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ammonium polyphosphate biocomposites
Accepted Manuscript Title: The effect of chitosan on the flammability and thermal stability of polylactic acid/ammonium polyphosphate biocomposites Author: Chen Xiaoyu Gu Xiaodong Jin Jun Sun Sheng Zhang PII: DOI: Reference:
S0144-8617(16)31300-5 http://dx.doi.org/doi:10.1016/j.carbpol.2016.11.035 CARP 11747
To appear in: Received date: Revised date: Accepted date:
6-4-2016 13-10-2016 11-11-2016
Please cite this article as: Chen, ., Gu, Xiaoyu., Jin, Xiaodong., Sun, Jun., & Zhang, Sheng., The effect of chitosan on the flammability and thermal stability of polylactic acid/ammonium polyphosphate biocomposites.Carbohydrate Polymers http://dx.doi.org/10.1016/j.carbpol.2016.11.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The effect of chitosan on the flammability and thermal stability of polylactic acid/ammonium polyphosphate biocomposites
Chen, Xiaoyu Gu, Xiaodong Jin, Jun Sun*, Sheng Zhang*
Key Laboratory of Carbon Fiber and Functional Polymers (Beijing University of Chemical Technology, Ministry of Education), Beijing 100029, China
*
Corresponding author. E-mail address: [email protected] (J. Sun), [email protected] (S. Zhang). 1
Highlights
Polylactic acid was blended with chitosan and APP. Chitosan together with APP significantly improved the flame retardancy of PLA. A clear synergistic effect was found between chitosan and APP. Both Chitosan and APP improved the crystallinity of the composites.
Abstract: This work reports our recent efforts on introducing chitosan (CS) in association with ammonium polyphosphate (APP) into polylactic acid (PLA) by melt blending to improve the flame retardancy of the biocomposites. The flammability of the composites was characterized by limiting oxygen index (LOI), UL-94 vertical burning test and cone calorimetry test (CONE). The results showed that the PLA sample containing 2% CS and 5% APP achieved the maximal LOI value of 33.1, passed the UL-94 V-0 rating, and decreased the peak heat release rate to 425.6 kW/m2. The morphology characterization of char residue by scanning electron microscope indicated a dense, homogeneous and continuous residue char could be formed by the presence of APP and CS in PLA. Fourier transform infrared spectroscopy and thermal gravity analysis suggested that CS could act as a novel carbon agent owning to its high content of carbon atoms and multi-hydroxyl groups, and the interaction between CS and APP could provide synergistic effects in improving the flame retardancy of PLA biocomposites. X-ray diffraction and differential scanning calorimetry results demonstrated that the presence of APP and CS could promote the crystallization of PLA.
Keywords: Polylactic acid, Flame retardancy, Chitosan, Synergistic effect
1. Introduction
2
Among all of the bio-based and biodegradable polymers, PLA has
attracted much
attention in the last two decades due to its excellent physical properties and high degree of transparency (Fox et al., 2014; Shan et al., 2012). However, the high flammability of PLA restrains its applications in automobiles, packing materials and electronic fields. Three main approaches that have been used to reduce the flammability of PLA include: modification of PLA chemical structure, surface treatments to the polymeric fibers/fabrics, and incorporation of flame retardants by melt blending (Bourbigot and Fontaine, 2010). Both chemical modified and surface treated PLA can provide excellent flame retardant durability, but the synthesis process is usually complicated and expensive. Blending with flame retardants has been developed into the most commonly used approach because of its ease of fabrication and high flame retardant efficiency. With a tremendous increase in environmental pollution and the pressure of strict law over the past years, halogen-contained flame retardants are gradually replaced by environmental friendly additives (Qian et al., 2013). In early studies, halogen-free flame retardants like phosphorus/nitrogen containing flame retardants (Deng, Huang, Yang and Wu, 2009), aluminum hydroxide (Nishida et al., 2005), expended graphite (Zhu et al., 2011) and nano-additives (Alexandre and Dubois, 2000; Bourbigo and Fontaine, 2009; Bourbigot, Fontaine, Duquesne and Delobel, 2008; Hapuarachchi and Peijs, 2010; Solarski et al., 2008) as well as intumescent flame retardants (IFR) have been successfully used in flame retardant PLA composites. IFR has been widely used in polymers as halogen-free additives for its excellent performance in protecting the polymer matrix and decreasing smoke release. However, the flame retardant efficiency of traditional IFR used in PLA was lower than that in other polymer matrix. More specifically, the traditional carbon source like pentaerythritol showed deteriorated char formation performance. During the past years, many efforts were made to finding new efficient carbon source such as cellulose, starch, β-cyclodextrin (Réti et al., 2008), polypseudorotaxan (Wang et al., 2013), hyperbranched polyamine (Ke et al., 2010) and kenaf (Shukor et al., 2014). There is a common advantage of those carbon sources, they not only improve the 3
flame retardancy of the composites, but maintain the environmental friendly character of PLA. Chitosan, produced by alkaline deacetylation of chitin (El-Tahlawy, 2008), was drawing lots of attention for its amino polysaccharide with multi-hydroxyl groups (Xiao et al., 2014). Chitosan has been widely used in biotechnology, biomedicine, food processing, wastewater treatment and so on because of its widespread availability, low price, nontoxicity, and biodegradability (Yang et al., 2016). CS has been widely used in the layer-by-layer assembly system for fabrics (Carosio, Alongi and Malucelli, 2012; Laufer, Kirkland, Cain and Grunlan, 2012). Positively charged chitosan was paired with anionic montmorillonite clay nano-platelets to protect polymer matrix. However, CS as a flame retardant additive in improving the fire performance of PLA has not been reported so far. In this paper, CS was introduced into PLA as a novel flame retardant additive. It was expected that CS could act as carbon source, whereas APP should act as acid source, releasing polyphosphoric acid at high temperature and promoting the char formation. The flammability of PLA composites was evaluated and analyzed. The flame retardant mechanism of CS and APP in PLA was proposed.
2. Experimental
2.1. Materials
PLA (3052D) with a specific gravity of 1.24 g/cm3 was supplied by Nature Works Company, America. CS (biochemical reagent) with a deacetylation degree of 80-95% was obtained from Sinopharm Chemical Reagent Company, China. APP (phase Ⅱ) was provided by Shandong Yingtai Company, China.
2.2. Preparation of flame retardant PLA sample
The formulations of the samples were listed in Table 1. All materials were dried at 80°C for 12 h before use. PLA, CS and APP were blended by a micro twin-screw 4
extruder (Wuhan Rayzong Ming Plastics Machinery Co., Ltd, China) with the speed of 40 rpm. The extrusion temperatures were fixed at 185°C, 190°C and 190°C from the hopper to die, respectively. Then the samples were pelletized and hot-pressed into standard plates (100×100×3 mm3, 100×10×3 mm3, and 125×13×3.2 mm3) for 25min under the condition of 190°C, 10 MPa.
