Long-term stability of UHMWPE fibers

Accepted Manuscript Long-Term Stability of UHMWPE fibers Amanda L. Forster, Aaron M. Forster, Joannie W. Chin, Jyun-siang Peng, Chiao-chi Lin, Sylvain Petit, Kai-Li Kang, Nick Paulter, Michael A. Riley, Kirk D. Rice, Mohamad Al-Sheikhly PII:

S0141-3910(15)00039-7

DOI:

10.1016/j.polymdegradstab.2015.01.028

Reference:

PDST 7572

To appear in:

Polymer Degradation and Stability

Received Date: 3 October 2014 Revised Date:

22 January 2015

Accepted Date: 27 January 2015

Please cite this article as: Forster AL, Forster AM, Chin JW, Peng J-s, Lin C-c, Petit S, Kang K-L, Paulter N, Riley MA, Rice KD, Al-Sheikhly M, Long-Term Stability of UHMWPE fibers, Polymer Degradation and Stability (2015), doi: 10.1016/j.polymdegradstab.2015.01.028. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Elsevier Editorial System(tm) for Polymer Degradation and Stability Manuscript Draft Manuscript Number: PDST-D-14-00767R2

Article Type: Research Paper Keywords: polyethylene fibers, artificial aging, body armor Corresponding Author: Dr. Amanda L Forster,

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Corresponding Author's Institution: NIST

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Title: Long-Term Stability of UHMWPE Fibers

First Author: Amanda Forster

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Order of Authors: Amanda Forster; Aaron M Forster; Joannie W Chin, Ph.D.; Jyun-siang Peng; Chiao-chi Lin; Sylvain Petit; Kai-Li Kang; Nicholas Paulter; Michael A Riley; Kirk Rice; Mohamad Al-Sheikhly

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Abstract: The performance of ultra-high molecular weight polyethylene (UHMWPE) fibers for ballistic protection is predicated on the development of a highly aligned molecular structure that allows the polymer to exhibit a superior strength in the axial direction of the fiber. However, even an ideal molecular structure will be subjected to degradation during use, which can reduce the high strength of these fibers, and impact their ability to protect the wearer. In this work, the long term stability of UHMWPE fibers are investigated and the activation energy for this mechanism was calculated to be approximately 140 kJ/mol, in agreement with previous reports. The inclusion of accelerated aging temperatures that encompass the alpha-relaxation temperature introduced physical effects in addition to oxidative degradation that complicate a simple explanation of the changes in properties. Assuming that the shift factors that were used in this analysis are correct, it would take approximately 36 years for the tensile strength of this UHMWPE yarn to fall by 30 % at 43 °C. Changes in the oxidation index of this material due to aging are also studied using Fourier Transform Infrared (FTIR) Spectroscopy, and no simple correlation between the retained strength and the oxidation index was found.

Detailed Response to Reviewers

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Dear Sir,  Please find below our response to the reviewer regarding Polymer Degradation and Stability  Manuscript number PDST‐D‐14‐00767.  Thank you very much for your time and consideration.  Sincerely,     Amanda Forster    Reviewer Comments 1‐4:  There is one issue I overlooked in the first review since the paper needed major revision. This  is the poor quality of the Abstract. It contains unnecessary general statements that are  irrelevant for this journal, gives little new information other than an activation energy (that  had been reported by others, albeit by a different method) and overlooks the significant  findings. It would be useful to include some of the significant findings that appear in the body  of the text and the conclusions.  I would expect to see the following:  1.Omission of the second sentence, which is fine for a general audience but adds nothing to  the science; 2.Inclusion of the observation that there is no simple correlation between the  retained strength and the oxidation index 3.The inclusion of the observation that accelerated  ageing temperatures that encompass the alpha relaxation introduce physical effects in  addition to oxidative degradation that complicates a simple explanation of the changes in  properties.  4.Inclusion of the statement from the text: If the shift factors are correct, it would take  approximately 36 years for the tensile strength of this UHMWPE yarn to fall by 30 % at 43oC.    Author’s Response to Reviewer Comments 1‐4:  Thank you for your comments.  A new abstract has been written that incorporates your  suggestions.  Please see the new abstract below.    The performance of ultra‐high molecular weight polyethylene (UHMWPE) fibers for  ballistic protection is predicated on the development of a highly aligned molecular  structure that allows the polymer to exhibit a superior strength in the axial direction of  the fiber.  However, even an ideal molecular structure will be subjected to degradation  during use, which can reduce the high strength of these fibers, and impact their ability  to protect the wearer.   In this work, the long term stability of UHMWPE fibers are  investigated and the activation energy for this mechanism was calculated to be  approximately 140 kJ/mol, in agreement with previous reports.  The inclusion of  accelerated aging temperatures that encompass the alpha‐relaxation temperature  introduced physical effects in addition to oxidative degradation that complicate a simple  explanation of the changes in properties.  Assuming that the shift factors that were used  in this analysis are correct, it would take approximately 36 years for the tensile strength  of this UHMWPE yarn to fall by 30 % at 43 °C.  Changes in the oxidation index of this  material due to aging are also studied using Fourier Transform Infrared (FTIR)  Spectroscopy, and no simple correlation between the retained strength and the  oxidation index was found.     

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  Reviewer Comment 5:  The discussion from the top of page 4 is not satisfactory as it fails to acknowledge that  UHMWPE cannot be treated as a pure alkane but in fact contains defect sites, unsaturation  and other impurities which are the source of the initiating free radicals leading to  hydroperoxidation. I would recommend they read their cited reference 40 and also part II by  these authors in the same journal (1997; 58: 41‐54). The statistical thermal fluctuation  argument for alkyl radical formation is one which I consider implausible and have not seen  put for any polymer by researchers in this field. If it is to be preferred over those above, then  it needs a reference.    Author’s Response to Reviewer Comment 5:  Thank you for your comments.  The discussion from page 4 was removed and the introduction  shortened and rewritten as suggested.  Please see the new text below:    Ultra high molecular weight polyethylene [1,2,3,4] (UHMWPE) is one of the two main  types of fibers currently used in ballistic‐resistant body armor.  UHMWPE is a long‐chain  polyolefin with a molar mass between 3 million and 5 million.  Its tensile strength is  reported to be approximately 40~\% greater than PPTA fiber [5] due to its high  crystallinity and highly oriented zig‐zag sp3 conformation.  Polyethylene has no  functional groups, resulting in superior chemical resistance as compared to other  materials [6].  A well‐publicized field failure in 2003 of a body armor based on the fiber  poly(p‐phenylene‐2,6‐benzobisoxazole), or PBO, has prompted our work to better  understand the long term stability of other classes of fibers used in body armor when  exposed to elevated temperatures [7,8,9].    Degradation of polyolefins initiates from thermal decomposition of hydroperoxides and  peroxides produced due to defects, unsaturations, and other impurities introduced  during processing [10].  The rate of degradation is dependent upon many factors such as  the number of free radicals generated, the presence of scavenger compounds (e.g.,  antioxidants), the presence of oxygen, and the degree of crystallinity of the polymer.   UHMWPE fibers used in soft body armor are protected from photo‐oxidation by a  protective fabric carrier, and exposure to ionizing radiation is not expected to occur  during general use. The likely routes to initiate degradation are thermal exposure (from  storage and wear) or mechanically‐initiated degradation from routine use of the armor.   Mechanical degradation initiates free radical formation and can occur at folds [11] due  to movement of the armor on the wearer. Once these free radicals are generated they  may follow the established free radical reactions such as hydrogen abstraction and  hydroperoxide formation resulting in main chain scission, reduction of molecular  weight, and oxidation.  Each of these short‐term exposures results in cumulative  damage that may lead to unforeseen performance reductions in the armor over time.   Mechanically‐induced oxidation is not a commonly explored route for artificial aging of  UHMWPE fibers since it can be difficult to quantify and control free radical formation via  this method. In this study, elevated temperature exposure will be used to accelerate 

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degradation, since the reaction pathways are assumed to be similar once free radicals  are initiated.  Costa and co‐authors have discussed thermal, photo, and mechano‐ oxidation of UHMWPE in detail, and their work includes a reaction scheme of how  oxidation occurs in this material [12,10].      Due to the highly oriented and crystalline microstructure of UHMWPE fibers, it is not  immediately clear that accelerating degradation via temperature will follow Arrhenius  behavior.  The goals of this study are threefold: first, to determine whether elevated  temperatures follow the constant acceleration assumption, next to determine whether  or not mechanical performance shifts to create a master curve, and finally to determine  how the calculated activation energy agrees with a previous study to determine  activation energy via elevated temperature exposure [4]. 

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  Reviewer Comment 6:  The author notes that the incorrect Figure 1 was uploaded and offers the replacement which  unfortunately also contains some shorthand abbreviations that are not chemically sensible.  eg in the centre of the figure: CH2(OO.) rather than ‐CH(OO.)‐. The paper does not really need  the level of complexity in this diagram as the point could be made by citing the references  above or replacing Figure 1 with a simplified scheme such as from PDST 2013; 98: 2383. Also  given the detailed oxidation product scheme (Figure 9) this could be omitted as it covers  similar ground.    Author’s Response to Reviewer Comment 6:  Thank you for your comments.  Figure 1 was omitted as suggested.    Reviewer Comments 7 and 8:  On page 18, from line 5 (in material not included in the original paper) it is claimed that: "The  absorption band around 1737 cm‐1 presents the combined absorption of the oxidative  products such as RO2H, ROH, ROOR..". Assuming that R still represents the alkyl group then  this is just not correct and I do not know what the authors have in mind here. Perhaps they  mean the products from the decomposition of peroxides and hydroperoxides.   8.  There is another incorrect statement in section 3.1.2, line 4: "Once activated the  antioxidant ester group does not participate in further hydrogen transfer reactions". As  shown in Figure 8, the ester group never participates in the stabilizing reaction; it is the  hindered phenol group.     Author’s Response to Reviewer Comments  7 and 8:  Thank you for your comments.  The text was reworded as suggested.  Please see new text  below:    The degree of oxidation increases with aging temperature and time, and is identified by  the formation of a new peak at 1737 cm‐1,  which is assigned to an ester.  Another  shoulder peak is observed at 1713 cm‐1, which is identified as a ketone.  At least two  peaks are superimposed to form a complex FTIR spectrum.  Both peaks were treated as 

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Gaussian functions and fitted as shown in Figure 5.  The absorption band around 1737  cm‐1 is not attributed to a single oxidation product.  This band is indicative of the  presence of oxidation products from the decomposition of peroxides and  hydroperoxides.  The baseline oxidation level for this material is evident by the presence  of a peak at 1737 cm‐1 and 1713 cm‐1 in the unaged material, as shown in Figure 6.  It is  possible that the peak at 1737 cm‐1 represents the hindered phenol of an antioxidant.  If  the antioxidant is present in the baseline material, there are implications for  interpreting the oxidation products. 

