Testing of Adhesives

The testing of adhesives comprises two aspects:

1) The adhesive as an individual compound

2) The performance in the actual adhesive joint that is formed between substrates Generally the following adhesive properties are determined:

— Solids content

— Rheological properties

— Shelf life or storage temperature range

— Mechanical properties: tensile strength, modulus, Shore hardness, and elongation to break

— Glass transition temperature

— Softening point

— Flammability and flash point

— Health and environmental classification

More important, however, is the performance of the adhesive in an actual joint. Standard test specimens are bonded under standard conditions and subjected to destructive tests. The load values of these tests give information on the adhesive strength between two bonded substrates. Standard destructive test methods are:

— Tensile test (EN 26922)

— Flatwise tensile shear test (EN 1465)

— Peel test (EN 1464/ISO 4578 and EN ISO 28510-1,2)

Standard destructive tests are, above all, tensile, shear, and peel tests on specially made, strip-form test specimens (Fig. 10). In general, short-overlap test strips are used for tensile shear tests and flat-bonded test strips for peel tests. Bending and impact tests occasionally are carried out on these or similar test specimen geometries. Apart from the adhesive itself, the material of the test specimen, surface treatment of the specimen before bonding (sandblasting, pickling, priming), dimensions, overlap surface area, the test speed, the test temperature, and the direction in which the force is applied, all affect the outcome of these tests. Accordingly, the nature and dimensions of the test specimens, the overlap length, the peeling angle, and also the test conditions are laid down in test standards and other guidelines. The test standards [DIN, EN, ASTM, ISO, BS, PSTC (Pressure Sensitive Tape Council)] are designed to give comparable results. Shear strength and tensile strength are expressed in N/mm2 (strain/joint area), peel strength in N/mm (strain/specimen width). In general, adhesive joints are more resistant to shear and tensile forces than to peel and bending forces.

A distinction can be drawn between:

— Static tests to failure under an increasing load

— Static long-term strain test below the breaking strain (creep)

— Dynamic tests with different loads and frequencies


The static test can be carried out with standard tensile testers that cover the appropriate force range, optionally with a data recorder to obtain a force plot. Dynamic tests require special vibrating devices with constant loads. After the destructive test further information can be gathered from the test specimen. The nature of the break is an important indication of the quality of a bond line. There are three types of failure:

— Cohesive failure: break in the bond line

— Adhesion failure: break between substrate surface and adhesive (poor adhesion)

— Substrate failure, break in the substrate, the bond line is stronger than the substrate (structural bonding)

A combination of the above breakage patterns is possible. This is then indicated by a percentage figure (e. g., 60 % cohesive, 40 % adhesive failure). The above tests are also performed with exposure of the test specimens to extreme environments. Bonds are preferably tested for their resistance to:

— Natural weathering

— Climate variations (cycling)

— Heat aging

— Humidity aging

— Corrosive atmospheres

— Salt spray mist (automotive)

— Underwater storage

— Submersion in various chemicals such as petroleum, acids, and alkalis

The test conditions are generally much more severe than required for the actual final use of the bonded species. However these exaggerated test conditions deliver valuable information on the durability of the bonded structure in real life conditions. A good understanding has been developed for forecasting the bond line dependability on the basis of the results of aging tests.

The testing of specimens bonded under laboratory conditions is used to compare the performance of adhesives and substrates or the combination thereof, especially in quality-control laboratories. Under real-life conditions laboratory results may not be
applicable. Surfaces of substrates, their geometry, curing temperatures, humidity, and many other factors may have an impact on the final bond quality. Therefore it is very important to perform a bond test in the final application even when the laboratory results were satisfactory. In many cases this is the only way to obtain information on the reliability of the bonded construction.

Testing of Anaerobic Adhesives. Testing standards and performance specifications for anaerobic adhesives and sealants have been established by government agencies and industrial organizations in several countries. In the United States there are military specifications for thread lockers, sealants, and retaining compounds. Many of these specifications are being superseded by commercial standards.

Mil-S-22473E, 12 April 1983 “Sealing, Locking and Retaining Compounds: (Single Component)” covers 15 of the earliest “Letter Grade” products. Specifications are set for color, viscosity, locking torque on 3/8-24 steel nuts and bolts and fluid tightness. The effects of immersion in a number of fluids, heat aging at 149 °С and hot strength at 149 °С (or 93 °С for some grades) are also measured. This specification calls for measurement of an “average locking torque” after 90, 180, 270 and 360° of turn.

MU-S-46163A, 12 July 1983 “Sealing, Lubricating, and Wicking Compounds: Thread-Locking, Anaerobic, Single-Component” covers nine grades of product for sealing (Type I), lubricating (Type II), and wicking (Type III). Specifications are set for color, viscosity, locking torque (break and prevailing torque) on 3/8-16 steel, zinc — and cadmium-plated nuts and bolts, fluid tightness, lubricity, and “wicking” into preassembled fasteners. The immersion, heat aging, and hot strength tests are similar to those specified in Mil-S-22473E.

Mil-R-46082B, 10 June 1983 “Retaining Compounds Single Component, Anaerobic” (Amendment 6, 9 January 1990) covers three types of retaining compounds which are tested with a pin and collar compressive shear specimen. The three types vary primarily in viscosity although there are also some differences in heat resistance and strength. These products are subjected to immersion, heat aging and hot strength tests similar to those described above.

In Britain the Ministry of Defence specification DTD 5628-5633 covers test procedures and performance requirements for a range of products. Five strength bands and four viscosities from penetrating to thixotropic are defined. The torque strengths, including breakloose torque, are tested on M8 nuts and bolts and the shear strength in 12 mm pins and collars [201].

For an improved method of detecting the breakloose torque in a threaded component bonded with an anaerobic thread locker/sealer, see [202].

A British Standard BS 5292 has been prepared which relates to the use of anaerobic sealants on gas appliances.

In Germany standards have been published describing the “Compression Shear Test” (DIN 54452), “Dynamic Viscosity Determination of Anaerobic Adhesives by Rotational Viscometer” (DIN 54453), “Initial Breakaway Test at Bonded Threads” (DIN 54455) and “Torsion Shear Test” (DIN 54455).

In the United States the Industrial Fastener Institute has published standards for “Test Procedure for Locking Ability Performance of Non-metallic Locking Element Type Prevailing Torque Lock Screws” and “Test Procedure for the Locking Ability Performance of Chemical Coated Lock Screws.” ASTM has published a “Standard Test Method for Shear Strength of Adhesives Using Pin-and-Collar Specimen” (ASTM D 4562-90, October 1990.)

Подпись: AdhesivesISO 10964 (August 15, 1993) “Adhesives — Anaerobic adhesives— Determination of torque strength of anaerobic adhesives on threaded fasteners” describes testing procedures for liquid and preapplied sealants.

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