Technical field

The present invention belongs to the technical field of polyurethane adhesives. In particular, the present invention relates to a polyurethane adhesive for the preparation of a sandwich panel used in 5G radome, to a sandwich panel for 5G radome produced therefrom, and to a method for preparing said sandwich panel for 5G radome.

Background art

In recent years, 5G communication technology has developed rapidly. In 5G millimeter wave communication base stations, the wave usually has a wavelength of millimeter wave band. Because of the short wavelength of such millimeter wave, the signal transmission is greatly influenced by the material type and structure of a sandwich panel used in the 5G radome. Therefore, a higher requirement is put forward on the material and structure of the sandwich panel used in 5G radome.

CN 111421937A describes a laminated material for 5G millimeter wave radome, which includes a skin layer and a core layer, wherein the skin layer is made by a thermoplastic resin material and there is an optional adhesive film present between the skin layer and the core layer. CN 108091998A describes a V-shaped radome which is obtained by co-curing an antenna, a skin layer and a filling core layer, wherein the skin layer and the filling core layer are bonded together with a cyanate ester adhesive film. CN104577324A discloses a method for designing a radome suitable for high-power broad band radar, which, among others, includes the selection of the materials for each layer of C-sandwich structure used in the radome wall.

On the other hand, although polyurethane adhesives are widely known and used, no attention is drawn to the effect thereof on the transmission properties of a sandwich panel used in 5G radome.

With the increasing development of 5G technique, there is an urgent need to find a polyurethane adhesive specifically used in the sandwich panel for 5G radome with low transmission loss.

The inventor has surprisingly found that polyurethane adhesive has significant influence on the dielectric constant (Dk) and the dissipation factor (Df) of the sandwich panel. Therefore, there is a need to provide an adhesive which has a sufficiently low Df combined with a low Dk while maintaining other favorable characteristics, such as light weight. In addition, there is a need to provide a sandwich panel used in 5G radome with low transmission loss in combination with good mechanical properties.

Summary of the invention

The first objective of the present invention is to provide a polyurethane adhesive which has a low dissipation factor (Df) of < 0.085, a low dielectric constant (Dk) of < 3.2, and a low density of <1 .2 g/cm ³ , for use in the production of a sandwich panel used in 5G radome.

It has now been found that, surprisingly, the polyurethane adhesive of the present invention provides an optimum balance between the transmission properties such as the Dk and Df values and the density property.

Accordingly, in one aspect, the presently claimed invention is directed to a polyurethane adhesive which is prepared by reacting:

(A) di- and/or polyisocyanates, with

(B) polyols containing at least one polyetherpolyol, optionally in the presence of

(C) chain extenders and/or crosslinking agents,

(D) blowing agents,

(E) catalysts, and/or

(F) additives and/or auxiliaries, wherein the at least one polyetherpolyol comprise oxypropylene groups in an amount of at least 50 wt%, based on the total weight of the polyetherpolyol.

In another aspect, the presently claimed invention relates to the use of the polyurethane adhesive of the present invention for the preparation of a sandwich panel used in 5G radome.

Another aspect of the present invention relates to a sandwich panel for 5G radome, wherein the polyurethane adhesive of the present invention is present in-between at least two adjacent layers of the sandwich panel and forms an adhesive film between them, and wherein the sandwich panel has a transmission of at least 89%, measured according to the ENCHI test method at a frequency of 6 GHz.

It is another object of the present invention to provide a method for preparing a sandwich panel for 5G radome, wherein at least two adjacent layers of the sandwich panel are bonded by using the polyurethane adhesive according to the present invention.

Detailed description of the invention

The present invention will now be described for purposes of illustration and not limitation. Except in the operating examples, or where otherwise indicated, all numbers expressing quantities, percentages, OH numbers, functionalities and so forth in the specification are to be understood as being modified in all instances by the term “about”.

Unless defined otherwise, all technical and scientific terms used herein have the meaning commonly understood by a person skilled in the art to which the invention belongs. As used herein, the following terms have the meanings ascribed to them below, unless specified otherwise. If hereinafter a group is defined to comprise at least a certain number of embodiments, this is meant to also encompass a group which preferably consists of these embodiments only. Furthermore, the terms "first", "second", "third" or "(a)", "(b)", "(c)", "(d)" etc. and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein. In case the terms "first", "second", "third" or “(A)”, “(B)” and “(C)” or "(a)", "(b)", "(c)", "(d)", "i", "ii" etc. relate to steps of a method or use or assay there is no time or time interval coherence between the steps, that is, the steps may be carried out simultaneously or there may be time intervals of seconds, minutes, hours, days, weeks, months or even years between such steps, unless otherwise indicated in the application as set forth herein above or below.

