Dimensional and Density Measurements

This type of testing includes only basic measurement techniques; essentially dimensional measurement coupled with weighing in some instances. Although superficially very simple, these tests require a good calibration regime if they are to be successfully accredited.

Another crucial issue is the correct conditioning of the sample with respect to temperature and humidity. It is not difficult to appreciate that the dimensions, and especially the weight per unit area/length, of a textile or leather sample will depend on temperature and particularly on relative humidity. Hence tests of this type normally require 24 hour conditioning of the sample under standard conditions. Additionally the test must be conducted under the same, standard and controlled, conditions. This ensures maximum comparability for results from different laboratories and a level playing field for commercially important measurements.

There are available rapid conditioning cabinets as shown left. These permit conditioning of fabrics in minutes, depending on the type and weight of fabric. For the purposes of accreditation it would be necessary to carry out some comparative studies, where tests are conducted after conventional 24 hour conditioning and after conditioning in the rapid system. There should be no significant difference in the results.

Demonstrating that conditioning of every sample has been correctly carried out requires careful management and recording of conditioning times, temperature and humidity. As discussed elsewhere laboratories will need to keep separate records of the laboratory temperature and humidity. If the start and end time of conditioning, and the time of the test, are recorded for each sample then an audit trail is created which can be checked to ensure that the general laboratory records confirm that the temperature and humidity were in specification during the relevant periods.

A useful way to create the necessary records is to attach a card to each sample as shown below. This is eventually filed with the results of testing that sample so providing an auditable record that the correct procedure was followed.

A dimensional measurement of the type being discussed may constitute a test in itself but the same issues arise when samples are being prepared for subjection to other tests. They must similarly be conditioned and measured with properly calibrated equipment. Obvious examples are when samples are being prepared for dimensional stability tests such a shrinkage in washing or, where test pieces of fabric, yarn or leather are being prepared for tensile testing. Even chemical tests require conditioning of the sample so that the weight used for analysis is measured under standard conditions of humidity.

Balance Calibration and Management

There is nothing special about the calibration and management of balances for textile and leather testing. Most work in this area requires a two place or, at most, a three place balance. The general guidance of the selected accreditation body on balance calibration and intermediate verification should be followed. There is some variability as between accreditation bodies on what they recommend in detail for balance calibration but the basic pattern is much the same. Typical requirements are explained below.

• Balances must be calibrated in situ. Ideally even movement of a balance within the laboratory requires it to be re-calibrated but most accreditation bodies are prepared to accept that modern balances can be moved occasionally. This dispensation does not, however, extend to allowing balances to be shipped to a calibration laboratory for calibration and then re-shipped to the laboratory. It is similarly not acceptable to buy a balance with a calibration certificate which reflects a calibration at the supplier’s premises, even if this calibration is fully valid and traceable.

• An annual service and calibration of the balance is required. The calibration must be fully traceable which means, in practice, having it conducted by an ISO 17025 accredited calibration service. Note that a calibration by an ISO 9001 certified body will not be acceptable. Some accreditation bodies allow longer periods than one year between balance calibrations but the laboratory should check with the body they intend to use.

• In principle a laboratory can carry out its own balance calibrations. If you wish to do this you must have a set of traceably calibrated weights, of course, but also a documented procedure for conducting the calibration. This must include a detailed description of how the calibration is carried out and details of how the uncertainty of the calibration is calculated. A typical approach to calibration, for a variety of balance types, appears in document LAB 14 available on the United Kingdom Accreditation Service UKAS) website (www.ukas.com). This document shows procedures to be followed and gives guidance on the appropriate class of weights for different balance resolutions. An example of an uncertainty calculation for balance calibration appears in the UKAS document M3003, also available on their website. If a laboratory does carry out its own balance calibrations it must have suitable arrangements for maintaining the calibration of the weights used. Provided the weights are treated carefully and used only for the annual calibration of balances a period of up to three years between calibrations is normally acceptable but the accreditation body should be consulted for guidance.

Dimensional Measuring Equipment

Textile laboratories will typically use steel rulers, measuring tapes and perhaps Vernier callipers. Additionally leather testing laboratories will have applications where thickness of hides may need to be measured using micrometers.

