materials

Article Coir Fibers Treated with Henna as a Potential Reinforcing Filler in the Synthesis of Polyurethane Composites

Sylwia Członka 1,*, Anna Str ˛akowska 1 and Agne˙ Kairyte˙ 2

1 Faculty of Chemistry, Institute of Polymer and Technology, Lodz University of Technology, −Stefanowskiego 12/16, 90-924 Lodz, Poland; [email protected] 2 Laboratory of Thermal Insulating Materials and Acoustics, Faculty of Civil Engineering, Institute of Building Materials, Vilnius Gediminas Technical University, Linkmenu St. 28, LT-08217 Vilnius, Lithuania; [email protected] * Correspondence: [email protected]

Abstract: In this study, coir fibers were successfully modified with henna (derived from the plant) using a high-energy ball-milling process. In the next step, such developed filler was used as a reinforcing filler in the production of rigid polyurethane (PUR) foams. The impact of 1, 2, and 5 wt % of coir-fiber filler on structural and physico-mechanical properties was evaluated. Among all modified series of PUR composites, the greatest improvement in physico-mechanical performances was observed for PUR composites reinforced with 1 wt % of the coir-fiber filler. For example, on the addition of 1 wt % of coir-fiber filler, the compression strength was improved by 23%, while the flexural strength increased by 9%. Similar dependence was observed in the case of dynamic-mechanical properties—on the addition of 1 wt % of the filler, the value of glass transition temperature increased from 149 ◦C to 178 ◦C, while the value of storage modulus increased by  ~80%. It was found that PUR composites reinforced with coir-fiber filler were characterized by better  mechanical performances after the UV-aging. Citation: Członka, S.; Str ˛akowska, A.; Kairyte,˙ A. Coir Fibers Treated Keywords: coir fibers; henna; polyurethane composites; UV-aging; mechanical performances with Henna as a Potential Reinforcing Filler in the Synthesis of Polyurethane Composites. Materials 2021, 14, 1128. https://doi.org/10.3390/ma14051128 1. Introduction Polyurethanes (PUR) represent a wide class of polymeric materials [1,2]. In recent Academic Editor: Nadezda Stevulova years, the market for polymeric materials has shown strong, sustained growth and provided an opportunity for the integration of new bio-based feedstock for improved sustainabil- Received: 6 February 2021 ity [3–9]. Agricultural waste has the potential to be utilized as renewable, bio-based fillers Accepted: 24 February 2021 in the synthesis of PUR composites [10]. The use of agricultural waste as functionalized Published: 27 February 2021 filler may not only improve the mechanical, physical, and combustion characteristics of PUR materials, but may also increase their biodegradability. Due to these positive and Publisher’s Note: MDPI stays neutral beneficial effects, it can be stated that the use of agricultural waste in the production of with regard to jurisdictional claims in published maps and institutional affil- PUR materials will promote a new application path in converting agricultural waste into iations. useful resources for creating a new class of green materials [11–13]. PUR materials derived from agricultural waste may offer solutions based on the main challenges of our times— the use of renewable raw materials, the economy and the preservation of resources, and minimization of the output of waste [14]. This approach creates an ideal scenario for the emergence of innovative opportunities under a rigid quality-control procedure that goes Copyright: © 2021 by the authors. from the raw material to advanced polyurethane materials for construction and structural Licensee MDPI, Basel, Switzerland. application. This article is an open access article distributed under the terms and In recent years, PUR composites reinforced with different natural fillers have been conditions of the Creative Commons evaluated [15,16]. For example, PUR composite foams reinforced with flax and jute fibers Attribution (CC BY) license (https:// were developed by Bledzki et al. [17]. The addition of flax fibers increased the mechanical creativecommons.org/licenses/by/ performances of PUR composites. PUR composites reinforced with cellulose filler derived 4.0/). from eucalyptus pulp were synthesized by Silva et al. [18]. According to the scanning

Materials 2021, 14, 1128. https://doi.org/10.3390/ma14051128 https://www.mdpi.com/journal/materials Materials 2021, 14, 1128 2 of 16

electron microscopy results, the incorporation of the filler up to 16 wt % resulted in a reduction in the size of composite cells and a deterioration of thermal conductivity. In another study, PUR composites were reinforced by selected amounts (5, 10, 15 wt %) of fly ash and wood ash, however, some deteriorations in mechanical performances were observed [19]. An opposite effect was observed by Olszewski et al. [20] in the case of PUR materials reinforced by the addition of glass and sisal fibers. In both cases, an improvement in the mechanical characteristics of PUR materials was observed. Cellulose derived from algal residue was selected as a reinforcing filler for PUR composites by Jonjaroen et al. [21]. According to the presented results, such developed PUR composites were characterized by greater apparent density, higher stiffness, and reduced loss modulus. Interesting results were reported by Li et al. in the case of PUR composites reinforced with fibers and alkali-treated bamboo fibers. The results showed that the alkali treatment of bamboo fibers resulted in better adhesion and interfacial interlocking between the fibers and PUR matrix, thus improving the mechanical performances of PUR composites [22]. Among different organic fillers, one of the most promising is coir fiber, which is extracted from shells. Coir fibers possess many advantages, such as versatility and biodegradability [23]. Coir fibers constitute a byproduct during the processing of coconut, and the extraction of coir fibers is very easy and inexpensive. The obtained coir fibers have a high content of lignin (~50%) and low content of cellulose (~35%) [23]. Due to this, the coir fibers exhibit good mechanical properties, low density, and great thermal conductivity, making them an ideal candidate for application in different composites, such as concrete [24], [25], unsaturated polyester [26], or epoxy compos- ites [27]. According to the literature, 40 million metric tons of produces about 2 million metric tons of coir fibers; however, just a small fraction of coir-fiber waste is further reused as a reinforcing filler in composites [28]. Therefore, it seems logical and fully justified to use coir fibers in polymeric composites such as polyurethane foams, which are widely used in the building and construction sectors. However, the main concern connected with the application of cellulosic fillers is their low thermal stability [29]. Thus, the surface modification of the cellulosic filler seems to be necessary. Previous studies have reported different methods of filler modification, which include silane treatment [30], acetylation [31], or physical impregnation with selected flame-retardant compounds [32] or polysilsesquioxanes [33]. In our study, we propose a new kind of polyurethane (PUR) composites reinforced with coir fibers treated with henna, which is extracted from the Lawsonia inermis plant, commonly known as the henna tree. The main component of henna is lawsone (a hy- droxynaphthoquinone), which possesses high antioxidative activity [34]. Besides this, henna contains various flavonoids, phenolic glycosides, quinoids, coumarins, xanthones, and [35,36]. It is expected that due to the outstanding properties of coir fibers treated with henna, such reinforced PUR composites will be characterized by improved selected physico-mechanical properties. Therefore the impact of the selected content of coir fibers treated with henna on the mechanical, thermal, and antioxidative properties of PUR composites will be evaluated in the following study.

