Heat sealing of semicrystalline quasi-amorphous polymers

Semicrystalline polymers can have predetermined amounts of their surfaces rendered quasi-amorphous by irradiation. Polymer surfaces which are so modified can display enhanced heat sealability to accept bonding to other materials.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to heat sealable semicrystalline polymeric 
materials and particularly to heat sealable semicrystalline polymeric 
materials having a quasi-amorphous surface layer of the same or similar 
polymeric material. 
2. Background of the Art 
The effects of actinic radiation on the degradation of polymer surfaces 
have been studied for many years. Prior to about 1970, this work was done 
with low intensity photolamps at wavelengths greater than 220 nanometers 
(nm). Numerous papers are available in the literature, typical of which 
are Day and Wiles, Journal of Applied Polymer Science, 16 175 (1972), and 
Blais, Day and Wiles, Journal of Applied Polymer Science, 17 p. 1895 
(1973). 
Between 1970 and 1980 the effects on polymer surfaces of ultra-violet (UV) 
lamps with wavelengths less than 220 nm were studied for lithography and 
surface modification purposes. Such studies are exemplified by Mimura et 
al., Japanese Journal of Applied Physics, 17 541 (1978). This work 
illustrates that long exposure times and high energies are required to 
cause photo-etching when UV lamps are used. U.S. Pat. No. 3,978,341 
(Hoell) teaches an apparatus for exposing polymeric contact lenses to a 
spark discharge producing 83 nm to 133.5 nm U.V. radiation to improve the 
wettability and adhesiveness of the lenses. 
In 1975 the excimer laser was discovered. An excimer laser is an excited 
dimer laser where two normally non-reactive gases (for example Krypton, 
Kr, and Fluorine, F.sub.2) are exposed to an electrical discharge. One of 
the gases (Kr) is energized into an excited state (Kr*) in which it can 
combine with the other gas (F.sub.2) to form an excited compound (KrF*). 
This compound gives off a photon and drops to an unexcited state which, 
being unstable, immediately disassociates to the orginal gases (Kr and 
F.sub.2) and the process is repeated. The released photon is the laser 
output. The uniqueness of the excimer laser is its high efficiency in 
producing short wavelength (UV) light and its short pulse widths. These 
attributes make the excimer laser useful for industrial applications. 
Kawamura et al., Applied Physics Letters, 40 374 (1982) reported the use 
of a KrF excimer laser at 248 nm wavelengths to photo-etch polymethyl 
methacrylate (PMMA), a polymer used in preparing photolithography resists 
for semiconductor fabrication. 
U.S. Pat. No. 4,414,059 (Blum, Brown and Srinivasan) disclosed a technique 
for the manufacture of microelectronic devices utilizing ablative 
photodecomposition of lithography resist amorphous polymers at wavelengths 
less than 220 nm and power densities sufficient to cause polymer chain 
fragmentation and immediate escape of the fragmented portions. The 
photodecomposition leaves an etched surface. The authors found that using 
an ArF excimer laser at 193 nm and with a 12 nanosecond pulse width, a 
threshold for ablatively photo decomposing poly(methylmethacrylate) resist 
material occurs at about a fluence of 10-12 mJ/cm.sup.2 /pulse. It is 
stated that large amounts of energy, greater than the threshold amount, 
must be applied before ablation will occur. The energy used must be 1) 
sufficiently great and 2) applied in a very short amount of time to 
produce ablative photodecomposition. 
U.S. Pat. No 4,417,948 (Mayne-Banton and Srinivasan) and a related 
publication, Srinivasan and Leigh, Journal American Chemical Society, 104 
6784 (1982) teach a method of UV photo etching poly(ethylene 
terephthalate) (PET). In these publications the authors indicate the 
mechanism of photo etching to be one of chain scission or bond breaking of 
surface polymer molecules by the high energy UV. Bond breaking continues 
in the presence of irradiation and the smaller units continue to absorb 
radiation and break into still smaller units until the end products 
vaporize and carry away any excess photon energy. This process results in 
small particles being ablated away, and various gases being evolved. The 
remaining surface material comprises molecules of low molecular weight 
(oligomers). Examining the PET repeating unit and the author's claim of 
bond scission, it is believed that the following occurs: 
##STR1## 
Indeed, in the Journal of the American Chemical Society article, the 
authors analyze for benzene and start detecting it at about the threshold 
for photodecomposition for PET; i.e., about 20 mJ/cm.sup.2 /pulse at 193 
nm. The authors also indicate that the photo etch process is accelerated 
in the presence of oxygen which seals the ends of the broken chain's 
fragments and prevents recombination of these fragments. 
Srinivasan, Journal of the Vacuum Society, B1, 923 (1983) reports the 
results of ablative photodecomposition of organic polymers through a 0.048 
cm diameter mask and states that a threshold exists for the onset of 
ablation and, for PMMA, that the threshold is 10 mJ/cm.sup.2 /pulse. He 
then goes on to state that one pulse at 16 mJ/cm.sup.2 gave an etch mark 
on PMMA while 50 pulses at 4 mJ/cm.sup.2 /pulse left no detectable etch 
marks. For PET and polyimide, the threshold began at about 30 mJ/cm.sup.2 
/pulse. However, for a satisfactory etch pattern the optimum fluence 
ranged from 100 to 350 mJ/cm.sup.2 /pulse. 
In Srinivasan and Lazare, Polymer, 26, 1297 (1985) Conference Issue, the 
authors report the photo etching of 6 X 12 mm samples of PET, PMMA and 
polyimide polymers with both continuous radiation at 185 nm from UV lamps 
and pulsed radiation at 193 nm from an excimer laser. The use of 
continuous low energy UV lamps causes photo oxidation of the polymer 
surface with a resultant increased oxygen to carbon ratio (O/C ratio) as 
determined by x-ray photoelectron spectroscopy (XPS) equipment, while the 
use of a pulsed high energy excimer laser, which produces chain scission 
in and ablation of the polymer surface, resulted in a lower O/C ratio as 
determined by XPS. The authors then go on to say "It may be pointed out 
that ablative photo decomposition is not exactly a method for the 
modification of a polymer surface at an atomic level since it totally 
eliminates the atoms at the surface and creates a fresh surface." 
U.S. Pat. No. 3,607,354 discloses the use of highly active hydroxybenzene 
solvents to delustre the surface of an oriented poly(ethylene 
terephthalate) film. The solvent acts to dissolve and swell the PET and 
remains in the surface layer. The chemical composition of the surface 
layer is different from that of the bulk polymer because of the presence 
of the very active solvents and the formation of large spherulitic 
crystals which interrupt the transmission of light through that layer is 
believed to occur. 
U.S. Pat. No. 4,568,632 (Blum, Holloway and Srinivasan) claims a method for 
photo etching polyimides. The process described uses a pulsed excimer 
laser at 193 nm. The stated incident energy required for photo ablation is 
much higher for polyimide than for PET. The values for the laser fluence 
threshold of PET was reported as about 30 mJ/cm.sup.2 /pulse while for 
polyimide it was reported as about 50 mJ/cm.sup.2 /pulse. An operative 
level was noted as about 50-100 mJ/cm.sup.2 /pulse for PET and 100-300 
mJ/cm.sup.2 /pulse for polyimide. The etch rate found for PET was 1000 
Angstroms for a fluence of 100-300 mJ/cm.sup.2 /pulse and for the 
polyimide was 750 Angstroms for 350 mJ/cm.sup.2 /pulse. 
Lazare and Srinivasan, Journal Physical Chemistry, 90, 2124 (1986) report 
on the study of surface properties of PET which have been modified by 
either pulsed UV laser radiation or continuous UV lamp radiation. The 
authors report on the high fluence ablation of PET as follows: 1) the PET 
irradiated surface is a layer of low molecular weight material, 2) the 
surface has a rough chemically homogeneous texture, 3) the surface has a 
high chemical reactivity characteristic of oligomers, and 4) the surface 
could be removed by washing in acetone. Since extremely low molecular 
weight fragments (oligomers) of PET are soluble in acetone, the authors 
assert this removal of the treated surface is indicative of the presence 
of low molecular weight material on the surface. The authors also report 
that the low intensity UV lamp treated PET surfaces would not wash off 
with acetone. This later article reports thresholds for ablation of PET at 
about 30-40 mJ/cm.sup.2 /pulse. 
Japanese Patent Publications JA 59-82380, JA 59-101937 and JA 59-101938 
(Kitamura, Veno and Nomura) describe the treatment of various polymers 
with many pulses from moderately high energy lasers for the purpose of 
increasing adhesion and forming a barrier layer to prevent plasticizer 
migration from within certain polymers. 
Bishop and Dyer, Applied Physics Letters, 47, 1229 (1985) extended the 
photoablation etching work of others to actually cutting through or 
slitting the polymer film by increasing the energy density of the laser 
beam by concentrating it at the film surface. 
The authors of the above references were studying the photodecomposition or 
photoablation process of UV radiation on polymer surfaces, without regard 
to whether the polymer was semi-crystalline or amorphous. The present 
invention does not produce substantial photodecomposition and little or no 
photoablation, and is concerned only with semicrystalline polymer surfaces 
produced by exposure to an energy regime different from those used in the 
prior art. 
"Polymer Interface and Adhesion", Souheng Wu, Published by Marcel Dekker, 
Inc., N.Y. and Basel, Chapter 5, page 206 indicates that when a polymer 
melt cools and solidifies, an amorphous surface is usually formed, 
although its bulk phase may be semicrystalline. This is at least in part a 
result of the presence of fractions or materials which are not readily 
accomodated in the crystalline structure being rejected to the surface. 
This amorphous surface is believed to be extremely thin, corresponding to 
only a few layers of molecules, and is of the order of no more than 2 or 3 
nm, and is generally less than 2 nm in thickness. 
U.K. Patent No. 1,579,002 discloses vacuum glow discharge treatment of 
polymeric surfaces to increase adhesion to that surface. The glow 
discharge (i.e., corona type discharge) in the vacuum reduces the 
yellowing typically resulting from corona discharge treatment by 75 to 
80%. The surfaces are heated to a temperature below the glass transition 
temperature or melting point during glow discharge treatment. 
U.S. Pat. No. 3,081,485 describes a process for heating and softening 
polymeric materials using electron-beam irradiation so that further 
mechanical treatment such as stretching and coating can be carried out. 
The energy densities used (e.g., column 2, line 15) are about two orders 
of magnitude higher than the energy densities used in the present 
invention. The energy levels described in U.S. Pat. No. 3,081,485 would 
cause ablation. The authors note on column 2, lines 26 ff. that small 
traces of irradiated material are evaporated during irradiation. Although 
the patent describes surface heating, the immediate depth of e-beam 
penetration (see column 3) appears to be greater than 150 microns. This 
form of energy would have equal effects on the bulk polymer and would not 
cause only surface modifications. 
U.S. Pat. No. 4,631,155 describes the surface modification of polymers by 
subjecting the surface to at least one pulse of intense electromagnetic 
radiation. The surface polymer is disoriented during the relatively long 
exposure to radiation. Disorientation is indicative of any amorphous 
surface. Very thick amorphous layers appear to be formed as indicated by 
the chloroform test described in column 5, and it is very likely that 
treatment under the conditions described in this patent would cause 
surface chemical changes. 
SUMMARY OF THE INVENTION 
The present invention provides a process for heat sealing at least one 
amorphized surface layer on semicrystalline polymers to another surface. 
Some of the special properties in the preferred semicrystalline polymers 
used in this invention are reduced optical reflectance and increased 
optical transmission, increased coating adhesion, increased auto-adhesion, 
and a non-yellowed (non-degraded) surface. The preferred polymeric article 
used in the present invention comprises a semicrystalline polymer having 
on at least one surface thereof areas having a depth of at least 5 nm of 
the same polymer composition in a quasi-amorphous state. The areas may be 
continuous or discontinuous.

DETAILED DESCRIPTION OF THE DRAWING 
The FIGURE graphically shows the effects of surface modification according 
to procedures for forming the article used in the practice of the present 
invention and shows effects of other known processes on properties on 
poly(ethylene terephthalate) film. 
The diagonal line represents constant energy density. That is, the number 
of pulses multiplied by the energy per pulse remains constant along that 
line. The shaded area (1) shows microtexturing of the surface which occurs 
with ablation and etching techniques. This tends to produce high bond 
strengths. The crosshatched area (2) shows surface modification according 
to the present invention wherein the properties of the surface can be 
controlled between strong (greater than 2000 g/linear inch), medium 
(1000-2000 g/linear inch) and weak (0-1000 g/linear inch) bonds. These 
bond strengths are for autoadhesion of the surfaces. 
DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides a process for heat sealing at least one film 
having at least one amorphized surface layer on a semicrystalline polymer. 
This surface is preferably formed by the irradiation of the polymer by 
radiation which is strongly absorbed by the polymer and of sufficient 
intensity and fluence to cause such amorphized layer. The semicrystalline 
polymer surface is thus altered into a new morphological state by 
radiation such as an intense short pulse UV excimer laser or short pulse 
duration, high intensity UV flashlamp. 
The quasi-amorphous surface layer or areas produced according to the 
practice of the present invention are generally and preferably 
substantially, essentially, or even totally free of polymeric 
decomposition debris which typically results from ablative processes as 
described in U.S. Pat. No. 4,417,948 and the articles of Srinivasan et al. 
noted above. 
The residual debris denoted above would be organic material having a lower 
oxygen/carbon ratio than the bulk polymer. Even if not visually observable 
in the amounts present, the debris itself would be yellower in color than 
the bulk material and would be more highly conjugated. The debris also 
tends to leave microscopically observable (at least 10,000, preferably 
20,000X) artifacts on the surface recognizable as debris and not merely 
texturing. With respect to poly(ethylene terephthalate), ablation produces 
a surface substantially soluble in acetone, while the preferred 
quasi-amorphous surface is not in acetone. 
The process of the present invention comprises 1) placing two surfaces in 
contact with one another, one of the surfaces comprising a semicrystalline 
polymer, the contacting surface of which has areas of the same polymer in 
a quasi-amorphous state, the areas having a depth of at least 5 nm, and 2) 
heating the areas of contact between said two surfaces sufficiently to 
cause bonding therebetween. The temperature used is, of course, dependent 
upon the polymers used, each polymer having different softening, melting, 
or fusing temperatures The temperature may be as low as 100.degree. C. 
although minimum temperatures of 110.degree. C. or 120.degree. C. are more 
common. The limit on temperatures at the interface should be lower than 
those that would degrade the polymer (generally lower than 400.degree. C.) 
or disrupt the semicrystalline structure or orientation (generally lower 
than 300.degree. C.). 
Pressure is of course desirable during the process to keep the surfaces 
intimately engaged during heat sealing. This may be accomplished by 
platens or rollers or the like. Pressure on the order of at least 5 
g/cm.sup.2 is desirable with pressures of at least 10 g/cm.sup.2 or 50 
g/cm.sup.2 being more preferred. 
In understanding the present invention, a number of terms and concepts 
should be appreciated. The treatment of the surface of semicrystalline 
polymeric materials according to the present invention does not add or 
substantially remove material from the surface. Residual solvent or 
residual low molecular weight reactants and additives may be volatilized 
during this treatment, but there is less than 0.1% or 1% degradation (to a 
volatile state) and/or volatilization of the bulk of polymeric material 
(within the amorphized layer or melted volume) having a molecular weight 
in excess of 10,000. The chemical modification of the polymer surface 
(e.g., oxidation, chain breakage) is minimal if there is any at all. Only 
a small amount of chain breakage occurs, without the generation of 
significant amounts (i e , greater than 1% or 0.1% by bulk weight) of 
materials volatilized during the process. 
The terms amorphous, crystalline, semicrystalline, and orientation are 
commonly used in the description of polymeric materials. The true 
amorphous state is considered to be a randomly tangled mass of polymer 
chains. The X-ray diffraction pattern of an amorphous polymer is a diffuse 
halo indicative of no regularity of the polymer structure. Amorphous 
polymers show softening behavior at the glass transition temperature, but 
no true melt or first order transition. 
The semicrystalline state of polymers is one in which long segments of the 
polymer chains appear in both amorphous and crystalline states or phases. 
THe crystalline phase comprises multiple lattices in which the polymer 
chain assumes a chain-folded conformation in which there is a highly 
ordered registry in adjacent folds of the various chemical moieties of 
which the chain is constructed. The packing arrangement (short order 
orientation) within the lattice is highly regular in both its chemical and 
geometric aspects. Semicrystalline polymers show characteristic melting 
points, above which the crystalline lattices become disordered and rapidly 
lose their identity. The X-ray diffraction pattern of semicrystalline 
polymers (or copolymers) generally is distinguished by either concentric 
rings or a symmetrical array of spots, which are indicative of the nature 
of the crystalline order. 
Orientation of the polymer is the directional alignment of the polymer 
chain (long-range order) or segments of the polymer (chain) within the 
polymer composition. In the quasi-amorphous state described in the 
practice of the present invention, it appears that the overall long-range 
order orientation or ordering of the crystal lattice remains in an 
apparent crystalline orientation. It also appears that there is, however, 
significant localized disordering along the chain (short-range order 
orientation). The quasi-amorphous form thus exhibits short-range order 
non-orientation or low orientation typical of amorphous phases while it 
exhibits long-range ordering typical of crystalline structures. These 
characteristics are observable and determinable by single analytic 
techniques or combinations of techniques such as X-ray diffractions, 
spectromicrophotometry, IRRAS, NMR, solvent extraction, and the like. 
The surface of the semicrystalline polymer is converted into its 
quasi-amorphous form by heating and rapid cooling of a determined amount 
of that surface. A determinable depth of the polymer composition is 
converted to the quasi-amorphous state. The conversion is referred to as 
"amorphizing." The thickness of the amorphized polymer, as measured from 
the surface downward into the bulk of the polymer, can be controlled. The 
polymer usually has a quasi-amorphous top surface having a depth of at 
least 5 nm, preferably at least 10 nm, more preferably at least 40 nm and 
most preferably at least 60 nm. The range of thickness for the 
quasi-amorphous phase or surface of the polymer may be from about 5 to 
10,000 nm, preferably 10 to 1,000 nm, more preferably 20 to 500 nm or 20 
to 100 nm and most preferably 20 to 250 nm, depending upon the ultimate 
use of the article. 
The surface quasi-amorphous layer is firmly adhered to the bulk of the 
semicrystalline polymer because of the in situ nature of the conversion. 
There can even be a discernible gradation zone between the quasi-amorphous 
and semicrystalline areas, although this is not always the case. That is, 
the transition can be very abrupt within the polymer. 
The portion of the surface area which is amorphized may be as small as 1% 
with some beneficial effects being noted. Generally it is at least 3%, and 
preferably 5 to 100% of the surface. More preferably at least 10% or 30 to 
100% of the surface is quasi-amorphous. These are percentages by surface 
area. 
In performing the process of making the quasi-amorphous surfaces of the 
present invention, the wavelength of the light or ultraviolet radiation 
and/or the polymer and/or absorbing dye in the polymer should be chosen so 
that the polymer composition exhibits an extinction coefficient greater 
than about 5,000. An absorption coefficient of 1/micrometer or preferably 
5/micrometer is preferred. The higher the extinction coefficient for any 
given wavelength, the thinner is the surface layer which resides in the 
optical path of the radiation, and correspondingly, the thinner is the 
surface layer which undergoes a morphological transition or 
"amorphization". The wavelength range of preferred interest is between 
about 180 and 260 nm, with the highest extinction coefficient being 
manifested at the shorter wavelengths. Preferably a coefficient of 
extinction of at least 10,000 is exhibited by the polymer at the 
wavelength of irradiation. 
When utilizing ultraviolet radiation (e.g., 193 nm), it is desired that the 
polyester film receives energy corresponding to a fluence of 3-25 
mJ/cm.sup.2 /pulse. At fluences of less than 3 mJ/cm.sup.2 /pulse, the 
effect of the radiation is not readily discerned. At fluences greater than 
25 mJ/cm.sup.2 /pulse, one begins to encounter excessive damage to the 
affected surface layer, such as vaporization (e.g., off-gassing) of low 
molecular weight products of photodegradation, substantial reduction of 
the molecular weight of the surface layer, and more extensive surface 
roughening. 
The radiation pulse duration, i.e., the pulse width, should be in the range 
of 10 nanoseconds to 100 microseconds to assure rapid excitation of the 
affected surface layer. 
The net effects of pulse width, coefficient of extinction, and radiation 
intensity are to produce a particular type of mechanistic events. First, 
and to a minor degree, there is a photolytic effect in which absorbed 
radiation energy causes random bond scission to occur in the 
semicrystalline polymer. This effect is desirably minimized in the 
practice of the present invention to minimize the damage to polymer 
properties caused by this effect. Indeed, operation of the present 
invention under ideal conditions has been found to cause some decrease in 
the oxygen-to-carbon ratio, but sensitive ellipsometric and infrared 
measurements have been unable to detect any significant loss of material 
as a result of proper radiation conditions. 
The second effect is a result of the unusual nature of the thermal 
excitation of the surface layer in the optical path of the radiation. Much 
of the absorbed light energy is translated into heat, with the heating 
cycle corresponding to the pulse width of the radiation. It is certain 
that instantaneous temperatures that exceed the normal melting point of 
the polymer (e.g., for poly(ethylene terephthalate that is about 
260.degree. C.) are reached throughout most of the affected volume, 
although an unusual thermal gradient may be produced within that volume 
because of the rapid attenuation of the incident energy due to light 
extinction by the polymer composition. The heat cycle thus corresponds to 
the pulse width, in a range of from about 10 nanoseconds to 100 
microseconds. After the heating cycle, the next phenomic concern is the 
ensuing cooling cycle. Because of the thin nature of the affected volume 
and its contact with ambient air at the surface and bulk material (which 
are usually at room temperature), it can be estimated that the surface 
probably cools down to the glass transition temperature (e.g., for 
poly(ethylene terephthalate) this is about 75.degree. C.) within 
microseconds. Once below this temperature, polymer chain conformations 
tend to be frozen. Considerations with respect to this unusually brief 
thermal cycle indicate that conformational changes available to the 
polymer chains remain highly restricted during the brief period while the 
affected surface area undergoes this excitation. Short segmental motions, 
e.g., of the `crankshaft` rotational type, have extremely short relaxation 
times, and it is expected that they may readily occur within the 
time-temperature regime created in the practice of the process of the 
present invention. The confirmation that such motions do indeed occur is 
provided by the IRRAS spectroscopic studies that show that there is a 
significant trans-to-gauche-conformer transformation in the surface layer 
which results from the irradiation of semicrystalline film (e.g., 
biaxially oriented poly(ethylene terephthalate)) with an ArF excimer 
laser. 
This type of conformational change requires the rotation of a short segment 
of the PET chain involving only a few carbon or oxygen atoms. Similar 
considerations indicate that it is highly unlikely that the pre-existing 
crystallites or crystal lattices in the affected surface layer undergo any 
major spatial rearrangements because this time-temperature regime 
precludes the type of long range translational and large chain segment 
rotational motions which would materially change the pre-existing packing 
arrangement within the crystal lattice. Thus, it strongly appears that the 
pulsed UV irradiation of PET (and probably all semicrystalline polymers 
having appropriate extinction coefficients) provides films having surface 
layers with a unique morphology (i.e., quasi-amorphous) in which the 
polymer chains are highly disordered over short segment lengths, but 
substantially retain the long-range order that existed between chains and 
over long segment lengths of those chains prior to excitation. Indeed, the 
excimer laser treatment of a thin film of thermally crystallized PET 
indicated that the original spherulitic structure remained intact, tending 
to affirm this description. 
The substantial trans-to-gauche-conformer transformation is a clear 
indication of short range chain conformation disordering, suggesting that 
although the crystallites may have undergone short range disordering, the 
longer range 3-dimensional packing order probably remains virtually 
intact. It is for this reason that the surface is referred to as 
quasi-amorphous since it has physical characteristics embodying some 
crystalline properties, and yet displays predominantly amorphous 
properties. 
The volume of polymer affected or converted (i.e., the affected surface 
layer) by the process of the present invention is defined as being in a 
`quasi-amorphous` state because the highly ordered registry of identical 
chemical moieties in adjacent folds of the chain-folded crystal lattice is 
largely destroyed, but the overall 3-dimensional architecture of the 
crystal lattice is preserved. Thus, the chemical disordering which occurs 
as a result of the radiation is characteristic of an amorphous state, 
while the retention of longer range geometric order resembles a 
pseudo-crystalline state. The layers or regions are neither totally 
amorphous nor totally crystalline in the classic sense of those words. In 
this specification where quasi-amorphous layers or regions produced in the 
practice of the present invention are discussed, those regions may be 
referred to as quasi-amorphous layers or regions because their chemical 
properties tend to resemble amorphous compositions rather than crystalline 
compositions, but amorphous and quasi-amorphous are distinctly different 
as noted in the description of quasi-amorphous materials given above. 
Quasi-amorphous is a state which is between semicrystalline and amorphous. 
It is more difficult to distinguish from a true amorphous state than a 
semicrystalline state, but a clear distinction can be drawn. 
The quasi-amorphous layer must, of course, be formed from a semicrystalline 
state. The semicrystalline state may be a uniaxially oriented film, 
biaxially oriented film, or contain grossly unoriented crystallites (e.g., 
spherulitic crystallites randomly distributed throughout the film). When 
such a semicrystalline film is converted by the process of this invention 
(in whole or in part, as on one surface only) to the quasi-amorphous form, 
the quasi-amorphous areas will appear to be amorphous except that they 
will retain a latent memory for the crystallite orientation. This is a 
definitive distinction from the true amorphous state. 
For example, oriented film will display anisotropy with respect to the 
absorption of infrared radiation (e.g. between 5,000 and 16,000 nm) in 
various directions in the film. Biaxially oriented film would most 
significantly display this anisotropy between the unoriented thickness 
dimension (e.g., the Z-axis) and the oriented length and width dimensions 
(e.g., the X- and Y-axes) of the film. When such an oriented film is 
quasi-amorphized according to the present invention to a state most 
closely resembling a true amorphous film (e.g., the entire thickness or a 
larger thickness is repeatedly treated without ablation of the film is 
quasi-amorphous), the film or layer will appear to be amorphous. However, 
the film or layer will not be truly amorphous because it will retain a 
latent memory for the crystallite orientation, in this case being 
evidenced by a latent memory for the anisotropic orientation of the 
original semicrystalline polymer. 
When this quasi-amorphous layer or film is heated to promote 
recrystallization, the film or layer will begin to regain its original 
crystallite distribution or in the case of oriented film, regain at least 
part of its anisotropic orientation. When a truly amorphous layer is 
reheated, it will not develop anisotropy. Where the semicrystalline 
polymer film originally contained grossly unoriented crystallites, 
reheating of the quasi-amorphous layer or film would return such a 
crystallite orientation to the layer or film. 
The process appears to work by the semicrystalline polymer's absorbing the 
energy of the irradiation within a limited depth of the irradiated 
surface. The energy is of sufficient intensity and duration to melt 
polymer, but of insufficient intensitiy and duration to evaporate, 
significantly chemically modify, or ablate polymer. When the irradiation 
stops, the melted polymer rapidly cools without recrystallization. No 
special cooling of the melted layer usually needs to be performed as the 
melted layer is usually sufficiently thin that ambient air and adjacent 
bulk polymer temperatures will cool it sufficiently rapidly. Forced 
cooling can be used on thicker layers if desired or can be used on thin 
layers to insure even more rapid cooling. 
The semicrystalline polymer should be able to absorb the irradiation used 
in the process. The more highly absorptive the polymer is of the 
radiation, the greater the concentration of the process to the surface of 
the polymer. In general, the polymer should be able to absorb sufficient 
energy to cause thermal softening or melting of the surface and yet not 
absorb radiation at such a high level as would cause ablation, excessive 
degradation, or volatilization of the polymer. For example, a polymer may 
absorb at least 5% of incident radiation in a 1 micron thick film when the 
radiation is applied at a rate of 1 Joule/cm.sup.2. Absorption of the 
radiation may be enhanced by the addition of radiation absorbing dyes and 
pigments to the polymer. These, and other, radiation abosrbing materials 
can have some noticeable effect at levels as low as 0.05% by weight, but 
can also be used at higher levels, even up to 90% by weight and higher. 
For example, a polymer used to modify a pigment may be treated after it 
has been combined with the pigment. A generally preferred range would be 
from 0.1 to 50% by weight for such radiation absorbing additives. 
The quasi-amorphous surface layer on the semicrystalline polymer base is 
unique because 1) it exists without substantial change of the surface 
chemical structure while the bulk properties of the polymer are unchanged, 
2) it has a lower softening temperature than the semicrystalline polymer, 
which lower softening temperature allows auto adhesion at a temperature 
below that at which the bulk film would autoadhere, 3) it is more easily 
swelled by organic solvents which allows a high degree of bond 
entanglement with itself and with other coatings and polymers, 4) the 
controlled depth of amorphization serves to limit the depth of solvent 
penetration and hence limits the effect of solvents on the quasi-amorphous 
layer, and 5) it has a reduced optical index of refraction which is graded 
from the bulk to the surface. 
