WO1993000400A1

WO1993000400A1 - Polyethylene blends for molding

Info

Publication number: WO1993000400A1
Application number: PCT/US1992/005214
Authority: WO
Inventors: Michael B. Biddle; Robert D. Villwock; Joseph M. Tanner
Original assignee: The Dow Chemical Company
Priority date: 1991-06-21
Filing date: 1992-06-19
Publication date: 1993-01-07

Abstract

Multicomponent polyethylene resin compositions which are processable using conventional molding techniques (e.g., rotational molding, injection molding and blow molding) have been discovered which offer improved IZOD impact strength of the molded part. Multicomponent polyethylene compositions comprising a uniform composition falling within the area of a polygon ABCD bounded by points A(66.7, 33.3, 0), B(33.3, 0, 66.7), C(0, 0, 100), D(0, 100, 0) and excluding the composition defined by line AB have improved room temperature notched IZOD impact strength over that calculated for the components using an additive rule. For three component polyethylene compositions, a first component (a) comprising a polyethylene resin having a density of about 0.92 g/cc and a melt index of about 1 g/10 minutes, a second component (b) comprising a polyethylene resin having a density of about 0.96 g/cc, and a melt index of about 1g/10 minutes, and a third component (c) comprising a polyethylene resin having a density of about 0.95 g/cc and a melt index of about 17 g/10 minutes are preferred. One of the components can also be replaced by a post consumer recycled plastic.

Description

POLYETHYLENE BLENDS FOR MOLDING

This invention relates to multicomponent polyethylene blend compositions for use in molding processes (e.g., rotational, injection or blow molding;. The multicomponent blend can incorporate a post consumer recycled plastic as one of the components.

Plastics, especially polyethylene, have been utilized in molding processes due to their thermoplastic nature. There are several important parameters for molding polyethylene, including polymer flow characteristics and end product strength properties (e.g., impact strength). Impact strength of articles molded from plastics is an especially important property when the article is subject to rough handling and repeated impact. Impact strength helps to prevent the article from prematurely rupturing.

The processing parameters of the molding process also create unique demands on the plastic. For example, plastics used in a rotational molding process need to be screened to a particular particle size for optimum flow and melting. For polyethylene, the typical particle size for use in rotational molding is approximately 35 mesh. The density and the molecular weight of the plastic are also important in a molding process, allowing the molded article to be homogeneous and uniform throughout, as well as supplying strength and modulus or stiffness. The many types of molding processes require different types of plastics. Density, and molecular weight of the plastic will vary depending upon the molding process and the desired end product properties.

The dominant molding processes are rotational molding, injection molding and blow molding. The largest volume consumer of these, the rotational molding process, forms hollow parts from plastic powders. Polyethylene is most often used in a rotational molding process, due to its low melting point and relatively high fabrication strength. Impact strength of the fabricated/rotomolded part continues to be a commercial need for success in areas in which rotomolding can be used.

Plastics, and particularly polyethylene, also have the advantage of being recyclable. Even though plastics account for only 7 weight percent of solid waste, they are viewed negatively by the public. This negative view and increasing landfill costs have led industry to investigate disposal alternatives. Some alternatives include degradation, chemical conversion, incineration, and recycling. Post-consumer materials, defined as those products generated by a business or consumer that have served their intended end uses, and that have been separated or diverted from solid waste for the purpose of collection, recycling, and disposition, represent a viable source of raw material for new product manufacture, if they can be processed appropriately. Household plastic waste, collected either at curbside by a refuse collection organization or at voluntary drop-off centers, is recognized as a primary source of post-consumer material to be used in the manufacture of recycle plastic-content products. However, post consumer recycled plastic has limited utility, due to the complex nature of the recycled plastic mix. The recycled plastic seldom resembles the original virgin plastic. The recycled plastic can contain different plastics, making fabrication into a useful article extremely complex. While collection of recycled plastic is a universal problem for the public, the useful disposition of these recycled plastics is becoming an increasingly serious p oblem.

