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Outdoor Textiles – an overview

 Abridged Notes from "Raw Fibre through to Finished Fabric" by Bradmill Outdoor Fabrics.

 Raw Materials

The principal raw materials used in the manufacture of canvas and coated or laminated synthetic fabrics are:

Canvas

  • Flax
  • Cotton
  • Jute
  • Polyester – staple (fibres of specific length) & filament (fibres of indefinite length)
  • Acrylic
  • Modacrylic
  • Polyvinyl Alcohol

Synthetic Outdoor Fabrics

  • Polyester
  • Nylon
  • Polyvinyl Alcohol
  • Polypropylene
  • Polyethylene

The term ‘canvas’ covers a very wide variety of relatively heavy fabrics used in apparel, furnishings, embroidery , sails and proofed outdoor fabrics.

Canvas in the industrial and outdoor fabrics field is synonymous with ‘duck’.  The term duck relates to heavy plain weave fabrics.  It originated with the heavy weight sailcloth produced in Britain in the 19th Century, which bore a stencil mark depicting a duck.

Canvas was woven originally from natural fibres but is now available in 100% synthetic and mixtures of natural and synthetic fibres.

Canvas may be supplied in raw or ‘loomstate’ form or may be finished in a variety of ways.  The most common types of finish are proofing by impregnation of water and rot resisting compounds, or coating with coloured pigment compounds which are also water and rot resisting.

Most finished canvas is air and vapour permeable – ie. it will ‘breathe’ and allow the passage of air and moisture but resist water penetration.

Flax

Flax and cotton are used traditionally in canvas manufacture.  Flax was utilized extensively in heavy duty transport tarpaulins, where its strength and abrasion resistant properties provided significant advantages over cotton.  However with the introduction of synthetic fibres, flax has been displaced with combinations of cotton and polyester which offer scope to provide lighter weight fabrics with equivalent or better properties, at lower cost. Flax canvases these days are generally confined to fabrics used for water bags and artist cloth.

Cotton

Cotton is ideally suited for use in canvas.  It has good general mechanical properties and it is especially suitable under wet conditions, provided it is adequately protected from microbiological degradation.

The fibre not only gains strength when wet but swells, thereby enhancing the water resistance of firmly woven canvases containing cotton.  An important feature of cotton is its ability to transmit water vapour, which prevents the formation of condensation within enclosures where high levels of moisture are present.

Jute

The lowest in cost of all natural textile fabrics, but its use in canvas manufacture is limited.  It has moderate strength, good dimensional stability and is a sound fibre for use in products not subjected to excessive moisture.  Under wet conditions it loses strength and will deteriorate rapidly when exposed to sunlight.  Its use in the canvas industry is generally confined to low cost livestock covers, casual furniture and utility products not subject to weathering.

Polyester

Polyester is used extensively in canvas and synthetic outdoor fabrics.  It is an economical material with excellent dimensional stability and mechanical properties.  It is tough, has high strength and modulus with good elastic recovery, abrasion resistant and with the exception of strong concentrated acids, it has good resistance to most common organic and inorganic chemicals.

Its’ resistance to ultra-violet degradation in outdoor exposure is only moderate and it should be protected with proofing compounds or suitable coatings. 

Polyester staple is mixed with cotton in ratios varying from 50/50 to 65/35 to produce blended polyester/cotton canvas.  Corespun yarns contain high tenacity polyester filament yarn, which is covered with a cotton sheath during the spinning process.

By comparison with 100% polyester staple yarn of equivalent count, polyester filament yarns have higher tensile strength due to their structure with continuous fibres of indefinite lengths (ie filaments) whereas polyester staple yarns rely upon inter fibre friction imparted by twist in their structure to provide strength.

Acrylic

Acrylic fibres are derived from acrylonitrile.

Co-polymers –contain 85% acrylonitrile, the remaining 15% being additives to provide good dyeing properties.

Homopolyers – fibres are 100% acrylonitrile, generally have higher strength and better heat stability than the co-polymers but are difficult to dye.

Acrylics have good resistance to weak akali, acids, fungal attack and generally adverse environmental conditions.  A notable feature is their good resistance to UV degradation.  This is enhanced by the addition of pigment colours into the solution from which the fibres are extruded.  Fibres coloured this way are known as solution or dope dyed acrylics.

Dope dyed acrylics have the highest light fastness properties of all textiles.  This makes them ideally suitable for fabrics which are permanently exposed to weathering such as fixed awnings and marine covers.  However the level of fastness depends on the colour itself and its depth.  Dark colours show no significant change after prolonger exposure.  Lighter colours will change in shade and may take on a greenish hue.

The conversion of fibre into yarns

Two principal methods of yarn formation

  • Spinning staple fibres
  • Extrusions of a chemical solution to make continuous filaments.

Fabric construction

Woven fabrics dominate the canvas and synthetic products field, where strength, dimensional stability and dense construction are of special importance.

Woven fabrics are constructed with yarns that are interlaced at right angles.  The yarns which run in the lengthwise direction are called warp and those running across the fabric are called weft.

Fabric Finishing - Colouration

Pigments differ from dyes, in that they are small particles that are insoluble in water.  When applied to the textile, pigments do not penetrate into the fibre but are mechanically bonded to the surface by a resin.

Pigments may be incorporated into the spinning solution of synthetic fibres prior to their extrusion.  These are referred to as ‘Dope’ dyed or ‘Solution’ dyed, and ensures the weathering fastness of acrylic fabrics.

The Measurement of Fabric Properties

 1.  Breaking Force

This is the measure of the tensile strength of the fabric. It is made in both warp and weft directions.

AS3567 stipulates testing of this parameter be in accordance with AS2001.2.3 – whereby a 50mm strip (frayed if possible or cut if fabric cannot be frayed) is subjected to a load at a constant rate of extension (for most case this will be 100mm/min).

The measure of force is taken when the fabric breaks, with an average of five tests representing the result that is expressed in Newtons (N) per 50mm strip.  Elongation of the fabric under the load can also be measured and from this the load/elongation characteristics of the fabric can be determined.

Generally speaking, the higher the density of the warp or weft threads in a given fabric type, the higher the breaking forces due to greater distribution load.

The minimum requirement of AS 3567 ranges from: Warp: 830 - 3000 N, Weft: 700 – 2600 N, depending on the fabric designation and mass per unit area.

2.  Tearing Force

AS 3567 requires canvas to be tested according to AS 2001.2.10, known as the Wing Rip test.  Here the fabric is cut in either the warp or the weft direction to form two tails or ‘wings’.  One wing is mounted in the upper jaw of the tensile tester and the other in the lower jaw.  The jaws are then moved apart at a constant rate of 100mm per minute.

The forces required to tear across the warp or weft threads are measured. The result for this test is taken by averaging the five highest readings over 50mm. The average of five results is taken to represent the tear strength.  It is measured in Newtons (N).

Tear strengths are significantly lower than tensile strengths of a given fabric.

NB the standard wing rip test does not adequately reflect the benefit of Ripstop constructions

3.  Resistance to Water Penetration

Hydrostatic Pressure AS2001.2.17

The test is conducted by clamping a specimen of fabric to form a diaphragm of 75mm diameter over a chamber that is connected to a vertically mounted glass tube by means of a flexible hose. The system is filled with distilled water and the glass tube is raised at a specified rate.  The reading is taken noting the height (calibrated in kPa) of water in the glass tube above the diaphragm, when three drops penetrate the fabric.

The minimum requirement of AS 3567, ranges from 3kPa to 6kPa, depending on the fabric designation and mass per unit area.

Cone Test AS2001.2.18

This test is designed to assess the resistance of textile fabrics to penetration of water under static conditions.

The test involves taking a 250mm square specimen, lightly folding to a cone shape and placing in a funnel that is suspended above a beaker.

250ml of distilled water is placed in the cone and left for a period of 18  hours, after which the amount of water (if any), which leaks through the cone into the beaker, is measured.

AS 3567 requires that proofed/coated canvases show zero cone leakage.

4.  Colourfastness

The assessment of colourfastness is done by comparing the textile in its original state and in its tested state, by assessing the staining of an undyed cloth brought into contact with the specimen.   Two standard scales called The Grey Scales are used to determine the change in colour of the test specimens and the degree of staining of undyed cloths.

The scales range from 1 to 5, with 1 representing the lowest level of fastness and 5 the maximum.

Colourfastness to light refers to a fabrics ability to resist or colour change when exposed to light.  The fabric is exposed under specific conditions to light produced by an MBTF lamp, (a mercury vapour, tungsten filament, internally phospor-coated lamp). A set of reference blue swatches (’blue standards’) is exposed at the same time. The colourfastness to light is obtained by comparing the degree of fading of the test sample with the blue standards. The Blue Scale consists of 8 gradations.

The requirements of AS3567 are – when tested to AS2001.4.21 (artificial light) the rating can be no less than 6.

5.  Colourfastness to Rubbing (crocking)

Measured by an instrument called a Crockmeter. The test involves having a standard undyed cotton cloth mounted on a peg, 18mm in diameter under a given load, and subsequently moved back and forth over the test specimen 10 times. The procedure includes both a wet and dry test.  The staining on the surface of the cloth and the change in colour of the specimen are evaluated against the Grey Scales.

6.  Water Repellency

Defined as the property of fabric to resist wetting.  A drop of water will rest on the surface of a water repellent fabric and may be shaken off without wetting it. Tested by means of the Spray Rating Test AS2001.2.16 in which 250ml of distilled water is sprayed on by gravity through a standard nozzle in to a 15cm diameter disc of fabric held at 45°.  After spraying the fabric is tapped sharply to remove the excess water and the surface is compared to a set of standards which range between 0 and 100. With 0 being complete wetting and 100 being no wetting or adhesion.

7.  Dimensional Stability

Shrinkage is an important property, and a small degree (up to 4%) due to the effects of weathering, is a desirable property in polyester cotton canvas fabrics used to provide weathering protection.  The consolidation of the fabric, which occurs during shrinkage, enhances its water resistance and the waterproof properties of canvas with residual shrinkage usually improve following exposure.

A small degree of shrinkage also helps to ensure that fabrics mounted on frames, such as fixed awnings and truck canopies, remain taut and well shaped.

Acrylic fabrics have relatively low elasticity and can tend to become ‘baggy’ after prolonged exposure.  It is generally recommended that they are undercut and stretched lightly over fixed frames.  However excessive stretch in not recommended as this can cause permanent distortion or may lead to reduction of waterproofness.

 

 

 

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