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PLASTICS FOR TANKERS

14th March 1958, Page 96
14th March 1958
Page 96
Page 99
Page 96, 14th March 1958 — PLASTICS FOR TANKERS
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Design Points to Watch

By a Special Correspondent

Modest Future for Polyester-Glass Fibre, But Greater Possibilities Offered by Epoxide Resins I.N any discussion on plastics as structural materials for road tankers, glass fibre reinforced polyesters come first to mind because of their already extensive use in general body-building. It is therefore proposed to review the broad prospects of resin-fibre laminates for this application before dealing with specific points about ' construction. •

The numerous industrial applications to which reinforced plastics have been put during the past decade cover a wide band in the spectrum of success and failure. One field which has produced a mixed crop of results is that of chemical-plant construction. It was at one time fashionable to include good corrosion resistance in the list of the outstanding properties of plastics. 'So far as freedom from rusting is concerned, the claim is unquestionable, but beyond this the implication can mislead.

Sometimes a Liability

Referring particularly to polyesterglass laminates, it may be nearer the truth to say that in chemical-plant work the performance of these materials under conditions of chemical attack is sometimes• a liability rather than an asset. The chemical engineer's standards of corrosion resistance may well be somewhat higher than those of the plastics technologist..

Furthermore, many of the early claims were made on the basis of relatively short exposure trials, in which the important effects of mechanical stress during exposure were not always considered. It must be admitted, however, that the current tendency, both here and abroad, is to temper enthusiasm with experience.

This, together with some limited but definite advances in polyester chemistry and laminating techniques, and an increasing interest in the use of epoxide resins, has redressed the balance somewhat. Even so, it must remain a general rule that any unfamiliar material should be subjected to a trial under comparable conditions n26 before being committed to a full-scale application under conditions Of stress and chemical exposure..

These considerations are important in the use of plastics for road tankers. Whether the primary •consideration is the effect of the tank material upon. the contents—milk, beer, critically pure chemicals and so on—or vice versa, the case for using reinforced plastics depends primarily upon their having economic and functional advantages over more traditional materials.

In chemical-plant . work, certain-. other plastics have met with more success. Polythene, p.v.c., rubber. and

• neoprene are well established for .piping, ducting and tank lining under the most arduous chemical conditions, particularly where corrosive inorganics are concerned. It would not be fair to omit reference to the 'phenolic compositions and lacquers which have long enjoyed widespread use in chemical plant.

These materials, however, do not generally rank as. structural materials in their own right for heavy-duty tank work. They either lack the strength and rigidity necessary for unsupported use in economically thin gauges, or cannot be made cheaply • into large curved shapes. • More interesting • are the possibilities of combining these highly resistant plastics with glass fibre polyester compositions, .so that the. resin-fibre component supplies the mechanical strength and a surfacing layer of thermoplastic the chemical protection.

The similarity to the established • techniques of glass-lining and rubberlining steel tanks will readily be seen.' Mention must, of course, be made of the recently announced success in the lamination of steel sheet with p.v.c., giving a combination of properties particularly valuable in tanker work.

• Turning now to the constructional aspects of tank building in plastics, it is tempting to envisage the fabrication of large shells by hand lay-up, these being subsequentlycombined, by resin-jointing, to give a " monolithic " structure, or by bolting around a single flanged joint. :The one-piece lay-up preferred for boatbuilding cannot, of course, be used for totally enclosed structures like road tanks.

Male moulds are to be preferred as they give a better surface finish to the interior; this is, of course,-the reverse of normal bodybuilding practice. . A • further reason for their use is the preference for stiffening members to be on the outside rather than the inside of the tank to minimize any tendency • for the wall element to flex away from the rib when under load.

Ribbing Indispensable The use of these reinforced ribs, , which may be of metal, wood or resinfibre composition, is standard practice in boatbuilding and is frequently met with in vehicle bodywork. In road tanks, built to contain many tons of liquid, carefully designed, ribbing, securely incorporated in the wall structure; is indispensable unless the basic wall thickness is to be increased beyond economic limits.

One method of construction, which not only provides a high degree of rigidity but may offer improvements in other respects, involves the use of standard sectional components for the sidewall, in conjunction with one-piece or sectional ends.

Sections of this type, bolted together . through drilled holes along external flanges, have proved to be a satisfactory arrangement for a. number of static tanks. They are especially suited to tanks of circular cross-section, for only one standard form is then required for the sidewall. A segment of the tank body r-ay be composed of from two or more sections, depending upon the diameter and the ribbing.

Each segment is staggered by half a section relative to its neighbours, -so that not mare than three sections Meet at any one point. Sectional construe.tion of oval tanks is less sinaple; requiring, up_ to three types of sidewall section for the best effect.

In designing a one-piece domed end, it is desirable to continue the sidewall cross-section a short distance before blending into the domed section. This helps to minimize any parting of the end-joint on outward flexure of the dome. A layer or gasket of some flexible composition, of which rubber, neoprene and p.v.c. are good examples, is desirable on all .mating surfaces to give a leak-proof seal.

Benefits of Sections Further advantages 'of. the sectional Method over the large-shell lay-up' are the relative ease of maintaining consistent laminate quality, economy in mould cost and in shop space, and the possibility of using replacement sections in repairing damage. This principle also Makes possible the use of hot-cured laminates from matched tools (in runs of suitable length) with consequent improvement in strength.

For the attachment of the tank to the vehicle, it may be found convenient to .employ a steel cradle integral with the chassis. This may be used in conjunction with over-strapping, or be bolted 'directly to stiffening members or to specially incorporated lugs. .

Multiple-compartment tanks present special problems in resin-fibre techniques, as the bulkheads or partitions are exposed to the contents on both sides. This limits the serviceability of hand lay-up components and generally rules out the use of bolted-section pari-itions. The one-piece partitions may require stiffening members incorporated in the laminate and should, in any case, be located at the junction of segments.

Pipework connections and hatches may either be incorporated during layup or fitted as separate components to the sections before assembly. There are indications that the use of a glasssurfacing tissue nearest the mould-face, giving a resin-rich skin, improves the chemical resistance of the laminate. Use of synthetic-fibre. Surfacing cloths may give similar improvements.

For the body of the laminate, there

is an advantage in using sandwich constructions, consisting of external layers of high-performance cloth, with cheaper, bulkier chopped-strand mat or -needleloom mat in between. Such combinations give better flexural strength and rigidity for a given material cost than does either type of reinforcement alone.

Only a grade of polyester recornmended for chennical exposure work should be employed, and the use of fillers, extenders and " flexible " resins avoided. Hand lay-up work should be post-cured by oven or radiant heating for the best mechanical and chemical properties. All -raw edges from cutting and drilling operations should be sealed by painting with resin.

The principal emphasis in this brief review has fallen naturally on polyester-glass fibre systems, and within the intrinsic limitations of these materials a modest future may be predicted for them in road tanker work. The use of epoxide resins, in spite of their ,present cost, will extend the range of application of resin-fibre techniques, because of their superior chemical properties.

Perhaps the most promising future in this type of work may ultimately be found in the combination, by laminating or post-coating, of highstrength structural materials with those plastics already shown by experience to have useful chemical resistance.

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People: Modest Future

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