WO2021048415A1

WO2021048415A1 - Polyethylene compositions having improved escr, impact resistance ad wear resistance, and also a process for preparing same

Info

Publication number: WO2021048415A1
Application number: PCT/EP2020/075559
Authority: WO
Inventors: Guy Debras
Original assignee: Gd Consulting And Polymers S.P.R.L.
Priority date: 2019-09-12
Filing date: 2020-09-11
Publication date: 2021-03-18
Also published as: BE1027573A1; BE1027573B1

Abstract

Rotomolded polyethylene articles, obtained starting from compositions comprising ultra-high molecular weight polyethylene (UHMWPE) or high molecular weight polyethylene (HMWPE), one or the other being used by dry blending with polyethylene obtained with a metallocene catalyst (mPE) and rotomolding process for preparing polyethylene compositions comprising (U)HMWPE/mPE taken in the form of pellets and obtained by melt blending a mixture of mPE taken in powder form with a (U)HMWPE powder or chopped UHMWPE fibers, these compositions being ultimately subjected to pressureless rotomolding.

Description

COMPOSITIONS OF POLYETHYLENE HAVING AN ESCR. IMPROVED IMPACT RESISTANCE AND WEAR RESISTANCE AS WELL AS A PROCESS

TO PREPARE THEM

Technical area

The present invention relates to polyethylene articles, in particular to 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).

The present invention also relates to polyethylene compositions comprising (U) HMWPE / mPE taken in the form of e † pellets 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 pressureless rofomolding. More particularly, the present invention relates to polyethylene compositions obtained by rofomoulding 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 significantly improved impact and wear resistance properties compared to those of the pheasant mPE † part of the composition. Technological background of the invention

Polyethylene (PE) is very often used in rotational molding and is able to prepare articles with a wide range of properties such as impact resistance, chemical resistance or 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 es † based on crack initiation and propagation through thickness.

Consequently, different types of polyethylene exist (crosslinked XLPE or bimodal HDPE) and its † 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 fracture than linear HDPE but on the other hand its stiffness is lower; while HDPE grades are † more susceptible to nicks because of the fact that they cannot deform under impact, and therefore cannot resist the propagation of a crack once it is damaged. es † 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 problem of notch sensitivity.

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 are formed, it has already been suggested to develop bimodal HDPE resins showing excellent mechanical properties e † which its † 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 bimodal H DPE grades prepared with Ziegler-Natta catalyst which are developed for pipe applications, these have a low molecular weight homopolymer part (approximately 25kDa molecular weight) e † a high molecular weight copolymer part (about 500kDa); the two parts having a similar weight. Hexene is often used there as co-monomer because it creates a high concentration of bound molecules when incorporated into the high molecular weight part

State of the art

These characteristics are described in several patents such as EP 1546253 of Exxon Mobil which mentions PE compositions with a first PE having an MI2 of between 0.4 e † 3.0g / l 0 'e † a density of between 0.910 e † 0.930g / cm3, e † a second PE having an MI2 of between 10 e † 30g / 10 'e † a density of between 0.945 e † 0.975 one or the other of these PE 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 values of ESCR and impact and wear resistance of UHMWPE resins are generally considered to be the consequence of a large number of intramellar bound molecules connected to adjacent lamellar layers. However, it is also known that UHMWPE resins cannot be rotomolded because of their high viscosity in the molten state 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, we can cite patent US7,456,242, 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 MI2 (g / 10 ') in the range 2.7 - 6.0.

As regards the UHMWPE resins prepared with a metallocene catalyst, which have already been proposed, 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 respect to the properties of ESCR, wear resistance while having easy processability.

There is therefore a general need for polyethylene compositions having improved ESCR, impact resistance and wear resistance as well as a process for preparing them.

