materials
Article A New Method of Diatomaceous Earth Fractionation—A Bio-Raw Material Source for Epoxy-Based Composites
Marta Dobrosielska 1, Renata Dobrucka 1,2,*, Michał Gloc 1, Dariusz Brz ˛akalski 3 , Marcin Szyma ´nski 4 , Krzysztof J. Kurzydłowski 1 and Robert E. Przekop 4,*
1 Faculty of Materials Science and Engineering, Warsaw University of Technology, ul. Wołoska 141, 02-507 Warsaw, Poland; [email protected] (M.D.); [email protected] (M.G.); [email protected] (K.J.K.) 2 Department of Non-Food Products Quality and Packaging Development, Institute of Quality Science, Pozna´nUniversity of Economics and Business, al. Niepodległo´sci10, 61-875 Pozna´n,Poland 3 Faculty of Chemistry, Adam Mickiewicz University in Pozna´n,8 Uniwersytetu Pozna´nskiego, 61-614 Pozna´n,Poland; [email protected] 4 Centre for Advanced Technologies, Adam Mickiewicz University in Pozna´n, ul. Uniwersytetu Pozna´nskiego10, 61-614 Poznan, Poland; [email protected] * Correspondence: [email protected] or [email protected] (R.D.); [email protected] (R.E.P.)
Abstract: The authors of this paper use an original method of diatomaceous earth fractionation, which allows for obtaining a filler with a specific particle size distribution. The method makes it possible to separate small, disintegrated and broken diatom frustules from those which maintained their original form in diatomaceous earth. The study covers a range of tests conducted to prove that such a separated diatomic fraction (3–30 µm) shows features different from the base diatomite (from 1 Citation: Dobrosielska, M.; to above 40 µm) used as an epoxy resin filler. We have examined the mechanical properties of a series Dobrucka, R.; Gloc, M.; Brz ˛akalski,D.; Szyma´nski,M.; Kurzydłowski, K.J.; of diatomite/resin composites, considering the weight fraction of diatoms and the parameters of the Przekop, R.E. A New Method of composite production process. The studied composites of Epidian 601 epoxy resin cross-linked with Diatomaceous Earth Fractionation—A amine-based curing agent Z-1 contained 0 to 70% vol. of diatoms or diatomaceous earth. Samples Bio-Raw Material Source for were produced by being casted into silicone molds in vacuum degassing conditions and, alternatively, Epoxy-Based Composites. Materials without degassing. The results have shown that the size and morphology of the filler based on 2021, 14, 1663. https://doi.org/ diatomaceous earth affects mechanical and rheological properties of systems based on epoxy resin. 10.3390/ma14071663 Elongation at rupture and flexural stress at rupture were both raised by up to 35%, and impact strength by up to 25%. Academic Editor: Debora Puglia Keywords: diatomite; bio-composites; mechanical properties; fractionation; purification of diatoma- Received: 8 March 2021 ceous earth; bio-raw materials Accepted: 23 March 2021 Published: 28 March 2021
Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in published maps and institutional affil- The engineering of new polymer-based composites with a reduced fraction of petroleum- iations. based material matrix is one of the trends in designing novel products containing natural raw materials. Due to their functional properties such as adhesion, flame resistance or chemical and thermal stability, epoxy resins are used in various sectors of industry. Despite having many benefits, they are fragile and have a low impact and crack propagation resistance, which considerably limits their scope of use. The curing of epoxy resins is Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. accompanied by a very low shrink, so a cured epoxy resin precisely represents the shape This article is an open access article and dimensions of the mold. This creates great possibilities to produce elements of different distributed under the terms and shapes [1]. However, it is still necessary to increase the toughness and strength of such conditions of the Creative Commons a material in order to satisfy the increasing demands of specific applications [2]. This Attribution (CC BY) license (https:// is despite the significant improvement of the already excellent resins by using them for creativecommons.org/licenses/by/ modification and as composite matrices [3,4]. Fillers, having a high surface area/volume 4.0/). ratio, are generally expected to show better mechanical properties. A high surface area is
Materials 2021, 14, 1663. https://doi.org/10.3390/ma14071663 https://www.mdpi.com/journal/materials Materials 2021, 14, 1663 2 of 29
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high surface area is usually associated with a small filler size and with extremely rough surfacesusually associated[5]. Diatoms with as biogenic a small filler materials size and have with a complex extremely porous rough hierarchical surfaces [5 ].structure, Diatoms whichas biogenic makes materials them micrometric have a complex fillers with porous a hi hierarchicalghly developed structure, specific which surface. makes It is them es- timatedmicrometric that diatoms fillers with produce a highly even developed up to 25% specific of organic surface. matter It is estimated in the ocean that and diatoms ap- proximatelyproduce even 25% up of to oxygen 25% of on organic the Earth matter [6]. in Thei ther ocean cell wall and is approximately saturated with 25% silica, of oxygenwhich hason thea great Earth potential [6]. Their as cell a raw wall material. is saturated The with principal silica, source which hasof bio-silica a great potential is the so-called as a raw diatomaceousmaterial. The principalearth (diatomite). source of It bio-silica is a mixture is the of so-called diatom diatomaceousfrustules with earth various (diatomite). sizes and It morphologies,is a mixture of including diatom frustules significant with variousamounts sizes of damaged and morphologies, and broken including structures significant being smalleramounts than of damaged2–3 µm. and broken structures being smaller than 2–3 µm. TheThe Earth’s Earth’s crust crust is is composed composed of of 93 93 elemen elements,ts, eight eight of of which which represent represent 99.5% 99.5% of of its its weight.weight. These These eight mostmost importantimportant elements elements are are oxygen, oxygen, silicone, silicone, aluminum, aluminum, iron, iron, calcium, cal- cium,potassium, potassium, and magnesium and magnesium (Figure (Figure1). We should1). We pointshould out point that out diatomaceous that diatomaceous earth is, earthon the is, oneon the hand, one a hand, representation a representation of the most of the common most common elements elements existing existing in theEarth’s in the Earth’scrust, and crust, on theand other on the hand other one hand of the one most of efficientthe most sources efficient of biogenicsources of silica biogenic with a silica great withpotential a great for potential commercial for commercial use. use.
FigureFigure 1. 1. ElementalElemental composition composition of of Earth Earth crust crust ( (AA)) and and diatom diatom frustule frustule ( (BB).). Diatomaceous earth contains 60–95% amorphous silica. Diatoms are one of the most Diatomaceous earth contains 60%–95% amorphous silica. Diatoms are one of the spectacular examples of biologically derived nano-structured materials [7]. Each diatom most spectacular examples of biologically derived nano-structured materials [7]. Each frustule has species-specific, regularly arrayed features: pores, ridges and protuberance [8]. diatom frustule has species-specific, regularly arrayed features: pores, ridges and pro- The skeleton of diatoms, known as the frustule or siliceous shell, is made of nano-size silica tuberance [8]. The skeleton of diatoms, known as the frustule or siliceous shell, is made of with a three-dimensional intricate framework [9]. The mechanical properties of individual nano-size silica with a three-dimensional intricate framework [9]. The mechanical prop- diatom frustules vary with the location of measurement, which is attributed to the different erties of individual diatom frustules vary with the location of measurement, which is at- stages of the bio-mineralization process [10]. The energy required to break the frustules tributedand the to area the of different fracture stages were further of the bio-mi shownneralization to be relatively process high, [10]. as the The cracks energy travelled required for toaround break 40the nm frustules through and silica the particlesarea of fracture of the diatomwere further frustules shown [11]. to be relatively high, as the cracksDue totravelled their peculiar for around characteristics, 40 nm through diatoms silica have particles been of recently the diatom proposed frustules to be [11]. em- ployedDue in to nanotechnology their peculiar ascharacteristics, natural fillers [12diatoms]. The varietyhave been of unique recently frustule proposed architectures to be employedwere attractive in nanotechnology materials for optical,as natural mechanical, fillers [12]. and The transport variety properties.of unique frustule Diatomaceous archi- tecturesearth has were been attractive modified materials in different for ways optica and,l, mechanical, after treatments and transport of different properties. kinds, used Dia- in tomaceousthe construction earth ofhas advanced been modified devices in for differen light harvesting,t ways and, photonics, after treatments molecular of separation, different kinds,sensing, used and in drug the construction delivery. Diatomaceous of advanced earth devices has alsofor light been harvesting, used in photo-catalysis, photonics, mo- the lecularsynthesis separation, of zeolites, sensing, removal and of drug dyes, delivery. wastewater Diatomaceous treatment, cement earth has production, also been filtration, used in photo-catalysis,nanotechnology, the chromatography, synthesis of zeolites, and mostly removal as anof adsorbentdyes, wastewater [13]. treatment, cement production,As mentioned filtration, above, nanotechnology, diatomaceous chro earthmatography, contains diatom and frustules mostly ofas variousan adsorbent sizes, a [13].lot of fractions that are mechanically damaged, as well as agglomerates and colonies. In the literature,As mentioned there are above, methods diatomaceous for purifying earth diatomaceous contains diatom earth with frustules the use of ofvarious thermal sizes, and athermo-chemical lot of fractions that treatments are mechanically [14]. Sun etdamaged, al. [15] cleaned as well diatomaceousas agglomerates earth and using colonies. sodium In thehexametaphosphate literature, there are as method a dispersants for purifying combined diatomaceous with centrifugation. earth with Other the use scientists of thermal [16] andused thermo-chemical cold and hot acidic treatments solutions. [14]. Various Sun et methodsal. [15] cleaned of the purificationdiatomaceous and earth separation using sodiumof diatomaceous hexametaphosphate earth before as itsa dispersant final use are combined known. with Hydraulic centrifugation. industrial Other separation scien- tistsmethods [16] used are among cold and the mosthot acidic efficient. solutions. These processesVarious methods use cyclones of the or hydrocyclonespurification and to separationachieve high of separationdiatomaceous efficiency. earth before However, its final their use main are purpose known. is Hydraulic to separate industrial sand and other inclusions [17].
