VIETNAM JOURNAL OF CHEMISTRY DOI: III 15625/0866-7144 2016-00251
54(2) 152-158 A P R I L S
STUDY ON CHANGE OF SOME CHARACTERS AND MORPHOLOGY 0 POLYETHYLENE COMPOUND EXPOSED NATURALLY IN
DONG HOI-QUANG BINH
Le Due Minh', Nguyen T h u y C h i n h \ Nguyen Thi T h u T r a n g ^ Nguyen Vn Giang^, T r a n H u u T m a M a i Due H u y n h ^ T r a n Thi M a i ^ Le D u e G i a n g , T h a i H o a n g ' '
'Faculty of Pedagogy Natural Sciences, Ha Tinh University, 447 26/3 street. Ha Tinh, J^etnam
^Institute for Tropical Technology, Vietnam Academy of Science and Technology
^Faculty of Chemistry, Vinh University, 182 Le Duan, Vinh, Nghe An, Vietnam Received 15 December 2015; Accepted for publication 25 April 2016
The degradation and stability of high density polyethylene (HOPE) compound depend strongly on geogr weather and climatic factore (solar radiation, temperature, humidity, etc.). From June 2014 to June 2015, the samples;^
HOPE compound were tested naturally on outdoor shelves in Dong Hoi sea atmosphere region (QuaugBmh provin*
for evaluation of change of their morphology and properties. The tensile property results shov( that tensile strength ani elongation al break of HOPE compound reduced significantly while then- Young modulus rose as increasing naunl testmg time After 12 months of exposure, the tensile strength and elongation at break of HOPE compound decre«i4*
47.63% and 1.39% in comparison with unexposed HDPE compound, respectively Scanning Electron M l c r o s 3 i (SEM) analysis on the surface of HDPE compound shows the number of cracks and size of cracks of the sampH mcrease and Ihe cracks become bigger and deeper when mcreasing natural exposure time. The Uiermo gravimeli analysis (TGA) and differential scanning calotimetiy (DSC) mdicate that the thermal stability of HDPE comoc steadily declines veraus natural exposure tune.
Keywords, HDPE, photo-degradation, natural exposure, tensile property, thermal property, morphology.
1. INTRODUCTION
HDPE is one of the most popular thermal plastics. Owing to its reasonable cost and low energy demand for processing, HDPE is ideal for many applications as packaging, bottles, films, pipes manufachire etc, HDPE has high ciystalline structure and tensile strength. Besides, HDPE offers a good barrier for humidity and resistance to abrasion and corrosion. Furthermore, HDPE is inertness die most of chemicals, and has high weather stability.
HDPE is a potential material for application in outdoor exposure, so its life time is determined by various environmental factors as solar radiation temperature, thermal cycling, humidity, pollutants' ulUa-violet (UV) radiation. In the recent years, many studies have been conducted to investigate various aspects of the weather stability of HDPE, Torikai et al.[l] have studied the photodegradalion of four commeroial polyethylene types by using a laboratory tester (samples were photo-irradiated with a Toshiba
H-400P medium pressure mercury lamp ( ; \ ^ 5 0 in vacuum and in air at 30°C, and they frequently turned over to give the same irradi dose on both sides). They concluded that densi crystallinity and crystal size of polyethylene j l important roles in its photostability. The study on change in structure and mechanical properties HDPE, which was natural tested during a Canadii^
winter were reported by Catrasco et al, [2], 1 ^ authors considered that the loss in crystallinity ail*
thermal fatigue caused the drastic decrease in impact energy of HDPE, Hoekstra et al, [3] have publish^, the fracture behavior, mechanical and morphologiciti properties of photo-oxidized HDPE in a Xenotestv^
For Charpy specimen which is a an HDPE g r a n u l ^ . supplied by DSM and about 350 ppm ' p h e n o 8 | antioxidanf was added to the material for t h e t w w stabilization, the authors observed a gradual increasJ in density of HDPE with exposure time and t b ^ concentration of carbonyl groups in the amoiphoi phase were decayed with the depth of the HDf sample.
VJC, 54(2) 2016
Up to now, the study on weather stability of HDPE compound m typical sea atmosphere condition for North Middle Region of Vietnam has not been presented. Therefore, this work focuses on the degradation of HDPE compound samples according to nawral exposure trnie in Dong Hoi sea atmosphere region (Quang Binh, Vietnam). The change of morphology, mechanical, thermal properties was determined and discussed,
2. EXPERIMENTAL 2.1. Materials
The materials used in this work were a HDPE (Daelim, Korea), density = 0.937 gem''; melting flow index, MFI,^^,^_,^^ = ,.20 g.min"', CaCOj powder (supplied by Minh Due Chemical Stockshare Co.), density = 2,7 g,cm'^
2.2, Preparatton of HDPE compound samples
The HDPE/CaCOa (wt./wt.) compounds were prepared by melt mixing in a Haake internal mixer at 160 C for 5 minutes. Immediately after melt mixing, the HDPE compounds were pressed by hydraulic heat press machine at a temperature of 160 °C the
pressure of 5 MPa to form sheets with thickness ' • • • f T K s p ' c ' r a from Ito 1.2 mm.
