Plastics Engineering 3E Episode 2 pptx

General Properties of Plastics 19 In many respects the stress-strain graph for a plastic is similar to that for a metal see Fig.. 1.3 Effect of material temperature on stress-strain beha

General Properties of Plastics 19

In many respects the stress-strain graph for a plastic is similar to that for a metal (see Fig 1.2)

At low strains there is an elastic region whereas at high strains there is a non- linear relationship between stress and strain and there is a permanent element

to the strain In the absence of any specific information for a particular plastic, design strains should normally be limited to 1% Lower values (-0.5%) are recommended for the more brittle thermoplastics such as acrylic, polystyrene and values of 0.2-0.3% should be used for thermosets

The effect of material temperature is illustrated in Fig 1.3 As temperature

is increased the material becomes more flexible and so for a given stress the

Fig 1.2 Qpical stress-strain graph for plastics

-20°C

20°C

50°C 70°C

Fig 1.3 Effect of material temperature on stress-strain behaviour of plastics

20 General Properties of Plastics material deforms more Another important aspect to the behaviour of plastics

is the effect of strain rate If a thermoplastic is subjected to a rapid change in strain it appears stiffer than if the same maximum strain were applied but at a slower rate This is illustrated in Fig 1.4

Strain (“A)

Fig 1.4 Effect of strain rate on stress-strain behaviour of plastics

It is important to realise also that within the range of grades that exist for a particular plastic, there can be significant differences in mechanical properties For example, with polypropylene for each 1 kg/m3 change in density there is a corresponding 4% change in modulus Fig 1.5 illustrates the typical variation which occurs for the different grades of ABS It may be seen that very often a grade of material selected for some specific desirable feature (e.g high impact strength) results in a decrease in some other property of the material (e.g tensile strength)

The stiffness of a plastic is expressed in terms of a modulus of elasticity Most values of elastic modulus quoted in technical literature represent the slope

of a tangent to the stress-strain curve at the origin (see Fig 1.6) This is often

referred to as Youngs modulus, E, but it should be remembered that for a plastic this will not be a constant and, as mentioned earlier, is only useful for quality

General Properties of Plastics 21

50

High-heat grade Medium-impact Hig h-impact -impact

40

Strain (%) Fig 1.5 Effect of grade on mechanical properties of ABS

Slope represents tangent, OrYoung‘s modulus ‘4/ /k sm

0 c’

Strain Fig 1.6 Tangent and secant modulus

control purposes, not for design Since the tangent modulus at the origin is sometimes difficult to determine precisely, a secant modulus is often quoted to

remove any ambiguity A selected strain value of, say 2% (point C’, Fig 1.6) enables a precise point, C, on the stress-strain curve to be identified The slope

of a line through C and 0 is the secant modulus npical short-term mechanical

22 General Roperties of Plastics properties of plastics are given in Table 1.5 These are given for illustration purposes For each type of plastic there are many different grades and a wide variety of properties are possible The literature supplied by the manufacturers should be consulted in specific instances

Table 1.5 Short-term properties of some important plastics Material

0.5

3.0 0.007 3.6 9.0 11.0

70

3 0.8

2.1 3.5 3.3 2.5 3.2

8.3 3.9 4.0

67

-

*On a weight basis, relative to polypropylene

Material Selection for Strength

If, in service, a material is required to have a certain strength in order to per€orm

its function satisfactorily then a useful way to compare the structural efficiency

of a range of materials is to calculate their strength desirability factor

Consider a structural member which is essentially a beam subjected to bending (Fig 1.7) Irrespective of the precise nature of the beam loading the

General Properties of Plastics 23

Fig 1.7 Beam subjected to bending

maximum stress, 0, in the beam will be given by

where /?I is a constant

But the weight, w , of the beam is given by

are taken into account and Tables 1.1 1 and 1.12 give desirability factors for a

range of loading configurations and materials

Material Selection for Stiffness

If in the service of a component it is the deflection, or stiffness, which is the limiting factor rather than strength, then it is necessary to look for a different desirability factor in the candidate materials Consider the beam situ- ation described above This time, irrespective of the loading, the deflection, 6,

General Properties of Plastics

24

will be given by

6=a1 (G)

where a1 is a constant and W represents the loading

The stiffness may then be expressed as

W

where a2 is a constant and again it is assumed that the beam width and length are the same in all cases

