United States Patent [19] [11] Patent Number: 4,519,964 Rosen [45] Date of Patent: May 28, 1985 [54] PROCESS FOR PREPARING PLASTIC 3,814,778 6/1974 Hosoda et a1. ................... .. 264/454 ARTICLES HAVING AN OUTER SHELL AND 3,984,451 10/1976 Weiss . INNER FOAM CORE 4,144,296 3/1979 Dickens ............................ .. 264/45.4 [75] Inventor: Haim Rosen, Tel-Aviv, Israel FOREIGN PATENT DOCUMENTS [73] Assignee: Rotoplas, Ltd., Jerusalem, Israel 2401034 7/1975 Fed Rep- of Germany - [ 21 ] A PP l. No.: 572,044 Primary Examiner—Phili p Anderson [22] Filed: Jan. 19, 1984 133331;} Agent, or Firm-Pollock, Vande Sande & [51] Int. Cl.3 ............................................ .. B29D 27/00 [52] US. 01. ................................ .. 264/454; 264/457; [57] ABSTRACT _ 264/46-6 A process for preparing plastic articles having an outer [58] Field of Search ................... .. 264/454, 46.6, 45.7 rigid Shell and an inner polystyrene foamed core’ [ 56 1 References Cited whereby pre-expanded polystyrene beads are foamed Us PATENT DOCUMENTS and fusecl inside a hollow thermoplastic she-ll, compris mg, coating the inside of the shell prior to introducing gllggnik -------------------------- the polystrene heads into the hollow shell, with a ther , , erg . 1 ~ - _ 3,309,439 3/1967 Nonweiler .1 ...... .. 264/454 mop asnc bondmg layer 3,455,483 7/1969 Inklaar ...... .. 264/454 X 3,505,137 4/1970 Kliene .......................... .. 264/457 X 6 Claims, 1 Drawing Figure FORM A SHELL OF POLYETHYLENE M OLD ON INNER SURFACE OF SHELL A THERMOPLASTIC POLYMER 1 FILL THE SHELL WITH POLYSTYRENE BEADS IEXPANDl BEADg US.‘ Patent , May28, 1985 1 4,519,964 FORM A SHELL OF POLYETHYLENE MOLD ON INNER SURFACE OF SHELL A THERMOPLASTIC POLYMER FILL THE SHELL WITH POLYSTYRENE BEADS l EXPAND BEADS 4,519,964 1 2 . Products prepared by the above method have one PROCESS FOR PREPARING PLASTIC ARTICLES serious defect in that the polystyrene foam is not HAVING AN OUTER SHELL AND INNER FOAM bonded to the outer shell and thus easily separates there CORE from to form air pockets. Such air pockets deform the shape of the body. Furthermore, they give an unpleas The present invention relates to a process for the ant feeling when pressed, as in the case of a windsurfer preparation of plastic articles having a thermoplastic when one steps on a section having an air pocket under» outer shell and an inner core of tightly bonded polysty neath. rene foam. It is therefore the object of this invention to provide More particularly the invention relates to a method a method for preparing articles having an outer shell of for preparing plastic articles by rotationally molding an thermoplastic polymer and a polystyrene foam core outer shell and compressing pre-expanded polystyrene which is tightly bonded to the shell. beads inside the shell to give a tightly bonded inner It is a further object of this invention to provide a foam core. method for preparing articles having a composite outer Plastic bodies having rotationally molded outer ther 5 shell comprising a dense layer and a rigid foam layer moplastic skins and inner foamed cores have been and an inner polystyrene foam core which is tightly known for some time. For example, US. Pat. No. bonded to the shell. 3,455,483 discloses a method for preparing rotationally The FIGURE is a diagrammatic representation of molded articles by ?rst molding an outer polyethylene our process for making plastic articles. skin and then an inner polyethylene foamed core. Simi We have discovered that in the process for making larly, U.S. Pat. No. 3,505,137 discloses a process for articles having an outer thermoplastic shell and inner rotationally molding hollow bodies reinforced with a polystyrene foam core, wherein the polystyrene core is foam core particularly suitable for withstanding high made by expanding pre-expanded polystyrene beads, if external pressure such as ?shing ?oats. US. Pat. No. there is applied a bonding layer unto the inner surface of 3,814,778 discloses yet another method for preparing 25 the shell prior to expanding the polystyrene beads, the rotationally molded composite foamed articles in a one resulting foam will adhere tightly to the shell. step operation. All these methods involve rotationally In particular we have found that if the inner surface molding the skin as well as the foam core which limits of the shell is coated with a layer of a copolymer se the method to materials which can be rotationally lected from ethylene-alkyl acrylates, ethylene-alkyl molded and in the presence of each other. Furthermore, 30 methacrylates and ethylene-vinyl esters prior to ex foams prepared by rotational molding as described in panding the polystyrene beads, a strong bond is formed the above patents have a generally high density. This is between the foam and shell. particularly the case where the article is a composite of The preferred process of this invention thus involves: a dense layer and foam. a. forming an outer shell by rotationally molding a There have also been known methods for preparing 35 thermoplastic material, rotationally molded plastic bodies having a thermoplas b. rotationally molding unto the inner surface of the tic skin and polystyrene foam core. Thus US. Pat. No. thermoplastic shell a bonding layer comprising a co 2,989,783 describes a method whereby polyethylene polymer selected from ethylenealkyl acrylates, ethy powder and a bag of thermoplastic material containing lene-alkylmethacrylates and ethylene-vinyl esters, expandable polystyrene beads are rotationally molded. 