Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Patent
1997-04-29
1998-02-10
Foelak, Morton
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
521 60, C08J 922
Patent
active
057169985
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELDS
The present invention relates to pre-expanded particles comprising a polypropylene resin and a .beta.-crystalline-nucleating agent, and a method of preparing thereof. And, further in detail, the present invention relates to pre-expanded particles of a polypropylene resin which contains a polypropylene crystal of .beta.-form wherein the percentage of the crystal of .beta.-form in the whole crystalline region is 5 to 100% by weight, which can lower substantially a peak temperature of a melting point, and relates to a process of preparing thereof. The pre-expanded particles thus obtained can provide a molded article superior in heat resistance, buffer property, strength, and rigidity.
BACKGROUND ART
Hitherto, the pre-expanded particles of a polypropylene resin particles have been well-known. For example, in JP-A-65734/1983, there is disclosed pre-expanded particles of polypropylene resin particles characterized in that a volatile foaming agent is contained in the polypropylene resin particle and pre-expansion is carried out by heating.
However, with respect to polypropylene homopolymer, block copolymer of propylene/ethylene and/or .alpha.-olefin, since the melting point of the resulting pre-expanded particles is so high as not less than 160.degree. C., fusion with each pre-expanded particle becomes insufficient and an excellent molded article through molding within dies cannot be obtained.
Also, with respect to a random copolymer of propylene/ethylene and/or .alpha.-olefin which has been used for expansion-molded articles conventionally, since the melting point of the obtained pre-expanded particle is relatively high, and approximately 130.degree. to 145.degree. C., pre-expanded particles which can be used for molding within dies at a lower temperature has been required.
Here, when the pre-expanded particles are used for molding within dies, lowering a steam temperature leads to lowering a steam pressure, which is economically advantageous in such points that a clamping force of a molding machine can be designed to be low and that the molding can be carried out by using the molding machine of a low clamping force, and, further, which is economical in such a point that an amount of vapor per a shot is little.
As processes of preparing pre-expanded particles, in general, a method called as beads expanding method is adopted. This method is a method wherein resin particles and a volatile expanding agent dispersed in water are introduced into a pressure-resistive vessel, the resin particles are partially melted by heating the water dispersion up to a temperature around the melting point of the resin (a part of the crystal of the resin remains not-melted), the resin particles impregnated with the expanding agent are expanded by releasing at a given temperature and pressure into a lower pressure region, for example, atmospheric pressure to give the pre-expanded particles.
In the beads expanding method as described above, since the polypropylene resin particles are heated up to a temperature around the melting point thereof and a part of the crystal remains not-melted, there appear two regions in the crystalline region of the obtained pre-expanded particles, one region being a recrystallized region yielded by cooling the molten resin portion and the other region being a crystalline region yielded by thickening of not-melted crystal. And, when the endothermic peaks of the pre-expanding particle are measured by means of a differential scanning calorimetry (DSC), two endothermic peaks derived from the above-mentioned two crystalline regions are observed. The higher temperature side of the two peaks is derived from the crystalline region of the crystal of .alpha.-form yielded by the thickening and the lower temperature side is the crystalline region of the crystal of .alpha.-form yielded by the recrystallization. However, the peak of lower temperature side is of approximately the same temperature as the melting point of the original resin particle (referring to FIG. 2 hereinafter).
Conventionally, th
Munakata Yasumitsu
Senda Kenichi
Foelak Morton
Kanegafuchi Kagaku Kogyo & Kabushiki Kaisha
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