Once again the beam weight will be given by equation (1.3) so substituting

for d from equation (1.7)

(1.8) Hence, the desirability factor, Df , expressed as maximum stiffness for

1/3

w = ( ~ 3 p / E

minimum weight will be given by

where E is the elastic modulus of the material in question and p is the density

As before a range of similar factors can be derived for other structural elements and these are illustrated in Section 1.4.6 (Tables 1.11 and 1.12) where the effect of material cost is also taken into account Note also that since for plastics the modulus, E, is not a constant it is often necessary to use a long- term (creep) modulus value in equation (1.9) rather than the short-term quality control value usually quoted in trade literature

Ductility A load-bearing device or component must not distort so much under the action of the service stresses that its function is impaired, nor must it fail by rupture, though local yielding may be tolerable Therefore, high modulus and high strength, with ductility, is the desired combination of attributes However, the inherent nature of plastics is such that high modulus tends to

be associated with low ductility and steps that are taken to improve the one cause the other to deteriorate The major effects are summarised in Table 1.6 Thus it may be seen that there is an almost inescapable rule by which increased modulus is accompanied by decreased ductility and vice versa

Creep and Recovery Behaviour Plastics exhibit a time-dependent strain response to a constant applied stress This behaviour is called creep In a similar fashion if the stress on a plastic is removed it exhibits a time dependent recovery of strain back towards its original dimensions This is illustrated in

General Properties of Plastics 25

Table 1.6

Balance between stiffness and ductility in thermoplastics

Effect on Modulus Ductility

Reduced temperature increase decrease

Increased straining rate increase decrease

Multiaxial stress field increase decrease

Incorporation of plasticizer decrease increase

Incorporation of rubbery phase decrease increase

Incorporation of glass fibres increase decrease

Incorporation of particulate filler increase decrease

1 Load is opplied instantaneously,

resulting in strain A 4 Sample recovers viscoelastically

to Point D

Fig 1.8 npical Creep and recovery behaviour of a plastic

Fig 1.8 and because of the importance of these phenomena in design they are

dealt with in detail in Chapter 2

Stress Relaxation Another important consequence of the viscoelastic nature

of plastics is that if they are subjected to a particular strain and this strain is held constant it is found that as time progresses, the stress necessary to maintain this strain decreases This is termed stress relaxation and is of vital importance

in the design of gaskets, seals, springs and snap-fit assemblies This subject will also be considered in greater detail in the next chapter

Creep Rupture When a plastic is subjected to a constant tensile stress its strain increases until a point is reached where the material fractures This is called creep rupture or, occasionally, static fatigue It is important for designers

26 General Properties of Plastics

to be aware of this failure mode because it is a common error, amongst those accustomed to dealing with metals, to assume that if the material is capable of withstanding the applied (static) load in the short term then there need be no further worries about it This is not the case with plastics where it is necessary

to use long-term design data, particularly because some plastics which are tough

at short times tend to become embrittled at long times

Fatigue Plastics are susceptible to brittle crack growth fractures as a result

of cyclic stresses, in much the same way as metals are In addition, because

of their high damping and low thermal conductivity, plastics are also prone to thermal softening if the cyclic stress or cyclic rate is high The plastics with the best fatigue resistance are polypropylene, ethylene-propylene copolymer and PVDF The fatigue failure of plastics is described in detail in Chapter 2

Toughness By toughness we mean the resistance to fracture Some plastics are inherently very tough whereas others are inherently brittle However, the picture is not that simple because those which are nominally tough may become embrittled due to processing conditions, chemical attack, prolonged exposure

to constant stress, etc Where toughness is required in a particular application it

is very important therefore to check carefully the service conditions in relation

to the above type of factors At mom temperature the toughest unreinforced plastics include nylon 66, LDPE, LLDPE, EVA and polyurethane structural foam At sub-zero temperatures it is necessary to consider plastics such as ABS, polycarbonate and EVA The whole subject of toughness will be considered more fully in Chapter 2

1.4.2 Degradation

Physical or Chemical Attack Although one of the major features which might prompt a designer to consider using plastics is corrosion resistance, nevertheless plastics are susceptible to chemical attack and degradation As with metals, it is

often difficult to predict the performance of a plastic in an unusual environment

so it is essential to check material specifications and where possible carry out proving trials Clearly, in the space available here it is not possible to give precise details on the suitability of every plastic in every possible environment Therefore the following sections give an indication of the general causes of polymer degradation to alert the designer to a possible problem

