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| M Suresh Babu, Sangeeta Baksi, G. Srikant & Soumitra Biswas
Abstract
The term ‘composite’ in the building materials industry is often used in a broad sense to describe woody materials bonded together by adhesives covering the entire gamut of plywood, oriented strand board, wafer board, particle board, fibre board and other types of panelling products. Historically, mankind has been familiar with several applications of composite materials for housing & building construction.
In the recent times, the world has seen a total transformation in materials technology and a vast variety of industrially produced composites have come into existence. These developments have mainly taken place to meet housing & building requirements of ever growing population, particularly in the developing world.
Search for stronger & stiffer fibres and feasibility to utilize widespread inorganic wastes and by-products have provided directions to the production and use of natural fibres such as coir, jute, sisal etc. as quite inexpensive and effective reinforcing fibres and hydraulic binders as alternative to conventional one.
Thermoplastic composites are expected to replace metals and thermosets in numerous applications as they offer remarkable advantages over these traditional materials. They are generally recyclable, offer higher impact strength and are environmentally acceptable as there are no solvent used or volatiles emitted during processing.
These vary from low-pressure processes such as vacuum molding, filament winding and pultrusion to high-pressure processes like co-molding, extrusion compression, or injection molding. Most thermoplastic composites can not only be disposed off, the post-industrial or even post consumer scrap can easily be converted to a valuable high performance moulding material.
Thermoplastic composite components achieve a typical weight saving of 40% over steel and 20% over aluminium. Innovative polymers and composites are dramatically broadening the range of applications and commercial production of thermoplastics.
Made from both well-established and novel polymers, materials such as long glass-fibre reinforced thermoplastics, wood-plastic composites and nanocomposites are experiencing high growth. Durable, light, environmentally-friendly and chemically stable, composites also offer a myriad of possibilities in terms of optimising materials to suit applications.
They have the ability to create more complex shapes both on-line and in subsequent forming operations. Most thermoset resins are relatively brittle, most thermoplastics are extremely ductile. This ductility gives the final composite greater impact resistance and damage tolerance.
Thermoplastic composites evolved from structural polymer composites. While structural polymer composites (e.g. epoxy and polyester thermosetting resins reinforced with continuous filaments or fibres greater than 10mm in length) had many beneficial properties such as low density, good mechanical properties combined with good insulation and environmental resistance, they suffered from chemical instability, i.e, the impregnated intermediate or prepreg has a limited shelf life.
Thermoplastic composites do not suffer from this problem as they use a thermoplastic matrix. The heating/cooling cycle can be repeated many times, thus giving the product an almost indefinite shelf life.
They have increased damage tolerances due to the tough nature of the matrix material. Thermoplastic composites can be split broadly into two broad categories as Glass Mat Thermoplastics (GMT) and Advanced Thermoplastic Composites (ATC) as follows.
Glass Mat Thermoplastics
GMT’s can use nearly any thermoplastic for the matrix, however, in practice choices have been limited to polyvinyl chloride, polypropylene, polyamide, polyesters, polycarbonate and polyphenylene sulphide, with polypropylene accounting for about 95% of commercial use. It is also a suitable material when service temperatures remain below 110°C.
When higher temperatures are encountered and service conditions are harsher, more expensive polyphenylene sulfide could be used. E-glass fibre in the form of chopped fibres, random chopped fibres or continuous mats are the most common reinforcing phase.
Due to the light weight and high toughness, GMT’s have been adopted by the automotive industry. Applications include seat frames, battery trays, bumper beams, load floors, front ends, valve covers, rocker panels and under engine covers.
Advanced Thermoplastic Composites
Originally, ATC’s used amorphous resins such as polyethersulphone and polyetherimide as the matrix. However where increased solvent resistance is required, semi-crystalline polymers such as polyether ether ketone and polyphenylene sulphide could be employed.
The continuous reinforcement phase may be in strand, woven, knitted or braided fabric forms and be made from carbon, aramid and/or S-, R- and E-glass. Carbon is the most popular material for higher temperature application, while E-glass dominates lower temperature applications.
Advanced Thermoplastic composites have found limited use in the aerospace industry as tougher composites. They are analogous to thermoset composites with fibre contents above 50% (v/v) with a highly aligned continuous fibre structure. Actual applications include missile and aircraft stabiliser fins, wing ribs and panels, fuselage wall linings and overhead storage compartments, helicopter fairings etc.
Recent R & D has produced many viable products that are finding commercial acceptance. Several standards writing and building code organizations are currently evaluating these new products and making decisions that will affect the speed of future development and application.
Thermoplastic composites, which combine wood flour or other natural fibres/fillers with thermoplastic matrices, were introduced to the US market in the early 1980’s.
Such natural fibre-thermoplastic composites find a wide array of applications in the building & construction industry such as door & window frames, decking material, railings for the parapet wall systems, furniture sections (park benches etc.) and others.
The use of woodfibre/plastic composites in building products provides an opportunity to depart from conventional frame and panel building systems. The wood filler increases the stiffness of the plastic but decreases the impact strength of the composite.
The addition of coupling agents and compatibilizers helps improve the inherently poor bonding between the hydrophilic wood filler and the hydrophobic polymer matrix and can help recover some of the impact strength. As performance is increased, true structural applications could be developed.
Thermoplastic composites can also cater to a number of industrial applications. All such products are fabricated by extrusion through properly designed dies. Thermoplastic composite is consistent & uniform in properties due to intimate compounding of resin & natural fibre/filler.
While positioned against wood, they score much better in terms of dimensional stability, no water absorption and hence, no swelling in moist weather, better fungal resistance, reduced thermal linear expansion etc. The workability of the composite is also quite good in terms of its ability to be sawn and nailed or screwed by conventional methods.
CURRENT SCENARIO
Thermoplastic natural fibre composites enjoy very good market demand in Western countries. In fact, demand for wood-plastic composites has grown by 35-40 % over the last five years. A recent study by Principia Partners (Exton, Pa., USA) has an estimated business volume of over US $600 million for 500,000 tons of composite material being consumed in the US market alone.
There exist over 50 companies in US producing & marketing wood composite products. Future projection for the material has been estimated at US $2.0 billion for North America & Western Europe by 2007. In the present scenario, the market for decking, window & door profiles has been continuously growing.
Most of the research has been undertaken with a focus on wood as a reinforcing element in thermoplastic composite; other locally available natural fibres/fillers need to be explored for applications development. The automotive industry is the main driver for the European long fibre reinforced thermoplastic market, which has been expanding faster than other markets.
New applications include under body panels, front-end modules, and door panels. While carbon nanofibres and nanotubes are currently under development, nanoclays are finding applications in thermoplastic resins for food packaging and in some automotive applications.
PROCESS TECHNOLOGY
The thermoplastic composite products are fabricated by compounding natural fibres or wood flour in a thermoplastic resin along with appropriate additives into pellets and subsequently extruding the pellets into various profiles. The process involves, raw material preparation, compounding, profile extrusion, tooling, cooling and finishing.
In general, wood flour is made from sawdust, planar shavings, sanding dust and scraps produced in wood recycling industries. Hardwood, softwood, even MDF waste are being used. Other cellulose sources may be used as well, namely, straw, flax, rice hulls, peanut hulls, coir, jute, bamboo dust, kenaf etc.
The most commonly used thermoplastic resins are polyethylene (PE), polypropylene (PP) or polyvinyl chloride (PVC). Additives such as coupling agents, UV stabilizers, pigments, fire retardants, lubricants, fungicides and foaming agents are added for increased performance level of the finished product.
Compounding is the first stage of manufacturing thermoplastic composite products. It is a process of feeding and dispersing of fillers and additives in the molten polymer resin - this can be done either in batch (i.e. internal & thermo kinetic mixers) or continuous mixers (i.e. extruders, kneaders).
Batch compounding system has few advantages over continuous one. In batch compounding, the processing parameters such as residence time, shear and temperatures, could be controlled very easily for better quality & performance of the end product. However, continuous compounding system provides uniform quality throughout.
The compounded material can either be immediately extruded into final form of the end product or made into pellets for future processing.
Many of the industries now prefer to produce a pelletized feedstock–this enables for further processing easily by other forming techniques (i.e. extrusion, injection and compression moulding), for obtaining the desired shape of the product. Extrusion produces continuous products/profiles.
However, injection & compression moulding techniques are used for producing complicated shapes/pieces with shorter length.
Extrusion is a continuous manufacturing process of heating, compressing and conveying a premixed blend of raw ingredients. The process encompasses shaping of the material by the extrusion dies, cooling of a product upon its exit from the die and down stream handling including cutting, embossing, sanding and other treatments.
This process saves the cost of machining and fabrication. Shape of the final product obtained would be replication of the die opening. The products/profiles made by this process can be cut into small pieces, moulded, routed, sanded etc., with the same tools, feeds and speeds as with wood.
Two constituents of the product i.e. wood and plastic have great synergy in nature: plastics expand & contract due to temperature changes and wood expands & contracts with humidity changes.
Adding wood to plastic significantly decreases thermal linear expansion, and the plastic mitigates moisture movement into the part. Because of the low degradation temperature of wood, only thermoplastic resins that melt or can be processed at temperatures below 2000 C are commonly used in the fabrication of wood thermoplastic composites.
There are various types of extruders presently available viz. single-screw extruders, counter-rotating twin-screw extruders and custom extruder set-ups.
A single-screw system is the least expensive type, but typically uses pre-compounded pellets, an added processing step that adds cost, and overall output is lower.
Twin-screw extruders are preferred for wood products because the mixing action blends the polymer and the wood filler together in a single process, with low screw speed and low-shear mixing, which keep temperatures down.
By using low viscosity resins temperatures within the screw could be brought down by reducing the shear forces created during mixing.
Advanced Engineered Wood Composites Center of University of Maine, Orono, USA has developed a system called "Woodtruder", which comprises a counter-rotating type twin extruder along with a separate single-screw extruder. This keeps wood feeding and drying separate from polymer melting until well down the barrel.
Strandex Corporation, Madison, Wisconsin, USA has been a major player in the licensing of wood composite technologies. The Strandex process uses a hot die mounted on to the extruder thus eliminating the need for hydraulic pullers, calibrators and cooling jackets.
The extruder profile is cooled by water spray as it exits from the die. As the profile shape remains intact while cooling, cost of machining is avoided. The profiles are run through a brushing tool while still warm to achieve a textured appearance.
However, selection of screw design & process equipment would depend on various factors such as raw materials (polymers, natural fibres, additives), processing parameters, end-use of the product etc.
RAW MATERIALS
Wood Flour: Wood flour is a finely ground wood cellulose. When the particle size is above 20 mesh or below 850 microns, the product generally is considered to be wood flour. Mesh size is the measurement of number of openings in a screen per linear inch.
The collected wood flour from various sources (sawdust, planar shavings, sanding dust and scraps) are hammer milled to form very fine powder, classified by the standard mesh size that it can pass through. Most wood-filled thermoplastic manufacturers specify flour in the 30–80 mesh range.
Bulk density of wood flour is relatively higher compared to wood fibres. The moisture content of wood has a significant effect on the processing and final composite product quality. Pre-drying the wood flour to less than 1% moisture content could be very helpful.
Wood flour with less moisture content is less likely to burn during compounding with thermoplastics. The chemical & physical properties of hard wood flour are given in Table. The particle size for other natural fibres/fillers such as pine needles, maple, oak, bamboo dust, jute and coir may vary from 10 – 80 mesh.
Hard Wood Flour- Chemical & Physical Properties
(Source: PJ Murphy Forest products Corporation, Montville)
Thermoplastic Resins:The choice of resin being used in thermoplastic composites depends on many factors. As wood has a tendency towards degradation at higher temperatures, the thermoplastic resins that can be processed below 2000C, should be selected for processing of composites.
The most commonly used thermoplastic resins are polyethylene (PE), polypropylene (PP) and polyvinylchloride (PVC).
PE is the cheapest one and has excellent toughness and impact strength, but the lowest in service temperature. The resin can be used both in virgin & recycled form.
All polyethylene grades (i.e. LDPE, MDPE, HDPE) are used for the manufacture of thermoplastic composites. These types of resins have tendency to stabilize easily against thermal degradation during the process. At present, many industries are using PE as resin matrix for fabricating exterior building components.
PVC is the strongest resin with the highest service temperature, but is the most brittle, measured by percent elongation. It has high stiffness capability, mechanical strength & weather resistance property. PVC resins are being used in thermoplastic composites for manufacture of window panels and now being used in decking as well.
PP has good service temperature characteristics and lowest density compared to PE & PVC. The thermoplastic composites made of wood polypropylene are best suited for automotive applications and consumer products and these products have recently been investigated for use in building profiles.
Additives: Thermoplastic composites contain not only the natural fibres/fillers and thermoplastic resin alone, but also many other additives in smaller quantities in order to bring better performance for the final product.
The additives such as coupling agents, pigments, lubricants, UV stabilizers, fungicides and foaming agents can be used for improved performance during the processing of thermoplastic composites. The main functions of additives in thermoplastic composites are given under:
In general, the wood filler increases the stiffness of thermoplastic but decreases the impact strength of the composite. The addition of coupling agents and compactabilizers helps improve the inherently poor bonding between the hydrophilic wood filler and the hydrophobic polymer and can help recover some of the impact strength.
Adding lubricants in thermoplastic composites, helps in increasing the flow of raw material blend, dissipation of heat generated during the process, reducing the viscosity of the blend at higher shear rate, promoting the dispersion of natural fibre fillers in the resin matrix and would also help in reducing the adhesion between resin & screw of the extruder, friction between resin & process equipment. Colour pigments, heat & light stabilizers are also added in thermoplastic composites for improving material properties.
Most of the research has been focused on maleated polypropylene (MAPP) and maleated polyethylene for its use in thermoplastics as compactibilizers for better results. The additives used in wood-filled thermoplastic composites are summarized as follows (Table)
Functions of Additives used in Thermoplastic Composites
Chemical Modification
The chemical constituents for most of the natural fibres, such as wood, coir, jute, flax, sisal etc., are cellulose, hemicellulose and lignin. They have certain drawbacks when they are exposed to adverse/ aggressive environmental conditions.
The natural fibres absorb moisture as the cell wall polymers contain hydroxyl and other oxygenated groups that attract moisture through hydrogen bonding. They are very much prone to swell/warp & shrink when exposed to moist & hot weather conditions respectively. They also burn, decay and degrade by acids, bases and ultraviolet radiation.
In order to develop composites with better mechanical properties and environmental performance, it is necessary to impart hydrophobicity to the fibres by chemical reaction with suitable coupling agents or by coating with appropriate resins.
Such surface modification of natural fibre would not only decrease moisture absorption, but would also concomitantly increase wettability of fibres with resin and improve the interfacial bond strength, which are critical factors for obtaining better mechanical properties of composites.
When natural fibres are exposed to outdoors, they undergo photochemical degradation caused by ultraviolet radiation. The degradation takes place primarily in lignin component, which is responsible for the characteristic color changes. As the lignin degrades, the surface becomes richer with cellulose content – this results in rough surface of the composite and also accounts for a significant loss in surface fibres.
Chemical Modification of Natural Fibres
Chemical modification can be defined as a chemical reaction between some reactive parts of the constituent of the natural fibre and chemical reagent, with or without a catalyst, to form a covalent bond between the two.
As the natural fibres result from the chemistry of cell wall components, modification of chemistry of the cell wall polymers can change the basic properties of a fibre. The chemicals to be used for chemical modification must be capable of reacting with lignocellulosic hydroxyls under neutral, mildy alkaline or acid conditions at temperatures below 1500C.
The chemical system should be simple and capable of swelling the structure to facilitate penetration. The complete molecule should react quickly with the lignocelluloses components yielding stable chemical bonds and the treated lignocelluloses must still possess the desirable properties of untreated lignocellulosics. The hydrophobic nature of reagent should be selected in this aspect.
Many chemical reaction systems have been investigated for the modification of natural fibres. The chemicals include anhydrides such as phthalic, succinic, malaic, propionic and butyric anhydride, acid chlorides, ketene carboxylic acids, different types of isocyanates, formaldehyde, acetaldehyde, difunctional aldehydes, chloral, phthaldehydic acid, dimenthyl sulphate, alkyl chlorides, beta- propiolactone, acrylonitrile, epoxides such as ethylene, propylene & butylenes oxide and difunctional epoxides.
Most of the research has been done on the reaction of acetic anhydride with cell wall polymer hydroxyl groups to give an acetylated fibre. Different types of natural fibres, such as wood, bamboo, bagasse, jute, kenaf etc. have been acetylated using a variety of procedures. Acetylation is one of the chemical modification procedures for natural fibres; there could be other procedures, which can be looked into.
PRODUCT PROPERTIES
Thermoplastic composites exhibit better chemical & mechanical properties when compared to conventional wood. These tailor-made composites have better dimensional stability and fungal resistance when exposed to moist weather and they exhibit low thermal linear expansion at adverse environmental conditions.
These composites have an ability to create more complex shapes amd the products could be painted, screwed, nailed with same conventional tools used for wood. The other features such as low cost, aesthetic elegance, environmental acceptability and durability in the long run are considered to be attractive features for opting these composites as alternatives. Various extruded composite profiles are shown in Fig.
However, the mechanical properties of end product depend on the factors such as raw material selection, formulation, manufacturing technique & processing parameters. Wood-derived fillers have several advantages compared with their inorganic counterparts, such as lower density and lower volumetric cost.
There are several variables to consider when using wood flour as a filler. Among them are moisture content, purity, particle size, and species. While the first three variables can be controlled by raw material selection and the manufacturing process, species availability is often regionally influenced.
It is important for the end user to be aware of the differences that species variations may have on their product. Increasing wood flour content & its particle size would also affect the performance of the composite. With increasing wood flour content, melt index decreases, mould shrinkage decrease, notched impact energy increases while unnotched decreases.
Flexural and tensile strength decreases, while modulus of elasticity increases and tensile percentage elongation decreases. The comparison of mechanical properties of wood flour-polypropylene composites with hardwood fibre composites is shown in Table
Mechanical Properties of wood-polypropylene composites
(Source: Forest Products Journal, June 2002,Vol. 52,No: 6, Pages: 10-18)
APPLICATIONS & MARKET:
Typical applications of thermoplastic composite products in various sectors are listed in the Table
Applications of Thermoplastic Composites
Products such as window & door panels, lumbers have already been inducted & widely used in the US market. Other applications of thermoplastic composites are also expected to grow significantly in the near future. Efforts are underway in European countries for commercial exploitation of thermoplastic composite technology.
Worldwide, around 100 companies have been identified, who are involved in manufacturing of thermoplastic composites. Majority of these are based in the USA and Japan. It is expected that the market would take off imminently, in the other parts of the world and would provide significant opportunities for industries interested to adopt the technology.
Opportunities for thermoplastic composites in coastal areas is gaining importance as lighting columns and utility poles. Thermoplastic composite columns are maintenance free and last longer than steel columns, making them considerably more cost effective on life cycle basis. Thermoplastic composite flagpoles conform fully with increasingly stringent health and safety legislation governing styrene emissions in the workplace and environment.
INTERNATIONAL TRENDS
Homogenous fibre distribution is a prerequisite that cannot be compromised in short fibre composites, while in long fibre composites, it is necessary to enhance composite reinforcing efficiencies that control tensile, impact and creep behaviour. In general, high shear processing is necessary to attain uniform fibre distribution, yet high shearing action results in significant breakage and poor composite strengthening efficiencies.
It is important to maximize uniformity of fibre dispersion and at the same time minimize fibre damage. The objective is to preserve fibre length by altering processing parameters, using minimal additives to improve fibre dispersion and stress transfer between matrix and fibre.
The market for continuous fibre reinforced thermoplastic composites has experienced exceptional growth in recent years with a global growth rate of 105% in last 5 years.
Growth rate in 2002 was 93%. Continuous fibre reinforced thermoplastic composites include a variety of products including unidirectional prepreg, fabric based prepreg, narrow tape, commingled fibre in roving and fabric forms, sheets, and rods.
Such thermoplastic composites has a history of about 20 years and it differs from discontinuous fibre reinforced thermoplastic composites in terms of fibre length. Historically, continuous fibre reinforced thermoplastic composites were used in niche applications in aerospace and defense market.
But in recent years, the market has expanded to include automotive, sporting, transportation, industrial and other applications. Common reinforcements used with thermoplastic composites are E-glass, carbon and aramid.
Resins typically selected are PPS, PEEK, polypropylene (PP), Nylon, PC, and PEI. Continuous fibre reinforced thermoplastic composites are even finding their way into furniture, fastener, medical, marine, and other higher performance applications. Airbus has projected to increase the use of thermoplastic composites by 20% every year.
The national Institute of Materials & Chemical Research, Ibarki, Japan has developed a new sheet production method for producing continuous fibre-reinforced thermoplastic composites.
In this method, continuous reinforced fibre and thermoplastic fibre are fed simultaneously to a machine which cuts, separates and mixes the fibres for continuous production of a mat uniformly mixed with both fibres.
Heat compression molding of laminated mixed mats takes only 30 seconds or less and allows free selection of the concentration of reinforcement fibres. This method can also be applied to hybrid type composites mixed with different reinforced fibres and functionally gradient materials with distributed concentrations
The Industrial Materials Institute (IMI), a division of the National Research Council of Canada (NRC), is working on the formulations, characterization, non-destructive analysis of polymer composites to optimize the performance through processing and control of their microstructure.
Optimization of composite process technologies lead to greater productivity with enhanced quality of composite component to obtain structural profiles or moulded parts with optimum mechanical performance.
Nowadays, ecological concern has resulted in a renewed interest in natural fibres based on lignocellulose, such as flax and hemp. The use of natural fibre as reinforcing elements for thermoplastics is gaining wide acceptance for various building & construction applications since several decades.
These fibres are an interesting alternative for the use of glass fibres in engineering polymer composites. In contrast to glass fibres these vegetable fibres are renewable, nonabrasive to processing equipment, can be incinerated and show less concern with safety and health.
In addition they exhibit excellent mechanical properties, especially when their low density and price in comparison to E glass fibres is taken into account. Due to their high moisture absorption and poor dimensional stability (swelling), as well as their susceptibility to rotting has hindered the successful application of these fibres in high quality products.
The first attempt was made in 1983, by American Wood Stock (now part of Lear Corporation in Sheboyan, Wisconsin, USA) in developing composite automotive interior substrates/panels with 50% wood flour compounded in polypropylene resin matrix by using Italian extrusion technology.
This composite material consists of 50 percent polypropylene with small amounts of other performance enhancing additives. The wood and plastic are blended together using a compounding twin-screw extruder and extruded into a flat sheet. The flat sheet is then post-formed into the substrate shape.
The advantage of this system is that overlays can be applied directly in the forming die. Later in 1990s, some of the US companies began experimenting with 50% wood flour compounded with polyethylene resin matrix, thereby producing various thermoplastic composite products. These products were used as deck boards, landscape timbers, picnic tables, industrial flooring etc.
Further in 1993, there was a dramatic increase in the utilization of wood thermoplastic composites. During that period, the products such as subsils for doors, windows, door profiles etc. entered the market.
There is still a great deal of basic research going on in many parts of the world in the area of natural fibre/thermoplastic composite materials.
This research includes the use of many different types of agro-based fibres, expanding the amount of agro-based fibres in the composites, developing better compatibilization systems, developing testing procedures and understanding performance through modeling.
Equipment development is also being carried out to produce more efficient mixing systems, faster through put and new methods of moulding/injection.
The European Commission Programme on biomaterials for non-food products, UK has been successful in developing thermoplastic composite products based on upgraded flax fibres with optimized mechanical and physical properties, improved durability and a good processability.
As a result, these materials could compete with commercially available glass fibre reinforced thermoplastic products, anticipating the growing demand for environmental friendly products offering tremendous market opportunities for the SME’s.
Extrusion of lignocellulosics-filled plastics for the automotive industry is well known and has been used for more than twenty years. Typical blending involves the plastic-filler/ reinforcement to be shear mixed at temperatures above the softening point of the plastics.
The heated mixture is then extruded into ‘small rods’ that are then cut into short lengths to produce a conventional pellet. The pellets can then be used in injection or compression moulded techniques. Manufacturers constantly look for more efficient and cost-effective ways to make durable products.
Many window and door manufacturers are looking seriously at woodfibre/plastic composites as an alternative to solid wood in clad components. The Andersen Corporation, USA has developed a technology to integrate woodfibre/plastic composites by utilizing their own in-plant waste.
Unlike other manufacturers that use polyethylene, Andersen uses the waste PVC generated in their cladding operations as a polymer base for their composites. Typically formulated from 60 percent PVC and 40 percent wood waste, the profiles are currently being used as sills with aluminium cladding. The composite material’s thermal coefficient of expansion is almost the same as that of aluminium.
Imhotep Ltd., Mansfield, England has developed a range of innovative process technology for the economical production of thermoplastic composite products. The novel technology makes the product stiffer and stronger with better performance thus bridging the gap between conventional extrusion and pultruded fibreglass. Such a technology based on Gel-Coated Pultrusion gives a unidirectional continuously reinforced thermoplastic composite profile with the outward appearance of an extrusion but with the mechanical performance of a conventional fibreglass pultrusion. The process allows reinforcements levels of between 0 and 60% by volume across the profile enabling a wide range of stiffnesses in the product. Although only simple profiles are being manufactured at the moment, the technology could easily be used in a wide range of thermoplastic extruded profiles to improve their performance.
Advantages of the process over conventional pultrusion are the profiles are reinforced with continuous fibre reinforcement to give higher stiffness and strength over the polymer or short fibre reinforced polymer alone. The process enables the production of complex components using a single conventional extruder.
Raw materials costs are lower as the process does not involve any intermediate product stages. The process is applicable to an extensive selection of polymers to give a wide range of component properties. As the glass fibres do not pass through the extruder, no abrasive damage to the barrel or screw occurs. The fibre volume fraction can be varied from 0 to 65% around the profile.
Profiles could be produced with a thick polymer coating to protect from splinters. The technology enables use of reclaimed or recycled plastics with minimal effect on the end product’s mechanical properties. Due to the polymer coating if the rods break, they do so in a benign way without producing long shards or splinters, which has gained a favourable response from customers using traditional fibreglass.
The thermoplastic composite does not suffer the rapid drop in load as seen in thermoset composites. After yield the thermoplastic composite appears to undergo plastic deformation, more commonly seen in pure thermoplastics and metals. Handling the product is also less problematic because of the polymer rich surface.
Carbon nanofibres (CNF) hold promise as reinforcement materials for novel nanocomposites. A range of composite systems have been investigated by the Department of Material Science & Metallurgy, University of Cambridge, UK using nanofibres of differing structure and diameter in a number of representative thermoplastic matrices, including commodity and high performance resins.
Compared with the conventional composites, the critical factors influencing composite performance are quality of the nanofiller, the nature of the filler dispersion and the strength of the filler-matrix interface. A high degree of dispersion could be achieved in all matrices using an intensive shear mixing protocol in conventional thermoplastic processing equipment such as injection moulding.
Use of a pre-oxidation treatment, however, can increase the interfacial bond strength, leading to an increase in stiffness and yield strength for certain systems.
Polypropylene composites with vapor-grown carbon nanofibres as reinforcement with an average diameter of 200 nm are very promising materials for engineering applications. Fibre purification and activation of functional groups were conducted by the Department of Mechanical Engineering & Materials Science, Rice University, Houston, Texas wherein amorphous carbon particles were successfully removed achieving high purity fibres.
Thermal physical analysis on the samples shows that the presence of fibres influence the morphology and crystallinity of the matrix. The decomposition temperature as well as the crystallization rate increases with increasing fibre content. The electrical resistivity of the prepared composites decreases 12 orders of magnitude providing a potential composite for ESD applications.
Achieving specific strength and stiffness two or more times that of existing composite materials could enable significant structural weight reductions for ballistic missile systems, single-stage-to-orbit vehicles, satellites, and aircraft. As prices of nanotubes come down, many other applications could be emerged for structure applications
The introduction of high strength/ modulus fibres, such as carbon or aramids, into the composite mix is seen as the preferred way to achieve the proper balance of physical properties for structural applications in the automotive industry. To date, however, this type of cost-effective way to manufacture large volumes of such composites has not been developed.
New DuPont technology presently, under evaluation, has the potential to significantly reduce pitch based carbon fibre prices. DuPont and Cambridge Industries are pursuing a program to evaluate these fibres for the automotive market.
Using high modulus fibres like carbon as the reinforcement, the composites would rely upon thermoplastic polyester as the matrix material. This will make it possible to use "post consumer" recycled polyester as the feedstock while simultaneously building recyclability into the resulting composites.
Key challenges in this program are to integrate the high modulus fibres into the composite in both oriented and isotropic form and to produce joints among parts that have the strength required for in-service performance in automotive applications.
Thermoplastic composites play an increasingly important role in the aircraft industry mostly in secondary aircraft structures such as interior parts. In primary structures thermoplastic composites are mostly used in combination with thermoset composites.
Delft University of Technology in the Netherlands is working on developing a primary structure part, a rudder, completely from thermoplastics. The materials used in the current rudder are an epoxy resin system, glass fibre and PVC foam. The current design is based on the sandwich principle, with the skins, the ribs and the spar all use a sandwich construction. It is difficult to use thermoplastic composites in sandwich construction.
Therefore, the rudder is being redesigned for it to be manufactured from thermoplastic composites. First step in the project at TU Delft is to redesign the Eaglet rudder so that a demonstrator could be build from thermoplastic materials, eliminating the sandwich parts in the design.
Materials that could be used are PEI and PPS thermoplastics with glass and carbon fibres. The initial research is based on developing and manufacturing one or more rudders completely from thermoplastic composite material. This rudder would then be installed in the Euro ENAER Eaglet research aircraft and would therefore be tested against stringent quality demands used in the aircraft industry.
Thermoplastic composites for structural applications have always lacked the mechanical properties of thermoset composites. Mainly, poor fibre impregnation and lack of fibre adhesion cause this performance gap. Recently, Dow Plastics has introduced FULCRUM technology based on a new thermoplastic matrix, (ISOPLAST, engineering thermoplastic polyurethane) and a new pultrusion process.
Reversal of the polymerization process in the melt stage is the key to the success of FULCRUM process. Not only the new process would yield equivalent mechanical properties similar to thermoset composites but toughness and damage tolerance could also be enhanced. Moreover, profiles can be processed at very high speeds, can be post formed and are also recyclable.
CONCLUSION
The technology for thermoplastic composites and its products has widely been demonstrated & utilized in many developed countries. There exists flexibility for manufacturers/developers for choosing appropriate raw materials, compactibilizers/ coupling agents, process technology, process equipment, process parameters for developing products for specific applications.
Utilization of natural fibres/thermoplastic waste as reinforcing element in thermoplastic composites, in a cost effective manner, has become an attractive feature for developing such composites for a wide gamut of applications.
These composites assume importance especially due to its capability of utilizing the natural fibre resources such as wood flour, rice husk, bamboo dust, jute, flax, coir, sisal, plastic waste etc. as reinforcing elements. The mouldability of plastic allows complex product design.
In addition, the process of pelletization & extrusion is environmentally benign as waste biomass and thermoplastics can be used to make these products. As India is endowed with abundant natural fibre resources and good expertise in polymers, exploring this technology would go a long way in creating new business opportunities in this area.
In view of the crucial need for developing indigenous capability in composite technology, the Advanced Composites Programme was launched by the >Technology Information, Forecasting & Assessment Council (TIFAC), Department of Science & Technology (Govt. of India).
The programme has been an attempt to enhance the utilisation & application of composite as an important performance material in various sectors and to improve upon the laboratory-industry linkages towards development & commercialisation. The programme has been an experiment to bring about a culture of development especially for the technology starved SMEs.
Poised with the successes of developing natural fibre based composites such as jute-coir composite boards, jute-thermoplastic composites for shoe components, bamboo composite laminates etc., the Advanced Composites Programme is seriously contemplating indigenous development & productionization of thermoplastic composite technology in partnership with an industry.
Dedicated development efforts would be necessary to explore the suitability of indigenous biomass viz. bamboo, jute, coir, pine needle etc.
For further information, please contact Mr. S. Biswas at advcomp@tifac.org.in
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For further details log on website :
http://tifac.org.in/index.php?option=com_content&view=article&id=533:thermoplastic-composites-a-new-business-avenue-&catid=85:publications&Itemid=952
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