Thai Hoang, et al.
before determining properties and morphology. The signed samples were MO, M3, M6, M9, M12 corresponding to the samples were exposed for 0, 3, 6, 9, 12 months, respectively,
2.4. Characterizations
The HDPE compound samples before and after natural exposure were characterized by Fourier Transform Inftared (FTIR) spectra (using a Nicolet/Nexus 670 spectrometer, USA) at Institute for Tropical Technology (ITT), Vietaam Academy of Science and Technology (VAST). The tensile properties of samples were conducted on Zwick Tensile 2.5 Machine (Germany) according to ASTM D63S standard at ITT, VAST. Field Emission Scannmg Electron Microscopy (FESEM) analysis was carried out by using a S-4500 SEM (Hitachi, Japan) at Institute of Material Science, VAST.
Thennal property studies were performed on a DTG- 60H and DSC-60 thermo-gravimetric analyzer (Shimadzu. Co, Japan) under argon gas from room temperature to 600°C with a heating rate of 10 C/min at Hanoi national University of Education, 3. RESULTS AND DISCUSSION
2,3, Natural exposure HDPE compound samples
The samples of HDPE compound were exposed starting from June 20' on outdoor testing shelves at the Natural Weathering Station of the Instimte for Tropical Technology in Dong Hoi sea atmosphere region (Quang Binh, Viemam). Inclining angle of shelf in comparison with the ground is 45" as typically shown m Fig 1 and total exposure time of the samples was 12 months.
The effect of natural exposure conditions on the MO, M3, M6, M9, M12 samples was evaluated by FTIR spectra as presented in Fig. 2.
^J'^ QMrt|Binii
Bj^^fHHp^HHHjjjj^B
Figure 1: View of outdoor exposure testmg shelves After eveiy duxe months, the samples were withdrawn and stored under standaid conditions
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Figure 2: FTIR spectra of HDPE compounds versus natural exposure time
In the FTIR spectrum ofMO sample, some peaks
charactenze for stretching and bending vibrations of
CH groups m HDPE were found at 2921 2854 1465
and 1380 cm-. Beside, a peak conesponding to out-
of-plane landing vibration of CH gmup appears at
725 cm . For die exposed samples, the FTIR spectra
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also indicate the peaks characterized for stretching and bending vibrations of CH groups in HDPE.
Interestingly, the absorption peak around 1735 cm"
characterizes for the stretching vibrations of carbonyl groups was seen clearly in FTIR spectra of the exposed samples. This is caused by the formation of the carbonyl groups such as ketone, lactone carbonyl and aliphatic ester occurring in photodegradation process of HDPE accordmg to the Norish 1 and Norish 2 reaction and mechanism has been well described in the scheme 1 of literatures [4].
HDPE
—CHj—CH^— C H — C H , — C H i —
— C H ; — C H ; — C — C H i - C H , —
— C H , - C H , — ^ — C H , — C H , —
Study on change of some chai^ictei,
increases as a A c t i o n of natural exposure time.;
significant increase of the CI was observed foT%
samples exposed from 0 to 6 months while from CS 12 months, the CI varies more slowly. The b varieties can be explained by the change of v in Dong Hoi during natural exposure time. ' climate in Dong Hoi exhibits clearly two s the hot season from March to November and i cold season from December to February (se 1). In the cold season, the temperature/month in Dong Hoi and the average sunny hours/month are low, reach to 19.6 "C and 76.3 hours, respectively. Therefore, the soli radiation is low, resulting in that the photodegradation of HDPE occurs slowly, leadingta that the CI increases slightly. In the hot season, tin average temperature/month and average sunny hours/month are higher, thus the samples have b«n affected by solar radiation more strongly. TTih caused the faster photodegradation of HDPE, so tl CI value increases significantly [5].