2.3. Measurements
The LOI value of the composites was tested by a Jiangning JF-3 oxygen index apparatus according to ISO 4589. The vertical UL-94 tests were implemented by a Jiangning CZF-3 apparatus according to ISO 9773 with a sample thickness of 3.2 mm. Mass flow rate (MFR) was measured by melt flow tester CEAST P/N 6941. The load weight was 2.16 kg and the melting temperature was 190°C. Flammability of the composites was characterized using a cone calorimeter from FTT, UK according to ISO 5660 with an incident flux of 50 kW/m2. Thermal gravity analysis (TGA) was undertaken by a TA SDT Q500 apparatus at a heating rate of 10°C/min under air atmosphere from ambient temperature to 600°C. The mass of specimen was approximately 5 mg. Scanning electron microscopy (SEM) was performed on a Hitachi S-4800 scanning electron microscope under the voltage of 20 kV. Differential Scanning Calorimeter (DSC) was carried out using a TA Instrument Q-2000apparatus. Each sample (about 5 mg) was first heated to 190°C at a heating rate of 10°C/min, and then held at 190°C for 5 min to eliminate the thermal history. The non-isothermal crystallization behavior was recorded with the cooling curve from 190 to 40°C at a cooling rate of 10°C/min. The second heating curve was recorded at a heating rate of 10°C/min from 40 to 190°C, exhibiting the trace of melting process. Fourier transform infrared (FTIR) spectra were recorded by a Nicolet Nexus 670 FTIR spectrometer under the resolution of 4 cm-1 in 128 scans by KBr disk with the wavenumbers from 4000 to 500 cm-1. Wide angle X-ray diffraction (XRD) patterns were collected by a D/max2500 5
VB21/PC diffractometer with Cu-Kα radiation (wavelength, λ=0.154 nm) in the range of 2= 5~70°.
3. Results and discussions
3.1. LOI and UL-94 test results
The LOI test results of PLA/CS samples are listed in Table 2. Neat PLA shows poor flame retardancy with a LOI value of 20.0 and the addition of CS alone can improve the flame retardancy of PLA to some extent. The LOI values of PLA/CS composites first increase with CS loading and reached its maximum value of 24.3 when CS content is 7%. However, further increase of CS decreases the LOI value. The LOI value drops to only 22.7 when CS content was 9%, which is much lower than that of PLA-C7 sample. This may be due to the decrease of viscosity caused by large CS loading, leading to more dripping and faster flame spreading. The viscosity of PLA containing different amount of CS can be measured by MFR test as shown in Fig. 1. It is observed that the addition of CS results in a decrease in the MFR value of PLA sample when the amount of additive is less than 7%. However, the MFR value increases sharply by the presence of 9% CS, indicating an obvious decrease in the melt viscosity. A possible explanation for this phenomenon is that the addition of 9% CS causes the degradation of PLA due to the existence of water, leading to the decrease in melt viscosity.
The flame retardant properties of PLA composites are further improved after loading with both CS and APP. The relevant results are also presented in Table 2. The LOI value of samples loaded with both APP and CS are observed to be higher than that of samples loaded with APP or CS alone under the same total loading of 7%. The LOI value is increased with the amount of APP. When the weight ratio of APP to CS is 5:2, the LOI reaches the highest value of 33.0. Compared with other materials reported in the literatures, CS shows higher flame retardant efficiency than 6
pentaerythritol and β-cyclodextrin (Feng, Su and Zhu, 2011). The enhancement of the flame retardancy of PLA may be due to the synergistic effect between APP and CS. The UL-94 test results are also shown in Table 2. Neat PLA burns out during the test and no UL-94 rating can be achieved. For PLA/CS samples, only V-2 rating is achieved due to the serious melt dripping which can ignite the absorbent cotton. All formulations containing both CS and APP reach V-0 rating, demonstrating the improvement of flame retardancy.
3.2. Cone calorimeter test
Fig. 2a shows the heat release rate (HRR) curves of different PLA samples from cone calorimeter test (CONE). The peak HRR value of neat PLA is 514.7 kW/m2, and the peak HRR value of PLA-A7 and PLA-C7 samples are 508.2 and 524.1 kW/m2 respectively, suggesting that the addition of APP or CS alone cannot reduce the HRR effectively. However, the peak HRR value of PLA-C2A5 sample decreases to 425.6 kW/m2, exhibiting a 17.4% reduction compared to that of neat PLA. It is worth noting that the HRR curve of PLA-C2A5 sample shows multi-peak phenomenon while only one peak can be observed for other samples. For PLA-C2A5 sample, the first peak observed after ignition is ascribed to the build-up of a foamed cellular layer on the polymer surface. Once formed, it shields the underlying polymer from the external heat exposure, and a plateau value appears in the curve. The second peak is assigned to the breakage of the intumescent char structure at higher temperatures, leading to further degradation of the substrate. Large amounts of combustible products are released leading to the increase in the HRR and formed the second peak. Fig. 2b shows the total heat release (THR) curves of different PLA samples. PLA-C2A5 sample presents the lowest THR value, indicating APP and CS inhibit the fully combustion of the sample and then reduce the THR value. Fig. 2c shows the weight loss curves of the composite samples. The amount of the char residue for the samples is summarized in Table 3. No residue char is obtained for neat PLA after combustion, and PLA-C7 sample remains only 0.6% char residue, 7
demonstrating that the incorporation of CS alone cannot significantly promote the char formation. For PLA-A7 sample, the amount of char residue increases to 3.1%, indicating an efficient char forming ability of APP. For PLA-C2A5 sample, the amount of char residue is further increased to 4.1%, which is much higher than the calculated result (2.4%), demonstrating the existence of synergistic effect between APP and CS in promoting the char formation. The amount of char residue is quite impressive considering only 7% flame retardants are loaded with PLA. It is proposed that CS can act as a carbon source because of its polysaccharide structure, while APP can act as acid source to catalyze the char formation during the burning process.
