Article pubs.acs.org/IECR
Melt Foamability of Poly(ethylene terephthalate)/Clay Nanocomposites Prepared by Extrusion Blending in the Presence of Pyromellitic Dianhydride Tian Xia,† Zhenhao Xi,† Xuefeng Yi,† Tao Liu,†,‡ and Ling Zhao*,† †
State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, People’s Republic of China ‡ Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, East China University of Science and Technology, Shanghai 200237, People’s Republic of China ABSTRACT: Exfoliated poly(ethylene terephthalate) (PET)/organoclay nanocomposites were prepared by extrusion blending method with different clay contents and molecular weights of raw PET. Pyromellitic dianhydride (PMDA) was extruded, together with PET and clay, to introduce long-chain branching to PET backbone and delaminate the clay layers. Although the molecular weights and viscoelastic properties of nanocomposites were much lower than those of foamable PMDA-modified PET, the melt foamability of nanocomposites was significantly improved by the well-dispersed clays due to the heterogeneous nucleation effect, enhanced nonisothermal crystallization rate, and so on. Foaming temperature windows with a width of 20−60 °C were explored for PET/clay nanocomposites with intrinsic viscosities of 0.67−0.94 dL/g, in which nanocomposites foams with cell diameters of 29−53 μm, cell densities of 6.5 × 107−6.9 × 108 cells/cm3, and expansion ratio of 10−50 could be controllably produced using supercritical CO2 as a blowing agent.
1. INTRODUCTION Poly(ethylene terephthalate) (PET) foams show good mechanical properties and high-temperature dimensional stability, which could be used as packaging materials and structure materials in wind energy, marine, and transportation applications. It is known that PET usually has low melt strength and elasticity, because of the low molecular weight and linear backbone, which cannot resist the intensive extensional deformations during cell expansion. The formed cells will merge into larger ones or rupture in the melt foaming process. The introduction of long-chain branching to the PET backbone is an effective way of increasing the melt elasticity via enhancing the possibility of entanglements in the polymeric melt.1,2 The foaming temperature window, as a critical parameter to evaluate the foamability of polymer, is broad only when the molecular weight of branched PET is high along with high melt viscosity and elasticity. In our previous work, the broad foaming temperature window was observed in batch foaming process with intrinsic viscosity (IV, which is an indication of the molecular weight of the PET) up to 1.36 dL/g.3 In continuous extrusion foaming processes, higher molecular weight was essential, which would deteriorate in the extruder, because of severe degradations. The branched PET had to be further polymerized in solid state for another 24 h, which cost much time and energy, and the IV value reached 1.5 dL/g.4 In recent years, nanoclays, such as montmorillonite, have been applied to improve the foamability of polymers, because of high aspect ratio, plate morphology, and natural abundance.5,6 The polymer/clay nanocomposites, in which the clays are dispersed in nano size by the intercalation of polymer chains into clay galleries, present attractive properties such as reduced gas permeability, flame retardance, and enhanced mechanical properties. The nanocomposites foams © 2015 American Chemical Society
are expected to possess the combined advantages of polymer foams and nanocomposites. In addition, nanocomposites offer some potential advantages in the foaming process (for example, more nucleation sites, lower diffusivity of blowing agent, and less cell collapse), which may broaden the foaming temperature window with lower melt elasticity. PET/clay nanocomposites could be fabricated by two methods: in situ polymerization and extrusion blending, and the latter method demonstrates several advantages in terms of preparation ease, economical cost, and compatibility with standard industrial process.7,8 Clay surface is hydrophilic in nature, which hinders its dispersion in the organic polymer phase. Ion exchange of the interlayer inorganic cations with an organic cation, such as quaternary alkylammonium salts, makes the clay surface hydrophobic and enhances the affinity between clay and polymer matrix,9 which could be used