36 Testing for knitted fabrics

S. Natarajan

1. Introduction

In order to assess the quality, the knitted fabrics are also subjected to physical testing such as thickness, pilling, abrasion resistance, bursting Strength, Dimensional Stability. However, the types of test carried out for knitted fabrics differ to some extent from the tests conventionally done for woven fabrics as the structures and end uses of knitted fabrics are different to those of woven fabrics. The tests are mainly done keeping in view

• To evaluate the quality aspects of the knitted fabrics
• To assess whether the quality of the fabric made is equivalent to the expected/predicted quality
• To assess whether the quality of the knitted fabric matches the required quality of the end product
• To assess the performance of the fabric during the end use
• To evaluate the comfort properties, feel and handle of the fabric/garment

2. Fabric Thickness Tester

The thickness of fabric is measured with a touching system, by lowering a plate of defined weight and size onto the fabric surface and detecting the height in which the plate stops. The thickness of fabric is normally measured according to EN ISO 5084 and ASTM D1777 standards. The principle of measurement of fabric thickness is based on “The precise measurement of the distance between two plane parallel plates separated by the cloth when a known pressure is applied and maintained on the plates.”

Determination of thickness of fabric samples in laboratory is usually carried out with the help of a precision thickness gauge. In this equipment, the fabric whose thickness is to be determined is kept on a flat anvil and a circular pressure foot is pressed on to it from the top under a standard fixed load. The Dial Indicator directly gives the Thickness in mm.

2.1 Specification of Fabric Thickness Tester:

• Range of measurement : 0 – 10 mm
• Least count of dial gauge : 0.01 mm
• Diameter of anvil : 50 mm
• Diameter of pressure foot (Interchangeable) : 10 mm & 25 mm
• Load on pressure foot : 10 g/sq. cm
• Throat depth : 22 mm

3. Pilling

Pilling is a fabric surface characterized by little pills of entangled fiber clinging to the cloth surface that considerably spoils the original appearance of a fabric. The pills are formed by rubbing action on loose fibres that are present on the fabric surface and also during washing.

Pill generation begins with a migration of fibres to the external surface of yarn, so that fluffiness emerges on the surface. Due to friction, this fluffiness gets entangled and forms somewhat spherical mass called pill. It remains suspended from the surface by long fibres called anchor fibres. Because of wear some pills fall off, causing the additional effect of loss of material.

• A garment is considered to be serviceable when it is fit for its particular end use.
• Stronger component in the blend aggravated its seriousness
• Higher breaking strength and lower bending stiffness of fibres present in the yarn results more pill formation.
• Low twist factor, higher hairiness, and loose fabric structure results easy and large pills (knitwear)

Pilling in the fabric can also be generated by artificial means, which can simulate actual wear over the long period. Pilling can be created by subjecting fabric samples to artificial abrasion in different apparatus and machines like pilling box machine shown in figure.

It is possible to assess the amount of pilling quantitatively either by counting the number of pills or by removing and weighing them. However, pills observed in worn garments vary in size and appearance as well as in number. The pilling resistance of the samples is assessed according to ASTM D4970 standard.

Counting the pills and/or weighing them as a measure of pilling is very time consuming and there is also the difficulty of deciding which surface disturbances constitute pills.

The more usual way of evaluation is to assess the pilling subjectively by comparing it with either standard samples or with photographs of them or by the use of a written scale of severity.

Most of pilling assessment scales are divided into five grades and run from grade 5, no pilling, to grade 1, very severe pilling

For this test, four specimens each 125mm X 125mm are cut from the fabric. A seam allowance of 12mm is marked on the back of each square. In two of the samples the seam is marked parallel to the wale direction and in the other two parallel to the course direction.

The samples are then folded face to face and a seam is sewn on the marked line. Each specimen is turned inside out and 6mm cut off each end of it thus removing any sewing distortion.

The fabric tubes made are then mounted on rubber tubes so that the length of tube showing at each end is the same. Each of the loose ends is taped with poly vinyl chloride (PVC) tape so that 6mm of the rubber tube is left exposed.

All four specimens are then placed in one pilling box. The samples are tumbled together in a cork-lined box. The usual number of revolutions used in the test is 18,000 which takes 5 hours. Some specifications require the test to be run for a different number of revolutions.

3.1 Assessment:

The specimens are removed from the tubes and viewed using oblique lighting in order to throw the pills into relief. The samples are then given a rating of between 1 and 5 with the help of the descriptions given in the table

4. Abrasion Resistance

Abrasion resistance is the ability of a fabric to resist surface wear caused by flat rubbing contact with another material. Abrasion may cause a loss of appearance by disturbing the surface of a fabric.

4.1 Factors affecting abrasion test

1. Type of Abrasion

This may be plane, flex or edge abrasion or a combination of more than one of these factors.

1. Type of Abradant

A number of different abradants have been used in abrasion tests including standard fabrics, steel plates and abrasive paper or stones (aluminum oxide or silicon carbide). The  severity as well as the type of action is different in each case. An important concern is that the action of the abradant should be constant throughout the test.

  C. Pressure

The pressure between the abradant and the sample affects the severity and rate at which abrasion occurs.

D. Speed

Increasing the speed of rubbing above that found in everyday use also brings the dangers of accelerated testing as described above.

E. Tension

It is important that the tension of the mounted specimen is reproducible as this determines the degree of mobility of the sample under the applied abradant. This includes the compressibility of any backing foam or inflated diaphragm.

F. Direction of abrasion

In many fabrics the, abrasion resistance in the length wise direction differs from that of the width wise direction. Ideally the rubbing motion used by an abrasion machine should be such as to eliminate directional effects.

4.2 Method of assessment

Two approaches have been used to assess the effects of abrasion:

A.    Abrade the sample until a predetermined end-point such as a hole, and record the time or number of cycles to this end-point.

B.     Abrade for a set time or number of cycles and assess some aspect of the abraded fabric such as change in appearance, loss of mass, loss of strength change in thickness or other relevant property.

The first approach corresponds to most people’s idea of the endpoint of abrasion but the length of the test is indeterminate and requires the sample to be regularly examined for failure in the absence of a suitable automatic mechanism. This need for examination is time consuming as the test may last for a long time. The second approach promises a more precise measurement but even when the sample has rubbed into a hole the change in properties such as mass loss can be slight.

4.3 Martindale Abrasion Tester

This apparatus is designed to give a controlled amount of abrasion between fabric surfaces at comparatively low pressures in continuously changing directions. The results of this test should not be used indiscriminately, particularly not for comparing fabrics of widely different fibre composition or construction

In this test, circular specimens are abraded under known pressure on an apparatus which gives a motion that is the resultant of two simple harmonic motions at right angles to one another. The fabric

Under test is abraded against a standard fabric. Resistance to abrasion is estimated by visual appearance or by loss in mass of the specimen.

4.4 Method of sample mounting on sample holder:

Four specimens each 38mm in diameter are cut using the appropriate cutter. They are then mounted in the modified specimen holders which stretch the knitted material thus effectively accelerating the test. A flattened rubber ball is pushed through the sample as the holder is tightened thus stretching it.

The test specimen holders are mounted on the machine with the fabric under test next to the abradant. A spindle is inserted through the top plate and the correct weight (usually of a size to give a pressure of 12kPa but a lower pressure of 9kPa may be used if specified) is placed on top of this.

The specimen is examined at suitable intervals without removing it from its holder to see whether two a hole appears or the material develops an unacceptable level of thinning.

5. Fabric Bursting Strength

Bursting strength is a method of measuring strength in which the fabric is stressed in all the directions at the same time and is therefore more suitable for materials such as knitted fabrics. During a test a fabric fails across the direction which has the lowest breaking extension. Tensile test is unidirectional and thus suitable for woven fabrics where definite warp and weft direction strength is measured. In case of knitted fabric, where no definite alignment of yarns/fibres is there, multidirectional force is required. These types of fabrics more likely to fail by bursting in service than it are to break by a straight tensile fracture.

Fabrics used in parachute, filters, sacks and nets are simultaneously stressed in all the directions during service. In service, a fabric is more likely to fail by bursting than by a straight tensile fracture;

Examples for real life application of bursting strength is stress present at elbows and knees of clothing.

5.1 Diaphragm of Bursting Test

ASTM D3786 standard describes a test in which the fabric to be tested is clamped over a rubber diaphragm by means of an annular clamping ring and an increasing fluid pressure is applied to the underside of the diaphragm until the specimen bursts. The operating fluid may be a liquid or a gas.

Two sizes of specimen are in use, the area of the specimen under stress being either 30mm diameter or 113mm in diameter. The specimens with the larger diameter fail at lower pressures (approximately one-fifth of the 30mm diameter value). However, there is no direct comparison of the results obtained from the different sizes. The standard requires ten specimens to be tested.

In the test the fabric sample is clamped over the rubber diaphragm and the pressure in the fluid increased at such a rate that the specimen bursts within 20±3s. The extension of the diaphragm is recorded and another test is carried out without a specimen present. The pressure to do this is noted and then deducted from the earlier reading. The following measurements are reported.