The formation of the char layer is believed to be principally responsible for the reduction of HRR during CONE tests (Murariu et al., 2010). The presence of the char acts as a barrier to isolate the heat and oxygen exchange. The more char residue formed during the combustion leads to the better barrier protection. It is reasonable to regard the flame retardant mechanism of PLA/CS/APP composites as a condensed phase mechanism in terms of CONE test results. The reactions between APP and CS can promote the char forming which will be further discussed in section 3.3. The condensed phase mechanism can be verified by comparing effective heat of combustion (EHC) of PLA-C2A5 sample to that of neat PLA. EHC can be obtained from HRR (kW/m2) divided by mass loss rate (g/s). It is believed that lower EHC is caused by more smoke release and the formation of gaseous intermediates which scavenges flame propagating free radicals (e.g.: OH·, H·) to inhibiting complete combustion to CO2 (Gilman, Kashiwagi and Lichtenhan, 1997; Sun et al., 2014). It is suggested that low EHC value indicates the flame retardant combination take effect through gas phase. Fig. 2d illustrates the EHC curves of different samples. It can be seen that the EHC value of PLA-C2A5 sample is slightly higher than that of neat PLA, indicating a classical condensed phase mechanism for PLA/CS/APP composites. 3.3. Thermal stability and crystallinity
The flame retarding effect between CS and APP on PLA shown in flammability 8
test results (Table 2 and Fig. 2) indicates that the synergistic effect mainly occurs in condensed phase. Further investigation has been under taken by analyzing TGA curves, and Fig. 3a shows the TGA curves of CS, APP, APP/CS (EXP) and APP/CS (CAL) samples. The corresponding theoretical curve is obtained based on the hypothesis that there is no reaction between the two components and calculated by linear combination of the weight loss of the components degraded separately under air atmosphere. It can be observed that the experimental and calculated curves are similar before 400°C, and the gap between the experimental curve and the calculated one increases progressively afterward. APP/CS (EXP) complex shows very high char forming ability. The experimental residue at 600oC is up to 58.6%, while the calculated residue is only 40.9%. At high temperature, APP degraded to release phosphoric acid, polyphosphoric acid and non-flammable gases. Those phosphorous containing acids act as catalysts contributing to the breaking of the bone chain, intramolecular or intermolecular dehydration and char forming of chitosan (Shukoret al., 2014). The differences between the experimental and calculated curves prove the synergistic effect between APP and CS. The thermal degradation behaviors of PLA, PLA-A7, PLA-C7 and PLA-C2A5 are compared in order to further investigate the synergistic effect between APP and CS, and the results are illustrated in Fig. 3b. All of the samples mainly perform a one-stage weight loss in the range of 300-400°C, and only slight difference is observed before 370°C. The weight of PLA and PLA-C7 samples decreases slowly after 370°C and end up with a small amount of char residue. For PLA-C2A5 and PLA-A7 samples, the amount of char residue is similar and much higher than that of neat PLA and PLA-C7 samples. In consideration of different APP additive amount (5%, 7%), it can be concluded that the addition of 2% CS promotes the char formation of PLA-C2A5 samples.
Fig. 4 presents the experimental and calculated TG curves of PLA-C2A5 sample. The calculated curve is calculated by linear combination from the experimental curves 9
of PLA-A7 and PLA-C7 curves based on their percentages under the same condition. All the samples show a one-stage weight loss between 300°C and 400°C. The decomposition temperature of PLA-C2A5 is lower than that of PLA-C2A5 (CAL) and neat PLA, indicating the adding of APP and CS catalyzes the degradation process of the sample at lower temperature. However, at 600°C, the amount of char residue for the experimental value increases to 6.0% which is higher than the calculated value of 4.7%. The 1.3% increase is worth noticed considering the total amount of addition is only 7%. The increase in the amount of char residue demonstrates the presence of APP and CS in PLA can promote the char formation. In order to get a better understanding of the degradation pathway and synergistic interaction between CS and APP, the decomposition products of CS/APP (2:5, wt. /wt.) at different temperatures (selected according to TGA curves) have been analyzed by FTIR. The obtained spectra are shown in Fig. 5, and the spectrum of residue char of PLA-C2A5 sample is also illustrated. At ambient temperature, the characteristic peaks of CS at 3065 cm-1 (-OH), 1691 cm-1 (C=O), 1435 cm-1 (CH), 1070 cm-1 (C-O-C) and APP at 3193 cm-1 (NH4+), 1252 cm-1 (P-O), 1019 cm-1 (P=O), 892 cm-1 (P-O-P) can be observed. When the temperature increases to 300oC, the peak of NH4+ disappears, which is due to the removal of NH3. For the spectrum at 350oC, the characteristic peaks are changed significantly. A new peak can be observed at 1168 cm-1, demonstrating the presence of P-O-C, which is formed by the dehydration reaction between polyphosphoric acid and chitosan. Another characteristic peak of P=O in APP shifts to 994 cm-1 because of the bonding of CS. Besides, the peaks at 1691 cm-1 and 1435 cm-1 disappear and new peaks appear at 1639 cm-1 and 1401 cm-1, which is corresponding to the formation of C=C and CH2 bonds. It can be proposed that the ring of chitosan starts to open after 350oC, and a carbonous char layer starts to be formed. With the increase of temperature, the intensities of peaks for C-O-C (1070 cm-1) and CH2 (1401 cm-1) turn to weak, indicating the ring opening of CS and formation of aromatic ring. At 600oC, peaks at 1635 cm-1 (C=C), 1146 cm-1 (P-O-C), 999 cm-1 (P=O), and 887 cm-1 (P-O-P) can still be found, which confirms that most phosphorus are left in the aromatic char residue. Compared with the spectra of 600oC and residue char of PLA-C2A5 sample, no significant difference can be observed, 10
demonstrating that PLA doesn’t participate in the formation of residue char, which is consistent with the TGA result (shown in Fig. 4).
Fig. 6 illustrates the proposed charring mechanism of APP and CS. The promotion of char residue for PLA/APP/CS samples can be attributed to the reaction between APP and CS. Based on the FTIR results discussed above, it is proposed that APP usually starts to decompose at about 300oC, releasing non-flammable gases such as ammonia and forming polyphosphoric acids. These acids can further react with the hydroxyl groups in CS via esterification. Similar mechanisms between polysaccharide hydroxyl and APP have been proved by previous works (Feng, et al., 2011; Li et al., 2009). When the temperature is higher than 350oC, CS starts to decompose via ring opening reaction accompanying with the removal of -NH2 groups. At high temperature (>400oC), the degradation products of CS and APP can self-condense to form a crosslinked structure with aromatic rings. After the rearrangement of the crosslinked structure, more stable carbonous char layers are formed.
The melting and crystallization behaviors of different PLA samples are analyzed by DSC. Fig. 7 presents the DSC curves after eliminating thermal-history, and Table 5 provides some key thermal data obtained from DSC curves. All the samples containing flame retardants presents double melting peaks: the peak at lower temperature indicates small crystals with cracks, and the other peak is formed through the recrystallized crystals melting process. The incorporation of APP or CS into PLA matrix exerts minimal effect on Tg, demonstrating the effect of the presence of APP or CS to the movement of chain segments is minimal. However, the crystallinity of the sample is greatly affected by the additives. The crystallinity of PLA-A7, PLA-C7 and PLA-C2A5 samples are 24.3%, 30.6% and 28.3% respectively, while neat PLA is usually amorphous with no crystallinity. The increase of the crystallinity leads to increased brittleness of the sample (Fox et al., 2014). Both CS and APP increase the crystallization temperature and the crystallinity in the role of nucleating agents. The rate of crystallization is increased by the presence of nucleating agents, whilst CS is more effectively than APP in promoting the formation 11
of crystals at lower temperature.
