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

Differential Scanning Calorimeter (TA Instruments, New Castle, DE). Uncured resin samples

were heated from 25 °C to 250 °C at 5 °C/min. The integrated value of the exotherm peak upon

cure was reported.

Linear shrinkage of the resins during cure was measured using ASTM D 2566-86. The samples

were cured for 2 hours at 90 °C, plus 2 hours at 150 °C.

2.3 Cured Resin Test Methods

Resin nanosilica content was determined using a 5 to 10 mg cured sample placed in a TA

Instruments TGA 500 thermo gravimetric analyzer (TA Instruments, New Castle, DE). Samples

were heated in air from 30 °C to 900 °C at 20 °C/min. The noncombustible residue was taken to

be the resin’s original nanosilica content.

The glass transition temperature (Tg) was measured using a Differential Scanning Calorimeter

(DSC) DSC Q2000 (TA Instruments, New Castle, DE) as the inflection point of the heat flow

(W/g) versus temperature graph using TA Instruments Universal Analysis Software.

Barcol hardness (HB) was measured according to ASTM D 2583-95 (Reapproved 2001). A

Barcol Impresser (Model GYZJ-934-1, available from Barber-Colman Company, Leesburg, VA)

was used to make measurements. For each specimen, between 5 and 10 measurements were

made and the average value was reported.

Fracture toughness was measured according to ASTM D 5045-99 using a compact tension

geometry, wherein the specimens had nominal dimensions of 3.18 cm by 3.05 cm by 0.64 cm

with W = 2.54 cm, a = 1.27 cm, and B = 0.64 cm. A modified loading rate of 1.3 mm/minute

(0.050 inches/minute) was used.

The tensile strengths, failure strains, and moduli of the resins at room temperature were measured

according to ASTM D638 using a Type I specimen. The loading rate was 1.3 mm/min (0.05

in/min). Five specimens were tested for each concentration level. The shear modulus was

calculated from the tensile modulus (E) and Poisson’s Ratio () which was measured using a

biaxial extensometer in the tensile test. The equation used to calculate the shear modulus (G) is:

G = E/[2(1+ ν)]. [1]

2.4 Preparation of Composite Overwrapped Pressure Vessels

Resin batches were prepared by combining the silica-containing epoxy resin with the curative

and mixing the batch manually for approximately 4 minutes until well blended. An MTHPA

(methyl tetrahydro phthalic anhydride) curative (Lindride 36Y from Lindau Chemicals) was

combined with the epoxy resin to achieve the stoichiometric ratio of 0.95 equivalents of

anhydride per equivalent of epoxy. Final silica contents of cured resins (epoxy and curative)

were 32.5, 18.9, and 10.6 wt%. A control sample containing no silica was also made. After

mixing the epoxy and curative, the resins were immediately placed into the coating bath on the

filament winding machine (ENTEC Composite Machines, Salt Lake City, UT). The resin

mixture for the control, 10.6 wt%, and 18.9 wt% silica resin vessels was not heated during the

coating operation. The resin for the 32.5 wt% silica resin vessel was heated to a temperature

between 35 and 43 °C during the winding process to reduce the viscosity and better control the coating weight of the resin on the carbon fiber. Aluminum liners (7.5-Liter) were mounted on

the filament winding spindle and 4 tows of T700-12K carbon fiber (Toray Carbon Fibers

America, Inc., Decatur, AL) were wound around the liners, with only a single spindle used at a

time, first in the hoop direction, then in the helical direction until the pattern was complete. The

same winding pattern was used for all the vessels in the hydroburst and impact/burst evaluations.

Three control vessels (resin with no silica), 2 vessels of 32.5 wt% silica resin, 3 vessels of 18.9

wt% silica resin, and 3 vessels of 10.6 wt% silica resin were prepared for the hydroburst test.

In the separate impact/burst evaluation, five control vessels and 2 vessels with 32.7 wt% silica

resin were prepared. The same liner type, fiber lot, and filament winding pattern was used for

the impact/burst evaluation as the hydroburst (no impact) study.

In the third pressure vessel study, the fatigue vessels were prepared with a substantially different

winding pattern resulting in a thicker and heavier composite overwrap layer, but utilized the

same 7.5-liter aluminum liner type and carbon fiber lot as the hydroburst and impact/burst tests.

