Bio-based nickel alginate and copper alginate films with excellent flame retardancy: preparation, flammability and thermal degradation behavior

Abstract

A bio-based nickel alginate film and copper alginate film were prepared via a facile ion exchange and casting approach. Their flame retardancy, thermal degradation and pyrolysis behavior, and thermal degradation mechanism were investigated systematically by the limiting oxygen index (LOI), vertical burning (UL-94), microscale combustion calorimetry (MCC), thermogravimetric analysis (TGA), thermogravimetric analyzer coupled with Fourier transform infrared analysis (TG-FTIR) and pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS). It was shown that the nickel alginate film had a much higher LOI value (50.0%) than those of the sodium alginate film (24.5%) and copper alginate film (23.0%). Moreover, the nickel alginate film passed the UL-94 V-0 rating, while the sodium alginate film and copper alginate film showed no classification. Importantly, the peak of heat release rate (PHRR) of the nickel alginate film in the MCC test was much lower than those of the copper alginate film and sodium alginate film. This indicated that the introduction of nickel ions decreased the release of combustible gases. TGA results showed that the addition of copper ions and nickel ions accelerated the thermal degradation of alginates and changed the thermal degradation mechanism of the alginates. TG-FTIR and Py-GC-MS results indicated that the pyrolysis of copper alginate and nickel alginate produced much less flammable products than that of sodium alginate in the whole thermal degradation process. Finally, a possible degradation mechanism for copper alginate and nickel alginate was proposed. The results of our study provide useful information for understanding the flame retardancy mechanism of alginate as well as for designing bio-based materials with excellent fire retardancy.

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Introduction

With the increasing trend for more stringent fire safety laws and regulations, demands for the decrease of fire hazards caused by highly combustible materials have obtained importance in past decades.¹ To reduce the flammability of highly combustible materials, a suitable flame retardant treatment can retard the ignition of these materials and decrease the flame spread, resulting in obviating fire hazards, loss of life and destruction of properties.^1–3

The energy crisis and increasing global warming have forced human beings to hunt for novel types of materials instead of petroleum-based materials.⁴ Biomass, which is a series of natural materials, is more sustainable than fossil-based materials, and use of it can decrease the emission of CO₂.⁵ In recent decades, the development of alginate has gained interest as an alternative source of biomass and a potential spinning feedstock.⁶ Alginate, a kind of biomass derived from algae,^7,8 is a polyelectrolyte and a linear copolymer composed of β-1,4-D-mannuronate (M) and α-1,4-L-guluronate (G) repeating monomeric units. Sodium alginate is soluble in water, forming very viscous solutions or jellies depending on the concentration.⁹ Gelation of alginate may take place by the interaction between carboxylate groups, hydroxyl groups and divalent or trivalent metal ions in aqueous solutions, forming strong, rigid and ordered structures, named hydrogels.^10,11 Because of its biocompatibility, relatively low price and abundance, alginate has many industrial uses in the food industry, textile printing industry, paper industry, wound dressings and drug formulation.^12,13

Due to its water solubility, sodium alginate is a desirable candidate for aqueous processing,¹⁴ and alginate fibers have been produced through a wet-spinning process.¹³ Calcium alginate fiber, copper alginate fiber, barium alginate fiber and zinc alginate fiber have better flame retardant properties and are inherently flame retardant materials.^15–18 The effects of these divalent metal ions on the flame retardant properties, thermal degradation properties and pyrolysis properties of alginate fibers were investigated by Xia et al.^15,16 and Zhu.¹⁸ The results indicated that the divalent metal ions improved the flame retardant properties of the alginate fibers, and decreased the gaseous products in the alginate pyrolysis process. However, they did not investigate the pyrolysis mechanism of other divalent metal alginates in detail, such as copper alginate and nickel alginate. In our previous work,^19–21 the flame retardancy, thermal degradation properties and pyrolysis properties of zinc alginate film, aluminum alginate film and iron alginate film were studied. The results indicated that the addition of zinc ions, aluminum ions and iron improved the flame retardancy, and that changed the thermal degradation process of alginate. Interest in the flame retardant properties and flame retardant mechanism of novel alginate materials is growing and leading to the development of alginate materials.

In the present work, the bio-based nickel alginate film and copper alginate film were prepared via a facile ion exchange and casting approach. Meanwhile, the flame retardancy, thermal degradation properties, thermal oxidative degradation properties and pyrolysis properties of these modified alginates were investigated by the limiting oxygen index (LOI), vertical burning (UL-94), microscale combustion calorimetry (MCC), thermogravimetric analysis (TGA), thermogravimetric analyzer coupled with Fourier transform infrared analysis (TG-FTIR) and pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS). The degradation mechanism of copper alginate and nickel alginate were speculated on according to the results of the MCC, TG-FTIR-MS and Py-GC-MS tests. Understanding the effects of copper and nickel ions on the flame retardant properties, thermal stabilities and pyrolysis properties would be useful to clarify the flame retardant mechanism of alginates. And these highly flame retardant alginate-based materials may be used in the textile industry in the future.

