High-Viscosity Fluid Transfer: Pump Selection for Adhesives & Sealants

Why Gear Pumps Dominate Adhesive & Sealant Transfer

Adhesives and sealants present one of the most demanding viscosity challenges in fluid transfer. Hot-melt adhesives are semi-solid at room temperature. Structural epoxy resins are thick, slow-flowing liquids. Construction sealants can exceed the viscosity of peanut butter. Traditional centrifugal pumps simply cannot move these fluids—they cavitate and fail.

Rotary gear pumps have become the industry standard for adhesive and sealant transfer precisely because they deliver consistent, reliable flow across the entire viscosity spectrum. Whether you're metering thin liquid adhesive at 100 cP or transferring thick polyurethane at 10,000 cP, a properly selected gear pump will perform reliably for years.

This guide walks engineers through viscosity fundamentals, why gear pumps excel in high-viscosity service, temperature management strategies, and practical sizing calculations for your specific adhesive application.

Understanding Viscosity: Fundamentals & Measurement

Viscosity is a fluid's resistance to flow—higher viscosity means thicker, slower-flowing fluid. In the adhesive and sealant industry, viscosity is commonly expressed in centipoises (cP) or centistokes (cSt), both measuring flow resistance at standard temperature (usually 40°C or 100°C).

Viscosity Units & Conversions

Centipoises (cP): The most common unit for adhesives and sealants. One centipoise equals one-hundredth of a poise (dyne·s/cm²). Water at 20°C = 1 cP (reference). Honey at room temperature = 2000–10,000 cP. Peanut butter = 150,000–250,000 cP.

Centistokes (cSt): Kinematic viscosity, accounting for fluid density. Conversion: cSt = cP ÷ density (g/mL). For most organic adhesives and sealants, cP and cSt are numerically similar, but always verify the reference temperature and density when comparing viscosity specifications.

Viscosity Index & Temperature Dependence

Viscosity decreases as temperature increases—a critical factor in adhesive pumping. A fluid with high viscosity index (VI) shows less change across temperature; low VI shows dramatic viscosity change. Most adhesives have VI of 50–150 (significant temperature sensitivity).

Example viscosity-temperature behavior: A typical epoxy resin at 20°C = 3000 cP; at 40°C = 1500 cP; at 60°C = 500 cP. A 40-degree temperature increase drops viscosity by a factor of 6. This is why temperature management is critical in adhesive transfer systems.

Non-Newtonian Fluids & Shear-Thinning

Many adhesives and sealants are non-Newtonian—viscosity changes with shear rate (pumping speed). Shear-thinning adhesives become thinner when pumped fast; shear-thickening (rare) become thicker. Most adhesives are shear-thinning, which means high-speed pumping reduces effective viscosity and improves flow. However, excessive shear can degrade adhesive chemistry, causing separation or foam formation. Moderate gear pump speeds (100–280 RPM) avoid excessive shear while maintaining practical flow rates.

Why Rotary Gear Pumps Excel with Viscous Adhesives

Centrifugal pumps generate flow by accelerating fluid radially using a spinning impeller. At high viscosity, the fluid resists acceleration—momentum transfer is minimal. The impeller spins but the fluid lags, creating cavitation voids inside the pump. Cavitation destroys impeller blades and produces zero useful flow. Centrifugal pumps simply cannot work with viscous adhesives.

Positive Displacement Design: The Gear Pump Advantage

Rotary gear pumps use mechanical gear action to physically move fluid forward. The drive gear meshes with an idler gear, and rotation forces fluid from inlet to outlet by reducing the chamber volume on the discharge side. Viscosity affects motor load (thicker fluid requires more torque) but does not affect flow generation—the pump delivers the same GPM whether fluid is thin or thick.

• Flow Rate Independence from Viscosity: GPM depends only on pump displacement (cc/rev) and RPM. Motor load increases with viscosity, but flow rate remains constant. A PA300 delivers 158 GPM at 280 RPM in both thin and thick adhesive.
• Self-Priming Capability: Gear pumps can prime themselves—pull fluid into the inlet without external priming. Thick, slow-flowing adhesives are difficult to prime in centrifugal systems but gear pumps handle this easily.
• Reliable at Low Flow Rates: Adhesive dispensing often requires precise, modest flow (5–20 GPM). Centrifugal pumps lose efficiency at low flow; gear pumps deliver accurate metered flow at any displacement and speed.
• Minimal Shear Degradation: Laminar flow pattern in gear pumps minimizes fluid shear, protecting shear-sensitive adhesives from separation or foam formation.

Common Adhesive & Sealant Applications

Hot-Melt Adhesives (HMA)

Hot-melt adhesives are thermoplastic polymers that melt at 150–190°C, allowing application as a liquid. At room temperature, they're solid (equivalent viscosity > 100,000 cP). When heated to operating temperature (180–200°C), viscosity drops to 500–3000 cP—pumpable range for gear pumps.

Typical applications: Packaging (box sealing, envelope closure), book binding, woodworking edge banding, appliance assembly, automotive interior assembly.

