Anrp00 ultra –thick cross section fiber reinforced composites

ULTRA –THICK CROSS SECTION FIBER REINFORCED COMPOSITES
Project Code: F98-D04
Investigators: Yong K. Kim, Leader, Armand F. Lewis, Alex J. Fowler
Graduate Student: Jonathan Reuss, David G. Pineault
ABSTRACT
A thermo-chemical resin cure computer model was developed that can approximate the heat
transfer as a composite cures. Previous studies have shown that pre-catalyzation of glass fabric
surfaces and impregnating the treated fabric layers with an unsaturated polyester resin matrix
exhibit a lower exotherm during the cure of thick composite assemblies. The mechanical
properties of these pre-catalyzed composites were found to be superior to conventional "bulk"
cured unsaturated polyester/glass fabric composites. Additional data on 120-ply, glass
fabric/unsaturated polyester composites confirm these results. The exotherm in pre-catalyzed
fabric (120-ply) composites averaged only 30C to 40C (higher than the curing temperature of
80C). This compared with the conventiona l bulk cure exotherm in similar composites of 80C to
85C above the cure temperature. The tensile, compressive and the shear strength were
significantly improved in these unsaturated polyester/catalyzed glass fabric composite systems.
New studies have been started using vinylester resins as the matrix resin. Here, the efficiency of
catalyzing the glass fabric with various levels of benzoyl peroxide (2% to 6%) catalyst was
examined. Tensile strength of the sample precatalized with 3% benzoyl peroxide showed the
maximum. It was decided that this 3% formulation will be used for subsequent catalyzing
reinforcement fabrics for vinylester thick composite fabrications.

PROJECT OBJECTIVE

The objective of this continuing project is to apply the pre-catalyzed yarn/glass fabric resin cure
methodology in the manufacture of thick fiber reinforced composites using vinyl ester resin
matrixes. This work is a continuation of research that was previously done employing
unsaturated polyester resin as the matrix. The thermo-chemical resin cure model previously
developed will be modified to accommodate the curing kinetics of vinyl ester resins. Thick
composite fabrication and testing experiments will be carried out to verify this model. Thick
walled flat laminate and filament wound tube specimens with various fiber orientations will be
used in these new experiments

INTRODUCTION

A method for pre-catalyzing yarns/glass fabric with benzyol peroxide was developed. These pre-
catalyzed fabrics were found to effective in fabricating thick cross-section laminar composites
National Textile Center Annual Report: November 2000 using unsaturated polyester resin matrices [1]. This procedure resulted in the creation of a laminate lay-up technique having a greater control of the unsaturated polyester resin’s cure rate. Here the cure reaction was initiated at the yarn’s surface and radiated into the uncatalysed bulk polyester resin. This resulted in geometrically leveling out the curing process and caused an overall slowing down of the cure reaction such that the exotherm in the reaction was minimized. Having this level of control of the cure reaction should now enable the fabrication of multiple ply (ultra-thick) composites. This methodology has been shown useful in creating (120-ply) layered unsaturated polyester resin yarn/glass based fiber reinforced composites [1]. Of importance, these composites had a much higher inter- laminar shear strength, compared to typical bulk resin polymerized laminates. This suggests that the fiber-to matrix adhesion is significantly increased in these pre-catalyzed yarn cure initiated systems. Furthermore, because of the low cure exotherm, and control of the resin curing process, this technique could presumably be used to create composite lay ups of any desired number of plies. The concept of pre-catalyzed yarn curing of composites is limited to resins that cure by a free radical curing mechanism. Free radical polymerization proceeds by the initiation of free radicals (electrons) and the subsequent diffusion of these free radicals throughout the bulk resin/monomer system. Hence, the mechanism of catalyzed yarn curing simply means that free radical initiation occurs at the composite’s reinforcing yarn surface. Free radical polymerization proceeds instantaneously and radiates out from the surface. In some early experiments, it was shown that the polymerization of uncatalyzed bulk polyester could occur outward to over 1 mm from the initiating catalyzed surface. This is the reason why in making catalyzed yarn lay ups, one can obtain excellent composite laminates by laying up catalyzed fabric at alternate plies, i.e. every other layer[1]. Overall the catalyzed yarn curing technique appears promising. However, so far the process has only been demonstrated for unsaturated polyester resin composites. One would like to apply this technique to using epoxy resin matrices. While epoxies are generally superior in mechanical strength and thermal stability to unsaturated polyesters as matrix resins, their chemical curing mechanism does not lend itself to the “catalyzed fabric” process. Epoxy resins cure by an addition process involving the diffusion of large curative molecules (polyamines, anhydrides, etc.). The epoxy cure process is much slower and occurs by a continuous build-up of molecular weight. Epoxies would not be readily adaptable to this pre-catalyzed reinforcing fabric technique. One type of composite matrix resin that would be adaptable to the catalyzed fabric curing process is the vinyl ester. In engineering properties, they are considered to lie between unsaturated polyesters and epoxies. Vinyl ester resins approximate an epoxy backbone with unsaturation points. It is appropriate that this project continue with a focus on vinyl ester resins as the composite matrix material. The goal of the project is to determine the properties of thick cross section composites made using pre-catalyzed glass fabric/yarn and a vinyl ester resin matrix material. The properties of the vinyl ester material will then be compared with composites made using unsaturated polyester matrices[1]. National Textile Center Annual Report: November 2000 EXPERIMENTAL
Glass/Polyester 120-ply pre -catalyzed fabric composites
Experiments on unsaturated polyester based pre-catalyzed glass fabric composites were
continued by the fabrication of 120-ply composites. The procedure was the same as for the 60-
ply composites reported on earlier [1]. This 120-ply composite lay up is shown in Figure 1.
Thermocouple insertions were included in this lay-up to record the exotherm. The cure
procedure was as previously reported. Several 120-ply laminates were prepared using pre-
catalyzed plies alternating with un-catalyzed glass fabric plies. These laminates are referred to
as being "partially" catalyzed.
Mechanical Testing of the Prepared Laminates from the 120-ply Composites

Tens ile test ASTM D-3039 [6], compression test (ASTM D695)[4], in plane shear test (ASTM
D3846)[5] and interlaminar shear test ASTM D2344[7] were used to determine the mechanical
properties of the fabricated composites. ASTM D-638 was also used for measuring the tensile
strength of the prepared laminates. This was done by cutting out at least 6 ASTM D638, Type
IV specimens from each sample [3]. The tests were carried out on Instron Model 5500 universal
testing machine at a cross- head speed of 0.2 inches/minute. The tensile strength was averaged
over the 6 replicate samples tested.



Figure 1. Lay up of a 120-ply composites (T1 to T7 indicate the location of thermocouples)
National Textile Center Annual Report: November 2000 Precatalyzing Solution Formulation for Vinylester Resin Curing.

Studies on vinylester resin based composites were started by conducting several tests with
varying amounts of initiator to determine the optimum amount of benzoyl peroxide needed in
these pre-catalyzing glass fabrics. The experiments were conducted on 9” X 9” 20-ply lay up
composites. The flat laminate sample ply arrangement is presented in figure 2. The procedure
involved “sizing” the surface of the glass fiber fabric with various amounts of benzoyl peroxide
based on the concentration of peroxide in the toluene/polystyrene solution used as the “sizing’
bath. In this series of experiments the benzoyl peroxide concentration in the “sizing” solution
varied from 2% to 6% by mass. The exact sizing procedure used was reported earlier[1][2]. The
uncatalyzed vinyl ester resin was applied onto the pre-catalyzed fabric at room temperature with
a paintbrush. The resin was allowed about one minute to impregnate the fibers before the next
laminate layer of catalyzed fabric was added. This is shown in Figure 2. The sequential lay-up
was continued until the 20th layer is added. At the 10th and 20th layer a type-k thermocouple is
added to monitor the temperature (seeFigure2). The uncured laminate was then placed in a flat
press heated to 80 oC and then 27 psi of laminate pressure applied. The laminate was then cured
in the press for 30 minutes at 80 oC and then post cured for a minimum of 12 hours at 80 oC.

Figure 2. Lay up of a 20-ply composite for precatalyzed glass/vinylester.

RESULTS AND DISCUSSION

Glass Fabric/ Polyester 120-ply Composites.
During the cure of the 120-ply (unsaturated polyester) composites, it was found that the
maximum temperature in the center of the pre-catalyzed composites was between 30 oC and 40
oC above the platen temperature of 80C. The bulk cured composite reached a temperature of
over 80 oC above the platen temperature. These data are shown in Figure 3. Clearly, pre-
catalyzed glass fabric unsaturated polyester matrix composites have a much lower exotherm
during cure.
National Textile Center Annual Report: November 2000 Fuly pre-cataylzed
P a r t i a l l y p r e - c a t a l y z e d Temperature (C)
Time (min)
Figure 3: Comparison between the center temperatures of the 120-ply composites.


Mechanical Properties Of the 120-Ply Composites
The mechanical testing results are presented in Figures 4, 5, 6 and 7. All the data showed weaker
center samples in the conventional (bulk) catalyst/resin composite. This difference between pre-
catalyzed and conventional composites has also been statistically verified. The weakness of the
conventional composite is a direct result of the exotherms that occurred within these bulk resin
catalyzed composites. Both of the pre-catalyzed fabric composites showed similar strengths
throughout their thickness, without any severely weaker samples. These results gained through
mechanical testing support the belief that pre-catalyzing the fabric prior to composite lay-up will
produce a better final product. Figures 4 through 7 show that the pre-catalyzed composites have
superior tensile strength, compressive strength, in-plane shear strength, and interlaminar shear
strength compared to conventional catalyst/resin mixed composites. Of additional significance is
the observation that the "partially" pre-catalyzed composites have mechanical strengths equal to
the "fully" catalyzed composite specimens.


National Textile Center Annual Report: November 2000 Tensile Strength (mPa) 20
Figure 4. Average maximum tensile strength composite comparison .
Compressive
Strength(mPa) 150
Figure 5. Average maximum compressive strength composite comparison.
National Textile Center Annual Report: November 2000 In-plane Shear Strength(mPa) 30
Figure 6. Average maximum in-plane shear strength composite comparison.
Interlaminar Shear Strength
Sample Number
Figure 7. Average maximum interlaminar shear strength composite comparison

National Textile Center Annual Report: November 2000 Effect of Initiator Concentration on the Tensile Strength of Vinyl Ester Composites

The precatalyzing solutions were formulated with 6 different concentrations of benzoyl peroxide.
Using these precatalyzing solutions, 20-ply glass fabric/vinylester composite plates were
fabricated. Tensile strengths of these samples indicate that within the concentrations of benzoyl
peroxide tested and the standard deviation of the data, there is no significant difference in tensile
strength among the samples fabricated with concentrations used (see Table 1).
Table 1. Specimen data from tensile test ASTM D638.

From these data it was decided that the optimum concentration of benzoyl peroxide to use in
these “sizing”(precatalyzing) treatments is 3 grams for each 100 ml of 5% polystyrene 95%
toluene solution.


FUTURE WORK
The next step is to implement the thermo-chemical cure computer model to predict the heat
transfer during the cure of vinylester/pre-caltalyzed glass fabric. Thick cross-section composites
of varying thickness using both pre-catalyzed and uncatalyzed glass fabric will be fabricated. It
will be examined if the same success can be achieved with vinyl ester as was achieved with
unsaturated polyester. Tensile test (ASTM D638)[3], compression test (ASTM D695)[4], and in
plane shear test (ASTM D3846)[5] will be used to verify the effectiveness of this approach for
the case of vinylester. Results from catalyzed and uncatalyzed composites will compared to
determine the usefulness of pre-catalyzation with vinyl ester resin matrices. After completing
the vinyl ester resins, the next area of investigation will be the fabrication of thick walled
composite tubes using the filament winding process. The computer controlled filament-winding
machine gives one the availability to produce ultra-thick cylindrical composites with different
fiber placements. Filament winding may be used to produce a variety of resin and fiber
combinations. Preparation and testing of these filament wound tubes will be carried out with
reference to comparing the results between unsaturated polyester and vinyl ester resins.
National Textile Center Annual Report: November 2000 CONCLUSIONS

A method for pre-catalyzing glass fabric has been developed for unsaturated polyester resin
matrices. This process results in greater controllability of the cure by slowing the reaction
enough to allow the laying up of an increased number of plies. Low exotherms are observed
when the pre-catalyzed fabric process is used. Mechanical tests demonstrate that these low
exotherm composites have superior mechanical properties to conventionally cured (bulk
catalyzed) unsaturated polyester/glass fabric composites.
A method for pre-catalyzing vinyl ester resin is being developed. Early results indicate that vinyl
ester matrix resins will also be adaptable to the pre-catalyzed glass fabric laminate fabrication. A
parametric study is now in progress involving the use of pre-catalyzed fabric in the fabrication of
an ultra–thick composite in a vinyl ester matrix. In this study we hope to show that precatalyzing
will significantly reduce any exotherm produced during its cure cycle. After completing the
analysis of precatalyzation using the vinyl ester resin system, the investigation concerned with
creating thick walled cylindrical composites using filament winding will begin.
REFERENCES

1. Reuss, Jonathan D. “Ultra Thick Cross Section Fiber Reinforced Composites.” Diss.
University of Massachusetts Dartmouth, 2000. 2. Kim,Y. K., A. F. Lewis and A. J. Fowler “F98-D04: Ultra thick Cross section Fiber Reinforced Composites”, Annual Report 1999, National Textile Center (1999) 3. ASTM 1999. “Standard Test Method for Tensile Properties of Plastics,” ASTM 2000 4. ASTM 1996 “Standard Method for Compressive Properties of Rigid Plastics,” ASTM 2000 5. ASTM 1985. “Standard Test Method for In-Plane shear Strength of reinforced Plastics,” ASTM 1996 Standards, Vol. 15.03: 195-197. 6. ASTM. 1989. “Standard Test Method for Tensile Properties of Fiber Reinforced Composites,” ASTM 1996 Standards, Vol. 15.03: 117-121. 7. ASTM. 1989. “Standard Test Method for Apparent Interlaminar Shear Strength of Parallel Fiber Composites by the Short Beam Method,” ASTM Standards 1996, Vol. 15.03: 43-45. National Textile Center Annual Report: November 2000

Source: http://www.ntcresearch.org/pdf-rpts/AnRp00/F98-D04.pdf