Article pubs.acs.org/IECR
Thermal Degradation and Flame Retardance of Biobased Polylactide Composites Based on Aluminum Hypophosphite Gang Tang,† Xin Wang,† Weiyi Xing,† Ping Zhang,§ Bibo Wang,† Ningning Hong,† Wei Yang,†,‡ Yuan Hu,*,†,‡ and Lei Song† †
State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, People's Republic of China ‡ Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute for Advanced Study, University of Science and Technology of China, 166 Ren’ai Road, Suzhou, Jiangsu 215123, People's Republic of China § State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, Southwest University of Science and Technology, 59 Qinglong Road, Mianyang 621010, People's Republic of China ABSTRACT: A series of flame retardant polylactide composites (FR-PLA) based on aluminum hypophosphite (AHP) were facilely prepared by melt blending method. The thermal behavior, flammability, and mechanical properties of FR-PLA composites were investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), the UL-94 vertical burning test, limiting oxygen index (LOI), cone calorimeter testing, microscale combustion calorimetry, mechanical testing, and dynamic mechanical analysis (DMA). TGA results showed that the FR-PLA composites presented higher char residue and reduced mass loss rate than neat PLA. The FR-PLA composite showed excellent fire resistance, from no rating of neat PLA to a V0 rating of FR-PLA containing 20 wt % aluminum hypophosphite in the UL-94 test. The microscale combustion calorimetry test showed that the heat release capacity, the peak of heat release and the total heat release of FR-PLA composites were significantly decreased with the increase of AHP content. The cone calorimeter test also confirmed that the addition of AHP resulted in a significant decrease in the peak heat release rate value of PLA/AHP composites compared with pure PLA. Additionally, the results from DSC and DMA tests indicated that the addition of AHP into polylactide significantly changed the crystallization behavior and storage modulus of polylactide. However, the addition of AHP decreases the tensile strength and elongation at break. The char after LOI testing was investigated by scanning electron microscopy and craterlike morphology was observed on the surface of the char. The thermal degradation process of FR-PLA composites was analyzed by real-time Fourier transform infrared spectroscopy.
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an intumescent flame retardant named SPDPM, and the sample with 25 wt % loading of SPDPM could reach a V0 rating and a high limiting oxygen index (LOI) value of 38 vol %.2 Other flame retardant additives such as aluminum hydroxide,11 expanded graphite,12 silica gel,13 and β-cyclodextrin14 have also been reported in past decades. However, these flame retardant additives need high loading to achieve good fire resistant effect, which will greatly affect the mechanical properties of the materials. Aluminum hypophosphite (AHP) has been widely used as an effective flame retardant for many engineering plastics, such as PET, PBT, and PA,15−17 which could achieve excellent flame retarded properties. As far as we know, few research reports have focused on flame retarded PLA with AHP. In this work, aluminum hypophosphite is used to prepare flame retardant PLA composites. The thermal, flame retardance, and mechanical properties of PLA composites were investigated by thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), microscale combustion calorimetry (MCC), cone calorimeter test, limiting oxygen index (LOI),
INTRODUCTION Polylactide (PLA) is one of the most promising candidates in the field of biobased plastics for its biodegradable property, and it can be produced by renewable sources, such as corn starch, potato starch, sugar beets, and other agricultural products.1,2 Moreover, PLA has excellent physical and mechanical properties and good transparency, which makes it a good candidate to replace traditional petrochemical plastics. Due to its excellent properties, PLA has been widely used in biomedical fields, household engineering, the packing industry, and so on.3−7 Unfortunately, the poor fire resistance of PLA restricts its further application and development in some important fields, especially in the electronics industry. Therefore, it is important and urgent to develop effective flame retardant systems for PLA. It is well-known that intumescent flame retardants (IFR) are efficient in some polymers and are widely used as halogenfree additives owing to their advantages of little smoke and low toxicity.8 An IFR system usually experiences an intense expansion and forms a protective char layer, thus well protecting the underlying material from the action of the heat flux or flame during combustion.9 Réti et al. introduced lignin and starch into an IFR system for improving the flame retardance of PLA.10 The results showed that the flame retarded PLA achieved a UL-94 V0 rating with the flame retardant at a loading level of 40 wt %. Zhan et al. synthesized © 2012 American Chemical Society
