METHOD FOR MANUFACTURING SANDWICH BUILDING ELEMENTS

Description

[0001] The invention relates to a method for producing sandwich construction elements from at least two spaced-apart building material panels with a polyurethane foam core arranged between the building material panels, as well as to the sandwich construction elements obtainable by the method.

[0002] Sandwich construction elements consisting of at least two spaced-apart building material panels, e.g., concrete slabs, with an intermediate foam core acting as an insulating layer are known. For example, WO 00 / 15685 and WO 2009 / 077490 describe a construction element with a rigid polyurethane foam as the insulating layer. Such sandwich constructions attempt to meet the requirements of modern building envelopes with regard to load-bearing capacity, thermal insulation, durability, and sustainability. To manufacture the sandwich construction elements, prefabricated rigid polyurethane foam panels are used, which are inserted by hand between the building material panels, or the foam cores are foamed between the concrete slabs, with the foaming occurring by pouring (see Bauingenieur 88, 412-419 and Composite Structures 121 (2015) 271-279). It is also known to laminate the polyurethane panels with aluminum foil.In this way, the polyurethane panels can be made diffusion-tight and more resistant to aging.

[0003] Ali Shams et al. in Composite Structures 121 (2015) 271-279 disclose the production of sandwich elements by pouring liquid polyurethane material between two concrete slabs fixed in a mold, closing the mold, and curing the foam.

[0004] The foam cores of conventional sandwich construction elements exhibit unavoidable anisotropy, meaning their mechanical properties vary with the orientation of the foam. This anisotropy results directly from the manufacturing process of the foam cores. Foams usually exhibit different mechanical properties in the main expansion direction (the direction of rise) than in a direction perpendicular to it. Cells grow differently in the direction of rise than perpendicular to the direction of rise. To achieve consistent mechanical properties regardless of orientation, foam cores with low anisotropy are desirable. Excessive anisotropy of the foam core can also impair the thermal insulation properties of the sandwich construction element.

[0005] The production times of sandwich construction elements manufactured using state-of-the-art technology are too long for effective industrial production. Since the production time for insulation production is significantly longer than the cycle time for the sandwich elements themselves, the current state of the art requires insulation to be performed as a process step outside of the sandwich element cycle, making production uneconomical. Furthermore, the pressure developed during foaming leads to cracks in the (brittle) building material panels, which reduces durability and the tensile strength of the foam to the building material panel, as well as increased diffusion of the cell gas from the foam. The result is a reduced insulating effect of the sandwich construction elements.

[0006] The present invention is therefore based on the object of providing sandwich construction elements whose foam core has reduced anisotropy with good insulation values, as well as a method for producing the sandwich construction elements which can be carried out with a short production time.

[0007] This object is achieved by a sandwich construction element made of at least two spaced-apart, substantially parallel to each other arranged building material panels with a polyurethane foam core between the spaced-apart building material panels, wherein the ratio of the highest measured compressive modulus of elasticity (according to DIN EN ISO 844) of the polyurethane foam core in a spatial direction oriented parallel to the building material panels to the compressive modulus of elasticity of the polyurethane foam core in a spatial direction oriented perpendicular to the building material panels is less than 1.7, preferably less than 1.5, and the foam core is obtainable by densifying the foam in the range from 1.1 to 2.5, wherein the densification is the quotient of the density of the foam in the cavity divided by the density of the freely foamed foam body.

[0008] The core density of the polyurethane foam core is preferably in the range of 20 to 100 kg / m 3 , in particular 20 to 80 kg / m 3 , and particularly preferably 30 to 60 kg / m 3 or 30 to 50 kg / m 3 . The core density (in kg / m 3 ) is measured from the center of the foam using a cube with an edge length of approximately 5 cm.

[0009] This object is further achieved by a method for producing sandwich construction elements from at least two spaced-apart building material panels with a polyurethane foam core, comprising the following steps: A) Mixing (a) at least one polyisocyanate component, (b) at least one component comprising at least one polyfunctional compound reactive towards isocyanates, and (c) a blowing agent by the high-pressure injection process; B) introducing the resulting mixture into a cavity between the spaced-apart building material panels, wherein the compression of the foam is in the range of 1.1 to 2.5, wherein the compression is the quotient of the density of the foam in the cavity divided by the density of the freely foamed foam body.

[0010] It has been found that foam cores produced using the high-pressure injection process with defined compaction exhibit lower anisotropy than freely foamed foam cores while maintaining good insulation values. For design reasons, the mechanical properties of sandwich construction elements in the thickness direction, i.e., in a spatial direction oriented perpendicular to the building material panels, are particularly important. In the sandwich construction elements according to the invention, the ratio of the highest measured compressive modulus of elasticity of the polyurethane foam core in a spatial direction oriented parallel to the building material panels to the compressive modulus of elasticity of the polyurethane foam core in a spatial direction oriented perpendicular to the building material panels is less than 1.7, preferably less than 1.5.Preferably, the ratio of the highest measured compressive modulus of elasticity of the polyurethane foam core in a spatial direction oriented parallel to the building material panels to the compressive modulus of elasticity of the polyurethane foam core in a spatial direction oriented perpendicular to the building material panels is from 0.58 to less than 1.7, in particular from 0.66 to less than 1.5. Typically, the spatial direction in which the highest compressive modulus of elasticity of the polyurethane foam core is measured is parallel to the direction of rise of the foam during production.

[0011] Percentages are to be understood as meaning that the sum of all percentages of the components of a formulation equals 100%. Unless otherwise stated, all percentages are based on the total weight of a formulation. The following statements refer both to sandwich components according to the invention and to the manufacturing process according to the invention, unless otherwise apparent from the context. Step A):

[0012] In step A), a polyisocyanate component (a) comprising at least one polyisocyanate (a1) is mixed with a component (b) comprising at least one polyfunctional isocyanate-reactive compound (b1) to form a polyurethane. For the purposes of the present invention, polyisocyanate (a1) is understood to mean an organic compound containing at least two reactive isocyanate groups per molecule, i.e., the functionality is at least 2. If the polyisocyanates used or a mixture of several polyisocyanates do not have a uniform functionality, the weight-average functionality of the component (a1) used is at least 2.

