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United States Patent n*i Vecchiotti mi 3,900,640 l45| Aug. 19, 1975 154 | HOLLOW, MULTI-LAYERED, CROSS-LINKED PLASTIC STRUCTURES AND PROCESS FOR PRODUCING SAME [75] Inventor: Camillo M. Vecchiotti, Ridgewood, NJ. [73] Assignee: Amerace Corporation, New York, N.Y. |22] Filed: Oct. 29, 1973 [21 | Appl. No.: 410,815 [52] [51 | 158 | U.S. Cl........................ 428/36; 136/181; 138/137; 138/146; 206/2; 220/DIG. 14; 264/310; 428/188; 428/420; 428/482; 428/500 Int. Cl.' ............... B65D 85/84; B65D11/20; B32B 27/36; B32B 27/32 Field of Search................... 220/83, 64, DIG. 14; 206/2; 161/188, 139, 231, 233, 247; 138/137, 146, DIG. 7 [561 3.109,763 3,388.007 3.391.823 3.408,234 3.471.359 3.491,799 References Cited UNITED STATES PATENTS 11/1963 6/1968 7/1968 10/1968 10/1969 1/1970 Finger................... ........... 161/231 X Fiandt................... ............... 206/7 x Tijms.................... ............ 161/160 X Ririe..................... ................ 136/181 Goldstein............. ............ 161/231 X Foil....................... ............... 138/137 3.5 28.542 3.556.816 3.561,493 9/1970 Miller........................................ 206/2 1/1971 Nughes.............................. 161/231 X 2/1971 Maillard............................. 161/188 X 1`rinutry Examiner--Harold Ansher Attorney, Agent, or Firm--S. Michael Bender; R. A. Craig; Gregory J. Battersby [57] ABSTRACT Hollow', multi-layered plastic structures are provided comprising a layer of thermoplastic polyester material and a layer of polyethylene, the polyethylene and polyester layers being cross-linked together at their interface. A process for producing such structures which comprises charging a mold with thermoplastic polyester material, rotating and heating the mold so as to fuse and form a layer of the polyester material on the inner surface of the mold, cooling the mold to a point below the melting point of the polyester but above the melting point of a subsequently added cross-linkable polyethylene material, stopping the ro tation and charging the mold with the cross-linkable polyethylene material, resuming the rotation of the mold and heating it to a temperature sufficient to fuse and cross-link the polyethylene with the polyester layer, cooling the mold to a point below the melting point of the polyethylene and removing the finished structure from the mold. 5 Claims, No Drawings 3,900,640 12 HOLLOW, MULTI-LAYERED, CROSS-LINKED PLASTIC STRUCTURES AND PROCESS FOR tating mold to a temperature below the melting point of said polyester but above the melting point of cross- PRODUCING SAME linkable polyethylene so as to crystallize said polyester BACKGROUND OF THE INVENTION layer, charging the slightly cooled but heated mold with 5 a sufficient amount of cross-linkable polyethylene to This invention is directed to hollow, rotation ally form a continuous coating on said polyester layer, ro molded, multi-layered plastic structures and to a tating said heated mold for a sufficient period of time method for forming such structures. More particularly, to form a polyethylene layer of predetermined thick this invention relates to rotationally molded, hollow ness on said polyester layer and to effect cross-linking structures formed from at least two plastic materials 10 across said layers at their interface, cooling said rotat having varying physical and chemical properties, one of ing mold to a point below the melting point of said which is preferably cross-linkable. polyethylene layer so as to solidify the layers, and re The use of plastic in forming relatively large contain moving a hollow, multi-layered structure from the ers which are useful for industrial applications, such as mold. for example, containers for large storage batteries, is 15 Accordingly, it is an object of this invention to pro known in the art. However, because the use of certain duce a hollow, multi-layered plastic structure in which plastics in forming such containers is economically im the respective layers are cross-linked together at their practical, and because of the poor physical or chemical interface. properties inherent in less expensive plastics, the use of It is a further object of this invention to provide a hol such materials in forming industrial scale structures has 20 low plastic structure having an outer thermoplastic not been universally accepted. For example, cases used polyester layer and an inner, cross-linked thermoplastic in manufacturing storage batteries which are installed polyethylene layer. in telephone exchanges are currently manufactured It is still further an object of this invention to provide from thermoset compositions rather than thermoplastic a process for producing hollow, multi-layered plastic materials. While such a composition is said to