682 CIIEAIISTRY AND INDUSTRY Jiily 24, l!l%

bituminous surfacing of one of tlie iuiiny types now available is superimposed on tlic concrctc. 111 mising concretes for road work the very large qu:tntitics of materials required, the length of tinic it takes to carry out the work, iind the variability of the British cliniatc all make i t extrcmely difficult to keep sufficiently rigid control of concrete niixtures. “ Especially is this the ciise with tthe amount of water to be used and this is of serious iniportancc as the struigtli of tlic resulting concrete is in no small nieasurc directly tlcpciidciit up011 it. With large stacks of siiiid and 1)rokcn stone it will be clear t,hnt, with chniigiiig writher the amount of water in t,he materid will w r y from day to day aiid hence the amount of water required to be :idded to make the correct mixture of coiicrete.” To meet this eerious practical difficulty s machine has been devised which, by iiicans of vibriitions, can bring the s t i d and stone to a constant water content before inixing. Thus, t,he report states, anot,her step forward has beeii made t o meet the demand for consi&ciitf, quality in road construction.

Turning to bituminous materials, the report points out that t,hese arc defornicd perniiinently under load and the amount of deformation depcnds upon the tinic of application of such load. Thc tendency to deformat.ion can be altered by varying both the viscosity (or hardncss) of the binder arid the griidirig of the irggregrite. 111 some types of bit~uniinous constructioii, crricks which lire foriiictl a t Ion- temperatures (~vl icn the binder is brittle uritlcr impact) hcal themselves in wiirni weather. Heating of the binder if necessary prior to iiiixing iiiay alter its characteristics. Should water be allo~ved to rericli aggregate which is not coniplctely coated with bitu- minous binder, disiiitcgrat,ioii ensues. It follows from the above t,hat load, tinic, teiiiperature, and other atmospheric influences (such iis rain) have an iinportnnt bearing on the design of bitriiiiinous coilstructions aiitl that the interaction of t,licse, one 011 the other, is complex.

The crushing strcngth of concrete ciiii be tirkeii 11s 11 useful criterion of its \ d u e ; the highcr tlic crushing strength, t.hc better the concrete. With bituminous construction, however, a high strength a t an arbitrary rate of loading and teinpernture may riot give a snt.is- factory road “ life.”

Attempts htrvc been made in the past t o find the relative life of different f o r m of bituminous construction from full-scale esperinicntal sectioiis of roads, but definite iiiforniation froill s m h tests is difficult t o obtain irnd occupies ni:iny years. The aim of the many labor&- tory investigations being carried out is to overcome this difficulty. But if the results of small-scale laboratory tests are to be correlatcd with full-scale road tests, the progress must of necessity be very slow. The alternative is to accelerate road conditions on some type of roiid- teatiug inacthe.

Three such maciiines :ire ill use at the Road Research Laboratory, which approximate in various degree8 to actual road conditions. Road machine No. 1 represents the smallest machine with which a full-sized pneumatic tyrc ciin be used for the tests. The track is some 5 ft. 6 in. in clinnieter, and the machine is novel in that the road ’’ itself is revolved under the wheel, whose axis is stcitioiiiiry. In No. 2 machine the track diameter is 38 ft., and the test road can be laid under normal rond conditions. The test with this machine is carried out with full-sized tyres uiicler their rated loads. Road machine No. 3 is the largest and approximates nearest to actual road conditions. With i t a full-sized lorry ie employed running a t its niuxiiiium speed and the road is again laid under noriual conditions. The nieaii diameter of the track is 110 ft., and the lorry is driven round this electrically, beiiig attached to a central post by a heavy steel girder. The use of road mechine No. 1, which is the rcniotcst from actual road conditions, in conjuiictiou with liiboriitory tests, should cliriiiiititc certain undesirable factors in roiitl coiistructioii arid narrow do . the field of suitable mixtures for surfacings or indicat 4rablc iiltcrcitions in thc methods of surface dressin d+ As a result of tliis work, tests 011 selected surfiiciiigs or thiu surfacings ore ciirried out in road machine No. 2, and lifter this further weeding out full- scale triiils are iiiadc with road iiiiicliiiie No. 3. With this ~nac l ine investigations arc bcing carried out on rcceut types of thin surfacing coats in which the prc- nlixed niiiterials arc laid to a tliickiicss of 2 in. to 1 in. This niethod of construction is iipplicablc to roads in which the surfacing coatiug is in good condition except for the development of unevenness. It is hoped that by its use frcquent surface dressing will be cliinioiited.

