Two suppliers can report nearly identical hot melt viscosity values and still be describing materials that behave differently. They may have tested at different temperatures, used different spindles or speeds, allowed different heating times, or reported a typical TDS value instead of an actual batch result.
ASTM D3236 gives buyers a common method for measuring the apparent viscosity of hot melt adhesives. The method name alone, however, does not make two numbers comparable. A usable report must identify the temperature, instrument, spindle, rotational speed, and result. For purchasing and quality control, the sample identity, batch number, thermal history, torque, and any departure from the method also matter.
A sound purchasing comparison starts with an ASTM D3236 hot melt viscosity result, complete Brookfield hot melt viscosity conditions, and an apparent viscosity report tied to the tested batch.
Quick answer: when are two viscosity values comparable?
Treat two hot melt adhesive viscosity results as directly comparable only when the following conditions are aligned:
| Comparison item | What should match or be disclosed | Why it matters |
|---|---|---|
| Test method | ASTM D3236 and the edition used | A method name without an edition or stated deviation is incomplete |
| Test temperature | The same temperature, based on the actual application window | Hot melt viscosity can change sharply with temperature |
| Instrument | Make, model, and torque range or instrument family | Instruments with different torque capabilities do not create the same measurement setup |
| Sample chamber | The same approved geometry and sample volume or mass | Geometry and fill level affect the spindle’s interaction with the sample |
| Spindle | The same spindle designation | Many hot melts are non-Newtonian, so spindle geometry can change the result |
| Rotational speed | The same rpm | Changing speed changes the test’s shear condition |
| Torque or scale position | Within the method’s usable range, preferably near the middle | A reading near the limit of the instrument is less useful for comparison |
| Equilibration and reading procedure | The same thermal stabilization and reading sequence | Hot melts need time to melt and reach a stable temperature |
| Sample condition | Representative lot, fresh portion, controlled storage and heating history | Aging, moisture, contamination, and prolonged heating can shift viscosity |
| Result status | Typical value, specification range, or batch result | These three numbers serve different purposes |
| Units | mPa·s or cP, stated clearly | 1 mPa·s is numerically equal to 1 cP |
If any of these fields is missing, the result may still help with preliminary screening, but it should not be used to rank suppliers or release a production lot without clarification.
What ASTM D3236 measures
ASTM D3236 is a rotational viscometer method for determining the apparent viscosity of hot melt adhesives and related coating materials under controlled elevated-temperature conditions. A molten sample is held in a temperature-controlled chamber while a spindle rotates in it. The instrument measures the torque required to maintain the selected speed and converts that response into viscosity.
The standard’s stated scope covers apparent viscosities up to 200,000 mPa·s at temperatures up to 175°C. Equipment may be capable of testing at higher temperatures and viscosities, but capability and method scope are not the same thing. A report at 180°C can be useful for a production process that runs near 180°C, yet the laboratory should state that it used an adapted condition if the temperature falls outside the stated scope of the edition cited.
The method is most relevant to relatively low-shear flow behavior. It can help buyers compare melt consistency and anticipate pumping, coating, bead formation, and general application behavior. It does not reproduce every shear condition inside a gear pump, narrow nozzle, fiber spray head, or high-speed coating die.
For a broader explanation of how viscosity works with bonding time, see Hot Melt Adhesive Viscosity, Open Time and Setting Time Explained.
Why the report says “apparent viscosity”
Many hot melt adhesives are non-Newtonian. Their measured viscosity can change when the spindle, rotational speed, or test geometry changes. The number therefore describes the material under a defined set of test conditions. It is not a universal constant that remains the same under every rate of flow.
This distinction explains a common purchasing problem. Supplier A may report 4,800 mPa·s with one spindle at 20 rpm. Supplier B may report 4,700 mPa·s with another spindle at 10 rpm. The values look close, but they were not measured under equivalent shear conditions. The 100 mPa·s difference cannot be treated as evidence that the products are equivalent.
