The present invention relates to a product made of crosslinked polyolefin foam.
Crosslinked polyolefin foams are flexible foams having relatively low densities, generally of less than 200 kg/m3, and the solid phase of which essentially consists of crosslinked polyolefins. The polyolefins most usually employed in this field are polyethylene (PE), polypropylene (PP) and ethylene/vinyl acetate copolymer (EVA).
Crosslinking allows the foams to be improved on many points, regarding their mechanical properties; finer cellular structures are obtained.
The crosslinking may consist of high-energy irradiation, such as by an electron beam, of a chemical reaction with agents of the peroxide type or with moisture by the incorporation of a silane side group into the backbone of the polyolefin. In the first two processes mentioned, reactive sites with free radicals are created on the carbon chains of the polyolefin molecules, for the purpose of subsequently forming intermolecular crosslinking bonds.
In the third process (crosslinking using moisture), the crosslinking is achieved by forming polyolefin-Si-O-Si-polyolefin intermolecular bonds.
Crosslinking using high-energy irradiation is noteworthy in its ability to produce perfectly uniform crosslinking on the surface; it allows high-quality foam sheets with a thickness as little as 0.2 mm to be manufactured.
The situation is the reverse in the case of chemical crosslinking by peroxides or with other agents and in the case of silane crosslinking, which processes are recommended for sheets having greater thicknesses, for example between 3 and 18 mm, since in these processes the crosslinking takes place throughout the thickness of the product in a homogeneous and uniform manner. On the other hand, these processes are much less suitable for very thin sheets since, on the microscopic scale, the dispersion of the crosslinking agents is less homogeneous than that which can be obtained by means of an electron beam.
Moreover, the expansion of crosslinked polyolefin foams occurs in three dimensions when no particular constraint is applied. It is important for the degree of crosslinking of the resin both at the start of the expansion and throughout its duration to be carefully controlled; the initial content of the compositions in terms of blowing agent and of crosslinking agent in the case of chemical crosslinking is, of course, tailored to this effect as well as to the durations and temperatures of the heat treatments.
Blowing agents fall within two main classes, namely volatile organic agents, such as chlorofluorocarbons (CFC), and chemical agents, such as azodicarbonamide. All blowing agents can be employed in accordance with the invention, according to all the well-known methods of implementation of the prior art comprising, depending on the case, extrusion operations and/or holding under pressure followed by expansion and/or heating, etc.
In the preparation of a crosslinked polyolefin foam, the start of the expansion usually takes place in an already partially crosslinked state of the resin. The purpose of this measure is to increase the viscosity of the resin, thus conditioning the regularity and fineness of the cellular structure finally obtained. In this case, the crosslinking continues during the expansion and, possibly, thereafter.
Nevertheless, the crosslinking may also be started during, or even after, the expansion (especially with the combination of a physical blowing agent, that is to say one that is active under the effect of expansion, such as isobutane, and of a silane crosslinking agent, mentioned above), without departing from the scope of the invention.
One of the drawbacks of crosslinked polyolefin foams known at the present time resides in their relative hardness. Their regular and fine cellular structure, having closed cells, gives them, however, good sealing properties, buoyancy, etc. However, in insulation applications especially for buildings, in forming seals, their mediocre compliance mentioned above constitutes a handicap, in particular in situations in which the foam product must conform to and/or plug up a complex relief having intricacies.
There is therefore a need to define suitable compliance characteristics, especially for the applications mentioned, and which nevertheless allow the cellular-structure and surface-appearance qualities of all crosslinked polyolefin foams to be maintained at a sufficient level.
This objective is achieved by the invention, the subject of which consists of a product made of crosslinked polyolefin foam, which has a compression of at most 7 N/cm2, measured according to the ASTM D 1667 standard with a degree of compression of 30% and a rate of compression of 10 mm/minute, for a density of at least 100 kg/m3, it being understood that this maximum compression may decrease for lower densities and increase for higher densities.
Preferably, the gel content of this product, defined as the percentage by weight of the insoluble fractionxe2x80x94in the form of dry extractxe2x80x94of a 50 mg specimen immersed for 24 hours at 120xc2x0 C. in 25 ml of xylene dried over a molecular sieve, is between 30 and 70%, especially between 40 and 60% and, particularly preferably, between 45 and 55%.
For some crosslinked polyolefin foams having a density of 100 kg/m3, compressions substantially less than 7 N/cm2 are unavoidably associated with a coarse cellular structure and an unacceptable deterioration in the surface appearance. In this regard, the claimed maximum compression has a universal character under the defined conditions.
Many types of compositions are compatible with the invention; two advantageous types may be mentioned.
A first type of initial composition comprises at least 20% by weight of a polyethylene or of an essentially linear ethylene copolymer having a density between 0.80 and 0.96 g/cm3.
Particularly preferably, the polyethylene or the ethylene copolymer used is obtained by metallocene catalysts and has a density of at most 0.92 g/cm3. With regard to the composition of these preferred polymers and, in particular, their physical characteristics and the nature of the comonomers, Patent Application EP 0,702,032 A2, which also describes their treatment by crosslinking and expansion, is incorporated here by way of reference.
A second type of preferred initial composition comprises at least 10% by weight of at least one polyolefin elastomer chosen particularly preferably from an ethylene/vinyl acetate copolymer, an ethylene/propylene/diene monomer terpolymer and a butyl rubber.
According to a preferred embodiment with regard to the practical considerations associated with the manufacture, storage, commercialization and use of the product, the latter is advantageously essentially flat, having a thickness of between 2 and 100 mm and, preferably, between 8 and 30 mm. It can be prepared continuously using well-known processes, these being known as the Toray process with the use of a heated bath of KNO3/NaNO2, the Toray and Sekisui processes using a vertical oven and a countercurrent of hot air, the most advantageous processes within the context of the invention being the Hitachi and Furukawa processes using a horizontal oven and three zones of gradual heating.
The continuous web of foam product thus manufactured may be collected in the form of wound rolls or cut into sheets, depending on the envisaged use and the physical characteristics of the product.
According to one particularly advantageous embodiment, at least one of the two faces of the essentially flat product is adhesively bonded to a support. In general, the support has substantially the shape of a sheet. The adhesion results, for example, from a coextrusion operation or is obtained progressively as the foam is being formed, especially by creep of the resin, followed by physico-chemical transformation of the latter (the formation of intermolecular bonds, expansion) or even, possibly, by a chemical reaction between the resin and the support. In certain cases, this adhesion does not prevent removal of the support after the foam has formed.
Moreover, it should be pointed out that a product according to the invention having a support on each of its two faces cannot in general be wound, unless at least one of the two supports has particular mechanical properties (elasticity, compressibility etc.).
Furthermore the subject of the invention is also the application of the product described above as a seal, especially for buildings and for transport vehicles (motor-vehicle door seal, etc.).
Other features and advantages of the invention will appear from the following example.