Effect of hygrothermal aging on mechanical and tribological behaviors of short glass-fiber-reinforced PA66

Abstract

Mechanical and tribological behaviors of polyamide 66 reinforced by various weight fractions of short glass fiber were investigated. All studied materials were subjected to accelerated hygrothermal aging tests. Three-point bending flexural tests were carried out to determine the mechanical behavior. Reciprocating tribotester was employed to determine friction and wear behaviors. Tribological tests were carried out, without lubrication and under ambient conditions. Ball on flat contact configuration was adopted for friction tests. Wear tests were carried out against an abrasive counterface. Results showed that flexural strength and elastic modulus increased when increasing glass fiber rate for all tested materials. In the contrary, the coefficient of friction and the wear resistance decreased. Results showed that the hygrothermal aging decreased the flexural features and increased the coefficient of friction. The effect of hygrothermal aging on the wear resistance depends on the reinforcement rate.

Keywords

PA66 glass fiber hygrothermal aging flexural properties friction wear

Introduction

Reinforced polyamides (PAs) are widely used in a variety of structures and applications, from automobile industry to biomechanics, such as stressed functional automotive and safety parts. These composites form an important class of tribomaterials. Fibers acting as a hard phase are frequently incorporated into PA matrix to improve their friction and wear performances. A number of researchers investigated the mechanical and tribological behaviors of divers PAs matrix as PA66,^{1

–11} PA6,^12
–14 and PA12¹⁵ reinforced with divers kinds of fibers under dry or humid conditions. Prediction of the durability of such composites is a very important issue for designers and users. Indeed, their use in a variety of conditions such as weather exposure and aggressive elements (sunlight, ultraviolet (UV) exposure, water, etc.) can enormously affect their durability. According to the severity of these conditions (high and/or low temperatures, water attack, UV exposure, saline atmosphere), different mechanisms may occur separately or simultaneously and decrease the performance of composite materials. Several researches investigated the effect of hygrothermal aging on the mechanical performance of glass-fiber-reinforced PA6.^16

–22 A small number of works were interested by the effect of aging on the mechanical and tribological behaviors of glass-fiber-reinforced PA66. Kim et al. studied the effect of hygrothermal aging on the mechanical behavior (by indentation test) and the friction and wear behaviors (using a block-on-ring-type tribometer) of 30% short glass-fiber-reinforced PA66.²³ Arif et al. investigated the damage mechanisms of injection molded PA66/short glass fiber 30 wt% composite using in situ scanning electron microscopy (SEM) mechanical tests on specimens conditioned under divers relative humidity contents.²⁴ Xin et al. studied the moisture diffusion and the hygrothermal aging effects on flexural behavior of pultruded glass-fiber-reinforced polymer laminates in bridge applications.^25,26 The present article investigates the effect of hygrothermal aging on the flexural, friction, and wear behaviors of unreinforced and short glass-fiber-reinforced PA66. The tribological properties of these materials are evaluated under dry reciprocating sliding conditions against a hard material as 100Cr6 for friction test and a silicon carbide (SiC) abrasive counterface for wear test.

Material and methods

Materials

The material investigated in this study is a short glass-fiber-reinforced PA66 (polyamide 6.6) with different weight fractions. To produce composites with 10 and 20 wt% glass fiber content, a combination of two commercial products was performed: PA66 (HERAMID A NAT) and PA66 containing 30 wt% glass fibers (HERAMID A NAT FV030) provided by Radici Group. These new reinforced materials are, respectively, designated PA66GF10 and PA66GF20. The commercial-reinforced PA is designated PA66GF30. The unreinforced material is designated PA66GF00. A glass type E with small diameter was employed to reduce micro-cracks and to have a very good mechanical performance. Samples for flexural and tribological tests were obtained by molding injection at 50% relative humidity. Plates have dimensions of 125 × 105 × 4 mm³. Detailed description of this composite was given in previous works.¹ All specimens were dried in an oven at 70°C during 24 h. Then cooled at room temperature in a desiccator before testing them.

Aging tests

Since natural aging tests are time-consuming, accelerated aging tests were generally used to reproduce the service conditions of the tested materials and to evaluate the durability of the used materials. The accelerated hygrothermal aging was conducted by immersing specimens in boiling distilled water during 8 h 30 min (Figure 1). The specimens were then removed from the water, blotted dry using a clean cloth before to be tested.

Figure 1.

A setup of accelerated hygrothermal aging test.

