Polymer Degradation and Stability 91 (2006) 3237e3244www.elsevier.com/locate/polydegstab

The effect of Zn, Al layered double hydroxide on thermaldecomposition of poly(vinyl chloride)

Zhi Ping Xu a,*, Susanta K. Saha b, Paul S. Braterman b,c, Nandika D’Souza c,*

a ARC Centre for Functional Nanomaterials, School of Engineering, University of Queensland, Brisbane, QLD 4072, Australiab Department of Chemistry, University of North Texas, P.O. Box 305070, Denton, TX 76203, USA

c Department of Materials Science and Engineering, University of North Texas, Denton, TX 76203, USA

Received 4 April 2006; received in revised form 22 June 2006; accepted 12 July 2006

Available online 14 September 2006

Abstract

Poly(vinyl chloride)/layered double hydroxide (LDH) composite was prepared by mixing 4 wt% Zn2AleCO3eLDH with PVC and fluxing at180 �C. The thermal decomposition behaviour of the LDHþ PVC composite in air and nitrogen environments was systematically investigated.We found that mixing Zn2AleCO3eLDH into PVC facilitates dehydrochlorination from ca. 300 to 270 �C but reduces the reaction extent toleave more chlorine on the polyene backbones both in air and N2. We have also found that at 400e550 �C, both in air and N2, LDH assiststhe formation of char-like materials and decreases the release of volatile hydrocarbons. From 550 to 800 �C, the char-like materials are mostlyretained in N2 while they are almost completely thermo-oxidized (burned) in air. Thus, addition of Zn2AleCO3eLDH to PVC does not increasethe thermal stability, but does promote charring to retard the generation of flame. The influence of LDH on PVC thermal properties has been alsoaddressed mechanically.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Layered double hydroxide (LDH); Polymer composite; PVC additive; Thermal stability; Flame retardant

1. Introduction

Poly(vinyl chloride) (PVC) is a widely used thermoplasticpolymer and finds applications in many fields, such as waterpipes, floor and roof tiles, packing films and sheets becauseof its easy processing and good mechanical properties [1].PVC contains 56.8% chlorine and thus shows high ignition-resistance and good flame retardancy. It is the chlorine, however,that makes PVC thermally and photochemically unstable bytaking part in an autocatalytic dehydrochlorination reactionunder heating and UV-light, which deteriorates PVC proper-ties [2,3]. Fortunately, a number of recent publications revealthat the thermal and photochemical stabilities of PVC can bereinforced by introducing various kinds of additives, such as

* Corresponding authors. Tel.: þ61 7 33469973; fax: þ61 7 33656074

(Z.P.X.). Tel.: þ1 940 565 2979 (N.D.).

E-mail addresses: zhipingx@cheque.uq.edu.au (Z.P. Xu), ndsouza@unt.

edu (N. D’Souza).

0141-3910/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymdegradstab.2006.07.006

metals, metal oxides, metal chlorides, metal hydroxides, metalcarboxylates, calcium carbonate, and clay materials [1e11].Among these additives, layered double hydroxides (LDHs),also known as anion-exchanging minerals, have attractedmuch attention in the recent search for efficient additives toenhance PVC thermal stability [8e11].

LDHs are a group of anion-exchanging materials contain-ing mixed metal hydroxide layers similar to those of brucite,Mg(OH)2. In these hydroxide layers, typically, up to one-thirdof divalent cations (M2þ: Mg, Zn, Ni, Co, Fe, etc.) can bereplaced by trivalent cations (M3þ: Al, Fe, Cr, etc.) [12e14].The resulting positive charges in the hydroxide layers due tothis replacement are balanced by exchangeable anions in theinterlayer space where there are typically some water mole-cules hydrogen-bonded to the anions and/or hydroxide groups.Thus LDHs can be described by the general formula[M2þ

1�xM3þ

x(OH)2]xþ[(Am�)x/m$nH2O]x�, where Am� repre-sents the exchangeable anion, such as CO3

2�, Cl�, SO42� and

various organic carboxylates, sulfates and sulfonates. Fischer

3238 Z.P. Xu et al. / Polymer Degradation and Stability 91 (2006) 3237e3244

et al. [9] found the considerable capability of HCl absorptionof Mg3ZnAl2eLDHs and their heat stabilisation effect onPVC. Wang and Zhang [10] and Lin et al. [11] reported signif-icant improvement of PVC thermal stability by incorporatingMgAleLDHeCO3 into PVC matrices, together with otherorganic salt stabilisers, such as Ca, Zn and Sn stearates.

