|Pultrusion of Composites - An Overview|
Atul Mittal & Soumitra Biswas
Since the early dawn of civilization, the strong and light material has always fascinated mankind for typical applications. The idea of combining two or more different materials resulting in a new material with improved properties exists from ages. It was discovered long ago that composite materials have the combined advantages with superior performance compared to each individual material.
For time immemorial, man has been using fibres along with a binder or matrix for stronger materials. The usage of plant fibres for strengthening & preventing bricks and pottery from cracking was practiced by the Pharaohs of Egypt and the ancient Incan & Mayan civilizations.
With the advent of modern civilization and development of scientific knowledge, there has been an upsurge in demand for developing newer materials for novel applications.
In fact, with the technological leaps in recent times, materials were required to perform in stringent conditions - high temperature & pressure, highly corrosive environment, higher strength but without much weight implications etc. which the conventional materials failed to service.
This triggered the needs for 'engineered material', devising material properties catering to the application needs. And the innovation was not limited to developing novel materials alone but it also addressed the method of manufacturing - improved processing techniques, effective use of energy while processing and more importantly with the least environmental impact. Advanced materials with combination of properties for specific end uses became a reality.
The composite materials belong to the aforesaid category of new materials, which are derived by combining two or more individual materials with intent of achieving superior properties over the ingredients. Thus composites can be broadly defined as materials that contain reinforcement (such as fibres or particles) supported by binder (matrix).
The composites typically have a discontinuous fibre or particle phase that is stiffer and stronger than the continuous matrix phase. Composites can be divided into classes in various manners. One classification scheme is to separate them accordingly to reinforcement forms: particulate-reinforced, fibre-reinforced or laminar composites.
Fibre reinforced composites can be further divided into those containing discontinuous or continuous fibres. Another commonly practiced classification is by the matrix used : polymer, metallic and ceramic.
A host of processes exist for the fabrication of composite components. Fibre-reinforced composites used in most high-performance applications are laminated with unidirectional layers at discrete angles to one another, thereby distributing the in-plane load in several directions.
A variety of fibre placement processes are used to achieve the desired combination of orientations. Some type of cure or moulding process follows the fibre placement process. The common fabrication techniques are listed below:
Filament winding is a semi-automatic manufacturing method for making fibre reinforced composite materials by precisely laying down continuous resin impregnated roving or tows on a rotating mandrel, that has the required shape. The mandrel can be cylindrical, round or of any shape that dose not have a reverse curvature.
The technique has the capacity to vary the winding tension, wide angle, or resin content in each layer of reinforcement until the desired thickness or resin content of the composite are obtained with the required direction of strength. A large array of products can be fabricated by this technique viz. storage tanks, pipes, pressure vessels, rocket engine cases, nose cones of missiles and other aerospace parts.
Resin Transfer Moulding
This is a low-pressure, closed mould semi-mechanized process. The process allows fabricating simple, low-performance to complex, high-performance articles in varied sizes. The fibre reinforcement, which may be preshaped is placed in the required arrangement in the cavity of a closed mould and a liquid resin of low viscosity is injected under pressure into the cavity, which is subsequently cured.
\ The main potential advantages of RTM can be summarized as the capability of rapid manufacture of large, complex, high-performance structures with good surface finish on both sides. It also permits the use of foam and other removable cores to yield three-dimensional parts and hollow components as well.
Resin transfer molding suffers from few limitations such as high glass ratios cannot be achieved and improperly placed reinforcement can cause dry spots or resin pools. Thus, resin transfer moulding is not recommended for part that will be highly stressed.
Pultrusion is a continuous, automated closed-moulding process that is cost effective for high volume production of constant cross section parts. Due to uniformity of cross-section, resin dispersion, fibre distribution & alignment, excellent composite structural materials can be fabricated by pultrusion.
The basic process usually involves pulling of continuous fibres through a bath of resin, blended with a catalyst and then into pre-forming fixtures where the section is partially pre-shaped & excess resin is removed. It is then passed through a heated die, which determines the sectional geometry and finish of the final product.
The profiles produced with this process can compete with traditional metal profiles made of steel & aluminium for strength & weight.
The pultrusion process has developed slowly compared to other composite fabrication processes. The initial pultrusion patent in the United States was issued in 1951. In the early 1950s pultrusion machines for the production of simple solid rod stock were in operation at several plants.
Most of these machines were the intermittent pull type. In the mid-1950s, continuous pull machines were available. The late 1950s were producing pultruded structural shapes and by 1970, there has been a dramatic increase in market acceptance, technology development, and pultrusion industry sophistication.
Pultrusion: Process Technology
The process begins when reinforcing fibres are pulled from a series of creels. The fibres proceed through a bath, where they are impregnated with formulated resin. The resin-impregnated fibres are preformed to the shape of the profile to be produced. This composite material is then passed through a heated steel die that has been machined precisely to the final shape of the part to be manufactured.
Heat initiates an exothermic reaction thus curing the thermosetting resin matrix. The profile is continuously pulled and exits the mould as a hot, constant cross sectional member. The profile cools in ambient or forced air, or assisted by water. The product emerges from the puller mechanism and is cut to the desired length by an automatic, flying cutoff saw.
a) Material In-Feed : Reinforcements are to be in a package designed for continuous feeding of the material. The continuous fibre creels are usually the first station on a process line. After the roving creels there is a creel meant for rolls of mats, fabric or veil. As materials travel forward toward the impregnation area, it is necessary to control the alignment to prevent twisting, knotting and damage to the reinforcements. This can be prevented by using creel cards or vinyl tubes.
b) Resin Impregnation/Material Forming :The impregnation of reinforcement with liquid resin forms the basis of every pultrusion process. A dip bath is most commonly used. In this process, fibres are passed over and under wet-out bars, which causes the fibre bundles to spread and accept resin.
A comb or grid plate is generally provided at the entrance and exit ends of the resin bath to keep the roving in alignment as they pass through the tank.
Forming is usually accomplished after impregnation, preforming fixtures consolidate the reinforcement and move them closer to the final shape provided by the die. A proper sizing of the preforming fixtures avoids excess tension on the relatively weak & wet materials, but also allows sufficient resin removal, avoiding too high hydrostatic force at the die entrance.
The commonly used materials for forming guides are Teflon, ultrahigh molecular weight polyethylene, chromium-plated steel and various sheet steel alloys.
c) Die Heating : Die heating is one of the critical process control parameters as it determines the rate of reaction, the position of reaction within the die, and the magnitude of the peak exotherm. Improperly cured material will exhibit poor physical and mechanical properties, yet may appear identical to adequately cured products. Excess heat inputs may result in products with thermal cracks or crazes, which destroy the electrical, corrosion resistance, and mechanical properties of the composites.
d) Clamping/Pulling Provision : A physical separation of 3 m (10 ft) or more between the die exit and the pulling device is provided in order to allow the hot, pultruded product to cool in the atmosphere or in a forced water or air cooling stream. Thus allows the product to develop adequate strength to resist the clamping forces required to grip the product and pull it through the die.
The pulling mechanism varies in design, but three general categories of pulling mechanism that are used to distinguish pultrusion machines are intermittent-pull reciprocating clamp, continuous-pull reciprocating clamp and continuous belt or cleated chain.
e) Cut-off Station :Every continuous pultrusion line requires a means of cutting product to desired length. Both dry-cut and wet-cut saws are available but regardless of design, a continuous grit carbide or diamond edged blade is used to cut pultruded products. The saw is clamped to the pultrusion product during the actual sawing operation.
Materials : One can use a wide variety of fibrous reinforcement and resin system to get a composite material with a broad spectrum of properties by pultrusion process. Since each fibre and resin material brings its own contribution to the composite, knowledge of raw material properties is the first step in designing a satisfactory composite product.
The reinforcement provides mechanical properties such as stiffness, tension and impact strength and the resin system (matrix) provides physical properties including resistance to fire, weather, ultraviolet light and corrosive chemicals.
a) Reinforcement types : Three characteristics must be considered when choosing reinforcements: first the fibre type (glass fibre, aramid and carbon); second the form (roving strands, mat & fabrics) and third the orientation.
The glass fibre continues to be the most widely used reinforcement, because they are readily available and comparatively cheaper. Electrical grade E-glass fibres, the most common, exhibits a tensile strength of approximately 3450 MPa and a tensile modulus of 70 GPa, but they have relatively low elongation of 3 to 4%.
A variety of fibre diameters and yields are available for specific applications. Surface sizing of glass fibres provides optimum impregnation and chemical bonding between the fibres and matrix resins, thus ensuring maximum strength development and retention.
S-glass fibre exhibits high tensile strength (4600 Mpa) & tensile modulus (85 Gpa) and is used for high-performance applications. The Carbon fibre exhibits tensile strength from 2050 to 5500 MPa and tensile modulus from 210 to 830 GPa with elongation of 0.5 to 1.5%. Carbon fibre has various unique properties like electrical conductivity, high lubricity and low specific gravity (1.8 versus 2.60 for E-glass).
Very tough composites having good flexural and impact strength can be fabricated by using Organic fibres such as aramids, having high tensile strength (2750 MPa) and modulus (130 GPa) along with elongations of up to 4%. Polyester fibres with appropriate binders have been used as a replacement for glass in applications that would benefit from increased toughness and impact resistance but where tensile and flexural strengths can be sacrificed.
b) Matrix Choice : The composite properties such as high-temperature performance, corrosion resistance, dielectric properties, flammability and thermal conductivity are determined exclusively by the properties of resin matrix.
Unsaturated Polyester resins are most commonly used in pultrusion. Orthophthalic, isophthalic acids or anhydrides, in combination with maleic anhydride and various glycols, are the basic elements. Pultrusion polyester must have the ability to gel and cure rapidly to form the strong gel structure required for release at the die wall.
Generally resins with the viscosities of 500 cP are used for pultrusion. Higher viscosity low-reactive monomer versions can be blended with additional styrene to suit the processing need. The styrene level must be properly maintained to achieve good cross-link structure without having residual (unreacted) styrene in the finished composite.
Polyester resins exhibit good corrosion resistance to aliphatic hydrocarbons, water, dilute acidic & alkaline environments. They do not perform well when exposed to aromatic hydrocarbons, ketons, and concentrated acids. A high degree of unsaturation in polyester chain exibits shrinkage up to 7% on curing.
This level can be reduced using fillers and low-profile additives. Composite based on polyesters retains high percentage of their electrical insulation properties even if used continuously at temperature up to 200oC. Though polyester supports combustion without modification, hence backbone bromination or the use of additives greatly improves its flammability and smoke generation properties.
The electrical properties of polyesters make them suitable for use as primary insulators in many high-voltage applications. Retention of electrical properties even at elevated temperatures has made polyester insulators the materials of choice in many applications.
The weatherability of polyester is fair to good. Additional protection is usually through a variety of ultraviolet absorption additives or using polyester surface veils and even painting (done after pultrusion)
Vinyl Ester Resins show better corrosion resistance and mechanical properties at elevated temperature, but are approximately 75% more expensive than polyester. These resins display greater toughness properties, such as inter laminar shear and impact strength.
The chemical structure of vinyl ester resins is such that the reaction sites are at the end of each polymer chain rather than along the chain resulting in rigid segments along the polymer backbone. This leads to lower-link density and high-temperature capability of these materials.
Epoxy Resins are expensive materials. They are suitable for increased continuous-use temperatures up to about 150oC. They are used when physical properties of the highest level, as well as elevated temperature property retention, are required. Their excellent electrical properties & corrosion resistance also qualify them for use in many commercial applications requiring superior performance at elevated temperatures.
Epoxy resins do provide increased flexural strengths and shear-strengths over polyester and vinylester systems. Epoxy resins are cured by stepwise reaction hence their reaction rate is very slow, which affects the productivity of pultrusion process.
Other resins - A variety of resin alternatives is also available for specific applications. The resins based on methyl methalcrylate although expensive than polyesters but could be used for their special properties viz. improved physical properties, high filler loading due to low viscosity, rapid processing speeds, smooth profile surfaces and improved flame retardancy and weathering characteristics.
Phynolic resins are also used in pultrusion owing to their high heat resistance and flame-retardancy/low-smoke characteristic. Phenolic resins are suitable typically for pultruding natural fibres such as jute.
A desire to improve toughness and post processing formability has lead to the use of thermoplastic resins. The engineering thermoplastic resins provide excellent heat distortion properties. The technology for impregnating fibres with thermoplastic resins includes hot-melt application and solvent solution impregnation.
Additives - By using various additives liquid resin systems can be made suitable to provide specific performance. Fillers constitute the greatest proportion of a formulation, second to the base resin. The most commonly used fillers are calcium carbonate, alumina silicate (clay), and alumina trihydrate.
Calcium carbonate is primarily used as a volume extender to provide the lowest-cost-resin formulation in areas in which performance is not critical. Alumina trihydrate is filler that is used for its ability to suppress flame and smoke generation.
Fillers can be incorporated into the resins in quantities up to 50% of the total resin formulation by weight (100 parts filler per 100 parts resin). The usual volume limitation is based on the development of usable viscosity, which depends on the particle size and the characteristics of the resin.
Special purpose additives include ultraviolet radiation screens for improved weatherability, antimony oxide for flame retardance, pigments for coloration, and low-profile agents for surface smoothness and crack suppression characteristics. Mould release agents (metallic sterates or organic phosphate esters) are important for adequate release from the die wall to provide smooth surfaces and low processing friction.
An important characteristic pertains to the curing of thermoset resins. The polyester vinyl ester and methacrylate systems are cured by the high-temperature initiation, followed by midrange accelerator and high-temperature completion. This contribution, which delivers the fastest processing speed, can also reduce resin pot life, especially in high ambient temperature.
a) Mechanical Properties - A broad spectrum of mechanical properties is provided by the selection of reinforcement types, style, form & proportion in combination with different matrix. The directionality of strength in a pultruded composite can be greatly influenced by substituting longitudinal reinforcement by random mat or directional fabrics. The absolute value of the specific property desired would depend on the fibre type chosen: glass, carbon, aramid, organic or natural fibres. Table 1 indicates the effect on select mechanical properties in the axial direction for various continuous fibres. Table 2 indicates the effect of orientation on mechanical properties of pultruded profiles.
Table 1 : Effect of Fibre Type on Select Mechanical Properties of Pultruded Profiles
Table 2 : Effect of Fibre Orientation on Mechanical Properties of Pultruded Profiles
Table 3 : Mechanical Properties of Pultruded Profiles Vs. Other Structural Materials
b) Physical Properties - Thermal conductivity of composites is affected by both matrix and fibre characteristics. Generally, the glass & organic fibre reinforced composites are excellent insulators for thermal and electrical environments.
The use of conductive carbon fibres, however, results in composites that exhibit thermal and electrical conductivity to some extent; this reduces their effectiveness as insulators but creates opportunities because of their static charge and heat dissipation characteristics.
Specific gravity is a key consideration when strength-to-weight ratios are important as in aircraft and aerospace applications. Carbon and aramid-reinforced composites excel because of their low specific gravities and high strength & stiffness characteristics.
The impact resistance of organic fibre reinforced composite is quite high, making them suitable for energy absorption applications. Fibre glass-reinforced composites are relatively poor in impact performance compared to organic fibres, but are superior to carbon-reinforced composites. Composites using carbon fibre rely on the toughness of the resin matrix for impact properties.
c) Chemical & Corrosion resistance characteristics of pultruded composites are attributed to the properties of resin used. Chemical & corrosion attack can occur at the product surface or at the end. The presence of a resin rich barrier layer on the surface provides greater degree of corrosion resistance.
To achieve a resin-rich surface, a synthetic veil or mat, typically of polyester fibre, is used on the surface of the products when pultruded. The layer can range from 0.15 to 1.00 mm thick, depending on the thickness of the material used.
The end cut of the profile is particularly vulnerable to corrosion because fibres are exposed to the environment. Therefore, it is a common practice is to dip-coat the end cuts of pultruded profile to seal them from corrosive attack.
If this is not done, the corrosion resistance of the fibre itself becomes an important consideration because the resin does not effectively protect the fibre from attack along the fibre-resin interface at open ends.
