A Statistical Analysis on the Effect of Antioxidants on the Thermal-Oxidative Stability of Commercial Mass- and Emulsion-Polymerized ABS

polymers

Article A Statistical Analysis on the Effect of Antioxidants on the Thermal-Oxidative Stability of Commercial Mass- and Emulsion-Polymerized ABS

Rudinei Fiorio 1,* , Dagmar R. D’hooge 2,3,* , Kim Ragaert 1 and Ludwig Cardon 1

1 Centre for Polymer and Material Technologies, Department of Materials, Textiles and Chemical Engineering, Ghent University, Technologiepark 915, 9052 Zwijnaarde, Belgium; [email protected] (K.R.); [email protected] (L.C.) 2 Laboratory for Chemical Technology, Department of Materials, Textiles and Chemical Engineering, Ghent University, Technologiepark 914, 9052 Zwijnaarde, Belgium 3 Centre for Textiles Science and Engineering, Department of Materials, Textiles and Chemical Engineering, Ghent University, Technologiepark 907, 9052 Zwijnaarde, Belgium * Correspondence: rudinei.ﬁ[email protected] (R.F.); [email protected] (D.R.D.); Tel.: +32-092-945-830 (R.F.); +32-093-311-772 (D.R.D.) Received: 27 November 2018; Accepted: 20 December 2018; Published: 25 December 2018

Abstract: In the present work, statistical analysis (16 processing conditions and 2 virgin unmodified samples) is performed to study the influence of antioxidants (AOs) during acrylonitrile-butadiene-styrene terpolymer (ABS) melt-blending (220 ◦C) on the degradation of the polybutadiene (PB) rich phase, the oxidation onset temperature (OOT), the oxidation peak temperature (OP), and the yellowing index (YI). Predictive equations are constructed, with a focus on three commercial AOs (two primary: Irganox 1076 and 245; and one secondary: Irgafos 168) and two commercial ABS types (mass- and emulsion-polymerized). Fourier transform infrared spectroscopy (FTIR) results indicate that the nitrile absorption peak at 2237 cm−1 is recommended as reference peak to identify chemical changes in the PB content. The melt processing of unmodified ABSs promotes a reduction in OOT and OP, and promotes an increase in the YI. ABS obtained by mass polymerization shows a higher thermal-oxidative stability. The addition of a primary AO increases the thermal-oxidative stability, whereas the secondary AO only increases OP. The addition of the two primary AOs has a synergetic effect resulting in higher OOT and OP values. Statistical analysis shows that OP data are influenced by all three AO types, but 0.2 m% of Irganox 1076 displays high potential in an industrial context.

Keywords: ABS; statistical analysis; FTIR; melt processing; thermal-oxidative stability

1. Introduction Electrical and electronic equipment (EEE) is extensively used worldwide and the production of these devices is increasing annually [1]. This results in a gradual increase in EEE waste (WEEE) generated after the end of life of these devices [1,2]. In 2016, 44.7 million metric tons of WEEE were produced worldwide and by 2021, it is expected that 52.2 million metric tons of WEEE will be created [1]. Manufacturing sustainable EEE is necessary, since the natural resources that are used to produce such devices are limited. Concerned about the increasing amount of WEEE, the European Parliament and the Council of the European Union [3] adopted the Directive 2012/19/EU, which establishes “measures to protect the environment and human health by preventing or reducing the adverse impacts of the generation and management of waste from EEE.” One of the required actions

Polymers 2019, 11, 25; doi:10.3390/polym11010025 www.mdpi.com/journal/polymers Polymers 2019, 11, 25 2 of 15 stated in this directive is associated with the recycling of WEEE, in which at least 55% of WEEE must be prepared for re-use and recycling starting in 2018. Based on life-cycle assessment studies, the benefits of plastics recycling for the environment are evident. The recycling of plastics from WEEE results in environmental impacts about 4 times lower than those from the disposal in municipal solid waste incineration plants, and up to 10 times lower than those found in the production of virgin plastics [4]. Moreover, when considering all plastics (not only from WEEE), the use of virgin plastics instead of the recycled ones can increase the environmental impact by almost 4 times [5]. Among the plastics found in WEEE, styrene-based plastics such as high-impact polystyrene (HIPS) and acrylonitrile-butadiene-styrene terpolymer (ABS) are frequently present [6–8], and can account for more than 80 m% of the total amount [6]. Therefore, recycling HIPS and ABS is particularly important for the environment. However, it is well known that post-consumer recycled (PCR) plastics usually have different physical properties compared to those of virgin plastics. Polymers can suffer thermal-mechanical degradation during processing as well as degradation during lifetime due to many factors in the environment (heat, oxygen, light, moisture, etc.) [9]. Processing HIPS and ABS can modify physical-chemical properties due to thermal-oxidative degradation processes accelerated in the polybutadiene (PB) rich phase, causing losses in mechanical properties, as well as aesthetic modifications [10–15]. In order to improve the performance of PCR plastics, it is a common practice to blend virgin and recycled polymers or to consider suitable additives [16]. Specific additives, such as antioxidants (AOs), can be mingled with both virgin and recycled polymers to achieve the required properties [4]. There are two main types of AOs: primary AOs, which act as radical scavengers, and secondary AOs, which act as hydroperoxide scavengers [14,17]. Figure1 shows a general scheme of the thermal-oxidative degradation of polymers without AOs (full lines) as well as in the presence of AOs (dashed lines). According to this scheme, a primary macroalkyl radical (R·) can be initially formed from a polymer chain segment (RH) due to, for example, heat (∆), shear stress (τ), metallic impurities, or a combination of these factors, among others. Peroxide radicals (ROO·) are formed by the reaction with molecular oxygen. Then, ROO· can form hydroperoxides (ROOH) upon abstraction of hydrogen from another RH segment thereby promoting the formation of new R· species and establishing an autoxidation cycle. ROOH can decompose to alkoxy and hydroxyl radicals, which can react with RH, also producing R· species [11,14]. The addition of primary AOs (e.g., sterically hindered phenols) inhibits radical chain reactions. Additional stabilization can be obtained if secondary AOs (such as phosphites) are added, which can reduce hydroperoxides to alcohol species [18]. A synergistic effect can be obtained if different AOs are used together [17]. The influence of AOs on the stabilization of specifically PB or PB-containing polymers has been studied by several groups. De Paoli et al. [19] investigated the effects of different hindered phenols and amine stabilizers on the photooxidation of butadiene rubber, and concluded that a very effective photostabilizer is octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (Irganox 1076), which is a fully sterically hindered monophenol. For thermal-oxidative degradation of a polystyrene impact-modified with PB, primary AOs such as Irganox 1076 and ethylenebis(oxyethylene)bis-(3-(5-tert-butyl-4-hydroxy-m-tolyl)-propionate) (Irganox 245), which is a partially hindered bis-phenol, inhibits degradation [14]. The influence of AOs on the thermal-oxidative stability of an ABS containing 25% polybutadiene was evaluated by Zweifel [14], and it was observed that Irganox 245 promoted higher stabilization than Irganox 1076. Földes and Lohmeijer [20] studied the performance of two commercial primary AOs (Irganox 1076 and Irganox 245) on the oxidative degradation of polybutadiene and concluded that, up to 2 m% of AO, Irganox 1076 presented better AO efficiency in oven aging (at 100 ◦C), whereas Irganox 245 showed higher efficiency in differential scanning calorimetry (DSC) experiments at 190 ◦C. Duh et al. [21] studied the oxidation of ABS (PB contents from 25 to 60 m%), using higher amounts of AO for ABS with higher PB content, and verified that the oxidation onset temperature was influenced by the AO, not by the quantity of PB. Polymers 2018, 10, x FOR PEER REVIEW 2 of 15

Based on life-cycle assessment studies, the benefits of plastics recycling for the environment are evident. The recycling of plastics from WEEE results in environmental impacts about 4 times lower than those from the disposal in municipal solid waste incineration plants, and up to 10 times lower than those found in the production of virgin plastics [4]. Moreover, when considering all plastics (not only from WEEE), the use of virgin plastics instead of the recycled ones can increase the Polymers 2019, 11, 25 3 of 15 environmental impact by almost 4 times [5]. Among the plastics found in WEEE, styrene-based plastics such as high-impact polystyrene Motyakin(HIPS) and and acrylonitrile-butadiene-styrene Schlick [22] investigated the effectterpolymer of a hindered (ABS) are amine frequently stabilizer present (HAS, [6–8], commercially and can knownaccount as for Tinuvin more 770;than 1 or80 2m% m%) of on the the total thermal amount degradation [6]. Therefore, of two ABSrecycling materials HIPS (one and prepared ABS is byparticularly mass polymerization important for containing the environment. 10 m% However, butadiene, it and is well the known other by that emulsion post-consumer polymerization recycled containing(PCR) plastics 25 m%usually butadiene) have different and observed, physical at properties ﬁrst sight, compared an unexpected to those faster of degradationvirgin plastics. of polymerPolymers samples can suffer that thermal-mech contained a higheranical degradation HAS content during (2 m%). processing These authors as well also as veriﬁeddegradation that theduring emulsion-polymerized lifetime due to many ABS factors showed in better the environment thermal stability, (heat, and oxygen, associated light, these moisture, results withetc.) the[9]. presenceProcessing of aHIPS cross-linked and ABS network can modify in the PB physical-c fraction, whichhemical is expectedproperties to promotedue to thermal-oxidative a reduction in the freedegradation volume and processes oxygen accelerated diffusion [22 in,23 the]. Suchpolybutadiene PB crosslinking (PB) rich also phase, occurs causing during aginglosses and in mechanical processing formingproperties, gels; as however,well as aesthetic the addition modifications of AOs can [10–15]. improve the stability, reducing crosslinking [14,24].

