WALKER, Richard (23 Blakesley Road, Wigston, Leicester LE18 3WD, GB)
BEANEY, Paul (9 Whitethorn Avenue, Newtownards BT23 8WT, GB)
WALKER, Richard (23 Blakesley Road, Wigston, Leicester LE18 3WD, GB)
| Claims 1. A method of extruding a unit of a recycled polymer feedstock material comprising at least the steps of: (a) providing the recycled polymer feedstock material; (b) passing the feedstock material of step (a) through an extrusion system; (c) applying ultrasonic energy to the feedstock material during step (b); (d) measuring one or more properties of the feedstock material and/or one or more processing conditions during the extrusion process, and either before step (c), after step (c), or both; (e) real time analysing the measurements of step (d); and (f) real time reviewing the application of ultrasonic energy in step (c) to control one or more properties of the feedstock material based on the analysis of step (e). 2. A method of modulating viscosity of a unit of recycled polymer feedstock material in an extrusion system comprising at least the steps of: (i) applying ultrasonic energy to the recycled polymer feedstock material in an extrusion system; (ii) measuring one or more properties of the feedstock material and/or one or more processing conditions during the extrusion process, either before step (i), after step (i), or both; (iii) real time analysing the measurements of step (ii); and (iv) real time reviewing the application of ultrasonic energy of step (i) to control one or more properties of the feedstock material based on the analysis of step (iii). 3. A method as claimed in Claim 1 , wherein step (e) further comprises comparing the real time analyses of the measurements of step (d) against pre-set processing conditions. 4. A method as claimed in Claim 1 , wherein step (e) further comprises comparing the real time analyses of the measurements of step (d) against model processing conditions. 5. A method as claimed in Claim 2, wherein step (iii) further comprises comparing the real time analyses of the measurements of step (ii) against pre-set processing conditions. 6. A method as claimed in Claim 2, wherein step (iii) further comprises comparing the real time analyses of the measurements of step (ii) against model processing conditions. 7. A method as claimed in any preceding claim, wherein the recycled polymer feedstock material is one or more of the group comprising: HDPE, PET, PP, LDPE, LLDPE, polystyrene. 8. A method as claimed in claim 7 wherein the recycled polymer feedstock material is HDPE. 9. A method as claimed in claim 8 wherein the recycled polymer feedstock material is PET. 10. A method as claimed in any one of the preceding claims wherein the one or more properties of the feedstock material measured during the extrusion process comprises one or more of the group comprising: viscosity, temperature, flow rate, power, torque, pressure, throughput. |
The present invention relates to methods and apparatus for polymer extrusion and extrusion compounding of recycled polymer material.
The extrusion of many plastic products is a large industry, and therefore an important industrial process. The optimisation of the processing conditions to maximise output is a key desire. However, controlling the quality of the final or extrudate material, and hence the final product, presents various problems.
There are many methods and apparatus for measuring the viscosity of an extrudate material in an extrusion system, including indirect viscosity measurements, side stream rheometry, in-line rheometry, in-line TOV viscometry.
There are also many suggestions for the process and feedback systems, such as that shown in GB1346095 and WO2008/040943 A2.
Where the feedstock for the extrusion is a known or standard material, such material often termed 'virgin polymer material'. Such material has the basic properties of the feedstock, such as its density and molecular weight. These properties are therefore considered 'known', such that such feedstock will act in a predictable manner through the extrusion process, and constant measurement of such properties is not required. However, there is an increasing desire to use extrusion processes for recycled plastics materials, whose properties will vary, and therefore whose properties cannot be readily predicted. The viscosity and remote measurements and feedback systems
mentioned above are only for measuring and controlling extrusion processes based on standard or regular feedstock materials, having standard or regular properties such as molecular weight throughout. For example, high-density polyethylene (HDPE) is a well known polymer material used for extrusion, and virgin HDPE material that is conventional feedstock has a melt flow index ("MFI", sometimes also termed 'melt flow rate' or 'melt flow index') anywhere in the range of 0.02 to 9g/10min for the various different grades available. For forming drainage pipes and the like, the MFI for the virgin HDPE material is generally in the range 0.25 - 0.3g/10min.
