Silane Grafting

Malaysian Polymer Journal (MPJ), Vol 2, No. 2, p 58-71, 2007

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Silane Grafting and Crosslinking of Metallocene-catalysed

LLDPE and LDPE.

A. A. Yussuf1, E. Kosior2 and L. Alban3

1 Industrial Research Institute Swinburne (IRIS), Swinburne University of Technology, 543-545 Burwood Road, Hawthorn, Vic.3122, Melbourne, Australia

2 Nextek Pty Ltd, 3 St. Thomas Street, Bronte, NSW 2024, Sydney, Australia

3 Royal Melbourne Institute of Technology, RMIT, Department of Civil and Chemical

Engineering – City Campus, 124 Latrobe Street, Vic. 3001, Melbourne, Australia

Corresponding author: [email protected] ABSTRACT:Two grades of metallocene-catalysed LLDPE and one grade of low-density polyethylene (LDPE) have been silane grafted and crosslinked by contact with water and their performance has been compared and evaluated. The effects of vinytrimethoxysilane (VTMOS) levels on the degree of crosslinking for each grade were also studied. It was found that increasing level of silane increases the degree of crosslinking. However, no significant increase of the gel content was observed beyond 1.5 phr of silane concentration. All grades studied are shown to crosslink adequately, albeit with differing sensitivities to the level of silane, curing time and temperature. However, metallocene-catalysed LLDPE grades performed better in terms of crosslinking than LDPE, and among the metallocene-catalysed grades; EG 8150 has achieved a higher degree of crosslinking under the same level of silane concentration, and same processing conditions. The influences of curing time and temperature on the degree and rate of crosslinking were investigated, and it was observed that increasing curing time and temperature had a significant effect of increasing both the degree and the rate of crosslinking for all grades. The mechanical properties of all crosslinked samples are reported and correlated with their silane content results, which shows the increase of tensile strength and decrease of elongation at break. Keywords: crosslinking, silane grafting, metallocene catalysed LLDPE, mechanical property, gel content. 1.0 INTRODUCTION Crosslinking is a widely used method for the modification of polymer properties. This process involves the formation of tridimensional structure-gels-causing substantial changes in material properties. In many applications there is a need to improve the properties of polyethylene, particularly with respect to heat deformation resistance, chemical resistance, stress cracking, and shrinkage. Crosslinking then is an obvious alternative to improve those properties and it is used today on a large commercial scale [1].


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Common methods of initiating crosslinking involve macroradical formation via thermal decomposition of peroxide [2-5], high-energy irradiation, gamma or electron beam [6-8], and grafting of silane groups, which form crosslinks via hydrolysis of silanol moieties [9-12]. Both radiation and peroxide crosslinking techniques require a high investment cost. In addition to that, the efficiency of radiation crosslinking decreases with thickness and thick sections are difficult to crosslink sufficiently, and in the case of peroxide crosslinking, speed is limited by the cure and the demands for a certain residence time in the heating zone. Silane crosslinking has become an alternative method and gained much attention in recent years because of its various advantages, such as easy processing, low capital investment, and favourable properties in the processed materials.

The silane crosslinking process involves at least two stages, which may be done together. In the first stage, a silane-grafted polymer is prepared using peroxide and vinyl alkoxysilane, and in the second stage, the silane-grafted polymer is crosslinked by exposure to hot water with catalyst. The crosslinking reaction involves hydrolysis of the alkoxy groups by moisture, followed by condensation of the hydroxyl groups to form stable siloxane linkage. The grafting step may be performed while the polymer is in either solution or the molten state. On the other hand, the crosslinking step is normally carried out after the polymer has been shaped into product and polymer is in a solid state. Figure 1 shows the free radical reactions producing silane grafted polyethylene (a) and mechanism of hydrolysis and crosslink formation through the condensation reaction of silanol groups (b). (a) (b)

Figure 1: Grafting (a) and crosslinking (b) reaction mechanism [13].

