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Microstructures and Properties of Materials Module Code: B11MS1

Responsible Person: Dr V. Arrighi Dr V. Arrighi, Dr A. Kraft.

Week 9 Index Theme of Week 9 : Polymer Processing. These are the topics that will be covered in Week 9. Click on each to navigate to page or choose the printable pdf version of this week's lecture material.

• Introduction • Polymer Processing 1 • Polymer Processing 2 • Polymer Orientation • Blends and Composites • Week 9 Tutorial and Revision List

Introduction into Polymer Processing

Polymers are either thermoplastics or thermosets.

Remind yourself of the basic difference and know examples of each. Thermoplastics are fabricated by moulding or shaping processes above their softening temperatures. In contrast, thermosets are formed by moulding monomers or oligomers and curing in situ.

This topic contains 3 sections -

(a) Processing methods 1 & 2 (study notes + animations)

(b) Polymer orientation (study notes)

(c) Blends and composites (study notes)

After working through the self-study material, you should be able to -

• describe, using sketchs, the following thermoplastic processes: extrusion and injection moulding, film blowing, blow moulding and compression moulding.

• describe the following thermoset processes: thermoforming, compression moulding, resin transfer moulding, RIM, prepregs.

• differentiate between cyclic and continuous processes, and give examples of products from each.

• appreciate what advantages occur through the use of reinforcing fibres and quantify the

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effect of fibre orientation on modulus.

• describe why it is desirable to induce chain orientation in processed articles and describe how to achieve and control this in practice.

• describe orientation in semicrystalline polymers and how the level of crystallinity may be controlled.

• appreciate the intrinsic orientation in liquid crystalline polymers.

• describe the mechanical properties of block or segmented copolymers and polymer blends in terms of domain structure.

Polymer Processing 1 Plastic material falls into two classes, thermoplastics and thermosets. Thermoplastics soften on heating, at Tm for semicrystalline polymers, at Tg for amorphous polymers. In contrast, thermosets are crosslinked and do not soften.

Major processes for thermoplastics These fall into the following categories : • Extrusion • Postdie processing such as film blowing, fibre melt spinning, sheet forming • Injection molding • Forming such as blow moulding, thermoforming, compression moulding. Thermoplastics are fabricated above their softening points by moulding or extrusion. Thermosets are fabricated in a mould prefilled with monomer or oligomer which is then crosslinked, usually by heating. In Polymer Processing 1, we concentrate on thermoplastics.

Extrusion The extruder is the main device used to melt and pump thermoplastics through a shaping device called a die. There are basically two types of extruders: single-screw and twin-screw. A single screw extruder consists of just one screw that rotates within a metallic barrel. The screw is kept well above the melting temperature (and, in case of an amorphous polymer, well above the glass transition temperature) of the polymer.

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Profiles are all extruded articles having a cross-sectional shape that differs from that of a circle, an annulus, or a very wide and thin rectangle (such as a flat film or sheet). To produce profiles for windows, doors etc. we need appropriately shaped profile dies. The cross-section of a profile die may be very complicated, as shown underneath.

The extrusion die shapes the polymer melt into its final profile. The die is located at the end of the extruder and can be used to extrude a variety of shapes : • Flat films and sheets • Pipes and tubular films for bags • Filaments and strands • Hollow profiles for window frames • Open profiles. Fibre spinning is used to manufacture synthetic fibres. The molten polymer is first extruded through a filter or "screen pack", which eliminates small contaminants. It is then extruded through a "spinneret", a die composed of multiple orifices which can have 1-10,000 holes. The fibres are then drawn to their final diameter, solidified (typically in a water bath or by forced convection) and wound-up.

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Annular (tubular) dies are used to make pipes or tubings. The polymer melt is separated into an annulus, enters the die land, and exits in a tubular shape. The complex design of such a die is shown underneath.