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  Reviewer Comment 9:  Figure 9 is also new and is generally OK but contains a decarboxylation reaction (d) of an in‐ chain hydroperoxide which is probably a multi‐step reaction and in any case the product: ‐‐‐ CH2 has no radical attached so is presumably the result of a disproportionation reaction and  should be: ‐CH=CH2. Again the authors need to carefully check over their schemes to  determine if they make sense.    Author’s Response to Reviewer Comment 9:  Thank you for catching this error.  It is supposed to be a CH3.  The figure was corrected as shown  below.   

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+ O2

CH2CH(O2)CH2

+

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CH2CH(O2)CH2

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CH2CH2CH2

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CH2CHCH2

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CH2CH(O)CH2

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CH2CH(O2)CH2 CH2CH(OH)CH2

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+

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H2O +

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CH2CH(O)CH2

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CH2CHCH2

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O     It seems that the authors have experience in radiation chemistry (based on cited literature) but  should note that the reactions possible in the absence of a radiation field are much restricted.   

*Revised manuscript with changes marked Click here to view linked References

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Long-Term Stability of UHMWPE fibers

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Amanda L. Forstera,∗, Aaron M. Forsterb , Joannie W. Chinb , Jyun-siang Pengb , Chiao-chi Linb , Sylvain Petitb , Kai-Li Kanga , Nick Paultera , Michael A. Rileya , Kirk D. Ricea , Mohamad Al-Sheikhlyc a

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Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD b Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD c Materials Science and Engineering Department, University of Maryland, College Park, MD

Abstract

The performance of ultra-high molecular weight polyethylene (UHMWPE) fibers for ballistic protection is predicated on the development of a highly aligned molecular structure that allows the polymer to exhibit a superior

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strength in the axial direction of the fiber. However, even an ideal molecular structure will be subjected to degradation during use, which can reduce the high strength of these fibers, and impact their ability to protect the wearer.

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In this work, the long term stability of UHMWPE fibers are investigated and the activation energy for this mechanism was calculated to be approximately

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140 kJ/mol, in agreement with previous reports. The inclusion of accelerated aging temperatures that encompass the alpha-relaxation temperature introduced physical effects in addition to oxidative degradation that complicate a simple explanation of the changes in properties. Assuming that the shift factors that were used in this analysis are correct, it would take approxi∗

Corresponding author Email address: [email protected] (Amanda L. Forster)

Preprint submitted to Polymer Degradation and Stability

January 14, 2015

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mately 36 years for the tensile strength of this UHMWPE yarn to fall by

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30 % at 43 ◦ C. Changes in the oxidation index of this material due to aging

are also studied using Fourier Transform Infrared (FTIR) Spectroscopy, and no simple correlation between the retained strength and the oxidation index was found.

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Keywords:

UHMWPE fibers, body armor, oxidation, activation energy, degradation,

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artificial aging 1. Introduction

Ultra high molecular weight polyethylene [1, 2, 3, 4] (UHMWPE) is one of the two main types of fibers currently used in ballistic-resistant body armor. UHMWPE is a long-chain polyolefin with a molar mass between 3

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million and 5 million. Its tensile strength is reported to be approximately 40 % greater than PPTA fiber [5] due to its high crystallinity and highly oriented zig-zag sp3 conformation. Polyethylene has no functional groups, resulting in superior chemical resistance as compared to other materials [6].

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A well-publicized field failure in 2003 of a body armor based on the fiber poly(p-phenylene-2,6-benzobisoxazole), or PBO, has prompted our work to

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better understand the long term stability of other classes of fibers used in body armor when exposed to elevated temperatures [7, 8, 9]. Degradation of polyolefins initiates from thermal decomposition of hy-

droperoxides and peroxides produced due to defects, unsaturations, and other impurities introduced during processing [10]. The rate of degradation is dependent upon many factors such as the number of free radicals generated, the

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presence of scavenger compounds (e.g., antioxidants), the presence of oxygen,

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and the degree of crystallinity of the polymer. UHMWPE fibers used in soft

body armor are protected from photo-oxidation by a protective fabric carrier, and exposure to ionizing radiation is not expected to occur during general

use. The likely routes to initiate degradation are thermal exposure (from

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storage and wear) or mechanically-initiated degradation from routine use of the armor. Mechanical degradation initiates free radical formation and can

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occur at folds [11] due to movement of the armor on the wearer. Once these free radicals are generated they may follow the established free radical reactions such as hydrogen abstraction and hydroperoxide formation resulting in main chain scission, reduction of molecular weight, and oxidation. Each of these short-term exposures results in cumulative damage that may lead to unforeseen performance reductions in the armor over time. Mechanically-

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induced oxidation is not a commonly explored route for artificial aging of UHMWPE fibers since it can be difficult to quantify and control free radical formation via this method. In this study, elevated temperature exposure will be used to accelerate degradation, since the reaction pathways are assumed

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to be similar once free radicals are initiated. Costa and co-authors have discussed thermal, photo, and mechano-oxidation of UHMWPE in detail,

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and their work includes a reaction scheme of how oxidation occurs in this material [12, 10].

Due to the highly oriented and crystalline microstructure of UHMWPE

fibers, it is not immediately clear that accelerating degradation via temperature will follow Arrhenius behavior. The goals of this study are threefold: first, to determine whether elevated temperatures follow the constant acceler-

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ation assumption, next to determine whether or not mechanical performance

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shifts to create a master curve, and finally to determine how the calculated

activation energy agrees with a previous study to determine activation energy via elevated temperature exposure [4].

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1.1. Methodology of Artificial Aging

Classically, researchers have extrapolated high temperature behavior to a lower temperature regime using the Arrhenius equation [13, 14, 15]. Time-

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temperature superposition may be used to shift data from different aging temperatures to create a master curve if the increase in temperature increases the rate of degradation by a constant multiplicative factor. When plotted on a logarithmic time graph, all data sets at the various temperatures should have the same overall curve shape. If the data from each elevated tempera-

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ture can be superposed at a reference temperature by multiplying the aging time by a constant, known as the shift factor, at , then the time-temperature superposition is generally regarded as valid. In order to verify the feasibility of time-temperature superposition, a plot of (log at versus 1/T ) should result

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in a linear dependence upon temperature. [13]. Previous work has focused on understanding the long term stability of

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bulk UHMWPE typically used in orthopedic joint replacement applications [16]. Chabba and coworkers performed artificial aging experiments on UHMWPE fibers, focusing on oxygen uptake as a marker of degradation and calculating the activation energy of this process as being approximately 120 kJ/mol. In this study, the focus is on tensile strength as an indicator of degradation in the fiber, and not oxygen uptake as in Chabba’s study. Tensile strength was selected as a metric for this study because it is generally considered to be 4

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an easily-studied parameter that is related to ballistic performance [17]. In

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order to better represent the aging environment for body armor, aging temperatures of 43 ◦ C and 65 ◦ C were chosen to simulate body temperature and

potential storage conditions (e.g., the trunk of a car), and 90 ◦ C and 115 ◦ C

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were used to accelerate degradation. 2. Experimental

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2.1. Sample Description

Gel spun UHMWPE continuous filament yarns were used in this study. Yarn samples were stored in dark ambient conditions when not being exposed. For the aging experiments, yarns were wound onto perforated spools and placed into ovens at 43 ◦ C , 65 ◦ C , 90 ◦ C , and 115 ◦ C under dry conditions for a predetermined period of time. A series of relative temperature/relative

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humidity dataloggers were used to monitor the consistency of the chamber temperature during the exposure period. 2.2. Mechanical Properties Measurement

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An Instron1 Model 5582 test frame equipped with a 1 kN load cell and pneumatic yarn and cord grips (Instron model 2714-006) was used to measure

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the breaking strength of UHMWPE yarns. The experiment was performed in accordance with ISO 2062 [18]. A 58.42 cm (23 in) yarn was given 23 twists (1 twist per 2.54 cm) on a custom designed yarn-twisting device, and the level of twist was maintained while transferring the yarn to the pneumatic grips. The gage length was 25 cm and the crosshead speed was 250 mm/min.

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The strength at break was recorded and each data point represents the mean

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of at least 7 trials. 2.3. Oxidation Measurement

Oxidation of UHMWPE was measured using Fourier Transform Infrared

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Spectroscopy. A Nicolet Nexus 670 Fourier Transform Infrared Spectrometer (FTIR) equipped with a liquid nitrogen-cooled mercury cadmium telluride (MCT) detector and a SensIR Durascope (Smiths Detection) attenuated total

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reflectance (ATR) accessory was used in the oxidation measurement. Consistent pressure on the yarns was applied using the force monitor on the Durascope. The final scans represent the average of 128 individual scans with a resolution of 4 cm−1 between 650 and 4000 cm−1 , respectively. Spectral analysis, including spectral baseline correction and normalization, was

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carried out using a custom software package developed at NIST to catalog and analyze multiple spectra. The spectra were baseline corrected and normalized using the peak at 1472 cm−1 , which was attributed to the CH2 bending mode. Typical standard uncertainties for spectral measurement are

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4 cm−1 in wavenumber and 5 % in peak intensity. For the purposes of evaluating the degree of oxidation, the overlapping peaks were deconvolved using

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the methods available in the instrument software package between 1712 and 1735 cm−1 . This peak ratio is known as the oxidation index and is commonly 1

Certain commercial equipment, instruments, or materials are identified in this paper

in order to specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the materials or equipment identified are necessarily the best available for this purpose.

