Polypropylene, rubbery phase

The preferred average particle size 1n HIPS was believed to be 0.8 ijm (J.). However, our current data indicate that a number average particle diameter of 1.05 urn and 0.5 -urn appear to be a preferred size for HIPS and rubber-toughened polypropylene (PP), respectively. The morphology of the rubbery phase in a toughened PP appears to be less complex, as evidenced in Figure 2 where the dark, also osmium-stained, phase is the styrene-butadiene rubber (SBR) particles. No PP occlusions were found in this material since it is a physical blend of SBR and PP. 

Figure 2. Rubbery phase morphology of a toughened polypropylene.

As noted, TPEs are either block copolymers or combinations of a rubber-dispersed phase and a plastic continuous matrix. The attribute contributed by the rubbery phase - such as butadiene or ethylenebutylene in an S-E-S or SEB-S styrenic block copolymer, or the completely vulcanized EPDM rubber particles in a polypropylene (PP)/EPDM EA - is classical elastomeric performance. The elastic properties of a rubber result from long, flexible molecules that are coiled in a random manner. When the molecules are stretched, they uncoil and have a more specific geometry than the coiled molecules. The uncoiled molecules have lower entropy because of the more restricted geometry and, since the natural tendency is an increase in entropy, the entropic driving force is for the molecules to retract. 

The comparatively low impact strength of many well-known polymers, such as PMMA, polystyrene and PVC, led to the production of rubber-modified thermoplastics with high impact strength. The best-known examples are high-impact polystyrene (HIPS) and ABS copolymer, where the rubbery phase is dispersed throughout the polymer in the form of small aggregates or balls. Other polymers that have been toughened in this way include PMMA, PVC, polypropylene, polycarbonate, nylons and thermosets such as epoxies, polyesters and polyimides. 

It is somewhat difficult conceptually to explain the recoverable high elasticity of these materials in terms of flexible polymer chains cross-linked into an open network structure as commonly envisaged for conventionally vulcanised rubbers. It is probably better to consider the deformation behaviour on a macro, rather than molecular, scale. One such model would envisage a three-dimensional mesh of polypropylene with elastomeric domains embedded within. On application of a stress both the open network of the hard phase and the elastomeric domains will be capable of deformation. On release of the stress, the cross-linked rubbery domains will try to recover their original shape and hence result in recovery from deformation of the blended object. 

In this section we will discuss the molecular structure of this polymer based on our results mainly from the solid-state 13C NMR, paying particular attention to the phase structure [24]. This polymer has somewhat different character when compared to the crystalline polymers such as polyethylene and poly(tetrameth-ylene) oxide discussed previously. Isotactic polypropylene has a helical molecular chain conformation as the most stable conformation and its amorphous component is in a glassy state at room temperature, while the most stable molecular chain conformation of the polymers examined in the previous sections is planar zig-zag form and their amorphous phase is in the rubbery state at room temperature. This difference will reflect on their phase structure. 

On the other hand, in the solid-state high resolution 13C NMR, elementary line shape of each phase could be plausibly determined using magnetic relaxation phenomenon generally for crystalline polymers. When the amorphous phase is in a glassy state, such as isotactic or syndiotactic polypropylene at room temperature, the determination of the elementary line shapes of the amorphous and crystalline-amorphous interphases was not so easy because of the very broad line width of both the elementary line shapes. However, the line-decomposition analysis could plausibly be carried out referring to that at higher temperatures where the amorphous phase is in the rubbery state. Thus, the component analysis of the spectrum could be performed and the information about each phase structure such as the mass fraction, molecular conformation and mobility could be obtained for various polymers, whose character differs widely. 

Polypropylene homopolymer (PP) is a widely used thermoplastic material, despite its brittle behaviour at either low temperature or high loading rates. Improvement in the fi acture toughness of PP can be achieved by either modifying the crystalline structure, or addition of a second phase material [16], The toughening effect and mechanisms of different second phase materials such as stiff fibres, soft rubbery inclusions (EPR, EPDM), and some mineral fillers have been analysed. Recent developments concern the effect of hybrid system consisting of rigid and rubbery inclusions. 

Accurate description of barrier films and complex barrier structures, of course, requires information about the composition and partial pressure dependence of penetrant permeabilities in each of the constituent materials in the barrier structure. As illustrated in Fig. 2 (a-d), depending upon the penetrant and polymer considered, the permeability may be a function of the partial pressure of the penetrant in contact with the barrier layer (15). For gases at low and intermediate pressures, behaviors shown in Fig. 2a-c are most common. The constant permeability in Fig.2a is seen for many fixed gases in rubbery polymers, while the response in Fig. 2b is typical of a simple plasticizing response for a more soluble penetrant in a rubbery polymer. Polyethylene and polypropylene containers are expected to show upwardly inflecting permeability responses like that in Fig. 2b as the penetrant activity in a vapor or liquid phase increases for strongly interacting flavor or aroma components such as d-limonene which are present in fruit juices. 

If the modulus of a dispersed rubbery component is only 1/1000 that of the surrounding rigid matrix, an air-filled structure is approximated. The two experimental points for foamed polypropylene in Figure 1 fit fairly well into the theoretical curves. It is apparent from Figure 1 that the modulus of a composite of known volume composition reflects clearly the phase distribution, i.e.y it indicates which component forms the continuous phase. The moduli may differ up to two decades after reversal of the continuous and the dispersed phases when the ratio of the component moduli is about 1000 1. 

Other polymers whose phase structure has been studied are polypropylene, poly(tetrahydrofuran), and polyurethanes. The detection of four components in the wide-line spectrum of polypropylene fibres was claimed, the narrowest being attributed to absorbed water. In the study on poly(tetrahydrofuran), the molecular orientation in drawn and rolled sheets was examined. The anisotropy of the H wide-line spectra were well-explained by uniaxiai and double orientations of crystallites. Assink observed two-component free induction decays in polyurethanes at room temperature, attributed to glassy and rubbery constituents. Immediately after quenching from 170 °C, a linear polyurethane showed a continuum of domain compositions, but after a few hours, the original two-phase structure was re-established. 

Minor phase rubbery olefinic polymer Matrix polypropylene... 

The Borstar PP process is based on the Borstar PE process. When homopolymers and random copolymers are produced, the reactor configuration consists of a propylene bulk loop reactor and a fluidized bed gas phase reactor operated in series. During hetero-phasic copolymer production, the polymer from the first gas phase reactor is transferred into a second smaller gas phase reactor where the rubbery copolymer is made. For bimodal rubber production a further gas phase reactor is necessary. Such a configuration allows for the production of polypropylenes with outstanding product properties. 

Figure 5.73. Phase images of polypropylene/ethylene propylene rubber (PP/EPR) blends of different compositions 85/15 wt.% (A), 70/30 wt.% (B), and 40/60 wt.% (C). The regions of dark contrast in the images are the rubbery EPR phase, and the PP exhibits bright contrast showing the change in morphology as expected with changes in concentration. (From Bar and Meyers [170] used with permission of the MRS Bulletin.)...

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