CS274520B1 - Method of high-liquid polypropylene production - Google Patents

Abstract

The polypropylene with high liquidity, high speed of crystallization of melt and antistatic treatment is produced by degradation of polypropylene with initial flow index ranging from 1 to 6.5 g/10 min., initiated by peroxides in presence of 0.3 to 1 percent by weight of silicon oxide with specific surface larger then 150 m<2>/g. The degradation is controlled by presence of 0.8 to 35 moles, related to moles of peroxide, diphosphite, or diphosphonite, whose atoms of phosphor are sterically separated.

Description

The invention relates to a process for the production of polypropylene (hereinafter referred to as PP) for injection molding, in particular to products of complex, thin-walled and high flow-to-wall-thickness ratio. Particularly preferred for this purpose are the so-called degraded types of PP, prepared 7, types prepared by polymerization.

Procedures for radical degradation of PP are described, for example, in Japanese Patent JA 5,135,996 or Belgian Patent BE 806,669.

Author's Certificate No. 229 766 describes the production of high-flow polypropylene by controlled degradation of polypropylene initiated by organic peroxide in the presence of a stabilizer system containing phenolic antioxidants, at least one third of which are sterically unhindered phenol stabilizers or combinations thereof with thiodipropionic or phosphite esters.

The toughness of the high-flow polypropylene is further increased if, according to the author's certificate No. 233 523, a polypropylene containing at least 20 wt. parts of a polymer having a flow index (IT) of less than 2 g / 10 min.

For some injection applications, such as the production of thin-walled containers with a wall thickness of only a few tenths, it is also possible to provide a high crystallization rate (the reason being the tendency to inject as short as possible) and in many applications extreme spray clarity.

If we increase extremely IT, crystallization may slow down. It is known, however, that this unfavorable property can be influenced by nucleation, whereby the addition of substances called nucleating agents can increase the number of nucleation centers and thus the overall crystallization rate of the polymer. The use of benzoic acid salts or derivatives thereof is known for this purpose (described, for example, in U.S. Patent No. 4,184,026).

Various sorbitol derivatives are currently widely used, as described, for example, in U.S. Patent Nos. 3,207,735 and 3,207,337.

The use of finely dispersed silica is also known (JA Patent Nos. 7,018,747 and 7,058,747). This agent is advantageous in terms of chemically internal character and the absence of odor or deterioration of the polymer color.

Exceeding certain concentrations of these substances may, however, lead to changes in the material properties (toughness reduction) that are not desirable in application. Efforts are made by nucleating agents to eliminate a decrease in the rate of crystallization due to a high degree of degradation under controlled degradation or the presence of other substances (eg antisols), while not adversely affecting controlled degradation and mechanical properties.

In many applications of PP, especially in the production of films and packaging, another unfavorable feature of PP is its low electrical conductivity, the tendency to generate electrostatic charges. This leads to the attracting of dust particles and thus to the soiling of the product. There may also be difficulties in handling charged bags or bowls. To eliminate this phenomenon, antistatic agents are applied and either external (applied to the finished product surface) or internal (incorporated into the edge and migrate to the product surface).

A number of antistatic agents are described in the patent literature. In addition to high molecular weight polar substances, it is primarily fatty acid amides and esters that are readily available. The problem of choosing a suitable antistatic agent for degraded types of PP is that these substances readily react with degradation initiators (peroxides) to form colored products.

In most PP packaging applications, one of the important requirements is the minimum yellowness of the material.

This property is influenced both by the granulation process and, above all, by the additive system, in which phenolic stabilizers, which are subject to reactions leading to the formation of colored, mostly yellow compounds, have a decisive influence. In many common injection-type applications or in fiber production (the use of PP natural pigment is minimal here)

CS 274 520 81, phenolic stabilizers have been successfully used, especially combinations of sterically hindered and non-hindered phenols, as described in the aforementioned U.S. Pat. 229 766 and 233 523. If we want to prepare the material for a special, narrow purpose, such as injection molding of thin-walled packaging, it must simultaneously meet a number of mutually and contradictory requirements. By this is meant a compromise between properties and fluidity, feedstock and yellowness and many others.

The question of yellowness precludes the already mentioned use of phenolic stabilizers in the production of degraded types of PP.

The use of higher molecular weight raw materials also leads to good mechanical properties in degraded types of PP, as previously described in MS. However, such feedstocks require higher doses of initiator peroxide to achieve a given IT level, leading both to a higher degree of side-reaction with other additives and to an increase in the volatile content of the granulate.

The task of finding the conditions of controlled PP degradation leading to a colorless antistatic product with a high nucleation rate was found in the process according to the invention.

