GB2034239A - Process for the Production of Low Weight Synthetic Plastics Bodies - Google Patents

Abstract

A low weight synthetic plastics body comprising a moulded casing and a foam core therein comprises the sequential introduction into a mould of a first liquid polymerizable monomer composition and a second liquid polymerizable monomer composition. The two compositions contain the same monomer component but the second composition additionally contains a blowing agent. The mould is rotated around at least one axis during the introduction of the first monomer composition thereinto and is optionally subjected to pivoting movement as it undergoes the rotation. When the second monomer composition is introduced into the mould (e.g. by injection), the mould being at a suitable temperature, foaming of the composition takes place simultaneously with polymerization of the monomer.

Description

SPECIFICATION Process for the Production of Low Weight Synthetic Plastics Bodies This invention relates to a process for producing shaped bodies of synthetic plastics material. In particular, this invention relates to a process for the production of shaped bodies of foamed plastics material having an unfoamed skin or shell therearound and in intimate contact therewith.

Injection moulding processes have not hitherto been found to be suitable for use in the production of shaped bodies of large dimensions, because it is necessary to keep the shaped body in the two-part mould used in the process for a considerable time until the shaped body has been completely cooled down and solidified or cured; this results in a reduction of the overall rate of production. Moreover, with large dimensioned shaped bodies there is the danger that the curved synthetic plastics material will become distorted so that the final product does not possess the desired shape. Further, a shaped body produced in such a manner is very heavy and requires a considerable amount of expensive synthetic plastics material for its production.

To reduce the weight of injection moulded synthetic plastics bodies, it has also been proposed to form an outer layer of the shaped body by injecting a polyvinylchloride paste into the mould to occupy surface regions thereof. As soon as this paste is cured, a foamable synthetic plastics composition is injected into the mould in such an amount that, on foaming, it completely fills the mould cavity. Although this process has the advantage that the consumption of material for a shaped body of particular volume is smaller and the shaped body produced is of lower weight than a massive shaped body produced by a conventional injection-moulding process. It has been found that only a relatively weak bond is produced between the outer layer of the shaped body and its foamed core.When such a shaped body is subjected to load stresses, the bond fails over large areas so that the outer layer of the shaped body is no longer reinforced or supported by the core to the required extent and therefore will break. Furtermore, the danger of distortion of the shaped body still exists.

According to the present invention, there is provided a process for the production of a low weight synthetic plastics body, which process comprises introducing into a mould through an opening therein a liquid polymerizable monomer composition and polymerizing said monomer in the mould as the mould is subjected to rotation around at least one axis, the mould being at a temperature at which the monomer composition remains liquid prior to undergoing polymerization at said temperature, thereby forming a hollow body contacting the inner wall of the mould, and subsequent to introduction of said liquid polymerizable monomer composition into the mould, introducing thereinto a further liquid polymerizable monomer composition, which composition contains the same monomer as the first introduced composition but additionally contains a blowing agent, allowing said further liquid polymerizable monomer composition to undergo foaming within the mould accompanied by polymerization of the monomer content thereof to fill said hollow body at least partially, and removing the finished shaped body from the mould.

The process of this invention thus involves the carrying out of two steps. In the first, a hollow body is produced which forms the outer layer of the shaped body to be produced and consists of a material which is able to resist the tresses to which the shaped body is liable to be subject.

Preferably the material used for the hollow body is a castable polyamide, which has the required strength and the monomer used in the liquid, generally melt, composition is thus an amide precursor, for example caprolactam. By subjecting the casting mould to suitable pivotal movements in addition to effecting rotating movements, it is possible to achieve a greater accumulation of material and hence a greater wall thickness at areas liable to be particularly subject to high stressing. Subsequently the hollow body is at least partially filled with the foamable synthetic plastics material which, when foamed, results in their being formed an outer shell partially or completely filled and thus reinforced by the foamed material.In view of the synthetic plastics material forming the core being produed from the same starting monomer as the hollow body forming the outer shell, an intimate and inseparable bond is achieved between the hollow body constituting the outer shell and the filling or core consisting of foamed material. A homogeneous shaped body is thus obtained whose rigidity is not much less than that of a massive shaped body formed from the same starting monomer but which had not been foamed. Hence, a shaped body produced by the process of this invention can easily withstand even high loads or stresses.Furthermore a shaped body can be produced by the process of the invention at a higher speed than with known processes, because solidification of the hollow body constituting the outer shell and foaming of its core or filling can, as a rule, be effected much more rapidly than solidification of a massive shaped body. Furthermore, when carrying out the process of the invention, there is no danger that the shaped body will become distorted. Therefore, shaped bodies can be produced by the process according to the invention for use in situations in which massive shaped bodies of synthetic plastics materials could not be used in view of their greater weight and/or in view of the need to maintain at all times a predetermined shape.

