WO1993023487A2

WO1993023487A2 - Topographical method

Info

Publication number: WO1993023487A2
Application number: PCT/US1993/003469
Authority: WO
Inventors: Michael A. Johnson; Shuichi Kitano; Akira Itoh
Original assignee: Minnesota Mining And Manufacturing Company
Priority date: 1992-05-05
Filing date: 1993-04-15
Publication date: 1993-11-25
Also published as: KR950701371A; EP0639212A1; JPH07506864A; CA2118017A1; WO1993023487A3; JP3417561B2; MX9302514A

Abstract

A method for imparting topographical features to a permanent substrate employing a sheet material comprising a thermosettable pressure-sensitive adhesive composition. The edges of the sheet material are caused to flow prior to the thermosetting reaction to provide smooth transition lines between an exposed surface of the sheet material and the permanent substrate.

Description

TOPOGRAPHICAL METHOD

FIELD OF THE INVENTION

This invention relates to a method for providing topographical features to a substrate.

BACKGROUND OF THE INVENTION

U.S. Patent No. 5,086,088 discloses a latent, thermosettable pressure-sensitive adhesive composition comprising an acrylate pressure- sensitive adhesive and an epoxy resin component which provides for the thermoset cure. The adhesive composition is disclosed as being useful to fasten roof molding to a car body.

BRIEF SUMMARY OF THE INSTANT INVENTION The instant invention provides a novel method for imparting topographical or protective features to a permanent substrate, comprising the steps of: a) providing a dissevered, hardenable sheet material having first and second major surfaces, comprising a latent, thermosettable pressure-sensitive adhesive throughout a major portion of its thickness, and exhibiting pressure-sensitive adhesive properties at the first major surface; b) contacting and adhering the first major surface of the sheet material to the permanent substrate or a temporary substrate leaving the second major surface of the sheet material exposed; c) substantially thermosetting and substantially hardening the sheet material in a manner permitting initial, controlled mass flow of the sheet material substantially in its thickness direction to provide a substantially smooth transition between the second surface of said sheet material and the permanent substrate or the temporary substrate to which it has been adhered; and d) in the event the hardened sheet material is adhered to the temporary substrate, removing the hardened sheet material therefrom and thereafter fastening (e.g., adhering) the first major surface of the sheet material to the permanent substrate. In a preferred method, the sheet material is adhered to the permanent substrate in the first instance.

The method of the invention finds particular utility in the application of sheet material to primed or unprimed metal automobile parts or bodies to seal metal parts or provide emblems or insignia or design elements such as trim. The resulting laminate is aesthetic since the hardened sheet material exhibits smooth, rounded edges relative to the substrate, and can be painted or otherwise decorated as desired to match or complement the remainder of the automobile.

Another application of the method of the invention is in the fabrication of signs or the like which are then adhered to the permanent surface by means of an adhesive distinct from the sheet material exployed in making the sign or other article.

DETAILED DESCRIPTION OF THE INVENTION The sheet material employed in the method of the invention comprises a latent, thermosettable pressure-sensitive adhesive. By "pressure- sensitive adhesive" is meant that the adhesive exhibits pressure-sensitive adhesive properties at the temperature at which the sheet material is contacted with the permanent substrate or temporary substrate in step b) of the method. Generally, the temperature involved in that step will be between ambient temperature and about 400°F. It is presently preferred that the adhesive exhibit pressure-sensitive adhesive properties at ambient temperature such as 22°C.

