Study on change of color and some properties of high density polyethylene/organo-Modified calcium carbonate composites exposed naturally at Dong Hoi - Quang Binh - Le Duc Minh

Vietnam Journal of Chemistry, International Edition, 55(4): 417-423, 2017 DOI: 10.15625/2525-2321.2017-00483 417 Study on change of color and some properties of high density polyethylene/organo-modified calcium carbonate composites exposed naturally at Dong Hoi - Quang Binh Le Duc Minh 1 , Nguyen Thuy Chinh 2 , Nguyen Vu Giang 2 , Tong Cam Le 1 , Dau Thi Kim Quyen 1 , Le Duc Giang 3 , Thai Hoang 2* 1 Faculty of Pedagogy Natural Sciences, Ha Tinh University, 447 26/3 street, Ha Tinh, Vietnam 2 Institute for Tropical Technology, Vietnam Academy of Science and Technology 3 Faculty of Chemistry, Vinh University Received 22 January 2017; Accepted for publication 28 August 2017 Abstract This paper presents the study on the UV-Vis spectra, change of color and some properties of high density polyethylene/organo-modified calcium carbonate (HDPE/m-CaCO3) composites exposed naturally in Dong Hoi district, Quang Binh province (Vietnam). From June 2014 to June 2016, the samples of HDPE/m-CaCO3 composites were tested naturally on outdoor shelves at Dong Hoi sea atmosphere region (at Dong Hoi, Quang Binh). The change of UV- VIS spectra, color and some properties of the HDPE/m-CaCO3 composites depend on geographic, weather and climatic factors (solar radiation, temperature, humidity, etc.). In the UV-VIS spectra, the band at 265 nm showed the formation of the carbonyl groups such as ketone, lactone carbonyl and aliphatic ester which were occurred in photo-degradation process of HDPE/m-CaCO3 composites. The results of color change indicated the surface of the samples of HDPE/m- CaCO3 composites was lightened continuously with increasing natural exposure time and increased in total color difference value and significant loss in both redness and yellowness. a*, b* values and electrical breakdown of HDPE/m-CaCO3 composites were decreased while their l*, E, dielectric constant and dielectric loss were increased with rising natural exposure time. Dielectric constant of HDPE/m-CaCO3 composites was in the range of 1.75 to 2.1 and dielectric loss of HDPE/m-CaCO3 composites went up from 1.7 to 3.2 for 0 to 24 months. The electrical breakdown of HDPE/m-CaCO3 composites reduced due to the decrease in the relative crystalline degree of the samples caused by the scission photo-degradation of HDPE macromolecules in HDPE/m-CaCO3 composites for natural exposure time. Keywords. HDPE/CaCO3 composites, photo-degradation, natural exposure, color change, electric properties, UV- Vis spectroscopy. 1. INTRODUCTION High-density polyethylene (HDPE) is currently the most widely used commercial polymer due to its superior mechanical and physical properties. However, its toughness, weather resistance, processability, and environmental stress cracking resistance are not good enough, which have thus limited its application in many high-technology fields. One measure to improve its properties is reinforcement with some fillers [1]. Inexpensive inorganic substances such as calcium carbonate (CaCO3), mica, wollastonite, glass fiber, glass beads, jute, and silica (SiO2) are widely used as fillers to improve mechanical and thermal properties of polymers in the plastic industry. In recent years micro-size fillers have attracted great interest, both in industry and in academia because they often exhibit remarkable improvement in properties of materials [2]. HDPE filled with mineral particles also improves dimensional stability, opacity, and barrier properties. CaCO3 is the largest volume mineral used in the polymer industry because of its low cost and abundance. It is available globally in a variety of particle shapes, purities, and sizes (macro, micro, and nano). However, because of its higher polar nature and higher surface areas, CaCO3 is difficult to disperse and stabilize in a polymer matrix. Poor dispersion and adhesion of filler lead to a composite with poor final physical properties [3, 4]. Therefore, organo-modification of surface of CaCO3 can help to improve the interaction and dispersion of CaCO3 into the polymer matrix [5-7]. VJC, 55(4), 2017 Thai Hoang et al. 418 The study on the degradability of linear polyolefins under natural exposure testing was reported by Telmo Ojeda [8]. This study showed that in less than one year of testing, the mechanical properties of all samples decreased virtually to zero, as a consequence of severe oxidative degradation, that resulted in substantial reduction in molar mass accompanied by a significant increase in content of carbonyl groups. Rui Yang et al. have studied the natural photo-oxidation of HDPE composites, with several inorganic fillers. They concluded that some inorganic fillers such as CaCO3 and wollastonite, can stabilize HDPE. The surfaces of the composites after natural exposure testing became rough and with cracks. A seriously damaged surface did not definitely correspond to a great oxidation degree. The remaining volatile oxidation products of the photo-oxidized composites were proven to be mostly a series of n-alkanes [9]. The study on the effect of natural exposure testing on tensile properties of kenaf reinforced HDPE composites was reported by A.H. Umar [10]. Due to better stiffness, Young modulus of HDPE composites is much higher than neat HDPE. The micro-cracking on the surface of HDPE composites can be observed after 200 hours of testing. Recently, we have studied the degradation and stability of HDPE/m-CaCO3 composites under natural weather condition on outdoor shelves in Dong Hoi sea atmosphere region (Quang Binh province) to evaluate the change of their morphology and properties. In the Fourier Transform Infrared spectra of the exposed samples, the absorption peak around 1735 cm -1 characterizes the stretching vibration of carbonyl group formed during natural exposure. The tensile strength and elongation at break of HDPE/m-CaCO3 composites were reduced significantly while their Young modulus, the number of cracks and size of cracks on the surface of the samples were increased with increasing natural exposure time. The melting enthalpy, relative crystalline degree of HDPE/m- CaCO3 composites were slightly increased during the first 9 months of natural exposure while their melting temperature and initial degradation temperature were decreased [11]. This study reports the results of change in UV- Vis spectra, color, electrical properties of HDPE/m- CaCO3 composites exposed naturally in Dong Hoi, Quang Binh. Here, we chose Dong Hoi, Quang Binh to investigate the change in properties and morphology of HDPE/m-CaCO3 because Dong Hoi has not only the sea climate but also draconic climate. This is typical climate at the sea atmosphere region in the north – middle provinces. The influence of natural exposure time and weather factors on the above changes HDPE/m-CaCO3 composites were evaluated and discussed. 2. EXPERIMENTAL 2.1. Materials The materials used in this work were a HDPE (Daelim, Korea) with melting flow index, kg16.2/C1900 MFI of 1.20 g.min -1 , and its density of 0.937 g.cm -3 ; CaCO3 powder with density of 2.7 g.cm -3 (Minh Duc Chemical Stockshare Co.) was modified by 0.5 wt.% of stearic acid in solid state using high intermixer (SHR-100A, Shanghai China) for 90 minutes at 60-65 o C and mixing speed of 750- 800 rpm. 2.2. Preparation of HDPE/m-CaCO3 composites The HDPE/m-CaCO3 (wt./wt.) composites were prepared by melt-mixing in a Haake internal mixer at 160 o C for 5 minutes at Institute for Tropical Technology (ITT), Vietnam Academy of Science and Technology (VAST). Immediately after melt- mixing, the HDPE/m-CaCO3 composites were pressed by hydraulic heat press machine at a temperature of 160 o C and the pressure of 5 MPa to form sheets with thickness from 1 to 1.2 mm. 2.3. Natural exposure of HDPE/m-CaCO3 composites The samples of HDPE/m-CaCO3 composites were exposed starting from June 2014 to June 2016 on outdoor testing shelves at the Natural Weathering Station of the Institute for Tropical Technology in Dong Hoi sea atmosphere region (Quang Binh, Vietnam). Inclining angle of the shelf in comparison with the ground was 45 degree as typically shown in Figure 1, and total exposure time of the samples was 24 months. Figure 1: View of outdoor exposure testing shelves at Dong Hoi sea atmosphere region VJC, 55(4), 2017 Study on change of color and some 419 After every three months, the samples were withdrawn and stored under standard conditions before determining their properties and morphology. The abbreviate samples were M0, M3, M6, M9, M12, M15, M18, M21, M24 corresponding to 3, 6, 9, 12, 15, 18, 21, 24 months of natural expose, respectively. 2.4. Characterizations 2.4.1. UV-Vis analysis UV-Vis spectra of HDPE/m-CaCO3 composites were recorded on a CINTRA 40 (USA) UV-Vis GBC scanning spectrophotometer in the range 200- 500 nm at ITT, VAST. 2.4.2. Color measurements The color parameters of HDPE/m-CaCO3 composites were determined by a ColourTec PCM (PSM TM , United State) according to ASTM D2244- 89 standard. The total color difference ( E) of the samples was calculated using the following equations. 