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Amorphous calcium carbonate in form of spherical nanosized particles and its application as fillers for polymers
Materials Science and Engineering A 477 (2008) 217–225
Amorphous calcium carbonate in form of spherical nanosized particles and its application as fillers for polymers K. Gorna a , M. Hund b , M. Vuˇcak c , F. Gr¨ohn a , G. Wegner a,∗ a
Max Planck Institute for Polymer Research, 55128 Mainz, Germany b KTC Consulting GmbH, 67434 Neustadt, Germany c Schaefer Kalk GmbH & Co. KG, 65582 Diez, Germany
Received 28 March 2007; received in revised form 7 May 2007; accepted 9 May 2007
Abstract The synthesis of amorphous calcium carbonate (ACC) via a liquid precursor to give spherical particles with monodisperse distribution of diameters in the range of 0.4–1.2 m has been optimized to the level to obtain multigram yields per batch. The synthesis was achieved by precipitation of ACC from a strongly alkaline solution of calcium chloride (CaCl2 ) at ambient temperature using the hydrolysis of water soluble dimethyl carbonate (DMC) as the internal source of CO2 . As ACC produced by this novel method contains a small fraction of bound water a drying process, namely annealing at 200 ◦ C for 6 h was developed. The water free powder was blended with conventional polyolefins (linear low density polyethylene (LLDPE), high density polyethylene (HDPE), polypropylene (PP), polystyrene (PS)) in a melt extrusion process. Blending with poly(lactic acid) (PLA) as a model of a biomedically relevant polymer was also achieved. While a homogenous dispersion of the dried amorphous calcium carbonate (DACC) particles in the polymer composites was easily achieved, the interaction between the polymer continuous phase and particle surface seems to be rather weak. Consequently, the physical properties of the blends having volume fractions from 10 to 40% of the filler behave as expected for non-interacting materials, e.g. the melt viscosity increases as predicted from Einstein’s law, the glass transition and melting temperature of the polymer matrices remain largely unaffected. The Young’s modulus did increase while tensile strength and elongation at break decrease. © 2007 Elsevier B.V. All rights reserved. Keywords: Amorphous calcium carbonate; Precipitation; Stabilization; Fillers; Compounding
1. Introduction Calcium carbonate is extensively used as an additive or modifier in paper, paints, plastics, inks, adhesives and pharmaceuticals, to mention but a few. New discoveries and refined processes in the plastic, paper and pharmaceutical industries call for high-end type of products consisting of particles, whose crystalline phase, morphology, size and distribution of sizes are strictly controlled and can be modulated according to specific requirements. Calcium carbonate is traditionally used in plastics as bulking agent to substitute the expensive polymers. All properties of the pure polymer are subject to change as a result of filling, and in fact a new material is created by blending a polymer with
∗
Corresponding author. Fax: +49 6131379100. E-mail address: [email protected] (G. Wegner).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.05.045
inorganics. The properties of the resulting composite material are determined by the properties of the components, namely type of polymer and filler, filler particle size, shape and modulus, the concentration of filler in the polymer matrix and the kind of interaction between the filler particles as well as filler particles and polymer host [1–4]. Calcium carbonate particles of an aspect ratio close to unity are expected to modify the viscosity of the polymer melt. Their good thermal conductivity contributes to the homogeneity of the melt and good dispersion in the polymer matrix [5]. Synthetic, so-called precipitated calcium carbonate (PCC) is commonly produced by a recarbonizing process in which natural calcium carbonate is decomposed to calcium oxide (lime) and carbon dioxide. Calcium oxide with water forms calcium hydroxide that reacts with carbon dioxide and as a result pure, synthetic calcium carbonate precipitates. Unfortunately, PCC synthesized by this traditional method usually comprises a mixture of various crystalline polymorphs with anisotropic,
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elongated particles having high aspect ratio. Particles are usually highly agglomerated and there is inadequate control over the distribution of their sizes. Recently, a new method to obtain amorphous calcium carbonate (ACC) in form of isotropic, spherical particles has been reported [6,7]. This process makes use of the base-catalyzed hydrolysis of dimethyl carbonate as a source of CO2 . The latter is homogenously produced in the solution and formation of a gas–liquid interface is avoided. This interface clearly serves as a nucleation site for crystalline PCC and must be avoided under all circumstances when one wants to produce ACC reproducibly. Consequently, ACC is precipitated in a very reproducible way as monodisperse spherical particles of sub-micrometer diameter. Additionally, it has been shown that ACC is formed in consequence of a liquid–liquid phase separation between a low concentrated and high concentrated calcium carbonate hydrate phase by which droplets are formed spontaneously in course of the binodal decomposition of the supersaturated solution. The droplets dehydrate rapidly and form colloidal particles of glassy, hydrated ACC which is stable for a long time in the dry state. Although preparation of the synthetic ACC from water under various reactive conditions has been already reported [8–15], only the method via hydrolysis of alkyl carbonate results in pure amorphous material without any crystalline contaminations. It is worth mentioning that ACC has received considerable attention in very recent years since it is believed that this metastable material plays an important role in biological mineralization. As has been shown by evaluation of sea urchin larvae, ACC is a precursor to calcite in the formation of spicules [16,17] and it serves as a kind of storage material from which later crystalline calcium carbonate is formed during the moulting period in Crustaceans [18]. One of the reasons why organisms use ACC as the precursor in formation of calcite or aragonite is that the disordered structure of ACC can be easily cast into any shape and that it has higher solubility as compared to the crystalline polymorphs [19–21] which makes it attractive as an intermediate source of the final crystalline material. The process of ACC synthesis via hydrolysis of alkyl carbonate has been well characterized and optimized with respect to the morphology of the material but only in a diluted system [6,7]. The use of higher concentrations of the necessary components in this process will determine the degree of supersaturation at which ACC will precipitate, and this, in consequence, will affect the properties of the final material. A higher concentration of the components can also favor precipitation of other modifications of calcium carbonate according to their solubility product [14,22–24]. Therefore, the following questions need to be addressed: How much can the concentration of the components be increased in order to maximize throughput and obtain ACC without any deterioration of its properties and to what extent do the conditions of high concentration affect the final material’s properties? Once ACC can be obtained in sufficiently large amount and quality its potential as a filler and modifier of polymers can be investigated. In the following study, we wish to describe how the reaction conditions for synthesis of ACC can be optimized to repro-
ducibly enhance the yield of spherical particles of defined size and size distribution. Since these particles contain bound water, a stabilizing process is reported which is able to remove water without disturbing size and shape of the particles. Finally, first experiments characterizing these particles as additives for polymers and properties of the resulting polymer/particle blends will be reported. 2. Experimental 2.1. Synthesis of ACC Calcium chloride dihydrate (Fluka) (CaCl2 ), dimethyl carbonate (Fluka) (DMC) and sodium hydroxide (Fluka) (NaOH) were used for precipitation of ACC as described [6,7]. Shortly, in this process CaCl2 and DMC are dissolved in MQ water at 20 ◦ C. Solution of NaOH is prepared in a separate glass vessel also in MQ water at 20 ◦ C. After quickly mixing both solutions hydrolytic decomposition of DMC takes place with release of CO2 and ACC starts to precipitate. The precipitate is isolated from the mother liquid by centrifugation, washed in acetone and subsequently dried in vacuum at room temperature. The precipitation of ACC was performed at increasing concentration of components ranging from 10 to 100 mM. In this work, equimolar concentrations of CaCl2 and DMC were used in all experiments. The following code will be used to identify materials obtained under different conditions: samples marked as 20–30, or 30–50 refer to the materials that have been synthesized using 20 mM of CaCl2 (DMC) and 30 mM of NaOH, or 30 mM of CaCl2 (DMC) and 50 mM of NaOH, respectively. 2.2. Compounding with polymers Linear low density polyethylene (LLDPE), high density polyethylene (HDPE), polypropylene (PP), polystyrene (PS), and poly(lactic acid) (PLA) were used to prepare composites with ACC synthesized using 20 mM of CaCl2 (DMC) and 30 mM of NaOH (ACC 20–30), for comparison, blends were also made with commercially available crystalline PCC (SCHAEFER PRECARB® 400 (Precarb 400) supplied by Schaefer Kalk GmbH & Co. KG). Before mixing the ACC 20–30 was dried in an air-vented oven at 200 ◦ C for 6 h to remove structural water. Dried amorphous calcium carbonate (DACC 20–30) or Precarb 400 powders were first mechanically mixed with polymer powders or pellets to achieve 10, 20 or 40 vol.% of CaCO3 in the composites. A twin-screw microextruder was used for compounding the fillers with the polymers. Pre-mixed calcium carbonate and polymer in a total amount of 5 g were fed into the extruder hopper and the composite material was squeezed through the extruder and was collected in the shape of bar. The operation temperature of the extruder was 230 ◦ C for the LLDPE, HDPE, PP, and 200 ◦ C for the PS and PLA. The extruding was performed at a screw speed of 100 rpm under nitrogen to avoid the degradation of the material.
