THERMOPLASTIC COMPOSITES IN CONSTRUCTION
Thermoplastic composites are extremely ductile materials that can be repaired even after they are put into service. They are highly resistant, strong and have the ability to remold and recycle. Such characteristics make thermoplastic composites a suitable building material. Generally, they are used for lightweight structures, storage tanks, frames and panels for doors and windows.
What are COMPOSITE MATERIALS ??
The composite material can be defined as a multiphase material, formed by the combination of materials differing in a composition which remain bonded together, but retain their identities & properties, without going into any chemical reaction.
The composite material doesn’t dissolve but they are completely merging with each other. They maintain an interface between each other & act in concrete to provide improved specific or synergistic characteristic not obtainable by any of the original component acting singly.
CLASSIFICATION OF THERMOPLASTIC COMPOSITES :
Composite materials are broadly classified into natural & synthetic composites. fig schematically shows the classification.
As seen in the phase composition which is generally alone in most composite ingredients is inserted into the other material known as matrix.
The classification based on the plastic matrix is thermoplastic & thermosetting.
CONSTITUENTS OF COMPOSITE MATERIALS.
The composite materials are made up of constituent materials which are grouped as
- FIBRE MATRIX INTERPHASE
It is also called as binder. The phase that receives the inserts in the phase composition is the continuous phase & is called as matrix.
Ex. – polymer, ceramics, Metals.
These are materials which basically give strength, stiffness & other mechanical properties to composite materials.
Ex. – Glass, Boron, Jute fibres.
The interphase between the fiber and matrix can be easily identified, it is the behaviour and property of interphase that generally control the properties of composites. The main role of interphase is to transmit and distribute the load on matrix to the reinforcement.
Fillers & additives
Fillers are added is polymeric matrix for following reasons.
1. Reduce cost
2. Increase stiffness.
3. Reduce mold shrinkage.
4. Control viscosity.
5. Produce smooth surface.
This fillers should be inert & their presence in a polymeric matrix.
Should not affect the processing & polymerization.
Definition – The phase that receive the inserts in phase composition is continuous phase & is called as matrix. It is also called as binders. The matrix material employed for fabrication of composites material are usually polymer & commonly called as Resin.
Functions – The function of matrix or binders are as follows.
a) It gives mouldability or shape to composite.
b) It makes the composite materials generally resistance to adverse environment.
c) It also protect the reinforcement from adverse environments.
Types – The materials used for the matrix are plastic, rubbers, ceramic & metals.
Plastic matrix based composite materials now constitute more than 95% of all composite materials in use today. Both Thermoplastic & Thermosetting materials are used as matrix materials.
Among these plastic material we are interested in the matrix made up of Thermoplastic composite with their inherent properties which can contribute to composite materials. Some of the thermoplastic materials are discuss.
LOW DENSITY POLYETHYLENE
The polyethylene is manufactured from pure ethylene as a result of high pressure methods.
These polymers have excellent properties that are required for matrix or binders this are.
Specific Gravity = 0.91 – 0.93
Processing temp = 180-2120 F.
Tensile strength = 0.9 – 2.5 x 103 psi
Impact strength = 0.2-12 ftlb/inch
High moisture Proofness.
Resistance to action of micro-organism.
It has high stability toward diverse Corrosive medium.
These PE is used in pipe which has been used for transportation various kinds of fluids because of its high resistance to corrosive materials.
HIGH DENSITY POLYTHYLENE
This polyethylene is manufactured from pure ethylene as result of low pressure methods. This polymer has excellent properties that are required for matrix or binders these are :
Mechanical properties :
Specific Gravity = 0.94 – 0.96
Tensile strength = 2.9-5.4 x 103 psi
Impact strength = 0.4 – 14 ftlb/inch
Processing temp = 175 – 2500F
Chemical Properties :
High moisture proof ness.
Resistance to action of micro-organism.
High stability towards diverse corrosive medium.
Polymerisation of propylene is carried out in presence of suitable heterogeneous catalyst to form polypropylene.
Polypropylene has been found to have rare combination of excellent physical, mechanical thermal, electrical & chemical properties with outstanding high temperature resistance. These properties are used in matrix.
Specific Gravity = 0.90 – 0.91
Tensile Strength = 4.5 – 6 x 103 psi
Impact strength = 0.4 – 12 ftlb/inch
Processing temp. = 225 – 300o F
The PP is probably lightest know industrial polymer.
