WO1995017294A1

WO1995017294A1 - Extrusion method for manufacturing orientated polymer products and a product manufactured according to the method

Info

Publication number: WO1995017294A1
Application number: PCT/FI1994/000563
Authority: WO
Inventors: Tommi Runar Borg
Original assignee: Tomcoat Oy
Priority date: 1993-12-15
Filing date: 1994-12-13
Publication date: 1995-06-29
Also published as: AU1244495A; FI935630A0; FI935630A

Abstract

A method for manufacturing disentangled and oriented products from thermoplastic raw material by extrusion and a product manufactured according to the method. Thermoplastic raw material is extruded through a die in at least two stages. In the first stage the material, which is over the crystallization temperature, is pushed under high pressure through a narrow channel. The great adhesion forces make it possible to achieve high internal shear rate of the material, whereby the molecule chains will disentangle and straighten in the flow. In a second stage the material will be cooled in a channel with wider cross section, whereby the shear rate is kept high enough to conserve the material structure obtained in the first stage, and a suitable cross-sectional form of the polymer flow is achieved. After extrusion the extrudate is quenched very rapidly, which will conserve a high degree of disentangling and stretched molecule chain orientation in a finished product or an extrudate ready for further stretching.

Description

EXTRUSION METHOD FOR MANUFACTURING ORIENTATED POLYMER PR_OD_UCT_S AND A PRODUCT MANUFACTURED ACCORDING TO THE METHOD.

The scope of invention is extrusion method with molecular disentangled, stretched and orientated products or extrudates by using as raw material thermoplastic polymers and method will be called Jet Extrusion. By stretching the extrudate bellow crystallization temperature the molecules can be ultraoriented freely. The products by using the method have wide range of uses. Product can be used as a construction material in the form of pipe, sheet, film, bar or filament or as heat shrinkable products in the form of pipe, sheet, film or injection moulded products or by extra stretching extrudate will be got high performance filament, bars film or sheet. Product can be crosslinked afterwards, when specially the tolerance of temperature and the properties of shrinkability will be better. At first will be focused to the prior of Art techniques. Mono- or biaxially orientated polymer products are manufactured by conventional extrusion processes as flat-, profile-, annular die or by filament spinning techniques an extrudate, and an orientation stretching will be done afterward. There is no be concerned to the molecular structure, but it is assumed to be principally in unoriented form. The orientation stretching of extrudate will be done in elevated temperature to achieve the orientation process lighter, but according the method the temperature of material will remain bellow the crystallization point. The stretching ratio is from 3:1 up to the natural maximum ratio 10:1. During manufacturing film or sheet, the orientation process will be done to two direction or biaxially. There is used a lot orientated products for example in the form of film, splice or ropes, bars and in pre- processes. Oriented products will have better mechanical and technical properties to the orientation directions of product. There have been done a lot of patents and patent applications in this field. In the pre-orientation scope is also included the technology of dies, will be found an example of method of manufacturing heavy articles, as orientated bars or pipes, where have been used also solid-state or hydrostatic extrusion, as there is described for example in the patent issue EP 0 038 798 Bl.

The melt orientation already done in die is described in the U.S. patent N:o 4,053,270. In this method the melt will enter during extrusion to the entrance, after that into the form of tapered conical inlet and formed entrance and the pattern of flow is said to form polymer orientation and by this way thus have risen the melting point, when it is exiting from the die. Outside of die still melt but solidifying bar is hauled off, because of preventing the typical polymer swell in die during exiting.

When the polymer melt is orienting already in die having low viscosity and after extrusion is doing quenching below the crystallization point of polymer, there will be retain the molecule orientation structure in solidified product, as have been described in the Finnish patent FI 874485 and FI 881067 "Menetelma kutistuvan tuotteen valmistamiseksi",(EP 0380547 B, US 5,053,174, CA 1,316,318, DE 38 90 850 C2, and in application Japan 63-508144, Korea 4863/89 and U.S.S.R 4614086.05). But very soon after invention arose some questions, why the tenacity of the product was nearly same with unoriented, and why during shrinking job there might appear holes in the product without any obvious reason, although the product should be crosslinked. The orientation was possible to discover moreover besides shrinkability by low longitudinal break elongation. All above methods do have similarity, that molecules are connected each other by entangling or they are coiled with each other, although they should be possibility in oriented form. The entanglementing of molecules in the extrudate is limiting the orientation stretching and good final result.

Especially HDPE polymer offers potential possibilities in manufacturing high performance filaments and profiles, because the yield strength of HP-filament can be at the level of diamond in range

19-25 GPa and in practice there have been achieved at 3.5GPa stage. The yield strength of high quality level steel may be at 3.5 GPa stage, but because of it's great density the weight have to be nearly at ten times at the yield strength of HP-polymer.

