Silane Crosslinked Polyethylene from Different Commercial PE’s: Influence of Comonomer, Catalyst Type and Evaluation of HLPB as Crosslinking Coagent

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DOI: http://dx.doi.org/10.1590/1516-1439.303214

Silane Crosslinked Polyethylene from Different Commercial PE’s: Influence of Comonomer,

Catalyst Type and Evaluation of HLPB as Crosslinking Coagent

_Renato Pereira de Melo_ _a_ _, Vinícius de Oliveira Aguiar_ _a_ _, Maria de Fátima Vieira Marques_ _a_

_aInstituto de Macromoleculas Professora Eloisa Mano, Universidade Federal do Rio de Janeiro – UFRJ,_

_Av. Horácio Macedo, s/n, CEP 21941-598, Rio de Janeiro, RJ, Brazil_

Received: June 16, 2014; Revised: February 4, 2015

In the present work, the synthetic route to obtain crosslinked polyethylene (PEX) was studied
using vinyltrimethoxy silane (VTMS) as crosslink agent and commercial polyethylene (HDPE and
LLDPE). Materials were processed with increasing amounts of VTMS (2.5, 5.0 and 7.5 wt.%).
After insertion of silane onto polymer chain in solid state, samples were molded by compression.
Gel content was used to measure the extension of crosslinking. Melting temperature and degree of
crystallinity were evaluated using differential scanning calorimetry (DSC). The microstructure of the
starting polyethylene was correlated with the effectiveness of crosslinking and the resultant thermal
and mechanical properties of the PEX. Hydroxylated liquid polybutadiene (HLPB) was employed to
improve crosslink efficiency, as well as mechanical properties of the material.

Keywords: _crosslinking, structure, crystallization, additives_

1. Introduction

The linking of linear polymer chains by covalent bonds
produces a tridimensional network. Lightly crosslinked
polymers possess only occasional crosslinks, but these prevent
the material deforming indefinitely under load. Between the
crosslinks, the molecular segments remain flexible. Thus,
at appropriate temperature, the polymer may be rubbery or
rigid; it may also crystallize if the segments can pack together
sufficiently closely. As the degree of crosslink increases,
segmental motion is progressively restricted until in, for
example, phenolic, amino or epoxy resins, only infusible,
rigid, amorphous state exists 1-3 .

Polyethylene crosslinking may be induced through
three different methods: radiation, peroxide and silane 4-8 . In
peroxide crosslinking, organic peroxide and polyethylene
are blended during extrusion process and free radicals are
formed, crosslinking the polymer chain to produce a network
during thermal decomposition 9 . One of the most usual
substances to crosslink polyethylene is dicumyl peroxide
(DCP), moderately stable in low-density polyethylene at
a temperature range between 130 - 150 °C. Generation of
primary radicals, attack of these radicals in the polymeric
chain by hydrogen capture and combination of two polymeric
radicals to form a carbon – carbon bond are showed in
Figure 1 10-12 . However, this crosslinking technique involves
high costs. Moreover, there are risks of pre-curing during
peroxide crosslinking 13,14 .

Silane crosslinking is defined by grafting of
vinyltriethoxysilane (VTES) or vinyltrimethoxysilane
(VTMS) in polyethylene with small amounts of peroxides as
initiators, followed by hydrolysis and condensation reaction of
generated silanol groups 15 . Crosslinking takes place between

these silanol groups, forming siloxane structure (-Si–O–Si-)
with water loss. This reaction is optimized with presence
of organotin components as dibutyltin dilaurate (DBTDL),
according to Figure 2 16 . The influence of some parameters on
the molecular structure of a polymer can lead to development
of new products, which can be tailored according to some
requirements. The creation of a crosslink network between
individual polymer chains leads to restriction of chain
movement. Hence, when heat or other form of energy is
applied, the basic network structure cannot deform and
some of the properties (e.g. dimensional stability, impact
strength, creep and abrasion resistance) are retained at
high temperatures. Those properties are important for the
applications of wire and cable coating, hot water tubing and
weathership 17-19 . The degree of silane crosslinking is of vital
importance in determining how well the properties of the
materials are maintained at elevated temperature 20 .

The aim of this work is to evaluate the synthesis of PEX
obtained from different starting polyethylenes using VTMS,
to compare the influence of PE microstructure on the gel
content, and on thermal and mechanical properties of the
final product. A new crosslinking coagent (hydroxylated
liquid polybutadiene – HLPB) was also evaluated 21 .

