Surface Chalking upon Weathering of Dark-Colored PVC Articles and Relevant Stabilizers

PVC items (38% carbon atoms w/w) are environmentally friendly as, unlike polyolefins (86% carbon atoms w/w), they are mainly based on chlorine, one of the most abundant elements on Earth and, so, less based on fossil resources. However, in the eco-design context, articles’ durability plays a crucial role, contributing to the enhancement of their sustainability. In this framework, the research on additives capable of increasing the weatherability of outdoor articles is essential. The theory section of the paper reviews the mechanisms of weathering that lead to PVC degradation and undermine the durability of items such as window frames or roller shutters. The weathering of PVC items is a complex phenomenon, involving photo-chemical and secondary chemical reactions, that yields the formation of conjugated polyene sequences underskin in the absence of oxygen and carbonyls in the surface. Here, the chain scission of the polymer backbone occurs, bringing about the disintegration of the surface of the item and causing the typical discoloration called chalking, especially evident in dark-colored articles. In the experimental section of the paper, the effect of different acid scavengers on item weathering has been studied using a natural outdoor and two accelerated exposures with xenon-arc and Q-UV testing devices. Results confirm that some acid scavengers are efficient in preventing chalking, but some are ineffective or even detrimental. Thus, the PVC formulations of durable articles upon weathering still depend on a complex choice of the appropriate ingredients, and several outdoor and indoor accelerated-weathering tests are needed to predict the articles’ lifetime.


Introduction 1.Scope
The durability of PVC items has gained increasing magnitude in order to save resources in the context of the eco-design of the articles [1,2].PVC articles have a primary role in this scenario as they are made up of a sustainable material contributing significantly to energy saving [3,4].However, despite their noteworthy durability, there is still the need to improve their stability, particularly outdoors, where degradation phenomena such as photodegradation, photooxidation, photo-catalyzed oxidation, photobleaching, side chemical reactions, and thermal degradation occur.Dark-colored articles are particularly susceptible to color change [5][6][7] and it was also considered that the colors of plastics may play a role in plastic photoaging [8].In this context, a key role is played by the PVC stabilizers necessary to preserve the resin against degradation in processing and during items' service life.Therefore, it is crucial to understand the chemical mechanism underlying the degradation phenomena to design more effective stabilizers.Many scientific papers have been published on the thermal degradation of PVC, and they are discussed in Refs.[9][10][11][12][13][14][15][16].Although many studies have been published, a complete review of outdoor-weathering mechanisms leading to chalking is still missing.

Theory 1.2.1. Introduction
As PVC is only transparent at wavelengths of more than 250 nm, the initiation is likely due to chromophores found in the PVC resin.However, polyenes rapidly become the main absorbing species among chromophores due to their high extinction coefficient [18].Only polyene sequences with more than four double bonds can interact with sunlight at the ground, as the atmosphere screens wavelengths shorter than about 290 nm (Table 1, [19]).When photons hit the PVC item, two kinds of reactions take place.The first generates conjugated polyene sequences and HCl [18].The chromophores absorb the photons, and one of the non-radiative pathways releases Cl radicals, starting the zip-elimination quickly before oxygen can become involved [18].This yields conjugate-polyene sequences and their crosslinking.The UV screen from polyene sequences protects the matrix from further degradation.
A second series of reactions involves oxygen, and it causes a severe photo-oxidation, bringing about chain scission and the formation of carbonyls (Figures 1 and 2).Zip elimination will dominate where oxygen is lacking.Therefore, it will take place underskin and is called photodegradation.The second series of reactions will take place in the proximity of the surface, where oxygen concentration is high; it is called photo-oxidation (Figures 1 and 2).Titanium dioxide can promote a strong photo-oxidation, and this is called photocatalyzed oxidation.Fillers, commonly found in the PVC compound, can react with HCl, impacting the weatherability of the article.When the photo or photocatalyzed Polymers 2024, 16, 1047 3 of 22 oxidation destroys the surface, it exposes the photodegraded underskin to light and oxygen, and the polyene sequences are oxidized through photobleaching [20].
is called photodegradation.The second series of reactions will take place in the proximity of the surface, where oxygen concentration is high; it is called photo-oxidation (Figures 1  and 2).Titanium dioxide can promote a strong photo-oxidation, and this is called photocatalyzed oxidation.Fillers, commonly found in the PVC compound, can react with HCl, impacting the weatherability of the article.When the photo or photocatalyzed oxidation destroys the surface, it exposes the photodegraded underskin to light and oxygen, and the polyene sequences are oxidized through photobleaching [20].
Figure 1 outlines the overall first stage of the PVC photodegradation and photo-oxidation mechanism leading to chain scission, polyene sequence formation, and crosslinking, depending on the exposure conditions [18].
As is evident from the calculated quantum yields (Table 2), the presence of oxygen also enhances chain scission and triggers the generation of peroxides and ketones [18].
Polymers 2024, 16, 1047 4 of 22 Figure 1 outlines the overall first stage of the PVC photodegradation and photooxidation mechanism leading to chain scission, polyene sequence formation, and crosslinking, depending on the exposure conditions [18].
As is evident from the calculated quantum yields (Table 2), the presence of oxygen also enhances chain scission and triggers the generation of peroxides and ketones [18].Table 2.Quantum yield of the primary process in the photodegradation of PVC (expressed as mmol/einstein −1 ), ref. [18].The reactions of Figure 1 are also described in Figure 2 in a simplified form with complete lines.This latter Figure, however, also outlines two subsequent degradation stages.

