A Discussion of the Radiant Infrared Process

Due to federal or local regulations, increasing numbers of printers and converters are considering alternatives to solvent-based ink systems.  Generally, the firms that have made the commitment and switched are pleased with the results gained from waterbase chemistry.  Firms that are negative about aqueous systems have experienced poor test results, probably with inadequate equipment, or else are concerned about abandoning known technology and learning new techniques.  The business faced with the greatest difficulty are those trying to use solvent and aqueous chemistry simultaneously.  They are reluctant to eliminate solvents and hesitant to commit to new methods.

Switching technologies need not be a catastrophe.  Time spent in selecting inks/coatings and drying equipment can provide financially rewarding results.  The purpose of this report is to explain alternate drying methods and to provide guidelines for drying equipment decisions.

Infrared Drying Systems are an efficient and appropriate means of drying aqueous inks, coatings, and adhesives due to the increased amount of BTUs delivered to the ink/coating surface, along with controlled air flow.  The "IDEAL" IR Drying System is referred to as a "SYSTEM", to differentiate it from IR heaters sold as dryers.  "IR Heaters" normally lack any air flow, certainly do not have controlled air flow, and have little or no temperature control capability.  Their appeal is low price, but the design normally results in low productivity.

IR Drying "SYSTEMS" can safely dry alcohol-based chemistry, either as a booster or as a primary dryer.  For safety reasons, IR dryers even systems designed with "general purpose electrical enclosures" are not recommended for drying volatile solvents comparable to those found in gravure chemistry.

The advantages of switching to waterbase chemistry are elimination of:

• Storage and handling solvents
• Explosion-proof storage rooms
• Need for explosion proof electrical equipment in press room
• Need for expensive solvent burn-off units

Benefits accrued from switching to waterbase chemistry are:

• Greatly reduced fire insurance premiums
• Reduction of personnel problems, i.e., health safety
• Logical method of achieving EPA and local codes and regulations
• Lower capital investment required
• Reduced taxes

Switching to aqueous chemistry not only requires changes to existing drying equipment, it will also necessitate selection of new inks or coatings appropriate

Benefits of Using Electric IR Are Numerous

New production techniques along with personnel training will have to be developed (obligatory steps for any new process.)

The financial benefits will more than offset the costs involved.  The resulting increased productivity in a healthier and safer environment will benefit both management and plant personnel.

To effectively dry waterbase inks/coatings/adhesives requires a drying technology that is an extension of standard drying methods.  The following explanation of drying techniques may assist in the decision process.

Laundry will not dry in a damp basement.  As an analogy, remember this statement when considering the importance of air movement relative to heating requirements.  Both air movement and heat are just as important for effective drying of waterbase inks and coatings.

Some ink/coating sales personnel are totally unfamiliar with infrared drying systems.  The information provided to them, with respect to drying time and temperature, is based on drying ink/coating swatches in a hot air oven, typically batch type.  Not being familiar with IR, it is easier to suggest a hot air system than to learn about infrared.  Some ink/coating people may have ill-fated drying experiences with IR heaters, the type without scrubbing or exhaust air.  Their reticence is understandable.

In general, ink and coating drying requires:

1. heat to vaporize the vehicle
2. impinged air to scrub the vapor laden air away from the substrate
3. exhaust to remove the vapors from within the enclosure

Vapor removal through scrubbing and exhaust is necessary to reduce the partial vapor pressure of the air within the enclosure below the partial vapor pressure of the air within the enclosure below the partial vapor pressure of the vehicle being vaporized in order to permit more evaporation.

Solvent-based formulations, due to solvent volatility, usually require little or no heat, relying mostly on air flow to remove the solvent vapors.  In many applications, room temperature air along with air flow caused by web travel is sufficient to dry.  In these cases, exhaust provides some assist, but is more for health and safety reasons rather than for drying purposes.

Waterbase (flexo, gravure, etc.) inks or coatings on the other hand require considerably more heat to vaporize the vehicle before the other elements of drying can be effective.  As an example, water requires 965 BTUs per pound to evaporate, whereas toluene needs only 155 BTUs per pound to evaporate.  It takes 6.23 times more energy to evaporate a pound of water as compared to a pound of toulene.  Solvents with lower boiling points require even less energy.  Water removal is the critical factor in drying speed, not the type of ink, i.e., flexo, gravure, etc.

