BE1027573B1 - COMPOSITION OF POLYETHYLENE HAVING IMPROVED ESCR, IMPACT RESISTANCE AND WEAR RESISTANCE AS WELL AS A PROCESS FOR PREPARING THEM - Google Patents

Abstract

Rotomolded polyethylene articles, obtained from compositions comprising very high molecular weight polyethylene (UHMWPE) or high molecular weight polyethylene (HMWPE), either of which is used as a dry mix with polyethylene obtained with a metallocene catalyst (mPE). There is also described a rotational molding process for preparing polyethylene compositions comprising (U) HMWPE / mPE taken in the form of pellets and obtained by mixing in the molten state a mixture of mPE taken in powder form with a powder. of (U) HMWPE or chopped fibers of UHMWPE, these compositions finally being subjected to pressure-free rotational molding. The molded articles thus obtained have both good ESCR properties as well as impact and wear resistance properties markedly improved compared to those of the mPE forming part of the composition, this improvement being of the order of double. those of the mPE.

Description

POLYETHYLENE COMPOSITIONS HAVING IMPROVED ESCR, IMPACT RESISTANCE AND WEAR RESISTANCE AS WELL AS A PROCESS FOR PREPARING THEM.

FIELD OF THE INVENTION PE2019 / 0088 The present invention relates to articles made of polyethylene, in particular objects obtained from compositions comprising very high molecular weight polyethylene (UHMWPE) or high molecular weight polyethylene (HMWPE), one or the other being used in dry mixture with polyethylene obtained with a metallocene catalyst (mPE). The present invention also relates to polyethylene compositions comprising (U) HMWPE / mPE taken in the form of pellets and obtained by mixing in the molten state a mixture of mPE taken in powder form with a powder of (U ) HMWPE or chopped fibers of UHMWPE, these compositions finally being subjected to pressure-free rotational molding.

More particularly, the present invention relates to polyethylene compositions obtained by rotational molding comprising the mixture composed of (U) HMWPE with mPE, this mixture being finally mixed with a powder of mPE, these compositions of the invention exhibiting good properties. of ESCR as well as impact and wear resistance properties markedly improved over those of the mPE forming part of the composition.

| | BE2019 / 0085 PRIOR STATE OF THE FIELD OF THE INVENTION Polyethylene (PE) is very often used in rotational molding and is able to prepare articles exhibiting a wide range of properties such as impact resistance, chemical resistance or even flexibility.

Water or agricultural product storage tanks are made with conventional linear polyethylene while chemical storage tanks or fuel tanks require much greater mechanical properties.

Indeed, for this type of application, the PE used must be resistant to breaking (in relation to the hardness of the material), while in thicker PE such as for fuel tanks or pipes, the fracture mode due to fracture is based on the initiation of the crack and its propagation through the thickness.

Therefore, different types of polyethylene exist (crosslinked XLPE or bimodal HDPE) and are used to manufacture by a process of rotational molding this type of articles for storage tanks or fuels.

It is also known that linear low density PE is more resistant to breaking than linear HDPE but on the other hand its rigidity is lower; while HDPE grades are more susceptible to nicks due to the fact that they cannot deform under impact, and therefore cannot resist the propagation of a crack once it is initiated. .

It is of course known that the presence of notches is not possible when using a rotational molding process to manufacture tanks.

This is the reason why the rotational molding industry proposes to use crosslinked PE (XLPE) in order to eliminate this notch sensitivity problem. This is because the macromolecular chains of XLPE are chemically linked or bridged together to form a three-dimensional structure. In this way, the energy released by the shock is dissipated in the three-dimensional network to avoid the problem of propagation. In order to avoid the known disadvantages of XLPE, such as non-recyclability, exposure to peroxides or their secondary products which form, it has already been suggested to develop bimodal HDPE resins showing excellent mechanical properties and which are applied for several decades for extrusion production for the latest generation of PE100 grade pipes.

It is also known that these PE100 grades show excellent resistance to slow crack propagation (SCG) but also to rapid crack propagation (RCP).

