Direct Carbonization Synthesis And Application Of Calcium Carbonate Whiskers

Calcium carbonate (CaCO3) whiskers, an environment-friendly whisker material,is characterized by wide raw material sources, good performance, nontoxic and low price, and widely used in various fields such as plastics, coating, rubber, friction, paper-making and pharmaceutics. The synthesis, surface modification and application of CaCO3 whiskers have attracted more and more researches' attention due to low costs, high whiteness, and contamination-free. However, such product still stays in reaserch stage in China, and there is a big gap compared with that in JP and US.Synthesis, and surface modification of CaCO3 whiskers were engaged in this subject, and modified whiskers were added into polypropylene to investigate the toughening and reinforcing effects.CaCO3 whiskers were direct synthesized at first time from CaO by carbonization method with soluble phosphate as controlling agents, and additives dosage, initial Ca(OH)2 solid concentration, reaction temperature, CO2 flow rate and agitation speed were investigated to achieve the optimum conditions. The results revealed that initial Ca(OH)2 solid concentration of 3.0wt.%,phosphate dosage of n(p/Ca) of 0.25,reaction temperature of 75℃, low CO2 flow rate of 16.2L/(h·kgCa(OH)2) and agitation speed of 2000rpm are beneficitial to formation of aragonite whiskers. Well crystallized CaCO3 whiskers with aragonite content of 88.3%,length of 23μm and aspect ratio of 15.3 were synthesized.The effect mechanism and whiskers growth law were suggested in terms of XRD, SEM, TEM, FTIR and TG-DSC characterization results and related theorys of crystal chemistry and phsical chemistry. It has been indicated that the most stable calcium phosphate in thermodynamics, hydroxyapatite (HA) with length of 80nm and width of 20nm more or less, is formed through the reaction of soluble phosphates and Ca(OH)2 before the introduction of CO2, and then [CO32-(OH)] enters the crystal lattices of HA to replace partially [PO43-] yielding B-carbonate HA in the early stage of CO2 introduction, which induces aragonite to heterogeneously nucleate as nucleation center thereafter, and grows into CaCO3 whiskers by the continuous stack of Ca2+ and CO32-, and other HA selectly adsorbed onto the flanks of CaCO3 to control crystal morphology.Wet surface modification was carried out with surfactants such as stearic acid, sodium stearate, zinc stearate and titanate, etc, and modification effects were characterized with activication index and contact angle. Surfactant types, dosage, initial slurry concentration, modification temperature, modification time, agitation speed, and drying temperature were studied to achieve the optimum conditions as following:better surfactant is sodium stearate, srufacant dosage of 3%,initial slurry concentration of 8wt.%,modification temperature of 80℃,modification time of 20min, agitation speed of 1500rpm, drying temperature of 100℃, and drying time of 3h. The activication index, contact angle and oil absorption value of CaCO3 modified under the optimum conditions are 100%,146.66°and 64ml/100g, and the dispersibility and fluidity of CaCO3 whiskers are improved largely.The modification mechanism of sodium stearate was suggested in terms of SEM, XRD, TG-DSC,and FTIR characterization results. It has been revealed that sodium stearate had not change the CaCO3 whiskers phase, and the insoluble (CH3(CH2)16COO)2Ca was generated by reactions of CH3(CH2)16COO- and Ca2+,Ca(OH)+,or CaHCO3+ onto the surface of CaCO3 whiskers when ethyl alcohol solution of sodium stearate was added into. Chemical adsorption layer as well as physical adsorption layer existed between CaCO3 and sodium stearate, the former is very strong and steady, while the latter is faint and easy to desorb.Modified CaCO3 whiskers of different additions were put into polypropylene to perform effects on mechanical property of composite materials. Cantilever notched impact strength is chosen to evaluate toughening, tensile strength and tensile modulus of elasticity are adopted to estimate rigidity, breaking elongation is introduced to appraise plasticity. Impact strength increased and then decreased, tensile strength slightly reduced, tensile modulus of elasticity increased and breaking elongation decreased with addition of whiskers. The properties of toughening and rigidity of composity materials were improved when certain CaCO3 whiskers were added.Research results above have guiding meaning to exploitation of the aboundant lime resources in China and industrialized production of CaCCO3 whiskers.