Method for treating a foamable plastic and a plastic treated according to said method

Method of treating a foamable resin and thereafter treated plastic

The invention relates to a method for treating a foamable plastic material, and a treated according to the method of plastic, wherein the plastic is überfuhrt from an initial state with a higher density by means of a blowing agent in a foaming operation to a foamed state of lower density.

Conventional foamable plastics usually consist of a base polymer, a blowing agent and additives. The plastic is melted, the blowing agent is activated by heat and zugefuhrte of plastic foams. As an example of other additives or colors plasticisers include that give the foamed plastic the desired color or impart a desired elasticity. A disadvantage of this foamable plastics is that they sag during the foaming process due to the gravitation and the foaming does not proceed characterized isotropic. One of the plastic given before the foaming structure or shape is lost usually because the melt is not sufficiently "stabilized" thereof.

The invention is therefore based on the object to avoid or mitigate this problem.

The object is achieved by a plastic which is crosslinked before foaming by at least a first cross-linking agent and during the foaming process by at least a second crosslinking agent.

Advantageous developments of the invention are shown in the subclaims.

The invention will be explained with reference to various exemplary embodiments for implementing the method according to the invention with reference to a drawing. Show it:

Fig. 1 is a side view of an apparatus for irradiating a plastic part;

Fig. 2 is a perspective view of an apparatus for irradiating a plastic part through a mask and

Fig. 3 is a perspective view of a treated by the process according to the invention the molded plastic part, which is arranged by means of a molded part in a frame of an automobile. - J -

According to the inventive method, a foamable plastic is crosslinked before foaming by a first crosslinking agent and during the foaming process by a second crosslinking agent.

A plastic which is not crosslinked, largely loses its shape or structure that the mold part from which the foam is formed, had during the foaming process. This is partly due to the major structural changes of the plastic during the foaming and the other on the gravity, under the influence of foam, in particular during cooling in or after the foam process, bags. To maintain the shape or structure during the foaming process, it is necessary to cross-link the plastic.

Through the use of a crosslinking agent polymer chains are destroyed in some areas in the plastic and form radicals within the same. The free ends of the polymer chains and the free radicals go with each a new bonds. so that it comes to a partial crosslinking of the polymers. If a such pretreated plastics heated thereof above the melting point, so softened the rigid structure of the plastic, and it forms a ..stabilisiertes material ", which has a higher viscosity than a melt. The higher viscosity is due to the partial pre-crosslinking of the plastic and causes the stabilized material having a certain form stability. If now the foaming triggered, for. example, by a wäπneaktivierbares propellant, the resultant from the stabilized material foam has substantially the shape or structure of the underlying molding. During the However, foaming loses the frothed material consists of the above reasons emeut a little, the shape or structure of the underlying molding. It is therefore necessary to obtain them by a second crosslinking agent which is activated during the foaming process. Under the influence of the second cross-linking agent are cross-linked, the polymer chains in the plastic material further to obtain the foamed molding of the plastic. The thus formed foam has substantially the shape of the underlying molding, has usually due to the nucleation more volume and is pored.

As crosslinking agents physical and chemical crosslinking agents are contemplated.

The physical crosslinking agent may be used in particular high-energy radiation, which preferably completely penetrates the plastic. The energy of the radiation should be chosen so that the mean free path of the radiation in to the plastic is greater than the material thickness of the plastic in the direction of incidence of the radiation. Preference is given to electron radiation, in particular with an energy between 100 keV and 10 MeV is used, but it can also be used as proton radiation. At sufficiently high energies and atomic beams or light nuclei can be used for irradiation. Furthermore suitable as radiation also Garnma- or X-rays.

When chemical crosslinking agents can peroxides, in particular organic, used. These are for example the plastic supplied in the molten state prior to the formation of a shaped part from the melt. Due to the radical decomposition of the peroxide, the plastic is partially crosslinked and there is a partially crosslinked molding. This is following used in an arrangement and foamed. Furthermore there are chemical crosslinking agents which can be activated during the foam process and the resulting foam teilvemetzen.

In a first Ausfuhrun SExample of the inventive method is a foamable plastic 1, in particular as injection molded molding, irradiated in Fig. 1 before the foaming with high-energy radiation from a radiation source 3 9 through a slit diaphragm 5. The thus pretreated plastic molding is both example used in the body of an automobile to seal cavities in the body.

If upon heating of the molded part, for example in a drying oven for drying the lactate marking of the automobile, the melting point of the plastic is reached, then the stabilized material forms due to the physical pre-crosslinking. At this temperature and the heat-activatable foaming agent is preferably activated so that the foaming process is initiated. During the foaming process the second chemical crosslinking agent is activated to crosslink the polymer chains of the plastic on. The second crosslinking agent may also be activated at the activation temperature of the blowing agent or at a higher temperature. The latter is particularly advantageous if a plastic is used, which is curable by heat and thus, the plastic is heated during the foaming on significantly higher than the melting point of the plastic temperatures.

