TECHNICAL MATERIALS III. PLASTICS

INTRODUCTION

The word plastic comes from the Greek word plastikos, which means "capable of being molded." The term expresses the main property of this material: its deformability and, therefore, ease to take virtually any shape.

Plastics are one of the most used materials today. Production and consumption have increased at a rate much higher than any other material. The first plastics were originated in 1840, but the plastic industry was born in 1865 when celluloid was produced from cellulose plus nitric acid. It was employed for a long time in the film industry. It was very elastic but flammable.

Later in 1909, bakelite and nylon appeared and many wood products or metals were substituted.

Our consumption of plastics is very high, just think when you buy a product, its plastic packaging and the plastic bag you use to carry it. A visit to a large shopping mall or hypermarket can give us an idea of ​​the magnitude of the consumption of this material, which often ends in the rubbish bin when we arrive home.

PROPERTIES OF PLASTICS

Plastics are made from petroleum, hard coal, natural gas and other organic elements where the carbon appears. This material has an intense use because of its properties:

• Plasticity: Plastics are very easy to work because they are highly deformable, which facilitates its industrialization, and the cost of the final product is cheaper.

• Electric conductivity: Conducts electricity very poorly.

• Thermal conductivity: Plastic conducts heat very badly; because of this, they are very good insulators.

• Chemical and atmospheric resistance: Plastics resist very well the attack of acids, and don't modify their properties. They are also very resistant to weather, sun, wind, rain, salt, etc.

• Mechanical resistance: Structurally plastics do not resist well to twist efforts and bend efforts, but can be used in mechanisms that do not require high mechanical stress. There are plastics that resist good compression efforts.

• Density:They are not dense (weigh little ).

• Elasticity: They are very elastic.

• Resistance to abrasion: Although some plastics are resistant to friction, in most of them abrasion causes rapid wear.

• Hardness: In general plastics are easily scratched.

• Fusion temperature: In the case of plastic it is very low, so their heat resistance is low also.

• Variety of form, colour, texture, appearance: There are thousands of variations and each year new plastics are produced. Because they are easy to work with, you can get many forms, some very complicated. Besides you can give them the desired colour and texture, which makes them extremely versatile.

• Recycled: Most plastics are easily recycled.

CLASSIFICATION OF PLASTICS (according to their molecular structure)

Considering the distribution of intertwining of macromolecules, they can be classified into three groups: plastics thermoplastics, thermosets, and elastomers.

THERMOPLASTICS: It is a type of plastic that can be heated, molded and cooled indefinitely. Because of this, they are the cheapest ones. They are the most common ones.
THERMOSETS: These plastics when heated for first time can be shaped but once cooled, their form stays permanently, i.e. we  can't  heating them again  to  give them a new form because they are degraded (because  of  the  destruction of the molecular bonds). They are not recyclable, resist heat and electricity. They are highly insulating
ELASTOMERS:  They are highly elastic and plastic are very adherent due to the great quantity of chemical bonds between macromolecules. They are not recyclable.

HOW TO MANUFACTURED PLASTIC PRODUCTS

To obtain the final product with the look we know, we need all an industrial manufacturing process, which can become very complex. There are many ways to obtain plastic. The most important way is the molding. The molding consists of giving a plastic the desired shape and size using a mold. The mold is a hollow piece in which the molten plastic is poured to acquire its form.

The most important processes for the transformation of plastics are:

• High pressure molding:
  • Compression molding.
  • Extrusion molding.
  • Injection molding.
• Low pressure molding:
  • Vacuum molding.
  • Blow molding
• Casting molding.

• Foam molding.

• Calendaring / calendering.

We are going to consider the characteristics of each of these processes

HIGH PRESSURE MOLDING

For this purpose, the plastics are pressed into the mold. Basically there are three types: compression, injection and extrusion.

• Compression. In this process, the plastic powder is heated and compressed between the two parts of a mold, by the action of a hydraulic press because the pressure required in this process is usually very high. This is the procedure used to obtain small pieces of bakelite, as heat insulating handles used in cookware.

• Extrusion. It is to mold products continuously, because the material is pushed by a screw through a cylinder ending in a nozzle, producing a strip of indefinite length. Changing the shape of the nozzle we can obtain bars with different profiles (cylindrical, square, hexagonal ...). The final product is screwed or chopped, depending on its future use. This process is also used for the manufacture of pipes, injecting air under pressure through an orifice at the tip of the nozzle. By regulating the air pressure, you can achieve tubes of different thicknesses.

• Injection. It is to introduce the plastic into a cylinder, where it is heated. Inside the cylinder there is a worm screw that acts as the piston of a syringe. When it is softened enough, the worm screw presses the product into a steel mold.The mold and the injected plastic are cooled by interior channels for flowing water. For economy and speed, injection molding is very suitable for the production of large series of parts. By this procedure, basins, buckets, housings, automotive components, etc. are manufactured.

