disentangling-high-strength-copolymer-aramid-fibers-to-enable

Disentangling High Strength Copolymer Aramid Fibers to Enable the Determination of Their Mechanical Properties

Traditionally, soft body armor has been made from poly(p-phenylene terephthalamide) (PPTA) and ultra-high molecular weight polyethylene. However, to ...

cutting-procedures-tensile-testing-ageing-flexible-unidirectional

Cutting Procedures, Tensile Testing, and Ageing of Flexible Unidirectional Composite Laminates

Many body armor designs incorporate unidirectional (UD) laminates. UD laminates are constructed of thin (&#60;0.05 mm) layers of high-performance yarns, ...

solution-blow-spinning-polymeric-nano-composite-fibers-for-personal

Solution Blow Spinning of Polymeric Nano-Composite Fibers for Personal Protective Equipment

Light-weight, protective armor systems typically consist of high modulus (&#62;109 MPa) and high-strength polymeric fibers held in place with an elastic ...

a-synthetic-methodology-for-preparing-impregnated-grafted-amine-based

A Synthetic Methodology for Preparing Impregnated and Grafted Amine-Based Silica Composites for Carbon Capture

Recently, there has been a significant effort towards reducing or mitigating CO2 emissions through the use of carbon capture materials for point source or ...

National Institute of Standards and Technology

Material Measurement Laboratory, National Institute of Standards and Technology

National Institute of Standards and Technology (NIST)

As carbon nanotube (CNT) infused hybrid composites are increasingly identified as next-generation aerospace materials, it is vital to evaluate their long-term structural performance under aging environments. In this work, the durability of hierarchical, aligned CNT grafted aluminoborosilicate microfiber-epoxy composites (CNT composites) are compared against baseline aluminoborosilicate composites (baseline composites), before and after immersion in water at 25 °C (hydro) and 60 °C (hydrothermal), for extended durations (90 d and 180 d). The addition of CNTs is found to reduce water diffusivities by approximately 1.5 times. The mechanical properties (bending strength and modulus) and the damage sensing capabilities (DC conductivity) of CNT composites remain intact regardless of exposure conditions. The baseline composites show significant loss of strength (44 %) after only 15 d of hydrothermal aging. This loss of mechanical strength is attributed to fiber-polymer interfacial debonding caused by accumulation of water at high temperatures.  acoustic and DC electrical measurements of hydrothermally aged CNT composites identify extensive stress-relieving micro-cracking and crack deflections that are absent in the aged baseline composites. These observations are supported by SEM images of the failed composite cross-sections that highlight secondary matrix toughening mechanisms in the form of CNT pullouts and fractures which enhance the service life of composites and maintain their properties under accelerated aging environments.

Enhanced durability of carbon nanotube grafted hierarchical ceramic microfiber-reinforced epoxy composites.

Microspectrophotometry (MSP) is a rapid, nondestructive technique for the analysis of color in textile fibers. This technique combines microscopy and ultraviolet (UV)/visible (Vis) spectroscopy, allowing for very small colored samples, like dyed textile fibers, to be analyzed directly and thereby eliminates the need for time consuming and destructive extractions. While MSP is generally accepted to be a nondestructive evaluation method, a loss of color during analysis, or photofading can occur. In this work, cotton fabric dyed with blue, yellow, and red direct dyes at different concentrations. Dye photofading during MSP examination was investigated by measuring the absorbance at a specific position on the fibers from these fabrics, periodically over the course of 30 minutes. Visible color loss and a reduction in absorbance was observed for all three colors, but was most pronounced for the fibers dyed red. A major goal of this study is to increase awareness of the photofading phenomenon when analyzing cotton fibers using MSP.

Photofading in cotton fibers dyed using red, yellow, and blue direct dyes during examination with microspectrophotometry (MSP).

This work demonstrates the synergy between the thermo-mechanical and humidity induced degradation as well as the oxidation reactions in the kink-banded areas of ultra-high molar mass polyethylene (UHMMPE) fiber-based laminates used in body armor. For aged materials, the energy-dispersive X-ray spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR) results reveal high concentrations of oxygen containing products, and the EPR results demonstrate the presence of the peroxyl radicals (RO ) in the kink-banded areas. After one year of dark ambient storage, very long-lived RO radicals were observed primarily in the samples exposed to ageing conditions of elevated temperatures, humidity, and mechanical stress. The total percentage of crystallinity, as measured by differential scanning calorimetry, of the kinkbanded fibers was unchanged, indicating that the degradation occurs primarily in the amorphous region, and may also involve recrystallization processes of the degraded chains. However, the most abundant orthorhombic crystalline phase decreases from 77 % to 70 %. This decrease in the orthorhombic structure leads to more diffusion of oxygen into the kink-banded region, enhancing the oxidation processes. No changes are observed in the monoclinic phase of the kinked fibers, which remained constant and constituted ~2 % of the total crystallinity.

Oxidation Reactions in Kink Banded Regions of UHMMPE Fiber-Based Laminates Used in Body Armor: A Mechanistic Study.