Thus, for example, the process can be used to produce centreboards (drop keels) or lee boards of sailing boats or to produce surfboards the specific gravity of which must be smaller than unity to enable them to be capable of floating. Further areas of application of the process of the invention are, for example, the production of rollers for conveyor belts, load carrying wheels or push rods of motorcars.

It is preferred when carrying out the process of this invention to rotate the mould, with the hollow shell contained therein, optionally accompanied by pivoting around at least one axis, when introducing the liquid monomer composition containing the blowing agent into the mould. This ensures that the synthetic plastics material for forming the core or filling will first adhere to the inner surface of the hollow body or shell and will become strongly bonded to this hollow body. The thickness of the layer of foam undergoing production can be allowed to grow gradually until opposed parts of this layer come together in the centre of the hollow body and the whole hollow body will be filled with a foam core of uniform density. Hence, a homogeneous filling of the hollow body with the synthetic plastics material is produced.

The liquid monomer composition forming the outer shell (or hollow body) and/or the liquid monomer composition containing the blowing agent for forming the core or filling can be poured into the mould. It is, however, preferred if these liquid monomer compositions are fed into the mould under pressure. This mode of operation both accelerates introduction of the monomer compositions for producing synthetic plstics material in situ in the mould and assists in ensuring that uniform distribution of such materials is produced.

For a better understanding of this invention and to show how the same can be carried into effect, reference will now be made, by way of example only, to the accompanying drawing, wherein: Figure 1 shows the arrangement of a mould during the formation of a hollow body or shell of synthetic plastics material therein when carrying out the process of this invention; Figure 2 shows the mould of Figure 1 during introduction into the hollow body arrangement within the mould of the liquid composition for forming the core or filling; and Figure 3 shows a section through the mould along line Ill-Ill of Figure 1.

The drawing will be referred to during the course of the following Example which illustrates this invention: Example A mould was constructed for producing a leeboard for a small sailing boat. Caprolactam melt available from BASF, Western German, under the product name "Ultralon" (Registered Trade Mark) and containing an activator and a catalyst for polymerization thereof, both available from BASF under the product name "Ultramid" (Registered Trade Mark) was supplied under a pressure of 0.5 bar into a two part mould consisting of two aluminium parts 1 and 2 (Figure 3). The melt entered the mould through an opening 3, passing through a steel tube 4 which extended into the interior of the mould.A hose 5 of silicone rubber connected the tube with a valve 6 which was, in a manner not shown in detail, fed by the components of the melt, which melt was heated to a temperature of 1 300C by means which are also not shown.

The mould 1, 2 was mounted both for rotation around its longitudinal axis 7 and for pivoting around the transverse axis 8. The mould was provided with heating means (not shown) by which the mould was kept at a constant temperature of 1400C. The melt polymerized within the mould to yield a polzamide of a molecular weight of about 1,000,000.

During introduction of the melt via the tube 4, the mould was rotated around its longitudinal axis 7 at a speed of about 100 revolutions/minute, so that the melt was subject to centrifugal forces such that the hot melt clung to the inner wall of the mould 1, 2, and underwent polymerization to an average molecular weight of 1,000,000 when completely polymerized. In this way, a hollow body 9 having an average wall thickness of 1.5 mm was formed which contacted the inner wall.

For this purpose, the heated mould was rotated for 5 minutes. By pivoting the mould 1, 2 around the transverse axis 8, the distribution of the amide melt over the inner surface of the mould was influenced so that a greater wall thickness of hollow body was obtained in individual areas thereof. This enabled the hollow body forming the outer shell of the shaped body to be produced to have a greater wall thickness at areas to be subjected to higher stresses.

The tube 4 was lubricated at the end thereof which entered the mould so that this tube could easily be removed before the hollow body 9 was substantially polymerized. After polymerization of the introduced melt was substantially complete a further steel tube 10 of smaller outer diameter than that of the tube 4 was introduced into the opening 3 (Figure 2). In this manner, an annular gap 11 was provided between the outer surface of the tube 10 and the wall of the opening formed within the hollow body 9 and resulting from the previous presence of the tube 4. The gap 11 allowed the air present within the hollow body 9 to escape when a foamable synthetic plastics monomer melt was introduced into the hollow body by means of the tube 10. The tube 10 was connected via a hose 12 of silicon rubber to a mixing valve 13 fed in a manner not shown in detail, with a molten monomer composition comprising a catalyst and an activator, all these components being the same as those used in the production of the hollow body 9. This melt mixture, however, contained a blowing agent as an additive, namely "Genitron" (Registered Trade Mark) available from Fisons Limited, Great Britain.