Choices of Methods & Drying Equipment

Usually, ovens or dryers designed for solvent applications will not provide satisfactory drying speeds with waterbase formulations due to insufficient heat availability and/or lack of controlled air flow.  Therefore, test results with aqueous chemistry on existing presses (with dryers designed for solvents) are generally poor.  Inadequate drying and slow operational speeds can be the result of insufficient moisture removal caused by lack of available heat and insufficient air flow.

Within practical limits, the temperature difference between heat source and substrates is inversely related to the dwell time required for drying.  The hotter the source, the shorter the drying time.  The larger temperature difference between source and target enables a greater energy flow.  The type of heat source utilized also affects heat transfer efficiency.

Typical heat sources are steam, gas and electric.  Drying systems are normally either hot air or infrared.  Direct gas fired systems are not usually found in gravure applications for safety reasons and are very inefficient due to very large stack losses.

Hot air systems, in general, are inefficient due to the number of heat transfer processes involved.  Every heat transfer process is responsible for some loss of energy.  ("HTP #n" denotes (H)eat (T)ransfer (P)rocess #(n)umber).  The heat source must heat the air, (HTP #1) which in turn is delivered through duct work (heat loss) to the drying chamber.  Air flow within the chamber determines the heat transfer efficiency of heat delivered to the substrate (HTP #2).  The substrate, in turn, vaporizes the ink/coating vehicle (HTP #3).

Making Energy Serve You Better

An Infrared Drying "System" will have electric IR elements that typically radiate medium wave energy directly to the surface of the ink or coating, a one step heat transfer process.  The IR energy induces the moisture in the ink or coating to vaporize.  Substrate temperature and relative humidity are hardly affected as compared to product delivered from a hot air dryer.

The ideal system will have IR elements with a fast warm up from a cold start (approximately 20 seconds or less) and immediate cool down capability (upon being de-energized).  The IR heaters are energized through the power supply whose control circuit is governed by press "RUN" and "STOP" circuits.  Upon web stoppage, the heating elements are de-energized through the power supply whose control circuit is governed by press "RUN" and "STOP".

The ink or coating surface temperature should be easily regulated by energizing the IR elements through an SCR.  The SCR in turn is controlled through a variable potentiometer with a graduated scale that can easily be readily and repeatedly set in discrete levels for diverse job conditions.  The SCR provides an energy efficient means of fine tuning heat requirements to the job.

A high pressure exhaust blower is needed to provide negative air pressure within the drying chamber for moisture control.  For energy conservation blowers should be controlled by the time delay relays to shut down after several minutes of cooling the web and purging the system.

Infrared Drying "Systems," in general, provide the following advantages overhot air systems:

• Thorough Drying
• Faster Production Speeds High Efficiency
• Compact Size
• Installation Ease
• Operation and Maintenance Simplicity
• Accurate and Repeatable Heat Control

Benefits accrued from Infrared Drying ``Systems" are:

• Increased Profitability
• Lower Capital Investment
• Lower Maintenance Costs

General guidelines in determining if IR drying with waterbased materials is tenable:

 Waterbase chemistry is available that:

1. is compatible with the substrate
2. compatible with other chemical components of the system
3. provides the needed characteristics, i.e. gloss, scuff or product resistance, adhesion, etc.

Clear coatings and all colors, especially light hues, are most efficiently drier with medium wave IR.  Although this range of materials is mot receptive long wave IR, the heat-up and cool down times of long wave units make substrate protection difficult.  Medium wave IR is therefore a suitable alternative.

Dark colors, i.e., black ink on white paper, are most efficiently dried t short wave IR.  Short wave, however, is practically useless on clear coating or light hues as most of the light energy is reflected.

The application thickness (wet) is relatively thin and measured in thousand not tenths of an inch.

The chemistry does not skin over and blister when exposed to IR. Typical this is an adhesive, rather than ink or coating limitation.

The material to be dried is an exposed surface with line of sight to the I source, and is not hidden between layers of substrate or by machine components.

Explosion proof electrical requirements are not specified for the press ( press room area where the system is to be installed.

The specifics of an application should be discussed with the ink/coating adhesive supplier as well as the IR drying system source.  If an incompatibilities exists, or the specific application is more suited for another drying method: then an alternate to IR drying should be selected.  Otherwise, the installation of an electric infrared "Drying System" should provide increased productivity, reduced personnel health and safety problems, and increased financial incentives for most users.