The Fourth International Conferenceon Woodfiber-Plastic Composites

Investigations of Species Effectsin an Injection-Molding-Grade,Wood-Filled Polypropylene

Mark J. BergerNicole M. Stark

AbstractThe utilization of wood flour as a filler in the

thermoplastics market is continually increasing.Thermoplastics processors are more often usingwood flour as a viable filler in their resin systems. Evenwith this growth, much remains to be learned aboutwood flour’s use, application, and its effects in apolymer composite mixture. Considering the qualityrequirements from today’s plastics processors, mate-rial-associated variables are important to understandand control. Being prepared for the potential variabil-ity, the processor can make an educated selection of

Berger:Technical Sales Representative, American Wood Fibers, Inc.,Schofield, Wisconsin

Stark:Chemical Engineer, Performance Designed Composites Group,USDA Forest Serv., Forest Prod. Lab., Madison, Wisconsin

The authors acknowledge the assistance of the Solid WasteRecovery Research Program, Univ. of Wisconsin, Madison,Wis.; the USDA Forest Serv., Forest Prod. Lab., Madison, Wis.;and Solvay Polymers, Inc., Deer Park, Texas.

materials, thereby bringing a quality product to themarket. A joint project with the USDA Forest Prod-ucts Laboratory explored some of the differences inmechanical properties of wood flour-filled polypro-pylene, resulting from variations in particle size dis-tribution and species. The wood species used in thisstudy were ponderosa pine, loblolly pine, maple, andoak. The mechanical properties tested were impactstrength, flexural strength, tensile strength, moldshrinkage, and specific gravity There were definitedifferences in the performances of various species ofwood fillers in a polypropylene matrix. This paperpresents the data and conclusions of the speciesvariation part of that study.

IntroductionWood flour, a unique filler in thermoplastics, is

building a reputation for providing mechanical prop-erty improvements to the plastics processor com-pared to unfilled thermoplastics. Research conductedby the USDA Forest Products Laboratory (FPL) andothers has shown the value of using wood flour

Berger and Stark • 19

derived from wood by-products as a reinforcing fillerin thermoplastics (3). The thermoplastics industry’sutilization of wood fillers is growing, and considerablecommercial activity has already begun.

After decades of using wood flour in the thermosetplastics industry thermoplastic processors have dis-covered the benefits of wood flour as a filler. Variouscommercially available products, including Trex™ ,Fibrex ™, Strandex™, Woodcom™, and others, areproving the usefulness of this versatile filler. Theemergence of these products and others shows thatthe use of wood flour has grown from extruded linealsto compounded pellets utilized in injection-moldingprocesses. Resins utilizing wood flour range frompolyolefins and PVC, to polystyrene, ABS, and EVA.Understanding the uses and benefits of wood flour asa filler will help the market to potentially expand toother unfilled resins.

Research at the FPL has shown that wood flour hassimilar performance characteristics compared to talcand other mineral fillers commonly used in ther-moplastics (2). One notable exception sets woodflour apart from mineral fillers: specific gravity. Woodflour-filled polypropylene can offer similar perform-ance with 10 to 20 percent less weight. Currentlyabout 200 million pounds of talc are used each yearin the United States. If wood flour replaces only 10percent of this market, 20 million pounds of addi-tional wood by-products that are normally discardedcan be recycled each year.

Wood represents the largest available natural andrenewable fiber resource, estimated at over 300,000million m3 worldwide(4). This equates to approximately180 million metric tonnes of wood. Each year, hundredsof species of wood are utilized in commercial prod-ucts, producing a wide mixture of post-industrialwood by-products. In 1993 alone, there was an esti-mated 39 million metric tonnes of wood waste recy-cled and sold to a variety of industries in the UnitedStates. Considering the total disposal burden, esti-mated at 69 million metric tonnes, an additional 30million metric tonnes of waste remains as a potentialresource available for processing wood fillers for theplastics industry (5).

Most of the wood filler research has been donewith standard commercially available grades of woodflour. Wood flour is produced from post-industrialby-products such as cut-offs, planer shavings, andsawdust. This material is processed and separatedinto various blended particle size distributions orgrades. Wood flour is therefore available in large

economical quantities, easily differentiated by speciesand particle size distributions.

There are many key variables to consider whenutilizing wood flour as a filler. Among them are:

● moisture content;● purity (virgin wood vs. contaminated wood);● particle size distribution; and● species.Specifications are normally set by the manufac-

turer. While the first three variables can be controlledby raw material selection and the manufacturingprocess, species availability is often regionally influ-enced. This creates a geographic variable for plasticsprocessors to consider.

To date, most research has been conducted with alimited number of wood species, and only a fewgrades of relatively wide particle size distributions. Tofully exploit the use of these wood fillers, and expandtheir use in the plastics industry the full effect ofwood species and particle size distribution must bedetermined. These factors are important because:1. Wood species availability can be regional, depend-

ing upon the primary wood manufacturer’s end-products.

