Flow Properties Influencing Processing Of Pp Biology Essay

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Extrusion is the most widely practiced process for forming polypropylene, accounting for some 46 of consumption in the USA Of this, the major share is taken by fiber and filament, two forms that are not readily identified with polypropylene in the public consciousness. Film, mainly for packaging and also not popularly recognized as polypropylene, is also a very significant extrusion product. Sheet and profile extrusion is of relatively minor importance. Almost one third of polypropylene is processed by injection molding. Other processes such as blow molding, thermoforming, calendering and so on, probably account for less than 5 of polypropylene consumption. These figures are only a guide; almost a quarter of polypropylene is processed by unspecified methods. Nevertheless, the relative proportions of the various processing methods are probably reliable.

Figure 3.1 Processing methods for polypropylene, USA, 1996. [12]

3.2 Flow Properties influencing processing of PP

Polypropylene is formed into articles almost exclusively by melt processes that rely on the flow of the melted material at elevated temperatures. Injection molding, blow molding, extrusion, and thermoforming are all examples of melt processing. An understanding of melt flow is essential for success with these processes.

The rheology of a thermoplastics melt is complex, being very dependent on temperature and shear rate. This means that the melt viscosity - the characteristic that makes flow easy or difficult - can vary widely in the melt condition. The two key points about the flow of thermoplastics are that the behavior is non-Newtonian and that viscosities are very high. These characteristics are dictated by the long polymer chain molecular structure of the materials. One practical consequence is that considerable force is required to make a plastics melt flow into a mold or through a die. This explains why plastics processing machinery and molds must be so robust and are costly.

To understand and control melt processes, it is necessary to define the way in which melt viscosity changes with temperature and shear rate. The shear rate is a measure of how fast the melt passes through a channel or orifice. A simple fluid such as water has a constant viscosity value regardless of shear rate. This is known as Newtonian behavior in which the fluid can be described fully by a single constant - the viscosity. By contrast, the viscosity of a plastics melt at a constant temperature changes markedly as the shear rate changes. This is non-Newtonian behavior. There is no single viscosity value. The viscosity value for a plastics melt must always be related to the shear rate at which it was determined and strictly it should always be referred to as the apparent viscosity, although this qualification is usually assumed rather than explicitly stated. An important consequence follows. For a viscosity value to be truly useful in determining how a process will turn out, it should be measured at about the same shear rate experienced in the process (Table 3.1). Unfortunately, this is not true of the most popular and widely available measure of melt viscosity, the melt flow rate (MFR) or melt flow index (MFI).

Table 3.1 Process shear rate ranges

Process

Shear rate

Injection Molding

High

Blow Molding

Medium to low

Extrusion

Medium to high

Thermoforming

Medium to low

Melt flow rate testing

Low

Capillary rheometer testing

Medium to high

The melt flow rate test is performed at a low shear rate so MFR figures will be at their least inaccurate for medium to low shear rate processes like blow molding and thermoforming, and will be most inaccurate for injection molding. The quoted MFR value is the weight of polymer melt flowing through an orifice in specified conditions, so the higher the MFR value, the lower the melt viscosity and the easier the material will flow (Table 3.2).

Even though MFR values are measured at an unrealistically low shear rate, it might seem that the test would accurately rank different materials for comparative ease of flow. Unfortunately, not even this can be guaranteed because of the varying degree of shear dependency shown by different materials and grades. The MFR test owes its continued survival mostly to tradition and the fact that it is cheap and easy to perform.

Table 3.2 Approximate relationship between MFR and polypropylene injection molding conditions.

MFR range

(g/10 min)

Injection pressure range

(bar)

Melt temperature range

(°C )

<4

600-1200

240-280

4-10

500-1000

220-250

10-20

500-1000

210 -240

>20

400-1000

200-230

More reliable viscosity measurements can be made with high-shear rheometers. Material testing is performed at shear rates similar to those experienced during extrusion or injection molding, so the resulting values have a direct bearing on process considerations (Figure 3.2). Even then, it is not a simple matter to depict flow behavior. A number of viscosity "models" have been developed to describe this behavior. The simplest version is known as the power law model. More elaborate and accurate models are based on higher order versions of the power law, or on the Carreau, Cross, or Ellis models. Flow data corresponding to these models is still not widely published but can usually be obtained on request from materials suppliers. Developers of flow simulation software maintain extensive databases of plastics flow data but these are not freely accessible.

Figure 3.2 Typical viscosity curves at 260°C for some PCD polypropylene grades. Key: 1 = Daplen BHC 5003 (blow molding grade) - MFR 0.4, 2 = Daplen CF 501 (calender film grade) - MFR 1.1, 3 = Daplen FSC 1012 (block copolymer molding grade) - MFR 5.0, 4 = Daplen PT 551 (thin wall molding grade) - MFR 19.0.

A third type of flow measurement is sometimes available, although its popularity appears to be waning. Spiral flow data is an attempt to relate flow information directly to the injection molding process by using an industrial molding machine to run the tests, in conjunction with a test mold in which a very long graduated flow channel is arranged in a spiral. The disadvantage is that reproducibility between different presses and molds is low. The spiral flow behavior of a thermoplastic is characterized simply by the flow length observed under prescribed conditions of temperature, pressure, and flow rate. Flow length measurements are specific to the test conditions and cannot be extrapolated to other circumstances. For example, there is no straightforward way to relate flow data taken on a 2 mm thick test mold to a practical molding of a different thickness. However, because the test is performed at process shear rates, it will reliably rank materials for ease of flow at the test condition (Table 3.3). Practical considerations of time and cost make the test unwieldy for exposing temperature and shear dependencies (Figure 3.3).

Table 3.3 Approximate flow range of polypropylene compared with other thermoplastics.

Polymer

Approximate flow length for 2mm wall thickness (mm)

Polyamide 6/6

810

Polypropylene

250-700

LD polyethylene

550-600

Polyamide 6

400-600

PBT

250-600

HD polyethylene

200-600

Acetal

500

Polystyrene

200-500

PMMA

200-500

PET

200-500

Polyamide 12

200-500

ABS

320

PVC-U

160-250

Polycarbonate

150-220

Figure 3.3 Spiral flow length of some reinforced Hoechst polypropylenes at 750 and 1130 bar injection pressure Key: a = non-reinforced base grade, b = Hostacom M4 N01 (40 talc), c = Hostacom G2 N01 (20 glass fiber),

d = Hostacom G3 N01 (30 coupled glass fiber).

A wide spread of polymer chain lengths is an inevitable consequence of the polymerization process, and it is this that creates within any polymer a range of molecular weights. The statistical distribution of these molecular weights is known as the molecular weight distribution, or MWD. The sensitivity of polypropylene melt viscosity to shear and temperature is largely dependent on its molecular weight distribution. This distribution can be controlled to an extent so that polypropylene grades may be produced in broad or narrow molecular weight distributions (Figure 3.4). The broad MWD product will be more shear sensitive than a narrow MWD grade (Figure 3.5). This exposes another shortcoming of the melt flow rate test. Broad and narrow MWD polypropylenes can have the same melt flow rate and the same average molecular weight but still vary significantly in processing.

Figure 3.4 Comparison of broad and narrow molecular weight distributions

Figure 3.5 Comparison of shear sensitivity for broad and narrow molecular weight distributions.

Easier flowing grades of polypropylene can be produced by deliberately promoting chain scission during the production of the polymer. Chain scission involves a breaking of the polymer chains in a mechanism akin to degradation. Unlike degradation, the method results in a predictable and reproducible degree of chain scission. The result is a polypropylene with a narrower molecular weight distribution and a lower melt viscosity (Figure 3.6). The narrower molecular weight reduces the sensitivity of melt viscosity to shear, particularly at higher shear rates (Figure 3.7). Materials produced in this way are known as controlled rheology (CR) grades, or sometimes as vis-broken (VB) grades (Figure 3.8). Homopolymers, random copolymers, and block copolymers are all available in controlled rheology versions.

Figure 3.6 Effect of vis-breaking on the molecular weight distribution of polypropylene.

Figure 3.7 Effect of vis-breaking on the melt viscosity and shear sensitivity of polypropylene.

Controlled rheology grades of polypropylene are generally used for fiber and film production where the improved draw-down results in faster production rates, and for injection molding parts that are difficult to fill, or where it is important to minimize distortion or warpage (Table 3.4).

Table 3.4 Principal characteristics of controlled rheology polypropylenes

Property

Advantage

Disadvantage

Narrower molecular weight distribution

Reduced warpage

More uniform shrinkage

Improved draw-down performance

Reduced melt strength

Reduced stiffness

Less reduction in viscosity at high shear rates

Shorter polymer chains

Reduced melt viscosity

Figure 3.8 Melt viscosity behavior of controlled rheology polypropylene compared with conventional

polypropylene. Key: BASF Novolen 100N = conventional homopolymer grade, BASF Novolen 1148 RCX = controlled rheology homopolymer grade.

3.3 - GFPP

3.3.1 - Glass fibers

Glass fibers are the most widely used reinforcement in thermoplastics. They are cost-effective, and a broad range of physical properties can be achieved for a large number of applications. Glass fiber reinforcements are strands of filaments drawn to various diameters between 3.8 and 18 μm, with letter designations of B to P. The number of filaments per strand, the configuration of the strand, and the fiber length-to-weight ratio can be varied, depending on the desired properties. [13, 14]

Most plastics are reinforced with E (electrical) glass, a borosilicate glass with poor acid resistance, fair alkali resistance, good moisture resistance, and excellent electrical insulation properties. Other types of glass can provide better alkali or acid resistance (C or chemical glass), good dielectric properties (D glass), or high strength for advanced composites (S glass). Glass fibers are usually coupled or coated with sizing in order to facilitate processing, minimize fiber breakage during processing, and provide compatibility with the polymer. Sizing agents are generally proprietary organic formulations designed for the particular resin. They consist of polymers (polyvinyl acetate, polyester, epoxy, starch) that form films to hold the fibers together, amine lubricants to impart lubricity, and silane, chrome, or titanate coupling agents for polymer-fiber crosslinking. [13, 14]

