Fibers and Polymers 2011, Vol.12, No.4, 451-456
DOI 10.1007/s12221-011-0451-3
Flame Retarding PC/ABS Resins having Superior Thermomechanical Properties Kwang Ho Sohn, Min Kwan Kim1, So Min Lee1, Byung Chul1 Ji1, Kwang Soo Cho2, 3 Kyungmoon Jeon , and Han Do Ghim *
Department of Fire Protection Engineering, Gimcheon College, Gimcheon, Gyeongbuk 740-704, Korea Department of Advanced Organic Materials Science and Engineering, Kyungpook National University, Daegu 702-701, Korea 2 Department of Polymer Science and Engineering, Kyungpook National University, Daegu 702-701, Korea 3 Department of Science Education, Gwangju National University of Education, Gwangju 500-703, Korea 1
(Received August 19, 2010; Revised February 3, 2011; Accepted March 6, 2011)
Abstract: Triphenyl phosphate (TPP) is well known to be one of the most effective flame retardants for acrylonitrile-
butadiene-styrene copolymer (ABS) and its blending resins, such as polycarbonate (PC)/ABS, among various phosphorousbased compounds. However, TPP can also play a role as a plasticizer, which decreases the mechanical properties of PC/ABS resins at high temperature. Furthermore considerable amount of TPP has to be evaporated during molding process due on its much lower evaporation temperature. To overcome these shortcomings, we tried to immobilize TPP by grafting on butadiene moiety of ABS. FT-IR analysis of prepared TPP-grafted ABS (ABS-g-TPP) comparing with TPP, ABS and their blend confirmed that chemical reactions happened between TPP and ABS resins and it was attributed to the graft reaction of TPP onto butadiene moieties. Prepared ABS-g-TPP resins were blended with PC at various compositions to be prepared as testing specimens by injection molding. The physical characteristics such as mechanical properties, thermal stability, and flame retarding properties of the PC/ABS-TPP graft copolymer were analyzed through Vicat softening temperature, IZOD impact strength, transmission electron microscope, and UL94 flame retardation tests. Results showed that PC/ABS-g-TPP resin takes better thermomechanical properties than the existing PC/ABS resins at relatively low additional TPP amounts. Keywords: Triphenyl phosphate, Flame retardant, PC/ABS resin, Thermomechanical property
much less. Evaporation of TPP during processing can degrade the final products by generating flow marks and so on. Polybutadiene (PB) has abundant C=C bonds in its backbone; one for every repeating units. Allen et al. [6] investigated the grafting of methyl methacrylate onto natural rubber at 60 oC with 14C marked benzoyl peroxide (BPO) as an initiator in benzene environment. They proposed that the graft site was formed by addition of initiator free radicals onto C=C bonds or abstraction of allylic hydrogen atoms of natural rubber. Brydon et al. [7,8] grafted styrene monomer on PB in benzene solution at 60 oC with BPO as initiator. They proposed a grafting mechanism of primary radicals attacking PB by abstraction of allylic hydrogen and found that the rate coefficient for the primary radicals attacking PB was lower than that for the primary radicals attacking monomer molecules. Abdel-Razik et al. [9,10] investigated the photoinduced graft copolymerization of acrylamide on butadiene moiety of ABS in the presence of benzophenone and 4-acetyldiphenyl, respectively. Irradiation of benzophenone or 4-acetyldiphenyl produced a radical that abstracted an allylic hydrogen and added to C=C bond of butadiene moieties of ABS to generate the radical site for grafting. Several researchers have studied and reported about grafting of maleic anhydride [11,12] and unsaturated carboxylic acid [13,14] on butadiene moiety of ABS through radical pathway by breaking C=C bond with initiating catalysts such as BPO and 2,2’-azo-bis-isobutyronitrile (AIBN). Aimin and Chao [12]
Introduction
Nowadays, it is obvious that safety takes precedence over functionality. From this point of view, phosphorous flame retardants are getting important than traditional halogen type counterparts [1,2]. Triphenyl phosphate (TPP) is one of the most widely used phosphorous flame retardants and remarkably effective for acrylonitrile-butadiene-styrene copolymer (ABS) [3]. Grand and Wilkie [4] have reported that the flame retarding mechanism of TPP is due to the formation of pyro phosphoric acid during thermal degradation, which acts as heat transfer barrier in condensed phase. Unfortunately, TPP is one of the most frequently used plasticizers for ABS; plasticizing effects reveal at over 2 phr [5]. In those studies, however, TPP should also be added over 10 wt% to obtain the adequate flame retardation. Moreover to obtain reasonable level of flame retardation, 10 to 15 phr of TPP should be added to the polymer composition. Therefore, flame retarding ABS and polycarbonate (PC)/ ABS (PC/ABS) using TPP as additive show lowered softening temperatures, which limit the application fields of these resins. On the other hand, considerable amount of TPP has to be evaporated during molding process due on its much lower evaporation temperature. Therefore the effective amount of TPP acting as flame retardant in polymer resin should be *Corresponding author:
[email protected] 451
452 Fibers and Polymers 2011, Vol.12, No.4
Kwang Ho Sohn et al.
