Impact of organic-inorganic color additive on the properties of ethylene-norbornene copolymer

Polymer Testing 82 (2020) 106290

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Material Properties

Impact of organic-inorganic color additive on the properties of ethylene-norbornene copolymer Anna Marzec a, *, Bolesław Szadkowski a, Małgorzata Ku�smierek a, Jacek Rogowski b, � ski c, Marian Zaborski a Waldemar Maniukiewicz b, Przemysław Rybin a b c

Institute of Polymer and Dye Technology, Faculty of Chemistry, Lodz University of Technology, Stefanowskiego 12/16, 90-924, Lodz, Poland Institute of General and Ecological Chemistry, Lodz University of Technology, Zeromskiego 116, 90-924, Lodz, Poland Institute of Chemistry, Jan Kochanowski University, Kielce, Poland

A R T I C L E I N F O

A B S T R A C T

Keywords: Aluminum-magnesium hydroxycarbonate Physico-chemical characterization Organic-inorganic pigment Polymer composite Photostability Flame retardancy

In this study, we developed a new organic-inorganic pigment (LP) by precipitation of dye molecules onto aluminum-magnesium hydroxide (LH). The morphology and physico-chemical properties of the pigment were characterized by X-ray diffraction analysis (XRD), secondary ion mass spectrometry (TOF-SIMS), 27-Al solidstate nuclear magnetic resonance (NMR) spectroscopy, scanning electron microscopy (SEM) and thermogravi­ metric analysis (TGA). The incorporation of the azo dye into the LH host caused an increase in the interlayer distances of the LP from 0.751 to 0.758 nm. The LP pigment also showed better thermal- and photo-stability than the pristine azo chromophore. The novel organic-inorganic additive was next applied as a filler at different concentrations (2 phr, 5 phr, 10 phr) to obtain colorful ethylene-norbornene (EN) films. The morphological, mechanical and flame-retardant properties of the composites were determined by XRD, SEM, dynamicmechanical analysis (DMA) and cone calorimetry tests (CCT). The EN films containing the LH/Azo dye demonstrated improved mechanical, barrier and flame-retardant properties. Compared to the neat EN copolymer and EN/LH composite, the EN/LP sample was found to be the most resistant to UV aging, as confirmed by FTIR and mechanical analysis.

1. Introduction Organic-inorganic hybrid materials have attracted increasing atten­ tion in recent years, due to their ability to combine the advantages of their organic and inorganic components [1,2]. This is reflected by the growing number of publications containing references to hybrid mate­ rials. Organic-inorganic materials with outstanding physico-chemical properties and multifunctionality represent a new field of basic research. Organic-inorganic colorants are considered to be one of the most promising groups of hybrid materials [3–7]. Dyes are organic compounds which are usually applied to ensure the color fastness or to change the color of a material [8]. However, in comparison to pigments, which are solid particles, they have many disadvantages, such as low melting points and high tendencies to migrate and degrade. This limits the application of dyes in many areas. Most of the disadvantages of dyes can be overcome by transforming the organic chromophores into the insoluble form of pigments. The organic-inorganic colorant group combines the most desirable

properties of organic pigments, such as their wide range of colors including colors with high intensity, with those of inorganic pigments, such as high resistance to light, temperature and organic solvents [9–11]. Stabilization and immobilization of organic dyes on inorganic hosts has been reported for different carriers, such as layered double hydroxides, gamma-alumina, silica, organo-modified silica, TiO2 and zeolites [12–20]. Many different clays, including bentonite, smectite clay or synthetic fluorinated mica, have also been identified as suitable inorganic matrices for organic components, based on their swelling properties and their characteristics as soft matter materials [21–24]. Generally, research has confirmed that these minerals are potential candidates for producing new colorants with vivid colors as well as high thermal and chemical stability. Organic-inorganic pigments offer an attractive alternative to conventional coloring agents, due to the possi­ bility of combining coloring ability with the functionality of polymer fillers. In recent years, several studies have been conducted on the synthesis and application of organic-inorganic pigments. Marchante et al. [25,26]

* Corresponding author. E-mail address: [email protected] (A. Marzec). https://doi.org/10.1016/j.polymertesting.2019.106290 Received 8 August 2019; Received in revised form 8 December 2019; Accepted 10 December 2019 Available online 16 December 2019 0142-9418/© 2019 Elsevier Ltd. All rights reserved.

