] and [Cl⁻].

pH	[H+]	[Cl−]	C [μS/mm]
1.56	0.02754229	0.02754229	1101.64
1.57	0.02691535	0.02691535	1077.37
1.58	0.02630268	0.02630268	1053.63
1.59	0.02570396	0.02570396	1030.40
1.60	0.02511886	0.02511886	1007.67
1.61	0.02454709	0.02454709	985.44
1.62	0.02398833	0.02398833	963.68
1.63	0.02344229	0.02344229	942.40
1.64	0.02290868	0.02290868	921.59
1.65	0.02238721	0.02238721	901.22
1.66	0.02187762	0.02187762	881.30
1.67	0.02137962	0.02137962	861.81
1.68	0.02089296	0.02089296	842.74
1.69	0.02041738	0.02041738	824.09
1.70	0.01995262	0.01995262	805.85
1.71	0.01949845	0.01949845	788.00
1.72	0.01905461	0.01905461	770.55
1.73	0.01862087	0.01862087	753.47
1.74	0.01819701	0.01819701	736.77
1.75	0.01778279	0.01778279	720.43
1.76	0.01737801	0.01737801	704.45
1.77	0.01698244	0.01698244	688.82
1.78	0.01659587	0.01659587	673.53
1.79	0.01621810	0.01621810	658.58
1.80	0.01584893	0.01584893	643.95
1.81	0.01548817	0.01548817	629.64
1.82	0.01513561	0.01513561	615.65
1.83	0.01479108	0.01479108	601.97
1.84	0.01445440	0.01445440	588.58
1.85	0.01412538	0.01412538	575.49
1.86	0.01380384	0.01380384	562.69
1.87	0.01348963	0.01348963	550.17
1.88	0.01318257	0.01318257	537.92
1.89	0.01288250	0.01288250	525.94
1.90	0.01258925	0.01258925	514.23
1.91	0.01230269	0.01230269	502.77
1.92	0.01202264	0.01202264	491.57
1.93	0.01174898	0.01174898	480.61
1.94	0.01148154	0.01148154	469.90
1.95	0.01122018	0.01122018	459.42
1.96	0.01096478	0.01096478	449.17
1.97	0.01071519	0.01071519	439.15
1.98	0.01047129	0.01047129	429.35
1.99	0.01023293	0.01023293	419.76
2.00	0.01000000	0.01000000	410.39
2.01	0.00977237	0.00977237	401.23
2.02	0.00954993	0.00954993	392.26
2.03	0.00933254	0.00933254	383.50
2.04	0.00912011	0.00912011	374.93
2.05	0.00891251	0.00891251	366.55
2.06	0.00870964	0.00870964	358.35
2.07	0.00851138	0.00851138	350.34
2.08	0.00831764	0.00831764	342.50
2.09	0.00812831	0.00812831	334.84
2.10	0.00794328	0.00794328	327.35
2.11	0.00776247	0.00776247	320.02
2.12	0.00758578	0.00758578	312.86
2.13	0.00741310	0.00741310	305.85
2.14	0.00724436	0.00724436	299.00
2.15	0.00707946	0.00707946	292.31
2.16	0.00691831	0.00691831	285.76
2.17	0.00676083	0.00676083	279.35
2.18	0.00660693	0.00660693	273.09
2.19	0.00645654	0.00645654	266.97
2.20	0.00630957	0.00630957	260.99
2.21	0.00616595	0.00616595	255.13
2.22	0.00602560	0.00602560	249.41
2.23	0.00588844	0.00588844	243.82
2.24	0.00575440	0.00575440	238.35
2.25	0.00562341	0.00562341	233.00
2.26	0.00549541	0.00549541	227.77
2.27	0.00537032	0.00537032	222.66
2.28	0.00524807	0.00524807	217.66
2.29	0.00512861	0.00512861	212.77
2.30	0.00501187	0.00501187	207.99
2.31	0.00489779	0.00489779	203.32
2.32	0.00478630	0.00478630	198.75
2.33	0.00467735	0.00467735	194.29
2.34	0.00457088	0.00457088	189.92
2.35	0.00446684	0.00446684	185.65
2.36	0.00436516	0.00436516	181.48
2.37	0.00426580	0.00426580	177.40
2.38	0.00416869	0.00416869	173.41
2.39	0.00407380	0.00407380	169.51
2.40	0.00398107	0.00398107	165.70
2.41	0.00389045	0.00389045	161.97
2.42	0.00380189	0.00380189	158.33
2.43	0.00371535	0.00371535	154.76
2.44	0.00363078	0.00363078	151.28
2.45	0.00354813	0.00354813	147.88
2.46	0.00346737	0.00346737	144.55
2.47	0.00338844	0.00338844	141.29
2.48	0.00331131	0.00331131	138.11
2.49	0.00323594	0.00323594	135.00
2.50	0.00316228	0.00316228	131.96
2.51	0.00309030	0.00309030	128.99
2.52	0.00301995	0.00301995	126.08
2.53	0.00295121	0.00295121	123.24
2.54	0.00288403	0.00288403	120.46
2.55	0.00281838	0.00281838	117.75
2.56	0.00275423	0.00275423	115.10
2.57	0.00269153	0.00269153	112.50
2.58	0.00263027	0.00263027	109.97
2.59	0.00257040	0.00257040	107.49
2.60	0.00251189	0.00251189	105.06
2.61	0.00245471	0.00245471	102.69
2.62	0.00239883	0.00239883	100.38
2.63	0.00234423	0.00234423	98.11
2.64	0.00229087	0.00229087	95.90
2.65	0.00223872	0.00223872	93.74
2.66	0.00218776	0.00218776	91.62
2.67	0.00213796	0.00213796	89.55
2.68	0.00208930	0.00208930	87.53
2.69	0.00204174	0.00204174	85.56
2.70	0.00199526	0.00199526	83.63
2.71	0.00194984	0.00194984	81.74
2.72	0.00190546	0.00190546	79.89
2.73	0.00186209	0.00186209	78.09
2.74	0.00181970	0.00181970	76.33
2.75	0.00177828	0.00177828	74.60
2.76	0.00173780	0.00173780	72.92
2.77	0.00169824	0.00169824	71.27
2.78	0.00165959	0.00165959	69.66
