The Seventh International Conference on Woodfiber-Plastic Composites ~ Hybrid Composites: Combining Cellulose Fibers and Wollastonite Mineral Fibers into a Nylon 6 Matrix Rodney E. Jacobson and Daniel F. Caulfield Abstract The objective of this research was to develop a tion. Attempts to maximize the composite proper- high purity cellulose/wollastonite pellet that could ties were not the focus of this research. Stable, then be accurately metered and feed into a labora- controllable processing characteristics and re- tory scale twin-screw extruder and compounded peatability of the twin-screw extrusion trials was with a nylon 6 resin. The major focus was targeted the goal. It is the authors’ opinion that this goal on a 20 percent cellulose/20 percent wolla- was accomplished. Future research will focus on stonite/60 percent nylon 6 composite. Limited re- maximizing composite properties and determin- search with nylon 6,6 resins was also attempted ing if cellulose fibers alone or in combination with and will be discussed briefly. A process for devel- mineral fibers can be compounded on larger com- oping a cellulose/wollastonite pellet was success- mercial scale equipment. Extreme care and pre- ful and 100 Kg were produced for twin screw ex- cise processing knowledge is needed to develop a trusion processing with nylons. The 100 Kg of commercial scale process that works. If this can- pellets were then compounded via a “low temper- not be accomplished, then cellulose fibers as rein- ature compounding” technique as discussed in de- forcement in any of the high melting point engi- tail elsewhere (1). Further information can be ob- neering thermoplastics may remain as a lab- tained in U.S. Patent #6,270,833 B1 (2). The oratory oddity. compounding process research was to determine if composites could be produced in small batches Introduction (50 Kg trial runs) on laboratory scale twin-screw Hybrid composites of glass fiber/wollastonite fi- extrusion compounding equipment. The result ber reinforced engineering thermoplastic (ETPs) was 125 Kg of composite material for injection are available for sale for use in commercial pro- molding and ASTM mechanical property evalua- duction applications. Blending various levels of glass to wollastonite or visa versa allows for tai- lored composites with properties high in strength (i.e., high glass content) or good dimensional sta- Jacobson: bility and warp resistance (i.e., high wollastonite President, AJ-Engineering, LLC content). Years of research and commercial devel- Caulfield: Research Chemist, USDA Forest Service, Forest Prod- opment has provided an excellent understanding ucts Laboratory, Madison, Wisconsin, USA of glass and wollastonite fiber reinforced ETPs, Jacobson and Caulfield ~ 271 but never a reduction in densities from utilizing percent alpha cellulose content. Wollastonite min- these two components. Chemical foaming agents eral fibers were provided by three suppliers, can be utilized and micro-cellular injection mold- VANCOTE W-50AS is a surface treated grade from ing can reduce weights. This density issue is one of R.T. Vanderbilt Company, Inc. Wollastonite-520S the reasons for utilizing cellulose in combination treated grade was provided by FIBERTEC, Com- with wollastonite fiber or glass fibers in reinforc- pany and an un-treated PG Grade from Intercorp, ing nylon 6 or other ETPs. Company. Ashlene Polymers Nylon 6 - 829LSW and If half of the wollastonite or glass fibers can be Ashlene Polymers Nylon 6,6 - 529L grades was used replaced with high purity cellulose in commercial as the base matrix polymer. grades of ETP hybrid composites, then an approx- Materials handling problems associated with imate weight saving of 5 to 8 percent can be real- low bulk density materials, such as cellulose fi- ized in a final automotive part, for example. Prop- bers, are a critical hurdle to overcome when com- erties of cellulose hybrid composites must meet pounding cellulose into nylon or other ETPs. The current mechanical property needs for applica- same problem exists with the hybrid composites. tions, raw material costs must be minimized, and Extensive details of pelletizing 100 percent HPK stable raw material supplies are necessary to enter fibers is reported in reference 1 and with some the automotive sector with cellulose hybrid ETP slight modifications to the overall moisture con- composites. Rayonier, Performance Fibers, Jesup, tent is all that was needed to progress further with GA has a scale-up processing plant for developing the hybrid composite research. Consistent feed TerraCel-10J, a 100 percent high purity pulp fiber rates into the extruder via a V-type feed hopper, pellet (3) for reinforcement fibers for the polymer such as an accurate feed hopper and potentially a industry. Small modifications to existing infra- K-Tron twin screw feed hopper with mixing pad- structure could lead to a 50 percent cellulose/50 dles, is an appropriate method of delivering cellu- percent wollastonite fiber pellet for hybrid com- lose/wollastonite pellets into a sidefeeder attached posite applications. The reader should take note to