2 2 2 * * *E L a b Where, L * = L * – L0; a * = a * – a0; b * = b * – b0; And L * is a measurement of brightness ( L * > 0 for light, L * < 0 for dark); a * is a measurement of redness or greenness ( a * > 0 for red, a * < 0 for green); b * is a measurement of yellowness or blueness ( b * > 0 for yellow, b * < 0 for blue); L * , a * and b * are the color parameters of the natural exposed sample; L0, a0 and b0 are the color parameters of the unexposed sample. For each sample, the color parameters were measured at ten different positions of the sample to obtain the average value. The above measurements were performed at ITT, VAST. 2.4.3. Electric properties The dielectric parameters of HDPE/m-CaCO3 composites (dielectric constant - ’ and dielectric loss - tan ) were measured at 1 kHz by TR-10C machine (Ando, Japan) according to ASTM D150 standard. The volume resistivity and surface resistivity were conducted on TR 8491 machine (Takeda, Japan) according to ASTM D257. The electrical breakdown was carried out on Til-Aii 70- 417 machine (Russia) according to ASTM D149-64 standard. The above experiments were performed at 25 o C and humidity about 60 % at ITT, VAST. 3. RESULTS AND DISCUSSION 3.1. UV-Vis spectra The UV-Vis spectra of HDPE/m-CaCO3 composites according to natural exposure time at Dong Hoi (Quang Binh) were presented in figure 2. The UV- Vis spectra showed an increase of the absorption intensity of HDPE in the composites between 200 and 300 nm wavenumber. In the UV-Vis spectrum of initial sample (M0 sample), there was one very strong absorption band at 226 nm. The absorption at 226 nm must be associated with the π – π* transition of the ethylenic group of the α,β-unsaturated carbonyl of impurity chromophores of the enone type in photo-oxidation degraded HDPE. The presence of these chromophores had been identified in the previous studies results [11]. For the exposed samples, the UV-Vis spectra also had the absorption band at 226 nm. Interestingly, the formation of a very broad absorption centred at 265 nm characterized for the carbonyl groups in HDPE when increasing natural exposure time. The results from the UV-Vis spectra indicated the formation of the carbonyl groups such as ketone, lactone carbonyl and aliphatic ester which were occurring in photo- degradation process of HDPE/m-CaCO3 composites. Figure 2: UV-Vis spectra of HDPE/m-CaCO3 composites according to natural exposure time The chain scission of the HDPE in the composites matrix by photo-oxidative degradation of the polymer via Norrish 1 and 2 reactions. If degradation of the carbonyl groups proceeds according to the Norrish 1 reaction, the formed free radicals can attack the polyolefin (scheme 1) [12], which may lead to termination via crosslinking or chain scission. If the degradation proceeds according to the Norrish 2 reaction, carbonyl groups and terminal vinyl groups are produced (scheme 2) and chain scission occurs [12]. The ketones, carboxylic VJC, 55(4), 2017 Thai Hoang et al. 420 acids, and vinyl groups are the three major functional groups that accumulate with the photo- degradation of HDPE macromolecules in HDPE/m- CaCO3 composites [13]. The formation of carbonyl groups and vinyl groups can be remarks of HDPE chain scission. HDPE CH2 CH CH2 O2, PE CH2 C CH2 O H OH CH2 C CH2 O H OH CH2 C CH2 O h CH2 C O + CH2 h h CH2 C O CH2 + CO; Scheme 1: Norrish Type 1 reaction for the photo-degradation of HDPE [12] HDPE CH2 CH2 CH CH2 CH2 O2, PE CH2 CH2 C CH2 CH2 O H OH CH2 CH2 C CH2 CH2 O h CH2 C CH3 O CH + h CH2 CH2 C CH2 CH2 O H OH h Scheme 2: Norrish Type 2 reaction for the photo-degradation of HDPE [12] 3.2. Color change The change of surface color of HDPE/m-CaCO3 composites depends on their structure and composition (the chemical composition change leads to the changes in electric, thermal, and color properties) [14]. The change in values for three color parameters ( L * , a * and b * ) as well as the total color change ( E) of the composites as a function of natural exposure time was displayed in table 1 and figure 3. Figure 3: The a * , b * , L * and E value of HDPE/m-CaCO3 composites according to natural exposure time The surface of the samples of HDPE/m-CaCO3 composites was lightened continuously, the L * and E values were increased with increasing natural exposure