Лако-красочные материалы — производство

Since most silicone adhesives and sealants are elastomeric in nature, their physical prop­erty testing often parallels classical rubber testing approaches. Common tests include durometer, tensile strength, elongation, and modulus. Several methods are available for the measurement of rubber properties, but the most commonly used are the American Society for Testing and Materials (ASTM) D-412, Test Method for Rubber Properties in Tension, and the ASTM C-661, Standard Test for Indentation Hardness of Elastomeric — Type Sealants by Means of a Durometer. These properties vary widely with the product and its intended application. Durometer measurements can range from Shore A less than 20 to over 50. Tensile strength ranges from less than 0.2 to greater than 5 MPa, and elongation varies from about 100 to 2000%.

Cure time testing tends to be somewhat subjective, but again there are methods available, such as ASTM C-679, Tack-Free Time of Elastomeric Sealants. Tack-free time is the curing time required for the product to develop a skin that is not damaged when subjected to application and removal of a plastic film. It is important in all cases to determine the cure characteristics of the product in actual working conditions. Since most silicones cure by reaction with moisture in the air, the sensitivity of cure time to humidity should be determined. Surface cure rates can usually be tailored to meet application requirements.

The rheological properties of adhesives and sealants are important in many applica­tions. When these products must be pumped or applied through automated equipment, the flow characteristics at pertinent shear rates are critical. Sophisticated rheological measure­ments can be performed to predict performance. The rheology of silicone adhesives and sealants can be tailored through adjustment of polymer viscosity, filler loading, and incorporation of various additives.

Often, only the extrusion rates of adhesives and sealants are measured, which is accomplished by subjecting the product to a given pressure and measuring its flow rate through a nozzle of known diameter (see ASTM C-1183, Extrusion Rate of Elastomeric Sealants). For many sealant applications, the sealant must not flow under its own weight in conditions of low shear rate. In this case, some measurement of ‘‘slump’’ is generally made. Several methods are available for measuring slump [see ASTM D-2202, Standard Test for Slump of Sealants, and ASTM C-639, Standard Test for Rheological (Flow) Properties of Elastomeric Sealants]. Again, it is important to determine the rheological performance of the product in the actual application.

Adhesion testing is a matter of some controversy. There is, however, a growing trend among manufacturers, specifiers, and standards organizations to move toward tests that better predict performance in application. The 180° peel adhesion test is often used as an internal quality control tool by manufacturers. This test allows for measurement of lot-to — lot consistency of products. The methodology for this test can be found in ASTM C-794, Test Method for Adhesion In-Peel of Elastomeric Joint Sealants. In its typical form this test involves placing a bead of the product onto the substrate, with a flexible member embedded in the product. The product is allowed to cure, and the member is then pulled away from the substrate. The force required for peel is noted, along with the mode of failure. The advantages of this test are that it is fast and lends itself well to subjecting the adhesive bond to environmental stresses such as hot-water exposure.

A series of tests that are better predictors of performance of sealants involves the preparation of tensile-adhesion joints or H pieces. In this case, the sealant bead is placed between blocks of the two substrates of interest and allowed to cure. This joint can then be pulled to destruction, with measurement of strength and failure mode. In addition, various environmental stresses may be applied, such as UV light exposure (weatherometry), water exposure, and cyclic movement. Testing in this way allows for improved prediction of movement capability and long-term performance. The methodology involved in this test­ing is included in the following methods: ASTM C-1135, Determining Tensile Adhesion Properties of Structural Sealants, and ASTM C-719, Test for Adhesion and Cohesion of Elastomeric Joint Sealants Under Cyclic Movement.

Any sealant or adhesive that is expected to perform in outdoor applications should be tested after exposure to light, heat, and water. For most forms of weatherometry, 500 to 1000 h is considered approximately the equivalent of 1 year outside in most climates (United States). If a minimum of 5 years of service is expected from the sealant, no less than 2500 h, and probably 5000 h in a UV fluorescent accelerated weathering machine should be used (as the conditioning cycle for rubber property testing). This is true for silicones and all other sealants that are expected to perform in such applications. This is a startling contrast to the 250 to 500 h used in most present standards (see Refs. 6 and 7).