SUMMARY OF THE INVENTION The present invention relates to an adhesion promoter additive, and to a coating composition suitable for use with a variety of plastic components, including thermoplastic polyolefin (TPO). The coating composition comprises the adhesion promoter additive having (a) at least one modified polyolefin and (b) at least one monohydric alcohol, at least one epoxy-functional silane, or a mixture thereof. Substrates painted with the coating composition are also disclosed. The composition of the present invention is particularly useful as a primer for plastic components in the automotive industry.

BACKGROUND OF THE INVENTION Plastics are used in wide variety of applications, including containers, household appliances, automobile parts and accessories, and other commercial items. With respect to the automotive industry, the use of plastic components has increased dramatically over the past several decades. The quest for lighter, more fuel efficient cars has led automotive manufacturers to utilize plastics in an increasing number of interior and exterior applications, such as airbag covers, bumpers, fascias, fenders, door panels, panel hoods, panel roofs, and panel trunk lids. Besides generally being low cost, plastics are also low weight, corrosion resistant, impact resistant, relatively easy to mold, and recyclable. Because of these and other advantages derived from their use, it is inevitable that more and more automotive parts will be produced from plastics.

It is often desirable to paint or otherwise coat automotive components, especially those of the exterior body, in order to decorate them or protect them from degradation in the face of sunlight, moisture, heat and cold. To achieve longer lasting and more durable parts, it is important for these coatings to be tightly adhered to the surface of the components.

One of the problems with the increasing use of plastic for automotive components is that polymeric substrates made from a variety of thermoplastic and thermosetting materials can have widely varying surface properties, such as surface tension, roughness, and flexibility, which can make strong adhesion of coatings difficult, particularly after aging or environmental exposure. Typically, in order to use a polyurethane primer on a plastic during automotive repair, the plastic must first be identified, e.g. type, flexibility, etc., by performing such activities as the "burn" test or by checking an identification system in order to determine the appropriate process for coating the plastic.

Thermoplastic olefin (TPO), an alloy of polypropylene and an elastomer, is one type of plastic commonly used in the automotive industry. The structure of this polymer results in a surface that most coatings will not wet out or adhere to. Therefore, the surface must be physically or chemically altered in order for it to be coated with most currently available coating technology, but each of these surface pretreatments has its own disadvantages. Plasma treatment or corona discharge employs ionized gases to oxidize the surface and can be difficult and costly. Flame treatment also oxidizes the surface but requires a precise distance of flame that is difficult to achieve on a production line. Etching is another way of preparing the surface. However, the etching chemicals can be toxic or the subject of environmental concerns.

Another, more common approach is painting a thin layer or "tie coat" of an adhesion promoting primer containing a chlorinated polyolefin resin (CPO) onto the surface to gain adhesion. However, these solvent borne chlorinated polyolefins may have limited solubility and require use of aromatic solvents such as xylene or toluene, which may be difficult to utilize without exceeding emission standards. Further, this multi-step process can be time consuming, more costly and is prone to omission. Therefore, it is desired to shorten this process by developing an additive that would provide the properties of adhesion to plastics, including TPO, so that both the need for a "tiecoat" and identifying the plastic, prior to coating, would be unnecessary steps. Such a process would be highly beneficial in the automotive industry and the automotive repair industry as well as other industries in which coated plastics are commonly used.

Some coating compositions that exhibit acceptable adhesion directly to polymeric materials, such as TPO, without the use of separate adhesion promoter layers or tie coats have been developed. For example, polyolefin diols have been used in coating compositions to improve adhesion to polymeric substrates without the use of separate adhesion promoter layers or tie coats. For example, U.S. Patent US 6001469, herein incorporated by reference, discloses a coating composition containing a saturated polyhydroxylated polydiene polymer having terminal hydrpxyl groups.

U.S. Patent US 4863988, also incorporated by reference herein, relates to a curable paint composition containing a curable film-forming resin and a chlorinated and graft-modified polypropylene.

However, there remains a need for a coating composition, incorporating an adhesion promoter additive therein, which has good adhesion directly to the surface of a wide variety of plastics. SUMMARY OF THE INVENTION It is an object of the present invention to provide an adhesion promoter additive that enhances the adhesion properties of a coating formulation, particularly with respect to plastic substrates.

It is a related object of the invention to provide a coating composition that will directly adhere to the surface of a plastic substrate without the need to first apply a tie coat to the substrate.

It is a further object of the invention to provide a coating composition that will form a film coating on a variety of plastic substrates without the need to identify the particular plastic substrate prior to applying the coating.

