Keratinous Fiber Based Intumescent Flame Retardant with

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Keratinous fiber based intumescent flame retardant with controllable functional compound loading Daeseung Jung, and Debes Bhattacharyya ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02756 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 7, 2018

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ACS Sustainable Chemistry & Engineering

Keratinous fiber based intumescent flame retardant with controllable functional compound loading Daeseung Jung* and Debes Bhattacharyya Centre for Advanced Composite Materials, Department of Mechanical Engineering, The University of Auckland, Private Bag 92019, Auckland, New Zealand

Keywords: Keratinous fibers, Intumescent flame retardants, Phosphoric acid, Amine phosphate, Polypropylene composites

---------------------------------------------------------------------------------------------------------------*Corresponding author: Tel: +64 9 9231255 Email address: [email protected] ----------------------------------------------------------------------------------------------------------------

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Abstract: An intumescent flame retardant working with various combinations of acid source, blowing agent and char former promises high-performance and low-toxicity. However, the research for more potent functional constituents and their best combination is still extremely important. Here we report on a novel way to use keratinous fibers as the host material for creating an effective flame retardant. Simple solution-based treatment to implant amine phosphate and phosphoric acid in the fiber through sequential monomer infiltration is found to be significantly effective for flame retardancy and reducing the flammability of polymeric materials. After the flame retardant fiber modification, polypropylene (PP) shows significantly improved flame retardancy to achieve V-0 grade and >70% reduced peak heat release rate in vertical burning and cone-calorimeter tests, respectively. We expect this strategy of converting the low-grade keratinous fiber to valuable flame retardant material to become a novel and attractive solution for achieving fire-safety, value addition and eco-friendly recycling goals at the same time.

Introduction Various types of flame retardants and different fire extinguishing mechanisms have been applied to reduce the high flammability of polymeric materials during the last few decades. However, researchers are still looking for more effective flame retardants to satisfy the increasing concerns for health and safety as well as better performance expectation.1-4 Conventionally, halogenated flame retardants have been widely used for commercial polymer owing to its high performance and low price. Nevertheless, the fundamental problems of the halogenated compounds, such as toxic gas evolution during combustion5 and potential endocrine disruption for human body6, have gradually forced this type of material out of the market.3, 5 While halogen-free flame


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retardants are becoming new alternatives, intumescent flame retardants have attracted increasing attention due to its tunable flame retardant properties from various combinations of the functional constituents.7,8 Upon flame exposure, the intumescent flame retardant (IFR) instantly start to produce a carbonaceous layer to stop further fire propagation. At the time, viscous carbonaceous liquid created from melted char former and the polymer matrix is converted to an expanded solid char by a blowing agent and acid source. Therefore, the main emphasis of the IFR research has been on finding the best parametric combination for better char formation.9 For high-performance IFR additive for polymeric materials, various approaches have been applied. Most practically, ammonium polyphosphate (APP) modification has been conducted by physical and chemical methods to complement its char forming property and low compatibility with polymer matrix.10,11 On the other hand, phosphorus compound modified nano-materials have been aggressively investigated because of the biggest advantage for improved flame retardancy with low filler contents.12,13 An utilization of natural resource as an IFR constituent also offers a good possibility for developing a flame retardant more compatible with human life.14,15 The similarity shown in the aforementioned strategies is that they aim to contain all types of IFR constituents (i.e. acid source, blowing agent and char former) in a single material. Ultimately, the content control of each constituent is essential for the optimized flame retardancy. However, any single molecular flame retardant developed so far cannot control the percentage of each constituent independently because the contents of the constituents are strongly dependent on the molecular structure of the final product.9 Not only that, the surface modification of nanomaterial has a limitation to provide a wide range of the content control because certain amount of IFR constituent is necessary for achieving practical flame retardant criteria. The flame retardant for bulk polymer composites requires a higher loading than that for coated one because it is diluted


