Multilayer rotational moulding

Abstract

A multilayer article prepared by rotational moulding can include a layer A that is a polyolefin (PO) based layer prepared from PO and a functionalised polyolefin (FPO), or a PO grafter to a non-polyolefin (NPO) in the form of a block copolymer (PO-g-NPO). A layer B can be a PO that is dissimilar from layer A, an NPO, a mixture thereof, and optionally an FPO or a PO-g-NPO. A layer C can be dissimilar from layer A and similar to or dissimilar from layer B. Layer C can be adjacent to layer A and/or layer B, can have good adhesion to layer A and/or layer B, and is not a blend of layer A and layer B. A method of preparing the multilayer article can include rotational moulding.

Claims

1. A multilayer article prepared by rotational molding comprising: (a) an outer layer prepared from a polyethylene composition comprising at least 40 wt. % of a first polymer component comprising a non-functionalized polyethylene and from 2 wt. % to 60 wt. % of a second polymer component wherein the second polymer component of the outer layer comprises a polyolefin (PO) grafted to a non-olefinic polymer (NPO) in the form of ((PO)g-NPO) and wherein the second polymer component comprises a functionalized polyethylene having reactive functional units with acid functions or corresponding salt or ester functions or with maleic anhydride functions; and (b) an inner layer prepared from a composition comprising a polyamide, wherein the polyamide is present in an amount ranging from 75 to 100 wt % of said composition of the inner layer, wherein the melting point of the polyethylene composition is at least 15 C. lower than the melting point of the polyamide, wherein the melting point of the polyethylene composition is less than 145 C., and wherein the melting point of the polyamide is at least 145 C.

2. The multilayer article of claim 1, wherein the ((PO)g-NPO) is polyolefin grafted to polyamide.

Description

LIST OF FIGURES

(1) FIGS. 1 to 5 are respectively the rotolog traces of examples 1 to 4 and of example 7. They each represent the peak internal air temperature (PIAT) expressed in degrees centigrade as a function of the cycle time expressed in seconds.

(2) FIG. 6 represents a two-skinned rotomoulded tank having an outer layer prepared from pure mPE and an inner layer prepared from PA-11.

(3) FIG. 7 represents micropellets of PA-11 resin obtained in conditions A.

(4) FIG. 8 represents micropellets of PA-11 resin obtained in conditions B.

Example 1

(5) A shot weight of 1.8 kg was used to produce a single skinned 3 mm thick moulding of a polyethylene composition consisting of 80 wt % of a polyethylene prepared with bis(n-butyl-cyclopentadienyl) zirconium dichloride and having a density of 0.940 g/cm.sup.3 and a melt flow index MI2 of 9 g/10 min, and of 20 wt % of a polyethylene gradted with anhydride sold under the name OREVAC by ATOFINA. A PIAT of 212 C. was achieved and the cycle time was 36.42 minutes. The rotolog trace is shown in FIG. 1.

Example 2

(6) A shot weight of 1.85 kg was used to produce a 3 mm thick single skinned moulding of a PA-11 resin sold by ATOFINA under the name RILSAN. As Nylon melts at a higher temperature than polyethylene, a PIAT of 239 C. was achieved and the cycle was completed in 48.9 minutes. The rotolog trace is shown in FIG. 2.

Example 3

(7) A two skinned moulding was produced as follows: a shot weight of 1.8 kg of the same polyethylene composition as in Example 1 was first dropped in the mould to constitute the outer layer; a shot weight of 1.8 kg of the same PA-11 resin as in example 2 was dropped in at a temperature of 133 C. to produce the inner layer.

(8) A PIAT of 245 C. was achieved and the cycle was completed in 63.7 minutes. The rotolog trace is shown in FIG. 3. The bond between the two layers was very good and no delamination was observed.

Example 4

(9) The same two skinned moulding as that of Example 3 was used except that the temperature at which the PA-11 resin was dropped in was of 170 C. instead of 133 C.

(10) A PIAT of 243 C. was achieved and the cycle was completed in 68.12 minutes. The rotolog trace is shown in FIG. 4. The bond between the two layers was very good over the flat surfaces. A small degree of delamination was observed at the sharp corners of the mouldings where the Nylon inner layer appears to have contracted more than the polyethylene composition during cooling.

Example 5

(11) A two skinned moulding was produced as in example 4 except that the shot weights of the polyethylene composition and of the PA-11 resin were both of 1.2 kg to produce two 2 mm layers. The PA-11 material was dropped into the moulding at 133 C. The moulding reached a PIAT of 241 C. and the cycle was completed in 45.57 minutes.

Example 6

(12) Example 5 was repeated except that the PA-11 resin was dropped into the moulding at a temperature of 175 C. instead of 133 C. The moulding reached a PIAT of 245 C. ant the cycle was completed in 50.12 minutes.

(13) These examples are summarised in Table I.

(14) TABLE-US-00001 TABLE I Mass Mass PA-11 Cycle PE PA-11 drop T PIAT time Example (kg) (kg) ( C.) ( C.) (min) 1 1.8 212 36.4 2 1.8 239 48.9 3 1.8 1.8 133 245 63.7 4 1.8 1.8 170 243 68.2 5 1.2 1.2 133 241 45.6 6 1.2 1.2 175 245 50.1

(15) For all the two skinned mouldings prepared according to the present invention, the adhesion between the polyethylene layer and the polyamide layer was excellent on flat surfaces. A minor amount of delamination was observed at the sharp angles in some of the tests.

