Facile fabrication of HDPE-g-MA/nanodiamond nanocomposites via one-step reactive blending
© Song et al.; licensee Springer. 2012
Received: 5 April 2012
Accepted: 23 June 2012
Published: 29 June 2012
In this letter, nanocomposites based on maleic anhydride grafted high density polyethylene (HDPE-g-MA) and amine-functionalized nanodiamond (ND) were fabricated via one-step reactive melt-blending, generating a homogeneous dispersion of ND, as evidenced by transmission electron microscope observations. Thermal analysis results suggest that addition of ND does not affect significantly thermal stability of polymer matrix in nitrogen. However, it was interestingly found that incorporating pure ND decreases the thermal oxidation degradation stability temperature, but blending amino-functionalized ND via reactive processing significantly enhances it of HDPE in air condition. Most importantly, cone tests revealed that both ND additives and reactive blending greatly reduce the heat release rate of HDPE. The results suggest that ND has a potential application as flame retardant alternative for polymers. Tensile results show that adding ND considerably enhances Young’s modulus, and reactive blending leads to further improvement in Young’s modulus while hardly reducing the elongation at break of HDPE.
With the increasingly rapid need for synthetic polymers due to their various advantages over conventional materials, their inherent flammability, however, is gradually limiting their many potential applications for safety considerations . Thus, it is rather crucial to explore nontoxic and environmentally friendly flame retardancy approaches for polymer materials . Among various polymers, polyolefin is much more flammable due to its saturated hydrocarbon structure and non-char-forming nature. Recently, nanocomposites have been reported to significantly reduce the flammability properties of polymers only by adding very low loading of nanoscale additives, offering an alternative to conventional flame retardants [1, 3, 4].
So far, layered silicates or clay [5–7], carbon-based nanomaterials like carbon nanotubes (CNTs) [1, 8–10], C60[11–13], and polyhedral oligomeric silsesquioxanes [14, 15] have been reported to improve significantly the thermal stability and flame retardancy of polymers. Recently, another carbon-based nanomaterial, nanodiamond (ND), has been widely used in the field of material science and engineering because of its unique properties such as the highest bulk modulus, high wear resistance, superior thermal conductivity and stability, excellent electrical insulating, and outstanding tribological properties[16, 17]. It has been reported that the addition of ND could improve the mechanical and thermal properties of polymeric materials such as epoxy resin , poly(vinyl alcohol) [17, 19], and poly(lactic acid) . Recently, Behler et al.  prepared composites nanofibers with high loading of ND via electrospun method which exhibited significantly enhanced tensile strength and Young’s modulus. Since CNTs and C60 can enhance the thermal stability and flame retardancy of polymer materials, thus ND, as their homogeneous carbon material, should also exhibit this capability. However, ND has not been employed as a potential flame retardant for polymeric materials like flammable polyolefins to date.
This communication will examine the effects of ND on thermal stability, flammability, and mechanical performances of polymers, especially for polyolefin. In addition, various spectroscopic techniques have found that the surface of ND have various functional groups, mostly oxygenated moieties such as -COOH (carboxylic acid), lactone, C = O (keto carbonyl), -C-O-C (ether), -OH (hydroxyl), and as such [17, 22]. Thus, to achieve a good dispersion of ND in polyolefin matrix, ND was modified by 3-aminopropyltriethyoxyl silane (APTES) to produce amine-functionalized nanodiamond (NH2-ND), and anhydride-grafted high density polyethylene (HDPE-g-MA) was chosen as the polymer matrix. By this way, we will create polymer nanocomposites with homogeneous dispersion of ND just by one-step reactive melt-blending strategy.
ND powder was purchased from Heyuan Zhonglian Nano-Tech, Co., Ltd., Heyuan City, Guangdong Province, China and produced by detonation of explosive compounds, including 2-methyl 1,3,5-trinitrobenzene and hexahydro-1,3,5-trinitro-1,3,5-triazine, under an inert atmosphere. Maleic HDPE-g-MA(CMG 9804) was purchased from Shanghai Kumho Sunny New Technology Development Co., Ltd., Minhang District, Shanghai, China with a maleic anhydride content of 0.9 wt.%. All chemical reagents like APTES, absolute ethanol, and other materials are received and used without further purification.
Fabrication of functionalized ND and its nanocomposites
Amine-functionalized nanodiamond (NH2-ND) was fabricated by the following procedure. Typically, 50 mg ND powder was dispersed in absolute ethanol via sonication for 30 min and then, an excess solution of APTES was slowly dropped and stirred at 80 °C for 8 h to complete the reaction. The NH2-ND was filtered and subsequently washed with ethanol for at least 5 times, which then was dried at 80 °C under vacuum for 12 h.
HDPE/ND nanocomposites were fabricated via a melt-blending method using a Thermo Haake mixer (Thermo Scientific, Boston, USA) with a rotation speed of 60 rpm at 180 °C for 15 min to enable the in situ reaction complete. Then, composites were compression-molded at 185 °C under 10 MPa into sheets with certain sizes and shapes for a particular test. HDPE composites with NH2-ND loading of 1.0 and 2.0 wt.% were designated as HPgND-1% and HPgND-2%, and HDPE/ND with 1.0 wt.% of ND was also prepared using the same protocol as a comparison named HPmND-1%.
