Process of high pressure curing with ultraviolet radiation

A method of molding a polymeric containing article by exposing the article to ultraviolet radiation and a substantially uniform pressure. The method includes placing the precursor article within a pressure vessel and substantially filling the vessel with a solid, flowable, polymer medium that is substantially ultraviolet transparent under pressure. The polymer medium is pressurized so as to produce a substantially, uniform predetermined pressure on the surface of the article and the article is exposed to ultraviolet radiation that passes through the polymer medium to cure the article. Alternatively, the polymeric article may be passed through the medium containing vessel during exposure to ultraviolet radiation and a substantially uniform pressure.

TECHNICAL FIELD 
The present invention relates to methods of molding polymeric containing 
articles with ultraviolet light under pressure. 
BACKGROUND ART 
There are a variety of methods of molding articles. These include 
autoclaves, hydroclaves and compression molding. Particularly for high 
temperature molding (e.g., polyimide resins) one isostatic molding process 
has shown great advantage over other methods. This method is described in 
commonly assigned European Patent Application No. 87630010.4 entitled 
"Molding Method and Apparatus Using a Solid, Flowable, Polymer Medium" to 
Kromrey. An article is molded by contacting it with a solid polymer 
medium, such as an especially low strength unfilled silicone rubber which 
is solid and able to flow readily. One characteristic of such a silicone 
material is that it tends to coalesce under pressure so that the 
interfacial boundaries between the particles are so essentially conformed 
that the compressed rubber becomes translucent instead of opaque. (E.P.A. 
No. 87630010.4, Col. 10, lines 55-62) Thermal expansion of the medium or 
mechanical force is used to create molding pressure and thereby provides a 
substantially uniform pressure on the article precursor. Various 
temperature and pressure cycles can be attained; constant high pressures 
can be maintained on the article precursor during cooldown, optionally 
aided by flowing of medium to and from a vessel in which the article 
precursor is being molded. The method is particularly adapted to molding 
filler or fiber reinforced thermosetting polymer composite articles. 
Typically, the above process cures composites or plastics over short 
periods of time. However, application of heat for rapid curing is often 
impractical or impossible. The higher temperatures typically needed for 
snap cures can cause severe warpage in laminates and can result in 
exotherms that can degrade polymers. An alternative to thermal curing is 
ultraviolet (UV) radiation curing. Although there can be a slight exotherm 
with a UV cure, the small exotherm can be controlled. Even in-depth UV 
curing does not result in the large thermal gradients that can develop 
with heat cures. 
Recent advances in formulating UV curing polymers have resulted in high 
performance resin matrices for rapid curing composites. These polymeric 
materials must have adequate optical transparency required for in-depth 
curing. The optical transparency constraint also precludes the use of 
opaque tooling including some conventional vacuum bag materials, 
particularly for thick structures. 
Accordingly, there is a constant search for UV composite curing processes. 
DISCLOSURE OF INVENTION 
This invention is directed to a method of molding a polymeric containing 
article comprising exposing the article to ultraviolet radiation and a 
substantially uniform pressure. The method comprises placing the precursor 
article within a pressure vessel and substantially filling the vessel with 
a solid, flowable, polymer medium that is substantially ultraviolet 
transparent under pressure. The polymer medium is pressurized so as to 
produce a substantially, uniform predetermined pressure on the surface of 
the article and the article is exposed to ultraviolet radiation that 
passes through the polymer medium to cure the article. 
Another aspect of this invention is directed to a method of molding a 
polymeric containing article, comprising transferring a polymeric 
containing article through a vessel and exposing the article to a 
substantially uniform pressure and ultraviolet radiation. The method 
comprises substantially filling a pressure vessel with a solid flowable 
polymer medium that is substantially ultraviolet transparent under 
pressure. The polymer medium is pressurized so that said medium is capable 
of producing a substantially uniform, predetermined pressure on the 
surface of the article to be molded. The article precursor is transferred 
through the pressurized vessel while passing ultraviolet radiation through 
the polymer medium sufficient to cure said article precursor. 
