Thermooxidation effects on the structuring of molten polypropylene clay nanocomposites

Transcription

1 Polymer International Polym Int 55: (2006) Thermooxidation effects on the structuring of molten polypropylene clay nanocomposites Yibin Xu, Jianmao Yang and Yuanze Xu The Key Laboratory of Molecular Engineering of Polymers, Ministry of Education, Department of Macromolecular Science, Fudan University, Shanghai , People s Republic of China Abstract: The storage modulus G plateau at low frequency, the solid-like state in molten nanocomposites, has been verified by comparative studies in air or nitrogen for polypropylene (PP) organoclay with compatibilizer (PPCH). It was found that the thermal oxidation does not only degrade the polymer chain but also enhances the structuring in PPCH. Well-designed rheology testing was able to distinguish the surface effect from the bulk responses. The inhomogeneity of clay distribution in samples was also studied using X-ray element analysis. We found that the apparent G plateau was mainly caused by diffusive oxidation starting from sample-free surface in oxidative environment and, finally, a charring layer at the free surface was gradually developed, which is responsible for the solid state or very high apparent G plateau. This structuring scheme agrees with the dehydrogenation mechanism of oxidation leading to crosslinking and volatilization of PP composites, while the participation of organoclay is essential. A silane coupling agent may retard this process by interacting with silicates Society of Chemical Industry Keywords: thermooxidation; nanocomposites; solid-like state; rheology; polypropylene; montmorillonite INTRODUCTION Polymer nanoclay hybrids have attracted much attention worldwide for over 10 years. 1 4 The outstanding thermal and mechanical properties of nanocomposites rely on extremely high surface and aspect ratios of layered silicate well dispersed in polymer media. Among many characterization approaches, e.g. X-ray diffraction for interlayer status, electron microscope for dispersion status, rheological characterization is sensitive and easy to observe in situ. 5 7 One commonly used characteristic for studying linear viscoelastic behavior for molten nanocomposites is the solid-like state, which describes the enormous increase in storage modulus G at lower frequencies, approaching a plateauinag frequency dependency curve like an elastic solid. Since the increase in G may increase by more than 1000-fold by just adding a small proportion of clay in many polymer organoclay systems, 2,6 this phenomenon has been studied at the nanoscale intensively both experimentally and theoretically. 8,9 When researchers have attempted to regard the solid-like state as an easy measure of dispersive quality of organoclay in polymer matrix, they have found that the time dependency of the G of the composite increased significantly with annealing time. 5,10 There have been many explanations, such as the necessary time of polymer diffusion into galleries of platelets, 11,12 or the further exfoliation and formation of a percolated network. 13 The nature of the interaction between platelets, whether it is a frictional or inter-plate networking force, is not very clear. 2,4,14 One obstacle to clarifying the problem is that qualified experimental data are still lacking. Many authors indicate that the data quality and reproducibility of rheological measurements are far from satisfactory and the reason is unknown. 15 As far as long-term experiments at high temperatures are concerned, the protective atmosphere must be taken into account. It has been recognized for long time that polypropylene (PP) or polyethylene (PE) are oxidized and degraded, but not seriously below 200 C. Careful rheological studies were often done under nitrogen atmosphere to reduce the thermooxidation of polymers; however, the purity of the nitrogen is not indicated in most cases. Some authors studied the stability of PP nanocomposites at high temperatures and found that nano-clay seems to play a role as a barrier, retarding the oxidation process. 16,17 No report has appeared concerning how the oxygen-containing atmosphere affects the further structuring or the formation of solid-like behaviors. This work carried out comparative experiments in air and nitrogen, verified the surface versus bulk effects and the inhomogeneity of clay distribution, and discusses the possible origin of oxidation influencing the nanostructuring in molten PP clay and means of stabilization. Correspondence to: Yuanze Xu, The Key Laboratory of Molecular Engineering of Polymers, Ministry of Education, Department of Macromolecular Science, Fudan University, Shanghai , People s Republic of China yuanzexu@fudan.edu.cn Contract/grant sponsor: NSFC, China; contract/grant number: Contract/grant sponsor: Major State Basic Research Projects; contract/grant number: 2003CB (Received 29 October 2005; revised version received 22 November 2005; accepted 16 January 2006) Published online 29 March 2006; Society of Chemical Industry. Polym Int /2006/$30.00