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2.3. Characterization of calcium carbonate The morphology and size of the calcium carbonate particles were characterized using a low voltage scanning electron microscope (Leo Gemini 1530) operated at 3 kV. IR-spectra were recorded using a Nicolet 730 FTIR spectrometer. Spectra of the samples dispersed in KBr powder were recorded in the diffusive mode (DRIF). The crystal form of calcium carbonate was evaluated by means of X-ray diffraction (Cu K␣ radiation). DSC measurements were carried out under nitrogen on a Mettler-Toledo DSC 30S. Thermogravimetry was done under nitrogen on a Mettler-Toledo ThermoSTAR TGA. The specific surface area of the particles was determined by the BET method (Micromeritics Gemini 2360 Analyser) using nitrogen adsorption. The samples were prepared for adsorption analysis by degassing (FlowPrep 060 Degasser) at 130 ◦ C for at least 3 h. The oil absorption was measured using diisononyl phthalate (DINP) by the rubout test similar to EN ISO 787-5. In this test, DINP is mixed with a sample and rubbed with a spatula on a smooth surface until a stiff putty-like paste is formed which does not break or separate. The oil absorption in g/100 g sample was calculated by Oil absorption =
DINP absorbed, g 100 sample weight, g
This value represents the quantity of oil required per unit weight of sample particles to completely saturate the calcium carbonate sorptive capacity. 2.4. Characterization of polymer composites The dispersion of the particles in the polymer matrices was visualized using a low voltage scanning electron microscope (Leo Gemini 1530) operated at 3 kV or using a high voltage scanning electron microscope (Zeiss DSM 962) operated at 15 kV. The extruded samples were fractured in liquid nitrogen and in the case of using the high voltage SEM, the sample surface was sputtered with a gold–palladium layer. Mettler-Toledo DSC 30S differential scanning calorimeter calibrated with indium was used for the evaluation of the thermal characteristics of the polymers and the composite materials. The measurements were carried out under dry, oxygen free nitrogen flowing at a rate of 40 ml/min to protect the materials from degradation. The weight of samples was in the range of 15–20 mg. The samples were first heated from 25 to 230 ◦ C, and then cooled to 25 ◦ C and finally heated a second time from 25 to 230 ◦ C at a rate of 10 ◦ C/min. The content of calcium carbonate in the polymers was assessed from thermogravimetric analysis using Mettler Toledo ThermoSTAR TGA-SDTA 851. The measurements were carried out in nitrogen at temperatures in the range of 25–800 ◦ C at a heating rate of 10 ◦ C/min. The mechanical properties of selected composites filled with 10 vol.% of calcium carbonate were evaluated by tensile tests carried out on a Instron 4200 equipped with a 0.1 or 1 kN load cell operating at a cross-head speed of 10 mm/min. For this measurement the samples in the shape of dog-bone
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(2 mm × 15 mm × (0.16 ± 0.02) mm) were cut out from the film prepared by melting the extruded bars in a hydraulic press at a temperature of 180 ◦ C and under pressure of 40 kN. At least four samples for each material were tested at room temperature. 3. Results and discussion 3.1. Precipitation of ACC from concentrated solution ACC was synthesized according the method [6] in which carbon dioxide is supplied to the solution of a calcium salt (CaCl2 ) as a result of alkaline hydrolysis of dimethyl carbonate (DMC). This method works in dilute conditions and allows to produce ACC as very regular spheres of narrow distribution of diameters. The diameter of particles oscillated around 700 nm when the reaction was run in diluted solution at 20 ◦ C and the precipitate was isolated after 20 min of reaction time. The morphology of the calcium carbonate particles obtained from solutions containing CaCl2 and DMC in a concentration ranging from 20 to 60 mM but constant concentration of NaOH (20 mM) are presented in Fig. 1. The average size of the particles decreased from 790 to 430 nm for the systems 20–20 and 60–20, respectively. The distribution of sizes became broader as the concentration of CaCl2 was increased. The particles maintain their spherical shape when produced from up to 60 mM of calcium chloride and DMC, further increase of CaCl2 concentration leads, however, to a material with rather ill defined particle shape and, in addition, a small amount of big spherical particles with diameter of up to 3–4 m are observed. FTIR analysis of the samples as prepared showed in all cases some features characteristic for ACC (data not presented). The presence of a broad carbonate out-of plane bending vibrational band at 863 cm−1 (v2), a symmetric stretch absorption at 1075 cm−1 (v1), broad but not split peak at 1440 cm−1 , and a broad absorption peak at around 3350 and 1640 cm−1 due to water molecules is seen [9,10,12,18]. For materials obtained at higher concentration of the reactants poorly developed absorption peaks characteristic for calcite could be observed (712 cm−1 ). XRD measurements confirmed that the material synthesized at a concentration of 60 mM of CaCl2 (DMC) showed XRD pattern characteristic for calcite and vaterite (Fig. 2). However, the XRD shows that the crystalline polymorphs are not the dominating component but a large part of the material remains still amorphous. This explains that the peaks detected in the FTIR spectrum are neither typically “amorphous” nor “crystalline” indicating the presence of poorly developed crystalline material. The induction time for precipitation characterized by the appearance of turbidity became shorter with increasing concentration of calcium chloride and DMC. A higher amount of dialkyl carbonate not only allowed to reach the sufficient amount of CO2 for the nucleation faster but also led to a high consumption of base at the beginning of the process which caused a sudden drop in pH. In consequence, the growth of the particles was suppressed as a result of gradual reduction of CO2 concentration. Irrespective of the higher concentration of components the yield of product decreased (Fig. 3) and the particle diameter became smaller. The reduction in the amount of base used in the pro-
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Fig. 1. SEM micrographs of ACC synthesized at increasing concentration of CaCl2 (DMC) from 20 to 60 mM, and constant concentration of NaOH (20 mM).