For coating wires (e.g. electrical wires), the design problems concern the need to provide uniform coating at the highest extrusion rate possible. Important process variables include pressure and feed rate. Important phenomena needing to be addressed are melt instability and die swelling.

Now look at the following videoclip to see an extruder in operation.

(998 kB)

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This video can be viewed with Windows Media Player. If you are unable to view the video with existing software, try to download Microsoft Windows Media Player. One of the most widely used methods for film forming is film blowing . Since the film is drawn upward, and expanded radially by air pressure, a biaxially oriented film is produced. Most plastic bags and wrapping films are made this way. The die is similar to that used for making pipes or tubings. The liquid polymer is forced upward through the spin annular slit, emerging into the atmosphere as a thin-walled continuous tube. The tube is then very rapidly hauled upwards by the pull rolls, expanded by internal pressure, and at the same time acted upon by cooling air jets which cause it to solidify at some tens of centimeters above the shaper lips. The bubble is collapsed and the film is rolled onto a wind-up roll. The stability of the bubble is crucial, resulting in a biaxially oriented film. The polymer of choice for making films is branched polyethylene (tension stiffening).

Now look at the second videoclip to see a film blower in action.

(2130 kB)

Injection moulding Injection moulding generally combines a reciprocating screw injection moulding machine with a mould. Injection moulding uses a machine somewhat like an extruder, but with a

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different screw action. In the first step, the hot screw rotates for a while and forms a metered flow of melt. In a second step, the screw ceases to rotate and acts as a ram, thus thrusting forward and injecting the polymer melt into the mould.

Examples of injection moulds are shown underneath.

The concept is simple, but the high pressures (up to 1000 atm) needed to fill a mould with molten polymer require heavy-duty engineering and hydraulics. Now have a look at the videoclip to see this operation.

(1933 kB)

Blow moulding Blow moulding shapes objects from an injected or extruded parison by inflating it inside a mould. It is a widely used technique for producing hollow containers (bottles). The precursor is known as parison and us produced by extrusion or injection molding. The polymer inflates in its softened state until it touches the walls of the cooled mould. The parison takes thus up the shape of the mould and cools. There are three variants of blow moulding : • Extrusion blow moulding (see also the following animation) • Injection blow moulding (see also the following animation) • Stretch-blow moulding (see also the following animation) Injection blow moulding : The parison is injection moulded onto a steel rod (station 1). The rod and the parison are then rotated to the second station (blow mould) to blow the parison to take up the mould shape. The road and the bottle are next rotated to the third station at which the mould opens and the bottle is ejected.

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Stretch-blow moulding : The highly sophisticated equipment involved causes the blown bottle to have a biaxial orientation in the wall. It makes use of a relatively low parison temperature, a stretch rod and high blow pressure. Rapid cooling locks the orientation. Its advantages are : • higher transparency (less light scattering), • improved permeation resistance, • higher rigidity and impact resistance. Polymers for stretch-blow moulding are: PET (polyethylene terephthalate), PVC (polyvinyl chloride), PP (polypropylene), PS (polystyrene), ABS (acrylonitrile-butadiene-styrene copolymer), PC (polycarbonate). Extrusion-blow moulding : A parison is first extruded into the open mould (A). The mould closes and the parison is inflated (B). When the parison has taken up the mould shape (C), the bottle is cooled and finally the mould opens (D).

Now have look at the videoclip.

(2130 kB) The last two techniques for thermoplastic objects involve softening of the polymer prior to pressing.

Thermoforming Thermoforming has advantages over injection moulding in the following cases : • Large formings, • Thin-wall packaging and

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• Short-run or prototype products. Disadvantages : • Polymer has to be preheated before warming, • Range of available shapes is limited, • Good detail is difficult due to low pressure difference, • Poor material distribution. In thermoforming a heated sheet of plastic is stretched over the female half of the mould. Shaping can be assisted by applying a vacuum under the sheet – vacuum thermoforming.

