Polymer powder compositions, particularly polyethylene powder compositions and objects to be made and made thereof

The invention relates to polymer powder compositions consisting of 1-99 wt. % of a polymer powder component A and 99-1 wt. % of a polymer powder component B, the average particle size of component A being smaller than that of component B, and to the preparation of hollow objects by rotational molding using these polymer powder compositions.

The invention generally relates to improved polymer powder compositions. 
The improvements reside in employing a controlled average particle size of 
the powdered polymer components of the composition. The polymer powders 
are preferably polyolefin powders and more specifically polyethylene 
powders. 
BACKGROUND OF THE INVENTION 
Polymer powders have been used for manufacturing articles by rotational 
molding, a process that is well known in the art. The powder to be 
rotationally molded melts against the hot mold wall and forms a layer 
thereagainst which will eventually be the outer surface of the molded 
object. The thus formed surface of the molded object may consist of a 
single layer, but may also consist of a number of layers. Foam structures 
between or against either a layer or layers may be applied to improve the 
insulating properties or to limit the weight of the objects. The surface 
layer and the foam laying thereagainst must adhere to each other. Now, 
this adhesion leaves much to be desired in the processing of polyethylene. 
The same is true with polypropylene. This is an acute problem in producing 
surf-boards or wind surf boards, made for the most part, by the rotational 
molding process wherein a wall of polyethylene, particularly of 
polyethylene having a high or medium density of at least 0.930 g/ml forms 
the outer surface, i.e. envelopes, the core of such boards. Polyethylene 
having a lower density may also be processed for this purpose. Generally 
speaking polyurethane foam forms the core of such boards. 
In use however, the polymer wall may be damaged and water may penetrate 
into the foam. Damage, will occur less rapidly when there is good adhesion 
between the wall and foam. In such circumstances accompanied by outer wall 
damage, water can then penetrate only into the foam in an area immediately 
under the damaged portion of the wall. If the wall-core adhesion is poor, 
then water may spread between the wall and foam and thus be absorbed 
throughout the object. Good adhesion is thus highly desirable for in 
particular wind surf boards as well as for other objects built up from 
similar components. There is, therefore, a widespread need for polymer 
compositions to satisfy this objective. 
There are other uses of hollow article made by rotational molding. So 
without filling the hollow article with a foam the article may be used as 
e.g. a container for organic or other liquids. It may be required that the 
inner layer has mechanical, physical or other properties that differ from 
the properties of the outer layer. Such a condition may also be imposed 
when the requirements for the wall of the hollow article cannot be met by 
one material. One can fulfill such conditions by making the hollow article 
from two or more layers constituting the wall of the hollow article. It 
has been disclosed to produce such articles in a two shot rotational 
molding process, molding first an outer layer, cooling the rotational 
mold, adding into the mold a second charge of polymer powder and molding 
an inner layer. The first and second charge can consist of different 
materials, but they can consist also of the same or closely related 
materials having different properties. So in order to make surf-boards one 
can make in a first shot an outer layer of stabilized polyethylene that 
releases easily from the mold and has good wheathering properties, whereas 
in a second shot an inner layer of the same or another grade of 
polyethylene, that has not been stabilized is produced. Due to the lack of 
stabilization said inner layer becomes somewhat oxidized during rotational 
molding and said oxidized polyethylene adheres well to a core consisting 
of polyurethanfoam. Such two shot processes are laborious and costly. 
The compositions described in U.S. Pat. No. 4,307,133 for coating metal 
pipes consisting of a polyolefin powder composition having from 0.5% to 
80% by weight of unstabilized polyethylene powder and from 20% to 99.5% by 
weight of a stabilized polyolefin powder are not suitable for producing 
hollow articles that are to be filled with e.g. a polyurethan foam. Such 
compositions adhere strongly to the metal object while the exterior of the 
coating exhibits some resistance to oxidation and other weathering 
influences. When such compositions are rotationally molded and filled 
subsequently with e.g. polyurethan foam they adhere strongly to the mold 
and mold release is very difficult or not possible at all whereas the 
adherance to the foamed core is unsatisfactory. 
Improved adhesion of polyethylene to metal substrates by mixing the 
polyethylene with oxidized polyethylene is proposed in U.S. Pat. No. 
3,639,189. The oxidized polyethylene is obtained by heating polyethylene 
in an oxygen atmosphere at temperatures that may range from 90.degree. C. 
to the crystalline melting point of the polyethylene until the desired 
degree of oxidation has been reached. The un-oxidized polyethylene is then 
mixed in the melt with the oxidized polyethylene and granulated. 
Subsequently the granulate is used as a starting material applying polymer 
layers to metal substrates. This proposal suffers from several drawbacks. 
The oxidation of polyethylene is laborious and significantly increases the 
cost of the composition used as starting material. In applications as for 
instance, manufacturing windsurf boards by rotational molding such a 
composition is not suitable, because the oxidized polyethylene reduces the 
resistance of the polyethylene composition against atmospheric influences 
and degradation. The decrease of the wheathering resistance is highly 
undesirable for an outer wall, such as the outer skin of a windsurf board, 
which is exposed to atmospheric influences in an outdoor environment. 
Now, it is true that this disadvantage might be combated by incorporating 
more stabilizing agents in the polyethylene composition. However, 
incorporating more such agents is just not practical since the product 
costs will significantly and undesirably escalate. Using compositions 
containing oxidized polyethylene to produce objects in a die or mold by, 
for instance rotational molding, has the additional disadvantage that such 
compositions also adhere to the mold. If non-oxidized polyethylene is 
used, then release agents are a necessity if the polyethylene enveloped 
article is to be released from the mold. Besides, despite the use of 
release agents difficulties will still occur in attempting to remove an 
oxidized-polyethylene-coated-article from a mold. 
Further attempts to solve these drawbacks include polyolefin compositions 
of stabilized and unstabilized polyethylenes wherein the focus is on 
controlling the crystalline melting points of the stabilized and 
unstabilized components. Such compositions do provide some improved 
adhesion to a substrate and improved release from the wall of the mold. 
However, imperfections are not infrequent. For instance, the adhesion of 
the polyolefin wall to a foamed core is sometimes unsatisfactory and the 
release from the mold is often very difficult. It has now been found that 
the results are dependent upon the processing conditions. It is supposed 
that the degree of separation of the stabilized and of the unstabilized 
polyolefin depends on the method and rate of heating of the mold and on 
other processing conditions. 
SUMMARY OF THE INVENTION 
It has now been found that according to the present invention articles can 
be rotationally molded, whose outer and inner surface show different 
properties by employing a polymer powder composition consisting of about 
1% to about 99% by weight of a polymer powder, component A and about 99% 
to about 1% by weight of a polymer powder, component B, wherein the 
average particle size of component A is smaller than the average particle 
size of the component B. 
DETAILED DESCRIPTION OF THE INVENTION 
The compositions according to the invention are particularly suitable for 
rotational molding, and the invention will also be elucidated with 
reference thereto, but the uses are not restricted to rotational molding. 
In rotational molding a quantity of a thermoplastic material is introduced 
in a mold that can rotate and/or oscillate about one or more axes. The 
mold is then heated to above the melting point of the plastic, and slowly 
rotated and/or oscillated to ensure a uniform distribution of the powder 
over the mold surface. It appeared that the present compositions 
consisting of a fine component A and a coarser component B separate on 
rotational molding. The outer surface layer of the molded wall is formed 
essentially from the fine component A, whereas the inner surface layer of 
the molded wall is formed essentially from the coarser component B. 
Analysis of a cross section of the wall shows that from outside to inside 
there is a gradual change from essentially component A to essentially 
component B. 
As elucidated hereinafter, the essence of the invention must be understood 
to consist in the difference in particle size of the two components so 
that separation of the components will occur when the present compositions 
are heated in a mold beyond the melting point of each component. 
It should be understood that the present invention contemplates in a broad 
sense polyolefinic powders having the aforementioned critical features. 
Therefore, the use of polyethylene in the examples is not intended to 
restrict the present invention. For instance by using a finer powdered 
stabilized component and a coarser powdered component, the objects of the 
present invention can also be obtained with other polymers. 
Therefore it should be clear that the present invention also relates to 
polymer compositions other than polyethylene. For reasons of simplicity, 
however, reference is made hereinafter only to polyolefin or polyethylene 
compositions. 
The polyolefin compositions according to the invention are preferably 
polyethylene compositions. In addition, polyethylene compositions with 
polypropylene or polypropylene compositions are eligible. Commercially, 
other than the polyolefins, only the polyisobutenes are of importance. 
