Primers BACKGROUND

Field

[0001] The present invention relates primers, which are particularly well suited to promoting cure and assisting in the bonding of difficult to bond substrates, such as those constructed of non-polar or highly crystalline materials or for promoting the cure of otherwise slow curing products.

Description of Related Technology

[0002] Some surfaces (substrates) are difficult-to-bond with adhesives, and are widely recognised, including non-polar or highly crystalline substrates.

[0003] Non-polar substrates typically have low surface energy that is due to an absence of polar groups on the surface to be bonded. Non-polar substrates are often constructed of polyolefins in particular linear polymers of simple olefins, such as polyethylene, polypropylene, polybutene, polytetrafluoroethylene and the like, including their copolymers; polyacetals, plasticised PVC (polyvinyl chloride), polyurethane, EPDM (ethylene-propylene diamine monomer) rubber, thermoplastic rubbers based on vulcanised polyolefins and the like.

[0004] Difficult-to-bond materials also include those with a waxy or paraffin-like surface character; a low critical surface tension of wetting; and which may have at their surface a weak boundary layer.

[0005] An adhesion-promoting primer is intended to promote adhesion in cases where curing (polymerization) occurs readily but where adhesion may not take place at all or a least to a desired extent.

[0006] Primer compositions may also promote cure of the curable composition. In some instances curable compositions may provide a desired degree of adhesion but achieve adhesion at an undesired rate. For example, the curable composition may cure to give sufficient adhesion but may do so only very slowly so that the substrates to be bonded must be kept in contact over a period of hours or even longer. While relatively long cure times are desirable in some instances, a shorter cure time without deleteriously affecting bond strength is desirable for many other purposes.

[0007] Application of a primer composition which comprises a primer component in a solvent to a substrate to be primed is typically by way of a brush or some such other applicator, to form a layer or coating on the surface. The coating is then typically dried, or allowed to dry for a period, before bonding with adhesive is attempted, and to ensure that the surface is properly primed. In this respect at least for adhesion-promotion primers the solvent is normally a volatile one, that is it typically evaporates readily at room

temperature leaving behind the primer component. The solvent does not therefore interfere with the curing process - acting only as an inert carrier solvent for placing the adhesion-promoting primer component on the substrate. For cure-promoting primers the solvent may be a polymerizable monomer or oligomer which may take part in the polymerization process during bonding. Accordingly, these latter solvents are not usually considered "inert" in the sense that they participate in the subsequent bonding process.

[0008] They do however still act as carrier solvents for the primer component. Monomeric or oligomeric solvents do not tend to evaporate to any appreciable extent after application so that they are not normally considered volatile.

[0009] The nature of such primer compositions can result in difficult handling for example unwanted spillage, run-on of the coating applied etc. It is desirable to provide an alternative form of primer composition.

[0010] Primer compositions typically include at least one priming compound or agent which is the primer component of the composition. Various primer components and primer compositions are described below.

[0011] Not withstanding the state of the technology it would still be desirable to provide alternative primer compositions to supplement existing options.

SUMMARY

[0012] In one aspect, the present invention provides a primer composition for priming a substrate for application of a curable composition. Such a primer composition comprises:

(a) a carrier component having a molecular weight in the range of 30 to 300, desirably 60 to 150 and a melting point in the range of 30 to 500 °C desirably 50 to 200 °C; and

(b) a primer component for the curable composition which is dispersible in the carrier component when the carrier component is in a melted state.

The primer composition can be applied to the substrate in liquid form and thereafter cooled to solidify thereon.

[0013] As used herein the term "cooled" includes both active cooling at a lower temperature and passive cooling to ambient temperature.

[0014] In general, the carrier component will be at a temperature greater than its melting point, i.e. it will be liquid, when the primer component is dispersed therein. This primer composition will then be applied hot to a target substrate and allowed to cool to form a primer film thereon.

[0015] Compositions of the invention can be applied in a liquid phase, but form a solid applied mass, for example a layer upon cooling in-situ. Advantageously, this makes a substrate to which the primer composition of the invention is applied dry to touch and easy to handle. They can be used in a pre-applied form where the substrate can be moved easily after application without the composition running off.

