Rubber facedTJ-34
25mm Square Vice Action Grip
A 25 mm square vice grip matches the usual bead width, holding the mesh-reinforced strip without letting it draw.
SpecificationsTesting standard
ASTM C794
Standard Test Method for Adhesion-in-Peel of Elastomeric Joint Sealants
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM C794 measures the peel adhesion of an elastomeric joint sealant to a substrate. A bead of sealant with wire mesh embedded in it is cured on the substrate, then peeled back at 180° at 50 mm/min. What matters most is not the force but the failure mode — whether the sealant let go of the substrate or tore within itself.
At a glance
- Test type
- Peel & adhesion — a bonded joint is pulled apart
- Published by
- ASTM
- Edition
- C794-18
- Material
- Adhesives, tapes & bonded joints
- Runs on
- Series 7200 and Series 9000
What the test does
A substrate panel is prepared exactly as the sealant manufacturer specifies — cleaned, and primed with the flash-off time the primer requires. A bead of sealant is applied to it, commonly 25 mm wide, with a wire mesh embedded in the sealant so that the strip can later be pulled without simply stretching. The assembly is cured fully. A short length is then freed from the substrate, the panel is clamped in one grip and the mesh-reinforced strip in the other, and the strip is peeled back on itself at 180° at 50 ± 5 mm/min. Force is recorded, and the peeled surfaces are examined afterwards.
What it measures, and why it matters
The method reports peel adhesion as force per unit width, and the proportion of the peeled area that failed at the substrate interface rather than within the sealant. The second of those is the real result. A sealed joint works when the sealant is bonded more strongly to the substrate than it is to itself, so that any overload tears the sealant rather than opening the bond — cohesive failure is the desired outcome, adhesive failure is not, and the recorded force cannot distinguish them. Because the method also covers water immersion and other exposures, it is the practical way to find out whether a bond that works dry survives the wet.
Specimen preparation
The specimen is made rather than cut, and almost everything that decides the result happens before any load is applied.
- Substrate
- The actual material the sealant will bond toGlass, aluminium, concrete and coated metal all behave differently. A result on one substrate says nothing about another.
- Surface preparation
- Exactly as the sealant manufacturer specifiesCleaning solvent, primer and flash-off time are the largest levers in the whole test.
- Wire mesh
- Embedded in the sealant beadIt reinforces the sealant so the strip can be pulled without simply stretching, which is what makes a peel possible at all.
- Bead width
- Commonly 25 mm
- Cure
- Full, at the specified conditionsSealants cure slowly and from the surface inward. Testing early gives a low result that says nothing about the product.
- Conditioning
- Standard, plus water immersion or other exposure as specifiedWater immersion is where most adhesion failures actually appear.
- Prepare a control set alongside every exposure set
- From the same batch and dayDakAdhesion is compared before and after exposure, and a control from a different batch makes the comparison meaningless.
The failure mode is the result. Cohesive failure — the sealant tearing within itself — means the bond is stronger than the sealant and is the desired outcome. Adhesive failure at the substrate means the bond is the weak link, whatever the recorded force.
Test speed
- Crosshead speed
- 50 ± 5 mm/min
- Peel angle
- 180°, the strip pulled back on itself
- Averaging
- Over a steady length after the start transient
- Record the trace shape
- Not only the averageDakPeel force that falls steadily along the specimen indicates a bond that varies — often a primer applied unevenly.
Calculations
Peel adhesion—
Peel adhesion = average peel force / bead width
- average peel force
- mean force over the steady peel region, N
- bead width
- width of the bonded sealant bead, mm
Reported as force per unit width. The average must come from the steady region, since the start of any peel is a transient.
Percentage adhesive failure—
Proportion of the peeled area that failed at the substrate interface
Estimated by inspection and reported alongside the force. A high peel force with substantial adhesive failure is a worse result than a lower force with none.
How the test runs
- 01Prepare substrate panels exactly as the sealant manufacturer specifies.
- 02Apply primer where required and allow the specified flash-off.
- 03Apply the sealant bead and embed the wire mesh in it.
- 04Cure fully at the specified conditions.
- 05Prepare a control set and an exposure set from the same batch.
- 06Apply water immersion or other exposure to the exposure set.
- 07Cut the bead to width and free a short starting length from the substrate.
- 08Clamp the substrate in one grip and the mesh-reinforced strip in the other.
- 09Peel at 180° at 50 mm/min, recording force.
- 10Average over the steady region.
- 11Inspect the peeled surfaces and estimate the percentage adhesive failure.
- 12Report force and failure mode together, for both control and exposed sets.
Grips and fixtures for this method
Standard 25 mm
Pneumatic Vice Action Grip
Pneumatic actuation gives a constant clamping force on a soft, tacky strip that a screw grip would keep bedding into during the peel.