STYRENE AND ACRY.LIC RESINS By A. RENFREW and DR. A. CARESS

Research Chemist and Research Manager respectively of Messrs. Mouldrite, Ltd., Billingham, co. Durham

(Paper discussed at the Symposium on Plastics held on January 29, 1937)

INTRODU$TION The styrene rind acrylic resilis: pepresent two iinportant

typerc of polyiiicrs whiclr criii be prepared by the hiking up in straight chains of siini)le iiioiioincrs. Styrcllc is B hydrocarbon, phcnyl etliylenc, and hila iiiucl~ in C O I I ~ I I ~ O I I with simpler alipliat ic iiiiRa$uruted hydrocarbons YucIl as butadicnc and nicthyl butadicne, the iiionoiiier of 011e of the most important synthetic rubber$. ‘l’he

acrylic resin nioiioiiiers, on the other liaiid, are esters. T h y iiiiry be rcgiirclcd 11s derivirtives of unsaturated acids, wliereas the otlicr important cliiss of polyincrizablo esters, the vinyl coiiipouiids, may be regarded as dcrivativcs of 1111 unsiituratcd nlcoliol.

Styrene iuid ricrylic resins h i v e in comiiioii unusual transprirency to liglit, tlicriiioplosticity, iiiid tlic property of rcwrting to t Iic I I ~ O I I ~ I I I P ~ ~ ( : foriii \vlirn tlestnictivcly

distilled. Tlic only iiiiportaiit iiicrnhcr of the styreitc or aroiiint,ic I~ydrocnrboii class is polystyrciic itself :iltliough polycyclic rind licterocyclic aiialogiies tire known. Acrylic rcsiiis Itiivc two iniiiii reprcsent:iti\w, polynict~ltyl acryIiit,e iind polyiiiethyl nictli:iciyli\te, :ilt.liough ninny higlicr Iioinologues exist and :ire of potential technical iinportniice. Of the plilstics iticii- t ioned, oidy polynwtliyl nictliacrylate is ~ n n ~ i ~ f i ~ c t ~ e d iri tliis country whilst po1ystyrcnc and polymethyl itcry- late iire iinportcd froin Ckmiiiiiiy. In thc short time i t t in>- disposal I propose to outline t h i r iipplictitioiis, properties, and probiiblc dcvclopiiieiit mid to rcfcr to rtiet hods of production, wit.11 speciiil reference to t,lic riI\V innterinls a t present used :tnd capiiblc of hii ig used i n tiiiics of nat~ionnl cnicrgciicy.

THE POLYMERIZATION REACTION 'I'he polyiiierizittion reiictioii in t,ltc styrelic cult1 itcrylic

groups dcpen~ls on t,hc opening of the doul)lc bonds in the rnoleculcs ;ind in the forriiiitioii of a large nunihcr of cnrboii t o c:irboii liiikiipcs. Not ;ill bodies coiitniniiig tloublc boiids ciin IJC polyincrizctl to resiiis, e.!g., fi-inethyl styrene iiiid isopropeiiyl 1)eiizciic both yicltl polyriicra wit,horit resinous cliiiriicteristics. Siiiiiliirly tlic crotoiiic :icitl est cry, ~vhicli iirc isomeric with t lie corresponding a-iiict~hyl (or iiicr hiicrylic) acid esters, :ml t.hc a-nlkyl itcrylic acid esters wlicrc tlic iilkyl got ip coiit:iins :it Iciist two c;irI)oti iitoiiis, :ire \rit~Iioiit interest as resin fnrrniiig hotlit#.<.

PREPARATION OF MONOMERS One of t l ie itiost. sntisfiictory tiietliods of ptpiirilig

styrene iiio~ioiner is by the cracking of etliyl benzene wliich can itsrlf he prepired f'roiii bciizcnc and etliyleiie by ii kind of Frictlel-Crafts rciict.ioii. In nnotlicr proccss the ct,liyl lwizeiw is first chloriniitcd and t,licn t rci\tctl to remove t Ilc c~lcrt~cnts of' Iiytlrochloric iicitl. 'J'lic csscnt.iiil riiw i n i i t eriiils, 1)ciizciic i111d ethylene, iirc readily