As used herein, the articles “a” and “an” refer to one or more than one (i.e. , at least one) of the grammatical object of the article. By way of example, “an element” means one element or more than one element.

As used herein, the term “about” is understood to refer to a range of numbers that a person of skill in the art would consider equivalent to the recited value in the context of achieving the same function or result.

Unless otherwise identified, the temperature refers to room temperature and the pressure refers to ambient pressure. Unless otherwise identified, all percentages (%) are “percent by weight". All values for molecular weight are based on weight average molecular weight, unless indicated otherwise.

In the following text, different aspects of the invention are defined in more detail. Each defined aspect may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

The polyurethane adhesive according to the claimed invention has a low Dk and Df values. The Dk and Df values have great impact on the transmission properties of the resulting sandwich panel. This low Dk and Df values allows a sandwich panel with several layers for use in 5G radome to be bonded together by applying the polyurethane adhesive of the claimed invention to each layer. The polyurethane adhesive according to the present invention typically have a Dk value of < 3.2, preferably < 3.1 , more preferably 3.0, the Dk value being determined by 2.45GHz split post dielectric resonator according to I EC 61189-2-721. The polyurethane adhesive according to the present invention typically have a Df value of < 0.085, preferably < 0.081 , more preferably <0.06, and even more preferably <0.05, the Df value being determined by 2.45GHz split post dielectric resonator according to I EC 61189-2-721.

Beside the improved Dk and Df values, the polyurethane adhesive of the invention offers light weight which are added advantages. For bonding a sandwich panel for use in 5G radome, it is crucial to reach the specific density. It has now been surprisingly found that the polyurethane adhesives of the claimed invention have reduced density, and thus allow the light weight of the sandwich panel.

The polyurethane adhesive according to the claimed invention has a density of <1.2 g/cm ³ , preferably <1.15 g/cm ³ , more preferably <1.12 g/cm ³ , measured according to ASTM D1875-03(2018).

According to the present invention, the polyurethane adhesive is prepared by reacting:

(A) di- and/or polyisocyanates, with

(B) polyols containing at least one polyetherpolyol, optionally in the presence of

(C) chain extenders and/or crosslinking agents,

(D) blowing agents,

(E) catalysts, and/or

(F) additives and/or auxiliaries, wherein the at least one polyetherpolyol comprise oxypropylene groups in an amount of at least 50 wt%, based on the total weight of the polyetherpolyol.

The suitable details of the above components (A)-(F) are provided herein as guidance to produce the polyurethane adhesive that will have the desired characteristic properties as described above. Those of ordinary skill in the polyurethane chemistry art will understand that a wide variety of materials are suitable for these components.

(A) Di- and/or polyisocyanates

The di- and/or polyisocyanate component (A) is an isocyanate functional component which forms urethane linkages when reacted with the hydroxyl groups of the polyol component (B). Suitable di- and/or polyisocyanates are known to those skilled in the art and include unmodified isocyanates, modified isocyanates, and isocyanate prepolymers. As used herein, the term “polyisocyanates” refer to those comprising three or more isocyanate functional groups. Such organic di- and/or polyisocyanates may include linear or branched, aliphatic, cycloaliphatic, araliphatic, aromatic, and heterocyclic polyisocyanates of the type described, for example, by W. Siefken in Justus Liebigs Annalen der Chemie, 562, pages 75 to 136. Examples of such di- and/or polyisocyanates include those represented by the formula,