The composition of the thermosettable pressure-sensitive adhesive contained in the sheet material is such that, when employed in the method of the invention, its modulus decreases permitting controlled flow of the sheet material at its edges resulting in smooth transition lines between the exposed surface of the sheet material and the substrate to which it has been adhered. By "controlled flow" is meant that there is no substantial change in the dimensions of the sheet material in its x- and y-planes defining the first major surface. Typically the sheet material will have been dissevered or configured in the desired shape and dimensions by a process such as die-cutting or knife or laser slitting resulting in sharp transition lines. After the above-described controlled flow has occurred, a latent curing or crosslinking chemistry is activated to harden the sheet material and the pressure-sensitive adhesive therein. Accompanying such hardening may be a loss of substantially all the pressure- sensitive adhesiveness of the sheet material. In a preferred method of the invention, the controlled mass flow and subsequent thermosetting reaction result from heating of the sheet material.

The sheet material used in the method of the invention comprises a latent thermosettable pressure-sensitive adhesive throughout a major portion of its thickness. Preferably, the sheet material comprises the adhesive throughout its entire thickness.

The thermosettable pressure-sensitive adhesive preferably comprises the photochemical reaction product of starting materials comprising i) a prepolymeric (i.e., partially polymerized to a viscous syrup typically between about 100 to 10,000 centipoises) or monomeric syrup comprising an acrylic or methacrylic acid ester; ii) an epoxy resin; iii) a photoinitiator; and iv) a heat- activable hardener for the epoxy resin. Such a composition may be coated and polymerized conveniently in a variety of thicknesses including relatively thick sections. The photopolymerizable prepolymeric or monomeric syrup contains an acrylic or methacrylic ester and optionally a copolymerizable reinforcing comonomer. The acrylic or methacrylic ester is a monofunctional acrylic or methacrylic ester of a non-tertiary alcohol, having from about 4 to about 12 carbon atoms in the alcohol moiety. Included in this class of esters are n-butyl acrylate, hexyl acrylate, 2-ethylhexyl acrylate, octyl acrylate, isooctyl acrylate, decyl acrylate and dodecyl acrylate. Mixtures of esters may be employed.

The copolymerizable reinforcing monomer, if employed, is preferably selected from the group consisting of monomers such as acrylic acid, isobornyl acrylate, N-vinyl pyrrolidone, N-vinyl caprolactam, N-vinyl piperidine, N,N-dimethylacrylamide, and acrylonitrile. Preferred reinforcing monomers are niteogen-containing such as those nitrogen-containing monomers listed above. The reinforcing monomer will generally be selected such that a homopolymer prepared therefrom will have a glass transition higher than a homopolymer prepared from the acrylic or methacrylic ester employed.

In the event that the prepolymeric or monomeric syrup comprises both an acrylic or methacrylic ester and a reinforcing comonomer, the acrylic or methacrylic ester will generally be present in an amount of about 50 to 95 parts by weight, and the reinforcing comonomer will be present in a corresponding amount of about 50 to 5 parts by weight. One skilled in the art will be able to vary the nature and amount of the reinforcing monomer to obtain the pressure-sensitive adhesive properties desired.

It may also be desirable to employ glycidyl methaerylate, glycidyl acrylate or another epoxy-functional monomer as a starting material together with the acrylic or methacrylic ester and reinforcing monomer, if employed. Such an epoxy-functional monomer, if employed, will preferably be present in an amount of about 0.1 to 10 parts by weight per 100 parts by weight of all monomers used.

The acrylic copolymers useful in the pressure-sensitive adhesive are very stable compositions. Because of their stability, the sheet material employed in the method of the invention may be subjected to the heat conditions required for curing the epoxy resin without degradation of the pressure-sensitive adhesive portion of the composition. Other types of pressure- sensitive adhesives might experience a partial or total loss of adhesion, causing detachment of the sheet material from the substrate such as a structural part to which it has been adhered prior to the conditions required, i.e., length and elevation of temperature, for thermosetting of the epoxy resin.

Further, both the photopolymerizable acrylic or methacrylic prepolymer or monomeric syrup and the photopolymerized polymer form a stable mixture with the epoxy resin.

