Potential Use of Melamine Phytate as a Flame-Retardant Additive in Chicken Feather-Containing Thermoplastic Polyurethane Biocomposites

Using waste materials such as chicken feathers (CF) and biobased flame-retardant additives including melamine phytate (MPht) has become an effective approach for environmentally friendly and sustainable production in recent years. This study explores the flame retardant effectiveness of MPht in thermoplastic polyurethane (TPU)-based biocomposites containing CF. The characterizations of the composites are performed through thermal gravimetric analysis (TGA), limiting oxygen index (LOI), vertical UL-94 (UL-94 V), and mass loss calorimetry (MLC) tests. According to the test results, the highest UL-94 V rating of V0, a LOI value of 29.4%, and the lowest peak heat release rate (pHRR) (110 Kw/m2) and total heat evolved (THE) (39 MJ/m2) values are obtained with the use of 20 wt % MPht. It is demonstrated that MPht acts as an effective flame-retardant filler through the formation of intumescent char in the condensed phase and flame dilution in the gas phase.


INTRODUCTION
Thermoplastic polyurethane (TPU), a multiphase block copolymer composed of hard and soft segments, can be synthesized from polyols and diisocyanates derived from petrochemical and/or renewable resources. Because of the variety of available monomers, there are many different grades of TPU with varying properties. The outstanding properties of flexibility, convenient mass production, recyclable properties, and good abrasion resistance can give rise to finding use in numerous industrial sectors of medical, automobile, construction, electrical, and electronic. However, some drawbacks of TPU including poor thermal stability, low mechanical strength, and high flammability restrict its wider application. To address these issues, polymer blend technology and various types of fillers are used with TPU. 1,2 About 40 million ton of feather-based waste materials is produced in a year. In recent years, finding unique and added value applications for them has attracted careful attention. Feather-based fibers are used in numerous applications including nonwovens, biochar, biodiesel, bioplastics, and composites owing to producing novel sustainable and ecofriendly materials with increasing environmental consciousness. 1,3,4 In the development of biocomposites, feather fibers, such as chicken feather (CF), have been utilized and referenced in previous detailed review articles. 4−9 Specifically, the incorporation of CF has been shown to enhance the mechanical properties of TPU in the literature. 1,10−13 CF mainly consist of keratin (90%), water (1.8%), and oil (1.3%). Keratins, which are built from amino acids, especially cystine (∼10 wt % total amino acids), contain heteroatoms including nitrogen, oxygen, and sulfur. They leave a remarkable amount of char during the combustion. 14 Accordingly, CF and keratin derived from CF have been utilized in the production of flame-retardant textiles and composites. 15,16 In the literature, the flame retardant properties of CF-containing polypropylene-, 17−20 TPU-, 21,22 and polyester resin-based 23 composites were improved using commercial flame retardants.
Several different additives including biomolecules were used to enhance the flame retardant properties of TPU-based composites. 24,25 Biomolecules including phytic acid derivatives have been considered environmentally friendly flame retardant fillers in various polymeric materials. 15,16,26,27 Phytic acid was used to modify boron nitride and silicon nitride, and the resulting modified compounds were used as flame-retardant additives in TPU. 28−30 Phytic acid has the ability to form complexes with organic cations including melamine via coordinative bonds and supramolecular interactions. 31,32 Melamine is classified in nitrogen-containing flame-retardant additives and is considered environmentally friendly due to the low evolution of smoke and low toxicity in the event of fire with the release of carbon dioxide, ammonia, and water. 33 The flame retardant effect of melamine phytate (MPht) was investigated in polypropylene 34−38 and poly(lactic acid). 39 In the current study, MPht is selected as a biobased additive to produce sustainable and ecofriendly TPU-based biocomposites with an enhanced flame retardant character. The thermal and flame retardant performances of the biocomposites are analyzed using thermogravimetric analysis (TGA), limiting oxygen index (LOI), vertical UL 94 (UL-94V), and mass loss calorimetry (MLC) tests.

Materials.
Polyether-based TPU with the commercial name PEARLTHANE CLEAR 15N85-CF was purchased from Brenntag (Istanbul, Turkey). It has a Shore A hardness of 86 and a density of 1.11 g/cm 3 . CF was supplied from local sources located in Bursa, Turkey. Phytic acid was purchased from Sigma Aldrich as a 50 wt % aqueous solution. Melamine with the trade name Melafine was purchased from DSM (Geleen, the Netherlands).

