CA1244574A - Method for reducing shrinkage of injection molded linear low density polyethylene - Google Patents
Method for reducing shrinkage of injection molded linear low density polyethyleneInfo
- Publication number
- CA1244574A CA1244574A CA000447203A CA447203A CA1244574A CA 1244574 A CA1244574 A CA 1244574A CA 000447203 A CA000447203 A CA 000447203A CA 447203 A CA447203 A CA 447203A CA 1244574 A CA1244574 A CA 1244574A
- Authority
- CA
- Canada
- Prior art keywords
- low density
- linear low
- density polyethylene
- copolymer
- ethylene
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
- B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
- B29C45/00—Injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould; Apparatus therefor
- B29C45/0001—Injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould; Apparatus therefor characterised by the choice of material
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
- B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
- B29C37/00—Component parts, details, accessories or auxiliary operations, not covered by group B29C33/00 or B29C35/00
- B29C37/005—Compensating volume or shape change during moulding, in general
-
- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
- C08L23/00—Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers
- C08L23/02—Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers not modified by chemical after-treatment
- C08L23/04—Homopolymers or copolymers of ethene
- C08L23/08—Copolymers of ethene
- C08L23/0807—Copolymers of ethene with unsaturated hydrocarbons only containing more than three carbon atoms
- C08L23/0815—Copolymers of ethene with aliphatic 1-olefins
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
- B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
- B29C45/00—Injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould; Apparatus therefor
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
- B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
- B29K2023/00—Use of polyalkenes or derivatives thereof as moulding material
- B29K2023/04—Polymers of ethylene
- B29K2023/06—PE, i.e. polyethylene
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
- B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
- B29K2995/00—Properties of moulding materials, reinforcements, fillers, preformed parts or moulds
- B29K2995/0037—Other properties
- B29K2995/0063—Density
-
- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
- C08L23/00—Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers
- C08L23/02—Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers not modified by chemical after-treatment
- C08L23/04—Homopolymers or copolymers of ethene
- C08L23/06—Polyethene
Landscapes
- Chemical & Material Sciences (AREA)
- Health & Medical Sciences (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Medicinal Chemistry (AREA)
- Polymers & Plastics (AREA)
- Organic Chemistry (AREA)
- Engineering & Computer Science (AREA)
- Manufacturing & Machinery (AREA)
- Mechanical Engineering (AREA)
- Compositions Of Macromolecular Compounds (AREA)
- Manufacture Of Macromolecular Shaped Articles (AREA)
- Injection Moulding Of Plastics Or The Like (AREA)
Abstract
METHOD FOR REDUCING SHRINKAGE OF INJECTION
MOLDED LINEAR LOW DENSITY POLYETHYLENE
ABSTRACT
Shrinkage following injection molding of linear low density polyethylene, e.g. copolymers of ethylene and butene, is reduced by intimately blending the linear low density polyethylene with high pressure low density polyethylene in an amount up to 50 weight percent of the blend.
MOLDED LINEAR LOW DENSITY POLYETHYLENE
ABSTRACT
Shrinkage following injection molding of linear low density polyethylene, e.g. copolymers of ethylene and butene, is reduced by intimately blending the linear low density polyethylene with high pressure low density polyethylene in an amount up to 50 weight percent of the blend.
Description
METrlOD FOR REOUCING SHRINKAGE OF INXCTION
_LDED LINEAR LOW _ENSI TY POLYETH`~LENE
This invention relates to a method for reducing shrinkage oF
injection molded linear low density polyethylene.
Injection molding of linear low density polyethylene produces articles with improved toughness and environmental stress cracl~ resistance, and can be effected with shorter molding cycles than possiDle with conventional high pressure low density polyethylene. However, when injection molded, the linear low density polyethylene is found to undergo excessive shrinkage as compare~ with conventional polyethylene. An object of the present invention is therefore to minimize or alleviate tnis proDlem.
Accordingly the invention resides in a method for reducing shrinkage i~ the injection molding of an article from a linear low density c~polymer of ethylene and a higher olefin having a density of less than 0.94 and a melt index of 30 to 150, comprising blending said linear low density copolymer with an effective amount of hlgh pressure low density polyethylene of from 3 to 8 weight percent of the blend and injection molding the resulting blend.