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by the polymer matrix.8 Therefore, it is particularly important to understand the effect of each constituent on the composite’s flammability in a quantitative manner and explore how to control the percentages of the constituents in the produced composite. From this point of view, we suggest a new method to implant IFR constituents in a keratinous fiber by sequential monomer infiltration along with simultaneous amine phosphate layer formation. In this research, the fiber plays a double role in the final product as a container to include various chemical substances and good IFR constituent on its own. Originally, sulphur-engaged cyclic amine and phenolic compound generated during keratinous fiber decomposition provide the fiber with a distinguishable flame resistance characteristic.16,17 In the previous research conducted by Ingham18, phosphoric acid treated wool fiber (PA wool) showed potential to be manufactured as a flame retardant fabric even with a small amount of phosphoric acid. Furthermore, the possible application of wool fiber alone in the APP/PP system as a flame retardant additive successfully replaced a certain amount of APP in the composite to pass vertical burning test requirements.15 However, in spite of the inherent advantages of the keratinous fiber, the usage of low-grade wool fiber is decreasing continually and billions of kilograms of feathers are wasted every year without recycling.19 The utilization of the cheap natural resource becomes not only a useful means for eco-friendly recycling but also an opportunity to provide cost-effectiveness for composite preparation.20 Under the given circumstances, here we have investigated the adequacy of the chemical substance implanted keratinous fiber as a flame retardant additive for PP using coarse wool fiber and chicken feather as the starting materials. Experimental section Materials. Scoured coarse wool fibers and chicken feathers were provided by Bloch & Behrens Ltd., New Zealand and Wallace Group Ltd., New Zealand respectively. Phosphoric acid (conc.:


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85 wt% in H2O), ethylenediamine and toluene were obtained from Sigma-Aldrich and used without any further purification. PP (K515, MFI: 19) was purchased from A. Schulman, Inc. Maleic anhydride modified PP (Licocene PP MA 6452) was provided by Clariant Ltd., New Zealand to be used as a compatibilizer. Commercial intumescent ammonium polyphosphate flame retardant (Exolit AP 766) was obtained from Clariant Ltd., New Zealand. Chemical treatment of keratinous fibers. Chopped keratinous fibers (mean fiber length: 2.4 mm) using a mechanical granulator (GR2020 granulator, MORETTO S.P.A, Italy) were immersed in various concentrations of phosphoric acid (PA) solution (solvent for the wool fiber treatment: water, for the chicken feather: ethanol) for 10min at room temperature; then they were vacuum filtered and dried at 35 °C in vacuum oven for 20 h. The PA wool was dried again at 110 °C until reaching a constant weight to completely remove the solvent and measure the weight difference between before and after the treatment. 35 g of the PA treated keratinous fibers was slowly immersed in ethylenediamine (EDA)/toluene (20 g/300 ml for wool fiber, 12 g/300 ml for chicken feather) solution keeping the solution temperature under 50 °C. Then, the solution temperature was maintained at 80 °C for 30 min to prepare ethylenediamine phosphate (EDAP) and PA implanted keratinous fibers. The resultant precipitates were vacuum filtered and dried at 110 °C until reaching a constant weight to completely remove the residual EDA. As a comparison, in-house EDAP was prepared according to the method of Kruger et al..27 Briefly, 8.07 g of phosphoric acid was added dropwise to 4.21 g of ethylenediamine with continuous stirring in an ice bath. The precipitated crystals were filtered and washed once with cold water and acetone subsequently. The final product was dried at 110 °C until reaching constant weight. Fabrication of treated keratinous fiber/ PP composite. The chemical treated keratinous fibers were melt-blended with PP in the presence of 5 wt% of maleic anhydride grafted PP (MA-g-PP)


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at 170 °C for 3 min at a speed of 70 rpm with an internal mixer (W 50 EHT, Brabender GmbH & Co, Germany) to obtain an 40% treated keratinous fiber/PP composite. The obtained composites were pelletized and then hot pressed into sheets at 175 °C for further characterization. Materials Characterization. Fourier transform infrared spectroscopy (FT-IR, Nicolet 6700 spectrometer, Thermo Electron Corp., USA) was used to investigate the surface functional groups of the modified keratinous fiber. The thermal property of untreated and treated fiber was measured with a thermogravimetric analyzer (TGA, Q5000, TA Instruments, USA). Approximately 5 mg of each sample was tested in an air atmosphere, from room temperature to 850 °C, with a heating rate of 10 °C min– 1. The morphology observation and elemental analysis of the fibers were carried out by field emission environmental scanning electron microscope (SEM, Quanta 200, FEI, USA) with energy-dispersive X-ray spectroscopy (EDS, EDAX Pegasus EDS detector, AMETEK. INC., USA). Phosphorus content of chemically treated fiber was measured with an elemental analyzer (Thermo Flash 2000, Thermo Scientific, USA). Flammability test of treated keratinous fiber/PP composite. Vertical burning test was performed according to ASTM D3801-10 (equivalent to UL-94 standard). Samples of 125 mm×13 mm×2.4 mm dimensions were prepared and preconditioned under 23 °C and 50 % relative humidity for 48 h. The tests were repeated five times for each sample and the average values were considered. The test results were classified into different grades, V-0, V-1, V-2 or no rating (NR). In addition, cone calorimeter (Fire Test Technology, East Grinstead, UK) was used to measure the fire behavior with constant heat exposure according to ASTM E1354-11. Samples of 100 mm×100 mm×2.4 mm were tested in a horizontal position with an external heat flux of 50 kW/m2. Aluminum foil was used for the sample holder preparation and samples were preconditioned at 23 °C and 50 % relative humidity for 48 h.


