Combustion properties and thermal degradation behaviors of biobased polylactide composites filled with calcium hypophosphite

Abstract

In this work, calcium hypophosphite (CaHP) was introduced into polylactide to prepare flame retardant polylactide composites (FR-PLA). Thermal gravimetric analysis (TGA) was used to investigate the thermal degradation properties of CaHP and FR-PLA composites. It was found that CaHP presented a different thermal degradation process in air atmosphere compared with that in nitrogen. The flame retardancy properties of the composites were evaluated by limiting oxygen index (LOI), underwriter laboratories 94 testing (UL-94), microscale combustion calorimetry (MCC) and cone calorimetry. LOI and UL-94 testing showed that the incorporation of CaHP can significantly improve the flame retardancy of FR-PLA composites. MCC and cone calorimeter tests showed that the values of heat release rate (pHRR) and total heat release (THR) of FR-PLA composites were significantly decreased compared with those of pure PLA. The thermal degradation processes of PLA and FR-PLA were researched by real time Fourier transform infrared spectra (RT-FTIR). The crystallization behavior of FR-PLA composites was investigated by differential scanning calorimetry (DSC). Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) revealed that the addition of CaHP promoted the formation of condensed char residue which contained aromatic structure and phosphorous-rich inorganic structure. The condensed char could inhibit the inner material from being attacked by the heat flux and flame during combustion, resulting in improved flame retardance of FR-PLA composites.

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1. Introduction

In recent years, aliphatic polyesters have attracted great interest due to their excellent merits of biodegradability and biocompatibility.^1–3 Among these polymers, polylactic acid (PLA) plays a dominant role because of it can be produced by renewable sources, such as corn starch, potato starch, sugar beets, and other agricultural products.^4,5 Moreover, PLA has excellent high mechanical strength, thermal plasticity, a high melting point, high degree of transparency and ease of fabrication, which makes it a good candidate to replace traditional petrochemical plastics.^6–8 Due to its excellent properties, PLA has been widely used in automotive component, electrical industry, building materials and aerospace industry.^9–12 However, because of its intrinsic chemical composition and molecular structure, PLA shows high combustibility and melt dripping which restricts its application in new areas. Therefore, the preparation of flame retardant PLA composites is a meaningful work.

Up to now, there are many research used to improve the flame retardancy of PLA. Such as phosphorous-containing,^13–15 silicon containing,¹⁶ and carbon containing materials,^17,18 inorganic materials,^19,20 and layered materials.²¹ However, it is hard to reach UL-94 V-0 rating at low addition level.

Recently, aluminum hypophosphite (AHP) has been used as a flame retardant, which has achieved effective flame retardance in many engineering plastics, such as PET, PBT and PA, which could reach excellent flame retardancy.^22–24 And also, Tang et al. reports that aluminum hypophosphite shows good flame retardancy in polylactide acid and FR-PLA composites with 20 wt% AHP loading could reach UL-94 V-0 rating.

Calcium hypophosphite has the same active ingredients, so it may show good flame retardancy in PLA composites. As far as we know, little work has been reported regarding to FR-PLA composites based on calcium hypophosphite. In the current work, calcium hypophosphite is blended with PLA to obtain flame retardant and environment friendly PLA composites (FR-PLA). The flame retardant and thermal stability of FR-PLA composites are investigated by limiting oxygen index (LOI), UL-94, microscale combustion calorimetry (MCC), cone calorimeter and thermogravimetric analysis (TGA). Differential scanning calorimetry (DSC) is used to reveal crystallization behaviors of FR-PLA composites. The residue char after cone calorimeter are further investigated by scanning electron microscopy (SEM) and Fourier transform infrared spectra (FTIR).

2. Experimental section

2.1 Material

PLA resin (Nature Works 4032D) in granular form was supplied by Cargill Dow Inc. (America). Calcium hypophosphite (CaHP) was supplied by Hanye chemical New Material Co., Ltd (China).

