Abstract
Cellulose derivatives containing associating hydrophobic groups along their hydrophilic backbone are used as rheology modifiers in the formulation of water-based spray paints, medicinal sprays, cosmetics and printable inks. Jetting and spraying applications of these materials involve progressive thinning and break-up of a fluid column or sheet into drops. Strong extensional kinematics develop in the thinning fluid neck. In viscous Newtonian fluids, inertial and viscous stresses oppose the surface tension-driven instability. In aqueous solutions of polymers such as Ethyl Hydroxy-Ethyl Cellulose (EHEC), chain elongation provides additional elastic stresses that can delay the capillary-driven pinch-off, influencing the sprayability or jettability of the complex fluid. In this study, we quantify the transient response of thinning filaments of cellulose ether solutions to extensional flows in a Capillary Break-up Extensional Rheometer (CaBER) and in a forced jet undergoing break-up using Rayleigh Ohnesorge Jetting Extensional Rheometry (ROJER). We also characterize the steady state molecular deformations using measurements of the flow-induced birefringence and excess pressure drop in an extensional stagnation point flow using a Cross-Slot Extensional Rheometer (CSER). We show that under the high extension rates encountered in jetting and spraying, the semi-dilute solutions of hydrophobically modified ethyl hydroxy-ethyl cellulose (hmEHEC) exhibit extensional thinning, while the unmodified bare chains of EHEC display an increase in extensional viscosity, up to a plateau value. For both EHEC and hmEHEC dispersions, the low extensibility of the cellulose derivatives limits the Trouton ratio observed at the highest extension rates attained (close to 10(5) s(-1)) to around 10-20. The reduction in extensional viscosity with increasing extension rate for the hydrophobically modified cellulose ether is primarily caused by the disruption of a transient elastic network that is initially formed by intermolecular association of hydrophobic stickers. This extensional thinning behavior, in conjunction with the low extensibility of the hydrophobically modified cellulose ether additives, makes these rheology modifiers ideal for controlling the extensional rheology in formulations that require jetting or spraying, with minimal residual stringiness or stranding.
Highlights
Multicomponent complex uids containing long polymer molecules are found to provide a much larger resistance to extensional ow than expected on the basis of their shear viscosity
We have recently developed a jetting-based rheometry technique, called the Rayleigh Ohnesorge Jetting Extensional Rheometer (ROJER) that provides access to the typically unchartered regime of very short relaxation time, low viscosity complex uids,[70,71] corresponding to the bottom right corner of Fig. 1
We use a hydrophobically modi ed ethyl hydroxyethyl cellulose ether with the degree of substitution (DS) occupied by the ethyl group given by DSEt z 0.8; the molar degree of substitution (MS) by ethylene oxide groups is MSEO z 2.5, and the degree of substitution by the hydrophobic sticker distributed along the chain is MSC14 z 0.008.15 The hydrophobically modified ethyl hydroxy-ethyl cellulose (hmEHEC) was provided in a puri ed form by Asa Soderlund, AkzoNobel Cellulosic Specialties and has a degree of polymerization DP z 800, which corresponds to a molecular weight Mw z 240 000 Daltons
Summary
Multicomponent complex uids containing long polymer molecules are found to provide a much larger resistance to extensional ow than expected on the basis of their shear viscosity. For a Newtonian uid, the extensional viscosity is a factor of three times larger than the shear viscosity (as was rst shown by Trouton1); it can be several orders of magnitude higher for polymeric uids.[1,2,3] A stretching or extensional deformation is established when streamwise velocity gradients are present in a ow. Polysaccharides or carbohydrate polymers[12,13,14] are among the most common constituents of industrial and natural complex uids and contribute to the measurable ow resistance in many extensional deformations, in uencing spraying and coating of paints,[15] inkjet printing, lament spinning,[16] porous media ow,[17] enhanced oil recovery,[18] and turbulent drag reduction as well as in natural systems such as synovial uid, mucus and deadly viscoelastic uids found in carnivorous plants.[19,20]
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
