Tian-Xiang Jina,
Xian-Yin Zhangb,
Yun-Feng Taob,
Dan Wangb,
Feng Chen*a and
Qiang Fu*a
aCollege of Polymer Science and Engineering, Sichuan University, State Key Laboratory of Polymer Materials Engineering, Chengdu 610065, China. E-mail: qiangfu@scu.edu.cn
bChengdu TALY Chemical Industrial Co., Ltd, Chengdu 610100, China
First published on 9th July 2015
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.
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.
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.
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 unit17,19) 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
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
Sample | Content of SA (mol%, based on diacid) | Content of HPPPA (mol%, based on diacid) | Molecular weight (Mw × 104) | Melt flow rate (g/10 min) | Tm (°C) | Tc (°C) | Xca (%) | Xcb (%) |
---|---|---|---|---|---|---|---|---|
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 |
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 |
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.
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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.
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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.
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 |
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.
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