In order to further understand the crystal structures of different PLA samples, wide angle X-ray diffraction analyses was performed. Fig. 8 displays the XRD patterns of PLA and PLA composites. It is observed that neat PLA shows a wide amorphous halo, and no distinct crystal peak is found in the diffraction patterns, which is consistent with DSC results. For the samples containing flame retardant, some crystal peaks can be observed. It has been reported that APP is crystallizable (Liu, et al., 2011), so the samples of PLA-A7 and PLA-C2A5 exhibit distinct crystal peaks at 14.5°, 15.4°, and 25.9°-30.4°, which is associated to APP. It has been discussed that the incorporation of APP and CS can improve the crystallization behavior of PLA. Two strong peaks at 16.5° and 18.9° can be detected in the curves of PLA composite samples containing flame retardants (Fig. 8), which is corresponding to the diffraction peak of α crystal of PLA. In a word, both APP and CS can promote the crystallization behavior of PLA.
3.4. Morphology of char
Fig. 9 shows the photos of the char residue of different PLA samples collected after CONE tests. Neat PLA and PLA-C7 samples leave trace of hard residues, whereas the char residue is loose for PLA-A7 sample. The compact char is observed only in PLA-C2A5 sample. The compact charred layers can efficiently protect the underlying polymeric materials from further degradation; as a result PLA-C2A5 sample exhibits the best flame retardant properties. Moreover, the inner char of PLA-A7 sample presents a loose and porous structure, while the inner char of PLA-C2A5 sample is quite dense with almost no big voids. APP can act as both acid source and blowing agent. For PLA-A7 sample, non-flammable gases like NH3 and H2O released by APP degradation process causes the clasp of the unstable char. However, by the presence of both APP and CS, the reaction between them stabilizes the char, resulting in a dense char structure.
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Fig. 10 shows the SEM images of the char residue of PLA composite. Similar to the digital pictures in Fig. 9, only the PLA-C2A5 sample presents a dense, homogeneous and continuous char structure with fewer voids and cracks. The char residue of PLA-C7 is very loose. For PLA-A7, the char residue is much denser than PLA-C7, but many big voids can be observed. The dense char plays an important role in hindering the flammable gases and heat exchange by providing a good flame shield for the underlying polymeric substrate. The incorporation of APP and CS together enhances the structure of char residue, resulting in lower heat release rate and higher LOI values.
4. Conclusions
CS has been successfully employed as a carbon source to enhance the char formation of PLA. CS/APP combination displays high efficiency in improving flame retardant properties of PLA. The LOI value can reach up to 33, and UL-94 can pass v-0 for the PLA composite containing 2% CS and 5% APP. It is found that there is an intensive interaction between CS and APP. The combination of APP and CS produces continuous and homogenous char residue. The intumescent protective layer formed by APP/CS significantly reduces the HRR and THR. Both CS and APP can act as nucleating agents to increase the crystallinity of PLA. It is suggested CS can also be applied as carbon agent in IFR formulations to other polymer matrix because of its inherent environmental friendly property and char-forming ability.
Acknowledgments
The authors would like to thank the National Natural Science Foundation of China (Grant No. 21674008) and the Fundamental Research Funds for the Central Universities (YS201402) for the financial support of this research.
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Fig. 1. MFR values for PLA/CS samples.
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Fig. 2. The HRR (a), THR (b) and Weight loss (c) curves of PLA, PLA-A7, PLA-C7 and PLA-C2A5 samples and EHC (d) curves of. PLA and PLA-C2A5 samples.
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Fig. 3.The TGA curves of APP, CS, APP/CS (EXP), APP/CS (CAL) samples (a); and PLA， PLA-C7, PLA-A7, PLA-C2A5 samples (b).
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Fig. 4. The TGA curves of neat PLA, PLA-C2A5 and PLA-C2A5 (CAL) samples.
19
Fig. 5. FTIR spectra of CS/APP samples at different temperatures and residue char of PLA-C2A5 sample
20
Fig. 6. Proposed charring mechanism of APP and CS
21
Fig. 7. DSC curves of PLA, PLA-A7, PLA-C7, and PLA-C2A5 samples.
22
Fig.8. XRD patterns of PLA, PLA-A7, PLA-C7, and PLA-C2A5 samples.
23
Fig. 9. Digital photos of the residue char after CONE test: PLA (a), PLA-A7 (b), PLA-C7 (c), and PLA-C2A5 (d).
24
Fig. 10. SEM images of the char residue for PLA-C7 (a), PLA-A7 (b) and PLA-C2A5(c).
25
Table 1 Formulations of PLA composite samples Sample
PLA/%
CS/%
APP/%
PLA PLA-C5 PLA-C7 PLA-C9 PLA-C5A2 PLA-C3.5A3.5 PLA-C2A5 PLA-A7
100 95 93 91 93 93 93 93
0 5 7 9 5 3.5 2 0
0 0 0 0 2 3.5 5 7
26
Table 2 The flame retardant properties of PLA/CS and PLA/CS/APP composite Samples PLA PLA-C5
LOI (%) 20.0 22.5
UL-94 *t
1/t2
(s)
Cotton ignition
rating
-/-
Yes
NR
12.5/20.6
Yes
V-2
PLA-C7
24.3
9.0/8.0
Yes
V-2
PLA-C9
22.7
9.3/12.5
Yes
V-2
PLA-C5A2
29.0
1.1/4.4
No
V-0
PLA-C3.5A3.5
30.0
1.7/3.6
No
V-0
PLA-C2A5
33.1
1.8/3.6
No
V-0
PLA-A7
27.0
1.0/2.0
Yes
V-2
*t1/t2 represent the after-flame time after the first and second 10 s flame application, respectively
27
Table 3 Char residue (600s) of the PLA composite samples samples
PLA
PLA-C7
PLA-A7
PLA-C2A5(CAL*)
PLA-C2A5(EXP)
Char residue/%
0
0.6
3.1
2.4
4.1
* CAL= 0.6×(2/7)+3.1×(5/7)=2.4
28
Table 4 Char residue (600°C) of the samples Samples PLA PLA-C7 Char residue/% (600℃)
1.8
1.9
PLA-A7
PLA-C2A5(CAL*)
PLA-C2A5
5.9
4.7
6.0
* CAL =1.8×(2/7)+5.9×(5/7)=4.7
29
Table 5 The data collected from DSC. Sample
Tg (°C)
Tc (°C)
Tm (°C)
ΔHc (J/g)
ΔHm (J/g)
Crystallinity (%)
PLA PLA-A7 PLA-C7 PLA-C2A5
59.9 59.9 59.8 59.9
113.1 118.1 114.5
141.4 150.0 149.6
0 22.7 28.6 26.4
0 23.6 27.3 26.7
0 24.3 30.6 28.3
Tg: Glass transition temperature, Tc: Cold crystallization temperature, Tm: Melting temperature, ΔHc: Enthalpies of the cold crystallization, ΔHm: Enthalpies of the melting.
