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).