[0016] Advantageously, the present invention removes the need to have an evaporating solvent as a carrier for the primer composition. This is very desirable from a handling, health and safety, and environmental point of view.

[0017] Compositions of the present invention when applied to a substrate form solid masses, which adhere well to the substrate. They have been demonstrated to form tenacious coatings on substrates.

[0018] The carrier component will desirably have a melting point (mp) in the range of 50 to 200 °C, for example 50 to 150 °C, such as 50 to 100 °C. All references to ranges in this specification include the indicated end points.

[0019] The carrier component of the primer composition of the present invention may be selected from the group consisting of acetoacetamide (mp 53 - 56 °C), tropolone (mp 50 - 52 °C), ethyl sulfone (mp 73 - 74 °C), diacetamide (mp 75.5 - 76.5 °C), dialkyl ureas (for example 1 , 1 -diethyl urea [mp 69 - 71 °C], 1 ,3 -diethyl urea) and combinations thereof. The carrier component of the primer composition of the present invention may comprise acetoacetamide. The carrier component of the primer composition of the present invention may comprise ethyl sulfone. The carrier component of the primer composition of the present invention may comprise diacetamide.

[0020] The primer component of the composition of the present invention may comprise at least one of:

(a) 1 , 5-diazabicyclo [4.3.0] non-5-ene; 1 ,8-diazabicyclo [5.4.0] undec-7-ene, or 1 , 5,7-triazabicyclo [4.4.0] dec-5-ene;

(b) triphenyl phosphine;

(c) ethylenediamine or derivatives of ethylenediamine;

(d) an imidazole; and,

of course, combinations thereof.

[0021] The primer compositions of the invention are particularly suitable for anaerobic compositions for example structural anaerobic materials. The primer component of the composition of the present invention may comprise an accelerator for an anaerobic adhesive composition. Thus, the primer component may comprise at least one of: (a) substituted thioureas;

(b) a compound having a sulphur-containing free radical source;

(c) toluidines, for example Λ/,/V-ib/salkyl-toludienes;

(d) aryl hydrazines, for example acetylphenylhydrazine (APH);

(e) /V-substituted dihydropyridines, for example /V-substituted-1 -2- dihydropyridines;

(f) a compound containing an oxidisable transition metal; or

(g) a compound containing one of,

and, of course, combinations thereof.

[0022] The primer component may comprise at least one of:

(a) substituted thioureas;

(b) a compound having a sulphur-containing free radical source;

(c) a compound containing an oxidisable transition metal; or

(d) a compound containing one of,

I «_

-N— C—

and, of course, combinations thereof.

[0023] For example, the primer component of the primer composition of the present invention may comprise benzoyl thiourea.

[0024] Unless otherwise indicated, the term aryl refers to an aromatic C ₅ -C ₆ o carbocyclic structure which is monocyclic or polycyclic (unfused or fused). The carbon atoms of the aryl residue may optionally be substituted one or more times, for example, with at least one of a cyano group, a nitro group, a halogen, C1-C10 alkyl, a C1-C10 ether, a C1-C10 thioether, a C1-C10 ester, C1-C10 ketone, C1-C10 ketimine, C1-C10 sulfone, C1-C10 sulfoxide, a C1-C10 primary amide or a C1-C20 secondary amide.

[0025] Unless otherwise indicated, the term alkyl embraces CrC ₅₀ unbranched alkyl, d- C ₅₀ branched alkyl and combinations thereof. [0026] The carrier component may comprise acetoacetamide and the primer component may comprise a substituted thiourea, a compound having a sulphur-containing free radical source, a compound containing an oxidisable transition metal, or combinations thereof. With reference to the primer composition of the present invention, a particularly advantageous combination is a primer component comprising benzoyl thiourea and a carrier component comprising acetoacetamide.

[0027] The primer component of the composition of the present invention may be present in a concentration of about 0.001 to about 30% by weight based on the total weight of the composition. The primer component may be present in a concentration of about 0.01 to about 12% by weight based on the total weight of the composition.

[0028] Another further advantage of primer compositions of the present invention is that adhesive compositions applied thereto demonstrate good CTV (cure through volume) when cured thereon.

[0029] In a further aspect, the present invention provides for a two-part adhesive system comprising:

(a) an adhesive composition comprising at least one curable monomer; and

(b) a primer composition according to the present invention.

