Thermoplastic Polyesters: It's a Good Time to Know Them Better . Plastics Technology. There’s more to TP polyesters than you think.
You may know PET, PBT, and PETG—but what about PCT, PCTG, PCTA, and PTT? If you’re not sure what they are, how their properties compare, and who sells them, we have the answers—and lots of new developments to report. Click Image to Enlarge. PCT polyesters are increasingly used in electrical devices because they offer short- term heat resistance between that of PBT and LCP. Photo: Du. Pont). Copolyesters are replacing glass in thick- walled bottles for personal- care and cosmetic products. Photo: Eastman). New compounds that give flawless reflective surfaces improve PBT’s prospects in auto headlamp bezels.
Photo: Bayer). PTT, a new kind of polyester, is a candidate for non- halogen FR electrical compounds that can compete with nylon. Photo: RTP). Polyester compounds containing recycled PET are a cost- effective solution for some electrical devices. Photo: BASF). Ask a molder or extruder of engineering resins to name a thermoplastic polyester, and most would probably come up with PET or PBT or maybe PETG. Most likely, a smaller number would be aware that there are at least three more members of the terephthalate polyester family that are commercially available, plus another one waiting in the wings. An even smaller minority of processors would know where these “hidden” members of the resin family fit in terms of cost/performance or where they can be obtained. Given their long track record in electrical, automotive, medical, and appliance parts as well as premium packaging (not to mention commodity packaging, which is beyond the scope of this article), what accounts for the fuzzy picture of polyesters in the marketplace today?
The lack of focus partly arises because TP polyesters encompass seven homopolymer and copolyester types, all based on ester chemistry and a terephthalic acid (TPA) backbone (see table). The different varieties share excellent chemical resistance and electrical properties, good dimensional stability and (if reinforced) high toughness- stiffness balance. Several have high clarity. But in varying degrees they are sometimes subject to poor thermal properties, appearance deficits, and susceptibility to moisture pick- up that makes drying a critical issue.
A second source of confusion arises from the marketing policies of some suppliers in recent years. Some deliberately avoid identifying specific types of polyesters and copolyesters. What’s more, in different end- use markets, the same resin from the same supplier may go by entirely different trade names. A third source of uncertainty stems from recent shifts in the polyester supply line- up. Virtually every supplier in the past year has divested or acquired a polyester product line or initiated a major restructuring of their polyester operations.
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While these moves are intended to lay the foundation for a stronger long- term supply base in polyesters, the immediate effect is to heighten uncertainty about just who supplies what. Further blurring the view is a key characteristic of polyesters themselves—their efficiency in accepting glass and mineral reinforcements, pigments, and other polymers. Around 9. 0% of polyester is compounded with fillers, reinforcements, and other additives. This blurs distinctions between different base resin properties.
Yet compounding is also responsible for new developments in flame- retardant and FDA- acceptable PBTs and a proliferation of alloys that combine polyesters with one another and with polycarbonate, ABS, and ASA. Three factors make this an opportune time to clarify the status of TP polyesters in today’s market. One is the numerous shifts in supply. Another is marketing efforts by competing resin producers to replace polyesters with other resins in target applications. Last but not least is a number of new technical developments: Polyester suppliers are responding to this competition with novel resins and alloys, including hydrolysis- resistant PBT grades that enhance performance in electrical devices (e.
Other options improve the decorative capabilities of PBT and engineering- grade PET. And PTT, a novel polyester, has recently been launched.
A versatile chemistry. Polyesters puzzle many would- be users because of their diversity. Workhorse homopolymers obtain their varied properties by combining TPA with different diols: PET and PBT are made from ethylene glycol (EG) and butanediol, respectively. North American PBT suppliers include GE Advanced Materials, Ticona, and Du. Pont (which use domestically made resin), BASF (domestic and imported resin), and Bayer (which uses imported resin). Du. Pont, Ticona, BASF, and Bayer also supply reinforced PET compounds. Although there is a long list of independent polyester compounders, few of them say they are active in product development today. A separate group of copolyesters combine TPA, EG, and a secondary diol, cyclohexanedimethanol, or CHDM.
