Pollen fillers for reinforcing and strengthening of epoxy composites
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Pollen grains have the potential to be effective plant-based biorenewable fillers in polymer matrices due to their high modulus, strength, chemical stability, and unique nanoscale architectures. In this work, we present evidence for the effectiveness of pollen as a reinforcing filler in epoxy matrices, characterized as a function of pollen loading and surface treatment. Composites prepared with unmodified native defatted ragweed pollen (D) displayed decreased mechanical properties and increasing glass transition temperatures (Tg) with increasing pollen loading. A soft interphase was observed to form around native pollen that is likely due to incompletely cured epoxy, resulting in decreased mechanical properties. However, pollen treated via a common base-acid (BA) surface preparation was a load-bearing, toughening filler in epoxy composites, displaying simultaneously increased tensile strength (by 47%) and strain at break (by 70%), improving interfacial morphology (absence of soft interphase), and increasing Tg at 10 wt% pollen loading. Elastic modulus improves by 14% with 10 wt% BA pollen loading, and fitting of the modulus with the Halpin-Tsai and Counto models results in an estimated pollen exine modulus of 8 GPa, the first reported pollen modulus measurement from composite studies. Improvements in mechanical properties in BA pollen versus D pollen likely result due to crosslinks with the epoxy matrix due to the presence of protic functional groups (hydroxyls or carboxyls) on the BA surface. BA-treated ragweed pollen shows promise as a toughening filler for imparting higher strength to polymers without increasing mass.
KeywordsPollen Polymer composite Waterborne epoxy
Polymer mechanical, thermal, and optical properties, among others, are often modified by the use of fillers [1, 2]. Filler materials for mechanical reinforcement in polymer composites range from inorganic calcium carbonate to organic carbon black and nanotubes [1, 2, 3, 4]. It is recognized that many types of filler particulates are denser than the matrix polymer, which increases the mass of final parts. In addition, some mined inorganics and carbon-rich fillers are not produced sustainably. Due to concerns over energy efficiency deriving from mass (for example in vehicles) and increased emphasis on sustainability, there is interest in lower-density, sustainably sourced filler alternatives.
Pollen exine has the potential to be an effective biorenewable filler due to its high strength, chemical stability, low density (when hollow) and unique architecture [5, 6, 7, 8]. Pollen is an ubiquitous natural material that is already sold as a product for medical purposes, and can be harvested in large quantities from sustainable non-food plant resources. Biologically derived particules including pollen, virus, and bacteria have attracted interest as templates for synthetic inorganic mimics that also possess properties such as magnetic forces and unique spectroscopic signatures [9, 10, 11, 12, 13]. Due to their complex biologically derived morphology, these mimics may provide unique function as fillers. In addition, pollen exine has been proposed as vehicles for the delivery of pharmaceuticals and vaccines [14, 15].
Pollen grains carry male gametes for plant reproduction, and they comprise an inner intine layer surrounded by a hard outer exine layer [16, 17]. The exine, composed of a substance called sporopollenin, is highly crosslinked and consists of fatty acid, phenylpropanoid, and phenolic substances. Sporopollenin is an extraordinarily stable natural organic substance [18, 19]. The monomers and macromers thought to comprise sporopollenin include aliphatic chains, aromatic cross-linkers (mainly cinnamic acids), ether cross-linkers, and esters. The composition of exine should support its compatibility with relatively polar polymers. Underneath the exine, and not exposed to the outer surface, lies the cellulose-rich intine. Cellular material contained in a pollen grain can be extracted readily by using well-known preparatory methods, resulting in a strong, hollow, and clean exine shell.
Only two studies have explored pollen grains as reinforcing filler, in the thermoplastics polystyrene, polycaprolactone, and polyvinyl acetate [6, 7]. These studies showed that surface treatment and surface functionalization of pollen was critical in tuning interfacial adhesion and mechanical properties of pollen-polymer composites, in order to make pollen an effective load-bearing filler. To date, the effectiveness of pollen fillers in thermosetting polymers has remained unexplored. Epoxy resins are widely used in adhesives, coatings, composites, electric systems, and aerospace applications. Epoxy resins can be crosslinked with curing agents containing multiple amine, hydroxyl, and carboxyl groups in order to form flexible or rigid materials. Due to increased legislative restrictions on organic solvent emissions, waterborne epoxy resins are becoming increasingly important in coatings and adhesives . Additionally, waterborne resins permit straightforward addition and dispersion of water-dispersible fillers.
