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CARBON FIBERS SURFACE AND ITS EFFECTS ON AN

INTERPHASE FORMATION FOR ULTIMATE ADHESION-

RELATED PERFORMANCES

Felix N. Nguyen1

, Yoshifumi Nakayama2

, Daigo Kobayashi2

, Toshiya Kamae2

, KenichiYoshioka

1

1Toray Composites (America), Inc., 19002 50

thAve E, Tacoma, Washington 98446, USA

2Toray Industries, Inc., 1515, Tsutsui, Masaki-cho, Iyogun, Ehime 791-3193, JAPAN

ABSTRACT

Adhesion between a carbon fiber and a resin matrix has been documented to substantially affectadhesion-related mechanical properties of carbon fiber reinforced polymer composite (CFRP)

such as tensile, flexural, and fracture toughness. Adhesion strength generally relies on theformation of an interphase between the carbon fiber and the resin matrix, and chemical

interactions within this interphase. Nanostructures on the fibers surface, chemicalfunctionalities introduced by a surface treatment, and a reactive sizing material with both the

fibers surface and the resin are essentially critical factors. This paper discusses the uniqueness

of an intermediate modulus (IM) and high modulus (HM) carbon fiber, their surface chemistryand sizing chemistry that altogether improve adhesion strength substantially. Furthermore, when

all factors involved in the formation of the interphase were fully optimized, a tensile strength of

CFRP normalized to fiber strength and fiber volume fraction could approach 100%. Thisremarkable breakthrough is being further developed for all CFRP systems at Toray Composites

(America) Inc.

1. INTRODUCTION

When bonding carbon fibers together by a resin matrix to create a fiber reinforced polymercomposite (CFRP), functional groups on the fibers surface is very critical. In addition, the bond

has to be durable as subjected to environmental and/or hostile conditions. Bond strength or force

per unit of interfacial area required to separate the (cured) resin from the fiber that were in

contact is a measure of adhesion. Maximum adhesion is obtained when a cohesive failure ofeither the resin or the fiber or both, as opposed to an adhesive failure between the fiber and the

resin, is mainly observed.

To make a strong bond, firstly oxygen functional groups on the pristine fibers surface must beintroduced; secondly an adhesion promoter is selected such that one end could covalently bond

to the oxygen functional groups while the other end can promote chemical interactions with thefunctional groups in the resin. Essentially, the adhesion promoter acts as a bridge connecting the

fiber to the bulk resin during curing. A surface treatment such as plasma, UV, corona discharge,

wet electro-chemical treatments is often used to introduce oxygen functional groups. Selection

of a type of surface treatment as well as a level of surface treatment depends on the surfacestructure of the pristine fiber, determined from a precursor type, a spinning process and a

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carbonization temperature. A successful surface treatment should provide a uniform distribution

of the oxygen functional groups on the surface without damaging the fiber and weakening it.The adhesion promoter material often called a sizing material or simply sizingorsize or in othercontext it might be called a surface finish. The sizing can be as simple as a silane or a polymer

to as complicated as a mixture of appropriate components to achieve adhesion-related properties

of the composite. Normally, the sizing is coated onto fibers via a wet chemical process which isin-line with the surface treatment process. Finally, the sized fibers are impregnated with the

resin, and the resulting composite is cured. Selection of a right curing method is also important.

A common method is to use an autoclave to produce the highest bonding quality. Yet, othermethods such as a thermal oven, a microwave oven, a UV light, or an electron beam could also

be used, but voids might be found in the composite and thus its performances are lower.

Ultimately, to achieve the strong bond, there certainly cannot be voids at the interface between

the fiber and the resin, i.e., there is sufficient molecular contact between them upon cured.