Certain multicomponent polyethylene compositions have now been discovered to have an improvement in room temperature notched IZ0D impact strength when formed into molded articles. The measured IZ0D impact strength of the molded article is greater than the calculated IZ0D impact strength of a molded article made from the multicomponent composition using an additive rule. The multicomponent polyethylene compositions contain two or more linear polyethylene components. Multicomponent polyethylene compositions comprising a uniform composition falling within the area of a polygon ABCD bounded by points A(66.7, 33-3. 0), B(33.3, 0, 66.7), C(0, 0, 100), D(0, 100, 0) and excluding the composition defined by line AB in FIGURE 1 have improved room temoerature notched IZ0D impact strength over that calculated for the components using an additive rule. For three component polyethylene compositions, a first component (a) comprising a polyethylene resin having a density of about 0.92 g/cc and a melt index of about 1 g/10 minutes, a second component (b) comprising a polyethylene resin having a density of about 0.96 g/cc, and a melt index of about • g/10 minutes, and a third component (c) comprising a polyethylene resin having a density of about 0.95 g/cc and a melt index of about 17 g/10 minutes are preferred. Linear ethylene/1-octene copolymers as component (a), linear ethylene homopolymers as component (b), and linear ethylene/1-propene copolymers as component (c) are especially preferred.

In another aspect, the invention is a method of improving the room temperature notched IZ0D impact strength, measured in accordance with ASTM D-256, of a molded article made from a multicomponent polyethylene composition comprising at least two polyethylene polymers, wherein said IZ0D impact strength is higher than that calculated by the additive rule. The method comprises:

(a) blending at least two linear polyethylenes in amounts sufficient to form a uniform composition falling within the area of a polygon ABCD bounded by points A(66.7. 33-3, 0), B(33-3, 0, 66.7), C(0, 0, 100), D(0, 100, 0) and excluding the composition defined by line AB in FIGURE 1, and

(b) molding said uniform composition to form a molded article having enhanced room temperature notched IZOD impact strength. The method of improving the IZ0D impact strength is especially effective when the composition falls w thin the area of a polygon EHCD defined by points E(33-3- 66.7, 0), H(33-3, 33-3, 33-3). C(0, 0, 100), D(0. 100, 0) in FIGURE 2, and most particularly effective when the composition falls within -o-

the area of a triangle EFD defined by points E(33-3, 66.7, 0), F(0, 66.7, 33-3). D(0, 100, 0) in FIGURE 3.

The molded article can be made in a molding process selected from the group consisting of: rotational molding, injection molding and blow molding. The improvement in room temperature notched IZOD impact strength is especially effective in a rotational molding process.

Post consumer recycled plastic, usually comprising at least about 90 weight percent polyethylene (such as recycled blow molded bottles and/or recycled molded rigid bottles) can also be incorporated into the three component composition as component (b).

Figure 1 is a compositional ternary diagram of three polyethylene polymers and shows operative weight percentages resulting in increased IZOD impact strength of a molded article over that obtained using an additive rule.

Figure 2 is a ternary diagram based on weight percentage of three polyethylene polymers and shows a preferred embodiment of this invention.

Figure 3 is a ternary diagram based on weight percentage of three polyethylene polymers and shows an especially effective embodiment of this invention.

Linear polyethylene is the preferred type of polyethylene useful in practicing the present invention Manufacture of linear polyethylene is disclosed, e.g., in U.S. Patent 4,076,698, incorporated herein by reference, and involves coordination catalysts of the

"Ziegler" type or "Phillips" type and includes variations of the Ziegler type, such as the Natta type.

These catalysts may be used at very high pressures, but may also (and generally are) used at very low or intermediate pressures. The products made by these coordination catalysts are generally known as "linear" polymers because of the substantial absence of branched chains of polymerized monomer units pendant from the

10 main polymer "backbone." Two types of linear polyethylene are suitable for use in the present invention: linear high density polyethylene (HDPE) and linear low density polyethylene (LLDPE) .

Linear high density polyethylene has a density

15 from 0.941 to 0.965 g/cc while linear low density polyethylene typically has a density from 0.88 g/cc to

0.94 g/cc. The density of the linear polyethylene is lowered by polymerizing ethylene along with minor amounts of alpha, beta-ethylenically unsaturated alkenes

20 having from 3 to 20 carbons per alkene molecule (e.g.,

1-propene, 1-dodecene), preferably 4 to 8 (e.g., 1- butene, 4-methyl 1-pentene, 1-hexene) and most preferably 8 carbons per alkene molecule (i.e., 1-

„2(5- octene). The amount of the alkene comonomer is generally sufficient to cause the density of the linear low density polymer to be substantially in the same density range as LDPE, due to the alkyl side chains on the polymer molecule, yet the polymer remains in the 30 "linear" classification. These polymers retain much of the strength, crystallinity, and toughness normally found in HDPE homopoly ers of ethylene, but the higher alkene comonomers impart high "cling" and "block" characteristics to extrusion or cast films and the hiαh "slip" characteristic inherently found in HDPE is diminished.

