By Cassandra Balentine
Part 3 of 4
Glass and ceramics can be difficult applications to print onto. This is primarily due to the natural surface energy and irregularities in either flat or cylindrical or molded options. Pretreatment is designed to offer a surface that can accept ink and ensure a solid bond.
As we discussed in parts one and two of this series type of glass, end use, and durability—think dishwashing—determine what inks and/or pretreatment should be utilized for the best results.
Pretreatment
It is almost always appropriate to pretreat glass prior to digital printing to the surface.
“Glass and ceramics treat very easily and many of the contaminations can be removed by the plasma itself, which eliminates additional cleaning steps,” offers Wilson Lee, director of business development, Enercon Industries.
The key to any type of bonding to a surface, whether you are using adhesives, coatings, or inks, is a consistent and repeatable high-energy surface. “That is what the plasma and flame surface pretreatment steps are designed to do. In the long term, plasma and flame plasma are the most economical and repeatable options,” adds Lee.
Michael Perrelli, marketing director, Innovative Digital Systems, agrees, adding that for optimal adhesion when UV printing, the answer is always to pretreat.
“I would not really recommend printing without using an adhesion promoter,” stresses Adam Tourville, director of sales, North America, Direct Color Systems. “If it is second surface print and going up against a wall or backing, I would still recommend an adhesion promoter. The thermal expansion and contraction of the glass can cause the ink to still come off because the chemical bond is almost not existent.”
“For us, it is about testing adhesion and verifying it fits the end use application. Is it an architectural panel, backsplash, shower remodel, art, or even a stained glass as its end use? All of these may impact the pretreatment decision,” shares Michael Hemmelgarn, territory sales manager, swissQprint America.
In addition to promoting ink adhesion, pretreatments offer other functions. For example, Sean Lanigan, president, Applied Surface Technologies, LLC (AST), suggests pretreatment anytime the printed surface needs to offer abrasion and scratch resistance or requires high speed filling—i.e. beer, liquor, cosmetics; must have durability for extended periods; needs to be ice and water durable—beer and liquor bottles; requires resistance to chemicals—lotions, soap, isopropyl alcohol, and things encountered in bathrooms for cosmetics; or needs to be dishwasher safe.
Bas Buser, consultant printing applications, Plasmatreat GmbH, says pretreatment does depend on the type of glass and whether or not a coating was applied on the glass at some point during its production.
Lanigan says one of the important reasons to use a surface treatment on glass containers is when cold end coatings are applied during the manufacturing process. “Cold end refers to the point after the glass is formed and during the cooling down stage, so it is the ‘cold end of the process,’ but it can mean when the glass is no longer warm,” he explains.
While cold end coatings work great to reduce scratches, they are hard to print on and even after treatment, they can create a surface that resists adhesion, warns Lanigan.
He shares that a majority of glass containers, such as bottles and drinkware, have cold end coatings applied to reduce scratching of the glasses against one another during shipping and to reduce scratches during high-speed filling. He says in these cases, the container may feel greasy or slippery, “although, this can depend on the coating and sometimes this can be difficult to identify.”
The most common cold end coating is DuraCote manufactured by Sun Chemical, but AP5 from Somec is also popular. “Some glass manufactures companies spray animal fat or oils onto the glass surface for the same reason and sometimes we see glassware that has both DuraCote or AP5 applied and then an oil applied for additional scratch resistance,” explains Lanigan.
He feels that for glass containers, cold end coatings represents the biggest problem faced by the industry. “DuraCote is permanent and can only be removed when the glass is heated up to 1,000 degrees Fahrenheit. This is rarely done because the cost to get glass up to 1,000 degrees Fahrenheit is very expensive.”
Lanigan points out that some digital print manufacturers don’t do a wonderful job of explaining adhesion issues up front. “We get calls from frantic decorators that have newly purchased digital printers where the ink can be removed just by wiping fingers across the ink with very little pressure—no fingernails necessary. The ink can also come off when submerged in water, without any wiping.”
When it comes to ceramics, many of the same rules apply as glass when you’re talking about pretreatment.
Most ceramics will have a glaze, which Tourville says makes the surface energy very low and almost chemically inert.
Lanigan adds that ceramics are colored and less susceptible to visual scratches so they are not often treated with DuraCote or AP5.
Pretreatment Considerations
There are many options for glass pretreatment, including flame, silane, plasma, corona, Pyrosil, lehr oven, and adhesion promotors, primers, spray, or clear coats.
“In all these cases we are looking to create a high energy and reactive surface so that the adhesion promoters or primers and/or inks bond better,” says Lanigan.
Hemmelgarn suggests pretreatments in the form of hand primers or digital jettable primer for printing to glass.