cess slowed down the hydrolysis process of DMC and created a different (decreasing) supersaturation level. Elfil and Roques demonstrated in their work [23,24] that homogenous nucleation occurs once the solubility product for amorphous calcium carbonate is surpassed. At such condition amorphous or calcium carbonate hydrates are initially formed, but as they are thermodynamically not stable, they transform to a mixture of crystalline calcium carbonate polymorphs. Which polymorph is eventually formed depends on the conditions of precipitation like temperature and the range of supersaturation achieved in the system [14,22,25]. In fact, this was observed in the present study as well. As the base concentration used in the reaction was too low, the supersaturation level rapidly changed in the course of precipitation and as a result, initially formed ACC started to transform into vaterite and calcite. A small fraction of spherical particles of vaterite with diameter in the range of 3–4 m was observed in the material obtained from systems deficient of base (60–20). In consequence of the results so far described the concentration of base was increased, even though a rough calculation showed that the increase of base concentration can favor the undesired precipitation of calcium hydroxide in the system. The product yield at various concentrations of components is pre-
sented in Fig. 3. It shows that in order to obtain more yield per batch it is essential to use higher concentrations of base as the higher pH enhances hydrolysis of the alkyl carbonate and this leads to production of sufficient amount of CO2 in the system. From the SEM micrographs presented in Fig. 4 it can be seen that the particle shape is strongly affected by the concentration of the components from which precipitation occurs. The higher is the solution concentration the smaller are the particles, which result. However, the smaller particles are more agglomerated (Fig. 4, 100–100). FTIR and XRD measurement reveal the amorphous nature of all the materials synthesized from solutions in which the initial concentration of NaOH was at least 30 mM. The concentration of the components determines not only the particle diameter but also the thermal characteristics of the material. Depending on the initial concentration of components ACC showed one or two distinct thermal transitions in the DSC curves (Fig. 5). The first broad endothermic transition is assigned to the release of water from ACC [6,7,13,16,18]. This transition was usually composed of two, more or less distinguishable peaks. A broad peak with maximum at around 100–120 ◦ C was more pronounced for the calcium carbonate precipitated at lower pH. The second narrow peak occurred in the range of 150–175 ◦ C. The position and shape of this second peak depended again on
Fig. 2. XRD pattern of calcium carbonate synthesized at 40 and 60 mM of CaCl2 (DMC) and 20 mM of NaOH; expected Brags reflections for calcite—solid lines, and vaterite—dashed lines.
Fig. 3. Yield of ACC at various concentrations of CaCl2 (DMC) and NaOH. Material obtained under conditions indicated by the arrow was selected for further studies in blending with polymers.
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Fig. 4. SEM micrographs of ACC synthesized at various concentration of CaCl2 (DMC) and NaOH.
the pH at which ACC had been precipitated. For higher pH the peak was located at a slightly higher temperature and became broader. The small exothermic transition in the DSC curves seen around 290 ◦ C seems to be related to the complete water release associated with the transition of ACC into calcite [6]. For the material precipitated at higher concentration of CaCl2 and DMC but lower concentration of base this transition, however, was not always detected (Fig. 5, system 20–20). There was also a decrease in the enthalpy of endothermal process related to the water release form the calcium carbonate precipitated at higher concentration but lower pH. Release of water observed by DSC corresponds to the weight loss observed by TGA. The amount of water detected by TGA measurement was in the range of 8–10% of total weight for materials synthesized using at least 30 mM of base. Calcium carbonate obtained from the system deficient of base contained much less water and its relative amount decreased to 2% as the concentration of CaCl2 increased. It should be stressed, however, that materials obtained under such conditions showed evidence to be partially crystalline. Similar relations between thermal characteristics and pH of the starting solution have been observed by others authors [14,15,26]. From the structure of the endothermal transition one
Fig. 5. DSC of ACC synthesized from solutions of 20 mM CaCl2 (DMC) at various concentrations of NaOH ranging from 20 to 50 mM.
may conclude that the water included in the material may have different environments. In other words, the water may be incorporated as bound water, may be coordinated to calcium ions in ACC and eventually may be occluded in ill-defined positions in the ACC structure [14,15,26]. Among different ACC particles precipitated from various concentrations of components for the further study we selected material synthesized using 20 mM of CaCl2 , DMC and 30 mM of NaOH with yield of about 30% (ACC 20–30). These conditions allow to produce ACC in form of non-agglomerated particles with average sizes ranging from 0.9 to 1.2 m depending on the volume of the reactor used. Material was not contaminated by the crystalline forms of calcium carbonate and showed a long time stability over 3 months of storage, even though, it is known that ACC is not a stable material, and with time, when exposed to humidity, spontaneously undergoes crystalline transformation. This process, however, can be induced thermally and the crystallization occurs, following some authors, by a solid-state transformation [16,26]. ACC 20–30 has been treated at temperatures ranging from 80 to 200 ◦ C in order to remove the structural water and to evaluate the progress of crystallization. Drying the material for 72 h at 80 ◦ C in air activated the crystallization of the material and XDR showed weak reflections of calcite. Still about 2% of water was present at the end of annealing as determined by TGA. The effect of temperature and time of heating on the amount of water remaining in the calcium carbonate is shown in Fig. 6. The higher the temperature and longer the time of heating the higher is the amount of water that was released. The loss of water was accompanied by transformation of the material into calcite (Fig. 7). The crystallization process during heating occurred only sluggishly even at higher temperature as can be seen from the XRD pattern. Even after 6 h at 200 ◦ C, still part of the material remained amorphous. It is important that the morphology of the particles did not change upon drying. Only the surface of the spheres became somewhat rougher as shown by SEM micrographs (Fig. 8(a)). A change of surface roughness of ACC particles resulting from the loss of water has been reported previously [16,26]. Some of the spherical particles exhibited a restructured surface and the presence of small particles inside the bulk material with diameter of around 30–40 nm that could be assigned to calcite. The crystal size evaluated from
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Fig. 6. The effect of temperature and time of heating on the amount of remaining water in the particles of calcium carbonate 20–30. The fraction of remaining water was determined by TGA.
Debye–Scherrer XRD patterns pointed to presence of crystals with diameter in the range of 25–35 nm. 3.2. Effect of calcium carbonate on the properties of composites The ACC 20–30 (e.g. Fig. 3) was used for compounding with a series of archetypical polymers, namely LLDPE, HDPE, PP, PS, and PLA. The ACC 20–30 as synthesized was first stabilized at 200 ◦ C for 6 h, meaning that the ACC lost the structural water and was partially crystallized to give calcite, without loss of its spherical morphology. The characteristics of this calcium carbonate after drying (DACC 20–30) and a commercial material (Precarb 400) used in this study as a reference are shown in Fig. 8, and Table 1. The two materials differ significantly in the shape of particles. DACC 20–30 particles have a regular spherical shape while the Precarb 400 consists partially of needles of rather high aspect ratio. Specific surface area and DIPN absorption (see Section 2) are slightly lower for DACC 20–30 compared to Precarb 400 but the surface accessibility is comparable.
Fig. 8. SEM images of (a) DACC 20–30 after drying at 200 ◦ C for 6 h, (b) Precarb 400.
The composites were prepared with a theoretical content of fillers of 10, 20 or 40 vol.%. The calcium carbonate powder was mixed with the polymers in a small twin extruder. The material after feeding was processed a few times trough the extruder in closed cycle to ensure god mixing of both phases—polymer and inorganic filler. The quality of the dispersion of the filler in the polymer matrix was evaluated from visual inspection of crosssections of the composites by SEM. Examples are shown in Fig. 9. Both types of fillers were used without any modification of their surfaces which could potentially improve the dispersion. Nevertheless, reasonable good distribution of calcium carbonate particles was achieved in all cases. A more detailed analysis of Table 1 Characteristics of the calcium carbonate fillers used for compounding with polymers Parameters
Fig. 7. The XRD of calcium carbonate 20–30 dried at 200 ◦ C for 6 h; expected Brags reflections of calcite—solid lines, and vaterite—dashed lines.
Inorganic fillers DACC 20–30a
Precarb 400b
Polymorph Particle size
Calcite/amorphous Spheres: 1.15 m
Specific surface (BET) DINP absorption
12.2 m2 /g 51 g/100 g
Calcite/aragonite Spindles/needles: 1.16 m × 0.20 m 8.4 m2 /g 68 g/100 g
a b
Preparation as described in the work, sample 20–30, see also Figs. 3 and 8a. Supplied by Schaefer Kalk GmbH & Co. KG.
