Process for surface-modifying polypropylene or polyethylene

A process for coating a polypropylene or polyethylene surface with a maleic acid-grafted polypropylene or polyethylene copolymer, and stretching the surface to increase permeability without damaging the coated surface. In this process, a dope containing the graft copolymer is applied to the polypropylene or polyethylene surface, e.g. by dip coating, at a temperature high enough to keep the copolymer solvated in the dope solvent; the surface is then dried. The permeability of the surface is improved by stretching the surface at least about 50% beyond its original area or length such that when the stretching force is released a residual strain of less than about 11% exists. The stretching operation may be done simultaneously with or subsequent to the coating operation.

BACKGROUND OF THE INVENTION 
This invention relates to a process for modifying the surface of 
polypropylene or polyethylene to improve surface adhesion properties, 
especially to a process for bonding a maleic acid-polypropylene or 
polyethylene graft copolymer to polypropylene or polyethylene. 
Polypropylene and polyethylene are common, relatively inexpensive, polymers 
used in fibers, films, and other articles. They have many useful 
properties, but are difficult to blend with or bond to other polymers. As 
a rule, these polymers do not readily adhere to other polymers. 
CELGARD.RTM. polypropylene fiber (made by Hoechst Celanese Corp., 
Charlotte, N.C.) is an example of a commercial polypropylene fiber. 
Block and graft copolymers are often used as compatibilizers. See, e.g., 
U.S. Pat. Nos. 3,483,273, 3,860,442, 4,081,424, 4,107,130, and 4,110,303, 
the disclosures of which are herein incorporated by reference. 
Polyolefins having functional monomers grafted thereon are known in the 
art. Acrylic acid grafted polypropylene, for instance, is produced by BP 
Chemicals, which sells this polymer as part of its POLYBOND.RTM. product 
line. This product is used as a reactive compatibilizer for preparing 
polypropylene-polyester and polypropylene-polyamide blends. However, 
acrylic acid grafting causes polyolefins to degrade significantly, 
reducing the intrinsic viscosity, molecular weight, and other properties 
of the polymer. Graft copolymers of this type are disclosed in U.S. Pat. 
Nos. 4,455,273 and 4,584,347, the disclosures of which are herein 
incorporated by reference. 
Hoechst Celanese Corporation makes HOSTAPRIME.RTM. HC 5, a maleic 
anhydride-grafted low molecular weight polypropylene coupling agent. 
Maleic anhydride grafted polyolefins are also available from BP Chemicals 
under the POLYBOND.RTM. tradename. These products are used in polymer 
blends to improve the compatibility of blended polymers, e.g. in 
polypropylene-polyamide blends. 
Pending U.S. patent application No. 622,563, filed Dec. 5, 1990, describes 
a graft copolymer comprising a polyolefin backbone or main chain, 
preferably polypropylene, having muconic acid groups pendant therefrom. 
The acid groups are bonded to the backbone of the polyolefin by free 
radical addition across a muconic acid double bond. 
SUMMARY OF THE INVENTION 
The present invention is a process for modifying a polypropylene or 
polyethylene surface, including coating with a graft copolymer comprising 
maleic anhydride- or maleic acid-grafted polypropylene or polyethylene, 
said process comprising applying a dope containing said copolymer to said 
surface, stretching said surface, and drying said surface. The surface is 
stretched at least about 50% of its original length in a manner that will 
produce no more than about an 11% residual strain. The process of this 
invention produces a porous copolymer coating bonded to said surface. The 
coated product has superior adhesion characteristics compared to uncoated 
polypropylene or polyethylene, and superior permeability compared to an 
unstretched coated product. 
The temperature of the coating process is important. The dope must be hot 
enough to keep the copolymer dissolved in the dope solvent until the 
copolymer bonds to the surface. 
It is an object of the present invention to provide a process for coating 
polypropylene or polyethylene with a graft copolymer. 
It is another object of the present invention to provide a process for 
modifying the surface of polypropylene or polyethylene to improve its 
adhesion characteristics. 
