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DIRECT FIBER FEEDING INJECTION MOLDING OF CARBON FIBER REINFORCED POLYCARBONATE COMPOSITES

Putinun Uawongsuwan, Hiroyuki Inoya and Hiroyuki Hamada, Department of Advanced Fibro-Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto, 606-8585,

Japan Hiroaki Ichikawa, Nihon Yuki Co., Ltd., 4-2-2 Higashifuchinobe, Chuo-ku, Sagamihara-

shi, Kanagawa, 252-0203, Japan

Abstract The new fabrication method by direct fiber feeding (DFF) injection molding was introduced in

this work as the new processing route for the production of short fiber reinforced polymer composite. The carbon fiber reinforced polycarbonate (CF/PC) composites with fiber loading content from 8 to 28.9 wt.% were successfully fabricated. The tensile properties of CF/PC composites fabricated by DFF process showed linear correlation and increased with increasing of fiber content. At the same fiber content, tensile properties of DFF composite were slightly lower than that of conventional injection molded composite. This process reduced the fiber breakage during conventional compounding.

Introduction A short fiber reinforced polymer (SFRP) composite usually consists of relatively short,

variable length and imperfectly aligned fibers distributed in a polymer matrix. The use of short fibers has the advantage of achieving substantial stiffening without compromising significantly the processability of the materials [1]. Carbon Fibers, having supreme characteristics, are adopted in wide varieties of uses because the fibers have low specific gravity, high specific tensile strength, high specific elastic modulus and attractive performances such as electric conductivity, heat resistance, low thermal expansion coefficient, chemical stability and self-lubrication property. Those features have been stimulating carbon fiber users to develop numerous kinds of applications. Carbon fiber reinforced plastics (CFRP) is superior to steel or glass fiber reinforced plastics (GFRP) in its specific tensile strength and specific elastic modulus (specific rigidity). Moreover, fatigue resistance of carbon fiber surpasses that of other structural material. Now there is a tendency to try to use the fiber for commercial, mass production cars in full scale in order to reduce production costs to a minimum practical level [2]. Polycarbonate is one of the most important technical thermoplastic because of its excellent heat resistance, outstanding impact strength and good dimension stability. Polycarbonate resin with reinforcement is one of today`s most versatile engineering materials. The addition of carbon fiber to polycarbonate results in an injection moldable composite with high tensile and impact strengths, modulus, creep resistance and fatigue endurance limits. Carbon fiber reinforced polycarbonate provides low volume and surface resistivity. With carbon fiber, flexural strength is increased threefold. Flexural modulus is increased by nearly seven times with carbon fiber reinforcement. Notched Izod impact strength values are among the very highest of all carbon fiber reinforced thermoplastic compounds [3-7].

Extrusion compounding and injection molding techniques are conventional methods of manufacturing thermoplastic composites. However, the extrusion compounding for preparing fiber filled polymer pellets, has a practical processing limit on the fiber attrition problem. Furthermore, the attrition of fibers can occur during injection molding process and in general, it is important to select the appropriate molding conditions to avoid the excessive fiber attrition [8-12]. Because most thermoplastic composite preparation routes lead to significant uncontrollable

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degradation of fiber length, several direct compounding technologies are developed to solve this issue. Pultrusion is often used for producing long fiber thermoplastic (LFT) pellets, which are then used for producing relatively long fiber reinforced polymers using injection molding. Produced by pultruding continuous fiber and resin into small diameter rods and then cutting into pellets based on the desired fiber length. The pre-compounded LFT can be supplied in highly loaded up to 70 percent fiber and can be diluted like a masterbatch to customize loading levels [13-15]. Alternatively, the development of direct long fiber thermoplastic (D-LFT) process, which combine compounding and molding processes in one system. This process is the compounding of fiber and resin at the molding stage, keeping the material hot and molding without the intermediate steps of making and reprocessing of pellets. The advantages of D-LFT technique are the gentle mixing and maximizing retained fiber length due to the feeding of compounded material in molten stage. The direct roving feed produces longer fibers and yields molded parts with higher structural performance. However, this D-LFT technology requires the large investment in the new in-line compounding unit, which is not suitable for commercially SFRP composites [16].