Polymer
In fact, the reaction goes further in a second stage (dashed lines in Figure 2), leading to photocleavage of peroxides and ketones and generating further initiating radicals [18].
The following paragraphs will detail the chemistry of photodegradation and photooxidation.

Photodegradation Initiation
Impurities and chromophores trigger photodegradation.Polyenes, hydroperoxides, and ketones are possible chromophores (Figure 3).As is evident from the calculated quantum yields (Table 2), the presence of oxygen also enhances chain scission and triggers the generation of peroxides and ketones [18].Table 2.Quantum yield of the primary process in the photodegradation of PVC (expressed as mmol/einstein −1 ), ref. [18].The reactions of Figure 1 are also described in Figure 2 in a simplified form with complete lines.This latter Figure, however, also outlines two subsequent degradation stages.

Polymer
In fact, the reaction goes further in a second stage (dashed lines in Figure 2), leading to photocleavage of peroxides and ketones and generating further initiating radicals [18].
The following paragraphs will detail the chemistry of photodegradation and photooxidation.

Initiation
Impurities and chromophores trigger photodegradation.Polyenes, hydroperoxides, and ketones are possible chromophores (Figure 3).Hydroperoxides generate radicals upon light irradiation (Figure 4, [21]) as efficiently as ketones do through the Norrish type I reaction (Figure 4, [21]).Polyene sequences, having a high extinction coefficient, are the most absorbent chromophores [18].Table 1 [19] gives the UV-visible absorptions of polyenes of different lengths.Hydroperoxides generate radicals upon light irradiation (Figure 4, [21]) as efficiently as ketones do through the Norrish type I reaction (Figure 4, [21]).Polyene sequences, having a high extinction coefficient, are the most absorbent chromophores [18].Table 1 [19] gives the UV-visible absorptions of polyenes of different lengths.Moreover, photodegradation studies in the presence and absence of oxygen showed the independence of the quantum yield of HCl on the irradiation time and the initial amount of unsaturation.That leads to the hypothesis of alkene-photo-sensitized degradation processes [18,22].In these processes, excited singlet polyenes deactivate by radiative and non-radiative processes.Only 1% of absorbed photons generate HCl due to the restricted mobility that favors radical recombination (reaction 2 of Figure 5) [18].
Moreover, photodegradation studies in the presence and absence of oxygen showed the independence of the quantum yield of HCl on the irradiation time and the initial amount of unsaturation.That leads to the hypothesis of alkene-photo-sensitized degradation processes [18,22].
In these processes, excited singlet polyenes deactivate by radiative and non-radiative processes.Only 1% of absorbed photons generate HCl due to the restricted mobility that favors radical recombination (reaction 2 of Figure 5) [18].

Cl
Cl Cl II + Cl

Polyene Formation
Cage reaction of Cl• with the adjacent CH 2 leads to termination and HCl evolution (Figure 6).
Moreover, photodegradation studies in the presence and absence of oxygen showed the independence of the quantum yield of HCl on the irradiation time and the initia amount of unsaturation.That leads to the hypothesis of alkene-photo-sensitized degrada tion processes [18,22].
In these processes, excited singlet polyenes deactivate by radiative and non-radiative processes.Only 1% of absorbed photons generate HCl due to the restricted mobility tha favors radical recombination (reaction 2 of Figure 5) [18].
the independence of the quantum yield of HCl on the irradiation time and the initia amount of unsaturation.That leads to the hypothesis of alkene-photo-sensitized degrada tion processes [18,22].
In these processes, excited singlet polyenes deactivate by radiative and non-radiativ processes.Only 1% of absorbed photons generate HCl due to the restricted mobility tha favors radical recombination (reaction 2 of Figure 5) [18].
In these processes, excited singlet polyenes deactivate by radiative and non-radiativ processes.Only 1% of absorbed photons generate HCl due to the restricted mobility tha favors radical recombination (reaction 2 of Figure 5) [18].

Cl
Cl Cl II + Cl

Initiation
The same initiation occurs in the presence or absence of oxygen, as the excited single polyenes (with very short lifetimes, about 10 −9 s) cannot be quenched by it (Figure 5, [18])

Peroxidation
The polyenyl radical I (contrary to the less stable radical II) reacts with oxygen (reac tion 12) to give peroxy radical IV.The attack on CH2 of the PVC chain yields radical I (reaction 13).The attack on the CHCl of the PVC chain yields radical III (reaction 14) Radical III does not have labile β-Cl and reacts with oxygen to give γ-chloroalkylperoxy radical V (reaction 15) (Figure 10, [18]).

Reaction of the Peroxy Radical
At low irradiations, radical V abstracts H from PVC (reaction 16).The same radica V, at higher irradiations, either reacts with another peroxy radical to give "termination (reaction 17) or reacts with another peroxy radical to give alkoxy radicals VI (reaction 18 (Figure 11, [18]).Peroxide obtained by reaction 17 is a photoreactive re-initiating site.

Photooxidation Initiation
The same initiation occurs in the presence or absence of oxygen, as the excited singlet polyenes (with very short lifetimes, about 10 −9 s) cannot be quenched by it (Figure 5, [18]).

Peroxidation
The polyenyl radical I (contrary to the less stable radical II) reacts with oxygen (reaction 12) to give peroxy radical IV.The attack on CH2 of the PVC chain yields radical II (reaction 13).The attack on the CHCl of the PVC chain yields radical III (reaction 14).Radical III does not have labile β-Cl and reacts with oxygen to give γ-chloroalkylperoxy radical V (reaction 15) (Figure 10, [18]).

