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A Novel Method to Prepare Zinc HydroxystannateCoated Inorganic Fillers and Its Effect on the Fire Properties of PVC Cable Materials

Ling Yang,1 Yuan Hu,1 Fei You,1 Zuyao Chen2 1

State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230026, Anhui, People’s Republic of China

2

Department of Chemistry, University of Science and Technology of China, Hefei 230026, Anhui, People’s Republic of China

Fire-retardant (FR) properties, including limiting oxygen index, peak rate of heat release, and smoke parameter have been measured and compared for unfilled and filled polyvinyl-chloride (PVC)-based cable formulations, containing 15 wt% amounts of uncoated and zinc-hydroxystannate (ZHS)-coated magnesium hydroxide (MH) and calcium carbonate (CaCO3) fillers at the same addition level. Of the uncoated fillers, MH was more effective at lowering flammability than CaCO3. When the ZHS coating was applied to MH and CaCO3, CaCO3 became the most effective additive at lowering PVC flammability and smoke output. POLYM. ENG. SCI., 47:1163–1169, 2007. © 2007 Society of Plastics Engineers

INTRODUCTION Poly(vinyl chloride) (PVC) is a kind of thermoplastic, which has been used in various aspects, such as building materials, sealing strip, wire, and cable, etc. However, a large quantity of black smoke is produced when PVC is forced to burn, which is a big problem in its applications where the fire hazard is a concern. There has been a great volume of literature on applications and mechanisms of flame retardancy and smoke suppression of PVC and this kind of halogen-containing polyesters [1–7]. Although antimony trioxide is efficient as a synergistic flame retardant in PVC compositions, it is found to increase the amount of smoke and toxic gases. Recent studies on the development flame retardants formation have used inorganic tin compounds such as zinc hydroxystannate and zinc stannate

Correspondence to: Yuan Hu; e-mail: [email protected] DOI 10.1002/pen.20482 Contract grant sponsor: China NKBRSF project; contract grant number: 2001CB409600; contract grant sponsor: National Natural Science Foundation of China; contract grant number: 50323005 and 50476026. Published online in Wiley InterScience (www.interscience.wiley.com). © 2007 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—2007

because they have less toxicity. Cusack et al. indicated that zinc hydroxystannate and zinc stannate could be used as highly effective flame retardants. Zinc hydroxystannate and zinc stannate have less toxicity than antimony compounds [1–3, 8, 9]. There is an increasing interest in the use of zinc hydroxystannate [ZnSn(OH)6], ZHS-coated fillers as fire retardant (FR) and smoke suppressant additives for polymeric materials, especially for halogen-containing polymers, because they have less toxicity. The application of a ZHS coating to various hydrated inorganic fillers, in particular aluminium hydroxide (ATH) and magnesium hydroxide (MH), allows a significant reduction to be made to the overall filler loading, with no loss in FR properties [1, 10]. Mg(OH)2 has the advantage of decomposing into magnesium oxide (MgO) and water at a relatively higher temperature (300 –320oC), thus allowing it to be processed in plastics, such as polypropylene (PP) and natural fiber-PPcomposites, for which Al(OH)3 (decomposition temperature: 200oC) is not thermally stable enough [11, 12]. ATH and MH provides effective flame retarding effects [11–13]: 1. When heated, the hydroxides release water vapor that cools the substrate to a temperature below that required for sustaining the combustion processes. 2. The water vapor released has also a diluting effect in the gas phase and forms an oxygen-displacing protective layer. 3. Producing an oxide (MgO or Al2O3)-coating during burning results in further flame retardant protection and less smoke generation.

Calcium carbonate has the similar thermal decomposition properties; it decomposes into calcium oxide (CaO) and carbon dioxide at a very high temperature (about 750°C) [14]. CaCO3 can often be used in flame retardant system as

TABLE 1.

PVC formulations (in parts by weight).