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separation methods are among the most efficient. These processes use cyclones or hy- Materials 2021, 14, 1663 drocyclones to achieve high separation efficiency. However, their main purpose 3is of to 26 separate sand and other inclusions [17]. We have proposed and described a method of fractioning and purifying raw dia- tomaceous earth which allows for the generation of a fraction with a narrow size distri- We have proposed and described a method of fractioning and purifying raw diatoma- butionceous earthof diatom which frustules. allows for the generation of a fraction with a narrow size distribution of diatomWe frustules.prepared degassed and non-degassed composites based on epoxy resin, which containedWe prepared a variable degassed amount and (0, 12, non-degassed 24, 36, 48, 60, composites 70% vol.) basedof base on and epoxy fractionated resin, which di- atomite.contained The a variableprepared amount systems (0, were 12, 24, cast 36, in 48, silicone 60, 70% molds, vol.) ofcross-linked base andfractionated and then sub- di- jectedatomite. to Themechanical prepared and systems functional were property cast in silicone testing molds, and imaging cross-linked with and the thenuse of subjected a scan- ningto mechanical electron microscope. and functional The property tests allowed testing us and to determine imaging with whether the use the of methodology a scanning elec- of compositetron microscope. production The tests is appropriate allowed us from to determine the perspective whether of the the methodology following variables: of composite the degassingproduction of is the appropriate prepared from system thes, perspective contents of of the the filler following (% vol.) variables: in the thetested degassing systems, of andthe preparedthe method systems, of preparing contents the of applied the filler mo (%difier vol.) (fractionated in the tested and systems, base anddiatomite). the method of preparing the applied modifier (fractionated and base diatomite). 2. Experimental Section 2.1.2. Experimental Materials Section 2.1. Materials For the tests, we used epoxy resins produced by Zakłady Chemiczne Organika Sarzyna:For theEpidian tests, we601 used (Nowa epoxy Sarzyna, resins producedPoland) with by Zakłady an epoxide Chemiczne equivalent Organika of 0.50–0.55 Sarzyna: mol/100Epidian 601g and (Nowa a viscosity Sarzyna, of Poland) 700–1100 with [mPa·s], an epoxide cured equivalent with triethylenetetramine of 0.50–0.55 mol/100 (curing g and a agentviscosity Z-1). of 700–1100Diatomaceous [mPa ·s],earth cured (Perma-Guard with triethylenetetramine, Bountiful, UT, (curing USA) agent was Z-1).derived Diatoma- from diatomiteceous earth deposits. (Perma-Guard, It was provided Bountiful, in UT,a powd USA)er wasform derived and used from as received diatomite for deposits. “base di- It atomite”was provided preparations in a powder and fractionated form and used according as received to the for procedure “base diatomite” described preparations further for “fractionatedand fractionated diatomite” according preparations. to the procedure It mostly described contains further the for “fractionatedcells of Aulacoseira, diatomite” a planktonicpreparations. freshwater It mostly diatom. contains They the cellshave of a Aulacoseira,perforated scutellum a planktonic specific freshwater to that diatom.species (FigureThey have 2). aDiatoms perforated have scutellum a mechanism specific that to thatallows species them (Figure to create2). Diatoms colonies have (chains). a mecha- The mechanismnism that allows (“zipper”) them tois createpresented colonies in Figure (chains). 3. The The mechanismdiversified size (“zipper”) of diatoms is presented is due to in theirFigure life3. Thecycle. diversified Along with size cell of diatomsdivision, is diat dueoms to theirshrink life until cycle. they Along are no with longer cell division, capable diatoms shrink until they are no longer capable of dividing. They sexually multiply, as of dividing. They sexually multiply, as a result of which their cells reach their maximum a result of which their cells reach their maximum size, which is followed by vegetative size, which is followed by vegetative division where the cells start to shrink. For this division where the cells start to shrink. For this reason, diatomaceous earth is a filler that reason, diatomaceous earth is a filler that features a high heterogeneity of particle size, features a high heterogeneity of particle size, which has already been mentioned (Figure2), which has already been mentioned (Figure 2), and a high structural heterogeneity (Figure and a high structural heterogeneity (Figure4)[6]. 4) [6].
FigureFigure 2. Base 2. diatomaceousBase diatomaceous earth containing earth containing both fractured both fractured and non-fracture and non-fracturedd frustules, frustules,with a domi withnant a content dominant of Aulaco- content seira. of Aulacoseira.
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10 µm 10 µm
Figure 3. The “zipper” mechanism for creating colonies. Figure 3. The “zipper” mechanism for creating colonies.
10µm 10µm
Figure 4. Pores present in the frustule wall. Figure 4. Pores present in the frustule wall. 2.2. Sample Preparation 2.2. Sample Preparation 2.2. SampleRaw diatomaceous Preparation earth contains broken, mechanically damaged structures which differRaw by thediatomaceous geometry of earth their contains inner structur broken, e, mechanicallyshape, and size. damaged For the structures purpose of which this study,differ byraw the diatomite geometry was of theirmechanically inner structure,structur mixede, with shape, water and and size. fractionated For the purpose with theof thisuse ofstudy, sedimentation. raw diatomite The was small, mechanically broken frustule mixed pieces with water were and partly fractionated removed, with along the with use biggerof sedimentation. agglomerates/colonies, The The small, small, brokenby broken suspending frustule frustule thepieces pieces diatomite were were partlyin water removed, and subjecting along with the biggersuspension agglomerates/colonies, to sedimentation. Basedby by suspending suspending on interacting the the diatomite diatomite gravitational in in water water forces, and particles subjecting of dif- the ferentsuspension sizes tosettledto sedimentation.sedimentation. out at different Based Based ratios. on on interacting interacting gravitational gravitational forces, forces, particles particles of different of dif- ferentsizesThe settled sizes fractionation settled out at out different atwas different ratios.performed ratios. by placing 1 kg of diatomite in a 50 dm3 tank, 3 3 addingThe 40 fractionationfractionation dm3 of demineralized waswas performed performed water by andby placing placingmixing 1 kg mechanically1 ofkg diatomite of diatomite infor a 10 50 in dmmin, a 50 tank,and dm leaving adding tank, 3 3 itadding40 for dm 3.5 40of h dmto demineralized cause of demineralized sedimentation water andwater of the mixing and heavier mixing mechanically precipitate mechanically forfraction 10 for min, PRE1 10 andmin, (Figure leaving and leaving 5). it The for nextit3.5 for h step3.5 to cause h was to cause sedimentationto pour sedimentation off the ofsuspension the of heavier the heavierfrom precipitate above precipitate the fraction precipitate fraction PRE1 toPRE1 (Figure obtain (Figure5 ).suspension The 5). next The next1step (SUS1) step was andwas to pour precipitate to pour off theoff 1 the suspension (PRE1). suspension SUS1 from wasfrom above mixed above the for the precipitate 10 precipitate min with to to40 obtain obtain dm3 suspensionof suspension water and 1 3 1then(SUS1) (SUS1) left and andfor precipitate24 precipitate h to obtain 1 1 (PRE1). (PRE1). SUS2 and SUS1 SUS1 PRE2. was was mixedSUS2mixed was forfor 10left10 minmin for with48with h 40for40 dm dmsedimentation3 of water and to obtainthenthen left left PRE3 for 24 (which, h to obtainonce dried, SUS2 was and marked PRE2.PRE2. SUS2 as tail was fraction left for 0a) 48 and h forforSUS3 sedimentationsedimentation which was left to forobtain 168 PRE3 h to obtain (which, tail once fraction dried, 0b. was The marked PRE2 was as tailfilled fraction with 40 0a) dm and 3 of SUS3 water, which mixed was for left 10 3 minforfor 168 168 and h h toleft to obtain obtain for 24 tail tailh tofraction fraction obtain 0b. 0b.SUS4 The The andPRE2 PRE2 PRE4 was was filled(which, filled with withonce 40 40 dmdried, dm3 of wasofwater, water, marked mixed mixed asfor tailfor 10 fractionmin10 min andand 1b). left left SUS4for for 24 was 24h hto left toobtain obtainfor 24 SUS4 h SUS4 to obtainand and PRE4 PRE4 tail fraction(which, (which, 1a.once once The dried, dried, PRE1 was was was marked marked filled with as tail 40 dmfractionfraction3 of water, 1b). 1b). SUS4 SUS4mixed was was for left left10 formin for 24 24and h hto left to obtain obtain for two tail tail hoursfraction fraction to 1a.obtain 1a. The The SUS5,PRE1 PRE1 whichwas was filled was filled with left with for40 3 3 dm40 dm of water,of water, mixed mixed for for10 min 10 min and and left leftfor fortwo two hours hours to obtain to obtain3 SUS5, SUS5, which which was was left leftfor 48 h to obtain tail fraction 2. The PRE5 was filled with 40 dm 3of demineralized water, for 48 h to obtain tail fraction 2. The PRE5 was filled with 40 dm3 of demineralized water, mixed48 h to for obtain 10 min tail and fraction left for 2. 