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Elecsound Fuse and Thermostat

Monday, 5. December 2011 3:19

Elecsound can offer types of Fuse and Thermostat, such as Fuse 3610, Fuse 5020, etc.
Thermal fuses are safety devices installed in electrical appliances which are designed to interrupt the circuit if the temperature rises to a dangerous level. Heat-producing devices such as stoves, microwaves, and hair dryers often have thermal fuses, and they can also be installed in a wide variety of other electrical circuits, such as light switches. These devices are usually intended as failsafes which will kick into place when other electrical safety measures such as circuit breakers fail to operate.
Below are some examples for your reference.
Fuse 3610
Fuse 5020
Fuse Holder
KSD301 Big Aluminium Cap
Auto Fuse

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Polymer types

Monday, 19. September 2011 1:59

Polymer science is a broad field that includes many types of materials which incorporate long chain structures with many repeated units. One useful way of categorising polymers for the requirements of electronic assembly is by functional behaviour. In the strictest sense these categories are not fixed, or even particularly precise, and you should be aware that some materials can fit into more than one category:

  • Elastomers are flexible or ‘rubbery’ materials which can readily be deformed, and return rapidly to almost their original shape and size once released from stress, thus making them able to form reliable seals. Natural and synthetic rubbers are common examples of elastomers
  • Plastics are materials which can be shaped or moulded under appropriate conditions of temperature and pressure, and then hold their shape. In contrast to elastomers, plastics have a greater stiffness and lack reversible elasticity
  • Some plastics, such as nylon and cellulose acetate, can be formed into fibres. These have different mechanical characteristics and are often regarded as a separate class of polymer.

The stress-strain relationships for these groupings are substantially different, as may be seen in Figure 1.


ASTM D-156611 defines an elastomer as a ‘macromolecular material that returns rapidly to approximately the initial dimensions and shape after substantial deformation by a weak stress and release of the stress.’ Such elongations typically exceed 100%.

The earliest-used elastomer was natural rubber, obtained from the sap of the rubber tree, which contains around 95% of a polymer whose repeating unit is isoprene. As polymer chemists evolved more and more polymers resembling natural rubber in properties, the term elastomer has grown to represent these materials, rubber being reserved for its original use.

Elastomers have three main functions in electronic assemblies:

  • to form an environmental seal
  • to provide mechanical strain relief
  • to give a means of conducting heat away from sources within the assembly.

Although used for many centuries in its raw form, a significant step forward was made when Charles Goodyear succeeded in ‘vulcanising’ natural rubber by heating it with sulphur to induce what is now understood to be cross-linking. The significance of the great performance improvement resulting from this treatment has led to the term ‘vulcanisation’ often being loosely used to describe the cross-linking of any elastomer.

Elastomers consist of long chain-like molecules, linked together to form a three dimensional network. Typically, an average of about 1 in 100 molecules are cross-linked: when this number rises to about 1 in 30, the material becomes more rigid and brittle. Most elastomers are thermoset materials, and cannot be remoulded, an exception being the class of materials known as ‘thermoplastic elastomers’.

The most common elastomers used in electronics are silicones (Section 0), which are supplied either as thick pastes which can be dispensed, or as fully cured preforms. Silicones may be chosen because of their chemical inertness or, more commonly, because of their thermal performance. Silicone elastomers may also ‘double’ as adhesives.

Moulded cured elastomers can also be supplied with conductive filler materials, such as silver, copper, nickel and graphite. These gasket materials are intended to combine an environmental seal with shielding against electromagnetic interference (EMI).


‘Plastic’ is a term which can cover a wide range of polymer materials, all of which can be moulded, for example to produce the body of a QFP component, the casing for a computer keyboard, the hand set of a mobile telephone or the encapsulant cover for a PLCC.

There are two main groups of plastic polymers, thermoplastics and thermosets:


Thermoplastics are supplied fully polymerised and remain permanently fusible, melting when exposed to sufficient heat, and potentially they can be recycled and reused.