time. The changes in E values for the samples were found to be consistent with the change in L * values. The results of color change indicated that the surface of the samples of HDPE /m-CaCO3 composites was faded continuously with increasing natural exposure time expressed by a constant increase in L * value and significant loss in both redness and yellowness. This phenomenon may be due to the change in morphology and existence of double bonds, chromophore groups and heterogeneous structures inside the HDPE macromolecules during photodegradation HDPE/m- CaCO3 composites. These groups affect the visible light absorbability, leading to the variation in visual color of the composites. The b * value of HDPE/m-CaCO3 composites was decreased significantly with natural exposure time. This decrease indicated a loss in yellowness. Two distinguished periods of lightness decrease: one between the third and ninth months (from September 2014 to March 2015) and another between the fifteenth and twenty-first months (from September 2015 to March 2016). After 3 and 9 months of natural exposure testing, the b * values of HDPE/m- CaCO3 composites were 0.86 and 0.26, respectively. Similarly, when natural exposure time was reached up to 15 and 21 months, the b * of HDPE/m-CaCO3 composites were -1.8 and -2.08, respectively. The winter and spring months were characterized by gradual increase of rainfall and decrease of solar radiation (table 2). The significant decrease of the b * value was observed for the samples exposed from 9 to 15 months and from 21 to 24 months. After 9 and 15 months of natural exposure testing, the b * of HDPE/m-CaCO3 composites were 0.26 and -1.80, respectively. When natural exposure time was reached up to 21 and 24 months, the b * of HDPE/m-CaCO3 composites are -2.08 and -2.85, respectively (table 1). In the summer, the average temperature/month and average sunny hours/month are higher, thus the samples have been affected by solar radiation more strongly. This caused the faster photo-degradation of HDPE/m-CaCO3 composites, thus, their b * values were decreased significantly. The average temperature, the relative humidity, the total rainfall and total hours of sunlight at Dong Hoi (Quang Binh) in the period from 2014-2016 were demonstrated in table 2. It is clearly seen that, from ninth to fifteenth months and from twenty-first to twenty-fourth months of natural exposure, the highest temperature is from 27.2 to 38.6 o C and 35.2 to 36.5 o C, total sunlight hours were quite high, 1208 and 493 hours, respectively. The high intensity of VJC, 55(4), 2017 Study on change of color and some 421 solar radiation could make a significant contribution to the photodegradation in amorphous part of HDPE/m-CaCO3 composites. Table 1: The change of a * , b * , L * and E * value of HDPE/m-CaCO3 composites according to natural exposure time Samples M3 M6 M9 M12 M15 M18 M21 M24 a * 3.27 2.63 2.33 2.05 1.71 1.41 1.21 1.11 b * 0.86 0.59 0.26 -0.86 -1.80 -1.96 -2.08 -2.85 L * 2.99 3.31 3.77 5.27 7.22 7.62 7.98 9.24 E 4.03 4.26 4.44 5.71 7.64 8.00 8.33 9.73 Table 2: Climate and weather database at Dong Hoi (Quang Binh) from June 2014 to June 2016 Times Ttb ( o C) Tx ( o C) R (mm) Rx (mm) Utb (%) E (mm) S (h) St (d) CC (d) 2014 June 30.9 39 78 41 67 163 191 22 0 July 30.1 37.5 85 31 71 137 220 12 0 August 29.6 38.5 132 60 72 134 176 11 0 September 29.6 38.5 132 60 72 134 176 11 0 October 25.6 32 605 189 87 57 129 0 0 November 24.2 30 344 160 88 48 106 0 0 December 19.2 25.8 160 48 82 70 35 0 0 2015 January 18.8 25 84 42 84 55 130 0 0 February 20.7 27.2 40 9 91 28 64 0 0 March 24.2 36.7 32 24 90 39 100 0 0 April 25.6 41 206 133 85 72 173 8 0 May 31 40.5 9 6 70 176 298 18 0 June 30.9 39.5 73 36 69 153 290 22 0 July 29.1 39.3 88 15 72 136 106 9 0 August 29.6 38.6 36 19 76 114 241 8 0 September 28.8 38.6 567 194 81 93 204 6 0 October 25.8 32.8 95 36 81 79 170 0 0 November 25.5 31 339 68 86 49 143 0 0 December 21.2 29.2 79 47 85 55 75 0 0 2016 January 19.8 27.3 70 44 89 35 48 0 0 February 17.6 35.2 8 4 80 70 82 1 0 March 20.6 28.5 16 4 89 36 80 0 0 April 25.7 40 53 36 87 53 169 3 0 May 28.4 36.5 75 38 80 92 244 3 0 June 31.0 38.5 119 63 70 117 260 13 0 Ttb, Tx: Average and highest temperature; R, Rx: Rainy total and highest rainy quantity in day; Utb: Average humidity; e: Steam quantity; S: Sunny hours; St: Storm; CC: Day numbers have drizzle. 