Solvent Exchange Technique Research Articles

Development and characterization of novel starch and alkyl ketene dimer microcellular foam particles

There is interest in replacing inorganic fillers in paper, coatings and plastics with renewable organic fillers to improve the economics, performance and environmental aspects of such products. Starch microcellular foam (SMCF) particles are promising materials in this regard. This research was undertaken to produce SMCF particles and characterize their morphology, optical properties and interaction with water. SMCF particles were produced using corn starch; which was solubilized in water. The foam structure was created by precipitating the starch using ethanol in a solvent exchange technique. The starch was also reacted with alkyl ketene dimer (AKD) wax at different levels in order to understand if the AKD could impart water resistance to the particles. Two methods were used to incorporate AKD: (1) the AKD was coated onto foam particles after foam formation, and (2) the AKD was blended with the starch before foam particle formation. Hexane extraction was used to purify a portion of each of the samples to determine if unreacted AKD existed with the starch matrix. Particles with porous structure and high brightness were developed for all combinations of starch and AKD. The brightness of the particles was significantly higher than that of the uncooked starch, presumably due to the development of a porous structure. For the AKD coated particles a minimum amount of AKD charged was needed to develop a high contact angle, low solubility, and low water retention value, higher levels of AKD in AKD coated particles did not further alter these properties. This indicates that the changes in properties with AKD level with coated particles is a surface phenomena. Starch AKD blends did not show any increased resistance to water relative to the particles with no AKD blended. This is due to the ethanol used in the precipitation process extracting the AKD, which was confirmed by elemental analysis. The particles, even with the 10% AKD coated or blended, lost their porous structure upon rewetting. Further work needs to be performed to more effectively hydrophobize the starch foams with AKD.

Aspects of the preparation of starch microcellular foam particles crosslinked with glutaraldehyde using a solvent exchange technique

Starch microcellular foam (SMCF) has been produced using a solvent exchange technique with a native corn starch crosslinked with glutaraldehyde. To produce a high specific surface area for the starch particles the cooked starch solution was reacted with glutaraldehyde and then precipitated under shear with ethanol. The relationship between the crosslinking density on the void structure of the SMCF has been studied. Characterization by NMR confirmed the crosslinking reaction. An increase in the glutaraldehyde concentration from 0 to 15 g glutaraldehyde/100 g starch was accompanied by a decrease in particle size and moisture content and an increase in brightness and specific surface area. Scanning electron microscope images of the SMCF particles show that the smallest average void diameter obtained was 0.182 μm at a 7.5 g glutaraldehyde/100 g starch. Four starch materials having different viscosity were prepared by hydrolysis of the native corn starch with a 1 N HCl/methanol system for different reaction durations to investigate the relationship between starch molecular weight and void structure of the SMCF. The starches were crosslinked with 15 g glutaraldehyde/100 g starch and precipitated with ethanol to form SMCF. Decreasing the starch viscosity decreased the brightness and specific surface area, and increased the particle size, void diameter and moisture content. Increases in stirring speed during the precipitation enhanced the properties of the SMCF particles. The effect of pressing the starch particles to form a pellet caused a collapse of the foam structure at pressures above about 6000 psi. The results indicate that the structure/chemistry of the starch material and the processing conditions can be controlled in order to produce particles with morphology and properties useful for light scattering applications. In particular, higher molecular weight and intermolecular crosslinking and high shear during precipitation have been found to enhance the foam formation.

Polymeric Multiporous Materials from Fibrillar Networks

Abstract Summary: Poly(vinylidene fluoride) (PVF2) forms thermoreversible gels with liquid alkyl diesters as well as with camphor which is solid at room temperature. The diesters are replaced by another low boiling solvent cyclohexane by solvent exchange technique while camphor is dried just by exposing the material in vacuum to yield highly porous materials. Nano pores are generated as a result of solvent removal from polymer‐solvent intercalates whereas macropores are contributed by percolation of polymer fibrils. The porosity thus created covers a wide range from 3 nm to 400 µm producing multiporous materials. Pores greater than 6 nm are measured by mercury intrusion porosimetry (MIP) and pores of diameter less than 6 nm is observed by N2 adsorption porosimetry. The dried samples show two melting regions, low temperature hump for porous portion and high temperature peak for bulk portion. The porous materials have the ability to absorb water‐soluble herbicides from around 10‐ppm aqueous solution as indicated in UV spectrophotometric experiment.

Complexation of trivalent rare earth elements (Ce, Eu, Gd, Tb, Yb) by carbonate ions

Carbonate stability constants for five rare earth elements (Ce3+, Eu3+, Gd3+, Tb3+, and Yb3+) have been determined at t = 25°C and 0.70 ± 0.02 M ionic strength through solvent exchange techniques. Estimated stability constants for Ce, Eu, and Yb are in close agreement with previous work. Analyses using Gd and Tb provide the first carbonate stability constants for these elements based on direct measurements. Our measured stability constants were used to estimate carbonate stability constants for the entire suite of REEs. Our Eu, Gd, and Tb carbonate stability constants demonstrate the existence of a “Gd-break”: Carbonate stability constants for Gd are smaller than those for Eu and Tb. In analogy to Gd concentration anomalies reported in field observations, Gd stability constant anomalies have been defined in terms of the difference log Lβn(Gd) − log {(Lβn(Eu) + Lβn(Tb))2}, where Lβn(M) = [MLn][M3+]−1[L]−n. Examinations of REE-organic stability constants demonstrate that 106 out of 125 organic ligands have negative Gd anomalies in their first stability constants. The magnitudes of negative Gd anomalies generally become greater with increasing magnitude in Gd-ligand stability constants. Field observations of positive anomalies in shale-normalized Gd concentrations can be explained in terms of REE scavenging by organic surface ligands, such as polyaminocarboxylic acids, which possess a more negative Gd anomaly than carbonate ligands. Our modeling efforts indicate that a mixture of strongly complexing organics such as polyaminocarboxylic acids, and weakly complexing organics such as mono- and dicarboxylic acids is consistent with the pattern of REE scavenging by marine particulate matter.

Effect of Drying Condition of Precipitated Gel on the Transparency of PLZT Ceramics

The relation between the drying condition of precipitated gels and the transparency of PLZT ceramics was studied. The drying condition affected the transparency of ceramics. The highest transparency was obtained when the precipitated gel was dried by the solvent-exchange technique using acetone. The transparency decreased when the calcined powder had a cohesive nature. In order to prepare an aggregate-free powder after calcination, it was necessary that the precipitated gel was coagulated in the organic solvent used in the drying procedure. If the gel was intensionally coagulated in aqueous solution of ammonium carbonate, the dried powder was non-aggregated, and transparent ceramics was made without using an organic solvent. The coagulated structure of gel in a solvent was retained after drying and calcination, so that a powder with a small amount of internal pores was obtained by grinding, because of its weak cohesive strength.

The suitability of solvent exchange techniques for studying the pore structure of hardened cement paste

An examination was made of the interaction between three organic solvents (methanol, propan-2-ol and ethanol) and samples of calcium hydroxide and of hardened cement paste. The techniques used were thermogravimetric analysis, X-ray diffraction, infra-red spectroscopy and mercury intrusion porosimetry. Although there was evidence of a reaction between the solvents (particularly methanol) and cement paste during thermal analysis, there was no indication of a reaction at room temperature. The pore size distribution of vacuum-dried cement samples was unaffected by methanol soaking, and it is concluded that solvent exchange techniques are suitable for the preparation of samples for pore structure determinations.

Studies on Gas Transport Through Dry Cellulose Acetate Membranes Prepared by Solvent Exchange Technique

Abstract The mechanism of gas transport through pores on the surface of dry cellulose acetate membranes under pressure was identified for membranes prepared by the solvent exchange technique using pure gas permeation rate data. The pure gases were helium, methane and carbon dioxide. The variables Involved in the membrane preparation are the shrinkage temperature, the first solvent, the second solvent and the combinations thereof. Different conditions of membrane preparation produce different pore sizes. Depending on this pore size, one of the following mechanisms becomes dominant: Knudsen, surface and size exclusion.

Studies on the solvent exchange technique for making dry cellulose acetate membranes for the separation of gaseous mixtures

AbstractThe effect of the pore size on the surface of the water‐wet reverse osmosis membrane and the solvents used in the solvent exchange process on the pore size of the resulting dry cellulose acetate membrane was studied with respect to the separation of a CO2/CH4 mixture. It has been found that there is a critical pore size on the surface of wet membrane that results in the smallest pore size on the dry membrane and, consequently, in the highest separation factor. Such critical pore sizes become greater when the boiling point of the second solvent is increased.

Adsorption of ions at the cellulose/aqueous electrolyte interface. Part 2.—Determination of the surface area of cellulose fibres

The surface areas of the fibres studied in Part 1, obtained by the method of negative adsorption, are reported. Corrections were applied for low surface potential using Gouy–Chapman theory and the charge/pH isotherms. The fibres were oxidised to increase the charge and surface areas obtained for oxidised and non-oxidised fibres were in good agreement. The surface area was also obtained for unbleached sulphite pulp which has a high charge. The results were compared with surface areas determined by B.E.T. nitrogen adsorption. All the pulps were initially in a ‘never-dried’ state and for the B.E.T. work they were specially prepared using solvent-exchange techniques such that all the water was replaced by dry pentane. The negative adsorption results for the unbleached sulphate pulp clearly showed the effect of overlapping double layers, and this was treated theoretically. The effect of beating the fibres, as in paper-making, on the surface area was also investigated by both methods as was the effect of drying the fibres once.

Porosity of swollen solvent-exchanged cellulose and its collapse during final liquid removal

Recording of the first benzene desorption from swollen cellulosic material dehydrated by the solvent exchange technique shows that a collapse of its porous structure is considerable during the removal of benzene and as a result the porosity of swollen cellulose is only partly preserved. A comparison of the first and subsequent isotherms indicates the extent of the collapse, which is expressed as a difference between the volumes of benzene sorbed at an arbitrarily chosen relative benzene vapor pressure of 0.98. As documented on various pulp samples swollen to different degrees in water, ethylenediamine and aqueous sodium hydroxide the absolute loss in pore volume is greater for the samples swollen to a higher degree. The porous structure of never dried pulp changes significantly upon drying due to permanent closure of larger pores. Mercerization of both never dried and dried pulp results in an increased volume of micropores. The efficiency of the solvent exchange dehydration in producing a highly porous aerogel of cellulose depends on the liquids used and is likely sensitive to the amount of unreplaced water and other polar liquids.

The fine structure of cotton fibre as revealed by swelling during methacrylate embedding

Water-swollen cotton fibres, accommodated to the embedding medium by means of a solvent exchange technique, were embedded in a mixture of methyl methacrylate and n-butyl methacrylate and sectioned with a diamond knife. Examination of the sections under the electron microscope showed that the swelling effect of the water on the fibres was subsequently enhanced by an enormous additional swelling that occurred during the polymerization of the embedding medium. This additional swelling is explained by the assumption that the polymerization reaction proceeds in the medium permeating the fibre at a faster rate than in the external medium. In consequence, monomer diffuses from outside into the fibre. Since the methacrylate embedding technique can disperse the cellulose to a great extent, it provides a suitable method for studying the structure of the cotton fibre.