CA1202773A - Alkene polymer composite heat storage material - Google Patents

Abstract

TITLE
Alkene Polymer Composite Heat Storage Material ABSTRACT OF THE DISCLOSURE
Heat storage composite consisting essentially of, on a weight basis, with the total being 100%, about 2-55% of an organic latent heat material, about 5-65% of a polyalkene having a molecular weight of at least 10,000, and about 15-90%
of a finely divided (particulate) solid having a melting point above and being substantially insoluble in the organic latent heat material and the polyalkene.

Description

TITLE
Alkene Polymer Composite Heat Storage Material E3ACKGROUND OF TH:E ~NV~;NlION
Field of the Invention This invention relates to composites which are useful a5 heat storage materials and which are comprised of an alkene polymer, a finel~ divided solid and an organic latent heat storage material.
Background As fossil fuels become scarce and more expensive, the use of alternate energy sources, such as solar energy and waste energy from industrial proces~es r becomes more attractive. Since the time periods during which such sources of energy are 15 available do not necessarily correspond to the time periods of energy n~d, energy storage plays an important role in the use of alternate energy sources.
Various types of heat storage are known 7 For example, heat can be stored in the form of the 20 sensible heat of a fluid such as water or the sensible heat of a solid such as stone or in the form of a combination of sensible heat and latent heat of a transitionl particularly the latent heat of fusion, using materials such as inorganic salt hydratesy 25 paraffin or organic polymers. The transition temperature of the latent heat material used must be below the temperature of ~he material from which heat is to be removed and stored, and equal to or above the temp~rature which is to be provided on removal of 30 heat from the latent heat material~
Latent heat materials undergoin~
liquid/solid phase transi~ions must be suitably containedO One way to accomplish this is to place the latent heat storage material in a container of CR-8157 35 suitable size, making allowance for temperature 3Z~73 expansion of the latent heat material. This approach using a container may give rise to a heat transfer problem when the heat is removed from ~he latent heat storage material. Solidification o the latent heat storage material occurs first on the walls of the container, and any additional heat removed must be conducted through the solid thus formed, the solid increasing in thickness as more of the liquid solidifies. Conversely, when heat is being stored in a solid latent heat material, heat transfer is inefficient because convection, which is requisite to efficient heat transfer, is hindered by the high viscosity of the liquid first formed from melted solid. Moreover, on cycling, the solid latent heat storage m terial may contract and pull away from the walls of the con~ainer, thereby fur~her decreasing the efficiency of heat conduction.
The art discloses attempts which have been made to solve some of these problems. For example, a granular form of latent heat material may be used, with the heat transfer medium being passed through a bed of such granules either to supply heat or to extract heat from the granules.
U.S. Patent 2,846,421 discloses A method for controlling the temperature of liquid phase reactions, ~or example, emulsion polymerization, by means of an encapsulated latent heat material, the capsule being formed from a metal or plastic~ the latent heat material being commonly available materials, including water, benzene, glycol, mercury, &lauber's salt and Wood' 5 metal. U.S. Patent 4,182,398 discloses a method for removing heat from a fluid by means of crystalline polyethvlene silane-grafted-crosslinked polymer pieces crosslinked to retain at least 70% of the heat of fusion of the 7~3 uncrosslinked crystalline polymer and suf~iciently crosslinked for the pieces not to stick together upon being cycled above and below the melting polnt of the polymer. U.S. Patent 4,221,259 discloses a method for storing heat by means of a ~usible substance, that is, a latent heat material, which is absorbed on a finely divided microporous carrier. Paraffin absorbed on active coal or coke in yrains or sticks is exemplified. Other latent heat materials which are disclosed are fusible mineral salts, metal hydrides, alloys, metal alloys, and polymers. U~S, Patent 4,003,~26 discloses a heat or thermal energy storage structure comprising a crosslinked polymeric resinous matrix having a plurality of substantially unconnected small closed ca~ities and a heat sink material encapsulated within the cavities. A similar type of heat storage composition is disclosed in U.K.
published Patent Application ~B 208603~A.
It is an object of this invention to provide a composite material which is suitable for heat storage. Another object is to provide such a material which can be cycled repeatedly between heat sink and heat source conditions without substantial deterioration~ Still another object is to provide such a material which can be fabricated readily from commonly available ingredients. Other objec-ts will become apparent h~ereinafter.
DISCLOSURE OF INVENTIO~
For ~urther comprehension o~ the invention and of the objects and advantages thereof, refererLce may be made to the following description and to the appended claims in which the various novel features of the invention are more particularly set forth.

'7~3 The invention resides in the composite consisting essentially of, by weight, with the total being 100%:
(a) about 2 55%, preferably about 30-40%, more preferably about 35%, o~ an organic latent heat materlal having at least one la~en~ heat transition, as a solid-solid transition(s) and/or a solid~liquid transition, in the temperature range 0-100C, and which, when liquid, wets the surface of the polyalkene of (b) and the ~inely divided solid of (c);
(b) about 5-65%, preferably about 15-25%, of a polyalkene having a molecular weight of at least lO,000, preferably at least lO0,000, molecular weiqht being weight averaye; and (c) about 15-90%, preferably about 35 55~, of a finely divided (particulate) solid such as a mineral, an organic material such as a polymer, a metal or a mixture thereof, said solid having a melting point above that of the materials of (a) and 20 (b) and being substantially insoluble in these materials.
The term "consisting essentially of" is used to specify the essential components of the composite of the i.nvention; the term is not intended to preclude the inclusion of other components which do not substantially. adversely affect the desirable properties of the composite of the invention, such other components thus being nonessential components of the composite.
The organic latent heat compounds which are useful in this invention have a latent heat transition(s); ~s a solid-solid transition(sj and/or a solid-liquid transition, singly or in combination, in the temperature range 0-100C. The preferred organic compounds have the highest latent heat capacity per gram when compared to their isomers.

Thus, organic compounds having unbranched sa~urated chains are preferred over organic compounds having branched chains, rings or unsaturated linkages. In addition, since it is necessary that the organic compound be compatible wi~h the alkene polymer, it is restricted to those organic compounds that possess a minimum of polar groups so that, when molten, it will wet the surface of the alkene polymer and the finely divided solid. In order for the liquid organic compound to wet the surface of the alkene polymer and the solid it must have a lower refractive index than those of the alkene polymer and the finely divided solid. It is also desirable that the organic compounds have a low vapor pressure in the working temperature range, that is, in the cycling temperature range as a heat sink and as a heat source, be nontoxic, possess little odor, and be thermally stable for a period of years. Preferred organic latent heat materials are paraffin wax and stearic acid.
It has been found that the heat storage capacity of the composite is equal to the sum of the sensible heat capacity of the components over the working temperature range and the latent heat capacity of the organic material undergoing solid-solid and/or solid-liquid transitions. The latent hea~ capacity of the composi~e is related only to the amount of organic material and is not influenced by the amount of polyalkene or type of finely divided solid. l'he solid provides the major portion of the heat conductivity necessary to conduct heat in and out of the composite, thereby making the composite useful as a heat storage element, and the solid must provide sufficient heat conductivity for this purpose. For example~ aluminum dust or flake

2~7~3 can be used as the solid ma~erial to provide good heat conductivity or it can be added to supplement the conductivity of a less conductive mineral or polymer. The polyalkene and the solid supply strength to the composite and also ~upply about one-half the sensible heat capacity.
The finely divided solid used in the composite of this invention can be a finely divided (particulate) mineral, polymer, or metal or a combination of these. Carbon is considered herein as a mineral. The size of the particles or agglomerates of particles is in the range o 1 ~m to 2 mm, passing a 10 mesh sieve (U.S. Sieve Series).
The latent heat storage composites of this invention can be prepared by three general procedures. In the first procedure a solution of organic latent heat material, for example, paraffin wax in any solvent that dissolves a reasonable amount of the organic latent heat material, but preferably with a solubility of at least 5% W/W7 iS slurried with a m;xture of finely divided solid and finely divided polyalkene. The solvent is removed and the resultant powder is pressed in an appropriate mold to give the desired shape. Preferably, the mold is heated above the melting point of the polyalkene since, in this case, it is not as critical that the polyalkene be finely divided. In the second procedure, molten organic latent heat material, for example, paraffin wax is added to the finely divided solid and the resultan~ mixture is then melt blended with a polyalkene. In the third procedure, a finely divided solid, polyalkene and an organic latent heat material, for e~ample, paraffin wax are melt b~ended together. The solid products recovered after carrying out the last two procedures can be remelted 7~3 and pressed into the desired shape. Al~ernatively, the solid products can be ground and the resulting powder can be pressed into ~he desired shape, optionally being heated above the melting point of the polyalkene. As a fuxther alternative, the products can be extruded into the desired shape.
The following examples are intended to illustrate but not limit the invention. Unless otherwise noted, all parts and percentages are by O weight and all tempera~ures are in degrees Celsius.
EX~MPLES 1-8 These examples demonstrate the preparation of composites using a mixture of hexane, finely divided (particulate) solid, polyalkene, and organic latent heat ma~erial. In each example, 10 g quantities of composite consisting of 4.7 g of mineral, 1.8 g of polyalkene, and 3.5 g of paraffin wax (m.p. 52-54C) were prepared by slurrying the components together in hexane which was heated to a temperature above the melting point of the wax but below the boiling point of the hexane (about 69C);
the mineral and polyalkene components are shown in Table I. The hexane was removed at 0.01 mm (1.3 Pa) pressure while heating at 95C for 2 h. The resultant powders were pressed at room temperature (20-25C) into cylindrical pellets at 5,000 psi (35,000 KPa). The pellets from Examples 1, 3, 5, and 7 were then pressed for 5 min at 3,000 psi (21,000 KPa) in a mold that was heated to 160C. The pellets were then cooled to 20-25C~ The pellets were weighed and measured. The pellets were then measured for creep crush according to ASTM D 2990-77~ The pellets were placed in a heatable press and subjected to a pressure of 2~ psi (140 ~Pa). The press temperature was varied cyclically, ~ h heating from ~g~æ~7~3 20C to 95~C, 8 h at 95C, 4 h cooling from 95~C to 20C, and 8 h at 2~C. After 72 h (3 complete cycles) creep crush was measured. The pellets were removed from the press. Any wax which was lost appeared as a solidiied meniscus around the base of the cooled pellet. ~his wax was carefully removed by scraping with a razor blade and the pellet was then weighed to determine the amount o wax lost during the creep crush tes~. The results appear in Table II. All wax loss was less ~han 1%. A negative wax loss indicate~ tha~ the mineral absorbed some water from the air and caus~d a sligh~ weight gain for the pellet. The ~ creep was less than 5%. A negative creep crush arises from relaxation of compression produced in the pellet during the preparation of the pellet.
In order to be useful as heat storage elements, these pellets should exhibit creep crush results of less than 20%, preferably less than 3~, and most preferably less than 1%.
Examples 1-8 show that shaped structures of the composite can be made either by cold-pressing the powder or by hot-pressing the powder above the melting temperature of the wax and the polyalkene and that, with either preparation, the composites have substantially the same good mechanical properties.

7~3 TABLE I
Ex. Mineral Polyalkene 1 Optiwhite~ clay 1220 Allied LPE*

5 3 Attagel** 50 n 4 n n Optiwhite~ clay Ultrahigh molecular weigh~ linear polyethylene 7 Attagel 50 n B n n *1220 Allied LPE is ultrahigh molecular weight polyethylene with a MW in the range 1-3x106 TABLE II
Example% Wax Lost % Creep 1 0.~ 2.515 2 0.22 2.199

3 0.25 1.343

4 -0.30 -2.721 0~39 4.129 6 0.23 - 1.0~5 7 -0.69 0.~79 8 -1.34 -2.716 3~
\

** denote~ trade mark Polyalkene pellets were cooled with liquid nitrogen and ground in a Janke and Kunkel* grinder t~
yield a fine powder~ The composites were prepared by slurrying together in hexane heated to a temperature above the melting point of the w~x but below the boiling point of the hexaneo 3.5 9 of paraffin wax (melt;ng point 52-54C), 1.8 9 of polyalkene powdPr, and 4.7 g of mineral. The hexane was removed at 0.01 mm ~1.3 Pa) pressure while heating at ~5C for 2 h.
~he specific polyalkenes and minerals used are shown in Table III. For each example the resulting powder was ground to approximately 10 ~m and a cylindrical pellet was pressed at 10,000 psi (69~000 RPa). The pellets of Examples 10, 12, 14 and 16 were heated to 165C and hot-pressed at tha~ ~emperature at a pressure of 1,500 psi ~10,000 RPa). Creep crush testing and determination of ~ wax loss after creep crush were done for each sample as described for ~xamples 1-8; the results are shown in Table IIIL
Example 9 is considered outside the invention. % Wax loss could not be determined because the sample was crushed and the particulate could not be recovered and weighed accurately. Although not wishing to be bound by this explanation, it is believed that the experiment failed because the polyalkene was not sufficiently finely divided and admixed with ~he mineral, as is necessary when the pellets are prepared by cold pressing.