The properties of the stabilized material can be controlled substantially above the applied energy dose, with the plastic material is irradiated. If a too low energy dose is used, new cross-linking points are not sufficiently generated, so that the desired effect is not achieved. At high doses of energy such a powerful form of networking is that the material is highly stabilized, whereby the foaming capacity is greatly reduced. However, if a dose of energy between these two extremes is used, the material is sufficiently stabilized to maintain the structure of the molded part also in the heated state, while maintaining a satisfactory foamability of the material.

The absorbed dose actually required depends on the base polymer, the blowing agent and the additives used, such as chemical Vemetzungsmit- stuffs, stabilizers, kickers and inhibitors from. When using an ethylene vinyl acetate (EVA) as the base polymer and azodicarbonamide as blowing agent enters the desired effect in an energy dose range between 1 and 10 kGy, in particular in the range 3-6 KGy.

The base polymer of the plastic used in the present embodiment, Fes is ethylene-vinyl acetate, the vinyl acetate content from 5 to 25 wt .-% makes up the EVA component. As a blowing agent azodicarbonamide is used which is activated by heat in the melt. In a preferred embodiment the blowing agent is introduced prior to irradiation in the plastic, however, it may also be supplied after the irradiation.

As shown in Fig. 2, portions of the plastic can be shaded by means of a mask 7 of the radiation 3. If you select the absorbed dose so that the non-shadowed by a mask areas of the plastic 1 record an inventive mean absorbed dose, the result is thus a molding which sections an networking. If the foaming process is triggered, foamed onto the irradiated region of the plastics material is substantially isotropic, whereas the foaming of the non-irradiated portion of the plastic material is affected by the gravitation. In this manner, the foaming of the molded part can be influenced or controlled in a desired manner.

In one variation, the mask is partially transparent to the radiation. If the molding is now exposed to a high dose of energy, as can be found in the area of ​​the plastic, which was exposed to a high dose of energy, a highly interconnected world. In the area of ​​the plastic, which was partially protected by the mask of the radiation, only an energy dose was recorded, which is in a central area. During the foaming process, the areas that have received an average dose of energy foam, isotropic, whereas the foaming process in the areas that have received a high dose of energy is more or less inhibited strongly. The latter areas may in particular as holding elements of a plastic molding are used in an array, because they still have a certain stability, even in the heated state.

In a variation of a holding element is present as a separate element from the same plastic or a material other than the irradiated plastic part and is fixed thereto to retain it by means of the retaining element for example. In one with the foamed plastic material to be sealed cavity, in particular in a body of a motor vehicle.

If the irradiated plastic from a precursor, such as raw material, powder or granules, prepared as in place of the irradiation of a finished part, the precursor to the used manufacturing a part or molded article are irradiated.

Is a molding of plastic with other mold parts, esp. Of plastic, secured to, to form a plastics composite part, they can be exposed as a whole or only in regions of the radiation.

Furthermore, it is possible to irradiate one or more plastic (shape) parts before they are assembled with other components to form a plastic composite part.

In a second exemplary embodiment of the inventive method a melt of a foamable plastics material is admixed with an organic peroxide, whereby the latter is partially crosslinked. However, this partial crosslinking takes place in a so-wrestle an extent that the following sprayed from the melt molding is sufficiently cross-linked before the foaming with high-energy radiation. In this way it is ensured that the following built-molding forms a sufficiently stabilized material in large heated. Because of the chemical and the physical crosslinking occurs during heating of the first mold part, the stabilized material. at this temperature the blowing agent is preferably activated to foam it. During the foaming process, the chemical crosslinking agent is reactivated, leading to a further cross-linking of the foam, some of the same until the solidification leads.

In a third embodiment, both the first and the second cross-linking agent, a chemical. The first cross-linking agent is activated at a temperature that is less than the temperature at which the second crosslinking agent is fourth Akt. If both means of a melt of the plastic material fed so at a suitable temperature control only the first cross-linking agent is activated and partially cross-linked following the melt. In a following step, a shown in Figure 3 form part 11 is injected from the melt, which is attached to a further mold part, in particular of a harder plastic. The thus formed composite plastic part is used for example in a portion of a frame 15 of an automobile. In a drying oven is of the following the mold part 11 forming foamable melted plastic, whereby the blowing agent is activated. The temperature in the oven is so high that now the second chemical linking agent is activated and the resulting foam crosslinked partially. A flow of the melt to a side wall of the frame portion 15 is prevented by the cross-linking of the foam and closes the cross-section of the frame portion completely. In this way, a composite of the foam and the steel of the frame portion 15 is formed, which provides additional stability in the region of the frame and provides a reliable seal of the cavity. The blowing agent for the plastic and / or other additives may be incorporated into any embodiment of the process before, during or even after the first crosslinking in the plastic.

- LIST OF REFERENCE NUMBERS

radiation

slit

mask

radiation source

Molding of foamable plastic

molding

Frame portion of an automobile

- claims