Often these elastomers are marketed in a modified form and have their 
principal uses in other fields. Furthermore, limited quantities of 
polybutene and poly-4-methylpentene-1 are marketed. These polymers also 
come within the scope of the invention. In addition to homopolymers, many 
copolymers are produced and may also be incorporated in compositions 
according to the present invention. It will be understood that the aims of 
the invention can be realized with a finer component A and a coarser 
component B consisting of any other thermoplastic resin. One of the 
components may even be a thermosetting resin. 
When the finer component A and the coarser component B are selected from 
two different polymers it is advantageous when component A has the lowest 
melting point. When the melting points of component A and component B 
differ only slightly, say less than 5.degree. centigrade, said condition 
becomes less serious, and one may succeed in realizing the effect of the 
invention when component A has a slightly higher melting point than 
component B. However it goes without saying that component A has 
preferably a lower melting point than component B. Different polymers will 
have different melting points. However closely related polymers e.g. 
copolymers showing only different ratio's of the same comonomers have 
usually different melting points. Ethylene homopolymers have generally 
higher melting points than ethylene copolymers, the melting point being 
the lower the higher the amount of comonomer in the polymer. The invention 
will now be further elucidated for polyethylene, but as the aforementioned 
shows, it will be clear that the invention is not limited thereto. 
Polyethylene is generally marketed in the form of pellets. For uses like 
rotational molding, however, it must be in granular form. Generally the 
particle size of rotational molding powders is smaller than 2 mm and 
preferably smaller than 1 mm. More specifically, the average particle size 
ranges between about 0.5 and about 1 mm, though It may even be somewhat 
smaller. Commercially, particle sizes smaller than about 0.3 mm are 
generally not used. Rotational molding powders are mostly obtained by 
grinding pellets. It is true that manufacture of polyethylene by a slurry 
process or gas phase process, gives granular ethylene polymers, but the 
morphological and rheological properties of such powders are generally 
poor. Therefore such powders are pelletized and subsequently ground. 
Preferably the present composition consists of a finer component A having 
an average particle size of 0.050 mm to 0.250 mm such that the average 
particle size of the finer component A is at least 0.050 mm smaller than 
that of the coarser component B. This aspect is essential. 
The average particle size is determined by sieve analysis in a manner known 
per se. The results of the sieve analysis is represented graphically in a 
Rosin-Rammler diagram, as described in, for example DIN 66145. The point 
on the line of the Rosin-Rammler curve corresponding with a sieve residue 
of 36.8% gives the average particle size. The slope of the line is a 
measure for the width of the particle size distribution. The particle size 
distribution is narrower as the slope increases. It is characterized by 
the factor of uniformity n. Thus, as the uniformity factor increases, the 
more uniform the powder and the narrower the particle size distribution. 
According to the present invention, the particle size distribution is of 
interest. With a wide particle size distribution in the polyolefin 
composition and a limited difference in particle size between the 
components A and B, the powder fractions at the bottom of the particle 
size distribution of the coarser component B may penetrate into the 
particle size distribution field of the finer component A. 
Though not to be regarded as a binding statement, it is posited that when 
the present compositions are employed in rotational molding the smallest 
particles are the most rapidly heated particles and therefore are the 
first to melt since their mass content per unit surface area is far 
smaller than the mass/area ratio for the coarser particles. The transfer 
of heat is proportional to the surface, but the amount of melting heat is 
proportional to the mass. On this basis it might indeed be assumed that 
the smallest particles are the first to melt. It was extremely surprising, 
however, that in this process a very distinct separation of the coarse and 
fine particles was obtained. As they are the first to melt, the fine 
particles will form on outer layer, i.e. surface-wall against the interior 
surface of the mold wall. Only after that the coarser particles begin to 
melt and will then form the inside part or layer of the wall. 
The processing of polyethylene, particularly of high-density polyethylene, 
takes place at temperatures above 140.degree. C., and for this reason the 
granulate is stabilized against thermal decomposition. The polyethylene is 
also stabilized both against oxidative attack and the influence of light, 
particularly of UV, so that objects made of the polyethylene will have 
good resistance against atmospheric influences. Stabilization against 
oxidative modification is also required to prevent rapid oxidative attack 
when the polymer contacts oxygen, for instance atmospheric oxygen, during 
the processing. After the polymerization of ethylene, small quantities of 
thermal and oxidative modification stabilizing agents are often added to 
protect the polymer while it is being further worked up. Then prior to 
pelletization, further quantities of stabilizing agents are added. If this 
last-mentioned addition is not made, a non-stabilized or hardly stabilized 
polyethylene will be obtained containing in any case less than 0.01% wt, 
more specifically less than 0.005% wt, stabilizing agents. 
If such a non-stabilized or hardly stabilized polyethylene is used for 
making objects in a die or in a mold, for instance by rotational molding, 
oxidative conversions will occur at those places where, at elevated 
temperatures, the polyethylene comes into contact with air. The mold 
mostly contains air, and during the heating of the mold a noticeable 
oxidative conversion of the polyethylene will then take place, such as 
oxidation, with or without chain breakage, cross-linking, and the like. 
Thus the adhesive properties of the polyethylene will improve. 
As a result of the presence of stabilizing agents, there will be no or 
hardly any perceptible oxidation of the stabilized polyethylene under the 
processing conditions. 
It is supposed that when a stabilized and an unstabilized polyethylene is 
used as finer component A with respect to coarser component B, and in the 
rotationally molded hollow form polyurethane is foamed, the adherance of 
the inner surface of the polyethylene form to the polyurethane foam is due 
to oxidized groups in the polyethylene. Such oxidized groups are capable 
of being formed during the processing in the non-stabilized or hardly 
stabilized component. This can easily be demonstrated through infrared 
analysis. For instance, an infrared analysis can be made to establish that 
each component can form a separate layer under the intended processing 
conditions. An infrared spectrum of the non-stabilized or hardly 
stabilized polyethylene will show a clearly perceptible band at 1650-1800 
cm.sup.-1 which is indicative of C.dbd.O bonds. With the stabilized 
polyethylene such a band is not or hardly perceptible. 
An infra-red study of a layer made by rotational molding of a composition 
according to the invention revealed that the presence of C.dbd.O groups on 
the inside of the wall was quite distinct. This is apparent from the 
occurrence of a band at 1650-1800 cm.sup.-1, whereas the outside of the 
wall showed no band at 1650-1800 cm.sup.-1. From this it may be concluded 
that the inside of the wall is oxidized, whereas on the outside of the 
wall the oxidation can be referred to as insignificant at most. Some 
oxidation of the outer surface cannot always be ruled out. In some cases a 
very weak band at 1650-1800 cm.sup.-1 will then be observed. Though some 
oxidation is permissible, it should be limited as far as possible. 
If polyurethane is foamed into a hollow form made by rotationally molding a 
non-stabilized or hardly stabilized polyethylene, the adhesion turns out 
to be so strong that breaking will occur in the foam and not on the 
interface when efforts are made to pull the polyethylene loose from the 
polyurethane foam. A non-stabilized or hardly stabilized polyethylene does 
have good adhesion to the polyurethane foam or to other substrates. 
However a non-stabilized layer will have insufficient resistance to cope 
with atmospheric influences, and objects made thereof will weather in an 
unacceptably short period of time. Such a layer will also undesirably 
adhere to the mold wall. It has been found that, despite the use of 
release agents, it is difficult to release such a layer from the mold. 
Now, good adhesion to the substrate occurs, while the molded object having 
an outer surface consisting essentially of the stabilized component is 
easily released from the mold. The outer surface of the wall is also very 
resistant against weathering. 
In the rotational molding of a composition according to the present 
invention wherein the component A is stabilized and component B is hardly 
stabilized the oxidation must be such as to provide proper adhesion to the 
substrate, but also the oxidation must not be such as to present 
difficulties in mold release and/or problems in the stabilization. This 
must be considered in determining the quantity of unstabilized component. 
Less than 1% wt of the hardly or unstabilized component will barely 
produce any effect and more than 80% wt is also undesirable. Preferably 
said compositions contain about 10% to about 30% by wt unstabilized 
polyolefin and in particular about 10% to about 30% by wt of a 
non-stabilized or hardly stabilized polyethylene. The stabilized component 
of these compositions, is preferably polyethylene which may, if desired, 
be a copolymer, containing minor amounts of at least one other olefin. 
Compositions consisting of a stabilized and an unstabilized component have 
been and are discussed here by way of example. It will be understood that 
there are many other embodiments within the scope of the invention. 
