PROCESS AND APPARATUS FOR PRODUCING BLOW MOLDED THERMOPLASTIC ARTICLES HAVING IMPROVED WELD STRENGTH

Abstract: Weld seams of parisons formed in an inert atmosphere during extrusion followed by blow molding with a blowing gas containing an oxidizing agent are extremely strong when compared with conventional seams formed in the presence of air. By sealing in an inert atmosphere and subsequently blowing the parison with a mixture of nitrogen and fluorine, a container is produced having both high weld seam strength and high levels of impermeability to non-polar hydrocarbons.

Description: PROCESS AND APPARATUS FOR PRODUCING BLOW MOLDED THERMOPLASTIC ARTICLES HAVING IMPROVED WELD STRENGTH TECHNICAL FIELD This invention relates to an improvement in a continuous process and an apparatus for manufacturing blow molded hollow articles and, in particular, a process and an apparatus for producing such articles having improved weld strength. BACKGROUND OF PRIOR ART It is well known to blow mold bottles and other hollow articles from thermoplastic materials by extruding the materials through an annular extrusion nozzle to form a tubular parison, blowing air through the parison to maintain it pin an open position and, after the parison has been enclosed within a mold and both of its ends have been welded or sealed together to form a weld seam, injecting into the parison a compressed fluid or blowing gas, e.g. water, air, nitrogen, argon and the like, while it is still in a molten state; see U.S. Pat. No. 3,168,207. It is also known to expand and simultaneously cool a thermoplastic preform or parison with, for example, LIN (liquefied nitrogen) so that both the expansion and cooling of the hollow article is done with the same medium; see British Pat. No. 1,397,513. A modification of this process is to blow air into the parison along with the LIN to intensify the expansion operation; see U.S. Pat. No. 4,021,517. Examples of thermoplastic materials or resins which have been employed in the production of blow molded articles include polymers and copolymers of polystyrene, polyacrylonitrile, polyvinyl chloride and polyolefins having at least one aliphatic mono-l-olefin with a maximum of 8 carbon atoms per molecule. The preferred types of thermoplastic materials for blow molding include polyolefin polymers and copolymers of ethylene, propylene, l-butene, l-pentene, 4-methyl-1- pentene, 3-methyl-l-butene and 3, 3-dimethyl-l-butene. The resulting blow molded bottles from the foregoing processes, particularly when polyolefins are used as the thermoplastic material, have the disadvantage of allowing the passage of non-polar solvents through the walls thereof. A post-treating method for improving barrier properties and rendering the blow molded polyolefin bottles relatively impermeable to the passage of such-solvents is described in U.S. Pat. No. 2,811,468. In this process, the surface of the previously formed blow molded bottle is fluorinated with pure fluorine or a mixture of fluorine and air or nitrogen. The resulting fluorinated containers have been found to be greatly improved in their barrier properties with respect to hydrocarbon solvents. A more efficient and economical process for obtaining hollow articles having improved barrier properties is described and claimed in U.S. Pat. No. 3,862,284, which process includes a step in which 0.1 to 10% by volume fluorine is blended into a fluid medium prior to expanding the parison within the closed mold to conform the parison to the contour of the mold. One embodiment of this process is described and claimed in German OLS 29 24 797 in which an extruded parison is continuously formed, the fluorine/inert gas mixture is introduced as the parison is sealed off from the outside atmosphere, the parison is introduced into the blow mold and an inert pressurizing gas is injected into the blow mold to expand the parison within the mold. As set forth in greater detail in the examples below, it was quite unexpectedly found that containers produced by the process of the German OLS readily failed at the weld seam formed by pinching off the ends of the parison as the mold assumes a closed position. Although not wishing to be bound by the explanation, it is believed that the ends would not properly fuse together because the fluorine confined within the parison reacted with its ends to form a crystal fusion point. As a result of this, the finished bottle became separated at its weld seam during a standard bottle drop test. In fact, the bottles made in this manner can be separated at the weld seam by squeezing the bottle between one's hands. The standard plastic bottle drop test comprises filling the bottle with water, capping the filled bottle with a metal closure and dropping the bottle from various heights onto a concrete floor. The height at which the bottle fails is recorded as evidenced by any noticeable split, tear or similar indication of weakness in the dropped container. It has been recently published to