Received: Revised: Accepted: Published: 12009
March 27, 2012 August 7, 2012 August 13, 2012 August 13, 2012 dx.doi.org/10.1021/ie3008133 | Ind. Eng. Chem. Res. 2012, 51, 12009−12016
Industrial & Engineering Chemistry Research
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The Underwriter Laboratories 94 vertical burning test (UL94) was performed using a vertical burning instrument (CFZ-2 type, Jiangning Analysis Instrument Co., China), and the specimens for testing were of dimensions 130 × 13 × 3 mm3. In the measurements, the samples were vertically exposed to a Bunsen burner flame for 10 s. If the samples were selfextinguished, another 10 s was employed. The classification of the samples was defined according to ASTM D3801. The cone calorimeter test was performed on a cone calorimeter (Fire Testing Technology, U.K.) according to ASTM E1354/ISO 5660. Each specimen (100 × 100 × 3 mm3) was wrapped in an aluminum foil and exposed horizontally to a 35 kW/m2 external heat flux. Differential scanning calorimetry (DSC) measurements were performed with a TA DSC Q800 under nitrogen condition at a flow rate of 50 mL/min. Samples of 5−10 mg were sealed in aluminum pans and heated to 200 °C and kept at this temperature for 3 min to eliminate thermal history. Then the samples were quenched to 0 °C at the maximal cooling rate. The treated samples were reheated from 0 to 200 °C with a heating rate of 10 °C/min. In this test, the glass transition temperature (Tg), cold crystallization temperature (Tc), melting temperature (Tm), cool crystallization enthalpy (ΔHc), and melting enthalpy (ΔHm) were obtained from the second heating scans. The melting enthalpy of 100% crystalline PLA was taken as ΔH0m = 93 J/g.18 After considering the effect of AHP loading (ϕ), crystallinity was calculated by eq 1:19,20
UL-94 vertical burning test, mechanical testing, and dynamic mechanical analysis (DMA). Moreover, real-time Fourier transform infrared (FTIR) spectroscopy was used to investigate the thermal degradation process of the composites. The char residue after LOI testing was investigated by scanning electron microscopy (SEM).
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EXPERIMENTAL SECTION Materials. PLA resin (Nature Works 4032D) in granular form was supplied by Cargill Dow Inc. (USA). Aluminum hypophosphite (AHP) was supplied by Qingzhou Yichao Chemical Co. Ltd., China. Preparation of FR-PLA Composites. PLA and AHP were dried at 80 °C overnight before use. The samples were prepared by a two-roll mixing mill (XK-160, Jiangsu, China) at 175 °C, and the rotation speed of the screw was 100 rpm. The extrudates were cut into pellets and then hot-pressed under 10 MPa pressure for 10 min to obtain 3.2 mm thick plaques. The detailed formulations of the samples are shown in Table 1. Table 1. Results of UL-94 and LOI Tests for Neat PLA and FR-PLA Composites composition (wt %)
UL-94. 3.2 mm bar
sample
PLA
AHP
LOI
t1/t2a (s)
dripping
rating
PLA PLA/10AHP PLA/15AHP PLA/20AHP PLA/30AHP
100 90 85 80 70
0 10 15 20 30
19.5 25.5 27.5 28.5 29.5
BCb 10.7/1.7 8.3/3.4 0.9/5.8 1.1/2.2
Y Y Y N N
NRc V2 V2 V0 V0
χc (%) =
ΔHm − ΔHc (1 − ϕ)ΔHm0
·100% (1)
The mechanical properties were measured with a universal testing machine (WD-20D) according to ASTM D-638 at temperature 25 ± 2 °C. The specimens were prepared by cutting strips 4.0 ± 0.1 mm wide and 1.0 ± 0.05 mm thick. The crosshead speed was 20 mm/min. An average of at least five individual determinations was obtained. Dynamic mechanical properties were measured with a DMAQ800 (TA, USA). The dynamic storage modulus was determined at a frequency of 10 Hz and a heating rate of 5 °C over the temperature range 25−140 °C. Real time Fourier transform infrared (RT-FTIR) spectra were recorded using a MAGNA-IR 750 spectrometer (Nicolet Instrument Co., USA) equipped with a ventilated oven having a heating device. The samples were mixed with KBr powders, and the mixture was pressed into a disk, which was then placed into the oven. The temperature of the oven was raised at a heating rate of 10 °C/min in air condition. RT-FTIR spectra were obtained in situ during the thermooxidative degradation of the cured sample.
