J. Chem. Eng. Chem. Res.
Vol. 1, No. 6, 2014, pp. 365-372
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 ([email protected])
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,
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.
Fig. 1
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 10-50 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
Silane grafting reaction and water crosslink reaction of HDPE.
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.
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. Materials and Methods
2.4 Characterization and Testing
2.1 Materials
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.
TM
HDPE (El-Lene H6105JU, SCG chemicals Co.
Ltd., Bangkok, Thailand) was used. It has MFI of 5.5
g·10 min-1 and melting temperature (Tm) 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-XRTM), with specific
surface area of approximately 65 m2·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
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 160-200 °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.
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
(Tc) and melting temperature (Tm) 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 (Td) 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 40-600 °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
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-CH2 and NH2 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
surface is possibly in the form of structure (I).
CHN elemental analysis of HNT-supported amine catalyst.
Samples
Amine-HNT
C/N = mmol·g-1 C/mmol·g-1 N.
a
treatment with APTES in the wave number range of
and this suggests that the amine catalyst on the HNT
3.1 HNT-supported Amine Catalyst
Table 1
Fig. 2 shows FTIR spectra of HNT before and after
wt %
1.47
Carbon content
mmol·g-1
1.23
wt %
0.26
Nitrogen content
mmol·g-1
0.19
C/N a
6.6
Silane Crosslinking of High Density Polyethylene as Catalyzed by Tin and Amine Catalyst
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 CH2
rocking of long chain hydrocarbon, the symmetric
C-H deformation of CH3, and the C-H deformation of
CH2, respectively. VTES has the characteristic peaks
at 1,170-1,160, 1,100, 1,075, and 970-940 cm-1 which
correspond to the –Si-OCH2CH3 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-OCH2CH3 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
369
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
Silane Crosslinking of High Density Polyethylene as Catalyzed by Tin and Amine Catalyst
370
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
24 h.
Catalyst
None
Tin
Amine
Content of gel in extrudate samples after leaving extrusion die exit (time = 0 h) and after crosslinking for 6, 10, and
0h
0.62 ± 0.05
11.40 ± 0.04
3.51 ± 0.52
% Gel content at different crosslinking time
6h
10 h
19.16 ± 1.38
44.78 ± 2.66
75.32 ± 0.59
78.65 ± 0.92
24.61 ± 0.78
45.54 ± 0.92
24 h
60.55 ± 4.22
77.82 ± 0.96
75.81 ± 1.89
Silane Crosslinking of High Density Polyethylene as Catalyzed by Tin and Amine Catalyst
371
Table 3 Crystallization temperature (Tc), melting temperature (Tm), decomposition temperature (Td), tensile strength,
modulus and elongation at break of uncrosslinked and crosslinked samples.
Samples
Catalyst
Uncrosslinked
None
None
Tin
Amine
Crosslinked
Thermal properties
Tc (°C) Tm (°C)
Td (°C)
110.0
131.7
491.4
110.0
131.7
491.4
109.2
132.4
493.5
110.1
134.4
475.2
3.3 Thermal and Mechanical Properties of XLPE
Table 3 shows the results from DSC, TGA, and
tensile testing. Uncrosslinked HDPE shows Tc at
110 °C, Tm at 132 °C, and Td at 491 °C. All
crosslinked samples which contain nearly 75% gel
exhibit similar Tc, Tm and Td 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.
Strength (MPa)
22.5 ± 0.6
27.4 ± 0.5
20.3 ± 0.4
27.6 ± 0.8
Elongation (%)
375.1 ± 22.5
82.1 ± 18.9
189.1 ± 94.8
35.1 ± 3.9
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.
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