Although some thermoplastics can have a crystalline microstructure, the essential feature of their structure is that there are relatively weak forces of attraction between the chains. These are overcome when an external force is applied (resulting in the plastic deforming) or when the material is heated, so that it becomes first soft and flexible and eventually a viscous melt. For each thermoplastic there is a specific temperature at which the material will start to distort, which is known as the ‘heat distortion point’. However, when the material is allowed to cool it solidifies again. This cycle of softening by heat and solidifying by cooling can be repeated more or less indefinitely and is the basis of most processing methods for these materials.

Thermoplastics are usually supplied in the form of granular feedstock, which is heated, melted and moulded, and removed from the mould only when it has cooled below its ‘heat distortion temperature’.

Examples of thermoplastics are polyethylene, poly(vinyl chloride), polystyrene, nylon, cellulose acetate, acetal, polycarbonate, poly(methyl methacrylate), and polypropylene.


A thermoset material is produced by a chemical reaction which has two stages. The first results in the formation of long chain-like molecules similar to those present in thermoplastics, but still capable of further reaction. This second stage of inter-linking the long molecules takes place at the point of use and often under the application of heat and pressure.

Since the cross-linking of the molecules is by strong chemical bonds, thermoset materials are characteristically quite rigid and their mechanical properties are not heat sensitive. Once cured, thermosets cannot again be softened by applying heat: if excess heat is applied to these materials they will char and degrade – as with eggs, once hard-boiled, they cannot be softened! Examples of thermosets are phenol formaldehyde, melamine formaldehyde, urea formaldehyde, epoxies, and some polyesters.

Thermoset raw materials are supplied in an uncured or partially cured state and fully cured during fabrication. The various stages of cure of a catalysed thermoset resin are known as ‘A-stage’ (uncured), ‘B-stage’ (partially cured), and ‘C-stage’ (fully cured). Many moulding compounds and laminating fabrics can be processed whilst in the B-stage, and some must be kept refrigerated until ready to use.

Compared with thermoplastics, thermosets are much less soluble in organic solvents, and have harder surfaces. Inherently somewhat brittle, thermosets can be combined with reinforcements such as fibre-glass to form very strong composites.

Many thermoset mixes produce an ‘exothermic’ reaction, that is they give off heat during curing. The amount of heat generated will depend on the material and the amount of catalyst used. This effect needs to be taken into account when processing thermosets, particularly when producing large castings.


Adhesives can be classified by the method used for curing, and a number of different mechanisms have been developed to suit different applications:

  • Anaerobic adhesives are single-component materials which cure at room temperature when deprived of contact with oxygen. The curing component in the adhesive will not react with the adhesive as long as it is in contact with oxygen. The capillary action of this type of liquid adhesive carries it into even the smallest gaps to fill the joint.
  • In ultraviolet curing adhesives, the chemicals which would initiate curing are present, but are bound together and are inactive until exposed to UV light. The degree of cure depends on the UV intensity, and in some applications light may be physically blocked from reaching the polymer. In order to resolve this ‘shadowing effect’ problem, UV curing adhesives may have secondary curing activation systems, often using heat to ensure that the cure is complete.
  • Anionic reactive adhesives polymerise when in contact with slightly alkaline surfaces. In general, ambient humidity in the air and on the bonding surfaces is sufficient to initiate curing.
  • Activation adhesives are two part adhesives, where the solvent-based activator is applied to the surfaces to be bonded, often by spraying. Polymerisation begins as soon as the adhesive is applied to the activated surface.
Adhesive types

The lack of a detailed understanding of the adhesion process has not hindered progress in developing very strong adhesives for most materials. The only problem is that the wide range of chemical structures makes it impossible to produce an adhesive which is compatible with all polymers. It is always prudent to check recommendations on suitable adhesives and surface preparation with the material manufacturers.

There are two main classes of adhesive for polymeric materials:

  • Solvent adhesives, which may be either a pure solvent which attacks the surfaces to be joined so that they fuse together, or a solvent containing some of the adherend material. This approach is used for polymers such as polystyrene and polymethyl methacrylate, the choice of solvent depending on practical issues such as the rate of evaporation
  • Organic adhesives based on rubbers or polymeric materials, which may be thermoplastic or thermosetting in nature.