Experimental section

Materials

Sodium alginate powder (M_n = 357 475, M_n/M_w = 1.392, M/G = 0.32) was supplied by Qingdao Mingyue Co. (Qingdao, China), and used as received. Nickel chloride and copper chloride were purchased from Sinopharm Chemical Reagent Co., Ltd (Wuhan, China), and were analytical grade reagents.

Preparation of alginate films

5 wt% sodium alginate aqueous solutions were prepared in a beaker with vigorous stirring at room temperature for about 4 h. The obtained solution was cast into a sheet on a glass plate and air-dried at room temperature for over 3 days to obtain a film. Then, the film was removed from the glass plate and was immersed in 5 mol L⁻¹ nickel chloride aqueous solution for 2 h. After coagulation, the film was washed with deionized water to remove the unreacted nickel chloride. Then the film was subjected to vacuum drying for 24 h before measurements. The thickness of the film was about 0.75 mm. Scheme 1 presents the preparation process for the nickel alginate film and copper alginate film. The preparation of the copper alginate film was done by the same process. The content of nickel ions on the alginate film was 12.95 wt%, and the content of copper ions on the alginate film was 15.32 wt%, which were obtained from the ICP test.

Scheme 1 Schematic illustration of the preparation and characterization of copper alginate (A) and nickel alginate (B) films.

The preparation process and energy dispersive X-ray spectroscopy (EDX) characterization of the copper alginate and nickel alginate films are shown in Scheme 1. The results of EDX indicated that the copper ions and nickel ions were dispersed uniformly in the film, and that the copper alginate and nickel alginate films were prepared successfully through the facile ion exchange and casting approach.

Measurements

Limiting oxygen index (LOI)

An XZT-100A oxygen index meter (Chengde, China) was utilized to measure the LOI value of the samples, and the sheet dimensions of the samples were 130 mm × 6.5 mm × 0.75 mm according to ASTM D2863-97. Each sample was run in three replicates.

Vertical burning (UL-94)

A vertical burning test instrument (CZF-2-type) (Jiangning, China) was used to investigate the vertical burning ratings, and the sheet dimensions of the samples were 130 mm × 13 mm × 0.75 mm according to ASTM D3801. Each sample was run in five replicates.

Thermogravimetric analysis (TGA)

A SDTQ600 (TA Instruments Co., USA) thermogravimetric analyzer was exploited to perform thermogravimetric analysis (TGA) of the samples with a heating rate of 10 °C min⁻¹. 4–6 mg of the sample was examined at a flow rate of 50 mL min⁻¹ from room temperature to 700 °C under air and N₂. Each sample was run in three replicates.

Microscale combustion calorimetry (MCC)

A microscale combustion colorimeter (Fire Testing Technology, UK) was utilized to study the flammability of the nickel alginate film, copper alginate film and sodium alginate film. 5 ± 0.05 mg of the sample was heated to 700 °C with a heating rate of 1 °C s⁻¹ in nitrogen, and the flow rate of N₂ was set to 80 mL min⁻¹. The volatiles, anaerobic thermal degradation products in the nitrogen gas stream, were mixed with pure oxygen with a flow rate of 20 mL min⁻¹ prior to entering a 900 °C combustion furnace. Each sample was run in three replicates.

Thermogravimetric analyzer coupled with Fourier transform infrared analysis (TG-FTIR)

The TG-FTIR measurements were evaluated by a TG analyzer (Perkin-Elmer STA 6000), which was coupled with a Fourier transform infrared spectrometer (Perkin-Elmer Frontier FTIR) outfitted with a gas cell. Gaseous products released from the TG analyzer were directly collected in the gas cell and analyzed immediately by FTIR spectrometry. About 10 mg of the sample was heated from room temperature to 700 °C with a heating rate of 20 °C min⁻¹ under a N₂ atmosphere, and the flow rate of N₂ was set to 50 mL min⁻¹. FTIR spectra of the gaseous products were acquired continuously with the baseline amended. The stainless steel pipe and the gas cell were preheated to 280 °C, in order to minimize secondary reactions and tar condensation. The resolution factor of the FTIR spectrometer was 2 cm⁻¹ and the spectral range was from 4000 to 450 cm⁻¹.

Pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS)

Analytical pyrolysis was carried out using a CDS Pyroprobe 5200 pyrolyser coupled with a GC-MS (Perkin Elmer Clarus 680GC-SQ8MS) system, and helium was used as the carrier gas. The pyrolysis chamber was filled with He and about 300 μg of the sample was loaded in the pyrolysis tube. The pyrolysis temperature of the furnace was 700 °C with a flash heating rate of 20 °C ms⁻¹, and the residence time for the sample in the furnace was 15 s, to make sure that most of the solid sample was pyrolyzed. The evolved volatiles were identified by GC-MS, with the conditions given as: the injector temperature was kept at 280 °C; the chromatographic separation was performed with a HP-5 capillary column (0.25 mm); the temperature of the chromatographic column was progressively increased from 40 °C (3 min) to 280 °C (5 min) with a heating rate of 10 °C min⁻¹; the mass spectra were obtained in electron ionization (EI) mode at 70 eV. With reference to the database of the NIST library, the yield of the compounds was determined by the characterized GC-MS spectra.

