PVC and PVC-VAc Nanocomposites: Negative Effects on Thermal

Chapter 7

PVC and PVC-VAc Nanocomposites: Negative Effects on Thermal Stability Downloaded by STANFORD UNIV GREEN LIBR on September 27, 2012 | http://pubs.acs.org Publication Date: November 24, 2005 | doi: 10.1021/bk-2006-0922.ch007

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Marco Zanetti , Simona Valesella, M a r i a Paola Luda, and Luigi Costa Università degli Studi di Torino, Dipartimento di Chimica IFM, Via P. Giuria 7, 10125 Torino, Italy Corresponding author: [email protected] *

Polymer layered silicate nanocomposites constitute a new class of materials with unique properties offering new technological and economic opportunities. The organic modification of clay, forming the so-called organoclay, opened the possibility to make nanocomposites with a wide range of polymers. However, in spite of the organic treatment of the clay, preparation of P V C nanocomposite via direct melt compounding still poses many problems due the rather low stability of P V C to the influence of heat. Intercalated PVC/organoclay and PVC-VAc/organoclay nanocomposites were prepared via direct melt compounding. The thermal degradation of P V C nanocomposites was studied in thermogravimetry where it was observed a destabilizing effect.

© 2006 American Chemical Society

In Fire and Polymers IV; Wilkie, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

Downloaded by STANFORD UNIV GREEN LIBR on September 27, 2012 | http://pubs.acs.org Publication Date: November 24, 2005 | doi: 10.1021/bk-2006-0922.ch007

76 Polymer layered silicate nanocomposites (PLSN) constitute a new class of materials with unique properties offering new technological and economic opportunities (1,2). In particular, the PLSNs demonstrated to be a promising material as far as the flame retardant. In literature many examples of flame retardant improvements due to the nanoscopic dispersion of clays in thermoplastic polymer are reported. It has been in fact assessed that polymers that normally bum fast without char formation (i.e. PP (3,4), PE (5,6) and EVA(7)) can bum slowly with char formation once they have been transformed in nanocomposites. It has been evidenced that the mechanism of flame retardant of PLSN acts essentially in condensed phase These papers show that the primary parameter responsible for the lower heat released rate (HRR) of the nanocomposites is the mass loss rate (MLR) during combustion, which is significantly reduced from those values observed for the pure polymer. This is due to the formation on the surface of a char-clay refractory material, which creates a protective shield for the polymer that slows down the flame feeding from the thermal decomposition in the nanocomposite. In the other hand a similar behavior is well known in the so-called char former polymer such as PVC. Exposed to heat, PVC eliminates the chlorine atoms as HC1 forming carbon carbon double bond ordered in polyenic sequences that evolve on heating to aromatized thermally stable charred structures through inter- and intra-molecular Diels-Alder reactions (8). Char forming reactions reduce the fuel feeding the flame and the residue formation creates a shield able to hinder the combustion cycle. In spite of this benefit, the diminishing of PVC flammability trough the nanocomposite formation represents an interesting challenge. The surface modification of clay with onium salt, forming the so-called organoclay, opened the possibility to make nanocomposites with a wide range of polymers. However, the limited thermal stability of alkyl ammonium cations and the processing instability of some polymers such as PVC pose many problems in the nanocomposite preparation via direct melt compounding as recently observed by Wan et al. (9). In this paper we report our studies on the thermal stability of PVC and PVCVAc nanocomposites prepared via melt blending with different organoclay. PVC is also one of the most widely used materials in electrical cable construction, especially in its flexible plasticized form, obtained by means of plasticizer. For this reason we studied also the thermal behavior of a flexible PVC formulation, containing epoxidized soybean oil as plasticizer. The nanocomposite formation has been verified by means of X-ray diffractometry (XRD). The thermal degradation behavior has been studied in thermogravimetry. 3,7


