A novel biodegradable phosphorus-containing copolyester with preferable flame retardancy and mechanical properties

Abstract

Phosphorus-containing poly(butylene succinate-co-3-hydroxyphenylphosphinyl-propionate) (PBSH) was synthesized through esterification and melt copolycondensation of succinic acid (SA), 1,4 butanediol (BD) and 3-hydroxyphenylphosphinyl-propionic acid (HPPPA). The chemical structure of PBSH was confirmed using Fourier transform infrared spectroscopy (FTIR) and ¹H NMR spectroscopy. It is found that the addition of HPPPA can improve the initial decomposition temperatures of copolyesters. The LOI values of the PBSHs were increased from 22.0% to 35.0% and the UL-94 ratings of vertical burning were improved to V-0 with a relatively low content of phosphorus containing comonomer. Furthermore, we used melt postpolycondensation to increase the molecular weights of the copolyester and found that a copolymer with enough high molecular weight can maintain its good mechanical properties for practical applications.

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1. Introduction

The rapid growth of plastics production in recent years is considered as a serious source of environment problems. As an alternative to conventional nonbiodegradable plastics, the use of biodegradable polymers has attracted great interest. Among these biodegradable polymers, poly(butylene succinate) (PBS) is expected to be one of the most economically competitive biodegradable polymers due to its outstanding merits of biodegradability, biocompatibility and processability.^1–7 PBS can also be widely applied in agriculture, forestry, civil engineering, and other fields. However, like other polyesters such as PET^8,9 and PBT,¹⁰ PBS is highly combustible, which restricts its end-use applications in the textile industry, for automotive components and in the electrical industry. Thus, it is still a necessary and urgent task to improve the flame retardant property of PBS.

In principle there are two methods used to prepare flame retardant polymer materials. One is to add flame retardant additives into polymer matrix during processing.^11–13 However, the composites made by this method have certain drawbacks. For example, some chemical compounds may be leached from the composites. Furthermore, in order to achieve good flame retardancy, exceedingly large amounts of flame retardants are necessary, which will also inevitably deteriorate the mechanical properties of PBS. Wang¹³ et al. prepared intumescent flame retardant/poly(butylene succinate) (IFR/PBS) blends by melt blending. They found that even if the IFR content was as high as 20 wt% the composite still did not pass UL-94 V-0 rating. Moreover, the tensile strength and elongation at break of the composite decreased by about 15% and 30%, respectively. The other strategy is to use some reactive flame retardants in terms of comonomers or chemical modifiers of the preformed homopolymers.^8,14 This method is more attractive because flame retardant will not migrate out of polymer matrix during processing and the flame retardant in the molecular chain generally have higher flame retardance efficiency than flame retardants incorporated into polymer matrix by blending. However, few work about biodegradable flame retarding copolyester was reported.

In this work, we incorporated 3-hydroxyphenylphosphinyl-propionic acid (HPPPA) into the polymer chains of PBS to synthesis a novel flame-retarding biodegradable copolymer by copolycondensation. HPPPA is a kind of phosphorus-based reactive flame retardant. As a halogen-free compound, it is environmental friendly and have a good flame retardant efficiency.^8,14 The chemical structure, thermal behaviours, flame retardant properties and mechanical performance of copolyesters were presented. Our results demonstrate that using a small amount of HPPPA as a comonomer is a simple way to improve flame resistant performance. However, because of the hindering effect of HPPPA comonomer, the molecular weight of copolymer is lower than that of neat PBS, which is a common problem in the system of phosphorus containing copolyester.^15,16 We also found molecular weight is one of the most critical factors affecting mechanical performance of PBS copolyester. To address this issue, we treat PBS and its copolymers by melt postpolycondensation. The results showed that both of the molecular weights and mechanical properties of the samples were improved evidently by this method. This work broadens the application field of PBS and also provides a feasible path for the development of biodegradable flame retarding copolyester.