Regarding the HDPE bimodal grades prepared with Ziegler-Natta catalyst which 9201910088 are developed for pipe applications, these have a low molecular weight homopolymer part (about 25kDa molecular weight) and a high molecular weight copolymer part ( about 500kDa); the two parts having a similar weight. Hexene is often used as a comonomer because it creates a high concentration of bound molecules when incorporated into the high molecular weight portion. These characteristics are described in several patents such as EP 1546253 of Exxon Mobil which mentions PE compositions with a first PE having an M of between 0.4 and 3.0g / 10 ° and a density of between 0.910 and 0.930g / cm3, and a second PE having an MI of between 10 and 30g / 10 ° and a density of between 0.945 and 0.975, one or the other of these PEs being a metallocene PE; we can also cite other patents such as US2009 / 0004417 from Borealis for a bimodal PE composition suitable for rotational molding or else EP3157967 from Total which also mentions a bimodal mPE suitable for rotational molding.

UHMWPE resins are also well known for their very high ESCR properties, high resistance to impact, abrasion and chemicals; these resins also show a high degree of entanglement and this is known to imply good mechanical properties for end applications.

The good ESCR and impact and wear resistance values of UHMWPE resins are generally considered to be the result of a large number of intralamellar bound molecules connected to adjacent lamellar layers.

But it is also known that UHMWPE resins cannot be rotomolded because of their high melt viscosity at the temperatures at which the process is carried out as well as their long sintering time.

Efforts have been made, as described in the patent literature, to improve the processability of UHMWPE resins, but this has never been used for rotational molding.

To this end, mention may be made of patent US Pat. No. 7,456,242, which claims PE compositions showing a polydispersity (MWD) of 2 to 4, a molecular weight in the range of

150,000 to 500,000 g / mol and a melting point of at least 100 ° C, these compositions having excellent wear resistance properties.

In the context of mPEs also having a polydispersity of 2 to 4 and a melting point> 100 ° C which are intended for rotomolded applications, these generally have an MR (g / 10 ”) in the range 2.7 - 6.0.

As regards the UHMWPE resins prepared with a metallocene catalyst, which have already been proposed PE2019 / 0085, they have given satisfaction in particular by lowering their entanglement density thus improving their processability. However, none of these compositions has really given full satisfaction with the properties of ESCR, wear resistance while having easy processability. Consequently, the present invention relates to new PE compositions, obtained by dry mixing, and consisting of a mixture between a UHMWPE / mPE resin taken in the form of pellets and an mPE resin taken in powder form, these new compositions showing a set of good properties including ESCR, good wear resistance, chemical resistance as well as impact resistance.

Another object of the invention resides in new PE compositions comprising a mixture of UHMWPE and mPE resins having better impact resistance than mPE resin while the latter does not have a low molecular weight part.

Another object of the invention is a reinforced mPE composition, comprising chopped fibers of UHMWPE as a reinforcing agent.

Yet another object of the present invention is to provide novel PE compositions comprising a UHMWPE resin and an mPE resin having good processability.

Yet another object of the invention resides in novel PE compositions comprising a UHMWPE resin and an mPE resin having a low surface tension in order to have an easy phase dispersion as well as a high interfacial adhesion which leads to no longer making necessary the. use of a compatibilizer.

A subject of the present invention is also a non-pressure molding process or a rotational molding process for producing these compositions comprising the UHMWPE and mPE resins.

Another object of the invention is also cited which is to implement a non-pressure molding process or a rotational molding process according to which the pellets of the HMWPE or UHMWPE / mPE resin are coated with the powder of the mPE resin.

These objects and other advantages of the invention will become more apparent with the following description. This set of objects of the invention can be explained as being a PE resin having improved properties including ESCR, wear resistance, chemical resistance and impact resistance with respect to the same properties observed on the mPE resin used.

DESCRIPTION PE2019 / 0088 The Applicant has now found that it was possible to provide compositions comprising UHMWPE obtained by rotational molding and showing excellent ESCR properties, high impact resistance as well as good wear resistance.

Indeed, the Applicant has unexpectedly found that when mixing a resin (HMWPE or UHMWPE) / mPE)) with an mPE powder and subjecting it to a rotational molding process or a non-pressure molding process It is possible to produce PE articles having excellent ESCR and wear resistance properties when this process is practiced under certain conditions.

In particular, it was found that resins obtained from a compound consisting of chopped fibers of UHMWPE with mPE, the latter being mixed with mPE powder, showed significantly improved impact resistance and as such could be considered as a composition of reinforced mPE.

According to the Applicant, the mPE powders are chosen from the grades of mPE intended for rotational molding and having an MI2 between 2 and 8 g / 10 ° and a density between 0.925 g / cc and 0.945 g / cc and a particle size between 100 and 600um and preferably an average size of the powder particles of between 300 and 400 µm.