Results and discussion

Flammability of the nickel alginate film and copper alginate film

LOI and UL-94. To investigate the flame retardancy of the nickel alginate film and copper alginate film, the LOI and UL-94 tests were utilized, and the relevant results are presented in Table 1. The LOI value of the nickel alginate film was 50.0, and it passed the V-0 rating in the UL-94 test, indicating that nickel alginate is an inherent flame retardant material without any addition of other flame retardants. However, the copper alginate film and sodium alginate film had low LOI values (23.0 and 24.5%, respectively) and no rating in the UL-94 test. These results indicate that the introduction of nickel ions into the alginate obviously improves the flame retardancy of the alginate. It can be seen from Table 1 that the LOI value of the copper alginate film is lower than that of the sodium alginate film. It was found that the copper alginate film smoldered without flame, while this phenomenon was not found for the nickel alginate film. Maybe this is the reason for the different flame retardancy properties of the nickel alginate film and copper alginate film. The reason for the different flame retardancy properties of the nickel alginate film and copper alginate film will be discussed in detail in the last section.

Table 1 Results of the LOI, UL-94 and MCC tests

Sample	LOI (%)	UL-94	PHRR1 (W g^−1)	TPHRR1 (°C)	PHRR2 (W g^−1)	TPHRR2 (°C)	Residues (%)
a Note: N.R. stands for no rating.
Sodium alginate	24.5	N.R.^a	105.0	230	31.4	453	47.4
Nickel alginate	50.0	V-0	12.1	265	18.1	447	31.8
Copper alginate	23.0	N.R.^a	41.1	221	—	—	35.0

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MCC. Microscale combustion calorimetry (MCC) is a pyrolysis-combustion flow calorimetry method, and has the dynamic capability to measure HRR and other related parameters, such as total heat release (THR) and heat release capacity (HRC); a few milligrams of samples are used, avoiding the sample preparation problems encountered for the cone calorimetry test.^22,23 HRR curves of nickel alginate, copper alginate and sodium alginate as a function of temperature are presented in Fig. 1, and the relevant data are listed in Table 1. From Fig. 1, nickel alginate and sodium alginate present HRR curves with two peaks; however, copper alginate shows a HRR curve with only one peak. The first sharp PHRR of sodium alginate appeared with a PHRR value of 105.0 W g⁻¹ at 230 °C, and the second one appeared with a PHRR value of 31.4 W g⁻¹ at 453 °C, which is much lower than the first one. It was noted that the results showed a distinct characteristic brought on by the addition of nickel ions. There was an obvious decrease in the first and second PHRRs of nickel alginate; the first PHRR value of nickel alginate was 12.1 W g⁻¹ at 265 °C, and the second one was 18.1 W g⁻¹ at 447 °C. For copper alginate, the PHRR was 41.1 W g⁻¹ at 221 °C, which is higher than that of nickel alginate. From Fig. 1 and Table 1, the first PHRR value was greatly reduced by the addition of nickel ions, from 105.0 to 12.1 W g⁻¹; however, the first PHRR value decreased to 41.1 W g⁻¹ by the addition of copper ions. This suggests that the addition of nickel ions might induce alginate to form residues, reducing the release of flammable gases, at lower temperatures; or the addition of nickel ions might induce alginate to produce larger amounts of non-flammable gases, such as CO₂ and H₂O, at a lower temperature. According to the results of the LOI and UL-94 tests, the flame retardancy of nickel alginate was better than that of copper alginate, while the HRR values of copper alginate were lower than those of nickel alginate at a higher temperature range. These results suggest that alginate possesses better flame retardancy if the addition of the metal ions obviously decreases the first PHRR value of sodium alginate.

Fig. 1 HRR curves of nickel alginate, copper alginate and sodium alginate as a function of temperature.

From Fig. 1, the HRR values of nickel alginate were lower than those of sodium alginate in the temperature range studied, which suggests that the addition of nickel ions increased the flame retardancy of the alginate. From Table 1, the residual amounts of nickel alginate and copper alginate, 31.8 and 35.0%, respectively, were lower than that of sodium alginate, 47.4%, while the flame retardancy of nickel alginate was much better than that of copper alginate and sodium alginate. This suggests that the addition of nickel ions accelerates the degradation of alginate to produce more non-flammable gases, such as CO₂ and H₂O, resulting in the better flame retardancy of nickel alginate.

Thermal degradation behavior of nickel alginate and copper alginate

In order to understand the effects of nickel ions and copper ions on the stability of alginate during its thermolysis, thermogravimetric analysis (TGA) tests of nickel alginate, copper alginate and sodium alginate were carried out at a heating rate of 10 °C min⁻¹ in an air and N₂ atmosphere.