Summary of the invention: New PE compositions are provided according to the present invention, comprising an HMWPE or UHMWPE resin and an mPE resin characterized in that (i) the mPE has an MI2 of between 2 e † 8 g / 10 'e † a density between 0.925 e † 0.945 g / cc e † a MWD between 2 e † 4, while (U) HMWPE resin has a density between 0.930 e † 0.950 g / cc e † a melting point between 130 e † 136 ° C e † (ii) the weight ratio between (U) HMWPE resin e † mPE resin is † between 80/20 e † 20/80.

These compositions are preferably obtained by dry mixing, for example by mixing between a UHMWPE / mPE resin taken in the form of shovels and / or powder and an mPE resin taken in the form of shovels and / or powder, these new compositions compositions showing good ESCR properties as well as good wear resistance. The compositions may be in one embodiment comprised of a mixture between a UHMWPE / mPE resin taken in the form of shovels and an mPE resin taken in the form of powder.

The compositions may in one embodiment consist of a mixture between a UHMWPE / mPE resin taken in the form of ef pellefs / or powder and an mPE resin taken in the form of ef pellefs / or powder.

Another object of the invention resides in novel PE compositions comprising a mixture of UHMWPE and mPE resins having better impact resistance than the 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 being necessary. the use of a compatibilisan †.

The present invention also relates to 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 molding process without pressure 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.

More particularly, in one embodiment of the method according to the invention, the following steps are implemented: a. 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; b. This homogeneous dry mixture is subjected to an extrusion to form pellets. vs. These pellets are mixed with an mPE resin taken in powder form d. This latter mixture is introduced into a rotomolding mold e. The rotomolded articles having the PE composition are recovered.

Finally, an object of the invention is to provide molded objects composed of at least a mixture of an mPE resin e † of a (U) HMWPE resin, these molded objects having significantly improved properties including properties of 'ESCR, wear resistance, impact resistance and phase dispersion by rotational molding of the aforementioned mixture with respect to the same set of properties observed on the mPE resin.

Advantageously, the compositions according to the present invention as well as the molded objects exhibit good resistance to chemicals compared to that observed for the mPE resin.

This set of objects of the invention can be explained as being a PE resin having improved properties including ESCR, wear resistance, resistance to chemicals and impact resistance compared to the same. properties observed on the mPE resin used.

Other embodiments of the compositions, the process and the molded articles are indicated in the appended claims.

Other characteristics, details and advantages of the invention will emerge from the description given below, without limitation e † in pheasant † with reference to the examples. Description of the invention

The Applicant has now found that it is possible to provide compositions comprising UHMWPE obtained by rotational molding e † 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 e † that it is subjected to a rotomolding process or a molding process without pressure, it is possible to produce PE articles with 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 a powder of mPE, showed significantly improved impact resistance e † as such can be considered to be 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 e † 8 g / 10 'e † a density between 0.925 g / cc e † 0.945 g / cc e † possibly a particle size between 100 e † όqqmiti e † preferably an average size of the powder particles of between 300 e † 400 pm.

Both UHMWPE e † HMWPE resins are made of polyethylene with a molecular weight between 500kDa e † 10,000 kDa, preferably between 600 kDA e † 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 e † 20/80; the mixture obtained is finally extruded in the molten state in order 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 the average size of the powder particles being approximately 300 to 400 mi.

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 itself † by mixing the mPE fluff with the HMWPE or UHMWPE resin or even by mixing mPE pellets with HMWPE or UHMWPE pellets. The mixtures thus obtained are finally introduced into a rotational molding e † subjected to a rotational molding process. As an example of operating conditions applicable for the rotational molding process, we can mention 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, one can also use chopped fibers of HMWPE or UHMWPE with the mPE to form a reinforced mPE resin which one can use in the form of pellets. 'it is combined with powder of a grade of mPE 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 under pellet form can be easily used in a rotational molding process for the manufacture of storage tanks soi † for agricultural products soi † for water or other automotive articles to improve properties mechanical.

In view of the uniformity of the compositions of mPE e † of 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 compatibilisan †.

