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Uniform Dispersion and Alignment of Short Fiber Composite Reinforcement

Description:

TECHNOLOGY AREA(S): Materials 

OBJECTIVE: Develop a method for the uniform dispersion and alignment of short fiber reinforcement in highly loaded composite materials. 

DESCRIPTION: There is an on-going effort to reduce the weight of Army vehicles to increase combat effectiveness, improve fuel efficiency and reduce the burdens associated with transporting fuel to the battlefield. Currently, there are many fielded Army vehicle parts that are made of aluminum or other metals that could potentially be replaced by lighter and stronger fiber composite materials. The anisotropic nature of the fiber reinforcements often requires the fibers to be highly aligned to obtain advantageous material properties. However, this imposes restrictions on the part geometries due to the need to preserve the continuity of long or continuous fibers. Strong curvatures or sharp angles would cause the fiber reinforcement to break, compromising mechanical properties. A method of addressing this problem is to produce prepregs of highly aligned (>90% of fibers within 5º of the same orientation), short (<5 mm), discontinuous fibers with high aspect ratios (i.e., fiber length divided by fiber diameter), and high fiber volume fractions (>45%). This strategy has two advantages: (1) theoretical models [1, 2] have shown that high alignment with high aspect ratios (approaching 1000) should produce materials that have material properties that approach those of their continuous counterparts and (2) the short fiber material should be readily formable (e.g., it could be stamp formed or compression molded in a manner similar to that of aluminum) due to its discontinuous nature. Despite the central importance of formability while maintaining properties, there is currently no commercially available method for achieving the high precision short fiber alignment mentioned above. There are at least two reasons for this. The first reason is the difficulty in creating a uniform dispersion of short fiber reinforcement in a resin or other fluid media (e.g., air, water) used for alignment. It is extremely difficult to prevent clumping or agglomeration among the short fibers in highly loaded resins due to the electrostatic interactions and dispersion forces that attract fibers to each other, and surface energy mismatches between the reinforcement and the dispersion media that prevent full wetting of the fibers. In addition to compromising the alignment necessary to attain the desired material properties, fiber agglomerates and clumps can impair filling of the resin, creating processing defects in the composite parts. A second reason is the challenge associated with uniformly aligning well dispersed short fibers in a consistent and reproducible manner. Some alignment techniques have shown promise, but they have suffered from the following drawbacks: (i) low fiber volume fractions, (ii) insufficient alignment, or (iii) long overall fiber lengths, which prevented them from achieving materials with properties that approach those of similar continuous materials with better formability. Some of these challenges are themselves associated with the dispersion problems mentioned above. Recently, it was demonstrated that specific patterns or alignments of particles in a fluid can be created through the use of arranged ultrasonic transducers [3, 4]. This was accomplished by developing a sufficient mathematical understanding of the forces generated from ultrasonic interactions that the resulting particle patterns could be predicted. Other researchers have had success in employing electromagnetic fields to accomplish similar controlled alignments (5). Given that it has been established that it is possible to create dispersed and organized patterns using external fields, it should be possible to develop a methodology of creating well-dispersed and highly aligned composites via chemical, acoustic, electromagnetic, or mechanical methods. A method of consistently creating uniform well dispersed and oriented short fiber reinforcement in highly loaded composite materials would not only enable the development of more flexible and inexpensive composite fabrication 

PHASE I: The offeror(s) shall develop a technique to (1) consistently disperse a short fiber (<5mm) reinforcement (e.g. carbon, or glass) in a medium without clumping or agglomeration and use this dispersion to (2) produce a highly aligned (>80% of fibers within 15º of the same direction) in a highly loaded (>30 vol% fiber) thermoplastic or thermosetting matrix (e.g., Nylon 6 or an epoxy resin). Offeror(s) should take care to address or counter the electrostatic interactions between fibers and surface tensions that promote agglomeration. Potential solutions for obtaining good dispersion include, but are not limited to, chemical modification of the fiber and matrix, ultrasonic dispersion, or utilization of EM interactions for dispersion. Potential solutions for alignment include, but are not limited to, fluid flow, (di)electrophoresis, and pneumatic techniques. The goal of phase I is to demonstrate an ability to consistently produce a 30vol% or higher fiber loaded composite sheet with a uniform alignment of short fiber reinforcement. The parts produced by said method should be a minimum of 0.5 mm thick and of sufficient lateral dimensions for a simple tensile test in the fiber direction. Adequate dispersion and alignment of the fibers should be confirmed via microscopic or non-destructive evaluation. Samples shall be provided to Army researchers for independent testing and validation. For Phase II to be awarded, the offers should also be able to articulate a technically viable path for the dispersion and alignment methods to be employed in a flexible composite manufacturing process such as stamp forming or compression molding. 

PHASE II: The offeror(s) shall expand the method in phase I to the development of 45vol% or higher short fiber composites with highly aligned fibers. Highly aligned is defined as 94% of the fiber reinforcement deviating by a maximum of 10° in alignment. The goal of Phase II is to demonstrate the methodology by producing two example parts. One example part is at least 1 mm thick having an angle feature that is >85º and the other is at least 1 mm thick and has a hemispherically shaped feature with a radius of about 2 inches. The offeror(s) shall measure the tensile modulus, tensile strength and short beam shear strength of flat plates of the produced material in a manner consistent with ASTM Standard D3039 and demonstrate variance of no greater than 10% in a set of ten samples. Offeror(s) shall provide additional example parts and test specimens to Army researchers for independent testing and validation. 

PHASE III: The offeror will adapt the dispersion methodology to as many fiber/matrix systems as possible, and develop commercial processes that employ the dispersion/alignment solution for the production of commercial composite parts. The offeror will begin to offer high fiber loaded short fiber composite parts for use in military ground vehicles, military autonomous vehicle, military rotorcraft, and commercial applications in automotive, aerospace, and robotics. 

REFERENCES: 

1: Fukuda, H. and T.-W. Chou, A probabilistic theory of the strength of short-fibre composites with variable fibre length and orientation. Journal of Materials Science, 1982. 17(4): p. 1003-1011.

2:  Lauke, B. and S.-Y. Fu, Strength anisotropy of misaligned short-fibre-reinforced polymers. Composites Science and Technology, 1999. 59(5): p. 699-708.

3:  Prisbrey, M

4:  Greenhall, J

5:  Vasquez, F

6:  and Raeymaekers, B, Ultrasound directed self-assembly of three-dimensional user-specified patterns of particles in a fluid medium. Journal of Applied Physics, 2017. 121: p. 014302

7:  Greenhall, J

8:  Homel, L

9:  and Raeymaekers, B, Ultrasound directed self-assembly processing of nanocomposite materials with ultra-high carbon nanotube weight fraction. Journal of Composite Materials, 2018.

10:  Ma, W-T

11:  Kumar, S

12:  Hsu, C-T

13:  Shih, C-M

14:  Tsai, S-W

15:  Yang, C-C

16:  Liu, Y-Y

17:  and Lue, S-J, Magnetic field-assisted alignment of graphene oxide nanosheets in a polymer matrix to enhance ionic conduction. Journal of Membrane Science, 2018. 563, p. 259-269

KEYWORDS: Composites, Manufacturing Processes, Short Fiber, Dispersion, Fabrication 

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