40 c. ?lling the shell with pre-expanded polystyrene US. Pat. No. 3,309,439 discloses a so-called “one shot” beads and introducing pressurized steam into the shell process wherein a free ?owing molding composition to expand and fuse the pre-expanded beads to form a comprising a compound containing chemically bound tightly adhering polystyrene foam core. water is incorporated in the rotational molding process. The method of rotational molding is well established, US. Pat. No. 3,984,451 teaches the use of expandable 45 and the outer shell for preparing articles according to polystyrene beads having less than 3 percent of blowing the process of this invention may be prepared in accor agent for preparing a foam by rotational molding. dance with this known art. Thermoplastics which can The foregoing patented methods have not been found be rotationally molded and which can form shells for to be entirely satisfactory in many cases. One of the this invention are, polyethylene, polypropylene, ethy disadvantages is their limitation to preparing articles of lene-propylene copolymers, PVC, polycarbonate, ther relatively small cross sections, where the foam core moplastic polyesters and others. In general, polyethyl between the outer skins is only a few inches thick. This ene and its copolymers are preferred. This outer shell limitation is due to the insulating properties of the foam can be any practical thickness, from 0.5-20 mm thick, which prevents proper heat transfer from the outer preferably 0.5-10 mm thick. surface to the inner core, thereby not enabling adequate The outer shell can also be a composite made from expansion of, for example, polystyrene beads. two or more layers of the same or different polymers. Therefore, where polystyrene foam is desired as the For example, a composite shell can be molded having a inner core of a rotationally molded article of substantial dense polyethylene layer and a second inner layer of cross section, the foam is prepared separately after the rigid foamed polyethylene. This type of shell is often molded article has been removed from the rotating 60 preferred, particularly when stiffness and dimensional mold by ?lling the hollow article with pre-expanded stability are required. The rigid foam layer of the com polystyrene beads and introducing steam into the hol posite shell can be any thickness desired, but in general low shell, thereby providing the necessary heat to ex it is practical only to thicknesses of less than 10-20 mm. pand and slightly fuse the beads into a foam body. This The bonding layer must be one that is compatible to latter operation is usually conducted in a separate mold some extent with the shell and the foamed polystyrene. whose purpose is to hold the shape of the article and not Preferred materials for this layer are copolymers of allow any distortions thereof during the expansion and ethylene with alkyl acrylates, alkyl methacrylates and fusion of the inner core. vinyl esters. This bonding layer should be thick enough 4,519,964 3 4 to bond the shell to the polystyrene foam. Practically and placed into a ?tting mold of concrete. Pressurized this is in the order of 0.2-3 mm, preferably 0.3-1 mm. steam (2-3 atrn.) was introduced into the bed of polysty Suitable copolymers of this type are ethylene rene beads in the hollow of the shell via pipes, perfo methylacrylate, ethylene-methyl methacrylate, ethy rated at intervals, to evenly distribute the steam. When lene-ethylacrylate, ethylene-ethyl methacrylate each the polystyrene was fully expanded the pipes were re-v containing from about 60% up to 95% ethylene and 5% moved and the windsurfer taken out of the mold. A to about 40% acrylate or methacrylate ester, and ethy cross-section of the windsurfer showed a fully foamed lene-vinyl esters of which ethylene-vinyl acetate (EVA) core of polystyrene tightly bonded to the shell. This is most preferred. The EVA usually has a 10% to 40% bond remained even after repeated compression. A vinyl content. Rotational molding grade copolymers of similar windsurfer ?lled with polystyrene foam but these types are available and suitable for- use in this without an ethylene-vinyl acetate bonding layer invention.