Q(NCO) _n in which n is a number from 2-5, preferably 2-3, and Q is an aliphatic hydrocarbon group containing 2-18, preferably 6-10, carbon atoms; a cycloaliphatic hydrocarbon group containing 4-15, preferably 5-10, carbon atoms; an araliphatic hydrocarbon group containing 8-15, preferably 8- 13, carbon atoms; or an aromatic hydrocarbon group containing 6-15, preferably 6-13, carbon atoms. Examples of suitable di- and/or polyisocyanates include, but are not limited to, ethylene diisocyanate; 1 ,4-tetramethylene diisocyanate; 1 ,6-hexamethylene diisocyanate; 1 ,12-dodecane diisocyanate; cyclobutane-1 ,3-diisocyanate; cyclohexane- 1 ,3- and -1 ,4-diisocyanate, and mixtures of these isomers; 1-isocyanato-3,3,5-trimethyl-5- isocyanatomethylcyclohexane (isophorone diisocyanate); 2,4- and 2,6-hexahydrotoluene diisocyanate and mixtures of these isomers; dicyclohexylmethane-4,4'-diisocyanate (hydrogenated MDI, or HMDI); 1 ,3- and 1 ,4- phenylene diisocyanate; 2,4- and 2,6-toluene diisocyanate and mixtures of these isomers (TDI); diphenylmethane 2,2’-, 2,4’-, and/or 4,4’-diisocyanate (MDI), the mixtures of monomeric diphenylmethane diisocyanates and of diphenylmethane diisocyanate homologs having a greater number of rings (polymeric MDI); naphthylene-1 ,5-diisocyanate; triphenylmethane-4,4',4"- triisocyanate; polyphenyl-polymethylene-polyisocyanates of the type which may be obtained by condensing aniline with formaldehyde, followed by phosgenation (crude MDI); norbornane diisocyanates; m- and p-isocyanatophenyl sulfonylisocyanates; perchlorinated aryl polyisocyanates; modified polyisocyanates containing carbodiimide groups; modified polyisocyanates containing urethane groups; modified polyisocyanates containing allophanate groups; modified polyisocyanates containing isocyanurate groups; modified polyisocyanates containing urea groups; polyisocyanates containing biuret groups; polyisocyanates obtained by telomerization reactions; polyisocyanates containing ester groups; reaction products of the above-mentioned isocyanates with acetals; and polyisocyanates containing polymeric fatty acid groups. It is also possible to use the isocyanate-containing distillation residues accumulating in the production of isocyanates on a commercial scale, optionally in solution in one or more of the polyisocyanates mentioned above. Those skilled in the art will recognize that it is also possible to use mixtures of the polyisocyanates described above.

Isocyanate-terminated prepolymers may also be employed in the present invention. In one embodiment, the isocyanate-terminated prepolymers may be a polymerization product of respective isocyanates themselves, i.e. , dimers, trimers or oligomers, or a reaction product of the isocyanate component and the active hydrogen-containing component to give an isocyanate functionalized prepolymer. Such active hydrogen-containing components have functional groups like hydroxyl groups, ester groups or amine groups. In a preferred embodiment, the prepolymers may be prepared by reacting an excess of organic polyisocyanate or mixtures thereof with a minor amount of an active hydrogen-containing compound as determined by the well-known Zerewitinoff test, as described by Kohler in "Journal of the American Chemical Society," 49, 3181 (1927). These compounds and their methods of preparation are well known to those skilled in the art. The use of any one specific active hydrogen-containing compound is not critical; any such compound can be employed in the practice of the present invention.

Preferably, the di- and/or polyisocyanate is selected from the group consisting of one or more isomers or homologues of diphenylmethane diisocyanate, polymeric diphenylmethane diisocyanate, and diphenylmethane diisocyanate based prepolymers.

The di- and/or polyisocyanate component preferably contains organic di- and/or polyisocyanates having a number average isocyanate (NCO) functionality of from at least 1.8 to 4.0, more preferably from 2.0 to 3.0, most preferably from 2.3 to 2.9. The NCO functionality of the isocyanate component may be a number ranging between any combination of these values, inclusive of the recited values. The isocyanate component preferably has a free isocyanate group content (NCO content) in the range of from 5% to 50% by weight, more preferably from 12% to 40%, most preferably from 20% to 35% by weight. The free NCO group content of the polyisocyanate component may be an amount ranging between any combination of these values, inclusive of the recited values.

(B) Polyols

The polyols according to the claimed invention comprises at least one polyetherpolyol which comprise oxypropylene groups in an amount of at least 50 wt%, based on the total weight of the polyetherpolyol. In a preferred embodiment, the at least one polyetherpolyol comprises oxypropylene groups in an amount of at least 70 wt%, preferably at least 80 wt%, and more preferably at least 95 wt%, based on the total weight of the polyetherpolyol. Besides the oxypropylene groups, the polyetherpolyol may comprise oxyethylene groups in an amount of up to 30 wt%, preferably up to 20 wt%, and more preferably up to 10 wt%, based on the total weight of the polyetherpolyol. It is predicted that polyetherpolyol comprising oxypropylene groups in an amount of less than 50 wt% may not give a polyurethane adhesive with desired properties.