Useful epoxy resins may be selected from the group of compounds that contain an average of more than one, and preferably at least two, epoxy groups per molecule. The epoxy resin preferably is either liquid or a semi-liquid at room temperature for handling purposes. Most preferred is a mixture of a liquid and solid resin. Representative examples include phenolic epoxy resins, bisphenol epoxy resins, hydrogenated epoxy resins, aliphatic epoxy resins, and halogenated bisphenol epoxy resins. Mixtures of epoxy resins may be employed.

Preferred epoxy resins include bisphenol epoxies with the most preferred epoxy resin being the diglycidyl ether of a bisphenol-A, formed by reaction of bisphenol-A with epichlorohydrin.

The epoxy resin will generally be present in an amount of about 25 to 120 parts by weight based on 100 parts by weight of the prepolymeric or monomeric syrup. The photoinitiator employed to polymerize the prepolymeric or monomeric syrup may be any conventional free radical photoinitiator activatable by, for example, ultraviolet light. An example of a suitable photoinitiator is 2,2-dimethoxy-l,2-diphenylethane-l-one (Irgacure™651 available from Ciba- Geigy Corporation). The photoinitiator will typically be employed in an amount of about 0.01 to 5 parts by weight per 100 parts of the prepolymeric or monomeric syrup.

The heat-activatable hardener is added to effect the curing of the epoxy resin under application of heat. The hardener may be any type, but preferably an amine type hardener that is selected from the group comprising dicyandiamide or polyamine salts. These are available from a variety of sources, e.g., Omicure™ available from Omicron Chemical and Ajicure™ available from Ajinomoto Chemical. The heat-activatable hardener will typically be employed in an amount of about 0.1 to 20 parts by weight, and preferably 0.5 to 10 parts by weight per 100 parts by weight of the prepolymeric or monomeric syrup. Sufficient hardener should be employed to achieve cure of the epoxy resin.

Because there are many points in, for example, an automotive painting cycle at which the sheet material may be used, the heat to which the sheet material is exposed may be insufficient to fully cure the epoxy resin. these cases, it may be advantageous to add an accelerator to the prepolymer blend, so the resin may fully cure at a lower temperature, or may fully cure when exposed to heat for shorter periods. Imidazoles and urea derivatives are particularly preferred in the practice of the present invention for use as accelerators because of their ability, as shown by the examples herein, to extend the shelf life of acrylic based materials containing uncured epoxy resin. The most presently preferred imidazoles for use in the present invention are 2,4-dian__no-6-(2'-methyl-imidazoyl)-ethyl-s-triazine isocyanurate, 2-phenyl-4- benzyl-5-hydoxymethylimidazole, hexakis (imidazole) nickel phthalate and toluene bis-dimethylurea. Such an accelerator may be employed typically in an amount of up to about 20 parts by weight per 100 parts by weight of the prepolymeric or monomeric syrup.

In order to provide a sheet material exhibiting the desired flow characteristics in response to heating it may be desirable to include a chain transfer agent in the starting materials used for preparing the thermosettable pressure-sensitive adhesive. Such inclusion facilitates obtainment of a lower molecular weight acrylic polymer having a broader distribution of molecular weight.

Other useful materials which can be blended into the thermosettable pressure-sensitive adhesive include, but are not limited to, fillers, pigments, fibers, woven and nonwoven fabrics, foaming agents, aπtioxidants, stabilizers, fire retardants, and viscosity adjusting agents. The sheet material employed in the method of the present invention is preferably prepared by premixing together the photopolymerizable monomers and the photoinitiator. This premix is then partially polymerized to a viscosity in the range of from about 500 cps to about 5,000 cps to achieve a coatable syrup. Alternatively, the monomers may be mixed with a thixotropic agent such as fumed hydrophilic silica to achieve a coatable thickness. The other ingredients such as the epoxy resin and heat-activatable hardener are then added to the syrup prior to photo-polymerization.

