Plastic and nonmetallic article shaping or treating: processes – Direct application of fluid pressure differential to... – Including application of internal fluid pressure to hollow...
Reexamination Certificate
2000-04-18
2004-02-03
McDowell, Suzanne E. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of fluid pressure differential to...
Including application of internal fluid pressure to hollow...
C264S328170
Reexamination Certificate
active
06685874
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to blow-molding processes used to form hollow articles of a blow-moldable resin. More specifically, the present invention relates to processes whereby a polybutylene terephthalate resin is blow-molded to form hollow articles thereof.
BACKGROUND OF THE INVENTION
Crystalline thermoplastic polybutylene terephthalate (PBT) resins have been used extensively as an engineering plastic in various fields owing to their excellent mechanical and electrical properties, as well as their physical and chemical characteristics. PBT resins have typically not been employed as a feedstock for blow-molding operations since the intermediate hollow parison that is formed during the blow-molding process is undesirably “drawn-down” due to the poor melt tension that conventional PBT resin exhibits. Thus, hollow blow-molded articles have typically not been produced from PBT resins. Instead, PBT resins are conventionally thought of as an injection-moldable resin from which hollow injection-molded articles may be produced.
Prior attempts at forming hollow articles of PBT resins by blow-molding techniques have included increasing the molecular weight of the PBT resin so as to obtain a corresponding increase in melt strength (melt tension) and melt viscosity, since greater molecular weight typically results in a greater melt strength for a given resin. However, merely increasing the molecular weight of PBT resin to thereby increase its melt viscosity does not achieve the needed melt strength sufficient to employ PBT resins in blow-molding processes. Furthermore, the addition of inorganic filler materials has also been attempted as a means to increase the melt viscosity and melt strength of PBT resins sufficiently to allow blow-molding of hollow articles. Again, however, simply adding an inorganic filler to PBT resins (even those having an increased molecular weight) does not achieve the necessary melt strength properties necessary to allow the resin to be processed by blow molding techniques. U.S. Pat. No. 5,232,773 to Itoh et al describes blow-moldable PBT resins comprised of a PBT base polymer and an effective amount of between 0.01 to 5 parts by weight (based on 100 parts by weight of the PBT base polymer) of at least one organosilane compound.
PBT resin is desirably improved for blow molding by enhancing the melt tension characteristics.
SUMMARY OF THE INVENTION
PBT resins are desirable for blow-molding processes which typically produce hollow articles. A blow-moldable PBT resins comprises a PBT base polymer and an effective amount of between 0.1 to 20 parts by weight (based on 100 parts by weight of the PBT base polymer) of polyethylene terephthalate (PET). PET is melt-blended with the PBT base polymer prior to blow molding.
The hang time of a tubular parison formed from a thermoplastic resin, for example, PBT, is controlled by providing a second higher melting thermoplastic resin, for example, PET, as an additive and blow molding the resulting resin at a temperature below melt temperature (Tm) of the higher melting component. In the case where the lower melting base resin is PBT and the higher melting additive resin is PET, the temperature of molding is below the Tm of the PET additive.
DETAILED DESCRIPTION OF THE INVENTION
When small quantities of a high melting point semi-crystalline resin are added to polymers, either amorphous or semi crystalline with lower melt temperatures, and subsequent processing is done at temperatures below the high Tm, significant enhancement to the base polymers's melt strength is achieved. Improved processing in processes such as blow molding, thermoforming and profile extrusion can be achieved where resin melt strength (resistance to sag) is important.
An example, the addition of small quantities of PET resin to PBT resin or PBT blends increases resistance to sag. By processing the resulting material above the PBT Tm but below the Tm of PET significant improvements in melt strength are observed and the material flow is adequate for the application. Other high Tm resins such as nylon 6,6, PPS etc., which are high melting semicrystalline materials, can be added to either amorphous or semicrystalline resins to get the desired effect.
The presence of the high melting point crystals is desirable. If the Tm of the high melting point material is exceeded during processing, the melt strength is lost. To date these materials have been utilized in profile extrusion and extrusion blow molding with excellent results. The level of high melting crystalline resin is dependent on the process being targeted and the specific resins being used. It is desirable to have a large temperature difference between the high melting point semi-crystalline resin and the low melting resin to provide a wide process temperature window possible.
Improved melt strength in traditionally low melt strength resins greatly expands the types of polymers that can be used in polymer conversion processes requiring improved melt strength. More resins are under experimentation to determine what other desirable combinations can be utilized. This technology potentially has a large impact on the thermoforming, blow molding and profile extrusion communities.
The polybutylene terephthalate base polymers used in the compositions of the present invention include polyesters comprised mainly of recurring butylene terephthalate units. In particular, the preferred base polymers are polyesters obtained by condensing 1,4-butanediol with terephthalic acid or its lower alcohol ester. The polybutylene terephthalate base polymer is not limited strictly to PBT homopolymers, but also includes copolymers mainly comprised of polybutylene terephthalate units. The term “copolymers” as used herein thus refers to polymers obtained by the polycondensation of terephthalic acid or its lower alcohol ester as the main dibasic acid component with 1,4-butanediol as the main glycol component in the presence of 40 molar % or less of an ester-forming monomer. The comonomer components usable herein include dibasic acid components such as isophthalic acid, orthophthalic acid, adipic acid, sebacic acid, succinic acid and oxalic acid as well as their lower alcohol esters; and glycol components such as ordinary alkylene glycols other than 1,4-butanediol, e.g. ethylene glycol, diethylene glycol, propylene glycol, trimethylene glycol, hexamethylene glycol, neopentyl glycol and cyclohexanedimethanol as well as aromatic diols, e.g. bisphenol A and ethylene oxide (2 mol) adduct of bisphenol A.
Hydroxy acids such as hydroxybenzoic acid and hydroxynaphthoic acid and their ester-forming derivatives may also be used as suitable comonomers. Furthermore, comonomers having halogen substituents are also usable and are effective in imparting flame retardancy to the resulting blow-molded articles.
In addition, copolyesters having a branched structure obtained by the polycondensation of a polyfunctional comonomeric compound having three or more reactive groups may also be employed as the PBT base polymer. The polyfunctional compounds usable herein include trimesic acid, trimellitic acid, pyromellitic acid and alcoholic esters thereof as well as glycerol, trimethylolethane, trimethylolpropane and pentaerythritol.
The PBT base polymer that may be used in the present invention is preferably one having an intrinsic viscosity (IV) within the range of 0.7 to 2.0, particularly 1.0 to 1.6. PBT resins having an intrinsic viscosity of less than 0.7 exhibit insufficient melt tension and cannot be blow-molded. On the contrary, an intrinsic viscosity exceeding 2.0 is unfavorable because the fluidity of the resin composition is poor. Furthermore, the extrusion moldability property of PBT resins having an IV of greater than 2.0 is impaired due to overloading of the extruder motor and pressure increases at the die.
The blow-moldable PBT resin compositions and the hollow blow-molded articles produced by such blow-molding processes include PET. The amount of the PET compound to be melt-blended with the PBT base polymer
Dunton Thomas Paul
O'Connell Lisbeth Lynn
General Electric Company
McDowell Suzanne E.
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