It has been found that the inclusion of at least one polyetherpolyol in the polyol component has unexpected benefits. The at least one polyetherpolyol employed in the practice of the present invention will generally have an average OH number of from 100 to 650 mg KOH/g, preferably, from 300 to 600 mg KOH/g, most preferably, from 350 to 500 mg KOH/g and an average functionality of from 1.0 to 4.5, preferably, from 1.5 to 4.0, most preferably, from 2.5 to 3.2. Preferably, the at least one polyetherpolyol has an average molecular weight Mw in the range from 90 to 5000 g/mol, preferably from 150 to 3000 g/mol, more preferably 300 to 1000 g/mol. This at least one polyetherpolyol may constitute at least 70% by weight of the components (B)- (F) of the present invention, but will generally be included in the components (B)-(F) in an amount of up to 95 wt%, preferably, in an amount of from 80 wt% to 90 wt%.

In a preferred embodiment, the polyols contain at least one polyetherpolyol according to the invention. The polyetherpolyol can be produced by known processes, for example by anionic polymerization using alkali metal hydroxides or alkali metal alcoholates as catalysts, and with addition of at least one starter molecule which comprises from 2 to 3 reactive hydrogen atoms, or by cationic polymerization using Lewis acids, such as antimony pentachloride or boron fluoride etherate, from one or more alkylene oxides having from 2 to 4 carbon atoms in the alkylene moiety. Examples of suitable alkylene oxides are tetrahydrofuran, propylene 1 ,3-oxide, butylene 1 ,2- or 2,3-oxide, and preferably ethylene oxide and propylene 1 ,2-oxide. Other catalysts that can be used are multimetal cyanide compounds, known as DMC catalysts. The alkylene oxides can be used alone, in alternating succession, or in the form of a mixture. Preference is given to use of mixtures of propylene 1 ,2-oxide and ethylene oxide, where amounts of up to 20 wt%, based on the total amount of alkylene oxides, of the ethylene oxide are used, and amounts of at least 50 wt% of the propylene oxide are used.

A starter molecule that can be used is water or di- or trihydric alcohols, such as ethylene glycol, 1 ,2- or 1 ,3-propanediol, diethylene glycol, dipropylene glycol, 1 ,4-butanediol, glycerol, or tri methylolpropane. In another embodiment, molecules having primary and secondary amine functionalities can be used as starter material. Other suitable polyols are polymer-modified polyetherols, particularly preferably graft polyetherols. Such polymer modified polyetherpolyols are also known as polymer polyetherols. These polymer polyetherols are described by way of example in WO 05/098763 and EP-A-250 351 , and are usually produced via free-radical polymerization of suitable olefinic monomers, such as styrene, acrylonitrile, (meth)acrylates, (meth)acrylic acid, and/or acrylamide, in a polyetherol serving as graft base. The side chains are generally produced via transfer of the free radicals from growing polymer chains onto polyetherols. The polymer polyetherol comprises, alongside the graft copolymers, mainly the homopolymers of the olefins, dispersed in unaltered polyetherol.

In one preferred embodiment, the monomers used comprise acrylonitrile, or styrene, preferably acrylonitrile and styrene. The monomers are optionally polymerized in the presence of further monomers, of a macromer, i.e., of an unsaturated polyol capable of free-radical polymerization, and of a moderator, and with use of a free-radical initiator, mostly azo compounds or peroxide compounds, in a polyetherol as continuous phase. This process is described by way of example in DE 111 394, US 3 304 273, US 3 383 351 , US 3 523 093, DE 1 152 536, and DE 1 152 537.

During the free-radical polymerization reaction, the macromers are concomitantly incorporated into the copolymer chain. This gives block copolymers having a polyether block and a polyacrylonitrile-styrene block; these act as compatibilizers at the interface between continuous phase and disperse phase. The proportion of the macromers is usually from 1 to 20% by weight, based on the total weight of the monomers used to produce the polymer polyol.

(C) Chain extenders and/or crosslinking agents

Chain extenders and/or crosslinking agents (e) can be used individually or preferably in the form of a mixture. Chain extenders and/or crosslinking agents (C) used comprise substances with a molar weight that is preferably smaller than 500 g/mol, particularly preferably from 60 to 400 g/mol, more preferably from 60 to 250 g/mol, where chain extenders have 2 hydrogen atoms reactive toward isocyanates and crosslinking agents have 3 hydrogen atoms reactive toward isocyanate.

