| Bentonite resource is rich and widely distributed in China,the total reserve of which is No.1 all over the world.Montmorillonite,the main composition of bentonite,has been widely used in various fields of national economy because of the unique structure and performance.The requirements of property vary with the applications.The rheological properties of montmorillonite such as dispersity,expansibility,suspension property,thickening behavior and thixotropy are influenced by many factors,including the genesis,layer charge,charge distribution,interlayer cations,anion,particle size,chemical component,purity and so on.And there was a certain correlation among the factors.As a result,it is difficult to develop and apply montmorillonite rheological agent.At present,the development and application of high quality montmorillonite rheological agents in China are not mature,the product category is single,and there is a big gap in performance,price and application solutions compared with similar foreign products,which greatly limits the application and promotion.Montmorillonite rheological agents used in high value-added fields are all monopolized by American and Japanese companies.In this paper,for the purpose of solving the problems including the dispersity of high layer charge montmorillonite,the suspension stability of artificial sodium montmorillonite,the clustering and slow hydration of conventional sodium montmorillonite,the pre-activation and low efficiency of thickening and suspension in alcohol-water mixing system,these methods were selected containing changing the type and number of interlayer cations in montmorillonite,reducing the steric resistance of interlayer water,and adjusting the layer charge and variable charge.Combining with the market demand and the actual application,the rheological properties and related action mechanism of montmorillonite with different layer charge types were studied,which is of great guiding significance to the development of high value-added montmorillonite rheological agents in the future,to promote the development and application of montmorillonite products,and to replace or surmount the similar products abroad.Three kinds of bentonite with different layer charge from Kazuo of Liaoning province,Xuanhua of Hebei province and Chifeng of Inner Mongolia were selected as representatives to prepare high purity montmorillonite with purity above 98.0%.The effects of layer charge,anion,heat treatment,complex treatment,high value cation between layers,ammonium ion,type and amount of reagent on the properties of montmorillonite,such as suspension,thickening,expansion,thixotropy,dispersion and hydration,were discussed.The differences of rheological behaviour of montmorillonite with different layer charge were investigated.The methods to improve the dilatability of high charge montmorillonite,to enhance the suspension stability of sodium montmorillonite,to improve the dispersion and hydration properties,the thickening and suspension efficiency of montmorillonite in alcohol-water mixing system were put forward.The influence mechanism of p H,electrolyte,different reagent types and modification methods on the rheological properties of montmorillonite,as well as the related action mechanism of thickening,thixotropy and expansion of modified montmorillonite,were analyzed.In addition,the effect of modified montmorillonite in applications such as pellets,latex paint,casting paint and cosmetics was evaluated.The effects of rheological properties and the requirements of layer charge,thixotropy index,thickening and suspension properties in different applications of montmorillonite were discussed.The innovation of this paper lies in:(1)To solve the problem of poor dispersion and suspension property of high layer charge montmorillonite,the new process of calcination at low temperature in advance and then sodium modified can greatly improve the property of montmorillonite.Compared with the traditional sodium modified method,the acid expansion capacity is increased by more than 1.5 times.(2)The method of acid and ammonium salt composite modification was selected to solve the problem that the high price ions in sodium montmorillonite were difficult to be washed out due to the excellent suspension property.The suspension stability,thickening property and swelling property were improved,and was better than that of the high purity natural sodium montmorillonite Kunipia G of Japan.This paper mainly draws the following conclusions:(1)The method by grinding with sodium modification was used to purify bentonite with different layer charge from Liaoning,Hebei and Inner Mongolia.The best purification parameters were obtained,and the three kinds of high purity montmorillonite prepared confrom the requirements of 2015 edition pharmaceutical montmorillonite standard.By XRD,SEM,XRF and Zeta potential test,and calculating the layer charge,the results show that the three kinds of montmorillonite from Liaoning,Hebei and Inner Mongolia are low,medium and high layer charge types in proper order.The lower the layer charge of montmorillonite,the lower the content of magnesium,the greater the electronegativity,and the better the dispersion and suspension performance.(2)The effect of anions on the rheological properties of montmorillonite was discussed.The results showed that the montmorillonite modified by sulfate,acetic acid,nitrate and chloride ions with the same cation had excellent thickening,thixotropy,and fast dispersion speed,which were better than those modified by carbonate.The silicate modified montmorillonite has excellent suspension swelling property and fast dispersion speed,but poor thixotropy and thickening,so it can be used to prepare low viscosity and high suspension product.