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prepared and characterized a series of dye-clay nano-pigments and studied their effects in linear low-density polyethylene (LDPE) and ethylene vinyl acetate (EVA) composites. They found that the polymer composites containing the organic-inorganic pigments exhibited better color characteristics and improved mechanical properties in comparison to the composites with conventional pigments. Other studies have shown that the application of organic-inorganic pigments results in the reinforcement of thermostable poly(hydroxyethylacrylate) composite [27]. Raha et al. [28] modified montmorillonite with non-ionic dyes, reducing migration of the dyes from polypropylene. Marangoni et al. [29] and Kang et al. [30] used layered double hydroxide intercalated with dye anions to produce polymer composites with improved me­ chanical properties as well as high fire resistance. In a previous work, we used mono and dicarboxylic azo dyes modi­ fied with aluminum-magnesium hydoxycarbonates to produce organicinorganic pigments [31], which were found to improve the perme­ ability and flame resistance of elastomer composites cured with different crosslinking systems. In other research, exposure tests showed a signif­ icant improvement in the photo-stability of ethylene-norbornene (EN) composites containing organic-inorganic pigments compared to an EN/dye sample [32]. The present paper reports the modification of aluminum-magnesium hydoxycarbonate with azo dye containing a carbonyl group and sulfur in its structure. This was expected to enhance the flame retardant properties of the organic-inorganic pigment in a polymer film. The chemical composition and morphology of the LH-based pigment were analyzed by X-ray diffraction analysis (XRD), secondary ion mass spectrometry (TOF-SIMS), 27-Al solid-state nuclear magnetic resonance (NMR) spectroscopy and scanning electron micro­ scopy (SEM). The pigment was further characterized by thermogravi­ metric analysis (TGA) and a solvent test was conducted to confirm its thermal stability and resistance to dissolution. The properties of the EN copolymer containing the organic-inorganic pigment were investigated using static and dynamic-mechanical (DMA) analysis, X-ray diffraction analysis (XRD) and the cone calorimetry technique (CCT). The results were compared to those for neat EN samples (EN, EN/LH, EN/Azo dye). The impact of the pigment on the resistance to UV aging of the EN copolymer was also evaluated.

2.2. LH/Azo dye pigment (LP) preparation method

2. Experimental

TOF-SIMS mass spectra were recorded using a TOF-SIMS IV sec­ ondary ion mass spectrometer (IONTOF GmbH, Germany). This appa­ ratus is equipped with a high mass resolution time of flight analyzer and Biþ 3 primary ion gun. Secondary ion mass spectra were obtained across an area of approximately 100 μm � 100 μm on the sample surface. The analyzed area was irradiated with pulses of 25 keV Biþ 3 ions with a 10 kHz repetition rate and an average ion current of 0.4 pA. The mea­ surement time was 30 s. Secondary ions emitted from the bombarded surface were mass separated and counted in a time of flight (TOF) analyzer. Measurements of 27Al Solid state Nuclear Magnetic Resonance (MAS NMR) were performed in a Bruker Avance III 400 WB spectrom­ eter operating at a resonance frequency of 104.26 MHz. The crystalline structure of the LH-pigment was analyzed by powder X-ray diffraction (XRD) analysis using a PANalytical X’Pert Pro MPD diffractometer (Malvern Panalytical Ltd., Royston, UK) in the Bragg–Brentano reflect­ ing geometry with (Cu Kα) radiation from a sealed tube in the range of 2θ ¼ 2–70� using a step length of 0.0167� . The thermal stability of the samples was investigated using a Mettler Toledo Thermogravimetric Analyzer TGA (USA). Powder samples of approximately 10 mg were placed in an aluminum oxide crucible and heated from 25 � C to 600 � C in an argon atmosphere, with a heating rate of 10 � C/min. The morphology of the surfaces of the studied powders and the tensile fracture surfaces of the polymer composites were observed using a LEO 1530 Gemini scanning electron microscope (Zeiss/LEO, Ger­ many). The specimens were coated with a carbon target employing the Cressington 208 HR system. The solvent resistance of the LH-pigment was determined in accordance to the PN-C-04406/1998 standard. Samples (0.05 g) of the LP pigments were immersed in 20 ml of different

The synthesis of LP/A pigment was performed according to the following procedure. First, a water solution of azo dye (2.0 g in 200 ml) with the addition of 10 ml of ethyl alcohol was subjected to 30 min of ultrasonication at room temperature. Then, the suspension was heated to 80 � C and aluminum-magnesium hydroxycarbonate (8.0 g) was added. The slurry was stirred continuously for 3 h, at a pH in the range of 8–8.5. Finally, the solid content was separated from the water solution by vacuum filtration and washed with deionized water to remove excess reactants, until a clear colorless filtrate was obtained. Hybrid pigment powder was produced by drying in an oven at 70 � C for 24 h. 2.3. Preparation of EN/LP composites Polymer compounds were prepared in a Brabender N50 measuring mixer at 120 � C with a rotors speed of 50 rpm. The formulations for the prepared EN composites were as follows (where phr means parts per hundred parts of rubber): � 100 phr of ethylene-norbornene copolymer – EN, � 100 phr of ethylene-norbornene copolymer filled with 2 phr organic-inorganic pigment (LP) – EN/2LP, � 100 phr of ethylene-norbornene copolymer filled with 5 phr organic-inorganic pigment (LP) – EN/5LP, � 100 phr of ethylene-norbornene copolymer filled with 10 phr organic-inorganic pigment (LP) – EN/10LP, � 100 phr of ethylene-norbornene copolymer filled with 5 phr aluminum-magnesium hydroxycarbonate (LH) – EN/5LH, � 100 phr of ethylene-norbornene copolymer filled with 0.75 phr azo dye – EN/Azo dye.