In this study, the novel processing technique for the fabrication of short fiber reinforced composite is introduced as the direct fiber feeding (DFF) injection molding process. This new processing technique is aimed to improve the fiber dispersion with minimum fiber breakage during molding. By direct feeding of continuous fiber into the barrel of injection molding machine, the fiber attrition during extrusion compounding will be eliminated. Furthermore, the reduction of material cost can be the most effective cost reduction route. This is a fundamental industry change that eliminates the compounding step and also the cost of reinforcing compounded pellet in the traditional composite market value chain. The effects of typical processing parameters on the processability of the new process are evaluated and discussed. The mechanical property of the composite fabricated by new technique is also compared with the common compounded pellet composites.

Experimental Materials

Polycarbonate (PC: grade lupilon S-3000) was used as the matrix with a melt flow index (MFI) of 16 dg/min, and it was manufactured by Mitsubishi Engineering-Plastics, Japan. The carbon fiber (CF: grade TR50S12L) with 1200 tex was used as reinforcing fiber, and it was manufactured by Mitsubishi rayon, Japan. Two carbon fiber-reinforced polycarbonate compounded pellets (PC-CF: grade lupilon CF2020 and CF2030) were used as the reference commercial pellet in order to compare the new process with conventional injection molding process. These two pellets were fabricated by Mitsubishi Engineering Plastics, Japan. The carbon fiber content were 20 and 30 % by weight, respectively.

Specimen preparation The direct fiber feeding (DFF) injection molding was introduced in this study. The schematic

drawing and photograph of DFF injection molding process are shown in Figure 1. The 75-tons injection molding machine (Sumitomo: model iM75) with vented barrel was used for the fabrication of testing specimens. The general vented barrel, which is typically used for releasing the volatile gasses of the hydroscopic materials, is adapted as the direct fiber feeding unit. The continuous fiber roving strands are guided into the vent of the devolatilizing unit of injection barrel and fed into the melt by the shearing motion of the screw during plasticization process. The carbon fiber roving strands were guided into the vent of devolatilizing unit of the barrel and fed into the melt by the shearing action of the injection screw during plasticization process. The number of carbon fiber roving was varied from 1, 2, 3, 4 and 5, respectively in order to control fiber loading content. The normal feeding hopper was replaced with the controllable feeding

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hopper in order to control the fed amount of matrix. Adjusting the screw speed of the controllable hopper varied the amount of matrix. The barrel temperature and mold temperature were set at 310 and 80 oC, respectively. The screw plasticization speed was fixed at 130 rpm.

Figure 1: Schematic drawing of direct fiber feeding injection molding process.

Testing Tensile tests were conducted on Instron universal testing machine (model 4206) in accordance with ASTM D638. The strain was measured by using strain gauge extensometer. The testing speed was 1 mm/min with at least five samples repeating test. Notched Izod impact tests were performed on the Digital Impact tester (Toyoseki) with 5.5 J pendulums in accordance with ASTM D256. At least five specimens were repeated for all tests.

Scanning electron microscopy Scanning electron microscope (JEOL: JSM5200) was conducted on the fracture surface to observe the failure surface. The specimen was mounted on aluminum holder and sputtered with gold for 6 minutes prior the observation.

Fiber length determination The middle part of specimen was cut and dissolved in the dichloromethane solution under room temperature for 1 hour. The dissolved polycarbonate was completely removed by using the dropper. The extraction process was repeated for 3 cycles. The solvent was completely removed again in vacuum oven at 80oC for 12 hours. The remained carbon fibers were cast on glass slide and observed by optical microscope in order to obtain the distribution of fiber length. The weight average fiber length of carbon fiber was defined by

𝑳𝑾 =𝑵𝒊𝑳𝒊𝟐

𝑵𝒊𝑳𝒊

where Ni is number of fiber at length Li

Results and discussion Effect of processing parameters on fiber loading content of composites

The minimum and maximum fiber loading content are 8 and 28.9 wt.%, respectively. The increasing of number of carbon fiber and reducing the feeding speed of matrix lead to the increasing of fiber loading content. The limitation of DFF injection molding process consists of both number of carbon fiber and matrix feeding speed. Figures 2 shows the comparison of weight average fiber length between the composites fabricated by using DFF and conventional injection molding process. The fiber lengths of compounded composites are much shorter than the DFF CF/PC composites. The average fiber lengths of CF/PC composites fabricated by DFF

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process are longer than 0.6 mm while the average fiber lengths of compounded CF/PC composites are 0.1 mm.

Figure 2: Weight average fiber length of carbon fiber of CF/PC composites as a function of carbon fiber content.