- C H , - C H , - C - C H , - C H i —
° " - C H , - C H , - p - C H , - C H , -
• C H , - C H ; - C H O + C H j - C H , —
\ /
caitoxylic acids lactones
/ \
- C H ; - C H i - C - C H , + H , C = C H - C H , - - C H , - C H ; - C -f C H , - C H , -
-CH=CH, + -^-CHj _ » CHjCOCHj
- C H , - C H = C H ; — ^ - C H - C H = C H ; 4
ao8 ao7 aoG 1 ao5
•5. ao4
•s ao3 ace aoi •
0
^^_. •:
^^--*^''^ X..
jf"""^ \X-^
X i
<f
- C - C H = C K ,
°|-
saturated ketones - C H = C H - C H a (vinylenes)
Scheme 1: Simplified photo-degradation mechanism of polyethylene.
Exposure time, manths Figure 3: CI value of HDPE compound versus •'
natural exposure time - 3.2. Tensile p r o p e r t y
The change in tensile properties of HDPE compounds is described through remained percent^, tensile strength (Aa), elongation at break (Ae) of ^ samples versus natural exposure time Is performed in figure 4. The tensile strength and elongation tf break of HDPE compounds have a decrease trend | increasing the natural exposure time. They d « Ii465 significantly during the first 3 months of c Whpm I ^«j T L exposure and then reduce more slowly. The abc
be due to fhe effect of weather factors such a c; - ,. , radiation, himiidity, temperature on the amorc Figure 3 displays that die CI of the samples region of HDPE as above u
To quantiiy relatively die carbonyl group content existed in the exposed samples, carbonyl index (Cl) was calculated using the following equation:
CI
VJC, 54(2) 2016 Thaiffoang eta
Table J2014 June July August September
October November December 2015 Januaiy Febmaiy March
April May June
- Climate database TlbCC)
30.9 30.1 29.6 29.6 25.6 24,2 19.2
18.8 20.7 24.2 25,6 31.0 30.9
Tx
CC)39 37.5 38.5 38.5 32.0 30.0 25.8
25.0 27.2 36.7 41.0 40.5 39.5
m Dong Hoi- Quang Binh from June 2014 to June 2015 R
(mm) 78 85 132 132 605 344 160
84 40 32 206 9 73
Rx (mm)
41 31 60 60 189 160 48
42 9 24 133 6 36
Utb (%)
67 71 72 72 87 88 82
84 91 90 85 70
E (mm)
163 137 134 134 57 48 70
55 28 39 72 176
69 1 153S (h) 191 220 176 176 129 106 35
130 64 100 173 298 290
St (d) 22 12 11 11 0 0 0
0 0 0 8 18 22 1
CC (d) 0 0 0 0 0 0 0
0 0 0 0 0 0 S^m » 2 % ° 1 " * f <=""^«-»; R, R.: Ramy total and highest ™ny quantity m day; U.: Average humidity;.
Steam quantity: S: Sunny hours; St: Stonn;CC; Day numbers have drizzle, - -e !,•
Figure 4: The remained percent of tensile strengdi, elongation at break of HDPE compound versus
natural exposure time
From data in table 1, it is clearly seen that, in the first 3 months of natural exposure, the average temperature is iitim 29,6 to 30.9 °C, total sunlight hours are quite high. 587 hours, respectively. The high mtensity of solar radiation could make a significant contribution to the photodegradation in amorphous part of HDPE. This can lead to the
formation of carbonyl, carboxyl, hydroperoxide groups and free reactive radicals in polymer chains, causing the occunence of chain scission, thus deteriorating mechanical properties of HDPE compounds. After Brst 3 months of natural exposure, the photodegradation mainly happens in crystallme part of HDPE and more difficuldy compared to the amorphous part of HDPE, Therefore, die tensile strengdi and elongation at break of HDPE compounds decrease slower.
The Young modulus of HDPE compounds has
an increase tendency widi increasing natural
exposure tune. After 3, 6, 9, 12 months of nahiral
exposure, die Young modulus of HDPE compounds
increase 6, 17, 36, and 56% compared to the
unexposed HDPE compound, respectively. These
resnhs demonstrate diat when increasing natural
exposure time, die samples lose dieir ductility and
become stiffer but brittle and weaker in tenns of
tensile strengdi. In addition, crosslinking of HDPE
molecules is happened due to die high rate of radical
tennination in die bulk of die polymer where oxygen
cannot gain access. Thus, diere is an observed
mcrease in die Young modulus.
VJC, 54(2) 2016 33. Morphology
As mentioned in [6], the photodegradation of a polymer material usually begins from the surface and then develops along the depth gradually. Hence the surface photo-degradation of the material has a direct influence on the bulk oxidation of the material. Figure 5 demonstrates the surface images of the samples before and after natural exposure.