[0013] Suitable polyisocyanates (a1) include the known aliphatic, cycloaliphatic, araliphatic, and preferably aromatic polyfunctional isocyanates. Such polyfunctional isocyanates are known per se or can be prepared by known methods. The polyfunctional isocyanates can, in particular, also be used as mixtures, so that component a) in this case contains various polyfunctional isocyanates. Polyfunctional isocyanates suitable as polyisocyanates have two (hereinafter referred to as diisocyanates) or more than two isocyanate groups per molecule.

[0014] Specifically, the following may be mentioned: alkylene diisocyanates having 4 to 12 carbon atoms in the alkylene radical, such as 1,12-dodecanediisocyanate, tetramethylene-1,4-diisocyanate, 2-ethyltetramethylene-1,4-diisocyanate, pentamethylene-1,5-diisocyanate, 2-methylpentamethylene-1,5-diisocyanate and preferably hexamethylene-1,6-diisocyanate;cycloaliphatic diisocyanates such as cyclohexane-1,3- and 1,4-diisocyanate and any mixtures of these isomers, 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (IPDI), 2,4- and 2,6-hexahydrotoluene diisocyanate and the corresponding isomer mixtures, 4,4'-, 2,2'- and 2,4'-dicyclohexylmethane diisocyanate and the corresponding isomer mixtures, and preferably aromatic polyisocyanates, such as 2,4- and 2,6-toluene diisocyanate and the corresponding isomer mixtures, 4,4'-, 2,4'- and 2,2'-diphenylmethane diisocyanate and the corresponding isomer mixtures, mixtures of 4,4'- and 2,2'-diphenylmethane diisocyanates, polyphenylpolymethylene polyisocyanates, mixtures of 4,4'-, 2,4'- and 2,2'-diphenylmethane diisocyanates and polyphenylpolymethylene polyisocyanates (crude MDI) and mixtures of crude MDI and toluene diisocyanates.;

[0015] Particularly suitable are 2,2'-, 2,4'- and / or 4,4'-diphenylmethane diisocyanate (MDI), 1,5-naphthylene diisocyanate (NDI), 2,4- and / or 2,6-tolylene diisocyanate (TDI), 3,3'-dimethyldiphenyl diisocyanate, 1,2-diphenylethane diisocyanate and / or p-phenylene diisocyanate (PPDI), tri-, tetra-, penta-, hexa-, hepta- and / or octamethylene diisocyanate, 2-methylpentamethylene-1,5-diisocyanate, 2-ethylbutylene-1,4-diisocyanate, pentamethylene-1,5-diisocyanate, butylene-1,4-diisocyanate, 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl-cyclohexane (isophorone diisocyanate, IPDI), 1,4- and / or 1,3-bis(isocyanatomethyl)cyclohexane (HXDI), 1,4-cyclohexane diisocyanate, 1-methyl-2,4- and / or-2,6-cyclohexane diisocyanate and 4,4'-, 2,4'- and / or 2,2'-dicyclohexylmethane diisocyanate.

[0016] Modified polyisocyanates, i.e., products obtained by chemically converting organic polyisocyanates and containing at least two reactive isocyanate groups per molecule, are also frequently used. Polyisocyanates containing ester, urea, biuret, allophanate, carbodiimide, isocyanurate, uretdione, carbamate, and / or urethane groups are particularly suitable.

[0017] The following embodiments are particularly preferred as polyisocyanates (a1) of component a): i) Polyfunctional isocyanates based on tolylene diisocyanate (TDI), in particular 2,4-TDI or 2,6-TDI or mixtures of 2,4- and 2,6-TDI; ii) Polyfunctional isocyanates based on diphenylmethane diisocyanate (MDI), in particular 2,2'-MDI or 2,4'-MDI or 4,4'-MDI or oligomeric MDI, which is also referred to as polyphenylpolymethylene isocyanate, or mixtures of two or three of the aforementioned diphenylmethane diisocyanates, or crude MDI which arises during the production of MDI, or mixtures of at least one oligomer of MDI and at least one of the aforementioned low molecular weight MDI derivatives; iii) Mixtures of at least one aromatic isocyanate according to embodiment i) and at least one aromatic isocyanate according to embodiment ii).

[0018] Polymeric diphenylmethane diisocyanate is particularly preferred as a polyisocyanate. Polymeric diphenylmethane diisocyanate (hereinafter referred to as polymeric MDI) is a mixture of dinuclear MDI and oligomeric condensation products, and thus derivatives of diphenylmethane diisocyanate (MDI). The polyisocyanates can also preferably be composed of mixtures of monomeric aromatic diisocyanates and polymeric MDI.

[0019] In addition to dinuclear MDI, polymeric MDI contains one or more polynuclear condensation products of MDI with a functionality of more than 2, especially 3, 4, or 5. Polymeric MDI is well known and is often referred to as polyphenyl polymethylene isocyanate or oligomeric MDI. Polymeric MDI is typically composed of a mixture of MDI-based isocyanates with different functionalities. Typically, polymeric MDI is used in a mixture with monomeric MDI.

[0020] The (weight-average) functionality of a polyisocyanate containing polymeric MDI can vary in the range from approximately 2.2 to approximately 5, in particular from 2.3 to 4, in particular from 2.4 to 3.5. Such a mixture of MDI-based polyfunctional isocyanates with different functionalities is, in particular, the crude MDI obtained as an intermediate in the production of MDI.

[0021] Polyfunctional isocyanates or mixtures of several polyfunctional isocyanates based on MDI are known and are marketed, for example, by BASF Polyurethanes GmbH under the name Lupranat ®<.

[0022] The functionality of the polyisocyanate (a1) is preferably at least two, in particular at least 2.2, and particularly preferably at least 2.4. The functionality is preferably from 2.2 to 4, and particularly preferably from 2.4 to 3.

[0023] The isocyanate group content of the polyisocyanate (a1) is preferably from 5 to 10 mmol / g, in particular from 6 to 9 mmol / g, particularly preferably from 7 to 8.5 mmol / g. Those skilled in the art are aware that the isocyanate group content in mmol / g and the so-called equivalent weight in g / equivalent are in a reciprocal relationship. The isocyanate group content in mmol / g is derived from the content in wt. % according to ASTM D-5155-96 A.

[0024] In a particularly preferred embodiment, component a) contains at least one polyfunctional isocyanate selected from diphenylmethane 4,4'-diisocyanate, diphenylmethane 2,4'-diisocyanate, diphenylmethane 2,2'-diisocyanate, and oligomeric diphenylmethane diisocyanate. Within the scope of this preferred embodiment, component (a) particularly preferably contains oligomeric diphenylmethane diisocyanate and has a functionality of at least 2.4.