There are also manufactured oriented products by solution spinning methods besides mechanical stretching of extrudates. The polymer molecules will be dissolved with adequate dissolvent, where used base polymer material do have very great molecule sizes, for example PE do have molecular weight from M_n>2*10⁵ up to 6*10⁶ and these materials are called many times HMWPE (high molecular weight PE), which do have also long molecular relaxation time over the point of crystallization temperature.

There have been achieved also good results in filament manufacturing with the variations of different solution methods, as has been described in US patent 4,137,393 surface-growth and gel spinning methods, which are described in US patents 4,344,908, 4,433,993 and 4,436,689. Single molecules have been separated from each other by a gel, after the procedure the filaments are spinned and stretched straight. There have been used also same idea in the modified melt-kneeding-process, which have been described in EP patent N:o EP 0 115 192 Bl, where is mixed paraffin wax with HMW polyethylene, which makes possible to extrudate the polymer blend and latter the wax is separated during the stretching work below the point of crystallization (130°C) temperature.

It is known also decades in hydrodynamics, that in oppositing¹ jet flow geometry molecules will orientate, the question still remained how to collect technically diverse orientated molecules as will now be explained. Specially chemical methods do have very slow throughput and that is why end products will be expansive.

The abstract to all above methods and the fact that molecules are is extrudate entangled coiled with each other before stretching or there is aimed by solvents to get molecules remotely separated, so that there is not possible to re-entangle and high degree of orientation should be possible. Also the product made by this way is also uniform without points of disturbance and points of many molecules entangled in same point, although the product is high degree of orientation at average, because these disturbance points could alter mechanical and heat shrinking properties of product. Also there has not be paid attention to chain-extension of molecules especially in injection moulding processes. The scope of this invention is to get a manufacturing method, by which there is possible to avoid above mentioned backsides and by which there have been made possible to get multi purpose and high level method of manufacturing by a simple way. This goal is achieved by a method according this invention, which is known as during the extrusion the polymer is extrudated through the die at least in two stages by the technique where the molecule with great size and low viscosity is pushed at adequate warm temperature to narrow channel, where molecules will disentangle and straighten, there will happen chain-extension and stretching at this apparent high shear rate or in other words in jet extrusion condition. In the second stage polymer is quenched in wider channel but still keeping at least the shear rate enough to maintain the construction of molecules achieved at the first stage and will be attained suitable form of profile before - will be performed quenching during exiting the extrudate, when is conserved the high degree of disentangling of molecules and stretched orientation in solidified finished product or extrudate for to wait pre-stretching.

The very novel idea of the invention is the fact that there is used the most modern results of latest research of the property of polymer disentangling during the defined conditions at higher temperature compared to standard orientation temperature.

At higher temperature the biggest molecules will have the property of easy movement and there is possible technically to disentangle without impossible or oversize level of pressure, and after above the temperature of polymer is cooled as close there is possible to reach technically above crystallization point, at this stage the relaxation time of molecule is longer and the needed small difference of temperature make possible to quench quickly bellow the crystallization temperature, especially the molecule do have normal longer relaxation time above all due to orientation. Also molecules are in disentangled form, which makes possible to achieve great stretching ratios. The process is also not depending strictly on the used method of extrusion, and there can be used all normal procedures, where can be reached the conditions described above.

The solution is exceptional in extrusion and die technology, there is typically at the latest after sharing and steering equally polymer flows, the , channels will become narrower or will stay at the most in same cross area before exiting. The reason for that is the difficulties in adjustment of extrudate wall. During using blended materials there can use with great molecular size polymer smaller size substance or can be blebded very different type chemical structures, and there is those circumstances also possible to extrudate and orientate very great molecular size polymers.

To help observe the invention, there have to examine a little theory: The inventor' 's earlier patents (FI 874485 and F 881067) were produced evidence with a classical theory as follows:

As to the desired properties of the product to be produced, it has been found that the raw material, such as HDPE, has critical properties (i.e. properties affecting the elastic deformation) such as a high molecular weight, broad molecular distribution, low melt viscosity and low density (these properties are dealt with e.g. in a BASF product brochure LUPOLEN R, 1986). With a steady flow, the elastic deformation can be anticipated and this can be illustrated e.g. by means of the following equations (Physics of Plastics, The Plastics Institute, London, 1965, p.272-282).

