Polymer Matrix Composites (PMC)

Reasons for the use of polymeric materials as matrices in composites

  Processing of PMCs need not involve high pressure and high temperature  i. The mechanical properties of polymers are inadequate for structural purposes, hence benefits are gained by reinforcing the polymers The equipment required for PMCs are much simpler

Disadvantages of PMC

   Low maximum working temperature High coefficient of thermal expansion- dimensional instability Sensitivity to radiation and moisture

Classification of Polymer Matrices



1. Thermoset



2. Thermoplastic- crystalline & amorphous

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3. Rubber

Thermoset

     Thermoset materials are usually liquid or malleable and designed to be molded into their final form prior to curing, has the property of undergoing a chemical reaction by the action of heat, catalyst, ultraviolet light, etc., to become a relatively insoluble and infusible substance.

They develop a well-bonded three-dimensional structure upon curing. Once hardened or cross-linked, they will decompose rather than melt. A thermoset material cannot be melted cured. and re-shaped after it is Thermoset materials are generally stronger than suited to high temperature temperature of the material. thermoplastic materials due to this 3-D network of bonds, and are also better applications up to the decomposition

  Some examples of Thermosets are: Polyester resin (used in glass-reinforced plastics/fibreglass ( GRP ))  Epoxy resin (used as an adhesive and in fibre reinforced plastics such as glass reinforced plastic and graphite-reinforced plastic )    Polyimides used in printed circuit boards and in body parts of modern airplanes Vulcanized rubber Phenolic

 Polyester polyester resins are generally copolymers of unsaturated polyesters with styrene Styrene is the crosslinking monomer and curing is effected by the use of an organic peroxide initiator which generates free radicals leading to the formation of 3-D network Are relatively inexpensive and have low viscosities, which is beneficial in many fabrication processes

  Epoxy is a thermosetting epoxide polymer that cures (polymerizes and crosslinks) when mixed with a catalyzing agent or "hardener".  More expensive and more viscous than polyester  Epoxies have a major advantage in that they are usually cured in two or more stages. This allows preforms to be pre-impregnated with the epoxy in a partially cured state  The pre-preg may be stores, before moulded into the final shape and then cured

Generally start as linear low molecular weight polymer, curing agents such as polyamides & polyamines were used as curing agents The mechanical properties depend on the particular resin system and the curing; generally epoxies are stiffer and stronger, but brittle than polyester Epoxies maintain their properties to higher temperature than polyester

Phenolic

   Produced by reacting phenol and formaldehyde, characteristics of the resin product depending on the proportions of the reactant and catalyst Good fire resistance An undesirable feature of phenolic resin volatile by-product are evolved during curing; hence high pressures are often necessary in composite production

Polyimides

 More expensive, less widely used than polyester and epoxies, but can withstand relatively high service temperature  The presence of ring structure, results in high stiffness, low CTE, and service temperature as high as 425C for several hours  Like other thermoset, polyimides are brittle where R′ and R″ are two carbon atoms of an aromatic ring.

   

Thermoplastic

is a plastic that melts to a liquid when heated and freezes to a brittle , very glassy sufficiently. state when cooled Most thermoplastics are high molecular weight polymers whose chains associate through weak van der Waals forces ( polyethylene ); stronger dipole dipole interactions and hydrogen bonding even stacking of aromatic rings ( ( nylon polystyrene ). ); or The bondings are easily broken by the cobined action of thermal activation and applied stress, that’s why thermoplastics flow at elevated temperature unlike thermosetting polymers, thermoplastic can be remelted and remolded.

 Thermoplastics can go through melting/freezing cycles repeatedly and the fact that they can be reshaped upon reheating gives them their name  Some thermoplastics normally do not crystallize: they are termed "amorphous" plastics and are useful at temperatures below the

T

g. They are frequently used in applications where clarity is important. Some typical examples of amorphous thermoplastics are PMMA , PS and PC .

 Generally, amorphous thermoplastics are less chemically resistant

    Depends on the structure of the thermoplastics, some of the polymeric structure can be folded to form crystalline regions, will crystallize to a certain extent and are called "semi-crystalline" for this reason. Typical semi-crystalline thermoplastics are PE, PP, PBT and PET. Semi-crystalline thermoplastics are more resistant to solvents and other chemicals. If the crystallites are larger than the wavelength of light, the thermoplastic is hazy or opaque. Why HDPE exhibits higher cystallinity than LDPE?

 Polyetheretherketone (PEEK) is a semicrystalline polymer having 20-40% crystallinity.

 It has a rigid backbones, which gives high Tg and Tm (Tg= 143C and Tm= 343C). Can be employed at temperature as high as 230C)  It is possible to blend two or more polymers to obtain a multi-phase product with enhances properties

Comparison of typical ranges of property values for thermoset and thermoplastics

    

Properties t/set

Young’s Modulus (GPa)1.3-6.0

Tensile strength(MPa) 20-180 Max service temp.(ºC) 50-450 Fracture toughness,K Ic (MPa 1/2 ) 0.5-1.0

t/plastic

1.0-4.8

40-190 25-230 1.5-6.0

Thermoplastics are expected to receive attention compared to thermoset due to:

 Ease of processing  Can be recycled  No specific storage  Good fracture modulus

Rubber

 Common characteristics;  Large elastic elongation (i.e. 200%)  Can be stretched and then immediately return to their original length when the load was released  Elastomers are sometimes called rubber or rubbery materials  The term

elastomer

the term rubber is often used interchangeably with  Natural rubber is obtained from latex from

Brasiliensis Hevea

tree which consists of 98% poliisoprena  Synthetic rubber is commonly produced from

butadiene butadiene

, spt

styrene-butadiene

(NBR) (SBR) dan

nitrile-

 To achieve properties suitable for structural purposed, most rubbers have to be vulcanized; the long chain rubber have to be crosslinked  The crosslinking agent in vulcanization is commonly sulphur, and the stiffness and strength increases with the number of crosslinks

PREPREG

 It is short form for pre-impregnation material  It is a semifinished product  It will be used in next processing technique to obtain a finish product  It can be produced from thermoset or thermoplastic matrix

Thermoplastic prepregs are getting attention due to:

   

Easy storage High toughness Fast & easy processing Can be recycled