The technology of silane grafted and crosslinked polymers is being successfully

exploited on a large commercial scale with polyethylene to produce XLPE for electrical cable insulation and water heating pipes [1].

R-O-O-R 2 RO

DECOMPOSITION OF PEROXIDE TO FREE RADIAL

CH 2 CH 2 CH 2 CH 2

POLYMERPEROXY RADICALR O

*

*

CH 2 CH

CH

CH 2 CH 2

CH 2

CH 2 CH 2 CHCH 2

CH 2 CH 2 CHCH 2

Si (OR) 3

H

Si (OR) 3

CH 2

CH2

*

*

P

H2C = CHSi(OR) 3

GRAFTING

CHAIN BRANCHING

(VTMOS)

-

+ P *

CH2 CH CH2 CH2

CH2 CH CH2 CH2

CH2

2

2

CH2 CH CH2 CH2

CH 2

CH2

OCH3

Si OCH3H3CO

H3CO OCH 3Si

OH

O

Si OCH3H3CO

CH2

CH 2

Si OCH3H3CO

CH2

CH2

H2C CH CH2 CH2

CATALYST , H 2O

-CH3OH

GRAFTED COMPOUND

CONDENSATION

REACTIONCATALYST, -H 2O

CROSSLINK

BETWEEN

POLYMERS

CH2

HYDROLYSIS

REACTION


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In the literature, a great number of papers on silane crosslinking have been published, dealing with very different aspects such as formulation, processing, reaction kinetics, thermal and mechanical properties. Sen et al. [13] investigated the kinetics of silane grafting and moisture crosslinking of polyethylene and ethylene propylene rubber and concluded that both the degree and rate of crosslinking are controlled by the concentration of catalyst and moisture. Cartasegna [14] reported on the molecular and structural parameters of ethylene/propylene rubber (EPR) and low density polyethylene (LDPE) on silane grafting and crosslinking and discussed how these parameters affect the degree of crosslinking. Bullen [15] studied the silane crosslinking behaviour of low-density polyethylene from different production routes and found that LDPEs can behave quite differently in their response to a standard silane crosslinking formula. However, in his study it was not possible to predict the sensitivity of a particular polyethylene to silane crosslinking. Shieh et al [16] have studied the heterogeneous molecular structure and morphology of the silane-grafted water crosslinked LDPE by using DSC fractionation method.

Metallocene or single-site catalysts are able to produce polyethylene and polyethylene copolymers with narrow molecular distributions and uniform distribution of short chain branches. The successful development of metallocene catalysed LLDPE was considered to be one of the most significant achievements in the polymer history for the past 20 years [17]. Metallocene-catalysed LLDPE are ethylene-octene copolymer, which combine the features of narrow molecular weight distribution (MWD), processability and good end-use performance for a variety of wire and cable applications [17]. This allows the polymer researcher and manufacturer to explore opportunities that were previously unreachable with traditional polyolefins. As for silane crosslinking of metallocene catalysed LLDPE, very few articles have been noted in the literature. Zhang et al. [18] investigated the effects of silane crosslinking of metallocene-based polyethylene on the mechanical and thermal properties and concluded that increasing silane concentrations increases tensile strength and degree of crystallinity decreases slightly with increase of degree of crosslinking. Sirisinha and Meksawat [19] studied the changes in properties of silane crosslinked metallocene ethylene-octene copolymer after prolonged crosslinking time.

Hence, the aim of this article is to study the influence of silane concentration, curing time and temperature on the degree and rate of crosslinking, and also to examine the link between the molecular structures of the metallocene catalysed LLDPE and crosslinking evaluations. Finally to compare the performances of metallocene catalysed grades to the conventional LDPE. This research will further extent the knowledge of silane crosslinking and will allow the potential of these materials to be evaluated as new generation materials for demanding applications such as cable sheathing. 2.0 EXPERIMENTAL 2.1 Materials

