Method for treatment of surfaces with ultraviolet light

CROSS-REFERENCES TO RELATED APPLICATIONS

Not Applicable

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a process for treating surfaces of substrates using ultraviolet light along with cooling of the surface to produce surface activation. In particular, the present invention relates to a preferred process for pretreating surfaces of substrates by providing water to provide the cooling of the surface between the radiation and the substrate. The process is particularly useful when ozone is used to treat the surface, particularly with the water, since ozone is six times more soluble in water than oxygen. The treatment enhances surface activation, allows for surface modification in shorter time periods and preferably increases the wetting characteristics.

(2) Description of the Related Art

Manufactured surfaces of substrates always contain undesirable compounds or additives that limit or reduce adhesion to an adhesive or paint film. Hence, surface preparation, which includes cleaning and activation of the surfaces, of polymeric, polymer composite or metal substrates is carried out prior to applying protective paint films or adhesive bonding. Surface preparation determines the mechanical and durability characteristics of the composite created. Currently the techniques used for surface preparation are mechanical surface treatments (e.g. abrasion), solvent wash and chemical modification techniques like corona, plasma, flame treatment and acid etching. Each of the existing processes have shortcomings and thus, they are of limited use. Abrasion techniques are found to be time consuming, labor intensive and have the potential to damage the adherent surface. Use of organic solvents results in volatile organic chemical (VOC) emissions. Chemical techniques are costly and are of limited use with regard to treating three dimensional parts, can be a batch process (such as plasma, acid etching) and need tight control.

The use of lasers for surface treatment is known in the art. The focussed beams of the lasers make it difficult to treat a large surface. U.S. Pat. No. 4,803,021 to Werth et al describes such a method. U.S. Pat. No. 4,756,765 to Woodroffe describes paint removal with surface treatment using a laser.

Plasma treatment of surfaces is known in the art. Relatively expensive equipment is necessary for such treatments and plasmas are difficult to control. The surfaces are treated with vaporized water in the plasma. Illustrative of this art are U.S. Pat. No. 4,717,516 to Isaka et al., U.S. Pat. No. 5,019,210 to Chou et al., and U.S. Pat. No. 5,357,005 to Buchwalter et al.

A light based process which cleans a substrate surface also creates a beneficial chemistry on the surface for adhesive bonding and paintability is described in U.S. Pat. No. 5,512,123 to Cates et al. The process involves exposing the desired substrate surface to be treated to flashlamp radiation having a wavelength of 160 to 5000 nanometers. Ozone is used with the light to increase the wettability of the surface of the substrate being treated. Surfaces of substrates such as metals, polymers, polymer composites are cleaned by exposure to the flashlamp radiation. The problem with the Cates et al process is that the surface of the substrate is heated to a relatively high temperature, particularly by radiation above 500 nanometers and relatively long treatment times. Related patents to Cates et al are U.S. Pat. No. 3,890,176 to Bolon, U.S. Pat. No. 4,810,434 to Caines; U.S. Pat. No. 4,867,796 to Asmus et al; U.S. Pat. No. 5,281,798 to Hamm et al and U.S. Pat. No. 5,500,459 to Hagemever et al and U.K. Pat. No. 723,631 to British Cellophane. Non-patent references are: Bolon et al., “Ultraviolet Depolymerization of Photoresist Polymers”, Polymer Engineering and Science, Vol. 12 pages 109-111 (1972). M. J. Walzak et al., “UV and Ozone Treatment of Polypropylene and poly(ethylene terephthalate)”, In: Polymer Surface Modification: Relevance to Adhesion, K. L. Mittal (Editor), 253-272 (1995); M. Strobel et al., “A Comparison of gas-phase methods of modifying polymer surfaces”, Journal of Adhesion Science and Technology, 365-383 (1995); N. Dontula et al., “A study of polymer surface modification using ultraviolet radiation”, Proceedings of 20th Annual Adhesion Society Meeting, Hilton Head, SC (1997); C. L. Weitzsacker et al., “Utilizing X-ray photoelectron spectroscopy to investigate modified polymer surfaces”, Proceedings of 20th Annual Adhesion Society Meeting, Hilton Head, S.C. (1997); N. Dontula et al., “Ultraviolet light as an adhesive bonding surface pretreatment for polymers and polymer composites”, Proceedings of ACCE'97, Detroit, Mich.; C. L. Weitzsacker et al., “Surface pretreatment of plastics and polymer composites using ultraviolet light”, Proceedings of ACT'97, Detroit, Mich.; N. Dontula et al., “Surface activation of polymers using ultraviolet activation”, Proceedings of Society of Plastics Engineers ANTEC'97, Toronto, Canada. Haack, L. P., et al., 22nd Adhesion Soc. Meeting (Feb. 22-24, 1999).

Non-pulsed UV lamps have been used by the prior art. These are described in: “Experimental Methods in Photochemistry”, Chapter 7, pages 686-705 (1982). U.S. Pat. No. 5,098,618 to Zelez is illustrative of the use of these types of lamps.

A disadvantage of the ultraviolet lamp treatments of the prior art is that they are time consuming and sometimes unreliable. To achieve suitable surface chemistries for adhesive bonding and painting purposes, exposure times for certain materials like polypropylene, thermoplastic olefins (TPO's) tend to be of the order of 5 to 60 minutes. In many cases there is a limit on the length of time to which one may expose the substrates to UV since there is a fear of degrading the substrate. There is a need for development of an environmentally friendly as well as cost effective and robust surface treatment process which can be used over a range of surfaces.

OBJECTS

It is therefore an object of the present invention to provide process and apparatus which are reliable and which can produce a highly active surface. It is further an object of the present invention to provide process and apparatus which are economical. These and other objects will become increasingly apparent by reference to the following description and the drawings.

SUMMARY OF THE INVENTION

The present invention relates to an improved method for modifying a surface of a substrate by generating optical energy having ultraviolet wavelengths and irradiating the surface of the substrate, preferably comprising a polymer, with the optical energy which comprises: irradiating the surface with the optical energy to modify the surface, wherein the wavelengths are between about 160 nanometers and 500 nanometers without higher wavelengths with cooling of at least the surface of the substrate. The cooling of the surface is to temperatures less than 100° C., preferably between 0 and 50° C.

