WO2014003692A1

WO2014003692A1 - Highly crosslinkable polyethylene composition

Info

Publication number: WO2014003692A1
Application number: PCT/TH2013/000027
Authority: WO
Inventors: Wannee SUTTHITAVIL; Walailuck KAMPHUNTHONG
Original assignee: Thai Plastic And Chemicals Public Co., Ltd.
Priority date: 2012-06-28
Filing date: 2013-06-27
Publication date: 2014-01-03

Abstract

This invention relates to a high crosslinkable polyethylene composition comprising a crosslinking booster 0.1-3% by weight of polyethylene composition where said crosslinking booster is a composition of triallyl cyanurate and oligomer butadiene liquid having number average molecular weight (M_n) in a range of 2,500 to 3,900. The objectives of this invention are to improve crosslinking efficiency, strength, heat resistance, elongation, processability, and active crosslinking reaction rate.

Description

HIGHLY CROSSLINKABLE POLYETHYLENE COMPOSITION

FIELD OF THE INVENTION

This invention relates to a high crosslinkable polyethylene composition.

BACKGROUND OF THE INVENTION Currently, crosslinked polyethylene homopolymer or copolymer (XLPE) has been applied in various products such as insulation for low to high voltage power cable, hot water pipes, and hoses in car engine compartments, and etc. This is due to a crosslink between polyethylene chain resulting in crosslinked structure with good mechanical strength and heat resistance. The crosslinked polyethylene can be prepared by three methods, which are: radiation crosslinking, silane crosslinking, and peroxide crosslinking. There has been an attempt to study composition for preparing crosslinked polyethylene as disclosed in patents.

Sweden Patent no. 199703844 discloses a composition for an electric cable, which comprises polyethylene or polyethylene copolymer, and various additives, such as peroxide agent and antioxidant (especially 2,2,6,6-tetramethylpiperidine substituted nitrogen atom). The amount of the additive does not exceed 5% by weight of the composition.

WO2008/097732 discloses polyethylene composition having crosslinking coagents selected from the group consisting of alkadienes, alkatridienes, alkatetradienes, and combinations thereof; including a method of making the same.

US Patent No. 6,143,822; 6,187,847; 6,228,917; and 6,656,986 disclose polyethylene composition having an antioxidant and a scorch retardant. For example, thioester or hindered amine is used as the antioxidant, and 1 , 1 -diphenyl ethylene (substituted or unsubstituted) or 4,4'- thiobis (2-methyl-6-t-butylphenol) or 2,2'-thiobis (6-t-butyl-4-methylphenol) is used as the scorch retardant. This crosslinked polyethylene can be prepared at high temperatures providing high productivity without any crosslinking occurrence during process. In addition, it provides high crosslink density and good stability property against ageing.

However, there are some limitations in processing crosslinked polyethylene composition, such as narrow processing temperature range, i.e. the processing temperature should not cause crosslinking reaction of polyethylene chain during mixing and/or processing. Moreover, the applied screw speed should not cause insufficient melting of polyethylene, or shear heating, which leads to a higher accumulated heat in the mixing chamber than the desired temperature. This is because the accumulated heat may cause crosshnking during mixing and/or processing. These processing conditions affect productivity, crosslinked structure of polyethylene, and properties of crosslinked polyethylene.

SUMMARY OF THE INVENTION

The objective of this invention is to develop polyethylene homopolymer or copolymer composition using peroxide crosshnking agent in order to improve crosshnking efficiency and processability. This processability is similar to neat polyethylene, i.e., having a wider processing temperature range, no crosshnking occurence during extrusion or injection. This leads to a good processability, high crosshnking reaction rate, and also better properties in term of strength, hot set and elongation.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a graph of torque versus time during mixing the crosslinked polyethylene by internal mixer at 120°C.

Figure 2 shows a graph of energy versus time during mixing the crosslinked polyethylene by internal mixer at 120°C.

Figure 3 shows a graph of torque versus time during mixing the crosslinked polyethylene by internal mixer at 160°C.