The use of coordination-type catalysts for polymerizing ethylene into homopolymers or copolymerizing ethylene with higher alkenes to make copolymers having densities above about 0.94 g/cc as defined in ASTM D-1248 (i.e., HDPE polymers) and/or for copolymerizing ethylene with higher alkenes to make copolymers having densities in the range of LDPΞ and medium density polyethylene (i.e., LLDPE copolymers) is disclosed variously in, e.g., U.S. 2,699,457; U.S. 2,862,917; U.S. 2,905,645; U.S. 2,846,425; U.S. 3,058,963 and U.S. 4,076,698, all of which are expressly incorporated herein by reference. The density of the linear polyethylene useful as component (a), (b) and/or (c) in the present invention is from 0.88 g-'cc to

The molecular weight of the linear polyethylene is indicated and measured by melt index according to

ASTM D-1238, Condition (E) (i.e., 190°C/2.16 kilograms). The melt index of the linear polyethylene useful as component (a), (b) and/or (c) in the present invention is from 0.05 grams/10 minutes (g/10 min) to 200 g/10 min and preferably from 0.5 g/10 min to 25 g/10 min.

The post consumer recycled plastic which can replace one of the components (usually component (b)) can comprise various thermoplastic polymers conventionally used in molding processes. Preferably, the post consumer recycle plastic comprises at least about 90 weight percent polyethylene, more preferably at least about 95 weight percent polyethylene, based on the total recycle plastic weight. Other thermoplastic polymers in the post consumer recycle plastic component include, e.g., polypropylene, polyester and polystyrene. A paper presented by M.L. Wu at The Society of Plastics Engineering VII International Conference on February 27, 1991 entitled "Plastic Recycling: Walnut Creek City Curbside Collection Pilot Study" describes typical mixed, or co-mingled, disposable recyclable plastics which were collected from households and characterized according to type as described in Table 1:

Table 1

Polyethylene, PP = polypropylene, PET = polyethylene terephthaiate, PS = polystyrene and PVC = poiyvinyl chloride. Municipal curbside plastic pick-up programs currently collect predominately co-mingled or segregated rigid plastics described above as categories 1, 2, and 4. Hand sortation is used in start-up programs to deliver separated plastics for recycle. However, for commercial operations, mechanized technology has been developed to clean and separate co-mingled rigid plastics according to specific gravity. Resultant cleaned, separated, ground rigid polyethylene "flake", is then extruded (compounded), devoiatilized. screened, and pelletized to form a homogenized recycle pellet to be used in formulation with compositions of the present invention to manufacture new molded products.

Recycled polyethylene from combined categories 1 and 2 also known as "mixed lights" (natural and pigmented blow molded, injection molded, and thermoformed HDPE and LLDPE rigids) will generally have the properties described in Table 2:

Table2

Recycled polyethylene from a more segregated combination of categories 1 and 2 (natural and pigmented blow molded HDPE bottles) will generally have the

Table3

Recycled polyethylene from category 1 (natural HDPE bottles) will generally have the properties described in Table 4:

Table4

Household film waste, described as category 3, is generally not collected in curbside pick-up programs, but rather specific film products are collected in voluntary collection programs. Grocery sacks (mixture of LLDPE and HDPE films) are currently being collected at grocery stores via voluntary consumer αrop-off programs. Collected grocery sacks are then shredded, cleaned, and separated (from paper contamination) via mechanized systems similar to those used for rigids. Resultant film "flake" is then generally extruded to be formulated with virgin resin to manufacture new products. Recycled polyethylene from grocery sack will generally have the properties described in Table 5: Table 5

The multicomponent polyethylene compositions the present invention can also contain minor amounts additives (e.g., antioxidants, pigments) which do not interfere with the molding of the polymer blend. Phosphites and phenolics (e.g., IRGAFOSTM 168 and IRGANOXTM 1010, respectively, both sold by Ciba-Geigy Corporation) are particularly effective antioxidants f use in the polyethylene compositions of the present invention. Other antioxidants commonly used in the polymer industry can be incorporated into the three component polyethylene compositions of the present invention.