of of of of of

After blending, the polymer composites were pressed between two steel plates under 15 MPa pressure at 120 � C for 10 min to obtain the final plate-shape samples. 2.4. Characterization techniques

2.1. Materials Ethylene-norbornene random copolymer (EN) – Topas Elastomer 140 (40 wt% bound norbornene content) – was purchased from TOPAS Advanced Polymers (Germany). Aluminum-magnesium hydroxycar­ bonate (LH) with high content of Al (Al/Mg weight ratio: 70/30) – PURAL® MG 30 – was provided by Sasol (Germany). Ethyl alcohol (95%), toluene (99.8%), acetone (99.9%), hydrochloric acid (37%) and n-butyl acetate (99.5%) were purchased from Sigma Aldrich (Germany) and used without purification. Other commercially available reagents for azo dye synthesis (3-hydroxy-2-naphthoic acid, 4-aminobenzenesul­ fonamide) were supplied by Sigma Aldrich (Germany), all with analyt­ ical grades. Azo dye (Fig. 1) was synthesized following a typical diazotization procedure described in the literature [33].

Fig. 1. Chemical formula of azo dye applied to obtain the organicinorganic pigment. 2

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organic solvents (acetone, toluene, n-butyl acetate, ethyl alcohol) and water for 24 h. Diffuse reflectance UV–Vis spectra were obtained using an Evolution 201/220 UV–Visible Spectrophotometer (Thermo Scienti­ fic, USA) in the spectral range of 1100-200 nm. Dynamic-mechanical tests were conducted using a DMA/SDTA861 analyzer (Mettler Toledo) under tension mode conditions. All the measurements were carried out with the following parameters: frequency 5 Hz; strain amplitude 4 μm; temperature range -80–80 � C; heating rate 2 � C/min. To determine the glass transition temperature of the composites, a maximum of tgδ ¼ f(T) was used. To estimate the photostability of the EN composites, the UV ageing process was simulated in a UV 2000 apparatus from Atlas. The measurement consisted of two alternately repeating segments with the following parameters: daily segment (ra­ diation intensity ¼ 0.7 W/m2, temperature ¼ 60 � C, duration ¼ 8 h) and night segment (no UV radiation, temperature ¼ 50 � C, duration ¼ 4 h). The color of the studied composites was measured using a CM-3600d spectrophotometer (Konica Minolta Sensing, Inc., Osaka, Japan) in the wavelength range of 360–740 nm. The total color change of the samples was determined based on the following equation: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi (1) ΔE ¼ ðΔLÞ2 þ ðΔaÞ2 þ ðΔbÞ2 ;

Ltd. During the flammability tests, squared specimens of the composites with dimensions of 100 mm � 100 mm � 2 mm were irradiated hori­ zontally with a heat flux of 35 kW/m2. 3. Results and discussion 3.1. Organic-inorganic pigment characterization results 3.1.1. Powder X-ray diffraction analysis (PXRD) The powder PXRD patterns of LH host, LH/azo dye, and azo dye are shown in Fig. 2, respectively. As seen from Fig. 2a, the hydrotalcite type phase was identified for LH host. The diffraction pattern consists of three sharp peaks at a low 2θ angle, equivalent to diffraction by planes (003), (006) and (009). The sharpness of the peaks suggests ordered and reg­ ular stacking of the LH layers. The interlayer distances (d003) and (d006), corresponding to 0.751 nm and 0.377 nm, are due to basal reflections indexed to a hexagonal crystal lattice with rhombohedral 3R symmetry. These values, together with other non-basal spacings at 0.152 nm (d110) and 0.149 nm (d113), are consistent with the typical XRD pattern of hydrotalcite-like compounds [36]. On the slopes of the sharp peaks, small broad peaks can be observed which indicate the presence of co-precipitated, poorly crystalline boehmite. After the addition of the azo dye to LH host, the diffraction pattern (Fig. 2b) becomes more complicated. The presence of characteristic basal and non-basal re­ flections confirm that the LH structure was retained. However, a slight shifting of the peak (003) occurs towards the lower angles of 2θ. Therefore in the organic-inorganic pigment the interlayer distance of (d003) increased from 0.751 to 0.758 nm. Also, a series of new peaks appear on the diffractogram. Some of these come from the azo dye, while the peaks at 2θ ¼ 8.02, 9.02 and 10.39� reveal the formation of a new crystalline phase. In order to confirm that the changes observed in the crystalline phase of the organic-inorganic pigment were the result of dye–host interactions, additional experiments were performed. Both components of the organic-inorganic pigment, i.e. the dye and the LH matrix, were separately subjected the same reaction conditions as during pigment formation (Sec. 2.2) and subsequently studied by XRD. The diffraction patterns obtained showed that the structure of the azo dye and LH remained almost unchanged in comparison to the structure of the dye and LH before treatment (SFig.1). Therefore, it was concluded that the new peaks which appeared on the diffraction pattern for the organic-inorganic colorant were the result of the formation of a new crystalline phase.

