HK1087976B - Method for making articles by cold compaction molding and the molded articles prepared thereby - Google Patents

Description

Method for manufacturing articles by cold forming and moulded articles obtained thereby

Cross reference to related applications

The applicant's priority of U.S. provisional patent application 60/445959, filed on 7/2/2003, the contents of which are incorporated herein by reference in their entirety.

Technical Field

The present invention relates to high molecular weight polyethylene (HMW-PE) and ultra high molecular weight polyethylene UHMW-PE) and to a method for manufacturing an article comprising HMW-PE and UHMW-PE by cold compaction. More specifically, the method of the present invention comprises the step of press forming a resin composition comprising HMW-PE or UHMW-PE and an inorganic acid scavenger. The invention also relates to moulded articles obtained by this process.

Background

HMW-PE and UHMW-PE fluff is typically made by a Ziegler-type catalytic slurry polymerization process. Several patents have disclosed general features of the HMW-PE and UHMW-PE production processes. For example US6486270 discloses a process for the preparation of high molecular weight polyethylene. US5587440 and EP645403 disclose a process for the manufacture of UHMW-PE. Recently, catalysts having high activity for the above production methods have been developed. An example of such a catalyst system is the reaction product of titanium tetrachloride and a trialkylaluminum.

After polymerization, a certain amount of residual catalyst by-products, such as chloride ions, will always leach from the catalyst and remain in the HMW-PE and/or UHMW-PE. In the presence of water, these residual catalyst by-products can form chlorine and hydrochloric acid, which can damage and corrode polymer processing equipment.

To reduce the likelihood of corrosion to equipment, small amounts of chlorine/acid acceptor or scavenger, typically about 0.01 to 5 weight percent, are typically added to the dry polymer during or after polymerization. Such acid scavengers widely used by manufacturers of HMW-PE and UHMW-PE are metal soaps. The most commonly used metal soaps used as acid scavengers are stearates such as calcium stearate and zinc stearate. In addition to acting as an acid scavenger, stearates can act as internal lubricants and mold release agents.

Articles formed from HMW-PE and UHMW-PE can be made by a one-step process involving high temperature compression molding, or by a two-step process involving cold compression molding followed by high temperature compression molding. In a high temperature compression molding process, HMW-PE or UHMW-PE powder is poured into a heated full pressure mold and then cooled under pressure. The cooled mould is then opened to obtain a fully sintered HMW-PE or UHMW-PE article. US6313208 provides an example of high temperature molding of synthetic resins. According to this patent, high purity hydrotalcite particles are mixed with a thermoplastic resin to be used as a heat stabilizer or an acid acceptor in a thermal molding process. Although US6313208 discloses HMW-PE and UHMW-PE as examples of thermoplastic resins that can be used for hot molding, none of the examples in US6313208 relate to the use of HMW-PE or UHMW-PE.

In contrast to hot moulding as described in US6313208, HMW-PE or UHMW-PE particles are press-formed into a preform, sometimes referred to as a preform, without heating, in cold-press forming. Optionally, the preform is then blended or molded with a second material, such as rubber or another plastic, and then sintered and pressed at elevated temperatures to form the final product.

It was previously believed that the addition of small amounts of stearates and other organic based additives to HMW-PE or UHMW-PE as acid scavengers during processing did not affect the cold compaction strength of molded articles comprising HMW-PE or UHMW-PE resins. Cold compaction Strength, also known as "green strength", is a known expression used in the art to indicate the mechanical strength necessary for a compacted powder to withstand mechanical handling after compaction and before sintering without damaging its fine structure (McGraw-Hill Dictionary of Scientific and Technical Terms, Second Edition, 1978).

However, the inventors have unexpectedly found that the internal lubricating properties of stearates significantly reduce the binding properties of HMW-PE and UHMW-PE particles during cold compaction. It has been found that the compression strength of the HMW-PE or UHMW-PE resin containing metal soaps is significantly lower than the neat resin. Pure resins are understood to include pure resins, i.e. resins which contain no additives but may contain residual catalyst by-products. The metal soap additive provides lubrication to the HMW-PE or UHMW-PE particles during pressing and the resulting preform is therefore particularly susceptible to breakage. Furthermore, since a small variation in the concentration of metal soap in the resin can result in a large variation in the cold compaction strength values of the article, it is particularly important to carefully control the addition of metal soap to the resin to avoid too much or too little addition.

