PTT - An innovative polymer and its versatile application in textiles

Abstract

CORTERRA PTT is the Shell trademark name for the aromaticpolyesters known as polytrimethylene terephthalate (PTT). CORTERRA PTT polymersare produced by the polycondensation reaction of purified terephthalic acid(PTA) and 1,3-propanediol (PDO). PTT competes with the other aromaticpolyesters - polyethylene terephthalate (PET) and polybutylene terephthalate(PBT) - as well as with nylon. CORTERRA PT is used in:

• Fibres for carpet and textiles
• Non-woven fabrics

Other emerging applications, including monofilament, filmand engineering thermoplastics (ETPs). PTT greatly reduces the amount ofacrolein generated during thermal decomposition of the polymer. Thisstabilisation is effective at temperatures well above the melt temperature andis active for days at these temperatures. 

Introduction:

CORTERRA Polymer is aromatic polyester known generically asPTT, (polytrimethylene terephthalate). PTT is produced by the polycondensationreaction of PTA (purified terephthalic acid) and PDO (1,3-propanediol) and hasunique properties as compared to the other aromatic polyesters, PET(polyethylene terephthalate) and PBT (polybutylene terephthalate).

The unique properties of PTT have been known for many years,but the polymer has not been commercially available because of the high cost ofproduction of the PDO raw material. Extensive research efforts has resulted ina cost-effective process to manufacture PDO. With this breakthrough inprocessing technology for PDO, CORTERRA Polymers are now commercially availablefor use in carpet and textile fibers monofilament, film, nonwoven fabric, andengineering thermoplastic applications.

CORTERRA 200 can be used by itself, as the base polymer forcompounds, or as a polymer modifier for many engineering thermoplasticapplications. Compounded engineering resins based on CORTERRA PTT polymerprovide physical properties that are equivalent to or better than those ofsimilar PBT compounds. Extrusion and injection molding conditions, andprocessing characteristics are very similar to those established for PBT. PTTcompounds generally exhibit higher tensile strength, flexural modulus and heatdeflection temperatures but with slightly lower impact strength than the PBTcounterparts.

Development of PTT Polymer:

CORTERRA Polymer is an aromatic polyester known genericallyas PTT, polytrimethylene terephthalate). PTT is produced by thepolycondensation reaction of PTA (purified terephthalic acid) and PDO (1,3-propanediol)and has unique properties as compared to the other aromatic polyesters, PET(polyethylene terephthalate) and PBT (polybutylene terephthalate).

The unique properties of PTT have been known for many years,but the polymer has not been commercially available because of the high cost ofproduction of the PDO raw material. Extensive research efforts by ShellChemicals** has resulted in a cost-effective process to manufacture PDO. Withthis breakthrough in processing technology for PDO, CORTERRA Polymers are nowcommercially available for use in carpet and textile fibers monofilament, film,nonwoven fabric, and engineering thermoplastic applications.

CORTERRA 9240 is a semi-dull, PTT homopolymer. The polymerproperties which provide resilience, elastic recovery, high bulk, soft hand,inherent stain resistance, and ease of dyeing, make it well-suited for carpetand textile fibers. CORTERRA 9240 has TiO₂ added as a dulling agentand is used when a bright or shiny luster is not desired in the end product.

Many of these same properties make CORTERRA 9240 a goodcandidate for nonwoven applications. CORTERRA 9240 can be compounded to provideperformance properties and processing characteristics that are similar to PBT.

Process Parameters:

CORTERRA 200 has been successfully compounded using both single-screw and twin-screw extruders. It is not necessary to dry PTT polymer before use in vented single-screw or twin-screw compounding machines as long as the moisture content is less than 0.2%. However, if the moisture content is greater than 0.2%, or the compounding equipment is not vented, it is recommended that the polymer be dried to a moisture level below 50 ppm to obtain optimum properties.

Typical drying equipment would include a dehumidifying air hopper dryer with regenerative desiccant beds using the following conditions:

Typical physical properties

Different classes in PTT Fibre

CORTERRA Polymer 200

CORTERRA 200 can be used by itself, as the base polymer for compounds, or as a polymer modifier for many engineering thermoplastic applications. Compounded engineering resins based on CORTERRA PTT polymer provide physical properties that are equivalent to or better than those of similar PBT compounds. Extrusion and injection molding conditions, and processing characteristics are very similar to those established for PBT. PTT compounds generally exhibit higher tensile strength, flexural modulus and heat deflection temperatures but with slightly lower impact strength than the PBT counterparts.

