Silane Crosslinking of High Density Polyethylene as Catalyzed by Tin and Amine Catalyst

Transcription

1 J. Chem. Eng. Chem. Res. Vol. 1, No. 6, 2014, pp Received: June 30, 2014; Published: December 25, 2014 Journal of Chemical Engineering and Chemistry Research Silane Crosslinking of High Density Polyethylene as Catalyzed by Tin and Amine Catalyst Kalyanee Sirisinha and Benchawan Sungmanee Department of Chemistry, Faculty of Science, Mahidol University, Salaya Campus, Phuttamonthon 4 Road, Salaya, Nakhon Pathom 73170, Thailand Corresponding author: Kalyanee Sirisinha (kalyanee.sir@mahidol.ac.th) Abstract: Silane crosslinking of high density polyethylene (HDPE) is industrially used for producing crosslinked materials for pipe applications. The process involves melt-grafting of vinyl silane onto polymer chains and crosslinking of the grafted polymer with water. Without the aid of catalyst, the process takes long time to achieve a high degree of crosslinking. Dibutyltindilaurate (DBTL) is frequently used as catalyst for the crosslink reaction. However, tin compounds are toxic to human life. This study aims at investigating the feasibility of using amine compound, i.e. aminopropyltriethoxysilane (APTES), as a more environmental friendly catalyst for silane crosslinking of HDPE. Effects of amine catalyst on the progress of silane crosslink reaction and properties of crosslinked products were explored and compared to those using tin catalyst. Halloysite nanotube (HNT) was used as catalyst supporting material and also as reinforcing filler. Fourier Transform Infrared (FTIR) Spectroscopy and CHN elemental analysis confirmed the attachment of APTES on the HNT surface. For both tin and amine catalyst systems, a high degree of crosslinking (75% gel) could be reached within 24 h of curing process. DBTL showed stronger catalytic effect than the amine compound.all crosslinked products showed significant improvement in tensile strength and modulus, and slight change in thermal behaviors. The properties of crosslinked materials can be tailored by controlling time of crosslinking, type of catalyst supporting material and base polymer used in catalyst masterbatch. Key words: Silane crosslinking, catalyst, high density polyethylene, properties. 1. Introduction Silane crosslinking is, nowadays, an industrial method widely used in producing crosslinked polyethylene (PE) for wire and cable, and pipe industries [1-3]. For those applications, materials with good mechanical and thermal endurance properties are needed. Silane technology consists of two steps as shown in Fig. 1. The silane grafting step is usually performed by means of reactive extrusion where polymer is melt-grafting with vinyl alkoxysilanes, e.g., vinyltrimethoxysilane (VTMS), and vinyltriethoxysilane (VTES). The crosslinking of silane-grafted polymer is then performed after shaping stage, in the presence of water or moisture. Previous studies have demonstrated that silane crosslinking of PE can occur through a mechanism involving hydrolysis of alkoxy groups of silane, and then condensation of these hydroxyl groups to form stable siloxane linkages [1-6]. The time of crosslinking may vary from a few hours to a few weeks, depending on several factors such as the extent of silane grafting, the temperature of water, and the type of polymer. The silane crosslinking rate can be accelerated with the aid of catalyst. Tin catalysts such as dibutyltindilaurate (DBTL) are commonly used for this purpose [4, 5]. Narkis et al reported that time for silane-curing of high-density PE (HDPE) at 80 C decreased significantly from 1000 to 10 h after DBTL catalyst was introduced [4]. Similar trend was found for silane crosslinked ethylene copolymers [5,

2 366 Silane Crosslinking of High Density Polyethylene as Catalyzed by Tin and Amine Catalyst 6]. The maximum gel of 70% was reached after 5 h of curing in hot water [6]. Dialkyl tin mercaptide was used as a catalyst in this case [6]. However, the use of tin catalysts for silane crosslink reaction has some drawbacks since tin compounds are toxic and have biological activity. They can cause liver and brain damage to human life [7]. Considerable efforts have been made in searching for non-toxic and more environmental-friendly silane crosslinking catalysts [8-10]. Kinetics study by Adachi et al showed that boron trifluoride monoethylamine complex provided much lower activation energy for the hydrolysis reaction of VTMS grafted ethylene-propylene copolymer (EPR-g-VTMS) than DBTL [8]. Catalytic mechanisms of sulfonic acid and amine compounds for silane water crosslink reaction of EPR-g-VTMS have also been reported by Adachi and Hirano [9, 10]. In the present study, amine compound i.e. aminopropyltriethoxysilane (APTES) is of interest to be used as a catalyst for silane crosslinking of HDPE.APTES is an aminosilane which has an amine group as the organo-functional groups, a propyl linkage between amine group and Si atom, and three ethoxy groups attached to the Si atom. These ethoxy groups are hydrolyzable and can condense onto the inorganic reinforcement surface. Therefore, APTES is frequently used as coupling agent to improve interfacial interactions between polymers and reinforcements in many polymer composites. APTES has been used also as Brönsted base catalyst for hydrolysis and condensation of organosilicate compounds [11]. Halloysite nanotube (HNT) is a type of aluminosilicate clay with hollow tubular structure. Its structure is much similar to the structure of kaolin and it is difficult to distinguish between them experimentally. The diameters of HNT are typically smaller than nm in outer diameter and 5-20 nm in inner diameter with 2-40 nm in length [12]. Due to various attractive characteristics of HNT such as high length to diameter ratio, nanoparticle size, relatively low hydroxyl group density on the surface, HNT has recently been used in many applications such as metal corrosion prevention, drug delivery system. Reviews on applications of HNT can be found in reference [13, 14]. In the fields of plastic industry, HNT has been used as filler for improving the mechanical properties, thermal stability, and flame retardancy for Fig. 1 Silane grafting reaction and water crosslink reaction of HDPE.

3 Silane Crosslinking of High Density Polyethylene as Catalyzed by Tin and Amine Catalyst 367 various nanocomposites of PE [15], polypropylene [16], polyamide [17, 18] and etc. This work aims at investigating the potential use of amine compound, i.e. APTES as a non-toxic catalyst for silane crosslinking of HDPE. HNT was used as acatalyst support and also as reinforcing filler for the HDPE nanocomposites. In the study, APTES was fixed onto the surface of HNT via silanization. Effects of amine catalyst on the progress of silane crosslink reaction and properties of HDPE crosslinked products were explored and compared to those using DBTL catalyst. Fourier Transform Infrared (FTIR) Spectroscopy and CHN elemental analysis were used for analyzing the HNT supported catalyst. The contents of crosslink gel in the products at different curing time were determined using solvent extraction. Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and tensile test were used for the determination of thermal and mechanical properties of the silane-crosslinked products. 2. Materials and Methods 2.1 Materials HDPE (El-Lene TM H6105JU, SCG chemicals Co. Ltd., Bangkok, Thailand) was used. It has MFI of 5.5 g 10 min -1 and melting temperature (T m ) of 132 C. Vinyltriethoxysilane (VTES), dicumyl peroxide (DCP) and aminopropyltriethoxysilane (APTES) were purchased from Aldrich Chemical Co., Milwaukee, WI. Dibutyltindilaurate (DBTL) used was in the form of masterbatch in linear-low density PE (LLDPE). The amount of DBTL was 0.2 wt% in the masterbatch. HNT filler (DRAGONITE-XR TM ), with specific surface area of approximately 65 m 2 g -1, was from Applied Minerals Inc., New York. It was dried in a hot air oven at 80 C for 24 h before use. 2.2 Preparation of HNT-supported Amine Catalyst APTES was fixed onto the HNT supporting material via silanization. 1 ml of APTES was added to 100 ml of ethanol/water solution (95:5) which was adjusted a ph to 5.0 by acetic acid in a round bottom flask. The mixture was stirred at room temperature for 15 min. 10 g of HNT was then added to the mixture and refluxed for 4 h at 80 C. The APTES treated HNT precipitates were filtered, washed with 95% ethanol/water solution, and then dried at 80 C for 24 h. 2.3 Preparation of Silane-crosslinked HDPE (XLPE) HDPE pellets were melt-grafted with 5 wt% silane, using 0.1 wt% DCP as initiator. The silane grafting reaction was carried out in a twin screw extruder (Prism TSE 16, Staffordshire, UK), using a screw speed of 30 rpm and temperature range of C. The grafted HDPE obtained was cooled and pelletized before melt-mixing with 20wt% catalyst masterbatch. Crosslinking of the silane-grafted products was performed by immersing the samples in hot water at 90 C for various durations to produce the samples of different amounts of crosslink. 2.4 Characterization and Testing FTIR was used here for characterizing the silane-grafted samples and the APTES treated HNT. Silane grafted PE films of 50 µm thick were prepared by hot pressing the polymers at 190 C. The FTIR measurements were performed on a Perkin Elmer System 2000 (Boston, MA). Spectra were recorded in transmission mode at room temperature with a resolution of 4 cm -1 over the range of 4000 to 600 cm -1. In the case of HNT-supported amine catalyst, the examination of APTES in the HNT structure was performed in diffuse reflection mode (DRIFTs) over the range of 4000 to 400 cm -1 wavenumber with a resolution of 4 cm -1. KBr was used as the background. Quantitative analysis of amine on the catalyst support was also carried out using CHN Elemental analyzer (PerkinElmer 2400 Series II CHNS/O Elemental Analyzer). The contents of carbon (C) and nitrogen (N), and the C/N ratio were determined.

