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Cone calorimetric study of copper-promoted smoke suppression and fire retardance of poly(vinyl chloride)
Polymer Degradation and Stability 82 (2003) 15–24 www.elsevier.com/locate/polydegstab
Cone calorimetric study of copper-promoted smoke suppression and fire retardance of poly(vinyl chloride) William H. Starnes Jr*, Robert D. Pike*, Jenine R. Cole, Alexander S. Doyal, Edward J. Kimlin, Jeffrey T. Lee, Philip J. Murray, Ronald A. Quinlan, Jing Zhang Departments of Chemistry and Applied Science, College of William and Mary, PO Box 8795, Williamsburg, VA 23187-8795, USA Received 25 February 2002; received in revised form 1 April 2003; accepted 6 April 2003
Abstract Copper-based smoke suppression additives for poly(vinyl chloride) (PVC) were tested for crosslinking capability in pyrolysis studies and for smoke suppression and fire retardance by the use of cone calorimetry. Crosslinking of PVC at 190 C was promoted by most of the additives without an obvious dependence on additive copper content or copper oxidation state. The copper additives (at 10 parts by weight per hundred parts of resin) proved to inhibit both smoke and heat evolution in burning PVC samples (both rigid and plasticized) in cone calorimetric studies. Mixed-metal oxides of copper were especially effective in this regard. Synergism in smoke suppression was noted for combinations of Cu3(MoO4)2(OH)2 and CuSnO3 in plasticized PVC. A 2:1 (w/w) mixture of Cu3(MoO4)2(OH)2 and CuSnO3 yielded a reduction in specific extinction area (a measure of smoke obscuration) of 64% and a reduction in total smoke release of 79% vs. the control sample. # 2003 Elsevier Ltd. All rights reserved. Keywords: Poly(vinyl chloride); PVC; Cone calorimeter; Smoke suppression; Fire retardance; Copper additives; Synergistic smoke suppressants
1. Introduction The problem of smoke and flame associated with poly(vinyl chloride) (PVC) is well-documented [1–3]. Although PVC is inherently nonflammable, its dehydrochlorination produces conjugated polyene segments, which in turn produce volatile aromatics, such as benzene, during pyrolysis. Combustion of these aromatics results in smoke formation and additional enthalpy input, thus promoting further polymer pyrolysis. An apparent key to limiting PVC smoke formation is minimizing the formation of volatile fuels through crosslinking, which produces nonvolatile char. Crosslinking in pyrolyzing PVC is typically accomplished through the use of high-valent metalbased smoke suppression additives, such as MoO3, which are actual or incipient Lewis acids and thus promote Friedel-Crafts alkylation reactions of PVC and polyenes. However, at elevated temperatures, these highvalent metal additives can also function as cracking cata* Corresponding authors. Tel.: +1-757-221-2552; fax: +1-757-2212715. E-mail addresses: [email protected] (W. H. Starnes), rdpike@wm. edu (R. D. Pike). 0141-3910/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0141-3910(03)00158-7
lysts, at least in certain cases. Cracking of the principally hydrocarbon char can produce volatile aliphatic organics, which act as efficient fuels. This situation results in less smoke, but increased flame [4,5]. Preliminary studies have shown that low-valent metals, such as Cu(0), can also promote crosslinking [1– 3]. However, in contrast to the Friedel-Crafts mechanism, the copper-promoted reaction appears to follow a reductive pathway such as that shown in Fig. 1. In addition, since copper is readily reduced thermally, the original PVC additive may be a compound of either Cu(I) or Cu(II). Copper(I) compounds are subject to valence disproportionation, which allows them to generate Cu(0) even in the absence of reducing agents. They are also often colorless, an obvious advantage in the formulation of uncolored PVC. However, Cu(I) complexes typically possess very limited thermal stability. Copper(II) compounds, on the other hand, can be thermally robust. In previous papers, we have demonstrated that a variety of coppercontaining compounds are rapid and efficient crosslinking agents for PVC at moderate temperatures without causing the concurrent formation of volatile byproducts [1–3]. The cone calorimeter is the successor instrument to the NBS smoke chamber [6,7]. It allows for the burning
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2. Experimental 2.1. Materials
Fig. 1. Possible pathway for the crosslinking of PVC via reductive coupling.
of sample plaques at relatively high heat flux values (such as 50 kW/m2) with the concomitant measurement of heat release and smoke temporal profiles. Thus, the cone calorimeter is an ideal instrument to evaluate realistically the combined smoke suppression and fire retardance behavior of polymer samples. Some of the important parameters that may be derived from cone calorimetry experiments are summarized in Table 1. By using cone calorimetry, Li and Wang [8–13] have recently studied the burning behavior of rigid PVC samples containing Cu2O, CuO, and MoO3 at heat fluxes of 25 and 50 kW/m2. Some of their major results for the flaming mode may be summarized as follows. Cuprous oxide shortens TTI and causes earlier smoke release than MoO3. Molybdenum(VI) oxide lengthens TTI and delays smoke evolution. However, both oxides show significant smoke suppression and fire retardance. In addition, synergism between Cu2O and MoO3 is evidenced in smoke suppression and enhanced char residue, but not in flame retardancy. In the current contribution, we report the cone calorimetric evaluation of a variety of coppercontaining additives for plasticized and rigid PVC.
The PVC was a powdered commercial product containing no additives. Palatinol 79TM-I plasticizer [tri(nheptyl, n-nonyl) trimellitate, or ‘‘7,9-trimellitate’’] was received from BASF Canada. Additive compounds Cu2O, CuCO3.Cu(OH)2, CuC2O4, Cu(O2CH)2.1.5 H2O, 2ZnO.3B2O3.3.5H2O, Sb2O3, and (NH4)4Mo8O26, as well as additive precursor compounds CuSO4.5H2O, Na2MoO4.2H2O, Na2SnO3.3H2O, Na2SiO3, H2PhPO3, H3PO3, HPh2PO2, H2PhPO2, H3PO2 (50% aqueous), NH4S2P(OEt)2, NaS2CNEt2.3H2O, PhSH, n-C12H25SH, Ph3CSH, Ph3PS, and P(OPh)3, were obtained from commercial sources and used as received. Additive compounds CuCl and CuBr were purchased from commercial sources and were purified by recrystallization from aqueous HCl and aqueous HBr, respectively. Hindered organophosphites were received from commercial sources (see Fig. 2). All copper-containing compounds were analyzed for copper content by atomic absorption, as previously described [1]. 2.2. Preparation of CuCl(BPP) and other copper(I) phosphite complexes Copper(I) chloride (1.00 g, 10.1 mmol) and tris(2,4-dit-butylphenyl) phosphite (6.54 g, 10.1 mmol) were suspended in 50 ml of CHCl3. The mixture was stirred at 25 C under N2 for 0.5 h, during which time it became less cloudy. After filtration, the solvent was removed under vacuum to leave a colorless oil, which solidified into a white solid under high vacuum (6.35 g, 8.51 mmol, 84%). Other complexes of CuCl or CuBr with P(OPh)3 (in C6H6), BPP (in CHCl3), MBPOP (in C6H6), BHTPD (in CH3CN), or CPD (in CH3CN) were prepared similarly. 2.3. Preparation of Cu(S2CNEt2), Cu(S2P(OEt)2), and CuSR To an ice-cold solution prepared from 25 ml of conc. aq. NH3 and 100 ml of H2O was added CuSO4.5H2O
Table 1 Cone calorimetry parameters Parameter
Abbreviation
Unit
Description
Time to ignition Specific extinction area (average and peak) Total smoke released Heat release rate (average and peak) Total heat released Mass loss rate (average and peak) Effective heat of combustion (average) Char yield
TTI SEAav, SEApk TSR HRRav, HRRpk THR MLRav, MLRpk EHCav CY
s m2 kg1 (unitless) kW m2 MJ m2 g s1 MJ kg1 %
Time of sustained ( >10 s) flaming Smoke produced per unit mass being volatilized Cumulative smoke produced Rate of heat release per unit sample area Cumulative heat energy released during flaming per unit sample area Mass of sample being volatilized per unit time Heat released per unit mass being volatilized Percent of sample mass remaining after burning
W.H. Starnes Jr et al / Polymer Degradation and Stability 82 (2003) 15–24
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EtOH gave a green solution. The solution was heated to just under boiling. A suspension of SnCl2.2H2O (0.490 g, 2.17 mmol) in 6 ml of EtOH was then added until the green color was discharged. The mixture was subjected to hot filtration, and a white precipitate formed on cooling overnight. It was isolated by filtration, washed in succession with EtOH and ether, and vacuum-dried (1.48 g, 1.51 mmol, 70%) [16]. 2.6. Preparation of Cu(Ph2PO2)2 and other copper(II) phosphinates and phosphonates Diphenylphosphinic acid, HPh2PO2 (1.25 g, 5.73 mmol), in 30 ml of MeOH and NaHCO3 (1.00 g, 11.9 mmol) in 30 ml of H2O were combined. Solid CuSO4.5H2O (1.46 g, 5.86 mmol) was added to the solution with stirring. A precipitate formed rapidly. The solid product was collected by filtration after 20 min. It was washed with H2O, EtOH, and ether in succession and then dried under vacuum (1.19 g, 2.39 mmol, 83%). 2 The Cu(II) complexes of PhPO2 3 , HPO3 , H2PO2 , and HPhPO2 were prepared similarly. Fig. 2. Hindered organophosphites. Reprinted with permission from Ref. [2]. Copyright 2001 American Chemical Society.
