Preparation of polyolefin based thermoplastic elastomers

Details Preparation of polyolefin based thermoplastic elastomers Preparation of polyolefin based thermoplastic elastomers

2001-10-23

2003-08-05

Seidleck, James J. (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C525S263000, C525S301000, C525S302000

Reexamination Certificate

active

06602959

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention concerns a polyolefin based thermoplastic elastomer which can be prepared without a separate vulcanization stage and which has polyacrylate as a dispersed phase and which has been achieved by polymerization of acrylate into the polyolefin matrix.
Thermoplastic elastomers are polymers which have the desirable processing properties of thermoplastics but have the same physical properties a vulcanized rubbers. This combination of properties generates materials having segments that are soft and elastic with low glass transition temperature (t
K
) and a rigid, eventually crystalline, segment with a high glass transition temperature or a high melting point. The rigid and soft segments must be thermodynamically incompatible so that they form separate phases. Unlike conventional rubber, thermoplastic elastomers do not need a separate vulcanizing stage and can be processed using methods normally used with thermoplastics, such as extrusion, injection molding and blow molding. In addition, thermoplastic elastomers can also be reprocessed, for example when recycling material from the processing stage.
Thermoplastic elastomers can be divided into two main groups, block copolymers and thermoplastic/elastomer blends. A well-known example of block copolymers, which are thermoplastic elastomers, is the anionically polymerized block copolymer of styrene and butadiene (SBS) and the hydrogenized form of the same (SEBS). When these polymers are at room temperature, the soft and elastic phase is the continuous phase and the rigid phase, polypropylene, is dispersed. Here, the rigid polystyrene gives the material its strength, but during processing the temperature is raised over the glass transition temperature of polystyrene when it melts and the material can flow. The SBS thermoplastic elastomer, however, has poor weather resistance because of the butadiene double bonds. In SBS and SEBS polybutadiene and its hydrogenated form is the continuous phase, consequently they both have low oil resistance. Additionally, SEBS is expensive and requires a complicated preparation method.
Examples of materials that belong to the group of thermoplastic/elastomer blends are blends of polypropylene and ethylene/polypropylene rubber or ethylene/polypropylene/diene rubber. In these blends the rigid polypropylene phase is the continuous phase and the soft phase is dispersed, giving the material good oil resistance properties. These blends are made by blending the two main components and various additives in an extruder. Stabile phase separation results from curing the dispersed rubber phase (see, for example, U.S. Pat. No. 4,594,390).
The current invention describes a method to produce a thermoplastic elastomer with a polyolefin as a continuous phase and a rubber-like polyacrylate as a dispersed phase. This product is made in a reactor where crosslinking, if needed, can occur during polymerization. Thus, no separate vulcanization stage is needed. The resulting product has very good weather and oil resistance properties because the polyolefin is the continuous phase and because the elastomer is a polyacrylate. Hence, the current invention provides a method to produce a polyolefin based thermoplastic elastomer with a dispersed polyacrylate phase and without requiring a separate vulcanization stage.
SUMMARY OF THE INVENTION
The object of the current invention is to provide a new thermoplastic elastomer comprising a polyolefin/polyacrylate blend that has the polyolefin as the continuous phase and the polyacrylate as a dispersed phase.
A further object of the invention is to provide a new thermoplastic elastomer which maintains its dispersed polyacrylate structure during processing due to crosslinking of the dispersed elastic polyacrylate phase to the continuous polyolefin phase during polymerization.
Another object of the current invention is to provide a method for preparation of the new thermoplastic elastomer without employing a separate vulcanization stage.
The current invention provides for a polyolefin based thermoplastic elastomer with a dispersed polyacrylate phase that is polymerized into the polyolefin matrix. The invention further provides for a method for its preparation without a separate vulcanization stage. The acrylate used in the current invention has elastic properties and a glass transition temperature that is below room temperature. The acrylate forms a dispersed phase in the polyolefin matrix and, because polymerization occurs by the free radical technique, part of the acrylate chains are crosslinked to adjacent polyolefin chains. This provides good adhesion between the continuous polyolefin phase and the dispersed polyacrylate phase. Crosslinking can be controlled using varying ratios of diacrylate and acrylate. This crosslinking is especially important in cases where low adhesion between the polyolefin matrix and the polyacrylate is expected, for example when homopolyethylene or polypropylene is used. Here, because of the crosslinking, the soft dispersed polyacrylate is maintained in its dispersed form during processing when the polyolefin melts and becomes fluid.
The material could be produced by some of the methods given in the patent literature in which monomers are polymerized by free radical polymerization techniques into polyolefin matrix, e.g. by the Finnish patent 88170. In principal the acrylate monomer, and optionally diacrylate monomer, and the initiator are absorbed into polyolefin particles. The impregnation temperature is low enough so that no decomposition of the initiator occurs, yet high enough so that the monomer and the initiator can penetrate into the polyolefin particles. When all of the monomer and initiator have been absorbed, the temperature is elevated and the initiator decomposes and initiates the polymerization of the acrylate. The polyolefin particles swell to some extent (depending on the amount of monomer added) during the impregnation, but maintain their particle structure. The polyolefin particle structure is also maintained during polymerization.
DETAILED DESCRIPTION OF THE INVENTION
Polyolefin
Useful polyolefins include high density polyethylene, low density polyethylene and linear low density polyethylene. The polyethylene can be a homopolymer or a copolymer. The co-monomer of ethylene can be vinyl acetate, vinyl chloride, propylene or some other &agr;-olefin, C
1
-C
7
-alkylacrylate and -methacrylate, glycidylacrylate and -methacrylate, dienes such as hexadiene-1,4, hexadiene-1,5, heptadiene-1,6, 2-methylpentadiene-1,4, octadiene-1,7, 6-methylheptadiene-1,5 and polyenes such as octatriene and dicyclopentadiene. Also ethylene-&agr;-olefin-polyene-terpolymeres are useful. Useful &agr;-olefins include propylene, butene, pentene, isoprene, hexene or their mixtures and useful polyenes include hexadiene-1,4, hexadiene-1,5, heptadiene-1,6, 2-methylpentadiene-1,4, octadiene-1,7, 6-methyl-heptadiene-1,5, octatriene, dicyclopentadiene. In cases where an ethylene copolymer is used, at least 50% by weight must be ethylene.
The polyolefin can also be comprised of polypropylene and its copolymers. Propylene copolymers must consist of over 50% by weight propylene and can be random- or block copolymers of propylene and ethylene. Also, other &agr;-olefins can be used as co-monomers and also dienes such as hexadiene-1,4, hexadiene-1,5, heptadiene-1,6, 2-methylpentadiene-1,4, octadiene-1,7, 6-methylheptadiene-1,5 and polyenes such as octatriene and dicyclo-pentadiene.
The polyolefin can be in any form, but preferably in the form of pellets with a diameter of 0.5-10 mm. Particle forms of the polyolefin facilitate after treatment washing and drying.
Acrylate monomer
Suitable monomers are acrylates and methacrylates whose polymers have low glass temperatures, that is, they are rubber-like at and below room temperature, preferably at temperatures below −20° C. The glass temperature of the polyacrylate specifies the lower operating temperature of the material; below the glass temperature the polyacrylate is rigid and inelastic and the elast

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