4 368 Silane Crosslinking of High Density Polyethylene as Catalyzed by Tin and Amine Catalyst Solvent extraction was used for the evaluation of crosslink gel content in various crosslinked samples. About 0.3 g of samples were wrapped in a stainless cage of 120-mesh and extracted in boiled xylene containing 1% of Irganox 1010 antioxidant for 6 h. After extraction, the cage was dried in a vacuum oven until constant weight. The content of gel in the crosslinked products was determined using Eq. (1). Final weight of dried gel Gel content % 100 (1) Initial weight of sample Thermal properties of crosslinked products were characterized using DSC and TGA. The crystallization (T c ) and melting temperature (T m ) of various samples were analyzed using a differential scanning calorimeter (Perkin-Elmer DSC-7, Boston, MA). Sample of about 7 ± 1 mg was heated under nitrogen atmosphere from 50 to 180 C at a scan rate of 20 C min -1. The temperature was then maintained at 180 C for 5 min before cooling to 50 C at the same rate. The decomposition temperatures (T d ) of the uncrosslinked and crosslinked samples were examined using Mettler Toledo SDTA851 TGA, (Schwerzenbach, Switzerland). Sample of about 5-8 mg was placed in an aluminum pan and heated over a temperature range of C at a rate of 20 C min -1 with a controlled nitrogen flow of 60 ml min -1. Tensile test was performed using an Instron Model 5569 tensile tester (Canton, MA), equipped with a 1 kn load cell. The dumbbell-shaped specimens were prepared by compression-molding at 190 C. The test was conducted at a crosshead speed of 50 mm min -1. Tensile strength, modulus, and elongation at break of specimens were determined. All reported results are the averages of at least five test specimens. 3. Results and Discussion 3.1 HNT-supported Amine Catalyst Fig. 2 shows FTIR spectra of HNT before and after treatment with APTES in the wave number range of 2000 to 1,200 cm -1. In this region, pristine HNT shows one peak at 1,630 cm -1, corresponding to the OH deformation of water [18, 19]. After treatment, HNT-supported amine catalyst reveals two additional peaks at 1,410 and 1,550 cm -1, corresponding to the Si-CH 2 and NH 2 groups of APTES. This evidence confirms the presence of amine catalyst on the HNT. To support characterization of silanization of APTES on HNT structure, CHN analysis was used to quantitatively analyze the content of carbon (C) and nitrogen (N) in the HNT-supported amine catalyst. Table 1 summarizes the CHN analysis results, including the contents of C, N, and C/N ratio. The HNT-supported amine catalyst of this study shows the C/N ratio of approximately 7. This information could be used to predict the structure of amine attachment on the filler surface. It is known that there are two important chemical groups on the tubular structure of HNT [18-20]. One of them is the Si-O-Si groups on the outer surfaces, and the other one is Al-OH present on the inner tubular surfaces and at the edges of the tubes. Al-OH is also found on external surface defects. These Al-OH groups are more important than Si-O-Si for silanization of APTES to HNT structure. Possible reactions of the Al-OH groups of HNT and APTES molecules are shown in Fig. 3. The structure (I) is the form which only one ethoxy group of APTES involves in the reaction. In this case, the calculated C/N ratio is equal to 7. For the structures (II) and (III), two and all three of the ethoxy groups react with HNT, respectively. From CHN analysis, the experimental C/N ratio is close to 7 and this suggests that the amine catalyst on the HNT surface is possibly in the form of structure (I). Table 1 CHN elemental analysis of HNT-supported amine catalyst. Samples Carbon content Nitrogen content wt % mmol g -1 wt % mmol g -1 Amine-HNT a C/N = mmol g -1 C/mmol g -1 N. C/N a

