CA1167218A - Process for the continuous extrusion forming of a plastic double-walled foam-core conduit - Google Patents

Abstract

08-12-0398 A PROCESS FOR THE CONTINUOUS EXTRUSION FORMING OF A PLASTIC DOUBLE-WALLED FOAM-CORE CONDUIT ABSTRACT OF THE INVENTION The invention relates to a process for the continuous extrusion forming of a thermoplastic double-walled, foam-core conduit by the sequen-tial steps of extrusion forming an inner plastic tube, evenly coating said inner tube with foam-core layer and extrusion forming and bonding an outer tube in contact with said foam-core layer, said outer tube being generally evenly spaced apart from said inner tube by said foam-core layer providing a plastic double-walled, foam-core conduit.

Description

~ 08-12-0398 OF A PLASTIC DOUBLE-WALLED FOAM CORE CONDUIT
__ BACKGROUND OF THE INVENTION
Plastic double-walled~ foam-core conduit having solid plastic inner and outer tubes and foamed core have been disclosed in published articles, e.g.~ "Modern Plastics"~
November, 1978 issue, pages 78-80.
Such pipes or conduits have been coextruded using a primary extruder for the foam-core layer or tube and a satellite ex~ruder for the outer and inner skin tubes.
The coextrusion die uses a feed block design with dual gate valves to controI flow to the inner and outer skin tubes which are formed slmultaneously around a foam-core layer or tube from the primary extruder in a common d~e.
Foam-core conduits have the advantage of being less dense without sacrificing properties, hence, savings in ; ~ ~ raw material and handling costs are realized.
;~ ~ As the art progessed the coextrusion process was ound to have certain limitations, in particular, the den-sity of the foam-core has been kept in the range of about 0.50 to 0.90 grams per cubic centimeter so that it would not collapse during its coextrusion with the inner and outer skins. The solid skin material is generally extruded at about 5 to 30Fo higher temperature than the foam-core 25 ~layer, hence, when the three streams passed through the ; common profile die, the foam layer can be collapsed by the hotter skins or ~low differentials.
h~re has de~eloped a need for even lower density foam :::: :

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core pipe to -further reduce raw material and energy costs consistent with optimized physical properties.
It is an objective of the present invention -to provide a continuous process for the extrusion forming of double-walled, foam-core conduits wherein the -Eoam-core layer has a density of 0.015 to 0.4 grams per cubic centimeter.
U. S. Patent 3,379,221 relates to double-wall plastic conduits having a porous cementious fill material as a core layer incorporated after said double wall profile has been formed. U. S. Patent 3,845,184 relates to a process for ex-trusion forming higher density foamed plastic ex-truda-tes having an annular profile and thin skins such that the tubu-lar extrudate is self-supporting. The known prior art then relates to double-walled plastic conduits having high den-sity foamed cores prepared by coextrusion.
SUMMARY OF TME PRESENT INVENTION
The present invention relates tG a process for the ex-trusion forming of a thermoplastic, double-walled, foam-core conduit having generally concen-tric inner and outer plastic tube walls generally evenly spaced apart and bonded by a plastic foam-core layer, the process comprising the con-tinuous and sequential steps of:
A. extrusion forming the inner tube in a first extrusion means, B. conveying the inner tube through a second extrusion means and extrusion coating an outer surface of the inner tube with an even coating of the foam-core layer having a den-ity of about 0.015 to 0.4 g./c.c., C. conveying the inner tube, coated with the foam-core layer, through a third extrusion means and extrusion forming the outer tube in contact with and bonded to the foam-core layer providing a plastic double-walled, foam-core conduit.

~i ~-BRIEF DESCRIPTION OF THE DRAWINGS
Figure l is a cross-sectional view of a plastic, double-walled, foam~core conduit having a generally annular shaped profile.
Figure 2 is a cross-sectional view of a plastic 7 double-walled~ foam-core conduit having a generally rect-angular shaped profile.
Figure 3 is a cross-sectional view of a plastic, double-walled, foam-core conduit having an irregular shaped profile.
Figure 4 is a schematic view of an extrusion line suitable for practicing the present invention.
Figure 5 is a cross-sectional view of a die assembly suîtable for forming an inner plastic tube as part of a first extrusion means.
Figure 6 is a cross-sectional view of a die assembly suitable for coating said inner tube with said foam-core layer in a second extrusion means.
Figure 7 is a cross-sectional view of a die assembly suitable for extrusion forming said outer tube in contact with and bonded to said foam core layer providing a double-walled, foam-core conduit.
PREFERRED EMBODIMENTS
This invention is broadly applicable for forming plastic, double-walled, foam-core conduits. The profile can take the shape of a conduit having a central bore which can be annular, round, square, retangular or of irregular shape.
Referring to the drawings, Figure l, a cross-sectional view of a plastic~ double-walled, foam-core conduit lO, having inner tube ll, outer tube 12, foam-core layer 13 and central bore 14, said conduit 10, having a generally annular shaped profile in cross section.
Figure 2, shows a cross-sectional view of a conduit 20 having inner tube 21, foam-core 22, and outer-tube 23, said conduit having a cross-section being generally rect-angular in profile.

- 4 - ~J8-12-0398 Figure 3, shows a cross-sectional view of a conduit 30, having inner tube 31, foam-core layer 32, and outer tube 33, central bore 34, said conduit having an irregular shaped profile in cross-section.
Figure 4, shows a schematic view of an extrusion line 40, for practicing the present invention having a first extrusion means 41, second extrusion means 42, and third extrusion means 43, said first extrusion means 41 having first extruder 4~ and first die 45, said second ex-trusion means 42 having second extruder 46 and second die 47, said third ex~rusion means 43, ha~ing ~hird ex~ruder 48 and third die 49, said first sizing means being shown as 401, said second sizing means being shown as 402, and said third sizing means being shown as 403.
Said inner tube 404 is extr~ded by said first extru-sion means 41, conveyed through said second extrusion means 42 and coated with said foam-core layer, said inner tube coated with said foam-core layer 40S, is conveyed through said third extrusion means 43, forming said outer tube in contact and bonded to said foam-core layer forming said double-walled, foam-core conduit 406 having an axial bore 407, said conduit 406, being pulled by pipe or con-~ duit puller 408.
:: Figure 5, shows a cross-sectional view of said first ; : 25 die 50, having flange 51 for attaching to said first ex-truder 44~ having an open channel S2, having the shape of said inner tube, said open channel formed by a first bush-ing 53, and centrally disposed filler piece 54, having optional filler piece extension 55 for carrying inner tube on exiting from die to said sizing means.
Figure 6, shows a cross-sectional view of die 60, adaptable for operating as said second die, having a flange 61, for attaching to said second extruder 46, having ~: an open channel 62, formed by a centrally disposed first : 35 fixed mandrel 63, forming one face of said open channel in cooperation with a second bus~ing 64, said first fixed ~ `..... mandrel ha~ing centrally disposed opening 65 having the ; general shape of said inner tube 66, for carrying said . - .

tube through said die 63, said first fixed mandrel 63, ex-tending partially through said die such that said inner tube becomes a first moving mandrel 66, on exiting said first mandrel 63 such that moving mandrel 66 becomes coated 5 with said foam-core layer on passing through die 60 feed with a melt or a foamable melt of said plastic fed through said open channel 62 of die 60.
Figure 7, shows a cross-sectional view of a die 70, adaptable for operating as a third die, having a flange 71, for attaching to said third extruder 48, having an open channel 72, formed by a centrally disposed second fixed mandrel 73, forming one ~ace of said open channel in coop-eration with a third bushing 74, said second fixed m~ndrel having a ~entrally disposed opening 75~ having the general shape o~ an inner tube 76 coated with a foam-core layer 77, said open channel 72 carrying a plastic melt 78 formed into outer tube 79 by said die 70, said outer tube being drawn down in contact with and bonded to said foam-core layer 77 by said inner -tube coated with said foam core layer as a second moving mandrel.
The plastic used in forming said double-walled, foam core~ conduit can be a thermoplastic formed from monomers selected from the group consisting of alkenyl aromatic, al-kenyl nitrile, maleic anhydride, butadiene, ethylene, pro-pylene, vinyl choride, acrylic, acrylate and mixtures - thereof.
The preferred plastic to be used in said conduit is polystyreneg styrene-acrylonitrile copolymer, styrene-maleic anhydride copolymer, butadiene rubber reinforced plastic thereof and mixtures thereof.
~ The rubber reinforced polymers of polystyrene are known as high impact polystyrene or HIPS polyblends. The rubber reinforced polymers of styrene-acrylonitrile poly-mers are known as ABS polyblends. The ABS polyblends can be prepared by the processes disclosed in U.S. Patents 3,509,237 and 3,509,238. The high impact polystyrene poly-blends (HIPS) can be prepared by the process disclosed in U.S.P.3,903,202.

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THE ABS POLYBLENDS
The interpolymers of the present invention of both the matrix and the graft superstrates consist at least princi-pally of a monovinylidene aromatic hydrocarbon (alkenyl aromatic) and an unsaturated nitrile (alkenyl nitrile), i.eO, such monomers comprise at least 50.0% by weight and preferably at least 75.0% by weight of the interpolymers.
Most desirably, such monomers comprise at least 90.0% by weight of the interpolymer and the usual commercial compo-sitions are substantially completely comprised of suchmonomers although minor amounts, i.e. a less than 5.0% by weight of other components such as chain transfer agents, modifiers, etc.~ may be included.
As will be readily appreciated, the int~rpolymers used for the graft superstrates should be compatible with the interpolymer of the matrix so as to obtain good properties which will require the presence of the similar monomers.
Most desirably, the superstrate interpolymers closely ap-proximate the chemical composition of the interpolymer of the matrix so as to obtain matching of the chemical prop-erties, and, accordingly, it is desirable that the super-strates of both graft copolymers closely approximate each other. In addition, it is believed that increased chemical bonding is thereby obtained with commensurate improvement in chemical properties. Moreover, by close matching of certain interpolymers used in the matrix and superstrate such as those containing aerylate, it is possible to ob-tain a high degree of translucency and substantial trans-parency. However, it wil~ be appreciated that deviations in the composition of the interpolymers of the matrix and ~ superstrates such as different monomers and/or ratios may `- be desirable for some applications ~nd thai some deviations may inherently occur as a result of process variables.
Exemplary of the monovinylidene ~romatic hydrocarbons which may be used in the inter~olymers are styrene; alpha-alkyl monovinylidene monoaromatic compounds, e.g. alpha-methylstyrene, alpha-ethylstyrene, alpha-methylvinyltolu-ene, alpha-methyl di~lkylstyrenes, etc.; ring-substituted ..

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- 7 - 0~-12-Q398 alkyl styrenes, e.g., vinyl toluene, o-ethylstyrene, p-ethylstyrene, 2,4~dimethylstyre~e, etc.; ring-substituted halostyrenes, e.g. o-chlorostyrene, p-chlorostyrene, o-bromostyrene, 2,4-dichlorostyrene, etc.; ring-alkyl, ring-halosubstituted styrenes, e.g. 2-chloro-4-methylstyrene,

2,6-dichloro-4-methylstyrene, e-tc.; vinyl naphthalen~;
vinyl anthracene, etc. The alkyl substituents generally have l to 4 carbon atoms and may include isopropyl and iso-butyl groups. If so desired, mixtures of such monovinyli-dene aromatie monomers may be employed.
Exemplary of the unsaturated nitriles which may beused in the interpolymers are acrylonitrileg methacryloni-trile, ethacrylonitrile and mixtures thereof.
Exemplary of the monomers which may be interpolymer-ized with the monovinylidene aromatic hydrocarbons and un-saturated nitriles are conjugated 1,3 dienes, e.g., buta-diene, isoprene, etc.; alpha-or beta-unsaturated monobasic acids and derivatives thereof~ e.g. acrylic acid, methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, methacrylic acid and the corresponding esters thereof, acrylamide, methacrylamide; vinyl halides such as vinyl chloride, vinyl bromide, etc.; vinylidene chloride, vinylidene bromide, etc.; vinyl esters such as vinyl ace-tate, vinyl propionate, etc.; dialkyl maleates or fumar-ates such as dimethyl maleate, diethyl maleate, dibutyl maleate, the corresponding fumara~es, etc., and maleic acid. As is known in the art, the amount or these comono-mers which may be included in the interpolymer will vary as the result of various factors.
In addition, the monomer formulation at the time of polymerization may include a preformed polymer or a par-tially polymerized mat~erial such as a partially polymer-îzed monovinylidene aromatic hydrocarbon or interpolymer thereof.
The polymerizable monomer mixtures contain at least 50% by weight of the monovinylidene aromatic monomer and - preferably at least 60~o by weight thereof. They also con-tain at least 5% by weight o~ the unsaturated nitrile and '7~
~ 8 - C8-12-0398 preferably at least 15% by weight thereof. From the stand~
point of highly a~vantageous commercial practice, the mono-mer formulations contain S0 to 95% and preferably 50 to 85%, by weight of the vinylidene aromatic hydrocarbon and 50 to 5%, and preferably 15 to 50%, by weight of the un-saturated nitrile.
The Matrix . .
As is well known in the art 9 the polyblend is produced by polymerizing the monomers in the presence of the pre-formed rubber. It is believed that a portion of the poly-mer formed grafts onto the preformed rubber since it is generally not possible to extract the rubber from the polymerized mass with the usual rubber solvents although some of the rubber polymer may not be in actual chemical combination with the polymer.
Since 100% grafting efficiency is not usually attain-able, at least a portion of the monomers polymerized in the presence of the preformed rubber will not chemically combine therewith so as to provide a matrix for the graft copolymers. This portion may be increased or decreased depending upon the ratio of monomers to rubber, the par-ticular monomer formulation, the nature of the rubber and the conditions of polymerization. Generally, interpoly-;~ mers prepared without the inclusion of rubber will be com-pounded with material from the graft polymerization reac-tions to obtain the desired composition.
; Any of the usual polymerization processes may bé used to effect polymerization of the ungrafted superstrate, i.e., mass suspension and emulsion, or combinations thereof. Such techniques are well known and are also described herein with respect to the graf-t copolymerization reactions.
;The Rubber Substrate Various rubbers onto which the interpolymer may be grafted during the polymerization in the presence thereof are utilizable as the substrate of the graft copolymer in-cluding diene rubbers, ethylenepropylene rubbers, acrylate rubbers, polyisoprene rubbers and mixtures thereof as well : ::