This melt was supplied at a temperature of 1 300C via the tube 10 into the cavity enclosed by the hollow body 9 and filled this cavity, substantially completely. Although not essential, it was expedient for the mould 1, 2 still heated to 1 400C to be rotated again around the axis7 at 1 10 revolutions/minute and oscillated around the axis 8 over an angle of 200 to both sides of the middle position. This rotation and oscillation of the mould represents preferred practice, but is not essential in all cases. The oscillation which was effected was slow, about 2 times/minute. Thus, the polymer melt supplied first clung or adhered to the inside wall surface of the hollow body 9 and bonded intimately with this hollow body. The polymer melt then expanded to fill the hollow body as the polymerization thereof proceeded.

The amount of molten monomer supplied was selected to be such that the desired volume of the hollow body 9 became filled with foamed material when this material was completely polymerized.

After the introduction of the molten monomer composition comprising the blowing agent was complete, the tube 10 was removed; this was facilitated by prior application of a lubricating coat to the inserted end of the tube. When foaming was complete, i.e. about 3 minutes from the moment of first introduction of the monomer melt into the hollow body 9, an excess of foamed material was seen to have emerged from the opening 3 of the mould 1, 2. Rotation and oscillation of the mould was continued during this period. After approximately 10 minutes the mould was opened and the shaped body was removed.

After re-assembling the mould parts and introducing the tube 4, the mould was ready for producing a further shaped body. It is advisable to coat the mould surfaces of the mould with a silicone lacquer before use.

To avoid the danger of the tubes 4 and 10 being blocked by cured plastics material, nitrogen was blown through these tubes after the introduction of the respective melt into the mould had taken place.

As mentioned above, the filling or core of the final shaped body need not fill the hollow body 9 completely. For many applications, for example for surfing boards, a hollow space may be left within the foamed core of plastics material. This hollow space may be filled later by a light weight filling foam, formed, in particular of synthetic plastics material, for example polyurethane.

The polymerization of the melt applied to mould inner surface to the outer shell or hollow body 9 and the polymerization and foaming of the core or filling may overlap each other. However, the process of this invention may also be carried out satisfactorily if, at first, the outer shell or hollow body 9 is completely polymerized and only then is the monomer melt for forming the foamed core introduced.

Claims (10)

Claims

1. A process for the production of a low weight synthetic plastics body, which process comprises introducing into a mould through an opening therein a liquid polymerizable monomer composition and polymerizing said monomer in the mould as the mould is subjected to rotation around at least one axis, the mould being at a temperature at which the monomer composition remains liquid prior to undergoing polymerization at said temperature, thereby forming a hollow body contacting the inner wall of the mould, and subsequent to introduction of said liquid polymerizable monomer composition into the mould, introducing thereinto a further liquid polymerizable monomer composition, which composition contains the same monomer as the first introduced composition but additionally contains a blowing agent, allowing said further liquid polymerizable monomer composition to undergo foaming within the mould accompanied by polymerization of the monomer content thereof to fill said hollow body at least partially, and removing the finished shaped body from the mould.

2. A process as claimed in claim 1, wherein the mould is subjected to pivoting movement as it undergoes said rotation.

3. A process as claimed in claim 1 or 2, wherein the mould is subjected to rotation about at least one axis of said further liquid polymerizable monomer composition is introduced thereinto.

4. A process as claimed in claim 3, wherein the mould is subjected to pivoting movement as it undergoes said rotation.

5. A process as claimed in any one of the preceding claims, wherein either or both of liquid polymerizable monomer compositions is/are introduced into the mould under pressure.

6. A process as claimed in any one of the preceding claims, wherein said compositions are melts.

7. A process as claimed in any one of the preceding claims, wherein said monomer is a polyamide-forming monomer.

8. A process as claimed in claim 7, wherein said monomer is caprolactam.

9. A process for the production of a low weight synthetic plastics body, substantially as described in the foregoing Example.

10. A synthetic plastics body which has been produced by the process claimed in any one of the preceding claims.