N.Balasubramanian Retd. Jt. Director, BTRA and Consultant 9869716298 Email Balasubramanian
Thermal bonding represents one of the important methods of making nonwovens which possesses many advantages.
It is an environmentally clean technology as no chemicals are used,
Has high production rates and requires low space.
No water pollution as in chemical bonding
The products made from cellulosic are free from allergy to skin. In products close to skin like diapers, sanitary napkins and medical applications it is preferred.
The products have a soft feel and are absorbent and permeable.
There is no weight loss after washing as in chemical bonding
Insulation products and composites can be made
It is ideal for making waddings and paddings used in winter clothing
However, this technology has not much headway in India. The reasons for this are there are no indigenous machine manufactures. Further, low melt fibres and powders have to be imported. In this paper, different methods of manufacturing various types of thermal bonded nonwovens are described. Influences of fibre properties of base and low melt fibre and process parameters on the properties of nonwovens are covered based on the research and development work carried out. Different types of bicomponent dibres and their merits over moncomponent low melt fibres are covered.
I
There are two major types of thermal bonding
Hot calender bonding
Through hot air bonding.
The former makes stiffer, thinner and stronger products and the latter loftier thicker and low strength products. For both types of products, batt preparation is the same. A small percentage of low melt binder fibre or powder is added to the regular fibre at mixing stage and the material is passed through opening and carding and crosslapping or airlaying techniques to form a batt. Belt calendar bonding makes products with characteristic between the two.
Calender Bonding
The batt is passed through the nip of a pair of calendar rollers (Fig 1) with pressure applied on top roller. One or both calender rollers are heated to a temperature above the melting point of binder fibre. The low melt fibre softens or melts to form bonds between the fibres. Heating of calender rollers is usually by oil. Oil, heated in a separate unit by gas burner or electric heater, is force circulated through the calender rollers by a centrifugal pump. Temperature regulators are provided in heating unit to ensure uniform temperature. Afterwards, the bonded material is passed through cooling rollers, known as quenching, to form a cohesive bonded fabric. If the material is wound without cooling, there will be unrelieved stresses caused by tension. This may lead to shrinkage during subsequent use. Both calendar rollers may be plain or one of them may be engraved. The product obtained with engraved roller will be softer as bonding takes place only at the engraved points but it will have lower strength. One or both rollers may be Teflon quoted to minimize sticking of material to hot roller. Instead of direct passage through nip, S passage around the bottom roller to the nip is sometimes done (Fig 1) for increasing heating time especially for heavy material. Calender roller diameter ranges from 40 cm to 70 cm, length from 1.5 to 5m and pressure from 100 to500 KN. Normal pressure range is 30 to 150 kp/cm Material speed depends upon the type of fibre and gsm and usually ranges from 25 to 100 m/min
.
To improve the bonding, some manufactures offer 3 calender rollers with double nip bonding (Fig 2 ). After passing through the nip of top and middle roller, the batt passes around the middle roller and emerges from the nip of middle bottom roller after a second bonding action. Top and middle rollers are usually heated while bottom roller is not heated. This system is used for heavier products like geotextiles and for giving special effects. In 3 calender bonding engraved rollers with different engravings can used in the top and bottom roller to product with enhanced designs.
Type of Loading
Since pressure is applied at the sides of calender rollers, there will be a deflection of the roller leading to more pressure at the sides than at the centre. To obtain uniform pressure across the width, either of the following options are used.
.Top and bottom rollers at kept at an angle
Adjustable calendar pressure can be obtained by X crossing of calendar rollers to get uniform pressure across the width. CX calenders are used to get varying amounts of offsetting as per the pressure. The offsetting is adjusted electrically by an encoder.
Top roller is made slightly convex.
As pressure is applied at the edges, the roller straightens out giving uniform grip over the width. But this method has the drawback that the extent of concavity cannot be varied, which is required with varying pressure.
Thermo hydrein calender by Ramisch Kleinwefers.