5 Silane Crosslinking of High Density Polyethylene as Catalyzed by Tin and Amine Catalyst 369 Fig. 2 FTIR spectra of HNT and APTES modified HNT. Fig. 3 Possible attachment of APTES on the HNT surface. 3.2 Silane Grafting and Crosslinking of HDPE Fig. 4 shows FTIR spectra of the unmodified HDPE and its grafted product in the frequency range of 600-1,600 cm -1. HDPE shows the characteristic peaks at 720, 1,370, and 1,465 cm -1 which assign to the CH 2 rocking of long chain hydrocarbon, the symmetric C-H deformation of CH 3, and the C-H deformation of CH 2, respectively. VTES has the characteristic peaks at 1,170-1,160, 1,100, 1,075, and cm -1 which correspond to the Si-OCH 2 CH 3 group of VTES (spectrum is not shown here) [21]. Additional absorption peaks are observed after introducing silane to the HDPE. The characteristic peaks at 1,170, 1,100, 1,075, and 960 cm -1 are of the Si-OCH 2 CH 3 group in the grafted polymer [22, 23]. Fig. 5 shows the plot of gel content of XLPE as a function of crosslinking time. The effect of different catalysts on silane-crosslinking of HDPE was compared. For all systems, the contents of gel increase with increasing the time of crosslinking process. Without catalyst, the rate of silane crosslink relation is approximately 2.5% h -1. A remarkable increase of the rate of crosslinking is found for the system using DBTL as a catalyst. A rate of 26.2% h -1 is resulted for the tin catalyst system. Similar finding on the effects of DBTL on silane crosslinking of HDPE has been reported by Narkis et al. [4]. Compared to tin compound, HNT-supported amine catalyst seems to be less effective. The results of Fig. 5 also show that the incorporation of untreated HNT filler to the grafted samples (without catalyst addition) causes a reduction in crosslinking rate of HDPE. In such filled composite, HNT may obstruct the polymer chain movement during the network formation. Therefore, a lower crosslinking rate is observed for the system using

6 370 Silane Crosslinking of High Density Polyethylene as Catalyzed by Tin and Amine Catalyst HNT-supported amine catalyst, compared to the use of DBTL. However, an advantage of HNT-supported amine system is that it produces the products with much lower gel content at extrusion die exit. As shown in Table 2, the crosslink gel in the extrudate products (immersing time of 0 h) is 11.4% for the tin catalyst system, and only 3.5% for the amine system. The presence of high gel content in the extrudates generates high pressure at a die exit and roughness on the surface of extrusion products. With the aid of tin catalyst, the gel content of 75% was reached only after 6 h of crosslinking. However, after conducting a crosslink reaction for 24 h, the similar gel content of 75% is resulted for both catalyst systems. Fig. 4 FTIR spectra of virgin HDPE and HDPE grafted with 5wt% of VTES. Fig. 5 Gel content of XLPE as a function of timeusing various catalyst systems. Table 2 Content of gel in extrudate samples after leaving extrusion die exit (time = 0 h) and after crosslinking for 6, 10, and 24 h. Catalyst % Gel content at different crosslinking time 0 h 6 h 10 h 24 h None 0.62 ± ± ± ± 4.22 Tin ± ± ± ± 0.96 Amine 3.51 ± ± ± ± 1.89