System requirements: Heated hose and manifold system (180–200°C). Insulated flexible hose prevents heat loss and cooling. Heated nozzles or spray guns maintain adhesive temperature at point of application.

Structural Epoxy Resins

Two-part epoxy resins used in aerospace, automotive, and industrial assembly are typically 1000–5000 cP (neat) or 5000–20,000 cP when thickened with fillers. Transfer requires metering accuracy to maintain proper resin-to-hardener ratio (often 2:1 or 4:1 by volume).

Typical applications: Aerospace structural bonding, automotive adhesive assembly, composite fabrication, electronics potting, industrial machinery bonding.

System requirements: Separate pump for resin and hardener, or dual-pump equipment. Static or dynamic mixers combine components before dispensing. Some epoxies benefit from gentle heating (40–60°C) to reduce viscosity without chemical degradation.

Polyurethane Adhesives & Sealants

Polyurethane (PU) adhesives range from liquid (400–1000 cP) to thick sealant pastes (10,000+ cP). Single-component PU (1K) is shelf-stable; two-component PU (2K) requires mixing. PU chemistry is sensitive to moisture, heat, and contamination.

Typical applications: Construction sealant (flooring, wall panels, roofing), automotive sealant, flexible bonding (dissimilar materials), wind turbine blade bonding.

System requirements: Moisture-resistant manifolds (PU reacts with water). Dry nitrogen purge recommended between operations. Ambient temperature operation (no heating needed). Careful inlet filtration—water contamination ruins adhesive.

Silicone Sealants

Silicone sealants for construction and industrial use are typically 5000–30,000 cP (paste consistency). Unlike epoxy or PU, silicones don't degrade with water and are ideal for damp environments. Cure rate is slow (24–48 hours to tack-free).

Typical applications: Building sealant (glass, metal, masonry joints), bathroom and kitchen sealing, automotive glazing, industrial weather sealing.

System requirements: Standard stainless steel manifolds (silicones don't attack stainless). Cartridge-based dispensing common (pump pushes cartridge plunger). Ambient temperature operation. Self-priming important due to extreme viscosity.

Water-Based Adhesives

Water-based adhesives (PVA, acrylic latex, protein-based) are typically 200–1000 cP—much thinner than organic adhesives. Used in woodworking, packaging, and nonstructural assembly. Viscosity is sensitive to temperature and humidity variation.

Typical applications: Woodworking assembly (furniture, cabinetry), paper lamination, cardboard box sealing, non-structural bonding.

System requirements: Standard stainless or ductile iron pumps suitable. Cooler operation than organic adhesives (room temperature). Foam formation possible with vigorous shearing—operate at moderate RPM. Inlet strainer protects pump from suspended solids.

Temperature Management & System Design

Temperature is the primary lever for controlling adhesive viscosity and pumpability. Even modest temperature changes (10–20°C) dramatically shift viscosity and motor load. Effective adhesive pumping systems maintain strict temperature control throughout the transfer path.

Temperature Windows for Common Adhesives

Heated Hose & Manifold Systems

Hot-melt adhesive systems require heated discharge hose and applicator manifold. Unheated hose loses temperature during transfer, causing adhesive to solidify before reaching the nozzle. Electric heating tape or water circulation jackets maintain hose temperature at 180–200°C. Common practice: thermostat-controlled heater on the discharge line maintains stable temperature & viscosity.

Chilling & Environmental Control

Conversely, some epoxy or polyurethane operations benefit from temperature reduction to slow cure and extend pot life. Cooling jackets on bulk tanks or immersion in ice-water baths lower temperature to 10–15°C. Careful—excessive cooling increases viscosity dramatically and may exceed pump motor capacity. Use thermostats to maintain specified temperature windows.

Storage & Shelf-Life Implications

Temperature variation during storage affects adhesive shelf life. Hot-melt adhesives are stable at room temperature but degrade if exposed to sustained heat (> 30°C over weeks). Epoxy resins shelf life is extended by cool storage (5–15°C). Polyurethane adhesives are sensitive to moisture and temperature fluctuation—stable storage extends pot life and viscosity consistency.

Pump Selection & Sizing for Adhesive Transfer

Pump selection for adhesive applications requires understanding three parameters: (1) required flow rate in the actual adhesive (not water equivalent), (2) viscosity of the adhesive at operating temperature, and (3) available motor horsepower and speed range.

Step 1: Determine Operating Viscosity

Identify the adhesive viscosity at the operating temperature you will maintain during transfer. Do not use room-temperature viscosity—it's misleading. Example: a hot-melt adhesive listed as "180 cP @ 180°C" is your target viscosity. An epoxy resin listed as "5000 cP @ 25°C" is your target if you maintain 25°C operation. Consult product datasheets or perform viscosity testing at your intended operating temperature.

Step 2: Calculate Required Pump Power

Motor power consumption in viscous fluids increases significantly. The NAPCO formula for estimating horsepower is:

Horsepower (approx.) = (GPM × PSI × 0.001 × viscosity_correction_factor) ÷ 500

Where viscosity_correction_factor = (1 + (viscosity_cP ÷ 1000) × 0.5) for viscosity above 100 cP. For practical sizing, consult NAPCO engineering with your adhesive viscosity, required GPM, and available motor horsepower.