2M

J = 5cRT,

Wherein

J = steady flow compliance, i.e. "recoverable deformation"

M = molecular weight c = concentration i.e. density

R = rate of shear T = temperature

On the other hand

I 2M j = =

G 5CRT, where G = elastic shear modulus, whereby the elastic shear strain gives

2SM

5cRT,

wherein S = shear stress.

It appears therefrom that a steady flow the elastic strain increases with increasing molecular weight and shear strain and with decreasing temperature. When the stress is removed by heating the product when it is used, the molecules resume their original configuration.

The above explanation is based on the estimation of Rouse-theory, that elastic tension is growing linearly. There is not taken account disentanglementing and chain-extension of molecules in higher shear rates. The perfect orientation degree of molecules P_t is structured;

1. The average direction of molecules, or the value of orientation function, which is called

(p₂(cosθ))_av

2. Coiling around the orientating axis of molecule)

3. Entangling with other molecules 4. Straightness of the backbone of molecule

5. The size, direction and degree of crystallinity

In other words although the value of orientation function (P₂(cosθ)) should be great and although there should take into account partially these other variables, single long molecules can make many 180° degrees turns to direction of orientation in melt flow which is still non crystallized polymer. The crystallization and the sig-zag form of molecules because of crystals will make still arise the complexity of matter.

The longest relaxation time is called λ_d. If the used strain rate (deformation rate) is γ, then the formula γλ_d<l is true during most melt flow. Now one have to attain, that the longest relaxation time λ_d is depending on the size of molecule and temperature and enclosed is a formula giving the indicative universal relaxation time λ

λ = CMⁿ _w where is a temperature dependend constant n = is a constant in range 1,0-

2,0

M_w= is a molecular weight of chain

There is maybe one reason for giving an indication only with approximately values of the relaxation time the fact that the complete orientation degree P_t can vary a lot. At high degree of orientation the constant C and especially constant n have to be much greater compared to low degree of orientation. The matter can be consider that higher degree of orientation level, the greater are the vectors of a single point to both directions and there is not so easy anymore to get loose chain during the molecule is tending to it's standard lower internal level of energy or it's coiled and entangled state. The situation is also similar, when the polymer is tending during quenching to se icrystallized state. It is well known, that during rapid quenching the average size of crystals become smaller and during the first days after extrusion there will happen partially reorganization of crystals. -This phenomena was found to be very strong in doing the orientated and heat shrinkable product according the patent application FI 874485. The stiffness of tape or sheet changed significantly during the first days or weeks after manufacturing or although the complete orienting degree P_t were quite low, the needed time was rise to achieve stabilized crystallization level. In non linear area during higher shear rates and when the formula γλ_d>l is true there is describing the tube model theory² made by Doi and Edwards well the procedure and during this function is happen disentangling and by this way the total orientation degree situation P_t is growing.

When the product of formula is still growing, the backbones of molecules are starting to make stretch and chain-extension because of straighten process, and there is needed a new theory to explain the phenomena³.

In the formula λ_d=3*Z*λ_eq is Z the number of entanglement points and λ_eq is relaxation time according Rousen theory to molecule.

There is also a relation λ_r ~ 4λ_eq, there is a result the product of new formula γλ_r>l, where both variables are in transform area and there will happen chain-extension. The relaxation time is λ ~4/3*λ/Z. There can be made a summary about formulas as follows:

γλ_d>l, chain disentangling occurs γλ_r>l, chain extension occurs

The next condition⁴ is said to hold for the chain extension.

e*λ>l; e*dt>l where e = strain rate λ = relaxation time dt= the time of macromolecule exposed to the flow field, Now we can see directly of based on the energy principle or entropy principle, that the whole function has a form of hysteresis and although the product γλ_d>l should decrease because of new lower shear rate, the situation will stay almost steady to the some range. Based on the above there can be seen that after the straighten of molecule backbone the cross section of flow channel can be extended carefully by keeping the formulas in force for example during quenching the polymer melt close above melting point or to preform profile product in die.

The product should be quenched bellow melting point already in die, but in practice this have to be done quickly after exiting the material from die, because now there is needed lower pressure to whole process. The cooling time t for the exiting extrudate have to be t<λ throughout to keep orientation condition, which is wanted to be orientated. There can be seen as one abstract, that as smaller is entangling density Z in base polymer, as easier there can be achieved molecule stretching.

There is used in actual equipment design and material selection the methods and calculation procedures typically used in rheology, however taking into account above explained formulas and models.

