Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in tubular or loop reactor
Reexamination Certificate
2000-10-03
2004-04-13
Harlan, Robert Deshon (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymerizing in tubular or loop reactor
C526S065000, C422S109000, C422S110000, C422S132000, C422S135000, C422S138000
Reexamination Certificate
active
06720393
ABSTRACT:
This invention relates to the determination of polymer characteristics. In practice, polymer is typically melt processed in onward production or ‘conversion’—and so measurement of Melt Flow Index (MFI) is of particularly relevance.
Key polymer properties, such as tensile strength, solubility, impact resistance and melt viscosity, are associated with (average) molecular weight. For a given polymer, a higher molecular weight generally affords greater resistance to impact. On the other hand, lower molecular weight polymers are useful for thin films or filaments.
Polymer conversion requires close control of melt viscosity—which is related to molecular weight, so that:
the polymer ‘melt viscosity’ (or melt flow index—MFI) is suitable for the conversion process (such as moulding or extrusion); and
a polymer product has desired physical characteristics.
To address this, most polymers are available in a range of average molecular weight grades. In practice, these grades are specified in terms of melt viscosity (MFI)—for example to 5% tolerance of target specification.
Melt viscosity is prescribed under a standard test regime, intended as ‘static’ and stable—and is routinely measured using a so-called ‘manual grader’, as specified in ASTM D 1238 standard; being ‘reported’ as ‘melt flow index’ (MFI).
However, the standard test is not readily achieved, or directly usable, for production control. The Applicants have devised an improved ‘dynamic’ test regime, which offers improved consistency and responsiveness—and admits the possibility of MFI as a polymer production control factor. Essentially, control of MFI requires control of average molecular weight during polymerisation.
In polymer production, rapid and reliable MFI measurement allows a more precise assessment of a polymerisation (reactor) output and so enables effective reactor process control—ultimately on-line and in real-time.
A polymerisation reaction typically generates polymer in a powder form, which is highly reactive and difficult to handle. This polymer powder is therefore commonly converted into a more stable, and ‘user-friendly’, granular or pelletised form, for collection, storage and onward transport to customer end users and re-processors.
Pelletisation or granulisation is typically undertaken in an extruder, as is any downstream (mechanical) mixing, blending and re-processing. Precise MFI knowledge during extrusion in such conversion and onward re-processing is just as relevant as in original polymerisation.
Overall, Aspects of the Invention Variously Address
(MFI) viscometry for ‘rapid-response’ determination of polymerisation reaction output;
(on-line) reactor control through (MFI) viscometry; and
post reaction powder-granule conversion and re-processor blending.
Polymerisation Catalytic Reaction Control
Polymers are commonly produced,ultimately as a fine powder particulate, by continuous catalytic polymerisation, initially in a gas and/or slurry phase.
Polymerisation is influenced, inter alia, by reaction time, temperature, pressure, monomer, catalyst and impurities concentrations in the reaction mixture.
A limited range of factors is available for such catalytic reaction control. Primarily, control of molecular weight is achieved through inhibition of chain growth. In turn, chain growth inhibition involves addition of a monomer inhibitor agent (a so-called Terminating Agent, Chain Transfer Agent or CTA), such as hydrogen.
More particularly, in (continuous) ‘catalysed’ polymerisation, to a target MFI specification, reaction is controlled by either:
the addition of a ‘chain terminator’, which reacts with a catalyst to stop polymerisation; or, more usually,
the introduction of a ‘chain transfer agent’ (CTA), which reacts with the growing polymer chain, to prevent further polymerisation—but leaves the catalyst able to start polymerisation of a new chain.
Thus, both MFI determination and hydrogen (inhibitor) input control can be used constructively to control molecular weight.
Control of polymer chain growth is a ‘delicate’ process, with a need to relate reactor chain transfer agent concentration to molecular weight. The relationship is process dependant—taking account of individual reactor and catalyst characteristics.
W09324533, EP002747 and U.S. Pat. No. 3,356,667 variously rely upon monitoring, and adjustment, of diverse reaction factors, such as temperature, pressure, CTA and/or monomer concentration—as a basis for computation and control of output polymer MFI. W09641822 adopts a predominantly mathematical approach—but a significant time correction, between sampling and MFI result, must be factored into the analysis.
None of this art addresses direct, on-line, viscometric measurement of polymer output MFI, as a continuous on-line control factor—which in turn justifies special visometric methodology and apparatus. Conventional viscometry is time-consuming, and performed off-line, commonly in laboratory controlled conditions.
Formative steps towards closed-loop control have been taken with the emergence of more reliable MFI measurement technology, such as the Applicants' own earlier work. An example is the Applicants' (model P5) viscometer and attendant interpolative (graphical) measurement technique—as taught in their UK Patent No. 2210466.
A polymerisation reaction (process) control aspect of the present invention places even greater reliance, upon responsive MFI measurement—and embraces refinements to the Applicants' (model P5) viscometer technology and attendant measurement technique.
Practical Difficulties of On-Line Process Control
In practice, it can be very difficult to maintain completely steady conditions in the reactor. Disturbances in changing grades to the are inevitable—with attendant MFI ‘drift’ from target specification. Frequent MFI measurements on polymer from the reactor guide adjustments in the flow of chain transfer agent, to correct any such drift.
Failure to compensate adequately for disturbances results in material properties outside prescribed MFI limits, and so one which must be sold at a substantial discount compared to ‘first (or consistently to specification) grade’ polymer.
These ‘downgrading’ (or off-specification) losses can be reduced by post-reaction blending of product and restricting the frequency of grade changes—but the capital equipment and attendant running costs are high and there are limits to the ‘spread’ of MFI's which can be dealt with in this way.
The number of ‘grade’ changes can be reduced by making larger-quantities of each grade at longer intervals, but the cost of the higher stock levels needed eventually outweighs the benefits. Despite this, between some 5-15% of product of conventional process control regimes is commonly downgraded by poor MFI control. Even ‘good’ product produced by this approach has a large ‘scatter’ of MFI.
In a typical case, the chance of a customer end user or re-processor receiving two successive batches at opposite extremes of the MFI range could be 1 in 26, rather than 1 in 1600 or more if effective MFI control were used.
Such poor performance is the result of inadequate, or inaccurate, understanding of the relationship between MFI and concentration (or flow) of chain transfer agents (or terminators). This applies to both ‘steady state’ and dynamic conditions.
System Errors
The relationship is often totally obscured by a series of system errors, such as:
Inaccurate measurement of MFI. Studies of the manual method by the ASTM (standards body), allied to industrial experience, have shown that a standard deviation of 5% in the middle of the operating range, rising to 15% at the upper and lower limits, can be expected for a single (given) reading.
Although other means of ‘measuring’ MFI exist which give lower scatter, the relationship between their values and MFI are neither simple nor consistent. This introduces an equally serious error.
Unrepresentative samples of reactor product.
In many cases the sampling device is incorrectly designed and/or situated, so that a sample drawn is not representative of reactor contents.
George Alan
Harper Peter
Dorsey & Whitney LLP
Harlan Robert Deshon
Porpoise Viscometers Limited
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