Two grades of metallocene catalysed LLDPE’s (EG 8150 and EG 8200), developed from single-site INSITE metallocene catalysed technology, were supplied by Dow Chemical Company, Australia, while low density polyethylene (LDPE), which was a commercial grade (LD0231), was supplied by Kemcor Australia. All additives, Vinyl Trimethoxy Silane (VTMOS), Trigonox 29B Peroxide (1.1-di-tert-butylperoxy-3.3.5-trimethylcyclohexane) and Dibutyl Tin Dilaurate (DBTDL) Catalyst, were supplied by MM cables, the project sponsor. Polymer properties are tabulated in Table 1


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Table 1: Properties of metallcene catalysed LLDPE and LDPE

Properties Polymer

grade Density (g/cm)

MFI (g/sec)

Octene content (%) MW

Crystallinity (%)

EG 8150 0.868 0.5 26 3.11x105 1.5 EG 8200 0.87 5 23 1.84x105 2

LDPE 0.932 0.2 --- ---- 43.6 2.2 Specimen preparation techniques for grafting and crosslinking.

In this study, the concentrations of silane used were varied from 0 to 2 phr (parts of reagent per hundred parts of polymer), while the concentrations of peroxide and catalyst were kept constant at 0.2 phr and 0.06 phr respectively. The grafting and crosslinking procedures were taken as follows.

Metallocene catalysed LLDPE and LDPE (pellets) were dried and preheated in a vacuum oven at 40°C overnight in order to minimise the moisture content of these materials. The pellets were then tumble blended with the requisite quantities of silane and peroxide in a stoppered flask for 15 minutes. The mixture was transferred to the sealed, nitrogen-gas- flushed, hopper of the Brabender extruder (type 832400). Barrel temperatures were 150°C, 170°C and 190°C working from the hopper, and the die was maintained at 200°C. A screw speed of 50 rpm was used; with residence time of five minutes and a melt temperature on exit from the die of 200°C-205°C, depending on the polymer type. Cold air was blown on to the extruded rod to chill it rapidly before granulation. The grafted compound was transferred into a plastic bag with nitrogen gas and kept in a desiccator to prevent premature moisture curing taking place. For the crosslinking step, the required quantity of dibutyl tin dilaurate (DBTDL) catalyst was added to the grafted material in a stoppered flask and immediately extruded generating a 2 mm thick sheet. All moisture crosslinking was conducted immediately after extrusion and was carried out by immersing the prepared sample sheets in a water bath maintained at different temperatures of 90°C, 60°C and 25°C for different periods of time. 2.3 Gel content measurements

The crosslinked and finished samples were removed from hot water and kept at ambient condition over night, after which the sample was ready to evaluate its degree of crosslinking. Degree of crosslinking was determined by measuring gel content or insoluble fraction of silane crosslinked samples after extraction. Weighed specimens (cut into a number of small pieces) were allowed to swell and the soluble portions were extracted in Toluene at 110ºC for 10 hours. The specimens were then dried to a constant weight in a vacuum oven at 70ºC for 5 hours. The percentage of the weight remaining, with respect to the initial weight, gave the gel fraction or gel content. The higher the degree of crosslinking, the higher the gel content of the sample. 2.4 Tensile testing

An Instron tensile tester (Series 4400) was used at a crosshead speed of 100 mm min-1 using a 2-kN load cell at room temperature. The specimens were prepared from crosslinked sheet extruded from a twin screw extruder, using dumb bell specimens according


62

to the ISO 37 type II standard. At least five specimens were tested for each batch of samples and the average of the results was taken. 3.0 RESULTS AND DISCUSSIONS 3.1 Effects of curing time and temperature on the degree of crosslinking

In order to study the effects of curing time and temperature on the degree of crosslinking, the crosslinking reactions were carried out at different temperatures (90ºC, 60ºC and 25ºC) in water for different time intervals (1 to 12 hours). The concentrations of additives used were kept constant for all grades i.e at 2 phr, 0.2 phr and 0.06 phr for silane, peroxide and catalyst, respectively.

