Classification of plastics

Classification of plastics

There are different ways to sensibly classify the multitude of plastics. One possibility is to classify according to use. For example, a distinction can be made between packaging films, plastics for molded parts, synthetic resin adhesives, insulating materials, paints, synthetic fibers, etc. However, the most common classification of plastics is based on their mechanical-thermal behavior and their formation reaction.
The division into these two classification groups provides information about the applicable manufacturing and processing method as well as the possible intended use.

Classification of plastics

Plastics are classified as follows:

• Classification according to mechanical-thermal behavior
  • Thermoplastics
  • Duroplasts (Duromere, Thermoduro)
  • Elastomers (elastes)
• Classification according to formation reactions
  • Polymerization
  • Polycondensation
  • Polyaddition

Based on their mechanical-thermal properties, plastics are divided into thermoplastics, thermosets and elastomers. If you look at the formation reaction of plastics, a distinction is made between polymerization, polycondensation and polyaddition.
In the following plastics technology scripts you will learn more about the classification according to mechanical-thermal behavior and about the formation reactions of plastics.

Thermoplastics

The special property of thermoplastics is their plastic deformability above their flow temperature range. Put simply, this means that thermoplastics can be melted again and their geometry changed (similar to metals).
Thermoplastics consist of long-chain, linear macromolecules that are only connected to one another by secondary valences. Within the group of thermoplastics, a distinction is made between:

• semi-crystalline thermoplastics and
• amorphous thermoplastics.

Semi-crystalline thermoplastic

In semi-crystalline thermoplastics, the macromolecules are arranged parallel to each other in certain areas. The polymer chains can also have lateral branches.

This is referred to as a semi-crystalline material because the molecules do not arrange themselves as regularly as is the case with steel, for example. In contrast, the resulting crystals are embedded in an amorphous environment. Common plastics have a degree of crystallinity between 20 and 80%. However, if the molecular chains have an unfavorable shape, crystallization is prevented. The macromolecules of the plastic melt then solidify amorphously (see amorphous thermoplastics). ( Classification of plastics )The reason for this is often side chains that are irregularly arranged like branches.
An amorphous and crystalline phase exists at the same time within this material.

Basically, the degree of crystallinity increases

• the more stretched the molecular chains are,
• the smaller the side chains are,
• the more regular the arrangement of the side chains is and
• the stronger the effectiveness of the secondary valences between the molecular chains.

Partially crystalline thermoplastics:
– PE polyethylene
– PP polypropylene
– POM polyoxymethylene (polyacetals)
– PA polyamide

Amorphous thermoplastic

In amorphous thermoplastics is the part of Classification of plastics , the individual molecular chains are tangled together in a completely random manner.
The temperature range in which an amorphous thermoplastic is solid is called the glass range. In this state, the plastic has similar properties to glass – it is relatively brittle and (in its pure form) transparent.
Amorphous thermoplastics also do not have a precisely defined melting point. When the temperature increases, the molecules become more mobile and the plastic changes from a solid to a flexible phase within a certain temperature range. However, the plastic does not (yet) become liquid. This only happens at even higher temperatures.

Amorphous thermoplastics:
– PS polystyrene
– PVC polyvinyl chloride
– PMMA polymethyl methacrylate
– PC polycarbonate
– CAB cellulose acetobutyrate
– CAP cellulose acetopropionate
– SAN styrene-acrylonitrile (example of a copolymer)
– ABS acrylonitrile-butadiene-styrene (example of a terpolymer)

Thermal behavior of thermoplastics

Depending on the temperature, there are four different states for thermoplastics. The states cycle through as follows as the temperature increases:

• Hard elastic condition
• Thermoelastic state
• Plastic state
• Thermal decomposition

The following areas are covered:

• EB = softening region, glass transition region
• Fl.B. = flow temperature range
• Eg = decomposition area

Hard elastic condition

In their hard-elastic state, thermoplastics are at low temperatures. In this state, the plastics are very brittle and glass-like. The macromolecules are very tightly connected to one another by secondary valences (hydrogen bonds, van der Waals forces). They have no mobility.

