WO2002050169A1

WO2002050169A1 - Method for compounding polymer with filler, and composite material containing polymer and filler

Info

Publication number: WO2002050169A1
Application number: PCT/FI2001/000890
Authority: WO
Inventors: Jouni Enqvist; Pirkko Järvelä; Hannu Minkkinen
Original assignee: Valtion Teknillinen Tutkimuskeskus
Priority date: 2000-12-21
Filing date: 2001-10-15
Publication date: 2002-06-27
Also published as: FI20002811A0; FI20002811A; FI114867B; AU2002210591A1

Abstract

In a method for compounding a polymer with a filler, the filler and the solid polymer are comminuted together mechanically. Talc used as the filler and polyethylene are ground together in solid state so that reactive or adhesive moieties are released from inside the talc, which react with reactive and adhesive moieties formed of the polyethylene in the same grinding. In the manufacture of a composite product, it is possible to use the same matrix polymer or another matrix polymer, such as polypropylene.

Description

Method for compounding polymer with filler, and composite material containing polymer and filler

The invention relates to a method for compounding a polymer with a filler, wherein the filler and a solid polymer are comminuted together mechanically. The invention also relates to a composite material containing a polymer and a filler.

It is generally known to reinforce a polymer matrix with a mineral filler. For example, a mineral filler is used to reinforce polyolefins so that the rigidity of the plastic is significantly increased. By the choice of the quantity and quality of the fillers, it is possible to control the properties of the obtained composite material in general.

However, it often happens that the impact strength of the plastic is reduced, if the rigidity of the plastic is improved with a filler. To improve the impact strength, the fillers are normally subjected to surface treatment, i.e. compatibilization, in which the aim is to functionalize the surface with organic chemical groups or coupling agents binding to the polymer matrix. This is typically performed by solution reactions to bring surface active compounds to react with the reactive groups of the fillers. Furthermore, it is known to use polymers to modify the surface of the fillers. For example, US patent 4,448,845 discloses a method, in which calcium carbonate is treated with a halogenized polar polymer to improve the physical properties, inter alia impact strength, of polyethylene filled with it. This will require the dispersion of the fillers in a liquid medium, to which the surface treatment compound is also admixed, after which the filler must still be separated from the liquid and dried.

Another idea, which is also presented in prior art (e.g. US 5,422,386 and GB 2099000), is to perform the polymerization already in the presence of the filler, wherein it is probable that, with at least some polymer/filler combinations and use of possible coupling agents, a very strong bond can be achieved between the polymer matrix and the filler, and also highly filled polyolefin composites can be obtained with the method, as mentioned in US patent 4,187,210. Technologically, such a method is demanding, it is not suitable for all polymerization methods, and the result is, in any case, a polymer which is produced in large batches and whose filler content is determined already at the polymerization stage and whose versatile uses are thus restricted.

Furthermore, an attempt has been made to improve the mixing of fillers and polymers by means of a so-called solid state reaction which is presented in US patent 4,440,916. The basic idea of the patent is to implement the reaction between solid substances between two substances, both of which are normally polymers. Polymer particles can be provided with free radicals by modifying them mechanically at cryogenic temperatures (lower than -78°C). The publication also mentions the possibility to bring a polymer, provided with free radicals by said method, to react with a non-polymeric paniculate solid substance, such as a filler, which must have a reactive moiety to react with the polymer. Examples mentioned include the reactions of titanium dioxide, aluminium dioxide and silica with several cryogenically ground polymer materials.

