WO2001089720A1

WO2001089720A1 - Solution polymerized vinyl resin for primer applications

Info

Publication number: WO2001089720A1
Application number: PCT/EP2000/011228
Authority: WO
Inventors: Jacobus Arno Schut; Robert Van Der Linden; Michael Theodorus Kroes
Original assignee: Corus Staal BV
Current assignee: Tata Steel Ijmuiden BV
Priority date: 2000-11-02
Filing date: 2000-11-02
Publication date: 2001-11-29
Anticipated expiration: 2003-05-02
Also published as: AU2001213940A1

Abstract

The invention relates to a primer for covering a metal surface such as the surface of steel, galvanized steel or aluminium, the primer comprising polymer chains to protect the metal surface and hydrolysable adhesion promoters to enhance the metal affinity of the primer, to a method of producing such a primer, and to the use thereof. According to the invention, the polymer chains comprise building blocks formed from vinyl monomers and linked together in the form of chains, a fraction θhy of the building blocks having a functional pendent group comprising a hydrolysable adhesion promoter. This provides a primer on the basis of polymers whose synthesis has substantially taken place in its entirety before the primer is applied. All the vinyl monomers are incorporated in the polymer chain as randomly as possible, although they need not all be equally reactive. The primer according to the invention provides a large degree of freedom in conferring desired characteristics.

Description

SOLUTION POLYMERIZED VINYL RESIN FOR PRIMER APPLICATIONS

The invention relates to a primer for covering a metal surface such as the surface of steel, galvanized steel or aluminium, the primer comprising polymer chains, which polymer chains comprise building blocks, formed from vinyl monomers, that are linked together in the form of chains, a fraction η_hy of the building blocks having a functional pendent group comprising a hydrolysable group. The invention also relates to a method of preparing said primer. The invention further relates to the use of the primer.

A primer is a coating which is applied to a material surface such as steel, galvanized steel or aluminium, in many cases as an undercoat to which a further organic layer, for example a top coat is to be applied. The requirements which a primer suitable for a wide range of applications must meet are often different from the requirements for a top coat; colour, gloss, resistance to external influences such as UV and wear resistance are less important, whilst the primer must have excellent adhesion characteristics in order to adhere both to the material underneath and to a possible further coating. The stability and resistance to water and acids must likewise be good. The material is often less expensive than a top coat. In addition, the primer should have a lower surface energy than the material underneath, should have a low viscosity, should be waterproof, be not soap-like and not cause undue cross-linking. However, depending on the characteristics which can be imparted to the primer and depending on the particular application, it is possible in certain cases for the primer also to act as the top coat.

Patent specification EP 0 196 645 Bl describes a general primer resin for organic materials such as wood, paper, plastic materials, and for inorganic materials for airplanes, buildings and automobiles. The known resin is a hydrolisable silyl group- containing vinyl resin. The main chain of the known resin is composed of a vinyl polymer in which at least one silicon atom is bonded to a hydrolysable group, for instance a silane, halogenated silane, or an alkoxysilane. The known resin is said to have improved adhesion, as a result of copolymerizing a (meth)acrylic acid higher alkyl esther to the main chain. The known resin is curable at ordinary temperature by exposure to the atmosphere, using the hydrolysable group to form a cross linked network structure.

A disadvantage of this known resin is that the kind of the hydrolysable group should be carefully selected, since the curing rates varies in accordance with the kind of the hydrolysable group.

When painting metal surfaces it is customary to coat the surface with a chromate- containing conversion layer before applying one or more organic coatings. Chromate- containing conversion layers are used, in particular, in the painting of steel and galvanized steel. They have excellent anti-corrosion and adhesion-enhancing characteristics. In addition, known primers often include chromium compound pigments.

However, the industry is increasingly being confronted with restrictions concerning the use of such chromium-containing compounds. There is therefore a need for alternative methods and media which can provide substitutes for the use of chromium-containing compounds for coating metal surfaces.

It is an object of the invention to provide an improved primer. Another object of the invention is to provide a primer of which specific characteristics such as applicability, durability, adhesion can be controlled a priori. It is a further object of the invention to provide a primer which can be applied without a conversion layer, e.g. a chromate conversion layer, to aluminium, steel or galvanized steel. Yet a further object of the invention is to provide a primer which is free from chromium and chromium compounds. Still another object of the invention is to provide a primer which offers a metal surface protection against corrosion. Yet another object of the invention is to provide a primer which is suitable as an undercoat to which various types of coatings will adhere.

According to a first aspect of the invention, one or more of these objects are achieved by means of a primer for covering a metal surface such as the surface of steel, galvanized steel or aluminium, the primer comprising polymer chains, which polymer chains comprise building blocks, formed from vinyl monomers, that are linked together in the form of chains, a fraction η_hy of the building blocks having a functional pendent group comprising a hydrolysable adhesion promoter to enhance the metal affinity of the primer, a fraction η_c, of the building blocks having a further reactive functional pendent group for cross linking to react with a curing agent. Herewith a primer is provided on the basis of polymers whose synthesis has substantially taken place in its entirety before the primer is applied. The hydrolysable pendent group acts essentially as an adhesion promotor. Because the polymer chain incorporates a certain fraction of such adhesion promoters, it has a number of anchoring sites having adhesive characteristics with respect to a metal surface, while the polymer chain provides excellent adhesion to a large number of different organic coating types by virtue of entanglement with the polymer chains from the coating. The polymer chain also incorporates a number of cross linking groups to react with a curing agent. Because, according to the invention, the hydrolysable adhesion promotors are not needed for cross linking, they can be optimized in their type and fraction to achieve the desired adhesion properties. Independently from that, the curing properties can also be optimized, for instance by choice of the type of the pendent group for cross linking and/or the fraction η.,. Additionally, the cross linking groups possibly improve the intercoat affinity with a top coat. The term "entanglement" for the purpose of this application relates to physical entanglement between polymer chains, said term thus being used in the art (see e.g.

"Polymer Entanglements" by R.P. Wool in Volume 26 of "Macromolecules" pp. 1564-

1596, 1993). The term "cross-linking" in the present application refers to chemical interaction between polymer chains.

In an embodiment, the primer further comprises a curing agent to react with said reactive functional pendent group for cross linking. After chemical curing, the curing agent is bonded to the reactive functional pendent group for cross linking, and the durability of the layer of primer is thereby improved, and its solubility reduced, without the necessity of sacrificing the hydrolysable adhesion promoters for this purpose. A further advantage is that the intercoat affinity to a certain number of types of further organic layers is improved.

Preferably, the hydrolysable adhesion promoter is essentially inert with respect to the further reactive pendent group and to the curing agent. The advantage of this primer is that the metal affimty has been uncoupled from cross-linkability. A polymer chain containing a relatively low number of adhesion promoter building blocks with respect to cross-linking building blocks is thus possible. Another advantage of this primer is that the resistance to water and acids is not impaired in the event of incomplete curing.

In an embodiment the fraction η_cl satisfies the relationship 0 < η_cl < 0.20. When the fraction of building blocks for cross linking exceeds 0.20, the primer suffers from fewer entanglements. Preferably 0.045 < η_c, < 0.20. When the fraction of building blocks for cross linking is more than 4.5 %, the primer has been found to be sufficiently durable. More preferably 0.045 < η_cl < 0.10. Herewith the entanglement properties are better optimized. In an embodiment, at least part of the curing agent is comprised as yet another reactive pendent group in the polymer chain. This ensures that the polymer chains are self-curing or self-cross linking.

In a particular embodiment, the reactive pendent group in the polymer chain being the curing agent is one from the same type as is the reactive functional pendent group for cross linking. This further ensures that both the cross linking building blocks and the curing building blocks are essentially inert for the adhesion promotors. Moreover, a similar degree of cross linking is achieved with fewer chemicals needed.

In an embodiment, the reactive functional pendent group in the polymer chain for cross linking comprises a group selected from groups with the formula -NH-(CH₂)_n- O-(CH₂)_mH; wherein n = 1, 2, 3, or 4, and m = 1, 2, 3, 4, or 5, or combinations thereof. This type of cross linking pendent group shows good self-cross linking properties in the form of ether-linkages, and also interacts well with polyester coatings, and at the same time is found to be essentially inert to the hydrolysable adhesion promotor. The number m determines the reactivity of the cross linking pendent group. When m exceeds 5, the reaction equilibrium does not sufficiently allow for the alkane group to dissolve, and hence the reactive CH₂-O is blocked. When m = 0, the reactive CH₂-O is not sufficiently blocked, so that curing already occurs uncontrollably at ambient temperatures. The number n determines the amount of spacing between cross linked chains. In a particular embodiment, the building block having a further reactive functional pendent group for cross linking is formed from an alkoxy-methylacrylamide or an alkoxy-methylmethacrylamide, by preference butoxy-methylmethacrylamide. Butoxy-methylmethacrylamide (nBuOMMA) deblocks at a temperature of between 140°C and 200°C, resulting in a curing temperature that is compatible with standard curing furnaces.

In another embodiment, the further reactive functional pendent group is an epoxy group and the curing agent comprises a dicarboxylic acid or an anhydride thereof, or in that the further reactive functional pendent group comprises an acid and the curing agent comprises an epoxy compound. This provides a primer in which the adhesion promoter is inert with respect to the further reactive group and to the curing agent. A further advantage of this embodiment is that chemical curing takes places at temperatures which are considerably higher than room temperature but not higher than temperatures achieved in production ovens. In yet another embodiment, the further reactive functional pendent group is an amine group or an amide group and the curing agent comprises a blocked isocyanate. This also provides a primer in which the adhesion promoter is inert with respect to the further active group and to the curing agent. A further advantage of this embodiment is that chemical curing takes places at temperatures which are considerably higher than room temperature but not higher than temperatures achieved in production ovens.

Preferably, the relationship M_bslM_e < η_hy < 4 _b/M_e and preferably 2 _bs/ _e < η_hy < 4 _bs/M_e applies, where _V _bs is the number average molecular weight of the building blocks incorporated into the polymer chains and M_e is a characteristic molecular weight where the polymer chains form circular loops, projected onto a plane. This ensures that the anchored polymer chains can entangle further polymer chains from the primer or from a possible further coating. This benefits the adhesion of the primer to a further organic layer. If approximately M_eIM_hs building blocks are present between two successive anchoring sites in the polymer chain, that section of polymer chain will be sufficiently long to adopt a fully closed loop shape. Such a loop enables cross-linking with other polymer chains from the primer or a possible further coating, which benefits adhesion of organic coatings to the primer. Using approximately MJ2M_bs building blocks between two successive anchoring sites, the polymer is itself capable of forming a half loop back to the metal surface, which according to one aspect of the invention is still sufficiently entangleable. Using _e/4M_bs, the adhesion of the two successive adhesion promoters will introduce a certain strain into the polymer chain, but it is sufficiently entangleable with other polymer chains from overlying primer layers or a possible further coating. A polymer chain which incorporates even more adhesion promoters will indeed adhere better to the surface, but its entangling capacity is too low to ensure adequate adhesion of a further coating.

Preferably, the fraction η_hy satisfies the relationship 0 < η_hy < 0.20, more preferably 0 < η_hy < 0.10, yet more preferably η_hy equalling about 0.02. It was found that a polymer chain which satisfies this provides adequate adhesion to a metal surface and also adequate entangling capacity to further polymer chains from the primer or from a possible further coating. A polymer chain which incorporates too many adhesion promoters will indeed adhere better to the surface, but its entangling capacity is too low to ensure adequate adhesion of a further coating. Conversely, a polymer chain containing too few adhesion promoters will adhere excellently to an organic coating, but there is insufficient adhesive interaction with the metal surface. Too high a content of hydrolysable adhesion promoters is at the expense of durability.

