Carminic Acid Stabilized with Aluminum-Magnesium ... - MDPI

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Carminic Acid Stabilized with Aluminum-Magnesium Hydroxycarbonate as New Colorant Reducing Flammability of Polymer Composites Anna Marzec 1, * , Bolesław Szadkowski 1 , Jacek Rogowski 2 , Waldemar Maniukiewicz 2 , 3 , Przemysław Rybinski 4 Dariusz Moszynski ´ ´ and Marian Zaborski 1 1

2 3 4

*

Institute of Polymer and Dye Technology, Faculty of Chemistry, Lodz University of Technology, Stefanowskiego 12/16, 90-924 Lodz, Poland; [email protected] (B.S.); [email protected] (M.Z.) Institute of General and Ecological Chemistry, Lodz University of Technology, Zeromskiego 116, 90-924 Lodz, Poland; [email protected] (J.R.); [email protected] (W.M.) Faculty of Chemical Technology and Engineering, West Pomeranian University of Technology, Szczecin, Piastów Ave. 42, 71-065 Szczecin, Poland; [email protected] Department of Management and Environmental Protection, Jan Kochanowski University, Zeromskiego 5, 25-369 Kielce, Poland; [email protected] Correspondence: [email protected]; Tel.: +48-426-313-209; Fax: +48-426-362-543

Academic Editor: Derek J. McPhee Received: 6 December 2018; Accepted: 2 February 2019; Published: 3 February 2019

Abstract: In this study, hybrid pigments based on carminic acid (CA) were synthesized and applied in polymer materials. Modification of aluminum-magnesium hydroxycarbonate (LH) with CA transformed the soluble chromophore into an organic-inorganic hybrid colorant. Secondary ion mass spectroscopy (TOF-SIMS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and UV-Vis spectroscopy were used to study the structure, composition, and morphology of the insoluble LH/CA colorant. Successful modification of the LH was confirmed by the presence of interactions between the LH matrix and molecules of CA. XPS analysis corroborated the presence of CA complexes with Mg2+ ions in the LH host. The batochromic shift in UV-Vis spectra of the organic-inorganic hybrid colorant was attributed to metal-dye interactions in the organic-inorganic hybrid colorants. Strong metal-dye interactions may also be responsible for the improved solvent resistance and chromostability of the modified LH. In comparison to uncolored ethylene-norbornene copolymer (EN), a modified EN sample containing LH/CA pigment showed lower heat release rate (HRR) and reduced total heat release (THR), providing the material with enhanced flame retardancy. Keywords: carminic acid; hybrid materials; aluminum-magnesium hydroxycarbonate; flame retardancy; ethylene-norbornene copolymer

1. Introduction Natural and synthetic dyes are used in a wide range of everyday products [1,2]. Their high solubility, as well as low thermal, chemical, and photo stability, strictly limit their use to certain materials and applications. Recently, interest has grown in new organic-inorganic colorants based on natural dyes [3–9]. This has led to the search for new pigments, formed by the immobilization of dyes in resistant substrates such clays, silicas, or zeolites [10–14]. Conventionally, hybrid pigments are obtained by the complexation of dye molecules with the metallic cations present in inorganic

Molecules 2019, 24, 560; doi:10.3390/molecules24030560

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Molecules 2019, 24,as x alumina or calcium carbonate. These forms of organic-inorganic pigments 2 of 15are substrates, such called lakes [15,16]. By combining natural or synthetic dyes with inorganic structures, it is possible to called lakes pigments [15,16]. Bywith combining natural or synthetic dyes with structures, it is possible to obtain hybrid enhanced chemical properties and inorganic a better thermal stability. obtain hybrid pigments with enhanced chemical properties and a better thermal stability. One of the oldest known coloring agents is carminic acid (CA), which is obtained from the female One of the oldest known coloring agents is carminic acid (CA), which is obtained from the Coccus laccae (Lacciferlacca Kerr) insect [17,18]. In its pure form, CA is extremely soluble in water female Coccus laccae (Lacciferlacca Kerr) insect [17,18]. In its pure form, CA is extremely soluble in and has a poor photo and thermal stability. Numerous studies have investigated ways to stabilize water and has a poor photo and thermal stability. Numerous studies have investigated ways to organic-inorganic pigments based on this chromophore. Pérez et al. [19] developed a new family of stabilize organic-inorganic pigments based on this chromophore. Pérez et al. [19] developed a new hybrid pigments bypigments the combination of γ-Al2 Oof chromophores, such as CA, alizarin, 3 and family of hybrid by the combination γ-Alorganic 2O3 and organic chromophores, such as CA, purpurin, curcumin, and betacyanins. Spectroscopic analysis formation of aluminum alizarin, purpurin, curcumin, and betacyanins. Spectroscopic proved analysisthe proved the formation of complexes between the aluminum mineral and particular organic groups (carboxylic acid, quaternary aluminum complexes between the aluminum mineral and particular organic groups (carboxylic ammonium, and β-keto enol) present the chromophores. Fournier et al. [20] reported the et stabilization acid, quaternary ammonium, and in β-keto enol) present in the chromophores. Fournier al. [20] of carminic using montmorillonite. Theyusing emphasized the role ofThey ketone and catechol reported acid the stabilization of carminic acid montmorillonite. emphasized the functions role of during the formation of the lake pigment. Guillermin et. al. [21] found that the adsorption complex ketone and catechol functions during the formation of the lake pigment. Guillermin et. al. [21] found of adsorptionmontmorillonite complex of CA with Al-pillared montmorillonite the photostability CAthat withthe Al-pillared enhanced the photostability of enhanced CA molecules. Velho et. al.of[22] CA molecules. Velho et. al.of [22] described the encapsulation of a series including CA, described the encapsulation a series of natural dyes, including CA, of in natural a silicadyes, matrix via the sol-gel in a silica matrix via the sol-gel process with the use of alkoxides. Colorimetric analysis revealed that process with the use of alkoxides. Colorimetric analysis revealed that silica-encapsulated natural silica-encapsulated natural dyes exhibited a higher resistance to discoloration under weathering dyes exhibited a higher resistance to discoloration under weathering conditions in comparison to conditions in colorants. comparison to non-capsulated colorants. non-capsulated Recently, several studies have been devoted to the adsorption or intercalation of synthetic dyes Recently, several studies have been devoted to the adsorption or intercalation of synthetic dyes based on the azo or anthraquinone chromophore into or onto layered double hydroxides [23–27]. based on the azo or anthraquinone chromophore into or onto layered double hydroxides [23–27]. This approach may lead to an enhanced resistance of organic dyes to solar irradiation, as well as an This approach may lead to an enhanced resistance of organic dyes to solar irradiation, as well as increased resistance to oxidative and thermal decomposition. However, there has still been little an increased resistance to oxidative and thermal decomposition. However, there has still been little research into the stabilization of natural dyes using aluminum-magnesium hydroxycarbonate (LH). researchIninto stabilization of natural dyes usingpigment aluminum-magnesium hydroxycarbonate (LH). thisthe work, we produced organic-inorganic by the precipitation of CA onto LH. The In this work, we produced pigment by the precipitation of CAincluding onto LH. The new new hybrid colorants were organic-inorganic studied using a range of experimental techniques, XRD, hybrid colorants were studied using a range of experimental techniques, including XRD, TOF-SIMS, X-ray TOF-SIMS, X-ray photoelectron spectroscopy, thermal analysis, SEM, and UV-Vis spectroscopy. The photoelectron spectroscopy, analysis, UV-Vis spectroscopy. chromostability chromostability of the newthermal LH pigments wasSEM, testedand in terms of their resistance The to thermal treatment of the and newtowards LH pigments was tested in terms of their resistance to thermal treatment and towards different different solvents. The hybrid pigment was added as a colorant to ethylene-norbornene solvents. The and hybrid was added a colorant to ethylene-norbornene copolymer thepigment flammability of the as resulting composite was investigatedcopolymer using the and conethe calorimeter test. flammability of the resulting composite was investigated using the cone calorimeter test.