It has been discovered that when a modified polyolefin is combined with a monohydric alcohol or an epoxy-functional silane, a novel adhesion promoter additive is formed that can be incorporated into a coating composition thereby allowing the composition to be coated directly onto many kinds of plastic substrates without the use of conventional pretreatment methods. An additional advantage was also discovered in that the additive also lessens the need to flexibilize the coating composition.

More particularly, the present invention relates to an adhesion promoter additive comprising: (A) at least one modified olefin; and (B) at least one component selected from the group consisting of monohydric alcohols, epoxy-functional silanes, and mixtures thereof.

The invention also relates to coating compositions comprising the adhesion promoter additive. One such coating composition comprises at least one hydroxyl- functional film forming polymer and at least one polyisocyanate compound as well as the adhesion promoter additive.

The invention also relates to a method of coating a plastic substrate by applying a coating composition comprising the adhesion promoter additive onto a plastic substrate and curing the coating composition and to plastic articles coated with such a coating composition.

Although particularly advantageous in the automotive industry, the present invention is also applicable to other industries as suggested above, which utilize coated plastic substrates.

DETAILED DESCRIPTION OF THE INVENTION The current invention relates to an adhesion promoter additive and to a coating composition comprising the adhesion promoter additive. In a preferred embodiment the coating composition additionally comprises at least one hydroxyl-functional film forming polymer and at least one polyisocyanate compound. The adhesion promoter additive comprises (A) at least one modified polyolefin and (B) at least one component selected from the group consisting of monohydric alcohols, epoxy- functional silanes, and mixtures thereof. Preferably, the amount of the adhesion promoter additive incorporated into the coating composition is in the range of 20 % to 40 % by volume, more preferably 25 % to 35 % by volume, based on the total volume of the coating composition.

The hydroxyl-functional film forming polymer preferably has a hydroxyl value between 50 and 300 mg KOH/g based on solids content of the polymer, and more preferably between 70 and 200 mg KOH/g. The number average molecular weight preferably does not exceed 6000 and more preferably does not exceed 4500. The polydispersity is preferably between about 1.1 to about 5 and more preferably between about 1.1 and about 3. The acid value of the polymer is preferably less than or equal to 50 mg KOH/g based on solids content of the polymer. Examples of suitable hydroxyl-functional film forming polymers include polyesters, polyacrylates, polyvinyl, polyurethanes, polycarbonates or polyamides. In one preferred embodiment, the polymer is a polyacrylate polyol.

The poiyisocyanate compound is a polyisocyanate having two or more, preferably two to four isocyanate groups. Examples of useful polyisocyanate compounds include toluene diisocyanate, methylene bis(4-cyclohexylisocyanate), isophorone diisocyanate and its isocyan urate or adducts, hexamethylene diisocyanate and its isocyanurate, biuret, urethdione, and allophanate, and meta-tetramethylxylene diisocyanate and the adduct thereof with trimethylolpropane.

The present coating composition further comprises an adhesion promoter additive, which comprises at least one modified polyolefin. Preferably, the amount of modified polyolefin in the additive is between 10 % by weight and 30 % by weight, based on the weight of the additive. One particularly preferred polyolefin is chlorinated polyolefin, also known as CPO. Chlorinated polyolefins suitable for use in the present invention preferably have a chlorine content between 15 % by weight and 60 % by weight, based on the total solid weight of the final chlorinated polyolefin, and more preferably between 18 % by weight and 23 % by weight. The chlorinated polyoiefin preferably has a weight average molecular weight between 5000 and 200000, more preferably, between 10000 and 40000. The chlorinated polyolefin may be solid, in powder or pelletized form, or in solution. Generally, any chlorinated polyolefin known to those skilled in the art to be an adhesion promoter may be used, for example chlorinated polypropylene, chlorinated polybutylene, chlorinated polyethylene, and mixtures thereof. One suitable chlorinated polyolefin is CPO-343-1, available commercially from Eastman Chemical Company of Kingsport, Tennessee, USA. Other suitable chlorinated polyolefins are described in U.S. Patent Nos. US 4997882; US 5319032; and US 5397602, herein incorporated by reference. Other suitable modified polyolefins include, for example, AP 440-1, a non- chlorinated polyolefin available commercially from Eastman Chemical Company of Kingsport, Tennessee, USA. More generally, if non-chlorinated, the modified polyolefin must have sufficient solubility for inclusion into a primer formulation, a low film formation temperature, and a high level of acid functional ization, i.e. an acid number of at least 40 mg KOH/g. Preferably, the modified polyolefin is capable of being heat cured.

