PVC, Polypropylene and Polyethylene - How Plastics are Used in the Home
What are Plastics?
"Plastic" is a term applied to any material which can be molded or bent into shape. So for instance soft toffee, modeling clay or red hot iron is plastic because it can be reshaped. In general though when we speak of plastic, we mean polymers or synthetic materials made from petrochemicals.
There are at least 10 different types of plastics used in the home, the type used depending on the application. Many of these plastics can be recycled and are marked with a symbol and numeric code indicating the plastic type.
What is Plastic Used For?
Well virtually everything! From the sixties onwards, plastic began to replace or supplement many items which were traditionally made from materials such as metal, wood, glass, stone, ceramic and natural textiles. Some examples:
- Textiles - Cotton, wool, silk, linen and hemp were the original textiles used from ancient times. We now have acrylic, polyester and nylon
- Footwear - Shoes originally had leather uppers and soles. PVC is now used for sneakers, and soles are made from PVC or polyurethane foam
- Windows and doors - Originally made from timber, they are now also made from PVC
- Food storage, eating and drinking utensils and bottles - They were originally ceramic, glass or metal but are now also made from polyester, polypropylene and polyethylene (Polythene)
- Electrical insulation - Originally rubber, now also PVC
- Furniture - Plastics such as polystyrene are used for stackable hard seating. Padded seating coverings are often made from PVC or "vinyl" and soft, flexible, polyurethane foam forms the internal padding
- Car and machine parts - Many parts in cars were made from metals such as steel, brass and rubber. These are now often replaced by plastics such as PVC, nylon and polystyrene
- Casings of electronic devices and appliances - Originally made from wood, metal or Bakelite, they are now generally made from polystyrene or polypropylene
- Fixtures and fittings in homes - Lighting, electrical fittings and bathroom fittings
What's So Great About Plastic?
- It's cheap and made from a relatively inexpensive raw material
- It's relatively strong and can be used for many applications
- Many plastics are inert so can be used in contact with solvents, water and other chemicals without suffering any ill effects
- Unlike organic materials such as wood and natural textiles, plastic doesn't decay when exposed to wet or damp conditions for long periods
- Thermoplastics can be molded into lots of different shapes using an industrial process called injection molding (basically forcing soft plastic into a mold under high pressure). So no machining is necessary as is the case with metals. Plastics can also easily be 3D printed
- Plastics can be easily colored
- Nylon, a tough and durable type of plastic can be used for moving parts in machinery without the requirement of lubrication (it has a relatively low coefficient of friction like Teflon)
- Plastic is non conductive of electricity
- Many plastics can be easily recycled by being melted down or shredded to make new products
- As a synthetic fiber, it can be more durable than natural fibers in textiles
What Are the Disadvantages of Plastic?
- Unlike scrap metals for instance, it is such a ubiquitous and cheap material that there has been very little incentive to recycle it until recent decades
- For some applications, plastics cannot be used because they lack the strength of metals. Composite materials such as carbon fiber (a fiber reinforced polymer) will eventually become more widely used
- While plastic parts are often used in machines and tools, they aren't as tough or durable as metal, so plastic parts can wear or break more easily
- Ultra Violet (UV) exposure from sunlight can cause plastics to crack and degrade over time unless special additives are added to the material during manufacture
- There are concerns about carcinogens used as fillers or trapped monomers leaching out of plastics used in food storage containers and drinking vessels
- It can't be used for applications where it is exposed to very high temperatures
What Are the Two Main Categories of Plastics?
Plastics can be broadly divided into thermoplastic and thermosetting types.
Thermosets are heated during manufacture and a non reversal chemical reaction takes place. Once they have cooled and set, they stay solid and can't be re-heated and re-molded. Examples are Bakelite, urea formaldehyde and vulcanization of rubber.
Thermoplastics can be reheated repeatedly without undergoing any chemical change. This allows them to be reshaped or recycled into new materials. Examples are PVC, polyethylene and polypropylene.
How is Plastic Made?