have the 25 structures which arc characterized by an outer layer desired chemical and load resistant properties required having superior physical properties and an inner layer for that particular use, cases formed from such hard having resistivity to chemical degradation said layers rubber arc quite heavy, are not flame resistant and have being cross-linked together at their interface. poor impact resistance. These and other objects and advantages of the pres Previous attempts to solve the apparent dilemma of 30 ent invention will become more apparent in the ensuing balancing cost with required chemical and/or physical description and appended claims. properties have included the formation of a multi-layer laminate of plastic materials, which are* mechanically or adhesively bonded to each other, one layer having the desired structural strength properties and the other 35 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The hollow, multi-layered structure according to this having resistivity to attack by chemicals. An example invention comprises an outer polyester layer and an of such a multi-layered structure may be found in U.S. inner polyethylene layer, both layers being cross-linked Pat. No. 3,391,823 to Tijms, which employs a layer of together at their interface. foamed polyethylene between a polyethylene layer and The polyester material utilized in forming these a polyester layer to effect a mechanical bond therebe 40 structures may be selected from any of the commer tween. Another example of such a structure is seen in cially available thermoplastic polyesters including lin U.S. Pat. No. 3,652,368, to Formo, which discloses a ear polyester resins such as a linear, highly polymerized method and a special apparatus for rotationally mold ester of tercphthalic acid and a glycol of the scries ing plastic materials in a manner which allows for a HO(CH >)/i OH wherein n is a whole number within the blending of the materials at their interface thus forming 45 range of 2 to 10. An important feature of the selected a mechanically cohesive bond therebetween. Other polyester is its melting point which must be higher than means of bonding dissimilar plastic materials together that of the cross-linkable polyethylene utilized in this have required surface treatments of each plastic layer invention. In a preferred embodiment the polyester ma by methods including flame, electrical discharge, chlo terial will have a melting point between about 400 and rination. or chemical oxidation. 50 about 500F. Filler, including fibrous materials such as SUMMARY OF INVENTION glass, quartz, cotton, asbestos, ramie, and sisal, may be added to the polyester to improve its strength and other It has now been discovered that hollow, multi-layered properties. In a preferred embodiment, the polyester plastic structures can be produced in which the respec 55 will be a thermoplastic, glass-fiber filled, polyester tive layers are bound together at their interface by based condensate polymer marketed by the Celanese cross-linking of the polymeric molecules. According to Plastic Company under its mark "Celanex". Although this invention, one of the layers is a thermoplastic poly the particle size of the polyester is not critical to the in ester. preferably a high melting polyester, while the vention, in general, the polyester material should be other is formed from cross-linkable polyethylene. The 60 finely divided powder and preferably, should have a process utilized in forming such structures comprises particle size ranging from about 30 to 40 mesh when charging a mold with a sufficient amount of polyester passed through a standard sieve. material to form a continuous coating of predeter The polyethylene material used in forming the sec mined thickness on the inner surface of the mold, heat ond layer of the above-indicated multi-layered struc ing the mold to a temperature above the melting point 65 ture must be cross-linkable with the polyester of the of said polyester, rotating the heated mold for a suffi first layer and accordingly, it must contain a cross- cient period of time to fuse said polyester and form a linking agent; for example, a peroxide such as dicumyl layer on the inner surface of the mold, cooling the ro- peroxide, 2,5-dimcthyl 2,5-di (t-butyl peroxy) hexane. 