2. Results 2. Results 2.1.2.1. X-Ray Photoelectron Spectroscopy X-Ray Photoelectron Spectroscopy(XPS) (XPS) X-ray photoelectron spectroscopy was used analyze chemical state surface X-ray photoelectron spectroscopy was used to to analyze thethe chemical state of of thethe surface of of thethe pure LH material, as of well of the with hybrid CA (Figures and 2,1).Table The formation of LHpure material, as well as theashybrid CAwith (Figures 1 and 2,1 Table The 1). formation of chemical chemical bonds CA and magnesium or aluminum ions the LH should structure bonds between CAbetween and magnesium or aluminum ions present in present the LH in structure beshould reflected be reflected as a shift of the core-level electron states of these elements. The X-ray photoelectron as a shift of the core-level electron states of these elements. The X-ray photoelectron spectra of the spectra of theregions bindingcorresponding energy regions corresponding to Mg 2p and Al 2p shown binding energy to Mg 2p and Al 2p are shown in are Figure 1. in Figure 1.

Figure 1. X-ray photoelectron spectra of Al (left(left panel) andand Mg Mg 2p transition (right panel) Figure 1. X-ray photoelectron spectra of 2p Al transition 2p transition panel) 2p transition (right acquired aluminum-magnesium hydroxycarbonate (LH) (LH) before (black line)line) andand after (red line) panel) for acquired for aluminum-magnesium hydroxycarbonate before (black after (red line) modification with carminic acid (20%). modification with carminic acid (20%).

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both pure LH and thehybrid hybridmaterial, material,the themaximum maximum of of the the XPS ForFor both thethe pure LH and the XPS Al Al 2p 2ppeak peakisislocated locatedatat a a binding energy of 74.4 eV. A binding energy close to 74 eV for the Al 2p line is attributed to binding energy of 74.4 eV. A binding energy close to 74 eV for the Al 2p line is attributed to aa series series of of aluminum compounds consisting of Al-O bonds [28]. The chemical shifts for these compounds aluminum compounds consisting of Al-O bonds [28]. The chemical shifts for these compounds were were minor. However, the Al 2p line positions for the Al2O3 type of bond are usually reported to be minor. However, the Al 2p line positions for the Al2 O3 type of bond are usually reported to be above above 74 eV. The asymmetry observed in this line on the low-binding energy side could indicate the 74 eV. The asymmetry observed in this line on the low-binding energy side could indicate the presence presence of an Al-O binding component with some contribution of hydroxyl groups. No notable of an Al-O binding component with some contribution of hydroxyl groups. No notable difference difference was observed between the XPS Al 2p spectra acquired for pure LH and those for the washybrid observed the XPS 2pbespectra acquired for pure and those for the hybrid with withbetween CA. Therefore, it Al may concluded that there wasLH no significant bonding between theCA. Therefore, it may concluded thatmolecules. there was no significant bonding between the aluminum ions in aluminum ions be in LH and the CA

LH and the CA molecules. LH/CA LH CA

XPS C 1s

294

292

290

288

286

Binding energy (eV)

284

282

LH/CA LH

XPS O 1s

280

540

538

536

534

532

530

528

526

Binding energy (eV)

Figure 2. X-ray photoelectron spectra of C 1s transition (left panel) and O 1s transition (right panel) Figure 2. X-ray photoelectron spectra of C 1s transition (left panel) and O 1s transition (right panel) acquired for aluminum-magnesium hydroxycarbonate (LH) before (black line) and after (red line) acquired for aluminum-magnesium hydroxycarbonate (LH) before (black line) and after (red line) modification with carminic acid (20%). XPS C 1s spectrum of pure carminic acid is depicted by modification with carminic acid (20%). XPS C 1s spectrum of pure carminic acid is depicted by dotted plot. dotted plot.

The XPS Mg 2p peak observed for pure LH is symmetrical and its maximum is located at a The XPS Mg 2p peak observed for pure LH is symmetrical and its maximum is located at a binding energy ofof50.9 line characteristic characteristicfor forMg-O Mg-O biding MgO binding energy 50.9eV. eV.The Theposition positionof of the the Mg Mg 2p 2p line biding in in MgO oxide hashas been reported energy range rangeofofbetween between50.4 50.4 and 50.8 [29,30]. oxide been reportedasasbeing beingin inthe thebinding binding energy eVeV and 50.8 eV eV [29,30]. TheThe respective positions of this line corresponding to MgCO and Mg(OH) bonds are given as being 2 respective positions of this line corresponding to MgCO33 and Mg(OH)2 bonds are given as being in the range of of 51.0 eVeV to to 51.4 hydroxycarbonate (LH) contains in the range 51.0 51.4eVeV[31]. [31].Aluminum-magnesium Aluminum-magnesium hydroxycarbonate (LH) contains magnesium ions coordinated with oxygen ions, as well as carbonate andand hydroxyl ions. Each of these magnesium ions coordinated with oxygen ions, as well as carbonate hydroxyl ions. Each of chemical environments can be presumed to have to thetoenvelope of the XPSXPS MgMg 2p 2p peak. these chemical environments can be presumed tocontributed have contributed the envelope of the peak. After modification with CA, the maximum of the Mg 2p line shifted to 51.4 eV. A similar shift of modification withreported CA, the maximum Mg 2p line shifted 51.4 eV. [32]. A similar the Mg After 2p line has also been in the caseofofthe a hybrid of LH withtoalizarin This shift shiftofwas the Mgto2phave line has also been in the case of a hybrid of LH alizarin and [32].magnesium This shift was assumed resulted fromreported the formation of bonds between dyewith molecules ions. assumed to have resulted from the formation of bonds between dye molecules and magnesium ions. A similar mechanism may occur in the present system. The prominent asymmetry on the high-energy maytooccur in the energy presentregion, system. The isprominent asymmetry the sideAofsimilar the Mgmechanism 2p line extends the binding which above the typical rangeonascribed high-energy side of the Mg 2p line extends to the binding energy region, which is above the typical to Mg 2p transition. It is possible that some differential charging of the LH particles takes place. range ascribed to Mg 2p transition. It is possible that some differential charging of the LH particles Figure 2 presents the XPS spectra of the C 1s and O 1s transitions for the considered samples. Two takes place. distinctive peaks are present in the XPS C 1s spectrum of aluminum-magnesium hydroxycarbonate Figure 2 presents the XPS spectra of the C 1s and O 1s transitions for the considered samples. (LH). The maximum of the first peak is located at 285.1 eV and is ascribed to “adventitious Two distinctive peaks are present in the XPS C 1s spectrum of aluminum-magnesium carbon” present on (LH). the surface of the sample. second peakat is very relatively hydroxycarbonate The maximum of the firstThe peak is located 285.1 eVbroad and isand ascribed to intense, with the center of gravity placed at 289.0 eV. This peak marks the presence of carbonate “adventitious carbon” present on the surface of the sample. The second peak is very broad and ions from theintense, aluminum-magnesium (LH) structure. the relatively with the center of hydroxycarbonate gravity placed at 289.0 eV. This peakAfter marks themodification presence of of aluminum-magnesium hydroxycarbonate with carminic acid, the XPS C 1s spectrum is changed carbonate ions from the aluminum-magnesium hydroxycarbonate (LH) structure. After the considerably. different local maxima can be distinguished. The most one located at modificationThree of aluminum-magnesium hydroxycarbonate with carminic acid, intense the XPS C 1s is spectrum 285.0 eV and considerably. is attributed Three to C-Cdifferent and C-H bonds. The origin of this transition corresponds is changed local maxima can be distinguished. The most intense onetoisthe locatedof at the 285.0 eV and isbond attributed to C-C and C-H acid bonds. The origin The of this transition presence aliphatic present in carminic molecules. second localcorresponds maximum is to the presence of the aliphatic bond present in carminic acid molecules. The second local maximum located at 286.5 eV and can be attributed to the presence of C-OH bonds. Several C-OH bonds are is located atcarminic 286.5 eV and be attributed to the this presence of C-OH bonds. Several C-OH are present in the acidcan molecule. Therefore, spectrum feature is considered as bonds an indicator of CA molecules. The XPS C 1s spectrum of pure carminic acid is shown for comparison in the left panel of Figure 2. The parts of the XPS C 1s spectra of the pure CA and LH/CA hybrid shown in the