Reaction of the Peroxy Radical
At low irradiations, radical V abstracts H from PVC (reaction 16).The same radical V, at higher irradiations, either reacts with another peroxy radical to give "termination" (reaction 17) or reacts with another peroxy radical to give alkoxy radicals VI (reaction 18) (Figure 11, [18]).Peroxide obtained by reaction 17 is a photoreactive re-initiating site.

Photooxidation Initiation
The same initiation occurs in the presence or absence of oxygen, as the excited singlet polyenes (with very short lifetimes, about 10 −9 s) cannot be quenched by it (Figure 5, [18]).

Peroxidation
The polyenyl radical I (contrary to the less stable radical II) reacts with oxygen (reaction 12) to give peroxy radical IV.The attack on CH 2 of the PVC chain yields radical II (reaction 13).The attack on the CHCl of the PVC chain yields radical III (reaction 14).Radical III does not have labile β-Cl and reacts with oxygen to give γ-chloroalkylperoxy radical V (reaction 15) (Figure 10, [18]).

Initiation
The same initiation occurs in the presence or absence of oxygen, as the excited single polyenes (with very short lifetimes, about 10 −9 s) cannot be quenched by it (Figure 5, [18])

Peroxidation
The polyenyl radical I (contrary to the less stable radical II) reacts with oxygen (reac tion 12) to give peroxy radical IV.The attack on CH2 of the PVC chain yields radical I (reaction 13).The attack on the CHCl of the PVC chain yields radical III (reaction 14) Radical III does not have labile β-Cl and reacts with oxygen to give γ-chloroalkylperoxy radical V (reaction 15) (Figure 10, [18]).

Reaction of the Peroxy Radical
At low irradiations, radical V abstracts H from PVC (reaction 16).The same radica V, at higher irradiations, either reacts with another peroxy radical to give "termination" (reaction 17) or reacts with another peroxy radical to give alkoxy radicals VI (reaction 18 (Figure 11, [18]).Peroxide obtained by reaction 17 is a photoreactive re-initiating site.

Reaction of the Peroxy Radical
At low irradiations, radical V abstracts H from PVC (reaction 16).The same radical V, at higher irradiations, either reacts with another peroxy radical to give "termination" (reaction 17) or reacts with another peroxy radical to give alkoxy radicals VI (reaction 18) (Figure 11, [18]).Peroxide obtained by reaction 17 is a photoreactive re-initiating site.

Reaction of the Alkoxy Radicals
Alkoxy radical VI either abstracts H from PVC (reaction 20), eventually decomposing to the corresponding ketone (reaction 21), or is stabilized by β-scission, giving ketones and either Cl• (reaction 22) or chain scission (reaction 23).These additional Cl• account for a 40% increase of θ HCl in the presence of oxygen (Figure 12, [18]).

Reaction of the Alkoxy Radicals
Alkoxy radical VI either abstracts H from PVC (reaction 20), eventually decomposing to the corresponding ketone (reaction 21), or is stabilized by β-scission, giving ketones and either Cl• (reaction 22) or chain scission (reaction 23).These additional Cl• account for a 40% increase of θ HCl in the presence of oxygen (Figure 12, [18]).

Reaction of the Alkoxy Radicals
Alkoxy radical VI either abstracts H from PVC (reaction 20), eventually decomposing to the corresponding ketone (reaction 21), or is stabilized by β-scission, giving ketones and either Cl• (reaction 22) or chain scission (reaction 23).These additional Cl• account for a 40% increase of θ HCl in the presence of oxygen (Figure 12, [18]).

Reaction of the Alkoxy Radicals
Alkoxy radical VI either abstracts H from PVC (reaction 20), eventually decomposing to the corresponding ketone (reaction 21), or is stabilized by β-scission, giving ketones and either Cl• (reaction 22) or chain scission (reaction 23).These additional Cl• account for a 40% increase of θ HCl in the presence of oxygen (Figure 12, [18]).

Reaction of the Alkoxy Radicals
Alkoxy radical VI either abstracts H from PVC (reaction 20), eventually decomposing to the corresponding ketone (reaction 21), or is stabilized by β-scission, giving ketones and either Cl• (reaction 22) or chain scission (reaction 23).These additional Cl• account for a 40% increase of θ HCl in the presence of oxygen (Figure 12, [18]).

Reaction of the Alkoxy Radicals
Alkoxy radical VI either abstracts H from PVC (reaction 20), eventually decomposing to the corresponding ketone (reaction 21), or is stabilized by β-scission, giving ketones and either Cl• (reaction 22) or chain scission (reaction 23).These additional Cl• account for a 40% increase of θ HCl in the presence of oxygen (Figure 12, [18]).

III (or II)
Russel mechanism chloro-radical generation.

Reaction of the Alkoxy Radicals
Alkoxy radical VI either abstracts H from PVC (reaction 20), eventually decomposing to the corresponding ketone (reaction 21), or is stabilized by β-scission, giving ketones and either Cl• (reaction 22) or chain scission (reaction 23).These additional Cl• account for a 40% increase of θ HCl in the presence of oxygen (Figure 12, [18]).