The adhesion promoter also comprises at least one component selected from the group consisting of monohydric alcohols, epoxy-functional silaπes, and mixtures thereof. The term "monohydric alcohol" is used throughout the specification to describe a linear, cyclic, aromatic, or branch-chained hydrocarbon having a single hydroxyl group located at one terminus of the molecule. Preferably, the amount of monohydric alcohol or epoxy-functional silane present is from 5 % by weight to 30 %> by weight of the additive. Monohydric alcohols useful in the invention contain between 8 to 18 carbon atoms, and preferably 12 to 18 carbon atoms. Examples of useful monohydric alcohols include cetylalcohol, tridecyl alcohol, tetradecyl alcohol, pentadecanol, hexadecanol, heptadecanol, and octadecanol, more preferably dodecylalcohol. Numerous additional monohydric alcohols may be used in the present invention.

Examples of suitable epoxy-functional silanes include but are not limited to (3- glycidoxypropyl) trimethoxysilane, (3-glycidoxypropyl) triethoxysilane, (3- glycidoxypropyl) tripropoxysilane, (3-glycidoxypropyl) dimethoxymethylsilane, (3- glycidoxypropyl) dimethylmethoxysilane. Other epoxy-functional silanes are known in the literature and are obtainable by procedures recognized in the art. Optionally, the coating composition of the present invention can also contain one or more dyes or pigments to provide color and/or hiding. Any conventional pigment known to those skilled in the art may be utilized, including inorganic pigments such as titanium dioxide, talc, mica, iron oxides, lead oxides, chromium oxides, lead chromate and carbon black/conductive carbon black, and organic pigments such as phthalocyanine blue and phthalocyanine green, as well as a variety of other color pigments. Preferably, the total amount of pigment in the coating composition will be between 15 % by weight and 60 % by weight, based on total solids. The specific amount of pigment may vary, but generally, the amount of pigment is such that requisite hiding is achieved at the desired film thickness and application solids.

In one embodiment of the current invention, the coating composition further comprises a flexibilizer to impart additional elasticity to the dried film and help prevent cracking of the film. The flexibilizer may be a polyester, a soft acrylic resin having a glass transition temperature below 1O0C, or preferably, a monofunctional alkane. Examples of suitable monofunctional alkanes include those having an alkyl chain of C8 -Ci8 of functionality X, where X can be OH, NH, NH2 or SH. Particularly preferred are those Ci2 -Ci8 compounds with OH functionality such as dodecyl alcohol, tridecyl alcohol, tetradecyl alcohol, pentadecanol, hexadecanol, heptadecanol and octadecanol. In one embodiment, the flexibilizer may be the same as the monohydric alcohol utilized in the adhesion promoter additive.

The present coating composition may, in addition to any solvents provided by other coating components, further include a solvent component. Non-limiting examples of suitable solvents include aliphatic solvents; aromatic and/or alkylated aromatic solvents such as toluene, xylene, and hydrocarbon solvents; alcohols such as isopropanol; esters; ketones; glycol ethers; and glycol ether esters. If desired, the coating composition of the present invention can contain other materials well known in the art of formulating surface coatings, such as, for example, surfactants, flow control agents, thixotropic agents, fillers, anti-gassing agents, organic co-solvents, additional film-forming polymers, polymeric microparticles, catalysts and other conventional additives.

The coating composition of the present invention may be made using techniques well known to those skilled in the art. The percent by weight solids content of the coating composition preferably varies from 19 % to 73 %.

The coating composition of the present invention is particularly suited for the coating of plastic substrates, including but not limited to thermoplastic polyolefins, polycarbonates, polyesters, polypropylene, polyurethanes and polyamides such as Nylon, ABS (acrylonitrile butadiene styrene copolymer), SMC (sheet molding compound), and RIM (reaction injection molding) urethane, and combinations thereof. Thermoplastic polyolefin substrates are preferred.

The present coating composition may be applied to the substrate by any conventional means including brushing, dipping, flow coating, spraying and the like but is most often applied by spraying. Conventionally known techniques and equipment for manual or automatic spraying and electrostatic spraying can be used. Although conventional application means are employed, one of the advantages of the present coating composition is that it can be deposited directly onto the surface of a plastic substrate without the need of a separate adhesion promoter, tie coat layer or corona pretreatment.