The product used in the practice of the present invention has 
characteristics and features which tend to be different from those of the 
products of prior art processes. For example, it has been noted that the 
depth of the quasi-amorphous areas is at least five (5) nanometers. This 
tends to be an inherent result of the process. The previously referenced 
work reported by Wu concerning truly amorphous surfaces generated by 
non-crystallizable fractions being forced to the surface produces very 
thin amorphous layers. The thickness of these layers is never more than 3 
nm and is usually less than 2 nm. Additionally, the chemical make-up of 
the surface region is significantly different from that of the bulk 
polymer because of the concentration of non-crystallizable fractions at 
the surface. The surface produced by this prior art phenomenon would have 
a weight average molecular weight more than 50% different from the weight 
average molecular weight of the associated bulk semicrystalline polymer. 
The surface produced by the practice of the present invention would have a 
difference of less than 50% between the weight average molecular weight of 
the surface quasi-amorphous layer and the bulk semicrystalline polymer. 
Another characteristic of the treated materials used in the present 
invention which sometimes can be observed but is unique to those articles 
of the present invention is the similarity between the molecular 
orientation of the surface quasi-amorphous layer and the semicrystalline 
polymer in bulk. Polymer orientation relates to the degree to which 
polymer chains are statistically or more predominantly oriented within the 
polymer. Ordinarily, when semicrystalline polymers are melted, the 
orientation in the amorphous and crystalline phase is randomized and is 
significantly different from the orientation of semicrystalline polymer. 
Observations of the amorphized surfaces in the practice of the present 
invention indicate that the orientation within the quasi-amorphous layer 
remains similar to that of the semicrystalline polymer. Microscopic 
examination under cross-polarizers shows that the orientation of the 
quasi-amorphous layer is similar to or indistinguishable by visual 
observation from the orientation of the semicrystalline polymer. The 
physical properties of the quasi-amorphous layer, such as its index of 
refraction, infrared absorption spectrum and solubility clearly show that 
the layer is in fact in an amorphous-like state. 
Corona discharge treatment of polymer surfaces does not necessarily render 
surfaces amorphous, but oxidizes the surface of the polymer. Corona 
treatment tends to have its most significant oxidative effect to a depth 
of about 2 nm. The corona treatment creates or adds functional groups to 
the polymer as a result of reactions with the environment in which the 
discharging is performed. For example, functional groups such as 
carboxylic groups, phenol groups, hydroxyl groups, carboxyl groups, and 
amide groups can be added to the polymer by the corona treatment. These 
groups would not be a direct product of the process of the present 
invention. Corona treatment of the amorphous surfaces of the present 
invention would generate such functional groups and would not necessarily 
crystallize the surface. Corona treatment also changes the optical density 
of the surface layer because of the formation of these new chemical 
materials in that surface. As compared to the bulk polymer, the optical 
density of the surface layer may increase by 0.2 within a 50 nm region of 
the visible portion of the electromagnetic spectrum (particularly in the 
yellow region). 
Both corona discharge and flame treatment significantly modify the chemical 
composition of the polymer in the surface regions treated. Corona 
discharge tends to crosslink or degrade the polymer, creating a higher or 
lower crosslink density in the surface than in the bulk polymer. The 
article of the present invention, unless further treated as by corona 
discharge, will have approximately the same crosslink density in the 
amorphous surface layer as in the bulk polymer region. This change in 
crosslink density can be observed in the surface layer by a reduced 
tendency or ability to recrystallize. Plasma, and ion implantation 
treatments have effects on the crosslink density similar to those 
generated by corona discharge. 
Flame treatment of polymeric surfaces (such as that reported in U.S. Pat. 
No. 4,568,632) is a much more destructive and chemical composition 
altering process than the process of the present invention. The patent 
describes the ablation of materials from the surface during treatment. 
This is probably the combined result of evaporation, oxidation, polymer 
chain breakage, and other destructive processes. This process would cause 
the formation of the functional groups described above and probably cause 
a significant overall change in the molecular weight and chemical make-up 
of the polymer on the surface, probably to a depth of about 2 nm. The 
flame treatment as presently practiced also causes a change in the optical 
density of the polymer on the surface due to the change in the chemical 
composition of that surface layer. That change in optical density is at 
least about 0.2. In the practice of the present invention, the 
quasi-amorphous layer produced on the surface has an optical density which 
is within 0.1, preferably within 0.08, more preferably within 0.05 and 
most preferably within 0.03 units of the bulk polymer. Additional 
treatment (e.g., corona discharge or coloration with dyes or pigments) 
could, of course, be used to change that value. But in the absence of dyes 
or pigments differentially distributed between the amorphous layer and the 
bulk layer, there should be little or no difference in optical densities. 
In the preferred fluence range of the present invention, the most notable 
result is the formation of a new morphological state of the polymer within 
the surface layer (i.e., a quasi-amorphous, deoriented or oriented glass) 
which resides in the optical path of the radiation and begins at the 
surface of the polymer. This morphological transition is attended by some 
extremely mild degradation, as attested by the diminution of the O/C ratio 
(XPS analysis and solvent extraction data). The failure to detect weight 
loss by infrared and ellipsometric measurements indicates that gas 
evolution is, at most, a minor event. Similarly, IRRAS spectra shows 
evidence of only a morphological rather than any chemical change. The 
change in the O/C ratio is quite different from that occurring with flame 
treatment or corona discharge where the atom/atom, oxygen/carbon ratio 
increases. This increase may be very small, but in most thorough 
treatments there is a change in the ratio of about 0.1 or 0.2. The O/C 
ratio may actually decrease in the quasi-amorphous layer as compared to 
the bulk polymer. 
The remarkable aspects of the preferred surface layer produced in this 
invention are: 1) its unchanged texture; 2) its unchanged optical 
absorption or scattering characteristics, and 3) its still appreciable 
molecular weight. Each of these aspects can be very important. For 
example, film roughness is very injurious in substrates for magnetic media 
because that roughness can be the limiting factor in the ultimate density 
of recorded information that can be achieved. Film yellowing or scattering 
(i.e., haze) on the other hand cannot be tolerated where the film is used 
as a substrate in the manufacture of imaging products, e.g., X-ray film. 
Finally, the absence of a major fraction of low molecular weight 
oligomeric products avoids the situation where subsequently applied 
functional coatings fail in use due to inherently poor adhesion or solvent 
resistance which stems from the weak boundary layer present at the 
coating/film interface. 
The quasi-amorphous surface of the polymer also reduces the reflectivity of 
that surface. Normal, smooth uncoated polymer films will have a 
reflectivity of 10% or more. Highly texturized polymer surfaces can reduce 
this reflectivity, but cannot present a smooth surface, that is a surface 
having no texture which is easily visible with a scanning electron 
microscope at 10,000x magnification. The polymer films of the present 
invention can provide smooth surfaces with reflectivities of 9% or less to 
550 nm light at 80.degree.-90.degree. incident angles. This is clearly 
shown in the Examples. 
The process of the present invention also tends to not modify the surface 
of the polymer in a topographic morphologic sense. The surface structure, 
before and after amorphizing, tends to be the same in the practice of the 
present invention. Surfaces with a high degree of surface roughness may be 
somewhat softened in their features, but will still tend to have rough 
surfaces. Smooth surfaces will be substantially unchanged with respect to 
the absence of features on their surface. Flame treatment would tend to 
greatly modify the surface features of the surface so treated. 
The process of producing this invention is an advance over prior methods of 
surface modification such as sputter etch, plasma, corona, chemical, flame 
and solvents because no vacuum is required, no contact with the surface is 
required, no chemistry is added to the treated polymer so that it is more 
likely to be recyclable, and there are no known environmental problems. 
