Process for improvement of adhesion between mineral fillers and thermoplastic polymers

The method according to the invention is based upon that the mineral filler, before being mixed with thermoplastic polymer, is coated with a thin layer of liquid ethylene oxide oligomer, optionally in presence of a solvent and then the solvent is evaporated at temperature 50.degree. to 200.degree. C. By the method according to the invention, a product with much better useful properties is obtained. The product is useful in the plastics industry.

The subject of the invention is a method for improvement of adhesion 
between granular mineral fillers and thermoplastic polymers. 
BACKGROUND OF THE INVENTION 
There is known the introduction of fillers to thermoplastic polymers in 
order to improve the useful properties but with improved useful 
properties, the processing properties deteriorate and vice versa. 
In the case of polyolefins the improvement of mechanical properties is 
achieved with great difficulty because of very low physical adhesion and 
lack of chemical adhesion between the polyolefin and the mineral filler. 
There are also known methods for improvement of adhesion, relying upon the 
change of physico-chemical properties of the grains of the filler. The 
simple method of preparation is to give "acidity" to the filler surface by 
coating the grains with aluminum or magnesium silicate compounds. See, J. 
Hodgkin, D. Solomon, J. Macromol. Sci. A8 (3) 635 (1974); D. Solomon - BP 
1228538 (1969). 
Also, from U.S. Pat. No. 4,186,123 (Kietzman) is known a process for 
coating fibrous minerals, such as asbestos, used as fillers for polymers. 
There is also known a method for surface preparation of the fillers by 
organotitanates. See, Plast.Tech. 22 (4) 71 (1976), Plast.Tech. 22 (4) 81 
(1976). For the fillers provided for the polyolefins, coatings made of 
triisopropylenoxytitanate are applied in a quantity of 0.5 to 3% w/w of 
the filler. 
Among the most popular organic coating substances may be ranked stearic 
acid, barium, calcium, sodium stearates and their compositions. See, 
Plast. Tech. 22 (4) 71 (1976), Plast.Tech. 22 (4) 81 (1976), Rev. Plast. 
Mod. 223, 8 (1975). The known coating media for mineral fillers are 
silanes, described in the publication Jap. Plastic Age, Sep.-Oct. 33 
(1975), Dow Corning Corp. (8-5-70), US-061505, Union Corp. (5-17-68) 
US-862027. 
Silanes of the general chemical formula R'Si(OR).sub.3 possess two types of 
functional groups, R' and OR. R' is usually a reactive organic 
group-amine, vinyl, epoxide, methacrylate, bonded to a silicon atom by 
short aliphatic chains but OR is a hydrolysable ether group. 
Through the OR groups, silanes are bound to the surface of the filler, 
whereas the functional R' groups react with the polymeric matrix. The 
known method for preparation of the filler surface is coating with a layer 
of polymer through polymerization of reactive monomer according to either 
a radical or a cation mechanism. Monomers such as styrene, pyridine, 
divinylobenzene, acrylic acid etc. are used. See, Jap. Plastic Age, 
Sep.-Oct. 33 (1975), J. Macromol. Sci A8 (3) 649 (1974), Asaki Chem. Ind. 
Co. Ltd. 29 (1967) JA 069210 US Polywood Champion Papers Inc. (8-21-70) US 
066107. 
Polymer coatings of the molecular weight of 500-800 make up to 3% of the 
filler weight and their thickness ranges from 20 to 30 .ANG.. 
The polymer coating may be applied on the surface of the filler in the 
separate process of preparation or in the presence of the polyolefin 
during mixing. In this last case, the amount of the catalyzer is used so 
as to initiate polymerization but not to cause cross-linking of the 
polyolefin. 
SUMMARY OF THE INVENTION 
According to the present invention, the method for improvement of adhesion 
relies upon that a granular mineral filler is coated by a thin layer of 
liquid ethylene oxide oligomer of molecular weight M.sub.w =100-800, 
optionally in the presence of a solvent. 
The thus prepared mass is dried at a temperature of 50.degree. to 
200.degree. C. During the processing, even at a temperature of 250.degree. 
C., ethylene oxide oligomer is not subject to decomposition and still 
remains liquid. 
Ethylene oxide oligomer is characterized by very good wettability of a 
range of mineral fillers that are granular such as talc, silica, chalk, 
kaolin, and quartz. Liquid state of the coating is especially preferable 
because of the possibility of reproducing the adhesive bonds after their 
breaking as a result of the application of extensive force. Also fatigue 
tests revealed considerable improvement of the properties of low-density 
polyethylene filled with the filler prepared by the method according to 
the invention. Applied optimal contents of the coating is from 1 to 10% 
w/w of the filler share in the mixture, depending on its particle size 
distribution. Simultaneously there exists the possibility of adjusting the 
mechanical properties of the material by changing the quantity of liquid 
ethylene oxide oligomer addition. The coating made of ethylene oxide 
oligomer is especially useful with regard to its low costs, availability 
and the uncomplicated procedure of coating the grains of the filler.

