Copyright 2011 by 3M. Published by the Society for the Advancement of Material and Process Engineering with permission. Nanosilica Concentration Effect on Epoxy Resins and Filament- Wound Composite Overwrapped Pressure Vessels Kristin L. Thunhorst, Andrew M. Hine, Paul Sedgwick, Mike R. Huehn 3M Composite Materials, Industrial Adhesives and Tapes Division Douglas P. Goetz 3M Corporate Research Materials Laboratory 3M Center St. Paul, MN 55144 ABSTRACT A study was undertaken to investigate the effect of nanosilica concentration on important epoxy neat resin and carbon fiber composite properties. In particular, the focus of the subject study is a resin appropriate for filament-wound carbon fiber composite applications. An experimental epoxy nanocomposite matrix resin was investigated at nanosilica loading levels from 0 to 33 % by weight. The resin was cured with a liquid anhydride curative (MTHPA, methyl tetrahydrophthalic anhydride, Lindride 6K and Lindride 36Y from Lindau Chemicals). The effect of silica concentration on neat resin properties was evaluated. The cured neat resin properties, including modulus, fracture toughness, and hardness, showed significant monotonic improvement with increasing nanosilica concentration. Desirable improvements in other properties such as reduction in cure exotherm and shrinkage were also quantified. Additionally, Type III carbon fiber composite overwrapped pressure vessels (COPV) were prepared via filament winding and were evaluated for improvements in burst pressure and fiber delivered strength. The results of this COPV study show that the increasing concentration of nanosilica in the filament winding matrix resin provided improvements in the burst pressure of the article and the total fiber delivered strength of the pressure vessels. Subsequent studies showed that high nanosilica concentration in the matrix resin of Type III carbon fiber COPVs also provided improvement in fiber delivered strength after impact damage as well as significantly improved cyclic fatigue life. 1. INTRODUCTION The objective of the current work is to investigate the effect of the concentration of nanosilica on epoxy matrix resin thermal and mechanical properties and to evaluate the performance in Type III carbon fiber overwrapped pressure vessel composites. 1.1 Role of the Fiber Matrix in Composite Overwrapped Pressure Vessels Composite Overwrapped Pressure Vessels (COPVs) are manufactured by filament-winding continuous fibers around a liner in directions designed to place the long directions of the fibers in the primary loading directions. This practice takes advantage of the excellent longitudinal stiffness and strength characteristics of structural fibers. In Type III (metal-lined, composite overwrapped) and Type IV (polymer-lined, composite overwrapped) pressure vessels, the use of carbon fiber is especially common in combination with thermosetting epoxy matrix resins.
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Copyright 2011 by 3M. Published by the Society for the Advancement of Material and Process
Engineering with permission.
Nanosilica Concentration Effect on Epoxy Resins and Filament-
Wound Composite Overwrapped Pressure Vessels
Kristin L. Thunhorst, Andrew M. Hine, Paul Sedgwick, Mike R. Huehn 3M Composite Materials, Industrial Adhesives and Tapes Division
Douglas P. Goetz
3M Corporate Research Materials Laboratory
3M Center
St. Paul, MN 55144
ABSTRACT
A study was undertaken to investigate the effect of nanosilica concentration on important epoxy
neat resin and carbon fiber composite properties. In particular, the focus of the subject study is a
resin appropriate for filament-wound carbon fiber composite applications. An experimental
epoxy nanocomposite matrix resin was investigated at nanosilica loading levels from 0 to 33 %
by weight. The resin was cured with a liquid anhydride curative (MTHPA, methyl
tetrahydrophthalic anhydride, Lindride 6K and Lindride 36Y from Lindau Chemicals). The effect
of silica concentration on neat resin properties was evaluated. The cured neat resin properties,
including modulus, fracture toughness, and hardness, showed significant monotonic
improvement with increasing nanosilica concentration. Desirable improvements in other
properties such as reduction in cure exotherm and shrinkage were also quantified. Additionally,
Type III carbon fiber composite overwrapped pressure vessels (COPV) were prepared via
filament winding and were evaluated for improvements in burst pressure and fiber delivered
strength. The results of this COPV study show that the increasing concentration of nanosilica in
the filament winding matrix resin provided improvements in the burst pressure of the article and
the total fiber delivered strength of the pressure vessels. Subsequent studies showed that high
nanosilica concentration in the matrix resin of Type III carbon fiber COPVs also provided
improvement in fiber delivered strength after impact damage as well as significantly improved
cyclic fatigue life.
1. INTRODUCTION
The objective of the current work is to investigate the effect of the concentration of nanosilica on
epoxy matrix resin thermal and mechanical properties and to evaluate the performance in Type
III carbon fiber overwrapped pressure vessel composites.