The surface properties of polymer films are of considerable importance to 
industry. These properties include adhesion, coefficient of friction, 
optical properties, wettability, and barrier properties. Modification of 
polymer surfaces to obtain these desired properties already can be 
realized by a number of different techniques. Many of these prior art 
processes can have adverse effects on the product, however. The more 
traditional "wet chemical" modification techniques, such as treatment with 
acids, amines, caustic, phenols or non-reactive liquids (i.e., solvents), 
have been successfully used to enhance the "wettability" and "bondability" 
of films and fibers. These chemical treatments can cause a temporary 
swelling of the polymer surface which results in a more reactive surface 
and on chemical evaporation this swelling subsides. These treatments can 
also result in a chemical modification of the surface by adding new 
substances, breaking the surface down to new substances, which also 
results in lower molecular weight polymer chains on the surface, or by 
cross-linking molecules on the surface. 
With the increasing concern over environmental and safety issues, industry 
has looked toward a number of non-chemical surface modification 
techniques. Alternative techniques such as treatment with corona, plasma, 
sputter etch, E-beam, heat, UV, and lasers have been used to modify 
polymer surface properties. All of these treatments affect polymer 
surfaces in a fairly gross manner. With the exception of E-beam and heat, 
they all result in a roughened surface caused by removing material, and 
they all result in chemical modifications to the surface which are much 
like the changes from wet chemical treatments. None of these treatments 
affects the crystallinity of the polymer significantly without creating 
new surface chemistry. Table 1 is a summary of how various treatments 
affect polymer surfaces. 
TABLE 1 
__________________________________________________________________________ 
TREATMENT OF POLYMER SURFACES 
Surface 
Surface Purpose of 
Treatment 
Texture 
Effect Treatment 
__________________________________________________________________________ 
Corona Rough to 
Remove Material 
Priming 
Smooth Add Material 
Enhance Wettability 
Bond Scission 
Improve Adhesion 
Plasma Rough to 
Remove Material 
Priming 
Smooth Add Material 
Enhance Wettability 
100-2000.ANG. 
Crosslinking 
Improve Adhesion 
Sputter-Etch 
Rogh Ablation Enhance Wettability 
100-2000.ANG. 
Change Chemistry 
Improve Adhesion 
Reduce Coefficient 
of Friction 
Reduce Optical 
Reflectance 
High Intensity 
No change 
Chain Scission 
Curing Surface Coats 
E-Beam Cross Linking 
Grafting 
Bulk Treatment 
Coating Adhesion 
Thick Layer 
High Energy 
Heat No change 
Chain Scission 
Enhance Printability 
(Flame) Change Chemistry 
Form Barrier Layer 
Oxidation 
Thick Layer 
High Intensity 
Rough to 
Ablation Priming 
UV Smooth Change Chemistry 
Enhance Wettability 
Improve Adhesion 
Etching 
Laser Rough to 
Ablation Etching 
(prior art) 
Smooth Change Chemistry 
Priming 
Enhance Wettability 
Improve Adhesion 
Laser No change 
Amorphize Thin 
Reduce Optical 
(Present Layer Reflectance 
Invention) Photolyze Increase Optical 
Transmission 
Improve Adhesion 
Improve Auto-Adhesion 
Reduce Coefficent of 
Friction 
Enhance e-Beam 
Grafting 
Barrier Layer 
Inc. Solubility of 
Crystalline Mtl. 
Grafting 
__________________________________________________________________________ 
Polymers generally can be either semicrystalline or amorphous. These 
categories are descriptions of the degree of ordering of the polymer 
molecules. Amorphous polymers consist of randomly ordered molecules. That 
is, the polymers are of low order or non-ordered and are independently 
surrounding and intertwined with other molecules. Semicrystalline polymers 
consist of a mixture of amorphous regions and crystalline regions. The 
crystalline regions are said to be more ordered and the molecules actually 
pack in some crystalline-like structures. Some crystalline regions may be 
more ordered than others. If crystalline regions are heated above the 
melting temperature of the polymer, the molecules become less ordered or 
more random. If cooled rapidly, this less ordered feature is "frozen" in 
place and the resulting polymer is said to be amorphous. If cooled slowly, 
these molecules can repack to form crystalline regions and the polymer is 
said to be semicrystalline. Some polymers are always amorphous. Some 
polymers can be made semicrystalline by heat treatments, stretching or 
orienting and by solvent inducement, and the degree of crystallinity can 
be controlled by these processes. 
One aspect of the uniqueness of the present invention is the reversal of 
the above crystallization process to transform a thin surface layer of 
semicrystalline polymer into a quasi-amorphous thin surface layer residing 
on nonaffected bulk semicrystalline polymer. 
There are two necessary conditions required of the radiation source to 
provide the treatment of the present invention. Both high intensity (high 
power per unit area) and high fluence (high energy density per pulse) are 
required of the radiation source. These requirements assure that a 
substantial amount of heat generated in the very thin surface of treatment 
stays in the surface. The effect of the radiation is to concentrate energy 
into the surface layer. Thermal diffusion into the bulk reduces this 
concentration of energy and makes the process less efficient. It is, 
therefore, desirable that only a small amount of heat be dissipated into 
the bulk of the polymer during irradiation. The more heat that is 
transfered to the bulk during the surface irradiation, the less efficient 
the process becomes until so much heat goes to the bulk that the process 
no longer works. Because of this requirement to rapidly heat only the 
surface layer and not the bulk of the polymer, conventional high intensity 
UV sources such as mercury arc lamps and common Xenon flash lamps with 
their inherently long pulse widths result in rapid diffusion of the 
thermal energy into the bulk polymer. This prevents a high concentration 
of energy being achieved at the surface. 
The UV excimer laser is capable of producing high intensity, high fluence 
radiation on the surface of the polymer to be treated. The polymer used 
with a UV excimer laser must be semicrystalline and UV absorbing at the UV 
laser wavelengths. The result of the laser pulse interacting with the 
surface is a combination of photolyzation and heating. In other words, the 
short intense pulse significantly heats the surface of the polymer, but 
not the bulk, above the polymer melting temperature, and some surface 
molecule chain scission occurs. During the brief time the surface region 
is heated above its melting temperature, the molecules can randomize 
themselves into a disordered condition and broken bonds reconnect, 
although not necessarily to the same end from which they were broken or to 
the same degree. The temporarily broken molecular bonds will assist this 
melting process. After irradiation the surface layer will rapidly cool, 
and "freeze" the new disordered layer into an amorphous structure. That 
is, the cooling rate is fast enough so the surface layer cannot 
recrystallize. The irradiation thus produces an amorphous layer on the 
bulk polymer which layer undergoes only a small change in molecular weight 
because of the recombination of bond scissions and no chemical changes 
such as the addition of ions. The surface texture undergoes no significant 
change because no material has been removed or ablated and both melting 
and cooling occur over a short period of time. 
The laser treated surface can be shown to be quasi-amorphous by a number of 
tests: 1) it washes off with solvents that only the amorphous form of the 
polymer is soluble in, 2) infrared reflection absorption spectroscopy 
(IRRAS) of the surface indicates the same pattern in the surface layer as 
is normally exhibited by the amorphous form of the polymer, and 3) thin 
film ellipsometry of the surface gives the same refractive index as does 
the amorphous form of the polymer. 