2. Materials and Methods 2.1. Materials The materials used in the synthesis of PUR composites were as follows: Polymeric diphenylmethane diisocyanate (Purocyn B) as isocyanate (Purinova Company, Bydgoszcz, Poland); polyether polyol (Stapanpol PS-2352, Stepan Company, Northfield, IL, USA); potassium octoate (Kosmos 75) and potassium acetate (Kosmos 33) as catalysts (Evonik Industry, Essen, Germany); silicone-based surfactant (Tegostab B8513, Evonik Industry, Es- sen, Germany); pentane and cyclopentane as blowing agents (Sigma-Aldrich Corporation, Saint Louis, MO, USA); sodium hydroxide (Sigma-Aldrich Corporation, Saint Louis, MO, USA); nanopowder henna (Sigma-Aldrich Corporation, Saint Louis, MO, USA); and coir fibers (local company, Lodz, Poland). Materials 2021, 14, 1128 3 of 16

2.2. Instruments and Methods The dynamic viscosity of PUR systems was measured using a Viscometer DVII+ (Brookfield, Berlin, Germany) according to ISO 2555 [37]. The cellular structure of PUR composites was determined using scanning electron microscopy (SEM) (JEOL JSM 5500 LV, JEOL Ltd., Peabody, MA, USA). The content of closed cells was determined based on SEM images according to ISO 4590 [38]. The apparent density of PUR composites was calculated following ISO 845. The compressive test and flexural test were performed ac- cording to ISO 844 [39] and ISO 178 [40] using a Zwick Z100 testing machine (Zwick/Roell Group, Ulm, Germany). The thermogravimetric analysis (TGA) was performed using an STA 449 F1 Jupiter Analyzer (Netzsch Group, Selb, Germany) in the function of the temperature (0–600 ◦C). The UV-aging of PUR composites was performed using a UV 2000 (Atlas, Berwyn, PA, USA)—the samples were UV-irradiated for 7 days (radiation inten- sity = 0.7 W m−2, temperature = 60 ◦C). The color characteristic of PUR composites was determined using a CM-3600d spectrophotometer (Konica Minolta Sensing, Inc., Osaka, Japan).

2.3. Synthesis of PUR Composites In the first step, coir fibers were alkali-treated following using NaOH solution (10% v/v). Subsequently, the alkali-treated coir fibers were functionalized with henna powder using a high-energy ball-milling process (60 min, 3000 RPM). The selected content of the developed coir-fiber filler (1, 2, and 5 wt %) was mixed with a polyol, water, surfactant, and catalysts using a mechanical stirrer (60 s, 2000 rpm). Subsequently, the polymeric diphenyl methane diisocyanate (pMDI) was added to the mixture and the PUR system was intensively mixed for another 30 s. The PUR composites were allowed to grow freely and left at room temperature for 48 h. The formulas of PUR composites are presented in Table1. The schematic steps of PUR composites preparation are presented in Figure1.

Table 1. Formulas of PUR composites developed in the study.

PUR_NEAT PUR_C_1 PUR_C_2 PUR_C_5 Component Parts by Weight (wt %) 1 STEPANPOL PS-2352 100 100 100 100 PUROCYN B 160 160 160 160 Kosmos 75 6 6 6 6 Kosmos 33 0.8 0.8 0.8 0.8 Tegostab B8513 2.5 2.5 2.5 2.5 Water 0.5 0.5 0.5 0.5 Pentane/cyclopentane 11 11 11 11

Materials 2021, 14, 1128 Coir-fiber filler (coir fibers treated with henna) 0 1 2 5 4 of 16

1 Parts by weight (in relation to the weight of polyol (STEPANPOL PS-2352)).

FigureFigure 1. 1.Schematic Schematic procedure procedure of of PUR PUR composite composite preparation. preparation.

3. Results and Discussion Scanning electron microscopy (SEM) of coir fibers before surface treatment and after the treatment with henna are presented in Figure 2. After the chemical treatment, the external surface of the coir-fiber filler seemed to be more rough than before the treatment. This may be connected with the alkali treatment, which removed the impurities and the that smooth the filler surface. Moreover, it is believed that such a rough surface was con- nected with the presence of henna molecules, which created a coating layer on the filler surface. It is believed that such developed filler will be more compatible with PUR struc- ture, providing better interfacial interlocking during PUR synthesis and avoiding the damages of PUR structure during the foaming process.

(a) (b) (c)

(d) (e) (f)

Figure 2. Optical and scanning electron microscope images of (a–c) pristine coir fibers and (d–f) coir-fiber filler treated with henna.

The size of filler particles was determined using the dynamic light scattering (DLS) method. According to the results (Figure 3a), the coir-fiber filler treated with henna had 60.2% of particles with an average size between 1.0 and 1.5 μm. Apart from this, 8.5% of particles were in the range of 0.7–0.8 μm, and only 5.5% of particles were bigger than 1.5 μm. It can be seen that the application of a high-energy ball-milling process resulted in the formation of coir-fiber filler with a more homogenous particle-size distribution. Before the treatment, the pristine coir fibers were characterized by a less-uniform distribution of par- ticle size, with different content of smaller and bigger particles. As shown in Figure 3b, most of the particles were in the 2.5–3.0 μm range; however, larger aggregates of coir-fiber filler with an average size of ~4.0 μm were also observed.

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Materials 2021, 14, 1128 4 of 16

Figure 1. Schematic procedure of PUR composite preparation. 3. Results and Discussion 3. Results and Discussion Scanning electron microscopy (SEM) of coir fibers before surface treatment and after the treatmentScanning electron with henna microscopy are presented (SEM) inof Figurecoir fibers2. After before the surface chemical treatment treatment, and after the theexternal treatment surface with of henna the coir-fiber are presented filler seemedin Figure to 2. be After more the rough chemical than treatment, before the the treatment. external surfaceThis may of bethe connected coir-fiber withfillerthe seemed alkali to treatment, be more whichrough removedthan before the the impurities treatment. and This the maywaxes be thatconnected smooth with the the filler alkali surface. treatment, Moreover, which it removed is believed the that impurities such a and rough the surface waxes thatwas smooth connected the with filler the surface. presence Moreover, of henna it molecules,is believed which that such created a rough a coating surface layer was on con- the nectedfiller surface. with the It ispresence believed of that henna such molecules, developed which filler created will be morea coating compatible layer on with the PURfiller surface.structure, It providingis believed better that such interfacial developed interlocking filler will during be more PUR compatible synthesis andwith avoiding PUR struc- the ture,damages providing of PUR better structure interfacial during theinterlocki foamingng process.during PUR synthesis and avoiding the damages of PUR structure during the foaming process.

(a) (b) (c)

(d) (e) (f)

Figure 2. Optical andand scanningscanning electron electron microscope microscope images images of of (a –(ca)– pristinec) pristine coir coir fibers fibers and and (d– f()d coir-fiber–f) coir-fiber filler filler treated treated with henna.with henna.