Compression moulding Compression moulding requires a mould and a matched pair of male and female dies. Its advantages are that (i) the polymer flows over a shorter distance which leads to reduced frozen-in stress and (ii) the polymer is not forced through a gate which can result in a reduction in mechanical properties. The following steps are involved in a compression moulding procedure : 1. A measured quantity of polymer is placed between the two halves of a mould. 2. The upper die is lowered and the polymer is compressed. 3. The polymer is cooled until it becomes solid. 4. The mould is opened and the part is removed.

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Polymer Processing 2

Polymers fall into two classes, thermoplastics and thermosets. Thermoplastics soften on heating and can be fabricated above their softening points. Thermosets are crosslinked, do not soften, and so have to be fabricated in a mould prefilled with monomer or oligomer (termed a prepolymer) which is then crosslinked, usually by heating.

Thermosetting plastics

The processing of thermosets differs from the processing of thermoplastics in several ways :

• The starting material is generally a prepolymer, e.g. an unsaturated polyester. • The mould temperature is typically about 100 °C higher than the barrel temperature. Therefore, the prepolymer cures inside the mould. • There are just two main processing methods : injection moulding and compression moulding.

Injection/compression moulding is much the same as for a thermoplastic. Since the prepolymer is generally liquid, this can be done at relatively low temperatures. Since the mould is hot, thermal curing (crosslinking) of the polymer takes place inside the mould. As a consequence, cycle times are longer than for thermoplastics to allow for the cure time.

Resin transfer moulding (view video clip underneath) is another process commonly used for thermosets.

(1255 kB)

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This video can be viewed with Windows Media Player. If you are unable to view the video with existing software, try to download Microsoft Windows Media Player.

Reaction injection moulding (RIM) is a process where (usually) two monomers are pumped first into a mixing chamber, and then to a mould where they react and polymerise and cure. Polyurethanes (from the reaction of a diol, usually a polyol, and a diisocyanate) are widely manufactured by this process, to produce quite large items such as car bumpers.

High capacity pumps are needed so that mixing and mould filling are rapid. The polymerisation reaction must be one which does not generate side products and must take place with minimal change in volume.

Reinforced RIM is where short glass fibres or mineral fillers are added to the polyol to produce reinforced RIM (RRIM) parts with enhanced stiffness and heat performance.

As well as polyurethane items, nylon and silicone articles (from ring-opening polymerisations) and two-component epoxy products can be made by RIM.

Foams

Injection moulding of foams is achieved in two steps

1. An inert gas is dispersed through the molten region directly before moulding by

• direct gas injection (usually N2) or • pre-blending of the resin with a chemical blowing agent.

2. Rapid injection of gas/resin mixture into the mould cavity causes the gas to expand "explosively". The material is thus forced in all parts of the mould.

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The resulting properties of the foam are :

• Very high rigidity/weight ratio, • Almost no orientation effect (uniform shrinkage), • Moulding of thick sections without sink marks.

Foamed articles are used for insulating containers and for packaging. The process above may be used with thermoplastics or thermosets.

Injection moulding of a foam (animation) (71kB)

Composites

Composites are manufactured by the following processes :

i) Pultrusion, a process used for making continuous filament-reinforced composite extruded profiles;

ii) Compression moulding, in particular for making glass-fibre reinforced thermoplastic mats (GMTs) and for sheet moulding compounding or SMC (SMC is a sheet of thermosetting plastics reinforced by glass fibres);

iii) Injection moulding of fibre-reinforced plastics (works for both thermoplastics and thermosetting plastics).

Pultrusion is a continuous process that produces full or hollow glass reinforced profiles of different shapes. Depending on the design, the glass ratio can vary from 30 to 70% by weight.

• Glass fibre is impregnated before the die, generally in an open bath system.

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• Fillers and additives are commonly incorporated into the resin to provide additional properties (such as fire retardants, cost reduction etc.) • The 3 next elements belong to the "pultrusion machine": The curing die shapes and polymerises the impregnated reinforcement. • The solid pultruded profile is clamped and pulled with a continuous belt or reciprocating system.