For the fatigue evaluation three each of the control vessels and those with 33.5 wt% silica resin

were prepared.

After the filament winding was complete for each individual study, the wet-wound vessels were

hung on a rack and were cured in an oven for 3 hours at 63 °C, then 2 hours at 91 °C, then an

additional 6 hours at 85 °C, and then were allowed to cool down over a period of approximately

3 hours to 27 °C.

2.5 Test Methods for Composite Overwrapped Pressure Vessels

Hydroburst testing was completed individually on each cured pressure vessel. The vessels were

filled with water and the pressure was increased such that the vessel burst pressure (ranging from

29.4 to 47.5 MPa) was reached in a time ranging from 50 to 140 seconds. The highest pressure

achieved in each vessel’s test was used to determine the vessel burst pressure and the calculated

fiber delivered strength (fiber translation efficiency).

Finite element analysis was performed on the vessels using a non-linear analysis with Algor FEA

software. Cured resin mechanical properties and burst pressure of the vessels were entered into

the model to determine the fiber delivered strength of each vessel.

To perform the impact/burst test, the vessels were impacted using a 4.5 kg steel rod

approximately 5.1 cm in diameter with a hemispherical end where it impacted the vessels. The

steel rod was dropped vertically from varying height (0.3, 0.6, 0.9, 1.8, and 3.7 m) onto the

middle of the sidewall of the empty vessel. No attempt was made to prevent the steel rod from

rebounding after initial impact, so that the rod struck the vessel multiple times for a single drop.

After impact, the depth of damage dent was measured using a straight-edge placed across the

dent. The vessels were then subject to the hydroburst test method.

Prior to the fatigue evaluation, autofrettage and a hydrostatic test were performed on the vessels.

The autofrettage pressure was 58.6 MPa with a 2 minute hold. The hydrostatic test consisted of

37.9 MPa with a 60 second hold time. To perform the fatigue (or cyclic) test on the pressure

vessels, the vessels were each filled with a glycol-inhibited water mixture (25/75 wt ratio of

glycol to water) and were subjected to pressure cycles between (not greater than) 3.1 MPa to the

upper cyclic pressure of 31.0 MPa at a rate of 10 cycles per minute or less. The minimum dwell

time in the pressure range between pressures of 27.9 and 31.0 MPa was not less than 1.2 seconds.

The test was terminated when the vessel no longer retained pressure. After 10,000 cycles, the

test was halted for approximately 1 week after which time the two remaining vessels which had

not leaked were started in the cyclic test again. After the test was re-started, the two remaining

vessels were run until they no longer retained pressure.

3. RESULTS AND DISCUSSION

3.1 Effect of Silica Concentration on Resin Properties

Tables 1-4 and Figure 1 summarize the data from the neat resin silica concentration study.

Table 1. Complex Viscosity for the Epoxy Resin/Curative Mixtures as a Function of

Temperature and Weight Percent of Nanosilica in the Resin

Temperature

(°C)

0 wt% Silica

(Pa-sec)

10 wt% Silica

(Pa-sec)

20 wt% Silica

(Pa-sec)

30 wt% Silica

(Pa-sec)

30 0.63 0.50 0.61 1.40

40 0.29 0.23 0.31 0.65

50 0.14 0.12 0.18 0.34

The viscosity of the matrix resin was increased through the addition of a high concentration of

nanosilica (30 wt%). Because mild heating (only a 10 °C increase in resin temperature) produced a viscosity for the 30 wt% silica resin comparable to the control sample, a gentle heat

source on the resin coating bath was the only process modification required to successfully use

the silica-containing epoxy resins in composite pressure vessel production.

The chemical composition of the epoxy resins containing nanosilica were slightly modified from

the control epoxy resin to minimize the viscosity impact of the addition of a relatively high

concentration of nanosilica. This modification is responsible for the slightly greater viscosity of

the control resin relative to that of the resins containing 10 and 20 wt% silica at 30 °C. The effect of the chemistry change in the resin is also noted subsequently to explain the glass

transition temperature differences between the control and nanosilica-modified samples.

Table 2. Shrinkage and Hardness as a Function of Weight Percent of Nanosilica

Silica

(Wt. %)

Shrinkage

(%)

% Shrinkage

Reduction

from Control Hardness (HB)

0 0.53 -- 38

10 0.47 11 46

20 0.43 18 51

30 0.37 30 59

Table 2 shows essentially linear trends of increasing hardness and decreasing shrinkage as the

amount of nanosilica in the neat resin sample is increased. These trends are due to the nature of

the inorganic silica being harder than the organic epoxy resin and it is not susceptible to

shrinkage during cure.