The size of filler filler particles was determineddetermined using the dynamic light scattering (DLS) method. According According to to the the results results (Figure (Figure 33a),a), thethe coir-fibercoir-fiber fillerfiller treatedtreated withwith hennahenna hadhad 60.2% of of particles particles with with an an average average size size between between 1.0 1.0 and and 1.5 1.5 μm.µm. Apart Apart from from this, this, 8.5% 8.5% of particlesof particles were were in the in the range range of 0.7–0.8 of 0.7–0.8 μm,µ m,and and only only 5.5% 5.5% of particles of particles were were bigger bigger than than 1.5 μ1.5m.µ Itm .can It can be beseen seen that that the the application application of of a ahigh-energy high-energy ball-milling ball-milling process process resulted resulted in thethe formation formation of of coir-fiber coir-fiber filler filler with with a more a more homogenous homogenous particle-size particle-size distribution. distribution. Before Before the treatment,the treatment, the pristine the pristine coir coirfibers fibers were were charac characterizedterized by a byless-uniform a less-uniform distribution distribution of par- of ticleparticle size, size, with with different different content content of ofsmaller smaller and and bigger bigger particles. particles. As As shown shown in in Figure Figure 33b,b, Materials 2021, 14, 1128 most of of the the particles particles were were in in the the 2.5–3.0 2.5–3.0 μµmm range; range; however, however, larger larger aggregates aggregates of coir-fiber coir-fiber5 of 16

fillerfiller with an average size of ~4.0 μµm were also observed.

40 40 (a) (b) 30 30

20 20 Fraction [%] Fraction [%] 10 10

0 0 0 2000 4000 6000 0 2000 4000 6000 Size [nm] Size [nm] Figure 3. Particle-size distribution of ( a) coir-fiber coir-fiber fillerfiller modifiedmodified withwith hennahenna andand ((bb)) pristinepristine coircoir fibers.fibers.

Ultraviolet–visible (UV–VIS) analysis of the coir-fiber filler treated with henna is pre- sented in Figure 4. An intense peak at 280 nm corresponded to C=O and O-H groups of lawsone [35,41]. The long tail of the band beginning at 370 nm was associated with the yellow/orange color of the lawsone [41].

Figure 4. UV–VIS spectrum of coir-fiber filler treated with henna.

The FTIR spectrum of coir treated with henna is presented in Figure 5. The obtained spectrum showed the presence of three intense peaks at 1220, 1360, and 1740 cm−1, which correspond to the C=C aromatic of henna [42]. This indicates that the coir fibers were suc- cessfully functionalized with the henna compound, which effectively covered the surface of the fibers.

Materials 2021, 14, 1128 5 of 16

40 40 (a) (b)

30 30

20 20 Fraction [%] Fraction [%] 10 10

0 0 0 2000 4000 6000 Materials 2021, 140, 1128 2000 4000 6000 5 of 16 Size [nm] Size [nm] Figure 3. Particle-size distribution of (a) coir-fiber filler modified with henna and (b) pristine coir fibers. Ultraviolet–visible (UV–VIS) analysis of the coir-fiber filler treated with henna is presentedUltraviolet–visible in Figure4. An(UV–VIS) intense analysis peak at of 280 the nm coir-fiber corresponded filler treated to C=O with and henna O-H groupsis pre- sentedof lawsone in Figure [35,41 4.]. An The intense long tail peak of theat 280 band nm beginning corresponded at 370 to nm C=O was and associated O-H groups with theof lawsoneyellow/orange [35,41]. colorThe long of the tail lawsone of the [band41]. beginning at 370 nm was associated with the yellow/orange color of the lawsone [41].

Figure 4. UV–VIS spectrum of coir-fiber filler treated with henna. Figure 4. UV–VIS spectrum of coir-fiber filler treated with henna. The FTIR spectrum of coir treated with henna is presented in Figure5. The obtained spectrumThe FTIR showed spectrum the presence of coir treated of three with intense henna peaks is presented at 1220, 1360, in Figu andre 1740 5. The cm −obtained1, which Materials 2021, 14, 1128 spectrumcorrespond showed to the the C=C presence aromatic of three of henna intense [42 ].peaks This at indicates 1220, 1360, that and the 1740 coir cm fibers−1, which6 were of 16

correspondsuccessfully to functionalizedthe C=C aromatic with of henna the henna [42]. compound,This indicates which that the effectively coir fibers covered were suc- the cessfullysurface of functionalized the fibers. with the henna compound, which effectively covered the surface of the fibers.

C=C

C=C C=C

FigureFigure 5. 5. FTIRFTIR spectra spectra of of coir coir fibers fibers treated treated with with henna. TheThe results results of of the the foaming foaming behavior behavior of PUR of PUR composites composites were were determined determined by measuring by mea- creamsuring and cream expansion and expansion times (Figure times 6). (Figure In this6). stud In thisy, on study, the addition on the additionof coir-fiber of coir-fiber filler, the creamfiller, thetime cream of PUR_C_1, time of PUR_C_2, PUR_C_1, and PUR_C_2, PUR_C_5 and increased PUR_C_5 from increased 40 s to from50, 53, 40 and s to 61 50, s, respectively,53, and 61 s, while respectively, the extension while thetime extension increased time from increased 268 s to 320, from 330, 268 and s to 365 320, s, 330, respec- and tively.365 s, respectively.These results These can be results explained can be based explained on the based following on the aspects. following PUR aspects. composites PUR reinforcedcomposites with reinforced coir-fiber with filler coir-fiber were synthesized filler were by synthesized chemical foaming by chemical with foaming carbon diox- with idecarbon (CO2 dioxide) as the foaming (CO2) as agent, the foaming which is agent, produced which in is the produced reaction inbetween the reaction water betweenand iso- cyanatewater and groups. isocyanate Since groups.the addition Since of the coir-fiber addition fillers of coir-fiber could affect fillers the could proper affect stoichiome- the proper stoichiometry of the reaction, the formation and releasing of CO was reduced, which try of the reaction, the formation and releasing of CO2 was reduced,2 which resulted in limiting expansion of the cells and extended processing times [43,44]. Moreover, the - ther expansion of the cells was additionally hindered by the increased viscosity of PUR systems containing coir-fiber filler. Therefore, the addition of the filler resulted in ex- tended processing times. Similar results can also be found in other studies [43,44].

450 65 61 425 60 400 53 55 365 50 375 50 330 350 320 45 325 40 Start time [s] time Start 40 300 Expansion time [s] 268 35 275

30 250 PUR_NEAT PUR_C_1 PUR_C_2 PUR_C_5

Figure 6. The results for processing times of the PUR systems.

The addition of coir-fiber filler decreased the maximum temperature (Tmax) measured during the PUR synthesis (Figure 7). On the addition of 1, 2, and 5 wt % of the coir-fiber filler, the value of Tmax decreased from 130 °C (for PUR_NEAT) to 125, 115, and 105 °C, respectively. This may be connected with the increased viscosity of the PUR systems with the addition of coir-fiber filler, which effectively reduced the efficiency of the reaction between functional groups of the components. Moreover, previous studies have shown that de- creased temperature of PUR reinforced with natural fillers may be connected with the fact

Materials 2021, 14, 1128 6 of 16

C=C

C=C C=C

Figure 5. FTIR spectra of coir fibers treated with henna.

