GB2035194A - Moulding foamed urethane plastics articles in a double coated mould - Google Patents

Abstract

Foam-forming mixtures having a "cream time" of less than 15 seconds, preferably less than 10 seconds and especially less than 5 seconds, are allowed to react and set in a mould provided with a metal coating incorporating polyfluorocarbon resinous particles and, covering the metal coating, a coating of a polyfluorocarbon resin. The process is of particular value in the "Reaction Injection Moulding" of microcellular polyurethane elastomers for car parts. Moulds are provided with, e.g. (i) a 30 mu Ni/PTFE coating itself covered with a 10 mu coating of PTFE and (ii) a 20 mu Ni/"FEP" (RTM) coating itself covered with a 10 mu coating of "FEP". The invention includes shaped articles so made.

Description

SPECIFICATION Process for the manufacture of shaped urethane plastics articles This invention relates to a process for the manufacture of shaped urethane plastics articles and to the articles so produced.

A variety of articles are made today by moulding urethane plastics materials. Such articles include car trim components such as arm rests and car fascia panels produced from semi-rigid foams or microcellular polyurethane elastomers by a "reaction injection moulding" (RIM) process. These articles are produced by assembling a mould of the requisite shape, usually formed in two or more sections, coating the mould surface with a release agent and injecting into the mould cavity a mixture comprising a polyisocyanate and a polyol which react to form a urethane plastics material. Setting and curing of the plastics material may be accelareted or completed by the application of heat to the mould and the moulded article then removed by separating the mould sections. The moulded article is often painted after its removal from the mould.

In some moulding processes, especially RIM processes, the release agents presently available are not entirely satisfactory. They are also inconvenient to use in that the mould surface has to be sprayed before every moulding and the moulded article has to be cleaned to remove contaminating release agent if it is to be painted.

According to the present invention we provide a process for the manufacture of shaped urethane plastics articles which comprises introducing into a mould and allowing to react and set a urethaneforming mixture having a "cream time" (as hereinafter defined) of less than 15 seconds, preferably less than 10 seconds and especially less than 5 seconds; the mould surface which contacts the urethaneforming mixture being provided, at least in part, with a metal coating having homogeneously distributed therein polyfluorocarbon resinous particles and, covering the metal coating, a coating of a polyfluorocarbon resin. The invention also includes the shaped articles so produced.

The types of urethane plastics which can be used vary considerably depending inter alia on the particular polyisocyanate and polyol chosen and the relative quantities used and other ingredients present which may include "chain-extenders" and "cross-linking agents". A "blowing agent", which may be water, will be used to produce a foam. When water is used, this reacts with the polyisocyante to form a polyurea and carbon dioxide which "blows" the plastics material into a foam. Thus the term urethane plastics materials, as it is hereinafter used, is taken to include materials which contain a sub stantial proportion of polyurea groups, as well as urethane groups, and also isocyanurate groups formed bytrimerisation of the polyisocyanate with a suitable catalyst.

"Cream time" is a term commonly used by polyurethane technologists to describe the time it takes a urethane-forming mixture to develop a creamy appearance once the ingredients of the mixture have been mixed. With experience it can be measured accurately to within one second by visual inspection.

The development of the creamy appearance corresponds with the beginning of nucleation, which is the process of forming gas bubbles within the mixture. The gas may be carbon dioxide, generated by reaction of isocyanate and water, or the vapour state of a low boiling liquid which is dissolved in the liquid ingredients. Nucleation begins when the gas concentration in solution exceeds the equilibrium saturation concentration.

While not wishing to be bound by any particular theory, we believe that urethane-forming mixtures which are relatively slow-reacting and which remain as low viscosity liquids for too long may penetrate the mould coating and cause sticking. It is therefore convenient to define mixtures which are suitable in the process of the invention by their "cream times" which reflect the speed at which the mixtures increase their viscosity and gel.

When the "cream time" of the urethane foamforming mixture is less than 15 seconds, preferably less than 10 seconds, the moulding process can be repeated many times without the moulded article sticking to the mould. Especially good 'multiple release' is obtained when the "cream time" is less than 5 seconds.

Some conventional urethane-forming mixtures have "cream times" in excess of 15 seconds. These include mixtures for forming cast elastomers and rigid and semi-rigid foams and tend to stick to the mould.

Urethane foam-forming mixtures having "cream times" less than 15 seconds are well known to urethane technologists. These include cold-cure flexible foams, microcellular elastomers and integral skin foams. Lower "cream times" are generally obtained by increasing the amount of catalyst used or by using a faster reacting diol "chain extender".

Articles having a metal coating containing polyfluorocarbon resinous particles are already known.

The coating may be applied by an electrodeposition process. Such a process is described, for instance, in UK Patent Specification 1424617 and Belgian Patent Specification 846 906.

Suitable metals for forming the coating include nickel, cobalt, silver, gold, platinum, copper, zinc and chromium.The preferred polyfluorocarbon resinous particles are polytetraflouroethlyene (PTFE) and the copolymer of tetrafluoroethylene and hexafluoropropylene which is marketed by Du Pont de Nemours under the name "FEP".

The metal coating is itself coated with a polyfluorocarbon resin. Suitable resins include those incorporated in the metal coating.

Good results have been obtained using moulds provided with (i) a 30,a nickel/PTFE coating which itself is covered with a 10fez coating of PTFE and (ii) a 20 ,a nickel/"FEP" coating which itself is covered with a 10 ,a coating of "FEP".

In the absence of the second resinous coating, 'multiple' mould release has not been obtained.

The present invention is of particular interest for the production of car body units and panels.