The decrease in the shrinkage of the resin during cure is desirable in several composite

manufacturing processes due to the consequent reduction in residual stresses in the cured

composite parts. Residual stresses can adversely affect composite product performance,

particularly in fatigue environments. In some cases, the composite part dimensions are critical to

the product function, so the reduction in shrinkage of the silica epoxy resin is beneficial.

The increase in resin hardness is also advantageous to reduce the scuffing and abrasion of the

cured product in some applications where aesthetics are important. Further, cured matrix resin

hardness may affect the damage tolerance of composite parts in cases where more intense

product damage (impact) is more likely than scuffing.

Table 3. Mechanical Property Data as a function of Wt. Percent of Silica in the Resin

Wt. %

Silica in

Resin

Modulus E

(MPa)

Modulus G

(MPa)

Strength

(MPa)

Strain at

Yield (%)

Maximum

Strain at

Break (%)

Fracture

Toughness

(MPa*m1/2

)

0 3151 1145 88.9 6 10 0.52

10 3503 1262 84.8 5 11 0.79

20 3985 1448 86.2 4 11 0.9730 4606 1717* 88.3 4 9 1.11

Tensile Properties

* This value was calculated from the tensile modulus using the Poisson’s ratio for a sample with

32.5 wt% silica instead of 30 wt% silica

302520151050

4600

4400

4200

4000

3800

3600

3400

3200

3000

1.1

1.0

0.9

0.8

0.7

0.6

0.5

Weight % Silica in Resin

Ten

sile

Mo

du

lus

(MP

a)

Fra

ctu

re T

ou

gh

ness

(M

Pa

*m

^1

/2)

Figure 1. Tensile Modulus (circles) and Fracture Toughness (squares) as a function of the weight

percent of silica in the resin

The data in Table 3 and Figure 1 show that the nanosilica is very effective at increasing the

modulus of the resin while simultaneously increasing the fracture toughness. This significant

improvement in both properties is of particular interest since additives or mechanisms which

increase resin modulus normally cause increased brittleness. Additionally, the additives which

are known to increase fracture toughness generally adversely affect the resin system modulus. It

is notable that the addition of the nanosilica has produced concurrent improvements in both of

these important properties. The improvement in fracture toughness might also be expected to be

beneficial in composite fatigue environments.

Table 4. Thermal Property Data as a Function of Weight Percent of Nanosilica in the Neat Resin

Silica

(Wt. %)

Glass

Transition

Temperature

Tg (°C)

Exotherm

(J/g)

% of

Control

Exotherm

0 134 293.5 --

10 114 269.1 92

20 119 234.4 80

30 119 180.0 61

The glass transition temperature is slightly decreased in the nanosilica-containing resins when

compared to the control due to the modification of the chemistry of the epoxy to provide

viscosity control.

As anticipated, the inclusion of the inorganic silica produced a significant reduction in the

exotherm experienced by the curing resin. When the reduced exotherm is considered in

combination with the shrinkage reduction experienced in the silica-containing resins, there is a

clear tendency to reduce shrinkage stress in cured composite articles.

Composite matrix resin properties are incorporated into finite element analysis to predict

performance in composite overwrapped pressure vessels. Generally the tensile and shear

modulus of the neat matrix resin are two parameters used in the models. The model analysis and

predictions are heavily dominated by the contributions of the carbon fiber which has a

significantly higher modulus than the epoxy matrix resin. With these factors in mind, a study

was undertaken to evaluate the effect of silica concentration in the epoxy matrix resin for carbon

overwrapped pressure vessels on the vessel performance. Several elements of pressure vessel

performance were evaluated including hydroburst performance, damage tolerance in the form of

impact then hydroburst performance, and finally, cyclic fatigue performance. Of the three Type

III pressure vessel evaluations, only the hydroburst performance was evaluated at multiple silica

concentrations in the epoxy matrix resin. The other two studies evaluated only a control sample

and one with high nanosilica concentration.