Photobleaching
Experiments in Ref. [20] were carried out, irradiating samples of PVC (with and without oxygen) with a UV lamp and then irradiating them with a lamp emitting in the solar spectrum region (i.e., 515 nm).In this second stage, a lowering of the absorbance was attributed to the photobleaching of conjugated double bonds by oxygen: in fact by far counterbalances double bond formation (Figure 14).Experiments in Ref. [20] were carried out, irradiating samples of PVC (with and without oxygen) with a UV lamp and then irradiating them with a lamp emitting in the solar spectrum region (i.e., 515 nm).In this second stage, a lowering of the absorbance was attributed to the photobleaching of conjugated double bonds by oxygen: in fact by far counterbalances double bond formation their formation (Figure 14).This kind of bleaching also occurs in the dark, and Figure 15 shows that the neat contribution of photobleaching is reduced by the absorbance increase due to polyenes formation the formation of polyenes, is subtracted.Figure 14 shows the extent of photobleaching (full lines) vs. dehydrochlorination (dashed line) under photolysis.The quantum yield of photobleaching is higher than dehydrochlorination one (θ photobleaching = 1.5 × 10 −2 vs. θ HCl = 0.9 × 10 −2 ).Θ photobleaching is not very dependent on the wavelength.Thus, it is similar regardless of the length of the conjugatedpolyene sequence (as will be schematically described in Figure 16).This kind of bleaching also occurs in the dark, and Figure 15 shows that the neat contribution of photobleaching is reduced by the absorbance increase due to polyenes formation.
Polymers 2024, 16, x FOR PEER REVIEW 9 of 24 Experiments in Ref. [20] were carried out, irradiating samples of PVC (with and without oxygen) with a UV lamp and then irradiating them with a lamp emitting in the solar spectrum region (i.e., 515 nm).In this second stage, a lowering of the absorbance was attributed to the photobleaching of conjugated double bonds by oxygen: in fact by far counterbalances double bond formation their formation (Figure 14).This kind of bleaching also occurs in the dark, and Figure 15 shows that the neat contribution of photobleaching is reduced by the absorbance increase due to polyenes formation the formation of polyenes, is subtracted.Figure 14 shows the extent of photobleaching (full lines) vs. dehydrochlorination (dashed line) under photolysis.The quantum yield of photobleaching is higher than dehydrochlorination one (θ photobleaching = 1.5 × 10 −2 vs. θ HCl = 0.9 × 10 −2 ).Θ photobleaching is not very dependent on the wavelength.Thus, it is similar regardless of the length of the conjugatedpolyene sequence (as will be schematically described in Figure 16).Figure 14 shows the extent of photobleaching (full lines) vs. dehydrochlorination (dashed line) under photolysis.The quantum yield of photobleaching is higher than dehydrochlorination one (θ photobleaching = 1.5 × 10 −2 vs. θ HCl = 0.9 × 10 −2 ).Θ photobleaching is not very dependent on the wavelength.Thus, it is similar regardless of the length of the conjugated-polyene sequence (as will be schematically described in Figure 16).The mechanism leading to dark bleaching involves a step where ground-state oxygen reacts with polyenes (reaction 1 of Figure 17), generating stable cyclic peroxides at room temperature.Their stability stems from very little HCl having evolved during oxygen bleaching in the dark.[20] (red mark means that the reaction is not happening).
O O

Cyclic peroxyde sequences
Figure 16.Amount of double bond reacting with oxygen vs. number of photons absorbed at various wavelengths (; x HCl evolved, ref. [20]).
The mechanism leading to dark bleaching involves a step where ground-state oxygen reacts with polyenes (reaction 1 of Figure 17), generating stable cyclic peroxides at room temperature.Their stability stems from very little HCl having evolved during oxygen bleaching in the dark.The mechanism leading to dark bleaching involves a step where ground-state oxygen reacts with polyenes (reaction 1 of Figure 17), generating stable cyclic peroxides at room temperature.Their stability stems from very little HCl having evolved during oxygen bleaching in the dark.[20] (red mark means that the reaction is not happening).
O O

Cyclic peroxyde sequences
. Cyclic peroxide formation, ref. [20] (red mark means that the reaction is not happening).
Unlike dark-bleaching, in photobleaching (Figure 18), polyenyl radical I is stable enough to react with oxygen (reaction 5).The resulting polyenyl peroxy radical may form cyclic peroxides (reaction 26).Then, a step-by-step polyene sequence-shortening follows, forming adjacent cyclic peroxides (reactions 7, 27).Reactions 26 and 27 also apply to shorter polyene peroxy radicals resulting from reactions 5, 11, and 28, thus accounting for the bleaching of polyenes with n < 14 (not absorbing the 515 nm radiation).The mechanism leading to dark bleaching involves a step where ground-state oxygen reacts with polyenes (reaction 1 of Figure 17), generating stable cyclic peroxides at room temperature.Their stability stems from very little HCl having evolved during oxygen bleaching in the dark.[20] (red mark means that the reaction is not happening).
O O  Studies on the generation of polyenes and carbonyl species in the presence of stabilizers have been performed and led to the conclusion that the (photo)oxidation is diffusion-controlled [23].This concept was completed by further studies on crosslink/chain-) polyene sequences, ref. [20].

Segmentation of Degradation Layers
Studies on the generation of polyenes and carbonyl species in the presence of stabilizers have been performed and led to the conclusion that the (photo)oxidation is diffusioncontrolled [23].This concept was completed by further studies on crosslink/chain-scission ratio, molecular weight, presence of polyenes, and carbonyl species along the thickness of an article exposed to sunlight (Figure 19, [24]).

Segmentation of Degradation Layers
Studies on the generation of polyenes and carbonyl species in the presence of stabilizers have been performed and led to the conclusion that the (photo)oxidation is diffusion-controlled [23].This concept was completed by further studies on crosslink/chainscission ratio, molecular weight, presence of polyenes, and carbonyl species along the thickness of an article exposed to sunlight (Figure 19, [24]).That experiment showed that there are three zones (roughly indicated by the different colors in Figure 20) along the thickness of a PVC article exposed to sunlight: 1.
A superficial zone (about 50 microns thick) with the predominance of oxidation products and chain scission: oxidation is oxygen diffusion-controlled in this region; chain scission results from oxidation reactions and is followed by chalking.