The degradation of a plastic occurs due to a breakdown of its chemical structure It should be recognised that this breakdown is not necessarily caused

by concentrated acids or solvents It can occur due to apparently innocuous mediums such as water (hydrolysis), or oxygen (oxidation) Degradation of plastics is also caused by heat, stress and radiation During moulding the mat- erial is subjected to the first two of these and so it is necessary to incorporate stabilisers and antioxidants into the plastic to maintain the properties of the material These additives also help to delay subsequent degradation for an acceptably long time

General Properties of Plastics 27

As regards the general behaviour of polymers, it is widely recognised that crystalline plastics offer better environmental resistance than amorphous plas- tics This is as a direct result of the different structural morphology of these two classes of material (see Appendix A) Therefore engineering plastics which

are also crystalline e.g Nylon 66 are at an immediate advantage because they

can offer an attractive combination of load-bearing capability and an inherent chemical resistance In this respect the anival of crystalline plastics such as PEEK and polyphenylene sulfide (PPS) has set new standards in environmental resistance, albeit at a price At room temperature there is no known solvent for PPS, and PEEK is only attacked by 98% sulphuric acid

Weathering This generally occurs as a result of the combined effect of water absorption and exposure to ultra-violet radiation (u-v) Absorption of water can have a plasticizing action on plastics which increases flexibility but ultimately (on elimination of the water) results in embrittlement, while u-v

causes breakdown of the bonds in the polymer chain The result is general deterioration of physical properties A loss of colour or clarity (or both) may also occur Absorption of water reduces dimensional stability of moulded arti- cles Most thermoplastics, in particular cellulose derivatives, are affected, and also polyethylene, PVC, and nylons

Oxidation This is caused by contact with oxidising acids, exposure to u-v,

prolonged application of excessive heat, or exposure to weathering It results

in a deterioration of mechanical properties (embrittlement and possibly stress cracking), increase in power factor, and loss of clarity It affects most thermo- plastics to varying degrees, in particular polyolefins, PVC, nylons, and cellulose derivatives

Environmental Stress Cracking (ESC) In some plastics, brittle cracking occurs when the material is in contact with certain substances whilst under stress The stress may be externally applied in which case one would be prompted to take precautions However, internal or residual stresses introduced during processing are probably the more common cause of ESC Most organic liquids promote ESC in plastics but in some cases the problem can be caused

by a liquid which one would not regard as an aggressive chemical The classic example of ESC is the brittle cracking of polyethylene washing-up bowls due

to the residual stresses at the moulding gate (see injection moulding, Chapter 4)

coupled with contact with the aqueous solution of washing-up liquid Although direct attack on the chemical structure of the plastic is not involved in ESC the problem can be alleviated by controlling structural factors For example, the resistance of polyethylene is very dependent on density, crystallinity, melt flow index (MFI) and molecular weight As well as polyethylene, other plastics which are prone to ESC are ABS and polystyrene

The mechanism of ESC is considered to be related to penetration of the promoting substance at surface defects which modifies the surface energy and promotes fracture

28 General Properties of Plastics

1.43 Wear Resistance and Frictional Properties

There is a steady rate of increase in the use of plastics in bearing applications and in situations where there is sliding contact e.g gears, piston rings, seals, cams, etc The advantages of plastics are low rates of wear in the absence of

conventional lubricants, low coefficients of friction, the ability to absorb shock and vibration and the ability to operate with low noise and power consumption Also when plastics have reinforcing fibres they offer high strength and load carrying ability Qpical reinforcements include glass and carbon fibres and fillers include PTFE and molybdenum disulphide in plastics such as nylon,

polyethersulphone (PES), polyphenylene sulfide (PPS), polyvinylidene fluoride (PVDF) and polyetheretherketone (PEEK)

The friction and wear of plastics are extremely complex subjects which depend markedly on the nature of the application and the properties of the material The frictional properties of plastics differ considerably from those of metals Even reinforced plastics have modulus values which are much lower than metals Hence metalkhennoplastic friction is characterised by adhesion and deformation which results in frictional forces that are not proportional to load but rather to speed Table 1.7 gives some typical coefficients of friction for plastics