Synthesis of Melamine Phytate.
MPht was synthesized using the starting materials melamine (25.2 g, 0.2 mol) and 50 wt % phytic acid solution (44 g, 0.033 mol). 0.2 mol melamine was dissolved in 400 mL of distilled water and heated up to 90°C. Phytic acid solution was added to the heated solution and stirred for 30 min. The precipitated offwhite MPht crystals were washed two times with distilled water and dried in an oven at 80°C for 24 h. The reaction yield was about 60%. The characterization of MPht was performed using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and TGA analysis.
2.3. Production of the Composites. CF were washed with hot soapy water (60°C) two times before the grinding process to remove the oil and other unwanted stains. After the washing process, CF was dried at 80°C for 16 h. CF with quills were ground with blade grinding (FRITSCH PULVERI-SETTE 19, Germany). After the grinding process, CF in the powder form were used in the composite production. The SEM images of ground CF with magnifications of ×100 and ×400 are shown in Figure 1a,b, respectively. As seen in Figure  1, barbs sustain their fiber form and quills were ground into small fragments. CF and MPht were dried in an oven at 80°C for 24h prior to the compounding process. The extrusion process was performed in a twin screw extruder (GULNAR MAKINA, Istanbul, Turkey) at 100 rpm with the temperature profile of 50, 165, 170, 175, 170, and 165°C from the hopper to the die. The samples for LOI, UL-94 V, and tensile tests were molded using a laboratory-scale injection-molding machine (DSM Xplore 12 mL Micro-Injection Molder, Netherlands). The barrel and mold temperatures and the injection pressure were constant at 190°C, 30°C, and 8 Bar, respectively. The samples for the MLC test were produced using a laboratory-scale hot-press (GULNAR MAKINA, Istanbul, Turkey) for 3 min at 160°C. The flame retardant performance of MPht was examined in a constant CF loading of 20 wt %. MPht was used in three different concentrations of 5, 10, and 20 wt %. For sample coding, the abbreviations TPU, CF, and MPht were used for thermoplastic polyurethane, chicken feather, and melamine phytate, respectively. The code TPU/CF/10 MPht indicates the sample containing 20 wt % CF and 10 wt % MPht.

Characterization Methods.
ATR-FTIR analysis was performed for melamine, phytic acid, MPht, heated MPht at 800°C, and the char residues remaining after the MLC test at an optical resolution of 4 cm −1 with 32 scans. TGA tests were carried out for each component (TPU, CF, and MPht) and the composites using a Hitachi-High Tech STA-7300 instrument with a heating rate of 10°C/min from room temperature to 800°C under a nitrogen flow of 50 mL/min. LOI values were examined using a Fire Testing Technology (FTT) Limiting Oxygen Index Analyzer instrument on the test bars of 130 × 6.5 × 3.2 mm 3 in size, according to the standard oxygen index test ASTM D2863. The UL 94 V test was performed on the test bars of 130 × 13 × 3.2 mm 3 according to ASTM D3801. The MLC test was carried out using Mass Loss Cone with a Square specimens with a dimension of 100 × 100 × 3 mm 3 were used. Tensile properties of the composites were analyzed at room temperature using a Devotrans GP/R testing machine equipped with a 5 kN load cell, following the ASTM D 638 standard.
The tests were carried out on dog bone-shaped samples (7.4 × 2.1 × 80 mm 3 ) at a crosshead speed of 50 mm/ min. The tensile strength and percentage elongation at break values were recorded, and the results were averaged over five samples with standard deviations. The microstructures of ground CF and the residual chars remaining after the MLC test, and the tensile fracture surfaces of the composites were examined with SEM (FEI Quanta 400F). The samples were coated with gold with a sputter coater to achieve the conductivity.

