Plastics

Overview of Polymers in Packaging

  • This lecture focuses on polymers as a packaging material, following previous studies on paper, paperboard, metal (steel and aluminum), and glass.

  • Plastics are the largest consumers, representing approximately 30% of the total plastics market.

Definition of Key Terms

  • Plastic: Describes the ability of materials to be molded or formed.

  • Polymer: Technically refers to substances made from many repeating units, but used interchangeably with plastic in this context.

Historical Development of Plastics

  • Gutta Percha (1843):

    • Developed in Malaysia from the latex of tropical trees, resembling rubber with higher resin content.

    • Common uses include knife handles and picture frames.

    • Etymology: "Gouda" translates to sap, and "percha" to strips of cloth.

  • Prophylactic Brush Company (Early 1950s):

    • Founded by Alfred Critchlow, manufacturing dyes and presses using materials like shellac and gutta percha for various items.

  • Celluloid Development (Civil War):

    • John Hyatt created celluloid from cellulose nitrate and camphor as a substitute for ivory billiard balls.

    • Influenced the later creation of cellophane.

  • Bakelite (Early 1900s):

    • Patented by Leo Baekeland; the first synthetic resin.

    • Applications include microwavable meal trays.

  • Polyethylene (1930s):

    • First developed in England; now the most widely used packaging plastic.

    • Over 13.3 billion pounds used in packaging applications per year.

Key Concepts in Plastics

  • Plastic Film vs. Sheets:

    • Plastic film: Material ≤ 0.001 inches thick (1 mil = 100 gauge).

    • Plastic sheet: Material > 1 mil thick.

  • Carbon Units and Bonding:

    • Carbon (C) as the basic unit in polymers has 4 bonding sites.

    • Ethane: C2H6 (single bond); Ethylene: C2H4 (double bond leads to losing 2 hydrogens).

  • Polymerization Process:

    • Addition Polymerization: Transformation of monomers into polymers under certain conditions with catalysts.

    • Examples: Polyethylene (from ethylene), Polyvinyl Chloride (PVC from vinyl chloride).

  • Types of Polymers:

    • Polyethylene (PE):

    • Most common plastic, used in various applications like garbage bags and kayaks.

    • Variants based on density: Low Density PE (LDPE), High Density PE (HDPE), Medium Density PE (MDPE), Linear Low Density PE (LLDPE).

    • Polypropylene (PP):

    • Known as "living hinge," use in bottle caps and containers.

    • Polyethylene Terephthalate (PET):

    • Common in beverage bottles and food containers.

    • Polyvinyl Chloride (PVC):

    • Used in plumbing, toys, and food wraps.

    • Polyvinylidene Chloride (PVDC):

    • Trade name: Saran Wrap; excellent oxygen and moisture barrier properties.

Polymerization Types

  • Basic Polymerization:

    • Same repeating units combined to create a polymer.

  • Copolymerization:

    • Involves combining two different materials, which can be:

    • Alternating: e.g., polyethylene and PVC alternating.

    • Random: No specific order of units.

    • Block: Grouped units of the same polymer alternating with different ones.

Organic Chemistry Foundations

  • Organic chemistry concepts help to understand polymer behavior, including the polarity of different materials in interaction with plastics.

  • Examples of basic organic molecules:

    • Ethylene: C2H4 (double bond between carbons).

    • Propylene: C3H6 (contains double bond, includes methyl group).

Polymer Density and Yield

  • Density Definition: Mass per volume, expressed in grams per cubic centimeter.

  • High Density vs. Low Density Polyethylene:

    • HDPE: Less branching, higher density; more closely packed molecules.

    • LDPE: More branching, lower density; less compact arrangement leads to higher yield in terms of area produced from raw material.

  • Yield Measurement: Important in assessing efficiency, noted in square inches per pound; LDPE generally exhibits higher yields than HDPE.

Types of Plastic Polymers

  • Thermosets vs. Thermoplastics:

    • Thermoset: Polymers that cannot be remolded after being set by heat.

    • Thermoplastic: Polymers that can be remolded upon reheating, often referred to as having "memory."

Plastic Processing Overview

  • General Processing Steps:

    1. Resin pellets are heated in an extruder, melted, and pumped through a die.

    2. The die determines the shape of the plastified material (e.g., slot or ring die).

    3. Film can be oriented by machine direction (MD) or cross direction (CD) based on processing.

  • Extruder Functionality:

    • The hopper feeds resin pellets into the heated barrel where they melt prior to being formed through a die.

Film Production Techniques

  • Slot Die Extrusion: Produces cast film in a single layer; utilizes chill rolls for cooling.

  • Blow Film Extrusion: Creates a hollow bubble of film which is biaxially oriented; involves an air inflation process.

  • Orientation Types:

    • MD: Stretching along the machine direction.

    • CD: Stretching across the machine direction.

    • Biaxial Orientation: Important for strength and elasticity in materials like shrink wrap.

Coextrusion Process

  • Coextrusion combines multiple layers of different materials to optimize performance features.

  • Example: Heat sealable polyethylene can be layered with materials with poor sealing properties, or to hide recycled material layers.

Molding Techniques

  • Injection Molding:

    • Used for small, precise parts such as cups; melted plastic is injected into molds.

  • Extrusion Blow Molding:

    • Parison (hollow tube) is formed and inflated into bottle shape; characterized by efficient production of handles.

  • Injection Blow Molding:

    • Involves creating a preform via injection before blow molding into final shape.

  • Thermoforming:

    • Involves heating a plastic sheet over a mold, creating dishes or blister packs through vacuum forming.

Advantages and Disadvantages of Plastics

  • Advantages:

    • Versatile and lightweight; unbreakable compared to glass; energy-efficient production.

  • Disadvantages:

    • Petroleum-based, non-renewable; negative environmental perception; recycling challenges; materials may react with certain products requiring careful selection.