Rheology is the study of flow and deformation of materials under applied forces which is routinely measured using various methods.
Rheology (Greek: rheos = flow or streaming, logos = word/science) is the study of deformation and flow of substances. Flow is the continuous deformation of a material under the influence of external forces. Various rheological measuring methods can be used to characterize coatings.
The quantities measured in rheological investigations are forces, deflections and velocities. Viscosity is the resistance of a liquid to forced, irreversible change of position of its volume elements.
Energy must be supplied continuously to maintain the flow of a liquid. Viscosity, the most frequently used rheological parameter, is calculated from the shear rate and the shear stress. Basic rheological parameters are explained in terms of the two-plate model (fig.).
The upper plate of area A is movable and the lower plate stationary. The plates are separated by a distance h which is filled with liquid. When a certain force F is applied the upper plate reaches a velocity v which is related to the shear stress.
The two characteristic values, shear stress, T ,and shear rate,
Y , can be derived from the two-plate model. The shear stress has the dimensions of pressure and is defined as the shear force F [N] per shear area A [m²] where the force is parallel to the surface. The unit is the Pascal (Pa) which is the same as N/m2.
The shear rate ( ) or velocity gradient is obtained from the velocity v [m/s] and the plate separation h [m]. The units are second-1.
Although the term viscosity is often used in connection with coatings, the term flow behavior would surely be better. Usually coatings show a flow behavior in which they become thinner, that is to say, the viscosity value drops under the influence of shear forces. This phenomenon, known as pseudoplasticity, will be dealt with in more detail later.
Applicational properties of coatings are associated with different shear rates (fig). When stirring (dispersing), low viscosities are necessary but storage should preferably take place at high viscosity so that the pigments are prevented from settling out. For spray application, the paint should have as low a viscosity as possible but as soon as the paint is on the surface it should assume a high viscosity to prevent sagging on vertical substrates. The flow behavior of paints under typical processing conditions can be ordered according to shear rate.
Viscosity describes the frictional forces in a system and thus the resistance of a liquid to flow. The viscosity is a measure of the viscous flow of a liquid. The larger the viscosity, the less flowable the liquid. The (shear) viscosity (dynamic viscosity) is obtained as the ratio of shear stress ( ) to shear rate ( ) The units are Pa · s-1
If the viscosity of a substance is constant at different shear rates, it is said to exhibit ideal or Newtonian behavior. Newtonian flow is generally found only with low molecular weight liquids such as water, solvents and mineral oils. In practice, most shearable systems have flow properties which depend on the shear rate.
If the viscosity decreases with increasing shear stress, the flow behavior is said to be intrinsically viscous, shear thinning or pseudoplastic. Most coatings and polymer solutions show pseudoplastic behavior.
Bingham fluids are viscoplastic but their flow properties are linear. After a minimum shear stress, they reach the yield point, , and begin to flow. Below this point they behave as an elastic body. Examples are ketchup and certain wall paints. , , and begin to flow. Below this point they behave as an elastic body. Examples are ketchup and certain wall paints.
These liquids only begin to flow when a particular stress is reached. This occurs, for example, if a loaded paint roller is pressed against a wall and rolled. At that moment, the shear force is sufficient for the paint to become liquid and wet the wall. As long as the paint is on the roller without any force applied it behaves like an elastic solid and does not drip from the roller.
- Newtonian behavior
- pseudoplastic (non-Newtonian) behavior
- Bingham behavior with yield point
- Thixotropic behavior
Materials whose viscosity increases with increasing shear stress are thixotropic or dilatant. Thixotropic behavior is shown by, for example, dispersions with high solids content or high polymer concentrations. Thixotropic behavior is not usually found in paints and coatings as it can lead to problems with processes involving pumping or stirring
Flow behavior can be shown diagrammatically in two ways: as a flow curve or a viscosity curve. A flow curve shows the dependence of shear force ( ) on shear rate ( ) (fig.) and a viscosity curve shows how the viscosity depends on the shear rate (fig.).
Rheological additives can be roughly separated into inorganic and organic thickeners or thickeners for solventborne or waterborne coatings.
Examples of modified inorganic thickeners for solventborne and waterborne systems include:
- synthetic lattice-layer silicate
- pyrogenic silica (sometimes organically modified)
Examples of organic thickeners for solventborne or waterborne systems include:
- cellulose derivatives
Examples of organic thickeners for waterborne coatings include:
- associative thickeners
- non-associative thickeners
- This list is not intended to be comprehensive.
Associative thickening involves non-specific interactions of hydrophobic end-groups of a thickener molecule both with themselves and with components of the coating. The thickener produces a reversible, dynamic network of thickener molecules and other components of the coating. The thickening effect is caused by interactions of the hydrophobic end groups of the thickener with other components of the formulation.
Non-associative thickening is thickening by an entanglement of water-soluble, high molecular weight polymer chains. The effectiveness of a thickener is mainly determined by the molecular weight of the polymer. Formulations thickened nonassociatively have pseudoplastic rheology with highly elastic properties. This produces good stabilization against settling out and low sagging even with high build coatings. Non-associatively thickened systems often have limited flowability. The high molecular weight of the thickeners can sometimes lead to compatibility problems such as depletion flocculation.
Associative thickening can produce rheology ranging from Newtonian to pseudoplastic. Associative thickeners are often used to adjust application properties such as spattering or brushing resistance. Formulations with associative thickeners often exhibit high gloss and good leveling.
Associative thickeners also consist of hydrophilic water-soluble or water-emulsifiable polymer components and can simultaneously act in a non-associative manner. Both thickening modes are exhibited by associative thickeners, albeit to different extents. Table shows thickeners commonly used for waterborne coatings and the way in which the thickening is produced.
- HEUR: Hydrophobically modified Ethylenoxide Urethane Rheology modifier
- HASE: Hydrophobically modified Alkali Swellable Emulsion
- HMPE: Hydrophobically Modified PolyEther
- HMHEC: Hydrophobically Modified Hydroxy Ethyl Cellulose
Polyurethane thickeners belong to the associative category. The molecular weight of polyurethane thickeners is from one to several powers of 10 lower than that of non-associative thickeners. Nonassociative thickening can therefore be ignored.
Chemistry of polyurethane thickeners
Polyurethane thickeners are water-soluble or water-emulsifiable polymers with a segmented structure. The middle section consists of one or more hydrophilic segments while the end-groups are hydrophobic. Linking of the middle segments with each other and with the end-roups occurs mainly via free hydroxyl groups of the segments with monoor poly-isocyanates forming the urethane structures which give their name to this class of products. The molecular weights of typical polyurethane thickeners lie between 15,000 and 100,000 g/mol.
Polyurethane chemistry opens up many possibilities for synthesizing thickeners with differing property profiles.
This can be achieved by, for example, varying:
- The type of hydrophobic end group
- The molecular weight of the thickener
- The hydrophilic character of the middle section by the choice of isocyanate and the molecular weight of the polyethylene glycol
- The middle section with hydrophobic dialcohols
- The branching of the thickener molecule by reaction with polyisocyanates or polyalcohols
the branching of the middle section by reaction with alkyl epoxides during manufacture of the polyethylene glycol
Further variations are possible during the formulation of the thickener as a waterborne preparation. Substances high in hydrophobic components require emulsifiers or solvents as formulation aids. The structure of the emulsifier co-determines the applicational properties of the thickener. A suitable choice of emulsifier allows the property profile of the thickener to be adjusted.
Mode of action of polyurethane thickeners
The associative thickening action of polyurethane thickeners can be explained as follows: interactions of the thickener molecules with each other and with the surface of dispersion and pigment particles produce a network (fig.).
Although the simplified model in fig. is helpful in explaining associative thickening clearly, the actual mechanisms of thickening by polyurethane thickeners are more complex.
Investigations on solutions of polyurethane thickeners in water show that the thickener molecules are present in mono-molecular form only in highly diluted solutions. With increasing concentration, the thickener molecules associate with each other to form loop micelles. The block-like structure of the thickener favors this phenomenon: the hydrophobic components of the molecule orient towards the interior of the micelle, the hydrophobic middle segments form the outer loop-shaped sheath.
At even higher concentrations, mutual interaction of the loop micelles results in two phase systems consisting of a freeflowing phase with low thickener content and a thickener gel. In the free flowing phase, the thickener molecules are dissolved individually or are present as loop micelles. The thickener gel consists of associates of thickener molecules. The cohesion of the gels is thought to stem from bridging and entanglement mechanisms by the micelles (figs).
If the concentration of thickener in aqueous solution is raised still further, the two phase system changes into a single phase thickener gel.
In paints, polyurethane thickeners are usually used at concentrations at which two-phase systems have been observed in aqueous solutions. It is therefore probable that the polyurethane thickener is also present in paints as a two-phase system. The thickener gel interacts via the hydrophobic groups with the surfaces of the pigments and binders. A refined model of the mechanism of associative thickening by polyurethane thickeners is shown in fig.
Polyurethane thickeners form a temporary network. The linkage points of the network consist of thickener molecules and thickener micelles adsorbed on the pigment or binder surfaces. It is characteristic that the linkage points of the network are constantly breaking and reforming. The system is thus highly fluid which explains the good flow and leveling characteristics of associatively-thickened paints.
The strength of the network and the resultant rheological properties of the system can be controlled via the hydrophobicity of the end groups. Long hydrophobic end groups exert strong interactions and ensure efficient thickening. To achieve comparable thickening with shorter end groups, significantly higher concentrations are required.
The length of the hydrophobic group influences not only the strength of the associative effect but also the kinetics of exchange and thus the rate at which the associative linkage points break and reform. When shear stress is applied to paints, the associative linkage points are broken. If the thickener is unable to reestablish the disrupted linkage points immediately, the associative network is weakened. There is then a shear-ratedependent loss in viscosity.
Long hydrophobic end groups possess a slow rate of exchange; i.e. the viscosity of thickeners with long hydrophobic end groups diminishes slowly under shear stress. The formulation becomes pseudoplastic. Short hydrophobic end groups exhibit a fast rate of exchange so that thickeners with such groups are effective even at high shear rates and result in formulations with Newtonian rheology.
Measuring viscosity with the relative viscometer
The volume of the sample is unknown; shear rate and shear stress cannot be quantified. Relative viscometers are suitable for comparative measurements of systems with similar rheology. In the coatings industry, relative viscometers of the Brookfield or Stormer spindle type are commonly used, usually at constant shear force.
Leveling, the flow behavior in a horizontal position, is determined with a leveling doctor blade. In this method, the coating is applied as five double lines with increasing film thickness from 100 to 1000 µm on a Leneta sheet. The proportion of lines which have coalesced is quoted according to a scale of 0 (no leveling) to 10 (very good leveling). It is also common practice to visually compare a dried paint film with a standard (fig.). The paint is usually applied in a manner resembling subsequent application conditions.
Measuring viscosity with the absolute viscometer
In absolute viscometers, the geometry of the measuring cell is known and the shear surface and plate separation are therefore also known. The shear stress, shear rate, and thus the viscosity can be calculated from the shear force and speed. The absolute viscometer is usually used to take measurements over a range of shear rates. The results are presented in flow curves or viscosity curves. Typical absolute viscometers are cone and plate or two-plate rotational viscometers (fig).
The Part II will be continued in the next edition