Process Technology. Process-

ing of natural fiber-reinforced

plastics in a single energy-op-

timized production process has

significant benefits: the sensi-

tive natural fibers are treated

gently, emission of odors dur-

ing processing and from the

finished parts is reduced –

and, since the intermediate step of pelletizing can be dispensed with when injec-

tion molding, energy savings of over 40 % compared to a two-step production

process are possible.

Energy-Efficient Processingof Natural Fiber-Reinforced

Plastics

No pellets, no semi-finished product – natural fiber-reinforced plastic parts can be produced directly by means of an energy-efficient process

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ERWIN BÜRKLE

GERHARD SCHEEL

LARS DARNEDDE

In recent years, growing environmen-tal awareness and the rising cost of rawmaterials have accelerated the search

for alternative materials. Renewable CO2-neutral materials that can help combatclimate change are of special interest. Atthe same time, the energy efficiency ofprocessing techniques is slowly undergo-ing a rethinking, not least of all becauseof increasing energy costs.

To date, energy losses in manufactur-ing processes have been accepted as a nec-essary evil for high-value production.This is true especially for processes thatinvolve changes in the material’s state.However, as KraussMaffei Technologies

GmbH, Munich, Germany, has estab-lished in investigations, depending on theparticular part, energy savings of up to40 % can be achieved already throughoptimized sequencing based on processintegration and shortened processchains. Thus, the goal is to replace or eveneliminate energy-intensive processes.

Comparison of ProcessingTechniques

When manufacturing components fromnatural fiber-reinforced materials, directprocesses – processes involving a singleheating step – are gaining in importance.In addition to greater energy efficiency,single-step processes offer the additionalbenefit of reduced emissions. To bringcompounding and part forming togeth-er, KraussMaffei fell back on two corecompetences and combined the continu-ous extrusion process with the discontin-uous injection molding process in a sin-gle machine.

Usually, polyolefins and PVC are em-ployed as the matrix when producingnatural fiber-reinforced compositesbased on thermoplastics. Both have a lowmelting point, which minimizes the riskof thermally degrading the temperature-sensitive natural fibers. Currently, themost important natural fibers for tech-nical applications are hemp, jute, kenaf,sisal and coconut fiber as well as variouswood fibers. For processing on the ex-truder, the fibers are generally present inthe form of the flour, chips, individualfibers, continuous fibers or pellets. Thematerial’s properties are adjustedthrough the addition of additives, for in-stance, to improve bonding of the polarnatural fibers to the nonpolar polyolefins[1]. Other common additives include UVstabilizers, impact modifiers and colormasterbatches.

Various processes are available to pro-duce fiber-reinforced resins, with a basicdistinction being made between one- andtwo-step processes (Fig. 1).

Translated from Kunststoffe 2/2009, pp. 39–44Article as PDF-File at www.kunststoffe-international.com; Document Number: PE110031

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Two-step Processes

With the aid of a mixing tool, an agglom-erate is created discontinuously in a dou-ble-jacketed, heated mixing chamber andsubsequently transferred to a coolingmixer where it is cooled and reduced insize. This process is suitable for both PVCand polyolefins.

As an alternative, pellets can be pro-duced continuously on a co-rotatingtwin-screw extruder with closely inter-meshing, co-rotating screws supplied bygravimetric metering units. Screw flightsopen in the axial direction ensure effec-tive degassing of the melt, allowing thenatural fibers to be added without pre-drying. Subsequent pelletizing takesplace on underwater or chilled-air pal-letizers.

Compared to a heating/cooling mixer,the intense mixing of the raw materialsimproves dispersion quality considerably.

Since the shear energy introduced is high-er, co-rotating twin-screw extruders arenot suitable for processing PVC as thematrix material.

The pellets produced by means of thesetwo methods can be processed further us-ing conventional techniques. The addi-tional thermal stress on the natural fibersmust, of course, be kept in mind.

One-step Processes

The combination of pre-drying naturalfibers and immediate subsequent pro-cessing on a counterrotating twin-screwextruder represents another alternative.The chamber principle employed in theextruder permits build-up of high pres-sure with little introduction of shear en-ergy. Because of the poor degassing char-acteristics of this equipment, the naturalfibers are pre-dried to a maximum resid-ual moisture level of 1 %. Since the ex-truder runs continuously, this method is

suitable for products such as sheet andprofiles.