It is a further object of the present invention to provide a polypropylene 
or polyethylene which is coated with a graft copolymer comprising 
polypropylene or polyethylene and maleic acid or anhydride units and has 
good permeability. 
It is an additional object of the present invention to provide a 
CELGARD.RTM. polypropylene fiber having improved adhesion characteristics 
and good oxygen permeability. 
Other objects and advantages of the present invention will be apparent to 
those skilled in the art from the following description and the appended 
claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In one preferred embodiment of the present invention, a CELGARD.RTM. 
polypropylene fiber (made by Hoechst Celanese Corp., Charlotte, N.C.) is 
coated with HOSTAPRIME.RTM. HC 5 maleic anhydride grafted low molecular 
weight polypropylene (made by Hoechst Celanese Corp., Charlotte, N.C.; 
melting point about 153.degree.-159.degree. C.), having at least about 4% 
by weight maleic acid content, according to the following procedure. 
A dope is made by dissolving the HOSTAPRIME.RTM. HC 5 copolymer powder in a 
solvent that is 80% xylene and 20% toluene by wt. at a temperature of at 
least about 100.degree. C. to make a dope having about 2-15% solids 
content by weight, preferably about 3-10%. As illustrated in FIG. 1, the 
CELGARD.RTM. polypropylene fiber 1 is passed through this dope 2 at a 
temperature of about 90.degree.-110.degree. C., preferably about 
90.degree.-100.degree. C. After the fiber 1 passes through the dope 2 it 
is dried in a drying apparatus 3 at about 100.degree.-140.degree. C. until 
substantially all the solvent has vaporized, producing a coated fiber 4. A 
mixing unit 5 helps maintain the dope at a uniform temperature. 
Cooling condensers or water jackets 6 serve to prevent solvent evaporation 
from the dope 2. These condensers do not touch the fiber 1, and must not 
cool the fiber 1 enough to cause complete precipitation of the polymer. 
The optimum temperature of the water or other coolant passing through the 
condensers 6 will depend on the system parameters, e.g., fiber speed, 
condenser dimensions, solvent, dope temperature, and the like. Typically, 
the water at a temperature of about 10.degree.-20.degree. C. is used. 
A tension force is applied to stretch or strain the fiber 1 by at least 50% 
of its length after the fiber 1 is dried. The fiber 1 is not stretched 
beyond the point where the residual strain would exceed 11% of the 
original length. The residual strain is the permanent increase in the 
length of the fiber 1 that remains after the tension force is released. 
The thickness of the coating may be varied as desired by controlling the 
solids content of the dope, and the residence time of the fiber in the 
dope bath (e.g., the fiber speed as it passes through, the length of the 
path the fiber travels in the dope bath, etc.). Generally, the thickness 
of the coating is about 0.1-25 microns, preferably about 0.5-10 microns. 
To achieve a strong bond between the copolymer and the polypropylene fiber, 
it is necessary to maintain the dope at a sufficient temperature until the 
solvent vaporizes so that the copolymer will not prematurely precipitate; 
premature precipitation as the fiber passes through and/or exits the dope 
bath leads to poor adhesion of the copolymer to the polypropylene. This is 
often a problem where there is a gap between the dope bath and the drying 
section of the apparatus, since the dope-coated fiber will cool below the 
coating temperature while passing through the gap. For example, the 
copolymer could be stable in the solvent at a temperature of about 
70.degree. C. but using such a temperature has been found to cause the 
copolymer to precipitate rapidly and adhere poorly where a gap of about 20 
mm exists between the dope and the drying apparatus. The lowest practical 
temperature that may be used will depend upon many factors, which may 
include: the solids content of the dope; the particular solvent and 
copolymer used in the dope; the air gap, if any, between the dope and the 
drying apparatus; the fiber tension; and the speed of the fiber. 
The temperature must never be high enough to cause any damage or 
decomposition of the fiber or the copolymer, nor to reach the boiling 
point of the solvent. However, it must be high enough to keep the 
copolymer in solution. Generally, a dope bath temperature of about 
85.degree.-110.degree. C. is suitable, preferably about 
90.degree.-105.degree. C. The drying column temperature should be at least 
as high and may be slightly higher, e.g. about 90.degree.-150.degree. C., 
preferably about 100.degree.-130.degree. C. The optimal temperatures for a 
given embodiment of the present invention depend on several variables, 
including the coating thickness, the coating speed, and the solid content 
of the dope. 