to explain the formation of polymer−particle network-like structure.10 Supercritical carbon dioxide has been used as a blowing agent for various polymers and their nanocomposites, because it is inexpensive, nontoxic, and environmentally benign. CO2 dissolution causes many changes in physical properties of polymer matrix (for example, crystallization and rheological properties), which may provide many opportunities for the manipulation of the foaming process.11 Moreover, the further exfoliation of clays was observed in the foaming process, because of the rapid depressurization, which forcibly peeled apart the clay platelets,12 and improved the enforcement effects of nanoclays. Received: Revised: Accepted: Published: 6922
April 28, 2015 June 24, 2015 June 26, 2015 June 26, 2015 DOI: 10.1021/acs.iecr.5b01583 Ind. Eng. Chem. Res. 2015, 54, 6922−6931
Article
Industrial & Engineering Chemistry Research
evaluate the dispersion of clay. The TEM specimens were prepared by microtome with a diamond knife. The rheological measurements were carried out using a Haake Mars III Rheometer (Thermo Fisher Scientific) in a parallel disk mode at 270 °C under nitrogen. Dynamic frequency sweeps were conducted over an angular frequency range of 0.1−100 rad/s in the linear viscoelastic region. Differential scanning calorimetry (DSC) (Netzsch, Model 204 HP) was used to detect the crystallization properties of nanocomposites under atmospheric N2 and compressed CO2. The sample was heated to 280 °C at a rate of 10 °C/min, held for 10 min to eliminate all crystals, and then cooled to 50 °C at a rate of 5 °C/min. To minimize the effect of the noise induced by compressed CO2 environment, at least two DSC measurements were conducted for each measurement to ensure the repeatability. 2.3. Preparation and Characterization of PET/Clay Nanocomposite Foams. A high-pressure autoclave with an internal volume of 115 mL was used in the foaming process and the batch foaming apparatus was described detailedly in our previous work.3 In a batch foaming process, 0.5 g of nanocomposite particles were loaded in the autoclave under CO2 atmosphere at 20 MPa and 270 °C for 20 min, in order to guarantee the completed melting of PET matrix and obtain a high diffusion rate of CO2. Afterward, the autoclave was cooled to a foaming temperature (Tf), repressurized to 20 MPa, and kept at the foaming condition for another 20 min. And the evaluated Tf value was in the range of 210−270 °C, as discussed later. The pressure then was quenched to ambient pressure via a maximum depressurization rate of 330 MPa/s. Finally, the autoclave was opened to remove the foam sample for subsequent analysis after cooling in an ambient-temperature water bath. A JEOL Model JSM-6360LV scanning electron microscopy (SEM) system was adopted to characterize the cell morphologies of nanocomposite foams. The samples were immersed in liquid nitrogen for 10 min and then fractured. The SEM scanned fractured surfaces with platinum coating. The number-average diameter of all the cells (D) was obtained through analysis of the SEM photomicrographs, using ImagePro Plus software (Media Cybernetics, Bethesda, MD (formerly in Silver Spring, MD)) and was calculated as shown below:
According to preliminary screening experiments, the physical polymer−particle network structure could not protect the cells alone from being destroyed by stretching force during cell expansion and the molecular chain entanglements in polymer matrix correlated to long-chain branching were necessary to obtain foamable PET/clay nanocomposites. Pyromellitic dianhydride (PMDA) was selected as the multifunctional modifier to introduce long-chain branching to the PET molecular chain in this work, since the melting point of PMDA, which is similar to the PET processing temperature, together with four functional groups ensure the reactions between PMDA and PET are fast.13 PET/clay nanocomposites were prepared by extrusion blending method with different molecular weights of raw PET and different clay contents in the presence of PMDA, and the properties of nanocomposites concerning foaming behavior, such as the morphology, crystallization, and rheological properties, were investigated thoroughly. The foaming temperature windows of PET nanocomposites were explored using batch foaming processes, which were improved significantly by clays.
