Core Materials for Sandwich Composite Constructions

1. Core Materials for Sandwich Composite Constructions

2. Sandwich structures An important composite construction for stiff and light applications MSK 2007-11-30 2

3. The sandwich concept Force Force

4. Refrigerated container for combi-traffic • Roof: 500 kg, 14 m x 3 m • Floor: 1500 kg, 14 m x 3 m • Vacuum injected integrated sandwich • Box Modul, Sweden

5. Selection criteria for core materials • Shear strength and shear stiffness during loading • Should withstand compressive loads to prevent buckling and wrinkling (especially for thin laminate skins • Good delamination strength between core and laminate skin • Good tensile strength

6. Sandwich under mechanical load

7. Other requirements • Low cost • Low density • Temperature resistance (low/high) • Chemical resistance • Moisture resistance • Good formability • Easy machining

8. Main core materials • Wood • Foamed plastics • Honeycombs

9. Balsa wood • Good mechanical propertes: (high compressive stiffness, crushing resistance, good cyclig load handling capacity) • Cheap • Good thermal resistance • Easy to process and use • High density • Absorbs large amounts of resin • Moisture sensitive

10. Foamed plastics • Made from synthetic polymers • The density can be varied • The fom can be applied as liquid in the mould, where it foams and cures to solid material • Very complex shapes can be prepared • Thermal stability is limited

11. Different types of foams • Polyvinyl chloride (PVC) • Cross-linked foams • Uncross-linked foams • Polystyrene (PS) • Polyurethane (PU) • Polyetherimide (PEI) • Polymethyl methacrylamid (Acrylic) • Styren-acrylonitril (SAN)

12. Applications for plastic foams Soft and flexible foams • Comfort cushioning, mattresses, packaging, wearing apparel Rigid foams • Thermal insulation, packaging, sandwich core materials

13. Many synonyms • Cellular polymers • Foamed polymers • Polymer foams • Expanded plastics

14. Foamed cell structures Extruded polystyrene Polyurethane foam Polyurethane foam High density plastic foam Magnification x 26 x 26 x 12 x 50

15. Definition • Cellular plastic is a plastic with a substantially decreased apparent density due to presence of numerous cells (voids) dispersed throughout its mass • A two phase gas-solid system, where the solid is the continious phase and composed of a polymer material

16. Open and closed cell foams • The gas phase is distributed in voids, pores or pockets called cells • If the cells are interconnected so that gas can pass from one cell to an other the foam is named open-celled • If the cells are discrete and the gas phase in each cell cannot move to an other cell through the cell walls the foam is termed closed-celled

17. Open-cell and closed-cell foams • Open-cell foams are used as membranes or filters • Closed-celled foams are used as rigid structural materials

18. Open cell formation Cell expansion to contact Cell formation Cell rupture to open cell Closed cell formation

19. Foamed plastics preparation • Expansion of a fluid polymer phase to a low density cellular state (this is the most important process, and only to be discussed here) • Foams can also be made by leaching out solids or liquids from a polymer, by sintering small particles, and by dispersing cellular particles in a polymer

20. Theory of expansion process Three steps: • Creation of small cells in a fluid or plastic phase • Growth of the cells to a desired volume • Stabilizing the cells into a cellular structure by physical or chemical means

21. Cell formation The cells are formed as a result of a pressure difference between the inside and the outside of the cell: Larger pressure inside cell than outside willgenerate cells!

22. Stabilisation of the foam • Chemical stabilisation • Cross-linking to rigid material • The speed of solidification can be adjusted by the used resin • Physical stabilisation • Solidification of a melted thermoplastic • The temperature for foam production can be adjusted by using different thermoplastics

23. Foam production methods • Pressure generation: • By increase of the internal pressure in the cells • Expandable formulations • Decompression: • By lowering the external pressure • Extrusion,injection moulding, compression moulding • Blowing agents: dispersing gas in the fluid state

24. Polystyrene • An important plastic for many products: • Rigid polystyrene: used in transparent products such as drinking glasses, packaging, …. • Foamed polystyrene: used in insulating materials, packaging protection material,… • Foamed polystyrene exists as: • Expandable formulations • Decompression formulations