All of this equipment must be calibrated by an ISO 17025 calibration service. Equipment such as rulers and tapes can retain their calibrations for up to five years if used carefully. They should, however, be inspected formally, and the inspection recorded, every six months for any signs of damage. However if any damage is noticed at any other time the equipment must be withdrawn from service immediately pending calibration checks or replacement.

Vernier callipers, specialist thickness gauges and micrometers must be fully traceably calibrated at least annually. It is advisable to verify them at least monthly using a set of gauge blocks. A set covering 5 to 50 mm in three or four steps is useful. These will normally be purchased with a calibration certificate but need not be re-calibrated. They can be used in essentially the same manner as balance check weights to confirm the consistency of callipers and micrometers, i.e. the values returned when using the working measurement equipment on the gauge blocks immediately after the annual calibration of the micrometer or callipers are recorded and the values returned at monthly checks compared with these original values. Drift should be no more than one digital interval. For example a set of Vernier callipers measuring an exactly 10 mm gauge block to 0.1 mm should give a result of 9.9 to 10.1 mm.

Some dimensional measurements such as picks per unit length in fabrics use low power microscopes or magnifying glasses fitted with scales used to define dimensions within which counts are made.

The simple type of pick counter (A) which consists of a square of defined dimensions, typically 25 mm or 1 inch, which can be viewed through a magnifying glass can be verified annually by means of a calibrated set of callipers. This can be done immediately the callipers have undergone their annual calibration. The main issue is to check that there has been no drift in the dimensions since the item was new. The longer type of pick counter (B) which is like a small travelling microscope can be checked by viewing a calibrated ruler, placed where the fabric would normally be, through the microscope and comparing the measurement from the ruler with that from the scale in the counter. This should be done annually and the measurements should agree to the nearest division, typically 1 mm. It is necessary to have a calibrated ruler specifically dedicated to this purpose and used for nothing else. Calibration references must be used only as such and not for routine measurement so the use of a working ruler, even if calibrated, would be invalid.

Cutting Templates

Many textile and leather testing procedures require a sample to be cut to a particular size and shape. It is convenient and common practice to have a metal template of the appropriate dimensions which is then cut around to generate the test piece. This is perfectly acceptable practice provided that the dimensions of the template are confirmed as within the necessary tolerance using a suitable piece of traceably calibrated measuring equipment, usually a calibrated ruler.

Additionally all templates should be inspected formally, and the inspection recorded, every six months for any signs of damage. However if any damage is noticed at any other time the template must be withdrawn from service immediately pending calibration checks or replacement.

Quality Control Sample

Laboratories should have samples of established composition and properties for use as quality controls for all tests. Such samples can be generated by preparing a large number of test pieces from a single sheet of leather or fabric. As the testing is almost invariably destructive the question arises as to whether the samples are homogeneous and how variations in the measurement can be distinguished from differences between samples taken from different areas of the sheet.

There is no complete answer to this question and, as a result any precision evaluation is bound to include a component from sample variability. Some control can be exercised, however, by adopting the following strategy for sheet materials.

In the case of fibres a similar strategy can be applied by spreading the fibre out on a table and sampling in the same manner.

In the case of yarns and threads as large a cone or reel as possible should be obtained. This should then be run off to generate up to 100 sample hanks of appropriate size for the test. Use every tenth sample taken regularly from the reel or cone for the initial establishment of the expected value and standard deviation. Store the other samples for regular quality control testing.

Quality control samples should be stored so as to protect them from dust and, ideally, in the controlled atmosphere section of the laboratory so they are maintained constantly under standard conditions.

A quality control sample should be run, as a minimum, as every twentieth test sample.

Mechanical Properties

This area of testing requires either a ‘universal testing machine’, effectively a tensile tester, or one of a range of specialist testing equipment to deal with issues such as bursting strength of fabrics, plastic film and leather sheet for clothing and upholstery applications.