Sample

PVC

DOTP

Pbst

ZHS-coated MH

PVC0 PVC1 PVC2 PVC3 PVC4

100 100 100 100 100

45 45 45 45 45

2 2 2 2 2

0 15 0 0 0

Physical blend of ZHS and MH 0 0 15 0

ZHS-coated CaCO3

Physical blend of ZHS and CaCO3

0 0 0 15 0

0 0 0 0 15

PVC2 relatives to PVC1, mass of ZHS divided by mass of MH are all 11.4, but they are physically blended. PVC4 relatives to PVC3 are the same condition.

a kind of inorganic fillers, such as in flame-retarded ABS system [12] and flame-retarded PP system [15]. In the present work, we evaluate the FR behavior of ZHS-coated MH and ZHS-coated CaCO3 in typical PVC cable formulations by a cone calorimeter and a thermal gravimetry analysis. The mechanical properties of these systems are evaluated by a Universal Testing Machine DCS-5000 (Shimadzu, Japan). EXPERIMENTAL PROCEDURES Materials Used Tin tetrachloride (SnCl4䡠5H2O, ⱖ99%), zinc chloride (ZnCl2, ⱖ98%), and hydrochloride (HCl, 36 –38%) were all purchased from Shanghai Hengxin Chemical Reagent. A suspension PVC (S1000, MW: 62,500) employed in this study was purchased from Qilu Petrochemical, Shandong Province, China. MH and CaCO3 fillers with an average particle diameter of 5– 8 ␮m were supplied by NanoTech Science & Technology Company, Beijing, China. The additives, lead stearate (Pbst) thermal stabilizer, and dioctyl phthalate (DOTP) plasticizer were provided by Haolong Chemical, Tianjin, China.

water, and dried in air at 100oC. The preparation of ZnSn(OH)6 (ZHS) is based on the following formal reaction: SnCl4⫹ZnCl2⫹6NaOH3 ZnSn(OH)6(s)⫹6NaCl. The dried cake was crushed using a pestle and mortar to give 111.4 g of a fine white powder ZHS-coated MH or ZHS-coated CaCO3. The rate of coating is defined as mass of ZHS divided by mass of inorganic fillers. The ratio of coating is ⬃11.4/100 g, i.e. 11.4%. The method to obtain physical blend of ZHS and MH or CaCO3 fillers is described as follows: 11.4 g ZHS and 100 g MH or CaCO3 were put in a rotating mixer of the Sigma type and blended for 15 min. The rotation direction was changed every 5 min to ensure distribution of the ZHS in the nano-CaCO3 filler. These inorganic fillers were incorporated into a flexible PVC cable formulation, given in Table 1. All the compositions were first melt compounded on a two-roll mill at 175oC, then the resulting mixtures were compression molded into sheets (3 and 1 mm thickness) using applied pressure of 10 MPa at the same temperature. Structural Characterization

Sample Preparation ZHS-coated fillers were prepared in the following manner. 100 g of magnesium hydroxide (MH, industrial product, 2M-C01, produced by Hefei HuaXing Chemical Engineering Factory) or 100 g of calcium carbonate (CaCO3, industrial product, KY-2, provided by Keyan Chemical Fire-retardant Materials (Hefei) Co. LTD.) was slurried in 600 ml of an aqueous solution containing of Sn4⫹ and Zn2⫹ ions, which was obtained by mixing SnCl4䡠5H2O (A.R.) and ZnCl2 (C.P.) in 100 ml distilled water with a molar ratio 0.04:0.04, with the addition of 1 ml of 0.1 mol/l hydrochloric acid to prevent quick hydrolyzation, and stirred to uniformity, then NaOH solid (A.R., reagents both from Shanghai Chemical Reagents Corporation) was added into the solution directly, and the resulting mixture was stirred for 2 h. The resulting solid product was separated from the solution by centrifugation, washed three times with distilled 1164


XRD was performed on a Japan Rigaku K/max-␥A Xray diffractometer with Cu K␣ radiation (␭ ⫽ 1.54178Å, 2␪ ⫽ 5– 65 du) at the scanning rate of 0.02 C/s. The transmission electron microscopy (TEM) images of the ZHS and ZHS-coated MH and ZHS-coated CaCO3 were obtained using a Jeol JEM-100SX transmission electron microscope with an acceleration voltage of 100 kV. The samples were dispersed in ethanol, agitated in an ultrasonic bath, and then deposited on a copper grid for observation. Thermal Analysis Thermal analysis (TA) including thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) was conducted with a Netzsch STA 409C thermoanalyzer instrument. About 15 mg of the specimens were heated from 50 to 750oC using a linear heating rate of 10oC min⫺1. All DOI 10.1002/pen

FIG. 1.