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Results and Discussion Figure 1 shows the general design concept of chemically treated keratinous fiber and its modification process of the coarse wool fiber as an example of typical keratinous fiber. The modification process is composed of two steps - in the first step, wool fiber was treated using aqueous phosphoric acid solution to infuse the phosphoric acid into the fiber. The absorbed phosphoric acid (PA) plays two different roles: i) it provides phosphate anion allowing a reactive amine to produce an amine phosphate in the following step, and ii) the remaining PA after the amine phosphate production acts as an acid source to accelerate the char formation upon flame exposure.

Figure 1. Schematic diagram of fabrication of FR wool and SEM images of A) untreated wool fiber surface, B) FR wool surface and C) SEM assisted EDS map image of FR wool crosssection.


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For the second step, following the PA treatment, ethylenediamine (EDA) infiltration process makes an ethylenediamine phosphate (EDAP) layer with a decreasing concentration gradient from the fiber surface to the core. Various kinds of reactive amines are available for the second step, such as EDA, melamine or ammonia as these chemicals easily combine with PA by an ionic bond. Among the reactive amines, EDA was chosen for this research because of its higher activity and the ease of control. After the two-step modification, the surface of the wool fiber was completely changed (Figure 1A and 1B). Small crystalline particles of EDAP (confirmed by FT-IR) cover the surface of the wool fiber after the modification. For further investigation to confirm the presence of phosphorus in the fiber after the EDA treatment, SEM assisted energydispersive X-ray spectroscopy (EDS) mapping technique was used in the fiber cross-section observation. The cross-section image of chemically treated wool fiber (FR wool), depicted in Figure 1C, clearly shows the phosphorus distribution inside the fiber with a higher concentration at the outer periphery of the fiber. This may be due to the stronger affinity between the PA and EDA. In addition, the EDS element analysis on a larger sample surface provides general information about the significantly changed phosphorus contents of the fiber after the modification (Figure S1). The aforementioned results imply that the contents of phosphorus and nitrogen of the FR wool fiber can be controlled separately by the initial concentrations of PA and EDA solution during the modification process. The excellent acid resistance of wool fiber21 is suitable as the container for the IFR constituents implantation and the amine phosphate shell on the fiber enhances its application as an IFR additive from a practical aspect. Furthermore, successful chemical treatment (Figure S3) and the following PP modification using chicken feathers confirm that other types of keratinous fibers are also suitable for this process. In the same line of view, the physical properties of FR wool are also affected by the concentrations of


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the modifiers. Figure S2 shows the ground FR wool (which may also be advantageous for certain applications) after high-concentration EDA treatment. In order to limit the effects of highly concentrated reactive amines, PA treatment could be conducted as the first step because the keratinous fiber is much stronger against acid than against alkali.22 Fast EDAP layer formation at the early stage of EDA treatment can control the adverse structural changes and the fiber properties. Figure S6 shows the mechanical properties of FR fiber/PP composites. The tensile modulus of every composite appears to be higher than that of neat PP but the strength decreases in all the cases. However, the FR wool/PP composite has a comparable tensile strength to that of the APP/PP composite in spite of the fact that the best filler content of the composite was yet to be properly determined. This raises the possibility of further strength improvement of FR fiber/PP composite. In order to investigate the surface functional groups and thermal property of the wool fiber before and after the treatment, FT-IR and TGA analyses were carried out. The FTIR spectra (Figure 2A) confirm that the EDAP was successfully deposited on the wool fiber surface. After the PA treatment, newly appeared P=O and P-O peaks at 1129 cm-1 and 990 cm-1, respectively, which is in agreement with the values reported in the literature29, showed the absorption of PA onto the wool fiber. The strong affinity between the amide groups in a wool fiber and PA allows the fiber to hold the PA by a weak molecular interaction.23,24 After the EDA treatment of the PA wool, the P-O peak shifted to 1014 cm-1 and new peaks appeared at 30002450 cm-1 ascribed to ethylenediamine cation (-CH2-CH2- and -NH3+)28, indicating the formation of EDAP.