where ΔL is the level of lightness or darkness, Δa is the relationship between redness and greenness and Δb is the relationship between blueness and yellowness. A universal testing machine (Zwick 1435, Roell) was used to measure the tensile properties of the EN/LP composites at a uniform crosshead speed of 500 mm min 1, according to the ISO 37 standard. The results for tensile strength were recorded as the averages of five tests. The aging factor (K) was calculated as the numerical change in the mechanical properties of the samples upon aging [34]: K¼

ðTS ⋅EB Þafter aging ; ðTS ⋅EB Þbefore aging

(2)

where TS is the tensile strength and EB is the elongation at break of the samples. The carbonyl index (CI) of the samples was determined from FT-IR spectra based on the following equation: CI ¼

AC¼O ; A CH2

(3)

where AC¼O is the area of the carbonyl absorption band in the range of 1800–1680 cm 1 and A-CH2- is the area of the internal reference band in the range of 3000–2800 cm 1, which is associated with C–H bending and stretching [35]. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy was conducted on a Thermo Scientific Nicolet 6700 FTIR spectrometer (USA). The barrier properties of the EN composites were evaluated based on the manometric method at room temperature. Permeability measurements were conducted using atmo­ spheric air. The barrier properties of the composites were determined based on the gas transmission rate (GTR) and coefficient of gas perme­ ability (P), which were calculated using the following two equations: GTR ¼

VC dp ⋅ ; R⋅T⋅PU ⋅A dt

3.1.2. Time of flight secondary ion mass spectrometry (TOF-SIMS) and solid state nuclear magnetic resonance of 27Al (NMR) The LH with added azo dye was investigated using time-of-flight secondary ion mass spectrometry. The presence of azo dye in the LH matrix was indicated by a series of peaks (Fig. 3a) characteristic for azo dye in the spectrum of the modified LH (Fig. 3b), which were not present in the pure LH. The assignment of particular peaks in azo dye spectrum is shown in Fig. 3b. The characteristic pattern of the peaks in Fig. 3b may be attributed to superimposed isotopic patterns of the molecular ion with a detached proton and the radical anion of the azo dye. Within this pattern, the peak at m/z 370 is assigned to (M H)- while the main ⋅contribution to the peak at m/z 371 is from the M radical anion. The 27Al MAS NMR spectrum of the LH mineral contains a sharp peak at 3.9 ppm which is assigned to octahedral Al (Fig. 4) [37]. After modification with dye, the intensity of the peak undergoes a change and shifts slightly to 3.6 ppm. We speculate that slight changes in the local Al environments may be related to the interactions with the azo chromo­ phore. This suggestion is consistent with the 27Al MAS NMR spectrum of the samples obtained in our previous studies, which displayed similar shifts after modification with anthraquinone and dicarboxylic azo dyes [38,39].

(4)

where VC is the volume of the low-pressure chamber [l], R is the gas constant at 8.31 � 103 [(l∙Pa)/(K∙mol)], T is temperature [K], PU is gas pressure in the high-pressure chamber [Pa], A is area permeation of gas through the sample [m2], and dp/dt is pressure change over time [Pa/s]. P ¼ GTR ⋅d;

(5)

where d is the thickness of the sample [m]. The flammability of the EN composites filled with new organicinorganic pigment and LH was estimated according to the PN-ISO 5600 standard using a cone calorimeter from Fire Testing Technology 3

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Fig. 2. The PXRD diffraction patterns of the studied samples: LH host (a), LP pigment (b) and azo dye (c).

Fig. 3. TOF-SIMS spectra of the studied samples: (a) LP pigment and LH host, (b) pristine azo dye.

3.1.3. Morphology investigation – scanning electron microscopy (SEM/ EDS) The morphology of the studied LH mineral, pristine azo dye and LP pigment was analyzed based on SEM micrographs, as shown in Fig. 5. The dimensions of the LH particles were in range of 400–700 nm up to, occasionally, several μm, with platelet thicknesses of around 10–30 nm. Fig. 5 c,d shows SEM images of the aluminum-magnesium hydroxycar­ bonate modified by the azo chromophore. A number of microscopic crystals can be seen on the LH surface, which are different from those in the pristine dye structure. This observation is consistent with the XRD pattern of the organic-inorganic pigment, which revealed the presence of new crystalline peaks. Moreover, the macrographs with EDS analysis revealed the presence of sulfur in the LH pigment, which was not detected in the unmodified LH host (Fig. 6). The absence of the nitrogen element in the EDS results can be explained by the presence of C atoms, for which the peak is located at the same position as nitrogen. Therefore, elemental analysis was employed to confirm the presence of nitrogen (which is also an element of the dye structure) and determine the con­ centration of the azo chromophore stabilized by the aluminum-

