Effect of humidity on the indentation hardness and flexural fatigue behavior of polyamide 6 nanocomposite

doi:10.1016/j.msea.2010.01.070

Materials Science and Engineering: A

Volume 527, Issue 12, 15 May 2010, Pages 2826-2830

https://doi.org/10.1016/j.msea.2010.01.070 Get rights and content

Abstract

Humidity affects the mechanical properties of polymers and their composites. Understanding the influence of humidity on the strength, stiffness and fatigue characteristics will aid in better product design. The effect of relative humidity (RH) on indentation hardness and flexural fatigue behavior of polyamide 6 nanocomposites is reported. Indentation hardness and indentation modulus of the material reduces up to ∼50% in the samples conditioned in water due to the plasticization and associated increased polymer chain mobility. Cantilever bending fatigue tests conducted at different relative humidity levels at constant displacement amplitude revealed increased fatigue life for polyamide 6 nanocomposites at high humidity. Hysteresis heating and molecular reorientation lowers the modulus during fatigue process and causes a reduction in the force amplitude at high humidity levels. The failure mechanisms at different humidity levels are discussed.

Introduction

Polymer nanocomposites are promising for structural applications due to their high stiffness-to-weight ratio compared to pristine polymer. Exfoliated layered silicate polymer nanocomposites exhibit remarkable improvement in mechanical properties such as flame retardancy, gas barrier properties, ionic conductivity, thermal stability, tensile properties and tribological behavior compared to pristine polymer even at very low nanoclay content [1], [2], [3], [4]. In many structural applications, components have to sustain not only various types of loads but also different types of environmental conditions. Temperature and humidity severely affect the mechanical properties of polymers and their composites [5], [6], [7]. Naval applications are examples where the components are exposed to extreme humidity levels, which will affect the chemical structure and there by the mechanical behavior of polymer and their composites [8], [9]. It is essential to understand the long-term behavior of components made of these materials under severe operating conditions for better product design.

The fatigue crack propagation behavior of nylon 66 containing different water content was reported by Bretz et al. [10]. At a constant stress intensity factor range, the fatigue crack propagation decreased as the water content was increased to 3%. The fatigue crack propagation rates at the moisture saturation level (8%) were higher than those observed in dry specimen. An increase in water content in nylon causes an increase in the slope of crack length versus number of cycles [11]. Fatigue properties of two engineering rubbers in air and water have been studied by Selden [12]. In polychloroprene rubber, a factor of two to three times lower crack growth rates was observed in water compared to that in air. Water was found to act as a plasticizer in polyhydroxy ester ether and lowers the room temperature tensile strength and modulus [13]. The strain at failure increased with increasing water content and a change in the mode of failure was reported. Brittle failure occurred when the moisture content was low. As the absorbed water content increased, the samples necked and exhibited extensive plastic deformation. Scaffaro et al. [14] reported the effect of humidity, temperature and ultraviolet on creep behavior of polyamide 6. The flexural fatigue life of the polyamide 6 is enhanced by the addition of nanoclay [15].

This paper reports the effect of relative humidity (RH) on the flexural fatigue behavior of dry polyamide 6 nanocomposite samples. The effects of humidity on mechanical properties of polyamide 6 nanocomposite were studied using the microindentation technique. The failure mechanisms of polyamide 6 nanocomposite at different humidities are discussed.

Section snippets

Test materials and characterization

The nanocomposite used in the present study was prepared using melt compounding of commercial grade polyamide 6 granules and hectorite clay (organically modified with a hydrogenated tallow quaternary amine complex) in a counter rotating twin-screw extruder. Prior to extrusion, the polyamide 6 pellets were dried at 333 K for 24 h. The required quantity of polyamide granules and nanoclay (5 wt.%) was fed into the extruder. The extrudate was cooled and then shredded into pellets. Prior to injection

Material characterization

It is well known that in many polymers the water absorption affects their properties [13], [14], [18]. The differential scanning calorimetry data of dry and wet polyamide 6 nanocomposite is shown in Fig. 4. The broad endothermic peak observed around 350 K in the case of wet samples is due to the evaporation of moisture absorbed by the sample. The melting temperature for the both dry and wet samples is nearly same. The initial part of the DSC data indicates that the water has been absorbed by

Conclusion

The effect of humidity on the indentation hardness and flexural fatigue behavior of polyamide 6 clay nanocomposites is investigated. The hardness and modulus of the polyamide 6 nanocomposite is reduced in humid atmospheres. Under displacement controlled condition, flexural fatigue life of polyamide 6 nanocomposite is increased with humidity. The increase in the surrounding humidity and absorption of water reduce the hardness and modulus, and improve the flexural fatigue life.

Acknowledgement

The authors would like to thank the Naval Research Board for financial support provided for this research project.

References (22)

D.R. Paul et al.
Polymer
(2008)
G. Srinath et al.
Mater. Sci. Eng. A
(2006)
J.R.M. d’Almeida et al.
Compos. Struct.
(2008)
D.P.N. Vlasveld et al.
Polymer
(2005)
G. Srinath et al.
Compos. Sci. Technol.
(2007)
J.L. Willett et al.
Polymer
(2002)
H.A. El-Hakeem et al.
Int. J. Fatigue
(1979)
S. St Lawrence et al.
Polymer
(2001)
R. Scaffaro et al.
Polym. Test.
(2008)
A. Ramkumar et al.
Compos. Sci. Technol.
(2008)

A. Hodzic et al.

Compos. Sci. Technol.

