High Performance and High Temperature Resistant Fibers Emphasis on Protective Clothing

I. INTRODUCTION

Faster, stronger, lighter, safer ... these demands are constantly being pushed upon today's researchers and manufacturers, including protective clothing - routine or specialized.

High performance and high temperature resistant fibers aid enormously in allowing products to meet these challenges. The markets and products which are facilitated by the use of these fibers go far beyond the scope and awareness of most people. This paper intends to provide a solid overview of the definitions, properties, products and end uses associated with some of the most common high performance and high temperature resistant fibers used today. It is stressed that not all high performance materials are presented.

Before exploring details these materials, it is important to define the parameters of high performance and high temperature resistant fibers.

High performance fibers are Synthetic fibers with a continuous operating temperature ranging from 375⁰ F to 600⁰ F or above.

The classification of high performance is less rigid and can be broken down into various segments. Generally speaking, fibers are said to be either

1. Commodity or

2. High performance.

Commodity fibers are typically used in a highly competitive price environment which translates into large scale high volume programs in order to compensate for the (often) low margins.

Conversely, high performance fibers are driven by special technical functions that require specific physical properties unique to these fibers.

Some of the most prominent properties of high performance fibres are:

Tensile Strength,

Modulus,

Elongation

Operating Temperature,

Limiting Oxygen Index and

Chemical Resistance.

Each fiber has a unique combination of the above properties which allows it to fill a forte in the high performance fiber spectrum.

For comparative purposes carbon, glass and high density polyethylene are also referenced. Although these fibers do not necessarily meet all of the requirements of the stated definitions, they commonly compete in the high performance market and should therefore be referenced.

The following presents some basic characteristics of each classification:

Commodity Fibers	High Performance Fibers
Volume Driven	Technically Driven
Price oriented	Specialty oriented
Large scale, line­  type production	Smaller batch-type  production

II. BASIC PROPERTIES

Specific (mass) stress

Specific stress is a more useful measurement of stress in the case of yarns as their cross-sectional area is not known. The linear density of the yarn is used instead of the cross-sectional area as a measure of yarn thickness. This allows the strengths of yarns of different linear densities to be compared. It is defined as the ratio of force to the linear density:

The preferred units are N/tex or mN/tex, other units which are found in the

industry are: gf/denier and cN/dtex.

Tenacity

Tenacity is defined as the specific stress corresponding with the maximum force on a force/extension curve. The nominal denier or tex of the yarn or fibre is the figure used in the calculation; no allowance is made for any thinning of the specimen as it elongates.

Tensile strength

Tensile strength is a measurement of the force required to pull something such as rope, wire, or a structural beam to the point where it breaks. The tensile strength of a material is the maximum amount of tensile stress that it can take before failure, for example breaking.

Tensile Strength is often the determining factor in choosing a fiber for a specific need (see chart 1).

A major advantage of high strength fibers over steel, for example, is the superior strength-to-weight ratio that such fibers can offer.

Para-aramid fiber offers 6-8 times higher tensile strength and over twice the modulus of steel, at only one-fifth the weight, but in applications where strength is not of paramount importance, other properties must be evaluated.

MODULUS

The slope of the first linear part of the curve up to the yield point is known as the initial modulus (Young's modulus) and it is the value generally referred to when speaking of modulus without qualification. Modulus as a general term means the slope of the force elongation curve and it is a measure of the stiffness of the material, that is its resistance to extension.

The higher the modulus of a material, the less it extends for a given force. If the curve is plotted in terms of stress against strain the units of modulus are the same as those of stress, that is force per unit area such as pascals. If the curve is plotted in terms of force against elongation the units of modulus are those of force/elongation and they depend on whether the elongation is measured in distance, percentage extension or strain.

The use of computer software to record and analyze force elongation curves means that the ways of specifying the modulus of a curve have to be clarified. It is no longer possible to lay a rule on the curve and judge the best position by eye. There are a number of possible moduli that may be

measured.

