Process for the production of fluorinated surfaces of polymers

A process for blow molding and fluorination of a hollow thermoplastic article includes bringing a blank to rest against the inside wall of the blow mold as a result of introducing an inert blow gas. An inert gas/reaction mixture is then introduced in the hollow article for fluorinating the inside surface of the hollow article. After the inside surface of the hollow article has cooled off, the inert gas/reaction gas mixture is blown in below the melting point of the thermoplastic material.

BACKGROUND OF INVENTION 
The invention relates to a process for the production of hollow articles 
having fluorinated inside surfaces consisting of thermoplastic materials. 
In addition to HDPE, also IDPE, PP, EPDM, PVC and similar materials are 
suitable as thermoplastic materials. 
In the automobile industry, fuel containers are increasingly produced from 
polymers, in particular, from high pressure polyethylene. These plastic 
fuel containers are lighter and cheaper than the conventional containers 
made from sheet steel. 
Moreover, they can be made without problems in a complicated shape so that 
the available space can be better utilized and the container volume can be 
increased. 
Plastic containers are, however, not completely tight for solvent motor 
fuel and readily volatile or gaseous substances since slight amounts 
continuously escape to the outside as a result of permeation. Recently, an 
effort was, therefore, made to considerably reduce the permeation rates. 
This can be successfully accomplished by exposing the inside surface of 
the container for some time to the influence of a fluorine-containing 
treatment gas. During this process, the surface is coated by elementary 
fluorine or also in the form of fluorocarbon compounds and 
fluorocarbonhydrogen compounds. Suitable reaction agents are also 
chlorofluoride, chlorotrifluoride, bromotrifluoride, fluorosulfonic acid 
and similar substances. The fluorine-containing layer considerably reduces 
the permeation rates. When the fluorine-containing treatment gas is used 
at the same time as blow medium in blow extrusion of containers, we speak 
of in-linefluorination. On the other hand, the fluorination conducted on 
already extruded containers is called off-line-fluorination. Such 
processes are known, for example, from German Patent No. 2 401 948 and 
German Patent No. 2 644 508. In addition to the in-line single step 
process according to German Patent No. 2 401 948, German Patent No. 3 523 
137 also describes an in-line two step process. 
The fluorination of plastic surfaces, however, not only influences the 
permeation behavior to a large degree but also the abrasion resistance, 
the chemical, thermal and mechanical strength, the adhesive behavior and 
the wettability are affected. The invention is, therefore, not only 
limited to the fluorination of the inside surface of plastic fuel 
containers. In the fluorination, the polymer surface is exposed to an 
attack by elementary fluorine. In the simplest case, for example, with 
polyethylene, an incremental radicalic substitution of the CH-bonds by 
CF-bonds takes place. 
Extensive in-house investigations have shown that, depending on the 
reaction conditions, very differently structured fluorinated layers are 
produced. In order to attain certain good and clearly reproducible surface 
effects for the above cited material behavior, the exact adherence to 
certain structure parameters of these fluorinate surfaces is extremely 
important. These are in the first place layer thickness, uniformity of the 
fluorine coating, distribution of the CH2-groups, CHF-groups and 
CF2-groups and the depth profile. 
SUMMARY OF INVENTION 
It is, therefore, the objective of the invention to provide an improvement 
in the barrier properties of fluorinated boundary layers for solvents, 
motor fuels, readily volatile or gaseous substances. 
In accordance with the invention, after the inside surface of the hollow 
article has cooled off, the inert gas/reaction gas mixture is blown in 
below the melting point of the thermoplastic material.