2. Experimental Procedure

_2.1. Material_

Polyethylenes HDPE and LLDPE’s were gently provided
by Quattor S.A., Brazil and their characteristics are presented
in Table 1. Dicumyl peroxide, DCP 98%, and vinyltrimethoxy
silane, VTMS 97%, were acquired from Aldrich Co. Ltd.,
USA. Hydroxylated liquid polybutadiene (HLPB) was

h d f P fl L d B il Dib l i dil

Figure 1. Scheme of the mechanism of peroxide polyethylene
crosslinking 11 .

Figure 2. Scheme of the mechanism of silane polyethylene
crosslinking 16 .

(DBTDL) was donated by Miracema-Nuodex Company,
Brazil. All of these reagents were used as received.

_2.2. Crosslinking reactions_

At first, polyethylenes were milled using a knife mill
model IKA-MF10 with rotation of 4000 rpm. Then, VMTS
(10.0% v/v), DCP (1.0% v/v) and DBTDL (1.0% v/v) were
dissolved in methanol and, after this, were mixed with milled
polymers, according to Table 1. For the samples with HLPB,
it was introduced with pure polyethylene and subsequently,
solutions were added. Finally, samples were molded by
compression at 180 °C and 10 MPa for 30 minutes for the

li ki i

_2.3. Gel content_

Gel content of silane crosslinked materials after solvent
extraction was calculated according to standard ASTM
D2765-01. Nearly 0.5 g of each sample was extracted in
refluxing xylene for 6h. After extraction, samples were
weighted before and after drying in a vacuum oven at 60 ºC
until constant weight. Equation 1 shows the calculation of
gel content of crosslinked samples 22 .

% gel content = [(final weight of sample) /
( initial weight of sample)] × 100 (1)

_2.4. DSC analysis and determination of degree of_
_crystallinity_

Samples with masses range from 3.0 - 4.5 mg were weighed
in an aluminum holder and heated at a rate of 10 °C/min
over a temperature range from 25 - 170 °C under nitrogen
in a DSC-TA instrument, Q -1000. Degree of crystallinity
was calculated using Equation 2:

enthalpy of the sample (J/g); ΔHm100 = melting enthalpy of

100% crystalline polyethylene (J/g), which was considered
to be 292 J/g 23 .

_2.5. Thermogravimetric analysis_

The thermogravimetric analysis (TGA) was performed in
heating mode from 25 - 700 °C at 10 °C/min under nitrogen.
Samples with approximately 10 mg were weighted and initial
temperature of degradation (Tonset), as well as maximum
degradation rate (Tmax), was determined.

_2.6. Mechanical properties_

Tests were performed following ASTM D638-5 standard.
Deformation of the specimens was fixed at 50.0 mm/min.
Dimensions of test bars for tension analysis were approximately
6.0 × 3.0 × 1.2 mm (length × width × thickness). For each
sample, six specimens were evaluated for reproducibility
of the results and standard deviations were calculated.
The tensile and elongation at yield and at break were also
obtained, as well as elasticity modulus.

3. Results and Discussion

After the crosslinking reactions of different LLDPE
(obtained by Ziegler-Natta or metallocene catalysts) and
HDPE with increasing amounts of silane, the samples were
analyzed according to the gel content and the results are shown
in Table 2 and Figure 3. It can be observed an increase of
gel content with increasing of VTMS amount, as expected
and also observed by some authors 18,19,24 . All samples have
reached the maximum value of degree of crosslinking when
VTMS concentration was 7.5 wt.%. At low amounts of VTMS
(2.5 wt.%, samples LLDPE 1 (butene as comonomer) and
LLDPE 2 (hexene as comonomer) showed similarly much
hi h d f li ki d i h 3

c = ∆∆ HH100mam

∆ H

Xc =

∆
=

∆∆ HH100mm (2)

Where Xc = degree of crystallinity (%); ΔHma

Where Xc = degree of crystallinity (%); ΔHma = melting

enthalpy of the sample (J/g); ΔHm100 = melting enthalpy of

Table 1. Characteristic of polyethylenes employed and conditions of crosslinking.