XRD patterns for prepared ZHS crystallites.

runs were performed in a nitrogen atmosphere at a flow rate of 50 ml min⫺1. Fire Testing Limiting oxygen index (LOI) determination was performed according to ASTM D2863. Test specimens of dimensions 100 ⫻ 6.5 ⫻ 3 mm3 were cut from pressed plates. UL-94 vertical burning tests were performed with a plastic sample of dimensions 130 ⫻ 13 ⫻ 3 mm3, suspended vertically above a cotton patch. The classifications are defined according to the American National Standard UL-94. The cone calorimeter experiments were carried out using a Redcroft cone calorimeter following the procedure defined in ISO 5660, on 3-mm-thick 100 ⫻ 100 mm2 plaques at a heat flux of 35 kW m⫺2. The quoted data are averaged from

FIG. 3. XRD patterns for prepared ZHS-coated CaCO3. The peaks marked with black square are the three strongest peaks of MH; the peaks marked with black spot are the three strongest peaks of ZHS.

three independent plaques, and the cone calorimeter data obtained were reproducible to within ⫾5%. RESULTS AND DISCUSSION Structure and Thermal Decomposition Behavior of ZHScoated MH and ZHS-coated CaCO3 The synthesized ZHS- and ZHS-coated MH and ZHScoated CaCO3 are all characterized by XRD (Figs. 1–3). The powder diffraction pattern of this product (Table 2) allows complete indexing consistent with the space group. The calculated lattice constants and the distribution of the diffraction line are in good agreement with the diffraction patterns of other ilmenite-like phases with reliably established composition and structure [16]. From the XRD patterns, we know that the prepared ZHS crystallites are relatively pure and perfect, when we make use of the same method to prepare ZHS-coated MH or ZHS-coated CaCO3, there is no evident interaction to cause the purity and structure change of the above-mentioned TABLE 2.

FIG. 2. XRD patterns for prepared ZHS-coated MH. The peaks marked with black square are the three strongest peaks of MH; the peaks marked with black spot are the three strongest peaks of ZHS.

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XRD pattern of ZnSn(OH)6.

No.

d (obs)

D (calc)

I/Imax (100)

h

k

l

1 2 3 4 5 6 7 8 9 10 11 12

4.5033 3.9000 2.7577 2.2665 2.3517 2.2516 2.0846 1.9500 1.7894 1.7441 1.5921 1.5011

4.4838 3.8833 2.7385 2.4487 2.3324 2.2325 2.0713 1.9372 1.7776 1.7338 1.5826 1.4924

14 100 65 11 10 26 3 22 3 61 45 3

1 2 2 0 3 2 3 4 3 0 4 5

1 0 2 1 1 2 2 0 3 2 2 1

1 0 0 3 1 2 1 0 1 4 2 1


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three inorganic compounds. The TEM images testified this conclusion (Fig. 4). The TEM images show that the prepared ZHS crystals on the surface of MH or CaCO3 particles do not grow into perfect cubes; the majority of the crystallites have distinct faces and smeared edges, while the smaller ones have a globular shape (Fig. 4a and 4b). MH and CaCO3 used in this work are sheets with the order 5– 8 ␮m (see Fig. 4c– 4f) and PKsp value are 10.74 and 8.54 respectively. In the process of core–shell forming, MH and CaCO3 sheets are the core and Zn2⫹ and Sn4⫹ capture the

FIG. 5.

The TGA and DSC curves for ZHS-coated MH.