A variant of the extrusion molding and the injection molding is blow plus extrusion plus injection molding, a procedure in which the plastic, as it leaves the nozzle, is compressed between the two halves of a mold, while pressurized air is injected. The plastic material is then adapted to the shape of the mold; and a hollow piece is obtained.

Synchronizing the opening and closing of the mold with the output speed of the material can be produced continuously and automated objects such as bottles, flasks and all kinds of hollow containers, impossible to obtain by another procedure.

Similarly, plastic bags can be manufactured. We need the plastic that is coming out of the injection machine to have very thin walls.

LOW PRESSURE MOLDING

The low pressure molding is used to shape plastic sheet by applying heat and pressure to suit a mold (no elevated pressures).

• Vacuum molding. It consists in making vacuum by absorbing air between the plastic sheet and the mold, so that it suits the shape of the mold. This type of molding is used for producing food packaging or containers reproducing the shape of objects that they contain.

• Blow molding. It involves applying air pressure against the plastic sheet to adapt the mold. This procedure is called blow molding. It is used to manufacture domes, hollow parts, etc.

CASTING MOLDING

The casting molding is the pouring of liquid plastic material into a mold, where it sets and solidifies.

The casting is useful for making few pieces, or when we can find inexpensive materials for the molds, such as plaster or wood.

Due to its slowness this method is not useful for the manufacture of large series of pieces.

FOAM MOLDING

It consists in introducing air or other gas within the plastic mass so that permanent bubbles are made. By this procedure polystyrene foam (Styrofoam), polyurethane foam PUR (foam rubber), etc. are obtained. With these materials we can manufacture mattresses, thermal insulation, packaging sponges, cycling and skating helmets, light panels and others.

CALANDARING

It consists of passing the plastic material through rollers that produce, by pressure, flexible plastic sheets of different thickness.

These sheets are used to make rubber, waterproof materials and thin plastic sheets.

VISUAL IDENTIFICATION OF PLASTICS

Plastics are difficult materials to identify.

Manufacturers use some abbreviations in products that allow us to know what type of plastic it is.

Plastics are usually mixed with additives and dyes which make it more difficult to identify.

However if we look at the packaging, we can see the recycling symbol with a number inside. This number identifies the type of plastic. Sometimes, instead of a number, there are capital letters that allow us to know the type of plastic it is.

To classify them in thermoplastics, thermosets and elastomers, we will look at their appearance and especially in their use. In this way the plastic "normal" will be thermoplastic, the elastic plastic will be elastomer and the plastic used as thermal and/or electrical insulation, the mayor thickness and with dull aspect will be thermostable.

RECYCLING OF PLASTICS

The use of plastics has spread so rapidly in recent years that their garbage is creating a problem worldwide.

To use plastics (and all products) in a sustainable way, we must make use of the three Rs rule: reduce, reuse and recycle.

REDUCE the consumption of plastics such as shopping bags, minimize the amount of packaging of products, make the thinnest packages...

REUSE whenever possible, such as reusable bags sold instead of using throwaway bags, using a leaky pot as flower pot...

RECYCLE: it consists of separating instead of throwing away, so that things can have a new utility. It is a process that depends on the type of recycling plant, the type of plastic and the citizen, who must separate them from other waste and deposit them into a special container.

There are two main types of recycling:

Mechanical recycling 

Mechanical recycling comes from:

• Yellow packaging containers, where the plastic is about 80% of waste and is very easy to separate.

• Standard garbage, which will have been previously separated in large plants.

In this process of recycling, plastic is chopped for later reuse. If all is well separated (all plastic is thermoplastic) we can use it to make new plastic objects, such as cubes (colour is often the mixture of the original colours or is stained with a dark tint).

Sometimes this material is mixed with sand to achieve structures with similar strength of concrete structures. Many urban posts -bollards- that are placed to prevent parking in cities, are filled with this material.

The mechanical process is the most evolved in the recycling of plastics, and also the most widespread. Chemical processes are not developed and implemented in all countries, but the good results and the big variety of recycled plastics they can work with, portend an important projection in the future.

Chemical recycling

It is a chemical process that breaks the molecules that make up the plastic in their starting components. It is a complex process and requires considerable amount of energy. It doesn’t exist in most of the waste treatment plants. On the other hand, thermosets and elastomers could be used in this process. They can’t be recycled by the above method.

This second option allows greater recovery of raw material than mechanical recycling processes. The waste we are currently generating could turn into fuel or raw materials in the future if this type of recycling processes evolve and are implemented in a majority.

PLASTICS AND SOCIETY

Since plastics are relatively inert, finished products pose no danger to the manufacturer or user. However, it has been shown that some monomers used in the manufacture of plastics cause cancer. The problems of the plastics industry are similar to those in the chemical industry in general.

Most synthetic plastics can not be degraded by the environment. Unlike wood, paper, natural fibres or even metal and glass, they don't rust or break down over time. Furthermore the recycling of food packaging presents sterilization problems, which are not reusable for the same purpose. In short, the disposal of plastic represents a big environmental problem.

Notes for printing