Therefore, on the basis that Al(OH)3 and ZnCl2 are known aseffective PVC additives [2] and that the PVC thermal stabilitycan be reinforced by various LDHs, a Zn2AleLDHeCO3

compound was prepared in the current research to investigatethe overall influence on the PVC thermal stability and charformation. Prior to being mixed with PVC, the hydrophilicZn2AleLDHeCO3 was made hydrophobic by coating witha small amount of surfactant anion, i.e. oleate [15], which isexpected to provide some synergy between the hydrophobicPVC and the hydrophilic carbonate LDH. The thermal decom-position behaviour of PVC with 4 wt% LDH was then system-atically examined by TGA under nitrogen and air, andcompared with that of pure PVC to understand the effect ofthe additive LDH.

2. Experimental

2.1. Preparation of Zn2AleLDHeCO3 coated with oleate

Zn2AleLDHeCO3 coated with oleate (cis-CH3(CH2)7

CH]CH(CH2)7COO�) was prepared as follows: 10 mmolof potassium oleate (8.0 g, 40% paste, Aldrich), 50 mmol ofNa2CO3 (Fisher Scientific, 99.8%) and 600 mmol of NaOH(31.4 mL, 50% NaOH solution, Alfa Caesar) were dissolvedin 18.2 Megohm Millipore deionized water (1.0 L) with gentleheating. Then a mixed salt solution containing 100 mmol ofAlCl3$6H2O (24.14 g, Aldrich, 99%) and 200 mmol of ZnCl2(27.20 g, Aldrich, 99%) was added slowly at room tempera-ture to the above alkaline solution under vigorous stirring,and aged at 90e95 �C with stirring for 2 h. After the mixturewas cooled, precipitates were collected and thoroughlywashed with deionized water via centrifugation, and then driedin an oven at 70 �C for 2 days.

2.2. PVCþ LDH sample preparation

An electrically heated and air-cooled 250 mL Brabender batchmixer with sigma blades was preheated to 180 �C. Rigid poly(vi-nyl chloride) (PVC) pellets (240 g, Geon 8700A natural, Aldrich)were added and fluxed at 30 rpm. Oleate-coated Zn2AleCO3eLDH (10 g) was gradually added, followed by continuous mixingfor 2e3 min to ensure adequate mixing. The composite wasremoved from the mixing bowl and granulated. Ninety gramsof granules were compressed in a heated Carver press andmoulded at 190 �C into 50 mm� 50 mm� 4 mm plaques.

2.3. Materials characterization

Powder X-ray diffraction patterns were collected on a pow-der X-ray diffractometer (XRD, Siemens F-series) with Cu Ka

(l¼ 0.15418 nm) at a scanning rate of 1.2�/min from 2q¼ 2�

to 2q¼ 70�. Powdered silicon was used as an internal cali-brant, and the d-spacing was calculated by applying the for-mula d¼ (d003þ 2d006)/2 to the (003) and (006) basalreflections. Infrared spectra were collected using KBr discson a PerkineElmer 1760X FTIR, combining 40 scans within4000e400 cm�1 at a resolution of 4 cm�1.

2.4. Thermogravimetric analysis (TGA)

Thermal decomposition studies were conducted on a Per-kineElmer Pyris TGA 6 analyzer under both nitrogen andair. The gas flow rate was fixed at 100 mL/min and the temper-ature was calibrated with calcium oxalate. The sample used ineach run was ca. 20 mg loaded on an Al2O3 crucible and theheating rate was varied at 5, 10, 20, 30, and 40 �C/min from30 to 800 �C. The differential TGA (DrTGA) curve wasderived from the sample weight percentage with respect tothe heating temperature.

3. Results and discussion

3.1. Physical features and composition of LDH

The LDH structure was identified as that of carbonate by itsX-ray diffraction pattern, in particular by its basal reflections(00l), as shown by curve A in Fig. 1. The Miller indices areindicated according to the conventional 3R symmetry, togetherwith the corresponding distance. The average interlayer

0 20 40 60 802 Theta (degree)

Relative in

ten

sity (a.u

.)