Thermoelastic state

In the thermoelastic state, thermoplastics are soft-elastic – sometimes rubber-elastic – and their shape can be changed. However, the plastic always returns to its original shape (the deformation is only elastic, not plastic). The cross-linking of the plastic here is more wide-meshed than in the hard-elastic state.

Thermoplastic state (elevated temperature)

In the thermoplastic state, secondary valences are no longer effective, which means that the molecular chains can slide past each other. In contrast to the thermoelastic state, deformations of the plastic are now retained – so they are plastic. However, if the temperature is reduced again, the plastic can return to its thermoelastic state – it now retains its current shape.

Thermal decomposition   (high temperature)

The state of thermal decomposition is characterized by the rupture of the molecular chains due to strong thermal movement. When the thermoplastic state is exceeded, the plastic initially becomes liquid and decomposes into its basic components when heated further.

processing

Possible processing methods for thermoplastics are:

• Injection molding
• extrusion
• Blow molding
• Foil bubbles
• Hot caulking
• Calendering
• Cutting processes (sawing, milling, grinding, turning, planning, drilling, etc.)
• Joining processes (gluing, welding)
• Thermal processing processes

Thermosets- Duromers

Thermosets (also known as Duromers and Thermodure) are plastics whose macromolecules are cross-linked by main valences (homeopolar bonds). The networking of the macromolecules in thermosets is three-dimensional (spatial), close-meshed and amorphous.

A special property of duromers is that they are not soluble in any solvent and do not show any signs of swelling with plasticizers. Within their application temperature, these polymer materials are brittle and glassy. After they have hardened (manufacturing process), they can no longer be plastically deformed.

Thermal behavior of thermosets

Depending on the temperature, state ranges can be determined for plastics (as well as thermosets) in which the material has specific properties. At room temperature, thermosets are in a hard-elastic state. This area is called the freezing area or glass transition area (glass point). If they are heated beyond this range, thermosets go directly into the area of ​​thermal decomposition (decomposition area).
This means that, in contrast to thermoplastics, duromers do not first go through the thermoelastic and thermoplastic state when heated before they undergo thermal decomposition.
This fact means that duromers cannot be processed using non-cutting, thermal deformation.

Hard elastic state (approx. room temperature and below)

In their hard-elastic state, thermosets are brittle. The macromolecules are very closely connected to each other by covalences and have no mobility. The hard-elastic state occurs approximately at room temperature and temperatures below.

Thermal decomposition (high temperature)

In the state of thermal decomposition, duromers are decomposed by the destruction of the main valences and the splitting off of molecular parts. The range of thermal decomposition begins at temperatures that exceed the hard elastic range – usually temperatures above 100°C.

Thermosets

• Polyurethanes
• Polyester resins
• Phenolic resins
• Epoxy resins
• Acrylate resins
• silicone resins

Processing methods for thermosets

Possible processing methods for thermosets are:

• Injection molding
• extrusion
• Pour
• Pulltrusion
• Prepreg production and processing
• Laminate production
• SMC process (Sheet Molding Compound)
• BMC process (Bulk Molding Compound)
• DMC process (Dough Molding Compound)
• PMC process (Powder Molding Compound)
• RIM process (Reaction Injection Molding)
• RRIM process (Reinforced Reaction Injection Molding)
• RTM process (Resin Transfer Molding)
• LFI process (Long Fiber Injection)
• IMD process (In-Mold Decoration)
• PUR molded parts/foams
• Stereolithography (rapid prototyping)

Elastomers, elasts

Elastomers (elastes) are plastics whose special property is their rubber elasticity. More precisely, these are plastics whose glass transition point is below the operating temperature. Elastomers are therefore dimensionally stable but elastic and return to their original shape after deformation.

The molecular chains of elastens are networked with each other in a wide-meshed and tangled manner using main valences. They are insoluble in all solvents, but are often subject to more or less pronounced swelling (since certain solvents become embedded between the molecular chains).