magnesium hydroxycarbonate (Table 1). The results show that modifi­ cation with 20% azo dye resulted in a lower nitrogen concentration (of approximately 1.98) than that which had been expected (2.26). 3.1.4. Thermal stability – thermogravimetric analysis (TGA) In order to investigate the incorporation of azo dye into the inorganic host, the LH and LH-based pigment were subjected to thermal treatment from room temperature to 600 � C in an argon atmosphere (Fig. 7). Thermal decomposition of the LH was divided into three main stages. The first step (below 100 � C) may be attributed to the elimination of the physisorbed water molecule. The next loss with a peak at around 200 � C was due to the removal of intercalated water molecules (dehydration) in the LH galleries. The third step, between 300 and 450 � C, can be ascribed to the deliberation of CO3 2 and the decomposition of metal hydroxide layers. The main loss weights of the organic-inorganic pigments were similar to that of the unmodified mineral. However, modification of the LH with dye affected its thermal stability. The mass loss peaks corre­ sponding to physisorbed water and dehydration shifted to higher tem­ peratures [40,41]. 4

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Fig. 4.

27

Al NMR spectra of LH mineral and prepared LP pigment.

Fig. 5. SEM images of unmodified LH mineral (a), azo dye (b) and the LP pigment (c,d).

The results of TGA show that the T5% weight loss temperature for LH/Azo dye (143 � C) was shifted to a higher temperature than that of the neat host (119 � C). It is difficult to distinguish the degradation process of the azo dye in the LH pigment, due to the overlap with the decompo­ sition of the mineral. The DTG peak of LH after modification shifted from 316 � C to 336 � C, suggesting that dehydroxylation and the

decomposition of the metal hydroxide layers were delayed. The LH/Azo dye also showed a significant increase in weight loss temperatures in the region corresponding to the liberation of hydroxyl groups and carbonate ions (300–500 � C). This most likely resulted from the incorporation of acidic dye into the LH matrix (Fig. 8). The mechanism of LH pigment formation is therefore similar to that observed in our previous study 5

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Fig. 6. SEM-EDS results obtained for (a) unmodified LH mineral and (b) LP pigment.

[42] and is most likely related to dye-metal interactions and partial intercalation of the dye into the LH layers (lower amounts of carbonate ions and increases in interlayer space).

Table 1 Elemental analysis data for unmodified LH mineral and the LP pigment. Compound

%C

%H

%Na

%Nb

LH LP

1.60; 1.55; 1.58 11.77; 11.92; 11.80

3.20; 3.24; 3.21 3.06; 2.96; 3.01

Absence 1.99; 1.99; 1.98

– 2.26

a b

3.1.5. Resistance to temperature and solvents The color stability of the LH/Azo dye and free azo dye were inves­ tigated at elevated temperatures by heat treating the powders in an oven at 200 � C and 250 � C for 30 min. The UV–Vis absorption was measured after each temperature step (Fig. 9). Neither sample showed significant changes in the spectra after heating at 200 � C. However, when the temperature was increased up to 250 � C the azo dye underwent decomposition and turned black, causing a significant variation in the absorbance curve. The color of the LP pigment after treatment at 250 � C changed only slightly. This means that the organic-inorganic pigment had better thermal stability than the unbounded dye. To evaluate the color stability of the LP pigment, it was submerged in solvents (toluene, ethanol, acetone, butyl acetate) and water for 24 h. As shown in Fig. 10, the pigment showed very good resistance to dissolu­ tion, especially in toluene, butyl acetate and water (colorless solvent after immersion time), whereas the free azo dye demonstrated a high degree of discoloration in all the studied media. The discoloration noticed in acetone and ethanol may be related to the higher sensitivity of the azo dye to these solvents and the presence of free dyes on the LH surface. The enhanced insolubility of the azo chromophore confirmed that the chemical stability of the LP pigment had been improved and that the dye had been effectively transformed into its pigment form.

Received. Expected.

Fig. 7. TGA/DTG curves for LH and LP samples.

3.2. Characterization of EN composites 3.2.1. Photostability of composites Many organic and inorganic pigments are well known for their 6

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Fig. 8. Schematic mechanism of pigment formation based on aluminum-magnesium hydroxycarbonate and azo dye.

