AirSprayTech Academy Certificate Program
2K and 3K Coating Systems for OEM Product Finishers
Article 05 of 24
Viscosity, Rheology, and Material Temperature
Material flow behavior influences every stage of a plural-component
process—from storage and pumping to metering, mixing, atomization,
film build, appearance, and cure.
Learning Objectives
After completing this article, the reader should be able to:
- Explain viscosity and rheology in practical production terms.
- Recognize Newtonian, shear-thinning, and thixotropic behavior.
- Describe how temperature changes material flow and reaction speed.
- Understand how viscosity differences affect pumping, metering, mixing, and ratio control.
- Identify appropriate methods for measuring and recording viscosity.
- Recognize why unauthorized solvent or reducer adjustment is not acceptable process control.
Viscosity Is More Than “Thick” or “Thin”
Viscosity describes a liquid’s resistance to flow. A high-viscosity
material resists movement more than a low-viscosity material under
the same test conditions.
In an OEM finishing operation, viscosity affects how easily a
component can be transferred, circulated, filtered, metered, mixed,
delivered through hoses, and atomized. It also affects sag
resistance, leveling, edge coverage, transfer efficiency, and final
appearance.
A viscosity number has meaning only when the measurement method,
material temperature, instrument, spindle or cup, test time, and
applicable units are identified.
Viscosity and Rheology Are Related but Different
Viscosity is a measured resistance to flow under
defined conditions. Rheology is the broader study
of how a material flows and deforms under different forces, times,
temperatures, and shear rates.
Two coatings can produce the same viscosity-cup time yet behave
differently in a pump, static mixer, hose, atomizer, or wet film.
This can occur because they respond differently when subjected to
shear.
A complete process evaluation should consider both the reported
viscosity and the material’s flow behavior under actual production
conditions.
Common Types of Flow Behavior
Newtonian or Near-Newtonian
A Newtonian liquid maintains approximately the same viscosity
as the shear rate changes at a constant temperature. Some
solvents, oils, and relatively simple coating liquids behave
approximately this way. A viscosity cup can be useful for
suitable Newtonian or near-Newtonian materials.
Shear-Thinning
A shear-thinning material becomes easier to move as shear
increases. It may appear thick at rest but flow more readily
through a pump, meter, mixer, nozzle, or rotary atomizer.
Pigmented and high-solids coatings frequently exhibit some
degree of shear-thinning behavior.
Thixotropic
A thixotropic material decreases in apparent viscosity while it
is being sheared and gradually rebuilds structure after the
shear is removed. This behavior can help a coating atomize and
level during application while rebuilding enough body to resist
sagging afterward.
Settling or Structure Change
Pigments and fillers may settle during storage or circulation
interruptions. A sample taken from the top of a container may
not represent the material entering the system. Improper
agitation can also introduce air, heat, moisture, or excessive
shear. Follow the coating manufacturer’s agitation requirements.
Temperature Changes Viscosity
For most liquid coatings, increasing material temperature reduces
viscosity, while decreasing temperature increases viscosity. The
amount of change depends on the specific formulation.
Material that performs correctly during a warm afternoon may be
difficult to pump or atomize after sitting in a cold storage area
overnight. Seasonal changes can alter pressure demand, meter
response, atomization, film build, and appearance unless temperature
is controlled.
A viscosity result without a recorded material temperature is
incomplete. Comparing readings taken at different temperatures can
lead personnel to diagnose a material or equipment problem that does
not exist.
Temperature Also Changes Reaction Speed
Heating a reactive coating can improve flow and reduce the pressure
required to move or atomize it. However, higher temperature can also
accelerate chemical reaction and shorten pot life, usable spray
life, or mixed-material residence time.
Temperature affects more than the material in the supply container.
Pumps, circulation lines, heaters, meters, mix manifolds, hoses,
atomizers, booth air, parts, and cure ovens can all change the
material’s temperature history.
Heating limits and control locations must be based on coating and
equipment documentation. Operators should not increase temperature
simply to make a difficult material flow without determining the
effect on reaction speed, pot life, safety, and film performance.
Component A and Component B May Behave Differently
The resin and hardener sides of a plural-component system frequently
have different viscosities, densities, flow rates, and temperature
responses.
|
Difference
|
Possible Process Effect
|
| Viscosity |
Different pressure losses, pump demands, valve response, and mixing behavior. |
| Density |
Different weight-to-volume conversion and material-settling behavior. |
| Required flow |
The low-flow component may require a smaller meter or more precise dosing valve. |
| Temperature response |
Components may not reach matching flow behavior when heated to the same temperature. |
| Shear sensitivity |
Circulation or pumping may change apparent viscosity differently on each side. |
The goal is not necessarily to make both components have identical
viscosity. The goal is to condition and deliver each component
within the range required for accurate metering and effective mixing.
Effects on Pump and Supply-System Selection
A pump must be able to move the material at the required flow and
pressure without damaging the coating or operating outside its
approved range.
Material properties influence:
- Pump type, displacement, pressure ratio, and cycle rate.
- Suction-line size and maximum lift distance.
- Need for gravity feed, pressure feed, ram assistance, or heated supply.
- Hose diameter, length, and pressure rating.
- Filter size and allowable pressure drop.
- Agitation and circulation requirements.
- Seal, hose, and wetted-material compatibility.
A pump selected only by maximum pressure may not deliver the required
material volume or may cycle too rapidly for dependable service.
Effects on Metering and Ratio Control
A metering device must operate within the flow, viscosity, pressure,
and material-compatibility range established by its manufacturer.
The best measuring technology depends on the coating and application.
