Category Chemistry

Did you know that paints and adhesives contain plastics? Paints are often made of three different chemicals. A ‘pigment’ provides the colour; a plastic holds the pigment in place and gives a shiny finish; and a ‘solvent’, usually white spirit, makes the paint runny and easy to use. When the paint dries, the solvent evaporates and only the pigment and plastic are left.

Pigments

The pigment is the color chemical in paint. It looks a certain color because it reflects some wavelengths of light and absorbs others. Traditionally, metal compounds (salts) are used to create different colors so, for example, titanium dioxide (bright white chemical often found in sand) is used to make white paint, iron oxide makes yellow, red, brown, or orange paint and chromium oxide makes paint that’s green. Black comes from particles of carbon (think what your burned toast looks like and you’re getting close to a color chemical known as “carbon black”). Different pigments are mixed together to make paint of any color you can imagine.

Binders

Pigments are typically solids, so you couldn’t use them to paint by themselves. They’d be difficult to apply, they wouldn’t spread evenly, they wouldn’t stick to paper or a wall, and they’d wash straight off if they got wet. That’s why paints also contain substances called binders. Their job is to glue the pigment particles to one another, but also to make them stick to the surface you’re painting. Some binders are made from natural oils such as linseed oil, but most are now made from synthetic plastics. Visualize the binder as an invisible skin of plastic with a colorful pigment dispersed through it and you can see just how paint gives a layer of protection.

Solvents

Mix a pigment and a binder and you get a thick gloopy substance that’s difficult to spread. Ever tried painting a wall with treacle? That’s what using a pigment and a binder is like. It’s the reason why paints have a third major chemical component called the solvent. As its name suggests, a solvent is something that dissolves something else. The solvent’s job is to make the pigment and binder into a thinner and less viscous (more easily flowing) liquid that will spread evenly (that’s why paint solvents are sometimes called thinners). Once the paint has spread out, the solvent evaporates into the air, leaving the paint evenly applied and dry beneath it. When you apply a really nasty paint and there’s a smell lingering for days while it dries, that’s the solvent evaporating into the air.

Strong glues like ‘superglue’ are made of thermosetting plastics called epoxy resins. They can stick metal, glass, china, and wood— in fact almost anything!

Laminates and perspex are both hard. Different plastics are needed to make flexible sheets. Carrier bags, light raincoats, shower curtains and food packaging are just some of the products made from plastic sheets.

Food and other articles are often ‘shrink wrapped’. The article is wrapped and sealed in a thin plastic film that has been heated, stretched and then cooled. Although the film stays stretched when it cools, if the wrapped article is passed through a hot tunnel, the plastic melts and shrinks back to its original size, wrapping the item very tightly.

Most Plastic bags are made from polyethylene – more commonly known as polythene, which is made from crude oil and natural gas, non-renewable resources.

The most common way to produce polythene bags is by blown film extrusion, also called the “tubular film process.”

In Blown film production process – polythene melt is extruded through an annular slit die, usually vertically, to form a thin walled tube. Air is introduced via a hole in the centre of the die to blow up the tube like a balloon into the tube causing it to expand and form a bubble. Mounted on top of the die, a high-speed air ring blows onto the hot film to cool it. The tube of film then continues upwards, continually cooling, until it passes through nip rolls where the tube is flattened to create what is known as a ‘ lay-flat’ tube of film. This lay-flat or collapsed tube is then taken back down the extrusion ‘ tower’ via more rollers. The lay-flat film is then either kept as such or the edges of the lay-flat are slit off to produce two flat film sheets and wound up onto reels. If kept as lay-flat, the tube of film is made into bags by sealing across the width of film and cutting or perforating to make each bag. This is done either in line with the blown film process or at a later stage.

Plastics are not always moulded into shapes — we often need sheets of plastic. Perspex is a thermosetting plastic, ideal for making windows and roof lights. Perspex is as clear as glass and does not break easily. It is made by pouring liquid plastic between two sheets of sealed glass. When the glass sheets are clamped together and passed through a hot oven, the plastic sheet hardens to form perspex.

Table tops and other surfaces are sometimes protected with sheets called ‘laminates’. Melamine is a laminate. This thermosetting plastic forms a very strong, tough surface. Melamine is resistant to heat and does not stain easily.

Laminate sheets are made by bonding together two or more layers of materials. Laminate sheets from plastics are glued to wood to make the surface aesthetic. These sheets are manufactured by laminating different types of papers with formaldehyde. The core sheet consists of Kraff paper with phenol formaldehyde (PF) and below it, a barrier paper is provided. Above the Kraff paper, a tissue paper is impregnated with FF melamine formaldehyde (MF) resin is provided which gives protection and also enhances abrasion resistance. Then, these laminates are bonded to wooden surfaces with suitable glue and pressing for some time finishes the process.

Moulding is the process of manufacturing by shaping liquid or pliable material using a rigid frame called a mold or matrix.

