tổng quan về vài không dệt công nghệ Thermal Bonded

6 See discussions, stats, and author profiles for this publication at https www researchgate netpublication326669036 THERMAL BONDED NONWOVEN AN OVERVIEW Article July 2018 CITATIONS 4 READS 15,248 5 authors, including Some of the authors of this publication are also working on these related projects my publication View project Tanveer Malik Shri Vaishanav Institute of Technology Science 25 PUBLICATIONS 22 CITATIONS SEE PROFILE All content following this page was uploaded by Tanveer Malik on.

THERMAL BONDED NONWOVEN-AN OVERVIEW

Article · July 2018

CITATIONS

4

READS 15,248

5 authors, including:

Some of the authors of this publication are also working on these related projects:

my publication View project

Tanveer Malik

Shri Vaishanav Institute of Technology & Science

25PUBLICATIONS    22CITATIONS    

SEE PROFILE

1/25/2011

THERMAL BONDED NONWOVEN – AN OVERVIEW

By:

1 Mr P.K.Roy

2 Mr.Tanveer Malik and

3 Mr.T.K.Sinha

2011

1 INTRODUCTION

There are three major bonding types, chemical bonding / thermal bonding/ mechanical bonding (needle punching) The development of the past few years has shown that the share of thermally bonded webs is growing steadily Thermal bonding is the most popular method of bonding used in nonwovens manufacture It offers high production rates because bonding is accomplished at high speed with heated calendar rolls or ovens Thermal bonding process has been used successfully with a number of thermoplastic fibers Among the various types of thermal bonding, point bonding is the most widely used technique

The first thermally bonded nonwovens were produced in 1940s The viability of the thermal bonding process is rooted in the price advantage obtained by lower energy costs However, the thermal bonding process also addresses the demanding quality requirements of the market place The development of new raw materials, better web formation technologies and higher production speeds have made thermal bonding a viable process for the manufacture of both durable and disposable nonwovens

Thermal bonding is successfully employed in bonding dry-laid, polymer laid and wet-laid webs as well as multi-layer materials The basic concept of thermal bonding was introduced by Reed He described a process in which a web consisting of thermoplastic and non-thermoplastic fibres was made and then heated to the melting or softening temperature of the constituent thermoplastic fibres followed by cooling to solidify the bonding area In the early development of thermal bonding, rayon fibres (the base fibre component) were blended with plasticized cellulose acetate or vinyl chloride (the binder fibre component) Typically; a carded web from a blend of base fibre and binder fibre was produced and hot calendared followed by cooling to solidify and bond the web structure The resulting thin, strong and relatively dense product was more akin to a paper product than a textile material Production costs for this material were very high, primarily because the available binder fibres were expensive Its applications were limited to products requiring a smooth surface, low porosity, high strength and lower thickness Given the product limitations and the high cost of such binder fibres, nonwoven producers continued to prefer latex bonding using chemical binders

The rising cost of energy and greater awareness of the environmental impact of latex bonding led to a change in direction A comparison of energy consumption by various web-bonding processes is given in Fig 1 which shows a considerable energy saving for the thermal bonding process The high production rates possible with thermal bonding and the significant energy savings as compared to chemical bonding, due to the absence of significant water evaporation during bonding, makes the process economically attractive In contrast to chemical bonding, the environmental impact of the process is also significantly reduced The growing market demand for disposable and durable products spurred developments in new thermoplastic and thermoset materials in the form of powder, films, webs, hot melt compounds

as well as improved production methods such as point-bonding calendars, through-air bonding and belt bonders This has greatly increased the diversity of products that can be manufactured

by the thermal-bonding process

2 PRINCIPLE OF THERMAL BONDING

Thermal bonding requires a thermoplastic component to be present in the form of a homofil fibre, powder, film, web, hot melt ores a sheath as part of a bicomponent fibre In practice, heat is applied until the thermoplastic component becomes viscous or melts The polymer flows by surface tension and capillary action to fibre-to fibre crossover points where bonding regions are formed These bonding regions are fixed by subsequent cooling In this case,

no chemical reaction takes place between the binder and the base fibre at the bonding sites When binders melt and flow into and around fibre crossover points, and into the surface crevices

of fibres in the vicinity, an adhesive or mechanical bond is formed by subsequent cooling Such

an adhesive bond is a physio-chemical bond at the interface of two dissimilar materials In the thermal bonding context, a mechanical bond is formed as a result of thermal shrinkage of the bonding material, which while in the liquid state encapsulates the fibre crossover points, In contrast, if it the binder fibre interface both components soften or melt, inter-diffusion and interpenetration of the molecules across the interface can occur and the interface may disappear This arises where compatible polymers are present with nearly comparable solubility parameters Bonds formed in this way may be called cohesive bonds

