Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2001-11-01
2003-03-25
Foelak, Morton (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C521S091000, C521S145000, C523S218000
Reexamination Certificate
active
06538050
ABSTRACT:
FIELD
The present invention relates to a method of foaming a polymer, in particular a fluoropolymer so as to produce a foamed article. In particular, the present invention relates to the use of zeolites as foaming agent to produce a polymer foam.
BACKGROUND
Foamed articles and in particular foamed polymer articles are well known in the art and have many applications. Foams are used for example for cushioning, insulation (thermal as well as sound), protection (packaging), weight reduction, impact absorption and thermal, chemical and electrical inertness. Such applications include for example wire insulation, coatings, tubes, films etc. Foamed polymer articles are typically produced from thermosetting foams, thermoplastic foams or elastomeric foams. Thermoplastic polymer foams can be made using expanded beads or conventional polymer processing techniques like extrusion, injection molding, reactive injection and mechanical blending. Foam extrusion, typically involves melting of the polymer in an extruder, adding a gas (or a compound that is in a gaseous state at extrusion temperature and standard pressure) (blowing agent) or a source of a gas, e.g. a chemical compound that produces a gas upon decomposition, and then extruding the molten thermoplastic polymer through a die to form a foamed structure. The process wherein a gas is used to foam the thermoplastic polymer is called physical foaming whereas the process wherein there is used a chemical compound which decomposes to produce the blowing agent is called chemical foam blowing. Often nucleating agents are also added to the molten polymer so as to improve the pore size and the homogeneity of the resulting foam.
Many different thermoplastic polymers are known to produce foamed polymer articles and these include for example polypropylene, polyethylene and polyester. Foamed polymer articles produced from thermoplastic melt processible fluoropolymers, i.e. polymers that have a partially or fully fluorinated backbone, are known as well. Such foamed fluoropolymers are of interest because of the superior heat resistance, chemical inertness, incombustibility, good dielectric properties, in particular insulating properties. For example, a foamed copolymer of tetrafluoroethylene and hexafluoropropylene, known as FEP, is particularly suitable for insulation of electrical wires such as data communication cables and coaxial cables because of the low dielectric constant and low dissipation factor associated with such foamed FEP polymers.
Processes for producing foamed polymer articles, including foamed fluoropolymers, have been disclosed in e.g. U.S. Pat. No. 5,726,214, U.S. Pat. No. 4,877,815 and U.S. Pat. No. 3,072,583. Additionally, U.S. Pat. No. 3,072,583 discloses the foaming of polyolefins using blowing agent and boron nitride as a nucleating agent. U.S. Pat. No. 4,764,538 discloses the use of boron nitride and certain inorganic salts as nucleating agents. U.S. Pat. No. 5,726,214 discloses the use of certain sulfonic and phosphonic acids as nucleating agents to foam a thermoplastic polymer through a physical or chemical foaming process. JP 08-12796 discloses the use of a combination of boron nitride and a zeolite as a nucleating agent in a composition to foam fluoropolymer resins.
A disadvantage of the processes of the prior art is that special equipment is needed to inject the gas when physical foaming is employed. When chemical foaming is used, the chemical foaming agent used may cause colored decomposition compounds to be formed and/or reaction with the polymer to be foamed may occur. Also, in order to produce foams of small cell size and of high homogeneity, nucleating agents need to be added to the composition, which may make the manufacturing more costly and less convenient.
It would be desirable to find an alternative way of foaming a thermoplastic polymer composition, which removes or mitigates one or more of the disadvantages of the prior art foaming processes. In particular, it would be desirable to find a method of foaming that can be easily practiced without the need for special equipment. Desirably, the new foaming process is easy and convenient and can be practiced in a cost effective and reliable way. Furthermore, it would be desirable to find foamed fluoropolymer articles that can be produced in an easy, convenient and cost effective way and that have good and/or improved properties, in particular that have improved dielectric properties.
SUMMARY
In accordance with the present invention there is provided a method of foaming a polymer comprising the steps of providing a composition comprising said polymer and a zeolite in an amount of 0.3% or more based on the weight of said polymer and extruding said composition.
Additionally, there is provided a foamed article comprising a foamed polymer comprising a zeolite in amount of at least 0.3% by weight based on the weight of polymer.
Finally, there is provided the use of a zeolite to foam a polymer, in particular a fluoropolymer.
It has been found that zeolites can be used on their own without the addition of a gas or a chemical foaming agent (i.e., a material that generates gas) to foam a thermoplastic melt processible polymer. This is surprising as the zeolites are not per se capable of decomposing into a gas and they are obviously not gas substances themselves. An interesting aspect of the use of a zeolite as the foaming agent is that it can simply be dry blended with the polymer or blended directly into the melt and upon extrusion of the zeolite containing polymer composition they will foam the polymer to a degree depending on the amount of zeolite contained. Of course the zeolite can be used in combination with typical foaming agents such as a gas or a chemical foaming agent but this is not necessary. Accordingly, the foaming of the polymer can be carried out on conventional extrusion equipment. Also, a foamed polymer produced with the process of the invention can be reintroduced into the extrusion equipment and will foam again without adding new or additional zeolite. Such is normally not possible with the known chemical foaming agent because in the known processes the chemical foaming agent will have been consumed in the foaming process. This provides an additional advantage for the process of the present invention because any waste of the foamed article can be conveniently recycled and used again to produce foamed articles. The zeolite is furthermore a substantially inert inorganic compound that can be used at high temperature without the risk of decomposing.
It has further been found that the foamed articles produced with the method of the invention may have very fine cells of high homogeneity. In particular, foams produced with the invention typically show closed cells distributed in a network of open cells. As a result, foams can be produced that have highly desirable properties. For example, foamed fluoropolymers produced with the method of the invention have excellent dielectric properties, in particular a low dielectric constant and a low loss factor or dissipation factor making such foamed fluoropolymers particularly suitable as insulating medium in for example wires, for example data communication cables such as plenum wires and high frequency cables such as coaxial cables.
Additionally, the foamed articles may find application as piezoelectric substrates or in tubes.
DETAILED DESCRIPTION
Zeolites used as foaming agent in the present invention are reversibly hydrated aluminum silicates that generally contain alkali or alkaline earth metal oxides which may be ion exchanged for other metals or for hydrogen. Zeolites for use in this invention may be synthetic zeolites as well as naturally occurring zeolites. The zeolite may be represented by the general structural definition:
M
x
[(AlO
2
)
x
(SiO
2
)
y
].mH
2
O
wherein M represents a cation such as H
+
, NH
4
+
, sodium, potassium, magnesium, and calcium of valence n, n is 1 or 2. x and y can vary from 1 to a large number, e.g. 5000 but usually has a value of 1 to 150. m represents the moles
Peters Erik
Weilandt Karl-Dieter
3M Innovative Properties Company
Foelak Morton
Lilly James V.
Szymanski Brian E.
LandOfFree
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