Fig. 9. UV–Vis spectra of the azo dye and LH-based pigment exposed to elevated temperatures.

outstanding performance in terms of photostability. In polymer com­ posites, they can exhibit a protective effect by absorbing and/or screening light energy, thereby extending the life of the polymer under harsh conditions. In this study, samples of EN, EN/LH, EN/Azo dye and EN/LP were irradiated with a UV source for 100, 200 and 300 h. The effect of the LP pigment on the stability of the EN copolymer under UV ageing conditions was then studied. First, the effects of 300 h of UV exposure on the mechanical properties of composites filled with 5 phr of LH mineral or LP were explored. As can be seen in Fig. 11, after 300 h of exposure the tensile strength (TS) of the neat copolymer decreased rapidly, from 40 MPa to 16 MPa, while for the pigmented sample only a slight reduction in TS (of approximately 10 MPa) was observed. The sample containing unmodi­ fied LH filler also exhibited considerable deterioration in terms of me­ chanical properties (TS from 40 to 20 MPa). Furthermore, the ageing factors determining the alterations in the mechanical properties after the UV exposure were found to be unchanged for the samples filled with LP pigment or azo dye, in contrast to the reference samples (EN and EN/ LH). This means that the unprotected compositions underwent consid­ erable degradation because of UV irradiation, while the sample con­ taining the LH-based pigment remained stable under the same conditions. This effect may be explained by the UV absorption capability of the LP pigment, which has also reported for other pigments in several other works [32,43,44]. The ageing stability of the studied EN composites was also monitored by FT-IR measurements, which provided important information

regarding structural changes in the samples (Fig. 11d). The most important region in the FT-IR spectra determining the presence of photooxidized products is in the range of 1700–1750 cm 1. Following the alterations in the intensity of this band, it is possible to estimate the degree of polymer degradation based on the carbonyl index (CI) [45]. The CI values determined for the studied composites confirmed the as­ sumptions drawn from the mechanical measurements. The light fastness of the colored samples was clearly superior to that of the unprotected samples. This was reflected by the quite rapid increase in the CI parameter determined for the EN and EN/LH samples during UV irra­ diation, which was due to the formation of carbonyl moieties, indicating a progressive ageing process. It is also important to note that the LP pigment exhibited markedly enhanced photostability in comparison to the unmodified azo chromo­ phore. This was proven using spectrophotometric measurements to determine the total color change (ΔE) of the samples in the CIE-Lab color system [46]. As shown in Fig. 11c, the ΔE values of the sample filled with azo dye were significantly higher than those for the organic-inorganic pigment after the same exposure time. This may be explained by the fact the dye molecules were protected by the inorganic matrix in the organic-inorganic pigment sample. 3.2.2. Dynamic-mechanical analysis (DMA) We also studied the dynamic-mechanical behavior of EN copolymers filled with azo dye, with an LH host and different amounts of LH-based pigment. The results are presented in Figs. 12 and 13. 7

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concentration of the LP pigment in the composite raised the storage modulus (E0 ) and loss modulus (E00 ) in comparison to the neat copol­ ymer. This means that this colorant showed some reinforcing activity, most likely due to the good pigment-polymer interactions and relatively uniform distribution of pigment particles within the polymer matrix. The presence of LP particles in the EN matrix contributed to increase the stiffness of the composite sample, by reducing the mobility of the macromolecule chains. It is also interesting to note that the pigment had a more pronounced effect on the dynamic properties of the EN copol­ ymer compared to unmodified dye or the unmodified LH filler, sug­ gesting superior filler-polymer compatibility. For example, the storage modulus at 20 � C for the EN/5LP sample was improved by around 141 MPa in comparison to EN/Azo dye and by around 83 MPa compared to EN/5LH (Table 2). These differences may be related to microstructuredependent matrix-filler interactions and suggest that the reinforcing effect dominates in the temperature region above glass transition. Further information about the bonding between the matrix and pigment can be provided by the tan δ vs temperature curves (Fig. 13). For all the filled compounds, the tan δ peaks were slightly reduced in intensity compared to the reference sample. This decrease in the tan δ peak for the filled samples was likely due to the restricted movement of the EN segments in the vicinity of the filler particles. In fact, the filler interfaces formed guarantee improved stress transfer from the polymer matrix to the filler. Moreover, the maximum of the tan δ peak in the DMA curve repre­ sents the glass transition temperature (Tg) of the composite materials. Fig. 13 shows that the Tg value determined for the unmodified EN copolymer was approximately 16 � C, and this is consistent with the re­ sults obtained in our other work on EN copolymers [47]. Nonetheless, it can be observed that, irrespective of the concentration, the application of unmodified LH filler or LP pigment had a negligible influence on this

Fig. 10. Digital images of LP pigment after 24 h of immersion in different solvents.