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present in the carminic acid molecule. Therefore, this spectrum feature is considered as an indicator ofMolecules CA molecules. The XPS C 1s spectrum of pure carminic acid is shown for comparison in the left 2019, 24, 560 4 of 15 panel of Figure 2. The parts of the XPS C 1s spectra of the pure CA and LH/CA hybrid shown in the binding energy range between 282 eV and 287 eV are very similar. This observation can be taken as binding energy rangeofbetween 282 eV and 287surface eV are very This observation as proof of the presence carminic acid on the of thesimilar. LH/CA hybrid sample.can At be thetaken binding proof of the presence of carminic acid on the surface of the LH/CA hybrid sample. At the binding energy of 289 eV, a shoulder is observed in the XPS C 1s spectrum of the LH/CA hybrid. Presumably, energy of 289 eV, a shoulder is observed in thethe XPS C 1s spectrum theLH LH/CA hybrid. this is a superposition of the XPS signal from carbonate ions inofthe structure andPresumably, the carboxyl this is a superposition of the XPS signal from the carbonate ions in the LH structure and the carboxyl functional groups from carminic acid. functional from carminic Figure 2groups also shows the XPSacid. O 1s spectrum for a pure LH sample as well as for the LH/CA Figure 2 also shows the XPS O 1s spectrum a purewith LH sample as for theeV. LH/CA hybrid. The envelopes of these spectra are very for similar, maximaasatwell about 532.5 This hybrid. binding The envelopes of these spectra are very similar, with maxima at about 532.5 eV. This binding energy energy region is typical for the presence of Al-O and C-OH bonds. Some asymmetry in the O 1s region is typical for the presence of Al-O and C-OH bonds. Some asymmetry in the O 1s peaks in the peaks in the region of low binding energy can be ascribed to the presence of Mg-O bonds. region of low binding energy can be ascribed to the presence of Mg-O bonds. Table 1. Quantitative evaluation of the surface composition of LA and LH-based pigments calculated Table 1. Quantitative evaluation of the surface composition of LA and LH-based pigments calculated based on XPS analysis. based on XPS analysis. Carbon Oxygen Magnesium Aluminum Sample Carbon Oxygen Magnesium Aluminum Sample At% At% LH 6 34 8 52 CA 68 32 LH 6 34 8 52 CA LH/CA 6812 32 25 9 54 LH/CA 12 25 9 54

A substantial increase was observed in the concentration of carbon atoms on the surface of the LH/CAA sample, in increase comparison to the LH sample. The ratios between concentrations substantial was observed in the concentration of carbon atomsthe on the surface of theof LH/CA sample, in comparison to the LH sample. ratios between the concentrations of aluminum aluminum and magnesium atoms were virtuallyThe identical. This effect may be explained by the and magnesium were virtually Thisby effect be explained by the on screening of Al screening of Al andatoms Mg atoms from the identical. LH structure the may CA molecules adsorbed top of the LH and Mg atoms from the LH structure by the CA molecules adsorbed on top of the LH structure. structure. 2.2. SecondaryIon IonMass MassSpectrometry Spectrometry(TOF-SIMS) (TOF-SIMS) 2.2. Secondary Figure 3 shows the TOF-SIMS spectrum of negative ions for pure CA. The pseudomolecular Figure 3 shows − the TOF-SIMS spectrum of negative ions for pure CA. The pseudomolecular ion ion (C22 H20 O and the fragmentation ion C H9 O8 − formed the most intense peaks in this 13 -H) (C22H20O13-H)− and the fragmentation ion C16H9O8− 16 formed the most intense peaks in this spectrum. spectrum. However, none of these ions appeared in the TOF-SIMS spectrum of the LH modified with However, none of these ions appeared in the TOF-SIMS spectrum of the LH modified with CA. This CA. This may indicate that the interaction between the CA molecules and the host matrix was so may indicate that the interaction between the CA molecules and the host matrix was so strong that strong that they could not be desorbed from the sample during TOF-SIMS spectrum acquisition. they could not be desorbed from the sample during TOF-SIMS spectrum acquisition.

Figure 3. TOF-SIMS spectra of (C22 H20 O13 -H)− ion emitted from carminic acid. Figure 3. TOF-SIMS spectra of (C22H20O13-H)− ion emitted from carminic acid.

2.3. X-Ray Diffraction Analysis (XRD) 2.3. X-Ray Diffraction Analysis (XRD) Figure 4a–c shows XRD patterns for CA, LH, and LH/CA. As can be seen from the diffraction pattern in Figure 4a, CA is anpatterns amorphous substance. InLH/CA. contrast,As thecan XRD of the sample Figure 4a–c shows XRD for CA, LH, and bepattern seen from theLH diffraction (Figurein4b) shows sharp andsubstance. symmetrical peaks at the lowXRD 2θ values, which areLH ascribed pattern Figure 4a,characteristic CA is an amorphous In contrast, pattern of the sample to diffractions by planes (003) and (006). These correspond to basal spacing and higher orderto (Figure 4b) shows characteristic sharp and symmetrical peaks at low 2θ values, which are ascribed diffractions [33]. The interlayer distances of (d003 ) and (d006 ) are 0.751 nm and 0.377 nm, respectively.


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Intensity (counts)

Molecules 2019,by 24, 560 15 diffractions planes (003) and (006). These correspond to basal spacing and higher5 of order diffractions [33]. The interlayer distances of (d003) and (d006) are 0.751 nm and 0.377 nm, respectively. These arecharacteristic characteristic for crystalline hydrotalcite-like compounds, which exhibit These distances distances are for crystalline hydrotalcite-like compounds, which exhibit hexagonal hexagonal lattices. On the slopes of the sharp peaks, small broad peaks can be observed, which may lattices. On the slopes of the sharp peaks, small broad peaks can be observed, which may have been have been caused by unrecognized crystalline impurities. After the the addition of CA, the caused by unrecognized crystalline impurities. After the addition of CA, basal reflection (003)basal of reflection (003) of the LH/CA pigment (Figure 4c) shifted slightly to a lower 2θ angle, but this not the LH/CA pigment (Figure 4c) shifted slightly to a lower 2θ angle, but this did not significantly did affect significantly the the distance the distanceaffect between layers.between the layers.

8000

(003)

6000

(006)

(009) (110) (113)

4000

c

b

2000

a 0 10

15

20

25

30

35

40

45

50

55

60

65

70 2Theta (°)

Figure4. 4.Powder Powder diffraction diffraction patterns Figure patterns for forCA CA(a), (a),LH LH(b), (b),and andLH/CA LH/CA(20%) (20%)(c). (c).

2.4. Thermogravimetric Analysis (TGA) 2.4. Thermogravimetric Analysis (TGA) The TG-DTA curves for pure CA, LH host, and the hybrid pigment were analyzed to investigate The TG-DTA curves for pure CA, LH host, and the hybrid pigment were analyzed to their thermal behavior. The thermal decomposition of LH/CA (10%) and LH/CA (20%) (Figure 5a,b) investigate their behavior. in The thermal decomposition (10%) and LH/CA (20%) can be seen to thermal have proceeded four weight loss steps. of TheLH/CA first, in a range from room (Figure 5a,b) can be seen to have proceeded in four weight loss steps. The first, in a range from room temperature to 100 ◦ C, corresponds to the loss of physically adsorbed water. The second weight temperature to ◦100 corresponds the lossofofintercalated physically adsorbed water. The second weight loss loss (100–215 C) °C, is related to the to removal water molecules (dehydration) in the ◦ (100–215 °C) is related to the removal of intercalated water molecules (dehydration) in the interlayered galleries. The relatively sharp third transition at 215–315 C is due to removal of hydroxyl interlayered galleries. The from relatively transition at 215–315 °Cpeak is due to removal of groups (dehydroxylation) the LHsharp layersthird as water molecules. The small at approximately ◦ − ◦ C)at hydroxyl groups (dehydroxylation) from the LH layers as water molecules. The peak 334 C can be attributed to the partial loss of OH in the brucite-like layer. The final stepsmall (350–450 − approximately 334 °Cwith canthe be liberation attributedoftocarbonate the partial loss of OH in the brucite-like The final is mostly associated ions inside the LH galleries and thelayer. beginning of step (350–450 °C) is mostly associated with the liberation of carbonate ions inside the LH galleries hydroxy layer decomposition [34]. This step is often characterized by the complete decomposition and the beginning hydroxy layer decomposition This is often characterized the of metal hydroxideof layers and the formation of metal [34]. oxides. In step the case of CA, the first massby loss complete decomposition of metal hydroxide layers and the formation of metal oxides. In the case is caused by the loss of water, whereas the maximum mass loss rate of the carminic chromophoreof ◦ Closs CA, the first mass caused by theofloss water, whereas maximum mass loss(105 rate◦ C) of in the occurred at 200 [35].isThe T5% values the of LH-based pigmentsthe were found to be lower ◦ ◦ comparison to the CA occurred chromophore (107 C) and hostof(119 C) (Table 2). It is interesting to carminic chromophore at 200 °C [35]. Thepure T5% LH values the LH-based pigments were found ◦ C) and LH/CA (20%) (335 ◦ C) were that the 20% weight loss temperatures forchromophore LH/CA (10%)(107 (334 °C) tonote be lower (105 °C) in comparison to the CA and pure LH host (119 °C) (Table ◦ C), probably as a result of CA shifted to a to higher than in the loss case of pure LH (329for 2).slightly It is interesting notetemperature that the 20% weight temperatures LH/CA (10%) (334 °C) and incorporation. However, the decomposition of carminic acid is not observed thermograms LH/CA (20%) (335 °C) were slightly shifted to a higher temperature than in on thethe case of pure LH for (329 theprobably hybrid pigment. Thisofbehavior can be explained by the fact that the main of weight loss peaks °C), as a result CA incorporation. However, the decomposition carminic acid isfor not the CA dye were similar to those for dehydroxylation and carbonate decomposition in the LH host. observed on the thermograms for the hybrid pigment. This behavior can be explained by the fact These correspond withfor thethe observations madesimilar in our previous [36], in which the that theresults main also weight loss peaks CA dye were to those works for dehydroxylation and decomposition peaks of organic chromophores incorporated in an LH matrix were covered by peaks carbonate decomposition in the LH host. These results also correspond with the observations made inorganicworks carrier[36], decomposition. insignaling our previous in which the decomposition peaks of organic chromophores incorporated in an LH matrix were covered by peaks signaling inorganic carrier decomposition.

