GB2100268A - Polypropylene pellets - Google Patents

Abstract

There are described thermoplastic polypropylene pellets of improved processability containing a free radical source prodegradant which on remelting the pellets by extruding or the like, will react to further reduce the molecular weight as well as the molecular weight distribution of the polymer to a point where high capacity production of quality fibers and extruded products can be obtained. The free radical material has a 1 DIVIDED 2 life in polypropylene in excess of 1 DIVIDED 2 minute at 190.56 DEG C.

Description

SPECIFICATION Polypropylene pellets This invention relates to polypropylene polymer pellets having improved processing characteristics for spinning, extruding and the like, as well as methods for obtaining them.

A good processing polypropylene polymer for fiber or film formation desirably has the following attributes: 1 ) the ability to be attenuated when molten without breaking -this allows high throughput production of fine filaments and thin films which have high strength relative to unattenuated products; and 2) the ability to be pumped through piping and capillaries and/or be attenuated as a fiber or film requiring a minimum of energy - this implies lower shear and extensional viscosities for the polymer melt.

It has been demonstrated that the first attribute (high attenuability) can be attained with a polypropylene polymer having a narrow molecular weight distribution (defined as the polymer weight average molecular weight divided by the polymer number average molecular weight). The second attribute (low shear and extensional viscosities) is attained with a lower weight average molecular weight polymer.

Ziegler-Natta catalysts presently used in the commercial production of polypropylene produce polymers having too broad a molecular, weight distribution coming out of the polymerization reactor for production of fine fibers or thin films.

Thus, low weight average molecular weight polymer out of such a reactor would have the desired low viscosity for processing, but would not be attenuable to the desired extent. Polypropylene suppliers have, therefore, found it necessary to make a very high weight average molecular weight polymer followed by a random molecular scission step (thermal or chemical degradation) which inherently narrows the molecular weight distribution while at the same time reduces the weight average molecular weight to the desired level.

Polypropylene is degraded chemically by addition of compounds that decompose forming free radicals. Chemical stabilizers added to polypropylene to enhance end-use stability may interfere with free radical generators.

However, it has been found that some free radical generator types of chemicals, such as the specific types of organic peroxides described in British Patent 1,442,681 for example, are minimally affected by commonly used stabilizers and are, thus, preferred prodegradants.

The degree to which the polymer can be degraded is limited, however, by the inability of the polymer producer to form pellets from very low viscosity polymers. Therefore, the polypropylene processor manufacturing films and fibers faces the problem of having to use a polypropylene not optimally suited for these applications. Thus, a need has been demonstrated for a polymer having high viscosity properties for pelletizing purposes and low viscosity properties for end use processing purposes.

Low viscosity polypropylene polymers desirable to processors cannot presently be pelletized commercially by polymer producers without producing an excess of "stringers" (pellets with long tails) that tend to plug producers' and processors' equipment.

It has been suggested that the polymer viscosity could be increased for pelletizing by running the pelletizer at a temperature just above the polypropylene polymer melting point to improve pellet cut. This can be done only at a low throughput to reduce the heat generated by shear forces in the pelletizing equipment and/or by cooling the molten polymer -- both adding considerably to the process cost and complexity.

It has also been suggested that the end use processor add additional chemical prodegradant to the polypropylene pellets to reduce the polymer viscosity to the desired level prior to fiber or film formation. However, there are several disadvantages to this approrach: 1) the peroxide prodegradants are fire/explosion hazards and require special handling procedures and equipment; 2) to be most effective, the peroxide must be uniformly dispersed within the polymer before it decomposes and reacts -- otherwise a polymer with variable viscosity may result with an even broader molecular weight distribution than the original polymer.The polymer producer, having access to specialized equipment and fine reactor flakes rather than pellets, is in a much better position to achieve this uniform distribution; 3) the processor's equipment may be damaged by a variable viscosity polymer; 4) the peroxide is more efficient as a prodegradant if well dispersed before reacting; and 5) the peroxide added to or on the pellets rather than within them acts as a lubricant in extruder feed sections reducing throughput for a given rpm.

The processor may also reduce the molecular weight by using very high temperatures to thermally degrade the polypropylene. However, these very high temperatures lead to: 1) reduced equipment life; 2) throughout limitations because of quenching restraints; 3) excessive energy consumption; 4) hazardous operating environments; and 5) additive problems.