Temperature resistance often plays an integral role in the selection of a fiber. Heat degrades fibers at different rates depending on the fiber type, atmospheric conditions and time of exposure. The key property for high temperature resistant fibers is their continuous operating temperature. Fibers can survive exposure to temperatures above their continuous operating temperatures, but the high heat will begin to degrade the fiber. This degradation has the effect of reducing the tensile properties of the fiber and ultimately destroying its integrity.

A common mistake is to confuse temperature resistance with flame retardant ability.

Flame retardant ability is generally measured by the Limiting Oxygen Index. LOI, basically, is the amount of oxygen needed in the atmosphere to support combustion.

Fibers with a Limiting Oxygen Index (LOI) greater than 25 are said to be flame retardant, that is there must be at least 25% oxygen present in order for them to burn.

The LOI of a fiber can be influenced by adding a flame retardant finish to the fiber. FR chemicals are either added to the polymer solution before extruding the fiber or added to the fiber during the spinning (extrusion) process.

In addition, impregnating or topically treating the fiber or the fabric, flame retardant properties are often added directly to fabrics (such as FR treating cotton fabrics).

Just as heat can degrade a fiber, chemical exposure, such as contact with acids or alkalis, can have a similar effect. Some fibers, such as PTFE (i.e. DuPont’s Teflon), are extremely resistant to chemicals. Others lose strength and integrity quite rapidly depending on the type of chemical and the degree of concentration of the chemical or compound.

III. FIBER FORMS AND PRODUCT FORMS

Fibers are available in several different forms. The most common forms used are:

Staple Fiber – filaments cut into specific lengths – usually spun into yarn

Chopped Fiber – coarser, cut to specific, often short, lengths to add to mixture

Monofilament – a single (large) continuous filament yarn – like fishing line

Multifilament – extruded continuously with many filaments in the bundle.

These basic forms of fiber are then further processed into one of four major converted forms. These converted forms can be categorized into four groups:

Spun yarn •

Knitted fabric

Woven fabric

Nonwoven fabric

Most are familiar with yarn, woven and knitted fabrics. Nonwoven fabrics may be another story. The most common types of nonwoven fabrics are – based on bonding and manufacturing processes - are:

Needle felts – the fibers are mechanically entangled with barbed needles

Dry-laid – chemical or thermal bond – many different forms, including

Direct formed - Spunbond and melt-blown (may be further bonded or combined)

Stitch Bond – sewn bond

Wet-laid – paper making process

Hydro-entangled (spunlace) – water jet entangled – mechanical bond

Many of the fibers are used in very similar end uses, but based on differences of specific properties, each fiber tends to find its own niche where it has an advantage over the others.

IV. FIBER PROPERTIES AND THEIR APPLICATIONS

ARAMID FIBRES

The name Aramid is a shortened form of "aromatic polyamide". They are fibers in which the chain molecules are highly oriented along the fiber axis, so the strength of the chemical bond can be exploited.

Perhaps the best known and most widely used of the aramid fibers (Nomex is familiar to many),

Meta-aramids are best known for their combination of heat resistance and strength.

In addition, meta-aramid fibers do not ignite, melt or drip; a major reason for their success in the FR apparel market.

In comparison to commodity fibers, meta-aramids offer better long-term retention of mechanical properties at elevated temperatures.

Meta-aramids have a relatively soft hand and tend to process very similarly to conventional fibers, giving them a wide range of converted products.

Meta-aramids are available in a variety of forms, anti-stat, conductive, in blends (with other high performance fibers), etc.

Teijin Conex HT high tenacity type meta-aramid has significantly higher tensile strength of other meta-aramids. This high strength allows it to bridge the gap between meta-aramid and para-aramid fiber when strength is the primary concern.