Deflection of calendar rollers is compensated by the application of thin roller shelf with internal pressure in the vicinity of the nip of calendar rollers1. The internal pressure is generated by a system of double piston bearing elements and oil from a hydraulic unit. The cylinder pistons produce the required force. With constant internal pressure, nip pressure is likely to be constant.
‘S’ roll by Andritz Kusters
In the ‘S’ roll, annular gap between rotating steel tube and fixed axle is separated by seals into two semi circular chambers. Hydraulic pressure by oil is applied in the chamber facing the nip in such a way that the pressure is in linear ratio to forces in cylinder. As a result deflection is automatically compensated and uniform and infinitely variable force is maintained across the full width. In the latest model direct drive is provided to this roll.
Texcol Hycon by Andritz Kusters
The deflection controlled calendar rollers have a flexiroll sleeve made of polyamide. The sleeve is supported by hydrostatic pistons. An oil film transfers infinitely adjustable forces to the sleeve to obtain uniform pressure across the width. In addition, pressing width can be easily adjusted by means of supporting elements.
Preheating
Preheating of web helps to achieve better bonding of inner layers. It also helps to keep calendar roller temperature lower thereby minimizing sticking tendency. Preheating is usually done by infra red heaters.
Area Bonded Nonwovens
Bonding with low melt fibre
Area bonded nonwovens refer to those made with both calendar rollers smooth A small proportion of low melt binder fibre is mixed with normal fibre and passed though batt formation machinery in the usual manner. The batt is then bonded by passing through calender rollers.
The effect of process parameters on such nonwovens are discussed below.
Calender Roller Pressure
Rakshit Patil and Balasubramanian2 carried out detailed investigations on effect of process parameters in thermal bonded viscose nonwovens made by mixing a small proportion of low melt fibre. With increase in calender roller pressure, thickness reduces initially at a faster rate, later slowly (Fig:3 ). Strength also increases initially up to critical pressure, beyond which improvements are marginal (Fig 4 ). Initial strength improvement is because of melting of binder fibres and bonding of the material. Beyond a critical pressure, fibres get flattened and damaged and strength improvements by bonding are offset by damage to fibres, Elongation reduces with pressure because of reduced fibre slippage due to bonding (Fig 5 ) Strength is higher and elongation lower in CD than MD with card – crosslapping because of preferential orientation fibres in cross direction. Bursting strength increases initially but later levels out because of stiffness by bonding.
Binder fibre concentration
With increase in binder fibre %, thickness reduces, strength improves and elongation reduces steeply (Figs 6, 7 and 8). Extent of bonding improves with increase in binder fibre % which explains the above results. But as binder fibre tenacity is much lower than normal fibre, increase of binder fibre % beyond a limit does not improve strength.
Temperature
With increase in temperature of calender, strength improves initially, afterwards levels off and drops beyond a certain value. Initial increase is because of better bonding. Increase of temperature beyond the melting point of binder fibre does not improve the bonding. On the contrary, it damages the fibre leading to reduction in strength.
Material Speed
With increase in material speed strength decreases as the time of contact at the nip reduces with lower bonding action.
Base Fibre properties
Winchester and Whitewell3 found that crimp, linear density and staple length are the important properties of viscose fibre affecting rupture, elasticity and handle of nonwoven. Finer fibre denier increases strength and reduces air permeability. Staple length increases wet strength, tear strength and abrasion resistance. Crimp increases elongation and reduces air permeability. Binder fibre types, % of binder fibre and bonding conditions have significant influence on the properties of nonwoven. Polyester binder fibre results in loftier and less stiff nonwoven than vinyon binder fibre. The findings of this work have however the limitation that thermal bonding has been done by hot plate pressing under static conditions and not by calender bonding
Type of binder fibre
Binder fibre with higher birefringence and higher crystallinity and molecular orientation forms weak and brittle bonds and result in lower strength4. This is because of insufficient polymer flow and fibrillation. On the other hand binder fibre with lower birefringence and less developed morphology results in better bonding. Further high production rates can be obtained with such fibres because of shorter time taken by fibre to soften and melt. Though these studies were made with point bonded nonwovens made from polypropylene, they should also hold good with area bonded nonwovens made by adding binder fibre to normal fibre.