XPS measurements of the treated surface indicates no significant chemical 
changes by addition. It also shows that a small O/C ratio change has 
occurred which indicates some small amount of surface decarboxylation. Gel 
permeation chromotography (GPC) shows only a small molecular weight 
decrease as compared to the untreated polymer. Water contact angle 
measurements show no change in the treated surface which means the surface 
has not been roughened significantly and that functionality groups have 
not been added. There is a slight texturing on an extremely fine scale. 
Shadow mask Transmission Electron Microscopy (TEM) indicates peaks and 
valleys on the surface of about 300 .ANG.. This may account for the 
improved slip properties of the treated surfaces of this invention. 
Early investigations of laser treatments of polymers were concerned with 
etching or ablation of the polymer and thus used laser intensites and 
fluences much higher than required for the present invention. These 
investigators found a fluence threshold for ablation which of course was 
different for each polymer treated. Below this threshold no ablation would 
take place. Investigation was never made to determine what actually was 
occuring at lower fluences. It has been found that like the fluence 
threshold for ablation, there is likewise a fluence threshold for the 
amorphization of this invention and it too varies with the polymer being 
treated. 
Because of its great commercial interest, the treatment of PET has been 
studied most extensively during the progress of the present invention. 
However, other polymers have also been studied. The following 
semicrystalline, UV absorbing polymers or copolymers thereof have been 
treated: polyesters (e.g., PET), nylon, urethanes, coating mixtures of 
poly(vinylidene chloride) on PET and poly(vinyl chloride) with UV 
absorbing plasticizer added. Polypropylene, polyethylene (e.g., 
polyolefins), polyvinyl chloride, polytetrafluoroethylene and 
polyvinylidene chloride, although semicrystalline, are not UV absorbing at 
wavelengths greater than 190 nm, and therefore, require one of the 
following: the addition of UV absorbing compounds, shorter wavelength 
lasers, or an energy source different than a UV laser. E-beam, x-rays, ion 
beams, and plasmas, if applied in sufficient intensity and fluence can 
work on these polymers. 
Polymethylmethacrylate and epoxies are already amorphous and so treatment 
is unnecessary and does not effect a differentiation between the surface 
and bulk polymer. 
The UV radiation source can be by excimer laser or flashlamps at 
wavelengths less than 320 nm. The pulse widths should be less than 100 
microseconds. Typical pulse widths are 7.5 microseconds for flash lamps 
and 10-80 nanoseconds for an excimer laser. 
EXAMPLES 
In the following examples all treatments were done using either a Model 
2460 laser by Questek, Billerica, Mass. or a Model 102E laser by Lambda 
Physik of Acton, Mass. These lasers give equivalent outputs for the 
purposes of treating polymer films. The lasers were operated with either 
Ar plus Fluorine gas at an emission wavelength of 193 nm or with Krypton 
plus Fluorine gas at an emission wavelength of 248 nm and with a system of 
cylinderical lenses to control the exposed area of the sample and thus the 
energy density of the beam striking the sample. Each system was calibrated 
using a Model ED500 power meter by Gentech, Ste-Fog, Qc, Canada. Pulse 
width was approximately 15 nanoseconds for both lasers. 
EXAMPLE 1 
This example describes the treatment of a surface of 0.1 mm (4 mil) thick 
biaxially oriented polyethyleneterephthalate (PET) film with no slip 
agents added. This film is available as product #OR8478400 obtainable from 
3M, St. Paul, Minn. After laser exposure each sample was measured for 
change in optical transmission at 550 nm using a Lambda 9 
Spectrophotometer from Perkin Elmer (Norwalk, Conn.) with a 10 second 
response time. Untreated film was used as a control and measured 88.25% 
optical transmission. The following data shows the change in % 
transmission from this control value. 
Table 2 shows the results and indicates an increase in optical transmission 
for PET films treated on one side at 193 nm and an apparent leveling off 
of the effect with increased fluence. This increasing and then leveling 
off is due to the depth of treatment increasing with increasing fluence. 
Also quite noticeable is the threshold effect wherein about 3 mJ/cm.sup.2 
/pulse fluence is required for the onset of this increased transmission. 
This fluence threshold is noticed on all effects measured for this laser 
treatment. 
TABLE 2 
______________________________________ 
Exposure % Change in 
Sample (MJ/cm.sup.2) 
Transmission (at 550 nm) 
______________________________________ 
A 1 0 
B 2 0 
C 3 .03 
D 3 .08 
E 3 .10 
F 4 .18 
G 4 .37 
H 4 .45 
I 5 .58 
J 5 .78 
K 5 .82 
L 6 1.1 
M 7 1.4 
N 8 1.28 
O 9 1.40 
P 9 1.44 
Q 10 1.38 
______________________________________ 
Laser treatment of polymer films does not significantly change the 
absorptivity of the film at wavelengths greater than 350 nm. Therefore, 
increased transmission of laser treated films is a result of reduced 
reflectivity of the film and measurement of either effect is equivalent. 
EXAMPLE 2 
The example is a repeat of Example 1 with the exception that the laser gas 
was a mixture of Kr and F and the output wavelength was 248 nm. 
The data indicated that there was no change in the optical transmission 
until fluence exceeded 5 mJ/cm.sup.2 There was an increase of transmission 
to a peak change of 1.5%, reached at 9 mJ/cm.sup.2. The shift of the 
fluence threshold to a higher value of about 5 mJ/cm.sup.2 /pulse (as 
compared to Example 1) which indicates a threshold dependence on the 
wavelength of the radiation used to treat the surface. This occurs because 
PET more efficiently absorbs 193 nm wavelength radiation than it does 248 
nm wavelength radiation. 
Excimer lasers operate efficiently at four different wavelengths: 193, 248, 
308, and 351 nm. Efficient modification of the polymer requires that most 
of the UV radiation be absorbed in the first few tenths of a micrometer of 
of the surface. PET intensely absorbs both 193 and 248 nm. The efficiency 
of the surface modification also depends on the photolytic activity of the 
UV. Since 193 nm is more strongly absorbed than 248 nm and has higher 
photolytic activity, 193 nm radiation is slightly more efficient for 
surface modification. The threshold for surface modification of PET by 
excimer laser radiation (15 nanosecond pulse width) is 3 to 4 mJ/cm.sup.2 
/pulse for 193 nm and 5 mJ/cm.sup.2 /pulse for 248 nm. 
Excimer lasers produce roughly twice as much power at 248 nm than 193 nm. 
Since the threshold for surface modification at 248 nm is almost twice 
that of 193 nm, the net efficiency of surface modification between the two 
wavelengths is nearly equal. Therefore, the choice of the operating 
wavelength can be based on other factors. 
The heat sealing process of the present invention can be best effected by 
heating the surface layer (the quasi-amorphous layer) during or 
immediately before contact with the surface to be bonded. This softens the 
layer, particularly when heated above Tg or the normal melting point. Upon 
cooling, the surface layer crystallizes and becomes indistinguishable from 
the bulk polymer, but is adhered to the other surface. 
EXAMPLE 3 
Samples of 0.1 mm (4 mil) PET as in Example 1 were treated with one 7.5 
microsecond pulse from an L-2695 flashlamp by ILC Technology, Sunnyvale, 
Calif., with a peak current of 1700 amperes, 25 Joules of stored energy 
and a lamp to sample distance of 1.0 cm. Optical transmission measurements 
were made on the treated sample with a Lambda 9 Spectrophotometer and 
showed an increase in transmission over the measurement range of 340 nm to 
700 nm and at 550 nm there was a 1.5% increase. This indicates intense 
short pulse UV rich flashlamps are also capable of forming amorphous 
surface on polymers. 
