Effect of Nickel-Containing Layered Double Hydroxides and

Article pubs.acs.org/IECR

Effect of Nickel-Containing Layered Double Hydroxides and Cyclophosphazene Compound on the Thermal Stability and Flame Retardancy of Poly(lactic acid) Xueying Shan,†,‡,§ Lei Song,† Weiyi Xing,† Yuan Hu,*,†,§ and Siuming Lo‡ †

State Key Laboratory of Fire Science, University of Science and Technology of China, and USTC-CityU Joint Advanced Research Centre, 166 Ren’ai Road, Suzhou, People’s Republic of China ‡ Department of Civil and Architectural Engineering, City University of Hong Kong, and USTC-CityU Joint Advanced Research Centre, 166 Ren’ai Road, Suzhou, People’s Republic of China § Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute of University of Science and Technology of China, 166 Ren’ai Road, Suzhou, People’s Republic of China ABSTRACT: Hydrotalcite-like anionic clays or layered double hydroxides with dodecylbenzenesulfonate anions (NiFe, NiAl, and NiCr LDH-SDS, where SDS denotes sodium dodecyl sulfate) were prepared by the coprecipitation method. Hexaphenoxycyclotriphosphazene (HPCP) is an effective cyclophosphazene flame retardant. The HPCP molecule contains a cyclic backbone consisting of alternating phosphorus and nitrogen atoms. LDH-SDS and HPCP were first used to prepare poly(lactic acid) (PLA) composites. One goal of this work was to compare the thermal stability and flame retardancy of PLA/ HPCP/LDH-SDS composites. The structures and properties of LDH-SDS materials were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). Thermal analysis and char residue analysis of PLA/HPCP/LDH-SDS composites suggested that NiFe, NiAl, and NiCr LDHSDS materials play different roles in improving the thermal stability and flame retardancy of PLA composites. The effective mechanism of LDH-SDS and HPCP in PLA composites is also hypothesized in this work.

1. INTRODUCTION Biodegradable polymers have attracted increasing attention because of their low environmental impacts. Among biodegradable polymers, poly(lactic acid) (PLA) plays a dominant role because its raw material, lactic acid, can be efficiently produced by fermentation from renewable resources, such as corn or sugar beet. When disposed properly, PLA is hydrolyzed to harmless and natural products. Moreover, it has good properties such as high mechanical strength, thermal plasticity, high melting point, high degree of transparency, and ease of fabrication.1−3 Thus, PLA has attracted extensive industrial interest, in biomedical as well as commodity applications. In fact, PLA has been used in service ware; grocery waste; composting bags; mulch films; and controlled-release matrixes for fertilizers, pesticides, and herbicides.4,5 However, because of its intrinsic chemical composition and molecular structure, PLA has low thermal stability, high combustibility, and melt dripping, which significantly restricts its expansion into new application areas.6,7 Therefore, high thermal stability and good flame retardancy are important requirements for PLA. To date, there have been few studies on improving thermal stability and flame retardancy of PLA. Additive-type flame retardants, such as phosphorus-containing,8−10 silicon-containing,11 or carbon-containing materials;12,13 inorganic materials;14−16 layered materials;−19 and additives with synergistic effects,20,21 are used in PLA. Unfortunately, it is hard to obtain a UL-94 V0 rating at low addition levels. Hydrotalcite-like materials, also known as layered double hydroxides (LDHs), are an important class of layered materials. © 2012 American Chemical Society

The LDH structure is described with the ideal formula [MII1−xMIIIx(OH)2]intra[Am−x/m·nH2O]inter, where MII is a divalent cation and MIII is a trivalent cation. Am− denotes an organic or inorganic anion with negative charge m. LDHs have a wide range of applications in many fields, for example, as catalysts or catalyst precursors,22,23 ion exchangers,24 adsorbents for environmental contaminants,25,26 and media for gas diffusion and adsorption.27,28 The interlayer spacing of LDHs can vary depending on the size of the intercalated anions. When heated above 600 °C, LDHs result in mixed oxides.29 Hexaphenoxycyclotriphosphazene (HPCP) is an effective cyclophosphazene flame retardant. The HPCP molecule has a cyclic backbone consisting of alternating phosphorus and nitrogen atoms. Thus, it has phosphorus−nitrogen synergetic effect and good thermal stability. In the present study, LDHSDS materials (where SDS denotes sodium dodecyl sulfate) containing nickel−iron, nickel−aluminum, and nickel−chromium were synthesized under similar conditions. They were first combined with HPCP to prepare PLA nanocomposites, and the thermal stability and flame retardancy of these nanocomposites were studied. These metals were chosen as representative of the range of trivalent metals that could be used in the formation of an LDH. Nickel was chosen as the divalent metal cation for all LDHs, as the goal was to compare the effect of trivalent metals Received: Revised: Accepted: Published: 13037

