Tech n ical p ape r – PCT Engineered Comparison of UV and EB Technology for Printing and Packaging Applications User Manual

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SEPTEMBER/OCTOBER 2008 RADTECH REPORT 33

Tech

ical P

ape

migration of the photoinitiator and

their fragments may be a concern.

EB curing of free-radical inks and

coatings requires inerting to displace

the atmospheric oxygen in the reaction

chamber of the cure unit. EB energy is

deposited more evenly throughout the

thickness of the ink and coating.

The absence of excess energy at the

surface does not allow curing reactions

to compete with oxygen termination

(Figure 1).

EB laminating involves irradiation

of the adhesive that is contained

between two layers of substrate. EB

laminating does not require inerting

because the substrates are generally

effective at preventing the diffusion of

oxygen into the adhesive layer.

Inerting is most commonly achieved

with nitrogen gas. Nitrogen serves to

displace oxygen from the reaction

chamber. The most common source of

the nitrogen gas is a tank of liquid

nitrogen. The liquid offers the high-

purity nitrogen and volume needed for

the curing process. Most modern EB

equipment is designed with nitrogen

knives to remove the surface boundary

layer of air. Optimized inerting systems

can reduce the amount of nitrogen that

is used.

Effect on Substrates

Since EB is ionizing radiation, it

may affect the thermal and mechanical

properties of substrates. EB affects

different polymer films in different

ways. References are available which

describe the effects. Fortunately, with

the relatively low dose (20 to 40 kGy)

used in most curing applications, the

effects are minimal and the films are

still fully functional for the intended

application. Another strategy to

minimize film damage is to use low

voltage in the range of 70 to 110 kV.

These voltages allow the beam to easily

penetrate the coating and ink layer

while minimizing the energy at the

inner (food contact) layer. This is

particularly important when the inner

layer is designed to be heat sealed when

the packaged is filled and sealed.

In some cases where porous

substrates (such as paper or cavitiated

films) are used, it can be advantageous

to use EB to cure materials which have

penetrated into the substrate.

EB’s effect on the substrate can be

beneficial. Cross-linking may enhance

the properties of some polyethylene-

based films. EB-induced ionization of

the film surface may result in

enhanced adhesion by grafting of the

ink or coating layer. EB can also

heat exposure. Most arc lamp-based

web systems include shutters to

prevent the web from burning when it

is stopped. Other strategies used to

minimize lamp heat effects on the

substrates include dichroic reflectors,

hot mirrors and chill drums.

EB is a cooler process compared to

UV. Some internal components of the

EB emitter (including the window)

utilize water cooling. Little heat is

transferred to the substrate which

allows most packaging films to run

without any effect on the dimensional

stability of the film. A chill drum may

be integrated into the EB unit for

potentially be used for simultaneous

curing and surface sterilization of the

food contact layer.

Since UV is non-ionizing radiation,

effects on the substrates are minimal.

Since grafting is not expected, a primer

layer may be needed for adhesion to

some films.

Heat Control

Mercury lamps used for UV curing

produce significant heat. This is due to

high temperatures needed to create

and maintain a plasma within the

quartz bulb. Approximately one-half

of the electrical energy input into

the lamp is converted to heat (IR)

energy. UV systems for printing and

packing applications are commonly

cooled by moving high volumes of air

over the lamp. Water-cooled lamps are

also available.

Many packaging films may be

adversely affected by heat from the

lamps. High-speed transport of the

substrate under the lamp minimizes

applications that are very sensitive to

heat. In this configuration the

substrate is in direct contact with the

chill drum during irradiation.

Food Packaging

UV-curable coatings and inks have

been used in food packaging applications

for many years. These applications are

possible with packaging designs that

include a functional barrier between

the ink or coating and the food. Taint

and odor problems can usually be

prevented by using properly formulated

UV-curable inks and coatings.

Photoinitiators and photoinitiator

fragments can be a source of concern

for migration, odor and taint. New

systems have been developed that

include polymeric photointiators,

reactive photointiators,

and oligomers

that contain a “built-in” photoinitiator

moiety.

Some of these systems have

been effective but may still lack cost/

performance properties needed for

practical applications.

In general, there does not tend to be a significant

difference in cost between UV and EB inks,

coatings and adhesives for printing and

packaging applications.