Weatherfastness

The pigments are manufactured either by reaction of the dianhydride with an amine or N,N -diaLkylation of the diimide. They are characterized by high tinctorial strength, excellent solvent stabiUty, very good weatherfastness, moderate brightness, and range in color from red to violet. An exception is the dianhydride which is not stable to alkah. 

Only a few azo metal complexes are available as pigments. Most of these are very lightfast and weatherfast. The chelating metal is usually nickel, and less commonly, cobalt or iron(II). 

Although of comparatively good light- and weatherfastness, solvent and migration resistance, only a few members of the isoindolinone and isoindoline families are commercially available as pigments. Chemically classified as heterocyclic azome-... 

The commercially leading member of this class, Anthrapyrimidine Yellow in very light white reductions affords a greenish to medium yellow with excellent weatherfastness. It lends itself primarily to application in industrial coatings such as automotive metallic finishes or to modify the shades of automotive finishes. 

Flavanthrone Yellow, the only commercially used flavanthrone, is a moderately brilliant reddish yellow. Excellent lightfastness and weatherfastness, combined with good solvent and migration resistance, make this pigment an attractive supplement to Anthrapyrimidine Yellow, mainly in the automotive finish industry. 

Commercial attention focuses on the derivatives of the pyranthrone molecule at a varying level of halogenation. Most are orange but others exhibit a dull medium to bluish red shade. Due to their good weatherfastness pyranthrone pigments are used for high grade industrial finishes. 

Lightfastness and weatherfastness of quinacridone pigments (Sec. 3.2) deteriorate in the order 2,9 - 3,10 - 4,11 substitution. It is assumed that decreasing the distance between substituents and NH function disturbs the formation of hydrogen bonds [14] a tendency which culminates in the very poor light and weather resistance of 5,12-N,N -dimethyl quinacridone. 

Metal complex formation considerably improves both lightfastness and weatherfastness in o,o -dihydroxyazo and o,o -dihydroxy azomethine pigments, usually compromised by distinctly dull color. 

There is no precise definition for the frequently used term application fastness of a pigment. It usually refers to the behavior exhibited by a finished product used in accordance with the specifications. The term may thus refer to a print, a coated object, or a plastic product, and the list of features ranges from properties such as lightfastness and weatherfastness to migration fastness and fastness to solvents. In this context, there is a certain amount of emphasis on features which play a role in connection with packaging materials and packed articles. 

Accelerated exposure equipment may also be used to test for weatherfastness in plastic materials [106], The natural destructive agents inherent in weather are approximated by filtering the radiation emitted by the xenon arc lamp and by spraying the sample with water under standardized conditions [106], Test programs are designed to relate to actual outdoor exposure to rain and humidity. In a standard program, a 3 minute wet cycle typically alternates with a 17 minute dry period. Weatherfastness tests are carried out and evaluated like lightfastness tests the black panel temperature and other parameters are the same in both procedures. 

Weatherfastness tests on coatings have shown that prolonging the dry and wet periods, i.e., extending the usual 17 minute dry cycle to a full 102 minutes and the wet period to 18 minutes, affords results which correlate much better with the Florida outdoor exposure tests. The humidity that penetrates a layer is known to interact with the various components in the coating, adversely affecting the mechanical properties of the material. The above mentioned cycle apparently corresponds much better with the climatic conditions in areas like Florida. If recirculated water is used in accelerated exposure equipment, suspended particles may adhere to the surface of the coating, producing a thin film. This obviously has a detrimental effect on the test results. 

Test specimens can only be compared in terms of lightfastness or weatherfastness if all samples show the same depth of shade. Up until a few years ago, such determinations were carried out at equal pigment concentrations. 

Lightfastness and weatherfastness of a pigmented system improve with the depth of shade i.e., the system becomes more stable as the pigment concentration in-... 

Since lightfastness and weatherfastness are properties of entire pigment-vehicle systems, additional components must be expected to have a more or less pronounced effect on the sensitivity of a material. In referring to test results, it is therefore necessary to mention the exact composition of the sample. In some systems, problems arise with peroxides, which may rapidly destroy organic pigments. UV absorbers, on the other hand, may improve the lightfastness of a system. 

Fig. 76 Effect of the particle size of P.Y.151 in alkyd-melamine resin systems on the weatherfastness. Organic pigment Ti02 = l 3.

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