Infrared spectrometry (IR, Vector-22 FT-IR, Bruker Corporation, Billerica, MA, USA) was used to characterize ND, NH2-ND, and the corresponding changes during reactive blending process. The morphology of ND, NH2-ND, and their dispersion were observed using transmission electron microscopy (TEM, JEM-1200EX, JEOL Ltd., Tokyo, Japan). Thermogravimetric analyses (TGA) was performed on a TA SDTQ600 thermal analyzer (TA Instruments, New Castle, DE, USA) at a scanning rate of 20 °C/min in air and N2 with a temperature range of 50 °C to approximately 800 °C. Cone calorimeter tests were performed using a dual cone calorimeter (Fire Testing Technology (FTT), London, UK) according to ISO 5660 at an incident flux of 35 kWm−2, and the size of specimens was 100 × 100 × 3.0 mm; each sample was performed in triplicate. The data reported herein from cone calorimetry are reproducible to within around ±10 %. Rheological properties were evaluated using an advanced rheological expanded system with parallel plate geometry of 25 mm in diameter.
Results and discussion
As shown in Figure 2B, for the host HDPE-g-MA, two strong absorption peaks located at 1,864 and 1,892 cm−1 are attributed to asymmetric stretching of anhydride carbonyl groups (O = C-O-C = O), whereas the major bands centered at 1,786 and 1,713 cm−1 correspond to the anhydride symmetric carbonyl groups and carboxylic groups stretching vibration, respectively in Figure 2B-c[25, 26]. The spectrum of HPmND does not show considerable differences when compared to the host polymer matrix, but the relative intensity of bands at 1,786 and 1,713 cm–1 changes which are most likely due to the superposition effect of carbonyl groups and to carboxylic groups from ND and those from HDPE-g-MA. However, these observations strongly demonstrate that only small changes in the molecular structure of HDPE-g-MA are brought by blending with ND even if some hydroxyl groups are on its surface in Figure 2B-d. In the contrary, by comparing the spectra for HDPE-g-MA and HDPE-g-MA/NH2-ND, significant changes of the IR absorptions occur in the carbonyl region, implying that a reaction has occurred between the MA and -NH2 groups upon blending for 15 min. Actually, both peaks at 1,860 and 1,892 cm−1 almost disappear, and the peak at 1,786 cm−1 reduces strongly in intensity and shifts to lower wave numbers for HPgND-1%. In response the relative intensity of peak at 1,713 cm−1 increases dramatically in Figure 2B-e. These findings strongly suggest the occurrence of a reaction between MA and NH2 groups with the formation of imide groups, in good agreement with the references [25, 27].
Detailed data obtained from cone calorimeter measurements at an incident flux of 35 kW/m 2
56 ± 1
660 ± 20
84.8 ± 0.5
0.054 ± 0.03
414 ± 10
62 ± 1
465 ± 15
79.5 ± 0.4
0.045 ± 0.02
320 ± 5
65 ± 1
77.1 ± 0.3
0.043 ± 0.02
343 ± 7
65 ± 1
480 ± 18
78.6 ± 0.5
0.049 ± 0.03
369 ± 8
Another important flame parameter, average specific extinction area (ASEA), should also be pointed out since it can evaluate the smoke-production capability during a fire and sometimes the smoke causes much more death than the heat in a real fire. The HDPE, HPmND-1%, HPgND-1%, and HPgND-2%, respectively, exhibit ASEA of 414, 320, 343, and 369 m2/kg, indicating that ND has certain capability of smoke suppression. Adding ND may cause the altering of the degradation and combustion mechanism of polymer and reduce the production of smoke particles. Whatever, this effect also contributes to improving the flame retardancy of polymer materials. As for the mechanism for both thermal degradation and flame retardancy of HDPE in the presence of ND, it is still being done presently and will be reported in the future.
HDPE-g-MA/amine-functionalized nanodiamond (NH2-ND) nanocomposites were facilely fabricated with homogeneous dispersion of ND particles via one-step reactive melt-blending strategy. The in situ reaction of MA groups in HDPE and NH2 groups in NH2-ND was responsible for the good dispersion of ND. The addition of pristine ND neither has obvious effect nor reduces the thermal stability and thermal oxidation stability of HDPE but NH2-ND enhances them due to good dispersion. Both ND and NH2-ND delay the time to ignition to different extent and remarkably reduce the heat release rate of HDPE, and the latter performs a little better in terms of enhancing the flame retardancy of polymers. Thus, nanodiamond will be a potential flame retardant for polyolefin or even other polymers. Adding both ND and NH2-ND slightly increase the yield strength and significantly enhance the Young’s modulus while without leading to the reduction of elongation at break of polymer host. Whatever, ND seems to display better flame retardancy than nano-CaCO3 and exhibit better mechanical reinforcing effects than nano-CB, since few related studies were reported. In addition, the color of nano-CB is also a big problem for some application where light color is required. Thus compared with other cheaper nanoparticles like nano-carbon black or CaCO3, ND exhibits comprehensive advantages in terms of improving the flame retardancy and mechanical properties despite its expensive price. Therefore, As-prepared composites will find potential applications such as in building, electric and electronic, and aerospace fields due to their improved flame retardancy and mechanical performances. Both flame retardancy and reinforcing mechanisms are still being investigated in detail and will be reported in the future.
The authors gratefully acknowledge the financial support of the Natural Science Foundation of Zhejiang Province of China (No.Y3100124), Scientific Research Foundation of Zhejiang Agriculture & Forestry University (No. 2351001088) and the Youth Innovation Team Foundation (2009RC03).
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