Fibres for Reinforcement in Composite Materials

Topics Covered

Fibre Types

Glass

E-Glass Fibre Types

Glass Fibre Designation

Aramid

Carbon

Fibre Type Comparison

Other Fibres

Polyester

Polyethylene

Quartz

Boron

Ceramics

Natural

Fibre Types

Glass

By blending quarry products (sand, kaolin, limestone, colemanite) at 1600°C, liquid glass is formed. The liquid is passed through micro-fine bushings and simultaneously cooled to produce glass fibre filaments from 5-24μm in diameter. The filaments are drawn together into a strand (closely associated) or roving (loosely associated), and coated with a “size” to provide filament cohesion and protect the glass from abrasion.

By variation of the “recipe”, different types of glass can be produced. The types used for structural reinforcements are as follows:

a.      E-glass (electrical) - lower alkali content and stronger than A-glass (alkali). Good tensile and compressive strength and stiffness, good electrical properties and relatively low cost, but impact resistance relatively poor. Depending on the type of E-glass the price ranges from about £1-2/kg. E-glass is the most common form of reinforcing fibre used in polymer matrix composites.

b.      C-glass (chemical) - best resistance to chemical attack. Mainly used in the form of surface tissue in the outer layer of laminates used in chemical and water pipes and tanks.

c.      R, S or T-glass – manufacturers trade names for equivalent fibres having higher tensile strength and modulus than E-glass, with better wet strength retention. Higher ILSS (interlaminar shear strength) and wet out properties are achieved through smaller filament diameter. S-glass is produced in the USA by OCF, R-glass in Europe by Vetrotex and T-glass by Nittobo in Japan. Developed for aerospace and defence industries, and used in some hard ballistic armour applications. This factor, and low production volumes mean relatively high prices. Depending on the type of R or S-glass the price ranges from about £12-20/kg.

E-Glass Fibre Types

E-glass fibre is available in the following forms:

a.      Strand - a compactly associated bundle of filaments. Strands are rarely seen commercially and are usually twisted together to give yarns.

b.      Yarns - a closely associated bundle of twisted filaments or strands. Each filament diameter in a yarn is the same, and is usually between 4-13μm. Yarns have varying weights described by their ‘tex’ (the weight in grams of 1000 linear metres) or denier (the weight in lbs of 10 000 yards), with the typical tex range usually being between 5 and 400.

c.      Rovings - a loosely associated bundle of untwisted filaments or strands. Each filament diameter in a roving is the same, and is usually between 13-24μm. Rovings also have varying weights and the tex range is usually between 300 and 4800. Where filaments are gathered together directly after the melting process, the resultant fibre bundle is known as a direct roving. Several strands can also be brought together separately after manufacture of the glass, to give what is known as an assembled roving. Assembled rovings usually have smaller filament diameters than direct rovings, giving better wet-out and mechanical properties, but they can suffer from catenary problems (unequal strand tension), and are usually higher in cost because of the more involved manufacturing processes.

It is also possible to obtain long fibres of glass from short fibres by spinning them. These spun yarn fibres have higher surface areas and are more able to absorb resin, but they have lower structural properties than the equivalent continuously drawn fibres.

Glass Fibre Designation

Glass fibres are designated by the following internationally recognised terminology: Table 1

Table 1. Demonstrates the internationally recognised terminology for glass fibre designation.

Glass Type

Yarn Type

Filament Diameter (μ)

Strand Weight (tex)

Single Strand Twist

No. of Strands

Multi-strand Twist

No. Turns/metre

E

C

9

34

Z

X2

S

150

E = Electrical

S = High Strength

C = Continuous

 

 

Z = Clockwise

S = Anti-Clockwise

 

 

 

Aramid

Aramid fibre is a man-made organic polymer (an aromatic polyamide) produced by spinning a solid fibre from a liquid chemical blend. The bright golden yellow filaments produced can have a range of properties, but all have high strength and low density giving very high specific strength. All grades have good resistance to impact, and lower modulus grades are used extensively in ballistic applications. Compressive strength, however, is only similar to that of E-glass.

Although most commonly known under its Dupont trade name ‘Kevlar’, there are now a number of suppliers of the fibre, most notably Akzo Nobel with ‘Twaron’. Each supplier offers several grades of aramid with various combinations of modulus and surface finish to suit various applications. As well as the high strength properties, the fibres also offer good resistance to abrasion, and chemical and thermal degradation. However, the fibre can degrade slowly when exposed to ultraviolet light.

Aramid fibres are usually available in the form of rovings, with texes ranging from about 20 to 800. Typically the price of the high modulus type ranges from £15-to £25 per kg.

Carbon

Carbon fibre is produced by the controlled oxidation, carbonisation and graphitisation of carbon-rich organic precursors, which are already in fibre form. The most common precursor is polyacrylonitrile (PAN), because it gives the best carbon fibre properties, but fibres can also be made from pitch or cellulose. Variation of the graphitisation process produces either high strength fibres (at ~2600°C) or high modulus fibres (at ~3000°C) with other types in between. Once formed, the carbon fibre has a surface treatment applied to improve matrix bonding and chemical sizing which serves to protect it during handling.