The results of the foaming behavior of PUR composites were determined by measuring cream and expansion times (Figure 6). In this study, on the addition of coir-fiber filler, the cream time of PUR_C_1, PUR_C_2, and PUR_C_5 increased from 40 s to 50, 53, and 61 s, respectively, while the extension time increased from 268 s to 320, 330, and 365 s, respec- tively. These results can be explained based on the following aspects. PUR composites reinforced with coir-fiber filler were synthesized by chemical foaming with carbon diox- Materials 2021, 14, 1128 6 of 16 ide (CO2) as the foaming agent, which is produced in the reaction between water and iso- cyanate groups. Since the addition of coir-fiber fillers could affect the proper stoichiome- try of the reaction, the formation and releasing of CO2 was reduced, which resulted in limitingresulted inexpansion limiting expansionof the cells of and the extended cells and extendedprocessing processing times [43,44]. times Moreover, [43,44]. Moreover, the fur- therthe further expansion expansion of the ofcells the was cells additionally was additionally hindered hindered by the by increased the increased viscosity viscosity of PUR of systemsPUR systems containing containing coir-fiber coir-fiber filler. filler. Therefore, Therefore, the addition the addition of the of filler the filler resulted resulted in ex- in tendedextended processing processing times. times. Similar Similar results results can can also also be befound found in inother other studies studies [43,44]. [43,44 ].

450 65 61 425 60 400 53 55 365 50 375 50 330 350 320 45 325 40 Start time [s] time Start 40 300 Expansion time [s] 268 35 275

30 250 PUR_NEAT PUR_C_1 PUR_C_2 PUR_C_5

Figure 6. The results for processing times of the PUR systems.

The addition of coir-fiber filler decreased the maximum temperature (T ) measured The addition of coir-fiber filler decreased the maximum temperature (Tmaxmax) measured during the PUR synthesis (Figure7). On the addition of 1, 2, and 5 wt % of the coir-fiber during the PUR synthesis (Figure 7). On the◦ addition of 1, 2, and 5 wt % of the coir-fiber◦ filler, the value of Tmax decreased from 130 C (for PUR_NEAT) to 125, 115, and 105 C, filler, the value of Tmax decreased from 130 °C (for PUR_NEAT) to 125, 115, and 105 °C, respectively. This may be connected with the increased viscosity of the PUR systems with Materials 2021, 14, 1128 respectively. This may be connected with the increased viscosity of the PUR systems7 with of 16 the addition of coir-fiber filler, which effectively reduced the efficiency of the reaction the addition of coir-fiber filler, which effectively reduced the efficiency of the reaction between between functional groups of the components. Moreover, previous studies have shown functional groups of the components. Moreover, previous studies have shown that de- that decreased temperature of PUR reinforced with natural fillers may be connected with creased temperature of PUR reinforced with natural fillers may be connected with the fact thatthe factduring that the during foaming the process, foaming some process, heat someis absorbed heat is by absorbed the filler by[45]. the A fillersimilar [45 explanation]. A similar mayexplanation be found may in our be found study inas ourwell. study as well.

160 3100 2800 130 125 130 2500 115 2200 105 1900 100 1600 1650

1300 Viscosity [mPa s] 70

Temperature [°C] 1000 1150 950 860 700 40 400 PUR_NEAT PUR_C_1 PUR_C_2 PUR_C_5

Figure 7. The results for temperature and dynamic viscosity of the PUR systems. The physico-mechanical properties of PUR composites are strongly affected by the The physico-mechanical properties of PUR composites are strongly affected by the rigidity of the PUR matrix and the cellular morphology of the composites. Previous studies rigidity of the PUR matrix and the cellular morphology of the composites. Previous stud- have shown that, owing to the relatively large size of filler particles, the addition of fillers ies have shown that, owing to the relatively large size of filler particles, the addition of may disrupt the closed-cell structure of PUR composites, thereby altering their uniform fillers may disrupt the closed-cell structure of PUR composites, thereby altering their uni- integrity [46,47]. Therefore, the evaluation of the impact of coir-fiber filler on the cellular form integrity [46,47]. Therefore, the evaluation of the impact of coir-fiber filler on the morphology of PUR composites seems to be necessary. As shown in Figure8a, the overall cellular morphology of PUR composites seems to be necessary. As shown in Figure 8a, structure of PUR_NEAT was quite uniform, with a high number of closed cells. On the the overall structure of PUR_NEAT was quite uniform, with a high number of closed cells. addition of coir-fiber filler (Figure8b–d), the closed-cell structure of PUR composites was On the addition of coir-fiber filler (Figure 8b–d), the closed-cell structure of PUR compo- still well preserved. Comparing all modified PUR composites, the highest number of sites was still well preserved. Comparing all modified PUR composites, the highest num- ber of broken cells and the most nonuniform size of the cells was exhibited by the PUR com- posite reinforced with 5 wt % of coir-fiber filler. When compared to PUR_NEAT, the per- centage content of closed cells decreased from 85.2% to 78.6%. This can be attributed to poor compatibility between the hydrophilic filler and the hydrophobic PUR matrix, which resulted in collapse of the cells and the formation of open cells [48]. A more uniform struc- ture was exhibited in the PUR composites reinforced with 1 and 2 wt % of coir-fiber filler. When compared with PUR_NEAT, the percentage content of closed cells increased slightly to 85.9 and 83.2% for PUR_C_1 and PUR_C_2, respectively. Moreover, with the addition of coir-fiber filler, the average size of the cells slightly decreased. The average size of the cells decreased from 490 μm (for PUR_NEAT) to 455, 440, and 430 μm for PUR_C_1, PUR_C_2, and PUR_C_5, respectively. This confirmed that the solid particles of the filler could act as a nucleating agent, changing the nucleation character from homo- to heterogeneous, and promoting the formation of a higher number of smaller cells [47,49,50].

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broken cells and the most nonuniform size of the cells was exhibited by the PUR composite reinforced with 5 wt % of coir-fiber filler. When compared to PUR_NEAT, the percentage content of closed cells decreased from 85.2% to 78.6%. This can be attributed to poor compatibility between the hydrophilic filler and the hydrophobic PUR matrix, which resulted in collapse of the cells and the formation of open cells [48]. A more uniform structure was exhibited in the PUR composites reinforced with 1 and 2 wt % of coir-fiber filler. When compared with PUR_NEAT, the percentage content of closed cells increased slightly to 85.9 and 83.2% for PUR_C_1 and PUR_C_2, respectively. Moreover, with the addition of coir-fiber filler, the average size of the cells slightly decreased. The average size of the cells decreased from 490 µm (for PUR_NEAT) to 455, 440, and 430 µm for Materials 2021, 14, 1128 PUR_C_1, PUR_C_2, and PUR_C_5, respectively. This confirmed that the solid particles8 of of16 the filler could act as a nucleating agent, changing the nucleation character from homo- to heterogeneous, and promoting the formation of a higher number of smaller cells [47,49,50].