For this reason, an apparent viscosity report must carry its test conditions with the result. “Viscosity: 4,800 cP” is incomplete. “Apparent viscosity: 4,800 mPa·s at 160°C, instrument model X, spindle Y, 20 rpm” is a reportable result that another laboratory can attempt to reproduce.
How an ASTM D3236 hot melt adhesive viscosity test is run
A laboratory should work from the controlled standard and its own approved procedure. The following summary helps a buyer understand the main controls without replacing the standard.
- Select the instrument, spindle, chamber, and speed range for the expected viscosity.
- Place a representative fresh portion of the adhesive in the sample container.
- Melt the sample at the selected temperature without unnecessary or prolonged heating.
- Align and level the instrument, insert the spindle, fit the insulating cover, and allow the sample to reach temperature equilibrium.
- Start at the lowest available spindle speed to reduce temperature gradients and unintended shear effects.
- Confirm the correct liquid level around the spindle, then reestablish temperature equilibrium.
- Continue low-speed rotation for 15 minutes after apparent equilibrium.
- Change to the selected measurement speed. The reading should fall within 10% to 95% of the torque scale, with the midpoint preferred when a suitable combination is available.
- Record readings at the required intervals until three consecutive readings meet the method’s stability rule.
- Average the accepted readings, convert them using the instrument procedure where required, and report the result with the temperature, instrument model, spindle, and rotational speed.
For routine purchasing, the buyer does not need to operate the viscometer. The buyer does need enough detail to judge whether the supplier followed a repeatable procedure.
Brookfield hot melt viscosity is not a complete test description
“Brookfield viscosity” usually means the supplier used a Brookfield rotational viscometer or rheometer. A Thermosel-type system is commonly used because it provides a small insulated chamber and controlled elevated temperature. That description is useful, but it still leaves several open questions.
Ask for:
- the complete viscometer model and torque range;
- the temperature-control system and chamber;
- the spindle designation;
- the rotational speed in rpm;
- the measured or controlled sample temperature;
- the torque percentage at the reported reading;
- the equilibration procedure;
- the individual readings or stable average;
- the sample mass or volume when this is part of the laboratory procedure;
- the calibration or verification status;
- any deviation from ASTM D3236.
The word “Brookfield” identifies an equipment family, not a single universal condition. Even two laboratories using Brookfield equipment can obtain results that are unsuitable for direct comparison if the spindle, speed, torque range, sample level, or heating procedure differs.
Temperature is usually the largest comparison error
Hot melt adhesive becomes easier to flow as its temperature rises. That is why a result at 180°C will normally be lower than a result from the same material at 160°C. A buyer cannot compare those values as though the temperature were a footnote.
Consider four supplier reports. The numbers are illustrative and are not product specifications:
| Supplier | Reported result | Can it be compared with Supplier A? | Reason |
|---|---|---|---|
| A | 4,800 mPa·s at 160°C, SC4-27, 20 rpm, 48% torque | Baseline | Conditions are disclosed |
| B | 4,500 mPa·s at 180°C, SC4-27, 20 rpm | No | The temperature differs and torque is missing |
| C | 4,900 mPa·s at 160°C, SC4-28, 10 rpm, 52% torque | Not directly | Spindle and speed differ |
| D | 4,750 mPa·s at 160°C, SC4-27, 20 rpm, 46% torque | Potentially | Core measurement conditions match; sample handling and result status still need checking |
Supplier D is the only reasonable direct comparison. Even then, confirm whether each number is a one-batch result, an average, a typical TDS value, or a release specification.
Test at a temperature that answers a production question
The best test temperature is not automatically the highest temperature supported by the instrument. It should relate to the supplier’s recommended application range and the buyer’s real process.
If a line normally runs at 160°C, a result reported only at 180°C does not show how the adhesive will behave at the normal setting. If the application system has separate tank, hose, and nozzle setpoints, record the actual adhesive temperature as closely as practical instead of relying only on the controller display.
Do not raise the application temperature solely to force a lower viscosity reading. Excess heat can accelerate oxidation, color change, odor, charring, and viscosity drift in thermoplastic hot melts. Reactive PUR also needs control of heated residence time and moisture exposure.