The effects of curing temperature and time on EG8150, EG8200 and LDPE are presented in Figures 2, 3 and 4 respectively. As illustrated in these figures the degree of crosslinking is controlled by the time and temperature. The degree of crosslinking increased dramatically with the increase in temperature and time of curing. Increasing temperature opens the network of polymer chain reducing density and increasing diffusion, and it also causes high mobility of water.

In Figure 2 for EG 8150, it is shown that the maximum gel content of 83% has been reached after two hours of curing in hot water at 90ºC and then no further gel content increase is observed, which means curing time of two hours at 90ºC is more than enough for this grade to reach the satisfactory degree of crosslinking. Almost similar gel content, 80%, is achieved at 60ºC for four hours of curing.

Crosslinking at 25ºC, however takes a relatively longer time (12 hours), gel content increases slightly and 50% gel content is achieved at two hours of curing, while it takes 12 hours to achieve about 72% gel content. For other metallocene catalysed grade, EG 8200 (Figure 3), the degree of crosslinking is lower compared to the other grade (EG 8150). As shown in Figure 3, the maximum gel content attained at 90ºC was 74% in two hours and 70% in four hours at 60ºC, and 65% in 12 hours at 25ºC.

Figure 2: The effect of curing time and temperature for EG8150

010

20304050

607080

90100

0 1 2 3 4 5 6 7 8 9 10 11 12

Curing Time (hours)

Gel

Conte

nt (

% )

25 Degree C

60 Degree C

90 Degree C


63

0

10

20

30

40

5060

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10 11 12

Curing Time (hours)

Gel

conte

nt (

% )

25 Degree C

60 Degree C

90 Degree C

Figure 3: The effect of curing time and temperature for EG8200

However, for LDPE as shown in Figure 4, has the lowest degree of crosslinking for all temperatures compared to the metallocene catalysed grades. For instance, for LDPE to reach the maximum gel content of 67% at 90ºC it takes four hours, and six hours at 60ºC. At ambient temperature, crosslinking of LDPE proceeds very slowly, and in 12 hours less than 40% gel content is achieved. From these results, it is clearly shown that the rate of gel formation for metallocene catalysed grades was high during the first hour of crosslinking at 90ºC water exposure, and maximum gel content of 70% was reached, while for LDPE gel content is less than 50%. Due to this LDPE requires high temperature and longer time of exposure in order to achieve higher gel content [20].

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10 11 12

Curing Time (hours)

Gel

Con

tent

( %

)

25 Degree C

60 Degree C90 Degree C

Figure 4: The effect of curing time and temperature for LDPE.

Metallocene catalysed grades have higher crosslinking performances than LDPE under the same processing conditions. This is due to different molecular structure of the resin. Metallocene catalysed grades have lower crystallinity, frequent short chain branching,


64

lower density and higher diffusion coefficient. All these aspects allow metallocene catalysed grades to have higher crosslinking rate than LDPE. Since grafted silane molecules only exist in the amorphous zone, a better silane crosslink distribution can be expected for the metallocene catalysed grades, which have lower crystallinity than LDPE, and the faster moisture diffusion into such a solid state structure will enable rapid moisture curing of metallocene grades. Similarly, Cartasegna [14] reported a higher crosslinking speed in an ethylene/propylene rubber system than in the LDPE, due to its lower crystallinity, and hence, higher diffusion rate of water.

It is interesting to note that the metallocene catalysed grades, which have similar crystallinity and density, perform differently. For instance, the EG 8150 grade achieves higher levels of crosslinking than the other grade, EG 8200. This would be attributed to difference in molecular weight and percent octene content, as shown in Table 1, EG 8150 has higher molecular weight and higher octene content than EG 8200. It is expected that the higher molecular weight resin will form a tighter physically crosslinked network than the lower molecular weight counterpart at similar crystallinity [21]. Also higher octene content means higher amount of amorphous phase in polymer, and as result, leads to a higher rate of water diffusion. The diffusion of water is faster in the amorphous domain than in the crystalline domain [22]. 3.2 Rate constant for crosslinking reaction

For determining the rate constant for all grades, crosslinking reactions were carried out at different temperatures (90, 60 and 25ºC) in water using a silane, peroxide and catalyst concentration of 2, 0.2 and 0.06 phr respectively.

