A-Side and B-Side Spray Foam: How the Chemicals Work Together

The two liquid components stay separated from the drums all the way to the spray gun. Transfer pumps feed them into a proportioner, the proportioner heats and meters them, and a dual heated hose carries them through separate internal passages. They meet in the gun’s mixing chamber immediately before application.

What is in each side

A-side: the isocyanate

The A-side of most professional spray polyurethane foam systems contains methylene diphenyl diisocyanate (MDI) and polymeric MDI. Installers usually just call it ISO. The isocyanate groups react with the polyols in the B-side to build the polyurethane polymer, and the A-side has its own drum, its own pump and its own passage through the hose.

Uncured isocyanates require professional handling, appropriate protective equipment, work-area controls and ventilation.

B-side: the formulated polyol resin

Contractors call the B-side “resin”, though it is a blend of several ingredients. It is a manufacturer-specific formulation built to produce a particular product, and it typically contains:

• Polyols — react with the A-side to form the polymer.
• Catalysts — control the reaction rate.
• Blowing agent — creates the expansion and the cellular structure.
• Surfactants — stabilize the forming cells.
• Additives — support product-specific processing and performance.
• Flame-retardant ingredients — contribute to tested fire performance.

Exact B-side ingredients vary between manufacturers and products, and flame-retardant ingredients slow ignition while the foam itself remains combustible.

From the drums to the finished foam

1. Separate drums. The liquids are stored apart and stay apart in the drum.
2. Transfer pumps. One dedicated pump per component moves it toward the proportioner.
3. Proportioner. Meters, heats and pressurizes both components to the manufacturer’s processing requirements.
4. Dual heated hose. Both components run through one hose assembly in separate internal passages.
5. Mixing chamber. The liquids meet and mix rapidly inside the gun. This is where the reaction begins.
6. Expansion and cure. The reacting material exits, expands against the prepared substrate, forms its cells and begins curing.

A spray rig makes the foam at the gun. It processes two liquids that become polyurethane during application, which is why the equipment settings matter as much as the product on the truck.

What happens when they meet

The isocyanate and the polyols react to form polyurethane. The reaction is exothermic — it releases heat — and at the same time the blowing system creates gas that expands the material into millions of small cells.

Expansion, surface cure and full property development are three related but distinct stages. Foam can look solid soon after application while the product is still developing its final properties. Cure time, ventilation and re-entry guidance come from the specific product manufacturer and the installing contractor.

Why ratio, temperature and pass thickness decide the result

The right drums are the starting point; correct installation is what delivers the product. Six things have to be inside the manufacturer’s approved range:

• Proportioning. The equipment has to deliver the components at the ratio specified for that system. Off-ratio material reacts and cures unpredictably.
• Material temperature. Affects viscosity, mixing and spray pattern; the requirement varies by product and by day.
• Pressure balance. Both components have to reach the mixing chamber at the pressures the system expects.
• Substrate condition. Clean, compatible, and within the manufacturer’s moisture and temperature range.
• Pass thickness. Within the approved range. Too thick in one pass traps heat and affects quality.
• Ambient conditions. Houston heat and humidity change material handling and substrate condition, sometimes on the same day.

Open-cell and closed-cell are different chemistries

Both use the two-component A-side/B-side concept, but they are different products. The formulations are designed to produce different densities, expansion rates, cellular structures and performance characteristics. Turning one into the other means a different set of drums.

Open-cell systems are formulated for a softer, lower-density structure that expands further. Closed-cell systems are formulated for a denser, more rigid structure, which is why they are specified for metal buildings, rim joists and anywhere water contact is possible.

What properly installed foam should show

• Consistent texture for that specific product.
• Proper adhesion to a prepared, compatible substrate, with the bond intact.
• Specified thickness, checked against the scope with a depth gauge.
• Expected expansion for the product, the settings and the conditions on the day.
• A firm, dry surface once cured. Lasting softness, tackiness or an abnormal odor justifies further evaluation by the installer or the manufacturer.
• Installation records. You are entitled to know which product went into your building, and to have the documentation for it.

Uncured chemicals and cured foam are different things

The raw A-side and B-side are reactive chemicals that need trained handling, protective equipment, work-area isolation and ventilation. Once properly reacted and cured, the material behaves differently from the liquids it came from.

That is why re-entry time is given per product and per job. Follow the requirements of the product manufacturer and the contractor responsible for the job. Safety Data Sheets explain the chemical hazards, Technical Data Sheets describe the product properties, and the application instructions explain how that system should be processed.

Ask your contractor for the product name, the manufacturer, the Safety Data Sheets, the Technical Data Sheets, the application instructions and written re-entry guidance for your installation. We will give you all of it on the first ask.

Across Greater Houston

The reaction is the same in every city; the building, the substrate and the conditions are what change. That is the part a local contractor is actually being paid for: correct product selection, surface preparation and installation control. See how spray foam works for the plain-English version, or the full FAQ.

Written for Phantom Foam by Cole Moore, owner and founder. Technical review by Josh Evans. Information should be confirmed against the documentation for the specific product used on your project.