Accordingly, it would be desirable to have a method of making an article from HMW-PE or UHMW-PE by cold compaction, wherein the HMW-PE or UHMW-PE is free of metal soap and the cold compaction strength of the article is not significantly reduced.

Disclosure of Invention

The present invention provides a method for making molded articles of HMW-PE or UHMW-PE without a significant reduction in cold compaction strength by cold compaction forming.

One aspect of the invention is to compress a powdered resin composition comprising a HMW-PE or UHMW-PE and an inorganic acid scavenger to form a molded article at a temperature below the melting point of the HMW-PE or UHMW-PE resin.

During the pressing process, the resin composition is subjected to a pressure of 50 to 6000 psi (3 to 413 bar). The pressing temperature is about 0-120 ℃. Optionally, the method further comprises sintering the molded article at an elevated temperature after pressing.

Examples of acid scavengers contemplated by the present invention include, but are not limited to, metal oxides, metal carbonates, silicates, and mixtures thereof. The concentration of acid scavenger in the resin composition is about 10-5000 ppm.

Advantageously, the molded articles produced according to the method of the present invention are characterized by improved cold compaction strength. Generally, the cold compaction strength of a moulded article made according to the method of the invention is not less than 75% of the cold compaction strength of an article made of a virgin HMW-PE or UHMW-PE, i.e.a polymer without additives. In some embodiments of the invention, the cold compaction strength of the article is not less than 90% of an article made from virgin HMW-PE or UHMW-PE. Also, the cold compaction strength of articles made according to the method of the present invention is significantly better than articles made from metal soap containing resins, with an increase in compaction strength of about 50-200%.

Drawings

FIG. 1 illustrates a mold assembly that can be used to practice the method of the present invention.

Detailed Description

Any type of HMW-PE or UHMW-PE may be used in the resin composition. The HMW-PE or UHMW-PE may be linear or branched, or have any particular characteristics that provide suitable properties to the desired molded article. The average molecular weight of the HMW-PE is 300000 g/mole to 1000000 g/mole as determined by viscometry. The average molecular weight of the UHMW-PE is at least 1000000 g/mol, in particular 2500000 g/mol to about above 10000000 g/mol, determined by viscometry. An example of a commercially available HMW-PE is Ticona grade GHR 8110 and examples of UHMW-PE are Ticona grade GUR 4120, 4130, 4150, 4170 and 2122 (available worldwide from Ticona LLC, Summit, NJ).

Methods for determining the molecular weight of polyethylene are described, for example, in CZ-Chemische Technik 4(1974), 129 et seq. ASTM D4020 provides additional information regarding the relationship between UHMW-PE molecular weight, intrinsic viscosity, and approximate viscosity average molecular weight.

Generally, the molecular weight range of HMW-PE in the present invention is 300000-500000, and the molecular weight range of UHMW-PE is 1000000-5000000. In one embodiment, HMW-PE and UHMW-PE having a bulk density of about 0.15 to 0.60 g/cc have been found to be particularly useful.

In one embodiment of the invention, the HMW-PE or UHMW-PE may be a linear or branched ethylene homopolymer. In another embodiment of the invention the HMW-PE or UHMW-PE is a copolymer of ethylene with other olefins such as propylene, butene, hexene or higher 1-olefins. The polymer may also be a blend of polyethylene with one or more other polymers such as polypropylene or polybutylene. However, the amount of non-ethylene monomer in the copolymer or the amount of non-polyethylene polymer in the blend should be less than 10% so that the physical characteristics of the predominant HMW-PE or UHMW-PE are not adversely affected by the other monomer or polymer.

