CHAPTER 7: POLYMER BLENDS AND COMPOSITES

Transcription

1 CHAPTER 7: POLYMER BLENDS AND COMPOSITES Polymer Science and Technology II November 18, 2015 The major problem in the application of polymers in engineering is their low stiffness and strength. When compared to metals; the moduli are ~ 100 times lower and strengths ~5 times lower 1

2 Homopolymer Moduli: GN/m 2 Strength: MN/m 2 Steel Moduli: 200 GN/m 2 Strength: 200 MN/m 2 Aluminium Moduli: 70 GN/m 2 Strength: 200 MN/m 2 Commodity Plastics The «Big Four» Commodity Thermoplastics PE PP PVC PS More than 85 % (by volume) of world plastic consumption belongs to the «Big Four» plastics. 2

3 Engineering Polymers (Plastics) The polymers that are used in the manufacture of premium plastic products where high-t resistance, high impact strength, chemical resistance and other special properties are required. Engineering Polymers (Plastics) Aliphatic polyamides (Nylon 6, Nylon 66) ABS resin Polycarbonate Polysulfones Poly (phenylene oxide) Polyacetals Engineering polyesters and fluoroplastics 3

4 Methods to increase the stiffness and strength of polymers Novel homopolymer design Crystallization Crosslinking Copolymerization Radiation IPN (Interpenetrating networks) structures Polymer blends and alloys Reinforced polymers Blending may be used POLYMER BLENDS to reduce the cost of an expensive engineering thermoplastic to improve the processability of a high-t or heat sensitive thermoplastic to improve impact resistance 4

5 BLENDING MISCIBILITY Commercial blends may be homogenous phase-separated a bit of both Type of polymer blend (homogenous or phaseseparated) will depend upon many factors such as; kinetics of the mixing process processing temperature presence of solvent or other additives However, the primary consideration for determining miscibility of two polymers is a THERMODYNAMIC ISSUE that is governed by the GIBBS free-energy considerations. 5

6 G m H m T Sm If G is positive at a given temperature, polymers in m the blend will separate into phases. For complete miscibility, two considerations are necessary: G must be negative m 2 nd derivative of G with respect to volume fraction m of component 2 ( ) must be greater than zero 2 2 G m ( ) 2 T, P 2 0 In general, polymer blends, which will separate at equilibrium into two mixed-composition, can exhibit a wide range of phase behaviour, including upper and lower critical solution temperatures. Two phases coexist The blend is miscible at all compositions Idealized phase diagram for a polymer blend 6

7 LCST behaviour is quite common for polymer blends compared to UCTS behaviour. LCST=240 o C. This means, if the blend is melt processed above 240 o C, phase separation occurs. UCST behaviour may be observed only in a solution which is a low MW solvent used. Recently, there has been interest in blends containing three-component polymers. (TERNARY BLENDS) PMMA-PEMA-Poly (styrene-co-acrylonitrile) (SAN) SAN is compatible (miscible)with PMMA and PEMA. But, PMMA and PEMA are immiscible. 7

8 Commercial Polymer Blends: Examples of Miscible Polymer Blends Polymer 1 Polymer 2 PS Poly (methyl vinyl ether) Poly (2,6-dimethyl, 1,4-phenylene oxide) PVC PCL Nitrile rubber PVF (poly vinylidene fluoride) PEMA PMMA Properties of Blends (Positive deviation from additivity) Dependence of miscible blend properties on composition -Properties of miscible polymer blends may be intermediate between those of the individual components (i.e. additive behavior) -In other cases blend properties may exhibit either positive or negative deviation from additivity. 8

9 Compatibilizers Mechanical properties of immiscible blends are often poor due to the inadequate interfacial strength between the dispersed phase and matrix. Additives to promote miscibility by reducing interfacial tension are called «compatibilizers» Reactive compatibilizers: They chemically react with blend components. Nonreactive compatibilizers: Block or graft copolymers of the blend homopolymers. Interpenetrating Networks (IPNs) IPNs are combinations of two or more polymers in network form. IPNs include PUs, PS, PEA, PMMA. Earliest commercialized IPNs used in many automative applications, consists of PP and EPDM (ethylene-propylene-diene terpolymer) 9

10 Sequential IPN swollen with styrene+dvb If no crosslinking agent is used for second polymer in the formation of sequential IPN Polymerizing the second monomer before the equilibrium sorption occurs When both polymers are synthesized and crosslinked simultaneously Semi-IPN (single network of the initial polymer) Gradient IPN Simultaneous IPN (SIN) First or second polymer Other chain polymer To eliminate copolymerization step polymerization 10

11 IPN structures are used for Soft contact lenses Ion exchange resins Pressure sensitive adhesives Controlled release of drugs Preparation of novel membranes Reinforced Polymers Polymer Composites Commercialization of composites began; Cellulose Fibers + Phenolic Resin Urea Resin Melamine Resin The most familiar composite material is Fiberglass R (1940) unsaturated polyester + glass fiber matrix 11