Before natural exposure testing, the sample surface is relatively smooth, only has some small cracks (MO sample). After 3 to 12 natural exposure months, tiiere are more cracks found on the surface of the exposed samples. When increasing natural exposure time, the number and size of cracks increases, the
Stt4^ on change of some character, cracks become bigger and deeper. As shown in.
SEM image of M3 sample, the sur&ce damage!
somehow slighter. Only few small cracks ' found on the surface of flie sample. There c observed some holes on the surface of the s after 6 to 12 natural exposure months, photodegradation of the samples can occur iri^
amorphous region before crystalline region b the higher density and regular structure in | crystalline region prevent permeability of c into structure cf the samples [4]. Therefore,'^
photodegradation of amorphous region in ^ samples to be main reason to form the holes on fl surface of these exposed samples.
Figure 5: SEM images of HDPE compound versus natural exposure time 3.4, Thermal property
Figure 6 perfonns DSC curves of HDPE compound versus natural exposure time. The melting temperature (T„,) of exposed and unexposed samples is almost constant, around 144 °C Table 2 exhibits a slight increase of melting entiialpy and relative degree crystalline during tiie fu-st 9 months of natural exposure. It may be attributed to tiie slight mcrease in the ciystallinity of HDPE compound which IS caused by tiie rearrangement in stnicture crystalline of HDPE, the appearance of sphenilite particles and tiie increase of carbonyl group content in the amorphous region after chain scission by photo-degradation of HDPE macromolecules.
However, after 12 natural exposure months, tiie relative crystalline degree of M12 sample is reduced nils can be explained by the photodegradation of
HDPE macromolecules continues in crystalline regions of HDPE.
Table 2: Melting temperature (Tn,), melting enthalpj^V \
(AHm) and relative degree crystalline (Xc) of HDPE compoundversus natural ejqjosure time Samples
MO M3 M6 M9 M12
T„,PE(°C)
144 143 144 143 145
AH,^re(J) 168.5 169.3 169.3 179.7 159.1
Xc(%) 57.4 57.7 57.7 61.2 54.2 Thermo gravimetric (TG) diagrams of samples versus natural exposure are in Figure 7 a the TG data are listed in table 3, The TG curv^
demonstrate first weight loss stage of the sample;
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observed at 300-465 "C. Then, die highest weight loss stage of die samples is at 465-500 °C, and fmally, small weight loss stage is at 500-600 "C, The results in figure 7 and table 3 show die initial diermo-degiadation temperahue, maximum dienno- degradation temperature, as well as remamed weight of die samples to have a decrease trend versus natural exposure time and aging temperature. This confums die influence of natural exposure time on decrease of average molecular weight and chemical durability of HDPE macromolecules [7].
100 200 300 100 Temperature, °c
Figure 6: DSC curves of HDPE compound versus natural exposure time Table 3: TG data of HDPE compound versus natural
exposure time
Samples
MO M3 M6 M9 M12
T •
°C 463 462 455 453 451
7
°C 467 465 459 461 460
Remained weight (%) at 400
ro
89.55 88.55 87.46 87.44 86.77
450 (°C) 56,75 55,92 54,82 53,60 52.27
500 (°C) 3.45 2.72 1.22 1.40 1.07
Where, T,,^: initial degradation temperature, TBU,: maximum de^adalion temperature 4. CONCLUSIONSSome remarks related to the change of IR character, morphology, mechanical, tiiermal properties of HDPE compound samples exposed naturally in Dong Hoi-Quang Binh as follows.
1. Caitonyl index of HDPE compound is increased versus natural e:qx)sure time.
2. From 6 montiis to 12 n^onths of natural exposure, tiie size of voids, cracks and holes on tiie surface and inside tiie structure of HDPE compound are b i ^ e r , larger and deeper hj comparison witii the origmal HDPE compound and tiie sample after 3
Thai Hoang, et al.
months of natural exposure.
3 . Tensile strength and elongation at break of HDPE compound are decreased quickly while its Young modulus is increased in tiie first 3 months of natural exposure.
4, Melting temperature of HDPE compounds is decreased, and their melting enthalpy, relative crystallme degree is slightly increased during tiie first 9 months of natural exposure. Initial thermode^adation temperature, maximum tiierraodegradation temperature, remained weight of HDPE compound are decreased versus natural exposure time and aging temperature.
Figure 7: TG curves of HDPE compound versus natural exposure
Acknowledgements. The authors thank Vietnam Academy of Science and Technology for supporting this research and National Center of Hydro- Meteorological Service for providing the weather and climate data.
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Corresponding author. T h a i H o a n g
Institute for Tropical Technology Vietnam Academy of Science and Technology 18 Hoang Quoc Viet, Cau Giay, Ha Noi, Vietnam E-mail: [email protected].