Analysis of the storage (E0 ) and loss (E00 ) moduli as a function of temperature can provide an insight into polymer-filler interactions and the dispersion of the fillers. It is clear from Fig. 12 that increasing the

Fig. 11. Changes in the properties of EN composites after UV irradiation: (a) aging factor, (b) tensile strength, (c) total color change, (d) carbonyl index. 8

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Fig. 12. Logarithm of storage and loss modulus versus temperature curves for EN composites.

reduction in the gas permeability parameter (P) from 6.9 (mol m/ m∙s∙Pa) for neat copolymer to 5.2 (mol m/m∙s∙Pa) or 3.3 (mol m/ m∙s∙Pa) for composites with higher pigment content. The enhanced barrier performance can be attributed mostly to the plate-like shape structure of the LH host, which may hinder the diffusion of the gas molecules and extend their transport route through the polymer matrix. 3.2.4. Flammability of composites – cone calorimetry According to the literature, many different hydroxides can act as non-toxic flame-retardant agents in various polymeric materials, such as PP, EVA or ABS resin [50–52]. We therefore investigated the effect of the LH-based pigment on the flammability of the EN composites using a cone calorimetry test (CCT). The results are presented in Fig. 15 and summarized in Table 3. The most important parameters providing information about the combustion dynamics of the tested materials are undoubtedly the heat release rate (HRR) and the total heat release (THR). Neat EN copolymer is clearly a flammable material, which has intense peaks for HRRMAX and THR parameters (Fig. 15). However, both the unmodified azo chromo­ phore and the LP pigment were found to have an ambiguous impact on the fire behavior of the EN composites. Firstly, it should be noted that unmodified azo dye considerably reduced the time to ignition parameter (tig), which may result in an increased rate of fire development. In the case of the LH-based pigment, the time to ignition parameter strongly correlated with the content of the pigment in the EN composite. A positive effect was observed for the sample containing 10 phr of the LHbased pigment, when the tig parameter reached 178 s (in comparison to the reference was 116 s). Furthermore, the HRR and THR values decreased steadily with increasing concentrations of LP in the compos­ ite. These results indicate that the flammability of the EN compounds was markedly reduced by the incorporation of the LP pigment. Another very important parameter determining the speed of fire development is the fire growth rate (FIGRA). The higher the value of this parameter, the faster the fire spreads. From Table 3, it can be seen that

Fig. 13. Tan δ versus temperature curves of EN composites.

parameter. 3.2.3. Barrier properties of composites It is known from the literature that the barrier properties of polymer composites can be enhanced considerably by the incorporation of impermeable lamellar fillers (such as hydrotalcite or montmorillonite), which alter the diffusion path of gas molecules [48,49]. In our study, the application of plate-like shape pigments also had a significant influence on the barrier performance of the EN copolymer, as was reflected in the decreased GTR and P parameters. From Fig. 14, it can be seen that even at such low concentrations as 5 or 10 phr, the LP pigment contributed to improve the barrier stability of the EN composites. The application of this colorful filler resulted in the deceleration of gas flow through the sample, as evidenced by the

Table 2 Storage and loss modulus values of EN composites at different temperatures, obtained by DMA analysis. Modulus Storage [MPa] Loss [MPa]

EN

EN/Azo dye

EN/5LH

EN/5LP

0 �C

20 � C

40 � C

0 �C

20 � C

40 � C

0 �C

20 � C

40 � C

0 �C

20 � C

40 � C

1300 204

290 71

114 18

1398 194

304 72

108 18

1947 283

362 94

128 16

1984 305

445 108

158 26

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Polymer Testing 82 (2020) 106290

Fig. 14. Gas transmission rate (GTR) and permeability coefficient (P) parameters determined for EN composites.

Fig. 15. Maximum heat release rate (a) and total heat release (b) versus time curves for the EN composites filled with azo dye, LH host and LP.

application of LP clearly reduces FIGRA. The results indicate that even 5 phr of the prepared pigment reduced this parameter by about 30% relative to the reference sample. Finally, it is also worth noting that the application of LP effectively reduced other parameters determining the fire resistance of EN com­ posites, such as MLR, EHC and MARHE. The lower flammability pa­ rameters observed for EN composites filled with LP mean that the studied organic-inorganic pigment could be used as a colorful flameretardant agent in polymeric materials. The lower flammability of the studied compounds can be explained in a twofold manner. On one hand, the layered minerals that act as an inorganic carrier in LP may produce a barrier effect, limiting heat transfer within the polymer matrix. Another reason for the improved flame retardancy may be the formation of a protective carbon layer during thermal decomposition of the pigment, which acts as a thermal shield reflecting thermal radiation. Neverthe­ less, the low content of LP in the composite and the presence of filler agglomerates in the polymer matrix mean that barrier effect may only occur in the initial phase of thermal decomposition.

Table 3 Summary of the cone calorimetry data for the EN composites. Flammability parameters

Polymer composition EN

EN/azo dye

EN/ 5LH

EN/ 2LP

EN/ 5LP

EN/ 10LP

tig (s) HRR (kW/m2) HRRMAX (kW/m2) tHRR (s) FIGRA (kW/m2s) THR (MJ/m2) EHC (MJ/kg) EHCMAX (MJ/kg) MLR (g/s) MARHE (kW/m2)

116 167 428 210 2.03 68 34 79 6 160

75 148 443 175 2.53 54 28 79 9 156

78 134 324 185 1.74 63 36 78 6 142

67 75 309 160 1.92 39 24 70 6 119

81 96 287 195 1.47 32 24 73 5 100

178 100 297 250 1.18 43 21 61 5 99

tig-time to ignition; HRR-heat release rate; tHRR-time to maximum heat release rate; FIGRA-fire growth rate.; THR-total heat release; EHC- effective heat of combustion; MLR-mass loss rate; MARHE-maximum average rate of heat emission.

