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Lec 4 glass

  1. 1. GLASS_ AS FOOD PACKAGING MATERIAL Ayesha Siddiqa PhD Scholar
  2. 2. PACKAGING MATERIAL_ GLASS • Glass is defined as “an amorphous inorganic product of fusion that has been cooled to a rigid condition without crystallizing” • For food packaging, bottles or jars are the types of glass packaging most often used, bottles being the primary use • In the US, 75% of all glass food containers are bottles • Glass is made primarily of silica, derived from sand or sandstone • For most glass, silica is combined with other raw materials in various proportions. For example, soda-lime glass, the glass typically used for food packaging, contains silica (68– 73%), limestone (10–13%), soda ash (12–15%), and alumina (1.5–2%).
  3. 3. WHY AND WHY NOT GLASS PLASTICS? • Glass is inert to a wide variety of food and non-food products, very rigid and strong against pressure, transparent, and non permeable (excellent barrier properties) • However, glass has disadvantages due to its heavy weight and fragility. • For food packaging, the fragility has caused some safety concerns such as the possibility of the presence of chipped glass in food products. • Glass for food packaging has declined over the last three decades, with glass losing market share to metal cans and, increasingly, to plastics. However, it still plays an important role in packaging
  4. 4. FORMING STEPS  Mixing  Melting  Forming Processes • Wide mouth press and blow • Narrow neck press and blow  Annealing surface treatment
  5. 5. MIXING AND MELTING • The glass making process begins with weighing out and mixing of the raw materials and introduction of the raw material to the glass melting furnace, which is maintained at approximately 1500 C • Cullet, broken or recycled glass, is also an important ingredient in glass production • In the melting furnace, the solid materials are converted to liquid, homogenized, and refined (getting the bubbles out). • At the end of the furnace, a lump of molten glass, called a “gob,” is transferred to the glass forming process.
  6. 6. GOB
  7. 7. FORMING PROCESSES • For food packaging, glass can be formed using;  Blow and-blow process  Wide-mouth-press-and-blow process  Narrow-neck-press-and-blow process
  8. 8. BLOW AND BLOW PROCESS • In the blow-and blow process, compressed air blows the gob into the blank mold of the forming machine and creates the shape of the parison • Then, the completed parison is transferred into the blow mold where air blows the parison to form a final shape.
  10. 10. WIDE-MOUTH-PRESS-AND-BLOW PROCESS • In the wide-mouth-press-and-blow process • Metal plunger is used to form the gob into the parison shape, instead of using air blowing. • As in the blow-and-blow process, the compressed air blows the container into its final shape • The press-and-blow process provides increased productivity, less weight, and more uniform wall thickness compared to the blow and- blow process.
  12. 12. NARROW-NECK-PRESS-AND-BLOW PROCESS • In the narrow-neck-press-and-blow process: • The overall process is similar to the wide-mouth-press but a much smaller metal plunger is used to make the parison shape • Beer or beverage bottles are common applications for the narrow-neck- press-and-blow process • Less than 38 mm of finish diameter is regarded as narrow mouth and over 38 mm is called wide mouth (Glass Packaging Institute, 2012).
  15. 15. SURFACE ANNEALING • Once the finished container is formed, it is transferred to a large oven known as a lehr for the annealing process • The function of annealing is to reheat and gradually cool the container in order to relieve the residual thermal stress • Surface coatings are often applied to the glass container for strengthening and lubricating the surface • Hot-end coatings are applied before the container enters the annealing oven (when the glass is still hot due to the previous forming process) Lehr (glassmaking machine)_ a lehr oven is a long kiln with an end-to-end temperature gradient, which is used for annealing
  16. 16. COATINGS • Hot end coatings consist of tin chloride (which reacts to form tin oxide) or organo-tin • At the hot end a very thin layer of tin(IV) oxide is applied either using a safe organic compound or inorganic stannic chloride. • These compounds are applied in vapor form and leave a rough high- friction surface on the glass container, which provides a good adhesive surface for the cold-end coatings • They also supply hardness, fill in minor cracks, and compress the glass surface.
  17. 17. CONT.. • After the glass containers are cooled, a cold-end coating is applied to increase lubricity and minimize the scratching of surfaces • These coatings typically consist of lubricants such as waxes, polyethylene, polyvinyl alcohol, and silicone • Since cold-end coatings make the glass surface more slippery, it is important to check the compatibility of the cold-end treatment with adhesives used in labeling
  18. 18. DEFECTS • Glass containers are 100% inspected; automatic machines, or sometimes persons, inspect every container for a variety of faults. Typical faults include small cracks in the glass called checks and foreign inclusions called stones • Other defects include bubbles in the glass called blisters and excessively thin walls. • Another defect common in glass manufacturing is referred to as a tear. • In the press and blow forming, if a plunger and moulder are out of alignment, or heated to an incorrect temperature, the glass will stick to either item and become torn. • In addition to rejecting faulty containers, inspection equipment gathers statistical information and relays it to the forming machine operators in the hot end.
  19. 19. CHECK • Computer systems collect fault information and trace it to the mould that produced the container • This is done by reading the mould number on the container, which is encoded (as a numeral, or a binary code of dots) on the container by the mould that made it • Operators carry out a range of checks manually on samples of containers, usually visual and dimensional checks.
  20. 20. INDICES OF FAILURE • Quality defects in glass containers • Cracks • Splits • Checks • Seams • Non-glass inclusions 20
  21. 21. INDICES OF FAILURE • Dirt • Spikes, bird cages, glass filaments • Freaks • Marks
  22. 22. INDICES OF FAILURE • Area of the bottle where they occur; • Sealing surface and finish area: off-set finish • Bulged finish • Broken finish • Corkage check 22
  23. 23. CONT.. • Neck ring seam • Dirty or rough finish • Bent or crooked finish • Seam on necking parting line • Bent neck 23
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  27. 27. TYPES OF GLASS • Type I : Borosilicate glass • Type II : Treated soda lime glass • Type III : Regular soda lime glass • Type IV : NP- general purpose soda lime glass 27
  28. 28. TYPES • Type I : Borosilicate glass • Least reactive • Substantial amount of alkali replaced by boric oxide • Higher ingredient like aluminum and zinc • Higher processing costs • Used primarily for more sensitive food
  29. 29. ADVANTAGES • Healthier and safer for consumers • NO stress about chemicals and contaminants • Only packaging material that has been designated as fully safe by the FDA • Chemically inert because of its natural composition • Recyclable 29
  30. 30. DISADVANTAGES • Glass can break and some of the broken glass might get into our food • Transporting glass may be hard • Glass is heavy and if there is too much glass IN A BOX • It is fragile • Expensive as compared to the plastic 30
  31. 31. DISADVANTAGES • Heavy to handle • Difficulty in transport • Harder to dispose • Expensive packaging equipment 31
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Notas do Editor

  • Annealing is a heat treatment process that changes the physical and sometimes also the chemical properties of a material to increase ductility and reduce the hardness to make it more workable.