25. Foamed (expandable) polystyrene • Two manufacturing processes: • Heating the polystyrene particles in the presence of a blowing agent and allowing the agent to penetrate the particle • Polymerising the styrene monomer in the presence of the blowing agent • Typical blowing agents: pentanes and hexanes • The obtained pre-foamed particles are then fused in a mould by steam treatment, which further expands the particles • Stabilisation is done by cooling to a rigid material (physical stabilisation)

26. Decompression foamed polystyrene • The polystyrene pellets are melt extruded together with a blowing agent in an extruder under high pressure and temperature • The melt is then forced through a die into atmospheric pressure, where the blowing agent is vaporised and forms cells in the polymer • At the same time the polymer is solidified by cooling into a rigid material • Low density foam sheets are typically formed by this method

27. Polyurethane foams • A condensation polymer of isocyanates and polyols • Many polyurethanes with different properties can be made, by selecting different types of monomers • Polyisocyanurate foams are a special type of low density, brittle rigid foams with a crosslinked structure • The reaction between an isocyanate and a polyol is a fast reaction, leading to long chain polymer networks • Foaming of the polyurethane is commonly done by in-situ blowing • Catalysts and other additives are commonly added to regulate polyurethane network formation and the foaming

28. Reaction between isocyanate and polyol Isocyanate Polyol (alcohol) As the isocyanate and the polyol as two reactive sites, a linear polymer is formed

29. In situ blowing • The isocyanate monomer reacts easily with water to form carbon dioxide (CO2) which foams the formed polymer • The foaming is often caused by the moisture in the air where the polyurethane is applied, for example in insulating foams for houses • Additional blowing can be achieved by added blowing agents, such as pentanes

30. Foam creation • After mixing all components, the polyurethane strats to polymerise to longer chains, which leads to increased viscosity • The foam bubbles are then strating to form, and expand to larger bubbles • At the end of the foam creation the cell walls are fully polymerised to stable structures • Thin walls will burst by the high pressure of the blwing agent thus leading to an open cell system • A typical reaction time is 200 to 300 seconds • To obtain a fully cured material additional post curing must often be done, this can be several hours or days

31. Polyurethane processes • One-shot process: all components are mixed together before foam creation • Prepolymer process: the polyurethane network polymer is first made, and the foam is created during application by in situ blowing agent (typically used for insulating foams)

32. Manufacturing • By a continious extrusion process or by a batch process • The rigid material is cut into sheets or the desired shape

33. Properties • Density: 32 kg/m3 to 800 kg/m3 • Use temperature up to 135 °C

34. High temperature foams • Polymethacrylimide (PMI) with trade mark ROHACELL • Used in applications where high temperature stability, chemical resistance, fire resistance and low smoke generation are requested • This is a closed-cell rigid structural foam • Main applications in aircrafts, ships and trains as sandwhich core material • See: www.rohacell.com for more info!

35. Polyvinyl chloride foams • Closed cell foams with good moisture resistance, good mechanical properties, low density, good solvent resistance, very good compatibility with adhesives, self-extinguishing if burned • Chemically the PVC foam is an interpenetrating polymer network (IPN) of PVC and a polyurethane (thermoset) • It behaves like a thermoplastics, and can be remoulded • Rigid foams are cross-linked, while ductile foams are uncross-linked (linear type) • Divinycell is the most important product made by Diab

36. Divinycell from Diab More info: www.diabgroup.com

37. Manufacturing process • The PVC and the polyurethane componenets are mixed and placed in a closed mould which is heated under pressure • Final cure is done in water bath where it is also expanded to wanted density • The obtained block is then cut into the required shape and dimensions

38. PVC foams properties • Density: 45 kg/m3 to 400 kg/m3 • 12 – 30 % shear elongation to break for rigid PVC foams • 40 % shear elongation to break for ductile PVC foams • Rigid foams have higher heat and solvent resistance than ductile foams

39. Temperature resistance

40. Honeycomb sandwich Stiff surface Benefits: low weight, good stiffness, high energy absorption, good impact behaviour, good flexural strength Porous core

42. Honeycomb sandwich core materials Glass fiber Carbon Aluminium Kevlar Perforated aramid

43. Honeycomb production