2. EXPERIMENTAL SECTION 2.1. Materials. Two types of PETs were kindly supplied by Sinopec Yizheng Chemical Fiber Co., Ltd.: bottle-grade BG80 and engineering plastics grade EP901, with IV values of 0.80 and 0.95 dL/g, respectively (corresponding number-average molecular weights of 26 300 and 33 800 g/mol). PMDA powders were purchased from Shanghai Aoke Industrial Co., Ltd. Organoclay Cloisite 30B (C30B) with bis-2-hydroxyethyl methyl tallow quaternary ammonium cations as surface modifier was commercially available from Southern Clay Products. PET particles, PMDA, and C30B were dried at 110 °C in vacuum overnight and then well blended with a given mass ratio. The content of PMDA was fixed at 0.8 wt % for all nanocomposites, and Table 1 showed the compositions and nomenclature of nanocomposites. Phenol and tetrachloroethane were of analytical purity and purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. CO2 (purity: 99.9% (w/ w)) and N2 (purity: 99.99% (w/w)) were purchased from Shang Chenggong Gases Co., Ltd. All gases and reagents were used without any further purification. 2.2. Preparation and Characterization of PET/Clay Nanocomposites. Extrusion blending was conducted in a Nanjing Giant SHJ-20 twin-screw extruder (screw diameter = 20 mm and length−diameter ratio of L/D = 30 (with L being length and D being diameter)), which was equipped with a 3 mm die and a feeder operated at a constant production rate of 3 kg/h. The extruder was performed at 240 rpm, to achieve uniform dispersion of the clay particles in the PET matrix. The temperature profile was 260, 275, 275, 275, 275, 270 °C from the hopper to the die. The extrudates were cooled in a water bath and pelletized. All extruded samples were dried in the vacuum oven at 110 °C overnight before characterizing and foaming. The IV value was measured using a Ubbelohde viscometer with a solvent mixture of phenol and tetrachloroethane (3/2, w/w) at 25 °C. X-ray diffraction (XRD) was conducted with a Rigaku Model D/Max 2550 diffractometer using Cu Kα radiation (λ = 1.54056 Å), and the X-ray generator was operated at 40 kV and 200 mA. The data were recorded over a range of 0.5°−10° with a step size of 0.02° and a step time of 0.2 s. A JEOL Model JEM-2100 transmission electron microscopy (TEM) system operated at 200 kV was used to
D=
∑ dini ∑ ni
(1)
where ni is the number of cells with a perimeter-equivalent diameter di. Cell density (N0) is defined as the number of cells per cubic centimeter of unfoamed nanocomposites and is calculated using the following equation: ⎡ n ⎤3/2 N0 = ⎢ ⎥ R ν ⎣A⎦
(2)
where n is the number of cells in the SEM photomicrograph, A the area of the photomicrograph (given in units of square centimeters (cm2)), and Rv the volume expansion ratio. Rv is defined as the ratio of the bulk density of the nanocomposites (ρ0) to that of the nanocomposite foams (ρf) and is determined by the equation ρ Rν = 0 ρf (3) and the densities of nanocomposites (ρ0) and their foams (ρf) were measured according to ASTM Standard D792-00, by 6923
DOI: 10.1021/acs.iecr.5b01583 Ind. Eng. Chem. Res. 2015, 54, 6922−6931
Article
Industrial & Engineering Chemistry Research means of weighing samples in water. ρf was determined with the help of a sinker and calculated as follows: ρf =
⎛ ⎞ a ⎜ ⎟ρ ⎝ a + w − b ⎠ water
(4)
where a is the actual mass of specimen in air without a sinker, w is the mass of the sinker totally immersed in water, and b is the mass of specimen and sinker completely immersed in water. ρ0 is determined without the sinker, and, in eq 4, w = 0 and b is the mass of only the specimen immersed in water.
3. RESULTS AND DISCUSSION 3.1. Preparation of PET/Clay Nanocomposites by Extrusion Blending in the Presence of PMDA. As shown Table 1. Nomenclature, Compositions, and IV Values of PET/Clay Nanocomposites nomenclature
PET
C30B content (wt %)
PMDA content (wt %)
IV (dL/g)
PETNC1 PETNC2 PETNC3 PETNC4
BG80 BG80 BG80 EP901
1 3 5 5
0.8 0.8 0.8 0.8
0.94 0.77 0.67 0.75
Figure 1. XRD patterns of PET/clay nanocomposites and C30B powders.
in Table 1, only PETNC1 with a clay content of 1 wt % had a higher molecular weight, indicated by the IV value, than that of raw PET attributed to the chain extension/branching reactions between the hydroxyethyl end groups of PET and anhydride groups of PMDA, as illustrated in our previous work.3 In the meanwhile, the hydroxyethyl group-containing clay surface modifier could also react with anhydride groups of PMDA and form a carboxyl group for each anhydride group, as reported by Weng et al.,14 which competed with the chain extension/ branching reactions. With increasing clay content, the molecular weight of PET matrix in nanocomposites obviously decreased. More PMDA was consumed by the clay surface modifier. Some of the C30B surface modifier unavoidably decomposed to α-olefins, amines, and other products, following the Hoffman elimination mechanism at high temperature.15 The degradation of PET was accelerated by the decomposer and, as a result, prevailed over the chain extension/branching reactions at high clay content. Compared with PETNC3, PETNC4 had a higher molecular weight, because of the higher IV value of raw PET. The IV values of all nanocomposites in this work were