During the processing the above compositions consisting of a fine component 
A that has been stabilized and a coarse component B that is not stabilized 
by, for instance, rotational molding, there may be some slight migration 
of stabilizing agents. At ambient temperature the migration will continue, 
albeit very slowly, so that in course of time a homogenous distribution 
may be brought about. The polyethylene layer is then uniformly stabilized. 
The total quantity of stabilizing agents present in the polyethylene 
composition must indeed suffice to preserve a sufficiently stabilized 
product. The above points must be taken into consideration in the 
preparation of the components of the present composition. 
Stabilized polyethylene generally contains at least 0.01% wt stabilizing 
agents and mostly at least 0.025% wt in all. The quantities of stabilizing 
agents incorporated in polyethylene are determined by the desired 
stability. Generally, more than one stabilizing agent is added to guard 
against different influences. Also many combinations of stabilizing agents 
are accompanied by synergistic effects. 
According to the present invention each component can be also composed of 
two or more polyolefins. For instance, in polyethylene compositions both 
low and high-density polyethylene may be used as starter material, 
however, preference is given to polyethylene having a density of at least 
0.930. 
The polymer components in the present compositions need not necessarily 
differ from each other only in particle size. Different grades of the same 
polymer can be used as well, for instance polyethylene grades having 
different melt indices, but different kinds can be used also, for instance 
an ethylene homopolymer and an ethylene copolymer, low and high-density 
polyethylene, polyethylene and polypropylene, and the like. Such different 
kinds of polymers generally also have different melting points. In 
rotational molding the kind of polymer having the lowest melting point 
will generally be the first to melt. Hence, if for the fine and for the 
coarse component polymers are used having different melting points, the 
fine component should preferably consist of the polymer having the lowest 
melting point. In this regard, patent application Ser. No. 298,253, now 
U.S. Pat. No. 4,440,899 the disclosure of which is incorporated herein by 
reference, describes compositions consisting of a non-stabilized or hardly 
stabilized component having a crystalline melting point which is at least 
1.degree. C. higher than that of the other component, which is stabilized. 
In the present compositions a polymer having a crystalline melting point 
which is at least 1.degree. C. higher than that of the fine component 
could be, if desired, employed as the coarse component. 
The compositions according to the invention and particularly the components 
which they are composed of may contain the usual additives such as 
colorants, fillers, and the like which are well known in the art. 
As explained before the present invention has been described with 
particular reference to the use of a composition of a fine stabilized and 
a coarse unstabilized component for the manufacture of hollow shapes by 
rotational molding. However, it will be understood that the invention and 
this disclosure encompass other compositions that consist of components of 
differing particle sizes. So one can select as a fine component a polymer 
that has good wheathering properties but is not impervious to organic 
liquids as hydrocarbons, and as a coarse component a polymer that is 
impervious to organic liquids but whose weathering properties can be 
rather poor. Containers for organic liquids can be made from such 
compositions. It is also possible to select one component from polymers 
that are impervious to one group of liquids and the other component from 
polymers that are impervious to another group of liquids such as non-polar 
organic liquids on the one side and water and polar liquids on the other 
side. 
The relative quantities of the components must enable to realize the 
desired properties. Less than 1% by weight of one of the components does 
generally hardly show any effect of said minor component. With amounts 
slightly over 1% the effect is generally rather limited. So the 
compositions contain preferably at least 10% and more preferably at least 
25% by weight of the minor component.

The invention is further elucidated by the following non-limiting examples: 
EXAMPLE I 
Polyethylene powder coloured red by kneading in 0.5 wt % iron oxide red 130 
B was mixed with polyethylene powder coloured yellow by kneading in 0.5.% 
wt cadmium yellow 1080. 
The red polyethylene has a melt index (ASTM D-1238 condition E) of 4.5 and 
a density of 0.941. 20% wt of the powder had a particle size of 0.3-0.4 
mm, 70% of the powder had a particle size of 0.4-0.5 mm and 10% of the 
powder had a particle size is larger than 0.5 mm. 
The yellow polyethylene was of the same grade as the red polyethylene. The 
particle size distribution of the powder was: 10% wt smaller than 0.125 
mm, 30% wt from 0.125 to 0.175 mm and 60% wt. from 0.175 to 0.25 mm. 
Mixtures were then prepared having 70 parts by weight red and 30 parts by 
weight yellow polyethylene and having 30 parts by weight red and 70 parts 
by weight yellow polyethylene. 
By rotational molding a hollow object was then made from these mixtures. 
