The invention relates to a container base (i.e. a closed bottom end of a container) of thermoplastic material, particularly aerosol container. It is customary to use the term aerosol container (or container for short) while the container is unfilled and to use the term aerosol dispenser for a filled and pressurized container. For the sake of simplicity only the term container will be used herein for both unfilled and filled containers.

Aerosol containers are mostly made of metal, eg aluminium or steel (tin plate). The invention relates exclusively to aerosol containers made of thermoplastic materials, particularly of a polyacetal (acetal resin), such as acetal homopolymer or acetal copolymer eg polyoxymethylene with melt flow index range 9-0 to 27.00 g/10 minutes), of a thermoplastic polyester, such as polyethylene terephthalate or polybutylene terephthalate, or a thermoplastic polyolefin, such as polyprop7lene.

The applicants made successful experiments with containers made of an acetal copolymer, namely that sold under the trade name Kematal, particularly Kematal M270 and Kematal 90, and Hostaform, particularly Hostaform 13031- The aim of the invention is to devise a base of a plastics aerosol container that has all the necessary properties expected from such a base, in particular good impact strength, which is dependent not only on the material used and its thickness (which need not be uniform) but quite significantly on its shape. Important is also creep resistance which is dependent upon the polymer type, time, temperature, internal pressure, the shape of the base and wall thickness.

Persons skilled in the art know that a number of characteristics of plastics materials, such as chemical resistance, permeation creep and impact strength, are fundamentally different from those of metals and, in fact, differ widely even between metals, such as aluminium and steel. Experience acquired from the testing and use of metal containers is therefore practically of no help with plastics, the characteristics of which differ from each other even more widely than is the case with metals. The basic requirements as regards non-refillable plastics aerosol containers are set out in British Standard BS 5597 published in 1978, a new edition of which relating specifically to plastics aerosols, is in preparation.

The invention provides a container base of thermoplastic material comprising an upright wall having at its lower end an inwardly inclined first wall portion, an inwardly domed bottom wall having at its perimeter an inclined second wall portion, and a stand-on wall which merges into and interconnects the first and second wall portions, all the walls and wall portions being continuous and integral, the container base having a longitudinal axis which is vertical when the stand-on wall is in contact with a horizontal imaginery stand-on plane, wherein the first wall portion extends to the stand-on plane at a first angle which is in the range of 42° to 79°•

The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which:

Figure 1 shows an axial section through a half of an aerosol container base according to the invention, Figure 2 shows a first graph,

Figure 3 shows a second graph, Figure 4 shows a third graph, Figure 5 shows a fourth graph, and Figure 6 shows a fifth graph. Figure 1 shows a preferred embodiment of an aerosol container base according to the invention made of a thermoplastic material. Only half of the base is shown in axial section, the other half being a mirrror image of the shown half, because the whole illustrated base (and also the associated container) is of circular cross-section. Other cross-sections are possible e.g. oval, triangular (preferably with convexly arcuate sides) or square-shaped. The base comprises an upright wall 1 having at its lower end an inwardly inclined first wall portion 2, an inwardly domed bottom wall 5 having at its perimeter an inclined second wall portion 4, and a stand-on wall 3 which merges into and interconnects the first and second wall portions. All the walls and wall portions are continuous and integral. The base has a longitudinal axis Y which is vertical when the stand- on wall is in contact with a horizontal imaginary stand-on plane X. As is apparent rom Figure 1 , the thickness of the upright wall 1 progressively increases from its top to the first wall portion 2, while the first wall portion 2, the second wall portion 4 and the bottom wall 5 are substantially of the same thickness.

An essential feature of the illustrated base is the magnitude of the angle «f at which the first wall portion 2 extends with respect to the horizontal stand-on plane on which the base rests.

The angle <ζ shown in Figure 1 has a great bearing on the stress level in the container wall as a result of internal pressurisation of the container.

likewise the performance of the container base under impact load P is effected by this angle o.

Various values of o ere analysed for maximum principal stresses. The angle Otis in the range of 42° to 79°, preferably 47° to 70°. A preferred specific value of the angle C is 56.5", particularly for acetal copolymer (polyoxymethylene) . A graph was plotted of the the angle 0 versus principal stress, and this graph is shown in Figure 2. The graph indicates a definite trend of the relationship, the curve having an apparent minimum value at the nadir of approximately 56.5°• When a horizontal line is drawn at stress 22 MPa, it cuts the curve at two points, namely 42° and 79° • Laboratory creep test work on containers moulded from acetal copolymer gave a relationship between burst pressure due to creep failure and time. The burst pressure can be directly converted into hoop stress. Figure 3 gives this relationship, in this instance at 5 °C - one of the recommended BAMA temperatures for creep tests. It is obvious that the higher the test pressure, the higher the resulting induced stress and the shorter the time duration to failure. If various stress levels are picked from the curve with their attendant failure time, these can be related to the extrapolated tensile creep curves in Figure 4» In general, the apparent failure strain occurs in the band of 2.5 to 2.8#. The values apparent from Figure 4 were derived from Kematal M270.

For five years' continuous use at a strain level maximum of 2.5$ (Figure 4) gives a maximum permissible stress of 14 MPa for Kematal M90 at 60°C A suitable production material is believed to be Hostaform 13031 , which has an increased creep modulus

of 33 . Thus the maximum long term allowable stress at 60°C will be 14 MPa x 1.38 = 19 MPa. This figure can be increased by 16$ to function at 55°C, i.e. maximum allowable stress for Hostaform 13031 at 5 °C and strain level 2. $ = 22MPa (19 x 1.16).

The graph shown in Figure 5 is a graph for the selection of the angle β at which the second wall portion 4 extends with respect to the horizontal stand-on plane X, as shown in Figure 1. The minimum value for the angle β is derivable from the graph in Figure 5- The value should be between 32.5° and 90°, preferably between 47° and 77°.

The graph in Figure 6 is similar to the graph in Figure 2, except that the X-coordinate indicates the pressure dome height h (in mm) of the bottom wall 5- The line at 22 MPa intersecting the curve gives for h a clear useful band of 9 mm to about 17*3 mm. These dimensions were established for a container base having an internal diameter D = 50 mm measured at the top of the upright wall 1. For containers of different diameter the heighth h would be analogically a corresponding fraction, ie 0.18 D to about 0-35 D (ie 18$ to about 35$), preferably 0.22 D to 0.31 D. A preferred specific value of h is 0.268 D, particularly for acetal copolymer.