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Figure 5. TGA (a) and DTG (b) profiles of CA, LH, LH/CA (10%), and LH/CA (20%). Figure5.5.TGA TGA (a) (a) and and DTG (b) and LH/CA (20%). Figure (b) profiles profilesof ofCA, CA,LH, LH,LH/CA LH/CA(10%), (10%), and LH/CA (20%). Table 2. Thermogravimetric analysis of hybrid pigments. Table 2. 2. Thermogravimetric Thermogravimetric analysis Table analysisofofhybrid hybridpigments. pigments. Thermal Stability Sample Thermal ThermalStability Stability Sample 1 ◦ 1 ◦ C) 1 Sample 1 (°C) T5% ( TC) T110% T20% 1 (◦ C) 20% (°C) TT10% (°C) ( T 5% 1 (°C) 10% 1 (°C) T5% T20%1 (°C) CA 203 260 CA 107 107 260 CA 107 203203 260 LH 119 205 329 LH 119 329 LH 119 205205 329 LH/CA (10%) 205 205 334 LH/CA (10%) LH/CA 105 105 334 (10%) 105 205 334 206 206 335 LH/CA (20%) LH/CA (20%) 105 105 335 LH/CA (20%) 105 206 335 1 Degradation 1 Degradationtemperatures temperatures 1

of 5, 10, and and20% 20%ofofsample. sample Degradation temperaturesofof5,5,10, 10, and 20% of sample

2.5. Color Stability of LH/CA Pigments 2.5. Color Stability of LH/CA Pigments 2.5. Color Stability of LH/CA Pigments Figure 6 shows the UV-Vis spectra of LH, CA, and LH/CA20%. The absorption spectrum of CA Figure 6 shows the UV-Vis ofstabilization LH, CA, andofLH/CA20%. Thematrix, absorption spectrum showed a maximum at 568 nm. spectra After the CA on the LH the band shiftedoftoCA Figure 6 shows the UV-Vis spectra of LH, CA, and LH/CA (20%). Thematrix, absorption spectrum of CA showed a maximum at 568 nm. After the stabilization of CA on the LH the band 573 nm, most likely as a result of the dye-metal interaction. This spectral region is assignedshifted to the to showed maximum atas 568 After the stabilization of CA on the LHspectral matrix, the bandisshifted to 573 nm, 573 nm,a most a nm. result of the interaction. This region assigned absorption oflikely CA, suggesting that CA dye-metal did not decompose after interaction with the LH host. Astoathe most likely as a result of the dye-metal interaction. This spectral region is assigned to the absorption absorption CA, suggesting that CA did not decompose with thered LHtohost. As of a result, the of modified LH/CA20% pigment showed a noticeableafter colorinteraction change, from dark violet. CA, suggesting that CA did not decompose after interaction with the LH host. As a result, the modified result, the modified LH/CA20% pigment showed a noticeable color change, from dark red to violet. Carminic acid is known to have three possible pKa values (2.8, 5.4, and 8.1). At pH 8, CA is LH/CA (20%) showed a noticeable color change, from dark5.4, redappear to violet. Carminic is Carminic acidpigment is known to have three possible pKa values (2.8, and 8.1).[37]. At pHBecause 8, acid CA is completely deprotonated and violet tri-anionic molecules known to have three possible pKa values (2.8, 5.4, and 8.1). At pH 8, CA is completely deprotonated and completely deprotonated and violet appearthe complexation [37]. Because aluminum-magnesium hydroxycarbonate usedtri-anionic in this study molecules is basic in nature, violet tri-anionic molecules appear [37]. Because used in this most likely occurs between the carboxylic and hydroxy-keto inhydroxycarbonate the CA structures and Mg aluminum-magnesium hydroxycarbonate usedaluminum-magnesium in this studygroups is basic in nature, the complexation study basicoccurs ininnature, complexation most occurs between thein carboxylic and hydroxy-keto ionsis present thebetween LHthe structure. It was concluded that the formation of organic-inorganic colorants most likely the carboxylic andlikely hydroxy-keto groups the CA structures and Mg groups in the CA structures and Mg ions present in the LH structure. It was concluded that seems similar to LH thatstructure. for lakeIt pigments, whichthat aretheobtained byofthe precipitation ofcolorants acid the ions present in the was concluded formation organic-inorganic formation of organic-inorganic colorants seems similar thatobtained forarrangement lake pigments, which areinorganic obtained by chromophores withthat saltsfor of alkaline earth [15,32]. Theto possible CA on an seems similar to lake pigments, which are by theof precipitation of acid carrier is presented in Figure 7. the precipitation of acid chromophores with salts of alkaline earth [15,32]. The possible arrangement of chromophores with salts of alkaline earth [15,32]. The possible arrangement of CA on an inorganic CA on an inorganic carrier is presented in Figure 7. carrier is presented in Figure 7.

Figure 6. UV-Vis spectra of LH, carminic acid, and LH/CA 20%.

Figure Figure6.6.UV-Vis UV-Visspectra spectraofofLH, LH,carminic carminicacid, acid,and andLH/CA LH/CA(20%). 20%.


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Figure 7. Proposed arrangement of carminic acid molecules in LH pigments.

