The invention relates to a foamable element for mutually coupling multiple components of a lamp. The invention also relates to a lamp assembly comprising at least two components of a lamp. The invention further relates to a method of mutually coupling multiple components of a lamp, using said foamable element. It is known in the art to manufacture lamp assemblies, such as fluorescent tubes, incandescent (low-energy) bulbs and discharge lamps, by affixing multiple components by means of a thermosetting cement. However, this cement has several drawbacks. The known cements have short shelf lives because of the need for solvents like trioxane or hexamethylene tetramine and other reactive components like aldehydes, ammonia, or metal hydroxides. Moreover, accurate application of these cements is difficult, and, as a result, a relatively large amount of cement must be used to adhere a glass lamp to a metal base. To eliminate these drawbacks of the known cement, the process of manufacturing these lamp assemblies can be improved by using a foamable element as disclosed in international application WO03/014203. By heating the lamp assembly for a certain time, the components are heated to a sufficient temperature, which causes the foamable element to expand, thereby securely affixing the lamp components. Although application of the foamable element yields significantly improved results as compared with application of the thermosetting cement, the known foamable element also has a major drawback. The method of using a conventional foamable element has the drawback that the lamp assembly must be positioned in an atmosphere having an increasing temperature to let the foamable material expand and thereby mutually couple the lamp components neighboring the foamed material. This overall heating of the lamp assembly can lead to deformations and other damage of certain lamp components which are sensitive to this elevated temperature during foaming. Moreover, exposure of these sensitive component to the foaming temperature for a certain period of time can reduce the life span of these components significantly. It is therefore an object of the invention to provide an improved foamable element which does not require significant heating of neighboring lamp components to be coupled by the foamable element during foaming. This object can be achieved by a foamable element as mentioned in the opening paragraph, and is characterized in that the foamable element is provided with heating means for heating said foamable element. By providing the foamable element with heating means, the heat required to expand the foamable element can be generated in its direct area. In this way, position-selective and direct heating of the foamable element can be achieved, without seriously heating other parts of the lamp assembly. Although the components neighboring the foamable element will be heated during operation of the heating means, the heat absorption of these components will be significantly small as compared with the heat absorption during the known foaming method with the known foamable element, which results in an improved life span and maintenance of the structure and material-related properties of these neighboring components. Moreover, the overall amount of energy required to sufficiently heat the foamable element is relatively smaller as compared with the amount of energy required in the method known in the art. Thus, by applying the heating means in (or directly on) the foamable element, the foamable element can be heated relatively efficiently and effectively without affecting other lamp components. Since the foamable element can be heated in a position-selective way, it is a further advantage that the freedom of choice as regards material, shape and format of the neighboring lamp components is many times greater than the freedom offered by the state of the art. It is imaginable that the heating means are provided with coupling means to allow electric coupling of the heating means to an external energy source, such as an electricity grid. However, in a preferred embodiment, said heating means are adapted to produce heat upon absorption of electromagnetic radiation. In this advantageous embodiment, the heating means can be activated in a wireless manner and relatively effectively by simply placing the heating means in an electromagnetic field preferably having specific characteristics. By absorption of the electromagnetic radiation by the heating means, the absorbed electromagnetic field energy will be converted into heat which is subsequently absorbed by the foamable element. This heat absorption will lead to a foaming (expanding) of the still unfoamed element. Said heating means preferably comprise at least one ring-shaped element. Said ring-shaped element may be formed as a circular ring, but it is also imaginable to apply e.g. a triangular or rectangular ring. A ring-shaped design of said heating means generates a relatively large freedom of choice as regards material, shape and format of both the neighboring lamp components and said foamable element. Moreover, due to this improved freedom of design, it is advantageous to apply ring-shaped heating means, because the outer edge or edges of the foamable element can be heated relatively intensively in this way. This outer edge or these outer edges are found to be of major importance to realize a solid, stable and lasting mutual coupling of two or more components of a lamp. In a preferred embodiment, said heating means form at least one conductive circuit. A conductive circuit is generally relatively suitable to absorb relatively large amounts of energy, which can be subsequently converted to heat to be absorbed by the foamable element. Metals, for example, copper, can be used to generate the conductive properties of the circuit. It is not necessary that the conductive circuit of the heating means is formed by a single physical element. This circuit, and thus the heating means, can also be formed by controlled distribution of conductive (metal) particles in the foamable element. In this embodiment, the heating means can be fully integrated in the foamable element. Generally, the conductive circuit is formed by the above-mentioned ring-shaped element, for example, a metal ring. In another preferred embodiment, the heating means are embedded in said foamable element. As mentioned