CA1195792A

CA1195792A - Drip irrigation tubing cross-linked by irradiation and extrudable tube forming composition

Info

Publication number: CA1195792A
Application number: CA000410351A
Authority: CA
Inventors: William J. Michie, Jr.
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1981-09-16
Filing date: 1982-08-27
Publication date: 1985-10-22
Also published as: AU546052B2; AU8791182A; SE8205287D0; SE8205287L; JPS5861138A

Abstract

ABSTRACT OF THE INVENTION

Drip irrigation tubing cross-linked by irradiation, the tubing having a burst strength at 80°C greater than about 3612 hours at 400 psi hoop stress and greater than about 8008 hours at 320 psi hoop stress. The tubing is formed by profile extrusion of a composition comprised of: (1) more than about 80% and less than about 97% by weight of a low pressure, low density hydrocarbon interpolymer; (2) more than about 2% and less than about 10% by weight of a copolymer of ethylene with a vinyl ester of C1-C30 monocarboxylic acid and other alpha olefins in minor concentrations providing a copolymer having a density between 0.91 and 0.94 gm\cm3; (3) more than about 0.01% and less than about 3% by weight of at least one ultraviolet stabilizer; and (4) less than about 0.5%
by weight of at least one anti-oxidant stabilizer which permits cross-linking by irradiation.

Description

7~%
;

1. Field of the Invention 3 This invention relates to an extrudable composition 4 capable of forming tubing, the composition comprised of a low pressure, low density (LP-~D) hydrocarbon interpolymer, a 6 copolymer of ethylene with a vinyl ester of Cl-C30 monocarboxylic 7 acid and other alpha olefins in minor concentrations, an ultra-8 violet (W) stabilizer and an anti-oxidant stabilizer.
9 2. Description of the Prior Art Drip irrigation tubing is well known. Such tubing is 11 useful for drip i-rigation, e.g., to dispense water to orchards 12 and row crops. The popularity of drip irrigation has grown since 13 it was first discovered some 30 years ago. Crops grown through 14 drip irrigation yield more frui~ and better quality fruit than other irrigation systems~
16 ~ Typically, drip irrigation tubing has a thic~ness from 17 about 0.1 mm to about 3.81 mm and an outside diameter from about 18 5 mm to about 51 mm. The tubing îs generally extruded from a 19 resin composition to form single and/or multiple chambered tubing. Generally, it is desirable for the resin composition 21 to have ease of both extrudability and vacuum sizing. It is 22 desirable for the tubing to exhibit good flexibility, pressure 23 capability and smooth and glossy surfaces. The tubing also mus~
24 have superior environmental stress crack resistance. Very important parameters are the pressure capabilities and resistance 26 to stress cracking.
27 Prior ar~ drip irrigation tubing has included off grade 28 blends of high pressure-low density polyethylene (HPLDPE)-29 ethylene vinyl acetate ~EVA) copolymer resins, specifically blends of EVA-HPLDPE copolymer having an EVA content of about 5%
. ~

1 EVA-high density polyethylene (HDPE)-HPLDPE interpolymer

2 having an EVA content of 8% and an HDPE content of 10%, the

3 balance being HPLDPE; and LP-LD polyethylene. However, all of

4 the aforementioned tubing materials suffered from one or more disad~antages, such as insufficient stress crack resistance or 6 press~re capabilities at elevated temperatures. LP-LD poly-7 ethylene provides an excellent drip irrigation tubing, but it 8 is harder to extrude than the other materials~ Other prior art 9 materials used for drip irrigation tubing include off grade film products such as HPLD polyethylene modified with rubbers, and 11 an ethylene-octene-l copolymer. mhe latter is difficult to 12 extrude, having a narrow molecular weight distribution which 13 leads to melt fracture during extrusion~
14 It is known to subject drip irrigation tubing to irradiation to cross-link unsaturated bonds within the product 16 in order to eliminate stress crack problems by creating an 17 infinite molecular weight polymer. For example, tubing has been 18 subjected to electron beam irradiation to receive a total dose 19 between 10 and ll M.rads. Other components`present in extrudabl~
compositions typically used for drip irriga~ion t~-bing include 21 a W stabilizer and an anti-oxidant stabilizer. In view of the 22 fact that the tubing is used outdoors and is exposed to sunlight, 23 a UV stabilizer is necessary. Carbon black is a popular W
24 stabilizer. Also, during extrusion, it is necessary to have an anti-oxidant to arrest polymer degradation via free radical 26 propagation.
27 3. Brief ~escription of the Invention 28 It now has been found that the following extr~dable 29 composition forms a drip irrigation tubing which, after being cross-linked by irradiation, has excellent burst strength at 31 80C: (1) more than about 80% and less than about 97~ by weight l ~ 57~
12~37 1 of a low pressure, low density hvdrocarbon interpolymer; (2) more 2 than about 2% and less than about 10% by weight of a copoly~er of 3 ethylene with a vlnyl ester of Cl-C30 monocarboxylic acid and 4 other alpha olefins in minor concentrations providing a copolymer having a density between 0.91 and 0.94 gm/cm3i 6 (3~ more than about 0.01% and less than about 3% by wei~ht of an 7 ultraviolet stabilizer; and (4) less than about 0.5~/0 by weight 8 of an anti-oxidant stabilizer which permits cross-linking by 9 irradiation.
4. Detailed Description of the Invention 11 LP-LD Hydrocarbon Interpolymer 12 Suitable LP-LD hydrocarbon interpolymers for the tube 13 forming compositions of the present invention include copolymers 14 of ethyLene or butene-l and one or more C3 to C10 alpha olefins.
These copolymers have a density of ~ 0.910 to ~ 0.940 grams/cm3 16 and preferably of about, ~ 0.915 to ~ 0.928 grams/cm3. These 17 copolymers can be made in a solution, slurry or gas phase process 18 well known to those skilled in the art.
19 The LP-LD hydrocarbon interpolymers used for the compositions of this invention should have a standard melt index 21 from about 0.1 t~ about 10 decigrams per minute and preferably 22 from abo~t 0.3 to about 1.2 decigrams per minute. `
23 The extrudable LP-LD hydrocarbon interpolymers employed 24 in the extrudable composltions of the present invention are normally solid materials, that is, solid at room temperature.
26 Any extrusion grade LP-LD hydrocarbon interpolymer can be used 27 in the compositions of the present invention. T~e term LF-LD
28 hydrocarbon interpolymer thus includes interpolymers of one or 29 more olefin(s) with each other, and/or up to about 30 weight pe ent of one or more monomer(s) which ~a copolymerizable wlth 3L~ 3~ 12637-C

such olefin(s). The olefins include those such as ethylene, pro-pylene, butene-l, isobutylene, pentene-l, 4-methyl-pentene-1, hexene-l, octene-l, nonene-l, decene-l, undecene-l, dodecene-l, tridecene-l, tetradecene-l, pentadecene-l, hexadecene-l, heptadecene-l, octa-decene-l, nonadecene-l, and eicosene. Interpolymers include one or more of such olefins and one or more other monomers which are inter-polymerizable with such olefins, such as other vinyl and diene ccmpounds, i.e., those having the group -C=C-.
Preferred copolymers are the ethylene copolymers such as ethylene/propylene copolymers, ethylene/butene-l copolymers, ethylene/
isobutylene copolymers, ethylene/pentene-l copolymers, ethylene/
4-methyl-pentene-1 copolymers, ethylene/hexene-l copolymers, ethylene/octene-l copolymers, and the like. Preferred ethylene interpolymers would include two or more of the followin~: propylene, butene-l, isobutylene, pentene-l, hexene-l, 4-methyl-pentene-1 and octene-l. Preferred butene-l interpolymers would include ethylene, propylene, hexene-l, 4-methyl-pentene-1 and octene-l as comonomers.
Preferred low pressure, low density ethylene copolymers for use in the present invention include those which may be produced in accordance with the procedures set forth in U.S. Patent 4,302,566 in the names of F.J. Karol et al and entitled "Preparation of Ethylene Copolymers in Fluid Bed Reactor", and the procedures set forth in U.S. Patent 4,302,565 in the names of G.L. Goeke et al and entitled "Impregnated Polymerization Catalyst, Process for Preparing, and Use for Ethylene Copolymerization" as well as 57~;~

procedures which will produce ethylene hydrocarbon copolymers with properties as heretofore described. U.S. Patent 4,302,566 corres-ponds to European Patent Application No. 79100953.3 which was opened to the public on October 17, 1979 as Publication No. 4645 and U.S. Patent 4,302,565 corresponds to European Patent Application No. 79100958.2 ~hich was opened to the public on October 17, 1979 as Publication No. 4647.
Other low pressure, low density ethylene hydrocarbon polymers preferred for use in the present invention are those which may be pr~pared as described in U.S. Patent 4,011,382, entitled "Pre-paration of Low and Medium Density Ethylene Polymer in Fluid Bed Reactor" by I.J. Levine et al.