30
S0144-8617(16)31300-5 http://dx.doi.org/doi:10.1016/j.carbpol.2016.11.035 CARP 11747
To appear in: Received date: Revised date: Accepted date:
6-4-2016 13-10-2016 11-11-2016
Please cite this article as: Chen, ., Gu, Xiaoyu., Jin, Xiaodong., Sun, Jun., & Zhang, Sheng., The effect of chitosan on the flammability and thermal stability of polylactic acid/ammonium polyphosphate biocomposites.Carbohydrate Polymers http://dx.doi.org/10.1016/j.carbpol.2016.11.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The effect of chitosan on the flammability and thermal stability of polylactic acid/ammonium polyphosphate biocomposites
Chen, Xiaoyu Gu, Xiaodong Jin, Jun Sun*, Sheng Zhang*
Key Laboratory of Carbon Fiber and Functional Polymers (Beijing University of Chemical Technology, Ministry of Education), Beijing 100029, China
*
Corresponding author. E-mail address: [email protected] (J. Sun), [email protected] (S. Zhang). 1
Highlights
Polylactic acid was blended with chitosan and APP. Chitosan together with APP significantly improved the flame retardancy of PLA. A clear synergistic effect was found between chitosan and APP. Both Chitosan and APP improved the crystallinity of the composites.
Abstract: This work reports our recent efforts on introducing chitosan (CS) in association with ammonium polyphosphate (APP) into polylactic acid (PLA) by melt blending to improve the flame retardancy of the biocomposites. The flammability of the composites was characterized by limiting oxygen index (LOI), UL-94 vertical burning test and cone calorimetry test (CONE). The results showed that the PLA sample containing 2% CS and 5% APP achieved the maximal LOI value of 33.1, passed the UL-94 V-0 rating, and decreased the peak heat release rate to 425.6 kW/m2. The morphology characterization of char residue by scanning electron microscope indicated a dense, homogeneous and continuous residue char could be formed by the presence of APP and CS in PLA. Fourier transform infrared spectroscopy and thermal gravity analysis suggested that CS could act as a novel carbon agent owning to its high content of carbon atoms and multi-hydroxyl groups, and the interaction between CS and APP could provide synergistic effects in improving the flame retardancy of PLA biocomposites. X-ray diffraction and differential scanning calorimetry results demonstrated that the presence of APP and CS could promote the crystallization of PLA.
Keywords: Polylactic acid, Flame retardancy, Chitosan, Synergistic effect
1. Introduction
2
Among all of the bio-based and biodegradable polymers, PLA has
attracted much
attention in the last two decades due to its excellent physical properties and high degree of transparency (Fox et al., 2014; Shan et al., 2012). However, the high flammability of PLA restrains its applications in automobiles, packing materials and electronic fields. Three main approaches that have been used to reduce the flammability of PLA include: modification of PLA chemical structure, surface treatments to the polymeric fibers/fabrics, and incorporation of flame retardants by melt blending (Bourbigot and Fontaine, 2010). Both chemical modified and surface treated PLA can provide excellent flame retardant durability, but the synthesis process is usually complicated and expensive. Blending with flame retardants has been developed into the most commonly used approach because of its ease of fabrication and high flame retardant efficiency. With a tremendous increase in environmental pollution and the pressure of strict law over the past years, halogen-contained flame retardants are gradually replaced by environmental friendly additives (Qian et al., 2013). In early studies, halogen-free flame retardants like phosphorus/nitrogen containing flame retardants (Deng, Huang, Yang and Wu, 2009), aluminum hydroxide (Nishida et al., 2005), expended graphite (Zhu et al., 2011) and nano-additives (Alexandre and Dubois, 2000; Bourbigo and Fontaine, 2009; Bourbigot, Fontaine, Duquesne and Delobel, 2008; Hapuarachchi and Peijs, 2010; Solarski et al., 2008) as well as intumescent flame retardants (IFR) have been successfully used in flame retardant PLA composites. IFR has been widely used in polymers as halogen-free additives for its excellent performance in protecting the polymer matrix and decreasing smoke release. However, the flame retardant efficiency of traditional IFR used in PLA was lower than that in other polymer matrix. More specifically, the traditional carbon source like pentaerythritol showed deteriorated char formation performance. During the past years, many efforts were made to finding new efficient carbon source such as cellulose, starch, β-cyclodextrin (Réti et al., 2008), polypseudorotaxan (Wang et al., 2013), hyperbranched polyamine (Ke et al., 2010) and kenaf (Shukor et al., 2014). There is a common advantage of those carbon sources, they not only improve the 3
flame retardancy of the composites, but maintain the environmental friendly character of PLA. Chitosan, produced by alkaline deacetylation of chitin (El-Tahlawy, 2008), was drawing lots of attention for its amino polysaccharide with multi-hydroxyl groups (Xiao et al., 2014). Chitosan has been widely used in biotechnology, biomedicine, food processing, wastewater treatment and so on because of its widespread availability, low price, nontoxicity, and biodegradability (Yang et al., 2016). CS has been widely used in the layer-by-layer assembly system for fabrics (Carosio, Alongi and Malucelli, 2012; Laufer, Kirkland, Cain and Grunlan, 2012). Positively charged chitosan was paired with anionic montmorillonite clay nano-platelets to protect polymer matrix. However, CS as a flame retardant additive in improving the fire performance of PLA has not been reported so far. In this paper, CS was introduced into PLA as a novel flame retardant additive. It was expected that CS could act as carbon source, whereas APP should act as acid source, releasing polyphosphoric acid at high temperature and promoting the char formation. The flammability of PLA composites was evaluated and analyzed. The flame retardant mechanism of CS and APP in PLA was proposed.
2. Experimental
2.1. Materials
PLA (3052D) with a specific gravity of 1.24 g/cm3 was supplied by Nature Works Company, America. CS (biochemical reagent) with a deacetylation degree of 80-95% was obtained from Sinopharm Chemical Reagent Company, China. APP (phase Ⅱ) was provided by Shandong Yingtai Company, China.
2.2. Preparation of flame retardant PLA sample
The formulations of the samples were listed in Table 1. All materials were dried at 80°C for 12 h before use. PLA, CS and APP were blended by a micro twin-screw 4
extruder (Wuhan Rayzong Ming Plastics Machinery Co., Ltd, China) with the speed of 40 rpm. The extrusion temperatures were fixed at 185°C, 190°C and 190°C from the hopper to die, respectively. Then the samples were pelletized and hot-pressed into standard plates (100×100×3 mm3, 100×10×3 mm3, and 125×13×3.2 mm3) for 25min under the condition of 190°C, 10 MPa.