When the coating composition is used as a primer composition, subsequent top coats such as conventional base coat - clear coat composites or conventional mono coat top coats can be applied to the primer coating. Still further, when used as a primer or an adhesion promoter layer, the coating composition can be air flashed before application of top coat layer(s) or, alternatively, can be pre-baked to a cured film prior to top coat application. Although one preferred embodiment of a coating composition has been described herein, the novel adhesion promoter additive of the present invention may be utilized as an additive in any conventional primer composition, preferably in primer compositions comprising isocyanates. It has been found that use of the adhesion promoter additive in primer compositions provides primers having good adhesion, not only to plastic substrates but also to metal substrates, such as steel substrates.

The present invention may be further illustrated by the following non-limiting Examples. All parts and percentages therein are by weight unless otherwise indicated or understood.

EXAMPLES

For the following Examples, adhesion promoter additives according to the present invention were prepared. The first adhesion promoter additive ("APA-1") was prepared by adding, in order, 30 parts by weight dodecylalcohol, 20 parts by weight CP 343-1, a chlorinated polyolefin adhesion promoter (25 wt % CPO in xylene) available commercially from Eastman Chemicals Company, and 50 parts by weight xylene to a suitable vessel and mixing under low agitation for 15 minutes. The second adhesion promoter additive ("APA-2") was prepared by adding, in order, 20 parts by weight of CP-440-1 , a non-chlorinated polyolefin adhesion promoter (25 wt % in xylene) available commercially from Eastman Chemicals Company, 75 parts by weight xylene, and 5 parts by weight of (3-glycidoxypropyl) trimethoxysilane to a suitable vessel and mixing under low agitation for 15 minutes. Two control systems were utilized in examples 1 and 2 for comparison purposes. Control system A is Primer PO, a CPO tiecoat composition commercially available from Akzo Nobel Coatings Inc. of Norcross, Georgia, USA. Control system B is Plastoflex, a primer composition, also available commercially from Akzo Nobel Coatings Inc.

The plastic panel substrates used in the following examples are Hifax CA186AC, a thermoplastic olefin (TPO) available commercially from Basell Polyolefins of Elkton, Maryland, USA; Xenoy 1102, a polycarbonate (PC)/polybutylene terephthalate(PBT)blend available commercially from General Electric Plastics of Pittsfield, Massachusetts, USA, and Phase Alpha Sheet Molding Compound (SMC) available commercially from Ashland Chemical Company in Dublin, Ohio.

EXAMPLE 1 A coating composition was prepared by adding, in order, 100 parts by volume of Colorbuild Primer, available commercially from Akzo Nobel Coatings Inc., 50 parts by volume of Colorbuild Sealer Hardener, also available commercially from Akzo Nobel Coatings, and 30 parts by volume of APA-1. These components were mixed together until homogenous and having a viscosity of 15-18 seconds on a #4DIN cup. This coating composition was then spray applied, separately, with a HPLV spray gun to each of the plastic panel substrates. The applied coating was allowed to flash for 15 minutes. Next, Autobase Plus, a base coat commercially available from Akzo Nobel Coatings Inc., was activated according the technical method required for use on plastic and then spray applied to each of the primed panels to hiding as denoted by coverage using black and white hiding sticker. The applied base coat was allowed to flash for 15 minutes until dry to a finger touch. Finally, Autoclear III, a clear coat commercially available from Akzo Nobel Coatings Inc., was activated per the technical data sheet for coating on plastics then spray applied to each of the base coats. These samples are hereto referred as Sample A.

Control A was supplied ready to spray and was applied to the plastic panel substrates and was flashed for 15 minutes. Control B was applied to the substrates as directed on the technical data sheet supplied with Control B and allowed to flash for 15 minutes. After flashing, Autobase Plus base coat and Autoclear III clear coat were applied as set forth above to all of the Control A and Control B panels.

Mandrel bend testing pursuant to General Motors Method 9503P at ambient temperature (0.5 inch mandrel) for the flexible TPO and Crosshatch adhesion testing pursuant to General Motors method 9071 P were conducted on the substrates. As can be seen in Table 1 below, the panels of Sample A performed equally well to the panels of Control A and Control B.

Table 1

System Adhesion (10 = best) Mandrel Bend Rating (10 = best) Initial (Ambient Temperature) TPO SMC PC/PBT TPO

Sample A 10 10 10 10

Control A 10 NA NA

Control B NA 10 10 NA EXAMPLE 2 A coating composition was prepared by adding 33 parts by weight of a polyester polyol with T9 less than 2O0C to 67 parts by weight of Colorbuild Primer and mixed with a spatula to form a modified Colorbuild Primer. 50 parts by volume of Colorbuild Sealer Hardener and 30 parts by volume of APA-2 were added to 100 parts by volume of the modified Colorbuild Primer. These components were mixed together until homogenous and having a viscosity of 15-18 seconds on a #4DIN cup. This coating composition was then spray applied, separately, with a HPLV spray gun to each of the plastic panel substrates. The applied coating was allowed to flash for 15 minutes. Next, Autobase Plus was activated according the technical method required for use on plastic and then spray applied to each of the primed panels to hiding as denoted by coverage using black and white hiding sticker. The applied base coat was allowed to flash for 15 minutes until dry to a finger touch. Finally, Autoclear III was activated per the technical data sheet for coating on plastics then spray applied to each of the base coats. These samples are hereto referred as Sample B.

Control A was supplied ready to spray and was applied to the plastic panel substrates and was flashed for 15 minutes. Control B was applied to the substrates as directed on the technical data sheet supplied with Control B and allowed to flash for 15 minutes. After flashing, Autobase Plus base coat and Autoclear III clear coat were applied as set forth above to all of the Control A and Control B panels.

All panels were allowed to age for seven days at ambient temperature. Mandrel bend testing pursuant to General Motors Method 9503P at ambient temperature (0.5 inch mandrel) for the flexible TPO and Crosshatch adhesion testing pursuant to General Motors method 9071 P were conducted on the substrates. As can be seen in Table 2, the panels of Sample B performed equally to the panels of Control A and Control B.

Table 2

System Adhesion (10 = best) Mandrel Bend Rating (10 = best) Initial (Ambient Temperature) TPO SMC PC/PBT TPO Sample B 10 10 10 10 Control A 10 NA NA 9 Control B NA 10 10 NA

EXAMPLE 3 Humidity resistance testing according to General Motors Method 4465P and chip resistance testing according to General Motors Method 9508 were performed on the TPO panels prepared as for Sample A and Sample B above. All panels were allowed to age for seven days at ambient temperature. Mandrel Bend testing according to General Motors Method 9503P at both ambient temperature and at - 200F was also conducted on the panels. The results are shown in Table 3 below. As can be seen in Table 3, the coatings comprising APA-1 and APA-2 had good retention and chip resistance. Table 3

EXAMPLE 4 Humidity resistance testing according to General Motors Method 4465P and chip resistance testing according to General Motors Method 9508 were performed on SMC and PC/PBT panels prepared as for Sample A and Sample B above. All panels were allowed to age for seven days at ambient temperature. The results are shown in Table 4 below. Note that all paint removal after humidity occurred between the base coat and the primer. No failure to the substrates was observed.

Table 4

EXAMPLE 5 In this example, a coating composition of the present invention was compared to currently available plastic primers. The plastic panel substrates used in this example are Hifax CA186AC, a thermoplastic olefin (TPO) available commercially from Basell Polyolefins of Elkton, Maryland, USA and Cyclolac AR4051, an acrylonitrile butadiene styrene (ABS) available commercially from General Electric Plastics of Pittsfield, Massachusetts, USA,

A coating composition was prepared using APA-1 as for Example 1 above. This composition is hereto referred as Sample C. Sample C was compared against SUA 4903 and DPX 801 , two primers available commercially from PPG Industries, and UPO7226, a primer available commercially from The Sherwin-Williams Company.

Sample C was spray applied, separately, with a HPLV spray gun to the plastic panel substrates. The applied coating was allowed to flash for 15 minutes. Next, Autobase Plus was activated according the technical method required for use on plastic to hiding as denoted by coverage using black and white hiding sticker. The applied base coat was allowed to flash for 15 minutes until dry to a finger touch. Finally, Autoclear III was activated per the technical data sheet for coating on plastics then spray applied to each of the base coats.

The comparative primers were applied to the substrates as directed on the technical data sheets supplied with the products. Autobase Plus base coat and Autoclear III clear coat were then applied as set forth above to all of the substrates coated with the comparative primers. Mandrel bend testing pursuant to General Motors Method 9503P at ambient temperature, relative humidity pursuant to General Motors method 4465P, chip resistance pursuant to General Motors method 9508P (using Gravelometer), and initial adhesion testing pursuant to General Motors method 9071 P were conducted on each of the coated substrates. The results are shown in Table 5 below.

Table 5

X: Crosscut, #: Crosshatch. Both reflect % adhesion retention