How to process LCP's.
Getting the best out of liquid-crystal polymers may require innovative processing techniques; new grades and alloys may help, too.
One of the interesting wagers on the future of plastics in the 1990's concerns the odds that liquid-crystal polymers (LCP's) will advance from their present status as a small-volume, high-priced, "exotic" niche material into the ranks of more widely used engineering resins.
While many industry observers have speculated that LCP's are poised for sharp growth in the 1990's, what makes the bet interesting is that its outcome will depend on new technologies that are only just beginning to emerge--technologies for LCP polymerization and compounding, and new techniques for molding and extruding LCP's so as to take best advantage of their unique properties.
First commercialized in late 1984, all currently available LCP's are random copolymers of aromatic polyesters, produced from a select group of monomers. Unlike most resins, LCP's have the ability to form highly oriented, fibrous crystalline chains under shear in the melt phase (see PT, Dec. '84, p. 82; April '85, p. 85).
Despite LCP materials' impressive thermal and physical properties, commercial utilization reportedly amounts to only 2-5 million lb/yr., primarily in small electrical surface-mount connectors and encapsulated devices, along with some chemical-processing applications and microwave cookware. LCP producers hope to move them into larger, more diverse structural parts in the '90s.
Wider utilization would help to bring down the cost of LCP's--typically $7-10/lb. But since high prices have been identified as one barrier to wider utilization, LCP suppliers are taking the initiative on seeking ways to reduce LCP costs and increase their value in use (cost/performance). One supplier anticipates bringing prices down below $5/lb by 1992.
Injection molding of glass-filled compounds have been identified by resin producers as the main thrust for processing of LCP's during the next 10 years. (Some work, as noted below, also is occurring in extrusion of unfilled LCP's.) Though often referred to as a "self-reinforcing" polymer, most material development efforts so far have concentrated on filled grades to minimize their anisotropic nature and improve transverse strength, reinforce weak weld lines, and also bring down the cost. Besides glass, other reinforcements used in LCP's include various minerals and carbon fibers.
Much of the materials and processing development work to address the LCP challenge already is beginning to unfold, with the promise of yielding major results during the next three years. On the material development side, LCP resin producers have offered PLASTICS TECHNOLOGY some glimpses into their proprietary research efforts toward making LCP's more useful engineering materials. Their goals for the 1990s are to reduce LCP prices, expand the range of product offerings, counter the drawbacks of the materials' greatest strength--their tendency toward high orientation, and balance the resins' outstanding thermal and mechanical properties with improved processability. Alloying and blending LCP's with other resins is being actively investigated as one means of achieving these goals.
As for processing development, the handful of experienced LCP molders (five of which were interviewed for this article) have compiled an extensive database on the unique behavior of these materials in the five years since they were introduced. They regard this as an "information platform," upon which they can proceed to more advanced fabrication methods. Tooling and part design, proper gating and venting, and gaining better understanding of melt-flow characteristics are vital areas demanding further work.
Along with optimizing their adaptations of existing processing methods to LCP behavior, processors believe that new types of processing technology are needed to more fully exploit the benefits (and minimize the drawbacks) of LCP's. One such production breakthrough may be the new "push-pull" injection molding technique from Klockner Ferromatik Desma GmbH in West Germany, unveiled at last year's K'89 exhibition in Dusseldorf (see PT, Jan. '90, p. 72). Some processors and resin suppliers also are conducting research on new extrusion techniques for LCP film and sheet. Complementing such developing fabrication advances is computer software that has proven successful for finite-element mold-filling and cooling analysis of LCP parts.
EASY TO PROCESS?
One of the most widely trumpeted attributes of these high-performance polymers is that they are "easy to process" (see PT, April '85, p. 85 for a 5-page LCP processing guide). Molders and resin producers have agreed on the ability of LCP's to easily fill thin-walled parts (down to 10 mills), their wide processing window for melt and mold temperatures, high dimensional stability and low shrinkage, and sharp shear-rate/viscosity curve and rapid transition in the mold from melt to solid, which results in quick cycle times.
However, there also has been a downside to processing LCP's: limited flexibility in mold gating configurations; restricted part size and geometry; and inability to fully exploit the material's superior physical properties or effectively orient the polymer melt to control weld-line weakness or undesirable anisotropic characteristics. It's also true that the higher the temperature capability of the LCP resin, the "stiffer" its processing characteristics. Molders and resin producers concur in an optimistic view that these limitations are all manageable factors, and that improvements will be forthcoming.
At least one LCP manufacturer asserts that concern over their anisotropic nature has been greatly overblown. John R. Dole, marketing manager and development specialist for Hoechst Celanese Corp., says anisotropy is a manageable condition that also occurs with other polymers: "Any crystalline engineering thermoplastic tends to be anisotropic. The anisotropy in LCP's can be diminished with increased levels of fillers, and also is dependent on the particular material grade and mold gating." He characterizes claims of LCP weld-line weakness as a similar case of overstatement, since other crystalline resins also cause problems with the knitting of melt fronts in a mold. However, not all sources interviewed shared Dole's conviction on these issues.
Moderate improvements in LCP resin quality have been noted recently by experienced processors. They told PLASTICS TECHNOLOGY that mold plate-out, material consistency, pellet size variation, and flow characteristics all had been typically mild problems when processing LCP's. However, even these slight difficulties apparently have been rectified during the last 12 months, as resin producers have introduced new grades and generations of material. Processors say current LCP grades afford them a wide molding-temperature window, with consistent, predictable cycle times and little tool wear or plate-out. "We've known Xydar since it was a puppy, and it's come a long way," says a spokesman for Cincinnati-based Performance Plastics Inc. "Weld lines are no longer a problem, and the material has much better flow properties for molding." Several molders also indicated that they worked closely with resin suppliers in fine-tuning the material to meet processing and end-use requirements.
Along with resin improvements, computer software for mold-filling analysis has been found to aid processors using LCP's. One such package is the TMconcept software system from Plastics & Computer Inc., Montclair, N.J. Ernest Bernhardt, president, says this expert system for injection mold-filling and analysis, shrinkage evaluation, and manufacturing cost optimization has been proven successful with LCP's and is now being used by two leading LCP materials producers--Amoco and Hoechst Celanese--to aid customers in mold and process design. Other suppliers of mold-analysis software--Moldflow, Inc., Advanced CAE Technology, and Unisys CAD/CAM, Inc.--all say they have achieved accurate results in analyzing flow of LCP's with their standard software.
Both Hoechst Celanese and Amoco plan to publish LCP processing and design guides this year. The guides will provide technical support to molders in the areas of material selection, processing methods, mold-filling analysis, and tooling and part design.
MOLD DESIGN CRUCIAL
Mold design is the most critical aspect of working successfully with LCP's, according to molders. This seems to be especially true because the majority of LCP applications are in tight-tolerance, very-thin-walled parts, as well as insert molding of pins and electrical connectors, which also demand exacting tolerances.
Gate placement with regard to a part's weld line is considered to be a crucial, though manageable, aspect of mold design. Processors say designing and positioning gates to move the knit line to a noncritical area of a part is a relatively easy task--and they recommend single gating whenever possible to minimize the problem. One also recommends edge gating over subgating whenever possible. "Mold and part design is the key to working with LCP's," according to Edward R. Christie, marketing manager for Pixley-Richards Inc., Plymouth, Mass., a molder of LCPs. "It's knowing where to put gates on a part," he adds, especially on long, thin components.
"Like any other highly crystalline material, LCP melt fronts have a hard time joining," according to Christie. "But relative to other crystalline resins, LCP's do show weakness at the weld lines." Most LCP molders feel this condition can be compensated for by tailoring mold design for non-critical placement of weld lines.
David P. Berkowitz, Pixley-Richards' regional accounts manager, says the anisotropic nature of the material is well understood and can be harnessed through mold design. He says that in designing molds for parts with complex geometries, LCP's can be made to approach a more isotropic nature, via structural support by arched shapes and ribs. "We can build in strength," he says. "The complex structure of parts can help to alleviate the material's anisotropic nature. To further compensate for the transverse weakness, we can mix up the flow patterns when molding. Glass fillers also add strength in the transverse direction of a part."
Brian Daly, plastics manufacturing engineer at Superior Electric Co., Bristol, Conn., says the high inherent dimensional stability of LCP's makes mold-design accuracy an essential consideration. The firm does its own mold design on precision stators and rotors encapsulated by LCP via insert molding. "The material has excellent dimensional stability," he says. "Shrinkage isn't a consideration with LCP. You get what you expect, so you need a high degree of accuracy in your mold design."
Daly says his most successful way of dealing with a high-tech material like LCP's has been to take a deliberately low-tech approach to processing. "We try to keep the process as simple as possible. We've stayed away from a high-tech, computer-control approach with LCP's."