Thermal degradation behavior in N₂

TGA and DTG curves of copper alginate, nickel alginate and sodium alginate in N₂ are shown in Fig. 2 and 3, respectively, and the relevant data are presented in Table 2. From Fig. 2 and 3, it can be observed that copper alginate and nickel alginate had three steps of weight loss, while sodium alginate had only one step of weight loss. The first step was from 133 to 204 °C for copper alginate, and from 156 to 204 °C for nickel alginate. The thermal degradation rates of copper alginate and nickel alginate were much higher than that of sodium alginate in this temperature range. Comparing the TGA and DTG curves of sodium alginate in this temperature range, it was noted that the addition of copper ions and nickel ions accelerated the thermal degradation of the alginates, resulting in the thermal degradation of the alginate at a lower temperature. This may be attributed to a decarboxylation reaction under the catalysis of the copper ions and nickel ions, and the fracture of glycosidic bonds, dehydration, and decarbonylation of the alginate, releasing CO₂, H₂O and other low molecular weight molecules. The maximum rates at the maximum rate degradation temperature (T_max) for copper alginate and nickel alginate, 10.9 and 5.8% min⁻¹, respectively, took place in this temperature range. The second step was from 204 to 319 °C for copper alginate, and from 204 to 350 °C for nickel alginate. The thermal degradation rates of copper alginate and nickel alginate were much lower than that of sodium alginate in this temperature range and sodium alginate started to degrade from 202 to 289 °C. For sodium alginate, this might be associated with the fracture of glycosidic bonds, dehydration, decarboxylation and decarbonylation of the alginate. For copper alginate and nickel alginate, the degradation step may be attributed to further decarboxylation reactions, the fracture of glycosidic bonds, dehydration and decarbonylation of the alginate. The third step was from 319 to 393 °C for copper alginate, and from 350 to 504 °C for nickel alginate. The thermal degradation rates of copper alginate and nickel alginate were much higher than that of sodium alginate. This may be attributed to the further degradation of fragments formed in the second degradation steps or the catalysis effect of copper ions and nickel ions.

Fig. 2 TGA curves of nickel alginate, copper alginate and sodium alginate in N₂.

Fig. 3 DTG curves of nickel alginate, copper alginate and sodium alginate in N₂.

Table 2 Thermogravimetric data of alginates studied in air and N₂

	Sample	Tonset (°C)	Tmax1 (°C)	Rate of Tmax1 (% min^−1)	Tmax2 (°C)	Rate of Tmax2 (% min^−1)	Residues (%)
							500 °C	600 °C	700 °C
In air	Sodium alginate	110	245	16.0	569	6.2	40.7	24.9	24.6
	Nickel alginate	161	170	5.5	491	8.0	26.5	22.9	21.1
	Copper alginate	143	156	11.8	361	15.2	25.7	25.4	24.2
In N2	Sodium alginate	106	242	15.1	—	—	41.7	40.4	38.1
	Nickel alginate	161	177	5.8	431	2.8	31.5	28.2	24.8
	Copper alginate	143	164	10.9	336	5.5	23.9	23.6	22.9

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The onset decomposition temperatures (T_onsets) of copper alginate and nickel alginate were 143 and 161 °C, respectively, which were much higher than that of sodium alginate, 106 °C, and T_onset was defined as the temperature at which 5% mass loss occurred. This behavior may be caused by the difference in the strength of chelation resulting from the partial ionic bonds between sodium and the divalent metal ions with water molecules. The initial weight loss for sodium alginate, from ambient temperature to 150 °C, was due to moisture evaporation and the elimination of water molecules. From Fig. 2 and 3, and Table 2, the T_max temperatures of copper alginate and nickel alginate, 164 and 177 °C, were lower than that of sodium alginate, 242 °C; however, the maximum rates at the T_max temperatures for copper alginate and nickel alginate, 10.9 and 5.8% min⁻¹, respectively, were much lower than that of sodium alginate, 15.1% min⁻¹. These results also indicated that the addition of copper ions and nickel ions accelerated the thermal degradation of the alginates to form the stable residues, decreasing the rate of degradation and the release of flammable gases. According to the results of the LOI test, the flame retardancy of nickel alginate was better than that of sodium alginate; however, the amount of the residues for nickel alginate was much lower than that for sodium alginate in the temperature range from 416 to 700 °C. The amount of the residues for copper alginate was also much lower than that for sodium alginate from 144 to 700 °C. The residual amount for sodium alginate at 700 °C was about 38.1%; however, the residual amounts for nickel alginate and copper alginate at 700 °C were 24.8 and 22.9%, respectively. These results indicate that the addition of nickel ions and copper ions accelerated the thermal degradation of the alginates, releasing larger amounts of non-flammable gaseous products or gaseous products with lower heats of combustion, such as CO₂ and H₂O. As a result, the flame retardant properties of nickel alginate were improved.