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used in other UHMWPE aging literature [19, 20, 21]. The oxidation index

work. 3. Results and Discussion

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3.1. Mechanical Properties of Artificially Aged Fibers

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will be described in further detail in the results and discussion section of this

Figure 1 shows the change in tensile strength as a function of aging at

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different temperatures. As anticipated, the reduction in tensile strength was lowest at the aging temperature of 43 ◦ C. Only a slight decrease in the tensile strength (2 %) was observed after one week of aging, with virtually no change after the first month of aging. A slow, steady deterioration in the tensile strength was observed throughout the long term aging study. The

9 %.

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study was ended at 102 weeks of aging, with a final tensile strength loss of

The reduction in residual tensile strength was more evident at the aging temperature of 65 ◦ C. The decrease in tensile strength after the first week

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was approximately 3.0 %, and in the first month it was 8 %. After 94 weeks of aging, the study was ended, with a final tensile strength loss of over 30 %. The reduction in tensile strength was even more apparent at the higher

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temperatures. The UHMWPE fibers lost 28 % of their initial tensile strength at the aging temperature of 90 ◦ C in the first week, and continuously decreased to 56 % after 17 weeks. Fibers exposed to the aging condition of 115 ◦ C lost 42 % of their original tensile strength after 1 week. The study continued for 17 weeks, after which the fibers had lost 52 % of their original tensile strength. However, given the rapid and catastrophic loss of tensile 7

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3.5

2.5

2.0

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Tensile Strength (GPa)

3.0

1.5

43 °C 65 °C 90 °C 115 °C

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Aging time (h)

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0

Figure 1: The decrease in tensile strength of artificially aged UHMWPE fibers at various temperatures over time.

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strength in the fibers aged at 90 ◦ C and 115 ◦ C, and the fact that the fibers

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were unconstrained during this experiment, it is possible that these fibers may have undergone a morphological change during elevated temperature exposure. The loss of orientation in the fiber, combined with scissions in the critical tie chains that are important in carrying load in UHMWPE fibers [22]

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could be partially responsible for the rapid loss in tensile strength (especially

at 1 week of aging time) in the fibers aged at 90 ◦ C and 115 ◦ C, rather than

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oxidative chain scission. DMTA temperature scans of unaged fibers were previously reported [23] and show reduction in the storage modulus by 30 % and 50 % at 90 ◦ C and 115 ◦ C, respectively, although the material remains 20 ◦ C away from the transition region. Shrinkage of highly drawn and oriented polyethylene has been widely reported [24, 1, 25]. Shrinkage was not directly measured or observed in this study, but remains a potential consequence

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of small changes in polymer morphology when working with such highly drawn fibers at elevated temperatures. Previous work [23] has shown that the UHMWPE fibers used in this experiment undergo an alpha-relaxation above 80 ◦ C, but degradation may be accelerated at higher temperatures.

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Gillen, et al. have also shown these methods to work for Nylon 6,6 material aging at elevated temperatures [14, 15, 13].

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In order to assess the long term stability of UHMWPE fibers, the master

curve at 43 ◦ C is presented in Figure 2. The lowest aging temperature represents the base use condition for the fibers (body temperature). A master curve was created by using 43 ◦ C as the reference temperature, and then horizontally shifting the higher temperature aging data until they superimposed smoothly to form a single curve [4, 26, 27]. The amount that each

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curve must be shifted is called the shift factor, at , and is used in Equation 1

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to determine the activation energy of the aging mechanism.

  tt Ea 1 1 ln at = ln = − − (1) t0 R T ref T Where at is the shift factor, t0 is the aging time of reference temperature,

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tt is aging time after shifting, Ea is the activation energy, R is the universal gas constant, and Tref is the reference temperature. The value of the acti-

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vation energy as calculated by Equation 1 is found to be 140 kJ/mol, with a standard deviation of approximately 15 % (based on the original error in the tensile strength measurements combined with the error in the shift factor calculation), which is approximately 20 kJ/mol higher than previously published results from Chabba [4], which were based on oxygen uptake. This difference is within the error of this measurement. A table of shift factors is

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given in Table 1 and the shift factors are plotted to show linearity in Figure 3. The strong linearity of the shift factor plot, even at the high temperatures where shrinkage may occur supports the elevated temperature aging methodology. Assuming that the master curve presented in Figure 2 is valid, one

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might predict that it will take approximately 36 years for the tensile strength

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of this UHMWPE yarn to fall by 30 % at 43 ◦ C. 3.1.1. Oxidation Analysis A representative FTIR spectrum of baseline, or unaged, UHMWPE fibers

is shown in Figures 4. The characteristic peaks at 2916 cm−1 and 2848 cm−1 are identified as sp3 C-H stretching, 1471 cm−1 and 1461 cm−1 are assigned to

C-H bending, and 731 cm−1 and 717 cm−1 are in-phase and out-of-phase C-H rocking, respectively. Hydroperoxides were not detected in the ATR-FTIR 10

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60

40

30

20

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Tensile Strength Loss (%)

50

10

0

6

8

10

12

14

16

18

ln time

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4

43 °C 65 °C 90 °C 115 °C

Figure 2: Master curve for aging of UHMWPE fibers at the reference temperature of 43 ◦ C. Error bars are not shown for clarity of presentation, the standard uncertainty in the data is approximately 5 %.

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1e+5

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y=1E+22e-16.07x R2=0.9884

1e+4

1e+2

1e+0

1e-1

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shift factor, aT

1e+3

1e+1

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2.8

3.0

3.2

1000/T (K-1)

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2.6

Figure 3: Plot verifying linearity of the shift factors. Shift factor has no units. Error bars are not shown for clarity of presentation, the standard uncertainty in the data is approximately 5 %.

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lnat

at

43

0

1.0

65

3.0

20.1

90

7.2

1339.4

115

9.1

8955.3

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Temperature ◦ C

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Aging

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Table 1: Shift factors for aging of UHMWPE.

spectra of the UHMWPE fibers during aging. Gugumus [28, 29, 30, 31] has previously shown that FTIR can be used to measure hydroperoxide in LDPE and HDPE films, and that hydroperoxide concentration decreases linearly with film thickness. Temperature has been shown to have a complicated effect on the formation and destruction of free and associated hydroperoxides,

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which may hinder our ability to resolve any hydroperoxides, even though they are expected to form.

The degree of oxidation increases with aging temperature and time, and is identified by the formation of a new peak at 1737 cm−1 , which is assigned to

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an ester. Another shoulder peak is observed at 1713 cm−1 , which is identified as a ketone. At least two peaks are superimposed to form a complex FTIR

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spectrum. Both peaks were treated as Gaussian functions and fitted as shown in Figure 6. The absorption band around 1737 cm−1 is not attributed to a single oxidation product. This band is indicative of the presence of oxidation products from the decomposition of peroxides and hydroperoxides. The baseline oxidation level for this material is evident by the presence of a peak at 1737 cm−1 and 1713 cm−1 in the unaged material, as shown in Figure 5.

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3.0

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baseline

2848 cm-1

2.5

2916 cm

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-1

-1

-1

1471 cm

717 cm-1

1461 cm

Absorbance

2.0

731 cm-1

1.5

0.5

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1.0

3000

2500

2000

1500

1000

-1

Wavenumber (cm )

AC C

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0.0 3500

Figure 4: FTIR spectrum of baseline, or unaged, UHMWPE fiber. The units of absorbance are arbitrary. The characteristic peaks at 2916 cm−1 and 2848 cm−1 are identified as sp3

C-H stretching, 1471 cm−1 and 1461 cm−1 are assigned to C-H bending, 731 cm−1 and 717 cm−1 are in-phase and out-of-phase C-H rocking, respectively. A small peak around 1125 cm−1 is unassigned.

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Oxidation would appear as a sharp peak at 1737 cm-1 Also a shoulder peak at 1713 cm-1

0.2

 

0.0

-0.2

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-0.4

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Absorbance (a.u.)

0.4

1900

1800

1700

1600

1500

-1

Wavenumber (cm )

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2000

Unaged UHMWPE fiber

Figure 5: FTIR spectrum showing baseline oxidation levels for unaged UHMWPE fiber. Standard uncertainty in these measurements is approximately 4 cm−1 .

15

0 .0 2 4

p e a k a t 1 7 1 2 .5 c m p e a k a t 1 7 3 6 .5 c m d a ta (1 w k ) f it

0 .0 2 0

-1 -1

re c o n s tru c te d fro m re c o n s tru c te d fro m

p e a k a t 1 7 1 2 .5 c m p e a k a t 1 7 3 6 .5 c m d a ta (4 w k ) fit

0 .0 2 4

f it to d a ta f it to d a ta

0 .0 2 0

0 .0 1 6

A b s o rb a n c e

0 .0 1 6

0 .0 1 2

-1 -1

re c o n s tru c te d fro m re c o n s tru c te d fro m

fit to d a ta fit to d a ta

0 .0 1 2

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A b s o rb a n c e

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0 .0 0 8

0 .0 0 4

0 .0 0 0

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0 .0 0 4

0 .0 0 8

0 .0 0 0

1 7 8 0

1 7 6 0

1 7 4 0

1 7 2 0

W a v e n u m b e r (c m

-1

)

1 7 0 0

p e a k a t 1 7 1 2 .5 c m p e a k a t 1 7 3 6 .5 c m d a ta (1 0 2 w k ) f it

0 .0 2 4

0 .0 2 0

-1

re c o n s tru c te d fro m re c o n s tru c te d fro m

1 7 6 0

1 7 4 0

W a v e n u m b e r (c m

1 7 2 0 -1

)

f it to d a ta f it to d a ta

A b s o rb a n c e

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0 .0 1 6

-1

1 7 8 0

0 .0 1 2

0 .0 0 8

0 .0 0 4

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0 .0 0 0

1 7 8 0

1 7 6 0

1 7 4 0

W a v e n u m b e r (c m

1 7 2 0 -1

1 7 0 0

)

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Figure 6: Curve-resolved FTIR spectra of UHMWPE fiber aged at 43 ◦ C for 1 week

(top left), 4 weeks (top right), and 102 weeks (bottom), scale enlarged to show 1680 to 1780 cm−1 region, where oxidation appears, more clearly. The units of absorbance are arbitrary. The shoulder of this peak is resolved to elucidate the formation of a new peak around 1713 cm−1 . Standard uncertainty in these measurements is approximately 4 cm−1 .