2.7. Preparation of Cu3(MoO4)2(OH)2 and other mixed metal oxides of copper(II)
(6.26 g, 25.1 mmol), thus forming a royal blue solution. Over a period of 45 min, solid NH2OH.HCl (3.89 g, 56.0 mmol) was added. Stirring overnight under a N2 purge produced a colorless solution of [Cu(NH3)4]+ [14]. By using a syringe, a solution of NaS2CNEt2.3H2O (7.04 g, 31.3 mmol) in 80 ml of H2O was added. A yellow-brown solid formed immediately. The solid product was collected via filtration; washed with H2O, EtOH, and ether in succession; and vacuum-dried. It was recrystallized by dissolving in CHCl3 and precipitating with ether (yield: 3.47 g, 16.4 mmol, 65%). The Cu(I) salt of S2P(OEt) 2 was prepared analogously, and the Cu(I) thiolates were prepared similarly by injecting the thiols dissolved in EtOH into an aqueous solution of [Cu(NH3)4]+.
A 250-ml aqueous solution of Na2MoO4.2H2O (21.6 g, 89.4 mmol) was added dropwise to a 250-ml aqueous solution of CuSO4.5H2O (33.5 g, 134 mmol). A pale blue-green precipitate formed immediately. The suspension was refluxed overnight, and the now pale green precipitate was collected by filtration. It was washed in succession with H2O, EtOH, and acetone and vacuumdried (15.9 g, 29.2 mmol, 65%) [17]. Other mixed metal oxides were prepared similarly. The aqueous mixture that produced CuSn(OH)6 was not refluxed.
2.4. Preparation of Cu(S2P(OEt)2)(SPPh3)2 A modified literature procedure was followed [15]. A suspension of Cu(S2P(OEt)2) (0.200 g, 0.804 mmol) and Ph3PS (0.473 g, 1.61 mmol) in 50 ml of CH2Cl2 was heated under reflux for 15 min. The resulting colorless solution was concentrated to a volume of 5 ml under vacuum. A white powder was precipitated by the addition of ether and vacuum-dried (0.487 g, 0.582 mmol, 72%). 2.5. Preparation of CuCl(SPPh3)3 Addition of CuCl2.2H2O (0.381 g, 2.23 mmol) to a suspension of Ph3PS (2.00 g, 6.79 mmol) in 50 ml of
2.8. Gelation studies These were carried out as previously described [1], except that 190 C rather than 200 C was used for the one-hour sample pyrolysis under a 50 ml/min Ar flow. All samples were run at least twice, and the reproducibilities of the gel yields and mass losses were found to be within 10% of the mean values. 2.9. Cone calorimetry Rigid plaques (1001003 mm3) containing ca. 45 g of PVC and 10 phr (parts by weight per hundred parts of resin) of an additive or additive mixture were prepared from blends that were mixed by grinding with a mortar and pestle. The plaques were made in a stainless steel mold with a Carver laboratory press (Model C) at 20,000 psi; mold temperature was raised within 7 min from ca. 65 to 150 C and then was held for 3 min at
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150 C. Plasticized plaques of the same size contained ca. 34 g of PVC, 30 phr of plasticizer, and 10 phr of additive. These plaques were molded as described above, except that the final hold time at 150 C was 4 min. The cone calorimeter was a Fire Testing Technology instrument. Plaques were burned in the flaming mode with spark ignition according to a standard test procedure (ASTM E 1354), using a heat flux of 50 kW/m2 and a horizontal receptacle that allowed samples to be immobilized with a stainless steel grid. In numerous replicate runs, reproducibilities were found to be within 10% for all of the properties studied.
3. Results and discussion 3.1. Choice of additive compounds In previous work [1], it was established that Cu(I) halides with coordinated phosphites and Cu(II) compounds of oxidizable anions (such as oxalate and formate) are potent crosslinking agents for PVC at 200 C. Copper(0) films and powders were shown to promote crosslinking even at temperatures of < 100 C. In addition, it was shown that copper-based additives could promote polymer crosslinking below the decomposition temperature of the additive. In the current extension of this work, several classes of copper compounds have been investigated as potential crosslinking additives. These include simple salts of copper(I), copper(I) thiolates, copper(I) halides with highly hindered phosphite ligands, salts of copper(II) with oxidizable anions, and mixed-metal oxides of copper(II). All copper com-
pounds were analyzed for copper content by using flame atomic absorption spectrophotometry and for decomposition point by using a melting point or TGA apparatus; see Tables 2 and 3. The simple salts of copper(I), CuCl, CuBr, and Cu2O, were obvious choices as potential additives, given their low price, high copper content, and in the case of the halides, lack of color. However, the halides are particularly moisture sensitive and tend to develop green color due to Cu(II) formation upon long-term exposure to moist air. This moisture instability was viewed as a potential polymer formulation problem. Copper(I) salts of sulfur-based anions are more stable toward moisture than simple halides. Therefore, Cu(I) salts of the following anions were synthesized: S2P(OEt) 2 (diethyl dithiophosphate), S2CNEt2 (diethyl dithiocarbamate), and thiolates PhS , n-C12H25S, and Ph3CS. These compounds were easily prepared on large scales by the addition of the alkali salt or acid form of the desired anion to aqueous Cu(NH3)+ 4 [14]. Copper(I) diethyl dithiocarbamate [18] is a tetrameric compound [19], and copper(I) diethyl dithiophosphate is probably also oligomeric, since the diisopropyl analog is known to be hexameric [20]. The Cu(I) thiolate compounds are widely regarded as being polymeric. Two presumably monomeric Cu(I) complexes of triphenylphosphine sulfide, Cu(S2P(OEt)2)(SPPh3)2 and CuCl(SPPh3)3 [16], were prepared. All of the foregoing sulfur-based compounds lacked significant color, thus making them potentially attractive as additive candidates. Although previous results had indicated that copper(I) halide organophosphite complexes were potent PVC crosslinking agents, it was found that upon expo-
Table 2 Copper(I) compounds prepared Compound
Color
Dec. point ( C)
Theo. Cu (%)
Expt. Cu (%)
CuCl CuBr Cu2O Cu(S2P(OEt)2) Cu(S2CNEt2) Cu(SPh) Cu(S-n-C12H25) Cu(SCPh3) CuCl(SPPh3)3 Cu(S2P(OEt)2)(SPPh3)2 CuCl(P(OPh)3) CuBr(P(OPh)3) CuCl(BPP) CuBr(BPP) (CuCl)2(BHTPD) (CuBr)2(BHTPD) CuCl(CPD) CuBr(CPD) CuCl(MBPOP) CuBr(MBPOP)
White White Rust red White Tan Pale yellow Cream Cream White White White White White White Cream Tan White White White White
400 450 >900 187 125 151 100 85 225 180 285 251 226 238 235 248 269 269 220 230
64.2 44.3 88.8 25.5 30.0 36.8 24.0 18.8 6.5 7.6 15.5 14.0 8.5 8.0 15.3 13.8 6.7 6.4 9.3 8.7
62.6 46.3 88.0 25.3 31.0 36.1 23.8 17.8 6.5 6.9 15.1 13.6 8.3 7.9 15.6 13.4 6.6 6.8 9.8 9.2
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W.H. Starnes Jr et al / Polymer Degradation and Stability 82 (2003) 15–24 Table 3 Copper(II) compounds prepared Complex
Color
Dec. point ( C)
Theo. Cu (%)
Expt. Cu (%)
Cu(C2O4) Cu(O2CH)2.1.5H2O CuCO3.Cu(OH)2 Cu(HPO3).1.5H2O Cu(PhPO3).1.5H2O Cu(H2PO2)2 Cu(HPhPO2)2 Cu(Ph2PO2)2 Cu3(MoO4)2(OH)2 CuSnO3 CuSn(OH)6 CuSi(OH)6
Pale blue Pale blue Blue-green Pale blue Pale blue Very pale blue Very pale blue Blue-violet Pale green Blue-grey Sky blue Pale blue
300 199 200 126 344 55a 99 299 >900 >900 >900 >900
41.9 35.2 57.5 37.3 25.8 32.8 18.4 12.8 35.0 27.6 22.4 32.8
41.0 34.7 55.2 37.5 26.2 33.0 18.1 13.3 34.9 25.7 21.9 32.5
a
Decomposes over several days at 25 C.
sure to moist air, simple phosphites [e.g., P(OPh)3] and their complexes readily hydrolyzed, thereby producing a phenolic-smelling gummy residue [2]. As a result, copper(I) complexes of the highly hindered phosphites [21] BPP, MBPOP, BHTPD, and CPD were prepared (phosphite structures are shown in Fig. 2). These complexes were found to be highly resistant to hydrolysis, even under elevated humidity conditions [2]. The use of Cu(II) salts was extended in the current study to include, in addition to salts of oxalate and formate, salts of phosphonate (HPO2 3 ), phenylphosphonate (PhPO2 phosphinate (H2PO 3 ), 2 ), phenylphosphinate (HPhPO2 ), and diphenylphosphinate (Ph2PO 2 ). These anions contain reduced forms of phosphorus. Thermal oxidation of phosphorus-based anions was expected to promote the reduction of copper. However, in the case of phosphorus anions containing P–H bonds, this internal redox reaction proved to be too facile and thus led to unacceptably low decomposition points (see Table 3). Finally, simple aqueous reactions were used to prepare the mixed copper molybdate, Cu3(MoO4)2(OH)2 (synthetic ‘‘Lindgrenite’’ [17]), as well as copper stannates and copper silicate. These materials were shown by thermogravimetry to have very high thermal stability, exhibiting less than 10% mass loss at 900 C. 3.2. Gelation studies Mixtures containing 10 wt.% of additive in PVC were produced by low-temperature grinding [1]. Gelation studies on the PVC/additive mixtures were carried out via solvent-free pyrolysis for 1 h at 190 C under flowing argon. The soluble portion of residues was removed by Soxhlet extraction with THF. Following extraction, the crosslinked gel residue was recovered, dried, and weighed. Since additive-free control samples show no gelation under these conditions, formation of a gel indicates that the additive is promoting PVC cross-
linking under these moderate conditions. As part of the same experiment, the sample mass lost due to volatilization during pyrolysis was determined. The gelation and mass loss results are shown in Table 4. None of the copper-containing additives caused polymer mass losses in excess of 10%, and most of the mass Table 4 PVC gelation and mass loss resultsa Additive, 10% by mass in PVC
Gel trials
Mass loss (%)
PVC gel yield (%)
None CuCl CuBr Cu2O CuCO3.Cu(OH)2 Cu(C2O4) Cu(O2CH)2.1.5H2O Cu(PhPO3).1.5H2O Cu(Ph2PO2)2 Cu(S2P(OEt)2) Cu(S2CNEt2) Cu(SPh) Cu(S-n-C12H25) Cu(SCPh3) CuCl(SPPh3)3 Cu(S2P(OEt)2)(SPPh3)2 CuCl(P(OPh)3) CuBr(P(OPh)3) CuCl(BPP) CuBr(BPP) (CuCl)2(BHTPD) (CuBr)2(BHTPD) CuCl(CPD) CuBr(CPD) CuCl(MBPOP) CuBr(MBPOP) Cu3(MoO4)2(OH)2 CuSnO3 CuSn(OH)6 CuSi(OH)6 (NH4)4Mo8O26
2 2 2 2 2 2 2 4 3 2 2 2 2 2 2 2 2 2 4 4 4 4 4 3 3 4 2 2 2 2 2
0.3 3.6 3.6 1.8 3.6 5.7 3.9 1.9 1.2 4.4 7.9 3.2 5.8 8.6 1.5 1.6 4.8 2.1 0.7 1.2 1.9 0.8 2.4 1.6 3.6 2.1 5.1 1.6 3.7 3.0 4.4
<5 62 60 72 52 50 65 69 86 <5 66 50 61 7 65 <5 82 69 60 50 63 63 68 45 39 45 74 70 93 84 60
a
Samples pyrolyzed at 190 C for 1 h under flowing argon.