FigureFigure 8. Scanning 8. Scanning electron electron microscope microscope images images of ( ofa,b (a) ,PUR_NEAT;b) PUR_NEAT; (c,d (c), dPUR_C_1;) PUR_C_1; (e,f (e) ,PUR_C_2;f) PUR_C_2; (g, (hg), hPUR_C_5.) PUR_C_5.

The impact of coir-fiber coir-fiber filler filler on the color of PURPUR compositescomposites was evaluatedevaluated using the colorimetric analysis. According According to to the the op opticaltical images images presented presented in in Figure 99,, onon thethe addition of the filler, filler, the overall color of the PURPUR compositescomposites became greener. This was connected with with the the fact fact that that henna henna contains contains a coloring a coloring matter—lawsone. matter—lawsone. According According to the to resultsthe results of color of color analysis analysis (Table (Table 2),2 ),on on the the ad additiondition of of the the filler, filler, the the value of totaltotal colorcolor change became higher. Moreover, the values of a ** andand bb *,*, whichwhich referrefer toto thethe /greenred/green and yellow/blue shades increased, indicating the more green and blue shadows of the and yellow/blue shades increased, indicating the more green and blue shadows of the PUR composites. PUR composites.

(a) (b) (c) (d)

Figure 9. OpticalOptical images of ( a)) PUR_NEAT; PUR_NEAT; ((b)) PUR_C_1;PUR_C_1; ((cc)) PUR_C_2;PUR_C_2; ((dd)) PUR_C_5PUR_C_5

Table 2. Color parameters of PUR composites measured before and after the UV-aging. L * a * b * ΔE * Sample Before UV-aging PUR_NEAT 14.3 63.4 −7.5 0 PUR_C_1 17.2 78.1 −3.7 14.6 PUR_C_2 22.5 78.3 −1.2 21.2 PUR_C_5 28.2 79.3 −0.2 25.2 After UV-aging PUR_NEAT 65.5 18.5 −4.1 50.2 PUR_C_1 55.4 31.1 −3.0 48.1 PUR_C_2 54.3 38.4 −2.9 42.5 PUR_C_5 51.1 49.6 0.5 41.5 ΔE *—total color change, L *—white/dark degree, a *—red/green shade, b *—yellow/blue shade.

Phytochemicals, especially phenols and flavonoids, are sensitive to UV-irradiation. The UV-irradiation can initiate a polymerization or degradation process of phytochemicals,

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Table 2. Color parameters of PUR composites measured before and after the UV-aging.

L * a * b * ∆E* Sample Before UV-Aging PUR_NEAT 14.3 63.4 −7.5 0 PUR_C_1 17.2 78.1 −3.7 14.6 PUR_C_2 22.5 78.3 −1.2 21.2 PUR_C_5 28.2 79.3 −0.2 25.2 After UV-Aging PUR_NEAT 65.5 18.5 −4.1 50.2 PUR_C_1 55.4 31.1 −3.0 48.1 PUR_C_2 54.3 38.4 −2.9 42.5 PUR_C_5 51.1 49.6 0.5 41.5 ∆E *—total color change, L *—white/dark degree, a *—red/green shade, b *—yellow/blue shade. Materials 2021, 14, 1128 9 of 16

Phytochemicals, especially phenols and flavonoids, are sensitive to UV-irradiation. The UV-irradiation can initiate a polymerization or degradation process of phytochemicals, leading to the deteriorationdeterioration of thethe physicalphysical andand mechanicalmechanical propertiesproperties ofof thethe compositescomposites containing plant fillers.fillers. Therefore, the impact of controlled UV-irradiation on thethe structuralstructural and physical properties of PURPUR compositescomposites waswas evaluated.evaluated. The optical images of PURPUR composites after the controlled UV-irradiation are presented in Figure 1010.. Based on this, it can be concluded that the addition of coir-fibercoir-fiber fillerfiller treatedtreated withwith hennahenna hadhad anan impactimpact on the color change of thethe PURPUR composites.composites. The optical analysis revealed that after thethe UV-irradiation, the overall color of PUR composites had changed from yellow to orange, and thisthis effecteffect waswas most most prominent prominent in in the the case case of PUR_NEAT.of PUR_NEAT. On On the additionthe addition of coir-fiber of coir- filler,fiber filler, the difference the difference in the in color the color was was not asnot intense. as intense. Confirmation Confirmation of these of these results results can can be foundbe found in thein the color-parameter color-parameter results results (Table (Tab2).le When 2). When comparing comparing all PUR all PUR composites, composites, the mostthe most significant significant change change in totalin total color color change change (∆ (EΔE *) *) was was observed observed forfor PUR_NEAT.PUR_NEAT. OnOn the addition of the coir-fibercoir-fiber filler,filler, thethe differencedifference inin ∆ΔEE ** waswas slightlyslightly reduced.reduced. AA similarsimilar trend was observed in the casecase ofof thethe lightnesslightness parameterparameter (L(L *)—on*)—on thethe additionaddition ofof thethe coir-fibercoir-fiber filler,filler, the valuevalue decreased. Moreov Moreover,er, the UV-aging increased the values of a * and b *, which refer toto thethe red/greenred/green and yellow/blueyellow/blue shades. shades. On On the the addition addition of coir-fiber coir-fiber filler,filler, thethedifferences differences between between the the parameters paramete measuredrs measured before before and and after after the UV-aging the UV-aging were slightlywere slightly reduced. reduced. This indicated This indicated that the that addition the addition of the coir-fiberof the coir-fiber filler treated filler treated with henna with effectivelyhenna effectively protected protected the PUR the composites PUR composites from UV-radiationfrom UV-radiation and acted and acted as an as antiaging an anti- compound.aging compound. Good resistanceGood resistance of PUR compositesof PUR composites against UV-irradiation against UV-irradiation should be should attributed be to the chemical composition of henna, which possesses strong antioxidant activity and attributed to the chemical composition of henna, which possesses strong antioxidant activity successfully reduces a natural discoloration of PUR composites. and successfully reduces a natural discoloration of PUR composites.

(a) (c) (e) (g)

(b) (d) (f) (h)

Figure 10. Optical images of ( a,b) PUR_NEAT; ((cc,,dd)) PUR_C_1;PUR_C_1; ((ee,,ff)) PUR_C_2;PUR_C_2; ((gg,,hh)) PUR_C_5.PUR_C_5.

Mechanical properties are an important parameter that determines the further appli- cation of PUR composites. The compressive strength of PUR composites depends signifi- cantly on their apparent density. According to the results presented in Table 3, the incor- poration of coir-fiber filler increased the apparent density of PUR composites, due to the incorporation of the filler with a certain weight. Depending on the content of coir-fiber filler, the value of apparent density increased from 35.9 kg m−3 to 37.9, 38.2, and 38.6 kg m−3 for PUR_C_1, PUR_C_2, and PUR_C_5, respectively. This may be explained by the following reasons. As discussed previously, PUR composites were synthesized in the foaming with CO2, which was formed in the reaction between water and isocyanate groups. Due to the filler incorporation, the viscosity of the PUR systems was increased, leading to a greater number of cells over a given volume. Therefore, the solid phases of PUR composites in- creased with the addition of the filler.