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Figure 2. A) FT-IR and B) TGA curves of untreated wool fiber, PA wool, FR wool and EDAP. C) Digital photo images of the wool fibers before and after the treatment. Furthermore, the complete agreement between FR wool and EDAP (in-house EDAP prepared from 1:1=EDA/PA molar ratio of PA and EDA solution) from the FT-IR curves clearly confirms the EDAP layer formation on the FR wool surface, which also agrees with the morphological change observed by SEM (Figure 1A and 1B). The EDAP and PA in the fiber (Figure 1B and 1C) impart the significantly different thermal property to the wool fiber. The TGA curves of untreated wool fiber, PA wool, FR wool and EDAP obtained in the air atmosphere are demonstrated in Figure 2B. Although the information from the thermal analysis, such as initial char formation, thermal oxidation and moisture absorption of the fibers, cannot reflect the flame


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retardant property of the fibers directly, they provide a useful understanding of the temperaturedependent intumescent behavior.9 The onset temperature of decomposition was defined as the point of 5% weight reduction (T5%) based on DTG curves (Figure S4). However, the range of 30~120 °C was not included in the analysis to eliminate the effect of moisture in the fiber. The in-house EDAP start to decompose at 224 °C which is very similar to that of commercial material31 and its TGA curve completely agrees with the result of a previous report.27 In spite of the two-stage char formation initiated at 255 °C and 465 °C, untreated wool fiber completely decomposed below 650 °C because the resultant chars were too week to survive at a temperature above 650 °C. After the PA treatment, the onset temperature of the first decomposition decreases from 255 °C to 214 °C because the absorbed PA accelerates the decomposition of the fiber.18 A rapid char formation follows between 214 and 420 °C and a further decomposition ensued at a considerably reduced decomposition rate, which resulted in 3.2 wt% residue at 850 °C. After the EDA treatment, the initial decomposition temperature of FR wool increased from 214 °C to 221 °C along with a further reduced decomposition rate, which made the final char residue increase to 14.8 wt%. The results show that the combination of EDAP and PA is more efficient than PA alone to prevent the thermal oxidation of the fiber. The aforementioned results suggest that the EDAP layer on the fiber not only provides the FR wool with a better thermal stability compared to PA wool over the functional temperature range but also enhances the thermal oxidation resistance in harmony with the implanted PA. In addition, the implanted EDAP also influence the moisture absorption property of the fiber due to their hydrophobicity. It is confirmed by comparing the weight loss at 30-120 °C ascribed to moisture desorption.25 While the PA wool reported about 8 wt% loss in that temperature range, the FR wool showed less than 2 wt% loss compared to that of the PA wool in spite of the equal phosphorus contents. A higher moisture


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content of the FR wool than that of EDAP shows that the moisture absorption property is still dependent on the percentages of PA, EDAP and wool in the fiber, in spite of the EDAP layer covering the outside surface of the FR wool. The images of the dried samples (Figure 2C) show the color change of the wool fiber (Figure 2C-left) to reddish yellow after the PA treatment (Figure 2C-centre) due to the thermal degradation. However, after the EDA treatment (Figure 2C-right), the color change is reversed back to white yellow similar to the original wool fiber because of the coated EDAP outside the wool fiber.

Figure 3. Surface image of A) untreated wool fiber. Plots of the content of B) absorbed PA in PA wool and C) combined EDA in FR wool (using 32% PA treated PA wool) as a function of the concentration. Surface images of D) 32% PA treated wool fiber, E) 32% PA and 3.6 EDA/PA molar ratio treated wool fiber and F) 32% PA and 18.8 EDA/PA molar ratio treated wool fiber.