The thermal stability of the LH-based pigments was assessed by heat treating the powders in an Figure 7. Proposed arrangement of carminic acid molecules in LH pigments. oven at 200 °C and 250 °C for 30 min. The UV-Vis spectra of the CA and hybrid pigments before and Figure 7. Proposed arrangement ofpigments carminic acid molecules LH pigments. The thermal stability of the LH-based assessed byinheat treating thesimilar powdersspectra. after thermal treatment are shown in Figure 8a,b. In was general, all samples revealed ◦ C and 250 ◦ C for 30 min. The UV-Vis spectra of the CA and hybrid pigments in an oven at 200 Differences were observed during heat treatment, especially in the case of pure CA (Figure 8a). At before after thermal are shown in Figure 8a,b. In general, all samples revealed similarin an thermal stability of treatment the LH-based pigments assessed heat treating the 200The °C, theand shape and λmax of the CA spectra were was changed, andby the intensity of thepowders absorption was spectra. Differences were observed during heat treatment, especially in the case of pure CA (Figure 8a). and oven at 200 °C and 250 °C for 30 min. The UV-Vis spectra of the CA and hybrid pigments before lower, indicating the start of thermal degradation. These results are in line with TG data, as the main At 200 ◦ C, the shapeare andshown λmax ofin theFigure CA spectra were changed,all and the intensity of thesimilar absorption after thermal treatment 8a,b. In general, samples revealed spectra. at decomposition peak for pure chromophore was also detected at around 200 °C. Further was lower, indicating the start of thermal degradation. These results are in line with TG data, asheating the Differences wereaobserved during heat in treatment, especially in the case pure CA (Figure complete 8a). At 250 °C caused significant variation the absorbance values, due to of carbonization, main decomposition peak for pure chromophore was also detected at around 200 ◦ C. Further and heating 200 °C, the shape andorganic λmax of dye the CA spectra were changed, and the intensity the absorption was degradation the structure (Figure 9). In values, contrast, UV-Vis of spectrum of the hybrid at 250 ◦ C of caused a significant variation in the absorbance duethe to carbonization, and complete lower, indicating start of thermal degradation. These results in line with TG was data, as the main degradation ofno the organic dye structure (Figure 9). In contrast, the spectrum of theobserved hybrid pigments showsthe change at 200 °C, and decomposition of are theUV-Vis chromophore only ◦ decomposition peak for pure chromophore was also detected at around 200 °C. Further heating pigments shows no change at 200 C, and decomposition of the chromophore was observed only when when the sample was heated to 250 °C (Figure 8b). It can be concluded that the color stability ofat the ◦ C (Figure 250 caused aunder significant variation in the8b). absorbance values, due tothe carbonization, and the sample was heated to 250 be concluded color stability between of thecomplete LHthe Mg LH°Cpigments elevated temperatures wasIt acan consequence ofthat strong interactions under elevateddye temperatures was a consequence of strongthe interactions between theofMg degradation ofCA themolecules, organic (Figure In contrast, UV-Vis spectrum theions hybrid ions pigments and the asstructure was confirmed by9).XPS analysis. and the CA molecules, as was confirmed by XPS analysis. pigments shows no change at 200 °C, and decomposition of the chromophore was observed only when the sample was heated to 250 °C (Figure 8b). It can be concluded that the color stability of the LH pigments under elevated temperatures was a consequence of strong interactions between the Mg ions and the CA molecules, as was confirmed by XPS analysis.

Figure 8. UV-Vis spectra carminicacid acidand andhybrid hybrid pigment pigment exposed temperatures: Figure 8. UV-Vis spectra of of carminic exposedtotodifferent different temperatures: (a) (a) carminic acid and (b) LH/CA (20%). carminic acid and (b) LH/CA 20%.

Figure 8. UV-Vis spectra of carminic acid and hybrid pigment exposed to different temperatures: (a) carminic acid and (b) LH/CA 20%.


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Figure 9. Color changes of carminic acid and LH/CA 20%. Figure 9. Color changes of carminic acid and LH/CA (20%).

Figure 10 shows that the stabilization of CA on the LH host raised the solvent resistance of the Figure 9. Color changes of carminic acid and LH/CA 20%. obtained LH pigments considerably, while pure solvent color after 24 h Figure 10 shows that the stabilization of CACA onturned the LHthe host raisedan theintense solventred resistance of the ofobtained immersion. This significant decrease in the solubility of CA canintense attributed to strong LH10pigments considerably, while pure CAon turned thehost solvent an red color after 24 Figure shows that the stabilization of CA the LH raised thebesolvent resistance ofhthe interactions with the inorganic host, indicating successful transformation of the organic of immersion. This significant decreasewhile in thepure solubility of CAthe cansolvent be attributed to strong interactions obtained LH pigments considerably, CA turned an intense red color after 24 h with the inorganic host, indicating successful transformation of the organic chromophore. chromophore. of immersion. This significant decrease in the solubility of CA can be attributed to strong interactions with the inorganic host, indicating successful transformation of the organic chromophore.

Figure10. 10.Digital Digitalimages imagesof ofcarminic carminic acid acid and (20%) after Figure and LH/CA LH/CA (20%) after 24 24 hhof ofimmersion immersionininwater water(a)(a)and and acetone(b). (b). acetone

Figure 10.Electron Digital images of carminic 2.6. Scanning Microscopy (SEM) acid and LH/CA (20%) after 24 h of immersion in water (a) and 2.6. Scanning Electron Microscopy (SEM) acetone (b). We also investigated the microscopic morphology of the hybrid pigments after modification with We also investigated the microscopic morphology of the hybrid pigments after modification CA. The results are presented in Figure 11a–e. In contrast to the pure host, some roughness appeared 2.6. Scanning Microscopy (SEM) with CA. The Electron results are presented in Figure 11a–e. In contrast to the pure host, some roughness on the LH surface after stabilization of the CA (Figure 11c,d). This is most likely associated with the appeared the LH surface the aftermicroscopic stabilizationmorphology of the CA (Figure is mostafter likelymodification associated We on also investigated of the11c,d). hybridThis deposition of carminic acid on the outer area of the LH structure. Unlike inpigments our previous studies [36], with the deposition of carminic acid on the outer area of the LH structure. Unlike in our previous with CA. The results are clearly presented in Figure 11a–e. Inmodified contrast surface. to the pure the CA structures are not distinguishable on the Thishost, is duesome to theroughness rather studies [36], the CA structures are not clearly distinguishable on the modified surface. This is due to appeared the LH surface after stabilizationwhich of thewas CAalso (Figure 11c,d).by This is most likely associated amorphic on nature of the organic chromophore, confirmed XRD results. the rather amorphic nature of the organic chromophore, which was also confirmed by XRD results. with the deposition of carminic acid on the outer area of the LH structure. Unlike in our previous studies [36], the CA structures are not clearly distinguishable on the modified surface. This is due to the rather amorphic nature of the organic chromophore, which was also confirmed by XRD results.

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Figure 11. SEM morphology of: (a,b) carminic acid; (c) LH/CA (10%); (d) LH/CA (20%); (e) LH. Figure 11. SEM morphology of: (a, b) carminic acid; (c) LH/CA (10%); (d) LH/CA (20%); (e) LH.

2.7. Colorimetric and Structural Analysis of EN/Hybrid Pigment Compounds 2.7. Colorimetric and Structural Analysis of EN/Hybrid Pigment Compounds

The color variation of the EN materials was studied in terms of L*, a*, and b* parameters (Table 3). The colordifference variation of ENcan materials was studiedthe in terms of L*,EN a*,samples and b* parameters (Tableand A considerable in the color be seen between reference (EN, EN/LH) A considerable difference in color can be seen between the acid reference EN12). samples EN/LH) of EN3). copolymer containing hybrid pigments and pure carminic (Figure After(EN, the addition and EN copolymer containing hybrid pigments and pure carminic acid (Figure 12). After the hybrid colorants and CA dye, EN composites showed a significant decrease in the L* value, which is addition of hybrid colorants and CA dye, EN composites showed a significant decrease in the L* the parameter indicating lightness. In comparison to the EN copolymer colored with carminic acid, value, which is the parameter indicating lightness. In comparison to the EN copolymer colored with hybrid pigments provide an EN composite rather violet in shade. Therefore, the greatest changes for carminic acid, hybrid pigments provide an EN composite rather violet in shade. Therefore, the EN/hybrid pigment composites were observed for the a* parameter, corresponding to red-green colors greatest changes for EN/hybrid pigment composites were observed for the a* parameter, in the CIE Lab system. corresponding to red-green colors in the CIE Lab system. Table 3. Color coordinates ofofthe LH,carminic carminicacid, acid,and and hybrid pigments. Table 3. Color coordinates theEN ENcomposites composites containing containing LH, hybrid pigments. Sample

Sample

L*

EN EN 85.19 EN/LH EN/LH 77.33 EN/CA EN/CA 56.77 EN/LH/CA (10%) EN/LH/CA (10%) 42.78 EN/LH/CA (20%)

EN/LH/CA (20%)

37.09

22.67

−2.27

∆E 8.19 46.87 49.60 53.30

L*—Lightness, a*—negative values for green and positive values for red, b*—negative values for L*—Lightness, a*—negative values for green and positive values for red, b*—negative values for and positive and positive values for yellow, ΔE—total color change. 1

1

1 1 Colorimetric ColorimetricParameters Parameters L* a* a* b* ∆E b* 85.19 0.83 4.78 77.330.83 −1.11 5.99 4.78 8.19 −1.11 56.77 34.58 20.58 5.99 46.87 34.58 20.58 42.78 25.51 −2.42 49.60 25.51 −2.42 37.09 22.67 −2.27 53.30

values for yellow, ∆E—total color change.

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Figure 12.12. Digital photographs copolymercontaining containing LH host, carminic acid, Figure Digital photographsofof ofethylene-norbornene ethylene-norbornene copolymer copolymer LH host, carminic acid, Figure 12. Digital photographs ethylene-norbornene containing LH host, carminic acid, andand hybrid pigments. and hybrid pigments. hybrid pigments.

