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

The Fourth International Conference on Woodfiber-Plastic Composites

Mark J. Berger Nicole M. Stark

Abstract The utilization of wood flour as a filler in the materials, thereby bringing a quality product to the thermoplastics market is continually increasing. market. A joint project with the USDA Forest Prod- Thermoplastics processors are more often using ucts Laboratory explored some of the differences in wood flour as a viable filler in their resin systems. Even mechanical properties of wood flour-filled polypro- with this growth, much remains to be learned about pylene, resulting from variations in particle size dis- wood flour’s use, application, and its effects in a tribution and species. The wood species used in this polymer composite mixture. Considering the quality study were ponderosa pine, loblolly pine, maple, and requirements from today’s plastics processors, mate- oak. The mechanical properties tested were impact rial-associated variables are important to understand strength, flexural strength, tensile strength, mold and control. Being prepared for the potential variabil- shrinkage, and specific gravity There were definite ity, the processor can make an educated selection of differences in the performances of various species of wood fillers in a polypropylene matrix. This paper presents the data and conclusions of the species Berger: variation part of that study. Technical Sales Representative, American Wood Fibers, Inc., Schofield, Wisconsin Introduction Stark: Wood flour, a unique filler in thermoplastics, is Chemical Engineer, Performance Designed Composites Group, building a reputation for providing mechanical prop- USDA Forest Serv., Forest Prod. Lab., Madison, Wisconsin erty improvements to the plastics processor com- The authors acknowledge the assistance of the Solid Waste Recovery Research Program, Univ. of Wisconsin, Madison, pared to unfilled thermoplastics. Research conducted Wis.; the USDA Forest Serv., Forest Prod. Lab., Madison, Wis.; by the USDA Forest Products Laboratory (FPL) and and Solvay Polymers, Inc., Deer Park, Texas. others has shown the value of using wood flour

Berger and Stark • 19 derived from wood by-products as a reinforcing filler economical quantities, easily differentiated by species in thermoplastics (3). The thermoplastics industry’s and particle size distributions. utilization of wood fillers is growing, and considerable There are many key variables to consider when commercial activity has already begun. utilizing wood flour as a filler. Among them are: After decades of using wood flour in the thermoset ● moisture content; plastics industry thermoplastic processors have dis- ● purity (virgin wood vs. contaminated wood); covered the benefits of wood flour as a filler. Various ● particle size distribution; and commercially available products, including Trex™ , ● species. Fibrex ™, Strandex™, Woodcom™, and others, are Specifications are normally set by the manufac- proving the usefulness of this versatile filler. The turer. While the first three variables can be controlled emergence of these products and others shows that by raw material selection and the manufacturing the use of wood flour has grown from extruded lineals process, species availability is often regionally influ- to compounded pellets utilized in injection-molding enced. This creates a geographic variable for plastics processes. Resins utilizing wood flour range from processors to consider. polyolefins and PVC, to polystyrene, ABS, and EVA. To date, most research has been conducted with a Understanding the uses and benefits of wood flour as limited number of wood species, and only a few a filler will help the market to potentially expand to grades of relatively wide particle size distributions. To other unfilled resins. fully exploit the use of these wood fillers, and expand Research at the FPL has shown that wood flour has their use in the plastics industry the full effect of similar performance characteristics compared to talc wood species and particle size distribution must be and other mineral fillers commonly used in ther- determined. These factors are important because: moplastics (2). One notable exception sets wood 1. Wood species availability can be regional, depend- flour apart from mineral fillers: specific gravity. Wood ing upon the primary wood manufacturer’s end- flour-filled polypropylene can offer similar perform- products. ance with 10 to 20 percent less weight. Currently 2. In general, the finer the wood flour, the more about 200 million pounds of talc are used each year processing is involved and the higher the cost. in the United States. If wood flour replaces only 10 This research had the objectives to determine: percent of this market, 20 million pounds of addi- ● the effects of different wood flour species in tional wood by-products that are normally discarded injection-molding-grade polvpropylene; and can be recycled each year. ● the effect of particle size, using ponderosa pine Wood represents the largest available natural and wood flour in an injection-molding grade of renewable fiber resource, estimated at over 300,000 polypropylene. 3 million m worldwide(4). This equates to approximately This paper examines the effect of species variation. A 180 million metric tonnes of wood. Each year, hundreds concurrent paper presented at this conference, given of species of wood are utilized in commercial prod- by Nicole Stark (pp. 134-143) will examine the ucts, producing a wide mixture of post-industrial effects of particle size (7). wood by-products. In 1993 alone, there was an estimated 39 million metric tonnes of wood waste recy- Experimental procedure cled and sold to a variety of industries in the United States. Considering the total disposal burden, esti- Materials mated at 69 million metric tonnes, an additional 30 The polypropylene used for all formulations, For- million metric tonnes of waste remains as a potential tilene 3907, was an injection-molding-grade homo- resource available for processing wood fillers for the polymer manufactured by Solvay Polymers, Inc. (Deer plastics industry (5). Park Texas), with a 36.5 melt flow index. Most of the wood filler research has been done The wood flours used were standard 40-mesh with standard commercially available grades of wood grades manufactured by American Wood Fibers, Scho- flour. Wood flour is produced from post-industrial field, Wisconsin: by-products such as cut-offs, planer shavings, and ● AWF-4020—ponderosa pine; sawdust. This material is processed and separated ● AWF-4010—maple; into various blended particle size distributions or ● AWF-404 1—loblolly pine, and grades. Wood flour is therefore available in large ● AWF-403l—oak.