Polypropylene exhibits high stiffness, hardness & tensile strength because of high crystallinity0.
Poly propylene is resistance to many chemicals such as alkali, acids.
It also has good moisture resistance.
Due to high strength rigidity, temp resistance & chemical resistance pp is suitable to used in the production of chemical & biological attacks.
NYLONS – 6 OR PERLON – L
A polyamide closely related is Nylon known as Perlon – L or Nylon – 6 is based on a polymeric fibre form only one constituent caprolactum NH – (CH)5 – CO giving polymer clip_image008 it is prepared by prolong heating of w – amino caprolactum at 250 – 278 C. Nylon is often used in thermoplastic composite because of presence of saturation or double bonds in its structures.
Mechanical Properties :
Specific Gravity = 1.14 Tensile Strength = 12.5 x 103 psi
Impact Strength = 2.0 – 5.0 ftlb/inch Processing Temp. =180 – 300 o F
Good Solvent Resistance
Good Biological Resistance
High Toughness and Elasticity
Nylon is used in making transmission belts, link belts.
These materials have been prepared by polymerisation at p-halothiophenoxide metal component both in solid state and in solution.
Condensation of p-chlorobenzene with element sulphur in presence of sodium sulphide. These are of two types, thermoplastic which is branched and viscous in nature. And this can be subsequently oxidised to give cross linked structure.
Mechanical Properties : Specific Gravity = 1.34
Tensile strength = 10 x 103psi
Processing temp = 5000 to 6000F
Heat resistance very high
This polymer with its very high heat & flame resistance & chemical & electrical resistance characteristic found its application in exhaust gas return value carburetor parts, ignition plates, Motor housing.
Fibres are the principle constituents in fibre reinforcement composite materials.
Definitions: – It is materials which give strength, stiffness & other mechanical properties to composite materials.
Function: – The reinforcement are used to improve the structural characteristics of materials. They can be continuous in form of fibre, filament or discontinuous in forms of (whiskers, flake or particular).
The reinforcement increases the ratio of strength to density & stiffness to density.
Improve formability & electrical prop.
It also increase resistance to corrosion fatigue, creep & repturs stress & reduce cost.
TYPES OF REINFORCEMENTS
Reinforcement can be classified in 5 types.
1. Mineral reinforcement: – The mineral fillers most commonly employed are CaCo3, Silica, mica tale clay, alumina etc.
2. Hybrid reinforcement – in hybrid composite two or more high performance reinforcements are combined.
3. Sandwich Reinforcement
Composite structure consisting of a thermoplastic core sandwich between two metal pieces or layers. Ex – steel – pp – steel Al – Nr – Al.
4. Metal filled thermoplastics
Although plastics are electrical & thermal insulators. They can be made conductor by introducing metallic or conductive fillers.
5. Organic & inorganic reinforcements
Inorganic or organic fibers such as carbon, glass, aramides etc have been widely used with plastic mainly to improve mechanical strengths & tensile modulus.
Now we will discuss some of the fibres used in reinforcements one by one.
Glass fibres are the oldest form of strength fibre used in composite structure materials. Continuous fibre are made by a growth extension process. In air craft aerospace & military application in which strength to wt. Ratio are critical constitute the fastest growing market for glass fibre reinforced materials.
The glass fibre for reinforcement is available in several forms like fibres rovings, chopped, strands, yarns & mate.
The principle ingredients in all glass fibres is silica and other oxide. The Na2O & K2O content in glass fibre is quite low which give them a better corrosion resistance to water as well as higher surface resistivity.
The glass fibres are availables in no. of grades for reinforcement purpose.
A particular form is choosen depending on the molding methods, properly to be improved & cost of final products.
A glass – cost effective, for general purpose. Easily attacked by moisture & alkaline.
E glass –these has high level electrical resistivity, surface resistivity & forming.
C glass – impact, high acid resistance
D glass – highest structural resistance properly when stiffness is more important than strength.
Application: These found application in pressure bottles, automobile bodies, for tubing & pipes & in sportsgoods.
Now a days carbon fibres finds its own place in the composite materials where weight reduction are valuable.
Many techniques have been tried for producing carbon fibre. All of them involve the heat treatment of carbon containing raw material usually in form of polymer fibre & carbonising them . Three raw materials generally employed for production of commercial products these are rayon, acryllic, & PAN fibre & fibre span pitch.
Carbon filled composite has electrical conductivities coupled with good mechanical prop.