As mentioned above, the flame retardant mechanism of nickel alginate may be a mixture of the condensed-phase mechanism and gaseous-phase mechanism.

Thermal oxidative degradation behavior in air

TGA and DTG curves of nickel alginate, copper alginate and sodium alginate in air are presented in Fig. 4 and 5, and the relevant data are listed in Table 2. From Fig. 2–5, the TGA and DTG curves in air are similar to those in N₂. Four stages were also found in the thermal oxidative degradation process for nickel alginate, copper alginate and sodium alginate as shown in Fig. 4 and 5. The first stage (about 3.2 and 5.0% weight loss for nickel alginate and copper alginate, respectively) was attributed to moisture evaporation from ambient temperature to 143 °C. The second stage for copper alginate (about 32.4% weight loss) was the main weight loss between 143 and 207 °C, and the second stage for nickel alginate (about 16.1% weight loss) was from 143 to 207 °C. The thermal degradation rates of copper alginate and nickel alginate were much higher than that of sodium alginate in this temperature range. This stage might be associated with the fracture of glycosidic bonds, dehydration, decarboxylation and decarbonylation of the alginate or the catalysis effect of copper ions and nickel ions. The first T_max temperatures occurred at 156 and 170 °C for copper alginate and nickel alginate, respectively, which were lower than that of sodium alginate in this temperature range. This also may be due to the catalysis effect of copper ions and nickel ions on the degradation of the alginate, which was similar to the thermal results in N₂. The third stage for copper alginate (about 20.5% weight loss) was from 207 to 301 °C, and for nickel alginate (about 25.0% weight loss) it was from 207 to 351 °C. This stage may be attributed to further chemical reactions of glycosidic bonds, dehydration, decarboxylation and decarbonylation of the alginate or the catalysis effect of copper ions and nickel ions. The fourth stage with the peaks at 361 and 491 °C for copper alginate (from 301 to 367 °C) and nickel alginate (from 351 to 525 °C), respectively, was due to oxidation of carbonaceous residues within inorganic solid particles. The weight losses were about 16.7 and 32.2% for copper alginate and nickel alginate, respectively, during this stage. The maximum rate at T_max for nickel alginate was 8.0% min⁻¹ at 491 °C; however, the maximum rate at T_max for copper alginate was 15.2% min⁻¹ at 361 °C. The weight losses were slight with further increasing temperature, and about 1.2 and 2.0% of the total weight losses were observed in the temperature range from 367 to 700 °C for copper alginate, and from 525 to 700 °C for nickel alginate, respectively. Finally, copper alginate and nickel alginate gave solid residues (carbonaceous residues within inorganic solid particles) equal to 24.2 and 21.1% of their original mass, respectively. Total weight losses of 75.8 and 78.9% for copper alginate and nickel alginate, respectively, were obtained in the thermal oxidative degradation process.

Fig. 4 The TGA curves of nickel alginate, copper alginate and sodium alginate in air.

Fig. 5 The DTG curves of nickel alginate, copper alginate and sodium alginate in air.

Understanding the thermal degradation behavior via TG-FTIR analysis

FTIR measurements were performed to observe the influence of copper ions and nickel ions on the thermal degradation behavior of the alginates. Gram–Schmidt curves of copper alginate, nickel alginate and sodium alginate are presented in Fig. 6. Absorbance information at different times (or temperatures) was obtained from the Gram–Schmidt curves of copper alginate, nickel alginate and sodium alginate. From Fig. 6, there are two peaks (at about 526 and 634 s) of absorbance intensities for copper alginate, three peaks (at about 493, 712 and 1265 s) for nickel alginate and one peak (at about 678 s) for sodium alginate. The spectra at 526 and 634 s for copper alginate, 493, 712 and 1265 s for nickel alginate and 678 s for sodium alginate are displayed in Fig. 7. The broad bands at 3950–3500 cm⁻¹ were related to O–H stretching vibrations of O–H groups and water,²⁴ the peaks at about 2358 and 669 cm⁻¹ were assigned to CO₂,²⁵ the peaks at about 2182 and 2102 cm⁻¹ were ascribed to CO,²⁶ and the peak around 1750 cm⁻¹ was attributed to the C=O stretching vibrations of carbonyl groups.²⁷ From Fig. 7, the spectrum at 493 s for nickel alginate shows that at the beginning of the thermal degradation for nickel alginate the main gaseous product was water. This may result from the release of crystal water and complex water. The evolved gases were mainly composed of H₂O (3950–3500 cm⁻¹) and CO₂ (2358 and 669 cm⁻¹) with increasing temperature. From Fig. 7, the addition of copper ions and nickel ions expedited the thermal degradation of the alginate, indicating that the addition of copper ions and nickel ions had changed the thermal degradation mechanism of the alginate.

Fig. 6 Gram–Schmidt curves of nickel alginate, copper alginate and sodium alginate by TG-FTIR.