Gear flow meters measure material directly and may provide rapid
response with suitable nonabrasive liquids. Other systems use pump
displacement or stroke sensing to determine delivered volume without
placing the measuring element directly in the coating.
Abrasive fillers can wear components. Very low-viscosity materials
can leak through clearances. High-viscosity materials can create
pressure drop or prevent the meter from operating within its
intended range.
Meter selection should be based on actual material properties and
production flow—not simply on the nominal pipe or hose size.
Pressure Does Not Correct Viscosity
Increasing pressure can force a difficult material through a
restriction, but it does not correct an unsuitable viscosity,
improper material temperature, blocked filter, undersized hose, or
incorrect equipment selection.
Excessive pressure can increase pump wear, hose stress, leakage,
overspray, atomization problems, heat generation, and safety risk.
When pressure demand rises, investigate the reason. Compare current
temperature, viscosity, filter condition, line restriction, flow
rate, valve operation, and material condition with the approved
production baseline.
Adding Solvent Is a Formulation Change
Adding reducer may lower viscosity, but it also changes the applied
formulation. It can affect solids content, volatile organic
compound emissions, film build, sag resistance, flash time, dry
time, cure, transfer efficiency, appearance, and regulatory
compliance.
The correct reducer, permissible amount, addition point, and method
of calculating the final mixture must be defined by the coating
manufacturer and controlled process documentation.
Operators should not add solvent until the material “looks right.”
Unauthorized reduction hides the original cause of a process change
and makes production records unreliable.
Measuring Viscosity Correctly
The selected test must be appropriate for the material. Different
instruments measure different aspects of flow and cannot be
substituted without technical justification.
|
Test Method
|
Appropriate Use
|
| Efflux viscosity cup |
Measures the time required for a suitable liquid to flow through a defined opening. |
| Rotational viscometer |
Evaluates apparent viscosity and can characterize shear-thinning or thixotropic materials. |
| Krebs or Stormer viscometer |
Measures coating consistency in Krebs Units where that method is specified. |
| In-line measurement |
Provides continuous or frequent process information when properly correlated and maintained. |
Viscosity-Cup Testing
When an approved viscosity cup is used:
- Use the exact cup type and orifice specified.
- Confirm that the cup is clean and undamaged.
- Prepare and agitate the material according to instructions.
- Obtain a representative sample without introducing bubbles.
- Measure and record the material temperature.
- Fill and operate the cup using the specified procedure.
- Use the defined endpoint for stopping the timer.
- Record the cup, seconds, temperature, material, batch, and time.
A result stated only as “22 seconds” is incomplete. It should identify
the cup and temperature, such as “22 seconds using the specified Ford
cup at the recorded material temperature.”
Establishing a Production Baseline
A useful process baseline records the conditions present when the
system produces acceptable work. This gives operators and
maintenance personnel a meaningful comparison when performance
changes.
The baseline may include:
- Component viscosity and test method.
- Material temperature at defined locations.
- Supply, circulation, and applicator pressures.
- Pump cycle rate and fluid flow.
- Filter type and pressure drop.
- Meter readings and ratio performance.
- Atomization settings and spray pattern.
- Wet-film thickness, appearance, and transfer efficiency.
Trends are often more useful than isolated readings. A gradual rise
in pressure or change in viscosity can reveal a developing problem
before production quality is lost.
Practical Troubleshooting Guide
|
Observation
|
Conditions to Investigate
|
| Pressure requirement increased |
Lower material temperature, blocked filter, restriction, solvent loss, settling, or changed material. |
| Poor atomization |
Viscosity, temperature, fluid pressure, atomizing air, tip or nozzle condition, and flow rate. |
| Ratio alarms at low flow |
Meter range, dosing-valve response, pressure balance, viscosity difference, and leakage. |
| Color or appearance varies by shift |
Material temperature, reduction practice, agitation, circulation time, and environmental changes. |
| Mixed material reacts too quickly |
Excess temperature, incorrect ratio, incorrect catalyst, contaminated material, or excessive mixed volume. |
Key Takeaways
- Viscosity values are meaningful only when the test conditions are identified.
- Rheology describes how material behavior changes with shear, time, and temperature.
- Increasing temperature usually lowers viscosity but can accelerate chemical reaction.
- Resin and hardener components may require different supply and conditioning strategies.
- Meters and pumps must be selected for the actual material properties and production flow range.
- Increasing pressure does not correct an unsuitable material condition.
- Adding reducer is a controlled formulation change—not a casual operator adjustment.
- Production records should include both viscosity and material temperature.
Knowledge Check
-
Why must material temperature be recorded with a viscosity reading?
-
What happens to a shear-thinning coating as shear increases?
-
Why can heating a reactive coating shorten its usable working time?
-
Does increasing fluid pressure correct an improperly conditioned coating?
-
When should ASTM D2196 be considered instead of a simple efflux-cup test?
Answer Guide
1. Viscosity normally changes with temperature, so readings taken at different temperatures cannot be compared reliably.
2. Its apparent viscosity decreases and it flows more readily.
3. Higher temperature can accelerate the chemical reaction between components.
4. No. Pressure may force the material through the system but does not correct its condition.
5. When the material is non-Newtonian, shear-thinning, or thixotropic and requires rotational-viscometer characterization.
Technical References and Industry Resources
Manufacturer products are referenced as technical examples and not
as endorsements. Use the current coating technical data sheet,
safety data sheet, equipment manual, approved process specification,
and facility quality procedures.
Professional responsibility: Do not change material
temperature, reducer content, agitation, circulation, pressure,
filtration, or viscosity limits without documented technical
authorization. When measured conditions fall outside the approved
process range, stop and determine the cause before continuing
production.
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