1. High tensile strength – wt. Ratio.
2. High tensile modulus – wt. Ratio.
3.Very low co-efficient of linear thermal expansion which provides dimensional stability.
4. High fatigue strength.
Disadvantage – low impact resistance, high electrical conductivity.
Applications – High temperature property of carbon fibre are being put to use in pump packing, bearing & breaks or breaks disc materials.
Boron fibre is also immerging as the principle constituents or high characteristic properties contain fibre.
These are prepared by reduction of boron tri-chloride in chemical vapour deposition process on hot tungsten or graphite filament.
1. these are of low density, high tensile strength, high modulus fibres.
2. these are extremely hard.
Disadvantages – The amorphous boron fibre has excellent properties but process is very costly.
Applications –These are highly suitable for aerospace industries, Automobile industries.
Boron fibres have been used in some sporting good equipments.
Now one of the immerging member in reinforcement fibre family is kevlar. These are introduce in 1972 commercially to replace steel in radial tyres.
These are mainly of two types kevlar 29 & kevlar 49.
These are low density strength armid fibre. Designed for ballistic protection slash & cut resistance, ropes, cables & coating fabrics for flashable & architechral fabrics. It is also used in production such as brakes & clutches.
Advantage – Light wt.- high strength & stifness
Resistance to stretch.
Vibration, damping & damage resistance.
Disadvantage – It is very costly.
Application: These are finding broad application in belts of radials car tyres, car cases, marine, automobile & other industrial applications.
Commercially jute is the most important vegetable fibre other than cotton. About 60% of world jute is produce in India.
These are cheap & easily available fibre. After the retting the fibre is removed from the stream by hand in a process known as stripping.
The jute fibre is 2.5 to 3 m long but ultimate fibre cell is 2.5 mm in length.
Now recent jute fibre reinforcement thermoplastic materials are used in fenching compounds, doors, decorative articles, tiles etc.
The interphase between the fibre & matrix can be easily identified, it is the behaviour & prop of interphase that generally control the prop of the composite. The proportion of composite cannot achieved by any of the components acting alone.
The main role of interphase is to transmit & distribute stress on matrix to the fibres & in desire orientation.
Localised stress are generally highest near the interphase which may be locus of premature feature of composite. The interphase must have appropriate character in order to provide necessary load transfer from matrix to reinforcements.
Hence their should be strong adhesion forces between matrix & fibre through interphase. This can also be achieved by coupling agents.
Coupling agents are defined as materials that improve the adhesion bonds of dissimilar surfaces. Coupling agents modify the interphase region to strengthen the organic & inorganic boundary layers by
A much positive attempt to increase the adhesion between polymer & fibre was linked them by covalent bond using coupling agents.
Ex – Titanates, Silanes.
Coupling agents are defined primarily as materials that improve the adhesive bond of dissimilar surfaces, this must involve an increase in true adhesion, but it may also involve better wetting, rheology & other handling properties. The coupling agent may also modify the interphase region to strengthen the organic & inorganic boundary layers. Since unsaturated polyester resins were originally the most common organic matrix material, and glass the primary reinforcement various unsaturated compounds of silicon & other elements were first tested as coupling agents; only unsaturated silanes & methacrylatochrome complexes, eg. Volan, have been important commercially.
Silane coupling agents function by modifying the interface between dissimilar phase for eg. in composites such as glass fibre-reinforced resins or mineral filled resins & elastomers, and in adhesive, caulk & sealant applications. Their use results in improved bonding & upgraded mechanical & electrical properties.
In general, the best coupling agents are those where the organizational group on silicon has max. reactivity with the particular thermoplastic resin.
Chemistry of Silanes:-
Silane coupling agents can be represented by formula
X = hydrolyzable group (typically alkoxy)
Y = Functional organic group (amino, epoxy, methacryloxy)
R = small aliphatic linkage -(CH,), that serves to attach the functional organic group to silicon (Si).
Bonding of silane coupling agents to surface hydroxy groups of inorganic components in accomplished by the SiX3 portion of the silane coupling agent, either directly or more commonly via it’s hydrolysis product – Si(OH) 3, Subsequent reaction of the functional organic group Y, with the organic component completes the coupling reaction & established a covalent bond – from the organic phase through the silane coupling agent to the inorganic phase.
This type of chemical bonding accounts for the good adhesion developed between the organic & inorganic components & the stability of the bond under adverse, environmental conditions.