The most versatile range of organic adhesives is that based on epoxy resins, and these are particularly widespread in electronics, although they are relatively expensive. The major advantages of epoxy adhesives are that:

  • No by-products are released on cure, so that only light contact pressure is necessary
  • They can be formulated to work well over a very wide temperature range
  • Epoxies have excellent resistance to moisture and chemicals
  • Shrinkage on cure is negligible, so that residual strains in the joint are small
  • Creep of the cured material is low
  • Epoxies can be cured at room or lower temperatures, although those cured at elevated temperatures are stronger.

There are however, the disadvantages that:

  • Care is needed when handling uncured epoxy resins
  • The shelf life of some formulations is limited
  • High temperature strength can only be achieved by sacrificing ductility.

Epoxy adhesives are sold either as two-part adhesives, where the epoxy resin is mixed with a catalyst just before use, or single-part materials, where the catalyst is incorporated during manufacture. Single-part adhesives are generally less reactive, needing to be heat-cured, and often require refrigerated storage to increase storage life.

Note that refrigerated materials generally need to be brought to room temperature before use, and it is unwise to try and accelerate this process. Remember to read the manufacturer’s recommendations on storage life both before and after thawing!

Other adhesives you may encounter are:

  • Nitrile rubber adhesives: usually copolymers of butadiene and acrylonitrile, these are good adhesives in their own right, but also combine with phenolic resins to produce very good structural adhesives
  • Resorcinol adhesives are particularly good for bonding thermosetting plastics, but with a few exceptions (ABS, nylon, acrylic) are not suitable for thermoplastics
  • Cyanoacrylate adhesives can produce very strong bonds very quickly, even between dissimilar materials. The polymerisation reaction is triggered off by water or other weak base on the surfaces to be bonded. Cyanoacrylate adhesives are quite difficult to work with, having a low viscosity and exhibiting a slight ageing effect.

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‘Tailoring’ Polymers

Monday, 19. September 2011 1:54

The range of polymer materials available is enormous, as slight changes in the chemical make-up of the monomers or the conditions of polymerisation can result in dramatic changes in the material characteristics of the end of processed polymer.

Polyethylene is an example of a polymer which can be used in a wide variety of applications because it can be produced with different forms and structures. The first to be commercially exploited was called low density polyethylene (LDPE), which is characterised by a high degree of branching, which forces the molecules to be packed rather loosely. The resulting low density material is soft and pliable and has applications ranging from plastic bags and textiles to electrical insulation.

By contrast, high density (HDPE) or linear polyethylene demonstrates little or no branching, so that the molecules are tightly packed and the plastic can be used in applications where rigidity is important, such as plastic tubing and bottle caps. Other forms of this material include high and ultra-high molecular weight polyethylenes (HMW; UHMW), which are used in applications where extremely tough and resilient materials are needed.

New materials can also be tailored by combining monomers with desirable properties. In some cases, these combinations are just physically mixed polymers, but more typically new ‘co-polymers’ are produced. Some types have a random structure of the constituent monomers, others may have a regular, repeating structure of the different materials:

  • A ‘block’ copolymer is made with blocks of monomers of the same type
  • A ‘graft’ copolymer has a main chain polymer built with one type of monomer, and branches made up of other monomers.

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Polymers Flammability

Friday, 16. September 2011 5:46

Flammability has little direct impact on any electrical or mechanical parameters (although additives can cause slight modification to the resin properties), but is extremely important from the point of view of safety. Polymers are used in many electronic devices, from radios to aircraft and the materials used in making the electronic circuits within these devices must not cause or dramatically assist combustion.

A cautionary note on nomenclature

Most people use the term ‘inflammable’ to refer to things which catch fire readily. The term ‘flammable’ is preferred in the technical literature, as being more correct, but means exactly the same.

The converse of both is ‘non-flammable’.

The most usual flammability tests are those specified by the American Underwriters’ Laboratories (UL). For historical reasons, related to the use of PCBs in racked systems, these are carried out on test strips of standardised dimensions clamped vertically, with a defined flame applied for 30s and then removed. In 94V-0, the highest UL grade, the polymer has to self-extinguish within 10s, with no drips of flaming molten material. Materials with lower specifications (94V-1, 94V-2) are allowed longer to self-extinguish.

