CONTROLLED CRACKING OF ALPHA-OLEFIN POLYMERS

Abstract: An improved process for the controlled reduction of average molecular weight and alteration of molecular weight distribution of C₃-C₈ alpha-monoolefin homopolymers or copolymers comprises adding a peroxide continuously at a programmed cyclic rate to the polymer or copolymer and heating the mixture in a melt extruder. The rate of peroxide addition is varied at a frequency with a period longer than the decomposition time of the peroxide but shorter than the passage time of the mixture through the extruder.

Description: This invention relates to an improved process for reacting C₃-C₈ alpha-monoolefin polymers or copolymers with peroxides to achieve controlled degradation or cracking thereof. For some commercial end-uses the melt-flow characteristics of C₃+ polyolefins, especially polypropylene, are not suitable because of the relatively high molecular weight (MW) of such polymer as it is originally produced in the polymerization reactor. Important end-uses where it has become accepted that the melt flow characteristics of such polymers must be substantially improved are in fibres and films. In view of this need, it has been shown in the past that higher melt-flow characteristics can be achieved by controlled chain scission, which in effect reduces the molecular weight of the longer and thus high molecular weight chains. The average MW is reduced lowering the melt viscosity. Furthermore, the molecular weight distribution (MWD) is significantly altered, primarily because of the reduction of the high MW fraction. Improvement of melt properties associated with reduction of melt elasticity results in reduced die swell in extrusion and reduced spin resonance in fibre spinning. Chain scission (cracking) is normally accomplished by oxygen and/or free radical sources, particularly peroxides. The basic concept of accomplishing such degradation by utilizing peroxides is disclosed in US-A 3,144,436, where a free radical initiator is introduced into a polymer melt in the absence of oxygen in a screw extruder. US-A 3,940,379 discloses a method for the degradation of propylene polymers by contacting a propylene polymer with oxygen and an organic or inorganic peroxide, melting and working the resulting mixture in a high shear zone, and recovering an essentially odour-free propylene polymer. US-A 4,061,694 discloses the manufacture of propylene moulding compositions of improved impact strength by subjecting block copolymers of ethylene and propylene to controlled oxidative degradation under conditions essentially similar to those of the preceding patent. A new term has been coined for such degraded or cracked polypropylene, that term being "controlled rheology" (CR) polypropylene. Although controlled rheology polypropylene has been commercially available for several years, its similarities and differences from "normal" or reactor polypropylene are just starting to be understood. CR polymers have a variety of advantages and disadvantages. The growing diversity of the polyolefin market is putting an increasing demand on polyolefin manufacturers for product grades to fit a large variety of processing behaviours as well as bulk mechanical properties. Increased control over MW and MWD in the manufacturing process is an important step in this direction. Typically, the polypropylene producers have focused on the single property, "melt flow", when manufacturing CR polypropylene for specific products. However, another property molecular weight distribution (MWD) is also critically important. As discussed by Brown et al in "Molecular Weight Distribution and its Effect on Fibre Spinning", Fibre World, Vol. 1, No. 2, pages 32-43 (March 1984), the three commonly used molecular weight averages are M n , M w , and M z . These are obtained by three different averaging methods, referred to as "number", "weight", and "z" and are based on ratios of successively higher moments of the MWD. The MWD itself can be defined by various ratios of these averages, as follows: Q = M w /M n R = M z /M w S = M z /M n In some cases, these are inadequate to express a detailed description of the MWD as they are based on averaging processes. In this case a detailed "SPECTRAL analysis" of the MWD is preferable, where separate segments of the MWD are specifically examined. Polypropylene homopolymers of different melt flow (MF) have roughly the same shape of MWD when prepared by the same reactor process. With a CR resin, this MWD changes. The three molecular weight averages are all reduced in the CR process. The fastest changing average is M z , while the slowest is M n . This is not surprising since the high molecular weight end of the MWD is the most changed by the CR process. Also not surprising is that S decreases faster than Q, which decreases faster than R. After the CR process takes place, an MWD becomes skewed due to the preferential loss of the high molecular weight components. For a completely random scission process, Q approaches a limiting value of 2.0, while R approaches 1.5. Rheological (melt flow) behaviour is very sensitive to the MWD, particularly to the high molecular weight portion of the MWD. Reduction of the high molecular weight portion of the MWD with corresponding increase of the medium or low molecular weight portions of the MWD is referred to as "narrowing" of the MWD. The difference between "narrow" and "broad" MWD can have profound effects on melt processibility. For example, for two polypropylenes with the same melt flow index and different breadth of MWD, the polypropylene with the narrow MWD will generally show a reduced shear sensitivity over a wider shear range than that with the broad MWD. In the past it has not been possible to achieve independent variability of melt flow and molecular weight distribution without blending together various CR polymers or using different polymers from different polymerization conditions. The applicants have discovered a new method that permits the preparation of increased melt flow products along with control over the desired molecular weight distribution, without