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loss values were below 5%. These results stand in contrast to those with highly Lewis acidic additives, such as FeCl3, which has been shown to cause mass losses of over 30% under similar conditions [1]. A plot of gel yield versus Cu content in the additive (Fig. 3) revealed no correlation. This result stands in contrast to the Cu loading correlations seen in the cone calorimetry results for rigid PVC (see below). Most of the additives studied promoted polymer gelation at 190 C. However, there were several notable exceptions. The strongest gel formers (giving gel yields of 570%) were mainly oxide materials, which are the most thermally stable of the compounds studied. This finding is quite significant, for it suggests that thermally stable and fairly inert mixed copper oxide additives might be potent smoke suppressants in PVC. Two of the additives with thiolate groups were reasonably good gel promoters. Other metal thiolates have long been used as heat stabilizers for PVC [22], their mode of action being (in part) that of thiolate for labile
Fig. 3. PVC gel yield versus % copper content in additive.
chloride substitution at unstable defect sites [23,24]. It seemed possible that the copper thiolates might increase the thermal stability of the PVC to such an extent that insufficient dehydrochlorination could occur at 190 C to form appreciable amounts of the polyenyl chlorides that lead to gel (see Fig. 1). However, our data do not support the general applicability of this hypothesis. 3.3. Cone calorimetry studies Cone calorimetric studies were carried out on PVC plaques containing 10 phr of additive, and in the case of plasticized plaques, 30 phr of 7,9-trimellitate. The cone calorimetry results for rigid and plasticized PVC samples are shown in Tables 5 and 6, respectively. It is instructive to compare cone results for metal-additivefree control samples of rigid and plasticized PVC. Since most plasticizers contain long aliphatic chains, they readily serve as fuel during fires. Comparison of the control results reveals the higher flammability of plasticized PVC. Thus, the plasticized PVC controls show shorter values for TTI (see Table 1 for definitions of parameters) and higher values for HRR, THR, MLR, and EHC. In addition, higher SEA and TSR values indicate that the plasticized samples produce more smoke than the rigid samples. Enhanced smoke formation is likely because of the higher quantity of the fuel available in plasticized PVC. Thus, the problems of flame and smoke in PVC are both exacerbated by the use of the plasticizer, as expected. It is also noteworthy that, at the 50 kW/m2 heat flux used in the study, neither control sample showed any residual char (CY=0%). The copper-based additives proved to be very effective smoke suppressants in the rigid PVC samples. Through the use of such additives, the smoke parameters SEAav and TSR were lowered by 20–89 and 36–82%, respec-
Table 5 Cone calorimetry results for rigid PVC Additivea
% metalb
TTIc (s)
TSRc
SEAc (m2/kg)
HRRc (kW/m2)
MLRc (g/s)
EHCc (MJ/kg)
None Cu2O CuCl CuBr [CuCl(BPP)] [CuBr(MBPOP)] [(CuBr)2(BHTPD)] [CuCl(CPD)] CuC2O4 Cu(O2CH)2.1.5H2O (NH4)4Mo8O26
0 88.8 64.2 44.3 8.5 8.7 13.8 6.7 41.4 35.2 61.1
98 311 346 316 225 183 145 171 251 229 202
3431 889 835 1084 2190 1998 1595 1527 627 737 2167
845 93 107 257 566 396 389 672 114 322 878
50.5 32.2 32.9 50.2 65.6 43.3 41.9 60.1 59.9 66.4 55.7
0.077 0.028 0.023 0.027 0.036 0.039 0.059 0.035 0.027 0.027 0.027
5.6 10.0 12.5 15.7 15.0 9.3 6.1 13.8 19.3 24.0 19.2
a b c
10 phr additive by mass in PVC. In the pure additive. See Table 1 for definitions.
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W.H. Starnes Jr et al / Polymer Degradation and Stability 82 (2003) 15–24 Table 6 Cone calorimetry results for plasticized PVCa Additivea
% metalb
TTIc (s)
TSRc
SEAc (m2/kg)
HRRc (kW/m2)
MLRc (g/s)
EHCc (MJ/kg)
None Cu2O CuCl CuBr [CuCl(BPP)] Cu(S2CNEt2) CuC2O4 Cu(O2CH)2.1.5H2O Cu3(MoO4)2(OH)2 CuSnO3 CuSi(OH)6 Zn3B4O9 Sb2O3 (NH4)4Mo8O26 CuMo/CuSn (2:1)e CuMo/CuSn (1:1)e CuMo/CuSn (1:2)e AOM/ZB (1:1)e Sb2O3/ZB (1:1)e AOM/Sb2O3 (1:1)e
0 88.8 64.2 44.3 8.5 30.0 41.4 35.2 70.2d 79.2d 47.3d 62.4d 83.5 61.1 73.2d 74.7d 76.2d 61.8d 73.0d 72.3d
76 145 177 142 118 96 97 120 90 194 77 74 91 196 121 149 173 47 159 277
6088 2715 2629 3299 2612 4923 2236 2541 2333 3305 4182 4693 7703 2414 1286 1670 1616 3936 3603 2026
1178 614 490 727 666 1122 524 490 697 1424 1095 1343 1542 823 425 544 538 1111 982 578
124.6 72.7 59.8 72.8 80.4 138.0 77.3 68.3 86.0 70.2 113.9 114.1 157.7 65.8 86.6 86.6 79.2 100.7 103.9 64.5
0.113 0.053 0.050 0.060 0.058 0.101 0.059 0.051 0.067 0.024 0.088 0.088 0.136 0.035 0.057 0.036 0.044 0.083 0.059 0.025
9.1 11.1 10.5 10.4 11.6 11.7 11.3 11.4 10.6 24.4 10.7 10.5 9.3 15.5 12.3 18.4 14.6 9.8 13.6 22.2
a b c d e
30 phr 7,9-trimellitate plasticizer and 10 phr additive by mass in PVC. In the pure additive. See Table 1 for definitions. Total metal content (for these purposes Si and B are regarded as ‘‘metals’’). CuMo=Cu3(MoO4)2(OH)2, CuSn=CuSnO3, AOM=(NH4)4Mo8O26, ZB=Zn3B4O9.
tively, in comparison to control data. As was found with gelation results, the smoke reduction is apparently independent of the initial oxidation state of copper in the additive. However, SEAav did appear to be roughly correlated to the copper content of the additive (see Fig. 4a). A similar trend was observed for TSR. The SEApk values showed a wider range, ranging from 70% reduction over the control to 150% enhancement. This result is not surprising, since a brief burst of intense smoke may occur for some samples in spite of an overall smoke reduction.
Significant fire retardancy was also evident in rigid PVC samples containing copper additives. Values for TTI were increased by ca. 50–250% versus the control value. Once again, TTI appeared to have a correlation to copper content (see Fig. 4b). The HRRav was lowered only slightly (if at all) through the use of copper additives, but the MLRav was lowered by some 25–70%. The MLRpk values were also favorably impacted by the copper additives with only one exception. The reduction in MLR is particularly important for both flame and smoke control, since both of these effects are associated
Fig. 4. Cone calorimetric trends in rigid PVC versus % copper content in additive: (a) SEAav trend, (b) TTI trend.