3.2 Effect of Silica Concentration on Pressure Vessel Hydroburst Performance

A series of studies were undertaken to evaluate the performance of the silica-containing epoxy

resins in Type III pressure vessels. The resins containing various silica concentrations were used

to wet the fibers and the pressure vessels were prepared by filament winding. After curing the

vessels, the various tests were performed including hydroburst, impact then burst, and fatigue.

For the hydroburst evaluation, three silica concentrations (10.6, 18.9 and 32.5 wt%) in the matrix

resin and a control composition (0 wt% silica) were evaluated. As noted in Table 5, 2 or 3

pressure vessels of each composition were prepared and evaluated. The hydroburst pressure was

used to calculate the fiber delivered strength for each vessel.

Table 5. The Hydroburst Pressure and Fiber Delivered Strength of Type III Pressure Vessels as a

Function of the Weight Percent Nanosilica in the Matrix Resin

Wt. %

Silica in

Resin

Hydroburst

Pressure

(MPa)

% Increase in

Avg. Burst

Pressure over

Control

Calculated

Fiber

Delivered

Strength

(MPa)

Average Fiber

Delivered

Strength (MPa)

Coefficient

of Variation

% Fiber

Strength

Improvement

vs. control

0 43.69 4981

0 43.31 -- 4925 5017 2.3 --

0 44.84 5147

10.6 43.40 4924

10.6 43.53 -1.0 4944 4937 0.22 -1.6

10.6 43.53 4944

18.9 44.17 5012

18.9 45.57 2.6 5214 5146 2.26 2.6

18.9 45.57 5214

32.5 45.98 5224

32.5 47.50 6.4 5441 5332 2.88 6.3

32.518.910.60.0

5500

5400

5300

5200

5100

5000

4900

Weight % Silica in Resin

Fib

er

Deliv

ere

d S

tren

gth

(M

Pa

)

Figure 2. Fiber delivered strength of the T700 fiber in the Type III composite overwrapped

pressure vessels as a function of the weight percent of silica in the matrix resin of the composite

The box plot in Figure 2 is made from the small data set in Table 5. Figure 2 represents

data such that the top and bottom boundary of the data boxes represent the high and low test

result for that nanosilica concentration, and the horizontal line within the box signifies the data

median. If two data points out of three overlap, no median is shown. If only two data points

exist, the horizontal line within the box represents the average of the data points for that

nanosilica concentration.

Statistical analysis of the data from Table 5 shows that there is a significant effect of silica

concentration on the fiber delivered strength of the composite in the pressure vessels. This result

is in contrast to the understanding of the role of the matrix on in-situ fiber strength, which

assumes no sensitivity to neat resin properties. In these experiments increases in burst pressure

were measured for COPVs that were identical in every way other than matrix properties.

Figure 3a. Photograph of the Type III pressure vessel with the control resin (no nanosilica) after

hydroburst test

Figure 3b. Photograph of the Type III pressure vessel with 32.5 wt% of nanosilica in the matrix

resin after the hydroburst test

Figures 3a and b show the resultant damage to the Type III pressure vessels after the hydroburst

test was completed. The rupture of the aluminum liner in the sidewall of the vessels is clearly

visible as is the damage and the explosive failure of almost all of the hoop wraps of the

composite in the case of the control vessel with no nanosilica in the matrix resin.

3.3 Effect of Silica on Pressure Vessel Impact/Burst Performance

The second pressure vessel performance evaluation completed in this series of studies was a

damage resistance evaluation in which the cured pressure vessels were impacted by a falling dart

of varying impact energy and then were subjected to hydroburst testing.

Table 6. The impact energy, damage depth, hydroburst pressure and fiber delivered strength for

Type III pressure vessels that had been impacted by a falling dart and then hydroburst as a

function of nanosilica content in the matrix resin

Wt. %

Silica in

Resin

Impact

Energy

(kg-m)

Damage

Depth (cm)

Hydroburst

Pressure

(MPa)

Calculated

Fiber

Delivered

Strength

(MPa)

Average

Fiber

Delivered

Strength

(MPa)

% of Control

Fiber

Strength*

% of Virgin

Fiber

Strength **

0 1.4 0.00 43.18 4908 97.7

0 2.8 0.13 41.29 4635 92.4

0 4.2 0.18 43.32 4928 98.2

0 8.3 0.28 40.44 4511 89.9

0 16.6 1.04 29.44 2944 100 58.7

32.7 16.6 0.76 37.05 3960 3816 130 71.6

32.7 16.6 0.81 35.00 3674 76.6

* The % of control fiber strength was calculated by dividing the average fiber delivered strength

of the vessels containing 32.7 wt% silica (3816 MPa) by that of the control vessel impacted with

the same impact energy (2944 MPa), and calculating a percentage

** The % of virgin fiber strength was calculated by dividing the fiber delivered strength of the

impacted then burst vessels by the same average value for a control vessel that was not impacted

(5017 MPa from Table 5).