The temperature of conditioning has a crucial effect on the absorption mechanism. Thus, Bergeret et al.²⁷ showed that the diffusivity of water within the polymer matrix is easier and faster at high temperature. Likewise, in a comparison of the diffusion coefficient at 20 and 70°C, Vlasveld et al.²⁸ found a difference of roughly 17 times smaller at 20°C than at 70°C. In order to highlight the temperature effect during moisture aging, Ray²⁹ denoted that temperature acts like an activator of the diffusion of the water molecules through the composite.

Mechanical tests

To evaluate mechanical properties, flexural tests were carried out using three-point bending test according to ASTM D-790.³⁰ Support span was 70 mm (Figure 2). The applied crosshead speed was 5 mm min⁻¹. The load was placed midway between the supports. Each sample was loaded until the core broke and their average is reported. For each material, three samples were tested.

Figure 2.

A setup of flexural test.

The samples were machined to a size of 105 × 10 × 4 mm³ from the rectangular plate. The flexural strength (σ_F), modulus (E_F), and strain to failure (ε_F) are given as follows:

σ_{F} = \frac{3 P_{max} L}{2 b h^{2}}

E_{F} = \frac{m L^{3}}{4 b h^{3}}

ε_{F} = \frac{6 D h}{L^{2}}

where L, b, and h are the span (length), width, and depth (or thickness) of the sample, respectively, m is the slope of the tangent to the initial straight-line portion of the load–deflection curve, D is the maximum deflection before failure, and P_max is the maximum load encountered before failure.

Tribological tests

Friction and wear tests were performed on a reciprocating tribometer (Figure 3). The test method and the apparatus architecture are described amply in previous works.^31,32 Friction tests were carried out using prismatic samples (15 × 15 × 4 mm³) sliding against a ball bearing steel 100Cr6 (ϕ 20 mm), illustrating a ball on flat contact configuration. Wear tests were carried out using prismatic samples similar to which employed for friction tests but sliding against an SiC abrasive paper. Samples for friction and wear tests were cut from the rectangular plate. The conditions of both friction and wear tests are summarized in Table 1.

Figure 3.

Reciprocating tribometer and illustration of the contact configuration.

Table 1.

Conditions of the friction and wear tests.

Parameters	Friction	Wear
Normal loads (N)	22.4/62.4	65.6
Hertzian pressure (MPa)	966/1359	—
Frequency of oscillation (Hz)	1	1
Average sliding speed (m s⁻¹)	0.01	0.056
Oscillating stroke (mm)	10	56
Sliding duration	8 h 30 min	12 min
Acquisition (s⁻¹)	32	—
Relative humidity (%)	40	40
Grade of the abrasive paper	—	50

The coefficient of friction µ was calculated as the ratio of the friction force and the normal force. The wear rate was calculated as the difference of weight of the sample before and after the wear test divided by the initial weight (in percentage).

Results and discussion

Effect of aging on the flexural behavior

Figure 4 shows flexural stress–strain curves of unreinforced and short glass-fiber-reinforced PA66 samples (10, 20, and 30 wt%). For each category, untreated, dried, and hygrothermal-aged specimens were compared. Results showed that decreasing moisture rate into composite improves flexural modulus and flexural strength for unreinforced and reinforced samples. Indeed, the lowest mechanical properties are obtained with hygrothermal aging (maximum water uptake), whereas the highest ones are obtained with dried samples. In PA thermoplastics, the polar amide groups generate strong interactions in the crystalline and amorphous phases, with hydrogen bonds being established between neighboring molecules. Despite their strong interactions, these hydrogen bonds exhibit a disadvantage as they can lead to water absorption.

Figure 4.

Flexural stress–strain curves versus aging conditions.

The water molecules in the amorphous phase can interact with the amide groups, which consequently weaken the preexisting interchain hydrogen bonds. Eventually, this can increase the chain mobility of the PA (plasticization effect). Meanwhile, swelling effect can also occur since a certain amount of water is absorbed by the PA.^33,34,35 The plasticization highly impacts the PA properties as it reduces its glass transition temperature.³⁶ It also influences the mechanical properties of the PA matrix such as a reduction of the strength and modulus.³⁷ The swelling effect of PA matrix can induce a mismatched fiber–matrix volume expansion and thus creating a residual stress, which in turn can reduce the fiber–matrix interfacial properties.^38,39 Table 2 recaps quantitatively the effect of aging on the maximum obtained flexural stress.

Table 2.

Effect of aging on the maximum obtained flexural stress (MPa).

