Translating Performance Testing to Field Performance in Conduit Adhesives

How does laboratory testing predict field performance?

Performance is directly related to product installation. Field installation adds complexity such as large temperature ranges, tight application space, short down-time window, and a fast cure time expectation. Performance testing must directly reflect the application parameters. Testing is built from the standardized test to reflect the field materials, use configurations, application constraints, and environmental conditions. Tests may be modified and new test methods developed to create standard performance criteria. These criteria can be used to improve and formulate better products.

The identity of a conduit adhesive is built through a series of tests. These tests are used first to develop an optimum product and then to quantify its performance. The first step is to define the field requirements:

1. Field application:

Packaging and kitting can simplify the adhesion process and minimize errors. Fewer measuring and mixing steps results in an efficient job with a high rate of success. Side-by-side or coaxial cartridges fitted with static mixers allow fast and clean application.

2. Adhesion to dissimilar materials:

Conduit adhesive is often used at material transition points. While laying conduit, HDPE (high density polyethylene) duct is often bonded to PVC (polyvinyl chloride) couplings. Other material transition points may involve steel, concrete or fiberglass bonded with, or in place of, these materials. A good conduit adhesive must have excellent adhesion to all these materials, regardless of the pairing.

3. Appropriate cure time:

Working time must be long enough to allow the assembly of parts, but cure time should be fast enough to get to the next step of the job in a timely fashion.

4. Extreme environmental and temperature performance:

While in-ground joints will experience limited extreme temperature swings, some joints are above ground. These joints can be exposed to very low and very high temperatures depending on the weather. In specialty applications, some joints may experience chemical exposure.

5. Pullout strength:

Once bonded, joints need to have the strength to stay together. The strength requirement depends on the specific size, geometry, and function of the joint.

Standardized Tests Are a Good Starting Point

Standardized testing organizations such as ASTM, ANSI, IEC, ISO, and ITU create test methods and technical standards by committees comprised of experts in the field. The ongoing development of standards is a rigorous process. These tests are a good place to start when designing a testing protocol. As an example, ASTM International has developed over 12,000 standards over 125 years. Membership is represented by over 140 countries.

Measuring Shear Strength and Adhesion Properties

ASTM D1002 is a test procedure for testing the shear strength of adhesives used to bond metals. This is often called “lap shear” and is easily adapted to examine a variety of adhesive requirements. In this test, two overlapping pieces of sheet material are bonded with adhesive. The two exposed tails are pulled apart with a measured tension. This tensile force is the shear stress on the bond.

The lap shear test isolates the interaction of the adhesive with the substrate sheet material. The geometry of the shearing force tends to minimize any distortion of the substrates that may skew the adhesion results.

While the ASTM standard describes the use of metal substrates in this test, substituting other, rigid substrates is a simple matter. The adhesive bond strength to common conduit and coupling substrates can be measured using this substitution. The test may be performed with a common material on both sides of the bond, or with dissimilar materials as is common in field situations. Results from this test may vary from thousands of pounds force for some adhesive/substrate combinations (particularly the metals) to hundreds of pounds force or less for other combinations (particularly plastics).

This straightforward force measurement can then be used as a control, or baseline, for repeated tests under different conditions.

Using Strength Tests to Determine Cure Rate

Lap shear tests may be useful to measure bond strength along a timeline to determine cure rate:

In this case, lap shears are performed at specific time intervals to illustrate progression towards maximum bond strength, which was determined with the original control samples. The example shows an adhesive that reaches 90% of its maximum strength within 70 minutes of bonding.

Retention of Shear Strength Can Be Used to Measure Chemical Resistance

Lap shears may be used to measure loss of bond strength after exposure to chemical reagents:

In this study, lap shear samples were submerged in a variety of chemical reagents for 28 days and then tested in comparison to the control samples.

Shear Strength Tests Can Measure Environmental Performance

In many cases, such as meeting UL requirements, lap shears can determine any change in bond strength following exposure to temperature and humidity extremes:

For heat aging, samples were conditioned for 1,000 hours at 104ºC. High humidity samples were conditioned at 60ºC, 95-100% RH for 7 days. Thermal cycling consists of 3 cycles of:
– 24 hours of water submersion at 25ºC
– 24 hours of conditioning at 60ºC
– 96 hours of conditioning at 35ºC and 90% relative humidity
– 8 hours of conditioning at -35ºC

Connecting Laboratory Testing to Pullout Performance

In theory, the bonding area of any size joint may be measured, and lap shear values are used to predict pull out strength for that joint. However, the geometry of the conduit to coupling bond is more complicated than that of a lap shear. The inner diameter of a coupling is tapered, narrowing to the center of the coupling. This affects the film thickness of the adhesive used. The taper is different for each coupling size, so the film thickness effect can vary for each situation. With larger conduit sizes, such as those 6 inches (150 mm) or larger, difficulty in fully seating the conduit into the coupling is common. At times, seating the pieces requires the use of a rubber mallet.

Pullout strengths are better measured directly. Photo 1 is an image of an experimental set up:

The following graph shows typical pullout strengths (both actual and projected from lap shears) for various sized HDPE conduits bonded to PVC couplings with Polywater® BonDuit®:

Pullout strength varies from a few hundred pounds force for small conduits to thousands of pounds force for the larger conduits. In every case, pullout force is maximized by proper preparation of the conduit and coupling, including abrading the bonding surfaces and cleaning with solvent wipes.

Concluding thoughts…What else can be tested?

Laboratory testing should reflect field use. A good conduit adhesive should adhere to all types of conduit materials, be easy to apply, have an appropriate cure time, withstand varied environmental conditions, and have excellent pullout strength. What else can be tested? It is possible to test different application conditions, surface preparation, and packaging changes.

Standard tests, as well as modifications on these tests, are a good way to develop products and understand performance. They can establish the quality and efficacy of a product. It is also advantageous to develop new test methods that better describe the actual use of a product, especially for innovative designs and brand-new product concepts. This rigorous testing creates product consistency and in turn, user trust.

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