The greater the impact energy imparted to the pressure vessel by the falling dart, the deeper the

permanent deformation of the sidewall of the vessel became (damage depth), and the more

compromised fiber delivered strength resulted (calculated fiber delivered strength). To

determine a level of impact energy appropriate for testing the silica-containing matrix resin,

several control vessels were built, damaged at a range of impact energy, and burst. Figure 4

shows the fiber delivered strength as a function of the impact energy of the falling dart for the

control vessels, and the fiber delivered strength of the vessels containing 32.7 wt% silica in the

epoxy matrix resin.

181614121086420

5000

4500

4000

3500

3000

Impact Energy (kg*m)

Fib

er

Deliv

ere

d S

tren

gth

(M

Pa

)

Figure 4. The calculated fiber delivered strength achieved by the pressure vessel as a function of

the impact energy of the falling dart for the control vessels with no silica (circles) and the vessels

containing 32.7 wt% silica in the matrix resin (squares).

Figure 5 shows the average fiber delivered strength for the control vessels (0 wt% silica) and the

vessels made with 32.7 wt% nanosilica in the epoxy matrix resin before and after being impacted

with 16.6 kg-m of energy by the falling dart. Figure 5 also clearly summarizes the performance

improvement seen in the pressure vessels containing nanosilica-modified matrix resin before

impact (from 5017 MPa in the control to 5332 MPa for the silica resin), and also the reduced

effect of the impact damage for the vessels containing the silica-modified epoxy matrix resin.

The ability for pressure vessels to withstand incidental impacts without significant property

degradation is important in a very practical sense for vessel use and installation in the field, as

well as for user safety for vessels which have long lifetimes and varied service requirements.

Control or Silica

Before or After Impact

SilicaControl

AfterBeforeAfterBefore

6000

5000

4000

3000

2000

1000

0

Avg

. F

iber

Del

iver

ed S

tren

gth

(M

Pa)

5017

2944

5332

3816

Figure 5. Average fiber delivered strength for control vessels (un-hashed boxes) containing 0

wt% silica and vessels containing 32.7 wt% silica (cross-hashed boxes) in the epoxy matrix resin

before and after being impacted with 16.6 kg-m of energy

3.4 Effect of Silica on Pressure Vessel Cyclic Fatigue Performance

The final phase of evaluation for this study was focused on cyclic fatigue performance of Type

III pressure vessels with three control vessels (0 wt% silica) and three vessels containing 33.5

wt% silica in the matrix resin. Fatigue performance is an important design criterion for long

service life pressure vessels and long design lifetimes can often require increased thickness in the

liner and/or in the composite overwrap of the Type III pressure vessel.

The fatigue failure mechanism for Type III pressure vessels is the development of a small crack

in the metal liner of the pressure vessel and a leakage of the gaseous or liquid contents. The role

of the composite overwrap during fatigue cycling is to reduce the cyclic strain range and

maximum strain experienced by the liner. During pressure cycling, matrix micro cracks and

some fiber breakage can occur. Any reduction in composite stiffness results in less constraint of

the liner so that it experiences greater cyclic strains. Therefore both initial composite stiffness

and resistance to fatigue damage are important factors in Type III pressure vessel fatigue

behavior.

Table 7. The number of cycles to failure and % average cycle improvement for Type III pressure

vessels for a control epoxy resin and 33.5 wt% nanosilica in the matrix resin

Wt. %

Silica in

Resin

Cycles to

Failure

Average

Cycles to

Failure

Coefficient

of

Variation

(%)

% Average

Cycle

Improvement

over Control

0 6214

0 6458 6,689 9.3

0 7394

33.5 8588

33.5 12400* 10,399 18.4 55.5

33.5 10210*

* These two tests were terminated at 10,000 cycles and testing was resumed several days later

The fatigue performance is characterized by taking an average number of cycles to failure.