The present invention also relates to a preferred improved method for modifying a surface of a substrate, particularly a polymer surface, by generating optical energy and irradiating the surface of the substrate with the optical energy with an optical power density of sufficient intensity to modify the surface, which comprises: providing water on the surface to be modified prior to irradiating the surface to modify the surface and then irradiating the surface with the optical energy to modify the surface, wherein the wavelengths are between about 185 nanometers and 254 nanometers.

The present invention further relates to an apparatus for modifying a surface of a substrate comprising: an optical energy source for generating optical energy having ultraviolet wavelength components ranging from about 160 to 500 nanometers without higher wavelengths; a cooling means for cooling at least the surface of the substrate; and an electrical power supply operably coupled to power the energy source.

The present invention also relates to a preferred improved method for modifying a surface by irradiating the surface with optical energy at an intensity sufficient to modify the surface forming the surface, which comprises: providing water on the surface to be modified prior to irradiating the surface and then irradiating the surface with the optical energy to photodecompose organic materials present on the surface, preferably with ozone dissolved in the water, wherein the wavelengths are between about 160 nanometers and 500 nanometers without higher wavelengths.

The present invention further relates to a preferred improved method for modifying a surface by irradiating the surface with pulsed optical energy at an intensity sufficient to modify the metallic surface, which comprises: providing water on the surface to be modified prior to irradiating the surface and then irradiating the surface with the optical energy to modify any organic materials present on the surface, preferably with ozone dissolved in the water, wherein the wavelengths are between about 185 nanometers and 254 nanometers without higher wavelengths.

The present invention relates to an apparatus for modifying a surface of a substrate comprising: an optical energy source for generating optical energy having ultraviolet wavelength components ranging from about 160 to 500 nanometers without higher wavelengths; an electrical power supply operably coupled to power the energy source; and means for providing water on the surface for modifying the surface of the substrate when irradiated.

In the present invention it was discovered that if higher wavelengths of light above about 500 nm are used, a thermoplastic surface of the substrate is heated to above the glass transition temperature. This allows the newly formed chemically active groups to rotate into the melted surface thus, decreasing the activity of the surface. By limiting the wavelengths produced by the UV lamp or by the use of filters, the light above 500 nanometers can be largely eliminated with the cooling.

It is also necessary to cool the surface to freeze the chemically active groups to the outside of the substrate. This is a very unexpected finding, since the prior art is concerned only with heating the surface. The cooling can be by a cooling gas flushed over the surface and/or by using short durations (3 minutes or less) of the ultraviolet light with liquid cooling.

The process of the present invention is cost effective for pretreatment of surfaces of polymers, polymer composites and metals prior to adhesive bonding or painting. The process creates beneficial surface chemistries for adhesive bonding or painting. The advantages of this process over the existing prior art is that the process is cheaper than chemical modification techniques such as plasma and is not a batch process as with plasma and acid etching. Further, the new process can be used to treat three dimensional parts where corona and flame treatments have difficulty in treating. The process can also be used to treat large surface areas quicker than flame treatment. In addition, the substrates are not as sensitive to degradation as when exposed to flame, such as with flame treatment. The process is environmentally friendly as compared to solvent wash, acid etching and mechanical abrasion techniques. The process is much cheaper than processes using UV exciter lasers which are cost intensive and work on the principle of ablating the surface layers or roughening the surface or amorphizing the top surface layers. In comparison to the existing ultraviolet lamp techniques, the current process reduces the process times for treating various substrate surfaces (thus making it cheaper, and avoiding degradation of the substrates) and achieves surface modifications which were not possible. This invention can be used to tailor the chemistry of the substrate surface by using other cooling reactivating vapors (ozone, ammonium and nitrogen) in between the substrate surface and ultraviolet light.

The process of the present invention uses one step for cleaning the surface and functionalizing the surface so that the materials can be painted without the use of adhesion primers or the use of costly and environmentally unfriendly treatment steps. Further, this process can be used prior to adhesive bonding of polymers, polymer composites and metals, to achieve good bond strengths.

The process can be used for treating carbon fibers and carbon whiskers prior to their use in composites, for instance. An alternate use of the process is in the packaging industry where polymers are printed with ink. This process is an easier, flexible or alternate way for them to create materials with different barrier properties for solvents.

The substance and advantages of the present invention will become increasingly apparent by reference to the following drawings and the description.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1

is a schematic side view of a conveyor system

10

for applying a spray of water droplets

14

onto a surface

12

A of a substrate

12

, then irradiating the substrate

12

with the water droplets

14

on it using a UV lamp

24

and then drying the substrate

12

to remove traces of the water droplets

14

with an electric heater

36

without melting the surface.

FIG. 2

is a schematic perspective view of a conveyor system

100

of a second embodiment for a substrate

104

wherein moist air is introduced into a chamber

108

containing the substrate

104

to place droplets

124

on the surface

104

A and then irradiated with a UV lamp

106

. The surface

104

A is then air-dried upon removal from the chamber

108

.

FIG. 3

is a schematic side view of a conveyor system

200

of a third embodiment with a substrate

202

with a layer of water

204

held in place by a dam

227

on surface

202

A while the surface

202

A is irradiated by a UV lamp

212

, then the substrate surface

202

A is air dried by air from a diffuser

224

.

FIG. 4

is a perspective view of an x-y movable table

302

of a fourth embodiment supporting a substrate

304

with water droplets

308

on surface

304

A so that the position of the substrate

304

is varied under the housing

306

containing lamp (not shown) as a function of time. The position of the housing

306

containing the lamp (not shown) can also be varied.

FIG. 5

is a schematic side view of a conveyor system

400

of a fifth embodiment with substrate

402

with a layer of water

410

on surface

402

A and with a transparent layer

412

to filter out infrared light. A cooling plate

404

is provided for the substrate

402

.

FIG. 6

is a schematic perspective view of a system

500

of a sixth embodiment wherein a substrate

502

with an opaque mask

504

having cutouts

506

forming the word “MASK” to the substrate

502

which are filled with water

508

. The cutouts

506

are irradiated with a UV lamp

510

.

FIG. 7

is a schematic representation of a PATTI tester

600

.

