User's guide to plastic

Chapter 1- Polymers and Plastics

Sometimes you get the question: What is the difference between polymer and plastic? The answer is simple: there is no difference, it's the same thing. The word "polymer" comes from the Greek "poly," which means many, and "more" or "meros," which means unity.

The online encyclopedia Wikipedia (www.wikipedia org) states the following: "Polymers are chemical compounds that consist of very long chains composed of small repeating units, monomers. Polymer chains are different from other chain molecules in organic chemistry because they are much longer than, for example, chains of alcohols or organic acids. The reaction that occurs when the monomers become a polymer is called polymerization. Polymers in the form of engineering materials are known in daily speech as plastics.

By plastic, we mean that the engineering material is based on polymers, generally with various additives to give the material the desired properties, such as colors or softeners. Polymeric materials are usually divided into rubber materials (elastomers). thermosets and thermoplastics."

Most polymers are synthetically produced, but there are also natural polymers such as natural rubber and amber that have been used by mankind for thousands of years.

Other natural polymers include proteins, nucleic acids, and DNA. Cellulose, which is the major component in wood and paper, is also a natural polymer.

In other words, plastic is a synthetically manufactured material composed of monomer molecules that bind to each other in long chains. If the polymer chain is made up solely of one monomer it is called polymer homopolymer.

If there are several kinds of monomers in the chain, the polymer is called copolymer. An example of a plastic that can occur both as homopolymer and copolymer is acetal.

Acetal is labeled POM (polyoxymethylene) and is mostly up-built of a monomer known as  formaldehyde. The building blocks (atoms) in formaldehyde are composed of carbon, hydrogen, and oxygen.

Most plastic materials are composed of organic monomers but may in some cases also be composed of inorganic acids. One example of an inorganic polymer is a silicone resin consisting of polysiloxanes, where the chain is built up of silicon and oxygen atoms.

Carbon and hydrogen are the other dominant elements in plastics. In addition to the aforementioned elements carbon (C), hydrogen (H), oxygen (o), and silicon (Si), plastics typically consist of another five elements: nitrogen (N), fluorine (F), phosphorus (P), sulfur (S), and chlorine (CI).

It is extremely rare to work with a pure polymer. As a rule, different additives (modifiers) are used to affect a material's properties. Common additives include:

lSurface lubricants (facilitate ejection)

lHeat stabilizers (improve the process window)

lColor pigments

lReinforcement additives such as glass or carbon fiber (increase stiffness and strength)

lImpact or toughness modifiers

lUV modifiers (e.g. to protect against UV light)

lFire retardants

lAntistatic agents

l Foaming agents (e.g. EPS, expanded polystyrene)

Thermosets

In thermosets as well as in rubber, binding can occur between the molecular chains, which is described "cross-linking." These cross-links are so strong that they do not break when heated--thus the material cannot be melted.

Thermosets occur in both liquid and solid form, and in some cases can be processed with high-pressure methods. Some common thermosets include:

lPhenolic plastic (used in saucepan handles)

lMelamine (used in plastic laminates)

lEpoxy (used in two-component adhesives )

lUnsaturated polyester (used in boat hulls)

lVinyl ester (used in automobile bodywork)

lPolyurethane (used in shoe soles and foam)

Many thermosets have excellent electrical properties and can withstand high operating temperatures. They can be made extremely stiff and strong with glass, carbon, or Kevlar fibers. The main disadvantages are a slower machining process and difficulties of material or energy recycling.

Thermoplastics

Thermoplastics have the advantage that they melt when heated. They are easy to process with a variety of methods, such as:

lInjection molding (the most common process method for thermoplastics)

lBlow molding (for making bottles and hollow products)

lExtrusion (for pipes, tubes, profiles, and cables)

lFilm blowing (e.g. for plastic bags)

lRotational molding (for large hollow products such as containers, buoys and traffic cones)

lVacuum forming (for packaging, panels, and roof boxes)

Thermoplastic can be re-melted several times. It is therefore important to recycle plastic products after use. Commodities can usually be recycled up to seven times before the properties become too poor. In the case of engineering and advanced plastics, a maximum of 30% regrind is usually recommended so that the mechanical properties of the new material are not significantly affected. If you cannot use recycled plastics in new products, energy recycling through incineration is often a suitable choice. There is however another option called chemical recycling, although this process has not yet become popular due to the high costs involved versus virgin manufactured material.

Amorphous and Semi-crystalline Plastics

As shown in Figure 3 plastics can be divided into two main groups depending on the plastic structure, i.e. amorphous or semi-crystalline. Glass is another common amorphous material in our environment, and metals have a crystalline structure. An amorphous plastic softens as glass does if you raise the temperature and can therefore be thermoformed. Amorphous materials have no specific melting point -instead we use the so-called glass transition temperature (Tg), when the molecular chains begin to move. Semi-cystalline plastics do not soften in the same way- -instead they change from solid to liquid at the melting point (Ts).

As a rule, semi-crystalline plastics cope better with elevated temperatures than amorphous plastics and have better fatigue resistance and chemical resistance. They are also not sensitive to stress-cracking. Semi-crystalline plastics are more like metal and have better spring properties than amorphous resins. Amorphous plastics can be completely transparent and can be thermoformed. They generally have less mold- and post-shrinkage and have less warpage than semi-crystalline plastics.

It is important that designers and processors of plastic products are aware of the type of material being used since amorphous and semi-crystalline materials behave differently when heated and require different process parameters.

Fig 11 and 12. Heating increases the specific volume linearly above and below the glass transition temperature (Tg) of the amorphous material. The semi-crystalline material also has a glass transition temperature as there are no plastics with 100% crystallinity. Around the melting point (Ts) the specific volume increases significantly. For acetal this is about 20%, which explains the high shrinkage with injection molding. Amorphous materials have no melting point and significantly less shrinkage. The energy required to raise the temperature one degree remains constant above the Tg of the amorphous material, as shown in the right-hand figure. The semi-crystalline material requires ? significant increase in energy to achieve the melting point, the so-called specific heat to convert the material from a solid to a liquid state. This causes problems for the injection molding processor, as it requires a large energy input when semi-crystalline plastics freeze in the nozzle or hot runners in the mold. Sometimes you have to take a blowtorch to melt the frozen slugs in the cylinder nozzle.