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It is possible that the peak at 1737 cm−1 represents the hindered phenol of

are implications for interpreting the oxidation products. 3.1.2. Role of Antioxidants in Polyethylene

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an antioxidant. If the antioxidant is present in the baseline material, there

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The antioxidants function is based on the hydrogen transfer mechanism,

by which the hydrogen atom of the antioxidants hydroxyl group transfers to the polyethylene carbon-centered, peroxyl, and alkoxyl radicals as shown in

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Figure 7 [32, 33, 34, 35, 36, 37]. The hindered phenol does not participate in hydrogen transfer reactions, although it may hydrogen bond to some of the oxidation products. The free radical produced from the above reactions is very stable with the alkoxide structure, and is similar to the vitamin E alkoxide radical [38].

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The peak analysis presented for 43 ◦ C in Figure 6 indicates an increasing of the shoulder at 1713 cm−1 . The peak at 1737 cm−1 appears to shrink with exposure time. This contradicts the production of multiple hydroxyl containing oxidation products that would increase this peak with time. One

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suggestion is the decrease in the peak at 1737 cm−1 is the consumption of an antioxidant ester. The oxidation analysis of 1713 cm−1 (discussed later) does

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not indicate an induction time for the production of the main ketone band that would be expected for the presence of an antioxidant. However, despite the fact that the oxidation products will continue to increase through various oxidation reactions in the amorphous structure, they will also decompose continuously to CO2, CO, and even OH radicals, as shown by the sequence of reactions in Figure 8 [39]. It should also be mentioned that these reactions take place in the limited amorphous part of the polymer and at the crystalline 17

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interfaces. These factors affect the interpretation of the FTIR spectrum in

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this region. In this material, the available oxygen is limited because the

diffusion of oxygen is much lower than the consumption due to the higher

crystallinity of this material. Currently, the exact nature of the changes at 1737 cm−1 are unknown and remain a convolution of potential degradation

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or protection mechanisms. 3.1.3. Oxidation Index

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The intensity of the ketone group generally increases with increasing aging time at 43 ◦ C. Thus, the degree of oxidation of UHMWPE fibers could potentially be quantified by introducing the oxidation index (OI), as used in other artificial aging applications for bulk UHMWPE [3, 21, 12]. For the purposes of this study, the peak at 1472 cm−1 was used as the reference peak, and the OI is the peak area at 1713 cm−1 divided by the peak area

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at 1472 cm−1 . Equation 2 defines oxidation index. Figure 9 and Figure 10 show the increase in OI over time for all aging temperatures. A1713cm−1 (2) A1472cm−1 The OI increased moderately at the lower aging temperatures of 43 ◦ C and

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OI =

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65 ◦ C. For these aging conditions, the OI increased from 0.0002 to 0.0243 and 0.0249, for 43 ◦ C and 65 ◦ C, respectively. For the higher aging temperatures, the OI increased more rapidly. After 17 weeks of aging at 90 ◦ C, the OI increased from 0.0002 to 0.0385, and for 17 weeks of aging at 115 ◦ C, the OI increased from 0.0002 to 0.0471. Figure 11 shows the relationship between tensile strength and OI, the higher the aging temperature, the faster the drop in tensile strength, regardless of the OI value. This result might be indicative 18

(a) hydrogen transfer mechanism

OH

R

O

O

+

C C

O

OH

O

O

O

O

C

hydrogen transfer mechanism

+

OH

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RO

O C

C

RO2H

+

ROH

O 4

4

(c)

+

C

C C

O

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hydrogen transfer mechanism

+

RH

4

4

RO2

+

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C

(b)

O

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O

O C C

O 4

AC C

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4

O

Figure 7: The hydrogen transfer mechanism by which a common antioxidant, which is likely present in the UHMWPE fiber, works to prevent oxidation [32, 33, 34, 35, 36, 37].

19

+ O2

CH2CHCH2

+

CH2CH(O2)CH2

(a)

CH2CH(O2)CH2

(b)

CH2CH2CH2

+

CH2CH(O2H)CH2

CH2CHCH2

CH2CH(O2H)CH2

CH2CCH2

+ H2O

(c)

CH2CH(O)CH2

+

CH2CH(OH)CH2

OH + 2

CH2 (d)

CH2CH(O)CH2

+ OH (e) (f)

CH2CH2CH2

+

CH2CHCH2

H2O +

CH2CH2CH2 CH2CH(O2)CH2

CH2CH(OH)CH2

CH3 +

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CH2CH(O2H)CH2

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O

CO2 +

CH2CH(O2)CH2

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CH2CHCH2

(g)

(h)

+

CH2CCH2

+ O2

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EP

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O

Figure 8: This series of reactions shows the formation of many of the common scission products due to the oxidation of polyethylene [39].

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0 .0 5

4 3 6 5 9 0 1 1 5

-1

c

o

o

c

c

0 .0 2

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1 7 1 3 c m

O .I . ( A

c

o

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0 .0 3

/A

1 4 7 2 c m

-1

)

0 .0 4

o

0 .0 1 0 .0 0 1 0 0

1 0 0 0

1 0 0 0 0

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A g in g tim e (h )

Figure 9: Changes in oxidation index (OI) for different aging temperatures as a function of time for UHMWPE fibers (log axis). Error bars represent standard combined uncertainty in the FTIR data.

previously.

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of changes in morphology at these higher aging temperatures, as described

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4. Summary and Future Work This work has demonstrated that exposure to elevated temperatures is a

valid method for aging UHMWPE fibers. When exposed to elevated temperatures for long periods of time, UHMWPE fibers lose tensile strength, and the Arrhenius model is applicable to predict this phenomenon. However, it is a challenge to identify one aging mechanism responsible for the loss of tensile 21

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0 .0 5

0 .0 3

0 .0 2

0 .0 1

0 .0 0

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O .I . ( A

1 7 1 3 c m

-1

/A

1 4 7 2 c m

-1

)

0 .0 4

4 0 0 0

8 0 0 0

1 2 0 0 0

4 3

o

c

6 5

o

c

9 0

o

c

1 1 5

o

c

1 6 0 0 0

EP

A g in g tim e (h )

Figure 10: Changes in oxidation index (OI) for different aging temperatures as a func-

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tion of time for UHMWPE fibers (linear axis). Error bars represent standard combined uncertainty in the FTIR data.

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T e n s ile s tr e n g th (G P a )

3 .0

4 3

o

C

6 5

o

C

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3 .5

o

C o

C

9 0 1 1 5

2 .5

2 .0

1 .0 0 .0 0

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1 .5

0 .0 1

0 .0 2

1 7 1 3 c m

-1

/A

0 .0 4 1 4 7 2 c m

-1

0 .0 5

)

EP

O .I . ( A

0 .0 3

Figure 11: The relationship between tensile strength and oxidation index at different aging temperatures for UHMWPE fibers. In all cases, as the OI increases, tensile strength

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decreases. Deviations from the overall in the 65 ◦ C data are not explained. Further

work will attempt to measure changes in morphology that may have resulted from this oxidation.

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strength because it is a convolution of oxidation and changes in fiber mor-

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phology. This is demonstrated by the behavior of the fibers exposed to 90 ◦ C and 115 ◦ C, which may undergo shrinkage and loss of orientation. Molecular spectroscopy verified that all samples were oxidized by the elevated temper-

ature exposure, however there is not a clear relationship between OI and

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the change in tensile strength. Therefore, it would be difficult to utilize this model to predict failure outside the accelerated environment of the labora-

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tory, unless these two contributing factors can be separately understood and their interactions accounted for in the model. A future paper will detail the changes in thermal properties and morphology of these fibers resulting from this aging experiment, and carefully consider the role of shrinkage. This work verifies that there is potential for oxidation and tensile strength loss in this material when it is exposed to elevated temperatures for long periods of time.

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These factors should all be considered when using these fibers in applications where this type of environment may be encountered.

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References

[1] Goavert L, Lemestra P. Deformation behavior of oriented uhmwpe

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fibers. Colloid and Polymer Science 1992;270:455–64. [2] Walley S, Chapman D, Williamson D, Morley M, Fairhead T, Proud W. High rate mechanical properties of Dyneema in compression. Proceedings of Dymat 2009 Conference 2009;:1133–8.

[3] Karacan I. Fibers and Textiles in Eastern Europe 2005;13:15–21.

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[4] Chabba S, Van Es M, Van Klinken E, Jongedijk M, Vanek D, Gijs-

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man P, et al. Accelerated aging study of ultra high molecular weight

polyethylene yarn and unidirectional composites for ballistic applications. Journal of Materials Science 2007;42:2891–3.

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[5] Greenwood J, Rose P. J Mater Sci 1974;9:1809–14.

[6] Chin J, Petit S, Forster A, Riley M, Rice K. Effect of artificial perspira-

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tion and cleaning chemicals on the mechanical and chemical properties of ballistic materials. Journal of Applied Polymer Science 2009;113:567– 84.

[7] NIJ . Status Report to the Attorney General of Body Armor Safety Initiative Testing and Activities. 2004.

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[8] NIJ . Supplement I: Status Report to the Attorney General of Body Armor Safety Initiative Testing and Activities. 2004. [9] NIJ . Third Status Report to the Attorney General on Body Armor

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Safety Initiative Testing and Activities. 2005. [10] Costa L, Luda M, Trossarelli L. Ultra high molecular weight polyethy-

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[11] Holmes G, Kim J, Ho D, McDonough W. The Role of Folding in the Degradation of Ballistic Fibers. Polymer Composites 2010;31:879–86.

[12] Costa L, Luda M, Trossarelli L. Ultra-high molecular weight polyethy-

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Stability 1989;24:137–68.

[16] Taddei P, Affatato S, Fagnano C, Toni A.

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molecular weight polyethylene and cross-linked polyethylene acetabular

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cups tested against roughened femoral heads in a hip joint simulator. Biomacromolecules 2006;7:1912–20.

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[17] Cunniff P. Dimensionless Parameters for Optimization of Textile-Based Armor Systems. In: 18th International Symposium on Ballistics. San Antonio, TX; 1999, p. 1302–10.

[18] ISO Standard 2062 Textiles- Yarns from packages: Determination of single-end breaking force and elongation at break. 1993.

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[20] del Prever E, M.Crova , Costa L, Dallera A, Camino G, Gallinaro P. Biomaterials 1996;17:873–8.