[0025] The viscosity of component a) used can vary within a wide range. Component a) preferably has a viscosity of 100 to 3000 mPa s, particularly preferably 200 to 2500 mPa s.

[0026] In a particularly preferred embodiment, component a) is a mixture of 1,4'-diphenylmethane diisocyanate with higher-functional oligomers and isomers (crude MDI), with an NCO content of 20 to 40 mass%, preferably 25 to 35 mass%, for example 31.5 mass% and an average functionality of 2 to 4, preferably 2.5 to 3.5, for example about 2.7.

[0027] In the polyurethane foam obtained, the polyisocyanate (a1) is generally present in an amount of 100 to 250 wt.%, preferably 160 to 200 wt.%, particularly preferably 170 to 190 wt.%, based in each case on the sum of components (a) and (b).

[0028] According to the invention, component b) comprises at least one polyfunctional isocyanate-reactive compound (b1). Polyfunctional isocyanate-reactive compounds are those that have at least two isocyanate-reactive hydrogen atoms, in particular at least two isocyanate-reactive functional groups.

[0029] The compounds (b1) used in component b) preferably have a functionality of 2 to 8, in particular of 2 to 6. If several different compounds are used as component b), the weight-average functionality of the compound (b1) is preferably 2.2 to 5, particularly preferably 2.4 to 4, very particularly preferably 2.6 to 3.8. Weight-average functionality is understood to mean the value that results when the functionality of each compound (b1) is weighted by the weight fraction of this compound in component b).

[0030] Polyols, and especially polyether polyols, are preferred as compounds (b1). The term polyether polyol is used synonymously with the term polyetherol and refers to alkoxylated compounds with at least two reactive hydroxyl groups.

[0031] Preferred polyether polyols (b1) have a functionality of 2 to 8 and hydroxyl numbers of 100 mgKOH / g to 1200 mgKOH / g, preferably 150 to 800 mgKOH / g, in particular 200 mgKOH / g to 550 mgKOH / g. All hydroxyl numbers in the present invention are determined according to DIN 53240.

[0032] In general, the proportion of the polyfunctional compound reactive towards isocyanates, based on the total weight of component b), is 40 to 98 wt.%, preferably 50 to 97 wt.%, particularly preferably 60 to 95 wt.%.

[0033] The polyetherols (b1) preferred in the context of component b) can be prepared by known processes, for example by anionic polymerization of one or more alkylene oxides having 2 to 4 carbon atoms with alkali hydroxides, such as sodium or potassium hydroxide, alkali alkoxides, such as sodium methylate, sodium or potassium ethylate or potassium isopropylate, or aminic alkoxylation catalysts, such as dimethylethanolamine (DMEOA), imidazole and / or imidazole derivatives, using at least one starter molecule which contains 2 to 8, preferably 2 to 6, reactive hydrogen atoms bonded, or by cationic polymerization with Lewis acids, such as antimony pentachloride, boron fluoride etherate or bleaching earth.

[0034] Suitable alkylene oxides include, for example, tetrahydrofuran, 1,3-propylene oxide, 1,2- or 2,3-butylene oxide, styrene oxide, and preferably ethylene oxide and 1,2-propylene oxide. The alkylene oxides can be used individually, alternately, or as mixtures. Particularly preferred alkylene oxides are 1,2-propylene oxide and ethylene oxide.

[0035] Component b) preferably comprises at least one polyether polyol having a hydroxyl number of 200 to 400 mgKOH / g, in particular 230 to 350 mgKOH / g and a functionality of 2 to 3. The above-mentioned ranges ensure good flow behavior of the reactive polyurethane mixture.

[0036] Component b) may further comprise at least one polyether polyol having a hydroxyl number of 300 to 600 mgKOH / g, in particular 350 to 550 mgKOH / g, and a functionality of 4 to 8, in particular 4 to 6. The aforementioned ranges lead to good chemical crosslinking of the reactive polyurethane mixture.

[0037] Examples of suitable starter molecules are: water, organic dicarboxylic acids such as succinic acid, adipic acid, phthalic acid and terephthalic acid, aliphatic and aromatic, optionally N-mono-, N,N- and N,N'-dialkyl-substituted diamines having 1 to 4 carbon atoms in the alkyl radical, such as optionally mono- and dialkyl-substituted ethylenediamine, diethylenetriamine, triethylenetetramine, 1,3-propylenediamine, 1,3- or 1,4-butylenediamine, 1,2-, 1,3-, 1,4-, 1,5- and 1,6-hexamethylenediamine, phenylenediamines, 2,3-, 2,4- and 2,6-tolylenediamine and 4,4'-, 2,4'- and 2,2'-diaminodiphenylmethane. Particularly preferred are the diprimary amines mentioned, for example ethylenediamine.

[0038] Other suitable starter molecules are: alkanolamines, such as ethanolamine, N-methyl- and N-ethylethanolamine, dialkanolamines, such as diethanolamine, N-methyl- and N-ethyldiethanolamine, and trialkanolamines, such as triethanolamine, and ammonia.

[0039] Preferably used are two or more polyhydric alcohols, such as ethanediol, 1,2- and 1,3-propanediol, diethylene glycol (DEG), dipropylene glycol, 1,4-butanediol, 1,6-hexanediol, glycerol, trimethylolpropane, pentaerythritol, sorbitol and sucrose.

[0040] Polyester alcohols with hydroxyl numbers of 100 to 1200 mgKOH / g can also be considered as compounds (b1).

[0041] Preferred polyester alcohols are prepared by condensing polyfunctional alcohols, preferably diols, having 2 to 12 carbon atoms, preferably 2 to 6 carbon atoms, with polyfunctional carboxylic acids having 2 to 12 carbon atoms, for example succinic acid, glutaric acid, adipic acid, suberic acid, azelaic acid, sebacic acid, decanedicarboxylic acid, maleic acid, fumaric acid and preferably phthalic acid, isophthalic acid, terephthalic acid and the isomeric naphthalenedicarboxylic acids.

[0042] Further information on the preferred polyether alcohols and polyester alcohols and their preparation can be found, for example, in the Kunststoffhandbuch, Volume 7 "Polyurethanes", edited by Günter Oertel, Carl-Hanser-Verlag Munich, 3rd edition, 1993.