[0030] In a preferred embodiment the adhesive composition of the two-part adhesive system of the present invention may comprise at least one anaerobically curable monomer.

[0031] The anaerobically curable monomer may be selected from the group consisting of (meth)acrylates, thiolenes, siloxanes, vinyls and, of course, combinations thereof.

[0032] The anaerobically curable monomer may be a (meth)acrylate monomer.

(Meth)acrylate monomers suitable for use as the (meth)acrylate component in the present invention may be chosen from a wide variety of materials, such as these represented by where G may be hydrogen, halogen or alkyl groups having from 1 to about 4 carbon atoms, and Ri may be selected from alkyl, cycloalkyl, alkenyl, cycloalkenyl, alkaryl, aralkyi or aryl groups having from 1 to about 16 carbon atoms, any of which may be optionally substituted or interrupted as the case may be with silane, silicon, oxygen, halogen, carbonyl, hydroxyl, ester, carboxylic acid, urea, urethane, carbonate, amine, amide, sulfur, sulfonate, sulfone and the like.

[0033] Additional (meth)acrylate monomers suitable for use herein include polyfunctional (meth)acrylate monomers, such as, but not limited to, di-or tri-functional (meth)acrylates like polyethylene glycol di(meth)acrylates, tetrahydrofuran (meth)acrylates and

di(meth)acrylates, hydroxypropyl (meth)acrylate ("HPMA"), hexanediol di(meth)acrylate, trimethylol propane tri(meth)acrylate ("TMPTMA"), diethylene glycol dimethacrylate, triethylene glycol dimethacrylate ("TRIEGMA"), tetraethylene glycol dimethacrylate, dipropylene glycol dimethacrylate, di-(pentamethylene glycol) dimethacrylate,

tetraethylene diglycol diacrylate, diglycerol tetramethacrylate, tetramethylene

dimethacrylate, ethylene dimethacrylate, neopentyl glycol diacrylate, trimethylol propane triacrylate and bisphenol-A mono and di(meth)acrylates, such as ethoxylated bisphenol-A (meth)acrylate ("EBIPMA"), and bisphenol-F mono and di(meth)acrylates, such as ethoxylated bisphenol-F (meth)acrylate.

[0034] Still other (meth)acrylate monomers that may be used herein include silicone (meth)acrylate moieties ("SiMA"), such as those taught by and claimed in U.S. Patent No. 5,605,999 (Chu), the disclosure of which is hereby expressly incorporated herein by reference.

[0035] Of course, combinations of these (meth)acrylate monomers may also be used.

[0036] The (meth)acrylate component should comprise from about 10 to about 90 percent by weight of the composition, such as about 60 to about 90 percent by weight, based on the total weight of the composition.

[0037] The inventive compositions may also include other conventional components, such as free radical initiators, free radical accelerators, inhibitors of free radical generation, as well as metal catalysts, such as iron and copper.

[0038] A number of well-known initiators of free radical polymerization may be

incorporated into the inventive compositions including, without limitation, hydroperoxides, such as CHP, para-menthane hydroperoxide, t-butyl hydroperoxide ("TBH") and t-butyl perbenzoate. Other peroxides include benzoyl peroxide, dibenzoyl peroxide, 1 ,3-bis(t- butylperoxyisopropyl)benzene, diacetyl peroxide, butyl 4,4-bis(t-butylperoxy)valerate, p- chlorobenzoyl peroxide, cumene hydroperoxide, t-butyl cumyl peroxide, t-butyl perbenzoate, di-t-butyl peroxide, dicumyl peroxide, 2,5-dimethyl-2,5-di-t- butylperoxyhexane, 2,5-dimethyl-2,5-di-t-butyl-peroxyhex-3-yne, 4-methyl-2,2-di-t- butylperoxypentane and combinations thereof.

[0039] Such peroxide compounds are typically employed in the present invention in the range of from about 0.1 to about 10 percent by weight, based on the total weight of the composition, with about 1 to about 5 percent by weight being desirable.