• The ultimate step is a cut-off system to cut the profile to the right length. Typical pultrusion speeds with thermoset resin systems are 0.5 to 2 meters per minute.

Pultruded profiles are used in many applications in electrical, building and consumer goods areas.

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Compression moulding of reinforced thermosets usually starts with a prepreg.

A prepreg is simply a combination of reinforcement fibres and a resin matrix. It is supplied to manufacturers in a form which is ready to use.

They are available in two different forms: unidirectional (tape, strips directional fibres) or as a woven fabric.

There are two different techniques for manufacturing prepregs.

Solution route : The resin matrix is applied to a roll of paper, the reinforcement is placed on top, and then the two are (part) cured in an oven. A further layer of paper is added on top to separate the layers of prepreg when they are rolled.

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Bath route : The fibres are passed through a bath of the matrix, which coats them, and then through the oven to (partly) cure the resin. Paper rolls are layered either side of the prepreg to protect and separate the layers of prepreg.

Prepregs are compression moulded in much the same way as thermoplastics but are allowed to cure fully in the mould. High strength items such as aircraft panels, wind turbine vanes, etc. are made this way.

The most common resin types used are cheap unsaturated polyesters, as well as the more expensive epoxy resins. Reinforcing fibres are commonly made of glass (that's the cheap option), carbon or a high-modulus polymer (such as Kevlar, which is much more expensive).

Polymer Orientation

The ability of a polymer to sustain a mechanical load depends on the strength of covalent bonds and the forces between the molecules. In an amorphous system much of the load is carried by van der Waals interactions and random coil entanglements between chains. However, if a substantial fraction of the polymer chains can be aligned in the load-bearing direction, a larger portion of the load can be transmitted to the main-chain covalent bonds.

Theoretical maximum modulus. We can estimate the maximum tensile modulus of a polymer if we make the assumption that the limit is the strength of an individual C–C bond. Consider a polyethylene chain. Its repeat unit is 0.15 × 10–9 m long and has a cross-section area of 18 × 10–20 m2. What force (F) is needed to give a 1% extension, i.e. an increase in bond length of 0.15 × 10–11 m?

The spectroscopic C–C vibrational force constant is 4.4 × 102 N m–1, so our extension requires 4.4 × 102 N m–1 × 0.15 × 10–11 m, i.e. F = 6.6 × 10–10 N.

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The modulus (E) of our C–C bond is, by the usual definition, force per cross section area per unit strain. For an extension of 1%, the strain equals 0.01, and so -

E = 6.6 × 10–10 N / 0.01 × 18 × 10–20 m2 = 3.7 × 1011 Pa

Experimental modulus. Real modulus values are significantly lower than this theoretical maximum, the following examples are typical of unoriented polymers.

Polymer E / 109 Pa

Polyethylene 1.1

Polypropylene 1.6

Polystyrene 3.3

Polyester 4.1

As a rule of thumb, the modulus for most polymeric materials is normally just above 1 Gigapascal (109 Pa) below the glass transition temperature. However, chain orientation can bring about a significant increase. For example, oriented high molar mass polyethylene has a modulus of 5.5 × 1010 Pa; this is more than a power of 10 improvement over unoriented PE.

Orienting polymer chains. Polymer chains are oriented by subjecting them to extensional strain (flow) in the melt, which occurs to some extent in most thermoplastic fabrication methods.

Significantly more orientation can be induced by mechanically drawing an extrudate (film, sheet or fibre) as it leaves the die and just before it solidifies — thus freezing-in extended chain conformations. This leads to uniaxial orientation in the draw direction. An example is shown schematically underneath.

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Biaxial orientation of polymer film can be achieved by drawing an extruded film in two directions at once, in the direction of extrusion as well as transverse to the direction of extrusion (see also blown film in polymer processing, section 1).