:ilcoliol, itsclf i t fcriiieiitiitioii product; of ~nol:isscs. IStllyl :~lcoliol is :it the ])resent time $1 very iinportniit, raw inittwinl for syiit.hrsis in the alipliat ic orgiiuic field :tiit1 I I ~ I S oh ious ;itl\-:l~ttc~ges wlicrc 11 s y i i t h i s from coiil is long iiiid difticult . St.yrciic has, t,liercforc, ii rcitsoilclbly souiitl riiw iniitcriiil Ijiisis, but, ti coal biisirj woulti be ~)ref'~riil)lv iu tirnc of w i r \vIicii supplies of ~noliisses might, be tlifiicult. t o oltiiin. T h e possibility of producing cthylciic i i i ot,hcr wiiys cxists : it is present in oil criickiiig giis iind tliere is ii liirgc totiil n1i10111lt in C O ~ C O V W ~ gns, h i t , like gold in suit water, in i i riitlicr tlilhtc forin. >let 11yI iicryhtc eintl mctliyl Incthncrylittc wliicli IVC Iiiivc :ilrciitly rnentioiicd as tlic two inost intportiiiit. iiiotioniws of t lie iicrplic rcsiiis iirc both econoiniciilly iiiiidc Iiy t lie siintiltitiicoiis dcliydriitioii nut1 Iiytlroly~is of Iiytlrosy nitrites. T h u s nicthyl ncryliitc nntl ~iietlipl i i w t hitcryliitc ;ire produced by tlic nctioii of mctliunol i i n d sull~liirric iicid 0 1 1 cthylciio cyiintiydriir iiiid iicctonc c~ycinliptlriii rcspectii\dp :-

ii\~iliiI~le, IwIIzciw froui co:tl-t:tr tit111 etliylcl1c fro111 ethyl

('I r2m m 2 c x + cxr,oir 4- i.r2so., .--5 C N , : CH.COOCl-F, 4- NII,llSO,

nlctlly1 ilcryliitc ( 4 1 i,qoi-r)(cs) m, -t cir:,oii + i.r2so., -+ cir, = c.(cir,ycoocir, + istr,irso,

I I I P I 11y1 iiic't Iiiicrylitte

l<thylene cynnhyclriii can he prepared froiii sodium cyitnirle and cthyleiie chlorhydriii wliich is formed directly from cthyleiic. dcctoiie cy:inhydrin is best made froni iicctolle and hydrocyaiiic acid. Both ethplenc inid i m t o n c can be inadc froin ethyl alcohol.

Of the four esseiitial raw Inatcrials : inethanol, hydrocyaiiic acid, acctoiie, aiid ethylene, tlic last-named has idready been dcd t with. Methanol is already iniidc from coal by tlic ~ ~ d I - k ~ i o \ ~ i i pressure synthesis from hydrogen aiid carbon iiioiiosidc cxt racted froin water-gas. Hydrocyaiiic acid is made froin nininonia, itself synt,Iiesizcd from the nitrogen of thc air, and hydrogen, agiiin , obtained froin cod as thc ultiinate raw mriterial. Acetone c : ~ be iiinde vin ethyl alcohol froiii ~iiolasscs, hut if there should be a shortage of rilw materials, it could bc ~iinde hy other methods I)iiscd cveutually on coal, q., froin the propylene iu oil criicking gases.

PREPARATION OF POLYMERS Castiiig is tlic eiirliest nictliod of polyiiieriaatioii and

in its crritlc forin the easiest. Sinioii cast styrene in 1839 iriid Caspary i i n d Tolleiis cast ally1 acrylntc iu 1873. l'urc inonoiiicrs can gciicrally be cast to clear resinous iniisses by iiicrely allowing to stand in sunlight. 1 he Iibsencc of rctlucing agents and t,hc preseiice of oxygen or of sortie substaiicc wliicli liberates it readily iire csscni in1 in low t cinperittiire polynieriaation. IIeiitiiig cilways i t c c c l ~ r a t ~ ~ polyiiicriziit ion, but iiicreiiscs the tendency to bubblc foriiiatioii wliich iiiust be supprcsscd in the coninicrcial production of cast polymer.

Polyiner powders suitable for inouliliiig can be riiade by ii variety of methods :

(1) Breiiking up ciist. sheets. (2) Polyinerixatioxi i i i solution follo\~*t!tt hy reilioviil

(3) Polynieriznt ioii i n i~qiicoiis ciiiiilsioii followed IJY precipitiitioii.