Ethylene Vinyl Ester Copol~er Suitable for this invention are > 2% and ~ 10% by weight of copolymers of ethylene with a vinyl ester of a Cl - C30 mono-carboxylic acid. Other alpha olefins also may be present in such copolymers in minor concentrations. The ethylene vinyl ester copolymers have a density between about 0.91 to about 0.94 gr~ms/cm3.
The carboxylic acid is preferably aliphatic, saturated and mono-carboxylic, e.g., vinyl propionate, vinyl hexoate, vinyl octoate, vinyl dodecanate, vinyl behenate or isopropyl acetate. The prefer-red ester is vinyl acetate in a concentration from about 2% to about 35%, preferably between about 23~/o and 35%.

57~

. I
I' 1 Ultraviolet Stabilizer 2 The tube forming compositions of the present invention ¦
3 contain at least one UV stabilizer. ~hese W stabilizers 4 are present in a wei~ht concentration of from about .01% to about~ 3~/0. The UV stabilizers are able to absorb ultraviolet 6 light more readlly than the olefin polymer in th-e composition 7 or are able to interact with and deactivate free radicals 8 immediately as they are formed in the composition.
g Suitable W stabilizers according to this invention include carbon black, derivatives of 2-hydroxy benzophenone, and 11 hydroxyphenylbenzotriazoles. Other W stabilizers which are 12 believed to be suitable for this invention include the ollowing:
13 2-hydroxybenzophenones such as 2,4-dihydrobenzophenone, 14 2-hydroxy-4-methoxybenæophenone, 4-(heptyloxy)-2-hydroxybenzo-phenone, 2-hydroxy-4-toctyloxy)benzophenone, 2-hydroxy-4-(2-16 hydroxyethoxy)benzophenone, 4-alkoxy~2-hydroxybenzophenone, 17 2-hydroxy-4-methoxy-5-methylbenzophenone, 5-benzoyl-4-hydroxy-2-18 methoxybenzene-sulfonic acid, 2-(2-hydroxy~4-methoxybenzoyl) 19 benzoic acid~2~21-dihydroxy-4-methoxybenzophenone~ 4-butoxy-2,2'-dihydroxybenzophenone, and 2~2'-dihydroxy-4-(octyloxy)benzophenone 21 2-(2H-benzotriazol-2-yl)phenols such as 2-(2H-benzotriazol-2-yl)~
22 p-cresol, 2-tert-butyl-6-(5-chloro-2H-benzotriazol-2-yl)p-cresol, 23 2,4-di-tert-butyl-6-(5-chloro-2H-benzotriazol-2-yl)phenol, and 24 2-(2H-benzotriazol-2-yl)4,6-di-tert-pentylphenol; phenyl esters such as phenyl salicylate, p-(1,1,3,3-tetramethylbutyl)phenyl 26 salicylate, resorcinol monobenzoate, bis(p-nonyl~henyl)terephtha-27 late, and bis(p-1,1,3,3-tetramethylbutyl)phenyl ~sophthalate;
28 nickel compounds such as bis[2,2'-thiobis-4-(1,1,3,3-tetramethyl-29 butyl)phenolato]nickel, and [2,2'-th-iobis[4-(1,1,3,3-tetrame~hyl-butyl)phenol]ato(2-)~butylamine)nickel.

I ` 12637 ~ 75~
., 1 Anti-Oxidant Stabilizer 2 The tube forming compositions of the present inveneion 3 also contain at least one anti-oxidant for the olefin polymer.
4 These anti-oxidants are present in stabilizingly effecci~e quantities. Such amounts are about 0.002 to 0.5, and preerably 6 about 0.05 to 0.12, percent by weight, based on~the weight of 7 the olefin polymer. The anti-oxidant stabilizers which may be 8 employed in the compositions of the present invention include 9 all those polyolefin anti-oxidants commonlv employed in olefin polymer based tube extrusion compositions. These materials 11 are such as are capable of providing anti-oxidant protection at 12 processing temperatures of the order of about 135C to 343C, 13 or higher.
14 Such anti-oxidant stabilizers include hindered phenols, such as p-hydroxyphenylcyclohexane; di-p-hydroxyphenylcyclohexane 16 dicresylolpropane; tertiary butyl para cresol; 2,6-di-tert- .
17 butyl-p-cresol; 2,4,6-tri-tert-butylphenol; octadecyl-3-(3,5-18 di-tert^butyl-4-hydroxyphenyl3propionate; tetra bistmethylene 3-19 (3',5'-di-tert-butyl-4'-hydroxyphenyl)propionate]methane;
1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl) 21 benzene; tris(3,5-di-tert-butyl-4-hydroxybenzyl)isocyanate;
22 1,3,5-tris(4-tert-butyl-3-hydroxy-2,6 dimethyl benzyl)^l,3,5-23 triazine-2,4,6-(lH,3H,SHO-trione; and bis-[3,3-bis-4'-hydroxy-3'-24 tert-butyl-phenyl)butanoic acid]-glycol ester, condensation pro-ducts of dialkylphenols with formaldehyde, reaction products of 26 phenol with styrene, 1,1'-methylene-bis(4-hydroxy-3,5-tert-butyl-~7 phenol), 2,2'-methylene-bis-(4-methyl-6-tert-but~lphenol), 2,6-28 (2-tert-butyl-4-methyl-6-methylphenol)-p-cresol, Phenylethyl-29 pyrocatechol, phenolisopropylpyrocatechol, 1,1,3-tris(2'-methyl-

5'-t'-butyl-4-hydroxy phenol)butane, 2,2-methylene-bis[6-(~-75~
1~637 1 methylcyclohexyl)-4-methylphenol], 1,3,5-trimethyl-2,4,6-tris-2 (3',5'-di-t-butyl-4-hydroxybenzyl]benzene and ~-naphthol; and 3 sulfur containing compounds such as 2,2'-thio-bis(4-methvl-6-4 tert-butylphenol), 4-4'-thio-bis(3-methyl-6-tert-butylphenyl), distearyl thiodipropionate and dilauryl thiodipropionate; and

6 quinoline based compounds such as polymerized 1,2-dihydro-2,2,4-

7 trimethylquinoline; 6-ethoxy-1,2-dihydro-2,2,4-trimethylquinoline;

8 and 6-dodecyl-1,2-dihydro-2,2,4-trimethylquinoline.

9 The preferred primary anti-oxidant stabilizers which are employed in the compositions of the present invention are 11 the three aforeme~ioned quinoline based compounds. They permit 12 cross-linking by irradiation and still operate as thermal 13 stabilizers.
14 The primary quinoline based anti-oxidants may be used individually or in various combinations with one another.
16 Other Additives 17 In addition to the LP-LD hydrocarbon interpolymer, 18 copolymer of ethylene with a vinyl ester of Cl-C30 monocarboxylic 19 acid, W stabilizer, and primary anti-oxidant(s), the`compositions of the present invention may contain other adjuvant materials 21 which are commonly employed in olefin polymer-based extrudable 22 tubing compositions. Such other adjuvants would include 23 fillers, pigments, lubricants, modifiers and similar materials.
24 The fillers which may be used in the olefin polymer-based extrudable compositions of the present invention are the 26 fillers which are commonly used with such polymers. The fillers 27 are used in amounts which correspond to about 1 t~ 20 percent 28 by weight, based on the weight of the olefin polymer. Such 29 fillers would include materials such as titanium dioxide, clays, diatomaceous earth, calcium silicates and others kno~m in the art.

I ~ 79~ 12637 1 The lubricants which ~re c~mo~ly employed in the 2 olefin polymer based extrudable compositions are the lubricants 3 which are cor~nonly used with such polymers. The lubricants are 4 used in amounts which correspond to about 0.02 to 0.3% by S weight of lubricant a8ent based on the weight o~ the olefin polymer. Example of such lubricants: fatty acid amides such 7 as stearamide, oleamide, behenamide and luramide. Other 8 lubricants include calcium, zinc, titanium, and other Grou~ I
9 or II metal stearates.
Preparation of Extrudable Compositions 11 The extrudable compositions can be prepared by several 12 methods which are well known to those skilled in the art. In 13 on~ method, the components are dry blended together in a roll 14 drum for 20 rninutes at room temperature. In another method, the lS components are compounded u~ilizing a Banbury batch mixer coupled 16 eo a Farrel Birmingham single screw extruder melt pump. The 17 components are mixed in the Banbury mixer for about 4 to 5 18 minutes, dropped at a temperature from about 125C to-about 19 185C and extruded through an extruder melt pump using a throat temperature of about 140C, a barrel temperature of about 150C
21 and a die temperature of about 175C.
22 - In still another method, the components are compounded 23 using a Banbury batch mixer and melt fed into a single screw 24 extruder and under water pelletized. The componen~s are mixed in 2~ the Banbury mixer for about 1.5 to 5 minutes, usually using three 26 cycles, two working cycles separated by a short non-working cycle 27 in ~hich the resin is turned over. It is then dropped at a 28 temperature from about 150C to about 190C and extruded ~hrough 29 the extruder using a barrel temperature of about 180C. The end of the extruder has a Tnany holed die plate where a rapidly !l i, ~ 57~

1 rotating blade cuts off the extrudate going through the die 2 plate to form pellets. These pellets are then used in the 3 extrusion of the tubing, 4 The process used for forming drip irrigation tubing fro S the above composition consists basically of a standard extruder 6 whose length may be from a 16 to 1 length diameter (L/D) ratio 7 to a 32 to 1 L/D ratio and any diameter required (i.e., 1", 8 2 1/2", 3 1/2", etc.) depending on output rate desired as is well 9 known in the industry. The extruder must be equipped with some form of an Archemeadian screw whe~her a standard conventional type 11 or a barrier type such as the ~addock mixing screw. The 12 Maddock mixing screw is the ~referred screw, however. The 13 extruder should be capable of infinite temperature settings' 14 from 125C to 280C and equipped with barrel coolin~ capabilities.
The polymer is fed into the extruder in pellet form and 16 is melted by the hot barrel and shear (i.e,, mechanical 17 dissipation from the screw in the form of heat) in order to 18 melt the polymer and deliver it in its molten stage to the die.
19 Profile Extrusion of Tubing In a process for forming drip irrigation tubing, a 21 polymer tubing forming composition is melted and then extruded 22 through an annular die. The die has a die gap greater than about 23 0.25 mm and less than about 5.0 mm, and preferably greater than 24 about 0,30 mm and less than 1.90 mm. The polymer tubing forming composition is extruded at a melt temperature between about 26 150C and about 240C. Extrusion takes place in a horizontal 27 or downward direction in an annular form. The annular extrudate 28 is usually drawn down to desired dimensions, then cooled in a 29 vacuum sizing tank which has water inside of it. The annular extrudate typically is drawn do~ to desired dimensions l ¦ via a vacuum sizing method which is a conventional technique, 2 ¦ well known in the art. This technique uses a vacuum sizin~
3 ¦ sleeve to size the tubing. This sleeve is in a water bath which 4 ¦ serves to quench and solidify the tubing in its proper shape.
5 ¦ The ubing is then pulled at a constant rate through a multi-6 contact puller and then wound up on a roll for irradiation.
7 The multi-contact puller provides the necessary tension to give 8 the wall thickness desired. This is also well known in the 9 ~rt.
The inside of the tubing is usually left exposed to ll atmospheric air pressure while ~he outside of the wall of the 12 tubing has a negative pressure from the vacuum sizing method 13 such that the tubing is pulled up against the vacuum sizing 14 sleeve from which it is given its final size. Dra~down ratios on the outside diameter, a ratio defined as the inside diameter 16 of the bushing divided by the outside diameter of the final 17 ~ubing, should be less than 2 to l. The wall thickness drawdown 18 ratio, as defined by the gap in the die divided by tXe ~hinnest l9 uall of the final tubing, should be less than 2.5 to l for quality tubing. In certain cases, the tubing may be perforated, 21 e.g., via exposure ~o laser beam, and then irradiated to cross-22 link the polymer tubing.
23 Physical Properties of the Drip Irrigation Tubin~ ¦
24 As hereinbefore noted, the physical properties ~hich are particularly significant for the successful use of thermoset 26 drip irrigation tubing include its stress crack resis~ance and 27 burst properties, especially at eleva~ed temperatures. With 28 respect to the latter, the burst properties at 80C are 29 particularly important. A tubing which possesses excellent environmental stress crack resistance (ESCR) and burs~ properties ~195~9~ 12637 l at ambient and 80C in general will be very satisfactory for 2 drip irrigation tubing applications worldwide.
3 The drip irrigation tubing of this invention have been 4 found to possess excellent ESCR and improved burst properties at 80~. The ESCR ensures that the tubing does not fail in the 6 field due to insert fittings or when subjec~ed to other mechanical 7 stresses such as a V-pinch in the tubing. A mathod commonly 8 practiced in the art to fasten tubing together is to insert the 9 tubing over a barbed fitting.
Drip irrigation is laid on top of the ground, usually ll in arid or semi-arid areas, where, in the summertime, the 12 temperature of the tubing will get to rather high temperatures, 13 up to about 80C. Since the inside of the tubing is still under l4 pressure, the tubing must possess pressure capabilities at 80C. Internal pressures are caused by water in the tube being 16 metered out at an individual plant. Usually, drip irrigation 17 tubing comes in a dual chambered tube having a main chamber and 18 a small chamber. The main chamber portion of the tubing will 19 hold the pressure. Only a few holes are drilled into the main chamber portion of the tubing so that the pressure therein is not 21 substantially reduced. Pressure in the small chamber is used 22 to meter water to the plant.
23 The drip irrigation tubing should have a very good 24 burst strength or burst stress. This is measured as a hoop 2~ stress, which is the actual stress in the tubing wall, defined 26 as the pressure times the quantity of the outside diameter of 27 the tubing minus the wall thickness which is then divided by two 28 times the wall thickness. Drip irrigation tubing formulations 29 which exhibit longer failure times at certain hoop stresses are generally recognized as better materials for this particular I ~ 2 l application at a particular temperature. Since drip irrigation 2 tublng is exposed to elevated temperatures only during the 3 extremely hot portion of a day (which is a fraction of the day or 4 a fraction of the number of hours that it is in ~ervice), improved burst properties of tubing at 80C for even a few hours translates 6 into a few days' more use. For drip irrigation tubing of this 7 invention, burst strength properties have been obtained when the 8 tubing did not fail after about 500 hours at 400 psi hoop stress 9 at 80C. Tubing that has failure times at 400 psi hoop stress at 80C of more than 500 hours is generally regarded as superior ll tubing. The drip irrigation ~ubing of this invention not only 12 achieves the aforementioned minimum requirements of burst strength 13 properties, but in fact far exceeds those minimum requirements.
14 Cross-Linking of the Extruded Tubin~
Cross-linkin~ of the extruded tubing can be effected 16 by a wide variety of methods and includes, but is not limited 17 to, ionizing and nonionizing radiation, rind chemical cross-18 linking through covalent, ionic and other types of bonds.
l9 One method of chemical cross-linking involves contacting the extruded tubing with a cross-linking agent 21 ¦1 in the presence of a free radical catalyst. Illustrative 22 !I cross-linking agents disclosed include compounds such as 1,4-23 butylene glycol diacrylate, tetraethylene glycol diacryla~e, 2~ polyethylene glycol diacrylate, me~hylene bisacrylamide and the like. Typical free radical catalysts disclosed are azobisiso-26 butyronitrile, benzoyl peroxide, 2,4-dichloroben~oyl peroxide, and 27 the like. ln addition to chemlcal cross-linking by means of a 28 free-radical catalyst, other methods can be employed. These 29 ¦ ~ethods are well known to those skilled in the art and include 3~ ' the use of peroxide catalysts.

~ -14-11~5~9Z 12637 l In addition to the aforementioned methods of effecting 2 cross-linking of the tubing, another method is to subject the 3 polymer tubing to sufficien~ ionizing radiation to cross-link 4 it. As used herein, the term "ioni~ing radiation" includes that radiation which has sufficient energy to cause electronic excita-6 tion ant/or ionization in the polymer molecules but which does 7 not have sufficient energy to affect the nuclei of the constituent 8 atoms. Convenient sources of suitable ionizing radiation are 9 gamma-ray producing radioactive isotopes such as Co60 and csl37 spent nuclear uel elements, X-rays such as those produced by ll conventional X-ray machines, and electrons produced by such means 12 as Van de Graaff accelerators, linear electron accelerators, 13 resonant transformers, and the like. Suitable ionizing radiation 14 for use in the present invention will generally have an energy level in the range of from about 0.05 ~1eV to about 20 MeV.
16 The irradiation.of.the non-cross-linked polymers can be 17 carried out in the air, in a vacuum, or under various gaseous l8 atmospheres. Any conventional method can be used to bring the 19 polymer into contact with the ionizing radiation. Suitable methods are well known and understood by those skilled in the art.
21 The following examples are illustrative of the present 22 invention and are not intended as`a limitation of the scope 23 thereof.
24 Example l Preparation of Polymer Resin 26 A low pressure, low density ethylene-butene-l copolymer 27 was prepared according to the procedure disclosed in South 28 African Patent Publication No. 79-01365, published September 22, 29 1980, entitled "Process for Makin~ :Film from Low Density Ethylene Hydrocarbon Copolymer" by W.A. Fraser et al. The properties of i¦ !

Si75aZ -1 the ethylene-butene-l copolymer were determined by the following 2 methods:
3 Density was determined according to ASTM D-1505. A
4 slo~ cooled (at 15C/min) plaque was prepared by using ASTM
D-192~, Condition C. Density is reported as gmsl cm3 .
6 Melt Index (MI) was determined according to ASTM
7 D-1238, Condition E. It was measured at 190C and 303kPa and 8 reported as grams/10 minutes.
9 Flow Index (HLMI) was measured according to ASTM
D-1238, Condition F. It was measured at 3030kPa and reported as 11 grams per 10 minutes.
12 Melt Flow Ratio (MFR) was calculated as Flow Index/
13 Melt Index.
14 Secant Modulus was determined according to ASTM D-882.
Stress crack resistance was measured according to the 16 following procedure: Tubîng samples were aged four weeks at 1? 70C and then inserted on barb fittings at 15% strain and put in 18 a 10% by volume nonylphenoxy poly(ethyleneoxy)ethanol aqueous 19 solution at 50C.
Yield strength percent elongation and ~ensile strength 21 I at break was measured on compression molded plaques made in 22 accordance with ASTM D-1928, Condition C. The properties were 23 tested according to ASTM D-638.
24 Long term burst properties were tested according to ASTM D-1598.
26 Short term burst properties (instant burst) were 2t ~easured according to ASTM D-1599.
28 Test for % crosslinking after irradiation consists of 29 immersing the cross-linked polymer in boiling decalin for six hou~s and then measuring the amount extracted. The amoun~ left is ~ 5~2 1 deemed to be the cross-linked fraction.
2 1 The ethylene-butene-l copolymer had the followin 3 properties: a melt index of 0.55, a MFR of 65 and a compound 4 density of 0.920.
S An ethylene vinyl ace~ate copolymer was used. The 6 .copolymer had a vinyl acetate content of 28% and a melt index 7 of 375. Also used was carbon black and an anti-oxidant, 8 polymerized 1,2-dihydro-2,2,4-trimethylquinoline.
9 Pre~aration of Extrudable Compositions The following compositions were compounded utilizing a 11 Banbury mixer which dropped as a molten mass into an extrusion 12 hopper for feedi.ng in~o an extruder as previously described.
13 Three extrudable compositions were prepared, and physical 14 properties of such compositions are set forth in Table I below:
TABI.E I
...
16 Composition 18 LPLDPE (wt%) 87.4 82.4 ~92.4 19 EVA (wt%) 5.0 10.0 Carbon Black Masterbatch* (wt%) 7.5 7.5 7.5 21 ¦ Polymerized 1,2-dihydro-2,2,4-22 ¦ trimethyl~uinoline 0.1 0.1 0.1 23 Melt Index (decigrams per 10 24 ¦ minutes) 0.52 0.62 0.4 25 1 _ 26 1 ;':Masterbatch consisted of the following compositi~n: 65% tubular l reactor HPLDPE having a 0.2 melt index and a density of 0.921, 28 and 35% of carbon blac~ having a maximum particle size of 45 ~m.

1 TABLE_I (con~'d) 2 I Com~osition 3 , A B C
4 ¦1 ~ensity (gm/cm3) 0.9388 0.9378 0.938~ ;
5 '~ ~ensilè Strength (psi x 10 3) 2.96 2 73 2.76 6 I Yield Strength (psi x 10 3) 1.59 1.42 1.69 7 Elongation at Break (%) 858 808 726 8 Secent Modulus (psi x 10 4) 3.28 2.9 4.35 g The extrudable compositions A-C were extruded uslng a @r~file extrusion process described herein as follows:
11 The pellet product made was fed into the hopper of a 12 (NRM) 2 1/2" 16 to 1 L/D ex~ruder equipped with a l~addock mixing 13 screw, barrel cool;ng and infinite ~emperature control settings 14 from 125C to > 300C. This extruder had three barrel zones whose lS temperatures were 193, 199 and 204~C respectively, one zone for thl , ~6' extruder head at 204C and one zone for the die at 204C. The ex-17 truder screw was rotated at 45 RPM resulting in output rates of 18 about 32 kg/hr. The molten polymer was then directly fed to a 19 straight through spider tape die in order to form the annular ex-trudate. The inside diameter of the die was about 31 mm while the 21 pin outside diameter was about 25 mm in diameter. The resulting 22 extrudate was then drawn down to about 17 mm outside diameter and 23 about 1.5 mm in wall thickness using a vacuum sizing method. The 24 vacuum sizing tank was equipped with a brass sizing sleeve about 5 tubing diameters long. It was slotted in order to allow the nega-26 tive pressure caused by the vacuum to pull the extrudate against 27 the sleeve. The cold water freezes the extrudate to this diameter 28 The vacuum sizing tank used between 2 and 6" of Hg as the vacuum.
29 The tank was 6' long. A multi-contact puller was used to pull the extrudate at a constant rate and thus yielding a constant wall d 1l95792 12637 1 ¦¦ thickness. The tubing was tested after at least two days delay.
2 ~ The three compositions shown in Table I were extruded 3 11 as described above into tubing, pressure tested both before and 4 11 after irradiation at 10 M.rads. and tested for stress crack 5 1I resistance and short and long term burst strength. The results 6 ¦¦ are summarized in Ta~le II below:
8 ~! T_BLE II
9 ¦¦ ~ Fro ComDosition