To improve properties such as thermal conductivity, low temperature thermal strain, and creep resistance of ultra-high molar mass polyethylene (UHMMPE) fibers, several researchers have previously undertaken efforts to crosslink these fibers using radiation. Ionizing radiation is commonly used to crosslink bulk UHMMPE in other applications, such as artificial joints. However, UHMMPE fibers differ from bulk UHMMPE in that they have a higher crystallinity (approximately 85% to 90%) and are very highly oriented during manufacturing in which the fibers are stretched 50 to 100 times their original length. Thus, the amorphous fraction of the UHMMPE fibers is also highly ordered. Several experiments were conducted to crosslink the UHMMPE fibers using both low dose rate (gamma) and high dose rate (electron beam) irradiation, all in the absence of oxygen. In all cases, the tensile strength of the fiber was greatly reduced by the irradiation. The oxidation index was also measured for the irradiated samples, and oxidation was not found to play a major role in the reduction of tensile strength in the fibers after irradiation. While this work did not achieve the desired result of improving the mechanical properties of the UHMMPE fiber, a significant result was found. The electron paramagnetic resonance (EPR) spectrum of the UHMMPE fibers was measured shortly after irradiation, and a mixture of allyl and alkyl radicals were detected. The irradiated samples were stored in dark ambient conditions for at least six years, then reexamined using EPR for free radical characterization. Surprisingly, the gamma-irradiated samples showed clear evidence of long-lived polyenyl radicals present in the material. Free radicals are very reactive species that will typically migrate to the surface of the crystalline domain and decay in a relatively short time through various reactions in the amorphous regions. It is hypothesized herein that due to the high crystallinity and large anisotropy of the highly drawn UHMMPE fiber, the polyenyl radicals were trapped in the crystal phase and were unable to migrate and decay. An experiment was performed to test this hypothesis, by which samples of the irradiated fibers were heated to temperatures above first the alpha relaxation and then melting point of polyethylene, and EPR measurements were taken. Results showed that the polyenyl radical signal persisted below the , but was rapidly eliminated upon melting of the crystals. These experiments support the hypothesis that the long-lived polyenyl radicals are trapped in the crystalline region of the polyethylene fibers.

Effect of Irradiation and Detection of Long-Lived Polyenyl Radicals in Highly Crystalline Ultra-High Molar Mass Polyethylene (UHMMPE) Fibers.

Carbon nanotube (CNT) grafted glass fiber reinforced epoxy nanocomposites (GFRP) present a range of stiffnesses (MPa to GPa) and length scales (m to nm) at the fiber-matrix interface. The contribution of functionalized CNT networks to the local and bulk polymer dynamics is studied here by using a combination of torsion dynamical mechanical thermal analysis (DMTA), positron annihilation lifetime spectroscopy (PALS), and neutron scattering (NS) measurements. DMTA measurements highlight a reduction in the storage modulus (') in the rubbery region and an asymmetric broadening of the loss modulus (″) peak in the -transition region. NS measurements show a suppressed hydrogen mean-square displacement (MSD) in the presence of glass fibers but a higher hydrogen MSD after grafting functionalized CNTs onto fiber surfaces. PALS measurements show greater free volume characteristics in the presence of the functionalized CNT modified composites, supporting the view that these interface layers increase polymer mobility. While NS and DMTA are sensitive to different modes of chain dynamics, the localization of functionalized nanotubes at the fiber interface is found to affect the distribution of polymer relaxation modes without significantly altering the thermally activated relaxation processes.

Multiscale Polymer Dynamics in Hierarchical Carbon Nanotube Grafted Glass Fiber Reinforced Composites.

Flexible Unidirectional (UD) composite laminates are commonly being used for ballistic-resistant body armor. These laminates comprise UD layers, each constructed by laminating thin layers of high-performance fibers held in place using very low modulus binder resins, with the fibers in each layer oriented parallel to each other. As these materials are used in body armor, it is important to investigate their long-term reliability, particularly with regards to exposure to temperature and humidity as these are known causes of degradation in other commonly used body armor materials. This work investigates the tensile behavior of a poly(p-phenylene terephthalamide), or PPTA flexible UD laminate aged for up to 150 d at accelerated conditions of 70 °C and 76 % relative humidity (RH). Tests were performed at three different crosshead displacement rates and three different gauge lengths. The effect of ageing on the mechanical properties of the material resulted in less than 10 % degradation in tensile strength, with a more significant reduction in longer specimens when tested at slower rates.

Tensile Testing of Aged Flexible Unidirectional Composite Laminates for Body Armor.

Soft body armor is typically comprised of materials such as aramid. Recently, copolymer fibers based on the combination of 5-amino-2-(p-aminophenyl) benzimidazole (PBIA) and PPTA were introduced to the body armor marketplace. The long-term stability of these copolymer fibers have not been the subject of much research, however they may be sensitive to hydrolysis due to elevated humidity because they are condensation polymers. Efforts to evaluate the impact of environmental conditions on fiber strength is very important for the adoption of these materials in armor systems. Three PBIA-based fibers were selected for the study, and were aged at 25 °C, 75 % RH; 43 °C, 41 % RH; 55 °C, 60 % RH; and 70 °C, 76 % RH for up to 524 d. Molecular spectroscopy, scanning electron microscopy, and single fiber tensile testing were performed to characterize changes in their chemical structure, tensile strength, and failure strain as a function of exposure time to different conditions. The fibers were all found to have some reduction in strength at high humidity conditions, with an approximately 14 % reduction for the copolymers and a 29 % reduction for the homopolymer. Molecular spectroscopy revealed some changes which suggest that hydrolysis of the benzimidazole ring is occurring at these elevated temperatures, possibly explaining the observed change in strength.

Effect of Elevated Temperature and Humidity on Fibers Based on 5-amino-2-(-aminophenyl) benzimidazole (PBIA).

The construction of ballistic-resistant body armor is experiencing an increasing use of flexible unidirectional (UD) composite laminates that comprise multiple layers. Each UD layer contains hexagonally packed high-performance fibers with a very low modulus matrix (sometimes referred to as binder resins). Laminates are then made from orthogonal stacks of these layers, and these laminate-based armor packages offer significant performance advantages over standard woven materials. When designing any armor system, the long-term reliability of the armor materials is critical, particularly with regard to stability with exposure to temperature and humidity, as these are known causes of degradation in commonly used body armor materials. To better inform future armor designers, this work investigates the tensile behavior of an ultra-high molar mass polyethylene (UHMMPE) flexible UD laminate that was aged for at least 350 d at two accelerated conditions: 70 °C at 76% relative humidity (RH) and 70 °C in a desiccator. Tensile tests were performed at two different loading rates. The mechanical properties of the material after ageing demonstrated less than 10% degradation in tensile strength, indicating high reliability for armor made from this material.

Effect of Aging on Unidirectional Composite Laminate Polyethylene for Body Armor.

In this work, different formulations of a room-temperature silicone composite backing material (SCBM) composed of polydimethylsiloxane (PDMS), fumed silica and corn starch were investigated using different characterization techniques, i.e., differential scanning calorimetry, thermogravimetry analysis, X-ray diffraction (XRD) and small-angle X-ray scattering, as a function of controlled relative humidity. At ambient relative humidities in the range of about 20-80%, the equilibrium water content in the SCBM ranges from approximately 4-10%, which is predominantly absorbed by the corn starch. This amount of water content has been shown to have minimal effect on thermal transition temperatures (melting and glass transition) of the SCBMs. The enthalpy of melting increases with increasing relative humidity, which reflects the heterogeneous semicrystalline structure of starch granules and the role of moisture in facilitating the formation of amylopectin double helices mainly in the imperfect crystalline regions. The thermal degradation of SCBM exhibits three major mass loss steps that correspond to dehydration, decomposition of corn starch and decomposition of PDMS. The XRD patterns reveal a characteristic diffuse peak for amorphous PDMS and an A-type crystallinity for the corn starch. The XRD results show no observable changes in the crystal type and crystallinity as a function of moisture content. Results from this work help clarify the fundamental structure-property relationships in SCBMs, which are important for future development of documentary standards, especially the handling and storage specifications of next-generation ballistic witness materials for body armor testing.

ArticleTotal : 4	Year
Publication title Cited by 4	2018
Publication title Cited by 5	2019
Publication title Cited by 5	2021
Publication title Cited by 5	2023

Article	Year
Enhanced durability of carbon nanotube grafted hierarchical ceramic microfiber-reinforced epoxy composites. Carbon\| PubMed ID: 29170562	2017
Photofading in cotton fibers dyed using red, yellow, and blue direct dyes during examination with microspectrophotometry (MSP). Forensic chemistry (Amsterdam, Netherlands)\| PubMed ID: 29251299	2017
Oxidation Reactions in Kink Banded Regions of UHMMPE Fiber-Based Laminates Used in Body Armor: A Mechanistic Study. Polymer degradation and stability\| PubMed ID: 30996496	2018
Effect of Irradiation and Detection of Long-Lived Polyenyl Radicals in Highly Crystalline Ultra-High Molar Mass Polyethylene (UHMMPE) Fibers. Polymers\| PubMed ID: 31137768	2019
Multiscale Polymer Dynamics in Hierarchical Carbon Nanotube Grafted Glass Fiber Reinforced Composites. ACS applied polymer materials\| PubMed ID: 32166225	2019
Tensile Testing of Aged Flexible Unidirectional Composite Laminates for Body Armor. Journal of materials science\| PubMed ID: 33041370	2020
Effect of Elevated Temperature and Humidity on Fibers Based on 5-amino-2-(-aminophenyl) benzimidazole (PBIA). SN applied sciences\| PubMed ID: 33043248	2020
Effect of Aging on Unidirectional Composite Laminate Polyethylene for Body Armor. Polymers\| PubMed ID: 36987127	2023
Temperature-insensitive silicone composites as ballistic witness materials: the impact of water content on the thermophysical properties. Journal of materials science\| PubMed ID: 37051189	2021

Amanda L. Forster

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