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with mass loss from the char. Thus CY was greatly enhanced by each of the additives. The EHCav, which is a measure of burning efficiency, was increased over that of the controls for most of the copper-additive-containing samples. This result probably points to the formation of less vapor-phase aromatics, which are poorly combustible and potent smoke-formers. Thus, combustion is reduced overall, but what burning does occur would be more efficient and therefore less smoky. The foregoing results for rigid PVC weigh strongly in favor of fairly simple and, therefore, copper-rich additives, irrespective of copper oxidation state. Thus, Cu2O, CuCl, and CuC2O4 all showed excellent performance. Copper(I) oxide, in particular, yielded the best values of all tested additives for SEAav, SEApk, HRRav, HRRpk, THR, MLRpk, and CY. The fire retardance and smoke suppression trends reported herein for Cu2O are in agreement with those of Li and Wang [8,9,11–13], with the exception of TTI trends. Li and Wang report that Cu(I) oxide actually shortens TTI, while the current findings show the opposite behavior. On the other hand, the relatively copper-poor complexes of the hindered organophosphites were found not to be particularly attractive as additive candidates. One of these additives, CuCl(BPP), after a relatively long induction period (TTI=225 s), caused a large burst of heat and smoke leading to elevated SEApk, HRRpk, and MLRpk results. It is difficult to rationalize these findings, since similar copper(I) phosphites did not behave in this way. Rigid PVC samples containing ammonium octamolybdate, (NH4)4Mo8O26 (AOM), were also studied for comparison to the copper results. This Mo(VI) compound is a commercially used smoke suppressant and probably functions via a Lewis-acid-catalyzed Friedel–Crafts coupling mechanism. The rigid PVC samples containing AOM produced high TTI and low MLRav values that indicate good fire retardance. The AOM additive was comparable to the better copper additives with respect to MLRav and EHCav results. However, the SEA and TSR values suggest fairly weak smoke suppression for AOM. A more extensive study was undertaken on plasticized PVC samples. Not surprisingly, the levels of both smoke suppression and fire retardance obtainable with these samples were generally less than those for rigid samples. Nevertheless, copper additives were again shown by the cone data to reduce smoke significantly, with TSR levels lowered by about 45–65% [omitting results for Cu(S2CNEt2) and CuSi(OH)6] and SEAav levels reduced by 40– 60% [omitting results for Cu(S2CNEt2), CuSi(OH)6, and CuSnO3]. In addition, fire retardance was also evident [again, excepting Cu(S2CNEt2) and CuSi(OH)6 but now including CuSnO3], with 20–155% increases in TTI and 30–50% reductions in HRRav. Mass loss from the solid phase was inhibited by copper additives, as indicated by MLRav reductions of 40–80% and char yields of 15–23% [omitting results for Cu(S2CNEt2) and
CuSi(OH)6]. However, for the plasticized samples, trends linking results such as SEA or TTI to Cu content were not evident. On the other hand, as was true for the rigid samples, neither additive copper oxidation state nor decomposition point appeared to have an influence on performance. Among the best copper additives for smoke suppression (based on SEA and TSR) were CuCl, Cu2O, CuC2O4, Cu(O2CH)2.1.5H2O, and Cu3(MoO4)2(OH)2. Excellent fire performance (based on TTI, HRR, THR, and MLR) but rather poor smoke suppression (based on SEA and TSR) was noted for CuSnO3. Among the worst copper-based additives were Cu(S2CNEt2) and CuSi(OH)6, both of which showed rather poor smoke and heat performance. The commercially used additives zinc borate, antimony(III) oxide, and AOM were tested individually and in concert for comparison to the copper compounds in plasticized PVC. Of these, only AOM offered any significant improvement over the better copper additives in some of the smoke and fire parameters. In fact, Sb2O3, when used alone, exacerbated both smoke and fire. Paired combinations of zinc borate, Sb2O3, and AOM were also studied and, as expected, yielded results markedly different from those for the individual additives. In particular, AOM and Sb2O3 formed a synergistic combination that lengthened TTI by ca. 260% and lowered SEAav by 51%, TSR by 67%, MLRav by 78%, and HRRav by 48% over the control values. All of these parameters are improved over those for either of the two additives alone, especially Sb2O3. Not surprisingly, the AOM/Sb2O3 combination finds application in commercial PVC formulations. Such synergism can be rationalized by examining the complementary modes of action of the two materials. While AOM is thought to function primarily in the solid state, Sb2O3 is considered to be a gas-phase fire retardant. It is noteworthy that this fairly Lewis acidic combination results in a high burning efficiency, as indicated by the elevated EHCav value (144% increase over the control). While the EHCav value is expected to be increased by a drop in volatile aromatics, a sharp increase in EHC might be indicative of Lewisacid-promoted char cracking or plasticizer dealkylation. Close inspection of the copper/molybdenum and copper/tin oxide additive data reveals an interesting dichotomy. The mixed Cu/Mo oxide Cu3(MoO4)2(OH)2 acts much like several other copper-based additives. Compared to the control sample, Cu3(MoO4)2(OH)2 significantly reduces smoke parameters TSR (by 62%) and SEAav (by 41%) but has a fairly modest impact on fire parameters TTI (18% increase) and HRRav (31% reduction). It also yields the most char of any additive in the plasticized formulation. On the other hand, the mixed Cu/ Sn oxide CuSnO3 greatly improves TTI (155% increase), HRRav (44% reduction), and MLRav (79% reduction). In fact, CuSnO3 shows fire performance strikingly similar to that of the AOM/Sb2O3 additive. However, the smoke
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23
Fig. 5. Trends in plasticized PVC as determined by cone calorimetry for Cu3(MoO4)2(OH)2 and CuSnO3 additive mixtures: (a) synergistic trends for SEAav and TSR, (b) additive trends for TTI and MLRav.
suppression of CuSnO3 alone is inferior to that of AOM/ Sb2O3, as CuSnO3 actually worsens SEAav (21% increase). In addition, this additive results in a particularly high EHCav value (168% increase). In view of the different behavior of the two additives, a synergism study was carried out by using a total of 10 phr of additive blends composed of Cu3(MoO4)2(OH)2 (CuMo) and CuSnO3 (CuSn) in plasticized PVC. Plots of SEAav and TSR versus additive composition are shown in Fig. 5a. These data reveal a very significant synergism for smoke suppression. The minimum values for SEAav (64% reduction over control) and TSR (79% reduction) were found for 2:1 CuMo/CuSn. These represent much better smoke suppression performance than was found with either additive alone or with 1:1 AOM/Sb2O3. The data for TTI and MLRav versus additive composition are shown in Fig. 5b. No synergism is indicated for these fire parameters. Both parameters favor the stannate. Thus, given the consistent smoke performance improvement in CuMo/CuSn mixtures, a 1:1 or 1:2 mixture might be more desirable because of improved fire retardance. Why does the synergism for smoke suppression occur with the copper additives? A conclusive answer to this question is, at present, unavailable. Nevertheless, we do know that synergism usually results from the operation of different mechanisms that are mutually reinforcing, and that several mechanisms of smoke suppression can be envisaged for the system under discussion. These include a Lewis-acid-promoted crosslinking caused by one or both of the additives themselves or by acids such as CuCl2, SnCl4, and MoO2Cl2 that may be formed in situ. Another possibility is reductive crosslinking brought about by Cu(0) [1], which could either be formed directly or result from the valence disproportionation of Cu(I): 2Cu(I)!Cu(II)+Cu(0) [1]. In this regard, it should be recalled that the condensed phase of burning PVC is well-known to be a strongly
reducing medium that converts higher-valent copper species into Cu(0) [25,26].
4. Conclusions Gelation data for PVC/copper-additive samples have strongly supported the theory that copper produces crosslinking in pyrolyzing PVC samples. Neither the original oxidation state of the metal nor the copper content of the additive appear to be particularly important in determining the degree of crosslinking. However, copper oxide compounds produce relatively high degrees of crosslinking. The cone calorimetry data for copper-based additives are very promising, not only with regard to smoke suppression, but also with respect to fire retardance. Plasticized formulations, which are especially vulnerable to burning and present a significant hazard, have shown significant improvement with respect to both flame and smoke through the use of copper-based additives, especially mixed copper oxides. In particular, mixtures of Cu3(MoO4)2(OH)2 and CuSnO3 show strong synergism for smoke parameters. The performance of these mixed additives is as good as or somewhat better than that of the commercially employed AOM/Sb2O3 mixtures. Moreover, it is very important to note in this connection that copper is generally regarded as being nontoxic to living systems [27] and is, in fact, an essential trace nutrient for humans [28], livestock [28], and plants [29]. Our studies of PVC smoke suppression and fire retardance by mixed copper oxides are ongoing.
Acknowledgements This research was supported in part by the National Science Foundation (Grant No. CHE-9983374) and the
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Camille and Henry Dreyfus Foundation (Grant No. TH-99-010). We thank R. Cozens of OxyVinyls L. P. for a generous gift of PVC powder, K. Shen of U. S. Borax Inc. for a sample of zinc borate, Dover Chemical Co. and Amfine Chemical Co. for samples of organophosphites, and BASF Canada for samples of plasticizer.
References [1] Pike RD, Starnes Jr WH, Jeng JP, Bryant WS, Kourtesis P, Adams CW, et al. Macromolecules 1997;30:6957–65. [2] Pike RD, Starnes Jr WH, Cole JR, Doyal AS, Graham PM, Johnson TJ, et al. ACS Symp Ser 2001;797:267–80. [3] Pike RD, Starnes Jr WH, Graham PM. In: Al-Malaika S, Golovoy A, Wilkie CA, editors. Specialty polymer additives: principles and applications. Oxford: Blackwell Science; 2001. p. 343–57. [4] Starnes Jr WH, Wescott Jr LD, Reents Jr WD, Cais RE, Villacorta GM, Plitz IM, et al. In: Kresta JE, editor. Polymer additives. New York: Plenum; 1984. p. 237–48. [5] Edelson D, Kuck VJ, Lum RM, Scalco E, Starnes Jr WH, Kaufman S. Combust Flame 1980;38:271–83. [6] Babrauskas V. Fire Mater 1984;8:81–95. [7] Babrauskas V, Mulholland G. Mathematical modeling of fires [ASTM STP 983]. Philadelphia: American Society for Testing and Materials; 1987. p. 83. [8] Li B, Wang J. J Fire Sci 1997;15:341–57. [9] Wang J, Li B. Polym Degrad Stab 1999;63:279–85. [10] Li B. Polym Degrad Stab 2000;68:197–204. [11] Li B, Wang J. J Vinyl Addit Technol 2001;7:37–42. [12] Li B. Polym Degrad Stab 2001;74:195–9. [13] Li B. Polym Degrad Stab 2002;78:349–56.