2.79	0.00162181	0.00162181	68.09
2.80	0.00158489	0.00158489	66.55
2.81	0.00154882	0.00154882	65.04
2.82	0.00151356	0.00151356	63.57
2.83	0.00147911	0.00147911	62.14
2.84	0.00144544	0.00144544	60.73
2.85	0.00141254	0.00141254	59.36
2.86	0.00138038	0.00138038	58.02
2.87	0.00134896	0.00134896	56.71
2.88	0.00131826	0.00131826	55.42
2.89	0.00128825	0.00128825	54.17
2.90	0.00125893	0.00125893	52.95
2.91	0.00123027	0.00123027	51.75
2.92	0.00120226	0.00120226	50.58
2.93	0.00117490	0.00117490	49.44
2.94	0.00114815	0.00114815	48.32
2.95	0.00112202	0.00112202	47.22
2.96	0.00109648	0.00109648	46.16
2.97	0.00107152	0.00107152	45.11
2.98	0.00104713	0.00104713	44.09
2.99	0.00102329	0.00102329	43.09
3.00	0.00100000	0.00100000	42.12
3.01	0.00097724	0.00097724	41.16
3.02	0.00095499	0.00095499	40.23
3.03	0.00093325	0.00093325	39.32
3.04	0.00091201	0.00091201	38.43
3.05	0.00089125	0.00089125	37.56
3.06	0.00087096	0.00087096	36.71
3.07	0.00085114	0.00085114	35.88
3.08	0.00083176	0.00083176	35.07
3.09	0.00081283	0.00081283	34.27
3.10	0.00079433	0.00079433	33.50
3.11	0.00077625	0.00077625	32.74
3.12	0.00075858	0.00075858	32.00
3.13	0.00074131	0.00074131	31.27
3.14	0.00072444	0.00072444	30.57
3.15	0.00070795	0.00070795	29.87
3.16	0.00069183	0.00069183	29.20
3.17	0.00067608	0.00067608	28.54
3.18	0.00066069	0.00066069	27.89
3.19	0.00064565	0.00064565	27.26
3.20	0.00063096	0.00063096	26.64
3.21	0.00061660	0.00061660	26.04
3.22	0.00060256	0.00060256	25.45
3.23	0.00058884	0.00058884	24.87
3.24	0.00057544	0.00057544	24.31
3.25	0.00056234	0.00056234	23.75
3.26	0.00054954	0.00054954	23.22
3.27	0.00053703	0.00053703	22.69
3.28	0.00052481	0.00052481	22.18
3.29	0.00051286	0.00051286	21.67
3.30	0.00050119	0.00050119	21.18
3.31	0.00048978	0.00048978	20.70
3.32	0.00047863	0.00047863	20.23
3.33	0.00046774	0.00046774	19.77
3.34	0.00045709	0.00045709	19.33
3.35	0.00044668	0.00044668	18.89
3.36	0.00043652	0.00043652	18.46
3.37	0.00042658	0.00042658	18.04
3.38	0.00041687	0.00041687	17.63
3.39	0.00040738	0.00040738	17.23
3.40	0.00039811	0.00039811	16.84
3.41	0.00038905	0.00038905	16.46
3.42	0.00038019	0.00038019	16.09
3.43	0.00037154	0.00037154	15.72
3.44	0.00036308	0.00036308	15.36
3.45	0.00035481	0.00035481	15.02
3.46	0.00034674	0.00034674	14.68
3.47	0.00033884	0.00033884	14.34
3.48	0.00033113	0.00033113	14.02
3.49	0.00032359	0.00032359	13.70
3.50	0.00031623	0.00031623	13.39
3.51	0.00030903	0.00030903	13.08
3.52	0.00030200	0.00030200	12.79
3.53	0.00029512	0.00029512	12.50
3.54	0.00028840	0.00028840	12.21
3.55	0.00028184	0.00028184	11.94
3.56	0.00027542	0.00027542	11.67
3.57	0.00026915	0.00026915	11.40
3.58	0.00026303	0.00026303	11.14
3.59	0.00025704	0.00025704	10.89
3.60	0.00025119	0.00025119	10.64
3.61	0.00024547	0.00024547	10.40
3.62	0.00023988	0.00023988	10.16
3.63	0.00023442	0.00023442	9.93
3.64	0.00022909	0.00022909	9.71
3.65	0.00022387	0.00022387	9.49
3.66	0.00021878	0.00021878	9.27
3.67	0.00021380	0.00021380	9.06
3.68	0.00020893	0.00020893	8.86
3.69	0.00020417	0.00020417	8.66
3.70	0.00019953	0.00019953	8.46
3.71	0.00019498	0.00019498	8.27
3.72	0.00019055	0.00019055	8.08
3.73	0.00018621	0.00018621	7.90
3.74	0.00018197	0.00018197	7.72
3.75	0.00017783	0.00017783	7.54
3.76	0.00017378	0.00017378	7.37
3.77	0.00016982	0.00016982	7.20
3.78	0.00016596	0.00016596	7.04
3.79	0.00016218	0.00016218	6.88
3.80	0.00015849	0.00015849	6.72
3.81	0.00015488	0.00015488	6.57
3.82	0.00015136	0.00015136	6.42
3.83	0.00014791	0.00014791	6.28
3.84	0.00014454	0.00014454	6.13
3.85	0.00014125	0.00014125	5.99
3.86	0.00013804	0.00013804	5.86
3.87	0.00013490	0.00013490	5.72
3.88	0.00013183	0.00013183	5.59
3.89	0.00012882	0.00012882	5.47
3.90	0.00012589	0.00012589	5.34
3.91	0.00012303	0.00012303	5.22
3.92	0.00012023	0.00012023	5.10
3.93	0.00011749	0.00011749	4.99
3.94	0.00011482	0.00011482	4.87
3.95	0.00011220	0.00011220	4.76
3.96	0.00010965	0.00010965	4.65
3.97	0.00010715	0.00010715	4.55
3.98	0.00010471	0.00010471	4.45
3.99	0.00010233	0.00010233	4.34
4.00	0.00010000	0.00010000	4.25
4.01	0.00009772	0.00009772	4.15
4.02	0.00009550	0.00009550	4.06
4.03	0.00009333	0.00009333	3.96
4.04	0.00009120	0.00009120	3.87
4.05	0.00008913	0.00008913	3.78
4.06	0.00008710	0.00008710	3.70
