Unformatted text preview: Unit 3 – Glass Packaging Glass is a major packaging material and in this unit we will discuss its history as a packaging material, its composition, how glass furnaces function, how glass containers are manufactured, and the characteristics of glass containers. 3.1 History of Glass in Packaging As stated in Unit 1, it was around 1,500 BC when hollow glass containers started being used for packaging applications in Mesopotamia and Egypt. In about 50 BC, the glass blowpipe was invented by the Romans creating the ability to blow round containers and increase the speed of production (Soroka 2002). In 1608, glass manufacturing began in Jamestown, VA. Manual glass manufacturing machines were invented in the 1800s, and by 1880 25% of glass containers were manufactured this way. The first automated bottle making machine was invented by Michaels Owens, of the Libby Company, in 1903. 3.2 Glass There are several characteristics that define what glass is. Glass is inorganic; therefore, it is not carbon‐based. It is formed by cooling from a liquid (molten) state. It is amorphous, meaning it does not crystallize. And it is brittle. Very basically, glass is made by melting sand (silicon dioxide), which requires very high temperatures to do. To lower the melting temperature, soda (sodium carbonate) can be added to the sand; the more soda that is added, the lower the melting temperature will be. To make glass objects, the ingredients are melted completely, formed into the desired shape from the viscous liquid, and allowed to cool and set. Glass gets harder when it is cooled and softer when it is heated (until it flows), but there is not a specific point or temperature where the transition between solid and liquid is defined. 3.2.1 Cystallinity Glass is amorphous because it solidifies from a molten state without crystallizing. Crystalline materials have a regular repeating atomic or molecular structure in all three spatial dimensions. The diagram below is a two‐dimensional representation of the chemical structure of one of the ingredients for glass, sand (silicon dioxide), which is crystalline. (The third dimension would be above and below the planes of this diagram, with each silicon atom bonding to a fourth oxygen atom.) The important thing to notice here is the regular repeating pattern, which is what characterizes crystallinity. 3. 1 Crystalline silicon dioxide (sand) SiO2 is a network former, meaning that instead of the SiO2 molecules remaining as individuals, they bond together and form a large crystalline network. When heat is applied, it disrupts bonds within the network, breaking down the crystalline arrangement and allowing the SiO2 molecules to flow away from their neighbors. Melting is the break‐up of the crystalline network while solidifying is the reformation of the crystalline network. Since glass is amorphous (non‐crystalline), it has a much more irregular arrangement and lacks the repeating pattern of sand. When it is heated, some of the bonds are disrupted, allowing the material to flow, but there is no defined point of transition from a solid to a liquid. Conversely, as the material cools, it gets stiffer and stiffer, but there is no clear point of solidification and it does not form in a crystalline arrangement. Here is a diagram of what the amorphous structure of silicon dioxide glass looks like: Amorphous silicon dioxide glass An important characteristic of amorphous glass is that it is transparent. Crystalline regions scatter light while amorphous regions allow light to pass through them. 3. 2 3.2.2 Brittleness When a material is defined as brittle, it means that it cannot be stretched or deformed very much without breaking. Brittle materials can also range from being weak to strong. The antithesis of brittle is ductile; therefore ductile materials can be stretched, bent, shaped, and deformed before breaking. Ductile materials include aluminum and many plastics. Brittle materials under stress tend to break in a single, sudden action where there is no observable deformation before failure. Compare how plastic and glass bottles would perform differently if dropped. The plastic bottle would deform and possibly break open, while the glass bottle would shatter instantly. 3.3 Glass Composition Glass is made of silica sand, soda ash, limestone, alumina, and cullet that are mixed and melted together with heat. Silica sand (silicon dioxide, SiO2) is the most abundant ingredient and serves as the network former. While it can be used alone to make glass, it requires a lot of energy to melt and it is difficult to work with. It is difficult to form, so high speed production lines are not effective. Additionally, it is difficult to “fine”, which is the term used for the process of removing air bubbles (seeds) from molten glass. To improve the ability for the SiO2 to melt and reduce its viscosity for forming, a fluxing agent is added, which is the soda ash in the form of sodium carbonate. It accomplishes this by breaking apart some of the Si‐O bonds. When it’s added to the furnace, it decomposes into sodium oxide (Na2O) and carbon dioxide. The CO2 bubbles out of the glass and helps mix it in the process. Now, melting temperature and viscosity have been improved, but glass in this form has poor chemical durability and is actually water soluble due to the sodium, which is not a good trait for glass containers. To prevent the glass from being water soluble, another ingredient needs to be added, which is the network stabilizer, limestone. This is in the form of calcium carbonate, which decomposes into calcium oxide (CaO) and CO2. Now the durability of the glass has been restored, but, again, the melting temperature and viscosity have been compromised. Additionally, “devitrification” can occur here, which is when glass turns into an opaque crystalline solid, meaning that it is no longer glass. One more ingredient needs to be added to prevent devitrification, alumina, which performs as an intermediate between a network modifier and a network stabilizer. Alumina is added in the form of aluminum oxide (Al2O3), but instead of adding it as a carbonate, it is added as a silicate, with some silicon oxide, typically from the mineral feldspar. This completes the main ingredient compositions for packaging glass. Here is a diagram of its final structure as “soda‐lime” glass: 3. 3 Soda‐lime glass structure Here is a graph showing the relationship of the ingredients used to make glass: http://gpi.org/glassresources/education/sustainabilityrecycling/section‐24‐glass‐