When moulding plastics, a powder or liquid polymer such as polyethylene or polypropylene is placed into a hollow mold so the polymer can take its shape. Depending on the type of process used, various ranges of heat and pressure are used to create an end product.

Many different objects are made from plastic — thin sheets for wrapping food, long strips for curtain rails, hollow bottles and complicated toys. With such a huge range of items made of plastic it is not surprising that there are many different ways of moulding plastic.

The Types of Plastic Moulding

       1. Rotational Moulding

Rotational Moulding, also called rotomoulding, is a manufacturing process for producing large hollow parts and products by placing a powder or liquid resin into a metal mould and rotating it in an oven until the resin coats the inside of the mould. The constant rotation of the mould creates centrifugal force forming even-walled products. Once the mould cools, the hardened plastic is removed from the mould.

Very little material is wasted during the process, and excess material is often re-used, making it economical and environmentally friendly.

Common Uses for Rotational Moulding

Rotational moulding is commonly used to make large hollow plastic products like bulk containers, storage tanks, car parts, marine buoys, pet houses, recycling bins, road cones, kayak hulls, and playground slides.

        2. Injection Moulding

Injection moulding is the process of making custom plastic parts by injecting molten plastic material at high pressure into a metal mould. Just like other forms of plastic moulding, after the molten plastic is injected into the mold, the mould is cooled and opened to reveal a solid plastic part.

Commonuses for Injection Moulding

Injection moulding is commonly used for making very high volume custom plastic parts. Large injection moulding machines can mold car parts. Smaller machines can produce very precise plastic parts for surgical applications.

         3. Blow Moulding

Blow moulding is a method of making hollow, thin-walled, custom plastic parts. It is primarily used for making products with a uniform wall thickness and where the shape is important. The process is based upon the same principle as glass blowing.

Blow moulding machines heat up plastic and inject air blowing up the hot plastic like a balloon. The plastic is blown into a mould and as it expands, it presses against the walls of the mould taking its shape. After the plastic “balloon” fills the mould, it is cooled and hardened, and the part is ejected. The whole process takes less than two minutes so an average 12 hour day can produce around 1440 pieces.

Common Uses for Blow Moulding

Blow moulding processes generate, in most cases, bottles, plastic drums, and fuel tanks.

        4. Compression Moulding

Compression moulding is done exactly like the name suggests. A heated plastic material is placed into a heated mold and then pressed into a specific shape. Usually, the plastic comes in sheets, but can also be in bulk. Once the plastic is compressed into the right shape, the heating process ensures that the plastic retains maximum strength. The final steps in this process involve cooling, trimming, and then removing the plastic part from the mould.

Common Uses of Compression Moulding

The best use of compression moulding is the replacement of metal parts with plastic parts. It is mostly used for small parts and products in very high volume. The automotive industry uses compression moulding heavily because the final products are very strong and durable.

Picture Credit : Google

There are many different types of plastic; they can all be divided into two main groups. These are called ‘thermoplastics’ and ‘thermosetting plastics’. Thermoplastics include polypropylene, polystyrene, polyester, acrylics and nylons. These types of plastic melt when heated and become solid again as they cool. Thermosetting plastics do not behave in this way. Bakelite and melamine are both thermosetting plastics.

• When plastics are made they are either in the form of thick liquid or solid granules. This material is called ‘raw plastic’.

• The granules are tipped into a container and heated until they melt. They may contain a colouring dye.

• The liquid plastic is then poured into a mould. The shape inside the mould is exactly the same shape as the finished article.

• Until this point the thermoplastics and the thermosetting plastics are treated in the same way.

Thermoplastics are often used for clothing and packaging material, both of which can be recycled. Thermosetting plastics are resistant to high temperatures. For this reason they are often used to make saucepan handles and ashtrays.

The strong polymer links of the thermosetting plastic mean that it holds its shape even when heated. Plastics of this kind cannot be melted down and used again easily.

THERMOSETTING PLASTICS

•  When the thermosetting plastic is heated, links form between the polymers in the plastic. They become joined together into a permanent structure.

THERMOPLASTICS

• The thermoplastic, however, has different types of links and can be melted down and used again.

Two main processes are used to produce plastics – polymerization and polycondensation – and they both require specific catalysts. In a polymerization reactor, monomers such as ethylene and propylene are linked together to form long polymer chains. Each polymer has its own properties, structure and size depending on the various types of basic monomers used.

In a polymerization reactor, pressure and heat cause ethylene molecules, mixed with other chemicals, to link together. In ethylene the molecules form a colourless inflammable gas. When the same molecules are linked together in big chains called polymers, the plastic polyethylene or polythene is made — one of the world’s most widely used plastics. Other chemical combinations can produce polymers of different lengths and structures.

Once the polymers have been formed, the hot liquid plastic is taken from the reactor, cooled, and cut into chips or pellets.

• Each molecule of ethylene is made up of two atoms of carbon and four of hydrogen.
• If another chemical, known as a ‘catalyst’, is added these molecules form a long chain. This is called ‘polymerization’.
• Polythene gets its name from ‘poly’ meaning ‘many’.