Portable Plural-Component Coating Systems for Contractors

Article 02 of 24

Component A, Component B, Mix Ratios, and Stoichiometry

The proportioner does not decide what ratio a coating needs. The coating formulator establishes the required relationship between the components, and the contractor must select, configure, verify, and operate equipment capable of maintaining that relationship.

The Central Principle

A plural-component coating performs as intended only when the correct components are delivered in the specified proportion and mixed thoroughly under the conditions established by the coating manufacturer.

Learning Objectives

After completing this article, you should be able to:

• Explain why A and B are identifiers rather than universal chemical descriptions
• Interpret common ratios such as 1:1, 2:1, 3:1, and 4:1
• Distinguish a ratio by volume from a ratio by weight
• Explain the practical meaning of stoichiometry
• Describe how density affects conversion between volume and weight
• Explain why pressure readings cannot replace a proper ratio check

Component A and Component B

Plural-component materials are commonly identified as component A and component B. These letters help distinguish the separate materials, but they do not tell the contractor what each material contains.

One manufacturer may identify the resin as component A and the hardener as component B. Another manufacturer may use the opposite arrangement. Some polyurethane and polyurea manufacturers assign the letter A to the isocyanate side, while terminology used by another supplier may differ.

The contractor must confirm component identity from the container label, product data sheet, safety data sheet, equipment setup instructions, and approved project documents. Hose color, pump position, or previous setup is not sufficient proof.

Never Assume That A Means Resin or B Means Hardener

Connecting a component to the wrong side can contaminate the complete fluid system, damage seals or equipment, create a hazardous reaction, and place uncured material on the project. Confirm every container before opening it or connecting a feed line.

Understanding a Mix Ratio

A mix ratio describes the required quantity of one component compared with another. A ratio written as 2:1 means two parts of the first listed component for every one part of the second listed component.

The ratio does not identify the unit of measurement. The product instructions must state whether the ratio is based on volume, weight, pump displacement, or another controlled measurement.

Total-Parts Calculation

To calculate the share represented by each component, add both sides of the ratio.

2 parts A + 1 part B = 3 total parts

A = 2 ÷ 3 of the mixed quantity

B = 1 ÷ 3 of the mixed quantity

For three gallons of mixed material at a 2:1 ratio by volume, the theoretical quantities are two gallons of A and one gallon of B. This example explains the ratio; it does not authorize hand mixing or field conversion when the manufacturer requires mechanical proportioning.

Ratio by Volume Versus Ratio by Weight

A ratio by volume compares occupied space. A ratio by weight compares mass. The two ratios are the same only when the components have the same density.

If component A weighs 10 pounds per gallon and component B weighs 8 pounds per gallon, equal volumes do not have equal weights. One gallon of A and one gallon of B would be a 1:1 ratio by volume but a 10:8 ratio by weight.

Contractors must not convert a published volume ratio into a weight ratio without accurate component-density information and an approved calculation. Use the ratio and measurement method stated by the coating manufacturer.

Practical Example: Why Density Matters

Assume a coating requires a 2:1 ratio by volume:

• Component A density: 10 pounds per gallon
• Component B density: 8 pounds per gallon
• Required quantity: 2 gallons A to 1 gallon B

Component A weight: 2 gallons × 10 pounds per gallon = 20 pounds

Component B weight: 1 gallon × 8 pounds per gallon = 8 pounds

The specified 2:1 volume ratio is therefore equivalent to 20:8 by weight, which simplifies to 2.5:1 by weight. Treating it as 2:1 by weight would produce the wrong mixture.

What Stoichiometry Means in the Field

Stoichiometry describes the quantitative relationship between substances participating in a chemical reaction. In practical contractor language, it means supplying the reactive groups in the relationship intended by the coating formulator.

When the components are delivered at the specified ratio, the formulation has the opportunity to develop its intended cure and performance. Too much or too little of one component can leave unreacted material in the film and change its physical or chemical properties.

The contractor does not calculate the coating's chemical formulation. The contractor's responsibility is to preserve the manufacturer's specified ratio throughout storage, conditioning, proportioning, mixing, application, and cure.

The Published Ratio Is an Operating Requirement

The mix ratio is not a suggestion that can be adjusted to make a coating spray more easily, cure more quickly, or remain workable longer. Adding extra hardener does not reliably make a plural-component coating “better” or produce a properly controlled faster cure.

If viscosity, pot life, cure speed, or application behavior is unsuitable for the project, stop and consult the coating manufacturer. Do not change the specified ratio unless the manufacturer provides written instructions authorizing the change.

Fixed-Ratio and Variable-Ratio Equipment

Fixed-Ratio Systems

A fixed-ratio system uses a specific relationship between pump displacements or metering components. Its mechanical arrangement is selected to deliver one established ratio. Changing the coating ratio may require different pump lowers, metering parts, or a different equipment configuration.