30The min PRE5 to obtain was filled PRE6 with (fraction 40 dm 4) and of demineralized SUS6, which was water, left mixed for 10 min and left for 30 min to obtain PRE6 (fraction 4) and SUS6, which was left formixed 48 hfor to 10obtain min andfraction left for3. The 30 min diatom to obtain precipitate PRE6 (individual(fraction 4) andfractions) SUS6, was which dried was with left for 48 h to obtain fraction 3. The diatom precipitate (individual fractions) was dried with a afor thin 48 hlayer to obtain at 80 °Cfraction to obtain 3. The a stable diatom weig precipitateht. The percentage (individual share fractions) of each was precipitate dried with is thin layer at 80 ◦C to obtain a stable weight. The percentage share of each precipitate is showna thin layer in Figure at 80 5. °C The to obtaintotal share a stable of the weig fractiht.ons The is percentage 95.7%, which share arises of each from precipitate the contents is shown in Figure5 . The total share of the fractions is 95.7%, which arises from the contents shownof extremely in Figure small 5. Thefragments total share of bio-silica of the fracti comingons isfrom 95.7%, the whichdiatoms arises that fromdid not the sediment contents of extremely small fragments of bio-silica coming from the diatoms that did not sediment inof extremelythe process small and fragmentswere rejected of bio-silica along with coming the operating from the liquid.diatoms The that morphology did not sediment of di- atomsin the processin each fractionand were was rejected characterized along with by scanningthe operating electron liquid. microscopy The morphology (SEM, Hitachi, of di- atoms in each fraction was characterized by scanning electron microscopy (SEM, Hitachi,
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Tokyo, Japan). On the basis of dynamic light scattering (DLS) (Figure 6) and SEM anal- yses (Figure 7), fraction 3 was chosen for further study, as explained in the “Discussion”. The developed method allows for the purification of raw diatomaceous earth by remov- Materials 2021 14 , , 1663 ing smaller and broken fragments of diatom frustules and by de-agglomeration (Figure5 of 26 6). Additionally, this method enables us to separate the fractions with bigger frustules (tail fraction 4) from those having a smaller geometric size (tail fraction 3); see Figure 6. in theThe process studied and composites were rejected of alongepoxy withresin the Epidian operating 601 liquid.(epoxy Thenumber morphology (mol/100g) of diatoms0.5–0.55, in viscosity each fraction at 25 °C was (mPa·s) characterized 800–1500), by scanningcross-linked electron with microscopyamine-based (SEM, curing Hitachi, agent Tokyo,Z-1 (triethylenetetramine, Japan). On the basis viscosity of dynamic at light25 °C scattering (mPa·s) 20–30; (DLS) (Figureamine number6) and SEM (mg analyses KOH/g) (minFigure 1100),7 ), fractioncontained 3 was different chosen quantities for further (0, study, 12, 24, as 36, explained 48, 60, 70% in thevol.) “Discussion”. of base and frac- The developedtionated diatoms method with allows an average for the purificationparticle size ofdistribution raw diatomaceous of approx. earth 10 µm by (Figure removing 8). smallerThe choice and of broken the resin/hardener fragments of system diatom was frustules dictated, and on bythe de-agglomeration one hand, by the necessity (Figure6 ).to Additionally,obtain a low viscosity this method of the enables mixture, us to and separate on the the other fractions hand, withby the bigger optimal frustules time of (tail its fractionfinal gelation. 4) from those having a smaller geometric size (tail fraction 3); see Figure6.
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FigureFigure 5.5. FractionationFractionation of of diatomaceous diatomaceous earth earth and and the the percentage percentage share share of each of each fraction fraction of diatomaceous of diatomaceous earth earth after afterfrac- fractionation.tionation.
Figure 6. Separation of particles of fractionated diatomaceous earth by size. Figure 6. Separation of particles of fractionated diatomaceous earth by size.
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FigureFigure 7.7. MicroscopicMicroscopic imagesimages ofof fractionatedfractionated diatomaceousdiatomaceous earthearth ((AA)) 0a, 0a, ( B(B)) 0b, 0b, ( C(C)) 1a, 1a, ( D(D)) 1b, 1b, ( E(E)) 2, 2, ( F(F)) 3, 3, ( G(G)) 4. 4.
The studied composites of epoxy resin Epidian 601 (epoxy number (mol/100g) 0.5–0.55, viscosity at 25 ◦C (mPa·s) 800–1500), cross-linked with amine-based curing agent Z-1 (triethylenetetramine, viscosity at 25 ◦C (mPa·s) 20–30; amine number (mg KOH/g) min 1100), contained different quantities (0, 12, 24, 36, 48, 60, 70% vol.) of base and fractionated diatoms with an average particle size distribution of approx. 10 µm (Figure8). The choice of the resin/hardener system was dictated, on the one hand, by the necessity to obtain a low viscosity of the mixture, and on the other hand, by the optimal time of its final gelation.
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Figure 8. Particle size distribution for (A) base and (B) fractionated diatoms.
The epoxy resin with diatom frustules was mixed mechanically in plastic vessel, using steel propeller (600 rpm, 10 min). The samples were cast in silicone molds, as in work of Zolghadr et al. [18]. The test samples were prepared according to EN ISO 527 [19] and EN ISO 178 [20]. The tests were performed after three weeks of samples curing at 20 ◦C.
2.3. Characterization Methods The size of the diatoms used to prepare the composites was measured with Master- sizer 3000 (Malvern Instruments Ltd., Malvern, UK). The measurements were made for the samples in water suspension (Hydro EV attachment). The parameters of measure- ments for wet samples were stirrer revolution speed: 2330 RPM, and ultrasound power: 70%. Fourier transform-infrared (FT-IR) spectra were recorded on a Nicolet iS50 Fourier transform spectrophotometer (Thermo Fisher Scientific, Walthan, MA, USA) equipped with a diamond ATR unit with a resolution of 0.09 cm−1. Spectra were collected in the 400–4000 cm−1 range, with 16 scans being collected for each spectrum. Contact angle analyses were performed by the sessile drop technique (5 µL) at room temperature and atmospheric pressure, with a Krüss DSA100 goniometer (KRÜSS GmbH, Hamburg, Ger- many). For flexural and tensile strength tests, the materials obtained were printed into type 1B dumbbell specimens in accordance with EN ISO 527-1:2012 and EN ISO 178:2010. Tests of the obtained specimens were performed on a universal testing machine INSTRON 5969 with a maximum load force of 50 kN (Instron, Norwood, MA, USA). The traverse speed for tensile strength measurements was set at 2 mm/min, and for flexural strength it was also set at 2 mm/min. A Charpy impact test (with no notch) was performed on an Instron Ceast 9050 impact machine according to ISO 179-1 [21] (Instron, Norwood, MA, USA). The density of the composites obtained was determined with the hydrostatic method using a Sartorius YCP04MS analytical balance at 20 ◦C with the use of distilled water (Sartorius, Göttingen, Germany). Thermogravimetry was performed using a NETZSCH 209 F1 Libra gravimetric analyzer (Netzsch Group, Selb, Germany). Samples of 5 ± 0.2 mg were cut from each composition and placed in Al21O 3 crucibles. Measurements were conducted under nitrogen (flow of 20 mL/min) in the range of 30–1000 ◦C and a 10 ◦C/min heating
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rate. The morphology and microstructure of the prepared composites were observed by scanning electron microscopy. Samples in the form of powders and molds were dusted with cupro-nickel using a GATAN duster. Observations were conducted with a Hitachi SU 8000 scanning microscope with an accelerating voltage of 5 kV to image the preparations (Hitachi, Ltd., Chiyoda, Tokyo, Japan). The imaging was performed in three SE modes. Viscosity tests were carried out with an MCRAnton Paar 302 rotational rheometer at a shear velocity range of 0.01–100 1/s, at room temperature (Anton Paar GmbH, Graz, Austria). A 1 mm measuring gap was used, and each measurement was performed three times.
3. Results and Discussion 3.1. Particle Size Measurements by Dynamic Light Scattering (DLS) It can be seen that the base diatomaceous earth contained a wide spectrum of particles sized from ~0.5 µm up to over 100 µm (Figure8). According to our previous research, for the species of diatoms used for this study, the particles smaller than 6 µm can be attributed to broken frustules (pieces thereof), while particles larger than approx. 40 µm are mostly agglomerates or colonies of frustules, as well as some single specimens of a bigger size [22]. It was observed that fraction 3 obtained by sedimentation of diatomaceous earth shows much narrower distribution of particle size, ranging from approx. 3 to 30 µm, with some additional fraction of broken frustule particles and dust particles (~1 µm) (Figure6) which, due to small size, are always present in the sedimented material together with traces of residual suspension liquid. It appears that a large portion of broken frustule particles and agglomerates of high diameter was discarded (Figure6). The results indicate that the proposed fractionation method allows for the obtainment of a raw material with a more uniform design and morphology, which thus provides specific properties of the finished product (Figure8).