2 YXu,JYang,YXu EXPERIMENTAL Materials Isotactic polypropylene (ipp, F280, Sinopec Shanghai Petrochemical Co. Ltd) with M w g mol 1 and M w /M n 5.44 was used as the matrix phase. Maleic anhydride grafted polypropylene (PP-MA, CMG9801, Shanghai Sunny New Technology Development Co. Ltd) of melt flow index (MFI) = 150 with maleic anhydride (MAH) = 1% was used as the compatibilizer. The intercalated organoclay montmorillonite (MMT) was supplied by Fenghong Clay Chemicals Co. Ltd. DK1B combined a quaternary ammonium salt with one C 18 and tri-methyl groups and DK4 contains a similar amine but with two C 18 chains. Systematic studies of the effects of composition of PP PP-MA MMT and processing conditions are described elsewhere. 10 The notation for selected samples in this paper denotes the composition of the PP clay hybrid (PPCH) nanocomposite, e.g. ipp 24 4 means PP (72 wt%), PP-MA (24 wt%) and organoclay DK1B (4 wt%). ipp 6 6 or ipp 9 9 means PP (88 or 82%), PP-MA (6 or 9%) and DK4 (6 or 9%). The PP-MA MMT ratio in ipp 24 4 is high to achieve the exfoliation of clay, as indicated by X-ray diffraction. All materials were dried at 80 C for 12 h before mixing. The PPCH was prepared through melt extrusion of the mixture of PP, PP-MA and the organoclay, using a twin screw extruder (PRISM PTS-16, HAAKE Co. Ltd) with corotating and intermeshing screws (D = 20 mm, L/D = 30) extruded at C. The residence time inside the twin-extruder was approximately 3 min. The extruded pellets were dried in a vacuum oven at 80 C for 12 h to remove residual moisture before sample disk preparation for rheological measurements. For stabilization we used silane coupling agent: ALink15, with the chemical formula H 2 NC 2 H 4 NHC 3 H 6 Si(OCH 3 ) 3, provided by GE Co. Shanghai. Characterizations The element distributions of silicone in the sample disks for rheological measurements were obtained by X-ray fluorescence analysis (model S4 Explorer, Bruker). The group changes by thermooxidation were studied using infrared spectra collected using an FTIR spectrometer produced by Nicolet (type Nexsus 470). The thermal stability and volatility were determined by thermogravimetric analysis (TGA) using a Perkin Elmer company Pyris 1 apparatus. Rheological measurements were performed using a strain-controlled rheometer (ARES, TA Instruments). Small-amplitude oscillatory shear measurements were performed using a set of 25 mm diameter cone and plate geometry with a cone angle of rad to keep homogeneous strain histories within the samples. Parallel plates were also used to verify the surface effect by thermo-oxidation effect by changing the gap. The sample disks (2.2 mm thick and 25 mm diameter) were prepared using a mixing molder (LMM, ATLAS Electric Devices Co.) with the extruded pellets at 200 C for about 5 min. The mold temperature was 140 C. Thehotgunovenoftherheometerconsumesa large amount of heating gas, which is also used as a protecting gas for the sample environment. For this purpose, pressurized N 2 with % was employed, generated from purified liquid nitrogen rather than normal industrial nitrogen gas. The dynamic sweep was carried out from high frequency to low, so that the possible sample history could be neglected. RESULTS AND DISCUSSION Comparisons of annealing in air or nitrogen Figure 1 shows the linear viscoelastic data of G, η of PPCH annealed in nitrogen and then in air. To maintain the experiment in a linear range, curves of G ipp/24/4-n 2-0h ipp/24/4-n 2-3h ipp/24/4-air-0.5h Figure 1. The comparison of linear viscoelastic data of G, η of ipp 24 4 with those of sample annealed in nitrogen and the sample subsequently annealed in air. All data at 190 C. 682 Polym Int 55: (2006)

