Лако-красочные материалы — производство

Adhesives are often used in applications where they are exposed to continuous or inter­mittent loads over long periods. It is difficult to duplicate such conditions in the labora­tory. Neither is adhesive testing and/or observation under actual service conditions a very feasible alternative. The designer is not usually able or willing to await the results of years of testing before using the adhesive, and to tie up testing equipment and space for such long periods would be prohibitively expensive. There are, however, companies, universi­ties, and other industry groups that have loading racks or other test systems where samples are exposed to dead weight or other loadings while exposed to ‘‘natural-weathering’’ conditions. It is advantageous, however, to have these backed up with (and an attempt made to relate them to) accelerated tests. These accelerated tests are generally experiments in which extreme conditions are used to increase the rate of degradation and deterioration of the adhesive joint. Although it is seldom possible to establish a one-to-one correlation between the rate of deterioration in the accelerated test and actual weather-aging condi­tions, it is hoped that the short-term tests will, at the very least, provide a relative ranking of adhesive-adherend pairs, surface preparation, bonding conditions, and so on, and/or provide some insight into relative expected lifetimes. As with all tests, the tester/designer should use all of his or her knowledge, common sense, and insight in interpreting the data.

Some accelerated tests are surprisingly simple and intended to give only highly qualitative information, while others have been formulated into standard tests intended to yield more quantitative results. Since heat and moisture, to which adhesive joints are commonly exposed, are environmental factors known to greatly influence adhesive dur­ability, most accelerated tests involve these two agents.

As an example of a simple qualitative test we would like to cite a test devised by the late E. Plueddemann of Dow Corning [28]. Plueddemann was perhaps the world’s fore­most researcher in the area of silane coupling agents, bifunctional compounds with one end of the molecule designed to react with oxygen or similar molecules on the substrate (e. g., an oxide layer on a metal) and the other end designed to react with the polymer in the adhesive [29-31]. In this way, a covalent ‘‘bridge’’ is developed between the adherend and the adhesive. One of the main goals of these treatments is to reduce moisture deterioration of the bond line. Accordingly, Plueddemann had need for a test to access quickly this aspect of the wide variety of silanes produced and differing substrates. He devised the following simple test for this purpose.

In his test, a thin film of adhesive on a glass microscope slide or a metal coupon is cured and soaked in hot water until the film can be loosened with a razor blade. There is usually a sharp transition between samples that exhibited cohesive failure in the polymer and those which exhibited more of an interfacial failure. Since the diffusion of water into the interface is very rapid in this test, the time to failure is dependent only on interfacial properties and may differ dramatically between unmodified epoxy bonds and epoxy bonds primed with an appropriate silane coupling agent. The time to debond in the hot water for various silane primers differed by several thousandfold when used with a given epoxy. In parallel tests, a thick film of epoxy adhesive on nonsilaned aluminum coupon showed about the same degree of failure after 2h in 70°C water as a silaned joint exhibited after more than 150 days (3600 h) under the same conditions.

The authors have, several times, heard Plueddemann express the opinion that he would be willing to guarantee that an adherend-silane-adhesive system that could with­stand a few months of exposure to the conditions of his accelerated test would last many decades under normal outside exposure conditions. He was quick to point out that this guarantee does not cover other types of deteriorations of the adherends or adhesive (e. g., corrosion or polymer degradation) and that because of his age and health, he would not be around to honor the guarantee. Nevertheless, he was very convinced (and convincing) that his test was an ‘‘acid test’’ much more severe than most practical adhesive joints would ever experience in their lifetime.

Perhaps, the best known test of this type is the Boeing wedge test, a form of which is standardized in ASTM D-3762. Figure 9 shows this type of specimen and a typical plot of results reported by McMillan, and his associates at Boeing [32,33]. Theidman et al. [34] have also used the-wedge to investigate coupling agents.