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~ 9 - 08-12-0~98 as interpolymers thereof with each other or other copoly-merizable monomers.
The preferred rubbers are diene rubbers or mixtures of diene rubbers, i.e., any rubbery polymers (a polymer having a second order transition ~emperature not higher than 0 centigrade, preferably not higher than -20 centi~
grade, as determined by ASTM Test D 746-52T) of one or more conjugated 1,3 dienes, e.g., butadiene, isoprene, piperyl-ene, chloroprene~ etc. Such rubbers include homopolymers and interpolymers of conjugated 1,3-dienes with up to an equal amount by weight of one or more copolymerizable mono-ethylenically unsaturated monomers, such as monovinylidene aromatic hydrocarbons ~e.g., styrene; an aralkylstyrene, such as the o-, m-, and p-methylstyrenes, 2,4-dimethyl~
styrene, the ar-ethylstyrenes, p-tert-butylstyrene, etc.;
an alpha-alkylstyrene, such as alpha-methylstyrene, alpha-ethylstyrene, alpha-methyl-p-methylstyrene, etc.; vinyl naphthalene, etc.); arhalo monovinylidene aromatic hydro-carbons (e.g., the o-, m-~ and p-chlorostyrenes, 2,4-di~
bromostyrene, 2-methyl-~-chlorostyrene, etc.); acryloni-trile; methacrylonitrile; alkyl acrylates te.g., methyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, etc.), the corresponding alkyl methacrylates; acrylamides ~e.g., acrylamide, methacrylamide, N-butyl acrylamide, etc.); un-saturated ketones (e.g., vinyl methyl ketone, methyl iso-propenyl ketone, etc.); alpha-olefins te~g., ethylene, propylene, etc.); pyridines; vinyl esters (e.g., vinyl acetate, vinyl stearate, etc.); vinyl and vinylidene hal-ides (e.g., the vknyl and vinylidene chlorides and bro-mides, etc.); and the like.
Although the rubber may contain up to about 2% of acrosslinking agent, based on the weight of the rubber-form-ing monomer or monomers, crosslinking may present problems in dissolving the rubber in the monomers for the graft polymerization reaction, particularly for a mass or sus-pension polymerization reaction. In addition, excessive crosslinking can result in loss of the rubbery character-istics. The crosslinking agent can be any of the agents conventionally employed for crosslinking diene rubbers, e.g., divinylbenzene, diallyl malea-te, diallyl fumarate diallyl adipate, allyl acrylate, allyl methacrylate, di-acrylates and dimethacrylates of polyhydric alcohols~ e.g., ethylene glycol dimethacrylate 9 etc.
A preferred group of rubbers are those consisting es-sentially of 75 to 100% by weight of butadiene and/or iso-prene and up to 25% by weight of a monomer selected from the group consisting of monovinylidene aromatic hydrocar-bons (e.g., styrene) and unsaturated nitriles ~e.g., acrylonitrile), or mixtures thereof. Particularly advan-tageous substrates are butadiene homopolymer or an inter-polymer of 90 to 95~ by weight butadiene and 5 to 10% by weight of acrylonitrile or styrene.
Various techniques are customarily employed for polymerizing rubber monomers including mass, suspension and emulsion polymeri~ation. Emulsion polymerization can be used to produce a latex emulsion which is useful as the base for emulsion polymerization of the graft copolymer.
5raft Polymerization Process The graft copolymers are prepared by polymerizing monomers of the interpolymer in the presence of the pre-formed rubber substrate, generally in accordance with con-ventional graft polymerization techniques involving sus-pension,e~ulsion or mass polymerization, or combinations thereof. In such graft polymerization reactions, the pre-formed ru~ber substrate generally is dissolved in the mono-mers and this admixture is polymerized to combine chemi eally or graft at least a portion of the interpolymer upon the rubber substrate~ Depending upon the ratio of mono-mers to rubber substrate and polymerization conditions, it is possible to produce both the desired degree of grafting of the interpolymer onto the rubber substrate and the polymerization of ungrafted interpolymer to provide a por-tion of the matrix at the same time.
Although the amount of interpolymer superstrate ~~ grafted onto the rubber substrate may vary from as little as 10 parts by weight per 100 parts of substrate to as much ~ ; .

11~7~18 as 250 parts per lO0 parts, and even higher, the preferred graft copolymers have a superstrate~substrate ra.io of about 30-200:100 and most desirably about 70-lS0:100. With graft ratios above 30:100, a highly desirable degree of S improvement in various proper-ties generally is obtained.
To minimize requirements for separate equipment, the same process of polymerization desirably may be utilized to prepare both sizes of rubber graft components, as well as ungrafted interpolymer or crystal for use as ~he matrix when required. Generally, the particle sizes of the graft copolymer can be varied by varying the size of the rubber substrate employed. For example, a rubber latex which will usually have a relatively small particle size, i.e., less than about 0.2 micron, may be creamed through the use of lS polyvalent metal salts to obtain agglomeration or coagula-tion of a number of the small rubber particles into a larger mass. During the grafting reaction, the polymeriz-ing monomers will graft onto this agglomerate and thus provide a graft copolymer of larger size. In addition~
seeding techniques during polymerization of the rubber and/or during the polymerization of the graft copolymers may be utilized to vary the size of the particles thus produced.
Chain transfer agents or molecular weight regulators also exhibit an effect upon the size of the graft copoly-mer produced, particularly in mass and suspension poly-merization reactions. The effect of the rate of addition of chain transfer agents will be referred to hereinafter.
The viscosity of the polymerizing mixture also tends to affect the condensate particle size of the polymers.
: TQ some extent, crosslinking and the ratio of the superstrate to substrate in the graft copolymer tend to a*fect the particle size of the graft copolymers by reason of an apparent tendency for the particles to aggregate or agglomerate as the amount of grafting and/or crosslinking becomes minor.
The graft copolymer particles produced in various polymerization processes may be agglomerated through ,~ .

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varicus techni~ues in the recovery .hereof such as during the coagulation and/or dewatering techniques. Heat and other conditions of polymerization such as catalysts mono-mer ratios, rate of addition o~ monomers, etc., also tend to affect the particle size of the graft copolymers pro-duced thereby.
However, different polymerization techniques may be utilized to produce the two different sizes of graft co-polymer particles by reliance upon inherent process characteristics. In practice, it has been found desirable to utilize an emulsion polymerization process to form the smaller graft particles and a mass suspension polymeriza-tion process to form the larger particles since highly spherical particles are produced within a relatively narrow size range. ~enerally, the gra~t copolymerization inher-ently produces crosslinking, and this may be enhanced by selec~ion of process conditions to ensure discreteness of the graft copolymer particles.
It will be appreciated tha-t both the large and small particle graft copolymer components may be provided by mixtures of two or more separately formed graft copolymers of distinctive properties to vary still further the bene-fi~s of the present invention. For example, the small particle graft copolymer may be a cocoagulation'of two dif~erent graft copolymer latices having difrerent super strate to substrate ratios, or the large particle graft copolymers may be formed by two different suspension prod-ucts with varying superstrate to substrate ratios.
The emulsion grafted diene rubbers have an average particle size diameter of about 0.005 to 0.30 microns, ;~ prefe~ably 0.01 to 0.25 microns, most preferably 0.10 to 0.20 microns. If the rubber particles are agglomerated beore grafting the average particle size diameter can be increased to 0~30 to 0.80 microns .in size and then grafted and stabilized at that size range. The mass-suspension ; prepared grafted rubber particles have an average particle size diameter of 0.80 to 2.0 microns~ preferably 0.90 to 1.5 microns. It has been found that the mass polymerized ~: .
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grarted rubber particles have occluded interpolymer inside the rubber particle as well as grafted interpolymer super-strate. The combined occluded and grafted interpolymer can be rom 0.1 to 5 parts per part of rubber whereas the grafted superstrate is contained in amounts of from about 0.10 to 2.~ parts per part of rubber.
Formation of the ABS Polyblend The polyblends can be blends of the emulsion grafted rubber copolymer particles with matrix interpolymer or blends of mass-suspension grafted rubber copolymer par-ticles with matrix interpolymer. A third type of blend can be used wherein two different emulsion grafted rubber copolymers having different amounts of grafted superstrate are blended with matrix interpolymers as in U. S. P.

3,509,238. The polyblends can also be blends of the emul-sion grafted rubber particles with mass-suspension grafted particles which are then blended with matrix interpolymer to form polyblends as in U. S. P. 3,509,237. The poly-blends can be prepared by dry blending the grafted rubber copolymers with the matrix interpolymers followed by melt colloiding in an extruder, banbury or roll mill at tem-peratures of 400 to 500F. (205 to 260C.).
The ABS poly~lends can contain 1.0 to 70% of the grafted rubber copolymers based on the weight of the poly-blend depending on the physical properties desired in theconduit.
HIPS POLYBLEND
The high impact polystyrene polyblends (HIPS) can be prepared by the process disclosed in U. S. Patent 3,903,202.
POLYMERIZ~E~LE MONOMER COMPOSITION
The monomer composition charged to the first reaction zone comprises at least one monoalkenyl aromatic monomer of the foFmula ' 7 ~a~

~' f = CH2 Ar where Ar is selected from the group consisting of phenyl, halophenyl alkylphenyl and alkylha].ophenyl and mixtures thereof and X is selected from the group consisting of hydrogen and an alkyl radical of less than three carbon atoms.
Exemplary of the monomers that can be employed in the present process are styrene; alpha-alkyl ~onovinylidene monoaromatic compounds, e.g. alpha-methylstyrene, alpha-ethylstyrene, alpha-methylvinyltoluene, etc.; ring-sub-stituted alkyl styrenes, e.g. vinyl toluene, o-ethylsty-rene, p-ethylstyrene, 2,4-dimethylstyrene, etc.; ring-sub-stituted halostyrenes, e.g. o-chlorostyrene, p-chlorosty-rene, o-bromostyrene, 2,4-dichlorostyrene, etc.; ring-alkyl, ring-halo-substituted styrenes, e.g. 2-chloro-4-methylstyrene, 2,6-dichloro-4-methylstyrene, etc. I~ so desiredg mixtures of such monovinylidene aromatic monomers may be employed.
In addition to the monomers to be polymerized, the formulation can contain catalyst where required and other desirable components such as stabilizers, molecular weight regulators, etc.
The polymerization may be initiated by thermal mono-meric free radicals, ho~ever, any free radical generating catalyst may be used in the practice of this invention in-cluding actinic irradiation. Conventional monomer-soluble ~; peroxy and perazo catalysts may be used. Exemplary cata-lysts are di-tert-butyl peroxide, benzoyl peroxide, lauroyl peroxide, oleyl peroxide, toluyl peroxide 5 di-tert-butyl diperphthalate, tert-butyl peracetate, tert-butyl perben-zoate9 dicumyl peroxide, tert-butyl peroxide isopropyl earbonate, 2,5-dimethyl-2,5-dimethyl-2,5-di(tert-butyl-peroxy) hexane, 2,5-dimethyl-2,5-di(tert-butylperoxy)`

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- 15 - 08-12-03~8 hexane-3, or hexyne-3, ~ert~bu-tyl hydroperoxide, cumene hydroperoxide, p~-menthane h~droperoxide, cyclopentane hydroperoxide, pinane hydroperoxide, 2,5~dimethylhexane, 2,5-dihydroperoxide, etc., and mixtures thereof.
The catalyst i5 generally included within the range of 0.001 to 3.0~ by weight, and preferably on the order of 0.095 to 1.0% by weight of the polymerizable material, de-pending primarily upon the monomer present.
As is well known, it is often desirable to incorpor-ate molecular weight regulators such as mercaptans, halides and terpenes in relati~ely small percentages by weight, on the order of 0.001 to 1.0% by weight of the polymerizable material. From 2 to 20% diluents such as ethylbenzene, ethyltoluene5 ethylxylene, diethylbenzene or benzene may be added to the monomer composition to control viscosities at high conversions and also provide some molecular weight regulation. In addition, it may be de-sirable to include relatively small amounts of antioxi~
dants or stabilizers such as the conventional alkylated phenols. Alternatively, these may be added during or after polymerization. The formulation may also contain other ad~itives such as plasticizers, lubrieants, colorants and non-reactive preformed polymeric materials whieh are suit-able or dispersible therein.
- 25 The Rubber Substrate Exemplary of the various rubbers onto which the polymerizable monomer formulation can be grafted during polymeri7ation in the presence thereof to produce the graft copolymers are diene rubbers, natural rubbers, ethylene-propylene terpolymer rubbers~ acrylate rubbers, polyiso-prene rubbers and mixtures thereof, as well as interpoly-- mers thereof with each other or other copolymerizable monomers.
The preferred substrates, however, are diene rubbers (including mixtures of diene rubbers), i.e., any rubbery polymer (a rubbery polymer having a second order transi-tion temperature not higher than 0 centigrade, preferably not higher than ~20 centigrade, as determined by ASTM