MFI is defined as the mass of polymer, in grams, flowing in 10 minutes through a capillary of a specific diameter and length by a pressure applied, according to ASTM D1238 and IS01 133.
It is the obvious intention of HDPE suppliers to supply their virgin polymer product with the minimum variational tolerance in properties such as MFI, so that the conventional maximum tolerance generally agreed for virgin HDPE material is +10%.
However, the properties of recycled polymer material obviously vary, not only within each batch, but from batch to batch also, so that the
assumptions used in prior art measurement and feedback extrusion systems cannot be used. Variable or inconsistent polymer feedstock material naturally has variable and unpredictable melt flow properties, and/ or a continuously variable and unpredictable amount of contaminant.
Reprocessed polymer can be inconsistent as it may contain different grades or types of polymer which have a range of thermal and physical properties and may or may not be immiscible. Other inconsistent polymer feedstocks include polymer materials with fillers added such as calcium carbonate or nanoclay, resulting in a material which may not be uniform throughout. Known contaminants in recycled polymer material include but are not limited to paper and metal foils. Other contamination may include a different polymer which is outside the specification of what is required in the extrusion material or blend, resulting in material properties outside of the material specification.
Thus, according to one aspect of the present invention, there is provided a method of extruding a unit of recycled polymer feedstock material comprising at least the steps of: (a) providing the recycled polymer feedstock material;
(b) passing the feedstock material of step (a) through an extrusion
system;
(c) applying ultrasonic energy to the feedstock material during step (b);
(d) measuring one or more properties of the feedstock material and/or one or more processing conditions during the extrusion process, either before step (c), after step (c), or both;
(e) real time analysing the measurements of step (d); and
(f) real time reviewing the application of ultrasonic energy in step (c) to control one or more properties of the feedstock material based on the analysis of step (e). In one embodiment, step (e) may further comprise comparing the real time analyses of the measurements of step (d) against pre-set processing conditions.
In one embodiment, step (e) may further comprise comparing the real time analyses of the measurements of step (d) against model processing conditions.
In this way, rapid variation of ultrasonic energy can be applied based on the outcome of steps (d) and (e) because of the possibly rapidly changing nature of the feedstock material and/or processing conditions. Such processing conditions include one or more of the group comprising:
temperature, time, screw speed, ultrasonic energy.
The outcome of step (e) may also change on or more of the processing conditions, and/or other processing operations or parameters such as barrel temperature.
It will be appreciated that where step (e) further comprises comparing the real time analyses of the measurements of step (d) against pre-set or model processing conditions, the real time review of the application of ultrasonic energy of step (f) will be based on the analysis and the comparison of step (e). According to a second aspect of the present, there is provided a method of modulating viscosity of a unit of recycled polymer feedstock material in an extrusion system comprising at least the steps of: applying ultrasonic energy to the recycled polymer feedstock material in an extrusion system;
measuring one or more properties of the feedstock material and/or one or more processing conditions during the extrusion process, either before step (i), after step (i), or both;
real time analysing the measurements of step (ii); and
real time reviewing the application of ultrasonic energy of step (i) to control one or more properties of the feedstock material based on the analysis of step (iii).
In one embodiment, step (iii) may further comprise comparing the real time analyses of the measurements of step (ii) against pre-set processing conditions.
In one embodiment, step (iii) may further comprise comparing the real time analyses of the measurements of step (ii) against model processing conditions.
It will be appreciated that where step (iii) further comprises comparing the real time analyses of the measurements of step (ii) against pre-set or model processing conditions, the real time review of the application of ultrasonic energy of step (iv) will be based on the analysis and the comparison of step (iii). The term "recycled polymer feedstock material" as used herein can be defined as a polymer feedstock material having a property such as melt flow index or a viscosity with a tolerance of +>10%. Optionally, +>15%, +>20%, +>25%, +>30%, +>35%, +>40%, +>45%, +>50% per unit.
