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Article

Enhancement of Flame Retardancy of Colorless and Transparent Semi-Alicyclic Polyimide Film from Hydrogenated-BPDA and 4,4′-oxydianiline via the Incorporation of Phosphazene Oligomer

by
**ao Wu
1,
Ganglan Jiang
1,
Yan Zhang
1,
Lin Wu
1,
Yanjiang Jia
1,
Yaoyao Tan
1,
**gang Liu
1,* and
**umin Zhang
2,*
1
Bei**g Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Bei**g 100083, China
2
School of Electrical Engineering, Bei**g Jiaotong University, Bei**g 100044, China
*
Authors to whom correspondence should be addressed.
Polymers 2020, 12(1), 90; https://doi.org/10.3390/polym12010090
Submission received: 29 November 2019 / Revised: 18 December 2019 / Accepted: 21 December 2019 / Published: 3 January 2020
(This article belongs to the Special Issue Advances in Flame Retardant Polymeric Materials)
Browse Figures
Versions Notes

Abstract

:
Enhancement of flame retardancy of a colorless and transparent semi-alicyclic polyimide (PI) film was carried out by the incorporation of phosphazene (PPZ) flame retardant (FR). For this purpose, PI-1 matrix was first synthesized from hydrogenated 3,3′,4,4′-biphenyltetracarboxylic dianhydride (HBPDA) and 4,4′-oxydianiline (ODA). The soluble PI-1 resin was dissolved in N,N-dimethylacetamide (DMAc) to afford the PI-1 solution, which was then physically blended with PPZ FR with the loading amounts in the range of 0–25 wt.%. The PPZ FR exhibited good miscibility with the PI-1 matrix when its proportion was lower than 10 wt.% in the composite films. PI-3 composite film with the PPZ loading of 10 wt.% showed an optical transmittance of 75% at the wavelength of 450 nm with a thickness of 50 μm. More importantly, PI-3 exhibited a flame retardancy class of UL 94 VTM-0 and reduced total heat release (THR), heat release rate (HRR), smoke production rate (SPR), and rate of smoke release (RSR) values during combustion compared with the original PI-1 film. In addition, PI-3 film had a limiting oxygen index (LOI) of 30.9%, which is much higher than that of PI-1 matrix (LOI: 20.1%). Finally, incorporation of PPZ FR decreased the thermal stability of the PI films. The 10% weight loss temperature (T10%) and the glass transition temperature (Tg) of the PI-3 film were 411.6 °C and 227.4 °C, respectively, which were lower than those of the PI-1 matrix (T10%: 487.3 °C; Tg: 260.6 °C)

Graphical Abstract

1. Introduction

Colorless and transparent polyimide (CPI) films have been paid ever-increasing attention in recent years due to their excellent combined thermal, optical, and dielectric properties [1,2,3]. CPI films have been thought to be good candidates for functional components for advanced optoelectronic fabrication, such as substrates for flexible display devices [4,5,6], flexible organic solar cells [7], and flexible heater for wearable sensors [8]; encapsulants for displays or electronic chips [9]; and other high-tech applications. Various CPI films, including the wholly aromatic ones containing fluoro-containing units [10,11,12] and the wholly alicyclic or semi-alicyclic ones containing alicyclic or aliphatic segments [13,14,15], have been well designed and developed in the past decades.
Among the CPI films, the semi-alicyclic ones usually possess superior optical transparency, mechanical toughness, dielectric properties, and molecular designability to their wholly aromatic counterparts [16]. However, in the practical applications for semi-alicyclic CPI films, flame retardancy is often one of the most important requirements with respect to the reliability and safety of the devices. The class of UL 94 (Underwriters Laboratories Incorporated, Bentonville, AR, USA) VTM-0 is usually required [17]. Actually, flame retardancy issues have rarely been addressed for wholly aromatic PI films due to their intrinsically inflammable nature caused by the rigid-rod and highly conjugated aromatic rings and imide units in their molecular structures [18,19]. However, these structural features for wholly aromatic PI films greatly deteriorate their optical transparency due to the formation of intra- and intermolecular charge transfer complexes (CTCs) among the electron-donating diamine units and the electron-accepting dianhydride units [20,21,22]. Due to the electron mobility in the formation of CTCs, visible light is highly absorbed by the PI film, resulting in colors from deep brown to dark yellow. Semi-alicyclic CPI films are designed by eliminating or prohibiting the formation of CTCs via incorporation non-conjugated alicyclic moieties into the molecular skeleton of CPIs, thus exhibiting very pale colors from pale yellow to colorless. Although the coloration issue is efficiently resolved for CPI films, this is usually accompanied by side-effects, mainly consisting of decreased high-temperature dimensional stability (high coefficient of thermal expansion, high-CTE), and sacrifice of flame retardancy. The current research and developmental hot topics for practical CPI films also focus on these two issues.
As for the flame retardancy issue of CPI films, the incorporation of flame retardants (FR) into the film matrix seems to be one of the most efficient pathways considering the feasibility with respect to technology and cost. However, the suitable FR for CPI films should be elaborately selected in view of the special requirements of colorless and transparent polymer films. First, the FR should not deteriorate the optical transparency of the CPI film matrix; therefore, those that could form nano-scale, preferably molecular-scale combinations with the CPI matrix are preferred. Secondly, the addition of FR cannot cause the yellowing of CPI film matrix, especially at high temperature; thus, those with good thermal stability and high-temperature oxidative resistance are preferred. Finally, the selected FR have to be compatible with the European Union (EU) RoHS (Restriction of Hazardous Substances) and WEEE (Waste Electrical and Electronic Equipment) directives; thus, those containing bromine and other harmful substances cannot be used [23]. Considering the requirements of FRs for CPI films mentioned above, the useable FRs are actually very limited.
In the current work, as one of our continuous series of works develo** high-performance CPI films for advanced optical applications, the enhancement of flame retardancy of one representative semi- alicyclic CPI film derived from hydrogenated 3,3′,4,4′-biphenyltetracarboxylic dianhydride (HBPDA) and 4,4′-oxydianiline (ODA) was performed by incorporation of an organic phosphazene (PPZ) oligomer FR. The effects of the addition of PPZ on the optical and thermal properties of the derived PI composite films were investigated in detail.