t1 and t2, average combustion times after the first and second applications of the flame. bBC, burns to clamp. cNR, not rated. a
Characterization. Thermal gravimetric analysis (TGA) was carried out using a Q5000 IR (TA Instruments) thermoanalyzer instrument. Samples were measured in an alumina crucible with a mass of about 5−10 mg. Composites in an open Pt pan were tested at temperatures ranging from room temperature to 750 °C with a heating rate of 20 °C/min. The onset decomposition temperature was defined as the temperature at which 5% of the original weight was lost. Tmax was defined as the temperature at which the samples processed the maximal weight loss rate. Microscale combustion calorimetry (MCC, Govmark) was used to analyze the combustion properties of the samples according to ASTM D 7309-7. For each sample 4−6 mg was heated from 100 to 600 at 1 °C/s in a stream of nitrogen flowing at 8 × 10−5 m3/min. The volatile anaerobic thermal degradation products in the nitrogen gas stream were mixed with a 2 × 10−5 m3/min stream of pure oxygen prior to entering a 900 °C combustion furnace. The MCC data obtained were reproducible to about 3%. The limiting oxygen index (LOI) was measured according to ASTM D2863 by an HC-2 oxygen index meter (Jiangning Analysis Instrument Co., China). The specimens used for the test were of dimensions 100 × 6.5 × 3 mm3. The morphology of the char residues after LOI tests was investigated with the use of a scanning electrical microscope (PHILIPS XL30E). The micrographs of the residues were obtained with a scanning electron microscope Inspect S at an accelerating voltage of 10 kV.
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RESULTS AND DISCUSSION Thermal Behavior. Thermogravimetric analysis (TGA) is an effective tool to evaluate thermal behavior for various polymers. The onset degradation temperature of samples which is evaluated by the temperature of 5 wt % weight loss (T−5%), the midpoint temperature of the degradation (T−50%), and the solid residue are obtained from the thermogravimetric (TG) curves; the temperature of the maximum weight loss rate (Tmax) of samples is obtained from the differential thermogravimetric (DTG) curves. The TG and DTG curves of aluminum phosphinate under nitrogen atmosphere are shown in Figure 1. In the nitrogen 12010
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AHP first decreases and releases phosphine and forms Al2(HPO4)3, and then part of phosphine is quickly oxidized into phosphoric acid at temperatures between 310 and 330 °C. At high temperatures of 450−650 °C, Al2(HPO4)3 degrades into aluminum pyrophosphate and releases water. Meanwhile, phosphoric acid dehydrates and releases water, forming polyphosphoric acid. Thermal degradation behaviors of PLA and FR-PLA composites in nitrogen condition are presented in Figure 2, and the related data are listed in Table 2. The onset
Figure 1. TGA and DTG curves of AHP under nitrogen and air conditions.