2.2 Preparation of FR-PLA composites

PLA resin and CaHP were dried at 80 °C overnight before used. All the samples were prepared by a twin-roll mixing mill (XK-160, Jiangsu, China) at 175 °C and the rotation speed of screw was 100 rpm. PLA was firstly added to the mill at the beginning of the blending procedure. After PLA was molten, the additives were then added to the matrix and processed about 10 min until a visually good dispersion was achieved. The mixtures after mixing processing were cut into pellets and then hot-pressed into sheets under 10 MPa for 10 min. The sheets were cut into suitable size specimens for fire testing. 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

Sample	Composition (wt%)		LOI	UL-94 3.2 mm bar
	PLA	CaHP		t1/t2^a (s)	Dripping	Rating
a t1 and t2, average combustion times after the first and the second applications of the flame.b BC, burns to clamp.c NR, not rated.
PLA	100	0	19.5	BC^b	Y	NR^c
PLA/5CaHP	95	5	24	12.9/2.0	Y	V-2
PLA/10CaHP	90	10	25	11.7/2.2	Y	V-2
PLA/15CaHP	85	15	25.5	13.4/2.2	Y	V-2
PLA/20CaHP	80	20	25.5	6.9/2.6	Y	V-1
PLA/25CaHP	75	25	26	5.3/3.0	Y	V-1
PLA/30CaHP	70	30	26.5	0.9/4.8	N	V-0

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2.3 Measurement and characterization

2.3.1 Thermogravimetric analysis. Thermal gravimetric analysis (TGA) was carried out using a Q5000 IR (TA Instruments) thermo-analyzer 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 temperature ranging from room temperature to 750 °C with a heating rate of 20 °C min⁻¹. The onset decomposition temperature was defined as the temperature at which 5% of original weight was lost. T_max was defined as the temperature at which the samples processed the maximal weight loss rate.

2.3.2 Limiting oxygen index. The limiting oxygen index (LOI) was measured according to ASTM D2863 by HC-2 oxygen index meter (Jiangning Analysis Instrument Company, China). The specimens used for the test were of the dimensions of 100 × 6.5 × 3 mm³.

2.3.3 UL94 vertical burning test. Underwriter Laboratories 94 vertical burning test (UL-94) was performed using vertical burning instrument (CFZ-2-type, Jiangning Analysis Instrument Company, China) and the specimens for test were of dimensions of 130 × 13 × 3 mm³. In the measurement, the samples were vertically exposed to a Bunsen burner flame for 10 s. If the samples were self-extinguished, another 10 s was employed. The classification of the samples was defined according to ANSL/UL-94-2009.

2.3.4 Microscale combustion calorimetry. Microscale combustion calorimetry (MCC, Govmark) was used to analysis the combustion properties of the samples according to ASTM D 7309-7. 4–6 mg of each sample was heated from 100 to 650 °C at 1 °C s⁻¹ in a stream of nitrogen flowing at 8 × 10⁻⁵ m³ min⁻¹. The volatile anaerobic thermal degradation products in the nitrogen gas stream were mixed with a 2 × 10⁻⁵ m³ min⁻¹ stream of pure oxygen prior to entering a 900 °C combustion furnace. The MCC data obtained were reproducible to about 3%.

2.3.5 Cone calorimeter. 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 m⁻² external heat flux.

2.3.6 Differential scanning calorimetry. Differential scanning calorimetry (DSC) measurement was performed with a TA DSC Q800 under nitrogen condition at a flow rate of 50 ml min⁻¹. 5–10 mg sample was sealed with an aluminum pans and heated to 190 °C and kept at 190 °C 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⁻¹. In this test, glass transition temperature (T_g), cold-crystallization temperature (T_c), melting temperature (T_m), cool-crystallization enthalpy (ΔH_c) and melting enthalpy (ΔH_m) were obtained from the second heating scans. The melting enthalpy of 100% crystalline PLA was taken as ΔH⁰_m = 93 J g⁻¹.²⁵ Considering the effect of CaHP loading (ϕ), crystallinity was calculated as eqn (1):^26,27

2.3.7 Mechanical properties test. The mechanical properties were performed by a universal test machine (WD-20D) according to ASTM D-638 at temperature at 25 ± 2 °C. The samples were prepared by cutting strips of 4.0 ± 0.1 mm wide and 1.0 ± 0.05 mm thick. The crosshead speed was set as 20 mm min⁻¹. An average of at least five individual determinations was obtained.

