Field of the invention

The invention relates to a thermoplastic compound generally designed for use in making products having a high strength as well as excellent sealing, tightness, and heat and weather resistance properties.

Background art

Silicone rubber, whether molded, deposited or extruded, either in liquid form (LSR) or in high consistency form (HCR) is known to be hardly bonded to the surface of articles made of thermoplastic materials such as polyolefins, polyesters, polyamides, polycarbonates, fluorinated resins, with an adequate adhesion force.

A number of technologies have been known to obviate this drawback.

One of these uses an adhesion "primer" (i.e. an activator) that is previously deposited on the thermoplastic material.

Another technology uses self-adhesive silicone.

A further technology uses oxidative surface treatments such as plasma or corona treatments.

Yet another technology includes the introduction of agents that act as an interphase between the thermoplastic material and the silicone rubber, to make them mutually compatible.

These prior art technologies have a plurality of drawbacks, which have discouraged widespread use of integrated processes such as co-molding and co-extrusion.

For example, the use of an adhesion primer requires solvent-based components to be deposited on the products.

This may be a difficult operation for articles with complex shapes and thus require special on-site arrangements for handling these chemicals.

A class of both liquid and high-consistency silicone rubbers is also known in the art as self-adhesive silicone rubbers which, due to their molecular structure and the presence of additives therein, ensure by themselves strong adhesion to certain thermoplastic materials. Nevertheless, this type of silicone rubbers has two basic problems, i.e. they are not suitable for the manufacture of food-grade articles, and they require complex arrangements for the manufacture of transformation equipment therefor, such as molds, due to their high inherent adhesion not only to thermoplastic materials, but also to metals.

Due to this characteristics, the molds must undergo very expensive anti- adhesion surface treatments, such as Teflon coating.

The use of oxidative treatments, such as the aforementioned plasma or corona treatments, causes partial degradation of the surface of the base thermoplastic material, as well as the formation of reactive groups such as hydroxides, hydroperoxides, carboxyls, which may chemically interact with the silicone rubber.

These treatments are not easily implemented in thermoplastic molding machines, and are typically poorly stable with time.

Finally, certain patent documents suggest the introduction of specific components into the thermoplastic material, which components are able to chemically react with the silicone rubber and also provide, at the same time, interaction with the thermoplastic part, such that they act as interphase agents to ensure adhesion between the two different materials.

Nevertheless, most of these materials are chemically complex, as taught for instance in US5366806, and their structure comprises at the same time epoxy, acrylic, vinyl, allyl, etc. functions, often in combination with trialkoxysilanes, as taught for instance in US 6207769.

Particularly, the presence of these alkoxysilane groups is not easy to handle because, when they react, for example, with the surface of mineral fillers or with polar groups within the thermoplastic material, they release 3 molecules of a low molecular weight alcohol, such as methanol or ethanol.

Therefore, at the processing temperatures of thermoplastic materials which are rarely below 200 °C, strong degassing is required to prevent the occurrence of processing problems, such as irregular flows, streaks, local decomposition. Disclosure of the invention

One object of the invention is to improve the prior art.

Another object of the invention is to provide a thermoplastic compound that is able to permanently join a thermoplastic material and a silicone material of any type, even of non-self-adhesive nature.

A further object of the invention is to provide a method of making a thermoplastic compound that can be used to make a composite article, also known as "thermoplastic composite", composed of a layer of thermoplastic material irreversibly joined to a silicone material not belonging to the class of so-called self-adhesive liquid silicones.

Another object of the invention is to provide a thermoplastic compound that can be used to form products suitable for every use, including food-grade products.

A further object of the invention is to provide a thermoplastic compound that is free of any structural defects, such as streaks and local decomposition.

Yet another object of the invention is to provide a method of making a thermoplastic compound that can avoid massive degassing processes, and hence avoid the occurrence of processing problems, such as irregular flows, streaks, local decomposition.

In one aspect the invention relates to a thermoplastic compound as defined by the features of claim 1 .

In another aspect the invention relates to a method of making a thermoplastic compound as defined by the features of claim 2.

Further aspects of the invention are defined in the dependent claims.

The invention affords the following advantages:

- providing a thermoplastic compound that comprises a layer of thermoplastic material intimately joined to a layer of silicone material of any type, possibly having a non-self-adhesive nature;

- providing a thermoplastic compound that is not restricted in its use and maintains adhesion between its two constituent materials unaltered.

Brief description of the drawings Further characteristics and advantages of the invention will be more apparent from the detailed description of a preferred, non-exclusive embodiment of a thermoplastic compound, which is shown as non-limiting example in the annexed drawing, in which:

FIG. 1 is a highly schematic view of a portion of a thermoplastic compound in a step that precedes crosslinking with a silicone;

FIG. 2 is a highly schematic view of a portion of a thermoplastic compound in a step of crosslinking with a silicone;

FIG. 3 is a schematic view that shows the components of the thermoplastic compound of the invention in separate relationship.

Detailed description of a preferred embodiment

Referring to the above mentioned figures, numeral 1 generally designates a thermoplastic compound, hereinafter briefly referred to as compound 1 .

The compound 1 comprises a base 2 made of a first amount of a thermoplastic material, and a coating 3 of a silicone material, permanently associated with the base 2.