EXAMPLE 4 
Samples of crystalline polyetheretherketone were treated as in Example 1 at 
various fluences. Optical reflectivity of the treated samples was measured 
at 550 nm with a spectrophotometer as in Example 1. The data indicated a 
reduced reflectivity with increased fluence from 16 to 24 mJ/cm.sup.2 
/pulse, from 14.74% reflectivity to 14.60% reflectivity. 
EXAMPLE 5 
Samples of PET were treated as in Example 2 at fluences from 1 to 6 
mJ/cm.sup.2 /pulse and measured for auto adhesion properties. A model 
12ASD heat sealer by Sentinal of Hyannis, Mass. at 350.degree. F., 20 psi 
sealing pressure and a 3 second dwell time was used to seal the treated 
surfaces to each other. Bond strength was measured by peeling the samples 
180.degree. apart by hand and judging the relative peel strength resulting 
from various fluences The data showed increased adhesion above a fluence 
of 4 mJ/cm.sup.2 /pulse. The film had slight adhesion without treatment, 
and increased to good adhesion at about 6 mJ/cm.sup.2 /pulse. 
These data are very similar to those for % increase in transmission of 
Example 2 and shows substantially the same fluence threshold. This 
strongly implies that the amorphous surface created by this invention 
causes both effects 
EXAMPLE 6 
Samples of PET were treated as in Example 1 and measured for autoadhesion 
properties. A model 12ASD heat sealer by Sentinal of Hyannis, Mass. at 
350.degree. F., 20 psi sealing pressure and a 3 second dwell time was used 
to seal the treated surfaces to each other. Bond strength was measured by 
peeling the samples 180.degree. apart by hand and judging the relative 
peel strength resulting from the various treatments. Since moisture is 
known to affect the bond strength of PET treated by other methods, each 
sample was tested both dry and under running water. Bond strengths were 
classified as follows: A weak bond was peelable without polymer film 
failure, a medium bond had a higher peel strength and occasional polymer 
film failure and a strong bond is not peelable and resulted in polymer 
film failure. These semiquantitative results are plotted in the FIGURE. 
It is well known in the literature of continuous wave, low to moderate 
intensity UV lamps, that surface modification of polymers is energy 
density insensitive. That is, if for example, 100 mJ/cm.sup.2 is required 
to modify a polymer in a certain manner, it doesn't matter if that energy 
density is obtained by using an intensity of 100 watts/cm.sup.2 for 1 
second or 50 watts/cm.sup.2 for 2 seconds and it has always been assumed 
that this was inviolate up to the energy region required for 
photoablation. The line indicating constant energy density of the FIGURE 
illustrates this conventional wisdom and is substantiated by experiments 
up to a certain fluence. 
The surprising discovery of this invention is that at a certain threshold 
fluence, in this case 3.5 mJ/cm.sup.2 /pulse, there is an enormous 
decrease in energy density required to produce auto adhesion. The 
explanation of this phenomenon is believed to be that at low fluences, 
auto adhesion is the result of oxidation of the surface layer, whereas 
above the threshold fluence an amorphous surface layer is created with a 
lower softening temperature than the bulk polymer which results in the 
increased auto adhesion. It can be seen that in the region of ablation or 
microtexturing, the auto adhesion for this polymer is also very strong. 
This is another surprising discovery of this invention and is due to a 
reduced softening temperature of structures generated on the polymer 
surface. 
From the FIGURE, it is apparent that amorphization can be achieved with one 
pulse if the fluence level is within certain ranges, and increasing the 
number of pulses at a particular fluence increases the depth of treatment 
until at too high a pulse count the polymer starts to photo degrade 
significantly. 
EXAMPLE 7 
Samples of PET as in Example 1 were treated with various exposure to a CW 
short wave UV from a 6 watt model ENF-26 Spectronics lamp of Westbury, 
N.Y. The lamp was placed directly on the polymer surface for 1 minute, 15 
minute, and 35 minutes. The exposed samples and an unexposed control were 
then sealed to themselves using a Model 12ASD heat sealer from Sentinal of 
Hyannis, Mass. set at 350.degree. F., 20 psi sealing pressure for 3 
seconds. Auto adhesion bond strength was measured by peeling the sealed 
samples 180.degree. apart by hand and judging the resulting relative peel 
strength. Samples were also tested under running water for moisture bond 
strength. The results are shown in Table 3. 
TABLE 3 
______________________________________ 
Exposure Adhesion Moisture Sensitivity 
______________________________________ 
0 min no adhesion NA 
1 min no adhesion NA 
15 min moderate adhesion 
yes 
35 min moderate adhesion 
yes 
______________________________________ 
Auto adhesion of PET from CW UV lamps is caused by surface oxidation and as 
can be seen gives a very different bond than PET laser treated as in 
Example 6. 
EXAMPLE 8 
A sample of PET was treated with short pulse UV flashlamps as in Example 3. 
A hand sealing iron at 145.degree. C. was used to bond two samples to each 
other for six seconds. The samples showed good adhesion by attempting to 
peel the sample apart with a 180.degree. hand pull. The bond was similarly 
tested under running water and was found to be moisture insensitive. 
EXAMPLE 9 
Samples of 0.038 mm (1.5 mil.) Nylon 66 from Allied Corp., Morristown, 
N.J., Product ID Capran-996 was exposed to one pulse of 25 mJ/cm.sup.2 as 
in Example 1. The samples were bonded to each other using a fiberglass 
covered hand sealing iron at 143.degree. C. for 6 seconds. Untreated 
control samples showed no auto adhesion while the exposed samples showed 
good adhesion by attempting to peel them apart with a 180.degree. hand 
pull. The samples were boiled in water for 15 minutes and there was little 
to no perceptible decrease in bond strength. 
EXAMPLE 10 
Samples of PET were treated at two pulses at 5 mJ/cm.sup.2 /pulse as in 
Example 1. These PET samples were bonded to the treated Nylon 66 samples 
of Example 9 using the same sealing conditions as Example 9. Peel tests 
using a 180.degree. hand pull indicated good adhesion between the samples. 
EXAMPLE 11 
Samples of a coated PET film were treated as in Example 1. The coating was 
0.002 mm (0.08 mil) of a solution of a copolymer of 75% polyvinylidene 
dichloride (PVDC) and 25% acrylonitrile and was coated on 0.0127mm (0.5 
mil) PET. This film product is available as Scotchpar 86096 from 3M, St. 
Paul, Minn. At about 130.degree. C. the coated side of this film is 
normally autoadhesive. This example shows the reduced temperature required 
to produce autoadhesion by first treating it with a UV laser. The samples 
were sealed to themselves at 110.degree. C., 20 psi and 3 seconds dwell 
time using a model 12ASD Sentinal heat seater from Hyannis, Mass. The 
bonds were tested using a 180.degree. hand pull and the results are shown 
in Table 4. 
TABLE 4 
______________________________________ 
Sample Fluence # Pulses Bonding Results 
______________________________________ 
A 3.0 5 slight 
B 3.5 5 tack 
C 4.1 5 excellent-film failure 
D 4.7 5 excellent-film failure 
E 5.5 5 excellent-film failure 
F 7.8 5 excellent-film failure 
G 10.4 5 excellent-film failure 
H Control 
0 0 easily peeled 
______________________________________ 
As can be seen, above the fluence threshold of about 4 mJ/cm.sup.2 /pulse, 
the bond strength was excellent and the peel test caused the film to fail. 
This example also shows that the of a U.V. absorber allows amorphatization 
of a normally non-UV absorbing crystalline polymer (PVDC).