No abstract
Stress relaxation, carried out in 85 : 15 methyl alcohol–water mixture at 60°C., of gelatin films crosslinked with oxystarch or difluorodinitrobenzene may be represented by the sum of three exponential rates of stress decay. Fresh formaldehyde‐crosslinked films relax rapidly and only require two such terms. The second term is shown to be due to the breaking of crosslinks. All excepting 10% of the broken crosslinks do not re‐form in unstressed positions as the formaldehyde is easily able to diffuse out of the layer. This 10% is suggested as being due to crosslinks involving a histidine residue. A large percentage of the broken oxystarch crosslinks do re‐form because the oxystarch is not easily able to diffuse out of the layer.
A description is given of the pipette and centrifuge methods of determining grain-size distributions by sedimentation, and results are presented for a number of barium sulphate powders. From the sizedistribution curves, the specific surfaces are calculated and compared with values obtained by air permeability, dye adsorption and a light-extinction method. The results for the dye-adsorption and air-permeability methods are shown to be in good agreement with each other, but do not always agree with the values calculated from the size distribution. It is suggested that this is due to aggregates, instead of discreet particles, settling in the sedimentation analysis, and this view is supported by electron-micrographs. The failure of the optical method is attributed to thc fineness of the powders.Barium sulphate is used in the photographic industry under the name of ' baryta ' or ' blancfure ' for coating paper before the application of the photographic-emulsion coat. The baryta, which is in the form of an aqueous slurry, is applied to the paper in admixture with gelatin, certain dyes, etc. A knowledge of the average particle size and of the particle-size distribution of the baryta is of great importance to the technician, as many of the coating properties, e.g. viscosity, surface quality of the coating, etc., are functions of the particle size.A study has been made of various methods for determining the particle-size distribution and the average particle size of such powders, and the results obtained by these methods have been compared.The powders which were to be examined consisted mainly of particles within the size range of 5 p to o * I p. Many methods, of different degrees of reliability, are available for particle-size measurement in this size range and, of these, the following were chosen for examination: (I) Sedimentation analysis (pipette).(3) Air permeability. 14) Dye adsorption.
Gelatin layers show minimum swelling when the temperature of the layer during drying is about 35°. When dried at a higher temperature, the layer is ‘so‐like’ and bonding does not occur until the layer is swollen or conditioned at a high relative humidity. Sol‐like layers of non‐de‐ionised gelatin when swollen in distilled water swell rapidly to a maximum value and then contract to a final value. De‐ionized sol‐like layers swell straight to this final value. The overshoot with the non‐de‐ionized layers is believed to be due to osmotic forces within the layers, which disappear when the salts are washed out. When conditioned in an atmosphere of high relative humidity, all layers afterwards show a reduced swell. The reduction is very large and very rapid for sol‐like layers. The results are explained by assuming that bonds formed during setting and drying are under no strain at the time of their formation and will tend to resist any compressive or tensile strains caused by drying or by increased swelling, respectively. The number and the strength of the bonds will be controlled by the drying conditions.
133 SummaryThe general relations between retardation and relaxation spectra previously derived by the author (3) are used for a discussion of the conditions under which one of the spectra contains a single line if the other is of the "box" type, i. e. differs from zero only within a finite interval. Zusammen/assungDie allgemeinen Beziehungen zwisehen Relaxationsund Retardationsspektren, welche friiher veto Vcrf. abgeleitet wurden (3), wcrden herangezogen, um zu unter-suchen, unter welehen Bedingungeu eines der Spektren cine Einzellinie enthalt, wenn das andcre ein Kastenspektrum ist. t~e]erences 1
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