Equipos electronicos de deteccion indirecta de fugas

Many features of the processing behaviour of PP may be predicted by consideration of thermal properties. The specific heat of PP is lower than that of PE but higher than that of PS. Therefore, the plasticising capacity of an injection moulding machine using PP is lower than when PS is used but generally higher than with HDPE. The plasticising capacity is defined as the amount of the material which can be melted and plasticised in the barrel in a given time in a given injection moulding machine.

Specific heat is a function of temperature below melting temperature. However, a significant rise in specific heat is observed near the melting point due to the partial crystalline nature of the polymer. However, the specific heat of the polymer melt is virtually independent of temperature. The specific heat, or more precisely the enthalpy, of the material controls the cooling of the artefact in the mould and predominantly the design of the cooling channels in the mould. The heat requirement for cooling of a PP artefact can be calculated from a graph such as that illustrated in Figure 10. To achieve faster cycles, mould cooling requirements should be considered from the beginning.

The cooling system should balance the heat flow from the part to ensure uniform part cooling and minimise residual stresses, differential shrinkage and warpage. Other thermal properties of PP are given in Table 16.

Table 16 Thermal properties of PP

Property Value Specific heat (J/g °C) at 23 °C 1.68

Specific heat (J/g °C) at 100 °C 2.10 Thermal conductivity at 20 °C (W/m K) 0.22 Linear coefficient of thermal expansion (/°C) 20–60 °C 10 x 10^-5

60–100 °C 15 x 10^-5 100–140 °C 21 x 10^-5

Figure 10 Specific heat of PP as a function of temperature 4.2.6 Thermal Conductivity

The lower thermal conductivity of PP and other plastics compared to metals, gives protection against external temperature changes and so PP could be used for insulation applications. However, the use of PP, unless foamed, as a primary insulating material is rather limited (owing to cost factors). PP has been used for food packaging of refrigerated foodstuffs due to its suitability for food applications rather than its suitability as an insulating material. Lower thermal conductivity limits the production cycles and can result in cooling strains in thick sections, which may lead to warpage of the article. Similar to other plastic materials, the conductivity of the PP is a function of density and foamed PP has lower conductivity than the unfoamed PP.

4.2.7 Thermal Expansion

The coefficient of thermal expansion is defined as the fractional change in length or volume of a material for a unit change in temperature. The coefficient of thermal expansion of plastics is considerably higher than metals, up to 6 to 10 times as high.

This difference in the coefficient of thermal expansion can lead to internal stresses and stress concentrations. Consequently, premature failure may occur. Thermal expansion in PP gives significant volume changes on melting. It thus shrinks by 1–2% in moulding, this must be allowed for when designing the tool. Mould shrinkage and thermal expansion values for PP are compared with other thermoplastics in Table 2.

The use of filler lowers the coefficient of thermal expansion considerably and brings the value closer to that of metals and ceramics (Section 4.3.6). The effect of thermal expansion on shrinkage, warpage and dimensional tolerances is discussed in Section 5.1.3.

4.3 Mechanical Properties

The mechanical properties of PP depend on several factors and are strongly influenced by the molecular weight. General observations suggest that an increase in molecular weight, keeping all other structural parameters similar, leads to a reduction in tensile strength, stiffness, hardness, brittle point but an increase in impact strength. This effect of molecular weight on the properties of PP is contrary to most other well-known plastics.

The properties of some PP grades with different melt flow indices and structure are compared in Table 12. It can be observed that an increase in mechanical properties is not necessarily reflected in a trend predicted only on the basis of molecular weight, and other structural parameters, particularly crystallinity, play a very important role. Hence, the prediction of the mechanical properties on the basis of molecular weight or melt flow rate should be treated with caution. Appropriate data for the properties of the material should always be consulted.

4.3.1 Short-term Mechanical Properties

A tensile test reveals that tensile force increases with increasing elongation, up to the yield point (Figure 11). After this, force initially decreases, i.e., the material can be further stretched with a smaller force. This is accompanied by a marked necking of the cross section of the test specimen. When this necking down has progressed along the entire length of the specimen, force increases again until elongation at break is reached.

