ORIENTED POLYESTER

Abstract: This invention relates to a process for producing oriented polyhydroxyalkanoate (PHA) that requires less stringent temperature control than previous processes. The process comprises melting the PHA, shaping preform from the melted PHA, stretching the preform a first time in at least one direction, and stretching the preform a second time. In one embodiment, the preform is cooled prior to stretching a second time. In another embodiment, the PHA is laminated with a polymer.

Description: ORIENTED POLYESTER This invention relates to oriented polyester and in particular to oriented shaped articles made of stereospecific polyhydroxyalkanoate (PHA). EP-A-0104731 describes a process of making oriented shaped articles, specifically fibre but possibly others such as film, by extruding a preform, effecting a controlled degree of crystallisation, drawing the article and increasing its degree of crystallinity. Such a process, while effective, is difficult to control and inconvenient if it is desired to change line speed. We have now discovered a procedure in which the need for such careful control is lessened and by which PHA film of attractive properties can be made. According to the invention a process of producing an oriented PHA article comprises: (a) melting PHA; (b) shaping preform therefrom; (c) allowing the preform to cool and solidify; and (d) stretching the preform to cause orientation of the PHA; and is characterised by: between steps (b) and (c) stretching the preform before it has fully solidified. The stretch between steps (b) and (c) will be referred to as "first stretch" and the stretch in step (d) as "second stretch." The first stretch takes place at a temperature less than the melting point of the PHA typically by 5-20°C, but by not more than 60°C. The temperature profile of the article is controlled to afford the required extent of first stretch. Conveniently this is effected by a current of gas, especially air, at a suitable temperature and flow rate. The time span of steps (b) and (c) and of first stretch are controlled to bring the preform to a level of crystallinity sufficient to avoid adhesion between PHA layers or between a PHA layer and a guiding surface such as a roller. The extent of first stretch is incomplete to permit further stretching in step (d) and is, in any event, limited by what is practicable in the incompletely solid condition of the PHA and the fact that it is not accompanied by significant orientation. Typically it is by a factor in the range 2 to 10 in at least one direction. First stretch may be in the machine direction or transverse direction or both. Preferably the preform is a tube and is flattened after solidification; and the first stretch is effected by pressure of fluid, suitably applied coaxially of the extrusion die and contained by the closed end of the tube between flattening rolls. Typical extents of first stretch are in the range 5 to 10 (machine direction) and 2 to 5 (transverse). The first stretched preform can be passed to step (d) immediately or after a convenient period, which can be up to 10 minutes but possibly longer depending on process conditions. Step (d) is effected by techniques differing according to the shape of the article being made. For fibre it simply consists in drawing the cast fibre away from a draw point faster than it is being fed to that point; a suitable draw ratio is at least 3, for example in the range 5 to 8. For flat film uni-axial forward stretching is analogous to fibre drawing; instead or, more usually, in addition, transverse stretching is practised, using stenter clamps applied to the sides of the advancing film. For hollow articles such as tubes transverse stretching may be by compressed gas. If desired, tube formed in step (b) may be slit to flat film before step (d) or before heating step for step (d). The temperature at which step (d) stretching is effected is preferably within 60 to 1 10°C of the PHA melting point. The degree of stretch in step (d) is preferably in the range 60-80% of the maximum possible, so as to provide for some residual elongation in the finished article. It appears that uni¬ axial stretching using the process of the invention gives substantial strength in the transverse direction without the need for much, if any, stretching in the transverse direction. The articles to which the invention is especially applicable are those having relatively thin regions, for example 0.01 to 2, especially 0.1 to 1, mm thick. The process can include co-extrusion or lamination of PHA with a different polymer; the web of such other polymer may be separated from the layer of PHA before the heating step or after the heating step and before step (d), but may be left in position if a duplex article is required and if that polymer has been chosen to withstand the conditions of the heating and stretching steps. Suitable PHAs comprise units of Formula I: -O-C m H n -O- where m is in the range 1-13 and n is 2m or (except when m is unity) 2m-2. Typically C m H n contains 2-5 carbon atoms in the polymer chain and the remainder (if any) in a side chain. In very suitable PHAs m is 3 or 4, n is 2m and especially there are units with m = 3 and m = 4 copolymerised together with respectively a C, and C 2 side chain on the carbon next to oxygen. Particular PHAs contain a preponderance of m = 3 units, especially with at least 70 mol % of such units, the balance being units in which m = 4. The molecular weight of the PHA is for example over 50000, especially over 100000, up to eg 2 x 10 6 . PHA containing only m = 3 units is referred to as PHB; PHA containing m = 3 and m = 4 units is the copolymer PHBV. PHBV preferably contains 4-30 especially 12-25, mol % of m = 4 units. The PHA can also be a blend of two or more differing in the value of m. A particular example contains: (a) PHA consisting essentially of units in which 2-5 mol % of units have m = 4, the rest m = 3; and (b) PHA consisting essentially of Formula I units in which 15-30 mol % of units have m = 4, the rest m = 3. The proportions in such a blend are preferably such as give an average m = 4 content in the range 12-25%. The PHA may be the product of a microbiological process. The microorganism may be wild or mutated or may have had the necessary genetic material introduced into it. Altematively the necessary genetic material may be harboured by a eucaryote, to effect the microbiological process. Microbiologically produced PHA is chiral (R) and stereospecific. Examples of suitable microbiological processes are the following: for Formula I material with m = 3 or m = partly 3, partly 4: EP-A-69497 (Alcaligenes eutrophus): for Formula I materials with m = 3, US4101533 (A. eutrophus ,. EP-A-144017 (A. latus); for Formula I material with m = 7-13, EP-A-0392687 (various Pseudomonas). The microbiological production of the PHA is preferably carried out in two stages: (a) aerobic growth of microorganisms and (b) aerobic fermentation of the resulting organisms in a medium containing a carbon source but deficient in at least one nutrient essential for growth. The deficient nutrient is preferably phosphate. The PHA is then separated from the PHA-containing cells by a harvesting process in which NPCM is solubilised leaving PHA of high purity, preferably at least 96, especially at least 98%. One preferred form of the process is shown in Figure 1, which is a schematic representation of the plant in which the production of oriented film may be carried out. Extruder 10 feeds into tubular die 12 fitted with coaxial air supply indicated schematically by 14. Die 12 is disposed in temperature-controlled enclosure 16 and feeds upwards towards the nip of flattening rolls 18 and 20, in open air above enclosure 16. From rolls 18, 20 the flattened preform passes around roll 22, then through a slitter/opener (not shown) on to roll 24 held at 70°C, which runs at the same speed as the feed of preform. From roll 24, however, the preform passes over fast roll 26 (also at 70°C) and through the nip of fast rolls 28, 30 and is stretched in the machine direction. If desired a transverse stretch can be effected after rolls 32, 34 by a stenter (not shown). The stretched film is then led out to storage. In a particular example the PHA was a BV copolymer containing 8 mol % of V and having a molecular weight about 400000. The extruder was a 20 mm laboratory unit feeding a 25 mm diameter circular die 12 in an upward direction. The atmosphere circulated about the extruded preform and through a nozzle in the centre of the die was initially at 25°C, permitting the emerging extrudate to widen to about 50 mm and to crystallise sufficiently in its upward passage to flattening rolls 18, 20 (time about 15 sec) to avoid self-adhesion. The main stretch in rolls 28, 30 was 300-400%. The resulting film was 15-20 μm thick. Typical properties of the product film were: Machine Direction Transverse Tensile strength MPa 90 80 Elongation to break % 130 150