Materials 2021, 14, 1128 9 of 16

Mechanical properties are an important parameter that determines the further applica- tion of PUR composites. The compressive strength of PUR composites depends significantly on their apparent density. According to the results presented in Table3, the incorporation of coir-fiber filler increased the apparent density of PUR composites, due to the incorporation of the filler with a certain weight. Depending on the content of coir-fiber filler, the value of apparent density increased from 35.9 kg m−3 to 37.9, 38.2, and 38.6 kg m−3 for PUR_C_1, PUR_C_2, and PUR_C_5, respectively. This may be explained by the following reasons. As discussed previously, PUR composites were synthesized in the foaming with CO2, which was formed in the reaction between water and isocyanate groups. Due to the filler incorporation, the viscosity of the PUR systems was increased, leading to a greater number of cells over a given volume. Therefore, the solid phases of PUR composites increased with the addition of the filler.

Table 3. Cell size, closed-cell content, and apparent density of PUR composites before and after UV-aging.

Before UV-Aging After UV-Aging Sample Cell Size Closed-Cell Cell Size Closed-Cell Apparent Density Apparent Density [kg m−3] [µm] Content [%] [µm] Content [%] [kg m−3] PUR_NEAT 490 85.2 35.9 505 75.4 31.3 PUR_C_1 455 85.9 37.9 470 76.2 33.5 PUR_C_2 440 83.2 38.2 455 75.2 33.7 PUR_C_5 430 78.6 38.6 450 70.2 35.8

The impact of the addition of coir-fiber filler on the σ10% of the PUR composites is presented in Figure 11a. When compared to PUR_NEAT, the value of σ10% increased by 23 and 21% for PUR_C_1 and PUR_C_2, respectively, and decreased by 4% for PUR_C_5. To eliminate the impact of apparent density on the value of σ10%, the compressive specific strength, defined as the ratio of σ10% to the apparent density of the PUR composites, was evaluated as well. Depending on the content of coir-fiber filler, the specific compressive strength increased from 6.7 kPa/kg/m3 to 7.8 and 7.6 kPa/kg/m3 on the addition of 1 and 2 wt % of coir-fiber filler, respectively, and then decreased to 6.0 kPa/kg/m3 with a further increase of coir-fiber filler to 5 wt %. An analog dependency is observed in the case of flexural strength (σF) (Figure 11b). When compared with PUR_NEAT, the value of σF increased by 9 and 5% for PUR_C_1 and PUR_C_2, respectively, and then decreased by 7% for PUR_C_5. Previous studies have reported that the mechanical characteristics of porous materials depend on interfacial interactions between filler surface and polymer matrix, as well as relatively uniform distribution of the filler in the composite structure [51]. A similar explanation can also be used in our study. The greatest improvement in mechanical characteristics was observed for PUR composites reinforced with 1 and 2 wt % of coir- fiber filler. Such a reinforcing effect may be connected with a uniform distribution of the coir-fiber filler in the PUR structure, as well as strong interphase adhesion between the functional groups of coir-fiber filler (mostly hydroxy groups of lawsone compound) and the PUR matrix. Therefore, the mechanical characteristic of the PUR composites was enhanced by a more rigid, cross-linked cellular structure. Furthermore, as discussed previously, the application of the high-energy ball-milling process resulted in the formation of smaller particles of the filler, which could be easily built in the PUR structure, showing a reinforcing function and effectively transferring an external load [52–54]. Owing to the higher stiffness of PUR composites, they were able to absorb more energy, effectively improving the mechanical resistance [55,56]. It has been shown that when the filler content was increased to 5 wt %, the mechanical properties were slightly decreased. According to previous studies, for a filler with an irregular shape, such as a cellulosic filler, the strength of the reinforced materials can be weakened by the filler’s insufficient ability to bear the stresses transferred from the matrix of the polymeric composites [55,56]. Besides, the high content of filler particles can result in poor interfacial adhesion, leading to the formation of partially separated microvoids between the polymer matrix and the filler surface, which Materials 2021, 14, 1128 10 of 16

prevent stress transfer. Due to the partial separation of the voids, the mechanical properties of PUR composites were deteriorated. According to the results presented in Figure 11c,d, the controlled UV-aging reduced the mechanical properties of the PUR composites. Interestingly, the most significant deterioration was observed in the case of PUR_NEAT. On the addition of coir-fiber filler, the mechanical properties were deteriorated as well; however, the difference was lower, as in the case of PUR_NEAT. The compressive strength decreased by ~36% for PUR_NEAT, while for the PUR composites reinforced with coir-fiber filler, the value decreased by ~28–34%, depending on the concentration of the filler. It seems that the solid particles of the filler may have acted as a physical barrier during the Materials 2021, 14, 1128 11 of 16 UV-aging and effectively supported the overall structure of the PUR composites, which resulted in better mechanical performances.

320 200 380 22 (a) 295 (b) 355 290 360 300 340 20 180 340 280 325 18 320 260 160 300 16 240 162 300 240 155 230 14 140 280 220 12

Flexural strength [kPa] 260 12.9

120 11.9 break Elongation at [%] 125 11.4 200 240 11.1 10

Compressive (parallel) [kPa] strength 115 Compressive (perpend.) strength [kPa] 180 100 220 8 PUR_NEAT PUR_C_1 PUR_C_2 PUR_C_5 PUR_NEAT PUR_C_1 PUR_C_2 PUR_C_5

240 190 260 16 (c) (d) 220 240 205 170 220 210 14 200 190 220 150 200 190 180 165 12 180 160 160 145 130 160 140 10 120 110 140 9.7

120 Flexural strength [kPa] 9.4 9.4 105 120 8 90 8.5 break Elongation at [%] 100 95 100 90 Compressive (parallel) [kPa] strength 80 70 Compressive (perpend.) strength [kPa] 80 6 PUR_NEAT PUR_C_1 PUR_C_2 PUR_C_5 PUR_NEAT PUR_C_1 PUR_C_2 PUR_C_5