The additive problems include: 1) excessive additive degradation, necessitating that more additive be added to the polymer than is required in the final product; 2) limited range of useable additives, requiring that more expensive or non-optimum additives be used; and 3) polymer piping, capillaries, dies, and the like plugging from the degradation products.

Additional information may be obtained by reference to prior patents. British Patent 1,442,681 to Chemie Linz describes a process for the preparation of polypropylene including degradation with peroxide prodegradants producing a narrow molecular weight distribution polypropylene polymer. U.S. Patent 3,867,534 to Baba et al describes the use of aliphatic perioxides as prodegradants for polypropylene and discusses problems related thereto but suggests that unreacted prodegradant is to be avoided. U.S.

patent 3,144,436 to Greene et al describes a process for degrading stereoregular polymers including the use of free radical initiators. In one embodiment a two-step method is described wherein there is controlled injection of the prodegradant into the melt zone of the extruder.

U.S. Patent 3,849,241 to Buntin et al and U.S.

Patent 3,978,185 also to Buntin et al describe meltblowing processes that are improved through controlled degradation of the polymers. U.S.

Patent 3,755,527 to Keller et al similarly describes advantages of polymer degradation.

According to one aspect of the invention there is provided polypropylene pellets containing a free radical source prodegradant having a half life in polypropylene in excess of one half minute at 1 90.560C (3750F) are preferably being present at at least about 0.01% by weight relative to the polymer.

According to a further aspect of the invention there is provided a method of processing polypropylene which method comprises incorporating within a high weight average molecular weight polypropylene at least one free radical source prodegradant having a half life in polypropylene in excess of one minute at 1 90.560C (3750F) and peiletizing the said polypropylene under conditions whereby a portion, preferable at least about 0.01% by weight relative to the polymer weight, of the said prodegradant remains available for further degradation after the pelletization.

According to a yet further aspect of the invention there is provided a method of processing polypropylene wherein the polypropylene pellets of the invention are extruded, under conditions causing the remaining prodegradant in the said pellets to substantially completely react, to produce a low viscosity polypropylene suitable for film and fiber forming.

According to a still further aspect of the invention there is provided blends of pellets according to the invention and polypropylene pellets not containing a free radical source prodegradant whereof the total prodegradant present is sufficient to degrade all the polypropylene present to a weight average molecular weight in the range of about 60,000 to about 13,000 with a molecular weight distribution of about 2.5 to about 4.5 upon complete reaction during further processing.

The present invention results from the discovery that when certain free radical generating chemicals that act as polypropylene prodegradants are added to the polymer and the pelletizing equipment operated in a specified manner, a portion of the chemical survives the pelletizing process. After extrusion to form pellets the reaction is interrupted and the remainder of the prodegradant will then react upon re-extrusion producing a polymer that processes well and produces films and fibers with excellent properties. The exact remaining percentage of prodegradant after pelletizing by the producer will vary depending upon pelletizing temperature, prodegradant residence time at this temperature, and type of prodegradant, but will preferably be more than half and up to 90% by weight of that originally added.Ideally, for pelletizing, no degradation takes place, but, as a practical matter, some prodegradant will initially react during pelletizing. The small amount reacting initially, as low as about 10%, only minimally reduces the polymer viscosity at the pelletizer, permitting well formed, free flowing pellets to be made. After pelletizing, residual prodegradant in an amount of at least 0.01 percent based on the weight of polymer is necessary for acceptable results to be obtained. Thus, the advantages of a two-step degradation addition method described above in connection with the prior art are retained, but the disadvantages are substantially eliminated.

The invention is applicable to the production and processing of polypropylene. It is also applicable to the processing of waste polypropylene material to permit reuse in film and fiber formation. Of course, as will be apparent to one skilled in the art, optimum operating conditions and concentrations will vary depending upon the properties of the polymer being used and the ultimate properties desired by the processor.

As produced, polypropylene generally has a high weight average molecular weight in the range of from about 250 M to 500 M and a molecular weight distribution of about 10 to 1 5.

For high speed spinning and fiber forming, the weight average molecular weight distribution is preferably about 2.5 to 4.5. However, when the molecular weight is reduced below about 130 M, the polypropylene resin cannot be easily commercially processed into pellets. The low viscosity polymer instead produces poorly formed pellets which are difficult to transport and handle.

Therefore, manufacturers prefer that the degradation of polypropylene prior to delivery be limited to produce a molecular weight not less than about 160 M. Many prodegradants are used to achieve this degree of degradation in pelletizing equipment, and most of them totally react under these conditions. The organic peroxide prodegradants decompose at different rates depending on temperature and environment. The rates of decomposition are defined in terms of half-life.