DETAILED DESCRIPTION 
FIGS. 1-4 are scanning electron micrographs (SEM) of HDPE at 10,000 
magnification from the inner fluorinated surface for 20 seconds with 9 bar 
pressure, 1.1% grams of F.sub.2 /N.sub.2 after flushing with 1 bar 
N.sub.2. The SEM of FIG. 1 is of the surface at 80.degree. C. FIG. 2 is at 
120.degree. C. FIG. 3 is at 140.degree. C. FIG. 4 is at 160.degree. C. 
Examinations with the scanning electron microscope have shown that a change 
in surface structure takes place as a function of the surface temperature 
of the investigated hollow articles. FIGS. 2 and 3 show an example of 
scanning electron microscope photographs with a 10,000 times enlargement 
of an HDPE surface at 120.degree. C. and at 140.degree. C. 
It was now discovered that the severe wrinkling (FIG. 3) begins at 
temperatures of the thermoplastic material which are close to or above the 
melting point. The specific surface is considerably increased by the 
wrinkled shape and tests have shown that, in spite of the increased 
fluorine content of the surface to 50-60 g/cm.sup.2, the permeation 
increases with a larger area. 
In contrast, it became evident that the specific surface does not increase 
for a fluorination below the melting temperature. As a result, the normal 
crystallinity of the thermoplastic material on the surface remains 
extensively unchanged. The oxidation of the plastic is almost completely 
limited to the substitution of the H-atoms by fluorine. When attacked by 
oxidation media, other decomposition reactions of the polymer are 
prevented by the process temperatures according to the invention. The 
fluorination in the temperature range of 50.degree. to 130.degree. C., 
preferably, 80.degree.-120.degree. C. according to the invention, in 
addition, has the advantage that at these temperatures an almost uniform 
temperature distribution exists on the inside surface of the hollow 
article, which allows for a uniform reproducible boundary layer 
production. 
Another advantage of the invention is that, in the temperature range 
according to the invention, the good morphological structure 
characteristics of the polymer surface are extensively retained. 
Polyethylene crystallizes when the melt is cooled. The long molecule 
chains in this process settle (folded) in very small crystallites. Low 
pressure polyethylene, also called hard or high density polyethylene 
attains a crystallinity degree of 60-80%. With an increasing 
crystallinity, yield stress, modulus of elasticity, stiffness, resistance 
to solvents and impermeability for gases and vapors increase. 
The good property characteristics of an undisturbed solidified 
(recrystallized) polymer surface are extensively retained since the 
fluorination reaction does not begin until the crystallites are formed. 
These differences can be clearly seen in FIGS. 1-4 as well as the 
crytallites and their cross-links. The surface of FIGS. 3-4 corresponds 
the most to the conventional surface structure of a container 
blow-extruded with an inert gas. If the fluorination is conducted at 
higher surface temperatures, therefore, above the crystallite melting 
temperature of the employed material, highly folded, amorphous structures 
having a much larger specific surface are produced after the polymer melt 
has cooled off. 
Example 
An HDPE blow type with MF/190/5=0.24 g/min.,density 0.944-0.948 g/cm, is 
processed on a blow-machine to plastic fuel containers 
(Kunststoff-Kraftstoff-Behaltern or KKB). The temperature of the blank is 
220.degree. C., blow medium is N.sub.2, 10 bar. The blown, dimension 
stable container having an average wall thickness of 4 mm is then cooled 
to a temperature of 110.degree. C. by interval purging with N.sub.2. 
Subsequently, a 1.1% F.sub.2 /N.sub.2 -mixture streams into the container 
until the pressure has increased to 10 bar. After 40 seconds action time, 
the inert gas/reaction gas mixture is discharged to waste disposal. 
A 60 1 plastic fuel container produced in this way has an average fluorine 
coating of 25 g/cm, and a permeation of 1.5 g/24 hours. The fluorinated 
inside surface of the plastic fuel container has a mat appearance. 
The invention is not limited to the indicated process parameters. The 
fluorination temperatures depend on the material of the hollow article. 
The reaction gas fraction can also be varied from 0.5 to 10%. The action 
time of the reaction gas may vary from 10 to 60 seconds. Next to N.sub.2 
other inert gases may also be used. 
Before the cooling agent is blown in, the pressure inside the hollow 
article is preferably reduced to 1 to 5 bar. This also applies before the 
reaction gas mixture is introduced. 
Investigations have shown that the invention results in an improved long 
term stability of the barrier layer. The permeation behavior with respect 
to polar fuel additives is, furthermore, clearly diminished. An efficient 
use of fluorine and corresponding reaction agents is, moreover, attained 
by the process according to the invention. 
An especially preferred and advantageous embodiment of the invention is the 
production of plastic fuel containers consisting of HDPE. 
SUMMARY OF INVENTION 
The invention relates to a process for blow molding and fluorination of 
hollow articles, in particular, plastic fuel containers consisting of 
HDPE. The invention resides in that the fluorination in the in-line 
process takes place after the hollow article has been produced whereby the 
temperature of the inside surface of the hollow article lies under the 
crystallite melting temperature of the employed plastic.