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What Reacts with What in Bisphenol A Polycarbonate/Silicon Rubber/ Bisphenol A Bis(diphenyl phosphate) during Pyrolysis and Fire Behavior? Eliza Wawrzyn,† Bernhard Schartel,*,† Henrik Seefeldt,† Andrea Karrasch,‡ and Christian J€ager‡ † ‡
BAM Federal Institute for Materials Research and Testing, Unter den Eichen 87, 12205 Berlin, Germany BAM Federal Institute for Materials Research and Testing, Richard-Willstaetter Strasse 11, 12489 Berlin, Germany ABSTRACT: The pyrolysis and flame retardancy of a bisphenol A polycarbonate/silicon rubber/bisphenol A bis(diphenyl phosphate) (PC/SiR/BDP) blend were investigated and compared to those of PC/BDP and PC/SiR. The impact modifier SiR consists mainly of poly(dimethylsiloxane) (PDMS > 80 wt %). The pyrolysis of PC/SiR/BDP was studied by thermogravimetry (TG), TGFTIR to analyze the evolved gases, and a Linkam hot stage cell within FTIR as well as 29Si NMR and 31P NMR to analyze the solid residue. The fire performance was determined by PCFC, LOI, UL 94, and a cone calorimeter under different external irradiations. The fire residues were studied by using ATR-FTIR as well as the additional binary systems PC + PDMS, PC + BDP, and BDP + PDMS, focusing on the specific chemical interactions. The decomposition pathways are revealed, focusing on the competing interaction between the components. Fire retardancy in PC/SiR/BDP is caused by both flame inhibition in the gas phase and inorganiccarbonaceous residue formation in the condensed phase. The PC/SiR/BDP does not work as well superimposing the PC/SiR and PC/ BDP performances. PDMS reacts with PC and BDP, decreasing BDP’s mode of action. Nevertheless, the flammability (LOI > 37%, UL 94 V-0) of PC/SiR/BDP equals the high level of PC/BDP. Indeed, SiR in PC/SiR/BDP is underlined as a promising impact modifier in flame-retarded PC/impact modifier blends as an alternative to highly flammable impact modifiers such as acrylonitrile butadienestyrene (ABS), taking into account that the chosen SiR leads to PC blends with a similar mechanical performance.
1. INTRODUCTION Bisphenol A polycarbonate (PC) has found extensive application because of its unique combination of properties. The wide utility of PC is due to its high heat distortion temperature, transparency, dimensional stability, and heat and strain resistance. Unfortunately, PC exhibits high notch sensitivity and the surface of the polymer can be easily scratched. This can be improved by adding an impact modifier, most commonly acrylonitrile butadienestyrene (ABS).1,2 However, because ABS burns heavily without residue and produces a lot of smoke, efforts are under way to replace it. Recently different impact modifiers were investigated in halogen-free flame-retarded PC blends.3,4 It was proposed that siliconeacrylate rubbers (SiR) are promising candidates to replace ABS, when a high amount of poly(dimethylsiloxane) (PDMS) is used.3,4 The fire retardancy may be improved with respect to flammability (limiting oxygen index (LOI) and UL 94 classification). Silicone-based additives generate smaller amounts of toxic substances in the case of fire.5 The most widely used polymer in the silicone industry is PDMS. PDMS exhibits elastic properties and thus works as an impact modifier in PC blends. Furthermore, when exposed to elevated temperatures under oxygen, PDMS leaves behind an inorganic silica residue. Silica residue may act as a protection layer for heat and mass transfer.6 Incorporation of PDMS in PC leads to residue of high thermal stability. Protection layers were observed due to the reaction of PC and PDMS that reduces the release of volatile fuel.7 Further, cross-linked poly(dimethylsiloxane)s have been found to be effective in preventing many thermoplastics from dripping during combustion. 8 r 2011 American Chemical Society
However, to provide sufficient flame retardancy, they need to be used in combination with other flame retardants. Commercially available halogen-free retardants for PC blends are aryl phosphates, among which bisphenol A bis(diphenyl phosphate) (BDP) plays an important role.9,10 The flame retardancy of BDP in PC combines gas-phase and condensed-phase modes of action. Phosphorus can be volatilized into the gas phase and acts through flame poisoning. Radicals such as HPO2•, PO•, PO2•, and HPO• are free radical scavengers that bond the oxygen or hydroxyl radicals. Thus they reduce combustion efficiency within the flame. Aryl phosphates also harbor the potential to initiate cross-linking reactions, and thus charring in the condensed phase. Indeed, BDP induces additional charring by cross-linking with PC.1114 In this work, the effect of using SiR as an impact modifier, mainly consisting of PDMS in PC/SiR/BDP, is discussed based on thermal analysis, flammability (reaction to the small flame), and fire behavior (forced-flaming behavior) investigations. The comprehensive multimethodical pyrolysis studies including the key results of solid-state NMR investigations on residues produced in an oven already published in detail15,16 led to the development of detailed decomposition pathways including the interactions between the distinct components; these explain the differences in the fire performance and flame retardancy mechanisms of PC/SiR/BDP in comparison to PC/SiR and PC/BDP. Received: August 26, 2011 Accepted: December 19, 2011 Revised: December 16, 2011 Published: December 19, 2011 1244