One temperature point or a viscosity-temperature profile?
A single value can support lot release when the product and process are already established. It is weaker for grade selection because it does not show how quickly the adhesive changes across the operating window.
For sample approval, request results at several relevant temperatures where practical, using a fresh representative portion for each measurement. The selected points should remain within the method scope unless the laboratory clearly reports an adaptation. A three-point profile can help the buyer see whether:
- the adhesive remains pumpable near the lower process limit;
- its bead or coating becomes too fluid near the upper limit;
- two candidates that match at one temperature separate at another;
- a small process-temperature change is likely to cause a large flow change.
ASTM D3236 allows observed results at several temperatures to support interpolation on a semi-logarithmic plot. Interpolation should use measured points that bracket the temperature of interest. Do not extrapolate far beyond the tested range and present the estimate as a measured result.
The profile is still a laboratory screen. The final process window must be confirmed on the production equipment with the actual substrate and application pattern. This is especially important on a high-speed hot melt adhesive line, where pump response, nozzle behavior, open time, compression time, and thermal residence interact.
The apparent viscosity report buyers should request
A useful report lets another qualified laboratory understand what was tested and how. The following format can be used for a supplier report, sample-approval record, or incoming inspection result.
| Report field | Required information |
|---|---|
| Supplier and product | Manufacturer, commercial grade, chemistry, color or form where relevant |
| Sample identity | Batch or lot number, manufacture date if available, package number, sampling date |
| Sample condition | Package status, storage condition, whether the sample was previously heated |
| Test method | ASTM D3236 and exact edition; list any deviations or adapted conditions |
| Instrument | Manufacturer, full model, torque range or instrument family |
| Temperature system | Chamber and controller used |
| Measurement temperature | Target temperature and recorded sample temperature or control tolerance |
| Spindle and chamber | Full spindle designation, chamber geometry, sample mass or volume as controlled |
| Rotational speed | rpm used for the final measurement |
| Torque | Percent torque or scale reading at measurement |
| Thermal procedure | Melt method, equilibration rule, time at temperature, test order for multi-temperature work |
| Readings | Individual stable readings and calculated average |
| Result | Apparent viscosity in mPa·s or cP |
| Result status | Development result, typical value, specification limit, or batch-release result |
| Quality status | Pass, fail, investigation, or information only |
| Traceability | Test date, operator, laboratory, instrument ID, calibration or verification status |
An apparent viscosity report should not hide the test conditions in a general laboratory SOP that the buyer cannot review. The result and the conditions that define it should travel together.
Example of a clear report line
Use a compact line in the TDS or COA, then attach the complete test record when required:
Apparent viscosity: [result] mPa·s at [temperature]°C, ASTM D3236-[edition], [instrument make and model], [spindle], [speed] rpm, [torque]% torque, batch [lot number].
The laboratory must replace every bracketed field with its real setup and result. An incomplete template is better than a polished report filled with assumed values.
Typical value, specification range, and batch result are different
A TDS often gives a typical value. This describes expected product behavior but may not be a release limit. A specification defines the range or limit used for acceptance. A COA should show the actual result for the delivered or manufactured lot when viscosity is a batch-release property.
| Data type | What it tells the buyer | What it should not be used for |
|---|---|---|
| Typical value | A general technical reference for the grade | Rejecting a batch unless it is also an agreed specification |
| Specification range | The contractual or quality acceptance window | Claiming every batch will equal the midpoint |
| Batch result | What the laboratory measured for the identified lot | Predicting the complete production performance by itself |
The product code, lot number, test method, and conditions should agree across the purchase specification, approved sample record, and COA. The Hot Melt Adhesive TDS, SDS and COA Buyer’s Guide explains how these documents serve different jobs and how to cross-check them.
