A/5. THERMOSET POLYMER MATRIX COMPOSITES

1.1. THE AIM OF THE EXERCISE

During the exercise a fiber reinforced thermoset matrix product will be produced from different glass reinforcements and polyester resin. The composite product will be produced by hand lay up, which is the most widely used and simple manufacturing technique.

1.2. THEORETICAL BACKGROUND

The engineering practice distinguishes three structural material groups: metals, polymers and ceramics. The composites are structural materials combining two or more of the basic materials. The composites represent the most up-to-date engineering structural materials. Their existence originates by recognizing that the loading conditions are not the same in every direction in space. These loading directions can be well determined in most of our technical components, parts and structures. In many case strength and stiffness must be some orders higher along these loading directions. This requires to locally reinforce the homogeneous structural materials with reinforcements having higher strength and/or modulus. The composite is: - a multiphase system (the constituents are separated by phase borders) - compound: consisting of at least two materials, which are - reinforcing material (typically fiber reinforcement) and - enclosing (embedding): matrix material, and can be characterized, that - between the reinforcing material having high strength and usually high modulus - and the lower strength matrix - the connection is excellent (adhesion properties), which - can be maintained on a high level of deformation and loading conditions. The reinforcing material provides the strength and stiffness. The matrix material holds the fiber bundles together, protects the fibers from the environmental and physical exposure and distributes the loads.

Composite: the polymer matrix based composite is a rigid material, which consists of at least two constituents: the low strength and density embedding matrix material, and the high strength and/or high modulus fiber type reinforcing material. Studying the arrangement of the constituents it can be concluded that the continuous (matrix) phase encloses the finely dispersed other phase (reinforcement), between the phases there is excellent adhesion which remains on high deformations.

1.2.1. Geometrical considerations

The reinforcing material of composites is typically (not only) fiber type. Above all, the engineering logic indicates the use of fibers, the product has to show high strength and/or stiffness in the loading directions. The additional reason for fiber reinforcement is to increase the specific area of adhesion, which is a determinant factor influencing the composite properties. The most important criteria of reinforced materials is the good adhesion between matrix and reinforcement, the larger the specific area of connection the better is the adhesion In this manner, the question is: when is the specific area (A) of a fiber the largest at a given volume (V)? The specific area in relation to the volume: A 2 ⋅ r 2 ⋅π + 2 ⋅ r ⋅π ⋅l = , V r 2 ⋅π ⋅l where, r is the fiber radius, and l the length. Rearranging we can derive the specific area: A 2 2 = + . V l r The equation will be large at the two extreme values: a) if l » r, or b) if r » l.

The first geometry is when the diameter is very small in comparison to the length, this is the geometry of fiber. The second cases are large diameters with small thicknesses, i.e. flakes, disk-shaped geometries. The reinforcing effect in given directions can be achieved with the fiber type reinforcements. In practice the l/d ratio (shape factor) has importance to discriminate short and long fiber composites. If l/d>50 the composite is long fiber reinforced, if the fiber length is smaller than the critical fiber length we call it short fiber composite. Critical fiber length is the fiber length when tensile load causes fiber fracture in a composite. Smaller fibers slip out from the matrix material without fracture.

Fig. 1 is used to determine the critical fiber length.

FIG. 1. FORCE EQUILIBRIUM OF A FIBERELEMENT

The force equilibrium: dσ σ ⋅ R 2π +τ ⋅2Rπdz = (σ + f Δz)R 2π . f f dz Simplifying and integrating the equilibrium we can derive that the stress increases proportionally with the coordinate z to an Lc/2 critical length (Fig. 2.):

σ R L z = f = c , 2τ 2 from which the critical fiber length in relation to the fiber diameter is:

L σ c = f . This is the „Kelly-Tyson” formula. D 2τ

FIG. 2. THE ARISING STRESS (σFM) ON THE FIBER-MATRIX INTERPHASE VERSUS CRITICAL FIBER LENGTH (LC)

The size effect also indicates the usage of fibers, because structural defects also cause reinforcement fracture in composite materials. If the probability of defects is a given number in an examined volume, than we get the most efficient reinforcing effect if we manufacture a fiber with very small diameters. So, we minimize the probability of a defect in a given length. This size effect can be easily proved by testing fibers with different diameter. Fig. 3. shows that in cases of d<10 μm the tensile strength increases significantly.