Since adhesives have long been used in the wood/lumber business, where outdoor exposure is inevitable, many of the standard accelerated tests were originally developed for these materials. Such tests are increasingly finding uses for other materials. The most common accelerated aging tests are:

1. ASTM D-1101, Standard Test Method for Integrity of Glue Joints in Structural Laminated Wood Products for Exterior Use. Two methods are outlined in this standard for using an autoclave vessel to expose the joint alternately to water at

vacuum pressure (ca. 635mmHg) and low temperature with a high-pressure stage (ca. 520 kPa), followed by a high-temperature drying stage (ca. 65°C cir­culated dry air). After the prescribed number of cycles (typically one or two), the samples are visually inspected for signs of delamination.

2. ASTM D-1183, Standard Test Methods for Resistance of Adhesives to Cyclic Laboratory Aging Conditions. This standard describes several different test procedures in which the joints of interest are subjected to cycles made up of stages at different relative humidities and temperatures, high-temperature drying cycles, and/or immersed in water for specified periods. The joints are then eval­uated by standard strength tests (lap joint, tensile, or other) to ascertain the extent of degradation in strength.

3. ASTM D-2559, Specifications for Adhesives for Structural Laminated Wood Products for Use Under Exterior (Wet Use) Exposure Conditions. Like ASTM D-1101, this test makes use of an autoclave-vacuum chamber to impreg­nate specimens with water followed by drying in a hot (65°C) air-circulating oven. A more quantitative measure of degradation is obtained in this test by measuring lap shear compressive strength and measuring deformation as well as visual evaluation to determine the extent of delamination.

4. ASTM D-3434, Standard Test Method for Multiple-Cycle Accelerated Aging Test (Automatic Boil Test) for Exterior Wet Use Wood Adhesives. This stan­dard describes the construction of apparatus to expose adhesive joints automa­tically to alternate boil/dry cycles. A typical cycle is composed of (a) submerging the specimen for 10 min in boiling water, (b) drying for 4min with 23°C air circulating at 1.75m/s, and (c) exposing the specimen for 57min to 107°C air circulating at 1.75m/s. At a prescribed number of cycles, 10 specimens are with­drawn and their tensile shear strength measured and compared to tests on sam­ples that have not been exposed to the accelerated testing conditions.

5. ASTM D-3632, Standard Practice for Accelerated Aging of Adhesive Joints by the Oxygen-Pressure Method. This test is intended to explore degradation in elastomer based and other adhesives that may be susceptible to oxygen degrada­tion. The practice involves subjecting specimens to controlled aging environ­ments for specified times and then measuring physical properties (shear or tensile strength or other). The controlled environment consists of elevated tem­perature (70°C) and high-pressure (2 MPa) oxygen.

6. ASTM D-4502, Standard Test Method for Heat and Moisture Resistance of Wood-Adhesive Joints. Rather than using an expensive autoclave as in D-1101 and D-2559, moisture aging in this test is accomplished in moist aging jars. The samples are exposed to prescribed temperature-humidity cycles in the jars heated in ovens. The strength of the aged samples is measured by standard methods and compared to similar virgin samples.

