Nonwoven Technology Conference 2002 – Ottawa 13-16th May

This conference, organised by Marketing Technology Service of Kalamzoo Michigan, USA , and held in a dull and chilly Ottawa, Canada, focussed on the air-lay process. A truly international audience of 200+ delegates was treated to a first-class collection of papers and a surprisingly informative tour of the new, but fully operational, Concert air-lay plant in nearby Gatineau Quebec.

At the start, the audience was saddened by the news of the death of John Mosgaard, the founder and President of Danweb Forming International A/S, and one of the best known speakers on the nonwoven conference circuit. He and his regular contributions to these industry gatherings will be sorely missed. At the end, to the great credit of the closing speaker and conference organiser Jim Hanson, the hall was as full as at the start.

Highlights

• Pre-formed airlaid cores still await a major diaper application. The need for a significant cost benefit over in-situ forming now seems to be accepted.
• After recent installations of new lines; 5 in the last year in the US alone, annual capacity now exceeds the estimated 350,000 tpa demand by about 130,000 tonnes.
• According to Reiter-Perfojet, 60gsm wipes made from 50/50 spunbond PP/Pulp and spunlaced would be half the price of current carded viscose/PES blends.
• Diaper production rates of 3000/min/machine were now possible if preformed cores replaced drum-formers.
• A major US wipes producer would be switching from airlaid to spunlace.
• More details of a new low-cost airlay former said to be capable of handling up to 50mm fibers at over 1 tonne/m/hr emerged.
• A new plaiting system for airlaid core promises significantly higher run-times than the current festooning or spooling systems.

Air-Laid Supply and Demand

Pricie Hanna (John R Starr Inc) predicted a 4.4% annual growth for airlaid nonwovens in wipes for the next 5 years. Sales would increase from 6.2 billion m 2 in 2001 to 7.7 in 2006.
• In 2001, half of all airlaid was latex bonded, a quarter thermal or multibonded and just over an eighth was the K-C Coform process. Hydrogen-bonded and spun-laced airlaid made up the remainder.
• By application, 55% went to wiping, 30% to pre-formed cores – mainly femcare and subfacings - and the rest into tabletop, medical, packaging etc.
• Reviewing nonwoven technologies used in wipes she pointed out airlaid is now in the second rank after spunlace: airlaid still dominates in the USA but spunlace dominates globally. (75% of all airlaid wipes, but only one third of all spunlace wipes, are used in the USA and Canada .)
• The trend to spunlace would continue, even in the USA where a major supplier was contemplating a switch from airlaid.
• “Airlace” – or hydroentangled carded fiber/airlaid pulp laminates was expected to grow rapidly to fill the perceived gap between airlaid and spunlace from both cost and softness aspects.
• Overcapacity would affect the industry for the next year. North America alone had added 5 lines (100,000 tonnes) in the last year, and while Buckeye and Concert were switching products to the new lines, there was underutilised capacity on the older machines.
• The main growth opportunities seemed to be in the less developed countries where a strong demand for modern femcare was arising. Unfortunately this demand was most likely to be met from production units closer to these markets: Mercosur , Turkey , Middle East , China being mentioned.
• To make matters worse, the US need for airlace would be met by European/Middle East imports in the near future. So airlaid profitability would suffer until supply and demand were more nearly in balance.

In response to questions, Ms Hanna commented that there was a lot going on in diaper cores that had yet to reach the public domain. The development of long-fiber airlaid technology to compete with carding looked a promising development (see later). Wet-toilet tissue would progress slowly. While this was a great disappointment to many, she felt K-C would use the plant to make products other than “Cottonelle” while the shortfall existed.

Airlaid History I

Bill Stevens (Consultant) reviewed the early development of airlaid technology, identifying Karl Kroyer as the visionary who first commercialised “dry-formed paper” and offered machinery for sale. Kroyer sold the technology to M&J in 1981, but in 1975, John Mosgaard left Kroyer and set up Scan-Web (renamed Dan-Web in 1981) to commercialise his own air-laying ideas. Independently, Honshu Paper (now part of Oji Paper) developed and commercialised a third process that they kept to themselves apart from licensing J&J.

Current airlaid capacity was put at 480,000 tonnes/year excluding the vertical operations of P&G, K-C, PGI etc, for a market demanding about 350,000 tonnes. This came from 50 production lines making 18 different product lines. For the future, Mr Stevens saw:

• Hydroentangled airlaid as a key area for growth with 6 companies operating the combined processes this year and a further 3 lines coming on stream in the next.
• Hydrogen or X-bonded (calendered without any other bonding agents) airlaid offering greater economy due to easier waste recovery.
• Baby diaper producers needing an even cheaper product before they would be willing to write-off current assets.
• Prospects for growth in agricultural markets at the expense of spunbond.
• Prospects for airlaid/meltblown combinations now that the K-C patents had expired.
• Products utilising the reactivity of cellulose in airlaid.
• The need for fully biodegradable products from renewable resources.
• Increased use in a variety of packaging applications.

In response to questions,
• The Merfin “Band-Aid” development failed because the latex bonded product proved too stiff compared with the incumbent needled viscose pads. (he thought that this would no longer be the case: latexes were now much better.)
• The “nit” problem had been solved mainly by the pulp suppliers developing pre-softened pulp for easy defibration.
• The main problem of the diaper core was now cost: reducing raw materials wastage was the key to its solution.

New Air-Lay system for Longer Fibers

James Westphal (Troika Nonwovens) introduced two new machines: a fiber mixing system applicable to any air-lay process and a new air-lay head capable of handling fibers up to 50mm in length at higher throughput than woodpulp. Both machines used the principle of rapidly rotating spiked rolls to keep the suspension of fiber in air “fluid”. In the air-lay head these replaced the static screen which has been the main bottleneck of the Kroyer-based systems.

The “GreenMat Star Fiber Mixer” was fitted into a line between the hammermill and the forming heads and was fed with pulp, recycle from the former, edge trim and bico fibers. In effect it replaced the transport fan, kept out of the hammermill those materials that could be damaged or reduce its efficiency and achieved very intimate mixing of all components. The payback came not only from the recycling element but from the ability to achieve strength specifications with lower levels of bicomponent fiber. On a 15000tpy line reducing the bico usage from 25% down to 15% would save $3.5M per year. ($350,000 per % bico eliminated)

The new air-lay head or Star former (USP 6233781-B1) had been developed by ME Consulting – Aarhus (ex Dan-Web) as part of a project with the Danish Department of Agriculture to process flax into insulation board. Here Mr Westphal, who had been retained to help commercialise it, revealed that when used in conjunction with the mixer described above it could handle a tonne of fibers/metre/hr irrespective of length in the 4mm to 50mm range. Woodpulp, for reasons unclear, was restricted to half this productivity. Basis weights claimed ranged from 25 to 3000gsm. Asked about handling SAP, he said the powder could be added between forming heads or to the mixer. The mixer tower as currently designed would also handle a tonne/hour of fiber.

With regard to fiber types processable, the gap setting of the star-forming rolls could be adjusted to suit the fiber. Cotton was not a problem. MD/CD ratios were typically 2:1. Troika is now working with a well-known producer of spunlace equipment to develop lightweight formers for nonwoven production.

Airlaid History II

Jesper Dobel (Danweb Forming International A/S) introduced the 3 rd generation air-lay drum former:
• Capable of giving “tabletop” quality free of nits and lumps
• 400 m/min operating speed
• 400 kg/m/hr output

Their pilot line was capable of 600m/min and they were now targeting 800m/min with 1000m/min thought to be ultimately achievable.

He claimed 50% of the installed airlaid capacity for Dan-Web and Dan-Web Technology compared with 43% for M&J.

Installations from 2000 on included:
• the 2.2m line at MAIN Spa in Italy .
• Concerts 2.7 m line in Canada .
• Orlandi's 3.6m former on the HE line.
• Rayonier.
• Kinsei.
• Nanking ( China ) 2.2m line.

Asked if diaper makers would install their own airlaid core line, Mr Dobel thought not: the technology does not integrate well with diaper production, and diaper makers would lose flexibility if they made their own.

Airless Spray Latex Bonding

Paul Scott (Vinamul Polymers) described the fruits of their collaboration with Nordson to develop an airless spray bonding system for uniform application of latex in concentrations up to 55% solids. Cross-cut spray nozzles were fed with latex at pressures between 100 and 1500 psi to give very precise surface coating (at ~50% solids), or penetration through the thickness of the pulp (at ~15% solids). Penetration depth could be adjusted by varying the dilution of the latex. The airless technology saved latex, reduced drying costs and reduced line contamination compared with conventional spraying. The system is already in commercial use on card-webs and spunlaid webs, and has been successfully tested on the M&J airlaid pilot line. The system worked best when heated latex (up to 130 0 C ) was sprayed, and this could mean the approach would work with low-melting polymers also.

Uniformity of application was checked by visual inspection of the spray pattern using special illumination. In the pilot trials, coloured latex was used.

Under-Utilised Technology?

Tony Butterworth (Consultant) dealt with the evolution of airlaid up to 1985, using expired patents to bring out a number of issues that are still relevant today:
• Fluidised bed technology is important. Fibers pass through vibrating screens much faster than through static screens. USP 4144619 describes a process where the brushes also vibrate the screen by beating it.
• USP 3859205 describes a device for separating a pulp/air mixture into three particle size fractions, the coarsest of which would be returned to the hammermill.
• USP's 3886629 and 3781150 cover the essentials of the superior Honshu forming system and starwheel disperser.
• USP's 3606175 and 3825381 (Kimberly Clark) describe air-laying direct from a pack of pulp sheets being ground away on a toothed drum.
• USP 3994047 describes an air-lay version of the dual-headbox Vertiformer.
• USP 4130915 describes the Fehrer K12 card with air-doffing operating on both pulp and fiber simultaneously.
• USP 3984898 describes Honshu 's method of air laying a viscose fiber/pulp sandwich with 3gsm viscose skins.
• The Rando Duo-Feed (USP's 3512218, 3918126) and the J&J Dual Rotor (USP's 3772739, 3740797, 3895089, 3963392) both deal with simultaneous processing of pulp and textile fibers to give homogeneous or two sided products, or multi-layer products with the textile fiber surfaces.
• Describing the J&J version of the Honshu technology, Mr Butterworth mentioned that the long-fiber could be layed onto rods above the forming wire giving a corrugated structure, the valleys of which could be filled with pulp.

Two-Layer Air-Lace Technology

Daniel Feroe (Rieter Perfojet) covered the difficulties of spun-lacing a pulp/long fiber mixture:
• The card web must be pre-bonded on a drum system to give it enough integrity to act as a filter for the pulp layer.
• The pulp needs to be hydroentangled on a flat-wire section with 8 to 10 injectors.
• The injectors work from low to higher pressure in easy steps to ensure that the pulp is entangled with minimal wash-through.
• These injectors need to be widely spaced to prevent any interference due to splashing.
• Even with these precautions, 4-5% of the pulp is lost through the wire and has to be removed - and discarded - before the water can be reused.
• An Air-lace line would require significantly more capital (€17.2M) than a card-only HE line (€9.2M). The additional cost arose for the pulp former and the more complex water filtration system.
• Such a line would be less flexible than a card-only line and in practise would have to be dedicated to very few similar products

So why bother? Because pulp is about one third of the price of viscose and great savings can arise by using it as a substitute. Compared with a 50/50 viscose/PET wipe costing €2.65/kg, Air-laced 50/50 Pulp/PET would be €1.89/kg.

Mr Feroe could not recommend trying to make wipe fabric with fiber either side of the pulp. This is because the first fiber layer would be too thin to act as a pulp filter.

Rieter Perfojet had also investigated combinations of their spunlaid process, airlaid pulp and hydroentanglement. Here the capital costs of a line were €21.7M but this approach gave the most efficient way of combining long-fiber and pulp at the lowest possible conversion cost (€1.32/kg for 50/50 spunlaid PP/Pulp at 60gsm).

In answer to questions, Mr Feroe added that at present there was no way of reclaiming the pulp washed through into the filter system. Spunlacing spunlaid webs was not a problem and needed only slightly more pressure than spunlacing staple. Finally all spunlaced systems needed chemical treatment of the water to prevent growth of microorganisms.

3000 Diapers/minute?

Mike Jelinsky (Martin Automatic) plotted the output of diaper machines over the last 30 years and found it roughly linear from 150 to 650 diapers/min. Extrapolating, he concluded that 850/min would be possible by 2010, but postulated that if cores could be delivered to the line in web form rather than being made in-situ, then 3000 diapers/min would be possible. He listed the other advantages of pre-formed cores as:

• Less waste for the diaper maker
• Easier start up after interruptions
• Energy savings, noise reductions and reduced levels of airborne SAP dust due to the elimination of hammer-milling.
• Lower capital costs: drum-forming now accounts for 15 to 20% of diaper line costs.

With regard to other possible bottlenecks on the diaper line:
• Adhesives application was capable of 3000 diapers/min now.
• Packing (15 diapers/pack) would already run at the 3000 rate.
• Process controls would work at the 3000 rate.

However, with current diaper orientation the 3000 rate could only be reached by making two diapers side-by-side on the same line, i.e. running at 1500 “doubles” per minute. Making diapers sideways on would make the 3000 rate possible on a singles basis.

Asked about the speed limit of running webs, Mr Jelinsky said that their Airnertia rolls would allow tension controlled unwinding at up to 2200 ft/min, at which speed the aerodynamic drag on the web became a significant source of uncontrolled tension. Dusting of the preformed webs would also become an issue at these speeds. Could drum formers be speeded up to allow 3000/minute? Maybe, but it would be cheaper to use preformed airlay.

Airlaid History III

Henning Skov Jensen (M&J Fibertech A/S) provided a catalogue of the new products possible with M&J forming technology, revisiting some themes of his presentations of the last couple of years:
• Binderless airlaid based on calendered long fiber pulps.
• Cotton-linter (70%) and Bico (1.5 den/3mm) airlaid
• Pulp/bico airlaid, spunlaced and through air bonded
• 100% pulp spunlaced from both sides (adequate dry strength but very low wet strength)
• High loft wadding based on 80% 6.7dtex, 6mm PET bonded with 20% of 4 dtex 6mm PET bico.

Line configurations now possible included one or more of thermal, latex, steaming, and calender bonding of single or multi-layer airlaid in combination with carding, spunlaying, meltblowing and glue lamination. The 5-layer diaper core concept, methods of reaching 2.5 tonnes/hr/m output and the Super-site concept were also mentioned.

17 installations of complete M&J lines were listed, along with the three air-formers on hydroentanglement lines at Albaad, Spuntech and Tenotex.

Frank Phillips of Source Options Consultants chaired most sessions.

In response to questions, Mr Skov Jensen said:
• M&J could guarantee profile control to better than +/- 3% over widths up to 5 m.
• Fiber losses when spun-lacing 100% pulp depended on the type of wire used.
• SAP levels could be 10-15% lower when preformed cores were used in diapers.
• SAP levels of up to 60% were possible if a carrier sheet was used.
• Nits could be recycled through the hammermill.

Why spunlace 100% pulp when hydrogen bonding was possible? Because spunlacing gave a much lower density product.

Would a fluff-free core be possible in future? Mr Skov-Jensen thought not.

Economics of Waste Recovery

John Cork (IBIS International Inc.) pointed out that the trend to larger machines using multibonding, or more specifically reduced levels of latex bonding, increased the scope for waste recycling on-site at the air-lay plant. A 20,000 tonne/year unit working at 90% material efficiency could be wasting 2000 tonnes, or about $1million, in pulp value per year.

Latex bond air lay (LBAL) trim and waste – typically 15-25% latex content - tended to gum-up the shredders used to breakup 100% cellulose products and could not be recycled economically. If, as in the thermal or multi-bonded products, levels of latex below about 5% were used then recycling on-line became possible. The key piece of equipment used was the sort of shredder common in other waste textile reprocessing operations. This could be obtained from Temafa or Hollingsworth set up to deal with the airlaid material. IBIS would provide a complete system to deal with all waste from a 20000 tonne plant for about $1M giving a best case payback of 1 year. This would require 2 operators per shift on a three shift system.

In response to questions he said the payback was based on recycling everything back into the process on site and valuing it at fresh pulp price, assumed to be $750/tonne. His reclaim units in diaper and femcare plants could however also separate and recover the SAP, and with TBAL, there was some value to be credited from the slight reduction in fresh bico needed when recycling was in use.

Asked how much of the reclaimed product could be recycled before product quality suffered, he guessed 10-15%. However he was aware of a diaper producer making diaper cores out of 100% IBIS-recycled diaper waste from a nearby larger diaper plant.

Calendering Airlaid

Johannes Walterfang (Eduard Küsters Maschinenfabrik GmbH & Co KG) described the functions of a calender as:
• To reduce the thickness of a web for the next processing step
• To calibrate, or define and set the final thickness of a fabric to meet a thickness specification
• To emboss and add surface interest and aesthetics to a fabric. Embossing could also add functionality e.g. guiding liquid flow.
• To polish one or both sides of a fabric
• To thermally bond a web, pre-heating being needed for thicker products such as airlaid.

Roll materials could be steel, rubber coated or textile coated, and the other key variables were speed, temperature and nip pressure (or gap setting). High nip pressures resulted in deformation of the calender rolls, a deflection of up to a millimetre being possible over a 3 metre wide roll, giving unacceptably high pressures at the edges compared with the middle. This could be compensated for by crowning the rollers, crossing them at a slight angle, bending them, or best of all by using Küsters “swimming roll” system. Here the bottom roll was filled with hydraulic fluid which could be pressurised to deform the surface shape to match the distortion of the top roll over a wide range of nip pressures. For use on air-lay machines, bearings and drive systems had to be protected from the dusty environment, and the rolls were often horizontally opposed to minimise strain on the weak web as it transferred into the nip. Asked about pressures used, 200 N/mm of nip was the maximum. Temperatures ranged from 80 to 250 0 C, spunbonds down to 6gsm were being calender bonded, and brushes could be fitted to keep the rolls free of deposits. Calenders with special engravings could be used to perforate card webs, but holes were normally regarded as a problem.

Fluff fiber Morphology

Denis Sens (Tembec Tartes S.A.- France) pointed out that the naturally hollow fibers extracted from wood had widely differing morphologies depending on whether the tree was deciduous or coniferous, grown in the cold north or the warm south, and whether pulp was made from forest thinnings and roundwood or from saw-mill waste.

The key aspects of pulp fiber morphology were length, curl and width (if collapsed) or diameter (if still hollow). Tembec now measure these properties using the Techpap Morfi LB 01 analyser which scans a flowing suspension of fiber at the rate of 4000 fibers in 3 minutes. Hollow wall thickness is important and this needs a separate evaluation on the Metso Fiber Lab 3 instrument.
• The faster growing southern softwoods have been shown to give longer fibers with thicker walls and hence fewer fibers per gram.
• Levels of curl are similar, but sulphite pulping gives higher curl than the Kraft process.
• Tembec Tartas now make different pulps from juvenile and mature southern pine, the mature giving longer (2.5mm versus 2mm), coarser (38 versus 28 mg/100m) fibers and the airlaid producer can select which is best or buy a blend of the two.

TT also makes specialities which they hope may prove of interest in higher value air-lay products:
• Cotton linter pulp (1.8mm fiber with 25mg/100m coarseness) normally used for cellulose derivative and speciality paper manufacture.
• Cold caustic extracted cellulose (a mercerised 1.8mm fiber with 34 coarseness and a high curl of 25% - up from 15%) normally used for cellulose acetate production and filter papers.
• Cross-linked Cellulose (2.3mm with 40 coarseness and 24% curl) made to stay bulky when wet.

Finer fibers give better softness, strength, rewet and printability. Coarser fibers give better acquisition rate, total capacity and porosity. Debonders make the fiber behave as if its finer while cross-linkers make it behave coarser. Hardwoods like eucalyptus tend to be too short and dusty for use in airlaid.

The Future of Festooning

Roberto Rahn (KorTec GmbH) compared the festooning systems developed by Gevas and KT Industries with the original KorTec process and a new KorTec process developed specifically for diaper core weights. The technology of festooning could be either single-lane (one endless ribbon per packed block) or multi-lane (as many ribbons as the width of the packer would take, splicing being used to join up the ends to create a single ribbon.) The multi-lane system required a rotary drum folder to build the pack, whereas the single-lane system could also use swivel-arm layering or KorTec's new plaiter machine.

Current single-lane systems were poor for narrow ribbons (30mm), while the multi-lane systems were not viable for heavyweights (>450gsm diaper core). KorTec's single lane plating system which used a complex cross-folding belt mechanism to build a much larger package than possible with swivel-arm or rotary drum folders appeared to solve both problems. The cross-folder operated with a 2.4 metre length between folds compared with 1.2 m for the swivel arm and 0.9m for the rotary folder. This roughly doubled the length per pack and halved the number of folds. Despite the apparent additional mechanical complexity, Mr Rahn claimed no higher investment (in €/m 2 /year) than required for the swivel arm system, but stressed that the equipment would vary widely depending on the precise range of products it had to handle.

For the future he saw the KorTec system being able to plait product directly into a container (did he mean a shipping container?), and the possibility of cores being delivered pre-cut to diaper size in special dispensers. Spooling was, he thought, poor in comparison to festooning and would decline.

Through-Air Drying

J.A.Villalobos (Air-Therm Corp.) related the problems he had encountered starting up the first commercial air-lay machines:
• Latex bonded fluff pulp has zero strength, so the transfer from the forming wire into the dryer was very troublesome.
• Flat-bed driers with low air pressure above the bed were essential.
• Dust in the recirculated air was a real problem because it discoloured and filtered out on the surface of the web.
• Honshu filtered all the air fed to the dryer to solve this problem, others used 100% fresh air with heat recovery.
• Heat exchangers (used for heat recovery) condensed plasticisers from the latex and clogged with dust. (Prone to fires.)
• Synthetic (as opposed to metal) dryer conveyors were preferred, with dryers being longer and cooler than might be expected. (low temperatures minimised dust discoloration)
• Air distribution and temperature control across the web were crucial. Closely spaced free-wheeling rollers under the conveyor created narrow distribution slots continuously cleaned by the slightly faster moving conveyor.
• Baffles above the bed formed matching slots.
• Baffles were usable below the bed, saving some roller costs, where the fabric was reasonably well bonded.
• Air temperature uniformity requires the use of line-burners - to release heat over a larger area - and air-mixers before the hot air enters the dryer.
• The key learning: It's cheaper to spend money at the beginning than to patch up the problems later.

Ultra-Low Melt Bico

Ida Pittman (KoSa) covered the use of a lower, broader melting range polyethylene obtained from Dow's constrained geometry catalyst process. She compared a bicomponent fiber with this new polymer as a skin on a polyester core (“LM”), with their popular T255 bico that has a similar core with an ordinary polyethylene skin:
• Fiber tensile properties were similar.
• The LM surface melts at 108 0 C – giving a bonding window 20-30 0 C lower than T255
• It flows faster and recrystallises at 91 0 C, giving lower dusting levels and higher strength.
• Like T255 it still requires an adhesion promoter for best bonding to pulp. (2 den 6mm LM with adhesion promoter gives 1600 g/in CD strength compared with 250 g/in for LM alone - 12% of fiber added to pulp in a 173 gsm airlaid bonded at 140 0 C.)
• Dry and wet CD strengths for LM were almost double those for T255 (conditions as above)

In response to questions Ms Pittman said the LM fiber was sold at a premium. It would find application in the processing of heat sensitive materials or when bonding needed to occur during a drying operation. She thought the fiber would also find application in calendaring processes. She could not reveal the adhesion promoters used.

High SAP Pre-formed Cores

Mark Bolyen (MTS Inc) reviewed data on airlaid diaper cores first given at the Aarhus NTC in 1999 adding new information on MTS's work with lighter weight structures. Here the thinking seemed to be to make a much thinner (100gsm) core, highly loaded with SAP and surfaced with a low-density bico acquisition layer to help contain the SAP and increase its strength. Multiple layers of this tested as a diaper core did not perform well because the SAP-containing part of the core would not pass fluid into the ADL below it. Wider widths of the composite could however be “ZS” folded in-line to give an effective 300gsm three-layer core. Such use of multiple layers of lighter webs would, Mr Bolyen thought allow much longer run times (more metres on any given spool or “brick”).

Some core-comments:
• Thermal bonding of cores does reduce their absorbency.
• Densification increases vertical wicking but reduces capacity and acquisition rate.
• Tissue/SAP laminates and SAP fiber/Bico composites gave the highest SAP concentrations in structures that worked, but these were too expensive.

Wet-Laid/Airlaid Combinations

Jim Hanson (MTS Inc) revisited the case for adding an air-lay former to a board machine to increase throughput and save energy in the container-board market. This time test data on the properties of various laminates made from ~200gsm of wet-laid northern softwood kraft at 25% moisture with 200 gsm of unspecified airlaid were provided. The laminates had been made at various laminating pressures in a slow calendering operation at 400 0 C.

The presence of airlaid generally increased the thickness but reduced the tensiles and burst strengths. Interestingly, and here mimicking the performance of some advanced composites, the product with an airlaid layer between two wet-laids more than doubled the Tabor stiffness compared with the results from 2 layers of wet-laid alone.