reported that BPO radical functions by removal of an allylic hydrogen atom and by addition on C=C of PB, whereas AIBN radical initiates the grafting by addition to C=C only. Our group has been in work for immobilization of volatile flame retardants by introducing covalent bonds between polymer backbone and additives to enhance the thermal and mechanical properties, as well as the processibility, of styrene copolymer and its blends. In this study, we grafted TPP onto ABS resins by using BPO as catalyst to ensure the immobilization of TPP onto butadiene moieties of ABS. TPP-grafted ABS (ABS-g-TPP) resins were blended with PC according to the preparative methods for commercially available flame retarding PC/ABS for further characterizations.
Experimental Reagent grades of PB (Aldrich Chemical, USA, w = 420,000) and TPP (Junsei Chemical, Japan; melting point 50-52 C) were used as received. Constituent units of PB were consisted as -1,4 addition (36 %), -1,4 addition (55 %), and 1-2 addition (9 %) according to the manufacturer. Tetrahydrofuran (THF) and BPO were purchased from Duksan Pure Chemical, Korea and used without further purification. Distilled water was of Milli-Q quality (Millipore, USA). Commercial grade ABS (GPABS, butadiene contents of 54.0 wt%), PC, styrene-acrylonitrile block copolymer (SAN), and polytetrafluoroethylene (PTFE) were provided from Cheil Industries, Korea. All the other chemicals used in this research were of HPLC grade and used as received. M
o
trans
Grafting of TPP
Graft reaction was performed using BPO and THF as initiator and solvent, respectively, in three neck flask equipped with a mechanical stirrer, a reflux condenser, and nitrogen gas inlet. Firstly to prepare TPP-grafted PB (PB-gTPP), 0.1 mol of PB was perfectly dissolved in 100 m of THF with equivalent amount of TPP at room temperature under nitrogen condition. After adding 0.005 mol/mol of BPO, reaction was sustained for 4 h at 85 C. Resulting PBg-TPP was precipitated in cold methanol bath with vigorous stirring. Purification was performed by repeating dissolvingprecipitation cycle for 3 times to remove any by-products and unreacted reagents. Specimens were oven-dried under vacuum at 65 C for 2 days. ABS-g-TPP was prepared through the same method with PB-TPP by using GPABS as base material. After above mentioned purification steps, ABS-g-TPP was filtrated at reduced pressure and dried under vacuum at 65 C for 2 days. l
o
PB
o
o
Test Specimen Preparation
g-TPP were processed in a Haake Plastic-Corder mixer at 230 C and 60 rpm for 7 min, followed by extrusion using twin screw extruder BA-19 (Bautek, Korea). The extruded filaments were immediately quenched in water and pelletized. Samples were molded in 2-mm-thick plate-type test specimens by using BabyPlast 6/10P. The size of the specimens was 100×10×2 mm. Table 1 shows the compositions for test specimens of this study. o
Materials
cis
Compositions for flame retarding PC/ABS test specimens Resin composition Additive Sample PC ABS SAN TPP PTFE (g) (g) (g) (phr) (phr) a Control 160 20 20 26 1 20 20 1 A 160 20b b B 160 20 20 18 1 20 16 1 C 160 20b b 20 14 1 D 160 20 E 160 20b 20 12 1 a GPABS and bABS-g-TPP. Table 1.