A

B

C

003

(0.7

66 n

m)

006

(0.3

80 n

m)

012

(0.2

60 n

m)

015

(0.2

30 n

m)

018

(0.1

95 n

m)

0,1,10 (0

.173

nm

)

0,1,11 (0

.163

nm

)

110

(0.1

54 n

m)

11

3 (0

.151

nm

)

D

Fig. 1. XRD patterns of LDH (A), PVC (B) and PVCþ LDH (C). Curve D was

compounded with 4% curve A and 96% curve B.

3239Z.P. Xu et al. / Polymer Degradation and Stability 91 (2006) 3237e3244

(0.763 nm) and lattice parameter a (0.308 nm) are in goodagreement with literature values for LDH carbonate [12e14]. The nominal particle thickness was estimated to be 30e40 nm, corresponding to 40e50 hydroxide layers, from themeasured widths of peaks (003) and (006) using theDebyeeScherrer equation [16].

The FTIR spectrum (Fig. 2A) further confirms the typicalphase of Zn2Al-layered double hydroxides. For example, thereare a broad band at 3440 cm�1 (nOH), a shoulder at around3000 cm�1 (CO3

2�-induced H-bonded OH stretching vibra-tion), a strong peak at 1363 cm�1 (n3 of CO3

2�) and bands at779 ðdOH�H2OÞ, 625, 553 and 430 (sharp and strong) cm�1

(MO vibrations and MOH bending) [17]. On the other hand,it is clear that oleate is adsorbed, as indicated by the character-istic CH2 stretching vibrations at 2924 (asymmetrical) and2853 cm�1 (symmetrical) and the corresponding COO�

stretching vibration at 1559 cm�1 [18] that overlaps with thebroad bending band of water, normally broad and featureless,centered at 1620 cm�1. The oleate anions are presumably ad-sorbed on the crystallite surface with the hydrophobic tailspointing outwards, because the XRD patterns do not exhibitany traces of diffractions due to the known oleate-intercalatedZn2AleLDH [18]. Thus the surface-adsorbed oleate in theLDH is well placed to provide a good linkage between the hy-drophilic LDH and the hydrophobic PVC when they are mixedinto a composite.

If the external (ab) surfaces are neutralized only by oleate,the absorption on the (ac) and (bc) faces is neglected, and

01000200030004000

Wavenumber (cm -1

)

Ab

so

rb

an

ce (a.u

.)

A

B

C

3440

2924

285

3

1559

1363

779

533

430

1242

1426

2850

2917

3480

Fig. 2. FTIR spectra of LDH (A), PVC (B) and PVCþ LDH (C).

each LDH crystallite consists of 40e50 hydroxide layers,it is expected that there would be 2e2.5% moles oleate permole Al in the LDH. This speculation is well matchedwith the nominal chemical formula Zn2Al(OH)6(CO3)0.48

(oleate)0.04$2H2O where [oleate]/Al ratio is 0.04. This for-mula leads to a 63.6% oxide residue, in good agreementwith the observed value of 62.4% at 800 �C found by TGAin air.

3.2. PVCþ LDH composite

When 4 wt% LDH was mixed into PVC matrix, the com-posite seems not much different from pure PVC physically.As can be noted in Fig. 1, the XRD pattern of the PVCþ LDHcomposite (curve C) does not show any traces of LDH phase.The absence of LDH phase in the PVCþ LDH compositeseems to indicate the ‘dissolution’ of LDH phase in the PVCmatrices, implying some degrees of LDH delamination inthe composite. This is because if no delamination occurs,the XRD pattern of physically mixed 4 wt% LDH and 96%PVC composite should be similar to curve D in Fig. 1 that con-tains obvious diffraction peaks of LDH. LDH delaminationis supposed to occur during the mixing of LDH and PVC at180 �C, similar to montmorillonite delamination when mixingwith polymer under the similar conditions [19]. Such mixingof LDH with PVC leads to the changes of thermal properties,as described below. In addition, the infrared spectrum is basi-cally identical to that of pure PVC (curve C vs. curve B inFig. 2).