March 5, 2012 June 7, 2012 September 18, 2012 September 18, 2012 dx.doi.org/10.1021/ie300589p | Ind. Eng. Chem. Res. 2012, 51, 13037−13045

Industrial & Engineering Chemistry Research

Article

on the properties of the polymer composites. The dodecylbenzenesulfonate anion was chosen as the spacer in the LDH, because the volume of this anion is sufficiently large. The objective of this study was to explore the effects of LDH-SDS and HPCP on the thermal behavior and flammability of PLA. In addition, the dispersion of LDH-SDS in the matrix and the char-layer materials of the composites was investigated. The effective mechanism of LDH-SDS and HPCP in PLA matrixes is also discussed in this article.

Table 1. Formulations of Samples and Flame Retardancy Characteristics of PLA/HPCP/LDH-SDS Composites flame retardancy

formulation of samples

2. EXPERIMENTAL SECTION 2.1. Materials. In the experiments, the following materials were used to synthesize LDH-SDS materials and prepare the composite samples: PLA (model 2002D, with a density of 1.27 g/cm3, a mold shrinkage of 2%, a notched impact strength of 1.9, a tensile strength of 69 MPa, and an elongation at break of 2%) was supplied by Polymer UNIC Technology Co., Ltd. (Suzhou, China). HPCP (CAS: 1184-10-7) was obtained from Zhangjiagang Chemical Co., Ltd. (Suzhou, China). Nickel(II) chloride hexahydrate (≥98 wt %), iron(III) chloride hexahydrate (≥99 wt %), chromium(III) chloride hexahydrate (≥99 wt %), sodium dodecylbenzenesulfonate (≥88 wt %), and sodium hydroxide (≥96 wt %) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). 2.2. Laboratory Equipment. A twin-roller mixer was obtained from Suyan Science and Technology Co., Ltd. (Changzhou, China). A semiautomatic molding press (YX-25D) was obtained from Zimmerli Weili Rubber & Plastic Machinery Co., Ltd. (Shanghai, China). A muffle furnace (SX2-4-10) was purchased from Hede experimental equipment factory (Shanghai, China). An ultrasonic cleaner (KH3200DE) was purchased from Kunshan Hechuang Ultrasonic Instrument Co., Ltd. (Kunshan, China). 2.3. Preparation of Dodecylbenzenesulfonate LDHs. The dodecylbenzenesulfonate-containing LDHs were synthesized by adapting the coprecipitation method reported by Manzi-Nshuti et al.30 The preparation was performed in a N2 atmosphere to exclude carbon dioxide. The basic strategy involved the addition of an MII/MIII metal salt solution (MII/MIII molar ratio = 2; MII = Ni2+ and MIII = Fe3+, Al3+, Cr3+) to a basic solution. In a typical experiment, in a three-neck flask under a flow of nitrogen, distilled water was boiled for 30 min and then cooled to room temperature. To this water sodium dodecyl sulfate (SDS) was added, and the mixture stirred until the SDS dissolved completely. Then, the metal salt solution and 1 mol/L NaOH solution were slowly added dropwise to the stirred SDS solution, with the pH maintained at the desired value by addition of NaOH solution. For NiFe, NiAl, and NiCr LDH-SDS materials, the pH values were adjusted to approximately 10, 9, and 8, respectively. The resulting slurry mixture was aged for 24 h at 70 °C, filtered, washed, and dried in a vacuum oven at 65 °C. These LDH-SDS compounds have similar structures. For the NiAl LDH-SDS, for example, the results obtained and calculated from elemental analysis are as follows: 29.2 wt % Ni, 7.4 wt % Al, 2.8 wt % S, 19.0 wt % C, and 5.0 wt % H. These results suggest the formula Ni1.81Al(OH)5.62(C18H29SO3)0.32·1.73H2O. All of the LDH-SDS compounds were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). 2.4. Preparation of Samples. The formulations of the samples are listed in Table 1. All materials were dried in an oven at 60 °C for 12 h before use. PLA composites were melted

sample

PLA (wt %)