There are tests that have been developed for use with solid polymer specimens that find some use with adhesives. Ultraviolet (UV) radiation (e. g., as present in sunlight) is known often to have detrimental effects on polymers. Accordingly, a popular accelerated weathering (aging) test, ASTM G-53, ‘‘Standard Practice for Operating Light — and Water — Exposure Apparatus (Fluorescent UV-Condensation Type) for Exposure of Nonmetallic Materials,’’ describes use of a ‘‘weatherometer’’ that incorporates UV radiation moisture and heat. These commercially available devices consist of a cabinet in which samples are mounted on aluminum panels, which in turn are stacked edgewise on a sloped rack along either side of the cabinet. These samples are then alternately exposed to two stages in periodic cycles: a condensation stage followed by a UV-drying stage. The first stage is accomplished by heating water in a partially covered tank below the specimens in the bottom of the cabinet. The specimens are maintained at a constant temperature (typically 30 to 50°C) which is lower than the water temperature. This results in moisture condensa­tion on the specimens. This stage might last for 1 to 4 h as selected by the operator. This is followed by the UV-drying stage. An array of special fluorescent light tubes are situated along each side of the cabinet parallel to the rack-mounted samples. The operator selects a temperature higher than the condensation temperature (typically 40 to 70°C) that the system automatically maintains in the cabinet for a fixed period (usually 1 to 40 h) while the specimens are exposed to the UV radiation. The weatherometer is equipped with a timer, a float-controlled water supply for the tank, and other controls, so that it can continuously cycle through this two-stage cycle for months or even years with mini­mum operator care. Samples are removed periodically and their strength measured by standard techniques for comparison with virgin samples and aged and virgin samples of other materials. Many adherends are opaque to UV light, and hence one might question the use of this weatherometer to explore weathering aging of adhesives used with such adherends. However, even here, having such a commercial automated device might be very useful. Both the condensation and radiation-heat curing stages are very analogous to environmental exposures experienced by practical joints. This, along with the automated and ‘‘standard’’ nature of the equipment, often makes the technique quite attractive. More important, there are problems where the UV part of the aging may be critically important. An example would be the bonding of a thin transparent cover film to a thin sheet containing the reflective elements in sheeting used to make reflecting road signs. This device would have obvious advantages in such cases and, indeed, has become the standard for use in evaluating weather durability in that industry.

It is recognized that most accelerated tests do not duplicate or even closely approx­imate actual service conditions. As a case in point, most joints will, in all probability, never be exposed to boiling water. It is hoped, however, that resistance to boiling for a few hours or days may provide some valid evidence (or at least insight) into the durability of a laminated part after years of exposure to high ambient humidity and temperature. While such accelerated tests are never perfect, they may be the only alternative to obser­ving a part in actual service for decades. The authors feel this philosophy is well stated in Section 1.1 of ASTM D-1183: ‘‘It is recognized that no accelerated procedure for degrad­ing materials correlates perfectly with actual service conditions, and that no single or small group of laboratory test conditions will simulate all actual service conditions. Consequently, care must be exercised in the interpretation and use of data obtained in this test.’’ ASTM D-3434 includes the following statement about its significance: ‘‘The test method assumes that boil/dry cycling is an adequate and useful accelerated aging techni­que.’’ Evaluation of long-term durability of adhesives in wood joints under severe service conditions, including extended exterior exposure, is a complex field, and no entirely reli­able short-term test is known to ensure that a new type of adhesive system will resist satisfactorily all of the chemical, moisture, microorganism, and solvent effects that such severe service may involve. Except for the effects of microorganisms and similar biological influences, this test method has proven very useful for comparison purposes to distinguish between adhesive systems of different degrees of durability to the usual temperature, moisture, and cyclic moisture conditions. It has proven very useful to distinguish between bond lines, made with adhesives of proven chemical and biological durability, that when properly used in production resist the mechanical and moisture effects that such joints must withstand in severe service over extended periods of exposure. It does not, however, in itself, assure that new types of adhesives will always withstand actual exterior or other severe service.

Other environmental related tests include ASTM D-904, Standard Practice for Exposure of Adhesive Specimens to Artificial (Carbon-Arc Type) and Natural Light; ASTM D-1828, Standard Practice for Atmospheric Exposure of Adhesive-Bonded Joints and Structures; ASTM D-1879, Standard Practice for Exposure of Adhesive Specimens to High Energy Radiation; and ASTM D-3310, Standard Test Methods for Determining Corrosivity of Adhesive Materials.