To To investigate the of the the hybrid hybridpigment pigmentparticles particles investigate thedispersion dispersionand andthe the morphology morphology of in in thethe ENEN matrix, scanning electron microscopy (SEM) was performed. The images obtained are presented matrix, scanning electron microscopy (SEM) was performed. The images obtained are presented in in Figure 13a,b. From thatthe theLH/CA LH/CA pigment retained Figure 13a,b. Fromthe theSEM SEMmicrophotographs, microphotographs, it it is is evident evident that pigment retained its its original plate-like shape, withwith sideside lengths as large as 400–700 nm after into the polymer original plate-like shape, lengths as large as 400–700 nm incorporation after incorporation into the matrix. Furthermore, the distribution and the compatibility of the hybrid matrix seem polymer matrix. Furthermore, the distribution and the compatibility of pigment/EN the hybrid pigment/EN matrix seem satisfactory, as no larger were agglomerates were observed. satisfactory, as no larger agglomerates observed.

Figure 13. SEM micrographs of EN composites containing LH/CA (20%) (a,b). Figure 13. 13. SEM SEM micrographs micrographs of of EN EN composites composites containing containing LH/CA LH/CA (20%) (20%) (a, (a, b). b). Figure

2.8. Flammability of EN/Hybrid Pigment Compounds

2.8. Flammability of EN/Hybrid Pigment Compounds

Polymer composites filled with the hybrid pigment were prepared. The flame retardancy of the Polymer filled with the hybrid pigment were prepared. The flame retardancy of thefire composites was composites evaluated using the cone calorimetry method. This technique approximates real composites was evaluated using the cone calorimetry method. This technique approximates real fire conditions and is considered to be an ideal tool for assessing the flammability of polymer composites. is considered to be an ideal tool for assessing the the flammability of rate polymer Theconditions available and flammability parameters of the cone calorimeter include heat release (HRR), composites. The available flammability parameters of the cone calorimeter include the heat total heat release (THR), effective heat of combustion (EHC), and mass loss rate (MLR). The release HRR and rate (HRR), total heat release (THR), effective heat of combustion (EHC), and mass loss rate (MLR). THR curves of the studied composites are given in Figure 14. The corresponding flammability data The HRR and THR curves of the studied composites are given in Figure 14. The corresponding is presented in Table 4. It can be seen that the neat ethylene-norbornene copolymer exhibited very flammability data is presented in Table 4. It can be seen that the neat ethylene-norbornene 2 . However, the incorporation of 5 phr poor fire resistance, with a heat peak ofwith 427.8 kW/m copolymer exhibited very poor release fire resistance, a heat release peak of 427.8 kW/m22. However, the LH/CA pigment into the ethylene-norbornene composite reduced its flammability, as evidenced by incorporation of 5 phr LH/CA pigment into the ethylene-norbornene composite reduced its theflammability, decrease in the HRR and THR parameters, by 55% and 44%, respectively, to the as evidenced by the decrease in the HRR and THR parameters,inbycomparison 55% and 44%, reference sample. Furthermore, the addition of hybrid pigment contributed to a slight reduction respectively, in comparison to the reference sample. Furthermore, the addition of hybrid pigment in thecontributed MLR parameter (from 6.9 to 6.5). It MLR is known, that (from the use inorganic hostthat (such to a slight reduction in the parameter 6.9oftoan 6.5). It is known, the as useLH) of is thought to promote the formation of an expanded carbonaceous char on the polymer, which prevents exposure to air. Therefore, the hybrid structure of the CA lakes may increase flame retardancy when


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an inorganic host (such as LH) is thought to promote the formation of an expanded carbonaceous 11 of 15 Molecules 2019,polymer, 24, x of 15 char on the which prevents exposure to air. Therefore, the hybrid structure of the CA11lakes may increase flame retardancy when applied in polymer composites. The increase in flame an inorganic (such as LH) iswith thought to LH promote the less formation of an expanded carbonaceous resistance for host the EN composite a pure host was pronounced in composite comparison with athe applied in polymer composites. The increase in flame resistance for the EN with pure char on the pigment, polymer, which preventsbyexposure to air. Therefore, the hybrid structureparameters. of the CA lakes EN/hybrid as evidenced higher values of considered flammability LH host was less pronounced in comparison with the EN/hybrid pigment, as evidenced by The higher may increase flame retardancy when applied in polymer composites. The increaseofintheflame higher efficiency of the hybrid parameters. colorants inThe terms of improving flame retardance values of considered flammability higher efficiency of the hybrid colorants in EN terms resistance for the EN composite with a pure LH host was less pronounced in comparison withand the copolymer may be attributed to the presence of an organic chromophore in the LH structure of improving flame retardance of the EN copolymer may be attributed to the presence of an organic EN/hybrid evidenced bywork higher values of considered flammability parameters. The was alreadypigment, observed as in our previous [36]. chromophore in the LH structure was already observed in our previous work [36]. of the EN higher efficiency of the hybridand colorants in terms of improving flame retardance copolymer may be attributed to the presence of an organic chromophore in the LH structure and was already observed in our previous work [36]. Molecules 2019, 24, 560

Figure 14.14. HRR (a)(a) and THR andEN/hybrid EN/hybrid pigment copolymer. Figure HRR and THR(b) (b)curves curvesof ofEN, EN, EN/LH, EN/LH, and pigment copolymer. Table 4. Flammability results recorded using a cone calorimeter.

Table 4. Flammability results recorded using a cone calorimeter. Figure 14. HRR (a) and THR (b) curves ofFlammability EN, EN/LH, and EN/hybrid pigment copolymer. Parameters Flammability Parameters Sample 3 4 4 Sample HRR 1 HRR 1 HRRMAX HRRMAX THR 2 EHC MLR MLR 2 (MJ/kg) 3 (MJ/kg) Table 4. Flammability results recorded using a cone calorimeter. EHC 2) THR (MJ/kg) 2) (kW/m (MJ/kg) (g/m2∙s) (kW/m2 ) (kW/m2) (kW/m (g/m2 ·s) EN 166.7 427.8 Flammability 68.1 Parameters33.6 6.9 EN 166.7 427.8 68.1 2 33.6 6.9 Sample EN/LH 89.9 1 347.4 61.1 32.9 3 6.6 4 MAX HRR HRR THR EHC MLR EN/LH 89.9 347.4 61.1 32.9 6.6 EN/hybrid pigment 75.3 2) 308.7 2) 38.5 23.5 6.52∙s) (kW/m (kW/m (MJ/kg) (MJ/kg) (g/m EN/hybrid 75.3rate; 2 Total 38.5 3 Effective heat 4 23.5 EN release 166.7 heat308.7 427.8 68.1 release; of combustion;33.6 Mass loss rate.6.9 6.5 pigment 1 Heat EN/LH 89.9 347.4 61.1 32.9 6.6 1 Heat release rate; 2 Total heat release; 3 Effective heat of combustion; 4 Mass loss rate. EN/hybrid 75.3 308.7 38.5 23.5 6.5 3. Materials andpigment Methods 1

Heat release rate; 2 Total heat release; 3 Effective heat of combustion; 4 Mass loss rate.

3.1. Raw Materials 3. Materials and Methods

3. Materials and Methods The LH carrier with an Al/Mg weight ratio of 70/30 was provided by Sasol GmbH (Hamburg, 3.1.Germany). Raw Materials The chromophore—carminic acid (CA) (Figure 15), as well as the organic solvents 3.1. Raw Materials toluene, cyclohexane, ethanol, acetone, all 70/30 of analytical grade,bywere from The LH carrier with an Al/Mgand weight ratio of was provided Sasolpurchased GmbH (Hamburg, The LH carrier with an Al/Mg weight of 70/30 was provided by Sasol GmbH (Hamburg, Sigma-Aldrich (Schnelldorf, Germany). Theratio ethylene-norbronene copolymer (EN) (40 wt% bound Germany). The chromophore—carminic acid (CA) (Figure 15), 15), as well as the organic solvents toluene, Germany). The chromophore—carminic acidAdvanced (CA) (Figure as well as the organic solvents norbornene content) was supplied by TOPAS Polymers (Frankfurt-Höchst, Germany). cyclohexane, ethanol, and acetone, all of analytical grade, were purchased from Sigma-Aldrich toluene, cyclohexane, ethanol, and acetone, all of analytical grade, were purchased from (Schnelldorf, Germany). The Germany). ethylene-norbronene copolymer (EN) (40 wt(EN) % bound norbornene Sigma-Aldrich (Schnelldorf, The ethylene-norbronene copolymer (40 wt% bound content) was supplied by TOPAS Advanced Polymers (Frankfurt-Höchst, Germany). norbornene content) was supplied by TOPAS Advanced Polymers (Frankfurt-Höchst, Germany).