FIG. 8

are graphs showing x-ray photoelectron spectroscopy (XPS; percent composition) and contact angle (cos &thgr;) measurements with water for polycarbonate. A: control; B: UV@t=2.25 min, d=2 cms; C: UV@t=2.25 min, d=1 cm; D: UV@t=2.25 min, d=2 cms, ozone from generator; E: UV@t=2.25 min, d=1 cm, and water droplets on surface and F: UV@t=2.25 min, d=1 cm, ozone from generator.

FIG. 9

is a graph showing C1s curve fits on XPS measurements of polycarbonate surfaces treated with UV and water.

FIG. 10

is a graph showing C1s curve fits on XPS measurements of polycarbonate surfaces treated with UV and ozone.

FIG. 11A

is an AFM image of a control (baseline) sample of polycarbonate.

FIG. 11B

is an AFM image of UV treated polycarbonate. UV treatment conditions were t=2.25 min, d=1 cm.

FIG. 12A

is a surface plot of the polycarbonate control sample AFM image (FIG.

11

A).

FIG. 12B

is a surface plot of the UV treated polycarbonate AFM image (FIG.

11

B).

FIG. 13

is a graph showing the effect of UV treatment on adhesive bond strength and contact angle with water (cos &thgr;) for polycarbonate.

FIGS. 14A

to

14

D are ESEM images of the molded surfaces of mechanical grade TPO (

FIGS. 14A and 14B

) and reactor grade TPO (FIGS.

14

C and

14

D).

FIGS. 15A and 15B

are ESEM images of the molded surfaces of toluene etched mechanical grade TPO (

FIG. 15A

) and reactor grade TPO (

FIG. 15B

) samples.

FIG. 16

is a graph showing the effect of UV treatment on wettability (cos &thgr;) and adhesive bond strength of reactor grade TPO (without UV stabilizers).

FIG. 17

is a graph showing the effect of UV treatment on adhesive bond strength of mechanically blended grade TPO (without UV stabilizers).

FIG. 18

are graphs showing the effect of UV treatment on wettability (cos &thgr;) and adhesive bond strength of polypropylene (without UV stabilizers).

FIGS. 19A

to

19

D are ESEM images of PATTI tested failure surfaces of baseline MTPO (without UV stabilizers).

FIGS. 20A

to

20

F are ESEM images of PATTI tested failure surfaces of UV treated MTPO (without UV stabilizers). V treatment conditions were t=4 min at d=2 cms.

FIGS. 21A

to

21

D are ESEM images of PATTI tested failure surfaces of baseline RTPO (without UV stabilizers).

FIGS. 22A

to

22

D are ESEM images of PATTI tested failure surfaces of ozone treated RTPO (without UV stabilizers). Ozone treatment conditions were t=30 min at d=2 cms.

FIGS. 23A

to

23

D are ESEM images of PATTI tested failure surfaces of UV treated RTPO (without UV stabilizers). The UV treatment was done in presence of water for t=4 min at d=2 cms.

FIGS. 24 and 25

are graphs showing contact angle measurements for the epoxy paint.

FIGS. 26 and 27

are graphs showing pull test measurements.

FIG. 28

is a graph showing contact angle change prior to and after UV/Ozone treatment

A

Baseline

B

t = 2 mins, d = 2 cm

C

t = 2 mins, d = 2 cm with ozone

D

t = 4 mins, d = 2 cm with ozone

E

t = 4 mins, d = 1 m with ozone

FIG. 29

is a graph showing wettability change presence/absence water during UV/Ozone treatment.

FIG. 30

is a graph showing the correlation of contact angle and adhesion change.

FIG. 31

is a graph showing the affect of water and ozone on adhesion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferably, the surface of the substrate is exposed to a UV flashlamp emitting the radiation in the required wavelength range (180 nm-500 nm). The substrate surface to be treated is preferably constructed of a polymer, polymer composite or a metal. Prior to exposing the substrate to UV radiation, water droplets or a sheet of water are preferably placed on the substrate surface. Process times are regulated by the distance of the UV lamp or flashlamp from the substrate surface, ambient temperature or condition and the extent of surface modification needed. The distance of the UV lamp or flashlamp from the substrate surface determines the intensity of UV radiation at the surface substrate and the time required to evaporate the water if present on the surface. Ambient conditions are important depending on whether air, nitrogen or ozone are present. Surface modifications are characterized using contact angle measurements which are done using a Rame-Hart goniometer apparatus with deionized water. The present process creates surfaces which wet better than if they were exposed solely to UV radiation (180 nm-500 nm), creates similar substrate surfaces in a shorter time than that achieved using only UV or ozone and is cheaper than using only UV or ozone.

In an alternate embodiment, a film of water is placed on a quartz or fused silica tray above the substrate to filter out longer wavelengths in infrared region (over ≈1200 nm). In this embodiment, only UV is allowed to interact with the substrate surface.

The process can also be used in a continuous process by either having a falling film of water on the substrate or by immersing the substrate in water and then exposing the substrate to UV radiation (FIG.

3

). The lamp could be moving over a row of substrates or the substrate surface could be moving under a bank of lamps (FIG.

4

). The lamps could be arranged to irradiate all surfaces, such as a tunnel of lamps. Alternatively, the current process can also be used in a continuous process by dispersing water in the form of fine droplets using an atomizer or any other technique between the lamp and the substrate surface (FIG.

1

). In any of the embodiments, either the lamp or the substrate surface can be moving. In addition, to use this process as a continuous process, air or ozone or other gases can be bubbled through water at various flow rates which may be introduced onto the substrate.