[14] Owsley DC, Castro CE. Org Synth Coll Vol 1988;6:916–8. [15] Drew MGB, Forsyth GA, Hasan M, Hobson RJ, Rice DA. J Chem Soc 1987;Dalton Trans:1027–33. [16] Tiethof JA, Hetey AT, Meek DW. Inorg Chem 1974;13:2505–59. These authors claim to form CuCl(SPPh3), rather than our observed product, CuCl(SPPh3)3. [17] Moini A, Peascoe R, Rudolf PR, Clearfield A. Inorg Chem 1986; 25:3782–5. [18] Victoriano LI, Corte´s HB. J Coord Chem 1996;39:231–9. [19] Hesse R. Ark Kemi 1962;20:481–505. [20] (a) Lawton SL, Rohrbaugh WJ, Kokotailo GT. Inorg Chem 1972;11:612–8. (b) Fackler Jr JP, Staples RJ, Liu CW, Stubbs RT, Lopez C, Pitts JT. Pure Appl Chem 1998;70:839–44. [21] Stevenson D, Stein D. In: Al-Malaika S, Golovoy A, Wilkie CA, editors. Chemistry and technology of polymer additives. Oxford: Blackwell Science; 1999 [Chapter 6]. [22] Bacaloglu R, Fisch MH, Kaufhold J, Sander HJ. In: Zweifel H, editor. Plastics additives handbook. 5th ed. Cincinnati: Hanser/ Gardner; 2000 [Chapter 3]. [23] Starnes Jr WH, Girois S. Polym Yearbook 1995;12:105–31. [24] Starnes Jr WH. Prog Polym Sci 2002;27:2133–70. [25] Lattimer RP, Kroenke WJ. J Appl Polym Sci 1981;26:1191–210. [26] Lattimer RP, Kroenke WJ. In: Voorhees KJ, editor. Analytical pyrolysis: techniques and applications. Woburn, MA: Butterworths; 1984. p. 453–73. [27] Ray SE. In: Richardson HW, editor. Handbook of copper compounds and applications. New York: Marcel Dekker; 1997 [Chapter 14]. [28] Cromwell GL. In: Richardson HW, editor. Handbook of copper compounds and applications. New York: Marcel Dekker; 1997 [Chapter 9]. [29] Gupta UC. In: Richardson HW, editor. Handbook of copper compounds and applications. New York: Marcel Dekker; 1997 [Chapter 10].
Cone calorimetric study of copper-promoted smoke suppression and fire retardance of poly(vinyl chloride) William H. Starnes Jr*, Robert D. Pike*, Jenine R. Cole, Alexander S. Doyal, Edward J. Kimlin, Jeffrey T. Lee, Philip J. Murray, Ronald A. Quinlan, Jing Zhang Departments of Chemistry and Applied Science, College of William and Mary, PO Box 8795, Williamsburg, VA 23187-8795, USA Received 25 February 2002; received in revised form 1 April 2003; accepted 6 April 2003
Abstract Copper-based smoke suppression additives for poly(vinyl chloride) (PVC) were tested for crosslinking capability in pyrolysis studies and for smoke suppression and fire retardance by the use of cone calorimetry. Crosslinking of PVC at 190 C was promoted by most of the additives without an obvious dependence on additive copper content or copper oxidation state. The copper additives (at 10 parts by weight per hundred parts of resin) proved to inhibit both smoke and heat evolution in burning PVC samples (both rigid and plasticized) in cone calorimetric studies. Mixed-metal oxides of copper were especially effective in this regard. Synergism in smoke suppression was noted for combinations of Cu3(MoO4)2(OH)2 and CuSnO3 in plasticized PVC. A 2:1 (w/w) mixture of Cu3(MoO4)2(OH)2 and CuSnO3 yielded a reduction in specific extinction area (a measure of smoke obscuration) of 64% and a reduction in total smoke release of 79% vs. the control sample. # 2003 Elsevier Ltd. All rights reserved. Keywords: Poly(vinyl chloride); PVC; Cone calorimeter; Smoke suppression; Fire retardance; Copper additives; Synergistic smoke suppressants
1. Introduction The problem of smoke and flame associated with poly(vinyl chloride) (PVC) is well-documented [1–3]. Although PVC is inherently nonflammable, its dehydrochlorination produces conjugated polyene segments, which in turn produce volatile aromatics, such as benzene, during pyrolysis. Combustion of these aromatics results in smoke formation and additional enthalpy input, thus promoting further polymer pyrolysis. An apparent key to limiting PVC smoke formation is minimizing the formation of volatile fuels through crosslinking, which produces nonvolatile char. Crosslinking in pyrolyzing PVC is typically accomplished through the use of high-valent metalbased smoke suppression additives, such as MoO3, which are actual or incipient Lewis acids and thus promote Friedel-Crafts alkylation reactions of PVC and polyenes. However, at elevated temperatures, these highvalent metal additives can also function as cracking cata* Corresponding authors. Tel.: +1-757-221-2552; fax: +1-757-2212715. E-mail addresses: [email protected] (W. H. Starnes), rdpike@wm. edu (R. D. Pike). 0141-3910/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0141-3910(03)00158-7
lysts, at least in certain cases. Cracking of the principally hydrocarbon char can produce volatile aliphatic organics, which act as efficient fuels. This situation results in less smoke, but increased flame [4,5]. Preliminary studies have shown that low-valent metals, such as Cu(0), can also promote crosslinking [1– 3]. However, in contrast to the Friedel-Crafts mechanism, the copper-promoted reaction appears to follow a reductive pathway such as that shown in Fig. 1. In addition, since copper is readily reduced thermally, the original PVC additive may be a compound of either Cu(I) or Cu(II). Copper(I) compounds are subject to valence disproportionation, which allows them to generate Cu(0) even in the absence of reducing agents. They are also often colorless, an obvious advantage in the formulation of uncolored PVC. However, Cu(I) complexes typically possess very limited thermal stability. Copper(II) compounds, on the other hand, can be thermally robust. In previous papers, we have demonstrated that a variety of coppercontaining compounds are rapid and efficient crosslinking agents for PVC at moderate temperatures without causing the concurrent formation of volatile byproducts [1–3]. The cone calorimeter is the successor instrument to the NBS smoke chamber [6,7]. It allows for the burning
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2. Experimental 2.1. Materials
Fig. 1. Possible pathway for the crosslinking of PVC via reductive coupling.
of sample plaques at relatively high heat flux values (such as 50 kW/m2) with the concomitant measurement of heat release and smoke temporal profiles. Thus, the cone calorimeter is an ideal instrument to evaluate realistically the combined smoke suppression and fire retardance behavior of polymer samples. Some of the important parameters that may be derived from cone calorimetry experiments are summarized in Table 1. By using cone calorimetry, Li and Wang [8–13] have recently studied the burning behavior of rigid PVC samples containing Cu2O, CuO, and MoO3 at heat fluxes of 25 and 50 kW/m2. Some of their major results for the flaming mode may be summarized as follows. Cuprous oxide shortens TTI and causes earlier smoke release than MoO3. Molybdenum(VI) oxide lengthens TTI and delays smoke evolution. However, both oxides show significant smoke suppression and fire retardance. In addition, synergism between Cu2O and MoO3 is evidenced in smoke suppression and enhanced char residue, but not in flame retardancy. In the current contribution, we report the cone calorimetric evaluation of a variety of coppercontaining additives for plasticized and rigid PVC.
The PVC was a powdered commercial product containing no additives. Palatinol 79TM-I plasticizer [tri(nheptyl, n-nonyl) trimellitate, or ‘‘7,9-trimellitate’’] was received from BASF Canada. Additive compounds Cu2O, CuCO3.Cu(OH)2, CuC2O4, Cu(O2CH)2.1.5 H2O, 2ZnO.3B2O3.3.5H2O, Sb2O3, and (NH4)4Mo8O26, as well as additive precursor compounds CuSO4.5H2O, Na2MoO4.2H2O, Na2SnO3.3H2O, Na2SiO3, H2PhPO3, H3PO3, HPh2PO2, H2PhPO2, H3PO2 (50% aqueous), NH4S2P(OEt)2, NaS2CNEt2.3H2O, PhSH, n-C12H25SH, Ph3CSH, Ph3PS, and P(OPh)3, were obtained from commercial sources and used as received. Additive compounds CuCl and CuBr were purchased from commercial sources and were purified by recrystallization from aqueous HCl and aqueous HBr, respectively. Hindered organophosphites were received from commercial sources (see Fig. 2). All copper-containing compounds were analyzed for copper content by atomic absorption, as previously described [1]. 2.2. Preparation of CuCl(BPP) and other copper(I) phosphite complexes Copper(I) chloride (1.00 g, 10.1 mmol) and tris(2,4-dit-butylphenyl) phosphite (6.54 g, 10.1 mmol) were suspended in 50 ml of CHCl3. The mixture was stirred at 25 C under N2 for 0.5 h, during which time it became less cloudy. After filtration, the solvent was removed under vacuum to leave a colorless oil, which solidified into a white solid under high vacuum (6.35 g, 8.51 mmol, 84%). Other complexes of CuCl or CuBr with P(OPh)3 (in C6H6), BPP (in CHCl3), MBPOP (in C6H6), BHTPD (in CH3CN), or CPD (in CH3CN) were prepared similarly. 2.3. Preparation of Cu(S2CNEt2), Cu(S2P(OEt)2), and CuSR To an ice-cold solution prepared from 25 ml of conc. aq. NH3 and 100 ml of H2O was added CuSO4.5H2O
Table 1 Cone calorimetry parameters Parameter
Abbreviation
Unit
Description
Time to ignition Specific extinction area (average and peak) Total smoke released Heat release rate (average and peak) Total heat released Mass loss rate (average and peak) Effective heat of combustion (average) Char yield
TTI SEAav, SEApk TSR HRRav, HRRpk THR MLRav, MLRpk EHCav CY
s m2 kg1 (unitless) kW m2 MJ m2 g s1 MJ kg1 %
Time of sustained ( >10 s) flaming Smoke produced per unit mass being volatilized Cumulative smoke produced Rate of heat release per unit sample area Cumulative heat energy released during flaming per unit sample area Mass of sample being volatilized per unit time Heat released per unit mass being volatilized Percent of sample mass remaining after burning
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17
EtOH gave a green solution. The solution was heated to just under boiling. A suspension of SnCl2.2H2O (0.490 g, 2.17 mmol) in 6 ml of EtOH was then added until the green color was discharged. The mixture was subjected to hot filtration, and a white precipitate formed on cooling overnight. It was isolated by filtration, washed in succession with EtOH and ether, and vacuum-dried (1.48 g, 1.51 mmol, 70%) [16]. 2.6. Preparation of Cu(Ph2PO2)2 and other copper(II) phosphinates and phosphonates Diphenylphosphinic acid, HPh2PO2 (1.25 g, 5.73 mmol), in 30 ml of MeOH and NaHCO3 (1.00 g, 11.9 mmol) in 30 ml of H2O were combined. Solid CuSO4.5H2O (1.46 g, 5.86 mmol) was added to the solution with stirring. A precipitate formed rapidly. The solid product was collected by filtration after 20 min. It was washed with H2O, EtOH, and ether in succession and then dried under vacuum (1.19 g, 2.39 mmol, 83%). 2 The Cu(II) complexes of PhPO2 3 , HPO3 , H2PO2 , and HPhPO2 were prepared similarly. Fig. 2. Hindered organophosphites. Reprinted with permission from Ref. [2]. Copyright 2001 American Chemical Society.