3 Structuring of molten pp/clay nanocomposites versus strain were made for each sample. The extrusion pelletized ipp 24 4 showed a linear range of strain >10%, while that of ipp 24 4 annealed in air had a linear range of strain 1%. Finally a strain of 1% was selected for all frequency sweep data in this work. It was found that the values of G, G and η for ipp 24 4 increased only slightly at low frequencies within a time of 3 h in nitrogen, while only 30 min afterwards in air, the curve of G increased remarkably. This increase of G to a plateau, a typical solid-state evolution, has been reported by many authors in air and nitrogen. 6,12 The differences in air and nitrogen have also been found in other PPCH systems. The results for PPCH systems with different contents of DK4-treated montmorillonite are shown in Fig. 2. Therefore, in our case, the oxidative atmosphere seemed to play a major role in the G increase; the bulk structuring in nitrogen was also observed, although to a lesser extent. All previous results are obtained using cone/plate geometry. Since the attack of oxygen was through the free surface, to trace the surface change we used parallel plate geometry. To estimate the contribution of surface layer, we refer to the basic equation of parallel plate rheometer and write the storage modulus G in small amplitude oscillation: G = 2H πr 4 T (1) where G is the storage modulus, T the torque in phase, the rotating angle amplitude, and the factor 2H/πR 4 contains the ratio of the gap H to the fourth power of radius. This high power indicates a much higher contribution from the skin layer. To distinguish the contribution of surface layer from that of the structure change in the bulk, a squeeze experiment was designed, as shown in Fig. 3. The initial frequency sweep produced curve A1. After 1 h annealing in air, curve A2 was measured; then, squeezing the plate gap H from 1.55 to 0.98 mm and cleaning the rim surface eliminated 37% of the sample; finally, a frequency sweep was done to make curve A3. If the G increase of curve A2 was from the bulk solid-state structuring, curve A3 should be equal to or above curve A2. The result shows curve A3 close to curve A1, the original curve. Therefore, the major change in air annealing is on the free surface part. We do not see this in nitrogen. (A) ipp/6/6-air- 0 h ipp/6/6-air- 1.5 h ipp/6/6-air-3 h ipp/6/6-n 2-0 h ipp/6/6-n h ipp/6/6-n 2-3 h (B) ipp/9/9-air-0 h ipp/9/9-air-1.5 h ipp/9/9-air-3 h ipp/9/9-n 2-0 h ipp/9/9-n h ipp/9/9-n 2-3 h Figure 2. The dynamic curves for PPCH during annealing in air or nitrogen. (A) For ipp 6 6; (B) for ipp 9 9. Polym Int 55: (2006) 683

4 YXu,JYang,YXu A2 A3 A Figure 3. The dynamic frequency sweep experiments of sample ipp 24 4 in air, 190 C, strain 1%, diameter 25 mm parallel plate. The line A1 is the result before annealing; A2, annealed for 83 min; then the surface layer was squeezed out and the line A3 was obtained with cleaned free surface. This experiment indicated the contribution of thermal oxidation near the free surface to G.After a long annealing period direct observation on a severely oxidized rim surface of the PPCH plate found the formation of light brown charring. The charring phenomenon has also been reported by other authors. 16,18,19 It is noted that the viscoelasticity measurement provides a more sensitive method at early stages. In this inhomogeneous case, G G a has only apparent meaning. To estimate the contributions of outer and inner concentric parts to the torque, T O and T I,wetake T = T I + T O = RI 0 RO 2πr 2 τ I dr + 2πr 2 τ O dr (2) R I Since the stress is the product of modulus and strain, thus: τ = G γ = G r (3) H Combining Eqns (1) (3), we have the apparent storage modulus: ( ) 4 G a = G O RI (G O R G I ) (4) O Therefore, if G O G I, the total modulus is determined by the modulus and thickness of the skin layer. To understand why the surface layer gradually shows a much higher G, rather than much lower as in the cases of usual thermal oxidation of polyolefin, we will go into the surface layer structure and its chemistry in more detail. Heterogeneity of PPCH samples after annealing To measure the clay distribution in the sample disk directly, samples were prepared as follows: six sample disks were placed between two parallel glass plates, then annealed in an oven of 190 Cfor5h to simulate the annealing in the rheometer. Then the disks were cut into five concentric circles, with only the rim surface layer, and the core parts were remelted to reshape sample disks for rheology and X-ray fluorescence analysis. In Fig. 4, we see the general decreases in viscosity and G after annealing due to the thermal degradation and oxidation compared with non-annealed ipp The curve s shape for core part did not change, while for the outer part the viscosity curve exhibited a yielding shape and G rose at low frequency, a sign of solid-like structuring. The element content of silicone was measured as shown in Table 1. Evident increase in Si following the sequence from core to surface was detected. In our PPCH