Table 2. TGA Data of AHP, PLA, and FR-PLA Composites under Nitrogen Condition
condition, the onset decomposition temperature (T−5%) of AHP is 334 °C, and the maximum mass loss rate occurs about 350 and 436 °C. The corresponding decomposition process of AHP could be described by eqs 7 and 7:
temperature at specific weight loss (°C)
a
PH3 and H2O are the main gas decomposition products at the first and second steps, respectively, leaving the residual product of Al4(P2O7)3. TG and DTG curves of AHP in the air condition are also shown in Figure 1. AHP presents a significantly different thermal behavior in the air condition compared to that in the nitrogen condition. It shows a sharp weight increase process between 310 and 330 °C with a Tmax at 321 °C. When the temperature further increases, AHP shows a slight decrease of mass in the range 450−650 °C and the char residue at 750 °C is 97.0%. The degradation behavior of AHP in air condition may be described as follows:
specimen
T−5% (°C)
T−50% (°C)
Tmax1 (°C)
Tmax2 (°C)
residuea
AHP PLA PLA/10AHP PLA/20AHP PLA/30AHP
334 353 344 336 332
− 387 388 389 393
350 − 351 351 351
436 392 392 392 395
74.2 0.2 8.2 14.8 22.4
At 750 °C.
degradation temperature (T−5%) of pure PLA is 353 °C, and those of the composites with 10, 20, and 30% AHP are 344, 336, and 332 °C, respectively. As the content of AHP increases, the value of T−5% is further decreased, indicating that the FRPLA composites have poorer thermal stabilities owing to the addition of AHP. Pure PLA displays a single Tmax at 392 °C during the thermal decomposition process. After the addition of AHP, another Tmax peak of FR-PLA composites around 351 °C is also observed, which is attributed to the decomposition of AHP. To further investigate the thermal properties of PLA and FRPLA, the composites are also analyzed by TGA in air condition, as shown in Figure 3, and the corresponding data are listed in Table 3. For pure PLA, T−5%, T−50%, and Tmax in air condition are reduced by 11, 11, and 12 °C compared to those in nitrogen
Figure 2. TGA and DTG curves of PLA and FR-PLA composites under nitrogen condition. 12011
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Figure 3. TGA and DTG curves of PLA and FR-PLA composites under air condition.
Table 3. TGA Data of AHP, PLA, and FR-PLA Composites under Air Condition temperature at specific weight loss (°C)
a
specimen
T−5% (°C)
T−50% (°C)
Tmax1 (°C)
Tmax2 (°C)
residuea
AHP PLA PLA/10AHP PLA/20AHP PLA/30AHP
− 342 334 332 330
− 376 371 373 373
321 − − 351 342
− 380 372 374 366
97.0 0.2 8.6 17.0 26.1
At 750 °C.
condition, indicating the oxygen promotes the degradation process of PLA molecules. Similar phenomena are also found in FR-PLA composites. This phenomenon could be explained as follows: when the TGA experiment is applied in nitrogen condition, only high temperature induces the degradation of the PLA chain; when the experiement is performed in air condition, the oxygen could oxidize the PLA chain, which promotes the degradation of PLA. Therefore, the samples show higher T−5%, T−50%, and Tmax values in nitrogen condition than those in air condition. It is interesting to find that in both the nitrogen and air conditions, the addition of AHP could significantly increase the char residues, indicating that the presence of AHP could promote the carbonization of PLA. Flammability of FR-PLA Composites. The LOI and the UL-94 vertical burning tests are widely used to evaluate the flammability of polymer materials. Table 1 shows the LOI values, UL-94 ratings, and dripping behaviors of all the samples. PLA is a flammable material with a low LOI value of 19.5%, and it shows no rating in UL-94 test. When 10 wt % AHP is added, the LOI value of PLA/10AHP is increased to 25.5%; however, this sample still cannot pass the UL-94 V0 rating. The sample with 20 wt % AHP addition achieves a higher LOI value of 28.5% and the UL-94 V0 rating. The photographs of the samples after LOI tests are shown in Figure 4. Pure PLA is of poor antidripping properties in the burning test. When 10 wt % AHP is added into the PLA matrix, dripping is partly inhibited. With the addition of 20 wt % AHP, dripping is completely suppressed. PLA/20AHP presents an intumescent char layer, as does PLA/30AHP. This intumescent char layer plays an important role that could effectively hinder mass and heat transportation and protect the inner materials from burning.21 Microscale combustion calorimetry (MCC) is an effective bench-scale measurement system for evaluating the combustion
Figure 4. Digital photos of samples after LOI tests: (a) PLA; (b) PLA/10AHP; (c) PLA/20AHP; (d) PLA/30AHP.