The dope solvent may be any suitable solvent having one or more components 
which is capable of dissolving the copolymer, does not cause undesirable 
reactions, and is easily vaporized at a temperature low enough to avoid 
damaging the polypropylene or copolymer. Organic solvents having low 
polarity are preferred. Xylene and toluene are useful for this purpose, 
and mixtures of these solvents have been found to be very good solvents in 
the process of the present invention. Preferably, the solvent comprises 
about 30-100% xylene and about 0-70% toluene. 
The fiber is stretched to enhance its permeability, e.g., to gases such as 
oxygen and the like. However, it must not be stretched so severely that 
the fiber is damaged. It has been found that the maximum amount of 
stretching that is allowable according to the present invention depends on 
the stretching temperature, force and speed; however, whenever the 
stretching causes a residual strain in excess of about 11% undesirable 
fiber damage occurs, i.e., the fiber surface becomes cracked. When the 
residual strain is less than about 11%, the stretched fiber has a uniform 
coating with enhanced permeability. 
The stretching operation may be done either at room temperature or at an 
elevated temperature, e.g., in an oven. Typically, the stretching 
temperature is in the approximate range of 25.degree.-150.degree. C., 
preferably about 50.degree.-125.degree. C. Although in the preceding 
embodiment the fiber is stretched after it is dried, it is also within the 
scope of the present invention to stretch the fiber during the coating 
process, or as part of the drying operation. 
The stretching rate and tension force may be varied. For example, a stretch 
rate of about 100%/minute is suitable, but faster and slower rates also 
may be used. The tension force must be sufficient to achieve the desired 
stretch; the minimum force needed will depend on the fiber being stretched 
and the other stretching conditions. Those skilled in the art will be able 
to determine a suitable stretch rate and tension force without undue 
experimentation. 
Although commercial polypropylene hollow fiber and copolymer have been used 
in the above embodiment, any polypropylene or polyethylene fiber, film, or 
other article that is capable of being stretched may be coated by this 
process, and many similar maleic acid-grafted copolymers may be used. The 
copolymer should contain about 0.5-10% by wt. of units derived from maleic 
acid, and a melting point of about 140.degree.-190.degree. C. These 
copolymers may be made by conventional methods for forming graft 
copolymers, e.g., by free radical addition of the maleic moiety to the 
polymer chain. The starting material for making such copolymers may 
include either maleic acid or maleic anhydride, or other maleic acid 
derivatives; the term "maleic acid-grafted" used herein encompasses all 
such copolymers. The chemical structure of these copolymers may be 
represented by the following: 
##STR1## 
where P represents a repeating polymer chain unit, M represents the maleic 
moiety, and X is the polymer unit to which M is bonded. 
Where the surface to be coated and stretched is not a fiber, but a film or 
other article, the surface may be stretched in a single direction, or in 
more than one direction, e.g. biaxially stretching a film. If the surface 
is stretched in more than one direction, the surface should be stretched 
at least about 50% its original area and the residual strain should not 
exceed about 11% of the original surface area. 
The stretched surface may be annealed in an oven to stabilize the surface 
structure. The annealing time may vary from a few seconds to hours. 
Preferably, the surface is annealed for from about 30 seconds to about one 
hour, more preferably from about one minute to about one hour. Suitable 
annealing temperatures include the temperatures that are suitable for 
stretching, e.g., approximately 25.degree.-150.degree. C., preferably 
about 50.degree.-50.degree. C. 
The surface-modified polypropylene or polyethylene obtained by the process 
of the present invention may be subsequently coated with another layer of 
material, e.g., another polymer, a layer of material useful for fluid 
separations, an abrasion-resistant layer, or any other desirable coating. 
The copolymer coating enhances the bonding of such subsequent coatings to 
the polypropylene or polyethylene article. These coatings may be applied 
by any means known in the art. 