2.3. Measurements
The LOI value of the composites was tested by a Jiangning JF-3 oxygen index apparatus according to ISO 4589. The vertical UL-94 tests were implemented by a Jiangning CZF-3 apparatus according to ISO 9773 with a sample thickness of 3.2 mm. Mass flow rate (MFR) was measured by melt flow tester CEAST P/N 6941. The load weight was 2.16 kg and the melting temperature was 190°C. Flammability of the composites was characterized using a cone calorimeter from FTT, UK according to ISO 5660 with an incident flux of 50 kW/m2. Thermal gravity analysis (TGA) was undertaken by a TA SDT Q500 apparatus at a heating rate of 10°C/min under air atmosphere from ambient temperature to 600°C. The mass of specimen was approximately 5 mg. Scanning electron microscopy (SEM) was performed on a Hitachi S-4800 scanning electron microscope under the voltage of 20 kV. Differential Scanning Calorimeter (DSC) was carried out using a TA Instrument Q-2000apparatus. Each sample (about 5 mg) was first heated to 190°C at a heating rate of 10°C/min, and then held at 190°C for 5 min to eliminate the thermal history. The non-isothermal crystallization behavior was recorded with the cooling curve from 190 to 40°C at a cooling rate of 10°C/min. The second heating curve was recorded at a heating rate of 10°C/min from 40 to 190°C, exhibiting the trace of melting process. Fourier transform infrared (FTIR) spectra were recorded by a Nicolet Nexus 670 FTIR spectrometer under the resolution of 4 cm-1 in 128 scans by KBr disk with the wavenumbers from 4000 to 500 cm-1. Wide angle X-ray diffraction (XRD) patterns were collected by a D/max2500 5
VB21/PC diffractometer with Cu-Kα radiation (wavelength, λ=0.154 nm) in the range of 2= 5~70°.
3. Results and discussions
3.1. LOI and UL-94 test results
The LOI test results of PLA/CS samples are listed in Table 2. Neat PLA shows poor flame retardancy with a LOI value of 20.0 and the addition of CS alone can improve the flame retardancy of PLA to some extent. The LOI values of PLA/CS composites first increase with CS loading and reached its maximum value of 24.3 when CS content is 7%. However, further increase of CS decreases the LOI value. The LOI value drops to only 22.7 when CS content was 9%, which is much lower than that of PLA-C7 sample. This may be due to the decrease of viscosity caused by large CS loading, leading to more dripping and faster flame spreading. The viscosity of PLA containing different amount of CS can be measured by MFR test as shown in Fig. 1. It is observed that the addition of CS results in a decrease in the MFR value of PLA sample when the amount of additive is less than 7%. However, the MFR value increases sharply by the presence of 9% CS, indicating an obvious decrease in the melt viscosity. A possible explanation for this phenomenon is that the addition of 9% CS causes the degradation of PLA due to the existence of water, leading to the decrease in melt viscosity.
The flame retardant properties of PLA composites are further improved after loading with both CS and APP. The relevant results are also presented in Table 2. The LOI value of samples loaded with both APP and CS are observed to be higher than that of samples loaded with APP or CS alone under the same total loading of 7%. The LOI value is increased with the amount of APP. When the weight ratio of APP to CS is 5:2, the LOI reaches the highest value of 33.0. Compared with other materials reported in the literatures, CS shows higher flame retardant efficiency than 6
pentaerythritol and β-cyclodextrin (Feng, Su and Zhu, 2011). The enhancement of the flame retardancy of PLA may be due to the synergistic effect between APP and CS. The UL-94 test results are also shown in Table 2. Neat PLA burns out during the test and no UL-94 rating can be achieved. For PLA/CS samples, only V-2 rating is achieved due to the serious melt dripping which can ignite the absorbent cotton. All formulations containing both CS and APP reach V-0 rating, demonstrating the improvement of flame retardancy.
3.2. Cone calorimeter test
Fig. 2a shows the heat release rate (HRR) curves of different PLA samples from cone calorimeter test (CONE). The peak HRR value of neat PLA is 514.7 kW/m2, and the peak HRR value of PLA-A7 and PLA-C7 samples are 508.2 and 524.1 kW/m2 respectively, suggesting that the addition of APP or CS alone cannot reduce the HRR effectively. However, the peak HRR value of PLA-C2A5 sample decreases to 425.6 kW/m2, exhibiting a 17.4% reduction compared to that of neat PLA. It is worth noting that the HRR curve of PLA-C2A5 sample shows multi-peak phenomenon while only one peak can be observed for other samples. For PLA-C2A5 sample, the first peak observed after ignition is ascribed to the build-up of a foamed cellular layer on the polymer surface. Once formed, it shields the underlying polymer from the external heat exposure, and a plateau value appears in the curve. The second peak is assigned to the breakage of the intumescent char structure at higher temperatures, leading to further degradation of the substrate. Large amounts of combustible products are released leading to the increase in the HRR and formed the second peak. Fig. 2b shows the total heat release (THR) curves of different PLA samples. PLA-C2A5 sample presents the lowest THR value, indicating APP and CS inhibit the fully combustion of the sample and then reduce the THR value. Fig. 2c shows the weight loss curves of the composite samples. The amount of the char residue for the samples is summarized in Table 3. No residue char is obtained for neat PLA after combustion, and PLA-C7 sample remains only 0.6% char residue, 7
demonstrating that the incorporation of CS alone cannot significantly promote the char formation. For PLA-A7 sample, the amount of char residue increases to 3.1%, indicating an efficient char forming ability of APP. For PLA-C2A5 sample, the amount of char residue is further increased to 4.1%, which is much higher than the calculated result (2.4%), demonstrating the existence of synergistic effect between APP and CS in promoting the char formation. The amount of char residue is quite impressive considering only 7% flame retardants are loaded with PLA. It is proposed that CS can act as a carbon source because of its polysaccharide structure, while APP can act as acid source to catalyze the char formation during the burning process.
