Rheology: Flow, Viscosity, and Deformation

Rheology Basics

• Rheology studies the flow and deformation of matter, especially in liquid form, but also in "soft solids" or solids responding with plastic flow under applied force.
• Viscosity is how a material manifests its resistance to flow, as viscous liquids (water, oil, solvents), viscoelastic liquids (milk, shampoo, paint), and elastic rigid solids.
• Rheology is important because the rheological response relates to texture and stability.
• A decreased viscosity is required for paint at application compared to when at rest.

Hooke's Law and Stress

• Rheology involves applying shear stress horizontally.
• Shear stress measures the force you apply, where height matters.
• Deformation is measured by shear stress.
• Stress is proportional to strain, connecting shear stress and shear strain.
• Hooke's Law: F = -kx Spring constant k describes spring stress.

Newtonian vs. Non-Newtonian Fluids

• Newtonian fluids have constant viscosity regardless of shear rate.
• Water exemplifies a Newtonian fluid.

Polymer Viscosity

• As polymer concentration increases, viscosity rises because polymers are elastic and recoverable; they can change conformation when stressed, leading to strain.
• Polymers drag each other along, giving a solid response.
• Viscosity control is linked to polymers.

Visco-elasticity of Polymers

• The Maxwell model indicates that polymers behave with both stress response and elastic components.
• Polymers are visco-elastic
• Visco-elasticity varies according to condition, which changes the samples behavior.
• Models viscoelasticity with springs (elasticity) and dashpots (viscosity).

Fluid Types

• Bingham plastics need a force to initiate pouring.
• Shear thinning is pseudoplastic.
• Shear thickening is dilatant.
• viscosity drops with shear thinning.
• Newtonian fluids have constant viscosity.

Rheometers

• Rheometers measure how shear stress relates to shear rate by measuring its viscosity.
• Common rheometer types include concentric cylinders, cone and plate, and parallel plates.
• A Peltier element maintains temperature.
• Apply slow shear rate to the sample, from that measurement determine viscosity and then shear rate is increased measure viscosity.

Viscosity Defined

• In simple terms, viscosity is how much a volume of liquids resists the flow
• Low viscosity sample does NOT resist flow much
• High viscosity sample DOES resist flow a lot

Shear Sweeps

• Shear sweeps involves 2 different hand creams.
• Samples can be measured based on storage, delivery, and end-use properties.

Measuring Properties

• Complex Modulus (G*) measures a material's stiffness, derived from how much a sample moves under force (measured in pascals).
• Modulus indicates resistance to movement.
• Tighter samples mean greater resistance to move, while more flexible ones mean less resistance
• Phase angle, measured in degrees, indicates sample properties: zero degrees means maximum and minimum shift.

Elastic and Viscous Behavior

• Purely elastic materials (solid-like) exhibit stress and strain in phase, with a zero phase angle.
• Purely viscous materials (liquid-like) display stress and strain 1/4 cycle out of phase, with a 90° phase angle.

Rheological Properties

• Elastic solids: Force (stress) is proportional to strain.
• Viscous fluids: Force (stress) is proportional to strain rate.
• Viscoelastic materials are in-between with most behaviors as they are not completely solid or liquid.

Rheology of Gels

• A 45° sample is both solid and liquid.
• Elastic solids have stress and strain in phase.
• Viscous fluids have stress and strain out of phase.
• In linear elasticity stress is proportional to strain

Advanced Modulus Concepts

• Rheology uses complex modulus and phase angle.
• Storage (elastic) modulus is G'.
• Loss (viscous) modulus is G".
• If G' > G", the phase angle is below 45° (solid-like).
• If G" > G', the phase angle is above 45° (liquid-like).
• G modulus still indicates toughness.

Fluid Behaviors

• Summary of General material behaviours as solids, liquids and gels
• Rheology is complex.
• Viscosity measures resistance to flow influenced by the polymer, particle, or polymer molecular weight.
• Materials have elastic and viscous components and can be measured by shear sweep and oscillation.
• Oscillation (G', G", phase angle) helps define flow properties as solid, liquid, or gel.

Surface Coatings

• Solvents in coatings can be solvent-borne or water-borne.
• A varnish is a clear coat drying exclusively by oxidation.
• Stain colours a substrate by penetrating it.
• Primer provides the first coat.
• Undercoat is any coat between primer and finish.
• Sealer is a liquid used to reduce the absorptive capacity of substrates before painting.
• Base coat is the first decorative coat before overcoating.

Surface Coating System Functions

• Surface coating serves these functions include decoration, protection, hygiene, communication, and illumination.
• The term ‘surface coating system’ encompasses paint and other materials.
• Coatings include aesthetic, protective, and hygienic properties, with some providing information or reflectance.

Paint Composition

• Paints have pigments to add color and hide the substrate being painted
• Pigments are insoluble while Inks are more viscous.
• "Raw Materials" are the components of paint are
• Film forming resins are used to make paints.
• Paint's properties depends on raw materials and how they combine.

Binders

• All binders are polymers either as solutions, emulsions, or dispersions
• Properties like adhesion, gloss, flexibility are determined by the binder
• Low Volatile Organic Compound (VOC) paint options are available

Paint Solvents

• Solvents can be water-based or solvent-based.
• Water-based paints are easy to clean, have low odors, dry quickly, retain color well, and are eco-friendly.
• Solvent-based paints provide excellent flow, high gloss, and are durable, but pose health and environmental concerns.

Solvents and VOCs

• VOC are the solvents in paints that help in viscosity and drying but emit through industrial sources.
• VOC is mainly used in solventborne paints with smaller amounts in waterborne.

Binders Defined

• Binders are resins or polymers; resins have lower molecular weights.
• Binders can be applied by step growth to form alkyds or chain growth for acrylic and vinyl copolymers.

Film Formation

• Cross-linking or curing is a process to increase the molecular weight which causes further chemical changes and is a stage of film creation
• As solvents evaporate, solutions become tacky after which they will absorb oxygen and 'cross link'
• The solvent evaporates and the coating coalesces to form a continuous film.

Solution Viscosity

• Binder and pigment particles are suspended or dispersed in liquid, similar to milk. The first step involves evaporation, then the water evaporates, the pigment and binder particles come closer and finally fuse to form a film. There is no cross-linking part to the drying of water based paints.

Binder Properties

• Key binder properties include hydrophilicity, glass transition temperature, particle size distribution, stabilization, and reactivity.
• Key variables affecting viscosity are the polymer solution and packing

Pigments and Extenders

• Pigments may not be soluble but is a substance that is used as a colorant.
• Dyes are soluble colorants that are known as “lake” when precipitated as a inorganic based
• Extenders can cheapen paint or add useful functions
• Colorants have optical properties.

Pigment

• White paint uses titanium oxide which has a very high refractive quality
• Common coatings on TiO2 are alumina, silica and zirconia.
• The differences in refractive index result in opacity

Pigment Properties

• Pigment size ranges from 0.2-0.3µm for optimal light scattering.
• Extenders: are carbonates, silicates, sulphates, or oxides.
• Extenders’ effect on viscosity is that kaolin clay has surface hydroxyl groups that creates Hydrogen bonds which create high viscosity
• Hydrogen bonding occurs between surface hydroxyls on kaolin and water molecules which is how it imparts thixotropy

Paint Formulation and Viscosity

• Lower viscosity for paints is achieved by the use of thickeners.
• Thickeners increase the overall viscosity and there are two types.

Thickeners Types

• Associative thickeners use surface active groups to interact with the surfaces of latex or extender/pigment particles in the paint
• Non-associative thickeners work primarily on space-filling

Surfactants and Dispersants

• Surfactant a contraction of “Surface Active Agents” and applies to any substance which interacts with surfaces
• Dispersant generally refers to an ionic surfactant while "Surfactant” generally refers to a non-ionic surfactant
• Electrostatic repulsion occurs via charged species while stability achieved through sterics.
• Paint are alkaline because Binder is usually acidic

Anti-foams and Biocides

• Anti-foams are composed of mineral oils and surface active materials.
• They reduce air bubbles, which can cause defects in films.
• They contain low HLBs and low water solubility to compromise surface tensions until it collapses Biocides are added to prevent bacterial and fungal growth in paint products

Solids Content

• Need to know the solids content of each raw material in order to work out the overall solids of the formulation.
• In paint formulating, it is often important to know the volume of the formulation which is solids rather than the mass.

Calculating

• Calculate: volume of non-volatiles/ volatiles.
• Density Of the paint is equal to the volume of raw material.
• PVC = 100 x volume of pigment and extenders/ volume of pigment, extenders and binder
• VOCs are solvents that help determine the viscosity, flow and drying time of paints.