PC was dried under vacuum at 120 C for 4 h before blending procedure to exclude the hydrolysis by remaining moisture. The mixtures of PC, SAN, TPP, PTFE, and ABSo
Characterization
Fourier Transformed Infrared (FT-IR) spectra were obtained by using a Galaxy 7020A FT-IR spectrometer (Mattson, USA) with 200 scans at 4 cm resolution. Crushed specimens were mixed with dry KBr (ratio 1:60) and pressed into a pallet using a macro KBr die kit. No significant changes were observed in the FT-IR spectrum of the PB-g-TPP and ABS-g-TPP after further purification, indicating that the procedure was effective. Structural changes of PB after grafting were evaluated by using DSX400 Solid State 400 MHz Proton NMR (Bruker, Germany). Thermal analyses of differential scanning calorimetry (DSC) and thermogravimetry analyzer (TGA) were also conducted. Morphological changes of ABS-g-TPP after graft reaction was analyzed by Transmission electron microscopy (TEM) (Hitachi-7600) at an accelerating voltage of 100 kV. The Izod impact strength was measured with 258-D Izod instrument (Yasuda, Japan) by ASTM D-256 method. The notches of specimens were cut by a notch instrument. Vicat softening temperature (VST) was measured with Vicat softening point tester (Ray Ran, UK) by ASTM D-1525 method. Flame retardancy tests were performed by using Flammability Tester QM500VA (Qmesys, Korea) equipped with Bunsen Burner by UL94 vertical flammability test method. -1
Results and Discussion Grafting of TPP on PB
Figure 1 shows the solid state H NMR spectra of PB and PB-g-TPP, respectively. From these spectra, peaks for CH 1
3
Fibers and Polymers 2011, Vol.12, No.4
Flame Retarding PC/ABS
453
Figure 3.
DSC thermograms of (a) PB, (b) PB-g-TPP, and (c) TPP.
Figure 4.
TGA curves of (a) TPP, (b) PB-g-TPP, and (c) PB.
Solid state 1H NMR spectra of specimens; (a) PB, (b) PB-g-TPP. Additional spectrum of upper left is enlarged image of PB-g-TPP at 6.5-8.5 ppm. Figure 1.
Figure 2.
FT-IR spectra of (a) PB, (b) TPP, and (c) PB-g-TPP.
C=C of trans-1,4 addition and for -CH=C of cis-1,4 addition moieties of PB are indicated at 1.6-1.9 ppm and 5.2-5.7 ppm, respectively. Peak at 4.6-5.0 ppm is assigned for CH =C of 1,2 addition moiety of PB. Those peaks at 1.4-1.6 ppm and 1.2-1.4 ppm are ascribed to alkane structure and C-CH -C of 1,2 addition moieties, respectively. For the spectrum of PBg-TPP, there are peaks at 7.0-7.5 ppm which are not observed for PB and these signals are assigned for the protons on -P-O-Ph originated from the grafted TPP. Calculated chemical shifts of protons on TPP are of a, 6.73 ppm; b, 6.99 ppm; and c, 7.09 ppm, respectively. Peak at about 7.9 ppm is attributed that from BPO bound on PB. FT-IR spectra of PB, TPP, and PB-g-TPP, respectively, are shown in Figure 2. There is a distinct peak for hydroxyl groups at 3300-3400 cm for PB-g-TPP, which cannot be observed for PB. Oxygen atoms can be supplied from 2
2
-1
initiator, as well as from P=O bonds of TPP. However, oxygen from BPO cannot be formed into hydroxyl groups on polymer structure by combining eliminated hydrogen. Therefore, it can be deduced that eliminated hydrogen atom will develop hydroxyl group by combining oxygen attached on phosphate atoms of TPP. However, these spectral differences can be developed from the TPP which is physically blended with PB resins as well as from the covalently bonded one. To exclude the possibility of physical blending, we also conducted thermal analysis. Figure 3 shows the DSC thermograms of PB, TPP, and PB-g-TPP, respectively. DSC analyses are carried out in nitrogen atmosphere at a rate of 10 C/min. TPP shows a sharp exothermic peak at about 50 C which is ascribed to the melting of TPP. Decomposition peaks of PB are appeared at about 375 C. There are not any exothermic peaks for PBg-TPP at about 50 C, which is regarded as an evidence for grafting, not blending of TPP with PB. There are new broad endothermic peaks at about 170 and 290 C for PB-g-TPP. These peaks are attributed for the structural changes of PB o