Kelnar et al.[23] proposed a method to characterize the rate of the crosslinking reaction, using the slope of the straight line in dependence on the logarithm of relative gel

!"

#$%

&

'

'

0

lnGG

GGt

(

( , where !G is maximum gel content , tG is gel content at time t , and

0G is initial

gel content (zero). By using the above method and gel content against curing time plots (section 3.1), the ln of relative gel vs. crosslinking time is plotted for all grades, and the slope is termed the “rate constant”.

Figures 5, 6 and 7 present the rate constant plots for the EG 8150, EG 8200 and LDPE respectively. As shown in these figures, the rate constant is controlled by temperature, the higher the temperature the higher the rate constants. All polymer grades have different rate constants, and as expected metallocene catalysed grades have a higher rate than LDPE. For instance, EG 8150 grade has the highest rates at all temperatures, which means this grade has a higher rate of crosslinking than the other grades. Therefore, metallocene catalysed grades requires less time for crosslinking than LDPE under the same processing conditions. The rate constants values for all grades are tabulated in Table 2. This result further proves and indicates that metallocene catalysed grades have better crosslinking than LDPE.


65

y = -0.524x

y = -1.354x

y = -2.234x

-6

-5

-4

-3

-2

-1

0

0 0.5 1 1.5 2

Crosslinking Time (hours)

Ln o

f R

ela

tive G

el

25 Degree C

60 Degree C

90 Degree C

Figure 5: Linear dependence of relative gel on crosslinking time for EG 8150

y = -0.4339x

y = -1.0208x

y = -2.185x

-6

-5

-4

-3

-2

-1

0

0 0.5 1 1.5 2


Ln o

f R

ela

tive G

el

25 Degree C

60 Degree C

90Degree C

Figure 6: Linear dependence of relative gel on crosslinking time for EG 8200


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y = -0.1744x

y = -0.6448x

y = -0.8932x

-6

-5

-4

-3

-2

-1

0

0 0.5 1 1.5 2


Ln o

f R

ela

tive G

el

25 Degree C

60 Degree C

90 Degree C

Figure 7: Linear dependence of relative gel on crosslinking time for LDPE

Table 2: Rate constants for crosslinking reaction of all grades

Rate constant (min-1) Polymer grades 25ºC 60ºC 90ºC

EG 8150 0.009 0.023 0.037 EG 8200 0.007 0.017 0.036

LDPE 0.003 0.011 0.015 3.3 Influence of silane concentration on the degree of crosslinking

Figure 8 shows the effects of the silane concentration used during grafting and crosslinking reaction on the gel content. The concentrations of silane were varied from 0 to 2 phr, while concentrations of peroxide and catalyst were kept constant at 0.2 phr and 0.06 phr respectively, and also the curing temperature of 60ºC was used for a period of four hours.

As shown in Figure 8, increasing silane content increases the gel content for all grades. Hence from these results, it is clear that increasing silane content induces a very dramatic change on the degree of crosslinking and similar observations have been reported by Zhang et al [18]. At lower levels of silane concentrations, i.e 0.5 phr, the gel content achieved for EG 8150 was almost 60%, while for EG 8200 that figure was approximately 40%. On the hand, for LDPE the gel content at lower silane concentrations (0.5 phr) was less than 30%. From these results it can be seen that the metallocene catalysed grades could achieve higher gel content at lower silane concentration than LDPE.

It must be noted that increasing silane concentration beyond 1.5 phr for all grades has no significant effect of increasing the gel content. The reason for this observation is attributed to the possible homopolymerisation of the vinyl silane group reducing the effective level of silane available for grafting.


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0

10

20

30

40

50

60

70

80

0 0.5 1 1.5 2 2.5

Silane Concentrations (phr)

Gel C

onte

nt (

% )

EG 8150

EG 8200

LDPE

Figure 8: Effect of silane concentration on gel content

As expected metallocene catalaysed grades perform better than LDPE at lower silane

content. The main reason for this is, the highly chain branched metallocene catalysed grades can be turned into a high crosslink density network at low silane grafting levels more readily than LDPE due to the high degree of substitution on the polymer backbone.

