Technical Field

[0001 ] The present invention relates to foam precursor compositions. In particular, it concerns a foam precursor composition comprisi ng an acrylic monomer or oligomer, a thiol component, and a borane initator. Such precursor composition can be used as I K, 1 .5 K and 2K compositions stored in aerosol type pressurized cans. The precursor composition of the present invention can be entirely isocyanate free. Background for the invention

[0002] Pressurized fluids stored in containers (cans or vessels) are used in a wide field of applications. Sprayable foams are used for several industrial applications as well as by hobbyists. Foams are, among others, used in hig h resi liency flexible foam sealing, rig id foam insulation panels, microcellular sprayable foam seals and gaskets, durable elastomeric wheels and tyres, electrical potting compounds, Spandex fibers, carpet underlay, hard plastic parts, and the like. At present, existing sprayable foam formulations are one, one and a half, and two component foams stored in aerosol cans and /or pressure vessels.

[0003] Probably the best known and most commonly used example of sprayable foam is polyurethane (PU) foam. A PL) foam formulation comprises a mixture of polyols, diisocyanates, liq uefied gases as blowing agents, and several additives. Upon srpaying, these froths are cu red by the reaction of the isocyanate terminated prepolymers with ambient moisture.

[0004] A first issue with PU foam formulations is that curing is triggered by the moisture present in the ambient ai r. This means that their crosslinking rates can vary from a rainy day to a sunny day, from generally humid regions to generally dry regions, and can even be useless in particularly dry atmospheres as can be encountered in continental or (semi-) desertic climates. [0005] A second, quite major disadvantage of PL) foam compositions is that isocyanates are toxic. Methylene Diphenyl Diisocyanate (MDI) is the isocyanate most commonly used in the production of PL) foams. This compound, although the least hazardous of the isocyanate groups, is still toxic, harmful by inhalation or ingestion, and also via skin contact. In addition, the compound is flammable and can also be explosive.

[0006] To solve this problem, DEI 031 1 607 proposes to react polyols with a stoechiometric excess of a monomer-free polyisocyanate and then to remove the excess di-isocyanate. Although quite effective, this solution requires an extra step for removing the excess diisocyanate and an additional thinning agent, methyl dimethyl ether, must be added to reduce the viscosity of the mixture.

[0007] Similarly, EP1 51 8874 discloses a one component foam (OCF) composition having a low monomer content, which is obtained from a specific mixture of poly-(phenylene methane poly-isocyanates) (PMDI), and comprising removal of the monomeric isocyanate by distillation. [0008] US200925381 9 discloses an OCF composition having a low content of monomeric MDI obtained by reacting a specific mixture of MDI and polymeric MDI having an average functionality of from 2.2 to 5.2. But the preparation of the specific mixture requires the extraction of mixtures of diisocyanates and polyisocyanates having an average functionality of greater than 2 and then freeing them from diisocyanates by distillation, which basically does not remove, but simply shifts the distillation step upstream of the process, compared with the previous disclosures.

[0009] WO2005 /007721 discloses an OCF composition having a low content of free MDI, obtained by the use of mixtures of NCO-terminated prepolymers obtained by reaction of polyols and diphenylmethane diisocyanate in stoechiometric excess which had been freed from monomers, and a number of polymeric MDI and diluents. Disadvantageous here are the extremely high viscosities of the starting materials in the absence of diluting monomeric MDI.

[001 0] In an attempt to make without any isocyanates, the use of Alpha Silane terminated PL) prepolymers (STP's) has been proposed. A well-known example is Soudafoam SMX(R) wh ich is an isocyanate-free fixing foam. Another exam ple of the incorporation of silane to avoid the toxic effect of isocyanate is presented in JP2000026648 which discloses a composition contain ing (i) an organic compound having a C = C double bond , (ii) a compound having an Si H group and (iii) a blowi ng agent and /or a compound having an OH group. Although isocyanate-free, the use of PL) foams based on Alpha Silane terminated PL) prepolymers in aerosol cans has several drawbacks, such as high viscosity, slow curing, some crack formation and a rather high cost.

[001 1 ] EP1 798255 discloses a process for preparing precursor mixtures of polymer foam to be packed in pressurized vessels and or aerosol cans. The polymer foam can be either a polyester foam, a foam based on cyclocarbonate oligomers and primary amine oligomers, or a foam based on acrylic oligomers. Biodiesel is included as latent hardener arid /or extender and/or liquid filler in polyurethane formulations, more particularly One Component Foams precu rsor mixtures. The improved formulation avoids long-term deterioration of flexibility and mechan ical strength of the foam. [001 2] US20041 76485 discloses a photo-polymerizable acrylic foam precursor composition for use in variety of indoor or outdoor sealing applications. Said composition comprises at least two acryl ic-based reactionable oligomers, one difunctional and the other at least trifunctional, and at least a first and a second radical producing photoinitiators.

[001 3] WO201 01 08920 discloses a pressurized can contain ing a polymer foam composition for use in one component and two component foams, said polymer foam composition comprising an unsatu rated resin and an oligomeric acrylic resin. Peroxide in itiators are proposed like benzoyl peroxide, methyl ethyl ketone peroxide, Cumene peroxide and Lauryl peroxide.

[001 4] WO9640528 discloses thiol-containing photosensitive polymeric compositions and foam compositions that are formed by mechanically frothing the polymeric composition and curing the resu lting froth with radiation to yield a foam composition. To trigger the crosslinking reaction the composition comprises a photoin itiator and /or a photosensitizer. [001 5] GB865651 discloses a polar vinyl monomer polymerized in the presence of a two- component catalyst system consisting of (a) an organo-boron compound having at least one direct carbon to boron linkage and (b) a hydroperoxide, including hydrogen peroxide. GB1 01 8991 discloses a coating composition comprising methyl methacrylate monomer, triethyl borane and a trace of iodine which can be formed into an aerosol spray with dichlorodifluoromethane. US201 01 78520 discloses (meth)acrylate-based polymerizable compositions and adhesive systems prepared therefrom, which include a alkylated borohydride or tetraalkyl borane metal or ammonium salt and a polymerizable siloxane. These compositions and adhesive systems are particu larly well suited for bonding applications which involve at least one low energy bond ing surface, for example, the polyolefins, polyethylene, and polypropylene.. GB1 1 1 1 603 discloses reinforced polymeric compositions comprising an alkyl methacrylate polymer and an inorganic reinforcing agent. The polymerization of the alkyl methacrylate polymer in presence of the reinforcing agent is in itiated by means of a free-radical catalyst, preferably a peroxy compound ; an activator may also be used, e.g . a boron hydride or borane (optionally complexed with various basic compounds, such as amines or triphenylphosphine, or with tetrahydrofuran) or a redox reducing component

[001 6] It is a main object of the present invention to provide a foam precursor composition wh ich can be used in 1 K as well as in 1 .5 K aerosol can systems for forming a foam. The curing of the foam composition is not dependent on ambient moisture and proceed even at low temperatures, below 0°C. By using (meth)acrylate oligomers, the foam precursor compositions of the present invention may comprise no isocyanate.

[001 7] These and other advantages of the present invention are described in continuation.

Summary of the invention

[001 8] The present invention is defined in the appended independent claims. Preferred embodi ments are defined in the dependent claims. In particular, the present invention concerns a polymeric foam precursor composition comprising the following components:

(a) A di- or tri-functional (meth)acrylate oligomer;

(b) a di- or higher-functional thiol compound, and (c) a borane compound as initiator.

[001 9] In a preferred embodiment, the (meth)acrylate oligomer is a polyester (meth)acrylate or a polyurethane acrylate, or mixture of both. The thiol compound preferably has the structure R(SH) _n , with n > 2, preferably n = 3 or 4, and is preferably pentaerythritol tetrakis(3-mercaptopropionate (TKT). The composition preferably comprises no isocyanates.

[0020] The borane compound can be an alkyl- or alkoxy- borane of formula (alkoxy)<3 -n) - B - (alkyl)n, with n = 0 to 3, and wherein alkyl and alkoxy each independently comprise between 1 and 1 4 C, more preferably the borane is selected among the group of trimethylborane, triethyl borane, tripropylborane, tributylborane, trihexylborane, trioctylborane, tridecylborane, tri tridecylborane, most preferably, the borane compound is selected among triethylborane, methoxydiethylborane, and tributylborane.

[0021 ] In a preferred embodiment, the composition further comprises one or more of the following components :

(d) A phenolic and/or phosphonic acid compound as stabilizer,

(e) A flame retardant, preferably TCPP;

(f) A surfactant, such as a silicone surfactant,

(g) A propellant such as LPG and/or dimethyl ether (DME).

(h) A diluent such as monoethylene glycol (MEG) for a half component.

[0022] The di- or tri-functional (meth)acrylate oligomer and di- or higher-functional thiol compound are preferably in the form of a stabilized thiol acrylate blend, further comprising a phenolic or phosphonic acid compound as stabilizer.

[0023] The composition is preferably stored in an aerosol can. To this effect, at least the di- or tri- functional (meth)acrylate oligomer and di- or higher-functional thiol compound should be stored in an aerosol can, with at least the borane compound being physically separated from the former.two components. The aerosol can should also contain a propellant. In a 1 .5K system, the borane compound optionally a diluent compound, such as monoethylene glycol (MEG) are stored in a separate compartment as half-component, and the di- or tri-functional (meth)acrylate oligomer and di- or higher-functional thiol compound are stored outside said compartment. The borane initiator is brought into contact with the di- or tri-functional (meth)acrylate oligomer and di- or higher-functional thiol compound upon spraying the composition out of the can through an aerosol nozzle. [0024] In a 1 K foam system the borane compound is microencapsulated such that the microcapsules are dispersed in a solution or suspension comprising the di- or tri-functional (meth)acrylate oligomer and di- or higher-functional thiol compound. The microcapsules are opened by bursting and/or tearing upon spraying the composition out of the can through an aerosol nozzle, so that the borane contacts the thiol acrylate blend.

[0025] It is preferred that the molar ratio of the total mercaptan units of the thiol compound to the total (meth)acrylic units of acrylate oligomer is comprised between 50 and 1 50%, preferably between 90 and 1 1 0%, more preferably between 95 and 100%. The borane initiator is preferably present in an amount comprised between 0.5 and 10 wt.%, with respect to the total weight of thiol compound and acrylate oligomer, preferably between 1 and 6 wt.%.

[0026] The present application also concerns the use of a bi- or higher-functional thiol compound and of a borane compound for curing a di- or tri-functional (meth)acrylate oligomer into a foam.

Brief description of the Figures

[0027] For a fuller understandi ng of the nature of the present invention, reference is made to the following detailed descri ption taken in conj unction with the accompanying drawings in wh ich :

Figure 1 : shows two embodiments of curing reaction of a foam according to the present invention.

Figure 2 : Shows a 1 .5 K foam system stored in an aerosol can.

Figure 3 : Shows a I K foam system stored in an aerosol can, with microencapsulated borane initiator.

Figure 4: Shows various stabilizers, (a) phosphonic acid, and (b) phenolic compounds. Figure 5 : Shows various boranes suitable as initiator according to the present invention. Figure 6: Shows the formula of a preferred thiol com pound : TKT. Detailed description of the invention

[0028] The cross-linking system proposed in the present invention as an alternative route to isocyanate-moisture curing is metal free click chemistry, in particular, it is thiol-X chemistry. The present invention concers a thiol-acrylate reaction system, defined by an acrylate oligomer (di- or tri-functional) to be cured with a di- or higher-fu nctional thiol compound with the aid of an accelerated initiator. In particular, the polymeric foam precursor composition of the present invention comprises at least the following components:

(a) A di- or tri-functional (meth)acrylate oligomer (2);

(b) a di- or higher-functional thiol compound (3), and

(c) a borane compound (4) as initiator.

[0029] The di- or tri-functional (meth)acrylate oligomer (2) can be a polyester (meth)acrylate or a polyurethane acrylate, or a mixture of both. For example, the (meth)acrylate oligomer can be produced by reacting maleic acid with a fatty acid diol yielding a fatty dimer polyol suitable as (meth)acrylate oligomer for the present invention. Bisphenol A based acrylate oligomers may be used instead or in combination with the foregoing oligomers, such as CN 104A75, CN 104A60, SR349, CN91 67, CN970A60, or CN 1 963available from Sartomer, or Bisphenol A glycerolate (1 glycerol/phenol) diacrylate from Sigma-Aldrich (CAS: 4687-94-9).

[0030] The di- or higher-functional thiol compound (3), preferably has the structure R(SH) _n , with n > 2, preferably n = 3 or 4. For example the following thiol compounds are suitable for the present invention:

• Pentaerythritol tetrakis(3-mercaptopropionate) (TKT),

• trimethylolpropane tris (3-mercaptopropionate) (CAS: 33007-83-9),

• Tris[2-(3-mercaptopropionyloxy)ethyl] isocyanurate (CAS: 361 96-44-8),

• Pentaerythritol tetrakis(2-mercaptoacetate) (CAS: 1 01 93-99-4). [0031 ] The di- or tri-functional (meth)acrylate oligomer (2) and di- or higher-functional thiol compound (3) are preferably pre-blended together with a stabilizer (5) to obtain a stabilized thiol- acrylate blend (3a), which is easier to handle than the two components separately. Best stabilized blends were obtained by using a phenolic and/or phosphonic acid compound as stabilizer. For example, pyrogallol, vinylphosphonic acid, phenylphosphonic acid, phosphorous acid (CAS: 1 3598- 36-2), (1 2-phosphonododecyl)phosphonic acid (CAS: 7450-59- 1 ), (3-Bromopropyl)phosphonic acid (CAS: 1 1 90-09-6), ethyl/buthyl phosphonic acid silica (no CAS nr. Sigma-Aldrich product nr:744794) can be successfully used to stabilize the thiol-acrylate blend (3a). Figure 4 illustrates examples of stabilizers, (a) phoshonic acid wherein R can be H, alkyl (preferably 2-9 C-alkyl), vinyl, or aromatic, and (b) phenolic stabilizers. [0032] Because many applications of such foams are in cavities, sheltered from any light, photo-initiators as proposed in the prior art were discarded as not satisfactory. Various in itiators were tested, including peroxides and toluidines. After an extensive screen ing it was finally found out that best results were obtained with a borane compound as initiator. For example, the borane compound (4) can be an alkyl- or alkoxy-borane of formula (alkoxy)3-n - B - (alkyl)n, with n = 0 to 3, and wherein alkyl and alkoxy each independently comprise between 1 and 1 4 C. In particular the borane compound may be selected among the group of trimethylborane, triethylborane, tripropylborane, tributylborane, trihexylborane, trioctylborane, tridecylborane, tri tridecylborane. Triethylborane, methoxydiethylborane, and tributylborane are preferred borane compounds. Examples of boranes suitable for the present invention are illustrated in Figure 5. [0033] One of the major advantages of the present invention is that stable foams capable of curing in less than 3 h, even in less than 1 h, regardless of the relative humidity, can be obtained with no isocyanate groups at all. In view of the high toxicity of isocyanate groups, this is h ig hly advantageous.

[0034] The polymeric foam precursor composition of the present invention may further comprise one or more of the following components:

• A flame retardant (6), preferably tris(2-chloroisopropyl) phosphate (TCPP);

• A surfactant (7), such as a non-ionic surfactant, preferably a silicone surfactant, such as Tegostab ^® available from Evonik Industries or Vorasurf available from Dow Chemicals.

• A propellant (8) such as LPG and /or dimethyl ether (DME).

· A diluent (9) for the borane compound initiator (4), such as monoethylene glycol

(MEG), for the half component (1 0) in a 1 .5 K system as will be discussed below.

[0035] The acrylate compound (2) and thiol compound (3), preferably in the form of a stabilized thiol-acrylate blend (3a) must be physically separated from the borane initiator (4) until curing is desired . This can be achieved by storing them in two separate containers thus forming a 2 K system. 2 K systems are, however, cumbersome to use and I K or 1 .5 K are preferred , in particular if stored in an aerosol can with a propellent (8) for spraying.

[0036] In a 1 .5 K foam systems the borane initiator (4) is added to the (stabil ized) thiol- acrylate blend (2 , 3 , 3a) shortly or immediately before the composition is dispensed from its container. As illustrated in Figure 2 the (stabilized) thiol-acrylate blend (2 , 3, 3a) is preferably stored in a pressurized aerosol can (1 ) which further contains a second, smal ler pressurized container containing the half component (1 0) comprising a borane initiator (4) and, if necessary, a di luent (9) such as monoethylene g lycol (MEG)). Any other additives such as flame retardants (6) or surfactants (7) may be contained either in the aerosol can or in the second, smaller container. It is generally preferred to store such additives in the main aerosol can. Pressurization of the aerosol can (1 ) and smaller container requires a propellant (8). Upon actuating the aerosol nozzle of the aerosol can, the half component (1 0) is released from the second container into the (stabil ized) thiol-acrylate blend (2 , 3, 3a), simultaneously or shortly before the foam forming composition is to be dispensed and applied. It is clear that once the aerosol nozzle is actuated, the aerosol can must be emptied and cannot be re-used at a later stage, since any com position remaining i n the can will cure in the can.