2.
A lower underskin zone (between 50 and 300 microns) with a predominance of conjugated polyenes.This region is dominated by crosslinking (resulting from C• radicals) and polyene growth.

3.
An undegraded core zone beyond 300 microns.In this region, photochemical reactions do not occur as the photons are screened by polyenes when they are increasingly generated after a specific time.
That experiment showed that there are three zones (roughly indicated by the different colors in Figure 20) along the thickness of a PVC article exposed to sunlight: 1.A superficial zone (about 50 microns thick) with the predominance of oxidation products and chain scission: oxidation is oxygen diffusion-controlled in this region; chain scission results from oxidation reactions and is followed by chalking.2. A lower underskin zone (between 50 and 300 microns) with a predominance of conjugated This region is dominated by crosslinking (resulting from C• radicals) and polyene growth.3.An undegraded core zone beyond 300 microns.In this region, photochemical reactions do not occur as the photons are screened by polyenes when they are increasingly generated after a specific time.The formation of the two superficial degraded layers occurs because: 1.At the beginning of the exposure, the irradiation layer (X1) is higher than the diffusion-controlled Oxidation layer (Xco) 2. Later on, polyene build-up provides a screen effect, and the irradiation layer (X1) becomes lower than the oxidation layer (Xco); this prevents any progression of the oxidation-front towards the core (Figure 21, [25]) Figure 21.Change of the thickness of irradiated layer X1 and oxygenated layer Xco, ref. [25].X1 = thickness where 90% of the irradiation is absorbed 1.At a certain point in time, the superficial layer is embrittled by oxidative chain scission and cracks producing chalking that is, naturally, mechanically removed from the surface.2. The polyenes, lacking their «protective layer», undergo photobleaching, shifting the boundary between degraded zones towards the core.The formation of the two superficial degraded layers occurs because: 1.
At the beginning of the exposure, the irradiation layer (X 1 ) is higher than the diffusioncontrolled Oxidation layer (Xco) 2.
Later on, polyene build-up provides a screen effect, and the irradiation layer (X 1 ) becomes lower than the oxidation layer (Xco); this prevents any progression of the oxidation-front towards the core (Figure 21, [25]).
That experiment showed that there are three zones (roughly indicated by the differ ent colors in Figure 20) along the thickness of a PVC article exposed to sunlight: 1.A superficial zone (about 50 microns thick) with the predominance of oxidation prod ucts and chain scission: oxidation is oxygen diffusion-controlled in this region; chai scission results from oxidation reactions and is followed by chalking.2. A lower underskin zone (between 50 and 300 microns) with a predominance of con jugated polyenes.This region is dominated by crosslinking (resulting from C• rad cals) and polyene growth.3.An undegraded core zone beyond 300 microns.In this region, photochemical reac tions do not occur as the photons are screened by polyenes when they are increas ingly generated after a specific time.The formation of the two superficial degraded layers occurs because: 1.At the beginning of the exposure, the irradiation layer (X1) is higher than the diffu sion-controlled Oxidation layer (Xco) 2. Later on, polyene build-up provides a screen effect, and the irradiation layer (X1) be comes lower than the oxidation layer (Xco); this prevents any progression of the ox dation-front towards the core (Figure 21, [25]) Figure 21.Change of the thickness of irradiated layer X1 and oxygenated layer Xc ref. [25].X1 = thickness where 90% of the irradiation is absorbed 1.At a certain point in time, the superficial layer is embrittled by oxidative chain scis sion and cracks producing chalking that is, naturally, mechanically removed from the surface.2. The polyenes, lacking their «protective layer», undergo photobleaching, shifting th boundary between degraded zones towards the core.1.At a certain point in time, the superficial layer is embrittled by oxidative chain scission and cracks producing chalking that is, naturally, mechanically removed from the surface.

2.
The polyenes, lacking their «protective layer», undergo photobleaching, shifting the boundary between degraded zones towards the core.
The sequential generation of dark polyene sequences, their bleaching, and eventually the erosion of the cracked surface triggers the cyclic change of color typical in outdoor exposure [26,27].
The thickness of the oxidation profile (X co ) increases with higher temperatures due to the higher diffusivity of oxygen, while it is independent of irradiation intensity.The depth of the max polyene concentration decreases with higher irradiation intensity and increases with higher temperature.
As the outer layer of PVC degrades, particles of pigment like titanium dioxide and fillers like CaCO 3 are released, producing a white powdery deposit; this phenomenon is known as "chalking."It is known that different colors may affect plastic photoaging by influencing its solar absorbance [8].In addition, as chalking makes the article look whitish, this phenomenon is more apparent in dark-colored samples than in the white-pigmented ones.This particular weakness of dark-colored articles is well known and is a genuine challenge for manufacturers [5][6][7].

Effects of Pigments and Fillers Effects of Titanium Dioxide Pigments
Figure 22 shows that titanium dioxide limits light penetration to 1-fifth.(Figure 22c).The fact that the thickness of the degraded layer is 1-third (Figure 22a for carbonyls, b for polyenes) [28] is due to the following reasons: 1.
UV light penetration decreases with time as polyenes build up.