Preferably, the building blocks without adhesion promoter are formed from one or more esters of acrylic acids or mefhacrylic acids or mixtures thereof. Polymers of (meth)acrylates have good mechanical and thermal characteristics and good durability. Moreover, very wide variation of polymer compositions is possible, as a very wide choice of (meth)acrylate monomers is commercially available. Excellent controllability of the desired primer characteristics is thus achieved. Save for the presence of functional pendent groups designed for this purpose, (meth)acrylates are non- or virtually non-hydrolysable, which inter alia benefits durability under damp and corrosion-accelerating conditions.

Preferably, at least part of the adhesion promoters comprises a hydrolysable silane group which is linked, via an alkane chain, to a building block in the polymer chain. This provides an adhesion-functional building block rendered durable in the simplest possible manner. The alkane chain prevents the presence of a hydrolysable bond between the silane group and the building block, so that the link from the silane group to the remainder of the polymer chain is stable in a hydrolysis process. No condensation catalyst is required while a metal surface is being covered with the primer according to the invention in this embodiment. A further advantage is that this also gives rise to a self-repairing effect of the anchoring sites. Preferably, the alkane chain contains one, two, three, four or five carbon atoms. In this range, the adhesive effect of the adhesion promoters has been found to work excellently. The alkane chain serves as a spacer to maintain the distance between the polymer chain and the metal surface. If the alkane chain in the pendent group is too long, the bonding between the polymer chains and the metal surface is impaired, thereby reducing durability. However, the shorter the alkane chain the less good are the adhesive characteristics of the polymer chains to a metal surface. The hydrolysable moiety of the adhesion promoter in that case, after all, is closer to the polymer chain backbone and possibly even closer than possible further pendent groups which are present in the polymer chain.

Preferably, the building block comprising an adhesion promoter is formed from γ-methacryloxypropyl-trimethoxysilane (MPS). Three carbon atoms in the alkane chain were found to afford a good compromise between durability and metal affinity. Moreover, this building block is free from chromium and other heavy metals. This monomer is commercially available.

In one embodiment, M_n in atomic mass units satisfies the relationship 10,000 < M_n < 20,000, preferably also the relationship _n < 15,000. With a molecular weight of about 15,000 the primer was found to be eminently well applicable to metal surfaces, in particular steel, more in particular galvanized steel, whilst the stability of the primer with respect to degradation and its durability are good. At this weight the primer proved eminently compatible, in actual application, with coatings applied in a manufacturer's painting line to coat steel strip, for example with the aid of a coil coater comprising e.g. roller coaters. Too high a molecular weight causes problems during application, whilst the stability of the primer with respect to degradation, the durability and the entangleability decrease if the molecular weight is too low.

In a preferred embodiment, the primer is characterized in that the glass transition temperature T_g of the primer after physical drying and, where necessary, chemical curing is between +5°C and +45°C. The glass temperature of a coating defines the temperature below which the coating is a hard and frangible material. Above it, the layer is leathery, rubbery or even liquid. A top coat having a T_g which meets this condition can readily be coformed in various forming operations to which steel is often subjected at room temperature, for example flanging, bending or deepdrawing, while the primer has sufficient mechanical strength for use of the coated steel in e.g. the construction or kitchen appliances sector. It was found that a primer/coating of this type satisfies the extreme demands made of coil coatings.

In a preferred embodiment, the glass transition temperature _T_g of the primer after physical drying and, where necessary, chemical curing is between +5°C and +15°C. For bending purposes _T_g is best kept below +15°C. According to a further aspect, the invention relates to a method of producing the above-described primer. The method according to the invention is characterized in that non-hydrolysable vinyl monomers and hydrolysable vinyl monomers are polymerized, with the aid of a free-radical initiator, by the vinyl monomers being added slowly, compared with the polymerization rate, to a reaction mixture which comprises a solvent and is low in monomers. This ensures that all the vinyl monomer types are incorporated as randomly as possible in the polymer chains, even though they need not all be equally reactive. The advantage of this is that the distribution of the hydrolysable adhesion promoters over a chain is as random as possible, so that the locations of the anchoring sites are well distributed over the entire length of the polymer chain. The chain itself is non- or virtually non-hydrolysable. This provides a durable polymer chain which is able, in equal measure over the entire chain, to enter into interaction with a substrate and a further organic coating. Preferably, the method according to the invention is characterized in that a glass transition temperature T_g of the primer can be adjusted by vinyl monomers of a first non- or virtually non-hydrolysable type and a second non- or virtually non-hydrolysable type being added to the reaction mixture as well as polymerizable adhesion promoters, the glass transition temperature of a polymer exclusively formed from vinyl monomers of the first type being higher, and the glass transition temperature of a polymer exclusively formed from vinyl monomers of the second type being lower, than a desired T_g. The advantage of adhering to the method according to the invention is that the T_g of the polymer chains is adjustable, independently of the concentration of adhesion promoters and/or building blocks comprising a different functional pendent group which is incorporated in the chains. Adjustment of the T_g is possible both by selecting the types of vinyl monomers and via the ratio in which the various vinyl monomers in the reaction mixture are added. Thus the moldability and the mechanical strength of the primer can be adjusted independently of the adhesion characteristics and further characteristics of the primer. Preferably, the ratio between the vinyl monomers of the first and the second non- or virtually non-hydrolysable type is chosen as a function of the composition of the primer. This provides a simple method by which the T_g of the primer can be adjusted independently of other characteristics. According to the method, the ratio of the first and the second vinyl monomers of non- or virtually non hydrolysable types is of lesser importance in the primer design.

Preference is given to an embodiment in which the vinyl monomer used of the first non- or virtually non-hydrolysable type is a vinyl monomer of which the polymer has a glass transition temperature between +50°C and +120°C, and the vinyl monomer of the second non- or virtually non-hydrolysable type used is a vinyl monomer of which the polymer has a glass transition temperature between -65°C and +0°C. This choice permits a wide spectrum of glass transition temperatures which are around or just below the temperatures which typically prevail on the surface of our earth indoors and in the air outside, particularly around room temperature. This choice also provides scope for corrections for the use of other functional groups in the polymer chains, such as an adhesion promoter or reactive cross-linking site, which may cause variations in the glass transition temperature.

In principle, (meth)acrylic acids and their esters, (meth)acrylates, having T_g in the range described, are well worth considering. Preferably, however, the vinyl monomer of the first non- or virtually non-hydrolysable type used is methyl methacrylate (MMA), and the vinyl monomer of the second non- or virtually non-hydrolysable type used is butyl acrylate (BA). These vinyl monomers are very readily available in large quantities. The glass transition temperatures of the respective corresponding homopolymers are +106°C and -54°C, thus providing a wide choice of T_g around temperatures which typically prevail on the surface of our earth indoors and in the air outside, and particularly around room temperature. Equally, there is scope to correct, via an adjustment of the ratio of MMA and BA, for the use of other functional groups in the chain, such as MPS whose homopolymer has a glass transition temperature of -29°C. In addition, this choice of vinyl monomers provides scope for correcting the T_g for possible cross-linking effects.

Preferably, the ratio MMA : BA of the amounts of MMA and BA which are added to the reaction mixture is in a range of from 1.0 to 3.0 and is preferably also lower than 2.3 and more preferably lower than 2.0. Thus a primer is achieved whose T_g is below, yet close to, room temperature. It has also been found that the thermal stability of the polymer chains increases as said ratio decreases. Both effects are advantageous. The ratio aimed for depends on the amount and type of further vinyl monomers which are incorporated into the polymer chains, and on the degree of cross- linking to be expected, since these quantities will affect the glass transition temperature, and the reaction rates and thermal stability.

In an advantageous preferred embodiment, part of the free-radical initiator is added to the solvent before the vinyl monomers are added to the reaction mixture, and the remainder of the free-radial initiator is added together with the vinyl monomers. This ensures an advantageous variation of the free-radical flux over time. It was found that if all the free-radical initiator together with the vinyl monomers is added to the reaction mixture, the free-radical flux increases excessively with reaction time, which has proven disadvantageous in terms of, inter alia, polydispersity in the weight distribution of the polymer chains. High polydispersity is detrimental to the use of the polymer chains in the primer. In principle, a large number of substances is suitable for use as free-radical iniator. Preferably, the free-radical iniator is of the azo type. These substances prove highly suitable in producing the polymers chains according to the invention. One advantage is that this free-radical initiator is free from heavy metals, which pollute the environment.

In addition, the invention relates to the use of the above-described primer as an adhesion-enhancing layer between an organic coating and a metal surface. The primer according to the invention is eminently suitable to serve as an intercoating, or adhesive layer, for a wide range of organic coatings. This is ascribed to a mechanism involving entanglement of loops of the polymer chains, with polymer chains from a possible further coating, which is possible because the primer polymer chains will anchor themselves only point by point to a substrate via the adhesion promoters. From the process point of view of a coating operation it is highly advantageous to be able to use one and the same primer to serve various coating systems.

Preferably, the organic coating is a polyester, a polyurethane (PUR), a poly(vinyl chloride) (PVC), a poly(vinylidene fluoride) (PVDF) or an acrylate. These coatings represent common coating bases which encompass a wide quality and price range. They each represent a separate field of application.

The primer according to the invention is particularly suitable for a metal surface which mainly comprises steel, aluminium or zinc. If the primer is used on galvanized material, the advantage is that the primer can be kept simple, as the zinc layer promotes the corrosion resistance of the underlying metal. Furthermore, the primer shows excellent adhesion to aluminium, steel or zinc (and galvanized steel), without intervention of a conversion layer, such as a chromate conversion layer, thus providing environmental advantages, inter alia. The primer is highly suitable for covering e.g. sections, tubes, metal strip, or metal plate material.

The invention in yet another aspect relates to the use of the primer as an organic finish layer. In certain applications and cases the primer on its own, without further overcoating, offers adequate protection of the substrate provided with said primer. In general it is the case that, the higher the primer's degree of cross-linking, the more suitable it is as a finish layer.

The invention will be explained in more detail on the basis of non-limiting examples and with reference to the drawing, in which

Figure 1 schematically depicts the function of an adhesion promoter in an ideal adhesion mechanism;

Figure 2 (parts a, b, c) schematically depicts the ideal adhesion of a polymer chain according to the invention to a surface; Figure 3 shows structural formulae of a few polymer building blocks;

Figure 4 is a diagram in which the measured average molecular weights are plotted against different ratios of measured polymer chain compositions; Figure 5 is a diagram in which measured thermal stability of terpolymers of MMA, BA and 10% MPS are plotted against the amount of BA;

Figure 6 is a diagram in which measured and estimated values of the glass transition temperature are plotted against different ratios of added numbers of monomers.

The polymer compositions referred to in the explanation are in all cases given in mol%.

