GB2116986A - Method for producing polymer mouldings - Google Patents

Abstract

Mouldings are produced by subjecting compositions comprising a polyolefin such as polypropylene and silane grafted EPR,optionally with a silanol condensation catalyst, to an injection moulding stage which is followed or preceded by moisture crosslinking. The injection moulded products, for example in the form of motor vehicle bumpers, show improved scratch resistance and impact strength, and a good surface finish.

Description

SPECIFICATION Method of producing polymer mouldings This invention relates to a method of producing crosslinked injection mouldings in which the crosslinking is carried out by a moisture curing technique prior or subsequent to the injection moulding stage.

It has long been known,forexampleform British Patent Specification 1 234034 to Midland Silicones Ltd,to effect crosslinking of organic polymers such as polyethylene by treating the polymer with a specific organo silicon compound and subsequently exposing the polymer to moisture.The organo silicon compound, for example vinyl triethoxysilane, is one which includes at least one silicon bonded atom or radical which is reactive with free radical sites generated in the polymer priortotreatment, and at least one silicon bonded hydrolysable atom or functional group. Ontreatmenttheorganosilicon compound is grafted to the polymer chain, and on subsequent exposure to moisture siloxane linkage formation occurs two give crosslinking ofthe polymer chains. Many developments of this basic method have been published; for example British Patent Specification 1 286460 to Dow Corning Ltd discloses the grafting ofspecificsilanesonto polyethylene or certain ethylene copolymers using a free radical catalyst.Crosslinking of the grafted material then takes place in the presence ofwater and optionally a silanol condensation catalyst.

The methods described have hitherto been used for producing, for example, extruded materials which are useful in electrical insulation or heat resistance applications, such as cable coverings or extruded sheets. Yet another disclosure (British Patent Specification 1 346 588) describes the production of a thin sheetorfilm of a crosslinked polyolefin such as polyethylene oran ethylene copolymer. Here, the graft composition is formed and then is converted to film orsheetform using film blowing equipment before being crosslinked with water.

Other substances which it is known to treat with certain silanesto make them moisture crosslinkable include polyvinylchloride, ethylene/alkyl acrylate copolymers, ethylene/vinyl acetate copolymers, ethylene/propylene and ethylene/butylene copolymers, and chlorinated polyethylene copolymers. Yet other published patents, for example British Patent 1 531 251 to Kabel and Metall-werke GutehoffnungshutteAG, describe the application of the silane grafting and moisture crosslinking technique to elastomeric materials, especiallythose used in the sheathing of lengths of longitudinally extended material, for example electrical cables ortubes.For example there is described a technique wherein the composition undertreatmentcomprises a major proportion of olefin polymer or copolymer, for example polyethylene, and a minor proportion of elastomericmodifier,forexample ethylene propylene rubber, EPDM or a butyl rubber. Such compositions are shaped by extrusion priorto crosslinking in the presence of moisture.

As far as can be ascertained, none ofthe prior art documents discloses the treatment method in which the silane grafted composition is shaped other than by extrusion orfilm blowing. It has now surprisingly been found that application of the silane grafting technique to certain elastomers results in composi tions which prior or subsequent to moisture crosslinking, may be injection moulded to form mouldings which, after moisture crosslinking, generally exhibit improved scratch resistance, heat resistance and impact strength.

Thus according to one aspect ofthe present invention there is provided a method of producing mouldings having improved scratch resistance, which method comprises providing a composition comprising a polyolefin and a silane grafted ethylene propylene rubber, injection moulding the composition to form a moulding of desired shape, and moisture curing the composition to form a crosslinked moulding having improved scratch resistance, said curing stage being performed prior or subse- quenttothe moulding stage.