In accordance with the invention a free radical source prodegradant having a half-life in polypropylene in excess of one-half minute at 3750F (1 90.560C) is added to a high molecular weight polypropylene reactor flake polymer in an amount sufficient to produce the final polymer properties desired by the polymer processor. If it is desired to use a prodegradant with a shorter halflife or allow a greater amount of prodegradant to make it through the pelletization step unreacted, it is also possible to inject the prodegradant into the molten polymer stream. As the prodegradant should be dispersed uniformly to be most effective, the injection should be followed by a mixing step.In general, the prodegradant should not interfere with or be interfered with by commonly used polypropylene stabilizers and should effectively produce free radicals that upon decomposition initiate polypropylene degradation.

The prodegradant should have a short enough half-life at the polymer processor's re-extrusion temperatures, however, so as to be essentially entirely reacted before exciting the extruder.

Preferably the prodegradant should have a half-life in the polypropylene of less than 9 seconds at 550"F (287.780 C) so that at least 99% of the prodegradant in the pellets reacts before 1 minute of extruder residence time at this temperature elapses. Such prodegradants include, by way of example and not limitation, the following: 2,5 dimethyl-2,5-his-(t-butylperoxy)-hexyne-3 and 4-methyl-4-t-butylperoxy-2-pentanone (e.g.

Lupersol 130 and Lupersol 120 available from Lucidol Division, Penwalt Corporation), 3,6,6,9,9 penta methyi-3-(ethyl acetate)- 1 ,2,4,5-textraoxa cyclononane (USP-138 from Witco Chemical Corporation), 2,5-dimethyl-2,5-bis-(t- butylperoxy)-hexane (e.g. Lupersol 101) and 1-3bis-(tert-butylperoxy-isopropyl)-benzene (Vulcup R from Hercules, Inc.). Of these Witco USP-138 and Lupersol 1 30 are highly preferred. Preferred concentrations of the free radical source prodegradants are in the range of from about 0.01 to 0.4 percent based on the weight of the polymers. Preferably the pelletizer is operated to retain at least 75% of the added prodegradant in the pellets.When subjected to extruding temperatures by the polymer user, the degradation of the polymer will resume and proceed to the extent desired, essentially completely reacting in the re-extrusion process.

Generally such extruder temperatures are in the range of from about 460 to 5500F (237.78--287.780C). Alternatively, these conditions may be obtained for degradation in the extruder die assembly.

In the following Example, provided to illustrate the pellets of the invention and their preparation, melt indices were determined using melt indexer (ASTM 1238) operated at 1 770F (80.560C) with a 2160 g. weight. Samples were allowed to heat to equilibrium for 5 minutes prior to testing. The melt index is equivalent to the grams exiting at 0.0825 inch (2.10 mm) diameter capillary in a period of 10 minutes. Throughout this specification percentages are by weight unless otherwise indicated. ("LUPERSOL" is a registered Trade Mark.) EXAMPLE 1 A polypropylene reactor flake was obtained that had a melt index of less than one. 0.275 weight percent Lupersol 130 was added to the flake and a homogeneous blend made.This blend was pelletized in pelletizing equipment operated at 375"F (1 90.560C) and the residence time was about 2 minutes. Calculations show about 22% of the peroxide had reacted. The pellets were tested and found to have a melt index of about 55.

Approximately 10% of the prodegradant in the pellets reacted in the melt indexer so that actual melt index may be considered to be in the 40-45 range for the pellets. This polymer was easily pelletized and gave polymer pellets equivalent to normal commercial pellets.

These pellets were then re-extruded at 4600F (237.780C) with an extruder residence time of about 3 minutes. The extrudate was then tested and found to have a melt index of about 550. To verify that the 4600F (237.780 C) extrusion step did not appreciably affect the melt index, the extrudate was re-extruded and the melt index increased from 550 to 580. Thus, about 95% of the melt index increase was due to the prodegradant in the pellets and about 5% due to the action of the extruder.

EXAMPLE 2 The same flake and equipment was used as in Example 1 except that 0.3% Lupersol 1 30 was added to the flake. The pellets were found to have a melt index of 45-50. Upon re-extrusion, the extrudate was found to have a melt index of about 660. As in Example 1 , the pellet cut was commercially acceptable.

EXAMPLE 3 Witco Chemical USP-138 was applied to the flake as in the method of Example 1 at a concentration of 0.35 weight percent. The blend was extruded at 3750F (1 90.560C) for an extruder residence time of about 2 minutes. The melt index of the extruded sample was found to be about 15. The sample was re-extruded at 4850F (251.670C) with a 3 minute residence time and the melt index was found to be 21 5. The flake without prodegradant added but processed in the above manner had a melt index of 1.7.

EXAMPLE 4 2% Lupersol 130 was blended with commercially available polypropylene pellets identified as Hercules PC-973. This blend was then extruded at 1 700C with a one minute residence time. Calculations show 98% of the peroxide remained unreacted in the extrudate.

Various percentages of the peroxide concentrate extrudate were then blended with polypropylene pellets. A calculated equivalent amount of pure peroxide was added to other pellets.

The viscosity characteristics of pellets of the invention are illustrated by way of example by the accompanying drawing in which: Figure 1 is a graph showing the relationship between reaction time and viscosity at extrusion temperatures for polypropylene with two prodegradant embodiments of the invention.

The "concentrate"/polypropylene blend and liquid peroxide/polypropylene blend of Example 4 above were extruded through a Brabender extruder at 4650F (240.560C) with a seven minute residence time. The viscosities exiting the extruder die tip were determined and are shown in Figure 1. They can be seen to be equivalent.

Thus, the invention includes prodegradant concentrates, pellets with high prodegradant concentrations, which can be added to nonprodegradant containing pellets to gain desired results. Concentrations of up to 5% by weight prodegradant can be formed with ease, and high concentrations are possible.

While it is not desired to limit the invention to any particular theory, the significance of certain prodegradant characteristics may be postulated.

From half-life determinations it can be shown that half-life reaction rate coefficients, k, approximately follow an Arrhenius relationship to give: 19,700 In k = - - + 40.4 for Lupersol 130 T 19,700 lnk=- + 41.6 for Lupersol 101 T where k = half-life reaction rate coefficient in polypropylene, mien.~, and T = temperature, OK.

Having determined k, the following equation may be used to find the amount'of unreacted prodegradant after a given time:

where CA = concentration of unreacted prodegradant; CAO= initial prodegradant concentration; and t = reaction time in minutes.

For example, after one minute at 41 00F (483 K) 50% of the original Lupersol 130 would be unreacted as compared to only 10% of Lupersol 101 under the same conditions.

In addition, it can be shown that the polymer viscosity exiting a piece of equipment can be predicted by the following equation:

where: y = polymer viscosity exiting the equipment after chemical degradation; = = viscosity the polymer would have had exiting without chemical degradation; K = chemical degradation efficiency coefficient; and CR = amount of prodegradant reacted upon exiting.

Since CR = CA CA, combining the above equations gives the following equation:

Thus, for a constant (KCAO) the ultimate polymer viscosity will be the same after a long reaction time regardless of prodegradant used. However, the relationship between viscosity and time will depend upon the half-life reaction rate coefficient, k. For example, FIG. 1 is a graph of exiting polypropylene polymer viscosity versus time based on KCA -0.005 poise-1 (a typical value e.g.

for Lupersol 130 or 101) and in initial polymer viscosity of 10,000 poise with the pelletizing/extrusion processes carried out at 3950F (201 .670C). It demonstrates that the "pelletizing" viscosity of the Lupersol 1 30 sample is about twice that of the Lupersol 101 sample at a normal pelletizing time range of 1 to 3 minutes although the ultimate viscosities would be about the same.

For a prodegradant with a short half-life, the viscosity of the polypropylene upon exiting a pelletizer at 3950F (201 .670C) after 1 minute residence time is only 67% of that if Lupersol 101 was used and only 30% of that of Lupersol 130 was used. Thus, the latter is preferred, although the others may be used.

Although, in the case of Lupersol 130, about 50% of the prodegradant may remain after pelletizing, because the initial addition level is quite low, there is little or no danger in handling the polymer. After re-extrusion there will be essentially no prodegradant remaining since typical processing conditions are at least 460cm (237.780C) at which the Lupersol 130 half-life coefficient is over 6 mien.~1. With an equipment residence time of only 2-1/2 minutes, for example, only 0.000017% of the peroxide in the pellets would remain in the extrudate. For example, if the polypropylene pellets had 0.2% Lupersol 1 30 as received from the producer, the processor's extrusion equipment was operated at 4600F (237.780C), and the extruder residence time was 2-1/2 minutes, the Lupersol 130 concentration In the polymer exiting the extruder would be less than 1 part per billion.

