Technical Field

The present disclosure relates generally to the field of adhesives, more specifically to the field of structural adhesive compositions and films for use in particular for bonding metal parts. More specifically, the present disclosure relates to a curable precursor of a structural adhesive composition and to a partially cured precursor composition. The present disclosure also relates to a method of bonding two parts and to a composite article. The present disclosure is further directed to the use of a curable precursor of a structural adhesive composition for construction and automotive applications, in particular for body -in-white bonding applications in the automotive industry.

Background

Adhesives have been used for a variety of holding, sealing, protecting, marking and masking purposes. One type of adhesive which is particularly preferred for many applications is represented by structural adhesives. Structural adhesives are typically thermosetting resin compositions that may be used to replace or augment conventional joining techniques such as screws, bolts, nails, staples, rivets and metal fusion processes (e.g. welding, brazing and soldering). Structural adhesives are used in a variety of applications that include general-use industrial applications, as well as high- performance applications in the automotive and aerospace industries. To be suitable as structural adhesives, the adhesives shall exhibit high and durable mechanical strength as well as high impact resistance.

Structural adhesives may, in particular, be used for metal joints in vehicles. For example, an adhesive may be used to bond a metal panel, for example a roof panel to the support structure or chassis of the vehicle. Further, an adhesive may be used in joining two metal panels of a vehicle closure panel. Vehicle closure panels typically comprise an assembly of an outer and an inner metal panel whereby a hem structure is formed by folding an edge of an outer panel over an edge of the inner panel. Typically, an adhesive is provided there between to bond the panels together. Further, a sealant typically needs to be applied at the joint of the metal panels to provide sufficient corrosion resistance. For example, US Pat. No. 6,000,118 (Biematetal.) discloses the use of allowable sealant bead between the facing surfaces of the two panels, and a thin film of uncured paint-like resin between a flange on the outer panel and the exposed surface of the inner panel. The paint film is cured to a solid impervious condition by a baking operation performed on the completed door panel. US Pat. No. 6,368,008 (Biemat et al.) discloses the use of an adhesive for securing two metal panels together. The edge of the joint is further sealed by a metal coating. WO 2009/071269 (Morral et al.) discloses an expandable epoxy paste adhesive as a sealant for a hem flange. A further hemmed structure is disclosed in US Pat. No. 6,528,176 (Asai et al.). Further efforts have been undertaken to develop adhesive compositions whereby two metal panels, in particular an outer and an inner panel of a vehicle closure panel, could be joined with an adhesive without the need for a further material for sealing the joint. Thus, it became desirable to develop adhesive systems that provide adequate bonding while also sealing the joint and providing corrosion resistance. A partial solution has been described in e.g. WO 2007/014039 (Lamon), which discloses a thermally expandable and curable epoxy -based precursor of an expanded thermo set film toughened foamed fdm comprising a mixture of solid and liquid epoxy resins, and which is claimed to provide both favorable energy absorbing properties and gap filling properties upon curing. Other partial solutions have been described in EP- Al-2 700 683 (Elgimiabi et al.) and in WO 2017/197087 (Aizawa) which disclose structural adhesive fdms suitable for forming a hem flange structure. Structural adhesive films or tapes typically suffer from lack of elasticity and insufficient tackiness which makes them only partially suitable for hem flange bonding. Further partial solutions have been described in US-A1- 2018/0127625 (Shafer et al.) which discloses a so-called structural bonding tape. Structural bonding tapes are generally suboptimal in terms of performance attributes such as e.g. adhesive strength, stability and corrosion resistance.

Without contesting the technical advantages associated with the solutions known in the art, there is still a need for a structural adhesive composition which would overcome the above- mentioned deficiencies.

Summary

According to one aspect, the present disclosure relates to a curable precursor of a structural adhesive composition, comprising: a) a thermally curable resin; b) a thermal curing initiator for the thermally curable resin; c) a radiation self-polymerizable multi-functional compound comprising a polyether oligomeric backbone and at least one free-radical (co)polymerizable reactive group at each terminal position of the oligomer backbone; and d) a free-radical polymerization initiator for the radiation self-polymerizable multi-functional compound; wherein the free-radical polymerization initiator is activated by visible light.

According to another aspect, the present disclosure is directed to a partially cured precursor of a structural adhesive composition, comprising: a) a polymeric material comprising the self-polymerization reaction product of a polymerizable material comprising a radiation self-polymerizable multi-functional compound; b) optionally, some residual free-radical polymerization initiator for the radiation self- polymerizable multi-functional compound; c) a thermally curable resin; and d) a thermal curing initiator for the thermally curable resin; and wherein the thermally curable resin(s) are (substantially) uncured; and wherein the free-radical polymerization initiator is activated by visible light.

In still another aspect of the present disclosure, it is provided a method of bonding two parts, which comprises the steps of: a) applying a curable precursor or a partially cured precursor as described above to a surface of at least one of the two parts; b) joining the two parts so that the curable precursor or the partially cured precursor (hybrid) structural adhesive composition is positioned between the two parts; and c) optionally, partially curing the curable precursor according of step a) by initiating the free-radical polymerization initiator for the radiation self-polymerizable multifunctional compound by irradiation with visible light, preferably at least visible blue light, thereby forming a partially cured precursor comprising a polymeric material resulting from the self-polymerization reaction product of the radiation self- polymerizable multi-functional compound; and/or d) substantially fully curing the partially cured precursor of step a) or c) by initiating the thermal curing initiator for the thermally curable resin, thereby obtaining a substantially fully cured (hybrid) structural adhesive composition and bonding the two parts.

According to yet another aspect, the present disclosure relates to the use of a curable precursor or a partially cured precursor as described above, for industrial applications, in particular for construction and automotive applications.

Detailed description

According to a first aspect, the present disclosure relates to to a curable precursor of a structural adhesive composition, comprising: a) a thermally curable resin; b) a thermal curing initiator for the thermally curable resin; c) a radiation self-polymerizable multi-functional compound comprising a polyether oligomeric backbone and at least one free-radical (co)polymerizable reactive group at each terminal position of the oligomer backbone; and d) a free-radical polymerization initiator for the radiation self-polymerizable multifunctional compound; wherein the free-radical polymerization initiator is activated by visible light.

In the context of the present disclosure, it has been surprisingly found that a curable precursor as described above is particularly suitable for manufacturing structural adhesive compositions provided with excellent characteristics and performance as to elasticity, tackiness, cold-flow, flexibility, handling properties and surface wetting in their uncured (or pre-cured) state, as well as to adhesive strength, ageing stability and corrosion resistance in their fully cured state. The curable precursor of a structural adhesive composition as described above have been surprisingly found to combine most of the advantageous characteristics of both the structural adhesive films and the structural bonding tapes known in the art, without exhibiting their known deficiencies.

It has further been discovered that, in some executions, the curable precursor as described above is provided with advantageous thixotropic properties, which makes it particularly suitable for the manufacture of multi-dimensional structural adhesive articles and objects, in particular via suitable printing techniques.

It has still surprisingly been found that the curable precursor as described above is particularly suitable for the manufacture of one-part curable precursors of structural adhesive compositions due to its outstanding chemical stability, and in spite of the dissimilar chemical nature of the hybrid curing system employed. One-part curable precursors are advantageous over two-part curable precursor compositions in that they do not require any specific delayed mixing steps or associated equipment, and typically provide more processing flexibility.

It has yet surprisingly been discovered that, in some executions, the curable precursor as described above is suitable for manufacturing structural adhesive compositions provided with excellent characteristics and performance as to adhesion to oily contaminated substrates, such as stainless steel and aluminum.

Without wishing to be bound by theory, it is believed that these excellent characteristics are due in particular to the presence of a specific hybrid curing system in the curable precursor involving both: a) a thermally-induced curing of a thermally curable resin, and b) a radiation-induced selfpolymerization of a multi-functional compound as specified above. Still without wishing to be bound by theory, it is believed that this dual/hybrid curing system involving two independent reactive systems, which have a different chemical nature and which co-exist in the curable precursor without interfering with each other, has the ability to form - upon complete curing - an interpenetrating network involving a polymeric material comprising the self-polymerization reaction product of a polymerizable material comprising the self-polymerizable multi-functional compound and a polymeric product resulting from the thermal curing of the thermally curable resin.

More specifically, the above-described hybrid curing system is particularly suitable to perform an overall curing mechanism involving a two-stage reaction whereby two polymer networks are formed sequentially.

In a first stage reaction (B-stage), the radiation self-polymerizable multi-functional compounds self-polymerize upon initiation by the free-radical polymerization initiator for the radiation self-polymerizable multi-functional compound, thereby forming a polymeric material comprising the self-polymerization reaction product of a polymerizable material comprising the self- polymerizable multi-functional compounds. Typically, the temperature T1 at which the free-radical polymerization initiator for the radiation self-polymerizable multi-functional compound is initiated is insufficient to cause initiation of the thermal curing initiator of the thermally curable resin. As a consequence, the first stage reaction typically results in a partially cured precursor, wherein the thermally curable resins are substantially uncured and are in particular embedded into the polymeric material comprising the self-polymerization reaction product of the polymerizable material comprising the self-polymerizable multi-functional compounds.

The first stage reaction which typically leads to a phase change of the initial curable precursor due in particular to the polymeric material comprising the self-polymerization reaction product of the self-polymerizable multi-functional compounds providing structural integrity to the initial curable precursor, is typically referred to as a film-forming reaction. Advantageously, the first stage reaction does typically not require any substantial energy input.

Furthermore, it is particularly advantageous when light of the visible spectrum of the light, in particular blue light, is used to activate the free-radical polymerization initiator, when compared to UV-light activation commonly utilized for initiating cure of adhesive in industrial applications. Not only is the penetration of visible light, in particular blue light, higher, it was also found that the reaction proceeds much faster than with UV-A-light. While the former has the advantage that thicker films may be generated, the latter has the advantage that films may be generally faster manufactured. Also, EHS concerns using UV light at manufacturing sites may be avoided.

The partially cured precursor may typically take the form of a film-like self-supporting composition or a multi-dimensional object having a dimensional stability, which makes it possible for it to be pre-applied on a selected substrate, in particular a liner, until further processing. The partially cured precursor is typically provided with excellent characteristics and performance as to elasticity, tackiness, cold-flow and surface wetting. The radiation self-polymerizable multi- functional compound comprising a polyether oligomeric backbone and at least one free-radical (co)polymerizable reactive group at each terminal position of the oligomer backbone is believed to play a critical role in obtaining the advantageous properties of the partially cured precursor as described above. Advantageously, the partially cured precursor may be appropriately shaped to fulfil the requirements of any specific applications.

The second stage reaction (A-stage) occurs after the first stage reaction and involves thermally curing the thermally curable resins upon thermal initiation by the appropriate thermal curing initiator at a temperature T2 which is typically greater than the temperature Tl. This reaction step typically results in forming a polymeric product resulting from the thermal curing of the thermally curable resins, in particular from the (co)polymerization of the thermally curable resins and the thermal curing initiators (or curatives) of the thermally curable resins.

The curable precursor of the present disclosure typically relies on the above-described dual/hybrid curing system involving two independent reactive systems activated with distinct triggering steps to ensure performing the above-described two-stage reaction in a sequential manner. Advantageously, the curable precursor of the present disclosure may be partially cured (or pre-cured) and pre-applied on a selected substrate before being finally cured in-place to produce a structural adhesive provided with excellent characteristics directly on the desired substrate or article.

As such, the curable precursor of the present disclosure is outstandingly suitable for bonding metal parts, in particular for hem flange bonding of metal parts in the automotive industry. The curable precursor of a structural adhesive composition as described herein is also outstandingly suitable for the manufacture of multi-dimensional structural adhesive articles and objects. Advantageously still, the curable precursor is suitable for automated handling and application, in particular by fast robotic equipment.

In the context of the present disclosure, the expression “radiation self-polymerizable compound” is meant to refer to a compound able to form a polymeric product (homopolymer) resulting from the radiation-induced polymerization of the compound almost exclusively with itself, thereby forming a homopolymer. The term “homopolymer” is herein meant to designate polymer(s) resulting exclusively from the polymerization of a single type of monomers.

In the context of the present disclosure still, the expression “thermally curable resin” is meant to refer to a resin/monomer able to form a polymeric product (heteropolymer) resulting from the thermally -induced (co)polymerization of the curable resins and the thermal curing initiators (or curatives) of the thermally curable resins. The term “heteropolymer” is herewith meant to designate a polymer resulting from the (co)polymerization of more than one type of monomers. In the context of the present disclosure, the expression “the thermally curable resins are substantially uncured” is meant to designate that less than 10 wt.%, less than 5 wt.%, less than 2 wt.%, or even less than 1 wt.% of the initial curable resins are unreacted.

The terms “glass transition temperature” and “Tg” are used interchangeably and refer to the glass transition temperature of a (co)poly meric material or a mixture of monomers and polymers. Unless otherwise indicated, glass transition temperature values are determined by Differential Scanning Calorimetry (DSC).

According to one typical aspect of the curable precursor of the disclosure, the free-radical polymerization initiator is initiated at a temperature Tl, wherein the thermal curing initiator is initiated at a temperature T2, wherein the temperature T2 is greater than the temperature Tl, and wherein the temperature Tl is insufficient to cause initiation of the thermal curing initiator which therefore remains substantially unreacted.

According to another typical aspect of the curable precursor of the disclosure, the temperature Tl is no greater than 90°C, no greater than 80°C, no greater than 60°C, no greater than 50°C, no greater than 40°C, no greater than 30°C, no greater than 25°C, no greater than 20°C, or even no greater than 15°C.

According to still another typical aspect of the curable precursor of the disclosure, the temperature Tl is in a range from -10°C to 85°C, from 0°C to 80°C, from 5°C to 60°C, from 5°C to 50°C, from 10 to 40°C, or even from 15 to 35°C.

According to yet another typical aspect of the disclosure, the temperature T2 is greater than 90°C, greater than 100°C, greater than 120°C, greater than 140°C, greater than 150°C, greater than 160°C, greater than 180°C, or even greater than 200°C.

According to yet another typical aspect of the disclosure, the temperature T2 is in a range from 95°C to 250°C, from 100°C to 220°C, from 120°C to 200°C, from 140°C to 200°C, from 140°C to 180°C, or even from 160°C to 180°C.

In another typical aspect of the curable precursor of the disclosure, the thermally curable resin and the radiation self-polymerizable multi-functional compound are (substantially) unable to chemically react with each other, in particular by covalent bonding, even when subjected to polymerization or curing initiation. In an exemplary aspect, the thermally curable resin and the radiation self-polymerizable multi-functional compound are unable to (substantially) chemically react with each other, when subjected to polymerization or curing initiation at a temperature of 23 °C.

Thermally curable resins for use herein are not particularly limited. Any thermally curable resins commonly known in the art of structural adhesives may be used in the context of the present disclosure. Suitable thermally curable resins for use herein may be easily identified by those skilled in the art in the light of the present disclosure. According to an advantageous aspect of the present disclosure, the thermally curable resin for use herein comprises at least one functional group selected from the group consisting of epoxy groups, in particular glycidyl groups.

According to another advantageous aspect, the thermally curable resin for use herein comprises at least one epoxy resin. Exemplary epoxy resins for use herein may be advantageously selected from the group consisting of phenolic epoxy resins, bisphenol epoxy resins, hydrogenated epoxy resins, aliphatic epoxy resins, halogenated bisphenol epoxy resins, novolac epoxy resins, and any mixtures thereof.

Epoxy resins are well known to those skilled in the art of structural adhesive compositions. Suitable epoxy resins for use herein and their methods of manufacturing are amply described for example in EP-A1-2700683 (Elgimiabi et al.) and in WO 2017/197087 (Aizawa).

In a particularly advantageous aspect of the disclosure, the thermally curable resin for use herein is an epoxy resin selected from the group consisting of novolac epoxy resins, bisphenol epoxy resins, in particular those derived from the reaction of bisphenol-A with epichlorhydrin (DGEBA resins), and any mixtures thereof.

Thermal curing initiators for the thermally curable resin for use herein are not particularly limited. Any thermal curing initiators for thermally curable resins commonly known in the art of structural adhesives may be used in the context of the present disclosure. Suitable thermal curing initiators for use herein may be easily identified by those skilled in the art in the light of the present disclosure.

According to one typical aspect of the disclosure, the thermal curing initiator for use herein is selected from the group consisting of rapid-reacting curing initiators, latent curing initiators, and any combinations or mixtures thereof. More typically, the thermal curing initiator for use herein is selected from the group consisting of rapid-reacting thermally-initiated curing initiators, latent thermally -initiated curing initiators, and any combinations or mixtures thereof.

According to an advantageous aspect of the present disclosure, the thermal curing initiator is selected from the group consisting of primary amines, secondary amines, and any combinations or mixtures thereof.

According to another advantageous aspect, the amines for use as thermal curing initiator for the thermally curable resin are selected from the group consisting of aliphatic amines, cycloaliphatic amines, aromatic amines, aromatic structures having one or more amino moiety, polyamines, polyamine adducts, dicyandiamides, and any combinations or mixtures thereof.

According to still another advantageous aspect of the disclosure, the thermal curing initiator for use herein is selected from the group consisting of dicyandiamide, polyamines, polyamine adducts, and any combinations or mixtures thereof. In a preferred aspect, the curing initiator of the thermally curable resin for use in the present disclosure is selected to be dicyandiamide.

In an advantageous execution, the curable precursor of the present disclosure further comprises a thermal curing accelerator for the thermally curable resin. Any thermal curing accelerators for thermally curable resins commonly known in the art of structural adhesives may be formally used in the context of the present disclosure. Suitable thermal curing initiators for use herein may be easily identified by those skilled in the art in the light of the present disclosure.