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Test D-746-52T) of one or more of the conju~ated, 1,3 dienes, e.g. butadiene, isoprene, 2-chloro~1,3-butadiene, 1 chloro,3-bu~adiene, piperylene, etc. Such rubbers in-clude copolymers and bloc~ copolymers of conjugated 1,3-dienes with up to an equal amount by weight of one or morecopolymerizable monoethylenically unsaturated monomers, such as monsvinylidene aromatic hydrocarbons (e.g. styrene;
an aralkylstyrene, such as the o-, m- and p-methylstyrenes, 2,4-dimethylstyrene, the arethylstyrenes, p-tert-butyl-styrene, ete.; an alphamethylstyrene, alphaethylstyrene,alpha-methyl-p-methyl styrene, etc.; vinyl naphthalene, etc.); arhals monovinylidene aromatic hydrocarbons (e.g.
the o-, m- and p-chlorostyrene, 2,4-dibromostyrene, 2-methyl-4-chlorostyrene, etc.~; acrylonitrile; methacrylo-nitrile; alkyl acrylates (e.g. methyl acrylate, butylacrylate, 2-ethylhexyl acrylate, etc.), the corresponding alkyl methacrylates; arcylamides (e.g. acrylamide, meth-acrylamide, N-butylacrylamide, etc.); unsaturated ketones (e.g. vinyl methyl ketone, methyl isopropenyl ketone, e~c,); alpha-olefins (e.g. ethylene, propylene, etc.), pyridines; vinyl esters (e.g. vinyl acetate, vinyl stear-; age, etc.); vinyl and vinylidene halides (e.g. the vinyl and vinylidene chlorides and bromides, etc.); and the like.
Although the rubber may contain up to about 2.0% o a crosslinking agent, based on the weight of the rubber-forming monome~ or monomers, crosslinking may present prob-lems in dissolving the rubber in the monomers for the graft polymerization reaction. In addition, excessive cross-linking can result in loss of the rubber characteristics.
A preferred group of rubbers are the stereospecific polybutadiene rubbers formed by the polymerization of 1,3-~ butadiene. These rubbers have a cis-isomer content of ; about 30-98% and a trans~isomer con-tent of about 70-2~ and generally contain at least about 85% of polybutadiene 35 formed by 1,4 addition with no more than about 15~ by 1,2 addition. Mooney viscosities of the rubber (ML-4, 212F.
can range from about 20 to 70 with a second order transi-tisn temperature of from about -50 to ~105C. as deter-:;
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mined by ASTM Test D-746-521'.
GRAFTED RUBBER PHASE
A monomer composition comprising at least one mono-alkenyl aromatic monomer having about 2-20% by weight OL a die~e rubber dissolved therein is charged continuously as a monomer-rubber solution to the initial reaction zone.
The monomer is polymerized at temperatures of about 110-14~C. in the first zone converting about 10-50% by weight of the monomer to a alkenyl aromatic polymer, already described, as a first polymer. At least a portion of the first polymer polymerized is grafted as polymer molecules to the diene rubber as a superstrate.
Although the amount of polymeric superstrate grafted onto the rubber substrate may vary from as little as 10.0 parts by weight to 100.0 parts of substrate to as much as 250.0 per 100.0 parts and even higher, the preferred graft copolymers will generally have a superstrate to substrate ratio of about 20 to 200:100 and most desirably about 30 to 150:100. With graft ratios about 30 to 150:100; a highly desirable degree of improvement in various proper-ties generally is obtained.
The remainder of the first polymer formed is dissolved in said monom~r composition as polymerized forming a mono-mer-polymer solution. The monomer-polymer solution or phase is incompatible with the monomer-rubber solution or ~; phase and phase separation is observed-by the well known Dobry effect. As the polymer concentratlon of the monomer polymer-phase increases and has a volume slightly larger than the monomer-rubber phase, the monomer-rubber phase disperses as rubber-monomer particles aided by the shear-ing agitation of the stirred first react;on zone.
The agitation must be significant and of high enough shear to disperse and size the rubber particles uniformly throughout the monomer-polymer phase. The intensity of the s~irring will vary with the size and geometry of the ini-tial reactor, however, simple experimentation with a given `" stirred reactor will establish the sufficient amount of stirring needed to insure the homogeneous dispersion of the ::
:
.

- 18 - 0~-12-0398 rubber particles throughout the monomer-polymer phase. The particle size of the rubber can be varied from a weight average par~icle diameter of from about 0.5 to 10 microns preferably from 0.5 to 5 microns to provide a balance be-tween the impact strength and the gloss of the rubber re-inforced polyblend~ Higher stirring rates and shearing agitation ean lower the size of the dispersed rubber par-ticle, hence, must be controlled to provide sufficient stirring to size the particles to the predetermined size needed and insure homogeneous dispersion.
At steady state polymerizationj in the initial poly-merization zone, the continuously charged monomer composi-tion containing 2 to 15% by weight diene rubber disperses almost instantaneously, under stirring, forming the rubber-monomer particles which on complete polymerization formdiscrete rubber particles. The conversion of monomers to polymers in the first reaction zone is controlled between 10~50% and must have a weight percent level that provides a polymer content in excess of the rubber content of the monomer composition to insure the dispersion of the mono-~ mer-rubber phase to a rubber-monomer particle phase having ; a predetermined size and being dispersed uniformly through-out the monomer-polymer phase.
The rubber particle becomes grafted with a first polymer in the first reaction zone which aids its disper-sion and stabilizes the morphology of the particle. During the dispersion of the rubber-monomer particles, some mono-mer-polymer phase is occluded within the particle. The total amount of occluded monomer-polymer phase and grafted polymer present in the particles can be from about 1 to 5 grams for each gram said diene rubber.
The dispersed rubber phase increases the toughness of the polymeric polyblend as measured by its Izod impact strength by Test ASTM D-256-56. It has been found that the impact strength of polyblends increase with the weight per-~; cent rubber dispersed in the polyblend in the range of 2 to 15% as used in the present invention. The impact strength is also determined by the size of the dispersed rubber par-.

- lg - 0~-12-0398 ticles, with the larger particles providing higher impact strength in the range of 0.5 to 10 microns measured as a weight average particle size diameter with a photosedi-mentometer by the published procedure of Graves, M. J., et.al., "Size Analysis of Subsieve Powders Using a Centri-fugal Photosedimentometer", British Chemical Engineering 9:742-744 (1964). A Model 3000 Particle Size Analyzer from Martin Sweets Co., 3131 West Market Street, Louis-ville, Kentucky was used.
The weight average diameter of the rubber particles also effects gloss with smaller particles giving high gloss and the larger particles giving low gloss to the fabricated polyblend article such as a molding or sheet product. One must balance impact strength and gloss re-quirements in selecting an optimum rubber particle size.
The range of 0.5 to 10 microns can be used with the range of 0.5 to 5 microns being preferred and 0.8 to 3 microns being most preferred for optimum impact strength and gloss.
It has been found possible to analyze the amount of totaI occluded polymer phase and grafted polymers. The final polymerized polyblend product (1 gram) are dispersed in a 50/S0 acetone/methyl ethyl ketone solvent ~10 ml.) whi~h dissolves the polymer phase matrix leaving the rub-ber phase dispersed. The rubber phase is separated from the dispersion by centrifuge as a gel and dried in a vacuum oven at 50C. for 12 hours and weighed as a dry gel.

. .
. . ,~

\
20 - 08-~2-0398 % Dry Gel _ Weight of dry gel in Polyblend Weight of polyblënd % Graft and Occlusions ) = % dry ~el ~ % rubber in Rubber ) Percent rubber~-Parts::~ by weight of graft polymer and occluded poly- ) = % dry ~el - % rubber mer per unit weight ) Percent rubber of rubbPr * Percent rubber determined by infra red ~pectrochemical analysis of the dry gel ** The present invention preferably has present about 0.5 to 5 grams of occluded and grafted polymer per gram of the diene rubber particle.

The swelling index of the rubber graft particles is determined by taking the dry gel above and dispersing it in toluene for 12 hours. The gel is separated by centrifuge and the supernatant toluene drained free. The wet gel is weighed and then dried in a vacuum oven for 12 hours at 50C., and weighed.

Swelling Index - welght o~ wet gel welght of dry gel .
As described earlier the amount of occlusions and graft polymer present in the rubber particle is present in the amount of about 0.5 to 5 part for each part of diene - rubber. The percent dry gel measured above then is the percent gel in the polymerized polyblend and represents the dispersed rubber phase having polymeric occlusions and polymeric graft. The percent gel varies with the percent ; rubber charged in the monomer composition and the total amount of graft and occluded polymer present in the rubber phase.

7~

Step (A) is carried out by extruding a melt of a plas-tic through an open channel of a first die of said first extrusion means, having the shape of said inner tube, form-ing a molten tube, carrying said molten tube through a first sizing means and cooling as said inner tube, said sizing means being a vacuum sizing means.
The extrusion means, dies, vacuum sizing means as shown in Figures 4~ 5 and 6 are commercially available from the Prodex, HPM Division, Xoehring Co., Mount Gilead, Ohio and can be adapted to the present process.
Step (B) is carried out by extruding a melt of a plas--tic, having present a blowing agent, through an open chan-nel of a second die of said second extrusion means as a foamed extrudate while passing simultaneously said inner lS tube through said second die, coating the outer surface of said inner tube evenly with said foamed extrudate as said foam-core layer followed by sizing and cooling said foam-core layer. Said open channel of said second die has a first fixed mandrel centrally disposed therein, said man drel forming one face of said open channel, said first fixed mandrel having a centrally disposed opening running axlally through said mandrel having the general shape of said inner tube for carrying said inner tube through said die, said first fixed mandrel extending partially through said die, such that said inner tube becomes a first moving mandrel, on exiting said first fixed mandrel and is evenly coated with said foamed extrudate on passing through said second die, said foam-core layer is sized to be radially uniform in thickness concentric with said inner tube.
Said foam-core layer is sized optionally with a second vacuum sizing means or mechanical sizing means.
Step (C) is carried out by extruding a melt of a ~ plastic through an open channel of a third die of said ; third extrusion means having the general shape of said profile while passing simultaneously said inner tube, ; having said foam core layer in place, through the open ; channel of said third die, extrusion forming said plastic melt as said outer tube in contact with said foam-core : ' .

~ 22 - 08-12-0398 layer, said outer tube being generally evenly spaced apart from said inner tube by said foam-core layer providing a double-walled, foam-core conduit. Said outer tube is sized optionally with a third vacuum sizing means.
S The foam-core layer is formed from an extrusion melt of said plastic having present a blowing agent selected from the group consisting of hexane, petroleum ether, C02, halogenated hydrocarbons or mixtures thereof in an amount of about 0.1 to 10.0 weight percent based on said plastic.
The foam-core layer ca~ also be formed from an ex-trusion m~lt of said plastic having present a chemical blowing agent selected from the group consisting of azodi-carbonamide, diazoaminobenzene, 1,3-di-phenyl triazine, 1~1 7 -azo-bis-formamide, 2-2'-azoisobutyronitrile, azo-hexa-hydrobenzonitrile, benzene sulfonylhydrazide~ sodium bi-carbonate, ammonium carbonate and mixtures thereof in an amount of about 0.1 to 10.0% by weight based on said plastic ~
The foam-core layer has a density of about 0.015 to 0.5 g/cc. wherein Step (B) is carried out with extrusion at a melt temperature of about 125 to 250C. and a pres-sure of about 60 to 270 atm providing a foam~core thickness of about 20 to 5000% of a combined wall thickness of the inner and outer tubes.
Steps (A) and (C) are carried out with extrusion at a melt temperature of about 175 to 260C. and a pressure of about 60 to 204 atm providing an inner tube tnickness o~ about 0.025 to 1.25 cm. and an outer wall thickness of about 0.025 to l.25 cm.
The bonding of the inner tube to the foam-core layer ;~ ~ can be earried out by heating the outside surface of said inner tube prior to coating with said fo~m-core layer in Step tB). The heating is carried out by heating only the outside surface with a suitable heater which can be, e.g., a heated sleeve forming an inner surface of said first mandrel such that the outside surace of said inner tube is heated to about 13Q to 230C~ prior to coating with said ; foam-core layer. The bonding of the foam-core layer to the :

outer surface of said inner .ube can be carried out by coating the outer surface of said inner tube with an active solvent for said inner tube prior to conveying through said second extrusion means, said active solvent can be se-lected, e.g., from the group consisting of methyl ethylketone, xylene, toluene, petroleum ether, hexane, pentane or mixtures thereof.
The following Example is illustrative of the process and is not to be construed as limiting the scope and spirit of the invention.

An ABS polyblend having a rubber conter.t of about 23~
was melt extruded through a first extrusion means at 425~.
(220C.) and a pressure of about 2000 psi (140 atm.) to form a inner tube, vacuum sized and cooled to an inside diameter of about 1.5" ~3.75 cm) and a wall thickness of about 100 mil (0.250 cm). The inner tube was conveyed by a pipe puller means from said first extrusion means through a second extrusion means wherein an ABS polyblend having a diene rubber content of about 4% dry blended with about 0.5~ talc~ 1.0% ionomer lubricant (Surlyn 1801*commercially available from DuPont, Wilmington, Del.) and o.1% gelatin, all said additives based on said ABS polyblend. Said dry-blend was melt extruded at about 275F. (135C.J and 2000 psi (140 atm.) while injecting about 5 parts of Freon 12*
~dichlorodifluoromethane commercially available from DuPont, Wilmington, ~el.) per 100 parts of ABS, into said melt in the second extruder of said second extrusion means.
An annular ~oam-core la~er was extruded in contact with the outer surface of said inner tube acting as a moving mandrel and allowed to foam radially to about 1" (~.5 cm) in thick-ness and a density of about 4 lbs./cu.ft. (0.064 gms./
c.c.). The inner tube having said foam-core layer was con-veyed through a heated annular sleeve sizing means, cooled and conveyed to a third extrusion means by said pipe puller, said third extrusion means extruding a melt of ABS
plastic under the same conditions and having the same prop-erties as extruded in said first extrusion means forming * Trademark an outer ~ube cf said ABS plastic having an inside diameter slightly larger than said first tube coated with said foam-core layer such that said first tube having said foam-core acts as a moving mandrel picking up said outer tube as it shrinks in contact with said foam-core layer and becomes bonded thereto. Said outer tube can be extruded at a rate slightly slower than said inner tube such that as said second tube shrinks in diameter to co~tact said foam-core layer, it is drawn down in diameter to contac~ and bond to said foam-core layer. The relative rates of extrusion of said inner and outer tubes are prede-termined so as to pro-vide an outer layer of a predetermined thickness in con-tact and bonded to said foam-core layer. Said outer tube was formed with a final thickness of about 0.100" (0.25~
cm) in contact and bonded to said foam-core layer providing a double-wal'ed foam-core conduit which was conveyed through a spray bath, cooled and cut to length. The ex-ample was carried out as a continuous operation such that said inner tube, said foam-core layer and said outer tube were formed sequentially but continuously providing a con-tinuous process for forming said conduit.
The plastic used in the inner and outer tubes and the foam-core layer of the double-walled conduit can be of the same plastic or different plastics can ~e used ~or the inner tube, the outer tube or the foam-core layer.
Several plastics suitable for the process and conduit have been disclosed supra. These are generally known as thermoplastics. The monomers disclosed include alkenyl aromatic, alkenyl nitrile, maleic anhydride, butadienes, ethylenes, propylenes, vinyl chloride, acrylic and acrylate or other vinyl or vinylidene type monomers which can be ` polymerized by free radical catalysts. These monomers can - ~ be used to prepare thermoplastics such as polystyrene, styrene copolymers having the comonomers of acrylonitrile, methacrylonitrile, esters of acrylic or methacrylic acid e.g., methyl acrylate, ethyl acrylate, methyl methacrylate, ethyl methacrylate, vinyl acetate, vinyl chloride, buta-diene, isoprene, chloroprene, ethylene, propylene ~nd ~ ~ 7 ~ 8 other olefins, maleic acid and its derivatives. The alkenyl aromatic monomers include those disclosed for ABS
and HIPS polyblends. The alkenyl aromatic monomers in-clude those disclosed for ABS polyblends. Other polymers that can be prepared from the disclosed monomers are poly-ethylene, polypropylene, polyvinyl chloride, polymethyl methacrylate, polyacrylic acid, polybutadiene, polymaleic acid and .heir copolymers with the monomer disclosed. The process of this invention can be adapted to the use of a wide variety of other conventional thermoplastics known in - the art, e.g., polycarbonates, polyesters, polyethers, polysulfones, polyamides, polyacetals or polyblends of such thermoplastics with ABS and HIPS for improved toughness.
Generally, such conduits are used for nonpressure--type sewer and drain pipe, utility ducts or conduits and DWV
(drain, water and vent) pipes.
Fire retardant properties may be incorporated in such conduits by conventional means such as incorporating halo-genated compounds, metal oxides and other inorganic fillers. Halogenated monomers can be used as comonomers in sufficient amounts to provide fire-retarding properties in the plastics used.
The blowing agents are not limited to those disclosed supra. Any conventional blowing agent can be used for forming the foam-core layer that is active within the temperature range which the plastic is extruded.
- The conduit configuration can be prepared such that - the inner and outer tubes have the same wall thickness or different wall thicknesses within the thickness ranges disclosed supra.
The conduit configuration can be varied. Increasing - the wall thickness of the inner tube generally provides .
greater impact strength for the conduit whereas increasing the thickness of the outer tube generally increases flex-ural strength or stiffness of the conduit. Stiffness canalso be increased by increasing the thickness of the foam-core layer. Hence, a range of physical properties can be ~ provided for double-walled, oam core conduit without : ' .

à;f~ Y~

changing overall density and dimensions of the conduit.

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.~

.

~, .

.

Claims (35)

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

1. A process for the extrusion forming of a thermoplastic, double-walled, foam-core conduit having generally concentric inner and outer plastic tube walls generally evenly spaced apart and bonded by a plastic foam-core layer, said process comprising the continuous and sequential steps of:
A. extrusion forming said inner tube in a first extrusion means, B. conveying said inner tube through a second extrusion means and extrusion coating an outer surface of said inner tube with an even coating of said foam-core layer having a density of about 0.015 to 0.4 g./c.c., C. conveying said inner tube, coated with said foam-core layer, through a third extrusion means and extrusion forming said outer tube in contact with and bonded to said foam-core layer providing a plastic double-walled, foam-core conduit.

2. A process of Claim 1 wherein said inner and outer tube walls are generally annular in profile providing a double-walled, foam-core pipe.

3. A process of Claim 1 wherein said inner and outer tube walls are generally rectangular in profile providing a double-walled, foam-core rectangular conduit.

4. A process of Claim 1 wherein said inner and outer tube walls are irregular shaped generally concentric profiles providing a double-walled conduit.

5. A process of Claim 1 wherein said plastic is formed from monomers selected from the group consisting of alkenyl aromatic, alkenyl nitrile, maleic anhydride, butadiene, ethylene, propylene, vinyl chloride, acrylic, acrylate and mixtures thereof.

6. A process of Claim 1 wherein said plastic is selected from the group comprising polystyrene, styrene-acrylonitrile copolymer, styrene-maleic anhydride copolymer, butadiene rubber reinforced plastic thereof and mixtures thereof.

7. A process of Claim 1 wherein step (A) is carried out by extruding a melt of said plastic through an open channel of a first die of said first extrusion means, having the shape of said inner tube, forming a molten tube, carrying said molten tube through a first sizing means and cooling as said inner tube.

8. A process of Claim 7 wherein said sizing means is a first vacuum sizing means.

9. A process of Claim 1 wherein step (B) is carried out by extruding a melt of said plastic having present a blowing agent, through an open channel of a second die of said second extrusion means as a foamed extrudate while passing simultaneously said inner tube through said second die, coating the outer surface of said inner tube evenly with said foamed extrudate as said foam-core layer followed by sizing and cooling said foam-core layer.

10. A process of Claim 9 wherein said open channel of said second die has a first fixed mandrel centrally disposed therein, said mandrel forming one face of said open channel, said first fixed mandrel having a centrally disposed opening running axially through said mandrel having the general shape of said inner tube for carrying said inner tube through said die, said first fixed mandrel extending partially through said die, such that said inner tube becomes a first moving mandrel, on exiting said first fixed mandrel, and is evenly coated with said foamed extrodate on passing through said second die.

11. A process of Claim 10 wherein said foam-core layer is sized to be radially uniform in thickness concentric with said inner tube.

12. A process of Claim 11 wherein said foam-core layer is sized with a second vacuum sizing means.

13. A process of Claim 1 wherein step (C) is carried out by extruding a melt of said plastic through an open channel of a third die of said third extrusion means having the general shape of said profile while passing simultaneously said inner tube, having said foam-core layer in place, through the open channel of said third die, extrusion forming said plastic melt as said outer tube in contact with said foam-core layer, said outer tube being generally evenly spaced apart from said inner tube by said foam-core layer providing a double-walled, foam-core conduit.

