Quick answer. Industrial buyers should specify PU glue by application viscosity, NCO content or mix ratio, density, solids, open time, cure profile, and bond performance on the actual substrate. Each value needs a test method, temperature, conditioning procedure, tolerance, and batch acceptance rule; a polyurethane adhesive TDS containing typical values alone cannot protect production.
The decisive specification depends on the chemistry. A moisture-curing 1K PU, two-component laminating adhesive, solvent-based shoe adhesive, and reactive PUR hot melt may all carry the polyurethane label, yet they require different dispensing equipment, process windows, and incoming inspection tests.
A useful RFQ therefore starts with the coating or bonding line: substrate, coat weight, line speed, application temperature, available curing time, and expected exposure. Procurement can then compare offers on measurable production cost per square metre instead of price per kilogram alone.
Translate the Production Line into Measurable Specifications
A supplier cannot select an industrial PU glue from a strength target alone. State the substrate pair, including grade and surface condition: PET film to aluminium foil, PVC to textile, split leather to rubber, painted steel to insulation panel, or wood at 8–12% moisture content.
Surface preparation changes the result. A lap-shear value obtained on grit-blasted aluminium cannot predict adhesion to steel carrying 20 mg/m² of forming oil. The test request should identify cleaning solvent, abrasion grade, primer, adhesive thickness, assembly pressure, cure temperature, and conditioning time.
Define the process window with numbers. A laminator running at 150 m/min and 1,300 mm web width has a different open-time requirement from a footwear line allowing 8 minutes of solvent flash-off before heat activation.
For each product family, procurement should request the following operating data:
- 1K moisture-cure PU: viscosity at 25°C, NCO content in %, skin time at stated temperature and relative humidity, cure depth in mm after 24 and 48 hours, and maximum permitted unopened storage time.
- 2K solvent-free or solvent-based PU: component viscosities, density of each component, mass and volume mix ratios, pot life, initial green strength, cure schedule, and residual monomer controls.
- Reactive PUR hot melt: viscosity at the specified application temperature, usually reported within a defined 100–140°C range, open time, recommended melter temperature, thermal exposure limit in the tank, and moisture-cure conditions.
- Shoe or contact adhesive: solids %, viscosity, flash-off time, heat-activation temperature, tack-retention window, and peel strength on the nominated upper and sole materials.
These are product-specific controls. Replacing PU adhesive with an epoxy or EVA hot melt would change the reactive-group test, cure mechanism, thermal history, and incoming inspection plan.
PU Glue Viscosity Must Include the Test Conditions
“Viscosity: 3,000 mPa·s” is incomplete. PU glue viscosity depends on temperature, shear rate, spindle geometry, rotational speed, sample conditioning, and elapsed time after mixing.
A purchasing specification could read “2,500–3,500 mPa·s at 25.0 ± 0.5°C, Brookfield RV, spindle 4, 20 rpm, tested after 30 minutes of temperature conditioning.” The numbers are an example acceptance band, not a universal PU value.
ASTM D1084 covers viscosity testing for adhesives and explains why the selected procedure must accompany the result. A supplier using a different spindle or temperature can produce a numerically different result from the same batch.
Viscosity affects more than pumping. On a roll coater, excessive viscosity can produce ribbing, poor levelling, and coat-weight variation across the web. In spray application, it can increase droplet size and leave dry edges; in a static mixer, it can raise back pressure and disturb the 2K ratio.
Very low viscosity can also create defects. Adhesive may penetrate porous foam or textile, leaving insufficient material at the bond line. On vertical assemblies, sag can reduce the specified 0.2–0.5 mm wet-film thickness before the parts are fixed.
Buyers should request a temperature-viscosity curve rather than one data point for heated applications. For a PUR hot melt dispensed at 120°C, results at 100°C, 120°C, and 140°C reveal the operating margin if a melter sensor drifts by 5–10°C.
For 2K systems, specify the mixed viscosity and the individual component viscosities. A 20:1 viscosity imbalance between components can make meter-mix accuracy harder to maintain, even if the nominal mass ratio is correct.
NCO Content, Solids, Mix Ratio, and Cure Chemistry
NCO content expresses the mass percentage of isocyanate groups available in an isocyanate component or prepolymer. It can influence equivalent ratio, moisture response, crosslink density, cure behavior, and the amount of hardener required, but a higher percentage does not automatically mean a stronger bond.