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2. EXPERIMENTAL SECTION PC/BDP, PC/SiR, and PC/SiR/BDP were investigated. In each of those materials poly(tetrafluoroethylene) (PTFE) was added to unbranched PC to prevent dripping. It was incorporated as a master batch consisting of a coagulated 1:1 mixture of styreneacrylonitrile copolymer (SAN) and PTFE. The composition of studied blends is given in Table 1. The SiR impact modifier used consisted of a PDMS/poly(n-butyl acrylate) (PBA) core enclosed by a shell of poly(methyl methacrylate) (PMMA). The overall composition is 82 wt % PDMS, 7 wt % PBA, and 11 wt % PMMA. All blends were compounded using a corotating twin screw extruder before injection molding the test specimen. All materials were provided by Bayer MaterialScience AG (Dormagen, Germany) as plates, bars, and granulate. Thermal analysis was performed by using thermogravimetry (TG; TGA/SDTA 851, Mettler Toledo, Germany) under nitrogen, at a heating rate of 10 K min1. TG was coupled with Fourier transform infrared spectrometry (TGFTIR; FTIR, Nexus 470, Nicolet, Germany) to analyze the volatile decomposition products. The temperature of the transfer line was held constant at 523 K and that of the gas cell at 533 K during measurement. The mass of all samples was 15 mg. Values for residue were taken at 1000 K. A Linkam hot stage cell (Linkam, U.K.) vertically mounted within an FTIR spectrometer (Nexus 470, Nicolet, Germany) was used to investigate the chemical changes in the residue during decomposition. Thin, free-standing sample films placed on potassium bromide (KBr) substrate were heated from 303 to 873 K at a heating rate of 10 K min1 under nitrogen. To prepare the samples, the blend was heated, and when it started to melt, it was pressed into thin films. These films were placed immediately into the Linkam cell. Solid-state 31P and 29Si NMR experiments were performed with a DMX 400 spectrometer (Bruker Biospin GmbH, Rheinstetten, Germany). The measurements were carried out at room temperature Table 1. Composition of Investigated Blends (in wt %)a
a
PC/BDP
PC/SiR
PC/SiR/BDP
PC
86.0
81.2
71.0
SiR
17.3
15.0
BDP
12.5
12.5
PTFE
0.9
0.9
0.9
Remaining wt % is other additives.
using magic angle sample spinning (MAS). Solid residues for solidstate NMR were prepared in a horizontal quartz tube furnace (inner diameter 38 mm, length 40 cm) under a nitrogen flow of 100 mL min1. The samples were placed in a quartz boat and heated to the set-point temperature (between 630 and 850 K) at a heating rate of 10 K min1. They were stored for 30 min at the setpoint temperature before cooling. Furthermore, the three binary systems PC + BDP, PC + PDMS, and PDMS + BDP and their residue after 70 wt % mass loss were investigated using attenuated total reflectance (ATR) FTIR (Nexus 470, Nicolet with “Smart Orbit Single Reflection Diamond ATR” tool). PC, BDP, and PDMS were provided by Bayer MaterialScience AG (Dormagen, Germany). PC + BDP and PC + PDMS were prepared by simple hand-mixing of 5 mg of the components in the crucible; 15 mg was used in the case of PDMS + BDP to ensure a sufficient amount of residue. The residues were obtained using TG at a heating rate of 10 K min1 and stopped after 70 wt % mass loss. Additionally, the residues of cone calorimeter measurements (external heat flux = 50 kW m2) were investigated by ATR-FTIR. The flammability (reaction to a small flame) of the blends was determined by both LOI (Stanton Redcroft FTA, U.K.) according to ISO 4589, and the standard UL 94 (FTT, U.K.) protocol according to IEC 60695-11-10. The sample size was 80 mm 10 mm 4 mm for LOI. For UL 94 two thicknesses of each material were measured: 120 mm 12.5 mm 1.6 mm and 120 mm 12.5 mm 3.2 mm. The forced-flaming behavior was investigated by a cone calorimeter (FTT, U.K.) following ISO 5660 under three different irradiations: 35, 50, and 70 kW m2. The sample size was 100 mm 100 mm 3 mm, and the samples were placed horizontally in the frame. The pyrolysis combustion flow calorimeter (PCFC; FTT, U.K.) was used to determine the heat of complete combustion of the pyrolysis gases. A sample of 5 mg was placed in the pyrolyzer and decomposed under nitrogen at a heating rate of 1 K s1. The maximum pyrolysis temperature was 1023 K. The volatile products were oxidized in a combustor at 1173 K. The flow was a mixture of 20/80 vol % oxygen/nitrogen.
3. RESULTS AND DISCUSSION 3.1. Pyrolysis: Mass Loss. In Figure 1 PC and the following blends of PC/BDP, PC/SiR, and PC/SiR/BDP are compared.
Figure 1. Mass and mass loss rate (under N2, heating rate = 10 K min1; triangles = PC/SiR/BDP, stars = PC/SiR, circles = PC/BDP, line = PC). 1245
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Table 2. Thermal Analysis of PC, SiR, BDP, PC/BDP, PC/ SiR, and PC/SiR/BDPa
Table 3. Characteristic FTIR Absorption Bands of PC/SiR/ BDP wavenumber/cm1
PC SiR BDP PC/BDP PC/SiR PC/SiR/BDP error T2 wt %/K
734 480 623
679
650
647
(2
Mass Loss I weight loss/wt % Tmax1/K
27.0 653
10.0 714
6.0 668
16.5 676
(1.0 (2
weight loss/wt % 72.2 42.8 95.3
62.5
47.5
60.0
(1.0
Tmax2/K
793
726
799
(2
794 760 754
Mass Loss III weight loss/wt %
27.0
26.5
(1.0
Tmax3/K
927
810
(2
mass/wt %
27.8 3.2
20.0
23.5
(1.0
Residue at 1000 K a
4.7
27.5
Under N2, heating rate = 10 K min1.
The TG results of investigated blends and of PC, SiR, and BDP are summarized in Table 2. PC as well as BDP decomposed in one single decomposition step. BDP is less stable than PC, and thus started decomposing much earlier. PC is a naturally charring polymer,11 which yielded 27.8 wt % residue, whereas pyrolysis residue for BDP amounted only to 4.7 wt %. SiR decomposed in three decomposition steps. The first step is attributed to the thermal decomposition of PMMA and PBA,17,18 whereas the second and third decomposition steps reflect the decomposition of PDMS.19 SiR started to decompose at 480 K and left only a small amount of residue of 3.2 wt %. PC/BDP and PC/SiR/BDP decomposed in two steps, whereas the decomposition of PC/SiR showed three processes. The best separation of decomposition steps was observed in PC/ BDP, which showed two distinct maxima in the mass loss rate. For PC/SiR/BDP the mass loss rate showed a plateau before the main decomposition step, and for PC/SiR a broad shoulder (third decomposition step) was observed after the main decomposition. For PC/BDP the first step is related to decomposition of BDP and the second to decomposition of PC. For PC/SiR/ BDP the first step is attributed to decomposition of the acrylic component of the blend (PMMA and PBA) as indicated by the decomposition temperature. Furthermore, as pointed out by the mass loss, the first decomposition step involves interactions between early decomposition of PC and decomposing SiR. At the plateau BDP began decomposing, whereas in the main step the decomposition of PC, PDMS, and BDP took place. For PC/SiR the first step reflected the decomposition of PMMA and PBA, in the main step PC and PDMS decomposed, and in the third step the decomposition products of PDMS were still released. The PC blend containing only SiR began the main decomposition step much earlier than the other blends investigated. The strong shift in the main decomposition step showed that PC is less stable in PC/SiR. It was proposed that the hydroxyl-terminated PDMSs may speed up the PC chain scission in comparison to PC.20 In PC/BDP, the PC component decomposed with the same maximum of decomposition temperature (Tmax2) as PC, and in PC/ SiR/BDP the stability of PC was even slightly improved. When PC is mixed with BDP, additional cross-linking is proposed via the transesterification reaction of rearranged PC and BDP;
2361, 669
CO2
3015, 1301 960
CH4 POaryl
2960, 1750, 1638, 1445,
acrylates (methyl methacrylate and
1025, 1168, 926
Mass Loss II (Main Decomposition Step)
assignment
butyl methacrylate)
3651, 1600, 1500
bisphenol A and phenol derivatives
1260, 1020, 810
hexamethylcyclotrisiloxane
1260, 1080, 810
octamethylcyclotetrasiloxane
consequently, charring is enhanced.9,12 Experimentally obtained residue of the PC/BDP blend was 3.0 wt % higher than the amount of residue expected for superposition of the char yields of each component. For the blend PC/SiR, lower residue was received than expected (3.2 wt %). SiR enhanced the decomposition of PC, resulting in the lowest residue among all investigated blends. Combining SiR with BDP in PC/SiR/BDP led to lower residue than for PC/BDP, but higher than for PC/SiR. This means that PDMS partly disturbs the cross-linking of PC and BDP. 3.2. Pyrolysis: Evolved Gas Analysis. The main bands detected in the gas phase and corresponding product identification are shown in Table 3. The main evolved products from PC decomposition are carbon dioxide (CO2) and bisphenol A, created via hydrolysis of carbonate linkages. Methane (CH4), carbon monoxide (CO), phenol, and its aliphatic derivatives are other main volatile products, which are created via chain scission of isopropylidene linkages.11,21 PC/BDP released the phosphorus species in the first decomposition step (in the gas phase POaryl vibration is detected), showing that BDP decomposes first. In the main decomposition step the CH4, CO2, CO, and phenol derivatives are evolved, indicating decomposition of PC. For the blend PC/SiR the absorption bands from methyl methacrylate and butyl methacrylate were identified in the first decomposition step, resulting from PMMA and PBA depolymerization into the monomers. Additionally, the absorption bands of alcohol, CO2, and aliphatic structures were detected in the first step. The literature18,22,23 describes that PMMA and PBA decompose as well, by the two following pathways: either via elimination of an alkoxide group with the release of alcohols, or via decarboxylation with the release of CO2 and aliphatic structures. The typical decomposition products from PC;CH4, CO2, and phenol derivatives;were detected in the second decomposition step. Furthermore, hexamethylcyclotrisiloxane and octamethylcyclotetrasiloxane were found. They were formed by the decomposition of PDMS.24 In the third decomposition step larger PDMS blocks decomposed; thus siloxanes and CH4 were observed in the gas phase. The decomposition gases at the maximum of the first decomposition step (36 min, ≈660 K) of PC/SiR/BDP showed that methyl methacrylate and butyl methacrylate were released (Figure 2). This indicates depolymerization of PMMA and PBA via β-scission, creating the monomers. There were also carbon dioxide (CO2) absorption bands and vibrations of aliphatic structures (RCH2R, RCH3) in the region around 2960 cm1, which suggests both early decomposition of PC interacting with decomposing SiR and the another decomposition pathway for PMMA and PBA such as decarboxylation. 1246
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Figure 2. (a) FTIR spectra of evolved gases in the first and second decomposition steps, (b) mass loss rate, and (c) product release rate of PC/SiR/BDP.