The term "unit" as used herein can be defined as one or more of the group comprising: weight, time. Examples of weight include per kg, tonnes or a multiple thereof such as 20 or 30 tonnes. Examples of time include per hour, per day or a multiple thereof.
As mentioned above, MFI is a well known property measurement for HDPE, and indeed other polyethylenes such as LDPE and LLDPE.
For polymer materials such as PET (polyethylene terephthalate), viscosity or intrinsic viscosity is a common measured property, and indeed melt flow rate is inversely proportional to viscosity. For PET, generally measured in decilitres per gram (dl/g), there are again ranges depending on the type of PET grade. 'Bottle grade' PET generally has an intrinsic viscosity range of 0.70 - 0.78 dl/g for flat water bottles, and 0.78 - 0.85 dl/g for carbonated soft drink grade.
Whilst the properties of virgin polymer materials used in extrusion processes are well known, the problem solved by the present invention is the variation in recycled polymer material due to the recycling of all the different grades of polymer material together. Thus, whilst PET
carbonated drinks bottles made of virgin PET material will have an intrinsic viscosity of a tightly defined range as exampled above, recycled material is formed from 'any' PET grade products. For example, textile fibre grade of PET can have an intrinsic viscosity of 0.4 dl/g, and monofilament grade PET can have an intrinsic viscosity of 2.00 dl/g. Such a variation in this property in the feedstock leads to difficulty in controlling the extrusion process in comparison with the minimum tolerance of the same property in virgin material. The skilled man is aware of the desire during any extrusion process to have close control over properties such as viscosity and temperature at one or more points along the extrusion barrel, so as to achieve most consistent final product having the desired final product properties. The solution of the present invention is to use ultrasonic energy and an analysis and review arrangement to accommodate variation expected in the recycled polymer feedstock material.
Applying ultrasonic energy to a polymer material in an extrusion system is known in the art and not further discussed in detail herein. Ultrasonic energy can be applied in an extrusion process or system through a number of apparatus, devices, units or systems, generally at one or more locations along the extrusion system, in particular along the extrusion barrel.
Ultrasonic energy can be applied with a variable frequency and/or amplitude, preferably amplitude over a range of 30-40 microns, and is instantly changeable and variable in line with the general process control. The nature of the ultrasonic energy, the application of the ultrasonic energy at one or more locations in the extrusion system, and the variation of ultrasonic energy are not limited in the present invention and will be understood by a person skilled in the art.
The measuring of one or more properties of the feedstock material during the extrusion process and/or in the extrusion system is also known in the art, and generally involves measurement of one or more of the group comprising: viscosity, temperature, flow rate, power, torque, pressure, and throughput. Thermocouples, sensors and the like for measuring such properties are known in the art, and not further described herein.
Analysis of the measurements carried out as described above are well known in the art, and are not further described in detail herein. Such analysis can be carried out without any comparison or reference, but is preferably carried out in comparison or reference with one or more models or programs.
Acquiring the measurements of the one or more process conditions of the extrusion system may comprise continuously acquiring the measurements of the one or more process conditions. Such measurements may be acquired using one or more analogue measuring devices.
Acquiring the measurements of the one or more process conditions of the extrusion system may comprise acquiring a measurement of the or each process condition at a plurality of sample times. This is referred to as sampling the or each process condition. Such measurements may be acquired using one or more digital measuring devices.
The measurements of the one or more process conditions may comprise temperature measurements of the extrusion system at one or more locations thereof. The temperature measurements may comprise temperature measurements from an extrusion barrel of the extrusion system at the one or more locations of and/or along the barrel.
The measurements of the one or more process conditions may comprise indications of the speed of rotation of at least one screw of the extrusion system. The indications of the speed of rotation of the at least one screw of the extrusion system may be generated by acquiring measurements of a voltage or current input to the at least one screw and using the voltage or current measurements to determine the speed of rotation of at least one screw.
