Polymer Composites IBased on McCrum, Buckley & Bucknall, “Principles of Polymer

Engineering”, Oxford Science, 2nd edition 1997

ΣΥΝΘΕΤΑ ΠΟΛΥΜΕΡΙΚΑ ΥΛΙΚΑ Ι

Σύνθετα υλικά

Σύνθετα είναι τα υλικά, τα οποία αποτελούνται από δύο ή περισσότερασυστατικά µέρη µε διαφορετική χηµική σύσταση τα οποία

διαχωρίζονται µεταξύ τους από µια συγκεκριµένη επιφάνεια

Το ένα, από τα συστατικά µέρη ονοµάζεται µήτρα, είναι συνήθωςχαµηλής πυκνότητας και η συµµετοχή του στο σύνθετο υλικό

εξασφαλίζει τη µέγιστη δυνατή εκµετάλλευση των ιδιοτήτων της

ενίσχυσης. Το δεύτερο συστατικό µέρος χαρακτηρίζεται ωςσυστατικό ενίσχυσης και προσδίδει στο σύνθετο υλικό

βελτιωµένες (συνήθως και κυρίως) µηχανικές ιδιότητες.

Introduction• Major problem in applications: Polymers: low

stiffness/elastic modulus (δυσκαµψία/µέτροελαστικότητα) and strength (όριο αντοχής)compared with metals (elastic moduli of metals x100 and strength x5)

Solutions: • (i) ingenious use of shape and design (e.g. ribs

and box section to increase stiffness and strength)

• (ii) the addition of reinforcing particles (σωµατίδια ενίσχυσης) or fibres (ίνες) within a polymer matrix (πολυµερική µήτρα) to form a composite material

Applications

• Automotive industry (e.g. F1)• Tyre industry (a tyre is reinforced at several

different levels)• Aerospace industry• Sports (competition Kayak: epoxy resin

(Εποξυδικές ρητίνες) with continuous (συνεχείς, µεγάλου µήκους) Kevlar (οργανικήίνα αραµιδίου) and carbon fibres (ίνεςάνθρακα, ανθρακονήµατα), tennis racket: carbon fibre reinforced nylon)

Attributes of a good reinforcing additive(πρόσθετο ενίσχυσης)

• Stiffer and stronger than the polymer matrix• Particle size, shape and surface characteristics

suitable for effective mechanical coupling(σύζευξη, πρόσφυση) (interfacial adhesion, συγκόλληση διεπιφάνειας) with the polymer matrix

• It preserves the desirable qualities of the polymer matrix (e.g. toughnessανθεκτικότητα/ολκιµότητα/δυσθραυστότητα)

• Low cost

Polymer matrices I: thermosets

• First reinforced plastics were based on thermoset(θερµοσκληρυνόµενα) polymers. Advantages:

(i) Prior to curing (crosslinking, διασύνδεση/διασταύρωση) the precursor (πρόδροµος)liquids have low viscosity (χαµηλό ιξώδες) which facilitates thorough wetting (διαβροχή) of reinforcing particles by the polymer

(ii) Inexpensive forming for large components(iii) High softening points with materials of only

moderate cost

Polymer matrices II: thermosets

• the most popular: thermoset polyesters(Πολυεστερικές ρητίνες) (versatile, inexpensive polymers) with glass-fibre (ίνες γυαλιού) reinforcement.

• Applications: storage tanks, pipes (σωλήνες), boat hulls(σκαρί, σκελετός πλοίου), and seating for public places(υλικό καθίσµατος θέσεων)

• Epoxies: for more demanding applications (superior mechanical performance, esp. toughness)

Polymer matrices III: thermoplastics, θερµοπλαστικά

• PP, nylon etc. • Advantage: forming by injection

moulding (µορφοποιήση µε έκχυση) or extrusion (εκβολή) which are the most economical processes when cheap and precise manufacture of large quantitiesof components

Fibres as reinforcements

• Forms: continuous bundles of fibres (συνεχείς, µεγάλουµήκους δέσµες ίνων), woven fabrics (πλέξεις ύφανσης), chopped fibres (κοντές ίνες) etc.

• They are initially manufactured as bundles of continuous filaments (νήµατα) (round cross-section, diameter~ 5-15 microns). The bundle (roving or tow) consists of 1,000 to 10,000 filaments. Characterised by the linear density (unit tex = 1g/km = 10-6 Kg/m)

• ∆ιάταξη-διευθέτηση των ινών σε ιστό, πλέγµα ή πακέτα(yarn, tow, roving), όπου οι µεµονωµένες ίνεςοµαδοποιούνται κατάλληλα ανά 100-100.000.

Fibre coating of size

• ειδικά κολλοειδή πρόσθετα (sizes) που δρουνως προστατευτικές επικαλύψεις καισυνεισφέρουν στην καλύτερη πρόσφυση ινών-µήτρας.