2. Experimental

2.1 Materials

Succinic acid(SA) (purity 98.0%) was supplied by Kemiou Chemical Reagent Corporation (Tianjin, China), while 1,4-butanediol (BD) (purity 99.0%), tetrabutyl titanium (purity 98.0%) were purchased from Kelongchemical corporation (Chengdu, China). 3-Hydroxyphenylphosphinyl-propionic acid (purity 99.0%) was purchased from Chengdu Ai Keda Chemical Technology Co., Ltd. All chemicals were of analytical grade and used without purification.

2.2 Synthesis of poly(butylene succinate-co-3-phosphinylpropionate) (PBSH)

A certain amount of HPPPA was firstly dispersed in 1,4-butanediol (BD) with vigorous agitation for 30 min at 90 °C. After cooling to room temperature, the solution was added into a home-made autoclave together with quantitative amount of succinic acid (SA) and tetrabutyl titanium as catalyst. With stirring, the reaction mixture was heated at 180 °C for 2.5 h under nitrogen atmosphere. Then, temperature was progressively increased to 225 °C, and pressure was reduced to 50 Pa for polycondensation. After about 2 h, the product was extruded into cold water from the reactor by compressed nitrogen. In our experiments, copolyesters (mole ratio of SA : HPPPA were 1 : 0; 99 : 1; 98 : 2 and 96 : 4) were synthesised with the same procedure and were labeled as PBS, PBSH-1%, PBSH-2% and PBSH-4%, respectively. The products were chopped into small pieces and dried in vacuum at 50 °C for 24 h. The schematic process of the reaction is presented in Scheme 1.

Scheme 1 Synthetic route for copolyester PBSH.

2.3 Melt postpolycondensation

Melt postpolycondensation was used to increase the molecular weights of the copolymer samples. The process was performed as follows: thin-films (0.3 mm) of PBS, PBSH-1%, PBSH-1%, PBSH-1% and PBSP-4% were prepared by hot-pressing. Then, these thin-films were put into a container and heated to 215 °C with high vacuum (20 Pa) for 8 h.

2.4 Characterization

GPC was performed at 35 °C on a Waters instrument, which was equipped with a model 1515 pump, a Waters model 717 auto-sampler, and a 2414 refractive index detector. CHCl₃ and polystyrene were used as the eluant and standard, respectively. The flow rate of eluant was 1.0 mL min⁻¹ and the concentration of samples was 0.25 mg ml⁻¹.

The FTIR spectra were recorded between 400 cm⁻¹ and 4000 cm⁻¹ on a Nicolet-560 infrared spectrometer. The sample was coated on a KBr plate.

¹H NMR spectra for all the samples were recorded on a Bruker AC-P 600 MHz spectrometer at ambient temperature in CDCl₃ solution with tetramethylsilane as the internal reference. The chemical structures of the copolymers were determined from their ¹H NMR spectra.

The crystallization behaviours of PBS and its copolyesters were studied by differential scanning calorimetry (DSC) with a PerkinElmer DSC Pyris-1. Samples were first annealed at 150 °C for 3 min to erase the thermal history, and cooled down to 0 °C at scan rate of 10 °C min⁻¹. Then the same sample was heated again to 150 °C at a rate of 10 °C min⁻¹.

Wide-angle X-ray diffraction patterns of samples were recorded with an X-ray diffractometer (Philips X'Pert X-ray diffractometer) with Cu-Kα radiation. The equipment was operated at room temperature at a scan rate of 2° min⁻¹ scanning from 5° to 45°. Before testing, the samples were annealed at 60 °C for 8 h.

The crystal morphology was studied using a polarizing optical microscope equipped with a hot stage. The crystallization of PBS and its copolyesters were observed at 75 °C and 85 °C, respectively. The samples were first annealed at 150 °C for 3 min to erase the thermal history, and then quenched to the designated temperature and kept at that temperature until crystallization finished.