The invention is more particularly described in the examples given below: 
EXAMPLE I 
2 g of ethylene oxide oligomer of an average molecular weight equal to 200 
is dissolved in 30 g of water and 100 g of kaolin is added with stirring 
till a thick mass is obtained. Then, the mass is dried in a dryer at a 
temperature of 80.degree. C. The thus prepared kaolin is mixed with 
polyethylene of a density of 0.92 g/ccm with a ratio of 6:4 and is 
granulated. A product of considerably improved mechanical properties is 
obtained, compared to the case of using non-prepared kaolin for filling. 
The results are shown below: 
______________________________________ 
Product obtained 
Product obtained 
according to 
according to 
Example 1 without 
Example 1 the oligomer addition 
______________________________________ 
modulus of elasticity 
107 115 
(N/m.sup.2 .times. 10.sup.6) 
tensile strength 
7.7 8.4 
(N/m.sup.2 .times. 10.sup.6) 
elongation at fracture 
75 30 
(%) 
impact strength 
106 63.8 
(J/m.sup.2 .times. 10.sup.2) 
______________________________________ 
EXAMPLE II 
The product is prepared as in Example I, substituting chalk for kaolin and 
using 10 g of ethylene oxide oligomer. The thus prepared chalk is mixed in 
a ratio of 5:5 with polyethylene of a density of 0.92 g/ccm. The material 
obtained possesses much better mechanical properties than in the case of 
using the non-prepared chalk. 
______________________________________ 
Product obtained 
Product obtained 
according to 
according to 
Example II without 
Example II the oligomer addition 
______________________________________ 
modulus of elasticity 
74.2 156 
(N/m.sup.2 .times. 10.sup.6) 
tensile strength 
6.5 7.6 
(N/m.sup.2 .times. 10.sup.6) 
elongation at fracture 
215 20 
(%) 
impact strength 
222.8 57.9 
(J/m.sup.2 .times. 10.sup.2) 
______________________________________ 
EXAMPLE III 
The product is prepared as in Example I, substituting quartz for kaolin and 
using 1 g of ethylene oxide oligomer. The material obtained possessed much 
better mechanical properties than in the case of using non-prepared quartz 
flour. 
______________________________________ 
Product obtained 
Product obtained 
according to 
according to 
Example III without 
Example III 
the oligomer addition 
______________________________________ 
modulus of elasticity 
90.2 135.4 
(N/m.sup.2 .times. 10.sup.6) 
tensile strength 
7.1 7.4 
(N/m.sup.2 .times. 10.sup.6) 
elongation at fracture 
90 50 
(%) 
impact strength 
139.3 92.2 
(J/m.sup.2 .times. 10.sup.2) 
______________________________________ 
The mixtures with non-modified fillers show large modulus of elasticity and 
low elongation at fracture values. The materials are brittle and 
non-ductile. 
The introduction of modifying agents like in Examples I, II and III causes 
the decrease of the modulus of elasticity, practically unchanged tensile 
strength and considerably increased elongation at fracture values. High 
values of marked impact strength (measure of brittleness) and high 
elongation at fracture are the result of the considerable increase in 
adhesion between the filler and the polyolefin as the result of the 
introduction of the modifying agent. The considerable increase of 
elongation at fracture causes the materials obtained from the modified 
fillers to be ductile and elastic. 
EXAMPLE IV 
The product is prepared as in Example I, substituting isotactic 
polypropylene of a density of 0.885 g/ccm for polyethylene and chalk for 
kaolin, and using 10 g of ethylene oxide oligomer. The material obtained 
has much better mechanical properties than in the methods using 
non-prepared chalk and non-filled polypropylene. This is seen from the 
data given below: 
______________________________________ 
weight ratio 5:5 
1 2 3 4 
______________________________________ 
PP+ chalk 940 16.8 90 48.5 
PP+ chalk + 570 15.8 420 79.7 
+10% oligomer 
PP 480 22.5 850 55.0 
______________________________________ 
where 
1 is the modulus of elasticity 
2--stress at yield point in N/m.sup.2 .times.10.sup.6 
3--elongation at fracture in % 
4--impact strength in J/m.sup.2 10.sup.2 
EXAMPLE V 
The product is prepared as in Example IV by mixing chalk prepared by the 
method according to the invention in a weight ratio of 6:4 with isotactic 
polypropylene of a density of 0.885 g/ccm. The material obtained has 
considerably better mechanical properties than in the case of non-prepared 
chalk and non-filled polypropylene. This is seen from the data given 
below: 
______________________________________ 
weight ratio 6:4 
1 2 3 4 
______________________________________ 
PP+ chalk 1200 16.9 30 31.4 
PP+ chalk + 550 11.8 328 63.0 
+10% oligomer 
PP 480 22.5 850 55.0 
______________________________________ 
where 1, 2, 3 and 4 are the same as in the table in Example IV.