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Halogen-free flame retardant plastics

Grades of polypropylene homopolymer and copolymer which are flame retardant, yet totally halogen-free, have been developed by Plascoat Systems. These are injection mould- ing grades offering non-drip self-extinguishing performance to UL94 V-O level.

Conventional FR polypropylenes are based on bromine or Chlorine-containing additives with antimony trioxide, which emit quantities ot'dense, toxic smoke when they are burned. This smoke is corrosive and therefore can cause severe damage, particularly to electrical installations and building

structures. In addition to much lower levels of smoke, which is non-

corrosive, the company claims better physical properties and a greater scope for pigmentation. Applications where halogen-free grades will be particularly relevant are trans- port systems (eg aircraft fittings, underground railway fittings), domestic appliances and eleetronic equipment.

To illustrate the physical properties and flame retardaney performances the following table compares properties of:

FRP 3001 Halogen-free retardant polypropylene homo- polymer A typical polypropylene homopolymer

FRP 3002 Halogen free flame retardant polypropylene copolymer A typical polypropylene copolymer A typical brominated flame retardant poly- propylene

Property

1.0 Tensile Yield Strength

(MN/m 2)

Elongation at Break (%) Notched Izod Impact

Strength (J/m) Limiting Oxygen Index UL94 Vertical Burn

Test at 3.2mm

FRP 3001 Halogen-free

PP Homopolymer 0.91 31

>200

39 32

VO

PP Homopolymer

FRP 3002 Halogen-free

PP Copolymer 0.91

22

>200

62

PP Copolymer

1.0 35

30

54 18

VO

1.35 25

>200

87

Bromine plus antimony based flame retardant

19

30

43 28

VO

PEEK grade launched for bearings

ICI has launched a new grade of its high performance polymer Victrex PEEK specially designed for bearing applications. It has been specially formulated to optimise the essential mechanical properties, particularly friction and wear, together with ease of processing. Designed for use in bushes, bearing cages, cams and rings, PEEK KX3 is also

being evaluated for under bonnet applications, such as piston scraper rings and skirts.

All bearing materials must have a suitable combination of properties relating to friction, wear and limiting PV (applied pressure x surface velocity). A low coefficient of friction is required, which varies with applied load, velocity and temperature. PEEK KX3 has a lower eoemeient of friction than porous bronze with graphite, resin-impregnated carbon and polyimide with graphite.

In plastic bearings, the primary wear mechanism is adhesive wear, charaeterised by fine particles of polymer being removed from the surface. PEEK KX3 has an extremely low wear factor and extremely high values of limiting PV.

Designing seals by computer

The development of a computer modelling system to predict seal performance is intended to enable Dowty Seals

to minimise the trial and error factor in seal design. The project has already achieved success in verifying the theoretical principles involved and is leading to a deeper understanding of the way that seal geometry affects per- formance under varying service conditions.

The foundation of the modelling system is finite element analysis, which investigates the behaviour of a material under stress by reducing its shape to an interlocking grid of

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small elements, each of which exhibits known and predict- able characteristics.

By using well-proven mathematical formulae, these known values can then be computed against variables such as variable stress to determine the performance of a given design under different operating conditions.

Thus far, the model, which is believed to be the first attempt to apply these concepts to the performance of

elastomeric seals, has produced results which are very close to those obtained by purely empirical test methods. Dowty is now looking at extending the work to include the evaluation of dynamic and temperature conditions.

The software used in this exercise is completely original, tailored specifically to the properties of sealing materials and the package is run in conjunction with the Prime-based CAD facility already available.

GRP boat-building without moulds

A new system for building GRP boats allows hulls to be produced without using moulds - which cost more to build than an actual hull- and is primarily intended to enable yards to produce 'one off' boats more profitably. The system is also cost-effective for production runs of up to 25 off.

Hultec hulls are built up on a frame of contour formers and H-section pultruded GRP battens. Glassfibre reinforcement, in the form of a new non-woven roving material that will not sag when applied, is then wetted out with polyester resin and laid up over the outer surface of the frame.

A remarkable degree of fairness is achieved and, therefore, subsequent reinforcement is able to be laid up on the inner surface of the hull. This minimises the amount of finishing work required on the outer skin.

Even before the inner skin is laminated, the hull is extremely rigid and strong enough to support the weight of several men. This means that there are no problems when handling semi-finished boats - in contrast to the relatively flimsy state of other types of no-mould hulls at a comparable stage of construction.

About 600 workboat and leisure craft designs are offered with the system by its creators, Farbond and Pultrex. Boatbuilders will be offered package deals consisting of pultrusion machinery, BIP Beetle polyester resin, glass fibre, tools, drawings and technical know-how.

Solid state hydrogen moni tor

Hydrogen monitoring is important, particularly in steel structures, since hydrogen is a primary product of the corrosion process. Free atomic hydrogen then permeates steel, where it can cause serious problems - blistering embrittlement and hydrogen-inducing cracking.

Existing hydrogen monitors include those containing liquid electrolyte, in the form of concentrated acid, or probes which rely on sacrificial corrosion to allow hydrogen to diffuse into the device prior to measurement.

As an alternative, Cormon has introduced a device based on a solid electrolyte, plus a working and reference electrode. The device gives an absolute measurement of hydrogen

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