Universal Testers

These instruments come in two types, single, left, and dual beam, right. They are used for a whole range of tasks ranging from simple tensile testing of fabrics, yarns and sheets of leather through to seam and stitch strength testing and applications such as button security.

Single beam instruments typically measure tension up to 5kN and are suitable for yarns and more delicate fabrics. Heavy fabric and leather applications will require a dual beam instrument which can operate up to 50kN.

When selecting a universal tester the following points should be taken into account.

• It is obviously necessary to purchase an instrument, or combination of instruments, which cover the necessary tension range. The obvious solution is to purchase an instrument with the widest possible range, e.g. up to 50kN. If this approach is contemplated be careful to check that accuracy is maintained at lower tensions. Ideally an accuracy of ±1% of the indicated load should be maintained at 1% capacity. This means that a 50kN machine measuring at 0.5kN has an accuracy of 0.005kN. Cheaper high capacity instruments may not achieve this performance so may be a false economy, requiring the purchase of a second instrument for low force applications.

• The machine should have the ability to measure the position of the cross head to within 0.01% of reading or 0.001mm, whichever is greater. This is effectively the parameter which determines the accuracy of any extension measurement.

• The cross head speed should be accurate to 0.005% of set speed. This characteristic is critical as many tests require a specific speed.

• Many instruments are supplied with the option of a dedicated controller or with a computer and specialist control and measuring software. The latter is usually the best choice as most systems permit extensive data handling and often export to other packages such as Microsoft Excel.

• Ensure the necessary grips are ordered with the machine. A typical set would include flat faced grips up to 150 mm wide for sheet materials, bollard grips for yarn and possibly specialist items such as grips for testing zip fasteners and button pull off. It is convenient to have pneumatically operated grips, especially for forces above 5kN. In this case a compressor, ideally low noise, will be required.

• Remember that the universal tester will need to be located in the temperature/humidity controlled area of the laboratory. It will also require a stable and spike free power supply so some kind of mains conditioner may be necessary.

Universal testers require at least annual calibration. This must be carried out by an ISO 17025 accredited calibration service. The calibration must encompass the following parameters.

Quality control samples should be available which can be run regularly to show that the test performance and instrument calibration remains acceptable. These should consist of a large number of retained samples taken from the same bulk sample. See elsewhere for guidance on generation and characterisation of quality control samples

Bursting Strength Testers

Bursting strength is a measurement widely applied to fabrics, paper, leather and all kinds of sheet and board materials. The basic principle is the application of hydraulic pressure to a known area of the sample through a rubber backing disc.

Bursting strength is a measurement widely applied to fabrics, paper, leather and all kinds of sheet and board materials. The basic principle is the application of hydraulic pressure to a known area of the sample through a rubber backing disc.When purchasing a bursting tester it is important to specify the area be tested as this differs, sometimes quite subtly between different standards. Calibration of the pressure sensor in the equipment is a specialised task and must be conducted by an ISO 17025 accredited calibration service. Annual calibration is appropriate. The best way to carry out checks between calibrations is by running routine quality control samples. Sheets of polythene are typically highly uniform so a large number of samples can be cut from the same sheet with reasonable confidence they will be closely similar in thickness and properties. One of these samples can be included with each batch of real samples as a quality control and trends in results monitored to detect any drift in calibration. For more detailed information on generating quality control samples follow this link.

Composition

Microscopy

Composition of mixtures of fibres can be determined qualitatively (identification) or even quantitatively by examination under a microscope. The system pictured is particularly sophisticated and includes digital imaging software to carry out a whole range of measurements automatically. Composition and fibre quality parameters can be determined.Simpler systems require entirely manual measurement against a microscope graticule. Fibre identification and quality requires skilled practitioners who have been well trained but there will always be some variation arising from the inevitable residual subjective element of the method.

A key element in obtaining accreditation for such methods will be performance of the laboratory and the individual staff in proficiency testing exercises. Proficiency testing should include independent determinations by all staff authorised for the method so that any internal inconsistencies can be detected in addition to estimating overall laboratory performance.

Microscopes will need to be checked on a three monthly basis using a traceably calibrated graticule so that validity of any dimensional measurements is established.