o
o
o
o
454 Fibers and Polymers 2011, Vol.12, No.4
due to the TPP-grafted moieties. TGA curves of TPP, PB-gTPP, and PB are presented in Figure 4. Remaining char of PB-g-TPP is increased over three times compared with that of PB. From this result, it can be deduced that immobilized TPP on PB moieties shows synergetic effects on formation of thermal barriers on PB-g-TPP which enhance the flame retarding properties at decreased amounts of TPP. TPP shows drastic weight loss at over 200 oC due to evaporation, which is a weak point of TPP when applied to the injection molding process of polymer blends resulting flowmarks on the surface of products and degenerating processibility. PBg-TPP decomposes rather earlier than PB; its stepwise weight loss implies that there is a structural change in polymer backbone which is ascribed to the TPP-grafted moieties.
Grafting of TPP on ABS
From above results, it can be deduced that TPP can be bound to butadiene moieties of ABS by radical pathway. To ensure the grafting sites, TPP was reacted with polyacrylonitrile (PAN) and polystyrene (PS), respectively, at the same condition for PB. Resulting PAN and PS were precipitated in cold methanol bath with vigorous stirring. Purification was performed by repeating dissolving-precipitation cycle for 3 times to remove any by-products and unreacted reagents. Resulting polymers were also oven-dried under vacuum at 65 oC for 2 days before spectroscopic analysis. There are not any spectral changes for resulting polymers and, therefore, it can be assumed that TPP can be grafted on butadiene moieties of ABS only. Figure 5 shows FT-IR spectra of
FT-IR spectra of (a) ABS, (b) TPP, (c) ABS/TPP blend, and (d) ABS-g-TPP. Figure 5.
Kwang Ho Sohn et al.
ABS, TPP, ABS/TPP blend, and ABS-g-TPP, respectively. There is also a distinct peak for hydroxyl groups for ABS-gTPP, which cannot be observed for ABS and ABS/TPP blend. From these results, it can be ensured that TPP can be grafted on butadiene moieties of ABS by our method.
Themomechanical Properties
Cumulated ignition times for PC/ABS test specimens measured by UL94 vertical flammability test method are shown in Figure 6. As well known, specimens are contacted methane flame during 10 sec for twice and cumulated ignition times should not exceed 10 sec to meet the V0 regulations of UL94 test. Specimens D and E showed 15.0 and 13.3 sec of cumulated ignition times, respectively, which are not adequate for V0 grading. However specimens A-C showed flame retarding properties enough for V0 grading. For the commercialized flame retarding PC/ABS resin (control), over 18 phr of TPP should be added for V0 grading. However, by adopting ABS-g-TPP, it can be reduced to 8 phr of TPP. This can be possible because of immobilized TPP molecules bound to butadiene moieties of ABS by covalent bonds. Morphological changes of ABS-g-TPP after graft reaction were compared with that of control (Figure 7(a)) by TEM and shown in Figure 7. There were not any significant differences in aggregation of PB particles which were polymerized in gas phase. From this result, it can be deduced that radical grafting of this study will not affect to the dispersion stabilities of PB particles. As well known, TPP can also be acted as plasticizer for PC/ABS, which reduces the thermomechanical properties of
Cumulated ignition times for PC/ABS test specimens measured by UL94 vertical flammability test method.
Figure 6.
Flame Retarding PC/ABS
TEM microphotographs of (a) control and (b) sample C, respectively. Figure 7.