2.3.8 RT-FTIR test. Real time Fourier transform infrared spectra (RT-FTIR) was recorded using the MAGNA-IR 750 spectrometer (Nicolet Instrument Company, 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⁻¹ in air condition. RT-IFTR spectra were obtained in situ during the thermo-oxidative degradation of the cured sample.

2.3.9 Char residue analysis. Some residues of the samples after cone calorimeter test were collected and further analyzed by scanning electron microscope (SEM). The SEM micrographs were obtained with a scanning electron microscope PHILIPS XL30E at an accelerating voltage of 20 kV. The specimens were sputter-coated with a conductive layer.

Fourier transform infrared spectroscopy (Nicolet 6700 FT-IR spectrophotometer) was employed to characterize the residues collected after cone calorimeter tests using thin KBr disc. The transition mode was used and the wavenumber range was set from 400 to 4000 cm⁻¹.

3. Results and discussion

3.1 Morphology and thermal stability of CaHP

Fig. 1 shows SEM images of CaHP particles. Fig. 1(a) presents CaHP particles at low magnification, it has a uniform size at about 10–40 μm. Fig 1(b) shows CaHP particles with a high magnification, from which we can found irregular surface, this may come from the processing technology.

Fig. 1 SEM images of CaHP.

Thermogravimetric analysis is used to further investigate the thermal behaviors of CaHP in nitrogen and air atmosphere. As showed in Fig. 2, CaHP presents onset degradation temperature at 383 °C, and the maximum mass loss rate occurs at 361 °C and 392 °C in nitrogen atmosphere. The corresponding decomposition of CaHP in nitrogen condition is similar to aluminum hypophosphate (AHP)²⁸ and it can be described as follows:

Fig. 2 TG and DTG curves of CaHP in nitrogen and air atmosphere.

PH₃ and H₂O are the main gas decomposition products at the first and second step,

respectively, leaving the residual product of Ca₂P₂O₇.

TG and DTG curves of CaHP in air atmosphere are also presented in Fig. 2, which are significant different from those in nitrogen atmosphere. CaHP presents a weight increase process at the range of 331–370 °C with a T_max at 363 °C. It presents inconspicuous mass loss between 370 and 750 °C. When the temperature further increase, it shows a slight decrease mass loss at the range of 750–800 °C. The char residue of the sample is 104.2 wt% at 800 °C. The thermo-oxidation degradation of CaHP can be described as follows:

CaHP firstly degrades and releases phosphine, forming CaHPO₄ in solid phase. The PH₃ gas quickly reacts with oxygen in gas phase and generates phosphoric acid. At the same time, CaHPO₄ further degrades into Ca₂P₂O₇, releasing water vapor. At the high temperature range, phosphoric acid dehydrates into polyphosphoric acid, releasing water vapor.

3.2 Thermal stability of PLA and FR-PLA composites

The TG and DTG curves of PLA and FR-PLA composites in nitrogen atmosphere are presented in Fig. 3, and the corresponding data are listed in Table 2. The onset degradation temperature (T_−5%) of virgin PLA is 353 °C, and T_−5% of FR-PLA composites with CaHP loading are 339–340 °C, which are lower than both CaHP and PLA, indicating some interaction is existed between CaHP and PLA molecular chain at high temperature. The pure PLA presents a T_max at 392 °C, while the peaks of PLA/CaHP composites are around 371–373 °C, indicating FR-PLA composites present lower thermal stability compared with pure PLA.

Fig. 3 TG and DTG curves of PLA and FR-PLA composites under nitrogen atmosphere.