Sample Starting PE Original thermal

properties

VTMS (wt.%) Code

Ziegler-Natta, 3.1 mol%
LLDPE 1

butene comonomer

Ziegler-Natta, 3.8 mol%
LLDPE 2

hexene comonomer

Metallocene, 1.7 mol%
LLDPE 3

hexene comonomer

HDPE Ziegler-Natta

0 BF0
Tm = 121.9 °C
2.5 BFS2

5.0 BFS5
Xc = 41%
7.5 BFS7

0 HF0
Tm = 123.9 °C
2.5 HFS2

5.0 HFS5
Xc = 36%
7.5 HFS7

0 MF0
Tm = 119.5 °C
2.5 MFS2

5.0 MFS5
Xc = 57%
7.5 MFS7

0 EI0
Tm = 132.2 °C
2.5 EIS2

5.0 EIS5
Xc = 65%
7.5 EIS7

VTMS – vinyltrimethoxy silane; PE mass = 2 g; VTMS/DCP/DBTDL = 1/10/15; BF – butene comonomer ; HF – hexene comonomer ; MF – hexene
comonomer (metallocene PE); EI – HDPE; 0 – no silane; S2 – 2,5 wt.% of VTMS; S5 – 5,0wt.% of VTMS; S7 – 7,5 wt.% of VTMS.

Table 2. Gel content and thermal properties of PEX samples.

Sample Gel Content

(%)

Xc (%) Tm (°C)

BF0 0 46 121.9
BFS2 61.7 34 119.0
BFS5 70.7 34 117.0
BFS7 83.7 33 113.1
HF0 0 36 123.9
HFS2 61.2 35 120.6
HFS5 66.3 31 114.4
HFS7 74.1 30 118.7
MF0 0 57 119.5
MFS2 9.1 44 118.4
MFS5 53.8 40 116.3
MFS7 60.1 45 118.1
EI0 0 65 132.2
EIS2 0.9 65 131.6
EIS5 16.5 50 131.6
EIS7 25.6 54 131.5

BF – butene comonomer; HF – hexene comonomer; MF – hexene comonomer
(metallocene PE); EI – HDPE; 0 – no silane; S2 – 2,5 wt.% of VTMS;
S5 – 5,0 wt.% of VTMS; S7 – 7,5 wt.% of VTMS.

(produced with metallocene catalyst). This is probably because
of the relatively higher amounts of extractable fractions
of low molecular weight chains with high comonomer
concentration, typical of the ZN copolymers, which favors
the attack of peroxides, producing high crosslinking reactions.
However, at higher VTMS amounts, gel content was higher
for LLDPE 1 than LLDPE 2, despite LLDPE 1 shows higher
degree of crystallinity.

To understand this behavior, the microstructure of both
ZN-LLDPE, with similar amounts of different comonomers
(3.1% of butene for LLDPE 1 and 3.8% of hexene for LLDPE
2) l d h fi h li h l l

Figure 3. Gel content of crosslinked samples.

although much higher Xc when compared with the second
one. This is because of the size of comonomer and different
sequence length distribution of monomers in the polymer
chain. The introduction of hexene in the polyethylene chain in
LLDPE2 will decrease sharply the degree of crystallinity of
the polymer because of its bulkiness, although the distribution
of monomers in the polymer chain is such that it leaves great
length sequences of ethylene in the chain, and therefore Tm
is higher. Larger sequences of ethylene difficult the attack of
peroxide and, as a consequence, lowers grafting reactions.

When density of polyethylenes are compared, it can
be noted only a small increase of HDPE gel content, when
compared to LLDPE’s, reaching no more than 25.6%
at maximum VTMS concentration. According to Wong
& Varrall 25, this behavior is due to the lack of tertiary
carbons along HDPE chains. On the other hand, LLDPE 3
(from metallocene catalyst), considering only low density
polyethylenes, showed less increase of gel content at the

il i 60 1 % h i

of HDPE but less than the other LLDPE. So, it can be
supposed that the chains of metallocene LLDPE are more
able to be grafted by silane than those of HDPE because of
the presence of tertiary carbons in its structure.

Other considerations must be taken according to the
polymer microstructure. Metallocene polyethylene, due to the
single-site nature of the catalyst, comprehends polymer chains
with narrow molecular weight and interchain compositional
distributions. This means that there is virtually any extractable
fraction with low MW and high content of comonomer, and
also any fraction containing chains with high sequences of
ethylene (with low comonomer incorporation) as in the case
of ZN-LLDPE, all polymer chains are very similar in length
and composition. Therefore, a low amount of comonomer
(such as 1.7% hexene) will deeply decrease the melting
temperature, producing high amounts of small crystals (due
to the lower sequences lengths of ethylene in the chain),
which explains the characteristic of this polymer grade.