US patent 4,124,562 also suggest the grinding of the filler in the pres- ence of a polymer consisting of a vinyl monomer, wherein the vinyl monomer must have at least one polar group capable of binding to the fresh surface of the filler material exposed during the mechanical processing. The vinyl polymer is preferably dissolved in a solvent before it is mixed with the filler, to make the polymer disperse well and bring it to a sufficiently good contact with the fresh surfaces of the filler particles during the grinding. For this purpose, a non-polar organic solvent is recommended. The polymer, with which the filler is brought to react, consists of monomers containing a polar group, such as acrylic acid, acrylates, metacrylic acid, metacrylates, styrene, acrylamide, and vinyl chloride, wherein the polymer can also be a copolymer of one of the above-mentioned monomers with -olefin. Fillers modified in this way can be used as fillers of polyolefins, wherein the polyolefin can be a homopolymer or copolymer of α-olefin (column 3, lines 1 to 9). Example 6 of said patent presents the manufacture of modified fillers by crushing talc with a laurylmethacrylate/diaminoethylmethacrylate copolymer or with a butylmethacrylate/dimethylaminoethyl/methacrylate copolymer. The obtained filler was used as a filler for a propylene/ethylene copolymer (with an ethylene content of 12 %). The substance ratios and the possible solvent are not mentioned in this example. This patent further emphasizes that when metal silicates and silicate minerals are used as filler materials, it is preferred to modify such fillers with especially an acrylate or methacrylate polymer, in which the alkyl group contains at least 6 carbon atoms (column 3, lines 40 to 48).

Up to date, it has not been known, how to improve the binding of filler materials to widely used ethylene polymers which are hydrofobic and non-polar.

It is the object of the invention to eliminate the above-mentioned drawbacks and to present a method for compounding a polymer with a filler, which can be applied to ethylene polymers, particularly to commonly used commercial polyethylene grades, without using particular chemicals to modify the fillers. To achieve this object, the method according to the invention is primarily characterized in that talc used as the filler material and a hydrofobic ethylene polymer are ground together in solid state in such a way that reactive or adhesive moieties are released from inside the talc, which react with reactive or adhesive moieties formed of the polymer in the same grinding.

By means of the invention, it is possible to considerably improve the compounding of talc with for example polyethylene or polypropylene, which has been particularly difficult for talc because of its inert nature. The invention can be applied in such a way that the first compounding is performed by grinding in the above-described way, and the obtained composite product of ethylene polymer and talc is used for compound- ing the same or similar ethylene polymer, e.g. ethylene polymer having the same chemical structure of the polymer chain, to prepare a composite material, product or piece. The composite product can also be used to compound a different ethylene polymer which is different in the chemical composition of the polymer chain. Another way is to use the composite product of an ethylene polymer and talc obtained in the grinding to compound another matrix plastic, particularly polypropylene, to which the ethylene polymer binds well. Depending on the matrix plastic, it is also possible to use still a third polymer component which is capable of coupling the matrix plastic and the ethylene polymer better to each other in the manufacture of a composite material, a composite product or a composite piece.

The composite material of the invention is, in turn, characterized in that it comprises an ethylene polymer and talc, bound to each other by means of reactive or adhesive moieties formed in grinding. Such a composite material, which is directly obtained from grinding, can be in the form of a solid semi-finished product to be used in the manufacture of a composite piece or product, wherein it is still mixed to a matrix polymer when the composite piece or product is made by a melt processing method. To it can also be added with one or more substances, for example the same, similar, or different matrix polymer, to make it into another semi-finished product to be used later for the manufacture of the actual composite piece or product. Thus, composite materials according to the invention also include semi-finished products which have been obtained by adding to the ethylene polymer/talc composite product obtained in the grinding one or more polymers, e.g. the actual matrix polymer and, if necessary, a coupling polymer to improve the adhesion between the actual matrix polymer and the ethylene polymer ground with talc. Such a solid semi-finished product containing an additional polymer or polymers can be a raw material whose qualitative and quantitative composition already corresponds to the final matrix poly- mer and filler composition of the composite piece or composite product.

In the following, the invention will be described in more detail with reference to the appended drawings, in which

Fig. 1 illustrates the structure of talc, and

Figs. 2 to 5 illustrate the properties of materials prepared by the method.

Talc, chemically magnesium silicate and, more precisely, having the chemical formula Mg₃Si4θ₁₀(OH)₂, contains a very inert O-Si-O-Si-O-Si- O chain structure which covers the reactive OH groups. Thus, talc can not react with commonly used compatibilizers, such as stearic acid or silane, to a significant degree, except for the OH groups at the ends of the chain structures in talc. Figure 1 shows the molecular structure of talc flakes or lamellae. The surface of the flake contains silicon atoms bound to oxygen atoms and no metal ions or reactive groups. Because of the lamellar structure of talc, the talc particles used in plastics are plate-like in such a way that most of the particle surface is relatively inert. Polar groups only exist at the edges of the flake-like particle, and even these have time to react, in connection with the normal grinding and processing of talc, with moisture and oxygen in the air, thereby impairing the effect of adhesive agents used in plastics filled with minerals.