system, the only source of Si was MMT. This indicates the accumulation of MMT at the surface layer. The higher concentration of Si could be due either to the diffusional migration of silicate platelets from internal to surface layer, which is a slow process taking hours, or to the clay accumulation at the surface when organic components volatilized due to oxidation and degradation. Chemical reaction involved Figure 5 shows FTIR spectra of the surface layer versus the core part of ipp 24 4 sample disks after annealing in air for 5 h, KBr pellets were made using ground sample. In the FTIR diagram, an obvious peak near 1700 cm 1 belonging to the carbonyl group for the surface layer appeared, while very little was observed for the core part, indicating the oxidation effects of oxidative environment on organic components. This result is in agreement with published work on oxidation of nanocomposites. 20 Accompanying thermal oxidation, weight loss was also observed for the annealed samples. In Fig. 6 TGA 684 Polym Int 55: (2006)

5 Structuring of molten pp/clay nanocomposites ipp/24/4 ipp/24/4-suface layer ipp/24/4-core Figure 4. The comparison of the storage moduli and dynamic complex viscosity of ipp 24 4, its surface layer and core of sample disks annealed for 5 h. Table 1. The element content of silicone in different part of sample disk after annealing Surface Middle Core Silicone content 0.941% 0.846% 0.734% Absorbance core part out part Wavenumbers (cm -1 ) Figure 5. FTIR spectra of layer near surface and core part of sample disk for ipp 24 4 after annealing in air for 5 h. results are presented for PPCH and its components in air or nitrogen. Weight loss was observed for all samples at high temperatures, to much greater degree in the air. We did not analyze the components of volatile parts. The following volatile products during the oxidation of PP were identified: water, formaldehyde, acetaldehyde, acetone, methanol, hydrogen, hydrogen peroxide, carbon monoxide and carbon dioxide. 17 Decomposition of the PP chain was also oxidatively sensitized and occurred by breakage of the weaker bonds at the polymer surface in the presence of chemisorbed oxygen. The peroxy radicals were formed by the propagating oxidative reaction at the reaction zone. The organoclay DK1B exhibited weight loss in air due to the evaporation of organic amine, while much less weight loss occurred in PPCH, indicating some reaction or interaction between PP, PP-MA and MMT. The stabilization of PP OMMT by nanoclay was studied using TGA. 16 It was found that the nanocomposites were more stable ( 50 C) in comparison to neat polymer, which delayed the volatilization of the products originating at the temperature of carbon carbon bond scission as in the pure PP PP-MA. Delayed thermal volatilization was also observed in a polystyrene-layered silicate nanocomposite at the NIST laboratory. 21 Volatilization may be delayed by the silicate layers dispersed in the nanocomposite. This is likely to be either due to physical chemical adsorption of the volatile degradation products on the silicate, 16 or the formation of a protective surface barrier layer consisting of accumulated clay platelets, which build up on the surface of the polymer melt and become the mass and heat transfer barrier. The reduction in heat release rate was also observed previously. 18 About the mechanism of the charring skin formation during the thermal oxidation, our results seem to be consistent with the following explanations: around C, oxygen attacks the carbon radical within the chain by H abstraction, resulting in two directions, the oxidative dehydrogenation forming volatile products and/or the double bond formed as a source of crosslinking. The decomposition of the amine silicate modifier leaves strong acid catalytic sites that can catalyze crosslinking, with increase an in charring precursor structures. 17,22 The complex combinations of crosslinking of PP and PP-MA with the layered silicate surface may form a percolated network, which grow to be a solid-like charring layer that is mainly responsible for the solid state, i.e. the high G plateau in an oxygen-rich environment. We tried to change this charring process using additives. One of the effective ways is to introduce silane coupling agent. Polym Int 55: (2006) 685