Thermal curing initiators and thermal curing accelerators are well known to those skilled in the art of structural adhesive compositions. Suitable thermal curing initiators and thermal curing accelerators for use herein and their methods of manufacturing are amply described for example in EP-A1-2 700 683 (Elgimiabi et al.) and in WO 2017/197087 (Aizawa).

In one advantageous execution, the thermal curing accelerator for use herein is selected from the group consisting of polyamines, polyamine adducts, ureas, substituted urea adducts, imidazoles, imidazole salts, imidazolines, aromatic tertiary amines, and any combinations or mixtures thereof.

In one preferred execution, the thermal curing accelerator for the thermally curable resin is selected from the group of polyamine adducts, substituted ureas, in particular N-substituted urea adducts.

In a particularly preferred execution of the disclosure, the thermal curing accelerator for the thermally curable resin is selected from the group of substituted urea adducts, in particular N- substituted urea adducts. In the context of the present disclosure, it has been indeed surprisingly discovered that the use of a thermal curing accelerator for the thermally curable resin selected from the group of substituted urea adducts, in particular N-substituted urea adducts, substantially improves the adhesion properties, in particular the peel adhesion properties of the resulting structural adhesive composition. Without wishing to be bound by theory, it is believed that the use of a thermal curing accelerator selected from the group of substituted urea adducts, in particular N-substituted urea adducts, beneficially impacts the overall curing profile of the curable precursor according to the present disclosure, which in turn translates into improved adhesion performance.

According to an advantageous aspect of the present disclosure, the curable precursor may further comprise a second thermally curable resin which is typically different from the (first) thermally curable resin as describe above.

In an advantageous aspect, the thermally curable resin for use in the present disclosure comprises at least one functional group selected from the group consisting of epoxy groups, in particular glycidyl groups. Advantageously still, the second thermally curable resin for use herein is an epoxy resin, in particular selected from the group consisting of phenolic epoxy resins, bisphenol epoxy resins, hydrogenated epoxy resins, aliphatic epoxy resins, halogenated bisphenol epoxy resins, novolac epoxy resins, and any mixtures thereof.

In a particularly preferred execution of the disclosure, the second thermally curable resin for use herein is an epoxy resin selected from the group consisting of hydrogenated bisphenol epoxy resins, in particular those derived from the reaction of hydrogenated bisphenol-A with epichlorhydrin (hydrogenated DGEBA resins), and any mixtures thereof. In the context of the present disclosure, it has been indeed surprisingly discovered that the use of a second thermally curable resin selected in particular from the group of hydrogenated bisphenol epoxy resins, substantially maintains or even improve the adhesion properties, in particular the peel adhesion properties of the resulting structural adhesive composition towards oily contaminated substrates. These specific curable precursors are particularly suitable to result into structural adhesive compositions having outstanding excellent oil-contamination tolerance towards, in particular oily contaminated metal substrates.

Exemplary oily contamination is for example mineral oils, and synthetic oils. Typical mineral oils include paraffinic mineral oils, intermediate mineral oils and naphthenic mineral oils.

According to a typical aspect, the curable precursor according to the disclosure comprises from 2 to 50 wt.%, from 2 to 40 wt.%, from 3 to 40 wt.%, from 5 to 30 wt.%, from 10 to 30 wt.%, or even from 15 to 30 wt.%, of the thermally curable resin(s), wherein the weight percentages are based on the total weight of the curable precursor.

Radiation self-polymerizable multi-functional compounds for use herein are not particularly limited, as long as they comprise a polyether oligomeric backbone and at least one free-radical (co)polymerizable reactive group at each terminal position of the oligomer backbone. Suitable radiation self-polymerizable multi-functional compounds for use herein may be easily identified by those skilled in the art in the light of the present disclosure.

Polyether oligomer backbones for use herein are not particularly limited, as long as they fulfill the above-detailed requirements. Suitable polyether oligomer backbones for use herein may be easily identified by those skilled in the art in the light of the present disclosure.

Without wishing to be bound by theory, it is believed that the radiation self-polymerizable multi-functional compounds as described above not only participate in forming the polymeric material comprising the self-polymerization reaction product of the self-polymerizable multifunctional compounds under the stage-B reaction described hereinbefore and which lead to a phase change of the initial curable precursor, but also formally act as a reactive diluent, rheological modifier and compatibilizer for the curable precursor, which in turn contributes to provide the curable precursor with outstanding flexibility characteristics and the stage-B reaction product with advantageous elasticity properties. The radiation self-polymerizable multi-functional compounds as described above are is also believed to beneficially impact the adhesion properties of the curable precursor, due in particular to the beneficial surface wetting properties provided in particular by the oligomeric poly ether backbone.

In one advantageous aspect of the present disclosure, the radiation self-polymerizable multifunctional compound for use herein comprises a polyether oligomeric backbone having a number average molecular weight of at least 2000 g/mol.

Unless otherwise indicated, the number average molecular weight of the radiation self- polymerizable multi-functional compound is determined by conventional gel permeation chromatography (GPC) using appropriate techniques well known to those skilled in the art.

In a beneficial aspect of the disclosure, the polyether oligomeric backbone for use herein has a number average molecular weight greater than 2000 g/mol, greater than 2500 g/mol, greater than 3000 g/mol, greater than 3500 g/mol, or even greater than 4000 g/mol.

In another beneficial aspect of the disclosure, the polyether oligomeric backbone has a number average molecular weight greater no greater than 10.000 g/mol, no greater than 9500 g/mol, no greater than 9000 g/mol, no greater than 8500 g/mol, or even no greater than 8000 g/mol.

In still another beneficial aspect, the polyether oligomeric backbone for use in the present disclosure has a number average molecular weight in a range from 2000 to 20.000 g/mol, from 2000 to 15.000 g/mol, from 2000 to 12.000 g/mol, from 2500 to 10.000 g/mol, from 2500 to 9.000 g/mol, from 3000 to 8500 g/mol, from 3500 to 8000 g/mol or even from 4000 to 8000 g/mol.

In yet another beneficial aspect of the disclosure, the polyether oligomeric backbone comprises (or consists of) a linear polyether oligomeric backbone.

According to an advantageous aspect, the (linear) polyether oligomeric backbone for use herein is obtained by copolymerization of tetrahydrofuran units, ethylene oxide units, and optionally propylene oxide units.

In an advantageous aspect, the at least one free-radical (co)polymerizable reactive group located at each terminal position of the polyether oligomeric backbone is selected from the group consisting of ethylenically unsaturated groups.

According to another advantageous aspect, the ethylenically unsaturated groups are selected from the group consisting of (meth)acrylic groups, vinyl groups, styryl groups, and any combinations or mixtures thereof.

In a more advantageous aspect of the disclosure, the ethylenically unsaturated groups for us herein are selected from the group consisting of methacrylic groups, acrylic groups, and any combinations or mixtures thereof.

In a particularly preferred aspect of the disclosure, the ethylenically unsaturated groups for use herein are selected from the group of methacrylic groups. Advantageously, the radiation self-polymerizable multi-functional compound for use herein is an ethylenically unsaturated compound.

According to one advantageous aspect of the curable precursor of the disclosure, the radiation self-polymerizable multi-functional compound for use herein has the following formula: wherein:

Y is a free-radical (co)polymerizable reactive group, in particular an ethylenically unsaturated group;

R ² is independently selected from the group consisting of alkylene groups having in particular from 2 to 6 carbon atoms; and n is an integer, which is in particular selected such that the calculated number average molecular weight of the radiation self-polymerizable multi-functional compound is of at least 2000 g/mol.

According to another advantageous aspect of the present disclosure, the radiation self- polymerizable multi-functional compound has the following formula: wherein: each R ² is independently selected from the group consisting of alkylene groups having in particular from 2 to 6 carbon atoms; and n is an integer in particular selected such that the calculated number average molecular weight of the radiation self-polymerizable multi-functional compound is in a range from 2000 to 20.000 g/mol.

In one particular aspect, n is selected such that the calculated number average molecular weight is at least 2000 g/mol, at least 3000 g/mol, or even at least 4000 g/mol. In another particular aspect, n is selected such that the calculated number average molecular weight is no greater than 20.000 g/mol, no greater than 15.000 g/mol, or even no greater than 10.000 g/mol. In still another particular aspect, n is selected such that the calculated number average molecular weight is between 2000 and 20.000 g/mol, between 3000 and 15.000 g/mol, or even between 3000 and 10.000 g/mol, where all ranges are inclusive of the end points. According to still another advantageous aspect of the present disclosure, the radiation self- polymerizable multi-functional compound has the following formula: wherein: n is an integer in particular selected such that the calculated number average molecular weight of the radiation self-polymerizable multi-functional compound is in a range from 2000 to 20.000 g/mol.