use an open parison purge with nitrogen to prevent air from interferring with the fluorination process in a batch-type blow molding operation; see "Plastic Engineering", September 1978, pages 40-43. However, in this process air in the atmosphere is in contact with the open parison prior to being pinched off from the extruder. This allows the possibility of air preventing an adequate weld after the pinch-off. It is recognized that nitrogen can be used as one of the gases in the hot welding process for welding two sheets of polyethylene film which is known to oxidize in the presence of air; see the "Plastics Engineering Handbook", McGraw Hill Publishing Company, pages 784 and 785, and "Plastics Engineering", April 1980. However, the fact that the welding process takes place entirely in an air atmosphere means that there is no recognition of combining this teaching of plastic welding with those of the blow molding prior art. In fact, if one were to combine such teachings, one would be directed to proceed in a contrary manner especially when one desires to produce a container that is impermeable to non-polar solvents. BRIEF SUMMARY OF INVENTION Contrary to that disclosed in the prior art, the process and apparatus of the present invention overcomes its disadvantages in producing blow molded articles that have welds that are considerably stronger than those of the prior art. In accordance with the present invention, a process is provided for the production of blow molded thermoplastic hollow articles which comprises the steps of: (1) extruding a thermoplastic material to form a tubular parison closed at on end, (2) introducing an inert gas into the open end of the parison, (3) closing a blow mold around the parison to seal off the open end, and (4) injecting a blowing gas containing an oxidizing agent into the closed parison until the parison conforms to the contour of the mold. The inert gas is selected from the group consisting of nitrogen, argon, helium xenon, carbon dioxide and neon. After the first two hollow articles or containers are recovered from the blow mold, each of the subsequent containers have improved weld strengths. The first and second containers from a given continuous blow molding operation have been found to have less weld strength than the subsequent containers. This is believed to be the result of oxygen in the air surrounding the open parison which prevents an adequate weld from occurring. After the first two containers have been produced, the interior of the subsequent closed parisons including the welds are formed completely in the absence of the surrounding atmosphere. The end of the parison opposite the extruder head, or the bottom end in a typical vertical extrusion operation, has been previously sealed closed in the presence of nitrogen as are all subsequent parisons during the continuous blow molding operation. The inert gas is forced under pressure during the pre-blow step (2) into the parison against this closed end until the blow mold is closed around the parison and the top end of the parison is sealed closed. The closure of the mold around the parison simultaneously pinches off and cuts through the resulting weld seam above the parison within the blow mold. This results in the inert gas being under a positive pressure within the closed parison. This is necessary to insure that the ends of the parison do not come into contact with an oxidizing agent before each end has been welded shut. Once the parison containing the inert gas under pressure is closed off from the surrounding atmosphere, the interior of the closed parison is injected with a blowing gas containing an oxidizing agent. This is done by piercing the outer wall of the parison by means of a blow pin or inflation needle and passing the blowing gas through openings in the needles until the parison conforms to the inner walls of the mold. The blowing gas in conventional blow molding operations is air and thus the most common oxidizing agent for the process of this invention is oxygen. However, the process of the present invention is particularly adapted to the embodiment in which the oxidizing agent is fluorine. Preferably, in this embodiment the blow gas that is used comprises about 0.1 to 10% by weight fluorine and the balance nitrogen, argon or helium. The avoidance of oxygen during the extrusion and pre-blow steps assures that no oxygen is present during the blowing step. The presence of oxygen has been found to be detrimental to the fluorine-containing blowing mixture; see "Plastics Engineering," September 1978, pages 40-43. In this embodiment the gaseous mixture is evacuated from the blow molded article while it is still in the mold by means described below in the DETAILED DESCRIPTION OF INVENTION. BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a partially schematic plan view of one side of a suitable blow molding apparatus for carrying out one embodiment of the process of the present invention; FIG. 2 is a partially schematic