HPCP (wt %)

LDH-SDS (wt %)

LOI (%)

UL-94

PLA1 PLA2 PLA3 PLA4 PLA5

100 90 90 90 90

0 10 8 8 8

− − 2NiFe 2NiAl 2NiCr

20 30 29 29 29

no rating V0 V0 V0 V0

and mixed in a twin-roller mixer at a rotor speed of 40 rpm and a temperature of 175 °C for 10 min. Sheets (100 × 100 × 3 mm3) of each formulation were obtained using a semiautomatic molding press. The thickness and size of the sheets were suitable for limiting oxygen index (LOI) and UL-94 tests. 2.5. Measurements. X-ray diffraction (XRD) measurements were performed using a Rigaku (Tokyo, Japan) D/MaxrA rotating anode X-ray diffractometer and graphite monochromator with a Cu Kα generator (λ =0.15418 nm). The Xray level was 40 kV/200 mA. The scanning step size was 0.01°. Transmission electron microscopy (TEM) images of the LDH-SDS materials and composites were obtained at 200 kV with a JEM-2010 electron microscope (JEOL, Tokyo, Japan), to investigate the crystal structures of the LDH-SDS materials and the dispersion of LDH-SDS in PLA/HPCP matrixes. LDH-SDS compounds were treated with ultrasonic dispersion with an ultrasonic cleaner. The PLA composites were ultramicrotomed with a diamond knife at room temperature to obtain ∼70-nm-thick sections. Thermogravimetric analysis (TGA) was conducted from 100 to 700 °C using a Q5000IR thermoanalyzer instrument under a flowing nitrogen atmosphere at a scan rate of 10 °C/min. The reproducibilities of the weight and temperature data from the TGA instrument were ±0.2% and ±1 °C, respectively. A limiting oxygen index (LOI) test was performed with an HC-2 oxygen index meter (LOI Analysis Instrument Company, Jiangning, China). The samples used for the test were 100 × 6.5 × 3 mm3 in size, and the test was performed according to standard ASTM D2863. A UL-94 burning test was conducted on 100 × 13 × 3 mm3 samples, and measurements were obtained with a CFZ-3 instrument (Burning Analysis Instrument Company, Jiangning, China). The tests were performed according to the vertical burning test standard. Differential scanning calorimetry (DSC) measurements were performed using a NETZSCH DSC instrument (204 F1 Phoenix) at a heating rate of 10 °C/min in a nitrogen atmosphere. The data reported were collected from the second scan. The procedure was as follows: Two scans from 20 to 190 °C were conducted, and the composites were cooled to 20 °C between the scans. The first scan was designed to erase the thermal history of the samples. The quantities of interest, namely, the glass transition temperature (Tg), cold crystallization temperature (Tc), enthalpy of cold crystallization (ΔHc), melting temperature (Tm), and melting enthalpy (ΔHm), were determined from the second scan. The degree of crystallinity was determined by subtracting ΔHc from ΔHm and considering a melting enthalpy of 93 J/g for 100% crystalline PLA.14 Scanning electron microscopy (SEM) with a tungsten filament (on an instrument made in the Czech Republic) was used to study the morphological features of char residue, which was 13038

dx.doi.org/10.1021/ie300589p | Ind. Eng. Chem. Res. 2012, 51, 13037−13045


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obtained in a muffle furnace at 650 °C for 10 min. The char was affixed to a copper plate. Laser Raman spectroscopy (LRS) was performed with a SPEX-1403 laser Raman spectrometer (Spectra-Physics, Mountain View, CA) at room temperature. Measurements were obtained using a back-scattering geometry with the 514-nm argon laser line. The scanning range was from 500 to 2000 cm−1. X-ray photoelectron spectroscopy (XPS) was performed with an ESCALAB 250 electron spectrometer (Thermo-VG Scientific Co.). The excitation source was an Al Kα line. The weight percentages of C, P, N, and O in char residue of PLA composites were measured by an inductively coupled plasma mass spectrometer (Plasma Quad 3).