Variable-Ratio or Electronically Controlled Systems

A variable-ratio or electronically controlled system may measure pump movement or material flow and regulate one or more components through its control system. These systems can offer flexibility, monitoring, alarms, data recording, or automatic interruption when operating limits are exceeded.

Neither design eliminates the need for correct setup, calibration, maintenance, and ratio verification. Electronic monitoring does not correct an empty container, blocked inlet, incompatible material, worn pump, leaking valve, or improperly configured machine.

Why Pressure Does Not Prove Ratio

Pressure indicates resistance to flow. It does not directly measure the quantity of material delivered. Two components can display similar pressures while flowing at different rates, and correctly proportioned components can operate at different pressures.

Pressure can be affected by:

• Material viscosity and temperature
• Pump displacement and mechanical condition
• Hose diameter, length, and internal condition
• Filters, valves, heaters, manifolds, and restrictions
• Spray-tip or mixing-chamber configuration

Gauges are important diagnostic tools, but a proper ratio check must use the approved equipment procedure.

Ratio Verification

A ratio check confirms whether the proportioner is delivering the required relationship between components. Depending on the equipment, verification may involve collecting the components separately, measuring pump output, weighing samples using verified density information, reading calibrated flow data, or using a manufacturer-designed ratio-check assembly.

A proper procedure should identify:

• Where the samples are collected
• Required material temperature and operating conditions
• Whether the result is evaluated by volume, weight, strokes, or measured flow
• Required sample size and test duration
• Permitted ratio tolerance
• Required documentation and corrective action

Never improvise a ratio test on pressurized equipment. Follow the exact procedure in the current equipment manual and coating manufacturer's written requirements.

Ratio Verification Is Not the Same as Mixing Verification

A proportioner may deliver the correct quantities while a damaged, missing, plugged, shortened, or unsuitable static mixer produces incomplete mixing. Ratio control and mixing quality must both be correct.

Ratio Tolerance

No field measurement is infinitely precise. The coating manufacturer and equipment manufacturer may establish an acceptable ratio tolerance. That tolerance is product- and system-specific.

A contractor should never create an unofficial tolerance or assume that a percentage accepted for one product applies to another. When the specification, product instructions, and equipment documentation do not agree, obtain written clarification before application.

What Can Cause an Off-Ratio Condition?

Possible Signs of Off-Ratio Material

• Unexpected color or color variation
• Material that remains soft, sticky, oily, or brittle
• Unusual gloss, texture, foaming, or surface appearance
• Cure that is faster or slower than expected
• Pressure imbalance or an equipment ratio alarm
• Unexpectedly high or low use of one component
• Failure to achieve specified hardness, adhesion, or chemical resistance

Visible appearance alone cannot establish that a coating is correctly proportioned. Some off-ratio material may initially appear acceptable.

Required Job Records

Ratio-control documentation should include, as applicable:

• Product name, component identification, batch numbers, and expiration dates
• Specified ratio and whether it is by volume or weight
• Component temperatures and conditioning information
• Equipment identification and configured ratio
• Calibration and ratio-check results
• Date, time, location, and person performing the check
• Alarm events, interruptions, corrective actions, and retesting
• Approximate quantities of A and B used during the work period

Key Takeaways

• A and B are component identifiers, not universal chemical descriptions
• A ratio must identify both the numerical relationship and the measurement basis
• Ratios by volume and weight are not interchangeable unless density is considered
• Stoichiometry is the chemical relationship established by the formulator
• Pressure readings do not independently verify material ratio
• Ratio verification and mixing verification are separate responsibilities
• The approved ratio must be verified and documented—not assumed

Technical References

• WAGNER TwinControl Electronically Controlled 2K Systems Operating Manual: Equipment operation, calibration, mixing-ratio conversion, alarms, flushing, maintenance, and troubleshooting. View the official manual
• Graco Ratio Check Kit Instructions 3A0421: Manufacturer-specific instructions illustrating ratio verification at a plural-component mix manifold. View the official instructions
• ASTM D1475—Standard Test Method for Density of Liquid Coatings, Inks, and Related Products: A recognized method for determining the density of liquid coating materials and components. View the ASTM standard page
• OSHA 29 CFR 1910.1200—Hazard Communication: Requirements addressing workplace chemical labels, safety data sheets, written hazard-communication programs, and employee information and training. View the OSHA standard

These references are provided for education and further study. Always use the current approved coating data, project specification, equipment manual, and test procedure applicable to the exact materials and equipment being used.

Professional responsibility: Never change a mix ratio, substitute a component, convert between volume and weight, or improvise a ratio-check procedure without written technical authorization. Stop application whenever correct component identity, material supply, proportioning, or mixing cannot be verified.

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