The above composition is coated onto a flexible carrier web, preferably a silicone release liner which is transparent to ultraviolet radiation, and polymerized in an inert, i.e., a substantially oxygen free, atmosphere, e.g., a nitrogen atmosphere. A sufficiently inert atmosphere can be achieved by covering a layer of the photoactive coating with a plastic film which is substantially transparent to ultraviolet radiation, and irradiating through that film in air as described in U.S. Pat. No. 4,181,752 (Martens et al.). The liners may then be removed when it is desired to use the resulting sheet material in the method of the invention.

The method of the present invention has a number of applications in industry. One utility of the method is in the automotive industry where it can be utilized in a process to seal metal joints in automobiles. By this process, one first prepares the sheet material such as by the above-described process. Subsequently, the sheet material would be applied over the joint to be sealed. Complete sealing and bonding would be obtained because the sheet material flows prior to hardening. As a result of the controlled flow of the edges of the sheet material, an aesthetic surface appearance is achieved. The exposed surface of the hardened sheet material can then be painted or otherwise decorated to match the automobile body.

An alternative application of the method of the invention is in the application of emblems or insignia or design elements to surfaces such as an automobile body. An example of an emblem or insignia is a logo of an automobile manufacturer. An example of a design element is trim to enhance and highlight auto body curvature and to provide protection to the primed metal substructure without the need for complex metal stamping to obtain the shape. In such a method, the sheet material is configured initially in the shape of the emblem or insignia or design elements desired such as by die-cutting. Practice of the method of the invention thereby provides an aesthetically pleasing emblem or insignia having smooth transition lines relative to the surface to which it has been bonded.

In still another application of the method of the invention, the substrate to which the sheet material is initially adhered is a temporary substrate such as a disposable liner. Subsequent to hardening of the sheet material in a fashion to provide the controlled flow of its edges, the hardened sheet material may be fastened (e.g., adhered) to the permanent substrate through the use, for example, of an adhesive system distinct from the sheet material itself since the hardened sheet material may be substantially devoid of pressure-sensitive adhesive properties. In this manner, the method of the invention may be used to apply configured, hardened sheet materials such as signs to surfaces such as wooden doors.

The invention is further illustrated by the following non-limiting examples in which all parts are expressed as parts by weight unless otherwise indicated.

Test Methods .

250°F. Shear Creep Flow Test: A % inch² piece of tape is placed at the top of a 2 inch by 6 inch anodized aluminum panel. A 2 inch by 1 inch piece of anodized aluminum weighing approximately 5.5g is placed on top of the tape and parallel to the base panel; contact is made by rolling twice with a 15 lb. wheel. A line is drawn to mark the initial position of the small aluminum piece. The panel is then hung vertically on a rack and placed in a 250°F. oven for 30 minutes. After this time, the rack is removed and allowed to cool. When cool, the panel is removed from the rack and the position of the small aluminum piece of marked by drawing a line. The difference between the initial position and the position after heating is measured in centimeters. If the tape is very meltable and the small aluminum piece falls off, a reading of > 15 cm will be noted.

Melt Flow Visual Observation: A 1 inch² piece of tape is placed on a steel panel which has been electro-coated with paint primer, such as PPG ED-3150 (from Advanced Coatings Technology, Inc., Michigan). The panel with tape is placed horizontally in a 250°F. oven for 30 minutes, then removed and allowed to cool to room temperature. Visual examination of the tape is made based on the following criteria:

1 = no flow, tape has square edges.

2 = some flow, tape has slightly rounded edges.

3 = more flow, tape has very rounded edges.

4 = more flow, tape is starting to become liquid. 5 = most flow, tape has become liquid.