Do not copy an arbitrary viscosity tolerance
A fixed tolerance such as ±200, ±300, or ±500 mPa·s may look strict, but it has no universal meaning. A 300 mPa·s window is wide for a 600 mPa·s product and tight for a 30,000 mPa·s product. It may also be narrower than the combined product and measurement variation that two laboratories can reliably reproduce.
ASTM D3236 includes a published precision statement. In the study behind it, the repeatability limit was 8.8% of the mean of two results, while the general between-laboratory reproducibility limit was 25.4%. The standard also estimates better reproducibility, about 15%, when laboratories compare at equal shear conditions. The study used a limited set of wax-based hot melts, so these figures provide method context rather than a ready-made specification for every EVA, polyolefin, PSA, APAO, polyamide, or PUR adhesive.
Build the purchasing tolerance from:
- data from representative development and production lots;
- the variation of the agreed test method in the supplier and buyer laboratories;
- the viscosity range that the application equipment can process reliably;
- the relationship between viscosity movement and actual bond or coating performance;
- a measurement-system study when the control is commercially important;
- a defined investigation rule for trends that remain inside the release limits.
A percentage range is often easier to interpret across different viscosity levels, but it still needs product-specific evidence. The accepted range should be written into the specification or purchase agreement rather than inferred from a TDS typical value.
Same-laboratory and cross-laboratory comparisons
Viscosity trending is usually strongest when the same laboratory uses the same instrument, spindle, speed, chamber, temperature procedure, and reporting rules over time. This reduces avoidable sources of variation.
If the buyer and supplier both test incoming material, run a correlation study before using the two laboratories to accept or reject shipments. Exchange split samples from the same well-mixed material, test them under the same written conditions, and compare the results. If the laboratories do not correlate, check temperature verification, spindle condition, alignment, sample level, torque range, heating history, instrument configuration, and calculation settings before blaming the adhesive.
Do not average conflicting supplier and buyer results into a compromise number. Resolve the method difference first, then retest under the agreed procedure.
Why two laboratories may obtain different results
1. The actual sample temperatures differ
Controller setpoint is not always the same as sample temperature. Heat loss, sensor position, chamber condition, and insufficient equilibration can create a meaningful difference.
2. Spindle or speed differs
For a non-Newtonian adhesive, the different shear condition can change apparent viscosity even when both laboratories are operating correctly.
3. Torque is too close to the measurement limit
ASTM D3236 calls for a scale position from 10% to 95%, with a result near the middle preferred when possible. A report should include torque so the buyer can see whether the setup suited the sample.
4. The sample level or volume is wrong
Too much or too little material changes the spindle immersion condition. Different chambers or incorrect fill levels reduce comparability.
5. The adhesive experienced different heating histories
A fresh sample and a sample held hot for several hours are not equivalent. Oxidation, volatile loss, polymer change, or reaction can move the result. For reactive PUR, moisture exposure and the elapsed time after melting require especially clear control.
Storage before testing also matters. Damaged packaging, heat exposure, or moisture can change the sample before it reaches the laboratory. Review the hot melt adhesive shelf-life and storage guide when a retained sample no longer matches a new delivery.
6. The sample is not representative
Contamination, surface material from an opened package, mixed lots, or an unrepresentative portion can create a false result. Sampling instructions should identify where and how the portion was taken.
7. The instrument or spindle needs verification
Spindle damage, residue, poor alignment, incorrect instrument settings, or an overdue verification can all affect the reading. The test record should identify the instrument so the issue can be traced.
How to interpret a viscosity shift without guessing
A change in viscosity is a signal, not a diagnosis. The same direction can have several causes.