According to yet another advantageous aspect of the present disclosure, the radiation self- polymerizable multi-functional compound has the following formula: wherein: a and b are integers greater than or equal to 1, the sum of a and b is equal to n, and wherein n is in particular selected such that the calculated number average molecular weight of the poly ether oligomer is in a range from 2000 to 20.000 g/mol.

In one particular aspect, n is selected such that the calculated number average molecular weight is at least 2000 g/mol, at least 3000 g/mol, or even at least 4000 g/mol. In another particular aspect, n is selected such that the calculated number average molecular weight is no greater than 20.000 g/mol, no greater than 15.000 g/mol, or even no greater than 10.000 g/mol. In still another particular aspect, n is selected such that the calculated number average molecular weight is between 2000 and 20.000 g/mol, between 3000 and 15.000 g/mol, or even between 3000 and 10.000 g/mol, where all ranges are inclusive of the end points.

According to yet another advantageous aspect of the present disclosure, the linear polyether oligomeric backbone is obtained by copolymerization of tetrahydrofuran units and ethylene oxide units, wherein the mole ratio of these monomer units is in a range from 1:2.5 to 1:5, or even from 1:3 to 1:4.

According to yet another advantageous aspect, the radiation self-polymerizable multifunctional compound for use in the present disclosure has a glass transition temperature (Tg) no greater than 20°C, no greater than 15°C, or even no greater than 10°C, when measured by Differential Scanning Calorimetry (DSC).

Without wishing to be bound by theory, it is believed that radiation self-polymerizable multi-functional compounds having a glass transition temperature (Tg) no greater than 20°C, no greater than 15°C, or even no greater than 10°C, are provided with advantageous viscoelastic characteristics which are in turn believed to beneficially contribute to providing the advantageous properties described hereinbefore with respect to the overall radiation self-polymerizable multifunctional compound.

In one advantageous aspect, the curable precursor of the present disclosure comprises no greater than 25 wt.%, no greater than 20 wt.%, no greater than 15 wt.%, no greater than 10 wt.%, or even no greater than 8 wt.%, of the radiation self-polymerizable multi-functional compound, wherein the weight percentages are based on the total weight of the curable precursor.

In another advantageous aspect, the curable precursor of the present disclosure comprises from 0.5 to 20 wt.%, from 0.5 to 15 wt.%, from 1 to 15 wt.%, from 2 to 15 wt.%, from 2 to 12 wt.%, from 2 to 10 wt.%, from 3 to 10 wt.%, or even from 3 to 8 wt.%, of the radiation self-polymerizable multi-functional compound, wherein the weight percentages are based on the total weight of the curable precursor.

The curable precursor according to the present disclosure further comprises a free-radical polymerization initiator for the radiation self-polymerizable multi-functional compound which is activated by visible light. Free-radical polymerization initiators for the radiation self-polymerizable multi-functional compound for use herein are not particularly limited, as long as they may be activated by visible light. Suitable compounds for use herein may be easily identified by those skilled in the art in the light of the present disclosure.

Preferably, the free-radical polymerization initiator is activated by light having wavelengths of at least 350 nm. Also, it is preferred that the free-radical polymerization initiator is activated by light having wavelengths of up to 750 nm. Accordingly, it is preferred that the free-radical polymerization initiator is activated by light having wavelengths in the range of from 350 nm to 750 nm, preferably from 380 to 700 nm, more preferably from 400 to 650 nm. That is, the free-radical polymerization initiator may be activated by light having wavelengths in the range of from 380 to 450 nm, and/or from 450 to 485 nm, and/or from 485 to 500 nm, and/or 500 to 565 nm, and/or from 565 to 590 nm, and/or from 590 to 625 nm, and/or from 625 to 700 nm. Particularly preferred herein is blue light, i.e., light having wavelengths from 450 to 485 nm. For instance, the free-radical polymerization initiator may be activated by light having wavelengths in the range of from (i) 450 to 485 nm, and optionally (ii) from 380 to 450 nm, from 485 to 500 nm, 500 to 565 nm, from 565 to 590 nm, from 590 to 625 nm, and/or from 625 to 700 nm. Thus, other wavelengths of the visible spectrum of light may be utilized.

According to a typical aspect of the disclosure, the free-radical polymerization initiator of the radiation self-polymerizable multi-functional compound is selected from the group consisting of Norrish type (I) free-radical polymerization initiators, Norrish type (II) free-radical polymerization initiators, and any combinations or mixtures thereof.

According to one advantageous aspect of the disclosure, the free-radical polymerization initiator of the radiation self-polymerizable multi-functional compound is selected from the group consisting of Norrish type (I) free-radical polymerization initiators, and any combinations or mixtures thereof.

According to a more advantageous aspect of the disclosure, the free-radical polymerization initiator of the radiation self-polymerizable multi-functional compound is selected from alpha- diketones and/or phosphinoxides, preferably from camphorquinone, acylphosphinoxide, phenyl- propane-dione, acrylphosphinoxide, dibenzoyl, 1 -phenyl- 1,2-propandione, and any mixtures and combinations thereof.

According to an even more advantageous aspect of the disclosure, the free-radical polymerization initiator for use herein is wherein the free-radical polymerization initiator of the radiation self-polymerizable multi-functional compound is camphorquinone.

In one typical aspect, the curable precursor of the disclosure comprises no greater than 10 wt.%, no greater than 8 wt.%, no greater than 6 wt.%, no greater than 5 wt.%, no greater than 4 wt.%, no greater than 2 wt.%, no greater than 1 wt.%, no greater than 0.8 wt.%, no greater than 0.6 wt.%, no greater than 0.5 wt.%, no greater than 0.4 wt.%, no greater than 0.2 wt.%, or even no greater than 0.1 wt.%, of the free-radical polymerization initiator of the radiation self-polymerizable multi-functional compound, wherein the weight percentages are based on the total weight of the curable precursor.

In another typical aspect, the curable precursor of the disclosure comprises from 0.01 to 10 wt.%, from 0.01 to 8 wt.%, from 0.02 to 6 wt.%, from 0.02 to 5 wt.%, from 0.02 to 4 wt.%, from 0.03 to 3 wt.%, from 0.03 to 2 wt.%, from 0.03 to 1.8 wt.%, from 0.03 to 1.6 wt.%, from 0.03 to 1.5 wt.%, from 0.03 to 1.4 wt.%, or even from 0.03 to 1 wt.%, of the free-radical polymerization initiator of the radiation self-polymerizable multi-functional compound, wherein the weight percentages are based on the total weight of the curable precursor.

In one particular aspect of the curable precursor according to the disclosure, the weight ratio of the radiation self-polymerizable multi-functional compound to the free-radical polymerization initiator of the radiation self-poly merizable multi-functional compound is a range from 200 : 1 to 5 : 1 , 180:1 to 8:1, 150:1 to 8:1, 100:1 to 8:1, or even from 100:1 to 10:1. It was also found that it is advantageous to employ at least one reactive diluent in the curable precursor of the present disclosure. This is believed due to the free-radical polymerization initiators as described herein may be better dissolved in the reactive diluent. Accordingly, it is preferred that the free-radical polymerization initiators as described are dissolved in the at least one reactive diluent before mixing with the other constituents of the curable precursor according to the present disclosure. Reactive diluents are known to the skilled person generally from the technical field of structural adhesives, in particular epoxy -resin based structural adhesives. Preferably, the at least one reactive diluent is selected from glycidyl ethers of linear or branched alkanol, preferably from diglycidyl ethers of linear or branched alkyldiols.

Another advantageous aspect of the present disclosure takes advantage of the fact that presently commercially available blue-light emitting diode lamps (LED) are all having a significant amount of radiation output also in the UV spectrum. However, by using optical brighteners, this particularly UV spectrum may be used for the present disclosure. Common optical brighteners are fluorescent compounds, i.e., they absorb light in the UV-spectrum, and emit light in the visible spectrum, in particular in the blue region. Thus, the UV light emitted by commonly used LED may be transformed into blue light which then activates the free-radical polymerization initiators as described herein. This may have the effect that less free-radical polymerization initiators as described herein may be needed, or that irradiation times may be reduced. Accordingly, it is preferred that the curable precursor as described herein further comprises at least one optical brightener. Preferably, the optical brightener is a fluorescent compound. The optical brightener preferably absorbs light in the ultra-violet range of light, preferably absorbs light having wavelengths in the range of from 290 to 490 nm. The optical brightener is preferably a fluorescent compound re emitting light in the visible range, preferably of wavelengths in the range of from 400 to 480 nm, optionally (ii) from 380 to 450 nm, from 485 to 500 nm, 500 to 565 nm, from 565 to 590 nm, from 590 to 625 nm, and/or from 625 to 700 nm. The at least one optical brightener may be contained in the curable precursor in an amount in the range of from 0.001 to 0.08 wt.-%, preferably from 0.005 to 0.05 wt.-%, more preferably from 0.008 to 0.03 wt.-%, based on the total weight of the curable precursor. Commercially optical brighteners of these types are known to the skilled person, examples of which are set forth in the experimental section of the present disclosure.

