GB2087247A

GB2087247A - Making thermoplastic road marking materials

Info

Publication number: GB2087247A
Application number: GB8036410A
Authority: GB
Original assignee: Clare R S & Co Ltd
Current assignee: Clare R S & Co Ltd
Priority date: 1980-11-13
Filing date: 1980-11-13
Publication date: 1982-05-26

Abstract

A liquid plasticiser is continuously dispersed through premixed dry components of a mix of thermoplastic road marking materials. Premixed dry components of a mix are fed via an output feed 20 from a mixing hopper 10 and a first input flow at 22 to a mixer vessel 23. A second input flow 24 of liquid plasticiser, e.g. plasticising oil, is fed to the mixer vessel 23 by a metering pump 25. Liquid plasticiser is dispersed into the vessel 23 via a sprinkler bar 27 extending across about 40% or more of the cross-section of the vessel. A stack 28 of rotatable substantially flat superposed blades, rotated at about 200 r.p.m., disperses the plasticiser throughout the premixed dry components to produce a flowable, non-liquid, essentially particulate final mix, which is removed from the vessel via an outlet 30. <IMAGE>

Description

SPECIFICATION Making thermoplastic road marking materials This invention concerns improvements in and relating to the manufacture of thermoplastic road marking materials.

Basically, in this country such materials for hotapplication as a liquid that subsequently sets are made to British Standard BS3262 and comprise light coloured aggregate, pigment, and extender, bound together with a thermoplastic resin, plasticised as required. Additional glass beads of suitable grading can be incorporated to improve visibility for night conditions. Requirements or what is acceptable, in or for other countries may vary but such variations conveniently represent the boundaries for constituents of materials of this invention.

It has become customary to manufacture such materials in batches by mixing and heating thereof to form a liquid that is run into containers and subsequently cools to a solid mass for later reheating at application as required. We are aware (see British Patent Specification No. 1474903) of one prior proposal to prepare a dry mix of all ingredients save for plasticising oil that is added only at the application site. A so-called dry mix is much easier to manufacture, but we feel that the added degree of skill and general operator-reliance required on-site for correct admixture of plasticising oil is undesirable and constitutes a retrograde step, especially as, in windy conditions, some of the finer elements of the mix are liable to be blown away if not over operators.

Accordingly, we now propose that a flowable, non-liquid, essentially particulate, mix be provided in a continuous operation wherein liquid plasticiser is dispersed through the other components of the mix in a stage whereat flow of such other components, usually in one combined premixed stream, and of plasticiser in a separate stream are controlled in a dependent manner to achieve the required overall mix. Such dispersion of liquid plasticiser renders the final particulate product less dry, though still flowable, and is less susceptible to wind on site, and can avoid any mixing in of plasticiser on site.

Mixing at speeds of 150-500 r.p.m. satisfactorily about 200 r.p.m. using a rotated plurality of flat spaced superposed blades has been found to achieve oil dispersal without undue heating in said stage. Advantageously, such blades are rotated in a region of a mixer vessel that is below a level at which a sensor is operative to open flows to the vessel, a further sensor being operative at a higher level to stop flows to the vessel. A single sensor that is displaced by the material may, of course, be used, say a pivotted mercury switch sensitive to a desired inclination, usually to the vertical. Plasticising oil is best added via a distributor head extending across about 40% or more of the cross-section of the vessel, so as to achieve a thorough and substantially even dispersion of the oil due in part to the rotation of the blade assembly acting on the mixture.

Thus, within such a vessel, the materials supplied thereto will be thoroughly admixed below, and usually within the lower depth of, the stack of inner blades and will be taken off through a bottom exit of the container, usually for bagging or filling of other containers. We have found that it is extremely important for the dwell time of the thoroughly mixed material in the vessel to be as short as possible, otherwise the action even of flat blades causes heating of the material, in a matter of second only, to temperature at which the material tends to soften and coagulate if not thermoset therein and, at best, provide a particulate final product that is less than readily flowable.

