Welcome to the World of Polymers!
In Design and Technology, "Polymers" is just a scientific word for what most of us call plastics. Polymers are everywhere—from the chair you are sitting on to the bottle you drink from. They are incredibly versatile because we can "engineer" them to be hard, soft, clear, or colorful. In this chapter, we will explore where they come from, the two main "families" of polymers, and how they are turned into the products we use every day.
1. Where do Polymers come from?
Most polymers are synthetic, meaning they are man-made. Their journey starts deep underground as crude oil. To turn oil into plastic, it goes through a few important steps:
1. Fractional Distillation: Crude oil is heated in a large tower. Because different parts of the oil have different boiling points, they separate into groups called "fractions."
2. Cracking: Large, heavy molecules are "cracked" into smaller, more useful molecules like ethene.
3. Polymerisation: These small molecules (monomers) are joined together in long chains to create a polymer.
The Life Cycle Assessment (LCA): Designers must consider the "cradle to grave" journey of a polymer. This includes extracting the oil, manufacturing the product, how it is used, and how it is eventually disposed of. Because polymers come from finite (limited) resources, thinking about their environmental impact is a huge part of being a good designer!
2. The Two Main Families
Polymers are divided into two main groups based on how they react to heat. A great way to remember this is the "Chocolate vs. Cake" analogy.
A. Thermoplastics (The "Chocolate" Group)
Think of a bar of chocolate. You can melt it, pour it into a mold, and it sets. If you don't like the shape, you can melt it again and start over. Thermoplastics work just like this. Their polymer chains are not chemically linked, so they can be reheated and reshaped multiple times. This makes them easy to recycle.
Specific Thermoplastics you need to know:
• Acrylic (PMMA): Often used as a glass substitute (like in car lights).
• High Impact Polystyrene (HIPS): Tough and used for things like vacuum-formed food containers.
• High Density Polythene (HDPE): Strong and stiff; used for milk bottles and pipes.
• Polypropylene (PP): Flexible and has great "fatigue resistance" (it can bend many times without breaking), used for folders and hinges.
• Polyvinyl Chloride (PVC): Can be rigid or flexible; used for raincoats and window frames.
• Polyethylene Terephthalate (PET): Lightweight and clear; used for fizzy drink bottles.
B. Thermosetting Polymers (The "Cake" Group)
Think of a cake mix. Once you bake it in the oven, it undergoes a chemical change. You can't "un-bake" it or melt it back into liquid batter. Thermosetting polymers have "cross-links" between their chains. Once they are heated and set, they are fixed forever. If you heat them again, they will char or burn rather than melt.
Specific Thermosetting Polymers you need to know:
• Epoxy Resin (ER): A strong adhesive used for bonding materials.
• Melamine-Formaldehyde (MF): Hard and heat-resistant; used for kitchen laminate surfaces.
• Phenol Formaldehyde (PF): Very hard and heat-resistant; used for saucepan handles.
• Polyester Resin (PR): Used with glass fibers to make GRP (Glass Reinforced Plastic) for boat hulls.
• Urea-Formaldehyde (UF): An excellent electrical insulator; used for white electrical plugs and sockets.
Quick Review: If it can be recycled by melting, it's a Thermoplastic. If it burns instead of melting when reheated, it's a Thermosetting polymer.
3. Material Properties
When choosing a polymer, designers look at two types of properties:
Physical Properties
• Absorbency/Resistance to moisture: Most polymers are excellent at resisting water (non-absorbent).
• Density: Polymers are generally lightweight compared to metals.
• Fusibility: This is how easily a material melts. Thermoplastics have high fusibility; thermosets do not.
• Electrical and Thermal Conductivity: Most polymers are insulators, meaning they do not let electricity or heat pass through them easily (think of the plastic coating on a wire).
Working Properties
• Strength: The ability to withstand force (like pulling or crushing).
• Hardness: Resistance to scratching or surface wear.
• Toughness: The ability to absorb energy and not break when hit suddenly (HIPS is very tough!).
• Malleability and Ductility: How easily the material can be shaped or stretched.
• Elasticity: The ability to bend and return to the original shape.
4. How are Polymers sold? (Stock Forms)
Manufacturers don't usually buy "finished" plastic. They buy it in standard stock forms to be processed later:
• Sheet: For vacuum forming or laser cutting.
• Rod: For turning on a lathe.
• Powder and Granules: The most common form for industrial machines (like injection molding).
• Foams and Films: Used for packaging or thin coatings.
Polymers are usually sold by length, width, gauge (thickness), and diameter.
5. Shaping and Forming Processes
How do we turn those granules into a product? There are many specialist techniques:
Wastage (Removing material)
• Sawing, Drilling, and Cutting: Basic ways to remove material using hand tools or machines.
• Turning: Using a lathe to create cylindrical shapes.
• Milling and Routing: Using a spinning tool to cut slots or shapes into the surface.
Shaping (Changing the form)
• Vacuum Forming: Heating a plastic sheet until soft and "sucking" it over a mold using a vacuum.
• Injection Moulding: Melting granules and forcing the liquid into a metal mold at high pressure. This is used for mass production (like LEGO bricks).
• Extrusion: Melting plastic and pushing it through a die to create long, continuous shapes like pipes or curtain rails.
• Blow Moulding: Blowing air into a tube of molten plastic to "inflate" it into a mold (used for bottles).
• 3D Printing (Addition): Building a product layer by layer from a digital file.
Quality Control Hint: When using a laser cutter, you ensure dimensional accuracy (getting the size exactly right) by selecting the correct power and speed settings for the specific polymer you are using.
6. Finishing and Protecting
Sometimes polymers need a little help to look better or last longer:
• Stabilisers: These are additives mixed into the polymer to help it resist UV degradation (sun damage). Without these, plastic left outside would become brittle and crack.
• Polishing: Using a buffing wheel to make the edges of acrylic crystal clear.
• Vinyl Decals: Applying sticky-back plastic graphics to the surface.
• Printing: Directly applying ink to the surface for labels or instructions.
Summary Key Takeaways
• Thermoplastics can be melted and recycled; Thermosetting polymers are set for life.
• Most polymers come from crude oil through fractional distillation and cracking.
• Injection moulding and extrusion are the heavy hitters of commercial manufacturing.
• Polymers are excellent insulators of heat and electricity.
• Always consider the environmental impact (Life Cycle Assessment) when designing with polymers!