Thermoplastic foams made with methyl formate-based blowing agents

IS 2 365 488 T3

DESCRIPTION

Thermoplastic foams made with methyl formate-based blowing agents.

Field of the invention

The present invention relates generally to foams in which mixtures or compositions of blowing agents are used and to processes for their preparation. More specifically, the present invention relates to thermoplastic polymeric foams in which mixtures of blowing agents based on methyl formate are used and which produce stable foams and to processes for their preparation.

Background of the invention

Polymeric foams are usually prepared using a pressure cast resin blowing agent and extruding the blend, after thorough mixing, through a suitable die in a low pressure atmosphere.

Chlorofluorocarbons and hydrochlorofluorocarbons have been used extensively in the past as physical blowing agents for the preparation of foams. However, the use of such blowing agents is likely to be prohibited due to their high ozone depletion potential (ODP). Some of these blowing agents can be replaced by hydrofluorocarbons (HFCs), which have zero ODP. However, HFCs are associated with a high global warming potential (GWP).

At present, the most frequently used physical blowing agents for the preparation of thermoplastic polymeric foams, such as alkenylaromatic polymeric foams (eg, polystyrene) or polyolefin polymeric (eg, polyethylene), are butanes and isopentanes. However, hydrocarbons with three or more carbon atoms are considered volatile organic compounds (VOCs) that lead to industrial haze. In addition, some compounds currently used in blowing agent compositions are Hazardous Air Pollutants (HAPs). The use of VOC and / or HAP for the preparation of polymeric foams is not environmentally preferred and imposes many limitations on the production process, thus significantly complicating and increasing the cost of production. Ethane is classified as non-VOC due to its very low photochemical reactivity. The photochemical reactivity of methyl formate is even lower than that of ethane and therefore can be considered to have a negligible propensity for industrial haze. Additionally, methyl formate is classified as non-HAP and exhibits zero ODP and close to zero GWP. Therefore, a mixture of methyl formate with any of the blowing agents currently in use can help offset the detrimental environmental impacts (ODP, GWP, HAP, VOC) associated with currently used blowing agents. US 3,914,191 describes the use of a methyl formate / trichloromonofluoromethane azeotropic mixture as a blowing agent in the production of polystyrene foam. US Pat. No. 6,753,357 describes the use of methyl formate to produce stable and rigid isocyanate / polyol-based polyurethane foams. Such foams are thermosetting foams prepared by a crosslinking and curing process, and the dimensional stability or instability conferred on the final foam product by the nature of the blowing agent is quite different than in the case of melt processed thermoplastic polymeric foams. Therefore, there is a demand for blowing agents in which methyl formate is used as the sole component or as one of the components of the blowing agent mixture to produce stable thermoplastic foams without compromising product quality in terms of appearance, the mechanical or compression resistance and the degree of insulation are concerned, and that allow to carry out an economical and versatile production process.

Summary of the invention

In accordance with one embodiment of the present invention, a preferred blowing agent mixture for the preparation of thermoplastic polymeric foams comprises methyl formate. The blowing agent mixture may further comprise at least one co-blowing agent. The co-blowing agent is a physical co-blowing agent (for example, an inorganic agent, a hydrocarbon, a halogenated hydrocarbon, an ether, an ester, an acetal, an alkanol, a carbonate, an amine, a ketone, water or any combination thereof), a chemical co-blowing agent, or combinations thereof. In a preferred embodiment, the thermoplastic polymeric foam is an alkenylaromatic polymeric foam. In a more preferred embodiment, the alkenyl aromatic polymer foam is a polystyrene foam. The preferred embodiment blowing agent mixture comprises 100 mole% methyl formate or may comprise any combination of methyl formate

ES 2 365 488 T3 with one or more co-blowing agents. According to another embodiment, a thermoplastic polymeric foam structure is prepared by a process comprising melting a thermoplastic polymer. An effective amount of a blowing agent mixture is dissolved in the polymeric melt. The blowing agent mixture comprises methyl formate and optionally at least one co-blowing agent. The co-blowing agent is a physical co-blowing agent (for example, an inorganic agent, a hydrocarbon, a halogenated hydrocarbon, an ether, an ester, an acetal, an alkanol, a carbonate, an amine, a ketone, water, or any combination thereof), a chemical co-blowing agent, or combinations thereof. An extrudate is formed that expands to produce the polymeric foam structure. For example, in accordance with a preferred embodiment of the present invention, the extrudate is initially transferred to an expansion zone and allowed to expand in the expansion zone.

According to a process of the present invention, a thermoplastic polymeric foam structure is produced by melting a thermoplastic polymer. An effective amount of a mixture of blowing agents is dissolved in the alkenyl aromatic polymer melt. The blowing agent mixture comprises methyl formate and optionally at least one co-blowing agent. The co-blowing agent is a physical co-blowing agent (for example, an inorganic agent, a hydrocarbon, a halogenated hydrocarbon, an ether, an ester, an acetal, an alkanol, a carbonate, an amine, a ketone, water or any combination thereof), a chemical co-blowing agent, or combinations thereof. An extrudate is formed which expands to produce the polymeric foam structure. For example, in accordance with a preferred embodiment of the present invention, the extrudate is initially transferred to an expansion zone and allowed to expand in the expansion zone.

The polymeric foam structure obtained by the process of the present invention is preferably a substantially closed cell structure and / or with dimensional stability. In a preferred embodiment, the alkenyl aromatic foam structure comprises a polystyrene polymer.

Brief description of the drawings

Figure 1 is a schematic flow diagram of the overall sequence of operations involved in the production of a foamed sheet with the blowing agent mixtures in accordance with one embodiment of the present invention.

Description of illustrative embodiments

The effectiveness of a blowing agent depends on its solubility in the polymeric melt and its ability to expand the polymer / blowing agent solution when such a solution is subjected to thermodynamic instability, such as when the solution exits a die attached to an extruder ( to provide the extrudate). The expansion of the extrudate depends on the difference between the glass transition temperature of the thermoplastic polymer Tg and the boiling point of the blowing agent Tb. In general, the solubility of the blowing agent in the polymeric melt depends on the difference between Tg and Tb (Tg-Tb); the smaller the difference, the greater the solubility. Since volatility is inversely proportional to Tb, this also implies that a more volatile blowing agent will exhibit, under the same conditions of temperature and pressure, a lower solubility than a less volatile blowing agent. Therefore, it appears that by mixing a less volatile blowing agent with a more volatile blowing agent, a foaming formulation with optimized solubility and expandability characteristics can be developed.

A mixture or composition of blowing agents is used in the foams and processes of the present invention to obtain a stable thermoplastic polymeric foam. The blowing agent mixture used in the present invention is based on methyl formate. Methyl formate is not classified as PAH and is the subject of a proposed standard by the US Environmental Protection Agency. (Federal Register, Volume 68, Number 170, September 3, 2003) for its classification as a non-VOC due to its negligible photochemical reactivity and thus its negligible contribution to industrial haze formation. Specifically, eliminating PAHs and minimizing the propensity for the formation of industrial haze in the production process and the resulting foam is not only safer and more environmentally friendly, but also allows to obtain a more economical and versatile process and product, avoiding this many of the drawbacks of currently used blowing agent compositions and processes.

Foamable resins in accordance with the present invention include melt-processable thermoplastic polymers, such as alkenylaromatic polymers, polyolefins, polycarbonates, polyacrylates, and others. The term "thermoplastic polymer" includes both amorphous and semi-crystalline polymers. Examples of

ES 2 365 488 T3 amorphous thermoplastic polymers include, but are not limited to, polystyrene, polycarbonate, poly (methyl methacrylate) and poly (phenylene oxide). Examples of semi-crystalline thermoplastic polymers include, but are not limited to, polyethylene, polypropylene, syndiotactic polystyrene, polyethylene terephthalate.

One embodiment of the present invention relates to alkenylaromatic polymers. The term "alkenylaromatic polymer" as used herein includes polymers of aromatic hydrocarbon molecules that contain an aryl group attached to an olefinic group with only double bonds in the linear structure, such as styrene or styrene homologues, such as α- methylstyrene, o-, m- and p-methylstyrene, α-ethylstyrene, o-, m-, p-ethylstyrene, 2,4-dimethylstyrene, α-vinylxylene and vinyl toluene. Alkenylaromatic polymers also include styrene homopolymers or styrene homologues (commonly referred to as polystyrene), copolymers of styrene, and rubber-toughened polystyrene (commonly referred to as high impact polystyrene, HIPS). Regarding the styrene copolymer, the comonomer can generally be any other ethylenically unsaturated material, such as the 1,3-conjugated dienes, for example butadiene, isoprene, alpha-beta-unsaturated monocarboxylic acids and derivatives thereof, for example acrylic acid. , methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate and the corresponding esters of methacrylic acid, acrylamide, methacrylamide, acrylonitrile and methacrylonitrile. If desired, blends of a styrene polymer with other polymers can be used, for example blends of a styrene polymer with poly (phenylene oxide). Preferably, the copolymers contain a predominant portion of styrene, for example greater than about 50% by weight styrene, more preferably greater than 75% styrene.

The blowing agent mixture comprises between about 1% and about 100% methyl formate. In one embodiment, the blowing agent mixture comprises 100% methyl formate. In another embodiment, however, the blowing agent mixture comprises less than 100% methyl formate and additionally comprises at least one co-blowing agent. The use of more than one co-blowing agent in the blowing agent mixture is contemplated. Such co-blowing agent (s) may be physical, chemical, or combinations thereof.

The co-blowing agent generally expands rapidly compared to a pure methyl formate blowing agent. The co-blowing agent can be an organic compound or an inorganic compound. Some non-limiting examples of physical co-blowing agents include, but are not limited to, inorganic agents, organic agents (e.g. hydrocarbons, halogenated hydrocarbons, ethers, esters, acetals, alkanols, carbonates, amines, and ketones), or any combination of the themselves.

Some suitable inorganic physical blowing agents include, but are not limited to, carbon dioxide, water, air, nitrogen, argon, xenon, sulfur hexafluoride, nitrous oxide, ammonia, silicon tetrafluoride, nitrogen trifluoride, boron trifluoride, and trichloride. boron, or any combination thereof. In an especially preferred embodiment, the inorganic agent is an inorganic gas such as carbon dioxide, nitrogen, argon, and air. An especially preferred inorganic gas is carbon dioxide. In another especially preferred embodiment, the inorganic agent is water.

Some examples of organic physical co-blowing agents that can be used in the present invention include, but are not limited to, hydrocarbons, halogenated hydrocarbons, liquids with polar groups such as ethers, esters, acetals, carbonates, alkanols, amines, and ketones, and combinations thereof. Examples of hydrocarbons include, but are not limited to, methane, ethane, propane, cyclopropane, normal- (n-) or iso-butane, cyclobutane, neopentane, and isopentane, or any combination thereof. Halogenated hydrocarbons include, but are not limited to, methyl fluoride, difluoromethane (HFC-32), trifluoromethane (HFC-23), perfluoromethane, chlorodifluoromethane (HCFC-22), methylene chloride, ethyl chloride, ethyl fluoride, 1,2-difluoroethane (HFC-152), 1,1-difluoroethane (HFC-152a), 1,1,1-trifluoroethane (HFC-143a), 1,1,2,2-tetrafluoroethane (HFC-134), 1, 1,1,2-tetrafluoroethane (HFC-134a), pentafluoroethane (HFC-125), perfluoroethane, 1,1-dichloro-1 fluoroethane (HCFC-141b), 1-chloro-1,1-difluoroethane (HCFC-142b), 1,1-dichloro-2,2,2-trifluoroethane (HCFC-123) and 1-chloro-1,2,2,2-tetrafluoroethane (HCFC-124) , Difluoropropane, 1,1,1-trifluoropropane, 1,1,1,3,3-pentafluoropropane (HFC-245fa), 1,1,1,2,3,3-hexafluoropropane (HFC-236ea), 1,1 , 1,2,3,3,3-heptafluoropropane (HFC-227ea), perfluoropropane, 2,2,4,4,4-pentafluorobutane (HFC-365mfc), perfluorobutane, perfluorocyclobutane and vinyl fluoride, or any combination of the themselves. Liquids with polar groups include, but are not limited to, ethers, such as dimethyl ether, vinyl methyl ether, methyl ethyl ether, dimethyl fluoroether, diethyl fluoroether, and perfluorotetrahydrofuran; amines, such as dimethylamine, trimethylamine, and ethylamine; ketones, such as acetone and perfluoroacetone; esters, such as ethyl formate and methyl acetate; acetals, such as methylal; carbonates, such as dimethyl carbonate; alkanols, such as ethanol, or any combination thereof.

IS 2 365 488 T3

The boiling point of methyl formate is 32 ° C. In another preferred embodiment of the present invention that is applied to alkenyl aromatic polymers, methyl formate is mixed with one or more physical co-blowing agents with a boiling point below 32 ° C.

Chemical co-blowing agents are compounds that undergo a chemical reaction, for example decomposition, to produce an inorganic gas, such as CO2 or N2. Non-limiting examples of suitable chemical co-blowing agents include azodicarbonamide, azodiisobutyronitrile, benzenesulfonyl-hydrazide, 4,4-oxobenzenesulfonyl-semicarbazide, p-toluenesulfonyl-semicarbazide, barium azodicarboxylate, N, N'-dimethyl-N-hydrazide compounds, N'drephthazine compounds, and other trihydrozine compounds. azo, N-nitroso, carbonate and sulfonylhydrazides. There are also various acid / bicarbonate mixtures that decompose into gases when heated. For example, mixtures of citric acid and sodium bicarbonate marketed under the name HYDROCEROL® can be used as chemical co-blowing agents.

The total amount of blowing agent mixture used depends on conditions such as the extrusion process conditions during mixing, the blowing agent mixture used, the composition of the extrudate and the desired density and associated properties such as degree of insulation, the weight / strength ratio, and the compressive strength of the foam article. The extrudate is defined herein as including the blend of blowing agents, polymeric resin (s), and other additives. For a foam having a density of about 0.016 to about 0.240 g / cm³, the extrudate typically comprises between about 18 and about 1% by weight of blowing agent.

The blowing agent mixture used in the present invention comprises 100% methyl formate or may comprise less than about 99 mole% methyl formate. The blowing agent mixture generally comprises between about 5 mole% and about 75 or 80 mole% methyl formate. The blowing agent mixture more typically comprises between about 20 or 25 mole% and about 60 mole% methyl formate. More specifically, the blowing agent mixture preferably comprises between about 20 or 25 mole% and about 50 mole% methyl formate.

If applicable, the blowing agent mixture generally comprises at least about 20 or 25 mole% co-blowing agent (s). The blowing agent mixture more typically comprises between about 80 or 75 mole% and about 40 mole% of co-blowing agent (s). More specifically, the blowing agent mixture preferably comprises between about 80 or 75% and about 50 mole% of co-blowing agent (s).

A nucleating agent or a combination of such agents can be used in the present invention for its advantages such as its ability to regulate alveolar formation and morphology. A nucleating agent, or cell size control agent, can be any conventional or useful nucleating agent. The amount of nucleating agent used depends on the desired cell size, the selected blowing agent mixture, and the desired foam density. The nucleating agent is generally added in amounts of from about 0.02 to about 2.0% by weight of the polymeric resin composition.

Some nucleating agents that are considered include inorganic materials (in the form of small particles), such as clay, talc, silica, and diatomaceous earth. For example, talc can be used in an amount of from about 0.25 to about 1.0% by weight of the polymer composition. Other examples of nucleating agents include organic nucleating agents that decompose or react at heating temperature inside an extruder to evolve gases, such as carbon dioxide and / or nitrogen. An example is a combination of an alkali metal salt of a polycarboxylic acid with a carbonate or bicarbonate. Some examples of alkali metal salts of a polycarboxylic acid include, but are not limited to, the monosodium salt of 2,3-dihydroxybutanedioic acid (commonly referred to as sodium hydrogen tartrate), the monopotassium salt of butanedioic acid (commonly referred to as potassium hydrogen succinate), trisodium and tripotassium salts of 2-hydroxy-1,2,3-propanetricarboxylic acid (commonly referred to as sodium and potassium citrate, respectively) and the disodium salt of ethanedioic acid (commonly referred to as sodium oxalate), or a polycarboxylic acid such as 2-hydroxy-1,2,3-propanetricarboxylic acid. Some examples of a carbonate or bicarbonate include, but are not limited to, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, and calcium carbonate.

The addition of mixtures of different nucleating agents is contemplated in the present invention. Some more desirable nucleating agents include talc, crystalline silica, and a stoichiometric mixture of citric acid and

ES 2 365 488 T3 sodium bicarbonate (the stoichiometric mixture presenting a concentration of 1 to 100% when the vehicle is a suitable polymer). The talc can be added in a vehicle or in powder form.

If desired, fillers, colorants, light and thermal stabilizers, antioxidants, acid neutralizers, stability control agents, flame retardants, processing aids, extrusion aids and additives can be used for foam preparation. sparkling.

Any of a number of suitable extrusion systems and procedures known in the art can be used in accordance with the present invention. An example of a suitable extrusion system and method includes, for example, a conventional tandem two extruder system in which each extruder has a single screw. Alternatively, a system of two tandem extruders in which the first extruder is a twin screw and the second extruder is a single screw can be used to extrude the foam article of the present invention. A single extruder with appropriate cooling can also be used in the present invention.

In accordance with a process of the present invention, granules of the thermoplastic polymer (eg, polystyrene) are mixed with a nucleating agent, such as talc. These materials are continuously fed into a hopper of an extruder. The feed composition is conveyed by a screw into a barrel of the extruder while the composition is mixed, compressed, heated, and converted to the molten form. Conversion to the molten form occurs before reaching an injection zone where the blowing agent is added. The blowing agent mixture of the present invention can be injected into the polymer composition at a point where the polymer is in a molten state (ie, beyond the feed zone). If the blowing agent mixture comprises more than one component (i.e., methyl formate and one or more co-blowing agents), each of the components can be injected into the polymer composition individually, both successively and simultaneously, and in any order. Alternatively, the components of the blowing agent mixture can be premixed and the mixture can be injected into the polymer composition.

After injecting the blowing agent mixture, the composition is continuously mixed under pressure to ensure a homogeneous solution of the resin and blowing agent mixture. The molten composition is then transported to the cooling zone, where further mixing takes place. After cooling, the composition is extruded into a dwell zone maintained at a temperature and pressure that prevent or inhibit foaming of the composition. The dwelling zone has (a) an outlet nozzle with an orifice that opens into a zone of lower pressure, such as atmospheric pressure, at which the composition foams, (b) means for closing the orifice without altering the foamable composition within the dwelling zone and (c) opening means to allow expulsion of the foamable composition from the dwelling zone. An example of a dwell zone is described in US Patent No. 4,323,528. Regardless of whether a dwell zone is used, the composition is then extruded through a die into a zone of lower pressure, such as atmospheric pressure.

According to one embodiment that is applied to alkenylaromatic polymers such as polystyrene, a system of two tandem extruders 10, shown in Figure 1, can be used for the extrusion of a foam article (for example, a sheet) of the present invention. The polymeric resin granules are mixed, if desired, with one or more additives (e.g., a nucleating agent and / or a stability control agent) to form a feed composition that is continuously fed into a hopper 11 of a first extruder 13. The feed composition is conveyed by a helical screw in a barrel of the extruder while the feed composition is mixed, compressed, heated and melted before reaching the blowing agent injection zone. The blowing agent mixture (comprising methyl formate and optionally including at least one co-blowing agent) is added at point 15. Thus, the blowing agent mixture of the present invention is injected into the polymer / additive composition (feed composition) at a point downstream of the feed zone where the polymer melts. If desired, the blowing agent mixture can be injected at other locations, including a second extruder.

After injection of the blowing agent mixture, the composition is continuously mixed in the first extruder 13. The outlet pressure of the first extruder 13 of the exemplary embodiment is generally in the range of about 13.8 MPa to about 27.6 MPa. The temperature of the first extruder 13 of the exemplary embodiment is generally in the range of about 204 to about 246 ° C. The composition is then transferred, at a pressure high enough so that the blowing agent mixture remains in solution, through a hollow adapter section 17 to

ES 2 365 488 T3 a second cooled tandem extruder 19. The molten composition is passed at low shear through the second chilled extruder, where cooling and further mixing are performed. The outlet pressure of the second extruder 19 of the exemplary embodiment is generally in the range of about 6.9 MPa to about 17.2 MPa. The temperature of the extrudate from the second extruder 19 of the exemplary embodiment is generally in the range of about 121 to about 160 ° C. In general, the temperature of the first extruder should be sufficient to melt the polymer and all additives and to promote efficient mixing. The temperature and pressure in the second extruder should be sufficient to keep the polymer and blowing agent mixture in the form of a homogeneous solution. It is understood that the temperatures, pressures and other conditions described may vary depending on the properties of the thermoplastic polymer used in the process. The specific conditions to be used are apparent to one of ordinary skill in the art.

The composition is then forced through an annular nozzle 21 in the form of an elongated blister or tube 23. In Figure 1, the foamable polymer is expanded and dumped onto a cylindrical surface of a cooling and gauge drum 25 and cuts to form a sheet 27. Sheet 27 is wound onto one or more bobbins 29. Alternatively, the foamable composition is forced through a nozzle of different configuration, such as a flat nozzle, and allowed to expand in the form of a sheet or board.

If the article produced is a sheet, the thickness can be up to about 1/2 inch. If the item produced is a board, the thickness is generally more than 1.27 cm. Articles produced from the extruded tube generally have a thickness of about 0.051 to about 0.645 cm.

Depending on the materials and procedures used, the resulting foamed article generally has a density of from about 0.016 to about 0.240 g / cm³; more typically, from about 0.032 to about 0.144 g / cm³. Furthermore, and in accordance with a preferred embodiment of the invention, the resulting foamed article possesses a substantially closed cell structure and is defined herein as a foam having greater than about 85% closed cells and, more typically, more than about 95% closed alveoli. Alternatively, and in accordance with another aspect of the invention, the resulting foamed article can be formed with 15% or more open cells, for example 20%, 25%, 30% or more open cells. Furthermore, the resulting foam structure can be controlled to comprise at least about 8 cells per cm, for example at least about 10, 12, 14, 16, 18 or 20 cells per cm.

The foam of the present invention can be used for insulation or as a construction material, in various containers and packaging systems, or as protective or flexible packaging. Generally speaking, foam sheets are used in both flexible and rigid packaging, while foam boards are used in protective packaging. In addition to foam sheets and boards, the present invention can take other forms, such as rods, tubes, or profiled elements.

Other uses for the foams of the present invention, as well as methods, apparatus, equipment, devices and systems suitable for their preparation.

The resulting foam of the present invention is desired to be "dimensionally stable". As defined herein, dimensional stability exists when the density or gauge of the foam deviates by no more than about 15%, preferably no more than 10%, and especially preferably no more than 5% of the density or gauge. foam after aging for 3 to 7 days.

Examples

Different blowing agents were tested, with the results shown below in Table 1. Specifically, various alkenylaromatic polymeric foams were prepared from comparative blowing agents and blowing agent mixtures according to the invention according to the extrusion process described generally in the present memory. It should be noted that in the different examples shown in table 1 the equipment was the same and operated in exactly the same way; the only variable was the mixture of blowing agents. All blowing agent mixtures according to the invention included methyl formate; the comparative blowing agent (s) did not include methyl formate.

Each of the alkenylaromatic polymeric foams was prepared on a tandem extrusion line using

ES 2 365 488 T3 6.35 and 8.9 cm single screw extruders, and the blowing agent was injected through a single port into the first extruder. The polymeric resin used was high temperature resistant polystyrene for general purposes, with a density of 1.05 g / cm³ and a melt index of 1.6 g / 10 min at 200 ° C and under a load of 5 kg. In addition to the blowing agents and polystyrene resin, talc was added in an amount of up to 2% by weight of the total foaming composition, which includes all blowing agent (s), polymeric resin (s) and additives.

TABLE 1

<td>Sample</td><td colspan="8">Blowing agent (s) used (% by weight)¹</td><td>Such- co</td><td>Density</td><td>Alve olos open rtos</td><td>Size alve olar year⁴</td><td colspan="2">% caliber variation⁵</td>

<td>Comp / inv²</td><td>Etan or</td><td>Propane</td><td>Isobutan or</td><td>n- Butan or</td><td>Isopentane</td><td>CO₂</td><td>FM³</td><td>H₂OR</td><td>% p¹</td><td>g / cm³</td><td> %</td><td>p.m</td><td>1 hour</td><td>7 d</td>

<td>Comp 1</td><td></td><td></td><td></td><td></td><td> 5,20</td><td></td><td></td><td></td><td> 1,8</td><td> 0,088</td><td> 1,8</td><td> 200</td><td> 0,8</td><td> 11,1</td>

<td>Comp 1</td><td></td><td></td><td></td><td></td><td> 3,96</td><td> 0,77</td><td></td><td></td><td> 1,0</td><td> 0,075 2</td><td> 1,0</td><td> 196</td><td> -7,9</td><td> 12,9</td>

<td>Inv 1</td><td></td><td></td><td></td><td></td><td></td><td></td><td> 4,30</td><td></td><td> 1,9</td><td> 0,13</td><td> 2,2</td><td> 187</td><td> -0,3</td><td> 5,6</td>

<td>Inv 2</td><td></td><td></td><td></td><td></td><td></td><td> 0,76</td><td> 3,32</td><td></td><td> 0,5</td><td> 0,09</td><td> 5,6</td><td> 209</td><td> 10,6</td><td> -3,9</td>

<td>Inv 3</td><td></td><td></td><td></td><td></td><td></td><td> 0,50</td><td> 1,95</td><td> 0,50</td><td> 0,7</td><td> 0,138</td><td> 23,5</td><td> 170</td><td> -0,3</td><td> 21,7</td>

<td>Inv 4</td><td> 0,56</td><td></td><td></td><td></td><td></td><td></td><td> 3,52</td><td></td><td> 0,7</td><td> 0,093</td><td> 2,2</td><td> 234</td><td> -1,1</td><td> 4,7</td>

<td>Inv 5</td><td> 1,00</td><td></td><td></td><td></td><td></td><td></td><td> 2,40</td><td></td><td> 0,6</td><td> 0,066</td><td> 1,8</td><td> 179</td><td> -0,1</td><td> 7,6</td>

<td>Inv 6</td><td></td><td> 1,22</td><td></td><td></td><td></td><td> 0,55</td><td> 1,97</td><td></td><td> 0,5</td><td> 0,07</td><td> 3,0</td><td> 210</td><td> -6,6</td><td> 9,0</td>

<td>Inv 7</td><td></td><td> 2,01</td><td></td><td></td><td></td><td> 0,37</td><td> 1,21</td><td></td><td> 0,3</td><td> 0,057 6</td><td> 1,5</td><td> 224</td><td> 0,1</td><td> 15,1</td>

<td>Inv 8</td><td></td><td> 2,67</td><td></td><td></td><td></td><td> 0,34</td><td> 0,34</td><td></td><td> 0,3</td><td> 0,056</td><td> 3,5</td><td> 254</td><td> 0,6</td><td> 9,5</td>

<td>Inv 9</td><td></td><td></td><td> 3,57</td><td></td><td></td><td> 0,35</td><td> 0,27</td><td></td><td> 1,0</td><td> 0,066</td><td> 1,8</td><td> 194</td><td> 0,1</td><td> 11,9</td>

<td>Inv 10</td><td></td><td></td><td> 3,04</td><td></td><td></td><td> 0,35</td><td> 0,74</td><td></td><td> 1,0</td><td> 0,064</td><td> 1,9</td><td> 197</td><td> -0,3</td><td> 13,2</td>

<td>Inv 11</td><td></td><td></td><td> 2,65</td><td></td><td></td><td> 0,31</td><td> 1,17</td><td></td><td> 1,0</td><td> 0,062</td><td> 0,9</td><td> 166</td><td> 0,5</td><td> 13,7</td>

<td>Inv 12</td><td></td><td></td><td> 2,03</td><td></td><td></td><td> 0,37</td><td> 1,82</td><td></td><td> 1,0</td><td> 0,078</td><td> 2,5</td><td> 183</td><td> -3,5</td><td> 22,1</td>

<td>Inv 13</td><td></td><td></td><td></td><td></td><td> 2,79</td><td> 0,78</td><td> 1,00</td><td></td><td> 1,0</td><td> 0,085</td><td> 3,0</td><td> 180</td><td> -5,5</td><td> 10,7</td>

<td>Inv 14</td><td></td><td></td><td></td><td> 2,63</td><td></td><td> 0,35</td><td> 1,20</td><td></td><td> 1,4</td><td> 0,075</td><td> 1,7</td><td> 163</td><td> -7,7</td><td> 18,4</td>

1. % by weight = (weight of one component) / (total weight of foaming composition, including all blowing agent (s), polymeric resin (s) and additives)

2. Comp - comparative example; inv - example according to the invention

3. FM - methyl formate

Four. The number of cells per cm of the extruded foam ranges from 82 to 165. The alveolar size (expressed in diameter) was determined from a scanning electron microscope image of the extruded sheet aged for at least 24 hours and then expanded in the z direction (along the thickness direction) in a bath. of oil at 115.5 ° C for 2 minutes, while compressing in the x and y directions; the number of alveoli per cm in these additionally expanded samples ranged from 43.3 to 82.5.

5. % caliber variation = 100 x (aged caliber - initial caliber) / initial caliber; the initial gauge was determined within 15 minutes of extrusion.

All of the above foams in Table 1 exhibited dimensional stability as no additional significant change in caliper was observed after aging for 3 to 7 days. It should be noted that all the foams in Table 1, except that of Example 2 according to the invention, showed post-extrusion growth. This one-way change is different from the conventionally used definition of dimensional stability according to which the foam cannot contract or expand over time. The compositions described herein provide stable foam structures, produced by an inexpensive and environmentally friendly process. Furthermore, in accordance with the present invention, a variety of foams can be formed having suitable and desired characteristics. For example, composition 3 according to the invention contains the highest percentage of open cells and is therefore advantageous since the flammability properties of the foam decrease as the percentage of open cells increases due to the rapid loss of the foam. flammable component (s) of the blowing agent mixture. In another example, compositions 2 to 5 according to the invention include components with the lowest and negligible impact on air quality. Comparative Example 2 is a typical formulation widely used in the preparation of leaves of

ES 2 365 488 T3 polystyrene foam or expanded beads. Other variants of this formulation, again very widespread, are those in which isopentane has been replaced by isobutane or n-butane. Examples 6 to 14 according to the invention demonstrate how foam sheets (and, by extension, expanded beads) with similar characteristics can be prepared using a formulation in which the use of the VOC hydrocarbon blowing agent has been greatly reduced. Furthermore, it should be noted that the total number of moles of the blowing agent in all the formulations in Table 1 is the same (approximately 0.07 moles per 100 g of total material processed). Obtaining foams with different densities thus simply reflects the v effective speed of the blowing agent mixture. It is obvious to one skilled in the art that foams with lower densities can be obtained by changing the composition of the mixture and enriching it in the component (s) of higher volatility, and that the density can be further reduced by using a higher number. moles of the blowing agent mixture. Each of the foams according to the invention in Table 1 gives rise to a foam that is stable and easy to manufacture and handle.