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Fig. 9. SEM micrographs of (a) PP filled with 40 vol.% of DACC 20–30; (b) PP filled with 40 vol.% of Precarb 400; (c) PLA filled with 20 vol.% of DACC 20–30; (d) PLA filled with 20 vol.% of Precarb.
the SEM micrographs, showed, however, that a better dispersion can be obtained for the spherical particles of DACC 20–30. Large aggregates were not observed indicating that the shear forces during the extrusion process were sufficiently large. In the case of Precarb 400 small aggregates of particles could be observed occasionally which were not broken up to yield individual primary particles during compounding. The elongated particles of this material tend to become aligned parallel to the direction of extrusion, although some needles can be observed lying normal to the preferred direction indicating that the shear forces were too low to align them perfectly. Moreover, comparison of the surface finishing of the extruded samples shows a higher roughness for samples filled with Precarb 400 as a consequence of the poor alignment of particles along the direction of extruding. Examination of fracture surfaces reveals the presence of voids between the calcium carbonate particles and the polymer matrix as a result of not optimized compatibility of the inorganic and organic components. Some of the particles are disattached from the polymer upon fracture. Slightly better adhesion of the polymer to the inorganic filler can be discerned when one inspects SEM micrograph from cross-section of PLA-based composites. One may speculate that the presence of lactic acid which could be released upon thermal degradation of PLA during extrusion in the composite acts as a binding agent of the polymer to the calcium carbonate surface. Although, the stabilization process of ACC caused its partial crystallization and appearance of small, most likely calcite crys-
tals inside of the spheres, the particle of DACC 20–30 withstood the shear forces upon extrusion and did not break up. The real content of inorganic filler in the compounded materials was estimated from TGA analysis and the results are summarized in Table 2. The experimentally determined fraction of calcium carbonate was in most cases consistent with the data of the initial mixture. This indicates that calcium carbonate does not decompose during processing. The composite materials obtained in this study were also characterized by DSC. LLDPE, HDPE and PP showed single melting endotherms and, upon cooling, crystallization peaks (Table 2). There is very little if at all effect of fillers on the thermal characteristics of the semicrystalline polyolefins indicating that the interaction between filler surface and polymer matrix is negligibly small. This is quite different for the case of PLA as matrix. The presence of both types of fillers decreases the glass transition and melting temperature in PLA proportional to the content of filler. Altogether, these data are not surprising as it is well known that calcium carbonate has a very weak nucleation effect in polymers [4,27]. The influence of the fillers on the tensile properties of composites is presented in Table 3. In general, the presence of both types of filler increased the Young’s modulus, decreased tensile strength and elongation at break, what is typically observed for the fillers with rather limited capacity of reinforcement [1–3]. The elongation at break decreased for all filled polymers, although, the decrease was lower for polymers filled with DACC
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Table 2 TGA and DSC data for the polymers and composites Material
Wt.% of fillera
Wt.% of fillerb
Hm (J/g)
Tm (◦ C)
Tc (◦ C)
Tg
HDPE HDPE10%DACC 20–30 HDPE 10% Precarb 400 LLDPE LLDPE 10% DACC 20–30 LLDPE 10% Precarb 400 PP PP 10% DACC 20–30 PP 40% DACC 20–30 PP 10% Precarb 400 PP 40% Precarb 400 PLA PLA 10% DACC 20–30 PLA 20% DACC 20–30 PLA 10% Precarb 400 PLA 20% Precarb 400
– 23.1 23.1 – 23.1 23.1 – 23.1 64.3 23.1 64.3 – 19.5 40.0 19.5 40.0
– 25.0 23.7 – 21.1 20.8 – 19.3 55.2 14.5 56.4 – 22.6 32.9 4.3 41.3
167.0 164.7 160.0 90.0 112.9 104.8 66.7 71.3 70.5 71.1 69.5 31.5 23.9 22.5 26.3 32.5
136.8 139.0 140.0 127.2 129.6 129.3 169.0 167.4 163.1 165.4 163.7 159.8 158.2 161.2 159.4 162.0
109.8 106.7 105.2 104.0 104.8 105.8 111.3 116.1 126.4 116.6 120.9 – – – – –
– – – – – – – – – – – 68.2 60.8 59.9 61.9 59.3
a b
Fraction of filler used in the mixture before compounding. Real fraction of filler after compounding estimated from TGA.
Table 3 Mechanical properties of selected polymers after extrusion and composites filled with 10 vol.% of DACC 20–30 or Precarb 400 Tensile strength (MPa) LLDPE LLDPE + DACC 20–30 LLDPE + Precarb 400 HDPE HDPE + DACC 20–30 HDPE + Precarb 400 PP PP + DACC 20–30 PP + Precarb 400
22 20 13 38 23 27 47 32 28
± ± ± ± ± ± ± ± ±
0.8 2.5 3.7 4 6.4 3.7 2.6 5 2.2
20–30. The spindle and needle-like particles of Precarb 400 seem to cause failure at very low elongation most probably due to the shape of the particles or as a result of inhomogeneous dispersion of the particles in the polymer matrix. Aggregated particles act as crack initiation sites under dynamic loading condition [4]. 4. Conclusions ACC in form of spherical particles of diameter between 0.4 and 1.2 m can be obtained by reacting calcium chloride in aqueous solution with carbon dioxide at a pH > 11 under the condition by which CO2 is homogenously supplied by hydrolysis of a water soluble dialkyl carbonate. Dimethyl carbonate was used as the source of CO2 in the present case. The droplets of initially strongly hydrated and fluid ACC are formed in consequence of a liquid–liquid phase segregation. Once formed they loose water and undergo a transition to glassy but still hydrated ACC. The nucleation of the particles of ACC can be controlled to give comparatively narrow size distributions. Among other factors, temperature of formation process and pH, the letter being connected to the scale of the hydrolysis of the dialkyl carbonate and supersaturation with regard to CO2 dissolved in the reaction medium play a major role. The ACC is metastable at room temperature and needs to be dried by a short heat treatment to
Tensile modulus (MPa) 161 169 168 631 823 844 798 990 1029
± ± ± ± ± ± ± ± ±
4 26 40 27 46 157 91 40 95
Elongation at fracture (%) 686 689 390 531 415 511 933 651 58
± ± ± ± ± ± ± ± ±
24 60 182 51 186 47 29 129 14
remove water around 200 ◦ C. Partial crystallization to give calcite may occur under these conditions; however, the form of spheres which are mostly non-aggregated remains unaffected. The reaction conditions for formation of this material have been optimized with respect to yield and scale as to obtain multigram quantities in a single batch at overall yield near 30% with regard to the starting material calcium chloride. ACC as prepared by this novel process may have interest as a high-end filler material for polymers. Thus, compounding of dried ACC with the archetypical polyolefins HDPE, LLDPE, PP, PS was tried to demonstrate its principal capabilities and potential. Moreover, compounding of PLA as a representation of a polymer for biomedical applications with dried ACC was studied as well. The particles of dried ACC maintain their identity during mixing and extrusion of a polymer host melt. They remain mostly individual and do not form irreversibly aggregates. In other words, the interaction between the ACC particles and the polymers is rather weak and the shear forces exerted onto the particle/polymer melt mixture suffice to establish quickly a rather homogenous dispersion. However, the particles as prepared in the above described precipitation process followed by a drying step do not have a specific interaction with the polymer matrix. Thus, reinforcing phenomena are barely seen with exception of
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PLA where thermal decomposition product of PLA may serve as modifier or compatibilizer with regard to ACC. Further work needs to investigate surface modification of the ACC particles in order to enhance their interaction with specific polymers, e.g. in order to improve the adhesion of the particles to the matrix as to carry specific functions into polymer/particle composite. This should be readily possible because ACC has a highly reactive surface and reacts quickly with all kinds of acidic species which are precipitated to their surface as insoluble calcium salts complexes.
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Acknowledgements The authors acknowledge Dr. Dorothee Benner-Luger from Schaefer Kalk GmbH & Co. KG for SEM measurements and Michael Steiert from Max Planck Institute for Polymer Research for XDR measurements. References [1] M. Xantos, in: M. Xantos (Ed.), Functional Fillers for Polymers, WileyVCH, Weinheim, 2005 (Chapter I). [2] B. Pukanszky, E. Fekete, Adv. Polym. Sci. 139 (1999) 109–153. [3] R.N. Rothon, Adv. Polym. Sci. 139 (1999) 67–107. [4] B. Pukanszky, J. Moczo Macromol. Symp. 214 (2004) 115–134. [5] A. Tanniru, R.D.K. Misra, Mater. Sci. Eng. (A) 425 (2006) 53–70. [6] M. Faatz, F. Gr¨ohn, G. Wegner, Adv. Mater. 16 (2004) 996–1000.
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Amorphous calcium carbonate in form of spherical nanosized particles and its application as fillers for polymers K. Gorna a , M. Hund b , M. Vuˇcak c , F. Gr¨ohn a , G. Wegner a,∗ a
Max Planck Institute for Polymer Research, 55128 Mainz, Germany b KTC Consulting GmbH, 67434 Neustadt, Germany c Schaefer Kalk GmbH & Co. KG, 65582 Diez, Germany
Received 28 March 2007; received in revised form 7 May 2007; accepted 9 May 2007
Abstract The synthesis of amorphous calcium carbonate (ACC) via a liquid precursor to give spherical particles with monodisperse distribution of diameters in the range of 0.4–1.2 m has been optimized to the level to obtain multigram yields per batch. The synthesis was achieved by precipitation of ACC from a strongly alkaline solution of calcium chloride (CaCl2 ) at ambient temperature using the hydrolysis of water soluble dimethyl carbonate (DMC) as the internal source of CO2 . As ACC produced by this novel method contains a small fraction of bound water a drying process, namely annealing at 200 ◦ C for 6 h was developed. The water free powder was blended with conventional polyolefins (linear low density polyethylene (LLDPE), high density polyethylene (HDPE), polypropylene (PP), polystyrene (PS)) in a melt extrusion process. Blending with poly(lactic acid) (PLA) as a model of a biomedically relevant polymer was also achieved. While a homogenous dispersion of the dried amorphous calcium carbonate (DACC) particles in the polymer composites was easily achieved, the interaction between the polymer continuous phase and particle surface seems to be rather weak. Consequently, the physical properties of the blends having volume fractions from 10 to 40% of the filler behave as expected for non-interacting materials, e.g. the melt viscosity increases as predicted from Einstein’s law, the glass transition and melting temperature of the polymer matrices remain largely unaffected. The Young’s modulus did increase while tensile strength and elongation at break decrease. © 2007 Elsevier B.V. All rights reserved. Keywords: Amorphous calcium carbonate; Precipitation; Stabilization; Fillers; Compounding
1. Introduction Calcium carbonate is extensively used as an additive or modifier in paper, paints, plastics, inks, adhesives and pharmaceuticals, to mention but a few. New discoveries and refined processes in the plastic, paper and pharmaceutical industries call for high-end type of products consisting of particles, whose crystalline phase, morphology, size and distribution of sizes are strictly controlled and can be modulated according to specific requirements. Calcium carbonate is traditionally used in plastics as bulking agent to substitute the expensive polymers. All properties of the pure polymer are subject to change as a result of filling, and in fact a new material is created by blending a polymer with
∗
Corresponding author. Fax: +49 6131379100. E-mail address: [email protected] (G. Wegner).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.05.045
inorganics. The properties of the resulting composite material are determined by the properties of the components, namely type of polymer and filler, filler particle size, shape and modulus, the concentration of filler in the polymer matrix and the kind of interaction between the filler particles as well as filler particles and polymer host [1–4]. Calcium carbonate particles of an aspect ratio close to unity are expected to modify the viscosity of the polymer melt. Their good thermal conductivity contributes to the homogeneity of the melt and good dispersion in the polymer matrix [5]. Synthetic, so-called precipitated calcium carbonate (PCC) is commonly produced by a recarbonizing process in which natural calcium carbonate is decomposed to calcium oxide (lime) and carbon dioxide. Calcium oxide with water forms calcium hydroxide that reacts with carbon dioxide and as a result pure, synthetic calcium carbonate precipitates. Unfortunately, PCC synthesized by this traditional method usually comprises a mixture of various crystalline polymorphs with anisotropic,
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elongated particles having high aspect ratio. Particles are usually highly agglomerated and there is inadequate control over the distribution of their sizes. Recently, a new method to obtain amorphous calcium carbonate (ACC) in form of isotropic, spherical particles has been reported [6,7]. This process makes use of the base-catalyzed hydrolysis of dimethyl carbonate as a source of CO2 . The latter is homogenously produced in the solution and formation of a gas–liquid interface is avoided. This interface clearly serves as a nucleation site for crystalline PCC and must be avoided under all circumstances when one wants to produce ACC reproducibly. Consequently, ACC is precipitated in a very reproducible way as monodisperse spherical particles of sub-micrometer diameter. Additionally, it has been shown that ACC is formed in consequence of a liquid–liquid phase separation between a low concentrated and high concentrated calcium carbonate hydrate phase by which droplets are formed spontaneously in course of the binodal decomposition of the supersaturated solution. The droplets dehydrate rapidly and form colloidal particles of glassy, hydrated ACC which is stable for a long time in the dry state. Although preparation of the synthetic ACC from water under various reactive conditions has been already reported [8–15], only the method via hydrolysis of alkyl carbonate results in pure amorphous material without any crystalline contaminations. It is worth mentioning that ACC has received considerable attention in very recent years since it is believed that this metastable material plays an important role in biological mineralization. As has been shown by evaluation of sea urchin larvae, ACC is a precursor to calcite in the formation of spicules [16,17] and it serves as a kind of storage material from which later crystalline calcium carbonate is formed during the moulting period in Crustaceans [18]. One of the reasons why organisms use ACC as the precursor in formation of calcite or aragonite is that the disordered structure of ACC can be easily cast into any shape and that it has higher solubility as compared to the crystalline polymorphs [19–21] which makes it attractive as an intermediate source of the final crystalline material. The process of ACC synthesis via hydrolysis of alkyl carbonate has been well characterized and optimized with respect to the morphology of the material but only in a diluted system [6,7]. The use of higher concentrations of the necessary components in this process will determine the degree of supersaturation at which ACC will precipitate, and this, in consequence, will affect the properties of the final material. A higher concentration of the components can also favor precipitation of other modifications of calcium carbonate according to their solubility product [14,22–24]. Therefore, the following questions need to be addressed: How much can the concentration of the components be increased in order to maximize throughput and obtain ACC without any deterioration of its properties and to what extent do the conditions of high concentration affect the final material’s properties? Once ACC can be obtained in sufficiently large amount and quality its potential as a filler and modifier of polymers can be investigated. In the following study, we wish to describe how the reaction conditions for synthesis of ACC can be optimized to repro-
ducibly enhance the yield of spherical particles of defined size and size distribution. Since these particles contain bound water, a stabilizing process is reported which is able to remove water without disturbing size and shape of the particles. Finally, first experiments characterizing these particles as additives for polymers and properties of the resulting polymer/particle blends will be reported. 2. Experimental 2.1. Synthesis of ACC Calcium chloride dihydrate (Fluka) (CaCl2 ), dimethyl carbonate (Fluka) (DMC) and sodium hydroxide (Fluka) (NaOH) were used for precipitation of ACC as described [6,7]. Shortly, in this process CaCl2 and DMC are dissolved in MQ water at 20 ◦ C. Solution of NaOH is prepared in a separate glass vessel also in MQ water at 20 ◦ C. After quickly mixing both solutions hydrolytic decomposition of DMC takes place with release of CO2 and ACC starts to precipitate. The precipitate is isolated from the mother liquid by centrifugation, washed in acetone and subsequently dried in vacuum at room temperature. The precipitation of ACC was performed at increasing concentration of components ranging from 10 to 100 mM. In this work, equimolar concentrations of CaCl2 and DMC were used in all experiments. The following code will be used to identify materials obtained under different conditions: samples marked as 20–30, or 30–50 refer to the materials that have been synthesized using 20 mM of CaCl2 (DMC) and 30 mM of NaOH, or 30 mM of CaCl2 (DMC) and 50 mM of NaOH, respectively. 2.2. Compounding with polymers Linear low density polyethylene (LLDPE), high density polyethylene (HDPE), polypropylene (PP), polystyrene (PS), and poly(lactic acid) (PLA) were used to prepare composites with ACC synthesized using 20 mM of CaCl2 (DMC) and 30 mM of NaOH (ACC 20–30), for comparison, blends were also made with commercially available crystalline PCC (SCHAEFER PRECARB® 400 (Precarb 400) supplied by Schaefer Kalk GmbH & Co. KG). Before mixing the ACC 20–30 was dried in an air-vented oven at 200 ◦ C for 6 h to remove structural water. Dried amorphous calcium carbonate (DACC 20–30) or Precarb 400 powders were first mechanically mixed with polymer powders or pellets to achieve 10, 20 or 40 vol.% of CaCO3 in the composites. A twin-screw microextruder was used for compounding the fillers with the polymers. Pre-mixed calcium carbonate and polymer in a total amount of 5 g were fed into the extruder hopper and the composite material was squeezed through the extruder and was collected in the shape of bar. The operation temperature of the extruder was 230 ◦ C for the LLDPE, HDPE, PP, and 200 ◦ C for the PS and PLA. The extruding was performed at a screw speed of 100 rpm under nitrogen to avoid the degradation of the material.
