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Flexible Bionanocomposites from Epoxidised Hemp Seed Oil Thermosetting Resin Reinforced with Halloysite Nanotubes Peter S. Shuttleworth, Ana Maria Díez-Pascual, Carlos Marco, and Gary Ellis J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b00103 • Publication Date (Web): 27 Feb 2017 Downloaded from http://pubs.acs.org on February 28, 2017
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The Journal of Physical Chemistry
Flexible Bionanocomposites from Epoxidised Hemp Seed Oil Thermosetting Resin Reinforced with Halloysite Nanotubes Peter S. Shuttleworth,* Ana M. Díez-Pascual,† Carlos Marco and Gary Ellis Institute of Polymer Science and Technology, CSIC, c/ Juan de la Cierva 3, 28006 Madrid, Spain
Supporting Information Placeholder ABSTRACT: Hempseed (Cannabis sativa L.) oil comprises a variety of beneficial unsaturated triglycerides with welldocumented nutritional and health benefits. However, it can become rancid over a relatively short time period leading to increased industrial costs and waste of a valuable product. The development of sustainable polymers is presented as a strategy where both the presence of unsaturation and peroxide content could be affectively utilised to alleviate both this waste and financial burden. After reaction with peroxyacetic acid, incorporation of halloysite nanotubes (HNTs) and subsequent thermal curing, without the need for organic solvents or interfacial modifiers, flexible transparent materials with a low glass transition temperature were developed. The improvement in thermal stability and both the static and dynamic mechanical properties of the bionanocomposites were significantly enhanced with the well-dispersed HNT filler. At an optimum concentration of 0.5 wt.% HNTs, a simultaneous increase in stiffness, strength, ductility and toughness was observed in comparison to the unfilled cured resin. These sustainable food-waste derived bionanocomposites may provide an interesting alternative to petroleumbased materials, particularly for low-load bearing applications, such as packaging.
Introduction The worldwide demand for bio-based products derived from renewable sources has dramatically increased over the last 10 - 15 years due to global awareness of the nonsustainable nature of petroleum products and their disposal, and the fluctuating price of crude oil. In this context, costeffective and sustainable polymeric materials that can replace those based on petroleum feedstock’s are actively being 1 sought. Of particular interest are vegetable oils that contain a broad range of unsaturated triglycerides that, by their very nature of high double-bond content, are susceptible to heat, 2 light and air, resulting in undesired peroxide formation. The maximum peroxide content allowed for human consumption 3-4 is 5 mmol/kg oil, and rancidity of these oils through oxidation not only imbues a large economic loss at the industrial level, but also generates waste of a rich chemical source. However, susceptibility to oxidation naturally lends itself to chemical modification, and there has been recent renewed interest in the development of epoxidized vegetable oil
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(EVO) based resins using oils, such as soybean, linseed, tung, corn and fish, etc. Among these, model systems based on epoxidised linseed and soybean oil have been used to help understand the properties of mixed triglyceride systems 6-7 typical of vegetable oils. These bio-based resins are sustainable, inexpensive and can easily substitute other petroleum derived polymer systems for certain low-load bearing applications. Thermosetting resins are highly-crosslinked polymers that are cured by heat, pressure, light radiation or a combination 8 of these. The double bonds present in the unsaturated fatty acids of the vegetable oils can be used as reaction sites for the crosslinking process, and the material properties are strongly dependent on the degree of cross linking: those with a high cross linking density exhibit good mechanical and 9 thermal properties. The curing process of plant oil based thermosets has been investigated in numerous studies using conventional crosslinkers including acids, anhydrides and 10amines, primarily sourced from non-renewable feedstocks. 