It is preferable to use simple glycols and triols with the molecular weights smaller than 400, particularly preferably from 60 to 300 and in particular from 60 to 150. Examples of suitable chain- extenders/crosslinkers that can be used include, but are not limited to, ethylene glycol, 1 ,3- propylene glycol, dipropylene glycol, 1 ,4-butanediol, 1 ,3-butanediol, triethanolamine, triisopropanolamine, tripropylene glycol, diethylene glycol, triethylene glycol, glycerol, and mixtures thereof. The most preferred chain-extenders/crosslinkers are liquids at 25° C. Although aliphatic-OH functional compounds, such as those just listed, are the most preferred chain- extenders/crosslinkers, it is also within the scope of the present invention to employ certain polyamines, polyamine derivatives, and/or polyphenols. Examples of suitable amines known in the art include diisopropanolamine, diethanolamine, and 3,5-diethyl-2,4-diaminotoluene, 3, 5- diethyl-2,6-diaminotoluene, and mixtures thereof. Examples of suitable isocyanate reactive amine derivatives include certain imino-functional compounds such as those described in EP 0284 253 and EP 0 359 456 and certain enamino-functional compounds such as those described in EP 0 359 456 having two or more isocyanate-reactive groups per molecule. Reactive amines, especially aliphatic primary amines, are less preferred due to their extremely high reactivity with polyisocyanates, but may optionally be used, if desired, in minor amounts.

(D) Blowing agents

Blowing agents (D) can also be present during the production of polyurethane adhesive. Blowing agents (D) that can be used comprise well-known compounds having chemical and/or physical effect or mixtures of these blowing agents. Chemical blowing agents are compounds which use reaction with isocyanate to form gaseous products, an example being water or formic acid. Physical blowing agents are compounds which have been emulsified or dissolved in the starting materials for polyurethane production and which vaporize under the conditions of polyurethane formation. By way of example, these involve hydrocarbons, halogenated hydrocarbons, and other compounds, for example perfluorinated alkanes, such as perfluorohexane, chlorofluorocarbons, and ethers, esters, ketones, acetals, and mixtures thereof, for example cycloaliphatic hydrocarbons having from 4 to 8 carbon atoms, or fluorocarbons. A preferred embodiment uses, as blowing agent, a mixture comprising at least one of said blowing agents and water, and in particular water as sole blowing agent. In a preferred embodiment, the blowing agents may comprise expandable microspheres which are in the form of hollow polymer microspheres filled with blowing agent, such as gas or low-boiling liquids, preferably hydrocarbons. Surprisingly, it has been found that this preferred embodiment makes it possible to not only reduce the density, but also optimize the Dk and Df values of the resulting sandwich panel.

Expandable microspheres are hollow micro-beads, which comprise a thermoplastic polymer shell (such as polyacrylonitrile or copolymers thereof) encapsulating hydrocarbon gas. When heated, the thermoplastic shell encapsulating hydrocarbon gas softens. Concurrent with the softening of the thermoplastic shell, the hydrocarbon gas expands and exerts an increasing pressure on the shell, resulting in an increase in the volume of the microspheres. The heat to which the material is exposed is sufficient to cause the thermoplastic shell to soften and simultaneously cause the enclosed gas to expand. The expandability of the microspheres may be indicated by TMA density [kg/m ³ ], determined by Mettler Toledo Stare thermal analysis system as a heating rate of 20°C/min. The TMA density here is the minimum achievable density at atmospheric pressure before collapse of the microspheres. Any thermally expandable microspheres can be used in the present invention. Expandable microspheres containing hydrocarbon, in particular aliphatic or cycloaliphatic hydrocarbon, are preferred. The term “hydrocarbon” as used herein is intended to include non-halogenated and partially or fully halogenated hydrocarbons. Examples of expandable microspheres suitable for use in this invention include, but are not limited to, EXPANCEL WU, EXPANCEL DU, EXPANCEL SL and EXPANCEL MB series, commercially available from AkzoNobel, and ADVANCELL EM, commercially available from Sekisui Chemical Company. Other expandable microspheres that are commercially available or known to those skilled in the art are also suitable for use in this invention.

In a preferred embodiment, the blowing agents are used in an amount of from 3 to 10 % by weight, preferably from 4 to 6 % by weight, based on the total weight of components (B) to (F).