blending, using a single degradation or cracking process. The present invention is directed to a process for reacting a homopolymer or copolymer of an alpha-monoolefin with 3 to 8 carbon atoms with from 0.001 to 1.0 parts by weight of a peroxide per 100 parts by weight of said homopolymer or copolymer and heating the resulting mixture in an extruder at a temperature of from 150°C to 300°C, characterized in that the rate of addition of said peroxide is cyclically varied at a frequency with a period longer than the decomposition time of said peroxide, but shorter than the passage time of said mixture through said extruder. The process of this invention allows for various embodiments of which the easiest operative modes are: 1. Sinusoidal variation of the peroxide addition rate around an average value. This will effect a general broadening of MWD. 2. Periodic pulsing of the peroxide addition rate above an average addition rate. This will result in low MW tailing of the polymers. 3. Periodic pulsing of the peroxide addition rate below an average addition rate. This will result in high MW tailing of the polymers. 4. Normal addition rate at zero level, periodic pulses of high addition rate. This will effect a minor amount of overcracked product (ultra-low MW) in the polymer. The present invention has particular application for propylene polymers useful for fibre application. Both melt flow (MF) and MWD of the polypropylene affect fibre properties. The effects of MF and MWD can be broadly related to processibility. In general, strength will decrease, and the draw ratio and processibility rate will increase as the MF increases. The effects of MWD on processibility are profound but are more difficult to define. Table 1 shows some general trends of properties as they relate to MF and MWD. These are generalizations, which may not always hold true for different processing conditions. Nevertheless, the table does provide some guidance for appropriate adjustment of MF and MWD: Table 1 TRENDS FOR POLYPROPYLENE RESINS PROCESSED UNDER IDENTICAL CONDITIONS In cracking a given polyolefin according to the prior art, a single steady rate of addition of peroxide in the continuous process results in a raising of the MF and narrowing of the MWD. With the exception of a general raising of processing speed, Table 1 shows that these changes in MF and MWD will produce opposite directions of changes, respectively, in other processing properties. Because of this, the advantage of cracking may therefore be reduced or even become a disadvantage for some aspects of processing. The MF and MWD cannot be independently controlled in the prior art, since for a given starting material, the final MWD is determined by the extent of cracking or by the final MF. The manufacturer is effectively "locked-in" to accepting a given set of processing properties dependent on his selection of desired MF. The primary advantage of the invention in this context is the capability of independent control of MWD for a given MF by use of an added degree of freedom, namely the cycling mode and/or frequency used in the peroxide addition. The polymers that are degraded or cracked according to this invention are homo- and copolymers of C₃ to C₈ alpha-monoolefins. Polypropylene is preferred. However, other polyolefins which can be processed according to the technique of this invention include propylene/ethylene impact copolymers, polybutene-1, poly-3-methylbutene -1, poly-4-methylpentene-1, propylene/4-methylpentene-1 copolymers, polyallomers and the like. With regard to the propylene polymer, starting melt flows are about 0.2 to about 20, preferably about 0.5 to about 3.0. A key aspect of the present invention is that the peroxide employed for cracking must have a decomposition time which is shorter than the passage time of the polymer/peroxide mixture through the melt extruder. For example, the passage time of a polymer through a 32'x1' Egan face-cutting extruder is about two minutes. Therefore, the peroxide employed with such a machine must have a decomposition time shorter than two minutes. Further, the rate of peroxide addition is varied at a frequency with a period longer than the decomposition time of the peroxide. Accordingly, it is generally preferred that the peroxide have a decomposition time (defined as the half-life at the extrusion temperature) of about 1 to about 30 seconds. If the peroxide has a half-life of less than one second, the peroxide is too unstable and can be decomposed before reacting with the polymer to the desired extent. If the half-life is more than about 30 seconds, then reaction may be incomplete and/or selection of the frequency of addition may be too constrained to achieve the desired results. Preferred peroxides are those which have relatively high decomposition temperatures and produce volatile decomposition products, the latter being relatively non-toxic and with minimal residual odour. The peroxide of choice is 2,5-dimethyl-2,5-di(t-butylperoxy)hexane (Lupersol 101 ® ). Other examples are dicumyl peroxide, di-t-butyl peroxide, t-butyl cumyl peroxide and 2,5-dimethyl-3,5-bis(t-butylperoxy)- hexene-3. The amount of peroxide and the cracking temperature depend upon the melt flow of the starting polymer and the desired melt flow of the final composition. Typical amounts of peroxide are between about 100 parts by weight per million parts by weight total polymer (ppmw) and about 10,000 ppmw, preferably between about 200 ppmw and about 1,000 ppmw. Typical cracking temperatures are between about 150°C and about 300°C, preferably between about 190°C and about 260°C. Preferably the peroxide is added to the polymer powder prior to feeding into the extruder. However, it is also possible to inject the peroxide into the polymer melt in the extruder. If it is added in this fashion, the point of injection should be downstream of the compression zone of the extruder. One possible arrangement for the injection pulsing involves the use of a 3-way valve. The 3-way valve is periodically actuated by a timed solenoid to provide the pulse of peroxide solution injection. Peroxide