~u An essential component of the injection molding resins used herein is linear low density polyethylene (LLDPE) which is the term applied to copolymers of ethylene and higher olefins, having a ~ensity of less tnan about 0O94~ Such materials are commercially available and tneir preparation is described in, for example, U.S. patents 4,076,698 and 4,205,221. Representative linear low density polyethylenes disclosed in patents and available in the prior art comprise copolymers of ethylene and higher alpha olefins, in particular, copolymers of ethylene and olefins containing 4 to 8 car~on atoms. Tne linear low density polymers which are particularly suitable for use herein have a relatively high melt index (MI) of 30 to 150, and preferably from about 30 to 70. The density of the polymers is generally below 0.94 and preferably in the range of 0.91 to 0.93.
A second essential component of the resin Dlends is a conventional high pressure low density polyethylene (HPPE~ which is prepared by high ? ' ,: i i:i 12~
pressure techniques using free radical initiation. In general the conventional polyethylene has a density of 0.91 to 0.94.
Certain blends of linear low density polyethylenes and conventional polyethylenes are disclosed in U.S. Patent 3,176,052. This patent is primarily concerned with obtaining better film properties from the blends and discloses that optical properties, particularly gloss, is improved with the blends compared to linear low density polyethylene itself. There is no disclosure or suggestion that reduced shrinkage of injection molded parts can be obtained with blends as disclosed herein.
Indeed, the lin2ar low density polyethylenes specified in this patent have a melt index of 0.01 to 10 which is considerably lower than preferred for use in this invention.
The blends suitable for use herein comprises up to about 50 weight percent of high pressure low density polyethylene, the remainder being linear low density copolymer of ethylene and a higher olefin. Generally, the conventional high pressure resin comprises from 3 to 30 weight percent of the composition. In a preferred aspect of the invention, the high pressure polyethylene comprises about 4 to 8 weight percent of the blend. In a further preferred aspect of the invention, the high pressure polyethylene comprises about 3 to 6 weight percent of the blend. The blending of the polymers can be accomplished in many ways familiar to those skilled in the art. A physical mixture of the two polymers in powder or pellet form is suitable for use in the lnjection molding apparatus. However, the benefits of blending are best achieved if the two resins are intimately mixed by milling or hot compounding above the melting point of the higher melting mixture. Thus, a homogeneous blend is particularly preferred.
The benefits of reduced shrinkage are particularly realized in the injection molding of parts to close tolerances. For example, in the production o~ lids for containers a shrinkage variation of several mils may result in a poor fit. Thus, the compositions disclosed herein yield the benefits of the excellent ohysical properties of linear low density 57~
F~2201 - 2a -polyethylene, while at the same time avoiding the dlsadvantage of its relatively poor shrinkage characteristics. The benefits are realized when the same molds which were previously used for injecl:ion molding of parts from conventional low pressure polyethylene are used with linear low 5 density polyethylene.
, ` ~2~i7~
Shrinkage is measured on injection molded parts in which dimensions are compared shortly after molding and several days later when it is assumed that substantially maximum snrinkage has occurred.
Typically, shrinkage is measured using a circumferential gauge such as manu~actured by the Starrett Corporation.
The shrinkage observed upon injection molding of the LLDPE/HPPE
blends used herein depends on many factors such as the method of blending and cycle time. However, shrinkage with these blenas generally is 75-50 percent or less than that observed witn LLDPE alonel and approaching the acceptably low shrinkage with HPPE alone. It is surprising that relatively low amounts of HPPE, e.g. 5 weight percent, should have such a profound effect on the shrinkage of the molded parts.
The invention is illustrated by the following Example in which all parts are by weight.
EXAMPLE
Linear high flow low density polyethylene (LLDPE) (Union Car~ide GRSN-7147~*, a copolymer of ethylene and butene) having a Melt Index (MI) of 50 and a density of 0.926 was injection molded into a container lid having a diameter of 6.03 inches using 6.1 and 7.1 second molding cycles. In a similar manner moldings were made using blends of the same linear low density polyethylene and 5 weight percent of a conventional high pressure low density polyethylene (HPPE) ("Mobil LPA 002"*), M~-25*, density 0.918. In one case a physical mixture of pellets o~ the two polyethylene resins was fed to the injection molding extruder. In the second case an intimate melt blend of the resins was prepared prior to feeding to the injection molding apparatus. Another molding was made using another conventional high pressure polyethylene ("Gulf 1410"*), MI 35 , d~nsity 0.923.
The circumference of the in~ection molded lids was measured a~ter ]. hour and agaln after 6 days, and the difference recorded. The results are summarized in the Table below.
* Trademark 5~
TABLE
Shrink Dif~erence (Mils) LLDPE HPPE Blencl Method (6.1 sec. cycle) (7.1 secO cycie~) Physical 5.3 4-4 Hot Compounded 4 . 7 2. 6 0 100 - 4 2 . 4 Gul~ 1410
_LDED LINEAR LOW _ENSI TY POLYETH`~LENE
This invention relates to a method for reducing shrinkage oF
injection molded linear low density polyethylene.
Injection molding of linear low density polyethylene produces articles with improved toughness and environmental stress cracl~ resistance, and can be effected with shorter molding cycles than possiDle with conventional high pressure low density polyethylene. However, when injection molded, the linear low density polyethylene is found to undergo excessive shrinkage as compare~ with conventional polyethylene. An object of the present invention is therefore to minimize or alleviate tnis proDlem.
Accordingly the invention resides in a method for reducing shrinkage i~ the injection molding of an article from a linear low density c~polymer of ethylene and a higher olefin having a density of less than 0.94 and a melt index of 30 to 150, comprising blending said linear low density copolymer with an effective amount of hlgh pressure low density polyethylene of from 3 to 8 weight percent of the blend and injection molding the resulting blend.
~u An essential component of the injection molding resins used herein is linear low density polyethylene (LLDPE) which is the term applied to copolymers of ethylene and higher olefins, having a ~ensity of less tnan about 0O94~ Such materials are commercially available and tneir preparation is described in, for example, U.S. patents 4,076,698 and 4,205,221. Representative linear low density polyethylenes disclosed in patents and available in the prior art comprise copolymers of ethylene and higher alpha olefins, in particular, copolymers of ethylene and olefins containing 4 to 8 car~on atoms. Tne linear low density polymers which are particularly suitable for use herein have a relatively high melt index (MI) of 30 to 150, and preferably from about 30 to 70. The density of the polymers is generally below 0.94 and preferably in the range of 0.91 to 0.93.
A second essential component of the resin Dlends is a conventional high pressure low density polyethylene (HPPE~ which is prepared by high ? ' ,: i i:i 12~
pressure techniques using free radical initiation. In general the conventional polyethylene has a density of 0.91 to 0.94.
Certain blends of linear low density polyethylenes and conventional polyethylenes are disclosed in U.S. Patent 3,176,052. This patent is primarily concerned with obtaining better film properties from the blends and discloses that optical properties, particularly gloss, is improved with the blends compared to linear low density polyethylene itself. There is no disclosure or suggestion that reduced shrinkage of injection molded parts can be obtained with blends as disclosed herein.
Indeed, the lin2ar low density polyethylenes specified in this patent have a melt index of 0.01 to 10 which is considerably lower than preferred for use in this invention.
The blends suitable for use herein comprises up to about 50 weight percent of high pressure low density polyethylene, the remainder being linear low density copolymer of ethylene and a higher olefin. Generally, the conventional high pressure resin comprises from 3 to 30 weight percent of the composition. In a preferred aspect of the invention, the high pressure polyethylene comprises about 4 to 8 weight percent of the blend. In a further preferred aspect of the invention, the high pressure polyethylene comprises about 3 to 6 weight percent of the blend. The blending of the polymers can be accomplished in many ways familiar to those skilled in the art. A physical mixture of the two polymers in powder or pellet form is suitable for use in the lnjection molding apparatus. However, the benefits of blending are best achieved if the two resins are intimately mixed by milling or hot compounding above the melting point of the higher melting mixture. Thus, a homogeneous blend is particularly preferred.
The benefits of reduced shrinkage are particularly realized in the injection molding of parts to close tolerances. For example, in the production o~ lids for containers a shrinkage variation of several mils may result in a poor fit. Thus, the compositions disclosed herein yield the benefits of the excellent ohysical properties of linear low density 57~
F~2201 - 2a -polyethylene, while at the same time avoiding the dlsadvantage of its relatively poor shrinkage characteristics. The benefits are realized when the same molds which were previously used for injecl:ion molding of parts from conventional low pressure polyethylene are used with linear low 5 density polyethylene.
