Viscosity

Detailed Description

Viscosity is a measure of a fluid's resistance to flow or deformation when subjected to stress. It represents the internal friction of a moving fluid and is a key physical property that affects how liquids behave during pouring, pumping, spraying, and other handling operations. In the context of a Safety Data Sheet (SDS), viscosity provides important information for handling, processing, and assessing potential hazards, particularly aspiration hazards for low-viscosity liquids.

The dynamic viscosity (η) of a fluid is defined by the relationship:

τ = η × (dv/dy)

Where:

τ is the shear stress (force per unit area)
η is the dynamic viscosity
dv/dy is the velocity gradient perpendicular to the direction of shear

Key concepts related to viscosity include:

Dynamic viscosity: The ratio of shear stress to shear rate, typically measured in pascal-seconds (Pa·s) or centipoise (cP).
Kinematic viscosity: The ratio of dynamic viscosity to density, typically measured in square meters per second (m²/s) or centistokes (cSt).
Newtonian fluids: Fluids whose viscosity remains constant regardless of shear rate (e.g., water, many oils).
Non-Newtonian fluids: Fluids whose viscosity changes with shear rate (e.g., ketchup, blood, polymer solutions).
Temperature dependence: For most liquids, viscosity decreases with increasing temperature.
Pressure dependence: For most liquids, viscosity increases with increasing pressure, though the effect is often small at moderate pressures.

Importance in Safety Data Sheets

Viscosity information in an SDS is important for several reasons:

Aspiration hazard assessment: Low-viscosity hydrocarbon liquids can pose aspiration hazards if swallowed.
Handling and processing: Affects equipment selection, pumping requirements, and handling procedures.
Spill response: Influences how quickly a substance will spread and what containment methods are appropriate.
Application methods: Determines suitability for spraying, brushing, dipping, or other application techniques.
Storage considerations: May affect sedimentation, separation, or flow characteristics during storage.
Transportation: Relevant for bulk liquid transport and potential hazards during loading/unloading.
Environmental fate: Affects mobility in soil and water systems.
Product identification: Can be a characteristic property for quality control and identification.

Measurement Methods

Several techniques are used to measure viscosity:

Method	Description	Typical Applications
Rotational Viscometers	Measures torque required to rotate a spindle in the fluid at a specified speed	Medium to high viscosity fluids, non-Newtonian fluids
Capillary Viscometers	Measures time for a fluid to flow through a capillary tube under gravity	Low to medium viscosity Newtonian fluids
Falling Ball Viscometer	Measures terminal velocity of a ball falling through the fluid	Transparent Newtonian fluids
Cone and Plate Viscometer	Measures torque required to rotate a cone against a flat plate with fluid in between	Small sample volumes, rheological characterization
Oscillatory Rheometers	Applies oscillatory shear to measure viscoelastic properties	Complex fluids, viscoelastic materials
Bubble Viscometer	Measures time for an air bubble to rise through the fluid	Quick field measurements
Flow Cups	Measures time for a specific volume to flow through an orifice	Paints, coatings, industrial quality control
ASTM Methods	Standardized procedures (e.g., ASTM D445, D2196)	Regulatory testing, specification compliance

Viscosity Units and Conversions

Viscosity can be expressed in various units:

Dynamic Viscosity Units

Unit	Symbol	Equivalent in SI Units	Common Applications
Pascal-second	Pa·s	1 Pa·s = 1 kg/(m·s)	SI unit, scientific applications
Centipoise	cP	1 cP = 0.001 Pa·s	Industrial applications, water = 1 cP at 20°C
Poise	P	1 P = 0.1 Pa·s	Older scientific literature
Pound-force second per square inch	lbf·s/in²	1 lbf·s/in² = 6,894.8 Pa·s	US engineering applications

Kinematic Viscosity Units

Unit	Symbol	Equivalent in SI Units	Common Applications
Square meter per second	m²/s	1 m²/s	SI unit, scientific applications
Centistoke	cSt	1 cSt = 10⁻⁶ m²/s	Industrial applications, petroleum products
Stoke	St	1 St = 10⁻⁴ m²/s	Older scientific literature
Square foot per second	ft²/s	1 ft²/s = 0.0929 m²/s	US engineering applications

Conversion between dynamic and kinematic viscosity:

Kinematic Viscosity (ν) = Dynamic Viscosity (η) / Density (ρ)

Viscosity Categories and Examples

Category	Dynamic Viscosity Range (cP at 20°C)	Classification	Examples
Very Low Viscosity	<10	Very Low	Water (1 cP), methanol (0.6 cP), gasoline (0.5-0.6 cP), acetone (0.3 cP)
Low Viscosity	10-100	Low	Light motor oil (50-100 cP), olive oil (80-100 cP), milk (3-10 cP)
Medium Viscosity	100-1,000	Medium	SAE 30 motor oil (150-200 cP), maple syrup (150-200 cP), liquid honey (2,000-10,000 cP)
High Viscosity	1,000-10,000	High	Ketchup (50,000-100,000 cP), molasses (5,000-10,000 cP), corn syrup (5,000-10,000 cP)
Very High Viscosity	>10,000	Very High	Peanut butter (250,000 cP), window putty (100,000+ cP), glass at room temperature (10²⁰ cP)

Types of Viscous Behavior

Newtonian Fluids

Fluids whose viscosity remains constant regardless of shear rate or shear history. The viscosity is a constant value at a given temperature and pressure.