CORTERRA Polymer 9200

CORTERRA 9200 is a clear, PTT homopolymer. The polymer properties which provide resilience, elastic recovery, high bulk, soft hand, inherent stain resistance, and ease of dyeing, make it well-suited for carpet and textile fibers. CORTERRA 9200 is used when a very bright and shiny luster is desired in the end product.

CORTERRA Polymer 9240

CORTERRA 9240 is a semi-dull, PTT homopolymer. The polymer properties which provide resilience, elastic recovery, high bulk, soft hand, inherent stain resistance, and ease of dyeing, make it well-suited for carpet and textile fibers. CORTERRA 9240 has TiO2 added as a dulling agent and is used when a bright or shiny luster is not desired in the end product.

Polymer Production

There are two routes to synthesis PTT: the transesterifi-cation of dimethylterephthalate (DMT) with PDO and the esterification route starting with terephthalic acid (PTA) and PDO - these are similar to PET synthesis. With the exception of the process temperature and catalysts, the synthesis of PTT follows more or less the same chemical rules as the PET process. As a consequence, it is possible, in general, to convert existing PET production facilities to produce PTT. The easiest way is to use an existing redundant PET batch-plant. In any case, it is necessary to have a separate PDO rectification unit available. Depending on the design of the polycondensation reactor, it is possible to produce PTT of an intrinsic viscosity, in a range of 0.65-0.85dl/g. More complicated, but still possible, is the conversion of a continuous polycondensation plant from PET to PTT. 

If the molecular weight of the PTT out of the melt-phase polycondensation is too low it is also possible to increase it afterwards by solid-state polycondensation (SSP). Technology and equipment for the SSP of PTT are again very similar to those of PET solid-state polycondensation processing. Finally, the best but most expensive solution is to build a polycondensation line that is tailor made for PTT. There are reliable equipment and matured melt-phase polycondensation technologies from leading engineering companies, including Zimmer AG and EMS-Inventa- Fischer to produce PTT, to high quality, on continuous lines. To reach the desired intrinsic viscosity in the melt-phase polycondensation, the finisher equipment, as used in PBT production, is useful for PTT. 

Origin for PTT Production

The production of PDO is key to the manufacture of PTT. Both process economy and PTT-polymer quality are strongly dependent on PDO purity. Especially aldehydes and ketones which are side products of the PDO and PTT production should be reduced to a very low level. However, this demand for PDO purity is similar to that for fibre-grade ethylene glycol, as it is used for PET production. Larger industrial facilities producing PDO, including DuPont use the acrolein process from Degussa and Shell, based on ethylene oxide, carbon monoxide and hydrogen. Recently, DuPont has announced the start up of PDO production, based on a biochemical route, using cornstarch as feed stock. DuPont has applied for a patent for this process. Because of high development activities in the field of PTT from some Japanese companies, such as ASAHI, TORAY and TEIJIN one can assume that PDO might soon available from the Japanese chemical industry. Finally, there are two main factors responsible for the velocity of further industrialization of PTT: highly effective polymer production and processing technology, and the price of PDO. 

When the raw material costs of PTT are similar to those of polyamides, we will see fast growth, especially in the application of PTT for fibre and filament spinning. For the spinning industry, the price should not significantly exceed the figure of PET+20 per cent. There is still some distance to travel because PDO is not a commodity and only a few companies are active in this chemistry.

Spinning of PTT Fibre:

Currently, PTT is extrusion spun. Because PTT, like PET, is sensitive to hydrolysis degradation, a drying process is necessary before extrusion. The drying temperature should not exceed 150C because at higher temperatures an oxidative destruction will occur. Unlike PET, PTT chips do not need to be crystallized before drying. When considering the adaptation of the spinning and winding process from PET to PTT, one has to take in account three properties:

• Melt Temperature
• Glass Transition Temperatures
• Intrinsic Elasticity

The lower melt temperature (by approximately 30C) means that there is a shorter time until the spin filament in the thread line is cooled down and, consequently, the quench-air adjustment and the cooling length dimension are different to the PET spinning process. The next important difference to PET is the lower glass transition temperature, which causes much faster cold crystallization. This significantly impacts on the development of the fibre morphology during solidification and cooling down. The spinning conditions of PTT are more comparable to PA6 than to those of PET. Table 1 presents a collection of some polymer and processing properties of PTT and PET. The intrinsic elasticity of PTT is directly connected to the unique molecular structure. Unlike all other known linear polyester polymers, PTT shows a repeatable elastic recovery of about 10-12 per cent. A team of British scientists recently discovered that the crystal structure of PTT is spiral shaped, which explains the elastic recovery of PTT fibres. 