properties of materials and only needs milligram quantities of the specimen. It uses an oxygen consumption calorimeter to measure the rate and amount of heat which is produced by complete combustion of the fuel gases generated during controlled heating of the samples.22 The heat release rate (HRR) curves of FR-PLA composites are shown in Figure 5, and the corresponding data are presented in Table 4. In the MCC test, HRR is the most important parameter for evaluating the fire hazard of materials, and a low HRR value indicates low flammability and low full-scale hazard.23,24 As can be observed, the peak heat release rate (PHRR) value changes little for PLA/ 10AHP compared to that of neat PLA. However, for PLA/ 20AHP and AHP/30AHP, PHRR values are reduced by 23.4 and 48.0%, respectively, compared to that of neat PLA. Meanwhile, the total heat release (THR) is reduced from 12.6 kJ/g for pure PLA to 9.2 kJ/g for PLA/30AHP, implying that the presence of AHP effectively reduces the heat release rate and total heat release. Moreover, it is interesting to find that the addition of AHP could increase the THRR value by about 5−10 °C, which is consistent with the earlier report.25 This may be attributed to that the degradation of AHP induces the formation of thermal stable aluminum phosphate and aluminum pyrophosphate, which functions as a barrier to hinder the degradation of PLA. 12012
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Figure 6. HRR curves of PLA and FR-PLA composites from cone calorimeter test.
Figure 5. HRR curves of PLA and FR-PLA composites from MCC testing.
Table 4. MCC Results of PLA and FR-PLA Compositesa specimen
HRC (J/g·K)
PHRR (W/g)
THR (kJ/g)
TPHRR (°C)
PLA PLA/10AHP PLA/20AHP PLA/30AHP
359 361 276 187
361 360 276 187
12.6 11.8 12 9.2
380 389 386 385
HRC, heat release capacity, ±5 J/g·K; PHRR, peak heat release rate, ±5 W/g; THR, total heat release, ±0.1 kJ/g; TPHRR, temperature at PHRR, ±2 °C. a
Cone Calorimeter Testing. The cone calorimeter is a useful bench-scale tool for testing flammability properties of materials in real-world fire conditions.26 In this study, the cone calorimeter test was used to investigate the influence of AHP content on the flammability of FR-PLA composites, and the corresponding data are shown in Table 5. Figure 6 shows the
Figure 7. THR curves of PLA and FR-PLA composites from cone calorimeter test.
Table 5. Cone Calorimeter Data for Each Sample at 35 kW/ m2 a sample
TTI (s)
Tp (s)
PHRR (kW/m2)
THR (MJ/m2)
PLA PLA/10AHP PLA/20AHP PLA/30AHP
57 42 29 36
155 85 70 70
549 363 301 244
62.3 60.4 60.2 52.7
sensitive to the acid species that result from the pyrolysis products of AHP, and thereby the addition of AHP catalyzes the degradation of PLA and brings the TTI value forward. Differential Scanning Calorimetry. DSC is widely used in evaluating crystallization and melting behaviors of polymers, which are important parameters for polymer processing. Figure 8 gives DSC thermograms during the second heating cycle, and the relevant data are summarized in Table 6. The glass transition temperature (Tg) and the melting temperature (Tm) of pure PLA are 59.6 and 165.8 °C, respectively, and no obvious cold crystallization temperature (Tc) peak is found. The addition of AHP decreases the glass transition temperatures of FR-PLA composites, while the melting temperatures are slightly increased. Obvious cold crystallization temperature peaks are observed in FR-PLA composites, and the cold crystallization temperature value of the PLA/30AHP slightly decreases compared with those of PLA/10AHP and PLA/ 20AHP. It comes out that pure PLA exhibits the lowest crystallinity degree (1.5%), whereas the incorporation of AHP leads to increased crystallinity, i.e., 11.2, 12.5, and 19.0% for PLA/10AHP, PLA/20AHP, PLA/30AHP, respectively. The addition of AHP into PLA acts as a nucleating agent, giving rise to the significant increases in crystallinity of the FR-PLA composites. In previous papers, some other fillers, including
TTI, time to ignition, ±2 s; PHRR, peak heat release rate, ±15 kW/ m2; TP, time to PHRR, ±2 s; THR, total heat release, ±0.5 MJ/m2. a