11 In particular, four main techniques have been reported regarding the preparation of EVO resins: epoxidation with organic and inorganic peroxides using a transition metal catalyst, epoxidation using halohydrines, epoxidation via molecular oxygen and in situ epoxidation with percarboxylic 12 acid. The first method involves a transition metal catalyst such as titanium grafted onto silica, which has low chemical 13 and mechanical stability and is costly. The second approach with halohydrines is environmentally unfriendly since it uses 14 hypohalous acids and salts as reagents, while epoxidation via molecular oxygen is considered more environmentally friendly and economically viable when silver is used as catalyst, although it only generates low levels of double bond 15 conversion and may result in vegetable oil degradation. Thus, the most suitable and efficient technique for epoxidation of vegetable oils appears to be the in situ reaction with percarboxylic acid using an acidic ion exchange resin as catalyst. Cannabis sativa L, commonly referred to as hemp, is a crop that presents considerable potential for the production of sustainable materials, and has long been used as a fibre th source for rope, clothing and other materials. In the early 19 16 century hemp was the world’s largest fibre crop, although its production has decreased due to the introduction of synthetic fibres. In addition to the fibre, oil is also produced from the plant, and is used as a
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nutrient rich feedstock. Use of hemp seed oil has seen a recent resurgence in its popularity not only due to its nutritional benefits, containing essential fatty acids, but also due to other reported health benefits, such as lowering cholester10ol, alleviating dermatitis and reducing high blood pressure. 11, 18 However, its beneficially high unsaturated fatty acids content makes it prone to oxidation / rancidity and it quickly becomes unsuitable for human consumption. Conversion of this product to needed alternative polymeric materials, rather than downgrading it to animal feed or disposal could be considered more environmentally and economically practical. Like most conventional epoxy thermosets, epoxidized hemp oil (EHO) and other EVO composites can be brittle 19 and require toughening. This can be achieved via blending with other polymers, although this approach can lead to a decrease in both mechanical and physical properties such as 20 modulus, strength and thermal stability. Motivated by recent developments in nanocomposite technology, toughening epoxies using inorganic nanofillers has been demonstrat21 ed. For example, the addition of naturally derived halloysite nanotubes (HNTs) to epoxy resins has been found to significantly increase their impact strength, whilst maintaining 22 both strength and stiffness. Given the geometrical similarity between halloysite nanotubes (HNTs) and carbon nanotubes (CNTs), HNTs are expected to be an effective impact modifier for brittle polymers at a lower cost. In addition, due to their multi-layered structure with only a few surface hydroxyl groups, HNTs present interesting surface chemical properties aiding dispersion within both polar and non-polar 23 polymeric matrices alike. This occurs through the for24 mation of H-bonds, or via weak secondary van der Waal’s 23 interactions. In this work we report for the first time the development and characterization of eco-friendly and sustainable cured epoxidized hemp oil (EHOC) / HNT bionanocomposites. The influence of HNT loading on the morphology, thermal and mechanical properties of the composites is investigated, and their structure-property relationships are discussed. These novel biomaterials show great potential to replace conventional petroleum-based composites for use in low-load bearing applications.
