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A common manufacturing process in the finishing of glass, ceramic, plastic, and metal components is the removal of residual water from the processed parts. In general, any process which solves this problem is called precision drying. There are a variety of methods to accomplish this task. The more common methods include evaporation, centrifugal drying, media absorbtion, and solvent drying -- aslso called displacement drying. There are positive and negative aspects to each choice:
- Evaporative (or hot air)Drying will dry the parts, but generally will allow any residue or nonvolatile material that was suspended or dissolved in the water to remain.
- Centrifugal Drying is usually effective only in removing gross amounts of water from free draining parts. A slight water film usually remains, which must be removed by other means.
- Media-absorbent Drying transfers moisture from the parts to a porous medium such as sawdust or corn-cobs, which then must be disposed of or dried through evaporation. The parts must then be further processed to remove residue deposited by the absorbent media.
- Solvent Drying mixes a solvent with the water and uses the physical characteristics of the solvent to trap the water, making it easy to remove. There are two types of solvent drying:
- Polar Drying uses a strongly polar solvent such as alcohol or acetone to bind with the excess water. As the polar solvent is removed, the water is removed with it. Cleverly designed, the solvent can be recovered and the water removed, allowing the solvent to be reused and costs kept to a minimum. Vertrel® XP is one such a polar blend and works very well in applications where there is not a lot of water to be removed.
- Displacement Drying uses a dense, heavy solvent to wet the surfaces of the parts and force the separation of the water from them, simply "floating" the water away. A hydrophobic solvent is required for this process, such as Vertrel® XDA. This process works best when large quantities of water need to be removed.
Solvent displacement drying is not new. In the days of CFCs, it was a common and widely accepted method of water removal prior to plating, coating and other surface treatments of plastics, highly-polished metals such as those found in jewelery and medical devices, mirrors, lenses, crystals, and ceramics. The method was particularly useful where reflective and refractive properties must be maintained. However, since the phase-out of CFCs and HCFC solvents, there have been few, if any, effective and affordable displacement drying solvents.
Cost-effective Vertrel® now re-introduces solvent-based displacement drying. Drying with Vertrel® operates on the displacement principle. The process uses (a) the high density and low surface tension (wetting ability) of Vertrel®, plus (b) a hydrophobic fluoro-surfactant additive that promotes water separation, and 9c) an anti-static additive to enhance the spot-free drying of electrically non-conductive parts such as glass lenses and mirrors. This mixture is called Vertrel® XDA.
Used in a 2- or 3-sump cascading process similar to a vapor degreaser, Vertrel® will effectively remove large or small quantities of water from metal, glass, or plastic parts and assemblies. The water, along with dissolved contaminants, adheres to the surface of parts in droplet form. When a wetted part is immersed in the Vertrel® XDA, the higher density, lower surface tension and unique properties of the fluorosurfactant allow Vertrel® XDA to penetrate beneath the water droplet. Buoyancy forces lift the agglomerated droplets away from the surface of the part. Like any mix of oil and water, in which the lighter oils rise to the surface, in displacement drying the water will rise over the heavier, hydrophobic Vertrel® XDA. Figure 24-1 illustrates this sequence.
Fig 24-1 -- Displacement Drying Conceptualization
Once on the surface, the water flows from the surface of the drying sump over a weir into a gravity water separator. There it is rejected from the dryer to the drain or to treatment, as required by local regulations.
As the parts are lifted out from the drying fluid they usually pass through the water layer (this is an equipment issue and such re-contamination can be minimized through clever equipment design). Any residual surfactant re-applied to the part from the water layer is removed by passing the parts through one or more cascading baths of Vertrel® XF. Upon removal, the parts are dry and spot-free.
Displacement drying equipment is commercially available and operate much the same as vapor degreasing equipment (except that water separation and removal capabilities are greatly enhanced). Figure 24-2 illustrates a typical displacement drying system.
Figure 24-2 -- Displacement Drying System
Displacement Drying Advantages
- Displacement drying depends on the use of a relatively high specific gravity drying fluid to encourage rapid displacement of water along with its water-soluble contaminants. At the low operating temperature (55°C / 130°F) of this system virtually no water evaporates from the wet parts. Therefore, water-soluble salts cannot be deposited on their surfaces. This permits water to float to the surface of the solvent, where it is continuously swept away by the equipment, enabling dried parts to be removed without passing through a layer of contaminated water. This combination of low-temperature boiling action, immiscibility with water, and high specific gravity is the major reason for the dry, spot-free condition of parts emerging from the Vertrel® XDA.
- Evaporative or hot air drying employs heat to remove water by changing it from a liquid to a vapor. This method requires large amounts of energy to operate because approximately 1,000 Btu (0.293 kWh) are required to evaporate each pound of water. Vertrel® XDA operates at a low operating temperature (55°C / 130°F) and consumes significantly less energy.