Fibers and Polymers 2011, Vol.12, No.4
455
flame retarding PC/ABS. Therefore, reduced TPP amounts for flame retarding PC/ABS resins can have positive effects on thermomechanical properties of specimens. Figure 8 shows the thermomechanical properties of test specimens; Figure 8(a) for Izod impact strength and Figure 8(b) for VST, respectively. Impact strengths of specimens gradually increase with decreasing TPP contents; Impact strengths of PC/ABS resins of V0 grading reached up to 34 J/m. This tendency can also be observed for the VST with drastic increase with decreasing TPP contents. We can prepare flame retarding PC/ABS resins of V0 grading with 110.9 oC of VST. These improvements of thermomechanical properties of PC/ABS resins are attributed to the reduced plasticizing effects of TPP for decreased amounts of free TPP molecules which are not bonded to butadiene moieties of ABS.
Conclusion TPP was successfully grafted on PB, a model polymer, by using radical catalyst. Structural changes of PB-g-TPP were evaluated by using solid state 1H NMR and FT-IR spectroscopy. From DSC and TGA analyses, the possibilities of simple physical blending of TPP and PB could be excluded. Based on these results, TPP was grafted on butadiene moieties of ABS by using the same reaction conditions. From FT-IR measurement, there is also a distinct peak for hydroxyl groups for ABS-g-TPP, which cannot be observed for ABS and ABS/TPP blend. From these results, it can be ensured that TPP can be grafted on butadiene moieties of ABS by our method. Prepared ABS-g-TPP was blended and extruded with the condition for commercialized flame retarding PC/ABS resins except for TPP contents; TPP contents were varied from 13 phr (control) to 6 phr. From the UL94 and thermomechanical tests, it was confirmed that flame retarding PC/ABS (V0 grade) with superior impact strength and VST can be prepared at 8 phr of TPP contents. These drastic increases in thermomechanical properties of flame retarding PC/ABS were attributed to the reduced amount of added TPP and, as a result, reduced plasticizing effects of flame retardant. From these results, it can be concluded that thermomechanical properties of flame retarding PC/ABS resins can be improved by immobilizing TPP by grafting on polymer backbones. Graft reaction mechanism will be discussed in elsewhere.
Acknowledgements This work was supported by the grant No. R01-2006-00010232-0 from the Basic Research Program of Korea Science & Engineering Foundation. Thermomechanical properties of flame retarding PC/ ABS test specimens; (a) Izod impact strength and (b) VST.
References
Figure 8.
1. P. Carty and S. White,
Polym. Degrad. Stab., 54,
379
456
Fibers and Polymers 2011, Vol.12, No.4
(1996). 2. R. Smith, P. Georlette, I. Finbery, and G. Reznick, Polym. Degrad. Stab., 54, 167 (1996). 3. C. E. Anderson, D. E. Ketchum, and W. P. Mountain, Fire Sci., 6, 390 (1998). 4. A. F. Grand and C. A. Wilkie, Fire Retardancy of Polymeric Materials, Marcel Dekker, New York, 2000. 5. G. Wypych, Handbook of Plasticizers, ChemTec Publishing, New York, 2004. 6. P. W. Allen, G. Ayrey, and C. G. Moore, J. Polym. Sci., 36, 55 (1959). 7. A. Brydon, G. M. Burnett, and G. G. Cameron, J. Polym. Sci. Polym. Chem. Ed., 11, 3255 (1973).
Kwang Ho Sohn et al.
8. A. Brydon, G. M. Burnett, and G. G. Cameron, J. Polym. Sci. Polym. Chem. Ed., 12, 1011 (1974). 9. E. A. Abdel-Razik, J. Photochem. Photobiol. A, 69, 121 (1992). 10. E. A. Abdel-Razik, M. M. Ali, M. Y. Abdelaal, and A. A. Sarhan, Polym. Plast. Technol. Eng., 35, 865 (1996). 11. R. Qi, J. Qian, Z. Chen, X. Jin, and C. Zhou, J. Appl. Polym. Sci., 91, 2834 (2004). 12. Z. Aimin and L. Chao, Eur. Polym. J., 39, 1291 (2003). 13. Z. F. Zhou, H. Huang, C. Y. Zhu, and N. C. Liu, J. Appl. Polym. Sci., 83, 1934 (2002). 14. Z. F. Zhou, H. Huang, and N. C. Liu, J. Polym. Sci. Polym. Chem. Ed., 39, 486 (2001).