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[22] Dijkstra D, Pennings A. The role of taut tie molecules on the mechanical properties of gel-spun UHMWPE fibres. Polymer Bulletin 1988;19:73–

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80.

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[24] Dijkstra D, Pennings A. Shrinkage of highly oriented gel-spun polymeric

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fibers. Polymer Bulletin 1988;19:65–72. [25] Menczel J, Prime R. Thermal Analysis of Polymers: Fundamentals and

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Applications. Wiley Interscience; 2009. [26] Cooper S, Tobolsky A. J Appl Polym Sci 1966;10:1837–44.

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[27] Cowie J. Polymers: Chemistry and Physics of Modern Materials. CRC Press; 1991.

[28] Gugumus F. Re-examination of the role of hydroperoxides in polyethylene and polypropylene: chemical and physical aspects of hydroperoxides in polyethylene. Polymer Degradation and Stability 1995;49:29–50.

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Stability 2002;76:329–40.

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[31] Gugumus F. Re-examination of the thermal oxidation reactions of polymers 3. Various reactions in polyethylene and polypropylene. Polymer Degradation and Stability 2002;77:147–55.

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and Stability 2001;71:75–82.

[33] Yang R, Liu Y, Yu J, Wang K. Thermal oxidation products and kinetics of polyethylene composites. Polymer Degradation and Stability

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2006;91:1651–7.

[34] Mallegol J, Carlsson D, Deschenes L. A comparison of phenolic an-

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tioxidant performanc in HDPE at 32-80 ◦ C. Polymer Degradation and Stability 2001;73:259–67.

[35] Barbes L, Radulescu C, Stihi C. ATR-FTIR Spectrometry Characterization of Polymeric Materials. Romainan Reports in Physics 2014;66:765– 77.

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[36] Ruvolo-Filho A, Pelozzi T. Correlation Between Onset Oxidation Tem-

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perature (OOT) and Fourier Transform Infrared Spectroscopy (FTIR) for Monitoring the Restabilization of Recycled Low-density Polyethylene (LDPE). Polmeros 2013;23:614–8.

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[37] BASF Technical Information Sheet for Irganox 1010. 2014.

[38] Wolf C, Krivec T, Blassnig J, Lederer K, Schneider W. Examination

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of the suitability of α-tocopherol as a stabilizer for ultra-high molecular weight polyethylene used for articulating surfaces in joint endoprostheses. Journal of Materials Science: Materials in Medicine 2002;13:185–9. [39] Woods R, Pikaev A. Applied Radiation Chemistry Radiation Processing.

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John Wiley & Sons, Inc.; 1994.

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*Revised manuscript with no changes marked Click here to view linked References ACCEPTED

MANUSCRIPT

Long-Term Stability of UHMWPE fibers

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Amanda L. Forstera,∗, Aaron M. Forsterb , Joannie W. Chinb , Jyun-siang Pengb , Chiao-chi Linb , Sylvain Petitb , Kai-Li Kanga , Nick Paultera , Michael A. Rileya , Kirk D. Ricea , Mohamad Al-Sheikhlyc a

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Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD b Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD c Materials Science and Engineering Department, University of Maryland, College Park, MD

Abstract

The performance of ultra-high molecular weight polyethylene (UHMWPE) fibers for ballistic protection is predicated on the development of a highly aligned molecular structure that allows the polymer to exhibit a superior

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strength in the axial direction of the fiber. However, even an ideal molecular structure will be subjected to degradation during use, which can reduce the high strength of these fibers, and impact their ability to protect the wearer.

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In this work, the long term stability of UHMWPE fibers are investigated and the activation energy for this mechanism was calculated to be approximately

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140 kJ/mol, in agreement with previous reports. The inclusion of accelerated aging temperatures that encompass the alpha-relaxation temperature introduced physical effects in addition to oxidative degradation that complicate a simple explanation of the changes in properties. Assuming that the shift factors that were used in this analysis are correct, it would take approxi∗

Corresponding author Email address: [email protected] (Amanda L. Forster)

Preprint submitted to Polymer Degradation and Stability

January 14, 2015

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mately 36 years for the tensile strength of this UHMWPE yarn to fall by

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30 % at 43 ◦ C. Changes in the oxidation index of this material due to aging

are also studied using Fourier Transform Infrared (FTIR) Spectroscopy, and no simple correlation between the retained strength and the oxidation index was found.

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Keywords:

UHMWPE fibers, body armor, oxidation, activation energy, degradation,

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artificial aging 1. Introduction

Ultra high molecular weight polyethylene [1, 2, 3, 4] (UHMWPE) is one of the two main types of fibers currently used in ballistic-resistant body armor. UHMWPE is a long-chain polyolefin with a molar mass between 3

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million and 5 million. Its tensile strength is reported to be approximately 40 % greater than PPTA fiber [5] due to its high crystallinity and highly oriented zig-zag sp3 conformation. Polyethylene has no functional groups, resulting in superior chemical resistance as compared to other materials [6].

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A well-publicized field failure in 2003 of a body armor based on the fiber poly(p-phenylene-2,6-benzobisoxazole), or PBO, has prompted our work to

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better understand the long term stability of other classes of fibers used in body armor when exposed to elevated temperatures [7, 8, 9]. Degradation of polyolefins initiates from thermal decomposition of hy-

droperoxides and peroxides produced due to defects, unsaturations, and other impurities introduced during processing [10]. The rate of degradation is dependent upon many factors such as the number of free radicals generated, the

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presence of scavenger compounds (e.g., antioxidants), the presence of oxygen,

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and the degree of crystallinity of the polymer. UHMWPE fibers used in soft

body armor are protected from photo-oxidation by a protective fabric carrier, and exposure to ionizing radiation is not expected to occur during general

use. The likely routes to initiate degradation are thermal exposure (from

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storage and wear) or mechanically-initiated degradation from routine use of the armor. Mechanical degradation initiates free radical formation and can

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occur at folds [11] due to movement of the armor on the wearer. Once these free radicals are generated they may follow the established free radical reactions such as hydrogen abstraction and hydroperoxide formation resulting in main chain scission, reduction of molecular weight, and oxidation. Each of these short-term exposures results in cumulative damage that may lead to unforeseen performance reductions in the armor over time. Mechanically-

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induced oxidation is not a commonly explored route for artificial aging of UHMWPE fibers since it can be difficult to quantify and control free radical formation via this method. In this study, elevated temperature exposure will be used to accelerate degradation, since the reaction pathways are assumed

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to be similar once free radicals are initiated. Costa and co-authors have discussed thermal, photo, and mechano-oxidation of UHMWPE in detail,

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and their work includes a reaction scheme of how oxidation occurs in this material [12, 10].

Due to the highly oriented and crystalline microstructure of UHMWPE

fibers, it is not immediately clear that accelerating degradation via temperature will follow Arrhenius behavior. The goals of this study are threefold: first, to determine whether elevated temperatures follow the constant acceler-

3

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ation assumption, next to determine whether or not mechanical performance

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shifts to create a master curve, and finally to determine how the calculated

activation energy agrees with a previous study to determine activation energy via elevated temperature exposure [4].

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1.1. Methodology of Artificial Aging

Classically, researchers have extrapolated high temperature behavior to a lower temperature regime using the Arrhenius equation [13, 14, 15]. Time-

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temperature superposition may be used to shift data from different aging temperatures to create a master curve if the increase in temperature increases the rate of degradation by a constant multiplicative factor. When plotted on a logarithmic time graph, all data sets at the various temperatures should have the same overall curve shape. If the data from each elevated tempera-

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ture can be superposed at a reference temperature by multiplying the aging time by a constant, known as the shift factor, at , then the time-temperature superposition is generally regarded as valid. In order to verify the feasibility of time-temperature superposition, a plot of (log at versus 1/T ) should result

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in a linear dependence upon temperature. [13]. Previous work has focused on understanding the long term stability of

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bulk UHMWPE typically used in orthopedic joint replacement applications [16]. Chabba and coworkers performed artificial aging experiments on UHMWPE fibers, focusing on oxygen uptake as a marker of degradation and calculating the activation energy of this process as being approximately 120 kJ/mol. In this study, the focus is on tensile strength as an indicator of degradation in the fiber, and not oxygen uptake as in Chabba’s study. Tensile strength was selected as a metric for this study because it is generally considered to be 4

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an easily-studied parameter that is related to ballistic performance [17]. In

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order to better represent the aging environment for body armor, aging temperatures of 43 ◦ C and 65 ◦ C were chosen to simulate body temperature and

potential storage conditions (e.g., the trunk of a car), and 90 ◦ C and 115 ◦ C

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were used to accelerate degradation. 2. Experimental

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2.1. Sample Description

Gel spun UHMWPE continuous filament yarns were used in this study. Yarn samples were stored in dark ambient conditions when not being exposed. For the aging experiments, yarns were wound onto perforated spools and placed into ovens at 43 ◦ C , 65 ◦ C , 90 ◦ C , and 115 ◦ C under dry conditions for a predetermined period of time. A series of relative temperature/relative

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humidity dataloggers were used to monitor the consistency of the chamber temperature during the exposure period. 2.2. Mechanical Properties Measurement

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An Instron1 Model 5582 test frame equipped with a 1 kN load cell and pneumatic yarn and cord grips (Instron model 2714-006) was used to measure

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the breaking strength of UHMWPE yarns. The experiment was performed in accordance with ISO 2062 [18]. A 58.42 cm (23 in) yarn was given 23 twists (1 twist per 2.54 cm) on a custom designed yarn-twisting device, and the level of twist was maintained while transferring the yarn to the pneumatic grips. The gage length was 25 cm and the crosshead speed was 250 mm/min.