M-aramid Properties	Value
Tenacity g/de	3.8-7.2
Elongation (%)	25-40
Limiting Oxygen Index	30
Chemical resistance	Mild-Good
Operating temperature	4000 F

  

Typical Applications for Meta-Aramid Fabrics (not an exhaustive list)

M-Aramid Fabric Form		Application
		� Automotive
Needlefelt		� Business machine parts � Cushion material
		� Hot gas filtration
		Safety & Protective clothing � Thermal insulation
		� Thermal spacers
		� Hot gas filtration
Woven fabric		� Loudspeaker components � Reinforcement: composites and rubber
		Safety & Protective clothing � Thermal insulation
		� Business machine parts
Wet-laid nonwoven		� Battery separators
	�	Heat shields
		� Business machine parts � Electrical insulation
Dry laid nonwoven		� Heat shields
		� Hot gas filtration � Laminate support base � Thermal spacers
Spunlace nonwoven		� High temperature filtration
		Safety & Protective clothing
		� Laminate support base

B. Para-aramid: Kevlar^® (DuPont), Twaron^® (Acordis), Technora^® (Teijin)

 Due to their highly oriented rigid molecular structure, para-aramid fibers have high tenacity, high tensile modulus and high heat resistance. Para-aramid fibers have similar operating temperatures to meta-aramid fibers, but have 3 to 7 times higher strength and modulus, making them ideal for reinforcement and protective type applications.

There are two types of Para-oriented aramid fibers:

Homo-polymer - Kevlar and Twaron

Co-polymer - Technora

Although para-aramids are high in strength, there is some problem with chemical resistance. Homopolymer para-aramids have relatively weak resistance to strong acids and bases.

Kevlar and Twaron, for instance, cannot be bleached with chlorine and are often not approved for food handling in protective gloves. The fine surface structure of Technora copolymer allows it to have much higher chemical resistance. Kevlar has new forms with increased strength and improved properties.

Co-polymer para-aramids have advantages with increased abrasion resistance and steam resistance – useful properties in many protective applications.

Typical properties of para-aramids are as follows:

	Properties	Value
	Tenacity g/de	22 - 26
	Modulus g/de	460-1100
	Elongation	2.4 – 4.4
	Continuous operating temperature	375o F
	Limiting Oxygen Index (LOI)	25 – 28
	Chemical resistance	Mild - Good

Appearances of Aramid Fiber

Fiber, Chopped fiber, Powder and Pulp

Aramid Properties | Aramid Fibers Properties | Properties of Aramid

Aramids share a high degree of orientation with other fibers such as Ultra high molecular weight polyethylene, a characteristic which dominates their properties.

General Properties of Aramid

• Good resstance to abrasion

• Good resistance to organic solvents

• Nonconductive

• No melting point, degradation starts from 500°C

• Low flammability

• Good fabric integrity at elevated temperature

• Sensitive to acids and salts

• Sensitive to ultraviolet radiation

• Prone to static build-up unless finished

Para-aramid | Para-aramid Fiber | Para-aramid Synthetic Fiber

• Para-aramid fibers such as Kevlar and Twaron, provide outstanding strength-to-weight properties

• High Young's modulus

• High tenacity

• Low creep

• Low elongation at break (~3.5%)

• Difficult to dye - usually solution dyed

Aramid, Uses

• Flame-resistant clothing

• Heat protective clothing and helmets

• Body armor[competing with PE based fiber products such as Dyneema and Spectra

• Composite materials

• Asbestos replacement (e.g. braking pads)

• Hot air filtration fabrics

• Tires, newly as Sulfron (sulfur modified Twaron)

• Mechanical rubber goods reinforcement

• Ropes and cables

• Wicks for fire dancing

• Optical fiber cable systems

• Sail cloth (not necessarily racing boat sails)

• Sporting goods

• Drumheads

• Wind instrument reeds, such as the Fibracell brand
• Speaker woofers

• Boat hull material

• Fiber reinforced concrete

• Reinforced thermoplastic pipes

• Tennis strings (e.g. by Ashaway and Prince tennis companies)

• Hockey sticks (normally in composition with such materials as wood and carbon)

Para-aramids are often blended with other fibers to impart some of their high strength properties to the blend or mix. A 60/40 blend of Kevlar and PBI, is the most widely used material for firemen’s premium turn out coats.

The Kevlar helps overcome some of the “textile” deficiencies (processing, strength) in the PBI; the PBI’s softness, moisture regain, and high temperature properties improves the performance characteristics of the Kevlar. And it reduces the cost of the otherwise expensive PBI fiber – over $70/lb.