Table 2 TGA data of PLA and FR-PLA composites under nitrogen atmosphere

Specimen	Temperature at specific weight loss (°C)			Residue^a
	T−5% (°C)	T−50% (°C)	Tmax (°C)
a At 800 °C.
PLA	353	387	392	0.2
PLA/5CaHP	339	369	373	5.6
PLA/10CaHP	340	370	372	10.3
PLA/15CaHP	340	371	372	15.3
PLA/20CaHP	340	371	371	19.4
PLA/25CaHP	340	372	371	24.0
PLA/30CaHP	339	373	371	28.2

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To further investigate the thermal stability of PLA and FR-PLA composites, the samples are also analyzed by TGA in air atmosphere and the corresponding data are listed in Table 3. As shown in Fig. 4, T_−5%, T_−50%, and T_max of pure PLA are 11 °C, 11 °C and 12 °C lower than those in nitrogen atmosphere. The similar phenomenon is also found in FR-PLA composites. This may come from that the oxygen could further promote the degradation of PLA molecular chain.

Table 3 TGA data of PLA and FR-PLA composites under air atmosphere

Specimen	Temperature at specific weight loss (°C)			Residue^a
	T−5% (°C)	T−50% (°C)	Tmax (°C)
a At 800 °C.
PLA	342	376	380	0.2
PLA/5CaHP	337	370	374	4.7
PLA/10CaHP	334	368	369	11.0
PLA/15CaHP	335	368	368	15.8
PLA/20CaHP	336	370	369	21.2
PLA/25CaHP	338	371	369	26.3
PLA/30CaHP	337	374	370	32.0

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Fig. 4 TG and DTG curves of PLA and FR-PLA composites under air atmosphere.

3.3 Flammability of PLA and FR-PLA composites

LOI and UL-94 vertical burning test are effective methods for evaluating the flammability of polymer composites. Table 1 lists the LOI values, UL-94 rating and dripping behaviors of all the samples. Pure PLA shows a LOI value of 19.5 vol%, it also presents no rating with serious dripping in UL-94 test, indicating poor flammability of PLA. When 5 wt% of CaHP is added, PLA/5CaHP presents a significant increased LOI value of 24 vol%, it also could pass UL-94 V-2 rating. When CaHP loading increase from 10 wt% to 25 wt%, the LOI value is sluggishly increased from 25 vol% to 26 vol%. Meanwhile, the samples only could pass V-2 or V-1 rating in UL-94 test and the dripping phenomena is also existed. To further increase of CaHP loading, PLA/30CaHP could reach UL-94 V-0 rating with a LOI value of 26.5 vol%, and the dripping is completely suppressed. The photographs of the samples after LOI tests are also presented in Fig. 5. Pure PLA shows serous dripping in the burning test. When 5–15 wt% of CaHP is added, dripping is partly inhibited. In these composites, intumescent char layer is formed when CaHP loading is increased to 20–30 wt%. The improved flame retardancy of FR-PLA composites results from the addition of CaHP. CaHP degrades and releases gas phosphine, which further reacts with oxygen to form phosphoric acid and polyphosphoric acid. Phosphoric acid and polyphosphoric acid play as acid resource, which promote the degradation of PLA molecular chain and the formation of intumescent char. This intumescent char could effectively hinder mass and heat transportation and protect the inner layer of the composites, thus could increase the flame retardancy of the composites.²⁹

Fig. 5 Digital photos of the samples after LOI test.

Microscale combustion calorimetry (MCC) is an effective bench-scale measurement to evaluate the combustion properties of materials and it only needs milligrams of specimen.³⁰ Heat release rate (HRR) curves of PLA and FR-PLA composites are presented in Fig. 6, and the corresponding data are listed in Table 4. As can be observed, pure PLA shows a pHRR value of 411 W g⁻¹. The pHRR values gradually decrease when the CaHP loading increase from 5 wt% to 30 wt%. Meanwhile, the total heat release rate (THR) value is reduced from 18.9 kJ g⁻¹ of pure PLA to 11.8 kJ g⁻¹ of PLA/30CaHP, with a reduction of 37.6%, indicating that the addition of CaHP could significantly decrease pHRR and THR values. Furthermore, it is interesting to find that T_pHRR values of FR-PLA composites are 2 °C to 5 °C higher than pure PLA, which is consistent with earlier reports.³¹ This may come from that the degradation of CaHP results in the formation of inorganic char which could plays as a barrier to hinder the degradation of PLA molecular chain.

Fig. 6 HRR curves of PLA and FR-PLA composites from MCC test.

