CA1303316C - System for forming hollow, porous pipe - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE A system for forming a hollow, porous pipe having a pipe wall containing uniform, spaced pores. The system has means for preheating a mixture of rubber particles and binder resin to a high temperature, and a pipe extruder having a barrel and die for forming the mixture into hollow, porous pipe. The formed pipe is used for irrigation and provides constant water-ing throughout its length.

Description

~3~33~i SYSTEM FOR FORMING HOLLOW POROUS PIPE

This invention relates to a system for the production of porous irrigation pipes having constant porosity along long lengths.

Back~round Art ~ s population steadily increase~, water becomes ' a more important and increa~ingly scarcer and more ex-pensive resource. ~griculture is one of the most import-ant uses of surface water. It is necessary to ~evelopmore efficient systems for delivering water to plants.
Surace watering tends to be wasteful since water that is not ahsorbed quickly enough runs off or evaporates, and the water that is absorbed must wet the soil until it reaches the roots, the water gathering system for most plants.
Surface irrigation systems must be removed and replaced each time the field is tilled or plowed for re-planting. Irrigation systems inter~ere witll mecl~anical harvesting and require substantial maintenance. ~bove ground waterlng should usually be conducted during the day since many plant~ are ~ubject to decay at night.
Furthermore, above-ground watering interferes with usage of recreat~onal areas 3uch as parks, athletic ~ields and golf courses. Suxface watering is non-specific in tllat the crop and weeds are both equally watered.
~ ecause o~ the limitations in a~ove-ground irri-gation, subsuxface irrigation system~ have been developed in which water is directly fed at an optimum subsurface depth to the roo~s of tlle crop being cultivated~ The pipe ~' ::

:~3~3;3~.~

must be inert to the soil environment, must be capable of withstanding hydrostatic pressure in the presence of hard objects such as rocks without collapse and prefer-ably is flexible, so that it does no~ ~uffer brittle failure an~ can be ben~ to ~ollow crop-line contou~s.
There are numerous agricultural applications for an irrigation pipe which leaks water slowly over its en-tire surface and length. Such pipes can be buried under-ground at levels appropriate for the partlcular crop being grown, and will supply water directly to the root system.
With proper controls, the water level in the soil can be maintained at near-optimum levels. With some crops, t~liS
has been shown to increase yields substantially.
~ porous irrigation pipe llas been produced from reclaimed tire rubber mixed witll a binder such as poly-ethylene. This mix~ure is extruded to form the pipe, and the water present within the hot extrudate vaporizes, pro-ducing the small pores through which water seeps under pressures of a few psi. While t}liS plpe is useful for some applications, it has several drawbacks for many large-scale agricultural uses. The most important problem witll the present product is its highly variable porosity. Some sections had no pores an~ other sections very large pores.
The rate at which water emerges from this product varies by 50 to 75 percent or more within a few feet along its length. If it were used with closely spaced plantings, such as densely paclced sugar cane plants, some areas would be overwatered, whlle others would be essentially dry.
Another problem is that the overall porosity of the pipe is poorly controlled from lot to lot. This causes severe engineering problems when one tries to de-sign a water system for a particular location. What is normally done is to use many pre3sure regulators through-out the system. This is expensive and furtl~er limits tl)e I)otential applications of the porous pipe material.
It has been discovered that the wide variation in ~3~J;33~1~

porosity is due to failure to control the moisture con-tent of the raw material~. The dry powder is somewhat hygroscopic and prior production systems disclo~ed by Turner in U.SO Patent No. 4,003,408; No. 4,110,420 and No. 4,168,799 relied on absorption of water by tile crumb material to provide the blowing or pore ~orming agent.
~lowever, the water content of each batch or portion of a batch varie~ with humidity, temperature, etc. of t~e en-vironment. Since the amount of water present in the ex-trudate is very important to the porosity o the final pipe, variations in the water con~ent of the feed will produce unacceptable variations of the product. Turner attempted to control exces~ water by venting the extruder but this did not e~fectively control variations in poro~ity.
The raw material is a mixture of fine powder~ and small amounts o~ oils. Such mater~als do not feed well in single-screw extruders. Moreover, uneven feedillg of the powders will produce variations in the density and thickness in the wall of the pipe. Since these factor~
are important to the poro~ity o the wall, uneven eeding in the extruder will result in further inconsistent leak rate~.

