Future Work/ReferencesAcknowledgements

Potential Void Mechanisms

Volatile Analysis

Void Content and Solvent Dynamics in Composite StructuresAndrew Hollcraft, Ryan Hackler, Tyler Kirkness, and David Rider, Department of Chemistry, Western Washington

University, Bellingham, WA

Introduction

Conclusions

Figure 1: Plot correlating average warp value obtained from composite parts and average

void content found via SEM

Figure 5: IR spectra of A-stage resin (left) and B-stage prepreg (right) collected every 48 minutes throughout a production cycle

Figure 6: IR spectra of phenol and ethanol C-O stretches in B-stage prepreg

Figure 9: Thermograms of B stage-to-C stage (top) and C stage-to-Char (bottom) transitions with

corresponding warp values

Figure 11: SEM micrograph of B-stage prepreg and corresponding void analysis image (via imageJ)

after binary filter and void outline

Void Investigation

BackgroundSandwich-structured composites are widely used in industry due to being lightweight, impact resistant, and inexpensive. They are composed of a honeycomb core sandwiched between two sheets of prepreg1.

Resin Modification

Impregnation of Fibers

Metering of Fibers

Curing @ 124°C

Characterization

Scheme 1: Method used to produce micro-scale prepreg to analyze the effects of various additives on void

content and volatiles during curing

Solvent content decreases throughout the production cycle in both A-stage resin and corresponding B-stage prepreg

Relative amounts of solvent changes throughout the production cycle, with little change in water content and a significant change in ethanol

The ratio of IR peaks attributable to phenol and ethanol correspond to a higher warp when a larger amount of ethanol is present

Three proposed void induction mechanisms include: Boiling Point3

Phase Separation Surface Tension4

Higher relative mass loss in low warp B-stage prepreg indicates solvent content affects void content

Jim Del PintoKevin BussardSean MitchellKalin KarichNicole HoekstraNicole LarsonCecile GrubbCharles WandlerErin Macri

Figure 8: Correlation plots for void content with surface tension (left) and boiling point (right) of additives used in prepreg

Void content plays a vital role in the resulting geometric distortion Volatile analysis identified residual solvent concentration from the initial formulation as

potential sources of voids Distribution of residual solvents was correlated to macroscopic distortion

Suggestion Non-solvents of the base polymer with high surface tension phase separate and nucleate

void creation Desirable in order to facilitate void production and reduce geometric

distortion

Produce Medium-scale C-stage prepreg samples with various void inducing additives and characterize resulting warp

Pilot scale prepreg development Determine best void inducing additive(s) and loading

such that void content satisfies warp and mechanical requirements

Novel resin systems synthesized at Western Washington University

1. Strek, T.; Jopek, H.; Maruszewski, B.T.; Nienartowicz, M. Phys. Status Solidi B 2013, 1-13.2. Hackler, Ryan A., Andrew T. Hollcraft, Tyler A. Kirkness, Nicole S. Larson, Nicole K. Hoekstra, and

David A. Rider. "Relief of Cure Stress in Prepreg Composites with Engineered Voids: A Solution to Warping in Composite Phenolic Resin/Fiberglass Laminates." Industrial and Engineering Chemistry Research; In Press (March 6, 2016).

3. Naganuma, T.; Naito, K.; Kyono, J.; Kagawa, Y. Comp. Sci. Tech. 2009 69, 2428-33.4. Liu, P.; Song, J.; He, L.; Liang, X.; Ding, H. J. Appl. Polym. Sci. 2009, 114, 811-7.

Figure 7: TGA-IR spectra of evolved gases during B to C stage curing Figure 12: Void distribution in

resulting composites

Scheme 2: Representative phenolic prepreg polymerization

Table 1: DSC on B stage formulated prepreg with tensile testing performed on C stage laminate

SEM analysis revealed a relationship between void content of the surface and average warp of the composite, which may be due to the voids acting as stress relievers during curing that allows for localized deformation as opposed to geometric distortion (known as warping)2.

Kevlar honeycomb

Phenolic fiberglass composite

Decorative face sheet

Additive = water

Additive = ethanol

Additive = none

Figure 13: Qualitative warpage observed in bench-top produced prepreg

Figure 2: Cartoon example of warpage observed in 2” by 4” industrially produced composites

Figure 3: Representative industrial production of prepreg

Figure 4: Top – Sequence of composite design, middle and

bottom – aerospace applications of these composites

Figure 10: Cartoon example of void creation; left – B stage prepreg with phase separated non solvent, right – resulting C stage prepreg with voids