Laboratories should retain a collection of samples covering the typical range of types and composition routinely measured. These can then be used as for regular quality controls and also as test materials for confirmation of competence of newly trained staff. (Hyperlink to staff training/competence).

Chemical Analysis

The chemical analysis of fibre mixtures and of leather samples to determine composition does not differ fundamentally from any other type of chemical analysis. The approach to preparing methods for accreditation is exactly the same as described on the chemical analysis section of this website [Hyperlink here to validation on chemistry site]. Almost invariably the methods chosen will be standard published methods so a full validation will not be required. Typically the methods for textiles are likely to be chosen from BS 4407, ISO 1833, ISO 5088, AATCC 20A and ASTM D629. The common analyses for leathers are Kjeldahl nitrogen (e.g. ASTM D2868) , water soluble matter (e.g. ASTM D2876), chlorides in mineral tanned leather (ASTM D4653) and hexane extract (ASTM D3495).

Validation for such methods will require at least a demonstration of bias (accuracy), precision and limit of detection. See the section on method validation for further discussion on this point. Quality control samples covering typical ranges of composition should be available

Contaminants and Restricted Substances

The list of substances banned or severely restricted as contaminates in textile and leather products increases every year. The recent REACH regulations have ensured a quantum leap in these controls as the list of Substances of Very High Concern expands.

The table below shows the tests of this type which are most likely to be encountered but is by no means exhaustive. In each case the method will require validation and quality control samples of typical matrices should be available.

Product Suitability

Tests in this category include a wide range of evaluations of raw materials, intermediate products and final wearable items against an extensive range of suitability criteria.

Only the common tests are covered here but the approach to preparing them for accreditation can be applied to any tests of this type.

Most of these tests require the use of a tightly defined piece of test equipment, e.g. abrasion tests, pilling, or the use of equipment set up to operate according to a specific programme, e.g. washing tests. It is, therefore, essential to confirm that purchased equipment has the dimensions, properties and operating parameters given in the test specification. Some accreditation bodies will accept that a piece of equipment which is specified by a reputable supplier as compliant with a specific set of standard test protocols may be used without detailed qualification. You should check the position of your chosen accreditation body as regards this policy. However after a period of use, normally one year, the equipment will need to be verified to establish that none of the critical parameters have changed.

Equipment which has to maintain specific temperatures and/or to operate on a time programme will require traceable calibration of both temperature and time. Temperature calibration of complex equipment such as reference washing machines cannot normally be carried out by the laboratory. Access to the relevant test points will typically require specialist knowledge and equipment.

Several of the well known suppliers of textile and leather testing equipment are accredited to ISO 17025 to calibrate such instruments. In order to order to keep calibration fees within reasonable bounds they will often visit a country, perhaps twice a year, and spend a period of weeks visiting local laboratories to carry out servicing and calibration. This enables the cost of international travel and accommodation to be shared between all of the laboratories. SDL-ATAS (www.sdlatlas.com) and James H Heal (www.james-heal.co.uk) provide such a service in many countries with indigenous textile industries.

In addition to issues of maintenance of specification and calibration of equipment with many of the tests being discussed the end result often depends upon an assessment carried out by a technician. For example colour fastness evaluations using grey scales. The issues which must be dealt with to ensure that the laboratory obtains consistent data without operator bias is dealt with under the section of the site on staff and training. The key to the approach is to ensure that staff undergo regular evaluation where all authorised staff for each test evaluate the same samples so that any bias can be detected.

Standardised Laundering and other Cleaning

These types of test are conducted in reference washing machines. There are two basic types. On the left is shown a machine which is effectively a high precision version of a domestic washing machine widely known as a Wascator. On the right is a Launder-ometer (the AATCC standard machine) or Rotawash type machine.

The Wascator actually carries out standardised washing cycles and is particularly suited to tests on complete garments. The Rotawash consist of a series of pots in which samples of cloth are placed and exposed to specific media for a pre-defined time and temperature. The Rotawash is suitable for fastness and dimensional stability tests on samples of fabrics. It can be used for exposure to a wide range of media including detergents, bleaches, brines and dry cleaning solvents.