According to one advantageous aspect, the curable precursor of the present disclosure comprises: a) from 5 to 40 wt.%, from 5 to 35 wt.%, from 10 to 35 wt.%, from 15 to 35 wt.%, from 15 to 30 wt.%, or even from 15 to 25 wt.%, of the thermally curable resin(s); b) from 0.1 to 20 wt.%, from 0.2 to 15 wt.%, from 0.2 to 10 wt.%, from 0.5 to 8 wt.%, from 1 to 6 wt.%, or even from 2 to 5 wt.%, of the thermal curing initiator for the thermally curable resin; c) from 0.5 to 20 wt.%, from 0.5 to 15 wt.%, from 1 to 15 wt.%, from 2 to 15 wt.%, from 2 to 12 wt.%, from 2 to 10 wt.%, from 3 to 10 wt.%, or even from 3 to 8 wt.%, of the radiation self-polymerizable multi-functional compound; d) from 0.01 to 10 wt.%, from 0.02 to 6 wt.%, from 0.02 to 5 wt.%, from 0.02 to 4 wt.%, from 0.03 to 2 wt.%, from 0.03 to 1.8 wt.%, from 0.03 to 1.5 wt.%, or even from 0.03 to 1 wt.%, of the free-radical polymerization initiator for the radiation self-polymerizable multi-functional compound; e) optionally, 0.05 to 10 wt.%, from 0.1 to 8wt.%, from 0.1 to 5 wt.%, from 0.2 to 4 wt.%, from 0.5 to 3 wt.%, or even from 1 to 3 wt.%, of a thermal curing accelerator for the thermally curable resin; f) optionally, a toughening agent; g) optionally, a thixotropic agent; h) optionally, at least one reactive diluent; and i) optionally, at least one optical brightener; wherein the weight percentages are based on the total weight of the curable precursor.

In an advantageous execution, the curable precursor of the present disclosure may further comprise a thixotropic agent. Any thixotropic agents commonly known in the art of structural adhesives may be formally used in the context of the present disclosure. Suitable thixotropic agents for use herein may be easily identified by those skilled in the art in the light of the present disclosure.

According to one advantageous aspect of the disclosure, the curable precursor further comprises a thixotropic agent which is typically selected from the group of inorganic and organic thixotropic agents.

According to another advantageous aspect of the disclosure, the thixotropic agent for use herein is selected from the group of particulate thixotropic agents.

According to a more advantageous aspect of the disclosure, the thixotropic agent for use herein is selected from the group of inorganic thixotropic agents, in particular silicon-based thixotropic agents and aluminium-based thixotropic agents.

According to an even more advantageous aspect of the disclosure, the thixotropic agent for use herein is selected from the group consisting of silica-based and silicate-based thixotropic agents.

In one very advantageous aspect, the curable precursor of the present disclosure further comprises a thixotropic agent selected from the group consisting of fumed silica particles, in particular hydrophilic fumed silica and hydrophobic fumed silica; silicates particles, in particular phyllosilicates, and any mixtures thereof.

In one particularly advantageous aspect of the disclosure, the thixotropic agent for use herein is selected from the group of organic thixotropic agents, in particular polyamide waxes, hydrolysed castor waxes and urea derivatives-based thixotropic agents.

In another particularly advantageous aspect of the disclosure, the thixotropic agent for use herein is selected from the group consisting of fumed silica particles, in particular hydrophobic fumed silica particles; silicate-based particles, in particular phyllosilicate particles; polyamide waxes, and any mixtures thereof.

According to an exemplary aspect of the present disclosure, the curable precursor comprises no greater than 20 wt.%, no greater than 15 wt.%, no greater than 10 wt.%, no greater than 8 wt.%, or even no greater than 5 wt.%, of the thixotropic agent, based on the overall weight of the curable precursor.

According to another exemplary aspect of the present disclosure, the curable precursor comprises from 0.05 to 20 wt.%, from 0.1 to 15 wt.%, from 0.5 to 10 wt.%, from 0.5 to 8 wt.%, from 1 to 6 wt.%, or even from 1 to 5 wt.%, of the thixotropic agent, based on the overall weight of the curable precursor.

The curable precursor of the disclosure may take any suitable form, as well known to those skilled in the art of structural adhesives. Suitable forms will depend on the targeted application and usage conditions.

In one advantageous aspect of the present disclosure, the curable precursor is in the form of a one-part (hybrid) structural adhesive composition. One-part curable precursors are advantageous over two-part curable precursor compositions in that they do not involve any specific delayed mixing steps, which usual require special precautions, additional process steps and specialized equipment. One-part curable precursors are usually characterized by providing more processing and formulation flexibility than their two-part counterparts. However, the present disclosure is not that limited.

Accordingly, and in an alternative aspect, the curable precursor of the present disclosure is in the form of a two-part (hybrid) structural adhesive composition having a first part and a second part, wherein: a) the first part comprises : i. the radiation self-polymerizable multi-functional compound; and ii. the thermal curing initiator for the thermally curable resin activated by visible light, in particular blue light; b) the second part comprises: i. the thermally curable resin; and ii. the free-radical polymerization initiator for the radiation self-polymerizable multi-functional compound activated by visible light, in particular blue light; wherein the first part and the second part are kept separated prior to combining the two parts and forming the (hybrid) structural adhesive composition.

According to another alternative aspect, the curable precursor of the present disclosure is in the form of a two-part (hybrid) structural adhesive composition having a first part and a second part, wherein: a) the first part comprises : i. the radiation self-polymerizable multi-functional compound; and ii. the free-radical polymerization initiator for the radiation self-polymerizable multi-functional compound activated by visible light, in particular blue light; b) the second part comprises: i. the thermally curable resin; and ii. the thermal curing initiator for the thermally curable resin; wherein the first part and the second part are kept separated prior to combining the two parts and forming the (hybrid) structural adhesive composition.

According to another aspect, the present disclosure is directed to a partially cured precursor of a structural adhesive composition, comprising: a) a polymeric material comprising the self-polymerization reaction product of a polymerizable material comprising a radiation self-polymerizable multi-functional compound; b) optionally, some residual free-radical polymerization initiator for the radiation self- polymerizable multi-functional compound activated by visible light, in particular blue light; c) a thermally curable resin; and d) a thermal curing initiator for the thermally curable resin; and wherein the thermally curable resin(s) are (substantially) uncured and are in particular embedded into the polymeric material comprising the self-polymerization reaction product of a polymerizable material comprising a radiation self-polymerizable multi-functional compound.

In a typical aspect of the partially cured precursor of a structural adhesive, the thermally curable resins are substantially uncured and are, in particular, embedded into the polymeric material comprising the self-polymerization reaction product of a polymerizable material comprising a radiation self-polymerizable multi-functional compound. In a typical aspect, the thermally curable resins are still liquid compounds embedded into the polymeric material resulting from the selfpolymerization of the radiation self-polymerizable multi-functional compounds, wherein this polymeric material represents a fully established three-dimensional network.

The partially cured precursor typically is a stable and self-supporting composition having a dimensional stability, which makes it possible for it to be pre-applied on a selected substrate, in particular a liner, until further processing. In particular, the pre-applied substrate may be suitably transferred to other production sites until final full curing is performed. Advantageously still, the partially cured precursor may be appropriately shaped or printed to fulfil the specific requirements of any selected applications. The partially cured precursor is typically provided with excellent characteristics and performance as to elasticity, tackiness, cold-flow and surface wetting.

According to a typical aspect of the partially cured precursor according to the disclosure, the polymeric material comprising the self-polymerization reaction product of the polymerizable material comprising the radiation self-polymerizable multi-functional compound is substantially fully polymerized and has in particular a degree of polymerization of more than 90%, more than 95%, more than 98%, or even more than 99%.

As the polymeric material comprising the self-polymerization reaction product of the radiation self-polymerizable multi-functional compound is substantially fully polymerized, this polymerization reaction has advantageously a fixed and irreversible end and will not trigger any shelf-life reducing reactions in the remaining of the curable precursor. This characteristic is believed to beneficially impact the overall shelf-life of the curable precursor.

In one typical aspect of the disclosure, the partially cured precursor has a shear storage modulus in a range from 1000 to 250.000 Pa, from 1000 to 200.000 Pa, from 2000 to 150.000 Pa, from 3000 to 150.000 Pa, from 3000 to 100.000 Pa, or even from 3000 to 80.000 Pa, when measured according to the test method described in the experimental section.

In one advantageous aspect, the partially cured precursor according to the disclosure has a glass transition temperature (Tg) no greater than 0°C, no greater than -5°C, no greater than -10°C, no greater than -15°C, or even no greater than -20°C, when measured by DSC.