EXAMPLE 1 One hundred parts of a mixture of n-butyl acrylate/N-vinyl caprolactam, with a monomer ratio of 80/20, was blended with 0.04 parts of Irgacure™651 photoinitiator and photopolymerized with an ultraviolet (UV) light source (Sylvania F20T 12BL) under a constant nitrogen purge to a viscosity of about 3000 cps. An additional 0.1 part of Irgacure™651 (available from Ciba- Geigy Corporation), 50 parts of Epon™1001 (diglycidyl ether of bisphenol A available from Shell Chemical Co.), 30 parts of Epon™828 (diglycidyl ether of bisphenol A available from Shell Chemical Co.), 5.94 parts of micronized dicyandiamide hardener, 5.04 parts of toluene bis-dimethyl urea (Omicure™24 available from Omicron Chemicals Inc.), and 0.05 parts of 3-mercaptopropionic acid as a chain transfer agent were added. The mixture was thoroughly mixed on a laboratory mixer for about 15 minutes and allowed to slowly roll on a ball mill mixer for approximately 16 hours. The mixture was then knife coated at a thickness of 40 mil (1.016 millimeters) onto a transparent silicone coated polyester liner having a thickness of about 0.05 millimeters and covered with a second similar polyester liner. The coated mixture was photopolymerized using UV light sources above and below the tape having intensities of 1.82 mWatts/cm² above and 1.73 below at the web as measured using a Uvirad 5 radiometer (Model No. VR365CH3) from E.LT. (Electronic Instrumentation & Technology, Inc., Sterling, VA). The total UV energy was 450 mJoules/cm².

The liners were removed from the adhesive sheet material obtained above and the sheet material was adhered to a steel sheet. The resulting laminate was then placed for twenty minutes in an oven maintained at 10 270°F. During this heating cycle the edges of the sheet material flowed to provide smooth transition lines between the apex of the sheet material and the steel sheet.

EXAMPLE 2

15 A sheet material was prepared according to the procedures of

Example 1 and was converted into a tape having a width of one-half inch (1.27 cm). The tape was placed in a recessed lap joint which was prepared by spot welding two pieces of steel together. A base coat ("NHU90394R" available from PPG, Pittsburg, PA) was applied directly over the tape and the lap

20 joint/tape laminate was placed in an oven maintained at 270 °F. for 20 minutes. A clear coat (DCT 5000 from PPG, Pittsburg, PA) was then applied and the lap joint/tape laminate was thereafter run through a second baking cycle as above. The resulting sealed lap joint had an aesthetically pleasing, painted appearance.

25 EXAMPLE 3

An adhesive sheet material was prepared using the ingredients (and amounts thereof) and procedures described in Example 1 except that no chain transfer agent was included, and the amounts of micronized dicyandiamide hardener and toluene bis-dimethyl urea added to the coatable

30 syrup were 10.8 parts and 9.02 parts, respectively. The sheet material was converted to a tape which was then adhered to a recessed lap joint and processed all as described in Example 2. During the process, the edges of the tape flowed, but to a lesser extent than resulted in Example 2, to provide smooth transition lines between the apex of the tape and the recessed lap joint.

EXAMPLES 4 - 6 The components which are listed in Table 1A were mixed together to prepare three formulations which varied only in the type of nitrogen-containing copolymerizable monomer used. The ratio of the butyl acrylate to the nitrogen-containing copolymerizable monomer in the syrup was varied so as to maintain the nitrogen-containing monomer on an equivalent molar basis. All amounts are amounts by weight.

Syruping: The acrylate components were blended with photoinitiator and photopolymerized with an ultraviolet (UV) light source under a constant nitrogen purge to produce partially polymerized "syrups". These syrups were then blended with the epoxy, epoxy curatives and other ingredients and were mixed until a solution was achieved.

Coating: These resulting formulations were degassed in a vacuum and fed into the nip of a knife coater between two transparent, biaxially-oriented polyethylene terphthalate films, the facing surfaces of which had a silicone release coating. The knife coater was adjusted to provide a coating thickness of approximately 40 mils. The coatings emerging from the knife coater were irradiated with a UV light source, exposing each side of the coatings to a total energy of 223 mj/cm² at an intensity of 1.29 mW/cm , to give pressure-sensitive transfer tapes. The properties of these tapes are shown in Table IB. TABLE1A

TABLE IB

EXAMPLE 7

As an additional characterization of these systems, the cured transfer tapes (140° C for 30 min) of Examples 4-6 were submitted for analysis by transmission electron microscopy. All the samples showed epoxy domains within a continuous acrylate phase, however, the average size of the majority of those epoxy domains was much larger (0.75 microns) for the Example 5 sample than for the Example 4 sample (0.18 microns) or the Example 6 sample (0.25 microns).

