Glass Properties and Production Quiz

Glass Packaging

• Glass (an amorphous, inorganic material) is formed by fusing and cooling materials to a rigid state without crystallizing (ASTM, 1999).
• It's a nonmetallic, inorganic subclass of ceramic materials.
• Glass is a supercooled liquid.

Advantages

• Chemically inert (doesn't react with food)
• Strong, resists pressure and weight
• Reusable and recyclable
• Impermeable to gases, aromas, moisture
• Barrier to microorganisms, insects, etc.
• Suitable for heat sterilization
• Good product display option in clear glass
• Long shelf life
• High customer appeal and acceptability
• Protects against physical damage

Disadvantages

• Breaks easily with rapid temperature changes
• Fragile and susceptible to shock
• Heavy
• In-plant breakage creates shard hazards in food

Chemical Structure

• Primarily composed of silica (~70-75%).
• Various other oxides (e.g. sodium oxide, potassium oxide, calcium oxide, magnesium oxide, barium oxide, lead oxide), affect properties. The specific oxides determine the type of glass (Flint, Amber, Green, Pyrex, Lead glass). See table 7.1 for specific compositions.
• Amorphous structure, with Si atoms making tetrahedral structures.
• Made through physical transformation above ~1450-1500°C.
• The structure is a-periodic (messy) → quick cooling.

Glass Properties - Mechanical

• Hard but fragile, susceptible to brittle fracture.
• Strong covalent bonds between silicon and oxygen atoms prevent plastic flow and stress absorption.
• Defects (e.g., cracks) can easily amplify stress.
• Glass objects are tested regarding internal pressure resistance (proportional to wall thickness) and vertical load strength (increases with glass weight and thickness).
• Design, like reducing the difference between the bottle neck and body diameter can improve strength as well as resistance to impact.
• See table 7.2 for typical physical properties.

Glass Properties - Thermal

• Amorphous material: no precise melting point, gradually softens.
• Cooling sets the amorphous structure.
• Lower transformation temperature is more economical but may involve lower temperature deformation conditions
• Silica has the lowest coefficient of thermal expansion of natural substances and can affect glass containers submitted to thermal treatment.
• Glass is a poor thermal conductor.
• Temperature changes, especially sudden changes (thermal shock), are more problematic with glass than compression because of its high sensitivity to tensile stresses.
• Stronger thermal resistance in compositions with boron and aluminum oxide (like Pyrex(R)).
• Potential surface treatments can contribute to better resistance.

Glass Properties - Electromagnetic

• Amorphous structure and chemical composition determine glass transparency.
• Pure silica has a UV cut-off at ~150 nm.
• Alkaline oxides enhance UV barrier and extend the higher wavelength range in the visible region for the application of colorings.
• See table7.3 for the ingredients influencing colors.
• Glass is essentially transparent to microwaves.

Glass Properties - Chemical Inertness

• Glass is generally inert to food and beverages.
• Minimal solubility in neutral or mild acidic pH (except under specific concentrations and temperature conditions with specific acids).
• Potential for acidic corrosion in glasses high in sodium and lead.
• Potential for alkaline corrosion in the glasses when in contact with alkaline media, where silica and the other network elements are progressively dissolved.

Glass Container Manufacturing - Glass Making

• Continuously charging raw materials into the furnace for melting and transformation into glass.
• Temperature gradient (e.g., 1250°C for start of melting, refined about 1500°C), then cooling to ~1150°C.
• Furnaces operate continuously.
• See Figure 7.4 for schematic furnace representation.
• Raw materials and their function in glass making are presented in table 7.5.

Glass Container Manufacturing - Container Manufacturing

• Two predominant types of forming machines: Blow and blow (B&B) & Press and blow (P&B)
• Two mold stages are used for both techniques:
• (1st) Mold: creates a preform (parison) with initial shape.
• (2nd) Mold: final shape reached by air or plunger pressure.
• Production time 10-12 seconds.
• See Figure 7.5 and 7.6 for specific process representations.

Glass Container Manufacturing - Post Blowing Operations

• High temperature of the gob (1100-1250°C) during manufacture, cools rapidly (~10 seconds) to ~500°C causes significant stress in the container and makes it fragile.
• The combination of non-equilibrium contact of the walls of the mold and the produced container and tools of the forming machine potentially induces defects on the glass product.

Operations to Strengthen Glass Containers

• Hot end: outer coating (e.g., tin or titanium compounds) with a thin metal oxide layer for filling microscopic cracks and surface defects.
• Little impact on transparency; high surface friction coefficient.
• Cold end: post-annealing process at ~540-550°C, which is just above softening temp.
• Spraying with a water solution of stearates, waxes, silicones or PE to reduce friction.

Chemical Toughening

• Producing a surface layer by using salt solutions in a heated furnace to replace sodium with lithium or potassium ions.
• Increases compressive strength of the surface layer to balance tensile stress in the interior.
• See page 19 for details.

Thermal Toughening

• Rapidly cooling the surface of the glass containers by blast of cool air or other methods.
• Compressive layer on surface.
• Balanced tensile layer in interior of the container; strengthens the entire container.

Pre-labeling

• Covering part of the bottle body with films, shrinks, or other material for impact and abrasion resistance.
• Protective coating that also contributes to a functional design.
• Acid polishing to chemically dissolve a layer of glass to remove existing defects.
• Surface crystallization for fracture propagation resistance.

Use of Glass Containers in Food Packaging

• Glass containers are used for liquid, solid, and semi-solid foods.
• Bulk delivery in palletized form.
• Depalletization, cleaning, filling, capping, heat treatment, labeling, and distribution.
• Temperature changes must be moderated to avoid drastic changes.
• Cleaning (often including water or detergent) before use and possible reused.
• Label attachments (usually paper or laminated film).
• Often packaged with plastic wraps and corrugated boxes.

Additional Notes

• Hot-filled products require an elevated temperature to prevent thermal shock upon filling or an extra tunnel.
• Filling of liquids often controlled with gravitational, vacuum, or pressurized systems.
• The process of sterilization and pasteurization may need to happen under controlled temperature to prevent thermal shock.
• Labels and secondary packaging (wraps and boxes) are important for end-use protection.

Test your knowledge on the properties, production, and characteristics of glass. This quiz covers the advantages of glass packaging, the melting process, and the various ingredients used in glass making. Perfect for students and industry professionals alike.