Paint drying ovens

One likes to see high Tm values for TPU hard segments, for this indicates strong virtual crosslinks which will allow the molded TPU parts to pass through paint-drying ovens and to stand in the hot sun without sag or distortion. Of course if Tm is too high, molding problems could be encountered. 

Table 5.2 shows further examples of dispersed phase coalescence in blends of PA dispersed phase in a less viscous PE or PS matrix. The data show that the mean PA particle size increases dramatically with simple heating under static conditions in the absence of any mechanism for morphology stabilization. The same coalescence can occur in molded parts of uncompatibilized polymer blends subjected to further thermal treatment after molding (fi.g., in a paint drying oven). The mechanical properties of these blends are quite poor. 

Estimate of the Heat Input for the Exhaust Air for Water Base Paint With No Solvent. The air exhaust rate required to provide a dry oven atmosphere is determined by the amount of water released into the oven. Using a figure of 10 cfm of air per pound of water per hour, the 570 pounds of water per hour will require approximately 5700 cfm of air at 70 F. 

Thanks to their low organic solvent content, the use of waterborne paints substantially reduces solvent emission from paint or spraying shops (spray cabin, ventilation zone, drying oven). 

Tunnel ovens can be used for stress relieving and annealing copper and its alloys at 500 to 900 F (260 to 480 C). Tunnel ovens are so common for paint drying that they are often assembled from standardized liber-lined, metal-encased sections that... 

It is essential to dry passivated surfaces promptly to protect them from moisture and atmospheric contamination. The drying may be carried out by blowing compressed air, which is easier and more economical, or by placing in the same oven as for the paint. Special care need be taken with hidden surfaces, such as in corners, bends and crevices, to ensure that there is no trapped moisture. 

It is necessary to choose the type of paint with care. If it skins-over too quickly in the oven the gases cannot escape and blistering results, but if setting is deferred for too long the paint will flow to the base of the article. Some experimentation is advisable and the conditions once established should be retained. Some users treat the sprayed coating with a passivator of 200 g sodium dichromate in 1 litre of 6 7o sulphuric acid and dry it before enamelling. 

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 ... 

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 . 

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. 

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