3.4. Electric properties 3.4.1. Dielectric parameters The frequency dependence of dielectric constant of HDPE/m-CaCO3 composites according to natural exposure time was shown in figure 4a. It can be seen that the effective dielectric constant of the M0 sample was very weakly dependent on frequency, which is the typical characteristic of non-polar VJC, 55(4), 2017 Thai Hoang et al. 422 polymers. The M0 sample contained non- dipolar units and there were not frequency characteristics in the range of 100-10 6 Hz. For the exposed samples, the interfacial polarization can cause an increase of dielectric constant when compared with the M0 sample. When the chains of HDPE in HDPE/m- CaCO3 composites were scissed, the free volumes could be decreased and may cause the increase of dielectric constant. Additionally, it was caused by the formation of the carbonyl groups such as ketone, lactone carbonyl and aliphatic ester occurring in photo-degradation process of HDPE/m-CaCO3 composites. When increasing natural exposure time, the charge carriers in composites were increased. This contributed to the rise of dielectric constant of the samples. The dielectric loss of HDPE/m-CaCO3 composites was increased with increasing natural exposure time and frequency because a higher frequency voltage can yield higher electrical conductivity as shown in figure 4b. Unlike the dependence of dielectric constant, an unclear correlation of dielectric loss which can be stated (the dielectric loss of the samples can increase or decrease when increasing natural exposure time) and immobility of charge carriers in the samples. There were two competitive factors that affect the dielectric loss of the samples such as hindrance in charge transport and the incorporation of charge. The incorporation of large volume fraction of interfaces and polymer chain entanglement which in turn cause immobility of charge carriers or reduction in electrical conductivity, and thus causing a reduction of dielectric loss. On the other hand, the agglomeration of volume fraction can also result in an apparent reduction of interface area of the samples. Therefore, the effect of immobility of charge carriers on reduction in electrical conductivity is far less important than the influence of charge carriers, which causes an increase of dielectric loss of the samples. 3.4.2. Electrical breakdown voltage The electrical breakdown voltage data of HDPE/m- CaCO3 composites were performed in table 3. The value of electrical breakdown voltage of the samples was decreased gradually with increasing natural exposure time. This observation is of vital importance for engineering application because the dielectric rupture always occurs at the weakest points. In other words, the real dielectric strength of the samples is determined by the weakest part of their insulation. Figure 4: Frequency dependence of dielectric constant (a) and dielectric loss (b) of HDPE/m- CaCO3 composites according to natural exposure time Firstly, when increasing natural exposure time, the relative crystalline degree of the samples was reduced. This can be explained by the scission photo-degradation of HDPE macromolecules in HDPE/m-CaCO3 composites leading to decrease crystalline regions of HDPE/m-CaCO3 composites as shown in previous research [11]. In the result, the intrinsic strength of the samples was decreased. Secondly, the mobility of charges in the HDPE/m- CaCO3 composite insulation is much higher with increasing natural exposure time. Therefore, the charges are wider distributed in the HDPE/m-CaCO3 composites and the screening effect is less pronounced. The above reasons make decrease of the electrical breakdown voltage of the composites according to natural exposure time (table 3). Table 3: Electrical breakdown voltage data of HDPE/m-CaCO3 composites according to natural exposure time Samples M0 M3 M6 M9 M12 M15 M18 M21 M24 E (kV/mm) 24.17 21.89 21.55 18.33 17.54 17.04 16.46 15.68 14.39 VJC, 55(4), 2017 Study on change of color and some 423 4. CONCLUSIONS In this work, the influence of climate and weather factors and natural exposure time at Dong Hoi (Quang Binh) on UV-Vis spectra, the change of color, and electric properties of HDPE/m-CaCO3 composites were investigated. The UV-Vis spectra showed the formation of carbonyl groups and vinyl groups in HDPE macromolecules of the composites by their photo-degradation. The surface of the composites was lightened continuously, the L* and E* values were increased with increasing natural exposure time. There was significant loss in both redness and yellowness of the composites. 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