4.07	0.00008511	0.00008511	3.61
4.08	0.00008318	0.00008318	3.53
4.09	0.00008128	0.00008128	3.45
4.10	0.00007943	0.00007943	3.37
4.11	0.00007762	0.00007762	3.30
4.12	0.00007586	0.00007586	3.22
4.13	0.00007413	0.00007413	3.15
4.14	0.00007244	0.00007244	3.08
4.15	0.00007079	0.00007079	3.01
4.16	0.00006918	0.00006918	2.94
4.17	0.00006761	0.00006761	2.87
4.18	0.00006607	0.00006607	2.81
4.19	0.00006457	0.00006457	2.74
4.20	0.00006310	0.00006310	2.68
4.21	0.00006166	0.00006166	2.62
4.22	0.00006026	0.00006026	2.56
4.23	0.00005888	0.00005888	2.50
4.24	0.00005754	0.00005754	2.45
4.25	0.00005623	0.00005623	2.39
4.26	0.00005495	0.00005495	2.34
4.27	0.00005370	0.00005370	2.28
4.28	0.00005248	0.00005248	2.23
4.29	0.00005129	0.00005129	2.18
4.30	0.00005012	0.00005012	2.13
4.31	0.00004898	0.00004898	2.08
4.32	0.00004786	0.00004786	2.03
4.33	0.00004677	0.00004677	1.99
4.34	0.00004571	0.00004571	1.94
4.35	0.00004467	0.00004467	1.90
4.36	0.00004365	0.00004365	1.86
4.37	0.00004266	0.00004266	1.81
4.38	0.00004169	0.00004169	1.77
4.39	0.00004074	0.00004074	1.73
4.40	0.00003981	0.00003981	1.69
4.41	0.00003890	0.00003890	1.65
4.42	0.00003802	0.00003802	1.62
4.43	0.00003715	0.00003715	1.58
4.44	0.00003631	0.00003631	1.54
4.45	0.00003548	0.00003548	1.51
4.46	0.00003467	0.00003467	1.47
4.47	0.00003388	0.00003388	1.44
4.48	0.00003311	0.00003311	1.41
4.49	0.00003236	0.00003236	1.38
4.50	0.00003162	0.00003162	1.34
4.51	0.00003090	0.00003090	1.31
4.52	0.00003020	0.00003020	1.28
4.53	0.00002951	0.00002951	1.26
4.54	0.00002884	0.00002884	1.23
4.55	0.00002818	0.00002818	1.20
4.56	0.00002754	0.00002754	1.17
4.57	0.00002692	0.00002692	1.14
4.58	0.00002630	0.00002630	1.12
4.59	0.00002570	0.00002570	1.09
4.60	0.00002512	0.00002512	1.07
4.61	0.00002455	0.00002455	1.04
4.62	0.00002399	0.00002399	1.02
4.63	0.00002344	0.00002344	1.00
4.64	0.00002291	0.00002291	0.97
4.65	0.00002239	0.00002239	0.95
4.66	0.00002188	0.00002188	0.93
4.67	0.00002138	0.00002138	0.91
4.68	0.00002089	0.00002089	0.89
4.69	0.00002042	0.00002042	0.87
4.70	0.00001995	0.00001995	0.85
4.71	0.00001950	0.00001950	0.83
4.72	0.00001905	0.00001905	0.81
4.73	0.00001862	0.00001862	0.79
4.74	0.00001820	0.00001820	0.77
4.75	0.00001778	0.00001778	0.76
4.76	0.00001738	0.00001738	0.74
4.77	0.00001698	0.00001698	0.72
4.78	0.00001660	0.00001660	0.71
4.79	0.00001622	0.00001622	0.69
4.80	0.00001585	0.00001585	0.67
4.81	0.00001549	0.00001549	0.66
4.82	0.00001514	0.00001514	0.64
4.83	0.00001479	0.00001479	0.63
4.84	0.00001445	0.00001445	0.62
4.85	0.00001413	0.00001413	0.60
4.86	0.00001380	0.00001380	0.59
4.87	0.00001349	0.00001349	0.57
4.88	0.00001318	0.00001318	0.56
4.89	0.00001288	0.00001288	0.55
4.90	0.00001259	0.00001259	0.54
4.91	0.00001230	0.00001230	0.52
4.92	0.00001202	0.00001202	0.51
4.93	0.00001175	0.00001175	0.50
4.94	0.00001148	0.00001148	0.49
4.95	0.00001122	0.00001122	0.48
4.96	0.00001096	0.00001096	0.47
4.97	0.00001072	0.00001072	0.46
4.98	0.00001047	0.00001047	0.45
4.99	0.00001023	0.00001023	0.44
5.00	0.00001000	0.00001000	0.43
5.01	0.00000977	0.00000977	0.42
5.02	0.00000955	0.00000955	0.41
5.03	0.00000933	0.00000933	0.40
5.04	0.00000912	0.00000912	0.39
5.05	0.00000891	0.00000891	0.38
5.06	0.00000871	0.00000871	0.37
5.07	0.00000851	0.00000851	0.36
5.08	0.00000832	0.00000832	0.35
5.09	0.00000813	0.00000813	0.35
5.10	0.00000794	0.00000794	0.34
5.11	0.00000776	0.00000776	0.33
5.12	0.00000759	0.00000759	0.32
5.13	0.00000741	0.00000741	0.32
5.14	0.00000724	0.00000724	0.31
5.15	0.00000708	0.00000708	0.30
5.16	0.00000692	0.00000692	0.29
5.17	0.00000676	0.00000676	0.29
5.18	0.00000661	0.00000661	0.28
5.19	0.00000646	0.00000646	0.27
5.20	0.00000631	0.00000631	0.27
5.21	0.00000617	0.00000617	0.26
5.22	0.00000603	0.00000603	0.26
5.23	0.00000589	0.00000589	0.25
5.24	0.00000575	0.00000575	0.25
5.25	0.00000562	0.00000562	0.24
5.26	0.00000550	0.00000550	0.23
5.27	0.00000537	0.00000537	0.23
5.28	0.00000525	0.00000525	0.22
5.29	0.00000513	0.00000513	0.22
5.30	0.00000501	0.00000501	0.21
5.31	0.00000490	0.00000490	0.21
5.32	0.00000479	0.00000479	0.20
5.33	0.00000468	0.00000468	0.20
5.34	0.00000457	0.00000457	0.19
5.35	0.00000447	0.00000447	0.19
5.36	0.00000437	0.00000437	0.19
5.37	0.00000427	0.00000427	0.18