(4) Polynieriaiitioii in the forin of liirge glol~ulcs suspciidcd in water.

(5) l'olyincrixiit ion in solvcnt for inoiionicr, but not for polyriicr. Tl~c inct,liod c~nploycd dcpeiids not oiily on cost coiisi- tlcrotions, but idso on t,lic ~iionoiner being used iinci on t,lic properties required in t,hc fiiiislicd polynicr. Solutioii ~~olynicriziit ion is ~)iirticularly c o n ~ ~ c ~ i i e ~ i t for the prepanit ioii of dopes iinil is carried out by dissolving t hc inoiio~ncr iii it suitablc sol\vmt and hcating generally under reflux iiiitl in the prcsciicc of a dissol\.cd catalyst. Emulsion polyiiicrixiit ion is cnrricd oiit by heat,ing well sf irred ~nistiircs of writer nut1 tno~~onicr in tlic presence of cmulsifyiiig tigents sucli iis ~~~iiiiioiiiuin olciitc. By t,liis incthod dispersions coutainiiig 60% of polynicr iind capiiblc of yielding clew filnis 011 cvirporiitioii of the wittcr ciiii be obtiiined. Thc triinspreiicy of poly- st.yrcnc is oxcclleiit i i i d its wiiter thorp t ion is almost zero, i i property wliicli (list i~igitislics it froin id1 other rcsiiis.

PROPERTIES OF THE POLYMERS

,l

of the solvent iintl disiiitcgr a t' 1011.

!rlic propcrtics of polystyrene in its wrious C O I ~ I I I C P - ciiil forins arc \rcll knowii. Thcy c ~ i be mouldcd either by iiijcci.ioii or compression to articles which, although rittlivr brittle iiiitl l iei l~lo to criize, posscss olcctrical

* . properties unequalled by any otlier resln iind liave good form stability. Little is kno\vn of tlic propcrties of tlic various heterocyclic nnd polycyclic annlogucs. ’J’hc various acrylic resins nrc better lilio\vn and i1rc niorc susceptiblc to clnssificntion siricc they form two homo- logous scries, thc polyacryliites and the polymctlia- crylntcs. As \vc risccnd each series the polyniers bccon~c softer and softer nnd finally censc to be solids. Thus the polylauryl esters of iicrylic iind mcthacrylic acid are syrupy liqnids. Polyniethiicrylic ncid esters arc invariably harder t hrin the corresponding polyiicrylic acid esters.

When we pass from the polyacrylic and met hncrylic esters of nionohydric nlcohols to those of polyhydric alcohols a marked chnngc in properties takes plncc. Solubility, clarity, and mouldnbility disnppenr, the products being infusible, opnqnc bodies nisolublc in organic solvents.

The general properties of the comnierciiilly ri\Tailablc polyacrylic and nicthacrylic acid esters iirc nlso well known. Polymet liyl acrylatc is distinguislied by its remarkable cliisticity iind toughness in film form. ‘l‘he softening point is, tiowever, low and there is no present likelihood of the polymer heing used ns n rohbcr sub- stitute or in the moulding industry. In cliirity iind light stability polymethyl ncrylntc is outstiindingly good. Polymcthyl nicthncrylnte is rcmtirkablc not only for its ext raordinnry opt icril properties, which we have alrendy mentioned its ii feiiturc coninion to the styrene and acrylic resins, but, d s o for its nicchnnical strength and its stn1)ility at ill1 tempcratures i ind hi~niiclities likely to be experienced nndcr the most ndversc climatic conditions. Thc polymer moulds \veil eit her by injection or compression. At its softening point it is much less fluid than polyst yrcnc iind, therefore, requires higher injection 1)re5snrcs.

APPLICATIONS Some of the appliciitions have been touched on in the

preceding discussion. Polystyrene finds niriny electrical uses because of its low power factor aud ease of injection. The softer acrylic resins iire used extensively in the textile industry and as Iiiniinntcd glciss intcrlayers. Thc harder oiies, in prticular, polymcthyl ~ncthcicrylritc, are finding inc rcahg I I S ~ S not only in thc forin of light- weight transparent sheets, but also, in thc form of lacquers and of inoulding powders, for tlic prcpiiriit ion of strong clear or colourcd iirticlcs of great, bciiuty. Tlic use of styrene and acrylic resins in cmulsion form is rapidly increasing. Although polyacrylic iicid ester emulsions are a t present most iniportiint polymetliacrylic acid ester emulsions iirc entering the field.