Table 4 MCC results of PLA and FR-PLA composites. (HRC: heat release capacity, ±5 J g⁻¹ K⁻¹; pHRR: peak of release rate, ±5 W g⁻¹; THR: total heat release, ± 0.1 kJ g⁻¹; T_pPHRR: temperature at pHRR, ±2 °C)

Specimen	HRC (J g^−1 K^−1)	pHRR (W g^−1)	THR (kJ g^−1)	TpHRR (°C)
PLA	404	411	18.9	396
PLA/5CaHP	406	410	16.6	401
PLA/10CaHP	378	384	17.2	400
PLA/15CaHP	356	360	13.4	400
PLA/20CaHP	325	332	13.6	400
PLA/25CaHP	323	328	14.9	398
PLA/30CaHP	286	290	11.8	398

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3.4 Cone calorimeter testing

Cone calorimeter is a useful bench-scale tool for testing flammability properties of materials in real-world fire conditions.³² In this study, cone calorimeter test is used to evaluate the influence of CaHP loading on the combustion behavior of FR-PLA composites, and the corresponding data are shown in Table 5. Fig 7 presents HRR curves of PLA and FR-PLA composites which different CaHP loading. Pure PLA shows a sharp HRR peak appears with a pHRR value of 549 kW m⁻². PLA/10CaHP presents a wide heat release process and its pHRR value is 339 kW m⁻², with a reduction of 38.2% compares with virgin PLA. The further increase CaHP loading to 20 wt% and 30 wt%, PLA/20CaHP and PLA/30CaHP show further decreased pHRR values of 296 and 263 kW m⁻², with a reduction of 46.1% and 52.1%, indicating CaHP could significantly decrease the thermal hazard of FR-PLA composites. Fig. 8 presents the total heat release (THR) of PLA and FR-PLA composites. At the end of burning, pure PLA shows total heat releases of 62.3 MJ m⁻². When 10 wt% of CaHP is added, the THR value of PLA/10CaHP decreases to 55.5 MJ m⁻². The further increase CaHP loading results in inconspicuous THR decrease in PLA/20CaHP and PLA/30CaHP. This may come from that the degradation of CaHP results in the release of phosphine gas which consume oxygen. The time to ignition (TTI) of virgin PLA is 57 s, while TTI values of FR-PLA composites decreases with the increases of CaHP loading. This may because that PLA molecular chain is sensitive to acid species, which results from the degradation of CaHP, thus the TTI values are brought forward.

Table 5 Cone calorimeter data for each sample at 35 kW m². (TTI: time to ignition, ±2 s; PHRR: peak heat release rate, ±15 kW m⁻²; T_P: time to pHRR, ±2 s; THR: total heat release, ±0.5 MJ m⁻¹)

Sample	TTI (s)	Tp (s)	pHRR (kW m^−2)	THR (MJ m^−2)	FPI	FGI
PLA	57	155	549	62.3	0.10	3.5
PLA/10CaHP	67	105	339	55.5	0.20	3.2
PLA/20CaHP	48	80	296	56.1	0.16	3.7
PLA/30CaHP	33	80	263	55.2	0.13	3.3

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Fig. 7 HRR curves of PLA and FR-PLA composites from cone calorimeter test.

Fig. 8 THR curves of PLA and FR-PLA composites from cone calorimeter test.

Fire propagation index (FPI) and fire growth index (FGI) are also used to evaluate the fire safety of FR-PLA composites. FPI is defined as the ratio of TTI to pHRR which indicates the difficult degree flashover of materials after it is ignited. The larger FPI value represents lower hazard of the material. In Table 5, FR-PLA composites all presents higher FPI value compares with virgin PLA, indicating lower hazard of these composites. FGI is defined as the ratio of pHRR and T_p, which reflects the spread speed of the fire. The larger the value of FGI, the higher hazard of the material. From Table 5, the FGI value of FR-PLA composites are only slight changed.

