Subject: Composite Materials Science and Engineering Subject code: 0210080060

1. Subject: Composite MaterialsScience and Engineering Subject code: 0210080060 Prof. C. H. XU School of Materials Science and Engineering Henan University of Science and Technology Chapter 7: Polymer Matrix Composites (PMCs)

2. Polymer Matrix Composites (PMCs) This chapter covers • Introduction to polymer matrix composites (PMCs) • Manufactures of PMCs • Some commercial PMCs

3. Introduction to PMCs The most common matrix materials for composites are polymeric. • Strengths and stiffness of polymer are low compared with metal and ceramics → improve properties by reinforcements • Processing of polymer matrix materials (PMCs): normal temperature and pressures → less degradation of the reinforcement during processing; simple equipment for processing • Disadvantages are low maximum working temperature, high coefficients of thermal expansion and sensitivity to radiation and moisture

4. Introduction to PMCs • Classification of the polymer for the polymers used for matrices of composites • Matrix: Thermosets ThermoplasticsElastomers Polyester resin (聚酯树脂) Acetyle Epoxy resin（环氧树脂） polypropylene Phyenolics (酚醛树脂) Nylon Bismaleimide Polycarbonate Polyimides (聚酰亚胺) Polyetheretherketone (PEEK) Most widely used matrix (¾) Less used matrix For large parts for small parts

5. Specific strength 103 s/r(MPa)/(Mg/m3) polyethylene 3 2 1 Aramid carbon Glass asbestors SiC Boron alumina 0 50 100 150 200 Specific modulus E/r (GPa)/(Mg/m3) Introduction to PMCs • Reinforcements (Fibers) • E-glass • Carbon • Aramid

6. Manufacturing Process of PMCs • Final Products • Hand (Manual) lay-up technique • Spray lay up • Vacuum bag molding • Filament winding • Pultrusion technique • Compression molding technique • Resin transfer moulding (RTM) • Reinforced reaction injection moulding Raw Materials 1) Fibers 2) Fabrics 3) Prepreg 4) Molding compound 5) Core structures

7. Manufacturing Raw Materials • 1) Fibers • 2) Fabrics • 3) Prepreg • 4) Molding compound • 5) Core structures

8. Prepreg（预浸料怌）Materials • Because many epoxy resins have a high viscosity, it is often difficult to mix the resin into the fabric and encapsulate (压入）the fibres. • Prepreg: continuous fiber, fabrics or mat, reinforcement preimpregnated（ 预浸渍的）with a epoxy resin that is only partially cured. • It can be stored up to more than 1 year. • Manufactures can use prepreg materials to make products without having to add any resin.

9. (b) (d) (c) (a) (e) Prepreg (a) Spooled fiber (b) A thin film of heated resin solution of relatively low viscosity is coated on release paper (c) Sandwich and press fibers between release sheets and carrier paper using heated rollers (d) Remove release paper (e) Spooled prepreg tape

10. Molding compound Molding compounds are made of short or long fibers impregnated with resins. The compound contains uncured thermosetting resins and uniformly distributed short fibers. They are raw materials for compression molding. A resin paste is placing on a polyethylene(聚乙烯） moving films, Continuous strands of glass fibers are chopped and dispersed over the moving resin paste. Top and bottom film with paste and fibers pass compaction rollers. Schematic of molding compound manufacturing

11. Core structure • Core materials is the central member of an assembly. • When bonded between two thin face sheets, it provides a rigid, lightweight component. • Composite structures are referred to as a sandwich construction.

12. Core structure • Two popular core structures are foam and honeycomb. • Materials of cores are aluminium, Nomex（聚酰胺） and thermoplastic honeycombs Aluminium Nomex thermoplastic honeycombs

13. Manufacturing Final Products Methods vary from simple labor intensive for one-offs to automated methods for rapidly producing large numbers of complex components. The method of product selected by a manufacture depends on factors, such as cost, shape of components and required performance

14. Basic steps in manufacturing composites There are four steps in manufacturing composites: • Impregnation浸渍: Fibers and resins are mixed together to form a lamina. • Lay-up铺层：Composite laminates are formed by placing fiber resin mixtures or prepreg at desired angles and at places where they are needed. • Consolidation 合并: creating initiate contact between each layer of prepreg or lamina, removing air, • Solidification (curing) 凝固: take a minute for thermoplastics or 120 min for thermoset. Vacuum or pressure is maintained during this period.