FUTURE DEVELOPMENTS The greatest disparity bctwecn the thcrniopl~istic

styrene and acrylic resins on t hc one lirind r ind the thcrniohnrdcning phenol rind urea-foriniildcliydcs on the other is iit present in cost of production. As we havc seen, 11o\vcvcr, they iirc a11 cripiiblc of bcing niiide from one of the chenpst rind most iibundunL of riiw materialr;, namely cod. The mmc cincrgeiicy which might accelcrntc tlic use of coiil iis fi basic riiw niiitcriiil and encourage the clevclopiiieiit of olternntivc Rynt hcsetl

for the niononicrs, would nndoubtetlly hasten tlic dcvelopnicnt of ncw npplici~tions .for thcsc resins, as, for instance, tlic IISC of tmnsparcnt plastics for acroplme windows nnd hoodings. Both factors-higher output, rind cheaper ultimatc mw niaterials--will rciict in the same \vny to lower costs of production, nnd although the complicated nature of the synthesis of styrcuc and iicrylic resins from coal will prevcnt costs cvcr bcconiing cst.ruinely low, thcy will undoubtedly reach n Ic \d at which in thc Ileilr fiiturc our clot,hcs, radio ant1 tclcvision sets, telephones and inany other of our Iionsc- hold requircnicnts will makc frec iisc of this class of t hernioplast in.

ELECTROSTATIC DUST SEPARATION TWO NOTABLE P O W E R STATION PLANTS

Further inforination has been madc availablc conceni- ing the two I‘ Lodge-Cottrell ” electrostatic plants now operating a t l h 1 s Hnll and Princes Street (Nechells) power stations (Uirniinghnm Corporiition) for thc removal of dust from the chimney gnses of pulvcriscd coal ficd boilers. This serious probleni has bccii successfully solved, and the remarkable guartrnteed dust contcnt of not niore than 0.10 grains per cubic foot in the gnscs, as dischnrged, hns been iniprovcrl upon in practice.

Hiinis I in l l now lins two I:irgc ncw brick chinincys 350 ft.. high by 20 ft. internal diumctcr a t tho top, ench \\~eighing 5200 tons, and one of these tnkcs all the combustion gases from fivc piil\wised fuel fired boilers after the dust has been scparat,ed in the precipitators.

!J!he clectrostntic plnnt, which is the lnrgest in Great Britiiin, supplied, as n t Ncchells, by Lodge Cottrell, Ltd., of Birmingham rind Lolidon (Bush Iioiise, W.C.2), comprises eight horizontal reinforced concrcte ‘. trcittcr ” chambers, operating in pnrallcl, each with its own inlct and outlet shut-off danipers, through which tho com- bustion gases pass” from tho boilers to the el!imney Eucli ‘ l treater ” hiis four baiiks of ciirthed electrodes of the firm’s pntent rod-curtain type, designed specially for free expansion cind cnsy rcniovnl of tho dust. Tho electrodes on which tlic dusl; is deposited cousist of long vertical rods closcly q m c d together, forming curtains hung in thc direction of tlic giis f!ow, allowing a clear unobstruct.cd passiigc. Between tliesc rows of

collecting ” electrodes nrc l1111ig ccntrully heavy t,wistcd squnrc section “ dischnrgc ” electrodes, niiiin- tnincd iit. ii high potcntinl, ribout, 50,000 volts.

!rhc (lust particles in tlic giis rcccivc i i iicgtitivc chiirgc and iirc rit oncc prccipitritcd on t.lic collecting rod curtains, froin wliich they tire continuously rcniovcd by nieiitis of ciutomiit.ically o~~erriting mecliitnicnl rapping gcnr, fornietl of spring-loiitlcd hiininiers opcrritccl by coinr;Iirifts, t,lic dust; fnlliiig into hoppers froni wliich it is rcinovcd by con~wyors. For supplying tlic liigli tciisioii currc~it~ six trnnsforincr-rectifier iiiiits are iiist iilled, Iiou~cd in it separiite control room, with t,lic nccessiiiy switch gew, riutoiiiiith devices, iind int,ru- mc11ts.

11 I W ~ large inild steel duct, up to 19 ft. dianietcr, conicys the combustion gases from the roof of the