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For most of the past century, applied ceramic labels (ACL) have been the most efficient method of putting a high quality label on a glass container. But as the 20th century drew to a close, concerns about the toxicity of the metals used to produce the most vivid colors drastically reduced the opportunities for ACL in North America and began rapidly eroding its use in Europe. Organic direct-to-glass inks and coatings, with their more vivid colors and higher efficiencies, promise a way to recapture these market opportunities. But the factors that govern the adhesion of organic materials to a glass surface are quite different from those affecting ACL. Successful decoration of glass has always demanded attention to the condition of the surface, but with organics, the condition of the surface is the controlling factor in determining the adhesion of the decoration.
Characteristics of Organic PolymersOrganosilanes are the materials of choice for promoting adhesion between polymers and glass surfaces (see Figure 1). The silanol groups formed by hydrolysis of the blocking alcohol (R) react with the glass surface to form a strong covalent bond. At the same time, organosilanes with a variety of reactive end groups (R’) are available for reaction with a variety of organic polymers, providing a bridge between the polymer and the glass surface.
But research in the adhesion of organic polymers to fiberoptic strands has shown that unlike the intimate fusing of a vitreous enamel, the polymer-to-glass bonding takes place only in discrete locations.1 This becomes especially crucial when considering that organic polymers are not 100% water barriers and that glass has a high affinity for water. Under the proper conditions, this can lead to water “pooling” in the areas between the silane bridges bonding the polymer and the glass. Over time, this water can dissolve the upper surface of the glass and lead to a general loss of adhesion.
This mechanism is likely the explanation for the behavior of organic decorations in caustic cleaning lines. Here, the mode of failure is quite different from that of ACL, where the color and density erode slowly over time. In the case of organic decoration, failure is sudden and complete, with the polymeric material de-bonding from the surface. The organic can usually be recovered from the caustic intact, and typically shows good inter-film integrity.
The combination of high temperature, which speeds the diffusion of caustic solution through the polymer, and the caustic solution itself, which accelerates the erosion of the underlying glass, is a set of conditions to which organic polymers on glass are particularly vulnerable. Durability of an organic decoration to caustic conditions can, however, be affected drastically by selection of the polymer. Certain polymer families—epoxies and urethanes, for example—are fundamentally more water resistant than other types. Selection of the proper polymer, and the thickness of the applied decoration, can have major effects on the rate at which water diffuses through the film and reaches the glass surface. If the rate of water penetration is sufficiently low to match the service conditions of the decoration, durability exceeding that of vitreous enamels can be achieved.
This is seen in the behavior of some organic decorations in household dishwashers. Here, the mildly caustic detergents do not have time during the dishwasher cycle to penetrate the polymer enough to disrupt the glass surface. After washing, drying releases the water from the film so the process has to start over in the next cycle. With the proper polymer selection, the dishwasher is never able to disrupt the glass surface enough to cause adhesion problems. However, polymers through which water diffuses easily will exhibit the same sort of complete adhesion loss seen in prolonged caustic bathes. A partial or incomplete cure will have the same effect, creating a polymer matrix that is more open and permeable than would otherwise be the case.
In cases where the decorated glass will be exposed to sunlight, special attention must be paid to the selection of the organic coating. The combination of sunlight and water is particularly severe since the ultraviolet component of sunlight can break down the polymer over time, leading to greater water permeability and increasing the potential for adhesion loss. The vulnerability of organic polymers to ultraviolet degradation varies widely, with some of the most chemically resistant polymers being the most sensitive to sunlight.
Cold End CoatingsThe situation becomes more complex when cold end coatings (commonly applied during bottle manufacturing to provide lubricity and abrasion resistance) are involved. Recent work at Pennsylvania State University using atomic force microscopy has characterized the distribution of tin oxide on the glass surface.2 It was found that between 30 and 50 coating thickness units, the glass surface was close to 100% covered with a layer of tin oxide, which also provided a significant increase in surface “roughness.” This increase in the three-dimensional nature of the surface may also increase the available bonding surface for the cold end coating or organic decoration, as well as substituting a tin surface instead of the more easily hydrolyzed glass surface.
The presence of a cold end coating also has the potential for disrupting the adhesion of an organic decoration. One widely used cold end coating, polyethylene, has been seen to interfere with the adhesion of organic decoration. One mechanism by which this occurs is by interfering with the ability of the decoration to effectively wet the surface. The “wetability” of a surface is typically measured by contact angle. While rigorously cleaned glass shows very low contact angles (around 8°), commercial glass is usually higher. The application of a polyethylene cold end coating increases the contact angle considerably, making the coated surface inherently more difficult to wet. The potential also exists for a cold end coating to form a barrier between the organic decoration and the underlying tin/glass surface, and may effectively prevent bonding.
Use of cold end coatings and organic decorations must be approached cautiously, because the potential for poor adhesion is high. Removing the cold end coating prior to decorating is typically the safest route, although care must be taken to ensure that the coating is completely removed, since partial removal could lead to spotty or intermittent adhesion failures. Cold end coatings can be re-applied after the organic decoration, since the organic-to-glass bond has already formed.
Determining the Cause of FailureIn cases where adhesion failure of an organic decoration does occur, the type of failure can help determine the cause. Gentle pressure against the edge of a printed decoration, tape pull testing or moderate face-to-face rubbing of decorated glass should not produce any visible marring or loss of adhesion. Failure by flaking, tearing or smearing may indicate a cure problem, either under cure or, in cases where the lehr temperature is too high, excessive heat partly degrading the polymer. In either case, the temperature of the glass in the lehr should be checked with heat tapes or thermocouples to ensure that the cure profile is in line with the organic decoration supplier’s recommendation. Large-scale adhesion loss—where the decoration de-bonds and leaves an intact free film—indicates that an effective polymer-to-glass bond has not formed. Interference by a surface contaminant or cold end coating is usually the root cause of this type of failure.
Similarly, an organic decoration should survive soaking in ice water for three days or hot water (180°F) for an hour without losing adhesion. While some softening is often seen in hot water, softening in cold water may indicate a cure problem. Any loss of hardness seen in hot water should recover on cooling to room temperature. Failure to do so also points to a cure problem. And, as discussed previously, adhesion loss leading to the removal of a free film is indicative of interference with the formation of the coating-to-glass bond.