heat release rate (HRR) curves of PLA and FR-PLA composites. It can be seen that pure PLA presents a sharp HRR peak appearing with a PHRR value of 549 kW/m2. For FR-PLA composites, the PHRR values of PLA/10AHP, PLA/ 20AHP, and PLA/30AHP are 363, 301, and 244 kW/m2, respectively, with reductions of 33.9, 45.2, and 55.6% compared with that of pure PLA. Figure 7 presents the total heat release (THR) curves of PLA and FR-PLA. At the end of burning, pure PLA releases a total heat of 62.3 MJ/m2. For the FR-PLA composites, the THR values exhibit a slight reduction compared with that of pure PLA, which is in good agreement with MCC results. The time to ignition (TTI) of virgin PLA is 57 s, while those of the FR-PLA composites shift to an earlier time. This phenomenon could be explained as that PLA is 12013
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strength, and PLA/20AHP and PLA/30AHP show tensile strengths of 40.6 and 32.4 MPa, respectively. Moreover, with the increase of AHP loading, the elongation at break of the FRPLA composites gradually decreases due to the rigid nature of AHP particles. Although the mechanical properties of the FRPLA composites deteriorate with the increase of AHP loading, the FR-PLA composites still show considerable mechanical properties for the application. Dynamic Mechanical Analysis (DMA). Figure 10 shows the storage modulus (E′) and tan δ versus temperature curves obtained from the DMA experiment. The storage modulus increases with the increase of AHP loading. The addition of 10 wt % AHP results in a 236% increase in the storage modulus at 25 °C and a 431% increase at 80 °C, indicating the improved rigidity of the FR-PLA composites, which may be because AHP is a kind of rigid filler acting as an effective strengthening agent. As can be observed from Figure 10b, the tan δ of pure PLA shows the highest damping. The incorporation of AHP significantly decreases the tan δ peak intensity compared to that of pure PLA. The decrease in intensity reflects an increased stiffness of the material at the glass transition region, but the glass transition temperature itself is nearly unaffected by the presence of AHP. This phenomenon indicates that the polymer chains do not have preferential affinity with flame retardants, since the onset temperature of segmental movement remains the same. Thermal Degradation Process. The thermal degradation process of pure PLA and PLA/30AHP is characterized by realtime FTIR. Figure 11a shows the decomposition process of pure PLA. The peak at 1750 cm−1 corresponding to the CO bond disappears at 350 °C, indicating that the ester group of PLA is pyrolyzed at this temperature. The wide peak ranging from 1300 to 1000 cm−1 is ascribed to the stretching vibration of the C−O−C bond, which is decreased at 350 °C. The stretching and bending vibrations of C−H at 2940, 2990, 1380, and 1460 cm−1 also disappear at 350 °C. The appearance of a new weak peak at 1610 cm−1 indicates that a CC bond is formed at 310 °C. Figure 11b shows the thermal decomposition process of the PLA/30AHP composite. It can be seen that three new peaks appear at room temperature compared to that of pure PLA: the peak at 2400 cm−1 corresponds to the stretching vibration of the P−H bond, the medium intensity peak at 817 cm−1 is assigned to the rocking mode of PH2, and the weak peak at 484 cm−1 corresponds to Al−O bond stretch modes.30 The intensity of these three peaks has a significant reduction at 310 °C, indicating that AHP begins to decompose at this temperature, which is consistent with the TGA results. With the temperature further increasing, the intensity of the peak at 1750 cm−1 ascribed to the CO bond reduces and disappears until 350 °C. The decrease of the CO bond is probably due to the formation of an OC−O−PO bond and unsaturated ester.2 The bands at 1080 and 1180 cm−1 are ascribed to the bending vibration of PH2 and the stretching vibration of the PO2 bond, respectively. When the temperature rises to 350 °C, these bands nearly disappear, meaning that the FR-PLA composite decomposes completely. At higher temperature (>350 °C), a new peak assigned to the stretching vibration of the P−O−P bond appears around 1100 cm−1, suggesting the formation of Al4(P2O7)3.16 Morphology of the Char. Figure 12 gives SEM images of char residue of the PLA/30AHP composite. Figure 12a displays a continuous char layer with some holes dispersed on the
Figure 8. Heat flow curves of PLA and FR-PLA composites from DSC testing.