77

Experimental


Materials The polymers used were ETINOX 630 produced by Aiscondel S.A. (Spain), which is a polyvinylchloride obtained by suspension polymerization, with Re value 65 and SC5710 produced by EVC (Italy), which is a poly(vinylcloride-covinylacetate) obtained by suspension polymerization, with K-value 57 ad a VAc content of 10.5% wt. The processing additives used for PVC were Naftomix TGRX530, Onepack of lead stabilizer and lubricant produced by Chemson Polymer Additive (Germany), tribasic lead sulphate (TLS) and lubricant Realube RL/105, both produced by Reagens Spa. (Italy). In order to prepare plasticized PVC epoxidized soybean oil (ESO) produced by Reagens S.p.A. Italia and containing 6.2-6.4% of oxiranic oxygen, trade name: Reagens EP/6, has been used. As nanofiller were used: Somasif ME 100, Co-Op Ltd Japan (FH) which is sodium-exchanged fluorohectorite-like synthetic silicate; Somasif MAE, Co-Op Ltd Japan (FH/DT) which is FH exchanged with a dimethyl ditallow ammonium cation (tallow: containing 70, 25, 4, and 1 mol % of C18, C16, C14, and C12 carbon chains, respectively); Cloisite 20A, Southern Clay products Texas (MMT/DT), which is a montmorillonite exchanged with a dimethyl ditallow ammonium cation.

Compounding PVC/clay composite and PVC/organoclay nanocomposites were prepared via direct melt compounding using a twin-screw extruder MD-30 (Bausano & sons), tailoring the extrusion profile to avoid any thermal degradation of the nanocomposites. The components were first heated to 110 °C in a pre-mixer and than cooled to room temperature. The twin-screw extruder operated with 7 heating zone: 175°C, 170°C, 170°C, 165°C, 175°C 160°C, 100°C. The mixing time calculated by speed screw (20 R.p.m.) corresponds to 8 min. The characteristics and the names of the materials are illustrated in Table I.

Characterization The interlayer spacing of the clay was studied by means of wide angle X-ray scattering (WAXS) using a Philips diffractometer with Co K radiation (X= 0.179 nm). The WAXS patterns of the thin films of the hybrids were obtained. a


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The interlayer distance was determined by the diffraction peak, using the Bragg equation. Thermodegradation was determined on approx. 10 mg samples in a TGA 2950 balance (TA Inc.) with alumina sample pan in a 60 cmVmin nitrogen flow (gas chromatography purity 99.999%) and with a 10°C/min heating ramp. Thermo-oxidation was determined in the same way in 60 cmVmin airflow.

Table I. Characteristics and the names of the materials Abbreviation PVC PVC/FH

PVC/FH-DT

PVC/MMT-DT

PVC-VAc PVC-VAc/FH PVC-VAc/FH-DT

PVC-VAc/MMT-DT

PVC-ESO

PVC-ESO/MMT-DT

Composition (%wt.) ETINOX630 (94.3) + TGRX530 (2.8) + TLS(2.8) + RL/105(0.1) ETINOX630 (90) + FH (4.5) + TGRX530 (2.7) + TLS (2.7) + RL/105 (0.1) ETINOX630 (90) + FH/DT (4.5) + TGRX530 (2.7) + TLS (2.7) + RL/105 (0.1) ETINOX630 (90) + MMT/DT (4.5) + TGRX530 (2.7) + TLS (2.7) + RL/105 (0.1) SC5710 (94.3) + TGRX530 (2.8) + TLS (2.8) +RL/105 (0.1) SC5710 (90) + FH (4.5) + TGRX530 (2.7) + TLS (2.7) + RL/105 (0.1) SC5710 (90) + FH/DT (4.5) + TGRX530 (2.7) + TLS (2.7) + RL/105 (0.1) SC5710 (90)+MMT/DT(4.5)+TGRX530 (2.7)+TLS(2.7)+RL/105 (0.1) ETINOX630 (73)+ESO(21 )+TGRX530(3)+TLS(3) +RL/105(0.1) ETINOX630 (70) + ESO(20) + MMT-DT (4.5) + TGRX530 (2.7) + TLS (2.7) + RL/105 (0.1)

T

e

yP Stabilized polymer Microcomposite

Nanocomposite

Nanocomposite

Stabilized polymer Microcomposite Nanocomposite

Nanocomposite

Stabilized and plasticized polymer Nanocomposite



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1 2

, 4

,

, 6

,

, 8 2

,

, 10

,

, 12

,

, 14

,

en

Figure 1. Comparison between the WAXS of MMT-DTwith the relative composites, based on PVC (PVC/MMT-DT) and PVC-VAc (PVC-VAc/MMT-DT).