, ` ~2~i7~
Shrinkage is measured on injection molded parts in which dimensions are compared shortly after molding and several days later when it is assumed that substantially maximum snrinkage has occurred.
Typically, shrinkage is measured using a circumferential gauge such as manu~actured by the Starrett Corporation.
The shrinkage observed upon injection molding of the LLDPE/HPPE
blends used herein depends on many factors such as the method of blending and cycle time. However, shrinkage with these blenas generally is 75-50 percent or less than that observed witn LLDPE alonel and approaching the acceptably low shrinkage with HPPE alone. It is surprising that relatively low amounts of HPPE, e.g. 5 weight percent, should have such a profound effect on the shrinkage of the molded parts.
The invention is illustrated by the following Example in which all parts are by weight.
EXAMPLE
Linear high flow low density polyethylene (LLDPE) (Union Car~ide GRSN-7147~*, a copolymer of ethylene and butene) having a Melt Index (MI) of 50 and a density of 0.926 was injection molded into a container lid having a diameter of 6.03 inches using 6.1 and 7.1 second molding cycles. In a similar manner moldings were made using blends of the same linear low density polyethylene and 5 weight percent of a conventional high pressure low density polyethylene (HPPE) ("Mobil LPA 002"*), M~-25*, density 0.918. In one case a physical mixture of pellets o~ the two polyethylene resins was fed to the injection molding extruder. In the second case an intimate melt blend of the resins was prepared prior to feeding to the injection molding apparatus. Another molding was made using another conventional high pressure polyethylene ("Gulf 1410"*), MI 35 , d~nsity 0.923.
The circumference of the in~ection molded lids was measured a~ter ]. hour and agaln after 6 days, and the difference recorded. The results are summarized in the Table below.
* Trademark 5~
TABLE
Shrink Dif~erence (Mils) LLDPE HPPE Blencl Method (6.1 sec. cycle) (7.1 secO cycie~) Physical 5.3 4-4 Hot Compounded 4 . 7 2. 6 0 100 - 4 2 . 4 Gul~ 1410
Claims (7)
1. A method for reducing shrinkage in the injection molding of an article from a linear low density copolymer of ethylene and a higher olefin having a density of less than 0.94 and a melt index of 30 to 150, comprising blending said linear low density copolymer with an effective amount of high pressure low density polyethylene of from 3 to 8 weight percent of the blend and injection molding the resulting blend.
2. The method of claim 1 in which said high pressure polyethylene comprises about 4 to 8 weight percent of said blend.
3. The method of claim 1 in which said linear low density copolymer has a melt index of 30 to 70.
4. The method of claim 1 in which said linear low density ethylene copolymer is a copolymer of ethylene and an olefin containing 4 to 8 carbon atoms, and having a density of from 0.91 to 0.93.
5. The method of claim 1 in which said linear low density ethylene copolymer is a copolymer of ethylene and butene having a density of 0.91 to 0.93.
6. The method of claim 1 in which said linear low density ethylene copolymer is a copolymer of ethylene and an olefin containing 4 to 8 carbon atoms, having a density of 0.91 to 0.93 and a melt index of 30 to 70 and in which said high pressure low density polyethylene comprises from 3 to 6 weight percent of said blend.