Below, we demonstrate for the first time the successful incorporation of pollen in epoxy matrices, in particular a waterborne epoxy formulation. Short ragweed pollen (A. artemisiifolia) is selected as a model pollen grain because of its natural abundance, unique “spiny” echinate surface morphology, and previously published studies on composites and adhesion [21, 22]. As received, defatted pollen is compared to pollen modified by a base-acid hydrolysis. The effectiveness of pollen as a reinforcing and strengthening filler in epoxy is characterized by mechanical properties, interfacial morphology, and glass transition temperature of pollen-polymer composites as a function of pollen loading and pollen treatment.
2 Experimental section
Defatted (D) short ragweed (A. artemisiifolia, Greer Laboratories) pollen grains were stored at 4 °C prior to use. Potassium hydroxide (KOH, EMD Millipore) and phosphoric acid (H3PO4, BDH chemicals) were used for subsequent pollen cleaning and surface treatment. Epoxy resin (Air Products and Chemicals Inc., Ancarez AR555) consisted of diglycidyl ether of bisphenol-A (DGEBA) suspended at 55 wt% solids in water stabilized by a nonionic surfactant. The nominal particle size in the suspension was D50 = 0.5 μm and the epoxy has a manufacturer-reported equivalent weight (EEW) of 550. Poly(oxypropylenediamine) (Air Products and Chemicals Inc., Anquamine 401) as a 70 wt% aqueous solution, with an amine hydrogen equivalent weight (AHEW) of 200, was diluted with an equal mass of DI water to reduce the viscosity prior to use.
2.2 Pollen cellular material extraction
A base-acid treatment procedure (BA) was used to clean the exine prior to incorporation as a filler in epoxy films [5, 6, 23]. Pollen was suspended in an aqueous solution (6 w/v%) of KOH for 24 h at 25 °C under constant stirring. Following this base treatment, the pollen was washed with hot water, ethanol, and dried, and was then dispersed in 85% H3PO4 for 7 days at 60 °C. The pollen was washed with hot water, acetone, ethanol, and then dried. Approximately 80% of the mass of the pollen was removed during the BA treatment.
2.3 Pollen-epoxy composite film preparation
Stoichiometric amounts of epoxy and amine were mixed at room temperature with the desired amount of D or BA pollen suspended in water in order to form composites of 0–15 wt% pollen loading. The mixture was magnetically stirred for 1–5 h depending on pollen concentration, with higher concentrations mixed for longer times. The mixtures were then precured for 0.5–2 h at room temperature until the viscosity of the mixture was suitable for casting. Precuring times were determined by visual inspection and increased with pollen concentration because greater amounts of water (introduced with the pollen suspension) were present at higher pollen concentration, diluting the reactive epoxy. The mixture was cast on glass plates with a doctor blade and allowed to dry in air for a short time until the mixture is not able to flow on the substrate. Then, films were then cured for 20 min at 100 °C. Samples were peeled off the substrates using a razor. Neat epoxy was prepared using the same protocol. Both D and BA pollen were incorporated into epoxy matrices. The composite samples are labeled as follows: Epoxy with native defatted pollen (E-D) and epoxy with base-acid-treated pollen (E-BA).
3 Results and discussion
3.1 FTIR analysis
BA-Epoxy displays several additional peaks versus BA pollen, indicating the strong interactions between BA pollen and epoxy. Peaks associated with the epoxide group appear at 3060 cm−1 due to C-H stretching and at 830 cm−1 due to C-O-C stretching . C-H stretches of aromatic groups in the epoxy molecules appear as a shoulder peak at 2925 cm−1 [27, 28, 29]. The C-C stretching of aromatics also appears at 1511 cm−1 and the C=C stretching of aromatic groups appears at 1459 cm−1 [27, 28, 30]. C-O-C stretching of epoxy ether groups appears at 1248 cm−1 and 1039 cm−1 [27, 28, 29]. Many of these peaks are lacking in D-Epoxy, and where new peaks are visible (1511 cm−1 and 1246 cm−1), they are relatively weak signals. The higher intensity of peaks and presence of numerous new peaks on BA-Epoxy versus D-Epoxy may be due to higher fractions of free hydroxyl and carboxyl groups more accessible for surface interactions on BA pollen versus D pollen. It is possible that the epoxide reacted with pollen surface hydroxyls or carboxyls, forming covalent linkages between the pollen and the epoxy matrix Fig. 1, shows that between BA and BA-Epoxy, the ratio of –OH (~ 3400 cm−1) groups to –CH2 (2930 cm−1) is unchanged, while the ratio of –COOH (1706 cm−1) to both –OH and –CH2 decreases. This may indicate the preferential participation of carboxyl groups in reaction with epoxide.