According to the invention, talc is ground together with a hydrofobic ethylene polymer, wherein the reactivity of both agents can simultaneously be improved. The grinding is performed in the solid state, for example in a ball mill or another suitable mechanical grinding device, by grinding the substances in an inert protective gas, such as nitrogen. The grinding can be performed at room temperature. In mechanical grinding, by the effect of the strong mechanical loading and the increase in the pressure and temperature caused by it, talc particles are disintegrated in the mixture of mineral particles and polymer particles, wherein reactive groups are exposed on the fresh plane of fracture. At the same time, the polymer chains in the polymer particles are split into polymer radicals which simultaneously react with the siloxyl radicals and magnesium oxide radicals formed in talc in the above-mentioned way. Another mechanism is the transfer of the generated fresh non- polar surfaces, forced by the impact energy caused by the grinding device, e.g. a ball mill, to a close contact with each other, wherein the polymer and the talc adhere to each other by Van der Waals bonds.

The invention provides quite novel possibilities for compounding hydrofobic and non-polar ethylene polymers, which are produced in large quantities and in common use, with talc which is an advantageous filler in the sense that it is soft, stiffening, harmless, and also inexpensive. An ethylene polymer refers to a polymer whose polymer chain consists solely or primarily of successive etylene monomers. In particular, the invention is suitable for coupling generally known commercial polyethylene grades and talc to each other. Ethylene polymers also include hydrofobic copolymers of ethylene with other monomers, particularly non-polar copolymers of ethylene with alkene hydrocarbon monomers, such as α-olefins. In these copolymers, the amount of the ethylene monomer is preferably at least 80 wt-%. It is also possible to use hydrofobic, relatively non-polar ethylene copolymers, in which the polyethylene chain contains a small amount (preferably not more than 20 wt-%, more preferably less than 15 wt-%) of a comonomer, whose side chain contains heteroatoms in addition to carbon atoms.

By means of the invention, it is possible to prepare a semi-finished product in which talc and the ethylene polymer are strongly compounded to each other, thanks to the solid state grinding. Such a semi- finished product can be a powder which is later used in the manufacture of composite products and pieces filled with a filler, particularly form pieces. To facilitate the handling or storage, the powdery or flaky product obtained in the grinding can also be processed by means of waxes or corresponding substances to be granular or otherwise less dusting. For the final manufacture of the composite products or pieces for example by injection moulding or extrusion, the talc/polymer mixture made by grinding together is preferably compounded with a polymer (typically the same or a similar polymer) to form a mixture having a suitable talc content. The mixing can be performed, for example, with a double-screw compounder or another device intended for the compounding of a plastic, in which the powder is mixed with the plastic in molten state under effective shearing to be sufficiently homogeneous for the use.

The ethylene polymer to which the talc is bound by the above- described grinding method, can be of the same type as the matrix polymer of the final product, wherein the mixture of polymer and talc is dosed in a suitable ratio with the same or a similar matrix plastic in the manufacture of a composite piece or product by a melt processing method, for example by injection moulding or extrusion. Another alternative is that in the mixture (compound) to be used in the manufacture of the final product or piece, a significant share of the matrix polymer is a polymer or a polymer mixture which is compatible with the ethylene polymer to which talc has been compounded in the above-described way by grinding. In this way, the adhesion of talc and the actual matrix polymer is improved by the ethylene polymer which adheres well to the actual matrix polymer and which has been reacted earlier with talc in solid state. The ethylene polymer compounded with talc can thus be well used e.g. with a polypropylene matrix in the manufacture of the final composite product or composite piece. It is also possible to use a copolymer of propylene.

Furthermore, it is possible to couple the ethylene polymer used in the processing of talc with the matrix polymer of the compounded mixture for example with a suitable block copolymer. The block copolymer, in which one end of the molecule chain is compatible with the ethylene polymer coupled to talc and other end of the chain is compatible with the matrix plastic, will act as a coupling agent between the phases, or as a bipolar "coupling polymer", improving the adhesion.