Tourville recommends Pyrosil and adhesion promoter for dishwasher safe or the best possible adhesion. “If it is an application that you do not need the best adhesion you can use a wipe on adhesion promoter by itself or with MAP gas flame treatment. Or you can sandwich a second surface print between two pieces of glass and that would protect the image as well.”
With glass, Lee recommends ensuring that your supplier is providing a consistent and clean surface. “If the surface is inconsistent or has excessive contaminants, then cleaning the surface before treatment is highly recommended. After this, any sort of plasma cleaning immediately before printing will optimize the surface for best print results.”
Again, similar guidelines are recommended for ceramics. “If the surface contains inorganic contamination, this needs to be removed or the printing failure rate will go up. After this step, using plasma or flame plasma is your best option. If the part is flat or the exposure to the treatment can be consistently controlled, either plasma or flame will work well. If the part is not flat, then flame is a better choice, Also, when the surface to be treated is larger, flame often becomes more economical,” shares Lee.
Lanigan also recommends the same pretreatments as glass but points out that ceramics don’t use as many cold end coatings. “In most cases, standard flame treatment along with an adhesion promoter or primer is enough.”
Other Considerations
There aren’t many cases where vendors suggest not pretreating glass and/or ceramics.
Lanigan feels that pretreatments shouldn’t be an issue since many of the processes are clear and cannot be seen. “There are some silane treatments and some primers that can be seen on clear glass and sometimes colored glass so it may be necessary to use a different process.”
The area where Lanigan says AST gets the most requests for help/advice is passing dishwasher testing on cylindrical drinkware and cold water/ice water submersion testing with cylindrical bottles and/or spirit containers. “You can imagine that if glass drinkware can pass 300-plus dishwashing tests or hours of ice water submersion tests, most other adhesion requirements are easily met.”
When it comes to durability, the only application Lanigan feels requires greater durability is exterior use—like an exterior glass sign. “I am not aware of any process using digital UV ink that could withstand direct exposure to the elements. This is when a laminated process or an insulated glass window would be used,” he shares.
Pyrosil is specifically recommended by the majority of ink manufacturers for increased bonding of UV inks to glass, says Lanigan, adding that AST is the exclusive North American distributor for Pyrosil SURALink.
He admits that there are other processes and applications where different treatments work just fine.
For example, standard flame treatment allows a good printing surface so there are some applications where this is all that is required. “Candles are a good example because they don’t have a lot of contact or wear on the surface,” shares Lanigan.
Silane treatment is available from many companies and is fairly easy to use. With this process, users heat glass up in an oven or with a flame and spray silane and a carrier agent, often water onto the surface. The carrier agent will evaporate leaving silane on the surface, which is very reactive and improves bonding.
Lanigan adds that plasma and corona treatment are used in some glass applications but not often as a pretreatment for printing. “Plasma is basically a gas electrically charged with freely moving electrons and is usually performed in a vacuum, making high-speed processes challenging. Corona treatment is also effective but the equipment gets exponentially more expense as the surface height increases. A two-inch treatment area on a shot glass is not overly expensive but increasing that to four inches on a pint glass or seven inches on a wine or liquor bottle is not commonly seen.
Pyrosil is a technology from SURA Instruments in Germany. Lanigan says it burns a proprietary liquid in a flame and silicon dioxide (SiO2) is deposited onto the surface. “The SiO2 is highly reactive and creates an excellent surface for bonding. In many situations, the Pyrosil by itself can meet many printing applications for scratch resistance and durability.”
“Flame, silane, plasma, corona, and Pyrosil solve surface issues on substrates, and this alone will improve printability by offering a high surface tension, so the ink lays flat. The quality and improvement in the glass and ceramic surface is not the same with each treatment. Nor is the consistency or repeatability of the process. Therefore, some surface treatments are generally better than others,” shares Lanigan.
In some cases, adhesion promoters or primers can be applied directly to the glass without prior surface treatment. “This is not the norm and requires that the glass be extremely clean, without contamination. Because of the manufacturing process of glass containers, the surface is always contaminated,” he continues.
To get the best durability and adhesion for glass, which would include resistance to scratch, abrasion, water, ice, chemicals, and dishwashing, Lanigan recommends pretreatment of the substrate using silane, plasma, corona, or Pyrosil in addition to using an adhesion promoter/primer.
Hemmelgarn points out that it is important to ensure any primer placed in non-image area wipes off clean without leaving any residue on the glass.
Promoting Proper Adhesion
A variety of pretreatment processes are applied to glass and ceramics prior to printing to ensure proper adhesion and durability.
In the fourth and final portion of this series we look at glass printing in action.
Mar2023, Industrial Print Magazine