Figure 1 shows an ideal adhesion mechanism for an alkoxysilane (2) having the general formula X-Si-(OR)₃, where X is a polymerizable group and R is a small group such as a methyl, ethyl or methoxyethyl group, to a metal surface (1). Under normal conditions, the metal surface is covered with hydroxylated metal oxides. In the example of Figure 1, use is made of a zinc surface. In virtually all cases, a zinc surface is substantially covered by zinc hydroxides. The metal surface is therefore itself, without the intervention of a condensation catalyst, capable of effecting to a sufficient degree a hydrolysis reaction in which the alkoxysilane bonds to the metal, with the formation of

-R-OH. The bonds are readily hydrolysed, in a reversible process. This results in a reduction of the surface energy, to the benefit of adhesion. (Halogenated) silanes and alkoxysilanes, as well as titanates and zirconates are known hydrolysable adhesion promoters. Hafhates, phosphonates and sulphonates have likewise been demonstrated, but are not yet commercially available.

Figure 2 shows how a polymer chain (3) of the primer according to the invention bonds to a metal surface (1). In Figure 2a, the fraction of building blocks comprising adhesion promoter (2) is too small to produce enough anchoring sites to the metal surface. If in addition the distance between two adjacent anchoring sites, measured in atomic mass units, greatly exceeds a critical value which to those skilled in the art is known as "entanglement molecular mass ( _e)", as is the case in the situation shown in Figure 2a, the polymer chains will primarily become entangled with themselves. Absorption of these polymer chains to the metal surface is therefore poor, on the grounds of entropy. The aim, however, is for the polymer chains to become entangled with further polymer chains from the primer or polymer chains from a possible further organic layer. This situation is schematically depicted in Figure 2b. As the adhesion promoter (2) is incorporated in a polymer chain (3) only in the correct amount, the polymer chains (3) have optimal entangling capacity with further polymer chains (4), whether from the primer or from elsewhere, owing to the loops present. If, as in Figure 2c, the fraction of building blocks comprising adhesion promoters is too high, a polymer chain will adhere too flatly against the metal surface and have insufficient volume to spare for entanglement with further polymer chains (4). The critical value _e depends, for example, on the polymer composition. R.P. Wool, in "Polymer Entanglements" (pp 1564-1596 of Vol. 26 of "Macromolecules", 1993), gives a theoretical algorithm via which M_e [lacuna] for a number of polymers comparable with the invention, e.g. PMMA, the homopolymer of MMA. It will be obvious that the precise value of M_e of the present polymer chains differs somewhat, owing to their composition, from the value for PMMA. The ratio MJM_bs in this algorithm depends on geometric degrees of freedom of the bonds in free space.

Figure 2 should merely be regarded as a schematic. In reality, the loops will look different, for example owing to anchoring and cross-linking, and the ideal layer thickness is 4 to 6 μm, i.e. much thicker than one polymer chain. In practise, the layer thickness may even amount to as much as about 20 μm.

Figure 3 depicts structural formulae of a few such building blocks of a primer polymer chain according to one embodiment of the invention as can be incorporated in the polymer chains. PMMA, the homopolymer of MMA (methyl methacrylate), has a glass transition temperature T_g of +106°C, PBA (the homopolymer of butyl acrylate) of -54°C, and PMPS (derived from MPS) of -29°C. In general it can be assumed that T_g is lower if there are more pendent groups and if the pendent groups are longer. It was found that the propyl group of MPS (three carbon atoms) provides an excellent spacer in collaboration with MMA and BA, by means of which the primer can be eminently well distributed homogeneously over a metal surface. This spacer of three carbon atoms is thought to distance the hydrolysable section of the adhesion promoter just slightly further from the polymer chain than e.g. the butyl group of the BA. Butoxy- methylmethacrylamide (nBuOMMA) is a preferred substance to form a (self-) cross- linking building block. The adhesion promoter-containing polymer chains can be produced in various ways. In many cases, e.g. in the case of (meth)acrylates, it proves advantageous to employ a free-radical coplymerization of the desired vinyl monomers under monomer- starved conditions to achieve a random distribution of the building blocks over a polymer chain. In so doing, the following procedure, for example, is adopted. Provided with about 1/3 of the total amount of free-radical initiator required, a solvent is brought to a reaction temperature of between 70° and 100°C. Butyl acetate is a suitable solvent. Of good utility as a free-radical initiator is a compound of e.g. an azo type, for example α,α'-azobisisobutyronitrile (AIBN). The vinyl monomers, together with the remaining 2/3 of the free-radical initiator, are then added so slowly to the reaction mixture as to result in an apparent deficiency of monomer, thus causing all the desired vinyl monomers, even the less reactive ones, to be incorporated as randomly as possible in the polymer chains. This allows a reasonable characterized molecular structure of the polymer chains to be obtained, so that no unduly long sections having different characteristics will be formed in the polymer chains. Optionally, further media can be added to the reaction mixture to further control the free-radical polymerization.

In the above procedure, the solvent has already been provided with a portion of the free-radical initiator before the vinyl monomers are added. This ensures an advantageous variation of the free-radical flux with time. On the other hand, it was found that if all the free-radical initiator is added to the reaction mixture together with the vinyl monomers, the free-radical flux increases excessively with reaction time, which has proven disadvantageous in terms of, inter alia, polydispersity in the weight distribution of the polymers.

The above reaction has been carried out in the laboratory, using different ratios of numbers of MMA, BA, and MPS monomers. To this end, 40 ml of butyl acetate was admixed with 1/3 of the total amount of AIBN and warmed to 90°C. Mixtures of the desired vinyl monomers and AIBN were added to the reaction mixture over a period of from 2 to 4 hours at a rate of 0.085 ml per minute, and the reaction mixture was then allowed to continue to react for one more hour. Finally, the polymer chains were separated from remaining monomer by precipitation.

The compositions of the polymer chains thus produced were determined by means of nuclear magnetic resonance spectroscopy (ΝMR) of ^!H and ¹³C, at a resolution of 300 MHz and 400 MHz, respectively, using deuterated solvents and chromium(III) acetylacetonate as a paramagnetic relaxant. The molecular weights of the polymer chains produced were determined with respect to polystyrene calibration samples by means of gel permeation chromatography (GPC), using BHT-stabilized THF as the eluent supplied at a rate of 1.0 ml per minute. The columns comprise a PLgel guard-B lOμm 50x7.5mm precolumn, followed by four PLgel mixed-B 300x7.5mm columns in series at 40°C.