We have therefore developed a particularly advantageous output mechanism having parallel feed for the mix of vessel to charge-vessels each of which is associated, with weighing means operative at a preset weight to close off a charging valve of that vessel at least until it is discharged via a discharge valve usually into a container dissociated therewith.

Preferably, the charge-vessels are each supported on pivots at end parts of upper and lower arms themselves pivotally supported medially at a frame and pivotally connected at their other end parts to a strain gauge. Then, only enough movement in pivot connections is required to operate the strain gauge.

Such a system and method hereof is generally entirely satisfactory for products that have a plasticiser oil content below 2% (including glass beads).

However, for products with a somewhat higher content of plasticiser oil, say 2.5% or more, may produce a mix that is rather "wet" for our desired purpose, namely a bagged flowable and reasonably "dry" mix whose constituents have no marked tendency to stick together after storage or during mixing.

Sprayable products tend to require a higher plasticiser content than products for screedapplication, but even some of the latter exceed 2%.

As such levels, if all of the plasticiseris liquid oil, the limits of acceptability herein can be exceeded at least in that constituents tend to stick together after a period of storage. Our proposals for overcoming that involve pelletisation of at least part of the overall constituents. Of the mix constituents some, particularly aggregates such as sand and perhaps also to a lesser extent filler such as amorphous calcium carbonate, have abrasive properties that could damage feed and metering means of pelletising apparatus and are thus best omitted from the pelletising, though some properties of fine grade filler has been found to be acceptable.At least 50%, preferably about 65% or more, of the pelletised constituents is thermoplastic resin, otherwise pelletisation will be difficult if not impossible by the preferred method of using a hot liquid mixture thereof that is metered and allowed or made to solidify, say as blobs on a conveyor surface. Importantly, however, to meet the problem mentioned above for liquid plasticisers at least some, and preferably only some, of that plasticiser is included in the pellets, the balance being added later by dispersion through the overall mixtureforthe earlier mentioned purposes hereof.

One embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic representation of material mixing apparatus hereof; Figure 2 is a plan section of a high speed mixer stage; and Figure 3 is a perspective view of an output weighing stage.

In the drawings, a mixing hopper 10 receives (via suitable feed means 11) the various ingredients of the final mixture excluding plasticising oil and serves thoroughly to admix them. A preferred inner mechanism comprises an orbital auger type where an auger blade indicated diagrammatically at 12 rotates about its axis and also bodily about the axis of the hopper 10 at an angle thereto. Relative rates of feed of individual ingredients to the hopper 10 can be preset in separate feed means 11 or a degree of premixing can be performed prior to the feed means 11.

The or each feed means 11 to the mixing hopper 10 is conveniently of a screw feed-type with a drive motor on-off controlled by signals from level.sen- sors 14, so as to keep the contents of the hopper 10 between upper and lower levels 16,17 associate therewith. A further lower level sensor 18 serves to operate an alarm or automatically to close down the apparatus pending investigation of the reason for the contents of hopper 10 going below level 17. A vibratertype exit device is shown at 19.

An output feed 20 for the mixing hopper 10 is shown as a screw feed type with a drive motor 21 and serves to provide a first input flow at 22 to mixer 23 for dry constituents of the final mix. A second input flow to mixer 23 is shown at 24 for plasticising oil. Avariable metering pump 25 is shown in oil supply line 26 and serves to preset oil flow relative to what is desired for the dry material input flow 22.

The mixer 23 serves thoroughly to disperse the plasticising oil from input 24 throughout the mixed dry constituents form input 22. To that end, the oil is dispersed and a sprinkler bar 27, flow via-the pump 25 being preferably by gravity from an oil storage tank. The dispensing bar 27 extends medial across about 40% of the cross-section of the mixer 23 and the latter has a stack 28 of substantially flat superposed blades 29-rotated at about 200 r.p.m. The combined effects ofthe rotating blades 29 and the extent of the dispenser bar 27 serve to achieve the desired thorough dispersion of plasticising oil.

