Nonwoven filtration media has existed for hundreds of years. Filtration market for nonwoven fabrics is especially broad and continue to grow.

Nonwoven is a term used to describe a type of material made from textile fibres which are not produced on conventional looms or knitting machines.
Nonwoven fabrics are products made of parallel laid, cross laid or random laid fibre webs bonded mechanically, chemically and thermally. For thermal bonding synthetic fibre and bi-component fibres are used.
The fibre web may be produced [Ref 1] in any of the following ways:

1. mechanically
2. aerodynamically
3. hydrodynamically
4. in an electrostatic field

Fig 1: Nonwoven filter

Filter can be divided into two groups: liquid phase and gaseous phase filter.

Liquid phase filter:
Fibre selection for liquid phase filter depends on what art chemicals need to filter and its temperature. For this type of filter mechanical or thermal or chemical bonding mechanisms are used. Filter can be made either from monofilament or from multifilament. The advantage of monofilament is filter pores are very fine to pass the big size particles.
Upper layer of this filter can be made from filament and lower level finer fibre can be used for better filtration [Fig 1].
The disadvantage of using adhesive bonded filter [Fig 2] is some portion of upper level is blocked with adhesive.
Liquid phase filters are used for chemical or water filtration.

Fig 2: Adhesive bonded nonwoven filter

Gaseous phase filter:
For this filter fibre webs are bonded by thermally. Gaseous phase filters are used for air filtration [Fig 3].

Fig 3: Air filter

Nonwoven filters should posses the following features:

Fibre selection for nonwoven filter:
Viscose (CV), polyester (PES), polypropylene (PP), polyacryle (PAN), m-Aramid (MPIA), polyimide (PI), polyphenylensulfid (PPS), polytetraflourethylen (PTFE), polycarbonate (PC), Glass (GL), metal (MTF), cotton (CO) and wool (WO) are used.

Table 1: Fibres and condition of filtration

++ very good, + good, 0 conditional,- bad

Percentage of various processes used for nonwoven filter production:

Very fine filtation:
The orientation of layer of fibre [Fig 4] in this case could be
Filament spun (0.4-12dtex) - fibre (0.8-44dtex) - filament spun - fine fibre spun (0.1-0.5dtex) - filament spun.

Fig 4: Filter for very fine filtration

Surface filtration:
Filament spun (0.4-12dtex) or fibre (0.8-44dtex) – weave (made of P84, glass fibre, nomex) of thickness 0.25-0.3mm  –  filament spun or fibre [Fig 5].

Fig 5: Filter for surface filtration

Nano fiber nonwoven  filter:
The fibers which have diameter below 1µm are used for nano fibre nonwoven filter.
Because of very fine fibre and fine pores fine particle can be filtered. This filter can be produced by thermal bonding.

Fibre surface modified nonwoven filter:
The fibre surface of filter can be modified with modified particles [Fig 6]. The surface modification makes the pores small between the fibres of filter.

Fig 6: Fibre surface modified nonwoven filter

Knitted or woven fabrics can also be used for filter production. Nonwovens are also used as medical textile (i.e. plasters, gauze dressing, bandages, absorbent pad, artificial skin, surgical gowns, and surgical masks) and in car (i.e. carpeting, upholstery backing, seat).  

References

1. Prof. Dipl-Ing. Dr. Radko krcma, Manual of Nonwovens, 1971.
2. Walter Fung and Mike Hardcastle, Textiles in automotive engineering, 2001.
3. A.R. Horrocks and S.C Anand, Hand book of technical textiles, 2000.
4. Robert Czajka, Development of medical textile market.
5. Joanna Grzybowska-Pietras  Jadwiga Malkiewicz, Influence of technologic parameters on filtration characteristics of nonwoven

Introduction

With an export value of over 79b USD in 2005-06, Readymade Garment (RMG) industry in Bangladesh is unique. This value is not only accounted for two third of total export but also responsible for direct jobs of 1.8 million people of whom 90% are women (Toai 2004) and indirect jobs of one million (Sattar et. al. 2006). In this massively important sector, one of the most burning issue for all the stakeholders, if not the most,  is compliance or code of conduct, which has been used synonymously.
So what is compliance? The exact definition could be somewhat vague but in general it could be said as ‘Conforming to a specification, standard or law that has been clearly defined.’ On the other hand International Labour Organization (ILO) defines corporate code of conduct as ‘Companies' policy statements that define ethical standards for their conduct.’ A code of conduct is a policy that defines the ethical standards to which an enterprise should comply. They vary greatly.  Thus codes of conduct for offshore suppliers bear little resemblance to the company codes that apply to employees in the home country.  Usually for a offshore supplier they are usually one or two pages long and outline very briefly to what standards a company should comply in the areas of non-discrimination, forced labor, harassment and abuse, health and safety, child labor, wages and benefits, hours of work, free right of association, and legally mandated benefits.
The ILO has outlined three various types:
• Compliance codes: directive statements giving guidance and prohibiting certain kinds of conduct
• Corporate credos: broad general statements of corporate commitments to constituencies, values and objectives
• Management philosophy statements: formal enunciations of the company or CEO’s way of doing business
For Bangladeshi apparel manufacturers compliance codes are of main importance. This sensitive issue that has been triggered so many events and discussions in last decades or so could be seen from two perspectives: international and home context. In following discussion we will find that how those two perspectives have channeled the growth of apparel industry of Bangladesh.

Compliance Issue: International Perspective

Increasing and widespread pressure, including the threat of boycotts and the potential risk to brand names, and perhaps the influence of an increasing awareness generally of corporate social governance, led to a sea change in behavior particularly in the 1990s. These changes in the corporate buyers have influenced the manufacturers in Bangladesh too. Thus the main compliance issues that faced by apparel industries in their more than two decades of journey were child labor and fire hazard.

Child Labor

In 1993, an American television newsmagazine "NBC Dateline" aired a documentary of young Bangladeshi children making apparels sold at Wal-Mart stores. This created public outcry and put pressure on Wal-Mart to cancel its contracts with Bangladeshi manufacturers. Seeing negative impression on Wal-Mart, other companies also informed their Bangladeshi partners about the use of child labor, which was bad for business.  There was also a proposed legislation US congress to restrict import of items made with child labor, potentially closing the apparel export to American market, which was largest in terms of value and quantity. 

Polyester prices remained unmoved in the last seven days in China. Fiber processors are waiting for lower rates after crude oil futures sharply decreased in New York. Fiber makers however reduced their production in order to maintain prices are current levels. Near-term price trends look more than ever unpredictable.

Polyester fiber prices remained mostly unchanged in their last seven days in China.
Demand stayed at a low level with fiber processors expecting some reduction in polyester prices after crude oil futures dramatically fell in New York.

Average price for staple fibers (1.4D*38mm) remained at 11,800-12,000 yuan per metric ton (ex-works) or about US$1.72-1.75 per kilo.

Although polyester intermediate prices are now declining on spot markets, polyester fiber prices may stay at the current level in the near future.

Producers of chips and fibers are currently generating operative losses and could resist any new fall in prices until contract prices of polyester intermediates will effectively drop.

Operating rates were already cut to 60-70% at fiber plants amid rising inventories. The current pressure from customers and lower raw material costs ahead may also force fiber makers in reducing their prices. Filament prices were already cut by 100 yuan in Qian Qing (Zhejiang), as reflected in our tables below. Price trends will probably depend on the level in crude oil prices in the near term, which is highly unpredictable.