LDH materials have high crystallinity, and the basal spacing (003 reflection) peak in the XRD pattern is at a high 2θ.31,32 Unmodified LDHs (NiFe, NiAl, and NiCr) have similar structures. For unmodified NiAl LDH, for example, the preparation method is similar to that for modified LDH-SDS except using SDS. The X-ray diffraction (XRD) pattern is presented in Figure 1. However, the crystallinity of modified LDHs is low.33 XRD patterns of LDH-SDS materials are also shown in Figure 1. The presence of an XRD peak (003 reflection) at a lower 2θ value implies the formation of an intercalated structure. The NiFe, NiAl, and NiCr LDH-SDS compounds have similar structures. XRD data suggest that layered materials are formed with basal spacings of 3.25 nm for NiFe, 3.45 nm for NiAl, and 2.91 nm for NiCr, consistent with the size of dodecylbenzenesulfonate. Modified LDHs with dodecylbenzenesulfonate anions are sufficient to cause expansion of the gallery space to permit the incorporation of the PLA matrixes. The morphology of unmodified LDH materials is uniform.31 When LDHs are modified by the coprecipitation method, the crystallinity is lower, so the morphology is less uniform. The structures of the LDH-SDS materials were characterized by TEM and SEM (Figure 2). The images of the LDH-SDS compounds exhibit a layer structure. The TGA and corresponding differential thermogravimetry (DTG) curves of the various LDH-SDS compounds and HPCP heated in a nitrogen environment at 10 °C/min from 50 to 700 °C are shown in Figure 3. The TGA results for LDH-SDS compounds and HPCP are summarized in Table 2. It can be noted that overall the thermal decomposition of these LDHSDS materials is very complex. However, there are some similarities when these materials are subjected to similar heating conditions. The mass loss below 350 °C can be attributed to the loss of external surface water and gallery water and to the partial dehydroxylation of the layers.30 Above 350 °C, all of the LDH-SDS materials show a main degradation step that can be

3. RESULTS AND DISCUSSION 3.1. XRD and TEM Images of LDH-SDS and TGA Characterizations of LDH-SDS and HPCP. Unmodified

Figure 1. XRD patterns of LDH-SDS and unmodified NiAl LDH.

Figure 2. TEM and SEM images of LDH-SDS materials. 13039



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Figure 3. Thermal stabilities of LDH-SDS and HPCP measured by TGA in a N2 atmosphere at 10 °C/min.

PLA3−PLA5 decreased to 29%, because metal oxides formed when the PLA/HPCP/LDH-SDS composites were ignited, and metal oxides have catalytic properties, making the PLA matrixes burn easily. All of the composites attained V0 ratings. 3.3. Thermal Analysis of PLA/HPCP/LDH-SDS Composites. The degradation of PLA can be attributed mainly to hydrolysis by trace amounts of water, zipper-like depolymerization, thermo-oxidative reactions, random main-chain scission during processing, and intramolecular transesterification resulting in the formation of monomers and oligomers.14 The thermal stability of PLA composites used to prepare samples is described in Figure 4. The 2 wt % loss temperature (T−2 wt %), the maximum mass loss temperature (Tmax), and the char residue at 700 °C are listed in Table 3. When PLA composites

Table 2. TGA Results for LDH-SDS and HPCP at 10 °C/min (N2)a sample

T−2 wt %(°C)

Tmax(°C)

char residue at 700 °C (wt %)

NiFe NiAl NiCr HPCP

85 83 65 331

416 461 438 426

47.9 45.8 50.2 0

a

T−2 wt %, temperature of 2 wt % mass loss; Tmax, temperature of maximum mass loss.

