Plastic Materials: Properties, Types, and Manufacturing Processes

**Plastic Materials: Properties, Types, and Manufacturing**

**Properties of Plastics**

Definition: Plastics are materials of organic origin, solid at room temperature, and easily molded by applying heat. They have a high molecular weight.

Most used: Polypropylene (PP)

Qualities:

• Weight reduction
• Noise damping
• Impact absorption
• Chemical resistance
• Corrosion resistance
• Thermal and acoustic insulation

Disadvantages:

• Low mechanical resistance
• Cannot withstand high temperatures
• Easily scratched
• Can easily capture moisture
• Sensitivity to sunlight
• Sensitivity to environmental conditions

**Chemical Processes**

Plastics are of organic origin and are chemical compounds formed by the union of several units. They can be formed by polymerization, polycondensation, or polyaddition. Additives are added to improve their properties.

**Polymerization**

1. Reaction Initiation: Monomers are activated by catalysts.
2. Reaction Time: The chain becomes large and stable.

Linear chains are obtained, and thermoplastic elastomers are obtained. This process forms PP.

**Polycondensation**

The union of different monomers forms the polymer. Water, alcohol, or ammonia are also obtained. This process forms polyurethanes and polyamides.

**Polyaddition**

This is the union of similar monomers. Thermostable materials are obtained, such as PUR.

**Chains**

• Linear: Thermoplastic
• Branched: Thermoplastics
• Crosslinked: Thermosets and elastomers

**Types of Plastics**

**1. Thermoplastics**

These can be welded by heat and shaped. They have initial hardness, and when cooled, they can be shaped in any way you want.

• Polyethylene (PE)
• Polypropylene (PP)
• Ethylene-propylene diene (PP/EPDM)
• Polyamide (PA)
• Polycarbonate (PC)
• Polyvinyl chloride (PVC)
• Acrylonitrile Butadiene Styrene (ABS)

**2. Thermosets**

These do not respond to heat and disintegrate at high temperatures. They are hard and fibrous materials that break by splitting. They are a combination of thermosetting resin and synthetic fibers.

• Epoxy resins (EP)
• Polyurethane (PUR)
• Glass fiber reinforced plastics (GRP)
• Unsaturated polyester resin (UP)

**3. Elastomers**

These support large deformations and recover their original form. They do not flow when heated and remain elastic until decomposition. Examples are rubber and PUR.

**Additives**

• Stabilizers: Provide protection against oxygen, heat, and light.
• Adsorbents: Block ultraviolet radiation.
• Removers: Nonstick, for easy extraction of the product obtained in molds.
• Plasticizers: Give flexibility to the polymer.
• Lubricants: Used for bad behavior in fusion and friction against the molds. They can be external or internal.
• Colors: Polymers have no color, so dyes must be added.

**Manufacturing Processes**

**Thermoplastics**

• Imprinting: Embossing process by applying heat and pressure.
• Rotary Mold: Obtain a uniform layer thickness by rotating a concave mold inside hot walls (250-300°C).
• Extrusion: Used for semi-finished products. The process takes place inside a cylinder subjected to temperature. A screw pushes the mass, compressing, softening, and shaping it.
• Extrusion Blow Molding: Extrusion process on two mold halves that bind when applying air pressure to fit the plastic walls of the mold.
• Injection: The most used process to make thermoplastic parts. The plastic is heated in a plasticizing cylinder and injected into a mold cavity to shape it.
• Calendering: Used for PVC sheets for car interiors. The material is heated and laminated between rollers to form a continuous sheet.
• Foaming: Used for foam. Air is introduced into the plastic, causing swelling, or a foaming agent is added to the plastic.

**Thermosets**

Reinforcement materials are mixed with resin to improve endurance. The most common reinforcement material is fiberglass, which can be in the form of tissues, mat, or roving.

• Manual Contact Molding: Apply a base of gel-coat resin, and once dry, apply polyester resin. Then, apply the glass reinforcement and use a metal roller to eliminate air bubbles. Finally, demold.
• Injection Molding with Resin: Liquid resin is injected into a mold at low pressure where the glass reinforcement is placed.
• Hot Injection Molding: