Polypropylene stiffness/toughness properties

Figure 12.4 Polypropylene-talc systems coveringa wide range of stiffness/toughness properties [28]. (With permission from the publisher.)...

Several approaches to compatibilizing PPE blends with commercial polyolefins (polypropylene, etc.) have been reported in the literature (Lee 1990 Kirkpatrick et al. 1989). Simultaneous compatibilization and impact modification of PPE/polypropylene blends was achieved by choosing selected types of styrene-ethylene/butylene-styrene block copolymers and PPE resin of low molecular weight (Akkapeddi and VanBuskirk 1992). A family of PPE/polypropylene alloys were commercially launched by G.E. in 2001, under the Noryl PPX trade name, and these are now sold by Sabic. Typical properties of a commercial PPE/PP blend are shown in Table 19.32. These PPE/PP blends are claimed to offer a balance of cost and performance between the TPOs and other higher-cost engineering thermoplastics such as nylons, modified PET, and PBT resins. Basically, the PPE/PP blends offer a balance of key properties stiffness, toughness, chemical, and heat resistance. 

The first step in selecting a material for a particular application is to define the properties that are essential to the performance of the component. In some cases, the requirements limit the choice to one or two materials. For example, expansion joints in some chemical installations require a combination of flexibility with resistance to extremely corrosive environments the bellows shown in Fig. 8.1 are made from PTFE, despite its high price and difficulties in processing, because no materials other than fluorocarbon polymers meet the requirements. On the other hand, several less expensive polymers have sufficient resistance to dilute sulphuric acid to make a satisfactory battery case toughened polypropylene is chosen for its combination of chemical resistance, stiffness, toughness, processability, and moderate price. 

Polypropylene polymers are typically modified with ethylene to obtain desirable properties for specific applications. Specifically, ethylene—propylene mbbers are introduced as a discrete phase in heterophasic copolymers to improve toughness and low temperature impact resistance (see Elastomers, ETHYLENE-PROPYLENE rubber). This is done by sequential polymerisation of homopolymer polypropylene and ethylene—propylene mbber in a multistage reactor process or by the extmsion compounding of ethylene—propylene mbber with a homopolymer. Addition of high density polyethylene, by polymerisation or compounding, is sometimes used to reduce stress whitening. In all cases, a superior balance of properties is obtained when the sise of the discrete mbber phase is approximately one micrometer. Examples of these polymers and their properties are shown in Table 2. Mineral fillers, such as talc or calcium carbonate, can be added to polypropylene to increase stiffness and high temperature properties, as shown in Table 3. 

Polypropylene block and graft copolymers are efficient blend compatibilizers. These materials allow the formation of alloys, for example, isotactic polypropylene with styrene-acrylonitrile polymer or polyamides, by enhancing the dispersion of incompatible polymers and improving their interfacial adhesion. Polyolefinic materials of such types afford property synergisms such as improved stiffness combined with greater toughness. 

By alternating layers of hard and soft materials, such as polystyrene and polypropylene, it is possible to achieve synergistic effects in properties, similar to polyblending. For example, a better combination of stiffness, ductility, and toughness can be obtained in polystyrene-polypropylene multilayer films than in either component alone. Such materials also display unusual optical behavior, such as total reflection, at certain wavelengths (Alfrey et al., 1969), because the films tend to reflect light in a manlier similar to the reflection of x-rays by crystals. 

The specific composition of the TPO blend produced depends on the balance of flexural modulus (stiffness) and impact toughness (drop impact and notched Izod) properties needed to meet the target performance specifications. In the formulation of TPO blends, the polypropylene is used normally as the major component, i.e., as the matrix phase, to provide the needed rigidity and thermal stability, while the elastomer dispersion provides the low-temperature impact toughness. A minor amount of a mineral filler such as talc provides additional stiffness and dimensional stability to the TPO. Hence, the levels of elastomer and mineral filler modifiers are carefully adjusted to achieve the desired balance of properties in the TPO. 

Heterophasic copolymer resins (so called because their morphology typically shows two or more phases) have lower stiffness and improved toughness at low temperature, down to -40°C (depending on the dispersed phase type and amount). These resins often demonstrate more complex thermal behavior (e.g., two or more melting points and reduced stiffness at elevated temperature). You can find examples of typical grades of polypropylene resins in Table 1.7. Chapter 2 describes propylene structure-property relationships that suit a variety of end-use applications. 

Particulate filled polypropylene (PP) is widely used in many fields of application. The major advantage of the introduction of fillers into PP is the higher stiffness of the composites, which is especially important in applications at higher temperatures. Increased stiffness is usually accompanied by decreased impact resistance, which is not always acceptable. The selection of an appropriate PP copolymer or the incorporation of additional elastomer may lead to the often desired simultaneous enhancement of stiffness and toughness. Incorporation of a filler into the polymer changes all properties at the same time, therefore an optimization of properties is required during development. The major benefits of modified PP are versatility and an exceptional price/volume/ performance ratio. 

Polypropylene has a rather high melting point, low density, good stiffness and toughness. These properties depend upon the degree of crystallinity and type and level of comonomer incorporated within the product. Polypropylene products may be compounded with rubber to modify their low temperature properties or with mineral fillers or glass fibres to increase stiffness and dimensional stability. 

P-phase within crystalline portion shows strong differences in mechanical properties as compared to common a-polypropylene, namely higher toughness and drawability, but lower stiffness and strength [7,8]. 

Polypropylene (PP) with predominant amount of P-phase within the crystalline portion of the material, i.e. P-nucleated polypropylene (P-PP), has been rated as an effective alternative to the common PP (a-PP) due to its specific properties. Material containing trigonal p-phase possesses significantly higher toughness and drawability, however, lower stiffness as compared to material with predominant monoclinic a-phase [1-4]. Excepting the physical properties, the supermolecular stracture plays also an important role in the process of degradation [5-8]. 

Thermoplastic polyolefin (TPO) is composed of polypropylene (PP) and elastomer. Blending elastomers with PP improves mechanical properties such as toughness, impact, and compression properties. However, this sacrifices the stiffness of the material. Thus, mineral-filler is used in TPO formrrlation to improves the stiffness and reduce the cost of the materials [11]. The TPO material used in this study, which composed of a specific mineral filler, is called the DTPO. 

---