3.5 Differential scanning calorimetry

DSC is a useful tool for analyzing the crystallization and melting behaviors of polymer, and the parameters are important for evaluating the processing properties of polymers. Fig. 9 shows the DSC thermograms during the second heating cycle and the corresponding data are listed in Table 6. Pure PLA presents glass transition temperature (T_g) and melting temperature (T_m) at 62.1 °C and 167.3 °C, respectively. It shows no obvious cold crystallization process, which is consistent with the relevant report.²⁸ In FR-PLA composites containing CaHP, the T_g values are not significantly changed, but the T_m value are slightly increased. Furthermore, the cold crystallization process is observed, which is consistent with previous literature. Pure PLA presents a crystallization degree of 3.5%, while that of FR-PLA are 3.2–4.2%, indicating the addition of CaHP shows little effect on the crystallization degree of FR-PLA composites. It is also found double melting peak in FR-PLA composites, this may results from the lamellar reorganization, and the melting peak at lower temperature can be ascribed to less organized crystals.

Fig. 9 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 °C to 200 °C with a ramp of 10 °C min⁻¹)

Specimen	Tg (°C)	Tc (°C)	ΔHc (J g^−1)	Tm (°C)	ΔHm (J g^−1)	χc (%)
PLA	62.1	—	—	167.3	3.4	3.5
PLA/5CaHP	62.7	109.4	25.2	170.8	28.9	4.2
PLA/10CaHP	62.8	110.0	26.1	170.7	29.2	3.7
PLA/15CaHP	62.4	109.3	24.3	170.2	27.3	3.8
PLA/20CaHP	62.6	109.3	22.2	170.1	24.6	3.2
PLA/25CaHP	63.2	110.2	20.8	172.0	23.7	4.2
PLA/30CaHP	62.7	108.5	19.2	170.0	21.6	3.7

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3.6 Mechanical properties

The values of tensile strength and elongation at break of FR-PLA composites with different CaHP loading are presented in Fig. 10. Pure PLA shows a high tensile strength of 65.3 MPa with elongation at break of 6.4%. When 5 wt% CaHP is added, the tensile strength and elongation at break of PLA/5CaHP are sharply reduced to 57.1 MPa and 2.7%, respectively. The further increase of CaHP loading results in gradually reduced tensile strength and elongation at break of FR-PLA composite. When 30 wt% CaHP is added, PLA/30CaHP shows tensile strength and elongation at break at 36.5 MPa and 1.0 wt%, respectively. Although the mechanical properties of FR-PLA composites deteriorate with the increase of CaHP loading, FR-PLA composites still present considerable mechanical properties for application.

Fig. 10 Effect of CaHP loading on the mechanical properties of FR-PLA composites.

3.7 Thermal degradation process

The thermal degradation process of pure PLA and PLA/30CaHP is characterized by real-time FTIR (RT-FTIR). Fig. 11(a) shows the thermal degradation process of pure PLA. The characteristic peaks at 1750 which corresponds to C=O bond disappears at 350 °C, indicating PLA molecular chain completely pyrolyzes at 350 °C. Furthermore, the peaks between 1000 cm⁻¹and 1300 cm⁻¹ can be assigned to stretching vibration of C–O–C bond, and also the stretching and bending vibrations of C–H bonds at 2940 cm⁻¹, 2990 cm⁻¹, 1380 cm⁻¹, and 1460 cm⁻¹ are disappeared at 350 °C, indicating the completely degradation of PLA.

Fig. 11 RT-FTIR spectra for the degradation process of pure PLA and PLA/30CaHP at different temperature: PLA (a); PLA/30CaHP (b).

Fig. 11(b) shows the decomposition process of PLA/30CaHP. Three new peaks are found compared with pure PLA. The new peak at 2380 cm⁻¹ is attributed to the stretching vibration of P–H bond, the bond at 814 cm⁻¹ corresponds to rocking mode of PH₂ and the weak peak at 467 cm⁻¹ is assigned to Ca–O bond stretch modes. The three peaks intensity is significantly reduced at the range of 330 °C, indicating the decomposition of CaHP. Meanwhile, the peak at 1750 cm⁻¹ is ascribed to C=O bond reduced and it disappears until 350 °C. The decreased C=O band may properly further formats the C=O–C–P band and unsaturated esters.⁵ The broad band at 1080 and 1180 cm⁻¹ belong to PH₂ bending vibration and stretching vibration of the PO₂ bond, respectively. When the temperature further increases, these bands almost disappear, indicating the complete decomposition of FR-PLA composites. At higher temperature, a broad new bond at about 1100 cm⁻¹ is formed, which can be assigned to the stretching vibration of P–O–P bond, indication the formation of Ca₂P₂O₇.

