WO1996031348A1

WO1996031348A1 - A method to reduce draw resonance

Info

Publication number: WO1996031348A1
Application number: PCT/EP1996/001503
Authority: WO
Inventors: François CHAMBON; Jean-François AGASSANT; Yves Demay; David Silagy; Stefan Bertil Ohlsson
Original assignee: Exxon Chemical Patents Inc.
Priority date: 1995-04-07
Filing date: 1996-04-04
Publication date: 1996-10-10
Also published as: GB9507324D0; AU5645696A

Abstract

This invention relates to a method to reduce draw resonance comprising selecting a polymer having a characteristic time of less than 0.4 seconds at 260 °C and a melt index of 4 g/10 min or less and drawing the polyolefin at a draw ratio of 30 or more and at an aspect ratio of about 0.8 to about 2.5.

Description

TITLE : A Method to Reduce Draw Resonance

Field of the Invention

This invention relates to methods for reducing draw resonance in polyolefin processes. In particular this invention relates to a method to reduce draw resonance in polyolefin film processing.

Background of the Invention

Polyethylene is widely used in many different processes, such as extrusion coating, film blowing, film casting, melt embossing and the like, to produce a wide variety of films, sheets and other products. In general these processes are limited by a defect known as draw resonance. Draw resonance is a time dependent phenomenon which, if not controlled, may severely limit the maximum line speed achievable in a processing method.

Draw resonance, (DR), sometimes also referred to as "surging", is probably best defined as: "a sustained periodic oscillation in the velocity and cross-sectional area of a drawing process, when the boundary conditions are a fixed velocity at the die exit and a fixed velocity at the take-off position" (US. Patent 4,339,507 to Kurtz, et al., 1982). It occurs when the draw ratio, D_r, exceeds a critical value, D_rc. In flat film forming processes, it is characterized by the occurrence of periodical fluctuations of the final film width and thickness as a function of time. The amplitude of these fluctuations will strongly increase beyond D_rc eventually leading to an actual breaking/tearing of the web. In film processes, such as cast films, cσextruded films and melt embossing, draw resonance was shown to be more prone to occur with linear-type polymers, i.e. LLDPE (linear low density polyethylene), PP (polypropylene), HDPE (high density polyethylene), PET (polyethylene- terephtalate), than with branched-type polymers, i.e. LDPE (low density polyethylene), (Roberts et al., Advances in Polymer Technology, Vol. 6. No. 1,65-71, 1986; Barq et al., Intern. Polymer Processing, Vol. V, No.4, 1990) . This suggests that melt elasticity and/or elongational rheology strongly influence the occurrence of draw resonance. At first sight, this is not a surprising result as it is consistent with fiber spinning models which predict a stabilization of the process with increasing melt elasticity until rupture is reached (Fisher and Denn, AIChE J., 22, 236, 1976; Demay and Agassant, J . Non Newt. Fluid Mechanics, 18, 187-198, 1985).

Several methods have been empirically devised in order to reduce draw resonance and/or delay its occurrence outside of the processing window. Examples include providing a gentle tensionmg of the web ( US Patent 4,608,221 to Kurtz et al., 1986) or, cooling the weo gradually instead of shock-cooling it in a water bath or on a chill roll (US Patent 4,486,377 to Lucchesi et al., 1984; Roberts et al., Advances in Polymer Technology, Vol. 6. No. 1,65-71, 1986) A special device, a so-called "draw resonance eliminator" has been specially designed to carry out this latter process.

In yet another methodology, "very short" drawing gaps are advised to eliminate draw resonance. This is realized by adding a heated draw roller, i.e at 110- 127°C for LLDPE, positioned at no more than 15cm froτ the extrusion die and used in conjunction with an air knife (UK Patent Application GB 2 124 139 A of Cancio et al., 1984). According to these authors, it is not clear. however, why the short draw gap method is effective. It is theorized that in both this and the draw resonance eliminator technique, the underlying reason for success is an increase of the melt elasticity during the drawing process. It is presumably achieved by partial crystallization of the polymer and apparently results in stabilization of the web.

Instead of modifying the process hardware to eventually affect the elasticity of the melt, an other approach to reduce draw resonance is to improve the design of the polymer itself. Blend compositions of LLDPE with LDPE are disclosed by Kurtz et al. in U. S Patent 4339507 (1982) which are claimed to overcome the deficiency in draw resonance and neck-in of neat LLDPE. In an attempt to avoid the blending step, other researchers have focused on altering the molecular weight distribution (I₁₀/l₂) of LLDPE. They found that the best method to reduce draw resonance for LLDPE is to narrow the MWD (US. Patent 4,780,264 in 1988 to Dohrer and Niemann; Dohrer and Niemann, ANTEC Proceedings, 1989) . In our view, this is an unsupported conclusion as it appears that these researchers did not probe or otherwise vary draw ratio (D_r ) . Instead, their tests "consisted of increasing the extruder rpm and the line speed concurrently to maintain 1.2 mil (30mm) thickness"; in other words keeping D_r constant. What Dohrer and Niemann actually determined is the maximum drawdown accessible at a fixed D_r . It is natural, under these conditions, to reach a higher line speed with a narrow MWD LLDPE than with a broad one. Hence, it should not be claimed that DR has been alleviated as there was no attempt to overpass D_rc in order to obtain a thinner film gauge.

In contrast to these empirical approaches, it is our purpose to now demonstrate that we have discovered a novel way to eliminate and/or delay draw resonance, in polyolefin processing, such as cast film processes, extrusion coating processes, melt embossing processes and the like. This is achieved by an optimization of the process geometry. Summary of the Invention

This invention relates to methods to reduce draw resonance, particularly the draw resonance that occurs during polyolefin film processing.

Brief Description of the Drawings

Figure 1 is a plot of the Aspect ratio versus the draw ratio for Example 1.

Detailed Description of the Invention.

This invention relates in part to the discovery that draw resonance can be controlled by carefully controlling the distance between die and chill roll, (also called the air gap), in relation to the die width.

In a preferred embodiment this invention relates to a method to reduce draw resonance comprising selecting a polymer having a melt index, as measured by ASTM 1238, condition E, ("MI") of 30 g/10min or less, preferably 15 g/ 10min or less, even more preferably 8 g/10min or less, even more preferably 4 g/10mir. or less and a characteristic time (λ_c) of 0.40 seconds or less at 260 °C, preferably a λ_c of 0.35 seconds or less at 260°C, even more preferaoly 0.30 seconds or less at 260 °C and drawing said polyolefin at a Draw Ratio (D_r ) of 30 or more, preferably at 30 to 35, even more preferably at 32 to 35, and using an Aspect Ratio ( A ) of about 0.8 to about 2.5, preferably 0.8 to about 2.0. Polymer Rheology - Characteristic time

Characteristic time λ_c is defined to be the product of the zero-shear rate viscosity, η₀, and the recoverable compliance,

that is λ_c = (Ferry, "Viscoelastic

properties of polymers" John Wiley & Sons, 3rd edition, 1980, incorporated by reference herein). It is obtained by first characterizing the polymer melt properties with small amplitude oscillatory shear (SAOS) measurements at different temperatures, i.e. 170, 200, 230, and 260°C. This characterization is accomplished, for example, with a Rheometrics Mechanical Spectrometer, RMS 800, in accordance to the principles of linear viscoelastic theory well known in the art. The isotherms of the dynamic storage and loss moduli, G'(ω ) and G"(ω), measured as a function of the frequency of oscillation, ω, are then shifted so to obtain a master curve (Mavridis and Shroff, Polymer Eng. Sci., Vol.32, No23, 1992, incorporated by reference herein). The master curve is then fitted with a generalized Maxwell model in accordance to the "parsimonious" technique of Baumgaertel and Winter (Rheologica Acta, 28, 511-519, 1989) and using the Innovative Rheological Interface Software (IRISä) v.2.5 developed at the University of Massachusetts, Amherst, USA by Winter and co-workers. For the purposes of this invention, λ_c is a direct output of "materials functions" in the IRIS software.

Draw Ratio (D_r ) is defined to be the ratio of the film velocity at the chill roll, i.e. line speed, to the film velocity at the die, i.e. extrusion speed, D_r = u_f /u_o . A close approximation to D_r is also obtained from the ratio of the initial film thickness, i.e. opening between die lips, to the final film thickness.

Critical draw ratio ( D_rc ) is defined to be the draw ratio at the onset of the draw resonance instability, i.e. for D_r < D_rc drawing is stable while for D_r > D_rc drawing is unstable.

Aspect ratio ( A ) is defined to be the ratio of the air gap height (X) to the half initial width of the film at the die exit (L_a) , A = X/L_o . The air gap height is taken from the die exit to the contact point between the polymer web and the chill roll. It may be measured with a caliper for example.

Preferred polymers that may be processed according to the invention preferably have a characteristic time of 0.40 seconds at 260 °C or less. Examples include all polymers that are drawn during formation into a product. Further examples include but are not limited to polyolefins which are homopolymers or copolymers of C₂ to C₃₀ α-olefins (for the purposes of this invention ethylene is defined to be an α-olefin). Polyolefins such as polyethylene and polypropylene homopolymes and copolymers are preferred examples. Likewise polyolefins such as linear low density polyethylenes, high density polyethylene, very low density poyethylene, narrow molecular weight distribution (Mw/Mn) polymers such as those available under the trade name EXACT^TM from Exxon Chemical Company in Houston Texas are also preferred. Additional prererred polymers that may be processed accoding to the invention include polymers of polar monomers. Preferred examples include polyesters and copolymers of ethylene or propylene with a polar monomer. Specific preferred examples include nylon polymers, polyethylene terephtalate polymers, ethylene vinyl acetate polymers, polyvinyl chloride polymers, ethylene acrylate polymers, and the like.

Particularly preferred examples include Escorene LL 1004YB a lineal low density polytthylene available from Exxon Chemical Company in Houston, Texas that has a melt index of 2.8g/10min and a density of 0.918g/cm³.

It is expected that when the method disclosed herein is used in conj unction with a device such as a " draw resonance eliminator" referred to above, then the benefits of the combination will be obtained.

Examples

Draw Resonance Measurement

The experiments were carried out on a COLLIN CAST coextrusion line supplied by Collin GmbH, Germany and which includes 3 single screw extruders (30mm diameter, 27 length to diameter ratio; 40mm diameter, 25 length to diameter ratio; 30mm diameter, 27 length to diameter ratio), a feedblock, an adjustable flat die of 280mm width, a 250mm hard chrome temperature controlled chill roll, and a double winder.

In these experiments, the extruders' rpm was set to 5, 13 and 26 for extruders I, II and III respectively, insuring an output of 5kg/h. The temperature control of each extruder was set to 160, 190, 22C, 220°C for zones 1, 2, 3 and 4 respectively.

The flat die opening was set to 1mm. Die positioning with respect to the chill roll was adjusted so that the web fell tangent to the chill roll. A caliper was used to determine the exact airgap height, X . The die temperature was set to 260°C while the chill roll temperature was regulated at 15°C.

In order to observe the changes in the velocity profile along the flow direction, marks were printed on the web using a DOMINO^® Solo inkjet printer supplied by Bopack S. A. The printing head was positioned underneath the die at about 50 to 100mm from the web, while a video camera was positioned on the opposite side of the web thus allowing to visualize the deformation of the printed pattern as well as the variations of the web width between die and chill roll.

For each value of the geometry aspect ratio (A ) , the draw ratio (D_r) was raised by increasing the chill roll speed incrementally. The critical chill roll speed, and therefore the critical draw ratio ( D_rc ) was determined as the speed at which periodic fluctuations of the width and/or velocity of the molten film in the flow direction are detected. This procedure allows to determine an experimental critical curve. The obtained results are shown in Figure 1 and Table 1. In Table 1, The maximum Dr obtainable is listed next to the given Aspect, A .

The error bars in Figure 1 indicate the inherent difficulty of precisely determining the onset of instability. The criterion used to define an unstable flow consisted in waiting two minutes after an increase of line speed and checking whether periodic variations are seen. Some times, after a small increase of the chill roll soeed, DR starts to initiate but vanishes atter a few seconds. In these instances the flow was considered as stable.

Density was measured by ASTM 1238, condition D.

Melt index was measured by ASTM 1238, condition E.

All documents described herein including any priority documents are incorporated by reference herein. As is apparent from the foregoing general description and the specific embodiments, while forms of the invention have been illustrated and described, various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited thereby.

Claims

Claims :

1. A method to reduce draw resonance comprising selecting a polymer having a characteristic time of less than 0.4 seconds at 260 °C and a melt index of 30 g/10 min or less and drawing the polyolefin at a draw ratio of 30 or more and with a process Aspect ratio of about 0.8 to about 2.5.

2. The method of claim 1, 2 or 3 wherein the draw ratio is from 32 to 35.

3. The method of any of the above claims wherein the polymer is a polyester, a polyethylene, a polypropylene, or a polyethylene terephtalate.

4. The method of any of claims 1 or 2 wherein the polymer is linear low density polyethylene, high density polyethylene, very low density polyethylene, ethylene vinyl acetate, polyvmyl chloride, ethylene acrylate or polypropylene.

5. The method of any of the above claims wherein the melt index is 8 g/ 10 mm or less .

6. The method of any of the above claims wherein the melt index is 4 g/ 10 mm or less.

7. A film formed by any of the above methods.