above, the heating means may be formed by multiple metal particles distributed (uniformly) in the foamable element, but may also be formed by, for example, a ring embedded at least partially in said foamable element. It is not required to embed the heating means completely in the foamable element, but the heating means are preferably surrounded substantially by the foamable element, because the heat transfer from the heating means to the foamable element can be maximized in this way. The foamable element per se is preferably ring-shaped. Since most lamp components have a ring-shaped outline (circular, triangular, etc.), it is advantageous to apply a ring-shaped foamable element to be able to optimize the mutual coupling of the lamp components. In a preferred embodiment, the foamable element is adapted to expand substantially in a radial direction. Generally, lamp components are assembled first by mutual, partial overlapping of these components, and secondly by affixing this telescopic orientation of the components. By positioning the foamable element in the space between the overlapping parts of the components, a relatively stable and solid coupling of the components can be achieved by radial expansion of the foamable element, thereby engaging the components to be coupled under a certain bias. The foamable element can be made of any material adapted to foam upon heating, but preferably comprises a blowing agent and a copolymer, wherein the copolymer is selected from the group consisting of ethylene vinyl acetate (EVA), ethylene methyl acrylate (EMA), ethylene butyl acrylate, ethylene ethyl acrylate, ethylene methacrylic acid and mixtures thereof. These compositions have foaming temperatures in the range of the intended application and can withstand prolonged use at elevated temperatures. The invention also relates to a lamp assembly as mentioned in the opening paragraph, and is characterized in that said components are mutually coupled by at least one foamable component as described above. Said components are preferably selected from the group consisting of a lamp base, a burner, a cover for said burner, and a bulb (sleeve). Besides these components, it is also conceivable to couple other lamp components by one of a multiple of foamable elements. The invention further relates to a method of mutually coupling multiple components of a lamp, using a foamable element as described above, the method comprising the steps of: a) assembling at least two components and at least one interposing foamable element, and b) causing the heating means to heat said foamable element, thereby expanding said foamable element and affixing said components. In step a), the foamable element is preferably positioned between the interior surface of one component and the exterior surface of a neighboring component. In another preferred embodiment, in step b), electromagnetic radiation is generated to heat the heating means which are sensitive to this radiation. In a particular preferred embodiment, the radiation has a frequency between 20 kHz and 1 MHz.

The invention will be elucidated by way of the following non- limiting examples of embodiments. In the drawing, Figure 1 is a cross-sectional view of a lamp assembly according to the invention. Figure 2 is a perspective view of an embodiment of an unfoamed element according to the invention. Figure 3 is a cross-sectional view of another embodiment of a foamable element according to the invention.

Figure l is a cross-sectional view of a lamp assembly 1 according to the invention. The lamp assembly 1 comprises a glass tube 2 which is securely affixed in a base 3 by means of a foamed (and cured) element 4. The foamed element 4 is provided with a metal ring 5 to improve the foaming process of the element 4. The metal ring 5 is sensitive to electromagnetic radiation, preferably electromagnetic radiation with a relatively high frequency. The metal ring 5 will absorb this radiation and convert this energy to produce heat which is subsequently transferred to the element 4 to cause it to foam. Optionally, a small amount of adhesive can be used to secure the coupling of the glass tube 2 and the base 3. The base 3 is provided with electric contacts 6 to connect the lamp assembly 1 to an electric power source. In this example, a part of a luminescent tube is shown. However, it is also conceivable to apply the foamed element 4 in other types of lamps, wherein even more than two, for example, three or four components can be mutually coupled. Figure 2 is a perspective view of an embodiment of an unfoamed element 7 according to the invention. The element 7 is ring-shaped and is substantially formed by an EVA or EMA-based copolymer composition. This composition is provided with a blowing agent to effect foaming and expansion of the foamable composition at an activation temperature from about 120 0C to about 250 0C. Suitable blowing agents will normally include azodicarbonamide and benzene sulfonyl hydrazide. Examples of suitable blowing agents are disclosed in international application WO 03/014203. The foamable composition further comprises metal particles 8 which are distributed uniformly through the copolymer composition. The metal particles 8 have such a distribution that the particles form a conductive circuit suitable for absorbing electromagnetic radiation to sufficiently heat the ring-shaped element 7. Figure 3 is a cross-sectional view of another embodiment of a foamable element 9 according to the invention. The foamable element 9 may have a circular ring shape or a rectangular design. In this example, the foamable element 9 comprises four rectangular conductive rings 10 suitable for heating the foamable element 9 to cause it to foam. The conductive rings 10 may either have a 2D flattened geometry or a 3D curved geometry. The conductive rings 10 are positioned in the foamable element 9 in such a way that particularly the most critical regions of the foamable element 9, i.e. the regions that are very important to establish a solid coupling of lamp parts, can be heated meticulously and intensively. It will be evident that the invention is not limited to the examples of embodiments shown and described here, but that numerous variants can be conceived by those skilled in the art and are possible within the scope of the appended claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.