	PA66GF00	PA66GF10	PA66GF20	PA66GF30
Untreated sample	42.6	51.8	66.9	80.7
Dried 70°C–24 h	65.6	79.4	90.6	108.9
Hygrothermal aging	30.2	44.6	51.8	63

Figure 5 illustrates three samples of short glass-fiber 30 wt%-reinforced PA66 (PA66GF30) after flexural test. As shown in Figure 5, the dried sample was broken (brittle fracture) during test, whereas the untreated and the hygrothermal-aged samples were deformed without fracture. The breaking of samples was encountered also with the other fractions of reinforcement for dried samples. For the untreated, hygrothermal-aged, and all unreinforced samples (with all aging conditions), no rupture was observed in the end of the flexural test. As seen in the photos, there is no significant change in the color of dried samples, but there is a swelling phenomenon due to water absorption for hygrothermal-aged samples.

Figure 5.

Aspects and modes of failure after flexural tests for PA66GF30 samples.

Effect of reinforcement on the flexural behavior

Figure 6 shows flexural stress–strain curves of untreated, dried, and hygrothermal-aged specimens. For each category, unreinforced and short glass-fiber-reinforced PA66 samples (10, 20, and 30 wt%) were compared. Results showed that increasing glass fiber weight fraction improved flexural modulus and flexural strength for aged and unaged samples. This improvement in the strength properties is in good agreement with literature data^4,40 and can be explained by the contribution of the glass fiber as a brittle and tough material.

Figure 6.

Flexural stress–strain curves versus glass fibers weight fractions.

Effect of aging on the friction behavior

Figures 7 to 9 show the variation of the coefficient of friction as a function of sliding time for unreinforced and short glass-fiber-reinforced PA66 samples (30 wt%).

Figure 7.

Coefficient of friction versus aging conditions for unreinforced samples (F = 22.4 N).

Figure 8.

Coefficient of friction versus aging conditions for unreinforced samples (F = 62.4 N).

Figure 9.

Coefficient of friction versus aging conditions for reinforced samples (F = 22.4 N).

For each category, dried and hygrothermal-aged specimens were compared. Results showed that decreasing moisture rate into composite decreases the stationary coefficient of friction for unreinforced and reinforced samples. Indeed, the lowest coefficient of friction is obtained with dried sample, whereas the highest one is obtained with hygrothermal aging (maximum water uptake). Results obtained in Figure 8 confirm the obtained results. Indeed, the same order of the coefficient of friction curves was found when normal load is augmented to 62.4 N. Results obtained in Figure 9 confirm, also, the obtained results. Indeed, the same order of curves of the coefficient of friction was found when the studied material is reinforced with glass fiber (30 wt%).

When PA66 absorbs water molecules, scission of the hydrogen bonding between amide functional groups takes place because of hydrophilicity of the polar amide functional groups in PA66.⁴¹ The weakening of water-absorbed PA66 becomes more severe at elevated temperatures at which the amide groups are decomposed to carboxyl, amine, and imide groups.⁴⁰ The change in the molecular structure of PA66 after water adsorption reduces its strength and dimensional stability^42

–46 and affects its friction and wear effects. Stuart and Briscoe⁴⁷ reported that water absorption increased the friction coefficient of PA66 by increasing the contact area. Table 3 recaps quantitatively the effect of aging on the maximum obtained coefficient of friction.

Table 3.

Effect of aging on the maximum obtained coefficient of friction.

	PA66GF00		PA66GF30
	22.4 N	62.4 N	22.4 N	62.4 N
Dried 70°C–24 h	0.6	0.58	0.21	—
Hygrothermal aging	0.66	0.6	0.23	—

Effect of reinforcement on the coefficient of friction

Figure 10 shows that increasing glass fiber content decreases the stationary coefficient of friction for aged and unaged samples. This behavior can be explained by the dominance of a friction mechanism relative to the other when increasing glass fiber rate. Indeed, the contact between the two antagonists will evolve from a sliding contact between a hard surface and another relatively soft, which promotes a soft abrasive friction mechanism, to a contact between two relatively hard surfaces, when the fiber content increases, favoring the predominance of a severe abrasive friction mechanism. Kim et al.²³ confirmed these results and showed that the short glass fibers in the PA66 composite decreased the coefficient of friction by producing contact plateaus with a high density of glass fibers on the composite surface.

Figure 10.

Stationary coefficient of friction versus glass fibers weight fractions for all aging conditions.