FIG. 1

shows a preferred system

10

of the present invention for irradiating a substrate

12

which has a coating of water droplets

14

. The substrate

12

is provided on a conveyor belt

16

supported for rotation on belt tensioning rollers

18

and

20

. Conveyor belt

16

moves from left to right as indicated by the arrow. At a first station A, a spray nozzle

22

disperses droplets

14

of the water onto the surface

12

A of the substrate

12

. The substrate

12

is then moved to station B where the surface

12

A is irradiated with UV light from a lamp

24

in a housing

24

A mounted in a hood

26

which is opaque to the light to prevent eye damage. The lamp

24

is controlled by a pulse modulator

27

and operated by a power supply

28

. Insulators

30

and

32

are provided for wires

34

leading from the hood

26

. Upon irradiating the surface

12

A, the substrate

12

is moved to station C where the surface

12

A is dried by an electric heater

36

operated by a power supply

38

. Insulators

40

pass wires

42

through the hood

26

. The substrate

12

is then removed taking care to keep surface

12

A clean. The surface

12

A is then painted or otherwise treated in a conventional manner (not shown). The hood

26

is provided with a blower

44

which removes volatilized products from the hood

26

through line

46

.

In operation, the system

10

moves the substrate

12

through stations A, B and C for treatment. The substrate

12

is provided with water

14

at station A. irradiated at station B, dried at station C and then removed from the system

10

for subsequent treatment.

FIG. 2

shows a system

100

wherein a conveyor belt

102

provides a substrate

104

to be irradiated by lamp

106

in housing

106

A. The conveyor moves from right to left as indicated by the arrow. The lamp

106

is powered by power supply (not shown) and pulse modulator (not shown) similar to power supply

28

and pulse modulator

27

as shown in FIG.

1

. The substrate

104

is surrounded by a chamber

108

which confines the substrate

104

and moist air is introduced via line

110

into the chamber

108

. The line

110

is supported from a vessel

112

containing water

114

. Air is bubbled into the water

114

by pump

116

. If designated, the water

114

or air is also ozonated by generator

118

. The chamber

108

is vented by blower

120

which draws the moisture laden air through the chamber

108

. Upon removal of the substrate

104

from the chamber

108

, the surface

104

A is air-dried by air directed at the surface

104

A by a blower (not shown).

In operation, the system

100

provides air on the surface

104

A of the substrate

104

. The substrate

104

can be cooled to encourage moisture to condense as droplets

124

on the surface

104

A. The substrate

104

is irradiated by the UV lamp

106

and then air-dried upon removal from the chamber

108

. The substrate surface

104

A is then painted or otherwise treated, taking care to prevent contamination.

FIG. 3

is a schematic view of a variation of the system

10

of

FIG. 1

wherein the system

200

is provided with a substrate

202

having a layer of water

204

on a surface

202

A which is spread on the surface

202

A. As before, the substrate

202

is supported on a conveyor belt

206

on tensioning rollers

208

and

210

. The belt

206

moves from left to right as indicated by the arrow. The substrate surface

202

A is irradiated by lamp

212

in housing

212

A through the layer of water

204

at station A. As before, the lamp

212

is provided in a hood

214

with a blower

216

connected by line

218

to the hood

214

. At station B an air blower

220

supplies dry air into the hood

214

via line

222

and diffuser

224

held in place by a dam

227

to dry the surface

202

A of the substrate

202

. The substrate

202

with dam

227

is then removed from the conveyor belt

206

. In station B, the drain

227

can be removed for drying by the air from blower

220

.

FIG. 4

shows a system

300

for moving a table

302

in the x-y direction. The substrate

304

under a housing

306

containing a lamp (not shown). The table

302

indexes the substrate

304

under the housing

306

. Droplets

308

of water are provided on the surface

304

A of the substrate

304

. Alternatively, the housing

306

can be moved as shown by the dotted lines. The lamp in the housing

306

is supplied by a power supply and pulse modulator (not shown). via wires

310

, as shown in FIG.

1

.

FIG. 5

shows a system

400

wherein the substrate

402

is mounted on a cooling plate

404

provided with a channel

404

A supplied with a coolant by a pump or compressor

406

and line

408

. A layer of water

410

held in place by a transparent layer or tray, such as quartz. The surface

402

A of the substrate

402

is irradiated through the layer

412

and water

410

by lamp

414

in housing

414

A. As before, the lamp

414

is powered by a power supply and modulated by a modulator (not shown) by lines

416

through insulators

418

. In this embodiment, the transparent layer

412

holds the water

410

in place. The filtering of the infrared light by the water

410

/layer

412

(quartz) assures that the substrate

402

does not overheat. This system

400

can be incorporated into any one of the other systems

10

,

100

,

200

or

300

.

FIG. 6

shows a system

500

wherein the substrate

502

is protected by an opaque mask

504

. The mask

504

has cutouts

506

spelling the word MASK which goes to the surface

502

A of the substrate

502

. The cutouts

506

are provided with water

508

. The mask

504

is irradiated with a lamp

510

in a housing

510

A powered by a power supply and pulse modulator (not shown) as in

FIG. 1

via lines

512

. In this embodiment, the surface

502

A is treated to make the cutouts

506

receptive to further treatment.

In the following Examples 1 to 12, a pulsed xenon lamp from Xenon Corporation, located in Woburn, Mass. is capable of providing the emission at wavelengths below 225 nm is used in the experiments. The materials used in the experiments were: (1) polypropylene based thermoplastic olefins (TPO), both reactor grade and mechanically blended with different amounts of UV stabilizers and sheet molding compound (SMC), both standardized and flexibilized, (2) aluminum alloy sheets (A1 5754, A1 5052 and A1 6061), (3) polycarbonate, and (4) vinyl ester. Polycarbonate was selected as a model base amorphous polymer. The experiments show that the nature of the substrate is an important factor in determining the extent of modification by UV radiation.

To study ozone's effect on the extent of surface modification, an ozone generator was included in the experimental setup. The other variables that play a role in the extent of modification of the substrate surfaces by UV are: distance of lamp from the substrate surface (d), exposure time (t), effect of humidity surrounding the substrate, intensity of lamp radiation, presence of UV stabilizers in the substrate, the nature of the substrate surface and cooling of the surface.

Normally, the environment between the lamp and the surface of the material being treated is normal ambient air. During some experiments, an external ozone generator (Ozotech, Eureka, Calif. 96097) was used to increase the concentration of ozone over the sample surface over what is generated in air by the UV light. The ozone flow rate used during experimentation was 30 std.cu.ft./hr. The other variables were the time of exposure, the distance between the sample and the UV source and the glass plate used to isolate the lamp tube from the sample. A pulsed lamp is preferred to prevent overheating of the substrate. Two types of glass plates were used: SUPRASIL™ (fused silica) and PYREX®. The glass plates act as filters and have different transmission characteristics. SUPRASIL™ transmits light in the UVC region and has a 10% transmission at 170 nm. PYREX® transmits light primarily in the UVB region (280 nm and higher) and filters the high frequency UVC radiation. Pyrex has 10% transmission at a frequency of 280 nm.