Figure 15. Chemical formula of carminic acid.

3.2. Hybrid Pigment Preparation Figure15. 15.Chemical Chemical formula formula of Figure of carminic carminicacid. acid.

3.2.3.2. Hybrid Pigment Preparation Hybrid Pigment Preparation Carminic acid hybrid pigments with two CA concentrations (10% and 20%) were synthesized in aquatic medium. The LH was used as a hybrid pigment template. The LH/CA pigment (for 10% CA)

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was prepared as follows: 1 g of CA was dissolved in 200 mL of pure water with the addition of ethanol (50 mL). This solution was subjected to ultrasonication for 30 min. Next, 9 g of LA was added to the mixture and then the reaction system was stirred continuously at an elevated temperature (80 ◦ C) for 3 h. The reaction product was filtered under a vacuum and then washed several times with deionized water until a colorless solution was observed. Finally, the hybrid pigment was dried in an oven at 70 ◦ C for 24 h under a static air atmosphere (Binder, Tuttlingen, Germany). 3.3. Characterization of Powders X-ray photoelectron spectroscopy (XPS) analysis was performed with the use of Mg Ka (hν = 1253.6 eV) radiation. A Prevac (Rogów, Poland) electron spectrometer for chemical analysis equipped with a Scienta (Uppsala, Sweden) SES 2002 electron energy analyzer was used, operating at constant transmission energy (Ep = 50 eV). The analysis chamber was evacuated to a pressure below 1 × 10−9 mbar. A powdered sample of the material was placed on a stainless steel sample holder. X-ray diffraction analysis (XRD) was performed using an Analytical Pert Pro MPD diffractometer (Malvern Panalytical Ltd., Royston, UK). The XRD patterns were collected in the Bragg-Brentano reflecting geometry with (Cu Kα) radiation, in the range of 2θ = 2–70◦ . The thermal stability of the prepared hybrid pigments was evaluated using thermogravimetric analysis (TGA). Thermal analysis was conducted on a Thermogravimetric Analyzer TGA (TA Instruments, Greifensee, Switzerand) in a temperature range from 25 ◦ C to 600 ◦ C, with a heating rate of 10 ◦ C/min. The measurements were processed in the presence of argon. The solvent resistance of the LH/CA pigments was determined based on the PN-C-04406/1998 standard. The hybrid pigment powders were immersed in 20 mL of water and acetone. Diffuse reflectance UV-Vis spectroscopy was performed using an Evolution 201/220 UV-Visible Spectrophotometer (Thermo Scientific, Waltham, MA, USA), with a spectral window from 1100 to 200 nm. The morphology of the pigments was investigated by scanning electron microscopy (SEM). The SEM micrographs were obtained using a LEO 1530 Gemini scanning electron microscope (Zeiss/LEO, Oberkochen, Germany). TOF-SIMS mass spectra were obtained using a TOF-SIMS IV secondary ion mass spectrometer (ION-TOF GmbH, Muenster, Germany). This apparatus is equipped with a high mass resolution time of flight analyzer and Bi3 + primary ion gun. Secondary ion mass spectra were recorded across an area of approximately 100 µm × 100 µm on the sample surface. The analyzed area was irradiated with pulses of 25 keV Bi3 + ions with a 10 kHz repetition rate and an average ion current of 0.4 pA. The measurement time was 30 s. Secondary ions emitted from the bombarded surface were mass separated and counted in a time of flight (TOF) analyzer. 3.4. Preparation and Characterization of Polymer Composites Ethylene-norbornene copolymer (EN) was used as a polymer matrix. The EN/hybrid pigment composites were prepared using a Brabender laboratory-scale measuring mixer N50 (Duisburg, Germany) at 110 ◦ C. Each EN compound was processed with a rotor speed of 50 rpm for 10 min. After mixing, the polymer blends were pressed between two steel plates at 110 ◦ C and under 15 MPa pressure for 10 min to obtain the final samples. The formulation of the prepared EN composites was as follows (phr—parts per hundred parts of rubber): 100 phr of ethylene-norbornene copolymer and 5 phr of hybrid pigment. The flammability of the EN composites filled with LH/CA pigment was examined using a cone calorimeter from Fire Testing Technology Ltd. (East Grinstead, UK) according to the PN-ISO 5600 standard. Squared specimens (100 mm × 100 mm × 2 mm) were irradiated horizontally with a heat flux of 35 kW/m2 . The characterization of color performance of polymer composites was fixed in the spectrophotometric measurements using the CM-3600d spectrophotometer from Konica Minolta

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Sensing, Inc. (Osaka, Japan). The spectral range of this experiment was 360–740 nm. Color characteristics of the vulcanizates were defined by the colorimetric coordinates: brightness (L*), red-green component (a*), blue-yellow component (b*), and total change of color (∆E). 4. Conclusions This paper has provided a detailed description of the production and properties of organic-inorganic pigments based on carminic acid (CA). Stabilization of CA on aluminum-magnesium hydroxycarbonate (LH) leads to pigments with an excellent resistance to acetone and water. This change may be explained by the formation of complexes between the dye molecules and magnesium ions. The formation of such complexes was confirmed by XPS studies, which showed that the position of the Mg 2p line maximum for the LH/CA composite shifted to a higher binding energy compared to that of pure LH. In respect to pure CA, the absorption band of the hybrid pigments in the visible region shifted towards higher wavelengths, producing a violet color and confirming metal-dye interaction. Moreover, the hybrid pigment obtained had an enhanced color stability under thermal treatment, as shown by UV-Vis measurements. The LH/CA pigment offers a promising solution for producing colored polymer composites with improved flame retardancy. Author Contributions: A.M. and B.S. designed the concept, processed the modification and characterization of samples, and wrote the manuscript; J.R., W.M, P.R, and D.M performed the experiments; M.Z. supervised the work; all authors discussed, edited, and reviewed the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

References 1. 2. 3. 4.

5.

6. 7. 8. 9.

10.

11.

Fleischmann, C.; Lievenbrück, M.; Ritter, H. Polymers and dyes: Developments and applications. Polymers 2015, 7, 717–746. [CrossRef] Castañeda-Ovando, A.; Pacheco-Hernández, M.D.L.; Páez-Hernández, M.E.; Rodríguez, J.A.; Galán-Vidal, C.A. Chemical studies of anthocyanins: A review. Food Chem. 2009, 113, 859–871. [CrossRef] Mahmud-Ali, A.; Fitz-Binder, C.; Bechtold, T. Aluminium based dye lakes for plant extracts for textile coloration. Dyes Pigm. 2012, 94, 533–540. [CrossRef] Deveoglu, O.; Cakmakc, E.; Taskopru, T.; Torgan, E.; Karadag, R. Identyfication by RP-HPLC-DAD, FTIR, TGA and FESEM-EDAX of natural pigments prepared from Datissca cannabina L. Dyes Pigm. 2012, 94, 437–442. [CrossRef] Kohno, Y.; Totsuka, K.; Ikoma, S.; Yoda, K.; Shibata, M.; Matsushima, R.; Tomita, Y.; Maeda, Y.; Kobayashi, K. Photostability enhancement of anionic natural dye by intercalation into hydrotalcite. J. Colloid Interface Sci. 2009, 337, 117–121. [CrossRef] [PubMed] Bourhis, K.; Blanc, S.; Mathe, C.; Dupin, J.-C.; Vieillescazes, C. Spectroscopic and chromatographic analysis of yellow flavonoidic lakes: Quercetin chromophore. Appl. Clay. Sci. 2011, 53, 598–607. [CrossRef] Itoh, T.; Yano, K.; Inada, Y.; Fukushima, Y. Photostabilized chlorophyll a in mesoporous silica: Adsorption properties and photoreduction activity of chlorophyll a. J. Am. Chem. Soc. 2002, 124, 13437–13441. [CrossRef] Lima, E.; Bosch, P.; Loera, S.; Ibarra, I.A.; Laguna, H.; Lara, V. Non-toxic hybrid pigments: Sequestering betanidin chromophores on inorganic matrices. Appl. Clay Sci. 2009, 42, 478–482. [CrossRef] Kohno, Y.; Kinoshita, R.; Ikoma, S.; Yoda, K.; Shibata, M.; Matsushima, R.; Tomita, Y.; Maeda, Y.; Kobayashi, K. Stabilization of natural anthocyanin by intercalation into montmorillonite. Appl. Clay. Sci. 2009, 42, 519–523. [CrossRef] Kohno, Y.; Senga, M.; Shibata, M.; Yoda, K.; Matsushima, R.; Tomita, Y.; Maeda, Y.; Kobayashi, K. Stabilization of flavylium dye by incorporation into Fe-containing mesoporous silicate. Microporous Mesoporous Mater. 2011, 141, 77–80. [CrossRef] Taguchi, T.; Kohno, Y.; Shibata, M.; Tomita, Y.; Fukuhara, C.; Maeda, Y. An easy and effective method for the intercalation of hydrophobic natural dye into organo-montomorillonite for improved photostability. J. Phys. Chem. Solids 2018, 116, 168–173. [CrossRef]

Molecules 2019, 24, 560

12.