Tensile strength [GPa]

Fiber diameter (μm) FIG. 3. FIBER STRENGTH VERSUS FIBER DIAMETER [2]

One more reason to use fibers as reinforcing material is the flexibility of the fibers. This can be proved when we have a look on the bending stiffness. It is known that the stiffness is determined by the product of Young’s modulus (E) and the moment of inertia (I). This second moment of area is: D 4π I = , 64 which is proportional of the fourth power diameter. On the other hand

E ⋅ I = M ⋅ R* , where M is the bending moment on the fiber and R* is the radius of curvature.

The compliance is the reciprocal value of the stiffness 1 1 = , E ⋅ I M ⋅ R*

1 M 1 * = ⋅ 4 , R E π ⎛ D ⎞ ⋅⎜ ⎟ 4 ⎝ 2 ⎠ resulting that the compliance is disproportionate to the fourth power fiber diameter. Practically this means the finer the reinforcing fiber the easier to lay it into sharp corners, it can be bend to smaller radiuses. This makes possible to manufacture complex shapes.

1.2.2. Reinforcing materials of polymer composites

In the composite manufacturing technologies natural (flax, hemp, sisal, etc.), mineral (ceramics, basalt, etc.), natural based manmade (viscose, acetate, etc.) and manmade (glass, carbon, aramide, HOPE, etc.) fiber types are common. In details we will discuss the manmade fiber types.

1.2.2.1. GLASS FIBER The glass as structural material belongs to the silicate material group. Manily it is from silicon-oxide, which gives 55-65 % of glass. It contains other metal-oxids and together with the silicone they conjugate a macromolecule with high cohesive enrgy primer atomic or ionoc bonds. From the glass melt a high strength fiber can be drawn through a special spinner, thousands of single fibers are bundled together to form a roving. The characteristic diameter of the single fibers are 8-17 μm. The glass fibers need surface treatment like other fiber types. On one hand a protection against the further mechanical processing steps (i.e. weaving) is required, this is the sizing. The sizing material has a provisional protective function and it holds the fibers together. On the other hand, a good connection should be provided between the fiber and matrix materials. This can be done by different epoxy compounds, vinylsilane, or phenolic coatings, these are the so called finishes. The glass fiber is the most widely used reinforcing fiber, it’s physical and mechanical properties are listed in Tab. 1.

Advantages: − cheap, − available in big amounts, − UV stable, chemically inert, electrical insulator. Disadvantage: − highly abrasive (at specific manufacturing technologies where friction is presented on the tool surface), − relatively high density, − brittle, − low Young’s modulus.

1.2.2.2. CARBON FIBER The versatile modes of connection and configuration of carbon atoms forming a carbon chain is a very interesting and well researched part of the material science. The engineering properties of the synthetic polymers are defined by the strength of the carbon-carbon bonds. The highest carbon-carbon linkage force is known in an extremely strict formation: the diamond by the covalent bonds with the best atomic conformation is a hardness etalon. Carbon black, which has high specific area and is a chemically bonded filler in elastomer matrix systems is also well known carbon structure. The carbon fibers utilize the graphite structure of carbon. The graphite structure provides excellent strength in the plane of the lamellas built up from hexagonal units. The carbon fibers utilize the strength of graphite together with its high modulus. Several polymer fibers can be used as a precursor in the carbon fiber processing. A precursor is successful if it does not melt or burn during the oxidation and graphitization process and the desirable graphite structure can be achieved. The length of oxidation and graphitization times can affect the fiber strength. The strength and the modulus of the precursor based fibers can be varied on a wide spectrum. From 1997 PAN (polyacrylnitryl) based carbon fibers are produced in Hungary by the ZOLTEK company.

Advantages: − low density, − high modulus, − high strength, − low coefficient of thermal expansion.. Disadvantage: − brittle, − high price.

1.2.2.3. ARAMID FIBER The aromatic polyamide (aramide) fibers gain their high strength because of the high orientation (stretching). Aramide fibers can be para- or meta-aramide according to the connection type. In practice the para type aramide fibers are well known (trade names: KEVLAR, TWARON, etc.), they exhibit high tensile strength. Their high strength and tensile strain made it possible to use them in elastomer based composites, i.e. braced tread tires. Using aramide fibers as reinforcement will lead to an excellent tough and impact resistant structure (i.e. bullet proof applications).