The foregoing and other objects, features and advantages will be apparent 
from the specification, claims and from the accompanying drawings which 
will illustrate an embodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 1 demonstrates schematically a method and apparatus according to the 
present invention wherein a polymer containing article precursor 1 (e.g., 
composite prepreg) is placed in a pressure vessel 3 (UV transparent (e.g. 
acrylic) where the vessel is between the radiation source 4 and the 
article precursor 1). Polymer medium 6 (ultraviolet transparent under 
pressure) is disposed between the article 1 and the radiation source 4. 
The polymer medium 6 may be in contact with more or less of the article 
precursor 1 as is desired. Typically, the surface area of the article 
precursor not in contact with the polymer medium is disposed (e.g., in 
contact) next to a tool 7 in order to provide (or maintain) a particular 
shape to the article. A pressurizer 9 (e.g., piston) applies the 
requisite, uniformly distributed medium pressure to the article precursor. 
However, the article is preferably pressurized via the thermal expansion 
of the polymer medium via heating elements 10. Such things as conventional 
pressure transducers 11 can be inserted in various places within the 
pressure vessel 3 to detect the requisite pressure. While any pressure can 
be used with the present invention, typically pressures up to 3000 pounds 
per square inch (psi) are required for molding such things as composite 
materials. The radiation source 4 provides UV radiation sufficient 
(described hereinafter) to cure the polymeric containing article. 
In FIG. 2, the pressure vessel 23 (preferably UV transparent) is 
substantially filled with a polymer medium (that is UV transparent under 
pressure) 26. The pressure control apparatus 28 provides pressure through 
a pressurizer 29 (e.g., piston) to the polymer medium. Alternatively, the 
thermal expansion of the polymer medium can provide the pressure. The 
article precursor 21 is transferred through a bushing 22 (which contains 
the polymer medium within the pressure vessel when an article is not being 
molded) located in an orifice 24 in a pressure vessel 23. As the article 
precursor 21 is drawn through the pressure vessel 23, it is exposed to a 
medium pressure by the polymer medium 26. This pressure can be uniformly 
distributed or have a gradient where the pressure is staged. This is 
particularly applicable to resin systems that can exhibit excess flow 
under initial high pressures (e.g., can't withstand initial high 
pressures). In addition, a radiation source 12 provides UV radiation 
sufficient to cure the polymeric containing article. The article exits 
through bushing 30. Extremely long sections can be made when a continuous 
pulling force is maintained. The speed of curing can be fast enough to 
process structural sections at a high rate (e.g., 12 or more inches per 
second). 
A preferred pressure causing/transferring solid flowable media is described 
in European Patent Application No. 87630020.4 entitled "Molding Method and 
Apparatus Using a Solid, Flowable, Polymer Medium" by Kromrey, the 
disclosure of which is hereby incorporated by reference and in commonly 
assigned U.S. application Ser. No. 829,048 now abandoned, Continuation 
Ser. No. 201,355 entitled "Molding Method and Apparatus Using a Solid 
Flowable, Polymer Medium", the disclosure of which is hereby incorporated 
by reference. 
The medium's responsiveness to temperature and pressure coupled with its 
flowability and solid nature at molding temperatures enable it to be 
useful. These properties cause the medium to produce an advantageous, 
substantially uniform, controllable pressure on the surface of the article 
precursor to be molded. In a typical embodiment of the invention, the 
polymer is an unfilled silicone rubber particulate of -4+30 U.S. mesh 
sieve size (4.7-0.42 millimeter (mm)), which when pressurized, is 
sufficiently self-compliant to coalesce as an essentially void-free medium 
at a pressure of the order of 69 kPa (10 psi). 
Typically, a silicone rubber is used as the pressurizing polymer. The 
preferred solid flowable polymer material is further described in U.S. 
Pat. No. 4,686,271 entitled "Hydraulic Silicone Crumb" by Beck et al, the 
disclosure f which is hereby incorporated by reference. The rubber is also 
an improvement on the type which is described in U.S. Pat. No. 3,843,601 
to Bruner. See also U.S. Pat. No. 4,011,929 to Jeram et al. The 
disclosures of the above patents are hereby incorporated by reference. 
Generally, the preferred materials are dimethylsilicones that have vinyl 
groups. They may be produced by conventional commercial procedures, 
including production from polysiloxanes using various vulcanizing 
techniques. Preferred materials which have been used thus far are the 
experimental unfilled silicone rubber materials designated as X5-8017, 
formerly No. 6360 B1 (more simply 8017 hereinafter), X5-8023 and X5-8800 
by the Dow Corning Corporation (Midland, Mich.). 