composition.html 3. 4 3.3.1 Minors “Minors” can also be added to the glass. These include colorants, fining agents, and decolorizers. Clear, or colorless, glass is referred to as “flint”. Colorants are used to add color to clear glass. The most popular colorants include: Iron, sulfur, and carbon for amber glass Ferrous sulfate and chrome oxide for green glass Cobalt oxide for blue glass Other colorants include: Manganese for violet glass Iron oxides for black glass Calcium fluoride for opal glass While some glass is colored solely for its appearance, some colorants serve a specific function. Amber glass provides protection from ultraviolet light and is used for products such as beer and pharmaceuticals. Green glass also provides UV protection, but not to the extent that amber glass does. Sodium sulfate, or salt cake, is the most common fining agent. It aids in melting the silica and reacts with carbon to release sulfur dioxide gas and CO2. As these gas bubbles float to the top of the molten glass, they collect smaller oxygen bubbles and carry them away with them. 3.3.2 Cullet There is actually one last ingredient that is used to make glass, cullet. Cullet is recycled glass that is cleaned and processed and used to make new glass. It aids in reducing the temperature required for melting, reduces emissions, and prevents old glass from going to waste. 3.4 Glass Production Glass is produced by adding raw materials into a furnace so that they melt and mix. The raw materials (the ingredients described above) are delivered to the manufacturing plant by rail or truck and stored in silos above the manufacturing floor, which is the level that the furnace is on. This way, gravity aids in feeding the materials into the mixing process. The “Batch House” is a part of the plant where the ingredients are mixed and added to the furnace. It is located near the silos at the input end of the furnace. The ingredients are measured from the silos onto a conveyor that feeds the “hopper” (this is automated with computer controls). The hopper mixes the ingredients with a little bit of water and then dumps the mixture into the furnace, where the water quickly evaporates. 3. 5 Batch House The glass furnace has multiple sections that include a melter, bridgewall, throat, and refiner, as well as one or more forehearths. The melter is the largest section, typically 50 ft long, 35 ft wide, and 6 ft deep, and is lined with fire brick which is a heat resistant ceramic. Here, the ingredient mix is exposed to heat and melted. Peak temperatures in the melter range from 2600‐2900°F. The end of the melter is the “bridgewall”. The bridgewall, also referred to as a shadow wall, directs the molten glass to the “throat” underneath it. The throat is where the glass moves from the melter to the refiner. The bridgewall forces all glass passing through the throat to come from the bottom of the melter. This is because undesirable impurities in the glass float to the top, where they get trapped against the bridgewall. This floating mixture is referred to as “slag” and is usually composed of dirt, stones, and other foreign material that get into ingredients. The refiner receives the glass as it passes through the throat. This glass is pure and the refiner performs the “fining” process and removes air bubbles by forcing them to float to the surface. Forehearths are ceramic lined troughs that convey glass from the refiner into glass container forming machines. The temperature of the glass is stabilized here so that it is the optimum temperature for forming containers. When glass enters the forehearth, it is about 2350°F and when it leaves it is about 2000°F. Colorant can also be added here, in addition to being added in the Batch House. Up to eight forehearths can be on one glass furnace, allowing each forehearth to design a different container simultaneously. Glass flows out of the forehearths through “feeders”, or “spouts”. 3. 6 3.5 Manufacturing Glass Containers The feeders, or spouts, are basically just holes at the bottom of the forehearth. Glass flows through them and is assisted with a plunger. The molten glass flows in a stream and “gobs” of it are cut off with gob shears as it flows through the feeder. The gob is caught by a chute and transported down by gravity into a blank mold. Each gob is used to form one container. Gob Formation 3. 7 Gob shape is affected by the action of the plunger (stroke length and speed) and the viscosity of the glass. Gob shape is important because it affects the precision of how the gob enters the glass container manufacturing machine. Each type of container has an optimum gob shape. Here are some examples: http://www.gpi.org/glassresources/education/manufacturing/section‐34‐forming‐process.html There is one IS (Individual Section) machine at the end of each forehearth that is used to form containers. These form the containers through controlled cooling and shaping of the glass. Each machine can have 4 to 20 sections and produce 1 to 4 containers simultaneously. IS machines have these independent multiple sections to facilitate maintenance and repairs without having to shut down the lines; glass production typically runs 24 hours a day every day of the year. The time to produce a container varies on the type, but a typical pop or beer bottle takes about 10 seconds. The gobs enter the molds on one side called the blank mold. This creates the preliminary form of the container, called the “parison”. The parison is transferred to the other side of the mold and formed into its final shape and size. There are two processes for forming glass containers: press‐and‐blow molding and blow‐and‐blow molding. 