associated with the decomposition of the intercalated dodecylbenzenesulfonate anion and further dehydroxylation of the layers, resulting in complete destruction of the LDH-SDS structure. It should be noted that the amount of char residue at 700 °C was found to be 45.8, 47.9, and 50.2 wt % for NiAl, NiFe, and NiCr, respectively. The thermal decomposition of HPCP is simple. HPCP is stable below 331 °C (T−2 wt %), and Tmax is 426 °C. Phosphoric acid, metaphosphoric acid, and polyphosphoric acid are formed. At 700 °C, HPCP is degraded completely. 3.2. LOI and UL-94 Tests of PLA/HPCP/LDH-SDS Composites. LOI and UL-94 vertical burning tests are widely used to evaluate the flame-retardant properties of materials. Table 1 also includes the LOI values and UL-94 results for PLA/HPCP/LDH-SDS composites. The LOI value of PLA1 is 20%, and that of PLA2 is 30%. LOI was improved significantly after addition of 10 wt % HPCP. However, the LOI values of

Table 3. TGA Results for PLA Composites at 10 °C/min (N2) sample PLA1 PLA2 PLA3 PLA4 PLA5 a

T−2 wt % (°C) (ΔTa) 348 310 270 302 316

(−38) (−78) (−46) (−32)

Tmax (°C) (ΔTa) 388 376 311 381 381

(−12) (−77) (−7) (−7)

char residue at 700 °C (wt %) 0 7.5 4.1 6.3 7.2

ΔT, difference between virgin polymer (PLA1) and its composites.

were melted and mixed at 175 °C for 10 min, there was no significant composition degradation (see T−2 wt % in Table 3).

Figure 4. Thermal stability of PLA/HPCP/LDH-SDS composites measured by TGA in a N2 atmosphere at 10 °C/min. 13040



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Compared to the results for PLA1, the values of T−2 wt % and Tmax decreased after HPCP and LDH-SDS were added, owing to the catalysis of acid and metal compounds (as demonstrated in Figure 3). ΔT of T−2 wt % and Tmax for PLA4 and PLA5 were smaller than the corresponding values for PLA3. In fact, the PLA3 values of T−2 wt %, Tmax, and char residue were the smallest of all of the PLA/HPCP/LDH-SDS composites, because of the strong catalysis of the NiFe metal compound. It should be noted that, when an LDH-SDS compound is added to the PLA/HPCP matrix, the char residue is decreased. Metal oxides are formed at 700 °C (as described above). This is helpful for further degradation of PLA/HPCP/LDH-SDS composites. From the TGA experiments performed in a nitrogen environment, the following trend was observed in PLA/HPCP/LDHSDS composites

Figure 5. DSC thermograms of different PLA composites (second heating from 20 to 190 °C at a ramp of 10 °C/min).

char residue:

The greater the char residue of PLA composites, the higher the thermal stability. An enlarged TGA image presenting the range from 300 to 700 °C is also shown in Figure 4. Figure 5 presents the DSC thermograms of PLA/HPCP/ LDH-SDS composites during the second DSC heating cycle. The results are summarized in Table 4. The specific thermal properties of the PLA matrix are altered after addition of HPCP and LDH-SDS. These additions result in significant changes in Tg (60 °C for PLA1) and Tm (Tm2 of 153 °C for PLA1). In addition, variations in crystallinity are

Table 4. DSC Results for PLA Composites sample

Tg (°C)

Tc (°C)

ΔHc (J/g)

Tm1 (°C)

Tm2 (°C)

ΔHm (J/g)

crystallinity (%)

PLA1 PLA2 PLA3 PLA4 PLA5

63 41 40 41 41

− 93 87 88 89

− 18.6 19.5 18.2 18.4

− 129 126 129 130

153 146 145 147 147

0.1 20.5 21.1 20.7 20.9

0.1 2.0 1.7 2.7 2.7

PLA5 > PLA4 > PLA3

Figure 6. TEM images of PLA/HPCP/LDH-SDS composites formed by melt blending. 13041



