Vectra A - Liquid Crystal Polyester - Supplier Data by Goodfellow

Vectra products are remarkable materials. Not only are they high performance thermoplastics with the properties normally associated with that group, they also have very distinctive additional properties resulting from their liquid crystalline nature. They have stiff linear molecules which readily organise themselves spatially into zones with the three dimensional order typical of solid crystals. 

In general, this occurs both in solution and in the melt and it is strongly promoted by shear or flow resulting in a high degree of orientation in the flow direction. In the case of these melt-processable or thermotropic (strict meaning : forms liquid crystal regions in the melt) polyesters, it is in the melt that this property is of practical importance, but in the case of Kevlar, which decomposes below its “melting point”, liquid crystals are formed in solution and are used to achieve its high degree of orientation and resultant properties. 

Morphology and Structure

One main consequence of this morphology is that the structure and properties of the Vectra A are highly anisotropic. The morphology is crudely similar to that of a fibre reinforced polymer but where the reinforcement is oriented molecules of the matrix material; they are often called self reinforcing polymers (SRP) as a result.

Design approaches based on the use of fibre reinforced polymers (FRP) can be applied, up to a point, to self reinforcing polymers ; it should also be noted that, in contrast to normal fibre reinforced polymers, the addition of fibres to self reinforcing polymers  (glass and carbon filled grades are relatively widely used) reduces rather than increases the degree of anisotropy.

The morphology of self reinforcing polymers  can also can be compared to that of wood and the idea of grain direction can be useful in thinking about its nature.

The Effect of Processing on Mechanical Properties

The mechanical properties of Vectra A products are extremely high in the direction of flow/molecular orientation and correspondingly low in the other dimensions.  Therefore, the properties of a melt-processed article depend very markedly on the details of the melt flow during its manufacture.

The outer layers of an extruded or injection moulded item will normally be uniformly and highly oriented parallel to the surface; in the core, at least of straightforward injection mouldings, "tumbling" of the melt usually occurs leading to molecular orientation roughly perpendicular to the general flow direction.

One consequence of this is that the common practice of machining prototype articles before making an injection moulding tool can lead to highly misleading results - the classic example is that of a flanged bobbin which, if machined from rod with its axis parallel to that of the rod, will have extremely weak flanges but when injection moulded will have strong, stiff flanges.

Furthermore, the highly fibrillar nature of the outer layers of semi-fabricated items leads to other problems if they are machined eg fibrillation, tear-out, poor surface finish and potential dimensional tolerance difficulties.  A further problem is the considerable risk of internal cracking or delamination within thick sections of Vectra A which is a particular risk in extruded rod which, roughly, increases with diameter.

hese three types of problem lead to a clear recommendation against both extrusion and machining from the polymer manufacturer and suppliers have to disclaim liability for the results of any attempted machining or internal cracking. 

However, this leaves would-be prototypers in a nasty “no-win” situation to which, short of making a “cheap” injection moulding tool, there is as yet no satisfactory solution.  Therefore, it is hoped that these observations will help end-users who see no option but to machine to do so on a basis of informed understanding and acceptance of the risks involved.

Mechanical Properties

With the exception of the very high impact strength of Vectra A products, which will be discussed in more detail later in this paragraph, published mechanical properties are broadly similar to those of glass fibre reinforced engineering polymers, but it follows from the previous paragraph that these properties depend to a much greater extent than usual on the details of the test specimen so they must be used with even more than normal caution.

This is even more true of measurement of impact strength, particularly notched, where results are dominated by whether/how the notch penetrates the highly oriented surface layers and it seems likely that conventional test results tend to underestimate impact strength.

Nevertheless, it is clear even from these values that self reinforcing polymers have very high impact strengths (not that much less than polycarbonate’s) and very much greater than those of conventional fibre reinforced engineering polymers whose strength and moduli self reinforcing polymers broadly match.

Furthermore, a high proportion of this toughness is retained down to liquid nitrogen temperatures at least.

Vectra A has good creep and fatigue properties but its wear characteristics, particularly for unfilled grades, are adversely affected by surface fibrillation.

Chemical Properties

The barrier properties of Vectra A products are excellent (at least comparable to those of PVDC) and its radiation, UV and chemical and hydrolysis resistances are very good.

It is insoluble in at least almost all solvents (and so very difficult to characterise scientifically) but attacked to some extent by strong acids and somewhat more by bases; however, even for bases a combination of elevated temperatures/long exposure times/quite high concentrations are needed before significant loss of properties occurs.

Coefficient of Thermal Expansion

Yet another of its remarkable properties is its low, though highly anisotropic, coefficient of thermal expansion (CTE) - to the extent that the coefficient of thermal expansion parallel to the flow direction has a (low) negative value. This rare characteristic is shared by the highly oriented fibres - carbon, Kevlar and ultra high modulus polyethylene.

In practice, the coefficient of thermal expansion can be varied within limits by selecting grade and moulding conditions and components produced that match the coefficient of thermal expansion of glass, ceramic, some metals and glass fibre/epoxy laminates; this is used in such devices as surface mounted electronic components.

Injection Moulding

There are other consequences of self reinforcing polymers morphology that favour injection moulding namely: low melt viscosity (once sheared), favouring long and complex melt flow paths; very low warpage and shrinkage, favouring production of high precision parts; very low heat of fusion, favouring fast cycle times. 

However, internal welds lines tend to be quite weak and it is most important that these are considered very carefully during design. 

Colour

The natural colour of Vectra A products is an opaque brownish-yellow and it often shows surface discolouration e.g. in swirls or marbling pattern.  Though this is a cosmetic “defect”, it is frequently coloured black to minimise it.

Applications

Applications of Vectra A appear to be in the fairly early stages of development but include mechanical and electronic components that require injection moulding to very close tolerances eg. complex electronic connectors.

Key Properties

The key properties of Vectra A products are tabulated below.

Table 1. Key properties

Chemical Resistance
Acids - concentrated Fair
Acids - dilute Good
Alcohols Good
Alkalis Fair
Aromatic hydrocarbons Good
Greases and Oils Good
Ketones Good
Electrical Properties
Dielectric constant @ 1 MHz 3.0
Dielectric strength ( kV.mm-1 ) 47 @ 1.5 mm
Dissipation factor @ 1 MHz 0.02
Surface resistivity ( Ohm/sq ) 4x1013
Volume resistivity ( Ohm.cm ) 1016
Mechanical Properties
Abrasive resistance - ASTM D1044 ( mg/1000 cycles ) 56
Coefficient of friction 0.12-0.14
Compressive strength ( MPa ) 70
Elongation at break ( % ) 3
Hardness - Rockwell M60
Izod impact strength ( J.m-1 ) 520
Tensile modulus ( GPa ) 2-10
Tensile strength ( MPa ) 55-165
Physical Properties
Density ( g.cm-3 ) 1.40
Flammability V0 @ 0.8 mm
Limiting oxygen index ( % ) 35
Radiation resistance - Alpha Good
Resistance to Ultra-violet Good
Water absorption - equilibrium ( % ) 0.03
Water absorption - over 24 hours ( % ) 0.02
Thermal Properties
Coefficient of thermal expansion ( x10-6 K-1 ) -5 to +75
Heat-deflection temperature - 0.45 MPa (°C ) 220
Heat-deflection temperature - 1.8 MPa (°C ) 180
Lower working temperature ( °C ) ~ -200
Specific heat ( J.K-1.kg-1 ) 1000
Thermal conductivity ( W.m-1.K-1 ) 0.18 @ 23
Upper working temperature ( °C ) 200-220
Properties Vectra A® - Liquid Crystal Polyester Fibre
Property Value
Material Vectran HS
Specific Modulus cN/tex 4800
Specific Tenacity cN/tex 210
Coefficient of thermal expansion x10-6 K-1 -ve.
Density g.cm-3 1.4
Extension to break % 3.3
Modulus GPa 65
Shrinkage @ 100 °C % =<0.5
Tenacity GPa 2.8

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