Beta-Crystalline Form of Isotactic Polypropylene and Method for Forming the Same

This invention relates to a beta-crystalline form of isotactic polypropylene having a high purity and thermal stability and a method of making the same. This invention also relates to the articles made of the beta-crystalline form of polypropylene. More particularly, by melt-crystallization in the presence of our newly discovered beta-nucleating agents, the beta-phase polypropylene is formed, which can be used for producing an article with high impact strength and a microporous films. This invention is also related to the newly discovered beta-nucleating agents, specifically, a two component beta-nucleating agent for polypropylene that has various advantages over beta-nucleating agents known in the art.

Isotactic polypropylene is capable of crystallizing in several crystal forms. The alpha or monoclinic form is the most stable and prevalent one. The beta or hexagonal form is occasionally found in commercially available polypropylene, usually at low levels.

The relative proportion ("K") of beta form crystalline in the polypropylene specimen can be determined by X-ray diffraction and expressed by the empirical expression: ##EQU1##

Wherein H.sub.110, H.sub.040 and H.sub.130 are the heights of the three strong peaks (100), (040) (130) of the alpha-form respectively and H.sub.B is the height of the strong beta-peak (300).

The K value is zero in the absence of the beta form and one if only the beta-form is present in the polypropylene.

In typical commercial products produced through injection molding, spinning filament and extruder shaping have spherulitic morphology, in which there is only a very small percentage of beta-crystallinity. Until recently, there has been no reliable way to produce enough beta-form for detailed studies. H. J. Leugering at Hoechst has reported that small amounts of a quinacridone colorant, permanent REd E3B, preferentially nucleated the beta-form. This compound is one of a few effective well known beta-uncleating agent. But even when polypropylene is crystallized from the melt in the presence of E3B, it is still difficult to obtain a material with a relatively high amount of beta-form. Ullmann and Wendorf have reported that the K value of polypropylene samples, crystallized in the presence of E3B, is less than 0.6. Moos and Hilger have succeeded in producing a sample with a K value as high as 0.85 by melt-extrusion of a special powder grade of polypropylene with E3B in a twin-screw extruder, but no further studies have been conducted. A method was disclosed in JP Kokai No. 57-172943 for producing the beta-form of isotactic polypropylene, which comprises mixing a quinacridone colorant with a polymerization catalyst, and then polymerizing the propylene in the presence of the foregoing mentioned mixed catalyst. The thus produced polypropylene was melt-extruded and then crystallized at a cooling rate of 10.degree. C./min. The sample was reported to contain 63-88% of beta-form, as measured by a differential scanning calorimeter (D.S.C.). The D.S.C. method, however, is not a precise method for measuring beta-form content in our opinion, based on our experience the content of beta-form measured by D.S.C. may be overestimated. Furthermore the beta-phase polypropylene thus obtained is red in color, thermally unstable and easily transform into the alpha-form during heating.

Another beta-nucleating agent described in the literature is the bi-sodium salt of O-phthalic acid and to a lesser degree isophthalic and terephthalic acids. These beta-nucleating agents are less effective than the E3B and no quantitative data for the K value is given.

The inventions of the present invention have unexpectedly found a more effective beta-nucleating agent for polypropylene which is able to produce a much higher content of beta-form polypropylene material under typical processing conditions.

One of the objects of the present invention is to provide a method for producing beta-form crystalline polypropylene. The method comprises mixing an effective beta-nucleating agent provided by this invention with commercially available polypropylene at a certain proportion and then melt-processing the mixture under typical conditions.

Another object of the invention is to provide a beta-crystalline form of isotactic polypropylene, wherein the proportion of beta-form crystalline, as expressed in a K value, lies between 0.85 and 0.93, as measured by X-ray diffraction.

A further object of the invention is to provide articles made of the beta-crystalline phase polypropylene material of this invention. The articles have a much higher impact strength and other properties that presently available commercial polypropylene does not provide.

It is another object of the present invention to also provide a two component beta-nucleating agent.

This invention provides a more effective beta-nucleating agent which is colorless, thermo-stable and non-volatile below 320.degree. C. By which in the presence of the foregoing beta-nucleating agent a colorless polypropylene with a much higher beta-form content can be obtained under typical processing conditions. The relative amount of beta-form, as expressed by a K value for samples crystallized at a temperature of between 25.degree.-135.degree. C. lies between 0.85 and 0.93. The mixing of polypropylene with beta-nucleating agent is quite simple and can be performed by a single screw extruder. Commercially available pellet grade polypropylene can be used for this purpose. The beta-phase polypropylene thus obtained has improved thermal stability and does not transform to the alpha-form prior to their melting.