FigureFigure 1.1. GeneralGeneral scheme scheme of of thermal-oxidative thermal-oxidative degradation degradation of polymerof polymer (depicted (depicted for segmentfor segment RH) RH) and itsand inhibition its inhibition by primary by primary and secondaryand second antioxidantsary antioxidants (adapted (adapted from from [14]). [14]). According to the above studies, the oxidative stability of ABS is inﬂuenced by the type of AO In order to improve the performance of PCR plastics, it is a common practice to blend virgin and as well as the polymerization method (leading to different PB amounts) in a non-trivial manner. recycled polymers or to consider suitable additives [16]. Specific additives, such as antioxidants Additional analyses are still necessary in order to evaluate the inﬂuence of the combination of (AOs), can be mingled with both virgin and recycled polymers to achieve the required properties [4]. different AOs, the polymerization method, and its interactions on the stability of ABS materials. There are two main types of AOs: primary AOs, which act as radical scavengers, and secondary AOs, The aim of the present work was to evaluate the effect of three commercially available AOs, of which which act as hydroperoxide scavengers [14,17]. Figure 1 shows a general scheme of the thermal- two were primary and one was secondary, and two types of ABS with similar mechanical properties, oxidative degradation of polymers without AOs (full lines) as well as in the presence of AOs (dashed either obtained by mass polymerization or emulsion polymerization, on the thermal-oxidative stability lines). According to this scheme, a primary macroalkyl radical (R·) can be initially formed from a and discoloration properties. A factorial analysis of variance (ANOVA) was conducted to evaluate the polymer chain segment (RH) due to, for example, heat (Δ), shear stress (τ), metallic impurities, or a effects of all factors and their interactions. To the best of the authors’ knowledge, a statistical analysis combination of these factors, among others. Peroxide radicals (ROO·) are formed by the reaction with has not yet been conducted, but it is essential to further identify the dominant contributors to oxidative molecular oxygen. Then, ROO· can form hydroperoxides (ROOH) upon abstraction of hydrogen from thermal degradation in an industrial context. another RH segment thereby promoting the formation of new R· species and establishing an 2.autoxidation Materials and cycle. Methods ROOH can decompose to alkoxy and hydroxyl radicals, which can react with RH, also producing R· species [11,14]. The addition of primary AOs (e.g. sterically hindered phenols) 2.1.inhibits Materials radical chain reactions. Additional stabilization can be obtained if secondary AOs (such as The materials used in this investigation were as follows: ABS Magnum 3453 Natural (Trinseo, Terneuzen, the Netherlands, obtained by mass polymerization), ABS Terluran GP-22 Natural (Ineos-Styrolution, Antwerp, Belgium, obtained by emulsion polymerization), Irganox 1076 [octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate] (BASF, Ludwigshafen, Germany, phenolic primary antioxidant), Irganox 245 [ethylenebis (oxyethylene)bis-(3-(5-tert-butyl-4-hydroxy-m-tolyl) Polymers 2019, 11, 25 4 of 15 propionate)] (BASF, phenolic primary antioxidant), and Irgafos 168 [tris(2,4-ditert-butylphenyl) phosphite] (BASF, phosphite secondary antioxidant). Both ABSs are general purpose injection molding grades with similar mechanical properties (Magnum 3453: density 1.05 g/cm3, melt volume rate 15 cm3/10 min (220 ◦C, 10 kg), tensile stress at yield (23 ◦C) 45 MPa, tensile modulus 2280 MPa, Charpy notched impact strength (23 ◦C) 20 kJ/m2; Terluran GP-22: density 1.04 g/cm3, melt volume rate 19 cm3/10 min (220 ◦C, 10 kg), tensile stress at yield (23 ◦C) 45 MPa, tensile modulus 2300 MPa, Charpy notched impact strength (23 ◦C) 22 kJ/m2)[25,26].

2.2. Sample Preparation and Analysis

2.2.1. Simultaneous Thermal Analysis (STA) All raw materials were simultaneously characterized by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC; STA 449 F3 Jupiter, Netzsch, Selb, Germany). Samples of 15 ± 5 mg were heated from 30 to 900 ◦C at a heating rate of 10 ◦C min−1, under nitrogen atmosphere (50 mL min−1) in a Pt-Rh pan.

2.2.2. Processing Before processing, ABS was dried at 80 ◦C for 4 h in a convection oven. Antioxidants (AOs) were used as received. The samples studied (see Table1) were prepared in a micro-compounder (Minilab II, Haake Thermo Fisher Scientific, Karlsruhe, Germany) at 220 ◦C, 100 rpm, and the mixtures were maintained in “cycle” in the machine for 3 min. Samples were codified as follows: m% of Irganox 1076/m% of Irganox 245/m% of Irgafos 168 / type of polymerization of the ABS (M—mass polymerization; E—emulsion polymerization). For example, sample 0.2/0/0.2/M contains 0.2 m% Irganox 1076, 0.0 m% Irganox 245, 0.2 m% Irgafos 168, and the ABS processes was obtained by mass polymerization. The total mass of each processed sample was 6 g. ABS and AOs were manually added to the micro-extruder during approximately 1 minute. After processing, samples were compression molded (220 ◦C, 1 min, 500 kPa) into films (thickness of 250 ± 25 µm) prior to the analysis. In total, 16 experiments were performed, enabling statistical analysis and the identification of the dominant contributors.

Table 1. Composition of the processed acrylonitrile-butadiene-styrene terpolymer/antioxidant (ABS/AO) combinations; M: mass; E: emulsion; numbers are m% in the order of the columns; a sufficient variation of the process variables is considered to allow for statistical analysis of the dominant factors.

Irganox 1076 Irganox 245 Irgafos 168 Experiment Sample Code Type of ABS (m%) (m%) (m%) 1 0/0/0/M 0.0 0.0 0.0 Mass 2 0.2/0/0/M 0.2 0.0 0.0 Mass 3 0/0.2/0/M 0.0 0.2 0.0 Mass 4 0.2/0.2/0/M 0.2 0.2 0.0 Mass 5 0/0/0.2/M 0.0 0.0 0.2 Mass 6 0.2/0/0.2/M 0.2 0.0 0.2 Mass 7 0/0.2/0.2/M 0.0 0.2 0.2 Mass 8 0.2/0.2/0.2/M 0.2 0.2 0.2 Mass 9 0/0/0/E 0.0 0.0 0.0 Emulsion 10 0.2/0/0/E 0.2 0.0 0.0 Emulsion 11 0/0.2/0/E 0.0 0.2 0.0 Emulsion 12 0.2/0.2/0/E 0.2 0.2 0.0 Emulsion 13 0/0/0.2/E 0.0 0.0 0.2 Emulsion 14 0.2/0/0.2/E 0.2 0.0 0.2 Emulsion 15 0/0.2/0.2/E 0.0 0.2 0.2 Emulsion 16 0.2/0.2/0.2/E 0.2 0.2 0.2 Emulsion

The two virgin ABSs (unmodiﬁed; no addition of AOs and unprocessed) were also compression molded into ﬁlms and their properties were evaluated for comparison. Polymers 2019, 11, 25 5 of 15

2.2.3. Fourier Transform Infrared Spectroscopy (FTIR) Fourier transform infrared spectroscopy (FTIR; Bruker Tensor 27, Billerica, MA, USA) analyses (for the spectra, see Supporting Information, Figure S1) for virgin ABS and processed samples were carried out in order to evaluate chemical modifications in the PB rich phase of ABS. Samples were studied in attenuated total reflectance (ATR) mode from 4000 to 600 cm−1. The absorbance peaks of 1,2 butadiene at 911 cm−1 and 1,4 butadiene at 966 cm−1 were compared with the absorbance peaks of styrene (phenyl) moieties at 1603 cm−1 and nitrile entities at 2237 cm−1 using the absorption ratios R1 to R4, as defined below (Equations (1)–(4)) [15,27,28]. Values of absorbance were determined using the baseline method. With the degradation of ABS, it is expected that the nitrile and phenyl absorbances do not change and that the diene content decreases gradually, as well as its absorption peaks [13,15].

