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existing grades could be used, hence avoid-ing the need to specify a special grades ofpolymer solely for the meltblown process.The work was therefore concentrated onpolymers with MFR in the range 3.4–16.0 g/10 min (PGA). This is in contrast withthe special grades with higher flow rates andtherefore lower viscosities offered by somepolymer suppliers for the meltblown process.The work was successfully concluded and theresult is a very versatile process from which awide range of structures and properties canbe achieved from materials that are not con-sidered conventionally to be suitable formeltblowing.

Meltblowing processMeltblowing is a process for producing fi-brous webs or articles directly from polymersor resins using high velocity air to attenuatethe filaments [1].The meltblowing process is shown schemati-cally in Fig. 2. The polymer melt is deliveredto a linear spinneret and extruded throughan array of fine holes. The emerging fila-ments are carried away immediately by thehot, high speed, primary airflow. This airflowquickly becomes turbulent, stretching andfragmenting the filaments. Cooler, secondaryair is drawn in to the airstream and the fila-ments are partially quenched by the time

New structures for resorbable polymers

New meltblown processes have been developed successfully and demon-strate the ability to produce narrow webs made from standard resorbablepolymers suitable for a wide range of biomedical applications, especially im-plantable devices. The plant, designed and built by FET, have the correctscale and output rates for products made from special bioresorbable poly-mers. Both the manufacturers of these biomedical products and the sup-pliers of resorbable polymers are interested in these developments.

Richard W. SlackFibre Extrusion Technology (FET), Leeds/UK

Research and development An intensive R&D project was embarked todevelop a method of producing nonwovens,especially targeted at processing resorbablepolymers such as those used in sutures. Theobjective was to produce a new range of ma-terials for use in medical devices. For the initial stages of the project an earliermeltblowing line was re-engineered and pro-cessing trials carried out with PGA. Once thekey design principals had been established, apurpose made pilot line was designed andbuilt (Fig. 1). After further refinement of the equipmentand processing conditions, the range of ma-terials was extended to include all the com-mon resorbable polymers such as PGA, PLAand their copolymers to be processed. Thesemedical grade polymers are of high value andefficient procurement would be assisted if

Nonwovens can be readily made from re-sorbable polymers following a new develop-ment by FET. The new processing technologyis an adaptation of the meltblowing processand makes it possible to convert existingmedical grade, resorbable polymers intononwovens in a single processing stage. Thenew process is versatile and capable of pro-ducing nonwoven structures with a verywide range of properties. The high porosityand high internal surface area of these mate-rials gives properties that are of potentialbenefit to the clinical repair of defective tis-sues. In particular they could be used to pro-vide the following functions:• scaffold for cell migration and growth• temporary mechanical support or separa-

tion• delivery of bioactives.Combinations of the nonwovens with othermaterials such as knits or films can provideadditional functionality and enhanced me-chanical properties. Possible areas of applica-tion include wound care, adhesion barriers,staple line reinforcement, hernia repair, or-thopedics including ligament and tendon re-pair, plus repair of other soft tissues.Suitable polymers are those used already formedical resorbable sutures such as PGA, PLAand their copolymers. But these have beenhitherto difficult to process using the stan-dard meltblown process because of theirhigh molecular weight and low melt massflow rate (MFR). New developments in the design and manu-facture of meltblowing equipment by FEThave enabled a single stage process frompolymer to web to be used successfully tomanufacture webs with a wide range ofstructures and properties from standard re-sorbable polymers.

Fig. 1 Meltblowing equipment

they land on the conveyor forming a contin-uous web. The filaments intermingle andfuse to each other to give the web integrity.Downstream processing such as calendaring,point bonding or laminating can be used tofurther develop the material properties. Therandom structure of resorbable nonwovensis shown in Fig. 3. The meltblowing process is much youngerthan the spunbonding process and theachievement of uniform drawing and theminimizing of waste at the edges of the webare still issues today. FET has shown that cor-rect design and control of the air flow cansignificantly reduce edge wastage. However,when considering the production of compar-atively narrow fibrous webs from high valuepolymers, the reduction of polymer waste es-pecially at the start and end of the produc-tion run is much more important.

Properties and applicationsThe range of resorbable polymers includesPGA, PLA and copolymers PLLA and PGLA.These polymers are made from lactides (lac-tic acid) and glycolides (glycolic acid). Theseingredients are made from natural materialssuch as starch and glucose. Provided care istaken during their manufacture, these mater -ials are not rejected by the human body – onthe contrary they are absorbed as a result ofhydrolysis. The time taken for this absorptionto occur depends upon many factors includ-ing: the initial molecular weight of the poly-mer and the inevitable losses during process-ing, conversion and sterilization. An advan-tage of the meltblowing process is that itproduces the nonwoven direct from the poly-mer granules in a single operation. This min-imizes opportunity for the materials to de-grade during manufacture and is in starkcontrast to other nonwoven processes suchas needle felt where many more processingsteps are required to achieve the same result.The processing of these resorbable polymersshows similarities to that for polyester al-

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though at lower temperatures. Polymershave to be dried before melting and the re-sulting textile materials have to be kept cooland above all dry to minimize degradationbefore use. Sterilization is a further processstage before use in or near the human body.Narrow webs made from these resorbablepolymers can be produced typically withproperties within the ranges shown in thetable below. Within these ranges, it is possi-ble to select materials that are appropriatefor the applications listed in the introduction.

New materials are being discovered and de-veloped all the time with outstanding abili-ties to improve medical care both during sur-gery and in cases where after care is evenmore important.

FET ProfileFET is a good solution for the plant requiredto produce webs made from available re-sorbable polymers using the meltblown pro-cess. The in-house pilot R&D facilities reducethe cost of misplaced investment and thesefacilities enable the customer to ensure thatthe available polymers can be spun to form aweb with the target properties before theplant is purchased and the polymer supplieris contracted.Much of the experience with resorbablepolymers has been gained already while sup-plying both mono- and multifilament plantsfor making sutures. All such plants are cus-tom-engineered to meet the requirements,both of the customer and the end-use appli-cation.

Reference[1] Artul Dahiya et al., web.utk.edu., April 2004

Fig. 2 Schematic diagram of

the meltblowing process

Fig. 3 Filament structure in PGA nonwovens

Fig. 4 Examples of fi lamentsize distributions formelt blow webs

Property Lower end Higher end of range of range

Filament size [μ] 2 30 Density [mg/ml] 20 250 Web thickness [mm] 0.2 10

The distribution filament sizes in meltblownnonwovens can be controlled by selection ofprocessing conditions. Filament diameterscan be as fine as 2 microns, which for PGA isequivalent to 0.04 denier. A selection of typ-ical filament size distributions are shown inFig. 4.