Figure 4 shows measured molecular weights of polymer chains produced in the laboratory, both as the number average ( J and the weight average (M_w) for various percentages of monomers expressed as MMA/BA/MPS. It can be deduced from the measured data of Figure 4 that molecules having M_n of between 10,000 and 20,000 atomic mass units can readily be produced. It appears that the smaller the ratio between added MMA and BA, the lower will be the polydispersity, defined as the ratio of M M_n. Low polydispersity benefits applicability of the primer according to the invention. An indication of the number of anchoring sites which are possible in polymer chains of this type, with the chains according to one aspect of the invention still being sufficiently entangleable, is from 2 to 4. If it is desirable to further improve adhesiveness to the metal surface while maintaining entangleability, one option is to provide an anchoring site with more than one adhesion promoter. Such clusters of adhesion promoters in the polymer chain increase the number of bonding sites to the metal surface, whilst a sufficient number of entangleable building blocks are present between the clusters. Figure 5 shows the decomposition temperature as determined by means of thermogravimetric analysis (TGA) for MMA/BA/MPS terpolymers, the amount of MPS being 10% in each case, as a function of the percentage of BA monomer added to the reaction mixture. In the course of the measurement, in a TGA-7 from Perkin-Elmer, the temperature was raised by 20°C per minute in air. The figure shows that the thermal stability increases with increasing amounts of B A in the reaction mixture: the onset of decomposition occurs at 270°C if no BA has been incorporated in the polymer chains, whereas the onset of decomposition in the case where only BA (and 10% MPS) has been incorporated in the polymer chains does not occur until 340°C is reached. The stabilizing effect of BA is thought to be due to the so-called unzipping mechanism, which occurs in methacrylates, being blocked as a result of acrylate incorporation. The typical temperatures which are achieved, for example, in a steel strip coating line oven, are around 200°C, so that the primer according to the invention is eminently suitable for use in e.g. an existing steel strip coating line.

The durability of the primer can be increased by chemically reactive functional pendent groups, which serve as a cross-linking site, being incorporated in the polymer chains. In conjunction with a curing agent which co-operates with two or more of these reactive cross-linking sites this produces cross-linking of the polymer chains. If required, partial curing is also possible. To this end, the primer can comprise such an amount of curing agent that not all the curable pendent groups are made to react. This is an important characteristic for a primer, because the durability of the primer layer is improved while at the same time the pendent groups left over improve the intercoat adhesion characteristics. Thus it is possible to establish that stability and intercoat affinity which is desirable for an application. A typical fraction of cross-linking building blocks which can be incorporated in the polymer chains is 0.05, but depending on the purpose and the use of the primer it is possible to deviate from this guideline figure. If the primer is to be used as a finish layer, for example, the aim will be to bind as many reactive building blocks as possible and at the same time allow a reasonably high fraction of the building blocks in the polymer chains to be curable.

Possible reactive cross-linking sites which can be incorporated next to the adhesion promoters in the polymer chains comprise hydroxy-functional pendent groups such as, for example, in (meth)acrylic acids and hydroxyethyl methacrylate (HEM A). These groups and the corresponding curing agents are also reactive, however, with the adhesion promoters, which is highly disadvantageous, particularly if the chains and/or curing agents contain an excess of cross-linking sites, as the adhesion promoters then become saturated, as it were, and are no longer available for anchoring to a metal surface.

In the laboratory, polymer chains IVIMA/BA/MPS/HEMA were produced at 70°C, monomers being added in proportions of 36/48/10/6 mol%. After drying under vacuum conditions over three days a cross-linked polymer was produced which is no longer soluble in THF. The presence of hydroxy-functional cross-linking sites is therefore not beneficial to, for example, the shelf life of the primer.

The curing agent and the cross-linking sites in the polymer chains are, according to one aspect of the invention, not reactive with the adhesion promoters. Thus the degree of cross-linking of the polymer chains is independent of the number of adhesion promoters in the chains. For this purpose, the polymer chains must preferably incorporate separate reactive cross-linking sites which are not hydroxy-functional. As adhesion promoters preferably do not contribute to the cross-linkability of the polymers and cannot react with the cross-linking reactive pendent groups, the adhesion promoters can be present in a lower concentration than the cross-linking vinyl monomers, so that the metal affinity and intercoat affinity are both maximally retained.

A first type of building block suitable for incorporation as a cross-linking site in the polymer chains comprise a pendent group with the formula -NH-(CH₂)_n-O- (CH₂)_mH; wherein n = 1, 2, 3, or 4, and m = 1, 2, 3, 4, or 5, or combinations thereof. Such a building block may for example be formed from an alkoxy-methylacrylamide or an alkoxy-methylmethacrylamide, such as butoxy-methylmethacrylamide. No separate curing agent is necessary with these pendent groups, since shows good self-cross linking properties in the form of ether-linkages resulting from a condensation reaction. If the number m is sufficiently high, the reactive group is blocked so that curing does not significantly take place during the shelf life.

Other building blocks that are suitable for incorporation as a cross-linking site in the polymer chains comprise an epoxy group. Examples of corresponding curing agents are dicarboxylic acid or dicarboxylic acid anhydride. These curing agents will not or virtually not cooperate with the cross-linking sites at room temperature, so that the primer will keep at room temperature, while the temperature at which the curing reactions take place are generally not above 230°C. Glycidyl methacrylate (GMA, see Fig. 3) is an example of such a building block comprising an epoxy group. It will be obvious to those skilled in the art that the parts played by curing agent and pendent group are invertible.