Examples: Water, most mineral oils, glycerin, simple organic solvents

Non-Newtonian Fluids

Fluids whose viscosity changes with shear rate or shear history. Several types exist:

Type	Behavior	Examples
Shear-Thinning (Pseudoplastic)	Viscosity decreases with increasing shear rate	Paint, ketchup, blood, polymer solutions
Shear-Thickening (Dilatant)	Viscosity increases with increasing shear rate	Cornstarch in water, some ceramic suspensions
Bingham Plastic	Requires minimum stress (yield stress) to flow, then behaves as Newtonian	Toothpaste, mayonnaise, drilling mud
Thixotropic	Viscosity decreases with time at constant shear rate, recovers when shear is removed	Some paints, printing inks, gelatins
Rheopectic	Viscosity increases with time at constant shear rate	Some lubricating greases, gypsum suspensions
Viscoelastic	Exhibits both viscous and elastic properties	Silly Putty, polymer melts, biological tissues

Viscosity and Aspiration Hazard

Low viscosity is a key factor in determining aspiration hazard, which is the potential for a liquid to be drawn into the respiratory system during ingestion or vomiting, potentially causing severe lung damage.

According to GHS criteria, hydrocarbon liquids with a kinematic viscosity ≤20.5 mm²/s (cSt) at 40°C are classified as Category 1 aspiration hazards. This includes many petroleum distillates, such as gasoline, kerosene, and some mineral oils.

The risk of aspiration is particularly significant for:

Low-viscosity hydrocarbons (e.g., petroleum distillates)
Substances containing surfactants that can reduce surface tension
Mixtures of varying viscosity where low-viscosity components may separate

For this reason, accurate viscosity information is critical in Section 9 of the SDS, especially for hydrocarbon-based products.

Factors Affecting Viscosity

Temperature

Temperature has a significant effect on viscosity:

Liquids: Viscosity typically decreases with increasing temperature, often following an Arrhenius-type relationship.
Gases: Viscosity typically increases with increasing temperature.

The temperature dependence of liquid viscosity can often be described by the Arrhenius equation:

η = A × e^(E_a/RT)

Where:

η is the dynamic viscosity
A is a pre-exponential factor
E_a is the activation energy for viscous flow
R is the gas constant
T is the absolute temperature

Pressure

For most liquids, viscosity increases with increasing pressure, though the effect is often small at moderate pressures. The pressure effect becomes more significant at very high pressures, such as those encountered in deep-sea environments or certain industrial processes.

Molecular Structure

The molecular structure significantly affects viscosity:

Molecular weight: Higher molecular weight generally results in higher viscosity.
Molecular shape: Linear molecules typically have higher viscosity than branched molecules of the same molecular weight.
Intermolecular forces: Stronger intermolecular forces (hydrogen bonding, dipole-dipole interactions) generally result in higher viscosity.
Functional groups: The presence of certain functional groups can significantly affect viscosity through their influence on intermolecular interactions.

Composition

For mixtures, viscosity depends on the composition and the interaction between components:

Solutions: Viscosity often varies non-linearly with concentration.
Suspensions: Viscosity increases with solid content and is affected by particle size, shape, and distribution.
Emulsions: Viscosity is influenced by the phase ratio, droplet size, and presence of emulsifiers.
Polymer solutions: Viscosity is highly dependent on polymer concentration, molecular weight, and solvent quality.

Examples of Viscosity Descriptions in SDSs

"Viscosity, kinematic: 4.5 mm²/s at 40°C"
"Dynamic viscosity: 1.002 mPa·s at 20°C"
"Viscosity: 80-120 cP at 25°C (Brookfield, spindle #2, 30 rpm)"
"Kinematic viscosity: <20.5 mm²/s at 40°C (aspiration hazard Category 1)"
"Viscosity: 15,000-20,000 cP at 23°C (non-Newtonian, shear-thinning)"
"Viscosity: Not applicable for solids"
"Dynamic viscosity: 35 mPa·s at 20°C; 15 mPa·s at 40°C"
"Viscosity: 45 seconds (Ford Cup #4 at 25°C)"

Regulatory Requirements

According to GHS and various regional regulations (EU CLP, US OSHA HazCom, etc.), viscosity should be indicated in Section 9 of the Safety Data Sheet as part of the description of basic physical and chemical properties. This information is particularly important for liquids.

For aspiration hazard classification under GHS, kinematic viscosity at 40°C is a critical parameter. Hydrocarbon liquids with kinematic viscosity ≤20.5 mm²/s at 40°C are classified as Category 1 aspiration hazards, requiring specific hazard statements and precautionary statements in Sections 2 and 3 of the SDS.

Best Practices

When reporting viscosity in an SDS:

Specify whether the value is dynamic viscosity (e.g., mPa·s, cP) or kinematic viscosity (e.g., mm²/s, cSt)
Clearly indicate the temperature at which the measurement was made
For aspiration hazard assessment, provide kinematic viscosity at 40°C
For non-Newtonian fluids, specify the measurement conditions (shear rate, measurement system)
For temperature-sensitive materials, consider providing viscosity values at multiple temperatures
Indicate the measurement method or standard used (e.g., ASTM D445, Brookfield viscometer)
For materials with variable viscosity (e.g., thixotropic fluids), provide appropriate ranges or additional information
For substances where viscosity is not applicable (e.g., solids, gases), clearly state this
Ensure consistency between viscosity information and aspiration hazard classification in the SDS
For complex rheological behavior, consider providing additional information in Section 16 of the SDS