Statement o~ the Invention Porous irrigation pipe having const~nt poro~ity along long lengths of pipe is disclosed in Canadian appln.
477,677, filed March 27, 1985, and assigned to applicant.
The improvea, porous irrigation pipe is made possible by accurate control of the watex content of the raw material and by providing the raw material in a form in which it ~eeds consistesltly and reliably to the ex-truder. Iligh ~ur~ace area crumb rubber and powder mix-tures have water contents varying from about 0. 2 percent by weight up to several percen~ water by weiyht and vary throughout the batch. The moi~ture content o ~he mater ial is con~rolled to not vary by more than *lO percent ~3g~

throughout the batch and to be at a value between 0.5 to 3 ~ercent by weight, preferably from about 0.75 percent by weight to 1.5 per~ent by wei~h~ of water.
The improved pipe is preEerably produced by pre-processing the raw material into a shaped pellet form.
This material feeds very consistently and reliably into a variety of types of ~ingle screw extruders. Thi~ makes it possible to produce the porous pipe in virtually any location where standard extrusion equipment ls available.
This reduces cost of shipping, production and installation of the porous pipe. Additionally, the moisure content of the pellets can be adjusted to predetermine~d, specific values depending on the desired porosity and leak rate. , The pellets are stored under water-excluding conditions such as in vapor barrier containers. ~he pellets are much less hygroscopic than the high surface area powder mater-ials or the prior art.
Another differenae in the production method~ is that the use of a non-vented extruder becomes poss~ble since water content ls known and there is no need to vent excess vapor pressure. Since the parameters of water con-tent and feed rate are controlled and the temperature is controllable, porosity can be controlled ~y preselection of water content of the pellets. ~lternately, ~ince all variables are controlled, porosity of a batch or run can be controlled by changing the temperatures in the extruder and die.
~ he pellet form of feed containing control~ed moisture content and pelletizing additive~ makes possible continuous production in high volume of porous irxigation pipe with very consistent leak rates on a variety of ex-truders anywhere in the world where the pipe is needed.
The porous pipe can be optimized or porosity, size ana strength for the intended application. In a~dition to the savings in shipping and production, and reduction in 35 the number of pressure regulators required, porous pipe 3 ~ ~ 3 produces irrigation sy~tems which in many cases will yield substantial increases in crop yields due to more accurate and unifonn watering cycle~, with particular ap-plications to undergroun~ drip or continuou~ irrigation of densely packed crops such as sugar cane.
Thougll the porous pipe produced by use of pellets having controlled moisture resulted in a significant in-crease in uniformity as determined by the consistency actor, Cv, the smoothness, strength and flexibility were not completely satisfactory and the flow rate wa~ still not as consistent as desired. Poor strength and flexi-bility result in ~racking or breaking of the pipe. Smooth-ness of the internal surface of the pipe causes lo~s of pressure through ~riction between the wall and the moving column of water and this limit~ the length of a run of pipe.
Pipe produced according to the process disclosed in the Turner patents is limited to run~ of about 500 feet due to poor linearity and high frictional loss.
Another pipe produced according to the Turner patents is much smoother but has such poor linearity that it is also limited to very short runs.
Porous pipe having excellent linearlt~ and much higher strength and smoothness has been produced accord-ing to the invention. Pipe runs can be increased by several factors up to runs of 2500 feet or more. Further-more, the pore structure is more regular and uniform and the pore~ are more evenly spaced. The inside wall i8 mUCII
smoother and has fewer pro~rusions than prior porous pipes.
The rubber particle~ are found to be more uniEormly coated with the binder resin and to be only connected b~ point contact.
These further improvements ~n properties of the porous pipe are believed due to further control of the parameters of the materials used and of the parameters practiced during the pelletiz~ng and extrusion ~teps.
The materials have been improved by the use of a fxesh 13~33~