Results and Discussion Nanocomposite morphology In figure 1 is reported the comparison between the XRD patterns of MMT-DT, PVC/MMT-DT and PVC-VA/MMT-DT. Mixing the organoclay with the PVC the dooi interlayer spacing is increased from 2.44 nm to 3.84 nm indicating the formation of an intercalated nanocomposite. The peak around 2theta=9, corresponding to a d spacing of 0.98 nm, is due to the presence of tribasic lead sulphate (TLS). A smaller interlayer spacing (3.53nm) was obtained using the PVC-VAc as polymer matrix. The amount of intercalated polymer is reasonably the same and the lower distance between the clay lamina is due to the higher chain flexibility caused by the presence of vinyl acetate pendant group.


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3.68

I 2

•

1 4

•

1 6

> 1

•

8

2

1 10

•

1 12

•

r

-

14

e [•]

Figure 2. Comparison between the WAXS of FH-DTwith the relative composites, based on PVC (PVC/FH-DT) and PVC-VAc (PVC-VAc/FH-DT).

Intercalated nanocomposites were obtained even using FH-DT, as shown in figure 2 where the intercalation was reached with an increasing of the d spacing of the organoclay from 3.42 nm to 3.74 nm and 3.68 nm for PVC and PVC-VAc respectively. In absence of organic treatment the same fluorohectorite was not able to reach the intercalation of the polymers as shown in figure 3. The WAXS pattern of PVC/FH and PVC-VAc/FH show the same peaks of the FH indicating that the silicate dispersed in the polymer matrix retained the stacked structure of the pristine clay. The WAXS of figure 4 indicates the formation of an intercalated nanocomposite even in the case of plasticized PVC. The presence of the plasticizer increased the intercalation grade of the polymer shifting the d i interlayer spacing of the nanocomposite to 4.0 lnm. 00



81

I

2

'

1 ' 1 4 6

1.23

0.95

—i

^—i

8 10 2 9[°]

•

1 12

'

r-

14

Figure 3. Comparison between the WAXS of FH with the relative composites based on PVC (PVC/FH) andPVC-Vac (PVC-VAc/FH).

2 0[°] Figure 4. Comparison between the WAXS of MMT-DT and the relative composite withplasticizedPVC (PVC-ESO/MMT-DT).



82

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,

100

,

,

200

,

,

300

,

,

,

400

,

500

,

,

1

600

Temperature (°C)

Figure 5. TGA in nitrogen flow of PVC (a), PVC/FH (b), PVC/FE-DT (c) and PVC/MMT-DT (d).

Thermal degradation In Figure 5 the thermogravimetry curves (TGA) under nitrogen flow of PVC, of the microcomposite (PVC/FH) and of the two nanocomposites (PVC/FH-DT and PVC/MMT-DT) are reported. As can be seen the thermal degradation process takes place with two main weight loss steps. In its earliest stages, the thermal degradation of PVC involves the sequential loss of hydrogen chloride molecules accompanied by the generation of conjugated polyene sequences. The PVC/FH behaves as the pure polymer: the presence of the microdispersed FH did not change the thermal degradation pathway of PVC, with the exception of the amount of residue corresponding to the clay added. In the same figure is possible to see that both nanocomposites shows a strong effect of destabilization reducing of 50°C the onset temperature of HC1 elimination. After the complete elimination of HCl (above 305°C) the nanocomposites behaves as the microcomposite. The organic treatment of organoclay exhibits a limited thermal stability, as we observed in a precedent work (10). The thermal decomposition of alkyl ammonium salts is known to take place