Moisture can be added in a variety of ways. The incoming air can be saturated with moisture to near 100% relative humidity by bubbling the air or nitrogen or another gas through water to saturate it. Alternatively, water can be sprayed onto the surface using a common atomizer prior to introducing the surface to the UV radiation under the lamp. The spraying process creates very small droplets, less than 1 micron in diameter on the surface. In addition, the sample can be covered with a very thin, continuous layer of water.

The experiments show that the treatment enhances the substrate's surface wettability, with the degree of enhancement depending on the substrate characteristics and the treatment processing conditions used. The substrates are characterized prior to and after UV treatment using contact angle measurements to determine wettability. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy with the attenuated total reflectance (FTIR-ATR) setup is used to characterize the surface chemical composition of the substrates. XPS results show an increase in the oxygen content of the polycarbonate substrates after UV treatment (FIG.

8

). Atomic force microscopy (AFM) is used to characterize and compare the control substrate surfaces with the UV treated surfaces (FIGS.

11

A and

11

B). Also, environmental scanning electron microscopy (ESEM) is being used to determine the effect initial substrate morphology has on UV treatment (

FIGS. 14A

to

14

D). Stability studies on the UV treated polycarbonate surfaces show that the surfaces are stable in time at room temperature. Adhesion measurements have been conducted using a pneumatic adhesion tensile testing instrument.

On exposure to various treatments the substrates were characterized for wettability, surface chemical composition, morphology and stability. Wettability was determined by measuring contact angles of de-ionized water using the Rame-Hart goniometer apparatus. Except where specified, the contact angles (&thgr;) were measured immediately after UV exposure. At least ten measurements of contact angles were taken for each sample and the averages are reported here. To correlate the change in wettability to a change in surface chemical composition, Fourier transform infrared spectroscopy (FTIR) with the ATR cell was used. Measurements using zinc selenide crystal at sampling depths of 1-2 microns and germanium crystal at a sampling depth of 0.5 microns were carried out. The FTIR-ATR spectra did not show any significant change in the polymer samples after UV treatment. Thus, it was concluded that the changes caused by UV on the polymers were confined to a few atomic layers near the surface and FTIR-ATR was seeing the polymer to depths of a few microns.

X-ray photoelectron spectroscopy (XPS) was used to characterize the substrate surfaces for a change in chemical composition after UV treatment. A Perkin-Elmer Physical Electronics PHI5400 ESCA Spectrometer equipped with both a standard Mg K&agr;

1,2

X-ray source, and a monochromated Al K&agr;

1,2

X-ray source, and an electron flood gun for neutralization was used. The instrument uses a 180° hemispherical energy analyzer operated in the fixed analyzer mode and a position sensitive detector. The instrument has variable apertures available from spot sizes of 250 &mgr;m to a rectangle of 1.5×5 mm. The optimum spot size for the conditions used in these experiments is the 1.00 mm diameter aperture. Resolution settings for collecting data are 89.45 eV for survey (wide window) scans, 35.75 eV for utility resolution and 17.90 eV for high resolution scans.

Use of the monochromatic source on a non-conducting sample necessitates the use of a neutralization source. This instrument utilizes a low energy electron flood gun. Prior to establishing the baseline chemistry of untreated polymers, the neutralizer operating conditions had to be optimized. This was accomplished by first aligning the neutralizer with mylar, followed by using these conditions on the baseline TPO and polycarbonate samples. The peak shape of the C1s was used to assess the neutralizer operation. An asymmetric C1s peak was initially observed. To determine if this was real or due to the neutralizer, the specimen was analyzed using the standard Mg source set to 150 W, 15 kV. Lower power was used to minimize damage that may occur when the non-monochromatic source is used. It was concluded that the asymmetry was an artifact of the neutralizer. The operating conditions were adjusted through several iterations until the C1s peak shape was symmetric. A molybdenum mask was employed to assist in neutralizing the charging occurring in the samples, where the mask attracts electron and causes a distribution of electrons to spread across the sample surface. Use of the mask has improved the reproducibility of neutralizing, allowing the same neutralizer setting to be used from sample to sample.

To determine whether surface topography had changed due to UV treatment, atomic force microscopy (AFM) of the polymer samples was done using a Nanoscope III made by Digital Instruments (

FIGS. 11A and 11B

. Along with AFM, environmental scanning electron microscopy (ESEM) was also used to characterize surface morphology prior to and after UV treatment (

FIGS. 14A

to

14

D and

15

A to

15

B). Also, ESEM was used to determine if there was any relationship between extent of modification and initial morphology of the substrate. The ESEM used for the morphological study was an Electroscan 2020.

A pneumatic adhesion tensile testing instrument (PATTI) was used to measure adhesion properties as a function of UV treatment (FIG.

7

). This instrument (PATTI-2A) is made by SEMicro division, M. E. Taylor Engineering, Inc. This instrument uses compressed inert gas to apply a continuous load to a 0.5 inch (outer diameter) aluminum pull stub which is bonded to the rest surface with an adhesive. Once the pull stub has been bonded and the adhesive cured, the stub is attached to a piston. The piston design assures uniaxial alignment with the pull stub axis for “true tensile testing”. A continuous load is applied perpendicular to the pull stub until failure occurs. The PATTI instrument and method conforms to ASTM D4541, “Pull-off strength of coatings using portable adhesion testers”.

EXAMPLE 1

Polymer samples (control and UV treated) were bonded to aluminum stubs using an epoxy structural adhesive, ARALDITE AW106 with hardener HV953 (from Ciba Geigy Corporation). 30 micron sized glass bubbles from 3M Corporation (S60/10000, 3M Scotchlite Glass Bubbles) were used to control the adhesive bond line thickness. In order to attain its full strength, the adhesive was allowed to cure for more than 15 hours at room temperature. Five or more samples were tested for control and each treatment condition. ESEM was used to characterize the polymer side of the PATTI test in order to determine the locus of failure. The following paragraphs summarize the results of the UV experiments for various polymers.

