Journal of Macromolecular Sciencew, Part B: Physics, 45:1135–1140, 2006 Copyright # Taylor & Francis Group, LLC ISSN 0022-2348 print/1525-609X online DOI: 10.1080/00222340600962650
Thermal Stabilities of Poly(Vinyl Chloride)/Calcium Carbonate (PVC/CaCO3) Composites PENG LIU, MINGFEI ZHAO, AND JINSHAN GUO Institute of Polymer Science and Engineering, College of Chemistry and Chemical Engineering, Lanzhou University, Gansu, People’s Republic of China Poly(vinyl chloride)/calcium carbonate (PVC/CaCO3) composites with micrometer or nanometer CaCO3 as fillers were prepared by the solution blending method. The thermogravimetric analysis (TGA) of the composite films conducted in N2 atmosphere showed that the addition of the CaCO3 fillers could improve their thermal stabilities. It was also found that the nanometer CaCO3 filler provided better thermal stabilities than the micrometer fillers even with a smaller amount. The mechanism of the improvements was investigated by a facile chemical analysis developed to examine the thermal stabilizing effect of calcium carbonate particles with different sizes in PVC/CaCO3 composites after the pyrolysis of the samples in an air atmosphere in an oven. Keywords poly(vinyl chloride)/calcium carbonate (PVC/CaCO3) composites, micrometer/nanometer filler, thermal stability
Introduction Poly(vinyl chloride) (PVC) is now one of the major thermoplastics in the world, with a large amount of PVC produced worldwide.[1] However, fluid plasticity and thermal stability of PVC are inferior to those of other commodity plastics such as polyethylene and polystyrene. Considerable improvement of the properties has been carried out by adding additives such as plasticizer, heat stabilizers, lubricants, fillers, and other polymers as well as copolymerization with other monomers.[1] It is well known that the thermal stabilities of polymeric materials can be improved by adding inorganic nanofillers.[2,3] PVC/inorganic nanocomposites based on silica,[4] titania,[5] layered silicate clays,[6 – 9] calcium carbonate,[10 – 13] ferromagnetic nanoparticles,[14] and vanadium pentoxide[15] have been reported previously. As active fillers that can react with HCl, calcium carbonate and metal hydroxides[16] or oxides[17] have attracted more attention. The HCl scavenging effect of particulate fillers such as Red Mud (RM), calcium carbonate (CaCO3), and dolomite on the thermal degradation of PVC was investigated by thermogravimetry/mass spectrometry (TG/MS)[18] and the degradation of a model polymer mixture (PVC/PS/PE) and a waste polymer mixture in Received 19 June 2006; Accepted 8 August 2006. Address correspondence to Peng Liu, Institute of Polymer Science and Engineering, College of Chemistry and Chemical Engineering, Lanzhou University, Gansu 730000, People’s Republic of China. E-mail:
[email protected]
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the presence of these HCl fixators was also studied using a cycled-spheres reactor.[19] The particle sizes and surface treatments of the CaCO3 particles on the interfacial structures, fusion, thermal, and mechanical properties of rigid PVC/CaCO3 composites were investigated.[20,21] In the present communication, a new method via facile chemical analysis was developed to examine the thermal stabilizing effect of CaCO3 particles with different sizes in PVC/CaCO3 composites.
Experimental Section Raw Materials The poly(vinyl chloride) (PVC) powders used were commercial products from Yanguoxia Chemical Factory, Gansu, China, and were used after being washed with ethanol for removal of the surface contamination and then being dried in vacuum at room temperature. The micrometer size CaCO3 particles (800 mesh and 3000 mesh) were obtained from Puyuan Chemical Co. Ltd. Hubei, China, and the CaCO3 nanoparticles (NPCC-300-SH, mean particle size: 40 nm) were obtained from Chaodong NanoMaterials Co. Ltd. Anhui, China. They were used without any pretreatment. Tetrahydrofuran (THF), sodium carbonate (Na2CO3), and other reagents used were all analytical reagent grade. Distilled water was used in all experiments.
Preparation of PVC/CaCO3 Composites The calculated amounts of CaCO3 particles were dispersed into 50 mL of THF via ultrasonic irradiation for 30 min. Then PVC powders were dissolved into the mixtures and the mixtures were stirred for another 20 min. The mixtures were poured into glass plates for the evaporation of the solvent in vacuum. The composite films with thickness of about 2 mm were obtained for the further experiments.
Thermal Properties Thermal degradation of the pure PVC and the PVC/CaCO3 composites were characterized by thermogravimetric analysis (TGA) performed with a Perkin-Elmer TGA-7 system (Perkin-Elmer Corporation, Norwalk, CT, USA) at a scan rate of 108C min21 to 8008C in N2 atmosphere. The mechanism of the improvements of the thermal stability of PVC was characterized by the following simple procedure: (1) Certain amounts of the PVC/CaCO3 composites were heated to 2808C at a rate of 108C min21 and then maintained for 30 min in air atmosphere. CaCO3 might absorb the HCl released during the thermal decomposition of PVC and CaCl2 was formed. (2) The pyrolysed samples were rinsed in small amounts of water for 24 h, three times, and the leachants were combined. The formed CaCl2 was dissolved and collected. (3) The saturated Na2CO3 aqueous solution was dropped into the leachants for the complete precipitation of Ca2þ as CaCO3. The precipitate was centrifuged and washed with water for removal the NaCl formed until the Cl2 ion could not be detected by AgNO3. The precipitates were dried in vacuum and weighed.