[0040] Stabilizers and inhibitors (such as phenols including hydroquinone and quinones) may also be employed to control and prevent premature peroxide decomposition and polymerization of the composition of the present invention, as well as chelating agents [such as the tetrasodium salt of ethylenediamine tetraacetic acid ("EDTA")] to trap trace amounts of metal contaminants therefrom. When used, chelators may ordinarily be present in the compositions in an amount from about 0.001 percent by weight to about 0.1 percent by weight, based on the total weight of the composition.

[0041] The invention further provides for a method of bonding a first substrate to a second substrate, the method comprising the steps of:

(i) priming at least one of the substrates by application of a primer

composition according to the present invention; and

(ii) applying an adhesive composition to the primed substrate and bringing the substrates together to form a bonded assembly.

[0042] As will be appreciated by a person skilled in the art the carrier component of the primer composition must be heated to a temperature corresponding to its melting point or above before the primer component can be dispersed therein, and the primer composition applied to a target substrate.

[0043] The at least one substrate to which the primer composition is applied may be heated prior to application of the primer composition. Advantageously, this may prevent premature solidification of the primer composition on the substrate.

[0044] With reference to the method of the present invention, the adhesive composition applied to the primed substrate may be an anaerobically curable adhesive composition.

[0045] In a further aspect, the present invention provides for a substrate having a primer composition according to the present invention applied thereto.

[0046] In yet a further aspect, the present invention provides for a cure product comprising a primer composition according to the present invention.

[0047] It will be appreciated by a person skilled in the art that the primer compositions of the present invention may additionally comprise conventional additives such as fillers, pigments, stabilisers, eic, subject to said additives not interfering with effective curing of the adhesive compositions applied thereto.

[0048] Where suitable, it will be appreciated that all optional and/or preferred features of one embodiment of the invention may be combined with optional and/or preferred features of another/other embodiment(s) of the invention.

DETAILED DESCRIPTION

[0049] It should be readily apparent to one of ordinary skill in the art that the examples disclosed herein below represent generalised examples only, and that other arrangements and methods capable of reproducing the invention are possible and are embraced by the present invention. [0050] The possibility of using low melting solid binders in primer compositions was investigated. For example, a solventless, pre-applied primer comprising BTU (benzoyl thiourea) would open up some attractive options. One approach would be to use a low melting material that would dissolve BTU and then cool on the substrate to give a dry film. Consequently we searched for relatively low mw species (100-200 - so that they would dissolve readily in a curable composition such as an anaerobic formulation) with a melting point between 50 °C and 60 °C that were inexpensive and of low hazard. The most promising candidate we found was acetoacetamide, which doesn't carry any risk/hazard phrases. We melted this at 60 °C, mixed it 9:1 with BTU and applied a thin coating of the mix to warm MS (mild steel) lap shears (we found it more difficult to make thin layers on cold lapshears). The mix cooled to a dry film that adhered very well to steel. The pre- applied primer performed very similarly in terms of fixture time and cure through volume to an acetone-based BTU primer when used with Loctite ® product 2701 - an anaerobic dimethacrylate ester composition suitable for threadlocking applications and available from Henkel Ireland, Tallaght Dublin 24.

Example 1

[0051] Acetoacetamide (3-Oxobutanamide) has a melting range of 52-54 °C and was investigated as a possible solid-state carrier for BTU in priming applications.

[0052] BTU was added to acetoacetamide in a ratio of 1 :9 and the mixture was placed in a 70 °C oven for 30 minutes. It was noted that after 30 minutes, the acetoacetamide appeared fully melted, however the BTU, with a higher melting point of -174 °C was still solid. Additional mixing for 2 hours in a 70 °C oil bath proved effective at dissolving the BTU in the acetoacetamide.

[0053] The optimum depth of the acetoacetamide/BTU layer was determined on GBMS using a "Doctor Blade" with preset depth markings:

[0054] Fixture across zero gap was attempted on a range of substrates using product Loctite® 326. Loctite® 326 is a liquid polyurethane methacrylate anaerobically curable composition. All Loctite® products are available from Henkel Ireland, Tallaght Business Park, Whitestown, Tallaght, Dublin 24. Primed Unprimed Substrate

Substrate (minutes) (minutes) Corrosion wetting

GBMS 3 10 - 20 Yes Good

Stainless Steel 6 -10 > 47 No Good

^* HDG Steel 3 - 5 10 - 15 No Poor

Aluminium T2024

T3

(Bare) 10 - 15 10 - 13 No Good

Aluminium T2024

T3

(alclad) 6 -14 > 60 No Poor

^* = Hot Dip Ga vanized

[0055] The acetoacetamide / BTU mixture corroded the GBMS surface within 5-10 minutes of application, however corrosion was not evident on the other substrates.