Advantages: − low density, − high strength, − good dynamical properties, − flexible, − flameproof. Disadvantage: − low resistance against environmental loading (UV and humidity sensitive) − low compression strength.

1.2.2.4. POLYETHYLENE FIBER From the ultra high molecular weight (UHMWPE) gel type polyethylene solution high strength and low density fibers can be drawn (these are the highly oriented polyethylene HOPE fibers, trade names: Spectra, Dyneema). Their wide spread usage is hindered by their limited heat resistance (max. 140 °C), and that PE exhibit low adhesive properties to other polymers. They can only fulfill the fiber-matrix adhesion requirement in case of special surface treatments.

The main properties of reinforcing fibers are listed in Tab. 1.1.

Tensile Young’s Strain at Density strength modulus break Fiber type ρ σ E ε [g/cm3] [GPa] [GPa] [%] Glass 2,5-2,8 3,2-4,6 70-85 1,8-5,7 Carbon 1,7-2 2-7 200-700 0,5-1,5 Aramid 1,44 2,8-3,8 60-130 2,2-4 Polyethylene (Spectra) 0,97 2,3-3,6 73-120 2,8-3,9 Steel wire 7,6 4 240 1,4

TAB. 1. MECHANICAL PROPERTIES OF THE REINFORCING FIBERS

1.2.3. Commercial forms of fibers

Commercially the reinforcing fibers are in 1D, 2D and 3D forms. The roving (bundle) and tape forms are the one dimensional types, the woven, knitted or non woven mats are in the 2D group. Reinforcements containing fibers at least 3 directions of the space are the so called 3D reinforcements, i.e.: stitched mats. By fiber arrangement orientation can be given to the reinforcement. Mats are containing fibers in every planar direction, this means there is no special distinct direction in the plane of the reinforcement. In unidirectional layers the fibers are aligned in one direction, mechanical properties are excellent only in the fiber direction. Reinforcements produced by weaving usually have two perpendicular roving directions, but multidirectional wovens exist also.

The mechanical properties of different commercial reinforcing structures are usually presented in polar diagrams. The tensile strength and Young’s modulus of the mentioned forms of reinforcements are presented in Fig. 4.

FIG. 4. POLAR DIAGRAM OF STRENGTH AND MODULUS OF DIFFERENT FORMS OF REINFORCEMENTS [3]

1.2.4. Matrix materials of polymer composite

Various polymers are used as matrix materials for composites. The polymer matrix materials can be divided into to basic groups: thermoplastics and thermosets. As their names show temperature is needed during the processing method. Thermoplastic parts (i.e. from polypropylene) can be manufactured by a melting – forming – cooling process. At the end of the process we get a linear structure (not crosslinked), which means the processing steps are irreversible. Thermoplastic based composites are usually reinforced with short fibers (1-5mm) and these are processed by injection molding or extrusion. Nowadays self reinforced polymers are also appeared, in which the fiber and the matrix material is the same thermoplastic. In these days mainly thermoset polymers are used as high performance composite matrix materials. The thermoset materials (i.e. epoxy, polyester, vinylester, polyimid) are changing from liquid to solid phase by an irreversible process, at the end we get a crosslinked structured. If this crosslinking process is done they can not be re-melted, however the temperature change greatly affects their mechanical properties. Their big advantage is that they are in a fluid phase at room temperature and for their processing only low or moderate pressure is required, and they are low cost materials. Unfortunately their recyclement is not fully solved yet. Polyester and epoxy thermoset matrix materials are used widely in industrial applications. Strength and stiffness of fiber reinforced composite structures are defined mainly by the fiber used in them, because the matrix materials have strength and modulus two orders lower than the reinforcement.

Viscosity is a measure of the resistance of a fluid to being deformed by either shear stress or extensional stress. It is commonly perceived as "thickness", or resistance to flow. Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction. Thus, water is "thin", having a lower viscosity, while vegetable oil is "thick" having a higher viscosity. 1.2.4.1 POLYESTER Polyester resin is the most widespread used thermoset matrix material, it can be built up from different acids, glycol and monomer. In general it is a viscous fluid, which contains polyester solved in a monomer (styrene). The styrene helps to lower the viscosity and promotes the crosslinking of the linear molecules without a by- product. The polyester resins can be stored only to a limited period because they tend to gel with time. As this time is too long to wait for during manufacturing the crosslinking process can catalyzed (catalyst) and accelerated (accelerator). After adding the initiator the crosslinking reaction starts. The initiator itself is not taking part in the chemical reaction, it is only catalyzing that. After a few minutes (this time depends on several factors) the crosslinking has started the system reaches the gel time, meanwhile an exoterm reaction occurs because of the chain polymerization. To the final hardening the temperature rises radically (Fig. 5.)

