The synthetic resin formed by the polymerization of propylene as the sole monomer is called polypropylene. While "polypropylene" has been used from time to time in the art to include a copolymer of propylene and a minor amount of another monomer, such as ethylene, the term is not so used herein.
The polypropylene of commerce is a normally solid, predominantly isotactic, semi-crystalline, thermoplastic polymer mixture formed by the polymerization of propylene by Ziegler-Natta catalysis. In such catalysis the catalyst is formed from an organic compound of a metal of Groups I-III of the Periodic Table, such as an aluminum alkyl, and a compound of a transition metal of Groups IV-VIII of the Periodic Table, such as a titanium halide. Typically the crystallinity of polypropylene thus produced is about 60% as measured by X-ray diffraction. As used herein, "semi-crystalline" means a crystallinity of at least about 5-10% as measured by X-ray diffraction. Also, the typical weight average molecular weight (Mw) of the normally solid polypropylene of commerce is 100,000-4,000,000, while the typical number average molecular weight (Mn) thereof is 25,000-600,000. The typical molecular weight distribution or polydispersity is about from 4 to 7. The typical melt flow rate (MFR) of most of the normally solid polypropylenes of commerce as-polymerized has ranged from less than 1 to about 20 dg/min as determined by ASTM D 1238. Most commonly, the melt flow rate is about 4 dg/min. Over about the last five years, polypropylenes having a melt flow rate, as-polymerized, of about 20 dg/min have become available. In the last half of 1986, a normally solid polypropylene was made commercially available having an as-polymerized MFR of from about 55 to about 430 dg/min, and weight average and number average molecular weight such that its polydispersity is about 4-5.
Fiber-reinforced polypropylene has been made by incorporating short fibers in as-polymerized, polypropylene having a MFR of up to 20 dg/min. Typically the amount of fiber has been 20-30% by weight of the total composition. A few have been made with 40% fiber content. As the amount of fiber reinforcement in this polypropylene is increased, the degree of warpage increases since the addition of the fiber leads to a decrease in the MFR, thereby making it necessary to increase the temperature and pressure used during the preparation of the fiber-reinforced product. This increased stress in the preparation process leads to the warpage problems. Hence, it is not surprising that heretofore fiber-reinforced polypropylenes with a 40% fiber content have been particularly difficult to mold.
Fiber mat-reinforced polypropylenes wherein the polypropylene is an as-polymerized typical MFR polypropylene and wherein a glass fiber mat is used as the fiber-reinforcing agent also are known. Such products are typically prepared by compression molding the glass mat with the polypropylene. However, these fiber-reinforced products cannot be used in injection molding processes since the glass mat does not flow.
Since the molecular weight distribution of the polypropylene of commerce having a MFR of up to 20 dg/min cannot be readily controlled in the reactor, and since the as-polymerized polypropylene produced cannot be spun into a fiber successfully in all fiber manufacturing processes, methods of visbreaking the polymer were developed to narrow the molecular weight distribution of the as-polymerized polypropylene and to increase its MFR. The most common method used today is to treat these as-polymerized polypropylenes of commerce with a free radical initiator, such as a peroxide, thereby initiating a free radical reaction. The narrower molecular weight distribution and higher MFR are a result of chain scissioning during the free radical reaction. The more chain scissioning which occurs under the particular reaction conditions, the narrower the molecular weight distribution and the higher the MFR of the resultant polypropylene. Polypropylenes having, as a result of visbreaking, a narrow molecular weight distribution, i.e., a polydispersity typically of about 2 to 3, and a high MFR, i.e., up to about 1000 dg/min, are known.
Unfortunately, the visbreaking process adversely affects the physical properties of the as-polymerized polypropylene starting material. This adverse effect increases as the extent of visbreaking is increased. Thus, the higher the MFR of a visbroken polypropylene, the more diminished are its physical properties so that it is generally weaker and less stiff than the as-polymerized polypropylene from which it has been prepared.
Visbroken polypropylene having a MFR of 50 dg/min or more is not practical for use in the preparation of fiber-reinforced polypropylene since it is tedious and difficult to take the polymer through the visbreaking operation as well as subsequent stranding and pelletizing operations which are required before the visbroken material can be compounded with fibers.