The second increase in deformation resistance is due to partial orientation of the macromolecules which strengthens the material. This typical behaviour of PP is similar to other ductile plastics. It can be seen from Table 12 that the mechanical properties of random and block copolymer grades are lower than the homopolymers for the same value of melt flow rate or molecular weight. The difference in their tensile stress/strain curves is highlighted in Figure 12.

Figure 11 A schematic tensile stress/strain curve for PP

Figure 12 Tensile stress/strain curves for different types of PP

It can be seen from Table 1 that the flexural modulus and tensile strength of PP is lower than most of the plastic materials except LDPE and HDPE. However, PP offers an advantageously high flexural modulus to cost ratio which makes it an ideal candidate for replacement material to many engineering plastics on the cost reduction basis.

The short-term stress/strain data of different grades of PP (and for other plastics) is of limited use and should only be used for pre-selection of material. In reality, plastic components are seldom designed and subjected to such high levels of strain as applied in short-term tensile tests. In addition, most of the cases of product failure are brittle in nature. Consequently, the long-term creep and fatigue properties of PP, discussed in Sections 4.3.3 to 4.3.5, are more important for structural applications.

4.3.1.1 The Effect of Test Speed

Like other viscoelastic thermoplastics, the mechanical properties of PP depend on the speed of the test. For instance, raising the speed of the test decreases the observed flexibility and increases the observed brittleness.

4.3.1.2 The Effect of Temperature

The stiffness of PP is a function of temperature. The variation of flexural modulus of different grades of PP as a function of temperature is shown in Figure 13. PP homopolymers are slightly stiffer than copolymers at room temperature. However, the difference between the two types is diminished as the temperature rises. The flexural modulus of elastomer-modified PP is significantly lower than the homopolymer or copolymer PP, and its service temperature is around 90 °C, much lower than that of homopolymer PP. PP becomes more ductile as the usage temperature increases, shown by an increase in elongation at break and decrease in ultimate tensile strength and yield stress.

Figure 13 Flexural modulus of different grades of PP as a function of temperature [2]

4.3.1.3 Time-temperature Superposition

PP is a viscoelastic material, and, consequently, its mechanical properties are strongly dependent on time, temperature, the level and type of applied stress and the testing speed. The apparent stiffness or elastic modulus of all plastics reduces with time under load due to the processes of stress relaxation and creep. Similarly, the modulus reduces with increasing temperature. In other words, the effect of time and temperature on the mechanical properties is interchangeable. The theory behind this behaviour of polymeric materials was given by Williams, Landel and Ferry. The detailed description of this theory can be found in standard textbooks [e.g., 14]. However, at this point, it will suffice to say that the effect of time during service could be simulated in the short-term using high temperature. This superposition of time and temperature could be used in practice to predict the durability of the products.

4.3.2 Impact Strength

The second-order transition temperature of PP homopolymer is –10 °C. This explains the drop in its impact strength at temperatures around 0 °C. The impact strengths of different grades of PP at different temperatures are given in Table 12. Several methods are used for measuring the impact strength of PP. However, none of the methods satisfactorily predict performance under conditions of end use. In the Izod or Charpy test, a notch is incorporated in the sample to concentrate stress; this normally leads to brittle failure. Impact strength is reduced as the notch gets sharper. Consequently, sharp corners in load-bearing sections must be avoided in the design of the article, as a general rule for all the plastics.

The impact strength of an article depends on the inherent molecular structure of the grade used and the morphology arising from the processing conditions. Changes in the

geometry of an item can have a major effect on its toughness rating. Impact strength increases with the molecular weight but more markedly with comonomer content. The most important way of improving the impact strength of PP is by incorporating a rubbery phase, as in heterophasic copolymers. Toughness increases rapidly with higher rubber content, and its transition from ductile to brittle failure occurs at lower temperatures.

One of the major reasons for the failure of PP artefacts is the brittle failure. This is mainly caused by the incorrect selection the PP grade, particularly the use of PP homopolymer in place of copolymer or use of wrong material at the moulding floor.

