Introduction to Polymers

The family of plastics materials is closely related to two other important families of manufacturing materials, textile fibres and elastomers (rubber), in that they are all based on carbon (organic materials) and they consist of extremely large molecules (polymers).

In comparison to other manufacturing materials (metals, glass and ceramics), polymeric materials have low density, low thermal conductivity, low electrical conductivity and are non-corrosive. However they are flammable materials and generally have lower stiffness and strength.

At one time the three classes of the polymer family were associated with quite separate sectors of industry. The textile industry has been around for thousands of years. Although rubber was known to the western world since the 18th century, it was not a serious industry until the early 19th century.

Likewise natural plastics materials (eg horn, shellac) had also been used for centuries but the modern plastics industry came into existence in the latter half of the 19th century. Today the global consumption of plastics (almost 280 million tonnes) considerably exceeds the consumption of natural and synthetic elastomer (rubber) put together (30 million tonnes)

Classification of Polymers According to Behaviour

In the early days, before polymer structure was fully understood, polymers were classified according to their observed behaviour and it was noticed that plastics could be further divided into two classes, thermoplastics and thermosetting plastics.

Polymer Synthesis

The method of converting monomer into polymer (polymerisation) has a bearing on the structure of the polymer and hence its properties.

In large-scale processes, the final form of the polymer is also of interest to users. Polymers are generally available as powders, granules, solutions and dispersions (latex). Some polymers are available in preform (sheet, block, rod, tube).

Polymerisation Mechanisms

Two main chemical mechanisms are used to produce polymers:

• Addition Polymerisation (Chain growth)
• Condensation Polymerisation (Step growth)
  

Addition Polymerisation (Chain growth)

Monomer molecules are added individually to a growing active species in a chain reaction to form large polymeric molecules (macromolecules). The reactive species may be radical, anionic or cationic. Addition polymerisation is characterised by a gradual growth in an average molecular size (although each polymer molecule is produced very quickly).

There are no by-products but the starting materials (monomer and initiator) must be very pure and oxygen excluded from the reaction if high molecular weight polymer is required. Polymerisation ceases if monomer is removed from the system.

Examples:

• polystyrene
• polyethylene
• polypropylene
• poly(vinyl chloride) (PVC)

Condensation Polymerisation (Step growth)

Monomer units condense to form large fragments which in turn condense to produce even larger fragments. Average molecular size increases exponentially. For commercial polymers with high molecular weight, it is essential to have pure starting materials, an exact balance of monomers and over 99% reaction.

In Elimination Condensation a small molecule (e.g. water) is produced at each reaction stage, causing problems in scale up processes. In Rearrangement Condensation, there is no by-product, which suits this polymerisation mechanism to in-situ moulding.

Examples:

Elimination Condensation:

• phenol-formaldehyde
• urea-formaldehyde
• melamine formaldehyde
• polyesters
• polyamides (nylon)

Rearrangement Condensation:

• epoxides
• polyurethanes

Functionality

Regardless of the mechanism of polymerisation, the molecular structure of a polymer (and hence its properties) is determined by the functionality of the monomers. Functionality is defined as the number of bonds a monomer can make with another monomer molecule in the polymerisation reaction.

Bifunctional monomers can link up to produce a linear polymer chain.

Trifunctional monomers react to produce three-dimensional networks.
Monomers of higher functionality also produce networks.

Mixtures of bifunctional monomers will form linear chain copolymers.

Mixtures of a bifunctional monomer with a small amount of trifunctional monomer will produce a loose network polymer.

Increasing proportions of trifunctional monomer will give tighter networks with increasing crosslink density.

Properties of Plastics Materials

1. Mechanical Properties

1.1 Stress v Strain

Polymers, like all materials, respond to an applied stress (force acting on unit area) by changing shape (deformation, expressed as strain = relative change in dimensions).

