Extending the properties of polypropylene(PP) of reinforcing it with glass, minerals, and natural fibers is critical performance-enhancing option. But the effort remains complex, with numerous cross-currents at work, buffeting the PP compounder and end-user alike.

PP is often described as "a least fussy resin" in terms of its willingness to mate with shorter long-glass fibers, talc, mica, and calcium carbonate particles, or the fast-expanding range of natural fibers (other than wood flour) now being explored or employed in many roles. PP's versatility is vastly extended by today's diverse extender-reinforces. For instance, special compounders draw on talc and CaCO³ to elevate stiffness, increase dimensional stability and hike thermal resistance in bumper fascia, electrical appliances and dishwashers. Reinforced PP is a key to replacing ABS, filled nylon, thermosets, and metals.

But the most momentous development in PP compounds is a novel group of base resins. They are high-crystallinity PP, nucleated PP, and metallocence PP, all enhancers of PP's conventional properties that allow typical filler levels to be cut by as much as 50%. The trend to these grades is strongest in Europe mostly in appliances and bumper fascia.

Fillers however, impose considerable penalties. Mineral fillers, for example, typically have over twice the density of neat PP, working against the grain in automotive markets, where emphasis is on lightweighting. In interior trim, deficiencies in gloss, odor, and scratch-resistance are linked to talcs.
As a counterpoint, compounding tools that stretch property envelopes are becoming readily available for PP. A notable one is long-glass-reinforced PP, which boosts toughness and stiffness of PP to unprecedented levels. Thus, LNP Engineering Plastics,Exton, PA, claims its long-glass Verton MFX structural PP offers typical notched Izod values of 5 to 6 ft-lb/in, plus a flexural modulus of 1 million to 1.5 million psi. Grades have a density 15% lower than that of glass-filled nylon.
Further, long-glass PP can be molded into parts using less complex and less costly process than the ones used for glass-mat-reinforced PP. Potential downsides are glass-fiber orientation, part distortion, surface quality debits, and a cost about double that of short-glass PP (and equal to that of short-glass nylon.)

Compounders have always used fillers to improve PP. The appliance and auto industries are inveterate users, along with oriented PP film extruders and thermoformers of microwave trays. Presently building panels are increasingly using talc filled PP in place of PVC in profiles, especially in Europe.

Estimates are that 25% of all PP used globally has a filler or reinforcer of some kind. Kline & Co., a Little Falls, NJ, consultant, pegs U.S. use of mineral reinforcers alone at 190,000 CaCO³ (30%). Underlying demand growth is estimated at 5% yr. Some 70,000 tonnes/yr of glass fiber is used as a reinforcer globally, with Europe using about half of that.

Bumper fascia are evidence of the role of today's novel PPs. Superior inherent stiffness and toughness lead to that reduced loadings of talc can achieve given levels of stiffness. Or a given loading can be translated into thinwalled parts, thus reducing costs and the inevitable tradeoffs (e.g., weight) linked to filler use. Montell and other high-crystallinity PP is now widely used in Europe's thinwall instrument panels and the trend is reaching the U.S. Europe's use of mineral fillers in PP is changing due to "new degrees of freedom" realized by high-crystallinity PP. Over the past five years, typically talc loadings have fallen about 50% to their present levels of around 12% for instrument panels (IP) and 8 to 10% for inner door panels where benefits include lower weight, better scratch resistance, and reduced cost.

Borealis has developed "multispot" technology for interior auto trim. This achieves a fabric-like effect by tailoring minerals and loading levels. An early application is in the Opel Astra. Talc-filled PP with a controlled coefficient of expansion is being used in a Class A bumper fascia for Mercedes-Benz. Borealis as well as Targor have launched scratch resistant PP compounds aimed at interior auto parts. Metallocene grades are being sampled as a potential tool for elevating stiffness and toughness a notch beyond that for high-crystallinity PP. But metallocene-PP's role in durables is unclear, given that its transparency appears irrelevant in target markets, while cost premiums are high.