14. A process of Claim 13 wherein said open channel of said third die having a second fixed mandrel centrally disposed therein, said second fixed mandrel forming one face of said open channel and having a centrally disposed opening running axially through said second fixed mandrel having the general shape of said inner tube, coated with said foam-core layer for carrying said coated tube through said third die, said second fixed mandrel extending through said second die such that said coated inner tube becomes a second moving mandrel on exiting said second fixed mandrel, extrusion forming and bonding said outer tube in contact with said foam-core layer, said outer tube being generally evenly spaced from said inner tube by said foam-core layer providing a double-walled, foam-core conduit.

15. A process of Claim 13 wherein said outer tube is sized with a third vacuum sizing means.

16. A process of Claim 1 wherein said foam-core layer is formed from an extrusion melt of said plastic having present a blowing agent selected from the group consisting of hexane, petroleum ether, CO2, halogenated hydrocarbons or mixtures thereof in an amount of 0.1 to 10.0 weight percent based on said plastic.

17. A process of Claim 1 wherein said foam-core layer is formed from an extrusion melt of said plastic having present a chemical blowing agent selected from the group consisting of azodicarbonamide, diazoaminobenzene, 1,3-diphenyl triazine, l,l'-azo-bis-formamide, 2-2'-azo-iso-butyronitrile, azo-hexahydrobenzonitrile, benzene sulfonyl-hydrazide, sodium bicarbonate, ammonium carbonate and mix-tures thereof in an amount of 0.1 to 10.0% by weight based on said plastic.

18. A process of Claim 1 wherein step (A) and step (C) is carried out with extrusion at a melt temperature of 185 to 260°C. and a pressure of 60 to 204 atm.

19. A process of Claim 1 wherein step (B) is carried out with extrusion at a melt temperature of 125 to 250°C. and a pressure of 60 to 270 atm.

20. A process of Claim 1 wherein said inner tube has a wall thickness of 0.025 to 1.25 cm., said outer tube has a wall thickness of 0.025 to 1.25 cm. and said foam-core layer has a thickness of 30 to 5000% of a combined wall thickness of said inner and outer tubes.

21. A process of Claim 1 wherein said inner tube is bonded to said foam-core layer in step (B) by coating the outer surface of said inner tube with an active solvent for said inner tube prior to conveying through said second extrusion means.

22. A process of Claim 21 wherein said active solvent is selected from the group comprising methyl ethyl ketone, xylene, toluene, petroleum ether, hexane, pentane or mixtures thereof.

23. A process of Claim 1 wherein said inner tube is bonded to said foam-core layer in step (B) by heating the outside surface of said inner tube prior to coating with said foam-core layer.

24. A thermoplastic, double-walled, foam-core conduit, said conduit comprising generally concentric spaced apart inner and outer tubes, said outer tubes being generally evenly spaced apart from said inner tube by a foam-core layer in contact and bonded with said inner and outer tubes, said foam-core layer being a foam of said plastic having a density of 0.015 to 0.4 g./c.c.

25. A conduit of Claim 24 wherein said plastic is formed from monomers selected from the group consisting of alkenyl aromatic, alkenyl nitrile, maleic anhydride, butadiene, ethylene, propylene, vinyl chloride, acrylic, acrylate and mixtures thereof.

26. A conduit of Claim 24 wherein said plastic is selected from the group comprising polystyrene, styrene-acrylonitrile copolymer, styrene-maleic anhydride copolymer, butadiene rubber reinforced plastic thereof and mixtures thereof.

27. A conduit of Claim 24 wherein said foam-core layer is formed from an extrusion melt of said plastic having present a blowing agent selected from the group consisting of hexane, petroleum ether, CO2, halogenated hydrocarbons or mixtures thereof in an amount of 0.1 to 10.0 weight percent based on said plastic.

28. A conduit of Claim 24 wherein said foam-core layer is formd from an extrusion melt of said plastic having present a chemical blowing agent selected from the group consisting of azodicarbonamide, diazoaminobenzene, 1,3-diphenyl triazine, 1,1'-azo-bis-formamide, 2-2'-azo-isobutyronitrile, azo-hexahydrobenzonitrile, benzene sulfonylhydrazide, sodium bicarbonate, ammonium carbonate and mixtures thereof in an amount of 0.1 to 10.0% by weight based on said plastic.

29. A conduit of Claim 24 wherein said inner tube has a wall thickness of 0.025 to 1.25 cm., said outer tube has a wall thickness of 0.025 to 1.25 cm. and said foam-core layer has a thickness of 30 to 5000% of a combined wall thickness of said inner and outer tubes.

30. A conduit of Claim 24 wherein said plastic is an ABS polyblend having 2 to 50% of a grafted diene rubber phase and 50 to 98% of a matrix phase, said grafted diene rubber phase having 10 to 250 parts of a superstrate graft per 100 parts of diene rubber substrate comprising alkenyl aromatic and alkenyl nitrile monomers, said matrix phase being a polymer comprising said monomers.

31. A conduit of Claim 30 wherein said alkenyl aromatic monomer is styrene and said alkenyl nitrile monomer is acrylonitrile, the ratio of styrene to acrylonitrile in said superstrate and said matrix being 50:50 to 90:10, said grafted diene rubber phase being dispersed in said matrix phase as rubber particles having a particle size of 0.10 to 10.0 microns.

32. A conduit of Claim 24 wherein said plastic is a HIPS polyblend having present 2 to 20% of a diene rubber phase moiety dispersed in a matrix polymer phase comprised of monoalkenyl aromatic monomer, said rubber phase being dispersed as rubber particles having a particle size of 0.10 to 10.0 microns grafted and occluded polymer of said alkenyl aromatic monomers in amounts of 1 to 5 parts per part of diene rubber.

33. A conduit of Claim 24 wherein said plastic is selected from the group consisting of polycarbonate, polyester, polyether, polysulfone, polyamide, polyacetals, HIPS and ABS or mixtures thereof.

34. A process for the extrusion forming of a thermoplastic, double-walled, foam-core conduit having generally concentric inner and outer plastic tube walls generally evenly spaced apart and bonded by a plastic foam-core layer, said process comprising the continuous and sequential steps of:
A. extrusion forming said inner tube in a first extrusion means, B. conveying said inner tube through a second extrusion means and extrusion coating an outer surface of said inner tube with an even coating of said foam-core layer having a density of about 0.015 to 0.4 g./c.c., C. conveying said inner tube, coated with said foam-core layer, through a third extrusion means and extrusion forming said outer tube in contact with and bonded to said foam-core layer providing a plastic double-walled, foam-core conduit, wherein said thermoplastic is selected from the group consisting of ABS, HIPS and polyvinyl chloride.

35. A thermoplastic, double-walled, foam-core conduit, said conduit comprising generally concentric spaced apart inner and outer tubes, said outer tubes being generally evenly spaced apart from said inner tube by a foam-core layer in contact and bonded with said inner and outer tubes, said foam-core layer being a foam of said plastic having a density of 0.015 to 0.4 g./c.c., wherein said thermoplastic is selected from the group consisting of ABS, HIPS and polyvinyl chloride.