15. Hand(manual) Lay-up Technique

16. Hand Lay-up Technique • A releasing agent, such as silicone is applied to the mould to allow easy removal the finished part. • The woven roving (fabric) or chopped strand mat (CSM) is laid in the mould. • Liquid thermosetting resin is mixed with a curing agent. The mixture is poured on and is then rolled or squeezed(压) evenly over the surface, with attention to removal air. This is done in layers until the part is done. • Curing is often done at room temperature, but hot air blowers and infrared lamps can accelerate the process.

17. Hand Lay-up Technique Advantages of hand lay-up technique: • Tooling can be made of any material that can withstand a small pressure. • Non-expensive (little capital equipment). • Highly skilled workers are not required. • Suitable for one-offs and large components Disadvantages of hand lay-up technique: • Labour intensive • Low reinforcement content of about 30 vol% and voids are common in matrix. • The strength of the materials tends to be poorer compared to other composite methods. • Resin might run when on non-horizontal surfaces, causing pooling of resign. In these cases higher viscosity resins are often used.

18. Spray Lay up Description:Fibre is chopped in a hand-held gun. Chopped fibres, resin, and catalyst are fed into a gun which spray of resin directed at the mould. The deposited materials are left to cure under standard atmospheric conditions. The sprayed composite has to be rolled to remove entrapped air and to give a smooth surface finish.

19. Spray Lay up • Main Advantages: • Low cost way of quickly depositing fibre and resin. • Low cost tooling. • Suitable for large components • Main Disadvantages: • Laminates （碾压层）tend to be very resin-rich. • Only short fibres are incorporated which severely limits the mechanical properties of the laminate. • Resins need to be low in viscosity to be sprayable. This generally reduces their mechanical/thermal properties.

20. Basic Processing Steps Release agent is applied to the mold Prepreg is laid on the mold surface until to applied all the prepreg sheets. Air between prepreg sheets is removed by using a squeezing roller after applying each prepreg sheet. Employing a vacuum bag in special equipment for curing (solidify the composite). (a) Vacuum forming (b) Autoclave (高压锅)molding (c) Pressure bagging After cooling, removing vacuum bag and take out parts. Vacuum Bag Molding

21. Vacuum Bag Molding in step 2 Prepregs commonly come in various widths (3 to 72 inches) and have a leathery feel. They are slightly tacky so that they will stick when formed. The prepreg materials degrade over time, and should be kept in cool environments.

22. Vacuum Bag Molding in step 2 Automated lay-up • The productivity of the hand lay-up can be automated using CNC machines. • Materials: prepreg tape and prepreg fiber • No limit to the size and relatively flat shape • Should be no gap, overlap or wrinkles （皱纹） • Automatic multi-axis fiber placement machine-dispensing numerous narrow individual tapes • Application: aerospace industry

23. Vacuum Bag Molding in step 3(a)-vacuum forming Vacuum forming uses atmospheric pressure to press the cloth tightly. This technique requires a vacuum bag, vacuum pump capable of pulling a significant vacuum (at least 25 inches of mercury) and various accessories. Thus vacuum forming has been mostly restricted to large commercial use.

24. Vacuum Bag Molding in step 3(b) -Autoclave Curing • The part is placed in the pressure vessel, and heated, Gas pressure (N2, about 5 MPa) is applied simultaneously. • The heat accelerates the curing of the thermoset, or melting of the thermoplastic resins. • Although autoclaves are expensive, they produce better parts, and can process many parts at the same time. • using for aerospace industry Basically an oven that also uses pressure.