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The strength at break was recorded and each data point represents the mean

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of at least 7 trials. 2.3. Oxidation Measurement

Oxidation of UHMWPE was measured using Fourier Transform Infrared

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Spectroscopy. A Nicolet Nexus 670 Fourier Transform Infrared Spectrometer (FTIR) equipped with a liquid nitrogen-cooled mercury cadmium telluride (MCT) detector and a SensIR Durascope (Smiths Detection) attenuated total

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reflectance (ATR) accessory was used in the oxidation measurement. Consistent pressure on the yarns was applied using the force monitor on the Durascope. The final scans represent the average of 128 individual scans with a resolution of 4 cm−1 between 650 and 4000 cm−1 , respectively. Spectral analysis, including spectral baseline correction and normalization, was

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carried out using a custom software package developed at NIST to catalog and analyze multiple spectra. The spectra were baseline corrected and normalized using the peak at 1472 cm−1 , which was attributed to the CH2 bending mode. Typical standard uncertainties for spectral measurement are

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4 cm−1 in wavenumber and 5 % in peak intensity. For the purposes of evaluating the degree of oxidation, the overlapping peaks were deconvolved using

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the methods available in the instrument software package between 1712 and 1735 cm−1 . This peak ratio is known as the oxidation index and is commonly 1

Certain commercial equipment, instruments, or materials are identified in this paper

in order to specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the materials or equipment identified are necessarily the best available for this purpose.

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used in other UHMWPE aging literature [19, 20, 21]. The oxidation index

work. 3. Results and Discussion

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3.1. Mechanical Properties of Artificially Aged Fibers

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will be described in further detail in the results and discussion section of this

Figure 1 shows the change in tensile strength as a function of aging at

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different temperatures. As anticipated, the reduction in tensile strength was lowest at the aging temperature of 43 ◦ C. Only a slight decrease in the tensile strength (2 %) was observed after one week of aging, with virtually no change after the first month of aging. A slow, steady deterioration in the tensile strength was observed throughout the long term aging study. The

9 %.

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study was ended at 102 weeks of aging, with a final tensile strength loss of

The reduction in residual tensile strength was more evident at the aging temperature of 65 ◦ C. The decrease in tensile strength after the first week

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was approximately 3.0 %, and in the first month it was 8 %. After 94 weeks of aging, the study was ended, with a final tensile strength loss of over 30 %. The reduction in tensile strength was even more apparent at the higher

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temperatures. The UHMWPE fibers lost 28 % of their initial tensile strength at the aging temperature of 90 ◦ C in the first week, and continuously decreased to 56 % after 17 weeks. Fibers exposed to the aging condition of 115 ◦ C lost 42 % of their original tensile strength after 1 week. The study continued for 17 weeks, after which the fibers had lost 52 % of their original tensile strength. However, given the rapid and catastrophic loss of tensile 7

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3.5

2.5

2.0

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Tensile Strength (GPa)

3.0

1.5

43 °C 65 °C 90 °C 115 °C

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1.0

4000

8000

12000

16000

20000

Aging time (h)

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0

Figure 1: The decrease in tensile strength of artificially aged UHMWPE fibers at various temperatures over time.

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strength in the fibers aged at 90 ◦ C and 115 ◦ C, and the fact that the fibers

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were unconstrained during this experiment, it is possible that these fibers may have undergone a morphological change during elevated temperature exposure. The loss of orientation in the fiber, combined with scissions in the critical tie chains that are important in carrying load in UHMWPE fibers [22]

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could be partially responsible for the rapid loss in tensile strength (especially

at 1 week of aging time) in the fibers aged at 90 ◦ C and 115 ◦ C, rather than

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oxidative chain scission. DMTA temperature scans of unaged fibers were previously reported [23] and show reduction in the storage modulus by 30 % and 50 % at 90 ◦ C and 115 ◦ C, respectively, although the material remains 20 ◦ C away from the transition region. Shrinkage of highly drawn and oriented polyethylene has been widely reported [24, 1, 25]. Shrinkage was not directly measured or observed in this study, but remains a potential consequence

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of small changes in polymer morphology when working with such highly drawn fibers at elevated temperatures. Previous work [23] has shown that the UHMWPE fibers used in this experiment undergo an alpha-relaxation above 80 ◦ C, but degradation may be accelerated at higher temperatures.

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Gillen, et al. have also shown these methods to work for Nylon 6,6 material aging at elevated temperatures [14, 15, 13].

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In order to assess the long term stability of UHMWPE fibers, the master

curve at 43 ◦ C is presented in Figure 2. The lowest aging temperature represents the base use condition for the fibers (body temperature). A master curve was created by using 43 ◦ C as the reference temperature, and then horizontally shifting the higher temperature aging data until they superimposed smoothly to form a single curve [4, 26, 27]. The amount that each

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curve must be shifted is called the shift factor, at , and is used in Equation 1

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to determine the activation energy of the aging mechanism.

  tt Ea 1 1 ln at = ln = − − (1) t0 R T ref T Where at is the shift factor, t0 is the aging time of reference temperature,

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tt is aging time after shifting, Ea is the activation energy, R is the universal gas constant, and Tref is the reference temperature. The value of the acti-

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vation energy as calculated by Equation 1 is found to be 140 kJ/mol, with a standard deviation of approximately 15 % (based on the original error in the tensile strength measurements combined with the error in the shift factor calculation), which is approximately 20 kJ/mol higher than previously published results from Chabba [4], which were based on oxygen uptake. This difference is within the error of this measurement. A table of shift factors is

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given in Table 1 and the shift factors are plotted to show linearity in Figure 3. The strong linearity of the shift factor plot, even at the high temperatures where shrinkage may occur supports the elevated temperature aging methodology. Assuming that the master curve presented in Figure 2 is valid, one

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might predict that it will take approximately 36 years for the tensile strength

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of this UHMWPE yarn to fall by 30 % at 43 ◦ C. 3.1.1. Oxidation Analysis A representative FTIR spectrum of baseline, or unaged, UHMWPE fibers

is shown in Figures 4. The characteristic peaks at 2916 cm−1 and 2848 cm−1 are identified as sp3 C-H stretching, 1471 cm−1 and 1461 cm−1 are assigned to

C-H bending, and 731 cm−1 and 717 cm−1 are in-phase and out-of-phase C-H rocking, respectively. Hydroperoxides were not detected in the ATR-FTIR 10

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60

40

30

20

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Tensile Strength Loss (%)

50

10

0

6

8

10

12

14

16

18

ln time

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4

43 °C 65 °C 90 °C 115 °C

Figure 2: Master curve for aging of UHMWPE fibers at the reference temperature of 43 ◦ C. Error bars are not shown for clarity of presentation, the standard uncertainty in the data is approximately 5 %.

11

1e+5

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y=1E+22e-16.07x R2=0.9884

1e+4

1e+2

1e+0

1e-1

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shift factor, aT

1e+3

1e+1

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2.8

3.0

3.2

1000/T (K-1)

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2.6

Figure 3: Plot verifying linearity of the shift factors. Shift factor has no units. Error bars are not shown for clarity of presentation, the standard uncertainty in the data is approximately 5 %.

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lnat

at

43

0

1.0

65

3.0

20.1

90

7.2

1339.4

115

9.1

8955.3

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Temperature ◦ C

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Aging

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Table 1: Shift factors for aging of UHMWPE.

spectra of the UHMWPE fibers during aging. Gugumus [28, 29, 30, 31] has previously shown that FTIR can be used to measure hydroperoxide in LDPE and HDPE films, and that hydroperoxide concentration decreases linearly with film thickness. Temperature has been shown to have a complicated effect on the formation and destruction of free and associated hydroperoxides,

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which may hinder our ability to resolve any hydroperoxides, even though they are expected to form.

The degree of oxidation increases with aging temperature and time, and is identified by the formation of a new peak at 1737 cm−1 , which is assigned to

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an ester. Another shoulder peak is observed at 1713 cm−1 , which is identified as a ketone. At least two peaks are superimposed to form a complex FTIR

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spectrum. Both peaks were treated as Gaussian functions and fitted as shown in Figure 6. The absorption band around 1737 cm−1 is not attributed to a single oxidation product. This band is indicative of the presence of oxidation products from the decomposition of peroxides and hydroperoxides. The baseline oxidation level for this material is evident by the presence of a peak at 1737 cm−1 and 1713 cm−1 in the unaged material, as shown in Figure 5.

13

3.0

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baseline

2848 cm-1

2.5

2916 cm

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-1

-1

-1

1471 cm

717 cm-1

1461 cm

Absorbance

2.0

731 cm-1

1.5

0.5

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1.0

3000

2500

2000

1500

1000

-1

Wavenumber (cm )

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EP

0.0 3500

Figure 4: FTIR spectrum of baseline, or unaged, UHMWPE fiber. The units of absorbance are arbitrary. The characteristic peaks at 2916 cm−1 and 2848 cm−1 are identified as sp3

C-H stretching, 1471 cm−1 and 1461 cm−1 are assigned to C-H bending, 731 cm−1 and 717 cm−1 are in-phase and out-of-phase C-H rocking, respectively. A small peak around 1125 cm−1 is unassigned.

14

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Oxidation would appear as a sharp peak at 1737 cm-1 Also a shoulder peak at 1713 cm-1

0.2

 

0.0

-0.2

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-0.4

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Absorbance (a.u.)

0.4

1900

1800

1700

1600

1500

-1

Wavenumber (cm )

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2000

Unaged UHMWPE fiber

Figure 5: FTIR spectrum showing baseline oxidation levels for unaged UHMWPE fiber. Standard uncertainty in these measurements is approximately 4 cm−1 .

15

0 .0 2 4

p e a k a t 1 7 1 2 .5 c m p e a k a t 1 7 3 6 .5 c m d a ta (1 w k ) f it

0 .0 2 0

-1 -1

re c o n s tru c te d fro m re c o n s tru c te d fro m

p e a k a t 1 7 1 2 .5 c m p e a k a t 1 7 3 6 .5 c m d a ta (4 w k ) fit

0 .0 2 4

f it to d a ta f it to d a ta

0 .0 2 0

0 .0 1 6

A b s o rb a n c e

0 .0 1 6

0 .0 1 2

-1 -1

re c o n s tru c te d fro m re c o n s tru c te d fro m

fit to d a ta fit to d a ta

0 .0 1 2

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A b s o rb a n c e

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0 .0 0 8

0 .0 0 4

0 .0 0 0

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0 .0 0 4

0 .0 0 8

0 .0 0 0

1 7 8 0

1 7 6 0

1 7 4 0

1 7 2 0

W a v e n u m b e r (c m

-1

)

1 7 0 0

p e a k a t 1 7 1 2 .5 c m p e a k a t 1 7 3 6 .5 c m d a ta (1 0 2 w k ) f it

0 .0 2 4

0 .0 2 0

-1

re c o n s tru c te d fro m re c o n s tru c te d fro m

1 7 6 0

1 7 4 0

W a v e n u m b e r (c m

1 7 2 0 -1

)

f it to d a ta f it to d a ta

A b s o rb a n c e

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0 .0 1 6

-1

1 7 8 0

0 .0 1 2

0 .0 0 8

0 .0 0 4

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0 .0 0 0

1 7 8 0

1 7 6 0

1 7 4 0

W a v e n u m b e r (c m

1 7 2 0 -1

1 7 0 0

)

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Figure 6: Curve-resolved FTIR spectra of UHMWPE fiber aged at 43 ◦ C for 1 week

(top left), 4 weeks (top right), and 102 weeks (bottom), scale enlarged to show 1680 to 1780 cm−1 region, where oxidation appears, more clearly. The units of absorbance are arbitrary. The shoulder of this peak is resolved to elucidate the formation of a new peak around 1713 cm−1 . Standard uncertainty in these measurements is approximately 4 cm−1 .