At the main decomposition step (50 min, ≈800 K) methane, CO2, and bisphenol A as part of phenol derivatives were released; these are the decomposition products from both PC and BDP. Another absorption band was detected, which was the POaryl vibration originating from BDP. Additionally, the absorption bands of hexamethylcyclotrisiloxane and octamethylcyclotetrasiloxane were detected, which belong to the pyrolysis of PDMS. PDMS decomposes mainly via an intramolecular transition state leading to cyclic oligomers, with tri- and tetrasiloxanes among the most abundant products.24,25 The higher stability of six-membered rings led to the larger amount of hexamethylcyclotrisiloxane. 3.3. Pyrolysis: Solid Residue Analysis of Binary Systems. Comparing the thermal decompositions of PC/SiR, PC/BDP, and PC/SiR/BDP leads to the question of what reacts with what in PC/SiR/BDP. To find out which interactions are possible in principle, the binary systems PC + BDP, PC + PDMS, and BDP + PDMS were studied with ATR-FTIR. The spectra obtained after 70 wt % loss of each binary system are presented in Figure 3. They are compared with the spectra of PC, BDP, and PDMS, respectively. It must be noted that ATR-FTIR spectra show vibrations which are not equal to absorption bands from the Linkam hot stage cell, since one is a reflection method and the other is a transmission method. The characteristic absorptions of PC observed in Figure 3 are the following: 1765 cm1, CdO stretching of carbonate group; 1600 and 1505 cm1, quadrant and semicircle vibrations of the aromatic ring, respectively; 1220 and 1158 cm1, asymmetrical OC(O)C stretching; 1187 cm1, (CH3)C(CH3) skeletal vibration. BDP showed the following absorptions: 1590 and 1485 cm1, CdC stretching bands of aromatic ring; 1187 cm1, deformation band of the quaternary carbon atom of the isopropyl group; 1296 cm1, stretching vibration of PdO from phosphate;
947 cm1, stretching vibration of POaryl. The spectrum after 70 wt % mass loss of PC + BDP showed a very broad peak around 1586 and 1430 cm1 from aromatic absorption bands and phosphate/phosphonatearyl vibration. The broadening of the 1187 cm1 peak indicated the decomposition of the carbonate group from PC and of the (CH3)C(CH3) group from both PC and BDP. The band at 947 cm1 strongly broadened and was shifted to 913 cm1, indicating cross-linking of the phosphorus with polyaromatic structures. The spectrum presented for solid residue of PC + BDP became more charlike, confirming interactions between PC and BDP, which corresponds well with the literature.12 PDMS’s characteristic absorptions of the asymmetric and symmetric vibrations of SiCH3 are 1257 and 784 cm1, respectively. The intensities of the double peaks at 1083 and 1008 cm1 corresponded to an SiOSi bond. The spectrum at 70 wt % loss of PC + PDMS showed that the 784 cm1 peak shifted to 792 cm1 and broadened. This change is ascribed to the creation of SiOC bonds due to the breaking of SiC linkages in the PDMS chain. This confirms the interactions between PC and PDMS. The spectrum after 70 wt % loss of PDMS + BDP showed big differences compared to initial PDMS + BDP (spectrum not shown). The peak at 947 cm1 broadened and partly shifted to 920 cm1. This broad shoulder, together with the new band at 834 cm1, was attributed to SiOP vibration, and the peak at 815 cm1 was attributed to SiOC vibrations in a different surrounding than in the case of PDMS + PC. Those new absorption bands created during decomposition prove that PDMS interacts with BDP. The investigation of the binary systems clearly proves that each component of PC/SiR/BDP can indeed react with the other two components. Thus pyrolysis of PC/SiR/BDP is determined by competing pathways and interactions. 1247
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Figure 4. FTIR spectra (transmission using the Linkam hot stage) of the residue of PC/SiR/BDP at (a) the initial stage, (b) 658 K, and (c) 798 K (all at the heating rate 10 K min1), as well as (d) after storage for 10 min at 970 K under N2.
Figure 3. ATR-FTIR spectra of (a) PC + BDP, (b) PC + PDMS, and (c) PDMS + BDP after 70 wt % loss, and initial PC, BDP, and PDMS.
