Abstract:Replicated composite optics is a promising technique to fabricate high-quality mirrors with reduced weight and processing time compared to conventional glass mirrors; however, the optical layer is organic and susceptible to environmentally-induced dimensional changes, specifically to moisture exposures. Generally, to enhance polymer stability, thermal curing is necessary to maximize the cure state. Because replications are bonded, thermal exposures generate residual stresses that degrade optical quality. In th… Show more
“…Even though the mechanical properties may still be acceptable for many structural applications, this distortion can negatively impact high-dimensional stability composite applications where nanometer resolutions are critical. 6,7…”
Section: Resultsmentioning
confidence: 99%
“…Even though the mechanical properties may still be acceptable for many structural applications, this distortion can negatively impact high-dimensional stability composite applications where nanometer resolutions are critical. 6,7 Figure 18.Stress measurements on cured composite TC410/M55J composites after elevated heat treatment exposure to 200°C. Each specimen shows increasing curvature due to residual stress build-up based on Tg differential between tool and bag side.…”
Section: Resultsmentioning
confidence: 99%
“…[3][4][5] In select composite applications, such as optical benches and replicated optics, even small variations in absorbed resin moisture content can translate into unacceptable dimensional changes. [6][7] Therefore, in order to overcome these limitations while maintaining similar processability, other thermosetting chemistries such as cyanate ester resins have been investigated and utilized in high-performance polymer matrix composites. 8 Cyanate esters are thermosetting polymers that have continued to replace epoxies as the primary matrix materials used in composites for space applications.…”
Siloxane-modified cyanate ester resins are ideal matrix materials for next-generation, high-precision composites used for radomes and satellite structures. They provide extremely low moisture uptake, excellent microcracking resistance, and protection from atomic radiation. However, cyanate ester resins have been shown to be susceptible to hydrolysis during cure, which may significantly impact both mechanical and thermal performance. In this investigation, we evaluate the cure kinetics and hydrolysis susceptibility of a siloxane-modified prepreg system (TC410/M55J). DSC tests verified that the Ea of polymerization for the TC410 system is 65 KJ/mol. Samples were also exposed to moisture during cure at several temperatures, and FTIR was used to follow changes in the carbonyl peak absorption intensity to compare the rate kinetics and activation energy for hydrolysis leading to carbamate formation (52 KJ/mol). DMA tests of composites exhibited significantly reduced Tg’s with increasing moisture levels during cure. TGA of these cured samples exhibited both a significant decrease in the onset of the thermal decomposition temperature as well as an increase in the relative degree of volatiles generated at low temperature. Flatwise tension strength tests showed a linear reduction in strength with decreasing Tg, at an equivalent trend to previous work on an M55J/RS3C system indicating a similar mechanism. However, the added margin provided from the higher initial FWT strength in the TC410 system allows for larger decreases in Tg before significant degradation occurs. Laminates manufactured on composite mandrels resulted in parts with larger decreases in the tool-side Tg of the part when compared to the bag-side Tg, with the differential depending on the initial moisture content of the tool. AFM was shown to selectively identify these carbamate-affected regions by showing that the recession behavior of these less cross-linked areas was greater than in areas where the resin was properly cured. Composites with increased carbamate formation resulted in increasing warpage of the part with elevated temperature exposure due to variations in stress relaxation. This effect should be considered when manufacturing precision, high-dimensional stability composite hardware.
“…Even though the mechanical properties may still be acceptable for many structural applications, this distortion can negatively impact high-dimensional stability composite applications where nanometer resolutions are critical. 6,7…”
Section: Resultsmentioning
confidence: 99%
“…Even though the mechanical properties may still be acceptable for many structural applications, this distortion can negatively impact high-dimensional stability composite applications where nanometer resolutions are critical. 6,7 Figure 18.Stress measurements on cured composite TC410/M55J composites after elevated heat treatment exposure to 200°C. Each specimen shows increasing curvature due to residual stress build-up based on Tg differential between tool and bag side.…”
Section: Resultsmentioning
confidence: 99%
“…[3][4][5] In select composite applications, such as optical benches and replicated optics, even small variations in absorbed resin moisture content can translate into unacceptable dimensional changes. [6][7] Therefore, in order to overcome these limitations while maintaining similar processability, other thermosetting chemistries such as cyanate ester resins have been investigated and utilized in high-performance polymer matrix composites. 8 Cyanate esters are thermosetting polymers that have continued to replace epoxies as the primary matrix materials used in composites for space applications.…”
Siloxane-modified cyanate ester resins are ideal matrix materials for next-generation, high-precision composites used for radomes and satellite structures. They provide extremely low moisture uptake, excellent microcracking resistance, and protection from atomic radiation. However, cyanate ester resins have been shown to be susceptible to hydrolysis during cure, which may significantly impact both mechanical and thermal performance. In this investigation, we evaluate the cure kinetics and hydrolysis susceptibility of a siloxane-modified prepreg system (TC410/M55J). DSC tests verified that the Ea of polymerization for the TC410 system is 65 KJ/mol. Samples were also exposed to moisture during cure at several temperatures, and FTIR was used to follow changes in the carbonyl peak absorption intensity to compare the rate kinetics and activation energy for hydrolysis leading to carbamate formation (52 KJ/mol). DMA tests of composites exhibited significantly reduced Tg’s with increasing moisture levels during cure. TGA of these cured samples exhibited both a significant decrease in the onset of the thermal decomposition temperature as well as an increase in the relative degree of volatiles generated at low temperature. Flatwise tension strength tests showed a linear reduction in strength with decreasing Tg, at an equivalent trend to previous work on an M55J/RS3C system indicating a similar mechanism. However, the added margin provided from the higher initial FWT strength in the TC410 system allows for larger decreases in Tg before significant degradation occurs. Laminates manufactured on composite mandrels resulted in parts with larger decreases in the tool-side Tg of the part when compared to the bag-side Tg, with the differential depending on the initial moisture content of the tool. AFM was shown to selectively identify these carbamate-affected regions by showing that the recession behavior of these less cross-linked areas was greater than in areas where the resin was properly cured. Composites with increased carbamate formation resulted in increasing warpage of the part with elevated temperature exposure due to variations in stress relaxation. This effect should be considered when manufacturing precision, high-dimensional stability composite hardware.
“…Ferrelli and others have shown for thermosetting resins that as the degree of cure is reduced, the level of equilibrium moisture uptake is increased for equivalent humidity exposure conditions. 8,9 Stokes has shown that moisture content will impact the thermo-mechanical stresses generated within fully cured phenolic composites. 10 Therefore, a lower cure state with increased moisture content may not only result in a depressed T g , but also in a significant strain response, that ultimately may contribute to delamination failures.…”
Section: Introductionmentioning
confidence: 99%
“…Finally, the cure state of a resin may influence the moisture absorption characteristics of the composite prior to use. Ferrelli and others have shown for thermosetting resins that as the degree of cure is reduced, the level of equilibrium moisture uptake is increased for equivalent humidity exposure conditions 8,9 . Stokes has shown that moisture content will impact the thermo‐mechanical stresses generated within fully cured phenolic composites 10 .…”
Phenolic resin is the material of choice used for composite liners in solid rocket motor nozzles. We investigate the effect of the cure conditions (94, 116, and 155 C) on vacuum only (VO) processed composite mechanical, thermal, and microstructural changes. Although the T g 's correlated with the processing conditions, mechanical properties showed unexpected non-linear degradation after 94 C processing. This difference is primarily due to the lower cure state of the
Replicated composite optics offer a route to manufacture precision mirrors for satellite applications in a fraction of time and with substantial cost and weight savings over conventional glass mirrors. However, the dimensional stability is a critical barrier to entry for utilizing these mirrors in UV/Vis space applications due to the organic nature of the optical surface and its susceptibly to environmentally-induced distortions that can cause deviations greater than SFE λ/20 (~32nm). Recently, advances in this technology have been achieved at The Aerospace Corporation by using UV cured replicating epoxy resins to produce replicated mirrors with relatively zero residual stress and high hygroscopic stability with RT processing. Elevated thermal stability, though, remains a critical issue as CTE mismatch between the composite and resin lead to residual stress formation which immediately degrades the optical quality and then causes further optical drift over time as stress relaxation occurs. In this paper, we demonstrate improved thermal stability of replicated mirrors by utilizing multiple replication layers where buried compliant layers accumulate residual stress and the top layer is an optimized high modulus resin that achieves a high-quality replication. As a result, thermal stresses incurred by elevated temperatures were reduced by more than 60% when measured via laser interferometry. Additionally, due to faster stress relaxation rates of the compliant layers, the CTE mismatch stress was removed in a matter of days versus years of a conventional single layer replication. This stack-up provides a route to mitigate stress and enhance replication stability.
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