Propylene polymer foams

Details Propylene polymer foams Propylene polymer foams

2001-10-02

2002-07-09

Foelak, Morton (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Cellular products or processes of preparing a cellular...

C521S079000, C521S143000, C521S144000, C521S134000, C525S304000, C525S305000, C525S376000

Reexamination Certificate

active

06417242

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to propylene polymer foams, methods for preparing the same, expandable compositions, and foamed articles.
BACKGROUND
Polypropylene can offer better impact properties than polystyrene, because polypropylene is a semi-crystalline polymer which has a glass transition temperature substantially below room temperature. In addition, polypropylene can offer good temperature stability and high chemical resistance. However, until now, the production of polypropylene foams has been limited because of its low melt strength and melt elasticity and this makes it difficult to be foamed in comparison to the other plastics. If the melt strength and the melt elasticity are too weak, as in the case of polypropylene, the cell walls separating the bubbles will be too weak to bear the extensional force that is generated during the foaming process and the bubbles will rupture very easily. As a result, foamed polypropylene products are generally characterized by high open cell content, which makes them unsatisfactory in many applications.
Extruded closed cell foams of polypropylene having a foamability factor of less than about 1.8 are disclosed in U.S. Pat. No. 5,527,573. Branched or lightly crosslinked polymers, such as may be obtained by chemical or irradiation branching or lightly crosslinking, including high melt strength polypropylene are used therein for making foams. In co-pending U.S. patent application Ser. No. 09/133,576 filed Aug. 13, 1998 and WO-99/10424 in-situ rheology modification of polyolefins is disclosed as applied to, among others, polypropylenes resulting in polypropylenes having advantageous melt processing properties. Foams are mentioned among the possible end uses or applications of such rheology-modified propylene polymers.
There is a continuous demand for propylene polymer foams of a relatively high closed cell content, which do not suffer from the bubble stability problem generally experienced with polypropylene foams. The foams desirably are capable of being manufactured at high foam production rates on conventional foaming equipment while using a relatively low amount of blowing agent.
SUMMARY OF THE INVENTION
The present invention is directed in one aspect to a foam comprising a coupled propylene polymer and having a density in the range of from 9.6 to 801 kilograms per cubic meter (kg/m
3
) (0.6 to 50 pounds per cubic foot (lbs/ft
3
)) and having either a foamability factor of more than 1.8 to less than 2.8 and an open cell content less than 20 percent (%), or a foamability factor of equal to or more than 2.8 and less than 15 and an open cell content of less than 50%. The invention includes in a further aspect a process for preparing a foam comprising heating a propylene polymer having a melt flow rate from 0.2 to 20 grams per 10 minutes (g/10 min) and a melt strength of at least 39 centiNewtons (cN), optionally mixed with a nucleating agent, to a molten state to produce a molten polymer material, and mixing said molten polymer material with a blowing agent under conditions to produce a foamed material having a density in the range of from 9.6 to 801 kg/m
3
(0.6 to 50 lbs/ft
3
), wherein the propylene polymer is obtained by a coupling treatment carried out before, during or after admixture with the blowing agent. According to yet a further aspect the invention relates to an expandable composition comprising a coupled propylene polymer having a melt strength of ≧39 cN at a drawability from 15 to 60 millimeters per second (mm/sec), and a melt flow rate from 0.2 to 20 g/10 min, and a blowing agent.
Finally, the present invention relates to articles comprising the foams according to the present invention. It has been found that excellent foams can be obtained from certain propylene polymers which due to their melt strength and melt drawability characteristics provide higher thermal collapse resistance, and therefore less open cell content and lower density than prior art polypropylenes.
DETAILED DESCRIPTION OF THE INVENTION
As used herein the “foam density” is determined by weighing a small representative piece of foam and dividing this by the volumetric displacement of the foam.
As used herein the terms “melt strength” and “drawability” refer to polymer melt properties and are measured at 190° C. according to the following procedure. Measure melt strength by using a capillary rheometer fitted with a 2.1 millimeter (mm) diameter, 20:1 die with an entrance angle of approximately 45 degrees. After equilibrating the samples at 190° C. for 10 minutes, run the piston at a speed of 25.4 millimeters per minute (mm/minute). The standard test temperature is 190° C. The sample is drawn uniaxially to a set of accelerating nips located 100 mm below the die with an acceleration of 2.4 millimeters per second per second (mm/sec
2
). The required tensile force is recorded as a function of the take-up speed of the nip rolls. The maximum tensile force attained during the test (at break) is defined as the melt strength and is expressed in cN. The limiting wheel velocity at break is the melt drawability and reported in units of mm/s. In the case of polymer melt exhibiting draw resonance, the tensile force and wheel velocity before the onset of draw resonance was taken as the melt strength and drawability, respectively.
As used herein the “average cell size” in millimeters is determined according to American Society for Testing and Materials (ASTM) D3576 Standard Test Method for Cell Size of Rigid Cellular Plastics.
As used herein the term “melt flow rate” refers to the melt flow rate of the polymer measured according to method ASTM D 1238L, at a temperature of 230° C. under a weight of 2.16 kg and is expressed in g/10 min.
As used herein the term “isotactic” refers to a degree of isotacticity as measured by C
13
NMR of at least about 50%.
As used herein, “propylene polymer” means propylene polymer selected from the group consisting of (a) homopolymers of propylene, (b) random and block copolymers of propylene and an olefin selected from the group consisting of ethylene, C
4
-C
10
1-olefins, and C
4
-C
10
dienes, provided that, when said olefin is ethylene, the maximum polymerized ethylene content is less than about 20% by weight, when said olefin is a C
4
-C
10
1-olefin, the maximum polymerized content thereof is less than about 20% by weight and when said olefin is a C
4
-C
10
diene, the maximum polymerized content thereof is less than about 20% by weight, (c) random terpolymers of propylene and 1-olefins selected from the group consisting of ethylene and C
4
-C
8
1-olefins, provided that the maximum polymerized C
4
-C
8
1-olefin content is less than about 20% by weight, and when ethylene is one of said 1-olefins, the maximum polymerized ethylene content is less than about 20% by weight, and d) impact propylene copolymers also referred to as heterophasic propylene copolymers where polypropylene is the continuous phase and an elastomeric phase is uniformly dispersed therein. Advantageously, the impact copolymers have at least about 5 weight percent, preferably at least about 10, preferably up to about 40, more preferably up to about 25 weight percent, and most preferably up to about 20 weight percent ethylene. The C
4
-C
10
1-olefins include the linear and branched C
4
-C
10
1-olefins such as, for examples 1-butene, isobutylene, 1-pentene, 3-methyl-1-buterie, 1-hexene, 3,4-dimethyl-1-butene, 1-heptene, 3-methyl-1-hexene, and the like. Examples of C
4
-C
10
dienes include 1,3-butadiene, 1,4-pentadiene, isoprene, 1,5-hexadiene, 2,3-dimethyl-1,3-hexadiene, and the like.
The term “isotactic propylene polymer” refers to a propylene polymer having preferably no or only a very minor percentage of comonomers polymerized in its structure and has in general an isotacticity of at least 50% as determined by
13
C NMR, more preferably an isotacticity of at least 52%, and most preferably of at least 54%. It preferably is a propylene homopolymer.
Surprisingly it has been found that a certain novel combinations of propylene polymer resin properties

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