3.4. Pyrolysis: Solid Residue Analysis of PC/SiR/BDP. Figure 4 presents the FTIR spectra of the solid residue of PC/SiR/BDP at different stages of decomposition. The spectrum of PC/SiR/BDP before decomposition showed mainly the characteristic absorption bands of PC: 3080 cm1, aromatic skeletal vibration; 1104 cm1, aromatic CH stretching vibration; 2967 and 2880 cm1, CH2 and CH3 asymmetric and symmetric vibrations; 1775 cm1, CdO stretching of carbonate group; 1591 and 1505 cm1, quadrant and semicircle vibrations of aromatic ring, respectively; 1226 and 1162 cm1, asymmetrical OC(O)C stretching; 768 cm1, symmetrical vibration of OC(O)C; 1195 cm1, (CH3)C(CH3) skeletal vibration.26 Characteristic absorptions of BDP which did not overlap with PC were 960 cm1, attributed to the POaryl stretching
vibration, and 1296 cm1, corresponding to PdO vibration from phosphate.27 An additional absorption band occurred at 1735 cm1: CdO stretching from the carbonyl group of acrylic components of SiR (PMMA and PBA). The typical absorptions of PDMS described in the literature are 1083 and 1010 cm1, asymmetrical and symmetrical SiOSi stretching, and 784 cm1, symmetric deformations of the SiC bond.25 They were also detected at the initial stage of PC/SiR/BDP decomposition. At 658 K the sharp peak at 1735 cm1 broadened. At that time the vibrations originating from the carbonate group of PC were reduced and broadened, indicating the cross-linking of PC with acrylic components. These reactions were already denoted in thermogravimetry results. The interaction of the monomer of PMMA with PC has been proposed before.3 However, when decomposition was going on, the broad shoulder at 1735 cm1 vanished completely. This means that after the first decomposition step the PMMA and PBA decomposed and vaporized completely, while vibrations from BDP and PDMS did not show any change. The IR pattern of PC/SiR/BDP differed significantly at 798 K. The decrease in the peak at 1775 cm1 and the occurrence of a new vibration at 1745 cm1 was due to the appearance of CdO stretching other than carbonate. Other new peaks were observed and were attributed to benzophenone. They occurred at 1660 cm1, CdO stretching vibration of carbonyl group from aromatic ketone, and at 1470 cm1, CdC vibration of aromatic ring. This product is created by KolbeSchmitt rearrangement of PC and subsequent dehydrogenation.21 PC also undergoes decarboxylation, leading to the creation of a diphenyl ether bond. It gave the absorption band at 1295 cm1, which, according to the literature,28 overlaps with PdO vibration from phosphate and phosphonate. The reduction and broadening of POaryl vibration indicated that BDP starts to cross-link. The new absorption band at 1430 cm1 belongs to phosphate/phosphonatearyl linkages. Furthermore, SiC absorption was reduced and SiOSi vibrations broadened, confirming the creation of silicate. Additionally, the sharp peak at 834 cm1 supported creation of the SiOP bond. At the end of decomposition a polyaromatization process took place, resulting in highly carbonaceous char described by very 1248
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Figure 5.
31
P NMR and 29Si NMR on solid residues of PC/SiR/BDP.
broad peaks at 1575 and 1430 cm1. Polyaromatic structures were cross-linked to phosphorus; therefore the band at 960 cm1 was shifted to 913 cm1. The broad band around 1125 cm1 indicated that a silicon oxide network was created during pyrolysis. The structural changes in BDP and PDMS during thermal decomposition of PC/SiR/BDP were additionally monitored by 29 Si and 31P NMR and are presented in Figure 5. The more detailed investigations using solid-state NMR on thermal and fire residues of PC/SiR/BDP blend were discussed recently.15,16 31 P NMR of the initial PC/SiR/BDP showed characteristically broad resonance at 20 ppm, attributed to BDP homogenously mixed in PC. After 45 wt % loss (≈750 K) a broadening of the BDP resonance was observed, and at 68 wt % loss (≈770 K) two new resonances appeared at 12 and 6 ppm. These signals indicated BDP decomposition via the formation of various phenyl phosphate esters. At higher temperatures (>790 K) the signals for these aromatic phosphate esters vanished. A new signal was observed at 0 ppm, which split into two resonances. These signals were attributed to orthophosphate anions with and without protons in their chemical environment. Extremely broad 31P resonances at 15 and 35 ppm were observed for temperatures >770 K and are interpreted as phosphonate esters.29 Additionally, at the end of decomposition (≈850 K) a chemical shift range (between 50 and 60 ppm) typical for crystalline silicon diphosphate SiP2O7 was detected. It was a minor product which contained only about 1% of the remaining phosphorus of the solid residue. The 31P NMR studies revealed that during pyrolysis aryl phosphates were still present in the residue. With increasing temperatures (>770 K) more and more inorganic phosphates were formed. Phosphates were converted to phosphonates directly substituted by aromatic species. The reduction of phosphate to phosphonate is provided by the carbonaceous char.16 At the end of decomposition a very small amount of SiP2O7 was found.