The silane which is used to provide the silane grafted rubber is one which includes an atom or functional group attached directly or indirectly to the silicon atom which is reactable with the rubber, and an atom orfunctional group which is hydrolysable on exposure to moisture. Many groups which are reactablewiththe rubberareknown,forexample vinyl, allyl, isopropenyl and isobutenyl; other nonreactive groups may also be attached to the silicon atom,forexample alkyl groups such as methyl and ethyl. The hydrolysable group which on contact with water leads to siloxane linkage formation and hence crosslinking of the rubber is preferably an alkoxy radical such as methoxy or ethoxy. However, other such functional groups, for example chlorine or acyloxy may be used.Preferred silanes for grafting onto the rubber (for example mechanically or by using a free radical generating compound) are vinyl trialkoxy silanes such as vinyl triethoxy silane and trimethoxy silane. It is stressed here, though, thatthis is a known and well understood field of art, and any organo silicon compound which serves to give crosslinking ofthe rubberthrough the silicon atoms may be used to provide the grafted ethylene proplyene rubber which is subsequently treated in accordance with the invention. The conditions used to form the grafted rubber are those conventionally employed in this technique. The extent of crosslinking produced in the composition depends on the amount of silane grafted onto the rubber. This amount may be controlled in orderto give a degree of control overthe physical propertiesofthefinal product.

Crosslinking of the composition before or after injection moulding in accordance with the invention is carried out by exposing the moulding to moisture, preferably in the presence of a silanol condensation catalyst. The mositure present in the atmosphere around the moulding may be sufficientto permit the crosslinking to occur, but artificial moistening or water immersion has been found to give an increased rate of crosslinking, especially at elevated tempera- ture. Preferably the crosslinking stage is performed at temperatures above 50"C, or more preferably in steam at tem peratu res of 100 C or more. Preferably the composition is injection moulded priorto crosslinking, although depending on the EPR or blend the curing stage may be carried outfirstto yield a crosslinked composition which still has sufficient flowto permit injection moulding. Such a composition may include, for example from 5 to 20% of EPR; it is known that compositions containing only EPR are notflowable enough to permit injection moulding.

Silanol condensation catalysts which are preferably incorporated in the composition which is injection moulded are well known in the art; there may be mentioned by way of example carboxylates such as dibutyl tin diluarate, diacetate and di octate, and certain titanium esters for example tetrabutyl titanate. Again, any silanol condensation catalyst which is effective may be used in accordance with this preferred embodiment of the invention, which is not intended to be limited in this respect.

The composition treated in accordance with the invention contains silane grafted ethylene propylene rubber (EPR) blended with a polyolefin or mixture of polyolefins (homo or copolymers) which may optionally also be silane grafted. The polyolefin may beforexample LDPE, HDPE, LLDPE, or EVA. Preferably, though, the polyolefin component of the composition is polypropylene. Depending on the precise composition which is selected,the moulding produced may have controlled impact strength. In any case it has been found that compositions comprising a polyolefin and silane grafted EPR may somewhat surprisingly be injection moulded, compared with the hitherto known extrusion and film blowing shaping stages.Furthermore, the injection moulded material, after moisture curing, preferably underthe effect of heat, has been found to exhibit markedly improved scratch resistance and impact strength, and also generally good heat resistance.

The product ofthis method, to which it will be understood the scope of this invention extends, is therefore a crosslinked injection moulding which is particularly suitable forcomponent parts of modern motorvehicles, such as car bumpers.

In a preferred embodiment the method uses a blend of silane grafted EPR, and polypropylene, togetherwith a quantity of silanol condensation catalyst. Forexamplethe blend may comprise from 1 Oto 60 wt% of silane grafted EPR, with the balance being a polyolefin such as polypropylene. It has been found that if the composition contains much in excess of 60 wt% then it may exhibit too low a flow rate to provide satisfactory injection moulding.

The following examples illustrate the invention: Examples 1 to 3 and Comparison Example 4 Silane grafted EPR was blended with polypropylene in a 1 litre Pomini internal mixer blender for 2 minutes as 180"C. Thereafter the blend was dumped, milled and extruded into pellet form. The pellets were then tumble blended with an amount of a silanol condensation catalyst (di-n-butyl-tin diluarate) in the form of a pelletised masterbatch with polypropylene and an antioxidant.The blended pelletsweresubse- quently injection moulded using a Boy injection moulding machineto produce a moulding which was then crosslinked in a sauna for 4 hours in the presence of water 100onto producethefinished article.

The method described above was repeated using varying amounts of EPR and polypropylene, and in one case (Comparison Example 4) using EPR which had not been silane grafted. The compositions which were subjected to the injection moulding and crosslinking steps are listed in the following Table 1. The numbers correspond to parts by weight.