In practical joints, adhesives are not always loaded statically or loaded for short periods of time. To help evaluate the performance of stressed adhesive joints as a function of time, tests have been developed to determine the response of adhesive joints to creep and cyclic loading. ASTM D-1780, Standard Practice for Conducting Creep Tests of Metal-to-Metal Adhesives, makes use of a deadload weight-lever loading frame to mea­sure creep of lap shear specimens. ASTM D-2793, Standard Test Method for Creep of Adhesives in Shear by Compression Loading (Metal-to-Metal), describes the construc­tions and procedures for use of creep test apparatus in which the sustained loading is maintained by springs. ASTM D-2294 is similar to D-2293, except that here the spring — loaded apparatus loads the lap specimen in tension. ASTM D-4680, Standard Test Method for Creep and Time to Failure of Adhesives in Static Shear by Compression Loading (Wood-to-Wood), describes the construction of a spring-loaded apparatus and testing procedures for a creep apparatus for use with relatively large wood specimens. ASTM D-2918 and D-2919 describe tests to measure the durability of adhesive joints in peel and lap shear, respectively. The tests and recommended fixtures are intended to hold specimens under sustained loadings while exposed to environments such as moisture, air, vapors, water, or other environments.

ASTM D-3166, Standard Test Method for Fatigue Properties of Adhesives in Shear by Tension Loading (Metal-to-Metal), provides procedures for testing and measurement of the fatigue strength of lap specimens. It makes use of conventional tensile testing machines capable of applying cyclic axial loads. Researchers have also made beneficial use of the concepts of fracture mechanics to evaluate the fatigue crack growth rate per cycle, da/dn, as a function of stress intensity factor. For this purpose, Mostovoy and Ripling [35], for example, have used fracture mechanics specimens similar to those described in ASTM D-3433. Fracture mechanics is discussed at the end of this chapter as well as in many books.

Adhesive joints are frequently exposed to sudden dynamics loads, and hence a knowledge of how adhesives react to impact loading is important for some applications. ASTM D-950, Impact Strength of Adhesive Bonds, describes sample configuration and testing apparatus for measuring the impact strength of adhesive bonds. The method is generally analogous to the Izod test method used for impact studies on a single material.

ASTM D-2295 describes apparatus that utilizes tubular quartz lamps to investigate failure of adhesive joint samples at high temperatures, and ASTM D-2257 outlines pro­cedures for testing samples at low temperatures (— 268 to — 55°C). ASTM also provides specific standards to investigate failure-related properties of adhesives that are less directly related to mechanical strength. Such properties include resistance to growth and attack by bacteria, fungi, mold, or yeast (D-4300 and D-4783), chewing by rodents (D-1383), eating by insects (D-1382), resistance to chemical reagents (D-896), and so on. It is enough to make the adhesive designer or researcher paranoid. Not only are stresses, temperatures, moisture, and age working against him or her, but now it appears that microorganisms and the animal kingdom want to take their toll on any adhesively bonded structure.

Although the primary purpose of this chapter is to discuss mechanical testing and strength of adhesive joints, the reader should be aware that ASTM covers a wide variety of tests to measure other properties. ASTM, for example, includes standard tests to measure the viscosity of uncured adhesives, density of liquid adhesive components, nonvolatile content of adhesives, filler content, extent of water absorption, stress cracking of plastics by liquid adhesives, odor, heat stability of hot-melt adhesives, ash content, and similar properties or features of adhesives.

Of particular interest to the adhesive technologist are surface treatments. ASTM has adopted standard practices for treating surfaces to better adhesives. ASTM D-2093, Standard Practice for Preparation ofPlastics Prior to Adhesive Bonding, describes physical chemical, and cleaning treatments for use on a wide variety of polymer adherends. D-2651, Standard Practice for Preparation of Metal Surfaces for Adhesive Bonding, describes tech­niques, cleaning solutions and methods, etchants or other chemical treatment, and so on, for metal adherends, including aluminum alloys, steel, stainless steel, titanium alloys, copper alloys, and magnesium alloys. ASTM D-2675 is concerned with the analysis and control of etchant effectiveness for aluminum alloys. ASTM D-3933 provides a standard practice for phosphoric acid anodizing of aluminum surfaces to enhance adhesion.