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2.3. Characterization of calcium carbonate The morphology and size of the calcium carbonate particles were characterized using a low voltage scanning electron microscope (Leo Gemini 1530) operated at 3 kV. IR-spectra were recorded using a Nicolet 730 FTIR spectrometer. Spectra of the samples dispersed in KBr powder were recorded in the diffusive mode (DRIF). The crystal form of calcium carbonate was evaluated by means of X-ray diffraction (Cu K␣ radiation). DSC measurements were carried out under nitrogen on a Mettler-Toledo DSC 30S. Thermogravimetry was done under nitrogen on a Mettler-Toledo ThermoSTAR TGA. The specific surface area of the particles was determined by the BET method (Micromeritics Gemini 2360 Analyser) using nitrogen adsorption. The samples were prepared for adsorption analysis by degassing (FlowPrep 060 Degasser) at 130 ◦ C for at least 3 h. The oil absorption was measured using diisononyl phthalate (DINP) by the rubout test similar to EN ISO 787-5. In this test, DINP is mixed with a sample and rubbed with a spatula on a smooth surface until a stiff putty-like paste is formed which does not break or separate. The oil absorption in g/100 g sample was calculated by Oil absorption =
DINP absorbed, g 100 sample weight, g
This value represents the quantity of oil required per unit weight of sample particles to completely saturate the calcium carbonate sorptive capacity. 2.4. Characterization of polymer composites The dispersion of the particles in the polymer matrices was visualized using a low voltage scanning electron microscope (Leo Gemini 1530) operated at 3 kV or using a high voltage scanning electron microscope (Zeiss DSM 962) operated at 15 kV. The extruded samples were fractured in liquid nitrogen and in the case of using the high voltage SEM, the sample surface was sputtered with a gold–palladium layer. Mettler-Toledo DSC 30S differential scanning calorimeter calibrated with indium was used for the evaluation of the thermal characteristics of the polymers and the composite materials. The measurements were carried out under dry, oxygen free nitrogen flowing at a rate of 40 ml/min to protect the materials from degradation. The weight of samples was in the range of 15–20 mg. The samples were first heated from 25 to 230 ◦ C, and then cooled to 25 ◦ C and finally heated a second time from 25 to 230 ◦ C at a rate of 10 ◦ C/min. The content of calcium carbonate in the polymers was assessed from thermogravimetric analysis using Mettler Toledo ThermoSTAR TGA-SDTA 851. The measurements were carried out in nitrogen at temperatures in the range of 25–800 ◦ C at a heating rate of 10 ◦ C/min. The mechanical properties of selected composites filled with 10 vol.% of calcium carbonate were evaluated by tensile tests carried out on a Instron 4200 equipped with a 0.1 or 1 kN load cell operating at a cross-head speed of 10 mm/min. For this measurement the samples in the shape of dog-bone
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(2 mm × 15 mm × (0.16 ± 0.02) mm) were cut out from the film prepared by melting the extruded bars in a hydraulic press at a temperature of 180 ◦ C and under pressure of 40 kN. At least four samples for each material were tested at room temperature. 3. Results and discussion 3.1. Precipitation of ACC from concentrated solution ACC was synthesized according the method [6] in which carbon dioxide is supplied to the solution of a calcium salt (CaCl2 ) as a result of alkaline hydrolysis of dimethyl carbonate (DMC). This method works in dilute conditions and allows to produce ACC as very regular spheres of narrow distribution of diameters. The diameter of particles oscillated around 700 nm when the reaction was run in diluted solution at 20 ◦ C and the precipitate was isolated after 20 min of reaction time. The morphology of the calcium carbonate particles obtained from solutions containing CaCl2 and DMC in a concentration ranging from 20 to 60 mM but constant concentration of NaOH (20 mM) are presented in Fig. 1. The average size of the particles decreased from 790 to 430 nm for the systems 20–20 and 60–20, respectively. The distribution of sizes became broader as the concentration of CaCl2 was increased. The particles maintain their spherical shape when produced from up to 60 mM of calcium chloride and DMC, further increase of CaCl2 concentration leads, however, to a material with rather ill defined particle shape and, in addition, a small amount of big spherical particles with diameter of up to 3–4 m are observed. FTIR analysis of the samples as prepared showed in all cases some features characteristic for ACC (data not presented). The presence of a broad carbonate out-of plane bending vibrational band at 863 cm−1 (v2), a symmetric stretch absorption at 1075 cm−1 (v1), broad but not split peak at 1440 cm−1 , and a broad absorption peak at around 3350 and 1640 cm−1 due to water molecules is seen [9,10,12,18]. For materials obtained at higher concentration of the reactants poorly developed absorption peaks characteristic for calcite could be observed (712 cm−1 ). XRD measurements confirmed that the material synthesized at a concentration of 60 mM of CaCl2 (DMC) showed XRD pattern characteristic for calcite and vaterite (Fig. 2). However, the XRD shows that the crystalline polymorphs are not the dominating component but a large part of the material remains still amorphous. This explains that the peaks detected in the FTIR spectrum are neither typically “amorphous” nor “crystalline” indicating the presence of poorly developed crystalline material. The induction time for precipitation characterized by the appearance of turbidity became shorter with increasing concentration of calcium chloride and DMC. A higher amount of dialkyl carbonate not only allowed to reach the sufficient amount of CO2 for the nucleation faster but also led to a high consumption of base at the beginning of the process which caused a sudden drop in pH. In consequence, the growth of the particles was suppressed as a result of gradual reduction of CO2 concentration. Irrespective of the higher concentration of components the yield of product decreased (Fig. 3) and the particle diameter became smaller. The reduction in the amount of base used in the pro-
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Fig. 1. SEM micrographs of ACC synthesized at increasing concentration of CaCl2 (DMC) from 20 to 60 mM, and constant concentration of NaOH (20 mM).