DETAILED DESCRIPTION OF THE EMBODIMENTS

These and other aspects, features and advantages will become apparent to those of ordinary skill in the art by reading the following detailed description, the drawings, and the appended claims.

This invention relates to a high crosslinkable polyethylene composition using peroxide crosshnking agent comprising:

(a) a polyethylene homopolymer or copolymer (PE) in an amount of 90-98% by weight of polyethylene composition where said polyethylene in this invention can be Low Density Polyethylene (LDPE) and/or Linear Low Density Polyethylene (LLDPE). Using PE less than 90% by weight, the obtained crosslinked polyethylene (XLPE) shows a decrease in tensile strength due to excessive crosslink density. Conversely, if using PE greater than 98% by weight, the obtained crosslinked polyethylene (XLPE) shows a decrease in tensile strength and hot set due to insufficient crosslink density to absorb force and resist heat distortion;

(b) a peroxide crosslinking agent in an amount of 1 -3% by weight of polyethylene composition where said peroxide crosslinking agent in this invention is a variety of alkyl peroxide, such as dicumyl peroxide (DCP). If using the crosslinking agent less than 1% by weight, the obtained crosslinked polyethylene (XLPE) shows a decrease in tensile strength and hot set due to insufficient crosslink density to absorb force and resist to the heat distortion. Conversely, if using the crosslinking agent greater than 3% by weight, the obtained crosslinked polyethylene (XLPE) shows a decrease in tensile strength due to excessive crosslink density. Moreover, it is possible for crosslinking occurrence during mixing and/or processing of crosslinked polyethylene composition; and

(c) a crosslinking booster, which is a compound having an active functional group chemically reacted with free radical at least one position. The appropriate crosslinking booster in this invention is a composition of two substances, which are:

- triallyl cyanurate, which is an accelerator for peroxide crosslinking system. As the cyanurate group acting as the electron withdrawing group, the double bond, to which is adjacent, provides a high active toward crosslinking reaction; and

- oligomer butadiene liquid with number average molecular weight (M_n) in a range of 2,500 to 3,900. It acts as an accelerator because it is more active in crosslinking reaction than polyethylene chain. Moreover, it also acts as the processing aid agent providing a wider processing temperature range and product appearance.

By using a crosslinking booster in an amount of 0.1-3%) by weight of polyethylene composition where the ratio of triallyl cyanurate to oligomer butadiene liquid is not less than 1 :8 and not greater than 1 :4 by weight. If using the amount of crosslinking booster less than 0.1% by weight and ratio less than 1 :8, no significant change in obtained polyethylene properties is observed in both mixing ability and/or processability. However, if using the amount of crosslinking booster greater than 3% by weight, this may lead to crosslinking occurrence during mixing and/or processing. Furthermore, if the ratio is greater than 1 :4, this will cause problem in preparation of crosshnked polyethylene due to excess of oligomer butadilene liquid, and also a migration of such substance to sample's surface as enviromental change such as at higher temperature.

The high crosslinkable polyethylene composition using peroxide crosslinking agent may further comprise an additive, such as scorch retardant, antioxidant, filler or a combination thereof in an amount of 0-5% by weight of polyethylene composition. Examples of suitable filler in this invention are Gumfree B52 and carbon black.

The high crosslinkable polyethylene composition according to this invention has an important development in selecting type and amount of crosslinking booster. This reveals significant advantages, which are an active to chemical reactions with free radicals, enhancing crosslinking site, increasing a balance of the crosshnked structure and obtained properties of crosshnked polyethylene. The ultimate outcome is that the crosshnked polyethylene composition has high crosslink density compared to the one without said composition at the same crosslinking reaction time. This means that the crosshnked polyethylene composition according to this invention can withstand high service temperature and also provide suitable mechanical strength and flexibility. As a result, it can also be used for flexible product applications. Another important advantage of this invention is a presence of oligomer butadiene liquid, which assists mixing in each processing steps such as mixing and/or processing, reducing viscosity of the molten polyethylene composition. Hence, the composition can flow easier and the used energy for mixing decreases.

Examples of crosshnked polyethylene preparation system compared among the system having a composition of both crosslinking booster, the system having one crosslinking booster, and the system without crosslinking booster are shown below.