Fig. 7 FTIR spectra of pyrolysis gaseous products emitted from copper alginate, nickel alginate and sodium alginate at the maximum evolution rate.

Several FTIR spectra of different functional groups, which contain H₂O, CO₂, CO, =C–H functional groups, C–H functional groups and C=O functional groups, are presented in Fig. 8 as a function of time. The specific wavenumbers of the spectral peaks for the main volatiles mentioned above are listed as follows: H₂O, 3736 cm⁻¹; CO₂, 2358 cm⁻¹; CO, 2180 cm⁻¹; =C–H, 3016 cm⁻¹; C–H, 2966 cm⁻¹; and C=O, 1750 cm⁻¹. According to the Beer–Lambert law, the absorption spectrum at a specific wave number is linearly dependent on the gas concentration.²⁸ Therefore, the variation of absorbance in the whole thermal degradation process reflected the concentration trend of the gas species. From Fig. 8, the volatile emissions of H₂O, CO₂ and compounds containing C=O functional groups were mainly concentrated at a lower temperature range; compounds containing =C–H functional groups, compounds containing –C–H functional groups, and CO were mainly produced at higher temperatures. The addition of copper ions and nickel ions expedited the release of H₂O, CO₂, compounds containing C=O functional groups, compounds containing =C–H functional groups and compounds containing C–H functional groups at a lower temperature range. The absorbance intensities of =C–H, C–H and CO for copper alginate and nickel alginate were higher than those of sodium alginate. This indicates that the addition of copper ions and nickel ions also accelerated the release of these compounds. However, the HRR values of copper alginate and nickel alginate were lower than those of sodium alginate at higher temperatures. This result shows that the heat of combustion values for these compounds were lower. The addition of copper ions and nickel ions mainly decreased the release of flammable gaseous compounds; therefore, they improved the flame retardant properties of the alginate.

Fig. 8 FTIR spectra of different functional groups as a function of time for nickel alginate, copper alginate and sodium alginate.

Understanding the thermal pyrolysis behavior via Py-GC-MS analysis and possible thermal degradation mechanism

In order to study the formation of the main components of the gaseous products in the copper alginate and nickel alginate pyrolysis process, pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) analysis on copper alginate and nickel alginate was carried out. By comparing the mass spectra fragmentation patterns of copper alginate and nickel alginate with the NIST library, the most likely identification of the gaseous compounds was obtained. The total ion intensities of the evolved gaseous products from copper alginate and nickel alginate pyrolysis are shown in Fig. 9. Peaks of the main products are denoted with numbers. More than 30 peaks for copper alginate and nickel alginate are displayed on the chromatograms, and a total of 21 main compounds for nickel alginate and 23 main compounds for copper alginate were identified, whose structures are summarized in Tables 3 and 4 (the peak numbers in Fig. 9 are accordant with the labels in Tables 3 and 4). From Fig. 9, and Tables 3 and 4, it can be observed that the main compounds in the nickel alginate pyrolysis process were as follows: carbon dioxide, furfural, pentanal, 2,3-butanedione, 1-hydroxypropan-2-one, benzene and so on; and the main compounds in the copper alginate pyrolysis process were as follows: carbon dioxide, furfural, pyruvic acid, trimethylolpropane, 2(5H)-furanone, 2-hydroxy-3-methyl-cylopent-2-enone and so on. From Fig. 9, and Tables 3 and 4, the main compounds in the nickel alginate and copper alginate pyrolysis processes were carbon dioxide and furfural, the yields of which were 60.3% for nickel alginate and 64.4% for copper alginate in total. Compared with the pyrolysis compounds of sodium alginate,²⁹ there were much fewer compounds in the nickel alginate and copper alginate pyrolysis process, suggesting that nickel ions and copper ions might have catalyzed the degradation process of the alginate. In general, these changes of the pyrolytic product components in the nickel alginate and copper alginate suggested different pyrolysis mechanisms to that of sodium alginate. Nickel ions and copper ions had changed the thermal degradation process of the alginate, favoring the decarboxylation and intramolecular esterification of alginate as opposed to the depolymerization of sodium alginate with further production of combustible volatile species at the beginning of the thermal degradation of alginate.

Fig. 9 Py-GC-MS detection of gas products evolved from pyrolysis of nickel alginate and copper alginate.

