GB2035336A - Method of producing a rubber- based product, preferably using waste rubbers - Google Patents

Abstract

Rubber crumb (e.g. from tyre waste) of size 20s mesh or finer is mixed in an amount up to 90% by weight with an adhesive and the mixture is cured, possibly under externally applied heat and or pressure. The preferred adhesives are urethane prepolymers which are isocyanate rich. Solid rubber products with Shore A hardness up to 85 can be produced, as well as porous materials.

Description

SPECIFICATION Method for the reclamation of waste rubbers Waste rubber, especially that from tyres is a big disposal problem to industry at present. Grinding and separation of metals and fibre content give a fine rubber crumb which can be incorporated into virgin rubber materials as an extender. The level of usage of the ground material is normally low to maintain reasonable physical properties, of the order of 15 to 20% of filler crumb in the finished product. Where physical properties are of lower order, slightly higher levels of filler crumb can be incorporated.

The invention proposes a method of producing a rubber-based product in which rubber crumb of size 20s mesh or finer is mixed with an adhesive in a percentage up to 90 by weight, and the mixture cured possibly under heat and or pressure.

Using this process, we have produced a range of products from resilient porous through to solid rubbers (i.e. those containing no voids). Hardnesses at fixed densities can be varied as can tensile strength, cut resistance, elongation, wear and abrasion characteristics. Filler levels of up to 90% can be used to give products with usable physical properties. The products can be cured to handleable condition rapidly and much more quickly than conventional vulcanising techniques using normal rubber practice.

In general, rubber crumb is mixed with a selected urethane prepolymer adhesive in a suitable mixer; water, cross-linkers and catalyst can also be added and the whole is mixed for the required period. The wetted mix, which is now warm is poured from the mixer, the consistency depending on the level of binder and varying from a damp powder through to a paste. The mixture can be pressed under various compressions to form new products ranging from porous spongy materials to hard solid rubbers, all starting from one basic mix. Cure can be effected by steam in the case of porous materials or by heat very rapidly in the case of solid materials, or the whole could be cured at ambient temperature over a period of hours or days, depending on the type of binder used, the level of crumb usage and whether or not catalysts have been used.The mixed material can be rolled out to give a bonded sheet of rubber particles of a porous nature and after up to seventy-two hours storage at ambient conditions, slabs can be further compressed with heat to give solid or almost solid rubber.

Whilst the adhesive could be of many types, the preferred binders are based on urethane prepolymers which are isocyanate rich and which can be cured by techniques known to those familiar with the art. Typical prepolymers can be prepared from polyether polyols ranging from diols in the range 500 to 300 molecular weight, triols in the range 1000 to 6000 molecular weight, or combinations of these diols and triols. The use of higher functionality polyols can confer stiffness on low molecular weight doils and other polyols, amines etc, functioning as crosslinkers.

In this aspect the binder types are well known to those familiar with the urethane technology applicable, principally elastomers.

It is, however, preferred to add higher functionality or low molecular weight crosslinkers or stiffeners separately and in one example this addition is also carried out using a prepolymer. A further modification is to use the combination of polyols and asocyanate in a one-shot system where the metred proportions of components are pre-mixed or in-line mixed, sprayed onto the rubber and the whole mixed thoroughly in a suitable mixer. Improved binders properties can be obtained using saturated polyester polyols of linear or branched variety and the properties of the finished product can be modified and improved in a similar fashion to that pertaining in conventional urethane elastomer technology.

All and any of these reactive hydrogan or hydroxyl containing components can be converted to binders using any suitable combination of isocyanates to produce preferably stable isocyanate rich prepolymers of handleable viscosity unless for practical reasons the one-shot approach is preferred. Such isocyanates can be drawn from those readily commercially available, for example T.D.I., crude M.D.I., pure M.D.I., and linear di-isocyanates. If required as a viscosity control, for ease of pumping, or for stability improvements, solvents can be added to the prepolymers. Since during mixing these solvents are to a very large extent driven off, it is preferred to use non-flammable solvents such as Genklene, even through these types have a lower "cutting" power than the more searching solvents such as Ketones etc.

The specific composition of the ground rubbers used were not known since the make-up of the crumb was that purchased as commercially available ground tyre rubber and shoe soling resin rubbers of the styrenated type. However, samples of ground waste rubber from a variety of sources have been recombined successfully covering butyls, S.B.R., natural, styrenated and E.P.D.M. combinations. Whilst the bulk of the examples used waste tyre crumb, it is not considered to be a specific requirement of the process but only illustrates the procedure.

Materials ground under ambient conditions (irregular shaped) and cryongenically ground (granular angular crumb) rubbers have been combined equally well. Finer mesh sizes e.g. 40's mesh, give the best solid rubberproducts.