3 BONDING PROCESS

Thermal bonding can be performed in several ways In through-air bonding, a hot fluid, air, is forced through a preformed web If the temperature of the fluid is high enough, the fibers may partially melt In this case, they form bonds where two or more fibers come into contact In infrared bonding (IR-bonding), infrared light provides the heat required to partially melt the fibers In ultrasonic bonding, friction between contacting fibers due to the application of ultrasound causes partial melting of the fibers In thermal point bonding, the preformed fiber web

is passed between heated calendar rolls The rolls may be smooth or embossed with a bonding pattern A uniform fabric requires uniform pressure, uniform temperature, and uniform input web Bonding occurs only where the fibers contact the heated rolls Therefore, on a smooth calendar roll, bonding occurs wherever fibers cross each other, while on an embossed calendar roll, bonding occurs primarily between the raised areas This results in bonding points or spots

In each of these processes, the underlying physics is the same, the fibers are heated, they form a bond, and they are subsequently cooled To keep the discussion tractable, the remainder of this discussion will be restricted to the formation of bond-points or spots via thermal point bonding, and the effect of the formation process on the resulting fabric properties

4 RAW MATERIALS

Thermally bonded fabrics are produced both from entirely thermoplastic materials and from blends containing fibres that are not intended to soften or flow on heating The non-binder component may be referred to as the base fibre component and commercially, a variety

of base fibre types are used The binder fibre component normally ranges from 5-50% on weight of' fibre depending on the physical property requirements of' the final product

4.1 Base Fibres Types

The base fibre contributes to key physical, chemical and mechanical properties of the fabric derived from the polymer from which it is constituted This influences dyeing characteristics, flame resistance, tensile and attritional properties, hydrolytic resistance, and biodegradability amongst many other properties The commonly used base fibres include natural fibres (regenerated cellulosic fibres, bast, vegetable and protein fibres such as wool), synthetic fibres (polyester, polypropylene, acrylic, nylon, aramid and many others), mineral fibres (e.g., glass and silica) and metallic fibres Sometimes the base fibre (carrier fibre) is the core of a bicomponent fibre, with the sheath component being the binder portion

4.2 Binder Materials

Binder components are produced in many different forms including fibre or filament (homogeneous or bicomponent sheath/core or side-by-side type melt-bonding fibres), powder, film, low melt webs, and hot melts The physical form of the binder affects its distribution throughout tile fibre matrix which has significant impact oil fabric properties The amount of binder also plays an important role in determining the properties of the resultant nonwoven fabric If the binder content is more than 50% of the total blend the fabric behaves like a reinforced plastic At a binder content of 10 % the fabric is a bulky, porous and flexible structure with relatively low strength To minimise energy costs it is desirable that binder fibres have a high melting speed, a low melting shrinkage and a narrow melting point range The most widely used thermoplastic binder polymers are given in Table 1

Table 1: Thermal transition point in common thermoplastic binder material

temperature CQ

Melting temperature (IC)

Decreasing the melt temperature of polymers, for instance PET, from 260 °C to 135-190

°C, requires the use of copolymers produced by polycondensation The melting speed of these copolymers is very high; hence the thermal shrinkage is reasonably low When thermoplastic fibres or powder, are used as binders, their melting temperature is significantly lower than the base fibres in the web, which helps to prevent thermal degradation In low melting temperature homopolymers, or copolymer binder fibres or powders, complete melting can occur and the polymer becomes a fluid If the viscosity of the molten polymer is sufficiently low, it flows along the surface of the base fibres and is collected at the fibre crossover points to form bonding points

in the shape of beads by subsequent cooling In webs composed of bicomponent fibres (of the sheath/core type) the sheath polymer does not need to completely melt but softens enough to form a bond However, if it does melt and flow, the bonding mechanism becomes similar to that

of homopolymer binder fibres The advantage of bicomponent fibres is that every crossover can

be potentially bonded and also since the physical structure of the core component is not

degraded, thermal shrinkage is minimised, web structure remains essentially intact and fabric strength is usually higher Binder fibres are selected by their suitability for the different thermal bonding processes

Many materials that are used as a binder for thermally bonded nonwovens can be apply

by following methods in production

• Binding fibers

• Binding powder

• Binding web

4.2.1 Binding Fibers

Single-component and bi-component fibers, as binder fibers, are most widely used in thermal bonding of nonwovens Single-component fibers are the least sophisticated and most economical because the fibers are often already in existence and low in cost The type bond that

is formed is dependent on several factors including fiber chemistry, morphology, linear density, staple length, crimp, and processing conditions The major disadvantage encountered when using

100 percent single-component fibers is the narrow temperature range that is necessary when thermal bonding If the temperature is too low, there is inadequate bond strength If the temperature is too high, the web will melt excessively and lose its identity as a web

When bi-component fibers are used to produce thermal bonded nonwoven, the acceptable temperature range for bonding may be as great as 25°C When thermal bonding, the high melting portion of the fiber maintains the integrity of the web, while the low melting point portion melts and will bond with other fibers at the fiber crossover points The product produced tends to have bulk and exceptional softness

4.2.2 Binding Powder

Powdered polymers are sometimes used in thermal bonding of nonwovens The most prevalent use is powdered polyethylene Powdered polymeric binders can be applied during web or batt formation or following web formation and pre-bonding A thermoplastic polymer with a low softening temperature is desirable that requires a short exposure to heat to melt and fuse the powder For ease of operation the thermoplastic powder should have a low melt viscosity and the transition from melt to solid should occur over the shortest possible temperature range Polymers such as polyethylene, low molecular weight polyamide and copolymers of vinyl chloride and vinyl acetate, are generally used This method of thermal bonding is limited by difficulties in obtaining polymers with a suitable range of particle sizes

to suit the base web Obtaining a uniform powder distribution throughout the web is also problematic Powder bonding is suited to lightweight webs where an open structure is required with a soft handle or in the production of reinforced, moulded products Applications include feminine hygiene, adult incontinence, medical and automotive products, wipes, computer disks, apparel and shoe composites

4.2.3 Binding Web

A very open-structured, low-melting-point thermoplastic fabric is placed between the webs and, during thermal bonding between the calendar rolls, the fabric melts completely bonding the webs together The nonwoven produced by this technique is soft and bulky

Thermoplastic coatings and hot melt print bonding have been used to a limited extent in controlled porosity filters, impermeable membranes and other items However, the use of this method of bonding is not expected to achieve a high level of importance

5 METHODS OF THERMAL BONDING

• Hot calendaring

• Belt calendaring

• Through-air thermal bonding

• Ultrasonic bonding

• Radiant-heat bonding, etc

5.1 Hot Calendaring

Thermal bonding relies on the use of heat energy to melt or soften one or more components of a web to achieve bonding There are different methods of applying heat energy

to the web and the heal transfer mechanism can take different forms; conduction, convection and heat radiation The widely used methods are discussed in this section Thermal calendar bonding is a process in which a fibrous web containing thermoplastic components (fibres, powders or webs) is passed continuously through a heated calendar nip that is created by two rolls pressed against each other Multi-nip calendars are also employed depending on the web weight and degree of bonding required Both rolls are internally heated to a temperature that usually exceeds the melting point of the binder components in the web to ensure there is sufficient heat transfer to induce softening at the prevailing line speed As the web passes between the at lender nip, fibres are both heated and compressed This causes the binder components of the web to become soft and tacky and induces polymer flow in and around the base fibres The fluid polymer tends to collect at fibre crossover or contact points and bonding sites are formed Cooling leads to solidification of the polymer and bonding

Calendar bonding is mainly applicable to light and medium-weight webs because the fibres in a thick web insulate heat from the interior of the structure leading to a temperature gradient and variation in the degree of bonding through the cross-section To increase the efficiency of the process, the web may be pre-heated immediately prior to calendar bonding sometimes by infra-red heaters Commercially, light-weight webs of 25-30 g/m2 for medical and hygiene applications and medium-weight webs of l00 g/m2 for interlining and filtration applications are thermally bonded using calendar bonding The degree of bonding depends on temperature, pressure and speed, which determines the contact or dwell time

There are three main types of hot calendaring

• Area bonding

• Point bonding

• Embossing

The properties of the fabric are influenced by the total bond area, which is normally expressed in percentage terms In practice, area bonding (100%) or point bonding (embossing)

is possible (<100%)

5.1.1 Area Bonding

This process involves the use of a calendar with a hot metal roll opposed by a wool felt, cotton or special composition roll Two, three or four roll calendars can be used, depending on the weight of the web to be bonded and the degree of bonding desired The three-roll calendar has the heated roll in the middle while the four-roll configuration has the heated rolls on the top and bottom, with the two composition roll in the middle The amorphous or co-polymeric binder fibers used in this process provide bonding at all cross-over points between the carrier and binder fibers The resultant product - commonly used in electrical insulation and coating substrates - is smooth, thin and stiff The material is always two sided, but this effect is most apparent in material processed through two and three roll calendars Four roll calendars minimize this effect

The application of heat from the outside produces a material whose inner area is less bonded than its outer surface This becomes more pronounced as the product weight increases beyond 35 g/m2 and can become detrimental unless corrective measures are taken These include increasing heat, slowing speed, or increasing the binder/carrier fiber ratio The two-roll calendar is used for low-to-medium weight products with light-to- medium bonding The three-roll calendar is used for special bonding and finish effects on a single surface The four roll calendar produces the widest weight range of materials because it provides more flexibility in the application of heat Area-bond hot calendaring is influenced by five factors:

• Heat

Bonding occurs at the surface of the metal roll, which obtains its heat by conduction from heated oil circulated through its center or from restrictive heating The composition rolls obtain their heat from contact with the heated metal roll Before the start of a production run, the roll stacks are operated until the composition rolls achieve dynamic heat equilibrium

• Pressure

Bonding occurs through simultaneous application of heat and pressure The heat causes the fiber binder to become thermoplastic The pressure enhances mechanical bonding by forcing the binder polymer to flow in and around the carrier fibers

• Speed

The speed at which the nonwoven passes through the calendar, combined with heat and pressure conditions, determines the degree of bonding in the nonwoven It also determines the throughput rate of the entire nonwoven line and is a critical factor in product cost The faster the rate, the lower is the cost This is the primary reason for the recent development of lower melting binders

• Roll combination

The only practical roll combination for area bonding is a metal roll-felt roll The metal roll applies the heat The surface resilience of the felt roll enables uniform application of pressure to all the minute surface thickness variations throughout the product

• Cooling rolls

The product is warm and thermoplastic as it leaves the calendar nip If the product were

to be wound while it was still hot, the tension applied to eliminate wrinkles would stretch the web and introduce unrelieved stresses This would lead to shrinkage whenever post-heat treatments were used A set of two cooling rolls placed immediately after the calendaring stage eliminates these unwanted side effects

5.1.2 Point Bonding

Point-bond hot calendaring is the main method of thermally bonding in disposables as diaper, sanitary products, and medical products This method involves the use of a two-roll nip consisting of a heated patterned metal roll and a smooth or patterned metal roll This second roll may or may not be heated, depending on the application In a typical production line, the web is fed by an apron leading to a calendar nip and the fiber temperature is raised to the point at which tackiness and melting cause fiber segments caught between the tips of engraved points and the smooth roll to adhere together The heating time is typically of the order of milliseconds The fabric properties are dependent on the process temperature and pressure and other parameters like the contact time, quench rate and calendar pattern Experimental results show that for a given nip line pressure and calendaring speed, the breaking strength reaches a maximum at a critical bonding temperature; on keeping the nip line pressure constant, the critical temperature was found to be a function of the calendaring speed

5.1.3 Embossing or Novonette System

This method is a figured or sculptured area-bond hot calendaring In this case, though, the area bonding is three-dimensional A "bulky but thin" product can be made in any pleasing or functional construction, depending on the faces of the embossing rolls The calendar roll combination has a male patterned heatable metal roll and a matching female patterned felt roll

In point bonding, webs are embossed by passing between an engraved calendar roller and a solid smooth roller This produces a fabric with an impression on one side only, while the other side remains smooth Both calendar rollers may be engraved with an identical pattern

of raised and depressed areas, so that a raised area on one roller matches a raised area on the other roller to create a highly compressed area However, two engraved rollers may be designed in such a way that the raised area of one registers with the depressed area of the other The main problem in this case is that exact positioning of both rollers is essential to achieve proper pattern definition which is difficult and expensive The Novonette pattern is a patented calendar system developed by the Kendall Co Two identical helically engraved steel rollers are used with lands and grooved areas Owing to this helical pattern on each calendar roller, a multiplicity of lands on one roller is constantly engaged with a multiplicity of lands on the other roller The pressure distribution is therefore even, and there is no need for exact positioning of the roller

The width of the lands and grooves can be varied, as well as the angle of the rollers, to alter the physical and aesthetic properties of the nonwoven fabric As the web passes through the heated rollers under pressure, a repeating pattern is formed as shown in Figure Three defined areas are shown; (i) the dark section represents the area where the land of one roller crosses the land of the other roller and maximum pressure is applied to produce a high degree of bonding, (ii) the grey block represents the area where the land of one roller passes over a groove of another roller and (iii) the white block is where the groove of one roller passes over the groove of the second roller Various factors influence the effect of the calendar associated with mechanical process conditions and web structure