Introduction

Together with wire and cable companies, resin and additive manufacturers must respond to emerging environmental, health, and safety issues with raw material and product innovations. The high rate of new product introductions and the significant amount of research and development on substitute materials bear testimony to the sector’s commitment to meeting the demand for “greener” wire and cable products. Even so, the wire and cable industry does make use of some questionable materials, several of which have drawn European Union attention and calls for use restrictions and outright bans. Among these are heavy metal derivatives and halogenated flame-retardant systems. Research into alternative flame retardants and a growing focus on alternatives to PVC have led to an increase in the use of polyethylene (particularly crosslinked) in some applications for which high thermomechanical qualities are required.

Silane pre-grafted crosslinkable compounds

The use of non-halogen flame-retardant thermoplastic and crosslinkable compounds for insulation and jacketing in flexible cords, appliance wires, building wires, and others is widespread in Europe and elsewhere.

Traditionally responsive to environmental health and safety issues, the Italian company Padanaplast SpA has developed a series of HFFR compounds. Of particular interest are the company’s silane pre-grafted crosslinkable compounds. Current work is focused on a new grade of crosslinkable silane pre-grafted HFFR compound for wire insulation in applications stipulating low emission of smoke and corrosive gas. CENELEC HD 22.1 and HD 22.9 require this material to pass vertical flame test IEC 60332-1 before it can be approved for H07Z-U/K cable construction. Good processability, high production speed rates, and self-curing in ambient conditions are essential properties for low-cost industrial implementation. These are, of course, in addition to the standard requirements. Drawing on the experience acquired with its Sioplas method crosslinking system, Padanaplast has made a determined effort to meet them all.

Materials described find typical application in single-wire cables for high-safety public building wiring

Studies carried out in the company’s laboratory led to a new system consisting of the pre-grafted compound Cogegum GFR/325 used together with catalyst CT/2 masterbatch. Cogegum GFR/315 and Catalyst CT/1 had already enjoyed many years of market success. Now, the new system offers an attractive solution for cable manufacturers who wish to enter the XL-HFFR market.

Laboratory rheological characterisation

The systems Cogegum GFR/325 + catalyst CT/2 and GFR/315 + catalyst CT/1 were submitted to a laboratory rheological characterisation in order to gather evidence on flowing and processing differences. In both cases the ratio 97:3 between grafted compound and catalyst masterbatch was used.

Tests were performed on this apparatus:

• 400 cm3 internal mixer equipped with roller rotors and melt temperature gauge;
• Single screw capillary extruder equipped with 1/10mm die, and melt temperature and pressure gauges.

Actual processing conditions were simulated, permitting exploration of the wide range of shear rates and mechanical stresses applied to material during processing.

Tests with internal mixer

Tests with internal mixer were carried out, setting 160°C as mantel temperature and chamber loading volume of 85%. Rotor speed was fixed at 20 rpm. Test duration was 15 minutes for estimating the torque differences between the two systems and to allow steady conditions for reaching the melt temperature. Comparison of the curves in Figure 1 reveals that the GFR/315+CT/1 system shows a torque about 30% higher than the GFR/325+CT/2 system when the minimum of the curve is reached. Besides the torque reduction, a melt temperature reduction also occurs (Figure 2).

Figure 1: Cogegum GFR/315 + CT/1 versus Cogegum GFR/325 + CT/2: torque

Figure 2: Cogegum GFR/315 + CT/1 versus Cogegum GFR/325 + CT/2: melt temperature

A practical consequence of the reduced friction and related overheating during compound processing is the reduction of prescorching phenomena which may occur due to the long period in the extruder at high temperature. Taking as a reference point the minimum of the torque curves, an increase of 10% occurs after two minutes for GFR/315+CT/1 and after 4.5 minutes for GFR/325+CT/2.

Tests with capillary extruder

Thermal profile was set at 160°C on the barrel and crosshead/die zone. Screw speed varied from 10 to 200 rpm. Figure 3 shows shear stress versus shear rate for both systems. Shear rate is imposed by the screw rotation speed. The resulting force, which depends on material viscosity, is expressed in the shear stress.

Figure 3: Cogegum GFR/315 + CT/1 versus Cogegum GFR/325 + CT/2: shear stress versus shear rate

GFR/325+CT/2 also demonstrates a considerable shear stress reduction vis-à-vis GFR/315+CT/1 over the entire shear rate range investigated. In the same way, melt pressure measured just before capillary (Figure 4) is greatly reduced. Both results are confirmed in industrial extrusion practice, where improved behaviour of the GFR/325+CT/2 system is observed even if the machinery has not been adapted for handling HFFR compounds.

Figure 4: Cogegum GFR/315 + CT/1 versus Cogegum GFR/325 + CT/2: melt pressure versus shear rate

Conclusion

Having acquired wide experience in the manufacture of silane crosslinkable HFFR compounds, the Italian company Padanaplast SpA committed itself to making these products available to the largest number of cable manufacturers. The introduction on the market of the new system Cogegum GFR/325 + catalyst CT/2 goes far toward fulfilling that pledge. The company’s goal is to offer a readily processable product with outstanding mechanical, flame-retardant, and ambient curing qualities. The new Cogegum GFR/325 + catalyst CT/2 system can offer a solution even if no line specifically specialised for HFFR extrusion is available. Building on its Cogegum GFR/315 + catalyst CT/1 system, which is still a reference point on the market, Padanaplast has taken a real step forward to make high-performance special cable production a possibility for an ever-wider customer base.