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The cavity transfer mixer: A blender for all seasonings
The Cavity Transfer Mixer: A Blender for all Seasonings
Ron S H i n d m a r c h
-
Rapra Technology Limited, Shawbury, Shrewsbury, Shropshire, SY4 4NR
Abstract The Cavity Transfer Mixer (CTM) invented by Dr G M Gale at Rapra Technology Limited, is described as a highly efficient add-on blending device for extruders. The basic theory and development work are described to illustrate the power law relationship which the device achieves in mixing compared with others achieving only linear relationships. Practical applications are described illustrating the very wide range of uses. These include: achievement of a constant temperature behind the die to give consistentprofile, completion mixing using masterbatch, improved electrical properties, reduction of nerve, improved output and direct injection of ingredients. The article concludes with a description of most recent modifications including separately driven CTMs to give maximum mixing efficiency.
Introduction Material blending is an essential part of very many industrial processes from food products through to steel and from pharmaceuticals to animal feeds. In some of these widely diverse industries batch or continuous processes operate which can be 'somewhat wanting' in terms of blending efficiency. This is particularly true in the polymer industry where materials are, in general, viscoelastic in nature, tend to be highly viscous and can be particularly difficult to blend effectively, especially when those materials are radically different in processing properties. It is not essential in all cases that blending be taken to an optimum in terms of distributing one ingredient throughout another but in some cases it is critical and can considerably alter the finished product performance if adequate distribution does not Occur.
Although single screw extruders have been in use for the extrusion of polymer profiles for over 100 years, problems relating to badly blended material, poor temperature gradients, variable pressures behind the die and, as a consequence, profile material of an inconsistent nature as finished product, still exist today. This situation can be directly related to the failure of screw and mixing element design to achieve little other than a linear relationship with regard to mixing or blending efficiency and a persistance in 'trial and error' developments over those years. During 1980 a new blending device was invented and patented called the Cavity Transfer Mixer (CTM) ~) for fitment as an add-on unit to the barrel and screw of existing extruders. This was developed from theoretical work, first outlined in 1950 by Spencer & Wiley t2! and later from experimental work by Ng and Erwin ~3). The development was, initially, to improve the blending efficiency of black masterbatch into polythene without the necessity of using internal mixers. An advantage over existing units was also seen in the ability to blend polymer to a degree where all the compound from the screw and CTM unit would be presented to a die in a very consistent manner. Profiles could therefore be produced to tighter standards and continuous processing could be that
MATERIALS & DESIGN Vol. 8 No. 6 N O V E M B E R / D E C E M B E R 1987
much more reliable. The CTM units worked very well when fitted to plain, pumping screws capable of generating high output rates. The original Rapra patent") stated that: "'Whereas the aforegoing description has been concerned particularly with the mixing of molten plastics and rubbers, the present mixer and process is also useful for mixing operations on other viscous fluids, such as soaps, doughs, clays and margarines". Subsequently, patents have been raised to cover improvements in soap processing t4 t0) for the processing of an edible fat containing product "~) and for the making of a shaped cross-linkable extruded polymeric producft2L To date over 150 CTM units have been manufactured in sizes between 18mm and 400mm internal diameter and the range of uses has extended well beyond the original expectations. This paper has been prepared to summarise the basic theory and development work, the range of uses and the future prospects for Cavity Transfer Mixers.
Theoretical Concepts Terminology The rather vague use of terms such as homogeneity, mixing and blending, etc, are not quite adequate when we are considering theoretical concepts unless we know exactly what is meant by each particular term. Mixing can be best considered to have two important but distinctive actions. dispersive : an operation which reduces the agglomerate size of the minor constituent to its ultimate particle size, eg compounding carbon black into polymer in an internal mixer. distributive: an operation which is employed to increase the randomness of the spatial distribution of the minor constituent within the major base with no further change in size of that minor constituent, eg distributing black masterbatch throughout a compound. This is an oversimplification when applied to practical mixing problems since both actions can occur simultaneously.
331
LL Fig 1
Maillefer section of a screw
Unfortunately, the term 'Mixing' is often used in the plastics industry to describe the distributive mixing of masterbatch into virgin polymer. This is something which the rubber industry would have termed 'Blending' since mixing to them is very much associated with the high shear compounding of powders into polymer in internal mixing machines. In effect the rubber industry describes dispersive mixing as 'Mixing' and distributive mixing as 'Blending'. Homogeneity will have been achieved when both the dispersive and the distributive elements of the mixing have achieved a fluctuation in average composition below a certain fixed and acceptable level. Homogeneity, in terms of extrusion, should encompass not only dispersion of ingredients but should consider the physical state of the compound in terms of its temperature and viscosity as it moves through the extruder.