The Rotawash is designed to obtain basic data on fastness of fabrics but does not seek to reflect the effects of a full washing cycle. For this type of test a Wascator is required. When purchasing equipment of this type laboratories need to be careful to ensure that it is capable of performing the precise test cycles required by their customers and specified in the relevant standards.

Both types of machine are usually supplied with a range of standard cycles already pre-programmed but also have the facility for the user to enter new programmes as necessary. They will all require an annual calibration by an accredited calibration service. The calibration must include the following.

• Verification of temperature sensors as reading accurately.

• Verification of timers.

• Verification of time/temperature programs.

• Verification of rotational speed.

• Verification of dimensions of pots in Rotawash equipment.

A wide range of test standard can be covered by these types of equipment. The some of the most common uses are shown below.

Light Fastness Testing

Most demand for light fastness testing now requires the use of a xenon lamp and is carried out in Xenotest equipment which is a self contained unit able to control temperature and humidity of samples in addition to carrying out the irradiation, see illustration on left. This permits extension of the testing to general weathering in addition to light fastness. Test standards which are relevant include ISO 105 parts B02, B04 and B06, AATCC Method 16 and a range of industry standards, e.g. M&S C9. The equipment can be applied to both textiles and to leather samples.Xenotest equipment will need calibration by an accredited calibration service on an annual basis. The main requirements for users is to monitor the number of hours which the lamp has been operated. Lamps must be changed at the interval recommended by manufacturers otherwise results will be invalid.Calibration must cover timers, temperature, humidity and rotational speed for sample carriers. Note that equipment of this type usually requires venting outside the laboratory so this should be borne in mind when selecting a location.

Composition of mixtures of fibres can be determined qualitatively (identification) or even quantitatively by examination under a microscope. The system pictured is particularly sophisticated and includes digital imaging software to carry out a whole range of measurements automatically. Composition and fibre quality parameters can be determined.Simpler systems require entirely manual measurement against a microscope graticule. Fibre identification and quality requires skilled practitioners who have been well trained but there will always be some variation arising from the inevitable residual subjective element of the method.

As with any light fastness equipment the key control is to ensure the lamp is used only for the number of hours recommended before changing. Most units have a counter to monitor hours of use so this is a simple record keeping exercise.

Each batch of samples should include a sample of humidity control fabric. This will fade to the same extent as the no 5 light fastness standard if the humidity is correctly set. It should also be noted that the blue wool fabric used as a light fastness standard is different for ISO/BS tests and for AATCC Test. Laboratories must ensure that the correct standard to meet the customer’s requirements is used.

Pilling and Abrasion Resistance

There are two common approaches to pilling measurements exemplified by the use of a pilling box, often referred to as ICI pilling and the Martindale pilling tester.

In the former samples of cloth are wrapped around tubes and tumbled in a cork lined box. In the latter a flat piece of fabric is rubbed by a weighted disc faced with a felt pad. The Martindale tester can also be used to carry out abrasion tests on fabrics and on leathers by choice of an appropriate facing for the disc.

Both testers will require annual traceable calibration of rotational speeds and timers. In addition the Martindale uses weights to apply specified pressure to the sample, usually 9 or 12 kpa depending on the standard being tested against. These will have to be checked to ensure they have not changed significantly in mass since installation.

Both systems can be calibrated in-house if the laboratory has a traceably calibrated stop watch and balance.

A major issue which arises at accreditation with the ICI pilling system is control of the properties of the cork linings of the boxes. The linings should be inspected at least monthly for any obvious deterioration. It is also advisable to have a quality control sample of established pilling properties and to run this with each batch of samples. The sample for control purposes can be generated in house. This approach will enable any deterioration in the cork linings sufficient to affect the results to be detected.

Crocking/Rubbing Tests

Annual calibration of crocking equipment will be required. This must cover the following parameters.

These checks can be done in house provided traceably calibrated balances, stop watches and linear measuring equipment are available.

Crocking tests can be applied to textiles and to leather. The relevant standards for fabrics are BS 1006 D02, ISO 105X12/D02 and AATCC 8/165. For leather ISO 11640, ASTM D5053 and IUF 450 are relevant.