Spinning and winding an elastic filament at high speed will prove more difficult than a non-elastic filament. During POY spinning, the cheese package might tighten or bulge, both of which are undesirable winding failures. Texturing of PTT-POY is, again under adapted conditions, is performed in a similar manner to PET-POY texturing. The last five years have brought a large number of patents related to PTT high-speed spinning processes. A number of those patents have come from Japan, where ASAHI is at the forefront. Any newcomer to PTT spinning should study the patent situation carefully. 

Application of PTT Fibre:

The applications of PTT are to be found mainly in the textile industry. Until now, all of the common fields or textile application, including filament yarns, stable fibre and bulked continuous filament (BCF) for carpets are in an intensive investigation phase. The amount of PTT fibres and filament in production worldwide is estimated to be beneath 50,000t/a.  

There are two main factors responsible for the velocity of further industrialization of PTT: highly effective polymer production and processing technology, and the price of PDO. 

Important driving forces for the development of PTT fibres and filaments are the elastic recovery, soft hand of the fibre and its ease of dyeing. PTT fibres can be dyed at 100C and ambient pressure, without the addition of carrier substances. Because of the lower melt temperature, melt-soluble and dispersible dyestuffs can be used. Dye fastness and light fastness of dyed PTT fibre are, despite the lower dyeing temperature, comparable to those of PET fibre. The combination of fast crystallization and elasticity makes PTT a well-liked candidate for BCF carpet yarn. BCF yarn made of PTT shows in excellent bulk resistance and appearance retention as well as elastic recovery and stain resilience. 

For now, most applications make use of PTTs high elasticity, in leisure and sportswear. Textiles made of PTT fibres combine the properties of common fibres, such as spandex, nylon, acrylic and PET. High stretch, bulk and softness are providing the potential to replace the expensive high elastic polyurethane fibres. Most of the mechanical properties, including tensile or bending strength, are similar to PET but they are also easy to dye. PTT fibres are providing a completely new tool to design consumer fabrics, with optimal processing and application values. Finally, the price of PTT will dictate the speed at which it replaces other fibres. The real earnings in PTT are hidden in large quantities.  

Comparison of polymer properties and process features for spinning

PTT and PET

Conclusion:

Unlike other introductions of new polymers to the industry, the introduction of PTT was accompanied by powerful marketing activities from the first. Shell has announced that it has a PDO production unit of 75,000t/a capacity on stream. Shells PTT polymer production is estimated to be in the range of 21,000t/a. Shells PTT activities are covered by the trade name Corterra. In 1998, DuPont acquired the PDO production of Degussa in Wesseling, Germany. This plant is producing about 12,000t/a PDO with the option to expand the capacity to 50,000t/a. DuPonts PTT activities are covered by the trade name Sorona.

The question of poly trimethylene terephthalate (PTT) is only appearing now and not 60 years ago, when it was invented, has many different answers. One reason is that at that time of invention, poly ethylene terephthalate (PET) was found to be the polymer with the broadest application potential in different industrial fields, including films for video and audio, textile fibre, technical films and packaging. Another reason was the limited avail-ability of 1,3-propanediol (PDO), which was expensive. However, PDO is now playing the major role in the industrial development of PTT. 

References:

1)       www.freepatentsonline.com/6495254

2)       www.intota.com/multisearch.asp

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4)       sciencelinks.jp/j-east/article

5)       www.fbi.gov/hq/lab/fsc/backissu/july2001/houck.htm

6)       www.wipo.int/pctdb/en/wo.jsp

7)       www.swicofil.com/polytrimethyleneterephthalate.html

8)       en.wikipedia.org/wiki/Polyethylene_terephthalate

9)       www.springerlink.com/index/AV1DWAWEDCQPHTC3.pdf

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