The selected grade of rubber crumb is put in the mixer and the prepolymer is added gradually during the mixing. After a suitable mixing period any further additions such as water, catalyst, crosslinkers etc., are sprayed onto the mixing mass and the mixing is continued. The temperature builds up and some polymerisation occurs at this stage. The time of mixing depends on mixer type, prepolymer concentration, use or not of additives particularly catalysts, the type of prepolymer, and whether its a "one shot" process or not. The mixture is dropped for curing, this could be into a transfer container or directly on to a conveyor feed to the curing stage. The composite mix is then cured under conditions necessary to give the product desired.

For example if a solid rubber is to be produced the mix is placed between heated compacting plates suitably treated with a release agent and the whole compressed. This can be a conventional press or a continuous belt press, conditions being controlled to give the required cure. If a lower density porous product is required then the mixture is compressed to the required density under pressures, and the curing can be effected in the case of porous products using steam injection or heat, or the materials can be cured at ambient conditions. Heating under pressure will cure the solid rubber to handleable sheets within minutes at up to 10 mm thickness using temperatures of the order of 160 to 200 degrees centigrade. As stated the porous products could be cured using injected steam in even shorter periods.If required the product can be partially cured in porous sheet form, to handleable stage with the final setting cure under pressure and heat, taking place even days later. When the mix (warm or hot) is held in bulk containers the heat of mixing will effect a quite substantial cure level within the lump of material and it is preferable therefore to increase the surface area to reduce this effect by spreading the mass into sheet form. When high speed impellortype mixer and 'Z' blade mixers are used effective mixing can be accomplished in a very short mixing time. For example with the Baker Perkins Dry Disperser, a good mix can be accomplished within one minute, even using a relatively high voscosity prepolymer, giving a temperature build up of up to 90 degrees centigrade within one minute.

Since in the freshly mixed state the coated rubber crumb will flow well under pressure, composites of the rubber with cotton belting or cords or metal inserts can be produced. These have different physical properties from the un-reinforced rubber product.

Porous rubber materials can be molded into cylindrical forms and these can be peeled to give continuous lengths. Solid sheets can also be split into different thicknesses using conventional heavy duty splitter equipment.

'Solid' rubber or composites can be used, for example, in shoe soling or heeling applications, and any other applications where flexible hard wearing rubber type products find use.

In order that the invention shall be clearly understood, a number of examples of methods of producing a rubber-based product will now be described. The basic methods described previously apply in general; more precise information is given for each example.

Example (1) A prepolymer was made from a 3500 molecular weight polyether triol and T.D.I. (80:20) to give a theoretical free N.C.O. content of about 9.5%. This was used as a binder for commercially available 40's mesh rubber crumb. Mixing was carried out in a 'Z' blade mixer.

40's Mesh Crumb - 75 pts. by wt.

Pre-polymer 9.5% free N.C.O. - 25 pts. by wt.

This was mixed for five minutes and then sprayed with water. (0.5%) and mixed for a further 10 minutes. The temperature of the mix had risen to 50 degrees centigrade. The mix was placed in a steel frame press, compressed under 25 Kgm/Cm2 and left to cure at ambient conditions overnight. The product was an apparently solid rubber with a Shore A hardness value of 75.

Example (2) A prepolymer was made from a branched saturated polyester polyol and T.D.I. 80:20 to give a theoretical 9.5% free N.C.O.

The polyester was Estolan P. 14 (Lankro Chemicals) 40's Mesh Crumb - 75 pts. by wt.

Polyester Pre-Polymer - 25 pts. by wt.

This was mixed as for example (1). The water was added and mix continued as before. The mix was compressed under heat (150 degrees centigrade) for 15 minutes to give a sheet 10 mm thick. The product was solid rubber with a Shore A hardness value of 78. Subjective testing showed this product to have better abrasion and cut/crack resistance than that of example (1).

Example (3) Ester pre-polymer mixed from Estolan P14 and T.D.l. 80:20, but in this case to give a 14% free N.C.O. content. The mixture was as for example (2). The product was a solid rubber with a Shore A hardness of 84.

Example (4) a pre-polymerwas made from Estolan P14 and T.D.I 80:20 to give an 18% free N.C.O. content prepolymer.

This was mixed as for example (2).

40's Mesh Rubber - 75.

Pre-Polymer - 25.

This mixing was carried out for 5 minutes and then sprayed with three parts by weight of a mixture of butane diol and water in a 5 to 1 blend. The mixing was continued and then the product was cured as in example 2, the product was a solid rubber less resilient and with poorer crack resistance than example 2 but rather harder; Shore A hardness 84.

