Achieving Durability on Glass Containers with UV Screen Inks
UV inks are able to withstand many dishwasher cycles, boiling, freezing, filling lines and aggressive scratch testing.
Market acceptance of ultraviolet (UV) screen printing technology has arrived in the glass and ceramic container industry as suppliers of equipment and inks have optimized the methods necessary to achieve the durability decorators and their customers expect. The first responsibility of suppliers is to make clear the requirements for achieving durability with UV technology on glass containers, but first a word about expectations.
UV inks are able to withstand many dishwasher cycles, boiling, freezing, filling lines and aggressive scratch testing. These inks are perfectly suited for applications such as amber beer bottles, liquor and wine bottles, pint and Champagne glasses, and more. The type of glass printed, quality of pretreatment, UV dose, ink film thickness and ink chemistry all contribute to the durability of the ink film. Some UV inks develop an immediate cure, which allows users to test adhesion immediately, while others require a 24-hr period to reach full cure. Where full cure is achieved immediately, it’s possible to palletize or box containers right away.
UV ink suppliers are happy to assist with best practices to meet specific durability requirements. One important factor to keep in mind is that UV inks are not melted into the actual glass container. Consider this: The paint on your car is incredibly durable, but would you use a knife to test it?
Cold-End Coatings and Necessary Pretreatment
Glass containers typically receive a spray coating after the annealing process. Referred to as the cold-end coating, this is most often a waterborne polyethylene emulsion. Cold-end coatings act as a slip coat to reduce or eliminate blocking and scuffing, but they also interfere with UV ink adhesion and must be treated prior to printing.
The most common and effective pretreatment method is Pyrosil®, which is a patented process that mixes a liquid silane into flame, depositing a layer onto the surface of the glass that acts as a primer (see Figure 1). Some pretreatment systems use a pre-burner to remove or soften the cold-end coating prior to a second burner that contains the Pyrosil mixture, while others achieve pretreatment without pre-burners. Pyrosil pretreatment increases the surface tension and allows for a proper bond to UV inks.
The industry relies on a standard water immersion test to determine whether the surface of the glass container has been properly pretreated. A glass container is dipped into a bucket filled with tap water to check the surface tension. An untreated or poorly treated surface will show immediate water reticulation, while a properly treated glass will show smooth water leveling (see Figure 2).
Properly treated containers will pass several standard industry tests, including the crosshatch/tape test and the pencil hardness test, as well as those designed to measure freezing, ice water and alcohol resistance.
The crosshatch/tape test (ASTM D3359-09) checks the level of adhesion to the substrate. First, the printed image is scored using a crosshatch cutting tool, and then a specific tape is applied. When the tape is pulled off, it is possible to quantify the amount of ink removed. With proper pre-treatment, a result between 5B (0%) and 4B (> 5%) is possible (values of 4B to 5B are considered passing).
Pencil Hardness Test
It is possible to measure the durability of the ink film surface with a pencil hardness between H1 and H6. The typical accepted range is between 3H and 5H.
Freezing, Ice Water and Alcohol Resistance
These tests are most commonly used in the spirits industry. Typical durations are shown below (all are followed by air drying for 30 min before crosshatch tape testing):
- Freezer durability: < 7 days with -20°C (68°F)
- Ice water durability: < 7 days
- Alcohol resistance: < 48 h (20 or 40°C, 68 or 104°F)
For home dishwashing units with a wash temperature of approximately 65°C (149°F) for 90 min, the scratch resistance is measured after every cycle while still wet. Normally, all UV-based inks that are designed for this application will survive a minimum of 300 cycles, with some exceeding 500 cycles.
In industrial dish washing units with a temperature of approximately 60°C (140°F) for 6 min, a UV-based ink decoration has to survive a minimum of 2,000 cycles. In practice under optimal conditions, it is possible to reach nearly 3,000 cycles.
UV curing options that have proven to be effective for glass containers include traditional mercury vapor systems, as well as UV/LED. Minimum requirements for UV dose must be met in order to obtain the durability required. These requirements vary depending on application and ink manufacturer; each application should be discussed in detail with knowledgeable technicians from ink and equipment suppliers prior to printing.
A Quality Solution
Decorating with UV ink technology is now standardized and accepted by the glass container industry. Every day and all around the world, millions of bottles and pieces of glassware are printed with UV inks and achieve repeatable, verifiable quality.
This article is intended to be a broad overview of the necessary requirements for UV decoration onto glass containers. In reality, each of the topics mentioned could warrant its own detailed article. While that is beyond the scope of this review, your equipment or ink supplier should be able to discuss these points in greater detail in order to provide the information needed to achieve the durability that you and your customers require.