may be returned to the storage tank on the "off" period. An effective means for pulsing might also be provided by simply rotating the valve at constant speed. In this case, pulses would be sinusoidally shaped rather than rectangular. A constant pressure device would be necessary in any case for normal or direct injection. To avoid excessive repeated pumping of a given portion of peroxide solution (which causes degradation) an excess of peroxide solution should be in the reservoir tank. Ideally, the volume of solution should be at least N times that actually used, where N is the ratio of off/on times. This would allow an average of one pumping cycle for each portion of peroxide. As used in the examples, melt flow is the amount (in grams per 10 minutes) of polymer which flows through a capillary with relatively small L/D ratio under ASTM D1238 Condition L at 230 °C. This measurement is done under constant stress. Neglecting end effects, the shear rate experienced by a polymer in a melt indexer with capillary of radius R is given by: where Q is the volumetric flow rate and R is 0.1045 cm. Thus polypropylene with a density of 0.738 at 230 °C and a melt flow value of 'X' will experience a shear rate of about: = 2.5(X)sec⁻¹ in the melt indexer. For most commercial polypropylenes (MF in the range of 1-20) the melt indexer relates to behaviour at moderately low shear rate (2.5-50 sec⁻¹). This is important to remember, since the area of real interest might be the shear flow behaviour at the spinnerette. This will generally be on the order of 10 to 1,000 times the shear rate experienced in the melt indexer, for typical fibre spinning applications. Even at the lower shear processes encountered in extrusion or injection moulding, shear rates may be many times that experienced in the melt indexer. Since polymers are highly non-Newtonian (viscosity is dependent on shear rate), melt flow index may be misleading in ranking processibility speeds of different product grades. Example This example describes a test of the peroxide cycled addition conducted at a plant scale level. The extruder used had twelve-inch diameter and produced 10,000 lbs of pelletized polypropylene per hour. Starting material was polypropylene with a melt flow of 1.5 and cracking was conducted to obtain a melt flow of 4.5±0.2. At this level of cracking, destruction of the higher MW fraction of the MWD is significant but not extensive. The increased MF was necessary to achieve the desired processing speed, yet the small changes in MWD resulted in some undesirable changes in processing behaviour (fibre-spinning application) when the conventional (non-pulsed) cracking technique was used. The use of the pulsed method succeeded in altering the MWD in the desired direction. Table 2 shows the MWD spectral analyses of the pulsed and non-pulsed cracked products obtained by high temperature gel permeation chromatography (GPC). The more detailed spectral analysis was required to detect MWD differences, since the Q, R and S values tended to obscure the subtle changes in the MWD caused by the pulsing technique. The pulsing technique used is the one described by normal addition rate at zero level, periodic pulses of high addition rate. This will effect a minor amount of overcracked product (ultra-low MW) in the polymer. Table 2 shows the spectral analyses for the two pulsing variations (pulsed-into-powder and pulsed-into-melt) and the conventional steady variation. Also given in the table are the melt flows as well as the values for Mn, Mw, Mz, Q, R and S for the three variations. The following conditions were used for delivery of a 50% mineral oil solution of peroxide 2,5-dimethyl-2,5-di- (t-butylperoxy)hexane to achieve the desired melt flow. STEADY INTO POWDER (Conventional): 22 ml/min. PULSED INTO POWDER: Peroxide injected into the powder crammer feed to the extruder, 1.4 seconds "on"; 23 seconds "off" at 55 ml/min. PULSED INTO MELT: peroxide injected into the extruder melt, 1.5 seconds "on"; 12.5 seconds "off" at 100 ml/min. Table 2 shows the spectral analyses to be clearly different for the three variations. Compared to the conventional "steady into powder", the "pulsed into powder" shows a definite shift to increased amount of higher molecular weights compensated by reduction in amount of lower molecular weights. A molecular weight shift is also apparent for the "pulsed into melt", but is more to an increase in the amount of the midrange molecular weight compensated by a reduction in the amount of lower molecular weights. Table 2

Claims: 1. A process for reacting a homopolymer or copolymer of an alpha-monoolefin with 3 to 8 carbons atoms with from 0.001 to 1.0 parts by weight of a peroxide per 100 parts by weight of said homopolymer or copolymer and heating the resulting mixture in an extruder at a temperature of from 150°C to 300°C, characterized in that the rate of addition of said peroxide is cyclically varied at a frequency with a period longer than the decomposition time of said peroxide, but shorter than the passage time of said mixture through said extruder. 2. The process according to claim 1 wherein said homopolymer is a propylene homopolymer. 3. The process according to claim 2 wherein said propylene homopolymer has a melt flow of from 0.5 to 20. 4. The process according to claims 1-3 wherein said peroxide has a half-life of from 1 to 30 seconds. 5. The process according to claims 1 to 4 wherein said passage time is less than 5 minutes. 6. The process according to claims 1 to 5 wherein said peroxide is mixed with said homopolymer or copolymer prior to feeding into said extruder. 7. The process according to claims 1 to 5 wherein said peroxide is added into the extruder separately from the injection of said homopolymer or copolymer. 8. The process according to claim 1 wherein the rate of addition of said peroxide is varied continuously. 9. The process according to claim 1 wherein the rate of addition of said peroxide is varied intermittently.

Inventors: DAVISON, SOL
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1985
Main classification: C08F8
IPC codes: C08F2, C08F8