The indications of the extrudate material throughput may comprise measurements of the rate at which the extrudate material is fed through the extrusion system. The measurements of the rate may be acquired using a feedrate sensor. The indications of the extrudate material throughput may comprise indications of the speed of rotation of at least one screw of the extrusion system.
The measurements of the pressure of the extrudate material at one or more locations of the extrusion system may comprise measurements from a pressure sensor placed in an end part of an extrusion barrel of the extrusion system. The end part of the barrel may be adjacent to a die of the extrusion system. The method may further comprise acquiring measurements of power and/or torque used by at least one screw of the extrusion system, either by direct or indirect measurement.
Reviewing the application of ultrasonic energy based on the analysis of the measurements as described above is also well known in the art to the skilled man, and includes any type or form of feedback processing, system or measuring, to allow the user to periodically and/or continuously understand the processing or system parameters or properties, and achieve best mode in relation to the desired extrudate material properties. For example, an ultrasound probe can be used to modulate the viscosity in response to changes in the recycled feedstock material properties. The intensity of the ultrasound signal can be adjusted to compensate for increases and decreases in the measured melt viscosity over time using the viscosity signal from the soft sensor as an input into a controller. This controller manipulates the input signal to the ultrasound probe to adjust the intensity of the ultrasound energy.
The present invention is particularly concerned with the mechanical recycling or direct circulation of polymer materials, which are able to recycle all or most grades of various polymer materials, in particular HDPE, PET, PP, as well as Polystyrene, LDPE, LLDPE and others known to those skilled in the art. One common method of mechanical recycling such polymer materials, which can include external material already used, as well as non-product internal material otherwise considered 'waste' in polymer material manufacturing, can involve a number of steps, including one or more purifications, decontaminations, gradings (e.g. by colour, weight, sort), sortings, selections, cuttings, siftings, grindings, rinsings, cleanings and other processing steps.
By way of example only, recycled polymer products can proceed by the sorting and selection for different colours, foreign polymers, foreign matter, removal of unwanted material such as film, paper, glass, sand, soil, metals, etc; prewashing; coarse cutting; further grading or sorting of close polymer materials such as LDPE and HDPE, further washing, and further cutting. A typical recycling end product comprises the material after one or more cuts, often in the form of 'flakes'. Flakes are a highly convenient form for providing a suitable material for subsequent use in an extrusion process or system. Such end products can then be used to form 'compounds' or final products such as pellets, which themselves can then be used to form final products. Indeed, it is possible to combine or blend recycling end products such as flakes and intermediate formed compounds such as pellets, to form further compounds and final products. The present invention is applicable to all such products and compounds, and their processing during extrusion or compound extrusion.
However, a measurable property of recycled products, such as MFI or intrinsic viscosity, will vary widely depending upon the amounts of different grades of recycled polymer materials provided to form the initial products such as flakes or pellets over weight, time, etc. Hence, the use of such products leads to forming a recycled polymer feedstock material having a property with a variation >10%, and typically significantly >10%, as this recycled polymer feedstock material is subsequently provided into an extrusion system or process.
In addition, many extrusion systems or processes work in a 'batch' arrangement, where it is intended to carry out the extrusion compounding process for a 'batch' of feedstock material, or until a 'batch' of extradate material has been provided. This is especially for recycled polymer extrusion. (There are no current extrusion processes based on recycled polymer material using a continuous or continuously supplied feedstock material, as opposed to the provision of feedstock material in one or more 'batches', generally being in the form of one or more feedstock bags or pouches. That is, there is a definitive batching into bags or pouches of the formed recycled polymer material prior to its distinct use in a recycling extrusion process.)
Because of this, as well as variation in a property of the recycled feedstock polymer material within a batch of provided material, there are also variations in such properties between batches. Such properties can vary even more significantly than within a batch, especially if recycled material is provided from different sources and/or over different time periods.