83

i—i—i—i—i—i—i—i—i—i—i—i 100

200

300

400

500

600

Temperature (°C)

Figure 6. TGA in airflow of PVC (a), PVC/FH (b), PVC/FH-DT (c) and PVC/MMT-DT (d).

with the Hofmann mechanism (11) leading to volatilization of amine and the corresponding olefin. As result strong protonic catalytic sites are created on the layer of the clay. Both radical and molecular mechanisms have been proposed to explain the HC1 elimination reaction of PVC as recently reviewed by Starnes (12). It is well known that the thermal degradation is accelerated by the catalytic effect of evolving HC1 and by this point of view the protonated sites of the clay layers may act as acidic catalyst able to accelerate the HC1 loss. With the exception of the weight loss at 500°C due to the combustion of the carbonaceous residue, the TGA in air (figure 6) do not show differences with those performed under nitrogen flow. In literature the major effect of thermal stabilization of nanocomposites has been observed in air flow where the organoclay layers demonstrate to be very effective in decreasing the oxidative reaction trough a shielding effect as well enhancing the char form reaction. As observed previously, the negative effects of the organoclay on thermal stability of PVC occur at temperature lower than the thermal oxidation temperature



84

Temperature (°C)

Figure 7. TGA in nitrogen flow ofPVC-VA (a), PVC-VA/FH (b) PVC-VA/FH-DT(c) andPVC-VA/MMT-DT(d).

t

frustrating the possible beneficial effect of the organoclay. There is, indeed, just a minimal effect of stabilization in the nanocomposites above 450°C. This effect is reached even by the microcomposite (PVC/FH). In Figure 7 the TGA under nitrogen flow of PVC-VAc, of the microcomposite (PVC-VAc/FH) and of the two nanocomposites (PVC-VAc/FHDT and PVC-VAc/MMT-DT) are reported. The PVC-VAc is characterized by a lower thermal stability if compared with PVC. The electron effect of the chlorine activates the vinyl acetate group enhancing the elimination of acetic acid. As observed in the case of PVC, the microcomposite (curve b) behaves similar to the polymer matrix (curve a) leaving an increased amount of residue corresponding to the silicate added. The nanocomposites show an enhanced thermal instability starting the weight loss at lower temperature than the PVCVAc. The onset temperatures are located around 200°C and are close to those observed for the PVC nanocomposites (figure 5). Above 305° C the nanocomposites behaves as the microcomposite. Performing the thermogravimetry under air flow (figure 8) the behavior of copolymer samples is very similar to that observed for the homopolymer



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-I

100

1

1

200

1

1

300

1

1

400

1

1—

500

Temperature (°C)

Figured TGA in air flow of PVC-VA (a), PVC-VA/FH (b), PVC-VA/FH-DT (c) and P VC- VA/MMT-DT (d).

samples in the same conditions. Below 400°C the weight loss of all the samples is the same observed under nitrogen flow. Above 450°C all the sample are subjected to the chain breaking as well to combustion phenomena. The combustion is associated to a rapid weight loss evidenced by a sharp peak in the derivative TG curve. PVC-VAc shows the combustion peak at 475°C while the nanocomposites (PVC-VAc/FH-DT and PVC-VA/MMT-DT) show this peak at 525°C, indicating a stabilization effect against the combustion. The microcomposite (PVC-VAc/FH) seems to be even more stable showing a combustion peak at 540°C. Concerning the plasticized PVC, as can be seen in figure 9, the presence of ESO increased the thermal stability of PVC as indicated by the increasing of the onset temperature from 246°C to 261°C. The formation of an intercalated nanocomposite affected the thermal stability of the polymer diminishing the onset temperature to 240°C that is, however, higher than the onset temperature observed for the PVC and PVC-VAc nanocomposites (220°C). Heating the samples in air flow (figure 10), the behavior of the plasticized PVC nanocomposite as well the plasticized PVC is the same observed in nitrogen flow up to 450°C. Above this temperature the nanocomposite shows a small stabilization effect probably due to the hindered escape of the volatile products caused by the clay layers.