3.2. Density of Composites Density is one of the most important physical properties of the composite material. Figures9 and 10 show the determined density of composites with different contents (0, 12, 24, 36, 48, 60, 70% vol.) of base and fractionated diatoms, which have or have not been degassed. Density was determined based on the formula below:
d = Vdiatom × ddiatom + (1 − Vdiatom) × depoxy
Vdiatom—volume fraction of diatomite; 3 ddiatom—density of solid diatomite excluding porous structures (considered 2 g/cm ); 3 depoxy—density of cured epoxy (measured to be 1.12 g/cm ). The bulk density of the base and fractionated diatoms is 0.271 g/cm3 and 0.249 g/cm3, respectively. For the degassed resin without diatoms, the average density was 1.131 g/cm3, and for the non-degassed resin, 1.080 g/cm3. For composites (both degassed and non- degassed) with base diatoms, we observed a slight growth in density (Figure9). A higher density was observed for the composites containing fractionated diatoms, both degassed and non-degassed (Figure 10). A system containing 70% vol. of diatomaceous earth has a density of nearly 1.3 g/cm3, which is almost 10% higher than in a system containing the same amount of base diatoms. Materials 2021, 14, 1663 9 of 26 Materials 2021, 14, 1663 10 of 29 Materials 2021, 14, 1663 10 of 29
Figure 9. Density of degassed and non-degassed epoxy resin containing base diatomite earth. FigureFigure 9. 9.Density Density of of degassed degassed andand non-degassednon-degassed epo epoxyxy resin resin containing containing ba basese diatomite diatomite earth. earth.
Figure 10. Density of degassed and non-degassed epoxy resin containing fractionated diatomite. Figure 10. Density of degassed and non-degassed epoxy resin containing fractionated diatomite. Figure 10. Density of degassed and non-degassed epoxy resin containing fractionated diatomite. An increase in density up to 1.3 g/cm3 was also observed by [5], who used centric 3 typeAn Andiatom increase increase frustules inin densitydensity as filler upup in to an 1.3 epoxy g/cm resin 3waswas (degassingalso also observed observed was by applied) by [5], [5 who], whowith used useda weightcentric centric typepercentagetype diatom diatom of frustulesfrustules 15%. It has asas fillerfillerbeen in observed an an epoxy epoxy that resin resin an increasing (degassing (degassing number was was applied) applied)of supplied with with afraction- weight a weight percentage of 15%. It has been observed that an increasing number of supplied fraction- percentageated diatoms of 15%. went It along has been with observed a linear thatgrow anth increasing in the density number of composites of supplied obtained. fractionated ated diatoms went along with a linear growth in the density of composites obtained. diatomsDeviation went from along the theoretical with a linear value growth calculated in the for densityfully dega ofssed composites composites obtained. with frus- Devi- Deviation from the theoretical value calculated for fully degassed composites with frus- ationtules fromcompletely the theoretical filled with value epoxy calculated (Figures 9 for and fully 10) arises degassed from composites the fact that with resin frustules does tules completely filled with epoxy (Figures 9 and 10) arises from the fact that resin does completelynot fully penetrate filled with into epoxythe diatom (Figures structure9 and and 10 )the arises nanostructure from the of fact the that diatom resin frustule. does not not fully penetrate into the diatom structure and the nanostructure of the diatom frustule. fullyWe can penetrate also observe into the silica diatom subs structuretructures andpresent the in nanostructure frustule holes of (Figure the diatom 4). Due frustule. to vis- We We can also observe silica substructures present in frustule holes (Figure 4). Due to vis- can also observe silica substructures present in frustule holes (Figure4). Due to viscosity,
Materials 2021, 14, 1663 11 of 29
Materials 2021, 14, 1663 10 of 26
cosity, despite the applied degassing process, the resin is not capable to penetrate into despite thediatom applied frustules, degassing since process, the holes the are resin covered is not by capable the substructures. to penetrate Diatomaceous into diatom earth is frustules, sincecharacterized the holes by are quite covered a low by specific the substructures. weight due Diatomaceousto its highly porous earth isstructure, charac- being terized by quitemuch alower low specific than that weight of neat due epoxy to its resin, highly but porous its main structure, component being is much nano- lower and micro- structured silica, which itself is characterized by a density of ~2 g/cm3. The frustules’ than that of neat epoxy resin, but its main component is nano- and microstructured silica, density is defined as 1.4–2.2 g/cm3 [23]. Therefore, along with a successful infusion of which itself is characterized by a density of ~2 g/cm3. The frustules’ density is defined frustules with the epoxy resin, a raise in the composites’ density is to be expected, along as 1.4–2.2 g/cm3 [23]. Therefore, along with a successful infusion of frustules with the with increasing loading; however, it is highly dependable on the fraction of air elimi- epoxy resin, a raise in the composites’ density is to be expected, along with increasing nated from the frustules during infusion. Additionally, mostly for base diatomite earth, loading; however, it is highly dependable on the fraction of air eliminated from the frustules with increasing loading, the effect of composition degassing on the material density be- during infusion. Additionally, mostly for base diatomite earth, with increasing loading, comes more substantial, as the diatomite traps significant amounts of air. Therefore, a the effect ofbigger composition difference degassing between degassed on the material and non-degassed density becomes samples more can be substantial, observed. More- as the diatomiteover, in traps general, significant higher amountsvalues of ofdensity air. Therefore, were observed a bigger for differencethe samples between with fraction- degassed andated non-degassed diatomite. This samples effect can can be be linked observed. to stronger Moreover, the thixotropic in general, effect higher of valuesmicron-sized of density wereparticles observed of broken for thefrustules, samples as withproven fractionated by (DMA) diatomite.rheology (Figure This effect 11), which can be in turn linked to strongercauses the the entrapment thixotropic of effect more of air micron-sized in the samples particles and bigger of broken differences frustules, between as de- proven bygassed (DMA) and rheology non-degassed (Figure samples11), which for inbase turn diat causesomite theearth. entrapment It can be seen of more that airwith low in the samplesloadings, and biggerviscosity differences is low and between barely degasseddependable and on non-degassed the diatomite samplestype or degassing for base diatomiteprocedure. earth. With It can increasing be seen that loading, with lowviscosity loadings, raises viscosity significantly is low and and the barely differences dependablebetween on the diatomitediatomite typetypes or become degassing substantial, procedure. as base With filler increasing imparts loading,much higher viscos- viscosity. ity raises significantlyA reduction andin the the viscosity differences of base between diatomit diatomitee earth-loaded types becomeepoxy after substantial, degassing as may be base filler impartsdue to the much agglomeration higher viscosity. of micron A-sized reduction particles in the during viscosity degassing. of base diatomite earth-loaded epoxy after degassing may be due to the agglomeration of micron-sized particles during degassing.