2.7. Preparation of Cu3(MoO4)2(OH)2 and other mixed metal oxides of copper(II)
(6.26 g, 25.1 mmol), thus forming a royal blue solution. Over a period of 45 min, solid NH2OH.HCl (3.89 g, 56.0 mmol) was added. Stirring overnight under a N2 purge produced a colorless solution of [Cu(NH3)4]+ [14]. By using a syringe, a solution of NaS2CNEt2.3H2O (7.04 g, 31.3 mmol) in 80 ml of H2O was added. A yellow-brown solid formed immediately. The solid product was collected via filtration; washed with H2O, EtOH, and ether in succession; and vacuum-dried. It was recrystallized by dissolving in CHCl3 and precipitating with ether (yield: 3.47 g, 16.4 mmol, 65%). The Cu(I) salt of S2P(OEt) 2 was prepared analogously, and the Cu(I) thiolates were prepared similarly by injecting the thiols dissolved in EtOH into an aqueous solution of [Cu(NH3)4]+.
A 250-ml aqueous solution of Na2MoO4.2H2O (21.6 g, 89.4 mmol) was added dropwise to a 250-ml aqueous solution of CuSO4.5H2O (33.5 g, 134 mmol). A pale blue-green precipitate formed immediately. The suspension was refluxed overnight, and the now pale green precipitate was collected by filtration. It was washed in succession with H2O, EtOH, and acetone and vacuumdried (15.9 g, 29.2 mmol, 65%) [17]. Other mixed metal oxides were prepared similarly. The aqueous mixture that produced CuSn(OH)6 was not refluxed.
2.4. Preparation of Cu(S2P(OEt)2)(SPPh3)2 A modified literature procedure was followed [15]. A suspension of Cu(S2P(OEt)2) (0.200 g, 0.804 mmol) and Ph3PS (0.473 g, 1.61 mmol) in 50 ml of CH2Cl2 was heated under reflux for 15 min. The resulting colorless solution was concentrated to a volume of 5 ml under vacuum. A white powder was precipitated by the addition of ether and vacuum-dried (0.487 g, 0.582 mmol, 72%). 2.5. Preparation of CuCl(SPPh3)3 Addition of CuCl2.2H2O (0.381 g, 2.23 mmol) to a suspension of Ph3PS (2.00 g, 6.79 mmol) in 50 ml of
2.8. Gelation studies These were carried out as previously described [1], except that 190 C rather than 200 C was used for the one-hour sample pyrolysis under a 50 ml/min Ar flow. All samples were run at least twice, and the reproducibilities of the gel yields and mass losses were found to be within 10% of the mean values. 2.9. Cone calorimetry Rigid plaques (1001003 mm3) containing ca. 45 g of PVC and 10 phr (parts by weight per hundred parts of resin) of an additive or additive mixture were prepared from blends that were mixed by grinding with a mortar and pestle. The plaques were made in a stainless steel mold with a Carver laboratory press (Model C) at 20,000 psi; mold temperature was raised within 7 min from ca. 65 to 150 C and then was held for 3 min at
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150 C. Plasticized plaques of the same size contained ca. 34 g of PVC, 30 phr of plasticizer, and 10 phr of additive. These plaques were molded as described above, except that the final hold time at 150 C was 4 min. The cone calorimeter was a Fire Testing Technology instrument. Plaques were burned in the flaming mode with spark ignition according to a standard test procedure (ASTM E 1354), using a heat flux of 50 kW/m2 and a horizontal receptacle that allowed samples to be immobilized with a stainless steel grid. In numerous replicate runs, reproducibilities were found to be within 10% for all of the properties studied.
3. Results and discussion 3.1. Choice of additive compounds In previous work [1], it was established that Cu(I) halides with coordinated phosphites and Cu(II) compounds of oxidizable anions (such as oxalate and formate) are potent crosslinking agents for PVC at 200 C. Copper(0) films and powders were shown to promote crosslinking even at temperatures of < 100 C. In addition, it was shown that copper-based additives could promote polymer crosslinking below the decomposition temperature of the additive. In the current extension of this work, several classes of copper compounds have been investigated as potential crosslinking additives. These include simple salts of copper(I), copper(I) thiolates, copper(I) halides with highly hindered phosphite ligands, salts of copper(II) with oxidizable anions, and mixed-metal oxides of copper(II). All copper com-
pounds were analyzed for copper content by using flame atomic absorption spectrophotometry and for decomposition point by using a melting point or TGA apparatus; see Tables 2 and 3. The simple salts of copper(I), CuCl, CuBr, and Cu2O, were obvious choices as potential additives, given their low price, high copper content, and in the case of the halides, lack of color. However, the halides are particularly moisture sensitive and tend to develop green color due to Cu(II) formation upon long-term exposure to moist air. This moisture instability was viewed as a potential polymer formulation problem. Copper(I) salts of sulfur-based anions are more stable toward moisture than simple halides. Therefore, Cu(I) salts of the following anions were synthesized: S2P(OEt) 2 (diethyl dithiophosphate), S2CNEt2 (diethyl dithiocarbamate), and thiolates PhS , n-C12H25S, and Ph3CS. These compounds were easily prepared on large scales by the addition of the alkali salt or acid form of the desired anion to aqueous Cu(NH3)+ 4 [14]. Copper(I) diethyl dithiocarbamate [18] is a tetrameric compound [19], and copper(I) diethyl dithiophosphate is probably also oligomeric, since the diisopropyl analog is known to be hexameric [20]. The Cu(I) thiolate compounds are widely regarded as being polymeric. Two presumably monomeric Cu(I) complexes of triphenylphosphine sulfide, Cu(S2P(OEt)2)(SPPh3)2 and CuCl(SPPh3)3 [16], were prepared. All of the foregoing sulfur-based compounds lacked significant color, thus making them potentially attractive as additive candidates. Although previous results had indicated that copper(I) halide organophosphite complexes were potent PVC crosslinking agents, it was found that upon expo-
Table 2 Copper(I) compounds prepared Compound
Color
Dec. point ( C)
Theo. Cu (%)
Expt. Cu (%)
CuCl CuBr Cu2O Cu(S2P(OEt)2) Cu(S2CNEt2) Cu(SPh) Cu(S-n-C12H25) Cu(SCPh3) CuCl(SPPh3)3 Cu(S2P(OEt)2)(SPPh3)2 CuCl(P(OPh)3) CuBr(P(OPh)3) CuCl(BPP) CuBr(BPP) (CuCl)2(BHTPD) (CuBr)2(BHTPD) CuCl(CPD) CuBr(CPD) CuCl(MBPOP) CuBr(MBPOP)
White White Rust red White Tan Pale yellow Cream Cream White White White White White White Cream Tan White White White White
400 450 >900 187 125 151 100 85 225 180 285 251 226 238 235 248 269 269 220 230
64.2 44.3 88.8 25.5 30.0 36.8 24.0 18.8 6.5 7.6 15.5 14.0 8.5 8.0 15.3 13.8 6.7 6.4 9.3 8.7
62.6 46.3 88.0 25.3 31.0 36.1 23.8 17.8 6.5 6.9 15.1 13.6 8.3 7.9 15.6 13.4 6.6 6.8 9.8 9.2
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W.H. Starnes Jr et al / Polymer Degradation and Stability 82 (2003) 15–24 Table 3 Copper(II) compounds prepared Complex
Color
Dec. point ( C)
Theo. Cu (%)
Expt. Cu (%)
Cu(C2O4) Cu(O2CH)2.1.5H2O CuCO3.Cu(OH)2 Cu(HPO3).1.5H2O Cu(PhPO3).1.5H2O Cu(H2PO2)2 Cu(HPhPO2)2 Cu(Ph2PO2)2 Cu3(MoO4)2(OH)2 CuSnO3 CuSn(OH)6 CuSi(OH)6
Pale blue Pale blue Blue-green Pale blue Pale blue Very pale blue Very pale blue Blue-violet Pale green Blue-grey Sky blue Pale blue
300 199 200 126 344 55a 99 299 >900 >900 >900 >900
41.9 35.2 57.5 37.3 25.8 32.8 18.4 12.8 35.0 27.6 22.4 32.8
41.0 34.7 55.2 37.5 26.2 33.0 18.1 13.3 34.9 25.7 21.9 32.5
a
Decomposes over several days at 25 C.