In another advantageous aspect of the disclosure, the partially cured precursor has an elongation at break of at least 50%, at least 80%, at least 100%, at least 150%, or even at least 200%, when measured according to tensile test DIN EN ISO 527. This particular property makes the partially cured precursor and the resulting structural adhesive suitable for automated handling and application, in particular by high-speed robotic equipment. More particularly, the partially cured precursor and the resulting structural adhesive of the present disclosure enables efficient automation of the process of forming a metal joint between metal plates. According to another aspect, the present disclosure relates to a structural adhesive composition obtainable by substantially fully curing the curable precursor as described above, in particular at a temperature T2 or greater.

In the context of the present disclosure, the expression “substantially fully curing the curable precursof’ is meant to express that more than 90 wt.%, more than 95wt.%, more than 98 wt.%, or even more than 99 wt.% of the overall amount of the thermally curable resins and the radiation self- polymerizable multi-functional compounds are polymerized/cured as the result of the polymerization/curing step(s).

In a typical aspect, the structural adhesive composition comprises an interpenetrating network involving the polymeric material comprising the self-polymerization reaction product of the polymerizable material comprising the radiation self-polymerizable multi-functional compound and the polymeric product resulting from the curing of the thermally curable resin.

The curable precursor or the partially or fully cured (hybrid) structural adhesive compositions of the disclosure may take or be shaped in any suitable form, depending on the targeted application and usage conditions.

According to one advantageous aspect, the curable precursor or the partially or fully cured structural adhesive composition of the present disclosure is shaped in the form of an elongated film. The elongated film shape is one conventional and convenient shape for the structural adhesive to be pre-applied on a selected substrate, in particular a liner, until further processing.

In one particular aspect of the disclosure, the elongated film for use herein has a thickness greater than 500 micrometres, greater than 600 micrometres, greater than 700 micrometres, greater than 800 micrometres, greater than 900 micrometres, or even greater than 1000 micrometres.

In another particular aspect of the disclosure, the elongated film for use herein has a thickness no greater than 500 micrometres, no greater than 400 micrometres, no greater than 300 micrometres, no greater than 200 micrometres, no greater than 100 micrometres, or even greater than 50 micrometres.

Although the elongated film shape may be convenient in many different applications, this specific shape is not always satisfactory for adhesively bond assemblies provided with complex three-dimensional configurations or topologies, in particular when provided with challenging bonding areas or surfaces.

Accordingly, the curable precursor or the partially or fully cured (hybrid) structural adhesive composition of the disclosure may - in another aspect - be shaped in the form of a three-dimensional object. Suitable three-dimensional object shapes for use herein will broadly vary depending on the targeted bonding application and the specific configuration of the assembly to bond, in particular the bonding area. Exemplary three-dimensional object shapes for use herein will be easily identified by those skilled in the art in the light of the present disclosure.

According to one exemplary aspect of the present disclosure, the three-dimensional object has a shape selected from the group consisting of circular, semi-circular, ellipsoidal, square, rectangular, triangular, trapezoidal, polygonal shape, or any combinations thereof.

In the context of the present disclosure, the shape of the three-dimensional object is herein meant to refer to the shape of the section of the three-dimensional object according to a direction substantially perpendicular to the greatest dimension of the three-dimensional object.

In yet another aspect, the present disclosure relates to a composite article comprising a curable precursor or a partially or fully cured (hybrid) structural adhesive composition as described above applied on at least part of the surface of the article.

Suitable surfaces and articles for use herein are not particularly limited. Any surfaces, articles, substrates and material commonly known to be suitable for use in combination with structural adhesive compositions may be used in the context of the present disclosure.

In a typical aspect, the article for use herein comprises at least one part, in particular a metal or a composite material part.

In an advantageous aspect, the composite article according to the disclosure is used for body- in-white bonding applications for the automotive industry, in particular for hem flange bonding of parts, more in particular metal or composite material parts; and for structural bonding operations for the aeronautic and aerospace industries.

According to still another aspect of the present disclosure, it is provided a curing system suitable for a (hybrid) structural adhesive composition, wherein the curing system comprises: a) a thermal curing initiator for a thermally curable resin; and b) a free-radical polymerization initiator for the radiation self-polymerizable multifunctional compound comprising a polyether oligomeric backbone and at least one free-radical (co)polymerizable reactive group at each terminal position of the oligomeric backbone.

All the particular and preferred aspects relating to, in particular, the structural adhesive composition, the thermally curable resin, the thermal curing initiator for the thermally curable resin, the radiation self-polymerizable multi-functional compound, the free-radical polymerization initiator for the radiation self-polymerizable multi-functional compound, the temperatures T1 and T2, and the curable precursor or partially cured precursor which were described hereinabove in the context of the curable precursor or the partially cured precursor, are fully applicable to the curing system for a structural adhesive composition. According to another aspect, the present disclosure is directed to a method of manufacturing a composite article comprising the step of using a curable precursor as described above or a partially cured precursor as described above.

According to yet another aspect, the present disclosure provides a method of manufacturing a (hybrid) structural adhesive composition, comprising the steps of: a) providing a curable precursor as described above; b) partially curing the curable precursor of step a) by initiating the free-radical polymerization initiator for the radiation self-polymerizable multi-functional compound by irradiation with visible light, in particular blue light, thereby forming a partially cured precursor comprising a polymeric material resulting from the selfpolymerization reaction product of the radiation self-polymerizable multifunctional compound; and c) substantially fully curing the partially cured precursor of step b) by initiating the thermal curing initiator for the thermally curable resin, thereby obtaining a substantially fully cured (hybrid) structural adhesive composition.

In the context of the present disclosure, the expression “substantially fully curing the partially cured precursor” is meant to express that more than 90 wt.%, more than 95 wt.%, more than 98 wt.%, or even more than 99 wt.% of the overall amount of the thermally curable resins and the radiation self-polymerizable multi-functional compounds are polymerized/cured as the result of the polymerization/curing step(s).

In yet another aspect of the present disclosure, it is a provided a method of bonding two parts comprising the step of using a curable precursor or a partially cured precursor as described above.

According to a particular aspect of the disclosure, the method of bonding two parts comprises the steps of: a) applying a curable precursor or a partially cured precursor as described above to a surface of at least one of the two parts; b) joining the two parts so that the curable precursor or the partially cured precursor (hybrid) structural adhesive composition is positioned between the two parts; and c) optionally, partially curing the curable precursor according of step a) by initiating the free-radical polymerization initiator for the radiation self-polymerizable multifunctional compound by irradiation with visible light, in particular blue light, thereby forming a partially cured precursor comprising a polymeric material resulting from the self-polymerization reaction product of the radiation self- polymerizable multi-functional compound; and/or d) substantially fully curing the partially cured precursor of step a) or c) by initiating the thermal curing initiator for the thermally curable resin, thereby obtaining a substantially fully cured (hybrid) structural adhesive composition and bonding the two parts.

According to an advantageous aspect of the method of bonding two parts, wherein the two parts are metal parts.

According to another advantageous aspect, the method of bonding two parts is for hem flange bonding of metal parts, wherein:

- the partially cured precursor is shaped in the form of an elongated film;

- the partially cured precursor film has a first portion near a first end of the precursor film and a second portion near the second end opposite to the first end of the precursor film;

- the first metal part comprises a first metal panel having a first body portion and a first flange portion along a margin of the first body portion adjacent a first end of the first body portion;

- the second metal part comprises a second metal panel having a second body portion and a second flange portion along a margin of the second body portion adjacent a second end of the second body portion; wherein the method comprises the steps of: a) adhering the partially cured precursor film to the first metal panel or second metal panel, whereby following adhering and folding, a metal joint is obtained wherein the partially cured precursor film is folded such that: i. the first portion of the partially cured precursor film is provided between the second flange of the second metal panel and the first body portion of the first metal panel, and ii. the second portion of the partially cured precursor film is provided between the first flange of the first metal panel and the second body portion of the second metal panel; and b) substantially fully curing the partially cured precursor by initiating the thermal curing initiator for the thermally curable resin, thereby obtaining a substantially fully cured (hybrid) structural adhesive composition and bonding the metal joint.

According to still another advantageous aspect of the method of bonding two parts, a side of a first edge portion of the first metal part is folded back and a hem flange structure is formed so as to sandwich the second metal part, and the curable precursor or the partially cured precursor as described above is disposed so as to adhere at least the first edge portion of the first metal part and a first surface side of the second metal part to each other.

Methods of bonding two parts, in particular for hem flange bonding of metal parts, are well known to those skilled in the art of structural adhesive compositions. Suitable methods of bonding two parts for use herein are amply described e.g., in EP-A1-2700683 (Elgimiabi et al.) and in WO 2017/197087 (Aizawa).

In a particular aspect of the present disclosure, the substrates, parts and surfaces for use in these methods comprise a metal selected from the group consisting of aluminum, steel, iron, and any mixtures, combinations or alloys thereof. More advantageously, the substrates, parts and surfaces for use herein comprise a metal selected from the group consisting of aluminum, steel, stainless steel and any mixtures, combinations or alloys thereof. In a particularly advantageous execution of the present disclosure, the substrates, parts and surfaces for use herein comprise aluminum.