plan view of the front of a suitable apparatus for carrying out one embodiment of the present invention and illustrates the first two steps of this embodiment; FIG. 3 diagramatically illustrates the third step of one embodiment of the present invention; FIG. 4 diagramatically illustrates the fourth step of one embodiment of the present invention and the step of evacuating the blowing gas from the blow molded article in another embodiment of the present invention; and FIG. 5 is a partially schematic plan view of a dual device for alternately injecting blowing gas into the parison and evacuating the blowing gas from the resulting container in still another embodiment of this invention. DETAILED DESCRIPTION OF INVENTION As shown in FIGS. 1 and 2, one apparatus suitable for carrying out the process of the present invention includes conventional extruder 1 for forming an expandable parison closed at its bottom end, rotary wheel multimold machine 2 having a plurality of blow molds with internal surfaces corresponding to the containers to be produced and into which each of the expandable parisons are separately and sequentially introduced, pipeline 3 for supplying nitrogen gas during the extrusion step or pre-blow step (2) and other means described in detail below for introducing a fluorine/nitrogen gaseous medium into the interior of the expandable parisons and means for evacuating the blowing gas from the resulting blow molded containers. In the embodiment of the present invention illustrated in FIGS. 1-4, a polyethylene tubular parison 4 is continuously extruded from extruder head 5 of extruder 1 positioned above rotary wheel, blow molding machine 2 operably mounted on frame 6 for forming bottles or other hollow articles of approximately 5 gallon capacity or less from the extruded parison. Parison 4 is maintained in an inflated position by a stream of commercially pure nitrogen (99.9% by volume) supplied through pipeline 3 to pressure the pre-blow system. The pressure for this system is established by means of regulator 6, preferably at a water pressure of at least 1 inch and up to about 5 inches. The nitrogen flow is shut-off when blow molder 2 is not in use by means of solenoid valve 6a. Parison 4 extends between right mold half 7a and left mold half 7b of bottle mold 10 and terminates in weld seam 11 formed during a previous blow molding operation. During the startup of blow molder 2, this seam can be formed by hand, Bottle molds 12 and 13, shown in phantom in FIG. 1, are simultaneously subjected to separate steps in the blow molding process as described below. The time limiting step for the blowing operation is that time it takes for the parison to become extruded to its full length between, for example, mold halves 7a and 7b. During the time of extrusion, the parison in the position shown for mold 12 is undergoing the blowing, and blowing gas evacuation steps. The mold halves open upon being rotated between the positions shown for molds 12 and 13. As the mold opens, the resulting blow molded bottle drops to a suitable conveyor (not shown) for transporting the bottle to deflashing and other finishing steps well known in the art. The mold halves (not shown) in the position shown for mold 13 are open awaiting rotation to a position under extruder 1 as shown in FIG. 2. Each half of the bottle mold is operably connected to its respective mold clamp cylinder for activation in a conventional manner. As illustrated in FIG. 2, hydraulic fluid flows through line 20 which is connected to right distributor shaft 21a within right distributor housing 22a and flows through channel 23, shown by a dotted line in FIG. 2 that is drilled into shaft 21a. Channel 23 is in fluid communication with a hydraulic fluid port 24. The alignment of an opening in port 24 with cap plug 25 in housing 22a permits the hydraulic fluid to flow through line 26 and hydraulic fluid inlet port 27 in mold clamp cylinder 28a and to close mold 10. In the closed mold position shown in FIG. 3, knife edges 29a and 29b meet to sever parison 4 above mold 10 forming weld seams 30a and 30b (FIG. 3). All of the elements on the left side of mold 10 with the exception of the blowing gas evacuation system described below are the mirror image of those elements on the right side of 10. After each interval of time equal to the extrusion time referred to above, a pneumatic system (not shown) activates a suitable prime mover (not shown) connected to right housing 22a and left housing 22b, which prime mover rotates right wheel 32a and left wheel 32b. This, in turn, rotates right and left mold clamp cylinders 28a and 28b mounted on wheels 32a and 32b, respectively. After the wheel 32a, rotates mold 10 in the position of mold 12 shown in FIG. 1, and mold 10 is closed, a pneumatic signal activates injection needle or blow pin 33. It is the rotation of housing 22a and housing 22b that opens and closes the hydraulic fluid ports in distributor shafts 21a and 21b, which shafts remain stationary. During the time interval illustrated in FIG. 4, a pneumatic signal opens valve 34 in line 35. This permits the blowing gas entering distributor shaft 21a via line 36 to flow through the shaft in a similar manner to that described above in connection with the hydraulic system. The same pneumatic signal that opens valve 34 simultaneously activates blow pin 33 and causes it to puncture the bottom wall of parison 4 to the right of weld seam 11 and to retract the blow pin after the blowing step. During the blowing step, blowing gas passes through channel 37, port 38, cap plug 39, valve 34, line 35, blow pin 33, and a plurality of openings (not shown) in blow pin 33 and into the interior of parison 4 for a period of time to cause the parison to conform to the contour of mold 10 as shown in FIG. 4. Throughout the blow molding operation the temperature of the interior surfaces of molds 10, 11 and 12 is controlled by cooling water. For example, the water is continuously circulated through inlet 40 on right half mold 7a, cooling coil 41 and outlet 42 in a similar manner to that described in U.S. Patent No. 3,891,371, which description in incorporated herein by reference. By maintaining a continuous circulation of water through each mold, the resulting blow molded bottle is suffacroa#, ly cooled at the end of the blowing gas step to be ejected from the mold without additional #llowan##e of time in the blowing cycle for cooling. In the embodiment in which the blowing gas contains a component which would be harmful if allowed to discharge into the surrounding atmosphere, i.e. such as a dilute fluorine-inert gas mixture, evacuation needle 43 pierces the bottom wall of parison 4, just to the left of weld seam 11 upon activation of the same pneumatic signal as that given to blow pin 33 and while parison 4 remains in a molten condition. It is not practical to pierce the neck of bottle 44 after it has been blown. Upon a separate pneumatic signal, valve 45 opens and the blowing gas within bottle 44 is evacuated through line 46 and line 49 in the reverse manner as that described above in connection with the blowing gas. The evacuated gas is passed to suitable exhaust facilities where it may be vented, incinerated or scrubbed. The nitrogen free of oxidizing gases may be recovered for recycle back to the blending system described below. FIG. 2 also illustrates an absorption system in which the evacuated gas in line 49 is passed in countercurrent flow to water in line 53 within absorption or scrubber column 54. The fluorine or other absorbed gas dissolves in the water and is discarded to an appropriate wastewater facility through line 55. Alternately, a caustic scrubber can be used in place of the water scrubber The nitrogen recovered in line 56 can be recycled for use in the pre-blow and/or blowing gas injection steps or it may be released to the atmosphere. To avoid the possiblility that a malfunction in blow molder 2 will release any harmful gas during operation into the atmosphere, an exhaust shroud (not shown) is provided and is connected to an exhaust fan (not shown). In addition, the exhaust shroud and fan serve to eliminate the nitrogen used in the pre-blow and for purging the mold, if necessary, from building up in the area surrounding the blow molder. A more complete discussion of such a shroud and fan system is found in an article entitled "Single-step Barrier Coating for Containers", Plastic Engineering, September 1978, pages 40-43, which discussion is incorporated herein by reference. After mold 10 has undergone the evacuation, mold halves 7a and 7b open under hydraulic pressure through lines 59a and 59a' and 59b and 59b' in the same manner described above and wheels 32a and 32b rotate mold 10 into the position shown for mold 12 and mold 13 into the position shown for mold 10 in FIG. 1. The continuous blow molding cycle set forth above is repeated. The separate ports within distributor shafts 21a and 21b for distributing the fluids used in the blow molding process are separated by packing rings, e.g. ring 60 shown in FIG. 2. A description of a preferred distributor housing and shaft construction and its operation can be found in U.S. Patent No. 3,891,371, which description is incorporated herein by reference. A blend system is shown in FIG. 1 for preparing a suitable blowing gas for the preferred embodiment of the present invention. Pure nitrogen, stored in a liquid form in tank 61, is vaporized in vaporizer 62 and blended with a commercially available higher concentration of fluorine in nitrogen mixture stored in cylinders 63, i.e. 10% by volume fluorine and 90% by volume nitrogen, in gas blender 64 to form the blowing gas mixture of the desired concentration ranging from about 0.1 to 10% by volume fluorine. Upon a proper setting of blender 64 to achieve the desired ratio of F2/N2, the gaseous mixture flows through line 36 as discussed above. Referring now to FIG. 5, dual device 70 comprises exhaust needle 72 having a