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The 29Si NMR showed that during decomposition of PC/SiR/ BDP three signal groups appear, corresponding to the D, T, and Q groups, respectively, where “D”, “T”, and “Q” denote an increasing number of oxygen atoms bonded to the silicon atom.30 A signal at 23 ppm belongs to PDMS; the characteristic 29Si resonances vary between 5 and 23 ppm for D, between 40 and 70 ppm for T, and between 90 and 120 ppm for Q groups.16 The PDMS chain fragments reacted with both bisphenol A units and phenyl groups, leading to the formation of D groups. As the temperature increased, these D groups were transformed into T groups. With increasing temperature the degree of condensation increased and finally the Q units were formed, which correspond to the amorphous silicate. This means that PDMS reacted with PC and BDP to create the silicon oxide network. 3.5. Pyrolysis: Decomposition Pathways. Based on all results obtained for mass loss, evolved gases, and solid residues, the decomposition pathways are proposed for PC/SiR/BDP. At the beginning of decomposition the PMMA and PBA of SiR decompose, releasing mainly monomers into the gas phase. Those monomers also react with PC, exchanging the carbonate group with an ester group.4 The newly created structures in the condensed phase are not stable, and all the acrylic components decompose completely with increasing temperature. Then the PC undergoes its normal decomposition pathways such as Kolbe Schmitt and Fries rearrangements, chain scission of isopropylidene linkages, and hydrolysis of carbonate linkages, which are described in detail in the literature.21 Some moisture presented in the original polymer is very detrimental to carbonate linkages, and it is responsible for chain scission (Figure 6). The hydroxyl group from bisphenol A derivative attacks the SiO bond of the PDMS chain and creates the phenyl silyl ether derivatives. This derivative gives the 29Si NMR signal at 13 ppm and belongs to D groups in the nomenclature of siloxanes. The pathways of formation of phenyl silyl ether derivatives during thermal degradation of PC/ silicone based composites were described already.20,31 Since there are plenty of newly developed siloxanes, they interact with each other in two different ways (Figure 6, pathways a and b), resulting in new structures (T2 and T4 groups) in which Si is surrounded by three oxygen atoms. T2 groups reacted further with PDMS finally giving T4 groups. T4 groups with water reacted to Q3, where silicon is surrounded by three oxygens and one hydroxyl group. The Q3 groups were further transformed to Q4 by the interactions of the OH groups. In this way the silicon oxide network was formed at high temperature. In the meantime, the BDP starts to decompose, releasing tetraphenyl phosphate and bisphenol A (Figure 7). Supported by the presence of PTFE, BDP undergoes hydrolysis and reacts with rearranged PC.12 The cross-linking of BDP and PC leads to char formation. BDP also reacts with PDMS. In this case, the hydroxyl group of phenol (or its derivative) attacks the silyl group of PDMS and, as a result, the phenylsilyl ether structure is formed (D group) (Figure 7). It reacts with another hydroxyl group from decomposed BDP, leading to a more cross-linked structure. Nodera et al. reported analogical reactions between bisphenol A and PDMS during combustion.7 They concluded that the condensed aromatic structures with siloxane units acted as an insulating barrier to reduce both radiant heat of flame and the diffusion of flammable degradation products into the combustion zone. With increasing temperature the more compact siloxanes were created, where the silicon atom is surrounded by an increasing number of oxygen atoms. The silyl group from PDMS also attacks phosphorus from BDP, leading to a formation of 1249
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Figure 6. Main PC decomposition pathways of PC/SiR/BDP, including the reactions between PC and PDMS. 31Si chemical shifts in parentheses according to the literature.16
POSi linkage. However, the very small amount of SiP2O7 found in the residue means that it is a minor reaction. The decomposition of PC/SiR/BDP blend finished with polyaromatic, graphite-like char, phosphoruscarbon complexes, and a silicate network. 3.6. Fire Performance. The heat release rate (HRR) patterns of PC/SiR, PC/BDP, and PC/SiR/BDP are shown in Figure 8 for different applied irradiations. The HRR patterns were typical for char-forming PC blends with the maximum HRR (pHRR) at the beginning of burning, with the plateaulike HRR afterward, and with the second small peak in HRR at the end of combustion attributed to char decomposition.32 For PC/BDP the shape of the HRR curve for 35 kW m2 differed significantly from those for PC/SiR and PC/ SiR/BDP. Moreover, the PC/BDP for 35 kW m2 had a very long time to ignition (tig). Before this blend started to burn at 35 kW m2 irradiation, a lot of pyrolysis gases evolved. After ignition the combustion of PC/BDP was immediately very intensive; therefore, after reaching pHRR the HRR curve dropped immediately. For higher heating rates this effect vanished. Larger tig for PC/ BDP in comparison to PC/SiR and PC/SiR/BDP for all external heat fluxes showed the influence of flame retardant. Strong flame inhibition led to the delay of ignition in PC/BDP. The biggest difference in tig was observed for the lowest irradiation applied. This corresponds also with thermogravimetry results (Table 2), in which PC/BDP was the last to begin decomposing. PC/SiR and PC/SiR/BDP show the same tig within the margin of error. The uncertainty, especially of tig, was somewhat greater than usual