Table 1.7 Coefficients of friction and relative wear rates for plastics

Material

Coefficient of friction

Relative Static Dynamic wear rate

to the extent where large troughs of melted plastic are removed Table 1.7

General Properties of Plastics 29 shows typical primary wear rates for different plastics, the mechanism of wear

is complex the relative wear rates may change depending on specific circum- stances

In linear bearing applications the suitability of a plastic is usually determined from its PV rating This is the product of P (the bearing load divided by the projected bearing area) and V (the linear shaft velocity) Fig 1.9 shows the limiting PV lines for a range of plastics - combinations of P and V above the lines are not permitted The PV ratings may be increased if the bearing

is lubricated or the mode of operation is intermittent The PV rating will be decreased if the operating temperature is increased Correction factors for these variations may be obtained from materialhearing manufacturers The plastics

with the best resistance to wear are ultra high molecular weight polyethylene

(used in hip joint replacements) and PTFE lubricated versions of nylon, acetal and PBT It is not recommended to use the same plastic for both mating surfaces

in applications such as gear wheels

Rubbing velocity, ( V ( d s )

Fig 1.9 Qpical P-V ratings for plastics rubbing on steel

30 General Properties of Plastics

L4,4 Special Properties

chains have a random configuration Inside the material, even though it is not

there is an increase in their relative movement This makes the material more

temperature, Tg9 below which the material behaves like glass Le+ it is hard and

not necessarily a low temperature This immediately helps to explain some of

the differences which we observe in plastics For example, at room tempera+

observe these materials in their glassy state Note, however, that in contrast, at

room temperature, polyethylene is above its glass transition temperature and so

a hard, brittle solid, Plastics can have several transitions

a hard, rigid, brittle statet

constant, As with many other properties of polymers it will depend on the

When the crystalline plastics have their temperature reduced they exhibit a

glass transition temperature associated with the amorphous regions At room temperature polypropylene, for example, is quite rigid and tough, not because

crystalline regions When i t is cooled below -10°C it becomes brittle because

the amorphous regions go below their Tg'

In the past a major limitation to the use of plastics materials in the engi- neering sector has been temperature T h i s limitation arises not only due to the

General Properties of Plastics 31

Material

Density heat conductivity therm exp (m2/s) Temp, operating,

0.24 0.52

0.25

0.25 0.16 0.14

0.17

0.2

0.2

0.032 0.032

reduction in mechanical properties at high temperatures, including increased

propensity to creep, but also due to limitations on the continuous working

temperature causing permanent damage to the material as a result of thermal and

32 General Properties of Plastics fibrous reinforcement, but the development of new polymer matrices is the key

to further escalation of the useful temperature range

Table 1.8 indicates the service temperatures which can be used with a range

of plastics It may be seen that there are now commercial grades of unreinforced plastics rated for continuous use at temperatures in excess of 200°C When glass

or carbon fibres are used the service temperatures can approach 300°C The other principal thermal properties of plastics which are relevant to design

are thermal conductivity and coefficient of thermal expansion Compared with most materials, plastics offer very low values of thermal conductivity, partic- ularly if they are foamed Fig 1.10 shows comparisons between the thermal conductivity of a selection of metals, plastics and building materials In contrast

to their low conductivity, plastics have high coefficients of expansion when compared with metals This is illustrated in Fig 1.1 1 and Table 1.8 gives fuller information on the thermal properties of plastics and metals

3Barm Concrpl~ blocks

Equivalent thickness of common

building and insulatm materials required

to achieve the same degree of insulaiion Fig 1.10 Comparative Thermal conductivities for a range of materials

Electrical Properties Traditionally plastics have established themselves in applications which require electrical insulation PTFE and polyethylene are

among the best insulating materials available The material properties which

are particularly relevant to electrical insulation are dielectric strength, resistance

and tracking

The insulating property of any insulator will break down in a sufficiently

strong electric field The dielectric strength is defined as the electric strength

(V/m) which an insulating material can withstand For plastics the dielectric strength can vary from 1 to loo0 MV/m Materials may be compared on the basis of their relative permittivity (or dielectric constant) This is the ratio of the permittivity of the material to the permittivity of a vacuum The ability of a