Thus it is apparent that there has been provided in accordance with the invention, a polymer composition maintaining easy pelletization for polymer producers while significantly improving the processor's ability to use it and a method for manufacturing the material that fully satisfy the objects, aims and advantages set forth above.

Claims (14)

1. Polypropylene pellets containing a free radical source prodegradant having a half-life in polypropylene in excess of one half minute at 1 90.560C.

2. Pellets as claimed in claim 1 wherein the said prodegradant comprises 2,5-dimethyl-2,5 bis-(t-butylperoxy)-hexyne-3 and/or 3,6,6,9,9 pentamethyl-3(ethyl acetate)-1 ,2,4,5-textraoxa- cyclononane.

3. Pellets as claimed in claim 1 or claim 2 wherein the said prodegradant is present at about 0.01% by weight relative to the polymer.

4. Pellets as claimed in any one of claims 1 to 3 wherein the said prodegradant is present at from about 0.01 to about 0.4% by weight relative to the polymer.

5. Pellets as claimed in any one of claims 1 to 4 wherein the said prodegradant is present in sufficient quantity to degrade the said polypropylene to a weight average molecular weight in the range of about 60,000 to about 130,000 with a molecular weight distribution of about 2.5 to about 4.5 upon complete reaction during further processing.

6. Blends of pellets as claimed in claim 1 and polypropylene pellets not containing a free radical source prodegradant whereof the total prodegradant present is sufficient to degrade all the polypropylene present to a weight average molecular weight in the range of about 60,000 to about 130,000 with a molecular weight distribution of about 2.5 to 4.5 upon complete reaction during further processing.

7. Polypropylene pellets containing unreacted free radical source prodegradant substantially as herein described.

8. Polypropylene pellets containing unreacted free radical source prodegradant substantially as herein described in any one of the Examples.

9. A method of processing polypropylene which method comprises incorporating within a high weight average molecular weight polypropylene at least one free radical source prodegradant having a half-life in polypropylene in excess of one-half minute at 1 90.560C and pelletizing the said polypropylene under conditions wherein at least a portion of the said prodegradant remains available for further degradation after the pelletization.

1 0. A method as claimed in claim 9 wherein the proportion of the said prodegradant remaining available for further degradation after pelletization is at least about 0.01% by weight relative to the polymer weight.

11. A method as claimed in either of claims 9 and 10 wherein the said polypropylene has a weight average molecular weight in the range of from about 250,000 to 500,000 and a molecular weight distribution of about 10 to 1 5 prior to addition of said prodegradant.

12. A method as claimed in any of claims 9 to 11 wherein the said prodegradant is added in an amount of from about 0.01 to 0.4% by weight relative to the polymer weight and wherein the portion of the said prodegradant which remains available after pelletization is up to 75% of the said amount of prodegradant.

13. A method as claimed in claim 12 wherein the said portion which remains available is up to 90% of the said amount of prodegradant.

14. A method as claimed in any one of claims to 1 3 wherein the said prodegradant is incorporated into the said polypropylene in sufficient quantity to degrade the said polypropylene to a weight average molecular weight in the range of from about 60,000 to 130,000 with a molecular weight distribution of about 2.5 to about 4.5 after said prodegradant has substantially completely reacted.

1 5. A method as claimed in any one of claims 9 to 14 wherein the said prodegradant comprises 2,5-dimethyl-2,5 bis-(t-butylperoxy)-hexyne-3 and/or 3,6,6,9,9-pentamethyl-3-(ethyl acetate)1,2,4,5-textraoxa cyclononane.

1 6. A method of processing polypropylene as claimed in claim 8 comprising the steps of, a) providing polypropylene having a weight average molecular weight in the range of from about 250,000 to 500,000 and a molecular weight distribution of about 10 to 15, b) incorporating within the said polypropylene a prodegradant comprising 2,5-dimethyl-2,5 bis-(tbutylperoxy)-hexyne-3 and/or 3,6,6,9,9penta methyl-3-(ethyl acetate)- 1 ,2,4,5-textraoxa cyclononane in an amount of from about 0.01 to 0.7% of the polymer weight, and c) pelletizing the said prodegradant containing polypropylene under conditions whereby at least about 75% of said prodegradant remains unreacted after pelletizing.

1 7. A method of processing polypropylene wherein polypropylene pellets as claimed in any one of claims 1 to 8 are extruded under conditions causing the remaining prodegradant in the said pellets to substantially completely react, to produce a low viscosity polypropylene suitable for film and fiber forming.