Sample 1 is a preparation of polyethylene composition having the crosslinking booster, which is a composition of triallyl cyanurate and oligomer butadiene liquid. The ratio of triallyl cyanurate to oligomer butadiene liquid equals to 1 :4 where the composition comprises:

1. polyethylene or polyethylene copolymer in an amount of 98.1% by weight of polyethylene composition;

2. dicumyl peroxide in an amount of 1.2% by weight of polyethylene composition; 3. crosslinking booster, which is a composition between triallyl cyanurate and oligomer butadiene liquid. The ratio of triallyl cyanurate to oligomer butadiene liquid equals to 1 :4 in an amount of 0.5% by weight of polyethylene composition; and

4. other additives in an amount of 0.2% by weight of polyethylene composition.

Sample 2 is a preparation of polyethylene composition having the crosslinking booster, which is a composition of triallyl cyanurate and oligomer butadiene liquid. The ratio of triallyl cyanurate to oligomer butadiene liquid equals to 1 :8 where the composition comprises:

1. polyethylene or polyethylene copolymer in an amount of 98.1 % by weight of polyethylene composition;

2. dicumyl peroxide in an amount of 1.2% by weight of polyethylene composition;

3. crosslinking booster, which is a composition of triallyl cyanurate and oligomer butadiene liquid. The ratio of triallyl cyanurate to oligomer butadiene liquid equals to 1 :8 in an amount of 0.5% by weight of polyethylene composition; and

4. other additives in an amount of 0.2% by weight of polyethylene composition.

Sample 3 is a preparation of polyethylene composition having only one type of the crosslinking booster, which is a triallyl cyanurate, where the composition comprises:

2. dicumyl peroxide in an amount of 1.2% by weight of polyethylene composition;

3. triallyl cyanurate in an amount of 0.5% by weight of polyethylene composition; and

4. other additives in an amount of 0.2% by weight of polyethylene composition."

Sample 4 is a preparation of polyethylene composition without crosslinking booster where the composition comprises:

1. polyethylene or polyethylene copolymer in an amount of 98.6%» by weight of polyethylene composition; 2. dicumyl peroxide in an amount of 1.2% by weight of polyethylene composition; and

3. other additives in an amount of 0.2% by weight of polyethylene composition.

The composition can be prepared via single and/or twin screw extruder by mixing polyethylene or polyethylene copolymer, dicumyl peroxide, crosslinking booster, and other additives at mixing temperature between 100 and 180°C.

Experiment 1 shows a comparison of mixing ability of crosslinked polyethylene composition for the samples 1 - 4 at 120°C. A change of torque, measured temperature in mixing compartment, and energy were further investigated.

From Figure 1, the curves of samples 1 and 2 (i.e. the system having the composition of crosslinking booster) illustrates similar trend to the one of sample 4 (i.e. the system having no crosslinking booster). This implies that the composition of crosslinking booster does not affect mixing ability of crosslinked polyethylene composition, and also does not accelerate the crosslinking reaction to occur before the specified time. This indicates an advantage over usage of only one type of the crosslinking booster (as shown in sample 3), which exhibits a change in torque during mixing, i.e. the scorch time (T_s) of sample 3 is less than others. By using the composition of crosslinking booster, it also illustrates less energy consumption during mixing (Em) than other systems (as shown in Figure 2). Properties of all samples prepared in Experiment 1 according to this inventio illustrates in Table 1.

Table 1 Comparative properties of all samples prepared in Experiment 1 according to this invention

Processability Crosslinking efficiency Gel content

Crosslinking time at at l20 °C at l60 °C

Sample 160 °C

T_s E_m Crosslinking Crosslinking Crosslinking

5 10 15 30

(min) (kJ) rate (N.m.mn ') density stability

Sample 1 17.3 80 24 51 4 70 85 88 89

Sample 2 17.3 73 28 55 4 78 88 95 95

Sample 3 14.2 89 N/A N/A N/A 0 80 88 95

Sample 4 17.3 91 15 50 11.9 0 30 60 67 Experiment 2 shows a comparison of crosslinking ability of crosslinked polyethylene composition for the samples 1 - 4 at 160°C for 15 mins. A change of torque, measured temperature in mixing compartment, and energy were further investigated.