Continuous and chopped strands are commonly used in polypropylene. Continuous strands are untwisted and wound onto a spool; chopped strands consist of continuous strands cut into lengths ranging from 1/8 in. (0.32 cm ) to greater than 1 in. (2.5 cm) in 1/4 in. (0.64 cm) intervals. Chopped and continuous strands are used in injection molding, at loadings of 5-30. Mats can be made from chopped or continuous strands. Polypropylene is the most common polymer used in glass mat reinforced thermoplastics (GMT). GMT polypropylene exhibits enhanced dynamic toughness, higher energy absorption upon impact, and an excellent stiffness to toughness ratio compared to short fiber reinforced polypropylene. As a result, GMT polypropylene can compete with other polymeric composites, aluminum, and steel. [15, 13, 16]

Glass fiber reinforced resins have high tensile strength, high stiffness and flexural modulus, and high heat deflection temperatures. With 40 glass fiber reinforcement, the heat deflection temperature of polypropylene at 1.82 MPa (264 psi) increases to 150°C (300°), compared to 60°C (140°F) for the unreinforced material. Impact strength decreases in glass-reinforced resins. The effects of increasing glass fiber content on flexural modulus, tensile strength, and Gardner impact strength are shown in Figure 4.6. The abrasiveness of glass fibers can damage machinery and tooling; hardened coatings on barrels, screws and tooling can minimize abrasion. [17, 18, 15, 19, 13]

Because glass fibers orient in the flow direction during injection molding, shrinkage is greatly reduced in the direction of flow; in the transverse direction, the shrinkage reduction is not as great. Distortion problems can result from the large difference in shrinkage values, and fiber reorientation can occur where two flow fronts meet, changing the direction of shrinkage. The orientation of glass fibers makes accurate predictions of shrinkage difficult and can cause warpage; wider manufacturing tolerances are required for glass fibers than for talc or glass sphere reinforcements. [17, 18,19, 13]

3.3.2 - ­Effects of Propylene Glass fiber reinforcement

Glass fibers are widely used for reinforcing polypropylene. The effect is to improve tensile strength, flexural modulus, and dimensional stability, and to raise the heat distortion temperature.

The drawbacks are reduced elongation at break, a reduction in electrical properties, and a tendency to distortion in injection moldings. Distortion arises from the flow orientation of reinforcing fibers during melt processing. The orientation process is more complex than is generally imagined, and is not uniform throughout the thickness of the part. The end result though, is that any departure from a random orientation of fibers will result in anisotropic properties (properties that vary with the flow direction). It is this variation that tends to cause distortion.

The effect of the glass fiber reinforcement can be considerably enhanced by the use of coupling agents that increase the bond between the fibers and the polypropylene matrix. The effect is to ensure a more efficient transfer of stress from the matrix to the fibers which are consequently more fully utilized. Polypropylenes reinforced with coupled glass fibers have greater stiffness and strength than uncoupled glass types (Figure 3.9, Figure 3.10, Figure 3.11).

Typical applications of glass reinforced polypropylene include automotive underhood applications and headlamp housings and washing machine components.

The length and diameter of the fiber have a bearing on the reinforcing effect. These two considerations are coupled in the aspect ratio - the ratio of fiber length to diameter. The critical aspect ratio is that at which the loaded fiber would be subject to its ultimate tensile strength. The average fiber aspect ratio is usually at least ten times greater than the critical aspect ratio. The situation is complicated by the tendency of compounding and processing to reduce the fiber length by mechanical fracture.

Studies show that fiber length after compounding and molding is almost independent of the initial length (Table 3.5). By reducing the fiber diameter, more fibers survive compounding and processing with an aspect ratio above the critical value. For semi-crystalline thermoplastics as a whole, the evidence suggests that tensile strength can be improved by 6 to 10 by reducing the fiber diameter from 13 microns to 10 microns.

Table 3.5 Effect of polypropylene processing on reinforcing glass fibers. [20]

Input strand length (mm)

Strand length after compounding (mm)

Strand length after injection molding (mm)

3.2

0.76

0.64

4.8

0.76

0.66

6.4

0.99

0.69

Greater improvements can be produced by reinforcing polypropylene with long fibers (Figure 12.38). The pellets are produced by a pultrusion technique that eliminates the fiber damage caused by conventional extrusion compounding. Because the fibers are oriented for pultrusion, it is possible to produce glass loadings up to 75 compared with a practical limit of about 50 for compounded short fibers. The fiber length before processing is effectively the same as the pellet length, and is typically 10mm to 12mm. Fiber damage and fracture does still occur in processing, but studies show that the effect is much less than had been presumed. Long-fiber reinforced polypropylene challenges for markets currently held by polyamides and other engineering plastics reinforced with short fibers. The properties are comparable while the cost and weight advantage is substantial.

Figure 3.9 Effect of 20 coupled and noncoupled glass fiber reinforcements on tensile strength

Figure 3.10 Effect of glass fiber reinforcement type and content on tensile strength of polypropylene.[1059]

Figure 3.11 Effect of glass fiber reinforcement type and content on heat deflection temperature of polypropylene. [21]

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