Characterization of Melamine
Phytate. ATR-FTIR analysis was used to confirm the synthesis of MPht, and the spectra of melamine, phytic acid, and MPht were compared. The related spectra are given in Figure 2a. As seen from Figure  2a, melamine displays characteristic peaks at 3470, 3421, 3330, 3110, 1625, 1532, 1430, 1180, 1020, 810, and 574 cm −1 . The peaks seen at 3470, 3421, and 3110 cm −1 stem from the typical symmetric stretching vibrations of the NH 2 group. The peak observed at 3330 cm −1 arises from the asymmetric stretching vibrations of the NH 2 group. The peak seen at 1625 cm −1 is attributed to the NH 2 group deformation. The other peaks observed at 1532, 1430, 1180, 1020, 810, and 574 cm −1 are attributed to the stretching and bending vibrations occurring in the triazine ring. 37,40 Phytic acid has characteristic peaks appearing at 3340, 3100, 1650, 1500, and 920 cm −1 . The peak seen at 3340 cm −1 is attributed to the stretching vibration of the −OH group. The peaks seen at 3100 and 1500 cm −1 are caused by C−H stretching and bending vibrations of the CH 3 group, respectively. The peaks observed at 1650, 1100, and 920 cm −1 are caused by the stretching vibrations of O−P−O, C− O−P, and P�O groups, respectively. 41,42 MPht exhibits the characteristic peaks of both melamine and phytic acid, which mask each other. The most notable difference is seen as the disappearance of the peaks seen at 3470 and 3421 cm −1 owing to the protonation of the NH 2 group (NH 3 + ) during the complex formation between melamine and phytic acid. The ATR-FTIR result is consistent with the literature. 34,37,39 The molecular structure of MPht is shown in Figure 3.
TGA and DTGA graphs of MPht are shown in Figure 2b, and the related data are given in Table 1. As seen in Figure 2b, MPht decomposes in four successive steps. In the first step (72°C ), evolution of the physically absorbed water occurs. Melamine salts decompose with two competing pathways of thermal dissociation and decomposition of each component via condensation reactions depending upon the parent acid type. 43 The phosphorus-containing acids favor the condensation of melamine as in the case of phytic acid. 43,44 The second degradation step takes place at 254°C with a shoulder at 280°C . In the second step, the sublimation and condensation of melamine and the dehydration and carbonization of phytic acid occur simultaneously. In the second step, a thermally stable melam structure up to 350°C is formed. Melam further undergoes successive condensation reactions in the third and fourth steps with the formation of melem (stable up to 450°C ) and melon (stable up to 600°C) with the release of NH 3 45 Finally, a thermally stable cyameluric ring formed. The structures of thermally stable condensates are shown in Figure  3. In the fourth step, pyrophosphate-and polyphosphate-based compounds are also formed with the degradation of phytic acid as well. 37,46,47 MPht leaves 37.4 wt % nitrogen and phosphorus-containing residue at 800°C. In order to indicate the residue structure, MPht is heated in an oven at 800°C for 30 min. After the treatment, the color of MPht turns black (Figure 2c). ATR-FTIR spectra of the residue are given in Figure 2c. The residue has characteristics peaks at 2200, 1620, 1412, 1220, and 880 cm −1 . The peak seen at 2200 cm −1 is attributed to the cyanate group. As stated in the literature, the cyameluric ring has characteristic peaks at 1620, 1412, and 800 cm −1 . The peaks seen at 1220 and 880 cm −1 stem from the P�O and P−O−P groups in the pyrophosphate and polyphosphate structures, respectively. 43  displays the TGA and DTGA graphs obtained experimentally as well as the calculated TGA curves. As seen from Figure 4, CF decomposes in two steps with the maximum decomposition rates at 68 and 311°C leaving 21.7 wt % residue based on heterocyclic amines and aromatic structures via crosslinking, cyclization, and aromatization reactions. 48,49 The first step is attributed to the removal of the physically absorbed water, while the second step taking place with successive overlapped degradation steps of keratin just starts from 200°C and ends at about 500°C via cleavage of the disulfide and peptide bonds and the aforementioned char-forming reactions. In the detailed analysis performed by Senoz et al., 48 and Brebu and Spiridon 49 showed that various aqueous and volatile combustible and noncombustible (water, ammonia, and carbon dioxide) decomposition products were formed during the decomposition.
Neat TPU decomposes in a single step, leaving 2.3 wt % carbonaceous residue. With the addition of CF into TPU, the composite degrades in two steps at 322 (shown as a shoulder) and 400°C. The first and second steps stem mainly from the degradation of CF and TPU, respectively. The addition of CF reduces the initial thermal stability (T 5% ) of TPU at about 40°C owing to the lower thermal stability of CF. The residue yield increases from 2.2 to 6.6 wt % due to the solid decomposition products of CF.
MPht-containing composites also undergo two-step degradation. The addition of MPht further reduces the T 5% value of the composites. The maximum degradation temperatures (T max1 and T max2 ) decrease as the added amount of MPht increases due to the formation of phytic acid during the decomposition of MPht. It is concluded that phytic acid accelerates the degradation of CF and TPU. The addition of MPht improves the residue yield as the added amount increases. The experimental residue yields are found to be higher than the calculated ones, indicating the presence of interactions among MPht, CF, and TPU. It is a well-known fact that phosphorus-containing additives promote the char formation in heteroatom-containing polymers including CF and TPU. Similar findings, reduction in thermal stability and enhancement in residue yield, are also observed in the literature with the use of phosphoric acid with CF, 17,19 phytic acid with silk/wool blend, 50 and phytic acid-modified boron nitrides in TPU. 28 Table 2. The performances of the composites are evaluated using time to ignition (TTI), peak heat release rate (pHRR), average HRR (avHRR), total heat evolved (THE)