Table 1 shows the effect of UV treatment with the xenon lamp on samples of polycarbonate where d=distance of lamp from the substrate surface and t=exposure time.

TABLE 1

Contact angle measurements on polycarbonate

Sample/Treatment Conditions

Water Contact Angle (degrees)

Control

78

UV @ t = 1 min, d = 2 cms

64

UV @ t = 1 min, d = 1.4 cms

49

UV @ t = 2.25 min, d = 1 cm

35

UV @ t = 2.25 min, d = 1 cm,

20

with water droplets on the

surface

UV @ t = 2.25 min, d = 1 cm

21

and ozone from ozone generator

It was observed that polycarbonate is significantly modified by UV radiation. From Table 1, it is also observed that the same synergistic effect of water is seen for polycarbonate as was noted for reactor grade TPO. Also, decreasing the distance between the lamp surface and the substrate or increasing the ambient ozone concentration was observed to enhance the surface wettability.

FIG. 8

is a plot of the carbon and oxygen content on the polycarbonate surface as a function of various UV treatment conditions. This was obtained using XPS. Also plotted in the same

FIG. 8

is the cosine of the contact angle (&thgr;) against the UV treatment conditions. With increase in the intensity of UV treatment (either due to decreasing distance between lamp and substrate or presence of water droplets on the surface or presence of ozone from ozone generator in the ambient) there is an increase in the oxygen content on the surface (increases from 15% to 31%) and also there is a corresponding increase in the wettability of the substrate (cos(&thgr;)) approaches a value of 1 which corresponds to &thgr; equal to 0 or the substrate being perfectly wettable). Thus, XPS is able to detect changes in surface chemical composition after UV treatment.

Chemical information indicating changes in the surface treated polycarbonates was further investigated by curve fitting the C1s spectra. Curve fitting defines and interprets the carbon chemistry as detected at the sample surface by allowing the user to distinguish overlapping features with a carbon is spectral envelope. Using a curve fit model, the surface chemical composition for control and various treated polycarbonate samples were compared.

FIG. 9

compares the effect of the distance from the UV lamp, and the presence of water during UV exposure on surface chemical composition. Upon surface treatment, oxygen functional groups have been added to the polycarbonate surface. There is a marked increase in the C—O, and oxygen functionality's such as C═O, COOH and COOR. The carbonate peak has decreased and this is consistent through all the UV treated polycarbonate samples.

FIG. 10

shows that UV used along with ozone also showed an increase in the oxygen functionality.

FIG. 10

also shows that there is no change in the polycarbonate chemistry within experimental error when exposed to ozone only for 5 minutes.

AFM was used to determine if UV treatment altered the substrate topography. The surface topography of polycarbonate was studied as a function of UV treatment.

FIGS. 11A and 11B

are a comparison of images obtained from AFM for control and UV treated polycarbonate. It was observed that the topography is different. The image for the UV treated polycarbonate shows regions which are lighter in gray scale. This corresponds to peaks on the substrate surface. An analysis of these images in terms of surface plots, show the UV treated polycarbonate surface to have a number of peaks (corresponds to white regions in gray scale) and troughs (corresponds to dark regions in gray scale). This is shown in

FIGS. 12A and 12B

. The frequency of these peaks and troughs was found to depend on the intensity of the UV treatment. Intensity increased with a decrease in the distance between the lamp and the substrate and also with the presence of water droplets on the substrate surface.

FIG. 13

shows the effect of UV treatment on wettability and bond strength for polycarbonate. UV and ozone treatments increased the wettability, and enhanced adhesive bond strengths. The extent to which the bond strength increases is dependent on the UV treatment condition and the nature of the polymer after UV treatment with bond strength increases of 200% for polycarbonate. Besides affecting bond strength, failure mode was observed to have changed from control samples to different UV treatment conditions. The locus of failure can be evaluated by characterizing the tested samples using XPS or ESEM. A main use of this process is as a surface pretreatment for polymers, composites or metals prior to the painting or adhesive bonding step. Thus, it was necessary to know what changes occurred on the UV treated surfaces as a function of time, temperature and moisture after the UV treatment. In Tables 2, 3 and 4, contact angle measurements for the preliminary time and moisture stability studies conducted on polycarbonate at room temperature are listed.

TABLE 2

Contact angle measurements on UV treated polycarbonate. UV

treatment was for t = 2.25 min at d = 2 cms

Water

Sample Conditions

Contact Angle (degrees)

control (analyzed immediately after UV)

51

aged for 2 hours at room temperature

47

aged for 4 hours at room temperature

49

aged for a day at room temperature

48

aged for 7 days at room temperature

48

rinsed in deionized water for 5 min

54

TABLE 3

Contact angle measurements on UV treated polycarbonate. UV

treatment was for t = 2.25 min at d = 1 cm.

Water

Sample Conditions

Contact Angle (degrees)

control (analyzed immediately after UV)

35

aged for 2 hours at room temperature

29

aged for 4 hours at room temperature

27

aged for a day at room temperature

31

aged for 7 days at room temperature

33

rinsed in deionized water for 5 min

54

TABLE 4

Contact angle measurements on UV treated polycarbonate. UV treatment

was for t = 2.25 min at d = 1 cm with water droplets on the surface.

Water

Sample Conditions

Contact Angle (degrees)

control (analyzed immediately after UV)

20

aged for 2 hours at room temperature

27

aged for 4 hours at room temperature

26

aged for a day at room temperature

28

aged for 7 days at room temperature

30

rinsed in deionized water for 5 min

55

It was observed that the contact angle made by de-ionized water on the substrates does not change significantly over the span of 7 days. This means that the UV treated polycarbonate surfaces are stable at room temperature. On rinsing the UV treated samples for 5 minutes in de-ionized water with slight agitation and then drying on a KIMWIPE®, the measured contact angles were found to be higher than the control sample for that particular UV treatment. These measured contact angles were lower than those obtained for the untreated polycarbonate (78%). From this it may be reasoned that though UV modifies polycarbonate, it might also create low molecular weight species on the surface which are dislodged during rinsing. More experiments need to be done to verify this hypothesis and determine the moisture stability of UV treated surfaces.

