This invention relates to microcellular polyurethane products that have application as engineering components such as spring assisters, buffers, vibration isolators and the like.

Conventional microcellular polyurethane products are manufactured by forming a prepolymer or quasi-prepolyer from the total or partial reaction of a high molecular weight polyhydroxy component such as a polyester or polyether with naphthalene di- isocyanate (NDI). The prepolymer or quasi- prepolymer is then reacted with water, a glycol, a catalyst together with appropriate additives to produce a microcellular material. There are a number of disadvantages with this process particularly associated with the use of NDI. The main disadvantages are:-

The limited storage life of the prepolymer formed by the reaction of the polyol with NDI.

The vapour pressure of NDI at the normal processing temperature is sufficiently high to give rise to the emission of toxic vapours.

The relatively long in-mould time required to produce the microcellular polyurethane which leads to high production cost and equipment cost.

The relatively small mixing ratio of 10:1 or less of the prepolymer to the chain extender mixture and high difference in viscosity of the two components, coupled with unstable prepolymer, prevents this material from being processed by modern and more efficient techniques such as high pressure impingement mixing and low pressure screw type injection machines in which the liquid mixture can be injected directly into closed moulds.

In order to overcome these disadvantages it has been proposed to use 4,4'-di-phenylmethane di- isocyanate (MDI) in place of the NDI. However, this proposal has not been successful because of high hysteresis losses and poor physical and dynamic properties of products produced to date using MDI. Attempts have been made to overcome the problems associated with the use of MDI, but to date these have only met with limited success. For example the following limitations have been noted:-

Mouldings of satisfactory quality products could only be obtained on low pressure

metering/dispensing machinery having a high speed stirrer injection type mixing head.

Satisfactory mouldings with a short in-mould time of 5 minutes or less could only be produced in a very limited part weight range and limited range of densities. Variation of mixing head speed or volume of dispensing machine did not overcome the problem.

Moulded products made by casting or pouring method and by high pressure impingement mixing had inferior compression set and inferior performance under dynamic compressive stress.

Optimum processing conditions and a highly efficient production process with low demould times was not compatible with the equivalent ratio of isocyanate group to isocyanate reacting compounds in the range of 0.85 to 1.5:1 required to achieve the desired physical and dynamic properties.

Given a certain level of blowing agent and catalyst level only a limited range of densities could be moulded from one compound, therefore, limiting the size and type of component which could be moulded from a given compound.

Generally, springs assisters, buffer and jounce bumpers made of microcellular polyurethane have highly complex internal and external configuration, frequently having deep undercuts on the innerbore which requires the material to acquire a very high green strength prior to the polymerification process being completed to enable the mould core pin to be withdrawn by stretching the component by over 300%, without splitting, within 2 to 5 minutes of the liquid material being poured or injected into the mould.

Spring assisters in motor vehicles suspension applications are also required to meet very precise stress strain characteristics, which means that the density and modulus of the material must be controlled within very close limits. With conventional processes this can only be achieved with a limited number of moulding techniques.

It has been discovered by scanning electron microscopy that most of the above disadvantages result from anisotropy of the microcellular foams produced using existing recipes and processes. An empirical definition of anisotropy is "A lack of spherical quality of the cells of the foam".

The present invention has been made in order to overcome the above disadvantages.

Careful attention to the reacting equivalency of the resin mixture, among other aspects of the invention, produces closer to ideal isotropic cell structure.

According to the invention there is provided a method of manufacturing a microcellular polyurethane comprising reacting a polymeric polyol having a molecular weight of at least 1000, a diol or diamine having from 2 to 20 carbon atoms and 4,4'-di-phenylmethane di-isocyanate (MDI) to produce a polyurethane having a density of from 0.3 to 0.7 g/cc.

In a preferred embodiment of the invention a prepolymer or quasi-prepolymer is formed from the polyπieric polyol which is preferably a diol, triol or tetrol having a molecular weight of at least 1000 and the MDI. The prepolymer is then reacted with a resin mixture formed from a pclymeric polyol which is preferably the same as the polymeric polyol used to form the prepolymer and the diol or diamine having 2 to 20 carbons. The resin mixture may also include additives such as chain extenders, catalysts, surface active agents, blowing agents and the like. The

equivalent weight of the resin mixture, expressed as mg of KOH per gram of material is preferably in the range 200 to 300.