[0043] Furthermore, in step A), at least one blowing agent (c) is added. The blowing agent can be contained in component a), but preferably in component b).

[0044] Blowing agents that can generally be used are those known to those skilled in the art, for example water and / or carboxylic acids, in particular formic acid, which reacts with isocyanate groups to release carbon dioxide (chemical blowing agents). Physical blowing agents can also be used. These are compounds that are inert to the feed components, are usually liquid at room temperature, and evaporate under the conditions of the urethane reaction. The boiling point of these compounds is preferably below 50°C. Physical blowing agents also include compounds that are gaseous at room temperature and are introduced into or dissolved in the feed components under pressure, for example carbon dioxide, low-boiling alkanes, and fluoroalkanes.The compounds are preferably selected from the group consisting of C 3 -C 5 alkanes, C 4 -C 6 cycloalkanes, di-C 1 -C 4 alkyl ethers, esters, ketones, acetals, fluoroalkanes having 1 to 8 carbon atoms, and tetraalkylsilanes having 1 to 3 carbon atoms in the alkyl chain, in particular tetramethylsilane. Examples include propane, n-butane, isobutane, cyclobutane, n-pentane, isopentane, cyclopentane, cyclohexane, dimethyl ether, methyl ethyl ether, methyl butyl ether, diethyl ether, methyl formate, dimethyl oxalate, ethyl acetate, acetone, methyl ethyl ketone, and C 2 -C 4 fluoroalkanes, which can be degraded in the troposphere and are therefore harmless to the ozone layer.Examples of propellants are trifluoromethane, difluoromethane, dichloromethane, 1,1,1,3,3-pentafluorobutane, 1,1,1,3,3-pentafluoropropane, 1,1,1,2-tetrafluoroethane, difluoroethane and heptafluoropropane, dichloromonofluoromethane, chlorodifluoroethanes, 1,1-dichloro-2,2,2-trifluoroethane, 1,1,1,3,3-pentafluoropropane, 2,2-dichloro-2-fluoroethane and heptafluoropropane as well as partially halogenated C 2 -C 4 -fluoroolefins such as trans-1,3,3,3-tetrafluoroprop-1-ene (HFO-1234ze), 3,3,3-trifluoro-1-chloroprop-1-ene (HFO-1233d), 2,3,3,3-tetrafluoroprop-1-ene (HFO-1234yf), FEA 1100 (1,1,1,4,4,4-hexafluoro-2-butene) and FEA 1200. The physical blowing agents mentioned can be used alone or in any combination with each other.

[0045] Preferred blowing agents are formic acid, halogenated hydrocarbons, partially halogenated fluorocarbons, water, or mixtures thereof.

[0046] The blowing agent is generally present in the polyurethane foam in an amount of 1 to 25 wt.%, preferably 2 to 10 wt.%, based in each case on the sum of components (a) and (b).

[0047] Preferably, at least one catalyst (d) is also added in step A). ​​The catalyst is generally present in component b), preferably together with the blowing agent (c). Catalysts used for producing the polyurethane foam cores are, in particular, compounds that greatly accelerate the reaction of the compounds (b1) of component (b) containing reactive hydrogen atoms, especially hydroxyl groups, with the polyisocyanates (a1).

[0048] It is advantageous to use basic polyurethane catalysts, for example tertiary amines such as triethylamine, tributylamine, dimethylbenzylamine, dicyclohexylmethylamine, dimethylcyclohexylamine, N,N,N',N'-tetramethyldiaminodiethyl ether, bis-(dimethylaminopropyl)-urea, N-methyl- or N-ethylmorpholine, N-cyclohexylmorpholine, N,N,N',N'-tetramethylethylenediamine, N,N,N,N-tetramethylbutanediamine, N,N,N,N-tetramethylhexane-1,6-diamine, pentamethyldiethylenetriamine, bis(2-dimethylaminoethyl) ether, dimethylpiperazine, N-dimethylaminoethylpiperidine, 1,2-dimethylimidazole, 1-azabicyclo-(2,2,0)-octane, 1,4-Diazabicyclo-(2,2,2)-octane (Dabco) and alkanolamine compounds such as triethanolamine, triisopropanolamine, N-methyl- and N-ethyldiethanolamine, dimethylaminoethanol, 2-(N,N-dimethylaminoethoxy)ethanol, N,N',N"-tris-(dialkylaminoalkyl)hexahydrotriazines, e.g. N,N',N"-tris-(dimethylaminopropyl)-s-hexahydrotriazine, and triethylenediamine.However, metal salts such as iron(II) chloride, zinc chloride, lead octoate and preferably tin salts such as tin dioctoate, tin diethylhexoate and dibutyltin dilaurate and in particular mixtures of tertiary amines and organic tin salts are also suitable.

[0049] Other suitable catalysts include: amidines, such as 2,3-dimethyl-3,4,5,6-tetrahydropyrimidine; tetraalkylammonium hydroxides, such as tetramethylammonium hydroxide; alkali metal hydroxides, such as sodium hydroxide; and alkali metal alkoxides, such as sodium methylate and potassium isopropylate; alkali metal carboxylates; and alkali metal salts of long-chain fatty acids having 10 to 20 carbon atoms and optionally pendant OH groups. Preferably, 0.001 to 10 parts by weight of catalyst or catalyst combination are used, based (i.e., calculated) on 100 parts by weight of component (b1). It is also possible to conduct the reactions without catalysis. In this case, the catalytic activity of amine-initiated polyols is utilized.

[0050] If a larger excess of polyisocyanate is used during foaming, other possible catalysts for the trimerization reaction of the excess NCO groups are isocyanurate group-forming catalysts, such as ammonium ion or alkali metal salts, especially ammonium or alkali metal carboxylates, alone or in combination with tertiary amines. Isocyanurate formation leads to particularly flame-retardant PIR foams.

[0051] Further information on the above-mentioned and other starting materials can be found in specialist literature, for example the Kunststoffhandbuch, Volume VII, Polyurethane, Carl Hanser Verlag Munich, Vienna, 1st, 2nd and 3rd editions 1966, 1983 and 1993.

[0052] Optionally, at least one chain extender (e) is also used in step A). ​​The chain extender is preferably used as a component of component b). Chain extenders are understood to be compounds that have a molecular weight of 60 to 400 g / mol and contain two isocyanate-reactive hydrogen atoms. Examples include butanediol, diethylene glycol, dipropylene glycol, and ethylene glycol.