2.
Small radicals such as OH• and Cl• diffuse beyond the irradiation layer.

3.
Photoreactions can be initiated by wavelength radiation close to the titanium dioxide cut-off (i.e., the limit between transmittance absorption and scattering, 365 nm).
Polymers 2024, 16, x FOR PEER REVIEW 13 of 24 The sequential generation of dark polyene sequences, their bleaching, and eventually the erosion of the cracked surface triggers the cyclic change of color typical in outdoor exposure [26,27].
The thickness of the oxidation profile (Xco) increases with higher temperatures due to the higher diffusivity of oxygen, while it is independent of irradiation intensity.The depth of the max polyene concentration decreases with higher irradiation intensity and increases with higher temperature.
As the outer layer of PVC degrades, particles of pigment like titanium dioxide and fillers like CaCO3 are released, producing a white powdery deposit; this phenomenon is known as "chalking."It is known that different colors may affect plastic photoaging by influencing its solar absorbance [8].In addition, as chalking makes the article look whitish, this phenomenon is more apparent in dark-colored samples than in the white-pigmented ones.This particular weakness of dark-colored articles is well known and is a genuine challenge for manufacturers [5][6][7].

Effects of Pigments and Fillers
Effects of Titanium Dioxide Pigments Figure 22 shows that titanium dioxide limits light penetration to 1-fifth.(Figure 22c).The fact that the thickness of the degraded layer is 1-third (Figure 22a for carbonyls, b for polyenes) [28] is due to the following reasons: 1. UV light penetration decreases with time as polyenes build up. 2. Small radicals such as OH• and Cl• diffuse beyond the irradiation layer.3. Photoreactions can be initiated by wavelength radiation close to the titanium dioxide cut-off (i.e., the limit between transmittance absorption and scattering, 365 nm).In addition, it is also well known that, despite its screening effect, titanium dioxide (if not adequately coated or in the anatase form) promotes polymer degradation [29].Furthermore, due to the PVC surface layer's degradation, titanium dioxide contributes to the chalking phenomenon described above, in the presence of water, as usually it happens in hot, humid climates such as Florida.On the other hand, chalking is not a common issue in hot, dry climates.This behavior is due to the typical mechanism through which TiO2 brings about the oxidation of the organic substances in the presence of water [18,30].

Effects of Fillers
Fillers also influence the degradation and chalking of weathered PVC articles.CaCO3 is an example of a filler capable of reacting with the HCl from PVC degradation to give the water-soluble calcium chloride.Its washout leaves holes, increasing the surface area subject to degradation and worsening the weathering.This phenomenon is increasingly evident if the filler amount rises [31].

HCl Effect on Carbonyl Formation and Inhibition
Schemes of Figures 1 and 2 do not involve HCl in the reactions in the presence of oxygen.However, it was observed that, in PVC photooxidation, there is a first autocatalytic phase in the formation and a subsequent inhibition phase for carbonyl species (Figure 23, [32]).As evident at short and long irradiation times, the initial auto-acceleration and the final auto-inhibition are favored when the thickness is high: the diffusion of a reactant from the sample to the atmosphere can thus control their kinetic behavior.In addition, it is also well known that, despite its screening effect, titanium dioxide (if not adequately coated or in the anatase form) promotes polymer degradation [29].Furthermore, due to the PVC surface layer's degradation, titanium dioxide contributes to the chalking phenomenon described above, in the presence of water, as usually it happens in hot, humid climates such as Florida.On the other hand, chalking is not a common issue in hot, dry climates.This behavior is due to the typical mechanism through which TiO 2 brings about the oxidation of the organic substances in the presence of water [18,30].

Effects of Fillers
Fillers also influence the degradation and chalking of weathered PVC articles.CaCO 3 is an example of a filler capable of reacting with the HCl from PVC degradation to give the water-soluble calcium chloride.Its washout leaves holes, increasing the surface area subject to degradation and worsening the weathering.This phenomenon is increasingly evident if the filler amount rises [31].

HCl Effect on Carbonyl Formation and Inhibition
Schemes of Figures 1 and 2 do not involve HCl in the reactions in the presence of oxygen.However, it was observed that, in PVC photooxidation, there is a first autocatalytic phase in the formation and a subsequent inhibition phase for carbonyl species (Figure 23, [32]).As evident at short and long irradiation times, the initial auto-acceleration and the final auto-inhibition are favored when the thickness is high: the diffusion of a reactant from the sample to the atmosphere can thus control their kinetic behavior.In addition, it is also well known that, despite its screening effect, titanium dioxide (if not adequately coated or in the anatase form) promotes polymer degradation [29].Furthermore, due to the PVC surface layer's degradation, titanium dioxide contributes to the chalking phenomenon described above, in the presence of water, as usually it happens in hot, humid climates such as Florida.On the other hand, chalking is not a common issue in hot, dry climates.This behavior is due to the typical mechanism through which TiO2 brings about the oxidation of the organic substances in the presence of water [18,30].

Effects of Fillers
Fillers also influence the degradation and chalking of weathered PVC articles.CaCO3 is an example of a filler capable of reacting with the HCl from PVC degradation to give the water-soluble calcium chloride.Its washout leaves holes, increasing the surface area subject to degradation and worsening the weathering.This phenomenon is increasingly evident if the filler amount rises [31].

HCl Effect on Carbonyl Formation and Inhibition
Schemes of Figures 1 and 2 do not involve HCl in the reactions in the presence of oxygen.However, it was observed that, in PVC photooxidation, there is a first autocatalytic phase in the formation and a subsequent inhibition phase for carbonyl species (Figure 23, [32]).As evident at short and long irradiation times, the initial auto-acceleration and the final auto-inhibition are favored when the thickness is high: the diffusion of a reactant from the sample to the atmosphere can thus control their kinetic behavior.This reactant may, thus, be a gas such as HCl, which can explain the initial autoacceleration with hydroxyl radical formation (Figure 24).This reactant may, thus, be a gas such as HCl, which can explain the initial autoacceleration with hydroxyl radical formation (Figure 24).HCl can also explain the final auto-inhibition.A charge transfer complex of HCl with polyenes (not photochemically active, different from polyenes themselves) is described in the literature (Figure 25).