FIG. 5. THE EXOTHERM HEAT EFFECT OF UNSATURATED POLYESTER [1] G GEL TIME; H HARDENING TIME

1.2.4.2. EPOXY The epoxi resins give the best properties from the thermoset resin family. Both their mechanical properties and environmental resistance are very good, that is the reason of usage in industries like aviation and aerospace. The epoxi resins have several advantages. On one hand they have low viscosity, which means they can be used at room temperatures to fill the available space. On the other hand they exhibit low shrinkage, meaning that the remaining stresses after crosslinking can be kept at a low level. Compared to the unsaturated polyesters the epoxies are also used with reinforcements or fillers. In case the reinforcement is identical, composites manufactured with epoxy resins exhibit better strength and stiffness properties than polyester composites, principally this is true if long term or cyclic loading is considered.

1.2.4.3. OTHER MATRIX MATERIALS Other matrix materials are also used for special applications: vinylesters (chemical resistance) furan resins (chemical and heat resistance), or acrylic resins (chemical resistance) are common. 1.2.5. Manufacturing methods of polymer composites

The layered structure provides the main technical advantage of the composite construction: the strength and stiffness of the whole product or part can be optimized with a low weight in a way that the best properties are in the preliminarily defined load directions. The design steps are defining the main loading directions, analyzing each layer according to failure criterions and to design the whole lay-up of the composite part. During designing the producibleness should be taken into account (costs, technology, length of serial, etc.), which determines the manufacturing process. One of the main questions is the handling of the crosslinking process: to convert the basic oligomer and monomer containing material to a fully crosslinked structure. Practically it is important that fully conversion occurs (which means that 100 percent of the reactive groups are converted) because physical, mechanical and chemical properties are greatly influenced by the conversion. The residual non- reacted monomer can significantly alter the properties of the composite product. Several processing method is available for manufacturing composite parts. During the exercise the hand lay up technology will be used to produce a sample.

1.2.5.1. HAND LAY UP Some ten years ago the first modern polymer composites were serial manufactured by hand lay up, by laminating layers on top of each other (i.e. the glass fiber reinforced polyester debarkation vessels in World War II). This technology is widely used because its low cost and ease of manufacturing. The hand lay up is the most cost effective in short series and prototype building polymer composite processing technique. The schematic of hand lay up is shown in Fig. 6. The surface of the positive or negative mold must be treated with a release agent, so the part can be easily demolded. The outer layer of the product is the gel coat, a resin rich layer containing fillers to protect the part against the environmental and mechanical effects (weather, UV, wear and scratch, etc.). After the gel coat the layers giving the sufficient mechanical strength and thickness are applied, the impregnation is done by with the help of brushes and rollers. The most important is to produce a well impregnated, air and void free wall thickness during the hand lay up process.

FIG. 6. BASICS OF HAND LAMINATION The final product can be built up from dozens of layers, but care must be taken when the matrix material is chosen (gel-time, exoterm heat). The wall thickness of the laminate can vary, it can contain local reinforcements, stringers, ribs, metal inserts or can be a sandwich construction. Curing (crosslinking) should take place according to the chosen resin system, usually at room temperature, or in some cases elevated temperatures can be used. It is advisable to use a postcuring (at elevated temperature) step, this helps the fully conversion of reactive groups. Finally, cutting, grinding and other postprocessing steps can be done.

1.2.5.2. OTHER MANUFACTURING METHODS The common characteristic of polymer composite processing methods is that from the liquid matrix and fibrous reinforcing material a crosslinked part can be molded at controlled temperature and pressure circumstances. Below the most important and wide spread composite manufacturing technologies are shortly introduced.

Spray-up is an industrialized variant of the hand lay up process. Through a special mixing and spraying head cut fiber and resin mixture can be sprayed up onto the surface of a mold. Parts with large dimensions can be molded in a cost effective way (i.e. ship hulls and decks, flat panels, etc.).