source o vulcanizsd crumb a~ contrasted with a reclaimed source. The vulcanized rubber utilized is a carbon~filled, vulcanized scrap recovered from flashing or other overruns during the manufacture of tires or other rubber products.
The scrap is carefully sized to a uniform particle size between -30 and 100 me~h containing no more than S~ ~ lO~i n~sl~
fines. It is much cleaner, firmer and stronger tllan ru~-ber crumb reclaimed from tires and tlle uni~orm particle size without fines provides uni~ormity in the shape, size and spacing of the pores. It also contributes to pro-viding a smoother inner wall without protruberances.
~ nother improved feature is theaddition of additives that increase the flow of the binder resin around the rubber particles. l'hls provides more even pores and more uniformly spaced pores. It further provides improvement in strengtll and flex~bility of the porous pipe. The more completely covered particles pro-vide a smoother inner wall with a substantially higher coefficient of friction.
It has further been dlscovered that in the present pipe that the pore formation is not oslly due to gen-eration of steam from the controlled amount of water present but that other volatiles are present 5uch a3 tlle decomposition products of sulfur containing compoullds/
mineral compounds that evolve carbon dioxide and other gasses present in the starting materials. The~e vola-tiles can result in variations in pore size. Tlle ~ncon-trolled volatiles are removed in the invention by heatillg the composition to a temperature high enough to boil off the volatiles be~ore forming the pellets, similarly by hea~ing the composition to the highest temperature ex-perienced during extrusion, usually 3S0F to 400~ 'he composition can be heated in the mlxing apparatus, such as a ~anbury mixer. The hot composition neea not be ex-truded to form pellets. The hot flowable compo~i~ion can be formed into a sheet in a two roll mill, cooled in air 13~33~i and diced ~o Eorm cube~, cylinder~, hexagons, etc.
These and many other features and attendant ad-vantages of the invention will become apparent as the invention becomes better under~tood by reference to the following detailed des~ription when considered in con-junction with the a~companying drawings.

Brief Description of the_Drawln~
Figure 1 iB a schematic view of a train of equip-ment 'or producing pellets in accordance with the invention:
Figure ~ i5 an enlarged view in elevation of a pellet;
Figure 3 is a partially broken-away view in elevation o a humidity-controlled storage container;
Figure 4 is a schematic vlew of a system for ex-truding porous pipe in accordance with the invention;
Figure 5 i~ a view in sect.ion taken alon~ line 5-5 of Figure 4;
Figure 6 i~ a schematic view of of an alternate apparatus for producing pellets5 Figure 7 is a photomicrograph of a prior art porous pipe at 20X magnification5 Figure 8 is a photoMicrograph o~ a porous pipe of the invention at 20X ma~nification;
Figure 9 i5 a photomicrograph oE a prior art porous pipe at 200X magnification; and Figure 10 is a photomicrograph of a porous pipe according to the invention at 200X magni~ication.

Detailed Descri tion of the Invention P
The pelle~izable mixture of the invention includes a major portion of elastomer in crumb forln, a minor amount, usually from 1.0 to 6.0 phr of a slip agent, pre~erably a mineral such a~ talc and 0.1 to 1.0 phr of a lubricant, such as a metal stearate. Coatin~ promoter~ ~uch as 0.001 to 0.1 phr of a citrate salt and an 0.1 to 1.0 phr of a -i3~?33~L~

carbon black may also be present.
The elastomer can be natural rubber wllich is cis-1,4-polyisoprene or ~ynthetic homopolymers of buta-diene or isoprene or their copolymers with minor amounts of 0.1 to 20 percent by weight of vlnyl monomers ~ucl~ as styrene, isobutylene or acrylonitrite. It is preferred that the elastomer be vulcanized. A ready an~ inexpensive source of prevulcanized crumb rubber is available a~ rub-ber reclaimed from automoblle tires aEter removal of tlle metal tire cords and metal reinforcement in the head.
The rubber ls ground into crumb particles no larger than those passing through a 10 mesh screen, preferably from 20 mesh to 60 mesh.
The binder resin is a thermoplastic material capable of softening at a telll~eratuxe below 3UUF so that yores will form during extrusion. ~r~.e resin must be sta~le to lon~term exl~osure to ~oil environ-ment and to Eertilizers, llerbicides or pestici~es see~ y into tl~e a~jacent soil ~r to Eertilizers,