3.3. Decomposition pathways of PVC

The TGA and DrTGA of pure PVC in N2 and air are shownin Fig. 3. There are two major stages for the weight loss duringthe heating in N2. The first weight loss starts at 270 �C, rea-ches a maximum rate at 299 �C, and ends at about 360 �C,with a total weight loss of 55.5% at 385 �C, at a heatingrate of 10 �C/min. The average weight loss at different heatingrates is 55.7% in this stage (Table 1). An oxidizing environ-ment (air) seems not to affect this decomposition behaviour.As shown in Fig. 3A, the two weight loss curves are almostsuperimposable at temperatures below 400 �C, leading to thesame weight loss (55.7% on average, Table 1). A detailed ex-amination of the DrTGA curves (Fig. 3B) reveals that there isa minor step on both sides, at 280 and 340 �C.

The second stage of weight loss spans a wide temperaturerange, mainly from 400 to 560 �C, but continues up to800 �C, resulting in a total average weight loss of 29.5% at800 �C in N2. The reaction in air, however, seems to involvemore individual steps and leads to more extensive weightloss (39.3%) at 800 �C (Table 1). A black residue is formedin both cases, but in air it contains only 5% of the originalweight, as opposed to 14.8% in nitrogen.

These results are consistent with many earlier studies[2,3,20e24] that have shown that the thermal decompositionof PVC undergoes these two stages of weight loss in nitrogen.

3240 Z.P. Xu et al. / Polymer Degradation and Stability 91 (2006) 3237e3244

The first stage is mainly attributed to dehydrochlorination withsubsequent formation of conjugated polyenes:

H

ClH

H

n

- HCl

nH

H(1).

It is suggested that the minor lower-temperature event at ca.280 �C is an initial reaction of dehydrochlorination whichprobably autoaccelerates the evolution of HCl and promotesitself to the maximum (299 �C) via two mechanisms [2,6]:(1) the initial formation of a double bond activates the subse-quent loss of HCl from an adjacent unit to form a conjugatedbond system; (2) the primary HCl molecule participates in theformation of a dehydrochlorination transition state which leadsto the formation of another HCl molecule. Ideally, the weightloss should be 58.4%, but the practical weight loss from our

(A)

0

20

40

60

80

100

120

0 200 400 600 800 1000Temperature (degree C)

Weig

ht p

ercen

tag

e

in Nitrogen

in Air

(B)

0 200 400 600 800 1000Temperature (degree C)

Relative w

eig

ht lo

ss rate (a.u

.)

in Nitrogen

in Air

299

299

472

460550

340

340

280

Fig. 3. TGA (A) and DrTGA (B) of pure PVC in N2 and air at a heating rate of

10 �C/min.

TGA data is 55.7% under both N2 and air (Table 1), indicatingthat ca. 5% chlorine is retained in the polyene backbones.

The second stage involves the scission of polyene se-quences by cracking, cross-linking, aromatization, dehydro-carbonation and charring in N2, evolving a wide range ofhydrocarbons, such as benzene, toluene, xylene, naphthalene,ethylbenzene, styrene, 1-butene, butane, pentane, and hexanewhile forming char-like materials [2,6]. Cheng and Liang [6]have reported that the evolution of a wide range of volatile hy-drocarbons starts at as low as 250 �C, and gives rise to a shoul-der at 340 �C in the DrTGA (Fig. 3B) [24]. The residue at800 �C is carbonaceous char, as IR does not show any CHvibrations at 2800e3000 cm�1. In air, however, some kindsof oxidation reactions are supposed to take place to produceCO2, CO, H2O and oxygen-containing organic compounds[2,25,26], resulting in a bump after 500 �C in the DrTGA(Fig. 3B). The oxidation burns off some char, and thus givesmuch less residue at 800 �C in air than in N2 (Fig. 3A).

3.4. Decomposition of LDHþ PVC composite

The TGA and DrTGA of LDHþ PVC composite in N2 andair are displayed in Fig. 4. In N2, weight loss similarly occursin two stages. The first stage starts at 260 �C, reaches a maxi-mum loss rate at 271 �C, and ends at about 300 �C, witha weight loss of 48.4% at 340 �C. This is much smaller thanthat in pure PVC (55.7%). The second stage mainly occursat 400e500 �C and continues to 800 �C, with an averageweight loss of 23.8%, also much smaller than that of purePVC (28.9%, Table 1).

