Polyme Analysis 2010 Part 10 pptx

INTERFACES IN COMPOSITES• Questions that need to be answered: – Are interfacial properties important at all?. – How can we measure interfacial adhesion in composites?. EXAMPLES OF PRACTI

INTERFACES IN COMPOSITES

• Questions that need to be answered:

– Are interfacial properties important at all?

– What is the role/effect of interfaces with respect to overall mechanical properties of composites?

– How can we measure interfacial adhesion in composites? – Can we develop analytical models that are useful for design purposes?

– Is it possible to tailor interfacial adhesion?

EXAMPLES OF PRACTICAL RELEVANCE

(VARIOUS FIELDS)

• Thin coatings on critical surfaces (reflective coatings on optical components, plating for corrosion protection, magnetic films for data storage…)

• Interfaces in composites (carbon-epoxy, glass-PP, SiC-SiC, PE-PE…)

• Interaction of biological systems or media with engineering

materials (tissue bonding of prostheses, dental implants,

artificial hips, cell adhesion [cells remain alive only if they

adhere to a substrate]…)

Wetting and adhesive strength

• Adequate wetting is a necessary (but not sufficient!) condition for good adhesion between a liquid and a solid surface

• The contact angle between a liquid droplet and a surface is an indication of compatibility between these

• If q < 90°, the liquid ‘wets’ the solid surface

• If q = 0°, the liquid ‘spreads’ on the surface (complete wetting)

• If q = 180°, the liquid ‘does not wet’ the surface

• In terms of surface energies:

θ

Surface energies (γ)

θ solid

(S = Solid, V = Vapor, L = Liquid)

The equilibrium wetting or contact angle q is dictated by the Young equation, obtained by a balance of horizontal forces:

γ SV = γ LV cosθ + γ SL

Thus, good wetting ( θ = 0 or small) arises when the surface energy

of the solid is equal to or greater than the sum of the liquid

surface energy and the solid-liquid interface surface energy

γ SV ≥ γ LV + γ SL

[Interface surface energies are difficult to measure (and may be

influenced by chemical reactions) but are often smaller than the values for the phases in air]

The lower the contact angle, the greater the wettability:

Liquid is attracted to

solid, less to itself

Indirect measurement can be made by immersing the fiber into the liquid

of interest and measuring the force of immersion or emersion (= Wilhelmy microbalance technique) A force balance permits the calculation of the contact angle if the fiber perimeter and surface energy of the liquid are known (see Bascom)

Sessile drop method Wilhelmy method Capillary-rise method

L i q u id G l a s s c o n t a i n e r

F i b r e

M i c r o b a l a n c e t e n s i o m e t e r K r ü s s

C o m p u t e r

D r i v e

s y s t e m

Fiber-matrix interfacial adhesion

• Two general methodologies:

– Indirect (or macromechanical) testing - focuses on the collective behavior of fibers in a polymer matrix [interfacial strength is

interpreted via simplistic approximations; fast but questionable data collection]

– Direct (or micromechanical) testing – probes interfacial behavior

of individual fibers interacting with a polymer matrix [more

fundamental information; variability within and between

techniques; issue of relevance to real-life composites (scaling-up)]

Indirect (or macromechanical) testing

1 The transverse tensile test

2 The short-beam shear (or interlaminar shear strength -ILSS) test

3 Asymetrical 4-point (Iosipescu) bend test

(And other macroscopic tests… )

CONCLUSION – They are simple to perform, but yield

ambiguous, inaccurate data