2~ growtJ~ regulators llerbici~es or ~esticides dispellse~
by dissolvil~g in tlle irrigation water. Tl~e res~
must ~e inert to tlle other componellts of t~e pi~e sucl as ~ e crum~ rub~er un~er extrusion conditiolls.
PQ1YVinY1 acetate is exclude~ from use SillCe it will react with the crumb rubber. Styrene polymers ill-cludin~ im~act ~olystyresle co~olymers are useful as are linear polyalllides SUCII as various l~yloll5, ~oly-vinyl-chloride, polypllel)eylelle oxide arl~ ~oly~llelleyl~lle sulfide polylllers.
Tlle most preferred group vf polymers are tlle linear pol~ners of alkenes of 2 to 4 cax~on atoms sucl~
as polyetllylene~ polypropylene or polybutel-e. l'llese polymers are unreactive in soil and in t1~e extrusiol-~arrel and l~ave lony se~mellts oE lil~eari~y ~)rovi~
crystalIine ~el~avior. Polyetllylenes llave lower .
. . .

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33~
g melting temperatures, are tougher an~ l~old sl~a~e ~e~er. Iligl~ dellsi~y ~olyetl~ylelles have densi~ies from about ~.94 to abou~ V.97 ym/cc, al)d porous pipe preyared witll all l~igll density ~olyethylene ~in~er are somewllat stiff, bri~tle and ~if~icult to extrude.
Low density polyetl~ylenes l~ave ~el)sities ~rom about 0.90 to 0.93 gm/cc, and ~orous pipe prepared witl~
low density polyetllylene binder are very flexi~le and can readily be bent to follow a desired path al~d are readily extruded. lhese pipes are very useful for above-ground irrigation. I~owever, wall s~iffness may not ~e adequate for subsurface syster.ls. 'l`he ~>ipe develops kinks in the bends and does not hold its shape. The optimum ~inder wllicll ~rovides a porous pipe which holds its shape without brittleness yet has adequate flexi~ility is com~osed of 50 percent to B0 percent by weight of hîgh density polyethylene, prefer-ably 60 percent to 70 percent to 20 percent to 50 per-cent by weight of low densit~ polyethylene, preferably 30 percent to 4Q percent. The polyetl-ylelle can be used in any com~lercial form such as powder, flake vr pellets.
Reclaimed polyethylene materials can also be us~d. q~le form and color of such materials have little ef~ect upon the product.
The slip agent aids in extruding the rubber bin-der mixture. Finely divided minerals other than talc can be utilized such as clay~, silicas, carbonates, or micas. The metal 3tearate lubricant can be ~elected from calcium, magnesium or zinc stearates, though cal-cium appears to provide the best porous pipe.
Referring now to ~igures 1-3, tlle crumb rubber, additives and ~inder are thoroughly dry blel~ded in blender 10 such as a ribbon blender or other ~uitable mixing device to form material which is fed to tlle 3S hopper 12 of the extruder 14. The m~xture is pelle-tized by being extruded into a die-face pelletizing ;;13~333L~i systen~. An extruder feedlng direc~ly into an under-water or water-ring pelletizer 16 is illustrated.
turn-screw extruder is ~referred tlloucJI~ ~ siJlgle-screw extruder e~uipped with a good cran~er can be utilized.
Strand pelletizers do not work well with the rubber-binder composition of the invention. The extruder is maintainéd at a tempera~ure o~ ~rom 320F to 40~F
and the die at a temperature from 250F to 325F by means of separate heating systems such as a set of heating j~ckets 18, 20 receiving separate flows of heated exchange fluias. ~l~he extruded strand material should have a bulk density after drying o~ at least 0.25 gm/cc and l-as a diameter from 3 to 20 llUll, pre-ferably 4 to 1~ n~. l'l~e strand is broken into lengths o~ 3 to 20 nm by means of a mechanical knife 22 immersed in the water bath ~4. The ~ater in the bath is cool, usually from 20F to 80F and as the extruded strand 26 enters the water 2b, it congeals and sets so that a thin blast of air from nozzle 30 breaks the strand 26 into pellets 32 which fall into the collector portion 34 of the water bath 24. The nozzle 30 is connected to an air supply 36 w~ich is pulsed by a controller 38.
The dispersion of pellets in water is fed from water bath 24 into a separator sucll as a cyclo~le separator 40. The water is recycled to the batl) ~4 through line 42 while the pellets 32 are delivered by conVsyor belt 44 to a dehumidifying drier 46 to dry the pellets to a preselected moisture contel~t between 0~5 to 2.5~ by weight depending on tl~e porosity desired. The conveyor be~t 44 carries tlle dried pellets 32 into a closed hopper 48 havillg a humidity controlled atmospllere whicll feed~ tl~e pellets into a storage container such a~ a poly-ethylene baq 50O The bag is closed with a secure closure such as a band 52 of metal and can be place~