EXAMPLE 2

UV treatment on reactor grade and mechanically blended TPO without UV stabilizers showed that wettability of reactor grade TPO could be improved while not that of the mechanically blended TPO. It was also shown that water droplets on the surface of reactor grade TPO enhanced the extent of modification and modification rates.

Table 5 compares the UV radiation exposure effects for reactor grade TPO's without UV stabilizers against those that contain UV stabilizers.

TABLE 5

Contact angle measurements on reactor grade thermoplastic olefin

Water Contact Angle (degrees)

TPO control + UV

Sample/Treatment Conditions

TPO control

stabilizer

baseline

93

101 

UV @ t = 4 min, d = 2 cms

85

94

UV @ t = 4 min, d = 2 cms

78

N.D.*

with water droplets on the

surface

UV @ t = 6 min, d = 2 cms

N.D.*

93

with water droplets on the

surface

UV @ t = 4 min, d = 2 cms

89

88

and ozone from ozone

generator

UV @ t = 4 min, d = 2 cms

76

82

and ozone from ozone

generator

*Not determined

It is noted that the initial value of the de-ionized water contact angle is higher for the TPO containing UV stabilizers. It was also observed that UV modifies the substrates containing the UV stabilizers, although to a slightly lesser extent. The contact angles achieved after UV treatment of the reactor grade TPO's are similar to those obtained using propane flame treatment (76°) on reactor grade TPO's.

From the above listed tables, it is noted that water has a synergistic effect with UV radiation for certain polymer substrates. Thus, some experiments were carried out to determine whether any other chemicals other than water, in the presence of UV, causes an increase in the substrate's wettability. Tables 6 and 7 list the preliminary results for the effect of droplets of different strengths of hydrogen peroxide solution on mechanical and reactor grade TPO substrate surfaces. Standard hydrogen peroxide solution is in water and is available at 30% strength. Preliminary conclusions from these experiments show that dilute solutions of hydrogen peroxide enhances UV modification slightly.

TABLE 6

Effect of hydrogen peroxide in the presence of UV on contact angle

measurements for mechanical grade thermoplastic olefin

Sample/Treatment Conditions

Water Contact Angle (degrees)

control

90

UV @ t = 6.75 min, d = 2 cms

93

UV @ t = 6.75 min, d = 2 cms,

88

with 8.5% strength hydrogen

peroxide droplets on the surface

UV @ t = 6 min, d = 2 cms, with

88

15% strength hydrogen peroxide

droplets on the surface

UV @ t = 6 min, d = 2 cms, with

84

6% strength hydrogen peroxide

droplets on the surface

TABLE 7

Effect of hydrogen peroxide in the presence of UV on contact angle

measurements for reactor grade thermoplastic olefin

Sample/Treatment Conditions

Water Contact Angle (degrees)

control

93

UV @ t = 4 min, d = 2 cms

85

UV @ t = 30 min, d = 2 cms

86

UV @ t = 4.5 min, d = 2 cms,

83

with 3% strength hydrogen peroxide

droplets on the surface

UV @ t = 5 min, d = 2 cms, with

85

6% strength hydrogen peroxide

droplets on the surface

UV @ t = 6.0 min, d = 2 cms,

88.5

with 15% strength hydrogen

peroxide droplets on the surface

FIGS. 14A

to

14

D are the ESEM images for reactor grade TPO and mechanical grade TPO samples. The images are of the top surface of the injection molded substrates. The ESEM images show that except for the reactor grade TPO's surface which appears to be rougher, there are no significant differences between the two grades of TPO. Also from

FIGS. 14A

to

14

D, it is not possible to distinguish the dispersed elastomer phase from the polypropylene matrix. Even when the ESEM was operated in the back-scattered mode, the two phases were not able to be distinguished from each other. To distinguish the elastomer phase from the polypropylene, the TPO samples were placed in toluene for different times at room temperature. Toluene selectively etches the elastomer phase, and one is able to distinguish the two phases. This is shown in

FIGS. 15A

to

15

B. The ESEM images are of the top surface of the injection id s molded TPO substrates. On toluene etching, it is observed that the resultant morphology for the two grades of TPO are different.

To determine whether UV treatment affected. adhesive bond strength, PATTI experiments were carried out on reactor and mechanically blended grades of TPO as shown in

FIGS. 16 and 17

.

FIG. 16

is a plot of adhesive bond strength and wettability versus treatment conditions. In reactor grade PTO, wettability in this figure is indicated by the cosine of the contact angle (cos(&thgr;)) made by de-ionized water. From

FIG. 16

, it is observed that for certain conditions the adhesive bond strength improved even if the wettability did not improve.

FIG. 17

is a plot of mechanically blended grade TPO's adhesive bond strength for different UV treatment conditions. Wettability has not been plotted in

FIG. 17

, since it was documented in the previous reports that wettability represented by the equilibrium contract angle did not change with UV treatment. Though wettability did not improve,

FIG. 17

shows that the adhesive bond strength improves.

Even if wettability did not change for certain UV treatment conditions, the adhesive bond strength showed improvement. The TPO's being studied here are made up of a polypropylene matrix in which an elastomer is dispersed. To identify the role played by the polypropylene matrix during UV treatments of TPO, PATTI tests were conducted on control and UV treated polypropylene samples.

FIG. 18

shows a plot of adhesion strength and wettability versus UV treatment conditions for polypropylene. It was observed that for the conditions studied, though the wettability did not significantly increase, the adhesive bond strength has improved after UV treatment.

The polypropylene and TPO samples contain an anti-oxidant package. These anti-oxidant materials may be preventing nascent oxygen from attacking the sites where surface bonds are broken by UV radiation. Due to this, wettability of the substrates might not have improved significantly. The other possible reason is that in polypropylene based materials, a competing mechanism like cross-linking of the substrate surface might be occurring. Due to this, the weak boundary layer might have cross-linked in polypropylene and hence the adhesive bond strength might have improved. Similarly in the case of injection molded TPO's, the dispersed elastomer migrates from surface and the surface (or skin) is dominated by polypropylene. Hence even for TPO's, it may be hypothesized that under some conditions surface cross-linking occurs. ESEM is also being used to characterize the failure surfaces of the PATTI tested samples and determine the locus of failure. XPS will be used to confirm the results obtained from ESEM. The following paragraphs discuss the same typical ESEM results obtained after scanning the substrate (polymer) side.

