Natural and Accelerated

Fluridone is a weak base with low water solubiUty. Sorption of fluridone increases with decreasing pH (436). Leaching of fluridone was not significant in field study, and the persistence has been determined to be less than 365 days. The degradation of fluridone appears to be microbial in nature, and accelerated breakdown of the herbicide occurs upon repeated appHcations (437). Fluorochloridone is shown to degrade by hydrolysis at pH 7 and 9, but not at lower pH. The half-Hves for this reaction are 190 and 140 days for pH 7 and 9, respectively. Breakdown by photolysis occurs rapidly with a half-hfe of 4.3 days at pH 7 (438). An HA is available for acifluorfen. 

The above tests for characterising coating properties necessarily continue to involve a certain amount of empiricism. The intelligent use of these tests, however, has shown that wide variations of physical and electrochemical characteristics of coatings as a function of composition may be obtained, and further, that significant changes in these characteristics, that can be measured before the usual evidence of failure appears, occur upon natural and accelerated ageing. 

Flowever, omitting to take account of the temperature of products used or stored in sunlight when selecting materials can lead to unexpected rates of deterioration or failure. Conversely, the temperatures reached during weathering tests, both natural and accelerated, need to be considered when assessing the results. 

Figure 18.1 Effect of Weather-Ometer exposure (0.35W/m2/nm) for color of materials [2]. From Weathering of polyester and copolymer sheeting, presentation given by D. R. Fagerburg at the Atlas School for Natural and Accelerated Weathering (ASNAW) course, Miami, FL, May 1999, and reproduced with permission of Atlas Electric Devices Company...

Fagerburg, D. R Weathering of polyester and copolyester sheeting, Presentation given at the Atlas School of Natural and Accelerated Weathering (ASNAW) Course, Miami, EL, 27-30 April, 1999, Atlas Electric Devices Company, Chicago, IL, 1999. 

Let us give some comparative examples of natural and accelerated weathering. 

When wood flour was first colored and then blended with HOPE, the respective acceleration factors were 8.6 and 7.7, respectively. In this case both natural and accelerated weathering were slower than those for nondyed wood flour, but the difference was not very significant for the accelerated weathering. Hence, a higher acceleration factor. 

Other studies of interest on polyolefins include determination of supramolecular structure, influence of rotational moulding, comparison of natural and accelerated weathering,and measurement of gel formation on photocrosslinking. 

The black panel temperature is measured with a thermocouple mounted on a flat black painted panel which is placed on the sample rack of tire Weatlier-Ometer. As such, the temperature registered is meant to indicate a maximum temperature of the specimens under tire same iiTadiation. Kockett, D., Factors of weathering. Presentation given at tire Atlas School of Natural and Accelerated Weathering (ASNAW) course, Miami, FL, 27-30 April, 1999, Atlas Electric Devices company, Chicago, IL, 1999. 

From the viewpoint of prediction of service lives, the photochemical deterioration processes of polymers used as paints and finishes are theoretically analyzed based upon unsteady state dynamics. Theoretical results are compared with experimental data under natural and accelerated exposure. Infrared spectra and scanning micrographs show that the deterioration proceeds continuously inwards from the surface, but differently with the exposure conditions. Parabolic (/t ) law was derived approximately for the increase in the depth of the deteriorated layer of polymers with time. Paying attention to the influence of the deterioration of polymeric finishes, the parabolic law involving a constant term was also derived for the progress of carbonation of concrete. These parabolic laws well predict the progress of deterioration and explain the protective function of finishes on reinforced concrete. 

It is important to remember when considering the results obtained for these materials that the re-mixes will not be identical with the original compounds produced in 1958 and this can be expected to have significant effect on the correlation between natural and accelerated results. As well as unavoidable differences in the polymers and compounding ingredients there was no way of ensuring that the states of cure achieved were identical. 

Most of the hardness results showed a relatively simple form of change with hardness increasing with time, but there were notable exceptions. After natural ageing all materials had either increased or changed very little so overall there is a general correlation between natural and accelerated results. 

The WLF predictions were in the wrong direction for 100% modulus of compounds K and W at 40 °C, probably because of the accelerated results changing direction with time. The same applies to compound D at 300%. The 300% modulus predictions were also in the wrong direction for compounds A, C, G and N. For compounds A and C changing direction in accelerated tests was again apparent but for compounds G and N the natural and accelerated results are simply different. 

Edidin A.A., C.W. Jewett, K. Kwarteng, et al. 2000. Degradation of mechanical behavior in UHMWPE after natural and accelerated aging. Biomaterials 21 1451-1460. 

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