Variants on the crocking test include rotary crocking. Typical equipment is shown left. The principle is similar to the to and for crocking but the peg is now rotated. This permits measurements on a specific part of the fabric so enabling the pattern on prints to be specifically tested. Relevant standards are ISO 105X16 and AATCC 116. Calibration is similar to that for to and for crocking. The weight/pressure, rotational speed, dimensions and profile of the peg plus any timers or counters need to be checked for compliance with the relevant test standard.

On the left is shown the Veslic tester for leathers. This is specifically designed to test leathers for tendency to transfer of dyes to clothing if rubbed up against garments. The relevant test standards are ISO 11640 and IUF 450.The principles is the same as the crocking testers already discussed and the same parameters will require annual calibration.

Perspiration Tests

Perspiration tests measure the tendency of textiles and leathers to transfer dyes to a white reference cloth treated with artificial perspiration. The perspirometer allows the reference cloth and the sample to be sandwiched at a known pressure. The whole unit is then placed in an oven for a time and at a temperature which depends on the test standard being applied.There are a number of standards for perspiration testing so the laboratory needs to be careful that they follow the one required by the customer. Difference include the mass of the weight applied, the composition of the artificial perspiration and the time and temperature in the oven. When purchasing a perspirometer particular care should be taken to ensure the relevant weight(s) are obtained. The AATCC tests require an 8 lb weight (3.63 kg) and the ISO tests 9 lb (4.1 kg), for example.

For textiles the standards of relevance are ISO 105, BS 1006, AATCC 15, 106, 107 and 165. For leathers IUF 426, ASTM D-2322 and EN ISO 11641 are relevant.

The key calibration issues with perspiration measurement are the actual dimensions of the perspirometer and the value of the mass applied. These parameters can, in theory, be checked in house with appropriate traceably calibrated equipment. In practice it is unlikely that a laboratory will have available a balance capable of verifying the 3-4 kg masses involved and an external calibration service with ISO 17025 accreditation will usually have to be used.

It is also necessary to check the calibration of the oven used to incubate the samples. Normally this operates at 37°C. It must be a fan type oven capable of maintaining the temperature set to within better than 1°C throughout its entire volume. Temperature calibration must be carried out annually. This must include calibration of any temperature readout and verification of the uniformity of temperature using either a multipoint thermocouple system or separate measurements at front back top and bottom of the oven. If the oven has a built in timer this will also need to be verified against a traceable stopwatch.

Colour Viewing Cabinets

Most colour fastness tests require the assessment of changes in colour or transfer of staining to a reference sample against a grey scale. In order to achieve consistency in results it is essential to have standardised lighting conditions so a colour matching or viewing cabinet will be required. This provides even illumination of a known colour temperature.Most cabinets make available a range of standards covering artificial daylight of various standards, fluorescent light and incandescent light. The intensity of the light

needs also to be varied to allow good appraisal of materials of different reluctances (lightness and darkness). The laboratory needs to ensure that the cabinet purchased can achieve the conditions needed in the standards to which it has to work as no cabinet is totally comprehensive in its coverage. Relevant standards are BS 950, ASTM D1729, DIN 6173 and ISO 3665. All have similar but not identical requirements.

The laboratory should have a traceably calibrated lux meter to enable the intensity and uniformity of the light in the cabinet to be checked. This should be done each month. The meter will need to cover the range 200 to 2500 lux at least. In practice most commercial meters have a much larger range then this. Typically very light materials are viewed at around 550 lux, medium light materials at 1000 to 1500 lux and very dark samples up to 2000 lux. The intensity of light at different points in the cabinet should not vary by more than 200 lux at 1000 lux. Note that this information, although typical, is for guidance only and the laboratory should be careful to study and to comply with the requirements of the test standards to which it is claiming to work.

Suppliers will give guidance on the lifetime over which lamps can be expected to maintain their performance. Lamps should be changed when they reach 80% of their projected life so as to ensure performance is maintained. In order to manage this it is useful to but a light cabinet with a timer fitted which records hours of usage.