Such synergy is often utilized in high performance fiber blends – one fiber contributing unique properties or improving characteristics of specialized materials – such as improved processing of otherwise difficult-to-handle fibers, or to reduce overall cost.

The following table shows typical applications, in fabric form, for para-aramids. The list is not exhaustive.

Needlefelt	� Cushion material Safety and protective clothing � Thermal insulation � Thermal barriers
Woven fabric	� Reinforcement: composites and rubber � Sporting goods � Thermal insulation
	� Mechanical rubber goods
	Safety and protective clothing � Ballistic application
Wet-laid nonwoven	� Friction materials
	� Heat shields
Yarn	� Reinforcement: composites and rubber � Sewing thread
	� Ropes and cables
	Safety and protective clothing
	(sewing thread)

C. Fluorocarbon fibers (PTFE): Teflon^® (duPont), Toyoflon^® (Toray)

PTFE (polytetrafluoroethylene) fibers offer a unique blend of chemical and temperature resistance, coupled with a low friction coefficient.

PTFE is virtually chemically inert, and is able to withstand exposure to extremely harsh environments.

The coefficient of friction for PTFE, the lowest of all fibers

Therefore the fiber is suitable for a wide range of applications such as bearing replacement material and release material when stickiness is a concern.

Due to low friction coefficient & low tensile strength,

It is difficult to process PTFE and blend PTFE with other fibers.

PTFE is breathable, porous membranes laminated to fabrics for protective uses.

The following properties area typical of PTFE materials

PTFE Properties	Value
Tenacity g/de	2
Elongation (%)	25
Limiting Oxygen Index (LOI)	95
Chemical resistance	Excellent
Friction coefficient	0.2
Operating temperature	500 (0F)

PTFE Form	Application
Needlefelt	� Automotive
	• Bearing replacement • Hot gas filtration
	• Release fabrics
Woven fabric	• Conveyor belts
	• Mechanical rubber goods
	• Gasket tape
Wet-laid nonwoven	• Battery separators
	• Heat shields
	• Liquid filtration
Monofilament	• Release fabrics
	• Filtration fabrics
Yarns	• Mechanical rubber good
	• Sewing thread
Membranes	• Filtration
	• Safety and Protective (vapor barriers, breathable membranes)

The following table lists typical applications for PTFE yarns/fibers.

D. Polyphelene Sulfide (PPS): Ryton^® (Amoco/Successor), Procon^® (Toyobo), Toray PPS^® (Toray)

-Moderate temperature resistance

-excellent chemical resistance.

-good flame resistance --high LOI.

PPS Properties	Value
Tenacity g/de	3.5 – 4.5
Elongation (%)	32 – 49
Limiting Oxygen Index (LOI)	34
Chemical resistance	Very Good
Operating Temp (0F)	500

-low moisture regain of PPS

-Unsuitable for use in protective apparel;

-uncomfortable hand,

-good chemical resistance makes it very attractive for industrial applications, especially for filtration.

The following represent typical applications for PPS. The list is rather short, but the applications are important.

Form	Application
Needlefelt	� Hot gas filtration
	� Liquid filtration
Woven fabric	� Laundry materials
	� Rubber industries

E. Melamine: Basofil^® (BASF)

-recently entered & one newest fibers; made a rapid impact. --its low cost

-a high operating temperature

-a high LOI

-used in hot gas filtration and safety and protective apparel markets.

-typical products are needled products

-yarns made from wrapped spinning techniques,

-recent advances have led to satisfactory ring spun yarns,

-blended with other fibers, such as para-aramids,

-suitable for weaving into products used by firemen

This development may lead the way to its adoption in other areas.

Basofil Properties	Value
Tenacity g/de	2.0
Elongation (%)	18
Limiting Oxygen Index (LOI)	32
Chemical resistance	Mild - Good
Operating temperature (0F)	400

-up to now limited range of application,

-rapidly growing, of on-going applications.

-promising for this high performance,

-low cost fiber

-suitable for a number of existing areas, especially as processing difficulties are overcome.         