Improved distributive mixing The basic forward motion of molten polymer in the channel of an extruder screw is laminar. Viewed in cross section, movement of polymer in the channel of a plain screw is by cross-channel circulatory flow in which shear rate is greatest at the periphery and practically zero at mid point of the channel width and two thirds of the channel depth. This point can be so far from surrounding metal surfaces, with deep flight screws, that heating by conduction is poor with the result that in this region it is difficult to homogenise the polymer. Therefore, any device which moves polymer from the outside to the inside should improve homogeneity. The Maillefer screw, Figure I, does perform this function in making all the material pass once at least over the dam into the next flight, Alternatives are interrupted flights and strategically placed pins; the simplest method is to terminate the flight and then restart it in the mid channel position. Similarly pins can be fitted to screw channels or barrel to disrupt the flow, Figure 2. However, all of these methods of improving the distributive mixing and homogeneity produce only linear improvements to the efficiency of the blending. The laminar flow although interrupted is not turned through the required angle which could 'double-up' the blending effect at each interruption. In simple terms what is required is a system which simulates the cutting and folding action which is used on rubber mills to blend compounds and distribute ingredients. Spencer and Wiley ~2)in 1950 showed that this system could be achieved by interruptions of simple shear which would give a power law relationship to this otherwise linear process. Yet developments of mixing elements appear to have missed this requirement and continued with linear mixing systems. The mechanism required by an extruder mixing device to provide this power law relationship to distributive mixing was clearly demonstrated in experiments by Ng and Erwin (3t based upon those concepts described by Spencer and Wiley.
332
Fig 2
Pin extruder, section of a screw
In the distributive mixing of two similar viscous lamina flow liquids, the degree of mix can be assessed by either the total interfacial area between them, or the striation thickness, the two being related. (Interfacial area) × (Mean striation thickness) × 2 = (volume of liquid)
(1)
From Spencer & Wiley growth of interfacial area in a fluid subjected to shear follows the formula: A/Ao =
V " I - 2 S Coso~ Cos/3 + (Coso0 2 S 2
(2)
where Ao is the original area, A the new area and a and are angles defining the orientation of the surface to the shear strain S From ( 1 ) since the volume stays constant A/Ao = ro/r
(3)
where ro is the original striation thickness and r the new striation thickness.
MATERIALS & DESIGN Vol. 8 No. 6 NOVEMBER/DECEMBER 1987
////J
[
E J Fig 3
Position o f the C T M on an extruder
For large unidirectional simple shear, as found in the metering section of a single screw extruder.
stages with an input of optimal orientation AdAo = (S/N) N
A/Ao = S C o s a
(6)
(4)
Where Afis the final area and N is the number of shearing
The derivative of equations 5 and 6 are covered in detail by Ng and Erwin ~3~. This concept was demonstrated by Ng and Erwin in experiments using an annulus formed between two concentric cylinders to apply predetermined amounts of shear to bands of coloured polyethylene. A/Ao was derived from measurements of striation thickness ratio following rotation of the inner cylinder to shear the polymer with repeated interruptions where segments were cut and replaced at right angles to the direction of shear. The results produced very close agreement with theory for the relationship of log (Ar/Ao) against log (total strain) for different values of N. To translate this theory into practice, the mechanism used in Erwin's experiments is similar in many respects to the process of plastic extrusion, cooling, granulating and reextrusion, repeated until the required degree of blending has
MATERIALS & DESIGN Vol. 8 No. 6 NOVEMBER/DECEMBER 1987
333
where S is the magnitude of shear strain As large shear orients interfaces parallel to shearing planes, further improvements in blending efficiency would be attained by each mixing section rotating the fluid so that the interfaces are more favourably orientated for the next shearing stage. From the equation ~4)the maximum effect will occur when Cos a = 1 If a shearing system has many mixing stages dividing the system into sections having large shear with equal magnitude, blending will be AJAo = (S/2N) (S/N) N~ = I/2(S/N) N
(5)
been attained. In a single pass extrusion, the incorporation of interrupted flights, barriers and pins, although a step in the right direction is not likely to provide either the complete cutting nor the favourable re-orientation required.' The idealised blending system would need an action such as that of die face cutting, a system involving alternative stationary and rotating parts. The turbine system devised by ICI and subsequently investigated by Martin and Kosel together with pin/slotted flight devices appear to be the only practical systems applied in the polymer industry which go towards meeting these theoretical requirements. The Cavity Transfer Mixer was developed and patented initially for plastics extrusion, based on those concepts outlined above and has been demonstrated by Gale n31 to exhibit a power law relationship to distributive mixing efficiency and flow patterns predicted from theoretical considerations.
The Cavity Transfer Mixer (CTM) The C T M is constructed having a rotor extension to the screw with hemispherical cavities overlapping similar cavities in a stator such that material is subjected to laminar shear but repeatedly re-aligned at approximately right angles during transfer to opposite cavities. Hence the name 'Cavity Transfer Mixer'. The device can be fitted to existing extruders (Figure 3) as an important economic feature. In order to demonstrate that the predicted blending action was occurring, a CTM was constructed with a transparent acrylic stator and an aluminium rotor. A liquid silicone polymer could then be fed continuously through the unit and a coloured polymer marker could be injected either continuously or intermittently into the first cavity row (Figure 4).
Fig 4
Acrylic model CTM Rotor
"• ;i'
°
..2
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t
°
~
2 ~ ' ~ - / \ N \
~
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t
\
I //
or cavity ...... bore ,nlect . . . . . . . iy
Striation deposited at Bottom of next cavity up
~ ( , ~ / 4~ \~',~ ~ ~ \\
~'~ Iniechon po,nt
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Stator
,vi! Striation e rotor and /l~i t . . . . through 90° X ~
3 ~
~ j
'\\
/i
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i I
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t\\
/
b Striation
,hroogh a,
\k~
~
4
Striation
,'
deposited each I f ~ l t, . . . . , o r - \\\ ~ . . . . ty p. . . . . through
Fig 5
334
"F~'~
X~
~
J
\ 6 /
~ ( ~ \
,
"
Y /
!