20 • Berger and Stark Formulations combined polypropylene with quanti- Testing ties of wood flour ranging from 20 to 60 percent by All testing was conducted according to ASTM weight (wt%). standards for plastics (1). The following ASTM test methods were used: Processing conditions ● Izod impact (notched andunnotched)—-D 256 Compounding was performed in a 32-mm Davis ● flexural properties—D 790 Standard (Pawcatuck, Conn.) corotating, intermesh- ● tensile properties—D 638 ing twin-screw extruder. The extruder has seg- ● shrinkage rates—D 955 mented screws with a length-to-diameter ratio of 32 b heat deflection (264 psi)—D 648 to 1. There are eight electrically heated, water-cooled Results barrel sections, with vent zones in the fourth and Figures 1 to 9 are graphs of the performance curves seventh sections. Power is supplied by a 15 -hp DC comparing the various species as fillers. These figures drive. A four-hole strand die is fitted to the discharge provide data on Izod impact (notched and unnotched), end. All materials were compounded with the same maximum flexural strength, flexural modulus of elas- screw configuration and the melt temperatures were ticity, maximum tensile strength, tensile modulus of kept relatively equal at 190°C. Discharge rates were elasticity, percent tensile elongation, mold shrinkage, between 20 and 40 kg/hr. Extrudate was cooled in a water trough and cut into pellets. A 60 percent wood flour-filled polypropylene master batch was pelletized and used for the 60 percent filled-compounds. This 60 percent filled master batch was subsequently used for preparing the other formulations. All pelletized materials were dried in an oven at 105°C for at least 24 hours before injection molding into standard ASTM specimens. The injection molder was a 33-ton Cincinnati-Milacron (Bata- via, Ohio) reciprocating screw-type injection molding press, using a 196°C melt temperature. The heavier loadings of wood flour, in 50 to 60 percent wood flour-filled polypropylene, resulted in higher viscosities (lower melt flow index). Therefore, higher injection molding tonnages were necessary to mold FIGURE 2. —Unnotched impact energy versus filler these samples compared to the filled resins with content. lower wood flour loadings.

FIGURE 3. —Maximum flexural strength versus percent FIGURE 1 . —Notched impact energy versus filler content. wood filler.

Berger and Stark • 21 FIGURE 4. —Flexural MOE versus percent wood filler. FIGURE 7. —Percent tensile elongation versus percent wood filler.

FIGURE 5. —Maximum tensile strength versus percent FIGURE 8. —Mold shrinkage versus percent wood filler. wood filler.