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It is possible that the peak at 1737 cm−1 represents the hindered phenol of

are implications for interpreting the oxidation products. 3.1.2. Role of Antioxidants in Polyethylene

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an antioxidant. If the antioxidant is present in the baseline material, there

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The antioxidants function is based on the hydrogen transfer mechanism,

by which the hydrogen atom of the antioxidants hydroxyl group transfers to the polyethylene carbon-centered, peroxyl, and alkoxyl radicals as shown in

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Figure 7 [32, 33, 34, 35, 36, 37]. The hindered phenol does not participate in hydrogen transfer reactions, although it may hydrogen bond to some of the oxidation products. The free radical produced from the above reactions is very stable with the alkoxide structure, and is similar to the vitamin E alkoxide radical [38].

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The peak analysis presented for 43 ◦ C in Figure 6 indicates an increasing of the shoulder at 1713 cm−1 . The peak at 1737 cm−1 appears to shrink with exposure time. This contradicts the production of multiple hydroxyl containing oxidation products that would increase this peak with time. One

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suggestion is the decrease in the peak at 1737 cm−1 is the consumption of an antioxidant ester. The oxidation analysis of 1713 cm−1 (discussed later) does

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not indicate an induction time for the production of the main ketone band that would be expected for the presence of an antioxidant. However, despite the fact that the oxidation products will continue to increase through various oxidation reactions in the amorphous structure, they will also decompose continuously to CO2, CO, and even OH radicals, as shown by the sequence of reactions in Figure 8 [39]. It should also be mentioned that these reactions take place in the limited amorphous part of the polymer and at the crystalline 17

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interfaces. These factors affect the interpretation of the FTIR spectrum in

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this region. In this material, the available oxygen is limited because the

diffusion of oxygen is much lower than the consumption due to the higher

crystallinity of this material. Currently, the exact nature of the changes at 1737 cm−1 are unknown and remain a convolution of potential degradation

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or protection mechanisms. 3.1.3. Oxidation Index

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The intensity of the ketone group generally increases with increasing aging time at 43 ◦ C. Thus, the degree of oxidation of UHMWPE fibers could potentially be quantified by introducing the oxidation index (OI), as used in other artificial aging applications for bulk UHMWPE [3, 21, 12]. For the purposes of this study, the peak at 1472 cm−1 was used as the reference peak, and the OI is the peak area at 1713 cm−1 divided by the peak area

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at 1472 cm−1 . Equation 2 defines oxidation index. Figure 9 and Figure 10 show the increase in OI over time for all aging temperatures. A1713cm−1 (2) A1472cm−1 The OI increased moderately at the lower aging temperatures of 43 ◦ C and

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OI =

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65 ◦ C. For these aging conditions, the OI increased from 0.0002 to 0.0243 and 0.0249, for 43 ◦ C and 65 ◦ C, respectively. For the higher aging temperatures, the OI increased more rapidly. After 17 weeks of aging at 90 ◦ C, the OI increased from 0.0002 to 0.0385, and for 17 weeks of aging at 115 ◦ C, the OI increased from 0.0002 to 0.0471. Figure 11 shows the relationship between tensile strength and OI, the higher the aging temperature, the faster the drop in tensile strength, regardless of the OI value. This result might be indicative 18

(a) hydrogen transfer mechanism

OH

R

O

O

+

C C

O

OH

O

O

O

O

C

hydrogen transfer mechanism

+

OH

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RO

O C

C

RO2H

+

ROH

O 4

4

(c)

+

C

C C

O

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hydrogen transfer mechanism

+

RH

4

4

RO2

+

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C

(b)

O

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O

O C C

O 4

AC C

EP

4

O

Figure 7: The hydrogen transfer mechanism by which a common antioxidant, which is likely present in the UHMWPE fiber, works to prevent oxidation [32, 33, 34, 35, 36, 37].

19

+ O2

CH2CHCH2

+

CH2CH(O2)CH2

(a)

CH2CH(O2)CH2

(b)

CH2CH2CH2

+

CH2CH(O2H)CH2

CH2CHCH2

CH2CH(O2H)CH2

CH2CCH2

+ H2O

(c)

CH2CH(O)CH2

+

CH2CH(OH)CH2

OH + 2

CH2 (d)

CH2CH(O)CH2

+ OH (e) (f)

CH2CH2CH2

+

CH2CHCH2

H2O +

CH2CH2CH2 CH2CH(O2)CH2

CH2CH(OH)CH2

CH3 +

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CH2CH(O2H)CH2

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O

CO2 +

CH2CH(O2)CH2

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CH2CHCH2

(g)

(h)

+

CH2CCH2

+ O2

AC C

EP

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O

Figure 8: This series of reactions shows the formation of many of the common scission products due to the oxidation of polyethylene [39].

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0 .0 5

4 3 6 5 9 0 1 1 5

-1

c

o

o

c

c

0 .0 2

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1 7 1 3 c m

O .I . ( A

c

o

SC

0 .0 3

/A

1 4 7 2 c m

-1

)

0 .0 4

o

0 .0 1 0 .0 0 1 0 0

1 0 0 0

1 0 0 0 0

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A g in g tim e (h )

Figure 9: Changes in oxidation index (OI) for different aging temperatures as a function of time for UHMWPE fibers (log axis). Error bars represent standard combined uncertainty in the FTIR data.

previously.

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of changes in morphology at these higher aging temperatures, as described

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4. Summary and Future Work This work has demonstrated that exposure to elevated temperatures is a

valid method for aging UHMWPE fibers. When exposed to elevated temperatures for long periods of time, UHMWPE fibers lose tensile strength, and the Arrhenius model is applicable to predict this phenomenon. However, it is a challenge to identify one aging mechanism responsible for the loss of tensile 21

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0 .0 5

0 .0 3

0 .0 2

0 .0 1

0 .0 0

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O .I . ( A

1 7 1 3 c m

-1

/A

1 4 7 2 c m

-1

)

0 .0 4

4 0 0 0

8 0 0 0

1 2 0 0 0

4 3

o

c

6 5

o

c

9 0

o

c

1 1 5

o

c

1 6 0 0 0

EP

A g in g tim e (h )

Figure 10: Changes in oxidation index (OI) for different aging temperatures as a func-

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tion of time for UHMWPE fibers (linear axis). Error bars represent standard combined uncertainty in the FTIR data.

22

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T e n s ile s tr e n g th (G P a )

3 .0

4 3

o

C

6 5

o

C

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3 .5

o

C o

C

9 0 1 1 5

2 .5

2 .0

1 .0 0 .0 0

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1 .5

0 .0 1

0 .0 2

1 7 1 3 c m

-1

/A

0 .0 4 1 4 7 2 c m

-1

0 .0 5

)

EP

O .I . ( A

0 .0 3

Figure 11: The relationship between tensile strength and oxidation index at different aging temperatures for UHMWPE fibers. In all cases, as the OI increases, tensile strength

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decreases. Deviations from the overall in the 65 ◦ C data are not explained. Further

work will attempt to measure changes in morphology that may have resulted from this oxidation.

23

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strength because it is a convolution of oxidation and changes in fiber mor-

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phology. This is demonstrated by the behavior of the fibers exposed to 90 ◦ C and 115 ◦ C, which may undergo shrinkage and loss of orientation. Molecular spectroscopy verified that all samples were oxidized by the elevated temper-

ature exposure, however there is not a clear relationship between OI and

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the change in tensile strength. Therefore, it would be difficult to utilize this model to predict failure outside the accelerated environment of the labora-

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tory, unless these two contributing factors can be separately understood and their interactions accounted for in the model. A future paper will detail the changes in thermal properties and morphology of these fibers resulting from this aging experiment, and carefully consider the role of shrinkage. This work verifies that there is potential for oxidation and tensile strength loss in this material when it is exposed to elevated temperatures for long periods of time.

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These factors should all be considered when using these fibers in applications where this type of environment may be encountered.

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References

[1] Goavert L, Lemestra P. Deformation behavior of oriented uhmwpe

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fibers. Colloid and Polymer Science 1992;270:455–64. [2] Walley S, Chapman D, Williamson D, Morley M, Fairhead T, Proud W. High rate mechanical properties of Dyneema in compression. Proceedings of Dymat 2009 Conference 2009;:1133–8.

[3] Karacan I. Fibers and Textiles in Eastern Europe 2005;13:15–21.

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[4] Chabba S, Van Es M, Van Klinken E, Jongedijk M, Vanek D, Gijs-

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man P, et al. Accelerated aging study of ultra high molecular weight

polyethylene yarn and unidirectional composites for ballistic applications. Journal of Materials Science 2007;42:2891–3.

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[5] Greenwood J, Rose P. J Mater Sci 1974;9:1809–14.

[6] Chin J, Petit S, Forster A, Riley M, Rice K. Effect of artificial perspira-

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tion and cleaning chemicals on the mechanical and chemical properties of ballistic materials. Journal of Applied Polymer Science 2009;113:567– 84.

[7] NIJ . Status Report to the Attorney General of Body Armor Safety Initiative Testing and Activities. 2004.

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[8] NIJ . Supplement I: Status Report to the Attorney General of Body Armor Safety Initiative Testing and Activities. 2004. [9] NIJ . Third Status Report to the Attorney General on Body Armor

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Safety Initiative Testing and Activities. 2005. [10] Costa L, Luda M, Trossarelli L. Ultra high molecular weight polyethy-

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lene: II. Thermal and photo-oxidation. Polymer Degradation and Stability 1997;58:41–54.