Figure 11. MechanicalMechanical performances performances of PUR composites measured ( a,b) before, and (c,d) after the UV-aging. 0 The dynamic mechanical properties (DMA), such as storage modulus (E′), and the glass-transition temperature (Tg)—determined as a maximum peak on the graph of tanδ glass-transition temperature (Tg)—determined as a maximum peak on the graph of tanδ in the function of temperature—were evaluated. The results for storage modulus are in the function of temperature—were evaluated. The results for storage modulus are pre- presented in Figure 12a. When compared to the PUR_NEAT, on the incorporation of sented in Figure 12a. When compared to the PUR_NEAT, on the incorporation of 1 and 2 1 and 2 wt % of coir-fiber filler, a significant improvement in E’ was observed. This wt % of coir-fiber filler, a significant improvement in E’ was observed. This may be ex- may be explained by the following reasons: The alkali treatment removed the impurities plained by the following reasons: The alkali treatment removed the impurities and waxes and waxes on a fiber’s surface, and due to this, the fiber’s surface became more rough, on a fiber’s surface, and due to this, the fiber’s surface became more rough, increasing the increasing the interphase adhesion between the filler’s surface and the polymeric matrix. interphase adhesion between the filler’s surface and the polymeric matrix. Furthermore, Furthermore, as discussed previously, on the incorporation of coir-fiber filler, the viscosity as discussed previously, on the incorporation of coir-fiber filler, the viscosity of the PUR of the PUR systems was increased, limiting the mobility of the polymer chains, which systems was increased, limiting the mobility of the polymer chains, which resulted in the resulted in the higher stiffness of the PUR structure. The incorporation of the coir-fiber filler higher stiffness of the PUR structure. The incorporation of the coir-fiber filler increased increased the cross-linking density of PUR, due to the formation of chemical bridge bonds thebetween cross-linking functional density groups of PUR, of the due coir-fiber to the formation filler and of isocyanate chemical groups.bridge bonds This between induced functionalthe reinforcement groups of effect the coir-fiber and improved filler and the isocyanate overall groups. mechanical This characteristicinduced the reinforcement of the PUR effect and improved the overall mechanical characteristic of the PUR composites. On the addition of 5 wt % of coir-fiber filler, the dynamic mechanical properties of the PUR com- posites were slightly decreased. This indicates that the addition of coir-fiber filler in a higher concentration resulted in the formation of a more flexible structure in the PUR composites. The results for E’ were in agreement with the results for the glass-transition temperature (Tg) (Figure 12b). When compared with PUR_NEAT, the value of Tg increased with the addi- tion of the coir-fiber filler. Similar to the results for E’, the greatest improvement in Tg was observed for PUR_C_1—the value of Tg increased from 149 °C (for PUR_NEAT) to 178 °C. Such improvement in Tg observed for the PUR composites may be connected with the presence of the filler particles, which were built into the PUR structure and were able to create the interlocks between the PUR cells. Due to this, more energy was needed to reach the Tg. The dynamic-mechanical properties of PUR composites subjected to the UV-aging were determined as well. According to the results presented in Figure 12c,d, after the UV-

Materials 2021, 14, 1128 11 of 16

composites. On the addition of 5 wt % of coir-fiber filler, the dynamic mechanical properties of the PUR composites were slightly decreased. This indicates that the addition of coir-fiber filler in a higher concentration resulted in the formation of a more flexible structure in the PUR composites. The results for E’ were in agreement with the results for the glass- transition temperature (Tg) (Figure 12b). When compared with PUR_NEAT, the value of Tg increased with the addition of the coir-fiber filler. Similar to the results for E’, the greatest ◦ improvement in Tg was observed for PUR_C_1—the value of Tg increased from 149 C ◦ (for PUR_NEAT) to 178 C. Such improvement in Tg observed for the PUR composites may be connected with the presence of the filler particles, which were built into the PUR Materials 2021, 14, 1128 structure and were able to create the interlocks between the PUR cells. Due to this,12 more of 16

energy was needed to reach the Tg. The dynamic-mechanical properties of PUR composites subjected to the UV-aging were determined as well. According to the results presented in Figure 12c,d, after the UV-aging, all PUR composites were characterized by lower Tg and a aging, all PUR composites were characterized by lower Tg and a reduced value of E’. Sim- reduced value of E’. Similar to the results for mechanical properties, the most significant ilar to the results for mechanical properties, the most significant decrease in dynamic-me- decrease in dynamic-mechanical properties was observed for PUR_NEAT—the value of Tg chanical properties was observed for PUR_NEAT—the value of Tg decreased from 149 to decreased from 149 to 121 ◦C, while the E’ decreased by ~38%. On the addition of coir-fiber 121 °C, while the E’ decreased by ~38%. On the addition of coir-fiber filler, the deteriora- filler, the deterioration of the properties was not as significant. For example, in the case of tion of the properties was not as significant. For◦ example,◦ in the case of PUR_C_5, the value PUR_C_5, the value of Tg decreased from 160 C to 149 C, while the value of E’ decreased of Tg decreased from 160 °C to 149 °C, while the value of E’ decreased by ~30%. by ~30%.

0.8 2 (a) (b) PUR_NEAT PUR_C_1 PUR_NEAT 1.6 0.6 PUR_C_1 PUR_C_2 PUR_C_2 PUR_C_5 PUR_C_5 1.2 0.4 tanδ [-] 0.8

0.2 0.4 Storage Modulus [MPa]

0 0 50 75 100 125 150 175 200 225 250 50 75 100 125 150 175 200 225 250 Temperature [°C] Temperature [°C]

0.8 2 (c) PUR_NEAT (d) PUR_NEAT PUR_C_1 PUR_C_1 1.6 PUR_C_2 0.6 PUR_C_2 PUR_C_5 PUR_C_5 1.2 0.4 tanδ [-] 0.8

0.2 0.4 Storage Modulus [MPa]

0 0 50 75 100 125 150 175 200 225 250 50 75 100 125 150 175 200 225 250 Temperature [°C] Temperature [°C] Figure 12. Dynamic-mechanical properties of PUR composites measured (a,b) before, and (c,d) after the UV-aging. Figure 12. Dynamic-mechanical properties of PUR composites measured (a,b) before, and (c,d) after the UV-aging. Thermal decomposition of the PUR composites was evaluated using thermogravi- Thermal decomposition of the PUR composites was evaluated using thermogravi- metric analysis (TGA) and derivative thermogravimetric analysis (DTGA). The results are metric analysis (TGA) and derivative thermogravimetric analysis (DTGA). The results are shown in Table4 and Figure 13. shown in Table 4 and Figure 13.

Table 4. The results of the thermogravimetric and derivative thermogravimetric analyses.

Char Residue T10% [°C] T50% [°C] T80% [°C] Sample Codes (at 600 °C) [%] Before UV-Aging PUR_NEAT 213 312 585 29.2 PUR_C_1 215 320 592 30.3 PUR_C_2 225 319 595 30.9 PUR_C_5 210 310 576 28.7 After UV-Aging PUR_NEAT 227 309 579 28.3 PUR_C_1 230 313 582 28.9 PUR_C_2 231 315 585 28.8 PUR_C_5 231 308 586 28.2

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Table 4. The results of the thermogravimetric and derivative thermogravimetric analyses.

◦ ◦ ◦ Char Residue T10% [ C] T50% [ C] T80% [ C] ◦ Sample Codes (at 600 C) [%] Before UV-Aging PUR_NEAT 213 312 585 29.2 PUR_C_1 215 320 592 30.3 PUR_C_2 225 319 595 30.9 PUR_C_5 210 310 576 28.7 After UV-Aging PUR_NEAT 227 309 579 28.3 PUR_C_1 230 313 582 28.9 Materials 2021, 14, 1128 PUR_C_2 231 315 585 28.8 13 of 16 PUR_C_5 231 308 586 28.2

100 0 (a) (b) 90 −0.001 80 −0.002 C] 70 o −0.003 60 −0.004 50 coconut filler coconut filler PUR_NEAT

Weight loss [%] loss Weight PUR_C_NEAT −0.005 40 PUR_C_1 [%/ weight Deriv PUR_C_1 PUR_C_2 30 −0.006 PUR_C_2 PUR_C_5 PUR_C_5 20 −0.007 0 100 200 300 400 500 600 0 100 200 300 400 500 600 Temperature [oC] Temperature [oC]

100 0 (c) (d) 90 −0.001 80 −0.002 C] 70 o −0.003 60 −0.004 50 PUR_NEAT PUR_NEAT Weight loss [%] Weight loss −0.005

40 PUR_C_1 [%/ weight Deriv PUR_C_1 PUR_C_2 PUR_C_2 30 −0.006 PUR_C_5 PUR_C_5 20 −0.007 0 100 200 300 400 500 600 0 100 200 300 400 500 600

o Temperature [oC] Temperature [ C]

Figure 13. TGA results for PUR composites measured (a,,b)) before,before, andand ((cc,,dd)) afterafter thethe UV-aging.UV-aging.