5.38	0.00000417	0.00000417	0.18
5.39	0.00000407	0.00000407	0.17
5.40	0.00000398	0.00000398	0.17
5.41	0.00000389	0.00000389	0.17
5.42	0.00000380	0.00000380	0.16
5.43	0.00000372	0.00000372	0.16
5.44	0.00000363	0.00000363	0.15
5.45	0.00000355	0.00000355	0.15
5.46	0.00000347	0.00000347	0.15
5.47	0.00000339	0.00000339	0.14
5.48	0.00000331	0.00000331	0.14
5.49	0.00000324	0.00000324	0.14
5.50	0.00000316	0.00000316	0.13
5.51	0.00000309	0.00000309	0.13
5.52	0.00000302	0.00000302	0.13
5.53	0.00000295	0.00000295	0.13
5.54	0.00000288	0.00000288	0.12
5.55	0.00000282	0.00000282	0.12
5.56	0.00000275	0.00000275	0.12
5.57	0.00000269	0.00000269	0.11
5.58	0.00000263	0.00000263	0.11
5.59	0.00000257	0.00000257	0.11
5.60	0.00000251	0.00000251	0.11
5.61	0.00000245	0.00000245	0.10
5.62	0.00000240	0.00000240	0.10
5.63	0.00000234	0.00000234	0.10
5.64	0.00000229	0.00000229	0.10
5.65	0.00000224	0.00000224	0.10
5.66	0.00000219	0.00000219	0.09
5.67	0.00000214	0.00000214	0.09
5.68	0.00000209	0.00000209	0.09
5.69	0.00000204	0.00000204	0.09
5.70	0.00000200	0.00000200	0.08
5.71	0.00000195	0.00000195	0.08
5.72	0.00000191	0.00000191	0.08
5.73	0.00000186	0.00000186	0.08
5.74	0.00000182	0.00000182	0.08
5.75	0.00000178	0.00000178	0.08
5.76	0.00000174	0.00000174	0.07
5.77	0.00000170	0.00000170	0.07
5.78	0.00000166	0.00000166	0.07
5.79	0.00000162	0.00000162	0.07
5.80	0.00000158	0.00000158	0.07
5.81	0.00000155	0.00000155	0.07
5.82	0.00000151	0.00000151	0.06
5.83	0.00000148	0.00000148	0.06
5.84	0.00000145	0.00000145	0.06
5.85	0.00000141	0.00000141	0.06
5.86	0.00000138	0.00000138	0.06
5.87	0.00000135	0.00000135	0.06
5.88	0.00000132	0.00000132	0.06
5.89	0.00000129	0.00000129	0.05
5.90	0.00000126	0.00000126	0.05
5.91	0.00000123	0.00000123	0.05
5.92	0.00000120	0.00000120	0.05
5.93	0.00000117	0.00000117	0.05
5.94	0.00000115	0.00000115	0.05
5.95	0.00000112	0.00000112	0.05
5.96	0.00000110	0.00000110	0.05
5.97	0.00000107	0.00000107	0.05
5.98	0.00000105	0.00000105	0.04
5.99	0.00000102	0.00000102	0.04
6.00	0.00000100	0.00000100	0.04
6.01	0.00000098	0.00000098	0.04
6.02	0.00000095	0.00000095	0.04
6.03	0.00000093	0.00000093	0.04
6.04	0.00000091	0.00000091	0.04
6.05	0.00000089	0.00000089	0.04
6.06	0.00000087	0.00000087	0.04
6.07	0.00000085	0.00000085	0.04
6.08	0.00000083	0.00000083	0.04
6.09	0.00000081	0.00000081	0.03
6.10	0.00000079	0.00000079	0.03
6.11	0.00000078	0.00000078	0.03
6.12	0.00000076	0.00000076	0.03
6.13	0.00000074	0.00000074	0.03
6.14	0.00000072	0.00000072	0.03
6.15	0.00000071	0.00000071	0.03
6.16	0.00000069	0.00000069	0.03
6.17	0.00000068	0.00000068	0.03
6.18	0.00000066	0.00000066	0.03
6.19	0.00000065	0.00000065	0.03
6.20	0.00000063	0.00000063	0.03
6.21	0.00000062	0.00000062	0.03
6.22	0.00000060	0.00000060	0.03
6.23	0.00000059	0.00000059	0.03
6.24	0.00000058	0.00000058	0.02
6.25	0.00000056	0.00000056	0.02
6.26	0.00000055	0.00000055	0.02
6.27	0.00000054	0.00000054	0.02
6.28	0.00000052	0.00000052	0.02
6.29	0.00000051	0.00000051	0.02
6.30	0.00000050	0.00000050	0.02
6.31	0.00000049	0.00000049	0.02
6.32	0.00000048	0.00000048	0.02
6.33	0.00000047	0.00000047	0.02
6.34	0.00000046	0.00000046	0.02
6.35	0.00000045	0.00000045	0.02
6.36	0.00000044	0.00000044	0.02
6.37	0.00000043	0.00000043	0.02
6.38	0.00000042	0.00000042	0.02
6.39	0.00000041	0.00000041	0.02
6.40	0.00000040	0.00000040	0.02
6.41	0.00000039	0.00000039	0.02
6.42	0.00000038	0.00000038	0.02
6.43	0.00000037	0.00000037	0.02
6.44	0.00000036	0.00000036	0.02
6.45	0.00000035	0.00000035	0.02
6.46	0.00000035	0.00000035	0.01
6.47	0.00000034	0.00000034	0.01
6.48	0.00000033	0.00000033	0.01
6.49	0.00000032	0.00000032	0.01
6.50	0.00000032	0.00000032	0.01
6.51	0.00000031	0.00000031	0.01
6.52	0.00000030	0.00000030	0.01
6.53	0.00000030	0.00000030	0.01
6.54	0.00000029	0.00000029	0.01
6.55	0.00000028	0.00000028	0.01
6.56	0.00000028	0.00000028	0.01
6.57	0.00000027	0.00000027	0.01
6.58	0.00000026	0.00000026	0.01
6.59	0.00000026	0.00000026	0.01
6.60	0.00000025	0.00000025	0.01
6.61	0.00000025	0.00000025	0.01
6.62	0.00000024	0.00000024	0.01
6.63	0.00000023	0.00000023	0.01
6.64	0.00000023	0.00000023	0.01
6.65	0.00000022	0.00000022	0.01
6.66	0.00000022	0.00000022	0.01
6.67	0.00000021	0.00000021	0.01
6.68	0.00000021	0.00000021	0.01