* denotes trade mark .

TABLE III
%

Creep % Wax Ex. Polyalk.ene* Mineral Crush Loss 9~B 301Satintone** #1 50.17 10 1~ n 1.82 --0.. 18 11 ~ Atta~el 50 -4.15 -0.36 12 n ~7 1.53 6.()7 13190~ Satintone #1 -1.16 0~12 10 14 n n 13 . 53 0 . 06 n Attagel 50 ~3~05 -0O71 16 n n 1.60 -0.74 *Both polyalkenes used are high molecular weight polyethylenes; ~B 301 is ~ercules* HB 301 polyethylene resîn ~MW = 1.5 x 106); 1900 is Hercules 1900 polyethylene resin (MW = 5 x 6) These examples demonstrate the preparation of composi~es by mel~ blending the components, and the use of polye~hylenes of differen~ densities~ In each example, 109~5 9 of paraffin wax, 131.5 g of mineral and 7~ 9 of polyethylene were blended at 160C, first in a Readco** M~xer and then in a two-roll rubber mill. Each sample had 0.3 g of Irganox~ 1010 antioxidant (pentaerythritol ester of 3,5-di-tert-butyl-4-hydroxyphenylacetic acid manufactured by Ciba Geigy) added to it during melt mixin~. The recovered materials were ground u~ing a Janke and ~unkel grinder cooled with liquid nitrogen to give a powder (about 10 ~m particle size).
Cylindrical pellets were prepared by pressing the powder at 10,000 psi (699000) kPa in a mold heated to 180C~ The pellets were weighed and tested for creep ~* denotes trade mark .1 crush as in Examples 1-8. ~he results after 72 h of creep crush testing are shown in Table IV, along with a description of the minerals and polyethylenes used. The free radical stabilizer is not detrimental to the mechanical properties o~ the resul~ing product.
TABLE IV
%

Polyethylene Creep % Wax Ex.Mineral (Aldrich) Crush Loss 17Attagel 50low density PE 6.71 6.3 18 n high density PE 1~76 3.1 195atintone #llow density PE 3.24 4.0 n high density PE 1.50 4.2 These examples demonstrate the use of polyethylene and isotactic ~olypropylene of intermediate melt flow and the use o a combination of charcoal and Satintone #l as the mineral. For each example, 317.8 9 of paraffin wax was melt blended with the quantities of mineral and polymer shown in Table V, using subs~antially the same procedure described in Examples 17-20. In each example r the recovered powder was pressed into a cylindrical pellet in an evacuated mold and heated to 200C for 30 min, cooled and examined for creep crush and wax loss as in Examples 1-8. The results are given in ~able V.

~2~73 TABLE V
%

Mineral Polyalkene Creep % Wax ~x.(~) (g) Crush Loss 21Satintone Du Pont 2.28 6.2 ~l (426.7) Alathon~
7030* (163.5~
22Satintone Du Pont 2~42 1~3 ~l ~499.4) ~lathon~
703~* (90.8) lO 23 Satintone Du Pont l.39 3.l ~l (372.3)/ Alathon~
Charcoal 7030* ~l63.5) (54.5) 24 Satintone Hercules 0.95 less ~l (426.7) Profax~ than 6523** 1163'5) 0-5 * Du Pont Alathon~ 7030, low density polyethylene resin.
**Hercules~ 6523 polypropylene resin.