The NCO equivalent weight can be estimated as 4,200 divided by NCO %. A component at 10.0% NCO has an NCO equivalent weight of approximately 420 g/eq; procurement still needs the polyol hydroxyl value and the supplier’s intended stoichiometry before changing a mix ratio.
NCO content must carry a test method and tolerance. For a tightly metered 2K laminating process, accepting 9.0–11.0% against a 10.0% nominal value may create a larger stoichiometric shift than the line validation allows. The manufacturer should define the validated release range and report the actual batch result on the certificate of analysis.
Free diisocyanate monomer is a separate measurement from total NCO content. The EU restriction for industrial and professional use sets training conditions where diisocyanates are present at a total concentration of at least 0.1% by weight; buyers can review the restriction summary through the European Chemicals Agency.
For workplace risk assessment, procurement and EHS teams should examine the SDS, ventilation method, application temperature, aerosol generation, and required protective equipment. The OSHA isocyanates resource describes respiratory and dermal exposure concerns associated with this chemical family.
Solids content controls the relationship between wet coat weight and dry adhesive deposition. A 60% solids adhesive applied at 100 g/m² leaves approximately 60 g/m² of non-volatile material before reaction effects, while a 100% solids system can target the same dry mass with less wet material.
Solids also exposes misleading price comparisons. If a 60% solids adhesive costs $2,400/ton, the simple cost per ton of non-volatile content is $4,000 before solvent handling and drying energy. A 100% solids alternative priced at $3,500/ton would have the lower material cost on that basis, although application and cure requirements still need validation.
For two-component adhesives, state whether the mix ratio is by mass or volume. A 100:50 volume ratio using components with densities of 1.10 and 1.25 kg/L converts to 110:62.5 by mass, or about 100:56.8; loading it as 100:50 by mass changes stoichiometry.
Pot life also needs a failure criterion. “Pot life: 40 minutes” should specify starting temperature, batch mass, ambient conditions, and endpoint such as viscosity doubling, gel formation, or loss of application quality.
Density, Bond Strength, Heat Resistance, and Consumption
Adhesive density converts purchase mass into applied volume. A glue with density 1.20 kg/L applied as a 100 µm uniform wet film consumes about 120 g/m², giving a theoretical coverage of 8.33 m²/kg before transfer loss.
If the line instead controls coat weight at 150 g/m² and adhesive costs $3,500/ton, direct adhesive cost is $0.525/m². At 5% transfer and purge loss, effective consumption becomes 157.5 g/m² and cost rises to about $0.551/m².
Density testing should state temperature because liquid volume changes with temperature. For 2K equipment, procurement should collect density for both components and check the meter’s calibration basis.
Strength data must match the joint’s failure mode. Lap shear is commonly reported in MPa, peel in N/25 mm or N/mm, cleavage in N/mm, and flexible laminate bonds may be evaluated through T-peel plus ageing.
A value such as “lap shear ≥8 MPa” is meaningful only with substrate alloy, surface preparation, overlap, bond-line thickness, cure schedule, test speed, and conditioning. Procurement should also request the failure mode: cohesive adhesive failure, interfacial failure, or substrate failure.
Initial strength can matter more than seven-day strength on fast lines. A panel process that removes fixtures after 20 minutes needs a quantified green-strength or handling-strength threshold at 20 minutes, not solely a final test after seven days at 23°C.
Heat resistance should be tested under load. A bond may retain acceptable room-temperature lap shear after heating yet creep several millimetres under a constant load at 80°C. Specify load, bonded area, temperature, duration, and maximum displacement.
Cold, water, hydrolysis, plasticizer, fuel, and cleaning-agent exposure should follow the end use. Flexible PVC bonding requires attention to plasticizer migration; footwear needs repeated flexing and hydrolysis assessment; retort laminates need temperature, pressure, food-simulant, and sealing-condition validation.
Flammability requirements belong in the application specification rather than a generic PU claim. For vehicle interior materials subject to FMVSS 302, the assembly and substrate construction must meet the applicable burn-rate requirement; an adhesive cannot be certified in isolation for every finished vehicle stack.