) is shown alongside the precision of the experimental measurements, which was quantified as the percentage error relative to the theoretical DHO conductivity.

Table 10. Chlorine concentrations from bubblers are reported for the formulation of Table 5. Internal method 4 was used. In column 2, the measured pH; in column 3, [HCl] was calculated from the measured pH. In column 4, chlorine has been calculated for pH values, assuming HCl is the only source of it. Column 5 shows the chlorine concentrations from IC. Column 6 reports the difference between values in columns 4 and 5.

Sample	pH	[HCl] Mol/L	[Cl−] mg/L	[Cl−] IC [mg/L]	Delta
F50.0	2.67	0.0021503	76.2	78.7	−3.26
F50.1	2.29	0.0051227	181.6	177.0	2.54
F50.2	2.29	0.0051227	181.6	175.8	3.21
F50.3	2.71	0.0019297	68.4	69.1	−1.02
F50.4	2.75	0.0017865	63.3	64.6	−2.04
F50.5	2.81	0.0015506	55.0	57.0	−3.67

reports the pH and [Cl⁻] measured applying internal method 4 to the formulations in Table 5: the precisely measured pH is in column 2, and the [Cl⁻] derived from HCl is in column 4. Column 5 gives the [Cl⁻] from IC measurements.

Table A4. IC measured the anions concentration of the first set of formulations (Table 5). Internal method 4 was performed in isothermal at 950 °C. LOQ according to ISO 10304-1: Br⁻ and NO²⁻ ≥ 0.05 mg/L; Cl⁻, F⁻, NO³⁻, PO₄³⁻ and SO₄²⁻ ≥ 0.1 mg/L.

Sample	[Cl−] mg/L	SD [Cl−] mg/L	[F−] mg/L	[NO2−] mg/L	[Br−] mg/L	[NO3−] mg/L	[PO43−] mg/L	[SO42−] mg/L
F50.0	78.72	2.91	0.10	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
F50.1	177.00	5.50	0.11	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
F50.2	175.78	6.76	0.11	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
F50.3	69.11	2.44	0.11	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
F50.4	64.63	2.50	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
F50.5	56.99	1.89	0.10	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ

and

Table A5. IC measured the cations concentration of formulations in Table 5. Internal method 4 was performed in isothermal at 950 °C. LOQ according to ISO 14911: Li⁺ ≥ 0.01 mg/L; Na⁺, NH₄⁺ and K⁺ ≥ 0.1 mg/L; Mg²⁺ and Ca²⁺ ≥ 0.5 mg/L.