10 ¦~ Instant Burse Before Irradiation 1575 1550 1795 12 D InsCant 8urst Aiter Irradiati~n 1800 1715 1830 14 ¦¦ Long-Term Pressure Test at 80C
15 ¦¦ (hrs) 16 ~at 400 psi before Irr. 216 124 125 17 ¦¦at 400 psi after Irr. ~3612 ~ 128 2826 1~ ¦at 320 psi before Irr. > 3208 ~1840 4760 L9 ¦at 320 psi after Irr. > 8008 > 5483 6696 '0 ¦ Crosslinking After Irradiation57 56 48

11 . I
2 Tubing ESCR (hours) (no 1000 lOoo 1000 I observable failures) at 3 1 15% strain in 10% by volume nonylphenoxy poly(ethyleneoxy) ethanol aqueoug solution at ~ r li 7~2 1 TABI.E III
2 Equipment: NRM 2 1/2" 16 to 1 L/D Extruder Tube 3 Formin~ Composition Zone 1 ~C) 193 193 193 6 2 (C) lg9 199 199 7 3 ~C) ~04 ~04 204 8 Hea~ (C) 204 204 204 9 Die (C) 204 204 204 Stock temperature (C~ 200 195 200 11 Amperes 11.0 10.6 10.6

12 RPM 45 45 45

13 Head Pressure (psi) 1000 975 1100 Ra~ kgs/hr) 32.6 32.3 32.6

Claims

WHAT IS CLAIMED IS:

1. An extrudable composition suitable for forming tubing via profile extrusion, the composition comprising:
(a) more than about 80% and less than about 97% by weight of a low pressure, low density hydrocarbon interpolymer;
(b) more than about 2% and less than about 10% by weight of a copolymer of ethylene with a vinyl ester of C1-C30 monocarboxylic acid and other alpha olefins in minor concentra-tions providing a copolymer having a density between 0.91 and 0,94 gm/cm3;
(c) more than about 0.01% and less than about 3% by weight of an ultraviolet stabilizer; and (d) less than about 0.50% by weight of an anti-oxidant stabilizer which permits cross-linking by irradiation without destabilizing.

2. A composition as defined in claim 1 wherein the low pressure, low density hydrocarbon interpolymer comprises ethylene-propylene copolymer, ethylene-butene-1 copolymer, ethylene-hexene-1 copolymer or ethylene-octene-1 copolymer or combinations thereof.

3. A composition as defined in claim 1 wherein the ethylene vinyl ester copolymer comprises ethylene vinyl acetate having a vinyl acetate content between about 23% and 35% by weight.

4. A composition as defined in claim 1 wherein the ultraviolet stabilizer comprises carbon black having a particle size less than 100 µm.

5. In a process for producing thermoset tubing having a thickness from about 0.1 mm to about 3.81 mm and an outside diameter from about 5 mm to about 51 mm wherein an extrudable composition is extruded into a tubing profile in a single screw extruder having barrel temperatures between about 150°C and 240°C, the extruded tubing being subjected to electron beam irradiation so as to receive a total dose from about 0.05 to about 40 Mrads, the improvement which comprises using as the ex-trudable composition (1) comprising:
(a) more than about 80% and less than about 97% by weight of a low pressure, low density hydrocarbon interpolymer;
(b) more than about 2% and less than about 10% by weight of a copolymer of ethylene with a vinyl ester of C1-C30 monocarboxylic acid and other alpha olefins in minor concentra-tions providing a copolymer having a density between 0.91 and 0.94 gm/cm3;.
(c) more than about 0.01% and less than about 3% by weight of an ultraviolet stabilizer; and (d) less than about 0.5% by weight of an anti-oxidant stabilizer which permits cross-linking by irradiation without destabilizing;
and operating at an extruder head pressure 7% to 11% below the extruder heading pressure necessary to extrude a Composition consisting essentially of low pressure, low density polyethylene having a melt index of about 0.55, a compound density of about 0.93 and a melt flow ratio of about 65.

6. A cross-linked and thermoset drip irrigation tubing made according to the process of claim 5, the tubing having a burst strength at 80°C greater than about 8612 hours at 400 psi hoop stress and greater than 8008 hours at 320 psi hoop stress.