Often, this interface is considered as a volumetric region or an interphase. The interphase canextend from the fibers surface a few nanometers up to several tens of micrometers, depending

on the chemical composition on the sized fibers surface, chemical interactions between the fiberand the bulk resin, and other chemical moieties migrating to the interface during curing [1]. The

interphase; therefore, has a very unique composition, and its properties are far different fromthose of the fibers surface and the bulk resin. Moreover, existence of high stress concentrations

in the interphase due to the modulus mismatch between the fiber and the resin often makes it

vulnerable to crack initiation. Such high stress concentrations could be intensified by chemicalembrittlement of the resin induced by the fiber, and local residual stress due to the thermal

expansion coefficient difference such that when a load is applied, a catastrophic failure of the

composite can be observed. As a result, an advanced interphase is critical to realize themaximum potential of carbon fibers in the composite.

The present paper introduces an innovative design methodology from Toray for such an

advanced engineering interphase (AEI) such that the resulting composite could reach a tensile

strength of 100% when normalized to strength of the carbon fiber and fiber volume (100%translation). The requirements for a stable carbon fibers surface and consistent surface chemical

functionalities as well as a stable sizing material for promoting a desired adhesion between the

fiber and a model resin are discussed.

2. EXPERIMENTATION

2.1. Materials

All PAN-based model carbon fibers were provided by Toray Industries, Inc. (Japan). Anintermediate modulus carbon fiber designated IM and a high modulus carbon fiber designated

HM were used for the study. A simple bisphenol-A epoxy resin system, R1, was used to study

the adhesion of the fibers to the resin while a model aerospace-grade resin, R2, was used toexamine tensile strength of the fiber composites.

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2.2. Methods

2.2.1. Surface Topography

A JEOL 7500F SEM equipped with a gentle beam technology and a Hitachi S-4800 SEMequipped with a deceleration beam technology both were used to examine and compare the

nanostructures on the fibers surfaces. Individual fibers were gently placed on an electrical tape

glued to an aluminum mount (also called stud or stub), and inserted into a sample stage in themachine. When a negative bias voltage was applied to the stage the primary electron beam was

slowed down, and hitting the surface at a desired energy; thus surface structures is preserved

without damaging the surface. The secondary electron beam generated from the surface is thenaccelerated to the detector. The sample was observed between 1-3mm working distance, and an

accelerating voltage of less than 1kV was used.

A SPI 3800N AFM from SII NanoTechnology Inc. was used to examine surface topography of

the fibers. Individual fibers were laid flat at least one fiber distance away from one another on a

clean silicon wafer. One end of each fiber was taped onto the wafer to keep them secure.

Tapping mode was performed with a silicon cantilever (SI-DF20) having a tip radius of 10nmand a catalog spring constant of 12N/m. After the laser was aligned, the tip was set onto the

fiber such that it was parallel to the fibers axis. The scan direction was perpendicular to the

fibers axis and at a speed of 1Hz.

2.2.2. Surface chemistry

Pristine IM and HM were oxidized by a proprietary wet chemistry method to introduce a desired

amount of oxygen functional groups onto their surfaces. An x-ray photoelectron spectroscopy

(XPS) technique was used to document a change in surface chemistry of the fibers. Severalconditions of an proprietary surface treatment method were performed. A Witec Alpha300

equipped with a digital pulse forced mode (AFM-DPFM), and a Veeco (now Bruker) DimensionIcon with PeakForce QNM were used to map an adhesion force between the tip and a location on

the carbon fibers surface. A description of PFM method was described by Marti et al. [2-3]

while details of PeakForce QNM can be found from the manufacturers website. When added to

an AFM System, the PFM extends the capabilities of the microscope beyond simply measuringtopography to include the investigation of all properties described by pulsed force and force-

distance curves. In addition to adhesion, other properties such as local stiffness, viscosity,

energy dissipation, contact-time, long range forces and many more can be analyzed and imagedsimultaneously along with topography. Yet, they are not in the scope of this study.

The electronics of PeakForce QNM is essentially similar to PFM. The electronics of the DPFM

include a high speed data-acquisition system, a freely programmable modulation generator and areal-time data evaluation module. With the storage of the complete measurement, extensive

post-processing data evaluation can be easily performed.

The aforementioned procedure in Section 2.2.1 was used to prepare samples and align the tip

onto a fibers surface. Once a suitable applied load (set point) on the surface was found to obtain

a stable a forc