There are several stages in the manufacture of plastic, but it all starts with crude oil and natural gas. There are three main stages:
Stage 1 - Crude oil (petroleum) and natural gas is extracted from ground
Stage2 - Intermediate products known as petrochemicals are made from crude oil and natural gas.
Stage 3 - Polymerization and other chemical processes are used to produce plastics from petrochemicals
Stage 1 - Crude Oil and Natural Gas, the Raw Materials For Plastic
The raw material for most plastics is crude oil, also known as petroleum. This is a thick black, brown, yellowish or greenish liquid, extracted from the ground using oil wells at land or at sea. Crude oil was formed when creatures in the oceans died and settled to the bottom of the ocean. Over millions of years, sand, shells and other stuff collected on top of them. As a result of heat and pressure, this organic material was transformed into oil.
Crude oil is a complex mix or soup of lots of chemicals and must be refined. Oil refineries are large industrial complexes which process thousands of barrels of oil daily. In a refinery the oil is cleaned of contaminates such as salt water. Several processes are then used to break the oil up into its constituent components, typical of which is fractional distillation. In this process, oil is heated and various compounds boil off at different temperatures and can be separated. This results in distillates such as gasoline, kerosene, lubricating oil, wax and asphalt (bitumen). Petrochemicals such as benzene, toulene and xylene or BTX are also produced here.
Natural gas is also used as a raw material. This is processed at a natural gas processing plant into a variety of different types of gases.
Stage 2 - Petrochemicals are the Intermediate Product
Petrochemicals are the intermediate chemicals in the production of plastics. Typical petrochemicals are ethylene, propylene, benzene, butadeine, toulene and xylene. Petrochemicals are produced in petroleum refineries, natural gas processing plants and by a process known as cracking.
Stage 3 - Polymerization
In this process, the petrochemical monomer raw product is converted into a long chain polymer or plastic. A typical petrochemical is ethylene gas with the formula C2H4. In the polymerization process, an activator known as a catalyst speeds up the chemical reaction. Lots of ethylene molecules join together into chains to form a polymer
Types of Plastic Used in the Home
There are lots of different types of plastics used in and about the home. The most common ones are:
- Polyethylene (Polythene)
- Poly Vinyl Chloride (PVC) also known as vinyl
- Polytetrafluoroethylene (Teflon)
- Polyesters, polyethylene terephthalate (PET) being the most common
How to Identify Plastic Type
Most plastic products or their component parts are marked with a plastic identification code symbol specifying the type, allowing ease of identification for recycling purposes.
There are many polyesters but the commonest type encountered in the home is PETE or Polyethylene terephthalate. PETE is spun into a synthetic fiber which is used to produce textiles. Polyester fiber is also mixed with natural fibers such as cotton to improve durability.
- Shirts, trousers, coats and other clothing, sometimes under the brand name "Terylene"
- Bed sheets
- Plastic bottles
- Insulating tapes
- Insulation in clothing and as padding in duvets, pillows etc
Products Made From PET
High density polyethylene has a high strength to weight ratio and also a high resistance to solvents, acids and other chemicals.
- Plastic bags
- Storage containers for food and other products
- Beverage bottles
- Toiletry bottles
- Fuel tanks on vehicles and garden/yard power equipment
- Containers for household cleaning chemicals
- Gasoline storage containers
- Storage containers for motor oil
- Blow molded cases for tools
Products Made From HDPE
Poly Vinyl Chloride (PVC)
Poly Vinyl Chloride is more commonly known by the abbreviation PVC or "vinyl". It comes in two forms rigid or flexible. The rigid form is used for structural work e.g door and window frames and the frame members of plastic greenhouses. The flexible form is widely used as electrical insulation, upholstery coverings and inflatable products.