Different recycling timings/processes/sources are usually based on different original recycled material, which can vary not only from day to day but site to site and country to country. Thus, the present invention is also able to overcome variation in one or more properties of recycled polymer feedstock material from different batches as well as within a single batch. One application for the present invention is the processing of recycled high density polyethylene into corrugated drainage pipes.
The present invention encompasses all combinations of various
embodiments or aspects of the invention described herein. It is
understood that any and all embodiments of the present invention may be taken in conjunction with any other embodiment to described additional embodiments of the present invention. Furthermore, any elements of an embodiment may be combined with any and all other elements of any of the embodiments to describe additional embodiments.
Embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings which are schematic representations of exemplary extrusion systems according to the present invention. In the Figures, the following notation is used: N = Screw speed
Tj = Temperature in barrel zone at point i
Pj = Pressure at point i
I = Current
T m = Melt temperature in the die
Referring to Figure 1 shows an extrusion system 2a having a polymer extruder 4. The double black lines represent a soft sensor loop 6.
A viscosity ('prediction') model 8 provides an estimate of the melt viscosity of a HDPE recycled feedstock material in the die of the extruder 4. This model incorporates a relationship between the extrusion conditions, an ultrasound input and the resulting melt viscosity. Based on the estimated melt viscosity and the screw speed of the extruder 4, a feedback model 10 estimates the barrel pressure in the extruder 4. An error signal generated by the comparison of the measured and estimated pressures indicates an error in the viscosity model if the feedback model 10 is accurate.
Therefore, this error signal is used to correct the estimated melt viscosity.
The crossed lines represent an ultrasound loop 12 also having a viscosity model 13. It is possible that the current drawn from an ultrasound device (such as the ultrasonic device shown in Figure 2) may itself be an indicator of viscosity and may be used to enhance or replace the soft sensor loop 6.
The viscosity determined by the soft sensor loop 6 is input to a controller 14, which uses one or more known viscosity models (not further
described) to determine an appropriate adjustment to the ultrasonic modulation probe to respond to fluctuations in the melt viscosity. Referring to Figure 2, there is shown an extrusion system 2b having a polymer extruder 4. In system 2b, extruder die pressure is manually preset by an operator. Measurement of actual die pressure is continually fed back via a controller 15 which controls an ultrasonic device 16, the intensity of ultrasonic energy applied by device 16 to the polymer melt being controlled to adjust polymer melt viscosity accordingly so that measured die pressure may remain as close as possible to that pre-set by the operator.
In this system feedstock material throughput may also be pre- set by an operator. Measurement from the extruder 4 of actual material throughput is continually fed back via controller 17 which is operable to control screw speed N, the screw speed being controlled so that feedstock material throughput may remain as close as possible to the target value pre-set by the operator.
The methods of the present invention are useable with a variety of industrial applications used to process variable materials such as injection moulding, thermoforming, extrusion and injection blow moulding and extrusion compounding.
Advantages of the present invention include:
9 reduced set-up time; the present invention requires minimal manual input of process variables and the process quickly adapts to the variability of feedstock and variability between batches of a feedstock material. • increased continuous automated processing of a highly-variable feed material thereby reducing downtime and the production of scrap/waste.
• enabling viscosity modulation to perform more effectively and accurately by relaying time/position and power transmission needs.
• allowing selective reduction in viscosity of un-melted polymers and plastics to match the overall flow character to the bulk of the melt, thus reducing energy and scrap costs.
• reducing the risk of degradation of the melt and helping to avoid production of large volumes of scrap/waste material.
• increasing the competitiveness of waste plastic processors by reducing process temperature and therefore reducing operating costs i.e. reduced energy consumption.
• limiting the potential for inclusions in a product that will produce structural defects and confer a poor aesthetic property.
• reducing the potential for poor surface finishes and poor mechanical properties that reduce both the quality and value of the product.
• providing production flexibility for feed materials with significant batch variation, minimising the production of scrap/waste.
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