3.2.5. Morphology and distribution of the filler in composite Scanning electron microscopy (SEM) was used to provide 10

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information regarding the morphology, arrangement and dispersion of the prepared pigment within the EN matrix. For this purpose, SEM pictures were taken using two different detectors, an InLens and SE2. Microphotographs depicting LP particles dispersed in the EN copolymer are shown in Fig. 16. In the SEM images, it is possible to identify clusters of layered ma­ terials showing satisfactory adhesion to the polymer matrix. This means that the layered LP pigments retained their plate-like shape after their incorporation into the EN copolymer and are relatively compatible with this medium. Moreover, in Fig. 16e it can be observed that the pigment particles form numerous small clusters were distributed uniformly within the polymer matrix. Such a distribution of LP pigment particles may have a positive effect on the mechanical properties of the EN copolymer and/or reduce gas permeability through the composite system.

To verify the alterations in the EN morphology and dispersion of the studied pigment in the EN matrix, X-ray diffraction analysis was per­ formed. The PXRD diffraction patterns obtained are shown in Fig. 17. The PXRD diffraction patterns of semicrystalline EN and its com­ posites samples show two sharp diffraction peaks whose intensities and widths reflect the frequencies and periodicities, respectively. The peak at about 2θ ¼ 21.5� observed on all diffractograms with an equivalent to Bragg d spacing of about 4 Å, corresponds to the van der Waals contacts of atoms. Dorigato et al. [53] reported similar XRD results for another cycloolefin copolymer, which confirms that peak at 2θ ¼ 21.5� is very characteristic for these types of polymers. Interestingly, in the XRD spectra of the EN composites filled with LP or unmodified LH, new peaks at 2θ ¼ 11.5� occur which are also present in the diffraction patterns of the pure LP and LH powder samples (See Fig. 17 c-d). However, other characteristic maxima from the applied fillers completely disappeared in

Fig. 16. SEM images showing a cross-sections of EN copolymer filled with 10 phr of pigment. 11

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Polymer Testing 82 (2020) 106290

Fig. 17. The PXRD diffraction patterns of EN (a) and its composites filled with 0.75% of azo dye (b), 5% of LH (c) and 5% of LP (d).

the PXRD patterns of the composite, indicating that both the LH and the LP pigment were relatively uniformly dispersed in the EN matrix. This is in agreement with the results of our SEM investigations. Moreover, the intensity of the characteristic peak at 2θ ¼ 21.5� was found to decreased after incorporation of the filler into the polymer matrix, this may be due to the reduction in the crystallinity of the material. However, no peak shift towards the lower 2θ angles was observed, which excludes the intercalation of EN macromolecules into the LH and/or LP gallery.

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4. Conclusions In this study, an organic-inorganic pigment (LP) with high resistance to dissolution and enhanced thermal stability was successfully prepared by precipitation of azo dye onto an aluminum-magnesium hydroxycar­ bonate host (LH). The diffraction pattern of the LP contained a set of reflections which did not belong either to LH or to the azo dye, sug­ gesting the formation of a new organic-inorganic pigment. The results of SEM proved that the LP pigment particles were quite uniformly dispersed in the EN copolymer. The storage modulus at 20 � C measured by DMA improved by around 20% when 5 phr of LP pigment was added. Moreover, the application of 5 phr of organic-inorganic pigment led to a reduction in the HRRMAX parameter of more than 30% relative to the neat EN copolymer. The EN composites colored with LP pigment also showed the highest UV resistance, as the ageing factor determining the changes in the mechanical properties after UV exposure was found to be unchanged, in contrast to reference samples (EN and EN/LH). Therefore, the obtained organic-inorganic pigments can be considered for use as multifunctional additives in polymers, positively influencing the appli­ cative performance of polymers while simultaneously providing the composites with high aesthetic qualities. CRediT authorship contribution statement Anna Marzec: Conceptualization, Methodology, Investigation, Data curation, Visualization, Writing - original draft. Bolesław Szadkowski: Conceptualization, Methodology, Investigation, Data curation, Visuali­ zation, Writing - original draft. Małgorzata Ku�smierek: Investigation, Visualization. Jacek Rogowski: Investigation, Writing - original draft. Waldemar Maniukiewicz: Investigation, Writing - original draft. � ski: Investigation, Writing - original draft. Marian Przemysław Rybin Zaborski: Supervision. References [1] L. Cao, X. Fei, T. Zhang, Y. Lu, Y. Gu, B. Zhang, Modification of C.I. Pigment Red 21 with sepiolite and lithopone in its preparation process, Ind. Eng. Chem. Res. 53 (2014) 31–37.

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