;
~5..// - --
Progression of a striation through each cavity
MATERIALS & DESIGN VoL 8 No. 6 NOVEMBER/DECEMBER 1987
Fig 6
Comparison of blending from a plain screw (left) and a CTM (right)
When transparent silicone polymers were used the flow paths and mixing mechanisms could be observed. The mixer was set up in a vertical plane and the rotor was separately driven at rotational speeds ranging from 2 to 16 rpm. Its diameter and length were 53 and 70mm respectively. A commercial liquid silicone was compounded on a Silverson L2R laboratory homogeniser with a 10% addition of the recommended curing agent and 3% of a fumed silica as thickening agent. After removal of air bubbles under a vacuum, the silicone was transferred to a positive displacement pump, which used a motor driven single stroke piston in a cylinder, analogous with the action of a syringe. A similar but coloured batch of silicone was prepared for injection from a tiny peristaltic pump into the first cavity
~",47
A Ig ( I)
t AST (:UR]N(~ ~LOW CLJRIN( l O W VISC~)~;IIY MII l i p 1~1 ~ h l ) O~ A AND I~ ( I M B I I N D ~ ) ! A A N I ) r~
C
row.
Silicone was pumped through at 100 ml/min with the rotor turning and the coloured silicone was injected at 1.5 ml/min until the colour was observed to have reached the top. The pumps and drive were then simultaneously stopped, the inlets were sealed and the silicone allowed to cure over a 24 hour period. The unit was then dismantled and the solid silicone casting removed. Sections were cut from the casting, photographed and flow patterns drawn (Figure 5). From these, it was shown that a predictable flow pattern was occurring and that this was causing a radical improvement in blending efficiency measured by reduction in striation thickness. The mechanism followed a pattern of repeated cycles of simple laminar shear, cutting, turning, stacking and shearing at right angles, conforming to theoretical considerations for good distributive mixing.
Practical Applieatlons
T,me mmu~es
Fig 7
Rheograph of Milled C, and CTM blended D, off-specification batches A and B
uniform viscosity of polymers, a very much more consistent profile can be produced. The early comparison of black and white chloroprene compounds being blended together at dissimilar temperatures to give a totally uniform profile (Figure 6) has not been improved. But factory trials over the two years have shown that a much more consistent profile can be produced because of these constant conditions.
Work-away and off-specification material
Experimental work carried out on the CTM with rubber showed that gaining a constant temperature behind the die was probably one of the most important attributes of the CTM and that it was a major development for the polymer industry. Since that paper in 1982 "4) subsequent work has confirmed that in gaining constant temperature and hence
When compound batches fail process control testing, an instruction is often given to the plant either to work away the material, perhaps at a 10% level in good compound or more generally to blend two batches together to bring the compound back on specification. This later remedy is fine if, in blending the two batches, the viscosity remains constant but quite often material breakdown occurs; profile extrusion to a constant specification then becomes difficult and
MATERIALS & DESIGN Vol. 8 No. 6 N O V E M B E R / D E C E M B E R 1987
335
Temperature and Viscosity
sometimes impossible with the blended batches. By use of a C T M unit, off-specification compounds have been blended into new stock compounds at levels indicated by Monsanto Rheometer cure curves (Figure 7) directly in the extruder obviating the necessity to use additional milling equipment. In all trials completely homogeneous material has been produced which has processed as original compound.
Completion mixing with masterbatch It has been customary in plastics processing to use masterbatch, usually to pigment profile. The same technique can be used with rubber and this has been illustrated by distributing black masterbatch into thermoplastic polyurethane with a C T M fitted to a plain screw.
Improved electrical consistency Consistency in electrical properties either for antistatic use or dielectric heating in polymer compounds has been a problem for very many years. Because the CTM is capable of blending two or more materials extremely efficiently, very consistent electrical properties can be produced. In our experiemental work reported ~15)in 1983 in Houston at the AC S meeting we were able to show that by using the C T M on the end of a plain screw more consistent electrical properties could be produced. These were for antistatic purposes and were compared with milling procedures and standard extruder screws. The longer the CTM the more consistent the electrical properties became.