Thermal-oxidative properties of PBS and its copolyesters were measured with a thermogravimetric analysis (TGA) instrument (Q500 TA Instruments) under an air atmosphere. The samples were heated from 25 °C to 700 °C at heating rate of 10 °C min⁻¹. For each thermogravimetric analysis, around 5 mg of sample was used.

The limiting oxygen index (LOI) measurements were performed on the Oxygen Index Flammability Gauge (HC-2C) according to ASTM D 2863-97. The test samples were moulded to a size of 120 × 6.5 × 3.2 mm.

The UL-94 vertical test was performed on the vertical burning test instrument (CZF-2) according to the ASTM D 3801 testing procedure. The test samples were moulded to a size of 125 × 12.7 × 3.2 mm.

Melt flow index (MFI) measurements were performed on a CS-127C MI meter from Subsidiary of Atlas Electric Devices Co. (USA). The measurement temperature was 150 °C, and the load was 2.16 kg.

An Instron universal testing machine was used to evaluate the tensile properties under a cross head speed of 10 mm min⁻¹. At least five samples were used for each measurement and the results were averaged to obtain a mean value.

3. Results and discussion

3.1 Characterization of poly(butylene succinate-co-3-phosphinyl-propionate) (PBSH)

The chemical structure of the prepared PBS and its copolymers were characterized using FTIR spectroscopy (Fig. 1). The characteristic absorptions of PBS can be observed at 2924 cm⁻¹, 1712 cm⁻¹, 1472 cm⁻¹ and 1155 cm⁻¹, which corresponds to the stretching vibrations of C–H, –C=O stretching vibration, C–H bending vibration and C–O stretching vibration in the ester group of PBS,^17,18 respectively. For the absorption spectrum of PBSH, in addition to the typical absorption bands of the PBS backbone, the bands corresponding to the vibration of the P=O and Ar–H appear at 1227.3 cm⁻¹ and 697.9 cm⁻¹, indicating that HPPPA are linked to the polymer chain during the polymerization process.

Fig. 1 FTIR spectrum of PBS and its copolymers.

The samples prepared in our work were also investigated by ¹H NMR spectrum. Fig. 2 shows the ¹H NMR spectra of PBSH-4%. The peaks of the PBS (δ = 4.13 ppm, –CH₂–, peak of 1,4-butanediol unit; δ = 1.72 ppm, –CH₂–, peak of 1,4-butanediol unit; δ = 2.64 ppm, –CH₂–, peak of succinyl unit^17,19) are clear observed. Compared with the spectrum of pure PBS, the extra peaks in the PBSH spectrum are supposed to be related to HPPPA. The peaks at 7.5–7.8 ppm are reasonably assigned to Ar–H of HPPPA.²⁰

Fig. 2 ¹H NMR spectra of PBSH-4%.

The signals at 4.10–3.60 ppm and 1.72–1.60 ppm are caused by the methylene of BD with different combinations of SA and HPPPA. The possible combinations of BD, SA, and HPPPA are concluded in Fig. 2. It should be noted that the peaks of HPPPA-BD-HPPPA can hardly be found in the ¹H NMR spectrum. That is probably because the proportion of HPPPA in the copolyester is far less than the proportion of SA so that the probability of finding a BD unit next to two HPPPA units is very low. Thereby, PBSH copolyesters are successfully obtained through a polycondensation process in our work.

GPC and melt flow rate measurements were utilized to determine the molecular weights of PBS and its copolymers (Table 1). It is found that the molecular weights of these samples decrease as the comonomer content increased. This is probably due to the hindering effect of bulky pendent group of HPPPA on the molecular weights of copolyesters.¹⁶

