Printing Paste Vehicles

 

 

By volume, typically the largest component of a thick film printing paste is the organic components, or vehicle. The vehicle is one of the most critical components of a printing paste even though it typically does not remain in the final fired component. Polymer Innovations, Inc. (PII) produces vehicles for a number of applications. These vehicles are formulated, produced, filtered and pre-tested at PII. In most cases these vehicles can be mixed with required metal and ceramic powders to yield pastes which can be designed to offer a variety of drying rates, residual tackiness levels, various solids and viscosity levels all with exceptionally low attack or distortion on ceramic tapes. Typically the vehicles are sold more concentrated than actually needed and the paste formula would also contain a thinner.

 

 

 

 

VS36H & VS36L

 

These products are based on a modified EC polymer designed to help with some of the weakness of EC based pastes. In particular they have been formulated for higher green adhesion to ceramic tapes after lamination and lower wet and dry ceramic tape distortion. These two products are identical except the ‘H’ uses higher molecular weight polymer while the ‘L’ contains lower molecular weight polymer. This means the ‘H’ has higher viscosity and the ‘L’ is lower viscosity and the two can be blended in various ratios to control final paste viscosity while keeping the organic solids amount in the paste constant. The preferred thinner is TS33 and typical formula weight usage level is approximately 10%.

 

 

 

 

Epoxy Vehicles

 

 

 

EV110H & EV110L and EV120H & EV120L are completely formulated, one part epoxies useful for the formulation of screen printing pastes. The vehicles are blend compatible within their own product group, for example: EV110H and EV110L are designed to be blendable while EV120H and EV120L are compatible to blend with one another. However, blends between EV120 and EV110 products may yield unknown results as the two product groups use different curing chemistries. The compromise in using one part versus two part epoxy systems is mostly related to curing temperatures required and shelf life/curing compromises. One part systems are convenient as the curing stoichiometry is correct and uniform from the factory. This is particularly convenient for screen printing operations were uniformity is more critical. The key to one part systems is to have a system which is relatively shelf life stable at normal storage temperatures yet cure quickly when a certain temperature is reached. The difference between EV110 and EV120 chemistries is in the curing chemistries.

 

Within each system there are ‘H’ and ‘L’ versions which refer to the molecular weight of the epoxy used and thus also the viscosity. ‘L’ versions have a solids content of 97% and use low molecular weight epoxy which will result in lower paste viscosities for a given epoxy solids content and a harder, higher crosslink cured product. This will provide the highest solvent resistance and hardest film. ‘H’ versions contain higher molecular weight epoxy and have higher viscosities even though the solids content is about 69.2%. The ‘H’ versions have lower crosslink density after curing and will result in a slightly lower but most likely still adequate solvent resistance. Also, the ‘H’ systems have a better screen printing nature with better printing and flow characteristics. Furthermore, the ‘H’ systems can be dried yielding a dry tack free print which can later be heated further and cured.

 

All of these systems are fully formulated with adhesion aids and rheology characteristics to make final pastes with the addition of desired fillers. With the abilities to blend ‘H’ and ‘L’ versions, vary the ratio with the filler, and thin further with solvents, this system makes it possible to use a wide range of fillers and filler loading levels.

 

 

 

EV110H & EV110L

lower temp curing

 

EV110 systems can cure at temperatures of 120°C or even lower. As a trade off shelf life is less, perhaps around 3 months at ambient temperatures and shelf life can be easily worsened by other additives used in the paste formulation. In particular, solvents or other liquid additives can dramatically shorten shelf life. For this reason the only recommended thinner for the EV1 system is carbitol acetate (monoethyl ether or diethylene glycol acetate) and other solvents would require testing before use to see the effect on shelf life of the paste.

 

 

EV120H & EV120L

higher temp curing

 

EV120 requires a minimum cure temperature of about 180°C for effective curing. Lower temperatures even for extended times may not result in an effective cure. As a result EV120 systems typically have long consistent shelf lives to the order of 6 months or more at normal ambient conditions. Also the EV120 system curing and shelf life are relatively insensitive to other additives (solvents in particular) added to the final formulated paste.

 

 

 

 

For Formulation Guidelines and more information on Epoxy Vehicles click here.

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