Effect of aging and reinforcement on the wear

Figure 11 shows the impact of aging on the wear rate. As shown, the effect of aging differs when varying glass fiber weight fractions so there is no unified effect on the wear rate for all reinforcement rates. Table 4 recaps quantitatively the effect of aging on the average obtained wear rate.

Figure 11.

Wear rate versus aging conditions for all glass fibers weight fractions.

Table 4.

Effect of aging on the average obtained wear rate.

	PA66GF00	PA66GF10	PA66GF20	PA66GF30
Dried 70°C–24 h	1.31	23.92	32.21	89.68
Hygrothermal aging	4.8	6.59	41.43	85.83

Figure 12 shows the impact of reinforcement on the wear rate. As shown, when the content of glass fiber is increased, the wear rate increases. These results are in agreement with those found by some researchers^9,11 but in disagreement with other works.^2,5,6,8,9 Kim et al.²³ contradict these results and showed that the short glass fibers in the PA66 composite improved the wear resistance by a strong reinforcement.

Figure 12.

Wear rate versus glass fibers weight fractions for all aging conditions.

When accessing a rate of 30%, there is a jump in the obtained wear rate values and the gap is more significant than which obtained for the other percentages. This wear behavior can be explained by an excessive fragility of this composite (PA66GF30) which facilitates the breakness under dynamic sliding.

Mechanisms of wear

Figure 13(a) shows the surface of PA66GF30 before the friction test. This micrograph indicates the presence of two phases: a blackish PA66 matrix and whitish short glass fibers.

Figure 13.

Typical SEM micrographs showing the effect of reinforcement and sliding time on the wear track: (a) PA66GF30 (neat surface), (b) PA66GF00 (30,600 cycles), (c) PA66GF30 (43,200 cycles), and (d) PA66GF30 (30,600 cycles). SEM: scanning electron microscopy.

Figure 13(b) to (d) illustrates typical SEM micrographs showing the effect of reinforcement and sliding time on the wear track. Zone A indicates the neat surface, zone B the track of wear, and zone C the wear debris. The arrow indicates the sliding direction, which is in these examples perpendicular to the fiber direction. The width of the abrasive wear track is smaller for unreinforced samples and increased when increasing sliding time. Indeed, the width of the wear track is about 1.3 mm for PA66GF30 and about 0.95 mm for PA66GF00 when the number of cycles is 30,600. When increasing the number of cycles to 43,200, the wear track increased to about 1.6 for PA66GF30. The SEM micrographs show that there is mostly an abrasive process of wear for reinforced and unreinforced specimens. Adherence of debris to metal counterface is reduced under very abrasive conditions. Larger asperities on metal counterface deform polymer surface, giving rise to ploughing, micro-cutting, and formation of abrasive wear tracks. For unreinforced nylon (PA66GF00), essentially plastic deformation of polymer occurs during sliding wear. For reinforced samples, glass fibers were worn by abrasive action of metal asperities. Particles of glass debris also combined to accelerate this abrasive action. Typically, the diameter of the glass fibers is reduced prior to fracturing into shorter lengths due to the effect of the alternating stress system followed by debonding from the polymer matrix.

Conclusions

The effect of hygrothermal aging on the flexural, friction, and wear behaviors of monolithic PA66 and glass-fiber-reinforced PA66 was investigated. Results showed that increasing moisture rate decreased the mechanical properties and increased the stationary coefficient of friction for reinforced and unreinforced samples. Hygrothermal aging has decreased the maximum flexural stress of PA66GF00, PA66GF10, PA66GF20, and PA66GF30 to about, respectively, 54, 44, 43, and 42% in comparison with drying treatment and 29, 14, 23, and 22% in comparison with untreated samples. In the contrary, hygrothermal aging has increased the stationary coefficient of friction to about 10% in comparison with drying treatment for PA66GF00 and PA66GF30 samples under a load of 22.4 N. When normal load was increased to 62.4 N, the difference was reduced to about 3.5%. Increasing glass fiber weight fraction improved, also, the flexural mechanical properties and decreased the stationary coefficient of friction for aged and unaged samples. Increasing the reinforcement rate increased the wear rate. Varying moisture rate using hygrothermal aging did not influence the wear rate similarly when varying glass fiber weight fraction.

Footnotes

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work supported by the Ministry of Higher Education and Scientific Research – Tunisia.

ORCID iD

Riadh Autay

References

Ghorbel

Saintier

Dhiab

, et al. Etude du comportement mécanique d’un polyamide 66 chargé de fibers de verre courtes. Mécanique Industrie 2011; 12(5): 333–342.