Compression molding is a large scale manufacturing method, in which heated hydraulic presses with precisely matching metal molds are used. The matrix and reinforcing materials are preliminary mixed together, mixing and feeding can be automated. Parts with short cycle times in long series are the typical compression molded products (i.e. automotive parts, covers, insulating parts, etc.).

Filament winding: resin impregnated endless fibers (rovings) are winded onto a rotating axially symmetric (usually cylindrical) mandrel (mold). The orientation of the fiber bundles can be preliminary calculated according to the loading. To promote demolding the mandrel must be tapered. Pressure vessels and pipes are the most usual filament wound products.

Pultrusion: The pultrusion is similar to the extrusion processing method used with thermoplastics. The main difference is that in pultrusion the impregnated continuous fiber reinforcement is pulled through the mold. This is the only continuous processing method in thermoset polymer composite manufacturing. Only one-dimensional products can be manufactured, such as profiles, beams, stringers, etc.

Liquid molding: Liquid molding is a group of manufacturing methods where reinforcement is laid in the mold and the resin is forced in with pressure (positive or negative). The resin flow impregnates the reinforcement. Parts with very good mechanical properties can be molded with this manufacturing method (i.e. aircraft parts, high performance products). 1.2.6. Sandwich structure

A sandwich structure is built up from two high strength and stiffness parallel skins (faces) and in between them low strength, weight and density core material can be found. The three layers must form one mechanical unit, usually the core material have higher thickness than the skins. The main advantage of sandwich constructions is high bending stiffness with low additional weight. The achievable mechanical properties of sandwich structures are shown in Fig. 7.

FIG. 7. MECHANICS OF SANDWICH STRUCTURES The sandwich structures are widespread within the composite industry. It is because the high strength composites have lower stability and bending stiffness in comparison with metal parts with the same dimensions. This is due to the fact that composites have the same strength as metals, but their modulus is usually much lower. This yields to stability problems in case of composite shell structures. The stability problem can be avoided by stringers, ribs or by the cheaper and more aesthetic sandwich construction. On the other hand, it is quite easy to manufacture the sandwich structure: - The two skins are manufactured in a form and the core material is bonded in between them. - The two skins are manufactured in a form and the core material is foamed in its final place. - Before the skin is cured core material is placed onto it and with pressure it is bonded to the laminate by the matrix material of the composite. Afterall the other skin can be bonded or laminated onto the part. - The skins are laminated onto the preliminary cut and shaped core material. Different kind of core materials are known, their basic materpuial can be aper, aluminum, polymer, wood, etc. Polymer cores, mainly foamed PUR and PVC are the most widespread. 1.3. EXERCISE DESCRIPTION AND TASKS

During the exercise glass fiber reinforced polyester composite parts will be manufactured by the hand lay up process.

The exercise: 1. Clothing of the protective wear. 2. Preparation of the tool. 3. Cutting and metering the reinforcement material. 4. Metering and initiation of the matrix material. 5. Hand lamination.

1.4. EQUIPMENTS AND MACHINES

o Protective wear, gloves, eye protection; o scissors, brush, roller; o scale.

1.5. THE MOST IMPORTANT TERMS IN HUNGARIAN, ENGLISH AND GERMAN

Hungarian English German gyanta resin s Harz héj, fedőlemez shell, skin e Deckschicht kompozit composite r Faserverbundwerkstoff maganyag core r Kernmaterial paplan mat e Fasermatte szálerősítés fiber reinforcement e Faseverstärkung száltartalom fiber content r Fasergehalt szendvicsszerkezet sandwich structure r Kernverbund szénszál carbon fiber e Kohlenstoff-Faser szövet woven structure, fabrics s Gewebe üvegszál glass fiber e Glasfaser

1.6. RECOMMENDED LITERATURE

1. Czvikovszky T., Nagy P., Gaál J.: Polimerek technológiája, Műegyetem kiadó, 2000 2. G. W. Ehrenstein: Faserverbund-Kunstoffe, Hanser Verlag, München, 1992 3. Hintersdorf: Műanyag tartószerkezetek 4. P. K. Mallick, S. Newman: Composite Materials Technology, Hanser Verlag, New York, 1990