Another promising approach in fillers is nanoparticles, or sub-micron clay particles that when dispersed well can beef up a composite's toughness, barrier and other properties. Montell and General Motors are working to apply these in autos, but skeptics view this as a long-term hope, due to the high cost of nanocomposites. Others say non-polar PP may be more resistant to efficient use of these particles than nylons (the focus of most development work).

Reinforced PPs have long played major roles in electrical appliances and white goods. While noting that talc loadings are down due to use of novel PP, mineral-filled grades are crucial to ensure stiffness, dimensional stability, and opacity.

Complex, load bearing parts like washing machine tubs, gears, and bearings are in rapid transition from metals to reinforced PPs, which offer excellent chemical resistance and facilitate parts integration. Europe's market, is divided into tiers: Chopped glass PP is used in the high-spin tub models (800 to 1500 rpm), while talc-filled PP is favored in lower-end (under 800 rpm) models. Visible parts like detergent trays, sieves, and fixtures use mostly talc filled PP. In the predominant top-loading washing machines in Europe, PP tends to discolor in visible parts, with some dire effects. In response, both Targor and Borealis have launched grades highly resistant to attack by detergents in hot water.

Europe is the leader in converting washing-machine tubs to short-glass-reinforced PP. That same trend is now apparent in the U. S. An advantage is gained in the alloying of multimineral and glass packages in PP as a way to balance cost with required strength or shrinkage levels. Ferro's Gapex reinforced PPs are also being tailored to reduce shrinkage, so they closely match rates of ABS, PC, and PC/ABS. As a result, existing molds can be used to run these PPs. The refrigerator door handle as a classic illustration of the "diversity of materials and processes" being trapped by PP compounders today. In the part, four top U.S. manufacturers use chopped-glass PP in coinjection (20% glass), gas-assist (10% glass) and foam.

Color matching is critical in small appliances, automotive, and other uses, and more complex when a reinforced PP is used. Case in point: the base color of talcs can vary widely between deposits and even within a deposit; producers have to use sophisticated blending to consistently match color. But Ferro, Hanna, A. Schulman Akron, OH, and others are drawing on extensive color expertise to supply solutions.

A decade ago, the favored practice of processors and OEMs was to buy pre-colored PP compounds. This obviated difficulties in handling and inventory of the low-bulk-density powders, but it imposed a considerable cost premium. In these cost- driven times, self-coloring of filled PPs (color concentrate fed directly at the extruder) has become an imperative for many large end-users (e.g., OEMS) willing to invest in necessary feeding equipment. M. A. Hanna's Max-batch line of filled and colored masterbatches, for instance, makes this feasible, while its Controlled Color Natural compounds are aimed at automotive injection molders, physical properties, and reduced inventory. Reinforced PP grades from M.A. Hanna are used by Nailite, Chicago, IL, to mold UV-stable architectural panels, and Crotty Corp., Quincy, MA, to ensure consistent color in sunroof visors.

For feeding low-aspect-ratio fillers (talc and CaCO³ ) directly, Hanna has a new masterbatch technology using letdown techniques comparable to those used to introduce color at the press. This allows end-users to buy lower-cost, unfilled base resin instead of a press-ready compound says Larry Stover, Hanna's technology vp. Upto 80% filler content is possible; cost savings are put at 5 to 20%.

Mineral fillers are improving dramatically, as finer, whiter, easier-to-color minerals appear, and chemically coupled, functionalized reinforcers are coming into play too. High-brightness talcs from China are used in both auto and appliance parts where color match is vital. Luzenac's Cimpact 610C and 710C grades, the first of these in concentrated forms, are said to improve handling, workplace safety, and dispersion. These realize cost saving: the 1 to 2¢/lb higher cost is outweighed by 20 to 40% higher compounding rate.