Molders like Daly add that mold temperature is not a critical factor in the process, he adds, as the material doesn't require a hot mold to develop crystallinity. One molder that uses primarily Hoechst Celanese's Vectra LCP says a typical processing window includes mold temperatures between 60 and 200 F, and melt temperatures between 520 and 580 F.
Predictive engineering and specially designed tools are keys to working with LCP's at Molex Inc., Lisle, Ill., a custom molder of electrical components. Robert Petrie, mold engineering manager, says Molex has established a working database on mold flow and tool design for LCP's. Because of the material's predictably low shrinkage rate, Petrie says his firm is able to rework and convert older tooling used for other resins to process LCP's. Molex overcomes weld-line problems through using single, end gates rather than multiple gating. As for dealing with LCP's anisotropic nature, he says Molex has devised ways to "play with the material's glass-fiber orientation," as well as include specially designed interrupters and deflectors in the tool to adjust flow patterns. "Successful processing of LCP's is very dependent on controlling mold flow, more than any other factor." To accomplish this, Molex relies on team meetings among leaders of the production, tooling and design departments, to collaborate on what Petrie calls predictive engineering.
Careful analysis of flow, with venting at the end of fill patterns, is one tip offered by Richard D. Caufman, manager of project engineering for Loranger Manufacturing Corp., Warren, Pa. In molding a 12-pin electrical connector for automotive ignition systems, Caufman said he prefers to keep vents away from the gates to prevent flash problems. "LCP's flow very well, and the material we use has a very low viscosity," he explains. "We found that we had a flashing problem when the material reached the vent, so we designed the vents at the end of the fill pattern." Nonetheless, Caufman points out that, in general, LCP's exhibit far less flash than most other engineering thermoplastics. He also says that compared with other high-temperature materials, such as PPS, LCP's create less wear on tools.
FAST CYCLES PRAISED
Processors agree that LCP's demonstrate superior capabilities in accurately filling molds for thin-walled parts, down to 10 mils. But they rate the materials' ability to provide fast cycle times as their greatest processing advantage. Molders explain that LCP's, because they don't need to be crystallized in the mold, require less cooling time. They noted that LCP's have a relatively low heat of fusion--the amount of heat energy needed to melt a resin, as well as the heat that must be drawn out to have the resin set up in a mold. LCP's maintain much of their ordered crystallinity in the liquid state, unlike other thermoplastics. As a result, less heat needs to be dissipated from LCP's before they set up in a mold.
A NOVEL INJECTION APPROACH
The one really new processing technique developed specifically for LCP's is Klockner's "push-pull" or "counterflow" process, noted above. This approach utilizes a twin-injector molding machine (such as is used for two-color work) with some control software modifications. As shown in the accompanying diagram (adapted from an article on the subject published in the November 1989 issue of Kunstoffe German Plastics), the part is gated at either end and may be filled by one injector alone or both together, producing a weld line in the middle. By either method, once the mold is initially filled, one injector screw advances while the other retracts, and vice versa in alternation. Thus, the melt is oscillated back and forth within the cavity, with some of it being temporarily sucked back into the injector barrel before being re-injected.
This reportedly has two beneficial effects: One is that the innermost core of the part is replenished with fresh, hot melt, achieving a higher degree of orientation in what is normally the least oriented region of the part, owing to relatively low shear and cooling of the melt. This results in higher flow-direction tensile strength than in a part injected normally from a single end gate. Second, reheating of the material during the push-pull phase is said to permit greater knitting of the flow fronts, effectively "erasing" the weld line, as shown by the higher strength attained. A patent application has been filed on this process. (See "Production of Technical Parts by Means of Counterflow Injection Molding," by L.M. Gutjahr and H. Becker in Kunstoffe German Plastics, Nov. '89, p. 8; published by Carl Hanser Verlag, Darmstadt, W. Germany.)
EXTRUSION AND ALLOYING R&D
Besides injection molding, research is also under way on new extrusion techniques for neat LCP films--for example, by Foster Miller Inc., an R&D contractor in Waltham, Mass. Although the work is proprietary, Richard W. Lusignea, manager of the Polymer & Composite Materials Div., says the program involves developing new extrusion die designs that create biaxial and multiaxial orientation in unfilled LCP sheet and film.
Extrusion of LCP sheet and thin films is at the semi-works scale of development at Hoechst Celanese Corp. One R&D project involves extruding LCP sheets for cross-ply lamination into structural composites. Other extrusion development work at Hoechst Celanese is aimed at taking advantage of LCP gas-barrier properties in coextruded thin packaging films. Du Pont is also working on extrusion techniques for its "amorphous" LCP, but details could not be obtained from the company. (See PT, Feb. '86, p. 23 for more on LCP extrusion.)