Fig 8
Natural robber tyre tread compounds with and without CTM CAVITY TRANSFER MIXER \
~
EXTRUDER BARREL
Reduction of nerve and increased output Trials which were carried out to examine the possibility of reducing nerve in some stocks, particularly natural rubber tyre compounds, very quickly changed to trials examining very positive increase in output. These illustrated the ability of the C T M to reduce the nerve in a natural rubber to the extent that the output was doubled in some cases. Some increase in temperature did occur but with very much higher and consistent output (Figure 8)
Direct injection of ingredients Recent developments have shown that direct injection of silane crosslinking agents together with peroxides can be carried out successfully to produce crosslinked polyethylene (PE) or ethylene propylene diene terpolymer (EPDM) The profile once produced requires only a period in hot water to complete the crosslinking. Figure 9 illustrates the laboratory set-up which has been used very successfully so far at Rapra. The main application of this process is for the production of cable coverings to resist higher temperatures and in the production of crosslinked PE pipe suitable for hot water use. Early in the development of the CTM, it was shown that particulate materials, such as pigments, could be injected into plastic films with a liquid carder and be fully distributed. In the rubber industry, the use of this technique with small powders - such as accelerators, antioxidants or curatives was an obvious choice for evaluation. Successful addition of an accelerator or curative at the last stage of product manufacture has meant that some companies have been able to alter their processing techniques to give them both greater storage times of their base compounds and process flexibility. The carders used were oils or plasticisers compatible with the compound. Bran and Luebbe reciprocating pumps set up in tandem have been used to feed the injector which is fitted directly into the head of the extruder before the mix passes into the CTM. The pumps cope well with liquids of relatively high viscosities but operation of the system is better if viscous liquids are heated, to lower their viscosity, and if a positive pressure is applied to the holding tanks which prime the pumps. This injection process has been extended for perfumes, liquid colour additions to cables and polyiso-
336
~..~ F-
Fig 9
SILCAT ]
BALANCE
Arrangement for silane injection
butylene (PIB) to polyethylene to produce cling film~xe. Companies using the injection system for colours have reported that because of the excellent distribution of the colour up to a 20% reduction in their loading has been possible giving considerable saving in material.
Limitations of the CTM The Cavity Transfer Mixer is a blending unit and will not break down agglomerates. Because of the low shear action, the unit is not capable of acting as a substitute for internal mixers such as the Shaw Intermix or the Bridge Banbury and will not be able to cope with powder additions to an extruder. Similarly, the C T M will not be able to improve badly dispersed compound, ie material which is full of large agglomerated particles such as carbon black, whiting or china clay. Mixing times cannot be reduced by substituting the CTM for part of the intensive mixing cycle. However, units have been added successfully to dump extruders to homogenise compound from internal mixers. When directly attached to the screw of an extruder, incorporation of a liquid or one carrying a particulate solid, is limited to a maximum of
MATERIALS & DESIGN Vol. 8 No. 6 NOVEMBER/DECEMBER 1987
@ @ @
Two-row r four-cavity Temperature blending, viscosity homogenisation, reduced nerve
Three-row, six-cavity Temperature blending, viscosity homogenisation, reduced nerve, injection of colour or liquids to 4%
Seven-row Tsix-cavity Ultimate blending Specific electrical properties
Twin screw PVC compounding
l
t
Independentl F driven Polymer blends, injection liquids to 15%
J
Fig 10 Variety of CTMs
MATERIALS & DESIGN Vol. 8 No. 6 N O V E M B E R / D E C E M B E R 1 9 8 7
337
Gear Pump
Extruder
-
?
I ] r~i,"l!--_
II
1 'ltColouran, r l/ i.Jj ]
Manifold
\ \
;J//'
I
-"< _d~,J"-"
i
I
I
Pressure Transducer
F
I_~ "
Extruder Drive Speed Control
+
°,.
Head
Ii
Fig 11 Schematic diagram of the Francis Shaw/PLCV process 5%; for higher loadings an independently driven unit is necessary. Heat build-up with stiff nervy stocks eg natural rubber tyre tread, can be a problem with multi-row CTMs. In these cases very short units are recommended. Specific electrical properties can best be achieved with longer C T M units ie up to 10 rows. This can be problematical if the compound is hard, causing high heat build-up. build-up.
CTM Modifications Independently driven CTMs (ie separated from the extruder) can be driven at substantially higher revs/min than the supply extruder to achieve greater efficiency. This enables a higher addition of injected material to be added and in the case of a simple oil this can be as much as 10-15%. Partly mixed compounds can be completed at the extrusion stage by using these units. Twin screw extruders used for compounding plastics from dryblend mixtures, eg rigid PVC, have been adapted with twin C T M units. Radical improvement in mixing quality has been reported. Heat build-up was a problem with CTMs which were 5 or 6 rows long when processing nervy stocks. Modifications have been made to reduce these CTMs to 2 or 3 rows with a reduced number of cavities in the circumference. Lower head pressures and decreased heat build-up were observed. Some CTM models available are illustrated in Figure 10.
338
Complete electric cables can now be made on just a pair of extruders applying up to five colours simultaneously; in addition to saving on hardware, the process cuts down substantially on colourant usage. Francis Shaw and its sister company PLCV have developed the system, for installation in a new computerised extrusion plant. The system works by splitting the extruder output and passing part of it through a Cavity Transfer Mixer where the colour is injected, then returning it to the crosshead where it forms a skin on the remainder of the uncoloured material which is coating the wire. (Figure 11)
Conclusion Over 150 Cavity Transfer Mixers have been in commercial use, some for 5 years, in polymer and non-polymer applications. Incorporation of liquids by injection has made it possible to blend highly reactive materials continuously. The range of distribution units covers short CTMs for producing more uniform temperature and viscosity distribution and copolymer blends, twin-screw units for compounding extruders and independently driven units for the addition of high quantities of liquids. The applications over the next five years are expected to extend to industries not yet using CTMs. Within the polymer industry an increasing number of companies are expected to evaluate and then use Cavity Transfer Mixers.
MATERIALS& DESIGNVol. 8 No. 6 NOVEMBER/DECEMBER1987
References
1. 2. 3.