Both UHMWPE and HMWPE resins consist of polyethylene having a molecular weight between 500kDa and 10,000 kDa, preferably between 600 kDA and 8,000 kDa. They can be prepared in the presence of any type of catalyst including, but not limited to, Ziegler and metallocene catalysts.

The starting mixtures (HMWPE or UHMWPE / mPE) can be prepared by mixing in the molten state at a temperature between 190 ° C and 220 ° C of the respective powders of HMWPE or UHMWPE with that of mPE in a ratio varying between 80 / 20 and 20/80; the resulting mixture is finally extruded in the molten state to obtain pellets.

It is then preferred to dry mix the pellets of (HMWPE or UHMWPE / mPE) with a grade of mPE intended for rotational molding taken in powder form with an average size of the powder particles of about 300 to 400um.

It was also found that it was not necessary to change the grade of mPE regardless of the choice that was made to prepare the mixture either by mixing the mPE fluff with the HMWPE or UHMWPE resin or even by mixing pellets. of mPE with pellets of HMWPE or UHMWPE. The mixtures thus obtained are finally introduced into a rotational molding mold and subjected to a rotational molding process. As an example of operating conditions applicable for the rotational molding process, mention may be made of a temperature of 230 to 260 ° C, a rotation speed of 3/1 to 5/1 with a residence time of 15 to 20 minutes. According to a preferred embodiment of the process for preparing the compositions of the invention, the pellets (UHMWPE or HMWPE / mPE) are coated with the mPE powder. It will be understood that other compounds or additives customary for this type of resins can be added in small amounts, such as in particular polyethylene glycol. As an alternative method to the preparation of the rotomolded compositions of the invention, it is also possible to use chopped fibers of HMWPE or UHMWPE with the mPE to form a reinforced mPE resin which is pelletized and which is combined. with powder of an mPE grade intended for rotational molding.

If necessary, the chopped fibers of HMWPE or UHMWPE can also be subjected to an oxidation pretreatment in order to improve the adhesion of the fibers to the PE matrix. The Applicant has also unexpectedly found that the dry mixtures of mPE powder with the compound (HMWPE or UHMWPE / mPE) taken, for its part, in the form of pellets, or the same types of mixture but the two compounds are taken in the form of pellets. Pellets can be readily used in a rotational molding process for the manufacture of storage tanks either for agricultural products or for water or other automotive articles to improve mechanical properties.

In view of the uniformity of the compositions of mPE and HMWPE or UHMWPE their surface tension values are low for easy phase dispersion, while their high interfacial adhesion properties lead to the unnecessary use of a compatibilizer.

In addition, the use of mPE having a narrow MWD and therefore a very low amount of low molecular weight is very interesting in order to achieve excellent welds during the production of storage tanks especially in the automotive field. According to the Applicant, the usual additives for this type of PE compositions can be added in the usual proportions.

The present invention is also described by means of the following examples which are intended to illustrate it but to limit it in any way.

Example 1-3 Several mixtures were prepared from mPE powder whose physical properties are shown in Table 1, (MI2, density, Bell ESCR, average particle size), with UHMWPE powder, whose physical properties are also shown in Table 1 (Molecular Weight, powder particle size). The weight fraction of UHMWPE in these mixtures was varied between 30 and 80%. These polymers were mixed in the molten state in a twin-screw extruder at a temperature of 220 ° C at a rotational speed of 80 rpm, to be finally extruded in the form of pellets.

The pellets obtained were mixed with amounts varying from 1 to 50% by weight of a powder of an mPE grade identical to the grade initially used in the above mixture.

The resulting mixture was introduced into a mold intended for rotational molding; this was heated in an oven at a temperature of 250 ° C with a rotation ratio of 4/1 for 18 minutes.

The molded article was then air-cooled for 15 minutes and its ESCR properties were then measured as shown in Table 1. It can be seen from this Table 1 that the ESCR value of 1 The molded object is about twice that of mPE, which is therefore a significant improvement in this property.

Example 4 A mixture was prepared from mPE powder whose physical properties are MI2 = 2.7g / 10 ° and density = 0.934g / cc, with chopped fibers of UHMWPE having a length of 3mm and a density of 0.930 g / cc.

The weight ratio of mPE resin to UHMWPE was 60/40. These polymers were mixed in the molten state in a twin-screw extruder at a temperature of 220 ° C at a rotational speed of 80 rpm, to be finally extruded as pellets.

The pellets obtained were mixed in a 50/50 weight ratio with a powder of an mPE grade identical to the grade initially used in the above mixture.