3.5.1 Press‐and‐Blow Molding Press‐and‐blow molding requires two molds, the blank mold and the blow mold, and is used to form wide‐mouth jars and cups. The steps are as follows: The gob from the forehearth falls down the chute into the blank mold. The gob is forced into the finish area of the blank mold with compressed air. A plunger enters the mold through the finish area and forms the parison by pressing the glass against the sides of the blank mold. This forms the “finish” of the container, which is the mouth of the container containing the threads. The parison is transferred to the second mold, the blow mold. Compressed air is blown through the finish of the parison forcing the glass against the walls of the blow mold. 3. 8 The container remains in the blow mold until it cools enough to hold its shape when removed. The container is removed from the mold and set on a conveyor belt where air, called wind, is blown to cool it. http://www.gpi.org/glassresources/education/manufacturing/section‐34‐forming‐process.html 3.5.2 Blow‐and‐Blow Molding Blow‐and‐blow molding also uses two molds, a blank mold and a blow mold, to create containers with narrow necks or mouths, like bottles and jugs. The steps are similar to press‐
and‐blow molding, except both steps are done with air, rather than with a plunger. The steps are as follows: The gob from the forehearth falls down the chute into the blank mold. The mold closes and a bubble of compressed air forces the gob in the mold and forms the finish. This is referred to as the “settle blow”. A tube is inserted into the finish and blows compressed air, forcing the gob to form to the walls of the blank mold, forming the parison. The parison is transferred to the blow mold. Air is forced through the finish to expand the glass against the walls of the blow mold. The container remains in the blow mold until it cools enough to hold its shape when removed. The container is removed from the mold and set on a conveyor belt where air, called wind, is blown to cool it. 3. 9 http://www.gpi.org/glassresources/education/manufacturing/section‐34‐forming‐process.html Comparison of wide‐mouth and narrow‐mouth parisons One of the advantages of press‐and‐blow over blow‐and‐blow is that it provides the ability for the glass to be distributed in the parison as desired, which allows for more control over glass thickness in the final container. Recently, a narrow neck press‐and‐blow process was developed, which is routinely used for beer bottles. Since there is better control over where the glass is located in the parison, it is possible to make bottles which are thinner and lighter while maintaining their required strength. This has given the glass industry a competitive edge. It is also notable that some glass containers, such as drinking glasses, don’t necessarily require the blow portion; they only need to be pressed. The limiting factor here is the ability of the plunger to be pulled out of the container. 3. 10 Forming glass containers using only “press” 3.6 Annealing When glass containers come out of the mold they are still extremely hot, despite being able to hold their shape. Rapid cooling of glass creates thermal stresses in the containers, making them prone to shattering. When they are on the conveyer, they do not cool evenly, so some parts are hotter than others, which create thermal stresses in regions of the containers. To alleviate this, the containers are sent through an annealing oven, or lehr. Here, they are reheated to a specific temperature to relieve the thermal stresses, and then cooled gradually so no new thermal stresses are introduced. 3.7 Coatings Coatings can also be added to the insides and outsides of containers during or immediately following annealing. These are used to create a layer of protection against scratches and also lubricate the container so it flows through the production line more smoothly. A bonding agent, such as tin or tetrachloride, is applied at the hot end, or front end, of the lehr. Friction reducing agents, such as polyethylene, waxes, and silicones, are applied at the cold end, or back end, of the lehr. The bonding coatings are used so the friction coatings will adhere better. 3.8 Inspection Once annealing and coating is complete, each container is inspected for defects. Those with defects are removed and added back into the production stream as cullet. Defect types include: Out of tolerance dimensions (thread height, finish diameter, body diameter, etc.) Chips Cracks Bubbles Color variation Birdswings – thin filaments of glass that form threads within the container 3. 11 3.9 Palletizing and Shipment Once the manufacturing of the containers is complete, they are packaged for delivery to the customers who fill them. The can be done with bulk handling or reshippers. For bulk handling, the containers are loaded in layers onto pallets, usually with sheets of paperboard in between layers. The pallets are stabilized with shrink or stretch film, or metal or plastic bands. Forklifts move the pallets into a warehouse for storage or load trucks to transport them to the customers. When reshippers are used, the containers are loaded directly into corrugated boxes which are then stacked onto pallets and restrained in the same manner as bulk handling. Once these reach the customers, the containers are filled, capped, and labeled and then loaded back into the same boxes for further shipment. 