6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 Viscosity [Pa.s] 2.0 1.5 1.0 0.5 12% BND 12% BD 12% FND 12% FD 70% BND 70% BD 70% FND 70% FD 0.0 0 102030405060708090100 Shear rate [1/s]
Figure 11. DMA rheology diatoms/epoxy resin composition, BND—base diatomite non degassed, Figure 11. DMABD—base rheology degassed, diatoms/epoxy FND—fracionated resin composition, non degassed,BND – base FD—fracionated diatomite non degassed, degassed. BD – base degassed, FND - fracionated non degassed, FD - fracionated degassed. 3.3. FTIR Analysis Figure 12 shows the FT-IR spectra of degassed composites containing base diatoms. For resin samples, we can observe characteristic bands at a wavelength of 2950–2800 cm−1 for C–H aliphatic bonds, 1450–1400 cm−1 for C–O bonds, 1258 cm−1, and 1100–650 cm−1 for different C–H bonds [24]. The strong band at 2962 cm−1 is typical for the vibration of −1 CH, CH2, and terminal methyl groups. The peak at around 864 cm can be assigned to 1,4-substitution of aromatic ring for epoxy resin. The band characteristics of ester groups, Materials 2021, 14, 1663 12 of 29
3.3. FTIR Analysis Figure 12 shows the FT-IR spectra of degassed composites containing base diatoms. For resin samples, we can observe characteristic bands at a wavelength of 2950–2800 cm−1 Materials 2021, 14, 1663 for C–H aliphatic bonds, 1450–1400 cm−1 for C–O bonds, 1258 cm−1, and 1100–65011 ofcm 26−1 for different C–H bonds [24]. The strong band at 2962 cm−1 is typical for the vibration of CH, CH2, and terminal methyl groups. The peak at around 864 cm−1 can be assigned to 1,4-substitution of aromatic ring for epoxy resin. The band characteristics of ester groups, −1 −1 representingrepresenting the C–O–C the C–O–C bond bond stretching, stretching, are foundare found at 1258 at 1258 cm cm−and1 and 1015 1015 cm cm−1 [25].[25]. For For degasseddegassed composites composites with with base base diatoms, diatoms, there there emerged emerged weak weak bands bands at at wavelengths wavelengths of − of 35663566 and 3304and 3304 cm 1cm, related−1, related to stretchingto stretching vibrations vibrations of of O–H O–H bonds bonds in in silanol silanol groups groups as as wellwell as in as hydrogen-bonded in hydrogen-bonded molecular molecular water water [ 26[26].]. WeWe alsoalso observed an an additional additional band band atat a a wavelength wavelength of 797 cm cm−1−, 1which, which is caused is caused by symmetric by symmetric bending bending vibrations vibrations of Si–O–Si of Si–O–Sibonds bonds [27]. [The27]. band The at band 1646 at cm 1646−1 is due cm− to1 isbending due to vibrations bending of vibrations H–O–H bonds of H–O–H of bound bonds ofmolecular bound molecular water [22]. water We [22additionally]. We additionally observed observed bands at bands a wavelength at a wavelength of 1015 ofcm−1, 1015 cmwhich−1, which could could be attributed be attributed to the to theasymmetric asymmetric stretching stretching of Si–O–Si of Si–O–Si bond, bond, while while the 661 the 661 cmcm−1 1belongbelong to to symmetric symmetric stretching stretching [28]. [28 ]. Transmittance [%] Transmittance
E P 48% base D 70% base D base diatom ite powder
4000 3500 3000 2500 2000 1500 1000 500 −1 Wave number [cm ]
Figure 12. FT-IR spectra of degassed composites containing base diatoms and base diatomite powder. Figure 12. FT-IR spectraBase D: of base degassed diatomite. composites EP: neat containing epoxy resin. base diatoms and base diatomite powder. Base D: base di- atomite. EP: neat epoxy resin. 3.4. Mechanical Properies As3.4. for theMechanical mechanical Properies properties of the obtained composites, we examined their tensile strength, three-pointAs for the flexural mechanical strength, properties and impact of the strength. obtained composites, we examined their tensile strength, three-point flexural strength, and impact strength. 3.4.1. Tensile Strength Figure 13 shows the results for the tensile strength (A) and elongation at break (B) of the prepared composites. The degassed samples of neat epoxy were characterized by a higher tensile strength than the non-degassed counterparts, that is 19.91 and 18.26 MPa, respectively. For base diatomite, at low filler concentration, a slight increase in tensile strength was observed. At higher loadings, a small decline was observed, down to 15.09 and 14.82 MPa for 70% degassed and non-degassed compositions, respectively. For the compositions with fractionated diatomite, the tensile strength reached higher values than for the composites with base diatomite, up to 22.78 MPa for 24% composition. As we know, the strengthening effect of the filler is determined by interfacial action forces, which largely depend on the break-up of the filler and the development of the surface. It proves that the elimination of agglomerated particles during diatomite preparation, as well as the degassing of the epoxy composition both have positive effects on the mechanical Materials 2021, 14, 1663 12 of 26
properties of the composites obtained. Tensile strength may be linked to the amount of resin introduced into the frustules’ internal structure during composition degassing, as observed in SEM images (see Microstructural analysis). Frustules with more resin introduced into their porous structure and penetrating inside their internal volume induce slightly more mechanical reinforcement, due to improved mechanical contact between the matrix and the filler. In the work [8] that studied calcined and natural frustules filled epoxy matrices, they observed variations of tensile elastic modulus and tensile strength with vol. % of frustules addition. In the work, two main tensile failure mechanisms were described for calcinated frustule filled epoxy composites: frustules debonding and frustules fracture/crushing, followed by partial pull-out. Similar failure mechanisms were observed with SEM imaging during this investigation. Debonding and pull-out may be the reason for small levels of difference in mechanical properties when comparing neat epoxy resin and differently filled composites thereof—the limited wetting of the filler surface by the matrix epoxy. The elongation at break was also investigated (Figure 13B). Due to the brittleness of epoxy resins being one of their main weak spots, as well as the sample preparation method, it was difficult to observe a particular trend for the elongation at break, as the results show a rather high standard deviation. The non-degassed resin without diatoms showed lower elongation at break (1.033%) than the degassed resin (1.520%). The samples containing base frustules without degassing showed slightly lower elongation at break than their counterparts with fractionated filler. It can be explained on the basis of the frustule agglomerates observed in the base diatomite earth forming defect sites. Additionally, for low loadings, the entrapped air in both systems also contributes to material failure at low elongation. Likely for this reason, the difference was poorly visible for degassed samples. In general, as expected, the degassed samples showed improved elongation at break when compared to the non-degassed ones. From the practical point of view, for the studied systems, it is important that even at high loadings the obtained composites show mechanical parameters comparable to those of the neat epoxy resin, being a reasonable option for more eco-friendly materials with a high content of biogenic filler.
3.4.2. Flexural Strength Figure 14 shows the maximum flexural stress and flexural modulus for degassed and non-degassed composites containing base (A) diatoms and fractionated (B) diatoms. The flexural stress for degassed and non-degassed epoxy resin was 42.48 MPa and 47.15 MPa, respectively. For degassed samples, the stress at three-point bending gradually increases at base diatom contents of 12 and 24% vol. to reach 53.09 MPa and 55.78 MPa, respectively, and then, at base diatom contents of 48, 60 and 70% vol., the stress gradually decreases to reach values down to 35.79 MPa. For degassed composites containing fractionated diatoms, the flexural stress reaches higher values than for the composites with base di- atoms, the maximum value of 61.87 MPa being reached for 48% vol. The non-degassed samples showed lower values of flexural stress than the degassed ones, both for base and fractionated diatoms. For the non-degassed samples, we observed much lower values regardless of the filler used. In general, the conclusions are similar to those for tensile tests and prove that both the purification of diatomite to the desired fraction containing single, undamaged frustules, and the degassing of the composition give the best results and improve flexural strength of the composites at the highest loadings, while leaving the flexural modulus similar to the base value. Additionally, for non-degassed composites, a decline of the flexural modulus was observed, as the air gaps increase the elasticity of the material. For degassed composites, the maximum value of flexural modulus is higher than for degassed composites. The initial value for resin with degassing was approx. 2.5 GPa. De- gassed composites with fractionated diatoms showed a higher maximum flexural modulus than those with base diatoms. This could be due to the uniformity of the raw material used. However, in both cases (base and fractionated diatoms), at 70% vol., the value of flexural Materials 2021, 14, 1663 13 of 26
modulus reached around 6.7 GPa. For non-degassed resin without a modifier, the value of flexural modulus was 3.2 GPa. The addition of 12 and 70% vol. of base diatoms caused a slight growth of the maximum flexural modulus up to 3.7 GPa. In other cases, the value was even higher than the initial value. The use of fractionated diatoms also did not cause Materials 2021, 14, 1663 any considerable increase in flexural strength. The addition of 36–60% vol. of fractionated14 of 30 diatoms caused a slight increase in the value, and the further addition of the modifier resulted in a significant deterioration of properties. The maximum value of flexural stress, existing at a 60% content of fractionated diatomite, is approximately 4.6 GPa.
40 40 base fractionated
35 35
30 30
25 25
20 20
15 15
Tensile strength [MPa] Tensile 10 strength [MPa] Tensile 10
5 5
0 0
D D D D D D D D D D D D D D D D D D D D D D D D D D D D N N N P N N N N N N N P % % % % % % N N N N % % % % % % E E 2 4 6 8 0 0 P 2 4 6 8 0 0 P % % % % % % % % % % % % 1 2 3 4 6 7 1 2 3 4 6 7 E 2 4 6 8 0 0 E 2 4 6 8 0 0 1 2 3 4 6 7 1 2 3 4 6 7 (A) 3.5 3.5 base fractionated
3.0 3.0
2.5 2.5
2.0 2.0
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1.0 1.0 Elongation at break [%]
0.5 0.5
0.0 0.0
D D D D D D D D D D D D D D D D D D D D D D D D D D D D P N N N N N N N % % % % % % P N N N N N N N E % % % % % % 2 4 6 8 0 0 % % % % % % E 1 2 3 4 6 7 2 4 6 8 0 0 P % % % % % % EP 2 4 6 8 0 0 1 2 3 4 6 7 E 1 2 3 4 6 7 2 4 6 8 0 0 1 2 3 4 6 7 (B)
FigureFigure 13. Tensile strength ( (AA)) and and elongation elongation at at break break (B (B) )for for degassed degassed (“D”) (“D”) and and non-degassed non-degassed (“ND”)(“ND”) compositescomposites containingcontaining basebase diatomsdiatoms andand fractionatedfractionated diatoms.diatoms.
The elongation at break was also investigated (Figure 13B). Due to the brittleness of epoxy resins being one of their main weak spots, as well as the sample preparation method, it was difficult to observe a particular trend for the elongation at break, as the results show a rather high standard deviation. The non-degassed resin without diatoms showed lower elongation at break (1.033%) than the degassed resin (1.520%). The sam- ples containing base frustules without degassing showed slightly lower elongation at break than their counterparts with fractionated filler. It can be explained on the basis of the frustule agglomerates observed in the base diatomite earth forming defect sites. Ad- ditionally, for low loadings, the entrapped air in both systems also contributes to material
Materials 2021, 14, 1663 14 of 26
The use of fractionated diatoms (degassed samples) in a polymer matrix caused a growth in flexural stress. The non-degassed samples showed lower flexural stress values than the degassed samples, both for base and fractionated diatoms, as the presence of air bubbles reduced the strength of the material. Due to degassing, we could see more clearly the strengthening impact of the filler. For the determination of the flexural modulus, the degassing of the sample was important. Non-degassed samples showed comparable (and in certain cases, reduced) flexural modulus values, which were caused by a high share of air and a weaker resin–filler interaction. Changes in flexural strength were observed only Materials 2021, 14, 1663 for degassed composites, where we observed a slight increase in the flexural modulus. The16 of 30 presence of air bubbles had a considerable impact on the results.