A preferred procedure for carrying out the invention to produce moulded products is as follows:-

Pre-heating (i) a prepolymer comprising the di-isocyanate and a polymeric polyol and (ii) a resin mixture of a polymeric polyol together with a diol or diamine having from 2 to 20 carbons, a chain extender, one or more catalysts, one or more surface- active agents, a blowing agent and other additives as may be required by technical or commercial considerations.

Metering under pressure the prepolymer (i) and the resin (ii) to a mixing apparatus.

Mixing the prepolymer (i) and the resin (ii) in the said mixing device either by a high speed stirrer enclosed in a chamber or by high pressure impingement mixing known as RIM technique.

Injecting the required amount of the liquid mixture at a given ratio into a closed or open heated mould.

Reacting the said component in the heated mould until a consistent polymer is formed which can then be demoulded after a comparatively short time, prior to curing for example in an oven.

The injection moulded component may require no after treatment other than to remove sprue.

The polymeric polyol is, as already stated, one having a molecular weight of at least 1000, preferably about 2000. Preferred polymeric polyols are difunctional polyester polyols having an equivalent weight of about 110, such as poly (hexamethylene) adipate, poly (butylene) adipate and poly (epsilon caprolactone), and polyether polyols such as poly (propylene) oxide and poly (tetramethylene) glycol. Combinations and co- polymers of more than one polymeric polyol can be used if desired.

The di-isocyanate used in the invention is preferably an alkyl or aryl substituted alkyl di-isocyanate such as 4,4'-diphenylmethane di- isocyanate (MDI). The MDI can be the pure form or one or more of the so-called modified forms. The amounts of di-isocyanate and polyol used are preferably in the molar range 2:1 to 5:1, more particularly 2.4:1 to 5:1 di-isocyanate to polyol.

The preferred range of isocyanate groups to isocyanate reacting compounds is from 0.85:1 to 1.15:1.

Additives which are preferably included in the reaction mixture include one or more catalysts which are preferably secondary or tertiary amines, organometallic compounds such as organic tin compounds. By choosing the appropriate catalyst the cure time in the mould can be varied from twenty minutes to about three minutes. Using the same catalyst type the density of the product can be adjusted within the range 0.35 to 0.70 g/cc. Particularly preferred catalysts include an amine salt sold under the name "Dabco" 33LV and an organotin compound sold under the name "Dabco" F12CL. Other additives include sufactants, preferably non- silicone surfactants (because use of silicone will detract from the achievements of isotropy of the cells), antioxidants, bacteriostatic compounds and pigments. In the Examples all parts or percentages are by weight.

EXAMPLE I

A quasi-prepolymer containing 20 to 22 wt% isocyanate group was prepared by the reaction between a polycaprolactone having an equivalent weight of

approximately 1100 and 4,4'-diphenylmethane di- isocyanate. The prepolymer was kept at a temperature of 42°C.

A resin mixture of the above polyester, 1,4- butane diol, water, tertiary amine catalyst, organometallic tin catalyst and a surface active agent was prepared and maintained at a temperature of 45°C. The resin mixture was formulated to give an equivalent weight of 270.

100 parts by weight of the quasi-prepolymer were reacted with 106 parts of the resin mixture using low pressure and high pressure metering/mixing machines. The mixture was injected into closed moulds at a temperature of 60°C to produce a spring aid and test plates.

EXAMPLE II

The process of Example I was repeated, but a polyol (resin) mixture having an equivalent weight of 230 and with 112 parts of prepolymer was mixed with 100 parts of polyol mixture (resin). Mouldings were produced as in Example I.

The density of the material was 250 kg/m ⁵ (moulded parts having a density of 450, 550 and

650 kg/m ³ could easily be produced from the same mixture with a demould time of less than 5 minutes). The mouldings were then post cured for 12 hours at 90°C.