[0053] The chain extenders (e) are generally used in an amount of 2 to 20 wt.%, based on the sum of components (a), (b), (c) and (d).

[0054] Optionally, at least one crosslinker (f) is also used in step A). ​​The crosslinker is preferably used as a component of component b). Alkanolamines, and in particular diols and / or triols with molecular weights of less than 400, preferably 60 to 300, are preferably used as crosslinkers.

[0055] Crosslinkers are generally used in an amount of 1 to 10 wt.%, preferably 2 to 6 wt.%, based on the sum of components (a) and (b).

[0056] Crosslinkers and chain extenders can be used individually or in combination. The addition of chain extenders and / or crosslinkers can be beneficial for modifying mechanical properties.

[0057] Component (b) may also contain other conventional additives (g), for example surface-active substances, stabilizers such as foam stabilizers, cell regulators, fillers, dyes, pigments, flame retardants, antistatic agents, hydrolysis inhibitors, fungistatic and bacteriostatic substances and mixtures thereof.

[0058] Suitable flame retardants are generally those known from the prior art, for example brominated ethers (Ixol), brominated alcohols such as dibromoneopentyl alcohol, tribromoneopentyl alcohol and PHT-4-diol as well as chlorinated phosphates such as tris-(2-chloroethyl) phosphate, tris-(2-chloroisopropyl) phosphate (TCPP), tris(1,3-dichloroisopropyl) phosphate, tris-(2,3-dibromopropyl) phosphate and tetrakis-(2-chloroethyl)-ethylene diphosphate.

[0059] Other liquid halogen-free flame retardants that can be used include diethyl ethanephosphonate (DEEP), triethyl phosphate (TEP), dimethylpropylphosphonate (DMPP), diphenyl cresyl phosphate (DPK) and others.

[0060] The flame retardants are generally used in an amount of 2 to 65 wt.%, preferably 5 to 60 wt.%, more preferably 5 to 50 wt.%, based on the sum of components (a) and (b).

[0061] The ratio of OCN groups to OH groups, the so-called ISO index, in the reaction mixture for producing the polyurethane foam according to the invention is 140 to 180, preferably 145 to 165, particularly preferably 150 to 160. This ISO index results in a polyurethane foam being obtained which has a particularly advantageous combination of low thermal conductivity and thermal stability.

[0062] Components (a) and (b) are mixed using the high-pressure injection process, either in the one-shot or multi-shot process. With this mixing principle, the components flow at high velocity into a mixing chamber, where they are mixed in a continuous flow, utilizing their kinetic energy. The mixing chamber is preferably operated in countercurrent. One or more mixing heads can be used, and the space between the building material panels can be divided into several cavities. Components (a) and (b) may contain organic solvents, but are preferably used without solvents. Components (a) and (b) are metered into the mixing chamber at a pressure of at least 100 bar, in particular at a pressure in the range of 100 bar to 300 bar. Further information on the high-pressure injection process can be found in specialist literature, for example, the Kunststoffhandbuch, Volume VII, Polyurethane, Carl Hanser Verlag Munich, Vienna, 3.Edition 1993.

[0063] The mixing of components (a) and (b) is generally carried out at a temperature in the range of 5 to 70 °C, in particular 10 to 50 °C. Step B):

[0064] The unfoamed mixture emerging from the mixing chamber is introduced into a cavity between two building panels. The surfaces of the building panels are spaced apart and arranged essentially parallel to one another, so that a cavity exists between the panels to accommodate the foam core. The upright or horizontal building panels are expediently held by external formwork. The spacing between the building panels is generally adjusted using spacers, e.g., made of plastic. The spacing is generally in the range of 1 to 30 cm, preferably 4 to 22 cm, in particular 8 to 20 cm, i.e., the thickness of the polyurethane foam core (insulating layer) is appropriate.

[0065] The amount of mixture introduced into the cavity depends on the size of the cavity. In general, the amount is calculated such that the total injection density is less than 100 kg / m 3 , in particular less than 80 kg / m 3 . Preferably, the total injection density is in the range from 20 to 100 kg / m 3 , more preferably 20 to 80 kg / m 3 , and in particular 30 to 60 kg / m 3 or 30 to 50 kg / m 3 . Total injection density is understood to mean the total amount of mixture introduced from step A), based on the total volume of the foam in the cavity.

[0066] The high-pressure injection process allows the mixture of components (a) and (b) to be prepared and introduced into the cavity in large quantities and in a short time. The process therefore contributes significantly to the economical production of sandwich components. In general, it is possible to introduce quantities of mixture in the range of 0.1 to 8 kg / s, preferably 1 to 8 kg / s, and especially 2 to 8 kg / s, into the cavity.

[0067] The building material panels are primarily made of inorganic mineral materials. Examples include concrete panels, gypsum boards, and geopolymer panels. All common cements, especially Portland cement, can be used for the production of the concrete panels, along with the usual additives. These cements also include latently hydraulic binders, such as industrial and synthetic slags, especially blast furnace slag, precipitated silica, pyrogenic silica, microsilica, metakaolin, aluminosilicates, or mixtures thereof. Gypsum boards are usually made of gypsum-containing materials, such as mortar gypsum, machine-made gypsum, stucco, etc.Geopolymers used for the production of geopolymer panels are inorganic binder systems that cure in aqueous alkaline media and are based on reactive, water-insoluble SiO 2 and Al 2 O 3 -based compounds, such as microsilica, metakaolin, aluminosilicates, fly ash, activated clays, pozzolanic materials, or mixtures thereof. Geopolymers are described, for example, in US 4,349,386, WO 85 / 03699, and US 4,472,199.

[0068] The building boards can also contain fibers, textiles, or reinforcement. Common materials such as plastic or metal can be used for the fibers, textiles, or reinforcement. These additives improve the tensile strength of the building boards.

[0069] The dimensions of the building material panels are available in a wide range, with sizes up to 3.5 m x 15 m and a thickness of up to 15 cm possible.

[0070] According to a preferred embodiment, at least one of the building material panels is coated at least partially, in particular over the entire surface, with a layer of primer on the side facing the cavity. Both building material panels are expediently coated with the primer.