Alloying and blending with other resins has been proposed as a means of realizing LCP properties at lower cost. LCP's apparently can serve as a "molecular reinforcement" in other polymer matrixes; some observers have suggested that the low viscosity of LCP's may enable them to serve as processing aids for other polymers.
However, LCP's may present some difficulties as an alloy partner because of their high crystallinity, which causes problems of compatibility, or adhering to other resins on a molecular level, according to Prof. Robert A. Weiss, of the Dept. of Chemical Engineering and Polymer Science at the University of Connecticut in Storrs. Weiss is the director of a federally funded, $3.8-million LCP research program at the university. He notes that LCP's have high interfacial tension, thus requiring wetting agents in order to blend with other polymers, much like sizing chemicals used for glass fibers in a composite matrix. Finding suitable compatibilizing and wetting agents for LCP's is one goal of resin suppliers' research.
One processor that's working independently on LCP alloying is Foster Miller. Lusignea says the firm has a $50,000 phase-one Small Business Innovation Research contract, which it won in February from The National Aeronautics and Space Administration (NASA). The project involves alloying LCP with a thermoplastic polyimide called LARC-TPI, developed by NASA's Langley Research Center in Norfolk, Va. The goals are to develop chemical and rheological compatibilization for LCP to function as a molecular reinforcement in the LARC polyimide, creating a "microcomposite."
"LCP's are generally unfriendly when it comes to alloys," says Lusignea.
"It's difficult to form a miscible blend with LCP's. Our work involves developing special compatibilization chemistry between the two resins. Aerospace applications for an LCP/polyimide alloy extruded sheet would be cryogenic fuel-tank liners and satellite solar collectors and reflector panels."
Target for completing phase one is August, when a phase-two contract for up to $500,000 may be offered. Foster Miller is seeking joint-development partners to assist in commercializing this technology.
Hoechst Celanese Corp. and Amoco Performance Products are the two best known suppliers of LCP's today. Du Pont entered the field in mid-1988, while several other contenders promise to unveil their materials soon, or have placed their development programs on hold. The following summary outlines their LCP R&D plans for the next few years.
* Hoechst Celanese: A boost in higher-temperature grades, with HDT's about 75 [degrees] F above existing levels, is one current research thrust for Hoechst Celanese's Vectra LCP line in the 1990s, according to John R. Dole, marketing manager and development specialist. The Vectra line currently has HDT's in the mid-to-high 400-500 F range. Attaining higher temperatures would involve using new monomers and polymerization techniques, says Dole.
Lower-molecular-weight LCP's is a second area of development under way at Hoechst Celanese. Dole says low-MW, highly filled (50-75%) LCP's will be needed for application such as insert molding and encapsulation of electrical/electronic components and wiring.
A third focus of R&D will be to expand the product range and tailor new LCP grades to satisfy specific application needs, through development of new polymers and blends, according to David W. Trott, Vectra senior product manager. The search for compatibilizing agents is a key element of that program.
* Amoco Performance Products: Since it already has the most heat-resistant LCP's on the market (with HDT's up to 675 F), Amoco Performance Products will focus its research efforts on improving the processing characteristics of its Xydar LCP line, according to marketing manager Daniel Love. Filler technology, including particle size and orientation, and wetting agents for carbon and glass fibers and mineral reinforcements, will be another research thrust for Amoco, he says.
Amoco plans to introduce a Xydar grade for under $5/lb by 1992. This third-generation material currently is in the second phase of test-market evaluations, Love says. He expects the company to make a final decision on this new material by the fourth quarter of this year. Love says this lower-cost grade would be glass-filled, with HDT around 500 F and a processing window similar to Amoco's current Xydar 300 series.
* Du Pont Co.: Du Pont, which fully commercialized its crystalline HX-4000 and "amorphous" HX-2000 LCP lines late last year, will introduce a new generation this year, says LCP business manager James Godfrey. The new type will be a crystalline, glass-reinforced material with a melt temperature of 608 F and glass-transition temperature of 345 F. It will have a HDT of 482 F @ 264 psi--midway between Du Pont's two existing product lines, while offering faster cycle times and a wider processing window. The new grade also will provide higher tensile strength (26,000 psi) and elongation (2.6%) than either of the two current grades, but with a lower flexural modulus (2 million psi).
Du Pont is the only supplier to have introduced what it calls an "amorphous" LCP (see PT, Aug. '88, p. 99; Sept. '88, p. 89). Godfrey explains this apparent contradiction in terms thusly: Amorphous LCP has a glass-transition temperature of 375 F, but no crystalline melting point. The material softens gradually over a temperature range, rather than having a sharp melting point, giving it suitable processing characteristics for film and sheet extrusion.
Godfrey says Du Pont is also working on applications in larger, structural engineered components that will use LCP's as a processing aid or reinforcement in other resin systems. "There is an upside to the fact LCP's don't wet out very well with other resins, and we feel we can take advantage of this aspect," he said, declining to explain further.
* Rhone Poulenc, Inc.: This French-based firm plans to introduce four grades of LCP to the U.S. next year, according to a company executive interviewed at the K'89 show (see PT Jan. '90, p. 92). Known as Rhodester C.L., the line includes one unfilled and three mineral-filled grades, with HDT's in the 446-518 F range. Rhodester material will be priced slightly higher than comparable Vectra grades, according to this spokesman.
* Eastman Performance Plastics: The company will begin market sampling of a new LCP in the third quarter. The unfilled resin has a melting point between 662 and 680 F, HDT above 572 F (at 66 psi), flexural strength above 20,000 psi, tensile strength over 30,000 psi, and a notched Izod impact strength above 6 ft-lb/in. The material, which has not yet been given a trade name, will be available in both filled and unfilled versions, and priced at or below existing LCP lines.
OTHER SUPPLIERS `ON HOLD'
Resin producers with LCP development programs "on hold" include ICI Advanced Materials (which previously introduced Victrex SRP resins); BASF Corp., Plastic Materials (Ultrax); Granmont Inc. (Granlar); and GE Plastics (no commercial introduction as yet). Most of these firms concede that the relative small market size, high material price and limited applications, as well as strength of the established commercial producers, has caused them to stay on the sidelines for now and reconsider their LCP development efforts.
Bayer AG in W. Germany, parent of Mobay Corp. in the U.S., cancelled its plans to introduce four new grades of LCP's, citing tepid market conditions and the existence of well-established producers (see PT Jan. '90, p. 93). This decision reversed plans the company had announced earlier in the year (see PT Oct. '89, p. 33). A Bayer executive interviewed at the K'89 show in Dusseldorf was candid about the reasons for his company's change in plans, saying LCP is a limited market already saturated by current producers. He did say Bayer would continue development on semifinished LCP products such as its Polystal line of tapes and reinforcements.
Patrick Collins, manager of high-performance products for GE Plastics, says his firm "basically has shelved" its plans for near-term commercial development of LCP's. GE has decided instead to start over and develop a new LCP thrust. "We decided we didn't want to come out with a `me-too' LCP product," says Collins. "We're looking for a unique technology and product line. We certainly think LCP's will be a market in the 1990s, and we think we could stimulate interest with a new LCP. But it's not something for the near future for us."
Granmont's efforts to develop and commercialize an LCP line have been stalled by ongoing reorganization moves at Montedison and uncertain market conditions, according to a spokesman. While he would not reveal details of the company's LCP development program, he said the resin, known as Granlar A, would be a low-cost LCP with an HDT above 600 F (see PT, June '88, p. 125).
The Blends & Advanced Materials unit of Dow Chemical Co. has initiated a research program into LCP's. A spokesman described the project as a "modest effort" still in the early stage of development. [Figure 1 to 2 Omitted]
PHOTO : The majority of parts made from liquid crystal polymers to date have been injection
PHOTO : molded, surface-mount electrical components, like these from Pixley-Richards. Processors
PHOTO : and resin producers seek to expand LCP applications in the 1990s.
PHOTO : High heat resistance and dimensional stability are among the most impressive properties of
PHOTO : LCP's. In this demonstration, Amoco's Xydar withstands a 2000 F burn-through test used to
PHOTO : qualify aircraft materials.
PHOTO : Mold design was identified by processors as the most crucial aspect to be considered when
PHOTO : working with LCP's. Part gating in order to locate weld lines in non-critical areas of the
PHOTO : component is one of the essential points in mold design.
PHOTO : Injection molding of glass-filled LCP's is expected to predominate in the 1990s. However,
PHOTO : several firms also are developing various extrusion applications for the material.
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|Title Annotation:||liquid-crystal polymers|
|Author:||Gabriele, Michael C.|
|Date:||Apr 1, 1990|
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