Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1999-08-19
2003-02-18
Michl, Paul R. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
active
06521709
ABSTRACT:
The present invention concerns polyolefin compositions comprising a crystalline propylene polymer, preferably a homopolymer, and a ultrahigh molecular weight polyethylene (hereinafter called UHMWPE) in form of particles.
It is known in the art to blend UHMWPE with polyolefins having lower (i.e. conventional) molecular weights in order to obtain compositions having improved processability in the molten state with respect to pure UHMWPE.
In fact, due to its extremely high molecular weight, UHMWPE cannot be practically processed by the melt-processing techniques usually employed for thermoplastic polymers, such as screw extrusion or injection molding.
On the other hand, the addition of the said polyolefins with lower molecular weight, like polypropylene of polyethylene, while improving the melt-processability can also influence, to a variable extent, the mechanical properties of the resulting compositions.
According to the prior art, said compositions are generally obtained by way of multistep polymerization processes comprising at least one step in which the UHMWPE is prepared and at least one step in which the polyolefin with lower molecular weight is prepared.
Also the relative order of the said steps can be critical, according to the teaching of the prior art. In the European published patent application No 318 190 it is explained that said polymerization processes are capable of producing compositions wherein the UHMWPE is present in form of fine particles having an average diameter of 20 &mgr;m or less, which constitutes an essential condition for the obtainment of the effect of the invention.
Actually, the only example reported in the European patent application (Example 1) shows that the average diameter of the UHMWPE particles obtained by way of the said multistep polymerization processes, in the presence of a typical Ziegler-Natta catalyst supported on magnesium dichloride, is significantly lower than 10 &mgr;m, namely around 3 &mgr;m.
Said particles are dispersed in the polyolefin with lower molecular weight to form a sea-island structure.
According to the disclosure of the cited European patent application, the polyolefin with lower molecular weight has an intrinsic viscosity of 0.1-5 dl/g and the UHMWPE is present in an amount of 10-40% by weight based on the total weight of the composition.
No data relating to compositions wherein the polyolefin with lower molecular weight is polypropylene are reported.
In fact the therein exemplified polyolefin with lower molecular weight is polyethylene, which exerts a stronger surface interaction with the UHMWPE with respect to polypropylene.
Similar compositions, but containing higher amounts of UHMWPE, namely from 40 to 98% by weight based on the total weight of the composition, are disclosed in the Japanese published patent application No 59(1980)-120605, wherein the polyolefin with lower molecular weight is selected from polypropylene or poly-1-butene.
Also in this case the compositions are prepared by way of a multistep polymerization.
Moreover, in Comparative Example 2 of the said Japanese patent application it is shown that unacceptable mechanical properties are obtained when the said compositions are prepared by mechanically blending a propylene homopolymer having a Melt Flow Rate measured under standard conditions (MFR at 230° C., with 2.16 kg load) of 76 g/10 min. with UHMWPE in powder form, by way of an extrusion apparatus.
In fact the obtained sample does not display an appreciable value of elongation at break (a 0 value is reported), while its yield strength is very low (half the value of pure UHMWPE, as demonstrated by a comparison with Comparative Example 1).
On the other hand, the examples of the Japanese patent application show that the MFR of the therein prepared compositions are low, as demonstrated by the fact that only in the case of Example 5 a measurable MFR value (namely 0.11 g/10 min.) is obtained by using a load of 2.16 kg (the temperature being 190° C.).
In the other examples it is necessary to use heavier loads in order to obtain appreciable MFR values.
Taking into account that in the said Example 5 the MFR value of the polypropylene component (measured under the cited standard conditions) is as high as 82 g/10 min. and that the amount of UHMWPE is relatively low, when compared with the definition of the invention of the Japanese patent application (namely, it is 53% by weight), it is clear that, according to the teaching of the said patent application, the addition of polyolefins with high MFR values is not capable of raising the MFR of the final compositions over a relatively low limit.
Obviously, the melt-processability of a polymer material is strongly influenced by the MFR, which should be kept relatively high in order to achieve the best degree of processability.
On the other hand, it would also be preferable not to raise too much the MFR of the polyolefin with lower molecular weight, in order to avoid a worsening of the mechanical properties of the same, to the detriment of the properties of the final compositions.
The above advantages are achieved by the polyolefin compositions of the present invention, which comprise from 10 to 95% by weight of a crystalline propylene polymer A) having a MFR value (measured under the previously said standard conditions, namely 230° C., 2.16 kg load, according to ISO 1133) of 60 g/10 min. or less, and from 5 to 90% by weight of UHMWPE B) in form of particles having a mean particle size of from 300 to 10 &mgr;m.
In fact, the MFR of the compositions of the present invention is not inferior, for comparable amounts of UHMWPE, to the values reported by the said Japanese patent application, even when the MFR of A) is remarkably lower than the MFR of many propylene polymers used in the examples of the Japanese patent application.
For instance, in the case of a composition according to the present invention wherein the concentration of UHMWPE B) is of 50% by weight, a MFR value of 0.2-0.3 g/10 min. at 230° C. and 2.16 kg can be easily achieved by using a component A) having a MFR value of 15 g/10 min.
It has been determined by the Applicant that, when measured at 190° C. instead of 230° C., the said MFR values are generally halved, so that the corresponding values at 190° C. are 0.1-0.2 g/10 min. Consequently the compositions of the present invention are particularly suited for the production of formed articles by means of injection molding.
Moreover, the said compositions display satisfactory values of tensile strength at yield, elongation at yield, heat distortion temperature and Vicat softening point, which are generally higher than the values of the UHMWPE B), of flexural modulus, and particularly high levels of abrasion and wear resistance, and low values of coefficient of friction, when compared with pure polypropylene.
When elevated values of elongation at yield and at break and high levels of impact, abrasion and wear resistance are required, the compositions of the present invention preferably comprise from 10 to 60%, more preferably from 25 to 50%, in particular from 25 to 45% by weight of component A) and from 40 to 90%, more preferably from 50 to 75%, in particular from 55 to 75% by weight of component B).
When elevated values of flexural modulus and Vicat softening point and high levels of melt-processability are required, the compositions of the present invention preferably comprise from more than 60 to 95%, more preferably from 65 to 90%, in particular from 65 to 80% by weight of component A) and from 5 to less than 40%, more preferably from 10 to 35%, in particular from 20 to 35% by weight of component B).
In addition, the compositions of the present invention can optionally comprise a reinforcing agent C), such as glass fibers, carbon fibers, mineral fillers (as silica or talc, for instance), in amounts ranging from 2 to 20% by weight, preferably from 4 to 17% by weight with respect to the total weight of the sum of components A), B) and C.).
Other additives commonly used in the art, like nucleating agents, stabilizers, pigments, can be present in the
Pitteri Silvio
Willems Miet
Basell Poliolefine Italia S.p.A.
Michl Paul R.
LandOfFree
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