Composite Technologies

Fiber Finish Systems

The use of the Adherent Technologies’ Axilan™ fiber finishes leads to 50-100% improvements in composite strength and stiffness as well as significantly enhancing the environmental durability (including moisture resistance and thermo-oxidative stability) of these composites. The result of these improvements is the potential for expanded use of composites in all arenas of the military, aerospace, industrial, and consumer applications. Adherent Technologies’ fiber finish systems are based on a patent-pending reactive coupling agent (Figure 1) that offers extraordinary compatibility and reactivity with virtually all matrix materials and many carbon and ceramic fibers used in military, aerospace and commercial composite applications.

Reactive coupling agent diagram

Schematic of reactive coupling agent function

In contrast to conventional fiber sizings, which are used primarily as handling aids, but do offer some enhanced interaction between the fiber and the matrix material, Adherent Technologies’ fiber finish systems serve primarily to address issues of interfacial bonding by creating direct chemical bonds between the fiber surface and the matrix material while still fulfilling the handling function of conventional sizings. This enhanced adhesion results in more effective load transfer between the fiber and the matrix in the composite, consequently leading to higher strength and stiffness as well as greatly improving moisture resistance by reducing absorption of water and crack propagation at the fiber/matrix interface.

The ability to achieve these higher mechanical properties is fundamentally important to the development of lighter, more efficient composite structures, which can in turn lead to greater agility, reduced deployment costs, and lower radar signatures, among other benefits. It is critical to note that the reactive coupling agent that is the heart of the fiber finish system is extremely versatile and can be used in conjunction with virtually any common matrix material used in carbon fiber reinforced composites. Matrix materials evaluated to date include vinyl ester, epoxy, phenolic, bismaleimide, and polyimide. This versatility offers enhanced compatibility and composite capability.

This technology is intended to replace the current commercial fiber sizing with a fiber finish that is 100% compatible with matrix resin, does not result in significant processing problems, and increases the resulting composite mechanical properties to near theoretical values.

Adherent Technologies’ Finish Features, Advantages, and Benefits

Customized to desired matrix resin Improved wetting and matrix compatibility Reduced processing cost
Reactive finish Chemical bond across fiber/matrix interface 50-100% improvement in composite strength and stiffness, enhanced environmental durability
Water based process resulting in no VOC emissions and no flammability Direct integration with existing manufacturing lines Low cost, low risk, quick to market process.
Reactive coupling agent Extremely versatile material Works with most composites so can be used on many carbon fiber product platforms

High-Temperature Finishes for Polyimide Matrix Composites

Adherent Technologies has recently developed a new finish system for carbon/polyimide composites to improve interface dependent properties and durability. High-temperature finish systems consisting of a reactive coupling agent, polyimide carrier, and solvent were developed. The reactive coupling agents have been shown to chemically bond to carbon fibers in prior work. An aqueous based finish was also developed by synthesizing a water soluble coupling agent and polyimide.

Unidirectional composite laminates were prepared from prepreg made with different ATI finish formulations, a commercial high-temperature sizing, and unsized carbon fiber. The laminates were processed with autoclave molding at 371°C (700°F) followed by an air circulating oven postcure at 399°C (750°F). Mechanical testing data is discussed below:

Dry Samples

Room temperature dry – similar for all fiber surface chemistries. The laminates with the finished fibers did show two times higher fracture toughness in interface sensitive tests (transverse flexure, short beam shear, compression).

Longitudinal (0°) Flexural Strength260 ksi
Transverse (90°) Flexural Strength24 ksi
Short Beam Shear Strength16 ksi
Compression Strength145 ksi

At 550°F in the dry condition the finished material is twice as strong and has 2-3 times the modulus and toughness compared to the commercially sized material in transverse flexure.

Moisture-Aged Samples

  • Reactive finished reduce the moisture absorption rate under elevated temperature and humidity conditions
  • 30% higher strength than commerically sized fiber
  • 400% higher toughness than commercially sized fiber.

Failure analysis using scanning electron microsocope (SEM) techniques showed the strength and toughness increases were due to superior interfacial bonding that lead to increased plastic deformation in the matrix.

SEM photomicrographs of 90° failure surfaces of moisture saturated laminates with commercial sizing (left) and 1.0% reactive finish (right). The flatter fracture surface observed for the commercially sized sample (upper left) relative to that for the reactive finish (upper right) is indicative of lower energy absorption and a lower fracture toughness. At higher magnifications, more bare fibers are apparent on the commercially sized laminate (lower left) than the 1.0% reactive finished laminate (lower right). In addition, more plastic deformation in the matrix resin is seen with the reactive finished laminate.