If the individual raw materials are fedinto a co-rotating twin-screw extruder,production of natural fiber-reinforcedcompounds and direct processing can al-so occur in a single operation – depend-ing on the equipment employed, eitherinto profiles and sheet through use of amelt pump and attached extrusion die, orinto molded parts through use of an in-jection molding compounder (IMC), acombination with an intermediate meltaccumulator, a special injection and aclamping unit.

Energy Comparison forthe Processes

To compare individual processes with oneanother in terms of energy consumed, theheat energy needed to produce a WPCprofile (wood plastic composite) was de-termined. The profile is produced frompolypropylene containing 60 % woodfibers with an assumed moisture contentof 12 %. Throughput is 200 kg/h.

In the following calculations, the ma-terials data for the wood fibers were in-corporated in two ways: once with the da-ta for kiln-dried wood, and then with thematerials data for water to account formoisture contained in the wood. It wasfurther assumed for the calculation thatthe moisture is driven entirely out of thewood fibers during processing, since thewater vaporizes at 100°C under the pres-sure conditions encountered in the ex-truder.

The product streams then result fromthe assumptions:

KraussMaffei Technologies GmbHKrauss-Maffei-Str. 2D-80997 MunichGermanyTel. +49 89 8899-3092Fax +49 89 8899-2592

KraussMaffei Berstorff GmbHAn der Breiten Wiese 3–5D-30625 HanoverGermanyTel. +49 511 5702-0Fax +49 511 5619-16

info@kraussmaffei.comwww.kraussmaffei.com

Manufactureri

Raw materials (polymer, natural fibers, additives)Raw materials (polymer, natural fibers, additives)

Profile, film, sheetProfile, film, sheet

Heat-coolmixer

Agglomerate Pellets Pellets Injectionmolding

compounder

Pre-drying of natural fibers Co-rotating twin-screw extruder

Counterrotating twin-screw extruder Melt pump Injection molding machine

Molded partMolded part

Fig. 1. When producing parts from natural fiber-reinforced resins, a basic distinction is drawnbetween one- and two-step processes. The latter involve pelletizing as an intermediate step (figures: KraussMaffei)

© Kunststoffe

Melt temperature

3,000

kJ/kg

2,000

1,500

1,000

500

00

Spec

ific

enth

alpy

50 100 150 200 250 °C 300

PolypropyleneWoodWater

Enthalpy Curves

Fig. 2. The specificenthalpies of the rawmaterials used de-pend on the tempera-ture [2, 3]. The maxi-mum melt tempera-ture is 200°C for pro-duction of both thecompound and profile

© Kunststoffe

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■ polypropylene 80 kg/h,■ wood fibers 120 kg/h,■ water 16.4 kg/h.The maximum melt temperature is200°C for production of both the com-pound and profile. The specific energy forthe raw materials used depends on thetemperature (Fig. 2); the amount of heatneeded to warm the raw materials fromthe infeed temperature (25°C) to the pro-cessing temperature can then be calculat-ed on the basis of this information:

where Δh = change in specific enthalpy.

hmQ Δ= ..

To produce a WPC profile using the two-step process, identical values of 61.7 kW forthe required heat energy are obtained forthe two processes considered. Assumingthat the compound absorbs no moistureduring intermediate storage, it is only nec-essary to warm the materials to the process-ing temperature of 200°C. Compared tothe single-step process, the higher energyrequirement of the counterrotating twin-screw extruder results from the fact that thestarting material is cold and must be melt-ed once again (Fig. 3).

When using direct extrusion to pro-duce WPC profiles, the wood fibers fed

into the counterrotating twin-screw ex-truder are pre-dried to a residual mois-ture content of about 1 %. The remain-ing raw materials are then heated in theextruder to the processing temperature.It is assumed that the pre-heated fibersare fed directly into the hopper on the ex-truder. Taking the melt pump needed forpressure buildup into consideration, theheat energy of 36.7 kW needed herematches that consumed by the extruderin the two-step process (Fig. 4).

It can thus be seen that, compared tothe two-step extrusion process, direct ex-trusion requires about 40% less heat en-

Raw materials (polymer, natural fibers)Raw materials (polymer, natural fibers)

WPC profileWPC profile

Heat-coolmixer

Co-rotatingtwin-screwextruder

Counterrotatingtwin-screwextruder

Counterrotatingtwin-screwextruder

61.7 kW0.3088 kWh/kg

61.7 kW0.3088 kWh/kg

Agglomerate

36.7 kW 36.7 kW

25.0kW 25.0kW

Pellets

Fig. 3. Production of WPC profiles by a two-step process consumes moreenergy than a one-step process. The higher energy consumption of thecounterrotating twin-screw extruder (compare Fig. 4) results from the factthat cold starting material must be melted once again

© Kunststoffe

Raw materials (polymer, natural fibers)Raw materials (polymer, natural fibers)

WPC profileWPC profile

Pre-drying of wood fibers

Co-rotatingtwin-screw

extruder

Counterrotatingtwin-screw

extruderMelt pump

36.7 kW0.1835 kWh/kg

36.7 kW0.1835 kWh/kg

14.8 kW 35.9 kW

21.9 kW 0.85 kW

Fig. 4. Compared with the two-step process, direct extrusion of WPC pro-files requires about 40% less heat energy. Depending on the efficiency ofthe manufacturing processes, the actual energy required will be greater.Since pre-drying of fibers is a low-efficiency process, it can be concludedthat direct extrusion is the most economical process

© Kunststoffe

Natural fibers Polymer

AdditivesVacuum

Atmosphere AtmosphereZSFE

Pressurebuildup Outgassing Venting Venting Plasticating

Feeding ofsolid components

Incorporation ofnatural fibers

Feeding ofnaturalfibers

M M

MM

Fig. 5. Schematic illustration:extrusion of natural fiber-con-taining compounds beginswith gravimetric metering ofthe polymer and additives,and ends with pressure build-up prior to pelletizing or themelt proceeding directly to amelt pump for further pro-cessing

© Kunststoffe

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ergy, energy which must be supplied inthe form of heat or power input. Depend-ing on the efficiency of the different man-ufacturing processes, the actual energy re-quired will be greater. Given that pre-dry-ing of fibers is generally a low-efficiencyprocess, it can be concluded that, in termsof energy consumption, direct extrusionis the more economical process.

Processing Method 1: Direct Extrusion

Direct processing of natural fiber-rein-forced compounds occurs in stages on a

co-rotating twin-screw extruder (Fig. 5).After gravimetric metering of the poly-mer and additives, the solid componentsare melted before a twin-screw feedingmechanism introduces the natural fibersinto the melt. The fibers are also meteredgravimetrically. Air and moisture arevented to the atmosphere by flowing inthe opposite direction. Once the naturalfibers have been incorporated into themelt, the moisture that is released can bevented at atmospheric pressure in a de-gassing zone. Finally, a vacuum degassingzone permits complete removal of low-molecular-weight components and any

residual moisture. Next, the pressureneeded for the downstream pelletizermust be built up – or the melt proceedsdirectly into a melt pump for further pro-cessing.

The characteristics of co-rotating twin-screw extruders offer several benefits forprocessing sensitive natural fibers, includ-ing ■ intense exchange of heat and material

as the result of continuous movementof the melt from screw to screw (Fig.6),

■ speed-independent throughput,■ almost complete self-cleaning and a

narrow residence time distribution asa consequence of the closely inter-meshing screw arrangement,

■ effective outgassing of the meltthrough use of screw lights open in theaxial direction and

■ the ability to provide an optimal con-figuration to meet the processing re-quirements thanks to the modularhousing and screw concept.

The ZE-R Series (manufacturer: Krauss-Maffei Berstorff) with its diameter ratioof 1.74 is ideally suited for natural fiberextrusion. Its deeply cut screw flights fa-cilitate removal of the moisture releasedfrom fibers in the form of water vapor.

Fig. 6. The character-istics of co-rotatingtwin-screw extrudersoffer several benefitsfor processing sensi-tive natural fibers

© Kunststoffe

Fig. 7. The natural fibers are incorporated into the melt with the aid ofkneading blocks. These are screw elements consisting of several coaxialkneading disks rotated (offset) with respect to one another

The following applies for the enthalpy change resulting from thermaland dissipation energy when considering the conversion of energy formaterial compounding

and for the specific energy conversion

By way of example, for a PP-flax composite with 20 wt.-% flax, the fol-lowing results for a two-step process involving intermediate pelletizing

For the machine configuration investigated, the following measuredvalues are obtained for the compounding step

Δh1 = 0.17 kWh/kg

and for injection molding

Δh2 = (0.34 + 0.32) = 0.65 kWh/kg

The following is then obtained for the two-step process

In contrast, the following holds for the one-step compounding/injectionmolding process

kgkWhq EP /49.032.017.0 =+=.

kgkWhq ZP /83.0)32.034.0(17.0 =++=.

21 hhqZP

Δ+Δ=.

hmQ

q Δ== ...

hmQQQ DisH Δ=+=....

One-step Compounding / Injection Molding Process

Continuous process (co-rotating twin-screwextruder)

Discontinuous injectionmolding process

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These extruders permit processing of nat-ural fibers with a moisture content of upto 12 % without the need for pre-drying.

Normally, the natural fibers are incor-porated into the melt with the aid ofkneading blocks (Fig. 7). These are screwelements consisting of several coaxialkneading disks rotated (offset) with re-spect to one another. Depending onprocess requirements, disk width, offsetangle and conveying direction vary. Be-cause of the narrow gaps between them,the closely intermeshing kneading disksgenerate high shear rates that acceleratedegradation of the natural fibers.

As an alternative, multi-process ele-ments that operate on a mixing chamberprinciple are available to incorporate thefibers (Fig. 8). These mixing chambers arebased on elements that convey in the for-ward and rearward directions, and the di-ameters of which are so small that they canbe used in parallel next to one another. Inaddition, openings in the screw flights en-sure more intensive mixing of the melt.The multi-process elements prevent in-

troduction of excessive amounts of shearenergy, thus protecting the natural fibers.

Processing Method 2: Direct Injection Molding

Combination of compounding andmolding in a single machine concept(IMC, Fig. 9) not only reduces the ther-mal load on the materials to be processed.It also raises the energy efficiency of theentire process (see box on p. 28).

Compared to the two-step process, theone-step process employing an injectionmolding compounder reduces energy con-sumption by about 40 %. Moreover, themore energy-efficient,continuously oper-ating co-rotating twin-screw extruder re-places the energy-intensive step of plasti-cating by means of a reciprocating screw.

Conclusions

Less is often more, since with this con-cept KraussMaffei is able to serve the en-tire value added chain with only two pro-

cessing techniques – from conversion ofthe raw materials into compounds orblends through the semi-finished prod-uct to the molded part. Elimination ofthe intermediate step of pelletizing pro-vides economical as well as technicalbenefits for both extrusion and injectionmolding. In addition to the approxi-mately 40 % lower energy consumption,the gentle processing of the sensitive nat-ural fibers as well as reduced noise emis-sions during production and later in themolded part can be mentioned as exam-ples. This opens up new perspectives forprocessors and users alike as they seekeco-friendly products. ■

REFERENCES

1 Endres, H.-J.; Hausmann, K.; Helmke, P.: Unter-suchung des Einflusses unterschiedlicher Haftver-mittler und Haftvermittlergehalte auf PP-Holzmehl-Compounds. KGK Kautschuk GummiKunststoffe (2006) 7/8, pp. 399–404

2 Frangi, A.: Brandverhalten von Holz-Beton-Ver-bunddecken. Research Report EidgenössischeTechnische Hochschule Zürich, 2001

3 GriguIl, U.: Zustandsgrößen von Wasser undWasserdampf in SI-Einheiten. Springer VerlagBerlin, Heidelberg, New York 1989

THE AUTHORS

DR.-ING. ERWIN BÜRKLE, born in 1942, is Manag-er – Advanced Development, New Technologies andInjection Molding Process Technology at KraussMaf-fei Technologies GmbH, Munich, Germany.

DIPL.-ING LARS DARNEDDE, born in 1979, is em-ployed as a process engineer for renewable resourcesin the plastics engineering development departmentat KraussMaffei Berstorff GmbH, Hanover, Germany.

DIPL.-ING. GERHARD SCHEEL, born in 1965, is re-sponsible for processing developments in the plasticsengineering development department at KraussMaf-fei Berstorff GmbH, Hanover.

Fig. 8. As an alterna-tive, multi-processelements that operateon a mixing chamberprinciple are avail-able to incorporatethe fibers (variousconfigurationsshown)

Fig. 9. Combination of compounding and molding in a single machine concept reduces the thermal load on the materials to be processed and also raisesthe energy efficiency of the entire process. Compared to two-step processing, both direct injection molding (IMC, left) and direct extrusion (ZE 60 R UTX,right) lower energy consumption by about 40 %

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