3.8 Residue analysis

In order to further investigate the flame retardancy, the char residues of some samples after cone calorimeter test are analyzed by SEM. Fig. 12 presents the SEM images of char residue of PLA/15CaHP (Fig. 12(a) and (b)) and PLA/30CaHP (Fig. 12(c) and (d)). As shown in Fig. 12(a), PLA/15CaHP shows a continuous char layer with many holes on the surface. The magnified image of Fig. 12(b) indicates that the char layer is coarse with many pores. Undoubtedly, it was not effective for the porous char residue to insulate heat and mass transportation. The further increase of CaHP loading results in the formation of char residue which can be observed in Fig. 12(c) and (d), the condensed char residue is formed without any pore. These results indicated that 30 wt% loading of CaHP can significantly improve the flame retardancy of FR-PLA composites, which is consistent with LOI and UL-94 testing.

Fig. 12 SEM images of residues from FR-PLA composites after cone calorimeter test: PLA/15CaHP (a and b); PLA/30CaHP (c and d).

Char residue of PLA/CaHP was further investigated by FTIR, and the corresponding spectrum is shown in Fig. 13. The peak at 3430 cm⁻¹ can be assigned to absorbed water. The weak peaks at 2920 and 2850 are ascribed to symmetrical and asymmetrical stretching of –CH₂– group, indicating that the introduction of CaHP could partly inhibit the degradation of PLA molecular chain at high temperature. The peaks at 1640 and 758 cm⁻¹ indicate that the formation of aromatic structure in degradation process of PLA/CaHP composites. The peak at 1270 corresponds to stretching vibration of P=O, and the bands between 1180 and 1000 cm⁻¹ belong to the stretching vibration of PO₂/PO₃ in phosphate–carbon complexes. The peak at 1060 cm⁻¹ can be ascribed to stretching vibration of P–O–P bond, indicating the oxidation process of CaHP in combustion process. The peak at 575 cm⁻¹ is belonged to Ca–O bond stretching modes.³³

Fig. 13 FTIR spectra of char residues from PLA/30CaHP after cone calorimeter test.

4. Conclusion

Flame retardant PLA composites (FR-PLA) based on CaHP were prepared by simple melt blending method. TGA test revealed that CaHP showed different thermal degradation behavior in air atmosphere compared with that in nitrogen condition, and the addition of CaHP could significantly increase the char residue of FR-PLA composites. The incorporation of CaHP could significantly improve the flame retardancy of FR-PLA composites. The LOI value of FR-PLA composite was improved to 26.5 vol% with 30 wt% CaHP loading compared with 19.5 vol% of virgin PLA. UL-94 test indicated that FR-PLA composites could pass V-0 rating compared no rating of pure PLA. MCC test revealed that CaHP could significantly decrease the peak at heat release rate (pHRR) and total heat release (THR), which presented good consistency with cone calorimeter test. DSC test revealed that the incorporation of CaHP did not change glass transition temperature (T_g) of FR-PLA composites, but slightly increased the melting temperature (T_m) value. Furthermore, obviously cold crystallization process and double melting phenomena were observed in FR-PLA composites. Char residue analysis test prompted that the incorporation of CaHP resulted in the formation of condensed char residue which contained aromatic structure and phosphorous-rich inorganic structure. The condensed char could inhibit the inner material from being attacked by the heat flux and flame during combustion, resulting in improved flame retardance of FR-PLA composites.

Acknowledgements

This research was supported by National Basic Research Program of China (973 Program) (2012CB719701), the National Natural Science Fund of China (no. 51206002), National Natural Science Found of China (no. 51303167), the Opening Project of State Key Laboratory of Fire Science of USTC (HZ2012-KF04), China Postdoctoral Science Foundation (no. 2013M531523) and University Natural Science Research Project of Anhui Province (KJ2012Z025).

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