CA1145272A - Gaseous diffusion resistant article and method utilizing same - Google Patents

Abstract

ABSTRACT

A gaseous diffusion resistant tube comprising a core (1) of a liquid resistant plastics material, and an outer layer(5) of a plastics impact-resistant material, and has an intermediate layer (3) of a gaseous diffusion resistant plastics material interposed between and bonded to the core and the outer layer. The outer layer and the core are preferably made of a cross-linked polyolefinic material such as cross-linked polyethylene. The intermediate layer is preferably selected such that it is an adhesive for both the core and outer layer materials and such that it has a measure of elasticity to compensate the relative coefficients of linear expansion between the intermediate layer and its adjoining layers.

Tubes in accord with this invention may be formed by co-extrusion with economy and precision and are particularly useful in central heating applications.

Description

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DESCRIP~IO~

~ he presen~ ~nvention relates to a gaseous diffusio~
re~istant tube compriai~s a core of a liquid-resi~tant plastlcs material and an outer layer of a plsstic~ impact r2sistant material. Partioularly it relates to su~h an arrangemsnt for use in hot liqu~a transfer pipes ~uoh as ; thoae utili~ed in cQntral heatin~, systems.
Plastics pipes, particularl~ those of oross-linked polyethylene are widely uaed for the trarsfer of hot l~quid~; partioularly heat tran~fer liquids for u8e i~
central heating ayEtems. At the temp~ra~ures at which the central he~ting ~y~tem3 operate, o~ygan from the air particularly~ diffu~es from the esterior of ~ald pipe ~to the w~ter or other heat exc ~ e liquid in the pipe. The o~y~en bosne by the liquid in the pipe co~tacts e~posed metal surfaces in the system and tends to oxid~3e the ~a~e '~

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particularly to ru~t~ Particles o~ xust a~d other o~idised metal~ then become detached and are tran~rerrsd~ ~uspended in the heat e~chang~ liquid9 to a constriction or other obstruction where they are deposited and oan event~ally cau~e blockages. Thus9 it is generally desira~le to proride oxygen diff~sion resista~t tubes for use in c~ntral haating ~y~tem~9 b~t for ~ariou~ rea~ons those currently a~ailable are not wholly satisfactoryc Xt is known, for e~ample ~rom 3ritish Patent ~o: 1~175~005, to provide a two-layer ~aseous di~fu~ion resistant tube made, for example,~ from polyethylene and nylon. Suoh an arrangement is not suitable for c~ntral heat~ng installations however be~ause R~ its linear a~pa~sion during heat cyoling. Wbera tha polye~hylene is cross-linked to pro~ide ~uitable dlmensioDal ~tability said o~ygen ro~istant nglon coating ovsrlyan~ ffi e ~ame ~annot be u~ed ~ucoe~s~ully becau~e its coefficie~t o~
linear exp~DEion iB EO differsnt from tbst o~ the cro~8-liD~ed polyethylene tube.
Similarly, it has been proposed~ ~or elample in ; ~ritish Pat~t ~o: 1~171,122 to pro~ide a ga~eOUB diffusio~
` re3i8tant tube ha~ing an ~ntermediate layer o~ a m~tallic : foil ~hi¢h both reinforces the tube ænd prevent~ ~a8eou~
di~u~io~. Such a~emblies are difficult and hanc~ :
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: ~ 4 -e~ps~aiva to prod~oe mainly because of the difficl~lty o~
bonal~g the estar$or ard interior pl~sti¢s layer~ to thQ
foil. Further the i~stallation of ~uch ~ubss i8 difficult beca~e of their i~herent ri31dity and because pla~tlc8 end fitt~ngs utili~ed in c~ntral heating syst~m~ are not generally ~uitabla for UBe with pipeB inoluding a metal in~ert.
~ ha present invention seeks to provide therefore a fle~ible ~aseous diffusion resistant tube whi¢h is u~able nnder heat cycling conditio~3, ~hich can be co-extruded~ and is easy to i~3tal.
~coording to the present imrention, therefore, ~hara : i~ provided a gaseou~ diffusion resistant tube comprising a core of a liquid resistant pla~3tic~ material, ana an outer layer of a plastict3 impaot~-resi~tant material, characteri~ed by æn intermediate la~er of a gasaous diff~on resis~ant pla~t~o~ material intexpo~ed between ana bondad ; bo the ¢ore a~d the a1ter layer. It is preferred that the core and thfl~ ou~er layer ha~e ~ub~tantially ~imilar ` 20 coeff~cien~ of ll~ear expansio~ g~a ~he inte~$ediate :~ layer ~s slightly alastic and ~dhesive to both the oore : &nd the out0r layer. ~he oore a~d/or ~he ouber layer ~ ~ay be mads of a c~oss-liDked polgole~i~ic material ~uah a~ cro8s ll~ked polyethylene which much improve~ the . .