Quench Rate
Higher quench rate increases the strength and elongation of the material. This is because of recrystallisation and lower crystal size. However, at high quench rates stress concentration takes place leading to brittleness and reduced strength.
Single nip and double nip
Though double nip point (Fig 2) should increase strength because double boding action, this is not always realized. As the material is in contact with hot middle roller for 1800 rotation, the binding points get stretched, especially with stiffer fibres like cotton and broken leading to lower strength particularly with thinner material. Double nip bonding is generally preferred with heavier fabrics.
Low melt monocomponent fibres
Low melt fibres are based on polyolefin (polyethylene and Polypropylene), polyamides or polyester. Polyvinyl chloride , vinyl acetate copolymers and cellulose acetate are also sometimes used as binder fibre. Modifications with commonomers and additives help to lower the melting temperature of normal synthetic fibres for development of low melt fibres.. The fibres are copolymers with less developed morphology and lower crystallinity. As a result, the fibres have low strength. A good low melt fibre should have lower melt viscosity and good affinity/ adhesive property to base fibres. Further, it should solidify quickly upon emerging from heat treatment. The melting point ranges from 1100 to 1500C. Grilene KE 150 and 170 copolyester (Foss), vinyl chloride vinyl acetate copolymer (Wacker MP), Exelto polyolefin fibre (Exelto), low melt polyamide (Premiere), FSI 101 polyethylene, FSI 0200 polyvinyl alcohol, 0502 ethylene vinylacetate, 0800 polyester fibre (Fibre Science.inc), tergal 190 PETP (Tergal) are some common low melt fibres. Biodegradable binder fibres have the merit that they permit recycling of nonwovens. Some important biodegradable fibres are cellulose acetate from celanese, polyactic acid (from corn) by Dow –Cargill, Bio PET by Dupont, PTAT copolyester by Eastman Chemicals,
Bicomponent fibres
Bicomponent fibres produce more uniform bonding and improved strength. Bicomponent fibres consist of two polymers, one of normal type and another low melt type made from the same spinneret with both components in the same filament.
Different types of bicomponent fibres are shown in Fig 9. Skin core, also known as sheath/core, types are shown at the left and right side of figure while side by side types are shown at the centre. In the skin core type, core is a normal and skin is made of low melt component. The two can be placed concentric as in left hand side or eccentric as in right hand side. In the side by side type, shown in centre, one half is normal and other half is low melt type. When the batt is heated to the melting point of low melt component, bonding takes place because of melting of this component. The main advantage of bicomponent fibre is uniform bonding throughout the material as the low melt component is part of the fibre. As a result, low variability in thickness, strength and elongation is obtained. Another advantage is a wider range of bonding temperature. Since the core is a normal material, strength of bonded material is higher. Further the product is softer compared to that made from mono component.
.With concentric skin core type, better strength is obtained while with eccentric skin core type higher bulkiness is obtained. Skin core type is also sometimes used for improving water absorbency, dyeability and soil resistance, these properties being provided by skin component. Side by side bicompomponent is used for making bulkier products because of the difference in shrinkage of the components. Trilobal type skin core type of bicomponent fibres are also available (Fig 10)
Trilobal
Fig 10 : Bicomponent fibre with trilobal crossection
Some of the common types of skin core fibres are
1 1100 amorphous coPET/PET
2.1800 amorphous coPET/PET
3. 1500 melt crystalline coPET/PET
4. PA/PET
5. PE/PP or PET
6. HDPE/PET
7. Easter coPolyester/PP
8. PETP/copolyester
Islands in Sea bicomponent
A number of pieces of one polymer (island) are placed in a sea of another polymer (Fig 11) . The island polymer has a higher strength and lower elongation than sea polymer. The sea has a lower melting point to ensure bonding without affecting the island. Polypropylene, nylon and polyester polymer usually form the island. Sea matrix are made of polyvinyl alcohol or water soluble polystyrene fibre.
Sea Island
Fig 11 : Islands in sea bicomponent fibre
Island/sea bicomponent fibre made from 75/25 N6 and PE resulted in stronger bonds than moncomponent N6 upon thermal bonding of spunbonds5. A strong interface occurs between sea and island polymers, the weaker PE holds the structure together and helps to transfer the stress to stronger islands.
