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Tuesday, February 17, 2009

Fundamentals of Resin in a Composite System

Roles Of Matrix Resin In Composite System

The purpose of the composite matrix is to bind the fibers together by virtue of its cohesive and adhesive characteristics, to transfer load to and between fibers, and to protect them from environments and handling.

Resins are divided into two major groups known as thermoset and thermoplastic.
Thermoplastic resins become soft when heated, and may be shaped or molded while in a heated semi-fluid state and become rigid when cooled.
Thermoset resins, on the other hand, are usually liquids or low melting point solids in their initial form.

To produce finished goods, these thermosetting resins are “cured” by the use of a catalyst, heat or a combination of the two.
Once cured, solid thermoset resins cannot be converted back to their original liquid form.
Cured thermosets will not melt and flow but will soften when heated (and lose hardness) and once formed they cannot be reshaped.

The most common thermosetting resins used in the composites industry are unsaturated polyesters, epoxies, vinyl esters and phenolics.
There are differences between these groups that must be understood to choose the proper material for a specific application.



Resin Dominated Properties

Matrix is the 'weak link' in the composite; especially because resins do not presently exist that allow utilization of the stresses that the fibers are able to withstand.

When the composite is under load, resins may micro crack and craze, form larger cracks through coalescence of micro cracks, debond from the fiber surface, and generally break down at composite strains far lower than desired

The matrix resin provides many essential functions;
-the matrix keeps the reinforcing fibers in the proper orientation and position so that they can carry the intended loads,
-distributes the loads more or less evenly among the fibers,
-provides resistance to crack propagation and damage,
-provides all of the interlaminar shear strength of the composite.

Matrix generally determines the overall service temperature limitations of the composite, and may also control its environmental resistance.



Problems with Polyester & Vinyl Ester Resin
The problems of attaining adequate adhesion to carbon and aramid fibers have discouraged the development of applications for polyester or vinyl ester composites that use these fibers.

Although there are applications of high performance fiberglass composites in military and aerospace structures, the relatively poor properties of advanced composites of polyester and vinyl ester resins when used with other fibers, combined with the comparatively large cure shrinkage of these resins, have generally restricted such composites to lower-performance applications.

The development of highly effective silane coupling agents for glass fibers allowed the fabrication of glass fiber reinforced polyester and vinyl ester composites having excellent mechanical properties and acceptable environmental durability


Thermoplastic vs Thermoset

Thermoset composites are hardened to permanent shape by an irreversible reaction (usually cross-linking).

One of the more common class of thermosets used for composite processing is epoxy.

Thermoplastics are also used but less frequently. This class of polymer will soften and melt at high temperatures and re-harden when cooled. This is due to the fact that thermoplastics are long chain polymers that are not cross-linked.

Thermosets are rigid and are usually stiffer, stronger and more brittle than thermoplastics.

Thermoplastics are often selected when high toughness is required.
Table below lists some properties of commonly used polymer matrix materials

Properties of typical polymer matrix materials

Saturday, February 14, 2009

Carbon Fibre Guitar

Everyone knows that carbon fibre is not cheap, but because of the properties such as light weight and excellent strength makes it an ideal choice for body panels and the like. Carbon fibre has been used in automotive, aerospace, sports and many more industries.

And you know what? Carbon fibre now available in music industry. Blackbird Guitar (blackbirdguitar.com) has been produce several type of cabon fibre gutar since 2007.

This carbon fibre guitar is so special because carbon fiber can be moulded to any shape. The designers weren't forced to build a traditionally flat-backed guitar just to suit the properties of the material. And the whole guitar weight just three pounds, despite being much tougher, more scratch-proof and waterproof than its wooden cousins.

Carbon Fibre Guitar by Blackbird Guitar

Wednesday, February 4, 2009

Composite Testing : Barcol Hardness

What is Hardness?
Hardness can be describe as the ability of a certain material (especially rigid plastic) to be indented. There are many method to use for determine the hardness of composite material such as Rockwell Hardness , Barcol Hardness etc.

Higer number of hardness indicate that the material is harder, which means it have more resistance to penetration by other material.

Barcol Hardness
Barcol Hardness of a certain composite material is determine by pressing the Barcol Hardness Impressor to the composite using hand. The hardness is indicated in the gauge meter at the impressor.

This Barcol Hardness test is very important to monitor the state of cure of the resin in a composite system. If the resin is not fully cure, this will effect the performance of that composite product. Thus, action should be taken to completely cure the resin such as : the time of the post-curing should be longer.

Barcol Hardness Test

Sunday, February 1, 2009

Composite Fabrication : Pultrusion

Pultrusion
Pultrusion process ia a continuous process, producing a profile of constant cross-section.
The fibres are pulled from a creel through a resin bath and then on through heated die.
Die completes the impregnation of fibre, controls resin content & cures to its final shape (as passes through die) and cured profile is automatically cut to length.
Fabrics may also be introduced into the die to provide fibre direction other than 0°.
A variant known as ‘pulforming’ allows for variation into cross-section where it pulls the materials through the die for impregnation, and then clamps them in a mould for curing (Process is non-continuous)


Pultrusion Process

Materials Options:
• Resins: Generally epoxy, polyester, vinylester and phenolic.
• Fibres: Any.
• Cores: Not generally used.

Main Advantages:
i) Can be a very fast i.e. Economic way of impregnating & curing materials.
ii) Resin content can be accurately controlled.
iii) Fibre cost is minimised, i.e. majority is taken from a creel.
iv) Structural properties of laminates can be very good since the profiles have very straight fibres and high fibre volume fractions can be obtained.
v)Resin impregnation area can be enclosed, i.e. limiting volatile emissions.

Main Disadvantages:
i) Limited to constant or near constant cross-section components
ii) Heated die costs can be high.

Typical Applications:
• Beams and girders used in roof structures, bridges, ladders, frameworks.


Pultrusion Machine