Technical Resource
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. Structural epoxy resins are thick, slow-flowing liquids. Construction sealants can exceed the viscosity of peanut butter. Centrifugal pumps lose flow quickly as these fluids get thicker.
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 moving thin liquid adhesive at 100 cP or transferring thick polyurethane at 10,000 cP, a properly selected gear pump will perform reliably.
This guide walks engineers through viscosity fundamentals, why gear pumps excel in high-viscosity service, temperature management strategies, and practical sizing steps 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 measured at a stated reference temperature.
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.
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 pump speeds, within the 280 RPM (3-inch) and 190 RPM (2-inch) maximums, avoid excessive shear while maintaining practical flow rates.
Adhesive and sealant plants rely on consistent, reliable bulk transfer.
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, so flow and pressure fall off as viscosity rises. That makes centrifugal pumps a poor fit for 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 flow stays near-constant regardless of viscosity.
- Near-Constant Flow Across Viscosities: GPM is set by pump size and speed. Motor load rises with viscosity, but flow stays near-constant. The PA300S delivers 158 GPM at 280 RPM.
- 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.
Common Adhesive & Sealant Applications
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). 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
| Adhesive Type | Operating Temperature | Limits |
|---|---|---|
| Structural Epoxy | 20–60°C (ambient to warm) | Below 10°C: very stiff; Above 80°C: premature cure |
| Polyurethane (1K, 2K) | 18–25°C (ambient) | Below 5°C: slow cure; Above 40°C: accelerated cure (reduced pot life) |
| Silicone Sealant | 15–30°C (ambient) | Below 10°C: very slow cure; Above 35°C: cure accelerates |
| Water-Based Adhesive | 18–25°C (ambient) | Freeze risk below 5°C; foam risk above 40°C |
Chilling & Environmental Control
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. Epoxy resin shelf life is extended by cool storage (5–15°C). Polyurethane adhesives are sensitive to moisture and temperature fluctuation: stable storage extends shelf life and keeps viscosity consistent.
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: 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: Required Pump Power
Motor load rises with viscosity. For sizing, send NAPCO your viscosity at operating temperature, flow rate, pressure and available motor horsepower. See the performance curves on the PA300S and PA200S product pages for brake horsepower at your pressure.
Step 3: Select Pump Model & RPM
NAPCO offers two primary models for adhesive transfer:
PA300C/PA300S: High-Volume Adhesive Systems
Delivers 158 GPM at 280 RPM maximum. For high-viscosity adhesives, run a slower speed to stay within motor torque limits.
Applications: Bulk adhesive and resin transfer, industrial assembly lines.
PA200C/PA200S: Moderate-Flow Adhesive Systems
Delivers 69 GPM at 190 RPM maximum.
Applications: Smaller-volume adhesive transfer, skid-mounted equipment.
Pressure Requirements
Pressure depends on the system. Long discharge lines and spray applicators need more pressure than gravity-fed or tank-metered dispensing. NAPCO pumps are tested to 100 PSI. Consult with your adhesive equipment manufacturer for specific pressure requirements.
Adhesive systems require precise flow control and temperature management for consistent product quality.
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 Separation in Adhesive
Excessive shear (high pump speed) or air entrainment causes foam. Reduce pump RPM to moderate levels. 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.
Rapid Gear Wear
High-viscosity service at elevated temperature can wear gears faster. Aromatic solvents in some epoxies can attack nitrile; choose Viton® gears for those. See the gear material guide.
System Design & Installation Best Practices
Inlet Strainer & Filtration
Adhesives often contain fine solid additives (fillers, colorants), and storage tank surfaces can build up an oxide layer. Install an inlet strainer sized for your fluid 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, never above the pump’s 100 PSI tested maximum.
Check Valve & Flow Control
A check valve prevents siphoning when the pump stops. To change flow from a positive displacement pump, change pump speed.
Discharge Line Sizing
Undersized discharge lines create backpressure, increasing motor load and heat generation. Size the hose for your flow rate and fluid 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® gears instead of nitrile. Polyurethane adhesives can cause swelling in some elastomers: verify gear 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: Gear 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
FAQ
What viscosity range do adhesives and sealants cover?
Adhesives and sealants run from thin water-based glues to thick sealant pastes, and any one product’s viscosity changes with temperature. Check the datasheet for viscosity at your operating temperature. A rotary gear pump is positive displacement, so flow stays near-constant as viscosity rises, unlike a centrifugal pump. Thicker fluid does take more motor torque, so size the drive for the viscosity you actually run.
Why do centrifugal pumps struggle with adhesives while gear pumps don’t?
A centrifugal pump moves fluid by spinning it with an impeller, and its flow falls off as the fluid gets thicker. A rotary gear pump carries a fixed volume from inlet to outlet on every turn, so flow stays near-constant regardless of viscosity or pressure. Thicker adhesive raises motor load, not displacement, which is why gear pumps suit viscous adhesive and sealant transfer.
How does temperature affect adhesive pumping?
Adhesive viscosity drops as temperature rises, so the same product can pump very differently in a cold plant and a warm one. Keep the adhesive inside the temperature window on its datasheet, and size the pump at that temperature. Nitrile gears are standard and rated to 240°F. Viton® gears are optional for aggressive chemicals and higher temperatures. Send NAPCO your fluid and operating temperature before you specify a pump.
Should I use a larger pump for high-viscosity adhesives?
Not always. Thick fluid usually calls for a slower pump speed, which lowers motor load. The PA300S and PA300C deliver 158 GPM at their 280 RPM maximum, and the PA200S and PA200C deliver 69 GPM at their 190 RPM maximum. Pick the model that reaches your flow rate at a speed your drive can handle in your fluid. Send NAPCO your viscosity at operating temperature, flow rate and pressure, and we’ll size the pump and drive.
Need Help Sizing a Pump for Your Adhesive or Sealant Application?
NAPCO’s engineering team specializes in high-viscosity adhesive and sealant transfer. Send us your adhesive type, viscosity (measured at your operating temperature), required flow rate, temperature range, and pressure requirements. We’ll recommend the optimal pump model, motor speed, and system configuration for reliable, consistent transfer.