2. Materials and Methods

2.1. Materials

Hydrogenated 3,3′,4,4′-biphenyltetracarboxylic dianhydride (HBPDA) was synthesized in our laboratory according to our previous work [9], or it can be purchased from ** and ignition of the cotton indicator were detected for the PI composite films in the UL 94 measurements. The LOI values of PI films are shown in Figure 9, together with the combustion conditions of PI films. Considering the deterioration of optical and thermal properties of the composite films with higher PPZ FR loadings, only PI-2 and PI-3, with the best combined properties, were tested in the flame retardancy evaluation. It can be seen from Figure 9 that the PI-1 film burned as soon as it was ignited, accompanied by sparks scattered around the ignition source. The film has a LOI value of 21.5%, which is a bit higher than that of the flammable polyethylene film (LOI: 18.0%) [34], and far lower than that of the wholly aromatic PI film, such as poly[N,N′-(oxydiphenylene)pyromellitimide] (Kapton®) film (LOI: 37.0%). Incorporation of PPZ FR apparently increased the LOI values of the PI films. PI-2 and PI-3 possessed LOI values of 24.8% and 30.9%, respectively, and exhibited self-extinguishing features during the combustion test. Black and tough char layers were left for both of the samples, especially for PI-3. As mentioned previously, the PI-3 film showed higher char yield at elevated temperature than those of the PI-1 matrix and the PPZ FR. The char layers undoubtedly acted as a barrier in the combustion and endowed the composite films with good flame retardancy and high LOI values. It has been well established that a thick and compact char layer is capable of effectively preventing the inner structure from being exposed to flame and releasing combustible gases [35]. To clarify the effects of residual char layers on the combustion behaviors of the films, the micro-morphology and elemental composition of the char layers after combustion test were detected.
Figure 10 depicts and compares the SEM images with energy dispersive X-ray (EDX) analysis. It can be clearly observed than many round or elliptical holes can be observed in the PI-1 residues after combustion. Furthermore, the diameter of the holes near the ignition source was apparently larger than that of those far from the ignition source. EDX testing showed that the residue was mainly composed of C and O elements. For PI-2 and PI-3, more condensed char layers were observed. The results of EDX showed that phosphorus-containing passivation layers formed on the surface of the residues. This indicates that the phosphorus components might migrate to the external char layer and accumulate in condensed phase during the combustion process, which might promote to form a compact barrier to delay heat and mass transfer and thus prevent the further ignition of the films. Therefore, based on the fact that the phosphorus-containing passivation layer enriched in the surface of residues during combustion, the flame retardancy of the current PI/PPZ systems mainly matches the mechanism of condensed phase [36].
The combustion behaviors of the PI films were further investigated and compared by the THR and HRR measurements, and the plots are shown in Figure 11 and Figure 12, respectively. According to the THR plots of the PI films shown in Figure 11, the THR values of the PI films decreased from 1.45 MJ/m2 for PI-1 to 0.99 MJ/m2 for PI-2, indicating the effective increase of the flame retardancy of the PI films via introduction of the PPZ FR. All of the PI films exhibited sharp peaks, with pHRR values from 76.6 to 98.9 kW/m2, as shown in Figure 12 and Table 3. By comparison, the PPZ-modified PI-2 and PI-3 films showed reduced pHRR values compared to those of PI-1. The combustion plots of the PI thin films agreed well with the literature [37], in which the characteristics of the current THR and HRR plots were attributed to the char-forming action during combustion. The current THR and HRR results are also in good agreement with the observation of char layers shown in Figure 10. Apparently, these inflammable and compact passivation layers provide the current PI composite films relatively good stability during the combustion test.
The smoke evaluation behaviors of the PI films were further investigated due to their high importance in possible fire scenarios. The smoke production rate (SPR) and rate of smoke release (RSR) plots of the PI films are illustrated in Figure 13 and Figure 14, respectively. It can be deduced from the figures that introduction of PPZ FR could reduce both the SPR and RSR values of the PI composite films. For instance, the peak values of PI-3 were 0.052 m2/s for SPR and 5.92 (m2/s/m2) for RSR, respectively, which are clearly lower than those of the pristine PI film (peak of SPR = 0.072 m2/s; peak of RSR = 8.13 m2/s/m2). This might be due to the good smoke suppression effects of PPZ FR [38]. Finally, the carbon monoxide production (COP) of the PI films during combustion was also investigated, and the results are shown in Figure 15. It can be seen from the figure that the introduction of PPZ FR did not obviously affect the CO release level of the PI films. However, the rate of CO production of the current PI films was much lower than that of the common polymers, such as polystyrene (PS) [39].