Table 1 Basic characteristics of PBS and PBSH samples

Sample	Content of SA (mol%, based on diacid)	Content of HPPPA (mol%, based on diacid)	Molecular weight (Mw × 10^4)	Melt flow rate (g/10 min)	Tm (°C)	Tc (°C)	Xc^a (%)	Xc^b (%)
a Determined by DSC.b Determined by WXRD.
PBS	100	0	11.0	13.4	109.9	64.3	48.9	56.0
PBSH-1%	99.0	1.0	4.5	24.1	107.0	58.0	45.6	54.0
PBSH-2%	98.0	2.0	4.5	29.2	102.0	44.9	41.0	51.3
PBSH-4%	96.0	4.0	3.6	37.4	97.2	28.3	36.5	46.6

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3.2 Thermal oxidative stability and flame retardancy

The flammability of polymers is related to their thermo-oxidative degradation behaviours at high temperatures. Therefore, thermal oxidative stability of PBS and its copolyesters in air were studied and results are showed in Fig. 3 and Table 2. The thermal degradation temperature of maximum weight loss (T_max) for pristine PBS and PBSH copolymers are not changed with the HPPPA content. PBSH copolymers show one more decomposition stage at about 350 °C than neat PBS. Since typical bond energies of P–C and C–C are 260 kJ mol⁻¹ and 349 kJ mol⁻¹,²¹ respectively, the bonding of phosphorous-carbon is susceptible to chain scission during thermal oxidative decomposition. Thus, the first weight-loss step mentioned above is mainly the decomposition of HPPPA units in polymer chains. Although the P–C bond in copolyester chain ruptures at a lower temperature than C–C of a pure PBS, the initial decomposition temperatures (T_5%) of samples are increased from 331 °C to 349 °C when HPPPA content reach 2 mol%. It is worth mentioning that the gap between melting point and decomposition temperature extend from 221 °C to 250 °C as the content of comonomer increased, which will effectively enlarge the temperature window for the processing of PBS.

Fig. 3 TGA thermograms of PBS and PBSHs in air atmosphere.

Table 2 Thermal degradation properties and flammability properties of PBS and PBSH samples

Sample	T5% (°C)	Tmax (°C)	LOI value (%)	Ignite the absorbent cotton	UL-94 rating
PBS	331	396	22.0	Yes	NR
PBSH-1%	336	388	23.5	Yes	V-2
PBSH-2%	349	399	28.5	Yes	V-2
PBSH-4%	347	394	35.0	No	V-0

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The LOI values and UL-94 results of the flame retardant PBS and its copolymer is presented in Table 2. Pure PBS exhibits a LOI value of 22.0% and is not classified in the UL-94 test. When 2 mol% HPPPA is added, the LOI value goes up to 28.5%, but this formulation still does not pass the UL-94 V-0 rating in vertical burning test. When HPPPA concentration reached 4 mol%, the LOI value can reach 35% and the copolymer can be placed in the UL-94 V-0 class. These results suggest that the flame retardancy of PBS is improved by introducing HPPPA comonomer. It should be noted that the dripping phenomenon still can be observed in the copolyester samples while the molten drops cannot ignite the absorbent cotton when the content of HPPPA reached 4 mol%. Furthermore, it is also found that flammability of the samples change a lot after adding HPPPA while their thermal stabilities are not improved evidently in the TGA tests. The reason was discussed as follows. In our system, HPPPA has two effects on the thermal stabilities of PBS. On one hand, C–P bond is less stable than common C–C bond. The bonding of C–P in HPPPA is susceptible to chain scission during thermal degradation and acts as a weak link. Thus, this effect will decrease the thermal degradation temperature of copolyester. On the other hand, as a type of phosphorus-containing flame retardant, HPPPA can degrade into polyphosphoric acid during thermal degradation which can scavenge active radicals and inhibit radical reactions in the gas phase, by preventing the radicals from attacking the carbon atom vicinal to the ether oxygen.^14,22–24 This effect can increase the thermal stabilities of copolyesters. The thermal stability of copolyester is affected by a combination of these two factors and thus the improvement of it is less evident than flammability.