It is important also to consider that, as crosslinking
process takes place in non-melt polymer and both LLDPE 3
and HDPE having high crystallinity index, maybe the water
penetration was not favorable to hydrolysis of methoxy
groups and condensation of hydroxyl groups to perform
intermolecular crosslinking was hindered.

To compare crosslinking performance between metallocene
LLDPE (MF) and Ziegler-Natta LLDPE’s (BF and HF), it
can be observed that, at lower VTMS content, crosslinking of
Ziegler-Natta LLDPE’s were higher than that of metallocene.
Moreover, at 7.5 wt% of VTMS, where all polyethylenes
reached higher gel content, metallocene LLDPE also
showed low degree of crosslinking, almost 20% lower. This
difference can be explained also by difference of crystallinity
degree of polymers that is higher for metallocene LLDPE.
Another reason can be attributed in terms of comonomer.
According to Table 2, metallocene LLDPE has lower content
of incorporated comonomer than Ziegler-Natta LLDPE’s,
which becomes difficult silane incorporation.

When crosslinked polymers synthesized by same catalyst
(Ziegler-Natta) with similar comonomer content (butene
and hexene) are compared, it can be noted that BF sample
(LLDPE with butene) showed higher crosslinking degree
than the one with hexene (HF sample), despite the higher
crystallinity of pure LLDPE (BF0). As BF0 has less ethylene
sequences length in chain, this can become favorable the
attack of peroxide, as well as silane grafting reaction.

Crystallinity results of crosslinked samples are shown
in Table 2 and Figure 4. In a general overview, increase of
VTMS content leads to reduction of degree of crystallinity
of silane-crosslinked samples. Curiously, lower variations
of Xc were attributed to Ziegler-Natta crosslinked samples
(BF and HF). They showed less reduction until 7.5 wt.% of
VTMS, which occurred because at higher VTMS amount, the
gel content was elevated. Otherwise, to HDPE and mainly
metallocene LLDPE, it can be observed higher variation in
Xc, as well as gel content.

When melting temperature of crosslinked samples are
analyzed, it can be seen in Table 2 and Figure 5, the decrease
of melting temperature with increase of silane content.
The same behavior was observed by Khonakdar et al. 26 .
C li k d ll ( ) d ( )

showed less significantly decrease in Tm when compared
with their correspondent pure samples. In HDPE, the low
degree of crosslinking (gel content) explains this behavior.
However, even with elevated degree of crosslinking (60%),
metallocene LLDPE still have long ethylene sequences
that can crystallize. So, the crosslinking in this polymer is
relatively more homogeneous.

Crosslinked Ziegler-Natta LLDPE’s (BF and HF) showed
higher decrease in melting temperature compared with
other samples. Considering that grafting and subsequent
formation of Si – O – Si bonds happens preferably in tertiary
carbons, more present in Ziegler-Natta LLDPE’s, it is more
influent in crystal growth, becoming them more defective
and decreasing the melting temperature.

In order to increase the properties of PEX, experiments
using an auxiliary crosslinking agent were performed. The
addition of hydroxylated liquid polybutadiene (HLPB) as a
crosslink coagent was analyzed in samples which resulted
in higher (BFS7) and lower (EIS7) gel content. Therefore,
the crosslinking reaction was performed adding different
contents of PBLH (5.0 and 10.0 wt.%) at the same experiment
conditions employed to compose those samples and variation

f l b d

Figure 4. Degree of crystallinity of crosslinked samples.

Figure 5. Melting temperature of crosslinked samples.

To the Ziegler-Natta crosslinked sample (BFS7), it
can be observed (Table 3 and Figure 6) that, with 5.0 wt.%
of HLPB incorporated in the formulation, gel content is
slightly higher than crosslinked sample without HLPB
(89.1% × 83.7%). No significant variation of gel content was
noted when 10.0 wt.% of PBLH was added. The increase
of gel content when HLPB is added can indicate that this
coagent may increase silane grafting through diminishing
the crystallinity, becoming easier water diffusion to form
silanol groups, allowing higher condensation of these groups
that can occur with grafted LLDPE chains, as well as HLPB
grafted chains.

Table 3. Gel content of BFS7/HLPB and EIS7/HLPB samples.

