Structural plastic with reinforcement has enhanced properties

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Polymer properties get enhanced when reinforced with mineral fillers and fibers. Ground minerals or wood fillers such as chalk, china clay, talcum and wood flour, etc in use for a very long time have a moderate reinforcing effect, but the processing is easy and the cost is generally low. From a recent development, nanofillers and Carbon NanoTubes (CNT) with nanometric sizes and high aspect ratios lead to a high reinforcing effect at a low level but the processing, a key factor for a good efficiency, is difficult.
Using fiber is a well-established technique; the most popular being glass fiber broadly used under various forms. The forms could vary as chopped ones dispersed into the matrix or continuous ones well ordered to customize the reinforcement. Carbon and agamid fibers are also used for more sophisticated applications. Natural fibers are developing for economical and environmental reasons. Cores of composites are diversified as wood or plywood flat pieces, polymer foams flat or shaped forms and honeycombs.
Reinforcement of plastics and polymer composites favour:

		Design freedom of structural parts of all shapes and sizes economically unfeasible with metals or wood.
		Weight reduction thanks to the low density and the opportunity to design ribs and other reinforcing elements. That leads to fuel savings in automotive and transportation sectors, labour and handling savings in building and civil engineering� reducing pollution
		Possibilities of selective reinforcement in the direction of the stresses by selecting particular composites or by part drawing.
		Faster manufacturing of increasingly complex and robust products

Structural is a fuzzy notion that must be interpreted according to the context. To be simple one can say that a structural component, by opposition to a decorative one, must support or contribute to support the various forces exerted on the part or device where it is involved or connected. It must also resist the environment to assume a "normal" lifetime. Most polymers can satisfy this definition according to the examined part and its application. For example:

		Foams, although they have very low modulus and strength, are involved in structural applications such as cores for composites or insulation. When used for tank or flooring insulation of warehouses or depots, they partially support the weight of full tanks or of heavy trucks or stored goods.
		Talc reinforced polypropylene with modest strength and modulus is broadly used for car bumpers.
		Glass fiber reinforced unsaturated polyester composites are used for boats Unidirectional composites with high strength and modulus are used for radomes, bridge reinforcements, giant silos or high-pressure tanks etc.
		Honeycomb reinforced composites are used for aeronautic structural elements. So, a new honeycomb made from a polyamide paper, Kevlar by DuPont, will be used in the new Airbus A380 to manufacture a variety of composites for flooring, interior walls, cabin racks, wing flaps, radomes and leading edges. Weight for weight, Kevlar is claimed five times stronger than steel and used as honeycomb core allows a super structural integrity while enabling weight savings of 30 to 50%.

Plastics and polymer composites compete with numerous metals, mainly aluminium, light weight and die-cast alloys, steel, and also stainless steel, titanium, special steels, copper, brass, tin, etc. and conventional materials such as wood, concrete, ceramic. All the markets are more or less penetrated: appliances, automotive, building, electricity & electronics, health & care, industry, medical & surgical sectors, office automation & communication, packaging, sports & leisure.
Among the myriad of parts or products let us randomly quote: absorbers, air-intakes, airbags, aquariums, armours, barriers, bars, bearings, blades, boards, boats, boots, bows, bumpers, car bodies, cartridges, circuit boards, cosmetic packaging, couplings, covers, cups, dishwasher components, disposable products, doors, enclosures, fenders, fittings, flooring, foamed cores, frames, gears, grilles, handles, hatches, headlights, housing, impellers, jaws, joints, knees, knives, lids, lift components, lighting, mallets, manifolds, masks, mountings, panels, pans, plates, ponds, powertrain components, pump parts, radiators, rods, rollers, seats, shafts, sheaths, skis, stairs, sunglasses, switches, tanks, trims, tubs, tumbler components, windows, wings�

The most frequent improvements concern the rigidity, the strength and the impact behaviour. Unfortunately, if rigidity is improved by fillers and reinforcements, impact behaviour can be lowered and inversely, if impact strength is enhanced rigidity can be weakened. A comparison of Modulus and Strength Range according to Fillers and Reinforcements schematically delimits a zone of general correspondence between modulus and strength: both values rise when the reinforcement shape factor increases in the following order ground fillers, short fibers (SF), "long" fibers (LF), semi- and continuous fibers and other composites.
Collateral effects are generally beneficial concerning:

		Improvement of the HDT
		Lower tendency to creep under continuous loading
		Possibilities of cost saving by decreasing the material cost used to obtain the same stiffening
		Improvement of thermal conductivity
		Reduction of the coefficient of thermal expansion
		Reduction of shrinkage

The infinite set of solutions obeys general rules managing the reinforcement level that depends on:

		The reduction of the basic size of the additive: nanometric fillers are more efficient than micrometric ones if mixing is efficient
		The shape ratio or length/diameter ratio. Efficiency increases in the following order: short, long, semi- and continuous fibers
		The arrangement of the reinforcements in the main direction of stresses
		The nature of the filler or reinforcement
		The surface characteristics of the additive
		The compatibility or binding of the reinforcement and the matrix

Reinforcement with mineral fillers is often limited. A few properties are improved and others can be altered but the cost is generally significantly decreased and the processing is easy. The amount of filler can be limited by the degradation of some properties, impact strength. For mineral and glass bead filled thermoplastics versus neat and glass fiber reinforced ones, generally:

		Tensile strength decreases below that of neat polymer
		Elongation at break is intermediate between neat and glass fiber reinforced polymer.
		Modulus increases and becomes intermediate between neat and glass fiber reinforced polymer.
		Impact strength decreases
		HDT increases and becomes intermediate between neat and glass fiber reinforced polymers.
		Coefficient of thermal expansion and shrinkage are reduced while thermal conductivity increases
		Price decreases but density increases

Wood plastic composites (WPC) are wood flour reinforced commodity thermoplastic compounds that are being fast developing as wood substitute because of the ease of processing, the shaping freedom and machining methods identical to those used for wood. The nanofillers are made up of:

		Elementary particles in platelet form of thickness in the order of the nanometer and diameter in the order of 100 nm.
		Primary particles made up by stacking several elementary particles. The thickness is about 10 nm.
		Aggregates of numerous elementary particles.

To exceed the usual filler reinforcement and to obtain a real nanocomposite it is necessary to destroy the primary particle structure during processing

		Either completely by dispersing the elementary particles into the macromolecules leading to a delaminated nanocomposite.
		Either partially, by intercalating macromolecules between the elementary particles leading to an intercalated nanocomposite.

The most popular nanofiller is a natural layered silicate, the montmorillonite that is subjected to specific treatments. The properties of the final nanocomposite depend on the nanocomposite treatments and the mixing efficiency. Table 4 displays reinforcement ratio examples of polyamide nanocomposites processed with various methods. The reinforcement ratio is the ratio of the nanocomposite performance versus the neat polymer one. All these data are only examples and cannot be generalized.

Carbon nanotubes are hollow carbon cylinders with hemispherical endcaps of less than 1 nm to a few nanometres in diameter and several microns in length. The aspect ratios are in the order of 1000 and more. The elementary nanotubes agglomerate in bundles or ropes that are difficult to disaggregate. The main properties are:

		Very unique modulus of the order of 1000 GPa and more.
		Very unique tensile strength of 50 000 MPa and more.
		A low density for reinforcement: 1.33 g/cm 3 .
		High electrical conductivities with a very high current density of the order of 109 A/cm 2 .
		High thermal conductivities of the order of 6000 W/mK.
		Very high cost:
	€1 million per kg in 2000. €100 000 and more per kg in 2002. €1 000 up to 400 000 per kg in 2004 €100 and more per kg expected in 2006

The production is expected to reach 300 tpa or more in 2007. The industrialization of these CNT is foreseen in a few years and in the polymer field could concern:

		The mechanical reinforcement: Easton Hockey, a manufacturer of advanced hockey equipment, has raised its stick game with NanoSolve� carbon nanotubes. The new composite stick increases in strength while reducing weight (10'15% increase in overall strength and toughness). Mitsui Chemicals is launching a new grade of carbon nanotube reinforced thermoplastic polyimide, Aurum CNT, with supplementary specific properties such as dust-reducing, and antistatic behaviour. Targeted applications are, for example, processing jigs for semiconductor or hard disk manufacturing, parts for hard disk drives.
		The electrical conductivity: Compounding with common polymer leading to extrinsic conductive polymers with nanotube levels lower than 1% for ESD, EMI compounds and ultra-flat screens. Italian compounder Lati introduces the new Latiohm CNT products using carbon nanotubes to achieve antistatic and conductive properties.
		The thermal conductivity for high thermally conductive polymers required for electronics.

The possibility to use additives magnifies the property range and the versatility of polymers and composites extending the application field. Among the broad range of reinforcing additives, common sub-micrometric fillers are less disturbing for the rheology and, consequently, for the ease of processing while having a certain reinforcing effect. They have been used for a long time without noticeable change in processing machine, tools and finishing operations. Mass produced parts are made with submicrometric filler reinforced plastics such as bumpers, appliance housing and components and so on. More specific parts with special functionalities or cheap goods also benefit from these techniques.

Nanofillers are promising with a reinforcing effect for very low filling amount but mixing of filler and polymer is a key parameter that is not always controlled. Carbon NanoTubes (CNT) are even more promising but mixing is even more difficult and costs are currently dissuasive. So they are only emerging as very special additives in applications, often apart from structural ones, mainly for their high electrical conductivity at low filling levels.

(Source: SpecialChem)