Several conditions need to be fulfilled for a photochemical reaction to proceed in crystals. Some of these conditions, for example, geometrical conditions, depend on the particular type of photochemical reaction, but the rest are common for all reactions. The mutual directionality of two neighbouring molecules determines the kind of product obtained. The influence of temperature on the probability of a photochemical reaction occurring varies for different types of photochemical reaction and different compounds. High pressure imposed on crystals also has a big influence on the free space and the reaction cavity. The wavelength of the applied UV light is another factor which can initiate a reaction and sometimes determine the structure of a product. It is possible, to a certain degree, to control the packing of molecules in stacks by using fluoro substituents on benzene rings. The crystal and molecular structure of 2,6-difluorocinnamic acid [systematic name: 3-(2,6-difluorophenyl)prop-2-enoic acid], C9H6F2O2, (I), was determined and analysed in terms of a photochemical [2 + 2] dimerization. The molecules are arranged in stacks along the a axis and the values of the intermolecular geometrical parameters indicate that they may undergo this photochemical reaction. The reaction was carried out in situ and the changes of the unit-cell parameters during crystal irradiation by a UV beam were monitored. The values of the unit-cell parameters change in a different manner, viz. cell length a after an initial increase starts to decrease, b after a decrease starts to increase, c increases and the unit-cell volume V after a certain increase starts to decrease. The structure of a partially reacted crystal, i.e. containing both the reactant and the product, namely 2,6-difluorocinnamic acid-3,4-bis(2,6-difluorophenyl)cyclobutane-1,2-dicarboxylic acid (0.858/0.071), 0.858C9H6F2O2·0.071C18H12F4O4, obtained in situ, is also presented. The powder of compound (I) was irradiated with UV light and afterwards crystallized [as 3,4-bis(2,6-difluorophenyl)cyclobutane-1,2-dicarboxylic acid toluene hemisolvate, C18H12F4O4·0.5C7H8] in a space group different from that of the crystal containing the in-situ dimer.
This article presents the technology of making an adhesive joint using two primers: Corrosion Inhibiting Primer BR127 (previously used, containing chromium compounds) and, as a potential substitute, Structural Adhesives Primer EW 5000 AS (which does not contain any compounds harmful to the environment). An adhesive film and a sol–gel primer were used to make the joint of two aluminum sheets, and various technologies were used for applying adhesion promoters. The mechanical properties of the prepared samples were tested using two test methods: wedge tests and shear strength tests. In both cases, the samples were aged in laboratory conditions in tap water, and in a climatic chamber (with increased temperature and humidity). The obtained results indicate that the best technology for preparing the joint using each primer is the technology that assumes heating the primer and hardening the adhesive film in one operation. The results of the strength tests indicate that the samples made using the EW 5000 AS primer have higher strength properties under all tested seasoning conditions compared to samples made using the BR 127 primer. It was also confirmed that the presence of moisture and/or water reduces the mechanical strength of the adhesive joints independently of the primer used. The results of the polymer coatings tests to protect the aluminum substrate against corrosion showed that the coatings are only effective for a certain period of time, and, as a result of the NSS test, after 480 h, all the samples were subject to corrosion.
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