In an oxidizing environment, the added LDH does notcause any major change for the first stage (weight loss of

Table 1

Weight loss (%) of PVC with/without LDH in N2 and air at different heating

rates

Heating rate

(�C/min)

PVC without LDH PVC with LDH (4 wt%)

W1 (T �C)a W2 (800 �C) W1 (T �C)a W2 (800 �C)

In N2

5 55.8 (380) 28.9 48.0 (330) 23.6

10 55.5 (385) 29.5 48.4 (340) 23.7

20 55.6 (400) 28.9 48.4 (350) 23.9

30 55.8 (415) 30.2 48.3 (370) 24.1

40 56.0 (425) 29.9 49.2 (390) 23.7

Averageb 55.7 29.5 48.5 23.8

Residuec 14.8 Residuec 27.7

In air

5 55.1 (380) 42.1 48.7 (330) 47.6

10 55.2 (385) 41.0 48.8 (340) 47.0

20 55.7 (400) 39.3 49.2 (350) 45.9

30 56.0 (415) 37.1 49.4 (370) 44.6

40 56.4 (425) 36.9 49.8 (390) 43.6

Averageb 55.7 39.3 49.1 45.5

Residuec 5.0 Residuec 5.4

a The weight loss (W1) was read at the temperature indicated in the

parenthesis.b The average weight loss was calculated from the five values at five heating

rates.c The residue (%)¼ 100 - W1 (Average) - W2 (Average).

3241Z.P. Xu et al. / Polymer Degradation and Stability 91 (2006) 3237e3244

49.1% with the peak temperature at 270 �C), but slightly de-lays the second stage when compared with pure PVC in airand composite material in N2 (Figs. 3B and 4B). The mostprominent effect of LDH in air is that the bump observed inpure PVC around 550 �C becomes much more pronounced(Fig. 4B). The average weight loss in the second stage is45.5%, more than that of pure PVC in air (39.3%), but the totalloss is close to 95% in both cases.

3.5. Effect of LDH on dehydrochlorination of PVC

As described above, mixing 4 wt% LDH into PVC matrixfacilitates PVC dehydrochlorination. Fig. 5 clearly shows themovement of the DrTGA peak from 299 to 271 �C when4 wt% LDH was mixed. Fig. 5 also shows that dehydrochlori-nation of LDHþ PVC composite occurs within a much nar-rower temperature range (260e300 �C). Fig. 5 further showsthat the initialization of dehydrochlorination of pure PVC is

(A)

0

20

40

60

80

100

120

0 200 400 600 800 1000Temperature (degree C)

Weig

ht p

ercen

tag

e

in Nitrogen

in Air

(B)

0 200 400 600 800 1000Temperature (degree C)

Relative w

eig

ht lo

ss rate (a.u

.)

in Air

in Nitrogen

271

270

470

466 553

Fig. 4. TGA (A) and DrTGA (B) of PVCþ LDH composite in N2 and air at

a heating rate of 10 �C/min.

spread out over the range from 250 to 285 �C while dehydro-chlorination of LDHþ PVC composite reaches its maximumrate almost immediately after initialization. The facilitationof PVC dehydrochlorination by LDH probably occurs throughcatalysis by chloride salts formed by reaction of the evolvedacid [8,9], schematically:

Zn2AlðOHÞ6ðCO3Þ0:48ðoleateÞ0:04$2H2O þ 7HCl / 2ZnCl2

þ AlCl3 þ 0:04ðoleic acidÞ þ 0:48CO2ðgÞ þ 8:48H2OðgÞð2Þ

The ZnCl2 and/or AlCl3 formed in reaction (2) make the de-composition autocatalytic by promoting dehydrochlorinationimmediately after initialization, lowering the overall reactiontemperature and causing the reaction to go to completenessin a narrow temperature range. This facilitation effect seemssimilar to that reported by Cheng and Liang [6], who foundthat 1 mol% of ZnCl2 mixed into PVC matrix increases theweight loss from 4% to 26% at 250 �C. As summarized byLevchik and Weil [2], ZnCl2 is assumed to catalyticallyreceive chloride ion from PVC to give a carbonium ion. Thecarbonium ion decomposes to alkene by giving off a protonthat combines with immediate ZnCl3

� to release HCl andrecover ZnCl2. It is our suggestion that AlCl3, freshly formed,functions catalytically as a similar Lewis base (ZnCl2 in thecurrent case) and thus co-facilitates PVC dehydrochlorination.

Table 1 shows that the average weight loss of PVCþ LDH(48.5e49.1%) is much smaller than that of pure PVC (55.7%)at different heating rates both in air and N2. According to re-action (1), 96% PVC should result in a theoretical weight lossof 56.1% (58.4%� 96%) or a practical weight loss of 53.5%

0 200 400 600 800 1000Temperature (degree C)

Relative w

eig

ht lo

ss rate (a.u

.)