[0071] A primer is understood here as a coating obtained by applying and curing a composition comprising an organic binder. The organic binder can be a physically curing or chemically curing binder. Physically curing binders are solutions of polymers in organic solvents and / or water. Curing then occurs by evaporating the water and / or the organic solvent. Chemically curable binders are monomeric, oligomeric, or polymeric compounds with chemically reactive groups that are incorporated into the composition in pure form or as a solution in water or a suitable organic solvent. The reactive groups then ensure, by means of a chemical reaction, that the organic binder cures into polymeric structures over a period of a few hours to 30 days.The organic binder can be introduced as a single-component system or as a two- or multi-component system. In single-component systems, mutually reactive chemical groups exist side by side in the system. Activation for the reaction then occurs via a switching or triggering mechanism, for example, by changing the pH value, by irradiation with short-wave light, by applying heat, or by oxidation with atmospheric oxygen. In two- or multi-component systems, the mutually reactive monomers, oligomers, or polymers are initially present separately. Only when the components are mixed is the organic binder activated, and the buildup of molecular weight can take place through chemical reactions. A combination of film formation and crosslinking can also occur in the organic binder.

[0072] Suitable organic binders are known to those skilled in the art; for example, polyurethanes, polyureas, polyacrylates, polystyrenes, polystyrene copolymers, polyvinyl acetates, polyethers, alkyd resins, or epoxy resins can be used. Physically curing binders are aqueous dispersions, for example, acrylate dispersions, ethylene-vinyl acetate dispersions, polyurethane dispersions, or styrene-butadiene dispersions. Suitable chemically curing one-component systems include polyurethanes or alkyd resins. Suitable chemically curing two- or multi-component systems include epoxy resins, polyurethanes, and polyureas. Organic binders that can exhibit a combination of film formation and crosslinking include post-crosslinking acrylate dispersions or post-crosslinking alkyd resin dispersions.

[0073] According to one embodiment, the primer comprises epoxy resins, such as bisphenol-based epoxy resins, e.g., bisphenol A, novolak epoxy resins, aliphatic epoxy resins, or halogenated epoxy resins. In epoxy resins, especially bisphenol-based or novolak epoxy resins, the organic binder is generally a monomer or oligomer, preferably with up to 4 units, containing at least two diglycidyl units. Curing occurs by adding a hardener, generally polyamines, such as 1,3-diaminobenzene, diethylenetriamine, etc.

[0074] According to one embodiment, the epoxy resins contain reactive diluents such as monoglycidyl ethers, for example glycidyl ethers of monohydric phenols or alcohols, or polyglycidyl ethers having at least two epoxy groups.

[0075] The coating to be applied to the building board may also contain conventional components such as solvents, defoamers, fillers, pigments, dispersing additives, rheology adjusters, light stabilizers, or mixtures thereof. The composition can be applied directly to the mineral substrate of the building board by spraying, squeegeeing, brushing, or rolling.

[0076] The primer is generally applied in an amount of 20 to 600 g / m 2< , e.g. 50 to 600 g / m 2<.

[0077] Further information on suitable epoxy resins can be found, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, vol. A9, page 547.

[0078] After the mixture of components (a) and (b) has been introduced into the cavity between the building panels, the mixture is expanded due to the action of the blowing agent to form a polyurethane foam. This results in a foam with a density in the range of 1.2 to 2.5, in particular 1.25 to 2.5. Density is the quotient of the density of the foam in the cavity divided by the density of the corresponding undisturbed (freely foamed) foam. The density of the foam can be controlled by the amount of foam added or by the amount of blowing agent. The total bulk density of the foam core is generally in the range of 20 to 100 kg / m 3 , preferably 20 to 80 kg / m 3 , and in particular 30 to 60 kg / m 3 or 30 to 50 kg / m 3 .

[0079] The foam is a rigid foam that generally completely fills the cavity. The process according to the invention enables the economical production of insulated sandwich construction elements with a short cycle time. It is therefore no longer necessary to carry out the insulation step outside of the cycle production of the sandwich construction elements themselves; instead, it can be done in-factory during the manufacture of the sandwich construction elements. Furthermore, it has surprisingly been shown that the process according to the invention makes it possible to dispense with reinforcement of the building panels because the strength of the sandwich construction elements is improved compared to sandwich construction elements produced without reinforcement according to the prior art. Finally, it has been shown that due to the stable bond between the concrete slabs and primer, spacers can be dispensed with, thus further improving the insulating effect.

[0080] The present invention therefore also relates to a sandwich construction element obtainable by the process according to the invention. In one embodiment, the foam of the sandwich construction element has a thermal conductivity in the range of 16 to 30 mW / m·K, in particular 22 to 28 mW / m·K. This is the thermal conductivity of the fresh foam, determined according to DIN EN 14318-1 after 1-8 days after 16 hours of preconditioning at (23 ± 3) °C and a relative humidity of (50 ± 10)%, measured in a spatial direction oriented perpendicular to the building material panels.

[0081] The sandwich construction elements can be used in the usual way as load-bearing wall elements, non-load-bearing wall elements, for example as facade cladding, and as ceiling elements.

[0082] The attached figures and the following examples illustrate the invention.

[0083] Fig. 1shows a definition of the spatial directions used to define PU foams.

[0084] Fig. 2 shows load-displacement curves of various sandwich elements.

[0085] With reference to Fig. 1 Two building material panels are spaced apart from each other and arranged essentially parallel to each other, so that a cavity exists between the panels that accommodates the foam core. Here, x denotes a spatial direction oriented perpendicular to the building material panels, z denotes the direction of rise of the foam, and y denotes a spatial direction oriented parallel to the building material panels and perpendicular to the direction of rise. Example 1

[0086] In the following examples, an epoxy primer was used, or in example d), a PU primer. In all tests, the primer was applied to the concrete by hand (brush or roller) and allowed to dry overnight before the PU reaction mixture was added.

[0087] The polyurethane foam (PU foam) used had a closed cell density of 91% and was manufactured on the basis of polymeric MDI and polyether polyol and water and / or HFC 245FA (1,1,1,3,3-pentafluoropropane) as blowing agent.

[0088] The following concretes were used: Concrete 1: cement-based (CEM I 52.5 R); compressive strength 55 MPa. Concrete 2: cement-based (CEM I 32.5 N); compressive strength 29 MPa. Concrete 3: cement-based (CEM III / B 42.5 NW / MS / NA); compressive strength 81.9 MPa

[0089] The compressive strengths (in a spatial direction oriented perpendicular to the building material panels) were determined according to DIN EN 1048. a) CO 2 diffusion with and without primer

[0090] For this test, concrete cubes with an edge length of 15 cm and concrete prisms (12 x 12 x 36 cm 3< ) were made of concrete 1.