for cone calorimeter values due to the strong deformation of the investigated materials. The residue of PC/BDP created a high, black char tower. The black color is attributed to carbonaceous char. For PC/SiR and PC/SiR/BDP white residues were obtained at the end of the measurement. The white color, especially for PC/SiR, belonged to silicon dioxide (SiO2), which accumulated on the surface of the fire residue. For PC/SiR less carbonaceous char formation was observed. The characteristic values obtained from the cone calorimeter and flammability results are summarized in Table 4. The lowest residue among all investigated materials was observed for PC/SiR. The interactions of PC and PDMS disturb the naturally occurring charring of PC. PC/BDP showed the highest residue for the irradiation of 50 kW m2. It is caused by the transesterification reaction between PC and BDP in the condensed phase, leading to stable inorganic-carbonaceous char. The reduced residue of PC/SiR/BDP in comparison to PC/BDP for 50 kW m2 shows that in the presence of PDMS less cross-linking of PC and BDP occurred. However, for 35 and 70 kW m2, PC/ SiR/BDP gave the same residue as PC/BDP within the margin of error. In general, the residues observed in the cone calorimeter at 50 kW m2 correspond well with the residues in thermal analysis. Figure 9 shows the ATR-FTIR spectra of the fire residues of the three investigated blends. For PC/BDP 985, 1225, and 1583 cm1 were detected. They are typical for highly carbonaceous char containing phosphorus complexes. The ATR-FTIR spectrum of PC/SiR showed mainly absorptions coming from the silicate network at 805, 1061, and 1208 cm1, and very small 1250
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Figure 7. Reactions between BDP and both PDMS and PC during pyrolysis of PC/SiR/BDP. 31Si chemical shifts in parentheses according to the literature.16
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Table 4. Cone Calorimeter Results (at Flameout) and Flammabilitya PC/BDP PC/SiR PC/SiR/BDP
Error
Cone Calorimeter (Irradiation = 35 kW m2) residue
29.0
19.9
30.2
(1.5%
THE THE/ML
50 1.9
62 2.3
57 2.3
(3 kW m2 (0.1 W m2 g1
pHRR
336
254
200
(17 kJ m2
TCOP/ML
0.11
0.05
0.07
(0.01
TSR/ML
108
98
98
(4 g1
tig
288
150
160
(32 s
χ
0.76
0.92
0.92
(0.03 2
Cone Calorimeter (Irradiation = 50 kW m ) residue
26.2
18.4
22.0
(1.1%
THE THE/ML
51 1.9
65 2.3
59 2.2
(2 kW m2 (0.1 W m2 g1
pHRR
253
316
279
(28 kJ m2
TCOP/ML
0.11
0.04
0.08
(0.01
TSR/ML
106
90
106
(9 g1
tig
81
42
44
(18 s
χ
0.72
0.94
0.86
(0.03
Cone Calorimeter (Irradiation = 70 kW m2) 25.6 48
16.6 62
26.2 52
(1.7% (1 kW m2
THE/ML
1.8
2.2
2.0
(0.1 W m2 g1
pHRR
282
369
274
(10 kJ m2
TCOP/ML
0.10
0.04
0.07
(0.01
TSR/ML
125
106
128
(6 g1
tig
47
22
25
(2 s
χ
0.70
0.88
0.80
(0.01
residue THE
Flammability LOI UL 94 (3.2 mm)
37.2 V0
29.7 V1
37.6 V0
(1.0%
UL 94 (1.6 mm)
V0
HB
V0
a
THE = total heat evolved, ML = mass loss, TCOP = total CO production, TCOP/ML = CO yield, TSR = total smoke release, TSR/ ML = smoke yield, χ = combustion efficiency.
Figure 8. Heat release rate (HRR) of PC/BDP (circles), PC/SiR (stars), and PC/SiR/BDP (triangles) (irradiation = 35, 50, and 70 kW m2).
vibrations at 1617 and 967 cm1 coming from condensed aromatic structures. The silicate network is created on the surface of the burning material, and thus high intensities of its vibrations were monitored by using the ATR-FTIR reflection method, which measures the surface of the material and is limited in depth penetration. The spectrum from PC/SiR/BDP also showed mainly absorptions from silicone at 805, 1093, and 1208 cm1, but also small vibrations from aromatic structures at 1617 and 945 cm1 coming from phosphorus complexes. Total heat evolved (THE) is a very important parameter to describe the fire behavior and thus fire retardancy of the materials. It gives information about the fire load available in the cone
calorimeter experiment and depends on the total fuel amount and the product χhc (=THE/ML), where χ is the combustion efficiency and hc is the specific heat of combustion of the volatiles.33 The highest THE was obtained for PC/SiR; this blend created the lowest residue, so more material was consumed, increasing the heat production. PC/BDP showed the lowest THE. Addition of SiR to PC/BDP blend increases the THE compared to PC/BDP, but THE was clearly lower than for PC/SiR. The THE for all blends remained constant for all external heat fluxes within the margin of error. The parameter that measures the flame inhibition effectiveness directly is the combustion efficiency (χ). The combustion efficiency was calculated by dividing the THE/ML obtained with the cone calorimeter by the heat of complete combustion of the pyrolysis gases (h°c) obtained with the PCFC (eq 1).34 χ¼ 1252
THE=ML h°c
ð1Þ
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Figure 10. Smoke release yield (TSR/ML) plotted against irradiation. Figure 9. ATR-FTIR spectra of cone calorimeter residues of PC/SiR/ BDP (triangles), PC/BDP (circles), and PC/SiR (stars).
h°c is determined by eq 2, where HRPCFC is the heat release per unit of original mass and μ is the char yield. h°c ¼
HR PCFC 1μ
ð2Þ
χ is always equal to or less than 1. A value of χ = 1 means total oxidation of the volatiles. Values less than 1 characterize incomplete combustion in the flame; thus reduction in χ indicates flame poisoning. PC/SiR showed the highest values for all irradiations applied, because there is no gas-phase mechanism. The lowest values were obtained for PC/BDP, which exhibits the best gasphase mechanism among all investigated blends. Using BDP creates several types of phosphorus radicals that react with highly reactive H• or OH• radicals during combustion. Observed flame inhibition was releated to the release of various phosphates and phosphonates to the vapor phase.35,36 This strongly limits the oxidation process and, as a consequence, gave lower heat release. Combining SiR and BDP in the PC/SiR/BDP blend led to lower flame poisoning compared to PC/BDP. The interactions between PDMS and BDP (Figure 7) influenced not only the condensed-phase mechanism of BDP, but also reduced the gasphase action of BDP. χ decreases with increasing irradiation, indicating an increased gas-phase action, especially for the PC/ SiR/BDP blend. CO and smoke release resulting from incomplete combustion constitute a fire hazard. Further, as a rule, flame inhibition goes along with an increase in CO yield (TCOP/ML). Since in PC/ BDP the phosphorus is the most effective at trapping radicals, thus inhibiting the oxidation process, the highest TCOP/ML was obtained. No gas-phase mechanism occurred in PC/SiR, so the lowest TCOP/ML was observed due to the more complete oxidation. Combining SiR with BDP in PC/SiR/BDP led to a TCOP/ML lower than PC/BDP, but higher than PC/SiR. For all blends the TCOP/ML remained constant within the margin of error with increasing external heat fluxes. Different results were found for the smoke release yield (TSR/ML). With increasing irradiation TSR/ML increased for PC/BDP and PC/ SiR/BDP, while for PC/SiR it remained constant within the margin of uncertainty (Figure 10). PC/BDP and PC/SiR/BDP showed higher values for high irradiations than did PC/SiR.
