Лако-красочные материалы — производство

The durability (long-term performance) ofa bonded joint depends on the properties of the adhesive, ofthe materials being joined together, and their surface pretreatment prior to bonding.

The true service life of an adhesive joint is influenced by a variety of factors such as climate, environment and mechanical stresses, which in common practice occur with a limited amount of predictability. Service conditions can vary widely, depending on the final application of the joint. In order to carry out an individual service life assessment it is necessary to make assumptions as to the ‘normal’ and ‘worst’ conditions that the adhesively bondedjoint willbe subjected to duringits application. These conditions are generally chosen so that the majority of adhesive joints will be at or below these conditions [5], although in some fields of application the basis for the assumption of load and service conditions is regulated by standards and perhaps building codes. As many of the joints will be located under conditions deemed less severe than ‘normal’, it is obvious that many products will achieve service lives greater than predicted.

The next step in assessing and predicting the service life of an adhesive joint is a sensitivity analysis, taking into account possible degradation mechanisms related to the chemical and physical properties of the adhesive and the adherent. The most common mechanisms that cause changes in the mechanical properties or irrevers­ible degradation of adhesive joints include:

• physical or chemical (e. g. thermal or UV) degradation of the bulk adhesive

• desorption or hydrolysis of superficial adhesive bonds under the influence of moisture

• loss of adhesion due to an interfacial weak boundary layer caused by, for example, a poorly cured adhesive (e. g. caused by acidity ofa surface after chemical pretreatment)

• dissolution and deterioration of superficial oxides or conversion coatings promoted by the chemical conditions inside the adhesive joint under climate exposure

• corrosion of the metal adherent in contact with corrosion-promoting media; examples are marine environments (high chloride levels) or industrial sites involving exposure to SO2,H2SorNOx, typically starting at the bond-line (bond-line corrosion)

• debonding under thermal cycling (e. g. freeze-thaw) due perhaps to mechanical thermal stress caused by mismatch in thermal expansion of the adhesive and the adherents

The standard experimental procedure in the assessment ofservice life ofadhesive joints is based on the comparison of characteristic mechanical and physical proper­ties before and after a certain period of aging.

If the aging conditions to assess the service life correspond directly to the expected service conditions, the designation ‘natural exposure’ has been widely accepted in case of adhesive joints subjected to outside weathering. In the absence of confirmed data on the relation of accelerated aging procedures to the performance in use, natural aging remains a major source for useful information on degradation mechanisms and service life projection. However, when climatic conditions of exposure are similar to those of the intended use, the period of natural exposure should be at least one-tenth of the anticipated service life in order to create useful data, and could be considerably longer. If the product is to be used in climatic conditions more severe than the natural exposure test site (e. g. higher ambient temperatures and/or radiation), then even significantly longer periods of natural exposure will be required to obtain equivalence. The analysis of aging tests should not only be limited

to the acquisition of data from mechanical tests but also include a careful examination and documentation of any changes in outer appearance, such as cracking or crazing, distortion, or change in failure patterns after testing according to ISO 10365 (Adhesives — Designation of the main failure patterns).

In spite of the fundamental importance of test results from natural exposure either under service conditions or under defined exposure conditions, there is a strong demand for accelerating aging tests to cut down the duration of tests and approvals, and to allow for a lifetime prediction beyond the actual time of test exposure. Therefore, testing following accelerated aging will generally form the basis of predictions of service lives.

The guidance document of the European Organization of Technical Approvals for the assessment of the service life of products differentiates three main groups of testing after accelerated aging: direct, indirect and torture tests.

• Direct testing is performance — or use-related, and links directly to measurement of the characteristic in question. Direct testing is often accomplished by using the actual joint geometry instead of a standardized specimen.

• Indirect testing relates to the measurement of properties which have a known relationship to performance in use. When using indirect tests it is critical that a proven correlation between the property measured and long-term performance has been established (i. e. the property measured must be significant in terms of performance).

• So-called ‘torture tests’ are short-term tests where the conditions are significantly more severe than the service conditions of the product; such tests may be used to negate the need for long-term aging. If the product passes the severe test, no further work needs to be done on that particular factor. If the productfails the test, it does not necessarily mean that it will not perform well, but additional testing over a longer period will be required under conditions closer to the service conditions (i. e. simulatory aging conditions) in order to establish the product’s credentials. Exam­ples ofadditional tests include water boil tests or salt spray tests with increased acidity.