Mouhmid

Imad

Benseddiq

, et al. A study of the mechanical behavior of a glass fiber reinforced polyamide 6, 6: experimental investigation. Polym Testing 2006; 25: 544–552.

Srinath

Gnanamoorthy

. Effect of short fiber reinforcement on the friction and wear behavior of nylon 66. Appl Composite Mater 2005; 12(6): 369–383.

Thomasson

. Micromechanical parameters from macro-mechanical measurements on glass reinforced polyamide 66. Compos Sci Technol 2001; 61: 2007–2016.

Kukureka

Hooke

Rao

, et al. The effect of fiber reinforcement on the friction and wear of polyamide 66 under dry rolling–sliding contact. Tribol Int 1999; 32: 107–116.

Byett

Allen

. Dry sliding wear behavior of polyamide 66 and polycarbonate composites. Tribol Int 1992; 25: 237–246.

Chen

Modi

Mhay

, et al. The effect of different metallic counterface materials and different surface treatments on the wear and friction of polyamide 66 and its composite in rolling–sliding contact. Wear 2003; 255: 714–721.

Chang

Zhang

, et al. On the sliding wear of nanoparticle filled polyamide 66 composites. Compos Sci Technol 2006; 66: 3188–3198.

Tewari

Bijwe

Mathur

, et al. Studies on abrasive wear of carbon fiber (short) reinforced polyamide composites. Tribol Int 1992; 25(1): 53–60.

10.

Demirci

Duzcukoglu

. Wear behaviors of polytetrafluoroethylene and glass fiber reinforced polyamide 66 journal bearings. Mater Design 2014; 57: 560–567.

11.

Yin

. Effect of rubber on tribological behaviors of polyamide 66 under dry and water lubricated sliding. Wear 2008; 265(3-4): 361–366.

12.

You

Deng

, et al. Tribological properties of solid lubricants filled glass fiber reinforced polyamide 6 composites. Mater Design 2013; 46: 809–815.

13.

Unal

Mimaroglu

. Friction and wear performance of polyamide 6 and graphite and wax polyamide 6 composites under dry sliding conditions. Wear 2012; 289: 132–137.

14.

Morioka

Tsuchiya

Shioya

. Influence of viscoelasticity on friction coefficient of abrasive wear for filler-dispersed polyamide 6. Wear 2015; 324–325: 17–26.

15.

Kim

Shin

Jang

. Tribological properties of short glass fiber reinforced polyamide 12 sliding on medium carbon steel. Wear 2012; 274–275: 34–42.

16.

Haddar

Ksouri

Kallel

, et al. Effect of hygrothermal aging on the monotonic and cyclic loading of glass fiber reinforced polyamide. Polym Composites 2014; 35(3): 501–508. DOI: 10.1002/pc.22688.

17.

Ksouri

De Almeida

Haddar

. Long term aging of polyamide 6 and polyamide 6 reinforced with 30% of glass fibers: physicochemical, mechanical and morphological characterization. J Polym Res 2017; 24:133.

18.

Ksouri

Guermazi

Haddar

, et al. Effects of processing steps and hygrothermal aging on mechanical performance of PA6GF30 composite: interfacial shear strength. Polym Composites 2016; 39(2): 504–512. DOI 10.1002/pc.23961.

19.

Taktak

Guermazi

Derbeli

, et al. Effect of hygrothermal aging on the mechanical properties and ductile fracture of polyamide 6: Experimental and numerical approaches. Eng Fract Mechan 2015; 148: 122–133.

20.

Seltzer

Frontini

Mai

. Effect of hygrothermal aging on morphology and indentation modulus of injection moulded nylon 6/organoclay nanocomposites. Compos Sci Technol 2009; 69: 1093–1100.

21.

Miri

Persyn

Lefebvre

, et al. Effect of water absorption on the plastic deformation behavior of nylon 6. Eur Polym J 2009; 45(3): 757–762.

22.

Ogunsona

Misra

Mohanty

. Accelerated hydrothermal aging of biocarbon reinforced nylon biocomposites. Polym Degradation Stability 2017; 139: 76–88. DOI: 10.1016/j.polymdegradstab.2017.03.013.

23.

Kim

Jang

Kim

. Friction and wear of monolithic and glass-fiber reinforced PA66 in humid conditions. Wear 2014; 309: 82–88.

24.