The rationale behind the choice of time-scales is that heating caused by fault over-current is the most common source of ignition, and fuse protection will operate within 30s. Although it is not realistic to expect freedom from burning while a source of ignition is present, the polymer should not remain alight afterwards. The tests are applied both to materials such as base laminates and solder resists separately and to their combination in the completed PCB.

The UL tests are practical tests on real products, and have been criticised on the grounds that they are biased towards testing PCBs rather than other components, results may be affected by factors such as surface roughness and sample thickness, and the test method is difficult to standardise and operator-dependent to some extent, being based on simple equipment. For this reason UL tests may not reflect the inherent flammability of the resins themselves.

A supplementary method, preferred by bulk polymer suppliers, is the so-called Critical Oxygen Index (COI), also known as the Limiting Oxygen Index (LOI). This is the lowest percentage of oxygen in a nitrogen stream passing a specimen under test in which the specimen will remain burning under specified conditions. The technique requires extensive laboratory facilities, but is more reproducible than the UL test. However, the user will probably argue that the COI result requires interpretation, whereas UL test relate directly to the practical application.

For most applications, fire retardant additives are included in the resins used to make board laminates whence, for example, the name ‘Fire Retardant version 4’, or FR-4. Note that this term applies to the base material of which the PCB is made, and does not describe the additives.

However, such additives (most commonly brominated compounds) frequently greatly increase the harmful by-products of combustion. Since the Falklands War and Kings Cross fire, end-users have become aware of the dangers from such smoke and fumes. There is consequently a fine balance to maintain between meeting a tight flammability specification and avoiding excessive use of possibly harmful materials.

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Mechanical Properties of Polymers

Friday, 16. September 2011 5:43


Unreinforced plastics have densities in the range 830-2200kg/m3. This compares with values of 2700kg/m3 for aluminium and 8000kg/m3 for stainless steel. Expanded polystyrene foam (used as a packaging and insulating material) can have a density as low as 10kg/m3, although structural foams usually have a density of at least 500kg/m3. The major advantage this low density gives plastics is that their strength/stiffness to weight ratios compare favourably with apparently much stronger materials. For example, on a stiffness to cost ratio there is little to chose between aluminium alloy, alloy steel, polypropylene and PVC. The choice of material can then be based on ease of manufacture and factors such as chemical or corrosion resistance and weatherability.

Strength and stiffness

An important property of materials is their response to the application of a force, which can be classified into two main types of behaviour:

  • Elastic materials return to their original shape once the force is removed
  • Plastic materials will not regain their shape and instead flow, behaving like a highly viscous liquid.

Most materials demonstrate a combination of elastic and plastic behaviour, showing plastic behaviour after an ‘elastic limit’ has been exceeded. This behaviour is strongly dependent on temperature.

Plastics are generally stronger in compression than in other modes, and the values for tensile and flexural properties in Table 2 appear to restrict the use of unreinforced plastics in load bearing applications. However, their favourable strength to weight ratios and manufacturing benefits compared with metals make plastics preferable in many applications, giving the designer the challenge of compensating for low strength and stiffness by good engineering design. Usually this involves devising shapes which will give a sufficiently rigid construction, whilst using thin sections to facilitate easy moulding and fast production.

Source: Crawford 1985 

Table 2: Comparative mechanical properties of selected structural materials
  unreinforced plastics aluminium stainless steel reinforced plastics
tensile strength 10-100 MPa 170 MPa 740 MPa 100-1700 MPa
modulus 0.2-3.5 GPa 70 GPa 210 GPa 3-150 GPa

Of course the strength and stiffness of plastics can also be increased significantly by reinforcement (with glass or carbon fibres), and it is this approach which is adopted for the printed wiring board laminate.

Note that the yield strength of any plastic varies considerably with temperature: a strength range of greater than 2:1 over the temperature range -20°C to +70°C is typical. As the temperature is increased, the material becomes more flexible, and so for a given stress the material deforms more.