Table 3 Analytical results of chemical constituents of pyrolysis products for nickel alginate at 700 °C

Label	tR (min)	Molecular formula	Name of compound	Molecular structure	Mw (g mol^−1)	Peak area (%)
1	1.57	CO2	Carbon dioxide	O=C=O	44	41.0
2	1.77	C5H10O	Pentanal		86	9.7
3	2.13, 3.43	C4H6O2	2,3-Butanedione		86	5.1
4	2.40	C2H4O2	Acetic acid		60	2.5
5	2.64	C4H6O	2-Butenal		70	1.0
6	2.78	C6H6	Benzene		78	3.1
7	2.88	C3H6O2	1-Hydroxypropan-2-one		74	3.7
8	4.48	C7H8	Toluene		92	1.8
9	5.78	C5H4O2	Furfural		96	19.3
10	6.49	C5H8O3	Acetoxyacetone		116	1.7
11	6.76	C5H4O2	5-Methylenefuran-2(5H)-one		96	0.6
12	7.32	C6H6O2	1-(Furan-2-yl)ethanone		110	0.8
13	7.59	C5H6O2	1,2-Cyclopentanedione		98	0.8
14	8.31	C6H6O2	5-Methyl furfural		110	0.6
15	9.00	C8H6O	Benzofuran		118	0.6
16	9.93	C7H8O	o-Cresol		108	0.9
17	10.29	C7H8O	p-Cresol		108	0.9
18	10.93	C9H8O	2-Methylbenzofuran		132	0.5
19	12.18	C10H8	Naphthalene		128	0.6
20	19.98	C17H34O2	Propan-2-yl-tetradecanoate		270	1.6
21	21.36	C16H32O2	Hexadecanoic acid		256	2.4

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Table 4 Analytical results of chemical constituents of pyrolysis products for copper alginate at 700 °C

Label	tR (min)	Molecular formula	Name of compound	Molecular structure	MW (g mol^−1)	Peak area (%)
1	1.49, 1.55	CO2	Carbon dioxide	O=C=O	44	34.2
2	1.75	C3H4O3	Pyruvic acid		88	6.1
3	2.12	C2H4O2	Acetic acid		60	2.5
4	2.77	C6H6	Benzene		78	0.6
5	4.48	C7H8	Toluene		92
6	4.54	C5H8O	Cyclopentanone		84
7	5.04, 7.26	C4H4O2	2(5H)-Furanone		84	2.5, 1.4
8	5.77	C5H4O2	Furfural		96	32.2
9	6.74	C5H4O2	5-Methylenefuran-2(5H)-one		96	0.7
10	6.96	C5H6O4	5-Oxotetrahydrofuran-2-carboxylic acid		130	1.0
11	7.56	C5H8O3	4-Hydroxy-2-methylenebutanoic acid		116	1.4
12	8.42	C5H4O2	Alpha pyrone		96	1.2
13	8.62	C6H6O	Phenol or 2-ethenylfuran		94	1.3
14	9.23	C6H8O2	2-Hydroxy-3-mtehyl-cylopent-2-enone		112	3.1
15	11.61	C8H18O	3-Methylheptan-3-ol		130	0.9
16	11.88	C7H8O2	1-(Furan-2-yl)propan-1-one		124	1.2
17	12.31	C9H16O2	3-Isopropyl-6-methyltetrahydro-2H-pyran-2-one		156	0.7
18	13.14	C8H12O2	5,5-Dimethylcyclohexane-1,3-dione		140	0.8
19	14.56	C6H14O3	Trimethylolpropane		134	4.2
20	17.42	C13H20O	7-Isopropyl-8-methylnona-3,5,7-trien-2-one		192	0.9
21	19.98	C17H34O2	Propan-2-yl-tetradecanoate		270	0.8
22	21.35	C16H32O2	Hexadecanoic acid		256
23	24.63	C18H26O3	2-Ethylhexyl-3-(4-methoxyphenyl)acrylate		290

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Considering the product distribution from the nickel alginate pyrolysis process, carbon dioxide, furfural, pentanal and 2,3-butanedione were the main products, the yields of which were 73.9% in total, and the yield of carbon dioxide, 41.0%, was much higher than that of furfural, 19.3%. According to the analysis of TG-FTIR and Py-GC-MS, the speculative thermal degradation mechanism of nickel alginate to partial fragmentation of nickel alginate was proposed through two main pathways. Fig. 10 presents the proposed degradation mechanism of nickel alginate. The speculative thermal degradation mechanism of nickel alginate is that the addition of nickel ions catalyzed decarboxylation and intramolecular esterification in the pyrolysis process of nickel alginate at the beginning of the thermal degradation of nickel alginate, with the evolution of H₂O and CO₂. Ni²⁺ has been used to catalyze cellulose decomposition,³⁰ and Ni²⁺ catalyzed the chemical reactions of decarboxylation (pathway A shown in Fig. 10) and esterification (pathway B shown in Fig. 10) of the alginate according to the different yields of carbon dioxide and furfural. For pathway A, the thermal degradation of the alginate with glycosidic bond cleavage to form 2,3,4,5-tetrahydroxypentanal happened with the increase of temperature, due to the lower thermal stability of the C–O bonds compared to that of the C–C bonds in the ring structure.³¹ 2,3,4,5-Tetrahydroxypentanal transferred to form a more stable structure, tetrahydro-2H-pyran-2,3,4,5-tetraol, under the catalysis of Ni²⁺, and under the catalysis of Ni²⁺, tetrahydro-2H-pyran-2,3,4,5-tetraol underwent a number of rearrangement, dehydration, decarbonylation, decarboxylation, scission and condensation reactions to yield furfural and H₂O. For pathway B, the thermal degradation of the alginate with glycosidic bond cleavage to form 3,4,5-trihydroxy-6-oxotetrahydro-2H-pyran-2-carbaldehyde took place with increasing temperature, and under the catalysis of Ni²⁺, 3,4,5-trihydroxy-6-oxotetrahydro-2H-pyran-2-carbaldehyde might undergo a number of decarbonylation, scission, decarboxylation, dehydration and rearrangement reactions to yield 2,3-butanedione, CO, CO₂ and H₂O. There were also a number of linear gaseous products, such as pentanal and 1-hydroxypropan-2-one, which were formed by a ring scission followed by decarbonylation and dehydration reactions.³²