SpecificationsWhat the report has to contain
- Reference to ASTM C794 and the edition
- Sealant identification, batch and cure schedule
- Substrate material and its surface preparation
- Primer used, and the flash-off time allowed
- Bead width and wire mesh details
- Conditioning, and any exposure applied to the exposure set
- Crosshead speed
- Average peel force and peel adhesion per unit width
- Percentage adhesive, cohesive and substrate failure
- Results for both control and exposed sets
- Mean and standard deviation
What the machine must be capable of
Very small forces measured well: peel forces are commonly between ten and a couple of hundred newtons over a 25 mm bead, so a load cell sized for structural work will report them as noise. Accuracy over that low range matters more than capacity. The crosshead must hold 50 mm/min steadily and have enough travel to peel a useful length after discarding the start transient. Grips need to hold a soft, tacky, mesh-reinforced strip without letting it draw, which favours a constant-force pneumatic grip over a screw-tightened one that keeps bedding in as the sealant relaxes under the jaws.
What goes wrong in practice
Testing before the sealant has fully cured gives a low result that describes the cure state rather than the product, and it is the commonest laboratory error. Failing to record the primer and its flash-off time makes a result unreproducible, and since primer is so often the difference between passing and failing, it is the first thing anyone investigating a failure will ask for. Reporting a peel force without the failure mode loses the most important part of the answer. And comparing an exposed set against a control from a different batch or a different day introduces a difference that has nothing to do with the exposure.
ASTM C794 or ASTM C719
| ASTM C794 | ASTM C719 | |
|---|---|---|
| Loading | Peel, at 180° | Cyclic extension and compression of a joint |
| Specimen | Bead on a substrate with wire mesh | A simulated joint between two substrates |
| Answers | Does the sealant stick, before and after exposure | Does the sealant survive joint movement |
| Duration | Short | Weeks, with cycling |
These test different things and both are normally required. C794 finds an adhesion problem quickly and cheaply; C719 finds whether the sealant can accommodate the movement the joint will actually see. A sealant can pass one and fail the other.
Questions we are asked about this test
What is ASTM C794?
It is the ASTM method for adhesion-in-peel of elastomeric joint sealants. A sealant bead with wire mesh embedded in it is cured on a substrate, then peeled back on itself at 180° at 50 mm/min. The method reports the peel force per unit width and, critically, the proportion of the peeled area that failed at the substrate rather than within the sealant.
Why is wire mesh embedded in the sealant?
Because an elastomeric sealant on its own would simply stretch when pulled rather than peel. The mesh reinforces the strip so that the load is delivered to the peel front instead of being absorbed by the sealant extending, which is what makes a controlled peel possible. Without it there would be no consistent peel line and no meaningful force to record.
Why does the failure mode matter more than the force?
Because they answer different questions. Cohesive failure — the sealant tearing within itself — means the bond to the substrate is stronger than the sealant, which is exactly what a sealed joint needs. Adhesive failure at the interface means the bond is the weak link and the joint will eventually open there, regardless of how high the recorded force was. A high force with substantial adhesive failure is a worse result than a lower force with none.
Why is water immersion part of the test?
Because that is where sealant adhesion actually fails. Many sealant-substrate combinations bond perfectly well when dry and lose adhesion after prolonged wetting, as water reaches the interface and displaces the bond. A joint in a building façade is wet for much of its life, so the dry result alone is not predictive. Comparing a control set with a water-immersed set from the same batch is the core of the method.
How important is the primer?
It is frequently the difference between passing and failing, and it is the most common cause of a disputed result. Primer type, coverage and flash-off time all matter, and a primer applied too thinly, too thickly or overcoated before it has flashed off will not perform. The report has to record what was used and how long it was left, because a C794 result without that information cannot be reproduced or acted on.
Does a result on aluminium apply to glass or concrete?
No. Adhesion is a property of the sealant-substrate pair, not of the sealant alone, and the differences between substrates are large. A sealant that bonds excellently to anodised aluminium may need a primer on concrete and a different one on coated steel. Each substrate in a project needs its own test, which is why sealant manufacturers publish adhesion data as a matrix rather than a single figure.
Is peel adhesion enough to qualify a sealant for a joint?
No. C794 answers whether the sealant sticks, quickly and cheaply, and it is the right first test. It does not answer whether the sealant can accommodate the movement the joint will see, which is what ASTM C719 addresses over weeks of cyclic extension and compression. Both are normally required, and a sealant can pass one and fail the other.
Running ASTM C794 on the Series 7200 and Series 9000
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | Very low — commonly 10 N to 200 N over a 25 mm bead width | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ASTM E4 over the working range | Verified to ASTM E4, and to ISO 7500-1 Class 0.5 |
| Gripping | Tensile grips holding the substrate and the embedded wire mesh, arranged for a 180° peel | Our peel and adhesion fixtures, built to the specimen |
| Environment | 23 ± 2 °C and 50 ± 5 % RH for standard conditioning; water immersion and other exposures are part of the method | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.