EP 0048590 A1 R S Spencer & R M Wiley, J Colloid Sci, 6, 133, (1951) N Y Ng & L Erwin, 37th Antec of the SPE, Vol 25, 241 (1979) 4.10 GB 2118055A, GB 2118056A, GB 2118057A, GB 2118058A, GB 2118450A, GB 2119666A, GB 2118854A 11. E P 0 1 9 8 533 A 12. EP 85104444.6
MATERIALS & DESIGN Vol. 8 No. 6 N O V E M B E R / D E C E M B E R 1987
13. 14. 15. 16.
G M Gale, Rapra Members, Report No 46 (1980) R S Hindmarch & G M Gale, presented at a meeting of the Rubber Division, American Chemical Society, Philadelphia, May 5-8th. 1982, Educational Symposium. R S Hindmarch, G M Gale & R H Norman. ACS Rubber Division, Houston, October 1983. 'Polybutenes in tackified Polyethylene Film'. A Quilley. B P Chemicals, Grangemouth.
339
Ron S H i n d m a r c h
-
Rapra Technology Limited, Shawbury, Shrewsbury, Shropshire, SY4 4NR
Abstract The Cavity Transfer Mixer (CTM) invented by Dr G M Gale at Rapra Technology Limited, is described as a highly efficient add-on blending device for extruders. The basic theory and development work are described to illustrate the power law relationship which the device achieves in mixing compared with others achieving only linear relationships. Practical applications are described illustrating the very wide range of uses. These include: achievement of a constant temperature behind the die to give consistentprofile, completion mixing using masterbatch, improved electrical properties, reduction of nerve, improved output and direct injection of ingredients. The article concludes with a description of most recent modifications including separately driven CTMs to give maximum mixing efficiency.
Introduction Material blending is an essential part of very many industrial processes from food products through to steel and from pharmaceuticals to animal feeds. In some of these widely diverse industries batch or continuous processes operate which can be 'somewhat wanting' in terms of blending efficiency. This is particularly true in the polymer industry where materials are, in general, viscoelastic in nature, tend to be highly viscous and can be particularly difficult to blend effectively, especially when those materials are radically different in processing properties. It is not essential in all cases that blending be taken to an optimum in terms of distributing one ingredient throughout another but in some cases it is critical and can considerably alter the finished product performance if adequate distribution does not Occur.
Although single screw extruders have been in use for the extrusion of polymer profiles for over 100 years, problems relating to badly blended material, poor temperature gradients, variable pressures behind the die and, as a consequence, profile material of an inconsistent nature as finished product, still exist today. This situation can be directly related to the failure of screw and mixing element design to achieve little other than a linear relationship with regard to mixing or blending efficiency and a persistance in 'trial and error' developments over those years. During 1980 a new blending device was invented and patented called the Cavity Transfer Mixer (CTM) ~) for fitment as an add-on unit to the barrel and screw of existing extruders. This was developed from theoretical work, first outlined in 1950 by Spencer & Wiley t2! and later from experimental work by Ng and Erwin ~3). The development was, initially, to improve the blending efficiency of black masterbatch into polythene without the necessity of using internal mixers. An advantage over existing units was also seen in the ability to blend polymer to a degree where all the compound from the screw and CTM unit would be presented to a die in a very consistent manner. Profiles could therefore be produced to tighter standards and continuous processing could be that
MATERIALS & DESIGN Vol. 8 No. 6 N O V E M B E R / D E C E M B E R 1987
much more reliable. The CTM units worked very well when fitted to plain, pumping screws capable of generating high output rates. The original Rapra patent") stated that: "'Whereas the aforegoing description has been concerned particularly with the mixing of molten plastics and rubbers, the present mixer and process is also useful for mixing operations on other viscous fluids, such as soaps, doughs, clays and margarines". Subsequently, patents have been raised to cover improvements in soap processing t4 t0) for the processing of an edible fat containing product "~) and for the making of a shaped cross-linkable extruded polymeric producft2L To date over 150 CTM units have been manufactured in sizes between 18mm and 400mm internal diameter and the range of uses has extended well beyond the original expectations. This paper has been prepared to summarise the basic theory and development work, the range of uses and the future prospects for Cavity Transfer Mixers.
Theoretical Concepts Terminology The rather vague use of terms such as homogeneity, mixing and blending, etc, are not quite adequate when we are considering theoretical concepts unless we know exactly what is meant by each particular term. Mixing can be best considered to have two important but distinctive actions. dispersive : an operation which reduces the agglomerate size of the minor constituent to its ultimate particle size, eg compounding carbon black into polymer in an internal mixer. distributive: an operation which is employed to increase the randomness of the spatial distribution of the minor constituent within the major base with no further change in size of that minor constituent, eg distributing black masterbatch throughout a compound. This is an oversimplification when applied to practical mixing problems since both actions can occur simultaneously.
331
LL Fig 1
Maillefer section of a screw
Unfortunately, the term 'Mixing' is often used in the plastics industry to describe the distributive mixing of masterbatch into virgin polymer. This is something which the rubber industry would have termed 'Blending' since mixing to them is very much associated with the high shear compounding of powders into polymer in internal mixing machines. In effect the rubber industry describes dispersive mixing as 'Mixing' and distributive mixing as 'Blending'. Homogeneity will have been achieved when both the dispersive and the distributive elements of the mixing have achieved a fluctuation in average composition below a certain fixed and acceptable level. Homogeneity, in terms of extrusion, should encompass not only dispersion of ingredients but should consider the physical state of the compound in terms of its temperature and viscosity as it moves through the extruder.