7. A molded article made by the method of claim 6.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US484,732 | 1983-04-13 | ||
| US06/484,732 US4485217A (en) | 1983-04-13 | 1983-04-13 | Method for reducing shrinkage of injection molded linear low density polyethylene |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1244574A true CA1244574A (en) | 1988-11-08 |
Family
ID=23925380
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000447203A Expired CA1244574A (en) | 1983-04-13 | 1984-02-10 | Method for reducing shrinkage of injection molded linear low density polyethylene |
Country Status (2)
| Country | Link |
|---|---|
| US (1) | US4485217A (en) |
| CA (1) | CA1244574A (en) |
Families Citing this family (16)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPH0629953B2 (en) * | 1986-06-11 | 1994-04-20 | 富士写真フイルム株式会社 | Photo Film Patron Container Cap |
| JPH0652401B2 (en) * | 1986-10-30 | 1994-07-06 | 富士写真フイルム株式会社 | Container cap for photographic film cartridge and method of manufacturing the same |
| US5082902A (en) * | 1988-07-22 | 1992-01-21 | Mobil Oil Corporation | Method for reducing cycle time and improving molded part impact energy and ESCR of linear high density polyethylene using a blend of two linear polyethylenes of different densities |
| US6025448A (en) * | 1989-08-31 | 2000-02-15 | The Dow Chemical Company | Gas phase polymerization of olefins |
| US6538080B1 (en) | 1990-07-03 | 2003-03-25 | Bp Chemicals Limited | Gas phase polymerization of olefins |
| US5278272A (en) | 1991-10-15 | 1994-01-11 | The Dow Chemical Company | Elastic substantialy linear olefin polymers |
| US5395471A (en) * | 1991-10-15 | 1995-03-07 | The Dow Chemical Company | High drawdown extrusion process with greater resistance to draw resonance |
| US5674342A (en) * | 1991-10-15 | 1997-10-07 | The Dow Chemical Company | High drawdown extrusion composition and process |
| US5582923A (en) * | 1991-10-15 | 1996-12-10 | The Dow Chemical Company | Extrusion compositions having high drawdown and substantially reduced neck-in |
| US5783638A (en) * | 1991-10-15 | 1998-07-21 | The Dow Chemical Company | Elastic substantially linear ethylene polymers |
| US5525695A (en) | 1991-10-15 | 1996-06-11 | The Dow Chemical Company | Elastic linear interpolymers |
| US5792534A (en) * | 1994-10-21 | 1998-08-11 | The Dow Chemical Company | Polyolefin film exhibiting heat resistivity, low hexane extractives and controlled modulus |
| US8097221B2 (en) | 2005-01-21 | 2012-01-17 | Multisorb Technologies, Inc. | Lamp assembly |
| DE602007003611D1 (en) * | 2007-05-02 | 2010-01-14 | Dow Global Technologies Inc | HIGH-DENSITY POLYETHYLENE COMPOSITIONS, METHOD FOR THE PRODUCTION THEREOF, INJECTION-MOLDED ARTICLES MADE THEREFROM AND METHOD FOR THE PRODUCTION OF SUCH ITEMS |
| FR3025130B1 (en) * | 2014-08-26 | 2016-12-23 | Faurecia Interieur Ind | METHOD FOR DIMENSIONALLY MONITORING A MOLDED PLASTIC PART AND CORRESPONDING MOLDED PIECE |
| CN112592530B (en) * | 2020-11-26 | 2022-07-12 | 金发科技股份有限公司 | Low-shrinkage polyethylene optical cable sheath material and preparation method and application thereof |
Family Cites Families (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CA566071A (en) * | 1958-11-11 | H. Dilke Max | Linear and branched chain polyethylenes | |
| US4076698A (en) * | 1956-03-01 | 1978-02-28 | E. I. Du Pont De Nemours And Company | Hydrocarbon interpolymer compositions |
| US3176052A (en) * | 1960-08-08 | 1965-03-30 | Du Pont | Blends of polyethylene and ethylene copolymers |
| JPS5952643B2 (en) * | 1977-01-27 | 1984-12-20 | 三井化学株式会社 | ethylene copolymer |
| US4243619A (en) * | 1978-03-31 | 1981-01-06 | Union Carbide Corporation | Process for making film from low density ethylene hydrocarbon copolymer |
| GB2019412B (en) * | 1978-04-07 | 1982-09-15 | Raychem Ltd | Cross-linked low density linear polyethylenes |
| US4337188A (en) * | 1979-12-17 | 1982-06-29 | Du Pont Canada Inc. | Polyolefin composition for manufacture of film having cling properties |
| US4339507A (en) * | 1980-11-26 | 1982-07-13 | Union Carbide Corporation | Linear low density ethylene hydrocarbon copolymer containing composition for extrusion coating |
-
1983
- 1983-04-13 US US06/484,732 patent/US4485217A/en not_active Expired - Fee Related
-
1984
- 1984-02-10 CA CA000447203A patent/CA1244574A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| US4485217A (en) | 1984-11-27 |
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