The desired final flowable particulate mixture with oil dispersed therethrough is taken at 30 from the bottom of the mixer 23 via slide valves 31 to respective weigh hoppers 32 of which there will be at at least two, each preferably as shown in greater detail in Figure 3. In Figure 3, a fairly narrow weigh hopper 32, suitable for filling bags when an exit valve (not shown but usually of slide action) is operated, is supported at both sides in pivots such as 33, an upper and lower pair of arms 34,35 and 36,37.

The arms 34to 37 are parallel and supported medially in pivots, such as 38, on a frame part, such as post 39. At their other ends, the arms 34to 37 are pivoted, as at 41, to a plate 42 carrying an extension arm 43 to which is attached at 44 a spring balance 45 also attached at 46 to the machine frame so that only minimal movement of the arms 34 to 37 is effective to operate the spring balance. Conveniently, the latter has a reference mark 47 at which its pointer 48 is sensed and the corresponding feed valve 31 is closed. Then the output valve of the weigh hopper can be operated to empty the weigh hopper into a bag orthe like and afterclosure of the inputvalve, the slide valve 31 will be reoperated, usually by a push-button switch 49 for a solenoid actuator 50 for the valve 31.

The multiple weigh hopper system just described constitutes an efficient means of emptying the mixer 23 and avoiding undue heating of materials therein due to the action of the blades 29, which can occur in a matter of seconds only.

Lastly, we refer to a further input 55 shown for the mixing hopper 10 and serving for entry of pellets in a preferred method of ours. Those pellets, to which a cofiled application relates have a resin low formulation different from that of the final mix so that some oil must be added at 24. However, the oil content of the pellets is sufficient to ensure that the amount of oil required at 24 cannot be such as to give rise to the final particulate material tending to compact or coagulate due to its fine oil content. The balance of constituents may be most conveniently added by as a single stream in a feed device 11. Then, premixing, not necessarily particularly thoroughly, can be done to individual formulae that allow the pellets to be added in appropriate proportions to cover a considerable range of final product mixes for single pellet formulation. A cofiled application gives further details of pellets formulations and mixes containing same, though we note here that it is considered advantageous for such mixes to require upto 2% oil dispersion therethrough, and preferably about 1.5% for all mixes, the remainder (to total upto about 31/2%} being accounted for by the pellet content of the mix.

Claims

1. A method of continuously dispersing a liquid plasticiser through dry components of a mix of thermoplastic road marking materials comprising dispersing the liquid plasticiserthrough the other components of the mix in a stage whereat flow of such other components and of plasticiser, in sepa rate streams, are controlled in a dependent manner to achieve the required overall mix.

2. A method as claimed in claim 1, wherein the flow of such other components is in one combined premixed stream

3. A method as claimed in claim 1, wherein the flow of plasticiser is controlled by way of a variable metering pump.

4. A method as claimed in any one of claims 1 to 3, wherein a plurality of flat spaced superposed blades are rotated in a mixer vessel to disperse the liquid plasticiser through the combined premixed stream of the other components of the mix.

5. A method as claimed in claim 4, wherein the blades are rotated at a speed in the range of 150-500 r.p.m.

6. A method as claimed in claim 5, wherein the blades are rotated at a speed of about 200 r.p.m.

7. A method as claimed in any one of the preceding claims, wherein the liquid plasticiser is added to the mixer vessel via a distributor head extending across about 40% or more of the crosssection of the mixer vessel, so as to achieve a thorough and substantially even dispersion of the plasticiser due in part to the rotation of the blades acting on the mixture.

8. A method as claimed in any one of claims 4 to 7, wherein the materials supplied to the mixer vessel are thoroughly admixed below the stack of inner blades and are taken off through a bottom exit of the mixer vessel.

9. A method as claimed in claim 8, wherein the materials are thoroughly admixed within the lower depth of the stack of inner blades.