3,900,640 or 2,5-dimcthyl -2,5-di (t-butyl peroxy) hexync -3. In polyethylene through the plug filling'opening. Follow a preferred embodiment, the polyethylene will also ing the addition of the cross-linkable polyethylene to contain a fire retardant agent such as, for example, any the mold and replacement of the filling plug in the of the well known halogcnatcd retardants. The cross- cover opening, the mold is indexed into an oven station linkable polyethylene should be finely divided, for ex 5 set at a temperature between about 425-450F where ample a 50 mesh powder and, preferably, have a melt ing point within the range 250`>-300<lF. A particularly it is rotated biaxially for a period between about 20-25 minutes. The speeds and ratio of rotation of the mold preferred fire retardant, cross-linkable polyethylene is during this period will be the same as those used in the marketed by U. S. I. Chemicals, a Division of National first heating stage. As the cross-linkable polyethylene Distillers and Chemical Corporation, under the mark 10 comes in contact with the crystallized or set surface of "Flamolin". the polyester it will fuse and become uniformly distrib The production of the above described structure ac uted over the surface of the polyester layer by the cen cording to this invention is accomplished in a rotational trifugal force of the rotating cycle. By maintaining the molding apparatus which is biaxially rotated. Any con mold in the oven station at the above indicated temper- ventional rotational casting equipment may be used for 15 ature for the previously suggested period of time, cross- such purposes, such as, for example, any of the equip linking between the polyester layer and the polyethyl ment described in the article by L. A. McKenna, "Ro ene layer will take place at their interface. It will be tational Molding", Modern Plastics Encyclopedia, noted that due to the temperature range maintained pages 780 et seq. (1967). Although the shape of the during this second oven stage there is no melting and mold forms no part of the present invention, it will be 20 consequently, no dimensional distortion of the polyes appreciated that should it be desirable to produce ter layer on the inner surface of the mold. At the end molded shapes such as a battery container, for exam of this oven cycle the mold will then be indexed to a ple, the mold will generally have a corresponding hol cooling chamber where it is cooled to a temperature low rectangular shape. Preferably, and in order to facil below the melting point of the polyethylene and to a itate loading of the mold and withdrawal of the molded 25 point which permits handling of the finished structure article therefrom it is provided at one end thereof with to remove it from the mold. This may be accomplished, a detachable cover including a generally smaller filling for example, by a cooling cycle which includes the use opening therein. Disposed in the filler opening in the of a fan for about 8 minutes followed by a water spray mold cover is a removable plug or similar such closure for about 10 minutes and the fan again for about 2 min means. The rate of rotation of the mold is not critical 30 utes. When the mold has been properly cooled it is to the invention, but it should generally be of a suffi transferred to an unloading station where the cover is cient rate to provide a uniform distribution of the fused unclamped and removed from the mold to allow ready thermoplastic material on the inner surface of the access to the multi-layered molded structure within. mold. Speeds of 12 rpm on the major axis and 15 rpm This molded product is removed from the mold and al on the minor axis with a 4:1 ratio of major to minor axis 35 lowed to cool to room temperature in an upright posi rotation have been found to be particularly effective in tion, usually overnight. In order to finish the resulting achieving such a uniform distribution. product, it may then be cut to its final desired dimen The process for producing the structures of this in sion or form, e.g., to produce a battery container the vention is carried out by first removing the cover with top of the molded article is cut off to produce a con its filling plug and charging the mold with the desired 40 tainer of the proper height. amount of thermoplastic polyester. In the usual case, The following example will further illustrate a spe the mold will be sprayed with a mold release agent cific embodiment of the invention. prior to its charging with the polyester which, in a pre ferred embodiment, will be pre-dried in a hot air circu lating oven. After the mold is filled, the cover and its 45 EXAMPLE A hollow, bi-layered, cross-linked plastic battery case plug are replaced and the mold is passed into the heat was produced according to the following process. An ing stage, usually an oven heated to a temperature be elcctroformed, nickel mold having an end opening de tween about 550F to about 650F, preferably sign and a removable Teflon filling plug in the cover for 600-625F, and rotated biaxially in that stage for a suf the end opening was mounted on a McNeil hot air rotoficient period of time to fuse the polyester and form a 50 cast machine No. 1500-88 (three spindle unit which is uniform coating in the mold. This is usually accom capable of rotating the mold biaxially. The mold had plished in about 20-30 minutes, preferably 25 minutes. been cleaned previously and sprayed with a commer At the end of this period the mold is indexed to a cool cially available mold release agent (FRECOTE No. ing chamber where biaxial rotation is maintained and the mold is cooled to a point below the melting point 55 33). 