Figure 11. Peak of heat release rate (pHRR) plotted against irradiation.
pHRR is generally expected to increase linearly with increasing external heat flux.32 This linear behavior was observed for PC/ SiR/BDP and PC/SiR. PC/SiR exhibited higher fire propagation than PC/SiR/BDP (Figure 11). This is caused by the absence of real flame retardancy in PC/SiR, not only because of the lacking gas-phase action, but also because of less effective char formation. PC/BDP and PC/SiR/BDP showed the same reduced pHRR for irradiations higher than 50 kW m2. The situation is different for the heating flux 35 kW m2: PC/SiR/BDP shows the lowest pHRR, whereas PC/BDP gives an extremely high value. The high pHRR for 35 kW m2 of PC/BDP was caused by a clearly delayed tig, followed by faster combustion of the overheated material. The flammability results of both LOI and UL 94 for two different thicknesses are presented in Table 4. The LOI of PC/ BDP was 37.2%, and a V0 rating was achieved for 1.6 and 3.2 mm sample thicknesses in UL 94. These excellent results were due to large residue created by the cross-linking of PC and BDP and phosphorus release in the gas phase. Less fuel is available for burning, so PC/BDP exhibits the best flame inhibition. In PC/ SiR no flame inhibition occurred in the gas phase and the carbonaceous charring of PC is somewhat reduced. The LOI 1253
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Industrial & Engineering Chemistry Research for PC/SiR was clearly lower than for PC/BDP (Table 4), but still higher than the values for comparable PC/ABS.4,12,37 Consistently PC/SiR achieved an HB classification for thinner samples and a V1 ratio for the thicker ones in UL 94. Therefore, PC/SiR performed clearly less effectively than flame-retarded PC/ BDP, but also better than comparable PC/ABS.4,12,37 The PC/ SiR/BDP yielded results in both flammability tests, LOI and UL 94 classification, that were equal to those of PC/BDP. Combining SiR, which mainly consisted of PDMS, with the flame-retardant BDP in PC/SiR/BDP leads to a V0 rating in UL 94 and to a high LOI value (37.6%). The LOI of the investigated PC/SiR/BDP blend is improved by 8.4% in comparison to PC/ABS/BDP.4,12,37
4. CONCLUSION A detailed investigation of the flame retardancy effect of PC/ SiR/BDP is presented. The impact modifier SiR consists of a high amount of PDMS and additionally of PBA and PMMA. PC/SiR/BDP exhibits the gas-phase and condensed-phase modes of action. In the vapor phase BDP works via flame inhibition, indicated by phosphate and phosphonate release in the evolved gases. Condensed-phase analysis shows that a part of phosphorus is incorporated into the char network via transesterification between BDP and PC. However, PDMS disturbs the BDP mode of action, both in the gas phase and in the condensed phase. Thus the overall findings of PC/SiR/BDP are not as good as superimposing the PC/BDP and PC/SiR performances. It was proved, using binary systems, that PDMS reacts with PC and BDP. PDMS in PC/SiR/BDP created the silicon oxide network via D, T, and Q groups observed using 29Si NMR. Detailed decomposition pathways are proposed for PC/SiR/BDP to explain the obtained results. The cone calorimeter measurements support the observations from pyrolysis studies that PDMS worsens the BDP action due to lower tig, CO yield, and higher THE and combustion efficiency for PC/SiR/BDP in comparison to PC/BDP. The fire residues showed that SiO2 is formed at the end of combustion, visually observed as the white layer. Using SiR (with high PDMS content) in PC/SiR/BDP gives flame retardancy as good as BDP in PC/BDP with respect to flammability tests (LOI and UL 94). V0 classification was achieved for both 3.2 and 1.6 mm specimen thicknesses; LOI > 36% was obtained. SiR which consists mainly of PDMS is proposed as a promising replacement for ABS in PC/impact modifier/BDP blends. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors gratefully thank Bayer MaterialScience AG, Germany, in particular Dr. V. Taschner, Dr. T. Eckel, and Dr. D. Wittmann, for providing the samples and financial support. We also thank D. Neubert and H. Bahr for supporting us within the experimental laboratory. Special thanks go to Dr. U. Braun for fruitful discussions and assisting with the measurements. ’ REFERENCES (1) Eckel, T. The most important flame retardant plastics. In Plastics Flammability Handbook; Troitzsch, J., Ed.; Hanser: Munich, 2004; pp 158172.
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