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(DMAP), purity > 99%, were obtained from Sigma-Aldrich. All reactants were used without further purification. Epoxidation reaction: Epoxidized hemp oil (EHO) was synthesized by reacting HO with peroxyacetic acid that was formed in-situ following approximately the procedure of 25 Manthey et al. Briefly, HO (50.3 g, 0.32 mol), AA (12.8 g, 0.21 mol) and the catalyst Amberlite IR-120 (7.5 g) were added to a 200 ml three-necked oil jacketed, flat bottomed vessel equipped with a magnetic stirrer (150 rpm) with the temperature maintained via an external programmable Julabo circulating temperature bath. The reactants were mixed for 5 min, then H2O2 (36.3 g, 0.32 mol) was added dropwise over a 1h period at 40 ºC. The reaction was then heated to 75 ºC, and maintained at this temperature for 5 h and then 85 ºC for a further 1.5h, all whilst maintaining stirring. Afterwards, the mixture was cooled to room temperature and washed with distilled water three times (cool, near-boiling, cool) to remove the residual peroxyacetic acid and subsequently filtered. The product was then centrifuged to separate the water/ EHO emulsion. To remove any residual water, the modified oil was further dried in a vacuum oven at 50 ºC for -1 12 h. Presence of a peak at 822 cm in the FTIR spectra corresponding to the C–O epoxide ring vibration, and the disappearance of the C=C stretching vibration and C-H stretching vibration of the cis-double bond (ESI, Fig. S1) confirmed the 26 success of the epoxidation reaction. EHO curing and nanocomposite preparation: EHO was mixed with DMAP at 120 ºC for 3 min using a mechanical stirrer. The mixture was then poured into silicone moulds and cured in an oven (VD 23 from BINDER GmbH, Germany) at 140 °C (or 120 ºC) overnight, to form the cured product EHOC. An analogous procedure was used to prepare EHOCbased nanocomposites containing 0.1, 0.5 and 1.0 wt% HNT. EHO and the corresponding amount of HNT were blended for 5 min at 300 rpm at room temperature in a planetary centrifugal mixer (Thinky Corporation AR-250 from GENTEC BENELUX, Belgium). DMAP was subsequently added to the mixture and blended and cured under the aforementioned conditions. Characterization techniques:
Experimental Materials and reagents: Cold pressed raw hemp oil (Cannabis sativa L), hereafter designated HO, was provided by Finolaky Seed and Oil, Finland. Its major properties are listed: iodine number = 160; saponification value = 193; d25ºC = 3 0.93 g/cm , chlorophyll content: 5-80 ppm. Its fatty acid composition is reported in the ESI section, Table S1. Halloysite nanotubes (HNT) were generously provided by Northstar Clay Mines LLC, UTAH, USA. They have an average tube diameter of 60 nm, an inner diameter of 30 nm, a 2 -1 typical surface area of 65 m g , with a pore volume of 1.25 ml -1 -3 g and density of 2.53 g cm . They contain 36.9% Al2O3, 48.0% SiO2, 2.5% Fe2O3 and traces of other metallic oxides, with a lower limit of impurities (LOI) of 13.8. The HNT were dried overnight in an oven to remove absorbed moisture. Analytical grade glacial acetic acid (AA) (C2H4O2) and hydrogen peroxide (H2O2) with minimum concentrations of 99.7% and 33%, respectively, were purchased from Panreac Química S.L.U., Spain. Amberlite IR-120 and 4-dimethylaminopyridine
Scanning electron microscopy (SEM): The surface morphology of the halloysite nanoparticles and fractured specimens were analysed using a SU8000 Hitachi SEM applying an acceleration voltage of 1.0 or 3.0 kV respectively. Samples were cryo-fractured in liquid nitrogen and then coated with a ~5 nm Cr over-layer to avoid charge accumulation during electron irradiation. Fourier transform infrared(FTIR): IR spectra of the films were recorded on a Perkin Elmer Spectrum One spectrometer equipped with a Universal ATR sampling accessory and -1 each sample analysed in the spectral range 4000-550 cm at a -1 resolution of 4 cm . FTIR analysis of the oxidation and subsequent drying of the unprocessed HO was obtained using a System 2000 infrared spectrometer (Perkin Elmer Ltd, UK) in the spectral -1 -1 range of 4000–670 cm , with a resolution of 4 cm . For this, a drop of the oil was placed on the small diamond ATR crystal
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The Journal of Physical Chemistry
of a High Temperature Golden Gate ATR accessory (Specac Ltd, UK) and maintained at 120 ºC for around 10 hours. Spectra were recorded every minute (30 accumulated scans) and analysed with Perkin-Elmer Spectrum Timebase® software. Raman: Raman measurements were performed in the Raman Microspectroscopy Laboratory of the Characterization Service in the Institute of Polymer Science & Technology, CSIC, Spain using a Renishaw InVia Reflex Raman system (Renishaw plc.,Wotton-under-Edge, U.K.). A laser wavelength of 785 nm was employed with a 20x objective and a power at the sample of