68 lbs. of CELANEX No. 3310 (glass-fiber filled polyester resin) was charged to the mold through its of the polyester but above that of the cross-linkable open end which was then closed and clamped tight. polyethylene. This cooling may be accomplished with The polyester resin was in finely divided powder form a fan and a cooling cycle of about 7-10 minutes, al having a particle size ranging from about 30 to about though, alternatively, other conventional heat ex 60 40 mesh. The charged mold was indexed into an oven change mediums may be used. At the end of the cool set at 620F where it was allowed to rotate biaxially for ing cycle the mold temperature will generally be be 25 minutes at speeds of 1 2 rpm major axis and 15 rpm tween about 280-350<'F and the polyester layer on the minor axis. These speeds produced a 4:1 ratio of major inner surface thereof will be crystallized or set. At the to minor rotation on the machine. The polyester was end of this first cooling cycle the rotational molding ap 65 thus fused and formed a uniform coating on the inner paratus will be stopped and with its cover maintained surface of the mold. At the end of this oven cycle the thereon the filler plug is removed to permit the mold to spindle was indexed with the mold to a cooling cham be charged with the desired amount of cross-linkable ber where the biaxial rotation was maintained and a fan 3,900,640 56 directed on the mold for 7 minutes to set the polyester polymeric material. layer within. The spindle was then indexed with the The above described process may be used in the pro mold to a load-unload station where rotation was duction of a wide variety of structures including tanks, stopped and the Teflon plug removed from the filling containers, vessels, barrels, and the like. Particularly opening in the mold end opening cover. Through this 5 useful multi-layered structures produced according to filler opening, 15 lbs. of cross-linkable polyethylene this invention serve as cases for industrial size batteries. (FLAMOLIN MF-15711) in finely divided powder Such a case will have an outer surface with the charac form of about 50 mesh was dumped into the mold and teristic structural strength and other properties of poly the filling plug replaced in the mold cover. Biaxial rota ester but the added feature of resistivity to chemical at tion was restarted and the spindle was then indexed 10 tack in its inner surface. with the mold to an oven station set at 450F where it I claim; was kept for 25 minutes so that a layer of polyethylene 1. A laminate comprising a first layer of a thermo was formed on and cross-linked to the polyester layer. plastic polyester and a second layer of a cross-linkable The speeds and ratio of rotation thereof were the same polyethylene, the melting point of the polyethylene as employed in the first heating station. At the end of 15 being lower than the melting point of the polyester, said the second oven cycle the mold was positioned in a layers being directly cross-linked at their interface. cooling chamber where it was subjected to a fan for 8 minutes followed by a water spray for 10 minutes and then a fan again for 2 minutes; thus lowering the tem 2. A laminate as defined in claim 1 wherein said poly ester contains a fibrous filler material. perature to a point below the melting point of the poly ethylene. Following this cooling, the mold was indexed 20 3. A laminate as defined in claim 1 wherein said poly ester is a linear, highly polymerized ester of tereph- to the unload station where the cover was unclamped thalic acid and a glycol of the scries HO(CH^)r OH from the mold and set aside. The mold was then tilted wherejn n is a whole number within the range of 2 to to a 30 angle whereupon the molded battery case 10. dropped out of the mold with a slight pulling force on 25 4. A lamiriatcr as defined in claim t in the form of a the case. After suitable cooling at room temperature, battery case comprising a first layer of glass-filled ther the molded case was cut to desired height. The polyes moplastic polyester and a second cross-linkable poly ter layer in the finished product had a thickness of ap ethylene layer, said layers being directly cross-linked at proximately one-fourth inch while the polyethylene their interface. layer was approximately 0.100 inch thick. 30 5. A laminate as defined in claim 1 wherein said poly As used in this description of the invention the term "multi" is intended to encompass two layers of layered ethylene contains a halogenated flame retardant agent. ' ***** 35 40 45 50 55 60 65