Plastic and nonmetallic article shaping or treating: processes – Mechanical shaping or molding to form or reform shaped article – Stretching or stretch forming
Reexamination Certificate
1995-09-25
2001-08-28
Morris, Terrel (Department: 1771)
Plastic and nonmetallic article shaping or treating: processes
Mechanical shaping or molding to form or reform shaped article
Stretching or stretch forming
C264SDIG007
Reexamination Certificate
active
06280676
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to extruded polymeric netting and more particularly to such netting in which at least some of the strands in at least one direction in the net, preferably all of the strands, are of an elastomeric polymeric material which may be stretch modified in accordance with this invention.
Extruded polymeric netting has been known and used for some time. U.S. Pat. No. 4,152,479 to Larsen and U.S. Pat. No. 3,252,181 to Hureau describe types of such net. These patents and their entire content are incorporated herein by reference. Extruded netting is netting in which the strands are extruded from a die, the joints therebetween being formed either within the die or immediately outside the lips of the die.
In the netting to which these patents relate, the extruded strands are comprised of orientable polymeric material. Orientation is a stretching process which can be applied to the net in the machine direction (MD) and/or the cross direction (CD) or transverse direction (TD). When oriented in only one direction, the net is said to be uniaxially oriented. When oriented in both directions it is said to be biaxially oriented.
The orientation process is applied to these types of net to significantly increase net size, both in width and length and to orient the molecules of the material in the strands from a random arrangement into a more ordered arrangement. Due to the nature of the polymeric material used heretofore in extruded netting, the net when stretched to a larger size remained essentially at the stretched or enlarged size. That is, such nets do not exhibit any significant recovery. The ordered arrangement obtained is desirable because it increases the strength to weight ratio of the net. Some of the more common materials used in such nets are polypropylene, nylon and linear low density and high density polyethylene.
It has recently become desirable to provide extruded net in which the strands or at least some of the strands in one direction or both directions are of elastomeric material in order to provide net exhibiting significant elasticity for a variety of purposes. One such net is described in co-pending application Ser. No. 08/295,635 entitled BICOMPONENT ELASTOMERIC NETTING which is assigned to the same assignee as is this invention. The content of this application is incorporated herein by reference.
Stretch orientation processing has not been applied to elastomeric net to modify elastic properties prior to this invention. It has not been believed that the orientation process would be useful with respect to extruded elastomeric nets since it does not provide the traditionally expected results and benefits.
In accordance with this invention, it has been discovered that stretching elastomeric strands in extruded netting produces beneficial modification of the elastic properties of the net. The invention most accurately is described herein as “stretch modification”. The same equipment as is used for orientation may be used for stretch modification when modified to accommodate the greater stretch involved. Such equipment is described in the aforenoted Larsen patent. Stretch modification may take place at room temperature (RT) or at elevated temperatures, similar to the known orientation process.
SUMMARY OF THE INVENTION
Stretch modification of extruded elastomeric net allows the elasticity level of the net to be engineered to predetermined specification for any desired recovery. It also allows stretch to be engineered likewise. This is accomplished by stretching the net in the MD and or CD directions under controlled conditions.
Definition of Thermoplastic Elastomer
The ASTM D 1566-66T definition of the term “elastomer” is “a macromolecular material that returns rapidly to approximately the initial dimensions and shape after substantial deformation by a weak stress and release of the stress”. A thermoplastic elastomer is a material that combines the processability of a thermoplastic resin with the functional performance and properties of a conventional thermoset rubber. In general, any thermoplastic elastomer can be used to produce this type of netting. They are generally covered by the six resin classes listed below.
Thermoplastic Elastomer (TPE) Types
There are generally considered to be six classes of commercially available TPE'S:
Styrenic Block Copolymers (SBC's)
The various SBC's include:
Styrene-Butadiene-Styrene(SBS)
Styrene-Isoprene-Styrene(SIS)
Styrene-Ethylene/Butylene-Styrene (SEBS)
Styrene-Ethylene/Propylene-Styrene (SEPS) (uncommon) Tradenames and producers include Kraton (SBS, SIS and SEBS) by Shell Chemical Co., Finaprene (SBS) by Fina Oil & Chemical, and Europrene (SBS and SIS) by EniChem Elastomers. Only Shell makes the SEBS resin (Kraton G).
Thermoplastic Olefins and Blends (TPO's)
Tradenames and suppliers of traditional TPO's include Polytrope (a blend of polypropylene and EPDM, a rubber) by A. Schulman and Telcar (also a blend of polypropylene and EPDM) by Teknor Apex. These are propylene/EPDM block copolymers. EPDM is Ethylene Propylene Diene Monomer.
A new subclass of TPO's are the VLDPE's (very low density), copolymers with a density of about <0.880 g/cm
3
. The elasticity of polyethylenes increases with decreasing density. Tradenames and suppliers of these include Exact by Exxon Chemical Co. and Engage by Dow Plastics.
Elastomeric Alloys
This class of TPE's consists of mixtures using two or more polymers that have received proprietary treatment to give them properties significantly superior to the simple blends of the same constituents. The two basic types are: Thermoplastic vulcanites (TPV's), such as Santoprene (polypropylene and crosslinked EPDM) by Advanced Elastomer Systems, Geolast (polypropylene or nitride rubber) by Monsanto and melt-processible rubbers (MPR's), such as Alcryn (polyvinylidene chloride and crosslinked polyvinylacetate copolymer) by Du Pont Co.
Thermoplastic Polyurethanes (TPU's)
Tradenames and suppliers include Pellethane (polyurethane with polyester, polyether, or polycaprolactone copolymers) by Dow Chemical and Estane by B.F. Goodrich.
Thermoplastic Copolyesters
Tradenames and producers include Hytrel (polyether-ester copolymer) by Du Pont Co., and Arnitel (polyether-ester copolymer) by DSM Engineering Plastics.
Thermoplastic Polyamides
Pebax (a block copolymer of polyamide and polyether) are made by Elf Atochem.
Various types of extruded netting may make use of the invention. For example, an extruded “square” netting that is extruded using either the “Hureau” process as aforenoted, yielding an all-elastomeric netting, or the “Bicomponent elastomeric netting” process as aforenoted, yielding either an all-elastomeric or a unidirectional elastomeric netting, or any other suitable netting process.
This extruded netting may then be uniaxially stretched in the machine direction in-line with the extruder, or biaxially stretched (essentially) using Larsen's process above noted, for example, to modify the properties of the net.
As already noted, extrusion of elastomeric nettings can often employ the same extrusion methods as those used for non-elastomeric nettings, i.e., orientable netting.
The three process steps of extrusion, MD stretch modification and TD stretch modification can be performed in two different ways:
1. Extrusion followed by MD and TD stretch at a biaxial orientor.
2. Extrusion followed by in-line MD stretch at ambient temperature with TD stretch at a (biaxial) orientor, such as that shown in the Larsen patent.
Extruded elastomeric nettings utilize thermoplastic elastomer resins as their raw material for at least some of the strands. This is possible as a result of the reversible crosslinking mechanism of thermoplastic elastomers. (Traditional rubber compounds, such as natural rubber, are vulcanized, resulting in non-reversible crosslinking.) This invention relates to extruded elastomeric netting, which in a secondary process step has been stretch modified. The combination of the use of thermoplastic elastomer
Leucadia, Inc.
Morris Terrel
Vidas Arrett & Steinkraus P.A.
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