Samples BFS7/HLPB

Sample Gel Content (%)

BF0 0
BFS7 83.7
BPB1 80.6
BPB5 89.1
BPB10 88.5

Samples EIS7/HLPB

EI0 0
EIS7 25.6
EPB5 60.0
EPB10 76.5

Regarding to HDPE crosslinked samples (EIS7),
according to the Table 3 and Figure 6, it can be noted a
continuous increase of gel content. With 10.0 wt.% of
HLPB, the amount of gel is almost 3 times higher than
crosslinked HDPE without HLPB. It can be supposed that
with the coagent insertion being added at the same time with
silane and catalyst, grafting of HLPB onto HDPE became
easier as a consequence of the hydroxyl reactive groups of
HLPB and also of the increasing distance between the linear
chains, reducing crystallinity of grafted material. So, it is
possible that water diffusion in the solid state was favored
also, allowing the increase of gel content.

To evaluate mechanical properties of the final products,
it was performed tension tests of pure Ziegler-Natta LLDPE
(BF0), as well as crosslinked Ziegler-Natta LLDPE (BFS7

- higher gel content overall) and crosslinked BFS7 samples
with 1.0, 5.0 and 10.0 wt.% of HLPB. Results are shown
in Table 4 and Figure 7. It can be observed an increase of
elastic modulus compared to pure polymer 27 and the decrease
tendency when HLPB is inserted. Elongation at break tends
to increase until 5.0 wt.% of HLPB in the PEX formulation.
It can be noted also that elongation at yield did not change
significantly, maybe because there is higher mobility and
flexibility of chains and better plastic fluency once that
possibly some extensions of chains did not crosslink.

Considering the low influence of crosslinking reactions
on degree of crystallinity in LLDPE and crosslinking taking
place in solid state, in fact it is expected reasonable distribution
of Si – O – Si bonds across grafted polymer chain and,
therefore, elastic modulus of crosslinked LLDPE would be
higher than pure LLDPE. However, incorporation of HLPB
at low concentration (1.0 wt.%) led to decrease of modulus
and with further increase to 5.0 and 10.0 wt.%, there was
an increase of this property. The increase of functional OH
groups from HLPB, as well as of water diffusion taking
place in amorphous regions, enhanced the crosslink density,
leading to higher modulus .

Regarding to thermal degradation (Table 5 and Figure 8),
an increase of Tonset was observed with grafting of pure
Ziegler-Natta LLDPE (BF0) and, in presence of HLPB, this
increase was even higher, what can be directly related to the
gel content of these samples. Thus, it can was confirmed that
the higher the gel content, the higher Tonset was found 19,28 .
Likewise, the maximum degradation temperature (Tmax) of
BPB10 sample (PEX + HLPB) was higher among these
materials and mass loss was the lowest found.

Table 4. Mechanical properties of crosslinked samples.

Sample Tensile Stress at

Yield (MPa)

Elongation at Yield

(%)

Modulus (MPa) Tensile Stress at

Break (MPa)

Elongation at Break

(%)

BF0 10.78 ± 0.63 29.13 ± 0.99 149.79 ± 13.20 22.50 ± 2.72 621.32 ± 79.76
BFS7 11.47 ± 0.77 21.65 ± 10.9 194.75 ± 31.06 16.61 ± 1.26 326.64 ± 47.29
BPB1 11.09 ± 0.61 44.29 ± 2.87 126.12 ± 15.42 14.13 ± 1.69 246.46 ± 46.92
BPB5 11.07 ± 0.80 43.17 ± 7.05 127.41 ± 22.90 13.78 ± 2.55 220.14± 133.97
BPB10 11.03 ± 0.79 22.24 ± 10.4 180.93 ± 23.53 13.66 ± 1.85 233.66 ± 122.15

BF0= HDPE, ZN; BFS7= HDPE with 2.5% of VTMS; BPB 1, 5 and 10 = BFS7 with 1 to 10% w/w of HLPB.

Table 5. Thermal properties of PEX’s samples.

Sample Tonset (°C) Tmax (°C) Total Loss (%)

BF0 424.4 458.1 99.5
BFS7 434.5 463.6 98.4
BPB10 444.2 469.7 98.2

Figure 8. Profiles of thermogravimetric analysis of the crosslinked
samples.

4. Conclusions

The silane method employed in the synthesis of PEX
starting from various grades of polyethylene under the
conditions studied proved to be feasible, as shown by
increasing of gel content of polyethylenes studied, the most
important parameter. Several variables were investigated:
starting polymer, influence of comonomer, catalyst-type and
crosslinking coagent. The Ziegler-Natta LLDPE with butene
resulted in better performance of crosslinking compared

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