Example 7

(16) A two skinned moulding was produced as follows: a shot weight of 1.8 kg of a pure polyethylene prepared with bis(n-butyl-cyclopentadienyl) zirconium dichloride and having a density of 0.934 g/cm.sup.3 and a melt flow index MI2 of 8.0 g/10 min was first dropped in the mould to constitute the outer layer; a shot weight of 1.8 kg of a pure metallocene-produced isotactic polypropylene (miPP) prepared with a bis-indenyl metallocene catalyst system and having a melt flow index MI2 of 15 g/10 min, was dropped in at a temperature of 133 C. to produce the inner layer.

(17) A PIAT of 245 C. was achieved and the cycle was completed in 63.7 minutes. The bond between the two layers was very good and no delamination was observed even though no functionalised polyolefin was used.

Comparative Example

(18) A two-skinned rotomoulded article was prepared under the same conditions as those of examples 1 to 7. The outer layer was a pure polyethylene prepared with ethylene-bis(tetrahydro-indenyl) zirconium dichloride and having a density of 0.934 g/cc and a melt index MI2 of 8 dg/min and the inner layer was PA-11. The finished article showed no adhesion at all between the two layers as can be seen in FIG. 6 representing a two-skinned rotomoulded tank having an outer layer prepared from pure PE and an inner layer prepared from PA-11.

Example 8

(19) A double skinned article was prepared from 1.8 kg of a blend comprising 85 wt % of polyethylene prepared with ethylene-bis(tetrahydro-indenyl) zirconium dichloride and 15 wt % of OREVAC and 1.8 kg of PA-11. PA-11 was dropped at a temperature of 150 C. and a PIAT of 219 C. was achieved. Forced air was used to cool the system from the starting temperature down to a temperature of 170 C., followed by water spray cooling at level 3 (18 litres/min) down to room temperature.

Examples 9, 10 and 11

(20) Example 8 was repeated except that water spray cooling was used from the start of cooling respectively at level 1 (6 litre/min) for example 9, at level 2 (12 litres/min) for example 10 and at level 3 (18 litres/min) for example 11.

(21) The PIAT and shrinkage results are summarised in Table II.

(22) TABLE-US-00002 TABLE II Example PIAT C. shrinkage % 8 219 1.57 9 214 1.67 10 216 1.65 11 216 1.87

(23) In these examples, the layers all showed excellent adhesion to one another and could not be separated by hand. Examples 8 and 9 however showed some slight shrinkage voids at the sharp corners of the finished product thereby indicating the importance of the cooling rate. Adequate mould design can also reduce the amount of shrinkage voids.

Example 12

(24) Two rotomoulded articles having a wall thickness of 6 mm were prepared. The first article had a 6 mm thick single layer and was prepared from polyethylene alone and cooled down with forced air. The cycle time was of 58 minutes. The second article had a 3 mm thick external layer prepared from the same PE as the other article and a 3 mm thick internal layer prepared from PA-11 and it was cooled down with water spray. The cycle time was of 42 minutes, therefore much shorter than that of the single layer article cooled down with forced air. The use of water spray to cool down the system was thus observed to be very beneficial for reducing the cycle time. Additionally, it was observed that the dimensional properties were essentially the same for one or two-layer articles: the shrinkage for both articles was of from 1.5 to 2%.

Example 13

(25) Micropellets of PA-11 have been successfully obtained from normal pellets (3.2 mm) with the Gala underwater system called Brabender/Leistritz/Maag/A5 PAC/TWS. It consists in a twin-screw extruder with A5 PAC 6 pelletiser, a gear pump and a molten polymer filtration.

(26) The experimental conditions were the as follows: 1) Conditions A: screw speed: 180 rpm, melt temperature: 250 C., feed rate: 100 kg/h, pelletizer speed: 4650 rpm, 24 holes of 0.8 mm diameter. 2) Conditions B: screw speed: 180 rpm, melt temperature: 250 C., feed rate: 100 kg/h, pelletizer speed: 4500 rpm, 120 holes of 0.65 mm diameter.

(27) The morphological features of the micropellets obtained in conditions A and B are illustrated in FIGS. 7 and 8. Average micropellet sizes were of from 1000 to 1200 m for conditions A and of from 800 to 1000 m for conditions B. The corresponding sieve results are: 1) for conditions A: S850: 99.6% S600: 0.4% 2) for conditions B: S850: 89.4% S600: 10.6%

(28) These micropellets of PA-11 were then successfully used for preparing two-layer rotomoulded articles. The external layer was produced from blends of metellocene-prepared polyethylene and OREVAC wherein the amount of OREVAC varied from 10 to 20 wt % and the internal layer was prepared from these micropellets of PA-11.

(29) Some additional experiments on two-layer rotomoulding were carried out using a system wherein the polyethylene was provided under the form of a powder and the polyamide was provided as powder or as pellets wrapped in a thermoplastic bag having a melting temperature slightly higher than that of the polyethylene, but below that of the polyamide. The PE melted first, coating the walls of the mould for producing the external layer, the thermoplastic pouch then melted, releasing the PA material used for producing the internal layer.