By Olivia Cahoon
Part 1 of 2
Ink primers, sometimes referred to as adhesion promoters, assist in proper ink adhesion to objects. Objects that require ink primers are generally untraditional, three-dimensional objects that require extra preparedness to ensure the ink sticks. Adhesion promoters are also used to prevent peeling and chipping.
Sticky Benefits
Printing rigid and super flexible substrates is sometimes difficult due to ink chemistry, the material, and printing methods. Adhesion promoters strengthen the bond between the ink and the substrate while stimulating adhesion.
Using adhesion promoters significantly improves the ink’s printability on difficult substrates. “With such a varying degree of quality found in substrates, adhesion promoters are increasingly necessary for UV-curable inks to stick to many substrates,” says Susie Mendelssohn, North American sales manager and international marketing manager, INX International Ink Co.
Curt Baskin, field marketing manager, Sun Chemical Corporation, agrees and says that adhesion promoters are popular because many plastics have sub-par adhesion when printing directly. Soft surfaces like polycarbonate are directly printed without adhesion promoters but not without the loss in brightness and light transmission.
“Printers can print on almost any surface with one machine when using an adhesion promote, which gives enormous flexibility,” suggests Pedro J. Martinez, CEO, Afford Inks. Using an adhesion promoter, he believes a small print shop can print virtually on any material. In some cases, promoters can even increase the optical density of the printers.
In addition, promoters also provide the ink with rub and scratch resistance. Without proper adhesion, ink may peel and chip off the printed object, adds Mendelssohn.
Unique Objects
Adhesion promoters are commonly used to print onto untraditional objects with curves like glass bottles and plastics. Baskin says this includes applications like appliances, medical parts, in-mold decorating, and in-mold labeling.
Materials that benefit vary. “Typically, adhesion promoters increase the ability of ink to fasten hard to substrates such as specific plastics, glass, metal, and tile,” explains Mendelssohn. Specifics include coroplast, lexan, polypropylene, PVC, and styrene.
Martinez believes that specific adhesion promoters are ideal for plastics, glass, and metal. There are three types—solvent-based, water-based, and UV-curable. He says the difference in the promoters lies in the chemical nature of the primers. “Some use mechanisms such as hydrogen bonding, whereas others use different mechanisms.”
Tips and Tricks
Adhesion promoters should be applied with proper safety equipment. This includes latex or nitrile gloves and safety glasses. Applying the promoter in a well-ventilated area to ensure the proper evacuation of fumes is also necessary. Lint-free cloths saturated with promoters are suggested to evenly wipe down the entire surface of the substrate. Printing room humidity should generally be between 40 and 75 percent.
Before printing, adhesion promoters should be dried onto the substrate for roughly ten minutes. After drying, it is suggested the object be printed as soon as possible and is printable for up to 24 hours after promoter application. For UV curing with adhesion promoters, printed surfaces should post cure for 24 hours.
Mendelssohn points out that using adhesion promoters on acrylics, especially dark colored acrylics, sometimes leave a light haze. Acrylic has a low smoke and burn rating and is often chosen for applications in museums and schools for display, exhibit, and signage industries.
Occasionally adhesion promoters just won’t work. According to Mendelssohn, some of the more inexpensive substrates available on the market, like plastics, have difficulty with ink adhesion using OEM, alternative, or third party inks. “Even the use of an adhesion promoter won’t be enough,” she admits.
Martinez warns that some primers may be affected by time and the application or the transparency may not be adequate to use on certain substrates, thus rendering the promoter ineffective.
Corona treatment is an alternative process to enhance plastic printability. It increases the surface energy of plastic films for improved adhesion of inks, coating, and adhesives. However, corona treatment may transfer static to the material. Luckily, adhesion promoters can address static issues if the inherent electrical energy doesn’t dissipate over time.
Adhesion Promoters
Advancements in digital print technology create demands for printing unique objects. Adhesion promoters strengthen the bond between the ink and the substrate so print providers can offer digitally printed ceramics, glass, and plastics. It’s important that print providers handle adhesion promoters with proper safety equipment like latex gloves in well-ventilated areas.
The second part of this series features a roundup of available adhesion promoters used in the industrial space.
Click here to read part two of this exclusive online series, Priming for Print
Ink Adhesion
Aug2017, Industrial Print Magazine