Table 6. DSC Data of PLA and FR-PLA Compositions (Second Heating from 0 to 200 °C with a Ramp of 10 °C/ min) specimen
Tg (°C)
Tc (°C)
ΔHc (J/g)
Tm (°C)
ΔHm (J/g)
χc (%)
PLA PLA/10AHP PLA/20AHP PLA/30AHP
59.6 57.4 57.5 55.9
− 98.3 99.1 93.6
− 26.0 20.2 18.2
165.8 166.0 166.2 166.5
1.4 35.4 29.5 30.6
1.5 11.2 12.5 19.0
starch, talc,27 cyclodextrin,28 and layered double hydroxide,29 have been reported that could significantly improve the crystallinities of polymers. The double melting peaks of PLA composites are attributed to the lamellar reorganization, and the melting peaks located at the lower temperature are ascribed to less-organized crystals. Mechanical Properties. Values of the tensile strength and elongation at break of the FR-PLA composites are shown in Figure 9. Neat PLA presents a high tensile strength of 61.1 MPa and an elongation at break of 3.7%. When 10 wt % AHP is added, the tensile strength is reduced to 42.8 MPa, with a reduction of 30% compared to that of pure PLA. Further increase in AHP loading results in a slight decrease in tensile
Figure 9. Effect of AHP loading on the mechanical properties of FRPLA composites. 12014
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Figure 10. Storage modulus and tan δ curves of PLA and FR-PLA composites from DMA testing.
Figure 11. RT-FTIR spectra for the degradation process of pure PLA and PLA/30AHP at different temperatures.
Figure 12. SEM images of residues from FR-PLA composites after LOI tests: (a) PLA/30AHP, 300× magnification; (b) PLA/30AHP, 2000× magnification.
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surface in the low magnification image. When observed in the high magnification photo (Figure 12b), the char residue is compact and some craterlike structure is observed which may be resulted from the gas products degraded from aluminum hypophosphite.
CONCLUSIONS
Flame retardant PLA composites with AHP were prepared by the melt blending method. The presence of AHP exhibited excellent flame retardant effect for PLA. The LOI of FR-PLA composites was increased to 28.5%, and the UL-94 V0 rating 12015
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Industrial & Engineering Chemistry Research
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was achieved with 20 wt % AHP loading. MCC results revealed that AHP significantly decreased the peak heat release rate and the total heat release (THR) for FR-PLA composites, which was in good agreement with the cone calorimeter test. In both nitrogen and air conditions, the addition of AHP dramatically increased the char residues. According to real-time FTIR and SEM analyses, incorporation of AHP into PLA played an important role in the formation of the compact and protective char layer, which protected the inner material from being attacked by the heat flux and flame during combustion.
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by National Basic Research Program of China (973 Program) (2012CB719701), the joint fund of Guangdong province and CAS (No.2010A090100017), the Fundamental Research Funds for the Central Universities (WK2320000014) and Opening Project of Southwest University of Science and Technology (No. 11ZXFK12).
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dx.doi.org/10.1021/ie3008133 | Ind. Eng. Chem. Res. 2012, 51, 12009−12016
