Glass Packaging Materials PDF

Glass packaging Glass is an amorphous, inorganic Advantages: product of fusion that has been – Chemically inert (no reaction with cooled to a rigid condition any food) without crystallizing (ASTM, – Strong, can resist intern...

Glass packaging Glass is an amorphous, inorganic Advantages: product of fusion that has been – Chemically inert (no reaction with cooled to a rigid condition any food) without crystallizing (ASTM, – Strong, can resist internal pressure 1999) and weight – Inorganic, nonmetallic, subclass of – Can be re-used and re-cycled ceramic materials. – Impermeable to gases, aromas and – Super-cooled liquid. moisture Disadvantages: – Barrier to microorganisms, insects, etc – Breaks with rapid temperature change – Can be heat sterilized – Fragile, poor shock resistance – Good product display in clear glass – Heavy – Long shelf life is possible – In-plant breakage carries danger of – High customer appeal and splinters in food acceptability – Good protection against physical damage Chemical structure  Chemical structure Almost ¾ is silicon. Made from physical transformation above 1450-15000C  crystalline structure is lost and the tetrahedra rearrange in a-periodic, messy, amorphous structure  quick cooling.  Glass properties – Mechanical property Hard, but fragile & easily susceptible to mechanical failure (brittle fracture) even though there is a strong covalent bond between silicon and oxygen. This unique characteristic is due to: – The continuous and rigid structure of silicate glass (the interconnected tetrahedra) does not permit any plastic flow of the material  no stress absorption  destructive effects of fracture. – The glass objects always have practically sharp cracks, flaws, or other superficial or internal defects even sometimes invisible  huge stress amplification at the tip. Finished glass objects are usually tested for: – Internal pressure resistance : proportional to wall thickness (e.g. from carbonated beverages, vacuum packing, high pressure used in heat treatments) – Vertical load strength : increases with glass weight and thickness. (e.g. in stacking of glass bottles, during closure application) Can also be improved with an appropriate design, e.g. reducing the difference between the neck and the body diameter. - Resistance to impact Glass properties – Mechanical property  Glass properties – Thermal property Amorphous material  no real melting temperature, but progressively softens achieving a liquid-like state. – Cooling process that fixes the amorphous structure into a solidified state is also not a sudden change of phase, but occurs over a temperature range Lower Tg  the glass is less expensive to produce as it melts at lower temperature and will have also a lower temperature of deformation. Silica: lowest coefficient of thermal expansion among natural substances  potential failures when a glass container, with a metal closure, is submitted to thermal treatment. Poor thermal conductor, but it has the ability to undergo a thermal shock, i.e. a sudden change of its temperature. – However, since glass is more sensitive to tensile stresses than to compressive ones  high vulnerability to sudden cooling Thermal strength is influenced by thickness (thinner glass is more resistant), chemical composition (the presence of boron and aluminum oxide, like in Pyrex© or Corning© glasses , increases the resistance), and possible surface treatments.  Glass properties – Electromagnetic property Amorphous structure and chemical nature of ingredients  determine the transparency of glass objects. Pure silica: UV cut-off at 150 nm, but alkaline oxide makes the UV barrier effective at higher wavelengths. VIS region: application of colorings – However, addition of colorings alters physical properties e.g. density and mechanical or thermal resistance Perfectly transparent to microwave  low energy dissipation (negligible reduction in energy as a microwave travels through the glass wall).  Glass properties – Chemical inertness Glass: inert to food and beverage contact. Amorphous materials are less reactive, and silica has little solubility at neutral or acidic pH values (except for concentrated HF and hot H3PO4 solution)  no chemical reactions and no relevant leaching phenomena. – However, it can also lead to some interactions phenomena with contacting aqueous phases. In acidic media: possible acidic corrosion in glasses rich in sodium (common silicate glasses) and lead glasses may allow some heavy metal leach out (first time contact) In alkaline media: the silica network is progressively dissolved together with all the remaining constituents  surface etching (alkaline corrosion)  depends on the pH value and the temperature  not likely to occur in real food/beverage contact. Other concern related with food contact: possible presence of glass fragments. Glass containers manufacturing – Glass making – Container manufacturing – Post blowing operations  Glass containers manufacturing – glass making Different elements have each a specific function in glass making. – Silica: fundamental structure of final material – Boron (B2O3) and partially aluminum (Al2O3): take part in the amorphous network and responsible for improved thermal and mechanical resistance. – Recycled glass (cullet): significant energy saving. It is already amorphous  less energy needed (easier to melt and mix with other ingredients) – At least 1/10 of the mixture should be from sodium carbonate (sodium silicate: quite soluble and tend to bloom on surface (affecting transparency) and about the same amount of calcium carbonate (soda-lime glass) ,which is less soluble, is also added. – Sulfate, nitrate, or sulfite of alkaline ions  to aid in melting and removing gas from the molten glass mass (the large quantity of CO2 released = environmental and technological problems). – Simple substances and metal oxides (less than 0.05%)  to get the right wanted color or the right level of colorlessness.  