Other building blocks suitable for incorporation as a cross-linking site in the polymer chains comprise, for example an amine group or an amide group as a reactive functional pendent group. Cross-linking sites of this type work eminently well in concert with curing agents like aromatic isocyanates such as diphenylmethane diisocyanate (MDI) and toluene diisocyanate (TDI), but also aliphatic isocyanates such as hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI). In the blocked state these curing agents do not or virtually not react at room temperature, so that the primer will keep at room temperature, while the temperatures at which the curing reactions take place are generally not above 180°C, which is beneficial for industrial processes. One example of a blocking agent is phenol. Epoxy compounds likewise react eminently well with amines or amides, but often at lower temperatures. There are also other reasons to incorporate as few hydrolysable groups as possible into the polymer chains. Hydrolysable groups are generally detrimental to durability, as the hydrolysis is reversible. At the same time they increase the sensitivity of the primer to water and acid.

In the description, the intercoat affinity is determined by the entangling capacity of the polymer chains, possibly assisted by the presence of unreacted cross-linking building blocks. The option of incorporating at least one further building block into the polymer chains, to improve the intercoat affinity even further, is not ruled out, however.

According to the invention, the glass transition temperature of the polymer can be adjusted, given a correct choice of monomers and their relative proportions in which they are added within the reaction mixture. The first step in so doing is to establish how much and which building block comprising an adhesion promoter shall be incorporated into the chain and how much and which cross-linking groups, and what degree of cross- linking is being aimed for. Given a correct choice of the relative proportion of a non- or virtually non-hydrolysable vinyl monomer whose homopolymer has a glass transition temperature which is higher than the intended T_g of the primer polymer, with respect to another non- or virtually non-hydrolysable vinyl monomer whose homopolymer has a glass transition temperature which is lower than the intended T_g of the primer polymer, it is possible to tune the T_gof the primer polymer to the desired value.

Preferably, the choice of the two abovementioned types of non- or virtually non- hydrolysable vinyl monomers is fixed. The proportion in which these two types of vinyl monomers are incorporated into the polymer chains will then be a defining parameter for the glass transition temperature. If these two types of monomers have not been assigned any further functional pendent groups, it is possible, according to one aspect of the invention, to adjust the T_g of the primer polymer independently of further functional characteristics. The literature discloses various approaches which can be used as an aid in establishing the correct amounts of monomers. Figure 6 shows estimated and measured values of T_g for different polymers which were produced under different relative monomer proportions. The estimation method used here is the Fox-Flory approach, the bars PMMA, PBA and PMPS forming values known from the literature. The measurements were carried out with the aid of a differential scanning calorimeter (DSC-7 from Perkin Elmer), primer samples being heated at a rate of 20°C per minute, after they had previously been fully cured at the same rate. It can be gathered from the figure that in practice a sizeable range of glass transition temperatures can be achieved, including the desired value which according to the invention is between +5°C and +15°C. In order to produce, according to the invention, a polymer having a T_g in the said range from MMA, BA and MPS, the ratio of the numbers of MMA monomers and BA monomers to be added to the reaction mixture is in a range of from 1.0 to 3.3, preferably being below 2.3 and more preferably lower than 2.0. The required ratio depends on the total composition of the polymer chains and on the degree of cross- linking to be expected and to a lesser extent on the molecular weights aimed for.

In the laboratory, adhesion tests and bending tests were carried out with galvanized steel provided with a layer of primer according to the invention. The steel had been galvanized by means of a hot-dipped galvanizing method. After the polymer chains had been separated from residual monomer by means of precipitation, they were dissolved in THF. The primer layers were applied from a 50% solution using a doctor blade and were then dried and cured at 200°C over a period of two minutes.

Adhesion was tested by means of the cross-hatch test according to the ASTM 1123 standard. Then the same substrate, in accordance with the ECCA 123 standard, was bent back by 180° in the cross-hatched area from the cross-hatch test, after which adhesion was tested again. The results of these experiments are set out in the following table on a scale of 6, in which "very poor" is denoted by 5 and "excellent" by 0.

Mixing ratios Measured τ_s Cross-hatch Result of

MMA BA/MPS proportions of (°C) test result 180° introduced into MMA/BA MPS in bending the reaction primer polymer test mixture chains

60/40/0 39/41/0 -11 2 2

50/40/10 62/34/4 +5 0 2

45/45/10 53/39/8 +11 1 1

40/40/20 41/17/41 +2 0 0

The measured compositions, given in the table, of the polymer chains were determined by means of the above-described NMR technique, while T_g was measured by means of the above-described DSC technique. What is found is that the polymer chains which comprise the adhesion promoter MPS exhibit better adhesion to the galvanized surface than a reference of polymer chains solely comprising MMA and BA. The table also shows that the bending characteristics are determined both by the T_g and by the fraction of adhesion promoters in the polymer chains; the higher T_s the better the adhesion at bending, but also the more adhesion promoters the better the adhesion at bending.

In another laboratory test, the following polymer chains were synthesised. The molecular weights were controlled by the amount of free-radical initiator that was added to the reaction mixture.

Primer Mixing ratios τ_{s n} M_w

MMA/BA/MPS/nBuMMA (°C) (a.m.u.) (a.m.u.) introduced into the reaction mixture

1 50/22/17/11 +32 7700 20100

2 50/22/17/11 +36 3840 13000

3 45/30/15/10 +30 4830 13350

4 42/40/10/8 +22 9180 22910

The glass transition temperature was measured before chemical curing by means of the above-described DSC technique, and the molecular weights M_a and _w were determined using the above-described gel permeation chromatography (GPC) technique.

Primer layers containing the above identified polymer chains were applied to samples of galvanized steel sheet to a thickness of 5 μm and a thickness of 10 μm, and one of each sample was cured at elevated temperature. Three temperatures were tested: 180°C, 210°C, and 240°C. After curing and cooling the samples, a polyester top coat was applied to a thickness of 20 μm, after which the samples were cured at a temperature between 225°C and 245°C.

Visual inspection showed attractive, shiny and transparent primer layers.

Adhesion was tested on all samples by means of the cross-hatch test according to the ASTM 1123 standard. The results are shown in the following table. Primer 5-μm layer 10-μm layer

180°C 210°C 240°C 180°C 210°C 240°C

1 1 1 1 1 1 1

2 1 0 0 3 3 3

3 1 1 1 1 1 3

4 1 1 0 1 0 0

As can be seen, the best primer is number 4, followed by 1, 3, and 2. This order reflects the ordering in molecular weight M_n. In none of the tests the top coat delaminated, showing there is excellent intercoat affinity. The cross-test values also show that very good adhesion to the metal is achieved.

After the cross-hatch test, the same substrates, in accordance with the ECCA 123 standard, were bent back by 180° in the cross-hatched area, after which adhesion was tested again.

Primer 5-μm layer 10-μm layer

180°C 210°C 240°C 180°C 210°C 240°C

1 0 3-4 1 2.5 0.5 2-3

2 1 1 0.5 1 2.5 2

3 0.5 2 2-3 0.5 2.5-3 2.5

4 1.5 0.5 2-3 1 2.5 1

A dependence on the glass transition temperature is observed.

Claims

1. Primer for covering a metal surface such as the surface of steel, galvanized steel or aluminium, the primer comprising polymer chains, which polymer chains

I comprise building blocks, formed from vinyl monomers, that are linked together in the form of chains, a fraction η_hy of the building blocks having a functional pendent group comprising a hydrolysable adhesion promoter to enhance the metal affinity of the primer, a fraction η_cl of the building blocks having a further reactive functional pendent group for cross linking to react with a curing agent.

2. Primer according to Claim 1, whereby the primer further comprises a curing agent.

3. Primer according to Claim 1 or 2, whereby the adhesion promoter is essentially inert with respect to the further reactive pendent group and to the curing agent.

4. Primer according to any one of the preceding claims, whereby the fraction η_cl satisfies the relationship 0 < η_cl < 0.20, preferably 0.045 < η_cl < 0.20, more preferably 0.045 < η_cI < 0.10.

5. Primer according to any one of the preceding claims, whereby at least part of the curing agent is yet another reactive pendent group in the polymer chain.

6. Primer according to Claim 5, whereby the reactive pendent group in the polymer chain being the curing agent is one from the same type as is the reactive functional pendent group for cross linking.

7. Primer according to any one of the preceding claims, whereby the reactive functional pendent group in the polymer chain for cross linking comprises a group selected from groups with the formula -NH-(CH₂)_n-O-(CH₂)_mH; wherein n = 1, 2, 3, or 4, and m = 1, 2, 3, 4, or 5, or combinations thereof.

8. Primer according to Claim 7, whereby the building block having a further reactive functional pendent group for cross linking is formed from an alkoxy- methylacrylamide or an alkoxy-methylmethacrylamide, by preference butoxy- methylmethacrylamide.

9. Primer according to any one of Claims 1 to 5 inclusive, wehreby the further reactive functional pendent group is an epoxy group and the curing agent comprises a dicarboxylic acid or an anhydride thereof, or in that the further reactive functional pendent group comprises an acid and the curing agent comprises an epoxy compound.

10. Primer according to any one of Claims 1 to 5 inclusive, whereby the further reactive functional pendent group is an amine group or an amide group and the curing agent comprises a blocked isocyanate.

11. Primer according to any one of the preceding claims, whereby the relationship M M_e < η_hy < 4M M_e and preferably 2M M_e < η_hy < 4 _bs/M_e applies, where M_hs is the number average molecular weight of the building blocks incorporated into the polymer chains and M_e is a characteristic molecular weight where the polymer chains form circular loops, projected onto a plane.

12. Primer according to any one of the preceding claims, whereby the fraction η_hy satisfies the relationship 0 < η_hy < 0.20, preferably 0 < η_hy < 0.10, more preferably equals about 0.02.

13. Primer according to any one of the preceding claims, whereby the glass transition temperature _T_g of the primer after physical drying and, where necessary, chemical curing is between +5°C and +45°C, preferably between +5°C and +15°C.

14. Method of producing the primer according to any one of Claims 1 to 12 inclusive, wherein a reaction mixture is provided which comprises a solvent and a free- radical initiator, to which the vinyl monomers are added slowly, compared with the polymerization rate, keeping the reaction mixture low in monomers, characterized in that part of the free-radical initiator is added to the solvent before the vinyl monomers are added to the reaction mixture, and in that the remainder of the free-radical initiator is added together with the vinyl monomers.

15. Method according to Claim 14, characterized in that a glass transition temperature T_g of the primer can be adjusted by vinyl monomers of a first non- or virtually non-hydrolysable type and a second non- or virtually non-hydrolysable type being added to the reaction mixture as well as polymerizable adhesion promoters, the glass transition temperature of a polymer exclusively formed from vinyl monomers of the first type being higher, and the glass transition temperature of a polymer exclusively formed from vinyl monomers of the second type being lower, than a desired T_g.

16. Method according to Claim 15, characterized in that the ratio between the vinyl monomers of the first and the second non- or virtually non-hydrolysable type is chosen as a function of the composition of the primer.

17. The use of the primer according to any one of Claims 1 to 13 inclusive as an adhesion-enhancing layer between an organic coating and a metal surface.

18. The use according to Claim 17, where the organic coating is a polyester, a polyurethane (PUR), a poly(vinyl chloride) (PVC), a poly(vinylidene fluoride) (PVDF) or an acrylate.

19. The use of the primer according to any one of Claims 1 to 13 inclusive as an organic finish layer.