The coating of a glass container is required to give the glass a smooth surface that resists scratching and other surface damage. Since glass bottles and jars often travel through conveyors, this coating stops the glass sticking together and prevents scratching from abrasive contact during transit. It is also known that water on the surface of uncoated glass contributes to container strength reduction. The glass chemically absorbs the water and forms bonds with silica which tends to weaken the bottle, particularly at the site of any flaw in the glass.

Problems with even minor flaws in the glass and degradation of bottle strength are particularly heightened with bottles that contain pressurised contents such as carbonated liquids, like beer, soda and sparkling wine. Bottles with such contents are under substantial internal pressure, and the weakened surface flaws can lead to bursting of the bottle even at relatively low internal pressure. Bursting can be triggered either spontaneously or as the result of a small impact load that would not have affected a container without a flaw in it. As well as losing the contents of the bottle, the bursting of the container also poses a threat of injury to anyone in the vicinity from glass fragments, not to mention the mess that has to be cleaned up.

The purpose of a coating is therefore to retain the strength of the glass from manufacture to the highest practical level by protecting the surface, and to minimise the occurrence of scratches and similar flaws during the post-forming process, thereby reducing degradation of bottle strength.

Contact issues

The container strength decreases as the glass articles come into contact with each other and with other surfaces in the course of manufacturing, packaging, filling and shipment. The glass coating therefore gives surface protection and reduces the magnitude of the degradation in container strength during these further processing steps as well.

Due to the nature of raw glass surfaces, abrasion occurs whenever two glass surfaces come into contact. Any subsequent scratches or flaws may cause a decrease in the strength of the glass, quite possibly reducing this to as little as 25% of its original strength. In fact, a good way to check for any problems with cold-end coatings is to subject the containers to internal pressure resistance testing at the end of the lehr and [at] the end of the inspection line. If there is a significant reduction in average bursting pressure then it is likely the coating is not being applied correctly and corrective action needs to be taken.

Other factors could be at play such as unintended metal-to-glass contact during transport, but without this exception, this comparative testing works well as a monitor for coating effectiveness.

Assessments such as the so-called “slip-test” and simple manual bottle-to-bottle abrasion testing (discussed later) generally provide adequate coating control. The lubricating effect of the cold-end coating enables bottles to ‘slip’ or roll past one another when coming into contact, which is required to smooth the transition of the bottles from the lehr exit. Here, bottle-to-bottle contact could easily cause the containers to stick together, as the bottles are funnelled into single line conveyors for automatic inspection followed by bulk packaging. Without the presence of a good quality coating, the bottles are liable to ‘stick’ or jam, blocking the entrance to the conveyors, and sustaining scuffing or other surface damage. The same goes for when the containers are run down the customers’ filling lines.

Current container coating technology

Under current operating methods, it is normal to have both a hot-end coating and a cold-end coating that work together to give the required overall coating performance. The performance required from the coatings is to provide permanency, low coating thickness, visual clarity, lubricity, abrasion resistance, corrosion resistance, resistance to hot water washing, compliance with food standards (where risk of internal contamination of the container is possible), and also not to adversely affect any label adherence and decorating or printing inks that are required in any secondary processing activities. And all at a very low cost. So, as you can see, we are not asking much from such a glass coating here, are we?

The coating at the hot-end simply but essentially provides a bonding agent for the cold-end coating to bind to the glass surface. This is achieved by exposing the hot glass to a vapourised tin compound which is applied shortly after the glass has been formed and transported along the hot-end machine conveyor, just prior to entering the lehr. At this stage the temperature of the glass container is typically between 400 and 600°C which is a good temperature range for the application of the tin compound.

The tin oxide deposited on the container then forms the bonding required for the cold-end coatings that will be applied after container annealing has taken place, typically at lower temperatures of between 50 and 200°C, depending on the particular cold-end coating being applied.

MTBC

It was common in the past for tin coatings to be applied using stannic chloride. This was applied to glass surfaces by exposing the heated glass to the vaporised tin compound. At the temperatures stated above, the tin compound was converted to stannic oxide almost immediately upon coming in contact with the heated glass. This has been replaced for many years now by the use of Monobutyltin Trichloride (MBTC) which comes in soluble form and is applied to the coating hood and then vapourised to apply to the glass container external surface.