25. Vacuum Bag Molding in step 3(c)-pressure begging Pressure bagging: A flexible bag is placed over the lay-up components on the mould. Inflation of the bag by compressed air forces the lay-up into mould. Cheap bag

26. Vacuum Bag Molding Advantages of vacuum baggingmould technique: • Because the resins (prepreg) are mixed by the manufacturer, the ratio of components is more closely controlled. The manufacturer also ensures better distribution of the resin in the cloth. • Automated machines can also be used to overcome efficiency problems Disadvantages of vacuum bagging mould technique: • Vacuum bagging is required to properly consolidate layers and remove voids. • Expensive curing ovens must typically be used. • It can be difficult to bag complex parts.

27. Filament Winding • This process is primarily used for hollow, generally circular or oval sectioned components, such as pipe-works, pressure vessels, storage tanks and aerospace parts. • Winding directions: Hoop or Helical winding

28. Wet winding: Fibre tows are passed through a resin bath before being wound onto a mandrel in a variety of orientations, controlled by the fibre feeding mechanism, and rate of rotation of the mandrel. • (b) The composite is cured at room temperature or elevated. • (c) After curing, the mandrel is extracted from the composite part and then reused.

29. Main Advantages: • This can be a very fast and therefore economic method of laying material down. • Resin content can be controlled by metering the resin onto each fibre tow through nips or dies. • Fibre cost is minimized since there is no secondary process to convert fibre into fabric prior to use. • Structural properties of laminates can be very good since straight fibres can be laid in a complex pattern to match the applied loads. • Main Disadvantages: • The process is limited to convex (凸起的) shaped components. • Mandrel costs for large components can be high. • The external surface of the component is unmoulded, and therefore cosmetically unattractive. • Low viscosity resins usually need to be used with their lower mechanical and health and safety properties.

30. Pultrusion • Fibres are pulled through a resin bath and then through a heated die. The die completes the impregnation of the fibre, controls the resin content and cures the material into its final shape. • This cured profile is then automatically cut to length. • Pultrusion is a continuous process, producing a profile of constant cross-section.

31. Pultrusion • Main Advantages: • This can be a very fast, and therefore economic way of impregnating and curing materials. • Resin content can be accurately controlled. • Structural properties of laminates can be very good since the profiles have very straight fibres and high fibre volume fractions can be obtained. • Resin impregnation area can be enclosed thus limiting volatile emissions. • Better surface finishing • Main Disadvantages: • Limited to constant or near constant cross-section components • Heated die costs can be high.

32. Compression molding process • Raw materials • Molding compounds • The mold is preheated to 1400C. • Put sheet molding compounds. • Close up-mould for applying pressure. • Keep 1-4 min, compounds become final shape

33. Compression molding process • Laminar structures (Molding compounds) • A laminar composite is composed of two-dimensional sheets or panels that have a preferred high-strength direction. • The layers are stacked and cemented together so the high-strength directions in the layers are different.

34. Compression molding process • Sandwich Panels(Molding compounds) • Sandwich panels, one class of the structure composites, consist of two strong outer sheets or faces, separated by a layer of less-densed material or core, which has less stiffness and lower strength. • The faces carry most of the inplane load, and the core separate the faces and carries load perpendicular to the faces.

35. Compression molding is very popular in the automotive industry because of its high volume capabilities. • Advantages: • Suitable for high volume products, mould cycle time is 1-4min. • Production of low-cost components and inexpensive. • High surface quality and good styling possibilities • Multiple parts can be consolidated into one single part. • Disadvantages: • Mould design is expensive. • The process is not suitable for making a small number of parts Car composite parts

36. Resin Transfer Moulding (RTM) (a) (b) (c) (a) A mould is filled with fibre and it is closed. (b) The mould is often in vacuum before injection. (c) Resin is injected at high pressure and high temperature (curing). The pressure of injection wets the fibres.

