Faced with an abundance of materials to choose from, plastics industries such as medical and food packaging are reaping the benefits of polypropylene (PP). Typically available at a lower cost, PP boasts clarity and chemical and impact resistance that contend with more expensive resins.

Produced by the polymerization of propylene monomer with or without co-monomers, PP has the lowest density of all the commodity thermoplastics and is available in a range of molecular weights depending on the production process and comonomer used. Due to PP's low density and impact/stiffness balance, finished parts often require less raw material than other competitive resins, making it one of the most cost-effective commodity plastics available.

There are three basic types of PP: homopolymer or unmodified general purpose PP, random copolymer and impact or rubber modified PP. The homopolymer PP possesses the highest rigidity while random copolymers provide transparency but have limited impact or toughness properties. Impact grades are not clear, but have impact resistance.

Seeking the expertise of leading manufacturers, distributors and fabricators, Plastics Machining & Fabricating asked about the following:

1. What advantages/disadvantages does polypropylene have compared to other leading thermoplastics?

2. What are some of the most common and unique uses for polypropylene?

3. What developments do you foresee in the next decade for polypropylene?

Polypropylene Properties: Advantages/Disadvantages
"Probably the most outstanding feature of polypropylene, and the one that leads to the high usage level and high activity in development, is low cost," said George Abd, vice president of compounding for Spartech Polycom in Washington, PA. "PP is a relatively simple molecule which is synthesized directly from abundant petroleum feedstocks. The low base cost has spurred investment in developing PP for new markets, and in developing new synthesis methods and modified forms."

"Polypropylene has an excellent balance of mechanical properties, melt flow, color stability and moisture barrier properties," said Sue Greer, director of order fulfillment at Witt Plastics in Greenville, OH.

PP's proportion mixes thermal, chemical and electrical properties with moderate strength. Because it has such a hard, high-gloss surface, it is ideally suited to environments where there is concern for bacteria build-up or build-up that can interfere with flow. "It is these properties that make it appealing for industries such as food, medical and beauty aids," said Greer. "These properties differentiate polypropylene from other thermoforming resins, making it unlimited in potential end uses."

"The single property where PP outperforms almost all other materials is chemical resistance," added Abd. "The saturated olefinic chains yield resistance to most oils and solvents, as well as water-based chemicals, soaps, and moderate acids and bases. Few other materials with the strength properties of PP, and certainly none in the same price range, can match the chemical resistance of PP."

The material's chemical resistance also defends against aqueous, salts or alkaline solutions.

PP features good insulating properties, but dielectric strength shows some degree of change at elevated temperatures. Excellent track, arc resistance and dielectric strength allow PP to be used extensively in electrical applications. It can also be modified to be conductive or antistatic.

As a homopolymer, PP can be used in temperatures ranging from 30F to 210F, depending on specific chemistry; copolymers in the range from -20F to 180F.

Abd lists PP's higher melting point (330F) as one of its advantages as compared to other thermoplastics such as ABS, polyethylene and polystyrene. Other benefits include: a lower cost per volume and lower specific gravity, and good fatigue resistance. "PP is useful at a wide variety of molecular weights, to allow proper viscosity for blow molding and sheet extrusion, right up to high-speed injection molding," he said.

Adding to the list of PP's benefits, Wayne Tomko, sales manager of S&L Plastics in Nazareth, PA, said, "In injection molding, it produces high-quality parts at high speed and it is the only low-cost resin capable of producing living hinges."

The material does have its disadvantages, though. Abd pointed out such flaws as its notch sensitivity in impact properties, which is a tendency to crack or split easily in areas with small radii. PP's clarity is poorer than amorphous resins such as polystyrene, polycarbonate or PET. Its chemical resistance also makes bonding difficult as well as decorating with paint, labels and ink. A low-melt viscosity yields a narrow processing window for thermoforming and blow molding and it tends to warp and shrink more than most other materials due to high crystallinity.

According to Tomko, PP's low temperature impact strength is poor as is its resistance to UV. "Mold-making tolerances must be closer because of its characteristic of flashing into small clearances," he added.

Applications
"Polypropylene is commonly spun into fibers, injection molded, cast into thin films, extruded into sheet and profiles and blow molded," said Abd. "Of these, the fiber market accounts for the largest share of polypropylene usage. Polypropylene has a high tensile strength when the molecules are oriented, and a relatively low melt strength; these are the primary requirements for fiber markets.

"The combination of excellent chemical resistance, a good balance of physical properties, and low cost makes PP an attractive material for automotive applications. With a melting point of 330F, PP can be reinforced for use in underhood applications, where temperature resistance is crucial. These include air duct work and covers for batteries and other components," he said.

Greer credits PP's clarity, microwave and freezer properties, impact properties, and FDA-approved status for its widespread use in the food, medical and beauty aid industries. She cites such applications as trays, cups, lids, clamshell packaging, sterilization trays, single-dose medication and other product packaging.

"What makes polypropylene unique is its potential to replace higher-priced, clear PVC and PETG applications," said Greer.

Abd added, "One of the most overlooked applications for PP is thin film, which is often oriented or biaxally oriented. These films are used in items such as candy wrappers and cigarette package wrappers. The extreme chemical resistance of PP leads to another unique application -- landfill liners and impoundment caps. Here, very thick extruded sheets are used to prevent landfill chemicals from leaching into the environment."

Other applications for molded PP include housewares, rigid packaging containers, toys, disposable medical syringes, video cassette cases, appliance housing/outdoor furniture and luggage. Non-woven applications include insulation wrap, disposable diapers, automotive interiors and medical textiles.

Growing Trends
The future of all thermoplastic resins mainly depends on what goes into them. Advancements in technology and additives allow the materials to strengthen their properties and boast features they did not have before. As Greer puts it, "Newer polypropylene resins are being developed for thermoforming that offer improved clarity and better processing windows."

Abd said he also foresees improvements in PP's future. "New catalyst technology will lead to the commercialization of PP materials with less low molecular weight material and fewer molecular 'mistakes.' This PP will have improvements in all properties, notably improved stiffness and heat resistance, and better taste and odor characteristics. The new catalysts will also lead to production of syndiotactic polypropylene, which has a higher melting point than isotactic PP.

"Novel combinations of different resins will also be possible, such as grafted PP and polystyrene. These improvements will broaden the reach of PP into new applications while retaining the cost advantages," he added.

John Dellevigne, president of HPG International in Somerset, NJ, added improved fire retardancy, impact modifications and foaming capabilities to the list of future possibilities for PP.

As PP looks to fortify itself, the products which utilize the material look to cash in on the newfound benefits. Obviously, the better PP gets, the more industries and applications it will attract.

"Applications served by the above improvements will be many," said Abd. "The reach of PP into packaging has just begun. This area may see the quickest growth."

Tomko took this idea a step further, saying, "Since there are many grades and types of PP, the range of possible products that could be manufactured from it are infinite." He went on to cite several grades of PP such as calcium carbonate filled, homopolymer, copolymer, clarified and talc-filled.

"Polypropylene use will continue to grow in thin-wall packaging, particularly in thermoformed containers," Greer said. "As the clarity of thermoformed polypropylene gets closer to that of polystyrene and PET, it will become the material of choice for most deli-type containers and lids because of its resistance to fats and oils and microwaveability.

"Also, new copolymer polypropylene resins are being developed for thermoforming that will challenge HDPE and injection-molded polypropylene for refrigerated dairy containers," she added.

Plastics Machining & Fabricating
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