Properties of Epoxy Systems
Written October 17, 2019
There are two main groups of epoxy screen printing inks: the single-component group and the two-component group. The single component system is made from an epoxy resin blended with heat curing resins. Amino resins (heat curing) such as urea-formaldehyde and melamine-formaldehyde, or phenolic resins such as phenol-formaldehyde or phenolic derivatives are commonly used. The amino resin blends are selected when color retention is important. Other resistant properties, with the exception of abrasion, are still maintained.
If abrasion resistance, as well as other resistant properties is important, then the phenolic resin blends are chosen with the understanding that color retention will lessen. Naturally, since the single-component group is made with heat curing resins, these inks must be baked. They cannot be air dried. The baking temperatures range from 121°C to 260°C (250°F to 500°F) with time periods of 90 seconds to 1 minute, respectively. Most manufacturers concede that multiple colors can be applied with a time period allowed between each color application for solvent evaporation, followed by a single color bake after the last color is applied.
An advantage of the single component system is that it is modified to printing viscosity with thinners or retarders as is a conventional ink. There is no need for special mixing scales or mixing formulas for catalyst (hardener). Control of storage temperature, however, is important for a stable, usable product. Given sufficient time, a single component system could gel if stored improperly. It need not be kept in a refrigerator, but by the same token, an outdoor paint shed would be disastrous. To ensure product performance, it is best to keep storage volume to a minimum.
Don’t over order! Baking temperatures and time periods recommended by the manufacturer refer not to oven temperature or how long the item is in the oven, but to the temperature of the substrate at which point the time cycle begins. Smoke charts, surface pyrometers or other instruments are excellent if you can afford them but are not required. Manufacturers can provide chemical or solvent testing procedures to determine the properties of a cured epoxy.
Oven recovery time and substrate mass can be established and recorded for consistent results. Most users of the single component systems today are captive and/or industrial plants (often equipped with convection ovens) producing parts or finished goods for aircraft, marine, appliance, electronic and automotive markets, to name just a few.
Two Component Type
The second type, and the most popular with job shop screen printers, is the two- component epoxy. Unlike the first epoxy mentioned above, this group requires the addition of a catalyst (hardener) by the screen printer before use. There are two methods for adding catalyst.
One is volume measurement and the other is by-weight measurement. Whichever method of measurement is chosen, it must be accurate. You cannot shortcut in either system. These systems offer both air dry and baking formulations. Air dry formulations are used where heat would cause the substrate to deteriorate.
A volume mix usually calls for mixing by fluid ounces, not by air dry ounce weight and certainly not by “one capful of catalyst to four capfuls of ink”.
Mixing of a volume system is best accomplished by using a minimum of two graduates marked for measurement in fluid ounces. Glass is best, since the level of both catalyst and ink can be seen and glass can be cleaned more completely after use. The graduate for measuring the catalyst should be shaped similar to a test tube with a base and have a capacity for two fluid ounces. The capacity of a graduate for ink should be eight fluid ounces. Do not buy large graduates unless you intend to mix epoxy in great volume. Measuring small amounts of catalyst in large capacity graduates is not accurate.
Formulas are usually one-part catalyst to four parts color. Do not attempt to get by with one graduate for both ink and catalyst. The procedure for mixing by volume is as follows. Thoroughly mix the ink and catalyst in their respective containers with separate, flat tipped spatulas or other non-contaminating mixing bars, until all settled solids are back in suspension. Pour catalyst and ink into their respective graduates to the desired level. Pour the measured catalyst into the measured epoxy ink. Mix thoroughly and allow to stand for a period of 30 to 45 minutes.
This time period is called “induction time.” During this period the catalyst begins to combine with the ink uniformly so that polymerization in the batch will be complete. Cured film failures are often traced to insufficient time allowed for induction, or a complete disregard for it. After induction time, the batch should then be transferred to a non-contaminating throw-away container. The graduates should then be thoroughly cleaned and stored. Graduates are inexpensive and should not be used for other than epoxy/catalyst measurements.
Mixing by Weight
To mix a by-weight system you will need a triple beam balance scale with measured capacity in grams. There are balance scales marked in ounces, but the gram weight scale is much more adaptable to formula variations. You will also need a minimum of two wide mouth glass beakers with fluid capacities of approximately 100 ml or 250 ml (4 or 8 ounces).
The triple beam balance scale consists of a platform or table on which the beakers are placed, an adjustable counterweight under the platform used for zeroing the scale, three separate beams with sliding weights for 10 gram, 100 gram and 500 gram measurement, and finally a zero balance line and mark.
If you have never used a by-weight system, you should mix the first batch by the manufacturer’s given formula. Let’s assume that this given formula calls for 7 parts of catalyst to 100 parts of color. (Formulas may vary according to each color and/or manufacturer.) If the batch obtained by mixing this formula was too great, and you only needed half that amount, for example, then the next time cut down the measurements to 3 1/2 parts of catalyst to 50 parts of color.
If the batch was too small, then increase it to perhaps 14 parts of catalyst to 200 parts of color, etc. Caution: Manufacturer’s formulations are accurate to 1%. When mixing less than the recommended formula in two separate beakers, it will be necessary to compensate for the amount of catalyst which clings to the beaker after it is poured into the color. The example of 3-1/2 to 50 given above, with compensation for lost catalyst, would be 4 to 50. When both color and catalyst are weighed in one beaker, compensation is not necessary.
Set the beam scale on a level surface. Slide all weights on the three beams back to zero.
Adjust the counterweight under the platform until the zero-balance line matches the zero mark. Next, place a beaker on the platform and move the beam weights until the zero balancing line and zero mark once again match. Be accurate! Do not accept any weight where the zero-balance line does not match the zero mark exactly. Then read the beaker weight and record it for future reference, and add the grams of catalyst to the beaker weight by moving the 10 gram weight to add 5 grams (not the full 7) to the beaker weight and begin adding catalyst.
By under-weighing each component at the start, you will become accustomed to the amount required to “tip the scale” and chances are that you will not over-weigh either component. After weighing 5 grams, move the 10-gram weight another 2 grams for the last addition of catalyst.
When weighing color, after establishing beaker weight, the procedure is the same in that you will move the weights to a point which will be less than the full weight.
The exception will be that you will be moving all three beam weights, so according to the above formula, your under-weight target should be 90-95 grams above beaker weight.
After weighing catalyst and color, combine the two and mix thoroughly. (Until you become more proficient, it is suggested that the two components be measured separately, then combined. Later as you become more expert, both components may be weighed in one beaker.)
Follow this with an induction time of 30-45 minutes, then transfer the mixture to a non-contaminating throw-away container for use. Mix only the amount needed, as any unused portion will only become useless and be then thrown away. Always remember to zero the balance scale on each use and after placing the empty beaker on the platform. Also, be sure to remove any ink spills from the scale and clean all beakers before storing.
There are two things to remember and/or establish when mixing either a volume mix or a by-weight system. The first is “pot life” and the second is the square foot coverage required during the printing period.
Pot life is the time limit established for a screen able viscosity. The pot life of a two component system, after induction time, will vary from 2 to 24 hours, depending upon the catalyst required for the specifications of the finished product.
If a short pot life catalyst is required by specification and the volume or square foot requirement per printing period is large, then it is best that small batches be made in succession to insure fresh, usable mixtures during the printing period. A long pot life catalyst under the same conditions would require fewer batches and each batch volume could be greater.
On the average, epoxy printing inks will yield 24.5 square meters to the liter (1000 square feet to the gallon).
But by improper calculations on your part, this figure could be far less. A good procedure for both volume and by weight systems (if more than one batch is required) is to have the catalyst and ink premeasured, but not mixed until within one hour of the time needed by the printer. This is enough time to mix the two components and allow for induction. Thinning or retarding for printing viscosity is done after induction time. Do not assume that alterations with solvents will extend pot life.
Catalysts
There are many catalysts available for use in epoxy resins. The selection by the manufacturer is determined by toxic level, color retention and resistance properties. Two types commonly used are polyamines and polyamides. The amine cured resins have good color retention and excellent resistance to oils, solvents, chemicals, humidity and salt spray, but have a higher degree of toxicity than the amides.
The amine catalysts usually have a shorter pot life and, therefore, have shortened curing times and lower baking temperatures. Amide cure resins sacrifice some color retention and chemical resistance for lower toxicity, greater adhesion, flexibility and impact resistance. Amide catalysts usually have longer pot life, longer curing times and higher baking temperatures.
A third type of catalyst, but not as common as the above, is polysulfide. This catalyst is similar to polyamides and further improves chemical resistance and flexibility—otherwise no additional benefits are gained. Most catalysts are room temperature cured.
However, manufacturers will call for baking or forced curing in order to achieve a specific result. If left to cure at room temperature, the full cure properties are not achieved until after 3 to 7 days. Product tests results on under cured film will, therefore, fail. This is another area which is commonly overlooked. Do not perform any tests before the prescribed air cure time has elapsed.
If certain finished film properties are required, the manufacturing chemist may blend several catalysts on the basis of their contribution, or stay with a given catalyst and either increase or decrease the proportion to color. The purpose of providing you with a large selection of catalysts is to make epoxy ink systems for almost every conceivable substrate.
Those substrates which are heat sensitive can be printed with a two component system which will air dry or at least cure at temperatures that would not affect the substrate. And at the opposite end of the list are those catalysts which would be used in cases where the substrate is not heat sensitive. In between are the combination properties of the extremes.
While there appears to be all the advantages in two component epoxy screen ink systems for the job shop, there are some characteristics to watch. Unlike the single component systems, the two component types will store quite well in the original, unopened container. After use, the ink or color will continue to store well, but the catalyst may absorb moisture and become ineffective.
This is another possible cause for failure following a successful run. For this reason, manufacturers will provide additional, fresh catalyst at no cost or at a small charge. Selecting a fast air cure (and, therefore, short pot life) catalyst may give you a completely polymerized, testable finish in three days, but the possible yellowing properties occurring on aging may cost you the entire job and/or customer.
Using a catalyst/color combination on washable glass that was designed for general application may result in adhesion failure. There are catalyst/color combinations designed specifically for finished products which will be subject to repeated washings or high humidity. As with conventional inks, there are special clears in epoxy systems for bronzing to prevent tarnishing.
These, too, will often call for a different catalyst and/or formulation. Epoxies are not suitable for use outdoors. While color retention may be excellent, they tend to chalk in a very short period (90-120 days). This is more noticeable in gloss finishes than in flat finishes. They should not be used for outdoor signs or displays.