12 Reinforced Polymers: High strength Low weight Uses of polymer composites: Automotive Marine Construction Electrical and electronics Aerospace and military Examples: 1) Competition kayak Epoxy resin: thermoset polymer matrix Lightness Excellent corrosion resistance due to water Economical construction of small batches Kevlar fibers + carbon fibres Essential strength and rigidity Very little cost in extra weight Very fast Manoeuvrable Light In small boats performance is critical low price more importance Glass fibre reinforced plastic (GRP) polyester 12

13 2) Tennis racquet (Nylon matrix+carbon fiber) Nylon 66 matrix Low density Economical construction by injection molding in large batches Polymer is moulded around the low T m metal alloy core 75 km/h 3) Rubber car tyre Reinforced at several different levels. at microscobic level - carbon black mixed with polymer increased stiffness, strength and wear resistance at macroscobic level - rigid cords (polyester fibres and/or steel wires) -to provide strength and stiffness in radial and circumferential direction 13

14 Reinforcing agents have following abilities/functions Must be stiffer and stronger than the polymer matrix It has good particle size, shape and surface character for effective mechanical coupling to the matrix It preserves the desirable qualities of the polymer matrix Mechanism of Reinforcement undeformed state under a tensile load Stress In the presence of reinforcing agent Strain Total strain in the matrix 14

15 The strength of the composite depends on the strength of the bond between particle and matrix. The more interface effective reinforcement Effectiveness of a reinforcement A/V ratio A: surface area of a particle V: its volume A/V to be as high as possible 15

16 - a>>1 fibre - a <<1 platelet Therefore two main classes of reinforcing agents; - fibres (glass fibres, carbon fibers) - platelets (mica and talk) fibre matrix m: mass of composite v= volume of composite m f : mass of fibres occupying a volume v f m m: mass of matrix occupying a volume v m Assuming there are no voids; m= m f + m m v= v f + v m v f f v v m m v m 1 f or f f 1 f ) ( density of composites m proportionsof fibre in the matrix by volume f m density of fibre density of matrix Reinforced polymers generally have low densities. Epoxy reinforced with 70% carbon fibres is only 1700 kg/m 3 (Density of water 1000 kg/m 3 ). 16

17 In practice, composite materials contain voids which comprise trapped air or solvents, etc. A void is source of weakness. A void content greater than 2 % poor fabrication A void % < 0.5 % high class «aircraft quality» fabrication POLYMER MATRICES Initially thermoset polymers 1) Thermoset polyesters - Inexpensive polymers - Used with glass-fibre reinforcements 2) Epoxies Preferred to polyesters - Superior mechanical performance - But higher cost than polyesters TODAY Reinforced thermoplastic materials Popular matrices: Semi-crystalline polymers PP and Nylon Major advantage: Forming is possible by normal injection moulding or extrusion. 17

18 Fibrous Reinforcement Glass fibers E-glass S-glass Oriental polymeric fibers (Aramid) Kevlar-Du Pont Company Carbon fibers Glass fibre is widely used one. E-glass SiO % CaO 17.5 % Al 2 O % B 2 O % MgO 4.5 % S-glass: higher modulus and strength C-glass: improved resistance to water and acids Glass fibers are manufactured by extruding molten glass at high linear velocity through a large number ( ) of holes in a platinium plate known as a «bushing». Then they cooled and solidify. O Structure of silica glass tetrahedrom Si O O O 18

19 Advantages of glass fibres: Resistance to high temperatures - softening point is 850 o C. Transparency to visible light takes the colour of matrix Isotropy such as thermal expansion is identical in axial and radial directions. Disadvantages: Susceptible to surface damage Carbon and Kevlar fibres are less widely used than glass fibers due to their relatively high cost. Best carbon fibers are produced from PAN (polyacrylonitrile) PAN is converted to graphite by controlled heating process. 19

20 Advantages of C fibers: Chemical inertness: resistance to moisture and common chemicals High electrical and thermal conductivity along the fibre axis Dimensional stability: thermal expansion is low and negative Disadvantage: Colour: black Oriented Polymer Fibres Aramid (Aromatic Polyamid) Fibres Aromatic groups Amide groups H H C O O C N O N O n Kevlar 49 : Poly (paraphenylene terephthalamide) (mostly used one) 20

21 The order of excellence carbon > aramid > glass Platelet reinforcement Talc: 3MgO.4SiO 2.H 2 O Mica: K 2 O.3Al 2 O 3.6SiO 2.2H 2 O Both are crystalline μm 1-5 μm thickness Low price Stiffness and strength Naturally-occuringmaterials, never obtained in pure form 21

22 Interfacial adhesion and coupling agents: Coupling agents low MW organic-inorganic compounds that promote adhesion between filler and matrix. Generally organofunctional silanes Engineering thermoplastic: PEEK (T m =334 o C) 30 % fiber loading Mechanical properties of composites are strongly influenced by the size type concentration dispersion of reinforcing agent (filler) interfacial tension between the matrix and filler 22

23 Composites are produced by a number of methods: Compression moulding Resin-transfer moulding Filament winding and pultrusion Filament winding operation 23

24 Pultrusion line Nanocomposites End of 1980s. Toyota researches developed first nanocomposites from Nylon-6 Clay nanofillers thickness: ~1nm aspect ratio (D/L): 10:1 1000:1 Commonly used clay nanofiller is montmorrilonite (mmt): naturally-occuring silicate 24

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