plurality of openings 73 around the periphery of the upper portion of needle body 74 and blow pin 75 concentric to exhaust needle 72 and having a plurality of openings 76 in needle 77 of blow pin 75. Conduits 81 and 82 are connected to exhaust needle 72 and blow pin 75, respectively and can be used in place of blow pin 33 and exhaust needle 43 shown in FIGS. 2-4. Upon activation of a timed pneumatic signal, air passes through valves 84 and 85 causing the upper portion of body 74 of needle 72, needle 77 of blow pin 75 to pierce the bottom of parison 4 on either side of seam 11 so that openings 73 and 76 are well within the parison. After the blowing gas through line 82 and openings 76 has caused the parison to assume the shape of the mold, air passes through valve 87 to retract blow pin 75 to its original position exposing openings 73. The blowing gas is evacuated through openings 73 and line 81 and is removed from the system in the manner discussed above. Air is then passed through valve 88 to retract exhaust needle 72 to its original position. The process of this invention is primarily designed to produce relatively small containers, i.e. those which are five gallons in capacity or less. It would be impractical to construct a blow molding machine for carrying out the process of this invention to manufacture containers much greater than five gallons. The invention is further illustrated by reference to the following examples: EXAMPLE 1 Tests were carried out to demonstrate the effect of pre-blowing at a pressure of about 1.5 inches of water with air (Test 1), a gaseous mixture comprising 1.5% by volume fluorine and 98.5% by volume commercially pure nitrogen as recommended in OLS 29 24 797 (Test 2) and commercially pure nitrogen (Test 3). High density pellets of polyethylene (0.92 gms./cc/) were fed to the extruder 1 operating at an average temperature of 2000C (about 3900F) and extruded through extruder head 5 maintained at about 2000C to form parison 4 closed at one end during a total elapsed time of 2 seconds as illustrated in FIG. 2. Each of these gases was respectively passed through line 3 during the extrusion of parison 4. The blowing step for each test in this example took 5 seconds using air as the blowing medium through blow pin 33 to produce a nominal 2 1/2 gallon jug. Water was circulated through cooling coils in the molds throughout the blowing molding operation. The blown containers of test 2 were purged with nitrogen and the gases were evacuated to a vent system prior to ejecting the containers from the mold to prevent contamination of fluorine in the surrounding atmosphere. This step took about 8 seconds. The containers were ejected from the mold and were each subjected to the same standard mold flashing removal and finishing steps. Each container had a bottom weld seam of approximately 0.011 cm. (0.0045 in.) thickness and an average wall thickness of approximately 0.074 cm. (0.029 in.) After each of tests 1, 2 and 3, the resulting jugs were filled with 2 gallons of water, capped with a metal cap and dropped on to a concrete floor from a height of 4 feet. The container undergoing test 2 failed by bursting at its weld seam. The containers from tests 1 and 3 were then dropped from 6 feet. The container of test 1 burst at its weld seam from this height. The container of test 3 blown in accordance with the process of the present invention was then dropped from heights of 15, 30 and 35 feet. At 35 feet, the container failed where the polyethylene sidewalls were the thinnest, but not at its weld seam. The results are tabulated in the table below where P equals passed and F equals failed at the weld seam. Test (1) (2) (3) No. of Height Trials of drop AIR gN2/F N 1 4 foot P F P 4 6 foot F - P 1 15 foot - - P 1 30 foot - - P 1 35 foot - - * *Failure occurred at side panel of container, but not at the weld seam. EXAMPLE 2 Tests were then carried out to see if the blowing gas introduced through blow pin 33 had any effect on the weld strength of the resulting container. It was found that no noticeable effect was recorded. In fact it was noted that when the blowing gas comprised a mixture of 1.5% by volume fluorine in nitrogen, the final container had the same high weld strength of the container for test 3 above and had high impermeability to non-polar hydrocarbons such as toluene. In a standard toluene impermeability test described in Example 1 of U.S. 3,862,284, the 2 gallon container retained only 5088 gms. of toluene after 30 days at 1000F. In contrast, the container of test 1, retained 8042 gms. of toluene for the same period. Mixtures of between 0.S and 2% by volume fluorine and the balance nitrogen were also found to result in a satisfactory container from a weld strength and impermeability standpoint. Various modifications to the process and apparatus described above are envisioned, for example the injection of the blowing gas can be supplemented by a provision of a vacuum circumjacent the outside of the parison.