Improved distributive mixing The basic forward motion of molten polymer in the channel of an extruder screw is laminar. Viewed in cross section, movement of polymer in the channel of a plain screw is by cross-channel circulatory flow in which shear rate is greatest at the periphery and practically zero at mid point of the channel width and two thirds of the channel depth. This point can be so far from surrounding metal surfaces, with deep flight screws, that heating by conduction is poor with the result that in this region it is difficult to homogenise the polymer. Therefore, any device which moves polymer from the outside to the inside should improve homogeneity. The Maillefer screw, Figure I, does perform this function in making all the material pass once at least over the dam into the next flight, Alternatives are interrupted flights and strategically placed pins; the simplest method is to terminate the flight and then restart it in the mid channel position. Similarly pins can be fitted to screw channels or barrel to disrupt the flow, Figure 2. However, all of these methods of improving the distributive mixing and homogeneity produce only linear improvements to the efficiency of the blending. The laminar flow although interrupted is not turned through the required angle which could 'double-up' the blending effect at each interruption. In simple terms what is required is a system which simulates the cutting and folding action which is used on rubber mills to blend compounds and distribute ingredients. Spencer and Wiley ~2)in 1950 showed that this system could be achieved by interruptions of simple shear which would give a power law relationship to this otherwise linear process. Yet developments of mixing elements appear to have missed this requirement and continued with linear mixing systems. The mechanism required by an extruder mixing device to provide this power law relationship to distributive mixing was clearly demonstrated in experiments by Ng and Erwin (3t based upon those concepts described by Spencer and Wiley.
332
Fig 2
Pin extruder, section of a screw
In the distributive mixing of two similar viscous lamina flow liquids, the degree of mix can be assessed by either the total interfacial area between them, or the striation thickness, the two being related. (Interfacial area) × (Mean striation thickness) × 2 = (volume of liquid)
(1)
From Spencer & Wiley growth of interfacial area in a fluid subjected to shear follows the formula: A/Ao =
V " I - 2 S Coso~ Cos/3 + (Coso0 2 S 2
(2)
where Ao is the original area, A the new area and a and are angles defining the orientation of the surface to the shear strain S From ( 1 ) since the volume stays constant A/Ao = ro/r
(3)
where ro is the original striation thickness and r the new striation thickness.
MATERIALS & DESIGN Vol. 8 No. 6 NOVEMBER/DECEMBER 1987
////J
[
E J Fig 3
Position o f the C T M on an extruder
For large unidirectional simple shear, as found in the metering section of a single screw extruder.
stages with an input of optimal orientation AdAo = (S/N) N
A/Ao = S C o s a
(6)
(4)
Where Afis the final area and N is the number of shearing
The derivative of equations 5 and 6 are covered in detail by Ng and Erwin ~3~. This concept was demonstrated by Ng and Erwin in experiments using an annulus formed between two concentric cylinders to apply predetermined amounts of shear to bands of coloured polyethylene. A/Ao was derived from measurements of striation thickness ratio following rotation of the inner cylinder to shear the polymer with repeated interruptions where segments were cut and replaced at right angles to the direction of shear. The results produced very close agreement with theory for the relationship of log (Ar/Ao) against log (total strain) for different values of N. To translate this theory into practice, the mechanism used in Erwin's experiments is similar in many respects to the process of plastic extrusion, cooling, granulating and reextrusion, repeated until the required degree of blending has
MATERIALS & DESIGN Vol. 8 No. 6 NOVEMBER/DECEMBER 1987
333
where S is the magnitude of shear strain As large shear orients interfaces parallel to shearing planes, further improvements in blending efficiency would be attained by each mixing section rotating the fluid so that the interfaces are more favourably orientated for the next shearing stage. From the equation ~4)the maximum effect will occur when Cos a = 1 If a shearing system has many mixing stages dividing the system into sections having large shear with equal magnitude, blending will be AJAo = (S/2N) (S/N) N~ = I/2(S/N) N
(5)
been attained. In a single pass extrusion, the incorporation of interrupted flights, barriers and pins, although a step in the right direction is not likely to provide either the complete cutting nor the favourable re-orientation required.' The idealised blending system would need an action such as that of die face cutting, a system involving alternative stationary and rotating parts. The turbine system devised by ICI and subsequently investigated by Martin and Kosel together with pin/slotted flight devices appear to be the only practical systems applied in the polymer industry which go towards meeting these theoretical requirements. The Cavity Transfer Mixer was developed and patented initially for plastics extrusion, based on those concepts outlined above and has been demonstrated by Gale n31 to exhibit a power law relationship to distributive mixing efficiency and flow patterns predicted from theoretical considerations.
The Cavity Transfer Mixer (CTM) The C T M is constructed having a rotor extension to the screw with hemispherical cavities overlapping similar cavities in a stator such that material is subjected to laminar shear but repeatedly re-aligned at approximately right angles during transfer to opposite cavities. Hence the name 'Cavity Transfer Mixer'. The device can be fitted to existing extruders (Figure 3) as an important economic feature. In order to demonstrate that the predicted blending action was occurring, a CTM was constructed with a transparent acrylic stator and an aluminium rotor. A liquid silicone polymer could then be fed continuously through the unit and a coloured polymer marker could be injected either continuously or intermittently into the first cavity row (Figure 4).