[0037] In this respect, I K foam systems are advantageous since a I K system can be dispensed from an aerosol can in several shots separated in time. This can be ach ieved by microencapsulating the borane initiator (4) in microcapsules (1 1 ) dispersed in the (stabi lized) thiol-acrylate blend (2 , 3, 3a). The microcapsules must open upon sprayi ng the composition through the nozzle in order to mix the borane initiator (4) with the (stabil ized) thiol-acrylate blend (2 , 3, 3a). This can be achieved either by blowing the microcapsule by the pressure difference between the inside of the microcapsules and the atmosphere or by shearing the microcapsules as they pass through the aerosol nozzle to tear them open. Examples of microcapsules which can be used to th is purpose are descri bed, e.g., i n WO201 1 003805 , US201 1 /0236498, GB241 6524, and WO03 /066209. The microencapsulated borane initiator (4, 1 1 ) generally does not require any diluent (9) as each particle comprises a small volume of initiator. [0038] The amount of thiol compound (3) relative to the amount of acrylate oligomer (2) depends on the basis of the number of mercaptan units with respect to the n umber of (meth)acrylate units and therefore depends on the respective functionalities of the thiol and acrylate compou nds. In particular, it is preferred that the molar ratio of the total mercaptan units of the thiol compound (3) to the total (meth)acrylic units of the acryliate oligomer (2) be comprised between 50 and 1 50%, preferably between 90 and 1 10%, more preferably between 95 and 1 00%. [0039] For a composition comprising 1 00 parts of both th iol compound (3) and acrylate oligomer (2) together, the borane initiator (4) is preferably present in an amount comprised between 0.5 and 10 parts, preferably between 1 and 6 parts, more preferably between 3.5 and 4.5 parts. [0040] For 1 00 parts of both thiol compound (3) and acrylate oligomer (2) together, the composition of the present invention may comprise between 1 and 20 parts, preferably 5 and 1 5 parts of a flame retardant (6) such as tris(2-ch loroisopropyl) phosphate (TCPP). It may comprise between 1 and 1 0 parts, preferably between 2 and 5 parts of a surfactant such as Tegostab ^® B8871 , B84503, B851 3, B851 7, available from Evonik Industries or Vorasurf 504 available from Dow Chemicals., and between 30 and 60 parts, preferably between 35 and 50 parts of a propellant (8), such as LPG and / or DME, the latter acting also as solvent for the (stabilized) th iol-acrylate blend (2 , 3, 3a). I n case a stabilized thiol-acrylate blend (2 , 3, 3a) is formed, it can be stabi lized by adding 1 0 to 20 parts of a stabilizer (6) such as phenolic and/or phosphonic acid to a 1 00 parts of both thiol compou nd (3) and acrylate oligomer (2) together. For 1 .5 K foam systems, it is important that the viscosity of the half component (1 0) -main ly the borane initiator (4)- be sufficiently low to ensure a homogeneous mix thereof with the main component -main ly the thiol-acrylate blend-. The viscosity of the half-component (1 0) can be controlled with a non reactive d iluent (9), such as monoethylene glycol (MEG). The weight ratio diluent (9) to borane (4) can be comprised between 0.5 and 5 , preferably between 1 and 3.

[0041 ] Table 1 lists an example of 1 .5 K foam composition according to the present invention.

Table Ί : Typical 1.5K aerosol foam formulation

[0042] The rigidity and other mechanical properties of the foam can be controlled by an adequate choice of the components. For examples, softer foams can be obtained with ester acrylates and harder foams with urethane acrylates, since the latter form a more rig id backbone than the former.

[0043] U nlike isocyanates which are activated by moisture, borane initators are activated with oxygen in the atmosphere. This has the great advantage that the cu ring kinetics of the foam is not dependent on the weather and climate of the place of application and is constant regardless of the moisture content of the atmosphere. Another great advantage is that the foam can cure also at temperatures below freezi ng point.