2. In general, the finer the wood flour, the moreprocessing is involved and the higher the cost.

This research had the objectives to determine:● the effects of different wood flour species in

injection-molding-grade polvpropylene; and● the effect of particle size, using ponderosa pine

wood flour in an injection-molding grade ofpolypropylene.

This paper examines the effect of species variation. Aconcurrent paper presented at this conference, givenby Nicole Stark (pp. 134-143) will examine theeffects of particle size (7).

Experimental procedure

MaterialsThe polypropylene used for all formulations, For-

tilene 3907, was an injection-molding-grade homo-polymer manufactured by Solvay Polymers, Inc. (DeerPark Texas), with a 36.5 melt flow index.

The wood flours used were standard 40-meshgrades manufactured by American Wood Fibers, Scho-field, Wisconsin:

● AWF-4020—ponderosa pine;● AWF-4010—maple;● AWF-404 1—loblolly pine, and● AWF-403l—oak.

20 • Berger and Stark

Formulations combined polypropylene with quanti-ties of wood flour ranging from 20 to 60 percent byweight (wt%).

Processing conditionsCompounding was performed in a 32-mm Davis

Standard (Pawcatuck, Conn.) corotating, intermesh-ing twin-screw extruder. The extruder has seg-mented screws with a length-to-diameter ratio of 32to 1. There are eight electrically heated, water-cooledbarrel sections, with vent zones in the fourth andseventh sections. Power is supplied by a 15 -hp DCdrive. A four-hole strand die is fitted to the dischargeend. All materials were compounded with the samescrew configuration and the melt temperatures werekept relatively equal at 190°C. Discharge rates werebetween 20 and 40 kg/hr. Extrudate was cooled in awater trough and cut into pellets.

A 60 percent wood flour-filled polypropylene mas-ter batch was pelletized and used for the 60 percentfilled-compounds. This 60 percent filled masterbatch was subsequently used for preparing the otherformulations. All pelletized materials were dried inan oven at 105°C for at least 24 hours before injectionmolding into standard ASTM specimens. The injec-tion molder was a 33-ton Cincinnati-Milacron (Bata-via, Ohio) reciprocating screw-type injection mold-ing press, using a 196°C melt temperature. Theheavier loadings of wood flour, in 50 to 60 percentwood flour-filled polypropylene, resulted in higherviscosities (lower melt flow index). Therefore, higherinjection molding tonnages were necessary to moldthese samples compared to the filled resins withlower wood flour loadings.

FIGURE 1 . —Notched impact energy versus filler content.

TestingAll testing was conducted according to ASTM

standards for plastics (1). The following ASTM testmethods were used:

●

●

●

●

b

Izod impact (notched andunnotched)—-D 256flexural properties—D 790tensile properties—D 638shrinkage rates—D 955heat deflection (264 psi)—D 648

ResultsFigures 1 to 9 are graphs of the performance curves

comparing the various species as fillers. These figuresprovide data on Izod impact (notched and unnotched),maximum flexural strength, flexural modulus of elas-ticity, maximum tensile strength, tensile modulus ofelasticity, percent tensile elongation, mold shrinkage,

FIGURE 2. —Unnotched impact energy versus fillercontent.

FIGURE 3. —Maximum flexural strength versus percentwood filler.

Berger and Stark • 21

FIGURE 4. —Flexural MOE versus percent wood filler.

FIGURE 5. —Maximum tensile strength versus percentwood filler.

FIGURE 7. —Percent tensile elongation versus percentwood filler.

FIGURE 8. —Mold shrinkage versus percent wood filler.

FIGURE 6. —Tensile MOE versus percent wood filler. FIGURE 9. —Heat deflection temperature versus per-cent wood filler.

22 • Berger and Stark

and heat deflection temperature tests. Table 1 pro-vides a summary of the properties and performanceof each wood-filled polypropylene formulation com-paring each species and unfilled polypropylene.

Discussion

Impact performanceThe ponderosa pine species (4020) exhibited the

best performance for both notched and unnotchedimpact tests, outperforming both hardwood species(oak and maple). All species provided an increasingnotched impact strength with increasing wood fillercontent, with a maximum strength at 40 to 50 per-cent filler loading. The loblolly pine showed the

lowest impact strengths, possibly showing less com-patibility with the polypropylene used. A discussedin other literature (2,6), fillers in general (includingwood flour) decrease the unnotched impact strengthof polypropylene.

Flexural propertiesAlll wood flour species provided an increase in the

flexural strength over the unfilled polypropylene,which supports how these materials are known tobehave. The 4041-60 formulation was the only ma-terial not to improve the stiffness of polypropylene.Figure 4 shows a linear relationship between theincrease in flexural MOE and increasing wood fillercontent. The curves for each species in Figure 3

TABLE 1. — Summary of properties and performance.

