WO2000040386A1

WO2000040386A1 - Rotomoulded polyethylene based composition

Info

Publication number: WO2000040386A1
Application number: PCT/EP1999/010433
Authority: WO
Inventors: Jacques Arnoux; Gérard MARCHIONINI; Josiane Rebollo
Original assignee: Bp Chemicals Limited; Bp Chemicals Snc
Priority date: 1999-01-05
Filing date: 1999-12-23
Publication date: 2000-07-13
Also published as: AU2539600A; FR2788063A1

Abstract

The invention relates to a rotomoulded polyethylene composition comprising a mixture of at least two different polyethylenes, characterized in that the first polyethylene consists of a polyethylene powder with an average particle size of less than 1 mm, the second polyethylene consists of polyethylene particles with an average particle size of more than 1.5 mm and the weight ratio between the first and second polyethylene in said composition is between 0.5 and 20.

Description

ROTOMOLDED COMPOSITION BASED ON POLYETHYLENE

The present invention relates to a rotational molded composition based on polyethylene. In particular, the present invention relates to a rotomolded composition based on polyethylene comprising a mixture of two different polyethylenes. The present invention also relates to a method for preparing rotational molding of a polyethylene composition according to the present invention.

The prior art describes rotomolded polyethylene compositions. For example, patent US5530055 describes a rotational molded composition comprising a mixture of polyethylene resin comprising (a) a small proportion of LLDPE with a density of 0.915-0.925 g / cc and a melt flow index of 12 g / 10 min. or less ; (b) a high proportion of polyethylene having a density of 0.93-0.945 g / cc and a melt index of 3-8 g / 10 min .; and (c) a small proportion of polyethylene of relatively high density (0.950-0.965 g / cc) and of melt index of 15 g / 10 min. or more; and an effective amount of nucleating agent to improve impact resistance.

The present invention results from the unexpected development of a rotational molded composition allowing, among other things, to respond to particular applications. Among these applications, mention will be made in particular of rotomolded hollow articles whose soundproofing and / or attachment characteristics to expandable plastic foams, such as for example polyurethane or polystyrene, are improved.

The present invention therefore relates to a rotomolded polyethylene composition comprising a mixture of at least two different polyethylenes characterized in that the first polyethylene consists of a polyethylene powder of size average particle size less than 1 mm, in that the second polyethylene consists of polyethylene particles of average particle size greater than 1.5 mm, and in that the weight ratio between the first and second polyethylene entering said composition is preferably between 0.5 and 20. Preferably, the present invention relates to a rotomolded composition based on polyethylene comprising a mixture of at least two different polyethylenes characterized in that the first polyethylene consists of a polyethylene powder of smaller average particle size at 1 mm and with a hot melt index (load of 2.16 kg / standard ISO 1133 - 1991: condition 4) between 1.5 and 15, in that the second polyethylene consists of polyethylene particles of size average particle size greater than 1.5 mm and hot melt index (load of 2.16 kg / standard ISO 1133 - 1991 condition 4) less than 1.5, in what the ratio of viscosities for a shear rate of 10 s ^"1 (measured according to standard ASTM-D3835-96, on a ROSAND capillary rheometer for a temperature of 190 ° C., a die diameter of 1.0 mm, a length / diameter ratio of 16 and an angle of 180 °) from the second to the first polyethylene is between 1.5 and 8, and in that the weight ratio between the first and the second polyethylene entering said composition is between 0.5 and 20, preferably between 1 and 5.

The first polyethylene therefore consists of a polyethylene powder of average particle size less than 1 mm and of hot melt index (load of 2.16 kg / standard ISO 1133 - 1991: condition 4) between 1.5 and 15 , preferably between 4 and 6. Preferably, the polyethylene powder has a particle size distribution such that more than 75% by weight of the powder has a particle size between 0.3 and 2 times the average particle size of the polyethylene powder. Preferably, the polyethylene powder has a particle size distribution such that more than 30% by weight, preferably more than 60% by weight, ideally more than 80% by weight of the powder has a size between 0.2 and 0.4 mm (ASTM D 1921-89 standard).

The second polyethylene therefore consists of polyethylene particles of average particle size greater than 1.5 mm and of melt flow index (load of 2.16 kg / standard ISO 1133 - 1991: condition 4) less than 1.5 , preferably between 0.1 and 1. Preferably, the polyethylene particles have a size distribution particle size such that more than 90% by weight of the particles has a particle size between 0.5 and 1.5 times the average particle size of the polyethylene particles. Preferably, the polyethylene particles have a particle size distribution such that more than 60% by weight, preferably more than 85% by weight, ideally more than 95% by weight of the particles have a size of between 2 and 4 mm ( ASTM D 1921-89).