a twin-screw extruder. Without accurate and that the suggested modifications at a Rayonier consistent feed rates, there will be problems asso- scale-up plant is my opinion and may not reflex the ciated with die face surge, torque (i.e., percent business/marketing plans of Rayonier Perfor- load) variations, and a twin-screw extrusion pro- mance Fibers. cess that is less stable during start-up, transition With the publication of the proceedings from phase, and steady-state conditions. Compounding the Seventh International Conference on Wood- cellulose fibers or cellulose fibers in combination fiber Plastic Composites, the 20-year mark will with mineral fibers is difficult with limited experi- have been crossed since Klason et al. (4) attempted ence. Removing all potential problems to the to compound cellulose flour and cellulose fibers process stability is critical to successfully produc- into nylon 6 resins in 1984. Technological advance- ing these composites in any ETPs. ments in polymer processing equipment and A brief review of the pelletizing process for cel- bridging the knowledge gap in processing tech- lulose fiber/wollastonite fiber pellets is reported. niques allows for the routine compounding of cel- Never dried cellulose fibers were shipped to the lulose fibers alone or in combination with mineral Forest Product Laboratory and stored in a 36°K 50 fibers into nylon 6 on a small laboratory 32 mm percent relative humidity cold storage room prior twin-screw extruder. The next and most critical to fiber pelletizing to prevent biological growth stage in the technical development is to scale up to and subsequent fiber degradation. The never a small commercial scale extruder and determine dried pulp fibers were shipped at 39 to 40 percent if the viscosity shear heating effects can be solids as per prior research criteria. Wollastonite controlled to produce ETP composites containing fibers were obtained in a dry form from the suppli- cellulose. ers. Research objectives were to develop a 50 percent cellulose fiber/50 percent wollastonite fi- Materials and Pellet Processing ber pellet, which could then be compounded into a The high purity cellulose fibers were provided by nylon 6 matrix at 40 percent loading level resulting Rayonier, Performance Fiber Division. The fibers in the 20 percent cellulose/20 percent wollaston- are a hardwood prehydrolyzed kraft (HPK) with 98 ite/60 percent nylon 6 composite. A Hobart batch 272 ~ Jacobson and Caulfield mixer with 60-liter capacity was used in the first lose/wollastonite blend through the die plate. As a step of the pelletizing process for creating the 50 note: a fiber dispersion agent such as Berocell percent cellulose/50 percent wollastonite fiber pel- 509HA could also have been used in the Hobart let. Prior research indicated that 40 percent solids mixer at low levels to aid during the compounding content was optimum for 100 percent cellulose fi- phase. Again, this would result in “fine tuning” the bers. Water was added during the Hobart mixing pellet process for optimum pellet quality. A 3-mm to accommodate for the dry wollastonite fibers so die plate with a 6 to 1 press way was used to pro- the total percent solids of cellulose and wolla- duce the hybrid composite fiber pellets. The 3 mm stonite was 40 percent. The results in the kahl pel- diameter pellets have a good ratio in size to the letizing process were a failure. Subsequent trial neat polymer pellets if a pre-blending stage is used runs decreased the amount of water so the resul- for twin-screw extrusion processing. In addition, tant batches were 45 percent solids, 50 percent sol- the 3-mm pellets provided more accurate feed ids, and finally 55 percent solids before optimum rates though a feed hopper verses larger pellet kahl pelleting was obtained. Surface interactions diameter as previously attempted in other re- with the cellulose hydroxyl radical and silane search efforts. treated wollastonite fiber maybe the reason for the After the kahl pelletizing process, the 50 per- shift in percent solids or particle size interactions, cent cellulose fiber/50 percent wollastonite fiber compactions factors, etc. A full factorial study was pellets were oven dried at 105°C to reduce the not conducted on the pellet process. In brief, based moisture content near 0.25 percent pellet moisture on experience of quality pellets that would feed ef- content. Moisture plays an extremely critical role fectively into the twin-screw extruder, the re- in the compounding sequence and great care must search moved forward at pelletizing the cellu- be taken to follow the nylon manufactures drying lose/wollastonite blend at 55 percent solids. The process and keep the total moisture content below researchers believe that fiber length, fiber surface 0.5 percent. If the pellets are exposed to moist am- chemistry, percent solids, and initial fiber prepara- bient conditions prior to the twin-screw extrusion tion play a critical role in fiber pelletizing related sequence, then care should be taken to re-dry the to quality and fiber damage during processing, (fi- pellets before extrusion.