Linear chain polymers (amorphous thermoplastics) can accommodate applied stresses by three different mechanisms of deformation:

Polymers are described as viscoelastic materials, the relative contributions of the three mechanisms depending on:

• temperature
• time
• polymer state (glass, rubbery state, melt)
• polymer structure
• repeat unit
• intermolecular attractions

Amorphous Thermoplastics
Linear chains

At very low temperatures (glassy state, below Tg) the only mechanism open to chains is the bond bending/twisting/stretching which requires a high stress to achieve deformation.

A high modulus (ratio of stress:strain) implies rigidity and hardness. Eventually at high stresses the polymer will be unable to respond further without falling apart and will break in a catastrophic manner.

At temperatures around Tg the polymer gains the additional freedom of rotation about the backbone and the rubberlike elastic mechanism will begin to predominate.

High strains will be achieved without high applied stresses (low modulus = low rigidity or hardness). At a particular strain level segmental movement and viscous flow combine to produce additional deformation without additional stress (YIELD).

Above Tg the polymer becomes softer, more elastic and, beyond the yield point, high strains can be achieved (NECKING) as the chains extend and slip over each other. Before final rupture the fully extended chains begin to show more resistance and the stress will increase (STRAIN HARDENING).

At much higher temperatures the viscous flow mechanism will dominate, giving very low modulus (very soft) and very high extensibility in the melt state.

Semi-crystalline Thermoplastics
Linear chains with secondary crosslinks

The inclusion of secondary crosslinks (forces of crystallisation) results in restricted deformation at all temperatures up to Tm, giving increased modulus (stiffness). This is particularly evident in the rubbery state.

Increasing levels of crystallinity will make the polymer stiffer, stronger but more brittle (lower strain at failure).

Crosslinked Elastomers
Linear chains with a few primary crosslinks

The inclusion of a few primary crosslinks will extend the rubbery state from below room temperature to high temperatures. The crosslinks limit the extent of the viscous flow mechanism and there is no melt region.

The crosslinks also ensure rapid recovery of the rubberlike deformation. Increasing the density of primary crosslinks will increase the stiffness and limit the extensibility before failure.

Thermoset Plastics
3-D network

With little opportunity of capitalising on segmental movement and chain slippage (due to the high concentration of primary crosslinks) thermoset plastics are very hard, have high stiffness but show brittle failure behaviour over a wide temperature range up to decomposition temperatures.

Mechanical Properties of Polymers

1.1 Stress v Strain (in Tension)

1.2. Impact Resistance

A polymer with a high energy to failure (high area under the stress-strain curve) is designated tough. This is important in impact stresses when chains have little time to accommodate the stresses.

Above the Tg, polymer systems show good toughness and impact resistance. Increasing concentration of crosslinks (primary and secondary) will reduce the impact resistance.

Brittle behaviour (failure below the yield point, with a low energy to failure) is encouraged by:

• decreasing temperature
• increasing speed of the applied stress
• inclusion of crystalline regions
• inclusion of primary crosslinks
• introduction of flaws or cracks

1.3. Creep Resistance

At extended time scales under stress the rubberlike elastic and viscous flow mechanisms will combine to produce steadily increasing strain with time.

The extent of creep is reduced by:

• decreasing temperature
• increasing crosslink density

2. Thermal Properties

Thermal properties relevant to the processing and end-use of plastics are:

1. Heat Capacity (Specific Heat)
2. Thermal Conductivity
3. Thermal Expansion
4. Glass Transition Temperature (Tg)
5. Melt Transition Temperature (Tm)
6. Heat Distortion Temperature
7. Continuous Use Temperature

The Heat Capacity (Specific Heat) is the heat flux required to raise the temperature of unit mass (1 kg) by one degree Celsius.