[0091] CO2 diffusion / carbonation was carried out at a CO2 content of 4%, a relative humidity of 57%, and a temperature of 20°C in accordance with Swiss standard SN 505 262 / 1 (Annex I). These values ​​are actively controlled in a carbonation chamber. The standard pre-storage for the test specimens after formwork removal involved immersion in water until day 3, followed by storage at 20°C and 57% RH for 25 days. This conditioning allows the concrete to dry, preventing excessive moisture from entering the carbonation chambers. 500 g / m2 of epoxy primer was applied to half of the test specimens.

[0092] To determine the carbonation depth, a slice is removed from the prisms and the new fracture surface is sprayed with phenolphthalein. The carbonated area does not change color, while the non-carbonated area takes on a pink color. The carbonation depth is determined at five locations per prism side. This results in 20 measurements per age. The carbonation depth is determined before the test specimens are placed in the chambers, as well as after 7, 28, and 63 days. Because the mortar carbonates very quickly, the carbonation depth was determined after 0, 7, 14, 21, and 46 days in the carbonation chamber. The carbonation coefficient KN was calculated as follows: dK = A + KS ⋅ t 1 / 2 KN = a• b• c• KS KN = Carbonation coefficient under natural conditions with a CO 2 content of 0.04 % mm / Jahr a = Conversion from 1 day to 1 year (365 / 1)1 / 2 = 19.10 b = Conversion factor from 4.0 to 0.04 Vol% CO 2 c= Correction factor for rapid carbonation material CO2 uptake coefficient KN mm / Jahr Uncoated concrete (without primer) 12.5 Concrete coated (with primer - 500g / m2) 0.0 b) Adhesive tensile values ​​with and without primer

[0093] Direct tensile bond strengths with and without primer show significantly higher strength for the concrete 2 test specimens with primer. A pre-expanded PU foam (block foam) placed between two concrete slabs on primer achieves tensile bond strengths of approximately 0.14 N / mm². A PU foam foamed between two concrete slabs without primer (HDI process; high-pressure dosing device; pressure >120 bar) with a density of 50 g / l achieves approximately 0.16 N / mm². A PU foam foamed with primer using the HDI process achieves approximately 0.20 N / mm². At higher PU foam densities and the use of a primer, the foam itself may crack, depending on the concrete strength.

[0094] Example I) with primer, concrete 2: With PU foam density 50 g / l: Failure of the PU foam at 0.20 N / mm 2< With PU foam density 100 g / l: Failure of the concrete specimen at 0.23 N / mm 2<

[0095] Example II) with primer, concrete 3: Concrete strength 81.9 MPa With PU foam density 90 g / l: failure of the foam at 0.32 N / mm 2< c) Load / deformation with and without high-pressure injection

[0096] To determine the load-bearing capacity of the sandwich element with a PU foam core between concrete slabs, load-displacement curves were measured under shear loading. Test specimens: cut from the sandwich elements. Dimensions: 25 x 25 x 2.5 cm concrete slabs, 15 cm foam thickness. The results are shown in Figure 2Graphically represented. When bonded PU block foam is used, the composite fails suddenly, causing the foam to delaminate from the sandwich element (dashed line). All foams applied using HDI exhibit ductile failure, meaning that a larger and more uniform deformation occurs under constant load, rather than a sudden failure. Dashed: Block foam as a panel with 50 g / l (compaction 1.0) glued with PU construction foam: max. load 15 kN and max. deformation 10 mm Black: Injection foam with 50 g / l (compaction approx. 1.5) with primer: max. load approx. 15 kN and max. deformation 20 mm Dot-dash: Injection foam with 30 g / l (compaction approx. 1.3) without primer: max. load approx. 10 kN and max. deformation > 40 mm Dots: Injection foam with 50 g / l (compaction approx. 1.5) without primer: max. load approx. 10 kN and max. deformation 25 mm d) Thermal conductivity with and without primer

[0097] The primers were an epoxy and a PU primer.

[0098] The sandwiches are made with two concrete specimens and a rigid PU foam core. The open sides are lined with vacuum packaging film in the mold. Dimensions concrete bowls: 20 x 20 x 2.0 cm Foam volume between the concrete bowls: 20 x 20 x 6 cm (2.4L)

[0099] The open sides of the mold are lined with VIP foil (vacuum insulation panel). PU foam panels are protected against the diffusion of cell gases by means of laminated aluminum foil with a thickness of 80 µm (reference). Cell gas exchange can also be prevented by using an epoxy primer (concrete system) with a thickness of 500 g / m². The results shown are measured thermal conductivities (TC) after accelerated aging, i.e., storage at 60 °C for 42 days. Thermal conductivity [mW / m·K] PU foam with VIP foil lamination 23,1 PU foam without lamination 26,7 Concrete element not pre-dried with epoxy primer 500 g / m 2< 23,1 Concrete element pre-dried with epoxy primer 500 g / m 2< 23,7 Concrete element pre-dried with PU Primer 500 g / m 2< 26,3 Concrete element pre-dried without primer 25,5 Example 2: Anisotropy investigations

[0100] Three test specimens were prepared. A volume of 20 x 20 x 6 cm³ (2.4 L), defined by 2 cm thick concrete slabs, was filled with polyurethane foam. The polyurethane foam (PU foam) used was made from polymeric MDI and polyether polyol, with formic acid, 1,1,1,3,3-pentafluorobutane, and 1,1,1,3,3-pentafluoropropane as blowing agents.

[0101] The "Compacted, RD 45" sample was foamed using high-pressure injection foam with a compression ratio of approximately 1.35. The total injection density (amount of liquid polyurethane material filled relative to the total volume of the foamed volume) was approximately 45 kg / m³.

[0102] The "Frei, RD 45" sample was filled with foam. The amount of blowing agent was reduced to achieve a total shot density of approximately 45 kg / m³.

[0103] The "Free, RD 38" sample was filled with cast foam. The composition of the liquid polyurethane material corresponded to the "Compacted, RD 45" sample; due to the lack of compaction, the total injection density was only approximately 38 kg / m³.