Table 4 : Physical & Chemical Properties of Pultruded Profiles Vs. Other Structural Materials
* Excellent with special additives
Advantages of Pultrusion
Pultrusion is the most cost-effective method for the production of fibre-reinforced composite structural profiles. It brings high performance composites down to commercial products such as light-weight corrosion free structures, electrical non-conductive systems, off-shore platforms and many other innovative new products. The advantages of pultruded FRP profiles are summarized in the following table :
Table 5 : Advantages of Pultrusion
(Source : Product Information Brochure; DK Fibre Forms, Pune, India)
(Source: The report on Putrusion & Pultruded Composites, June 1992, published by the Composite Development Corporation, West Wareham, MA, USA)
Table 6 : Pultruded Product Characteristics
Pultruded sections are well-established alternative to steel, wood and aluminium in developed countries and are fast catching up in other parts of the world. Structural sections have ready markets in oil exploration rigs, chemical industries etc. A survey of international patents establishes the wide applications & techniques used to pultrude various profiles.
A quick patent search at the USPTO (United States Patents & Trademark Office) website in the internet was carried out to understand the latest technology trends. These are summarized in the following section:
Kabushiki Kaisha Seiko Sho, Kobe, Japan in their patent (US patent #5700417) in 1997 describes a pultrusion process for preparing fibre-reinforced composite rod, for use as a strength member by pulling continuous fibres through a bath containing a radiation-curable composition (comprising a monomer, which is polymerizable under the effect of ultraviolet (UV) radiation, a polymer which is dissolved or dispersed in the monomer and a photoinitator) and exposing the impregnated fibres to UV radiation to effect polymerization of the monomer thereby producing a fibre reinforced composites.
In a US patent (#5647172) of 1997 by Mr. Stanley Rokicki, Toronto, Ontario, Canada describes a process of fabricating pultruded fibreglass framing sections.
A closure assembly is provided with framing sections being pultruded having thin walls, sufficiently thin so as to require reinforcing of the pultrusion proximate at the ends and at predetermined positions to allow for (i) joining adjacent pultrusion together (ii) assembling hardware to the pultrusion (iii) monitoring the pultrusion when assembled in an opening.
A US patent (# 5556496) granted in 1996 to Mr. Joseph E Sumerak, Solon, OH describes a method of producing a pultrusion product having a variable cross-section at selected intervals along the length of the article using a specially adapted temperature controllable pultrusion die.
In a US patent (# 5783013) of 1995, Owens-Corning Fibreglass Technology Inc., Summit, IL described the use of multiple resin system for performing resin injected pultrusion.
The method comprises impregnating the interior layers of a pultrusion reinforcement pack with a first resin material, adding the external layers to form a pultrusion pack and impregnating the pack with a resin material substantially non identical to the first resin material.
The Industrial Technology Research Institute, Hsinchu, Taiwan in their patent (US # 5424388) in 1995 describes a pultrusion processing method for long fibre-reinforced nylon composites. The method combines the nylon anionic ring-opening polymerization technology & the pultrusion process.
An active caprolactam sodium salt catalyst composition is formed by reacting melt nylon 6 monomer (i.e. caprolactum) with sodium hydride which is then mixed with co-catalyst composition (formed by mixing melt caprolactum and a polymeric co-catalyst) to give a low viscosity reaction mixture.
The mixture is then charged into a closed impregnating tank to impregnated preheated & dried reinforced fibre, which is immediately pulled into a hot mould to form a finished product of long fibre-reinforced nylon composite.
A US patent (# 5741384) granted in 1994 to Hoechst AG, Germany describes the process for making glass fibre-reinforced composite using thermoplastic matrix. This involves drawing a glass fibre strand in an agitated aqueous powder dispersion of a coupling agent, the powder is melted onto the glass fibre strand and the preheated fibre strand is subsequently impregnated with polyolefins by means of melt pultrusion.
The process of pultruding Nylon composites was patented (US patent # 5374385) by Monsanto Europe S.A., Brussels, Belgium in 1994.
The inventors used a process for the reactive pultrusion of a reinforcing material and a polyamide matrix. Reinforcing material is impregnated with a polyamide forming reaction mixture (comprising at least one monomer, an initiator and a catalyst) and then pultruded through a die while the polyamide forming reaction mixture is polymerized to form a polyamide matrix, wherein the temperature of the material being pultruded is at least about at the lower end of the melting range of the polyamide.
A US patent (# US 5264060) granted in 1993 to Aluminium Company of America, Pittsburg, PA describes a method for making a fibre-reinforced resin sheet comprising an array of plurality of resin film layers and fibre layers.