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dimenæional stability of ~he tu~e at elsvated t~mperatures.
: M~ter~als for the int~rmed~ate layer ~uoh as poly~inyl-aloohol(pVAL)are partic~larly ~seflul in the practioe of ~hi~ in~ention~ but are not, in ~hemselvea, impsct-resis~Ent S and henoe ~n8t be protected by an impaot resistant layer.
~he impact-re~istant layer in such oircumstances need have ~ no diffusion re~istant qualitiss and henoe~ if intrinsioall~
i~ ~t~ong>may be applied as a comparatively thLn layer.
Polyethylene in linear or cro~-linked polymerio rorm ia an example o~ such an impact-resistant layer.
~nother e~ample of a partioula.rly desirable ~aseou~
diffu~ion resistant material is polyacrylonitrile (P~
Othsr materials which may be u~ed as the ~mpact-resistant layer inolude oopolymers of aorylonitrile and thermo-.~ 15 plastics pol~esters9 pol~amideæ or chlorin~ted PVC.
Ib~ partioular gaseous diff~ion resistant layer ~ororporation wlth ~he plastios material o~ the oore will in ge~ea l ba selected depe~dent upon ~he atmo~phere in which ~he e~en*ual pipe i~ to be u~ed. ~hu~, in a normal ; 20 o~ntral heati~ ~y~tem ~t is most i~porga~t to prsnant ~he diffusio~ of o~g~a into the ~eat e~obange llquid, : ~ow~er, in o~rtain laboratory appli¢atioD~ the ga080~
d~ffusion re~lstant material i~ seleoted to inhlbit tho diffbsion of halogan8 such a~ chloriae or o~her orgEnio ga~es~

7~ 1 Some materials such as PVALwhen uaed in the gaseous diffu~ion-re~i~t~nt layer also act as an adh si~e 2nd c~n be uti~i~ed, as stated above, to adhere the sore to the outer impact-resi~tant layer selected.
~owever, when the imRact-resistant layer or the exterior ~ur~ace of the core does not readily sdhere to the g3seous diffuaion-resistant layer a ~eparate adhesive can be used.
Where pos~ible it ia desirable to coa ally co-e~trude sequentially the core, the diffusion-re~istsnt layer~ and the impact-resistant layer. In some applicatio~a it is co~venient to utilize the same material, eg:
cross-linked polyeth~lene a8 both the impact resistant layer and the core. ~hi~ ensures that the coef~icie~ce of linear e~pansio~ of the core snd th~
outer layer are aimilar; ~here P~AI~or example is u~ed a~ the int~rmediate layer itc slight elastlcity compensates for lts ~llgh~ly di~fe~ent coefficient of linRar eæpsnsion-Some ~mbodime~t~ of th~ i~vention will now be de~cribed by way of illu~tration only a~d with rafarffnce to the~ollowing e~amp~es and dr~win4s.
In the accompanyin~ drawin4~
Figures 1 and 2 represent diagrammatio oros~-~3ctions ~hrough ssgme!nts o~ oxy~n di~fusion re~ista~t tubes o~ th~
~n~ention.

~5Z~.2 Fi~ure 1 ~ho~ a cross-linked polyethylene (PEX) tube 1 ~used to an.osygen dif~usio~ resista~ layer of PVAL3bhich in turn i~ ~used to an impact re~istant layer of linear polyethylene per se 5; and Figure 2 show~ a PEX tube 1 ha~ing an adhesive interla~er 2~ and 4~ disposea respactively between the PEX tube 1 and an o~ygen dlPfusion resistant layer PA~ 3 and between the ~aid layer 3 and a polye~hylene impact re~istsnt layer 5.
Where the materials forming layers 3 and 5 Gannot be eoe~truded with the tube 1 they may be applied by spraying, dippin~ or wTappi~g a~ required. qhe thioknes3 of the layer 3 should be suoh as to reduoe osygen diffu0ion by at least 1/5th of that of the oore . .
~ 15 maberial, ana it is preferred t~t the thicknes~ of the .
.` layer ~ is between l/lO0 and l/lO of ~he kall thiokne~

o~ ~he co~e.