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exfoliated. Figure 6 presents TEM images of PLA3−PLA5 at high and low magnification. When PLA composites are melted and mixed at 175 °C, HPCP can be dispersed in a PLA matrix uniformly, because its melting temperature is low (Tm = 116 °C, but no composition is degraded, as demonstrated in Figure 3). When 2 wt % LDH-SDS compounds are added to the PLA/ HPCP matrixes, all of the images exhibit good dispersion, which suggests intercalation or exfoliation. The XRD patterns of PLA/ HPCP/LDH-SDS composites are presented in Figure 7. The 003 reflection peaks of the LDH-SDS compounds disappear. Combining XRD with TEM results confirms the PLA/HPCP/ LDH-SDS systems as nanocomposites. 3.5. Analysis of Char Residue. Figure 8 shows the morphologies of char residues. The residues were collected by burning the samples in a muffle furnace at 650 °C for 10 min. When the samples were enlarged 200 times (Figure 8a), many holes were evident in the char layer of PLA2. This structure is conducive to gas diffusion and heat transfer, which makes the sample burn easily. A much denser and more intumescent char layer was formed in PLA3−PLA5 than in PLA2. This structure could provide a much better physical barrier to the spread of flammable gases, prevent heat transfer, and further protect the underlying polymeric substate from attack by heat flux in a flame. Thus, PLA/HPCP matrixes have better flame retardancy after LDH-SDS compounds are added. PLA3 char residue was denser than that of PLA4 or PLA5. When the images of the samples were enlarged 2000 times (Figure 8b), many metal oxide particles were evident in the char layer of PLA3, owing to the deposition of metal oxide particles on the surface of the char layer. Representative LRS images of char residue are shown in Figure 9 (PLA1 had no char residue). The areas of the peaks were obtained by the Gaussian method, which is used for multiple peaks. The results are summarized in Table 5. The peak at approximately 1607 cm−1 (peak 1) is attributed to the stretching vibration of CC in the ordered carbon, whereas the peak at approximate 1360 cm−1 (peak 2) is attributed to amorphous carbon. It should be noted that the area ratio of peak 1 to peak 2 varied, being 0.31, 0.26, 0.29, and 0.27 for

evident. These results further suggest that, under the investigated DSC procedure, addition of HPCP and LDH-SDS to PLA could not overcome the slow crystallization kinetics of the PLA matrix, which can be attributed to the high content of D isomers, as reported elsewhere.34 Two different crystal structures result in double melting peaks. When HPCP was added to PLA, Tg and Tm decreased significantly. However, LDH-SDS did not induce further changes (DSC results for PLA2−PLA5). We obtained similar results for the PLA/HPCP/LDH-SDS composites. From the DSC experiments conducted in a nitrogen environment, the following trends were observed in PLA/HPCP/ LDH-SDS composites Tg and crystallinity:

PLA5 = PLA4 > PLA3.

It seems that, in the best case, PLA3 (PLA/HPCP/NiFe composite) presents only a low crystallinity occurring in all PLA/HPCP/LDH-SDS composites at the start of the second DSC heating cycle, according to our calculations. 3.4. Dispersion of LDH-SDS in PLA/HPCP/LDH-SDS Composites. TEM was used to determine the dispersion of LDH-SDS compounds in the PLA/HPCP/LDH-SDS composites, as well as the description of the material as intercalated or

Figure 7. XRD patterns of PLA/HPCP/LDH-SDS composites.

Figure 8. SEM images of samples collected by burning PLA/HPCP/LDH-SDS composites in a muffle furnace at 650 °C for 10 min: (a) low magnification (×200), (b) high magnification (×2000). 13042



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are burned in a muffle furnace at 650 °C. Because metal oxides experience the strong catalysis, the ordered carbon decreases. From the LRS experiments on PLA/HPCP/LDH-SDS composites, the following trend was observed peak 1/peak 2 ratio:

The chemical components of the char residue for PLA composites were investigated by XPS, as illustrated in Figure 10. The peaks at 284.6 eV are attributed to CH and CC in aliphatic species. The peaks of the P 2p spectra at approximately 134.2 eV can be attributed to the pyrophosphate, polyphosphate, or NPO (incomplete degradation of HPCP). For the N 1s spectra, peaks at around 401.2 (or 400.4) eV can be observed and assigned to the nitrogen in NPO. The peaks at approximately 532.9 (or 532.6) eV are attributed to O in POP, COP, or COH groups.35 The results are summarized in Table 6.

Figure 9. Representative LRS results for char residues.