Segmented pie structure
This is shown in Fig 12. There are 16 segments where alternate wedges are made of nylon and polyester. The fibres are carded and with the help of high pressure jet of air or water passed through the web fibres are split. The resulting entanglement results in a strong fabric. The reason for using polyester and nylon in the wedges is that they have a poor adhesion to each other and so under the action of jet of water, splitting easily occurs. Pie wedge fibres are used to make micro fibres. The main application for this is in synthetic suedes and leather and in technical wipes.
Thermal Bonded Cotton nonwovens
Thermal bonding has several merits for application to cotton for making nonwovens. Because of its lower length, cotton is difficult to needle punch. Chemical bonding causes allergy and is not suitable for close to skin and medical products. Thermal bonding is therefore widely used for making cotton nonwovens. The factors affecting thermal bonding of cellulosic fibres has been reviewed earlier by Desai and Balasubramanian1.
Area Bonded
Since low melt fibres are imported and not available easily, studies were made to make cotton thermalbonded nonwovens by using locally available polypropylene (PP) fibre as a binder fibre6. Melting point of PP fibre is 1600 to 1700 and calendar rollers are heated to this point. As in the case normal low melt binder fibre, thickness reduces with increase in PP. Absorbency reduces with increase in PP content (Fig 13) but the rate of reduction is more with light weight nonwovens.. Absorbency is higher with heavier products except at high PP levels. Strike through time, which measures the time taken by distilled water to pass through nonwoven, increases with increase in PP (Fig 14). Strike through time is also more with heavier weight nonwovens Rewet property, which determines the ability of nonwoven to keep the skin of wearer dry, increases with PP with low weight and reduces with PP with heavier weight nonwoven (Fig 15).
Absorbancy Strike Trough
Fig 14 : Effect of binder fibre% on strike through time, sec Rewet
Fig 15 : Effect of Binder fibre % on rewet %
Point bonded cotton nonwovens
Point bonded nonwovens refer to those made with engraved rollers. They have much lower strength than area bonded as only part of the area is bonded but are softer. Use of ordinary cellulose acetate(OCA), plasticized cellulose acetate (PCA), easter/PP bicomponent and PE/PET bicomponent as binders in thermal spot bonding of cotton has been investigated byH.Rong7,8, High strength absorbent cotton thermal spot bonded can be produced using 50% Easter/PP bicomponent fibre as binder. Easter/PP bicomponent fibre enables bonding at lower calender temperature and gives higher strength than Easter unicomponent Shape of the bond point becomes smoother and well defined as bonding temperature increases. Cellulose acetate as binder fibre has the merit of biodegradability as against synthetic binder fibre. However cellulose acetate has high melting temperature and this poses difficulty when used as binder. This can be overcome by pretreating it with water. This lowers the softening temperature of cellulose acetate and improves significantly strength of cotton/cellulose acetate thermal bonded nonwovens9.Melting point of cellulose acetate can also brought down by treatment with solvent vapours 10,11. Higher strength and bonding at lower temperature can be obtained as a result. Cotton bonded with cellulose acetate has good biodegradability and could be easily degraded by microbial attack. Investigations are reported on cotton based thermalbonded nonwovens for short wear-cycle applications. made with cotton on one side or both sides with 41 – 75% content have excellent wicking rates, wetting, absorbency and retention properties with handle similar to hydroentangled or knitted products12. Thorough mixing of binder fibre with cotton is required to get optimum tensile properties13. Nonwoven mouldable automobile carpets can also be made by blending cotton, kenaf and flax with biodegradable binder polymers PTAT and PTAT/PP, Biopet and PE/PET, PLA and PCA bicomponent fibre. Such products have the advantage of being environmentally biodegradable14 and trimmings made during manufacture can be reused. Statistical prediction equation has been developed for determining optimum process conditions to achieve peak loads of cotton thermal bonded nonwovens15.