FIGS. 19A

to

19

D are the ESEM images for PATTI tested baseline mechanical grade TPO. These images are taken of the polymer side after the PATTI test. It is observed that the failure is clean and it seems to occur at the adhesive-polymer interface. The condition of the TPO for this test resulted in poor bond strength.

FIGS. 20A

to

20

F show typical ESEM images for UV treated mechanical grade TPO. These images have been taken from the edge of the adhesive joint to the center of the joint. One observes that the failure mode has changed after UV treatment of the TPO. From the images in

FIGS. 20A

to

20

F, one observes polymer pull-out. This is seen to be more near the edge of the adhesive joint. A hypothesis on how the failure occurred may be: that the crack initiated near the edge of the adhesive joint below the interface and then propagated inward towards the interface. Hence one sees more polymer pull-out near the edge of the adhesive joint.

FIGS. 21A

to

21

D are ESEM images of PATTI tested baseline reactor grade TPO samples. The images taken are of the polymer side of the PATTI test. It is observed that the failure is clean and the condition corresponds to poor adhesive bond strength of reactor grade TPO.

FIGS. 22A

to

22

D show the ESEM images of the ozone treated reactor grade TPO samples which have been PATTI tested. It is observed that the failure surfaces seem different from that of the baseline sample of reactor grade TPO and also bond strengths registered for ozone treated TPO are higher than for baseline reactor grade TPO. Since glass beads which were used as spacers to control adhesive bond line thickness and also epoxy (adhesive used) residue are not observed on the polymer failure side, one can postulate that the failure is occurring in the interphase.

FIGS. 23A

to

23

D show ESEM images of the polymer failure side of UV treated reactor grade TPO. It is observed that the failure surface appears to be different from that of ozone treated or the baseline reactor grade TPO. It is seen from

FIGS. 23A

to

23

D that the failure has occurred in the polymer substrate. Thus, the failure mode has changed after UV treatment. XPS may be used to determine the exact location of the failure surface.

Overall, from the PATTI tests it was observed that UV treatment caused enhancement in the adhesive bond strength and also a change in the failure mode. PATTI tests and a preliminary ESEM analysis of failure surfaces revealed that mechanically blended TPO and reactor grade TPO failed differently. This may be correlated to the morphology of the TPO which in turn is a function of how they are manufactured and processed. PATTI tests carried out on polypropylene highlight that UV treatment enhances bond strength.

The wettability of polymeric substrates like polycarbonate, reactor grade thermoplastic olefin (TPO), polypropylene and vinyl ester matrix was increased after UV modification. This was indicated by a decrease in the contact angle of de-ionized water. The extent of UV modification was found to be dependent on distance (d) between lamp and substrate (i.e. intensity of radiation falling on the substrate), exposure time (t), ozone concentration, presence of water droplets, presence of UV stabilizers and nature of the substrate.

The presence of water droplets on the substrate surface was found to enhance the UV modification of certain polymers as well as increase the UV modification rates. This was found to be true for both reactor grade TPO and polycarbonate. The extent of improvement for polypropylene was to a lesser extent, while for mechanical grade TPO, the presence of water did not make any difference.

Experiments with hydrogen peroxide solution were undertaken to explore the effect of the presence of other fluids during UV radiation of polymeric substrates. The presence of hydrogen peroxide solution droplets on substrate surfaces of reactor and mechanical grade TPO's and polypropylene was found to affect UV treatment. Preliminary findings showed that the lower the concentration of hydrogen peroxide solution, the larger was the UV modifications of the substrate surfaces.

Besides wettability and surface chemical composition being altered, AFM of UV treated polycarbonate samples showed a change in surface morphology (FIG.

12

B). This change was seen as an increase in the number of peaks and valleys as a function of the intensity of UV treatment condition.

The adhesive bond between the polymer substrate surface and the aluminum stub showed varying strengths for different UV treatment conditions. In most cases, the UV treated samples showed higher bond strengths than baseline samples. The increase in adhesive bond strength was as high as 200-600%. This was found to be the case for polycarbonate, reactor and mechanical grade TPO, and polypropylene samples.

UV treatment was found to be stable over time for polycarbonate. On washing/rinsing these UV treated samples in de-ionized water, the substrate surfaces did change their contact angles. Yet, the resultant contact angle was found to be smaller (or surface more wettable) than those of the baseline samples.

ESEM of TPO has shown that the morphology of reactor grade TPO might be different from mechanical grade TPO. This might explain why UV treatment shows a different effect on them. ESEM has also been used to characterize polymer surfaces after PATTI testing to determine nature and locus of failure.

EXAMPLE 3

This example shows the effect of adding supplemental ozone to the environment under the UV lamp on tire rubber. Rubber samples were cut into 1 inch×1 inch squares prior to treatment. The distance between the sample surface and the UV lamp was kept at 20 mm. Treatment times ranging from 30 seconds to 4 minutes were evaluated. After treatment, the contact angle of pure deionized water with the rubber was measured using a contact angle goniometer (Rame-Hart Inc.). All the contact angle measurements were performed one (1) hour after treatment.

The treatment was evaluated for different environments like air, ozone and water for various times. The ozone was fed into the treatment area directly from the ozone generator via a tube. To keep the sample temperature low and prevent thermal damage to the polymer, the samples were exposed to UV light in stages. This is off and on pulsed where the sample is exposed for a certain amount of time, the light is turned off and the sample is allowed to cool. This step is repeated till the required amount of total exposure is achieved.

The various treatment conditions used and the results are tabulated in Table 8. A reduction in contact angle was observed in all samples exposed to UV light. The reduction in contact angles was seen to be directly dependent on the treatment time.

TABLE 8

Various UV Treatment Conditions Used for Cured

(Vulcanized) Tire Rubber and Resulting Contact Angles

Treatment Conditions

Contact

Distance

Time

Environment

Angle

Temperature

untreated

121 

25 C