These polyethylene particles can, for example, come from regrind polyethylene or, preferably, from polyethylene granules.

The Applicant has unexpectedly found that the rotational molded compositions in accordance with the present invention have soundproofing and / or bonding properties with expandable plastic foams, such as for example polyurethane or polystyrene, which are particularly improved, while also preserving their other physical characteristics such as impact or tensile strength.

The present invention also consists of a rotomolded part, the composition of which is based on polyethylene and comprises a mixture of at least two different polyethylenes and is characterized in that the first polyethylene consists of a polyethylene powder of average particle size less than 1 mm, in that the second polyethylene consists of polyethylene particles of average particle size greater than 1.5 mm, and in that the weight ratio between the first and the second polyethylene entering said composition is between 0.5 and 20, preferably between 1 and 5.

Preferably, the present invention also consists of a rotomolded part, the composition of which is based on polyethylene and comprises a mixture of at least two different polyethylenes and is characterized in that the first polyethylene consists of a polyethylene powder of medium particle size size less than 1 mm and of melt flow index (load of 2.16 kg / standard ISO 1133 - 1991: condition 4) between 1.5 and 15, in that the second polyethylene consists of polyethylene particles of average particle size greater than 1.5 mm and hot melt index (load of 2.16 kg / standard ISO 1133 - 1991 condition 4) less than 1.5, in that the ratio of viscosities for a rate of shear of 10 s ^"1 (measured according to standard ASTM-D3835-96, on a ROSAND capillary rheometer for a temperature of 190 ° C., a die diameter 1.0 mm, a length / diameter ratio of 16 and an angle of 180 °) from the second to the first polyethylene is between 1.5 and 8, and in that the weight ratio between the first and the second incoming polyethylene in said composition is between 0.5 and 20, preferably between 1 and 5. Preferably, the internal face of the rotomolded part has a specific surface at least 50%, preferably at least 80% greater than that which would have a rotomolded part made up only of the first polyethylene as described above.

In fact, the surface of the internal wall of the part obtained by rotational molding preferably has roughness of several millimeters whose appearance is reminiscent of chips whose implantation and orientation are carried out in a completely random manner.

Photographs of products obtained in accordance with the present invention are reproduced in Figures 1 to 3 attached.

This increase in specific surface area of the internal wall of the rotomolded part is very useful for many applications including, among other things, noise reduction and the attachment of expanded products sometimes used to obtain insubmersibility or thermal insulation.

Successful rotational molding is characterized by a perfect distribution of these chips over the entire internal surface of the rotational molded part. As for the external surface of the rotomolded part, it has an appearance similar to that which a rotomolded part made up of the first polyethylene would have.

The rotational molding composition in accordance with the present invention may also include the usual additives used in rotational molding such as crosslinking agents, swelling agents, nucleating agents and / or manufacturing aids such as antioxidants, UN stabilizers, neutralizing agents , mold release agents and anti-static agents.

The rotational molding composition in accordance with the present invention may also comprise the usual additives used in rotational molding such as crosslinking agents, swelling agents, nucleating agents and / or manufacturing aids such as antioxidants, UV stabilizers, neutralizing agents , mold release agents, pigments and antistatic agents. Preferably, the rotomolded composition according to the present invention will be free from any swelling agent. An exemplary embodiment of the rotomolded objects according to the present invention consists in loading the polyethylene mixture of the invention inside a mold. Two arms supporting the mold thus loaded allow the mold to be animated with a double rotational movement. At the same time, the assembly is introduced into an oven which heats the polyethylene mixture to a temperature higher than the melting temperature of the two polyethylene as measured according to standard ASTM D2117 in order to melt the polyethylene mixture. The polyethylene mixture is distributed homogeneously on the walls of the mold mainly by gravity and / or runoff. At the end of the time necessary for this operation, the mold is, for example, sent, still in rotation, to a cooling enclosure where it is subjected to:

1. an air blower

2. a sprinkling of water

3. an air blower again

The mold finally leaves the machine and the part is removed from the mold manually. The time and temperature conditions used obviously depend on the thickness of the rotomolded product and the thermal sensitivities of each of the polyethylenes.

Preferably, according to the present invention, the rotational molding temperature is between 250 and 300 ° C and the rotational molding time is between 2 and 4 minutes per mm of thickness of the product to be rotomoulded. The following examples illustrate the present invention.

Examples

A rotomoulded part is produced which comprises 77.8% by weight of a ground powder of medium density linear polyethylene copolymer and 22.2% by weight of average molecular weight polyethylene particles. The ground powder of linear medium density polyethylene copolymer has an average particle size of 0.3 mm; more than 80% by weight of the powder has a particle size between 0.12 and 0.5 mm; this polyethylene is also characterized by a hot melt index (load of 2.16 kg / standard ISO 1133 -

1991: condition 4) of 4. The average molecular weight polyethylene particles have an average particle size of 3.25 mm; more than 95% by weight of the particles have a particle size between 2.5 and 4 mm; this polyethylene is also characterized by a hot melt index (load of 2.16 kg / standard ISO 1133 - 1991: condition 4) of 0.2.