Torture tests should be used with extreme caution — the analogy that ‘boiling cannot accelerate the brooding ofan egg’ has often been cited. The application oftorture tests assumes a profound knowledge ofthe performance ofthe material in question under the conditions proposed. Any aging conditions applied and subsequent testing should be questioned concerning their relation to the ‘basic’ phenomena observed on site.

Although accelerated aging may be carried out in a number of different ways, the most appropriate method will depend on the type of adhesive and adherent material, and the intended use of the adhesive joint. In general, simulatory aging methods are used in which the aging conditions attempt to simulate natural conditions, usually with only a moderate acceleration of the factors. Some examples of simulatory aging methods include:

• heat aging

• water resistance

• freeze-thaw

• corrosion and chemical resistance

• artificial weathering

The international standard EN/ISO 9142 (Adhesives — Guide to the selection of standard laboratory ageing conditions for testing bonded joints), contains a comprehen­sive overview of applicable accelerated aging conditions.

Because the presence of mechanical stress during aging can have a significantly accelerating effect, standards such as ISO 14615 (Adhesives — Durability of structural adhesive joints — Exposure to humidity and temperature under load) take the interaction of climate exposure of adhesive joints under load into consider­ation. Susceptibility to fatigue crack growth under hot humid conditions is one of the major concerns for the durability assessment of adhesively bonded metallic joints. The recently published standard EN 15190 (Structural adhesives — Test methods for assessing long-term durability of bonded metallic structures) therefore specifies test procedures for determining the long-term durability of an adhesive system subjected to environmental and fatigue loads. The procedures are based on measurement of the crack growth rate and resistance to crack propagation through the adhesive layer in DCB-type specimens under an applied mode I opening cycling loading.

In specific application areas, such as the automotive industry, accelerated aging procedures have been established on the basis of experience. The standardized test procedure VW PV 1200 consists of cycles between -40 °C and 80 °C at a relative humidity of 95%, and promotes damage mechanisms related to moisture and temperature gradients. The VDA test procedure 621-415, named after the German Association of Automotive Manufacturers, consists of several test cycles with expo­sure to a corrosive chlorine environment, humidity and phases where the specimens are being stored at room temperature.

For general applications, the following overview may serve to suggest accelerating aging cycles as being both applicable and relevant to the major mechanisms of degradation:

• Exposure to moisture: Aging in climate chambers at 95% relative humidity (no condensation on the surface of specimen) and 60 °C or 80 °C for 300-1000 h, depending on the intended application.

• Exposure to water or other liquids: Immersion at 40 °C, 60 °C or 80 °C for typically 500-1500 h, depending on the intended application. Note that this type of exposure stimulates degradation under elevated temperature and high humidity, but does not promote corrosive attack due to the lack of oxygen in the immersion bath unless constantly aerated.

• Corrosion in a chlorine environment: Standard cabinets for corrosion testing create a salt spray environment at 35 °C. Exposure times typically range from 300 to 500 h, depending on the intended application.

• Aging at elevated temperature: Aging at elevated temperature in the absence of moisture especially promotes oxidation, embrittlement and thermal degradation.

Typical temperatures depend on the intended application. For adhesive bonds under outside weathering temperatures of 80-120 ° C are common, depending on the geographical area of application.

• Thermal cycling. Accelerated aging tests under cyclic change of temperature include the freeze-thaw transition to consider the detrimental effect of water penetration and icing. Temperatures typically span from -20 ° Cor -40 ° Cto60 ° Cor80 ° C with 95% relative humidity during the warm period of the cycles. Typical numbers of cycles range from 100 to 300. It should be considered that rapid transitions between high and low temperature limits may induce thermal stress beyond levels repre­sentative of outside weather conditions, for example.

• UV-exposure. Sophisticated test cabinets such as the Xenotest or Weatherometer (WOM) are available to simulate aging conditions, combining moisture, tempera­ture and UV radiation. The test duration depends on the irradiation doses to be expected in a specific application, and typically ranges from 500 to 3000 h. Clearly, such tests should mainly be considered if one or both adherents is/are transparent to the applied spectra of radiation.

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