Another Dow Corning silicone rubber, No. 93-104, without its ordinary 
fillers (called "93-104" herein, nonetheless) is useful. The Polygel 
C-1200 silicone rubber Stauffer Chemical Company, Westport, Conn. USA), 
believed to be essentially the material which is described in the Bruner 
U.S. Pat. No. 3,843,601 is also useful with the present invention. 
Other preferred materials are the vinylmethylsiloxane-dimethylsiloxane 
(VMS-DMS) polymers such as Dow Corning No. X5-8026 as described in 
commonly assigned copending application Ser. No. 907,946 entitled "High 
Temperature Solid Flowable Polymer Medium and Method of Molding Using the 
Same", the disclosure of which is hereby incorporated by reference as it 
is usable at high temperatures, (e.g., 316.degree. C. (600.degree. F.) 
482.degree. C. (900.degree. F.)). 
Most silicone rubbers are temperature limited for long term use, e.g., 
typically up to about 232.degree. C. (450.degree. F.). However, silicone 
resins of the vinylmethylsiloxane and silphenylene types have been 
successfully tested up to about 482.degree. C. (900.degree. F.). Fillers 
and other adulterants (such as the metal particulates described below) can 
be included with and within the medium, provided the essential behavior 
properties are maintained. 
The preferred 8023 silicone rubber is characterized by low strength and 
high friability. By "high friability" is meant there is such low strength 
that moderate size solids tend to disintegrate into smaller particulates 
when subjected to modest mechanical forces, even rubbing between the 
fingers. The 8017 material has a Shore A hardness of less than 1 (Shore 00 
hardness of 50-55) and compressive strength of the order of 70 kPa when 
measured on a 2.5 cm square by 1.27 cm thick specimen, and upon a 
compression deformation of about 40%, it shears into smaller particles. 
This behavior is contrasted with that of more conventional rubbers which 
have higher strength, greater resistance to deformation and greater 
elongation to failure. It has also be observed that preferred polymer 
useful with the present invention forced through a small orifice, or 
through a 1.1 cm diameter pipe as described below, has a tendency to 
disintegrate into smaller particulate. By example, it is found that over 
time, a nominal 30 mesh size powder having about 50 weight percent 
retained on a 40 mesh screen will change to one having only about 25 
weight percent retained on 40 mesh. 
The aforementioned behavior of the polymer media enables the fabrication of 
intricately shaped composite polymer parts with uniform properties under 
the controlled and independent application of uniform pressure and 
temperature. In one embodiment of the invention, the polymer has a Shore A 
hardness of less than about 15, typically less than 8, and desirably less 
than 1; the compressive strength is less than 1 MPa, and desirably less 
than 0.2 MPa. 
The ability of the inventive medium to flow under molding pressure is 
believed to be especially reflective of the properties of a good medium. 
This characteristic allows redistribution of the medium both within and to 
and from the vessel; it enables control of the absolute level and 
variability of the pressure. And tests show it is that which distinguishes 
the materials of the present mediums from those which have been used 
heretofore in the pressure pad molding technique. The flowability property 
can inferentially be seen to be analogous to viscosity. But there is no 
evident standard test known for measuring this property of importance to 
the invention and therefore a test apparatus was created as described 
above comprised of a cylinder having downwardly movable piston to test the 
polymer portion of the medium. The cylinder is filled with the rubber or 
other medium being tested. A replaceable pipe extends from the side of the 
cylinder and discharges rubber onto a weighing scale, the weight being 
recorded as a function of time and the pressure applied to the rubber as 
measured by a transducer. The pipe is a smooth stainless steel tube of 1.1 
cm inside diameter and nominally 32-64 RMS (root mean square) surface 
finish. The pipe length is chosen as desired, with 7.6 cm and 15.2 cm 
being preferred. 
Thus, generally it can be said that the polymer will have flowability, 
i.e., mass transport can occur when molding pressures are applied. The 
preferred polymer, when tested in the apparatus described above using 10.3 
MPa (1500 psi) and a 15.2 cm (6 inch) pipe, has a flow rate of at least 
0.6 g/s, typically 6 g/s, and desirably more than 25 g/s. 