- Vertrel® XDA also offers these other advantages:
- Removes water in pH ranges between 5 and 9
- Removes hard water
- Is easily rinsed because of lower drying agent concentrations
- Drying agent has limited water solubility and has no vapor pressure, so it will remain in the boil sump
- Reduces cycle time by elimination of air drying step
A Note About the Anti-Static Additives
The use of an anti-static additive in the drying fluid in combination with the surfactant may be necessary when drying electrically non-conductive parts such as glass lenses and mirrors. The main function of the additive is to prevent spots and stains.
Vertrel® XF is a key ingredient in Vertrel® XDA. Often used as a dielectric fluid, Vertrel® XF is a poor conductor of electric current and will not easily dissipate static charge. meanwhile, the boiling and general circulation of the drying fluid creates a static charge, particularly in the latter stages of the drying process when most of the water has been removed from the system. This charge collects on the surface of the non-conductive liquids and can prevent the release of the last water droplets, which then might dry in-place. This would result in water spots and stains.
The answer to this problem is the addition of an anti-static additive to the Vertrel® XDA. The additive is a polar compound soluble in Vertrel® XF which increases the conductivity and dissipates the static charge. The anti-static additive is only slightly soluble in water, and it has some volativity that makes it effective in both the drying and rinse stages of the process.
Following is the suggested procedure for drying:
- Prepare the drying machine by loading it with Vertrel® XDA drying agent boiling at 55°C / 130°F).
- Place the parts in an open-mesh holder and lower them into the primary drying sump. Usually, a 1- to 3-minute immersion is sufficient depending on the size, configuration and other characteristics of the parts.
- Agitate the holder up and down a few times to help dislodge any trapped water between parts. Drying cannot take place if water is trapped inside small cracks and crevices or if the orientation of the parts traps water underneath cup-like cavities. These problems often are solved simply by shaking or rotating the parts in the primary bath. Also, make certain that any parts with recesses or cups are oriented so released water can float upward, towards the surface of the bath.
- Remove the parts from the Vertrel® drying agent and hold them over the primary sump to permit excess solvent to drain back into sump.
- Lower the parts into the rinse sump. Again, agitate the holder. If the equipment employs a second rinsing sump, repeat the rinsing step.
- At the end of the rinse, hold the parts in the vapor zone until any dripping stops. At that time, remove the parts from the dryer. The parts will be dry, spot-free and ready for further processing.
Procedural Notes:
- Ultrasonics in the primary and/or rinse sumps can increase the efficiency of the drying process.
- Spray rinses are not normally recommended with Vertrel® because of the increased solvent losses such systems always entail. However, in certain drying applications a spray rinse in the vapor zone increases the efficiency of the final rinsing and drying. Train techncians well before allowing them to use the spray nozzle. Spraying always should be done slowly and beneath the vapor blanket to minimize emissions.
- In addition, Vertrel® is not recommended for the displacement drying of fibrous or porous parts, as they present special problems in both water displacement and excessive solvent carryout. For details, contact Micro Care Tech Support.
Required Equipment and Chemistries
Engineers should be aware that any vapor degreaser could be used for precision drying. However, to be truly cost-effective companies should consider purchasing new or retrofitted vapor containment technology. Modern vapor containment systems have higher freeboard and a secondary set of low temperature (-29°C / -20°F) condenser coils to greatly reduce vapor losses. This investment will enable the safe and economical use of Vertrel® XDA.
Vertrel® XDA is not designed to operate in isolation. Two different fluids are required:
- Vertrel® XDA -- This is the drying agent. It is used in the boiling sump of the drying machine. It consists of Vertrel® XF plus a fluorosurfactant additive and an anti-static additive to give the water-displacing action. It is used only for the initial charge of the drying sump and recirculating tank system, and for make-up.
- Vertrel® XF Fluid -- This chemistry is used to charge the rinse sumps. It is used to rinse residual drying agent from the parts coming from the boiling water removal sump. Thereafter, condensed Vertrel® XF vapors from the boiling sump will act to keep the fluid in the rinse sumps clean and at the proper level.
Both fluids are nonflammable and offer a drying system compatible with most plastics, elastomers, metals and other materials of construction (see Tables 3 and 4, below). The displaced water, along with its water-soluble contaminants, will automatically float out of the drying sump and down the drain. The Vertrel® drying bath will retain its effectiveness over long periods of use.
Due to the limited solubility of surfactant in water (75 ppm), some surfactant loss can be expected. This can be compensated for by adding Vertrel® XDA when additional solvent is needed in the drying sump. The surfactant is effective over a wide range of concentrations (0.02 to 0.2%).
The compatibility of materials with Vertrel® XDA is virtually the same as with Vertrel® XF. Both products have a broad compatibility range. Long-term compatibility data simulating exposure of equipment construction materials is available from DuPont upon request.
Elastomer swelling and shrinking will, in most cases, revert to within a few percent of original size after air drying. Swell, shrinkage, and extractables are strongly affected by the compounding agents, plasticizers, and curing used in the manufacture of plastics and elastomers. Therefore, prior in-use testing is particularly important.
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