Point Bonded Nonwovens from synthetics
One of the calendar rollers is engraved to make point bonded nonwovens for making smoother products. Both calendar rollers can also be engraved to produce novonette pattern in the product. A helical pattern of lands and grooves on both rollers results in a diamond point. Point bonded nonwovens can be made from polypropylene fibres without addition of low melt fibre.
Fibre Orientation
Fibre orientation in the input material, known as orientation distribution function (ODF), has marked influence on tensile properties of such material. Random orientation of fibres in the input material results in maximum strength because of improved bonding arising from higher number of cross over points16. With preferentially oriented fibres, number of bonds will be less and strength will be lower. ODF also determines the nature of break of nonwoven. Fabrics tear across the preferred fibre direction when load is applied in machine or cross direction. ODF determines the structural changes and deformation that take place during loading of nonwoven. Bonding temperature has influence only on the point of failure16,17. ODF and anisotrophy of bond pattern determine the load elongation behavior of nonwovens. Orthotropic theory has been used for predicting the effect of mechanical conditions in nonwovens with preferred orientation distribution. Theory shows good agreement with experimental results18.
Bond point Geometry
Bond point geometry, area of the tip, concentration and % bond point area are some of the critical factors that influence properties of point bonded nonwovens. Generally bond point area should be 10 – 25 % of total area and bond point concentration should be 100 to 500 per square inch.
Important bond point parameters are the length and width of the bond point tip, and side angle (Fig 16). Very low bond point concentration results in low strength and poor life of product. Smith19 et al; found that concentration beyond a point will result in stiff products. Lower bond point concentration increases elongation and decreases flexural rigidity of polyethylene point bonded nonwovens. Increase in angle of bond point increases tenacity. Measured bond area is found to be higher than engraved roll pattern possibly because of transmission of temperature at the periphery. Computer based study however shows that with increase in % bond area, tensile strength, energy to break and breaking elongation increase except with weak bond points20 Numerical method, based on finite element analysis, for prediction of nonwoven tensile behavior from bond point design and process parameters has been attempted by Mueller and Kochmann21. Rapid modeling of different point geometries and layouts is possible using this method and this will help manufacturers in developing design.
Process Parameters
Warner22 showed that unlike in area bonded, calender roller pressure does not have much effect on strength with point bonded material. Pressure however aids in compacting the web and facilitates plastic flow at melting temperature and reduces thickness. Melting temperature of polypropylene is increased by 150 with application of pressure due to clapeyron effect. Heat of deformation under pressure also increases the temperature substantially. Melting temperature at the centre of web is more important than that at the nip of calender rollers in order to get a good bond. However, heat transfer from calender nip to the centre of fabric is insufficient to increase temperature to melting point as conduction heating plays only a limited role in transferring heat from nip of rollers to the centre of fabric. Some diffusion also occurs but its contribution is much lower than flow. Because of these reasons, pressure does not have much effect on strength. The main function of pressure is to facilitate flow of polymer which mechanically locks the fibres upon solidification. Bond area, Bond size and bond temperature have got maximum effect on tensile properties23,24,, . Tensile modulus and shear increases with temperature and bond area23 . Fig. 17 shows the effect of temperature on strength of point bonded nonwovens
Temperature
Fig 17 : Effect of temperature on strength of point bonded nonwovens
Shrinkage of fabric also increases with temperature25. Polypropylene melts over a range of temperatures and not at one specific value and this range decreases with increase in process speed. If bonding is done at low temperature, fibres can be teased out of the bonded portion. Breakage of nonwoven then occurs due to inter-fibrillar slippage. At high temperature beyond melting point, fibre damage occurs and the bonds are also brittle and weak at the periphery. With such material failure occurs at the periphery of bond spot by breakage of fibres.
