Patent Application
20080136061
The present disclosure relates to injection molding, and more particularly to the forming of polycarbonate compounds in the presence of a supercritical fluid.
Polycarbonates, such as, for example, bisphenol A carbonate, are excellent engineering grade thermoplastics for many purposes. Such compounds tend to have high strength, stiffness, and toughness over a wide temperature range. They can be colored, compounded, and thermally formed in a variety of melt forming processes such as thermoforming, extrusion, compression and injection molding. The most often noted disadvantages to their use are limited chemical resistance, susceptibility to stress cracking, and notch sensitivity. In particular, polycarbonate compounds tend to have poor resistance to benzene, toluene, chlorinated hydrocarbons, heptane, ethyl acetate, and strong acids and bases. The ester linkage that connects the monomer units in a polycarbonate compound is hydrolysable, which renders the molecule susceptible to attack by hot water.
It is known to attempt to optimize the chemical resistance of the surface of a polycarbonate article when shaping by, for example, injection molding by operating running the mold at a substantially greater temperature than would be used for other injection moldable polymers. For example, the art teaches that injection molds for polycarbonate should be operated at least 180° F., and 200° F. and higher is more typical. Although the use of higher mold temperature increases mold cycle time undesirably, the chemical resistance does increase, presumably by reducing the amount of locked in stress adjacent to the surface of the finished part. The art would be greatly benefited by a method of shaping polycarbonate compounds that would further improve its chemical resistance, desirably without unduly increasing mold cycle time. Polycarbonate is also known to be able to exhibit semi-crystalline features, when prepared by slow evaporation from solvent or by long term heating at 180° C. While this morphology tends to increase resistance to chemical attack, the optical clarity of the polycarbonate decreases. Increasing chemical resistance of polycarbonate without introducing crystalline morphology is also desirable.
The present disclosure provides a process for shaping polycarbonate that results in parts having substantially greater chemical resistance than traditional processes. The process uses startlingly low mold temperatures, but uses them in concert with the introduction of supercritical fluid into the molten polycarbonate before it reaches the mold. Further, it has been discovered that maintaining pressure on the cooling part for a period of time further increases the chemical resistance.
In one aspect, the disclosure may be thought as a method for forming shaped parts from polycarbonate. Following the method includes providing a mold having at least one cavity, such that the mold has a means for precisely controlling the surfaces of the mold core and cavity in contact with injected polymer. Then molten polycarbonate is mixed with a supercritical fluid to form a mixture. This mixture is injected into the mold while using the cooling means to cool the cavity so that its temperature is no more than 150 degrees F.
In the several figures of the attached drawing, like parts bear like reference numerals, and:
a illustrates the fracture of a control part prepared according to conventional molding techniques; and
b illustrates the fracture of an experimental part prepared according to the method of the present disclosure.
Referring now to
An injection mold was prepared as a single cavity cold runner mold, shaped so as to form the part illustrated in
An injection molding run was performed with the mold, with polycarbonate resin commercially available as Bayer Makrolon 6555 from Miles Polymers Division of Pittsburg, Pa., being used as the polymer being molded. The mold was fed by a reciprocating single-screw type extruder, commercially available from Guelph, of Ontario, Calif., which was operated at a pressure of 2600 psi (17.9 MPa) in 18 second molding cycles. The mold was supplied with cooling fluid at 50 degrees F. These parts served as control samples in the experiment of Example 3 below.
Plastic parts were fabricated as described in example 1, except for the following particulars. Supercritical nitrogen was melt mixed into the molten polycarbonate to the extent of 0.3 percent nitrogen, and the molding cycle time was 13.8 seconds. Compared to the process of example 1, about 5% less raw material was processed per cycle, the parts having a solid skin layer with a cellular core. The internal morphology of the parts produced according to this example is illustrated in the micrograph of
A three point bending device was prepared, with the support points being 3 inches (7.62 cm) apart, and otherwise dimensionally convenient for the holding of the parts produced in examples 1 and 2. Parts according to examples 1 and 2 were placed into the bending device and a displacement of 0.015 inch (0.38 mm) strain from the horizontal was induced. While maintaining this strain, the parts were placed into a heptane/ethylacetate mixture (2:1 by weight), and a timer was used to identify when a crack visible to the naked eye first formed for each sample. Ten samples prepared according to example 1 had a mean time to crack formation of 28 seconds. Ten samples prepared according to example 2 had a mean time to crack formation of 164 seconds. This result indicates that the use of supercritical fluid can act to improve the chemical resistance in polycarbonate. It was noted that the type of fracture presented by the control and the experimental parts were quite different.
An injection mold was prepared as a four-cavity hot runner mold, shaped so as to form telephonic wire connector commercially available as the DPM-Body Top component from 3M Company of St. Paul, Minn. The mold was adapted so as to be able to inject supercritical nitrogen fluid into the molten polymer as a processing adjunct. A designed experiment of twenty-four injection molding runs were performed with the mold, with polycarbonate resin commercially available as Bayer Makrolon 2658 from Miles Polymers Division of Pittsburg, Pa. being used as the polymer being molded. The tenth part produced in each run was tested in the fixture and according to the protocol of Example 3, and the time to crack appearance according to that test is reported in the Table 1 below.
A designed experiment was performed to follow up on the particularly noteworthy improvement in chemical resistance revealed in run 24 above. The four-cavity hot runner mold used in Example 4 was used to injection mold parts from polycarbonate resin commercially available as Bayer Makrolon 2658 from Miles Polymers Division of Pittsburg, Pa. The pressure during the hold was 2600 psi (17.9 MPa). Supercritical nitrogen was added in the amount of 0.1 weight percent, and once again the tenth part produced in each run was tested in the fixture and according to the protocol of Example 3, and the time to crack appearance according to that test is reported in Table 2 below.
Lower processing temperatures and longer hold pressures are associated with optimum chemical resistance. It should be noted that the mold cooling fluid temperature was not able to be lowered due to limitations in the mold hot manifold design.
Various modifications and alterations of the present disclosure will be apparent to those skilled in the art without departing from the scope and spirit of this disclosure, and it should be understood that this disclosure is not limited to the illustrative embodiments set forth herein. The claims follow.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US06/02991 | 1/27/2006 | WO | 00 | 8/6/2007 |
| Number | Date | Country | |
|---|---|---|---|
| 60652449 | Feb 2005 | US |