In allall cases,cases, aa three-stagethree-stage decompositiondecomposition ofof thethe PURPUR compositescomposites waswas observed.observed. TheThe ◦ firstfirst stage of of decomposition decomposition (T (Tmax1max1) occurred) occurred in the in therange range of 200–230 of 200–230 °C andC referred and referred to the tomoisture the moisture absorption absorption of cellulosic of cellulosic filler [5 filler7–60] [57 and–60 decomposition] and decomposition of the ofurethane the urethane bond bond [57,58]. The second stagemax2 (Tmax2) of the PUR-composite decomposition occurred [57,58]. The second stage◦ (T ) of the PUR-composite decomposition occurred in the inrange the of range 300–350 of 300–350 °C, and shouldC, and be should assigned be to assigned the thermal to the pyrolysis thermal of pyrolysis hard segments of hard of segmentsPUR, which of PUR,resulted which in resultedthe formation in the formationof alcohol of and alcohol isocyanate and isocyanate groups groups[61–63]. [ 61When–63]. When compared with PUR_NEAT, Tmax2 increased on the addition of 1 and 2 wt % of compared with PUR_NEAT, Tmax2 increased on the addition◦ of 1 and 2 wt % of coir-fiber coir-fiber filler—the value of Tmax2 increased from 312 C (for PUR_NEAT) to 320 and filler—the◦ value of Tmax2 increased from 312 °C (for PUR_NEAT) to 320 and 319 °C, re- 319 C, respectively. Due to the more porous structure, the value of Tmax2 decreased spectively. Due to the more porous structure, the value of Tmax2 decreased slightly to 310 slightly to 310 ◦C on the addition of 5 wt % of coir-fiber filler. The third stage of thermal °C on the addition of 5 wt % of coir-fiber filler. The third stage of thermal decomposition decomposition of the PUR composites was mostly connected with the combustion of the of the PUR composites was mostly connected with the combustion of the composites, as well composites, as well as thermal decomposition of the cellulosic derivatives hemicellulose as thermal decomposition of the cellulosic derivatives hemicellulose and lignin [64,65]. When compared with PUR_NEAT, the PUR composites reinforced with 1 and 2 wt % of coir-fiber filler were more thermally stable—the value of Tmax3 increased from 585 °C (for PUR_NEAT) to 592 and 595 °C, respectively. The observed thermal-degradation behavior indicated that filler particles delayed the temperature of thermal decomposition by acting as a mass transport barrier and superior insulator to the volatile compounds, which were released during the thermal decomposition [66]. Besides, the thermal decomposition of PUR composites may be additionally reduced due to the higher cross-linking structure of the composites and the additional chemical bond between the functional groups of coir- fiber filler and isocyanate groups. The obtained results were in agreement with the results of the mass of char residue of the PUR composites obtained at 600 °C. Compared to PUR_NEAT, the mass of char residue increased from 29.2% to 30.3 and 30.9% for PUR_C_1 and PUR_C_2, respectively, and then decreased slightly to 28.7% for PUR_C_5. Based on this, it can be concluded that during the thermal-decomposition process, the filler particles could act as radical scavengers, while the higher cross-linking structure of the PUR Materials 2021, 14, 1128 13 of 16

and lignin [64,65]. When compared with PUR_NEAT, the PUR composites reinforced with 1 and 2 wt % of coir-fiber filler were more thermally stable—the value of Tmax3 increased from 585 ◦C (for PUR_NEAT) to 592 and 595 ◦C, respectively. The observed thermal- degradation behavior indicated that filler particles delayed the temperature of thermal decomposition by acting as a mass transport barrier and superior insulator to the volatile compounds, which were released during the thermal decomposition [66]. Besides, the thermal decomposition of PUR composites may be additionally reduced due to the higher cross-linking structure of the composites and the additional chemical bond between the functional groups of coir-fiber filler and isocyanate groups. The obtained results were in agreement with the results of the mass of char residue of the PUR composites obtained at 600 ◦C. Compared to PUR_NEAT, the mass of char residue increased from 29.2% to 30.3 and 30.9% for PUR_C_1 and PUR_C_2, respectively, and then decreased slightly to 28.7% for PUR_C_5. Based on this, it can be concluded that during the thermal-decomposition process, the filler particles could act as radical scavengers, while the higher cross-linking structure of the PUR composites effectively reduced heat transfer through the composite structure and improved their thermal resistance [66]. According to the results presented in Figure 13c,d, the UV-aging affected the thermal stability of the PUR composites; however, the differences between the maximum temperatures for non-aged and UV-aged PUR composites were not as significant.

4. Conclusions In this study, coir fibers were successfully modified with henna (Lawsonia inermis plant) using a high-energy ball-milling process. In the next step, the developed filler was used as a reinforcing filler in the production of rigid polyurethane (PUR) foams. The impact of 1, 2, and 5 wt % of coir-fiber filler on the structural and physico-mechanical properties were evaluated. The obtained results revealed that depending on the filler content, an improvement or deterioration of the examined properties was observed. Among all modified series of PUR composites, the greatest improvement in physico-mechanical performances was observed for the PUR composite reinforced with 1 wt % of the coir-fiber filler. For example, on the addition of 1 wt % of coir-fiber filler, the compression strength was improved by 23%, while the flexural strength increased by 9%. Similar dependence was observed in the case of dynamic-mechanical properties—on the addition of 1 wt % of the filler, the glass-transition temperature increased from 149 ◦C to 178 ◦C, while the value of the storage modulus increased by ~80%. Interestingly, it was found that the PUR composites reinforced with coir-fiber filler were characterized by better mechanical performances after the UV-aging. Moreover, the incorporation of low thermally-stable cellulosic filler did not deteriorate the thermal stability of the PUR composites. When compared with the reference PUR composites, all series of modified PUR composites exhibited a similar degradation pattern. This led to the conclusion that the application of henna protected the coir-fiber filler and the reinforced PUR composites from UV-aging and high temperatures.

Author Contributions: Methodology, S.C., A.S., A.K.; investigation, S.C., A.S., A.K.; data curation, S.C.; writing—original draft, S.C.; writing—review and editing, S.C.; visualization, S.C. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data sharing not applicable. Conflicts of Interest: The authors reported no conflict of interest related to this study. Materials 2021, 14, 1128 14 of 16

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