3.3 Crystallization behaviors

The non-isothermal melt-crystallization behaviours for PBS and its copolymers are shown in Fig. 4 and Table 1. In cooling process (Fig. 4a), it is obvious that, when the comonomer content increased, the exothermic trace become wider and shifts towards lower temperatures. That is because HPPPA is probably a kind of non-crystallizable comonomer in our copolymer system and will be excluded from the crystalline phase, which increases the energy barrier for critical nucleus formation and causes changes in the interfacial free energy of the forming crystals.

Fig. 4 DSC cooling (a) and second heating scans (b) of the PBS and copolymers at a rate of 10 °C min⁻¹.

The subsequent melting curves for PBS and its copolymers after non-isothermal crystallization are shown in Fig. 4b, it is apparent that, the peak of melting temperature decrease with the increase of comonomer content. This result is expected, because the crystals formed at lower temperatures will generally have lower lamellar thickness than those crystallized at higher temperatures. It is worth noting that the double melting peak and the small exothermal peak just prior to the final melting of neat PBS gradually disappear as comonomer content increases. The possible origin of the two phenomena has been investigated by many workers.^25–27 According to their research results, both of the double melting peak and the small exothermal peak are ascribed to the melting and recrystallization of the crystallites with low thermal stability. In our research system, the crystallization rate and lamellar thickness of the copolymer is depressed with increasing comonomer content. Therefore, copolymers will melt at a relatively lower temperature and cannot have enough time to recrystallization during heating process. Thus, melting and recrystallization related phenomena can hardly be seen in the samples with high comonomer content.

3.4 Crystal structures

WAXD was also performed to study the crystal structure and calculate the crystallinity degree (X_c) (Table 1). As shown in Fig. 7, which shows the WAXD patterns of the copolymers and homopolymer, we can see that PBS forms a monoclinic crystal lattice and the diffraction peaks from (020) and (021), (110) and (111) crystallographic planes are observed at 19.7°, 22.0°, 22.6° and 28.8°, respectively.^28,29 For PBSH samples, no new peaks appear and the peak positions of the PBSH almost coincide with that of pure PBS, which indicates that the crystal structure of PBSH is the same as that of PBS. As a comonomer, most HPPPA units are excluded from the crystalline region which is composed of SA and BD units. Furthermore, the diffraction peak positions shift slightly with HPPPA content increased, which indicates that HPPPA comonomer induces a distortion of the PBS crystalline lattice.

Fig. 5 WAXD patterns of PBS and the copolymers.

Fig. 6 Spherulitic morphologies obtained at different crystallization temperatures: (a) PBS at 75 °C, (b) PBSH-1% at 75 °C, (c) PBSH-2% at 75 °C, (d) PBSH-4% at 85 °C, (e) PBS at 85 °C, (f) PBSH-1% at 85 °C, (g) PBSH-2% at 85 °C, (h) PBSH-4% at 85 °C.

Fig. 7 Mechanical properties of neat PBS and its copolyesters before and after melt postpolycondensantion.

The degree of crystallinity, calculated by XRD and DSC data is presented in Table 1. As the HPPPA unit content in the copolyester increases, the degree of crystallinity decreases obviously. This tendency can be attributed to the reduction in homopolymer crystallite size due to the presence of the comonomer unit.³⁰

The spherulitic morphologies of PBS and its copolymers were obtained from POM and results are presented in Fig. 5. Parts a–d of Fig. 5 show the spherulitic morphologies of PBS and its copolymer isothermally crystallized at 75 °C. The spherulites of all samples show a negative feature, since quadrants 1 and 3 of the Maltese cross show yellow colours and 2 and 4 show blue colours. Besides, banded spherulites are observed in copolyesters. When crystallization temperature increases to 85 °C (parts e–h), instead of ring banded morphologies, clear branching texture can be observed in neat PBS. For copolymers, the ring banded morphology become more distinct and band spacing also increases as crystallization temperature rising.