The details of fiber length distribution of CF/PC composites are presented in Figures 3. The DFF CF/PC composites show broader distribution curve of fiber length from 0.05 to 4.05 mm (Figure 3a) while the compounded composites have the length between 0.05 to 0.45 mm (Figure 3b). In general the fiber breakage occurs during extrusion compounding process. The carbon fibers inside compounded pellet are further broken at the screw pre-plasticization zone during injection molding process. Therefore in DFF process, the continuous fibers were fed into injection barrel from the vented area, which ignore the fiber breakage from extrusion compounding process. The longer retained fiber length is achieved in final product by using DFF process for CF/PC composites.

(a)

(b)

Figure 3: Comparison of fiber length distribution of CF/PC composites; (a) DFF CF/PC, (b) compounded CF/PC.

Tensile properties In order to evaluate the effect of compounding process, the mechanical properties of

composite fabricated by DFF process are compared with the conventional injection molded composites. The tensile properties of CF/PC composites as a function of fiber loading content are shown in Figures 4. The increasing trend of tensile modulus and strength with the increasing

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of fiber content is observed. Moreover tensile properties show a linear correlation with carbon fiber content. The tensile strength of compounded composites is higher than DFF composites at similar fiber content. Therefore from the SEM micrograph of fracture surface in Figure 5, there is no evidence of the difference in interfacial bonding between carbon fiber and polycarbonate matrix from the two difference composites. In addition, for both composites, the clean surfaces of fibers are observed which revealed that the matrix is completely debonded.

(a)

(b)

Figure 4: Tensile properties of CF/PC composites as a function of carbon fiber content; (a) Tensile modulus, (b) Tensile strength.

(a)

(b)

Figure 5: SEM micrograph of fracture surface of tensile tested specimen CF/PC composites; (a) DFF composite, (b) compounded composite.

The SEM micrographs of the fracture surface of DFF and compounded CF/PC composites are shown in Figure 6. It is clear that the carbon fiber in compounded composite shows good distribution along overall cross-sectional surface of specimen (Figure 6b). Since the fiber was shorted and dispersed after the pellet making process, the distribution of fiber after injection molding is mainly depended on the development of flow inside the mold. However, as the flat fiber roving used in direct fiber feeding injection molding process consisted a number of carbon fiber, the distribution of fiber could be also depended on several factors such as the development of flow, screw rotation speed, screw rotation time, the melt viscosity of polymer and the ease of separation of the fiber from roving. The distribution of carbon fiber of DFFIM composites is poorer than the normal pellet injection molded composite. From Figure 6a, it is clear that the long fiber length portions align in the transverse direction in the case of DFF specimens. The poor distribution and orientation of carbon fiber of the DFF composites is the

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main reason that the tensile strength of DFF CF/PC composites is lower than the compounded CF/P C composites at similar fiber content.

(a)

(b)

Figure 6: SEM micrograph of fracture surface tensile tested specimen of CF/PC composites; (a) DFF composite 21.8 wt.% carbon fiber, (b) compounded composite 20 wt.% carbon fiber.

Izod impact strength The impact strength is one property of short fiber reinforced thermoplastic, which is very

important for various applications. Polycarbonate has very high impact resistance when compared with other thermoplastics. The incorporation of carbon fiber into polycarbonate matrix decreases the impact strength of polycarbonate as presented in Figure 7. The reduction of impact strength of the composites may result from the poor interfacial adhesion between reinforcement and matrix. It is known that the interfacial bonding strength are strongly influenced the impact properties of composite materials. Impact energy is dissipated by debonding, fiber and/or matrix fracture and fiber pull out. This is already revealed by SEM micrograph in Figure 5. The impact strength of DFF CF/PC composites increase with the increasing of carbon fiber content and then decreased when fiber content exceed 26 wt.%. Furthermore, at similar fiber content, the DFF CF/PC composites show higher impact resistance when compared with the compounded CF/PC composites. As previous discussed that the DFF CF/PC composites have longer retained fiber length after molding. The fiber length has a strong effect on impact strength of short fiber reinforced composites. In addition, from the SEM micrograph of impact fracture surface in Figure 8 shows the large aggregation area of carbon fiber in DFF composite when compared with the compounded composite. The aggregated fiber influence the impact resistance of CF/PC composite by increased the crack propagation distance, which resulted in higher impact energy.

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Figure 7: Izod impact strength of CF/PC composites as a function of carbon fiber content.

(a)

(b)

Figure8: SEM micrograph of fracture surface impact tested specimen of CF/PC composites; (a) DFF composite 21.8 wt.% carbon fiber, (b) compounded composite 20 wt.% carbon fiber.

