![]() The shear flow develops by step wisely increasing the shear rate, whereby viscosity evaluations can be performed as soon as the shear stress becomes constant through reaching a steady state. In rheometry, the dynamic viscosity is obtained through the application of a shear flow in a thin film of the considered fluid. However, viscosity measurements based on glass capillary viscometry may have certain limitations specifically, these measurements can be carried out only at a predefined, constant shear rate, and the registration of the flow time is often performed manually, making the method work-intensive and time-consuming.Īlternatively, the intrinsic viscosity of cellulose solutions can be determined by measuring the dynamic viscosity via rotational rheometry. This method has been widely applied, due to its relatively straightforward measurement procedure. By using the Mark-Houwink-Sakurada equation, the viscosity-average degree of polymerisation (DP v) is finally obtained as a function of the intrinsic viscosity. From the kinematic viscosity, the intrinsic viscosity of the solution is determined next. The procedure is based on glass capillary viscometry, which enables to determine the kinematic viscosity of the dilute cellulose solution in terms of the flow time that is needed for the fluid to pass through a capillary tube. ![]() For this purpose, cadmium triethylene diamine dihydroxide, cuprammonium hydroxide or cupriethylenediamine can be used. More often, the DP is evaluated by means of viscosity measurements on a dilute cellulose solution that is prepared by dissolving cellulose samples in a solvent. Size exclusion chromatography provides the molar mass distribution of cellulose, and consequently, the distribution of the DP and the polydispersity index. In the literature, different methodologies have been proposed to assess ageing of paper samples. The extent of paper degradation is typically quantified in terms of the Degree of Polymerisation (DP), defined as the average number of monomeric units in the cellulose molecules, and its accurate determination plays an important role in paper ageing studies. This phenomenon has important consequences for the structural integrity of paper, as a decrease of the molecular weight is directly related to a reduction of the mechanical properties of cellulose, e.g., the tensile strength. Hydrolysis reactions cause cleavage of the cellulose molecular chains through breakage of the glycosidic linkages, thereby resulting in a reduction of the average length of the cellulose chains, and thus in a reduction of the molecular weight. The agreement in measurement results further demonstrates that rheometry is an easy-to-use, accurate and efficient alternative for DP v measurements by glass capillary viscometry.Īcid-catalysed hydrolysis is one of the main factors governing the degradation of paper. At a longer ageing time, both experimental methods reveal a non-linear decrease in time of DP v, and a non-linear increase in time of the number of cellulose chain scissions, which indicate that the cellulose ageing process is realistically captured. The measurement results regarding the age-dependency of DP v and the number of cellulose chain scissions are compared to those obtained by glass capillary viscometry, showing a very good agreement. ![]() The rheometry method has been applied to 4 hygrothermally aged cellulose paper samples and the unaged counterpart. Also, rheometry measurements are less work-intensive than glass capillary viscometry measurements. Further, rheometry typically requires considerably less paper mass per test than glass capillary viscometry, which makes the method attractive for paper degradation studies with limited sample availability. Rheometry allows to measure the viscosity for a range of shear rates, which results in multiple DP v evaluations per sample, and thus in statistically representative data from an individual test. The proposed methodology relies on the application of a shear flow in a thin film of cellulose solution to measure its dynamic viscosity, from which DP v can be subsequently derived in a straightforward fashion. The current study proposes an efficient, alternative method to evaluate DP v of cellulosic paper, which is based on rotational rheometry. DP v can be deduced from the viscosity of dilute paper solutions, as typically measured through glass capillary viscometry. In paper degradation studies, the viscosity-average degree of polymerisation (DP v) is often used as a key indicator of the extent of degradation of cellulosic paper.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |