The invention relates to a composite material for hot molding. In particular, the material is convenient to mold paddings, e.g. of helmets, or generally cushions to absorb shocks.

A compound material is known, e.g. from WO2012140473 or EP0585965, that thanks to the fact that it contains synthetic hollow microspheres can advantageously be used for many applications. EP0585965 uses it to lighten the final product, while WO2012140473 exploits the expanding characteristics thereof during molding to better copy the mold cavity.

However, the material is not without drawbacks.

A first problem is that the material is highly hydrophilic. Its intrinsic porosity allows water to penetrate and stagnate into all of its volume, often with adverse consequences. Think e.g. when the material forms the padding of a helmet, which is in contact with sweat, rain or wash water. If subjected to high temperature (about 80 °C) the material swells up to +20% of its initial volume, making the helmet useless. Even a long transport in container under the sun would be just enough to create the same conditions of humidity and temperature to the same dimensional defect, which is obviously limitative.

One solution could be coating the molded material with a waterproofing layer. The most practical way is to spray or smear the coating, which has, however, always a solvent that enters inside the material and unfortunately actually behaves as the water that one wants to impede: substantially the material swells under the pressure of the solvent.

A second problem of the material is its poor resistance to fire. It is not taken into account that the microspheres contain flammable gas, therefore the overall speed of flame propagation is always high enough to not succeed the current safety tests. It is evident how restrictive this flaw is, and involves an immediate ban for many applications of the material, otherwise very favorable. For this material, and for all the other equivalents of its such as e.g. polystyrene, the current anti-fire solution is to coat it with a fire resistant layer or remove it altogether.

Despite the danger of fire, the microspheres find application in various sectors. E.g. US7563485 describes the production of so light a fishing line that it floats on water. It has an outer coating made of plastisol PVC with non-expanded microspheres dispersed inside, just to make them expand during the firing process and decrease the overall density of the line.

US7563485 does not disclose the use of foamed particles.

In sum, in the prior art there is no material of the above type that is resistant to fire and / or unresponsive to water.

The main object of the invention is to propose a material of the abovementioned type that has one or more of these advantageous characteristics.

The base material to be molded is preferably compound by weight by 20- 50% of expanded particles and 80-50% of non-expanded particles, the particles being made of plastic material, of closed-shape, hollow and filled with (flammable) gas. These values guarantee advantageous performance and weight, and suitable to applications, particularly the excellent results of impact absorption and lightness. The expanded particles are essential for the invention, and serve as binder or filler for the other non-expanded particles.

In fact, the expanded particles are the filler element (filler) and act as binder preventing that the other expandable particles (not expanded yet), which are heavier, precipitate by gravity on the bottom of the mold and are clustered. Instead, this way the expanded particles maintain the expandable particles suspended in all the material and in a uniform manner. That is why the presence of expanded and unexpanded microspheres ensures the density homogeneity of the whole piece, thereby ensuring uniformity of the mechanical performance.

The particles are generally spherical in shape and are very small ( 10-40 um in diameter). Note, however, that the size is not essential.

As particles, plastics microspheres, filled with gas, can be used.

An advantage of the material is that it gives the molded product shape memory. When the product undergoes a deformation, the expanded and unexpanded little spheres compress or expand in its mass. Thanks to the elasticity of the spheres, when the stress ceases every sphere goes back to its original state, and thus the material regains the starting shape. Note also that the molded product reacts to a second deformation in the same way as the first one, with obvious advantages for safety and shock response repeatability. Another advantage of the material is to have a lower density than that of EPS. Lighter items can be produced, volume being equal, or the adsorbing capacity for the same volume can be boosted. To solve the first problem, according to a first aspect of the invention before the molding a liquid elastomer may be added to the base material. The advantages of the elastomer liquid are:

- to be able to mix all of the microspheres before molding obtaining a kind of paste in which they are, after mixing, homogeneously dispersed or dispersible. By coating each particle one has the guarantee that the material is homogeneous, and a protective film is created in all the material against the entry of moisture, that would make it fragile. A powder elastomer would fail in this sense, preventing the molding; because it would not end up coating all the particles but it will locate between them, and would tend to gather on the bottom of the mold. The liquid elastomer gives warranty of total coating, powder does not. The elastomer increases the resistance of the material, which without is fragile;

- to convey water resistance and flexibility to the material;

- to be able to give a desired color to the molded product;

- to coat a whole particle with a minimum quantity of elastomer;

- to isolate after molding the base material by water, avoiding infiltration and thus solving the first problem;

- to form a matrix which incorporates the base material dispersed in it, so as to neutralize the intrinsic brittleness of the base material molded alone;

- to confer anti-flame properties to the molded product mitigating also the second problem.

The elastomer may be atomized and sprayed over the particles, to wet them all and evenly.

As liquid elastomer, one can use

- preferably liquid PVC (powder PVC + a plasticizer, e.g. Plastisol®), which has the advantage of not containing water and to cure forming a solid and compact molded piece. By plasticizer it is meant a plasticizer for plastics, a substance added to the material to decrease the stiffness of the polymer so as to allow processing thereof at room temperature or at temperatures sufficiently low so as not to risk the thermal degradation of the polymer during processing. As a plasticizer one can use: organic phosphates, organic esters of the phosphoric acid, phthalates, adipates, sebacates, esters of fatty acids, esters of adipic acid, of sebacic acid and of other fatty acids, esters of glycerine; - a polyurethane, e.g. like Cellasto® or Elastollan® of BASF Company. The selection criterion is the viscoelastic ratio and hardness after molding, paying attention to its poor stability in temperature (a window of about 20 °C);

- rubber or caoutchouc, but paying attention to their content of water, being emulsions;

- a silicone, e.g. BLUESIL® of Bluestar company. The criterion of choice is the viscoelastic ratio and hardness after molding, paying attention though to the poor tendency to make the microspheres expand during molding.

It is preferable that the elastomer allows the microspheres of the material to expand during molding, and expand with them in the same way (same firing and expansion speed). The polymer then would rather be liquid or semi-liquid until its cooling; this behavior is guaranteed by the PVC and in part by the polyurethane.

Advantageously the material can be used as an expanding core in the molding of the composite material (e.g. carbon or glass fiber, aramid), because it is not subject to shrinkage and therefore does not allow the composite, at the end of molding, to shrink. When the PVC is consolidating definitively (gelling) the expanding microspheres are still pushing the composite against the walls of the mold, indeed compensating the volume change in carbon and thus eliminating shrinkages thereof.

A composition by weight with good features in the final molded material is:

liquid elastomer, preferably in the form of plasticizer and powder PVC: between 50 and 95%;

expanded and unexpanded microspheres: between 5 and 50% (where the % of expanded microspheres is about 25% and the % of unexpanded ones is about the remaining 75%).

With the material of the invention there is no more absorption of water, and therefore there is no longer any bulging.

To solve the second problem, according to a second aspect of the invention an anti-flame inorganic substance or element may be added to the base material before the molding. The anti-flame substance or element is advantageously selected inorganic to avoid affinity with the microspheres, so as to let them expand freely without slowing the expansion thereof and therefore compromising their performance. If the anti-flame substance interacted with the microspheres and/ or bound to the microspheres, the anti-flame-property could fail.

As anti-flame inorganic substance or element one may use a phosphate and/ or preferably tri-hydrated alumina, which performs well with the microspheres when the firing is over. When tri-hydrated alumina heats, it releases water that slows propagation of a flame.

Other usable elements are those based on boron or borates.

In general for the anti-flame substance or element one may use

- a halogen-based substance, and/or

- a substance comprising molecules capable of releasing water during combustion (such as tri-hydrated alumina and derivatives of boric acid).

A composition by weight found to be effective is:

plasticizer + PVC: 80 to 90%, preferably 84%,

expanded and unexpanded microspheres: 8 to 16%, preferably 12% (in which the % of expanded microspheres is about 25% and the % of expanded one is 75%),

anti-flame substance: 2 to 6%, preferably 4%.

A typical hot molding cycle (firing) for the material of the invention is as follows:

1. a quantity of material is deposited in a mold;

2. the mold is closed; the material is brought to about 180 °C for about 2-5 minutes (the firing temperature in the mold may range from 100 °C to 200 °C);

3. the material solidifies; in particular the (plasticizer + PVC) becomes gel;

4. the material is allowed to cool before removing it from the mold.

The firing temperature should be chosen depending on the speed of desired expansion for the particles (it is proportional). E.g. for the composite's molding it is preferred a temperature toward the lower bound of the range, because the curing of the resin, which occurs more slowly, is waited for. The flame speed for a material according to the invention was measured experimentally. From 160 mm/min of a conventional material it drops to 70 mm/min.

The material may include both the anti-flame substance and the liquid elastomer, to obtain a material which solves or mitigates simultaneously the two above-mentioned problems.

A third aspect of the invention relates to an expanding core insertable into a mold to mold an object in composite material, e.g. comprising carbon, comprising, or consisting solely of, the material according to any one of the previous variants.