271

299

472

462

340

Fig. 5. DrTGA curves of PVC (bold line) and LDHþ PVC (thin line) in N2 at

a heating rate of 10 �C/min.

3242 Z.P. Xu et al. / Polymer Degradation and Stability 91 (2006) 3237e3244

(55.7%� 96%). According to reaction (2), absorption of HCland release of CO2 and H2O should add 0.8% mass to the res-idue. Thus the weight loss anticipated in practice for96%PVCþ 4%LDH composite is 52.7%. The observed valueof around 49% implies that in the presence of LDH, ca. 12%of the total organic chlorine is retained in the polyenebackbones.

In the dynamic TGA studies, we have found that the activa-tion energy is almost a constant when the weight loss is below50% (pure PVC) or 45% (LDHþ PVC), as shown in Fig. 6,being 90e100 kJ/mol [20,21,24,27]. There is no noticeabledifference in the activation energy between N2 and air inthis weight loss range. In general, the addition of LDH in-creases the activation energy by 5e15 kJ/mol at the reactionfraction of 0.2e0.45, relative to pure PVC. This seems incon-sistent with the fact that more chlorine is retained on the poly-ene backbones. Our explanation for this is that the facilitationof dehydrochlorination by LDH or its products leaves morechlorine randomly isolated on the polyene backbones or aromaticrings so that this chlorine is eliminated with more difficulty.

3.6. Effect of LDH on dehydrocarbonation

In pure PVC, as mentioned previously, dehydrocarbonationsets in immediately after dehydrochlorination, giving theshoulder at 340 �C (Figs. 3B and 5). Various volatile hydrocar-bons are detectable with GC analysis [6], presumably due tocross-linking and cracking of the freshly formed linear poly-enes into volatile aromatic species and more conjugated solidcomplexes [3,22,23]. The cross-linking is supposed to

0

50

100

150

200

250

300

350

0 0.2 0.4 0.6 0.8 1Reaction fraction

Activatio

n en

erg

y (kJ/m

ol)

Fig. 6. Relationship of activation energies [27] with thermal decomposition

progress of PVC (bold lines; filled square: in N2 and blank square: in air)

and PVCþ LDH (thin lines; filled triangle: in N2 and blank triangle: in air).

dominate within 370e430 �C since the release of volatile hy-drocarbons is not obvious. On the contrary, the obvious weightloss over 430 �C suggests that cracking to release volatilearomatics is predominate. Seemingly, dehydrocarbonation isslightly facilitated by the added LDH in N2 as the DrTGApeak moves from 472 to 462 �C (Fig. 5). However, the weightloss is much less for LDHþ PVC composite (23.8%) than forpure PVC (29.5%), and the char-like material (residue) at800 �C is almost doubled for composite (27.7%) as for purePVC (14.8%) (Table 1). As can be calculated, the retentionof carbon at 800 �C is 38.5% [¼14.8/(24.0/62.5)] for purePVC while this increases to 62.1% [¼(27.7�4.8 (metalchlorides))/(96%� 24.0/62.5)] for LDHþ PVC composite ifassuming that all the residues are carbonaceous as well asmetal chlorides. Therefore, dehydrocarbonation is retarded toa high degree by adding 4 wt% LDH into PVC matrix in

(A)

0 200 400 600 800 1000

0 200 400 600 800 1000

Temperature (degree C)

Relative w

eig

ht lo

ss rate (a.u

.)

(B)

Temperature (degree C)

Relative w

eig

ht lo

ss rate (a.u

.)

Fig. 7. DrTGA of PVC (A) and PVCþ LDH (B) at different heating rates in

air. The heating rate is 5, 10, 20, 30, and 40 �C/min, corresponding to the first

peak from left to right. Note that parts of the curves over 400 �C were artifi-

cially shifted up for clear comparison.