The thermoplastic material, which may be selected from the group comprising polyolefins, copolymers, polyesters, polyamides, polycarbonates, as well as from families and equivalents thereof, is added with an additional amount of oligomer means, which contain reactive functional groups, referenced 4 and 5 respectively in the figures.

The reactive functional groups 4 comprise a second amount of oligomers 4A containing hydride groups 4B and the functional groups 5 comprise a third amount of oligomers 5A containing vinyl groups 5B.

The coating 3 of silicone material contains usual silicone chains 6 having crosslinkable vinyl groups 6A and silicone crosslinking agents 7 containing hydride groups.

A catalysis step 9 by a salt or an organometallic platinum complex, e.g. platinum-catalyzed hydrosilylation, causes the reactive functional groups 4 and 5 of the oligomers to react with the silicone chains 6 and with the silicone crosslinking agents 7 thereby forming unalterable and stable chemical bonds 10 which inseparably bond together the base 2 and the coating 3, as schematically shown in Figure 2, thereby forming the thermoplastic compound 1 of the invention.

If needed, during preparation of the base 2, i.e. during addition of the reactive functional groups 4 and 5, an optional fourth amount of a mineral filler may be added, which is specially designed to partially absorb liquids and adjust availability thereof.

The thermoplastic compound 1 of the invention is preferably provided in the form of a two-layer body, with the base 2 and the coating 3 perfectly adhering to each other in such a manner as to pass the peel tests prescribed for these thermoplastic components.

The following Table 1 shows the results of a peel test, i.e. the resistance to forces applied tangent to the surfaces of the thermoplastic compound of the invention, as compared with the resistance values of reference thermoplastic compounds.

It will be noted that the adhesion force values are expressed in Newton per cm (N/cm) for each reference thermoplastic compound.

No Type Thermopla Si-H Si-vinyl Mineral Adhesion stic Fluid Fluid filler

(% by (% by (% by (N/cm) weight) weight) weight)

Ί Ref PBT 02

2 Example PBT 4 - 8

3 Example PBT - 4 - 10

4 Example PBT 2 2 2 18

The reference test 1 confirms a zero adhesion of silicone rubber to polybutylene terephthalate (PBT).

Examples 2 and 3 include the oligomer compounds of the invention individually and show their partial ability of promoting adhesion of silicone rubber to PBT. Example 4 includes both types of oligomers of the invention at the same time and shows the synergism of the components, leading to much higher adhesion values than in the preceding examples.

The method of producing the thermoplastic compound 1 comprises the steps of melt mixing a first base amount of a known thermoplastic material, which may be selected from the group comprising polyolefins, polyesters, polyamides, polycarbonates, with an additional amount of oligomer means containing reactive functional groups, namely a silicone oligomer containing hydride groups and a silicone oligomer containing vinyl groups, using mixing means, such as a single- or twin-screw extruder.

More in detail, the hydride groups may be selected from silyl hydride groups associated with the terminals of a chain, or associated along a chain, whereas the vinyl groups may be selected from linear, branched or cyclic vinyl groups.

Then the method includes the steps of homogenizing and pelletizing the mixed thermoplastic material at such a processing temperature as to ensure that it will melt or soften, to thereby obtain a thermoplastic material that is able to chemically adhere to a solid or liquid silicone adapted to be crosslinked by polyaddition, i.e. by catalysis by a salt or an organometallic platinum complex, and that is able to transfer the hydrosisylation reaction between the vinyl groups within the polymeric chain of the silicone polymer and an appropriate crosslinking agents containing hydride groups.

The method further includes the step of chemically bonding the thermoplastic material treated as described above with a coating of a silicone material, using a catalyst.

During treatment of the base thermoplastic material with the oligomers, a mineral filler, e.g. quartz or fumed or precipitated silica is added, to adjust the fluidity of the material and absorb silicone fluids.

In the production of the thermoplastic compound of the invention, the amounts of the components are as follows:

the first amount, or reference amount, is deemed to be the amount of the known base thermoplastic material.

A second amount of oligomers containing hydride groups ranges from zero to ten percent of the first amount.

A third amount of oligomers containing vinyl groups ranges from zero to ten percent of the first amount.

A fourth amount of a mineral filler ranges from zero to fifty percent of the first amount.

An illustrative, non-limiting example of a composition for preparing the base thermoplastic compound 2 to allow bonding thereof with a coating 3 of silicone material is as follows:

- 94% polybutylene terephthalate (PBT);

- 2% BLUESIL FLD 621 V 3500 fluid, manufactured by Bluestar Silicones (as the component containing vinyl groups);

- 2% MH FLUID MH15-EU fluid manufactured by Momentive Advanced Materials (as the silicone component containing hydride groups);

- 2% AEROSIL 300 manufactured by Evonik (silica mineral filler).

These components are cold-mixed together in a turbo mixer and then homogenized and pelletized in an extruder at a temperature of 280 °C.

The invention has been found to fulfill the intended objects.

The invention so conceived is susceptible to changes and variants within the inventive concept.

Also, all the details may be replaced by other technical equivalent elements. In its practical implementation, the materials in use and the amounts of each material may be changed as needed, without departure from the scope as defined by the following claims.