From Figure 3, it shows that the samples 1 and 2 have a faster crosslinking rate than samples 3 and 4, i.e. the system having the composition of crosslinking booster exhibits greater crosslinking acceleration than the one having only one type of crosslinking booster or the one without crosslinking booster. Besides, it shows a better stability of crosslinking structure as presented in Table 1.

Experiment 3 shows a comparison of gel content of crosslinked polyethylene composition according to ASTM D2765. To observe a change in gel content of crosslinked polyethylene, the experiment was conducted using compression moulding at 160°C for 5, 10, 15 and 30 mins, respectively.

From Experiment 3, solely 5 mins after crosslinking reaction, the samples 1 and 2 show gel content greater than 70% whereas no gel occurs in the Sample 3 (i.e. the system having only one type of crosslinking booster) and the sample 4 (i.e. the system having no crosslinking booster). That is, the system having the composition of crosslinking booster has greater crosslinking density and more active in crosslinking reaction than other systems as presented in Table 1.

Experiment 4 shows a comparison of properties of crosslinked polyethylene composition, which are mechanical properties before/after ageing, hot set, shrinkage, and physical property according to international standard in Table 2.

Table 2 Comparative mechanical and physical properties of all Samples according to this invention

Properties

Sample Tensile Elongation Hot set Shrinkage Surface

strength (MPa) (%) (%) (%) appearance

Sample 1 280 · 590 : 30 smooth

Sample 2 290 588 25 0 smooth

Sample 3 247 545 25 0 not smooth

Sample 4 260 600 65 0 not smooth The samples 1 and 2 provide better thermal resistance more than 50% compared to the sample 4 (lower hot set value indicates better resistance in heat distortion). Moreover, when comparing the same hot set value i.e. sample 2 and sample 3, it is found that sample 2 (i.e. the system having the composition of crosslinking booster) has a higher tensile strength and elongation than sample 3 (i.e the system having only one type of crosslinking booster) nearly 20% and 10%, respectively. This indicates that the obtained crosslinked structure using the composition of crosslinking booster has a better flexibility and strength than the obtained crosslinked structure using only one type of crosslinking booster.

It will be appreciated that, although specific embodiments of the present disclosure have been described herein for purposes of illustration, various modifications or alteration may be made without departing from the spirit and scope of the disclosure. Accordingly, the disclosure is not limited except as by the appended claims.

Claims

CLAIMS What is claimed is:

1. A high crosslinkable polyethylene composition comprising:

(a) a polyethylene in an amount of 90-98% by weight of polyethylene composition;

(b) a peroxide crosslinking agent in an amount of 1-3% by weight of polyethylene composition; and

(c) a crosslinking booster characterized in that it is a composition of triallyl cyanurate and oligomer butadiene liquid and is present in an amount of 0.1-3% by weight of polyethylene composition.

2. The high crosslinkable polyethylene composition according to claim 1 , further comprising an additive 0.1-5% by weight of polyethylene composition where said additive is selected from the group consisting of scorch retardant, antioxidant, filler or ¾ combination thereof.

3. The high crosslinkable polyethylene composition according to claim 1 or 2, wherein the polyethylene additive is selected from the group consisting of low density polyethylene (LDPE), linear low density polyethylene (LLDPE), or copolymer of low density polyethylene (LDPE) and linear low density polyethylene (LLDPE).

4. The high crosslinkable polyethylene composition according to one of any preceding claim 1 to .3, wherein the peroxide crosslinking agent is dicumyl peroxide (DCP),

5. The high crosslinkable polyethylene composition according to one of any preceding claim l to 4, wherein number average molecular weight (M„) of said oligomer butadiene liquid is in a range of 2,500 to 3,900.

6. The high crosslinkable polyethylene composition according to one of any preceding claim 1 to 5, wherein ratio of said triallyl cyanurate to said oligomer butadiene liquid is not less than 1:8 and not greater than 1 :4 by weight.