SUMMARY OF THE INVENTION The present invention provides a process for the production of high-flow polypropylene by peroxide-initiated polypropylene degradation and controlled by a phosphorus compound stabilization system, wherein the polypropylene having an initial flow index of 1 to 6.5 g / 10 min. % degrades with peroxide in the presence of 0.8 to 35 moles of diphosphite and / or diphosphonite, the phosphorus atoms of which are sterically separated, based on moles of peroxide, and 0.3 to 1 wt. fatty acid monoglycerides; and

0.1 to 0.5 wt. silica having a specific surface area greater than 150 m / g based on degraded polypropylene.

Phosphorus-containing stabilizers, usually in the form of phosphites, are known as effective stabilizers for polyolefins, although the detailed mechanism of their action is not explained in detail.

Their effectiveness depends on the structure, but they are all known as efficient hydroperoxide decomposers.

It is also expected to act as metal deactivators which otherwise act as oxidation catalysts in polyolefins.

The original types of phosphite stabilizers are phosphoric acid esters containing only one phosphorus atom per molecule. Examples thereof are trisnonylphenyl phosphite or tris (2,4-di-tert-butylphenyl) phosphite.

A certain disadvantage of phosphites, however, depending on their structure, is hydrolysis. This property can be influenced (favorably) both by the substitution of the phenolic component of the phosphites and by the suitable coupling of two phosphorus atoms in one molecule to form diphosphites or diphosphonites.

In addition to their high efficiency as a processing stabilizer, phosphites are also characterized by a synergistic effect in relation to light stabilizers and contribute significantly to preventing the formation of undesirable color changes in the polymer, typically characterized by a degree of yellowness in PP.

Thus, it would appear that the phosphorous-containing stabilizers are ideal, versatile, stabilizers for processing polyolefins, and the question of their structure and their relationship to other additives in the system are minor matters. When working with such systems where we deliberately carry out radically initiated reactions in the primary processing (granulation) process itself, the question of the antagonistic effect of initiators and phosphites as radical reactors is extremely important. An example of such deliberately performed reactions is the aforementioned controlled PP degradation initiated by the radicals resulting from the breakdown of organic peroxides. In this case, simple phosphites enter the process negatively in that they reduce its efficiency by reacting with the initiator. As a result, a higher concentration of initiation peroxide is required to achieve the desired IT.

However, when using diphosphite or diphosphonite stabilizers containing sterically separated phosphorus atoms, it was surprisingly found that (used at the same end of CS 274 520 B1 as simple phosphites) the adverse reaction was suppressed and achieved (at the same initiator concentration) higher final IT degraded PP. However, the high stabilizing effect of primary and secondary processing of degraded PP is maintained when diphosphites are used.

By spherical separation of phosphorus atoms is meant a structure wherein in the molecule the phosphorus atoms are separated by a group of atoms containing at least one carbon atom and forming rings. Examples of diphosphites with spherically separated phosphorus atoms include tetrakis (2,4-di-tert-butylphenyl) 4,4´-biphenylenediphosphite, distearyl petaerythritol diphosphonite and bis (2,4-di-tert-butylphenyl) pentaerythritol diphosphonite. For the antistatic treatment of polymers, oxyalkylated alkyl amines are most commonly used, where oxyalkyl is usually oxyethyl. The use of oxyethylated amides, alcohols and fatty acids is known (P. Caučík et al. Plastic Additives, Chapter 5, pp. 296-34, Alfa Bratislava 1985).

Another important group is the esters of glycerin and higher fatty acids (C 12-16).

A monoester should be used for antistatic use. Its mixture with glycerin for antistatic treatment is described, for example, in DE patent No. 2,153,438. Authentic Certificate No. 236 037 discloses for the antistatic treatment of PP the use of a reaction product of glycerin and tallow fatty acids containing at least 60% monoglycerides. The advantage of these substances is that it is essentially a product close to natural substances, thus presenting a minimum hygiene hazard.

The nucleation effect of finely dispersed silica is determined by its high specific surface area (of the order of 100 m / g) and porosity (of the order of cm). In the pores of the reagent, polymer residues can survive the melting process and become nucleation centers even when the melt is cooled down at low subcooling. The preparation of degraded PP according to the invention can be carried out in single- or twin-screw extruders designed to guarantee good material homogeneity and at temperatures customary for PP processing (190 to 270 ° C). The starting material may be PP in the form of powder or granules, and the granulate may comprise all or part of the stabilizing system. The raw materials used in the examples are specified as follows:

Polymers

Polymer AND homopolymerní isotactic PP with IT 0.4 g / 10 min Polymer II homopolymerní isotactic PP with IT 1.1 g / 10 min Polymer III homopolymerní isotactic PP with IT 4.5 g / 10 min Polymer IV homopolymerní isotactic PP with IT 8 g / 10 min Polymer in homopolymerní isotactic PP with IT 6.5 g / min