(E) Catalysts

Catalysts for the polyurethane-forming reactions of organic polyisocyanates are well known to those skilled in the art. These catalysts are described by way of example in “Kunststoffhandbuch, Band 7, Polyurethane” [Plastics Handbook, volume 7, Polyurethanes] Carl Hanser Verlag, 3rd edition 1993, chapter 3.4.1. Catalyst(s), where used, is/are preferably introduced into the reaction mixture by pre-mixing with the polyol component. As catalysts (F), conventional catalysts as known in the field of production of polyurethanes can be applied. Mention may be made by way of example of amidines, such as 2,3-dimethyl-3,4,5,6-tetrahydropyrimidine, tertiary amines, such as triethylamine, tributylamine, dimethylbenzylamine, N-methyl-, N-ethyl-, and N-cyclohexylmor- pholine, N,N,N',N'-tetramethylethylenediamine, N,N,N',N'-tetramethylbutanediamine, N,N,N',N'- tetramethylhexanediamine, pentamethyldiethylenetriamine, tetramethyldiaminoethyl ether, bis(dimethylaminopropyl)urea, dimethylpiperazine, 1 ,2-dimethylimidazole, 1-azabicyclo[3.3.0]oc- tane, and preferably 1 ,4-diazabicyclo[2.2.2]octane, and alkanolamine compounds, such as triethanolamine, triisopropanolamine, N-methyl- and N-ethyldiethanolamine, and dimethylethanolamine. It is also possible to use organometallic compounds, preferably organotin compounds, such as tin(ll) salts of organic carboxylic acids, e.g. tin(ll) acetate, tin(ll) octoate, tin(ll) ethylhexoate, and tin(ll) laurate, and the dialkyltin(IV) salts of organic carboxylic acids, e.g. dibutyltin diacetate, dibutyltin dilaurate, dibutyltin maleate, and dioctyltin diacetate, and also bismuth carboxylates, such as bismuth(lll) neodecanoate, bismuth 2-ethylhexanoate, and bismuth octanoate, or a mixture thereof. The organometallic compounds can be used alone or preferably in combination with strongly basic amines. Mixtures of metal catalysts and of basic amine catalysts are optionally used as catalysts. Preferably exclusively amine catalysts are used.

Preferably, the catalysts are applied in an amount of 0.01 to 2% by weight, preferably 0.1 to 1% by weight, each based on the total weight of components (B) to (F).

(F) Additives and/or auxiliaries

For the purposes of the present invention, additives and/or auxiliaries may also be used. Preferably, the additive and/or auxiliary component is added to the polyol component (B). According to an exemplary embodiment, a portion of the additive and/or auxiliary component is added to the polyol component before the reaction mixture is formed and another portion is separately added to the reaction mixture. According to another exemplary embodiment, the additive and/or auxiliary component in its entirety is added to the polyol component before the reaction mixture is formed.

The additive and/or auxiliary component may include, but are not limited to, fillers, water scavengers, deaerating agents, surface-active substances, wetting agents, flame retardants, smoke suppressants, bonding agents, dyes, pigments, hydrolysis inhibitors, antistatic agents, antioxidants, UV stabilizers, minor amounts of viscosity reducing inert diluents, fungistatic and bacteriostatic substances, combinations of these, and also any other known additives from the art. Preferred additives are fillers, water scavengers, deaerating agents, bonding agents, UV stabilizers, antioxidants and/or wetting agents. The additive and/or auxiliary may be used to modify the properties of the adhesive, for example, to control the wetting behavior, viscosity, storage life, sagging, moisture resistance, etc. The additives used herein are known and used in the polyurethane chemistry art for producing polyurethane adhesives.

Typically, a water scavenger is a material which is capable of adsorbing water. Preferred water scavengers are zeolite and/or calcium oxide. The wetting agents may be used to improve the spreadability of the adhesive on the components to be bonded. The deaerating agents may be added to reduce the formation of bubbles or to reduce sagging while bonding the components. Preferred fillers are selected from the group consisting of silica, titanium dioxide, sulphates, silicates, carbonates, kaolin, chalk, mica, quartz, talc, glass beads, ceramic spheres, and mixtures thereof. The fillers can be used individually or as mixtures and are advantageously added to the reaction mixture in amounts of from 10 to 30 % by weight, preferably from 15 to 25 % by weight, more preferably from 15 to 20 % by weight, based on the total weight of components (B) to (F).

To prepare the polyurethane adhesive of the invention, the components (A) to (F) are preferably mixed at temperatures of from 15 to 70°C and in particular from 20 to 50°C. In a preferred embodiment, components (B) to (F) are premixed to form the polyol component, which is then mixed with the isocyanate component (A).

The stoichiometry of mixing polyurethane-forming formulations, containing an organic di- and/or polyisocyanate and a polyol component is often expressed by a quantity known in the art as the isocyanate index. The index of such a formulation is simply the equivalence ratio of the NCO groups of di- and/or polyisocyanates (A) to the sum of the reactive hydrogen atoms of components (B) and optional component (C). This quantity is often multiplied by 100 and expressed as a percent. Preferred isocyanate index values in the mixing activated formulations, which are suitable for use in the practice of the present invention range from 95 to 150. A more preferred range of the isocyanate index values is from 105 to 130.

A further object of the present invention is to provide the use of a polyurethane adhesive according to the invention for the preparation of a sandwich panel used in 5G radome.