cess slowed down the hydrolysis process of DMC and created a different (decreasing) supersaturation level. Elfil and Roques demonstrated in their work [23,24] that homogenous nucleation occurs once the solubility product for amorphous calcium carbonate is surpassed. At such condition amorphous or calcium carbonate hydrates are initially formed, but as they are thermodynamically not stable, they transform to a mixture of crystalline calcium carbonate polymorphs. Which polymorph is eventually formed depends on the conditions of precipitation like temperature and the range of supersaturation achieved in the system [14,22,25]. In fact, this was observed in the present study as well. As the base concentration used in the reaction was too low, the supersaturation level rapidly changed in the course of precipitation and as a result, initially formed ACC started to transform into vaterite and calcite. A small fraction of spherical particles of vaterite with diameter in the range of 3–4 m was observed in the material obtained from systems deficient of base (60–20). In consequence of the results so far described the concentration of base was increased, even though a rough calculation showed that the increase of base concentration can favor the undesired precipitation of calcium hydroxide in the system. The product yield at various concentrations of components is pre-
sented in Fig. 3. It shows that in order to obtain more yield per batch it is essential to use higher concentrations of base as the higher pH enhances hydrolysis of the alkyl carbonate and this leads to production of sufficient amount of CO2 in the system. From the SEM micrographs presented in Fig. 4 it can be seen that the particle shape is strongly affected by the concentration of the components from which precipitation occurs. The higher is the solution concentration the smaller are the particles, which result. However, the smaller particles are more agglomerated (Fig. 4, 100–100). FTIR and XRD measurement reveal the amorphous nature of all the materials synthesized from solutions in which the initial concentration of NaOH was at least 30 mM. The concentration of the components determines not only the particle diameter but also the thermal characteristics of the material. Depending on the initial concentration of components ACC showed one or two distinct thermal transitions in the DSC curves (Fig. 5). The first broad endothermic transition is assigned to the release of water from ACC [6,7,13,16,18]. This transition was usually composed of two, more or less distinguishable peaks. A broad peak with maximum at around 100–120 ◦ C was more pronounced for the calcium carbonate precipitated at lower pH. The second narrow peak occurred in the range of 150–175 ◦ C. The position and shape of this second peak depended again on
Fig. 2. XRD pattern of calcium carbonate synthesized at 40 and 60 mM of CaCl2 (DMC) and 20 mM of NaOH; expected Brags reflections for calcite—solid lines, and vaterite—dashed lines.
Fig. 3. Yield of ACC at various concentrations of CaCl2 (DMC) and NaOH. Material obtained under conditions indicated by the arrow was selected for further studies in blending with polymers.
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Fig. 4. SEM micrographs of ACC synthesized at various concentration of CaCl2 (DMC) and NaOH.
the pH at which ACC had been precipitated. For higher pH the peak was located at a slightly higher temperature and became broader. The small exothermic transition in the DSC curves seen around 290 ◦ C seems to be related to the complete water release associated with the transition of ACC into calcite [6]. For the material precipitated at higher concentration of CaCl2 and DMC but lower concentration of base this transition, however, was not always detected (Fig. 5, system 20–20). There was also a decrease in the enthalpy of endothermal process related to the water release form the calcium carbonate precipitated at higher concentration but lower pH. Release of water observed by DSC corresponds to the weight loss observed by TGA. The amount of water detected by TGA measurement was in the range of 8–10% of total weight for materials synthesized using at least 30 mM of base. Calcium carbonate obtained from the system deficient of base contained much less water and its relative amount decreased to 2% as the concentration of CaCl2 increased. It should be stressed, however, that materials obtained under such conditions showed evidence to be partially crystalline. Similar relations between thermal characteristics and pH of the starting solution have been observed by others authors [14,15,26]. From the structure of the endothermal transition one
Fig. 5. DSC of ACC synthesized from solutions of 20 mM CaCl2 (DMC) at various concentrations of NaOH ranging from 20 to 50 mM.
may conclude that the water included in the material may have different environments. In other words, the water may be incorporated as bound water, may be coordinated to calcium ions in ACC and eventually may be occluded in ill-defined positions in the ACC structure [14,15,26]. Among different ACC particles precipitated from various concentrations of components for the further study we selected material synthesized using 20 mM of CaCl2 , DMC and 30 mM of NaOH with yield of about 30% (ACC 20–30). These conditions allow to produce ACC in form of non-agglomerated particles with average sizes ranging from 0.9 to 1.2 m depending on the volume of the reactor used. Material was not contaminated by the crystalline forms of calcium carbonate and showed a long time stability over 3 months of storage, even though, it is known that ACC is not a stable material, and with time, when exposed to humidity, spontaneously undergoes crystalline transformation. This process, however, can be induced thermally and the crystallization occurs, following some authors, by a solid-state transformation [16,26]. ACC 20–30 has been treated at temperatures ranging from 80 to 200 ◦ C in order to remove the structural water and to evaluate the progress of crystallization. Drying the material for 72 h at 80 ◦ C in air activated the crystallization of the material and XDR showed weak reflections of calcite. Still about 2% of water was present at the end of annealing as determined by TGA. The effect of temperature and time of heating on the amount of water remaining in the calcium carbonate is shown in Fig. 6. The higher the temperature and longer the time of heating the higher is the amount of water that was released. The loss of water was accompanied by transformation of the material into calcite (Fig. 7). The crystallization process during heating occurred only sluggishly even at higher temperature as can be seen from the XRD pattern. Even after 6 h at 200 ◦ C, still part of the material remained amorphous. It is important that the morphology of the particles did not change upon drying. Only the surface of the spheres became somewhat rougher as shown by SEM micrographs (Fig. 8(a)). A change of surface roughness of ACC particles resulting from the loss of water has been reported previously [16,26]. Some of the spherical particles exhibited a restructured surface and the presence of small particles inside the bulk material with diameter of around 30–40 nm that could be assigned to calcite. The crystal size evaluated from
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Fig. 6. The effect of temperature and time of heating on the amount of remaining water in the particles of calcium carbonate 20–30. The fraction of remaining water was determined by TGA.
Debye–Scherrer XRD patterns pointed to presence of crystals with diameter in the range of 25–35 nm. 3.2. Effect of calcium carbonate on the properties of composites The ACC 20–30 (e.g. Fig. 3) was used for compounding with a series of archetypical polymers, namely LLDPE, HDPE, PP, PS, and PLA. The ACC 20–30 as synthesized was first stabilized at 200 ◦ C for 6 h, meaning that the ACC lost the structural water and was partially crystallized to give calcite, without loss of its spherical morphology. The characteristics of this calcium carbonate after drying (DACC 20–30) and a commercial material (Precarb 400) used in this study as a reference are shown in Fig. 8, and Table 1. The two materials differ significantly in the shape of particles. DACC 20–30 particles have a regular spherical shape while the Precarb 400 consists partially of needles of rather high aspect ratio. Specific surface area and DIPN absorption (see Section 2) are slightly lower for DACC 20–30 compared to Precarb 400 but the surface accessibility is comparable.
Fig. 8. SEM images of (a) DACC 20–30 after drying at 200 ◦ C for 6 h, (b) Precarb 400.
The composites were prepared with a theoretical content of fillers of 10, 20 or 40 vol.%. The calcium carbonate powder was mixed with the polymers in a small twin extruder. The material after feeding was processed a few times trough the extruder in closed cycle to ensure god mixing of both phases—polymer and inorganic filler. The quality of the dispersion of the filler in the polymer matrix was evaluated from visual inspection of crosssections of the composites by SEM. Examples are shown in Fig. 9. Both types of fillers were used without any modification of their surfaces which could potentially improve the dispersion. Nevertheless, reasonable good distribution of calcium carbonate particles was achieved in all cases. A more detailed analysis of Table 1 Characteristics of the calcium carbonate fillers used for compounding with polymers Parameters
Fig. 7. The XRD of calcium carbonate 20–30 dried at 200 ◦ C for 6 h; expected Brags reflections of calcite—solid lines, and vaterite—dashed lines.
Inorganic fillers DACC 20–30a
Precarb 400b
Polymorph Particle size
Calcite/amorphous Spheres: 1.15 m
Specific surface (BET) DINP absorption
12.2 m2 /g 51 g/100 g
Calcite/aragonite Spindles/needles: 1.16 m × 0.20 m 8.4 m2 /g 68 g/100 g
a b
Preparation as described in the work, sample 20–30, see also Figs. 3 and 8a. Supplied by Schaefer Kalk GmbH & Co. KG.