Plastics have higher heat capacities than metals. Specific Enthalpy represents the heat required to raise 1 kg from room temperature to the melt processing temperature. Semi-crystalline thermoplastics have much higher specific enthalpies due to the latent heat associated with the melting stage

With a poor mechanism of transmitting energy by molecular vibrations plastics have very low Thermal Conductivities compared to metals. This is a useful property in end applications but can cause problems in processing, during heating and cooling stages.

Coefficients of Thermal Expansion are greater for plastics, which accounts for some of the dimensional stability associated with plastics. The greater freedom above the glass transition temperature (Tg) results in higher values of thermal expansion (elastomers).

Because of the orderly close packing in semi-crystalline thermoplastics there is a greater expansion at the melt transition (Tm) compared to amorphous thermoplastics (and a greater shrinkage cooling down from the melt state). The addition of fillers with low thermal expansion reduces the overall expansion values.

Softening of amorphous thermoplastics beyond acceptable functional levels is generally associated with the glass transition temperature (Tg).

Semi-crystalline thermoplastics can retain functional rigidity up to the melt transition temperature (Tm) but may be deformed under high loads at lower temperatures. To convey design information on heat softening, the Heat Distortion Temperature (Heat Deflection Under Load) is generally quoted for two different stress (load) levels to recognise the range of possible service conditions.

As temperatures increase the rate of thermal decomposition of polymers also increases, with subsequent deterioration in mechanical properties. A short time at a higher temperature has the same effect as a long time at lower temperature. The continuous use temperature combines heat softening and decomposition.

Unless otherwise specified the Continuous Use Temperature (CUT) refers to temperatures at which a 50% reduction in tensile strength occurs in 20,000 hours

3. Optical Properties

Organic polymers usually do not contain chemical groups (chromophores) that absorb in the visible region of the spectrum. Hence, most polymers are colourless in their natural state. Colour in polymeric products is supplied by additives (dyes or pigments).

Semi-crystalline thermoplastics, consisting of crystalline regions of higher density (and hence higher refractive index) and amorphous regions of lower density, will result in scattering of incident light and loss of transmission (haze).

If the crystallites are:

• oriented in one direction ;
• or if they are of a similar density to the amorphous regions
• or if the crystallites have dimensions smaller than the wavelength of the light
• the amount of scattering will be greatly reduced.

Amorphous thermoplastics will have high transparency.

Oriented plastics, eg in biaxially oriented polypropylene film or stretch-blow-moulded polyethyleneterephthalate (PET) bottles, will show high transparency even though highly crystalline.

Additives of different refractive index from the polymer can also lead to light scattering giving translucent or opaque products.

4. Electrical Properties

Organic polymers normally do not contain any charge carrying species, such as free electrons as found in metals or ions in electrolytes, and are useful as electrical insulators.

At very high voltages (high electrical stress) polymers will break down catastrophically.

In very high frequency electrical fields (kHz to MHz) the dipoles in some polymers are unable to keep up with the alternating field and electrical energy is transformed into heat. Although this can be a problem in the telecommunications field the phenomenon can be used to heat weld certain plastics (eg plasticised PVC).

Rubbing two polymers together can create static electrical charges which are slow to leak away to earth and cause dust collection and other problems.

5. Environmental Properties

Environmental properties relate to the changes taking place in structure and properties of polymers when exposed to a range of external agencies:

5.1 Heat

All organic materials break down (decompose) when subjected to heat energy through thermal dissociation of chemical bonds. Usually the rate of decomposition increases with increasing temperature. A long time at a low temperature gives much the same result as a short time at a high temperature.

Thermal decomposition is more noticeable in polymers than in other organic chemicals because even less than 0.1% decomposition can result in a 50% reduction in molecular weight, with disastrous consequences for mechanical properties that depend on molecular size.

The harmful effects of thermal decomposition can be minimised using heat stabiliser additives.

5.2 Radiation

Most forms of radiative energy can lead to decomposition of polymers but the most serious forms are ultra-violet radiation (in sunlight) and high energy radiation (X-rays, beta-rays and gamma rays).