These examples demonstrate the effect of varying the amount of paraffin wax on ~ creep crush, % wax loss on thermal cycling and latent heat capacity. The amounts of materials used are ~hown in ~able VI; Satintone #l was used as the mineral and ~he polyalkene used was Aldrich* isotactic polypropylene. The paraffin wax (mp 52-54C) was weighed into a l L beaker and then melted on a steam bath . The polypropylene pellets were added ~ followed 30 by the mineral. The mixture was well stirred, cooled, and melt blended on a ~cwo-roll blender at 185CI, The mixture was then cooled with liquid nitrogen and ground at 20, 000 rpm in a Janke and ~unkel grinder to yi21d a fine powder. The powder 35 * denotes ~rade mark ,~L A~ 7~ 7 3 from each example was pressed at 100,000 psi (690,000 ~Pa) for 20 min at 25~C in~o a cylindrical pellet.
The pellets were subject0d to a 72 h creep crush test as in Examples 1-8. The results are shown in Table VI. All pellets had a wax loss of less than 0.1%.
The latent heat was measured for two heating and cooling cycles and the average latent heats given in Table VI are, therefore, the average of 4 determinations.
TABLE VI
Avg.
Paraffin ~ Latent Wax Mineral Polyalkene Creep Heat Ex. (9~ (~) (9) Crush (cal/g) 2~ 20 144 36 0.22 4.7 26 40 124 n 0 . 569 . 5 27 60 104 " 1.4813 . 6 2S 70 94 1~ 0 . 0618 . 6 29 80 84 n 3 . 5520 . 6 EXRMPLE:S 30~34 Examples 25-29 were repeated using Hercules Profax~ 6523 polypropylene in place of Aldrich isotactic polypropylene~ The compositions of the composites are shown in Table VII. The pellets were examined for creep crush properties (shown in Table VII) and wax loss. All had wax losses of less than 0.1%. The latent heat was measured for two hea~ing and cooling cycles and the average latent heats given in Table VII are, therefore, the average of 4 determinations.

TABLE VI I
Avg.
Paraffin ~ Latent Wax Polyalkene Mineral Creep Heat Ex. (g) (g)(g) Crush (cal/g) 18 72 -1.34 3.0 31 20 n ~;2 ~0 . 39 8 .6 32 30 n 52 --1.32 13.6 33 35 n 47 0 . 26 13 . 9 3~ 40 " 42 1~00 18.0 This example demonstrates ~he sensible and latent heat storage by a melt hlended composite and demonstrates a utility for ~he material, namely, heating water.
A 151.87 g sample of composite prepared according to ~he procedure described in Example 31 was pressed in an unheated (25C) mold at 10,000 psi ~69,000 KPa) ~v form a disk 2'~ dia. x 1" ~hick (5.08 cm dia. x 2.54 cm thick). The disk was heated in a water bath to 95.1C for 1 h. A Dewar flask contain;ng 372.49 g of water was thermally preequilibrated at 32.8C. The warm disk was removed from the water bath and transferred to the Dewar together with 16.73 9 of water, also at 95.1C. Five minutes later the temperature of the water in the Dewar was 41.2~C. The bomb constant for ~he Dewar was previously found to be 95.38 cal/C, From these measurements and the known composition of the composite, a sen~ible heat capacity of 0.272 cal/g~C
was de~ermined. This compares with a calculated heat capacity of 0.276 cal/gC for a composite con~ain;ng 62~ Satintone ~1 clay, 18~ polypropylene, and 20%
paraffin wax having sensible heat capacities of 0.179 cal/gC, 0.44 cal/~C and 0.43 cal/~C, respec~ively, '7~