According to another aspect, the present disclosure relates to a metal part assembly obtainable by the method(s) as described above.

In yet another aspect of the disclosure, it is provided a (continuous) process of manufacturing a (shaped) curable precursor of a (hybrid) structural adhesive composition, comprising the steps of: a) providing a mixing apparatus comprising a reaction chamber and an extrusion die; b) providing a curable precursor of a (hybrid) structural adhesive composition as described above; c) incorporating and mixing the curable precursor of the (hybrid) structural adhesive composition in the reaction chamber of the mixing apparatus, thereby forming an extrudable composition; d) pressing the extrudable composition of step c) through the extrusion die, thereby forming an extrudate of the curable precursor of the (hybrid) structural adhesive composition; e) optionally, shaping the extmdate; f) optionally, cooling down the extmdate of step d) or e); and g) self-polymerizing or allowing the self-polymerization reaction of the radiation self- polymerizable multi-functional compound in the extmdate.

According to an advantageous aspect of the process of manufacturing a curable precursor of a structural adhesive composition, the free-radical polymerization initiator is initiated at a temperature Tl, wherein the thermal curing initiator is initiated at a temperature T2, wherein the temperature T2 is greater than the temperature Tl, and wherein the temperature Tl is insufficient to cause initiation of the thermal curing initiator. According to another advantageous aspect of the process, the step of incorporating and mixing the curable precursor of the (hybrid) structural adhesive composition in the reaction chamber of the mixing apparatus thereby forming an extrudable composition, is performed at a temperature no greater than temperature T2, and in particular no greater than temperature Tl.

According to still another advantageous aspect of the process, the temperature Tl is no greater than 90°C, no greater than 80°C, no greater than 60°C, no greater than 50°C, no greater than 40°C, no greater than 30°C, no greater than 25°C, no greater than 20°C, or even no greater than 15°C.

According to still another advantageous aspect of the process, the temperature Tl is in a range from -10°C to 85°C, from 0°C to 80°C, from 5°C to 60°C, from 5°C to 50°C, from 10 to 40°C, or even from 15 to 35°C.

According to yet another advantageous aspect of the process, the temperature T2 is greater than 90°C, greater than 100°C, greater than 120°C, greater than 140°C, greater than 150°C, greater than 160°C, greater than 180°C, or even greater than 200°C.

According to yet another advantageous aspect of the process, the temperature T2 is in a range from 95°C to 250°C, from 100°C to 220°C, from 120°C to 200°C, from 140°C to 200°C, from 140°C to 180°C, or even from 160°C to 180°C.

In still a further beneficial aspect of the disclosure, the step of forming an extrudate of the curable precursor of the (hybrid) structural adhesive composition involves a calendering process step, in particular a calendering process step whereby the extrudable composition is guided into the nip gap of two counterrotating rolls.

In still a further beneficial aspect of the disclosure, the process of manufacturing a curable precursor of a structural adhesive composition further comprises the step of shaping the extrudate of step c) in the form of a three-dimensional object, and wherein the step of shaping the extrudate is in particular performed simultaneously with the step of pressing the extrudable composition through the extrusion die.

In still another beneficial aspect of the process, the three-dimensional object for use herein has a shape selected from the group consisting of circular, semi-circular, ellipsoidal, square, rectangular, triangular, trapezoidal, polygonal shape, or any combinations thereof.

In another alternative aspect of the process, the three-dimensional object for use herein is shaped in the form of an elongated fdm.

According to a particularly beneficial aspect of the process, the mixing apparatus for use herein is selected from the group consisting of single- and multi-screw extruders, and any combinations thereof. In the context of the present disclosure, it has been surprisingly discovered that the use of single- and multi-screw extruders is particularly beneficial in the process of manufacturing a (shaped) curable precursor of a (hybrid) structural adhesive composition as described above. Single- and multi-screw extmders have been found to provide excellent mixing performance and characteristics for the initial ingredients and raw materials of the curable precursor as described above, which in turn provide excellent extrudability characteristics of the extrudable composition, in particular when used in combination with an extrusion die.

According to another beneficial aspect of the process, the mixing apparatus for use herein is selected from the group consisting of single screw extmders, twin screw extmders, planetary roller extmders, and ring extmders.

According to a more beneficial aspect of the process, the mixing apparatus for use herein is selected from the group consisting of co-rotating multi-screw extmders and counter-rotating multiscrew extmders.

According to an even more beneficial aspect of the process, the mixing apparatus for use herein is a twin-screw extmder, in particular a co-rotating twin-screw extmder.

According to an alternatively beneficial aspect of the process, the mixing apparatus for use herein is a planetary roller extmder comprising in particular a center spindle and multiple planetary gear spindles with center spindle and planetary gear spindles featuring a screw like geometry.

According to yet another aspect, the present disclosure is directed to a process of manufacturing a (hybrid) structural adhesive article, comprising the steps of: a) applying a curable precursor as described above onto a substrate; b) partially curing the curable precursor of step a) by initiating the free-radical polymerization initiator for the radiation self-polymerizable multi-functional compound, thereby forming a partially cured precursor of a (hybrid) structural adhesive article comprising a polymeric material resulting from the selfpolymerization reaction product of the radiation self-polymerizable multifunctional compound; c) optionally, substantially fully curing the partially cured precursor of a (hybrid) structural adhesive article obtained in step b) by initiating the thermal curing initiator for the thermally curable resin, thereby obtaining a substantially fully cured (hybrid) structural adhesive article; and d) optionally, removing the partially cured precursor of a (hybrid) structural adhesive article obtained in step b) or the substantially fully cured (hybrid) structural adhesive article obtained in step c) from the substrate. Application of the curable precursor in step a) may be carried out by means of a printing technique, application by doctor blade, application by extrusion through a nozzle, preferably by means of a printing technique.

In the context of the present disclosure, it has been surprisingly discovered that the curable precursor as described hereinbefore is particularly suitable for being applied on various substrates by various printing techniques, including conventional and less conventional printing techniques. Without wishing to be bound by theory, it is believed this excellent compatibility for printing techniques is due to the advantageous viscosity and rheological properties of the curable precursor as described above.

In an advantageous aspect of the process of manufacturing a structural adhesive article, the structural adhesive article for use herein is in the form of a two-dimensional object or three- dimensional object.

In one more advantageous aspect of the process, the three-dimensional object for use herein has a shape selected from the group consisting of circular, semi-circular, ellipsoidal, square, rectangular, triangular, trapezoidal, polygonal shape, or any combinations thereof.

In another more advantageous aspect of the process, the three-dimensional object for use herein is shaped in the form of an elongated film, a continuous or discontinuous bead, a designed shape or figure, a readable text or logo, and any combinations thereof.

According to yet another aspect of the present disclosure, it is provided a process of printing a (hybrid) structural adhesive composition, comprising the steps of: a) applying a curable precursor as described above onto a substrate using a printing technique; b) partially curing the curable precursor of step a) by initiating the free-radical polymerization initiator for the radiation self-polymerizable multi-functional compound by irradiation with visible light, in particular blue light, thereby forming a partially cured precursor comprising a polymeric material resulting from the selfpolymerization reaction product of the radiation self-polymerizable multifunctional compound; and c) optionally, substantially fully curing the partially cured precursor of step b) by initiating the thermal curing initiator for the thermally curable resin.

In one typical aspect of these processes, the printing technique for use herein is selected from the group consisting of inkjet printing, screen printing, gravure printing, flexographic printing, offset printing, thermo-driven printing, piezo-driven printing, pressure-driven printing, robocasting, direct ink-writing, 3D printing pen, xyz robotic dispensing, micro dispensing printing, displacement-driven dispensing printing, pneumatic-driven dispensing printing, and any combinations thereof.

In a particular advantageous aspect of these processes, the printing technique for use herein is selected from the group consisting of micro dispensing printing techniques, in particular inkjet printing, more in particular piezo-driven (multi-dimensional) inkjet printing.

In the context of the present disclosure, it has been indeed surprisingly discovered that the curable precursor as described hereinbefore is particularly suitable for being applied by micro dispensing printing techniques, in particular using piezo-driven inkjet printing devices controlled by a piezo actuator.

According to still another aspect, the present disclosure relates to the use of a curable precursor or a partially or fully cured (hybrid) structural adhesive composition as described above, for industrial applications, in particular for construction and automotive applications, more in particular for body-in-white bonding applications for the automotive industry and for structural bonding operations for the aeronautic and aerospace industries.

In a preferred aspect of the disclosure, the curable precursor or the partially or fully cured (hybrid) structural adhesive composition as described above is used for bonding metal or composite material parts, in particular for hem flange bonding of metal or composite material parts in the automotive industry.

According to yet another aspect, the present disclosure relates to the use of a curable precursor or a partially cured precursor as described above, for bonding metal or composite material parts, in particular for hem flange bonding of metal or composite material parts in the automotive industry.