The multicomponent polyethylene compositions can be made by dry blending, melt blending discrete polymers, or by in-situ reactor polymerization. Dry blending pellets can be accomplished by any convenient -eans, e.g., tumble blending weighed portions of pelle _n fiber packs. Since pellet stratification can occur in dry blends while in transit, melt blending is preferred, especially to ensure uniformity of the composition. Melt blending can be accomplished by feeding a dry blend (made as described above) into an extruder/compounder line equipped with a pelletization system (e.g., an underwater pelletizer, or a water trough through which the reextruded strands are cooled and later chopped into pellets). A double pass through the extruder system is preferred to ensure compositional uniformity.

Blends formed by in-situ reactor polymerization can be made by side-arm extrusion of one or more of the polyethylene polymers into another of the polyethylene polymer stream just prior to pelletization, or the blends can be made by operating multiple reactors such that the polymers are each made and blended immediately after polymerization, before or after removal from the reactor(s), but prior to pelletization in a single continuous system. Such multiple reactor operation is disclosed, for example, in U.S. Patent 3,914,342 (incorporated herein by reference). There are several ways known to skilled artisans for making in-situ polymerized reactor blends, and the invention is not limited to any one of the methods.

Example 1

A three component polyethylene blend was prepared according to the percentages described in Table

6. Table 6 also lists the properties of each of the 3 components, for later calculational use:

Table6

The blend was prepared by dry blending the components, and then double pass compounding at 220°C in a one inch diameter single screw extruder using an 80 mesh screen and having feed, transition and metering sections in the screw. The screw was run between 80 revolutions per minute (rpm) and 100 rpm. The extrudate was pelletized through a single strand die, a cooling trough and a strand cutter. The resultant pellets were dry blended and reextruded as before, thus completing the double pass.

The pelletized blend was molded into a plaque according to ASTM D-1928 and the room temperature notched IZOD impact strength was measured using the technique described in ASTM D-256. At least 10 specimens were broken and averaged for each reported value. The predicted IZOD impact strength of the blend using the additive rule was calculated as follows: ( (Percentage of component 1 x IZOD impact strength of component 1) + (Percentage of component 2 x IZOD impact strength of component 2) + (Percentage of component 3 x IZOD impact strength of component 3)) ÷ 100 = IZOD impact strength of the blend.

For the three component blend of Example 1, the calculation is:

((16.7 x 0.50 ft-lbs/in) + (66.6 x 9.97 ft-lbs/in) + (16.7 x 5.31 ft-lbs/in)) ÷ 100 = 7.69 ft-lbs/in.

Tensile yield and tensile break elongation were determined according to ASTM D-638. At least 6 specimens were tested and averaged for each reported value. Two separate blending trials of the same formulation, made into molded plaques, had the properties described in Table 7:

Table 7

Thus, the three component blend had an average impact strength (12.42 foot-pounds/inch) which was greater than that obtained by calculating the IZOD impact strength using the additive rule (i.e., 7.69 foot-pounds/inch) and greater than that of any of the individual components. Examples 2-6

The polyethylene polymers described in Example

1 were blended in varying percentages as in Example 1 and made into molded plaques for physical property tests. Table 8 lists the composition percentages as well as the resultant room temperature notched IZOD impact strength of the molded plaques.

Table 8

Average o 4 tr a s tota o at east spec mens

**Average of 2 trials (total of at least 20 specimens)

***1 trial (total of at least 10 specimens)

The area of a polygon bounded by points A(66.7, 33.3, 0), B(33-3, 0, 66.7), C(0, 0, 100), D(0, 100, 0) in FIGURE 1 (excluding the composition defined by line AB and excluding the corner points, where the composition was essentially a single component), graphically shows both two component and three component compositions for these three specific polymers which had IZOD impact strengths higher than that obtained by using the additive rule.

The area within the polygon defined by the points E(33.3, 66.7, 0), H(33-3, 33-3, 33-3), C(0, 0, 100), D(0, 100, 0) in Figure 2 graphically shows preferred ranges of compositions for these three specific polymers which, when blended and made Into a molded article, had room temperature notched IZOD impact strengths greater than that calculated using the additive rule.

The area within the triangle defined by points E(33.3, 66.7, 0), F(0, 66.7, 33.3), D(0, 100, 0) in FIGURE 3 graphically shows both two component and three component compositions for these three specific polymers which are most preferred for molding into articles. These compositions had room temperature notched IZOD 0 impact strengths higher than that obtained by any one of the components.