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L3~33~6 -11 `

in an outer protective container ~uch as a box or a barrel 54. The dried pellets con~a~n a uniform mo~ture content which can be accurately controlled and the mois-ture content i9 stable ~or ex~ended perlods. The pellets have a much smaller surface area than t~e prior powder materials and are humldity stable without storage for short periods of time.
Referring now to Figure 6 an alternate system for forming pellets compri~es a Banbury mixer 100 COII-taining a cavity 102 which receives the rubber-binder mixture. Tlle intensive mixing of the charge 104 by rotors 106 and resistance rods 107 rai3es the temperature to about 400F. The volatiles genexated are remove~ througl a vent 108. TI~Q cavity can be heated, i needed, by he~t-ing the walls 110 of the cavity by means of resi~tancerod heaters 107 powered by power supply 109 or a ~hell receiving a Elow of heat exchange fluid.
The degasses composition is flowed from the out-let 112 into the nip 114 of a two roll sheet extruder 116 to form a sheet 1187 ~fter the sheet ha~ cooled in air it is fed under dicer head 120 of the dicer 122 to form pel-lets 124 wllich are dried in drier 126 beEore being col-lected and sealed in bags 128.
Referring now to Figure 4, the pellets 32 are fed to hopper 60 of a pipe extruder 62. Tlle hopper ila~
a lid 61 to isolate ~he feed from the environment~ The extruder preferably contains a single low pressure screw 64 and has a length to diameter ratio of at lea~t 24/1, pre~erably at least 35/1. The compression ratlo of the feeding section to the metering section can be rom 1.5/1 to 2.2/1. The diameter of the barrel 66 i~ suitable to produce pipes having outside diameters from 2 to 10 in-ches, usually from 3 to 6 inches. Mixing pins are to be avoided since the crumb rubber can foul these elements.
The process i~ operated at a temperature high .~ ~ 3~ ~ 3 enou~h to mel~the binder resin but below the mel~ing temperature of the elastomer. Good temperature control of the barrel and expecially of the die 6B is required usually to within ~5~F. A mora uniform porous p.ipe is prepared by providing an increasing temperature profile over the length of extruder ~2. Separate heating jackets 70, 72, 74 can surround the feeding, transition and metering section~, re~pectively, oE tlle extruder barrel 66. Each jacket receives a separate flow of heat exchange fluid. The feeding section can be heated to 340F - 360F (Tl~, the transition ~ection from 360F - 370F (T2), and the metering section from 365F to 375F ~T3).
The die 80 is also provided with a separate temperature control. A suitable d~e is shown in Figure 5 o~ U.S. Patent No. 4,168,799. The die contains an outlet or~fice 72 ~n front of which is mounted a mandrel a4 for formlng the bore 86 of the porous pipe 88. The mandrel may be remvvable to vary the wall thickness of the pipe. The thickness is selected depending on desired flow rate, leak rate and wall strength to avoid collap~e. Wall thick-ness is usually from 0.1 to 2.0 inches. In U.S.
Patent 4,168,799, the die i~ chilled to a temperature of from 40F to 80F in order to avoid forming an impermeable skin on the surface of the pipe, and the barrel is vented to remove excess pressure.
}lowever, in accordance with the present inventior-, the barrel need not be vented and the die is heated to a preselected temperature from 240 F to 300F to control porosity of the porous pipe. An ar-nular jacket 90 receives a flow of preheated ~leat ex~
change fluid ~1`4).
~9 the screw 64 is rotated by motor 92, the feed moves forwardly and the b~nder re~in melts. T~e water vaporizes and the expanding bubbles of ~team .