10 80 (A) base base 9 70 8
7 60
6 50 5 40 4
3 30
2 20 Flexural modulus [GPa] Flexural modulus 1 Max. flexural stress [MPa]
0 10
D D D D D D D D D D D D D D D D D D D D D D D D D D D D N N N N N N N N P N N N N N N P % % % % % % (B) % % % % % % E 10 P E 2 4 6 8 0 0 80 P % 2 % 4 % 6 % 8 % 0 % 0 % % % % % % E 1 2 3 4 6 7 E 2 1 4 2 6 3 8 4 0 6 0 7 2 4 6 8 0 0 1 2 3 4 6 7 1 2 3 4 6 7 9 70 8 fractionated fractionated 7 60
6
tress [MPa] tress 50 5 40 4
3 30
2 20 Flexural modulus [GPa] Flexural modulus 1 Max. flexural s
0 10 D D D D D D D D D D D D D D D D D D D D D D D D D D D D N N N N N N N N N N N N N N P % % % % % % P % % % % % % E E P % 2 % 4 % 6 % 8 % 0 % 0 P % 2 % 4 % 6 % 8 % 0 % 0 E 2 1 4 2 6 3 8 4 0 6 0 7 E 2 1 4 2 6 3 8 4 0 6 0 7 1 2 3 4 6 7 1 2 3 4 6 7
Figure 14. Flexural modulus and max. bending stress for degassed and non-degassed (“D” and “ND”) composites containing base (A) and fractionated (B) samples diatoms. Figure 14. Flexural modulus and max. bending stress for degassed and non-degassed (“D” and “ND”) composites con- taining base (A) and fractionated3.4.3. Impact (B) samples Strength diatoms. For impact strength (Figure 15), two main reasons for decreased values of non- degassedFor composites degassed shouldcomposites, be considered. the maximum One value of them of flexural is the presence modulus of is airhigher bubbles than for introducingdegassed mechanical composites. discontinuities, The initial value and for the resin other with one degassing may be the was lower approx. bonding 2.5 GPa. betweenDegassed the matrix composites and the with filler, fractionated which in turn diatoms causes showed tearing ofa higher the frustules maximum from flexural the resinmodulus and increased than those crack with propagation base diatoms. during This impact. could Impact be due strength to the is uniformity usually lower of the for raw non-degassedmaterial used. samples; However, this is in caused both bycases the (base presence and offractionated air bubbles, diatoms), which have at 70% a signifi- vol., the cantvalue impact of on flexural mechanical modulus properties. reached A around higher impact6.7 GPa. strength For non-dega is shownssed by theresin samples without a modifiedmodifier, with the fractionated value of flexural diatomite, modulus which was may 3.2 be GPa. caused The by addition a more evenof 12 distributionand 70% vol. of of diatombase diatoms particles. caused Most a of slight the analyzed growth of samples the maximum have an flexural impact strengthmodulus rangingup to 3.7 from GPa. In 2 toother 5 kJ/m cases,2. The the values value grow was alongeven higher with increasing than the initial filler content.value. The The use best of results fractionated were di- atoms also did not cause any considerable increase in flexural strength. The addition of 36%–60% vol. of fractionated diatoms caused a slight increase in the value, and the fur- ther addition of the modifier resulted in a significant deterioration of properties. The maximum value of flexural stress, existing at a 60% content of fractionated diatomite, is approximately 4.6 GPa. The use of fractionated diatoms (degassed samples) in a polymer matrix caused a growth in flexural stress. The non-degassed samples showed lower flexural stress values than the degassed samples, both for base and fractionated diatoms, as the presence of air bubbles reduced the strength of the material. Due to degassing, we could see more clearly the strengthening impact of the filler. For the determination of the flexural modulus, the degassing of the sample was important. Non-degassed samples showed comparable (and in certain cases, reduced) flexural modulus values, which were caused by a high share of
Materials 2021, 14, 1663 15 of 26
attained by degassed samples containing 60% vol. of diatomite (base and fractionated: 6 Materials 2021, 14, 1663 and 9 kJ/m2, respectively). In general, the impact strength was comparable17 of to 29 the base
material regardless of the filler loading, and only sample degassing played a visible role in increasing the material durability.
10 10 base fractionated 9 9 ]
8 2 8 ] 2 7 7
6 6
5 5
4 4 Impact strength[kJ/m Impact strength [kJ/m 3 3
2 2
1 1 - - D D D D D D D D D D D D D D D D D D D D D D D D D D D D N N N N N N N N N N N P % % % % % % P N % N % % N % % % E P 2 4 % 6 8 % 0 % 0 % E P 2 4 6 8 0 0 1 % 2 3 % 4 6 7 1 % 2 % 3 % 4 % 6 % 7 % E 4 8 0 0 E 4 8 0 0 2 2 6 4 6 7 2 6 1 3 1 2 3 4 6 7
Figure 15. Impact strength for degassed and non-degassed composites containing base and fraction- ated diatoms. Figure 15. Impact strength for degassed and non-degassed composites containing base and frac- tionated diatoms.In the measurements, we observed a large dispersion of results. It may be due to the following reasons: (a) the structural heterogeneity of the composite; (b) the use of silicone In moldsthe measurements, for casting, whosewe observed edges a leave large defectsdispersion in the of results. composite It may (as be confirmed due to the by SEM, followingthe reasons: edges of (a) the the samples structural were heterogene highly irregular);ity of the (c)composite; the presence (b) the of use air bubbles,of silicone marking molds forthe casting, start of whose cracking edges (the leave presence defects of in air the bubbles composite (both (as for confirmed degassed by and SEM, non-degassed the edges ofcomposites) the samples at were random highly spots irregular); of the samples); (c) the presence (d) cracking of air ofbubbles, the agglomerates marking the for base start of diatomite.cracking (the Therefore, presence in of further air bu tests,bbles we(both may for consider degassed whether and non-degassed it is appropriate com- to use a posites)silicone at random mold spots to prepare of the epoxysamples); resin (d composites.) cracking of the agglomerates for base di- atomite.3.5. Therefore, Water Contact in further Angle tests, we may consider whether it is appropriate to use a silicone mold to prepare epoxy resin composites. Wettability is a physical feature of materials that specifies their interaction with liquids, 3.5. Watermostly Contact water, Angle and determines the basic material properties such as adhesion. The contact angle is an important parameter for determining the hydrophobic or hydrophilic nature of Wettabilitypolymers. is The a physical contact anglefeature test of was materials conducted that onspecifies samples their shaped interaction as dumbbells. with liq- We tested uids, mostlythe top water, and bottom and determines parts of the the dumbbells. basic material The angle properties value forsuch each as typeadhesion. of composite The was contact givenangle asis an the important average value parameter obtained for determining in the measurements the hydrophobic (Figure 16or). hydrophilic Example pictures nature of polymers. a droplet are The presented contact angle in Figure test 17was. The conducted samples on of degassedsamples shaped and non-degassed as dumb- resin bells. Wewithout tested diatomsthe top and showed bottom hydrophilic parts of the properties. dumbbells. As weThe know, angle 85% value of for diatomite each type consists of of composite was given as the average value obtained in the measurements (Figure 16). SiO2. The stable tetrahedral structure of SiO2 makes diatomite a hydrophobic material [29]. ExampleHowever, pictures dependingof a droplet on are the presented degree of purification,in Figure 17. diatomite The samples is available of degassed with hydrophilicand non-degassedor hydrophobic resin without surface diatoms properties showed [30 hy].drophilic Composites properties. modified As with we know, diatomaceous 85% of earth diatomiteshowed consists hydrophobic of SiO2. The properties, stable tetrahedral both in the structure top and of bottom SiO2 parts.makes Thisdiatomite was except a for hydrophobicsystems material with degassing[29]. However, that containeddepending 12on andthe degree 24% vol. of purification, of base diatoms diatomite and 12% of is availablefractionated with hydrophilic diatoms. Theor hydrophobi values of contactc surface angle properties in this case [30]. are Composites below 90◦. Hydrophobicmodi- fied withproperties diatomaceous of the testedearth systemsshowed increasedhydrophobic along properties, with the growing both inconcentration the top and bot- of modifier tom parts. This was except for systems with degassing that contained 12 and 24% vol. of base diatoms and 12% of fractionated diatoms. The values of contact angle in this case are below 90°. Hydrophobic properties of the tested systems increased along with the growing concentration of modifier in the samples and showed higher values for the sys-
Materials 2021, 14, 1663 16 of 26
Materials 2021, 14, 1663 19 of 30
in the samples and showed higher values for the systems containing fractionated diatomite. Materials 2021, 14, 1663 19 of 30 We may therefore conclude that the incorporation of diatomite into the epoxy resin matrix resulted in a considerable increase in the overall hydrophobicity. 120 120 top without degassing bottom without degassing (A) (B) 120 120 top without degassing bottom without degassing 110 (A) 110 (B)
110 110 100 100
100 100 90 90
90 90
Contact angle [°] 80 80
Contact angle [°] 80 80 70 70
70 70
60 60
P F F F F F F - P F F F F F F - B B B B B B - B B B B B B - E 60 E 60 % % % % % % % % % % % % % % % % % % % % % % % % 2 4 6 8 0 0 2 4 6 8 0 0 2 4 6 8 0 0 P F F F F F F - 2 4 6 8 0P 0 F F F F F F - 1 2 3 4 6 7 B1 B2 B3 B4 B6 B 7 - 1 2 3 4 6 7B 1B 2B 3B 4B 6B 7 - E E % % % % % % % % % % % % % % % % % % % % % % % % 120 2 4 6 8 0 0 2 4 6 8 1200 0 2 4 6 8 0 0 2 4 6 8 0 0 top1 degassed2 3 4 6 7 1 2 3 4 6 7 bottom1 2 degassed3 4 6 7 1 2 3 4 6 7 120 120 top degassed bottom degassed 110 110
110 110
100 100 100 100
90 90 90 90
80 80 Contact angle [°] 80 80 Contact angle [°]
70 70 70 70
60 60 - P B B B B B B F F F F F F - P F F F F F F - 60 B 60 B B B B B - E E % % % % % % % % % % % P % F F F F F F - % % % % % % % % % % % % - 2 4 6 8 0 0 B 2 B 4 B 6 B 8 B 0 B 0 - P B B2 B4 B6 B8 B0 F0 F F F F F - E 2 4 6 8 0 0 1 2 3 4 6 7 1 2 3 4 6 7 1 2 3 4 6E 7 1 2 3 4 6 7 % % % % % % % % % % % % % % % % % % % % % % % % 2 4 6 8 0 0 2 4 6 8 0 0 2 4 6 8 0 0 2 4 6 8 0 0 1 2 3 4 6 7 1 2 3 4 6 7 1 2 3 4 6 7 1 2 3 4 6 7
Figure 16. Contact angleFigure bottom 16. Contact(A) and angletop (B bottom) for degassed (A) and and top non-degassed (B) for degassed composites and non-degassed containing compositesbase (“B”) dia- contain- toms and fractionatedFigure 16.ing (“F”) Contact base diatoms. (“B”) angle diatoms bottom ( andA) and fractionated top (B) for (“F”) degassed diatoms. and non-degassed composites containing base (“B”) dia- toms and fractionated (“F”) diatoms. (A) (B) (A) (B)
Figure 17. Droplets applied on the bottom part of the dumbbells for samples containing 70% vol. (A) with non-degassed base diatoms, and (B) with non-degassed fractionated diatoms.
Materials 2021, 14, 1663 17 of 26
3.6. Changes in Weight/Rate of Changes with Increasing Temperature (TG/DTG) Analysis TG thermograms and DTG for degassed and non-degassed composites containing base (A,B) and fractionated diatomite (C,D) (Figure 18). In the samples obtained, we measured the changes in weight (TG) and the rate of these changes along with increasing temperature (DTG). The decomposition of both fractions of diatomaceous earth in the atmosphere takes place in a single stage. This is due to the decomposition of the epoxy resin itself. The addition of diatomite, regardless of the fraction used, increases thermal stability of resin–filler systems. Temperature at a 5% weight loss is, in each case, lower for degassed samples than for non-degassed samples, by nearly 10% maximum (for 48% fractionated samples), whereas the samples containing fractionated diatoms generally Materials 2021, 14, 1663 21 of 30 show lower temperature values at a 5% weight loss than base diatoms. Temperature at the maximum weight change rate is at a similar level for all the analyzed systems, and ranges from 375 to 381 ◦C.
100 A base 90 80 EP D EP ND 70 12% D B 12% ND 60 24% D 50 24% ND 36% D 40 36% ND DTG [%/min]
Mass loss [%] Mass 48% D 30 48% ND 20 60% D 60% ND 10 70% D 70% ND 0 100 200 300 400 500 600 700 800 900 base Temperature [°C] 100 200 300 400 500 600 700 800 900 Temperature [°C]
100 C fractionated 90 EP D 80 EP ND 70 12% D D 12% ND 60 24% D 50 24% ND 36% D 40 36% ND DTG [%/min] DTG
Mass loss [%] Mass 48% D 30 48% ND 20 60% D 60% ND 10 70% D 0 70% ND 100 200 300 400 500 600 700 800 900 fractionated Temperature [°C]
100 200 300 400 500 600 700 800 900 Temperature [°C]
FigureFigure 18. 18. ChangesChanges in in weight weight (TG) (TG) and and the the rate rate of of changes changes with with increasing increasing temperature temperature (DTG) (DTG) curves curves for for degassed degassed and and non-degassednon-degassed composites composites containing containing base base diatoms diatoms ( (AA,,BB)) and and fractionated fractionated diatoms diatoms ( (CC,,DD).).
3.7. Microstructural Analysis We have analyzed the side, fracture and meniscus of the samples. For both degassed and non-degassed samples, we observed air bubbles, which are smaller for degassed samples. The observations of sample surfaces (Figures 19–21) allowed us to conclude that the surface is rough. The superficial irregularity decreased along with the growing con- centration of diatoms in the system. On the surface of non-degassed samples, we can clearly see air bubbles, while in the degassed systems the air bubbles are either small or do not exist at all.
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3.7. Microstructural Analysis We have analyzed the side, fracture and meniscus of the samples. For both degassed and non-degassed samples, we observed air bubbles, which are smaller for degassed samples. The observations of sample surfaces (Figures 19–21) allowed us to conclude that the surface is rough. The superficial irregularity decreased along with the growing concentration of diatoms in the system. On the surface of non-degassed samples, we can clearly see air bubbles, while in the degassed systems the air bubbles are either small or do not exist at all. Observations of sample fractures (Figures 22–24) have shown that the analyzed surface is mostly homogeneous. In non-degassed samples, we can observe small air bubbles located on the entire fractured surface as well as larger bubbles located in the upper section of the sample (closer to the meniscus). The fractured surface of degassed samples shows a similar degree of uniformity regardless of diatomite concentration. The samples containing Materials 2021, 14, 1663 fractionated diatoms feature air bubbles that are slightly bigger than those in the samples 21 of 29
containing base diatomite.
FigureFigure 19. 19.SEM SEM images images of the of surface the surface of samples of samp withles 36% with vol. 36% of diatoms vol. of (diatomsA)—base ( deg,A)—base (B)—base deg, (B)—base nono deg, deg, (C)—fractionated (C)—fractionated deg, (deg,D)—fractionated (D)—fractionated no deg. no deg.
Figure 20. SEM images of the surface of samples with 48% vol. of diatoms (A)—base deg, (B)—base no deg, (C)—fractionated deg, (D)—fractionated no deg.
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Materials 2021, 14, 1663 19 of 26 Figure 19. SEM images of the surface of samples with 36% vol. of diatoms (A)—base deg, (B)—base no deg, (C)—fractionated deg, (D)—fractionated no deg.
Figure 20. SEM images of the surface of samples with 48% vol. of diatoms (A)—base deg, (B)—base Materials 2021, 14, 1663 Figure 20. SEM images of the surface of samples with 48% vol. of diatoms (A)—base deg,22 (B of)—base 29 no deg, (C)—fractionated deg, (D)—fractionated no deg. no deg, (C)—fractionated deg, (D)—fractionated no deg.
Figure 21. SEM images of the surface of samples with 70% vol. of diatoms (A)—base deg, (B)—base Figure 21. SEM images of the surface of samples with 70% vol. of diatoms (A)—base deg, (B)—base no deg, (C)—fractionated deg, (D)—fractionated no deg. no deg, (C)—fractionated deg, (D)—fractionated no deg. Observations of sample fractures (Figures 22–24) have shown that the analyzed surface is mostly homogeneous. In non-degassed samples, we can observe small air bubbles located on the entire fractured surface as well as larger bubbles located in the upper section of the sample (closer to the meniscus). The fractured surface of degassed samples shows a similar degree of uniformity regardless of diatomite concentration. The samples containing fractionated diatoms feature air bubbles that are slightly bigger than those in the samples containing base diatomite.
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Materials 2021, 14, 1663 23 of 29
FigureFigure 22. 22.SEM SEM images images of of the the fractures fractures of samplesof samp withles with 36% 36% vol. vol. of diatoms: of diatoms: (A)—base (A)—base deg, deg, (B)—base Figure 22. SEM images of the fractures of samples with 36% vol. of diatoms: (A)—base deg, no(B deg,)—base (C)—fractionated no deg, (C)—fractionated deg, (D)—fractionated deg, (D)—fractionated no deg. no deg. (B)—base no deg, (C)—fractionated deg, (D)—fractionated no deg.
Figure 23. SEM images of sample fractures: 48% vol. of diatoms: (A)—base deg, (B)—base no deg, Figure 23. SEM images of sample fractures: 48% vol. of diatoms: (A)—base deg, (B)—base no deg, Figure(C)—fractionated 23. SEM images deg, of (D sample)—fractionated fractures: no 48% deg. vol. of diatoms: (A)—base deg, (B)—base no deg, (C)—fractionated deg, (D)—fractionated no deg. (C)—fractionated deg, (D)—fractionated no deg.
Materials 2021, 14, 1663 21 of 26 Materials 2021, 14, 1663 24 of 29
Figure 24. SEM images of the fractures of samples with 70% vol. of diatoms: (A)—base deg, Figure 24. SEM images of the fractures of samples with 70% vol. of diatoms: (A)—base deg, (B)—base (B)—base no deg, (C)—fractionated deg, (D)—fractionated no deg. no deg, (C)—fractionated deg, (D)—fractionated no deg.