3.2 Interfacial morphology
D pollen may also affect the crosslinking ability of the amine. The supernatant of amine solutions mixed with D pollen displays a strong color shift, from yellow to dark brown (Fig. S5, supporting information). In the BA pollen mixtures, where intracellular material is absent, no color shift occurs. D pollen mixed with only water extracted some intracellular material, but still remained essentially clear, so extracted intracellular material alone does not account for the strong color shift. Films cured with unaltered amine and the altered D-amine immediately after mixture were transparent. However, films cured with D-amine after 3 days became opaque (Fig. S6). It is well known that amines can strongly bind to lipid structures, such as phospholipids and cellulose, which are present in D pollen intracellular material within the D pollen [31, 32, 33]. One potential explanation is that the intracellular material is released from within the pollen grain and strongly binds with the amine crosslinker overtime, a possibility to be explored below.
3.4 Mechanical properties
As shown in Fig. 5b, a better fit is provided by the Counto model. The largest percent errors were 2.7% and 1.8% for the Halpin-Tsai and the Counto model respectively. The Halpin-Tsai model resulted in a fitted modulus of 8170 MPa and the Counto model returned a fitted modulus of 8005 MPa for BA pollen versus the measured 2499 MPa of the neat epoxy polymer. Thus, modeling indicates that ragweed pollen has a high modulus value, exceeding the neat epoxy by a factor of 3.2, supporting its utility as a reinforcing filler.
3.5 Thermal properties
3.6 Density of materials
The density of D pollen was measured with pycnometry to be 1.305 g/cm3, and the BA pollen density was found to be 1.165 g/cm3. Intracellular material in D pollen likely makes it denser than hollow BA-treated pollen. However, native D pollen was considerably less dense than some widely used mineral-based fillers, such as talc (ρ = 2.75 g/cm3) and calcium carbonate (ρ = 2.71 g/cm3), and the density of D pollen is closer to those of carbon nanotubes (ρ = 1.3–1.4 g/cm3), cellulose (ρ = 1.5 g/cm3), and starch (ρ = 1.5 g/cm3). The shells of BA-treated pollen have a lower density than all of these fillers. The pycnometry method was also used to determine the neat epoxy and pollen-epoxy composite densities. Neat epoxy’s density was measured as 1.214 g/cm3, comparable to previously measured densities of epoxy .
E-D and E-BA film densities were determined to be 1.208 g/cm3 and 1.221 g/cm3, respectively. Thus, unlike many inorganic fillers, pollen does not significantly increase the density of the epoxy matrix. BA pollen decreased the material density by ~ 0.5% at a 10 wt% loading versus neat epoxy. However, E-BA mechanical properties appear to still be increasing at 10 wt% pollen loading. Thus, it is likely that further decreases in density are possible while still enhancing the mechanical properties of the cured epoxy. Also, interesting future work may involve sealing BA pollen and trapping air within the shells, which would allow for larger density reductions.
The efficacy of ragweed pollen as a filler on the thermal, interfacial, and mechanical properties of epoxy-pollen composites was investigated. Composites prepared with as received ragweed pollen (D) displayed decreased mechanical properties with increasing pollen loading due to the presence of a soft interphase. D pollen is unable to crosslink with epoxy, likely due to a lower amount of protic functional groups on the exine surface. However, D pollen composites still showed increased glass transition temperatures (Tg) with increasing pollen loading. Pollen treated via a base-acid (BA) surface preparation becomes a load-bearing filler in epoxy, displaying simultaneous stiffening and strengthening coupled with an improved interfacial morphology and increased Tg with increasing pollen loading. Modeling the elastic modulus gains with both the Halpin-Tsai and Counto models resulted in an estimated pollen exine modulus of 8 GPa, the first reported pollen modulus measurement from composite studies. Improvements in mechanical properties in BA pollen versus D pollen likely result due to crosslinks with the epoxy matrix due to the presence of protic functional groups (hydroxyls or carboxyls) on the BA surface. BA pollen displayed higher compatibility with epoxy than with the thermoplastics previously studied (polystyrene, polycaprolactone, and polyvinyl acetate) [6, 7], and with no additional surface functionalization step. Finally, BA pollen was found to lower slightly the composite density relative to neat epoxy. These results indicate that pollen is a promising filler for creating high strength, light-weight polymer composites and merits further study, for example, investigating additional matrices with potential compatibility with pollen or incorporating pollen of different species in polymers in order to elucidate the effect of filler microstructure on the wetting and adhesion of fillers.
We would like to thank the Renewable Bioproducts Institute (Georgia Institute of Technology) and the Air Force Office of Scientific Research (Grant # FA9550-10-1-0555) for financial support of this research.
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