7 Silane Crosslinking of High Density Polyethylene as Catalyzed by Tin and Amine Catalyst 371 Table 3 Crystallization temperature (T c ), melting temperature (T m ), decomposition temperature (T d ), tensile strength, modulus and elongation at break of uncrosslinked and crosslinked samples. Samples Catalyst Thermal properties Tensile properties T c ( C) T m ( C) T d ( C) Strength (MPa) Modulus (MPa) Elongation (%) Uncrosslinked None ± ± ± 22.5 None ± ± ± 18.9 Crosslinked Tin ± ± ± 94.8 Amine ± ± ± Thermal and Mechanical Properties of XLPE Table 3 shows the results from DSC, TGA, and tensile testing. Uncrosslinked HDPE shows T c at 110 C, T m at 132 C, and T d at 491 C. All crosslinked samples which contain nearly 75% gel exhibit similar T c, T m and T d to those of uncrosslinked HDPE. These results confirm the previous findings [2, 24-26] that silane crosslink occurs mainly in the amorphous region and leaves the crystalline phase unchanged or changed very slightly. The introduction of silane crosslink network hardly affects the thermal behaviors of the materials. Tensile properties of uncrosslinked and various silane-crosslinked samples are demonstrated also in Table 3. All XLPE exhibits higher tensile strength and modulus, and lower elongation at break, compared to the uncrosslinked sample. The crosslinked composites using HNT-supported amine catalyst seem to have the highest modulus among the other systems. This is mainly due to the effects of both stiff crosslink network and HNT filler present in the systems. For the case of XLPE prepared using tin catalyst, the strength and modulus of the samples are lower than the others since the base polymer of catalyst masterbatch is LLDPE. 4. Conclusions Silane grafting of HDPE was carried out by means of reactive extrusion. The grafted products were then crosslinked by immersing the molded products in hot water. DBTL and HNT-supported amine catalysts were investigated for their effects on silane crosslinking of HDPE. The extraction results showed that the content of crosslink gel in XLPE increased with increasing the time of crosslink reaction. Rate of crosslinking was much higher when using DBTL as a catalyst, compared to amine catalyst. However, an advantage of HNT-supported amine catalyst was that it produced the products with much lower gel content at an extrusion die exit. All XLPE exhibited similar thermal properties to the virgin HDPE. Silane crosslink occurred mainly in the amorphous portion of the HDPE. The tensile properties of XLPE products were found to depend not only on the time of crosslinking but also on the type of catalyst supporting material and base polymer used in catalyst masterbatch. Acknowledgment The Rubber Technology Research Centre (RTEC), Faculty of Science, Mahidol University is gratefully acknowledged. References [1] J. Morshedian, P.M. Hoseinpour, Polyethylene cross-linking by two-step silane method: A review, Iran. Polym. J. 18 (2009) [2] Y.T. Shieh, J.S. Liau, T.K. Chen, An investigation of water crosslinking reactions of silane-grafted LDPE, J. Appl. Polym. Sci. 81 (2001) [3] W. Kamphunthong, K. Sirisinha, Structure development and viscoelastic properties in silane-crosslinked ethylene-octene copolymer, J. Appl. Polym. Sci. 109 (2008) [4] M. Narkis, A. Tzur, A. Vaxman, H.G. Fritz, Some properties of silane-grafted moisture-crosslinked polyethylene, Polym. Eng. Sci. 25 (1985) [5] M. Palmlöf, T. Hjertberg, Catalysis of the crosslinking reactions of ethylene vinyl silane copolymers using carboxylic acids and DBTDL, J. Appl. Polym. Sci. 72 (1999)

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