4. Conclusions

The endeavors of increasing the flame retardancy of PI (HBPDA-ODA) film while maintaining its intrinsic optical and thermal properties were successfully achieved by incorporation of PPZ oligomer FR at a loading amount below 10 wt.%. The PI-3 composite film filled with 10 wt.% PPZ FR showed the best comprehensive properties among the developed colorless and transparent PI composite films, including high flame retardancy, good thermal stability, and good optical properties. The good combined properties make the PI-3 film a good candidate for advanced optoelectronic applications.

Author Contributions

Conceptualization, J.L.; Methodology, X.W.; Investigation, X.W. and G.J.; Data curation, X.W., Y.Z., L.W., Y.T., and Y.J.; Writing—original draft preparation, X.W.; Writing—review and editing, J.L.; Supervision, X.Z.; Funding acquisition, J.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Key Technology Research and Development Program of Shandong: 2019JZZY020235. Fundamental research funds of China University of Geosciences, Bei**g: 2652017345.

Acknowledgments

The financial supports from the Shandong Key Research and Development Program (No. 2019JZZY020235) and Fundamental Research Funds of China University of Geosciences, Bei**g (No. 2652017345) are gratefully acknowledged.

Conflicts of Interest

The authors declare no conflict of interest.

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Polymers 12 00090 g001 550
Figure 1. Preparation of PI-1 (HBPDA-ODA) and PI/PPZ composite films (PI-2~PI-6).
Figure 1. Preparation of PI-1 (HBPDA-ODA) and PI/PPZ composite films (PI-2~PI-6).
Polymers 12 00090 g001
Polymers 12 00090 g002 550
Figure 2. FT-IR spectra of PI films.
Figure 2. FT-IR spectra of PI films.
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Polymers 12 00090 g003 550
Figure 3. XPS spectra of PI films.
Figure 3. XPS spectra of PI films.
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Polymers 12 00090 g004 550
Figure 4. XRD spectra of PPZ (FP-100) and PI films.
Figure 4. XRD spectra of PPZ (FP-100) and PI films.
Polymers 12 00090 g004
Polymers 12 00090 g005 550
Figure 5. Optical properties of PI films. (a) CIE Lab color parameters of PI-1 and PI-4; (b) apparent transparency of PI films (thickness: ~50 μm).
Figure 5. Optical properties of PI films. (a) CIE Lab color parameters of PI-1 and PI-4; (b) apparent transparency of PI films (thickness: ~50 μm).
Polymers 12 00090 g005
Polymers 12 00090 g006 550
Figure 6. UV-Vis spectra of PI films. (film thickness: ~50 μm).
Figure 6. UV-Vis spectra of PI films. (film thickness: ~50 μm).
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Polymers 12 00090 g007 550
Figure 7. Thermal decomposition behaviors of PI films in nitrogen. (a) TGA; (b) DTG.
Figure 7. Thermal decomposition behaviors of PI films in nitrogen. (a) TGA; (b) DTG.
Polymers 12 00090 g007
Polymers 12 00090 g008 550
Figure 8. DSC plots of PI films.
Figure 8. DSC plots of PI films.
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Polymers 12 00090 g009 550
Figure 9. LOI values of PI films (insert: combustion of PI films).
Figure 9. LOI values of PI films (insert: combustion of PI films).
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Polymers 12 00090 g010 550
Figure 10. SEM images and EDX patterns of char layers in PI films after ignition. (a) PI-1; (b) PI-2; and (c) PI-3.