Silane modification of the organic-inorganic interface also produces changes in other properties that may, at times, be more important than the final adhesion cross the interphase.
The interface, or interphase region, between polymer & filler involves a complex interplay of physical & chemical factors related to composite performance. The total coupling mechanism involves all of these inter-related areas.
Under ideal conditions a treated filler wets out & disperses readily in the plastic with minimum viscosity. The treatment protects the filler against abrasion & cleavage during mixing & in the final composite.
Coupling agents are molecular bridges at the interface between two dissimilar substrates, usually but not limited to an inorganic filler and an organic polymer matrix.
Titanium derived coupling agents ones with free protons at the inorganic interface, resulting in the formation of organic monomolecular layers on the inorganic surface.
Typically, titanate – treated inorganics are hydrophobic, organophilic, and organic functional & therefore exhibit enhanced dispersibility & bond with the polymer or organic phase.
When used in filled polymers, they improve impact strength, exhibit melt viscosity lower than that of virgin polymers at inorganic loading above 50% & enhance mechanical properties during aging.
Chemistry of Titonates :-
Reactivity is possible with diverse substrates such as CaCO3, BaSO4, carbon black, ceramics, phthalo & lake pigments, cellulosics, peroxides & aramid & carbon fibres as well as with mineral & metal oxides derived inorganic chemical compounds.
Tetrafunctional compounds based on organometallic titanium (Ti) or Ziroconium (Zr) & Silicon (Si) make useful coupling agents because the central atom’s tetravalency is conductive to electron sharing. Each generic type has inherent natural limitations eg.
When coupling to metal substrate (M) the hydrolytic stability & strength of the Ti-O-M bond is superior to the Si-O-M bond.
However, when coupling to silica, the extra strength of Zr-O-Si superior to the Si-O-M bond. However, when coupling to silica, the extra strength of Zr-O-Si or Si-O-Si is often preferable to Ti-O-Si bond.
ORIENTATION & COMPOSITION OF REINFORCEMENT
The mechanical strength of reinforce plastic component is largely dependent on amount arrangement & type of reinforcement fibre in resin matrix.
1. Unidirectional Orientation.
All fibres are arranged in one direction for a lamina containing unidirectional orientation, the composite material has highest strength & modulus in longitedinal direction. However in transverse direction its strength & modulus are very low. Application found in fishing rods.
Ladders, hockey sticks etc.
2. Bidirectional Orientation.
In this type fibres are arranged in two directions usually normal to each other for a lamina containing bidirectional orientation the strength & modulus can be varied by employing different amount as well as different types of fibre in longitudinal & transverse direction. For a balanced lamina, these properties are the same in both directions.
Application in boat, swimming pull etc.
3. Multidirection or Randomly directed orientation
In this type of orientation, fibres are generally distributed randomly in all direction. Thus in this type of orientation strength & modulus are equal in all direction.
Application: machine housing & Helmets.
The amount of reinforcement that can be used is related to the orientation of the reinforcements & percentage loading of reinforcements.
In unidirectional orientation case the % of the reinforcement with resin is 25% & reinforcement is 75%.
In bidirection 50% resin & 50% reinforcement & in case of randomly directed reinforcement 25% & 75% resin.
APPLICATION OF THERMOPLASTIC COMPOSITES
1. THERMOPLASTIC COMPOSITE ROOF LIGHT, DOMES & SHEET.
These are used in construction of domes of hemispherical, pyramid cal or of any shape are used in houses, offices shopping complexes etc. These are light in wt.& requires very light frame works. These allow sun light to pass through it but shield protect the interior from dust, rain etc.
2. STORAGE TANKS.
Storage tanks are used for house hold, factory & offices. These are light wt. Leak proof, corrosion resistance oxygenic & strong & durable. These are easy to clean & install.
This are used in bath tub wash basin, shower & shower stalls are widely use now a days. These have great advantage over ceramics like light in wt., unbreakable, having minimum joints, easy to clean & mainly available in many attractive colours.
4. DOORS & WINDOWS
This are used in window pannel doors. They provide excellent water roofing. They are light in wt.strong & available in many colour. ( pultrusion or continuous laminating technique ).
5. READY TO ASSEMBLE CABINS
Due to adequate thermal insulation this are used in temporary shelters.
This are used as flooring or tiles.
7. IN PUBLIC SEATING SYSTEMS.
The chairs & benches are used for outdoor uses. This can also be made fire retardants.