Polymers having improved processability

Details Polymers having improved processability Polymers having improved processability

1998-04-17

2001-07-03

Truong, Duc (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

From carboxylic acid or derivative thereof

C528S355000, C528S495000, C528S50200C, C528S503000, C264S045200, C264S045800, C264S045900

Reexamination Certificate

active

06255445

ABSTRACT:

This invention concerns poly(lactones), particularly poly(caprolactones), and more particularly poly(lactones) having improved processability, especially permitting the poly(lactone) to be film-blown.
Poly(lactones) and particularly poly(caprolactones) have a number of desirable properties, including good biodegradability, tensile strength and toughness, and a relatively low melting point. These properties make the use of poly(lactones) particularly desirable in the polymer industry. Many potential applications for poly(lactones) require the poly(lactone) to be in the form of a film. However, poly(lactones) are commonly produced in non-laminar form, for example in block, powder or granular form, amongst others. It is therefore necessary for poly(lactones) to be processed to produce the desired film. One method commonly employed for processing polymers into a film is film-blowing, where a polymer melt is extruded through a circular die, and the resultant tube of extrudate is inflated and cooled by a gas, commonly air, to produce a bubble. The bubble is then flattened to produce a double thickness or tubular film, which may be used as such, or may be cut and opened out to form a single thickness film.
Several methods to produce films comprising a poly(lactone) have been proposed in the art. European patent application no. 0 003 846 discloses that a composition comprising a poly(lactone) that is suitable for film-blowing can be produced if a lactone is polymerised with a polyfunctional acrylate, followed by subsequent cross-linking of the polymer by exposure to radiation. Goldberg et al in a paper entitled “Degradation of Polycaprolactone Films in Soil and Compost Mediums” teach that an 80,000 molecular weight poly(caprolactone) could be film-blown under given conditions. Japanese patent application no. 6-143412 also teaches that linear 80,000 molecular weight poly(caprolactonel could be film-blown using a specifically designed film-blowing process including the use of a water cooling ring appropriately positioned on the bubble. The process is complicated, and requires stringent control of a number of process variables. The process is also restricted to apparatus comprising the water cooling ring, and a blow-up ratio of 1:1.5 to 2. In the course of practical studies leading up to the instant invention, it was found that it was not a simple matter to film-blow linear 80,000 molecular weight poly(caprolactone), and even with strict control of the film-blowing process, good results were difficult to achieve. Attempts to film blow such linear poly(caprolactones) produced an unstable bubble, excessive longitudinal stretching or collapse of the tubular melt at the die and welding together of the film at the nip rollers.
It is an object of one aspect of the present invention to provide poly(lactones) having improved processability and/or which avoid or ameliorate the problems associated with the production of films by film-blowing of prior art poly(lactones).
It is a second object of further aspects of the present invention to provide a process for film-blowing poly(lactones).
According to one aspect of the present invention, there is provided a poly(lactone), characterised in that:
a) the poly(lactone) is initiated with an initiator system comprising at least one polyfunctional initiator,
b) when shear viscosity is measured at some temperature in the range of from 35 to 45° C. above the melting temperature of the poly(lactone), the poly(lactone) has a shear viscosity of at least 6 kPas at 0.1 s
−1
shear rate and no greater than 3.5 kPas at 100 s
−1
shear rate, and
c) the poly(lactone) has a shear viscosity activation energy of greater than 40 kJ/mol.
According to a second aspect of the present invention, there is provided a process for film-blowing a poly(lactone), characterised in that:
a) the poly(lactone) that is film-blown is initiated with an initiator system comprising at least one polyfunctional initiator,
b) when shear viscosity is measured at some temperature in the range of from 35 to 45° C. above the melting temperature of the poly(lactone), the poly(lactone) that is film blown has a shear viscosity of at least 6 kPas at 0.1 s
−1
shear rate and no greater than 3.5 kPas at 100 s
−1
shear rate, and
c) the poly(lactone) that is film blown has a shear viscosity activation energy of greater than 40 kJ/mol.
According to a third aspect of the present invention, there is provided a polymerisation process for the production of a poly(lactone) by reaction between a lactone and an initiator system, characterised in that:
a) the initiator system comprises at least one polyfunctional initiator, and
b) the reaction is continued until the poly(lactone) produced thereby has a shear viscosity when measured at some temperature in the range of from 35 to 45° C. above the melting temperature of the poly(lactone) of at least 6 kPas at 0.1 s
−1
shear rate and no greater than 3.5 kPas at 100 s
−1
l shear rate, and a shear viscosity activation energy of greater than 40 kJ/mol.
For the avoidance of doubt, all molecular weights quoted herein are expressed in g/mol, unless otherwise stated.
Poly(lactones) that can be employed in the present invention comprise a backbone having a repeating unit of general chemical formula:
[—O—(C═O)—(CR′R″)
n
—]
and are commonly derived from the ring opening polymerisation of lactones having the general chemical formula:
wherein R′ and R″ are independently hydrogen atoms or alkyl groups having up to 12 carbon atoms and n is from 3 to 7, and preferably n is 5. In many embodiments the total number of carbons in R′ and R″ is 0 or 1 to 4, and preferably at least one of R′ and R″ is a hydrogen atom. Particularly preferably, the poly(lactone) is a poly(&egr;-caprolactone).
The poly(lactones) of the present invention exhibit significant shear thinning properties, ie at low shear rates, they have a viscosity significantly higher than the viscosity at high shear rates. The poly(lactones) are selected to have a viscosity at low shear rates such that when the poly(lactones) are film-blown, the viscosity is high enough to produce a dimensionally stable tubular melt and/or bubble. However, the viscosity is preferably not too high, because the higher processing temperature required for the poly(lactones) can cause excessively delayed solidification of the poly(lactone) in the bubble, thus causing bubble instability and can also cause incomplete solidification of the poly(lactone) at the nip rollers resulting in welds in the interior of the bubble. A high viscosity can also increase the tendency of the poly(lactones) to suffer from melt-fracture, which results in a film having an unacceptable appearance.
The shear viscosity of the poly(lactones) when measured at some temperature in the range of from 35 to 45° C. above the melting temperature of the poly(lactone) at a shear rate of 0.1 s
−1
(hereinafter called low-shear viscosity) is at least 6 kPas, often at least 8 kPas, and preferably at least 10 kPas. The low-shear viscosity is usually less than 120 kPas, often less than 90 kPas, more often less than 30 kPas, and preferably less than 20 kPas. Particularly preferably, the low-shear viscosity is in the range of from 11 kPas to 18 kPas, especially from 12 kPas to 15 kPas. The polyllactones) often exhibit the defined low-shear viscosity over at least half, and preferably all, of the temperature range.
The viscosity at high shear rates is preferably not too high, or the power consumption of the extruder may be excessive, and also there may be an unacceptable temperature rise during the extrusion process, although to some extent, these disadvantages can be ameliorated by modifications to the extruder, such as alterations of the screw geometry. The shear viscosity of the poly(lactones) when measured at some temperature in the range of from 35 to 45° C. above the melting temperature of the poly(lactone) at a shear rate of 100 s
−1
(hereinafter called high-shear viscosit

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