Claims: What is claimed is: 1. A process for blow molding an article which comprises the steps of: (1) extruding a thermoplastic material to form a parison closed at one end, (2) introducing an inert gas into the open end of the parison, (3) closing a blow mold around the parison to seal off the open end thereof, and (4) injecting a blowing gas containing an oxidizing agent into the closed parison until the parison conforms to the contour of said mold. 2. The process of Claim 1 wherein said inert gas of step (2) is a commercially available nitrogen containing at least 99.9% by volume nitrogen. 3. The process of Claim 1 wherein said oxidizing agent of step (4) is oxygen or fluorine. 4. The process of Claim 1 wherein said blowing gas of step (4) comprises a mixture of about 0.1% to 10% by volume fluorine and the balance being an inert gas. 5. The process of Claim 4 wherein step (5) is included and comprises evacuating the blowing gas from the resulting blow molded article after step (4) without contaminating the surrounding atmosphere. 6. The process of Claim 5 wherein step (6) is included and comprises scrubbing the fluorine from the balance of the evacuated blowing gas from step (5) and recovering the resulting fluorine-free inert gas. 7. The process of Claim 6 wherein said fluorinefree inert gas from step (6) is recycled for use in step (2) and/or in step (4). 8. The process of Claim 6 wherein said fluorinefree inert gas from step (6) contains at least 99.9% by volume nitrogen. 9. A process for blow molding a container which coinprises the steps of extruding a thermoplastic material to form a parison closed at one end thereof, introducing an inert gas into said parison as it is eXtruded to maintain said parison open and free from other gases, closing a mold around said parison so that it is closed at both ends, expanding said parison with a blowing gas containing an oxidizing agent to conform the parison to the interior surface of the mold and to thereby form the container, cooling the container, removing the container having improved weld strength from the mold and repeating the method. 10. In an apparatus for blow molding an article comprising in combination an extrusion means for continuously forming an expandable parison of a thermoplastic material which parison is closed at one end, a plurality of blowing molds having internal surfaces corresponding to the contour of the article to be produced and into which each expandable parison is sequentially introduced and the other end of the parison is sealed from the surrounding atmosphere and blow pin means for injection into the interior of the parison enclosed within said blowing mold, the improvement for preventing failure of the weld seam at each end of the parison, which comprises first means for introducing an Inert gas into the parison as it is being formed and second means in fluid cqflmunication with said blow pin mtaps for supply-ing said pin with a gaseous blowing gas containing an soxidiginq agent to conform the parison to said internal #urf#c#e of said moldy 11. The apparatus of Claim 10 wherein said oxidizing agent is oxygen or fluorine. 12. The apparatus of Claim 11 wherein blending means is in fluid communication with said second means for blending fluorine or a fluorine-containing gas with an inert gas to serve as the blowing gas. 13. The apparatus of Claim 12 wherein said blowing gas comprises a mixture of about 0.1 to 10% by volume fluorine and the balance being an inert gas. 14. The apparatus of Claim 12 wherein evacuating means are provided for evacuating the blowing gas from the resulting blow molded article. 15. The apparatus of Claim 14 wherein said blow pin means and said evacuating means are combined into a dual device for alternating between injecting and evacuating functions. 16. The apparatus of Claim 15 wherein absorption means are provided for absorbing the fluorine from the evacuated blowing gas.

Inventors: BARBANEL, JOSEPH, MAYNARD, FLOYD, MILCETICH, FRANCIS ANTHONY
Applicants: AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1982
Main classification: B29C49
IPC codes: B29B11, B29C49, B29C65, B29C47