MBTC is preferred as it is much less aggressive than the stannic chloride which was not popular for health reasons. I don’t know the details of the health concerns associated with the use of it, but I do remember having a bad burn on my forearm after making contact with a deposit of material on a metal girder in the hot end of one plant when I was new to the industry (many years ago!). When I reported that to the on-site doctor at the time, it was attributed to a deposit from stannic chloride that had not been contained within the hood extraction system. How true that is, I will never really know. However it was well-known that the decomposition products of such hot-end coating compositions produced corrosive vapours. Needless to say, it certainly etched the name ‘stannic chloride’ in my memory (the markings on my arm have long since disappeared).

I can certainly recall special precautions being exercised to minimise the health hazards to personnel operating the coating equipment, such as breathing apparatus and specific protective clothing. This also still applies with the use of MBTC. The extraction systems are more effective nowadays, although not sufficient to avoid a pungent smell close to coating hoods if the hood or extraction is not set up correctly.

Checking thickness

The residence time of each container in the coating area is sufficient to deposit an oxide film that is of the order of 0.5µ in thickness. Thicker coatings are undesirable as they cause the glass to give an unsightly iridescent appearance, often termed in the industry as ‘blooming’.

Periodic checks for the thickness of the hot-end coating are carried out as part of the manufacturing process, although the units of measurement are commonly known as CTUs. This stands for Coating Thickness Units and is not a universal standard measure of thickness as such, but one defined by the American Glass Research Inc equipment used to measure it. However, this proves sufficient as a process control measure.

Also, if any of the tin oxide gets onto the finish of the glass containers – and if these are subsequently given screw caps or crowns in a different metal, this results in cap or crown corrosion problems even during normal expected shelf life. Therefore, it is undesirable to have any significant amount of hot-end coating detected on the finish of the container. Coating measured above 10 CTUs on the finish is considered excessive and corrective action will be required to reduce it to less than 10 CTUs. Regular checks [should be] made to adequately control the level of coating on the finish.

Cold-end coating

The cold-end coating material to be bonded to the hot-end coating at the cold-end may be selected from a large range of materials/chemistries. The specific material/chemical is selected to provide the required properties for the future processing of the container. The compositions of such coatings include polyethylene waxes, acrylic-ethylene copolymers, complex stearates, fatty acids, polyurethanes, vinyl copolymers, and silicones. However, the current basic range of types of cold-end coating most commonly used are stearate spray (water soluble used for pharmaceutical containers), oleic acid vapour (often used where required for good labelling performance), and polyethylene spray (the most permanent kind of coating giving best container strength retention).

For the spraying technique, the spray is delivered onto the container from above and sometimes below the lehr belt. Care has to be taken so that the inside of the bottle is not sprayed/contaminated, which could cause so-called ‘foaming’ during filling – and subsequently contaminate the filled product.

One of the properties that is measured at this stage is lubricity. This is measured by determining the angle at which the top bottle in a triangle of three horizontally stacked bottles will start to slide when the bottom supporting platform is tilted upwards. Uncoated bottles will reach an angle of 35° to 40° before sliding, whereas a good cold-end coating will let a dry bottle slide at between 8° to 16°.

Quality control

Bottles treated with cold-end coating are also tested with a scratch test. There are machines that can calibrate and measure the effectiveness of the applied coatings, such as the scratch test machine which abrades the surface of one glass against the surface of a similar bottle at increasing loads until a scratch becomes evident. However, it is common to simply manually carry out this test and, in this case, it should be hard to create a scratch by manually rubbing one container against another. These approaches to testing cold-end coatings in the manufacturing process are adequate for quality control purposes.

For lightweight one-trip containers, these articles require a more permanent coating in the form of polyethylene. This is used in combination with the narrow neck press and blow (NNPB) forming process to give the thinnest and most consistent glass wall thickness necessary to guarantee the required duty strength of the container, at minimum weight.

