Document Sample
                         Alejandro Ureña, María R. Gude, Silvia G. Prolongo
      Dept. of Materials Science and Engineering. Escuela Superior de Ciencias Experimentales y Tecnología.
                          Universidad Rey Juan Carlos.28933 Móstoles (Madrid), Spain.

KEYWORDS: carbon nanofibers, nanoreinforced adhesive, epoxy, carbon fiber laminate


Mechanical methods for joining structures made with fiber reinforced polymeric composite
(FRC) usually cause stress concentrations and damage the continuous reinforcing fibres,
which reduce the overall load capacity of the structure. Adhesive joints are structurally more
efficient; having important advantages related with a higher flexibility of construction and
with the reduction the overall cost of manufacture. In this case, the integrity of the composite
structure is determined by the quality the adhesive joint, which depends on the joint design,
the substrate surface treatment and on the properties and application conditions of the
adhesive. If some of these factors fails, the joint will become the weakest part of the structure.

One way of improving behaviour of adhesive joints is to enhance the mechanical, chemical
or/and physical properties of the adhesives. Extensive attention is been paid to the
incorporation of nano-fillers to engineering polymers in order to enhance their properties such
as stiffness, toughness, electric and thermal conductivities, barrier properties, etc (1). Beside,
during the last years, the incorporation of these nano-fillers to the FRC matrices is been
proposed as an innovative solution for the improvement those composites properties which
are more depended of the matrix (i.e. transversal and interlaminar properties). The
combination of a polymeric nanocomposite with a FRC results in a multiscale composite
which combines reinforcement sizes from nanoscale to micron size fibers (2). This alternative
may be also applied to manufacture nanoreinforced adhesives for joining carbon FRC.

Present research study the development of an epoxy adhesive reinforced with carbon
nanofibers (CNFs) for joining carbon fibers reinforced laminates. Influence of the
nanoreinforcement characteristics (CNF contents and funtionalization stage) on the adhesive
properties (viscosity and dynamic mechanical properties) was evaluated. Beside, the
mechanical properties of the nanoreinforced adhesive joints, in comparison with unreinforced
ones, were evaluated under different conditions of surface preparation.

                                 EXPERIMENTAL AND RESULTS
The adherents used in this investigation were unidirectional ten-play laminates [0]10 of carbon
fiber/epoxy made by INTA using pre-impregates (Hexcell Hexply 8552/33%/268/IM7)
staking in an autoclave process. In order to get good bond strength and durability, it is
required to roughen the contact surface. For it, a peel ply fabric of polyester (Release Ply-C)
was added as the last layer in the lay-up of the composite part to be bonded. Monomer of the
epoxy resin used as adhesive was the diglycidyl ether of bisphenol A (DGEBA), with 178
g/epoxy equivalent and the curing agent was 4,4'-diaminodiphenylmethane (DDM). CNFs
were manufactured by Grupo Antolín with GANF1® denomination, having a diameter in the
range of 20 y 47 nm, and an average length of 35 µm. They were used in two different stages:
as-received and functionalized using the own curing agent of the adhesive (DDM).

Nanoreinforced adhesives were prepared with different CNFs contents (0.25, 0.5, 1 and 3 wt
%) by dispersing them in chloroform and adding the epoxy monomer under sonication (45
min at 45 ºC). After the elimination of dissolvent, heating at 80 ºC at vacuum during 24 h,
curing agent was added and adhesive was directly applied on adherent surfaces forming a
0.62±0.09 mm thickness layer. Joined specimens were cured for 3 h at 150 ºC and then post-
cured for 1 h at 180ºC.

Dinamomechanical thermal analysis (DTMA) of nanoreinforced adhesives showed maximum
increases in storage modulus of 20 %, when both as-received and functionalized CNFs were
used up to 1 wt % of reinforcement grade. Contents higher than 1 wt % favoured
agglomeration, decreasing mechanical properties of adhesive. Beside, although an increase in
viscosity of the epoxy resin was appreciated by the incorporation of the nano-filler, values
kept in the admissible range if the CNF contents were lower than 3 wt %. Single lap shear
tests (ASTM D5868) were performed on 25x100 mm composite specimens with a 25x25 mm
bonded area. Results of these tests showed that the best performances correspond to the
adhesive nanoreinforced with 0.5 wt % of as-received CNFs, which provide increases of shear
strength up to 15 % in relation with unreinforced adhesive. However, in all cases, SEM
analysis of the failed surfaces indicated that the failure was actually adhesive, consequence of
a limited adhesion between the adhesives and the adherent (figure 1).

         a)                                              b)

        Fig. 1: Fractographies of un-reinforced (a) and 0.5 wt % CNF (b) reinforced adhesive joints

Acknowledgements: Ministerio de Educación y Ciencia of Spain (MAT2007-61178)


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      nanotubes on the mechanical properties of epoxy matrix composites - A comparative
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   2. Dean D., Obore A.M., Richmond S., Nyairo E. “Multiscale fiber-reinforced
      nanocomposites: Synthesis, processing and properties”. Comp. Sci. & Tech. Vol. 66
      pp.2135-2142, 2006.