sure to moist air, simple phosphites [e.g., P(OPh)3] and their complexes readily hydrolyzed, thereby producing a phenolic-smelling gummy residue [2]. As a result, copper(I) complexes of the highly hindered phosphites [21] BPP, MBPOP, BHTPD, and CPD were prepared (phosphite structures are shown in Fig. 2). These complexes were found to be highly resistant to hydrolysis, even under elevated humidity conditions [2]. The use of Cu(II) salts was extended in the current study to include, in addition to salts of oxalate and formate, salts of phosphonate (HPO2 3 ), phenylphosphonate (PhPO2 phosphinate (H2PO 3 ), 2 ), phenylphosphinate (HPhPO2 ), and diphenylphosphinate (Ph2PO 2 ). These anions contain reduced forms of phosphorus. Thermal oxidation of phosphorus-based anions was expected to promote the reduction of copper. However, in the case of phosphorus anions containing P–H bonds, this internal redox reaction proved to be too facile and thus led to unacceptably low decomposition points (see Table 3). Finally, simple aqueous reactions were used to prepare the mixed copper molybdate, Cu3(MoO4)2(OH)2 (synthetic ‘‘Lindgrenite’’ [17]), as well as copper stannates and copper silicate. These materials were shown by thermogravimetry to have very high thermal stability, exhibiting less than 10% mass loss at 900 C. 3.2. Gelation studies Mixtures containing 10 wt.% of additive in PVC were produced by low-temperature grinding [1]. Gelation studies on the PVC/additive mixtures were carried out via solvent-free pyrolysis for 1 h at 190 C under flowing argon. The soluble portion of residues was removed by Soxhlet extraction with THF. Following extraction, the crosslinked gel residue was recovered, dried, and weighed. Since additive-free control samples show no gelation under these conditions, formation of a gel indicates that the additive is promoting PVC cross-
linking under these moderate conditions. As part of the same experiment, the sample mass lost due to volatilization during pyrolysis was determined. The gelation and mass loss results are shown in Table 4. None of the copper-containing additives caused polymer mass losses in excess of 10%, and most of the mass Table 4 PVC gelation and mass loss resultsa Additive, 10% by mass in PVC
Gel trials
Mass loss (%)
PVC gel yield (%)
None CuCl CuBr Cu2O CuCO3.Cu(OH)2 Cu(C2O4) Cu(O2CH)2.1.5H2O Cu(PhPO3).1.5H2O Cu(Ph2PO2)2 Cu(S2P(OEt)2) Cu(S2CNEt2) Cu(SPh) Cu(S-n-C12H25) Cu(SCPh3) CuCl(SPPh3)3 Cu(S2P(OEt)2)(SPPh3)2 CuCl(P(OPh)3) CuBr(P(OPh)3) CuCl(BPP) CuBr(BPP) (CuCl)2(BHTPD) (CuBr)2(BHTPD) CuCl(CPD) CuBr(CPD) CuCl(MBPOP) CuBr(MBPOP) Cu3(MoO4)2(OH)2 CuSnO3 CuSn(OH)6 CuSi(OH)6 (NH4)4Mo8O26
2 2 2 2 2 2 2 4 3 2 2 2 2 2 2 2 2 2 4 4 4 4 4 3 3 4 2 2 2 2 2
0.3 3.6 3.6 1.8 3.6 5.7 3.9 1.9 1.2 4.4 7.9 3.2 5.8 8.6 1.5 1.6 4.8 2.1 0.7 1.2 1.9 0.8 2.4 1.6 3.6 2.1 5.1 1.6 3.7 3.0 4.4
<5 62 60 72 52 50 65 69 86 <5 66 50 61 7 65 <5 82 69 60 50 63 63 68 45 39 45 74 70 93 84 60
a
Samples pyrolyzed at 190 C for 1 h under flowing argon.
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W.H. Starnes Jr et al / Polymer Degradation and Stability 82 (2003) 15–24
loss values were below 5%. These results stand in contrast to those with highly Lewis acidic additives, such as FeCl3, which has been shown to cause mass losses of over 30% under similar conditions [1]. A plot of gel yield versus Cu content in the additive (Fig. 3) revealed no correlation. This result stands in contrast to the Cu loading correlations seen in the cone calorimetry results for rigid PVC (see below). Most of the additives studied promoted polymer gelation at 190 C. However, there were several notable exceptions. The strongest gel formers (giving gel yields of 570%) were mainly oxide materials, which are the most thermally stable of the compounds studied. This finding is quite significant, for it suggests that thermally stable and fairly inert mixed copper oxide additives might be potent smoke suppressants in PVC. Two of the additives with thiolate groups were reasonably good gel promoters. Other metal thiolates have long been used as heat stabilizers for PVC [22], their mode of action being (in part) that of thiolate for labile
Fig. 3. PVC gel yield versus % copper content in additive.
chloride substitution at unstable defect sites [23,24]. It seemed possible that the copper thiolates might increase the thermal stability of the PVC to such an extent that insufficient dehydrochlorination could occur at 190 C to form appreciable amounts of the polyenyl chlorides that lead to gel (see Fig. 1). However, our data do not support the general applicability of this hypothesis. 3.3. Cone calorimetry studies Cone calorimetric studies were carried out on PVC plaques containing 10 phr of additive, and in the case of plasticized plaques, 30 phr of 7,9-trimellitate. The cone calorimetry results for rigid and plasticized PVC samples are shown in Tables 5 and 6, respectively. It is instructive to compare cone results for metal-additivefree control samples of rigid and plasticized PVC. Since most plasticizers contain long aliphatic chains, they readily serve as fuel during fires. Comparison of the control results reveals the higher flammability of plasticized PVC. Thus, the plasticized PVC controls show shorter values for TTI (see Table 1 for definitions of parameters) and higher values for HRR, THR, MLR, and EHC. In addition, higher SEA and TSR values indicate that the plasticized samples produce more smoke than the rigid samples. Enhanced smoke formation is likely because of the higher quantity of the fuel available in plasticized PVC. Thus, the problems of flame and smoke in PVC are both exacerbated by the use of the plasticizer, as expected. It is also noteworthy that, at the 50 kW/m2 heat flux used in the study, neither control sample showed any residual char (CY=0%). The copper-based additives proved to be very effective smoke suppressants in the rigid PVC samples. Through the use of such additives, the smoke parameters SEAav and TSR were lowered by 20–89 and 36–82%, respec-
Table 5 Cone calorimetry results for rigid PVC Additivea
% metalb
TTIc (s)
TSRc
SEAc (m2/kg)
HRRc (kW/m2)
MLRc (g/s)
EHCc (MJ/kg)
None Cu2O CuCl CuBr [CuCl(BPP)] [CuBr(MBPOP)] [(CuBr)2(BHTPD)] [CuCl(CPD)] CuC2O4 Cu(O2CH)2.1.5H2O (NH4)4Mo8O26
0 88.8 64.2 44.3 8.5 8.7 13.8 6.7 41.4 35.2 61.1
98 311 346 316 225 183 145 171 251 229 202
3431 889 835 1084 2190 1998 1595 1527 627 737 2167
845 93 107 257 566 396 389 672 114 322 878
50.5 32.2 32.9 50.2 65.6 43.3 41.9 60.1 59.9 66.4 55.7
0.077 0.028 0.023 0.027 0.036 0.039 0.059 0.035 0.027 0.027 0.027
5.6 10.0 12.5 15.7 15.0 9.3 6.1 13.8 19.3 24.0 19.2
a b c
10 phr additive by mass in PVC. In the pure additive. See Table 1 for definitions.
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W.H. Starnes Jr et al / Polymer Degradation and Stability 82 (2003) 15–24 Table 6 Cone calorimetry results for plasticized PVCa Additivea
% metalb
TTIc (s)
TSRc
SEAc (m2/kg)
HRRc (kW/m2)
MLRc (g/s)
EHCc (MJ/kg)
None Cu2O CuCl CuBr [CuCl(BPP)] Cu(S2CNEt2) CuC2O4 Cu(O2CH)2.1.5H2O Cu3(MoO4)2(OH)2 CuSnO3 CuSi(OH)6 Zn3B4O9 Sb2O3 (NH4)4Mo8O26 CuMo/CuSn (2:1)e CuMo/CuSn (1:1)e CuMo/CuSn (1:2)e AOM/ZB (1:1)e Sb2O3/ZB (1:1)e AOM/Sb2O3 (1:1)e
0 88.8 64.2 44.3 8.5 30.0 41.4 35.2 70.2d 79.2d 47.3d 62.4d 83.5 61.1 73.2d 74.7d 76.2d 61.8d 73.0d 72.3d
76 145 177 142 118 96 97 120 90 194 77 74 91 196 121 149 173 47 159 277
6088 2715 2629 3299 2612 4923 2236 2541 2333 3305 4182 4693 7703 2414 1286 1670 1616 3936 3603 2026
1178 614 490 727 666 1122 524 490 697 1424 1095 1343 1542 823 425 544 538 1111 982 578
124.6 72.7 59.8 72.8 80.4 138.0 77.3 68.3 86.0 70.2 113.9 114.1 157.7 65.8 86.6 86.6 79.2 100.7 103.9 64.5
0.113 0.053 0.050 0.060 0.058 0.101 0.059 0.051 0.067 0.024 0.088 0.088 0.136 0.035 0.057 0.036 0.044 0.083 0.059 0.025
9.1 11.1 10.5 10.4 11.6 11.7 11.3 11.4 10.6 24.4 10.7 10.5 9.3 15.5 12.3 18.4 14.6 9.8 13.6 22.2
a b c d e
30 phr 7,9-trimellitate plasticizer and 10 phr additive by mass in PVC. In the pure additive. See Table 1 for definitions. Total metal content (for these purposes Si and B are regarded as ‘‘metals’’). CuMo=Cu3(MoO4)2(OH)2, CuSn=CuSnO3, AOM=(NH4)4Mo8O26, ZB=Zn3B4O9.
tively, in comparison to control data. As was found with gelation results, the smoke reduction is apparently independent of the initial oxidation state of copper in the additive. However, SEAav did appear to be roughly correlated to the copper content of the additive (see Fig. 4a). A similar trend was observed for TSR. The SEApk values showed a wider range, ranging from 70% reduction over the control to 150% enhancement. This result is not surprising, since a brief burst of intense smoke may occur for some samples in spite of an overall smoke reduction.