OH⫺1 (from NaOH) and form ZnSn(OH)6, which deposits at the surface of the sheets to form a shell. From Fig. 4c and 4d, we can find that the shell of MH-core is asymmetrical, and lager of ZHS particles bunch up, but the shell of CaCO3-core is symmetrical. The theoretical pyrolysis temperature of ZHS is higher than 200oC but lower than 280oC [17]. There is a thermal decomposition peak at about 260oC (Fig. 5 and Fig. 6). The data confirm that the shell is ZHS. Figures 7 and 8 show the TGA curves of pure inorganic fillers, coated inorganic fillers, and physical blend of ZHS and inorganic fillers. The TG curves show that the first degradation stages (thermal decomposition of ZHS) of physical blend of ZHS and MH or CaCO3 take place at lower temperature and without any clear plateau to separate from the second degradation stages (thermal decomposition of MH or CaCO3). Perhaps, vigorous shearing strength in the blending process induced strong interaction between ZHS and MH or CaCO3 particles and changed their physical properties.

FIG. 4. The TEM image of prepared inorganic fillers. (a) and (b) TEM images of prepared pure ZHS; (c) and (d) TEM images of ZHS-coated MH; (e) and (f) TEM images of ZHS-coated CaCO3.

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FIG. 6.

The TGA and DSC curves for ZHS-coated CaCO3.

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FIG. 7. TGA curves of pure MH, ZHS coated MH particles, and physical blend of ZHS and MH particles.

Thermal Degradation of PVC Formulations The thermal decomposition behavior of PVC/inorganic fillers will be discussed and compared with that of bulk PVC. Three parallel tests for each sample were carried out under the same heating rate. The average deviation of temperature is within 1.5oC, and the average deviation of yield of charred residue is within 1.2%. The TGA curves are shown in Fig. 9. The 5% loss temperature (T⫺5%) and the 50% weight-loss temperature (T⫺50%) and residue at 750°C are listed in Table 3. In the TGA curves, we can see that the main step of PVC degradation is from 250 to 320oC, attributed to the main chain reaction leaving behind conjugated polyene sequences in the polymer. The second step occurs about 430oC, and it may be assigned to the degradation of the carbonaceous residue formed during the first step [18, 19]. All the char residues at 750oC of the PVC formu-

FIG. 9.

lations containing ZHS and MH or CaCO3 are increased. The T⫺5% temperatures of PVC3 and PVC5 (containing physical blend of ZHS and MH or CaCO3) are reduced, because the metals in ZHS attract chlorine, weaken the COCl bonds in PVC, and hinder benzene formation by forming chloride with HCl [20, 21]. But, the T⫺5% temperatures of PVC1 and PVC4 (containing ZHS coated MH or CaCO3) are increased. Fire Properties Unmodified PVC is self-extinguishing and will not undergo sustained combustion because of its high chlorine content (58.7%). However, organic plasticizers, which are commonly added to promote flexibility and processing, make PVC combustible. When PVC plaque was burned in cone calorimeter, it extinguished after the first ignition; after a second ignition, there was a flash ignition and burn out. The reduction of the heat release rate (HRR), especially the peak HRR measured by cone calorimeter, has been found to be the most convincing evidence of the efficiency of a flame retardant. Figure 10 shows the changes in the HRR for the PVC cable formulations with 15 wt% ZHScoated inorganic fillers. The corresponding cone calorimeter data are presented in Table 3. Although each of the fillers reduces HRR to some extent, the presence of a ZHS coating on the filler results in further reductions. There is a 34% reduction in the peak HRR for the PVC3 sample containing TABLE 3. Compound

FIG. 8. TGA curves of pure CaCO3, ZHS coated CaCO3 particles, and physical blend of ZHS and CaCO3 particles.

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TGA curves of used PVC cable formulations.


The average TGA results of used PVC formulations. T⫺5% (°C)

T⫺50% (°C)

Residue at 750°C (%)

255.5 263.3 253.2 263.0 253.2

288.8 312.1 308.1 307.4 298.7

26.1 29.6 28.5 27.5 31.1


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FIG. 10. HRR curves for PVC cable formulations. PVC0, control sample; PVC1, coated MH; PVC2, physical blend of ZHS and MH; PVC3, coated CaCO3; PVC4, physical blend of ZHS and CaCO3.