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Fig. 9. SEM micrographs of (a) PP filled with 40 vol.% of DACC 20–30; (b) PP filled with 40 vol.% of Precarb 400; (c) PLA filled with 20 vol.% of DACC 20–30; (d) PLA filled with 20 vol.% of Precarb.
the SEM micrographs, showed, however, that a better dispersion can be obtained for the spherical particles of DACC 20–30. Large aggregates were not observed indicating that the shear forces during the extrusion process were sufficiently large. In the case of Precarb 400 small aggregates of particles could be observed occasionally which were not broken up to yield individual primary particles during compounding. The elongated particles of this material tend to become aligned parallel to the direction of extrusion, although some needles can be observed lying normal to the preferred direction indicating that the shear forces were too low to align them perfectly. Moreover, comparison of the surface finishing of the extruded samples shows a higher roughness for samples filled with Precarb 400 as a consequence of the poor alignment of particles along the direction of extruding. Examination of fracture surfaces reveals the presence of voids between the calcium carbonate particles and the polymer matrix as a result of not optimized compatibility of the inorganic and organic components. Some of the particles are disattached from the polymer upon fracture. Slightly better adhesion of the polymer to the inorganic filler can be discerned when one inspects SEM micrograph from cross-section of PLA-based composites. One may speculate that the presence of lactic acid which could be released upon thermal degradation of PLA during extrusion in the composite acts as a binding agent of the polymer to the calcium carbonate surface. Although, the stabilization process of ACC caused its partial crystallization and appearance of small, most likely calcite crys-
tals inside of the spheres, the particle of DACC 20–30 withstood the shear forces upon extrusion and did not break up. The real content of inorganic filler in the compounded materials was estimated from TGA analysis and the results are summarized in Table 2. The experimentally determined fraction of calcium carbonate was in most cases consistent with the data of the initial mixture. This indicates that calcium carbonate does not decompose during processing. The composite materials obtained in this study were also characterized by DSC. LLDPE, HDPE and PP showed single melting endotherms and, upon cooling, crystallization peaks (Table 2). There is very little if at all effect of fillers on the thermal characteristics of the semicrystalline polyolefins indicating that the interaction between filler surface and polymer matrix is negligibly small. This is quite different for the case of PLA as matrix. The presence of both types of fillers decreases the glass transition and melting temperature in PLA proportional to the content of filler. Altogether, these data are not surprising as it is well known that calcium carbonate has a very weak nucleation effect in polymers [4,27]. The influence of the fillers on the tensile properties of composites is presented in Table 3. In general, the presence of both types of filler increased the Young’s modulus, decreased tensile strength and elongation at break, what is typically observed for the fillers with rather limited capacity of reinforcement [1–3]. The elongation at break decreased for all filled polymers, although, the decrease was lower for polymers filled with DACC
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Table 2 TGA and DSC data for the polymers and composites Material
Wt.% of fillera
Wt.% of fillerb
Hm (J/g)
Tm (◦ C)
Tc (◦ C)
Tg
HDPE HDPE10%DACC 20–30 HDPE 10% Precarb 400 LLDPE LLDPE 10% DACC 20–30 LLDPE 10% Precarb 400 PP PP 10% DACC 20–30 PP 40% DACC 20–30 PP 10% Precarb 400 PP 40% Precarb 400 PLA PLA 10% DACC 20–30 PLA 20% DACC 20–30 PLA 10% Precarb 400 PLA 20% Precarb 400
– 23.1 23.1 – 23.1 23.1 – 23.1 64.3 23.1 64.3 – 19.5 40.0 19.5 40.0
– 25.0 23.7 – 21.1 20.8 – 19.3 55.2 14.5 56.4 – 22.6 32.9 4.3 41.3
167.0 164.7 160.0 90.0 112.9 104.8 66.7 71.3 70.5 71.1 69.5 31.5 23.9 22.5 26.3 32.5
136.8 139.0 140.0 127.2 129.6 129.3 169.0 167.4 163.1 165.4 163.7 159.8 158.2 161.2 159.4 162.0
109.8 106.7 105.2 104.0 104.8 105.8 111.3 116.1 126.4 116.6 120.9 – – – – –
– – – – – – – – – – – 68.2 60.8 59.9 61.9 59.3
a b
Fraction of filler used in the mixture before compounding. Real fraction of filler after compounding estimated from TGA.
Table 3 Mechanical properties of selected polymers after extrusion and composites filled with 10 vol.% of DACC 20–30 or Precarb 400 Tensile strength (MPa) LLDPE LLDPE + DACC 20–30 LLDPE + Precarb 400 HDPE HDPE + DACC 20–30 HDPE + Precarb 400 PP PP + DACC 20–30 PP + Precarb 400
22 20 13 38 23 27 47 32 28
± ± ± ± ± ± ± ± ±
0.8 2.5 3.7 4 6.4 3.7 2.6 5 2.2
20–30. The spindle and needle-like particles of Precarb 400 seem to cause failure at very low elongation most probably due to the shape of the particles or as a result of inhomogeneous dispersion of the particles in the polymer matrix. Aggregated particles act as crack initiation sites under dynamic loading condition [4]. 4. Conclusions ACC in form of spherical particles of diameter between 0.4 and 1.2 m can be obtained by reacting calcium chloride in aqueous solution with carbon dioxide at a pH > 11 under the condition by which CO2 is homogenously supplied by hydrolysis of a water soluble dialkyl carbonate. Dimethyl carbonate was used as the source of CO2 in the present case. The droplets of initially strongly hydrated and fluid ACC are formed in consequence of a liquid–liquid phase segregation. Once formed they loose water and undergo a transition to glassy but still hydrated ACC. The nucleation of the particles of ACC can be controlled to give comparatively narrow size distributions. Among other factors, temperature of formation process and pH, the letter being connected to the scale of the hydrolysis of the dialkyl carbonate and supersaturation with regard to CO2 dissolved in the reaction medium play a major role. The ACC is metastable at room temperature and needs to be dried by a short heat treatment to
Tensile modulus (MPa) 161 169 168 631 823 844 798 990 1029
± ± ± ± ± ± ± ± ±
4 26 40 27 46 157 91 40 95
Elongation at fracture (%) 686 689 390 531 415 511 933 651 58
± ± ± ± ± ± ± ± ±
24 60 182 51 186 47 29 129 14
remove water around 200 ◦ C. Partial crystallization to give calcite may occur under these conditions; however, the form of spheres which are mostly non-aggregated remains unaffected. The reaction conditions for formation of this material have been optimized with respect to yield and scale as to obtain multigram quantities in a single batch at overall yield near 30% with regard to the starting material calcium chloride. ACC as prepared by this novel process may have interest as a high-end filler material for polymers. Thus, compounding of dried ACC with the archetypical polyolefins HDPE, LLDPE, PP, PS was tried to demonstrate its principal capabilities and potential. Moreover, compounding of PLA as a representation of a polymer for biomedical applications with dried ACC was studied as well. The particles of dried ACC maintain their identity during mixing and extrusion of a polymer host melt. They remain mostly individual and do not form irreversibly aggregates. In other words, the interaction between the ACC particles and the polymers is rather weak and the shear forces exerted onto the particle/polymer melt mixture suffice to establish quickly a rather homogenous dispersion. However, the particles as prepared in the above described precipitation process followed by a drying step do not have a specific interaction with the polymer matrix. Thus, reinforcing phenomena are barely seen with exception of
K. Gorna et al. / Materials Science and Engineering A 477 (2008) 217–225
PLA where thermal decomposition product of PLA may serve as modifier or compatibilizer with regard to ACC. Further work needs to investigate surface modification of the ACC particles in order to enhance their interaction with specific polymers, e.g. in order to improve the adhesion of the particles to the matrix as to carry specific functions into polymer/particle composite. This should be readily possible because ACC has a highly reactive surface and reacts quickly with all kinds of acidic species which are precipitated to their surface as insoluble calcium salts complexes.
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