| Observation | Possible causes to check | Production questions |
|---|---|---|
| Result is lower than baseline | Higher actual test temperature, different spindle or speed, formulation variation, thermal degradation, contamination, incorrect sample amount | Does the adhesive drip, penetrate too far, lose bead height, or create squeeze-out? |
| Result is higher than baseline | Lower actual test temperature, different test geometry, oxidation, moisture reaction in PUR, contamination, aging, formulation variation | Does pump pressure rise, wetting fall, stringing increase, or coating become uneven? |
| Result drifts during repeated readings | Temperature is not stable, sample is changing during heating, air or contamination is present, instrument setup is unstable | Does the material also change during long production holds? |
| Supplier and buyer results disagree | Method, equipment, sample, or calibration difference | Can both laboratories test split samples under one written condition? |
| COA passes but the line fails | Viscosity is not the failure driver, or the laboratory condition does not represent the line | Check substrate, temperature at application, open time, pressure, deposit, equipment condition, and end-use test |
Avoid accusing a supplier of formulation substitution from one viscosity result. Preserve the sample, review the complete test record, repeat the measurement, and compare retained material before drawing a conclusion.
Viscosity does not prove adhesive quality by itself
ASTM D3236 can show whether two molten samples have similar apparent viscosity under defined conditions. It does not prove that they have the same formulation or will deliver the same bond.
Two products with similar viscosity can differ in:
- open time and setting time;
- initial tack and green strength;
- adhesion to coated board, plastic, foam, textile, wood, metal, or nonwoven material;
- heat creep and low-temperature flexibility;
- thermal stability during a long molten hold;
- color, odor, charring, and cleaning behavior;
- final peel, shear, or tensile performance;
- moisture-cure behavior for reactive PUR;
- food-contact, electrical, or other project-specific compliance.
Viscosity is one part of grade approval. Buyers should test candidate materials on the actual line and substrates using written pass and fail criteria. The hot melt adhesive sample testing guide provides a practical structure for trials before bulk purchase.
A buyer’s comparison workflow
Step 1: define the process window
Record the adhesive chemistry, application method, tank, hose and nozzle settings, real substrate temperature, line speed, deposit geometry, assembly delay, compression time, and expected stop conditions. If the project is still at chemistry-selection stage, compare the main types of hot melt adhesives before fixing a viscosity target.
Step 2: request comparable laboratory data
Give every supplier the same target temperatures and reporting template. Ask for the exact method edition, instrument, spindle, speed, torque, sample condition, and result status.
Step 3: screen the documentation
Separate typical data from batch results. Flag reports with no temperature, no spindle or speed, an out-of-scope condition presented without explanation, or a viscosity number copied unchanged across several lots.
Step 4: test representative samples
Use the same substrates, equipment, application pattern, deposit, and conditioning for each candidate. Record viscosity as part of the trial, but judge the grade against production and end-use requirements.
Step 5: establish the approved baseline
Keep the approved sample or a retained sample from the approved lot. Document the full apparent viscosity condition and the range supported by product and method data.
Step 6: control commercial deliveries
Require the agreed batch result on the COA when viscosity is a release property. Trend results by lot. Review shifts before they reach the rejection limit if the movement is persistent or matches a production change.
Step 7: define what triggers revalidation
Examples include a new formulation, raw-material change, manufacturing-site change, revised test method, new substrate, faster line, different nozzle, lower application temperature, or new end-use requirement.
Copy-ready request for an apparent viscosity report
Please provide the apparent viscosity data for the proposed hot melt adhesive grade and identify whether each value is a typical TDS value, specification limit, or actual batch result. Report the exact ASTM D3236 edition, test temperature, instrument make and model, torque range, chamber, spindle, rotational speed, percent torque, sample amount, sample and heating history, equilibration procedure, individual stable readings, calculated average, unit, lot number, test date, and any deviation from the method. Please include results at [selected temperatures] relevant to our application window of [range]°C. If any requested temperature falls outside the stated scope of the cited method edition, identify it as an adapted condition. Also provide the recommended application range, maximum molten hold guidance, and the viscosity release specification proposed for commercial lots.
This request forces all suppliers to answer the same technical question. It also creates a record that purchasing, quality, and production teams can use during sample approval.