Thermally Aged Samples

Accelerated Testing:

  • 36% lower weight loss relative to commercially sized material
  • 20% higher strength relative to commercially sized material

Long term aging at 600°F (1000 hr, tested @ 550°F):

  • 20-40 percent better strength and toughness retention in longitudinal and transverse flexure

Reactive Finish Systems for Carbon/Vinyl Ester Composites

Adherent Technologies has developed a reactive finish system that is compatible with carbon/vinyl ester composites that traditionally have difficulties in meeting theoretical performance targets. In contrast to conventional fiber sizings, ATI’s reactive finish systems offer direct chemical bonding between both the fiber and the matrix material of the composite through a proprietary reactive coupling agent. The use of the Adherent Technologies’ fiber finishes leads to 50–100% improvements in composite strength and stiffness as well as significantly enhancing the environmental durability (including moisture resistance and thermo-oxidative stability) of these composites.

In these programs, unidirectional laminates containing several types of fiber surfaces (unsized, commercially sized, and reactive finishes of varying formulations) were prepared, characterized for microstructure, and mechanically tested using interface sensitive test methods (transverse flexure, short beam shear) and a fiber dominated test (longitudinal flexure), and characterized for failure mode. Data are summarized in the following tables.

Table I. Longitudinal (0°) Flexural Properties of Carbon/Vinyl Ester Wet Wound Laminates

Laminate Type Ultimate Strength (psi) Young's Modulus (psi) Deflection at Failure (in.)
Unsized 139,211 ± 7.0% 15,192,700 ± 4.7% 0.202 ± 4.6%
Commercial Size 152,623 ± 8.9% 15,113,900 ± 9.3% 0.205 ± 4.0%
ATI VE Finish 137B 133,652 ± 21.6% 12,769,800 ± 26.8% 0.209 ± 7.3%
ATI VE Finish 139A1 143,914 ± 4.3% 14,495,000 ± 1.1% 0.199 ± 8.5%
ATI VE Finish 139A2 164,683 ± 5.1% 14,729,900 ± 3.3% 0.237 ± 3.4%
ATI VE Finish 142A 163,520 ± 1.5% 14,073,800 ± 1.8% 0.235 ± 4.3%

Table II. Transverse (90°) Flexural Properties of Carbon/Vinyl Ester Wet Wound Laminates

Laminate Type Ultimate Strength (psi) Young's Modulus (psi) Deflection at Failure (in.)
Unsized 3,541 ± 6.9% 584,017 ± 8.6% 0.007 ± 6.9%
Commercial Size 3,359 ± 6.8% 622,058 ± 12.4% 0.006 ± 11.6%
ATI VE Finish 137B 8,715 ± 6.2% 778,074 ± 4.6% 0.014 ± 9.4%
ATI VE Finish 139A1 10,726 ± 7.1% 925,625 ± 4.2% 0.014 ± 6.3%
ATI VE Finish 139A2 8,672 ± 13.3% 804,847 ± 2.6% 0.012 ± 14.4%
ATI VE Finish 142A 10,813 ± 4.5% 814,711 ± 4.6% 0.015 ± 3.5%

Table III. Short Beam Shear Properties of Carbon/Vinyl Ester Wet Wound Laminates

Laminate Type Shear Strength (psi) Standard Deviation (psi) Coefficient of Variation (%)
Unsized 6,801 310 4.6
Commercial Size 7,364 403 5.5
ATI VE Finish 137B 8,748 125 1.4
ATI VE Finish 139A1 8,531 95 1.1
ATI VE Finish 139A2 9,699 143 1.5
*ATI VE Finish 139A2 10,681 153 1.4
*ATI VE Finish 142A 12,219 171 1.4

Results show that the reactive finishes substantially improve the carbon/vinyl ester interface-dominated mechanical properties compared to the unsized or commercially sized materials. In addition, the toughness of the interface in shear as seen in the area under the load-deflection curves is substantially increased.

Failure mode analysis shows that interfacial failure dominates the unsized and commercially sized materials, whereas resin cohesive failure dominates the ATI finished materials. This is a strong indication that the desired chemical bonding is occurring at the carbon/vinyl ester interface through the reactive finish.

Unsized material Commercially sized material Reactive finished
Unsized Commercially sized Reactive finished