The formation of the char layer is believed to be principally responsible for the reduction of HRR during CONE tests (Murariu et al., 2010). The presence of the char acts as a barrier to isolate the heat and oxygen exchange. The more char residue formed during the combustion leads to the better barrier protection. It is reasonable to regard the flame retardant mechanism of PLA/CS/APP composites as a condensed phase mechanism in terms of CONE test results. The reactions between APP and CS can promote the char forming which will be further discussed in section 3.3. The condensed phase mechanism can be verified by comparing effective heat of combustion (EHC) of PLA-C2A5 sample to that of neat PLA. EHC can be obtained from HRR (kW/m2) divided by mass loss rate (g/s). It is believed that lower EHC is caused by more smoke release and the formation of gaseous intermediates which scavenges flame propagating free radicals (e.g.: OH·, H·) to inhibiting complete combustion to CO2 (Gilman, Kashiwagi and Lichtenhan, 1997; Sun et al., 2014). It is suggested that low EHC value indicates the flame retardant combination take effect through gas phase. Fig. 2d illustrates the EHC curves of different samples. It can be seen that the EHC value of PLA-C2A5 sample is slightly higher than that of neat PLA, indicating a classical condensed phase mechanism for PLA/CS/APP composites. 3.3. Thermal stability and crystallinity
The flame retarding effect between CS and APP on PLA shown in flammability 8
test results (Table 2 and Fig. 2) indicates that the synergistic effect mainly occurs in condensed phase. Further investigation has been under taken by analyzing TGA curves, and Fig. 3a shows the TGA curves of CS, APP, APP/CS (EXP) and APP/CS (CAL) samples. The corresponding theoretical curve is obtained based on the hypothesis that there is no reaction between the two components and calculated by linear combination of the weight loss of the components degraded separately under air atmosphere. It can be observed that the experimental and calculated curves are similar before 400°C, and the gap between the experimental curve and the calculated one increases progressively afterward. APP/CS (EXP) complex shows very high char forming ability. The experimental residue at 600oC is up to 58.6%, while the calculated residue is only 40.9%. At high temperature, APP degraded to release phosphoric acid, polyphosphoric acid and non-flammable gases. Those phosphorous containing acids act as catalysts contributing to the breaking of the bone chain, intramolecular or intermolecular dehydration and char forming of chitosan (Shukoret al., 2014). The differences between the experimental and calculated curves prove the synergistic effect between APP and CS. The thermal degradation behaviors of PLA, PLA-A7, PLA-C7 and PLA-C2A5 are compared in order to further investigate the synergistic effect between APP and CS, and the results are illustrated in Fig. 3b. All of the samples mainly perform a one-stage weight loss in the range of 300-400°C, and only slight difference is observed before 370°C. The weight of PLA and PLA-C7 samples decreases slowly after 370°C and end up with a small amount of char residue. For PLA-C2A5 and PLA-A7 samples, the amount of char residue is similar and much higher than that of neat PLA and PLA-C7 samples. In consideration of different APP additive amount (5%, 7%), it can be concluded that the addition of 2% CS promotes the char formation of PLA-C2A5 samples.
Fig. 4 presents the experimental and calculated TG curves of PLA-C2A5 sample. The calculated curve is calculated by linear combination from the experimental curves 9
of PLA-A7 and PLA-C7 curves based on their percentages under the same condition. All the samples show a one-stage weight loss between 300°C and 400°C. The decomposition temperature of PLA-C2A5 is lower than that of PLA-C2A5 (CAL) and neat PLA, indicating the adding of APP and CS catalyzes the degradation process of the sample at lower temperature. However, at 600°C, the amount of char residue for the experimental value increases to 6.0% which is higher than the calculated value of 4.7%. The 1.3% increase is worth noticed considering the total amount of addition is only 7%. The increase in the amount of char residue demonstrates the presence of APP and CS in PLA can promote the char formation. In order to get a better understanding of the degradation pathway and synergistic interaction between CS and APP, the decomposition products of CS/APP (2:5, wt. /wt.) at different temperatures (selected according to TGA curves) have been analyzed by FTIR. The obtained spectra are shown in Fig. 5, and the spectrum of residue char of PLA-C2A5 sample is also illustrated. At ambient temperature, the characteristic peaks of CS at 3065 cm-1 (-OH), 1691 cm-1 (C=O), 1435 cm-1 (CH), 1070 cm-1 (C-O-C) and APP at 3193 cm-1 (NH4+), 1252 cm-1 (P-O), 1019 cm-1 (P=O), 892 cm-1 (P-O-P) can be observed. When the temperature increases to 300oC, the peak of NH4+ disappears, which is due to the removal of NH3. For the spectrum at 350oC, the characteristic peaks are changed significantly. A new peak can be observed at 1168 cm-1, demonstrating the presence of P-O-C, which is formed by the dehydration reaction between polyphosphoric acid and chitosan. Another characteristic peak of P=O in APP shifts to 994 cm-1 because of the bonding of CS. Besides, the peaks at 1691 cm-1 and 1435 cm-1 disappear and new peaks appear at 1639 cm-1 and 1401 cm-1, which is corresponding to the formation of C=C and CH2 bonds. It can be proposed that the ring of chitosan starts to open after 350oC, and a carbonous char layer starts to be formed. With the increase of temperature, the intensities of peaks for C-O-C (1070 cm-1) and CH2 (1401 cm-1) turn to weak, indicating the ring opening of CS and formation of aromatic ring. At 600oC, peaks at 1635 cm-1 (C=C), 1146 cm-1 (P-O-C), 999 cm-1 (P=O), and 887 cm-1 (P-O-P) can still be found, which confirms that most phosphorus are left in the aromatic char residue. Compared with the spectra of 600oC and residue char of PLA-C2A5 sample, no significant difference can be observed, 10
demonstrating that PLA doesn’t participate in the formation of residue char, which is consistent with the TGA result (shown in Fig. 4).
Fig. 6 illustrates the proposed charring mechanism of APP and CS. The promotion of char residue for PLA/APP/CS samples can be attributed to the reaction between APP and CS. Based on the FTIR results discussed above, it is proposed that APP usually starts to decompose at about 300oC, releasing non-flammable gases such as ammonia and forming polyphosphoric acids. These acids can further react with the hydroxyl groups in CS via esterification. Similar mechanisms between polysaccharide hydroxyl and APP have been proved by previous works (Feng, et al., 2011; Li et al., 2009). When the temperature is higher than 350oC, CS starts to decompose via ring opening reaction accompanying with the removal of -NH2 groups. At high temperature (>400oC), the degradation products of CS and APP can self-condense to form a crosslinked structure with aromatic rings. After the rearrangement of the crosslinked structure, more stable carbonous char layers are formed.
The melting and crystallization behaviors of different PLA samples are analyzed by DSC. Fig. 7 presents the DSC curves after eliminating thermal-history, and Table 5 provides some key thermal data obtained from DSC curves. All the samples containing flame retardants presents double melting peaks: the peak at lower temperature indicates small crystals with cracks, and the other peak is formed through the recrystallized crystals melting process. The incorporation of APP or CS into PLA matrix exerts minimal effect on Tg, demonstrating the effect of the presence of APP or CS to the movement of chain segments is minimal. However, the crystallinity of the sample is greatly affected by the additives. The crystallinity of PLA-A7, PLA-C7 and PLA-C2A5 samples are 24.3%, 30.6% and 28.3% respectively, while neat PLA is usually amorphous with no crystallinity. The increase of the crystallinity leads to increased brittleness of the sample (Fox et al., 2014). Both CS and APP increase the crystallization temperature and the crystallinity in the role of nucleating agents. The rate of crystallization is increased by the presence of nucleating agents, whilst CS is more effectively than APP in promoting the formation 11
of crystals at lower temperature.