Further description of the polymer is given below. A particulate elastomer 
is typically used in the practice of the invention. When the 8017 polymer 
is used as particulate solids, prior to the application of pressure the 
particulates are spaced apart at the article precursor surface. But when 
pressure is applied, the particles self-comply and coalesce into a 
continuous void-free body. Because of this and their inherent resilience, 
a uniform hydraulic-like pressure is applied to the article precursor 
surface. Tests show that the 6360 material will tend to coalesce upon the 
application of moderate compressive pressure, of the order of 70 kPa; at 
this point the interfacial boundaries between the particles are so 
essentially conformed that the compressed rubber becomes translucent 
instead of opaque. The 8017 material has a true density of 0.97 g/cc, an 
apparent bulk density of 0.5 g/cc as a -30 mesh size powder, and it is 
compressed to a coalesced translucent material with a density of 0.94-0.97 
g/cc by the application of about 70 kPa. (Further compression of captured 
material, in the range 70 kPa to 13.8 MPa, shows it to have about 0.4% 
volume change per 10 MPa.) Under the above-described coalesced condition, 
there is believed to be little void, or gas (except absorbed gas) 
contained in the interstices between the particulates. 
Thus, the preferred material, when used in particulate form, will be 
self-compliant and will tend to coalesce as an apparent void-free body 
below a pressure of 350 kPa, preferably 240 kPa; more preferably about 69 
kPa. 
Based on various molding tests and material property measurement, desirable 
results have been associated with medium having low strength, the ability 
to self-comply under molding level pressures, and the ability to flow and 
exhibit hydraulic-like behavior. Other silicone rubbers than 8017 have 
been used up to the date of this application, and it is within 
contemplation that there are still other organic polymers and other 
materials which are either known or can be developed which will carry out 
the essential features of the invention. To characterize the desirable 
properties associated with the good molding results, comparative tests 
have been run on various rubbers, in molding trials on actual composite 
articles, in the flow test apparatus described, and in standard apparatus. 
Tests run on the granular 8017 material showed a maximum variation in 
pressure of as low as 2% at about 6.9 MPa nominal pressure; other useful 
materials produced pressure uniform within 10%. Addition of molten metal 
matrices does not adversely affect the above-cited property. 
The usefulness of the materials is also evaluated according to the 
integrity of a molded finished product, it being well established that 
inspection will commonly show areas of low density or cracking where the 
proper application of pressure and temperature has not been achieved, 
during either the heating or cooling cycle. 
The polymer medium is also required to be substantially transparent to UV 
radiation at the elevated pressures that the polymer containing article 
precursor is molded. Substantially transparent as referred to in this 
application refers to sufficient transparency such that sufficient 
radiation is transmitted to cure the resin (e.g., activate a UV sensitive 
catalyst). Typically, this is at least about 10% but varies with many 
factors (e.g., polymer-media, UV source intensity, pressure, media purity, 
any entrained gases not removed by vacuum treatment, thickness of the 
part, etc.). Below about 10%, there is typically not enough intensity to 
cause curing within a reasonable time frame. It is especially preferred 
that the transmission is above about 33%. As stated below, UV radiation in 
particular refers to that portion of the spectrum that is required for the 
curing of the particular polymer being molded. 
The ultraviolet radiation used in the practice of this invention 
corresponds to that needed to cure the particular polymer that is being 
molded. This is typically about 200 nm to about 400 nm in wavelength 
because above about 400 nm, insufficient energy is available to activate 
the catalyst (initiator) and below about 200 nm, the radiation is outside 
the range of absorption of conventional photoinitiators (catalysts). It is 
preferred that it is about 280 nm to about 400 nm since below about 280 nm 
little, if any, transmission has been observed for the above-described 
silicone polymers and above about 400 nm, there is little increase in 
percent transmission. FIG. 3 depicts two graphs of transmission (%) (y) 
vs. wavelength (nm) (x). Two polymers described earlier Dow Corning 
X5-8800(A) and X5-8023(B) were tested in a round cuvette. The cuvette was 
filled halfway with polymer and compacted with a round bolt (that fit 
snugly in the cuvette) at hand pressure. The hand pressure was released 
and the effective compacting pressure was substantially reduce. The 
cuvette was placed in the spectrophotometer and the scans in FIG. 3 were 
taken. 
