1) Field of the Invention
The present invention relates to uncrosslinked polyethylene particles to be expanded and uncrosslinked polyethylene expanded particles obtained therefrom. 
2) Description of the Prior Art
A method of molding expanded particles of uncrosslinked straight-chain low-density polyethylene (hereinafter abbreviated to LLDPE) in a mold is suggested in Japanese Patent Publication No.  10047/1985, but in the case of expanded particles whose base resin is a polyethylene such as LLDPE, unless the expanded particles are given expandability for pressure pretreatment, satisfactory secondary expansion in a mold is not effected and it is difficult to obtain good molded products because the heating temperature range at the molding of the expanded particles in which a good molded product can be obtained is narrow and the adjustment of satisfactory heating to a proper heating temperature at the molding of the expanded particles is difficult and because of the crystalline structure of the polyethylene.  Therefore, when uncrosslinked straight-chain low-density polyethylene expanded particles are molded, generally, a technique is used wherein, prior to the molding, the same expanding agent gas or an inorganic gas, such as air, is further added to increase the internal pressure.  However, this technique wherein the same expanding gas or an inorganic gas is further added to expanded particles is attended with the problem that not only the equipments are massive but also the number of steps increases, leading to a high production cost of the molded products.  Further, in general, in the case of polyolefin resin expanded particles, even if the internal pressure is increased by further adding an inorganic gas or the like to give expandability to the expanded particles, since the pressure in the particles easily escapes, it is difficult to keep the expandability for a long period of time, and therefore in order to obtain excellent molded products by the conventional technique, after giving the internal pressure, the expanded particles have to be used in a short period of time, so that the molder cannot produce molded products easily by using simply the expanded particles directly supplied from the producer of the expanded particles. 
As means of solving these problems, the applicant suggested, as LLDPE expanded particles capable of being molded without specifically giving an internal pressure, LLDPE expanded particles having a crystalline structure wherein two endothermic peaks appear in a DSC curve obtained by differential scanning calorimetry (hereinafter referred to as DSC), the DSC curve being obtained by heating 1 to 10 mg of the pre-expanded particles to 220.degree.  C. at a rate of temperature rise of 10.degree.  C./min by a differential scanning calorimeter, and the energy of the endothermic peak on the higher temperature side is 5 J/g or over (Japanese Patent Application Laid-Open No.  1741/1989). 
However, in LLDPE particles that are a raw material of such LLDPE expanded particles, the proper expanding temperature range suitable for expansion is very narrow, and therefore it was quite difficult to obtain expanded particles securely wherein two endothermic peaks appear in the above DSC curve and the energy of the endothermic peak on the higher temperature side is 5 J/g or over. 
Thus, the applicant suggested a method wherein use is made of, as raw material resin particles of expanded particles, LLDPE particles having a crystalline structure in which a double peak appears in the DSC curve so that LLDPE expanded particles having a double peak in the DSC curve can be obtained securely (Japanese Patent Application Laid-Open No.  43206/1990). 
However, even when the LLDPE particles having a crystalline structure wherein a double peak appears in the DSC curve are used, the expanding proper temperature range for obtaining expanded particles in which a double peak appears in the DSC curve is not very wide and it is hard to say that intended expanded particles can be produced securely.  In addition, halogenated hydrocarbon expanding agents conventionally used as excellent expanding agents cannot be used because there is the problem of destroying the ozonosphere and in the case of inorganic gas expanding agents that are conventionally not used as expanding agents for uncrosslinked LLDPE, there is the problem that it is difficult to securely obtain uncrosslinked LLDPE expanded particles having a double peak at a high expansion rate. 
Further, even LLDPE expanded particles wherein a double peak appears in the DSC curve of the expanded particles and that can be molded without pressurizing pretreatment are attended with problems to be solved in view of the moldability.  That is, if the energy of the endothermic peak of the double peak on the higher temperature side is great, the fusibility between the expanded particles at the time of molding is poor and the expandability of the expanded particles is low.  Thus, if the molding temperature is increased in order to make good the expandability and fusibility of the expanded particles, such a problem arises that the surface of the expanded molded product is melted to make the state of the surface degraded.  On the other hand, if the energy of the endothermic peak on the high temperature side is small, even at the usual molding temperature the dimension of the molded product is degraded since the molded product is high in shrinkage.  Thus, if the molding temperature is lowered, the degradation of the dimensional accuracy can be prevented, but the fusibility between the expanded particles becomes poor and a good molded product cannot be obtained. 
Thus, in the case of the conventional expanded particles, in addition to the problem that it is difficult to obtain expanded particles having a high expansion rate when inorganic gases are used as expanding agents, there is the problem that it is difficult to obtain a good molded product excellent in the state of the surface, in the fusibility, in the secondary expandability, and in the dimensional accuracy.