CS245874B1 - Thermoplastic based on polypropylene and polyethylene - Google Patents

Abstract

The solution relates to a thermoplastic based on polypropylene and polyethylene with an inorganic filler. It contains 16 to 76% by weight of polypropylene with a melt index higher than 10, characterized by a polydispersity coefficient of less than 5, 8 to 57% by weight of polyethylene characterized by a melt index lower than 1 g/10 min. and 20 to 60% by weight of micro-ground limestone with an average particle size of 1 to 5 μm. The thermoplastic is usable for injection molding of articulated products with good toughness.

Description

(54) Thermoplastic based on polypropylene and polyethylene

The solution relates to a thermoplastic based on polypropylene and polyethylene with an inorganic filler. It contains 16 to 76% by weight of polypropylene with a melt index higher than 10, characterized by a polydispersity coefficient of less than 5, 8 to 57% by weight of polyethylene characterized by a melt index lower than 1 g/10 min. and 20 to 60% by weight of micro-ground limestone with an average particle size of 1 to 5 μm.

Thermoplastic is suitable for injection molding of complex products with good toughness.

The invention relates to a composite material formed by a mixture of polypropylene with polyethylene and an inorganic filler.

For many applications, such as in the automotive industry, easily processable thermoplastic materials with both high stiffness and toughness are required.

Of the known thermoplastics, tough polystyrene comes closest to meeting these requirements, but it is unusable due to easy damage in the environment of gasoline and oils.

Injection-molded types of polypropylene do not meet the above requirements, since their stiffness is relatively low, their toughness is low, and their large shrinkage anisotropy does not allow the required degree of accuracy to be achieved.

Polypropylene filled with inorganic fillers, for example micro-ground limestone, only meets the requirement of stiffness, but does not achieve the necessary toughness and flowability. Although the low flowability can be compensated by processing at higher temperatures and pressures, the limiting factor is thermal stability. Highly flowable composite materials can be prepared using polypropylene, the structure of which has been modified by degradation in the presence of peroxide, but these materials have low toughness for mechanically demanding products.

According to AO No. 197 043, it is possible to prepare materials having both high stiffness and toughness by using ternary mixtures of polypropylene with polyethylene and inorganic filler. If conventional (reactor) types of polypropylene are used for the preparation of these mixtures, the presence of high molecular weight polyethylene and inorganic filler causes a large reduction in flowability, making these materials difficult to use for injection molding of segmented products.

The problem of toughness is related to the presence of a sufficient number of long flexible macromolecules that create bonds between individual particles of the solid phase, that is, between the crystallites of the polymer itself and between the filler particles. In order to achieve the desired toughness, it is necessary that a certain proportion of flexible high-molecular chains characterized by a molecular weight greater than 10 ^fi be present in the semi-crystalline polymer. On the other hand, it is known that polymers having such a high molecular weight are difficult to process.

If mixtures of degraded polypropylene characterized by a narrow molecular weight distribution and high fluidity are used with high molecular weight polyethylene, it is possible to produce mixtures that have both good flowability and good toughness. From the point of view of melt viscosity, this case can be compared with a mixture of a liquid plasticizer with a high molecular weight polymer. It is known that mixtures containing 40% by weight of high molecular weight polymer have a rubbery-elastic character, mixtures containing 30% by weight of this polymer already represent well-fluid solutions. A similar case is observed in the case of mixtures of degraded polypropylene with high molecular weight polyethylene. For example, the addition of 15% by weight of high molecular weight polyethylene causes only a small increase in the viscosity of the degraded polypropylene melt. In practice, however, in the case of unfilled polymers, the addition of high molecular weight polyethylene cannot be used to improve the toughness of polypropylene, because during melting cooling, the two separately crystallizing phases separate. As was shown in AO No. 197 043, the addition of an inorganic filler prevents the separation of the two phases.

Such a process is also suitable for modifying high-flow polypropylene.

The subject of the invention is a thermoplastic based on polypropylene and polyethylene, the supramolecular structure of which is modified by an inorganic filler, which contains 16 to 76 percent by weight of polypropylene with a melt index higher than 10 g/10 min. with a polydispersity coefficient less than 5, 8 to 57 percent by weight of polyethylene with a melt index lower than 1 g/10 min. and 20 to 70 percent of 1 to 5 .«m.

