Edwin TafelmeierLaboratoryManager

Definition of Shelf Life

Everything ages. In addition to wear andtear there may also be changes withinmaterials themselves. Most metals forinstance show signs of corrosion. Astime passes many polymer materialslike coatings and inks also experienceproperty changes. Shelf life defines theperiod of time a material or thing willremain in a state suitable for the originallyintended purpose. Thus the term shelflife is also defined by the requirementsa material has to meet.

Printers do expect that a certain typeof printing ink will show reproducibleresults. Naturally this can only be ex-pected on the provision that the otherconditions remain unchanged as well.Are the prints applied on the same sub-strate? Are the adjustments of the prin-ting equipment the same? In thesecases the best possible exact reproduc-tion can be achieved using a newlyopened original ink container. Most dif-ficult applications are only printed thisway - unsealed containers are not used.Any inks left over after printing the orderor at the end of a shift are disposed of.These aspects are not of great signifi-cance in technical screen applicationswith their standardized processes.

SHELF LIFE OF INKS INUNSEALED CONTAINERS

For less difficult processes this may beconsidered to be a waste of materialand therefore inks in unsealed contai-ners are used up. Shelf life of openedcontainers can be improved

by tightly re-closing the containersimmediately after use, and storing thecontainers in a cool environment.Also never put left-over inks back intothe original container, it is better tostore them in separate containers.

Inks stored in unsealed containers canbe used as long as the resulting printsshow the required results. Un-fortunately it is not possible todetermine an exact expiration datefor opened ink containers. This is amatter of individual experience.

Aging is a continuous process.Properties of a printing ink are surelyno different one day before and oneday after the expiration date. Asmanufacturer, however, we have todetermine a period of time from thedate of filling during which weguarantee the properties of our goods.Here we can only assume controlledconditions in unopened originalcontainers, all influences after openingare beyond our direct control.

Influences in storage are eitherreversible or not. During storage youmay experience a non-homogenousdistribution of ink components such assettlement of pigments, floating ofadditives on the surface etc. Suchchanges are reversible and can easilybe reversed by thoroughly stirring theink before use.

Customers often want to know,

how long they can store an ink

after opening the container for the

first time.

Provided ink containers are stored

properly we can guarantee pro-

cessing properties of our inks until

the expiration date on the con-

tainers. This is different though for

ink containers customers have

already opened.

It is not possible to give a general

answer regarding shelf-life of

inks in unsealed containers. The

reasons why are explained below.

Separation*

Edwin TafelmeierLaboratory Manager

+49 911 64 22-241 +49 911 64 22-283edwin.tafelmeier@sunchemical.com

Thorough stirring of inks before use ismainly recommended to obtain thedefined original rheology. Generallyscreen printing inks thin by shearing,meaning that the viscosity is reducedby shearing powers like those triggeredby stirring. Often a loose structure isdestroyed which will rebuild itself againover longer periods of time. Experts callthis phenomenon the thixotropicproperty.

In order to obtain reproducible resultsa printing ink has to be in its predefinedstate.Inks which have thickened due tosolvent loss by evaporation can bethinned with suitable additives.However, after thinning the ink will nolonger be in its original delivery state.This is no problem for printers as longas they use suitable and recommendedthinners.

Most UV curing inks do not contain anyevaporating substances. UV thinnersare only used to adjust inks to optimalprinting viscosity. However, UV inks arereactive systems which may polymerizeprematurely under unfavourableconditions. Clear UV-varnishes oftenmay already skin on the surface due tolight scattered by regular daylight. UVinks which have unintentionally gelledalso are no longer in their original state.Even though you have no solventevaporation UV ink containers must beresealed tightly after opening.

Adjusting Inks by Addition of

Auxiliary Agents

Our product data sheets contain a lot ofinformation about combinations of theinks with various additives. Thisinformation is based on many years ofexperience. However, we cannotconsider all possible circumstances.Actually experiences of the printersthemselves are also very valuable. If indoubt only prepare as much ink as youreally need for a print job.

A general reply to the question howlong an ink will remain stable in acontainer which has already beenopened would be: as long as your printswill have the requested and requiredproperties. However, such an answeris not very helpful.

If in doubt, contact our laboratory

and technicians. We will try to find a

suitable answer tailor-made to your

special needs. In screen technology

you will often find that there are no

general answers. But that again is

the reason why our work is always

so interesting!

Can Inks Still Be Used After

Their Expiration Date?

Usually our solvent based screen printinginks have a shelf life of 5 years. UV-curing inks can generally be storedfor 1 year. Naturally there are also ex-ceptions. Always refer to the informationgiven on our labels. We have checked20 year old solvent based inks in un-opened cans and after thorough stirringthe inks were still ok. Some UV-curinginks can still be processed after 3 years.Still we do not recommend to processinks which are beyond their expirationdate. Printers will process such inks attheir own risk, without any manu-facturer’s liability. Some old inks mayalso contain substances which are -according to recent regulations - nolonger wanted or allowed. For instance5 years ago, phthalates were still usedin varnishes and coatings - today theyare banned in most applications. Re-cently manufactured inks will alwayscorrespond to present stipulations andregulations.

Floating of white pigmentin a mixed ink*

Bénard cells (metallic ink)*

Flooding of particleswith lower specific weight*

*Will not show in the printed image afterhomogenization/stirring