• Hold the fibres together as a coherent bundle before incorporation into the polymer matrix

• Protect the fibre surface from mechanical and environmental damage

• Provide chemical bonding between fibres and matrix in the final composite

Glass Fibres I (υαλονήµατα)• Η δοµική τους βάση είναι κυρίως τα οξείδιαπυριτίου αλλά και τα οξείδια του ασβεστίου, βορίου, αλουµινίου, κ.ά.. Θεωρούνται από τα πιοφθηνά ενισχυτικά υλικά.

• By far the most widely used fibre reinforcement: moderate price /desirable properties

• Glass: different types: all based on Silica, πυριτία (SiO2) but also contain smaller quantities of other inorganic oxides.

• E-glass (most commonly used)• S-glass (superior mechanical properties)• C-glass (improved resistance to attack by water

and acids)

Glass Fibres II: Pros and Cons

• Resistance to high temperatures (softening point 850 C)

• Transparency (διαφάνεια) to visible light• Isotropy (e.g. thermal expansion is identical in

axial and radial directions)• Susceptible (επιδεκτικές) to surface damage:

rubbing or by the action of moisture which gradually dissolves certain of the oxides present at the fibre surface. So strength of fibre can decrease dramatically (x0.5)

Carbon Fibres I

• Less widely used, increasingly more important/ higher cost but they have superior properties. Especially because they are not only stiff and strong but also of light weight.

• The best carbon fibres are prepared by polyacrylonitrileπολυακρυλονιτρίλιο (PAN) fibres. PAN converted to graphite by carefully-controlled heat-treatment operations. Final graphite should have high degree of crystal orientation.

• However, the graphitic crystal microstructure of commercial carbon fibres contains always some disorder and microvoids.

Carbon Fibres II: Features

• Chemical inertness (resistant to moisture and chemicals)

• High electrical and thermal conductivity along the fibre axis

• Dimensional stability (axial thermal expansion is extremely low and negative)

• They are black and impart this colour to the composite

• Highly anisotropic• Complete alignment difficult

Kevlar

• Eg. Kevlar 49: Poly(paraphenyleneterephthalamide)

• Aramid (αραµίδιο) polymers: (i) rod-like rigid polymers because of rigid benzene rings in the backbone ii) orient and pack together very well and bond firmly to their neighbours because of the amide groups (hydrogen bonds)

Role of the Interface-Shape of reinforcing particles [πετάλιο και ίνα]

Platelet Reinforcement

• Minerals: – Talc 3MgO·4SiO2·H2O,– Mica: K2O·3Al2O·6SiO2·2H2O (muscovite form)

• Both crystalline with a layered structure (φυλόµορφηδοµή). By crushing (σύνθλιψη) and grinding (λείανση, άλεσµα): particles of small plates: 10-1000 microns across and 1-5 microns in thickness

• Lower cost (1/5) of that of the common plastics but greater stiffness and strength

• If platelets are aligned parallel to each other they provide reinforcement in all directions in the plane

Forming of Reinforced Plastics

• Process not only shape the component; it also positions the reinforcing particles and fixes their orientation(προσανατολισµό)

• Usually the product has directionality (διευθυντικότητα) and shows anisotropy in its physical properties

Μονοδιευθυντικά

σύνθετα

Processes

• Pultrusion (Μορφοποίηση µε εµβάπτιση):

• Filament winding (Περιέλιξη νήµατος):

Χειροποίητη στρώση (hand lay-up)Χύτευση θαλάµου πίεσης (pressure bag)Χύτευση σε διαιρετό καλούπι (matched die-moulding)Συνεχής µορφοποίηση ελάσµατος (sheet moulding process, SMC)Συνδιέλαση (co-extrusion)Μορφοποίηση µε χύτευση (casting process)

Rule of Mixtures

Composite material of mass m occupies a volume υ.

It contains a mass mf of fibres occupying volume υf, and a mass mm of matrix occupying volume υm, then

Volume fraction of fibre:

Volume fraction of matrix:

From these equations one easily deduce that the density ρ of the composite is connected to the densities ρf of the fibre and ρm of the matrix by the following relationship

mf

mf

and m m mυ υ υ

= +

= +

ff

υφ

υ=

mm

υφυ

=

m f1φ φ= −

mf f f(1 )ρ φ ρ φ ρ+ −=

Rule of mixtures

Calculating Costs

The cost of a composite = cost of constituent materials + the cost of compounding them

Suppose that the cost per unit mass of fibres is Cf and the cost per unit mass of the matrix Cm and the cost of incorporation per unit mass of composite (compounding) is Ci; if the total cost per unit mass of composite is C, then the cost for mass m is

imm

fff

f

immff

)1(

or

CCCC

mCmCmCmC

+−+=

++=

ρ

ρφ

ρ

ρφ