It should be kept in mind that short-term strength and stiffness values give a guide to the nature of the plastic, but should not be used in design calculations because plastics are sensitive to temperature and tend to creep and recover. As a general rule the long term strength of plastics decreases as the service temperature increases.


Plastics exhibit a wide range of behaviour when subjected to impact forces. Whilst acrylic and polystyrene are brittle and fracture like glass, some plastics are very tough: polycarbonate is virtually unbreakable and is widely used in vandal-proof outdoor light fittings and safety helmets; nylon is used in industrial gears; ABS is used for crash padding in cars.

Unfortunately, comparing impact behaviour is difficult because different test methods use sample geometries with varying degrees of stress concentration. As the ranking of materials depends on the test method, designers have to be very careful when choosing a plastic on the basis of its tabulated impact strength, to ensure that the results quoted were obtained in a test which simulates closely the service conditions of the product.

In general, plastics will become brittle if they are subjected to sub-zero temperatures and/or contain severe stress concentrations, such as sharp corners and notches, and these conditions should be avoided whenever possible. Other factors known to lead to embrittlement include the use of excessively high melt pressures during injection moulding, which causes high residual stresses. Other areas of weakness in moulded plastics include weld lines and regions of highly oriented material, which will be strong in the orientation direction but relatively weak in the transverse direction. A common method of improving the impact strength of brittle plastics, such as polystyrene, is to add rubber fillers, but this is usually at the expense of strength and stiffness.


Two types of fatigue failure are observed:

  • ‘Creep failure’ or ‘static fatigue’ which occurs when a thermoplastic is subjected to a constant load for a prolonged period. The higher the stress, the shorter will be the time to failure.
  • More conventional fatigue failure, when parts are subjected to the repeated action of a relatively low stress which would not cause failure in a single application. Studies have shown that fatigue cracks can be initiated and propagated in plastics, just as in metals, although cracks generally develop within the wall thickness of the moulding rather than start from the surface.

Depending on the application, failure may be the result of less than a complete fracture. For example, when stressed, transparent plastics develop crack-like features (‘crazes’) and other plastics may whiten, a change in appearance which may end their useful life.

Operating temperature

The glass transition temperature (Tg) is the temperature at which the links in the polymer start to weaken and become more flexible. At temperatures above Tg, sections of the polymer backbone are relatively free to move, whereas below Tg their motion becomes frozen, with only small scale molecular motion remaining. Over a temperature range of only 5–10°C, the material thus becomes more rubber-like rather than rigid (or ‘glass-like’). This phase transition is reversible and will leave no permanent effect on the polymer unless it is broken or allowed to distort whilst in the abnormal state.

A knowledge of Tg is essential in selecting polymers, which are compounded to produce the required Tg for the application. In general, elastomers have values of Tg well below room temperature, and structural polymers have values above room temperature. One example is of poly(vinyl chloride) (PVC), which is normally a brittle solid at room temperature, with a Tg of 83°C. It is therefore suitable for use for domestic cold water pipes, but not for hot water. However, by adding a small amount of plasticizer, the Tg can be lowered to –40°C, turning the PVC into a material which is soft and flexible at room temperature, and can be used for applications such as garden hoses. But even a plasticized PVC hose can become stiff and brittle in a severe winter, in the same way that natural rubber becomes brittle when cooled in liquid air, which is below its glass transition temperature of –70°C!

In general, the maximum continuous operating temperature is determined for thermoplastics by the heat distortion point, or for thermosets by the glass transition temperature (Tg), as above these temperatures the polymer is not sufficiently mechanically stable. The glass transition temperature is particularly significant in the case of:

  • Thermoset materials used to make printed wiring boards, where the mechanical strength is lower above Tg, which can lead to distortion and warping during unsupported reflow soldering
  • Encapsulation resins, where exceeding Tg can cause excessive stresses on embedded components. This is caused by differences in the rate of expansion, as resins above Tg typically show large increases in TCE.
Elastomer properties

Elastomers are affected by the environment more than other polymers. Thermal ageing usually increases cross-linking and the chain length of the elastomer, thus increasing stiffness and hardness and decreasing elongation. Radiation has a similar effect.