The use of a sufficient (supercritical) concentration of filler prevents the separate crystallization of both polymers. In this case, high molecular weight polyethylene preferentially sorbs onto the filler and creates a tough boundary layer around the filler particles. At the same time, the filler prevents the formation of large spherulites, which act as stress concentrators and whose formation is very easy, especially in highly fluid types of polypropylene. The critical concentration of filler can be imagined as such that, with perfect dispersion, the distance between individual filler particles will be smaller than the space required for the formation of spherulites.

For example, assuming a critical distance between filler particles of l _klit — 10 μη, with an average particle size of dgo = 3 μΐη, the critical filler concentration is 6.33% by volume or 16.8% by weight, respectively. It is obvious that for smaller particles the critical filler concentration will be smaller, but here its perfect dispersion causes problems.

Polypropylene, characterized by a melt index higher than 10 and a polydispersity coefficient, which is the ratio of the weight M _v and the number M _n molecular weight, less than 5, is produced by peroxidic decomposition of reactor types.

Dispersion of filler in a low-viscosity environment of degraded polypropylene is very difficult due to the difficulty of transferring the necessary mechanical energy from the kneading device to the polymer melt and from there to the filler aggregates. Therefore, it is advantageous to proceed in such a way that a high concentration of high molecular weight polyethylene is added. The concentrate prepared in this way is then diluted to the optimal final filler concentration with highly fluid degraded polypropylene.

Homogeneous mixing of degraded polypropylene with a concentrate containing high molecular weight polyethylene and a portion of degraded polypropylene can be achieved in a conventional injection molding machine equipped with a proportional dosing device for both components. In this way, a wide range of materials can be prepared, differing in stiffness, flowability and toughness, and can thus meet the wide requirements of individual applications.

The following example will better illustrate the nature of the invention. The parts and percentages given in the example are by weight.

Example 1

The following was simultaneously dosed into the hopper of the Werner & Pfleiderer ZSK 53 twin-screw mixing extruder:

— 21.5 parts of high molecular weight linear polyethylene characterized by a flow index IT] _!M1 = 0.14 g/10 min.

— 21.5 parts of polypropylene prepared by degradation in the presence of peroxide and moderator, which was characterized by a flow index IT _23O = 15.0 g/10 min. and a polydispersity coefficient of 4.8 (I).

— 57.0 parts of micro-ground limestone, characterized by an average particle size of 2.7 µm, graded below 10 µm, surface-treated with 0.3% stearic acid.

Kneading took place at a temperature of 230 °C, the resulting string was chopped into granules (IJ).

Mechanical properties were evaluated for injection-molded bodies, with a mixture containing 70 parts of granulate II and 30 parts of polypropylene I being dosed into the hopper of the injection molding machine. Flowability was evaluated using a mold in the shape of an Archimedean spiral with a trapezoidal cross-section of 3x5 mm, mold temperature 50 °C, melt temperature 250 degrees Celsius, pressure 7 MPa.

The table compares the processing and mechanical properties of the material produced in this way (A) with a sample in which polypropylene I was replaced by conventional injection-molded polypropylene with a flow index of 3.0 g/10 min (B). The properties of polypropylene I filled with 40% of the same micro-ground limestone (C) and the properties of unfilled polypropylene I (DJ:

Property Unit

Flow Index g/10 min, Flowability cm Flexural modulus of elasticity GPa Notch toughness kj. m~ ² From material comparison And (according to this invention) with material B follows, that at achieving the same stiffness and practically equal-

valuable toughness is achieved by significantly better flowability. Material C has practically the same flowability compared to material A.

A B About D 8 2 12 15 40 32 42 56 2.4 2.4 2.8 1.4 10.5 12.0 3.5 2.6 volatility, slightly higher rigidity, however substantive-

The advantage of material D is only higher flowability, but this material has significantly lower stiffness and toughness.

Claims (1)

SUBJECT Thermoplastic based on polypropylene and polyethylene, whose supra-molecular structure is modified by an inorganic filler characterized in that it contains 16 to 76% by weight of polypropylene with a flow index greater than 10, characterized by a polydispersity coefficient of less than 5, 8 to 57 I INVENT WHY. by weight of polyethylene, characterized by a flow index of less than 1 g / 10 min. and 20 to 60% by weight micronized limestone, characterized by an average particle size in the range of 1 to 5 μη !.