Step 3: Select Pump Model & RPM

NAPCO offers two primary models for adhesive transfer:

PA300C/PA300S — High-Volume Adhesive Systems

Delivers 158 GPM @ 280 RPM (at low viscosity). For high-viscosity adhesives, reduce RPM proportionally to stay within motor torque limits. At 100 RPM with 5000 cP epoxy resin, this pump delivers ~56 GPM with manageable motor load.

Applications: Bulk adhesive metering, high-throughput dispensing, spray applicators, industrial assembly lines.

PA200C/PA200S — Moderate-Flow Adhesive Systems

Delivers 69 GPM @ 190 RPM (at low viscosity). Smaller displacement makes it ideal for precision metering (5–30 GPM) where flow accuracy matters. At lower RPM (50–100 RPM), this pump delivers 18–36 GPM of thick adhesive without excessive motor load.

Applications: Precision adhesive metering, small-scale manufacturing, laboratory dispensing, skid-mounted equipment.

Pressure Requirements

Most adhesive dispensing operates at low pressure (< 50 PSI) because the application is gravity-fed or metering from a pressurized tank. High pressure is needed only for long discharge lines or spray applicators. NAPCO pumps are rated to 150 PSI, providing ample margin. Consult with your adhesive equipment manufacturer for specific pressure requirements.

Troubleshooting High-Viscosity Adhesive Systems

Low Flow Output / Pump Stalling

Check inlet temperature. Low viscosity operation assumes room temperature; if temperature drops even 10°C, viscosity increases dramatically and motor load may exceed capacity. Verify heating system (if equipped) is maintaining target temperature. If temperature is correct, the pump may be undersized for the viscosity. Reduce required flow rate or select a larger pump with lower RPM operation.

Excessive Motor Current / Overheating

Motor draws excessive current when pumping thick adhesive at high RPM. Solution: reduce pump speed (lower RPM = less motor load) or reduce system pressure if possible. Verify the adhesive viscosity is not higher than expected—consult product datasheets or perform viscosity testing.

Foam or Separator Formation in Adhesive

Excessive shear (high pump speed) or air entrainment causes foam. Reduce pump RPM to moderate levels (50–150 RPM for thick adhesives). Ensure inlet strainer is clear and inlet line is below fluid level (no air suction). Install a degasser or separator tank downstream to release trapped air before application.

Hot-Melt Adhesive Solidifying in Hose

Inadequate hose heating. Verify heating tape or water jacket is maintaining hose temperature > 180°C along the entire discharge path. Unheated sections allow adhesive to solidify. Thermostats may be set too low—increase setpoint to 190°C. Check for thermostat failures.

Rapid Seal or Gear Wear

High-viscosity operation combined with elevated temperature (epoxy thermal exotherms, hot-melt heat) can degrade seals. Verify seal material is compatible with your adhesive chemistry. Nitrile seals may fail with aromatic epoxy resins; Viton is recommended. For hot-melt systems, ensure seals are rated for the operating temperature range. See seal material guide for recommendations.

System Design & Installation Best Practices

Inlet Strainer & Filtration

Adhesives often contain fine solid additives (fillers, colorants) or build up oxide layer on storage tank surfaces. Install a 100–150 micron inlet strainer to prevent gear damage. Clean strainers at each maintenance cycle.

Pressure Relief Valve

Set relief valve 10–20% above maximum operating pressure to protect pump from overpressure if discharge line becomes blocked. For typical adhesive systems at < 50 PSI, set relief to 60 PSI.

Check Valve & Flow Control

A check valve prevents siphoning when the pump stops. Flow control valves enable metering of adhesive without changing pump speed. Proportional control valves provide variable flow while maintaining stable pressure.

Discharge Line Sizing

Undersized discharge lines create backpressure, increasing motor load and heat generation. Maintain discharge velocity < 8 ft/sec. For a PA300 at 50 GPM (typical high-viscosity operation), a 1.5-inch hose is appropriate. Consult hose manufacturers for specific sizing based on flow rate and viscosity.

Material Compatibility

Most adhesives are compatible with stainless steel (SS) or ductile iron pump housings. Some epoxy resins with aromatic solvents may attack elastomers—specify Viton seals instead of nitrile. Polyurethane adhesives can cause swelling in some elastomers—verify seal compatibility with your specific adhesive product. Consult NAPCO engineering for materials questions.

Related Technical Resources

• Adhesives & Sealants Industry Solutions
• How to Size a Rotary Gear Pump
• What is a Rotary Gear Pump? Technical Fundamentals
• Nitrile vs. Viton Gears: Seal Material Selection
• PA300C Ductile Iron Pump Specifications
• PA200C Ductile Iron Pump Specifications
• PA300S Stainless Steel Pump Specifications
• PA200S Stainless Steel Pump Specifications
• Engineering & Technical Support