The following non-limiting examples illustrate several embodiments of the 
present invention. However, these Examples are only intended as 
illustrative, and the scope of the present invention is not limited to the 
embodiments illustrated herein; the scope of the present invention 
encompasses the entire subject matter covered by the appended claims. 
EXAMPLE I 
HOSTAPRIME.RTM. HC 5 maleic anhydride-grafted polypropylene was dissolved 
in an 80/20 (w/w) xylene/toluene solvent at 107.degree.-115.degree. C. 
with stirring in an amount calculated to achieve a dope solution 
comprising 10% HOSTAPRIME by weight. This dope remained stable at 
100.degree. C. 
The dope was placed in a container or reservoir that was temperature 
controlled and almost completely sealed. CELGARD.RTM. polypropylene fibers 
(m.p. about 180.degree.-190.degree. C.) were coated with the dope 
(103.degree. C.) by passing the fiber through this dope reservoir at 
speeds of 3 and 6 meters/min. The fiber passed out of the reservoir and 
into a heated drying column (105.degree. C.) where the solvent was driven 
off. Condensers maintained at 16.degree. C. were located at the reservoir 
openings, e.g., between the reservoir and the drying column. The air gap 
between the condensers and the drying column was 3-5 cm. A tension force 
was applied using a 50 gram weight. 
Table I summarizes the results, showing the effect of coating speed on 
fiber permeance. The permeances were measured at an oxygen pressure of 20 
psi; the sample length was 6 cm. For comparison, data on uncoated 
CELGARD.RTM. fiber is also included. 
TABLE I 
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Coating Speed Permeance Fiber Denier 
(M/min) (O.sub.2 cm.sup.3 /min) 
(g/9000M) 
______________________________________ 
6 2.1 161 
3 4.8 147 
Uncoated CELGARD 
60-70 151 
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SEM (Scanning Electron Microscopy) pictures revealed that both coated 
fibers have 3-dimensional interconnected porous surface structures. The 
higher permeance of the 3 M/min fiber can be attributed to greater 
stretching, as evidenced by the lower denier, due to a longer residence 
time in the coating apparatus. 
EXAMPLE II 
Fibers were prepared according to Ex. I at a coating speed of 3 M/min. 
After the fibers were coated and dried, they were stretched to varying 
degrees at room temperature (about 23.degree. C.) and a stretch rate of 
100%/min. As in Ex. I, the oxygen permeance at 20 psi of 6 cm lengths of 
fiber were measured and the fibers were studied by SEM. The results are in 
Table II. 
TABLE II 
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Stretch (%) 
O.sub.2 Perm. (cm.sup.3 /min) 
SEM Remarks 
______________________________________ 
0 4.8 3-D interconnected 
particles; uniform 
distribution 
50 8-11 same as above 
75 16-25 same as above 
100 23-42 visible crack 
125 34-38 
150 25-35 distorted 3-D 
porosity; highly 
cracked surface 
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EXAMPLE III 
Coated fibers were prepared as in Ex. II, except that the stretching was 
done in a temperature-controlled oven at various temperatures. After 
stretching, the fiber was annealed in the oven at the stretching 
temperature for one minute. Elevated temperatures generally proved to 
reduce the residual strain; it appears that elevated temperatures reduce 
fiber damage due to high speed stretching. SEM data showed no significant 
change in the surface structure where the residual strain was less than 
11%. The residual strain results are presented in Table III. 
TABLE III 
______________________________________ 
Residual Strain (%) at Various Stretching Temperatures 
Stretch 25.degree. C. 
50.degree. C. 
75.degree. C. 
100.degree. C. 
______________________________________ 
50 3.33 4-5 4-5 5 
75 6.5-9 
100 15-18 10-11 7.5 10 
125 25-30 
150 34-37 23-25 11-15 13-20 
______________________________________ 
Many variations and equivalents of the present invention will occur to 
those skilled in the art. The present invention is not limited to the 
embodiments illustrated or described, but includes all the subject matter 
within the spirit and scope of the appended claims and of the foregoing 
disclosure.