Experimental
As degradation generates cracking and the final effect is the chalking of the surface, this mainly affects the color variation of dark-colored articles.For this reason, several dark-green samples were prepared, as this color is particularly affected by weathering.
In the presence of pigments with high light fastness, the color change is due mainly to the chalking from the degraded surface rather than the degradation of the pigment itself.As shown above, carbonyl formation is limited to the surface as well ( § 1.2.5.).Therefore, as HCl seems to play a role in the carbonyl formation ( § 1.2.7.), samples with an increasingly higher loading of acid scavengers were prepared to study their effect on surface degradation of the dark-green-colored samples.Several acid scavengers are available and among them are hydrotalcites (also exchanged with different metals such as zinc, lithium, tin, titanium, and zirconium), hydrocalumite, ettringites, dawsonites, garnets, calcium and magnesium hydroxides, zeolites, metal carboxylates, organic phosphites, and epoxy compounds.Their general mechanism of action is well known and reviewed in the literature [33].

Sample Preparation
The formulations in Table 3 were prepared by adding the indicated acid scavengers to the base formulation, prepared as described above, into a dry blend obtained using a PlasMec (Lonate Pozzolo, Varese, Italy) laboratory high-speed heater and cooler mixer HCl-catalyzed hydroperoxide decomposition, ref. [32].
HCl can also explain the final auto-inhibition.A charge transfer complex of HCl with polyenes (not photochemically active, different from polyenes themselves) is described in the literature (Figure 25).This reactant may, thus, be a gas such as HCl, which can explain the initial autoacceleration with hydroxyl radical formation (Figure 24).HCl can also explain the final auto-inhibition.A charge transfer complex of HCl with polyenes (not photochemically active, different from polyenes themselves) is described in the literature (Figure 25).

Experimental
As degradation generates cracking and the final effect is the chalking of the surface, this mainly affects the color variation of dark-colored articles.For this reason, several dark-green samples were prepared, as this color is particularly affected by weathering.
In the presence of pigments with high light fastness, the color change is due mainly to the chalking from the degraded surface rather than the degradation of the pigment itself.As shown above, carbonyl formation is limited to the surface as well ( § 1.2.5.).Therefore, as HCl seems to play a role in the carbonyl formation ( § 1.2.7.), samples with an increasingly higher loading of acid scavengers were prepared to study their effect on surface degradation of the dark-green-colored samples.Several acid scavengers are available and among them are hydrotalcites (also exchanged with different metals such as zinc, lithium, tin, titanium, and zirconium), hydrocalumite, ettringites, dawsonites, garnets, calcium and magnesium hydroxides, zeolites, metal carboxylates, organic phosphites, and epoxy compounds.Their general mechanism of action is well known and reviewed in the literature [33].

Sample Preparation
The formulations in Table 3 were prepared by adding the indicated acid scavengers to the base formulation, prepared as described above, into a dry blend obtained using a PlasMec (Lonate Pozzolo, Varese, Italy) laboratory high-speed heater and cooler mixer

Experimental
As degradation generates cracking and the final effect is the chalking of the surface, this mainly affects the color variation of dark-colored articles.For this reason, several dark-green samples were prepared, as this color is particularly affected by weathering.
In the presence of pigments with high light fastness, the color change is due mainly to the chalking from the degraded surface rather than the degradation of the pigment itself.As shown above, carbonyl formation is limited to the surface as well (Section 1.2.5).Therefore, as HCl seems to play a role in the carbonyl formation (Section 1.2.7), samples with an increasingly higher loading of acid scavengers were prepared to study their effect on surface degradation of the dark-green-colored samples.Several acid scavengers are available and among them are hydrotalcites (also exchanged with different metals such as zinc, lithium, tin, titanium, and zirconium), hydrocalumite, ettringites, dawsonites, garnets, calcium and magnesium hydroxides, zeolites, metal carboxylates, organic phosphites, and epoxy compounds.Their general mechanism of action is well known and reviewed in the literature [33].

Sample Preparation
The formulations in Table 3 were prepared by adding the indicated acid scavengers to the base formulation, prepared as described above, into a dry blend obtained using a PlasMec (Lonate Pozzolo, Varese, Italy) laboratory high-speed heater and cooler mixer combination.A sample without any acid scavenger is not presented as, in its absence, PVC samples would not have been obtained, as it is a key component of thermal stabilizers too.#19 #20 1 From PQ Silicas BV, Eijsden Netherlands. 2 Acid scavenger whose patent is pending. 3Acid scavenger whose patent is pending. 4Stearic acid coated ground milled brucite from Lehmann & Voss & Co., Hamburg, Germany. 5Seastab 510 From Sea Water Chemical, Fukuoka, Japan comprises (Ca, Mg)(OH) 2 , SiO 2 and may have, for example, about 67.2 weight percent CaO, 1.09 weight percent MgO, and 3.26 weight percent SiO 2 .
The following protocol was used for the blending: • Ambient temperature: Start low-speed, add all components, and switch to high speed   3) at increased loadings.3) at increased loadings.