Nucleating agents (hereinafter abbreviated as NA)

NA I

SiO ₂ with a specific surface area of 60 m / g

NA II ₂

SiO ₂ with a specific surface area of 150 m / g

NA III

SiO ₂ with a specific surface area of 350 m / g

NA IV benzoic acid

NA V phthalic acid

Antistatic agents (hereinafter abbreviated as AA)

AA I ethoxylated alkylamine, commercial product Hostastat FA 14 (Hoechst, Germany)

AA II condensate of fatty acids and triethanolamine, commercial product Tribustol LT (Association for Chemical and Metallurgical Production, Ústí n. Labem)

CS 274 520 BI

AA III condensation product of glycerin and fatty acids, containing at least 90% of monoglycerols, commercial product Polynol A Extra, Severočeské tukové závody, Ústí n. Labem

Stabilizers

AND

2,6-di-tert-butyl-4-methylphenol

II

2,6-dimethyl-4-dodecapropylphenol

III tetrakis (2,4-di-tert-butylphenyl) 4,4'-biphenylenediphosphite

IV distearyl pentaerythritol diphosphonite

In bis (2,4-di-tert-butylphenyl) pentaerythritol diphosphonite

VI tris (2,4-di-tert-butylphenyl) phosphite

Peroxide

In all cases 2,5-dimethyl-2,5-di-tert-butylperoxyhexane (hereinafter referred to as PI) was used.

Excipients

Calcium stearate (CST) excipient was used in all examples.

Solids for measuring mechanical properties were prepared from the materials prepared according to the recipes listed in Table 1. The bodies were prepared by pressing at 230 ° C‘ and conditioned at 21 + 1 ° C for 14 days before measurement.

The measurement was performed according to the following Czechoslovak standards or procedures specified below:

ČSN 64 0605 - Tensile test of plastics

Specification: molded body, speed 50 mm / min, test specimen type 2 (reduction 1/5 thickness 1 mm).

ČSN 64 0612 - Determination of impact and notch toughness of plastics by Charpy method

Specification: test specimen No. 3, notch type A, pendulum velocity at the moment of impact 2.9 m / s.

Tensile impact strength - the test is carried out according to DIN 53 448 - Schlagversuch modified as follows:

1. Test specimen as for plastic tensile test (see above)

2. Oonstat N tester modified, hammer stroke 90 °.

Measurement of yellowness

Determination of yellowness was performed according to ČSN 64 3050 article 25. (method of contractual standards).

Volatile fraction

The determination was carried out according to ČSN 64 0227, art. 27.

Half-life of surface charge discharge

The half-life of the surface charge was measured on a Polystat 1 instrument in which the measured sample is charged by a corona charger. The measured quantity is the surface potential (V) related to the charge in relation Q = C. V, where C is capacity. The charge half-life is the time taken for the surface potential to drop to 1/2 of its original value. Samples were injection molded (1 mm thick plates) and conditioned for 4 days in 56% relative humidity prior to measurement.

CS 274 520 81

In all examples, the materials were manufactured using the following equipment:

single screw extruder with a length (L) to diameter (D) ratio of 20 and a compression ratio of 4/1. The screw speed was 80 rpm. Unless otherwise indicated, the temperatures in the individual zones (from the hopper) were 210, 220, 220 ° C, and the nozzle temperature was 210 ° C. In all examples, 0.05 parts of CST was added to 100 parts.

It is clear from the examples that:

1. The use of raw materials with an IT input of less than 1 g / 10 min significantly increases the volatile fraction.

2. The use of antistatic agents based on monoglycerides of fatty acids in combination with nucleation with SiO ₂ leads to products with relatively low yellowness, rapid crystallization, rapid discharging of surface charge, good mechanical properties.

3. The use of phosphonite stabilizers with sterically separated phosphorus atoms leads to a further improvement in yellowness with low peroxide consumption.

Claims (1)

SUBJECT OF THE INVENTION A process for producing high-flow polypropylene by peroxide-initiated polypropylene degradation and controlled by a phosphorus compound stabilization system, characterized in that the polypropylene having an initial flow index of 1 to 6.5 g / 10 minutes is degraded by peroxide in the presence of 0.8 to 35 moles of diphosphite; % of diphosphonite, the phosphorus atoms of which are sterically separated, based on moles of peroxide, and 0.3 to 1 wt. monoglycerides of fatty acids and 0.1 to 0.5 wt. % of silica with a specific surface area greater than 150 m / g based on the graded polypropylene.