Conclusions

The new fabrication method by direct fiber feeding injection molding was introduced in this study. The carbon fiber reinforced polycarbonate composites were successfully fabricated by the new process. The limitation of processing window was depended on several parameters such as number of feeding fiber roving, feeding rate of matrix, screw rotation speed and injection temperature. The minimum and maximum fiber loading content were 8 and 28.9 wt.%, respectively. This process reduced the fiber breakage during conventional screw compounding and resulted in longer length of retained fiber. The tensile properties of CF/PC composites processed by DFF method showed linear correlation and increased with increasing of fiber loading content. The tensile strength of composite fabricated by DFF process was slightly lower than that of conventional injection molded composite at similar fiber loading content due to the poor orientation and distribution of carbon fiber. On the other hand, impact strength of DFF composites is higher than the compounded composites. The new injection molding technology is a simple process without the requirement of extrusion compounding step and it can be easily adapted to existing injection molding equipment. This technology provides a unique route to incorporate cost reduction into their business strategy. In addition, the fiber can be surface modified with suitable chemical treatment to improve the interfacial adhesion between reinforcing fiber and matrix prior to molding process, which is more convenient than conventional extrusion compounding process.

Notched Notched

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References

1. Fu, S.Y., B. Lauke, Y.W. Mai, Science and engineering of short fiber reinforced polymer composites, Woodhead Publishing Limited, (2009).

2. Morey, B., Carbon Fiber on Its Way?. Manufacturing Engineering, 147:3, 81-91 (2011).

3. Sullivan, R.A., Automotive carbon fiber: Opportunities and challenges, JOM, 58:11), 77-79 (2006).

4. Zushi, H., T. Odai, I. Ohsawa, K. Uzawa, J. Takahashi, Mechanical properties of CFRP and CFRTP after recycling, In Proceedings of 15th international conference on composite materials (ICCM-15), 1-10 (2005).

5. Fukui, R., T. Odai, H. Zushi, I. Ohsawa, K. Uzawa, J. Takahashi, Recycle of carbon fiber reinforced plastics for automotive application, In Proceedings of 9th Japan international SAMPE symposium, 44-49 (2005).

6. Goto, T., T. Matsuo, K. Uzawa, I. Ohsawa, J. Takahashi, Study on optimal automotive structure made by CFRTP, In Proceedings of the 18th International Conference of Composite Materials (ICCM-18), 1-4 (2011).

7. Potschke, P., T.D. Fornes, D.R. Paul, Rheological behavior of multiwalled carbon nanotube/polycarbonate composites, Polymer, 43:11, 3247-3255 (2002).

8. Lunt, J.M., J.B. Shortall, The effect of extrusion compounding on fiber degradation and strength properties in short glass-fiber reinforced nylon 6,6. Plast Rubber Process, 4:4, 108-114 (1979).

9. Von Turkovich, R., L. Erwin, Fiber fracture in reinforced thermoplastic processing, Polym Eng Sci, 23:13, 743-749 (1983).

10. Bigg, D.M., Effect of compounding on the properties of short fiber reinforced injection moldable thermoplastic composites, Polym compos, 6:1, 20-28 (1985).

11. Franzen, B., C. Kalson, J. Kubat, T. Kitano, Fiber degradation during processing of short fiber reinforced thermoplastics, Compos, 20:1, 65-76 (1989).

12. Ramani, K., D. Bank, N. Kraemer, Effect of screw design on fiber damage in extrusion compounding and composite properties, Polym Compos, 16:3, 258-266 (1995).

13. Truckenmuller, F., H.G. Fritz HG, Injection molding of long fiber-reinforced thermoplastics: A comparison of extruded and pultruded materials with direct addition of roving strands, Polym Eng Sci, 31:18, 1316-1329 (1991).

14. Markarian, J., Long fibre reinforcement drives automotive market forward, Plast Addit Compound, 7:3, 24-29 (2005).

15. Markarian, J., Long fibre reinforced thermoplastics continue growth in automotive, Plast Addit Compound, 9:2, 20-24 (2007).

16. Schemme, M., LFT–development status and perspectives, Reinf Plast, 52:1, 32-39 (2008).

17. Garnier, M., In-line compounding and molding of long-fiber reinforced thermoplastics (D-LFT): Insight into a rapid growing technology, In : Proceedings of ANTEC 2004 Chicago, Illinois, 3500-3503 (2004).