Elastomers are sensitive to oxidation and in particular to the effects of ozone. Ultraviolet radiation acts in a similar way to ionising radiation, so some elastomers do not weather well. Environmental effects are especially noted on highly stressed parts. Some elastomers are particularly affected by hydrolysis, and complete chemical reversion has been experienced, where the polymer depolymerises back to a liquid state.

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Polymer materials

Thursday, 15. September 2011 1:22


Epoxies first became commercially available in Germany in the 1930s, and by the 1950s began to be used as coatings and casting resins despite their high price. Today, PCBs using epoxy monopolise the marketplace, and epoxies are widely found in structural adhesives, structural composites and fibre-glass.

Epoxies may be cured at room temperature using amine catalysts, or cured at up to 180°C using amines or anhydrides to give superior thermal and physical properties. Most laminates are cured at around 150°C under pressure.

The most important epoxies are the epichlorohydrin-bisphenol-A types. Bisphenol-A is prepared by reacting phenol and acetone; epichlorohydrin is produced directly from the petroleum fraction propylene. In the presence of a catalyst, the two react to form a long chain molecule having carbon atoms and benzene rings interspersed along the backbone (Figure 1). In each molecular unit there are two pairs of carbon atoms, each pair of which shares an oxygen atom to form the epoxide group. It is this pair which opens in the presence of a curing agent to form the cross-linked, cured epoxy resin.


Acrylics are transparent, light-stable resins that have been manufactured since the early 1930s. The clarity and weather-resistance of cast acrylic explains its major commercial application for glazing, and acrylic fibres are used both for fabric and fibre optic applications.

Poly(methyl methacrylate) (PMMA) is a linear carbon-chain compound with a relatively low softening point, which has a higher strength than most thermoplastics, although it is brittle. It has low moisture absorption and is resistant to alkalis, dilute acids, detergents and greases, but will dissolve in alcohols, ketones and chlorinated solvents.


Other polymers have a skeleton of carbon atoms: silicones are unique in that the skeleton is of silicon. Silicones range widely in molecular weight, and have a corresponding range of properties between light oils, greases and elastomers. Silicone resins cure to form solid but flexible materials which are impervious to moisture and have relatively high thermal conductivity.

Care has to be exercised in the selection and use of silicone elastomers:

  • Certain types produce acetic acid during cure, which can have an adverse effect on some types of metallisation
  • Silicone fluids (monomers and low molecular weight polymers) are very effective release agents (which can make bonding problematic) and inhibit solder wetting
  • Acrylics and urethanes will dewet on silicone surfaces, so that silicone elastomers cannot be used in assemblies intended for subsequent conformal coating.

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Initiating cure

Thursday, 15. September 2011 1:16

The curing reaction, which is the final stage of polymerisation for cross-linked thermoset polymers, is usually induced by adding a catalyst or applying heat/pressure. Most reactions are however ‘exothermic’, that is they generate heat themselves. Whilst this may make process control difficult when large quantities of resin are being handled, in typical electronics applications the resin quantities are smaller, so an external source of energy is needed.

Oven heating is relatively inefficient, as both oven and air are heated, rather than just the workload, and the process relies on heat being conducted from the surface to the bulk of the material. Alternative curing methods used include infrared, microwave, and high-frequency heating, which all rely on the material absorbing electromagnetic energy, yet allowing sufficient penetration for the internal parts to be cured.

  • Infra-red processing is used mostly to drive solvents from coatings, rather than cure coatings of 100% solids content.
  • Microwave processing has been used to cure inks, coatings and moulded parts, and fabricate foamed plastics as well as resin-impregnated fabric.
    The electronics industry also uses ultraviolet (UV) and electron beam processing in the manufacture of PCBs, conformal coatings, adhesives, coil lead encapsulants, solder masks, and resists, and in ink processing.
  • Ultra-violet light involves exposing the material to radiation with wavelength from 200-400nm, generated by either medium-pressure mercury lamps or microwave-excited lamps. The cure efficiency is affected by the extent to which the resin transmits UV light, and thickness is usually limited to 0.25mm.
  • Electron beam processing uses electrons which are emitted from a heated filament in an evacuated chamber, are accelerated and deflected by electrostatic and magnetic fields, and directed toward their target through a thin foil window.