Fig 4
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MATERIALS & DESIGN VoL 8 No. 6 NOVEMBER/DECEMBER 1987
Fig 6
Comparison of blending from a plain screw (left) and a CTM (right)
When transparent silicone polymers were used the flow paths and mixing mechanisms could be observed. The mixer was set up in a vertical plane and the rotor was separately driven at rotational speeds ranging from 2 to 16 rpm. Its diameter and length were 53 and 70mm respectively. A commercial liquid silicone was compounded on a Silverson L2R laboratory homogeniser with a 10% addition of the recommended curing agent and 3% of a fumed silica as thickening agent. After removal of air bubbles under a vacuum, the silicone was transferred to a positive displacement pump, which used a motor driven single stroke piston in a cylinder, analogous with the action of a syringe. A similar but coloured batch of silicone was prepared for injection from a tiny peristaltic pump into the first cavity
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Silicone was pumped through at 100 ml/min with the rotor turning and the coloured silicone was injected at 1.5 ml/min until the colour was observed to have reached the top. The pumps and drive were then simultaneously stopped, the inlets were sealed and the silicone allowed to cure over a 24 hour period. The unit was then dismantled and the solid silicone casting removed. Sections were cut from the casting, photographed and flow patterns drawn (Figure 5). From these, it was shown that a predictable flow pattern was occurring and that this was causing a radical improvement in blending efficiency measured by reduction in striation thickness. The mechanism followed a pattern of repeated cycles of simple laminar shear, cutting, turning, stacking and shearing at right angles, conforming to theoretical considerations for good distributive mixing.
Practical Applieatlons
T,me mmu~es
Fig 7
Rheograph of Milled C, and CTM blended D, off-specification batches A and B
uniform viscosity of polymers, a very much more consistent profile can be produced. The early comparison of black and white chloroprene compounds being blended together at dissimilar temperatures to give a totally uniform profile (Figure 6) has not been improved. But factory trials over the two years have shown that a much more consistent profile can be produced because of these constant conditions.
Work-away and off-specification material
Experimental work carried out on the CTM with rubber showed that gaining a constant temperature behind the die was probably one of the most important attributes of the CTM and that it was a major development for the polymer industry. Since that paper in 1982 "4) subsequent work has confirmed that in gaining constant temperature and hence
When compound batches fail process control testing, an instruction is often given to the plant either to work away the material, perhaps at a 10% level in good compound or more generally to blend two batches together to bring the compound back on specification. This later remedy is fine if, in blending the two batches, the viscosity remains constant but quite often material breakdown occurs; profile extrusion to a constant specification then becomes difficult and
MATERIALS & DESIGN Vol. 8 No. 6 N O V E M B E R / D E C E M B E R 1987
335
Temperature and Viscosity
sometimes impossible with the blended batches. By use of a C T M unit, off-specification compounds have been blended into new stock compounds at levels indicated by Monsanto Rheometer cure curves (Figure 7) directly in the extruder obviating the necessity to use additional milling equipment. In all trials completely homogeneous material has been produced which has processed as original compound.
Completion mixing with masterbatch It has been customary in plastics processing to use masterbatch, usually to pigment profile. The same technique can be used with rubber and this has been illustrated by distributing black masterbatch into thermoplastic polyurethane with a C T M fitted to a plain screw.
Improved electrical consistency Consistency in electrical properties either for antistatic use or dielectric heating in polymer compounds has been a problem for very many years. Because the CTM is capable of blending two or more materials extremely efficiently, very consistent electrical properties can be produced. In our experiemental work reported ~15)in 1983 in Houston at the AC S meeting we were able to show that by using the C T M on the end of a plain screw more consistent electrical properties could be produced. These were for antistatic purposes and were compared with milling procedures and standard extruder screws. The longer the CTM the more consistent the electrical properties became.
Fig 8
Natural robber tyre tread compounds with and without CTM CAVITY TRANSFER MIXER \
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Reduction of nerve and increased output Trials which were carried out to examine the possibility of reducing nerve in some stocks, particularly natural rubber tyre compounds, very quickly changed to trials examining very positive increase in output. These illustrated the ability of the C T M to reduce the nerve in a natural rubber to the extent that the output was doubled in some cases. Some increase in temperature did occur but with very much higher and consistent output (Figure 8)
Direct injection of ingredients Recent developments have shown that direct injection of silane crosslinking agents together with peroxides can be carried out successfully to produce crosslinked polyethylene (PE) or ethylene propylene diene terpolymer (EPDM) The profile once produced requires only a period in hot water to complete the crosslinking. Figure 9 illustrates the laboratory set-up which has been used very successfully so far at Rapra. The main application of this process is for the production of cable coverings to resist higher temperatures and in the production of crosslinked PE pipe suitable for hot water use. Early in the development of the CTM, it was shown that particulate materials, such as pigments, could be injected into plastic films with a liquid carder and be fully distributed. In the rubber industry, the use of this technique with small powders - such as accelerators, antioxidants or curatives was an obvious choice for evaluation. Successful addition of an accelerator or curative at the last stage of product manufacture has meant that some companies have been able to alter their processing techniques to give them both greater storage times of their base compounds and process flexibility. The carders used were oils or plasticisers compatible with the compound. Bran and Luebbe reciprocating pumps set up in tandem have been used to feed the injector which is fitted directly into the head of the extruder before the mix passes into the CTM. The pumps cope well with liquids of relatively high viscosities but operation of the system is better if viscous liquids are heated, to lower their viscosity, and if a positive pressure is applied to the holding tanks which prime the pumps. This injection process has been extended for perfumes, liquid colour additions to cables and polyiso-
336
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butylene (PIB) to polyethylene to produce cling film~xe. Companies using the injection system for colours have reported that because of the excellent distribution of the colour up to a 20% reduction in their loading has been possible giving considerable saving in material.
