Лако-красочные материалы — производство

The usual function of an aging test should be to challenge the product artificially in an accelerated manner in one or more environments that can be expected, so that either after prolonged storage or after long periods of use its behavior can be predicted.

The popular accelerated aging temperature used to predict natural aging is 65°C (150°F), with the adhesive-coated products either in roll or sheet form independently supported in a controlled humidity environment, such that they occupy less than a quarter of the available space to ensure ample circulating air. Typical commercial products are exposed to an 80% humidity using a saturated reagent grade ammonium sulfate bath [24], while for electrical products this is taken up to 90% using a bath of 37% by weight solution of glycerin [25]. The time period to predict behavior after 1 year of natural aging varies from 4 to 7 days at 65°C (150°F), depending on the authority, the change in physical characteristics before and after high temperature aging being compared. A reduction in physical characteristics of less than 10% after the aging test is normally considered a satisfactory performance.

The chemical makeup of many pressure sensitives makes them prone to oxidation, and while antioxidants can compensate for this, it is still necessary to evaluate whether the antioxidant is functioning satisfactorily. Any test to evaluate resistance to oxidation becomes a comparative test in change of performance, usually of adhesion and tack, the severity of the test depending on individual requirements for the finished product. One simple but effective test is to expose the adhesive to 120° C (250° F) in a forced-air oven for various periods of time, evaluating samples before and after heat exposure, a 1-hour exposure being a good starting point. A more severe test is the use of an oxygen bomb, at lower temperatures, with high pressure oxygen. Again, a good starting point is an overnight (16 hour) test at 38°C (100°F) and 300 psi.

Exposure to ultraviolet (UV) light can have similar degrading effects, either from sunlight or even from exposure to fluorescent lighting. The need for stability to UV light will depend on the adhesive end use, many adhesives never encountering such conditions. Similar to the oxidation test, the adhesive should be exposed to a controlled source of UV light, typically around 300 periods, with the samples under test mounted on a slowly rotating turntable to ensure uniform exposure, the adhesion and tack exposure being compared [26]. Commercial UV light sources are available. The source should be far enough away from the samples, usually around 18 to 30in. (450 to 750mm), to eliminate a possible secondary exposure to excessive heat from the light source; certainly exposed to no more than 50°C (120°F). The UV intensity at the test surface should be determined with a suitable UV light meter, adjusting the UV source location if necessary to ensure the same UV exposure for each test cycle. One proposed intensity is 2250 pW/cm2 at 12 in. (300 mm) from the source with the samples at 18 in. (450 mm). As the UV source will deteriorate with time, it will be necessary to replace it periodically. The time of exposure is dependent upon the marketing objectives for that product, but one can expect the test period to be as prolonged as 180 h.

An alternative UV exposure test method is the use of a commercial weatherometer [27], in which carbon arcs are used as the UV source, the samples being mounted on a carousel rotating around the source. The carbon arcs need changing frequently, to provide the necessary exposure time, again the time of exposure depending on individual prefer­ence. The weatherometer has the added advantage of being able to provide in addition a water spray at chosen intervals, to simulate outdoor exposure.

Degradation by heat, resulting in breakdown of the various polymer structures is another cause of loss of properties, as can occur during the processing of hot melt systems, or in adhesive systems intended for high temperature applications. Here, the effect will not be seen so much in a change of tack or adhesion (these characteristics maybe even improv­ing), but in deterioration of the ability of the adhesive to resist shear forces, which can be dramatic. The actual change in molecular weight and its distribution can be determined by gel permeation chromatography [28]. The lower-molecular-weight polymer systems as used in hot melt pressure-sensitive adhesives can be handled with conventional equipment and technology, but the much higher molecular weight of other natural and synthetic rubber-based systems will require more sophisticated equipment, including the use of heated columns.