Elastomers are often also referred to as rubber, although this is just a type of plastic from the group of elastomers. Elastomers are used primarily when elastic properties are required. Typical examples of this are seals, rubber bands, tires, etc.

Properties of elastomers

Elastomers have the following properties:

• They are rubber-elastic
• They are not fusible
• They do not have any thermoplastic properties (an exception to this are thermoplastic elastomers)
• They are not soluble in solvents
• Solvents can cause swelling

Entropy elasticity

Entropy elasticity is another word for the rubber elasticity of elastomers. More precisely, entropy is a measure of the disordered position of molecular chains.

Explanation:

In the unloaded and unstretched state, the molecules of an elastomer are in a disordered, but statistically probable position. If the elastomer is stretched due to an external force, the molecules inevitably leave this position. This puts you in a more orderly position, but one that is statistically less likely. If the external force is removed again, the molecules return to their original position due to the thermal movement.
A similar effect occurs with gas, which after compression becomes evenly distributed within the existing space due to the random movement of the gas atoms. This is also an entropic effect, also known as entropy elasticity.

Thermal behavior of elastomers

A particularly typical property of elastomeric plastics is the increase in their elasticity with increasing temperature. The reason for this increase in elasticity is the increase in the energy available. If the temperature drops, it will eventually reach such a low level that the energy is no longer sufficient for the necessary mobility of the molecules. Here elastomers are in a hard-elastic state.

Depending on the temperature, elastomers can assume three different states. As the temperature increases, the following states pass through:

• Hard elastic condition
• Thermoelastic state
• Thermal decomposition

Hard elastic state (below room temperature)

At low temperatures – temperatures below the glass transition point – elastomers are in a hard-elastic state, in which they are very brittle and glass-like. The macromolecules are very closely linked to one another by many secondary valences (hydrogen bonds, van der Waals forces) and very few main valences.

Thermoelastic state (at room temperature)

The thermoelastic state is the area in which elastomers are used. This condition occurs approximately at room temperature. Elastomers here are rubber-elastic, the molecular chains are networked with a wide mesh. The plastic can be reversibly deformed in its thermoelastic state – meaning it always returns to its original shape on its own.

Thermal decomposition (high temperature)

At high temperatures, elastomers – like all other plastics – are destroyed. In this state of thermal decomposition, the main valences tear apart due to the strong thermal movement.

• Elastomers: Rubber elasticity occurs at temperatures below 20°C
• Thermoplastics: Rubber elasticity is present above 20°C up to the decomposition temperature.

use

Elastomers are used where their characteristic properties are important, ie especially when rubber-elastic behavior is necessary in the operating temperature range. Typical areas of application for elastomers are tires, seals (O-rings), rubber bands and the like.

List of elastomers

• Butadiene rubber BR
• Butyl rubber IIR
• Ethylene vinyl acetate EVA
• Ethylene propylene diene rubber EPDM
• Fluororubber FPM or FKM (brand name: Viton)
• Natural rubber (gum arabic) NO
• Polyurethane PUR (also exists as a thermoset)
• Chloroprene rubber CR
• Acrylonitrile/chlorinated polyethylene/styrene A/PE-C/S
• Ethylene-ethyl acrylate copolymer E/EA
• Ethylene-propylene copolymer EPM
• Acrylonitrile/butadiene/acrylate A/B/A
• Acrylonitrile/methyl methacrylate A/MMA
• Isoprene rubber IR
• Polyisobutylene PIB
• Polyvinyl butyral PVB
• Styrene-butadiene rubber SBR
• Vinyl chloride/ethylene VC/E
• Vinyl chloride/ethylene/methacrylate

Thermoplastic elastomers

Thermoplastic elastomers , which are also less commonly referred to as Elastoplasts or TPE , are plastics that behave similarly to classic elastomers as long as there is room temperature. However, when heat is applied, they can be deformed and therefore exhibit thermoplastic behavior.

The properties of thermoplastic elastomers

Conventional elastomers are spatial network molecules that are chemically networked with a wide mesh. A solution to these cross-links is not possible without decomposition of the material.

In thermoplastic elastomers, on the other hand, elastic polymer chains are integrated into thermoplastic material. They can be processed using purely physical processes using shear forces, high heat, and subsequent cooling. The parts produced in this way have rubber-elastic properties, even though no chemical crosslinking is necessary through vulcanization, which requires a lot of time and high temperatures. Repeated exposure to shear force and heat allows thermoplastic elastomers to melt and deform again.

Conversely, this means that thermoplastic elastomers have reduced thermal and dynamic resilience compared to conventional elastomers. The former therefore represent less of a further development and more of a supplement to classic elastomers, which make further applications possible. Because they combine the advantages of processing thermoplastics with the many material advantages of elastomers.

In some areas, physical crosslinking points are present in thermoplastic elastomers (crystallites or secondary valence forces). They dissolve when heated, while the macromolecules remain intact and do not decompose. These materials are therefore significantly easier to process than classic elastomers. It is also possible to repeatedly melt them down as plastic waste and reprocess them.

The material properties of thermoplastic elastomers change non-linearly over time and temperature. The two measurable core variables, compression set and stress relaxation, are particularly relevant here. The raw material is more expensive and, compared to ethylene propylene diene rubber (EPDM), they have less good material properties in the short term. However, in terms of long-term behavior, thermoplastic elastomers have an advantage in this regard.

The processing process for thermoplastic elastomers is very similar to that of conventional thermoplastics. Cycle times that are almost as short are therefore possible. Thermoplastic elastomers are increasingly being used to produce seals in car bodies and components. They can be extruded, injection molded and suitable for blow molding. As a rule, the cover is ready to use.

Classification of thermoplastic elastomers

Depending on the internal structure, they are divided into copolymers and elastomer alloys.

TPE – copolymers

Copolymers are used either as static or block copolymers. The first type consists of a crystallizing main polymer that physically crosslinks. One such copolymer is, for example, polyethylene. The degree of crystallization of the main polymer is reduced by a comonomer that is randomly incorporated along the chain. This can be vinyl acetate, for example. As a result, the crystallites (hard phase) in the finished material no longer have direct contact with each other. For this reason, they act as isolated crosslinking points, as in conventional elastomers.

In block copolymers, a sharp separation of the hard and soft segments in a molecule occurs. Below a certain temperature, the material in a TPE separates into a discontinuous and a continuous phase. When the former exceeds its glass transition temperature, it acts as a crosslinking point. This temperature is significantly below the later application temperature.

TPE – elastomeric alloys

Elastomer alloys are mixtures of finished polymers, also called polyblends. This means that the plastic consists of several types of molecules. By varying the mixing ratio and the additives, it is possible to obtain exactly the right material for numerous applications. The following types can be distinguished:

• TPE-A or TPA such as PEBAX are thermoplastic copolyamides
• TPE_E or TPC such as KEYFLEX are thermoplastic polyester elastomers.
• TPE-O or TPO like PP are thermoplastic elastomers based on olefin.
• TPE-S or TPS, such as Kraton and Styroflex, are styrene block copolymers.
• TPE-U or TPU, for example Elastollan, are thermoplastic elastomers based on urethane.
• TPE-V or TPV, such as PP/EPDM, are thermoplastic vulcanizates or cross-linked thermoplastic elastomers based on olefin.

Advantages of thermoplastic elastomers

They behave like classic elastomers at room temperature, but – unlike them – they can be deformed under heat. These are mostly copolymers that consist of both a soft elastomer component and a hard thermoplastic component. The properties of the materials lie between those of elastomers and thermoplastics.
A popular advantage of such plastics is their ability to be welded to create waterproof joints. Classification of plastics Classification of plastics Classification of plastics Classification of plastics Classification of plastics Classification of plastics Classification of plastics Classification of plastics Classification of plastics