. ~y wa~ of this e~mpleg the following l~yers ~ere : all applled to samples of a PEg tube. Said ~ube had an . .
ou~0r diameter of 20 mm a~d an inner ~all thiokne~s of

2 mm. ~ch ~ample wa~ te~ted ~or o~gen diffusion against a ~imilar control ~ample which had either no coating -` thereo~ or o~ly a thin ooatin~ of an adhesi~e wlth no o3ygen diffu~ion resi~tant oapabilities.

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~I?he re~ult~ are ~et forth in the followin~ table:

~E 1 ~iDpa¢t a~d o:qgen difu~ion resi~ta~t sample~ ~rith and 5 . withou~ separate impact resi~tant layer.

O~YG~N Sli2ARAq!E IMpA CT COMP~ I!IVE
M~ L 0~ D:~EIJSI0~ RESIS~ D~ellSI0 !I!R~:NSE IR PIPE R~SIS~ L~ER L~ ~

10 1) 2mm PEX 20~ PVAL 0.3 mm P:C ~ 1/5th oo~trol 2) 2mm P~ 20 f~ P.~N 0.3 mm PE C 1/5th control

3) 2mm PEI~ 40~(~ PV~L 0,3 mm PE < 1/10th co~trol

4) 2mm Pl~ 40~( p~ 0.3 m~n PE ~1/10th co~trol oopolymer 15 5) 2mm PEX 40 0.3 mm PE ~1/5~ con~
chlorinated PYC O
It ca~ thus bs ~een ~at partiouiarly ~dvaD~ageolls re~ults ca~
be aohieved with a lsyer ~etween 20 alld 40~4 thi~ik a~ Pv~L
or P~ over a PE~ tube~ will re~oe ox~gen diff~ion 20 to less ~han 1/lOth. a~ the diffusion e~ected throu~h a PEX tube with a 2 mm wall thiCkZ~B~ ~d an outer diameter o~ 20 Dlm.
3!he use of tubes made in a¢cord with tho l~vention has been fo~d in practioe to con~iderably re~oe the 25 i~stance of co~rosio~ in oe~ral heatin~ tems and thi~

- lead~ ~o a reduction in the re~ired æerYiclng frequencies.
~ ecause the diffu~ion resistant layer~ can be ~o~med during the production of no~mal ~E~ tubes, ~ubes according to the i~ven~ion can be produced for much tke Bame co~t

5 - a~ that of ordinary PEX tube~. Consequently, o~yg~n diffu~ion re~is~ant ~ipes ¢an be utili~ed in the installa~ion of oeDt~al heating sy~t~ms without a significant e~tra co~t and with mate~ial long te~m benei`it~ It wlll also be appreciated that the comparati~ely thin non-structural ox~ge~ dif~usion layer allows of a greater fle~ibility than with metal foil o~ygen diffusion resistant layers and allows a close match of cosfficients of linear e~p~nsion by virtue of the s1ight elasticity of said oxygen dif~u~io~
re~istant layer.
Tube~ of this in~ention have a materially improvod long term structNral inteerity when utilizea in centIal heat1n~ in3tallatioD~.

Claims (8)

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

1, A coextruded gaseous diffusion resistant tube of high dimensional stability comprising: a liquid resistant core and an impact resistant outer layer both formed of a polymeric material having substantially identical coefficients of linear expansion and a continuous gaseous diffusion resistant intermediate layer formed of an adhesive polymeric material interposed therebetween, said adhesive material having a coefficient of linear expansion within the limits of its elasticity over the temperature range of intended use thereby to prevent delamination in response to heat cycling.

2. A tube according to claim 1 wherein the core at least is made of a cross-linked polyolefinic material.

3. A tube according to claim 2 wherein the outer layer is also made of a cross-linked polyolefinic material.

4. A tube according to either of claims 2 or 3 wherein the cross-linked polyolefinic material is cross-linked polyethylene.

5. A tube according to claims 1 to 3 wherein the intermediate layer is selected from polyvinyl-alcohol, polyacrylonitrile, or polyvinylidene chloride.

6. A tube according to any one of claims 1 to 3 wherein the intermediate layer has a thickness of between 60 µ and 10µ.

7. A method for the production of a tube according to claim 1 which comprises sequentially co-extruding the core layer, the intermediate layer and the outer impact resistant layer in coaxial array such that said layers are fused together at their interfaces.

8. A central heating system incorporating a tube as claimed in any one of claims 1 or 2 or made by the method of claim 7.