In yet another aspect, the present disclosure relates to the use of a curable precursor or a partially or fully cured (hybrid) structural adhesive composition as described above, for forming a curable precursor or a partially or fully cured (hybrid) structural adhesive composition shaped in the form of an article as described hereinbefore.

In yet another aspect, the present disclosure relates to the use of a curable precursor of a (hybrid) structural adhesive composition as described above, for (hybrid) structural adhesive printing.

In yet another aspect, the present disclosure is directed to the use of a curing system as described above for the manufacturing of a (hybrid) structural adhesive composition as described above. EXAMPLES

The present disclosure is further illustrated by the following examples. These examples are merely for illustrative purposes only and are not meant to be limiting on the scope of the appended claims.

Test Methods:

Preparation of the formulations for testing:

The curable precursor compositions are prepared from an extruded mixture of a one-component or a two-components (Part B and Part A) formulation. The preparation of the various components is described hereinafter. After a homogeneous mixture is achieved, the formulations are extmded between two liners producing a fdm having a thickness of about 0.3 mm on a 75 micrometer Slphan PET liner by using a knife coater. The samples are then placed under a 10x10 cm Opsytec lamp set to 100 % power, irradiating the adhesive precursor film for 30 to 60 seconds, which initiates the first reaction step (B-stage reaction step) resulting in a partially cured precursor adhesive film. The resulting tape is then cut into shape and applied to the surface of the test panel for further testing in the manner specified below.

Preparation of the test samples for OLS and T-Peel Tests:

The surface of OLS and T-peel samples (steel, grade DX54+ZMB-RL1615) are cleaned with n- heptane and in case of oily contaminated samples, coated with 3g/m ² of the testing oil (PL 3802-39S commercially available from Fuchs Petrolub AG, Germany). The test samples are left at ambient room temperature (23 °C +/- 2°C, 50% relative humidity +1-5%) for 24 hours prior to testing and the OLS and T-peel strengths are measured as described below.

1) Overlap Shear Strength (OLS) according to DIN EN 1465.

Overlap shear strength is determined according to DIN EN 1465 using a Zwick Z050 tensile tester (commercially available by Zwick GmbH & Co. KG, Ulm, Germany) operating at a cross head speed of 10 mm/min. For the preparation of an Overlap Shear Strength test assembly, the shaped B-stage tape is placed onto one surface of a prepared test panel. Afterwards, the sample is covered by a second steel strip forming an overlap joint of 13 mm. The overlap joints are then clamped together using two binder clips and the test assemblies are further stored at room temperature for 4 hours after bonding, and then placed into an air circulating oven for 30 minutes at 180 °C. The next day, the samples are tested directly. Five samples are measured for each of the examples and results averaged and reported in MPa. 2) T-Peel strength according to DIN EN ISO 11339.

T-Peel strength is determined according to DIN EN ISO 11339 using a Zwick Z050 tensile tester (commercially available by Zwick GmbH & Co. KG, Ulm, Germany) operating at a cross head speed of 100 mm/min. For the preparation of a T-Peel Strength test assembly, the shaped B-stage tape is directly placed onto the middle of the T-peel test panel. The second test panel surface is then immediately bonded to the first forming an overlap joint of 100 mm. The samples are then fixed together with clamps and first stored at room temperature for 12 hours, and then placed into an air circulating oven for 30 minutes at 180 °C. The next day, the samples are tested directly. Three samples are measured for each of the examples and results averaged and reported in Newtons (N).

Raw materials:

In the examples, the following raw materials are used:

Eponex 1510 is a hydrogenated bisphenol epoxy resin, commercially available from Hexion Specialty Chemicals GmbH, Iserlohn, Germany.

DEN 431 is an epoxy resin, commercially available from DOW Chemical Pacific, The Heeren, Singapore.

Epikote 828 is an epoxy resin, commercially available from Hexion Specialty Chemicals GmbH, Iserlohn, Germany.

Epikote 1004 is an epoxy resin, commercially available from Hexion Specialty Chemicals GmbH, Iserlohn, Germany.

IK EBSA is a one-component epoxy-based structural adhesive comprising: a) from 55 to 65 wt.% of bisphenol-A-(epichlorhydrin) epoxy resin (having a number average molecular weight Mn no greater than 700 g/mol; b) from 1.0 to 5.0 wt.% of bisphenol A; and c) from 1.0 to 2.5 wt.% of glycidyl neodecanoate, and which is obtained from 3M Deutschland GmbH, Neuss, Germany. Amicure CG1200 is a dicyandiamide-based latent curing initiator for epoxides, commercially from available from Evonik, Allentown, PA, USA.

Dyhard UR500 is a urea-based curing accelerator for epoxides, commercially available from AlzChem Trostberg, Germany.

Amicure UR2T is a urea-based curing accelerator for epoxides, commercially available from Evonik, Allentown, PA, USA.

Ancamine 2441 is a polyamine-based curing accelerator for epoxides, commercially available Evonik, Allentown, PA, USA.

D-6000-DMA is a dimethacrylate polyether oligomer having a number average molecular weight of about 6000 g/mol, and which is obtained from 3M Espe GmbH, Germany. KaneAce MX 257 is a toughening agent, commercially available from Kaneka Belgium N.V., Westerlo, Belgium.

KaneAce MX 153 is a toughening agent, commercially available from Kaneka Belgium N.V., Westerlo, Belgium.

Paraloid EXL 2650J is a toughening agent, commercially available from IMCD Benelux N.V., Mechelen, Belgium.

Camphorquinone is a free-radical polymerization initiator activatable via visible light, in particular blue light, commercially available from Sigma- Aldrich, St. Louis, USA.

Shieldex AC-5 is a silica based anti-corrosive agent, commercially available from Grace GmbH, Germany.

MinSil SF20 is a fused silica filler, obtained from the 3M Company, USA.

Aerosil R202 is a fumed silica surface-treated with polydimethylsiloxane, commercially available from Evonik GmbH, Germany.

Dynasylan GLYEO is a silane-based adhesion promoter agent, commercially available from Evonik GmbH, Germany.

/7-Toluene sulfonate, commercially available from Sigma- Aldrich, St. Louis, USA.

/7-Toluene sulfinate, commercially available from Sigma- Aldrich, St. Louis, USA.

Blankophor™ FB-28 is a fluorescent optical brightener, commercially available from Sigma- Aldrich Chemie GmbH, Taufkirchen, Germay.

Tinopal OB CO is a fluorescent optical brightener, commercially available from BASF, Germany. Heloxy BD is a reactive diluent, commercially available from Hexion Specialty Chemicals GmbH, Iserlohn, Germany.

Examples:

Preparation of Example 1

The exemplary 1 -component curable composition according of Example 1 is prepared by combining the ingredients from the list of materials of Table 1. KaneAce MX 257, KanAce MX 153, Eponex 1510 and DEN 431 are first placed in a small beaker and mixed together using a planetary highspeed mixer (DAC 150 FVZ Speedmixer, available from Hauschild Engineering, Germany) stirring at 3500 rpm for 1 minute. Aerosil R202 is added and mixed into the resins under vacuum at 1500 rpm for 2 minutes using a high-speed vacuummixer ‘ ARV-130 Thinky Mixer, available from Thinky Corporation, USA). Thereafter, Sil Cell 32, Shieldex AC-5 and MinSil SF20 are subsequently added and blended into the mixture by mixing at 3500 rpm for 1 minute. Then, Dynasylan GLYEO is added, followed by Amicure CG 1200, Dyhard UR500,D-6000-DMA as well as Camphorquinone dissolved in some acetone. In Table 1, all concentrations are given as wt.%. Table 1: Composition of ex. 1

Preparation of Examples 2 to 3

The general protocol of example 1 was followed. However, camphorquinone was not provided as solution in acetone. Rather, masterbatches of camphorquinone (ex. 2) and of camphorquinone and FB-29 (ex. 3 and 4) were prepared by dissolving the ingredients in Heloxy BD. The compositions of the masterbatches corresponding to ex. 2 to 4 are summarized in table 2. The wt.-% of camphorquinone and FB-28, respectively, correspond to the total amount of adhesive precursor. Table 2: Compositions of ex. 2 to 4

Preparation of pre-cured adhesive precursor films

The exemplary 1 -component curable compositions of Examples 1 to 4 were then processed on a knife coater to yield films of about 300 micrometer thickness using a 75 micrometer Silphan liner. The films were then exposed to the spectrum of a 10x10 cm Opsytec lamp set to 100 % power for 30 to 60 seconds, respectively, without further temperature control of the specimen. The films were then judged with regard to the curing status, i.e. whether a fully cured or a weak film was obtained. Ex. 1 yielded a fully cured film after 60 seconds. Ex. 2, 3 and 4 yielded fully cured films after 60 s or 30 s.

Overlap shear strength ( OLS ) and failure mode evaluation

The films according to ex. 1 to 4 were then evaluated regarding OLS and failure mode. The results are summarized in table 3. Table 4: OLS and failure modes.