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Figure 1. TGA curves of the pure PVC and the PVC/CaCO3 composites.
Results and Discussion The TGA curves of the pure PVC and the PVC/CaCO3 composites are given in Fig. 1. The first thermal degradation onset temperature (Tonset) and the maximum thermal degradation temperature (Tmax), defined as the temperature at which the weight losing rate is maximum, are summarized in Table 1. The weight losses of the PVC/CaCO3 composite films at about 1508C are assigned to the release of the adsorbed solvent and moisture. The weight losses at about 3008C are assigned to hydrochloride acid (HCl) gas generated due to the thermal decomposition of PVC or CO2 and H2O generated due to the reaction between CaCO3 and HCl. The weight losses above 4508C are assigned to release of H2O, CO2, and benzene during the thermal decomposition of the de-hydrochlorinated residues.[22] It was found that the addition of the 40-nm CaCO3 particles (.2%) enhances the thermal stability of PVC; 1% of 40 nm CaCO3 particles could act the same role as 5% of 3000-mesh CaCO3 particles. This indicated that the nanoparticles could result in Table 1 Thermal stabilities of the pure PVC and the PVC/CaCO3 composites Sample PVC powder PVC/CaCO3-800 PVC/CaCO3-3000 PVC/CaCO3-40 nm-1 PVC/CaCO3-40 nm-2 PVC/CaCO3-40 nm-3
CaCO3 size and amount added (weight %) None 800 mesh, 5% 3000 mesh, 5% 40 nm, 1% 40 nm, 2% 40 nm, 3%
Tonset (8C)
Tmax (8C)
270.2 265.2 275.6 274.0 278.1 278.1
293.1 287.2 289 290.6 295.6 299.7
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Scheme 1. The zipper dehydrochlorination of PVC.
higher thermal stabilities than the micrometer particles, the same as reported previously.[23] Furthermore, the thermal degradation onset temperature (Tonset) and the maximum thermal degradation temperature (Tmax) increased with increasing amounts of the CaCO3 nanoparticles added. It is well known that PVC thermal degradation is the result of a “zipper dehydrochlorination” process that involves allylic chlorides as intermediates and is catalyzed by hydrochloric acid (Scheme 1). The degradation process can be stopped by PVC stabilizers, which either react with allylic chlorine atoms (primary stabilizers) or scavenge hydrochloric acid (secondary stabilizers). Scavenging HCl cannot stop the degradation process completely as it is diffusion controlled. However, HCl scavenging considerably reduces the rate of degradation and avoids the very fast process that eventually causes PVC blackening (catastrophic degradation).[19] The CaCO3 particles can consume the HCl gas and avoid the self-accelerating effect when it is used as filler. To confirm it, the pure PVC powders and the PVC/ CaCO3 composites were pyrolyzed at 2808C, at which temperature the de-hydrochlorination reaction begins, in air atmosphere for 30 min and the products were chemically analyzed. The results showed that the release of the HCl gas generated decreased after the addition of CaCO3 particles and the release decreased with the increasing amounts of the CaCO3 particles added (Table 2). As high as 80% CaCO3 nanoparticles reacted with the hydrochloride acid (HCl) gas generated during the thermal decomposition of PVC and the percentage increased with the increasing of CaCO3 nanoparticles added. However, only about 30% of the CaCO3 particles reacted with HCl for the PVC/ micrometer CaCO3 composites. This is due to the larger surface area of the nanoparticles. The CaCO3 nanoparticles with larger surface area adsorbed more HCl gas and
Table 2 Chemical analysis of the samples pyrolyzed Sample PVC powder PVC/CaCO3-800 PVC/CaCO3-3000 PVC/CaCO3-40 nm-1 PVC/CaCO3-40 nm-2 PVC/CaCO3-40 nm-3
Weight loss (%)
CaCl2 obtained (mg/g sample)
CaCO3 reacted percentage (%)
62.47 59.8 58.33 61.34 59.47 58.15
— 55.1 55.5 11.1 22.2 33.4
— 27.59 32.50 65.76 73.02 81.59
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more effectively avoided the self-accelerating effect. Thus it can be concluded that the CaCO3 fillers act as thermal stabilizers. The mechanism of this can be summarized as follows: (1) Thermal decomposition of PVC:
(2) Adsorption of HCl by CaCO3 particles: CaCO3 þ 2HCl ¼ CaCl2 þ CO2 h þ H2 Oh (3) Detection of CaCl2 generated: CaCl2 þ Na2 CO3 ¼ CaCO3 i þ 2NaCl
Acknowledgments This work was supported by the Natural Science Foundation of Gansu Province (3ZS041A25-002) and the Interdisciplinary Innovation Research Fund For Young Scholars, Lanzhou University (LZU200302).
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