Sample	[Li+] mg/L	[Na+] mg/L	[NH4+] mg/L	[K+] mg/L	[Mg2+] mg/L	[Ca2+] mg/L
F50.0	0.01	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
F50.1	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
F50.2	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
F50.3	0.01	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
F50.4	0.01	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
F50.5	0.01	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ

, reported in Appendix E, show anions and cations concentrations found in the solutions performing internal method 4 on the formulations in Table 5. The Al, Zn, and Sb values for all formulations in Table 5 are lower than the detection limit of the ICP-OES and therefore are not reported. These extremely low elements’ concentrations indicate that evaporation phenomena are negligible.

Table 11. pH and conductivity of the compounds in Table 5 are shown, performing internal method 5 (40 min at 800 °C and further 20 min at 800 °C). The mean values, coefficients of variation, and standard deviations are reported. The calculated conductivity utilizing the DHO equation is presented with the precision of experimental measurements and quantified as the percentage error relative to the theoretical DHO conductivity. The pH and conductivity of the first run [5] are reported in Table A6.

Formulation	F50.0	F50.1	F50.2	F50.3	F50.4	F50.5
pH	2.51	2.29	2.38	3.20	3.57	3.27
SDpH	0.10	0.11	0.08	0.08	0.09	0.10
CVpH [%]	4.0	4.8	3.4	2.5	2.4	3.1
Conductivity [μS/mm]	135.7	219.7	174.2	30.7	11.6	24.9
SDc	6.3	5.4	8.7	2.5	0.2	1.2
CVc [%]	4.6	2.5	5.0	8.1	1.7	4.8
Accuracy [%] from DHO model	5.9	3.9	0.2	11.6	−4.9	7.3
Conductivity [μS/mm] (DHO)	128.1	211.4	173.8	27.5	12.2	23.2

displays the compounds’ pH and conductivity in Table 5, performing internal method 5. The conductivity determined through pH using the DHO equation in Appendix C and Appendix D, is presented alongside the accuracy of experimental measurements, which is expressed as the percentage deviation compared to the theoretical DHO conductivity.

Table 12. Chlorine concentrations from bubblers are reported for the formulations in Table 5. Internal method 5 was used. In column 2, the measured pH; in column 3, [HCl] was calculated from the measured pH. In column 4, chlorine has been calculated for pH values, assuming HCl is the only source of it. Column 5 shows the chlorine concentrations from IC. Column 6 reports the difference between column 4 and 5 values.

Sample	pH	[HCl] Mol/L	[Cl−] mg/L	[Cl−] IC [mg/L]	Delta
F50.0	2.51	0.0030584	108.43	113.82	−4.97
F50.1	2.29	0.0050933	180.57	176.10	2.47
F50.2	2.38	0.0041687	147.79	147.25	0.37
F50.3	3.20	0.0006339	22.47	21.23	5.55
F50.4	3.57	0.0002780	9.85	7.42	24.67
F50.5	3.27	0.0005339	18.93	17.41	8.03

reports pH (multimeter) and [Cl⁻] through IC, which were measured applying internal method 5 on the formulations in Table 5. It shows pH (column 2), the [HCl] calculated from pH (column 3), and the [Cl⁻] derived from HCl (column 4). Column 5 gives the [Cl⁻] from IC measurements.

Table A7. IC measured the anions concentration of the first set of formulations (Table 5). Internal method 5 was performed, and the thermal profile of EN 60754-1 was applied. LOQ according to ISO 10304-1: Br⁻ and NO²⁻ ≥ 0.05 mg/L; Cl⁻, F⁻, NO³⁻, PO₄³⁻ and SO₄²⁻ ≥ 0.1 mg/L.

Sample	[Cl−] mg/L	SD [Cl−]	[F−] mg/L	[NO2−] mg/L	[Br−] mg/L	[NO3−] mg/L	[PO43−] mg/L	[SO42−] mg/L
F50.0	113.82	2.32	0.10	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
F50.1	176.10	4.23	0.11	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
F50.2	147.25	2.66	0.11	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
F50.3	21.23	1.21	0.11	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
F50.4	7.42	0.57	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
F50.5	17.41	1.00	0.10	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ

and

Table A8. IC measured the cations concentration of the first set of formulations (Table 5). Internal method 5 was performed, and the EN 60754-1 thermal profile was used. LOQ according to ISO 14911: Li⁺ ≥ 0.01 mg/L; Na⁺, NH₄⁺ and K⁺ ≥ 0.1 mg/L; Mg²⁺ and Ca²⁺ ≥ 0.5 mg/L.

Sample	[Li+] mg/L	[Na+] mg/L	[NH4+] mg/L	[K+] mg/L	[Mg2+] mg/L	[Ca2+] mg/L
F50.0	0.01	0.10	<LOQ	0.10	<LOQ	<LOQ
F50.1	<LOQ	0.10	0.12	0.10	<LOQ	<LOQ
F50.2	<LOQ	0.10	0.19	<LOQ	<LOQ	<LOQ
F50.3	0.01	0.13	<LOQ	0.19	<LOQ	<LOQ
F50.4	0.01	0.10	<LOQ	0.19	<LOQ	<LOQ
F50.5	0.01	0.17	<LOQ	0.33	<LOQ	<LOQ

, reported in Appendix E, show the anions and cations concentration in the solutions performing internal method 5 of formulation in Table 5. The Al, Zn, and Sb values for all formulations in Table 5, detected by ICP-OES, are lower than the LOQ according to ISO 11885. These extremely low elements’ concentrations indicate that the evaporation phenomena are negligible.

Table A9. IC measured the anions concentration of the formulations in Table 6. Internal method 4 was performed, and the EN 60754-2 thermal profile was applied. LOQ: Br⁻ and NO²⁻ ≥ 0.05 mg/L; Cl⁻, F⁻, NO³⁻, PO₄³⁻ and SO₄²⁻ ≥ 0.1 mg/L.