6 YXu,JYang,YXu Weight% ipp/24/4-air-190 C ipp/24/4-n C ipp/24/4-alink-air-190 C PPMA-air-190 C PPMA-N C DK1B-air-190 C Time (min) Figure 6. Isothermal TGA curves for PPCH and components at 190 C. Sample heating time to 190 C was 7 min with a heating rate of 20 Cmin ipp/24/4+0.1wt%alink15 without annealing annealing 420 min ipp/24/4 without annealing annealing 320 min Figure 7. The effect of coupling agent on dynamic properties of PPCH. lgg, versus log ω for ipp 24 4 samples with or without coupling agent ALink15 and before or after annealing at 190 C. The effect of silane coupling agent In Fig. 7 the dynamic frequency sweeping results of sample ipp 24 4 containing the stabilizer ALink15 are compared with ipp 24 4 as reference. As we can see, although the annealing time of the sample containing silicane coupling agent is longer than that of ipp 24 4 in air, the storage modulus G plateau is much lower than that of ipp 24 4 and the charring is hard to see. Therefore, the stabilizer is effective. The possible explanation we propose is that the aminosilane couples and shields the catalytic centers on the surface of organoclay and, thus, retard the reaction of crosslinking and charring. The volatilization from olefin oxidation and stearic ammonium decomposition in organoclay are also retarded by aminosilane, as we see from the TGA results in Fig. 6. Further, clarifying the detailed mechanism of coupling agent is of interest for the nanocomposite stabilization. CONCLUSIONS The thermal oxidation will not only degrade the polymer chain but will also enhance the structuring, the G plateau for PPCH systems. To study the rheology, it is important to distinguish the surface effect from the bulk responses. Rheological tests by controlling atmosphere, sample annealing and squeezing are useful to trace the surface effect. In oxidative atmosphere, the solid-like state is mainly 686 Polym Int 55: (2006)

7 Structuring of molten pp/clay nanocomposites caused by diffusive oxidation starting from the free surface of the sample. In pure nitrogen, the formation of G plateau is much slower and weaker. In the annealing process, the oxygen-containing environment will gradually form a charring skin at the free surface. It is responsible for the high G and low linear range and, thus, the low break strain in the strain sweep. The charring layer behaves like a brittle networking structure of crosslinked polyolefin and silicate, rich in silicate layers. This structuring scheme agrees with the dehydrogenation mechanism of oxidation, leading to crosslinking and volatilization, while the participation of organoclay is essential. Some silane may retard this process by interacting with silicate surface. This work is limited to PPCH systems. However, from the previous mechanism discussions, similar effects should be observed more or less in other polymer hybrids. ACKNOWLEDGEMENT The authors are grateful to the support from the NSFC of China (no ), and Major State Basic Research Projects (2003CB615604), and the helpful communications with Dr Minglong Yao. REFERENCES 1 Usuki A, Kawasumi M, Kojima Y, Okada A, Kurauchi T and Kamigaito O, JMaterRes.8:1174 (1993). 2Sinha Ray S and Okamoto M, Prog Polym Sci 28:1539 (2003). 3 Giannelis EP, Krishnamoorti R and Manias E, Adv Polym Sci 138:107 (1999). 4 Krishnamoorti R and Yurekli K, Curr Opin Colloid Interface Sci 6:464 (2001). 5 Solomon MJ, Almusallam AS, Seefeldt KF, Somwangthanaroj A and Varadan P, Macromolecules 34:1864 (2001). 6 Galgali G, Ramesh C and Lele A, Macromolecules 34:852 (2001). 7 Rajkumar V, Bhavjit SG, Junseok L, Sravanthi M, Changmo S, Joey LM et al., Annual Technical Conference 62: 2932 (2004). 8 Dolgovskij MK, Lortie F and Macosko CW, Proceedings of the 4th Pacific Rim Conference on Rheology, Shanghai, pp (2005). 9 Chow WS, Ishak ZAM and Karger-Kocsis J, J Polym Sci: Part B Polym Phys 43:1198 (2005). 10 Xu YB, Dissertation, Department of Macromolecular Science, Fudan University (2004). 11 Maiti P, Nam PH and Okamoto M, Macromolecules 35:2042 (2002). 12 Li J, Zhou CX, Wang G and Zhao DL, J Appl Polym Sci 89:318 (2003). 13 Ren J, Silva AS and Krishnamoorti R, Macromolecules 33:3739 (2000). 14 Xu YZ and Xu YB, Chin J Polym Sci 23:147 (2005). 15 Giannelis EP, Adv Mater 8:29 (1996). 16 Zanetti M, Camino G, Reichert P and Mulhaupt R, Macromol Rapid Commun. 22:176 (2001). 17 Zhang S and Horrocks AR, Prog Polym Sci 28:1517 (2003). 18 Morgan AB, Harris RH, Kashiwagi T, Chyall LJ and Gilman JW, Fire Mater 26:247 (2002). 19 Qin HL, Zhang SM, Zhao CG, Feng M, Yang MS, Shu ZJ et al., Polym Degrad Stabil 85:807 (2004). 20 Tidjani A, Polym Degrad Stabil 87:43 (2005). 21 Gilman JW, Kashiwagi T, Giannelis EP, Manias E, Lomakin S, Lichtenhan JD et al., in Fire Retardancy of Polymers. The Royal Society of Chemistry, Cambridge, p. 203 (1998). 22 Pandey JK, Reddy KR, Kumar AP and Singh RP, Polym Degrad Stabil 88:234 (2005). Polym Int 55: (2006) 687