The EPR which was used in the above preparative technique was an ethylene propylene copolymer containing 77 wt% ethylene and having an ML (1 + 8) value at 127 C of 50. The silane grafted EPR was produced in a preliminary stage by mixing 100 parts by weight ofthe EPR with a solution of 0.2 parts peroxide in 3.0 parts silane, using an internai mixer.

Grafting was carried out in the mixerfor4 minutes at 140 C160 C. the peroxide used as free radical catalyst was dibenzoyl peroxide,and the silane was vinyl trimethoxy silane.

The crosslinked mouldingswerethen subjected to a variety of standard tests, the results being given in the following Table 2. Identical tests were performed on an ungrafted crosslinked polyolefin thermoplastic elastomer material containing no catalyst, and these results are also listed in Table 2 as Comparison Example 5. Certain tests were also carried out on the ungrafted EPR alone, and on the polypropylene alone.

From Table 2 it may be seen thatthe method of the invention permits the production ofcrosslinked injection mouldingswhich aresuperiorto uncrosslinked injection moulded materials (Comparison Examples 4 and 5) in so far as those characteristics required of, say, certain motorvehicle components are concerned. Thus, the products of the invention generally have much improved scratch resistance and impact strength, higher softening point and higher surface hardness than the materials which have hitherto been used for manufacturing vehicle bumpers. Furthermore, subjective tests show that mouldings produced in accordance with the invention have resulted in generally superior surface finishes than have hitherto been achieved.

TABLE 1 Example 1 2 3 4 Si-EpR 48 50 60 - EPR - - - 50 PP 48 50 40 50 Catalyst 7.5 7.5 7.5 7.5 Vulcan P 4 - - EPR = ethylene propylene rubber (ungrafted) Si-EPR = ethylene propylene rubber grafted with vinyl trimethoxy silane PP = polypropylene Vulcan P = a propylene homopolymercontaining 40% carbon black ("VULCAN" is a Registered Trade Mark).

Catalyst = a silanol condensation catalyst in the form of a masterbatch containing 97 parts PP, 2 parts Irganox B 215 (a process and long term antioxidant), and 1 part active catalyst (di-n-butyl tin diluarate), such that 7.5 parts "catalyst" = 0.075% active catalyst in the composition. ("IRGANOX" is a Registered Trade Mark).

TABLE 2 Example 1 2 3 4 5 EPR PP Colour black - - - black - Specific gravity 0.900 - - - 0.890 - Flow (before) - 5.0 0.84 23.0 15-18 2.4 60 (after) 10.0 10.4 1.65 28.4 - - Hardness (instant) 59 - - - - - (15 sec) 49 - - - 32 - Tensile strength 20.0 - - - 18.0 - Stiffness 400 400 260 395 300-350 - 1400 Scratch resistance 650 650 450 350 250 - Softening point 124 - - - 68 - Impact strength 24 24 28 24 22 - Failure Ductile Ductile Ductile Ductile Ductile - Flow = MFR (SON, 230"C) in units of g/l0min. (before) = before addition of catalyst (after) = after addition of catalyst Hardness = shore D value Tensile strength = of injection moulding, in MPa Stiffness = secant flexural modulus, in MPa Scratch resistance = Erichson test, in units of grms Softening point = Vicat (1kg) test, in C Impact strength = dart drop test using 2 kg falling weight (BS 2782, 306 B modified height of drop) at -30-C, in units of energy (J) Failure = mode of failure in dart drop test on 2mm plaques

Claims (9)

1. A method of producing a moulding having improved scratch resistance which comprises providing a composition comprising a polyolefin and a si lane grafted ethylene propylene rubber, injection moulding the composition to form a moulding of desired shape, and moisture curing the composition to form a crosslinked moulding having improved scratch resistance, said curing stage being performed prior or subsequent to the moulding stage

2. A method according to claim 1 wherein the moisture curing is carried out on the moulding subsequenttothe moulding stage.

3. A method according to claim 1 or 2 wherein the composition includes a silanol condensation catalyst.

4. Amethod accordingtoclaim 1,20r3wherein the polyolefin is also silane grafted.

5. A method according to any one of the preceding claimswhereinthe polyolefin is polypropylene.

6. A method according to claim 1 substantially as herein described.

7. A method according to claim 1 substantially as described with referenceto Example 1,2 or 3.

8. A moulding when produced by the method according to any one of claims 1 to 7.

9. A moulding according to claim 8 when in the form of a motorvehicle bumper.