Visible light has insufficient energy to dissociate the typical bonds found in polymers but additives can act as sensitisers.

Decomposition of polymers by radiation is essentially a surface effect and has little effect on bulk properties such as stiffness but can dramatically alter properties such as impact strength. One of the main outcomes of uv decomposition is discolouration (usually yellowing).

Light stabilisers operate as screens for uv light or interfere with the associated chemical reactions.

5.3 Chemicals

The environmental effects of chemicals can either be a reversible physical process (solvent attack, moisture absorption, permeability) or an irreversible chemical reaction. As with most chemical and physical processes the effect is usually accelerated as the temperature and concentration of the chemical is increased.

Solvent Resistance
Polymers are most strongly attacked by organic solvents that are chemically similar (similar polarity, similar solubility parameter).

Because of the size of polymer molecules true solution is a slow process, accelerated by heating. Attacks take the form of swelling and mechanical weakening. In some cases additives can be selectively extracted.

Crosslinked polymers and semi-crystalline thermoplastics show the best resistance to organic solvents.

Water Resistance
Some polymers, eg polyamides (nylons) can absorb up to 10% water, depending on humidity and temperature, with corresponding increase in dimensions, reduction in modulus but improvement in impact resistance.

The presence of moisture at melt processing temperatures can lead to breakdown of the polymer (hydrolysis) with deterioration in mechanical properties. Hence certain polymers, eg polyamides and polycarbonates, must be thoroughly dried before processing.

Oxidation
Atmospheric oxygen can cause serious chemical damage, particularly in hydrocarbon polymers. The chain reaction can lead to discolouration and reduction in molecular weight (degradation). Oxidation can be accelerated in the presence of uv light. Incorporation of antioxidants can minimise the effects of oxidation.

Biological Organisms
Most synthetic polymers have a high resistance to biological organisms (bacteria, enzymes, fungus, mould) although some additives can be attacked. Naturally occurring polymers ( cellulose, natural rubber) are more prone to biodegradation.

The concept of modifying synthetic polymers to make them more biodegradable and hence be disposed of in a manner similar to vegetation and waste foodstuffs has not found much support because of the uncertainty of speed of breakdown and the release of unknown chemicals into the food chain.

Weathering
Weathering of polymers involves a combination of heat, uv radiation, oxygen and moisture, the net effect on the polymer properties depending on the relative contribution of each.

Combustion: The high temperature oxidation of polymers is a complex process and considered one of the major weaknesses of polymers as manufacturing materials. Fire performance is assessed by a number of factors:

All of the above factors can be influenced by the structure of the polymer and the presence of fire performance additives.