Orientation can also take place in a post-processing step (cold drawing). Most polymer fibres are treated in this way, alignment of the chains occurs during "necking" and plastic deformation of the drawn fibre.

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Orientation in liquid-crystalline polymers. Polymers whose chains comprise liquid-crystalline (LC) units spontaneously align in the melt (or in concentrated solution) during processing. On cooling (or solvent loss), an aligned solid polymer results. LC polymers are used for injection moulded articles and fibre production. LC fibres can be used as fibre reinforcement in other polymers. A well-known example is Kevlar, a polyamide in which the aromatic groups are all linked in the 1 and 4 (or para) position.

Orientation in semicrystalline polymers. In semicrystalline polymers such as polyethylene and polypropylene, nylons and polyesters, the effects of drawing can be recognised on a second length scale, that of the crystalline domains.

Drawing aligns the crystalline regions forming the fibrilar structure below -

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Summary. Orientation can occur at two length scales in polymers.

In amorphous systems only chain orientation occurs. In semicrystalline polymers both chains and crystalline regions can be aligned. In both amorphous and semicrystalline systems the result of orientation is the same, i.e. it leads to an increased strength in the direction of orientation. In uniaxially oriented material the modulus at right angles to the direction of draw is generally reduced and the material is weaker in that direction. This is relatively unimportant in fibres where the load is always applied along the fibre. However, it is a disadvantage in films, and biaxially drawn films are therefore often preferred in most applications.

Orientation is generally induced by (hot or cold) drawing the polymer as a deliberate processing step in the manufacture of films and fibres. Flow-induced orientation can sometimes occur in injection moulded items. This may not always be desirable as it can give different properties in different regions of the product.

Orientation in liquid-crystalline polymers is self-induced, but may also be enhanced by drawing.

Blends and Composites

The presence of a second phase provides an additional length scale (i.e. that of the filler or heterophase) at which material properties may be controlled. The term "filler" is usually used for particulate inorganic materials, the term "heterophase" is more general, and often refers to another polymeric component. The resulting composite/blend properties are generally a combination of those of the two components.

Semicrystalline polymers. These can be regarded as composites of amorphous (lower modulus) domains and crystalline (higher modulus) domains, and so here the "filler" and polymer are chemically identical. Control of the amount of crystallinity can be achieved

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by changing tacticity, branching and/or by incorporating a non-crystallisable comonomer. For example, in polyethylene (PE) the service modulus can be tuned by altering the crystallinity in the sample.

A polyethylene with low crystallinity (also called low-density polyethylene, LDPE) is used for flexible packaging whereas high density polyethylene (HDPE), which has a higher degree of crystallinity, is used for more rigid items such as plastic containers, washing-up basins etc. The synthesis of these two types of polyethylene is described later on (see Ziegler-Natta Polymerisation). The service temperature range for polyethylene is from about –30 ºC to 80 ºC, that of isotactic polypropylene (PP) extends from about –10 ºC to 120 ºC. Polyesters and nylons are semicrystalline polymers with even higher upper limits to their service ranges (up to ~200 ºC). After all, you have to be able to iron textiles made from these materials without your polymer melting under the hot iron.

Lower service limits are dictated by the glass transition of the amorphous component of a "composite", at which brittleness occurs, and the higher limit determined by the onset of crystal melting.

Block copolymers. Thermodynamically immiscible blocks (A and B) self-aggregate to form nanometer sized domains. The resulting microstructure depends on the relative volume fractions of blocks A and B. Examples are shown underneath and the 3-dimensional structure of the phases, though regular, can be rather complicated and for the non-specialist almost unpredictable.

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Let's take an example in which block A is polystyrene (which has a high Tg of around 95 °C) and block B is polybutadiene (with a low Tg of –70 °C). When polystyrene is the major component, it forms the matrix and carries most of the load, with dispersed spherical domains of polybutadiene acting as rubbery "shock absorbers". When polybutadiene is the major component, the dispersed glassy polystyrene domains act as crosslinkers. At intermediate compositions, other morphologies occur to give a range of intermediate modulus values.