The motive behind its development and use has been the need to develop more environmentally compatible processes. Radiation processing requires less process time and less energy, is environmentally safe, and does not require elevated temperatures, so that it can be used with heat-sensitive parts or components. However, a limitation is that equipment capital costs are high.


Although many chemicals accelerate the polymerisation process, there are exceptions. For example, oxygen can inhibit both UV and electron beam processes.


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Polymer structure

Wednesday, 14. September 2011 2:27

Side-branched polyethylene

However, the number of side branches may be varied by changing the temperature and pressure under which polymerisation is carried out. Even small variations in the number of side branches can cause appreciable changes in mechanical properties such as elastic modulus, creep resistance, and toughness.

Polymers with a high degree of branching are called dendrimers. Often in these molecules, branches themselves have branches, which tends to give the molecule an overall spherical shape in three dimensions.

Both linear and side-branched polyethylene behave similarly when heated: they melt and flow like a liquid, and when cooled will crystallise. This liquid-solid transition is ‘reversible’, and may be repeated time and time again. Such polymers are referred to as ‘thermoplastic’ (or thermal-flow) polymers.


As well as the bonds which hold monomers together in a polymer chain, many polymers form bonds between neighbouring chains. These bonds can be formed directly between the neighbouring chains, or two chains may bond to a third common molecule. For polyethylene, cross-linking can be induced by treating the polymer with ionising radiation, but in other materials cross-linked networks are more frequently produced by chemical reactions triggered by heating in the presence of a catalyst.

Though not as strong or rigid as the bonds within the chain, these cross-links have an important effect on the polymer. Extensive cross-linking effectively turns the entire specimen into one giant molecular network (Figure 5), in which the cross-links inhibit movement: the network (and consequently the shape of the specimen) are ‘set’. This is the origin of the term ‘thermoset’, which is explained in more detail in Polymer types.

Molecular structure of cross linked polyethylene

Addition and condensation polymers

A very large number of monomers may be polymerised. The resulting polymers fall broadly into two categories, depending on the chemical mechanism by which polymerisation takes place. These are illustrated in Figure 6 & Figure 7: ‘addition polymers’ grow by adding ‘unsaturated’ monomers to the growing chain; in ‘condensation polymers’, the bonding is accompanied by the elimination of small molecules such as water.

Some common addition polymers

Some common condensation polymers

There is no relationship between the polymerisation mechanism and whether the resulting polymer is thermoplastic or thermosetting. The form of polymer produced depends instead on the capacity of the monomer to make external bonds, regardless of how these are made.

Mechanism for polymerisation Form of polymer Classification
Addition Linear Thermoplastic
Condensation Linear, with side-branches
Some cross-linking Elastomer
Fully cross-linked Thermoset

The number of bonds which each monomer molecule can make is referred to as the ‘monomer functionality’:

  • With a functionality of 2, a linear polymer is the only outcome – think of people holding hands! Branching can only be produced by extreme conditions, such as high pressure/temperature or chemical treatment, which induce side reactions
  • A functionality of 3 will allow branching
  • A functionality of 4 gives the ability to form cross links. The isoprene molecule which is the basis of natural rubber is an example where this functionality is provided by two double bonds.

Whatever the mechanism (or mechanisms) of bonding between monomer molecules, the higher the monomer functionality, the more cross-linked will be the network produced. For a given monomer functionality, the characteristics will also be affected by the size of the monomer molecule: the smaller the molecule, the shorter the distance between bonds, and the tighter the resulting network.

Naming polymers

In general, a polymer with an unspecified number of monomers is named by adding the prefix ‘poly’ to the name of the ‘constitutional repeating unit’ (CRU), so the polymer formed by combining styrene monomers is called polystyrene. Where the monomer name is made of two or more words, these are separated from the prefix ‘poly’ with parentheses, as in poly(vinyl acetate), which is the common ‘PVA’ adhesive used in building applications. There is a complete set of nomenclature rules which apply to monomers containing more than one sub-unit, but these become too complicated for this module!