Mechanism of Breakage
At high temperatures, a steep morphological gradient is found at the boundary of bonding area. Rapid loss in molecular orientation occurs from bridging fibres to bond edge, as a result of which most breakages occur in this region during breakage of nonwoven26. Fibres close to the bond periphery have a lower strength and elongation than in rest of unbonded area. Unequal strain distribution is therefore found in spot bonded nonwovens and as result only 10 – 15 % of fibre strength is realized. However, with polyethylene point bonded nonwovens fail due to disintegration of bond area rather than fibre breaking at perimeter19.Studies with polarised laser microspectroscopy showed that birefringence decreases to half its original value at the bonding point27,28 while it is unaltered in the non-bonded portion. Higher bonding temperature and longer bonding time result in larger differences in morphology at bonding region. While density at bond point is lower than that at bridge fibres, in polyester, opposite trend is found in polypropylene. Image acquisition equipment have been used by some workers to examine structural changes of thermal point bonded nonwovens during deformation29.This enables determination of orientation distribution, bond spot strain, unit bond repeat pattern strain as a function of macroscopic deformation.. Mode of application of deformation influences type of failure of material. Structural changes and microscopic deformation of bond spot are determined by initial orientation distribution function of the material17. Temperature of bonding has little influence on this. To get a clue to the low fibre strength utilization in thermal bonding, thermal bonding of a pair of fibres was carried out by passing them through a hot calendar roller30. This showed fibre strength reduces significantly upon bonding. The reduction is primarily because of heating and not because of pressure. This study confirmed that stronger bonds are formed with lower birefringence fibres even at lower temperatures.
Morphology of Fibre
Morphology of fibre has a marked influence on properties of thermal bonded material. Fibres with higher crystallinity and higher molecular orientation form weak and brittle bonds. This is the consequence of poor polymer flow compounded by fibrillation of fibres2,31 .Fibres with lower crystallinity and lower molecular orientation form stronger bonds and stronger fabrics. This is reinforced by a study where polypropylene fibres with varying molecular orientation were made on a pilot extrusion plant and made into point bonded nonwovens by Andrreassen et al;32Wei et al;25. Tensile strength of fabrics made from such fibres is found to be primarily influenced by molecular weight distribution and molecular orientation as determined by the conditions under which fibre is made primarily. Tensile strength of nonwoven increases with decreasing draw ratio and increase in extrusion temperature, though these conditions result in lower fibre strength. Fibres with low orientation or higher elongation also result in higher strength and flexural rigidity of nonwoven because of better load sharing33, . While increase of strength is desirable it should not lead to higher flexural rigidity as it makes the product stiff and board like. Changes in thickness due to bonding with poorly oriented fibre are lower25. T196 polypropylene fibre which has a sheath core structure with a lower refractive index at sheath, lower modulus and higher elongation than normal T101 performs better and result in higher strength in nonwoven. This is because T196 fibre produces stronger bonds at lower temperature, where strength degradation is minimum34.
Computer simulation
Computer model and simulation techniques have been used to predict stress strain relationship of point bonded polyester nonwovens. Bond layout, fabric density, orientation and curl of fibre and bond tensile properties were fed to the computer and a program used to determine stress strain curve during deformation35.Computer simulation program restricted to two dimensional nonwovens has been developed by Britton36 et al;. Mechanical properties of fibres laid in a web and held together by some means are first programmed. The changes in the fabric system, as it undergoes distortion, are computed. In another study an image simulation system is used to obtain an image of nonwoven using web density, fibre properties, unit cell size and bond properties and the effect of structural and process variables on the mechanical properties of nonwoven was examined37. Demirci38 et al; have developed a simulation technique to visualise a nonwoven composed of nonuniformly oriented fibres and bond points made from deformable fibres. Dynamic response of such a simulated material to process parameters bond points is determined using finite element software. Such studies have the merit that optimum process parameters can be obtained without extensive experimental investigations.
Testing Conditions
Effect of specimen length and width on strength of nonwovens in conventional tensile test and cyclic loading have been investigated by Hou et al;39. Longer specimen length generally result in higher modulus but sample width has insignificant effect. Cyclic loading reveals that the material is initially elastic but later on shows plastic behahiour.