ByBy observing observing the the wall wall of of the the mold mold that that contained contained the the tested tested samples samples ( Figures(Figures 25 25–27),–27), wewe found found a significanta significant heterogeneity heterogeneity in itsin its structure, structure, which, which, in the in results,the results, was was also also present pre- insent the in composite the composite samples. samples. In most In of most the examinedof the examined systems, systems, the mold the walls mold have walls irregular have ir- projectionsregular projections which are which due to are the due fact to that the the fact samples that the adhere samples to the adhere silicone to molds.the silicone As themolds. diatomite As the concentration diatomite concentration in the system in the raises, system the surfaceraises, the roughness surface roughness drops. Systems drops. thatSystems contain that over contain 48% vol.over of 48% diatomaceous vol. of diatomaceous earth showed earth a lowershowed inclination a lower toinclination adhere to to theadhere silicone to the mold silicone during mold sample during preparation, sample preparation, so the mold so walls the inmold those walls systems in those seems sys- totems be smoother.seems to be Non-degassed smoother. Non-degassed systems show systems the presence show the of airpresence bubbles. of air Their bubbles. size dependsTheir size both depends on diatom both concentrationon diatom concentrat in the systemion in the and system on filler and preparation. on filler preparation. Samples withSamples base with diatoms base without diatoms degassingwithout degassing have bigger have air bigger bubbles, air bubbles, while the while samples the samples with fractionatedwith fractionated diatoms diatoms have air have bubbles air bubbles that are muchthat are smaller. much Alongsmaller. with Along the growingwith the quantitygrowing of quantity diatoms of in diatoms polymer in matrix, polymer the matrix, share of the bubbles share of is usuallybubbles lower.is usually In addition, lower. In inaddition, all the tested in all samples,the tested whether samples, degassed whether ordegassed non-degassed, or non-degassed, we observed we thatobserved diatoms that werediatoms torn offwere the torn polymer off the matrix, polymer leaving matrix, a clear leaving mark, a orclear were mark, sheared or were and partially sheared leftand inpartially the matrix. left Thisin the proves matrix. that This the proves place wherethat the a place diatom where frustule a diatom is bonded frustule with is resin bonded is morewith exposedresin is more to damage exposed than to the damage structure than of th basee structure resin. This of base arises resi fromn. This low arises adhesion from betweenlow adhesion the mineral between and the organic mineral parts and of organic the composite. parts of Additionally,the composite. as Additionally, mentioned in as Results—Mechanicalmentioned in Results—Mechanical Properties, in all Properties, the tested samplesin all the we tested observed samples the supersaturationwe observed the ofsupersaturation resin into the diatomic of resin structureinto the diatomic (Figure 28structure on the right)(Figure as 28 well on asthe the right) mark as it well left afteras the debondingmark it left (Figure after debonding28 on the left). (Figure We have28 on noticed the left). that We the have resin noticed partially that penetrated the resin into par- thetially micropores penetrated in diatominto the frustule, micropores creating in diatom mechanical frustule, links creating with the mechanical polymer matrix. links with the polymer matrix.
Materials 2021, 14, 1663 22 of 26 Materials 2021, 14, 1663 25 of 29
Materials 2021, 14, 1663 25 of 29
Figure 25. SEM images of mold wall with 36% vol. of diatoms: (A)—base deg, (B)—base no deg, Figure 25. SEM images of mold wall with 36% vol. of diatoms: (A)—base deg, (B)—base no deg, Figure(C)—fractionated 25. SEM images deg, (ofD )—fractionatedmold wall with no36% deg. vol. of diatoms: (A)—base deg, (B)—base no deg, (C(C)—fractionated)—fractionated deg, deg, (D (D)—fractionated)—fractionated no no deg. deg.
Figure 26. SEM images of mold wall with 48% vol. of diatoms: (A)—base deg, (B)—base no deg, FigureFigure(C)—fractionated 26. 26.SEM SEM images images deg,of (ofD mold)—fractionatedmold wallwall withwith 48%no48% deg. vol. of diatoms: diatoms: ( (AA)—base)—base deg, deg, (B (B)—base)—base no no deg, deg, (C)—fractionated deg, (D)—fractionated no deg. (C)—fractionated deg, (D)—fractionated no deg.
Materials 2021, 14, 1663 23 of 26 Materials 2021, 14, 1663 26 of 29 Materials 2021, 14, 1663 26 of 29
FigureFigure 27. 27.SEM SEM images images of of mold mold wallwall withwith 70%70% vol.vol. of diatoms: ( (AA)—base)—base deg, deg, (B (B)—base)—base no no deg, deg, Figure(C)—fractionated 27. SEM images deg, of (D mold)—fractionated wall with 70% no deg. vol. of diatoms: (A)—base deg, (B)—base no deg, (C)—fractionated deg, (D)—fractionated no deg. (C)—fractionated deg, (D)—fractionated no deg.
10 µm 10 µm
Figure 28. SEM image of diatoms filled with resin (right) and the mark after diatom debonding (left). Figure 28. SEM image of diatoms filled filled with resin (right) andand thethe markmark afterafter diatomdiatom debondingdebonding ((leftleft).).
Materials 2021, 14, 1663 24 of 26
4. Conclusions The tests were aimed to experimentally verify the assumptions on the technology of the fractionation of raw diatomaceous earth to obtain resin–diatomite composites for a specific fraction of diatom frustule structures. We have verified the impact of fractionation as a function of diatom content as well as of process factors such as the degassing of samples. The applied methods of separating diatoms by fractionation and sedimentation enables the removal of diatom agglomerates and a large part of fractured frustules and dusts. It is possible to separate diatom frustule fractions with a specific size distribution. The effective density of diatoms is higher than the density of resin, so the density of obtained composites grows along with the increasing weight share of diatoms. What is important, however, is that the relation between density and weight share of diatoms is not linear and depends on the size of diatom frustule and on the degassing degree of the mixtures. Non-fractionated diatoms increase resin viscosity, which hinders its effective degassing. Samples obtained by degassing technology and those with fractionated diatoms featured higher density, which proves that the tested composites can be considered as a system of three phases: diatoms + resin + pores filled with gas. Based on calculations, we have estimated the expected density of composites with a theoretical 100% filling level, which has shown that with the applied method of preparing composites, diatoms are not completely filled with resin. At the same time, based on SEM, we can conclude that this is not required to obtain good wetting of the filler surface with resin, plus it allows us to obtain lighter materials. The degassed composites (so those with fewer pores) featured higher tensile strength. Their mechanical properties were also affected by the method of diatom preparation. Samples with fractionated diatoms showed higher tensile strength values than composites with base diatoms, but the effect was more noticeable for non-degassed systems. The slight differences in the strength of samples containing different amounts of filler could be caused by the separation of diatoms from resin during tension (debonding, pulling out). This was similar for elongation at break, which was higher for composites with fractionated diatoms, especially for non-degassed systems. As we know, the strengthening effect of the filler is determined by interfacial action forces, which largely depend on the break-up of the filler. Since diatoms after fractionation are smaller than diatoms as delivered (they do not contain the agglomerates imaged by SEM), the composites based on them feature higher tensile strength. An important observation is that the composites maintain good strength parameters despite a high degree of filling (even for 70% vol.). The addition of fractionated diatoms to the matrix in sample degassing technology affects the hydrophobicity of their surface. The effect is due to the formation of surface microstructure in a diatom frustule– epoxide resin arrangement. The addition of diatoms (whether base or fractioned) caused an increase in the thermal stability of resin–filler systems. The comparative tests carried out (without degassing and degassing) allowed us to conclude that the presence of gas in the composite does not result from the degassing method (which could be a methodological objection in the process of evaluating our results) but is related to the release of gas from the inside of the diatom during the cross-linking process (system heating up). Studies have shown that standard methods adopted as good industrial practice are not sufficient to remove the air contained in diatom shells. The specificity of diatom frustules’ morphology is unusual for commonly used fillers. They have a hierarchical porous structure (nanopores, micropores and macro pores). It is necessary to use other factors, such as auxiliary means, but this is not the subject of this work. Tests have shown that diatom frustules can be used as a biologically friendly filling phase of natural origin in polymer composites, which allows for the reduction in the consumption of production with a high degree of chemical treatment such as epoxy resins.
Author Contributions: Conceptualization, R.E.P.; measurements, evaluation and interpretation, M.D., D.B., R.D., R.E.P.; SEM measurements, M.G.; diamaceous earth sedimentary process, M.S.; writing—original draft, R.D., R.E.P., D.B., M.D.; writing—review and editing, M.D., D.B., R.E.P., R.D.; supervision. R.D., R.E.P., K.J.K. All authors have read and agreed to the published version of the manuscript. Materials 2021, 14, 1663 25 of 26
Funding: This work was financially supported in the frame of the project “Advanced Biocomposites for Tomorrow’s Economy BIOG-NET”, FNP POIR.04.04.00-00-1792/18-00, project is carried out within the TEAM-NET programme of the Foundation for Polish Science co-financed by the European Union under the European Regional Development Fund. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data available in a publicly accessible repository. Acknowledgments: Acknowledgement for Andrzej Witkowski for consultation regarding the identi- fication of diatom species in the tested raw material. Conflicts of Interest: The authors declare no conflict of interest.
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