A combination of good mechanical and surface properties with very good flow and favourable price makes PC/ LCP blends attractive to designers of parts for the rapidly growing markets of telecommunications and data processing. Because of their special flow properties, liquid crystal polymers (LCPs) are well suited for the production of thin-walled, low-warpage parts with excellent mechanical properties and outstanding soldering resistance, says M. Romer of Hoechst AG, the individual responsible for the development of Vectra products. The thermotropic nature of LCPs (the molecules orient specifically based on temperature) is responsible for both flowability and strength.
LCPs are now being used as a blending component for engineering plastics. The LCPs are added in quantities of 10 to 30 wt-%, but they do not mix with the base polymer. Shear stress during injection moulding converts the LCP particles into long LCP fibrils that act as reinforcing fibers.
Morphology and Material Properties
One such blend consists of LCP in a matrix of polycarbonate (PC). As with all blends of immiscible plastics, the properties of this material depend considerably on the morphology and thus on the processing parameters. During injection moulding, the rigid, rod-shaped LCP molecules mainly orient themselves in the direction of flow. This is why a polycarbonate reinforced with LCP will attain mechanical properties that are basically comparable to those of fiber-reinforced polycarbonate. PC/LCP blends, however, have much better flow properties than fiber-reinforced polycarbonate. The inherently good flow properties of LCPs help to improve flow in the blends.
In order that the PC/LCP blends will actually attain these good strength values, the interface between the fibrils and the PC matrix must be able to transmit shear force. Because the two phases do not mix, however, the maximum shear force that can be transmitted is generally low. This situation can be markedly improved by adding special compatibilizers to the PC/LCP blend. They also stabilize the blends overall property level and, by improving its flowability, create a larger processing window.
Polycarbonate-rich PC/LCP blends have very good mechanical properties on account of their morphology. Because the LCP fibrils orient themselves with the direction of flow, PC/LCP blends have better mechanical properties in this direction than in the transverse direction.
However, the differences in properties in the two directions on the one hand and the performance in thinner wall thickness on the other are much less pronounced in PC/LCP blends than in pure LCP. Since the polycarbonate base material is amorphous, an article that is injection moulded from the blend will have very high dimensional stability and narrow tolerances at low shrinkage.
PC/LCP blends have a much higher weld strength than LCP (which has a low value), and it may be as much as 80 percent that of unreinforced polycarbonate.
Processing of PC/LCP blends
PC/LCP blends are readily processed on standard injection moulding machines fitted with a three-section screw. Owing to the polycarbonate content, the blends must be predried at 120øC for 4 to 5 hours. The temperature profile on the screw barrel should increase gradually from 270øC in the feed section to 300øC (310øC max) in the nozzle section. The mould temperature should be between 90 and 120øC. Higher temperatures, e.g., 140øC, considerably improve the strength of weld lines and reduce weld line visibility, but they also lengthen the cycle time.
The processing characteristics of PC/LCP blends have been studied in injection moulded housings for mobile telephones. If a high injection rate and low hold pressure are chosen, as for LCP, the housings can be produced with a very short cycle time. The appearance is usually unsatisfactory as the parts exhibit a fine surface structure originating from the high degree of LCP fibrillation.
If the parameters for polycarbonate are applied (low injection rate, high holding pressure), the housings have a flawless appearance. Microscopic studies show that the LCP is extensively fibrillated in such housings as well, but the high holding pressure causes the LCP fibrils near the surface to be covered and so they remain invisible. Therefore, machine setup technicians should use the processing parameters for polycarbonate to process PC/LCP blends into housings that are required to have high-quality surfaces.
Thin-wall Part Demands
PC/LCP blends are particularly advantageous for mouldings with a wall thickness less than .8 mm. Because they flow so well, there is no need to raise the mould temperature to avoid the filling problems typical of such parts; consistent shot size is attained.
Good flowability and a high tensile E modulus make PC/LCP blends ideal for thin-wall housings with high-quality surfaces. Housings of this kind are required in rapidly growing numbers for mobile and cordless telephones and the numerous portable devices used in telecommunications technology.
PC/LCP blends also lend themselves to the production of metric connectors, which have good dimensional stability because of their low internal stress. For this application, though, the blends must be flameproofed. Although LCP is inherently flame retardant, the high polycarbonate content of PC/LCP blends means that they will attain a UL rating of V-0 only if halogen-free flame-retardants are added.
There are also economic reasons for using PC/LCP blends in the electrical and electronics applications described above. For instance, the wall thickness of a housing made from a PC/ABS blend - costing about DM 8/kg (US$ 5.30/ kg) - is currently about 1.6 mm. A housing of .8-mm wall thickness with comparable properties can be produced from a PC/LCP blend - DM 16/kg (US$ 10.60/kg) - at no extra cost. Given that the outside dimensions are the same, the crucial advantage lies in gaining an extra 1.6-mm-depth inside. Thus this housing could accommodate more powerful or more cost-favourable internal features.
Since PC/LCP melts have low abrasion properties, the injection moulds may be made of hot-work steel or, if a long service life is required, of tool steel with a Rockwell C hardness >56.Contact Information: Hoechst Ag, Marketing, Vectra, D-65926 Frankfurt am Main, Germany. Phone: (49) 69.305.3463. Fax: (49) 69 305 16345