13.

14.

15. 16. 17. 18.

19. 20. 21.

22. 23.

24. 25. 26.

27. 28. 29. 30. 31. 32.

33.

14 of 15

Kohno, Y.; Shibata, Y.; Oyaizu, N.; Yoda, K.; Shibata, M.; Matsushima, R. Stabilization of flavylium dye by incorporation into the pore of protonated zeolites. Microporous Mesoporous Mater. 2008, 114, 373–379. [CrossRef] Kohno, Y.; Kato, Y.; Shibata, M.; Fukuhara, C.; Maeda, Y.; Tomita, Y.; Kobayashi, K. Fixation and stability enhancement of beta-carotene by organo-modified mesoporous silica. Microporous Mesoporous Mater. 2016, 220, 1–6. [CrossRef] Berlier, G.; Gastaidi, L.; Uqazio, E.; Mietto, I.; Iliade, P.; Sapino, S. Stabilization of quercetin flavonoid in MCM-41 mesoporous silica: Positive effect of surface functionalization. J. Colloid. Interface Sci. 2013, 393, 109–118. [CrossRef] [PubMed] Christie, R.M.; Mackay, J.L. Metal salt azo pigments. Color. Technol. 2008, 124, 133–144. [CrossRef] Kennedy, A.R.; Stewart, H.; Eremin, K.; Stenger, J. Lithol Red: A systematic structural study on salts of a sulfonated azo pigment. Chem. Eur. J. 2012, 18, 3064–3069. [CrossRef] [PubMed] Borges, M.E.; Tejera, R.L.; Diaz, L.; Esparza, P.; Ibáñez, E. Natural dyes extraction from cochineal (Dactylopius coccus). New extraction methods. Food Chem. 2012, 132, 1855–1860. [CrossRef] Pozzi, F.; van den Berg, K.J.; Fielder, I.; Casadio, F.A. A systematic analysis of red lake pigments in French impressionist and post-impressionist paintings by surface-enhanced Raman spectroscopy (SERS). J. Raman Spectrosc. 2014, 45, 1119–1126. [CrossRef] Perez, E.; Ibarra, I.A.; Guzman, A.; Lima, E. Hybrid pigments resulting from several guest dyes onto γ-alumina host: A spectroscopic analysis. Spectrochim. Acta A 2017, 172, 174–181. [CrossRef] Fournier, F.; Viguerie, L.; Balme, S.; Janot, J.M.; Walter, P.; Jaber, M. Physico-chemical characterization of lake pigments based on montmorillonite and carminic acid. Appl. Clay Sci. 2016, 130, 12–17. [CrossRef] Guillermin, D.; Debroise, T.; Trigueiro, P.; de Viguerie, L.; Rigaud, B.; Morlet-Savary, F.; Balme, S.; Janot, J.M.; Tielens, F.; Michot, L.; et al. New pigments based on carminic acid and smectites: A molecular investigation. Dyes Pigm. 2019, 160, 971–982. [CrossRef] Velho, S.R.K.; Brum, L.F.W.; Petter, C.O.; dos Santos, J.H.Z.; Simuni´c, S.; Kappa, W.H. Development of structured natural dyes for use into plastics. Dyes Pigm. 2017, 136, 248–254. [CrossRef] Tang, P.; Xu, X.; Lin, Y.; Li, D. Enhancement of the thermo- and photo-stability of an anionic dye by intercalation in a zinc-aluminum layered double hydroxide host. End. Eng. Chem. Res. 2008, 47, 2478–2483. [CrossRef] Chakraborty, C.; Dana, K.; Malik, S. Intercalation of perylenediimde dye into LDH clays: Enhancement of photostability. J. Phys. Chem. C. 2010, 115, 1996–2004. [CrossRef] Sun, Z.; Jin, L.; Shi, W.; Wei, M.; Duan, X. Preparation of an anion dye intercalated into layered double hydroxides and its controllable luminescence properties. Chem. Eng. J. 2010, 161, 293–300. [CrossRef] Bauer, J.; Behrens, P.; Speckbacher, M.; Langhals, H. Composites of perylene chromophores and layered double hydroxides: Direct synthesis, characterization, and photo- and chemical stability. Adv. Funct. Mater. 2003, 13, 241–248. [CrossRef] Maruyama, S.A.; Tavares, S.R.; Leitao, A.A.; Wypych, F. Intercalation of indigo carmine anions into zinc hydroxide salt: A novel alternative blue pigment. Dyes Pigm. 2016, 128, 158–164. [CrossRef] Kameshima, Y.; Yasumori, A.; Okada, K. XPS and X-ray AES (XAES) study of various aluminate compounds. Hyomen Kagaku 2000, 21, 481–487. [CrossRef] Fournier, V.; Marcus, P.; Olefjord, I. Oxidation of magnesium. Surf. Interface Anal. 2002, 34, 494–497. [CrossRef] Fotea, C.; Callaway, J.; Alexander, M.R. Characterisation of the surface chemistry of magnesium exposed to the ambient atmosphere. Surf. Interface Anal. 2006, 38, 1363–1371. [CrossRef] Ardizzone, S.; Bianchi, C.L.; Fadoni, M.; Vercelli, B. Magnesium salts and oxide: An XPS overview. Appl. Surf. Sci. 1997, 119, 253–259. [CrossRef] Marzec, A.; Szadkowski, B.; Rogowski, J.; Maniukiewicz, W.; Moszynski, ´ D.; Kozanecki, M.; Zaborski, M. Characterization and properties of new color-tunable hybrid pigments based on layered double hydroxides (LDH) and 1,2-dihydroxyanthraquinone dye. J. Ind. Eng. Chem. 2018, 70, 427–438. [CrossRef] Cavani, F.; Trifirò, F.; Vaccari, A. Hydrotalcite-type anionic clays: Preparation, properties and applications. Catal. Today 1991, 11, 173–301. [CrossRef]

Molecules 2019, 24, 560

34.

35.

36.

37.

15 of 15

Kanezaki, E. Effect of atomic ratio Mg/Al in layers of Mg and Al layered double hydroxide on thermal stability of hydrotalcite-like layered structure by means of in situ high temperature powder X-Ray diffraction. Mater. Res. Bull. 1998, 33, 773–778. [CrossRef] Norman, N.; Bartczak, P.; Zdarta, J.; Tylus, W.; Szatkowski, T.; Stelling, A.L.; Ehrlich, H.; Jesionowski, T. Adsorption of C.I. Natural Red 4 onto Spongin Skeleton of Marine Demosponge. Materials 2015, 8, 96–116. [CrossRef] Szadkowski, B.; Marzec, A.; Rybinski, ´ P.; Maniukiewicz, W.; Zaborski, M. Aluminum-magnesium hydroxycarbonate/azo dye hybrids as novel multifunctional colorants for elastomer composites. Polymers 2019, 11, 43. [CrossRef] Trigueiro, P.; Pereira, F.A.R.; Guillermin, D.; Rigaud, B.; Balme, S.; Janot, J.M.; Santos, I.M.G.; Fonseca, M.G.; Walter, P.; Jaber, M. When anthraquinone dyes meet pillared montmorillonite: Stability or fading upon exposure to light? Dyes Pigm. 2018, 159, 384–394. [CrossRef]

Sample Availability: Samples of the compounds are available from the authors. © 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).