The surface finish of articles moulded by this process is excellent for e.g. painting without further surface preparation, in contrastto e.g. polyesterSMC, which yield poor surface finishes.

The invention is further illustrated by the following Examples in which parts, percentages and ratios are by weight unless otherwise stated.

Example t A steel strip measuring 15 x 2 < x 1# mm3 was provided with a nickel coating, 30 # thick, having homogenously distributed therein PTFE resin particles. A 10 thick coating of PTFE covered the nickel coating.

The strip was suspended in a plastic container and a urethane foam-forming mixture, prepared as described below, was dispensed into the container encapsulating the strip.

The urethane foam forming mixture was prepared by feeding separately to a mixer 122 parts of an isocyanate mixture of a uretonimine modified pure MDI and a prepolymer prepared from a mixture of pure MDI, diethylene glycol, 1,3- butylene gylcol and 1,2-propylene glycol and 121.75 parts of a polyol blend and mixing the isocyanate mixture and polyol blend in the ratio of 1:1.

The polyol blend was prepared by mixing together the following ingredients: A polyethertriol which is a 15% ethylene oxide tipped oxypropylated glycerol (MW approximately 5300 OH value 32 mg kOH/g: 75% primary OH groups). 100 parts; Ethylene glycol 21 parts; DABCO 0.7 parts; and Dibutyl tin dilaurate 0.05 parts; The water content of the polyol blend was 0.15 parts; The foam-forming mixture had a cream time of 3 seconds.

After the foam had set and cured, it was removed from the container. The steel strip was easily pulled end-on from out of the encapsulating foam. No foam was stuck to the strip.

This procedure was repeated seven more times.

Each time the strip was easily released from the foam without sticking.

Example 2 The procedure of Example 1 was repeated except that the steel strip used had a nickel coating, 20 # thick, having homogenously distributed therein "FEP" resin particles. A 10,a thick coating of "FEP" covered the nickel coating.

After each of eight mouldings the strip was easily released from the foam without sticking.

Forthe purposes of comparision, the procedure of Example 1 was repeated except that the following steel strips were used in turn: (a) a steel strip having a nickel coating, 30 CL thick, having homogenously distributed therein PTFE resin particles; (b) a steel strip as described in (a) which had been sintered for 4 hours at2500C; and (c) a steel strip as described in (a) which had been sintered for 1 hour at3600C.

Strip (a) showed good release on one side only after two mouldings but foam began to stick along the edges and over about half of the better side on further mouldings.

Some foam was found to stick at one side of strip (b) but this could be scraped off easily after eight mouldings. The other side showed poor release.

Some foam was found to stick to one side of strip (c) after two mouldings but this could be scraped off easily. After four mouldings this side failed to release. The other side showed poor release.

The results show that the steel strips used in Examples 1 and 2 which had a coating of a polyfluorocarbon resin applied to a metaliresin impregnated first coat have superior release properties to the strips in which no additional outer polyfluorocarbon resin coating is applied.

Example 3 A urethane foam-forming mixture, prepared as described in Example 1, was dispensed into a steel mould whose internal dimensions were 35 x 35 x 0.6 mm3. The mould was provided with the same twin coating as the steel strip in Example 1.

After the foamed moulding had set and cured, the mould was opened and the moulding was removed.

This procedure was repeated twenty nine times.

The mouldings were released easily on each occasion. Only a small amount of foam was found sticking in joints in the mould.

The mould temperature built up quickly after about five mouldings to approximately 70 C.

A further 30 mouldings were made using a foamforming mixture having a cream time of 7 seconds prepared by feeding separately to the mixer 102.24 parts of the isocyanate mixture used in Example 1 and 125.75 parts of a polyol blend and mixing the isocyanate mixture and polyol blend in the ratio of 1.

23:1.

The polyol blend was prepared by mixing together: The polyethertriol of Example 1 100 parts; 1,4-butane diol 25 parts; DABCO 0.7 parts; and Dibutyl tin dilaurate 0.05 parts.

The water content of the polyol blend was 0.15 parts.

In each case the mouldings were easily released from the mould.

A further 40 mouldings were carried out using a foam-forming mixture which was the same as that used for the previous 30 mouldings except that additional catalyst was added to reduce the cure time.

Again, in each case, the mouldings were easily released from the mould.

Claims (8)

1. Processforthe manufacture of shaped urethane plastics articles which comprises introduc ing into a mould and allowing to react and set a urethane-forming mixture having a "cream time" (as herein defined) of less than 15 seconds, the mould surface which contacts the urethane-forming mix ture being provided, at least in part, with a metal coating having homogeneously distributed therein polyfluorocarbon resinous particles and, covering the metal coating, a coating of a polyfluorocarbon resin.

2. Process as claimed in claim 1 in which the cream time is less than 10 seconds.

3. Process as claimed in claim 1 in which the cream time is less than 5 seconds.

4. Process as claimed in any one of claims 1 to 3 in which the polyfluorocarbon resinous particles distributed in the metal coating are polytetrafluoroethylene (PTFE).

5. Process as claimed in any one of claims 1 to 3 in which the polyfluorocarbon resinous particles distributed in the metal coating are the copolymer of tetrafluoroethylene and hexafluoropropylene.

6. Process as claimed in any one of claims 1 to 3 in which the metal coating is a 30 it nickel/PTFE coating and the polyfluorocarbon resin coating is a 10 cm coating of PTFE.

7. Process as claimed in claim 1 substantially as herein described with reference to any one of Examples 1 to 3.

8. Shaped articles whenever produced by a process claimed in any one of claims 1 to 7.