- Doors, windows and greenhouse frames
- Upholstery covering, chair seat coverings
- Beach balls, footballs and other inflatable products
- Fascia and soffit
- Downpipes and gutters
- Waste and water pipe
- Insulation for electrical wire
- Electrical conduit
- "Vinyl" records
Products Made From PVC
Low Density Polyethylene (LDPE)
Low Density Polyethylene is a soft flexible plastic widely used for making clear plastic bags and sheeting. It has a higher resilience than HDPE, which basically means that it can be stretched or deformed more without cracking or ripping. You can tell the difference between an LDPE and HDPE bag by scrunching them up. HDPE bags make lots of crackly noise. It is ideal also for snap on lids as it can deform and seal around the rim of the container.
- Plastic bags
- Clear flexible plastic sheeting, e.g. for polytunnels
- Soft flexible snap on lids
- Shrink wrap
- Waterproof linings of cardboard juice and milk cartons
Products Made From LDPE
Polypropylene is a tough, flexible plastic and used for applications which require these properties. It also resistant to fatigue which means that it can be repeatedly deformed or strained without cracking. PP also has good resistance to acids and solvents. Polypropylene is able to withstand higher temperatures than HDPE and so is used for applications where a product must be sterilized or heated, e,g. kettles and dishwasher proof kitchenware. It is also used for containers which contain dairy products such as butter, as it can withstand the heat given off by products during industrial filling processes.
A disadvantage of PP is that it degrades when exposed to heat and UV present in sunlight, so additives must be used if products are required to be long lasting in sunshine
- Beverage bottles
- Dish washer proof plates and food storage containers
- Living hinges on lids
- Food storage containers
- Dairy product containers
- Toiletry bottles
- Coal sacks
- Ratchet straps
Products Made From Polypropylene
Polystyrene has two main advantages. It is a stiff and rigid polymer, and unlike other plastics which are translucent or opaque, polystyrene can be made clear. So it is ideal for applications which require these characteristics such as CD and display cases.
- CD cases and display boxes
- Casings for kitchen appliances, TVs and Radios, torches and other electrical gadgets
- Seed trays
- Blister packing
- Disposable cutlery
- Disposable ballpoint pens
- Expanded polystyrene (Styrofoam) is used as insulation, as a packing material and for disposable cups
Products Made From Polystyrene
Other Plastics Used in the Home
Several polymers which don't have their own specific recycling number are classed under category 7 as "other plastics". Not all of these are recyclable
Other plastics used in the home are:
- Nylon - Tough and durable and used for ladies tights, sports clothing and moving parts in machines
- Polycarbonate - This is tough and durable and can be bent and deformed to a large extent without cracking. It is used for making sunglasses, safety glasses, CDs and as synthetic "glass" in greenhouses
- Acrylonitrile butadiene styrene (ABS) - Another tough plastic with high impact resistance. It is used for making Lego bricks, carrying cases and electrical boxes
- Urea formaldehyde - Commonly used for sockets, plugs, switches and other electrical fittings
- Polyurethane - Used in foam form as a padding in cushions and upholstery, air filters on lawn mower engines and as packing. Some air and fuel lines are made from this material. Hard foam soles on footwear are also polyurethane based
- Polytetrafluoroethylene (PTFE) - This plastic has a very low friction coefficient, i.e. it is "slippy". It is known by the brand name Teflon as the non stick coating on cooking utensils. It is also used in tape form for sealing the threads on plumbing joints
- Polymethyl methacrylate (PMMA) or Acrylic - Used as a synthetic fiber in sweaters and as clear sheeting or pipe under the trade names "Perspex", "Plexiglas", "Acrylite" and "Lucite
- Nitrile rubber - one of the many types of synthetic rubbers. These are used for a variety of products including gloves, o-rings, sealants, fuel lines and water hoses in vehicles, wellingtons and anything which requires a flexible, waterproof material
Older plastics used in the home were:
- Bakelite This is a thermosetting plastic, used in the past for electrical fittings, electrical insulation, door knobs and saucepan handles. Because it is a thermoset, it doesn't melt or deform when subjected to high temperatures.
- Celluloid Used for making dolls, musical instruments, pens, table tennis balls, cutlery handles and as a veneer for clocks and furniture
Other Plastics Used in the Home
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