Claims: WHAT IS CLAIMED IS: 1. A process for producing oriented polyhydroxyalkanoate (PHA), comprising: melting the PHA; shaping preform from the melted PHA for a suitable time; stretching the preform a first time in at least one direction by a suitable factor under suitable conditions of time and temperature; and stretching the preform a second time to a suitable degree at a suitable temperature. 2. The process of Claim 1 further comprising cooling the preform for a suitable time prior to said second stretching. 3. The process of Claim 1 further comprising maintaining the temperature of said first stretching by applying a current of gas under suitable conditions of temperature and flow rate. 4. The process of Claim 3 wherein the gas is air. 5. The process of Claim 1 wherein said first stretching comprises applying a fluid at a suitable pressure. 6. The process of Claim 1 wherein the temperature of said first stretching is 5 °C to 20 °C below the melting point of the PHA. 7. The process of Claim 1 wherein the temperature of said first stretching is not more than 60 °C below the melting point of the PHA. 8. The process of Claim 1 wherein the factor of said first stretching is 2 to 10. 9. The process of Claim 1 wherein the temperature of said second stretching is within 60 °C to 110 °C of the melting point of the PHA. 10. The process of Claim 1 wherein the degree of said second stretching is 60% to 80% of maximum. 11. The process of Claim 1 wherein the PHA is a product of a microbiological process. 12. The process of Claim 1 wherein the PHA consists of repeating units of: - O - C m H n - CO - wherein m is between 1 and 13 and n is 2m or 2m- 1. 13. The process of Claim 1 wherein the PHA is polyhydroxy-butyrate-co-valerate. 14. The process of Claim 1 wherein the preform is a tube. 15. The process of Claim 1 further comprising laminating the PHA with a polymer.

Inventors: WALDOCK PAUL, WALDOCK, PAUL
Applicants: MONSANTO CO, WALDOCK PAUL, MONSANTO COMPANY, WALDOCK, PAUL
Application year: 1996
Main classification: B29C55
IPC codes: B29C55, B29K67, B29L7, C08J5, C08L67