The acid scavenger in the resin composition is an inorganic acid scavenger. It has been surprisingly found that acid scavengers selected from the group consisting of metal oxides, metal carbonates, silicates and mixtures thereof have particularly suitable acid scavenging properties. Examples of suitable metal oxides are zinc oxide and magnesium oxide. Examples of metal carbonates are sodium carbonate and calcium carbonate. The metal carbonate may also be a hydroxy metal carbonate such as dihydroxy aluminum sodium carbonate or hydrotalcite. A suitable silicate is aluminium silicate.

Advantageously, only small amounts of acid scavenger are required in the resin composition. Typically, the amount of acid scavenger is in the range of about 10 to 5000ppm by weight, with a more typical amount being in the range of about 100 to 300 ppm. The concentration of acid scavenger in the resin can vary so long as the amount of acid scavenger used is sufficient to remove free acid and/or chlorine in the HMW-PE or UHMW-PE for cold compaction and any subsequent processing. The concentration of acid scavenger will depend on the particular application and scavenger chosen.

In addition to the HMW-PE or UHMW-PE and the inorganic acid scavenger, the resin composition may optionally contain other additives including, but not limited to, fillers, plasticizers, pigments, UV stabilizers, antioxidants, antistatic agents, and other commonly used additives that do not affect the properties required for the intended use of the molded article.

Generally, the resin composition is in a powdery form before pressing. However, in some applications it may be desirable for the resin composition to be in the form of fine particles. The acid scavenger and optional additives can be mixed into the HMW-PE or UHMW-PE using known techniques. For example, the polymer can be mixed with the acid scavenger using a ribbon blender, high speed mixer, pelletizer, extruder, or other techniques known to the skilled artisan.

The resin composition is typically pressed at room temperature, such as at about 60F to about 80F, without the need for heating. However, if the room temperature is too low, the pressing may be performed at a temperature above room temperature, as long as the pressing temperature is below the melting temperature of the HMW-PE or UHMW-PE. The melting temperature depends on the polymer properties such as molecular weight and degree of branching. The melting temperature of HMW-PE is typically in the range of 100 ℃ to 145 ℃ (R.P.Quirk, M.A.A.Alsamaraie, Institute of Polymer science of Akron; Akron, Ohio). UHMW-PE has no melting point per se and its melting temperature is considered to be the crystalline melting temperature of the polymer, which is the peak temperature of the Differential Scanning Calorimetry (DSC) scan known to the skilled person. The crystalline melting temperature for UHMW-PE, whether homopolymer, copolymer or blend, is generally in the range of 100 ℃ to 145 ℃.

The pressure to which the resin composition is subjected may depend on the properties of the composition and the use of the article. In practice, the pressure during pressing is typically between 50 psi and 5000 psi.

The pressing process may be carried out in a single step under given conditions, or may be carried out in a plurality of separate pressing steps, each of which is carried out under different conditions, such as a series of different pressures and corresponding pressure holding times. The article may optionally be sintered at elevated temperatures after pressing. In general, the skilled person is familiar with apparatus and methods for cold compaction of polyethylene.

Advantageously, the inorganic acid scavenger used in the process of the present invention is more effective than the stearate salt. In some embodiments, molded articles made from UHMW-PE resin compositions comprising an acid scavenger exhibit greater than 40% less corrosion of iron test specimens than stearate-containing resin articles, wherein the iron test specimens are molded in PE blocks and steam treated. Furthermore, the molded articles produced according to the method of the present invention are characterized by improved cold compaction strength, i.e., green strength.

The press strength of the preformed article is measured in a manner known to the skilled artisan. For example, DIN ISO527/1 and DIN EN20527/2 describe methods for obtaining the cold compaction strength of articles by bending tests. DIN ISO178 provides another test method for testing cold compaction strength.

Example 1

The following examples illustrate the process of the present invention. Advantageously, the molded articles made by this method are characterized by improved cold compaction strength compared to articles made from HMW-PE or UHME-PE containing a metal soap, e.g., a metal stearate such as calcium stearate. Although the examples illustrate cold compaction of UHMW-PE, similar conditions apply to HMW-PE.