Glass containers manufacturing – glass making All ingredients are continuously charged in the furnace  melted  transformed into glass. Gradient of temperature: start to melt around 12500C  refined at about 15000C (for better removal of the gases)  molten glass is then cooled to about 11500C (better temperature for forming hollow objects) Furnaces work non-stop! Glass containers manufacturing – glass making  Glass containers manufacturing – container manufacturing 2 predominant types of forming machines: – Blow and blow (B&B) – Press and blow (P&B) In both techniques, 2 different molds are used in 2 subsequent steps: – 1st mold (blank mold): the gob is transformed into a preform (parison) which has just a sketch of hollowness with the top and inside surface of the finish already formed. In B&B: blowing pressured air into the gob; for producing bottles and narrow neck containers. In P&B: pressing a plunger in the gob; (narrow neck press & blow)- less thermal inhomogeneities. – 2nd mold: the final shape of the container is reached by blown air, pushing the glass against the metal surface of mold. Total production time: 10-12 seconds (from the time the gob falls in the blank mold to the time the formed package exits)  slower compared to other food packaging containers. Glass containers manufacturing – container manufacturing  Glass containers manufacturing – post blowing operations Temperature of falling gob is 1100 – 12500C while at the exit, it is less than 5000C  there is a dramatic rapid cooling during short time (about 10 seconds)  strong stresses in the formed food container  fragility. Non-equilibrated contact of the parison and formed container with the mold’s wall and tools of the machine  superficial defects  critical for container’s integrity..  Operations to strengthen the glass containers – Hot end: Outer surface coating, able to fill up the microcracks, the flaws and all the surface defects Spraying a solution/suspension of tin/titanium compounds (inorganic or organic; most widely used: tin tetrachloride) maintained at 500-6000C  pyrolysis due to high temperature and presence of water vapor  coating the surface with a very thin layer of metal oxide (min: 3 μg/cm2) + release of HCl gas. Little effects on transparency, but increases friction coefficient of the containers  Operations to strengthen the final containers, after the bottle/jar exits the mold: (cont’d): – Annealing: thermal treatment to remove the tensile and compressive stresses concentrated in the glass because of the rapid cooling during forming. Along the path of the tunnel, the temperature is raised to 540-5500C (just above softening temperature), held constant for a couple of minutes then cooled slowly well below the softening point and finally cooled quickly to 35-400C. – Cold end: before the end of the annealing process, when the T reaches 120oC, the outer surface the container is sprayed with a water solution containing stearates, waxes, silicones, PE, or other organic substances – to anchor an organic layer on the surface modified by the hot-end treatment and reduce the coefficient of friction to make containers more resistant. – Chemical toughening: production of a surface layer by salt solution spray or by a molten salt bath in a heated lehr (temperature-controlled furnace), to substitute sodium with lithium or potassium ions  result in chemical ion exchange on the surface of the glass (outside and possibly inside)  production of surface layers of 15-20 µm  increased compressive strength. – Lithium reduces the thermal expansion coefficient of the glass surface  the surface contracts less than the interior when the glass cools  compression layer on the surface. – Potassium ions: bigger atomic radius than sodium  better strength. – Thermal toughening: reduction of surface temperature at much faster rate than the interior by blast of cool air  compressive layer on the surface is balanced by tensile layer in the interior  increased compressive strength (for thick and strong glass.) – Pre-labeling: covering a large part of the bottle body with a wrap to reduce the adverse effects of possible impacts (and also for decorative purposes)  cushioning effect  reduce the risk of abrasion during handling, reduce the noise on filling lines, retain the fragments in the glass if it should break. Shrink films e.g. PVC or polyolefin. Body sleeve made of foam or homogenous plastic. – Acid polishing: chemically dissolving (etching) a layer of glass at the surface to remove the faults that might have appeared during container making. Not really used commercially due to the danger of chemicals used (hydrofluoric acid) - Crystallization: homogenous melting of nucleating agents (titanate, zirconate, and fluoride particles = glass ceramics) into the glass  followed by subsequent precipitation  prevents fracture propagation  better mechanical reliability. However, they are not suitable for food containers.  Use of glass containers in food packaging Food: Liquid, solid, semi-solid. Delivered from glass manufactures to food processors in bulk palletized form shrink-wrapped. In food processors: – Depalletization  cleaning  filling  capping  in-bottle heat treatment  labeling  distribution. – The containers should not be exposed to drastic temperature change The limit in heating phase is 600C, while in the cooling cycle, the limit is 400C. – Cleaning before filling: air blowing, warm water rinse, washing with detergent solution, or combination of them. Returned containers for reuse are washed with caustic soda solution  In food processors (cont’d): – For hot-filled products, the container temperature needs to be increased in the cleaning step or a separate heating tunnel to prevent thermal shock. – Filing of liquid foods in to glass containers is usually based on level control and operated under the force of gravity, vacuum, pressure, or their combination. – Closures are applied for hermetic sealing, usually in integrated system with filling. – Heat processing of filled glass containers, e.g. pasteurization and sterilization, should be controlled in temperature to avoid abrupt thermal shock. – Temperature after cooling phase is desired to be around 400C to allow evaporation of moisture on the surface of closure and bottle. – Labels in paper or laminated film are attached onto sealed bottles by proper adhesive. – Secondary packaging: plastic wrap, corrugated box.