Red flags in a hot melt viscosity report
Pause the comparison and request clarification when the report shows any of the following:
- viscosity without a temperature;
- temperature without instrument, spindle, or speed;
- “Brookfield method” with no complete setup;
- ASTM D3236 named but a temperature above the stated scope is not identified as an adaptation;
- no lot number on a document presented as a COA;
- the same exact result repeated across many lots without individual test evidence;
- a very low or very high torque reading;
- a result reported as “pass” without the agreed specification;
- a typical TDS value presented as an actual batch result;
- changed test conditions between the approved sample and commercial shipment;
- a supplier tolerance that has no connection to the product’s process window or measurement capability;
- no explanation of how reactive PUR samples are protected from moisture and controlled for heated time.
Frequently asked questions
What is ASTM D3236 hot melt viscosity?
It is the apparent viscosity of a molten hot melt adhesive measured with a controlled rotational viscometer procedure under ASTM D3236. The result is expressed in mPa·s and must be linked to the test temperature, instrument model, spindle, and rotational speed.
Is ASTM D3236 the same as a Brookfield viscosity test?
Not automatically. Brookfield identifies an instrument manufacturer and common equipment family. A laboratory can use suitable Brookfield equipment to run ASTM D3236, but a report that says only “Brookfield viscosity” does not confirm that the complete ASTM procedure and reporting requirements were followed.
What is the difference between viscosity and apparent viscosity?
Apparent viscosity is the viscosity measured under a defined spindle, speed, geometry, and temperature. The term is used because many hot melts are non-Newtonian and can give different values when those conditions change.
Are cP and mPa·s the same?
They are numerically equivalent for dynamic viscosity: 1 cP equals 1 mPa·s. A report should still state the unit so the document is unambiguous.
Can I compare a result at 160°C with one at 180°C?
No. Temperature has a strong effect on hot melt viscosity. Ask both suppliers to test at the same relevant temperature and under the same spindle and speed conditions.
Can an ASTM D3236 report use 180°C?
The stated scope of ASTM D3236-15(2021) extends to 175°C. A laboratory may have equipment that can test at 180°C or above, and the result may be useful for the application. The report should identify the higher-temperature condition as an adaptation rather than implying that instrument capability expands the method’s stated scope.
What torque range should the test use?
ASTM D3236 specifies readings within 10% to 95% of the torque scale and prefers a spindle and speed combination that places the result near the midpoint when possible. The report should include the actual torque or scale reading.
Is a lower hot melt viscosity always better?
No. Lower viscosity may support easier pumping, fine spray, or thin coating, but it can also cause excessive penetration, squeeze-out, dripping, or loss of bead height. The correct range depends on equipment, deposit geometry, substrate, timing, and final performance.
Does matching viscosity prove two adhesives are equivalent?
No. Similar viscosity does not prove equal open time, setting speed, adhesion, heat resistance, thermal stability, compliance, or final bond performance. Use viscosity to control melt flow consistency, then complete a production and end-use trial.
Should viscosity be listed on every COA?
It should appear when the buyer and supplier have agreed that viscosity is a batch-release property. The COA result must identify the lot and use the same approved test condition. If viscosity is not a routine COA item, the purchase agreement should state how often it is tested and when the buyer can request the record.
How should PUR hot melt viscosity be tested?
Use the agreed ASTM D3236 condition while controlling moisture exposure, package opening, sample transfer, melt time, and total heated time. A reactive PUR can change while hot if it contacts moisture or remains heated too long, so the report should record the timing and handling procedure.
How many temperatures should buyers request?
One agreed temperature can support routine lot control. For initial grade comparison, several points across the relevant application window give a more useful viscosity-temperature profile. Use measured points within the method scope or label adapted conditions clearly.
Approve the method before approving the number
An ASTM D3236 hot melt viscosity value becomes useful when the conditions behind it are fixed and traceable. Buyers should compare the same temperature, instrument configuration, spindle, speed, torque range, thermal procedure, and result type. They should also keep viscosity in its proper role: a control for molten flow behavior, not a substitute for line trials or bond-performance testing.
Before ordering bulk adhesive, place the approved method, viscosity range, COA requirement, retained-sample rule, and investigation procedure in the purchasing specification. That turns a single laboratory number into a repeatable supplier-control tool.
