Polypropylene (PP)
The repeating unit for polypropylene (PP) is shown in Figure 7.8. The presence of the pendant CH3 group permits the formation of three different types of PP. These three types of molecules, called stereoisomers, differ in the way the atoms are spatially arranged about the backbone carbons. The three arrangements (stereoisomers) are illustrated in Figure 7.9, where the differences in the arrangements can be seen by focusing on the backbone carbon to which the pendant group is attached. Note that this backbone carbon has three other carbons attached to it—the carbon in the pendant group and the two adjacent backbone carbons. Carbons to which three other carbons are attached are called tertiary carbons. The other backbone carbon in the PP repeating unit is not a tertiary carbon because it has only two carbons attached to it. Carbons to which only two other carbons are attached are called secondary carbons. (The terms tertiary and secondary are introduced to assist in identifying which of the two types of backbone carbon is being referred to in the discussion.) The tertiary carbon is key in understanding stereoisomerism.
In the isotactic configuration, which is represented in Figure 7.9a, the pendant group is always attached to the tertiary carbon on the same side. This results in a very regular structure. (This arrangement can be compared to a line of people who are all facing the same direction, each holding a balloon in his/her right hand.) Isotactic comes from words meaning "same" (iso) and "hand or touch" (tactic). Another arrangement of the atoms is shown in Figure 7.9b. In this arrangement the pendant group regularly alternates from one side to the other side of the tertiary carbon. (This is like the line of people holding the balloon alternately right hand and left hand all the way down the line.) This arrangement is called syndiotactic, which implies that a set pattern exists in the arrangement. In the third arrangement of the atoms, the pendant group is attached to the tertiary carbon in a random fashion. (This is like a line of people where some have the balloon in their right hand and some have it in their left hand, but no particular order is established.) This arrangement is called atactic, which implies that there is no pattern.
The differences in properties between isotactic, syndiotactic, and atactic PP are quite pronounced and arise from the way in which the polymer molecules can pack together. Only the isotactic arrangement allows the molecules to pack tightly into crystalline structures. In the syndiotactic and atactic arrangements, the methyl pendant group (CH3) is too large to allow for tight packing and crystalline regions are not formed. Hence, isotactic PP is much more rigid and strong in comparison to the rubbery nature of syndiotactic and atactic PP. The only PP of commercial importance is the highly crystallized isotactic arrangement, and the remainder of this discussion will be limited to this type.
In order to obtain the regular arrangement of atoms required to make isotactic PP, a catalyst is used to force this arrangement during the polymerization of the polymer. Such catalysts are called stereoregular. The Ziegler-Natta catalyst used to produce HDPE is of this type. Other types of stereoregular catalysts (called high-selectivity catalysts) have now been developed and are increasingly used to produce PP, in part because the ability to control the shape and length of the polymer is even better with the new catalysts. Therefore, commercial grades of PP are made using Ziegler-Natta or some other stereoregular catalyst. Metallocene catalysts are a new group of stereoregular catalysts which have proven effective in stereospecific polymerizations of several different polymer types.
Figure 7.8 Polymer repeating unit for polypropylene.
Figure 7.9 Different types of polypropylene that depend upon the arrangement of groups attached to the carbon (stereoisomerism).
It is not surprising that PP and PE, especially HDPE, have similar properties and compete for many of the same applications. However, PP and PE differ in some important respects, and these differences have led to preferences for one or the other in various applications. PP is stiffer than PE, so in applications requiring flexibility (such as wire coating), one of the PE materials would be used. On the other hand, if greater stiffness is needed, PP is the preferred resin. This is especially true if the application also requires abrasion resistance or hardness, such as for gears, toys, automotive battery cases, and seats for stacking chairs. The resistance to environmental factors is similar for PP and PE. PP is somewhat more susceptible to UV and oxidative degradation than is PE but is more resistant to stress cracking than PE. Hence, crosslinking of PP for improved ESCR is not practiced commercially, partially because the electron beam radiation degrades the PP.
PP has a higher glass transition point and a higher melting point than PE (except for UHMWPE). This means that processing temperatures are generally higher, but it also means that service temperatures are higher. Sterilizable medical devices, dishwasher-safe food containers, and appliance parts are often made of PP for this reason. A very important property difference that has led to many applications for PP is its superior resistance to cracking from mechanical stresses. PE materials will readily blush and craze when subjected to bending, but PP will not. Applications requiring this property include carpets, ropes, strapping tape, and molded items incorporating integral hinges. (Integral hinges or living hinges are formed when the hinge of a container is formed from the same material as the container itself and is molded as one piece. The hinge is usually a region of the part that is thinner or narrower so that bending will preferentially occur in that region.) An example of a container with a living hinge is shown in Photo 7.3.
The superior stiffness of PP over PE and the low price of PP compared to the engineering plastics have led to its use in some structural applications. If additional stiffness or strength is needed, reinforcements can be added to PP. For instance, the addition of 30% short fiberglass reinforcements can double the tensile strength and impact resistance of PP. Impact modifiers can be added to further improve impact strength, especially for low-temperature applications where PP is less impact resistant than HDPE. EPDM, the copolymer of PP, PE, and a diene monomer, has improved impact properties and much greater elongation than either PP or PE. Fillers (such as calcium carbonate or talc) are often added to PP up to about 30% concentration by weight. The filled plastic has improved stiffness, mold shrinkage, and cost. Many molded automotive parts have been converted from thermoset materials to filled PP because PP can be molded into very complex shapes using fast molding cycles and still retain the dimensional stability that was previously provided by the thermoset materials.
Photo 7.3 Polypropylene container using living hinges. (In the form of a medicine container).