Absorbance at 911 cm−1 R1 = (1) Absorbance at 1603 cm−1

Absorbance at 966 cm−1 R2 = (2) Absorbance at 1603 cm−1 Absorbance at 911 cm−1 R3 = (3) Absorbance at 2237 cm−1 Absorbance at 966 cm−1 R4 = (4) Absorbance at 2237 cm−1

2.2.4. Oxidation Onset Temperature (OOT) For the evaluation of the presence or effectiveness of AOs in a polymer, the determination of the oxidation onset temperature (OOT) is also relevant. OOT is a relative measure of the degree of oxidative stability of a material studied at certain conditions under an oxidative atmosphere; the higher the OOT value, the more stable the material [29]. The OOT of the samples was evaluated in a simultaneous thermal analyzer (STA 449 F3 Jupiter, Netzsch, Selb, Germany) based on the ASTM E2009 standard [29] (for the results, see Supporting Information, Figure S2). All the virgin and processed samples were investigated (samples of 4 ± 0.3 mg, obtained from films of 250 ± 25 µm). The experiments were conducted under an oxygen flow of 50 mL min−1. A Pt-Rh pan was used, and the samples were studied from 50 to 500 ◦C using a heating rate of 10 ◦C min−1. The OOT was determined as the onset of the exothermic oxidation signal observed in the DSC curves. The exothermic oxidation peak temperature (OP) was also evaluated (for the results, see Supporting Information).

2.2.5. Yellowing Index (YI) The color properties of the samples were investigated with a spectrophotometer (Mercury, Datacolor, Lawrenceville, NJ, USA) using a D65 light source and 10◦ viewing angle. All color measurements were obtained from the compression molded ﬁlms (approximately 250 ± 25 µm). The yellowing index (YI) was obtained by CIE tristimulus values X, Y, and Z according to the CIELAB color system [30]. The YI was determined according to the following:

100 × (1.3013X − 1.1498Z) YI = (5) Y

2.2.6. Statistical Analysis A factorial design 24 (four factors, two levels of each factor) was conducted to evaluate the influence of each factor (Irganox 1076, Irganox 245, Irgafos 168, and ABS type; see Table1) and their interactions, employing analysis of variance (ANOVA) [31]. More specifically: factor ‘A’—Irganox 1076 content (levels: 0 or 0.2 m%); factor ‘B’—Irganox 245 content (levels: 0 or 0.2 m%); factor Polymers 2018, 10, x FOR PEER REVIEW 6 of 15 Polymers 2019, 11, 25 6 of 15 A factorial design 24 (four factors, two levels of each factor) was conducted to evaluate the influence of each factor (Irganox 1076, Irganox 245, Irgafos 168, and ABS type; see Table 1) and their ‘C’—Irgafosinteractions, 168 employing content analysis (levels: of 0 variance or 0.2 (ANOVA) m%); and [31]. factor More ‘D’—type specifically: of factor ABS ‘A’ (levels: – Irganox mass- or emulsion-polymerized1076 content (levels: 0 ABS).or 0.2 m%); The valuesfactor ‘B’ of – the Irganox factors 245 ‘A’, content ‘B’, ‘C’,(levels: and 0 ‘D’or 0.2 were m%); codified factor ‘C’ according – to theIrgafos level 168 of content each factor: (levels:− 01 or for 0.2 the m%); lower andlevel factor and ‘D’ +1– type for of the ABS higher (levels: level mass- (for or example, emulsion- sample polymerized ABS). The values of the factors ‘A’, ‘B’, ‘C’, and ‘D’ were codified according to the level 0/0.2/0/E uses −1 for ‘A’, +1 for ‘B’, −1 for ‘C’, and +1 for ‘D’, whereas sample 0.2/0/0/M uses +1 of each factor: −1 for the lower level and +1 for the higher level (for example, sample 0/0.2/0/E uses −1 − − − forfor ‘A’, ‘A’,1 +1 for for ‘B’, ‘B’, −1 for ‘C’, ‘C’, and and +1 for1 for ‘D’, ‘D’). whereas sample 0.2/0/0/M uses +1 for ‘A’, −1 for ‘B’, −1 for ‘C’,Two and replications −1 for ‘D’). of each experiment (which reflects sources of variability both between runs and within runsTwo [replications31]) were done.of each Since experiment each replication, (which reflects consisting sources of of variability 16 experiments both between as shown runs and in Table 1, waswithin done runs with [31]) a time were interval done. Since of approximately each replicatio 20n, days,consisting replications of 16 experiments were considered as shown as in blocks Table in the statistical1, was done analysis with aiming a time interval to reduce of approximately nuisance factors. 20 days, The orderreplications of the were mixtures considered within as each blocks replication in wasthe randomized. statistical analysis The results aiming obtained to reduce were nuisance compared factors. using The the order Tukey’s of the test, mixtures which within indicates each if there is areplication significant was difference randomized. between The theresults average obtained results were of populationscompared using [31 ];the a conventionalTukey’s test, which significance levelindicates (α) of 0.05if there was is considered. a significant difference between the average results of populations [31]; a conventional significance level (α) of 0.05 was considered. 3. Results and Discussion 3. Results and Discussion 3.1. Simultaneous Thermal Analysis (STA) 3.1. Simultaneous Thermal Analysis (STA) TGATGA results results for for the the rawraw materials (virgin (virgin ABS ABS by M by and M andE; 3 AOs) E; 3 AOs)are presented are presented in Figure in 2a. Figure It 2a. It isisobserved observed thatthat the polymers polymers show show higher higher onsets onsets of weight of weight loss lossthan thanthose those of the of AOs, the since AOs, AOs since AOs ◦ havehave lower lower molar molar masses. masses. No No significant significant weightweight loss was observed observed below below 250 250 °C.C. Figure Figure 2b2 showsb shows the DCSthe results DCS forresults the for raw the materials. raw materials. It is possible It is possib to identifyle to identify the rigid the phase rigid glassphase transition glass transition temperature ◦ (Tg)temperature of each ABS, (Tg close) of each to 110ABS, C.close The to recorded 110 °C. The melting recorded points melting (Tm points) of the (T additivesm) of the additives are in accordance are ◦ within theaccordance data provided with the bydata the provided manufacturer by the manufacturer (Tm of Irganox (Tm of 1076: Irganox 50–55 1076: C50–55 (black); °C (black); Tm of T Irganoxm of Irganox◦ 245: 76–79 °C (red); Tm Irgafos 168: ◦183–186 °C (green)) [32–34]. Hence, upon processing ◦ 245: 76–79 C (red);Tm Irgafos 168: 183–186 C (green)) [32–34]. Hence, upon processing (220 C), all materials(220 °C), all are materials in the moltenare in the state. molten state.

100 Irganox 1076

Irganox 245 Up Endo Irgafos 168 80 Virgin ABS Emulsion Virgin ABS Mass Irganox 1076

Irganox 245 40 Mass (%) Mass Heat (W/g) Flux

20 Irgafos 168 2 Emulsion ABS Mass ABS 0

0 200 400 600 800 50 100 150 200 Temperature (°C) Temperature (°C)

(a) TGA (b) DSC

FigureFigure 2. (2.a) ( Thermogravimetrica) Thermogravimetric (TGA) (TGA) results; results; and and (b ()b Differential) Differential scanning scanning calorimetry calorimetry (DSC)(DSC) results forresults the raw for materials the raw materials studied. studied.