Applications
Light to medium weight products in the range 30 – 100 gsm are made from calendar thermal bonded nonwovens. Main applications are
1. Interlining
2. Coverstock for diapers
3. Filter material
4. Sanitary napkins
5. Wipes
6. Tea bags
7. Trims for automobile
8. Geotextiles
9. Surgical gowns and caps
10. Face masks
11. Automobile headliner
12. Wipes
13. Shoe composites
14. Computer disc
Belt Calendering
In belt calendaring nonwoven batt is passed between a heated roller/drum and a heat resistant silicone coated blanket. Pressure is applied at the exit end by the guide roller on the hot roller. Nonwoven is in contact with the hot roller for longer time of 1- 10 sec and pressure is much lower, up to 9 N (9kp/cm), compared to hot calender bonding. The products are thicker, flexible, permeable and weaker than calender bonded material. By varying the speed, temperature and pressure special effects can be given. Patterned bonding can be obtained by using a patterned blanket. Heated drum from 0.5 to 2 m, width up to 6 m and speeds up to 100 m/min are available. Machines with 2 heated drums in tandem are available where fabric is bonded on one side in first drum and on other side in next.
Embossed calender Bonding
The web is passed though a pair of calendar rollers, one of which is male patterned metal roller and other matched female patterned felt roller. Many geometrical shapes including designs are used in embossed roller (Fig 18).
Embossed calendar bonding is used for making baby wipes, coverstock, automobile products and special fabrics and clothings
Through Hot Air Bonding
Through hot air bonding is used to produce loftier products used as waddings in quilts, blankets and winter clothing and insulation material. The batt made out of normal and a small % of low melt fibre is passed over a slotted traversing lattice and is taken through a hot air chamber. The batt can also be guided between two belts through the chamber. Distance between the belts is adjustable as per the product specifications. Monocomponent low melt fibre or bicomponent fibres can also be used as binder fibre. Hot air at melting temperature of low melt fibre is drawn though the batt from the bottom as well as top as shown in Fig 19 through suction by a fan. Binder fibre melts and forms droplets on normal fibre and bonding takes place. Hot air chamber is in number of sections based on the capacity. Positive pressure from one side and negative pressure from opposite side is provided in each section to ensure even passage of hot air through the material. The direction of air supply is adjustable separately in each section. Air supply to the chamber is both from left and right to ensure uniform temperature. The same air after passage though batt is reheated and used for conserving energy. Temperature measuring units are placed in each chamber to facilitate monitoring and control. Temperature across the width is maintained with an accuracy of ± 1.5 % . Direct heating systems by gas or indirect heating by steam or thermal oil is used for heating air. The material then passes through a cooling chamber and a bulky product emerges. When products of lower width than the belt are made, systems are provided for lateral closing of air to minimize loss of air. Products made by this method are lofty, open, soft absorbent and porous. Width range is from 2 to 7m and material speed is up to 100 m/min.
Fleissner uses a perforated drum, over which batt is taken round. Hot air is sucked from the end of drum through a radial fan through the perforations and passed through the batt to form a bonded material (Fig 20). Low melt fibre melts flows to the point of contact between fibres and forms the bond. After bonding the material becomes loftier and bulky. The material is soft, absorbent, resilient and has insulation value. Flow of air can be adjusted as per requirement. Working width up to 7m and fabric thickness up to 50 mm and speeds up to 1000 m/min are claimed.
Circular drum
Fig 20 : Through Hot air bonding over perforated Drum
Products made by through hot air bonding can be given cold calendering to make more compact products. Such products are softer, flexible and more extensible than hot calendered nonwovens.
Struto Technology
Struto technology consists of vertical lapper to make batt from card web. This is then taken through a hot air chamber for bonding. Vertical arrangement of fibres is achieved by this technology which improves loftiness. Thickness up to 40mm and production rates up to 100 m/min are possible.Jirsak40 et al ; found that compression properties of high lofts made from batts prepared from vertical lapper were significantly better than those made from conventional crosslapper. Further hollow polyester fibres bonded with newly developed binder fibres from Teijin using Struto technology have compression property close to polyurethane foam.
Air laying
Air laying method of batt preparation has also a 3 dimensional random arrangement of fibres. With through hot air bonding, these products will be loftier than card crosslapping batts.