3.10 North American Glass Industry Currently, there are 69 glass manufacturing plants in North America; 50 are in the US, 4 in Canada, and 15 in Mexico. Glass container manufacturing declined significantly in the 1980s and 1990s due to competition from other materials, primarily plastics and aluminum, but it has remained stable for the past several years. In 1980, the EPA reported that 13.2 million tons of glass containers were discarded; in 1990 it dropped to 9.2 million tons, and in 2007 in was 8.3 million tons. Glass is used abundantly in the food, beverage, pharmaceutical, and cosmetics industries. Here is a diagram illustrating its use by market sector: 3.11 Advantages of glass compared to other packaging materials: Moderate cost Low coefficient of thermal expansion Inherently strong, especially in compression Rigid, so shape won’t change during shipping, handling, under vacuum or pressure Resistant to most chemicals Non‐permeable, tasteless, and odorless 3. 12 o don’t have to worry about products damaging glass or glass damaging products inside o allows for longer shelf life Microwave safe Transparent, which allows for product visibility (flint glass) Can provide UV protection (amber and green glass) Stable at high temperatures allowing for hot‐filling and retorting Has upscale, premium image and provides perceived value Recyclable 3.12 Disadvantages of glass compared to other packaging materials: Heavy Manufacturing requires large, costly facilities and equipment Brittle and easy to break 3.13 Leaching While glass is generally very inert, leaching can be a serious problem when it comes to parenteral (intravenous) drugs. Since sodium helps the transfer of fluid in the body, sodium leaching from glass can become a serious problem for these types of drugs. These types of glass packaging have special compositions and treatments to prevent leaching. 3.14 Thermal Stress Borosilicate glass, which is glass that contains boron, is resistant to thermal shock. This glass is common for containers that need to be heated and cooled. Examples include Pyrex containers. 3.15 Strength Since glass is a brittle material, it is very strong in compression but weak in tension. In its natural state, it is actually stronger than steel, with failure occurring around 150,000 psi. However, when glass has scratches on its surface, its strength can be reduced significantly. Since glass receives many fine scratches during manufacturing, the strength of the final containers is about 15,000 psi. Other factors that affect glass strength include: Distribution and thickness of glass throughout the container Container shape Type of load (compression vs. tension) Scratches reduce glass strength by acting as stress concentrators. Here is an image representing a glass surface with a scratch: 3. 13 If you were pulling the ends of this bar away from one another or trying to push them down (creating tension), you would concentrate the stress at the notch of the scratch. Deeper scratches will have more stress concentration. If you do the opposite, and press the ends together or upward, you create compression, which is much less likely to create a failure. Glass can fail in compression though. Compressive force pushing down on the tops of bottles creates tension in the shoulders, which is where failure typically occurs. The shape of the shoulder is a major determining factor in how much top load a bottle can withstand. More gradual shoulder shapes reduce the concentration of tension and increase the strength of the bottle. This is why heavier containers, like wine bottles, have more tapered shoulders and smaller bottles, such as for fragrances, can be square. Here are some examples of bottle shoulder shapes and the loads they can withstand before failure: Strength of Shoulder Shapes 3. 14 3.16 Parts and Nomenclature of Glass Containers Here is a drawing of the parts of a glass container and their respective nomenclature. Many of these terms are interchangeable with plastic containers as well and are common terms in the packaging industry. We’ve already talked about the finish and shoulder, and the body, sidewalls, and heel are fairly obvious. The transfer bead is the part that the IS machine uses to hold the bottles when transferring from the blank mold to the blow mold, and then to the conveyor belt. The mold seam is the signature left behind on the container from where the two sides of the mold meet. The bottom plate parting line is also a signature left from the bottom section molding sections. Other features of glass containers are located on the bottoms and include stippling and identifying marks. Stippling is the bumps on the bottom, which limit contact area between the container and the surfaces it is on during manufacturing so that it does not cool too quickly and develop excess thermal stresses. The sides of containers can also have stippling to protect the side walls from getting scratched; this way, only the bumps acquire stress concentrations, not the containers themselves. Glass container bottoms also have identifying marks showing which company made the bottle and even which mold was used. 3. 15 Stippling Identifying Marks I encourage you to examine some different glass containers and identify and compare these parts and all of the attributes discussed in this unit on them. If you are interested in learning more about glass packaging, visit the Glass Packaging Institute website at: http://www.gpi.org 3. 16 ...
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- Spring '08
- Glass, glass containers