15 wt% ZHS-coated CaCO3, compared with that for pure PVC0 containing 2 wt% Pbst stabilizer only, whereas the reduction is 26, 20, and 19% for the PVC1, PVC2, and PVC4 samples, respectively. Meanwhile, the flame parameters such as the specific extinction area (SEA) (a measure of the smoke yield) decreased significantly with the use of ZHS-coated inorganic fillers. As seen in Table 4, UL-94 V-0 was achieved at 15 wt% loading of all kinds of inorganic fillers, and in each case, it is clearly evident that the presence of the ZHS coating dramatically increases LOI. The LOI value of PVC3 (containing ZHS-coated CaCO3) is high up to 33.7. These results show that the flame-retardant efficiency of ZHS-coated fillers are always better than that of uncoated fillers just automatically mixed together. Furthermore, ZHScoated CaCO3 have better efficiency for PVC cable formulations in reducing the value of HRR than that of ZHScoated MH. The primary parameter responsible for the decreased HRR of the composites is the mass loss rate (MLR) during combustion, which was significantly reduced compared with those values observed for pure PVC (just containing stabilizers) (Fig. 10 and Fig. 11). These MLR data essentially mirror the HRR data. The consistency of the HRR and the MLR confirms that the flame-retardant mech-

FIG. 11. MLR curves for PVC cable formulations. PVC0, control sample; PVC1, coated MH; PVC2, physical blend of ZHS and MH; PVC3, coated CaCO3; PVC4, physical blend of ZHS and CaCO3.

anism of the inorganic fillers depends on a condensed-phase process. This observation is consistent with the previously reported work, which highlights the condensed phase charpromoting activity of ZHS in chlorinated polymers [3, 7]. The TEM morphology shows that the shell of MH-core is asymmetrical, and lager of ZHS particles bunch up, leading to part of surface of MH being exposed, but the shell of CaCO3-core is symmetrical, and the surface of CaCO3 sheet are all packed by a even ZHS layer, as shown in Fig. 12. When the coated fillers are used into PVC cable formulations, the interaction effect between PVC polymer matrix and ZHS-coated MH includes two kind: one kind is the interaction between PVC matrix and MH surface, the other is the interaction between PVC matrix and ZHS surface; whereas, the interaction effect between PVC polymer matrix and ZHS-coated CaCO3 just one kind, i.e., the interaction between PVC matrix and ZHS surface. And, the even coating of active ZHS on CaCO3 particles’ surface improved CaCO3 particles’ dispersion in the polymer matrix; at the same time, CaCO3 has higher thermal decomposition temperature, and so the oxygen-displacing protective layer formed by CaCO3 are stable than that by MH and results in further flame-retardant protection.

TABLE 4. Flammability performance of the used flexible PVC cable formulations. Sample code

LOI (vol%)

UL 94 testing

Peak HRR (kW m2)

Peak SEA (m2 kg⫺1)

Char yield (%)


27.4 31.6 28.8 33.7 28.3

V1 V0 V0 V0 V0

177.7 132.3 141.7 117.6 143.6

1237.6 835.9 1027.4 959.6 1041.4

13.4 25.6 19.1 29.9 19.2

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FIG. 12. Scheme of the prepared ZHS-coated inorganic fillers. (a) ZHScoated MH; (b) ZHS-coated CaCO3.

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CONCLUSIONS 4⫹

The preparation of ZHS-coated inorganic fillers by Sn and Zn2⫹ capturing OH⫺1 (from NaOH) in solution directly and coprecipitating on the surface of MH or CaCO3 was successful. The flammability characteristics of ZHS-coated MH and ZHS-coated CaCO3 used in rigid PVC systems have been investigated. The coated inorganic fillers not only reduce the HRR but also suppress the emission of smoke, and compare with uncoated fillers at the same addition level; they have a more efficient effect. The flame retardance induced by ZHS-coated CaCO3 is more effective than that induced by ZHS-coated MH, which promotes the formation of char in the condensed phase and provokes the extinguishing of the flame in the gas phase, especially induced a 34% reduction in the peak HRR.

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