Accordingly, in one aspect, the present invention provides a sandwich panel for 5G radome, comprising: 1) an inner skin layer,

2) at least one first reinforcement layer overlying the inner skin layer;

3) a core layer overlying the at least one first reinforcement layer;

4) at least one second reinforcement layer overlying the core layer; and

5) an outer skin layer overlying the at least one second reinforcement layer; wherein the polyurethane adhesive according to the present invention is present in-between at least two adjacent layers of the sandwich panel and forms an adhesive bond between them, and wherein the sandwich panel has a transmission of at least 89%, measured according to the EN- CHI test method at a frequency of 6 GHz.

In a preferred embodiment, the sandwich panel according to the invention has a transmission of at least 92%, preferably at least 94%.

In one embodiment, the inner and outer skin layer each has a thickness of from 0.2 to 1 mm, and each comprises a polycarbonate (PC), a polyethylene terephthalate (PET), a polybutylene terephthalate (PBT), a polypropylene (PP), or combinations thereof, preferably PC. In one embodiment, the sandwich panel comprises a third and fourth reinforcement layer. In a further embodiment, the sandwich panel comprises further reinforcement layers. In another embodiment, the first and second reinforcement layer (and optionally the third and fourth reinforcement layer) each has a thickness of from 0.1 to 0.2 mm, and each comprises at least one reinforcing substance selected from the group consisting of glass fibers, glass mats, carbon fibers, polyester fibers, aramid fibers, nylon fibers, and basalt fibers. In a further embodiment, the core layer has a thickness of from 2 to 3 mm, and comprises at least one foams selected from the group consisting of a polyurethane (Pll), a polypropylene (PP), a polyethylene terephthalate (PET), a polystyrene (PS), and a polyvinyl chloride (PVC), preferably Pll.

In one embodiment, the polyurethane adhesive according to the present invention is applied to form a bond film with a thickness of 0.05-0.3 mm, preferably 0.05-0.2 mm, more preferably 0.05- 0.1 mm.

In one embodiment, the sandwich panel according to the invention has a density of < 0.9 g/cm ³ , preferably < 0.8 g/cm ³ , more preferably < 0.7 g/cm ³ , even more preferably < 0.65 g/cm ³ and still more preferably < 0.65 g/cm ³ , measured according to ASTM D1875-03(2018).

In one embodiment, the sandwich panel according to the invention has a density of < 0.9 g/cm ³ , preferably < 0.8 g/cm ³ , more preferably < 0.7 g/cm ³ , even more preferably < 0.65 g/cm ³ and still more preferably < 0.65 g/cm ³ , measured according to ASTM D1875-03(2018).

In one embodiment, the sandwich panel according to the invention has an impact strength of at least 10 KJ/m ² , preferably at least 13 KJ/m ² , more preferably at least 14 KJ/m ² , even more preferably at least 15 KJ/m ² , and still more preferably at least 18 KJ/m ² , determined acording to IOZD Notched ASTM D256-2010E1. In one embodiment, the sandwich panel according to the invention has a flexural strength of at least 15 MPa, preferably at least 18 KJ/m ² , more preferably at least 20 KJ/m ² , determined acording to DIN EN ISO 178.

In one embodiment, the sandwich panel according to the invention has a transmission of at least 89%, preferably at least 90%, more preferably at least 93%, and even more preferably at least 93.5%, measured according to the ENCHI test method at a frequency of 6 GHz.

In one embodiment, the sandwich panel according to the invention has no break in the Ball-drop- ping test, which is carried out at a temperature of -40 °C , with a ball having a weight of 500 g and from a height of 1 meter. The test is performed at a corner of the panel product.

In another aspect, the present invention further provides a method for preparing a sandwich panel for 5G radome according to the present invention, comprising

1) disposing at least one first reinforcement layer on an inner skin layer,

2) disposing a core layer on the at least one first reinforcement layer,

3) disposing at least one second reinforcement layer on the core layer, and

4) disposing an outer skin layer on the at least one second reinforcement layer; wherein at least two adjacent layers of the sandwich panel are bonded by using the polyurethane adhesive according to the invention.

The present invention is further illustrated, but is not to be limited, by the following examples.

Examples

The following materials were used:

Polyols

Polyol 1 : Glycerin-started polyoxypropylene polyol, with an OH value of 400 mg KOH/g, a hydroxyl functionality of 3, and a molecular weight Mw of 420 g/mol.

Polyol 2: Glycerin-started polyether polyol comprising propylene oxide unit (25.3 wt%) and ethylene oxide unit (72.5 wt%), with an OH value of 42 mg KOH/g, a hydroxyl functionality of 2.66, and a molecular weight Mw of 3550 g/mol.