Fig. 10 Proposed thermal degradation mechanism of nickel alginate pyrolysis.

As discussed above, the compounds containing nickel ions had a catalysis effect on the nickel alginate pyrolysis process, yielding lower molecular weight and more stable products, such as CO₂ and H₂O. This improved the flame retardant properties of the nickel alginate.

Considering the product distribution from the copper alginate pyrolysis process, carbon dioxide and furfural were the main products, the yields of which were 66.4% in total, and the yield of carbon dioxide, 34.2%, was similar to that of furfural, 32.2%. According to the analysis of TG-FTIR and Py-GC-MS, the speculative thermal degradation mechanism of copper alginate was proposed to proceed by partial fragmentation of copper alginate through one main pathway. The proposed degradation mechanism of copper alginate is shown in Fig. 11. The speculative thermal degradation mechanism of copper alginate is that the addition of copper ions catalyzed the decarboxylation in the pyrolysis process of copper alginate at the beginning of the thermal degradation of the copper alginate, with the evolution of CO₂. Cu²⁺ is a Lewis acid,³³ which catalyzes the chemical reaction of decarboxylation of the alginate according to the production of almost the same yield of carbon dioxide and furfural. For the catalysis of the decarboxylation reaction, the thermal degradation mechanism was similar for both nickel alginate and copper alginate and carbon dioxide and furfural were produced through a lot of chemical reactions. However, according to the production of almost the same yields of carbon dioxide and furfural, intramolecular esterification might not take place at the beginning of the thermal degradation for copper alginate. This result suggests that the addition of copper ions may not catalyze the intramolecular esterification of the alginate, which is different from nickel ions.

Fig. 11 Proposed thermal degradation mechanism of copper alginate pyrolysis.

As discussed above, the compounds containing copper ions and nickel ions had a catalysis effect on the copper alginate and nickel alginate pyrolysis process, yielding incombustible products and products with lower heat of combustion values, such as CO₂ and H₂O. This improved the flame retardant properties of the copper alginate and nickel alginate. The thermal degradation mechanism of the copper alginate and nickel alginate are different, and this causes the different flame retardant properties of copper alginate and nickel alginate. However, more investigations are needed to further clarify the effects of copper ions and nickel ions on the alginate pyrolysis processes and the flame retardant properties of alginates.

Conclusions

Significant differences in the flame retardancy, thermal degradation properties, thermal oxidative degradation properties and pyrolysis properties were observed for the copper alginate film, nickel alginate film and sodium alginate film. Compared with those of the copper alginate film and sodium alginate film, the nickel alginate film possessed the best flame retardant properties. The results of MCC indicated that the addition of copper ions and nickel ions mainly decreased the release of combustible gaseous products at lower temperatures and thus decreased the HRR values; however, the PHRR value of the nickel alginate was lower than that of the copper alginate. TGA results showed that the thermal stabilities of the copper alginate and nickel alginate were worse than those of the sodium alginate, and the amount of residual char was lower than that of the sodium alginate at higher temperatures. The addition of copper ions and nickel ions accelerated the thermal degradation of the alginate and changed the thermal degradation mechanism of the alginate. TG-FTIR and Py-GC-MS results indicate that the pyrolysis of the nickel alginate and copper alginate produced much fewer flammable products than that of sodium alginate. This suggests that the nickel ions and copper ions had catalyzed the degradation process of the alginate. In general, these changes of the pyrolytic product components for the nickel alginate and copper alginate suggested different pyrolysis mechanisms from that of sodium alginate. The addition of nickel ions mainly accelerated the decarboxylation and esterification at the temperature that the nickel alginate began to degrade; however, the addition of copper ions mainly catalyzed the decarboxylation at the beginning of the thermal degradation of the copper alginate. The addition of nickel ions and copper ions significantly decreased the release of gaseous products, including incombustible and combustible gases at lower temperatures. This improved the flame retardancy of nickel alginate and copper alginate. The results of our study provide useful information for understanding the flame retardancy mechanism of alginates as well as for designing bio-based materials with excellent fire retardancy.

Acknowledgements

This work was funded by the National Natural Science Foundation of China (51203126) and China Scholarship Council (CSC: 201308420380).

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