Figure 10. SEM images and EDX patterns of char layers in PI films after ignition. (a) PI-1; (b) PI-2; and (c) PI-3.
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Polymers 12 00090 g011 550
Figure 11. THR plots of PI films.
Figure 11. THR plots of PI films.
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Polymers 12 00090 g012 550
Figure 12. HRR plots of PI films.
Figure 12. HRR plots of PI films.
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Polymers 12 00090 g013 550
Figure 13. SPR plots of PI films.
Figure 13. SPR plots of PI films.
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Polymers 12 00090 g014 550
Figure 14. RSR plots of PI films.
Figure 14. RSR plots of PI films.
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Polymers 12 00090 g015 550
Figure 15. COP plots of PI films.
Figure 15. COP plots of PI films.
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Table
Table 1. Optical properties of PI films.
Table 1. Optical properties of PI films.
PIPPZ (wt.%)λ (nm) aT450 (%) bL* ca* cb* cHaze
PI-1029183.696.11−0.232.571.21
PI-2529281.195.10−0.544.213.79
PI-31029475.094.87−0.273.565.03
PI-41529527.292.370.164.1527.67
PI-5202959.587.950.113.4293.83
PI-6252981.186.980.094.65100.00
aλ: Cutoff wavelength; bT450: Transmittance at the wavelength of 450 nm with a thickness of 50 um; cL*, a*, b*, see 2.2. Characterization methods.
Table
Table 2. Thermal properties of PPZ FR and PI films.
Table 2. Thermal properties of PPZ FR and PI films.
SamplesPPZ (wt.%)Tg (°C) aT10% (°C) bTmax (°C) bRw700 (%) c
PPZ100ND d381.3438.28.2
PI-10260.6487.3527.710.0
PI-25244.8470.2501.717.1
PI-310227.4411.6448.958.3
PI-415220.9405.5413.256.3
PI-520219.8405.1407.155.2
PI-625207.3406.6405.050.8
aTg: Glass transition temperature; bT10%: Temperatures at 10% weight loss; cRw700: Residual weight ratio at 700 °C in nitrogen; d Not detected.
Table
Table 3. Combustion data of PI composite films.
Table 3. Combustion data of PI composite films.
PIUL 94t1
(s)		t2
(s)		t1 + t2
(s)		Drip** aIgnition bLOI c
(%)		THR d
(MJ/m2)		pHRR e
(kW/m2)
PI-1Not VTM-2ND fNDNDYesYes21.51.4598.9
PI-2VTM-011819NoNo24.81.4877.3
PI-3VTM-015015NoNo30.90.9976.6
a If drip** in the UL 94 measurements; b If the cotton indicator was ignited in the UL 94 measurements; c limited oxygen index; d Total heat release; e Peak of heat release rate; f Not detected.

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Wu, X.; Jiang, G.; Zhang, Y.; Wu, L.; Jia, Y.; Tan, Y.; Liu, J.; Zhang, X. Enhancement of Flame Retardancy of Colorless and Transparent Semi-Alicyclic Polyimide Film from Hydrogenated-BPDA and 4,4′-oxydianiline via the Incorporation of Phosphazene Oligomer. Polymers 2020, 12, 90. https://doi.org/10.3390/polym12010090

AMA Style

Wu X, Jiang G, Zhang Y, Wu L, Jia Y, Tan Y, Liu J, Zhang X. Enhancement of Flame Retardancy of Colorless and Transparent Semi-Alicyclic Polyimide Film from Hydrogenated-BPDA and 4,4′-oxydianiline via the Incorporation of Phosphazene Oligomer. Polymers. 2020; 12(1):90. https://doi.org/10.3390/polym12010090

Chicago/Turabian Style

Wu, **ao, Ganglan Jiang, Yan Zhang, Lin Wu, Yanjiang Jia, Yaoyao Tan, **gang Liu, and **umin Zhang. 2020. "Enhancement of Flame Retardancy of Colorless and Transparent Semi-Alicyclic Polyimide Film from Hydrogenated-BPDA and 4,4′-oxydianiline via the Incorporation of Phosphazene Oligomer" Polymers 12, no. 1: 90. https://doi.org/10.3390/polym12010090

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