Limitations of the CTM The Cavity Transfer Mixer is a blending unit and will not break down agglomerates. Because of the low shear action, the unit is not capable of acting as a substitute for internal mixers such as the Shaw Intermix or the Bridge Banbury and will not be able to cope with powder additions to an extruder. Similarly, the C T M will not be able to improve badly dispersed compound, ie material which is full of large agglomerated particles such as carbon black, whiting or china clay. Mixing times cannot be reduced by substituting the CTM for part of the intensive mixing cycle. However, units have been added successfully to dump extruders to homogenise compound from internal mixers. When directly attached to the screw of an extruder, incorporation of a liquid or one carrying a particulate solid, is limited to a maximum of
MATERIALS & DESIGN Vol. 8 No. 6 NOVEMBER/DECEMBER 1987
@ @ @
Two-row r four-cavity Temperature blending, viscosity homogenisation, reduced nerve
Three-row, six-cavity Temperature blending, viscosity homogenisation, reduced nerve, injection of colour or liquids to 4%
Seven-row Tsix-cavity Ultimate blending Specific electrical properties
Twin screw PVC compounding
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Independentl F driven Polymer blends, injection liquids to 15%
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Fig 10 Variety of CTMs
MATERIALS & DESIGN Vol. 8 No. 6 N O V E M B E R / D E C E M B E R 1 9 8 7
337
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Fig 11 Schematic diagram of the Francis Shaw/PLCV process 5%; for higher loadings an independently driven unit is necessary. Heat build-up with stiff nervy stocks eg natural rubber tyre tread, can be a problem with multi-row CTMs. In these cases very short units are recommended. Specific electrical properties can best be achieved with longer C T M units ie up to 10 rows. This can be problematical if the compound is hard, causing high heat build-up. build-up.
CTM Modifications Independently driven CTMs (ie separated from the extruder) can be driven at substantially higher revs/min than the supply extruder to achieve greater efficiency. This enables a higher addition of injected material to be added and in the case of a simple oil this can be as much as 10-15%. Partly mixed compounds can be completed at the extrusion stage by using these units. Twin screw extruders used for compounding plastics from dryblend mixtures, eg rigid PVC, have been adapted with twin C T M units. Radical improvement in mixing quality has been reported. Heat build-up was a problem with CTMs which were 5 or 6 rows long when processing nervy stocks. Modifications have been made to reduce these CTMs to 2 or 3 rows with a reduced number of cavities in the circumference. Lower head pressures and decreased heat build-up were observed. Some CTM models available are illustrated in Figure 10.
338
Complete electric cables can now be made on just a pair of extruders applying up to five colours simultaneously; in addition to saving on hardware, the process cuts down substantially on colourant usage. Francis Shaw and its sister company PLCV have developed the system, for installation in a new computerised extrusion plant. The system works by splitting the extruder output and passing part of it through a Cavity Transfer Mixer where the colour is injected, then returning it to the crosshead where it forms a skin on the remainder of the uncoloured material which is coating the wire. (Figure 11)
Conclusion Over 150 Cavity Transfer Mixers have been in commercial use, some for 5 years, in polymer and non-polymer applications. Incorporation of liquids by injection has made it possible to blend highly reactive materials continuously. The range of distribution units covers short CTMs for producing more uniform temperature and viscosity distribution and copolymer blends, twin-screw units for compounding extruders and independently driven units for the addition of high quantities of liquids. The applications over the next five years are expected to extend to industries not yet using CTMs. Within the polymer industry an increasing number of companies are expected to evaluate and then use Cavity Transfer Mixers.
MATERIALS& DESIGNVol. 8 No. 6 NOVEMBER/DECEMBER1987
References
1. 2. 3.
EP 0048590 A1 R S Spencer & R M Wiley, J Colloid Sci, 6, 133, (1951) N Y Ng & L Erwin, 37th Antec of the SPE, Vol 25, 241 (1979) 4.10 GB 2118055A, GB 2118056A, GB 2118057A, GB 2118058A, GB 2118450A, GB 2119666A, GB 2118854A 11. E P 0 1 9 8 533 A 12. EP 85104444.6
MATERIALS & DESIGN Vol. 8 No. 6 N O V E M B E R / D E C E M B E R 1987
13. 14. 15. 16.
G M Gale, Rapra Members, Report No 46 (1980) R S Hindmarch & G M Gale, presented at a meeting of the Rubber Division, American Chemical Society, Philadelphia, May 5-8th. 1982, Educational Symposium. R S Hindmarch, G M Gale & R H Norman. ACS Rubber Division, Houston, October 1983. 'Polybutenes in tackified Polyethylene Film'. A Quilley. B P Chemicals, Grangemouth.
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