Industry standard coefficient of variation for the fatigue test is approximately 20%. The fatigue

performance resulting from this evaluation showed that the vessels containing silica-modified

matrix resin provided, on average, a 55% increase in the cycle performance of the pressure

vessel. As an illustrative example, in an application where a vessel might be expected to endure

300 pressure cycles per year for this pressure level, the control vessel from the example above

would be expected to have a lifetime of 22 years. For the same vessel design, same

manufacturing process, and same carbon fiber type, the vessels made with the silica-modified

matrix resin would be expected to have a lifetime of 34 years. The implications of the increased

service life are to permit vessel manufacturers to re-design their pressure vessels and remove

composite content to retain an equal service life to an existing product, or to offer a product of

similar design, certified for a considerably longer service life. It is also significant to note that

sometimes vessels designed specifically for fatigue life specify carbon fiber with a higher

modulus, such as intermediate modulus fiber, to achieve fatigue design targets. Generally, the

higher modulus carbon fiber is also more expensive than standard modulus fiber. The use of the

nanosilica-modified matrix resins offers pressure vessel designers another tool to balance product

weight, life, performance and cost.

4. SUMMARY AND CONTEXT

The effect of nanosilica concentration on matrix resin properties and in composite overwrapped

Type III pressure vessels was studied. Dramatic concurrent enhancements in both resin modulus

and fracture toughness are correlated with increases in the nanosilica concentration in the matrix

resin. Other desirable resin properties are also improved by the incorporation of silica in the

matrix resin such as exotherm and shrinkage reduction upon cure and the increase of hardness in

cured samples. The desirable mechanical property improvements were translated into

improvements in finished composite articles such as Type III pressure vessels with only very

slight modification of the production process.

The incorporation of silica up to 33.5 wt% in the matrix resin resulted in Type III pressure

vessels demonstrating at least 6% increase in fiber delivered strength measured by hydroburst

testing, 30% improvement in fiber delivered strength measured by hydroburst testing after

impact damage, and 55% improvement in cyclic fatigue life in comparison to control samples.

The current work has led to the development of 3M™ Matrix Resins 4831, 4832 and 4833 for

filament winding applications. Silica-modified prepreg resins have shown significant

improvements in compression-dominated composite properties [3,4]; these have been

commercialized as 3M™ Prepregs 3831 and 3832. The present study shows improvement in a

carbon fiber tensile-property dominated application, and extends the consideration to fatigue and

impact. Other related work demonstrates the composite performance improvements and

processing benefits achieved with 3M™ Matrix Resin 5831 and 5832 for pultrusion applications

[5].

5. FUTURE WORK

The nanocomposite matrix technology described here is being extended into additional resin

systems for prepreg and liquid composite processing, offering new and exciting opportunities for

composite designers to take advantage of matrix resin technology to address their most

challenging product requirements.

6. ACKNOWLEDGEMENTS

We would like to thank Rachel Wilkerson for neat resin fracture and tensile modulus testing.

7. REFERENCES

1. Mao, C.S., Yang, M.F., Hwang, D.G., Wang, H.C. “An estimation of strength for composite pressure vessels.” Composite Structures 22 (1992): 179-186.

2. Thesken, J., Murthy, P. L.N., Phoenix, S.L. “Composite Overwrap Pressure Vessels: Mechanics and Stress Rupture Lifing Philosophy.” NASA TM-2009-215683, 2009.

3. Hackett, S.C. “The Effect of Nanosilica concentration on the enhancement of epoxy matrix

resins for prepreg composites.” Proc. Fall Society for the Advancement of Materials and

Process Engineering Technical Conference, Salt Lake City, UT, Oct. 11-14, 2010

4. Hackett, S.C. et al. “Improved carbon fiber composite compression strength and shear stiffness through matrix modification with nanosilica.” Proc. American Society for Composites 25th

Annual Technical Conference. Dayton, OH, Sept. 20-22, 2010.

5. Thunhorst, K.L., Goetz, D.P., Hine, A.M, Sedgwick, P. “The effect of nanosilica matrix modification on the improvement of the pultrusion process and mechanical properties of

pultruded epoxy carbon fiber composites.” Proc. American Composites Manufacturers

Association, Ft. Lauderdale, FL, Feb. 2-4, 2011.