3.2.3.2. Fourier Fourier Transform Transform Infrared Infrared SpectroscopySpectroscopy (FTIR) (FTIR) FTIRFTIR results results for for the theR R1–1–RR44 absorption ratios ratios of ofthe the samples samples (Equation (Equation (1)–(4)) (1)–(4)) are shown are shown in in FigureFigure3 (black 3 (black and and grey grey symbols), including including error error bars. bars. Samples Samples from Table from 1 Table(processed1 (processed even if they even if do not contain AO) are organized in a descending sequence according to their experimental average they do not contain AO) are organized in a descending sequence according to their experimental R1–R4 absorption ratio value. For reference, virgin unprocessed ABS results are also shown in Figure average R1–R4 absorption ratio value. For reference, virgin unprocessed ABS results are also shown 3 as the first two entries (blue: M; fuchsia: E), showing that the mass-polymerized ABS has likely a in Figurelower 3PB as content the ﬁrst despite two entries the mechanical (blue: M; properties fuchsia: E), of showingthe ABS thatmaterials the mass-polymerized studied being similar ABS has likely a lower PB content despite the mechanical properties of the ABS materials studied being similar according to the manufacturers [25,26]. Gesner studied ABSs with different PB content, and observed that a higher R2 indicates a higher PB content [35]. In agreement with this statement one can observe that the processed samples containing emulsion-polymerized ABS tend to present higher R1–R4 (black symbols) absorption ratios than the samples based on ABS obtained by mass polymerization (grey symbols). Polymers 2018, 10, x FOR PEER REVIEW 7 of 15

according to the manufacturers [25,26]. Gesner studied ABSs with different PB content, and observed that a higher R2 indicates a higher PB content [35]. In agreement with this statement one can observe that the processed samples containing emulsion-polymerized ABS tend to present higher R1–R4 (black symbols) absorption ratios than the samples based on ABS obtained by mass polymerization Polymers(grey2019 symbols)., 11, 25 7 of 15

2.0 Experimental ABS Emulsion 2.8 Experimental ABS Emulsion Experimental ABS Mass 2.6 Experimental ABS Mass 1.8 Predicted Predicted Virgin ABS Emulsion 2.4 Virgin ABS Emulsion 1.6 Virgin ABS Mass Virgin ABS Mass 2.2

1.4 2.0

1.2 1.8 1.6 1.0

1(911/1603) 2 (966/1603) 1.4 R R 0.8 1.2

0.6 1.0

• s n E E E E ¤ E E E / / / / / M M / / M M M M M • s o / / / / / / / s n ¤ 0.8 E E E E E E E i M M M M M M M M a 0 2 0 2 E 0 2 2 s / / / / / / / / . / . / / 2 2 . . 0 0 2 / 2 0 o / / / / / / / s . . / / . . / 0.4 i l a 0 2 2 0 E 2 0 2 M 2 0 2 0 0 0 0 0 0 2 0 2 0 2 / 0 2 M 0 0 0 2 0 0 2 s / . . / / . / . u . / . / / / / / / l . / . / . / . / / / . / / . 2 0 0 2 0 0 0 0 0 0 M 0 2 0 0 0 2 0 n 0 2 0 0 0 2 2 0 u . / / . / / / / / i m / . / / / . 0 2 . / 2 0 0 / 2 0 / . / / / . / 0.6 / . . / / . / n 0 2 0 0 0 2 2 0 0 g 0 0 2 2 0 0 0 0 0 i m / . / / / . . / 0 0 2 2 0 / 0 2 E 0 0 0 0 2 0 2 / / . . / / . r / . . / i / . / . g 0 0 2 2 0 0 0 0 0 2 0 0 0 E 0 0 0 0 2 2 0 n 0 0 2 0 r / . . / i . i / . . / V . 2 0 0 0 n g 0 . 0 0 0 2 0 V i . r i 0 g 0 r i V V Sample Sample (a) R1 (911/1603) (b) R2 (966/1603)

2.6 3.8 Experimental ABS Emulsion Experimental ABS Emulsion Experimental ABS Mass Experimental ABS Mass 2.4 3.6 Predicted Predicted Virgin ABS Emulsion 3.4 Virgin ABS Emulsion 2.2 Virgin ABS Mass Virgin ABS Mass 3.2 2.0 3.0

1.8 2.8

1.6 2.6 2.4

3 (911/2237) 1.4 4 (966/2237) R R 2.2 1.2 2.0 1.0 1.8 • s n E E E E E ¤ E E • M M M M M M M s n ¤ s / / / / / / / E E E E E E E o / / / / / / / / / / / / / / M M M M M M M i s o / / / / / / / 2 0 0 2 2 E 0 2 M a 2 0 2 2 0 2 0 i s . / / . . / / . / a 2 0 0 2 0 2 E 2 M 0.8 . / . . / . / . / / . / . / . 2 2 0 2 0 2 / 0 l 1.6 s . . / . / . / 0 2 2 0 0 0 0 0 0 l M 0 2 0 0 0 0 2 u / . . / / / / / / M 0 2 2 0 0 0 0 0 0 / . / / / / . u / . . / / / / / 0 0 2 0 0 0 / 2 / / . / / / . 2 0 0 0 2 0 2 0 0 n 2 i m . / / / . / . / 0 0 0 2 2 / 0 n 2 0 0 0 2 2 0 0 0 / / . / . . / i m . / / / . . / / 2 0 0 2 2 0 / 0 . / / . . / / g 0 0 2 2 0 0 0 0 0 E 0 0 0 2 0 0 2 0 r / . . / g E 0 0 2 2 0 0 0 0 / . / . / . . / 0 0 0 0 0 2 2 i r / / . . 2 0 0 0 i n 2 0 0 0 i . n 2 0 0 0 V . . 2 0 0 0 V i . g 0 0 0 r g 0 i r i V V Sample Sample (c) R3 (911/2237) (d) R4 (966/2237)

FigureFigure 3. Measured 3. Measured (black (black (emulsion-polymerized (emulsion-polymerized AB ABS)S) and and grey grey (mass-polymerized (mass-polymerized ABS)) ABS)) and and predictedpredicted (red) (red) FTIR FTIR absorption absorption ratios ratios for for samplessamples presented presented in in Table Table 1,1 including, including additionally additionally the the virginvirgin unprocessed unprocessed ones: ones: blue: blue: mass, mass, and and fuchsia: fuchsia: emulsion-polymerized emulsion-polymerized ABS; ( aa)) RR11 (911/1603); (911/1603); (b ()b ) R2 (966/1603);R2 (966/1603); (c) R3 (911/2237);(c) R3 (911/2237); (d) R (d4)(966/2237); R4 (966/2237); Equations Equations (1)–(4) (1)–(4) forfor definitions;definitions; predictions predictions based based on on Equations (6)–(9); R3 and R4 data are most suited for statistical analysis; entries from Table 1 Equations (6)–(9); R3 and R4 data are most suited for statistical analysis; entries from Table1 ranked ranked from high to lower absorption ratios (left to right); data highlight a lower polybutadiene from high to lower absorption ratios (left to right); data highlight a lower polybutadiene content for content for mass-polymerized ABS, consistent with the lower blue (virgin unprocessed) point. mass-polymerized ABS, consistent with the lower blue (virgin unprocessed) point. The ANOVA results for both R1 and R2 absorption ratios indicate that the only statistically Thesignificant ANOVA factor results is the factor for both ‘D’ (typeR1 and of ABS;R2 absorption p-values <0.0062), ratios explaining indicate that the alignments the only statistically of the significantpredicted factor values is the(red factorsymbols ‘D’ in (typeFigure of 3a–b ABS; – Equationsp-values (6) <0.0062), and (7)). explainingThe replications the alignments(blocks) in the of the predictedanalysis values of variance (red symbols of R1 and in R Figure2 showed3a,b—Equations a p-value of 0.0148 (6)and and (7)).0.0370, The respectively, replications indicating (blocks) in the analysisnuisance of factors variance influenced of R1 andthe resultsR2 showed in each ablock.p-value of 0.0148 and 0.0370, respectively, indicating nuisance factors influenced the results𝑅1 in each = 1.104 block. + 0.154D (6)

𝑅2R1 = 1.7221.104 ++ 0.139D0.154D (7) (6)

R2 = 1.722 + 0.139D (7)

However, the ANOVA results for the R3 absorption ratio indicate that several factors and interactions are statistically signiﬁcant: ‘B’ (Irganox 245 content; p-value of 0.0401), ‘D’ (type of ABS; p-value <0.00001), ‘AC’ (Irganox 1076 and Irgafos 168; p-value of 0.04875), and ‘BD’ (Irganox 245 and type of ABS; p-value of 0.04346). The ANOVA results for the R4 ratio show that the signiﬁcant factors and interactions are as follows: ‘B’ (Irganox 245 content; p-value of 0.01434), ‘D’ (type of ABS; p-value <0.00001), ‘AD’ (Irganox 1076 and type of ABS; p-value of 0.01212), and ‘BD’ (Irganox 245 and type of ABS; p-value of 0.0478). The replications (blocks) showed a p-value of approximately 0.001 for both R3 and R4. The predicted values presented in Figure3c,d (red symbols) were obtained from Equations (8) and (9): Polymers 2019, 11, 25 8 of 15

R3 = 1.622 + 0.015A + 0.041B − 0.008C + 0.039AC + 0.296D + 0.041BD, (8)

R4 = 2.522 + 0.061A + 0.086B + 0.315D + 0.088AD + 0.067BD. (9)