Properties used to characterise plastics materials

PROPERTY	DEFINITION	UNITS	TYPICAL VALUES
1. Physical
1.1 Density	Mass per unit volume at 23oC (water = 1000 kg m-3)	kg m-3, g cm-3	Solid polymers 800 – 1500 kg m-3 0.8 – 1.5 g cm-3Cellular polymers 20 – 750 kg m-3
2. Mechanical
2.1 Stiffness (Rigidity)	Modulus of Elasticity (stress/strain) [stress = force acting on unit cross sectional area; strain = relative change in dimension eg length] Modulus of Elasticity in tension Modulus of Elasticity in compression Flexural Modulus (in bend)	N mm-2, GN m-2, GPa stress: N mm-2, MN m-2, MPa strain: no units	0.4 – 8.0 GN m-2   0.1 – 10 GN m-2
2.2 Yield Stress	Stress at Yield Point	N mm-2, MN m-2, MPa	10 – 100 MN m-2
2.3 Tensile Strength	Stress at failure under tensile load	N mm-2, MN m-2, MPa	10 – 100 MN m-2
2.4 Compressive Strength	Stress at failure under compressive load (also stress at 0.1 % compression)	N mm-2, MN m-2, MPa	4 – 300 MN m-2
2.5 Flexural Strength	Stress at failure under bending load	MN m-2, MPa	10 – 130 MN m-2
2.6 Elongation at Break	Percentage increase in length at break under tensile load	%	0.4 – 600 %
2.7 Creep	Slow change in dimensions over a long period of time under load Creep strain	No units
2.8 Impact strength	Energy to cause failure under an impact force a) Charpy (notched and unnotched) b) Izod (notched and unnotched) c) Falling weight	mJ mm-2 (kJ m-2) J/m notch J	1 – 16 kJ m-2 10 – 800 J/m
2.9 Hardness	Indentation of a ball indentor under a static load Shore D – hard plastics Shore A – soft plastics and rubber Rockwell – various scales	arbitrary scale 100 = infinitely hard 0 = infinitely soft	R 20 – M130
2.10 Abrasion resistance	Loss of material under set abrasive conditions	mg/cycle
3. Thermal
3.1 Coefficient of linear expansion	Proportional increase in length for 1oC rise in temperature	K-1 (oC-1)	2 – 20 x 10-5 K-1
3.2 Thermal conductivity	Rate of heat transfer through unit thickness to maintain 1oC temperature difference	W m-1 K	0.6 – 5.2 x 10-3 W m-1 K
3.3 Melt temperature	Temperature at which the polymer melts (semi-crystalline thermoplastics only)	oC	30 – 380 oC
3.4 Softening point	Temperature at which the modulus (stiffness, rigidity, hardness) Drops below an arbitrary acceptable level a) Vicat Softening temperature b) Heat Deflection under load (0.45 MPa 03 1.8 MPa)	oC oC	50 – 300 oC 30 – 250 oC
3.5 Continuous Use Temperature	Maximum temperature to give life of x hours under service conditions	oC	50 -250 oC
3.6 Oxygen Index	Minimum fraction of oxygen in a nitrogen/oxygen mixture required to sustain burning	No units	0.15 – 0.80
3.7 Flammability	Behaviour towards standard fire performance tests e.g. Underwriters Laboratory UL 94
4. Electrical
4.1 Volume Resistivity	Resistance between opposite faces of a unit cube	ohm cm (or log ohm cm)	1010 – 1018 ohm cm ( 10 – 18)
4.2 Surface Resistivity	Resistance between electrodes on surface	ohm
4.3 Relative Permittivity	Ability to store electricity in a capacitor	no units (measured at different frequencies)	2.0 – 7.0 at 108 Hz
4.4 Dissipation Factor	Indication of heat production in a high frequency field	no units (measured at different frequencies)	0.0002 – 0.35
4.5 Dielectric Strength (Electric Strength)	Indication of ability to stand up to high voltages		10 – 30 kV mm-1
5. Chemical Resistance
5.1 Water Absorption	Percentage increase in weight (or volume) after exposure to water e.g. 1 day total immersion at 70 oC 7 days at 50 % relative humidity	%	0 – 3.3 % (24 hours at 23 oC)
5.2 Chemical Resistance	Measured as volume (or weight) increase after exposure to chemical Or effect on mechanical properties
6. Processability
6.1 Drying	Time and temperature to remove moisture	hours at x oC	1 – 16 hours at 60 – 120 oC
6.2 Processing Temperature	Temperature for extrusion (die temp) or injection moulding (nozzle temp)	oC	180 – 350 oC
6.3 Mould Temperature	Recommended mould temperature	oC	10 – 120 oC
6.4 Melt Index (Melt Flow Rate)	Mass of polymer extruded in 10 minutes from a standard piston extruder	g / 10 min	0.1 – 50 g / 10 min
6.5 Melt Viscosity	Ratio of shear stress to shear strain (varies with shear strain rate)	Pa s
6.6 Mould Shrinkage	Difference between moulding dimension and corresponding mould dimension, relative to mould dimension	No units but expressed as %	0.4 – 3.0 %

This article was originally written by Dr Charlie Geddes