Table 6. XPS Results for PLA Composites

Table 5. LRS Results for PLA Composites sample PLA2 PLA3 PLA4 PLA5

peak 1 (cm−1) 1607 1610 1607 1600

peak 2 (cm−1) 1364 1371 1360 1356

area of peak 1(×10−5) 1.27 1.30 0.86 0.69

area of peak 2 (×10−5)

PLA4 > PLA5 > PLA3

area ratio (peak 1/peak 2)

4.09 5.01 2.99 2.51

0.31 0.26 0.29 0.27

sample

C 1s (wt %)

P 2p (wt %)

N 1s (wt %)

O 1s (wt %)

P/C

N/C

PLA2 PLA3 PLA4 PLA5

52.08 32.43 50.49 49.36

7.57 11.37 6.94 8.00

3.03 2.76 2.62 2.79

37.32 53.44 39.96 39.85

0.145 0.351 0.137 0.162

0.058 0.085 0.052 0.057

From the XPS experiments conducted in a nitrogen environment, the following P/C and N/C trends were observed

PLA2−PLA5, respectively. Ordered carbon can improve the thermal stability of a system, so the PLA2 result is optimal. As discussed in the context of the LOI and UL-94 tests, metal oxides are formed when PLA/HPCP/LDH-SDS composites

P/C:

PLA3 > PLA5 > PLA2 > PLA4

N/C:

PLA3 > PLA2 > PLA5 > PLA4

Figure 10. Representative XPS results for char residues. 13043



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Notes

It should be noted that the comparison of the weight percentages for PLA composite char residues follows a different order. When PLA/HPCP/LDH-SDS composites were combusted in a muffle furnace, HPCP broke down into phosphorus-containing acid and nitrogen-containing gases (such as NH3 and NO2). A cross-linking structure was formed between acid species and PLA matrixes that acted as a superior thermal insulator and mass-transport barrier to protect the underlying materials from further burning and reducing their heat release. Nitrogen-containing gases had a different loss. Higher P/C and N/C ratios are useful for improving the thermal stability of the system. As shown in Figure 8, the PLA3 (PLA/HPCP/NiFe composite) char residue had the greatest stability.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by joint funding from the National Basic Research Program of China (973 Program) (2012CB719701), National Natural Science Foundation of China (No. 51036007), and Opening Project of Southwest University of Science and Technology (No. 11ZXFK12).

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4. MECHANISM DISCUSSION When PLA/HPCP/LDH-SDS composites are melted and mixed at 175 °C, there is no significant composition degradation. HPCP is uniformly dispersed in the PLA matrix, because the melting temperature of HPCP is low, whereas LDH-SDS compounds are partially intercalated and partially exfoliated in the PLA matrix (as shown in Figures 6 and 7). The layers of LDH-SDS are used as a physical barrier that can decelerate heat and mass transfer between the gas and the condensed phases, so that the LDH-SDS compounds protect the underlying material from further burning and reducing its heat release. There are esterification reactions between PLA (or its degradative products) and polyol phosphate compounds (degraded from HPCP during the combustion process, as shown in Figure 3), which are catalyzed by acid species or metal oxides. This is useful for improving the thermal stability of the matrixes. 5. CONCLUSIONS Hydrotalcite-like anionic clays or layered double hydroxides (LDH-SDS) of the general formula [MII1−xMIIIx(OH)2]intra[(C18H29SO3)x·H2O]inter, with MII = Ni and MIII = Fe, Al, Cr, were prepared and first used to form PLA/HPCP/LDH-SDS composites. The properties of the LDH-SDS materials were characterized by XRD, TEM, SEM, and TGA. The morphology, thermal stability, and flame retardancy of the PLA/HPCP/ LDH-SDS composites were investigated and compared. After the LDH-SDS compounds were added, all of the composites gave a UL-94 V0 rating at low addition levels. For PLA composites containing LDH-SDS materials, TGA measurements confirmed that the PLA/HPCP/NiCr composite had slightly more char residue. DSC levels suggested that, in the best case, the PLA/HPCP/NiFe composite presented only a low crystallinity, and SEM levels demonstrated that its char residue was densest. In addition, XPS revealed that this composite exhibited the highest P/C and N/C ratios. The largest ratio of ordered carbon for the PLA/HPCP/NiAl composite was evidenced by LRS. All of the TEM images showed that the LDH-SDS materials have good dispersion in PLA/HPCP matrixes. Thus, thermal analysis and char residue analysis of the PLA/HPCP/ LDH-SDS composites confirmed that NiFe, NiAl, and NiCr LDH-SDS materials play different roles in improving the thermal stability and flame retardancy of PLA composites.

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Effect of Nickel-Containing Layered Double Hydroxides and

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