F. PBO: Zylon^® (Toyobo)

Poly-phenylene benzobisoxazole is another new entrant to the high performance organic fibers market.

-outstanding thermal properties

-almost twice the tensile strength of conventional para-aramid fibers.

-high modulus makes it an excellent candidate for composites reinforcement.

-Due to its high LOI, PBO has over twice the flame retardant properties of meta-aramid fibers.

-still in its pilot plant stages, with commercial production just coming on stream.

PBO Properties	Value
Tenacity g/de	42
Modulus g/de	1300
Elongation (%)	3.5
Continuous operation temp.	550-600 (oF)
Limiting Oxygen Index (%)	68
Chemical resistance	Mild-Good

 

The following lists some of the possible areas of application for PBO materials.

Form	Application
Woven Fabric	� Reinforcement composites and rubber
	� Sporting goods � Thermal shields
	Safety and protective clothing � Ballistic applications � Mechanical rubber goods
Needlefelt	� Aluminum spacers
	� Heat shields

 G. PBI: PBI (Celanese)

Polybenzimidazole is an organic fiber

-excellent thermal resistant properties

-a good hand.

-PBI does not burn in air and

-does not melt or drip.

-high LOI coupled

-good chemical resistance

-good moisture regain

-an excellent fiber for fire blocking end uses such as safety and protective clothing and flame retardant fabrics.

-physical properties are relatively low,

-PBI is processed on most types of textile equipment. -blends well with other materials such as carbon and aramid fibers and is most often done for performance reasons as well as cost.

-had significant success in the fireman's apparel market where,

-blended in a 60/40 para-aramid/PBI mixture,

-it has become the standard “premium” material.

-PBI’s characteristic gold color blends well with other materials for a pleasing appearance.

-main drawback is its very high price – over $70 per pound.

PBI Properties	Value
Tenacity g/de	2.7
Modulus g/de	32
Elongation (%)	29
Continuous operation temp. (OF)	482
Limiting Oxygen Index (%)	41
Chemical resistance	Good - Excellent

Typical applications for PBI include the following:

Form	Application
	� Thermal insulation
Needle felt	Safety and protective clothing
	� Fire blocking
Woven Fabric	� Thermal insulation
	Safety and protective clothing

H. Polyimide (PI): P-84^® (Inspec)

P-84 is a polyimide fiber developed by Lenzing AG (Austria) and now produced and marketed by a spin-off company, Inspec  Fibres GmbH in Austria.

-a high operating temperature

-very good flame retardant properties

-good chemical resistance.

-a unique multi-lobal irregular cross section.

- irregular structure offers greater surface area than a conventional round cross section,

-achieved widespread recognition in the hot gas filtration market.

-Due to its high price, use is limited to areas where extreme emission controls are necessary.

-has  protective clothing market  in Europe.

P-84 Properties	Value
Tenacity g/de	4.2
Elongation (%)	30
Continuous operation temp.	500(0F)
Limiting Oxygen Index (%)	38
Chemical resistance	Good

Typical applications for P-84 polyimide fabrics include the following:

                                                            

 I. Carbon Precursor: Lastan^® (Asahi)

Lastan is a flame-retardant fiber made by pyrolytic carbonization of a modified acrylic fiber.

Carbon precursor fibers are partially carbonized fibers which transform into carbon or graphite fiber when they undergo further carbonization in an inert atmosphere at high temperature.

-a high operating temperature

-excellent flame resistance.

-has limited abrasion resistance,

-it is often blended 50%/50% with para-aramid fibers creating a strong durable product still having an LOI of 45.

-Due to its soft hand, Lastan fiber is desirable in apparel applications

-as well as certain industrial applications.

Lastan ® Properties	Value
Tenacity g/de	2
Elongation (%)	15
Continuous operation temp. (OF)	392
Limiting Oxygen Index (%)	60
Chemical resistance	Mild
Electrical resistance	108 – 1010 Ù cm

Typical applications for carbon precursor fabrics include the following:

Form	Application
Woven Fabric	� Welding blankets � Aluminized fabrics
	� Thermal barriers
	Safety and protective clothing
	� Welding blankets
Needlefelt	� Thermal barriers
	Safety and protective clothing
Dry Laid Nonwovens	� Aluminized fabrics - S&P

J. Carbon fiber: PAN (polyacrylonitrile) and Pitch based                        

(C₃H₃N)_n

1. Any of various thick, dark, sticky substances obtained from the distillation residue of coal tar, wood tar, or petroleum and used for waterproofing, roofing, caulking, and paving. 2. Any of various natural bitumens, such as mineral pitch or asphalt.)

There are different categories of carbon fibers based on modulus, tensile strength, and final heat treatment temperature.

In the carbonization process, temperature exposures range from 1000⁰ C to 2000⁰ C, each different level of exposure creating a different property for the fiber. For example, high-modulus type is processed at 2000⁰ C, 1500⁰ C for high strength type, and 1000⁰ C for low modulus and low strength type.

The main carbon fibers are made from Polyacrylonitrile (PAN) based and pitch based, and is well known for their composite reinforcement and heat resistant end uses.

Carbon Fiber Properties	PAN	PITCH
Tenacity g/de	18-70	14-30
Modulus g/de	1640 - 3850	1000 -5850
Elongation (%)	0.4-2.4	0.2 – 1.3
Continuous operation temp. (0F)	570 - 1000	570 – 1000

 Carbon fibers find application in many forms and many areas. Some include the following: 

          K. Glass:

Glass is an inorganic fiber, which is neither oriented nor crystalline. Glass fibers were one of the first “man-made” fibers, commercialized in the late 30’s.

-Widely used as insulation (glass batts in home insulation and industrial insulation in mats and fabric form).

-widely used in reinforcing thermoplastic composites in products from circuit boards to boat hulls.

-High temperature filtration is another high volume use.

The ingredients normally used in making glass fibers are: silicon dioxide, calcium oxide, aluminum oxide, baron oxide, plus a few other metal oxides.

Glass types:

A -alkali-containing glass composition.
AR - alkali-resistant for reinforcing cement.
C -chemically-resistant glass composition.
E -standard uses, this composition has high electrical resistance.

 HS magnesium-alumina-silica glass. High strength.
S -composition similar to HS glass.

The following chart is representative of the properties of various glass fibers.

Properties	E-glass	AR-glass	S-glass
Tensile Strength (g/de)	35	46	35
Modulus (g/de)	524	1250	620

Elongation (%)	4.8	2	5.4
Refractive index	1.547	1.561
Density (g/cm3)	2.57	2.68	2.46
Coefficient of thermal expansion (107 0C)	50-52	75	23-27
Dielectric(1010Hz) Constant	6.1-6.3

Typical glass applications include:

Form	Application
	� Automotive
Woven Fabric	� Filtration
	� Reinforcement - plastic/rubber/cement � Thermal insulation
	� Printed circuit boards – electrical
Needlefelts	� Aircraft and aerospace � Cushion material
	� Filtration
	� Thermal insulation and spacers � Acoustic insulation

L. High Density Polyethylene - HDPE: Spectra^® (Honeywell), Dyneema^® (Dyneema)

HDPE fibers offer strength similar to that of para-aramids.

Developed in Japan by Dyneema, and known throughout the world as Dyneema, except in the US where the process is licensed to AlliedSignal and is known as Spectra.

-Light in weight,

-the fiber has a specific gravity of less than 1,

-tough yet lightweight products can be made, including rope and cordage that floats as well as soft and semi-rigid body armor and in

-cut resistant materials such as gloves that are lighter than competitors, reducing fatigue in use.

-high tenacity,

-HDPE fibers have very good abrasion resistance and

-excellent chemical and electrical resistance.

-fibers are inherently “slick” and difficult to adhere to, a drawback in some areas but not of concern in others.

-can be bleached and sterilized and used for food handling gloves, among others.

-low melting points, however, so their continuous operating temperature is a relatively low 250⁰ F.

HDPE Fiber Properties	Value
Tenacity g/de	30
Elongation (%)	3
Continuous operation temp. (OF)	250
Modulus g/de	1400
Chemical resistance	excellent

Typical applications and forms of HDPE fibers include:

Form	Application
Yarns	� Marine ropes and cordage
	� Sail cloth
	� Marine
Woven Fabric	Safety and protective products � Reinforcement of composites (sport, pressure vessels, boat hulls, implants) � Medical

V. CONCLUSION

High performance fibers and high temperature resistant fibers offer numerous advantages over traditional materials. Higher strength, lighter weight, higher operating temperatures and flame-retardant ability are some of the most prominent features of these fibers. These outstanding properties create opportunities to manufacture products that historically could not be made due to technical constraints. The protective clothing area is one of those markets.

Each of these fibers discussed their limitations. It is not as easy to take these materials “off the shelf” except for a few well-distributed ones. Surely, some are more readily available than others -- the aramids, HDPE, for instance -- but most are less so and should be considered as engineering or specialized materials to be used where their properties are paramount. Review thoroughly each fiber for the properties it brings to the product.

High performance fibers allow companies to enter niche markets, which typically provide higher profits as well as strong barriers to entry for the competition. Even in the high performance area, many markets have become "commodity" applications, particularly the aramids in protective clothing. The protective clothing market will continue to bring new opportunities for high performance fibers as the fiber manufacturers expand their current product lines as well as create new and exciting specialized materials.

Filament technical yarns

There have been many types of filament yarns developed for technical applications, such as reinforcing and protecting. The reinforcing technical yarns have either high modulus, high strength, or both. Yarns for protecting applications can be resistant to safety hazards such as heat and fire, chemical and mechanical damage. There are many types of technical filament yarns used in various applications; it is only possible, therefore, to list just a few yarns here that are popularly used in the development of some technical textile products.

Aramid filament yarns

Aramid fibre is a chemical fibre in which the fibre-forming substance is a long chain synthetic polyamide where at least 85% of the amide linkages are attached directly to two aromatic rings. Nomex and Kevlar are two well-known trade names of the aramid fibre, owned by Du Pont. Aramid fibres have high tenacity and high resistance to stretch, to most chemicals and to high temperature. The Kevlar aramid is well known for its relatively light weight and for its fatigue and damage resistance. Because of these properties, Kevlar 29 is widely used and accepted for making body armour. Kevlar 49, on the other hand, has high tenacity and is used as reinforcing material for many composite uses, including materials for making boat and aircraft parts. The Nomex aramid, on the other hand, is heat resistant and is used in making fire fighters’ apparel and similar applications.

Aramid yarns are more flexible than their other high performance counterparts such as glass and Kevlar, and thus are easier to use in subsequent fabric making processes, be it weaving, knitting, or braiding. Care should be taken, though, as aramid yarns are much stronger and much more extensible than the conventional textile yarns, which could make the fabric formation process more difficult.

Glass filament yarns

Glass is an incombustible textile fibre and has high tenacity too. It has been used for fire-retardant applications and also is commonly used in insulation of buildings. Because of its properties and low cost, glass fibre is widely used in the manufacture of reinforcement for composites. There are different types of glass fibres, such as Eglass, C-glass, and S-glass. E-glass has very high resistance to attack by moisture and has high electrical and heat resistance. It is commonly used in glass-reinforced plastics in the form of woven fabrics. C-glass is known for its chemical resistance to both acids and alkalis. It is widely used for applications where such resistance is required, such as in chemical filtration. The S-glass is a high strength glass fibre and is used in composite manufacturing.

Glass filament yarns are brittle compared with the conventional textile yarns. It has been shown that the specific flexural rigidity of glass fibre is 0.89mNmm2 tex-2, about 4.5 times more rigid than wool. As a result, glass yarns are easy to break in textile processing. Therefore, it is important to apply suitable size to the glass yarn to minimise the interfibre friction and to hold the individual fibres together in the strand. Dextrinised starch gum, gelatine, polyvinyl alcohol, hydrogenerated

vegetable oils and non-ionic detergents are commonly used sizes.

When handling glass fibres, protective clothing and a mask should be worn to prevent skin irritation and inhalation of glass fibres.

v      Carbon filament yarns

Carbon fibres are commonly made from precursor fibres such as rayon and acrylic. When converting acrylic fibre to carbon, a three-stage heating process is used. The initial stage is oxidative stabilisation, which heats the acrylic fibre at 200–300°C under oxidising conditions. This is followed by the carbonisation stage, when the oxidised fibre is heated in an inert atmosphere to temperatures around 1000°C. Consequently, hydrogen and nitrogen atoms are expelled from the oxidised fibre,

leaving the carbon atoms in the form of hexagonal rings that are arranged in oriented fibrils.The final stage of the process is graphitisation, when the carbonized filaments are heated to a temperature up to 3000°C, again in an inert atmosphere. Graphitisation increases the orderly arrangement of the carbon atoms, which are organised into a crystalline structure of layers.These layers are well oriented in the direction of fibre axis, which is an important factor in producing high modulus fibres.

Like the glass yarns, most carbon fibres are brittle. Sizes are used to adhere the

filaments together to improve the processability. In addition to protecting operatives against skin irritation and short fibre inhalation, protecting the processing machinery and auxiliary electric and electronic devices needs to be considered too, as carbon fibre is conductive.

v     HDPE filament yarns

HDPE refers to high density polyethylene. Although the basic theory for making super strong polyethylene fibres was available in the 1930s, commercial high performance polyethylene fibre was not manufactured until recently. Spectra, Dyneema, and Tekmilon are among the most well-known HDPE fibres. The gel spinning process is used to produce the HDPE fibre. Polyethylene with an extra high molecular weight is used as the starting material. In the gel spinning process, the molecules are dissolved in a solvent and spun through a spinneret. In solution,

the molecules which form clusters in the solid state become disentangled and remain in this state after the solution is cooled to give filaments.The drawing process after spinning results in a very high level of macromolecular orientation in the filaments, leading to a fibre with very high tenacity and modulus. Dyneema, for example, is characterised by a parallel orientation of greater than 95% and a high level of crystallinity of up to 85%.This gives unique properties to the HDPE fibres. The most attractive properties of this type of fibre are: (1) very high tenacity, (2) very high specific modulus, (3) low elongation and (4) low fibre density, that is lighter than water.

HDPE fibres are made into different grades for different applications. Dyneema, for example, is made into SK60, SK65 and SK66. Dyneema SK60 is the multipurpose grade. It is used, for example, for ropes and cordage, for protective clothing and for reinforcement of impact-resistant composites. Dyneema SK65 has a higher tenacity and modulus than SK60. This fibre is used where high performance

is needed and maximum weight savings are to be attained. Dyneema SK66 is specially designed for ballistic protection.This fibre provides the highest energy absorption at ultrasonic speeds.

Table 3.1 compares the properties of the above mentioned filament yarns to steel.

v      Other technical yarns

There have been many other high performance fibres developed for technical applications, among which are PTFE, PBI, and PBO fibres.

PTFE (polytetrafluoroethylene) fibres offer a unique blend of chemical and temperature resistance, coupled with a low fraction coefficient. Since PTFE is virtually chemically inert, it can withstand exposure to extremely harsh temperature and chemical environments.The friction coefficient, claimed to be the lowest of all fibres, makes it suitable for applications such as heavy-duty bearings where low relative

speeds are involved.

PBI (polybenzimidazole) is a manufactured fibre in which the fibre-forming substance is a long chain aromatic polymer. It has excellent thermal resistance and a good hand, coupled with a very high moisture regain. Because of these, the PBI.

Fibre is ideal for use in heat-resistant apparel for fire fighters, fuel handlers, welders, astronauts, and racing car drivers.

PBO (polyphenylenebenzobisoxazole) is another new entrant in the high performance organic fibres market. Zylon, made by Toyobo, is the only PBO fibre in production. PBO fibre has outstanding thermal properties and almost twice the strength of conventional para-aramid fibres. Its high modulus makes it an excellent material for composite reinforcement. Its low LOI gives PBO more than twice the flame-retardant properties of meta-aramid fibres. It can also be used for ballistic

vests and helmets