A pair of belts, which are interposed respectively between the upper surface of the array & the die and the lower surface of the array & the die, moves through the die. The resin film layers are heated to a temperature of less than about 500F to 1000 F below resin film layers' melting point.
Owens-Corning Fiberglass Technology Inc., Summit, IL in its patent (US patent # 5286320), granted in 1991, described an innovative approach to continuously pultrude a composite sandwich structure.
The process comprises arranging fibre reinforcement materials on the surface of a preformed foam core, applying liquid resin to the reinforcement materials on the surface of the foam cure, heating the surface region of the foam cure to a temperature of at least 1000C to convert water in the foam core to steam, thereby causing water vapour pressure expansion of the foams, and using the expansion of the foam cure to subject the liquid resin to increased pressure.
The process of curing the pultruded composites was patented (US patent # 4861621) by a Japanese company in 1988. The inventors used a UV curable resin mixture and pulled the impregnated continuous reinforcing material through a pipe die having a fixed cross-section.
The die is made of a material capable of transmitting UV rays, which is provided on its inner surface with Silicone or fluorine compound capable of transmitting UV rays and which resists sticking with the UV curable resin.
Pultrusion has now made a foray into a whole lot of industrial & structural applications. Internationally pultruded FRP profiles are being used more & more to substitute conventional structural materials like steel, aluminium & wood for better properties. A list of select manufacturers of pultruded products is as follows :
Note : Information on Member companies of the European Pultrusion Technology Association (EPTA) can be accessed from : www.pultruders.com
Assessing the importance of composites as an advanced performance material in various sectors such as railways, automobiles, building & construction, marine, medical etc., the Advanced Composites Mission was conceptualized by the Department of Science & Technology (DST) and Defence Research & Development Organization (DRDO).
The Mission-mode activities are being implemented by the Technology Information, Forecasting & Assessment Council (TIFAC), an autonomous organization under DST.
While considerable expertise in composite technology exists in India in the national laboratories (NAL, VSSC-ISRO, CSIR, DRDL etc.) and academic institutions, the commercial exploitation of composites is yet to catch up with the international advancements.
The Advanced Composites Mission aims to improve upon the laboratory-industry linkages towards application development & commercialization.
Among a wide array of composite product development, the Advanced Composites Mission has instituted a project focussing on improvement of pultrusion technology and fabrication of products.
The project on 'Development of FRP Pultruded Profiles' has been launched in partnership with M/s. Sucro Filters Pvt. Ltd., Pune with technology support from the National Chemical Laboratory, Pune.
The project aims to develop & commercialize products like gratings, solid rods for electrical insulation, cable trays, ladders etc. The project activities would enable the fabrication of pultruded profiles with excellent surface finish of international standard and Class - I flame retardancy.
The optimization of process parameters & resin formulation with combination of catalysts such as BPO, TBPB & Centamox with varied loading in resin vis-à-vis pultrusion speed could achieve a speed of 1.0-1.5 m/min. for different sections. Efforts are now on to fabricate various products based on pultruded profiles for market seeding towards commercialization.
There has been a few excellent success stories in Indian industrial sector in developing indigenous technologies. In this context, the UNDP assisted project on pultrusion of jute composites (jute + phenol formaldehyde) for structural members (angles, beams, channels etc.) by M/s. TIPCO Polymers, Vapi (Gujarat) merits mention.
Other companies such as M/s. Kemrock Industries, Vadodara in collaboration with M/s. Creative Pultrusions of UK, M/s. Uniglass Industries Pvt. Ltd.-Bangalore, DK Fibre Forms-Pune have entered the field in recent times for catering to Indian & overseas market for pultruded profiles.
Today there is a tremendous interest worldwide in the pultrusion industry. Further technological developments in equipment, raw materials & applications would support its steady growth in the future.
The amount of energy required to fabricate FRP composite materials for structural application with respect to conventional materials such as steel & aluminium is lower and would work for its economic advantage in the end. The pultruded products have already being recognized as commodity in the international market for construction.
Apart from typical pultruded structural sections, innovative thermoset composite products as well as thermoplastic composites would go a long way in developing new application areas thus enhancing its market reach.
India with an excellent knowledge base in resins, catalysts & curing systems coupled with an adequate availability of various raw materials can certainly carve out a niche in the upcoming technology of pultrusion.