3243Z.P. Xu et al. / Polymer Degradation and Stability 91 (2006) 3237e3244

N2, as also indicated by the positive D at various temperaturesin Table 2. In our opinion, LDH or its products probably cat-alyze the cross-linking to densely aromatic complexes. Char-ring suppresses the cracking and releases less volatilehydrocarbons when a Zn based LDH is used, which is similarto the function of ZnCl2 in PVC [2,6]. We have further notedthat the residue char-like materials from pure PVC are verybrittle while those from PVCþ LDH composite show amuch higher hardness. The dynamic TGA studies reveal thatthe activation energy of dehydrocarbonation abruptly increasesto 200e250 kJ/mol [23,24], compared with 100 kJ/mol for de-hydrochlorination (Fig. 6) with or without LDH. This indicatesthat dehydrocarbonation becomes much more difficult.

In the air environment, the formation of char-like materialsis also promoted by LDH in the medium temperature range of400e550 �C. As shown in Figs. 4B and 7B, LDH obviouslyseparates dehydrocarbonation and/or oxidation into two dis-tinct processes. The first process (400e500 �C) has an activa-tion energy of ca. 250 kJ/mol, the same as that for pure PVC(Fig. 6) [21,24]. As listed in Table 2 with the positive D, theresidue of PVCþ LDH composite in 400e550 �C in air ismuch higher than that of pure PVC at the same temperature,indicating that more char-like materials are generated in thepresence of LDH in this process. It is also noted that the res-idue amount after this process (28.8% at 530 �C) is quite closeto that (27.7%) of PVCþ LDH in N2 at 800 �C. Thus, it issuggested that this process is dehydrocarbonation, accompa-nied by cross-linking and charring to form char-like materials,like in N2 [26]. Presumably, cross-linking and charring areenhanced while dehydrocarbonation is suppressed to some de-gree by LDH or its products, thus leading to a less weight lossin this process.

The second process, having a much smaller activationenergy (200e100 kJ/mol, Fig. 6), tentatively attributed to theoxidation [23,24,26], very quickly loses almost all char-likematerials in 550e800 �C, especially for LDHþ PVC compos-ite (Table 2). The derivatives of LDH during PVC decomposi-tion, i.e. the mixed chlorides, are suggested to catalyze thethermo-oxidation (combustion) of char-like materials in air

Table 2

Residue (%) in N2 and air at different temperatures at a ramp of 10 �C/min

Temp.

(�C)

In N2 In air

PVC LDHþ PVC Da PVC LDHþ PVC Da

400 43.8 49.1 5.3 44.4 49.6 5.2430 40.7 46.0 5.3 43.3 48.5 5.2

450 35.4 42.1 6.7 38.8 45.5 6.7

470 27.9 36.8 8.9 31.2 39.8 8.6490 20.7 33.1 12.4 25.8 35.1 9.3

510 17.6 31.7 14.1 22.7 32.2 9.5

530 16.7 31.0 14.3 19.5 28.8 9.3

550 16.4 30.5 14.1 16.4 21.6 5.2580 16.2 29.9 13.7 13.2 10.3 �2.9

600 16.0 29.6 13.6 11.8 6.6 �5.2

650 15.8 29.0 13.2 8.8 5.0 �3.8

700 15.5 28.6 13.1 6.7 4.8 �1.9

800 15.0 27.9 12.9 3.8 4.2 0.4

a D was calculated from the residue of LDHþ PVC minus that of PVC.

at temperature over 550 �C [28]. This may be the reasonwhy the residue of LDHþ PVC over 550 �C is less than thatof pure PVC, i.e. the negative D (Table 2).

4. Conclusions

In conclusion, mixing 4 wt% surfactant-treated Zn2AleCO3eLDH into PVC produces both positive and negativeeffects on the thermal stability of PVC to some degree. First,LDH facilitates dehydrochlorination at a lower temperature inboth N2 and air. However, it helps retain much more chlorinein the polyene backbones, which is good to retard the flameformation during heating. Secondly, LDH reduces the subse-quent dehydrocarbonation and helps to form more char-likecarbonaceous materials, particularly in N2, showing the poten-tial that Zn2AleCO3eLDH is a good fire/flame retardant. Fi-nally, LDH catalyzes the thermo-oxidation (or combustion) ofthe char-like materials in air at temperatures over 550 �C.

Acknowledgements

The authors thank the Robert A. Welch Foundation (GrantB-1445) and the University of North Texas Faculty ResearchFund, for support. The authors also thank Dr Amit Dharia,Transmit Technology Group for use of his Brabender andcompression press. Dr Xu appreciates the support from theARC Centre for Functional Nanomaterials funded by the Aus-tralia Research Council under its Centre of ExcellenceScheme.

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