Significant fire retardancy was also evident in rigid PVC samples containing copper additives. Values for TTI were increased by ca. 50–250% versus the control value. Once again, TTI appeared to have a correlation to copper content (see Fig. 4b). The HRRav was lowered only slightly (if at all) through the use of copper additives, but the MLRav was lowered by some 25–70%. The MLRpk values were also favorably impacted by the copper additives with only one exception. The reduction in MLR is particularly important for both flame and smoke control, since both of these effects are associated
Fig. 4. Cone calorimetric trends in rigid PVC versus % copper content in additive: (a) SEAav trend, (b) TTI trend.
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with mass loss from the char. Thus CY was greatly enhanced by each of the additives. The EHCav, which is a measure of burning efficiency, was increased over that of the controls for most of the copper-additive-containing samples. This result probably points to the formation of less vapor-phase aromatics, which are poorly combustible and potent smoke-formers. Thus, combustion is reduced overall, but what burning does occur would be more efficient and therefore less smoky. The foregoing results for rigid PVC weigh strongly in favor of fairly simple and, therefore, copper-rich additives, irrespective of copper oxidation state. Thus, Cu2O, CuCl, and CuC2O4 all showed excellent performance. Copper(I) oxide, in particular, yielded the best values of all tested additives for SEAav, SEApk, HRRav, HRRpk, THR, MLRpk, and CY. The fire retardance and smoke suppression trends reported herein for Cu2O are in agreement with those of Li and Wang [8,9,11–13], with the exception of TTI trends. Li and Wang report that Cu(I) oxide actually shortens TTI, while the current findings show the opposite behavior. On the other hand, the relatively copper-poor complexes of the hindered organophosphites were found not to be particularly attractive as additive candidates. One of these additives, CuCl(BPP), after a relatively long induction period (TTI=225 s), caused a large burst of heat and smoke leading to elevated SEApk, HRRpk, and MLRpk results. It is difficult to rationalize these findings, since similar copper(I) phosphites did not behave in this way. Rigid PVC samples containing ammonium octamolybdate, (NH4)4Mo8O26 (AOM), were also studied for comparison to the copper results. This Mo(VI) compound is a commercially used smoke suppressant and probably functions via a Lewis-acid-catalyzed Friedel–Crafts coupling mechanism. The rigid PVC samples containing AOM produced high TTI and low MLRav values that indicate good fire retardance. The AOM additive was comparable to the better copper additives with respect to MLRav and EHCav results. However, the SEA and TSR values suggest fairly weak smoke suppression for AOM. A more extensive study was undertaken on plasticized PVC samples. Not surprisingly, the levels of both smoke suppression and fire retardance obtainable with these samples were generally less than those for rigid samples. Nevertheless, copper additives were again shown by the cone data to reduce smoke significantly, with TSR levels lowered by about 45–65% [omitting results for Cu(S2CNEt2) and CuSi(OH)6] and SEAav levels reduced by 40– 60% [omitting results for Cu(S2CNEt2), CuSi(OH)6, and CuSnO3]. In addition, fire retardance was also evident [again, excepting Cu(S2CNEt2) and CuSi(OH)6 but now including CuSnO3], with 20–155% increases in TTI and 30–50% reductions in HRRav. Mass loss from the solid phase was inhibited by copper additives, as indicated by MLRav reductions of 40–80% and char yields of 15–23% [omitting results for Cu(S2CNEt2) and
CuSi(OH)6]. However, for the plasticized samples, trends linking results such as SEA or TTI to Cu content were not evident. On the other hand, as was true for the rigid samples, neither additive copper oxidation state nor decomposition point appeared to have an influence on performance. Among the best copper additives for smoke suppression (based on SEA and TSR) were CuCl, Cu2O, CuC2O4, Cu(O2CH)2.1.5H2O, and Cu3(MoO4)2(OH)2. Excellent fire performance (based on TTI, HRR, THR, and MLR) but rather poor smoke suppression (based on SEA and TSR) was noted for CuSnO3. Among the worst copper-based additives were Cu(S2CNEt2) and CuSi(OH)6, both of which showed rather poor smoke and heat performance. The commercially used additives zinc borate, antimony(III) oxide, and AOM were tested individually and in concert for comparison to the copper compounds in plasticized PVC. Of these, only AOM offered any significant improvement over the better copper additives in some of the smoke and fire parameters. In fact, Sb2O3, when used alone, exacerbated both smoke and fire. Paired combinations of zinc borate, Sb2O3, and AOM were also studied and, as expected, yielded results markedly different from those for the individual additives. In particular, AOM and Sb2O3 formed a synergistic combination that lengthened TTI by ca. 260% and lowered SEAav by 51%, TSR by 67%, MLRav by 78%, and HRRav by 48% over the control values. All of these parameters are improved over those for either of the two additives alone, especially Sb2O3. Not surprisingly, the AOM/Sb2O3 combination finds application in commercial PVC formulations. Such synergism can be rationalized by examining the complementary modes of action of the two materials. While AOM is thought to function primarily in the solid state, Sb2O3 is considered to be a gas-phase fire retardant. It is noteworthy that this fairly Lewis acidic combination results in a high burning efficiency, as indicated by the elevated EHCav value (144% increase over the control). While the EHCav value is expected to be increased by a drop in volatile aromatics, a sharp increase in EHC might be indicative of Lewisacid-promoted char cracking or plasticizer dealkylation. Close inspection of the copper/molybdenum and copper/tin oxide additive data reveals an interesting dichotomy. The mixed Cu/Mo oxide Cu3(MoO4)2(OH)2 acts much like several other copper-based additives. Compared to the control sample, Cu3(MoO4)2(OH)2 significantly reduces smoke parameters TSR (by 62%) and SEAav (by 41%) but has a fairly modest impact on fire parameters TTI (18% increase) and HRRav (31% reduction). It also yields the most char of any additive in the plasticized formulation. On the other hand, the mixed Cu/ Sn oxide CuSnO3 greatly improves TTI (155% increase), HRRav (44% reduction), and MLRav (79% reduction). In fact, CuSnO3 shows fire performance strikingly similar to that of the AOM/Sb2O3 additive. However, the smoke
W.H. Starnes Jr et al / Polymer Degradation and Stability 82 (2003) 15–24
23
Fig. 5. Trends in plasticized PVC as determined by cone calorimetry for Cu3(MoO4)2(OH)2 and CuSnO3 additive mixtures: (a) synergistic trends for SEAav and TSR, (b) additive trends for TTI and MLRav.
suppression of CuSnO3 alone is inferior to that of AOM/ Sb2O3, as CuSnO3 actually worsens SEAav (21% increase). In addition, this additive results in a particularly high EHCav value (168% increase). In view of the different behavior of the two additives, a synergism study was carried out by using a total of 10 phr of additive blends composed of Cu3(MoO4)2(OH)2 (CuMo) and CuSnO3 (CuSn) in plasticized PVC. Plots of SEAav and TSR versus additive composition are shown in Fig. 5a. These data reveal a very significant synergism for smoke suppression. The minimum values for SEAav (64% reduction over control) and TSR (79% reduction) were found for 2:1 CuMo/CuSn. These represent much better smoke suppression performance than was found with either additive alone or with 1:1 AOM/Sb2O3. The data for TTI and MLRav versus additive composition are shown in Fig. 5b. No synergism is indicated for these fire parameters. Both parameters favor the stannate. Thus, given the consistent smoke performance improvement in CuMo/CuSn mixtures, a 1:1 or 1:2 mixture might be more desirable because of improved fire retardance. Why does the synergism for smoke suppression occur with the copper additives? A conclusive answer to this question is, at present, unavailable. Nevertheless, we do know that synergism usually results from the operation of different mechanisms that are mutually reinforcing, and that several mechanisms of smoke suppression can be envisaged for the system under discussion. These include a Lewis-acid-promoted crosslinking caused by one or both of the additives themselves or by acids such as CuCl2, SnCl4, and MoO2Cl2 that may be formed in situ. Another possibility is reductive crosslinking brought about by Cu(0) [1], which could either be formed directly or result from the valence disproportionation of Cu(I): 2Cu(I)!Cu(II)+Cu(0) [1]. In this regard, it should be recalled that the condensed phase of burning PVC is well-known to be a strongly
reducing medium that converts higher-valent copper species into Cu(0) [25,26].
4. Conclusions Gelation data for PVC/copper-additive samples have strongly supported the theory that copper produces crosslinking in pyrolyzing PVC samples. Neither the original oxidation state of the metal nor the copper content of the additive appear to be particularly important in determining the degree of crosslinking. However, copper oxide compounds produce relatively high degrees of crosslinking. The cone calorimetry data for copper-based additives are very promising, not only with regard to smoke suppression, but also with respect to fire retardance. Plasticized formulations, which are especially vulnerable to burning and present a significant hazard, have shown significant improvement with respect to both flame and smoke through the use of copper-based additives, especially mixed copper oxides. In particular, mixtures of Cu3(MoO4)2(OH)2 and CuSnO3 show strong synergism for smoke parameters. The performance of these mixed additives is as good as or somewhat better than that of the commercially employed AOM/Sb2O3 mixtures. Moreover, it is very important to note in this connection that copper is generally regarded as being nontoxic to living systems [27] and is, in fact, an essential trace nutrient for humans [28], livestock [28], and plants [29]. Our studies of PVC smoke suppression and fire retardance by mixed copper oxides are ongoing.
Acknowledgements This research was supported in part by the National Science Foundation (Grant No. CHE-9983374) and the
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Camille and Henry Dreyfus Foundation (Grant No. TH-99-010). We thank R. Cozens of OxyVinyls L. P. for a generous gift of PVC powder, K. Shen of U. S. Borax Inc. for a sample of zinc borate, Dover Chemical Co. and Amfine Chemical Co. for samples of organophosphites, and BASF Canada for samples of plasticizer.
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