37. Resin Transfer Moulding (RTM) Advantages: • Very large and complex shapes can be made efficiently and inexpensively. • Production times are very short compared to lay-up. • Better surface definition (smooth) than lay-up. • Inserts and special reinforcements are easily added. • Part consistency is good. • Worker exposure to toxic chemicals is reduced. Disadvantages: • Mould design is complex. • Material properties are good, but not optimal. • Reinforcement may move during injection, causing problems

38. This process was used to make car body panels. The fibre in the mould can be any that holds its shape during the injection. Layers are often stitched, and bonded. Inserts/ribs/etc can easily be put into the mould before it is closed. Most resins can be used, but low viscosities are useful. parts

39. Variation of RTM – Reinforced reaction injection moulding (RRIM) • The RRIM process is similar to RTM. • A mould is filled with fibre and closed • In the PRIM process, two resins A and B are mixed in a mixing chamber just before injecting in the mould. • Liquid jets is filled into fibers in closed die. • Cycle time is short (1-2 mins) due to part of cross-linking process of polymer done during mixture.

40. Comparison of costs & process efficiency of PMC production processes

41. Glass fibre-reinforced polyesters the largest proportion of the market Less cost, lower strength, less heat and weather resistant, shrinkage of 4-8% during curing Glass fibre-reinforced epoxy Better properties, shrinkage of 1-5% during curing. Good bond between matrix and glass fiber Some Commercial PMCs- Glass reinforced thermoset composites(玻璃钢） Properties of unidirectional glass reinforced epoxy and polyester

42. Reinforcement: Glass fiber or fabric Coupling agent (VOLAN) enhances the wetting and bond between glass and polyesters Mechanism of coupling (a) Volan hydrolyze （水解） (b) Absorb water on the surface of glass (c) Reaction between volan-volan and volan - the absorbed water’ glass (d) Drying (e) Bond between C=H2 and polyesters VOLAN (b) (a) (c) (d) Some Commercial PMCs- Glass reinforced thermoset composites(玻璃钢）

43. Some Commercial PMCs- Glass reinforced thermoset composites(玻璃钢） Water reduces the strength of the composites. The reasons are • Water degrades glass fibers in composites (Water reaches fiber via defects in the matrix.) • Water acts as plasticizer (可塑剂）. • Water reduces glass transfer temperature (Tg) of matrix The effects of water on glass transition T (Tg) of six cured epoxy resins ●caculated ○experimental The strength of epoxy laminates versus time exposed to boiling water ( 4 coupling agents)

44. Some Commercial PMCs- carbon or aramid reinforced thermoset composites • Carbon fiber-reinforced thermoset composites: better mechanical properties; High specific strength; High cost; • Aramid fibre-reinforced composites: the best impact resistance; high specific strength. • Application: air space industry Mechanical properties of epoxy with various fibers Impact characteristics of fiber reinforced epoxies

45. Some Commercial PMCs- Polyetheretherketone (PEEK) matrix composites • Growing interest in thermoplastics matrix composites • Ease of fabrication • Reusing offcuts and scrap (零碎) • Polyetheretherketone (thermoplastic) (PEEK) • Relative cost thermoplastics • good mechanical properties • Simi-crystalline polymer, Good toughness • glass-transition temperature of 143℃ • Reinforced PEEK composites • Less absorption of water • Resistance to fuel, hydraulic fluid, lubricants, solvents; (exception of strong sulphuric acid solutions.) • Use at temperature as high as 120℃

46. Some Commercial PMCs- Polyetheretherketone (PEEK) matrix composites • Mechanical properties of PEEK at high temperature Effects of T on stiffness & strength of PEEK matrix composites Creep modulus against time

47. Some Commercial PMCs- Polyetheretherketone (PEEK) matrix composites • Fatigue (疲劳）crack growth (da/dN) obeys Paris-Erdogan equation da/dN=A△Kim, • where △Kiis stress intensity factor range, A and m are constants. • Linear relationship between log(da/dN) and log (△Ki). • The composites have the better fatigue crack growth resistance

48. Some Commercial PMCs- Rubber matrix composites Polymer matrix (rubber) + carbon black particles • Enhance tensile strength, toughness, tear and abrasion resistance of rubber. • Application: automobile tires, conveyer belts

49. Further Reading: Text Book: Composite Materials: Engineering and Science (pages179-207). Reference book: Introduction to Materials (page 241-283) Other reference: Lecture note 7