Materials Science Quiz: Radiation and Properties

Physical Properties of Packaging Materials

• Packaging materials' behavior under physical stress/treatment is studied.
• Chemical structure modifications are not involved.
• Different categories of physical properties exist:

Thermal Properties

• Thermal: describes a material's behavior to heat.
  • Thermal conductivity, thermal capacity, specific heat, heat of formation, heat of combustion, transition temperatures, sealing properties, thermal strain.
  • Heat transport phenomena occur during thermal treatment, long-term storage with temperature variations, and package fabrication.
  • Thermal conductivity is the tendency of a solid to exchange heat by conduction.
  • Fourier's Law describes the rate of heat transfer.
  • Typical thermal conductivity values vary based on material. Metals have high thermal conductivity due to vibrating metal ions.
  • Different thermal conductivity affects the heating/cooling rate in processes.

Thermal Properties - Heat Capacity (Cp)

• Measures the amount of heat needed to change the temperature of a body by one unit.
• Common units: kcal °C⁻¹ or J K⁻¹.
• Specific heat (kcal kg⁻¹ °C⁻¹) is used for unit mass.
• Amorphous solids usually have higher heat capacity than crystalline materials.
• Specific heat values vary based on the packaging material.

Thermal Properties - Thermal Expansion

• Coefficient of linear expansion (α) and volume expansion (β): measures relative changes in length and volume due to temperature changes.
• Coefficients depend on the temperature.
• Temperature changes in packaging can lead to expansion/contraction and have consequences like breaking of glass jars in constrained spaces.
• Linear thermal expansion coefficients (per 1°C) vary based on material: Polypropylene, Rubber, Aluminum, Polyester, Steel, Glass, Wood, Ceramic.

Thermal Properties - Tolerable Thermal Range

• Lowest limit: maximum fragility temperature during commercial distribution.
• Highest limit: temperature causing physical distortion or chemical change.
• 'Service temperature' of packaging is often different from the raw material, due to manufacturing methods (crystallization, orientation, additives).

Thermal Properties - Transition Temperatures (Tm, Tg)

• Temperatures where a substance changes state under standard pressure.
• Melting point, glass transition, and crystallization temperature are vital for packaging materials.
• Adjustments in crystallization degree affects crucial packaging properties of plastics, ceramics, and metals (e.g., diffusional, mechanical, thermal properties, strength, fragility, ductility).

Thermal Properties - Melting Point (Tm)

• Transition from solid to liquid state.
• Essential for both inorganic and organic packaging materials.
• Crucial for materials intended to solidify (liquid or semi-solid) during package preparation.
• Sharp melting temperatures usually only in materials with true crystalline organization

Thermal Properties - Glass Transition Temperature (Tg)

• Temperature marking substantial changes in molecular chain mobility.
• Below Tg, materials show minimum mobility, significant free volume reduction, and high fragility.
• Around Tg, materials show unusual thermal sensitivity.
• Tg is important for polymers, affecting whether they are rubbery (flexible) or glassy (rigid) at ambient temperature.

Electromagnetic Properties - Transmittance/Absorption Spectra

• Materials' behavior with light irradiation.
• Absorption/transmission spectra quantify light energy absorbed by specific wavelengths.
• IR, UV-VIS radiations affect materials' properties like color, atomic structure, food/product protection.

Electromagnetic Properties - Refractive Index (n)

• Ratio of light speed in vacuum to the medium.
• Higher refractive index means slower light speed.
• Used to characterise light passing and bending characteristics of packaging materials. Relative refractive indices are also important.

Electromagnetic Properties - Transparency

• Aesthetic characteristic measured as the transmitted radiation (of specific wavelengths).
• Measured for transparent materials like plastic films and glasses as an aesthetic indicator and for optical/protection reasons for light-sensitive foods.
• Beer's Law quantifies transparency.

Electromagnetic Properties - Haze

• Opacity degree (transparent materials, e.g., plastic films and glass).
• Percentage of light deviating by more than 2.5° from the initial angle as a result of scattering in a sample.
• Haze values for different materials are also measured.

Electromagnetic Properties - Gloss

• Shininess attribute of surfaces (transparent or opaque).
• Ratio of emitted/reflected light in a specific direction relative to the initial light.
• Used in evaluating and measuring smoothness.

Electromagnetic Properties - Behavior to Ionizing Radiations

• Some electromagnetic radiation can cause ionization in materials.
• Effects like fragmentation of macromolecules and reticulation can occur.
• These processes can affect materials' characteristics like mechanical resistance, permeability, migration/degradation issues, and are important in packaging sterilization.

Electromagnetic Properties - Behavior to Microwaves

• Microwaves (between 300MHz and 30GHz) and their effects on materials/packaging.
• Behavior depends on material composition and impurities (like residual moisture).
• Absorption by polar molecules (water, ions) produces heat, reaching melting temperatures in some cases which isn't always desired.

Mechanical Properties - General

• How a material reacts to applied force(s).
• Factors: type, magnitude, direction and application time.
• Material's response (e.g., stiffness, fragility, burst strength, elasticity, coefficient of friction). In packaging, this is crucial for design and production. Key factors affecting the characteristics include natural surface roughness, surface treatment, processing additives (lubricants, plasticizers), undesirable friction. Useful for packaging, optimization, transport.

Mechanical Properties - Density

• Mass per unit volume to evaluate concentration.
• Key measurement used to evaluate packaging materials.
• Can be measured in bulk (kg/m³) accounting for voids & solids (e.g., plastic foams, expanded polymers, corrugated boards).

Mechanical Properties - Coefficient of Friction

• Resistance force between surfaces sliding on each other.
• Packaging/printing machinery need these considerations for efficient automated handling & processing.

Mechanical Properties - Strength Properties - Tensile Strength

• Maximum stress a material can sustain, often tested in "dog bone" shape specimens.
• Measures elongation with consistent stress.
• Elastic region shows reversible deformation.
• Plastic region shows permanent deformation.
• Related parameters like elastic modulus & maximum breaking point are also essential for evaluating packaging materials' strength and behavior under stresses.

Mechanical Properties - Strength Properties - Tear Strength

• Resistance of materials (e.g., flat packaging, plastic films) to tearing.
• Notches (micro-holes, micro-bubbles, sudden thickness changes), act as stress concentrate locations causing reduced tear strength.

Mechanical Properties - Strength Properties - Burst Strength

• Resistance of materials to bursting.
• Pressure values required to rupture the material when applying controlled, increasing pressure (water, compressed air).
• Useful for evaluating performance, potentially for highlighting possible hidden defects (e.g., in glass bottles, plastic films).

Mechanical Properties - Strength Properties - Creep Resistance and Response to Dynamic Stresses

• Creep: material deformation over time under constant stress (e.g., stacked packages on a shelf, internal forces in pressurized containers).
• Dynamic stresses include varying intensity forces (e.g., impacts, vibrations) in production and distribution.
• Important to evaluate protection against shocks and vibrations, important in packaging design, for durability. Key factors including external and internal fragility, can be evaluated in measurements like impact resistance.

Mechanical Properties - Cushioning Properties

• Ability to protect internal contents from shocks and vibrations.
• Spreading stress over a greater area and reducing the force per unit area (to reduce stress) important factors for design.