In the following the method according to the invention will be described by means of examples with reference to the attached figures, wherein Fig 1 illustrate equipment and die arrangements, wherein

1A) is circular profile or filament die

IB) is annular die for films and proflies 1C) is arrangement for equipment according the example during doing sheet by annular die by cutting longitudinally after shaping and after opening it will be rolled. Fig. 2 is showing calculated values to product γλ_d in using method according the new method. There are also with for comparison other calculated maximum values to γλ_d for standard extrusion methods with use of conventional raw materials. In the figure the name "Jet Extrusion" is meaning values according new method and those values have to multiply xlOO in other words the "Jet Extrusion" has the maximum product value over 400.

There will be examined firstly the operation illustrated in fig. 1A describing circular filament or bar profile die 1, into which the melt polyolefin material HDPE at 180°C temperature, MI/21,6 = 9 MI/2,16 = 0,05, M„ = 261.800 is incoming from extruder 21 illustrated in fig. 1C via the body of die 20 to the conical entrance 2 having about 500bar internal pressure.

By the right measuring of jet channel 3 there will be get situation in force, which is describing formulas γλ_d>l γλ_r>l and in jet flow 4 is happen disentanglementing of molecules and chain extension. The length of jet channel 3 has to be at enough amount, so that the jet flow 4 should become calibrated, but there have to find out that too great length will take quickly the pressure which is only available and powered by extruder and in such case the process will be terminated. By the rigth measuring of diameter of jet channel 3 there will come pseudoplastic shear rate about at 1000/s. The organization of molecules will stay similar also later, as there is taken into account the formulas e*λ>l and e*dt>l to keep in force in measuring the cooling channel 5 . There is more time in wider cross sectional channel to use for cooling polymer for example to 150°C temperature made external 6 by means of air, oil or water. The cooled material arriving into the forming die 7, the viscosity is greater and the diameter of channel have to be wider compared to jet channel 4, to prevent the pressure to rise too high in the forming die 7. By cooling more the adhesion stress on the walls of forming die 6 is preventing the plugging of polymer and will generate more the orientation. The exiting bar or filament 8 is pulled if necessary more by puller to achieve right dimensions to product or extrudate 9 and will be quenched as soon as possible by cold water or gas during the polymer extrudate 8,9 being already outside of die. It the quenching of extrudate will be succeed throughout according the formula t<λ, there will be high degree of orientation and homogeneity of the extrudate, the size of single crystals will be also small.

Because the apparent viscosity of material will decrease in the square root to the shear rate, there will not consumed the most amount of the available pressure, but usually more is used most part of pressure in cooling section. By altering the properties on polymers, dimensions and processing parameters is possible to manufacture a product, where the formulas will be in power and the integrated orientation degree P_t of manufactured product is high. If the density of polymer entanglementing Z is high, there will not necessary achieved the chain extension formula into power during process, but in that case there can be stretched the extrudate with known techniques and will be achieved with this way extremely high degree of orientation.

The case will be discussed to the annular die shown in (FIG. IB), the material is coming shown in (FIG 1C) from the body of die 20 between sleeve 11 and pin 12. The gap in jet channel 13 for jet flow 14 is smaller because of annular conic. There is not separate cooling reservoir 15, but the channel is extending with similar form to the forming part 16 and if there is wanted to rise throughput, there have to construct in sleeve 11 outside of cooling reservoir 15 channels for oil cooling or outside fan with accurate thermistor control to increase cooling effect. The exiting tubular material in melted form 17 will be sucked with vacuum or blown with over pressure inside against the wall of cold calibration tube 18. By means of the blow ratio, it is possible to affect small transverse orientation, which is needed and enough in use heat shrinking works for manage with sheet of film products. Small transverse shrinking will take off possible wrinkles from heat shrinking film or sleeve. The cold water in container 19 is cooling the calibration tube 18.

The above function is explained from the beginning according the FIG 1C: Thermoplastic raw material (ethylene plastic HDPE, density 0.948 g/cm³, melt viscosity descriptive of melt flow properties = 9 with a load of 21.6kg, carbon black granulate concentration 1.5%) is usually purchased in granular form i.e. as granulates. In FIG. 1C, the raw material is introduced into a screw extruder 21 (speed of rotation of the screw r = 35/min; diameter D = 60 mm; L/D 25; running temperature in the screw cylinder 22 varies between 130-190°C in rising manner, temperature at the die body 20 and die 23 or in the beginning of die in FIG IB 170°C, wherefrom the thoroughly plasticized molten plastic substance is extruded into die shown in FIG IB with the pressure of 470bar and the die has unnormal design form (the gap =0,8mm, sleeve 12 diameter = 46,5mm, pin 11 diameter****** 51,7mm), and the gap between sleeve and pin in cooling section 15 and forming section 16 is 2,6mm, and material is in forming section 16 at temperature 150°C. One have to find that extruder 21, the whole body 29 and annular die 23 or FIG IB have to start to operation at about 50°C higher temperature to make possible in the die IB and in jet channel 13 the jet flow. High shear rate will decrease the viscosity of material, which make possible to use lower temperatures in extruder 21 and die 23 or in FIG IB die during process to achieve temperatures used in production.