3.5 Mechanical properties

As a significant property for practical application, the mechanical performance of PBSH samples was investigated. According to the aforementioned results, it is hard to obtain high molecular weight phosphorus-containing copolymer by the general melt polymerization. Therefore, in order to further improve its molecular weight, the samples was subjected to solid-state polycondensation. The melt flow rates of samples before and after treatment are present in Table 3. It can be seen that the molecular weights of the samples increase dramatically after processing. For the samples have not been treated by melt postpolycondensation, as illustrated in Fig. 6a and Table 3, both of tensile strength and elongation at break are decreased as increasing the comonomer content. The decrease of crystallinity is one of the reasons lead to the drop of mechanical strength. Beyond that, the existence of the weak P–C bond³¹ and the low molecular weights of the copolymer samples also can reduce the mechanical performance of the samples. Compared with the low molecular samples, both of tensile strength and tensile strain are increased when molecular weight increases. This phenomenon is more evident in samples with high comonomer content, such as PBSH-2% and PBSH-4%, which probably because the changes of molecular weights of these two samples are larger than the other samples. Furthermore, for the samples with similar molecular weights and different monomer contents, both of tensile strength and strain are decreased as comonomer content increases. The above results indicate that the mechanical performance of PBSH is impaired synthetically by crystallinity, molecular weight and the comonomer content. Nevertheless, the tensile strength of PBSHc-4% can reach to 30 MPa and elongation at break can reach about 150%, which can still meet end use requirements of PBS. It is worth noting that the sudden drop of tensile strength after yielding point observed in low molecular weight samples disappears in higher molecular weight samples.

Table 3 The melt flow rate and mechanical properties of PBS and its copolymers before and after melt postpolycondensantion

Sample	Characterization	Melt flow rate (g/10 min)		Tensile strength (MPa)		Elongation at break (%)
	Postpolycondensantion	Before	After	Before	After	Before	After
PBS		13.4 ± 0.4	1.8 ± 0.1	31.1 ± 0.1	47.2 ± 1.9	308.0 ± 7.1	310.2 ± 9.1
PBSH-1%		24.1 ± 1.3	2.1 ± 0.1	30.4 ± 0.6	40.9 ± 2.1	200.9 ± 5.8	216.5 ± 8.2
PBSH-2%		29.2 ± 1.9	2.3 ± 0.1	27.5 ± 0.7	39.8 ± 3.0	69.3 ± 3.0	199.3 ± 7.1
PBSH-4%		37.4 ± 2.8	2.8 ± 0.2	25.7 ± 0.9	31.2 ± 2.5	26.1 ± 4.7	155.7 ± 5.8

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As we know that the drop of tensile strength after yielding point in the tensile curve is explained by the onset of permanent or plastic deformation of the chains brought about by the resolved shear stress.³² For the high polymer weight samples, the entanglement density of the polymer matrix is probably high enough to impose a strong constraint on the sliding of polymer chains. Therefore, the tensile strength will not drop obviously after yielding point. Significantly, this mechanical property is generally preferred in real-world usage since it can avoid the sudden collapse of PBS products caused by the drop of tensile strength under mechanical stress.

4. Conclusions

Poly(butylenesuccinate-co-3-hydroxyphenylphosphinyl-propionate acid) (PBSH) was synthesized through esterification and copolycondensation of succinic acid (SA), 1,4-butanediol(BD) and 3-hydroxyphenylphosphinyl-propionic acid (HPPPA). Upon the incorporation of HPPPA, the onset decomposition temperatures (T_5%) of PBS increase from 331 °C to 347 °C. Moreover, the LOI values of the PBSHs are increased to 35% and the copolyester can also achieve a UL-94 V-0 rating in vertical burning test. The mechanical properties of PBSH were also investigated. We found that the addition of HPPPA can decrease the tensile strength and elongation of PBS. To solve this problem, melt postcondensation was applied to increase the molecular weight of the copolyester. The result shows that the mechanical performance of copolyesters is evidently improved after melt postcondensation. The resulting novel flame-retardant polyesters will find important applications in nonwoven textiles, automotive components and other fields.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 21274095 and Grant No. 51173112).

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