The beta-nucleating agent, provided by this invention is formed by the mixing of components A and B. Component a is an organic dibasic acid, such as pimelic acid, azelaic acid, o-phthalic acid, terephthalic and isophthalic acid and the like. Component B is an oxide, hydroxide or an acid salt of a Group II metal e.g. magnesium, Calcium, Stronium and Barium. The acid salt of Component B may come from inorganic or organic acid such as carbonate, stearate etc. Component B may also be one of the additives of polypropylene, that already is present in the pellet material.

The proportion of component A is in the range of 0.0001-5% by weight, most preferably 0.01-1 wt %, the proportion of component B is 0.0002-5% by weight, most preferably 0.05-1%, during mixing. The beta-form of the polypropylene produced according to this invention has improved mechanical properties as shown in Tables 1 and 2. The mechanical properties of the alpha and beta-form of polypropylene shown in Table 1 were tested at room temperature.

Beta-phase polypropylene has a lower yield strength and a higher impact strength in comparison with alpha-phase polypropylene of the same grade. Another feature of the beta-phase polypropylene is that it transforms into the alpha-form, with some voids in it, when the beta-form sample is stretched. Therefore a microporous film can be obtained simply by drawing the beta form of the isotactic polypropylene.

The unique properties of the beta-phase polypropylene lend themselves to a variety of potential applications, including high impact resistant molding articles, microporous films, synthetic paper, rough surface film having good oil impregnation for electrical equipment.

5 Kg of commercially available pellets of polypropylene homopolymer 1300 (melt flow rate 1.5 gm/10 min), supplied by Xiangyang Chemical works were used as a starting material. The beta-nucleator was prepared by mixing 5 gm of azelaic acid with 5 gm of calcium carbonate. The materials were processed by premixing the polypropylene pellets with the beta-nucleator powder, followed by melt-processing using a single screw extruder at 200.degree. C. The extrudates after quenching in cold water were cut into pellets, then isothermally crystallized from the melt. The formation of the beta form, in addition to the alpha-form, as expressed by the K-value was observed at a crystallization temperature range from 23.degree. to 140.degree. C. The results obtained are summarized in the following table.

The experiment of Example 1 was repeated except that the beta-nucleator was prepared by mixing 5 gm of pimelic acid with 5 gm calcium carbonate. The K value of the sample, crystallized at a temperature range of 25.degree.-130.degree. C., was between 0.85 and 0.93.

The experiment of Example 1 was repeated except that the beta-nucleator was prepared by mixing 5 gm of terephthalic acid with 5 gm calcium oxide. The K value of the sample, crystallized at a temperature range of 25.degree.-130.degree. C., was between 0.85 and 0.93.

The experiment of Example 1 was repeated except that the polypropylene sample used was a block-copolymer grade 1330 (melt-flow rate 3.0 gm/10 min), supplied by Siangyang Chemical Works. The K value of the sample, crystallized at a temperature range of 23.degree.-120.degree. C., is shown in the following table.

The experiment of Example 1 was repeated except that component B of the beta nucleator was 5 gm barium oxide. The K value of the sample thus obtained was between 0.85 and 0.93 at crystallization temperatures from 25.degree. to 120.degree. C.

The sample prepared according to Example 1 was injection molded into notched specimens for impact testing. When they were tested at 23.degree. C., they exhibited an average value of impact strength of 17.6 kg.cm/cm.sup.2. Whereas the alpha form of the same grade sample was 6.8 kg.cm/cm.sup.2.

The polypropylene pellet sample, prepared according to Example 1, was first extruded into a thin film by a flat-die extruder, then crystallized during cooling. The film was then stretched uniaxially or biaxially at a temperature range between 60.degree. and 140.degree. C. During the stretching process the film was converted into a microporous film which was opaque and was of greatly reduced apparent density. The film was heat-treated in a tensioned state to stabilize the void structure. The resultant film had a whiteness of 95% and was printable on both sides by oil based inks and also by typewriter ribbon ink.

It should be understood that various changes and modifications to the preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention and without diminishing its attendant advantages. It is therefore intended that such changes and modifications be covered by the appended claims.