In addition, the use of mPE having a narrow MWD e † therefore a very low amount of low molecular weight is very interesting for performing 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 not to limit it in any way. Example 1 -3.-

Several mixtures were prepared from mPE powder with physical properties shown in Table 1, (MI2, density, Bell ESCR, mean particle size failure), with UHMWPE powder, with physical properties being † also shown in Table 1 (Molecular Weight, powder particle flaw).

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 exfruder at a temperature of 220 ° C at a rotational speed of 80 rpm, to be finally exfruded in the form of pellets.

The shovels obtained were mixed with amounts varying from 1 to 50% by weight of a powder of a grade of mPE 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 one temperature of 250 ° C with a rotation ratio of 4/1 for 18 minutes.

The molded object 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 the molded object is about twice that of the mPE, which therefore constitutes a significant improvement in this property.

Example 4.-

A mixture was prepared from mPE powder with physical properties its † MI2 = 2.7g / 10 'e † 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 in the form of pellets.

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

The resulting mixture was introduced into a mold intended for rofomoulding; this was heated in an oven at a temperature of 250 ° C with a rotation ratio of 4/1 for 20 minutes. The molded object was then air cooled for 18 minutes and its impact resistance properties were then measured as shown in Table 1.

We can see from this Table 1 that the impact resistance value of the molded object is more than double that of the mPE, (65 kJ / m2 against 30 kJ / m2) therefore constituting a significant and totally unexpected improvement in this property.

It is of course understood that the present invention is in no way limited to the embodiments described above and that many modifications can be made thereto without departing from the scope of the appended claims.

Table 1.-

Claims

1. PE compositions, 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 / l 0 'and a density of between 0.925 and 0.945 g / cc and an MWD between 2 and 4, while the resin (U) HMWPE has a density between 0.930 and 0.950 g / cc and a melting point between 130 e † 136 ° C e † (ii) the weight ratio of the resin (U) HMWPE e † mPE resin is † between 80/20 e † 20/80.

2. A PE composition according to claim 1, exhibiting having both improved properties for ESCR, wear resistance, chemical resistance as well as impact resistance with respect to the same set of properties observed on. mPE resin.

3. PE compositions according to claim 1 or claim 2, characterized in that the mPE resin is obtained by polymerization of ethylene in the presence of a single-site catalyst, preferably a metallocene catalyst, e † the resin ( U) HMWPE is obtained by polymerization of ethylene in the presence of a Ziegler-Natta catalyst itself † a single-site catalyst, and preferably a metallocene catalyst.

4. PE compositions according to claims 1 to 3, characterized in that the molecular weight of the HMWPE or UHMWPE resin is between 500 kDa e † 10,000 kDa e † preferably between 600 kDa e † 8,000 kDa.

5. PE compositions according to any one of claims 1 to 4, characterized in that they show a increase in the value of the ESCR relative to that of the mPE resin initially used in the starting mixture.

6. PE compositions according to any one of claims 1 to 5, 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 mixture of departure.

7. PE compositions according to any one of claims 1 to 6, characterized in that they show an improvement in the phase dispersion thus making it possible to avoid the use of a compafibilisan †.

8. Reinforced PE compositions according to any one of claims 1 to 7, characterized in that the reinforcing agent is chosen from the group of HMWPE e † UHMWPE resins as well as chopped fibers of a UHMWPE e † resin. used in a proportion by weight with the mPE resin of between 80/20 e † 20/80, this mixture then being subjected to rotational molding.

9. Process for preparing the PE compositions as claimed in any one of claims 1 to 8, characterized in that: a. 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; b. Subjecting this homogeneous dry mixture to an extrusion to form pellets c. Mix these pellets with an mPE resin taken in powder form d. Introduce the latter mixture into a rotational molding e. Recover the rotomolded articles having the composition PE.

10. 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.

11. Molded articles composed of at least a mixture of an mPE resin and a (U) HMWPE resin, as described in claims 1 to 8, these molded articles having significantly improved properties of ESCR, strength. to wear, of impact resistance e † of phase dispersion, characterized in that said mixture is subjected to rotational molding as claimed in claims 8 e † 9