Device

A 60 kilokilogram Wickert press with a Regoplas refrigerator unit (WickertPress Type WLP 600/3/3);

the diameter of a die cavity of the full-pressure carbon steel matched die is 14 mm;

aluminum plate (0.2 mm thick); and

ulmia Type 1708 circular saw.

Procedure

The mold was assembled and an aluminum plate was placed at the bottom of the mold cavity. A resin composition (90 grams) comprising UHMW-PE powder and one of the above given inorganic acid scavengers was poured into the mold cavity of the mold assembly. The powder was laid flat until the surface was smooth and then covered with a second aluminum plate and a cover mold. No release agent was added. Both the mold and the resin composition were at room temperature.

The powder filled mold assembly was placed in the center of a Wickert press and a pressure of 136 bar (1972 psi) was applied for 2 minutes. After 2 minutes, the pressure was not released and the pressure was raised to 373 bar (5409 psi) and held for 15 minutes. During pressing, the temperature was controlled at 20. + -. 2 ℃.

After 15 minutes, the platen table was opened and the mold assembly removed from the press. The mold assembly is then disassembled and the pressed article is removed. The articles were left at room temperature (23 ℃) for 1 hour before the cold compaction strength was determined. A circular saw was used to cut out a sample (about 120 mm. times.12.5 mm. times.8 mm) from the article.

Shown in fig. 1 is a die assembly, in order from top to bottom:

a. cover mould

b. Aluminium plate

UHMW-PE powder

d. Aluminium plate

e. And (4) a die bottom plate.

Table 1 below illustrates that the articles made according to example 1 have improved cold compaction strength and reduced corrosivity. Pure UHMW-PE was used as baseline for comparison. The pure UHMW-PE polymer has an Elongation Stress (ES) of 0.43 MPa and a particle size (d)₅₀) 125 microns and a bulk density of 0.38 g/cc. Cold compaction strength was determined using a UTS model 10T Universal testing Machine (UTS test system GmbH) according to the procedures of DIN ISO527/1 and DIN EN20527/2, revisions.

The corrosivity of each sample was determined using the following test procedure. A narrow steel strip (containing 0.07% carbon and 25 mm in length) was cleaned with acetone and then the cleaned strip was placed on top of 3 g of polyolefin powder in a mould (5 cm in diameter). An additional 3 grams of polymer was added to the steel bar face, the mold was closed, heated to 250 ℃ over 10 minutes, and then pressurized at 5 bar for 50 minutes. Thereafter, the mould was cooled to 40 ℃ over 10 minutes at 25 bar and the steel strip was separated from the pressurised polymer and treated with steam for 1 hour. The increase in weight between the weight of the final corroded product and the weight of the starting cleaned steel strip was taken as a measure of the corrosivity.

TABLE 1

Corrosive nature of additive concentration¹Cold compaction strength

[ppm] [％] [kPa]

As can be seen in table 1, the cold compaction strength of the article made with the pure, additive-free UHMW-PE was 1650 kpa. However, these articles are highly corrosive to iron coupons, up to 0.20% due to the presence of unneutralized residual catalyst in the UHMW-PE polymer. The addition of both metal soaps and non-lubricating inorganic acid scavengers to the resin can reduce corrosion compared to neat polymer. However, inorganic acid scavengers have a much better effect than metal soaps in reducing corrosion. In addition, the cold compaction strength of the inorganic acid scavenger containing resin is significantly higher than that of the metal soap containing resin.

Although the resin compositions comprising aluminosilicate and stearate have similar low corrosivity, the cold compaction strength of molded articles made from stearate-containing resins is significantly inferior to molded articles made from aluminosilicate-containing resins according to the present invention. The cold compaction strength of molded articles made from the resin composition comprising dihydroxy aluminum sodium carbonate and hydrotalcite is similar to or superior to that of molded articles made from virgin materials, while the corrosivity is significantly lower than virgin resins.

As shown in table 1, it is noteworthy that the amount of the inorganic acid scavenger used in the UHMW-PE resin composition is half the amount of the metal soap. That is, the concentration of the inorganic acid scavenger in the resin composition was 250ppm, and the concentration of the metal soap was 500 ppm.