Infrared microscopy and gel permeation chromatography can quickly identify the source of the problem.

4.3.2.1 Falling Dart Impact Test

The falling weight or dart drop test method simulates actual day-to-day abuse and can be carried out either on standard laboratory specimens or on the articles themselves.

Failure may occur in various ways ranging from brittle to ductile failure (Figure 14).

Particular care must be taken to avoid the brittle failure by proper selection of grade. At temperatures below –20 °C, elastomer-modified PP is more impact resistant than PP copolymer and homopolymer.

Ductile Bructile Brittle

Figure 14 Example of ductile, bructile or brittle material failure Reproduced with permission from Shell, © Shell, website: www.basell.com

4.3.2.2 Notched Impact Strength

PP copolymer has higher impact strength than homopolymers even at low temperature (Figure 15). Higher molar mass provides better impact and notched impact strength

above 0 °C. Elastomer-modified PP shows high notched impact strength even at temperatures below 0 °C.

Figure 15 Typical notched impact strength of PP as a function of temperature

4.3.2.3 Tensile-impact Strength

Impact strength tests permit no differentiation between specimens undergoing the test without failure. In this respect, the tensile-impact strength test is superior. Other test variables such as notch sensitivity, loss factor and specimen thickness are eliminated in the tensile-impact strength test. In addition, tensile-impact strength tests can be used for very thin specimens. The tensile-impact strength test consists of a specimen-in-head type of set up. In this case, the specimen is mounted in the pendulum and attains full kinetic energy at the point of impact. One end of the specimen is mounted in the pendulum and the other end be gripped by a crashing member, which travels with the pendulum until the instant of impact. The energy to break by impact in tension is determined by kinetic energy extracted from the pendulum in the process of breaking the specimen. The superior impact properties of elastomer-modified PP are, once again, observed.

4.3.3 Creep

PP is a viscoelastic material and, like all other thermoplastics, it exhibits creep (or cold flow). Creep is the deformation or total strain which occurs after a stress has been applied. Its extent depends on the magnitude and nature of stress, the temperature and time for which the stress is applied. Over a period of time, PP undergoes deformation, even at room temperature and under relatively low stress. After removal of stress, a moulding more or less regains its original shape, depending on the time under stress and

magnitude of stress. Recoverable deformation is known as elastic deformation and permanent deformation as plastic deformation.

Typical creep curves plot deformation or creep against time on logarithmic scale for a range of loads or stresses. This basic creep data could be used to plot isochronous stress/strain curves, isometric stress curves or creep modulus as a function of time. In an isochronous graph, stress is plotted against strain at a constant series of time intervals (Figure 16). In isometric curves, stress or strain is plotted as a function of time for a series of constant strains or stresses. Creep modulus curves are the time-dependent value of modulus (Figure 17). As the properties of polymers are a function of temperature, these curves can be produced at different temperatures. This type of data is available from the raw material suppliers in most of the cases. However, sometime the creep data for the conditions which the component might observe in service are not available, hence the data is extrapolated to the required conditions. Care should be exercised in extrapolating the data to higher temperatures or longer durations outside the experimental creep data range.

Copolymer type and melt flow rate also influence the creep behaviour. Copolymer grades of PP have substantially lower creep modulus than the homopolymer grades. PP has a similar modulus to high density PE, but its resistance to creep is much better and, at a equivalent time under similar load, the creep modulus of PP is more than that of high density PE. However, the creep resistance of amorphous plastics is much better than the semi-crystalline plastics such as PP and PE. Creep resistance of PP could be further improved by addition of fillers or reinforcements. The creep behaviour of moulded artefacts is affected by the residual stress or orientation effect in the moulded article.

Figure 16a Isochronous stress/strain curves of PP at 23 °C

Figure 16b Isochronous stress/strain curves of PP at 80 °C

Figure 17 Tensile creep modulus of PP as a function of time under stress (T1 = 23 °C, T2 = 65 °C and T3 = 110 °C)

PP shows different responses to different stresses or combination of stresses. For PP, it is reported that, up to strains of 0.8%, stress is proportional to strain measured during the creep tests. Up to this level of deformation, stress/strain behaviour under both compressive and flexural stress can be approximately calculated from the tensile creep tests.

4.3.4 Fatigue

An alternative case to creep, where the deformation of the material is measured as a function of time at a constant stress, is fatigue (or stress relaxation). In this case, the material is subjected to constant strain, the relaxation in the stress in the component is measured as a function of time. This scenario occurs in press fits, springs, interference fits, screws and washers, etc., which during service undergo stress relaxation. A typical stress relaxation curve for PP is shown in Figure 18. A significant relaxation in the tensile modulus occurs over the 10 year period depicted in this graph (logarithmic scale), the value dropping from around 1,000 N/mm² to around 350 N/mm².

Figure 18 Tensile stress relaxation modulus for PP homopolymer at 23 °C

4.3.5 Dynamic Fatigue

Materials subjected to cyclic loads or stresses fail at a point far below the ultimate strength measured in short-term mechanical tests. The cyclic loads may be caused by periodic or intermittent loading in on-off situations. It is well known that the amorphous plastics are more susceptible to fatigue than semi-crystalline plastics such as PP. But it should be noted that the semi-crystalline materials also suffer from dynamic fatigue and the stress level decreases significantly as the number of cycles increases, though semi-crystalline materials do not undergo the ductile to brittle transition of amorphous materials. The stress levels for cycles to failure for different plastics are compared in Figure 19. This figure clearly demonstrates that the amorphous plastics (PC and ABS)

are seriously prone to fatigue related problems. However, most semi-crystalline plastics show a similar slope in the fatigue strength curve. A notable exception is acetal resin which shows a transition. However, it should be noted that PP is not a very stiff plastic, hence the safe stress level for PP under fatigue conditions is very low.

Figure 19 Stress levels for cycles to failure for different plastics

Fatigue data is usually published in the form of Wohler curves where stress or strain amplitude is plotted against the number of cycles to failure on a logarithmic scale.

Dynamic fatigue is a complex issue. However, the following common observations can be made:

• Fatigue strength decreases with increasing temperature.

• Fatigue strength is sensitive to stress concentration such as notches or sharp corners.

• Fatigue strength depends on the stress frequency. The effect of fatigue at low frequencies is much more severe. In other words, at low frequencies the failure could occur earlier than that predicted using high frequency tests.

4.3.6 Mechanical Properties of Filled Grades

The properties of filled or reinforced grades of PP are heavily influenced by the type and amount of the filler. For example, the density of a heavily filled grade can be up to 50% higher than the unfilled material. In the next paragraphs, the effect of fillers or reinforcing agents on the properties of PP is explained. The typical properties of filled grades of PP are given in Table 17.

It can be seen from the table that the mechanical properties of filled grades of PP are substantially modified by the presence of filler. Depending on the type and content of filler or reinforcement, PP may even show a brittle failure at low applied strains at low testing rates. The improvement/reduction in the tensile strength of the filled grades is marginal (with the exception of 30% glass fibre reinforced PP with coupling agent) due to stress concentration effects. However, the modulus is significantly improved on addition of fillers and reinforcements, particularly for glass fibre reinforced grades with a suitable coupling agent.

The impact properties of the glass fibre reinforced grades are reduced. However, reinforced copolymer grades provide good low-temperature impact properties but at the expense of rigidity. The notched impact strength of the glass fibre reinforced grades is better due to blunting of the crack propagation mechanism.

Reinforced grades of PP have a distinctly higher surface hardness than the non-reinforced grades; hardness varies according to the type of reinforcing material and its proportion by weight. The reinforced materials can be used for sliding elements but the wear of other materials in contact may be very high.

The crystalline melting temperature and glass transition temperature of reinforced PP is not substantially different to those of the unreinforced grades. However, substantial changes in HDT values are observed. Reinforced PP grades have reduced specific heat values since the reinforcing materials have considerably lower values than the base polymer.

The coefficient of linear expansion, to a large degree, is dependent on the orientation

The coefficient of linear expansion, to a large degree, is dependent on the orientation