Polyol 3: Propylene glycol-started polyoxypropylene polyol comprising propylene oxide unit (96.1 wt%) only, with an OH value of 55 mg KOH/g, a hydroxyl functionality of 1.93, and a molecular weight Mw of 190 g/mol.

Polyol 4: Propylene glycol-started polyoxypropylene polyol, with an OH value of 104 mg KOH/g, a hydroxyl functionality of 1.98, and a molecular weight Mw of 1070 g/mol.

Crosslinker: Glycerin

Blowing agent: EXPANCEL DLI120 from Nouryon

Catalyst: POLYCAT SA 8 from Evonik Specialty Chemicals (Shanghai) Co. Ltd.

Additive 1 : addovate LP from BASF

Additive 2: SILICONE ANTIFOAM from Univa

Additive 3: Aerosil 200 from Evonik Additive 4: Vetec from Sigma-Aldrich (Shanghai) Trading Co. Ltd.

Isocyanates:

LUPRANATE M20S from BASF

Measuring and test methods:

Dk and Df: I EC 61189-2-721 , using 2.45GHz split post dielectric resonator

Transmission: ENCHI@6GHz

Impact strength: IOZD Notched ASTM D256-2010E1

Flexural strength: DIN EN ISO 178

Ball-dropping test: under -40 °C, with a ball of 500 g and from a height of 1 meter

Density: measured according to ASTM D1875-03(2018) standard test method for density of adhesives in fluid form

Examples 1-4: Preparation of the polyurethane adhesive

Inventive Examples 1-4 were prepared according to the components for forming polyurethane adhesive as specified in the following table 1. The amounts of the respective materials are given in part by weight. Each of the polyurethane adhesives includes B-component, which includes therein polyol components, crosslinker, catalyst, blowing agent and some additives, and A- component, which is isocyanate.

The B-component was prepared by mixing the respective materials as specified in table 1 by agitation and then making the mixture stand untill the agitating air bubbles were removed. The thus-prepared B-component was then injected into the B tank of the polyurethane low-pressure machine, and the A-component was added into the A tank, with the temperatures of the B tank and the A tank being set as 20-30 °C, respectively. Then, the B-component and the A-component were mixed by speedmixer at 2000 rpm for about 1 min to give the product of the polyurethane adhesive. The cured adhesive was cut for Dk and Df test. The results were summarized in the following table 1.

An epoxy adhesive commercially available from Huntsman under the trade name of Araldite 2015 was used as a comparative example.

Table 1

PO ratio means weight percentage of oxypropylene groups within the polyetherpolyol based on the total weight of the polyetherpolyol.

Inventive Examples 1-4 realized the desired compromised Dk and Df values and the density of <1.12 g/cm ³ . Furthermore, the Dk and Df values for the adhesive in Exp. No. 4 combined with the glass fibre (GF) were <4.17 and <0.04, respectively. Production of the sandwich panel

The general procedure for producing the sandwich panel by using the polyurethane adhesive according to the invention will be described below. Example 5: Pll foam+GF/Epoxy+PC An epoxy adhesive was commercially available from Huntsman under the trade name of Araldite 2015.

A spread PC film with a thickness of 0.75 mm was used as an inner skin layer, on which was disposed a glass-fiber layer with a thickeness of 0.15 mm as a first reinforcement layer. Then, a Pll foam with a thicknes of 2.5 mm, as a core layer, was disposed on the glass-fiber layer by using the epoxy adhesive in a thickness of 0.1 mm. Another glass-fiber layer with a thickeness of 0.15 mm, as a second reinforcement layer, was disposed on the Pll foam, which was then covered by a PC film with a thickness of 0.75 mm as an outer skin layer, to give a sandwich board with a surface area of 1 m ² . The thus-prepared sandwich board was baked under a temperature of 80-90 °C with 20 Kg of gravity applied for 2 hours to be fully cured, which was then cooled down at room temperature for properties tests. The final thickness was 3.6 mm.

Example 6: Pll foam+GF/PU+PC

The procedure of example 5 was repeated, except that the epoxy adhesive was replaced with the polyurethane adhesive of example 4. The final thickness was 3.5 mm.

The mechanical properties of the sandwich panel prepared above are summarized in Table 2.

Table 2

The foregoing examples of the present invention are offered for the purpose of illustration and not limitation. It will be apparent to those skilled in the art that the embodiments described herein may be modified or revised in various ways without departing from the spirit and scope of the invention. The scope of the invention is to be determined by the appended claims.