Due to restrictions in the function of the NNPB process, not all containers can be produced this way and recently the approaches to producing lightweight blow-blow (LWBB) have been a focus for such containers. For example, where corkage control is required in the mouth of wine bottles, as well as where bottle shapes are required which are not suitable for NNPB production.

Generally, however, glass containers are strongest when freshly formed and after the controlled cooling (annealing) process has been carried out. It is well known that flaws on the surface of a glass container such as chips, cracks, scratches and other surface imperfections result in significant reduction in the strength of the bottle.

The flaw sites on the surface of a bottle are stress concentrators, and breakage tends to occur at such stress concentration locations and often in combination with the glass surface profile of the container at the location of the stress concentrator. This is true even at a micro level. Therefore, the more perfect the quality of the glass surface we can make a container, the stronger it will be. Current coating technology can only serve to preserve the strength of the container that we have produced.

Future coating technology and uber light-weighting

Having discussed the current widespread application of best coatings technologies, which in combination with the NNPB forming process produce the lightest containers possible, we are now at a juncture where we need a step change to push the boundaries of reducing container weight-to-capacity ratios beyond what we can currently conceive is possible.

The reason for this is the worldwide focus on manufacturing operations becoming carbon neutral. Not only that, it is a large part of the supply chain for drinks companies, who also want to reduce the environmental impacts and scope 3 emissions associated with their packaging.

With the primary purpose of the glass container being a holding pot for its contents, it is clearly more efficient in function if the volume of material used to perform this function is minimised. This means less glass, less drain of the earth’s resources, and less energy required to manufacture and provide this basic function. In fact, the lighter a container is made for the given function of holding a specific amount of content, the more benefits that are also realised – in terms of less energy required in transportation from the manufacturing plant to the filling line, and subsequent distribution after filling.

The future will therefore [involve] improved glass coatings and radical light-weighting of glass containers. Both of these will provide significant challenges.

Adapting to new requirements

We have already discussed the basic requirements of coatings which need to be met, in addition to that we are looking for strength enhancement qualities of a coating, not just preservation of post-forming strength. Therein lies the main challenge, coupled with delivering on the other qualities of the current coatings. This could mean a different hot-end coating, a different cold-end coating, or different coatings overall. In an ideal world there would also just be one coating. This could be applied at either the hot end or cold end, or even during the annealing process.

Elimination of the use of hot-end surface treatment is an added bonus if that can be achieved, because it will free up some space on the hot-end conveyor which will allow the future installation of more hot-end inspection equipment. In addition, the hot-end coating process requires expensive exhaust systems and filters or even scrubbers to remove the potentially toxic vapours produced from the decomposition products used in the hot-end coating compositions.

The method of application of the new coating will have to keep up with production speeds, or be applied post production as a form of secondary processing. However, the latter would be a challenge for some plants due to lack of space for additional production equipment.

Assuming we eventually see such a new ‘super-coating’, which enhances glass strength and enables much thinner glass to [be used for container manufacture], we are then faced with the challenge of ensuring the consistency of this glass’ wall thickness. (Keep in mind that such a coating also must fulfil the many other functions stated earlier in this article.)

The manufacture of containers with thin and consistent glass wall thickness is a mighty challenge in itself, but one that has to be dealt with concurrently in order to realise the full package (no pun intended) of having uber-lightweight yet strong glass containers. The shape of the container will have a significant effect on what will be possible in this regard, with the more basic shapes being the easiest to achieve in the first instance. Progressing to the more complex shapes and features will bring more challenges for any step change in light-weighting.

Discussion about glass distribution control issues of uber-lightweight containers is a topic in its own right; a story that will have to be written about in hindsight at some stage; at least if we are to realise the benefits of any future strength-enhancing coatings that prove their worth.

UK-based industry-backed research and technology organisation Glass Futures’ forthcoming R&D pilot plant in St Helens will provide an ideal testing ground for developing and proving new coating technologies as well as a platform to develop new production methods for manufacturing the uber-lightweight containers for which the new ‘super-coating’ will serve. Anyone with ideas on such future coatings and container development would be welcomed in discussions for testing and development work.