EXAMPLES 8-11

The components listed in Table 2A were mixed together to prepare four formulations which differed in the non-polar monomer used (BA or IOA), and the nitrogen-containing monomer used (NNDMA or NVC). The formulations were syruped and coated with the procedure described in Examples 4-6. The coated mixture was photopolymerized using UV light sources (positioned above and below the tape) having intensities of 1.1 mW/cm² to 2.5 mW/cm². The total UV energy was 797 mJ/cm². The properties of these tapes are shown in Table 2B.

TABLE 2A

TABLE 2B

EXAMPLES 12-16 The components listed in Table 3A were mixed together to prepare five formulations with increasing amounts of NNDMA in combination with NVC in the syrup. The formulations were syruped and coated using the procedure described in Examples 4-6. The coated mixture was photopolymerized using UV light sources (positioned above and below the tape) having intensities of 1.1 mW/cm² to 2.5 mW/cm². The total UV energy was 396 rnJ/cm². The properties of these tapes are shown in Table 3B.

TABLE3A

TABLE 3B

Electro-coated steel panels coated with paint primer available as ED-3150 from PPG Industries.

EXAMPLE 17 A formulation was prepared generally according to the procedures of the previous Examples using the ingredients (and amounts) listed in Table 4 below.

TABLE4

Claims

WHAT IS CLAIMED IS:

1. A method for imparting topographical or protective features to a permanent substrate comprising the steps of: a) providing a dissevered, hardenable sheet material having first and second major surfaces, comprising a latent thermosettable pressure-sensitive adhesive throughout a major portion of its thickness, and exhibiting pressure-sensitive adhesive properties at said first major surface; b) contacting and adhering said first major surface of said sheet material to said permanent substrate or a temporary substrate leaving said second major surface of said sheet material exposed; c) substantially thermosetting and substantially hardening said sheet material in a manner permitting initial, controlled mass flow of the sheet material substantially in its thickness direction to provide a substantially smooth transition between said second surface of said sheet material and said permanent substrate or said temporary substrate to which it has been adhered; and d) in the event said hardened sheet material is adhered to said temporary substrate, removing said hardened sheet material therefrom and thereafter fastening said first major surface of said sheet material to said permanent substrate.

2. A method according to Claim 1, wherein step b) involves the contacting and adhering of said first major surface of said sheet material to said permanent substrate.

3. A method according to Claim 1, wherein step b) involves the contacting and adhering of said first major surface of said sheet material to said temporary substrate.

4. A method according to Claim 1, wherein step c) is accomplished by heating said sheet material.

5. A method according to Claim 4, wherein step c) is accomplished by heating said sheet material to a temperature sufficient to decrease its modulus and thereby permitting said controlled mass flow, followed by heating said sheet material to a higher temperature resulting in substantially thermosetting and substantially hardening said sheet material.

6. A method according to Claim 5, wherein step c) is accomplished by a continuous increase in temperature of said sheet material.

7. A method according to Claim 1, further comprising the step of painting said second major surface of said sheet material prior to step c).

8. A method according to Claim 1, further comprising the step of painting said second major surface of said sheet material subsequent to step c).

9. A method according to Claim 1, wherein said thermosettable pressure-sensitive adhesive exhibits pressure-sensitive adhesive properties at ambient temperature.

10. A method according to Claim 1, wherein said sheet material comprises said thermosettable pressure-sensitive adhesive throughout substantially its entire thickness.