Sample	[Cl−] mg/L	SD [Cl-]	[F−] mg/L	[NO2−] mg/L	[Br−] mg/L	[NO3−] mg/L	[PO43−] mg/L	[SO42−] mg/L
REA3	98.70	1.18	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
REA4	70.94	1.20	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
REA5	97.07	2.78	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
REA6	64.05	1.30	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
REA7	206.06	6.23	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
REA8	197.12	8.23	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
REA9	42.81	2.41	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
REA10	62.05	1.23	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ

and

Table A10. IC measured the anions concentration of the formulations in Table 6. Internal method 5 was performed, and the EN 60754-1 thermal profile was applied. LOQ: Br⁻ and NO²⁻ ≥ 0.05 mg/L; Cl⁻, F⁻, NO³⁻, PO₄³⁻ and SO₄²⁻ ≥ 0.1 mg/L.

Sample	[Cl−] mg/L	SD [Cl-]	[F−] mg/L	[NO2−] mg/L	[Br−] mg/L	[NO3−] mg/L	[PO43−] mg/L	[SO42−] mg/L
REA3	64.47	1.93	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
REA4	60.81	2.43	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
REA5	23.44	1.23	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
REA6	2.98	0.45	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
REA7	173.64	2.45	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
REA8	192.44	3.25	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
REA9	10.33	1.00	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ
REA10	25.00	1.25	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ

, reported in Appendix E, refer to formulations of Table 6; in particular, they display the results of internal methods 4 and 5, applying IC through anion column exchange. The Al, Zn, and Sb values for all formulations in Table 6, detected by ICP-OES in both internal methods, are lower than the LOQ according to ISO 11885, as in the previous cases. Therefore, they are not reported. These extremely low elements’ concentrations indicate that evaporation phenomena are negligible.

Table 13. The pH and conductivity measured from cable compounds from the market. FG16OR16, where G16 is HEPR, R16 is a flame-retarded PVC jacket compound, S18 and R18 are, respectively, PVC insulation and jacket compounds from FS18R18 cable, and S17 is an insulation compound from a single wire FS17 cable. FG16OR16, FG18R18, and FS17 are Italian non-harmonized cables. The PVC plenum compound is a flame-retarded jacket compound developed to meet the requirement of the US standard of NPA 0262 [40], containing 30 phr of ammonium octa molybdate (AOM). TPU-FR is a flame-retarded jacket made up of TPU polyether based with melamine in the intumescent system. Internal method 4 is performed.

Formulation	R16	S18	R18	S17	PVC Plenum	TPU-FR
pH	2.67	2.40	2.65	2.36	2.38	8.20
SDpH	0.13	0.11	0.10	0.09	0.11	0.22
CVpH [%]	4.9	4.8	3.9	3.8	4.8	2.7
Conductivity [μS/mm]	88.4	171.9	94.3	189	243.4	32.2
SDc	1.2	5.4	4.2	9.0	10.0	1.3
CVc [%]	1.4	3.1	4.5	4.8	4.1	4.0
Conductivity [μS/mm] (DHO)	89.7	167.6	92.3	181.3	173.8	n.a.
Accuracy [%]	−1.45	2.57	2.17	4.25	40.0	n.a.

reports pH and conductivity measurements from cable compounds from the market.

4. Discussion

4.1. Species Found in Solutions

Formulations F50.0–F50.5 in Table 5 do not have ATO, and F50.3, F50.4, and F50.5 contain acid scavengers at high temperatures acting in the condensed phase. F50.1 and F50.2, having, respectively, ATH and MDH as not efficient acid scavengers, give the highest acidity at 950 °C (Table 9, pH and conductivity). F50.0 with GCC reaches 2.67 pH, and F50.3–5 with the performant acid scavengers are slightly better than F50.0, which means that at high temperatures, even the excellent acid scavengers (Winnofil S, AS-1B, and AS-6B) suffer the fast kinetic of HCl evolution from zip elimination, as seen in Ref. [5]. Table 10 reports the chlorine concentration in mg/L in column 5 from IC measurements and the comparison with [Cl⁻] (column 4) derived from pH measurements, assuming a complete dissociation of HCl and the absence of other acidic species in the bubblers from side reactions during the combustions. The delta in column 6 indicates clearly that both measurements are comparable. That can be explained from the data in Table A4, where only Cl⁻ is detected, and HCl is acidity’s driving force. The formulation in Table 5 has no bromine, nitrate, nitrite, orthophosphate, or sulfate source.

Regarding the six cations, the possible candidates are some chlorides. Specifically, the chlorides that can be formed through the reaction between some ingredients in the stabilizer one pack, fillers, and flame-retardant fillers are mainly ZnCl₂, CaCl_2, and MgCl₂. ZnCl₂, in formulations F50.0–5, is only yielded from the reaction between a calcium organic stabilizer (COS), a calcium zinc stabilizer, and HCl. The quantity of Zn in all compounds is enough to be seen in the solution in case of evaporation and, therefore, potentially higher than the LOQ of ISO 11885. The data evidence the absence of Zn²⁺ in the solution, which is probably because ZnCl₂ hydrolyzes, yielding ZnO in the ashes and HCl. The CaCl₂ derives from the reaction between HCl with CaCO₃ and calcium soaps in the stabilizer one pack. Obviously, it remains in the condensed phase without evaporation, which occurs massively around the boiling point over 1900 °C. However, it can also hydrolyze over 900 °C, leaving CaO in the ashes and releasing HCl in the gas phase. MgCl₂, which has a boiling point of around 1600 °C, comes from MDH. It is prone to be hydrolyzed between 450 °C and 550 °C, leaving MgO and HCl [41,42]. ATH does not react at all with HCl (which explains the scarce HCl scavenging efficiency of formulation F50.2). Therefore, the data clearly indicate the absence of Zn, Al, Ca, and Mg ions in the bubbling devices, indicating that in formulations F50.0–F50.5, the conductivity is massively affected by HCl only.