(3)The dispersion and suspension properties of montmorillonite can be effectively improved by calcining at low temperature in advance and then sodium modified.The crystal structure was maintained while the adsorbed water and interlayer water of montmorillonite were removed by low temperature heat treatment,and the steric resistance was reduced.It also promoted the iron divalent be oxidized to ferric iron in montmorillonite,which reduces the interlayer charge and further increases the expansibility of montmorillonite.Especially for the montmorillonite with high layer charge,the improvement effect is remarkable.The acid expansion capacity increased from 32 m L/g of traditional sodium montmorillonite to 80 m L/g.(4)The effects of urea and oxalic acid on the rheological properties of montmorillonite were investigated.The results showed that both of them could increase the viscosity and expansibility,and reduce the filtration loss.Moreover,There was synergism between urea and oxalic acid.The results of pellet application showed that the pellets prepared by urea modified sodium montmorillonite can significantly improve the performances in terms of compressive strength,drop strength and pellet amount,and the dosage could be reduced from 1.5% to 1.0%.The quality of the pellets met the requirements and the production cost was effectively saved.(5)The natural-like sodium montmorillonite was prepared by removing the high valence cations,then washing them with water and been sodium modified.The results showed that the content of calcium oxide in montmorillonite can be reduced from more than 2.00% to less than 0.50% by acid and ammonium salt composite modification,for the purpose of reducing the influence of high valence cations on the suspension stability of montmorillonite.The viscosity and 2 g expansion capacity of this natural-like sodium montmorillonite were twice that of traditional sodium montmorillonite,and the properties were better than that of the high purity natural sodium montmorillonite Kunipia G from Japan.(6)The rapidly dispersed hydration montmorillonite were prepared.The results showed that the sodium montmorillonite modified by sodium sulfate,sodium silicate and coupling agent were all of the rapid dispersion and hydration grade with less than10 min.High thixotropic rheological agent can be prepared by sodium sulfate modified montmorillonite with the thixotropic index of above 7.0 and 5% viscosity of 770 m Pa·s.The low viscosity and high suspension rheological agent can be prepared by sodium silicate modified montmorillonite with the thixotropic index of below 5.0 and 5%viscosity of 110 m Pa·s.The high viscosity rheological agent can be prepared by coupling agent grafting sodium montmorillonite with 4% viscosity of 5240 m Pa·s and the thixotropic index of 5.0-7.0.The results of application in latex paint showed that the rapid dispersion and hydrated montmorillonite can obviously improve the suspension property of latex paint,and effectively avoid the problem of after thickening with sodium montmorillonite,and the application effect of coupling agent grafting montmorillonite was the best.(7)The hydration and expansion mechanism of montmorillonite is analyzed,which can be divided into crystalline layer expansion and osmotic expansion.The water adsorbed by montmorillonite is filled layer by layer in the form of water molecular layers.The filling of a large number of water molecules weakens the interlayer bonding force,increases the interlayer spacing,and results in volume expansion.When the interlayer spacing increases to a certain extent,it turns into external expansion.Due to the difference in the concentration of ions inside and outside the crystal layer,the osmotic pressure is different.Water molecules enter the interlayer,and the cation diffuses into water,forming double electric layer,generating repulsion force,increasing the interlayer spacing and causing expansion.The mechanism of rapid dispersion is also based on the double electric layer theory.A small amount of electrolyte compresses the double electric layer,which reduces the interlamellar force and speeds up the dispersion rate.After complete dispersion,the local ion concentration decreases and the rheological properties recover.(8)The coupling agent grafted montmorillonite used as cosmetic emulsifier has the characteristics of self-thickening and self-emulsification.Compared with the conventional emulsifier,it is suitable for a wide p H range,good electrolyte resistance and temperature stability,no need for additional thickener,and is not limited by HLB theory,with good compatibility.It has excellent emulsifying effect on all kinds of oils such as mineral oil,vegetable oil,ester,silicone oil,etc.At the same time,it also has functions of thickening,whitening,repairing and improving skin feeling of montmorillonite.The appropriate amount of montmorillonite is 1-5%.(9)Modified montmorillonite used for both alcohol and water was prepared.The results showed that the thickening and suspension property of the medium and high layer charge montmorillonite in the mixture system of alcohol and water is poor.The thickening and suspension properties in water and alcohol of low charge lithium montmorillonite can be improved by modification with lithium oxalate,lithium fluoride,attapulgite and coupling agent.The application experiments in alcohol-based foundry coating showed that the best sample was 2.0% dodecyl triethoxy silane coupling agent grafted the lithium modified mixture of montmorillonite and attapulgite,with the dosage as low as 2.0%.The suspension rate of 24 h was more than 95%,and 7d suspension rate was more than 80%.It won't produce hard precipitation in long-term storage,and the effect is superior to similar products in market. |
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