[0104] From the resulting foam bodies, parallelepipeds were cut in three spatial directions on a square base measuring 5 x 5 cm2 and 50 mm thick. The mechanical properties of the test specimens in the thickness direction of the parallelepipeds were investigated according to DIN EN ISO 844.

[0105] The compressive strength [N / mm 2< ] and compressive modulus of elasticity were determined at 10% compression / min.

[0106] The thermal conductivity was determined according to DIN EN 12667. The results are summarized in the following table (std. dev. = standard deviation). Table Compacted, RD45 Free, RD45 Free, RD38 Perpendicular to the surface layer (x) Perpendicular to the surface layer (x) Perpendicular to the surface layer (x) Test characteristic mean Staffw. Unit mean Staffw. Unit mean Staffw. Unit Compressive strength / stress 0.085 0.004 N / mm 2 0.092 0.008 N / mm 2 0.046 0.003 N / mm 2 compression 10.0 0.0 % 7.1 2.4 % 10.0 0.1 % Compressive modulus of elasticity 2.7 0.34 N / mm 2 2.82 0.40 N / mm 2 1.18 0.03 N / mm 2 Thermal conductivity 21.8 0.0 mW / m*K 22.4 0.0 mW / m*K 21.9 0.0 mW / m*K Parallel to the direction of ascent (z) Parallel to the direction of ascent (z) Parallel to the direction of ascent (z) Test characteristic mean Staffw. Unit mean Staffw. Unit mean Staffw. Unit Compressive strength / stress 0.100 0.018 N / mm 2 0.208 0.002 N / mm 2 0.122 0.002 N / mm 2 compression 7.9 1.8 % 4.4 0.2 % 4.3 0.3 % Compressive modulus of elasticity 2.66 0.56 N / mm 2 6.99 0.06 N / mm 2 3.95 0.26 N / mm 2 3.Spatial direction / width (y) 3.Spatial direction / width (y) 3.Spatial direction / width (y) Test characteristic mean Staffw. Unit mean Staffw. Unit mean Staffw. Unit Compressive strength / stress 0.093 0.014 N / mm 2 0.117 0.015 N / mm 2 0.102 0.005 N / mm 2 compression 6.7 2.9 % 9.0 1.0 % 6.9 2.6 % Compressive modulus of elasticity 2.54 0.52 N / mm 2 2.95 0.80 N / mm 2 2.87 0.17 N / mm 2

[0107] The "Compacted, RD 45" sample exhibits low anisotropy (ratio of compressive modulus of elasticity parallel to the rise direction / perpendicular to the facing layer = 1.18) and a good insulation value of 21.8 mW / m*K. The "Free, RD 45" sample exhibits comparable compressive strength perpendicular to the facing layer, but exhibits a poorer thermal insulation value. The "Free, RD 38" sample exhibits insufficient compressive strength perpendicular to the facing layer.

Claims

1. A sandwich component composed of at least two building material plates which are arranged essentially parallel to one another at a distance from one another and have a polyurethane foam core between the spaced building material plates, wherein the ratio of the greatest measured compressive modulus of the polyurethane foam core in a direction oriented parallel to the building material plates to the compressive modulus of the polyurethane foam core in a direction oriented perpendicular to the building material plates is less than 1.7, wherein the compressive modulus is measured according to the description, and the foam core is obtainable by compaction of the foam in the range from 1.1 to 2.5, where the compaction is the ratio of the density of the foam in the hollow space divided by the density of the free-foamed foam body.

2. The sandwich component according to claim 1, wherein the core density of the polyurethane foam core is in the range from 20 to 100 kg / m3.

3. The sandwich component according to claim 1 or 2, wherein at least one of the building material plates is provided at least partly with a primer on the side facing the polyurethane foam core.

4. The sandwich component according to any of the preceding claims, wherein the building material plates are made of concrete, geopolymers or gypsum plaster.

5. The sandwich component according to any of the preceding claims, wherein the fresh foam of the sandwich component has a thermal conductivity in the range from 16 to 30 mW / m·K, in particular from 22 to 28 mW / m·K, measured according to the description.

6. A process for producing sandwich components composed of at least two building material plates which are at a distance from one another and have a polyurethane foam core, comprising the following steps: A) mixing of (a) at least one polyisocyanate component, (b) at least one component which comprises at least one polyfunctional compound which is reactive toward isocyanates and (c) at least one blowing agent by the high-pressure injection process; and B) introduction of the mixture obtained into a hollow space between the spaced building material plates, where the compaction of the foam is in the range from 1.1 to 2.5, where the compaction is the ratio of the density of the foam in the hollow space divided by the density of the free-foamed foam body.

7. The process according to claim 6, wherein the mixing of the components (a) to (c) is carried out in a mixing chamber at a pressure of at least 100 bar, in particular at a pressure in the range from 100 bar to 300 bar.

8. The process according to claim 6 or 7, wherein a blowing agent selected from among C3-C5-alkanes, C4-C6-cycloalkanes, di-C1-C4-alkyl ethers, methyl formate, formic acid, acetone, fluorohydrocarbons, partially halogenated fluoroolefins, chlorofluorocarbons, carbon dioxide, water and mixtures of two or more thereof is used.

9. The process according to any of claims 6 to 8, wherein at least one catalyst for the reaction of the polyisocyanate component with the polyol component is added in step A).

10. The process according to any of claims 6 to 9, wherein the amount of the mixture introduced into the hollow space in step B) is such that the overall injected foam density is less than 100 kg / m3, in particular less than 80 kg / m3, where the overall injected foam density is the total amount of mixture from step A) which is introduced divided by the total volume of the hollow space to be filled with foam.

11. The process according to any of claims 6 to 10, wherein the amount of mixture introduced into the hollow space in step B) is in the range from 0.1 to 8 kg / s.

12. The process according to any of claims 6 to 11, wherein at least one of the building material plates is provided at least partly with a primer on the side facing the hollow space.

13. The process according to claim 12, wherein the primer is based on a physically setting binder and / or a chemically curing binder.

14. The process according to claim 13, wherein the primer is based on a binder selected from among an epoxy resin, post-crosslinking acrylate dispersions or post-crosslinking alkyd resin dispersions.

15. The process according to any of claims 12 to 14, wherein the primer is applied in an amount in the range from 20 to 600 g / m2.

16. The process according to any of claims 6 to 15, wherein the building material plates are made of concrete, geopolymers or gypsum plaster.