Hence, from Figure3c,d it follows that R3 and R4 absorption ratios (reference peak: absorbance of nitrile groups at 2237 cm−1) are more sensitive to modifications in 1,2 butadiene and 1,4 butadiene −1 absorptionsPolymers peaks2018, 10, x than FOR PEERR1and REVIEWR2 (reference peak: absorbance of styrene moieties at 16038 of cm15 ). Previous studies indicated an increase in FTIR absorption spectra at approximately 1600 cm−1 for photodegradedHowever, ABS, the and ANOVA it was associated results for (see the FigureR3 absorption1) with theratio formation indicate ofthat carbonyl several groupsfactors and [ 27 ,36]. Theseinteractions carbonyl moieties are statistically can be relatedsignificant: tothe ‘B’ oxidative(Irganox 245 degradation content; p-value of the of PB0.0401), fraction ‘D’ (type [11], andof ABS; also to the presencep-value of<0.00001), primary ‘AC’ AOs, (Irganox since 1076 both and primary Irgafos AOs 168; investigatedp-value of 0.04875), in this and work ‘BD’ exhibit (Irganox C=O 245 and groups. type of ABS; p-value of 0.04346). The ANOVA results for the R4 ratio show that the significant factors The simultaneous change of absorptions around 1600 cm−1 therefore obscured the modifications of PB and interactions are as follows: ‘B’ (Irganox 245 content; p-value of 0.01434), ‘D’ (type of ABS; p-value phase for R1 and R2. From the Tukey’s multiple comparison test (not shown here), and considering the <0.00001), ‘AD’ (Irganox 1076 and type of ABS; p-value of 0,01212), and ‘BD’ (Irganox 245 and type of experimentalABS; p-value average of 0.0478). values The of Rreplications1 and R2 absorption(blocks) showed ratios, a p no-value difference of approximately was observed 0.001 for among both the −1 samples.R3 Thisand indicatesR4. The predicted that the usevalues of the presented absorption in Figure peak at 3c,d 1603 (red cm symbols)as reference were peakobtained is insensitive from to indicateEquations changes (8) and in the (9): PB fraction of the ABSs due to thermal-oxidative degradation, since the FTIR −1 absorption at 1600𝑅3 cm = 1.622also + changes 0.015A + [27 0.041B,36]. − 0.008C + 0.039AC + 0.296D + 0.041BD, (8) Figure4 shows the Tukey’s test results for R3 and R4 absorption ratios, aiming to clarify which samples showed undeniable𝑅4 = 2.522 differences + 0.061A between + 0.086B the + 0.315D average + 0.088AD results obtained.+ 0.067BD. One can observe (9) that there is a clear statistical difference among samples according to the type of ABS used. No difference was observedHence, among from Figure all samples 3c,d it follows from mass-polymerizedthat R3 and R4 absorption ABS ratios (see Figure(reference3), peak: implying absorbance that the of nitrile groups at 2237 cm−1) are more sensitive to modifications in 1,2 butadiene and 1,4 butadiene change of the PB phase is limited. For emulsion-polymerized ABS, only the sample 0/0/0.2/E absorptions peaks than R1 and R2 (reference peak: absorbance of styrene moieties at 1603 cm−1). showed statistically different values, presenting a lower R3 and R4 value in comparison with sample Previous studies indicated an increase in FTIR absorption spectra at approximately 1600 cm−1 for 0.2/0.2/0.2/Ephotodegraded and alsoABS, a and lower it wasR3 associated value related (see Fi togure the 1) sample with the 0/0.2/0/E. formation of If carbonyl one solely groups relates degradation[27,36]. withThese the carbonyl changes moieties in the can PB phase,be related one to expects the oxidative at first degradation sight that theof the sample PB fraction 0.2/0.2/0.2/E [11], wouldand show also higher to the absorptionpresence of ratiosprimary and AOs, consequently since both primary a higher AOs thermal-oxidative investigated in this stability. work exhibit However, the additionC=O groups. of AOs The did simultaneous not cause change a statistically of absorptions significant around difference 1600 cm− in1 therefore the absorption obscured ratios the in comparisonmodifications to the sample of PB phase 0/0/0/E. for R This1 and could R2. From imply the that Tukey’s degradation multiple is comparison not dominant test in(not the shown PB phase (or veryhere), limited). and considering Nevertheless, the experimental these insights average could values still alter of R if1 and more R2 repeat absorption experiments ratios, no are difference conducted, −1 other analysiswas observed properties among are the considered samples. This (see indicates discussion that below),the use of a lowerthe absorption AOs content peak isat used1603 (cm<0.2 as m% ), reference peak is insensitive to indicate changes in the PB fraction of the ABSs due to thermal- the melt processing time is higher (>3 min), or the melt processing temperature is higher (>220 ◦C). oxidative degradation, since the FTIR absorption at 1600 cm−1 also changes [27,36]. The variation of the preceding process parameters is out of the scope of the present work.

2.2 3.0 Tukey's Test Tukey's Test 2.1 2.9 2.0 2.8 1.9 2.7 1.8

1.7 2.6

1.6 2.5 3 (911/2237) 4 (966/2237)

R 1.5 R 2.4 1.4 2.3 1.3 2.2 1.2 • • ¤ ¤ E E E E E E E E E E E E E E / / / / / / / M M M M M M M / / / / / / / M M M M M M M / / / / / / / / / / / / / / 2 0 0 2 0 2 E 2 M 2 0 0 2 2 E 0 2 M . / / . / . / . 2 2 0 2 0 2 / 0 . / / . . / / . 2 0 2 2 0 2 / 0 . . / . / . / . / . . / . / 0 2 2 0 0 0 0 0 0 1.1 0 2 2 0 0 0 0 0 0 2.1 / . . / / / / / 0 0 2 0 0 0 / 2 / . . / / / / / 0 2 0 0 0 0 / 2 / / . / / / . / . / / / / . 2 0 0 0 2 2 0 0 0 2 0 0 0 2 0 2 0 0 . / / / . . / / 2 0 0 2 2 0 / 0 . / / / . / . / 0 0 2 0 2 2 / 0 . / / . . / / / / . / . . / 0 0 2 2 0 0 0 0 0 0 0 2 2 0 0 0 0 0 / . . / 0 0 0 0 0 2 2 / . . / 0 0 0 2 0 0 2 / / . . / . / . 2 0 0 0 2 0 0 0 . 2 0 0 0 . 2 0 0 0 . . 0 0 0 0 Sample Sample (a) Tukey’s test for R3 (911/2237) (b) Tukey’s test for R4 (966/2237)

FigureFigure 4. Tukey’s 4. Tukey’s test test for for the the twotwo most sensitive sensitive FTIR FTIR ratios ratios from fromFigure Figure 3: (a) R3:(3 (911/2237);a) R3 (911/2237); (b) R4 (b) R4(966/2237). (966/2237).

3.3. OxidationFigure Onset 4 shows Temperature the Tukey’s (OOT) test results for R3 and R4 absorption ratios, aiming to clarify which samples showed undeniable differences between the average results obtained. One can observe that OOT results are presented in Figure5a, following the same color coding as in the Figure3 ranking there is a clear statistical difference among samples according to the type of ABS used. No difference againwas as the observed processed among samples all samples in Table from1, frommass-polymerized high to lower ABS stability, (see Figure and again 3), implying adding that the the virgin change of the PB phase is limited. For emulsion-polymerized ABS, only the sample 0/0/0.2/E showed statistically different values, presenting a lower R3 and R4 value in comparison with sample 0.2/0.2/0.2/E and also a lower R3 value related to the sample 0/0.2/0/E. If one solely relates degradation with the changes in the PB phase, one expects at first sight that the sample 0.2/0.2/0.2/E would show higher absorption ratios and consequently a higher thermal-oxidative stability. However, the

Polymers 2019, 11, 25 9 of 15 unprocessed data. A different ranking from the results in Figure3 occurs, highlighting that a sole focus on the PB content is insufficient. The ANOVA results show that the factors ‘A’ (Irganox 1076), Polymers 2018, 10, x FOR PEER REVIEW 9 of 15 ‘B’ (Irganox 245), and ‘D’ (type of ABS) as well as the interaction ‘AB’ (between Irganox 1076 and Irganoxaddition 245) of AOs significantly did not affected cause a the statistically OOT of ABS; sign theseificant factors difference and the in interaction the absorption exhibit ratiosp-values in lowercomparison than 0.01. to the Irgafos sample 168 0/0/0/E. (factor This ‘C’) could shows imply a p-value that degradation of 0.077 and is was not not dominant considered in the significant PB phase for(or thevery OOT limited). results. Nevertheless, From the ANOVA these resultsinsights of co OOTuld data,still Equationalter if more 10 is repeat obtained, experiments presenting are an acceptableconducted, correlationother analysis coefficient properties (R) are of 0.853, considered as is also (seereflected discussion by below), the relatively a lower good AOs agreement content is betweenused (<0.2 experimental m%), the melt and processing predicted resultstime is observedhigher (>3 in min), Figure or5a. the melt processing temperature is

higher (>220 °C). The variation◦ of the preceding process parameters is out of the scope of the present = + + − − work. OOT C 189.19 7.551A 4.915B 3.578AB 3.781D (10)

230 Experimental ABS Emulsion 200 Experimental ABS Mass Tukey's Test 220 Predicted Virgin ABS Emulsion 195 210 Virgin ABS Mass

200 190

190 185 180

170 180

160

Oxidation Onset Temperature (°C)Onset Oxidation 175 Oxidation Onset Temperature(°C)Oxidation Onset 150 • s n E E E E E E E ¤ M M M M M M M • s / / / / / . / o / / / / / / / E E E E E E E ¤ i M M M M M M M 2 0 0 2 2 0 2 M E / / / / / . / a 2 2 0 2 0 0 2 / / / / / / / s . / / . . / . / / . . / . / / . 2 0 0 2 2 0 2 M E l 2 2 0 2 0 0 2 0 2 0 0 0 2 0 0 0 . / / . . / . / / M 0 0 0 0 2 2 0 . . / . / / . u / . / / / . / / / 140 / / / / . . / 170 0 2 0 0 0 2 0 0 0 0 0 0 0 2 2 0 2 0 2 0 2 0 0 0 0 / . / / / . / / / n 2 / / / / . . / i m 0 . 2 2 / 0 . 0 / 0 . / / / / . / . . / / / 2 0 2 0 2 0 0 0 0 2 0 2 2 0 0 0 g 0 2 0 2 0 0 0 0 0 . / . / . / / / / E 0 2 0 0 0 2 0 . / . . / / / r / . . / / . / . 0 2 0 2 0 0 0 0 0 i 0 2 0 0 0 2 0 2 0 0 0 / . . / n 2 0 0 0 / . / . i . V . 2 0 0 0 2 0 0 0 0 . g 0 . r 0 i 0 V Sample Sample (a) OOT (b) Tukey’s test for OOT

Figure 5. Oxidation onset temperatures (OOTs) for the samplessamples studied, with a ranking from high to low stability (left to right); (a) OOT results (color coding as in Figure 33;; EquationEquation (10)(10) forfor prediction);prediction); ((b)) Tukey’sTukey’s test test for for OOT OOT results; results; data data indicate indicate an undeniablean undeniable higher higher stability stability for mass-polymerized for mass-polymerized ABS. ABS. From Figure5a, one can observe that the processing of both virgin (unmodified) ABS materials (last3.3. Oxidation two entries) Onset causes Temperature a very strong(OOT) decrease in OOT in comparison with the virgin unprocessed ABS materials (first two entries). This reduction in OOT is likely related to the consumption of AOs addedOOT by theresults polymer are presented manufacturers in Figure during 5a, thefollowing original the manufacturing same color coding and at leastas in partlythe Figure due to3 theranking degradation again as ofthe the processed PB phase samples [14]. The in additionTable 1, fr ofom primary high to AOs lower promotes stability, an and increase again inadding polymer the stabilityvirgin unprocessed of the studied data. samples. A different The stabilizationranking from effect the results of the in primary Figure AOs3 occurs, was expected,highlighting as itthat was a alsosole verifiedfocus on in the previous PB content studies is insufficient. [14,19,20]. The The OOT ANOVA values results observed show for that samples the factors containing ‘A’ (Irganox at least one1076), of ‘B’ the (Irganox AOs studied 245), areand close ‘D’ (type to those of ABS) observed as well for as virgin the interaction (unprocessed) ‘AB’ ABS,(between further Irganox indicating 1076 thatand Irganox the individual 245) significantly amount of affected 0.2 m% canthe OOT be critical. of ABS; Equation these factors (10) predicts and the thatinteraction the highest exhibit OOT p- values arelower found than in 0.01. samples Irgafos containing 168 (factor both ‘C’) Irganox shows 1076 a p-value (factor of ‘A’) 0.077 and and Irganox was not 245 considered (factor ‘B’) (samplessignificant 0.2/0.2/0/M, for the OOT 0.2/0.2/0.2/M, results. From 0.2/0.2/0/E,the ANOVA andresults 0.2/0.2/0.2/E). of OOT data, Samples Equation containing 10 is obtained, only the secondarypresenting AOan acceptable (Irgafos 168; correlation factor ‘C’, coefficient 0/0/0.2/M (R) and of 0/0/0.2/E)0.853, as is also show reflected lower OOT by the values relatively than thosegood observedagreement for between the samples experimental containing and one predicted (or both) results primary observed AOs. Given in Figure that the 5a. OOT is obtained at the beginning of the oxidationOOT °C process,= 189.19 it is + expected 7.551A + that 4.915B primary − 3.578AB AOs (radical − 3.781D scavengers) affect the OOT(10) values more than the secondary AOs do, as the latter act as hydroperoxide scavengers. Secondary AOs From Figure 5a, one can observe that the processing of both virgin (unmodified) ABS materials do not operate in the first steps of the thermal-oxidation process (where OOT is identified) but can (last two entries) causes a very strong decrease in OOT in comparison with the virgin unprocessed interfere in subsequent degradation steps (see Figure1), which explains Irgafos 168’s lack of influence ABS materials (first two entries). This reduction in OOT is likely related to the consumption of AOs on OOT, according to the ANOVA results. added by the polymer manufacturers during the original manufacturing and at least partly due to In addition, the type of ABS (factor ‘D’) significantly influences the OOT results. The ABS the degradation of the PB phase [14]. The addition of primary AOs promotes an increase in polymer obtained by mass polymerization presents a higher thermal-oxidative stability than the stability of the studied samples. The stabilization effect of the primary AOs was expected, as it was emulsion-polymerized ABS. This higher stability for the mass-polymerized ABS may be related also verified in previous studies [14,19,20]. The OOT values observed for samples containing at least to the expected lower PB content of such samples (see the FTIR data in Figure3), and it is likely also one of the AOs studied are close to those observed for virgin (unprocessed) ABS, further indicating that the individual amount of 0.2 m% can be critical. Equation (10) predicts that the highest OOT values are found in samples containing both Irganox 1076 (factor ‘A’) and Irganox 245 (factor ‘B’) (samples 0.2/0.2/0/M, 0.2/0.2/0.2/M, 0.2/0.2/0/E, and 0.2/0.2/0.2/E). Samples containing only the secondary AO (Irgafos 168; factor ‘C’, 0/0/0.2/M and 0/0/0.2/E) show lower OOT values than those

Polymers 2019, 11, 25 10 of 15 Polymers 2018, 10, x FOR PEER REVIEW 10 of 15 relatedobserved to thefor differencesthe samples in containing the morphology one (or of both) the PB primary phase ofAOs. studied Given ABS that types. the OOTGiaconi is obtained et al. [37 at] observedthe beginning that theof the morphology oxidation process, of the PBit is phase expected found that for primary mass AOs and (radical emulsion-polymerized scavengers) affect ABS the materialsOOT values is different. more than A mass the polymerization-basedsecondary AOs do, as ABS the showslatter aact ”salami” as hydroperoxide morphology, scavengers. in which theSecondary butadiene AOs disperse do not phase operate shows in the inclusions first steps of of styrene-acrylonitrile the thermal-oxidation copolymers process (where (SANs) OOT inside is theidentified) dispersed but PB can particles, interfere whereas in subsequent an emulsion-polymerized degradation steps (see ABS Figure shows 1), bulkwhich rubber explains particles, Irgafos with168’s almost lack ofno influence SAN inclusions on OOT, according in the butadiene to the ANOVA phase [ 37results.]. The differences in morphology can interfereIn addition, in the thermal-oxidative the type of ABS resistance (factor ‘D’) of the sign studiedificantly ABSs, influences since the the inclusions OOT results. of SAN The phase ABS inobtained the butadiene by mass particles polymerization can inhibit presents oxidation a higher of the thermal-oxidative rubber phase, increasingstability than the the OOT emulsion- values. Notepolymerized that the presentABS. This results higher are stability at first sightfor the contradictory mass-polymerized to the work ABS may of Motyakin be related and to Schlickthe expected [22], inlower which PBthe content emulsion-polymerized of such samples (see ABS the presented FTIR data higher in Figure thermal 3), stability, and it is even likely containing also related a higher to the PBdifferences content than in the the morphology mass-polymerized of the PB ABS. phase This of stud divergingied ABS behavior types. Giaconi can be relatedet al. [37] to observed the presence that ◦ ofthe a hinderedmorphology amine of stabilizerthe PB phase (HAS) found in that for work, mass the and thermal emulsion-polymeriz aging of the samplesed ABS at onlymaterials120 C is, anddifferent. the different A mass stabilizerpolymerization-based amount (1 or ABS 2 m%). shows Additionally, a ”salami” morphology, those authors in which did not the investigate butadiene thedisperseunmodified phase ABSshows. Figure inclusions5b shows of styrene-acryloni the Tukey’s testtrile results copolymers for the OOT(SANs) experiments. inside the dispersed Almost all PB samplesparticles, show whereas statistically an emulsion-pol equivalentymerized OOT values, ABS despite shows thebulk fact rubber that an particles, increasing with trend almost (from no right SAN toinclusions left) can bein the observed butadiene overall phas fore [37]. the redThe points differences (predicted in morphology values) in can Figure interfere5a, highlighting in the thermal- the differenceoxidative betweenresistance measurements of the studied andABSs, detailed since the statistical inclusions analysis. of SAN Importantly, phase in the an butadiene exception particles is the increasecan inhibit in OOT oxidation for samples of the containingrubber phase, Irganox increasing 1076 (left the regionOOT values. of Figure Note5a). that the present results are Figureat first6 sightshows contradictory the OP results to for the the work samples of Motyakin investigated—with and Schlick the same[22], in ranking which as the in Figureemulsion-5a, frompolymerized high to lowABS stabilitypresented (left higher to right)—and thermal stability, the respective even containing Tukey’s a testhigher results. PB content The ANOVA than the resultsmass-polymerized shows that theABS. four This main diverging factors behavior ‘A’ (Irganox can be 1076),related ‘B’ to (Irganoxthe presence 245), of ‘C’a hindered (Irgafos amine 168), andstabilizer ‘D’ (type (HAS) of ABS in )that significantly work, the affect thermal the aging oxidation of the peak samples (p-values at only lower 120 than °C, 0.005).and the Moreover, different thestabilizer interaction amount ‘AB’ (1 (between or 2 m%). Irganox Additionally, 1076 and those Irganox authors 245; didp-value not investigate of 0.00226) the and unmodified the interaction ABS. ‘ABD’Figure (between 5b shows Irganox the Tukey’s 1076, Irganox test results 245, andfor typethe OOT of ABS; experiments.p-value of Almost 0.0237) alsoall samples influence show OP. Fromstatistically these results, equivalent Equation OOT (11)values, is obtained, despite the leading fact that to a an very increasing good prediction trend (from (R= right 0.966), to left) further can highlightingbe observed the overall need for of multiplethe red analyses.points (predicted values) in Figure 5a, highlighting the difference

between measurements◦ and detailed statistical analysis. Importantly, an exception is the increase in OOT for samplesOP C containing= 206.71 + Irganox5.647A 10+765.109B (left −region1.628AB of Figure+ 1.478C 5a). − 4.997D + 1.116ABD (11)

240 220 Experimental ABS Emulsion Tukey's Test Experimental ABS Mass 230 Predicted 215 Virgin ABS Emulsion Virgin ABS Mass 220 210

210 205

200 200 Oxidation Peak(°C)Oxidation

Oxidation (°C) Peak 190 195

180 190

• s n E E E E E E E ¤ M M M M / M / M / M / / / / s o / / / / / / / i a 2 0 0 2 2 0 M 2 E 0 2 2 2 . 0 / 0 / 2 . . / / . / s / . . . / / . • l E E E E E E E ¤ 0 2 0 0 0 2 0 0 0 M M M M M M M M 0 2 0 0 / / / / / / / u 2 0 0 / . / / / . / / / / / / / / / / . / / / . / / 2 0 0 2 2 0 M 2 E 2 0 2 0 2 0 0 0 0 0 2 2 2 0 0 2 170 n 0 0 2 2 0 2 0 185 . / / . . / / . / i m . / . / . / / / / / . . . / / . / / . . / . / 0 2 0 0 0 2 0 0 0 g 0 2 0 2 0 0 0 0 0 2 0 0 2 0 0 E 0 0 / . / / / . / / / 2 2 0 / 0 . 0 0 . / . / / / . / / r . . / / i 2 0 2 0 2 0 0 0 0 2 0 0 0 0 2 0 n 2 0 0 2 2 . / . / . / / / / 0 0 0 . / / . . / . / V i . 0 2 0 2 0 0 0 0 0 0 g 0 2 2 0 0 / 0 . 0 0 . / r . . / / i 2 0 0 0 0 0 2 0 . . V 0 0 Sample Sample (a) OP (b) Tukey’s test for OP

FigureFigure 6. 6.Oxidation Oxidation peak peak (OP) (OP) for for the the samples samples studied; studied; (a )(a OP) OP results results (color (color coding coding again again as inas Figurein Figure3, ranking3, ranking from from high high to lowto low stability stability (left (left to to right)); right)); (b ()b Tukey’s) Tukey’s test; test; consistent consistent with with Figure Figure5, 5, the the data data highlighthighlight a a high high stability stability for for mass-polymerized mass-polymerized ABS. ABS.

FromFigure Equation 6 shows (11),the OP it isresults possible for the to predictsamples that investigated—with the samples with the the same highest ranking OP as values in Figure for each5a, from ABS arehigh those to low containing stability the(left three to right)—and AOs simultaneously, the respective implying Tukey’s that test an results. interpretation The ANOVA solely onresults statistical shows analysis that the offour absorption main factors data ‘A’ (see (Irganox Figures 1076),3 and ‘B’4) (Irganox or OOT 245), data ‘C’ (see (Irgafos Figure 168),5) is and not ‘D’ recommended.(type of ABS) Similarsignificantly to the effectaffect observedthe oxidation for the peak OOT ( values,p-values Figure lower6a than shows 0.005). that theMoreover, processing the interaction ‘AB’ (between Irganox 1076 and Irganox 245; p-value of 0.00226) and the interaction ‘ABD’ (between Irganox 1076, Irganox 245, and type of ABS; p-value of 0.0237) also influence OP. From these

Polymers 2019, 11, 25 11 of 15 of both unmodified ABS types causes a significant decrease in OP due to, for example, PB-phase degradation during processing and that the addition of primary AOs, as well as the type of ABS, influences the OP values. However, the secondary antioxidant (Irgafos 168, factor ‘C’) shows a significantPolymers 2018 influence, 10, x FOR on PEER the REVIEW OP, a phenomenon not observed in OOT for this AO. The influence of11 theof 15 secondary AO is observed in the oxidation peak values but not in OOT since the OP occurs at the maximumresults, Equation stage of (11) the oxidationis obtained, process, leading and to a it very is expected good prediction that at this (R point = 0.966), significant further amountshighlighting of hydroperoxidesthe need of multiple are being analyses. formed (see Figure1), which allows the secondary AO (a hydroperoxide scavenger)OP to °C be effective.= 206.71 Furthermore, + 5.647A + 5.109B the mass-polymerized − 1.628AB + 1.478C ABS − showed4.997D + higher 1.116ABD OP values than(11) those found for the emulsion-polymerized ABS, considering the same AO composition, indicating From Equation (11), it is possible to predict that the samples with the highest OP values for each that the amount of PB and the morphology of PB phase interfere in the thermal-oxidative stability ABS are those containing the three AOs simultaneously, implying that an interpretation solely on of ABS, as well as for the OP. From Figure6b (Tukey’s test), it follows that samples containing at least statistical analysis of absorption data (see Figures 3 and 4) or OOT data (see Figure 5) is not one primary AO show higher OP values, whereas the addition of the secondary AO alone results in recommended. Similar to the effect observed for the OOT values, Figure 6a shows that the processing lower OP values. Hence, despite the fact that the secondary AO significantly affects the OP values, of both unmodified ABS types causes a significant decrease in OP due to, for example, PB-phase the primary AOs have a more pronounced effect on OP. degradation during processing and that the addition of primary AOs, as well as the type of ABS, 3.4.influences Yellowing the Index OP (YI) values. However, the secondary antioxidant (Irgafos 168, factor ‘C’) shows a significant influence on the OP, a phenomenon not observed in OOT for this AO. The influence of the The yellowing index (YI) results are shown in Figure7, ranking the data in Table1 from low to secondary AO is observed in the oxidation peak values but not in OOT since the OP occurs at the higher values (right to left) and complementing this data set with the data for virgin ABS. The ANOVA maximum stage of the oxidation process, and it is expected that at this point significant amounts of results for the yellowing index (YI) show that the type of ABS (factor ‘D’; p-value lower than 0.0002) hydroperoxides are being formed (see Figure 1), which allows the secondary AO (a hydroperoxide and the interaction between Irganox 1076 and Irganox 245 (‘AB’; p-value lower than 0.0271) are scavenger) to be effective. Furthermore, the mass-polymerized ABS showed higher OP values than significant. The replications (blocks) for ANOVA of the YI show a p-value <0.00001, indicating a strong those found for the emulsion-polymerized ABS, considering the same AO composition, indicating difference concerning the replications. From these results, and after removing the effect between that the amount of PB and the morphology of PB phase interfere in the thermal-oxidative stability of replications (blocks), Equation (12) is obtained. A lower correlation coefficient (R = 0.744) was obtained ABS, as well as for the OP. From Figure 6b (Tukey’s test), it follows that samples containing at least for this equation, mainly related to nuisance factors (e.g., possible differences in the cleanliness of one primary AO show higher OP values, whereas the addition of the secondary AO alone results in the micro-extruder). lower OP values. Hence, despite the fact that the secondary AO significantly affects the OP values, YI = 11.16 − 0.593A + 5.551B − 1.211AB + 2.433D (12) the primary AOs have a more pronounced effect on OP.