When carbon fibre was first produced in the late sixties the price for the basic high strength grade was about £200/kg. By 1996 the annual worldwide capacity had increased to about 7,000 tonnes and the price for the equivalent (high strength) grade was £15-40/kg. Carbon fibres are usually grouped according to the modulus band in which their properties fall. These bands are commonly referred to as: high strength (HS), intermediate modulus (IM), high modulus (HM) and ultra high modulus (UHM). The filament diameter of most types is about 5-7μm. Carbon fibre has the highest specific stiffness of any commercially available fibre, very high strength in both tension and compression and a high resistance to corrosion, creep and fatigue. Their impact strength, however, is lower than either glass or aramid, with particularly brittle characteristics being exhibited by HM and UHM fibres.

 

Table 2 - Strength and modulus figures for commercial PAN-based carbon fibres

Grade

Tensile Modulus (GPa)

Tensile Strength (GPa)

Country of Manufacture

Standard Modulus (<265GPa) (also known as ‘High Strength’)

 

T300

230

3.53

France/Japan

T700

235

5.3

Japan

HTA

238

3.95

Germany

UTS

240

4.8

Japan

34-700

234

4.5

Japan/USA

AS4

241

4.0

USA

T650-35

241

4.55

USA

Panex 33

228

3.6

USA/Hungary

F3C

228

3.8

USA

TR50S

235

4.83

Japan

TR30S

234

4.41

Japan

Intermediate Modulus

(265-320GPa)

 

T800

294

5.94

France/Japan

M30S

294

5.49

France

IMS

295

4.12/5.5

Japan

MR40/MR50

289

4.4/5.1

Japan

IM6/IM7

303

5.1/5.3

USA

IM9

310

5.3

USA

T650-42

290

4.82

USA

T40

290

5.65

USA

High Modulus (320-440GPa)

 

M40

392

2.74

Japan

M40J

377

4.41

France/Japan

HMA

358

3.0

Japan

UMS2526

395

4.56

Japan

MS40

340

4.8

Japan

HR40

381

4.8

Japan

Ultra High Modulus (~440GPa)

 

M46J

436

4.21

Japan

UMS3536

435

4.5

Japan

HS40

441

4.4

Japan

UHMS

441

3.45

USA

Fibre Type Comparison

Comparing the properties of all of the fibre types with each other shows that they all have distinct advantages and disadvantages. This makes different fibre types more suitable for some applications than others. Table 3 provides a basic comparison between the main desirable features of generic fibre types. ‘A’ indicates a feature where the fibre scores well, and ‘C’ indicates a feature where the fibre is not so good.

Table 3 – Basic comparison between the main desirable features of generic fibres.

Property

Aramid

Carbon

Glass

High Tensile Strength

B

A

B

High Tensile Modulus

B

A

C

High Compressive Strength

C

A

B

High Compressive Modulus

B

A

C

High Flexural Strength

C

A

B

High Flexural Modulus

B

A

C

High Impact Strength

A

C

B

High Interlaminar Shear Strength

B

A

A

High In-plane Shear Strength

B

A

A

Low Density

A

B

C

High Fatigue Resistance

B

A

C

High Fire Resistance

A

C

A

High Thermal Insulation

A

C

B

High Electrical Insulation

B

C

A

Low Thermal Expansion

A

A

A

Low Cost

C

C

A

 

Other Fibres

There are a variety of other fibres, which can be used, in advanced composite structures but their use is not widespread. These include:

Polyester

A low density, high tenacity fibre with good impact resistance but low modulus. Its lack of stiffness usually precludes it from inclusion in a composite component, but it is useful where low weight, high impact or abrasion resistance, and low cost is required. It is mainly used as a surfacing material, as it can be very smooth, keeps weight down and works well with most resin types.

Polyethylene

In random orientation, ultra-high molecular weight polyethylene (UHMWPE) molecules give very low mechanical properties. However, if dissolved and drawn from solution into a filament by a process called “gel spinning”, the molecules become disentangled and aligned in the direction of the filament. The molecular alignment promotes very high tensile strength to the filament and the resulting fibre. Coupled with their low S.G. (<1.0), these fibres have the highest specific strength of the fibres described here. However, the fibre’s tensile modulus and ultimate strength are only slightly better than E-glass and less than that of aramid or carbon. The fibre also demonstrates very low compressive strength in laminate form. These factors, coupled with high price, and more importantly, the difficulty in creating a good fibre/matrix bond means that polyethylene fibres are not often used in isolation for composite components.

Quartz

A very high silica version of glass with much higher mechanical properties and excellent resistance to high temperatures (+1000°C). However, the manufacturing process and low volume production lead to a very high price (14μm - £74/kg, 9μm - £120/kg).

Boron

Carbon or metal fibres are coated with a layer of boron to improve the overall fibre properties. The extremely high cost of this fibre restricts it use to high temperature aerospace applications and in specialised sporting equipment. A boron/carbon hybrid, composed of carbon fibres interspersed among 80-100μm boron fibres, in an epoxy matrix, can achieve properties greater than either fibre alone, with flexural strength and stiffness twice that of HS carbon and 1.4 times that of boron, and shear strength exceeding that of either fibre.

Ceramics

Ceramic fibres, usually in the form of very short ‘whiskers’ are mainly used in areas requiring high temperature resistance. They are more frequently associated with non-polymer matrices such as metal alloys.

Natural

At the other end of the scale it is possible to use fibrous plant materials such as jute and sisal as reinforcements in ‘low-tech’ applications. In these applications, the fibres’ low S.G. (typically 0.5-0.6) mean that fairly high specific strengths can be achieved.

 

Source: SP Systems

 

For more information on this source please visit SP Systems

 

Date Added: Oct 25, 2001 | Updated: Jul 12, 2013
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