Odor is a growing issue for talcs in autos and also in interior and hot-air ducting parts. But a new Luzenac coating technology is said to thermally stabilize talc particles, thus greatly reducing potential odor.

Specialty Minerals, Bethlehem, PA, reports that calcined talcs offer an improvement in scratch resistance viz-a-viz conventional ones, and are most often used in blends to keep cost down. The launch of Microtuff AGD, a family of densified fine talcs that beef up thermal performance and appearance, are easy to handle, and boost compounding rates by up to 300%.

Long-glass-reinforced PP has already made inroads in performance roles against engineering resins globally. Current uses are auto mirror brackets, battery supports and truck cooling fans. Industrial applications include power tool housings, vibration absorbers for snowboards and mountain bikes as well as loudspeaker cones.

A benefit noted by LNP is its ability to harmonize a materials system when supporting new design. A tool maker in East Greenwich, RI, has developed a coil-fed screw gun that allows 167 screws to be installed automatically. This taps not only LNP's glass-fiber PP for housing and adapter, which yields weight savings and cost reduction, but also its lubricated, reinforced nylon, optimizes mating with the PP part. Growth for LNP's materials is pegged at 30% /yr in Europe and about 12% in the U.S. In automotive, shift to larger parts and integrated modules is favorable to long glass PP. Findlen cites front-end assemblies, IPs, and loadbearing floor systems. A recent addition to LNP's line includes a "cosmetic black" grade, MSX UV Black, a 30% glass grade for exterior (weatherable) door handles. Breakthroughs have occurred in combining long-glass PP with variants of injection molding. One is injection compression molding, a gentle process that retains the length and integrity of long glass fibers. Another new approach is structural foam mold in which structures of 10 to 35% glass PP and a 65 to 90% endothermic foam core are made. And gas-assist techniques are being applied in glass-reinforced PP.

After prevailing in a legal fight to practice long-glass reinforcement,RTP Co., Winona, MI, is reaffirming its role. The company supplies Europe directly from Beaune, France. RTP's grades are "excellent replacements for metal, particularly diecast parts where costly or time-consuming secondary steps are used," says the company. RTP puts emphasis on chemical coupling of long glass in its RTP 100CC series. Tensile strength of 30% coupled-glass in said to top an unmodified PP by 180%, and a conventional glass-reinforced PP by 50%.

Appryl Composites, a venture of Atochem and Multibase, St-Laurent DuPont, France, focuses Prylex long-glass fiber PP on automotive use. It supplies grades from a 4000-ton/yr facility in Dieuze, France, for use in bumper beam battery support, and non-visible interior parts. Appryl has a long-fiber (12-mm) grade for visible interior trim, countering a prior handicap. Typically, long fibers tend to migrate to surfaces, impairing appearance. A novel additive, however, distributes fibers at the core rather than at the surface, holding them. Appryl's cites a success. Initially, the interior storage bin frames in the Renault Scenic used short-glass fiber PP with carpet overlay, a costly approach. But using long-glass PP allowed molding of the part without the carpet.

Conversion from glass-mat-reinforced PP to long-glass PP is being witnessed in Europe, being driven by better overall economics, plus weight reduction and parts integration. Growth could be pegged at 15 to 20%/yr. Shortage-glass PP, meanwhile has good prospects too, according to Montell, which offers the material. Its cost is about half that of long-glass PP. Montell has introduced a blow moldable version, using one of its high-melt-strength PPs as the base resin.

Natural fiber use in Europe is driven by environmental considerations and to a lesser extent its lower density viz-a-viz talcs, says Targor's Sondern. He does note deficits for jute, flax, kenaf (hibiscus), and other natural fibers: they tend to degrade at or near processing temperatures typical for pp, and they absorb moisture. A study of the potential of wheat and rice straw states availability and cost of crop residues are favorable. Their use of fillers has a special appeal in the developing regions, such as China and Africa. A potential use is in building panels.