.
.
, ~3V33~6 ~orm a network of pores 96 extending from t~le bore 86 to the surface 9B of tlle porous pipe. The pipe 88 can be extruded througll the die 6~ into the ambient arld enters a chilling bath 100 colltainillg S water at a temperature of about 25~ to 50F before being pulled onto rewind stand 102. The chiller bath usually has a lengtll of at least 40 feet.
The invention will now be illustxated by the following specific examples of practice.
Tlle dry matexials were mixed in a ribbon blender and ed into the hopper of a twin-screw ex~
truder heated to 360F - 390F with a 5 n~ die heated to 300F. The water bath was maintained at 35F -40~F and the S ~n strand was chopped into approx~mately ground pellets about 8 - 9 l~n in d~ameter by an air knife. ~fter drying the pellets had a density of 0.275 gm/ml.
Example l The following mixture was pelletized aod 20dried to 0.7S percent moisture content:
Crumbed ~ire ~ubber ~4~ Mesh) lO0 lb.
Low Density Polyet~ylene 35 lb.
Finely Powdered Talc 3 lb.
Zinc Stearate .25 lb.
~5 These pellets were extruded in an unvented single screw extruder into porous pipe with an ID of 0.55 inch and a wall thickne~s of 0.2 inch. Tlle extruder ~emperatures were:
Extruder (all zones) 350 F.
Gate 3~0 F.
Spider 3 3 5~F .
Dis 335F
This porous pipe had the follo~lng poro3ities ~t lO psi:
0.27~.02 GPM/lO0 Linear Feet 0.11~.003 GPM/lO0 Square Feet ~3V~3~i Example 2 This same pelletized raw material of Example 1 was extruded under the ~ame conditions, except tl~at the die temperature was 290F. This pipe had the ~ollowing porosities at 10 p8i.
O.l9+U.17 GPM/100 Linear Peet 0.076+0.007 GPM/100 Square ~eet Example 3 Example 1 was repeated except that 35 lb.
of high density polyet~lylene was substituted for the low density polyethylene binder. The pellets were more difficult to extrude and the pipe was more brittle and less flexible.
Example 4 15Crumbed Tire Rubber ~40 Mesh) 100 lb.
~ligh Density Po~yethylene25 lbo 10w Density Polyetllylene10 lb.
51ip Agent-Talc 3 lb~
Lubricant-Calciwn Stearate0.25 lb.
The formulation was proces~ed into pellets ~nd dried to contain 1.0 percent moi~ture. The pellets were extruded in a 2.5 inch diameter, ~4/1 L/D, Prodex ~ingle-screw extruder. The extruder was equipped Witll a PVC
~ype screw, which had a compression ratio of 1.9/1, and a circular pipe die wi~h a land-lengtll of 16/1. Temperatures in the exkruder were maintained at 340~F - 360F. Tile gate, spider and die tempera-tures were adjusted to yield an extrudate having the ~emperatures ~hown below. Porous pipes having a wall thlckness of 0.165 incll were produced having the ollowing propertie~:

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~3~33~;

~PP~OXI~*E LE~K ~TE
Extrudate GPM/100 linear feet Tem~erature 0.5" 1.0"
(F.) GPM/100 sq ft ID Pipe ID Pipe 250-260 0.10+0.02 0.25~0.04 0.50+D.~
275-285 0.20~0.03 Ø50+0. oe 1. 00+~.16 300-320. 0.40+0.06 1.00~0.1~ 2.00+~.32 340-360 0.80+0.12 2.00+0.3~ 4.00+0.64 Tlle pellet material fed smootl~ly and tlle porous pipe had good compression strength, yet was flexible. The pipe had uniform poros~ty along its length. The con~istency o~ leak rate i~ measured by determininy the amount of Plow of one foot in-crements over 50 feet of pipe to determine the consistency factor, Cv, -~the ~tandard deviation/
flow xate.
For most prior commereial porouq pipes, the Cv achievable is from 0. 25 to 0. 5. Por most appl~-cations, a Cv of 0.2 is preferre~ and for densely packed plants such as sugar cane, a Cv of U.l is necessary to re~uce no-growtll in overwet or dry areas of irrigation.
A one-half inch I.D. porous pipe produced in accordance with the invention havlng a flow rate of 1 gpm/100 linear feet at 10 psi pressure has a Cv of 0.1 to 0.15 and a porous pipe havin~ a flow rate of 0.25 gpm/100 linear feet has a measured Cv of 0.05 to 0.1.
Porous pipe ~aving improved properties was pro-duced by utilizing coating promoters a~ follows:
Example 5 Virgin scrap rubber was sieved into -30 mesll with no more than 5% fines ~100 mesh. 72.5 parts by weight of thisrubber wa mixed with 27.5 parts by wei.ght o~
a mixture of 30~ low dens~ty polyethylene having a melt index of 2 or less and 9o% of a high density ~)3~

polyethylene llaving a melt index oE 0.5 or less. 0.1 phr ~ carbon black, 0.1 phr of calclum stearate and 0.01 phr of ammonia citrate were added and the mixture Eed to Banbury mixer and mixed for several minutes at 400F witl venting to remove volatiles.
The hot mixture was pas6ed into the nip of a two roll mill to form a sheet and air cooled be~ore dicing into cubes. The cubes were dried in an oven to a moisture content of 0.75 weight percent.
The pellets were extruded in an unvented sillgle screw extruder at 340F to 360F under the conditions of Example 4 to form a porous pipe with ~ 3/32 inch wall.
The linearity ~Cv) at 10 p5i was determined for pipes with leak rates as follows:
Table 1 Cv Flow ratel q~h/100 ft.
0.1 to 0~12 20 0.125 to ~.175 40-50 0.15 to 0.20 50-60 0.175 to ~.225 80~100 ~ t higller leak rates, the pore sizes are larger and there is also a wider variation in pore size. The internal wall and pores of the porouq pipe oE Example 5 and o two pipes produced according to the Turner patellts were examined. Pllotomicrographs of these pipes are shown in Figures 7-10. The improved smoothness of the pipe of the invention is apparent.
The degree of coating of the rubber particles is also apparent since Eree uncoated rubber particles are apparent in the 200X photo of Figure 10. The unevenness of the prior art pipes causes more friction. The pores are more even since the binder resin evenly flows over the surface of the particles and the particles adllere only at points of contact where the binder resin br~dges and connects the particles.