The oven temperature was set at 275.degree. C. The period of rotation was 
16 minutes, the wall thickness of the mold was 3 mm. 
The wall formed was yellow on the outside with only a few hardly 
perceptible red spots. The inside was red with only a few hardly 
perceptible yellow spots. In cutting the wall and observing the cross 
sections it was found that the separation was so good that the differences 
in the composition of the two mixtures could not be seen on the exterior 
surfaces. The cross sections did show that one composition contains far 
more red polyethylene than the other. 
The two components colored with two contrasting colors clearly demonstrated 
the separation of the components and the formation of the desired layers 
during the rotational molding process. 
EXAMPLE II 
Polyethylene powder coloured red by kneading in 0.5 wt % iron oxide red 130 
B was mixed with polyethylene powder coloured yellow by kneading in 0.5 
wt.% cadmium yellow 1080. 
The red polyethylene had a melt index (ASTM D-1238 condition E) of 4.5 and 
a density of 0.941. It contained only 0.004 wt.% 
octadecyl-3-(3.5-di-tert.butyl-4-hydroxyphenyl)propionate. 20 wt.% of the 
powder had a particle size of 0.3-0.4 mm, 70% of the powder had a particle 
size of 0.4-0.5 mm and 10% of the powder had a particle size is larger 
than 0.5 mm. 
The yellow polyethylene was of the same grade as the red polyethylene, but 
was stabilized with 0.25 wt.% 2-hydroxy-4-n-octoxy-benzophenone and 0.05 
wt.% octadecyl-3-(3.5-di-tert.butyl-4-hydroxyphenyl)propionate. The 
particle size of the powder was: 10 wt.% smaller than 0.125 mm, 30 wt.% 
from 0.125 to 0.175 mm and 60 wt.% from 0.175 to 0.25 mm. 
Mixtures were prepared with 70 parts by weight of red and 30 parts by 
weight of yellow polyethylene and with 30 parts by weight of red and 70 
parts by weight of yellow polyethylene. 
By rotational molding a hollow object was made from these mixtures. The 
oven temperature was set at 275.degree. C. The period of rotation was 16 
minutes, and the wall thickness of the mold was 3 millimeters. Release 
from the mold was easy. 
Next, polyurethane was foamed within the hollow object. Subsequently, cross 
sections having a surface area of 5.times.6 cm were cut from the object so 
that blocks of polyurethane foam were obtained which were covered with a 
polyethylene skin on two sides. These blocks were subjected to a tensile 
test with a drawing speed of 1 cm/min. 
The adhesive force of the polyethylene to the polyurethane could not be 
established, because breaking occurs in the polyurethane foam, but was at 
least 0.260 N/mm.sup.2. 
COMATIVE EXAMPLE A 
Example I was repeated, but now both the red polyethylene and the yellow 
polyethylene contains only 0.004 wt.% 
octadecyl-3-(3.5-di-tert.butyl-4-hydroxyphenyl)propionate. 
Release from the mold was difficult. 
The adhesive force of the polyethylene to the polyurethane could not be 
established because breaking occurs in the polyurethane foam, but was at 
least 0.260 N/mm.sup.2. 
COMATIVE EXAMPLE B 
Example II was repeated, but now the particle size and the particle size 
distribution of the red polyethylene were the same as those of the yellow 
polyethylene. 
Upon cutting of the wall and examination of the cross sections it was found 
that no separation of the red and the yellow polyethylene had taken place. 
Release from the mold was difficult. 
The determination of the adhesive force presented problems because the 
polyethylene skin had already started to come off when the polyurethane 
blocks were fixed in the clamps of the tensile strength tester. Upon 
drawing, the polyethylene had already come loose from the foam before a 
measurable value could be measured. The plane of partition was clean, 
without any polyurethane foam adhering to the polyethylene. 
COMATIVE EXAMPLE C 
Example II was repeated, of the yellow, stabilized polyethylene 5 wt.% had 
a powdered particle size &lt;250 .mu.m, 20 wt.% had a particle size of 
250-300 .mu.m, 45 wt.% had a particle size of 300-400 .mu.m, 25 wt.% had a 
particle size of 400-450 .mu.m and 5 wt.% had a particle size of 450-500 
.mu.m. 
Release from the mold was less easy than in Example II. 
The adhesive force of the polyethylene to the polyurethane foam is 0.10 
N/mm.sup.2.