CHDM is made by Eastman Specialty Chemicals and Korea’s SK Chemicals. It is at least four times more costly than the diols used in PET and PBT, so CHDM- based copolyesters command sizeable price premiums. The glycol- modified copolyesters are PETG (less than 5. CHDM) and PCTG (more than 5. CHDM). PCT, a homopolymer of TPA and 1. CHDM diol, offers elevated heat resistance. Eastman recently sold the PCT family to Du.
Pont Engineering Polymers. Du. Pont also has rights to some reinforced compounds of PCTA, an isophthalic acid- modified PCT.
In recent years, Eastman has used only the generic labels “polyester” and “copolyester” to describe all its different materials. Since until recently Eastman was the only producer of PCT, PCTG, PETG, and PCTA, its generic labeling may have somewhat obscured perception of the variety of copolyesters and their individual attributes. Electronic uses in flux. Demand for polyester resins and compounds in North America in 2. SRI Consulting, in Menlo Park, Calif. Joel Levy, author of the report, breaks down the polyester market at 6.
PBT (alloys included), 1. PET, and 2. 2% copolyesters (of which PETG makes up 8. Domestic demand is expected to grow at 5% to 6%/yr over the next few years.
Much of the polyester—especially PBT—molding activity is migrating to the Asia- Pacific region, which is where a large share of recent capacity expansion is occurring. More than 5. 0% of demand for PBT and filled or reinforced PET is in molded electrical connectors, switches, and relays for automobiles and, to a lesser extent, printed circuit boards. This market is in transition as new forces drive up temperature requirements—notably miniaturization, increased crowding of automotive components under the hood, and newer lead- free soldering methods. One response by polyester suppliers is a major push to upgrade connector materials so they are not made obsolete and replaced by competing resins. Equally important are efforts to exploit new technologies that drive polyesters into entirely new applications. Du. Pont recently purchased from Eastman the technology and marketing rights to reinforced Thermx PCT and reinforced PET, as well as PCTA compounds. Du. Pont views PCT as broadening its overall engineered thermoplastics palette for electrical devices.
David Glasscock, Du. Pont’s high- temperature materials manager for the Americas, says PCT gives Du. Pont a crystalline polyester with short- term thermal- stress performance bridging the gap between PET and liquid- crystal polymers (LCP). This is evident in the HDT for a 3. PCT, which is 2. 62 C/5. F at 2. 64 psi. That is an advantage in the performance zone between PET (2.
C HDT) and LCP (2. C). PCT’s heat resistance suits it for electrical devices requiring infrared- reflow and surface- mount soldering, both involving elevated operating temperatures. Tyco Electronics in Greensboro, N. C., has adopted a 3. PCT (Thermx CGT3. PBT in an automobile power- seat connector that was converted to IR soldering.
PCT compounds reportedly also provide outstanding electrical properties, and good arc- tracking resistance relative to competing materials. Arc- track resistance opens opportunities in relays and switches for two fast- proliferating automotive applications—hybrid- electric and 4. V vehicles. Du. Pont is working on new PCT grades and alloys to exploit this potential. Another response to changes in the automotive connectors market is the launch of hydrolysis- resistant PBTs by BASF, Ticona, Bayer, Du. Pont and others. These compounds modify both the polymer and additives to increase resistance to degradation under elevated temperature and humidity. Mitchell Lee, BASF’s polyesters product manager, views these compounds as critical to PBT’s continued success in automotive connectors, since higher moisture resistance translates into improved warp- and heat resistance.
They allow those making connectors to meet global standards without redesign and retooling of existing connectors for a different polymer. BASF is working to reduce the cost premium for hydrolysis- resistant PBT. Ticona recently added impact- modified, glass/mineral- reinforced, and alloy grades of hydrolysis- resistant Celanex PBT. One of them, Celanex 3. HRT with 4. 0% glass offers toughness along with exceptional ability to bond to epoxy adhesives.
Bayer Polymers has recently emphasized PBT grades offering isotropic shrinkage for high- density plug connectors, whose dimensional tolerances are critical, and for higher stress- crack resistance in distribution terminal housings. Starting next Jan. Bayer will restructure its polymer and chemical businesses into two companies. That move will sever ties between its Pocan PBT business, which will go to the new Lanxess Corp., and its Makro blend PC/PBT alloys, inherited by the new Bayer Material. Science AG. LNP Engineered Plastics, previously a minor factor in PBT, is seeking a larger role in electronics markets.