~3V33~i Physical propertie~ of the pipe of the inven-tion with a 3~32 incll wall were compared to two commercial porou~ pipes having 3/16 inch walls prepared accordin~
to tlle Turner patents.
Exam~le 5 Turner I Turner II
TenRile strength tp~i) 1175 795 650 ~longation (%) 90 100 llO
Linearity - Cv (typ.) .1 .3 .25 Fluid Friction Coef ~est) llO 100 80 Coating of Rubber (~) g5-9890-95 75-80 The porous pipe of the ~nventlon i3 ~tronger, smootherand itis ea~ier to control properties during man-ufacture than the Turner process. The ten~ile strengths and elongations were measured at the break point of standard test samples using the ASTM D638 method so the values presented are real and directly comparable. They clearly show that the pipe of the invention with a 3/32 inch wall is stronqer than the Turner pipe with a 3/16 inch wall. The linearity values are averages of pub-lished test result~ by the manufacturers, universlty of Florida and U.C. Davis and tests conducted by the ln-ventor.
The ~luid friction coefficient is used ~o calcu-late the Ios5 of pressure in a pipe due to fri~tion be-tween the water and the wall. The higher the number, the less frictional loss i8 encountered. ~hu~, higller numbers are better. In porous pipes, the rictional ` loss is combined with the linearlty function to determine how long a run vf plpe can be u~ed in the field.
The Turner II pipe is limited to runs of about 500 feet due to it~ poor linearlty and high frictional loss. The Turner I pipe ~s much smoother than the Turner II pipe, but it~ poorer linear~ty also limits it to very short runs. Calculations indicate with the good linear-~36~33~6 ity and low frictional lo~s should make it pos0ible to run the porous pipe of Example 5 at runs of at least 2500 Fee~.
The data values were carried out according to "Agricultural Engineering Extension Report 84-2", by A.G. Smajstria and D.S. Ilarrison, University of Florida, Gainsville, FL 32611, January, 1984.
The particle coatinq value~ were determined from the SEM pictures. They are based upon counting -the coated and uncoated rubber particles on the inside surface of the tube samples. The SEM pictures clearly show some other things, though. The pore ~tructure of the porous pipe of the invention is much more regular and evenly spaced than any of the other samples. The pipe oE the invention is also much smoother, and has very ew large protrusions. These diEferences are believed to be due to the mixing/pelletizing step, the addition of additives and use of evenly sided rubber particles which control the pore size, and the control of the moisture content in the raw material.
It i~ to be realized that only preferred embodi-ments of the invention have been described and that num-erous substitutions, modification~ and alternations are permissible without departing from the spirit and scope of the invention as defined in the fol~owing clalms:

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Claims (10)

THE EMBODIMENT OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A system for forming a hollow, porous pipe having a pipe wall containing uniform, spaced pores comprising in combination:
means for preheating a mixture of rubber particles and binder resin; and a pipe extruder in cooperative relationship with said preheating means and having a barrel and die for forming the mixture into hollow, porous pipe with substantially uniform spaced pores;
said preheating means comprising means for preheating said mixture of rubber particles and binder resin to a high temperature of at least the temperature of the pipe extruder to remove volatiles from the mixture, and including a mixer having a closed mixing chamber for receiving the mixture, independent heating means applied to said chamber for heating the mixture to said temperature and vent means connected to said chamber to remove the volatiles from said chamber.

2. A system according to claim 1, in which the pipe extruder contains means for heating the barrel and independent means for heating the die.

3. A system according to claim 2, in which the barrel and die of the extruder contains a plurality of zones and separate and independent means are provided for heating each zone.

4. A system according to claim 3, in which the separate and independent heating means comprise jackets surrounding the barrel and the die for receiving flows of heat exchange fluid.

5. A system according to claim 1 in which the rubber particles are vulcanized, carbon filled, rubber in which the size does not vary by more than ? 10%.

6. A system according to claim 5 in which the binder resin is a polyolefin.

7. A system according to claim 6 in which the polyolefin is polyethylene.

8. A system according to claim 7 in which the polyethylene contains less than 5 phr of carbon black.

9. A system according to claim 8 in which the polyethylene also contains less than 0.5 phr of a citrate salt.

10. A system according to claim 1, further including means for preparing the mixture of rubber particles and binder resin comprising blender means for mixing binder resin and rubber particles and for heating the mixture and forming the mixture into sheet or strand form;
means for cooling the sheet or strand material;
and means for severing the sheet or strand material into particle form.