The tubular profile extrudate 24 exiting the forming die 24 is then calibrated by rapid cooling with cooling tube 25, so that the shape and outer diameter of the plastic pipe 24 or film are determined. Alter cooling container 26 and haul off machine 27 the pipe is cut open in cutter with rotating blade 28 and is rolled on reel to rolls 29. The manufacturing speed is I4kg/h and the product thickness is 0,9mm. The width of products can be manufactured up to 1000mm wide and there can be choice blow ratios in range -50%..-...+50% compared to the diameter of die. When there is need as strong material longitudinally as possible, there is not of course done the transverse stretching.

In this example the product is heat shrinkable sheet, which is shrinking uniform without making holes also without cross linking procedure, but the heat tolerance is still limited. Also yield strength and degree of orientation have been risen.

When is manufactured the film extrudate for further orientation process, there have to only increase the haul off speed. Thicknesses can be in the range 0,02mm-10mm.

The above examples are only indented to illustrate the basic idea of the invention and keeping on the track and using the given principles there are only at the beginning of applying the invention and with the method there do not have principal limitation to come closer of theoretical maximum values of molecules.

There are also other orientable raw materials suitable as exhibited here in examples. Also the use of extruder is not limited to screw type extruder, but , for high needed pressures are disk- drum- and ramextruders and melt pumps or combination of those are worth attention possibilities for use as extruder.

1. Mackley, M.R. and Keller, A, Phil. Trans. Roy. Soc, London, 278, 1276, 29, (1975).

2. M. Doi ja S.F. Edwards, J. Chem. Soc, Faraday Trans.II, 74, 1789, 1802, 1818 (1978). 3. D. Pearson, E. Herbolzheimer, N. Grizzuti, ja G. Marrucci, J. of Polymer Science: Pat B; Polymer Physics, Vol.29, 1589-1597 (1991).

4. Mackley, M. R. and Sapsford, G.S. In: Developments in oriented polymers (Ed I.M. Ward), Applied Science Publishers, London, (1982).

Claims

C L A I M S

1. Method for manufacturing disentangled and oriented products from thermoplastic raw material by extrusion, characterized in that - during extrusion plastic raw material is extrudated at least thought two stage die by the manner that at first the material being over at crystallization temperature the orientable polymer is pushed under high pressure to narrow channel and great adhesion forces make possible to achieve high internal shear rate of material, where molecules will disentangle and for chain there maybe also will happen straighten, extension and stretching in the apparent high shear rate flow and in the second stage material will be cooled in a wider cross section channel keeping the shear rate enough high to conserve the material structure gotten in the first stage and will be achieved suitable cross-sectional form of polymer flow before - after extrusion the extrudate is quenched very soon, where will be conserved high degree disentangled and possible chain stretched orientation in solid form in finished product or in extrudate ready for further stretching.

2. Process according to the claim 1 characterized in that there is used annular, profile, flat, monofilament and fiber dies and methods.

3. Process according to the claims 1 and 2 characterized in that temperature in the end part of die is kept as low as possible to increase adhesion forces.

4. Process according to the claims 1-3 characterized in that the product can be stretched longitudinally or expanded to the transverial direction of hollow profile soon after exiting the die.

5. Process according to the claims 1-4 characterized in that product can crosslinked chemically or irradiation to improve mechanical, chemical, heat tolerance and heat shrinkable properties.

6. Process according to the claims 1-5 characterized in that cooled product can be stretched further to improve orientation.

7. Process according to the claims 1-6 characterized in that there can be used as a raw material polymer blends.

8. Process according to the claims 1-7 characterized in that cooled product can be expanded also afterwards.

9. Process according to the claims 1-8 characterized in that product can be prewarmed to make easier afterwards done stretching or expanding.

10. Process according to the claims 1-8 characterized in that starting the flow process in extruder die there should use higher temperatures at the beginning compared to production temperatures.

11. Process according to the claims 1-3 characterized in that extruder can not only be screw extruder, but also diskextruder or drumextruder or ramextruder.

12. Product manufactured according the claims 1-3, characterized in that two or more layers of product have been laminated together, and between layers have been used reinforcing material such as fiber net between layers. Fiber net or other reinforcing material can be made from glass, aluminum or other metals.

13. Product manufactured according the claims 1-3 and 12, characterized in that final product can be in tubular form.

Nomenclature: