Indian Journal of Fibre & Textile Research Vol. 30, December 2005, pp. 3 84-388

ESEM study of tensile behaviour of spunbonded bicomponent fibre nonwovens

Qufu Wei a Heriot-Watt University, Scottish Borders Campus, Galashiels, TD l 3HF, UK

and X Q Wang

�nhui University of Technology & Science, Wuhu 24 1 000, P R China

Received 26 April 2004; revised received and accepted 29 December 2004

The tensile behaviour of polyethylene/polypropylene (PEIPP) bicompomnent fibre spun bonded nonwovens has been studied using an environmental scanning electron microscope (ESEM). A tensi l e stage, mounted in the ESEM, was used to examine the dynamic process of the PE/PP bicompomnent fibre spunbonded nonwovens at different stages of deformation. The visual information obtained through the ESEM provides clear evidence of relevant mechanism of nonwoven deformation. The study shows that the ESEM is a powerful tool for examining the dynamic tensile behaviour of different materials.

Keywords: Biocomponent fibre, Nonwoven, Polyethylene, Polypropylene, Tensile behaviour

IPC Code: Int. CI.7 D06H3/00, GOI N3 3136

1 Introduction Spun bonding is one-step web forming technology,

which involves deposition of spun filaments onto a collecting conveyor i n a uniform random manner followed by bonding the fibres. S ince the fabric production is combined with the fibre production, the process is generally more economical than the processes using staple fibre to make nonwovens 1 . The demand for high performance nonwovens in the wide range of applications has promoted innovations in spunbonding process. B icomponent fibre technology has been successfully applied in the spunbonding process to produce nonwovens with unique properties and better performance2.

Spun bonded bicomponent fibre nonwovens are composed of two polymers. A lower melting polymer acts as a sheath covering over a higher melting core. Bicomponent fibre technology app lied i n spunbonding process has been developed to improve the properties of spurbonded nonwovens for high performance applications. These new applications require the nonwovens to be engineered very carefully and precisely since their failure could have fatal consequences (e.g. civil engineering).

"To whom all the correspondence should be addressed. Present address: School of Textiles & Clothing, Southern Yangtze University, Wuxi 2 1 4 1 22, China. Phone: 2877790; Fax: +86-5 1 0-59 l 3200; E-mail: qfwei @sytu.edu.cn

To better understand bicomponent fibre on its properties, i t i s necessary to gain as much information as possible about the fibre microstructure at all stages of production and hence better structure/property relationships. Our objective i s to develop a technique to examine the damage progression and failure strength of the bicomponent fibre spunbonded nonwovens from the knowledge of constituent fibres and the fibre web structure.

Environmental scanning electron mIcroscope (ESEM) is a new development in the microscopy technology, which is able to image dry, wet, oily and outgassing samples in their natural state due to the presence of gas molecules i n the chamber3. Specimens observed in the ESEM do not need to be coated with a conductive l ayer, thereby ESEM offers the possibility to perform in situ mechanical testing and observations. I n the present study, the installation of a tensile stage into the chamber of an ESEM was used to examine the process of tensile deformation of the bicomponent fibre spunbonded nonwovens at high magnification and in real time.

2 Materials and Methods 2.1 Materials

Popyethylene/polypropylene (PE/PP) bicomponent spundonded nonwovens were used for the study. The lower melting polymer PH can function as a sheath binder covering oyer the higher melting core of PP.

WEI & WANG: TENSILE BEHA VIOUR OF SPUNBONDED BICOMPONENT FIBRE NONWOVENS 385

The materials obtained from production l ine were characterized in the laboratory based on BS EN 29073 standards. The detail s of the materials used are given in Table 1 .

2.2 Equipment

The instrument Philiphs ESEM XL 30, which is specifically designed to be able to examine the detail s of samples in their natural state by means of a differential pumping system and gaseous . secondary electron detector, was used for the study. The differential pumping system enables the electron gun and upper parts to be held at high vacuum of 1 0-6_ 1 0-7 Torr, while the pressure in the specimen chamber can be maintained up to 20 Torr (ref. 4).

The gaseous secondary electron detector amplifies the signal generated by secondary electrons. The electrons collide with the gas molecules in the chamber, resulting in emission of more electrons and ionisation of the gas molecules as they travel through the gaseous environment. The positively charged gas ions are attracted to the negatively biased specimen and offset charging effects. Therefore, an ESEM is able to examine wet, o i ly and outgassing samples without coating5 .

A tensile stage (Ernest F Fulham, 1 00 lb), fitted within the ESEM, was used to test the specimens. The tensile stage was mounted i n the observation chamber. Samples were mounted horizontally and then clamped to a pair of jaws. When the tensile force was applied, a dual threaded leadscrew drove the jaws symmetrically in opposite directions, keeping the sample centred in the field of view. A computer interface and data logging were used to record the results, which were presented i n the form of load extension curves.

2.3 Experimental Procedure

The nonwoven specimens were cut i nto the size of 40mmxl Omm (machine direction -MD) and then mounted on the tensile stage in the ESEM chamber. The microscope was pumped down and flooded with water vapour up to 0.5 Torr pressure, leaving the specimen relatively dry. The beam voltage was kept as low as 20 keV i n order to avoid beam damage to the materials.

Tensile tests were performed at room temperature with an extension rate of 5 .7 mmlmin. The deformation process and fracture surface morphology were video recorded in real time.

Table I-Bicomponent fibre nonwovens

Sample Nonwoven A Non:-voven B

Material Bonding

PP(70% )IPE(30%) Through-air

PP(70% )IPE(30%) Through-air

Mass, g/m2

Thickness. mm Fibre diam. llm

1 00

1 00 0.54 45-55

- A

,--Z '-"d '" 0 .....:J

90 80 70 60 50 40 30 20 1 0

0 0

-- 8

1 0 20

200 1 .20 45-55

30 Elongation (%)

Fig. I -Tensile curves

3 Results and Discussion 3.1 Tensile Behaviour

40

The tensile curves for Nonwoven A and Nonwoven B are shown in Fig. 1 . Significant stress develops at very low elongation. The force i ncreases sharply as the fibre web is stretched. Final fai lure of these specimens occurs at a lower average elongation « 30%). Although the tensile strength of Nonwoven B is much higher than that of Nonwoven A, both have very similar elongation of about 25%. Thi s implies that both the fabrics fol low the similar breaking mechanism. The dynamic deformation process was fol lowed by the ESEM observation.

3.2 Development of Nonwoven Deformation

The development of nonwoven deformation was observed i n the ESEM in real time. As shown in Fig.2(a), the Nonwoven A i s the fibrous web with a random distribution of fibre orientations. It can be observed that at high magnification the fibres are bonded at the fibre i ntersections [Fig.2(b)] . The ESEM images also reveal that the fibres have rough surfaces, which is believed to be caused by the heating and cooling during the manufacturing process.

386 I DIAN J . FI13RE TEXT. RES. , DECEMBER 2005

Fig. 2-Structure of Nonwoven A at ( A ) low (x49) and ( 13 ) high (x4()( ) ) magn i fications

Fig. 3-Te ns i le deformation i n sample Nonwoven A [ (A) at 1 0% elongation, (8) at 1 5% elongat ion, (C) at 20% elongation, and ( 0 ) at 25% e longat ion j . Scale bar 500 �m

As the extension is applied, the fibres in the fabric start moving towards the loading direction, The series of the ESEM images (Fig. 3) show the progress of tensile deformation of Nonwoven A. The randomly distributed fibres in the web are tightened and strengthened as the loading force is applied [Fig,3(a)] .

As the extension is increased, the deformation at the bonding points restrains the movement of bonded fibres, resulting in the fibre bending in the web [ Fig .3 (b) ] , Some weak bonds are broken and the fibres move towards the loading d i rection as the extension i s further increased. The fibres s l ip past over one

WEI & WANG: TENSILE B E H A V I O U R OF SPUN BON DED L3 1COM I'O N ENT FI f3 R E NONWOVENS ]H7

another from the bonded posi t ions and move towards

the load ing d i rection IFig.3(c ) ] . The web col l apses

with the further i ncrease in extens ion . Web fai l u re

i n i t iates by fi bre breakage at the weakest bondi ng

poi n t and rapid ly expands across the web, res u l t i n g i n

the fast movement of fi bres a long the load i ng

d i rect ioll I Fig . 3 (d ) ] . I t can be c learly seen that the

fibre orientat ions have becll s ign i ficant ly changed .

The ES EM i mages ( Fig . 4) reveal t he debonded

surfaces at fi bre i ll lcrscct ions . Bonded l aycrs arc

Fig. 4-Morphology of the debonded surfaces I (A) peeled part of

PE sheath. and ( B ) core of polypropy lene J Fig. 5-Tensi le deformation in sample Nonwoven B l (A) at 0% e longation, ( B ) at 1 5% e longation, and (e) at "25 % elongati()11 I

388 INDIAN J. FIBRE TEXT. RES., DECEMBER 2005

either peeled off from the other fibres at the bonding points [Fig.4(a)) or taken off by the other fibres [Fig.4(b)] . It can be seen that the separation of the sheath-bonding layer is the major mechanism of web deformation. Fig.4(b) also reveals ·the smooth surface of the PP core, indicating that there is no interaction between the sheath and the core. It is believed that the strength would be improved if the bonding exists between the sheath and the core.

The ESEM observation also indicates that the Nonwoven B has the similar behaviour under loading force (Fig. S) . Fig. Sea) reveals that the Nonwoven B has the dense fibre distribution due to the higher mass per square meter. The random distribution of fibres in the web with bonding at fibre intersections and the rough surfaces can also be seen i n the image. The deformation shown in Fig.S (b) clearly shows the behaviour of fibres i n the web under force. The collapsed web [Fig.S(c)] shows the separation of bonding layers and the smooth PP core fibres.

4 Conclusions This study has explored the use of the

environmental scanning electron microscope (ESEM) for the dynamic examination and observation of the tensile behaviour of the bicomponent fibre nonwovens. The abi lity of the ESEM to fol low dynamic events under a variety of conditions gives new insight i nto the kinetics of structure formation, interfaces and structure rearrangement that are important for the processing and product development in textile research and engineering.

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