Results and Discussion
As it is evident from Figure 26, both natural-outdoor and accelerated-weathering data are in accordance and show that higher loadings of some acid scavengers are more effective than others against photodegradation (e.g., Acid Scavenger One-pack A and Acid Scavenger One-pack B), while some of them are almost ineffective (e.g., simple Magnesium di-hydroxide, or simple Zeolite 4A), or even detrimental as a trivial (Ca, Mg)(OH) 2 , SiO 2 .
Increased loadings of Zeolite 4A and, in particular, Magnesium di-hydroxide, has no impact on color change during exposure, which is evident in all three exposure types.
An increased loading of (Ca, Mg)(OH) 2 , SiO 2 , is even increasingly detrimental for the color change when it raises from 1 phr to 4 or 8 phr.This is probably due to the reaction of HCl with calcium, generating water-soluble CaCl 2 .As in the case of CaCO 3 , outlined in Section 1.2.6,CaCl 2 brings water into the matrix and makes the surface more prone to photo-oxidation.Furthermore, it is washed out during natural outdoor rain times and in accelerated-weathering devices during their wet cycles, leaving holes, cavities, and an uneven surface.Thus, the increased surface area and a stronger photo-oxidation worsen the weathering (Section 1.2.6).As in the case of CaCO 3 , this phenomenon is increasingly evident as the amount rises.This adverse effect probably offsets the beneficial HCl scavenging mechanism.
Zeolite 4A provides slightly higher protection at 4 and 8 phr, while an increase in Magnesium di-Hydroxide loading does not.
On the contrary, well-balanced acid-scavenger packages like Acid Scavenger One-Pack A and Acid Scavenger One-Pack B provide increasingly stronger protection against weathering as their loading increases.In particular, Acid Scavenger One-Pack B provides the strongest protection effect, lowering Delta E when it is increased from 4 to 8 phr.On the contrary, Acid Scavenger One-Pack A effect is leveled off at a concentration above 4 phr.
It is noteworthy that milder (and closer to reality) weathering methods allow for better discrimination between the performance of formulations than accelerated ones do.For example, this is evident for Acid Scavengers One-Pack A where the difference in performance between its 2 and 4-8 phr is well evident in natural weathering.On the contrary, it is less noticeable in the more aggressive xenon-arc tester, and almost not evident in the far more aggressive QUV tester.Regarding Acid Scavenger One-Pack B, natural weathering well discriminates among the performance of its 1, 4, and 8 phr loading.This gap is narrower in xenon-arc tester where the difference between 1 and 4 phr is negligible.In the QUV test apparatus, the Acid Scavenger One-Pack B loading effect is not evident at all.The slightly improved protection of Zeolite 4A at higher loading is not evident at all, not only in the most accelerated QUV tester, but also in the intermediate accelerated xenon-arc tester.
This emphasizes the importance of extended testing in the real field for a better understanding of the weathering performance, while accelerated-weathering tests should be used only for screening purposes.From a practical point of view, an accelerated screening QUV test would have been helpful to discriminate between evidently detrimental additives (e.g., (Ca, Mg)(OH) 2 , SiO 2 ) and beneficial ones (e.g., Acid Scavenger One-Pack B), but only natural outdoor exposure has been used to investigate the loading effect of the latter.

Conclusions
Acid scavengers are remarkable substances designed to prevent thermal degradation and scavenge HCl during the thermal decomposition and combustion of low-acidity PVC compounds for cables [38][39][40].For these reasons, they already are crucial ingredients in formulating PVC compounds.
This paper shows another, somehow new aspect of some acid scavengers in preventing the drawbacks of exposure of the items to the outdoors, where thermal degradation and photodegradation, photooxidation, photo-catalyzed oxidation, and chemical reactions bring about phenomena affecting not only the aesthetic but also the mechanical properties of the articles.

Figure 19 .
Figure 19.Synoptic comparison along the thickness of the sample during photodegradation: depth distribution of chain scission n over crosslink x ratio (a); depth distribution of average molecular weight Mw and Mn (b); depth distribution of polyene concentration (c); depth distribution of carbonyl concentration (d), ref. [24].

Figure 19 .
Figure 19.Synoptic comparison along the thickness of the sample during photodegradation: depth distribution of chain scission n over crosslink x ratio (a); depth distribution of average molecular weight Mw and Mn (b); depth distribution of polyene concentration (c); depth distribution of carbonyl concentration (d), ref. [24].

Figure 21 .
Figure21.Change of the thickness of irradiated layer X 1 and oxygenated layer X co , ref.[25].X 1 = thickness where 90% of the irradiation is absorbed.

Figure 22 .
Figure 22.Degradation product profile for carbonyl and polyenes ((a,b), respectively) and irradiation profile (c).Blue squares: with titanium dioxide, Orange dots: without titanium dioxide, ref. [28] (yellow arrows indicate the difference in distance from the surface of the variable in function of the presence or absence of titanium dioxide).

Figure 22 .
Figure 22.Degradation product profile for carbonyl and polyenes ((a,b), respectively) and irradiation profile (c).Blue squares: with titanium dioxide, Orange dots: without titanium dioxide, ref. [28] (yellow arrows indicate the difference in distance from the surface of the variable in function of the presence or absence of titanium dioxide).

Polymers 2024 , 24 Figure 22 .
Figure 22.Degradation product profile for carbonyl and polyenes ((a,b), respectively) and irradiation profile (c).Blue squares: with titanium dioxide, Orange dots: without titanium dioxide, ref. [28] (yellow arrows indicate the difference in distance from the surface of the variable in function of the presence or absence of titanium dioxide).

Figure 26 .
Figure 26.Color variation (Delta E) upon natural outdoor, accelerated Xenon-arc, and accelerated Q-UV exposure of samples in the function of different acid scavengers (listed in Table3) at increased loadings.

Table 3 .
Type and loading of tested acid scavengers vs. formulation numbers of Figure26.