In order to further understand the crystal structures of different PLA samples, wide angle X-ray diffraction analyses was performed. Fig. 8 displays the XRD patterns of PLA and PLA composites. It is observed that neat PLA shows a wide amorphous halo, and no distinct crystal peak is found in the diffraction patterns, which is consistent with DSC results. For the samples containing flame retardant, some crystal peaks can be observed. It has been reported that APP is crystallizable (Liu, et al., 2011), so the samples of PLA-A7 and PLA-C2A5 exhibit distinct crystal peaks at 14.5°, 15.4°, and 25.9°-30.4°, which is associated to APP. It has been discussed that the incorporation of APP and CS can improve the crystallization behavior of PLA. Two strong peaks at 16.5° and 18.9° can be detected in the curves of PLA composite samples containing flame retardants (Fig. 8), which is corresponding to the diffraction peak of α crystal of PLA. In a word, both APP and CS can promote the crystallization behavior of PLA.
3.4. Morphology of char
Fig. 9 shows the photos of the char residue of different PLA samples collected after CONE tests. Neat PLA and PLA-C7 samples leave trace of hard residues, whereas the char residue is loose for PLA-A7 sample. The compact char is observed only in PLA-C2A5 sample. The compact charred layers can efficiently protect the underlying polymeric materials from further degradation; as a result PLA-C2A5 sample exhibits the best flame retardant properties. Moreover, the inner char of PLA-A7 sample presents a loose and porous structure, while the inner char of PLA-C2A5 sample is quite dense with almost no big voids. APP can act as both acid source and blowing agent. For PLA-A7 sample, non-flammable gases like NH3 and H2O released by APP degradation process causes the clasp of the unstable char. However, by the presence of both APP and CS, the reaction between them stabilizes the char, resulting in a dense char structure.
12
Fig. 10 shows the SEM images of the char residue of PLA composite. Similar to the digital pictures in Fig. 9, only the PLA-C2A5 sample presents a dense, homogeneous and continuous char structure with fewer voids and cracks. The char residue of PLA-C7 is very loose. For PLA-A7, the char residue is much denser than PLA-C7, but many big voids can be observed. The dense char plays an important role in hindering the flammable gases and heat exchange by providing a good flame shield for the underlying polymeric substrate. The incorporation of APP and CS together enhances the structure of char residue, resulting in lower heat release rate and higher LOI values.
4. Conclusions
CS has been successfully employed as a carbon source to enhance the char formation of PLA. CS/APP combination displays high efficiency in improving flame retardant properties of PLA. The LOI value can reach up to 33, and UL-94 can pass v-0 for the PLA composite containing 2% CS and 5% APP. It is found that there is an intensive interaction between CS and APP. The combination of APP and CS produces continuous and homogenous char residue. The intumescent protective layer formed by APP/CS significantly reduces the HRR and THR. Both CS and APP can act as nucleating agents to increase the crystallinity of PLA. It is suggested CS can also be applied as carbon agent in IFR formulations to other polymer matrix because of its inherent environmental friendly property and char-forming ability.
Acknowledgments
The authors would like to thank the National Natural Science Foundation of China (Grant No. 21674008) and the Fundamental Research Funds for the Central Universities (YS201402) for the financial support of this research.
13
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Fig. 1. MFR values for PLA/CS samples.
16
Fig. 2. The HRR (a), THR (b) and Weight loss (c) curves of PLA, PLA-A7, PLA-C7 and PLA-C2A5 samples and EHC (d) curves of. PLA and PLA-C2A5 samples.
17
Fig. 3.The TGA curves of APP, CS, APP/CS (EXP), APP/CS (CAL) samples (a); and PLA， PLA-C7, PLA-A7, PLA-C2A5 samples (b).
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Fig. 4. The TGA curves of neat PLA, PLA-C2A5 and PLA-C2A5 (CAL) samples.
19
Fig. 5. FTIR spectra of CS/APP samples at different temperatures and residue char of PLA-C2A5 sample
20
Fig. 6. Proposed charring mechanism of APP and CS
21
Fig. 7. DSC curves of PLA, PLA-A7, PLA-C7, and PLA-C2A5 samples.
22
Fig.8. XRD patterns of PLA, PLA-A7, PLA-C7, and PLA-C2A5 samples.
23
Fig. 9. Digital photos of the residue char after CONE test: PLA (a), PLA-A7 (b), PLA-C7 (c), and PLA-C2A5 (d).
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Fig. 10. SEM images of the char residue for PLA-C7 (a), PLA-A7 (b) and PLA-C2A5(c).
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Table 1 Formulations of PLA composite samples Sample
PLA/%
CS/%
APP/%
PLA PLA-C5 PLA-C7 PLA-C9 PLA-C5A2 PLA-C3.5A3.5 PLA-C2A5 PLA-A7
100 95 93 91 93 93 93 93
0 5 7 9 5 3.5 2 0
0 0 0 0 2 3.5 5 7
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Table 2 The flame retardant properties of PLA/CS and PLA/CS/APP composite Samples PLA PLA-C5
LOI (%) 20.0 22.5
UL-94 *t
1/t2
(s)
Cotton ignition
rating
-/-
Yes
NR
12.5/20.6
Yes
V-2
PLA-C7
24.3
9.0/8.0
Yes
V-2
PLA-C9
22.7
9.3/12.5
Yes
V-2
PLA-C5A2
29.0
1.1/4.4
No
V-0
PLA-C3.5A3.5
30.0
1.7/3.6
No
V-0
PLA-C2A5
33.1
1.8/3.6
No
V-0
PLA-A7
27.0
1.0/2.0
Yes
V-2
*t1/t2 represent the after-flame time after the first and second 10 s flame application, respectively
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Table 3 Char residue (600s) of the PLA composite samples samples
PLA
PLA-C7
PLA-A7
PLA-C2A5(CAL*)
PLA-C2A5(EXP)
Char residue/%
0
0.6
3.1
2.4
4.1
* CAL= 0.6×(2/7)+3.1×(5/7)=2.4
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Table 4 Char residue (600°C) of the samples Samples PLA PLA-C7 Char residue/% (600℃)
1.8
1.9
PLA-A7
PLA-C2A5(CAL*)
PLA-C2A5
5.9
4.7
6.0
* CAL =1.8×(2/7)+5.9×(5/7)=4.7
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Table 5 The data collected from DSC. Sample
Tg (°C)
Tc (°C)
Tm (°C)
ΔHc (J/g)
ΔHm (J/g)
Crystallinity (%)
PLA PLA-A7 PLA-C7 PLA-C2A5
59.9 59.9 59.8 59.9
113.1 118.1 114.5
141.4 150.0 149.6
0 22.7 28.6 26.4
0 23.6 27.3 26.7
0 24.3 30.6 28.3
Tg: Glass transition temperature, Tc: Cold crystallization temperature, Tm: Melting temperature, ΔHc: Enthalpies of the cold crystallization, ΔHm: Enthalpies of the melting.
30