On the other hand, less chain extended LDPE requires more silane grafting, and

therefore longer crosslinking time to achieve a satisfactory network structure. Therefore, due to these facts metallocene catalysed LLDPE have higher rate and higher degree of crosslinking than LDPE. 3.4 Influence of silane concentration on the mechanical properties.

The samples were prepared using different amounts of silane content, while the concentrations of peroxide and catalyst were kept constant at 0.2 phr and 0.06 phr respectively. All samples were immersed in hot water at 60ºC for a period of four hours.

The effects of silane concentration on mechanical properties are shown in Figure 9 and 10. As shown in Figure 9, tensile strength increases gradually with increasing silane concentration from zero to 1.5 phr, and reaches a maximum of 38±1.0 MPa, 34±0.85 MPa and 27±0.9 MPa for LDPE, EG 8150 and EG 8200 respectively. However, a further increase of silane concentration from 1.5 to 2.0 phr has no significant influence on tensile strength. This is consistent with the influence of silane concentration on the gel content discussed in the previous section (3.3). These results indicate that crosslinking increases tensile strength of the crosslinked polymers; this is due to the extensive network and the formation of silane crosslinking structure, which is a very stable network due to the high bonding energy of the Si-O-Si crosslink.


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0

5

10

15

20

25

30

35

40

45

0 0.5 1 1.5 2 2.5


Tensile

Str

ength

(M

Pa)

EG8150EG8200LDPE

Figure 9: Effect of silane concentration on the tensile strength for all grades.

On the other hand, Figure 10 shows the correlation between percent elongation at break and silane concentrations for all grades. As shown in that figure increasing silane concentration decreases elongation at break slightly. This is unlike the peroxide crosslinking where the presence of a crosslink network generally results in a drastic decrease in elongation at break [19].

0

200

400

600

800

1000

0 0.5 1 1.5 2 2.5


Ele

nogation a

t bre

ak (

%)

EG 8150 EG 8200LDPE

Figure 10: Effect of silane concentration on the percent elongation for all grades.


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4.0 CONCLUSIONS

Different grades of polymers (metallocene catalysed LLDPEs and LDPE) have been successfully grafted and crosslinked by using silane crosslinking technique. The performances of each grade has been evaluated and compared. From the results and discussions, the following main conclusions have been drawn: 1. Degree of crosslinking is a function of curing time and temperature, level of silane concentrations, and percent crystallinity. 2. Metallocene catalysed LLDPE have higher degree of crosslinking than LDPE under the same processing conditions. Among metallocene catalysed LLDPE grades; EG 8150 grade has higher degree of crosslinking than that of EG 8200. 3. The molecular parameters such as percent crystallinity, density, molecular weight and percent octane content were found to have a significant influence over the degree of crosslinking and physical performance of the different grades of polymers used. 4. Increasing silane concentration was found to give the optimum degree of grafting and subsequent crosslinking. However, increasing silane concentration beyond 1.5 phr has no significant change on the degree of crosslinking for all grades. 5. The rate constant for crosslinking reaction is controlled by curing temperature. The higher the temperature the higher the rate constant. Metallocene catalysed LLDPE grades have higher rate constant than LDPE at all temperatures. The range of rate constants for metallocene catalysed LLDPE grades from 25ºC to 90ºC are between 0.009 to 0.036 min-1, and those for LDPE are between 0.003 to 0.015 min-1. 6. The effects of crosslinking on the mechanical properties has been studied, and it was found that increasing silane concentration increases tensile strength for all grades and slightly decreases percent elongation.

5.0 ACKNOWLEDGMENTS

The authors wish to express their gratitude to the MM cables an Australian leading cable company for funding this research project. 6.0 REFERENCES [1] Rado, R. and Zelenak, P., 1992. Applications of crosslinked polyethylene.

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Silane Grafting