General Properties of Plastics 33

Fig 1.1 1 lsrpical thermal propexties of plastics

material to resist the flow of electricity is determined by its volume resistivity, measured in ohm m Insulators are defined as having volume resistivities greater

than about 104 ohm m Plastics are well above this, with values ranging from about 108 to 10l6 ohm m These compare with a value of about ohm m for copper Although plastics are good insulators, local breakdown may occur due

to tracking This is the name given to the formation of a conducting path (arc) across the surface of the polymer It can be caused by surface contamination (for example dust and moisture) and is characterised by the development of carbonised destruction of the surface carrying the arc Plastics differ greatly

in their propensity to tracking - PTFE, acetal, acrylic and PP/PE copolymers offer very good resistance

It is interesting to note that although the electrical insulation properties of

plastics have generally been regarded as one of their major advantages, in recent

years there has been a lot of research into the possibility of conducting plastics

This has been recognised as an exciting development area for plastics because electrical conduction if it could be achieved would offer advantages in designing against the build up of static electricity and in shielding of computers, etc from electro-magnetic interference (EM) There have been two approaches - coating

or compounding In the former the surface of the plastic is treated with a conductive coating (e.g carbon or metal) whereas in the second, fillers such

as brass, aluminium or steel are incorporated into the plastic It is important that the filler has a high aspect ratio (1ength:diameter) and so fibres or flakes of metal are used There has also been some work done using glass fibres which

are coated with a metal before being incorporated into the plastic Since the fibre aspect ratio is critical in the performance of conductive plastics there can

34 General Properties of Plastics

be problems due to breaking up of fibres during processing In this regard ther- mosetting plastics have an advantage because their simpler processing methods cause less damage to the fibres Conductive grades of DMC are now available

with resistivities as low as 7 x ohm m

Optical Properties The optical properties of a plastic which are important are refraction, transparency, gloss and light transfer The reader is referred to BS

4618:1972 for precise details on these terms Table 1.9 gives data on the optical properties of a selection of plastics Some plastics may be optically clear (e.g acrylic, cellulosics and ionomers) whereas others may be made transparent These include epoxy, polycarbonate, polyethylene, polypropylene, polystyrene, polysulphone and PVC

Table 1.9 Typical properties of plastics

Refractive Light Dispersive Material index transmission power

extinguishing, slow burning, $re retardant etc have been employed to describe

their behaviour under such standard test conditions, but could never be regarded

as predictions of the performance of the material in real fire situations, the nature and scale of which can vary so much

Currently there is a move away from descriptions such as jre-retardant

or self-extinguishing because these could imply to uninformed users that the

material would not bum The most common terminology for describing the flammability characteristics of plastics is currently the Critical Oxygen Index (COI) This is defined as the minimum concentration of oxygen, expressed as volume per cent, in a mixture of oxygen and nitrogen that will just support combustion under the conditions of test Since air contains 21% oxygen, plastics

having a COI of greater than 0.21 are regarded as self-extinguishing In practice

a higher threshold (say 0.27) is advisable to allow for unforeseen factors in a particular fire hazard situation Fig 1.12 shows the typical COI values for a range of plastics

General Properties of Plastics 35

COI -

Fig 1.12 Oxygen Index Values for Plastics

Permeability The low density of plastics is an advantage in many situations but the relatively loose packing of the molecules means that gases and liquids can permeate through the plastic This can be important in many applications

such as packaging or fuel tanks It is not possible to generalise about the

performance of plastics relative to each other or in respect to the performance

of a specific plastic in contact with different liquids and gases

Some plastics are poor at offering resistance to the passage of fluids through them whereas others are excellent Their relative performance may be quantified

in terms of a permeation constant, k, given by

(1.10) where Q = volume of fluid passing through the plastic

d = thickness of plastic

A = exposed area

t = time

p = pressure difference across surfaces of plastic

The main fluids of interest with plastics are oxygen and water vapour (for

packaging applications) and C02 (for carbonated drinks applications) Fig 1.13

and Fig 1.14 illustrate the type of behaviour exhibited by a range of plastics

In some cases it is necessary to use multiple layers of plastics because no single plastic offers the combination of price, permeation resistance, printability, etc required for the application When multi-layers are used, an overall permeation constant for the composite wall may be obtained from

(1.11)

1.4.5 Processing

A key decision in designing with plastics is the processing method employed The designer must have a thorough knowledge of processing methods because