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

22 • Berger and Stark and heat deflection temperature tests. Table 1 pro- lowest impact strengths, possibly showing less com- vides a summary of the properties and performance patibility with the polypropylene used. A discussed of each wood-filled polypropylene formulation com- in other literature (2,6), fillers in general (including paring each species and unfilled polypropylene. wood flour) decrease the unnotched impact strength of polypropylene. Discussion Flexural properties Impact performance Alll wood flour species provided an increase in the The ponderosa pine species (4020) exhibited the flexural strength over the unfilled polypropylene, best performance for both notched and unnotched which supports how these materials are known to impact tests, outperforming both hardwood species behave. The 4041-60 formulation was the only ma- (oak and maple). All species provided an increasing terial not to improve the stiffness of polypropylene. notched impact strength with increasing wood filler Figure 4 shows a linear relationship between the content, with a maximum strength at 40 to 50 per- increase in flexural MOE and increasing wood filler cent filler loading. The loblolly pine showed the content. 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 flexural 4. Hardwood flours provide an improvement in ten- strength in the range of 30 to 40 percent filler. sile properties and heat deflection temperature Hardwoods appeared to outperform softwoods in over softwood flours. flexural strength. 5. Compared to other species, ponderosa pine wood flour provides the maximum blend of mechanical Tensile properties property enhancements when used as a filler in Oak and maple provided the optimum tensile prop- Fortilene 3907 polypropylene. erties. In general, the curves indicate an increase in This study was able to effectively show there are tensile strength with wood filler loading, and a maxi- differences in mechanical properties of wood flour- mum tensile MOE obtained in the 50 percent loading fllled polypropylene, depending on the species of range. Loblolly pine provided the least improvement wood flour. Availability of wood species will vary in tensile strength. All wood flours showed a decrease geographically both in the United States and around in tensile elongation with increased loading. the world. This is an important consideration when Mold shrinkage designing a product utilizing wood flour as a filler. With the basic understanding of the variables associ- Values reported for 4020-40 and 4010-40 agree ated with wood flour as a filler, including species and with published sources (2). All wood fillers that were improvements in quality, consistency can be success- tested provided reduced mold shrinkage with in- fully obtained. creased filler percentage. Again, loblolly pine proved the least effective in enhancing this property, showing This research defined mechanical property curves an average of 15 percent higher shrinkage than the dependant on the percentage of wood flour loading. other wood species tested. Mold shrinkage seems to It compared polypropylene filled with various species level off in the 0.3 percent range at the highest wood of wood at various wood filler loadings. The data filler loadings. Compared to unfilled polypropylene, should be helpful to designers looking for the most wood proves to be an effective means of dramatically appropriate loading when utilizing wood flour as a reducing mold shrinkage. filler. This research also validates past work done on Heat deflection temperature wood flour-filled polypropylene, proving the poten- In general, wood fillers improve the heat deflection tial of wood flour as a filler in competing with other of unfilled polypropylene. Heat deflection tempera- mineral fillers such as talc and calcium carbonate. The ture increases with increasing wood-filler loading, potential for wood flour to gain market share as a with a leveling-off appearing at the 60 percent filler thermoplastic filler shows promise for utilizing more level. Oak and maple provide the greatest heat deflec- wood by-products, improving our ability to recycle tion, with 25 to 45 percent improvement compared worldwide. It allows us to fully utilize a renewable to the softwood species tested. resource, extending the use of nonrenewable polymers. Conclusions Finally it is important to understand the research The following conclusions can be made for the done here was not concerned with optimizing the wood flour species compared as fillers in polypropy- properties of these wood-filled formulations. No lene in this study: coupling agents, additives, or modifiers were added 1. Wood species is an important, “controllable” vai- to improve the properties tested. Many questions still able to consider when utilizing wood flour as a filler. remain from plastics processors on wood-filled poly- 2. Generally increasing filler loading results in: mers, such as UV stability, flammability, colorants and increased notched Izod impact-strength; pigmentation, aging, and appropriate coupling agents, decreased unnotched Izod impact strength; additives, and modifiers. The data presented is an increased flexural and tensile MOE; excellent baseline to be used for further research on decreased tensile elongation percentage; the aforementioned questions. Continuing this re- decreased mold shrinkage; and search and expanding it will improve the know-ledge increased heat deflection temperature. base of wood flour as a filler in thermoplastics. 3. Wood flour provides an excellent design option as Literature cited a filler when impact properties are not a main design consideration.

24 • Berger and Stark Berger and Stark • 25

Fourth International Conference on Woodfiber-Plastic Composites

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

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

Forest Products Society 2801 Marshall Court Madison, WI 53705-2295 phone: 608-231-1361 fax: 608-231-2152 www.forestprod.org The opinions expressed are those of the authors and do not necessarily represent those of the USDA Forest Service or the Forest Products Society.

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