TESTING OF ADHESIVES
Adhesives are tested by manufacturing people to :
1. Assist in selecting an adhesive for a particular use
2. Monitor the quality of an incoming product.
3. Confirm the effectiveness of the bonding process.
The adhesive selection process usually concerntrates on tests of joint strength. The goal is to determine which of several adhesives is the most suitable. The quality of incoming adhesives is primarily assessed by checking adhesvie properties such as viscosity, percent, solids, infrared spectrum, and bond strength. Destructive tests of joint strength are usually used to verify that the bonding process, for example, cleaning, mixing, curing, etc., has been correctly performed.
TYPES OF TESTS
A. General Requirements
Most adhesive tests of interest to manufactures can be considered tests of the adhesive material or tests of adhesive bonds. The vast majority of these tests are based upon procedures approved by the American Society of Testing and Materials (ASTM). An entire volume of their test specifications is devoted to adhesive testing (1). Whether ASTM tests or tests of your own design are used, it is imperative that the test conditions be rigorously specified in order to make comparative judgements based upon test results. Surface cleaning and other preparation, joint geometries, method and extent of mixing, method of application, fixtures utilized, and cure conditions are among the obvious variables that can effect joint strength. Repetitive tests must be carried out under identical conditions if ueful reults are to be obtained.
B. Material Tests
The most commonly used tests for properties of adhesive materials measure viscosity, shelf life, pot life, tack, cure rate, and percent solids.
1. Viscosity
Viscosity is defined as the resistance of a liquid material to flow. Adhesive viscosity is an indication of how easily pumped or spread the product will be. Viscosity may also reflect errors in compounding or excessive age of the material. Adhesives may be Newtonian, thixotropic, or nonflowable liquids. Viscosity measurements for free-flowing Newtonian or near-Newtonian adhesives are usually based on one of the following methods,
a. Volume flow cups. Volume flow cups measure the time required for a given volume of adhesive to flow through an orifice in the bottom of a metal cup containing an accurately known volume of material. Cup size and orifice diameter are chose so that the flow-out time is not more than approximately 60-90 sec. Results are reported as the time in seconds required for various volume flow cups
b. Rotating spindle method. The rotating spindle method, which is used for products which range in viscosity form 50 to 200,000 cP. measure sthe rotational resistance encountered by a metal spindle of a particular size spinning at a predetermined rate (1). The test is accurate and fairly rapid. It can be adapted to prouduction-floor conditions although it is most conveniently performed in a laboratory.
The Viscosity of thixotropic adhesives is usually determined by ASTM Standard D-2556 (1). Thixotropic materials exhibit a viscosity which is shear-rte dependent. the viscosity is determined at several different shear rates., usually with an electrically driven spindle, disk, T-bar, or coaxial cylinder rotated in the adhesive. A plot of apparent viscosity versus rotational speed is prepared, and from this plot the apparent viscosity associated with the particular rotation speed and spindle shape is obtained.
c. Extrusion. The viscosity of nonflowable products is often determined on the production floor by an extrusion test. A test of this type typically utilizes a Semco 440 nozzle attached to standard extrusion cartridge filled with the adhesive to be tested. Air at a pressure of 90-95 psi (630-665 kPa) is used to extrude a bead of the product. Viscosity is reported as the extrusion rate in grams per minute based on the average of three 10-sec extrusion trials.
2. Shelf life
The shelf life of an adhesive is usually determined by noting changes in viscosity or in bond strength which occur after periods of storage. The usual methods of determining viscosity or bond strength are used.
Time and temperature of storage, as well as the size and nature of storage containers should be agreed upon in advance by the adhesive user and supplier. There is no universally accepted change in viscosity or average bond strength which denotes a nonusable product. Any decision relating to this question should be based upon agreement reached by the supplier and user of the adhesive at the time the original adhesive selection is made.
3. Pot life
The pot life of an adhesive is that period of time over which it is actually usable. Pot life may very from minutes to hours and is clearly an important parameter. Methods of determining pot life, which, like the shelf life tests, are based upon changes in either viscosity or bond strength. Any accurate method of determining viscosity or of testing for bond strength may be used. Decisions concerning the rejection of a product because of changes in viscosity or bond strength should be based upon agreement reached between the adhesive supplier and user at the time the adhesive is originally selected. To simply say that a given percent change in viscosity makes the product unusable is not sensible. Considerable prior testing is usually required to make such a determination.
4. Tack
Tack is that characteristic of an adhesive which results in the easy sticking of an adhesive-coated surface to another surface upon contact. High-teck adhesives are those which are “sticky”. A number of tack measuring devices have been used. Many of them measure the force required to separate two surfaces, one of which has been coated with the adhesive, after they have been in contact over a specified area at a specified pressure for a specified time.
Another type of tack test measures the distance that a steel ball will roll over an adhesive coated surface after the ball has traveled down an inclined plane . The higher the tack, the shorter the roll will be.
5. Cure rate
It is often desirable to know how the strength of a bond will vary with the rate of cure. A recommended practice for determining the bond strength at intervals of two fifths, three fifths, four fifths, five fifths, and six fifths of the prescribed cure time. The actual bond strength may be determined with a standard ASTM tensile test. Results are commonly reported as a graph of average breaking load versus cure time.
6. Percent solids
The solids content of an adhesive should be checked to ensure that formulation or dilution errors have not been made. Solids are usually determined by weighting a sample of the adhesive and then heating or curing it until a constant weight is obtained. Percent solids is equal to the sample weight before heating or curing multiplied by 100 and divided by the sample weight after heating or curing.
C. Joint Tests
A numbers of tests of finished bonds have proved to be of value in checking the assembly process. If bond strength suddenly decline using good ahesives, it normally indicate that something has gone awry with the surface preparation or the assembly process. Not all of the following tests need to be utilized in every situation. Lap shear and impact tests are probably the most commonly used, but the others might be chosen if they more accurately reflect the most critical stresses to which a particular product might be exposed in service.
1. Lap shear
Lap shear testing is very widely used to determine adhesive bond strength. The test which utilizes specimen is easy to carry out and is, therefore, widely used. Lap shear generates peel or cleavage sources at the ends of the bond and is, thus, a stringent measure of bond strength. Test results are reported in failing load per unit area.
2. Tensile tests
A number of tests, of which the ASTM D-897 standard is the most common, have been devised for determining the tensile strength of an adhesive bond. In all cases the specimen is loaded in nominally pure tension. Test results will be significantly lower if cleavage forces are allowed to develop in the loaded samples.
3. Peel
Peel tests measure the force required to peel a flexible substrate away from rigid or flexible material. The climbing drum peel test, which is widely used in the aircraft industry to determine bond strengths involving honeycomb sandwich structures. In all cases, it is important to specify the peel rate. Peel strength are reported in load per unit bond width.
4. Cleavage
Adhesive bonds are usually tested for resistance to cleavage failure. The bonding surfaces are 1 x 1 in. and the test requires that both adherends be rigid. The specimen is usually loaded at 500-800 lb./min. The results are reported in breaking load per unit area.
5. Impact
Impact testing measures the force required to break an adhesive bond when one adherend is rigidly held and the other is impacted, typically be a pendulum. It is often difficult to achieve reproducible results with impact testing and, as a result, the test is not widely used in production situations.
6. Fatigue
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