Generally, the radiation is maintained for under about 10 minutes and for 
larger throughput under about 10 seconds. However, the exposure time 
depends upon a variety of factors such as the transparency of the polymer 
medium, pressure vessel and polymer containing article, the thickness of 
the article, the type of polymer used in the article and the pressure 
employed. The elevated pressures referred to above are preferably above 
about 5 psi because below about 5 psi, the media has not coalesced 
sufficiently to allow transmission. It is especially preferred that the 
pressure is above about 20 psi as above that pressure, UV transmission is 
increased significantly. 
The polymer has only been characterized according to the properties of some 
currently available materials; the data are insufficient to establish that 
the totality of measured properties in combination are necessary. On the 
contrary, to a significant extent, it is believed there is a redundancy 
and that they independently characterize the invention. 
The pressure vessel used can be virtually anything that can provide support 
and/or structural support to the polymer medium and/or article. It is 
required that that portion of the vessel that the UV radiation must pass 
through to cure the article is substantially transparent to UV radiation, 
or at least that portion of the UV spectrum that provides the curing. The 
vessel need not be UV transparent if a UV source is disposed within the 
vessel in proximity to the article. When using transparent vessels, 
exemplary materials include quartz and acrylic pressure vessels. However, 
even transparent vacuum bags, when coupled with a tool (optionally in an 
autoclave) to provide support can work in the practice of this invention. 
In this embodiment, the polymer medium is placed above or between the 
article and the bag in an autoclave or the article is bagged and the 
polymer medium is placed outside the bag (vessel) and container in a 
second vessel (e.g., autoclave). 
It is desirable that the vessel contain mainly the desired polymer medium 
and the article being molded. However, it will be appreciated that there 
will be allowed other objects, particles and materials to be contained 
within the medium. While the material is described as being essentially 
void-free during molding, such reference is to the absence of spaces 
between the individual pieces of the medium, and is not a limitation on 
such occasional voids as may be in the cast or formed polymer piece due to 
the nature of its manufacture. 
The polymer containing article precursors of this invention are preferably 
composites and solid polymeric articles. These articles must contain a 
polymer or initiator that can be cured upon exposure to UV radiation. The 
preferable polymers will be those that are susceptible to in-depth UV 
curing. By in-depth is meant typically about 0.64 cm (0.25 inch) to 
typically about 7.62 cm (3.0 inch). In addition, the preferable polymers 
are those susceptible to UV fast cures. By UV fast cures is meant cures 
that take place in from less than about 1 second to about 2 to 3 minutes. 
Examples of these polymers include polyesters, vinyl esters, and 
silicones. The fibers used in composites typically detract from the UV 
transparent nature of the composite. However, for examples, some 
reflection will occur off of fiber surfaces and aid in-depth curing. Thus 
exemplary fibers such as quartz, glass and ceramic, etc. when incorporated 
in resins such as polymer, vinyl ester and silicone do not prohibit the 
curing. The thickness of the article as well as its composition will 
determine the UV radiation exposure time needed. 
The present invention enables particularly good control over the pressure 
to which the article is subjected. Because the medium is solid, the 
article being molded need not be sealed in a manner which is impervious to 
gas or liquid, greatly alleviating problems with prior art methods such as 
bagging. The articles produced are considerably more uniform in 
properties, especially when of complex shape, compared to articles 
produced by the prior art method, for example. Because the medium is 
flowable and allows the mass contained within the pressure vessel to be 
varied during molding, the method surmounts problems associated with 
vacuum bagging requirements used in such things as autoclave and 
hydroclave techniques. 
Finally, the UV transparent nature of the polymer medium eliminates the 
problem of opaque pressure vessels (e.g., vacuum bags) while facilitating 
the curing of thick structures under high pressures. Thus, this invention 
makes a significant advance in the field of molding particularly the field 
of fast care composite molding. 
It should be understood that the invention is not limited to the particular 
embodiment shown and described herein, but that various changes and 
modifications may be made without departing from the spirit or scope of 
this concept as defined by the following claims.