35 Experimental ABS Emulsion 30 Experimental ABS Mass Predicted 25 Virgin ABS Emulsion Virgin ABS Mass 20

0 Yellowing Index -5

-10

• -15 s n ¤ E E E E E E E / / / / M / / M / M M M M M s o / / / / / / / i a 2 2 E 0 2 0 0 2 M . . / / . 0 / / 2 . 2 0 / 2 0 2 s . l / . . / . / M 0 0 0 2 0 0 2 0 0 u / / / . / 2 / . 0 / 0 2 / 0 0 0 -20 . / / . / / / 2 0 0 0 2 2 0 0 0 n 2 i m . / / / . 0 . / 2 / 0 0 / 0 2 / . / / / . . g 0 2 0 0 0 0 2 0 0 E 0 0 2 2 0 0 0 r / . / . i / . . / n 0 0 2 0 . 0 0 0 2 V i . 0 g 0 r i

V Sample FigureFigure 7. 7.Yellowing Yellowing index index (YI) (YI) for for thethe samplessamples studiedstudied inin Table1 1 (ranking (ranking fromfrom lowlow toto high, right to toleft) left) and and the the two two unprocessed unprocessed virgin onesones (outer(outer left);left); again,again, color color coding coding is is as as in in Figure Figure3; 3; consistent consistent withwith the the data data in in Figures Figures5 and 5 and6, more 6, more yellowing yellowing takes takes place place for for emulsion-polymerized emulsion-polymerized ABS. ABS.

3.4. Yellowing Index (YI) The yellowing index (YI) results are shown in Figure 7, ranking the data in Table 1 from low to higher values (right to left) and complementing this data set with the data for virgin ABS. The ANOVA results for the yellowing index (YI) show that the type of ABS (factor ‘D’; p-value lower than 0,0002) and the interaction between Irganox 1076 and Irganox 245 (‘AB’; p-value lower than 0,0271) are significant. The replications (blocks) for ANOVA of the YI show a p-value <0.00001, indicating a strong difference concerning the replications. From these results, and after removing the effect

Polymers 2019, 11, 25 12 of 15

From Figure7 and Equation (12), one can observe that the processing of both ABS types causes an increase in the YI. The type of ABS largely influences the YI and the virgin ABS obtained by mass polymerization presents a lower YI than the virgin emulsion-polymerized ABS. This difference in the YI is again related to the higher amount of PB phase of the ABS based on emulsion polymerization. It should be noted that Zweifel [14] reported that styrene-acrylonitrile copolymers (SAN) become discolored above 220 ◦C even in the absence of oxygen due to reactions of the acrylonitrile comonomer, and the combination of Irganox 1076 with Irgafos 168 suppresses discoloration during processing. However, the data analysis in the present work (see Figures3 and7) indicates that the PB significantly contributes to the increase in the ABS YI. In addition, the multiple comparison Tukey’s test for the YI results (not shown here) indicate no significant difference among all processed samples. This is probably related to a variation in the cleanliness of the equipment before each processing, which generated an additional variance for the YI results.

3.5. Summary of the ANOVA Table2 summarizes the results for the different degradation properties obtained in the present work according to ANOVA (for the results, see Supporting Information). For clarity, the reader is reminded that (−1) and (+1) represent the level of each factor which results in the higher predicted value for each property. By jointly investigating these properties, the relevance of more individual and interaction parameters is evident. It is observed that the type of ABS affects all properties and is thus very significant for the interpretation of the degradation data. The emulsion-based ABS (‘D’ = +1) provides higher values for the R1–R4 absorption ratios (more PB phase), as well as higher YI. The mass-polymerized ABS shows higher OOT and OP values after melt processing, indicating better thermal-oxidative stability. The addition of both Irganox 1076 and Irganox 245 results in higher OOT and OP values, and there is a synergistic effect with the use of these primary AOs, as can be deduced from the significant interaction AB. However, this effect can be related to the higher amount of primary AOs in the polymer when using Irganox 1076 and Irganox 245 simultaneously. The addition of 0.2 m% of Irganox 1076 is however better than adding 0.2 m% of Irganox 245. The secondary AO has a significant effect on OP, increasing the thermal-oxidative stability. Only the OP displays a relevance for all AOs and several interactions, as witnessed by the significance of the interaction ‘AB’ and ‘ABD’.

Table 2. Summary of the ANOVA results for the studied degradation properties, based on the experimental data in Figures3–7 and Equations (1)–(12); S: signiﬁcant; I: insigniﬁcant; ( −1) or (+1): level of each factor which results in the higher predicted value for each property; M/E: −1/+1.

Main Factor (Code) Signiﬁcant Interaction(s) Property Irganox 1076a Irganox 245a Irgafos 168b Type of ABS (‘A’) (‘B’) (‘C’) (‘D’) R1 I I I S (+1) - R2 I I I S (+1) - R3 I S (+1) I S (+1) AC; BD R4 I S (+1) I S (+1) AD; BD OOT S (+1) S (+1) I S (−1) AB OP S (+1) S (+1) S (+1) S (−1) AB; ABD YI I I I S (+1) AB a Primary antioxidant (AO); b Secondary AO.

4. Conclusions Processing of unmodified emulsion- and mass-polymerized ABS (220 ◦C) significantly degrades the polybutadiene (PB) phase, contributing to the reduction of the oxidation onset temperature (OOT) and the oxidation peak temperature (OP), as well as to the increase in the yellowing index (YI). Therefore, antioxidants (AOs) should be added in sufficiently high amounts during the original processing to reduce thermal-oxidative degradation. It can be postulated that this results Polymers 2019, 11, 25 13 of 15 in post-consumer recycled plastics—such as those originating from waste electrical and electronic equipment (WEEE)—with better physical-chemical properties. The relative PB phase amount variation for processing at 220 ◦C can be assessed by dedicated Fourier transform infrared spectroscopy (FTIR). The styrene (phenyl) absorption peak at 1603 cm−1 changes simultaneously with the thermal-oxidative degradation of the PB rich phase, whereas the nitrile absorption peak at 2237 cm−1 can be seen as insensitive to this degradation. Therefore, the nitrile absorption peak is recommended as reference peak in order to evaluate chemical changes in the PB content. Due to a lower PB content, the ABS samples obtained by mass polymerization display higher thermal-oxidative stability than those samples obtained with emulsion-polymerized ABS, despite the fact that both virgin ABS types show similar mechanical properties. Hence, the use of an ABS polymerized by a mass process in the manufacturing of EEE can generate WEEE ABS with lower changes in physical-chemical properties than WEEE obtained from emulsion-polymerized ABS. The addition of the two primary AOs studied (Irganox 1076 and Irganox 245) enhances the thermal-oxidative stability, increasing both OOT and OP, as well as causing a reduction in the YI. A synergetic effect is observed with the combination of the two primary AOs, resulting in higher OOT and OP values. The addition of the secondary AO increases OP, further increasing thermal-oxidative stability. This OP is influenced by all AO types and several interactions, as can be deduced from the ANOVA predictive equation. Further statistical analysis of the current data set however suggests that the use of 0.2 m% of primary AO, in particular Irganox 1076, displays a high potential, which is economically attractive.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4360/11/1/25/s1, Figure S1: FTIR spectra, Figure S2: Oxidation onset temperature and oxidation peak. Author Contributions: Conceptualization, R.F. and L.C.; Methodology, R.F.; Validation, R.F. and D.R.D.; Formal Analysis, R.F. and D.R.D.; Investigation, R.F.; Resources, L.C.; Data Curation, R.F.; Writing—Original Draft Preparation, R.F.; Writing—Review and Editing, R.F., D.R.D., K.R., and L.C.; Visualization, R.F., D.R.D., and K.R.; Supervision, L.C.; Project Administration, L.C.; Funding Acquisition, L.C. Funding: This research was funded by the European Union’s Horizon 2020 Research and Innovation Program, grant number 730308. Acknowledgments: The authors are grateful to Arne Rüdiger (SITRAPLAS GmbH) for the supply of the materials used in this work. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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