The formulations REA01-10 follow the behavior of F50.0–5, confirming that Sb is not found in the solution. That is because SbCl₃ reaching the bubblers gives the following reaction:

SbCl₃ + H₂O → SbOCl + 2HCl

(1)

where SbOCl, insoluble in water, makes Sb not accessible for ICP-OES after filtering procedures, but in any case, it is not available as a free ion capable of affecting the conductivity.

The data obtained through method 5 (Table A10) show the same trends. HCl drives pH and conductivity. The only difference between the two sets is in the concentration of HCl in solutions of formulations containing potent acid scavengers: they release less HCl in the bubblers when the heating conditions are milder (ramp of 20 °C/min) and at a lower temperature (800 °C vs. 950 °C) [5,43].

4.2. Statistical Approach

Assuming an HCl scavenging mechanism in the condensed phase and negligible contribution of the evaporation of some chlorides in the gas phase, HCl is the primary driver of acidity and conductivity. In this condition, the correlation model between conductivity and pH should follow Equation (A4) in Appendix C, according to the DHO theory (DHO model). Appendix C explains in detail the DHO model, and Appendix D explains the conductivity and pH values for the ideal solutions of HCl following the DHO model. The theoretical values predicted by the DHO model are presented in Table 9, Table 11 and Table 13, which are juxtaposed with experimental ones and their accuracy as a metric for the deviation from the model.

Examination of the data in Table 9 and Figure 1a reveals that the accuracy of the conductivity of the investigated formulations remains below 12% in all instances, closely aligning with the predicted by the DHO model.

Building upon the considerations outlined in Section 4.1, Table 9 and Figure 1a highlight formulations characterized by evident HCl scavenging in the condensed phase, with no discernible chloride evaporation, in which HCl drives the acidity and conductivity. In addition, with no information on their constituents, PVC jackets and insulations from the market exhibit analogous behavior (Table 13 and Figure 1b). This observation strongly suggests a condensed-phase mechanism that aligns well with the values predicted by the DHO model. An exception is noted in the plenum compound, demonstrating a deviation of approximately 40%. This anomaly can be rationalized by AOM, which liberates ammonia over 200 °C. [44] The ensuing reaction with HCl introduces an airborne NH₄Cl into the bubblers, exerting a pronounced influence on both pH and conductivity synchronously. In conclusion, the measure of the accuracy as a metric for the deviation from the DHO model can be a valid and rapid screening which can be completed with pH and conductivity measurements in order to understand if the HCl is the driver of the acidity, the mechanism acts in the condensed phase and if there is any interference of chlorides or other substances evaporating during the combustion.

4.3. The Idiosyncrasy of EN 60754-2, EN 50525-1 and EN 50620

Compounds based on polymers different from PVC with different thermal degradation patterns show different correlations between pH and conductivity. A typical example of this is the measurement of flame-retarded TPU polyether, which shows pH 8.20 and conductivity 32.2 μS/mm (Table 13). A cable made with this compound would bring a classification a₃. Measurements revealed the effect of ammonia evolving from melamine, effectively becoming a gas-phase acid scavenger and affecting pH but also conductivity pushed over the limit of 10 μS/mm for the class a₂. That is a bizarre case where such a cable would be in class a₃ (the worst case for acidity) and not halogen free according to the halogen assessment of annex B of EN 50525-1. That also explains the derogation introduced in EN 50620, where the conductivity limit was enhanced to 40 μS/mm as the best compromise between conductivity and flame retardancy.

5. Conclusions

Many standards based on tube furnaces are available to assess indirectly and/or quantify HCl released during the combustion. The paper reviews are featured in the first part, highlighting their current status, where they are used, what they determine, their weaknesses, and the flawless. The case of TPU jackets developed, according to EN 50620 and containing melamine-based intumescent systems as flame retardants, clearly suggests the importance of reviewing the halogen-free assessment in EN 50525-1 and the additional classification for acidity in CPR. In this context, IEC EN 60754-3 based on ion chromatography will probably play a crucial role, and this paper highlights its potentiality.

Indeed, the paper showed how IC is a potent detection method to quantify halogen species and other anions or cations dissolved in solutions from the PVC compound combustion. The quantification of [Cl⁻] through IC aligns with the [Cl⁻] derived from pH measurements, assuming all H comes from the wholly dissociated HCl. Furthermore, it has been shown that IC can be easily performed at the low concentrations indicated in the IEC EN 60754-3 and even at higher concentrations, making the standard a valid substitute for EN 60754-2 and EN 60754-1.

Then, IC and ICP-OES were used to see the main actors driving acidity and the conductivity of the solution in the bubblers. In the process of combustion of CPR compounds for cables performing a ramp to 800 °C or in isothermal at 950 °C, it has been shown that chlorine is the dominant species in the solutions, and no evaporation of substances capable of generating electrolytes occurs. In this way, the solution in the bubblers behaves as a standard solution of HCl following the DHO equation for HCl. Plenum PVC compounds containing AOM behave differently. Here, the decomposition of AOM brings more electrolytes, further increasing the solution’s conductivity. The deviation from the DHO model here is high (more than 40%), which is a clear indication of the scavenging in the gas phase of HCl and the yielding of products capable of severely affecting the conductivity of the solutions in the bubblers.

Finally, the results clearly demonstrate that a straightforward measurement of pH and conductivity and an assessment of their deviation from the DHO equation can serve as a rapid screening method. This approach accounts for the evaporation of chlorides and the formation of acidic, basic, or neutral species in the gas phase, which can influence pH and conductivity to varying degrees compared to HCl.