As well as the shape of the molecules, the way they are arranged in a solid is an important factor in determining the properties of a thermoplastic polymer. One distinction made is between ‘crystalline’ and ‘amorphous’ materials:

  • Crystalline materials, such as table salt, ice and most metals, have a highly ordered and regular structure, with their molecules arranged in repeating patterns. Applying heat turns them from solid to liquid over a narrow range of temperature
  • Amorphous materials, such as glass, have molecules arranged randomly and in long chains which twist and curve around one-another, so that they have much less long-range internal structure. As the temperature is increased, these gradually turn from brittle solid-like materials to viscous liquids.

These differences can be seen when the materials are viewed between crossed polarizers, crystalline melting leading to striking changes in optical properties.

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The Polymer Molecule

Wednesday, 14. September 2011 2:22

The example polyethylene is the simplest hydrocarbon polymer, formed when ethylene gas is heated at high pressure in the presence of a suitable polymerisation initiator. The number of ethylene molecules which form the polymer molecule (‘n’ on the diagram) is usually of the order of 10,000, but may be as high as a million or as low as a thousand.The

polyethylene molecule

Different materials require different methods of preparation of the monomer, and slightly different techniques to induce polymerisation, but the starting point is the monomer, or small groupings of monomers, usually in a liquid state. The factor which initiates the polymerisation process can be different for different materials. Examples are heat, pressure, light, or a chemical additive. The chemical additive that starts this chain reaction is called an ‘initiator’ or ‘promoter’. Since an initiator greatly speeds up the reaction rate, it is often referred to as a ‘catalyst’, though this is not strictly correct, because it is usually consumed in the reaction.

Different polymerisation conditions can be used to tailor the length of the molecule, which is proportional to the number ‘n’. It indicates how most physical properties of a polymer vary according to the molecular weight.

Dependence of physical properties on molecular weight

It is important to realise that when we talk about the number of monomers per polymer molecule, that this is an average, about which there is substantial variation. And, illustrates with its analogy of piles of stones of the same average weight, we need to know whether some molecules are significantly different from the norm.

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Polymer basics

Tuesday, 13. September 2011 2:36

A polymer (from the Greek ‘poly’, meaning many, and ‘meros’, meaning part) is a long molecule consisting of many small units (‘monomers’) joined together. A typical polymer may include tens of thousands of monomers and, because of their large size, polymers are classified as ‘macromolecules’. Typically formed by interlinked carbon atoms, polymers are commonly found in nature, in many forms of animal and plant life. One of the most common natural polymers is cellulose, a building material for all vegetation; a more specialist substance, but one which is more readily recognised as a polymer, is natural rubber.

For centuries people have used polymers in the form of oils, tars, resins, and gums. However, it was not until the industrial revolution that the modern polymer industry began to develop. In the 1830s, Charles Goodyear succeeded in producing a useful variant of natural rubber through a process known as ‘vulcanisation’, and some 40 years later, celluloid (a hard plastic formed from nitro-cellulose) was successfully commercialised.

The earliest artificial polymers were made by modifying natural polymers: the first totally synthetic polymer was Bakelite, produced in 1907 by combining phenol and formaldehyde. The two substances which the Belgian-born American chemist Baekeland united produced a third with properties far different from either of the originals. The giant Bakelite molecule is extremely complex; it consists of a network of benzene rings, each joined to another at one, two or three of its six corners by a chemical group of two atoms of hydrogen and one of carbon. The dark brown colours of much early electrical hardware and the 1930’s radio were both Bakelite. As what is now called ‘phenolic’, this type of resin is still in use, for example, in making low-cost PCBs.

Progress in polymer science was slow until the 1930s, when materials such as vinyl, neoprene, polystyrene, and nylon were developed. The wide acceptance of these revolutionary materials began an explosion in polymer research that is still going on today. Modern polymers tend not to be natural compounds, but are manufactured mainly from oil-based raw materials in large scale chemical processing factories which are usually sited close to oil refineries in order to minimise the transport requirements, as large volumes of polymer materials are required for products such as paints, adhesives, elastomers, and fabrics.

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