EXAMPLE III

A quasi-prepolymer containing 14 to 16 wt% isocyanate groups was prepared by the reaction between a polyether glycol (PTMEG) having an equivalent weight of approximately 1100 and 4,4'- diphenylmethane di-isocyanate. The quasi-prepolymer was kept at a temperature of 40°C.

A resin mixture of the above polyether 1,4- butane diol, water, tertiary amine catalyst, organometallic tin catalyst and a surface active agent was prepared and maintained at a temperature of 40°C. The resin mixture was formulated to give an equivalent weight of 260.

112 parts by weight of the quasi-prepolymer were reacted with 100 parts of the polyol mixture using low pressure and high pressure metering/mixing machines. The mixture was injected into closed moulds at a temperature of 60°C to produce a spring aid and test plates.

The density of the material was 260 kg/m ³ . (Moulded parts having a density of 450, 550 and 650 kg/m ³ could easily be produced from the same mixture with a demould time of less than 5 minutes).

The mouldings were then post cured for 6 hours at 90°C.

EXAMPLE IV

A quasi-prepolymer containing 14 to 16 wt% isocyanate group was prepared by the reaction between a polyethylene/butylene adipate having an equivalent weight of approximately 1050 and 4,4'-diphenylmethane di-isocyanate. The prepolymer was kept at a temperature of 40°C.

A mixture of the above polyester linear and branched, 1,4-butane diol, water, tertiary amine catalyst, organometallic tin catalyst and surface active agent was pre-poured and maintained at 40°C. The above mixture was formulated to give an equivalent weight of 240.

100 parts by weight of the quasi-prepolymer mixture were reacted with 85 parts of the polyol mixture using low pressure and high pressure metering/mixing machines. The mixture from the

machine was injected into closed moulds at a temperature of 60°C to produce a spring aid and test plates.

The density of the free foam was 230 kg/m ^J . (Moulded parts having a density of 450,550 and 650 kg/m ³ could easily be produced from the same mixture with a demould time of less than 5 minutes). The mouldings were then post-cured for 16 hours at 90°C.

The products made in accordance with the above Examples were maintained at ambient temperature for a week and then the physical properties thereof were measured. The results are shown in the following table.

EXAMPLE 1 2 3 4 TEST METHOD

Free Foam Density kg/m ³ 250 ?50 260 230 DIN 53420

Mould Part Density kg/m ³ 450 450 450 450 DIN 53420

Tensile Strength MPa 5.5 6.0 4.5 5 DIN 53571

Elongation at Break % 450 430 400 400 DIN 53571

Tear Strength N/mm 14.4 16 14.0 17.0 DIN 53515 Compression Set 22 hrs at 50% comp. at

70 C % 7 7.5 10.0 9.4 DIN 53572

Rebound Resilience % 60 55 50 35 BS 903

Part A8/B

Dynamic fatigue test.

A dynamic fatigue test was carried out on a moulded product as illustrated in the accompanying drawing which shows an axial section through a typical spring assistor for a motor vehicle suspension. The part was made in each of the Example formulations. The part density was approximately 500 kg/m ³ .

The test comprised 200,000 compression cycles at 2HZ at a load of 4KN. On completion of test, the part showed that the permanent set did not exceed 5% and there was no evidence of any splits or cracks, internally or externally.

With the invention microcellular polyurethane products and particularly vehicle suspensions, spring aids and jounce bumpers can be obtained in any desired internal and external configuration using high or low pressure liquid injection moulding technique, casting or othei fluid moulding techniques.

In particular, an advantage of the invention is that by the use of any of the conventional polyurethane dispensing machinery, articles can be produced by introducing the liquid mixture into

closed moulds, thus atmospheric contamination by di-isocyanates can be more easily controlled and also the production cycle time can be reduced without incurring other processing penalties.

Moreover the invention permits the production of a wide range of part weights and configurations over a range of densities from 0.45 to 0.70 cc without any changes or adjustment to the process parameters or to the reacting mixtures.

Together with these advantages, the physical and dynamic properties of the moulded products are of a level which have not been achieved hitherto with MDI as the isocyanate.