•  Mean bursting strength – kN/m2
•  Mean bursting distension -mm

6. Dimensional Stability

Knitted fabric is so infamous for its tendency to change size and shape in wear and washing. This most challenging demerit of knitted structures is because of their lower dimensional stability, owing to the instability in their loop dimensions. There are several factors which may be responsible for the instability of loop dimensions and consequently the poor dimensional stability of knitted fabrics. Fabric shrinkage may result due to fiber swelling under moist or wet conditions and relaxation of internal stresses which the fibers may undergo during different manufacturing phases. Yarns having higher spinning tensions result in knitted fabrics with inferior dimensional stability. Knitting tensions and fabric wet processing parameters also affect the dimensional stability of knitted structures.

6.1 Measurement of Dimensional Stability

It is recommended that samples are preconditioned at a temperature not greater than 50oC with a relative humidity of between 10% and 25%. Then fabrics samples are conditioned in a standard atmosphere of 20 ± 2o C and 65 ± 2% relative humidity for 24 hours. The samples are marked with three sets of marks in each direction, a minimum of 50 cm apart and at least 10 cm from all edges as shown in Figure. After measurement the samples are subjected to washing according to AATCC TM-135 standard and the procedure for conditioning another 24 hours to bring them in to the same state they were in when they were marked. They are then re-measured on a flat smooth surface and the percentage dimensional change calculated. The mean dimensional Change and direction is reported.

Shrinkage/Expansion %          = Initial measurement – final measurement    X 100

Initial measurement

The change in the fabric dimensions is expressed as a percentage of the initial length and width to change in fabric dimensions occurs in specified conditions. When dimensional change results in increase of the specimen dimensions then it is termed as expansion. When dimensional change results in decrease of the specimen dimensions then it is termed as shrinkage.

Spirality

It is necessary that the wale on the knitted fabric be perpendicular to the course. However, the wales are not always perpendicular to the course and skew to the right or left, forming a spirality angle as seen in Figure

Spirality is a major problem of knit fabrics which is produced in circular knitting machines. Relaxation of torsional stresses cause dimensional distortions and instability in the knitted loop constructions. The effect of machine gauge, yarn and fabric properties on the spirality of single jersey knit fabrics has been analyzed by some researchers. There have been few researches regarding the effect of knit structures on the spirality of the fabric.

Spirality is determined by placing a protractor on the smooth fabric surface with its base-line along the course and reading the angle between the wale line and a line 90°perpendicular to the course line

7. Air permeability

Air permeability is defined as the volume of air in liters which is passed through 100 cm2 (10 cm x 10 cm) of the fabric in one minute at a pressure difference of 10 mm head of water Air permeability is often used in evaluating and  comparing  the ‘breathability’ of various fabrics (coated and uncoated) for such end uses as raincoats, tents and uniform shirting Air permeability is an important factor in the comfort of a fabric as it plays a role in transporting moisture vapour from the skin to the outside atmosphere.

The air permeability of fabric depends on the shape and value of the pores and the inter thread channels, which are dependent on the structural parameters of the fabric

Since knitted fabrics have a loop structure, they have more pores than woven fabrics; therefore, in general, the air permeability of knitted fabrics is higher than that of woven fabrics of the same weight.

A circle of fabric is clamped into the tester and through the use of a vacuum, To obtain accurate results in the test, edge leakage around the specimen has to be prevented by using a guard ring or similar device. The pressure drop across the guard ring is measured by a separate pressure gauge. the air pressure is made different on one side of the fabric. Airflow will occur from the side with higher air pressure, through the fabric, to the side with the lower air pressure. From this rate of air flow, the air permeability of the fabric is determined.

8. Conclusion

Knitted structures offer several advantages over woven fabrics including better stretch and recovery, resilience, porosity, air permeability, softness, and warmth. Due to better extensibility in structure, knitted fabrics provide better fit and comfort to the wearer.

Consumer interest in apparel items made from knitted fabrics has increased at a rapid pace during the past few decades.

In apparel design and garment manufacturing, fabric characteristics are usually dictated by a specified end-use. Understanding the relationship between the fabric end-use and fabric properties becomes fundamental for classification, selection, search, and purchase control of apparel fabrics. Appearance properties are very important in all classes of fabrics. Appearance retention is directly related to the longevity and serviceability of fabrics. A fabric may lose its aesthetic appeal due to wear, which is a combined effect of several factors like abrasion, repeated laundering, the application of forces in dry and wet states.

REFERENCES and URLs

1. Saville B.P. “Physical Testing of Textiles”, Woodhead publishing -UK, 1999.
2. Booth J. E. “Principles of Textile Testing” Butterworths, 1996
3. Sadhan C. Ray, “Fundamentals and Advances in Knitting Technology”, WPI Publishing, March, 2012
4. http://nptel.ac.in/courses/116102029/1