Fibre properties and process parameters
Type of fibre, % of low melt fibre, temperature of hot air and duration of exposure are some of the important parameters that should be optimized to get most desirable high- loft. Desai and Balasubramanian41 investigated the influence of fibre crimp and type and process parameters on properties of high loft nonwovens from polyester fibres. Hollow and high crimp fibres result in thicker and bulkier product but loose their bulkiness most rapidly with cyclic loading compared to normal fibres. Hollow fibre high lofts are more compressible than normal fibre material and at the same time have higher insulation value. Increase of low melt fibre content and duration of bonding reduces specific compressibility and resilience (Fig 21 and 22). Thickness of the high loft is an important parameter that determines thermal resistance. Thermal insulation improves with thickness. The specific stress of such material increases with low melt fibre content, temperature and duration of exposure. The products are found to have good uniformity. If needle punching is done prior to through hot air bonding, a higher strength is obtained but the product becomes thinner and stiffer.
Resilence
Fig 21 Resilence vs low melt fibre% Compressibility
Fig 22 : Specific Compressibility vs low melt fibre %
Hong42 et al found that PP through air bonded material has lower friction coefficient and withstands abrasion compared to polypropylene thermal bonded, tencel spunlace and cotton spunlace and therefore can be used as coverstock material for diapers. Its quick drying characteristic is another advantage. However consumers prefer cellulosic spunlace compared to surfactant treated polypropylene. Studies have been made to make nonwovens from kenaf by through hot air bonding43.Through hot air bonded kenaf has a soft feel and is lofty. If needle punching is done prior to hot air bonding, the product becomes stiffer. A statistical approach was employed to determine optimum bonding process parameters for getting minimum pore size of through hot air bonded nonwoven by Wang and Gong44. Lin45 et al; showed that bulk density of nonwoven has more influence on thermal conductivity and limiting oxygen index than thermal consolidation and processing methods. Nonwovens with good thermal insulation and low flammability can be made from FR polyester hollow fibre and low melt polyester fibre by thermal pressing of batt made out of them. Modelling thru air bonding process showed that the time needed to heat and melt the fibre decreases with increase in porosity of product and velocity of hot air46. Fibre orientation factor has maximum influence on bond formation. Bonding temperature and fibre diameter have a small influence on time required for bonding. Computational fluid dynamics modeling has been employed to examine the effect of fibre properties and process parameters on properties of bonded product47. Fibre orientation in the input material has a marked influence on the anisotropy of strength with through hot air bonded material48. These products have a good stability and elasticity. A model was developed to estimate initial tensile response of such products based on orientation averaging and Poisson’s ratio.
Applications
Medium weight products in the range 70 to 200 gsm are made by through hot air bonding. Common applications are
1. Wadding or padding used in winter clothing
2. Quilting
3. Filters
4. Seat cushions
5. Sound proofing material
6. Composites
Thermal Bonding using powder
Thermal bonding is also carried by using a low melt powder in place of fibre. Powders from polyethylene, low density polyamide, vinyl acetate/chloride copolymers are commonly used . The powder is added in the mixing stage in a controlled manner. It is also sprayed on the web or nonwoven fabric for making powder coated and mouldable products like automobile carpets (Fig 23). In airlaying fibre abd powder are dropped on the laying lattice.After application of powder the material passes through an infra red heating chamber and passes through a pair of calender rollers to form laminated products which can then be moulded to shape. Powder bonding can be made on one side (Fig 23) or both sides (Fig 24) Powder bonding has a wider range of temperature for bonding than low melt fibre.
BR>
An LDPE film or thermoplastic fabric can also be introduced over the material before calendaring. Powder bonder is suitable in open structures or mouldable materials and composites. The powder should withstand washing and chemical treatments and should not emit any fumes or odour during use.
Applications
1. Interlings for collar and outer wear
2. Shoe inlays
3. Automobile carpets
4. Insulation material
5. Building material
6. Waddings and quilts
Radiation Bonding
Infra red lamps are used to provide the radiant energy to bond the Nonwovens with binder. While binder is melted there is no harmful effect on base fibre. Bonding takes place when material comes out of radiation chamber. This method has the advantage of low energy and capital costs.
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