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Figure 3 shows the surface images observed by SEM as well as plots of infused PA and EDA contents in the wool fiber as a function of PA and EDA concentrations. After the PA treatment in 32% PA aqueous solution, the shape of the wool fiber was changed from the untreated state (Figure 3A) to be smoother (Figure 3D). In spite of the high PA concentration, PA wool still maintained its shape which meant that the structural degradation after PA treatment was not severe. From the earlier literature, it is well known that weak acid easily combines with wool fiber via weak molecular interaction as well as the diameter increase of the wool fiber.21,24 The tendency has been confirmed in our experiment: the weight of absorbed PA in the fiber drastically increases proportionally to phosphoric acid concentrations (Figure 3B). The PA absorption rate increases significantly at a lower PA concentration but gradually decreases as the PA concentration increases because the active site of wool fiber is saturated by the excess amount of PA. Among various factors affecting the PA absorption, the concentration of PA solution dominantly affects the PA absorption by the wool fiber rather than treatment temperature or time. Furthermore, in order to avoid the severe structural damage of the fiber, mild process condition appears to be more desirable during the PA infiltration. After the 32% PA solution treatment, the treated wool fiber reported 43% weight increase which meant that 43% of PA was absorbed into the fiber. This assumption was based on minimal weight loss of the wool fiber during the treatment confirmed by the SEM image (Figure 3D) and the elemental analysis result (Figure S5). However, EDA treatment on the PA wool makes a significant change in fiber property due to the strong alkalinity of EDA. Although the EDAP layer developing on the outside of the fiber (Figure 3E and 3F) relieves the influence of alkalinity, increasing EDA concentration can reduce the fiber flexibility and eventually, makes the fiber brittle to be pulverized (Figure S2). After the EDA treatment, the core of the fiber shows significantly


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different morphology (Figure 3E and 3F) which appears to be more brittle compared to that of PA wool (Figure 3D). According to Harris,22 alkali destroys the disulphide linkage that binds the protein constituents in a wool fiber and hence reduces the strength of the fiber. This explains the reason for FR wool becoming brittle. The pulverized FR wool could be useful to achieve better distribution in the polymer matrix for improved flame retardancy but it may also reduce the overall composite strength due to fiber damage and reduced aspect ratio. Prolonged reaction at a higher concentration of EDA might be necessary for a complete conversion but it is often not desirable in order to avoid the adverse effects of strong alkali. The phosphorus content in the FR fiber was measured by the elemental analysis of the samples whose PA contents were known. It confirmed that there was still the same amount of phosphoric acid left in the final product (Figure S5). The difference between the calculated P content based on weight change after each treatment and measured P content by elemental analysis was less than 0.5 % for both wool fiber and chicken feather. This means that it is reasonable to calculate the EDAP and PA content on the basis of weight change during the process. Therefore, if the EDA solution with 3.6 EDA/PA molar ratio were used for 32% PA treated PA wool, the resulting FR wool would contain 38% EDAP and 13% PA. The various combinations of each IFR constituent using the aforementioned example can provide a good opportunity for improving the flame retardant property of a composite material while maintaining the mechanical properties within the acceptable range. In order to evaluate the improved flame retardancy performance of the PP after adding chemically treated keratinous fibers, vertical burning and cone calorimeter tests were conducted. The effects of the IFR constituent percentages in the treated fiber on the flammability of the composite product were investigated at the same time. For this purpose, three types of treated fiber were prepared with coarse wool fiber and chicken feather containing the same amount of EDAP but


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different PA contents. Subsequently, each fiber was melt-blended with PP in order to prepare 40% chemically treated fiber/PP composites.

Figure 4. Captured images during the vertical burning test with A) pure PP, B) 40% wool/PP, C) 40% FR wool L-PA/PP, D) 40% FR wool H-PA/PP, E) 40% FR CF H-PA/PP and F) 20% APP/PP composite. G) Heat release rate curves of pure PP and modified PP composites. Char image of H) 40% FR wool L-PA/PP, I) 40% FR wool H-PA/PP and J) 20% APP/PP composite after the cone-calorimeter test.


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The first composite (Figure 4C) denoted as 40% FR wool L-PA/PP, contained 14% EDAP and 26% wool fiber with

Keratinous Fiber Based Intumescent Flame Retardant with

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