Rotational molding is carried out in accordance with the description given above with the following conditions:

- cooking temperature: 270 ° C.

- cooking time: 15 minutes.

- cooling time: 4 minutes (air) + 3 minutes (water) + 8 minutes (air).

- 1400 g of powder; 400 g of particles. The thickness of the rotomolded tank obtained is approximately 4.3 mm when the roughness represented by the granules is not taken into account.

In order to measure the soundproofing quality of the rotomolded tank thus obtained, we use the internal method described as follows: We use a sound level meter (example: type 2225 from the brand Bruel Kajer) in position "peak measurement" which gives the sound intensity in dB "at the top of the noise peak".

A circular plate 15 cm in diameter is cut in the upper part (cover) of the tank. The tank is placed on a glass wool plate. A timer is placed under the tank. The measurement of the ringing noise of this timer is made with the sound level meter placed at approximately 50 cm, at the level of the tank (see diagram in Figure 4). The noise measurement carried out on the rotomoulded part according to the present invention indicates a value of 82 dB.

By way of comparison, the noise measurement carried out on a reference rotational molded tank, consisting solely of the polyethylene powder described above and obtained in an identical manner to the method described in the example above, indicates a value of 86 dB, a loss of almost 4 dB.

Claims

claims

1. Rotational molded composition based on polyethylene comprising a mixture of at least two different polyethylenes characterized in that the first polyethylene consists of a polyethylene powder of average particle size less than 1 mm and of melt flow index (load of 2.16 kg / ISO standard 1133 - 1991: condition 4) between 1.5 and 15, in that the second polyethylene consists of polyethylene particles of average particle size greater than 1.5 mm and of melt index hot (load of 2.16 kg / standard ISO 1133 - 1991: condition 4) less than 1.5, and in that the weight ratio between the first and second polyethylene entering said composition is between 0.5 and 20.

2. A rotomolded composition according to claim 1 characterized in that the viscosity ratio for a shear rate of 10 s ^"1 (measured according to standard ASTM-D3835-96, on a ROSAND capillary rheometer for a temperature of 190 ° C, a die diameter of 1.0 mm, a length / diameter ratio of 16 and an angle of 180 °) from the second to the first polyethylene is between 1.5 and 8.

3. Rotomolded composition according to claim 2 characterized in that the weight ratio between the first and the second polyethylene used in said composition is between 1 and 5.

4. Rotomolded composition according to any one of the preceding claims, characterized in that the first polyethylene consists of a polyethylene powder with an average particle size less than 1 mm and with a hot melt index (load of 2.16 kg / standard) ISO 1133 - 1991: condition 4) between 4 and 6.

5. rotomolded composition according to claim 4 characterized in that the first polyethylene consists of a polyethylene powder whose particle size distribution is such that more than 75% by weight of the powder has a particle size between 0.3 and 2 times the average particle size of the polyethylene powder.

6. rotomolded composition according to claims 4 or 5 characterized in that the first polyethylene consists of a polyethylene powder whose particle size distribution is such that more than 30% by weight, preferably more than 60% by weight, ideally more 80% by weight of the powder has a size between 0.2 and 0.4 mm (standard ASTM D1921-89).

7. rotomolded composition according to any one of the preceding claims, characterized in that the second polyethylene consists of polyethylene particles with an average particle size greater than 1.5 mm and with a hot melt index (load of 2.16 kg / ISO standard 1133 - 1991: condition 4) between 0, 1 and 1.

8. A rotomolded composition according to claim 7 characterized in that the second polyethylene consists of polyethylene particles whose particle size distribution is such that more than 90% by weight of the particles has a particle size between 0.5 and 1, 5 times the average particle size of the polyethylene particles.

9. A rotomold composition according to claims 7 or 8 characterized in that the polyethylene particles have a particle size distribution such that more than 60% by weight, preferably more than 85% by weight, ideally more than 95% by weight of the particles have a size between 2 and 4 mm (standard ASTM D 1921-89).

10. Rotomolded composition according to any one of the preceding claims, characterized in that it is free from any swelling agent.

11. Rotomolded part, the polyethylene-based composition according to any one of the preceding claims.

12. Rotomolded part according to claim 11 characterized in that the internal face of the rotomolded part has a specific surface at least 50%, preferably at least 80% greater than that would have a rotomolded part consisting only of the first polyethylene (face external).