The test data for the molded articles provided by the present invention demonstrate that cold compaction strength is reduced with little or no reduction in cold compaction strength and corrosion is greatly reduced when compared to virgin resins at lower concentrations of acid scavenger. Lower concentrations of acid scavenger can reduce raw material costs. In addition, the articles made by the process of the present invention do not discolor due to the presence of the acid scavenger.

According to another embodiment, the molded or preformed article prepared according to the method of the present invention is subjected to an optional sintering step. Sintering methods and equipment are well known to the skilled person. The process involved in the sintering step will depend on the particular application or intended use of the molded article.

Advantageously, one or more layers of resin or rubber containing parts or elements may be compounded, added or coated onto the surface of the molded article made by the cold pressing method of the present invention. Examples of such rubbers include, but are not limited to, Styrene Butadiene Rubber (SBR), EPDM, Neoprene (CR), Natural Rubber (NR), Isobutylene Isoprene (IIR), and Chloro Isobutylene Isoprene (CIIR). A crosslinking agent such as sulfur or peroxide may be used to internally crosslink the resin or rubber to form a stronger bond between the polyethylene and the resin.

In this embodiment, after the preformed article has been formed by cold compaction, the thermosetting material may be applied to or contacted with the preformed article, and heated and pressurized. It is advantageous to form a strong bond between the resin and the polyethylene. After cooling, the sintered article is removed from the mold and is suitable for its intended use. The bond strength between UHMW-PE and SBR is much greater than the bond strength between SBR and lower molecular weight HDPE.

When metal soaps are used as acid scavengers in the preparation of the preformed articles, the thermosetting resin and the preformed article do not successfully fuse together to form a new article. In contrast, when the acid scavenger of the present invention is used in place of a metal soap, the two materials can be successfully fused and a strong bond can be formed between the two layers due to the absence of internal lubrication.

Without being bound by theory, it is conceivable that the high bond strength between the HMW-PE or UHMW-PE and the resin is due to the formation of entanglements between the chains of polyethylene and resin. Upon application of heat, the metal soap migrates to the polyethylene surface and acts as a grease and lubricant, thus preventing bonding.

Many modifications and variations of the present invention are possible in light of the above teachings and, therefore, such modifications and variations are within the scope of the appended claims, in addition to the specific details set forth above.

Claims (10)

1. A method of making an article by cold compaction molding, wherein the method comprises forming the article by compressing at a temperature of 0 ℃ to 120 ℃ and a pressure of 50 to 6000 psig a resin composition comprising a mixture of high molecular weight polyethylene having a molecular weight of 300000-500000 g/mol or ultra high molecular weight polyethylene having a molecular weight of 1000000-5000000 g/mol and an inorganic acid scavenger selected from metal oxides, metal carbonates, silicates and mixtures thereof at a concentration of 100-2500 ppm.

2. The method of claim 1, further comprising sintering the article after pressing.

3. The method of claim 1, wherein the resin composition is in powder form prior to pressing.

4. The process according to claim 1, wherein the metal oxides, metal carbonates and silicates are selected from the group consisting of oxides, carbonates and silicates of magnesium, calcium, zinc or aluminum and mixtures thereof.

5. The process according to claim 1, wherein the metal carbonate is a hydroxy metal carbonate.

6. The process according to claim 1, wherein the metal oxides, metal carbonates and silicates are selected from the group consisting of hydrotalcite, dihydroxy aluminum sodium carbonate, calcium carbonate, zinc oxide, magnesium oxide and mixtures thereof.

7. A molded article prepared according to the method of any one of claims 1-6.

8. The molded article according to claim 7, wherein the article is characterized in that the article has a compression strength not lower than that of an article made of virgin polyethylene.

9. The molded article according to claim 7, wherein the cold compaction strength of the article is not less than 75% of the cold compaction strength of an article made from virgin polyethylene.

10. The molded article according to claim 7, wherein the cold compaction strength of the article is not less than 90% of the cold compaction strength of an article made from virgin polyethylene.