Arif

Meraghni

Chemisky

, et al. In situ damage mechanisms investigation of PA66/GF30 composite: effect of relative humidity. Composites Part B 2014; 58: 487–495.

25.

Xin

Liu

Mosallam

, et al. Moisture diffusion and hygrothermal aging of pultruded glass fiber reinforced polymer laminates in bridge application. Composites Part B Eng 2016; 100: 197–207.

26.

Xin

Liu

Mosallam

, et al. Hygrothermal aging effects on flexural behavior of pultruded glass fiber reinforced polymer laminates in bridge applications. Construct Build Mater 2016; 127: 237–247.

27.

Bergeret

Pires

Foulc

, et al. The hygrothermal behaviour of glass-fibre-reinforced thermoplastic composites: a prediction of the composite lifetime. Polym Testing 2001; 20:753–763.

28.

Vlasveld

DPN

Groenewold

Bersee

HEN

, et al. Moisture absorption in polyamide-6 silicate nanocomposites and its influence on the mechanical properties. Polym 2005; 46(26) 12567–12576.

29.

Ray

. Aging on interfaces of glass and carbon fibers reinforced epoxy composites. J Colloid Interface Sci 2006; 298(1): 111–117.

30.

ISO 690 Standard, ASTM D-790-07. Standard Test Method for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulation Materials. PA: American Society for Testing and Mater, 1997.

31.

Autay

Kchaou

Dammak

. Friction and wear behavior of induction hardened ISO 42CrMo4 low-alloy steel under reciprocating sliding conditions. Proc I Mech E Part J Eng Tribol 2015; 229(2): 115–125.

32.

Autay

Kchaou

Dammak

. Experimental investigation of the tribological behavior of carbon and low-alloy steels sliding against HSS. Mechanics Industry 2015;16: 109.

33.

Bergeret

Ferry

Ienny

. Influence of the fibre/matrix interface on aging mechanisms of glass fibre reinforced thermoplastic composites (PA-6,6, PET, PBT) in a hygrothermal environment. Polym Degradation Stability 2009; 94:1315–1324.

34.

Monson

Braunwarth

Extrand

. Moisture absorption by various polyamides and their associated dimensional changes. J Applied Polym Sci 2008; 107(1): 355–363.

35.

Hassan

Rahman

Yahya

. Moisture absorption effect on thermal, dynamic mechanical and mechanical properties of injection-molded short glass-fiber/polyamide 6, 6 composites. Fibers Polym 2012; 13(7): 899–906.

36.

Jia

Fraenkel

Kagan

. Effects of moisture conditioning methods on mechanical properties of injection molded nylon 6. J Reinforced Plastics Composites 2004; 23(7): 729–737.

37.

Bradley

Grant

. The effect of the moisture absorption on the interfacial strength of polymeric matrix composites. J Mater Sci 1995; 30: 5537–5542.

38.

Gautier

Mortaigne

Bellenger

. Interface damage study of hydrothermally aged glass-fibre-reinforced polyester composites. Composite Sci Technol 1999; 59: 2329–2337.

39.

. Tensile properties of short glass fiber and short carbon fiber reinforced polypropylene composites. Composites A 2000; 31: 1117–1125.

40.

Stuart

. The application of Raman spectroscopy to the tribology of polymers. Tribol Int 1998; 31: 687–693.

41.

Mikolajewski

Swallow

Webb

. Wet oxidation of undrawn nylon 66 and model amides. J Applied Polym Sci 1964; 8: 2067–2093.

42.

Vachon

Rebenfel

Taylor

. Oxidative degradation of nylon 66 filaments. Textile Research J 1968; 7: 716–728.

43.

Freeman

Mantell

Davison

. Mechanical performance of polysulfone, polybutylene, and polyamide 6 / 6 in hot chlorinated water. Solar Energy 2005; 79: 624–637.

44.

Valentin

Paray

Guetta

. The hygrothermal behavior of glass fibre reinforced PA66 composites: a study of the effect of water absorption on their mechanical properties. J Mater Sci 1987; 22: 46–56.

45.

Stuart

Briscoe

. Surface Plasticization of nylon 6, 6 by water. Polym Int 1995; 106(38): 95–99.

46.

Carrascal

Casado

Polanco

, et al. Absorption and diffusion of humidity in fiber glass-reinforced polyamide. Polym Composites 2005; 26(5): 580–586.

47.

Goncalves

Poulsen

Ogilby

. Mechanism of the temperature - dependent degradation of polyamide 66 films exposed to water. Polym Degradation Stability 2007; 92: 1977–1985.