86

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Temperature (°C)

Figure 9. TGA in nitrogen flow of PVC plasticized with soy oil (PVC-ESO) and the plasticized PVC nanocomposite (PVC-ESO/MM-DT)

Temperature (°C)

Figure 10. TGA in air flow of PVC plasticized with soy oil (PVC-ESO) and the plasticized PVC nanocomposite (PVC-ESO/MM-DT).



87 Concerning the plasticized PVC, as can be seen in figure 9, the presence of ESO increased the thermal stability of PVC as indicated by the increasing of the onset temperature from 246°C to 261°C. The formation of an intercalated nanocomposite affected the thermal stability of the polymer diminishing the onset temperature to 240°C that is, however, higher than the onset temperature observed for the PVC and PVC-VAc nanocomposites (220°C). Heating the samples in air flow (figure 10), the behavior of the plasticized PVC nanocomposite as well the plasticized PVC is the same observed in nitrogen flow up to 450°C. Above this temperature the nanocomposite shows a small stabilization effect probably due to the hindered escape of the volatile products caused by the clay layers.

Conclusion Nanocomposites of PVC, PVC-VAc and plasticized PVC were prepared by melt compounding using a twin-screw extruder. As observed in WAXS analysis all the nanocomposites showed an intercalated morphology while the absence of an organic treatment of the clay did not lead to a nanocomposite formation. A destabilizing effect due to presence of the intercalated organoclay was observed in all cases in thermogravimetry. Polymer layered silicate nanocomposites have been demonstrated to posses enhanced properties compared to the virgin polymers. In the field of flame retardant the benefits of nanocomposites are usually associated to an enhanced stability to thermal oxidation in the temperatures range between 150 and 350°C. This statement seems to be inapplicable to PVC where the formation of a nanocomposite leaded to a lower thermal stability of the polymer. PVC is a char former polymer and the absence of advantages deriving from nanocomposite formation demonstrate that the improved thermal oxidative stability, observed for other non char former thermoplastic polymer, originates from a chemical effect on the thermooxidation pathway of the polymer matrix.

Acknowledgement Authors wish to thank Mr Eraldo Bausano and Dr. Riccardo Sangiorgio at Bausano & sons for their kind help and use their facilities.


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References


1 2

Giannelis, E. P. Adv. Mater. 1996, 8, 29-35. Zanetti, M.; Lomakin, S.; Camino, G. Macromol. Mater. Eng 2000, 279, 19. 3 Gilman, J. W.; Jackson, C. L.; Morgan, A. B.; Harris, R.; Manias, E.; Giannelis, E. P.; Wuthenow, M.; Hilton, D.; Phillips, S. H. Chem. Mater. 2000, 12, 1866-1873. 4 Zanetti, M.; Camino, G.; Canavese, D.; Morgan, A. B.; Lamelas, F. J.; Wilkie, C. Chem. Mater 2002, 14, 189-193. 5 Zhang Jinguo; Wilkie, C. A. Pol Deg Stab 2003, 80, 163-169. 6 Zanetti, M.; Costa, L. Polymer 2004, 45, 4367-4373. 7 Zanetti, M.; Kashiwagi, T.; Falqui, L.; Camino, G. Chem. Mater. 2002, 14, 881-887. 8 Starnes, W. H. In Developments in Polymer Degradation-3; Barking, Ed.; Applied Science Publisher:, 1981, p 152. 9 Wan, C.; Zhang, Y.; Zhang, Y. Polymer Testing 2004, 23, 299-306. 10 Zanetti, M.; Camino, G.; Reichert, P.; Mulhaupt, R. Macromol Rapid Comm 2001, 176-180. 11 Jhon March Advanced Organic Chemistry; McGraw-Hill Kogakusa Ltd: Tokyo, 1977. 12 Starnes Jr.W.H. Progress in polymer science 2002, 27, 2133-2170.


PVC and PVC-VAc Nanocomposites: Negative Effects on Thermal

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