The resulting mixture was introduced into a mold intended for rotational molding; this was heated in an oven at a temperature of 250 ° C with a rotation ratio of 4/1 for 20 minutes.

The molded article was then air cooled for 18 minutes and its impact resistance properties were then measured as shown in Table 1.

It can be seen from this Table 1 that the impact resistance value of the molded object is more than double that of mPE, (65 kJ / m2 against 30 kJ / m2) therefore constituting a significant and totally unexpected improvement in this property. . Table 1 mPE | mPE mPE mPE mPE UHMWPE | UHMWPE | | UHMWPE | MI2 density blend | Bell Yopoids | dimension | Mw dimension Ypds Rotomolded (2/10) | (g / cm3) | ESCR on | average | (g / mol) average in the Bell ESCR (10% mixture | particle particle mixture (10% Igepal Igepal) | (um) (um) 4 .940 150 70 360 180 30 230 LL VE moe | EE

2.7 | .934 1080 50 320 180 50> 2000 LL EN NE [me EE 935 560 20 405 190 1090 OIL hmm) TT | | mPE | mPE mPE mPE mPE Fibers Fibers Fibers Mixture MR density | resistance | % wt | dimension | chopped chopped chopped Rotomolded (g / 10 ”) | (g / cm3) | in dansle | average | UHMWPE | UHMWPE | | UHMWPE | Shock resistance mixture | particles | Mw length (% wt impact (kJ / m2) (um) (g / mol) fiber in (kJ / m2) (mm) mixture

2.7 | .934 <30 320 3 40 65 | OI bee 7 7

Claims (10)

1. PE compositions having both improved properties for ESCR, wear resistance, chemical resistance as well as impact resistance compared to the same set of properties observed on mPE resin, the so-called. composition comprising an HMWPE or UHMWPE resin and an mPE resin characterized in that (i) the mPE has an MI2 of between 2 and 8 g / 10 ° and a density of between 0.925 and 0.945 g / cc and an MWD of between 2 and 4, while (U) HMWPE resin has a density of 0.930 to 0.950 g / cc and a melting point of 130 to 136 ° C and (ii) the weight ratio of (U) HMWPE resin to mPE resin is between 80/20 and 20/80. 2. PE compositions according to claim 1 characterized in that the mPE resin is obtained by polymerization of ethylene in the presence of a single-site catalyst, preferably a metallocene catalyst, and the (U) HMWPE resin is obtained by polymerization. ethylene in the presence of either a Ziegler-Natta catalyst or a single site catalyst, and preferably a metallocene catalyst. 3. PE compositions according to claims 1 or 2, characterized in that the molecular weight of the HMWPE or UHMWPE resin is between 500 kDa and 10,000 kDa and preferably between 600 kDa and 8,000 kDa. 4. PE compositions according to any one of claims 1 to 3, characterized in that they show an increase in the value of ESCR relative to that of the mPE resin initially used in the starting mixture. 5. PE compositions according to any one of claims 1 to 3, characterized in that they show a significant increase in the value of the wear resistance compared to that of the mPE resin initially used in the starting mixture. 6. PE compositions according to any one of claims 1 to 3, characterized in that they show an improvement in phase dispersion, thus making it possible to avoid the use of a compatibilizer. 7. Reinforced PE compositions according to any one of claims 1 to 6 characterized in that the reinforcing agent is chosen from the group of HMWPE and UHMWPE resins as well as chopped fibers of a UHMWPE resin and used in a proportion weight with the mPE resin of between 80/20 and 20/80, this mixture then being subjected to rotational molding. 8. Process for preparing the PE compositions as claimed in any one of claims 1 to 7, characterized in that: a UHMWPE or HMWPE resin is dry mixed with a rotational molding grade of an mPE resin, these two resins. being used in powder form; o Subject this homogeneous dry mixture to an extrusion to form pellets o Mix these pellets with an mPE resin taken in powder form o Introduce the latter mixture into a rotomolding mold o Recover the rotomolded articles having the PE composition. 9. The method of claim 8 characterized in that the rotational molding is carried out at a temperature of 230 to 260 ° C, a rotational speed of 3/1 to 5/1 and for residence of 15 to 20 minutes. 10. Molded articles composed of at least a mixture of an mPE resin and a (U) JHMWPE resin, as described in claims 1 to 7, these molded articles having significantly improved properties of ESCR, strength. to wear, impact resistance and phase dispersion, characterized in that said mixture is subjected to rotational molding as claimed in claims 7 and 8.