Segmented copolymers. These are commonly polyurethanes (PUs), with chains composed of alternating "hard" and "soft" segments. Spandex, for example, contains poly(tetramethylene oxide) soft segments and aromatic urethane units (made from diisocyanates such as MDI and TDI) forming polar hard segments which associate through H-bonding and act as cross-links. In effect, PUs are phase-separated composites of polar and less polar segments. Segmented copolymers are often elastomeric materials with good creep resistance. Control of properties arises from the wide choice of diisocyanates and diols available to the PU industry. The modulus-temperature behaviour is like that of block copolymers above.

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Polymer blends. A two-phase morphology results also if you blend (= mix) two different polymers that are not thermodynamically miscible. In principle, the behaviour is similar to block copolymers. However, in contrast to block copolymers, there are no covalent bonds linking the phase domains; the mechanical properties of such blends are therefore more often than not inferior. Compatibilising additives (e.g. AB block copolymers where block A is compatible or identical to one component of the blend, block B to the other) usually have to be added to strengthen the interface between the domains.

Filled polymers. Almost all commercial plastics are filled! A low-cost inorganic filler, such as chalk, talc or silica is added, both to reduce cost and improve the modulus. The filler particles are normally of a size of the order of a few micrometers, although recently nano-sized inorganic fillers have become available for commercial use. A lot of fillers are surface-modified to provide good dispersion and to improve adhesion to the polymer matrix. Fillers generally have higher moduli than polymers, so the modulus of a filled polymer lies between that of the polymer matrix and that of the filler.

Fibre-reinforced composites. By incorporating fibres of higher modulus than the matrix improved strength is obtained. Fibres may be inorganic (glass fibre, boron) or organic (carbon fibre, high-modulus polymer). The modulus of such a composite depends on both the volume fraction of the fibre, and on fibre orientation with respect to the direction of the applied load. The polymer matrix can be a thermoplastic, in which case they are coextruded with short fibres. Fibre-reinforced thermoset polyurethanes are produced by reactive injection moulding (RIM) of diol, isocyanate and fibre. Other reinforced thermosets, such as polyesters or epoxies, normally incorporate somewhat longer fibres of the order of centimeters and are typically used for large items such as car bodies and aircraft panels.

Modulus of a filled composite

The modulus of a composite (Ec = σc/εc) can be estimated from that of the filler (Ef) and that of the polymer matrix (Em). Ideally a composite will be a uniform dispersion of filler

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in a matrix, but two extreme model morphologies can be envisaged; (a) the load is shared by filler and matrix "in series" or (b) is shared "in parallel" -

(a) Series model, the stress (load) is the same in filler and matrix, σc = σ f = σm

The total strain of the composite is assumed to be the volume fraction (V) sum of the filler and matrix strains -

εc = σc/Εc = Vf εf + Vmεm = Vf(σf /Εf) + Vm(σm /Εm)

1/Ec(series) = Vf /Εf + Vm /Εm

(b) Parallel model, the strain is the same for filler and matrix, ε c = εf = εm

The total stress carried by the composite is assumed to be the volume fraction sum of the stresses carried by filler and matrix -

σc = εcΕc = Vfσf + Vmσm = Vf(εfΕf) + Vm(εmΕm)

Ec(parallel) = Vf Εf + Vm Εm

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The modulus of most real composites lies between these two bounds. For fibre-reinforced composites the parallel model is the more appropriate, but the orientation of the fibres has to be taken into account -

Ec = k Vf Ef + VmEm

where k is an efficiency factor depending on fibre orientation with respect to the tensile load direction –

Fibre orientation Stress direction k

All parallel Parallel to fibres 1

All parallel Perpendicular to fibres 0

Random in 2-D Any direction in 2-D 3/8

Random in 3-D Any 1/5

Note that, in practice, it is difficult to have more than 60% fibres in a composite.