1.1 Role of the Fiber Matrix in Composite Overwrapped Pressure Vessels
Composite Overwrapped Pressure Vessels (COPVs) are manufactured by filament-winding
continuous fibers around a liner in directions designed to place the long directions of the fibers in
the primary loading directions. This practice takes advantage of the excellent longitudinal
stiffness and strength characteristics of structural fibers. In Type III (metal-lined, composite
overwrapped) and Type IV (polymer-lined, composite overwrapped) pressure vessels, the use of
carbon fiber is especially common in combination with thermosetting epoxy matrix resins.
The stiffness and strength of the fiber greatly exceed those of the matrix resin. For example,
carbon fibers having a tensile modulus of about 200 to about 550 GPa are available, while the
tensile modulus of a typical matrix resin is one to two orders of magnitude smaller, e.g., the
tensile modulus of epoxies is about 2.5 to 4.5 GPa. Because of the much greater stiffness of the
fibers relative to the matrix resin, load in the fiber direction of the composite overwrap is carried
mostly by the fibers. Therefore the strength in the fiber direction is dominated by the fiber
strength and best utilization of the high fiber strengths is achieved by orienting the fibers in the
primary loading directions of composite pressure vessels.
Optimum design of composite pressure vessels requires efficient utilization of the constituent
materials, especially the fiber. The cost, weight, and strength of a Type III or IV pressure vessel
are all dominated by fiber utilization. Typically the fiber used is more expensive by weight than
the matrix resin constituent, as well as being of higher density. The density of carbon fibers is
about 1.8 g/cc, whereas the density of common matrix resins are lower, e.g., about 1.2 g/cc for
epoxy resins. In addition, the processing time (and cost) required to fabricate a pressure vessel as
well as its weight are advantageously affected by reducing the amount of fiber used. Optimum
design for a pressure vessel dictates achieving the required strength using the minimum amount
of fiber.
The dominance of the fiber strength on pressure vessel burst strength without regard to the
properties of the matrix resin is acknowledged in the literature concerning pressure vessel design.
Mao et al. [1] propose a method for estimating the fracture strength of a composite pressure
vessel assuming that the entire load is carried by the fibers. Thesken et al. [2] acknowledge the
industry recognition of the dominance of the fiber properties on pressure vessel strength and
negligible contribution of matrix when they state, “Following common filament winding design
practice, no strength nor stiffness is ascribed to the resin.”
1.2 Introduction to Matrix Resin Modification
Previous studies have shown that the mechanical properties of the matrix resin, particularly
epoxies, can be positively affected through modification with nanosilica [3,4] in resins that are
appropriate for prepreg composite applications. Properties such as resin modulus, fracture
toughness, and compression strength were shown to monotonically increase with increasing
nanosilica incorporation into the matrix resin. These neat resin properties for the modified
prepreg resins have been shown to extrapolate into significant mechanical property improvement
in the final composite products.
1.3 Introduction of the Current Work
The current study investigates the effect of nanosilica concentration on liquid epoxy resins
appropriate for processes such as filament winding. The effect of the nanosilica concentration on
important neat resin mechanical properties such as tensile modulus, fracture toughness, hardness,
shrinkage, exotherm and viscosity are explored.
With the historical backdrop of the predicted relative unimportance of the matrix resin properties
on the final performance of the pressure vessels, the current work was dedicated to determine the
effect of modification of the matrix resin with nanosilica on both the neat resin properties as well
as the translation of those resulting properties into Type III pressure vessels. The three
performance aspects of Type III pressure vessels explored included fiber delivered strength
(evaluated through hydroburst performance), fiber delivered strength in vessels which had been
damaged by impact (followed by hydroburst), and cyclic fatigue life.
2. EXPERIMENTATION
2.1 Materials and Sample Preparation of Neat Resin
Resin samples were generated by dilution of an epoxy blend suitable for filament winding having
49.4 wt% silica of nominal particle size 81 nm. The MTHPA curative (Lindride 6K, Lindau
Chemicals, Columbia, SC) used to prepare the neat resin samples was combined with the epoxy
resin to achieve a stoichiometric ratio of 0.95 equivalents of anhydride per equivalent of epoxy.
Final silica contents of cured resins (epoxy and curative) were 30, 20, and 10 wt% for the neat
resin study. A control sample containing no silica was also made. The epoxy resin and curative
were combined prior to being poured into appropriate molds for curing of samples for neat resin
tensile testing, and determination of hardness, density, glass transition temperature, and fracture
toughness. The samples were cured in a forced air oven first for 2 hours at 90 °C and then for an
additional 2 hours at 150 °C.
2.2 Uncured Resin Test Methods
Rheological analyses of these nanosilica-epoxy resin/curative systems were conducted on an
ARES rheometer (TA Instruments, New Castle, DE) in parallel plate dynamic mode.
The cure exotherm was obtained using a modification of ASTM D 3418-08 using a Q2000