Example (5) Ester prepolymer. Mix proportions as for example 2, but with ambient cure and at different levels of compaction to give products ranging from 0.5to 0.9 S.G. The products were porous, resilient composites with Shore A hardnesses ranging from 30 to 70.

Example (6) The prepolymer was as used in example 2, but the ratio was reduced. Compression loads were also reduced to give porous products.

40's Mesh Rubber Crumb - 9O pts. by wt.

Prepolymer - 10 pts. bywt.

This was mixed and then 0.25% water was sprayed and compressions carried out on this mixture to give densities ranging from 0.6 to 1.05 S.G. The products were porous with reasonable properties in the finished product above 0.7 S.G.

Example (7) The adhesive was as for example 2.

40's Mesh Rubber Crumb - 82.5 pts. by wt.

Prepolymer - 17.5 pts bywt.

This was compressed at different levels of compression and acceptable properties were obtained from 0.5 S.G. upwards. The hardness, tensile and crack resistance properties improving with density.

Example (8) As example 2 but using cryogenically ground rubber crumb at a nominal 30's mesh. This was rubber tyre shredded materials ground in a cryogenical process and the products were compressed at various levels. In the case of the solid rubber material, the Shore A hardness was 78.

Example (9) The prepolymer as in example 2 but mixed in a Baker Perkins Dry Disperser with a one minute mixing time.

No water was added and the samples were compressed some four hours after mixing.

40's Mesh Rubber Crumb - 75 pts. by wt.

Prepolymer - 25 pts. by wt.

The solid rubber products had a Shore A hardness of approximately 80.

Example (10) 30's Mesh ru bber from window screen sealer trims were blended with a high prepolymer content in the following proportions: 30's Mesh Rubber Crumb - 60 pts. by wt.

Prepolymer (the ester type as example 2) - 40 pts. by wt.

The mixing was carried out for 15 minutes and then cold-rolled to approximately 3 centimetres thickness. It was left under these conditions at ambient for 72 hours, and then hot pressed at 180 degrees centigrade and 40 kilogrammes per centimetre square. The product was a solid rubber with a Shore A hardness of 80.

Example (11) The trial was carried out using a Francis Shaw "Rococure" Rotary Press and the binder used was that as for example 1. The mixture of 75 pts by wt. rubber crumb with 25 pts. by wt. binder was made in the laboratory and the mix placed in the press where the curing was carried out using a temperature of 160 and a pressure of about 50 P.S.I. The speed was controlled to give 3 minutes in the heated/pressure area. The product was an effectively solid rubber with a density of about 1.13 and a Shore A hardness of 65 to 70.

Example (12) Curing rates for the standard example 2 were studied. The mixtures were prepared as for example 2 and then cured on the heated plattens to give a nominal 9 mm thick sheet.

(i) 180 deg. cent. 2 minutes at pressure plus the excess time in releasing. The sample had some delamination and blistering.

(ii) 180 deg. cent The sheet from (i) was further compressed and had a further 2 minute heating,-the cure was then found to be quite acceptable.

(iii) 180 deg. cent. 3 minutes under pressure plus the excess time to release the cure was good, there was no blistering or delamination in evidence.

(iv) 190 deg. cent. 2.5 minutes plus excess time to release, the cure was good in patches, but there were signs of imcomplete cure with blisters and some delaminations.

(v) 200 deg. cent. for 2 minutes. Extensive delamination was visible and blistering due to incomplete cure.

This cure for a solid rubber product of thickness approximately 9 mm can be effected without catalysts using one type of prepolymer in well under 5 minutes for this thickness of sheet.

It can be seen that these methods make available commercially viable processes for the reclaimation of waste rubber, in particular tyres.

Claims (10)

1. A method of producing a rubber-based product in which rubber crumb of size 20s mesh or fines is mixed with an adhesive in a percentage up to 90 by weight, and the mixture is cured.

2. A method as claimed in claim 1 wherein the curing step takes place under the influence of externally applied heat and/or pressure.

3. A method as claimed in claim 1 or 2 in which the adhesive is a urethane prepolymer.

4. A method as claimed in claim 3 wherein the prepolymer is isocyanate rich.

5. A method as claimed in claim 3 wherein the prepolymer is prepared from polyether diols and/or triols.

6. A method as claimed in claim 5 wherein higher functionality polyols are used as crosslingers.

7. A method as claimed in any preceding claim wherein the cure and starting materials are adjusted to give a product which is effectively solid and with a Shore A hardness between 65 and 85.

8. A method as claimed in any preceding claim wherein the rubber crumb is that available commercially as ground waste and-tyre-rubber.

9. A method of producing a rubber-based product substantially as herein described.

10. A rubber-based product produced by a method as defined in any of claims 1 to 9.