[11] Holmes G, Kim J, Ho D, McDonough W. The Role of Folding in the Degradation of Ballistic Fibers. Polymer Composites 2010;31:879–86.

[12] Costa L, Luda M, Trossarelli L. Ultra-high molecular weight polyethy-

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bility 1997;55:329–38.

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lene: I. Mechano-oxidative degradation. Polymer Degradation and Sta-

[13] Bernstein R, Derzon DK, Gillen KT. Nylon 6.6 accelerated aging stud-

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Polymer Degradation and Stability 2005;88:480–8.

[14] Gillen K, Wise J, Clough R. Novel techniques applied to polymer life-

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Stability 1989;24:137–68.

[16] Taddei P, Affatato S, Fagnano C, Toni A.

Oxidation in ultrahigh

molecular weight polyethylene and cross-linked polyethylene acetabular

EP

cups tested against roughened femoral heads in a hip joint simulator. Biomacromolecules 2006;7:1912–20.

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[17] Cunniff P. Dimensionless Parameters for Optimization of Textile-Based Armor Systems. In: 18th International Symposium on Ballistics. San Antonio, TX; 1999, p. 1302–10.

[18] ISO Standard 2062 Textiles- Yarns from packages: Determination of single-end breaking force and elongation at break. 1993.

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[19] Li S, Burstein A. J Bone Surg Am 1994;76:1080–90.

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[20] del Prever E, M.Crova , Costa L, Dallera A, Camino G, Gallinaro P. Biomaterials 1996;17:873–8.

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[22] Dijkstra D, Pennings A. The role of taut tie molecules on the mechanical properties of gel-spun UHMWPE fibres. Polymer Bulletin 1988;19:73–

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[24] Dijkstra D, Pennings A. Shrinkage of highly oriented gel-spun polymeric

TE D

fibers. Polymer Bulletin 1988;19:65–72. [25] Menczel J, Prime R. Thermal Analysis of Polymers: Fundamentals and

EP

Applications. Wiley Interscience; 2009. [26] Cooper S, Tobolsky A. J Appl Polym Sci 1966;10:1837–44.

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[27] Cowie J. Polymers: Chemistry and Physics of Modern Materials. CRC Press; 1991.

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[29] Gugumus F. Re-examination of the thermal oxidation reactions of poly-

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mers 1. New views of an old reaction. Polymer Degradation and Stability 2001;74:327–39.

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and Stability 2001;71:75–82.

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2006;91:1651–7.

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tioxidant performanc in HDPE at 32-80 ◦ C. Polymer Degradation and Stability 2001;73:259–67.

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[36] Ruvolo-Filho A, Pelozzi T. Correlation Between Onset Oxidation Tem-

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perature (OOT) and Fourier Transform Infrared Spectroscopy (FTIR) for Monitoring the Restabilization of Recycled Low-density Polyethylene (LDPE). Polmeros 2013;23:614–8.

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[37] BASF Technical Information Sheet for Irganox 1010. 2014.

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of the suitability of α-tocopherol as a stabilizer for ultra-high molecular weight polyethylene used for articulating surfaces in joint endoprostheses. Journal of Materials Science: Materials in Medicine 2002;13:185–9. [39] Woods R, Pikaev A. Applied Radiation Chemistry Radiation Processing.

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EP

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John Wiley & Sons, Inc.; 1994.

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Figure

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ACCEPTED MANUSCRIPT

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SC

3.5

TE D

2.5

EP

2.0

1.5

AC C

Tensile Strength (GPa)

3.0

43 °C 65 °C 90 °C 115 °C

1.0

0

4000

8000

12000

Aging time (h)

16000

20000

Figure

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ACCEPTED MANUSCRIPT

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SC

60

TE D

40

30

EP

20

10

AC C

Tensile Strength Loss (%)

50

43 °C 65 °C 90 °C 115 °C

0

4

6

8

10

12

ln time

14

16

18

Figure

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ACCEPTED MANUSCRIPT

SC

1e+5

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y=1E+22e-16.07x R2=0.9884

1e+4

TE D

1e+2

1e+0

EP

1e+1

AC C

shift factor, aT

1e+3

1e-1

2.6

2.8 1000/T (K-1)

3.0

3.2

Figure

3.0

baseline

2848 cm-1 2916 cm-1

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2.5

SC

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ACCEPTED MANUSCRIPT

1461 cm-1

0.5

0.0 3500

731 cm-1

TE D

1.0

717 cm-1

EP

1.5

AC C

Absorbance

2.0

-1

1471 cm

3000

2500

2000

1500

Wavenumber (cm-1)

1000

Figure

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Oxidation would appear as a sharp peak at 1737 cm-1 Also a shoulder peak at 1713 cm-1

0.2

-0.2

EP

0.0

TE D

 

AC C

Absorbance (a.u.)

0.4

SC

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ACCEPTED MANUSCRIPT

-0.4

2000

1900

Unaged UHMWPE fiber

1800

1700

Wavenumber (cm-1)

1600

1500

Figure

ACCEPTED MANUSCRIPT

p e a k a t 1 7 1 2 .5 c m p e a k a t 1 7 3 6 .5 c m d a ta (1 w k ) f it

-1

re c o n s tru c te d fro m re c o n s tru c te d fro m

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0 .0 1 6

TE D

0 .0 1 2

EP

0 .0 0 8

0 .0 0 4

AC C

A b s o rb a n c e

f it to d a ta f it to d a ta

SC

0 .0 2 0

-1

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0 .0 2 4

0 .0 0 0 1 7 8 0

1 7 6 0

1 7 4 0

W a v e n u m b e r (c m

1 7 2 0 -1

)

1 7 0 0

Figure

ACCEPTED MANUSCRIPT

-1

re c o n s tru c te d fro m re c o n s tru c te d fro m

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0 .0 1 6

TE D

0 .0 1 2

EP

0 .0 0 8

0 .0 0 4

AC C

A b s o rb a n c e

fit to d a ta fit to d a ta

SC

0 .0 2 0

-1

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p e a k a t 1 7 1 2 .5 c m p e a k a t 1 7 3 6 .5 c m d a ta (4 w k ) fit

0 .0 2 4

0 .0 0 0 1 7 8 0

1 7 6 0

1 7 4 0

W a v e n u m b e r (c m

1 7 2 0 -1

)

1 7 0 0

Figure

ACCEPTED MANUSCRIPT

-1

re c o n s tru c te d fro m re c o n s tru c te d fro m

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0 .0 1 6

TE D

0 .0 1 2

EP

0 .0 0 8

0 .0 0 4

AC C

A b s o rb a n c e

f it to d a ta f it to d a ta

SC

0 .0 2 0

-1

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p e a k a t 1 7 1 2 .5 c m p e a k a t 1 7 3 6 .5 c m d a ta (1 0 2 w k ) f it

0 .0 2 4

0 .0 0 0 1 7 8 0

1 7 6 0

1 7 4 0

W a v e n u m b e r (c m

1 7 2 0 -1

)

1 7 0 0

Figure

ACCEPTED MANUSCRIPT (a) hydrogen transfer mechanism

OH

O

O

+

C

O

C O

O

OH

O

EP

+

TE D

hydrogen transfer mechanism

C

+

ROH

O 4

O

O C

C

AC C

RO

RO2H

C C

4

(c)

+

O

SC O

O

C

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RO2

hydrogen transfer mechanism

OH

RH

4

4

+

+

C

C

(b)

O

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R

C

O

4

O 4

Figure

ACCEPTED MANUSCRIPT + O2

CH2CH(O2)CH2

+

(a)

CH2CH(O2)CH2

(b)

CH2CH2CH2

+

CH2CH(O2H)CH2

CH2CHCH2

CH2CH(O2H)CH2

CH2CCH2

+ H2O

(c)

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CH2CHCH2

O

+

(f)

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H2O +

CH2CH2CH2 CH2CH(O2)CH2

CH2CH(OH)CH2

+ OH (e)

CH2CHCH2

CH2CHCH2

(g)

(h)

CH2CCH2

+

+ O2

EP

TE D

O

AC C

2

CH2CH(O)CH2 CH2CH2CH2

+

CH2CH(OH)CH2

OH +

CH2 (d)

SC

CH2CH(O2H)CH2 CH2CH(O)CH2

CH3 +

CO2 +

CH2CH(O2)CH2

Figure

ACCEPTED MANUSCRIPT

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0 .0 5

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0 .0 3

EP

TE D

0 .0 2 0 .0 1

AC C

O .I . ( A

1 7 1 3 c m

-1

/A

1 4 7 2 c m

-1

)

SC

0 .0 4

4 3 6 5 9 0 1 1 5

0 .0 0 1 0 0

1 0 0 0 A g in g tim e (h )

1 0 0 0 0

o

o

c o

c o

c c

Figure

ACCEPTED MANUSCRIPT

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0 .0 5

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0 .0 3

EP

TE D

0 .0 2

0 .0 1

0 .0 0

AC C

O .I . ( A

1 7 1 3 c m

-1

/A

1 4 7 2 c m

-1

)

SC

0 .0 4

4 0 0 0

8 0 0 0

1 2 0 0 0

A g in g tim e (h )

1 6 0 0 0

4 3

o

c

6 5

o

c

9 0

o

c

1 1 5

o

c

Figure

ACCEPTED MANUSCRIPT

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3 .5

SC M AN U

C

6 5

o

C

9 0

o

C

1 1 5

o

C

TE D

2 .5

o

EP

2 .0

1 .5

AC C

T e n s ile s tr e n g th (G P a )

3 .0

4 3

1 .0 0 .0 0

0 .0 1

0 .0 2 O .I . ( A

0 .0 3 1 7 1 3 c m

-1

/A

0 .0 4 1 4 7 2 c m

-1

)

0 .0 5

AC C

EP

TE D

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SC

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LaTeX Source Files Click here to download LaTeX Source Files: UHMWPEaging.bib ACCEPTED MANUSCRIPT

AC C

EP

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LaTeX Source Files Click here to download LaTeX Source Files: UHMWPEaging ACCEPTEDpart1.tex MANUSCRIPT