Application of melted acetoacetamide to GBMS did not result in corrosion.

[0056] Wetting ability was poor on HDG steel and again on alclad aluminium.

[0057] Shear strengths with Loctite® 326 (1 hr) on primed substrates were recorded as follows;

[0058] Initial results after 2 weeks @ 45°C did not show any degradation in cure ability of the acetoacetamide/BTU layer, when bonded with Loctite® 326 over 1 hour Room Temperature Cure (RTC).

Acetoacetamide "On-Part Life Testing" [0059] A stainless steel (SS) lapshear substrate was coated with Acetoacetamide / BTU and aged at 45°C & 40°C / 98% relative humidity (RH). Bonds were prepared post ageing with Loctite® 326 across 500 micron gap.

[0060] The results in the above table illustrate that there was very little degradation in activity.

Example 2: Other low melting point carrier molecules for BTU

[0061] Tropolone (2-Hydroxy-2,4,6-cycloheptatrien-1 -one) [1], ethyl Sulfone [2],

Diacetamide [3] & 1 , 1 -Diethyl Urea [4] were evaluated as possible solid state BTU carriers. BTU was dissolved into the above carriers at 10% wt. The resulting mixtures were coated onto SS and bonded with Loctite® 326 across 500 micron gaps. The 1 hr strengths are as follows:

Bond strength with Tropolone carrier -> 3 N/mm ²

Bond strength with ethyl Sulfone carrier -> 7 N/mm ²

Bond strength with Diacetamide carrier -> 10 N/mm ²

Bond strength with 1 ,1 -Diethyl Urea carrier -> 10.6 N/mm ²

1 2 3 4

[0062] Tropolone (2-Hydroxy-2,4,6-cycleheptatrien-1 -one), ethyl Sulfone, Diacetamide, & 1 ,1 -Diethylurea were evaluated and found to be good solid state primer carriers, exhibiting good BTU dissolution and producing results varying from 3 - 10.6 N/mm ² on primed stainless steel with product Loctite® 326 across 500 micron gap. [0063] Further comparative work was undertaken on "as received" mild steel, GBMS & Aluminium (T2024-T3 bare) substrates. Diacetamide and 1 ,1 -Diethylurea proved to be the most effective molecules, giving good strengths on all substrates.

Cure strengths of BTU in solid carriers

Example 3: Acetoacetamide plus Alternative Anaerobic Accelerators

PDHP

[0064] N-phenyl -2-propyl -3,5-diethyl -1 ,2-dihydropyridine (PDHP) was added to melted acetoacetamide at 15% wt and applied as a thin coating on GBMS. When solidified, this primed substrate was tested against control combinations of Loctite® 533, which is a rapid curing surface-activated acrylic adhesive that is used in conjunction with an activator. All Loctite® products are available from Henkel Ireland, Tallaght Business Park, Whitestown, Tallaght, Dublin 24. Initial tests showed that Loctite® 533 would not cure on GBMS without addition of a primer. Cure strengths in the presence of the primer are illustrated below.

Product 533 Shear Strength on primed GBMS

Acetylphenylhydrazine (APH) [0065] APH at 1 wt% in acetoacetamide was prepared and tested as a primer on GBMS with a model formulation of threadlocker Loctite® 270 upgrade in which APH was omitted. Loctite® 270 is a dimethacrylate based acrylic adhesive that is designed for the permanent locking and sealing of threaded fasteners. All Loctite® products are available from Henkel Ireland, Tallaght Business Park, Whitestown, Tallaght, Dublin 24. Initial tests showed that there was no cure on GBMS without addition of a primer. Cure strengths in the presence of the primer are illustrated below

Product 270 upgrade Shear Strength on primed/unprimed GBMS after 24 hour RTC

[0066] The words "comprises/comprising" and the words "having/including" when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps or components but do not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0067] It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.