(2004)

Cited by (45)

Hydrothermal aging of short glass fiber reinforced polyphenylene sulfide composites and property prediction
2023, Polymer Degradation and Stability
In this paper, hydrothermal aging behavior of short-cut glass fiber reinforced polyphenylene sulfide composites (PPS/GF) were studied. The water adsorption behavior can be well described by Fickian law and have strong correlation with mechanical properties. The microbond test joint with morphology observation by scanning electron microscope (SEM) and separate evaluation about matrix, fiber and interface unveil that the mechanism of hydrothermal aging of PPS/GF composites and deterioration of interface play the dominant role in degradation of macroscopic mechanical properties. Finally, the life and mechanical properties prediction model of the hydrothermal aged PPS/GF composites were established based on the collected aging data and Arrhenius formula.
Moisture-induced changes in the mechanical behavior of 3D printed polymers
2022, Composites Part C: Open Access
Citation Excerpt :
For the Nylon specimens immersed in DI water at 21 °C, the percentage crystallinity decreased from approximately 22% to 12% after 14 days, indicating a moisture-induced breakdown of the molecular chains in both the amorphous and crystalline regions of the polymer, see Table 3a. During the moisture absorption process, water first agglomerates in the easy-to-penetrate amorphous regions [73] as diffusion into the crystalline phase is a bit more difficult in the early stages [74] and only occurs after prolonged immersion, with potential decreases in crystallinity. From the results, 7 days of immersion is sufficient time for the water molecules to enter and disrupt the crystalline regions.
Commonly used 3D printing polymers, such as nylon and polylactic acid (PLA), can experience significant material degradation when exposed to moisture and high temperatures for extended periods. Understanding the extent of their degradation is critical to their long term use and can also inform end-of-life strategies, such as recycling or recovery. Specimens of nylon, carbon fiber-reinforced nylon composite, and PLA, three popular 3D printing materials, were 3D printed and immersed in deionized water at 21 °C and 70 °C to evaluate the effect of moisture and temperature on their mechanical and chemical properties. Water absorption rates were much higher at elevated temperatures and, at both temperatures investigated, the nylon-based materials absorbed up to 10 times more water than PLA. A direct relationship between moisture absorption and reduction in flexural properties was observed for the nylon-based specimens, with the flexural modulus of nylon decreasing by as much as 60% after 7 days of immersion. PLA, however, displayed negligible mechanical property degradation after immersion at 21 °C, but showed substantial physical degradation after being immersed in water at 70 °C for 7 days. Analogous changes in chemical structure and crystallinity were observed via Fourier transform infrared spectroscopy and differential scanning calorimetry.
Structure of polyamide 6/poly(ethylene terephthalate) blends under high cooling rate and shear stress and their moisture-sensitive properties
2020, Polymer
Polyamide 6 (PA6) with low moisture-sensitivity is realized by blending with poly(ethylene terephthalate) (PET), the crystal structure and morphology formed under high cooling rate and shear stress are also investigated. A pronounced reduction in the maximum water content from 9.7 to 5.6 wt% accompanying the variation of the diffusion coefficient from 1.2 × 10⁻⁶ to 5.2 × 10⁻⁷ mm²/s with the increasing of PET content is achieved. The dielectric constant decreases from 12.3 to 6.2 (@10³ Hz) and the dielectric loss decreases from 0.25 to 0.13 (@10³ Hz) with the increasing of PET content. The structural characterization confirms that the hydrogen-bonding interactions between PA6 and PET play a major role in the reduction of moisture-sensitivity of PA6 as well as that the evolution of the polymorphism of PA6 that the high shear stress facilitates the formation of α-form while the high cooling rate promotes the generation of γ-form.
Chain Mobility and Progress in Medicine, Pharmaceuticals, and Polymer Science and Technology
2020, Chain Mobility and Progress in Medicine, Pharmaceuticals, and Polymer Science and Technology
Fatigue behavior of nanoparticle-filled fibrous polymeric composites
2020, Fatigue Life Prediction of Composites and Composite Structures
In this chapter, new models, Nano-NSDM and Nano-EFAT based on the micromechanical theory, normalized stiffness degradation approach, and energy method were established to predict the fatigue life of nanoparticle-filled fibrous composites based on the experimental data of fibrous composites without nano-fillers and they were evaluated under different cases: epoxy resin modified by silica nanoparticles, chopped strand mat/epoxy composites, nanoparticle/chopped strand mat/polymer hybrid nanocomposites, and thermoplastic nanocomposites. The obtained results in each case are in very good agreement with the experimental observations with negligible errors. In the last section, the displacement-controlled flexural bending fatigue behavior of nanocomposites was considered. An experimental test setup, called BFM-110, was designed and manufactured to investigate the flexural bending fatigue. Then, by means of the testing device, the effects of adding synthesized graphene nanosheets and carbon nanofibers (independently and simultaneously) on the flexural fatigue behavior of epoxy polymer were investigated.
Assessment of nano-scale tribological and mechanical properties of flexible transparent polymers based on atomic force microscopy
2019, CIRP Annals
Transparent polymers are often used in various devices that are fabricated by nano-imprinting lithography (NIL). One of the main concerns in NIL is regarding the interfacial interaction between the mold and the polymeric part. In this work, the tribological and mechanical properties of PET and PDMS were systematically assessed with respect to temperature using an AFM. The nano-scale properties obtained were used to analyze the stress distribution of a polymeric part during NIL. These results are expected aid in optimization of NIL process to achieve higher precision and durability of polymeric parts used in bio and energy devices.

View all citing articles on Scopus

View full text

Effect of humidity on the indentation hardness and flexural fatigue behavior of polyamide 6 nanocomposite

Abstract

Introduction

Section snippets

Test materials and characterization

Material characterization

Conclusion

Acknowledgement

Polymer

Mater. Sci. Eng. A

Compos. Struct.

Polymer

Compos. Sci. Technol.

Polymer

Int. J. Fatigue

Polymer

Polym. Test.

Compos. Sci. Technol.

Compos. Sci. Technol.