Oven drying systems

Oven drying systems often require highly specialized equipment and production units for application and drying. Such systems are mainly processed in industrial operations. At the present time, the majority of industrial coatings are oven drying (i.e., heat set) systems. 

Conditions to be met in oven drying enamels depend also on the composition of the binder. Paint systems containing melamine-formaldehyde or urea-formaldehyde resins, for instance, harden by polycondensation with other resins, such as epoxy resins, short-oil alkyd or acrylic resins at elevated temperatures. Baking is carried out at temperatures between 100 and almost 200°C and may last from a few minutes to more than an horn. A general trend towards energy conservation has shifted public attention towards binders which require low baking temperatures. 

In recent decades, considerable efforts have been made to restrict or completely eliminate solvent emission during oven drying. Intensive research has focused on developing paint systems which contain very little solvent or none at all [5]. A variety of approaches are possible systems that have found their way to practical use tend to present a special challenge in terms of pigment selection  

Apart from the O.E.M. sector water reducible systems are gaining growing importance with high quality industrial paints and refinish systems as well. 

Due to the very stringent EU regulations on VOC emissions [11], European countries are leading with respect to water reducible systems. 

---

These requirements considerably restrict the selection of pigments for oven drying systems. The complex nature of the problem makes it necessary to submit any pigment which is basically applicable to a pilot experiment under the exact processing conditions. 

In practical application, most types of P.B.15 1 are completely fast to overcoating in oven drying systems. The pigments are suitable candidates for powder coatings, for instance for acrylate or polyurethane-based systems, in which they do not exhibit plate-out (Sec. 1.6.4.1). 

P.R.252 provides yellowish to medium red shades and is recommended particularly for use in architectural paints. The pigment shows very poor fastness to a number of organic solvents which are commonly used in paints, a deficiency which largely precludes it from being used in oven drying systems. Regarding lightfastness and weatherfastness, the only available type with coarse particle sizes performs somewhat better than the much more yellowish P.O.5. 

Chiral salen chromium and cobalt complexes have been shown by Jacobsen et al. to catalyze an enantioselective cycloaddition reaction of carbonyl compounds with dienes [22]. The cycloaddition reaction of different aldehydes 1 containing aromatic, aliphatic, and conjugated substituents with Danishefsky s diene 2a catalyzed by the chiral salen-chromium(III) complexes 14a,b proceeds in up to 98% yield and with moderate to high ee (Scheme 4.14). It was found that the presence of oven-dried powdered 4 A molecular sieves led to increased yield and enantioselectivity. The lowest ee (62% ee, catalyst 14b) was obtained for hexanal and the highest (93% ee, catalyst 14a) was obtained for cyclohexyl aldehyde. The mechanism of the cycloaddition reaction was investigated in terms of a traditional cycloaddition, or formation of the cycloaddition product via a Mukaiyama aldol-reaction path. In the presence of the chiral salen-chromium(III) catalyst system NMR spectroscopy of the crude reaction mixture of the reaction of benzaldehyde with Danishefsky s diene revealed the exclusive presence of the cycloaddition-pathway product. The Mukaiyama aldol condensation product was prepared independently and subjected to the conditions of the chiral salen-chromium(III)-catalyzed reactions. No detectable cycloaddition product could be observed. These results point towards a [2-i-4]-cydoaddition mechanism. 

P.R.48 2 is less commonly found in paints. In paints, like in other areas of application, the calcium salt performs like the barium lake. Both are, for instance, equally fast to overcoating. The list of suitable application areas for both pigments is the same. P.R.48 2 is also used in oven drying paints, nitro paints, and in similar systems. Besides, it is also found in emulsion paints. While barium and calcium salts exhibit equal lightfastness in full shades, there is a considerable difference in white reductions. Increasing amounts of Ti02 render P.R.48 2 much more sensitive to light than P.R.48 . 

In oven dyring and air drying systems, for instance, P.R.48 4 equals step 7 on the Blue Scale, while its weatherfastness after one year of exposure matches step 4-5 on the Gray Scale. Its shade darkens upon weathering. In such deep shades the pigment satisfies more stringent requirements and may even be used in applications such as automobile refinishes. 

Biomass accumulation and its distribntion are usually determined after a prolonged period of exposure to pollutants. At harvest, the plant is divided into its constitutive organs, the fresh weight of each is recorded and the material is then oven-dried for 1-2 h at 105°C and at 70°C for further 2-3 days, preferably using an air circulation system. Drying time can vary depending on type of tissue. 

Under an atmosphere of dry nitrogen gas, the dropping funnel is replaced by a fractional distillation apparatus provided with a 10-cm Vigreux column and a 5-cm water-cooled condenser. The system is heat-dried under a flow of nitrogen gas with an electric heat gun or may be assembled from oven-dried glassware. The mixture is then heated over a period of 20-30 min until the silicone oil bath reaches 130-140 °C. During this time, the initial yellow precipitate is partially dissolved and the mixture refluxes smoothly. The silicone oil bath is maintained at this temperature for an additional 30 min. Heating is then increased over the period of 1 h to achieve distillation of the product the final oil temperature reaches 200 to 215 °C. 

Occasionally, granular silica (supplied either by the Ottawa Silica Co. or Fisher Scientific Co.) was added to the plutonium solution for various periods before centrifugation to determine its effect on the colloidal system. The silica was first dry sieved, washed thoroughly, then oven dried at 120 °C. the particle sizes ranged from 280 to 390p. Other chemicals used were reagent grade. 

Charge an oven-dried, two-necked, round-bottomed flask (100 mL), containing a stirring bar, with CuCN (3.1 g, 35 mmol) and LiBr (3 g, 35 mmol). Fit the flask with a gas adapter (with stopcock) and a septum. Connect the flask to a manifold system and subject it to three evacuation/inert gas refill cycles. Add dry, distilled THF (15 mL) and stir the mixture until the salts dissolve. Cool the solution to -20°C. 

---