It is also possible that the talc/ethylene polymer blend is used as such in the manufacture of the final composite product or piece, wherein the portions of talc and the matrix polymer are the same in the mixture obtained by grinding and in the finished product.

The mixture of ethylene polymer and talc obtained by grinding, and the above-mentioned matrix polymers and possible coupling polymers can be mixed with each other to make a solid semi-finished product which can be used later for the manufacture of the actual composite product or composite piece by a melt processing method.

The proportion of talc in the final composite product or piece may vary depending on the use, for example from 5 to 40 wt-%.

The invention will be illustrated by means of the following tests which do not restrict the invention. 50 % of polyethylene (HE 8253, supplied by Borealis) and 50 % of talc were ground in a ball mill under a protective nitrogen gas. In reference tests, polyethylene and talc were ground separately. The used talc quality M20SL was talc in whose particle size distribution (Sedigraph 5100) the top cut (D98%) is 25 μm, the average particle size (D50%) is 6 μm and the content of particles smaller than 2 μm is 13 wt-%. For the talc quality M30, the corresponding values are 35 μm, 9 μm and 8 wt-%, respectively. Of the materials, test pieces were made in a polyethylene matrix plastic (same grade) by injection moulding (manu- facturing standard ISO 294) as follows (contents in wt-%):

1. polyethylene only

2. polyethylene 80 % and unground talc M20SL 20 %

3. polyethylene 80 % and unground talc M30 20 % 4. polyethylene 60 % and talc M30 and polyethylene ground together according to the invention (50:50) 40 %

5. polyethylene and separately ground talc M30 20 %

6. polyethylene and separately ground polyethylene (3:2) 80 % and unground talc M20SL 20 %

Figure 2 shows the tensile strength values of test pieces made of materials processed in different ways (test ISO 527). The tensile strength of the piece (No. 4) made by the method according to the invention was of the same order as that of the piece (No. 3) filled with unprocessed talc. The best result was obtained with the piece (No. 5) which was filled with separately ground talc.

Figure 3 shows the tensile moduli of the same pieces. Of the filled pieces, the piece (No. 4) made according to the invention has the smallest tensile modulus.

Figure 4 shows the effect of different processing methods on the impact strength (test ISO 179) at room temperature. Here, the most distinctive effect of the invention is manifested, because the impact strength of polyethylene filled with talc is considerably improved, when the filler is processed according to the invention. The difference to the piece (No. 3) compounded with the same talc grade M30 in a corresponding ratio (80 % polyethylene, 20 % talc) is more than threefold. Furthermore, it can be observed that the grinding of polyethylene only (piece No. 6) improves the impact strength.

Figure 5 shows the impact strength results at the temperature of -20°C. Here, the effect is even more dramatic, because the impact strength of the piece (No. 4) made according to the invention is more than four times that of the reference piece (No. 3) made with the same mixture ratio. This means that the product made according to the invention can be used in products, whose use temperatures will be, at least for some time, below 0°C.

Claims

Claims:

1. A method for compounding a polymer with a filler, wherein the filler and the solid polymer are comminuted together mechanically, charac- terized in that talc used as the filler and a hydrofobic ethylene polymer are ground together in solid state in such a way that reactive or adhesive moieties are released from inside the talc, which react with reactive or adhesive moieties formed of the polymer in the same grinding.

2. The method according to claim 1 , characterized in that the grinding is performed in an inert protective gas.

3. The method according to claim 1 or 2, characterized in that the grinding is performed in a ball mill.

4. The method according to any of the preceding claims, characterized in that the hydrofobic ethylene polymer is a polyethylene grade.

5. The method according to any of the preceding claims, character- ized in that a composite product containing a matrix and a filler is made by compounding the obtained blend of filler and ethylene polymer to a matrix polymer of another kind.

6. The method according to claim 5, characterized in that the obtained blend of filler and ethylene polymer is compounded to polypropylene.

7. The method according to any of the claims 1 to 4, characterized in that the composite product containing a matrix and a filler is made by compounding the obtained blend of filler and ethylene polymer to the same or a similar matrix polymer, with which the filler has been ground, for example to a matrix polymer having the same chemical structure of the polymer chain.

8. The composite material containing polymer and filler, characterized in that it comprises an ethylene polymer and talc bound to each other by means of reactive or adhesive moieties formed in grinding.