10. A method as claimed in claim 8, wherein the materials are taken off for bagging or filling of other containers.

11. A method as claimed in any one of claims 8 to 10, wherein the thoroughly mixed material remains in the mixer vessel for as short a dwell time as possible to avoid heating the material to temperatures at which the material tends to soften and wagulate and thus less than readily flowable.

12. A method as claimed in any one of claims 8 to 11, wherein the thoroughly mixed material is taken off the mixer vessel through an output mechanism having parallel feed to charge-vessels, each of which is associated with weighing means operative ata preset weight to close off a charging valve of that vessel at least until it is discharged usually into a container associated therewith.

13. A method as claimed in claim 12, wherein the charge-vessels are each supported on pivots at end parts of upper and lower arms themselves pivotally supported medially at a frame and pivotally connected at their other end parts to a strain gauge operable by movement in the pivot connections.

14. A method as claimed in any one of claims 1 to 7, wherein the liquid plasticiser is a plasticiser oil.

15. A method substantially as herein described with reference to and as illustrated in the accompanying drawings.

16. A dry mix of thermoplastic road marking materials having a free plasticiser oil content of upto 2% when made by the method as claimed in any one of claims 1 to 14.

17. An apparatus for continuously dispersing a liquid plasticiser through premixed dry components of a mix of thermoplastic road marking materials comprisng a mixer vessel having a first inlet for the premixed dry components, a second inlet for liquid plasticiser, the second inlet being in communication with means for dispensing the liquid plasticiser within the mixer vessel, means for dispersing the liquid plasticiserthroughoutthe premixed dry components within the mixer vessel, and an outletfor removing the final mixture with liquid plasticiser dispersed therethrough from the mixer vessel.

18. An apparatus as claimed in claim 17, wherein the first inlet comprises a screw feeder operable by a drive motor for feeding the premixed dry components from a mixing hopper to the mixer vessel.

19. An apparatus as claimed in claim 17, wherein the second inlet comprises a liquid plasticiser supply line connecting a supply tank to the mixer vessel, the supply line having means actuated by fluid pressure located therewithin for together with gravity moving liquid plasticiser from the supply tank to the mixer vessel.

20. An apparatus as claimed in claim 19, wherein the means actuated by fluid pressure is a variable metering pump.

21. An apparatus as claimed in claim 17, wherein the means for dispersing the liquid plasticiser within the mixer vessel comprise a sprinkler bar or distributor head extending across about 40% or more of the cross-section of the mixer vessel.

22. An apparatus as claimed in claim 17, wherein the means for dispersing the liquid plasticiser throughout the premixed dry components comprises a stack of rotatable substantially flat superposed blades, the stack being arranged beneath the first and second inlet.

23. An apparatus as claimed in claim 22, wherein the blades are rotatable at a speed in the range of 150-500 r.p.m.

24. An apparatus as claimed in claim 22, wherein the blades are rotatable at a speed of about 200 r.p.m.

25. An apparatus as claimed in claim 17, wherein the outlet comprises an outlet mechanism having an arrangement of slide valves which are electromechanically operable, the output mechanism having parallel feed to at least two charge-vessel or weigh hoppers, each of which is associated with weighing means operative at a preset weight to close off a particular slide valve of that vessel at least until it is discharged via a discharge valve usually into a container associated therewith.

26. An apparatus as claimed in claim 25, wherein the charge-vessels or weigh hoppers are each supported on pivots at end parts of upper and lower arms (themselves pivotally supported medially at a frame and pivotally connected at their other end parts to a strain gauge operable by movement in the pivot connections.

27. An apparatus as claimed in any one of claims 17 to 22, wherein the liquid plasticiser is a plasticiser oil.

28. An apparatus substantially as herein described with reference to and as illustrated in the accompanying drawings.

29. A dry mix of thermoplastic road marking materials having a free plasticiser oil content of upto 2% when made in the apparatus as claimed in any one of claims 17 to 27.