Use a Procurement Table, COA Limits, and Change Control
The table below converts common polyurethane adhesive specifications into purchase controls. Target values must come from line trials and finished-assembly validation, rather than copying a competitor’s TDS.
| Specification dimension | Production effect | Minimum test definition | Recommended purchase control |
|---|---|---|---|
| Viscosity | Pumping pressure, coating pattern, penetration, levelling | mPa·s, temperature, instrument, spindle, rpm, conditioning time | Approved range on every batch COA; incoming verification at the same conditions |
| NCO content | Equivalent ratio, cure response, crosslink formation | Mass %, analytical method, repeatability, sampling point | Batch result and validated tolerance for isocyanate component or prepolymer |
| Density | Volume conversion, mix calibration, coverage per kilogram | g/cm³ or kg/L at a stated temperature | COA range where metering or coat-volume control depends on it |
| Solids content | Dry deposition, solvent load, oven demand, material yield | %, test temperature, heating time, sample mass | Batch limit plus wet-to-dry coat-weight calculation |
| Mix ratio | Stoichiometry, cure, residual reactive material | Mass or volume basis, tolerance, component densities | Meter calibration and ratio alarm limit documented during trial |
| Open time or pot life | Assembly window, mixer flushing, scrap risk | Temperature, humidity, batch size, endpoint | Minimum value tied to maximum line interruption and assembly time |
| Bond performance | Joint load capacity and failure location | Substrates, preparation, thickness, cure, test speed, ageing | Incoming or periodic test on approved production substrates |
| Shelf life | Inventory exposure and requalification risk | Packaging, storage temperature, unopened condition | Minimum remaining life at delivery, such as 75% of labelled life |
A TDS is a product-description document, so many figures are typical rather than guaranteed. Convert critical figures into an agreed product specification: nominal value, upper and lower limits, method, sampling frequency, and response to an out-of-specification result.
The COA should contain actual batch results for release tests. Appearance statements such as “clear” or “amber” should use an agreed colour scale if colour can affect a transparent laminate or pale footwear material.
Request the current TDS, SDS, batch COA format, shelf-life statement, storage conditions, packaging options, and applicable regulatory declarations. ISO management certificates describe a management system; they do not prove that a particular adhesive will bond a customer’s untreated TPU or corona-treated PET.
For manufacturer-direct sourcing, ask who controls polymerisation, blending, filling, and final release. Direct technical access becomes commercially useful when the producer can adjust a validated viscosity window, solids level, open time, package size, or component ratio and then lock the approved formulation through change control.
A qualification plan can begin with a 20–50 kg engineering trial, followed by a 200–500 kg line trial and then a production batch, depending on consumption and equipment hold-up. These quantities are planning examples; the RFQ should ask the supplier to quote actual sample quantity, MOQ in tons, production lead time in days, and export packaging.
Require written notification before changes to raw-material source, catalyst, solvent, stabilizer, manufacturing site, test method, or packaging contact material. A six-month notice target gives buyers time to run ageing or customer approval tests, but the final period should match the supply agreement.
Buyers comparing PU glue and adhesive product categories should send the actual line conditions with the inquiry. That permits a manufacturer to propose a base chemistry, customisation range, test plan, and documentation package without inventing a universal grade.
FAQ
Q: Which polyurethane adhesive specification should procurement check first?
Start with the application method and viscosity under actual operating conditions. Then check cure mechanism, open time, and bond performance on the production substrates; the lowest-price product fails commercially if the existing pump cannot meter it or the assembly window is too short.
Q: What is the difference between a TDS, SDS, and COA?
A TDS describes product properties and processing guidance, often with typical values. An SDS communicates hazards, handling, exposure controls, and transport information, while a COA reports release results for one manufactured batch.
Q: Is higher NCO content better in PU glue?
No universal ranking exists. NCO content must be evaluated with hydroxyl functionality, equivalent ratio, catalyst package, moisture exposure, flexibility target, and free-monomer level; changing it can alter cure speed, modulus, pot life, and residual reactive groups.
Q: Why do two suppliers report different PU glue viscosity for similar samples?
They may use different temperatures, spindles, rotational speeds, shear histories, or conditioning times. Ask both laboratories to repeat the measurement under the same ASTM D1084 procedure and report the complete setup.
Q: Can adhesive density be ignored if purchasing is by kilogram?
No. Density determines litres delivered per kilogram, affects volume-ratio conversion in 2K equipment, and changes theoretical coverage. At a fixed 100 µm wet thickness, density of 1.10 kg/L consumes about 110 g/m², while 1.25 kg/L consumes about 125 g/m².
Q: How should buyers validate a customised PU adhesive?
Freeze the trial formulation, test it on production substrates, run the intended coat weight and line speed, record initial and aged performance, and agree COA limits before the first commercial order. Retain labelled samples from the approved trial and initial production batch so later deviations can be compared against physical reference material.