together with a latent heat capacity of 52 cal/g for the paraffin wax.
EXPERIMENTS 1 and 2 These showings demonstrate the failure of composites prepared from paraffin wax and high molecular weight polyethylene when the finely divided solid is omitted. The composi~es were prepared using the hexane slurry technique as in Examples 1-8, except that in each experim~nt 3.5 g of paraffin wax and 6.5 g of high molecular weight polyethylene were used. Hercules HB 301 polyethylene was used in Experiment 1 and Hercules 1900 polye~hylene was used in Experiment 2. The hexane slurry was evaporated at 0.02 mm (2.7 Pa) for 2 h at 20C and the recovered powder was pressed into cylindrical pellets at 10,000 psi (69,000 RPa~. The pellets, tested for creep crush as in Examples 1-8, were completely crushed.
EXPERIMENTS 3 to 6 These experiments demonstrate the use of isotactic polypropylene and various amounts of paraffin wax without the finely divided solid.
Hercules Profax~ 7523 was ground to about a 10 ~m particle size using a Janke and Kunkel liqu;d nitrogen cooled grinder. The resultant powder was slurried wikh a hexane solution of paraffin wax. The hexane was removed as in the examples and a cylindrical pellet from the powder recovered from each experiment was pressed at 10,000 psi (69,000 KPa) at 20C for 5 min. The compositions are shown in Table VIII. The pellets of ~xperiments 4 and 6 were placed in a mold, heated to 200C, and hot-pressed at 800 psi (5,500 KPa) for 4 min. The polymer appeared to be fused i.n ~hese two pellets~
The results of creep crush tests and the ~ wax loss are shown in Table VIII. The materials of '7~3 Experiments 4 and 6 would not be suitable for heating storage elements because the heat conductivities are too low.
TABLE VIXI
Paraffin %
Wax PolyalkeneCreep % Wax Expt~(g) (g) Crush Loss 3 305 6.5 Crushed 4 3.5 6.5 2054 1.3 4.5 5.5 Crushed 6 4.5 505 17.99 0.4 Example 25 was repeated eY.cept that the amount of Satinkone $1 was 164 g and no paraffin wax was added. The % creep crush was 0.51 and the average latent heat was 0.
Example 30 was repeated except that the amount of Sa~intone #1 was 82 g and no paraffin wax was added. The % creep crush was -0.01 and the average latent heat was 0.
Example 30 was repeated except that the amount of paraffin wax was 100 9 and no mineral or polyalkene was added. Creep crush could not be determined because the wax melts under the tes~ and ~5 the average latent heat was 51.2.
BEST MODE FOR CARRYIN~ OUT THE lNv~NlION
The best mode presently contemplated for carrying out the invention is represented by Examples ~5 ~o 34.
INDUSTRIAL APPLICABILITY
The industrial applicability of heat sink/heat source composikes is well Icnown and is adequately discussed in the baclcground section of this specification. The composites of this invention provide an improvement over the artO

Disclosed and claimed in a copending commonly assigned Canadian Application No. 449,426 of A.G. Anderson and E.G. ~Ioward, Jr. filed simultaneously herewith is an invention which also is directed to a composite heat storage material, the composite consisting essentially of an organic latent heat material and a filled ethylene polymer which is prepared by polymeriziny the monomer(s) in the presence of particulate filler so that substantially all of the polymer is deposited on filler and substantially all o the -filler has polymer deposited thereon. The invention herein resides in an improvement over the invention of the commonly assigned application in that it has been discovered, surprisingly, that a useful composite, that is, one which can be cycled repeatedly without loss of the organic latent heat material, for example, paraffin wax, can be prepared by carefully admixing the requisite components, thus avoiding the less economical step of first preparing filled ethylene polymer.

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Claims (10)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Composite consisting essentially of, by weight, with the total being 100%:
(a) about 30-55% of an organic latent heat material having at least one latent heat transition, as a solid-solid transition(s) and/or a solid-liquid transition, in the temperature range 0-100°C, and which, when liquid, wets the surface of the polyalkene and the finely divided solid of (b);
(b) about 45-70% of a polyalkene having a molecular weight of at least 10,000 and a finely divided (particulate) solid having a melting point above and being substantially insoluble in the organic latent heat material and the polyalkene, the polyalkylene being at least 5% by weight and the finely divided (particulate) solid being at least 15% by weight of the combined weights of organic latent heat material, polyalkylene and finely divided (particulate) solid.

2. Composite of Claim 1 wherein the finely divided solid is selected from a mineral, a polymer, a metal and mixtures thereof.

3. Composite of Claim 2 consisting essentially of 30-40% of (a), 15-25% of polyalkylene and 35-55% of finely divided solid.

4. Composite of Claim 3 wherein the amount of (a) is about 35%.

5. Composite of Claim 1 wherein the organic latent heat material is paraffin wax.

6. Composite of Claim 1 wherein the organic latent heat material is stearic acid.

7. Composite of Claim 1 wherein the size of the finely divided solid is in the range 1 µm to 2mm.

8. Composite of Claim 1 in the form of compressed pellets exhibiting a creep crush according to ASTM D 2990-77 of less than 20%.

9. Composite of Claim 8 wherein the creep crush is less than 3%.

10. Composite of Claim 1 wherein the molecular weight of the polyalkene is at least 100,000.