Comparative Examples 7-10

(Not examples of the invention) 5 Multicomponent polyethylene blends were prepared as described in Example 1 and made into molded plaques for physical property tests. The blend compositions and the resultant room temperature IZOD impact strength of the molded plaque is listed in Table 0 9 :

Table 9

'Average o 2 tria s

** 1 trial (total of at least 10 specimens) Thus, when the polyethylene polymers were blended in proportions which lie outside the area of Figure 1 defined by a polygon which is formed by points A(66.7, 33.3, 0), B(33.3, 0, 66.7), C(0, 0, 100), D(0, 100, 0) in FIGURE 1, or when the compositions were blended in proportions as defined by line AB in Figure 1, the multicomponent polyethylene compositions had room temperature IZOD impact strengths less than that obtained by the additive rule. Thus, these 0 multicomponent polyethylene compositions are not examples of the invention.

Example 11

A three component blend comprising two virgin 5 polyethylene polymers and a post consumer recycled plastic was prepared according to the percentages and properties described in Table 10:

Table 10

'Comprising plastics escr e y categor es 1 an Ta le 3 herein) The blend was prepared by dry blending in 2500 gram batches and then compounded on a 1 inch diameter Killion single screw 30:1 L/D extruder, as described in

Example 1. The blend was passed through the extruder system twice for better mixing.

This blend was roto-molded into a rectangular box having a wall thickness of about 0.125 inches, thus demonstrating that blends of the present invention can also incorporate post consumer recycled plastic and maintain utility. A molded plaque made from the blend according to ASTM D-1928, when tested in accordance with

ASTM D-256, had the properties described in Table 11:

Table 11

Blend melt index Blend Tensile yield Tensile IZOD impact break strength

(g/10 density stress elongation (foot¬ minutes) (g/cc) (psi) (percent) pounds/inch)

1.45 0.9319* 2296- 810' 12.35*

* Average of 2 trials

The three component blend had an average room temperature notched IZOD impact strength of 12.35 foot¬ pounds/inch which was greater than that obtained by calculating the IZOD impact strength using the additive rule (i.e., 7.46 foot-pounds/incn) .

The multicomponent blends disclosed herein, whether made from virgin polymers or incorporating recycled plastic, are useful for manufacturing many molded articles, including refuse containers, roll-out refuse carts, trash bins and toys.

Claims

1. An improved molded article made from a multicomponent polyethylene composition characterized by the molded article having a room temperature notched IZOD impact strength measured in accordance with ASTM D- 256 greater than a calculated room temperature notched IZOD impact strength of an article made from the multicomponent composition using an additive rule.

2. The molded article of Claim 1, wherein the molded article is made by rotational molding, injection molding or blow molding.

3. The molded article of Claim 2 made by a rotational molding process.

4. The multicomponent polyethylene composition used in Claim 1 comprising a uniform composition of a three component linear polyethylene falling within the area of a polygon ABCD bounded by points A(66.7, 33.3, 0), B(33.3, 0, 66.7), C(0, 0, 100). D(0, 100, 0) and excluding the composition defined by line AB in FIGURE 1.

5. The multicomponent polyethylene composition used in Claim 1 comprising a uniform composition of a three component linear polyethylene composition falling within the area of a polygon EHCD defined t/ points E(33.3, 66.7, 0), H(33.3, 33.3, 33.3), C(0, 0, 100), D(0, 100, 0) in FIGURE 2.

6. The three component linear polyethylene composition of Claim 4 wherein a first component (a) is

10 a linear low density polyethylene, a second component (b is a linear ethylene homopolymer and a third component (c) is a linear polyethylene.

7. The multicomponent composition of Claim ό

15 wherein (a) is an ethylene/1-octene copolymer and (c) is an ethylene/1-propene copolymer.

8. The composition of Claim 4 wherein one of the components is a post consumer recycled plastic. 20

9. A method of improving the room temperature notched IZOD impact strength, measured in accordance

_^-- with ASTM D-256, of a molded article made from a 25 multicomponent polyethylene composition comprising at least two polyethylene polymers, wherein said IZOD impact strength is higher than that calculated by the additive rule, said method comprising the steps of: 3C (a) blending at least two linear polyethylenes in amounts sufficient to form a uniform composition falling within the area of a polygon ABCD bounded by points A(66.7, 33-3, 0), B(33.3, 0, 66.7), C(0, 0, 100), D(0, 100, 0) ana excluding the composition defined by line AB in FIGURE 1, and (b) molding said uniform composition in a molding process to form a molded article having enhanced room temperature notched IZOD impact strength.