Berger and Stark • 23

follow a similar pattern, showing a maximum flexuralstrength in the range of 30 to 40 percent filler.Hardwoods appeared to outperform softwoods inflexural strength.

Tensile propertiesOak and maple provided the optimum tensile prop-

erties. In general, the curves indicate an increase intensile strength with wood filler loading, and a maxi-mum tensile MOE obtained in the 50 percent loadingrange. Loblolly pine provided the least improvementin tensile strength. All wood flours showed a decreasein tensile elongation with increased loading.

Mold shrinkageValues reported for 4020-40 and 4010-40 agree

with published sources (2). All wood fillers that weretested provided reduced mold shrinkage with in-creased filler percentage. Again, loblolly pine provedthe least effective in enhancing this property, showingan average of 15 percent higher shrinkage than theother wood species tested. Mold shrinkage seems tolevel off in the 0.3 percent range at the highest woodfiller loadings. Compared to unfilled polypropylene,wood proves to be an effective means of dramaticallyreducing mold shrinkage.

Heat deflection temperatureIn general, wood fillers improve the heat deflection

of unfilled polypropylene. Heat deflection tempera-ture increases with increasing wood-filler loading,with a leveling-off appearing at the 60 percent fillerlevel. Oak and maple provide the greatest heat deflec-tion, with 25 to 45 percent improvement comparedto the softwood species tested.

ConclusionsThe following conclusions can be made for the

wood flour species compared as fillers in polypropy-lene in this study:1. Wood species is an important, “controllable” vai-

able to consider when utilizing wood flour as a filler.2. Generally increasing filler loading results in:

increased notched Izod impact-strength;decreased unnotched Izod impact strength;increased flexural and tensile MOE;decreased tensile elongation percentage;decreased mold shrinkage; andincreased heat deflection temperature.

3. Wood flour provides an excellent design option asa filler when impact properties are not a maindesign consideration.

24 • Berger and Stark

4.

5.

Hardwood flours provide an improvement in ten-sile properties and heat deflection temperatureover softwood flours.Compared to other species, ponderosa pine woodflour provides the maximum blend of mechanicalproperty enhancements when used as a filler inFortilene 3907 polypropylene.

This study was able to effectively show there aredifferences in mechanical properties of wood flour-fllled polypropylene, depending on the species ofwood flour. Availability of wood species will varygeographically both in the United States and aroundthe world. This is an important consideration whendesigning a product utilizing wood flour as a filler.With the basic understanding of the variables associ-ated with wood flour as a filler, including species andimprovements in quality, consistency can be success-fully obtained.

This research defined mechanical property curvesdependant on the percentage of wood flour loading.It compared polypropylene filled with various speciesof wood at various wood filler loadings. The datashould be helpful to designers looking for the mostappropriate loading when utilizing wood flour as afiller.

This research also validates past work done onwood flour-filled polypropylene, proving the poten-tial of wood flour as a filler in competing with othermineral fillers such as talc and calcium carbonate. Thepotential for wood flour to gain market share as athermoplastic filler shows promise for utilizing morewood by-products, improving our ability to recycleworldwide. It allows us to fully utilize a renewableresource, extending the use of nonrenewable poly-mers.

Finally it is important to understand the researchdone here was not concerned with optimizing theproperties of these wood-filled formulations. Nocoupling agents, additives, or modifiers were addedto improve the properties tested. Many questions stillremain from plastics processors on wood-filled poly-mers, such as UV stability, flammability, colorants andpigmentation, aging, and appropriate coupling agents,additives, and modifiers. The data presented is anexcellent baseline to be used for further research onthe aforementioned questions. Continuing this re-search and expanding it will improve the know-ledgebase of wood flour as a filler in thermoplastics.

Literature cited

Berger and Stark • 25

Fourth International Conference onWoodfiber-Plastic Composites

May 12–14, 1997The Madison Concourse HotelMadison, Wisconsin

Sponsored by the USDA Forest Service in cooperation with theAmerican Plastics Council, the University of Wisconsin, theUniversity of Toronto, the Cellulose, Paper, and Textile Division ofthe American Chemical Society, and the Forest Products Society.

Forest Products Society2801 Marshall Court Madison, WI 53705-2295phone: 608-231-1361fax: 608-231-2152www.forestprod.org

The opinions expressed are those of the authors and do notnecessarily represent those of the USDA Forest Service orthe Forest Products Society.

Copyright © 1997 by the Forest Products Society.Proceedings No. 7277ISBN 0-935018-95-6

All rights reserved. No part of this publication may bereproduced, stored in a retrieval system, or transmitted, inany form or by any means, electronic, mechanical, photo-copying, recording, or otherwise, without prior writtenpermission of the copyright owner. individual readers andnonprofit libraries are permitted to make fair use of thismaterial such as to copy an article for use in teaching orresearch. To reproduce single or multiple copies of figures,tables, excerpts, or entire articles requires permission fromthe Forest Products Society and may require permissionfrom one of the original authors.

Printed in the United States of America.

9711500