ACS Omega
http://pubs.acs.org/journal/acsodf Article and THE/TML (total mass loss) ratio, and residue yield. HRR curves of the composites are shown in Figure 5. The digital photographs and SEM images (100× magnification) of the samples remaining after the MLC test are shown in Figure 6. As seen in Figure 5, TPU burns rapidly, giving a sharp HRR peak after ignition, and leaves 1.8% carbon-based residue. With the addition of CF, the HRR curve shifts left due to the reduction in the TTI value. The reduction in the TTI value arises from the lower thermal stability of CF with respect to TPU. Accordingly, the required amount of combustible volatile compounds for ignition reaches in a short time. A similar trend is observed with the use of protein-based fibers in polypropylene 19 and epoxy resin. 51 With the addition of CF, a little amount of char is formed and accumulated on the sides of aluminum foil as seen in Figure 6. Accordingly, the protective function of the char structure is not good, and no significant change is observed in the pHRR value. However, the THE value reduces about 14% due to the improved char formation. No meaningful change is observed with the addition of CF in the THE/TML value that gives information about the gas-phase action of additives.
With the addition of MPht, HRR curves become more plateau-like with prolonged combustion time. The addition of MPht causes reduction in TTI, pHRR, avHRR, THE, and THE/TML values. The lower TTI value with respect to TPU/ CF arises from reduction in thermal stability as shown in TGA analysis in detail. The pHRR value reduces about 60, 66, and 69% with the addition of 5, 10, and 20 wt % MPht, respectively. The reduction in the pHRR value stems mainly from the decrease in the fuel source with improved char formation and the barrier effect of the compact char structure with a remarkable intumescent character, as seen in Figure 7. The THE value reduces steadily at about 13, 20, and 28% with increasing MPht amount. The reduction in the THE value is mainly attributed to the reduction in the fuel source. The reduction in the fuel source arises from three factors of the reduced TPU content with increasing MPht amount, enhanced char formation, and incomplete pyrolysis due to the barrier effect of the foamed char structure. In order to indicate charforming interactions and incomplete pyrolysis, ATR-FTIR analysis is performed for the residues. The related spectra are shown in Figure 7. As seen from Figure 7, all residues have the same characteristic peaks. The characteristic peaks seen in heated MPht at 800°C (see Figure 2) are also observed in the residues at 2350, 1590, 1400 1240, and 890 cm −1 . It is concluded that a thermally stable cyameluric ring is also

Tensile Properties.
Tensile testing is carried out to understand the mechanical properties of the composites. The representative stress−strain curves of the composites are depicted in Figure 9. As seen in Figure 9, TPU has a tensile strength of 25.5 ± 1.5 N/mm 2 and percentage strain of 525 ± 35 with strain hardening character. Reductions in tensile strength and percentage strain are observed with the addition of CF. However, the composite still has strain hardening character. Tensile strength and percentage strain are reduced at about 22 and 50%, respectively. With increasing amount of MPht, the gradual reductions in tensile strength and percentage strain are observed. It is thought that the formation and the growth of the pores around filler particles (barbs, large quill fragments, and MPht) give rise to premature failure of TPU. The strain hardening character diminishes when the added amount of MPht reaches 10 wt %. The composite containing 20 wt % MPht fails in a brittle manner. Tensile strength is reduced at about 23, 29, and 38% with respect to only CF-containing composites with the addition of 5, 10, and 20 wt % MPht, respectively. Tensile fracture surfaces of the composites give valuable information related with the observed trend. The photographs of the tested tensile specimens and the tensile-fractured surfaces of the composites with a magnification of ×100 are shown in Figure 10. As seen from Figure 10, TPU undergoes plastic deformation, and stress-induced whitening is observed up to 10 wt % MPht addition. Embedded and debonded barbs (seen as circular small holes) and quills (seen as large holes) are observed on the fracture surfaces. The large quill fragments and debonding due to the weak interfacial adhesion between CF and TPU cause reduction in tensile strength.

CONCLUSIONS
In the current study, the synthesized MPht was used as a biobased flame-retardant additive in CF-containing TPU composites. The effect of the MPht amount on the thermal and flame retardant characteristics of the composites was examined using TGA, LOI, UL 94V, and MLC tests. According to TGA analysis, the addition of CF and MPht reduced the thermal stability of TPU. MPht favored the char formation, and the residue yield increased as the added amount increased. According to the flammability test results, the LOI  value steadily increased as the added amount of MPht increased. The V2 rating in the UL 94 V test was obtained in 5 and 10 wt % MPht-containing composites. The highest UL-94 V rating of V0 was achieved with the addition 20 wt % MPht. According to the MLC test results, the addition of MPht improved the fire retardant performance of the composite as the added amount increased with low pHRR, THE, and THE/TML values. In brief, MPht was highly effective in CF-containing TPU composites. It had a dual flame retardant effect observed in the condensed and gas phases via a heat sink, enhanced intumescent char formation, and flame dilution. According to tensile test results, the addition of CF and MPht gives rise to reduction in tensile strength and percentage strain.